Guest-host interactions in zeolites as studied by NMR spectroscopy: implications in synthesis,catalysis and separations

The intermolecular forces between occluded organic molecules and the atoms that bound the pores of zeolite structures determine the catalytic activity and diffusivity of the guest molecules. These organic-inorganic forces are also a crucial aspect of the process of structure-direction in zeolite synthesis. A molecular understanding of guest-host interactions can often be deduced by using NMR spectroscopy to study the rotational motion of the occluded molecules. This contribution is an overview of systems where NMR, particularly deuterium NMR, has been used to quantify the forces which exist between occluded organic molecules and the zeolite structure. Systems that are relevant to synthesis, separations, and catalysis are discussed. The works presented show that NMR is a powerful tool for studying guest-host interactions in zeolitic materials. A molecular level understanding of the nature and relative importance of the various forces discussed should lead to a more rational basis for the selection and development of zeolitic materials used in catalytic and adsorption applications. This revised version was published online in August 2006 with corrections to the Cover Date.     

Carbonium ion formation in zeolite catalysis

Based on data ofn-butane conversion over HZSM5, different transition states are proposed for the Brønsted acid catalyzed reactions of alkanes involving carbonium ions as intermediates. Hydrogen-deuterium exchange, dehydrogenation and cracking are proposed to proceed via pentacoordinated carbonium ions that are stabilized by the zeolite lattice.     

Interactions of chemical species with acid sites in zeolites

A detailed understanding of reaction pathways in zeolite solid acids requires knowledge about the structure and thermochemistry of all chemical species which could potentially exist along the reaction coordinate. Because some high-silica zeolites, such as H-ZSM-5, consist of a collection of nearly identical, Brønsted-acid sites, equal in concentration to the Al content, the 1 : 1 stoichiometric adsorption complexes formed by various molecules when they interact with the acid sites are well defined and relatively easy to characterize using temperature-programmed desorption, microcalorimetry, and NMR. This paper briefly reviews what is known about the reactivity, thermochemistry, and structure of complexes formed by amines, pyridines, imines, alcohols, thiols, olefins, aldehydes, ketones, and nitriles. It is shown that the thermochemistry and structure of the complexes are affected by both local effects (proton transfer or hydrogen-bonding interactions) and nonlocal effects (van der Waals interactions with the zeolite cavity). However, solvation effects in zeolites are very different from that found in acidic solutions so that the use of gas-phase reference conditions provides a much better starting point for understanding adsorption complexes, and therefore acid-catalyzed reactions, in the zeolite.     

在不同构型分子筛催化剂中吸附和扩散的分子模拟

基于巨正则蒙特卡洛和分子动力学,对NH₃-SCR反应体系中吸附质分子（NO与NH₃）在不同拓扑结构沸石分子筛（LTL、FER、LEV、BEA、MOR、FAU、CHA和MFI）上的吸附和扩散特性进行系统研究。结果表明,对于全硅分子筛而言,其分子筛的拓扑结构影响NO与NH3在分子筛上的吸附,综合吸附量及吸附作用能发现,MFI和LEV分子筛对NO具有较优的吸附特性;MFI和BEA分子筛对NH₃ 具有较优的吸附特性。研究了Si与Al物质的量比对BEA分子筛吸附性能影响,结果表明,随着Si与Al物质的量比降低,分子筛自由体积逐渐增加,进而有助于分子筛催化剂对NO和NH₃的吸附。采用分子动力学模拟计算NO与NH₃在不同构型全硅分子筛上的扩散系数,发现具有三维直通道且孔径较大的分子筛催化剂有利于NO和NH₃在其孔道内部的扩散,MFI虽然具备三维孔道结构,但由于存在Z型交叉通道,一定程度阻碍了反应物分子的扩散。     

A link between reactivity and local structure in acid catalysis on zeolites

The extent to which spatial constraints influence rates and pathways in catalysis depends on the structure of intermediates, transition states, and active sites involved. We aim to answer, as we seek insights into catalytic mechanisms and site requirements, persistent questions about the potential for controlling rates and selectivities by rational design of spatial constraints around active sites within inorganic structures useful as catalysts. This Account addresses these matters for the specific case of reactions on zeolites that contain Br?nsted acid sites encapsulated within subnanometer channels. We compare and contrast here the effects of local zeolite structure on the dynamics of the carbonylation of surface methyl groups and of the isotopic exchange of CD4 with surface OH groups on zeolites. Methyl and hydroxyl groups are the smallest monovalent cations relevant in catalysis by zeolites. Their small size, taken together with their inability to desorb except via reactions with other species, allowed us to discriminate between stabilization of cationic transition states and stabilization of adsorbed reactants and products by spatial constraints. We show that apparent effects of proton density and of zeolite channel structure on dimethyl ether carbonylation turnover rates reflect instead the remarkable specificity of eight-membered ring zeolite channels in accelerating kinetically relevant steps that form *COCH3 species via CO insertion into methyl groups. This specificity reflects the selective stabilization of cationic transition states via interactions with framework oxygen anions. These findings for carbonylation catalysts contrast sharply the weak effects of channel structure on the rate of exchange of CD4 with OH groups. This latter reaction involves concerted symmetric transition states with much lower charge than that required for CH3 carbonylation. Our Account extends the scope of shape selectivity concepts beyond those reflecting size exclusion and preferential adsorption. Our ability to discriminate among various effects of spatial constraints depends critically on dissecting chemical conversions into elementary steps of kinetic relevance and on eliminating secondary reactions and accounting for the concomitant effects of zeolite structure on the stability of adsorbed reactants and intermediates.     

Zeolite Beta with hierarchical porosity prepared from organofunctionalized seeds

Beta zeolite with hierarchical porosity has been obtained by a new synthesis strategy, based on perturbing the growth of the zeolite crystals by functionalization of the zeolitic seeds with organosilanes in order to hinder and prevent their further aggregation and agglomeration. As a consequence, a secondary porosity in the supermicropore region has been generated in zeolite Beta, leading to a considerable increase in both the total surface area and the pore volume of the material. The enhancement of the textural properties can be controlled by changing the silanization agent molecular size and its quantity added to the synthesis medium. This type of hierarchical Beta zeolites presents interesting applications as catalysts in reactions involving bulky molecules. Thus, their catalytic activity in the catalytic cracking of LDPE has been found to be strongly enhanced compared to a standard Beta zeolite sample, due to the higher accessibility to the acid sites caused by the presence of the secondary porosity.     

Adsorption of benzothiophene on Y zeolites investigated by infrared spectroscopy and flow calorimetry

Diffuse reflectance infrared spectra of benzothiophene adsorbed on different Y zeolites reveal that the cations and protons in the zeolites are the sites responsible for the adsorption of benzothiophene. On NaY, benzothiophene was molecularly adsorbed on the cations through the electrophilic interaction between the cations and the thiophenic rings. On the transition metal ion exchanged NiY and CuY zeolites, because of the presence of the d-electrons in the cations, the thiophenic rings interact with the cations to form the π-complexes through the σ–π electron donations. In the presence of hydroxyl species in the zeolites, the adsorbed sulfur compounds attach to the protons molecularly via the electrophilic interaction and undergo the opening of the thiophenic rings depending on the acidity of the zeolites and the adsorption amount. The apparent heat of adsorption of benzothiophene in normal octane on the Y zeolites determined by flow calorimetry shows that the adsorption strength based on the measured heat for each mole sulfur adsorbed on the Y zeolite is in the order of CuY > NiY > NaY  USY. For USY, due to the endothermic breakage of the thiophenic ring of benzothiophene induced by the acid sites of the zeolite, the apparent heat of adsorption is similar to that obtained from the adsorption on NaY. This work demonstrates that the transition metal ion exchanged zeolites exhibit excellent properties for sulfur adsorption because of the formation of the π-complexes and that the acidity of the zeolites is not advantageous for sulfur removal due to the strong adsorption and decomposition of the adsorbed species.     

DFT Cluster Modeling of Molecular and Dissociative Hydrogen Adsorption on Zn2+ Ions with Distant Placing of Aluminum in the Framework of High-Silica Zeolites

The problem of various cationic positions in zeolites with high Si/Al ratio in the framework is discussed. The statistical distribution of aluminum in the lattice of pentasils makes probable appearance of the structures with distant placing of two aluminum atoms. Cations, localized at such sites, should be very strong Lewis acids that are highly active in different chemical reactions. An example of such a site is considered for two Zn²⁺ ions stabilized in the zeolite fragment represented by two adjacent five-membered rings sharing the common edge. DFT calculations of molecular and dissociative hydrogen adsorption by such sites are in agreement with experimental results. Adsorption of dihydrogen by zinc ion at such sites results in an unusually large low-frequency shift of H–H stretching vibrations indicating essential activation of adsorbed H₂ molecule. The calculated path of heterolytic dissociative adsorption of dihydrogen and of the proton migration to the distantly placed basic oxygen of such acid-base pair are in agreement with the previously published DRIFT experimental data.     

Flexibility in zeolites:29Si NMR studies of ZSM-5 frame transitions

Zeolites are most often perceived as rigid solids. Recent evidence has demonstrated that the temperature or the adsorption of molecules whose dimensions approach the pore dimensions induce changes in the solid structure, i.e., a flexing of the solid lattice. For pentacil zeolites, as ZSM-5, a transition between monoclinic and orthorhombic forms of the crystalline structure is found with a change in temperature or the adsorption of ring containing molecules. We find that the temperature at which this transition occurs depends on the Si/Al ratio within ZSM-5 and the discrete, reversible transition can be measured by²⁹Si NMR, confirming prior X-ray studies. A similar dependence is found for ZSM-11. Phosphorous modification of the ZSM-5 does not change the transition temperature; however, steam treating of the zeolite does. The implications of these measurements to the flexibility and to the potential for transport is discussed. An analogy between transport and flexibility in one-dimensional polymers and that in the three-dimensional, open, solid network of zeolites is suggested.We wish to thank Union Carbide, Mobil, Haldor Topsøe, Johannes Lercher, peter Jacobs and Mark Davis for generously providing samples for this study.     

A practicable mesostructured MFI zeolitic catalyst for large molecule reactions

A microspherical mesostructured zeolite (MMZ-5) with MFI-topological frameworks is employed. In-situ FT-IR spectra of pyridine and 2, 6-di-tert-butylpyridine have indicated that almost all of the acid sites are located on the zeolitic external surface, which are favorable for access of reactive large molecules. This key effect, along with the reduced diffusion and steric limitation, has been proposed to be main the reason for the enhanced catalytic activity, selectivity and stability exhibited in benzylation of aromatics with benzyl chloride. The need for optimized characterization of the mesoporous surface acidity for different aromatics is highlighted.     

A comparative study of the sorption of benzene and phenol in silicalite, HAlZSM-5 and NaAlZSM-5 by computer simulation

The sorption of benzene and phenol in silicalite, HAlZSM-5 and NaAlZSM-5 has been studied comparatively with the Cerius² software from MSI using Monte Carlo simulations. As a test of the simulation method, the well-studied system of benzene–silicalite was simulated first. The results show that the Henry constant and the isosteric heat of adsorption of benzene in silicalite are in good agreement with experimental data from the literature obtained by different methods. The open force field “Burchart–Dreiding” is proved to be suitable for simulation of the adsorption of aromatics in ZSM-5 type zeolites. The Henry constants of the systems mentioned above between 273 and 673 K, the isotherms at temperatures of 473 and 673 K, the sorption sites as well as the interaction energies between the guest molecules and the host zeolite frameworks have been obtained. The differences in the adsorption behavior between benzene and phenol in silicalite, HAlZSM-5 and NaAlZSM-5 are attributed to the differences in Coulomb interactions.     

Modeling of key reaction pathways: Zeolite catalyzed alkylation processes

A six lump kinetic model that considers the key reactions for the zeolite catalyzed alkylation process is presented. The influence of different reactions and rate limiting steps on reactor performance is examined by coupling an appropriate reactor model that accounts for different back-mixing on reactor scale, with a zeolite particle model which accounts for the diffusion inside the zeolite pore, the alkylation reaction, and zeolite deactivation. Model predictions are compared with experimental results and lead to conclusions that hydride transfer and oligomerization reactions are the key kinetic steps affecting the overall performance of zeolite catalyzed alkylation processes. It is suggested that higher alkylate yield and longer zeolite activity are achieved by increasing the intrinsic hydride transfer rate and the ratio of feed isobutane to n-butene (P/O) concentration. For a given P/O feed ratio, achieving close to plug flow for isobutane and high back-mixing for n-butene further enhances local P/O ratio and yield. Furthermore, optimal zeolite catalyst design should consider the egg shell type of Brønsted acid site distribution and a lower silicon to alumina (Si/Al) ratio.     

Sorption of nitrogen,oxygen, and argon in Cd (II) exchanged zeolite X: volumetric equilibrium adsorption and grand canonical Monte Carlo study

The adsorption of nitrogen, oxygen and argon has been studied in cadmium (II) cations exchanged zeolite X at 288.2 and 303.2 K. Experimentally measured adsorption isotherms are compared with theoretically calculated data using grand canonical Monte Carlo (GCMC) simulation. Nitrogen showed higher adsorption capacity and selectivity than oxygen and argon in these zeolite samples. The cadmium exchanged zeolite X was showed that increased adsorption capacity for nitrogen, oxygen, and argon with increase in Cd (II)-exchange levels, indicating as charge density increases adsorption capacity also increase. Isosteric heat of adsorption data showed stronger interactions of nitrogen molecules with cadmium cations in zeolite samples. These observations have been explained in terms of higher electrostatic interaction of nitrogen with extra framework zeolite cations. The selectivity of oxygen over argon is explained in terms of its higher interaction with Cd (II)-exchanged zeolites than argon molecules. The selectivity of N₂/O₂ of cadmium-exchanged zeolite X is better than only sodium containing zeolite-X. Heats of adsorption and adsorption isotherms were also calculated using GCMC simulation algorithm. Simulation studies expectedly show the proximity of nitrogen molecules to the locations of extra framework sodium and cadmium cations.     

Dynamic Monte Carlo simulations of binary self-diffusion in ZSM-5

Dynamic Monte Carlo (DMC) simulations are used to examine binary diffusion in the zeolite ZSM-5. Diffusion in zeolites is strongly influenced by dynamic and static heterogeneity. The former describes the concentration dependent influence of diffusing molecules on each other within the restricted environment of the zeolite pore network; the latter relates to the presence of adsorption sites of different strengths, on which molecules adsorb for different average times, before moving on to a neighboring adsorption site. DMC simulations are an effective tool to study the influence of both forms of heterogeneity on diffusion. The self-diffusivities, as determined from DMC, strongly depend on the pore network topology, the average residence times of both species on each type of site, and the concentrations of those species. The Maxwell–Stefan (MS) approach is known to predict diffusion in silicalite-1, the statically homogeneous version of ZSM-5, rather well. However, it is not rigorous in predicting the self-diffusivity in the presence of any form of strong static heterogeneity. The capability of the MS approach to model the self-diffusion of binary mixtures in ZSM-5 is compared to DMC results, which serve as a benchmark. Alternative theories, which account for the discrete sites of the lattice, are considered for their ability to predict the self-diffusivity in binary mixtures, namely, the effective medium approximation (EMA) and a mean field theory (MFT). The EMA is shown to be a promising method in situations where the MS approach cannot work due to the presence of strong correlation effects.     

NOx adsorption on MnO2/NaY composite: an in situ FTIR and EPR study

The NO, NO/O₂, and NO/O₂/H₂O adsorption on MnO₂/NaY (5 and 15 wt.% MnO₂) composite catalyst and NaY has been studied by means of in situ FTIR and EPR spectroscopy at elevated temperatures and during heating under reaction-like conditions. NO adsorption and co-adsorption of NO and O₂ on NaY and MnO₂/NaY proceeds via oxidation of NO forming NO₂⁻ and NO₃⁻ species. Whereas the manganese dioxide preferably acts as oxidising agent, the zeolite stores the NO_x species as nitrite and nitrate ions in the solid. In the presence of oxygen, the nitrate formation is enhanced due to additional oxidation of NO through gaseous oxygen leading to NO₂. Dimerisation of NO₂ to N₂O₄ and following disproportionation of the latter causes the formation of NO⁺ and NO₃⁻ species which are associated with nucleophilic zeolitic oxygen and especially alkali cations of the zeolite, respectively. The presence of oxygen facilitates reoxidation of Mn²⁺ which keeps more Mn ions in the active state. Pre-adsorbed water and higher amounts of water vapour in the feed hinder the NO adsorption by blocking the adsorption sites and shift the nitrate formation to higher temperatures. The quantities and thermal stability of the nitrates formed during NO and NO/O₂ adsorption differs which points to a different mechanism of nitrate formation. In the absence of gaseous oxygen, nitrates are formed by participation of only lattice oxygen. In the presence of oxygen, nitrate formation by dimerisation and disproportionation reactions of NO₂ dominates. The manganese component of the composite catalyst supports the oxidation of NO to nitrite and subsequently to nitrate. During this process Mn⁴⁺ is reduced to Mn²⁺ as evidenced by in situ EPR measurements.     

Effects of guest poly(ethyleneglycol) homopolymer on the order-to-disorder transition of hydrophobically modified poly(ethyleneglycol) (C12E25) micellar solution: PEG molecular weight dependence and SCF simulation

Effects of the molecular weight of guest poly(ethyleneglycol) (PEG) on the order-to-disorder transition of micellar aqueous solution of hydrophobically modified ethyleneglycol oligomer, poly(ethyleneglycol) monododecylether (C12E25) have been investigated by means of linear-viscoelastic measurements, and differential scanning calorimetry. Added PEG having a chain length similar to or even higher than that of micellar corona PEG can be incorporated into the micellar lattice of C12E25 aqueous solution with maintaining the body-centered-cubic (bcc) lattice structure. The order-to-disorder transition temperature decreases with increasing concentration of guest PEG at a fixed concentration of C12E25. The decrement of transition temperature is increased with increasing molecular weight of guest PEG. Self-consistent field (SCF) calculation shows that the higher molecular weight guest chains are more excluded from the micellar corona chains to be localized between the micellar particles, which enlarge the lattice distance more pronouncedly than the shorter guest chains do. The unstability of micellar lattice, i.e. the depression of transition temperature, may be explained in terms of guest chain localization in the lattice.     

Selective isomerization of n-butenes into isobutene over aged H-ferrierite catalyst: nature of the active species

The nature of the active species responsible for butene isomerization over aged HFER samples is reexamined in the light of the change in the product yields at very short time-on-stream and of the reversible and irreversible increases in weight of the zeolite during the reaction. At very short time-on-stream, the selectivity of butene isomerization is that expected from a dimerization-cracking process, in particular simultaneous formation of isobutene, propene and pentenes. A rapid decrease of all the yields is observed with time-on-stream; however, for isobutene but not for the other products, the initial decrease is followed (after 10 minutes-on-stream) by an increase. The decrease in the yield can be related to the formation of carbonaceous compounds (``coke') which block the access to the pores, while the increase in isobutene yield can be explained by the development of a new isomerization mode which is very selective to isobutene. This new mode could be catalyzed by carbonaceous compounds and/or by reaction products which are shown to be retained inside the pores during the reaction. It is proposed that at short time-on-stream the increase in isobutene yield is due to an autocatalytic reaction, n-butene isomerization occurring on t-butyl carbenium ions formed by adsorption of isobutene molecules (which are slowly desorbed from the pores) on the protonic sites of the zeolite. At long time-on-stream, the active species would be benzylic carbocations formed from carbonaceous compounds trapped in the pores near the outer surface of the crystallites. This revised version was published online in July 2006 with corrections to the Cover Date.     

Physicochemical aspects of host-guest compounds

The macro properties of crystalline inclusion compounds depend on their structures. Their thermal stabilities are a function of the strength and the directionality of the various nonbonded interactions occurring in the host-guest assembly. Their lattice energies, as measured by the method of atom-atom potentials, correlate with the thermodynamics of the guest-release reactions and the selectivity that a given host displays for a particular guest. The kinetics of solid-host:vapor-guest reactions and of guest exchange are important in our understanding of catalytic processes. Crystal engineering, in which materials of predetermined properties may be synthesized, is still at the empirical stage.     

ADSORPTION KINETICS AND EQUILIBRIA OF BASIC DYES ONTO ZEOLITE IN SINGLE AND BINARY COMPONENT SYSTEMS

Adsorptive removal of Basic Blue 3 (BB 3) and Basic Red 18 (BR 18) by a clinoptilolite-type natural zeolite from their single-component solutions has been studied in the temperature range of 298–328 K. Experimental equilibrium results are well described by the Freundlich and the Langmuir isotherm models. The model parameters obtained for single-solute systems at 298 K have been used for the prediction of adsorption isotherms in binary dye solutions using multicomponent isotherm models. Competitive adsorption results between BB 3 and BR 18 in binary solutions satisfactorily fit the extended Freundlich, extended Langmuir, and modified Langmuir models. A site distribution function that gives information about affinity of adsorption sites for competing species in binary systems has been mathematically calculated using the Freundlich parameters. Time-dependent results for single and binary dye solutions have been analyzed according to a pseudo-second-order kinetic model based on chemisorption and a McKay model assuming two resistance diffusion processes, respectively. The diffuse reflectance FT-IR spectra indicate that the dyes are adsorbed via electrostatic interactions in external rings of clinoptilolite by replacing the zeolitic water in a single system, and they diffuse through the liquid associated with zeolitic water in TO₄ tetrahedra (T: Al, Si) in a mixed solution.     

On some dynamic peculiarities of the charge transfer with adsorption and attractive interactions

Complex electrode reactions with adsorption of intermediate product and with attractive interactions accounted for by Frumkin adsorption isotherm may show up a variety of unusual steady states (ss) characterized by ss adsorption isotherms. Under symmetry of kinetic equations the single peculiarity to be expected in electrochemical adsorption and desorption reactions is hysteresis responsible for current discontinuities in potential sweep experiments and similar discontinuity of immittance data recorded for potential or frequency sweeps. For asymmetry of partial reactions, as for charge transfer coefficients different then 0.5 or unequal charge numbers in partial reactions, the steady-state isotherms get unusual forms described in the chemical dynamics literature as “mushrooms” or “isolas”, this last term referring to disjoined branches of the ss isotherm. In potential scan experiments, these isotherms present double hystereses and irreversible switches between steady states’ branches at high and at low surface coverage.