Transition State Shape Research Articles

Acid‐Catalyzed Cyclization of Terpenes Under Homogeneous and Heterogeneous Conditions as Probed Through Stereoisotopic Studies: A Concerted Process with Competing Preorganized Chair and Boat Transition States

Based on stereoisotopic studies and beta-secondary isotope effects, we propose that the acid-catalyzed cyclization of geranyl acetate proceeds through a concerted mechanism. Under heterogeneous conditions (zeolite Y confinement), a preorganized chairlike transition state predominates, whereas under homogeneous conditions the boat- and chairlike transition states are almost isoenergetic. For the case of farnesyl acetate, we propose that under homogeneous conditions a concerted dicyclization occurs with a preorganized boat-chair transition state competing with the chair-chair transition state. Under zeolite confinement conditions, the chair-chairlike dicyclization transition state is highly favorable. The preference of chairlike transition states within the cavities of zeolite Y is attributed to a transition state shape selectivity effect.

The Effect of Confined Space on the Growth of Naphthalenic Species in a Chabazite‐Type Catalyst: A Molecular Modeling Study

AbstractMethylation reactions of naphthalenic species over the acidic microporous zeolite with chabazite topology have been investigated by means of two‐layered ab initio computations. Large cluster results combined with van der Waals contributions provide thermodynamic and kinetic results of successive methylation steps. The growth of fused bicyclic species is important as these can act as hydrocarbon pool species within the methanol‐to‐olefin (MTO) process, but ultimately leads to the deactivation of the catalyst. The influence of the confined space of the zeolite pore on the resulting transition state or product shape selectivity is investigated in detail.

Bent out of shape

Reactants require a certain amount of energy to react — but what kind of energy? Chemical dynamics simulations predict that vibrationally exciting reactants can promote 1,3-dipolar cycloaddition reactions by bending them into the correct transition state shape.

Effect of pore size and acidity on the coke formation during ethylbenzene conversion on zeolite catalysts

The present work provides solid-state 13C NMR spectroscopic evidence that the zeolites acidity and the pore size strongly affect the catalytic behavior of ethylbenzene disproportionation and coke formation. The medium-pore zeolite H-ZSM-5 ( ca. 0.56 nm) and the large-pore zeolite H-Y ( ca. 0.74 nm) used in this study have exclusively Brønsted acid sites, but with different acid strength. Due to the transition state shape selectivity of ethylbenzene disproportionation, ethylbenzene transalkylation on H-Y takes place at low reaction temperature without side-reactions. On H-ZSM-5, dealkylation/realkylation was observed and generation of alkylcarbenium ions resulted in secondary reactions. These alkylcarbenium ions initiate coke formation on zeolite H-ZSM-5 via oligomerization, six-ring closure, and aromatization of alicyclic hydrocarbons. Ethylbenzene disproportionation on large-pore zeolites Y is an attractive reaction due to its low reaction temperature, high selectivity without side-reactions, and low coke formation. Medium-pore zeolite H-ZSM-5 showed higher reactivity for dealkylation of ethylbenzene and protolytic cracking of light alkanes due to its narrow channels and stronger Brønsted acid sites.

Insight into the Mechanisms of the Ethylbenzene Disproportionation: Transition State Shape Selectivity on Zeolites

The direct experimental evidence shows that ethylbenzene disproportionation is a transition state shape selective reaction on zeolites: a bimolecular reaction mechanism via diphenylethane-mediated pathway on large-pore zeolites X and Y (ca. 0.74 nm) and a monomolecular reaction mechanism on medium-pore zeolites ZSM-5 (ca. 0.56 nm) via the ethoxy-mediated intermolecular ethyl group transfer. The lifetime of bulky diphenylethane species was prolonged by a fine-tune of FAU-zeolites, which makes this transition state detectable by 13C MAS NMR spectroscopy. Due to tunable catalytic properties and pore shapes, zeolites are promising catalysts toward emulating the efficiency and selectivity in desired reactions.

Synthesis of nonsteroidal drugs with anti-inflammatory and analgesic activities with zeolites and mesoporous molecular sieve catalysts

A series of molecules with nonsteroidal anti-inflammatory and analgesic properties were synthesized through a chemical route involving the oximation of acetophenone derivatives, followed by the solid acid-catalyzed Beckmann rearrangement to give the corresponding amides. Microporous and mesoporous molecular sieves, as well as a delaminated (ITQ-2) zeolite, were studied as catalysts, and excellent activity and selectivity for amides were achieved when accessibility of the reactants to the active sites and the surface polarity of the catalyst were optimized. When the number and size of ring substituents increase in the product, only ITQ-2 and MCM-41 give high activities and selectivities. The order of activity observed, when the size of the oxime increases, can be reversed with respect to the obtained conventionally, because of a transition-state shape selectivity in the pores, where the bulkier substituents in the anti-position are impeded from migrating.

Computational elucidation of the transition state shape selectivity phenomenon.

The most commonly cited example of a transition state shape selective reaction, m-xylene disproportionation in zeolites, is examined to determine if the local spatial environment of a reaction can significantly alter selectivity. In the studied reaction, ZPE-corrected rate limiting energy barriers are 136 kJ/mol for the methoxide-mediated pathway and 109 to 145 kJ/mol for the diphenylmethane-mediated pathway. Both pathways are likely to contribute to selectivity and disfavor one product isomer (1,3,5-trimethylbenzene), but relative selectivity to the other two isomers varies with pore geometry, mechanistic pathway, and inclusion of entropic effects. Most importantly, study of one pathway in three different common zeolite framework types (FAU, MFI, and MOR) allows explicit and practically oriented consideration of pore shape. Variation of the environment shape at the critical transition states is thus shown to affect the course of reaction. Barrier height shifts on the order of 10-20 kJ/mol are achievable. Observed selectivities do not agree with the transition state characteristics calculated here and, hence, are most likely due to product shape selectivity. Further examination of the pathways highlights the importance of mechanistic steps that do not result in isomer-defining bonds and leads to a more robust definition of transition state shape selectivity.

π-Face diastereoselection: stereochemistry and reactivity of reduction reactions on conformationally rigid substrates: 5-X-adamantan-2-ones and trans-10-X-decal-2-ones

Stereochemistry and relative rates k ax and k eq of reduction reactions on title compounds have been measured under four different reaction conditions (NaBH 4 in i-PrOH, LiBH 4 and NaAlH 4 in THF and LiAlH 4 in Et 2O). Reaction sensitivities show that the Group III element has a prominent role in determining the structure of the transition state shape.

Supramolecular origins of product selectivity for methanol-to-olefin catalysis on HSAPO-34.

Ethylene selectivity in methanol-to-olefin (MTO) catalysis is related to the number of methyl groups on benzene rings trapped in the nanocages of the preferred catalyst HSAPO-34. By correlating the time evolutions of the catalysts' 13C NMR spectra and the volatile product distribution following abrupt cessation of methanol flow, we discovered that (in the absence of other adsorbates) propene is favored by methylbenzenes with four to six methyl groups but ethylene is predominant from those with two or three methyl groups. We substantially increased ethylene selectivity by operating at lower methanol partial pressures or higher temperatures, either of which reduces the steady-state average methyl substitution. As a step toward a kinetic analysis of the MTO reaction on HSAPO-34, we treated each nanocage with a methylbenzene molecule as a supramolecule capable of unimolecular dissociation into ethylene or propene and a less highly substituted methylbenzene. Addition of a water molecule to a nanocage containing a methylbenzene produces a distinct supramolecule with unique properties. Indeed, co-feeding water with methanol significantly increased the average number of methyl groups per ring at steady state relative to identical conditions without additional water, and also increased ethylene selectivity, apparently through transition state shape selectivity.

The Origin of the C7-Hydroconversion Selectivities on Y, β, ZSM-22, ZSM-23, and EU-1 Zeolites

We carry out experimental hydroconversion tests of n-heptane paraffin on five different zeolites: Y, β, ZSM-22, ZSM-23, and EU-1. The experimental selectivities clearly depend on the catalyst pore structures. Through force field simulations, we investigate adsorption and diffusion properties as well as model cyclopropane intermediate stabilities inside each zeolite framework. In open structures such as Y and β, no shape transition state selectivity or diffusion limitation are taking place. In the more restricted 10-membered ring (MR) channels of ZSM-22, ZSM-23, and 10-MR windows of EU-1, we show that the selectivities in branched isomers and cracked products are explained by the combined effects of transition state restriction and product diffusion limitations. In particular, the selectivities in monomethyl-branched products are correlated to their relative diffusion barriers, in agreement with earlier results of E. B. Webb III et al. (1999, J. Phys. Chem. B103, 4949). The explanation for this result has its foundation in the degree of symmetry of the monomethyl-branched paraffin. In ZSM-22 and ZSM-23, the 2,2- and 3,3-dMC5 are not formed on account of transition-state shape restrictions, whereas a slight amount of 2,3- and 2,4-dMC5 is formed and can diffuse out of the pores. In the EU-1 sieve, the large side pockets behave as the open Y or β structures, where all isomerization reactions can take place without restriction, but only the faster diffusing 2,3-dMC5 and 2,4-dMC5 products are released. The slower diffusing multibranched 2,2-dMC5 and 3,3-dMC5 paraffins are transformed via methyl shift or cracked inside the pockets producing a higher amount of cracked products in EU-1 than in ZSM-22 or ZSM-23.

Desproporcionamento de tolueno sobre zeólitas tipo mordenita <FONT FACE=Symbol>-</FONT> atividade e seletividade na obtenção de xilenos

A sodium mordenite zeolite (Na-MOR) was synthesized and modified by dealumination with chloridric acid solution (H-MOR). X-Ray Diffraction (XRD), Inductive Coupled Plasm (ICP) and Scanning Electron Micrograph (SEM) techniques were used for sample characterization. The zeolite catalytic behavior was evaluated through toluene disproportionation at 435°C. It was verified that mordenites were very selective for the disproportionation reaction and the samples with higher aluminum content showed larger initial activity, however, these samples showed too a higher deactivation velocity due to a blockage of the unidimensional porous system of the zeolite by coke deposits. The selectivity to xylene isomers was practically not influenced by the Si/Al ratio and changed with the time on stream, due to coke formation. Transition state shape selectivity inside the mordenite pores is also discussed.

Monodimensional tubular medium pore molecular sieves for selective hydroisomerisation of long chain alkanes: n-octane reaction on ZSM and SAPO type catalysts

n-octane has been hydroisomerised at 523 K on ZSM-12, 48 and 22, SAPO-5, 11, 31 and 41 containing Pt–Pd metal function. The materials were prepared with hydrothermal methods, and characterized. Selectivities to C8 isomers ranged from 99 to 50% maximum yield around 80% being reached on SAPO-11, 31 and 41, ZSM-22 and 48. Among the various C8 isomers 2-MeC7 was preferentially formed. Such selectivity was interpreted in terms of transition state shape selectivity at the pore mouths. To further corroborate these interpretations the reaction of 2-MeC7, 3-MeC7, 4-MeC7, 2,5-diMeC6, 2,2,4-TMP on the previous catalysts was also investigated. Determination of the respective reaction rates of these molecules, as well the product distribution analysis allowed to reinforce the concept of transition state shape selectivity at the pore mouths.

The effect of crystal size of SAPO-34 on the selectivity and deactivation of the MTO reaction

The effect of intracrystalline diffusion on the conversion of methanol to olefins (MTO) over SAPO-34 of crystal sizes in the range 0.25–2.5 μm has been investigated at 425°C in a TEOM (tapered element oscillating microbalance) reactor. The 0.25 μm crystals resulted in a relatively large amount of dimethylether (DME) escaping the pores of SAPO-34 before being converted to olefins, hence giving lower olefin yields. The 0.4–0.5 μm crystals gave the largest capacity of olefin formation. The 2.5 μm crystals resulted in diffusion limitation in methanol conversion, with lower olefin yields as a result. The coking rates increased with increasing crystal size at low coke contents, levelling off at a lower level for the largest crystals than for the smaller crystals at high coke contents. The selectivity to different olefins was independent of the crystal size at relatively low coke contents. The selectivity on the fresh catalysts was controlled kinetically, and an effect of transition-state shape selectivity on the selectivity to olefins was induced by coke. The positive effect of coke on the ethene formation was explained in terms of a transition-state shape selectivity mechanism, as the formation of olefins with different sizes went through differently sized intermediates and the reactions with larger intermediates deactivated more quickly.

A study of shape selectivity in ethylation/disproportionation of ethylbenzene on ZSM-5 zeolites using a continuum and a Monte Carlo method

The ethylation and the disproportionation of ethylbenzene on ZSM-5 zeolites yield a mixture of the three isomers ortho-, meta- and para-diethylbenzene. The two reaction systems were simulated on the basis of an adsorption/diffusion/reaction-model using both a continuum and a Monte Carlo approach. Para-selectivity, i.e. the fraction of the para-isomer in the isomer mixture, depended on several factors: the Thiele modulus of ethylation/disproportionation, the primary product distribution, and the relative diffusivity of diethylbenzene isomers. The simulation results obtained by both methods were in good agreement. The nature of the shape selectivity could be discussed in detail. By comparing experimental results from literature with these simulations, it was concluded that in the ethylation para-selectivity can be mainly attributed to product rather than transition state shape selectivity. On the contrary, for the disproportionation it is more reasonable to ascribe the observed para-selectivity to transition state shape selectivity. On the basis of the simulations, some suggestions to enhance the para-selectivity for different cases were proposed.

Influence of Coke Deposition on Selectivity in Zeolite Catalysis

The selectivity of a complex reaction depends on a number of factors, such as the reaction mechanism, operating conditions, catalyst properties and catalyst deactivation. The present work discusses how the selectivity of a complex reaction depends on the formation of coke. For zeolite catalysts, changes in selectivity can be a result of intrinsic selectivity effects or shape selectivity effects. A method is suggested to analyze a complex reaction system with deactivation caused by coke formation, and different cases of selectivity change during deactivation of zeolites are discussed. Transition-state shape selective deactivation is proposed as a mechanism in addition to the deactivation mechanisms suggested by Guisnet and Magnoux (Guisnet, M.; Magnoux, P. Appl. Catal. 1989, 54, 1). By variation of the space velocity, the selectivities to the main products are measured as a function of conversion (optimum performance envelopes). Selectivities at given conversions can then be compared for results obtained with different contact time and with varying degree of catalyst deactivation (space velocity loops). Selective or nonselective deactivation is thereby distinguished. This type of selectivity plot is applied to two different types of reaction, i.e. ethene oligomerization over HZSM-5 and methanol conversion to light olefins (MTO) over SAPO-34. The selectivity of ethene oligomerization was affected only by the decrease in conversion due to coke formation; hence, this is an example of nonselective deactivation. Selective deactivation was found for methanol conversion over SAPO-34.

Catalytic synthesis of methanol, higher alcohols and ethers

Methanol, higher alcohols and ethers are synthesized on a large scale over heterogeneous catalysts with specific functionalities. When copper is the key component of the methanol and higher alcohol catalysts, alkali promotion gives rise to dramatic enhancements of yields due to non-dissociative activation of CO and promotion of aldol coupling with reversal of oxygen retention. Ethers are formed from alcohols over solid acid catalysts, both organic sulfonic resins and inorganic compounds such as zeolites, by a preferential surface S N2 pathway. This is exemplified by 18O retention in reactions of two different alcohols, chirality inversion where asymmetric carbon is involved, and kinetics that reflect competition of the reactant alcohols for surface acid sites. Transition state shape selectivity is exemplified in the reaction of 2-pentanol and ethanol in H-ZSM-5 and selectivity due to orthogonality of the S N2 pathway to main channel direction in the formation of dimethyl ether in H-mordenite.

Dealumination of zeolites Part VIIIAcidity and catalytic properties of HEMT zeolites dealuminated by steaming

S. Morin, A. Berreghis, P. Ayrault, N. S. Gnep and M. Guisnet, J. Chem. Soc., Faraday Trans., 1997, 93, 3269 DOI: 10.1039/A702538F

Rearrangement reactions of 1,2-alkene oxides over zeolites: Improved selectivity in aldehydes by the use of silanated offretites

The liquid-phase rearrangement of 1,2-epoxy-octane over various zeolites (HY, H-beta, H-offretite) is reported. Octanal is obtained as the main product, but other isomerisation products are formed together with condensation derivatives. The latter are produced by a consecutive aldol reaction of the aldehyde. The use of an offretite, the external surface of which was selectively silanated, totally suppresses the formation of the aldol derivatives. Such a result, which leads to a significant increase of the selectivity in octanal, is a consequence of transition-state shape selectivity.

Selective isopropylation of naphthalene over zeolites

The isopropylation of naphthalene with isopropylbromide over a series of H mordenites and Y zeolites has been studied. In both cases, a high β-selectivity is observed in the monoalkylation and dialkylation reactions (selective formation of 2-isopropylnaphthalene and a mixture of 2,6- and 2,7-iisopropylnaphthalenes, respectively). Over H mordenites, the β-selectivity is explained as the result of transition-state shape selectivity. Over Y zeolites, the β-selectivity is due to a thermodynamic equilibrium favorable to the formation of 2-isopropylnaphthalene from 1-isopropylnaphthalene. The use of zeolites modified by silanation using chemical vapor deposition leads to an improvement of the β-selectivity by reducing the amount of trialkyl derivatives that are formed on the external surface.

Isomerization and disproportionation of m-xylene : Selectivities Induced by the Void Structure of the Zeolite Framework

The conversion of m-xylene was performed on acid zeolites with ten- or twelve-membered rings, such as ZSM-5, ZSM-12, ZSM-48, OFF, MOR, FAU, Ω, β and L. All of these zeolites have distinctly different crystallographic structures with different shapes and dimensions of the intracrystalline cavities. This diversity of pore structures allowed to determine to what extent the pore shape and dimensions influence the selectivity of the reaction of m-xylene. The p/o selectivity in the isomerization of m-xylene is always high for ten-membered ring zeolites with crystals bigger than 1 μm. The moderate selectivity for p-xylene formation in some zeolites with twelve-membered rings is due to transition-state shape selectivity. The selectivity for disproportionation versus isomerization is very low for ten-membered ring zeolites and allows them to be distinguished easily from the twelve-membered ring zeolites, which show much more disproportionation. In the group of twelve-membered ring zeolites the disproportionation/isomerization selectivity does not correlate at all with the effective size of the intracrystalline cavities. The disproportionation/ isomerization selectivity is determined by the strong isolated acid sites which are present in cavities that allow the formation of at least one of the possible bimolecular transition-state complexes for disproportionation. The distribution of the trimethylbenzene isomers reflects important features of the shape of the pores, the presence and the position of larger spaces in lobes, cages or intersections adjacent to the twelve-membered ring channels of large pore zeolites.