Quantum-chemistry of zeolite acidity

Reactivity descriptors in acid catalysis: acid strength, proton affinity and host–guest interactions

Acid strength and proton affinity, the independent properties of catalysts and molecules, are incomplete descriptors of because cations and conjugate anions reorganize their charges as they interact as bound intermediates and transition states.

Stability of bound species during alkene reactions on solid acids

This study reports the thermodynamics of bound species derived from ethene, propene, n-butene, and isobutene on solid acids with diverse strength and confining voids. Density functional theory (DFT) and kinetic data indicate that covalently bound alkoxides form C-C bonds in the kinetically relevant step for dimerization turnovers on protons within TON (0.57 nm) and MOR (0.67 nm) zeolitic channels and on stronger acids HPW (polyoxometalate clusters on silica). Turnover rates for mixed alkenes give relative alkoxide stabilities; the respective adsorption constants are obtained from in situ infrared spectra. Tertiary alkoxides (from isobutene) within larger voids (MOR, HPW) are more stable than less substituted isomers but are destabilized within smaller concave environments (TON) because framework distortions are required to avoid steric repulsion. Adsorption constants are similar on MOR and HPW for each alkoxide, indicating that binding is insensitive to acid strength for covalently bound species. DFT-derived formation free energies for alkoxides with different framework attachments and backbone length/structure agree with measurements when dispersion forces, which mediate stabilization by confinement in host-guest systems, are considered. Theory reveals previously unrecognized framework distortions that balance the C-O bond lengths required for covalency with host-guest distances that maximize van der Waals contacts. These distortions, reported here as changes in O-atom locations and dihedral angles, become stronger for larger, more substituted alkoxides. The thermodynamic properties reported here for alkoxides and acid hosts differing in size and conjugate-anion stability are benchmarked against DFT-derived free energies; their details are essential to design host-guest pairs that direct alkoxide species toward specific products.

Probing zeolites by vibrational spectroscopies

This review addresses the most relevant aspects of vibrational spectroscopies (IR, Raman and INS) applied to zeolites and zeotype materials. Surface Brønsted and Lewis acidity and surface basicity are treated in detail. The role of probe molecules and the relevance of tuning both the proton affinity and the steric hindrance of the probe to fully understand and map the complex site population present inside microporous materials are critically discussed. A detailed description of the methods needed to precisely determine the IR absorption coefficients is given, making IR a quantitative technique. The thermodynamic parameters of the adsorption process that can be extracted from a variable-temperature IR study are described. Finally, cutting-edge space- and time-resolved experiments are reviewed. All aspects are discussed by reporting relevant examples. When available, the theoretical literature related to the reviewed experimental results is reported to support the interpretation of the vibrational spectra on an atomic level.

New method for the temperature-programmed desorption (TPD) of ammonia experiment for characterization of zeolite acidity: a review

In this review, a method for the temperature-programmed desorption (TPD) of ammonia experiment for the characterization of zeolite acidity and its improvement by simultaneous IR measurement and DFT calculation are described. First, various methods of ammonia TPD are explained, since the measurements have been conducted under the concepts of kinetics, equilibrium, or diffusion control. It is however emphasized that the ubiquitous TPD experiment is governed by the equilibrium between ammonia molecules in the gas phase and on the surface. Therefore, a method to measure quantitatively the strength of the acid site (∆H upon ammonia desorption) under equilibrium-controlled conditions is elucidated. Then, a quantitative relationship between ∆H and H0 function is proposed, based on which the acid strength ∆H can be converted into the H0 function. The identification of the desorption peaks and the quantitative measurement of the number of acid sites are then explained. In order to overcome a serious disadvantage of the method (i.e., no information is provided about the structure of acid sites), the simultaneous measurement of IR spectroscopy with ammonia TPD, named IRMS-TPD (infrared spectroscopy/mass spectrometry-temperature-programmed desorption), is proposed. Based on this improved measurement, Brønsted and Lewis acid sites were differentiated and the distribution of Brønsted OH was revealed. The acidity characterized by IRMS-TPD was further supported by the theoretical DFT calculation. Thus, the advanced study of zeolite acidity at the molecular level was made possible. Advantages and disadvantages of the ammonia TPD experiment are discussed, and understanding of the catalytic cracking activity based on the derived acidic profile is explained.

A THEORETICAL STUDY ON ADSORPTION OF THIOPHENE, BENZENE, AND HEXANE IN Na-, Ag-, AND Cu-EXCHANGED MORDENITE

Density functional theory method was applied to investigate adsorption of thiophene, benzene, and hexane in Na- , Ag- , and Cu -exchanged mordenite (MOR). A cluster model containing 16 tetrahedral atoms was constructed as structure model of MOR. The charge-balancing metallic ions ( Na+ , Ag+ , and Cu+ ) were introduced to produce the Lewis acid site (LAS). The acidic strength was estimated by calculating adsorption energy of NH3 in the model. The equilibrium configurations, Mulliken charge populations, and adsorption energies of thiophene, benzene, and hexane in models were calculated and analyzed. For the adsorbates investigated, interaction strengths followed the decreasing order of Cu –MOR > Ag –MOR > Na –MOR that agreed well with their Lewis acidic strengths. The interaction strengths followed the order of thiophene ≈ benzene > hexane in Na –MOR, and thiophene > benzene > hexane in Cu –MOR and Ag –MOR. Moreover, the interaction mechanisms of thiophene and benzene in Cu –MOR and Ag –MOR were investigated by the natural bond orbital (NBO) analysis. Furthermore, similar calculations were carried out for the adsorption of thiophene, benzene, and hexane in the naked Na+ , Cu+ , and Ag+ for comparison.

Theoretical simulations elucidate the role of naphthalenic species during methanol conversion within H-SAPO-34

The role of naphthalenic species during the methanol-to-olefins (MTO) process in a silicoaluminophosphate zeolitic material exhibiting the chabazite topology (H-SAPO-34) has been studied from first principles. These species could either act as active olefin-eliminating compounds or as precursors for deactivating species. Results incorporating van der Waals contributions for finite large clusters point out that successive methylation steps of naphthalenic compounds are feasible. The calculated intrinsic activation barrier is relatively independent of the number of methyl groups already attached on the aromatic compound and is approximately 140 kJ mol(-1). The influence of the composition of the catalyst and hence the acidic strength on the intrinsic chemical kinetics was investigated in detail through comparison with the isostructural high-silicon material. Apparent chemical kinetics, starting from adsorbed methanol on the acid site, were also computed. The initiation steps of the side-chain route starting from a trimethylated naphthalenium ion were also examined. The actual side-chain methylation exhibits a high barrier and hence this mechanism involving methylated naphthalenes is not expected to be an active ethene-eliminating route in H-SAPO-34.

A computational study of methanol-to-hydrocarbon conversion — Towards the design of a low-barrier process

Computational quantum chemistry has been employed to examine the production of ethylene with methanol-to-hydrocarbon (MTH) processes via a carbon pool mechanism. We find that the M05-2X functional performs well for the types of reactions that are involved. The methylation reactions of the aromatic cocatalyst are the most energy-demanding steps in the process. For the subsequent production of C2H4, we have identified a low-energy pathway that involves multiple methyl shifts, followed by concerted deprotonation and C2H4 elimination. The substitutions of the Al and Si atoms in the participating Si–OH–Al moiety of zeolite catalysts with Ga and Ge do not lead to lower barriers for the methylation reactions, nor does the use of a more electron-rich aromatic cocatalyst. However, we find that the use of two cocatalysts, a nucleophile and an aromatic carbon pool, can provide an overall low-energy pathway for the MTH process.

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 Brønsted 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.

Concentration dependence of the methane structure in silicalite-1: a molecular dynamics study using the Møller-Plesset-based potential

Møller-Plesset perturbation-based potentials have been used in molecular dynamics simulations to examine methane diffusion in silicalite-1. The simulation box contains 2 unit cells of silicalite-1 and varying loading numbers (n(ld)) from 1, 2, 3, to 4 methane molecules per intersection, corresponding to 8, 16, 24, and 32 molecules per simulation box, respectively. Consistent with the previous study, a preferential diffusing path for methane is close to the channel axes. The structure of the methane molecules in the silicalite-1 pore was exhibited in terms of the methane-methane radial distribution function (RDF) in which the first peak appears at 6.4 A for n(ld) < or = 2, becomes a broad maximum at n(ld) = 3, and splits into two sharp peaks centered at 6.4 and 8.6 A at n(ld) = 4. This fact can be clearly described by an intensification of the methane density in the straight and zigzag channels but a decrease in the intersection when the loading increases. These features of the observed RDFs are in contrast with the previous report using a molecular dynamics force-field potential in which the RDFs for all concentrations show first maxima at approximately 4 A. The analysis of the relative residence times of methane at different sites inside the silicalite suggests that the zigzag channel is the most favored location. The computed self-diffusion coefficients as well as the heat of adsorptions are in reasonable agreement with the available values.