Hydroisomerization and hydrocracking of linear and multibranched long model alkanes on hierarchical...

This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institution

and sharing with colleagues.

Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third party

websites are prohibited.

In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further information

regarding Elsevier’s archiving and manuscript policies areencouraged to visit:

http://www.elsevier.com/authorsrights

http://www.elsevier.com/authorsrights

Author's personal copy

Catalysis Today 218– 219 (2013) 135– 142

Contents lists available at ScienceDirect

Catalysis Today

jou rn al hom epage: www.elsev ier .com/ locate /ca t tod

Hydroisomerization and hydrocracking of linear and multibranchedlong model alkanes on hierarchical Pt/ZSM-22 zeolite

Johan A. Martensa,∗, Danny Verboekendb, Karine Thomasc, Gina Vanbutselea,Javier Pérez-Ramírezb,∗∗, Jean-Pierre Gilsonc,∗ ∗ ∗

a Center for Surface Chemistry and Catalysis, KU Leuven, Kasteelpark Arenberg 23, B-3001, Heverlee, Belgiumb Department of Chemistry and Applied Biosciences, Institute for Chemical and Bioengineering, ETH Zurich, Wolfgang-Pauli-Strasse 10, CH 8093, Zurich,Switzerlandc Laboratoire Catalyse et Spectrochimie, ENSICAEN, Université de Caen, CNRS, 6, bd Maréchal Juin, 14050, Caen, France

a r t i c l e i n f o

Article history:Received 14 December 2012Received in revised form 6 March 2013Accepted 23 March 2013Available online 25 May 2013

Keywords:Pore mouth catalysisHydrocrackingHydroisomerizationHierarchicalZSM-22Pristane

a b s t r a c t

Hydroisomerization and hydrocracking using bifunctional zeolite catalysts with hydrogenation-dehydrogenation next to the Brønsted acid functionality are at the heart of key refinery processesconverting heavy petroleum fractions to high grade lubricants and fuels. Hierarchical zeolites presentinga network of auxiliary mesoporosity in addition to their native microporosity are known to improveaccessibility to their active sites. In this paper we assess the impact of hierarchization by demetallationof the well-established hydroisomerization catalyst Pt/ZSM-22 on the reaction pathways of the n-decane,n-nonadecane and pristane (2,6,10,14-tetramethylpentadecane) model molecules. Detailed analysis ofreaction products and assessment of acid site accessibility in conventional and hierarchical ZSM-22 sam-ples highlight the contributions of acid sites in pore mouths and micropores to the skeletal rearrangementand cracking reactions.

© 2013 Published by Elsevier B.V.

1. Introduction

Desilication is an established post-synthesis modification pro-cedure used on a wide variety of medium pore size zeolites [1,2].It generates mesopores inside a zeolite crystal by interruptingthe micropores, thus shortening diffusion paths. The impact onacid catalyzed reactions in zeolites is well documented [3–11].In hierarchical zeolites prepared by desilication, the enhancedintracrystalline diffusivity of the reacting molecules does notalways lead to enhanced catalytic activity due to changes of theacidity of the zeolite. In the particular case of TON type zeolitessuch as NU-10 and ZSM-22, with narrow one-dimensional ellip-tical 10-membered ring (10-MR) channels, the rearrangement ofthe framework aluminum is so dramatic that (extra-framework) Alspecies deposited on the external surface of the crystals block theaccess to the micropores to molecules as small as N2 [12–14]. Evenfor a native ZSM-22, activation and the preservation of aluminum inthe framework is critical [15]. The pore blocking species produced

∗ Corresponding author. Tel.: +32 16321610.∗ ∗ Corresponding author. Tel.: +41 446337120.

∗ ∗ ∗ Corresponding author. Tel.: +33 231452815.E-mail addresses: [email protected] (J.A. Martens),

[email protected] (J. Pérez-Ramírez), [email protected] (J.-P. Gilson).

during the alkaline treatment need to be removed by a subsequentmild acid leaching step [12,13]. Previous catalytic investigations ofhierarchical TON type zeolite dealt with relatively small moleculessuch as o-xylene [14], n-octane [13] and 1-butene [16]. In arecent communication we showed that heavier alkanes such asn-nonadecane and pristane benefit more strongly from hierarchiza-tion [17]. Bifunctional hierarchical ZSM-22 showed an enhancedactivity and exceptional selectivity in hydroisomerization of thesebulky substrates. Whether the acid catalyzed turnovers take placeinside the micropores, or with the reactants adsorbed at the poreopenings – the so called pore mouth and key-lock catalysis – hasbeen a matter of debate [18–22]. The mode of action of a ZSM-22catalyst will depend strongly on the size of the reacting moleculesand the reaction conditions. While short alkanes will rapidly diffusein the micropores to react even deep inside the zeolite crystals, asthe length (and the heat of adsorption) of the reactants increases,they will experience severe diffusion constraints and will tend toreact preferentially on the acid sites encountered shortly after pen-etrating a pore. The reaction scheme of pristane, in the absenceof shape selectivity, on an ultrastable Y zeolite and silica-aluminacatalyst is quite remarkable in the sense that the different methylgroups of the molecule show different reactivity [23]. In the presentfull article we explore the impact of hierarchical porosity on theshape selective reaction scheme of these model molecules on con-ventional and hierarchical ZSM-22 zeolite.

0920-5861/$ – see front matter © 2013 Published by Elsevier B.V.http://dx.doi.org/10.1016/j.cattod.2013.03.041


136 J.A. Martens et al. / Catalysis Today 218– 219 (2013) 135– 142

Table 1Porosity and acidity of the ZSM-22 zeolites [17].

Parent Hierarchical

Si/Al ratio (mol/mol) 38 50Vmicro (cm3/g) 0.07 0.05Vmeso (cm3/g) 0.17 0.70Smeso (m2/g) 41 203Bpyridine

a (�mol g−1) 208 113B2,6-lutidine

a (�mol g−1) 16 61

a Adsorption temperature: 150 ◦C; desorption temperature: 200 ◦C.

2. Experimental

2.1. ZSM-22 zeolite samples

The synthesis and characterization of parent (conventional) andhierarchical ZSM-22 zeolite samples are detailed in a previouspublication [17]. The parent ZSM-22 zeolite consists of elon-gated agglomerates of fused nanorods [24]. During desilication,the nanorods are separated and the spacing created generates themesopores [17]. The properties of the two samples are summa-rized in Table 1. The parent and hierarchical sample show oppositetrends in the evolution of their Brønsted acidity probed by pyridine(total number of sites located in the micropores and on the meso-porous surface) and 2,6-lutidine (number of sites located only onthe mesoporous surface). This is the typical behavior [13]. Duringdesilication and acid washing there is a substantial redistributionof accessible aluminum species [13].

For obtaining bifunctional properties, 0.3 wt.% of platinum isloaded on the acid zeolites by incipient wetness impregnationwith aqueous Pt(NH3)4Cl2, followed by drying, pelletizing by com-paction to the 125–250 �m size range for catalytic experimentswith n-decane, and 250–500 �m with n-nonadecane and pristane.Earlier the two samples were shown to have similar platinum dis-persion [17].

2.2. Catalytic experiments

n-Decane hydroconversion is performed in a high-throughput reactor with 50 mg of catalyst pellets [25] underthe following conditions: Ptot = 0.45 MPa; PH2 /PHC = 214;W/Fn-C10

◦ = 1400 kg s mol−1. Pristane and n-nonadecane arediluted to a concentration of 1 mol% in n-heptane, vaporized andconverted in a fixed bed reactor loaded with ca. 0.5 g of catalystpellets under the following conditions: P = 0.45 MPa, PH2 /PHC = 13and W/FC10

◦ = 4600 kg s mol−1 [23]. Prior to reaction the catalystsare pretreated under oxygen at 400 ◦C with a heating rate of5 ◦C/min, flushed with nitrogen, and then reduced with hydrogenat the same temperature. The reaction products are analyzed on-line with GC. The conversion is increased stepwise by increasingreaction temperature at constant contact time.

3. Results and discussion

3.1. Catalytic activity

The evolution of the conversion with temperature for n-decane,n-nonadecane and pristane highlights the impact of hierarchiza-tion of ZSM-22 on catalytic activity (Fig. 1). With n-decane thedesilicated zeolite is less active than its parent, requiring a higherreaction temperature to reach an equivalent conversion level. Inprevious work on n-octane hydroconversion under severe reactionconditions, at a pressure of 5 MPa and a reaction temperature of300 ◦C, the hierarchical specimen was slightly more active than theconventional zeolite [13]. Diffusional limitation is accentuated athigh temperature and pressure. Interestingly, the reactivity orderis reversed with the heavier n-nonadecane, and especially withthe tetrabranched pristane molecule. The parent ZSM-22 sampleshows, by pyridine adsorption, a concentration of 208 �mol g−1 ofBrønsted acid sites and 113 �mol g−1 for its hierarchical deriva-tive (Table 1). Conversely, the concentration of Brønsted acid

Fig. 1. Model alkanes conversion at increasing reaction temperature on conventional (thin lines) and hierarchical (bold lines) Pt/ZSM-22 catalysts. Reaction conditions forn-decane: P = 0.45 MPa; PH2 /PHC = 214; W/FC10

◦ = 1400 kg s mol−1. Reaction conditions for n-nonadecane and pristane: P = 0.45 MPa; PH2 /PHC = 13; W/FC19◦ = 4600 kg s mol−1.


J.A. Martens et al. / Catalysis Today 218– 219 (2013) 135– 142 137

Fig. 2. Yield of skeletal isomers (iso) and cracked products (cr) at increasing conversion for the three model alkanes on conventional and hierarchical Pt/ZSM-22 catalyst(reaction conditions of Fig. 1).

sites located on the mesoporous surface, measured by 2,6-lutidineadsorption [26], is 16 �mol g−1 for the parent against 61 �mol g−1

for its hierarchical derivative. The enhanced catalytic activity ofthe hierarchical version of ZSM-22 for the heavy n- and iso-C19molecules suggests that these molecules are converted over thereadily accessible Brønsted acid sites probed selectively by 2,6-lutidine rather than acid sites located inside micropores probedonly by pyridine.

3.2. Hydroisomerization versus hydrocracking

Isomerization yield curves display a maximum because of theconsecutive hydrocracking reaction (Fig. 2). In earlier work onn-octane hydroconversion on the same zeolites a net gain ofisomerization yield was observed with the hierarchical ZSM-22variant [13]. In the present work a similar remarkable improvementof the hydroisomerization yield of n-decane is obtained (Fig. 2).On the parent zeolite the isomerization yield reaches a maximumof ca. 67%, which is the common behavior [27,28]. After hierar-chization this maximum yield reaches 82%, which is exceptionallyhigh. In particular, the formation of dibranched skeletal isomers isenhanced (Fig. 3). In the conversion of n-nonadecane, the parentzeolite already reaches a very high maximum hydroisomerizationyield of 88%, further increasing to 92% after hierarchization (Fig. 2).The yield of dibranched skeletal isomers peaks at 70% at a very highconversion level (Fig. 3). Pristane is also first isomerized and consec-utively cracked (Fig. 2). The isomer yield is much lower than withn-C19 but, again, the hierarchical zeolite outperforms its parent.The parent and hierarchical ZSM-22 zeolites have the same plat-inum loading and a similar platinum dispersion [17]. The classicinterpretation of an enhanced hydroisomerization yield is a bet-ter balance between metal and acid functions [29]. In a desilicated

ZSM-22 there are two environments for acid sites complicating theinterpretation of catalytic behavior in terms of acid to metal bal-ance. The shortening of the 10-MR channels by segmenting thecrystals and disconnecting the nanorods is expected to improve

Fig. 3. Yield of monobranched (mono) and multibranched (multi) skeletal isomersfrom n-decane and n-nonadecane at increasing conversion for the three modelalkanes on conventional and hierarchical Pt/ZSM-22 catalyst (reaction conditionsof Fig. 1).



Fig. 4. Distribution of methylnonane positional isomers obtained from n-decanehydroisomerization over conventional and hierarchical Pt/ZSM-22 catalysts (reac-tion conditions of Fig. 1).

intimacy between platinum nanoparticles spread over the meso-pores [17] and acid sites inside the zeolite micropores. ZSM-22zeolite is a special case as molecular shape selectivity contributes toa suppression of hydrocracking [18–22]. All authors agree that theexceptional hydroisomerization performance of ZSM-22 is due tothe shape selective suppression of hydrocracking reactions insidethe narrow micropores. In the next section a deeper analysis ofthe reaction products will pin down the origin of the exceptionalhydroisomerization performance of hierarchical ZSM-22.

3.3. Detailed analysis of reaction products

3.3.1. n-Decane hydroconversionThe selectivity for the individual skeletal C10 isomers during n-

decane hydroisomerization is known to be a particularly sensitiveprobe for shape selectivity effects [30,31]. In Fig. 4, the methyl-nonane isomers distribution is plotted against isomerization yield,the upper limit of the range corresponding to the maximumisomerization yield. Initially, the conventional as well as the hier-archical ZSM-22 favor 2-methylnonane formation, and suppressthe formation of 4- and 5-methylnonane. Close to the maximumisomerization yield, the methylnonane positional isomers compo-sition converges to the internally equilibrated composition. Therefined constraint index CI◦, defined as the ratio of 2- over 5-methylnonane formation at 5% hydroisomerization, reflects thepore width [30,31]. Hierarchization causes the CI◦ to drop from14.5 to 8.0, still is in the range of 10-MR zeolites (CI◦ > 2.2). 10-MRzeolites are known to completely suppress the formation of skele-tal isomers with ethyl and larger side chains [30,31]. The parentZSM-22 zeolite does not form ethyloctanes at all, a common prop-erty of 10-MR zeolites. However, on the hierarchical sample at 5%hydroisomerization, the monobranched C10 isomers contain 3.6%ethyloctanes shifting hierarchical ZSM-22 at the transition between10- to 12-MR zeolites (Fig. 5).

Among the dibranched isomers, 2,7-dimethyloctane is themolecule with the methyl side chain positions furthest apart. Onthe parent ZSM-22 zeolite the 2,7-dimethyloctane content of the

Fig. 5. Ethyloctane content of monobranched skeletal isomers from n-decane at 5%hydroisomerization yield.

Reference data from [30,32].

dibranched isomer fraction at 5% dibranched C10 isomer yield is44.8%, a typical result [31]. After desilication, it decreases to 16.7%.Thus the increased hydroisomerization yield after hierarchization(Fig. 2) is accompanied by a partial loss of selectivity for the favoredisomers. Such an observation leads to the following tentative expla-nation for the marked gain in hydroisomerization yield. On theparent ZSM-22, the hydroisomerization yield is limited by the inter-nal thermodynamic equilibrium among n-decane and its preferredskeletal isomers (2-methylnonane, 2,7-dimethylnonane) [18]. Thehierarchical ZSM-22 zeolite catalyst, having less selective sitesenabling the formation of other positional isomers, favors a higherisomerization yield. The effect is akin to a previous observationwhere mixing different 10-MR zeolites, offering a variety of shapeselective environments, resulted in an enhanced hydroisomeriza-tion yield [33].

ZSM-22 is known to lower hydrocracking because its narrowpores hinder the diffusion and formation of dibranched and tri-branched skeletal isomers [18–22]. Such skeletal isomers with twoor more side chains offer the fastest hydrocracking pathways viaa �-scission of their derived alkylcarbenium ions. It is surprisingthat the availability of acid sites with a wider variety of shapeselective environments on the hierarchical zeolite, inferred fromthe formation of more atypical reaction products, does not favorhydrocracking. On the contrary, in the hierarchical catalyst, hydro-cracking is even more suppressed than on conventional ZSM-22,although the multibranched isomers that are sensitive to crackingare formed more abundantly (Fig. 3).

The hydrocracking pattern at 35% hydrocracking yield is pre-sented in Fig. 6. On the parent zeolite the distribution of crackedproducts is slightly asymmetric, with on a molar basis the C3 frag-ments exceeding the C7’s and C4’s exceeding C6’s, indicative ofsecondary cracking. On the hierarchical zeolite, complementaryfragments are formed in equal amount. A further observation isthe lower content of C3 + C7 fragments in the cracked productsafter hierarchization (Fig. 6). They typically originate from parentmolecules with methyl groups at C2 positions (2-methylnonane;2,7-dimethyloctane) and the formation of these isomers is lessfavored, vide supra.

There are arguments for the hydrocracking occurring inside themicropores. Secondary cracking is typical of zeolites with monodi-mensional channels [34]. If cracking occurs inside the micropores,the segmentation of the ZSM-22 crystals after mesopore introduc-tion leads to shortened diffusion paths, facilitating the evacuationof primary fragments therefore lowering secondary cracking. Tounderstand hydrocracking reaction pathways, analyzing the com-position of a light fraction is appropriate because it undergoesless secondary isomerizations. These would obscure the relationwith the cracking of the parent molecules. There is a significant



Fig. 6. Molar yields of cracked products expressed per 100 mol of model molecule cracked at 35% hydrocracking yield on conventional ZSM-22 (thin lines) and hierarchicalZSM-22 catalyst (bold lines). Reaction conditions of Fig. 1.

difference between the conventional and hierarchical ZSM-22 inthe composition of the C5 cracked product fraction (Fig. 7). Onthe conventional ZSM-22, the C5 fraction is composed of ca. 60%n-pentane, and 40% isopentane till ca. 90% hydrocracking. On thehierarchical ZSM-22 isopentane represents ca. 55% and n-pentane45% of the C5 fraction (Fig. 7). Hydrocracking becomes signif-icant after the maximum hydroisomerization yield is obtained(Fig. 2). Under those conditions the internal composition of methyl-nonanes (Fig. 4) as well as the dibranched isomers (not shown)is already equilibrated. Isopentane is formed by cracking of, atleast, dibranched isodecanes. The higher isopentane content in thecracked products from the hierarchical catalyst should then bedue to the higher content of dibranched isomers in the isodecanes(Fig. 3). Because of the high crystallinity and micropore volume ofthe hierarchical zeolite, their micropores are only shortened com-pared to the parent. The transition state shape selectivity should,

therefore, be identical. If the micropores in the two versions ofthe ZSM-22 generate a same fairly linear type of cracked products,the excess of isopentane in the hierarchical catalyst should haveanother origin. The Brønsted acid site distribution, monitored byIR spectroscopy of adsorbed basic probes, and the textural anal-ysis (Table 1) bring further insight in the catalytic results. Thehierarchical zeolite presents a considerably larger number of poremouths compared to the conventional zeolite, reflected by a five-fold increase in mesopore surface area and fourfold increase ofBrønsted acid sites located on this mesopore surface (probed selec-tively by 2,6-lutidine). Conversely, in the hierarchical zeolite, theBrønsted acid sites located inside the micropores decreases by a fac-tor of four (Table 1). These observations validate the proposal thatskeletal isomerization and multibranched isomer hydrocrackingoccur in pore mouths, while cracking into linear fragments mostlyoccurs inside the micropores. Therefore, the desirable textural

Fig. 7. Composition of the C5 cracked product fraction as a function of hydrocracking yield of model alkanes on conventional and hierarchical ZSM-22 catalyst. Reactionconditions of Fig. 1.



Fig. 8. Distribution of methyloctadecanes obtained in n-nonadecane hydroisomer-ization on conventional and hierarchical ZSM-22 catalyst. Reaction conditions ofFig. 1.

changes brought by a sequential base-acid treatment of a parentZSM-22 zeolite are further amplified by a concomitant modificationof the spatial acid site distribution. The conjunction of these twoeffects will also explain the significant selectivity changes observedwith bulky molecules such as n-nonadecane and pristane.

3.3.2. n-Nonadecane hydroconversionn-Nonadecane is isomerized consecutively in monobranched

and multibranched skeletal isomers (Fig. 3). The distribution ofmethyloctadecanes is shown in Fig. 8. At low conversion, 2-methyloctadecane is the dominant isomer, as commonly observedin long n-alkane hydroisomerization on ZSM-22 catalysts [34,35].The preference for 2-methyloctadecane is most pronounced onthe parent ZSM-22 (Fig. 8). With increasing hydroisomerization ofthe n-nonadecane feed, the methyloctadecane composition evolvestoward its internally equilibrated composition, reached when theisomerization yield is at its maximum. The difference in mono- andmultibranched isomer yield between the two ZSM-22 samples isless pronounced than with n-decane. However, the hydrocrack-ing product distributions on the two catalysts are very differentas revealed by the composition of the C5 fraction (Fig. 7).

On the parent zeolite the cracked product distribution is skewedtoward C3–C6, indicative of extensive secondary cracking (Fig. 6).The greater extent of secondary cracking in the conversion of n-C19 compared to n-C10 on the same parent zeolite is ascribed tothe larger size of the primary fragments (C3–C16 vs. C3–C7), leadingto their slower diffusion and increased residence time in the longmicropores. On the hierarchical zeolite the cracked product distri-bution is more evenly spread over the carbon numbers and thereis less over-cracking of long fragments. The branchiness of the C5fraction is different on the two zeolites. On conventional ZSM-22n-pentane is dominant, while on hierarchical ZSM-22, isopentaneis the main isomer (Fig. 7). By analogy with n-decane, the forma-tion of an entirely different C5 product slate from a similar C19skeletal isomer mixture on the two zeolites can be explained by

a greater number of pore mouths promoting skeletal isomeriza-tion and hydrocracking, and a lower contribution of hydrocrackinginside micropores.

3.3.3. Pristane hydroconversionThe pristane (2,6,10,14-tetramethylpentadecane) isomers elute

from the GC column in an unresolved envelope of overlappingpeaks; a detailed identification or even grouping according todegree of branching is, therefore, not possible. Insight in thereaction pathways should come from a detailed analysis of thehydrocracked products. On the parent zeolite only two significantcracked fragments are formed, viz. C3 and C4. This shows that oncea pristane isomer cracks, the fragments, smaller and less branchedundergo further cracking to end up as short fragments (Fig. 6).While it is unlikely that the tetrabranched pristane molecule and itsisomers can diffuse in ZSM-22 channels, fragments can, explainingthe behavior.

Hierarchization of ZSM-22 has a strong impact on pristanehydrocracking, and leads to a more symmetrical hydrocrackingpattern (Fig. 6). Because a detailed analysis of the skeletal iso-mers from pristane is lacking, we used indirect ways to investigatethe reaction scheme [23]. Hydrocracking through �-scission leadsto two fragments which together have one branching less thanthe parent molecule. When a tetrabranched pristane molecule iscracked in two fragments, the total number of branchings in thetwo fragments is 3, corresponding to 1.5 branchings per molecule.Secondary cracking is dealt with in the same way, further reducingthe branchiness of the fragments. Thus the average branchinessof the fragments can be used to verify whether pristane under-went a change in branchiness before cracking (Table 2). Given theexcessive secondary cracking on the conventional ZSM-22 (Fig. 6),the estimation is performed for the hierarchical zeolite only. Theexperimental branchiness value is very close to the expected valuein absence of a change of branchiness of the parent moleculesbefore cracking (Table 2). A same observation was made earlierwith ultrastable Y zeolite, and silica-alumina [23] highlighting thatin pristane hydrocracking the hierarchical ZSM-22 behaves like amesoporous material (silica-alumina) or an hierarchical large porezeolite (ultrastable Y).

Since pristane and its isomers undergo only methyl shift beforecracking, the cracking process can be analyzed even deeper. Pris-tane carries two outer (at C2 and C14) and two inner (at C6 and C10)methyl groups. Even when these positions are altered, scissionsnext to one of the outer methyl groups are distinguished from theinner ones [23]. Hydrocracking through �-scission of a C-C bondwith one of the carbon atoms carrying an outer methyl side chainleads to the formation of a linear and a tribranched fragment, while�-scissions involving tertiary C-atoms carrying a central methylgroup lead to a monobranched and a dibranched fragment [23]. Theratio of the sum of yields of monobranched plus dibranched crackedproducts to linear plus tribranched cracked products as a functionof hydrocracking conversion of pristane on hierarchical ZSM-22 anda reference ultrastable Y zeolite and silica-alumina from ref [23] ispresented in Fig. 9. At low hydrocracking conversion, this ratio isvery high on the hierarchical ZSM-22, about 10 times higher thanon the references. The ratio drops and converges to the referencecatalysts around 40% hydrocracking yield. Even then there remainsa large preference for scissions involving the central methyl groups.This remarkable behavior of pristane on hierarchical ZSM-22 pro-vides strong evidence for an earlier postulate about hydrocrackingin pore mouths, stating that �-scission modes involving penetra-tion of a branched part of the molecule into the micropores areunlikely [36], also in agreement with adsorption studies [37–41].According to that model of �-scission in pore mouths [36] the sym-metrical pristane molecule with branching on both sides at a C2position has no access to the micropores, which is confirmed here



Table 2Branchiness of hydrocracked products from pristane on hierarchical ZSM-22 catalyst.

T (◦C) Hydrocracking yield Average branchiness of cracked products

% Mol/mol pristanecracked

Experimental In absence of change ofbranchinessa

200 4 2.46 1.4 1.3205 8 2.33 1.4 1.3215 23 2.29 1.3 1.3220 34 2.33 1.4 1.3225 57 2.29 1.4 1.3230 71 2.22 1.3 1.3235 86 2.24 1.2 1.3

a Assuming absence of changes of branchiness of pristane skeletal isomers undergoing hydrocracking.

Fig. 9. Ratio of the sum of yields of monobranched plus dibranched cracked prod-ucts to linear plus tribranched cracked products against hydrocracking conversionof pristane on hierarchical ZSM-22 and reference silica-alumina and ultrastable Yzeolite catalysts from ref [23]. Reaction conditions of Fig. 1.

by the very low reactivity of the extremities of the molecule inhydrocracking (Fig. 9).

The high reactivity of pristane and n-nonadecane on the hierar-chical ZSM-22 (Fig. 1) demonstrate that pore mouth catalysis canbe effective and responsible for high catalytic activity. The availablekinetic models of hydrocracking based on pore mouth catalysis,and key-lock catalysis involving penetration of a long moleculewith its extremities in two pore openings [42–44] can be extendedto multibranched alkanes representative of future renewablealkanes.

4. Conclusions

This work adds substantially to the fundamental understand-ing of zeolite catalysis and paves the way to ever more rationaldesign of new generations of hydroisomerization and hydrocrack-ing catalysts. Detailed analysis of hydrocracking products fromlinear and tetrabranched model molecules provides evidence thatskeletal isomerization occurs preferentially in the pore mouths,while hydrocracking occurs both in pore mouths and in the micro-pores. ZSM-22 micropores contribute by producing mostly linearand multiple times cracked fragments. The superior hydroiso-merization performance of hierarchical ZSM-22 is the result of adual reorganization of the spatial distribution of Brønsted acidity,namely: i) a lower number of acid sites in micropores resultingin less hydrocracking and ii) an increased number of acid siteslocated in pore mouth favoring skeletal isomerization. Hierarchiza-tion through demetallation induces significant desirable effectson the catalytic performance of bifunctional ZSM-22 zeolites inhydroconversion of large alkanes. An already excellent hydroiso-merization yield is further increased; in particular, the yield of

multibranched isomers is substantially enhanced. These advanta-geous properties are typically those required in dewaxing processesby isomerization to lower the pour point of lube oils while main-taining the yield and viscosity index (VI) of these valuable products.By decreasing the extent of secondary cracking of linear (n-C10 andn-C19) as well as multibranched alkanes (pristane), the formationof undesired short alkane molecules is minimized. In the particularcase of ZSM-22, it appears that desilication, followed by a mild acidwashing to clear its external surface of detrimental Al species block-ing the pore mouths, is so far a unique and affordable tool to designsuperior hydroisomerization catalysts. Moreover, our current workimplies that even better catalytic performances could be reached byfine tuning the relative extent of the textural and acid site modifica-tions, i.e. locating acid sites where they are the most needed. Sucha strategy can be done in synergy with single-event microkineticmodeling; such detailed modeling already proved [15] its valuablecontribution in testing scenarios to design superior hydroisomer-ization catalysts. Further work is needed, and currently in progress,to find whether these effects are specific to ZSM-22 or can beextended to other 10-MR zeolites.

Acknowledgments

JAM acknowledges the Flemish government for long-termstructural funding (Methusalem) and the Belgian governmentfor supporting interuniversity attraction poles (IAP-PAI). ENSI-CAEN and KU Leuven acknowledge CNRS and FWO Vlaanderenfor supporting their collaboration in the frame of a LEA (Labora-toire Européen Associé). The Swiss National Science Foundation(projects 200021-134572 and 200021-140496) is acknowledged.

References

[1] J. Pérez-Ramírez, C.H. Christensen, K. Egeblad, C.H. Christensen, J.C. Groen,Chemical Society Reviews 37 (2008) 2530–2542.

[2] D. Verboekend, J. Pérez-Ramírez, Catalysis Science & Technology 1 (2011)879–890.

[3] C. Christensen, K. Johannsen, I. Schmidt, C. Christensen, Journal of the AmericanChemical Society 125 (2003) 13370–13371.

[4] M. Bjorgen, F. Joensen, M.S. Holm, U. Olsbye, K. Lillerud, S. Svelle, AppliedCatalysis A: General 345 (2008) 43–50.

[5] Q. Lei, T. Zhao, F. Li, L. Zhang, Y. Wang, Chemical Communications (2006)1769–1771.

[6] K.P. de Jong, J. Zecevic, H. Friedrich, P.E. de Jongh, M. Bulut, S. van Donk, R.Kenmogne, A. Finiels, V. Hulea, F. Fajula, Angewandte Chemie InternationalEdition 49 (2010) 10074–10078.

[7] M.S. Holm, E. Taarning, K. Egeblad, C.H. Christensen, Catalysis Today 168 (2011)3–16.

[8] K. Na, C. Jo, J. Kim, K. Cho, J. Jung, Y. Seo, R.J. Messinger, B.F. Chmelka, R. Ryoo,Science 333 (2011) 328–332.

[9] X. Zhang, D. Liu, D. Xu, S. Asahina, K.A. Cychosz, K. Varoon Agrawal, Y. Al Wahedi,A. Bhan, S. Al Hashimi, O. Terasaki, M. Thommes, M. Tsapatsis, Science 336(2012) 1684–1687.

[10] S. Mitchell, N.-L. Michels, K. Kunze, J. Pérez-Ramírez, Nature Chemistry 4 (2012)825–831.

[11] D. Tzoulaki, A. Jentys, J. Pérez-Ramirez, K. Egeblad, J.A. Lercher, Catalysis Today198 (2012) 3–11.



[12] D. Verboekend, A.M. Chabaneix, K. Thomas, J.-P. Gilson, J. Pérez-Ramírez, Crys-tEngComm 13 (2011) 3408–3416.

[13] D. Verboekend, K. Thomas, M. Milina, S. Mitchell, J. Pérez-Ramírez, J.-P. Gilson,Catalysis Science & Technology 1 (2011) 1331–1335.

[14] C. Sá Couto, P. Matias, E.T. Santos, A. Fernandes, I. Grac a, J.M. Lopes, M.F. Ribeiro,European Journal of Inorganic Chemistry 26 (2012) 4190–4199.

[15] I. Choudhury, K. Hayasaka, J. Thybaut, C.S. Laxmi Narasimhan, J.F. Denayer, J.A.Martens, G.B. Marin, Journal of Catalysis 290 (2012) 165–176.

[16] P. Matias, C. Sá Couto, I. Grac a, J.M. Lopes, A.P. Carvalho, F. Ramôa Ribeiro, M.Guisnet, Applied Catalysis A: General 399 (2011) 100–109.

[17] J.A. Martens, D. Verboekend, K. Thomas, G. Vanbutsele, J.-P. Gilson, J. Pérez-Ramírez, ChemSusChem 6 (2013) 421–425.

[18] J.A. Martens, W. Souverijns, W. Verrelst, R. Parton, G.F. Froment, P.A. Jacobs,Angewandte Chemie International Edition 34 (1995) 2528–2530.

[19] A. van de Runstraat, J.A. Kamp, P.J. Stobbelaar, J. van Grondelle, S. Krijnen, R.A.van Santen, Journal of Catalysis 171 (1997) 77–84.

[20] T.L.M. Maesen, M. Schenk, T.J.H. Vlugt, J.P. de Jonge, B. Smit, Journal of Catalysis188 (1999) 403–412.

[21] J.A. Martens, G. Vanbutsele, P.A. Jacobs, J. Denayer, R. Ocakoglu, G. Baron, J.A.Munoz Arroyo, J. Thybaut, G.B. Marin, Catalysis Today 65 (2001) 111–116.

[22] T.L.M. Maesen, R. Krishna, J.M. van Baten, B. Smit, S. Calero, J.M. Castillo Sanchez,Journal of Catalysis 256 (2008) 95–107.

[23] G. Burnens, C. Bouchy, E. Guillon, J.A. Martens, Journal of Catalysis 282 (2011)145–154.

[24] K. Hayasaka, D. Liang, W. Huybrechts, B.R. DeWaele, K.J. Houthoofd, P. Eloy,E.M. Gaigneaux, G. van Tendeloo, J.W. Thybaut, G.B. Marin, J.F.M. Denayer, G.V.Baron, P.A. Jacobs, C.E.A. Kirschhock, J.A. Martens, Chemical Engineering Journal13 (2007) 10070–10077.

[25] W. Huybrechts, J. Mijoin, P.A. Jacobs, J.A. Martens, Applied Catalysis A: General243 (2003) 1–13.

[26] F. Thibault-Starzyk, I. Stan, S. Abelló, A. Bonilla, K. Thomas, C. Fernandez, J.-P.Gilson, J. Pérez-Ramírez, Journal of Catalysis 264 (2009) 11–14.

[27] S. Ernst, J. Weitkamp, J.A. Martens, P.A. Jacobs, Applied Catalysis A: General 48(1989) 137–148.

[28] J.A. Martens, R. Parton, L. Uytterhoeven, P.A. Jacobs, G.F. Froment, Applied Catal-ysis 76 (1991) 95–116.

[29] F. Alvarez, F.R. Ribeiro, G. Perot, C. Thomazeau, M. Guisnet, Journal of Catalysis162 (1996) 179–189.

[30] J.A. Martens, M. Tielen, P.A. Jacobs, J. Weitkamp, Zeolites 4 (1984) 98–107.[31] J.A. Martens, P.A. Jacobs, Zeolites 6 (1986) 334–348.[32] N. Kasian, G. Vanbutsele, K. Houthoofd, T. Koranyi, J.A. Martens, C.E.A.

Kirschhock, Catalysis Science & Technology 1 (2011) 246–254.[33] G. Hastoy, E. Guillon, J.A. Martens, Studies in Surface Science and Catalysis 158

(2005) 1359–1366.[34] M.C. Claude, J.A. Martens, Journal of Catalysis 190 (2000) 39–48.[35] M. Claude, G. Vanbutsele, J.A. Martens, Journal of Catalysis 203 (2001)

213–231.[36] W. Souverijns, J.A. Martens, G.F. Froment, P.A. Jacobs, Catalysis 174 (1998)

177–184.[37] J.F. Denayer, W. Souverijns, P.A. Jacobs, J.A. Martens, G.V. Baron, Journal of

Physical Chemistry B 102 (1998) 4588–4597.[38] R. Ocakoglu, J.F. Denayer, J.A. Martens, G.V. Baron, in: G.U. Lee (Ed.), Proc. of the

1st Topical Conference on Nanometer Scale Science and Engineering, November4–9, 2001, Reno, Nevada, USA, 2002, pp. 220–227.

[39] R. Ocakoglu, J.F. Denayer, G.B. Marin, J.A. Martens, G.V. Baron, Journal of PhysicalChemistry B 107 (2003) 398–406.

[40] C.S. Laxmi Narasimhan, J.A. Martens, G.V. Baron, J.F. Denayer, Journal of Catal-ysis 218 (2003) 135–147.

[41] J.F. Denayer, R. Ocakoglu, W. Huybrechts, J.A. Martens, J. Thybaut, G.B. Marin,G.V. Baron, Chemical Communications (2003) 1880–1881.

[42] J.F. Denayer, G.V. Baron, G. Vanbutsele, P.A. Jacobs, J.A. Martens, Chemical Engi-neering Science 54 (1999) 3553–3561.

[43] C.S. Laxmi Narasimhan, J. Thybaut, G.B. Marin, P.A. Jacobs, J.A. Martens, J.F.Denayer, G.V. Baron, Journal of Catalysis 220 (2003) 399–413.

[44] C.S. Laxmi Narasimhan, J. Thybaut, J.F. Denayer, G.B. Marin, G.V. Baron,J.A. Martens, P.A. Jacobs, Chemical Engineering Science 59 (2004)4765–4772.

Hydroisomerization and hydrocracking of linear and multibranched long model alkanes on hierarchical...

Documents

Transcript of Hydroisomerization and hydrocracking of linear and multibranched long model alkanes on hierarchical...