Engineering Zeolites for Catalytic Cracking to Light Olefins - ACS

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Engineering zeolites for catalytic cracking to light olefins Vincent Blay, Benoit Louis, Ruben Miravalles, Toshiyuki Yokoi, Ken A. Peccatiello, Melissa Clough, and Bilge Yilmaz ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02011 • Publication Date (Web): 17 Aug 2017 Downloaded from http://pubs.acs.org on August 17, 2017

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Engineering zeolites for catalytic cracking to light olefins Vincent Blay1*, Benoît Louis2, Rubén Miravalles3, Toshiyuki Yokoi4, Ken A. Peccatiello5, Melissa Clough7, Bilge Yilmaz8 1

Departamento de Ingeniería Química, Universitat de València, Av. de la Universitat, s/n, 46100 Burjassot, Spain

2

Laboratoire de Synthèse Réactivité Organiques et Catalyse, Institut de Chimie, UMR 7177 CNRS, Université de Strasbourg, 1 rue Blaise Pascal, 67000 Strasbourg cedex, France

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Centro de Tecnología Repsol, C/ Agustín de Betancourt s/n, 28935 Móstoles, Spain

Institute of Innovative Research, Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan 5 7

Peccatiello Engineering, Catalytic Cracking Solutions LLC, NM 87035, USA

BASF Refinery Catalysts, 11750 Katy Fwy. Ste. 120, Houston, TX 77079, USA 8

BASF Refinery Catalysts, 25 Middlesex-Essex Tpk., Iselin, NJ 08830, USA *Corresponding author: [email protected]

Abstract: Propene is a key building block for the petrochemical industry whose demand is increasing strongly in recent years, even faster than that of ethene. The availability of propene is limited and therefore efforts to optimize its production are being pursued. On the occasion of the 75th anniversary of the first FCC unit, we analyze some recent advances that have been achieved in the understanding and development of zeolites aiming to increase the production of light olefins as petrochemical building blocks by means of catalytic cracking. We discuss a selected group of emerging strategies in zeolite engineering that have great prospects for research and that we consider could impact the sector in the near future. These include advances in crystal engineering and hierarchization achieved through bottom-up and top-down approaches, composite materials, tuning of the location of active sites among the different

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crystallographic positions available, and, importantly, how to characterize these modifications and their impact on the catalysis. Finally, we survey what advances are actually being implanted into the current industrial practice and conclude with a reflection on the future of zeolite research to satisfy light olefin demand. Keywords: propene, catalytic cracking, olefin cracking, zeolites, ZSM-5, FCC, hierarchization, aluminum distribution

1. Introduction: light olefins markets Light olefins (ethene, propene and butenes) are key raw materials for the petrochemical industry, an industry of around 500 billion dollars and which is expected to grow to over 958 billion by 2025 (1). Ethene is relatively cheap and can be produced from light hydrocarbon streams by means of appropriate refining processes, including steam cracking. Ethene can react with high tonnage compounds (such as chlorine, hydrogen chloride, oxygen…) to obtain products of higher added value (ethene oxide, ethene glycols, ethanol, ethoxylates, PVC, ethylbenzene, ethanolamine…) and less subproducts than with the rest of olefins. . Ethene production in 2013 exceeded 130 Mtpy (Nexant, 2014) and was predicted to grow to 157 Mtpy in 2017 (IHS, 2013). This growth is caused mainly by a strong polyethene demand from growing economies. Hence, although Asia is already the greatest ethene producer worldwide, it is forecasted to show the greatest growth in demand and to remain the greatest net ethene importer. In its turn, propene is the second largest building block in petrochemistry, with an annual production around 100 Mtpy (2). Propene has traditionally been obtained from


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refining and steam cracking processes but in the last years these sources can no longer satisfy its energetic demand (forecasted to grow at an average 4.5 % per year in 20162020, which will be 0.7 % higher than the average annual growth rate for ethene, according to IHS), especially from China, given the global changes in the trends from gasoline to diesel and ethanol and with lighter cracking feedstocks derived from shale gas in the US. Consequently, technologies more selective towards propene have been developed —among which the so-called propene on purpose technologies— to supplement the traditional processes of thermal and catalytic cracking. Demand for propene, as it is the case for ethene, is led by the production of polymers in addition to other products (polypropene, acrylonitrile, propene oxide, acetone…). Butenes have a total production around 132 Mtpy, growing at around 4 % per year. From these, around 30 Mtpy correspond to isobutene produced in the refinery (FCC, coker…) and, to a lesser extent, in the steam cracking of liquids and butane dehydrogenation. Isobutene is mostly used as raw material for alkylate production (47 %) and production of high octane additives for gasoline blending (41 %), including MTBE and ETBE, with only a minor percent devoted to petrochemistry. This difference increases with the growing addition of bioethanol to gasoline, since more alkylate is required in the gasoline pool compared to XTBE additives to limit Reid vapor pressure of the blend. Also, with the processing of light tight oils (LTOs), refineries, especially in the United States, exhibit a higher need for high octane blending components given the paraffinic nature of these feeds. The situation is similar with 1- and 2-butene. These factors may pose a misbalance with petrochemical demand in the future, hence, again, highlighting the interest of exploring novel possibilities of light olefins production. The trend towards lighter feedstocks at advantageous prices impacts directly on the availability of valuable co-products such as propene and butenes, which also show a


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strong demand in the expanding markets. It is therefore justified the current interest in more flexible, efficient and selective cracking catalysts for light olefin production, particularly propene, as we will see along this review. In section 2 we cover advances related to a better understanding of the mechanism of catalytic cracking. Next, we identify four main directions in zeolite research efforts with potential to improve the selectivity to light olefins of zeolites in cracking catalysts, which are discussed section 3. These include the testing of novel structures as they were firstly synthesized, which has been steadily performed for decades. The introduction of mesoporosity to create structured or hierarchical materials has also been studied for long, although much more advanced methods are available today to engineer zeolite crystals. The incorporation of other active species into the zeolite has been extensively explored, usually deposited as extraframework metal oxides. More elaborated approaches are those in which heteroatoms are isomorphically substituted into framework positions. Most recently, increasing emphasis has been put in trying to locate the active acid centers in specific crystallographic positions to maximize transition state shape selectivity during the catalysis. Finally, incremental advances in reactor and process engineering also play an important role in petrochemistry-oriented fluid catalytic cracking (FCC) units, on which selected results are presented in section 0, along with a discussion on commercial catalysts. On May 25th, 1942 —75 years ago—, the first FCC unit (PCLA#1), was started up 75 years ago, in Baton Rouge, Lousiana. In 1967 —50 years ago—, Argauer, Olson and Landolt applied for the patent GB1161974, which discloses the first synthesis of ZSM-5 by using tetrapropylammonium hydroxide (TPAOH) as the organic structure-directing agent (OSDA). In this same year, patent US3308069 by Wadlinger, Kerr and Rosinski, also colleagues at Mobil Oil, was published, concerning the preparation of Beta zeolite


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in the presence of tetraethylammonium hydroxide (TEAOH) as the OSDA. Since then, researchers on cracking and zeolites have shown an extreme resilience and ability to innovate, only paralleled by the increasing challenges they are presented. There is no doubt that both progresses and challenges will keep ensuing at an accelerated pace as we leap into the future. On the occasion of these anniversaries, we hope that this review will contribute someway to stimulate and improve the efficiency of research in catalytic cracking to keep catering for the demands of the society as it has been doing for so many years.

2. Mechanism of catalytic cracking over acid zeolites Before the 90s, little attention was paid to the selective production of light olefins by means of catalytic cracking because steam cracking offered reliable operation to cater for the demand of light olefins. Light olefins produced in cracking operations were regarded as co-products (3). However, steam cracking suffers from multiple limitations: high reaction temperature (over 800 °C, requiring expensive construction materials and leading to very high energy consumption), inflexibility of the product slate and, in particular, low propene/ethene (P/E) ratios (which are strongly determined by the feed composition), high investment costs to benefit from economy of scale, etc. (4) By contrast, catalytic cracking can operate at much lower temperatures (often at 500-600 °C) with the consequent energy savings, and the product slate and P/E ratio can be selected with a proper catalyst design and operation variables (5). Therefore, catalytic cracking is contributing importantly to satisfy the changing light olefin demands of today.


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Catalysts used in cracking processes in the refining industry are made up of several components. An acid zeolite is key to achieve high activity and selectivity to light olefins and therefore enormous research efforts have been devoted to understand its working mechanism since its introduction, which is a need to enable rational improvements. 2.1 Hydrocarbon activation Seventy years ago, Bloch, Pines and Schmerling observed that n-butane isomerized to isobutane under the influence of pure aluminum chloride only if HCl was present (6). They proposed that the ionization step took place through initial protolysis of the alkane, as evidenced by the formation of traces of hydrogen in the initial stage of the reaction. Whereas all studies involving isomerization, cracking and alkylation reactions over solid acids agree that carbocations are key intermediates, the mode of their formation from barely reactive hydrocarbons is still under debate. Due to this everlasting controversy concerning the initial step in hydrocarbon conversion over solid acid catalysts, several pathways can be found in the literature: protolysis (7), hydride abstraction by a pre-existing carbenium ion (8), hydride abstraction by a Lewis acid (911), oxidation by traces of metal impurities, and protonation of alkene impurities or of those formed through thermal cracking (12). Nowadays, it is generally accepted that this first mode of alkane activation occurs in many cases through the Haag-Dessau mechanism, also called monomolecular or α-protolytic cracking mechanism (13, 14) (Fig. 1). It involves the formation of high energy pentacoordinated carbonium-like species. This carbonium-like intermediate collapses, thus forming a carbenium ion with


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the concomitant formation of either hydrogen (dehydrogenation) or methane/ethane (cracking).

Figure 1. Protonation of saturated alkanes over acid zeolites.

The catalytic cracking of hydrocarbons under relevant conditions can proceed through a monomolecular and/or a bimolecular mechanism, Fig. 2. The monomolecular mechanism is dominant in medium-pore zeolites (namely ZSM-5) (15-18), with high Si/Al ratio (19) over strong protons well dispersed throughout the zeolite (low acid site density) (20), at a low partial pressure of reactants (21), low concentration of alkenes in the reaction environment (22), high temperature and low conversion (15, 17, 21). The bimolecular pathway prevails at mild temperatures (usually below 350 °C), whilst at higher temperatures (450-650 °C) typical of industrial operation, the monomolecular mechanism can become predominant. Under these conditions several authors reported that the coverage of the catalytic surface is low, the reaction follows an apparent reaction order close to 1 with respect to the feed, and the selectivity is barely affected by the aluminum concentration or the feed partial pressure, although the cracking activity


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is proportional to the aluminum concentration in the catalyst (23-25). The bimolecular mechanism involves the formation of carbenium-like species by hydrogen transfer to pre-existing (shorter) chemisorbed intermediates and their cracking by β-scission to yield shorter olefins.

Figure 2. Monomolecular and bimolecular cracking mechanisms of alkanes over acid zeolites.

Sommer et al. have further demonstrated the occurrence of the monomolecular cracking mechanism of C3-C4 alkanes at even lower temperatures, i.e. 150-200 °C (12). Fig. 3 presents the distribution of initial products in isobutane cracking in a recirculation experiment over H-ZSM-5 zeolite at 200°C. Remarkably, hydrogen is formed after few minutes on stream before secondary products (C1-C3) and tertiary products (isopentane). Using an elegant study by GC and on-line mass spectrometry in which all the hydrons


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from the solid catalyst were exchanged by deuterons, Sommer et al. demonstrated that the origin of hydrogen could neither be due to a chromatographic effect nor to a conventional dehydrogenation mechanism (12). Fig. 1 illustrates the superacid-like activation of isobutane over acid zeolite observed at low temperature. Indeed, H2 and CH4 were detected over H-zeolites, whilst HD and CH3D were formed over D-zeolites as primary products in the cracking reaction. These reaction products cannot be accounted by the classical oligomerization and β-scission mechanism, in which the smallest alkane cracking product should be propane (26). In line with Olah’s chemistry and the Haag-Dessau mechanism, the results indicate that the σ-bond protolysis by strong acid sites is the first step in hydrocarbon activation over zeolite catalysts even at mild temperatures (200 °C) (27). However, it is important to point out that the degree of conversion remains very low and that the amount of strong Brønsted acid sites involved in this monomolecular activation was less than 1% of the total number of Brønsted acid sites. Naturally, however, at higher conversions the bimolecular catalytic cycle takes over for further conversion (28).


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Figure 3. Isobutane cracking (recirculation experiment) over HZSM-5 zeolite at 200°C, conditions: 8.91 g H-ZSM-5 (Alsi-Penta), 60 NmL isobutane, 190 NmL N2.

2.2 Kinetics of cracking Very recently, the kinetics of monomolecular cracking, a topic of great interest, has been subject to considerable debate. The various reactions involved in the cracking of alkanes rely both on the strength and the local structure of Brønsted acid sites in zeolites. The activity of Brønsted acid sites in zeolites is strongly affected by the ratelimiting step of the reaction. Monomolecular cracking of alkanes proceeds via protonation as the rate determining step. This reaction is influenced by the size and shape of the pores, which impact the heat of adsorption. Previous studies support that the differences in the heat of adsorption, thus the size and shape of the pores, are dominant factors in the cracking activity of Brønsted acid sites (29, 30). Accordingly, the observed cracking rates must include, in addition to the intrinsic cracking rate constant, kint, an adsorption term that accounts for the physisorption and protonation (i.e. chemisorption) of the reacting molecule. Thus, under monomolecular cracking conditions, the observed cracking rate takes the form:

r = L ∙ k ∙ p = L · ∙ · p where kap is the apparent rate constant per active site measurable in kinetic experiments. Notice that kint is only accessible if Kads, the adsorption equilibrium constant, has been determined by other means, usually by adsorption experiments at low temperature (21, 29, 31-34). The apparent cracking constant thus exhibits the following dependence with temperature:

= ∙ = exp −

# # exp · exp − " exp " ℎ


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where ∆Hads and ∆Sads are the adsorption enthalpy and adsorption entropy, whereas ∆H# and ∆S# are the activation enthalpy and entropy, respectively. The intrinsic activation energies for various alkane chain length and zeolites (including ZSM-5, MOR, USY, BEA, FAU) were found to be relatively constant with small deviations (21, 29, 31, 32, 34), although this keeps being matter of debate (see section 3.3). Iglesia and co-workers reported that the concentration of adsorbed alkanes of different chain length did not change substantially due to the compensation between adsorption enthalpy and adsorption entropy. They postulated that differences in cracking rates would be dominated by the pre-exponential factor of the intrinsic rate constant (activation entropy) (35-37). However, computational experiments raised controversy about whether the differences were due to changes in activation entropy or activation enthalpy (38). Very recently, Li et al. conducted an operando IR study of adsorption while cracking light alkane molecules (327-437 °C, X < 2%) (38). Their results confirm that monomolecular cracking rates of alkanes are chiefly controlled by their intrinsic activation entropies at high temperatures, which has just been demonstrated for the dehydration of cyclohexanol as well (39). These activation entropies are significantly lower than those obtained by combining low temperature adsorption experiments and classical activity measurements, although they remain much higher than those currently predicted by calculations. This is partly due to the need to consider the reference state of the adsorbed phase in simulations and to the definition of the adsorbed phase, and thus the configurational space used to determine its entropy, which defines the initial reactant state of the cracking reaction. An approach using a cutoff radius of 5 Å around the Al atom by Janda et al. (40, 41) led to some improvement but did not solve the


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discrepancy, which may also require the consideration of the directionality of the hydrogen bond (38). Despite this controversy, which notably concerns the validity of computational experiments, satisfactory kinetic modeling of conventional cracking experiments had been achieved long before. Kinetic lumping is a widely used approach, although the parameters are not fundamental and thus may not be suitable for extrapolation and upscaling. A preferable modeling approach is that based on single-events (42). In this case, all possible elemental reactions are generated by a computer algorithm, and the number of kinetic parameters is greatly reduced by grouping them into reaction families depending on the reactant and product carbenium ion (tertiary-secondary, tertiaryprimary, secondary-secondary, secondary-primary…). Moreover, only the steps of cracking, dimerization or isomerization are considered as rate-determining; protonation, deprotonation and desorption are considered in equilibrium. The system of differential equations for cracking of olefins can be further simplified by assuming that isomerization reactions are in equilibrium, too. Thus, the model has recently been applied successfully to the cracking of hexene and pentene olefins (43, 44). Li et al. also observed that selectivities to products are not dependent on temperature (up to 700 K) for a specific probe molecule (38). Similar results have been reported by other groups and for different types of zeolites (29, 30, 45, 46). The selectivities to light alkenes could also be distinguished assuming that prevalent mechanism is α-protolytic cracking. The differing ratios C3=/C2= and C3=/C4= with increasing carbon atoms showed that the initial feed plays a key role on the product distribution. Moreover, the authors identified that product selectivities differed based on the activation entropies (38). They reason that activation entropy is highly dependent on the carbon atom which is common for C-C bond cleavage and the vicinal C-C bond that will belong to the product olefin.


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2.3 Extraframework aluminum species Another important question concerning the mechanism of catalytic cracking over acid zeolites is the effect of extraframework aluminum species (EFAl). EFAl have been identified in different H-zeolites, mostly by sophisticated

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Al MAS NMR techniques

(47-50). Although the precise nature of EFAl is not well established, different species have been recently proposed, being either in tetrahedral, pentahedral or, more often, octahedral state: cations (AlO+, Al(OH)2+), neutral species up to a certain degree of oligomerization (51, 52), and hydroxyaluminates (Al(OH)3, AlO(OH)) (53). EFAl species can be localized in the vicinity of framework aluminum (54) or outside the zeolite channels in the form of 20-50 nm long amorphous particles (55). These long EFAl species were also recently observed in the ZSM-5 structure (56). Smaller EFAl species are currently associated to enhanced hydrocarbon conversion over zeolites compared to that in EFAl-free HY zeolite (57). Regarding ZSM-5 zeolite, steaming treatment, often mild, led to the formation of EFAl and yielded improved activity in acid catalysis, as shown by Hölderich and co-workers, for the electrophilic aromatic substitution of benzene with nitrous oxide (48, 58). Several groups have related the zeolite activity to Lewis acid sites formed by coordinately unsaturated aluminum species present in the channels (59-61). EFAl species formed in zeolites are often afford an enhancement of the zeolites activity in many acid-catalysed processes, including cracking. Despite a lack of knowledge in their exact nature, those species often had Lewis acidity associated and proved to be beneficial, even vital, in many acid-catalysed reactions (48, 62-66). The improvement in catalytic performance by EFAl species in the cracking of alkanes remains a longstanding debate in zeolite science and several effects have been proposed: 13 Environment ACS Paragon Plus

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(i) the possibility of EFAl acting itself an active site for hydrocarbon activation (53), (ii) no detectable aluminum species or invisible aluminum (67), (iii) inductive influence of EFAl on Brønsted sites, (iv) enhancing acidity of Brønsted sites by affecting electron density of neighbouring OH group (68), (v) change in the heat of adsorption of reactants (69), (vi) increasing the amount of highly diluted Brønsted sites during small alkane cracking (70), (vii) Mota et al. explained the catalytic improvement by EFAl through stabilization of the conjugate base by the zeolite framework (through deprotonation reaction) (71), (viii) Lercher et al. suggested the polarization of hydrocarbon bonds, especially of tertiary σ C-H bond in isobutane over La-exchanged Y zeolite (72), and ix) confinement effects created by the size and shape of zeolite voids over the transition states during the reaction (73, 74) could also be favored by EFAl. Indeed, each reaction pathway (related to a specific transition state/intermediate in FAU zeolite) shows different slopes in their Gibbs energies, i.e. alkoxides, π-complex and carbenium ion present each different entropic contributions, therefore temperature could affect Gibbs energy (75). A similar debate exists concerning the effects of rare earth (RE) ion exchange in FAU zeolite (see section 3.3). Thus, although extra framework aluminum (EFAl) species have been seriously characterized and even quantified by HRTEM, Al MAS NMR techniques and synchrotron measurements or modelled by quantum chemistry, judging from the numerous models and assumptions found in the literature, it appears that the debate on the exact nature and effect of EFAl species is not solved yet.

3. Recent advances in zeolite science relevant to catalytic cracking 3.1.Novel zeolite structures We identify several directions of research in zeolite synthesis. On the one hand, small pore zeolites are mostly aimed to maximize selectivity to light olefins, isomerization to 14 Environment ACS Paragon Plus

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linear hydrocarbons or direct alkylation to certain positions. On the other hand, large and extra-large pore zeolites have been traditionally sought after for the processing of bulky molecules, especially as crude oils become heavier and also, more recently, for the processing of plastics (76). However, most of these materials involve costly syntheses and, importantly, limited thermal and hydrothermal stability which prevents their use in heavy-duty, high-tonnage FCC operations (77). Delaminated zeolites may overcome some of these limitations, as their stability can be high while retaining the ability to process large molecules (78, 79). Especially of interest are those synthesis procedures which allow a structured crystal growth at different length scales, which could be favorable to the preparation of technical bodies for commercial use. In the section 3.2 we will review some interesting advances in hierarchical zeolites. Another class of zeolites are so-called multipore zeolites (80, 81). In these materials, the presence of pores of different sizes could affect the diffusion of certain molecules through different pores (so-called molecular traffic control) or the location of active sites in certain crystallographic positions, hence impacting the catalytic results. Nevertheless, with the objective to maximize the selectivity to light olefins in catalytic cracking, interest in small pore zeolites is recovered (82). Below we comment some recent examples from the literature to illustrate these different trends. Jung et al. observed that ferrierite is affected by severe mass transfer limitations in the cracking of n-octane, leading to low conversion in spite of a moderate amount of strong acid sites (83). One-dimensional mordenite is deactivated quickly by pore blockage, whereas large pore Y zeolite suffers from very fast deactivation by coke deposition in the cracking of n-octane (83). Gong’s group developed a seed-assisted method to prepare pure EU-1 (EUO) zeolite with high Si/Al ratio (84). EUO framework has a monodimensional 10-MR pore system


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periodically interrupted by 12-MR side pockets. The thus prepared materials were studied in the cracking of n-hexane (85). At a given conversion, selectivities to propene, butenes, ethane and methane increased with Si/Al ratio. On the other hand, selectivities to propene, butenes and butane decreased when increasing contact time. Increasing temperature increases methane, ethane and ethene. Interestingly, EU-1 leads to higher yields to propene than ZSM-5, comparable ethene + propene molar yields, and much lower BTX (85). Although it is not studied in detail, their results suggest us that ZSM48 might lead to very low BTX and P/E. In particular, ZSM-48 is also a 1D 10MR zeolite but it does not have side pockets like EU-1, which would affect its transition state shape selectivity. Gounder and Iglesia reported different rate constants for monomolecular reactions among zeolites (MFI, FER, MOR) and among HMOR samples with similar Si/Al ratios but different origin (37). Measured activation energies for dehydrogenation were consistently higher than for cracking. Rate parameters varied among the zeolites due to their difference in transition states in their energy and entropy relative to the gas phase alkanes. Added to that, the terminal to central alkane cracking rate ratio was enhanced with increasing acidity. Similar observations were reported by Schallmoser et al. (86). Moreover, the substitution of acidic H+ ions by Na+ within 8-MR have shown that cracking reactions prevailed when the acidity decreased. Generally, cracking and dehydrogenation rates are higher in 8-MR than in 12-MR, which is related to the corresponding activation entropies (see section 3.4). A drawback of many 1D zeolites is that they suffer from high crystal aspect ratios (length to width ratios). Muraza et al. managed to lower the crystal aspect ratio of ZSM23 (MTT) from 400 to 5 by microwave-assisted synthesis, which was attributed to a faster nucleation (87). However, the thus prepared samples had low accessible area.


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Both micro- and mesopore volumes could be increased through a microwave-assisted alkaline treatment, increasing catalytic lifetime and preserving selectivities in the cracking of n-hexane (88). Total selectivity to light olefins obtained was around 60 % at X = 85 % (T = 650 °C, x0 = 0.25), with a P/E ratio over 2 and very low selectivity to BTX (< 4 %) (88). Corma and co-workers proposed IM-5 zeolite as a cracking catalyst for light olefins production under high severity conditions (650-700 °C, steam catalytic cracking) (89). This zeolite has a central 2D channel system connected through 10-MR along [010] to another 2D channel system on either side (90). They observed that IM-5 zeolite could lead to equivalent propene yields as a ZSM-5 zeolite of similar Si/Al (Si/Al = 15) with increased ethene production. This was attributed to the stronger acid sites and tight pores of IM-5 zeolite. However, IM-5 zeolite possesses a very large channel intersection and this also led to increased BTX formation at the expense of butenes in the cracking of n-heptane (89). MCM-22 is a 2D 10 MR zeolite containing a peculiar system of 12 MR supercages which are exposed to the crystal surface as hemicages. It has been extensively studied for aromatics conversion reactions and recently its potential for cracking to light olefins has been explored as well. Notably, Wang et al. prepared MCM-22 zeolites with different aluminum contents by direct synthesis (Si/Al = 19 and 34) and by modification of the Al content in a lamellar precursor (Si/Al = 66 and 100) (91). The direct synthesis sample (Si/Al = 19) is also subjected to dealumination with ammonium hexafluorosilicate (AHFS) at different concentrations. Cracking of n-hexane on these materials is explored in a wide range of conditions. Very interestingly, an apparent first order cracking rate constant is almost proportional to the concentration of acid sites at 650 °C, but depends more strongly on this concentration at 450 °C (potencies elevated


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to 1.3 and 1.8, respectively), Fig. 4a. This suggests us the prevalence of mono- and bimolecular cracking, respectively. Higher temperatures lead to increased selectivities to ethene and BTX at the expense of butenes, ethane, propane, and butanes, whereas selectivity to propene remains constant. Modification of contact time suggests that BTX are formed mainly from propene and butenes. Importantly, selectivities are compared as a function of a wide range of acid site concentrations at a fixed conversion of 80 %, Fig. 4b. A decrease in the acid amount led to a slight increase in propene and butenes at the expense of ethene and BTX. Remarkably, the dealuminated MCM-22 (Si/Al = 62) allowed to maintain a constant high selectivity to propene (ca. 40 C-%), which was higher than H-ZSM-5 and H-Beta catalysts with similar Si/Al ratios, even at a high nhexane conversion of 95 %, Fig. 4c. Moreover, it showed a catalytic lifetime comparable with H-ZSM-5 and longer than H-Beta catalyst, Fig. 4d. In addition, this dealuminated sample showed better performance than the post-synthesized MCM-22 sample (Si/Al = 66), although they had similar acid amounts. This could be due to the lower amount of Lewis acid sites and to the decrease in the strength of acid sites (evidenced by Pyr-IR results) in the AHSF-dealuminated MCM-22.


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Figure 4. a) Dependence of apparent first-order n-hexane cracking rate constants on the acid site density of H-MCM-22 at different reaction temperatures (a: 450 °C, b: 550 °C, c: 650 °C); b) product selectivities at a constant n-hexane conversion of ca. 80 %, c) selectivity to C3= over ZSM-5, and d) catalytic lifetime dealuminated MCM-22 and Beta with comparable Al content (T = 650 °C, pn-hexane = 6 kPa. Reprinted from (91) with permission from Elsevier. Additionally, as mentioned above, multipore zeolite systems may be of particular interest when molecules of substantially different sizes intervene in the reaction, e.g. bulky hydrocarbons and light olefins. This is particularly interesting given the bifurcation in demand from gasoline towards light olefins and diesel (92).


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MCM-68 is a zeolite presenting 12-MR straight channels interconnected by tortuous 10MR. It also has a supercage (18-MR x 12-MR) accessible only though 10-MR channels. Inagaki et al. reported that a dealuminated MCM-68 (Si/Al = 51) exhibited higher selectivity to propene than ZSM-5 at comparable conversion, temperature and amount of acid sites in n-hexane cracking (93). The results suggest that dealumination may occur preferentially in the straight 12-MR channels (see section 3.4), improving the selectivity of the resulting material. However, the conventional hydrothermal synthesis had the drawback of too long crystallization time (as long as 12-16 days). More recently, Inagaki, Kubota et al. reported two different synthesis methods which remarkably shorten the crystallization period, including the conversion of a FAU-type zeolite with the aid of an OSDA (94) and an OSDA-free hydrothermal synthesis with the aid of seed crystals (95). In n-hexane cracking, the OSDA-free MCM-68 showed the best catalytic performance among three different MCM-68 zeolites after post-synthetic modification (96). Alternatively, mixtures of different zeolites can prove valuable to increase yields both to lighter and heavier products. A particular example is a mixture of ITQ-33 (18 MR x 10 MR) with ZSM-5, yielding more middle distillates and propene than the system USY/ZSM-5 (97). More zeolite structures have been tested as catalysts or co-catalysts to improve propene production. We refer to selected references for an overview of other zeolite structures, such as SSZ-24, ITQ-7, ITQ-7, NU-86, NU-87, IM-5, ITQ-21 or ITQ-33 (77, 98). The literature suggests that there are zeolite topologies which can offer higher selectivity to light olefins in the cracking of alkanes or longer olefins than MFI when compared at similar acid site concentration and feed conversion. In particular, since bulky transition states leading to BTX are allowed inside ZSM-5 crystals, frameworks


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with tighter pores and/or smaller intersections have the potential to restrict these reactions and possibly provide higher yields to light olefins and longer catalytic lifetimes (99). In Table 1 we present some frameworks selected from the IZA Database of Zeolite Structures highlighting this tighter confinement (90). In addition to these, novel structures keep being explored. Foster and Treacy have predicted thousands of different low-energy frameworks in their Atlas of Prospective Zeolite Structures (100, 101), compared to the 232 frameworks that have been reported so far in the Database of Zeolite Structures (90).

Table 1. Selected zeolites and crystallographic values (90).

Zeolite

Framework

Channel system

MCM-68

MSE

3D

SSZ-74

-SVR

3D

Y

FAU

3D

Mordenite

MOR

1D

CIT-1

CON

3D

SU-15

SOF

3D

ZSM-5

MFI

3D

IM-5

IMF

3D

Ferrierite

FER

2D

MCM-22

MWW

2D

ZSM-23 ZSM-22 EU-1 ZSM-48

MTT TON EUO *MRE

1D 1D 1D 1D

Natrolite, scolecite

NAT

3D

Channel / MR, Å

FD /T 1000 Å-3

ddiffusea / Å

dincludedb / Å

16.4

7.09

6.59

17.2

4.65

5.85

13.3

7.35

11.24

17.0

6.45

6.7

15.7

5.6

7.45

16.4

4.42

5.14

18.4

4.7

6.36

17.5

5.44

7.34

17.6

4.69

6.31

15.9

4.92

9.69

18.2 18.1 17.1 19.7

5.07 5.11 4.99 5.59

6.19 5.71 7 6.36

17.8

4.38

4.52

12 6.4x6.8 10 5.2x5.8 10 5.2x5.2 10 5.5x5.7 10 5.2x5.9 10 5.2x5.6 12 7.4x7.4 12 6.5x7.0 8 2.6x5.7 12 6.4x7.0 12 5.9x7.0 10 4.5x5.1 12 4.4x9.7 9 4.3x4.8 10 5.1x5.5 10 5.3x5.6 10 5.5x5.6 10 4.8x5.4… 10 4.2x5.4 8 3.5x4.8 10 4.0x5.5 10 4.1x5.1 10 4.5x5.2 10 4.6x5.7 10 4.1x5.4 10 5.6x5.6 9 2.5x4.1 8 3.9x2.6


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SAPO-34, SSZ-13

CHA

3D

8 3.8x3.8

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15.1

3.72

7.37

*

Partially disordered type material. Maximum diameter of sphere that can diffuse along a, b or c directions b Maximum diameter of sphere that can be included a

To access these structures, research on OSDAs must continue. A reaction-tailored OSDA could mimic the transition state that one would like to stabilize (102). In addition, attention must be paid to the framework flexibility or entropy. This is beautifully exemplified by the reversible transformation between interrupted -OKO and condensed OKO zeolite, a process demonstrated to be entropy-driven (103). The very rigid OKO zeolite can stabilized by tuning the composition. Namely, heteroelements such as Al and Ge can accommodate longer T-O distances and a wider range of T-O-T angles. In particular, the variability of Si-O-Si angles is what provides the necessary flexibility in 97 % of the known zeolite frameworks, while 3 % of them require larger tetrahedra such as AlO4 or GeO4 (104). However, as Vogt and Weckhuysen pointed out in a recent review (77), in addition to the framework design and acid strength, cost of synthesis and hydrothermal stability can impede commercial application of many of the new zeolite structures proposed for catalytic cracking. Many of the novel materials explored in the laboratory lack proper studies of deactivation and regenerability (105) to start considering their possible comparison with ZSM-5 at a larger scale.

3.2.Crystal engineering zeolites Residence time of molecules inside the zeolite crystals can exceed the turnover frequency at which the desired catalysis occurs, leading to reduced catalytic effectiveness and, most importantly, increased chances of undesired reactions, such as hydrogen transfer, overcracking, olefin interconversion, coking… (106-109). To reduce


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this residence time, either the net diffusion rate of molecules in the crystals is increased, which can be achieved by engineering connected pore systems of different sizes (hierarchization), or diffusion lengths are shortened by synthesizing smaller crystals (nanocrystals) (110, 111). In addition, if large molecules are to be processed, acid sites accessible by mesopores are necessary (112). Crystal engineering can be achieved during synthesis, by means of templating or repetitive branching, among others, or by means of different postsynthetic acid, base or mechanochemical treatments. Bottom-up methods Perhaps the most straightforward method to introduce mesoporosity in zeolite crystals was the use of hard templates. Nanoparticles such as carbon nanotubes or polymers, can be included in the synthesis and be occluded in the zeolite crystals. After careful removal, mesoporosity in the zeolite crystals can be revealed. Other approaches have been the preparation of lamellar zeolites (113), the use of amphiphilic organosilanes (113-116), or the crystallization of organosilanized protozeolitic units (117), in addition to the postsynthetic methods mentioned in the next section. A recent advance in the synthesis of hierarchical materials has been the synthesis of microporous materials with ordered mesoporosity. Even if it is unclear whether such improved order will prove cost-effective at the commercial scale compared to less ordered mesopores, they are systems suitable for research (118). At present, ordered meso-microporous materials can be synthesized i) by means of zeolite growth confined in an ordered hard template (so-called three-dimensionally ordered mesoporousimprinted zeolites, 3DOm-i) (119, 120), ii) by use of appropriate dual soft templates (surfactant-driven synthesis) (116) or iii) by repetitive branching at the nanoscale (121, 122).


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3DOm-i may be less appealing for scalability and their mesopores are a result of extraparticle voids. On the other hand, dual template syntheses are receiving increasing attention (123), although they do not necessarily lead to ordered mesoporosity. This is also the case for repetitive branching, also referred to as rotational intergrowth or twinning. Repetitive branching occurs in the synthesis in many zeolite systems due crystal growth under non-equilibrium conditions and, if properly controlled, can lead to ordered meso-microporous materials, as shown in Fig. 5. In particular, the thickness of the layers and the distance between branching points are amenable to modification by proper synthesis conditions, among which the OSDA (121, 124, 125). However, it should be taken into account that mesopore volumes are not the only factor that can control diffusion rates. For instance, the connectivity of mesopores (see below), the presence of acid sites (126), and, of particular relevance in these materials with outstanding surface to volume ratios, the effect of surface resistances to diffusion (127) may become relevant in state-of-the-art hierarchical materials, in addition to considerations at the technical scale of these catalysts (see section 0). In parallel, acid strength and distribution crystal-wise has been demonstrated to remain largely unaltered in hierarchization procedures (128, 129) but it may be affected when moving to very small crystals with lengths of just a few unit cells (130). Recently, outstanding findings from the group in Caen led to revisited strategies to design FAU-type zeolites without any template use (131). Indeed, nanosized faujasite crystals (10-15 nm) with exceptional properties, narrow particle size distribution, high crystalline yields and micropore volumes (0.30 cm3 g−1), Si/Al ratios adjustable between 1.1 and 2.1 and excellent thermal stability were prepared. The presence of mesoporosity in between the nanocrystals led to achieve superior performance in the dealkylation of 1,3,5-triisopropylbenzene. Mintova et al. also succeeded in the template-free synthesis


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of single EMT-type crystals having 6–15 nm in size (132). The downsizing of zeolite nanocrystals to few unit cells, without any template use, remains a smart approach to maximize the diffusion of reactants and products within the zeolite channel as well as favoring the adsorption / desorption steps onto external active sites.

Figure 5. SEM images of hierarchical silicalite-1 octahedron synthesized by repetitive branching at the nanoscale. Reproduced from (125) with permission of The Royal Society of Chemistry.


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Top-down methods Apart from bottom-up synthetic approaches, top-down or postsynthesis methods have been developed to effectively alter the crystal structure. Steaming is the most extended postsynthesis procedure applied to zeolites, whereas treatments with bases are well described in the literature and continue being refined. Interestingly, the use of fluoridebased treatments is demonstrating further richness in what traditionally were considered single crystals. Finally, a brief mention to mechanochemical processes is made as well. The possibilities of combining treatments are also promising to tune trimodal pore size distributions (133) and for the preparation of hollow ZSM-5 and Y zeolites by preferential removal of Si-rich (134) or Al-rich (135) zones of the crystal, respectively (see section 3.4). Dealumination can be achieved by means of hydrothermal, thermal, acid or mechanical treatments. Removal of Al does not necessarily involve modifications at the crystal level (see section 3.4), however, the most classic method of steaming used in the ultrastable Y zeolite (USY) manufacture can lead to the generation of mesopores, often evidenced by the presence of cavitation in the N2 adsorption isotherms at 77 K over USY (136-138). Upon extraction of Al from the framework, hydroxyl nests are formed, which can move and coalesce under the steaming conditions. However, mass-transfer limitations along the crystal seem not to be alleviated efficiently by this procedure (139). This is because most of the generated mesopores are not connected to the outer surface of the crystal (140), which is a prerequisite for efficient hierarchization (137, 141). Steaming of other zeolites, although of potential interest (142), has been less explored. In the case of high-silica zeolite ZSM-5, dealumination was proven to be nonrandom but to preserve preferentially Al pairs that could be compensated by Co(II), in


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contrast to isolated Al sites that remained charged-balanced by Na+ and were more prone to dealumination (143). Desilication methods (those leading to a preferential removal of silicon in zeolite structures), often based on NaOH, have been widely applied to generate mesopores in zeolite crystals with suitable Si/Al ratios. In fact, concentration of framework Al (144, 145) and of framework defects (146, 147) are major factors affecting the results of desilication, in addition to the conditions of the treatment (144). For instance, Milina et al. observed that treatments of ZSM-5 zeolite (Si/Al = 40) with increasing NaOH concentrations up to 0.4 M led to increased mesopore surface (148). These mesopores were accompanied by a higher Lewis acidity of the materials. However, treatment at higher NaOH concentrations reduced Brønsted acidity, increased the amount of nonacidic Al and reduced micropore volume. A subsequent acid wash partially removed Lewis and non-acidic Al. Desilication methods have been generally applied to many zeolite structures. For instance, Muraza et al. studied desilication of ZSM-12 zeolite with NaOH under microwave irradiation (149). Desilication notably increased mesopore volume at the expense of micropore volume, suggesting the deposition of debris. Since NMR did not evidence an increase in the EFAl at 0 ppm, the deposits must be due to amorphous silica. In general, acid treatment leads to preferential dealumination. It also rapidly decreases Lewis acidity by removing EFAl species. A decreased concentration of Al can yield improved selectivity to propene and butenes, higher stability and less hydrogen transfer and aromatization products (150, 151). Desilication with NaOH reduces Brønsted acidity and increases Lewis acidity. Selectivity to propene and butenes is often increased, but stability may increase or decrease depending on the generation of mesopores, the deposition of debris and the increase in Lewis acid sites. These Lewis


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acid sites may favor propane dehydrogenation and olefin aromatization (152). Thus a mild acid treatment after desilication of zeolites may prove beneficial (153). However, similar results can be achieved by tuning the Si/Al ratio in the synthesis. Therefore real benefits upon modification of the crystals need careful consideration if one suspects that the Al content or distribution may also be affected (25, 154). Compared to desilication, demetallation with NH4F is as a new tool that allows the removal of Si and Al at equal rates (i.e. the parent and its hierarchical derivatives keep the same Si/Al ratios). It is applicable to many zeolite structures whatever their initial Si/Al (155, 156), including SAPO molecular sieves (157). Specific fluoride species, such as HF2-, generated in-situ in small concentration, promote a dissolution starting at the interfaces between intergrown crystals and grain boundaries. Dissolution then progresses inside apparently single crystals and creates a relatively uniform mosaic of mesopores with well-defined sizes, shapes, and structures (158). This points to the preexistence of imperfectly oriented nano-domains resulting from an imperfect aggregation of zeolite nuclei during synthesis (Fig. 6), a seemingly widespread phenomenon in zeolites. Although the connectivity of the mesopores is not very good, this kind of information may prove very valuable to optimize both the synthesis and hierarchization of zeolite materials.


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Figure 6. Zeolite growth process (A) and its dissolution in fluoride medium (B) revealing the mosaic structure of zeolite crystals. Taken from (158). Copyright 2016 Wiley-VCH.

From a practical viewpoint, surfactant-templating is an important postsynthetic procedure which allows the introduction of mesoporosity in zeolite crystals with a precise control of the size and amount of mesopores (137, 159-161). It involves the treatment of the zeolite in a surfactant-containing mildly basic solution for a controlled time and temperature. Importantly, the generated material preserves crystallinity, Si/Al ratio and generates interconnected mesopores (162). The treatment has been demonstrated on different zeolites, including FAU, MFI, CHA, *BEA and MOR. Sometimes it can be more effective after a pretreatment with acid, or, at least for MFI, if the starting material already contains some mesopores (163). Micelles of the surfactant end up forming within the zeolite crystals, which rearrange through what seems a very dynamic process (164). Upon removal, these micelles leave mesopores of well-defined shapes and good connectivity to the crystal external surface (165). Interestingly, under certain conditions it can lead to some ordering in the mesopores generated. By contrast,


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if the conditions are not optimized, what may be obtained is a surfactant-assisted desilication, with a decrease in the Si/Al of the material. Finally, some mention to mechanochemical treatments should be made. The difficulty in these is the amorphization of the zeolite and, particularly, its dealumination, which may be unequal along the crystal volume. Inagaki et al. milled ZSM-5 zeolites in water using ZrO2 beads of 300 µm (166). They reduced crystals size from 1-2 µm down to 50100 nm. However, crystallinity was partially lost. A combination of collidine adsorption with FTIR of CO pointed to alumina deposits on the crystal surface. However, a hydrothermal treatment with a proper solution of sodium silicate can lead to recrystallization (166). Alternatively, the amorphous deposits on the crystal surface upon mechanochemical treatment can be removed with an alkaline solution, which was found to dramatically increase the specific activity of the resulting ZSM-5 catalyst in the cracking of cumene (167). Nevertheless, the application of mechanochemical treatments to zeolites is not restricted to postsynthesis crystal modifications. In spite of the currently almost phenomenological use of mechanochemistry and other nonconventional chemistry in zeolite science, highly promising advantages for zeolite synthesis and for the introduction of framework and extraframework species have been the topic of an excellent recent review (168) . Composite materials In addition to the hierarchical zeolite systems discussed above, ordered mesoporous materials have usually lacked the hydrothermal stability and acid strength typical of crystalline zeolites, although they have shown improved ability for cracking large size molecules. However, their application to reactions less demanding in acid strength, such as olefin cracking, might be possible if longer stabilities are achieved. Progress in these


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materials has led to the development of micro/mesoporous zeolite composites, which hold great promise for cracking catalysis (169). These comprise a mesoporous material wherein zeolite crystals are embedded, thus improving their acid strength and shape selectivity. These composites can be achieved by i) direct-synthesis, incorporating protozeolites or zeolite nanocrystals to the surfactant-driven synthesis of the mesophase, Fig. 7 (170); ii) by partial dissolution and recrystallization in the presence of surfactant (171), or by iii) zeolitization of preformed mesoporous materials. This last approach is difficult given the thin wall thickness of mesoporous materials, often leading to phase segregation unless steam assisted crystallization is used.


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Figure 7. TEM images of a core-shell composite silicalite-1/SBA-15 with a welldefined mesostructured shell. Adapted from (170) with permission of The Royal Society of Chemistry. Micro-characterization The application of more sophisticated crystal engineering strategies is suitably being complemented by more advanced characterization techniques. In particular, microimaging of concentration profiles inside single catalyst particles by micro-spectroscopy and micro-diffraction methods (77) offer great prospects to improve our understanding and design of cracking catalysts. In particular, the popularization of tomographic techniques in micro-XRD and electron diffraction (172, 173) is increasing our power to solve complex structures (174, 175) and to understand the 3-dimensional intricacies of the catalyst operation. For instance, it has been demonstrated that, in equilibrium cracking catalysts (E-cat), Fe tends to deposit on the external surface, Ni tends to appear as a shell, whereas V tends to penetrate deeper in the catalyst particles. Thus, Ni is most responsible for macropore constraining and clogging and, along with Fe, can also cause the agglutination of particles (77). On the other hand, micro-spectroscopic measurements are starting to allow coupling this information with in-situ activity measurements (77, 176, 177). The recent development of a photo-spectroscopic method with an off-the-shelf DSLR camera and a selective stain to determine E-cat age distribution (178) evidences the room for innovative solutions, particularly when it comes to technical bodies and the commercial scale of cracking technology (see section 0).


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3.3.Introduction of heteroatoms The most classic heteroatoms incorporated to cracking catalysts are La and Ce for the preparation of rare earth-exchanged Y zeolite (REY). Nd, Pr and Ca are also often incorporated in smaller amounts. RE ions migrate to the small cages of Y zeolite (Fig. 8a, (179, 180)) and reduce dealumination and loss of crystallinity under hydrothermal conditions by generating bridges with the oxygen atoms thereof. Moreover, the extent of the stabilization depends on the degree of exchange (181). Cation exchange initially takes place at the hexagonal prism and sodalite cages. According to some authors, this should polarize the zeolite framework, increasing the acid strength of acid sites in the supercages and their TOF in alkane cracking (179, 182). The TOF increases when reducing the ionic radius of the rare earth dopant, which would agree with a stronger polarization (183, 184), Fig. 8b. As a result of RE-exchange, higher rates of hydrogen transfer reactions are maintained in the equilibrium catalyst (E-cat), favoring gasoline yields but decreasing its olefinicity and research octane number (RON) (185). In addition, higher hydrogen transfer results in the saturation of light olefins. Moreover, rare earth may act as a vanadium-trap by forming REVO4 (186). Alternatively to the polarization explanation, which would modify the acid strength of the zeolite and thus the intrinsic TOF for cracking, other authors argue that differences in deprotonation energies (DPE) among microporous aluminosilicates do not differ noticeably. By contrast, the different solvating microenvironments within the zeolite voids would differ among different zeolite frameworks, within a given zeolite depending on the different T-locations, and also for a given acid site depending on the cations exchanged in neighbor positions. This would lead to changes in the adsorption free energy that justify the differences observed in the apparent activation energies in cracking alkanes (187).


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In our view, the controversy in the interpretation of cation-exchanged USY zeolite is possible because of the uncertainties found in the determination of adsorption enthalpies and of deprotonation energies. This allows supporting both the view that acid strength differs and that adsorption enthalpy differs enough to justify the differences seen in the TOF of these materials. Thus, as shown in Fig. 8c, a deprotonation energy (DPE) difference of around 20 kJ/mol would be needed to explain the difference between both USY samples, which some authors found justifiable with modification of a given zeolite by experimental and computational methods (188, 189). It is clear that confinement is a determinant factor allowing zeolites to catalyze highly acid demanding reactions and must be prevalent when comparing zeolites that exert a highly stabilizing solvation compared to others in which the van der Waals interactions are weaker, Fig. 8c (21). This, however, need not be in contradiction per se with the (additional) thesis that the exchange of cations (often in positions inaccessible to reactants) could subtly affect the acid strength of acid sites nearby, leading to differences in the intrinsic activation energy that, nonetheless, turn out noticeable when it comes to TOF values. Rare earth modification of ZSM-5 has also been explored. In this medium pore zeolite, however, rare earths are likely to deposit on the crystal outer surface, sometimes reducing the formation of aromatic subproducts in cracking processes. Among recent works, Jang et al. studied the impregnation of ZSM-5 zeolite (Si/Al = 25) with ceria and lanthana (190). Ceria stays on the zeolite external surface forming small crystals and compensating some Brønsted acid centers, but does not block zeolite pores. Ceria exhibits oxidation states Ce+3 and Ce+4, provides some mesopores for weak methanol adsorption but it is not catalytically active. In its turn, La enters the zeolite pores, hampering the access of bulky molecules like o-xylene, and compensates Brønsted centers throughout the crystal, reducing its activity in the MTO reaction. Other zeolite


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structures have also been modified with La. For instance, Masuda et al. impregnated zeolite ZSM-23 with different La concentrations (191). Incorporation of La slightly affects the Si/Al ratio and reduces the acidity of the material. At higher loadings, however, Brønsted acidity is recovered as new protons are generated by hydrolysis at high temperature. La addition diminished the activity of the material, although its stability might have improved. In particular, volume reduction due to dealumination by steaming was reduced upon La incorporation.


ACS Catalysis

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Figure 8. a) Exchange positions in Y zeolite (180). b) Change in conversion at a given contact time in octane cracking (T = 500 °C, p = 0.019 bar) over HY partially exchanged with rare earth and alkaline earth elements. Reprinted from (192) with permission from Elsevier. c) Adsorption equilibrium constant for protonated methanol dimer as a function of DPE of Keggin POM clusters and zeolites (DPE for zeolites is an averaged value). Reprinted from (73) with permission from Elsevier.


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ACS Catalysis

Improvements in catalytic activity upon modification by alkaline earth elements have also been reported. This is particularly interesting given the scarcity of rare earth elements and the dependence on imports from China (193, 194), which led to the rocketing of rare earth prices few years ago, Fig. 9. In Y zeolite, Ba and Ca would tend to localize in the sodalite cage (sites I’ and II, Fig. 8a), possibly polarizing the framework and increasing the acid strength of the Brønsted centers in the supercage and the activity in octane cracking (182) (see above). Interestingly, however, Mg, which possesses a smaller ionic radius than Ba or Ca, might occupy preferentially position III in the supercage (192). There, it would operate, as pointed by NH3-FTIR, as a strong Lewis center, activating the hydrocarbon by hydride abstraction, in contrast to Ca or Ba. Accordingly, the apparent activation energy for the cracking on this material was the lowest and a significant evolution of H2 with conversion was observed (192). In fact, magnesium was reported to yield dehydrogenative cracking of butane similarly to Co (195). Alternatively, differences in selectivity to dehydrogenation or cracking catalyzed by protons were proposed to depend mainly on activation entropies (187), although no straight correlation with protons amount has been found for dehydrogenation over MFI (38), which would support the involvement of other catalytic centers.


ACS Catalysis

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Figure 9. Price evolution for La2O3 (a) and CeO2 (b) relevant for FCC catalysts (source: Asian Metal).

Rahimi et al. recently reviewed the modification of ZSM-5 to produce light olefins from alkanes, by incorporation of other elements and modification of the Si/Al ratio (196). They observed that the modification with La, Co, Ni, Fe or Zn, the increase of the Si/Al ratio or the operation with steam decreased the BTX formation. Incorporation of other rare earth elements also afforded a reduction in the aromatics produced (197). Nevertheless, the authors concluded that modification by phosphorus yields the best equilibrium between increasing the catalyst stability and affecting its acid properties for cracking of ZSM-5 (196, 198). Interestingly, since phosphorus migration into the zeolite framework may encounter diffusional limitations (199), its incorporation under


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ACS Catalysis

hydrothermal conditions may afford a more uniform distribution (200). Phosphorus deposited on zeolites coexists in several forms, whose proportions can vary with the treatment conditions (199): 1) a large part of phosphorus precursor is often deposited on the zeolite crystal surface, forming more or less long chains of phosphates upon calcination. These do not interact with the zeolite but they can affect the accessibility to its inner porosity and react with alumina matrices in technical catalysts. 2) Phosphoric acid (or steam) will leach extraframework aluminum species often leading to its deposition as amorphous AlPO4 on the crystal surface. 3) A portion of phosphorus enters the zeolite structure reversibly interacting with tetrahedrally coordinated framework aluminum. This changes Al coordination to six-fold and suppresses the associated Brønsted acidity (preferentially interacts with strong acid sites) but improves the lattice integrity (199). Importantly, however, upon heat treatment a small portion of the phosphorus can promote the breaking of Si-O-Al bonds and the creation of Al-O-P bonds. This Al can present different interactions with phosphorus leading to bulky SAPO interfaces, with four-, five- or six-fold Al coordination (201, 202), which exhibit a high resistance to dealumination and would reduce the space available for bimolecular reactions at channel intersections, thus increasing selectivity to propene (203). Overall, it increases selectivity to light olefins by favoring monomolecular cracking over bimolecular cracking, mainly because of a decreased concentration of acid sites and a reduction in the pore space available. On the other hand, even though modification of acid strength may not affect substantially the selectivity in alkane cracking under usual conditions, its impact on activity is being increasingly demonstrated (177, 188, 189, 192, 204, 205). Phosphorous modification remains as the most widely used ZSM-5 modification for cracking in industrial practice. However, in spite of the great diversity of P sources investigated in the academia (206, 207) and


ACS Catalysis

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those covered in patents (208), phosphoric acid is the one most commonly employed in commercial practice (209). The introduction of transition metals, such as Fe, Zn or Ga, affects the mechanism and yields of cracking, including the activation step. Whereas Brønsted acid sites tend to catalyze C-C cracking reactions, Lewis acid sites seem to catalyze preferentially C-H dissociation and dehydrogenation by enhancing recombination and desorption of hydrogen (196, 198, 210). Catalysis by Lewis sites may work in tandem with that on Brønsted sites, for instance, in the oligomerization of olefins (211). The reduced amount of Brønsted acid sites also tends to decrease selectivity to BTX. Recently, Ga has been impregnated into nano-ZSM-5, which was further structured as hollow fibers by means of coaxial electrospinning and tested in n-butane cracking (212, 213), Fig. 10. Ga2O3 accelerated the dehydrogenation of n-butane at lower temperatures but also catalyzed the subsequent aromatization of the light olefins formed.


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ACS Catalysis

Figure 10. a) SEM image of 0.3% Ga2O3/ZSM-5 hollow fibers after calcination at 550 °C; b) conversion of n-butane (x0 = 0.05) as a function of temperature at constant WHSV. Taken from (213), CC BY 4.0.

Pereira et al. have studied the impregnation of Ni in zeolites for catalytic cracking, which enabled a great increase in isobutane cracking activity (214). This would occur through a bifunctional mechanism of dehydrogenation followed by acid-catalyzed cracking. The group also studied Ni impregnation of ferrierite and mordenite zeolites (215). Over mordenite, nickel accelerated hydrogen transfer reactions and hence coke formation in its larger channels and cavities, leading to very fast deactivation. Over ferrierite, where Ni seems to be predominantly as small NiO particles, it accelerated n-


ACS Catalysis

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hexane cracking rates. However, Ni wet impregnation of ZSM-5 was found to actually decrease n-hexane cracking rate. The results might involve, in addition to the formation of NiO nanoparticles active in dehydrogenation, the partial exchange of some Brønsted acid sites to an extent dependent on the structure and impregnation conditions (216). Selectivities in n-hexane cracking were largely unaffected as these would result from the acid catalyzed cracking of the same carbenium-like species, be they formed through unimolecular protolysis of the alkanes or by dehydrogenation and subsequent protonation (215). However, higher selectivity to ethene was enabled in isobutane cracking due to the competing oligomerization-cracking reactions upon formation of butenes (215). Effects similar to those exerted by Ni have been reported upon incorporation of Co (217) and Cr (218, 219) on ZSM-5 zeolite. In addition to these, other interesting modifications such as Ag introduction(220) or fluorination (221, 222) have been carried out and tested recently in the cracking of alkenes. Additionally, it should be mentioned that the introduction of heteroatoms in the zeolite catalyst is not limited to postsynthesis modifications. Certain heteroatoms can be introduced into the synthesis gel and they may occupy framework positions in small amounts under certain conditions. The isomorphic substitution of small amounts of certain metals in framework positions of zeolites has been demonstrated for B, Ti, Ga, and Fe (223-225). This has enabled major advances in redox catalysis, although its impact on acid catalyzed catalytic cracking has not attracted much interest. Overall, although isomorphic substitutions would affect the acid strength of the associated Brønsted acid sites or introduce distinct Lewis acid sites, their stability in framework positions definitely limits their prospects for catalytic cracking under severe conditions. However, they may demonstrate improved selectivities in the conversion of aromatics (226) or alkenes under milder conditions.


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ACS Catalysis

Researchers at Chiyoda Corporation modified ZSM-5 by introducing Ga and Fe in the synthesis gel (227). Brønsted sites associated to Fe would have a lower acid strength (228). However, as it is the case with B, their stability in framework positions is low under cracking conditions (229). In the end, Fe only seems to slightly reduce the acid amount and strength (230), if not hampering access to the zeolite crystal. In its turn, Lewis sites associated to Ga would catalyze dehydrogenation of alkanes (231), leading to increased propene yields and somewhat less propane and ethane (227). A patent issued in 2015 and transferred to Sinopec discloses the preparation of a WZSM-5 based catalyst and its application (232). The W-containing ZSM-5 can be directly synthesized in an alkaline system. The X-ray diffractograms show that the unit cell size of the molecular sieve is expanded, pointing to the incorporation of W into the framework. The W-ZSM-5-based catalyst is used for the cracking reaction of light olefins, leading to higher catalytic activity and higher ethene and propene selectivity. Another patent, from the University of King Fahd Petroleum & Minerals (233), claims the use of a Mn-modified ZSM-5 zeolite as an additive for the production of light olefins in the FCC process. The metal is incorporated into the zeolite by impregnation or by ion exchange, followed by drying and calcination to yield the final Mn-modified ZSM-5 zeolite. This metal-modified ZSM-5 zeolite was tested along with a FCC base catalyst at different weight ratios. The results demonstrate a significant increase in the production of light olefins, particularly propene, and a decrease in the paraffins/olefins ratio in the catalytic cracking of heavy oils.

3.4.Tuning Al siting and distribution during synthesis The location of Brønsted acid sites within zeolites influences catalytic rates and selectivities due to diverse intra-channel environments which guide the stabilization of


ACS Catalysis

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transition states to different extents (36). One of the most prototypical examples is that of mordenite zeolite. In this section, recent examples on the effect of different aluminum siting or distributions on the catalytic behavior are surveyed, as well as the very interesting strategies that are being developed to affect this distribution and that could enable improved catalysts for catalytic cracking. Synthetic methods

Distinguishing between Si and Al at a given crystallographic position is not a trivial task, particularly for structures exhibiting high number of different T sites (12 in the case of MFI, 5 for FER, and 9 for BEA*). However, it has been consistently demonstrated that the location of Al among the different sites is not a random process but is affected by several factors during the zeolite synthesis (234, 235). This can be advantageously used to preferentially locate Al atoms, and hence catalytic sites, in different positions of the framework that favor the desired reaction (236).

Such a strategy was first used to affect the location of Al atoms in ferrierite. Pyrrolidine is used conventionally as the OSDA for this zeolite, but Pinar et al. demonstrated that the introduction of a second OSDA, tetramethylammonium (TMA), occupied some of the cages and affected the position of pyrrolidine in the remaining cages (237). Pyrrolidine could form strong H bonds with O atoms in the framework, thus introducing Al in specific positions. This would not be possible with TMA, resulting in an increased accessibility of molecules to the catalytic protons. The location of the OSDAs in the structure was solved by combining the Rietveld method with Fourier differences maps. Materials prepared following such approach have shown remarkable differences in mxylene and 1-butene isomerization (238). Another approach based on the use of different OSDAs in the synthesis was also applied to affect Al siting in RTH-type


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ACS Catalysis

zeolites (239). The authors concluded that Al was preferentially located in the larger pores when using bulkier 1,2,2,6,6-pentamethylpiperidine or 1-methylpiperidine as OSDAs, whereas a more uniform Al distribution among T sites was obtained when using smaller pyridine or an OSDA-free method, showing differences with time on stream in the MTO reaction.

A similar strategy was later applied to the industrially prominent ZSM-5 zeolite. Yokoi et al. synthesized several ZSM-5 zeolites with Si/Al = 50 using different OSDAs (TPAOH, dipropylamine, cyclohexylamine, hexamethylenimine) with or without Na cations (240). By means of

27

Al MQMAS NMR they identified at least five

crystallographically different Al species, with their relative proportions varying substantially among the samples. The constraint index (CI) has also been used to estimate the distribution of acid sites in the micropores. The location of acid sites was investigated based on the difference in the transition-state shape-selectivity through the cracking of n-hexane and 3-methylpentane (3-MP). They concluded that ZSM-5 synthesized with TPAOH in the absence of Na cations would have a higher proportion of Al in the channel intersections as compared to those synthesized with TPAOH and other organic cations in the presence of Na cations. They also observed catalytic differences among the samples in 3-methylpentane cracking (3-MP). In particular, the ZSM-5 synthesized with TPAOH in the absence of Na cations was the most active sample and it showed a lower selectivity to CH4 + C2H6 + H2 characteristic of monomolecular cracking and a lower activation energy. This sample would have a higher proportion of Al in the channel intersections as compared to those synthesized with organic cations and presence of Na cations. These results also agree with those of toluene disproportionation, which would occur through bulky diphenylmethane intermediates.


ACS Catalysis

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Additionally, it was hypothesized that ZSM-5 synthesized OSDA-free could lead to more Al situated in the sinusoidal and straight channels, affecting its resistance to dealumination by acid (241). Nevertheless, various syntheses of ZSM-5 using TPAOH as OSDA in the absence and presence of Na cations (242) showed that Al atoms are predominantly located at the channel intersections in ZSM-5 zeolite. Besides, the regulation of Al siting in ZSM-5 can also be achieved by adjusting the silicon and aluminum source in the synthetic gel. Recently, Liang et al. reported the synthesis of ZSM-5 by using silica sol and tetraethyl orthosilicate with TPAOH in the presence of Na cations (243). By means of vis−DRS of Co(II) ions and 27Al MAS NMR, the authors concluded that the framework Al is enriched in the sinusoidal and straight channels in ZSM-5 synthesized using silica sol. This favored a longer lifetime in the MTO reaction and a more selective propene formation through the olefin-based reaction cycle. Very recently, Yokoi et al. reported that the use of pentaerythritol (PET) as a porefilling agent in combination with Na cations in the synthesis of ZSM-5 with Si/Al = 25 led to a unique Al distribution in the MFI framework (244). In this case, considering the size of the PET molecule, it would occupy channel intersections, while Na+ species would be located at straight and sinusoidal channels. Moreover, since PET has no charge (unlike TPA cations), Al3+ species would be located near Na+ species. This would lead to the preferential location of Al atoms at straight and sinusoidal channels rather than channel intersections in the MFI framework. The ZSM-5 material with a higher proportion of Al in the channels showed a lower selectivity to coke in the nhexane cracking and MTO reactions, probably due to a stronger steric hindrance for the coke precursors to grow within the narrow straight and/or sinusoidal channels of the MFI framework.


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ACS Catalysis

Although these kind of results are just starting to be explored for catalytic cracking, TOFs for monomolecular cracking of light alkanes over different ZSM-5 zeolites have already been shown to differ due to changes in the pre-exponential factor of the intrinsic rate constant (activation entropy), which can depend on the location of the OH groups in the zeolite pores (38). More precisely, a decreasing TOF with increasing Si/Al has also been reported by Janda and Bell (236), who related it to an increasing fraction of acid sites located in the intersection of the channels. Iglesia et al. showed that monomolecular cracking and dehydrogenation of propane and n-butane occur preferentially within 8-MR constrained side pockets in MOR structure, where transition states and adsorbed reactants are partially confined (34, 37). Such configurations lead to entropy gains that compensate the weaker binding of partially confined structures to produce lower free energies transition states within those sidepockets. For n-alkanes, monomolecular dehydrogenation reactions showed greater specificity in the latter 8-MR locations than cracking and also exhibited higher activation barriers, predominantly because (C-H-H)+ species involved in transition states for dehydrogenation reactions are less stable than (C-C-H)+ carbonium ions in cracking transition states (proponium, n-butonium) (245, 246). Activation entropies were also shown higher for n-alkane dehydrogenation than for cracking, being consistent with transition states of higher energies (37). It seems therefore plausible that reactions involving the latter transition states, with fully formed ion pairs, benefit from entropy gains thanks to partial confinement within 8-MR MOR side pockets. The electrostatic field generated by these entropy benefits resemble those for both protontransfer and electron transfer reactions in solvated systems, leading to molecular and charge reorganizations and, hence, being essential in stabilizing the ion pairs formed upon charge transfer.


ACS Catalysis

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The siting of heteroatoms can also affect the flexibility of the zeolite and its diffusional properties. Hong’s group studied the so-called “fibrous” natrolite gallosilicates (NAT framework type). They observed that by increasing the crystallization temperature or time the degree of long-range ordering of Si-Ga increased (247), apparently through an intraframework migration mechanism (248), and could thus be controlled. They studied the ion exchange ability of these materials at a fixed Si/Ga ratio, leading to remarkable findings (249): i) gallosilicate natrolites only exchange monovalent cations, as opposed to the results with high alumina zeolites. ii) Exchange is hindered as the size of the cation increases but also when the degree of Si-Ga ordering increases (Table 2). Since the Shannon ionic diameters of the larger diffusing cations were greater than the crystallographic diameters of the pores, the authors hypothesized that changes in the flexibility of the framework must be key. In more disordered natrolites, a higher fraction of germanium is located in the low multiplicity T1 site (which is only bound to tetrahedra in the same natrolite chain). This would increase the framework flexibility since Ge is larger and more electropositive and therefore bonds to T1 tetrahedra become more ionic and less directional. iii) Calcination of natrolite causes a closing of the 9and 8-membered ring pores upon dehydration. Ion exchange is thus severely hindered unless the zeolite is able to deform to a large extent. In the limit case, the most ordered zeolite was found stable up to 800 °C and it was no longer able to uptake He or H2 as a microporous material. Table 2. Ion exchanges in sodium gallosilicate natrolites. Reproduced from (249). Na-GaNAT-I

Na-GaNAT-II

Na-GaNAT-III

0.29

0.55

0.97

Li+

40

12

1

+

99

99

84

Long-range order parameter Degree of exchange / % K


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ACS Catalysis

Rb+

99

66

6

+

16

Engineering Zeolites for Catalytic Cracking to Light Olefins - ACS

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