Transition State Shape Selectivity Research Articles

Transition State Shape Selectivity in Theoretical Prognosis for Zeolites Catalytic Activities on Example of Reaction Between Acrylonitrile and Piperylene

Transition State Shape Selectivity in Theoretical Prognosis for Zeolites Catalytic Activities on Example of Reaction Between Acrylonitrile and Piperylene

Bio-fuel production from hydroconversion of hexadecane over Pt/SAPO-11 catalysts

The hydroconversion of normal renewable diesel fractions appears as a pivotal process in the production of bio-fuel derived from lipids. The adaptability of bio-fuel product distribution to align with market demand necessitates the utilization of catalysts with superior performance in long-chain alkane hydroconversion. The hydroconversion performance of the model compound hexadecane over Pt/SAPO-11 catalysts was comprehensively investigated. The accessibility α of B acidity in Pt/SAPO-11 catalysts was determined using Py and DMPy-IR, and its impact on hexadecane hydroconversion was addressed. This study revealed that the shape selectivity mechanism in long-chain alkane hydroconversion on Pt/SAPO-11 catalysts involves the combination of the transition state and product shape selectivity catalysis. Pt/SAPO-11-S1, with the largest accessibility factor α, reached unprecedented maximum yield levels of 68 %, 87 %, and 37 % for multibranched C16H34 isomers, total C16H34 isomers, and jet fuel fractions, respectively. Additionally, Pt/SAPO-11-S1 exhibited excellent performance for isomerization and jet fuel production in actual renewable diesel hydroconversion. It demonstrates the potential of Pt/SAPO-11 catalysts featuring accessible B acidity in renewable diesel hydroconversion.

Interaction of the transition state of the Claisen rearrangement with surface of carbon nanotubes to predict catalytic activity

The possibility of using the transition state shape selectivity concept for control of selectivity of reaction was previously demonstrated by us using the example of 1,3-dioxanes formation by Prins reaction.The extension of the applied approach to other reactions is of theoretical and practical interest. The aim of our work was to study the interaction of the frozen transition state of Claisen rearrangement with the surface of nanotubes of various structures to predict their catalytic activity.Claisen rearrangement of allylphenyl ether was chosen as a model reaction, and carbon nanotubes with different diameters were chosen as a porous materials. The calculated dependence of the stabilization energy on the nanotube diameter has an extreme character with pronounced maxima at a diameter of 10.2–10.4 Å. The obtained results allow to predict the catalytic activity of nanotubes with a diameter of 10–11 Å in Claisen rearrangement of allylphenyl ether.

On the product and transition-state shape selectivities in 2-heptene isomerization

Isomerization of n-alkanes is industrially applied to improve the octane number of gasoline. The origin of shape selectivity and influence of micropore confinement in this reaction have not been fully understood. In this work, periodic DFT calculations have been performed to examine the mechanism for n-heptene isomerization in H-BEA and H-MFI. Three reaction pathways, including the protonated cyclopropane, edged-protonated cyclopropane, and alkyl shift mechanisms, have been examined. It is found the formation of smaller di-branched isomers is determined by product shape selectivity, but that of relatively bulky isomers is kinetically relevant and can be accounted for by transition-state shape selectivity.

Promoting hydroisomerization selectivity using channel axis reduced ZSM-48 fabricated by a combined bead-milling and porogen-assisted recrystallization method

ZSM-48 is a unidimensional zeolite that is frequently used as acid component of bi-functional hydroisomerization catalyst. As selectivity towards unwanted cracking products increases with retention time of isomerized products inside micropores, shortening their retention time inside the micropores by reducing channel length offers an effective way to improve isomerization selectivity. Direct synthesis of unidimensional zeolitic crystals with reduced channel axis is particularly difficult, since they habitually grow into rod-like shape along their channel direction. Post-synthetic bead-milling is a cost effective, mechanochemical method to decrease crystal size of zeolites from micro meter to nano meter scale, but suffers from amorphization or partial destruction of zeolitic crystals. Consequently, a post-milling recrystallization process is often entailed to remedy the destructed domains, which, nonetheless, readily causes secondary crystal growth and formation of bulky crystals that compromises the effect of bead-milling. Here, we show that porogen-assisted recrystallization can overcome the problem to fabricate channel axis reduced ZSM-48 nanocrystals. The Si/Al ratio is tunable between 30 and 200 and the acidity is largely preserved after recrystallization. The catalytic advantages of the obtained material are demonstrated in catalytic n-heptane hydroisomerization. The obtained ZSM-48 nanocrystal with Si/Al ratio of 100 outperforms the rest samples and shows enhanced isomerization yield (72% vs 62%) with respect to the parent sample. By combined diagnoses of the impact of diffusion length on product selectivity (product shape selectivity) and kinetic studies, selectivities to monomethyl branched isomers (2-methyl hexane and 3-methyl hexane) and small bulky dimethyl branched isomers (2,3- and 2,4-dimethyl pentane) are attributed to product shape selectivity, whereas the formation to bulky dimethyl branched isomers (2,2- and 3,3-dimethyl pentane) is hindered mainly by transition-state shape selectivity. The combined destructive-constructive synthetic protocol and the revelation of the origin of shape-selectivity in n-heptane hydroisomerization are useful for hydroisomerization catalyst design.

Transalkylation of C10 aromatics with 2-methylnaphthalene for 2,6-dimethylnaphthalene synthesis: High-efficiently shape-selective & synergistic catalysis over a multifunctional SiO2-Mo-HBeta catalyst

A new process for the low-cost synthesis of 2,6-DMN, the high-efficiency transalkylation of C10 aromatics (C10A) with 2-MN in H2 atmosphere was developed depending on a multifunctional SiO2(Ⅱ)-3.0%Mo-HBeta catalyst with three kinds of active sites located only inside its 12-ring channels. Firstly, the high-efficient synergistic catalysis from metal-Mo monomer and Lewis & Bronsted acid-sites significantly improved transalkylation, attributed to a surface behavior (involving hydrogen spillover) may producing the active hydrogen-species which markedly enhance the proton-transfer efficiency in this proton-acid catalytic reaction. Secondly, the hydrogenation & subsequent ring-opening etc. side-reactions of naphthalene series and the formation of multi-alkylnaphthalene byproducts were almost completely suppressed due to both the passivation of the external surface of catalyst and the transition-state shape-selectivity & the diffusion-constraint effect inside pores. Besides, both the reasonable catalytic-hydrogenation (from metal-Mo monomer) immediately converting light olefins (from dealkylation) into alkanes and the transition-state shape-selectivity suppressing the formation of larger-molecular multi-alkylnaphthalenes in channels, effectively prevented from the coking-deactivation of catalyst. As a result, the 2-MN conversion of above 75.3% and the 2,6-DMN yield of above 9.7% were obtained during the 50 h on-stream reaction. Our work may provide a promising model for rationally designing the elegant catalyst for a complicated conversion reaction.

Presituated “coke”-determined mechanistic route for ethene formation in the methanol-to-olefins process on SAPO-34 catalyst

Control of product selectivity is a central yet challenging issue in catalysis chemistry, both in academia and industry. The ever-increasing market demand for ethene, together with relatively low ethene selectivity, necessitates the upgrading of the current methanol-to-olefins (MTO) industry to enhance ethene selectivity. We report here an operable strategy, 1-butene “precoking” technology, by which ethene selectivity has been unprecedentedly enhanced while the catalyst lifespan has been conserved. We also made effects to provide more insight into the long-standing controversies regarding the mechanistic origin of the increase in ethene selectivity. With the aid of a 12C-/13C-methanol switch experiment, MD calculations, and FTIR analysis, it was demonstrated that ethene increase is neither simply—or solely—caused by the configurational diffusion effect, nor due to the transition-state shape selectivity or methylnaphthalene species, but rather to the spatial siting of the presituated “coke”. The relatively evenly distributed presituated coke extends the reaction zone toward the near-core of the SAPO-34 crystal, elongating the diffusion trajectories of molecules and allowing more acid sites to be utilized. The diffusion-hindered higher olefinic intermediates tend to evolve to active aromatic species, which, in turn, lead substantially to ethene. This mechanistic understanding could be also applicable to explaining ethene increase with time on stream (TOS) on parent SAPO-34.

On the mechanism for ethyl transfer and removal from ethylbenzene during commercial xylene isomerization

Abstract Ethylbenzene (EB), a structural isomer of xylenes, is present in feedstocks for commercial xylene isomerization. EB cannot be economically separated from the close boiling xylenes by distillation. It must be converted to lower and/or higher boiling byproducts that can, to prevent its buildup in the xylene isomerization unit recycle loop. Several routes are available for converting EB, including ethyl transfer to another aromatic via transalkylation, and EB dealkylation. A mechanism initiated by hydride abstraction has often been cited for ethyl transfer over large pore molecular sieves, based on old data developed for liquid phase conversion over HF/BF3 catalysts which are strong Lewis acids. However, ethyl transfer occurs readily over large pore molecular sieves that contain only Bronsted acid sites, and little or no Lewis acidity via a mechanism in which the ethyl group never leaves the first ring before being transferred to a second. I propose reconsideration of an SN2 mechanism instead. Also, a published mechanism proposed for EB dealkylation/realkylation that occurs over medium pore sieves is corrected to show ethylene as a stable intermediate and expanded to provide the mechanism for EB dealkylation over catalysts comprising medium pore sieves and a mild hydrogenation catalyst. The role of dual bed catalysts that employ a combination of transition state and product shape selectivity to further reduce xylene loss is explained.

Modification of the acidic and textural properties of ZSM-5 zeolite by using double mineralizers in synthesis and its catalytic performance in the conversion of methanol to propene

A static hydrothermal approach was adopted to synthesize nanosized SiO2-ZSM-5 zeolite in the media of F−-OH− with double mineralizers, using tetraethoxysilane, sodium aluminate, and tetrapropylammonium hydroxide as the silicon source, aluminum source, and template agent, respectively. The physical and chemical properties of the synthesized ZSM-5 zeolites were characterized and their catalytic performance was evaluated in the conversion methanol to propene (MTP); the effect of F−/Al2O3 molar ratio on the catalytic performance of synthesized H-ZSM-5 was investigated. The results indicate that an increase in the F−/Al2O3 molar ratio of the synthesis mixture leads to an increase in the surface content of microcrystalline SiO2, accompanying with a decrease in the relative crystallinity, surface area, pore volume, and acid strength and density. With a F−/Al2O3 molar ratio of 12, the SiO2-ZSM-5 zeolite exhibits the best catalytic performance in MTP, with a selectivity of 45% to propene and a propene/ethene (P/E) ratio of greater than 10. It is further hypothesized that the transition state shape selectivity plays an important role in determining the product selectivity in MTP.

Crystallization of ATO silicoaluminophosphates nanocrystalline spheroids using a phase-transfer synthetic strategy for n-heptane hydroisomerization

Changing size and shape of zeolitic crystals brings about enhanced diffusion properties and related promotion of catalytic performances. This work puts forward a phase-transfer synthetic strategy to generate ATO nanocrystalline silicoaluminophosphates spheroids under tumbling crystallization using aluminum isopropoxide dissolved in toluene and other ingredients (fumed silica/phosphoric acid/Di-n-hexylamine) dissolved in water as precursors. Toluene and aqueous phase formed a Pickering emulsion like structure during hydrothermal crystallization. Highly uniform crystalline spheroids of weighted 55 nm with strong acidity and high surface area were obtained, which exhibited a high isomer yield up to 79% in n-heptane hydroisomerization. Comparisons of product distribution and kinetic studies suggest that monomethylbranched isomer selectivity is controlled by product shape selectivity, whereas dimethylbranched isomer selectivity is affected by combined transition state and product shape selectivity. The increase in selectivity for ATO spheroids is attributed to an expansion of intrinsic reaction controlled temperature regime and an improved diffusion property.

Diffusion of Trimethylbenzenes, Toluene, and Xylenes in UWY Zeolite as a Catalyst for Transalkylation of Trimethylbenzenes with Toluene

A molecular dynamics study has been carried out on the diffusion of trimethylbenzene (TMB), toluene, and xylene molecules in transalkylation of TMB and toluene in UWY zeolite, containing crossing 10-ring and 12-ring channels. Two models of UWY have been employed, the pure silica and an acidic form of UWY including Al and Bronsted sites, using a recently parameterized general force field for zeotypes, which reproduce pore diameters within ca. ±0.2 A. Molecular traffic has been observed from the result of TMBs using almost exclusively the 12-ring channels and the preferential location of p-xylene and toluene in the 10-ring channels at high loading (reaction conditions). From the three different 10-ring channels, only p-xylene and toluene can fit in the two smallest channels, whereas m-xylene and 1,2,4-TMB can also diffuse in the largest 10-ring channel. An in-depth analysis of transition-state shape selectivity has been performed, showing that all transition states of transalkylation of toluene and TMBs can...

Transformation of limonene into p-cymene over acid activated natural mordenite utilizing atmospheric oxygen as a green oxidant: A novel mechanism

Natural mordenite originated from volcanic soils in Greek islands was treated with sulfuric acid aqueous solutions of different concentrations and solid mass/solution volumes. The samples were characterized using various techniques (N2-physisorption, XRD, ATR-FTIR, SEM-EDS, TEM, Microelectrophoresis, Equilibrium pH) and tested in the transformation of limonene into p-cymene in the presence of atmospheric air at various temperatures and reaction times. The acid treatment is causing the removal of sodium oxide located inside the framework micropores and the small inter-fiber mesopores. This, in turn, increases drastically the BET specific surface area and unmasks negatively charged surface sites which are transformed into acid sites by adsorbing H+/H2O+ ions. The relatively low extent removal of Al3+ ions does not disturb the framework of natural mordenite nor its fibrous morphology. The development of micropores and small mesopores surface area and surface acid sites are transforming the catalytically inactive natural mordenite into very active catalysts. The increase in the conversion and the yield of p-cymene follows the increase in the BET specific surface area. A novel mechanism was experimentally established involving a catalytic step followed by a non catalytic one. The first step involves adsorption of limonene on acid sites via the exocyclic double bond to form the more stable tertiary carbenium ion from which terpinolenes, terpinenes and “polymeric species” are formed. The “transition state shape selectivity” manifested by the catalysts studied does not allow the formation of intermediate disproportionation products. In the second step, catalyst-free aromatization and “polymerization” of terpinolenes and terpinenes were found to occur. The aromatization was proposed to proceed by abstraction of an allylic hydrogen resulting to free radical (R) followed by combination with O2 and radical chain propagation to yield allylic peroxides ([ROO]) which by elimination of (HOO) lead to the production of p-cymene. Moreover, high molecular weight compounds may be formed as radicals are combined to alkyl and peroxyl dimmers that may be polymerized. The experimental parameters concerning the acid treatment and reaction conditions were optimized for maximizing the amount of the produced p-cymene keeping as low as possible the amount of the produced polymeric species.

Product shape selectivity of MFI-type, MEL-type, and BEA-type zeolites in the catalytic hydroconversion of heptane

The influence of product shape selectivity on the bifunctional conversion of n-C7 by zeolite catalysts is investigated. Three different zeolite catalysts with different pore sizes (MFI-type, MEL-type, and BEA-type zeolites) have been investigated experimentally. For all three catalysts, n-C7 is isomerized to monobranched isomers which are further isomerized into dibranched isomers, and these dibranched molecules are converted into cracking products. More dibranched isomers and less cracking products are produced by BEA-type zeolite compared to MFI-type and MEL-type zeolites and clear differences are observed in the distribution of dibranched isomers produced by different catalysts. Molecular simulation is used to compute the adsorption isotherms and free energy barriers for diffusion of dibranched isomers in MFI-type, MEL-type, and BEA-type zeolites. Combining simulation results and experimental observations, it is shown that product shape selectivity can explain the distribution of dibranched molecules while transition state shape selectivity fails to do so. For the medium-pore zeolites (MFI-type and MEL-type zeolites), free energy barriers for diffusion of dibranched molecules are significant. For MFI-type and MEL-type zeolites, the dibranched molecule that has to overcome lower diffusion barrier is produced with a higher yield and the distribution of dimethylpentane molecules is determined by their diffusion rate. It is shown that there is almost no free energy barrier for the diffusion of any of these molecules in BEA-type zeolite. As BEA-type zeolite imposes no free energy barrier for diffusion of any of dibranched isomers, the distribution of dibranched isomers is very close to the equilibrium distribution in the gas phase. Due to the limited mobility of dimethylpentanes within the pores of MFI-type and MEL-type zeolites, most of the dimethylpentane molecules are trapped inside the zeolite and undergo consecutive cracking. Dimethylpentane molecules diffuse sufficiently fast in the large pores of BEA-type zeolite and transfer to the gas phase, before consecutive reaction converts these molecules into cracking products. Moreover, the effect of the MFI-type crystal size on the production of dibranched isomers is investigated. The yield of dibranched isomers reduces by increasing the size of the crystal and larger part of dibranched isomers are cracked as the crystal size of MFI-type is increased.

Production of olefinic diesel, lubricants, and propylene

HEU-1 zeolites with different SiO2/Al2O3 ratios (60–530) were in-situ synthesized and systematically evaluated in n-hexane catalytic cracking reaction. The influencing factors including SiO2/Al2O3 ratio, weight hourly space velocity and reaction temperature were investigated to optimize the light olefin yield, especially propylene. Compared to HZSM-48 and HZSM-5 zeolites with similar SiO2/Al2O3 ratios, HEU-1-300 (SiO2/Al2O3 = ca. 300), for example, displays much higher yield of ethylene plus propylene than HZSM-48 and comparable with HZSM-5. Furthermore, the propylene yield (35.6%) on it is higher than HZSM-5 by 9.0 percentage points, thus leading to higher propylene/ethylene ratio of 2.1. This excellent cracking performance of HEU-1-300 is ascribed to the combination of its moderate Brönsted acidity, due to the optimal SiO2/Al2O3 ratio, and unique framework topology. Moreover, both HEU-1 and HZSM-48 zeolites present much lower aromatics selectivity in the gas effluent than HZSM-5. For HEU-1 zeolite, it displays significant product shape selectivity due to its confined channel system, even though the large void space favors the formation of large intermediate species or aromatics precursors, while there is transition state shape selectivity for HZSM-48 structure which possesses no enough void space available to accommodate the large and complex hydrocarbons. HEU-1 topological structure also has pronounced effect on the formation and oxidation of coke deposition.

Catalytic cracking of n-hexane over HEU-1 zeolite for selective propylene production: Optimizing the SiO2/Al2O3 ratio by in-situ synthesis

HEU-1 zeolites with different SiO2/Al2O3 ratios (60–530) were in-situ synthesized and systematically evaluated in n-hexane catalytic cracking reaction. The influencing factors including SiO2/Al2O3 ratio, weight hourly space velocity and reaction temperature were investigated to optimize the light olefin yield, especially propylene. Compared to HZSM-48 and HZSM-5 zeolites with similar SiO2/Al2O3 ratios, HEU-1-300 (SiO2/Al2O3=ca. 300), for example, displays much higher yield of ethylene plus propylene than HZSM-48 and comparable with HZSM-5. Furthermore, the propylene yield (35.6%) on it is higher than HZSM-5 by 9.0 percentage points, thus leading to higher propylene/ethylene ratio of 2.1. This excellent cracking performance of HEU-1-300 is ascribed to the combination of its moderate Brönsted acidity, due to the optimal SiO2/Al2O3 ratio, and unique framework topology. Moreover, both HEU-1 and HZSM-48 zeolites present much lower aromatics selectivity in the gas effluent than HZSM-5. For HEU-1 zeolite, it displays significant product shape selectivity due to its confined channel system, even though the large void space favors the formation of large intermediate species or aromatics precursors, while there is transition state shape selectivity for HZSM-48 structure which possesses no enough void space available to accommodate the large and complex hydrocarbons. HEU-1 topological structure also has pronounced effect on the formation and oxidation of coke deposition.

Control of the Al Distribution in the Framework of ZSM-5 Zeolite and Its Evaluation by Solid-State NMR Technique and Catalytic Properties

The effects of the organic structure-directing agents (OSDAs) and Na cations for the synthesis of ZSM-5 on the location of Al atom in the framework as well as the acidic and catalytic properties were investigated. To achieve these purposes, ZSM-5 zeolites were synthesized by using four kinds of OSDAs including tetrapropylammonium hydroxide cations, dipropylamine, cyclohexylamine, and hexamethylenimine with or without Na cations. In situ FT-IR spectroscopy using CO as probe molecule was applied to the evaluation of the acid property of the ZSM-5 zeolites. The location of Al atoms was examined by high resolution 27Al MAS and MQMAS NMR techniques. 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. Furthermore, the cracking of various types of paraffins and the conversion of aromatic compounds were conducted to clarify the acid site distributions.

Liquid phase dehydration of 1-indanol: Selective synthesis of indene over microporous acid catalysts

The selective dehydration of 1-indanol in liquid-phase was studied at 363 K and 2 bar, over solid acid catalysts, and using cyclohexane as a solvent. Mesoporous SiO2–Al2O3 with a high density of strong Lewis acid sites produced di-1-indanyl ether and other heavy products apart from indene. In contrast, the overall selectivity to indene with beta zeolite (HBEA, Si/Al = 12.5), HZSM-5 (Si/Al = 20) and dealuminated mordenite zeolite (HMOR, Si/Al = 80) was almost 100%. The transition state shape selectivity and a moderate density of both Lewis and Brønsted acid sites were crucial to achieve high selectivity to indene. In spite of the high overall selectivity, the final yield in indene with HBEA was only 55%. This was attributed to the formation of heavy compounds inside the large cavities of HBEA, which cannot diffuse through its micropores. In contrast, yields in indene higher than 90% were obtained with HMOR and HZSM-5. It was also observed that the yield in indene increased with time even after reaching the 1-indanol total conversion, which indicates the slow diffusion of the reactant and product molecules inside of the microporous network of HMOR and HZSM-5. Therefore, a process involving liquid phase dehydration of 1-indanol, employing one of these two zeolites as a catalyst would provide: 1) high yield in indene; 2) easy separation of product from solvent; 3) easy separation of the catalyst from the reaction medium.

1,2,4-Trimethylbenzene disproportionation over large-pore zeolites: An experimental and theoretical study

While 1,2,4-trimethylbenzene (TMB) disproportionation is one of the potential technologies for p-xylene production, its reaction intermediates have been neither experimentally observed nor identified yet. Here we present GC–MS evidence that not only pentamethylated diphenylmethane (5mDPM) derivatives but also hexamethylated diphenylmethane (6mDPM) ones are serving as key reaction intermediates of 1,2,4-TMB disproportionation over large-pore zeolites. We also propose a new bimolecular diphenylmethane-mediated reaction pathway for the formation of tetramethylbenzenes (TeMBs) and xylenes over large-pore zeolites based on the GC–MS results obtained. Comparison with the GC–MS results from LaNa-Y after transalkylation reactions of three different TeMB isomers with three xylene and three TMB isomers, respectively, allows us to rationally identify most of the seven 5mDPM and three 6mDPM derivatives at their isomer level. A combination of GC–MS and DFT calculation results demonstrates that the bimolecular 1,2,4-TMB disproportionation over LaNa-mordenite, a practically one-dimensional 12-ring zeolite with regard to this reaction, is a new example of transition state shape selectivity in zeolite catalysis.

Theoretical Investigation of the Isomerization and Disproportionation of m-Xylene over Medium-Pore Zeolites with Different Framework Topologies

The strain energies of three trimethylated diphenylmethane (3mDPM) and six tetramethylated diphenylmethane (4mDPM) isomers serving as the main reaction intermediates of m-xylene isomerization and disproportionation over eight medium-pore zeolites with different framework topologies have been determined theoretically in order to elucidate the effects of zeolite pore structure on this aromatic transformation. Although the strain energies of 3mDPM and 4mDPM derivatives in MCM-22, TNU-9, and NU-87, all of which have large 12-ring cavities/channels, are always lower than 40 kJ mol–1, some of them in cavity-free ZSM-5, ZSM-57, and TNU-10 are characterized by the strain energies higher than 40 kJ mol–1. In particular, all the species in ZSM-22 and ZSM-23 with narrower one-dimensional 10-ring channels have the strain energies much higher than 40 kJ mol–1. On the other hand, the energy difference (<30 kJ mol–1) between the (dimethylphenyl)methylium ion and the transition state for formation of the tetramethylated benzenium-type carbenium ions was calculated to be much lower than the energy barrier (183 kJ mol–1) to the hydride transfer from the reactant molecule. The overall results of this study clearly show that transition-state shape selectivity is responsible for the formation of 3mDPM derivatives, as well as of slightly larger 4mDPM ones, in medium-pore zeolites.

Unraveling Diffusion and Other Shape Selectivity Effects in ZSM5 Usingn-Hexane Hydroconversion Single-Event Microkinetics

Potentially dominant factors governing the shape selectivity in n-hexane hydroconversion over a Pt/H-ZSM5 catalyst were evaluated by means of single-event microkinetic (SEMK) model regression against experimental data. The observed product distribution could be adequately modeled, and a corresponding physically meaningful interpretation could be made only when accounting for intracrystalline diffusion limitations for each hexane isomer involved in the reaction network, rather than considering physisorption effects or transition-state shape selectivity. Simultaneous diffusion and reaction inside the catalyst crystallites were expressed via Fick’s second law, while the alkane Fick diffusion coefficients were assessed by explicitly accounting for mixture nonideality effects. A 3-fold lower diffusion coefficient was found to be required for 3-methylpentane compared with 2-methylpentane to explain the typically high selectivity toward the latter alkane. Once formed inside the catalyst crystallite, dimethylbuta...