Utilization of ZSM-5/MCM-41 composite as FCC catalyst additivefor enhancing propylene yield from VGO cracking

B. R. Jermy • M. A. B. Siddiqui • A. M. Aitani •

M. R. Saeed • S. Al-Khattaf

Published online: 23 July 2011

� Springer Science+Business Media, LLC 2011

Abstract A micro-mesoporous ZSM-5/MCM-41 com-

posite molecular sieve (ZM13) was synthesized and tested

as an FCC catalyst additive to enhance the yield of pro-

pylene from catalytic cracking of vacuum gas oil (VGO).

The catalytic performance of the additive was assessed

using a commercial equilibrium USY FCC catalyst (E-Cat)

in a fixed-bed micro-activity test unit (MAT) at 520 �C and

various catalyst/oil ratios. MCM-41, ZSM-5 and two ZSM-

5/MCM-41 composites were systematically characterized

by complementary techniques such as XRD, BET, FTIR

and SEM. The characterization results showed that the

composites contained secondary building unit with differ-

ent textural properties compared to pure ZSM-5 and MCM-

41. MAT results showed that the VGO cracking activity of

E-Cat did not decrease by using these additives. The

highest propylene yield of 12.2 wt% was achieved over

steamed ZSM-5/MCM-41 composite additive (ZM13)

compared with 8.6 wt% over conventional ZSM-5 additive

at similar gasoline yield penalty. The enhanced production

of propylene over composite additive was attributed to its

mesopores that suppressed secondary and hydrogen trans-

fer reactions and offered easier transport and accessibility

to active sites. Gasoline quality was improved by the use of

all additives except MCM-41, as octane rating increased by

6–12 numbers.

Keywords FCC additive � Catalytic cracking �Gasoline � Propylene � Light olefins � ZSM-5 �Composite molecular sieve � MCM-41

1 Introduction

Fluid catalytic cracking (FCC) process has been a major

source of incremental propylene in which heavy feedstocks

such as vacuum gas oil or residual oil are cracked into value-

added lighter products (LPG and gasoline). Currently, about

30% of the world’s propylene is supplied by refinery FCC

operations, 58% is co-produced from thermal steam cracking

of naphtha or other feedstocks, and the remaining 12% is

produced on-purpose using metathesis, propane dehydro-

genation and high-severity FCC processes [1].

Special FCC process designs and catalysts have been

developed to selectively increase propylene production and

ultimately provide a technology for full light olefins and

aromatics petrochemical integration. Moreover, the addi-

tion of ZSM-5 to FCC catalyst has been one of the efficient

methods for improving propylene yield because it offers

refiners a high degree of flexibility to optimize the pro-

duction output of their FCC units [2, 3]. ZSM-5 additives

crack gasoline range olefins to light olefins, mainly pro-

pylene and butenes. The amount of ZSM-5 crystal added to

USY-based FCC catalysts has increased with values as

high as 25–30 wt% are reported in some commercial FCC

units.

There have been many studies on the effect of ZSM-5

additives on hydrogen transfer reactions and FCC products

selectivities. Incorporating active components such as

phosphorus and metals in the additive formulation that aid

in the conversion of heavier molecules and optimization of

additive formulations with high ZSM-5 levels, are some of

B. R. Jermy � M. A. B. Siddiqui � A. M. Aitani �M. R. Saeed � S. Al-Khattaf (&)

Center of Research Excellence in Petroleum Refining and

Petrochemicals, King Fahd University of Petroleum and

Minerals, Dhahran 31261, Saudi Arabia

e-mail: skhattaf@kfupm.edu.sa

123

J Porous Mater (2012) 19:499–509

DOI 10.1007/s10934-011-9499-0

the approaches for overcoming the loss in activity of FCC

catalyst systems [4–9]. The utilization of other types of

zeolites as FCC additives such as MCM-22 [10], ITQ [11],

beta [12, 13], mordenite [13], MCM-68 [14] and bi-func-

tional ZSM-5/MgAl2O4-based additive [15] were discussed

in other papers. Our recent investigation [16] has demon-

strated the importance of both acidity and mesoporosity on

the performance of mesoporous ZSM-5 additive in

enhancing FCC propylene. Mesopores ensure an optimal

accessibility and transport of VGO feed and products,

while the zeolite micropores induce the preferred shape-

selective properties [17].

Considerable research is undergoing towards controlled

extraction of ZSM-5 through alkali treatment to produce

extended mesoporosity. A mesoporous hierarchical ZSM-5

zeolite was prepared by desilication in aqueous solutions of

tetrapropylammonium or tetrabutylammonium hydroxides

[18]. The silicon dissolution in these hydroxides was much

slower than NaOH, making the demetallation process

highly controllable. Groen et al. [19] showed that an

optimum amount of alkaline treatment of ZSM-5 zeolite

led to a combined porous material with increased meso-

porosity and preserved microporosity. An optimal treat-

ment of commercial ZSM-5 (SiO2/Al2O3 = 37) in

0.2 M NaOH at 65 �C for 30 min resulted in a spectacular

increase in mesopore surface area from 40 to 225 m2/g

(*450%) and a relatively small decrease in microporosity

(25%) [20]. In order to overcome limitations of single

microporous or mesoporous molecular sieves, several

approaches have been developed to form micro/mesopor-

ous composites such as ZSM-5/MCM-41 [21, 22], ZSM-5/

MCM-48 [23], MCM-22/ZSM-35 [24], ZSM-5-SAPO-5/

MCM-41 [25]. The advantages of such composite materials

comprise different pore structure configuration and acidity

distribution.

In this work, attempt has been made to develop a simple

route for the synthesis of a micro/mesoporous composite

ZSM-5/MCM-41 material through ZSM-5 dissolution in

0.7 M NaOH using cetyltrimethylammonium bromide

(CTAB). The composite molecular sieves along with pure

ZSM-5 and MCM-41 were investigated as FCC catalyst

additives for enhancing propylene yield. The catalytic

performance of these additives was evaluated in a fixed-

bed MAT unit for the cracking of hydrotreated Arabian

Light vacuum gas oil under FCC conditions.

2 Experimental

2.1 Materials

The base catalyst used in this study was a commercial

equilibrium FCC catalyst (E-Cat) obtained from a domestic

refinery. It is based on USY zeolite with a surface area of

135 m2/g and a pore volume of 0.23 cm3/g. The E-Cat was

calcined at 500 �C for 3.0 h before further use. HZSM-5

was a conventional zeolite having 27.0 SiO2/Al2O3 molar

ratio (procured from CATAL, UK), whereas MCM-41 and

the two composite ZSM-5/MCM-41 zeolites were prepared

in our lab. For the preparation of the final additive, Cataloid

AP-3 (which contains 75.4 wt% alumina, 3.4 wt% acetic

acid, and water as balance) was used as an alumina binder.

The calcined additives were added at 10 wt% with E-Cat

prior to catalyst characterization and MAT evaluation.

2.1.1 Synthesis of pure MCM-41

MCM-41 material was synthesized by hydrothermal

method according to Beck et al. [26]. 10.6 g of sodium

metasilicate (Aldrich) and 0.95 g of aluminum nitrate were

dissolved in 60 g of water and then thoroughly stirred until

a clear solution was obtained. 3.36 g of CTAB was dis-

solved in 20.0 g of ethanol. The mixture of sodium meta-

silicate and aluminum sulfate was added to this solution

drop-wise. The resultant mixture was stirred for 3.0 h, and

then the pH of the resulting gel was adjusted to 11.0 with

4.0 N sulfuric acid followed by stirring for 3 h. This

homogenous solution was transferred into an autoclave and

heated to 140 �C in static conditions for 12 h.

2.1.2 Synthesis of ZSM-5/MCM-41 composite (ZM13)

Two grams of HZSM-5 was disintegrated in 55.0 mL of

0.7 M NaOH (pH 13.0) by gradual heating at 100 �C for

24 h in the presence of CTAB (4.45%). The mixture was

cooled down and then the pH was adjusted to 9.0 through

the addition of dilute sulfuric acid (2.0 N). The mixture

was then stirred for 24 h and then aged at 100 �C for 24 h

to form a ZSM-5/MCM-41 composite. The solid product

was filtered, washed thoroughly using distilled water, dried

at 80 �C overnight, then calcined at 550 �C for 6 h to

remove the surfactant. The obtained composite was ion-

exchanged thrice with 0.05 M NH4NO3 solution at 80 �C

for 3 h then calcined at 550 �C for 4 h. The composite

sample was subjected to steaming (100% steam) in a fixed-

bed for 4 h at 650 �C and atmospheric pressure. The steam

treatment makes it possible to determine the stability of the

additive (ST-ZM13) under the severe conditions present in

the FCC regenerator.

2.1.3 Synthesis of ZSM-5/MCM-41 composite (ZMST2)

Two grams of commercial HZSM-5 was stirred at 100 �C

for 2 h in 55.0 mL of NaOH (0.7 M NaOH) in the presence

of 4.45% cetyltrimethylammonium bromide (CTAB). The

mixture was cooled down and then the pH was adjusted to

500 J Porous Mater (2012) 19:499–509

123

9.0 through the addition of dilute sulfuric acid (2 N). The

mixture was then stirred for another 24 h and then aged at

100 �C for 24 h. The solid product was filtered, calcined at

550 �C for 6 h to remove the surfactant. The obtained

composite was ion-exchanged thrice with 0.05 M NH4NO3

solution at 80 �C for 3 h then calcined at 550 �C for 4 h.

2.2 Catalyst characterization

Atomic absorption measurement was carried out with a

Perkin Elmer Analyst 100 using a mixed gas of acetylene-

N2O-air. X-ray diffraction (XRD) patterns of the samples

were recorded in a Rigaku Miniflex II diffractometer utiliz-

ing nickel filtered CuKa radiation k = 1.5406 A at 40 kV

and 30 mA. The textural properties of the additives were

characterized by N2 adsorption measurements at 77 K, using

Quantochrome NOVA 1200 adsorption analyzer. Samples

were out-gassed at 350 �C under vacuum (10-5 Torr) for 2 h

before N2 physisorption. The BET specific surface areas

were determined from the desorption data in the relative

pressure (P/Po) range from 0.06 to 0.2. The total pore volume

was calculated from the desorption branch of isotherm, using

the Barrett-Joyner-Halenda (BJH) method. Micropore vol-

ume was determined by t-plot method, whereas mesopore

volume was calculated as the difference between total pore

volume and micropore volume.

The shape and size of zeolite crystals were determined by

scanning electron microscopy (SEM) using Jeol, JSM-

5500LV system. Infrared spectroscopy of adsorbed pyridine

was used to determine the types of acid sites present. The

measurements were carried out using a Fourier Transform

Infrared (FTIR) spectrophotometer (Nicolet 6700), using the

self-supported wafer technique. The concentrations of Lewis

and Bronsted acid sites were determined using the extinction

coefficients for pyridine e(B) = 1.67 ± 0.1 cm lmol-1 and

e(L) = 2.22 ± 0.1 cm lmol-1) [27]. The samples, in the

form of a self-supporting wafer (ca. 5 mg/cm2), were

obtained by compressing a uniform layer of powder. The

wafer was then placed in an infrared vacuum cell equipped

with KBr windows and pretreated under vacuum (ca.

10-3 Torr) at 450 �C for 1 h. The adsorption temperature of

pyridine was 170 �C. Samples were then evacuated at the

same temperature for 20 min.

2.3 Catalytic evaluation

The feed used in all MAT runs was an Arabian Light hy-

drotreated vacuum gas oil (VGO) procured from a Saudi

Aramco domestic refinery. The properties of VGO are listed

in Table 1. The catalytic cracking of VGO was carried out

in a fixed-bed microactivity test (MAT) unit, manufactured

by Sakuragi Rikagaku, Japan according to ASTM D-3907

and D-5154 test methods. For each MAT run, a full mass

balance was obtained. If the material balance was\96% or

[102%, the test was repeated. All MAT runs were per-

formed at a cracking temperature of 520 �C and a time-on-

stream of 30 s. Conversion was varied by changing catalyst/

oil (C/O) ratio in the range of 1.5–5.0 g/g. This variable was

changed by keeping constant the amount of VGO (1.0 g)

and changing the amount of catalyst.

A thorough gas chromatographic analysis of the gaseous

products was conducted to provide detailed yield patterns and

information on the selectivity of the catalyst/additives being

tested. Gaseous products (dry gas and LPG) were analyzed

using two Varian gas chromatographs equipped with 50 m

(0.32 mm diameter) Alumina Plot capillary column and FID/

TCD detectors. Coke on catalyst was determined by a Horiba

carbon analyzer. For liquid products, three different cuts were

considered: gasoline (C5, 221 �C), LCO (light cycle oil,

221–343 �C), and HCO (heavy cycle oil, ?343 �C). The

weight percentage of liquid products was determined by a

simulated distillation GC equipped with 10 m (0.53 mm

diameter) RTX-2887 capillary column and FID detector

according to ASTM D-2887. Gasoline composition was

determined using a Shimadzu GC system that was configured

to give paraffins, olefins, naphthenes and aromatics (POINA)

distribution. The GC was equipped with 50 m (0.15 mm

diameter) BP-1 PONA capillary column and FID detector

and. Conversion was defined as the sum of yields for dry gas

(H2 and C1–C2), LPG (C3-C4), gasoline, and coke.

3 Results and discussion

3.1 XRD results

The XRD patterns of MCM-41, ZSM-5, and ZSM-5/MCM-

41 composites (ZM13, ZMST2 and steamed ZM13) are

Table 1 Properties of Arabian light hydrotreated vacuum gas oil feed

Property Value

Density (g/cm3) (15 �C) 0.896

Sulfur (ppm) 300

Nitrogen (ppm) 170

Saturates (wt%) 59

Aromatics (wt%) 40

Residue (wt%) 0.8

Simulated distillation (�C)

Initial boiling point 308

5% 348

25% 376

50% 420

90% 507

Final boiling point 568

J Porous Mater (2012) 19:499–509 501

123

shown in Fig. 1. The pattern of MCM-41, Fig. 1a, showed

characteristic diffraction patterns similar to that of a typical

mesoporous MCM-41 structure [28]. The d spacing value

of MCM-41 (d10 0 = 4.06 nm) was higher than that of

siliceous MCM-41 (d10 0 = 3.84 nm) indicating frame-

work substitution of aluminum in MCM-41 structure [29].

Figure 1b shows a typical XRD diffraction pattern for

ZSM-5 zeolite.

The XRD pattern of ZM13 composite comprised two

types of patterns corresponding to hexagonal MCM-41 and

ZSM-5. The crystallinity of ZSM-5 was partially retained

with ordered MCM-41 as composite zeolite. The presence

of diffraction peaks at low angle region (2o–5o) and higher

angle (8o–50o) indicates mesophase formation and intact-

ness of ZSM-5 structure at this alkaline condition. The

ZM13 composite sample was found to be highly crystal-

line, compared to earlier reported synthesis of ZSM-5/

MCM-41 composite [30]. Figure 1d shows the XRD pat-

tern of ZM13 after steaming at 650 �C for 4 h. It shows

characteristic diffractions peaks similar to that of

hexagonal pore channels signaling retainment of meso-

phase along with microporous ZSM-5.

The pattern of the other ZMST2 composite sample,

shown in Fig. 1e, exhibited hexagonal symmetry which is

characteristic of MCM-41, but without any Bragg peaks

indicative of a zeolite phase. In this sample, the completely

dissolved ZSM-5 was used for the synthesis of MCM-41-

type mesoporous material.

3.2 Nitrogen-adsorption isotherms

The textural properties of the samples, presented in

Table 2, were determined from nitrogen adsorption iso-

therms. Surface area and pore volume were the highest for

MCM-41 followed by ZMST2 composite. Figure 2 shows

the adsorption isotherms for MCM-41, ZSM-5, ZM13,

ST-ZM13 and ZMST2 samples. The specific surface area,

micropore area, pore diameter (PD) and the total specific

pore volume (TPV), are presented in Table 2. The

adsorption isotherm for MCM-41, Fig. 2a, showed a typi-

cal type IV of the IUPAC classification with steps of

capillary condensation in the primary mesopores. The

isotherm for ZSM-5, Fig. 2b showed typical pattern char-

acteristics of micropores.

The ZM13 sample, Fig. 2c, showed a unique charac-

teristics adsorption isotherm pattern switching from a

typical type IV to an intermediate between type IV and I.

The textural properties presented in Table 2 show that the

character is unique for this zeolite composite different from

those of MCM-41 and ZSM-5. The mesoporous surface

area increased from 284 to 468 m2/g. The pore diameter

increased to 2.4 nm compared to ZSM-5 but remain less

than that of MCM-41. Such a transformation of type I N2

isotherm of purely microporous sample into a combined I

and IV isotherm in the NaOH treated sample is attributed

due to the hierarchical porous system [31]. Song and Yan

observed similar larger type-H4 hysteresis loop for ZSM-5/

MCM-41 sample at relative pressure P/P0 [ 0.45, which

they attributed to extensive structural defective holes [22].

Fig. 1 XRD patterns for: (a) MCM-41, (b) ZSM-5, (c) ZM13,

(d) ST-ZM13, and (e) ZMST2

Table 2 Physical properties of pure zeolites and ZSM5/MCM-41 composites

Zeolite/composite dspacing (nm) a0 (nm)a PD (nm)b TPV (cc/g)c Vmicro (cc/g) Vmeso (cc/g) SA (m2/g)d

MCM-41 4.06 4.68 4.4 0.73 0.10 0.63 660

ZSM-5 3.85 – – 0.29 0.04 0.25 284

ZM13 4.06 4.68 2.4 0.25 0.18 0.07 468

ST-ZM13 3.50 4.04 2.2 0.26 0.13 0.14 365

ZMST2 3.86 4.45 2.2 0.72 0.34 0.38 527

a a0 hexagonal unit cellb PD pore diameterc TPV total pore volumed SA: BET mesoporous surface area (Hexagonal lattice parameter (a0) was calculated by a0 = 2 9 d(100)/H3)

502 J Porous Mater (2012) 19:499–509

123

The effect of steaming on the textural properties of ZM13

showed that the mesoporous structures were quite well

preserved with considerable uniformity in the pore size

distributions. The surface area reduced from 468 to

365 m2/g, while pore diameter decreased from 2.4 to

2.2 nm. The hydrothermal of the composite can be attrib-

uted to the presence of zeolite building units in the com-

posite material.

The isotherm of ZMST2 sample, Fig. 2e can be classi-

fied as typical type-IV isotherms with a sharp inflection at

relative pressure P/P0 = 0.3–0.4, characteristic of capillary

condensation of mesoporous materials and the isotherm

uniformity indicates the unique mesopore-size distribution

The difference in the textural property and the acidity

could be attributed to the differences between the alumi-

nosilicate fragments from alkali-treated ZSM-5 zeolite and

the amorphous silica–alumina species used for MCM-41.

The ZMST2 synthesized from aluminosilicate fragments

with ZSM-5 structure units have slightly varied textural

property, where high surface area of 527 m2/g, compara-

tively lower pore diameter of 2.2 nm and larger pore vol-

ume of 0.72 cm3/g were observed.

3.3 SEM images

Figure 3 shows the SEM images of the four samples that

have different morphological structure. Figure 3a shows the

irregular crystal morphology of MCM-41, whereas hexag-

onal shaped crystals are visible for ZSM-5. The composite

samples showed a unique aggregated crystals like mor-

phology, that is clearly different from those of parent ZSM-

5 and MCM-41 particles. ZM13 showed the formation of

mesopores over ZSM-5 structure whereas ZMST2 showed a

unique aggregated crystals like morphology.

Fig. 2 Nitrogen adsorption isotherms for: (a) MCM-41, (b) ZSM-5,

(c) ZM13, (d) STZM13, and (e) ZMST2

Fig. 3 SEM images of: a MCM-41, b ZSM-5, c ZM13 and d ZMST2

J Porous Mater (2012) 19:499–509 503

123

3.4 FTIR measurement

Figure 4 shows FTIR spectra of the four samples. The

spectra of MCM-41, Fig. 4a, showed a broad absorption

band between 3,150 and 3,700 cm-1 due to asymmetric

stretching of –OH group, corresponding to water and

hydroxyl functional group. The absorption bands at 1,201,

1,082 and 777 cm-1 represent the asymmetric stretching

vibration of outer SiO4 tetrahedron, the asymmetric

stretching vibration of inner SiO4 tetrahedron, and the

symmetric vibration of SiO4 tetrahedra outer links [32],

respectively. An absorption band around 941 cm-1 is

assigned to the metal incorporation (Si–O–Al) into the

framework of the mesoporous material [33]. In the case of

pure ZSM-5, Fig. 4b shows a characteristic vibration of

framework OH groups from Brønsted acid sites is seen

around 3,680 cm-1. The absence of a band at 3,720 cm-l

indicates high crystalline material with negligible amounts

of terminal silanols.

The spectra of the two composite samples showed a

comparatively less intense band around 3,400 cm-1, which

was observed due to the as-symmetric –OH stretching. In

Fig. 4, ZM13 composite showed the presence of five-

membered rings of zeolitic subunits around 551.5 cm-1

similar to that of pure ZSM-5 [34]. The absorption band at

444 cm-1 represents the deformation vibration of silicon

oxygen and aluminum oxygen bond T–O bend of com-

posite. The FTIR spectra of steamed ZM13 (ST-ZM13)

showed the characteristics bands of ZSM-5 and MCM-41

composite. The presence of less intense hydroxyl vibra-

tions around 3,500 cm-1 as shown in Fig. 4d indicates the

occurrence of dehydration, and the withstanding capacity

of more severe steaming condition.

The FTIR spectra of ZMST2, Fig. 4e, showed a com-

paratively less intense band near 550 cm-1, indicative of

five-membered ring subunits. In all samples, a complete

removal of surfactant was seen with the disappearance of

three absorption bands at 2,923, 2,852 and 1,489 cm-1

assigned to asymmetric, symmetric stretching and bending

mode of C–H group of surfactant [35].

3.5 Acidity measurement

Figure 5 shows FTIR spectra of pyridine chemisorbed on

the five samples (a) MCM-41, (b) ZSM-5 (c) ZM13,

(d) ST-ZM13, and (e) ZMST2 at 150 �C. The spectra

exhibited a clear distinction between Bronsted (B) and

Lewis (L) acid sites. The data of acid sites density are

reported in Table 3. All samples displayed both Lewis and

Bronsted acid sites, however, the extent of these sites

depended significantly on the nature of these samples. A

clear difference was seen between ZSM-5, MCM-41 and

the composite samples.

ZMST2 composite sample showed the highest acid sites

density (0.80 mmol/g) owing to its low SiO2/Al2O3 ratio,

while the density of acid sites for ZSM-5 was the lowest

(0.66 mmoles/g). ZMST2 showed the highest Lewis to

Bronsted (L/B) ratio (10.42), whereas ZSM-5 displayed the

lowest L/B ratio (0.46). The concentration of Bronsted sites

decreased in the following order: ZSM-5 [ ZM13 [ ST-

ZM13 [ MCM-41 [ ZMST2. These results explain the

low acid density of MCM-41 compared to ZSM-5 despite

the fact that both have relatively close SiO2/Al2O3 ratios.

3.6 VGO conversion and yields

The conversion versus C/O ratio over E-Cat and E-Cat/

additives is shown in Fig. 6. The plots show linear rela-

tionship between conversion and C/O that confirms the

apparent second-order kinetics of VGO cracking over all

Fig. 4 FTIR spectra of: (a) MCM-41, (b) ZSM-5, (c) ZM13, (d) ST-

ZM13, and (e) ZMST2

Fig. 5 Pyridine adsorbed FTIR spectra of: (a) MCM-41, (b) ZSM-5

(c) ZM13, (d) ST-ZM13, and (e) ZMST2

504 J Porous Mater (2012) 19:499–509

123

E-Cat/additives [36, 37]. Over E-Cat base catalyst, the

conversion increased from 55 to 73% as C/O ratio was

varied from 1.5 to 4. At high C/O ratio, all additives

showed higher conversion compared with E-Cat despite the

dilution effect of the additives. The conversion order

obtained for the four additives (ZM13 [ ZMST2 [ MCM-

41 [ ZSM-5) is explained by the different total acidity

shown in Table 3.

Figure 7 shows the changes in product yields (selectiv-

ity curves) for propylene, ethylene, LPG, gasoline, dry gas,

and coke as a function of conversion over E-Cat and E-Cat/

additives. Except for gasoline, all product yields were

higher over E-Cat/additives compared with the base cata-

lyst (E-Cat). The results showed an increase in HCO yields

at a constant conversion of 75.0% and only a slight

decrease in LCO yield for all E-Cat/additives. By them-

selves, the absolute yields obtained in MAT testing of any

catalyst are of little value (Table 4). Rather, it is the

comparison of yields between the additives at constant

conversion that provides a means for distinguishing them.

Using the selectivity curves developed for each additive,

yield ratios of selected components (dry gas, LPG, and

gasoline) were determined at constant conversion to truly

distinguish the performance characteristics of additives.

3.6.1 LPG and light olefins yields

LPG (C3–C4) yield increased linearly with conversion over

all E-Cat/additives as shown in Fig. 6. It increased by 6.0

wt% over E-Cat and MCM-41 (10–16 wt%) compared to

an increase of 11 wt% over ZSM-5 and ZMST2 (24–35

wt%). ZM13 showed the highest increase in LPG yield at

higher conversion with about 15 wt% drop in corre-

sponding gasoline yield. LPG olefinicity was the highest

over steamed ZM13 and the lowest over MCM-41. There

was slight increase in olefinicity with increased conversion

over E-Cat. While LPG yield over E-Cat was about 17 wt%

at constant conversion (75%), it increased by two folds

over all E-Cat/additives except for MCM-41 additive.

However, the composition of LPG was different. Over

ZSM-5 and ZM13, LPG comprised 18% propane compared

to 5% over MCM-41 and ZMST2. Butanes content was the

highest over ZSM-5 at 30% compared with 4% each for

MCM-41 and ZMST2, and 24% over steamed ZM13.

The difference in LPG enhancement for the various

additives can be explained by comparing the hydrogen

transfer coefficient (HTC) which is defined as the ratio of

butanes to butenes [38–40]. Hydrogen transfer is a bimo-

lecular reaction that requires the reactants to be in close

proximity to a strong acid site. It reflects the hydrogen

transfer activity of the additives. The HTCs of ZSM-5 and

ZM13 were similar, which explains the similar enhance-

ment of light olefins over steamed ZM13. The steamed

ZM13, which yielded higher light olefins, showed the rel-

atively low HTC of 0.8. The high yield of light olefins

obtained over steamed ZM13 is attributed to its low HT

activity which is due to its lower acidity, highly accessible

pore structure and rapid elution of primary products due to

its mesopores.

The yield of C2–C4 light olefins increased with

increasing conversion over E-Cat and E-Cat/additives as

shown in Fig. 8. At constant conversion of 75%, maximum

light olefins yield was obtained over ZM13 additive (24.8

wt%) compared with E-Cat (11.9%). ZSM-5 and ZMST2

additives yielded equal amount of light olefins at 19.5%

followed by MCM-41 at 13.4%. As illustrated in Table 4

and Fig. 8, the yields of propylene and ethylene showed the

Table 3 Acidity of the zeolites and composites

Additive sample SiO2/Al2O3

molar ratio

Lewis acidity, L

(mmol/g)

Bronsted acidity, B

(mmol/g)

Total acidity: L ? B

(mmol/g)

L/B ratio

MCM-41 78.0 0.53 0.19 0.72 2.78

ZSM-5 27.0 0.21 0.45 0.66 0.46

ZM13 24.0 0.24 0.35 0.59 0.68

ZMST2 26.0 0.73 0.07 0.80 10.42

ST-ZM13 26.0 0.32 0.27 0.59 1.18

Co

nver

sio

n, %

C/O, ratio

Fig. 6 Conversion versus C/O for E-Cat and E-Cat/additives: (opentriangle) E-Cat; (filled diamond) ZSM-5; (open square) MCM-41;

(open circle) ZM13; and (filled circle) ZMST2

J Porous Mater (2012) 19:499–509 505

123

largest increase in light olefins fraction. However, only the

composite additives showed an increase in butenes yield.

The increase in light olefins yield is attributed to the con-

version of reactive gasoline-range species (mainly iso-

paraffins and olefins).

Over E-Cat, propylene yield increased from 3.0 to 5.0

wt% upon increasing conversion from 55.0 to 84.0%.

ZM13 showed the highest increase in propylene yield from

5.0 to 12.2 wt% for a conversion between 55.0 and 84.0%.

For ZSM-5 and ZMST2, propylene yield was in the range

of 8–9 wt%. The increase in propylene yield was the lowest

over MCM-41 additive (3.0–5.0 wt%). ZM13 showed a

propylene yield enhancement of more than 100% com-

pared with E-Cat. Propylene yield increased from 5 wt%

over E-Cat to 12.2 wt% over ZM13. ZMST2 and ZSM-5

additives enhanced propylene yield by 4.0 wt% each, while

MCM-41 increased propylene yield by 2.0 wt%.

The rapid increase in ethylene yield with increasing

conversion was similar for all E-Cat/additives, however,

this increase was much higher compared with E-Cat

(Fig. 7). At constant conversion, ethylene yield increased

by 3.0 wt% over ZM-5 and ZM13, 1.0 wt% each over

MCm-41 and ZMST2 and 2.1 wt% over steamed ZM13.

On the other hand, the increase in butenes was significant

over steamed ZM13 (?4.0 wt%), however, the other three

additives did not show any significant increase in butenes

yield.

3.6.2 Dry gas and coke yields

The yield of dry gas (H2, C1–C2) varied between 1 and 2

wt% over E-Cat within the conversion range of 65–75%

(Fig. 7). All E-Cat/additives showed a linear increase in

dry gas yield with increasing conversion. Dry gas yields

over ZSM-5 and ZMST2 were the highest at 3–7 wt%

reflecting the increase in ethylene yield. ZM13 and MCM-

41 yielded 1–3.5 wt% dry gas, respectively. The other

fractions of dry gas (C1 and C2) did not show any signif-

icant change over all E-Cat/additives. The cracking

mechanism ratio (CMR), which is defined as the ratio of

Yie

ld, w

t.%

Conversion, %

Yie

ld, w

t.%

Conversion, %

Yie

ld, w

t.%

Conversion, %

Yie

ld, w

t.%

Conversion, %

Yie

ld, w

t.%

Conversion, %

Yie

ld, w

t.%

Conversion, %

Fig. 7 Product yields over

E-Cat and E-Cat/additives:

(open triangle) E-Cat; (filleddiamond) ZSM-5; (open square)

MCM-41; (open circle) ZM13;

and (filled circle) ZMST2

506 J Porous Mater (2012) 19:499–509

123

dry gas to iso-butane, is a measure of protolytic cracking to

beta scission cracking [12, 40]. Dry gas products reflect

protolytic cracking while iso-butane reflects products

formed by beta scission of branched hydrocarbons. The

low value of CMR for both E-Cat and ZSM-5 indicates that

beta scission cracking mechanism was dominant compared

to protolytic cracking.

Coke yield over E-Cat increased from 0.4 to 2.7 wt%

within the conversion range studied, as shown in Fig. 7. At

75% conversion, coke yield over all E-Cat/additives was in

the range of 1.1–1.8 wt%. Steamed ZM13 gave the highest

increase in coke yield of 2.4 wt%.

3.6.3 Gasoline yield and composition

Gasoline yield increased with increasing conversion over

E-Cat and for all additives. Highest gasoline yield was over

MCM-41 and E-Cat. It increased from 42.0 to 57.0 wt%

with increasing conversion from 55.0 to 84.0%, respec-

tively. For ZSM-5 and ZMST2, gasoline yield increased

from 25 to 35 wt%, while it increased slightly over ZM13

from 40 to 42 wt%. Over both E-Cat and MCM-41, gasoline

yield at 75% conversion was 55 wt% (Table 4) compared to

about 35 wt% over ZSM-5 and steamed ZM13. For ZMST2

additive, gasoline yield was about 48 wt%.

Figure 9 presents the composition of gasoline fraction

(n-paraffins, iso-paraffins, olefins, naphthenes and aromat-

ics) at 75.0% conversion over E-Cat and E-Cat/additives.

Over E-Cat, gasoline comprised 55.0% of the total cracked

products. Its composition was 4.0% n-paraffins, 30% iso-

paraffins, 10% olefins, 12% napththenes, and 44%

Table 4 Comparative MAT data at constant conversion (75%) over E-Cat and E-Cat/additives at 520 �C

Product yields (wt%) Base E-Cat E-Cat/10 wt% additive

MCM-41 ZSM-5 ZM13 ZMST2 Steamed ZM13

Dry gas 1.4 2.4 4.6 5.1 2.5 3.8

H2 0.08 0.09 0.14 0.11 0.07 0.10

C1 0.4 0.64 0.48 0.58 0.57 0.64

C2= 0.5 1.0 3.4 3.6 1.2 2.4

C2 0.4 0.68 0.57 0.76 0.64 0.69

LPG 16.7 17.2 33.3 32.5 23.0 32.5

C3= 5.0 5.4 8.6 9.6 8.4 12.2

C3 0.8 0.81 6.2 5.5 1.00 2.2

C4= 6.4 7.1 6.9 8.1 8.8 10.2

n-C4 0.1 0.52 2.6 2.3 0.60 1.1

i-C4 4.5 3.4 9.0 7.0 4.20 6.8

C2 = –C4= 11.9 13.5 18.9 21.3 18.4 24.8

Gasoline 55.0 54.5 35.0 36.0 48.0 36.6

LCO 19.5 13.6 18.2 11.6 14.0 13.5

HCO 5.2 10.5 6.5 12.0 11.0 11.5

Coke 1.1 1.1 1.5 1.8 1.1 2.4

HTCa 0.7 0.6 1.7 1.1 0.5 0.8

CMRb 0.3 0.7 0.5 0.7 0.6 0.6

Gasoline RONc 75 75 87 79 80 81

a HTC Hydrogen transfer coefficient (nC4 ? iC4)/C4=b CMR Cracking mechanism ratio (dry gas/iC4)c RON Research Octane Number by GC PIONA

0

2

4

6

8

10

12

14

16

Ethylene Propylene Butenes C2-C3 Olefins

Yie

ld, w

t.%

E-cat

ZSM-5

MCM-41

ZM13

ZMST2

ST-ZM13

Fig. 8 Light olefins yield at constant conversion of 75% over E-Cat

and E-Cat/additives

J Porous Mater (2012) 19:499–509 507

123

aromatics. There was not much change in the n-paraffins

content of gasoline over all E-Cat/additives. More than half

of the iso-paraffins were cracked over ZSM-5, ZM13 and

ZMST2 as its content decreased from 30.0 to about 13.0%.

The lowest decrease of 4.0% in iso-paraffins was observed

for MCM-41. The amount of iso-paraffins over steamed

ZM13 composite did show any change compared with

E-Cat indicating some isomerization function in this

additive.

Although about 70% of gasoline-range olefins were

cracked over ZSM-5 compared with 4% over ZMST2,

more olefins were formed over MCM-41. As discussed

earlier, this was attributed to the higher hydrogen transfer

activity (HTC) of ZSM-5 compared with other additives

[41]. MCM-41 showed higher olefins content while the

composite samples (ZM13, ST-ZM13 and ZMST2) did not

crack much of the gasoline range olefins.

Among various gasoline hydrocarbon groups, naphthenes

did not show much significant change over all E-Cat/addi-

tives. As ring cracking was not possible at the reaction con-

ditions employed [42], naphthenes were slightly decreased

over ZSM-5 additive due to dehydrogenation reactions.

Gasoline aromatics content increased over ZSM-5 and

ZMST2 additives. There was a 15–30% increase in aro-

matics over ZMST2 and ZSM-5 compared with E-Cat. For

MCM-41 and steamed ZM13 composite, aromatic content

in gasoline dropped by 10 and 5%, respectively, with no

change over ZM13. Aromatics were formed by cyclization

and dehydrogenation of iso-paraffins and to some extent by

the dehydrogenation of naphthenes. This explains the high

iso-paraffins content in both MCM-41 and steamed ZM13

compared with other additives.

The increase in aromatics in some additives was asso-

ciated with a corresponding increase in research octane

number (RON) that was determined by GC method. While

gasoline RON was 75 over E-Cat, it increased over all

E-Cat/additives except MCM-41. An improvement of 12

numbers was obtained over ZSM-5. Low enhancement in

octane rating was observed for ZM13, ZMST2 and steamed

ZM13 (80). Although incremental aromatics over ZMST2

were higher compared with ZSM-5, the increase in RON

over steamed ZM13 is attributed to the higher content of

iso-paraffins compared with ZSM-5 additive.

4 Conclusions

In this study, we have reported the synthesis and utilization

of ZSM-5/MCM-41 (ZM13) composite as an FCC catalyst

additive to enhance propylene yield. The composite zeolite

was synthesized through a simple route comprising ZSM-5

dissolution in 0.7 M NaOH using CTAB. Propylene yield

increased from 5.0 wt% over E-Cat (base case) to 12.2 wt%

over steamed ZM13 composite additive compared with 5.4

wt% over MCM-41 and 8.6 wt% over ZSM-5. Enhanced

mass transport was a critical issue for the increase in pro-

pylene and other light olefins yields over the composite

additive. Olefins-consuming hydrogen transfer reactions

were suppressed and secondary reactions were reduced due

to shorter residence time and rapid elution of primary

cracking products. PIONA results showed that the loss in

gasoline yield was mainly due to the cracking of gasoline

range iso-paraffins and olefins in some additives. The

advantage of using ZSM-5/MCM-41 composite as an FCC

additive appeared in the relatively high-octane value for the

gasoline fraction. For all additives except MCM-41, gas-

oline octane number increased by 6–12 numbers due to the

significant increase in aromatics level, mainly over ZSM-5

additive.

Acknowledgments The authors acknowledge the financial support

provided by King Abdul Aziz City for Science and Technology

(KACST) for this research under Refining & Petrochemicals Program

of the National Science, Technology & Innovation Plan (NSTIP;

09-PETE85-4). We are also grateful for the support from Ministry of

Higher Education, Saudi Arabia for establishing the Center of

Research Excellence in Petroleum Refining & Petrochemicals at King

Fahd University of Petroleum & Minerals (KFUPM).

References

1. J. Yim, Presented at the 11th Asia Petrochemical Industry Con-

ference (APIC), May 2011, Fukuoka, Japan

2. M. Hamayel, A. Aitani, M.R. Saeed, Chem. Eng. Technol. 28,

923 (2005)

3. A. Lappas, C.S. Triantafillidis, Z. Tsagrasouli, V. Tsiatouras, I.A.

Vasalos, N. Evmiridi, Stud. Surf. Sci. Catal. 142, 807 (2002)

4. A. Aitani, T. Yoshikawa, T. Ino, Catal. Today 60, 111 (2000)

5. T.F. Degnan, G.K. Chitnis, P.H. Schipper, Microporus Meso-

porous Mater. 35–36, 245 (2000)

6. D. Wallenstein, R.H. Harding, Appl. Catal. A: General 214, 11

(2001)

0

10

20

30

40

50

60

70

80

n-Paraffins i-Paraffins Olefins Naphthenes Aromatics

Yie

ld, w

t.%

E-cat

ZSM-5

MCM-41

ZM13

ZMST2

ST-ZM13

Fig. 9 Gasoline composition at constant conversion of 75% over

E-Cat and E-Cat/additives

508 J Porous Mater (2012) 19:499–509

123

7. J. Arandes, I. Torre, M.J. Azkoiti, J. Erena, M. Olazar, J. Bilbao,

Energy Fuels 23, 4215 (2009)

8. X. Gao, Z. Tang, H. Zhang, D. Ji, G. Lu, Z. Wang, Z. Tan, J. Mol,

A. Catal, General 325, 36 (2010)

9. J. Wenbin, C. Beiyan, S. Ningyuan, S. Haitao, Z. Yuxia, China

Pet. Proc. Petrochem. Tech. 12, 13 (2010)

10. A. Corma, J. Martinez-Triguero, J. Catal. 165, 102 (1997)

11. A. Corma, J. Martinez-Triguero, C. Martinez, J. Catal. 197, 151

(2001)

12. Z. Liu, J. Fu, M. He, M. Li, Prepr. Pap. Am. Chem. Soc. Div. Fuel

Chem. 48, 712 (2003)

13. A.F. Costa, H.S. Cerqueira, J.M. Ferreira, N.M. Ruiz, M.C.

Menezes, Appl. Catal. A: General 319, 137 (2007)

14. S. Inagaki, K. Takechi, Y. Kubota, Chemical Comm. 46, 2662

(2010)

15. X. Xu, X. Ran, Q. Cui, C. Li, H. Shan, Energy Fuels 24, 3754

(2010)

16. M.A.B. Siddiqui, A. Aitani, M.R. Saeed, N. Al-Yassir, S. Al-

Khattaf, Fuel 90, 459 (2011)

17. S. van Donk, A.H. Janssen, J.H. Bitter, K. de Jong, Catal. Rev.

45, 297 (2003)

18. S. Abello, A. Bonilla, J. Perez-Ramırez, Appl. Catal. A: General

364, 191 (2009)

19. J.C. Groen, W. Zhu, S. Brouwer, S. Huynink, F. Kapteijn, J.A.

Moulijn, J. Perez-Ramirez, J. Am. Chem. Soc. 129, 355 (2007)

20. J.C. Groen, L.A. Peffer, J.A. Moulijn, J. Perez-Ramirez, Colloids

Surfaces A: Physicochem. Eng. Aspects 241, 53 (2004)

21. M.L. Goncalves, L.D. Dimitrov, M.H. Jordao, M. Wallau, E.

Urquieta-Gonzalez, Catal. Today 133–135, 69 (2008)

22. C. Song, Z. Yan, Asia-Pac. J. Chem. Eng. 3, 275 (2008)

23. C. He, P. Li, J. Cheng, H. Wang, J. Li, Q. Li, Z. Hao, Appl. Catal.

A: General 382, 167 (2010)

24. X. Niua, Y. Songa, S. Xiea, S. Liua, Q. Wanga, L. Xu, Catal.

Lett. 103, 211 (2005)

25. J. Qi, T. Zhao, X. Xu, F. Li, G. Sun, J. Porous. Mater. 18, 69

(2011)

26. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge,

K.D. Schmitt, C.T. Chu, D.H. Olson, E.W. Sheppard, S.B.

McCuller, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc. 114,

10834 (1992)

27. C.A. Emeis, J. Catal. 141, 347 (1993)

28. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck,

Nature, London 359, 710 (1992)

29. R.B. Borade, A. Clearfield, Catal. Lett. 31, 267 (1995)

30. S. Inagaki, M. Ogura, T. Inami, Y. Sasaki, E. Kikuchi, M. Mat-

sukata, Microporus Mesoporous. Mat. 74, 163 (2004)

31. J. Perez-Ramırez, C.H. Christensen, K. Egeblad, C.H. Christen-

sen, J.C. Groen, Chem. Soc. Rev. 37, 2530 (2008)

32. K.H. Schnabel, G. Finger, J. Kornatowski, E. Loffler, C. Peuker,

W. Pilz, Microporous Mater. 11, 293 (1997)

33. M. Selvaraj, A. Pandurangan, K.S. Seshadri, P.K. Sinha, K.B.

Lal, App. Catal. A: General 242, 347 (2003)

34. Y. Liu, W. Zhang, T.J. Pinnavaia, Angew. Chem. Int. Ed. 40,

1255 (2001)

35. A. Palani, N. Gokulakrishnan, M. Palanichamy, A. Pandurangan,

Appl. Catal A: General 304, 152 (2006)

36. S. Al-Khattaf, H.I. de Lasa, Can. J. Chem. Eng. 79, 341 (2001)

37. K.A. Mahgoub, S. Al-Khattaf, Energy Fuels 19, 329 (2005)

38. M.A. den Hollander, M. Wissink, M. Makkee, J. Moulijn, Appl.

Catal. A: General 223, 85 (2002)

39. X. Zhu, S. Liu, Y. Song, L. Xu, Appl. Catal. A: General 288, 134

(2005)

40. A.F. Wielers, M. Vaarkamp, M.F. Post, J. Catal. 127, 51 (1991)

41. J. Dekun, L. Shuyuan, D. Fuchen, C. Yaoling, China Petroleum

Proc. Petrochem. Tech. 11, 10 (2009)

42. J. Scherzer, Catal. Rev. Sci. Eng. 31, 215 (1989)

J Porous Mater (2012) 19:499–509 509

123