Catalytic cracking of hydrocarbons over modified ZSM-5 zeolites to produce light olefins A review

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Applied Catalysis A: General 398 (2011) 1–17

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Applied Catalysis A: General

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atalytic cracking of hydrocarbons over modified ZSM-5 zeolites to produce lightlefins: A review

azi Rahimi, Ramin Karimzadeh ∗

epartment of Chemical Engineering, Tarbiat Modares University (TMU), Jalal Al Ahmad Highway, P.O. Box 14155-4838, Tehran, Iran

r t i c l e i n f o

rticle history:eceived 11 October 2010eceived in revised form 17 February 2011ccepted 5 March 2011vailable online 11 March 2011

a b s t r a c t

Steam cracking of hydrocarbons has been the major source of light olefins for more than half a century.The recent studies have reported that ethylene and propylene can also be produced through the crackingof hydrocarbons over modified ZSM-5 zeolites in a considerable amount.

This paper highlights the important current ideas about acid-catalyzed hydrocarbon cracking that hasresulted in high yield of ethylene and propylene. Light olefin production via catalytic cracking of various

eywords:ight olefinsatalytic crackingSM-5lement modificationewis/Brønsted acid sites

industrial feedstocks, ranging from heavy hydrocarbons to ethane, over modified ZSM-5 zeolites, hasbeen reviewed in the present paper. Furthermore, the influence of various employed promoters, i.e.,alkali and alkaline earth, transition, rare earth elements, and phosphorus, on the chemical properties ofthe modified ZSM-5 and the performance of resulting catalyst in enhancing the selectivity to light olefins,have been addressed. Moreover, the influences of different factors, including the zeolite acidity, Si/Al ratioand the temperature, on the light olefin production and the reaction scheme have been specified. The
role of incorporated element in the catalytic cracking mechanism is also summarized.
© 2011 Elsevier B.V. All rights reserved.

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22. Summary of historical aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23. Why zeolites? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34. Producing light olefins via catalytic cracking of various hydrocarbons over modified ZSM-5 zeolites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

4.1. Heavy feedstocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34.2. Naphtha range feedstocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

4.2.1. C8–C6 alkanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54.3. C4 alkanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4.3.1. Alkaline earth metal modified ZSM-5 zeolites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64.3.2. Transition metal modified ZSM-5 zeolites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64.3.3. Rare earth (RE) element modified ZSM-5 zeolites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64.3.4. Phosphorus modified ZSM-5 zeolites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64.3.5. HZSM-5 zeolites with different Si/Al ratios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4.4. Hydrocarbons lighter than C4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84.5. Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

5. Effect of element modification on the performance of ZSM-5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95.1. Acidity of zeolites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.2. Alkali and alkaline earth metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.3. Transition metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.4. Rare earth elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.5. Phosphorus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
∗ Corresponding author. Tel.: +98 21 82883315; fax: +98 21 88006544.E-mail address: [email protected] (R. Karimzadeh).

926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.apcata.2011.03.009

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2 N. Rahimi, R. Karimzadeh / Applied Catalysis A: General 398 (2011) 1–17

6. Mechanism of catalytic cracking over zeolites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136.1. The factors determining the dominant mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136.2. The role of incorporated element in the catalytic cracking mechanism. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

7. Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148. Future prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

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References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. Introduction

Steam cracking of hydrocarbons has been the major source ofight olefins and diolefins for more than half a century. The yieldf ethylene and propylene varies between 24–55% and 1.5–18%,espectively depending greatly on the feedstock type and oper-ting condition [1–4]. It is the first energy-consuming process inetrochemical industry in spite of the improvements that haveeen made in this process. It requires high reaction temperature00–880 ◦C that accounts for 40% of the total energy consum-

ng every year in the entire petrochemical industry and resultsn high amount of CO2 emission [5–7]. The development of tech-ologies that not only maximize energy and resource-savings butlso minimize CO2 emissions is stimulated by global environ-ental issues. Moreover, the additional disadvantages of steam

racking are the limited control over the P/E (propylene to ethy-ene) ratios in the olefins coming from steam crackers and itsirect dependence on the feed type, whilst the demand for propy-

ene is growing faster than that for ethylene [3,8–10]. Hence,he processes capable of controlling the ratio of light olefinsre preferred [3,8–10]. The catalytic cracking of hydrocarbonsas attracted researchers’ attention to overcome the mentionedisadvantages.

The catalytic cracking of various types of hydrocarbons haveeen investigated over the modified HZSM-5 zeolites in order tonhance light olefin production. The reaction occurs at 550–650 ◦Chat is about 200 ◦C lower than steam cracking, and the yields ofthylene and propylene are high enough to compete with the steamracking products. It is noteworthy that the effect of feed type onhe light olefins ratio is not as prominent as the steam crackingnd the ratios of P/E could be controlled by adjusting the acid type,cid strength, and acid distribution, i.e. Lewis/Brønsted (L/B) acidites, as well as the operating condition. Moreover, researchers havettempted to establish the fundamental technologies for catalystesign and preparation in order to commercialize the industrialrocesses in this field.

In the present paper, light olefin production via catalyticracking of various industrial types of hydrocarbons, i.e., heavyeedstocks, naphtha range, and gaseous feedstocks, over modifiedSM-5 zeolites has been reviewed; furthermore, the latest activitiesccomplished in this regard are highlighted. The review has merelyocused on ZSM-5 type zeolite because the promising results of lightlefin production have been mostly achieved over this pentasil typetructure. In other words, this paper does not cover all the differ-nt catalytic cracking researches carried out using various types ofydrocarbons and/or zeolite structures.

In addition, the function of various employed promoters on thehemical properties, and the performance of the resulting catalystn enhancing the selectivity to light olefins have been addressed.

oreover, the influence of different factors such as, the zeolitecidity, the Si/Al ratio and the operating condition on the reactioncheme has been specified. It also summarizes some relevant ideas
egarding the role of incorporated elements in the catalytic crackingechanism in order to shed light on this process. It is notewor-
hy that the catalytic dehydrogenation processes are not includedn this paper and it also excludes cycloalkanes, unsaturated andromatic hydrocarbons.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2. Summary of historical aspects

Great effort has been dedicated to the researches on developinga novel process that can overcome the deficiencies of steam crack-ing. Different types of catalysts (basic catalysts, transition metaloxide catalysts and acidic catalysts) have been studied in the cat-alytic cracking of hydrocarbons to achieve higher yields of lightolefins [3,5,11].

The use of metal oxides such as CaO, A12O3, SrO, MgO, TiO2,MnO2, Mn2O3, ZrO2, K2O, and In2O3, in olefin production has beeninvestigated since 1960. An ethylene yield of 24–40% and a propy-lene yield of 15–22% have been obtained over metal oxide catalystsdepending on the feed type and the reaction condition [11–17].The calcium aluminate (12CaO–7Al2O3) loaded with K2CO3, andpotassium vanadate (KVO3) on alumina creates the most promisingresult. Compared with the conventional steam cracking, the yieldof ethylene from naphtha was enhanced by 5–10%, at 770–820 ◦C[11,13,14,18].

In spite of using the catalyst, the reaction temperature is notreduced considerably, and the higher yield of ethylene is notprobably a result of catalytic ethylene production. However, itis due to the improved heat transfer because of surface/volumeeffect [18,19] and the enhanced rate of coke and heavy hydro-carbons gasification reactions, which produce high amount ofCO and CO2 [11,13,20] about 5–20% [3]. This concept (catalyticreforming of gasified coke) have been also applied in combina-tion with steam cracking as coatings on the wall of pyrolysisreactors [21,22], e.g. Linde/Veba’s thermocatalytic PYROCAT pro-cess [23] and/or tube inserts [20,24–27], in order to lessen thetendency of carbon to deposit on metal surface and increaselight olefin production. The processes of VNIIOS, Toyo engi-neering, and LG petrochemical have been developed based onthe metal oxide catalysts [11], but the catalyst deactivation atelevated reaction temperatures, great amount of steam consump-tion [3], and high rate of coke formation are critical issuesthat have decreased the energy efficiency of these processes[11].

The most active and widely investigated catalysts for hydro-carbon cracking are solid proton-donor acids, including zeolites,which are the key components of industrial petroleum crackingcatalysts [28]. Light olefin production through catalytic crackingof hydrocarbons using zeolites was firstly studied by Pop et al.over bifunctional Ag/Cu/Co–mordenite–Al2O3, at 600–750 ◦C [29].They obtained ethylene and propylene yields of 8–41% and 11–17%,respectively, in the catalytic cracking of raw materials from n-butane to oil cuts (boiling point >500 ◦C) [29]. Early studies onzeolitic catalysts resulted in low ethylene (15–27%) but high propy-lene (15–50%) and aromatic (11–34%) yields, at lower temperaturescompared with the conventional steam cracking process [3].

To our knowledge, few reports exist concerning the catalyticcracking of hydrocarbons over zeolites with the aim of light olefinproduction before 1990. Less attention has been dedicated to cat-
alytic production of light olefins specifically ethylene, over zeolitesbecause of the fully developed industrial steam cracking process[30]. The recent researchers (after 1990) have focused on formu-lating a catalyst that could improve the selectivity to both ethyleneand propylene in catalytic cracking of hydrocarbons.

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. Why zeolites?

Zeolitic materials are being extensively used in three major cat-lytic reactions in the area of petroleum refining, petrochemicalsnd pollution control [31]. Zeolites, e.g. ZSM-5, are active catalystsnd/or supports for a variety of reactions such as cracking, alkyla-ion, aromatization, isomerization of hydrocarbons, etc., owing toheir activity, shape selectivity [30,32–36], ion-exchanging prop-rties and special pore structure, such as the tri-dimensionalicro-pore topology and large specific surface area in ZSM-5

37–42]. Zeolites have not yet been employed for on-purpose olefinroduction through catalytic cracking of hydrocarbons because ofhe existing well-established industrial steam cracking process;evertheless, they play an important role in the olefin indus-ry, especially in the processes that are under development ormprovement, in order to meet the requirements for more efficientechnologies [30].

. Producing light olefins via catalytic cracking of variousydrocarbons over modified ZSM-5 zeolites

The production of light olefins through catalytic cracking of dif-erent types of industrial feeds, i.e. heavy feedstocks, naphtha range,nd gaseous feedstocks, are reported in the following sections.

.1. Heavy feedstocks

Steam cracking is still the most reliable and efficient commercialrocess for the production of light olefins, specifically ethylene [43].he primary sources of propylene have been as a byproduct fromteam cracking and refinery fluid catalytic cracking (FCC) units. Theonventional FCC technology results in the yield of propylene andthylene about 3–6 wt.% and 1–2 wt.%, respectively [11,30]. Cur-ently, about 30% of the world’s propylene is supplied by refineryCC operations, 60–65% is co-produced from steam cracking, andhe remaining is produced on-purpose using metathesis or propaneehydrogenation processes [9,43,44]. The projected growth ratef propylene demand is estimated at 4–5%/year [44]. At present,he FCC modified processes (FCC family techniques) accounts forne third of the total propylene supply; thus, it is indispensableo optimize or improve propylene production in these processes2,10,43,45–48].

The addition of ZSM-5 to an FCC catalyst is one of the effi-ient methods for improving the yield of light olefins [49,50]ecause it provides refiners with a high degree of flexibility to opti-ize the production output [51,52]. Utilizing HZSM-5 as a hybrid

atalyst, results in minimizing hydrogen transfer reactions, so itinders bimolecular reactions; consequently, enhances crackingver hydrogen transfer due to its shape selective sinusoidal poretructure [52,53]. Moreover, its acidity significantly influences onelectivity, given that high acidity enhances the cracking of higholecular weight olefins into the gasoline range, whereas low acid-

ty enhances the isomerization of olefins, especially those of 4–6 Ctoms [52]. When ZSM-5 is added to the FCC unit, the main reac-ions it catalyzes are C+

5 olefin isomerization and cracking; thus,he yield of C−

5 increases while that of C+7 olefins and paraffins

ecreases. The high C=3 /C=

4 ratio is observed over ZSM-5 additivesith enhanced activity [54].

The FCC family technologies, such as MAXOFIN developed byellogg Brown & Root, Inc. (KBR) and Mobil Technology Com-
any [9,55], UOP’s PetroFCC [56], UOP’s Fluidized Light Olefinatalytic Cracking (FLOCC), ABB Lummus’ Selective Catalytic Crack-
ng (SCC), RIPP/Sinopec’s Deep Catalytic Cracking (DCCI & II) [10],IPP/Sinopec’s and Stone & Webster’s Catalytic Pyrolysis ProcessCPP) [57–59], and Grace Davison’s HPFCC (High Propylene Fluid

lysis A: General 398 (2011) 1–17 3

Catalytic Cracking) [60], have been commercialized and industrialplants are established based on the mentioned technologies. Vac-uum gas oil (VGO), VGO blended with residual oil, cocker gas oil,deasphalted oil, and atmospheric residue are among the industrialfeedstocks used in these technologies [2,11,45,61,62]. The yield ofethylene in the first five mentioned processes is typically less than6 wt.% [11] because of the fast aromatization reactions, but they canyield up to 15–25 wt.% propylene [2,11,48,59], while in the formerones the combined yield of ethylene and propylene above 40% hasbeen obtained [11,57,58].

4.2. Naphtha range feedstocks

Naphtha is currently one of the dominant feedstocks usedworldwide in steam cracking [2]. In a typical naphtha cracker,ethylene and propylene yield varies in the range of 25–32.4% and15.3–16.5%, respectively, with regards to the operating condition[1]. It is one of the most attractive feedstocks for investigating theproduction of light olefins via catalytic cracking because it behavesas an intermediate product of heavy oil catalytic cracking, and itsstudy could also provide further understandings of the FCC pro-cess [63,64]. It can also take advantage of the present facilities thatare used for steam cracking [63]. The properties of typical naphthafeedstocks are presented in Table 1 [3].

Yoshimura et al. conducted a comprehensive research study onthe catalytic cracking of naphtha to produce light olefins underthe national project called “Development of a next-generationchemical process technology” that was undertaken from 1995 to1999, in Japan. They investigated oxidative cracking over basiccatalysts (aerobic reaction), oxidative and non-oxidative crackingover transition metal catalysts (aerobic/non-aerobic reaction) andcatalytic cracking over solid acid catalysts (non-aerobic reaction),and achieved promising results over a newly developed zeolite-based La2O3-P/ZSM-5 (Si/A1 = 200) catalyst. The developed catalystcould create both a sufficient stability and high yield of ethylene(34 wt.%) and propylene (23 wt.%), at 650 ◦C (in the presence ofsteam), which is about 10% higher than the conventional steamcracking process operated at around 820 ◦C [3]. Furthermore, thefeasibility study for naphtha processing capacity of 3000 tons/dayindicated that the fixed-bed type catalytic cracking unit can reduceenergy consumption by more than 20% per ton of ethylene pluspropylene and hence decrease CO2 emission by approximately20% [3].

It was observed that the La loading (Section 5.4) was apparentlythe major cause of the lower aromatics formation [3] which wasdifferent from the results obtained by Wei et al. to some extent.They observed that over La/ZSM-5 (Si/A1 = 72), more than 50% ofnaphtha was transferred to BTX indicating that hydrogen transferis more prominent. While, the lower yield of BTX and the higheryields of light olefins were achieved over the other catalysts mod-ified with the incorporation of P specially and the other elements:Mg, Ca, and Cu (Table 2). The modification of ZSM-5 zeolites withCo, Ni, Fe and Zn suppressed the BTX generation, although theyield of ethylene and propylene also decreased to some extent [5].Nevertheless, they also achieved the best results over P-La/ZSM-5 (Si/A1 = 72) catalyst with C=

2 + C=3 yield of about 57–60%, at the

beginning 10 h. The test of stability (at various times on stream)indicated that over P-La/ZSM-5, the C=

2 + C=3 yield was kept higher

than 50% for 30 h, while the yield of 47% at the beginning 1 h of thereaction decreased to 31%, over 10 h on HZSM-5 [5]. The catalyticcracking reactions under the steam condition (without N2 stream)
compared to the results without steam, produced more light olefinsover the sample of P/, Mg/, Ca/, Sr/, Cu/ZSM-5, especially for P/ZSM-5 (C=
2 + C=3 = 57%); furthermore, the BTX yield decreased greatly.

This suggested that the associated steam improves the yield of lightolefins by suppressing the generation of BTX and light paraffins (i.e.


Table 1Properties of typical naphtha feedstocks.

Light naphtha Naphtha Heavy naphtha

Specific gravity (15/4 ◦C) 0.643 – –Av. molecular weight (g/mol) 74.6 – 106Sulfur (wt ppm) 30 – –

Carbon number (wt.%)C4 7.5 – –C5 59.8 – –C6 30.6 – –C7 2.1 – –

Composition (wt.%)n-Paraffins 53.1 30.82 22.0Isoparaffins 40.3 20.63 32.2Naphthenes 5.5 33.79 19.8Aromatics 1.1 3.61 13.6

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Olefins 0.1Others 0Hydrogen content (wt.%) 16.4Ref. [3]

ethane, ethane, propane), which are thought to be generated fromydrogen transfer [5].

The synergistic effect of lanthanum and phosphorous isoncluded to be prominent to obtain promising results over P-a/ZSM-5 for light olefin production. Adopting both the rare earthnd phosphorus to modify the zeolite might result in the formationf RE–P–O oxides that precipitate on the outer surface of zeolitend cover some parts of surface acid sites [65]. Lanthanum alsotabilizes P species in the zeolite from leaching in steam at highemperatures. The modification of zeolite with rare earth metalnd phosphorus altogether, reduces the surface acidity owing tohe interaction of rare earth cation and phosphate anion [65,66].he distinct influences of rare earth elements and phosphorus onSM-5 zeolites are reported in Sections 5.4 and 5.5 of this paper,espectively.

The fluorinated HZSM-5 zeolites (F/HZSM-5) could be an effec-ive catalyst for the catalytic cracking of naphtha to light olefins. The
-Modified HZSM-5 samples were prepared by immersing HZSM-(Si/Al = 46) zeolite in an aqueous solution of NH4F. Feng. et al.
bserved that the maximum propylene and ethylene yields reached6.4% and 20.2%, respectively, at 600 ◦C (over 0.1F/HZSM-5 which

able 2atalytic cracking of naphtha feedstock over modified ZSM-5 zeolites to produce light ole

Zeolite Yield (wt.%) Temp. (◦C)

C=2 C=

3 C=4 BTX

HZSM-5 34 22 NA 8 65010%La/ZSM-S 38 24 NA 3 6502%P-10%La/ZSM-5 32 30 NA 2.5 650HZSM-5 23 22 8 23 650P-La/ZSM-5 28 32 NA NA 650P-La/ZSM-5 27 23 NA NA 650Mg/ZSM-5 18 19.5 11 11.5 650Ca/ZSM-5 10 25 13.5 13.5 650Sr/ZSM-5 18.5 26.5 12.5 11.5 650La/ZSM-5 22 20.5 6.5 26 650Cu/ZSM-5 20 28 9 15.5 650P/ZSM-5 27 29 7 15 650HZSM-5 22.9 25.6 NA NA 675HZSM-5 19 18.5 NA NA 650HZSM-5 15 27 8.9 25.5 600F/HZSM-5 20.2 36 7.3 21.8 600F/HZSM-5 27 33.8 3.5 23.9 650

A: Not available.3]: Light naphtha/steam/nitrogen (m3/min) = 2.0/5.6/13.2, steam/feed = 0.64 (wt.%), W/F5]: Ion-exchange, steam to naphtha ratio = 0.7–1.4, residence time (s) = 1.8.7]: Naphtha/steam (w/w) = 2, WHSV = 2 h−1.64]: TOS = 10 min.

8.39 11.52.76 0.9– –[5] [7]

possessed the largest Brønsted acid amount) that were 7.3 and 4.3%higher than those over the parent HZSM-5 zeolite [63]. The FT-IRof pyridine adsorption on F/HZSM-5 revealed that the modificationwith fluorine remarkably enhances the acid amount of HZSM-5.The BET surface area and the pore volume of F/HZSM-5 increased,mainly due to increase in the micropores. The fluorine modifica-tion not only regulates the pore characteristics of HZSM-5 zeolitesbut also modulates the amount of acid sites, especially the den-sity of Brønsted acidity which could be favorable for obtaining ahigh conversion of naphtha and high selectivity to light olefins[63]. Mao et al. found that F-ion-exchanged ZSM-5 zeolite shows anenhanced surface acidity because of the formation of new Brønstedacid sites and the strengthening of some acid sites of the parent zeo-lite. They ascribed it to the proton attack of the zeolite surface bythe chemisorbed H+F− ion pair. Under these preparation conditions,the zeolite framework structure is fully preserved [67].

Han et al. carried out the catalytic cracking of heavy naph-
tha for the selective formation of light olefins over various typesof zeolites, i.e. HZSM-5 (Si/Al = 20, 25, 40, and 75), H-mordenite(Si/A1 = 12.5), H-beta (Si/Al = 150), and SAPO-11. The yields of C=
2 +C=

3 over H-mordenite, H-beta, and SAPO-11, were 11.2%, 15.2%, and

fins in a fixed bed reactor.

Catalyst (cm3) N2 (cm3/min) Feed (cm3/min) Si/Al Ref.

NA 5.6 2.0 200 [3]NA 5.6 2.0 200 [3]NA 5.6 2.0 200 [3]

4 NA 8 72 [5]4 NA 8 72 [5]4 NA 8 45 [5]4 NA 8 72 [5]4 NA 8 72 [5]4 NA 8 72 [5]4 NA 8 72 [5]4 NA 8 72 [5]4 NA 8 72 [5]

NA NA NA 20 [7]NA NA NA 20 [7]

750 (mg) 400 0.3 46 [64]750 400 0.3 46 [64]750 400 0.3 46 [64]

(weight of catalysts (kg)/volumetric feed rate (m3/s) = 1.44 g s/m3.

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3.3%, respectively, while that over HZSM-5 (20) was 37.5% (naph-ha/steam (w/w) = 2, weight hourly space velocity (WHSV) = 5 h−1,= 650 ◦C). Among the various types of catalysts, HZSM-5 was found

o be the most effective catalyst for light olefin production. Theybtained the maximum yield of C=

2 + C=3 about 48.5 wt.% (C=

2 /C=3 =

.90) over HZSM-5 with the lowest Si/Al = 20 ratio [7]. The propy-ene can be produced selectively through the catalytic cracking ofeavy naphtha over HZSM-5 without the loss of total olefin yield7].

Hayim Abrevaya investigated the cracking of naphtha rangelkanes and naphthenes over a series of 8–12-MR (member ring)eolitic catalysts to verify the relationship between the zeo-ite topology and naphtha cracking activity. The highest yieldsf light olefins were obtained over 10-MR zeolites such aserrierite at 650 ◦C with 83% naphtha conversion and 54 wt.%electivity to the ethylene and propylene. The relative activi-ies and selectivities indicated that the activity ranking for theatalysts at 650 ◦C was: beta (Si/Al = 66), ZSM-22 (Si/Al = 62) > ZSM-3 (Si/Al = 89) > EU-1 (Si/Al = 70) > ZSM-35 (Si/Al = 19), ferrieriteSi/Al = 71) > Y (Si/Al = 81) [68]. The C=

2 + C=3 selectivity ranking for

he catalysts was: ferrierite > ZSM-22, ZSM-23 > ZSM-35, Y > EU-> beta [68]. 12-MR zeolites favor bimolecular hydride transferathways, which eventually increase the selectivity to light alkanesnd coke at the expense of ethylene and propylene [68].

The fluidized-bed catalytic cracking of naphtha over a ZSM-5-ased catalyst yielded up to 48% ethylene and propylene at 680 ◦C69]. The newly developed Advanced Catalytic Olefins (ACO) pro-ess combines the KBR fluidized catalytic cracking reactor systemith a proprietary catalyst formulated by SK Corporation & Koreaesearch Institute of Chemical Technology (KRICT). The processelectively converts naphtha and the heavier feedstocks such aserosene and LGO [60], to large quantities of C=

2 + C=3 and yields

oughly 65% olefins with P/E ratios up to 1/1 which is about 15–25%ore C=

2 + C=3 than a typical naphtha cracker on a relative basis,

epending on the operating conditions [70]. The ACO demo plantvaluation with 60,000 tons/year capacity is scheduled to be per-ormed by the end of 2010 [60], and the first commercialized ACOrocess is planned to be implemented at the Number 1 Ethylenenit of SK Corporation in Ulsan, by 2012 [70,71]. The process islaimed to reduce energy needs by 20% [72] and initial investmentosts by 30% [71].

The results of light olefin production in catalytic cracking ofaphtha feedstock over modified ZSM-5 zeolites are summarized

n Table 2. All the experiments have been carried out in a fixed bedeactor.

.2.1. C8–C6 alkanesCatalytic cracking of the typical components of naphtha such

s n-octane, n-heptane, n-hexane, methyl hexane, etc., have beenidely investigated over zeolites not only to elucidate the perfor-ance of catalysts and the reaction mechanism but also to create

urther improvement in FCC process [63,73]. Some relevant worksre reported in this subsection.

Yoshimura et al. investigated the reactivity of severalodel compounds in the catalytic cracking over 10%La/ZSM-5

Si/Al = 200). For n-paraffins, an increase in the carbon numberesults in an enhancement in the olefin yield. Furthermore, ashe number of methyl substitutes increased the conversion andlefin yields decreased. This decrease in reactivity is linked to thencreased stability of carbenium ions, i.e., the order of the reactivity

as: n-paraffins > methyl substituted isomers > dimethyl substi-
uted isomers [3].
Jung et al. studied the influence of pore structure andcidity of various zeolite topologies in the catalytic crack-ng of n-octane [74]. The NH3–TPD results depicted that therder of strong acid site numbers is: beta (Si/Al = 13) < faujasite

lysis A: General 398 (2011) 1–17 5

(Si/Al = 2.65) < ferrierite (Si/Al = 8.5) < MCM-22 (Si/Al = 10) < ZSM-5(Si/Al = 25) < mordenite (Si/Al = 10); however, the strength of strongacid sites increases with the order of: beta ≈ faujasite < MCM-22 ≈ ZSM-5 < ferrierite < mordenite. The order of the conversionover zeolite catalysts at 1 h nicely coincides with that of the acidstrength and the amount of strong acid sites, if the faujasite and fer-rierite catalysts were eliminated because of their rapid deactivationand mass transfer restriction (at 500 ◦C). The steric hindrance andthe limitation on mass transfer of reactant and products in ferrieritepores cause the low conversions even though it has a mediumamount of strong acid sites. On the contrary, the conversion of n-octane over faujasite is extremely low due to rapid deactivation ofthe catalyst because of the enough space provided for a severe car-bon deposition in faujasite supercages. One-dimensional pores ofmordenite are easily blocked even with a small amount of carbondeposit (the conversion decreased from 80% to 20% during 4 h reac-tion) [74]. On the other hand, the catalyst life of zeolites is largelydependent on their pore structures. Almost the same conversionsat 1 h and 5 h over ZSM-5 catalyst indicate a negligible deactivation.The ZSM-5 large number of strong acid sites and sinusoidal porescause high and constant catalytic activity that accelerates the crack-ing of n-octane and suppress the formation of large intermediates.The conversion of n-octane is largely dependent on the numberof strong acid sites, while the zeolite pore structure determinesthe rate of catalyst deactivation due to carbon deposit [74]. Fur-thermore, Jung et al. concluded that the acidity is the predominantfactor in determining the conversion level and product composi-tion, so a high conversion and a high yield of alkenes are obtainedover ZSM-5 zeolite with a large amount of strong acid sites [40].They found that by increasing the concentration of employed alkalisolution, the conversion of n-octane sharply decreased. The maxi-mum yield of C=

2 + C=3 was about 37% (80% conversion) [40].

Altwasser et al. verified the influence of zeolite pore size andspatial constraints on the contribution of monomolecular andbimolecular mechanisms in the catalytic cracking of n-octane.A comparison between 8-, 10-, and12-MR zeolites with similarframework aluminum contents, crystal sizes and ammonium ionexchange degrees clearly reveals that with decreasing the poredimensions the relative importance of monomolecular cracking isstrongly increases as compared to that of bimolecular cracking. Theproduct distribution at similar conversions shifted with decreas-ing the pore diameter of the zeolitic catalysts from medium-chain(C4–C6) to short-chain (C1–C3) hydrocarbons [75].

Comparing the catalytic cracking of n-heptane on various typesof zeolites indicates that a large pore HUSY zeolite is three timesmore active for cracking but decays some eight times faster thanHZSM-5 [76]. Theoretical calculations based on estimating the mag-nitude of steric hindrance predicted that hydride transfer wouldbe most restricted in ferrierite and Theta-1; however, the lowerheptane adsorption capacities suggested that ZSM-5 would be thebest compromise in terms of both high activity and selectivity.The results of the simulation indicated that the zeolites with 12-MR pores (beta and mordenite) or with cage structure (chabaziteand erionite) exert a smaller steric influence on the adsorbedmolecules/transition states than zeolites with 10-MR pores (ZSM-5, Theta-1and ferrierite) do [77]. HZSM-5 and H-ferrierite resultin higher alkene selectivity (80% conversion) than H-MCM-22, H-mordenite, H-beta, and HY in the catalytic cracking of n-heptane.The Ca2+ or Ba2+ exchanged HZSM-5 and H-ferrierite gave the high-est selectivity (60%) to alkenes at 70% conversion. The concentrationof Brønsted acid site does not affect the selective alkene formation
but the hindrance of the bimolecular hydride transfer in the Ca2+ orBa2+-exchanged ferrierite enhances the selectivity to alkenes evenat high conversions [78].
The mechanism of catalytic cracking of n-hexane, as a modelcomponent, over zeolites has been widely studied [79–82] but the

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igh yield of light olefins has not been addressed in literature. Theata obtained in the catalytic cracking of n-hexane over K anda impregnated ZSM-5 zeolites indicated that the enhanced smalllefin selectivity can be achieved over K/HZSM-5 and Ba/HZSM-5,nd the formation of aromatics will be reduced compared to HZSM-. C2–C4 olefin selectivities reached 54% at the conversion of 94.4%,t a temperature of 873 K [83]. Furthermore, Wei et al. observed thathe yield of light olefins increased in catalytic cracking of n-hexanever HZSM-5 zeolites ion-exchanged with alkali solution [6]. Theracking of n-hexane catalyzed by HZSM-5 zeolites dealuminatednder mild steaming conditions enhances the yield of H2 comparedith the fresh catalyst. The addition of 1-hexene to n-hexane as a

eed led to an increase of the activity of the fresh zeolite, whereast had just the opposite effect on the activity of dealuminated sam-les [84]. Maia et al. concluded that the bi-functional calcined nickelSM-5 (Si/Al = 25) zeolite catalyst improves the selectivity towardhe light olefins. Nickel was introduced in ZSM-5 zeolite by twoifferent methods, i.e., wetness impregnation and ionic exchange;he later improved the formation of light olefins. The method of

etal introduction influences on n-hexane cracking activity andhe selectivity to light alkenes [85].

Catalytic cracking of n-hexane over a series of calcinated ZSM-12amples elucidated that the higher ratios of L/B acid sites increasehe cracking activity [86]. The catalytic activity of MgAPO-11 wasvaluated for hydrocarbon catalytic cracking using n-hexane. Ithows a moderate activity for n-hexane catalytic cracking conver-ion in the reaction temperature range of 450–550 ◦C [87].

.3. C4 alkanes

C4 alkanes have been widely investigated for light olefin pro-uction as well as kinetic studies over zeolites. They are valuablerobe reactants because they are small and symmetrical enough to

imit the number of possible products while allowing enough differ-nt reaction pathways to be the representative of alkane cracking28,88]. The recent researches depict that catalytic cracking of C4lkanes over the modified ZSM-5 zeolites results in high yield ofight olefins that is reported in this subsection.

.3.1. Alkaline earth metal modified ZSM-5 zeolitesWakui et al. investigated the effect of alkaline earth metal (i.e.

g, Ca, Sr, Ba) modification on the performance of HZSM-5 in theatalytic cracking of n-butane. The zeolites were modified by theddition of carbonate salt (AE(CO3)) of each alkaline earth to thequeous solution used for the synthesis of the zeolite. The higherield of ethylene and propylene was observed by using the alkalinearth modified ZSM-5 zeolites compared with the non-modifiedZSM-5 (Table 3). The favorable results (C=

2 + C=3 � 4.5%) were

btained over Ba/ZSM-5 [89]. The stability of the modified catalystas not reported by the authors [90].

.3.2. Transition metal modified ZSM-5 zeolitesThe performance of Fe/HZSM-5 [91] and Cr/HZSM-5 [92]

eolites in the catalytic cracking of isobutane (for light olefin elab-ration) was firstly investigated by Lu et al. A very high catalyticctivity and selectivity in the catalytic cracking of isobutane wasttained over Fe/HZSM-5 (Si/Al = 32) and Cr/HZSM-5 (Si/Al = 32)atalysts loaded with a trace amount of 0.010 mmol/g Fe [91] and.004 mmol/g of Cr [92], at 625 ◦C (incipient-wetness impregna-ion). The yield of total olefins (C=

2 + C=3 + C=

4 ) was 65.6% and 56.1%ver Fe/and Cr/HZSM-5 zeolites, respectively. The high yields of
thylene and propylene reached 24.6% and 32.4% over Fe/HZSM-5eolites at 625 ◦C (Table 3) [91]. The presence of a small amount ofe or Cr could be favorable for the dehydrogenation of isobutaneo isobutene which will be easily cracked to light olefins [91,92].he selectivity of butene will be enhanced in the dehydrogenative
lysis A: General 398 (2011) 1–17

cracking of C4 alkanes occur over cobalt oxide-impregnated HZSM-5 (5%Co/ZSM-5) [89]. Moreover, the ZSM-5 zeolites (Si/Al = 200)modified with transition metals, such as Cu, Zn, and Ag increasethe yield of aromatics and reduce the C=

2 + C=3 yield in catalytic

cracking of C4 alkanes [3].

4.3.3. Rare earth (RE) element modified ZSM-5 zeolitesThe effects of rare earth on the structure, acidity, and cat-

alytic performance of HZSM-5 zeolite were investigated by Wanget al. with the aim of light olefin production. The performanceof RE (La, Ce, Pr, Nd, Sm, Eu, Gd)/HZSM-5 (Si/A1 = 64.1) catalysts(impregnating with an aqueous solution of RE(NO3)3) was evalu-ated in the catalytic cracking of mixed C4 alkanes [93]. The optimalpropylene yield at 600 ◦C reached 25.9%, 25.2%, and 25.2% overNd/HZSM-5, Ce/HZSM-5, and La/HZSM-5, respectively. The besttotal alkene yield was achieved over Ce/HZSM-5 (57.2 wt.%) at600 ◦C, and Nd/HZSM-5 (60.0 wt.%), at 650 ◦C [93]. The ratio ofP/E differs obviously according to the reaction temperature andthe type of incorporated rare earth element (Table 3). The resultsrevealed that RE-modified HZSM-5 catalyst significantly enhancesthe selectivity to light olefins; thus, the yield of olefins increasesand the yield of aromatics decreases greatly in the catalytic crack-ing of butane. The loading of the rare earth metals in HZSM-5sample promotes the performance of HZSM-5 (Section 5.4) in thecatalytic cracking of butane [93]. Moreover, Wakui et al. reportedan overall C=

2 + C=3 yield of 48 wt.% at 650 ◦C in the catalytic crack-

ing of n-butane over La impregnated HZSM-5 (Si/Al = 200) catalysts[94]. The yield of C=

2 + C=3 rises to 58% at 650 ◦C by suggesting

a dehydrogenation-cracking double-stage reaction with employ-ing a Pt–Sn type dehydrogenation catalyst and a cracking catalyst(RE-loaded HZSM-5) [95]. In the proposed process, n-butane isfirstly dehydrogenated to n-butene (1- and 2-butene) over thePt–Sn/Ca–ZnAl2O4 as a dehydrogenation catalyst (loaded at theupper part of the reactor), then n-butene is successively convertedto ethylene and propylene over 10%Pr/ZSM-5 as a cracking catalyst(loaded at the lower part of the reactor) [95].

4.3.4. Phosphorus modified ZSM-5 zeolitesAlthough phosphorus modification is the most commonly used

method employed to improve HZSM-5 zeolites performance, thecatalytic cracking of C4 alkanes for producing light olefins overP-modified (incipient-wetness impregnation with an aqueous solu-tion of (NH4)2HPO4) HZSM-5 catalyst is seldom reported so far[96]. Jiang et al. investigated the efficiency of P-modified HZSM-5(Si/Al = 25) in the catalytic cracking of mixed C4 alkanes to increasethe yield of ethylene and propylene [96]. At the temperature of650 ◦C, the maximum yields of propylene and ethylene reachedto 25.6 and 33.9 wt.%, which were higher than those over par-ent HZSM-5 by 7 and 4.5 wt.%, respectively [96]. On the contrary,Wakui et al. observed that ethylene and propylene yields dimin-ished over P/ZSM-5 (2 wt.% P-loading) in the catalytic cracking ofn-butane at 650 ◦C; however, the conversion over P/ZSM-5 wasrather stable for 5 h. They observed that the loaded phosphoruscreated a negative effect on the cracking activity (Table 3) [94]. Any-way, the favorable results were achieved over 2%P–10%La/HZSM-5(Si/Al = 200) zeolite prepared by impregnating with the aqueoussolutions of La(CH3COO)3·1.5H2O and (NH4)2HPO4, respectively[94]. The results of light olefins in the catalytic cracking of C4alkanes over modified ZSM-5 zeolites (fixed bed reactor) are sum-marized in Table 3.

4.3.5. HZSM-5 zeolites with different Si/Al ratiosAcidity of the zeolites is largely dependent on their Si/Al molar

ratios and the employed modifying promoters [74]. In general, theextremely high hydrothermal stability is closely related to the highSi/Al ratio of ZSM-5 zeolite [41,69]. The low Al content in ZSM-

N. Rahimi, R. Karimzadeh / Applied Catalysis A: General 398 (2011) 1–17 7

Table 3Catalytic cracking of C4 alkanes over modified ZSM-5 zeolites to produce light olefins.

Zeolite samples Yield (wt.%) Temp. (◦C) Catalyst (g) N2 (cm3/min) Feed(cm3/min)

Si/Al Ref.

C=2 C=

3 C=4 BTX

HZSM-5 15 12 6 20 600 1 4.9 n-C4: 8.2 197 [90]Mg/ZSM-5 25 16 3 16 650 1 11.5 n-C4: 5.5 200 [90]Ca/ZSM-5 20 20 5 7 650 11 4.9 n-C4: 2.7 200 [90]Sr/ZSM-5 12.5 15.5 4 2 650 11 4.9 n-C4: 2.7 206 [90]Ba/ZSM-5 25 20 6 6 650 11 4.9 n-C4: 2.7 189 [90]HZSM-5 21.3 25.1 9.1 14.5 625 200 38 i-C4: 2 32 [91]Fe/ZSM-5 24.6 32.4 8.6 15.8 625 200 38 i-C4: 2 32 [91]Fe/ZSM-5 19 33 11 11 600 200 38 i-C4: 2 32 [91]Fe/HZSM 29.8 21.7 3.4 17.8 600 200 38 i-C4:2 50 [98]Fe/HZSM 28.0 25.0 5.2 14.2 575 200 38 i-C4:2 50 [98]Fe/HZSM 23.3 24.8 6.6 9.8 550 200 38 i-C4:2 80 [98]Cr/HZSM-5 30.8 26 5 19.1 625 200 38 i-C4: 2 32 [92]Cr/HZSM-5 26.2 28.4 7.3 13.8 600 200 38 i-C4: 2 32 [92]Cr/HZSM-5 26.1 26.9 6.7 10.9 550 200 38 i-C4: 2 150 [98]Cr/HZSM-5 30.1 23.5 4.6 19.1 575 200 38 i-C4: 2 150 [98]Cr/HZSM-5 31.9 18.6 2.6 22.2 600 200 38 i-C4: 2 150 [98]Cr/HZSM-5 28.5 23.6 6.7 16.2 575 200 38 i-C4: 2 80 [98]Cr/HZSM-5 31.2 20.0 3.0 19.3 600 200 38 i-C4: 2 80 [98]HZSM-5 29.3 16.3 2.9 23.3 600 300 (i-C4: 50%, n-C4:50%) + N2 = 40 64.1 [93]La/HZSM-5 27.9 23.7 3.8 9.4 650 300 (i-C4: 50%, n-C4:50%) + N2 = 40 64.1 [93]Ce/HZSM-5 27.2 25.2 4.9 4.4 600 300 (i-C4: 50%, n-C4:50%) + N2 = 40 64.1 [93]Ce/HZSM-5 33.8 19.7 2.0 7.9 650 300 (i-C4: 50%, n-C4:50%) + N2 = 40 64.1 [93]Pr/HZSM-5 25.6 22.3 5.4 13.3 600 300 (i-C4: 50%, n-C4:50%) + N2 = 40 64.1 [93]Nd/HZSM-5 21.7 25.9 6.6 9.8 600 300 (i-C4: 50%, n-C4: 50%) + N2 = 40 64.1 [93]Nd/HZSM-5 27.7 25.0 7.2 12.6 650 300 (i-C4: 50%, n-C4: 50%) + N2 = 40 64.1 [93]Sm/HZSM-5 31.5 22.7 2.8 11.1 650 300 (i-C4: 50%, n-C4: 50%) + N2 = 40 64.1 [93]Eu/HZSM-5 26.7 22.0 4.8 14.6 625 300 (i-C4: 50%, n-C4: 50%) + N2 = 40 64.1 [93]Gd/HZSM-5 24.2 19.8 3.2 8.1 625 300 (i-C4: 50%, n-C4: 50%) + N2 = 40 64.1 [93]HZSM-5 27 13 2 21 650 0.5–1 5.7, Steam = 12.5 n-C4: 2.8 200 [94]10%La/ZSM-5 29 19 4 5 650 0.5–1 5.7 Steam = 12.5 n-C4: 2.8 200 [94]ZSM-5 25 14 3.5 20 650 NA 5.7, Steam = 12.5 n-C4: 2.8 200 [3]10%La/ZSM-5 28 19 5.5 4.5 650 NA 5.7, Steam = 12.5 n-C4: 2.8 200 [3]HZSM-5 26 14 2.5 20 650 1 n-C4 + N2 + steam = 21.0 200 [95]10%La/ZSM-5 30 18.5 6 4 650 1 n-C4 + N2 + steam = 21.0 200 [95]10%Pr/ZSM-5 32.5 20 5 5.5 650 0.5 n-C4 + N2 + steam = 21.0 200 [95]HZSM-5 27.5 10 NA NA 650 300 (i-C4: 50%, n-C4: 50%) + N2 = 40 25 [96]HZSM-5 29 15 NA NA 600 300 (i-C4: 50%, n-C4: 50%) + N2 = 40 25 [96]0.5P/HZSM-5 33.9 20 NA NA 650 300 (i-C4: 50%, n-C4: 50%) + N2 = 40 25 [96]0.5P/HZSM-5 25 22.8 NA NA 600 300 (i-C4: 50%, n-C4: 50%) + N2 = 40 25 [96]1P/HZSM-5 20 25.6 NA NA 650 300 (i-C4: 50%, n-C4: 50%) + N2 = 40 25 [96]1P/HZSM-5 17.5 23.3 NA NA 600 300 (i-C4: 50%, n-C4: 50%) + N2 = 40 25 [96]2%P/HZSM-5 6 7 4 NA 650 0.5–1 5.7, Steam = 12.5 n-C4: 2.8 200 [94]La-P/HZSM-5 31 21 5 4 650 0.5–1 5.7, Steam = 12.5 n-C4: 2.8 200 [94]

n-C4: n-butane.i-C4: iso-butane.[90]: Time on stream (TOS) = 5 h.[[[

5flltr5ti

lC5St5ac

3]: W/F = 2.86 g s/ml.94]: Steam/hydrocarbon (w/w) = 1.4.93]: TOS = 1 h; WHSV = 8000 cm3 h−1 g−1.

framework limits the extent of aluminum extraction from theramework structure and reduces the occurrence of framework col-apse during steam treatment [69], so the stability of the crystalattice enhances with increasing Si/Al ratio [41,97]. On dealumina-ion, the number of acidic centers decreases, but the acidity of theemaining centers increases up to a degree of dealumination of ca.0%. The opposite effects of the concentration of acid centers andheir acid strength are superimposed, so that maximum reactivitys reached at a certain SiO2/Al2O3 ratio [97].

The effects of Si/Al ratio on the performance of ZSM-5 catalyst foright olefin formation have been merely investigated by Lu et al. in4 alkane catalytic cracking. They studied several systems of HZSM-, Fe/HZSM-5 and Cr/HZSM-5 zeolites with the different ratios of
i/Al (25, 38, 50, 80, and 150). The NH3–TPD results indicated thathe total acid amounts, density, and strength of HZSM-5, Fe/HZSM-, and Cr/HZSM-5 zeolites obviously decrease, while those of weakcid amounts enhance with reducing the Si/Al molar ratios. It wasoncluded that when the ratio of Si/Al was less than 50, the three
systems of HZSM-5, Fe/HZSM-5, and Cr/HZSM-5 catalysts with thesame ratio of Si/Al gave similar and high isobutane conversions.However, these three systems of catalysts possess different alter-ing tendencies of isobutane conversions for the ratios of Si/Al ≥ 80.The higher ratios of Si/Al (≥80) are beneficial to improve the yieldsof light olefins (C=

2 + C=3 + C=

4 ) over Fe/HZSM-5 and Cr/HZSM-5zeolites (Table 3) [98]. The results reported on the catalytic per-formances of HZSM-5, Fe/HZSM-5, and Cr/HZSM-5 zeolites withdifferent molar ratios of Si/Al, are not in agreement with each otherat the mentioned temperatures (550–600 ◦C); furthermore, it is notpossible to specify the optimum Si/Al ratio that favors the yield ofeach light olefin, i.e. ethylene, propylene, and butene. For exam-ple, at the temperature of 550 ◦C, the highest yield of light olefins
(59.8 wt.%) over Cr/HZSM-5 zeolite was achieved using Si/Al ratioof 150, while over Fe/HZSM-5 and HZSM-5 the ratios of 80 and50 resulted in the highest yields of light olefins (54.6 wt.% and50.8 wt.%, respectively). Generally, the yield of BTX abated in allthe samples by increasing the Si/Al ratios, so hydride transfer and

8 d Catalysis A: General 398 (2011) 1–17

al

itotTtnsiaoZaacFtt(c7

4

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romatization are suppressed by keeping the number of acid sitesow.

The influence of Si/Al ratio on light olefin production is alsonvestigated in the catalytic cracking of other feedstocks reported inhis paper. Jung et al. observed that enhancing the Si/Al molar ratiof ZSM-5 (25, 75 and 100) zeolites induces a significant decrease inhe conversion of n-octane, at the temperature range of 200–600 ◦C.he selectivity for olefin slowly increases with elevating the reac-ion temperature regardless of their Si/Al molar ratio. Thus, theumber of strong acid sites influences on the conversion con-iderably, but not in selectivity [74]. Abrevaya reported that byncreasing the Si/Al ratio from 24 to 66, hydride transfer and arom-tization reactions are lowered [68]. Zhu et al. studied the impactf Si/Al ratio in the catalytic cracking of butene to propylene overSM-5 zeolites, and proposed that both the propylene selectivitynd the overall yield of propylene and ethylene increased withscending the Si/Al2 ratio [99]. Furthermore, the stabilities of theatalysts are effectively prolonged with enhancing the Si/Al ratios.or the ZSM-5 (Si/Al2 = 50), the conversion decreased from an ini-ial value of 96.6–67.5%, i.e. 29% lower in 10 h of running, while forhe ZSM-5 (Si/Al2 = 366) the conversion changed from 84.5 to 81.4%a decrease of only 3%) [99]. In contrast to the aforementioned con-lusions, Han et al. observed that as the Si/A1 ratio decreased from5 to 20, the yield of C=

2 + C=3 increased from 29.3% to 37.5% [7].

.4. Hydrocarbons lighter than C4

Propane dehydrogenation (PDH) is an industrialized process forhe production of propylene [30] that employs different types ofatalysts, e.g., Platinum–tin-based (Pt–Sn) promoters supported onlumina [100,101], silica, ZSM-5 [102–105], SAPO-34 [106,107],tc. The oxidative dehydrogenation (ODH) of ethane and propanever different types of catalysts including modified zeolites suchs Ga2O3/HZSM-5 [108], ZnO/HZSM-5 [109], etc., is another routeor light olefin production that has been widely studied [110–116].he mentioned processes are out of the scope in the current paper.

To the best of our knowledge, the non-oxidative catalytic crack-ng of propane and ethane has not been resulted in high conversionf light olefins. However, the transformation of propane overodified zeolites (e.g., Pt, Co, Zn, Ga, etc.) has been intensively

eported in literature [117–120]. The main feature of the pro-osed conversion mechanism involves propane dehydrogenation,ollowed by oligomerization and aromatization [121,122]. Metalsnduce the dehydrogenating properties of the material whereascid sites are responsible for the oligomerization of the dehy-rogenation products (bifunctional mechanism) [119]. Propanectivation mechanism and cracking for light olefin production overa oxide and Al oxide modified HZSM-5 zeolites depicts that thereference for C–H bond activation over Ga cations results in highropylene selectivities of more than 85% over Ga+ modified HZSM-[120,123]. The faujasite structure zeolites type-X and the USY

xchanged with copper, iron, and platinum have been used inropane transformation at 400 and 550 ◦C. The highest propaneehydrogenation activity has been achieved with the platinum-xchanged X zeolite (propylene yield ∼11.2%, selectivity ∼31%).SY zeolites show high cracking capability and relatively low dehy-rogenation activity [119].

Ethane aromatization has been also investigated extensivelyver zeolites modified with metal cations such as Ga and Zn124]. The initial stage is the dehydrogenation of ethane that pro-eeds with subsequent oligomerization and dehydrocyclization
125,126]. Different ideas regarding the nature of metal cations onhe catalyst surface, as well as the effect of the incorporated ele-
ent including the enhancement of ethane conversion, increasinghe initial formation of olefins, and retarding the deactivation byoke formation have been proposed in literature. The mechanistic

Fig. 1. Typical propylene/ethylene ratios from steam cracking [70,130].

considerations such as the occurrence of a bifunctional mechanismover the modified zeolite and/or competing of the reactants and theintermediates for the same acidic sites are also another issues [127].The theoretical calculations have been focused on the mechanismof ethane dehydrogenation and the nature of intermediates andtransition states over modified zeolites. The probability of the alkylrupture of the ethane C–H bond or the carbenium activation havebeen predicted by estimating the activation energies [128,129].

4.5. Concluding remarks

One of the advantages of catalytic cracking process, comparedto the conventional steam cracking, is that the feedstocks type isnot as prominent as steam cracking. A flexible product distribution,such as more light olefins, BTX or both of them could be achievedover modified ZSM-5 catalysts with a suitable strength and densityof acidity in the catalytic cracking of hydrocarbons (Tables 2 and 3).Another improvement is that the P/E ratio is adjustable by choosingthe appropriate catalyst (Tables 2 and 3).

Unfortunately, in the steam cracking process, ethane feedstockdoes not typically give enough propylene to be recovered. Theamount of propylene produced from propane, butane or even naph-tha cracking is less than half that of the ethylene yields as shownin Fig. 1 [70].

The data reported in Tables 2 and 3 depict that the ratio of P/Ediffers significantly, according to the reaction temperature and thetype of incorporated element. The catalytic cracking process hasovercome the low P/E ratio disadvantage of steam cracking; giventhat, the ratios are higher than 0.58 in all the modified zeolites, andvary in the range of 1.8–1.33 over F-, Fe-, Cr-, P-modified HZSM-5zeolites. The promoters that accelerate dehydrogenative crackingof hydrocarbons, such as F, Cr and Fe, are shown to increase theratio of P/E. Fig. 2 shows the variation of P/E (1.7–0.9) ratios versusthe modified HZSM-5 zeolite samples in the catalytic cracking ofC4 alkanes. The ratio of P/E can be altered by adjusting the zeo-lite structure and the acidity, i.e., type, strength, and distribution[131]. In most of the samples, the yield of ethylene increases butthe propylene yield decreases by elevating the temperature. Theselectivity for propylene is generally dependent on the extent ofthe reaction: the selectivity for propylene is usually high at lowconversion, while that for ethylene becomes higher with increasingthe conversion.

In the catalytic cracking of C alkanes, the maximum ethy-
4lene yields are achieved over 10%Pr/ZSM-5 [95], Ce/HZSM-5 [93],and 0.5P/HZSM-5 [96] zeolites, at 650 ◦C (32.5–33.9 wt.%), whilethe highest propylene yield is observed over Fe/HZSM-5 [91], at600–625 ◦C (∼32–33 wt.%). The yield of ethylene (∼31–31.9 wt.%)

N. Rahimi, R. Karimzadeh / Applied Cata

0

10

20

30

40

50

60

e/ZSM-5

/HZSM-5

/HZSM-5

/HZSM-5

a/ZSM-5

/HZSM-5

Ethy

lene

, Pr

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)

Propylene

Ethylene

Fz

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ttts

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ig. 2. Propylene/ethylene ratios in catalytic cracking of C4 alkanes over modifiedeolites.

ver Cr/HZSM-5 zeolite at 600 ◦C is also favorable. However, theighest total olefins as well as C=

2 + C=3 yields have been obtained

ver Fe/HZSM-5 and Cr/HZSM-5. In the catalytic cracking of naph-ha, the highest yield of propylene is observed over F/HZSM-536–33.8 wt.%), while that of ethylene is produced over La/HZSM-and La-P/HZSM-5 (38–32 wt.%); La-P/HZSM-5 is the most stable

atalyst with the highest yield of C=2 + C=

3 (62 wt.%).Investigating the yield of light olefins versus the corresponding

i/Al ratios (Table 3) elucidates that they do not follow any trend,enerally. The highest yields of total olefin are observed over theide range of Si/Al ratios from 32 to 200. It is noteworthy that

he operating conditions are not identical in the reported data;onsequently, comparing these data does not specifically elicit thempact of this property. Nevertheless, the values of BTX yield haveot exceeded 5 wt.% at the highest Si/Al ratios (200), and as thei/Al ratio increases the yield of BTX decreases over the modifiedZSM-5 zeolites. Higher Si/Al ratios hinder the bimolecular hydro-en transfer reactions.

The important job in designing a catalyst is to optimizehe balance between the number/strength of acid sites and theype/amount of metals in the modified H-ZSM-5 for maximiza-ion of light alkenes with relatively low coke formation and catalysttability.

. Effect of element modification on the performance ofSM-5

The effect of ZSM-5 zeolite modification on the selectivity ofavorable products can be studied considering various approaches.he appropriate catalyst formulation (modification of ZSM-5) is onef the most important factors that influences directly on the yield ofight olefins. The effect of various elements on the chemical prop-rties of ZSM-5 zeolite, and their impacts on light olefin productionre reported in this section.

A complete and detailed review of all modifications, which zeo-ites may sustain has been published by Kühl [132]. The topic isomprehensive enough to be excluded from the scope of the currentaper. The acid strength of zeolite varies over a wide range by theodification (e.g., ion exchange, impregnation, partial dealumina-

ion, and isomorphic substitution of the framework Al and Si atoms)ith multivalent cations. Brønsted acid centers are generally the

atalytically active sites of H-zeolites [97,132]. Steaming is proba-
ly the oldest modifying treatment, since it has been known sincehe mid 1960s. By steaming or acid extraction, part of the zeolitean be dissolved or restructured leaving a mesoporous system inddition to the microporous one, which may be of importance in theransport of reactants or products and thus the conversion of rela-
lysis A: General 398 (2011) 1–17 9

tively large molecules which may be present in a feedstock [133].The hydroxyaluminate species formed by steaming are potentialLewis acid sites, since the environment of the corresponding alu-minums is octahedral [39].

5.1. Acidity of zeolites

The global acidity of a given zeolite is the result, amongstother things, of two parameters characteristic of Brønsted sites,i.e., their density (or their concentration) and their strength [39].The density of Brønsted acid sites decreases when the Si/Al ratioof the zeolitic framework increases [39,41,133,134], since the ion-exchange capacity corresponds to the Al3+ content of the zeolites[41,97]. The zeolites are classified according to increasing Si/Al ratioand the associated acid/base properties (Table 4).

The strength of the Brønsted sites depends on the interactionbetween the proton and the zeolitic framework or the environmentof the framework Al [39]. A completely isolated Al tetrahedronwill create the strongest type of Brønsted acid site [134]. The twomain parameters governing the acid strength of the Brønsted sitesare the structural characteristics of the zeolite and its chemicalcomposition. The structural characteristic of the zeolite is relatedto the proton lability that depends on the angle formed betweenthe two adjacent tetrahedra T at the oxygen carrying the proton[39]. Regarding the chemical composition factor, the acid strengthdepends on the number of Al atoms (aluminate tetrahedral) thatare adjacent to a silanol (silicate tetrahedron) group [39,41,97]. Thehighest proton-donor strengths are exhibited by zeolites with thelowest concentrations of AlO−

4 tetrahedra such as HZSM-5 and theultrastable zeolite HY [41,97].

The strength of the Brønsted acid sites (intrinsic acidity) is alsoaffected by isomorphic substitution [133,134]. The acid strengthdepends on the type of heteroatom; gallium or iron zeolites aremuch less acidic than aluminum zeolites. Boron-substituted for Siin pentasil ZSM-5 zeolites was observed to yield very weak aciditystrength [133,135], although, it results in an excellent catalyst forisomerization [136]. Ga substituted ZSM-5 samples was shown toexhibit both acidic and dehydrogenation properties [136].

The Lewis acid sites are related to the formation of positivelycharged oxide clusters or ions within the porous structures of thezeolites [137]. They are usually associated with extra frameworkAl (EFAL) species formed by extraction of aluminum from the lat-tice [39,134,138], or metal ions exchanged for the protons of acidsites. These metal cations together with the adjacent frameworkoxygens will act as Lewis acid/base pair, and may polarize bonds inreacting molecules [137]. It has been also suggested that the extra-framework trivalent Al species (formation of hydroxyaluminatespecies in the microporosity) reduce the concentration of Brønstedacid sites in the framework, simultaneously [39] increase their acidstrength if the initial Si/Al ratio is not too high (<7) [39,79,120]. Ingeneral, the presence of Lewis acid sites could increase the strengthof the nearby OH Brønsted acid sites, due to an inductive or a syner-gistic effect between the Brønsted and the Lewis acid sites [39,79].

5.2. Alkali and alkaline earth metals

Exchanging zeolites with a less electronegative charge-balancing cation such as Cs+ and/or occlusion of alkali metal oxideclusters or alkali metal clusters in zeolite cages makes the zeo-lites basic. Basic zeolites contain either Lewis base sites associatedwith the framework oxygen ions, or Brønsted base sites linked
to basic hydroxyl groups, or both. The former is influenced bythe negative charge of the oxygen, while the latter depends onthe nature of extra framework cation present in the zeolite. Thebase strength of the zeolite rises with increasing the aluminumcontent of the framework (i.e. ZSM-5 < mordenite < L < Y < X) and


Table 4Classification of acidic zeolites according to increasing Si/Al ratio [97].

Si/Al ratio Zeolites Acid/base properties

Low (1–1.5) A, X Relatively low stability of lattice;Low stability in acids;High stability in bases;High concentration of acid groups ofMedium strength

Medium (2–5) Erionite, chabazite, chinoptilolite, mordenite, Y

High (ca. 10–∞) ZSM-5; Relatively high stability of the lattice;

r(at

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tfotlatWsltambttsrom

DealuminatedErionite, mordenite, Y

educes with decreasing the ionic radius of the exchanged catione.g. Li < Na < K < Rb < Cs). It is also influenced by the exchange of Alnd/or Si ions in the zeolite framework for Ga and/or Ge, respec-ively [134].

One of the roles of the alkaline earth metals is the modifica-ion of acid character by weakening acid strength or reducing acidites [90]. The NH3–TPD spectra of the alkaline earth modifiedZSM-5 catalysts indicated that the strong acid sites are trans-

ormed to weak acid sites by the introduction of alkaline earthetals [90]. The amount of strong acid sites reduces with the alka-

ine earth modification by increasing the ionic radius with therder of nonmodified (H form) Mg > Ca > Sr > Ba [90,97]. Bariums the strongest Lewis base in alkaline earths series resulted in theighest yield of light olefins [90]. Tynjala et al. proposed that theapid decrement of strong Brønsted acid sites with increasing theegree of Ba ion-exchange is due to quite negligible effect of thelank–Hirschler mechanism. The ion-exchange of zeolites with di-nd trivalent metal cations leads to the formation of metal hydrateations M(H2O)2+

n and M(H2O)3+n , respectively. H MAS NMR spec-

ra showed that the intensity of the Brønsted acid site signal alsoecreased as the degree of ion-exchange increased [139].

The modification of ZSM-5 zeolite catalysts with the elementsnhancing the surface basicity decreases the readsorption of theasic compounds of the cracking products, such as ethylene, propy-

ene, and butenes, and this is apparently the major cause of theower aromatics and higher light olefin formation [3]. On the otherand, the alkaline earth metal modified HZSM-5 zeolite suppressesydrogen transfer reaction and stimulates the dehydrogenativeracking by controlling the HZSM-5 acid character and dehydro-enation activity to improve the yield of light olefins [89].

The alkali treatment of zeolites induces a significant change inheir acidity and pore structure because silica and alumina of theramework are dissolved in alkaline solution [40]. The dissolutionf alumina removes aluminum atoms from the zeolite framework;hus, results in the loss of strong acid sites and destruction of zeo-ite structure [40]. The thermal and hydrothermal stabilities of thelkali-treated ZSM-5 zeolites are slightly deteriorated because ofhe introduction of mesopores caused by the desilication [140].

eak alkali solutions dissolve a small amount of silica and alumina,o create mesopores [6,40,141] by enlarging the micropores of zeo-ites, while strong alkali solutions destroy their crystal structure;herefore, convert them to amorphous materials with macroporesnd extremely small acid sites [40]. Ogura et al. reported that theicroporous structure remained unchanged in Na/ZSM-5 zeolite,

ut the acidic property altered very little quantitatively or qualita-ively [141]. The textural properties analyzed by Wei et al. indicated
hat the surface area and pore volume of micropores decrease toome extent [6]. Alkali modified zeolites, usually Na and K, are notecommended by some authors, because of the deactivating effectf them in most of the catalytic reactions and hydrocarbon transfor-ations [36,38]. The alkali treatment is not effective in enhancing
High stability in acids;Low stability in bases;Low concentration of acid groups ofHigh strength

the overall selectivity to alkenes, but the selectivity for propylene ishigh because of the rapid elution of primary cracking products [40].On the contrary, it was observed that the loss of strong acid sitesdue to alkali treatment suppresses the production of longer alkanessuch as hexane and heptane by reducing further oligomerization[40]. Wang et al. deduced that the role of K and Ba is to enhancethe dehydrogenation activity of the catalysts and minimize thebimolecular hydrogen transfer reaction, which is responsible forthe saturation of small olefins and the formation of aromatics [83].

5.3. Transition metals

Transition metals have partially occupied d-orbitals the sym-metry of which is suitable for formation of chemical bonds withneutral molecules. These metals also have several stable oxidationstates and different coordination numbers as a result of the changesin the number of d-electrons [142]. The catalysis by organometalliccompounds is based on activation of the substrates by coordinatingit to the metal, which lowers the activation energy of the reactionbetween substrates. Organometallic catalysts also include specificligands besides the atom or group of metal atoms. A very large num-ber of different types of ligands can coordinate to transition metalions. Once the ligands are coordinated, the reactivity of the met-als may change dramatically. The rate and selectivity of a givenprocess can be optimized to the desired level by controlling the lig-and environment [142]. It is known that transition metals createnew Lewis acid sites in HZSM-5 zeolites. The simultaneous pres-ence of Brønsted and Lewis sites could increase the acid activityof zeolites and influence on the selectivity of the catalytic crackingproducts. The nature of Lewis sites, and how they impact catalystperformance in catalytic cracking of hydrocarbons are still not fullyunderstood [134].

The modification of ZSM-5 zeolites with transition metalsadjusts the catalyst acidity. Lu et al. observed that the acidic amountof the Fe/HZSM-5 catalysts firstly increased with the introductionof a small amount of Fe, but it decreased with further enhanc-ing the quantity of Fe(III). The higher loading amount of iron wasproposed to lessen the catalytic cracking activity of Fe/HZSM-5 cat-alysts obviously, that might be due to the aggregation of a majorityof iron into oxo-iron compounds with large domain size, whichcould cover the outer surface of the zeolite catalysts, or combinewith acid sites [91]. The NH3–TPD analysis results of Cr/HZSM-5indicated that the density of acid sites (weak and strong) enhancewith increasing the loading amount of Cr(III), while the strengthof strong acid sites obviously reduces [92]. It was postulated thatthe majority of Cr species in the catalysts are uniform, isolated,
well dispersed, and almost oxidized to Cr(VI) after calcinations.The catalytic cracking reactivity of Cr/HZSM-5 catalysts decreasesby rising the loading amount of Cr, that might be due to the exis-tence of a majority of Cr(III) with large domain size, which couldcover the outer surface of the zeolite catalysts, or combine with

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cid sites [92]. Enhancing the quantity of Fe or Cr in HZSM-5 leadso increasing the selectivity to aromatic hydrocarbon and decreas-ng the total olefins, because of improving further oligomerizationnd cyclization reactions [91,92]. A partially Cu-exchanged HZSM-provides with an optimum balance between dehydrogenation

ctivity of the metal and acid function of the shape selective HZSM-zeolite, when employed as FCC catalyst additive for improving theield of light olefins. Lappas et al. found that a small amount of cop-er ions, as hydrated Cu(I) or Cu(II) cations in ion-exchange sites,inders framework dealumination during steaming compared toZSM-5 or the Ga- and Cr-modified samples. The total numberf acid sites of the Cu/HZSM-5 sample was also higher than theest of the samples [51]. Kwak et al. observed that the additionf Zn by chemical vapor deposition (CVD) decreased the intensityf the Brønsted acidity, but it did not cause the apparent forma-ion of Lewis acid sites, unlike Ga/HZSM-5 catalysts [143]. Theeported results depict that Co [89], Cu, Zn, and Ag [3] loadedites on HZSM-5 zeolites accelerate the dehydrogenative crack-ng [89] and cyclo-dehydrogenation reaction [3]. Zhang et al. foundhat the hydrogen transfer capability of the transition metals sup-ressed in the presence of phosphorus, and the selectivity to lightlefins were increased because of its dehydrogenation activity131].

As the aromatization reaction starts with dehydrogenation stepnd the formation of small olefins, the results obtained in thiseaction are also applicable for studying the impact of elementodification in improving the yield of light olefins. The incorpo-

ation of transition metal ions into zeolites leads to interestingifunctional catalysts [85] in which metal and acid centers canct simultaneously [97]. The transition metals, such as Ni, Co, Zn,u, Cr, and Mn, introduced into pentasil type zeolites are generallysed as active components for hydrogenation or dehydrogenation144,145] or aromatization reactions [121,122,131]. The hypothe-is of dehydrogenation enhancing role of Lewis acid in the initialtages of the cracking reactions have been supported by variousesearches. It results in the olefin formation that can initiate, or ateast enhance the cracking reaction [8,138].

The mechanism of hydrocarbon transformation over a modi-ed zeolite could involve both Lewis and Brønsted acid sites [84].he activation on a zeolite matrix should occur largely on strongewis acidic centers, while secondary cracking, on strong Brønstedenters [146]. It is also probable that both the incorporated ele-ent and zeolite protons intervene in hydrocarbon activation step

143,147]. In a true bifunctional catalyst such as Ga2O3/ZSM-5,he dehydrogenation and aromatization mainly occurs on metals,hile oligomerization, cyclization, dealkylation, transalkylation,

nd isomerization of alkyl aromatics occur on zeolite [42]. Tran-ition metals, e.g. Zn and Ga cations, are the driving force forehydrogenation by combining the H-atoms that form during thectivation of C–H at Brønsted acid sites due to the cyclic reductionnd oxidation of Zn and Ga species [148]. Altogether, the com-ination of Lewis sites for the dehydrogenation of the paraffiniceedstock with Brønsted acid sites for the cracking of subsequentigher olefins to light olefins, could enhance the yield of light olefins8].

.4. Rare earth elements

Rare earth is a generic name used for the 14 metallic elements ofhe Lanthanide series, which contain the atomic numbers from 57
hrough 71 plus Scandium (At. #21) and Yttrium (At. #39). Theselements occupy a unique place in the periodic chart. They are therst elements where the increasing atomic number results in thelling of the inner electron shell after an outer shell has been filled,ausing a high similarity in chemical properties [149].
lysis A: General 398 (2011) 1–17 11

Rare earth cations such as La3+ and Ce3+ have been successfullyemployed in FCC so called zeolite Y (REY) catalysts to improve theability of the catalyst to withstand high temperature, leading to thecatalytic stability [11,150], increasing catalyst activity and gaso-line selectivity [11,93]. A higher concentration of acid sites will befound in the rare earth exchanged catalyst due to inhibiting the dea-lumination of the zeolite; consequently, both the activity and thehydrothermal stability of the catalyst will be improved [149]. Theeffects of rare earth elements on Y zeolites are still controversial. Onone hand, they shift the cracking selectivity due to the reduction ofhydrogen transfer to produce higher percentages of olefins, whichwill increase gasoline octane number [150]. On the other hand, as aresult of the greater number of active sites, both the primary crack-ing and hydrogen transfer reactions that occur within the zeoliteare enhanced [149].

The influence of RE modification on the catalytic performancesof ZSM-5 zeolite is also debatable. On one hand, some researchershave reported that the introduction of a rare earth metal ioninto HZSM-5 has a great impact on its acidic properties [93,151].Hartford et al. observed that lanthanum exchanged ZSM-5 zeo-lite alters the acidity by lowering the number of strong acid sitesand increasing the number of weak acid sites (Table 5), but itdoes not reduce the effective free pore volume nor it enlargesteric hindrances for the passage of linear molecules throughthe pores of the catalyst [151]. Moreover, Wang et al. illus-trated that the total amount of acid sites of light RE-modifiedHZSM-5 samples increases, compared to the unmodified HZSM-5 sample, except for the Gd-modified sample [93]. The orders ofthe amount of weak acid, strong acid, and total acid total acidhave been observed as follows: Sm >Nd > Pr > Eu > Ce > La > HZSM-5 > Gd; Nd >Eu > La > Sm > Ce > Pr > Gd > HZSM-5; Nd > Sm > Eu > Pr> Ce > La > HZSM-5 > Gd (Table 5) [93]. On the contrary, Xue et al.deduced that adding La to HZSM-5 zeolite decreases the totalamount of Brønsted acid sites in the zeolite. This elimination wasprobably caused by the exchange of La3+ ion with bridged hydrox-yls in zeolite; although, some new acid sites might be generated bysplitting the coordinated water molecules with La3+ ions. The den-sity of Brønsted acid sites in La/HZSM-5 decreases with the increasein La loadings [66]. The acid amount of RE/HZSM-5 after NH3–TPDmeasurement is listed in Table 5.

The presence of the empty f orbital in rare earth metals is essen-tial to modify the acidity of HZSM-5 zeolites because it can at leastprovide some positions for the formation of Lewis acidic sites [93].The detailed results of the FT-IR spectra of adsorbed pyridine on theRE-modified HZSM-5 zeolites are reported in Table 6. The resultsindicate that Brønsted and Lewis acid sites coexist on all the stud-ied samples. The modification of HZSM-5 zeolite with rare earthmetals not only amend the amount of acid sites but also alters theacid type, i.e., the ratio of L/B [93].

On the other hand, it is generally accepted that the introduc-tion of RE elements modifies the basicity of ZSM-5 zeolite as REoxides usually possess some basic characters [93]. Yoshimura et al.and Wakui et al. observed that the acid amount and strength werealmost the same before and after the La loading (10 wt.% loading)indicating that the modification of HZSM-5 with La did not affectthe acidic character of the catalyst [3,94]. In contrast to the acid-ity, considerable changes were observed for the basicity measuredby CO2–TPD. The amount of CO2 adsorption calculated from thepeak at 70 ◦C enhanced as the quantity of La loading increased, sug-gesting the generation of basic sites on the surface as a result ofLa-loading [3]. Wakui et al. illustrated that as the amount of La-loaded increases, the olefin adsorption over La/HZSM-5 zeolites
decreases; thus, the main reason for the higher yield of ethy-lene and propylene is inhibiting the bimolecular reactions due tothe suppression of olefin adsorption [89,95]. The introduction ofRE elements modifies the basicity of ZSM-5 zeolite [3,93]; hence,


Table 5Acid amount of RE/HZSM-5 zeolites after NH3/TPD measurement.

Zeolite sample Weak acid peak Strong acid peak Total acid amount(mmol g−1)

Si/Al Preparation method Ref.

Temp. (◦C) Acid amount(mmol g−1)


HZSM-5 234 0.206 436 0.322 0.528 64.1 [93]La/HZSM-5 236 0.211 425 0.370 0.581 64.1 Impregnation [93]Ce/HZSM-5 236 0.218 435 0.364 0.582 64.1 Impregnation [93]Pr/HZSM-5 239 0.245 425 0.353 0.598 64.1 Impregnation [93]Nd/HZSM-5 236 0.253 427 0.422 0.675 64.1 Impregnation [93]Sm/HZSM-5 233 0.290 439 0.367 0.657 64.1 Impregnation [93]Eu/HZSM-5 234 0.238 427 0.373 0.611 64.1 Impregnation [93]Gd/HZSM-5 239 0.194 425 0.332 0.526 64.1 Impregnation [93]5%La/HZSM-5 225 0.35 460 0.31 0.66 40 Ion-exchange [152]25%La/HZSM-5 235 0.38 440 0.24 0.62 40 Ion-exchange [152]HZSM-5 232 0.135 422 0.449 35 [67]4%La/HZSM-5 232 0.248 422 0.405 35 Impregnation [67]

[ RE (La[ w/wt lanth2

dpo

5

hReaslocpsatLttowze[saazaspa

TA

93]: The series of RE/HZSM-5 catalysts contain 7.54 wt.% of different kinds of light152]: Two lanthanum-exchanged forms of the catalyst, containing 0.13% and 0.60%akes place, with three monovalent ammonium ions being replaced by one trivalent5%, respectively [152].

ecreases the readsorption of the basic compounds of the crackingroducts, such as ethylene, propylene, and butenes, which is onef the major causes of higher light olefin formation.

.5. Phosphorus

Phosphorus modification is the most widely used method thatas been applied for improving HZSM-5 zeolites performance [96].esearchers at Mobil where among the first to recognize the ben-fits of phosphorus [49] for improving the hydrothermal stabilitynd shape selectivity of ZSM-5 zeolites [49,152,153]. Phosphorustabilizes the lattice aluminum ions by retarding aluminum fromeaving the zeolite framework; thus, it hinders structural changesf zeolite that significantly results in enhancing the stability of theatalyst [3,5,49,152,153]. Degnan et al. have summarized that phos-horus reduces the initial acidity of the zeolite, but after severeteaming produces a zeolite that retains a larger fraction of itscidity [49]. The stabilization of cracking activity is entirely dueo the improved retention of framework aluminum in ZSM-5 [49].ercher et al. proposed that the phosphorus species interact withhe oxygen of the bridging hydroxyl groups by replacing the pro-ons on them and result in the cleavage of the Al–O bonding and thepening of the zeolite framework [154]. When phosphorus reactsith zeolite, the hydroxyl connected with aluminum atoms on the

eolite surfaces is replaced by P–OH, so the acid strength is weak-ned because the strength of P–OH is weaker than that of Al–OH154]. Xue et al. used various experimental techniques to demon-trate the influences of hydrothermal treatment on the Brønstedcid sites of the HZSM-5 impregnated by phosphorus. The inter-ction of phosphorus species with silanol nests exposed on the
eolite (due to dealumination) creates new hydrothermally stablecid sites during the steam treatment [152]. The new acid siteseem to be related to the phosphorus entering into the zeoliticosition left by dealumination and stabilized by some extra-latticeluminum. The entry of phosphorus into the zeolitic framework
able 6mount of Brønsted (B) and Lewis acid (L) sites determined by pyridine adsorption for RE

Zeolite sample Amount of acid sites (�mol g−1) (200 ◦C)

B L B + L L/B

HZSM-5 246.5 68.9 315.5 0.28Nd/HZSM-5 202.2 100.7 302.9 0.5Ce/HZSM-5 251.9 88.9 340.8 0.35Gd/HZSM-5 160.6 59.3 219.9 0.37

, Ce, Pr, Nd, Sm, Eu, Gd) [93].La, respectively, were prepared. Assuming that a stoichiometric exchange process

anum ion, these masses correspond to lanthanum-ammonium exchanges of 5% and

protects the residual framework aluminum against further dealu-mination. The integrated effect causes the hydrothermally stableacid sites in P/HZSM-5 [152]. The Phosphorous–zeolite interac-tion is also proposed to stabilize the framework aluminum pairsby extra-framework cationic species formed via protonation oforthophosphoric acid [155].

Altogether, the modification of HZSM-5 zeolite with phospho-rus decreases Brønsted acidity [5,153,156]; and/or converts strongBrønsted acid sites of HZSM-5 into weak Brønsted acid sites thatincreases the density of the weak Brønsted acid sites withoutchanging the overall acid–base properties [154,157]. On the otherhand, it is reported that the concentration of both Brønsted andLewis acid sites are significantly decreased in P/HZSM-5 zeolites, atthe same desorption temperature. Brønsted and Lewis acid sitesdropped with increasing the desorption temperature (from 100to 300 ◦C) [158]. It is suggested that during calcinations, some ofthe Brønsted acid sites are neutralized by attaching P species, andsubsequent hydrothermal treatment of P/HZSM-5 converts the Pspecies to more condensed phase which block pores of zeolitereversibly [153]. Acid amounts of P/HZSM-5 zeolites after NH3/TPDmeasurement are reported in Table 7.

Reducing the Brønsted acidity of P/HZSM-5 provides a path-way for reaction to occur with enhanced light olefin production[5]. The introduction of P in HZSM-5 increases the selectivity topropylene and butenes in the catalytic cracking of n-decane [155].The incorporated phosphorous species are responsible for both theenhanced acid stability, and the significantly improved catalyticperformance in the cracking of C4 olefin to propylene and ethy-lene [152]. Zhao et al. supported the idea that the P-impregnatedHZSM-5 zeolites improve the propylene selectivity greatly and
result in excellent anti-coking ability (lifetime exceeding 800 h ona single process) in the catalytic cracking of C4 olefin [158]. Liuet al. concluded that P/HZSM-5 leads to higher yields of light olefin(i.e., ethylene and propylene) in the catalytic cracking of naphtha[153].
/HZSM-5 and HZSM-5 zeolite samples at different desorption temperatures.

Amount of acid sites (�mol g−1) (350 ◦C) Ref.

B L B + L L/B

213.7 48.7 262.2 0.23 [93]195.6 46.3 242.0 0.24 [93]242.0 72.4 314.4 0.30 [93]132.4 43.9 176.4 0.33 [93]

N. Rahimi, R. Karimzadeh / Applied Catalysis A: General 398 (2011) 1–17 13

Table 7Acid amount of P/HZSM-5 zeolites after NH3/TPD measurement.

Zeolite sample Weak acid peak Strong acid peak Total acid amount(mmol g−1)

Si/Al Preparation method Ref.



HZSM-5 232 0.135 422 0.449 0.584 35 [67]0.9P/HZSM-5 232 0.221 422 0.394 0.615 35 Impregnation [67]1.8P/HZSM-5 232 0.205 422 0.157 0.362 35 Impregnation [67]3P/HZSM-5 232 0.109 422 0.055 0.164 35 Impregnation [67]HZSM-5 150–220 0.303 425 0.149 0.452 25 [96]0.1P/HZSM-5 150–220 0.318 420 0.148 0.466 25 Impregnation [96]0.5P/HZSM-5 150–220 0.299 415 0.124 0.423 25 Impregnation [96]1P/HZSM-5 150–220 0.303 415 0.075 0.378 25 Impregnation [96]

Table 8Amount of Brønsted (B) and Lewis acid (L) sites determined by pyridine adsorption for P/HZSM-5 and HZSM-5 zeolite samples at different desorption temperatures.

Zeolite sample Amount of acid sites (�mol g−1) (200 ◦C) Amount of acid sites (�mol g−1) (350 ◦C) Ref.

B L B + L L/B B L B + L L/B

98

5

ma(ltitioy[

6

mrTfr

lcacmcvebomc

eBia

strong proton sites of low density [68,159,164], at a low concen-tration of alkenes in the reaction environment [28,77], low partialpressure of reactants [39,47,81,159,165], high temperature and lowconversion [46,88,159,165]. The ZSM-5 zeolite improves the pro-

RH2+

Carbonium ion

R2+

H+

desorption

RH R1H

HZSM-5 524.4 49.4 574.8 0.00.1P/HZSM-5 501.6 89.7 591.3 0.10.5P/HZSM-5 534.9 52.9 587.8 0.11P/HZSM-5 296.8 43.7 340.5 0.1

On the other hand, the introduction of phosphorus not onlyodulates the amount of acidic sites and the percentage of weak

cidic sites in total acidic sites but also regulates the ratio of L/BTable 8), which are favorable for obtaining the high yield of propy-ene in the catalytic cracking of C4 alkanes. The results showed thathe new basic sites generated on the surfaces of P/HZSM-5 catalystsnhibit the adsorption of intermediate olefins and lower the forma-ion of aromatics. The results of the P/HZSM-5 catalyst stability testndicated that there was an insignificant decrease in the conversionf the mixed C4 alkanes after 10.5 h continuous reaction, while theield of ethylene plus propylene exhibited a slight enhancement96].

. Mechanism of catalytic cracking over zeolites

The present paper does not intend to go through the suggestedechanisms of hydrocarbon catalytic cracking as some extensive

eviews have been already published in this field [28,138,159–161].he catalytic cracking reaction scheme and the influence of variousactors on the contribution of each mechanism have been summa-ized in this section.

Alkanes can be activated by two mechanisms: monomolecu-ar and bimolecular mechanisms. In the monomolecular (protolyticracking) mechanism, proposed by Haag and Dessau in 1984 [162],lkanes are protonated to form carbonium ion transition states thatan undergo either C–C bond cleavage yielding alkanes (includingethane and ethane) or C–H bond cleavage yielding hydrogen and

arbenium ions. These carbenium ions subsequently form alkenesia back donation of a proton to the zeolite. The formation ofthylene is possible via this mechanism (Fig. 3) [28,68,77]. In theimolecular mechanism, cracking takes place when branched sec-ndary and tertiary alkylcarbenium ions derived from the feedolecule are cleaved by a single beta-scission into smaller alkyl-

arbenium ions and alkenes (Fig. 4).
The formation of primary alkylcarbenium ions is unlikely, so
thylene, ethane and methane are not observed via this mechanism.imolecular hydride transfer reactions between alkylcarbenium

ons and feed alkanes lead to chain reactions that produce shorterlkanes thus limiting the yield of olefins to 50 wt.% [28,77].

477.8 28.7 506.5 0.06 [96]506.3 25.3 531.5 0.05 [96]477.8 27.6 505.4 0.06 [96]255.6 19.5 275.1 0.08 [96]

6.1. The factors determining the dominant mechanism

The protolytic cracking mechanism allows the formation oflarge quantities of ethylene and propylene from naphtha rangereactants, provided that the bimolecular hydride transfer andaromatization reactions, which will consume ethylene and propy-lene are hindered [68]. The zeolite structure (e.g. the pore sizeand shape) and chemical properties (i.e. the promoter type,the acidity, and Si/Al ratio), as well as the operating conditiondirectly influence on the contribution of the mentioned reactionschemes in the system. The monomolecular mechanism is dom-inant in the medium pore shape selective zeolites, i.e. ZSM-5[28,46,68,75,77,81,88,138,163], with high Si/Al ratio [74,138] over

alkene

Fig. 3. Haag–Dessau cracking mechanism for an alkane molecule (RH) proceedingvia a carbonium ion transition state [161].

14 N. Rahimi, R. Karimzadeh / Applied Cata

RH R1H

H-transfer

alkene

β -scission

R1+

R+

Fig. 4. Classical cracking mechanism for an alkane molecule (RH) consisting ofa[

dassssvopi

liroloshatsi[

ooowasHr

6m

rm

hydride transfer step to a smaller carbenium ion (R1+) followed by �-scission

161].

uction of light olefins, particularly propylene. Its shape selectivityffects on the catalyst performance in two ways. (1) Reactant stateelectivity (RSS), so that the linear and monobranched paraffinictructures and even more the olefinic structures are converted intomaller fragments (C4

−) in the microporosity. (2) Transition stateelectivity (TSS) which acts in two ways in the ZSM-5: first, it pre-ents the formation of the transition state needed for the transferf hydrogen; second, it hinders the formation of coke in the micro-orosity of the zeolite. The role of shape selectivity is often more

mportant than their acidity [33].The various reactions rely in different ways on the strength and

ocal structure of the Brønsted acid sites in zeolites. The activ-ty of Brønsted acid sites in zeolites is strongly affected by theate-limiting step of the reaction. The monomolecular crackingf alkanes proceeds via protonation of the alkane as the rate-imiting step. This reaction is influenced by the size and shapef the pores that impact the heat of adsorption [165]. Previoustudies support this idea and declare that the differences in theeat of adsorption, and thus the size and shape of the pores,re the dominant factors in determining the cracking activity ofhe Brønsted acid sites [79,165,166]. High Brønsted acid site den-ity decreases the activation energy [167]; consequently, resultsn high rate of hydride transfer and stable alkylation catalysts167,168].

The rate-limiting step in dehydrogenation of alkanes is the des-rption of intermediate species, that is determined by the stabilityf these species [169]. This stability depends on the local structuref the Brønsted acid site and the Si/Al ratio of the zeolite frame-ork [165]. Thus, changes in zeolite structure and such properties

s the Si/Al ratio, framework ionicity, and framework flexibility,trongly influence the stability of the intermediate species [165].igher Si/Al ratio of zeolite decelerates the rate of hydride transfer

eactions [168].

.2. The role of incorporated element in the catalytic cracking
echanism
Determining the precise role of incorporated element in theeaction steps is not simple, especially when the nature of the activeetal species such as Ga remains unclear [123]. The effect of zeolite


modification on the catalytic cracking is still under investigation,from the mechanistic point of view. Some relevant ideas regardingthe role of incorporated element in the catalytic cracking reactionscheme have been summarized in this subsection.

Alternative to the conversion of hydrocarbons over Brønstedacid sites is their activation over Lewis acid sites. A distinct dif-ference exists in the product distribution between Brønsted andLewis sites. The former mainly catalyze C–C cracking reactionswhile the latter exhibits a strong preference for C–H activation andto a greater extent dehydrogenation [123]. The further cracking ofthe carbenium ion resulted from C–C cleavage increases the olefins-to-paraffin ratio compared to unity for Brønsted acid sites [8]. Whenbimolecular hydride transfer is limited as for HZSM-5, one predictsolefin-to-paraffin ratios of about 2 [163]. Doronin et al. consideredthat activation on a zeolite matrix should occur largely on strongLewis acidic centers, while secondary cracking, on strong Brønstedcenters [146]. Biscardi et al. [170] and Yu et al. [118] suggestedthat the predominant role of extra-framework cations like Zn andCo in Zn/HZSM-5 and Co/HZSM-5 is to enhance the recombinativedesorption of hydrogen, that increases the rate of dehydrogenationcompared to HZSM-5. This process consumes the surface hydro-gen pool available for the reverse hydrogenation reactions, and itincreases the overall rate of propylene conversion to aromatics. Znand Co cations promote not only alkane dehydrogenation but alsothe subsequent removal of hydrogen as H2, leading to unsaturatedintermediates required in cyclization steps [118,170]. Feller et al.proposed that the activation of propane over Ga-containing zeo-lites occurs via a pseudo-protonated cyclopropane intermediateinvolving both protons and Ga+ species [8,168]. The recent exper-imental and theoretical work have shown that Lewis sites such asGa+ species activate paraffins via heterolytic dissociation, over theLewis acid–base pair formed by the univalent Ga+ ion and the zeo-lite oxygen anion [123], that differs from the activation of paraffinsover HZSM-5 via a carbonium ion mechanism [120]. Kazansky et al.concluded that hydrocarbon activation on HZSM-5 modified by Znand Ga cations proceeds via two distinctly different mechanisms.The role of Zn2+ in Zn/ZSM-5 is the heterolytic dissociative adsorp-tion of ethane forming an alkyl group and a Brønsted acid site,while Ga2+ results in dissociative adsorption of hydrocarbon [171].Furthermore, Hensen et al. illustrated that over Ga cations, het-erolytic dissociation of propane takes place on the Lewis acid basepair formed by the Ga+ ion and the zeolite oxygen anion that leadsto high propylene production. Similarly, propane gives propyleneover the acid–base pair of GaO+ via a lower energy barrier [120].Rane et al. suggested that the synergy between Ga dehydrogenationsites and Brønsted acid sites improve the dehydrogenation rate; thehigh acidity of the zeolitic proton facilitates hydrogen recombina-tion and concomitant removal of product olefin from the Ga activesites. The promoter cations play a key role in the dehydrogenationof paraffins, and Brønsted acid protons catalyze the oligomerizationof the olefins produced [8].

7. Conclusion

Light olefin production through catalytic cracking of hydro-carbons has attracted researchers’ attention to overcome thedisadvantages of the conventional steam cracking, such as thehigh reaction temperature, the great amount of CO2 emission,and the low P/E ratios (0.4–0.6). The use of a shape selective, tri-dimensional micropore structure HZSM-5 zeolite with the large
specific surface area has led to promising results in the catalyticcracking of different types of hydrocarbons, i.e., heavy feedstocks,naphtha range feedstocks, and C4 alkanes, at 550–650 ◦C; that isabout 200 ◦C lower than the steam cracking. The ethylene yield of28–34% and propylene yield of 23–29% have been attained depend-

d Cata

ibPar

ahdsiAfcaaacocaitaots

bt[Ma

HssTmtftr

8

ooNdhtumsp

aknatcc


ng on the feed type and the operating condition that is improvedy 7–10%, compared with steam cracking. Furthermore, the ratio of/E could be improved to 1.8–0.6 by adjusting the acidity, i.e., thecid type, density, strength, and distribution (L/B), as well as theeaction temperature.

The appropriate modification, including the selection of a suit-ble promoter, the amount of loading and the modification methodas direct impact on the ZSM-5 performance and the productistribution. The promoter controls the pore characteristics andhape-selectivity of ZSM-5 zeolite, as well as the acidity; moreover,t influences on the catalyst activity and hydrothermal stability.lkaline and rare earth elements cations not only increase the sur-

ace basicity that results in reducing the readsorption of the basicompounds of the cracking products, such as ethylene, propylene,nd butenes, but also enhance the dehydrogenation reaction, whichre the major causes of light olefin formation. Transition metalslter the acid distribution, i.e., the ratio of L/B, regulate the con-entration, and amend the strength of Brønsted acid sites. The rolef Lewis acid sites in accelerating the dehydrogenation reactionombined to a synergistic effect that exists between the Brønstednd the Lewis acid sites, hinder the hydrogen transfer reaction, andmprove the dehydrogenative cracking that consequently improvehe yield of light olefins. Rare earth elements not only amend thecid strength and distribution but also enhance the surface basicityf ZSM-5 catalyst. The modification of ZSM-5 with phosphorus ishe best compromise in terms of both improving the hydrothermaltability as well as adjusting the zeolite chemical properties.

The higher Si/Al (>80) ratios in ZSM-5 framework are beneficialecause of the well-established impact of low Al content in limitinghe extent of aluminum extraction from the framework structure69] and thus, increasing the stability of the crystal lattice [41,97].

oreover, the higher Si/Al ratios hinder the bimolecular reactionsnd prevent from the formation of BTX.

The mechanism of hydrocarbon transformation over a modifiedZSM-5 zeolite could involve both Lewis and Brønsted acid sites,

o the reactant is under the influence of two different types of sitesimultaneously: the incorporated element and the zeolite acid sites.he synergy between Brønsted acid sites and the sites created viaodified elements is one of the essential factors in determining

he dominant mechanism in the catalytic cracking reaction. Theactors such as the zeolite chemical and physical properties andhe operating condition also verify the contribution of the favorableeaction scheme in the system.

. Future prospects

Naphtha has thus far been employed as the main feedstock forlefins productions. However, due to the sudden rise in the pricef oil and shortage of current light crude, many nations in Europe,orth America, and the Middle East have paid attention to pro-ucing light olefins via alternative potential feedstocks such aseavy crudes. Meanwhile, the investment in plant construction inhe petrochemical industry has focused on the utilization of nat-ral gas due to economical aspects. Although, it is expected thatarket demands for ethylene can be met by natural gas decompo-

ition technology, but in the near future there will be a shortage ofropylene supply [10,60].

According to the mentioned reasons, future technologies suchs the catalytic cracking will be required to fulfill forthcoming mar-et demand. Therefore, the catalytic cracking process should satisfy
ot only the production of high amount of light olefins (ethylenend propylene) from low grade crude as a feedstock but also con-rollable ratios of P/E [60,70]. Thus, it is anticipated that the thermalracking process will be gradually transformed to a catalytic pro-ess such as ACO that can efficiently produce both ethylene and
lysis A: General 398 (2011) 1–17 15

propylene, reduce the reaction temperature, and adjust the productdistribution [7,60,63,68,70]. Most of the currently used units andequipment in steam cracking processes can be utilized for this cat-alytic cracking process; hence, the new process has a high potentialfor commercialization.

However, large-scale production of light olefins using catalyticcracking has some obstacles. The catalyst should have enough activ-ity, selectivity and stability for the production of light olefins; theother problem could be the production cost. During the last decade,research scientists have focused on developing a number of mod-ified HZSM-5 zeolites with tuned acidities and optimized controlof the catalyst pore size to maximize the selectivities, and mini-mize the by product and coke formation. The suitable catalysts forthe catalytic cracking of paraffins should contain strong acid sitesto initiate the cracking reaction by producing reactive intermedi-ates and the specific pore structures to suppress the formation oflarge hydrocarbons. The high hydrothermal stability of the cata-lysts is also indispensable because the reaction is operated at hightemperatures in the presence of steam [74]. The shape selective, tri-dimensional HZSM-5 zeolite with high Si/Al (>80) ratios has provento be the best choice for light olefin production. Furthermore, theimpact of phosphorus on enhancing the hydrothermal stability ofthe given catalyst is well-established, but it is not easy to spec-ify the other concomitant promoter, exactly. Rare earth elementscould be appropriate candidates because they can amend both theZSM-5 acidity and basicity; thus, the dehydrogenation is acceler-ated, and the readsorption of olefinic basic products is hindered,simultaneously. Anyway, the La-P/HZSM-5 (Si/Al = 200) zeolite hasbeen so far the best compromise in terms of both the high selec-tivity to light olefins and the appropriate stability for the catalyticcracking of naphtha feedstock. The promoters that accelerate dehy-drogenative cracking of hydrocarbons, such as Cr and Fe, are shownto increase the ratio of P/E in the catalytic cracking of C4 alkanes.Finally, to put the catalyst and the catalytic process to practical use,the following studies are needed: (a) test of catalytic performanceon a pilot scale unit, (b) measurement and estimation of the cata-lyst life, (c) formation of catalysts with low pressure drop, and (d)optimization of the catalyst regeneration method [3].

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Catalytic cracking of hydrocarbons over modified ZSM-5 zeolites to produce light olefins A review

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