Computational chemistry of zeolite catalysis

\u3cp\u3eThis chapter presents an introductory overview of the basic concepts, power, capabilities, and limitations of modern quantum-chemical techniques for studying reactivity and chemical properties of zeolites. The subjects discussed here will include the methodological aspects of computational chemistry crucial for modeling extended chemical systems as well as recent relevant examples of application of computational methodologies for developing new concepts of zeolite reactivity. Emphasis will be made on the use of computational approaches for unraveling molecular-level phenomena underlying catalytic properties of zeolites.\u3c/p\u3

Computational approach in zeolite science

This chapter presents an overview of different computational methods and their application to various fields of zeolite chemistry. We will discuss static lattice methods based on interatomic potentials to predict zeolite structures and topologies, Monte Carlo simulations for the investigation of adsorption phenomena, molecular dynamics technique to model diffusion processes in micropores and electronic structure calculations to study chemical reactivity of zeolitic materials. Various methodologies will be illustrated by the state of the art examples from recent literature

Origin of enhanced Brønsted acidity of NiF-modified synthetic mica-montmorillonite clay

\u3cp\u3eThe Brønsted acidity of synthetic mica-montmorillonite (SMM) clay was studied by periodic DFT calculations. Different structural models were compared to determine the Brønsted acidity of protons of the SMM clay based on (i) isomorphous substitution of Si\u3csup\u3e4+\u3c/sup\u3e by Al\u3csup\u3e3+\u3c/sup\u3e in the tetrahedral silicate layer and additional NiF-doping (ii) in the platelets and (iii) at the edge terminations of the clay platelets. The acid strength was judged from the computed adsorption energies of ammonia and pyridine. The SMM acidity is mainly determined by the composition of the clay platelets. The strongest acidity is found in structures in which octahedral [AlO]\u3csup\u3e+\u3c/sup\u3e is replaced by [NiF]\u3csup\u3e+\u3c/sup\u3e adjacent to tetrahedral [Si-(OH)-Al] moieties in the tetrahedral layer. For the Brønsted acid sites in the interlayer of SMM, modification with either Ni\u3csup\u3e2+\u3c/sup\u3e or F\u3csup\u3e-\u3c/sup\u3e in the octahedral layers has only a minor influence on the acidity. Our data indicate that Brønsted acid sites, properly modified in the second coordination shell by electron-withdrawing F, in the interlayer and at defect sites at the edges of clay platelets (intralayer sites) can contribute to the enhanced acidity in NiF-modified SMM. Although the predicted acidity of SMM by ammonia adsorption is higher than that of faujasite zeolite, the reactivity judged from propene protonation demonstrates that zeolites are more reactive than clays. This difference seems to be the result of the curved nature of the micropores of zeolites, which stabilizes the transition states for an acid-catalyzed reaction more than flat surfaces of clays do.\u3c/p\u3

Multi-site cooperativity in alkali-metal-exchanged faujasites for the production of biomass-derived aromatics

\u3cp\u3eThe catalytic Diels–Alder cycloaddition–dehydration (DACD) reaction of furanics with ethylene is a promising route to bio-derived aromatics. The reaction can be catalyzed by alkali-metal-exchanged faujasites. Herein, the results of periodic DFT calculations based on accurate structural models of alkali-metal-exchanged zeolites are presented, revealing the fundamental roles that confinement and the nature of the exchangeable cations in zeolite micropores have in the performance of faujasite-based catalysts in the DACD reaction. Special attention is devoted to analyzing the effect of functional substituents on furanic substrates (furan, 2,5-dimethylfuran, 2,5-furandicarboxylic acid) on the catalyst behavior. It is demonstrated that the conventional reactivity theories of the Diels–Alder chemistry based on simplistic single-site Lewis acidity and substituent effects do not apply if catalytic processes in the multiple-site confined environment of zeolite nanopores are considered. The nature and cooperativity of the interactions between the multiple exchangeable cations and the substrates determine the reaction energetics of the elementary steps involved in the DACD process.\u3c/p\u3

The mechanism of glucose isomerization to fructose over Sn-BEA zeolite : a periodic density functional theory study

The isomerization of glucose to fructose in the presence of Sn-containing zeolite BEA (beta polymorph A) was studied by periodic DFT calculations. Focus was placed on the nature of the active site and the reaction mechanism. The reactivities of the perfect lattice SnIV site and the hydroxylated SnOH species are predicted to be similar. The isomerization activity of the latter can be enhanced by creating an extended silanol nest in its vicinity. Besides the increased Lewis acidity and coordinationflexibility of the Sn center, the enhanced reactivity in this case is ascribed to the reaction environment that promotes activation is ascribed to the reaction environment that promotes activation of the confined sugar intermediates through hydrogen bonding. The resulting multidentate activation of the substrate favors the rate-determining hydrogen-shift reaction. These findings suggest the important role of defect lattice sites in Sn-BEA for catalytic glucose isomerization. Sn-BEA for catalytic glucose isomerization

Molecular aspects of glucose dehydration by chromium chlorides in ionic liquids

A combined experimental and computational study of the ionic-liquid-mediated dehydration of glucose and fructose by CrII and CrIII chlorides has been performed. The ability of chromium to selectively dehydrate glucose to 5-hydroxymethylfurfural (HMF) in the ionic liquid 1-ethyl-3-methyl imidazolium chloride does not depend on the oxidation state of chromium. Nevertheless, CrIII exhibits higher activity and selectivity to HMF than CrII. Anhydrous CrCl2 and CrCl36¿H2O readily catalyze glucose dehydration with HMF yields of 60 and 72¿%, respectively, after 3 h. Anhydrous CrCl3 has a lower activity, because it only slowly dissolves in the reaction mixture. The transformation of glucose to HMF involves the formation of fructose as an intermediate. The exceptional catalytic performance of the chromium catalysts is explained by their unique ability to catalyze glucose to fructose isomerization and fructose to HMF dehydration with high selectivity. Side reactions leading to humins by means of condensation reactions take predominantly place during fructose dehydration. The higher HMF selectivity for CrIII is tentatively explained by the higher activity in fructose dehydration compared to CrII. This limits the concentration of intermediates that are involved in bimolecular condensation reactions. Model DFT calculations indicate a substantially lower activation barrier for glucose isomerization by CrIII compared to CrII. Qualitatively, glucose isomerization follows a similar mechanism for CrII and CrIII. The mechanism involves ring opening of D-glucopyranose coordinated to a single Cr ion, followed by a transient self-organization of catalytic chromium complexes that promotes the rate-determining hydrogen-shift step

Multinuclear gallium-oxide cations in high-silica zeolites

Periodic DFT calculations of the stability of mononuclear and oligonuclear Ga-oxo cations in mordenite (MOR) have been carried out. Independent of the aluminium distribution in the zeolite framework the stability of cyclic Ga 2O22+ ions is much higher than that of the isolated GaO+ (gallyl) ions in a high-silica mordenite (Si/Al = 23) model. As to the location of such dimers, favorable tetrahedral coordination environment of Ga dominates over the necessity to compensate the positive extraframework charges directly with proximate negative framework charges. Charge alternation can occur in Ga2O2/MOR models in which positive charges of the cationic complex are separated from the framework anionic sites. Oligomerization of four isolated gallyl ions in a MOR model with Si/Al = 11 results in the formation of cubic Ga4O44+ ions. Also in this case direct interaction of the cluster is limited to two anionic sites, while two other framework [AlO2] - units are significantly remote. Binuclear sites are argued to account for the enhanced activity of oxygenated gallium-exchanged high-silica zeolites in alkane dehydrogenation. These sites, however, tend to decompose via water desorption upon the catalytic reaction resulting in less reactive reduced Ga+ ions. As per predictions from the quantum-chemical calculations, the experimental results show that the high alkane dehydrogenation activity can be maintained by in situ hydrolysis of the reduced extraframework Ga species. © The Owner Societies 2009

Fuelling the hydrogen economy:scale-up of an integrated formic acid-to-power system

\u3cp\u3eTransitioning from fossil fuels to sustainable and green energy sources in mobile applications is a difficult challenge and demands sustained and highly multidisciplinary efforts in R&D. Liquid organic hydrogen carriers (LOHC) offer several advantages over more conventional energy storage solutions, but have not been yet demonstrated at scale. Herein we describe the development of an integrated and compact 25 kW formic acid-to-power system by a team of BSc and MSc students. We highlight a number of key engineering challenges encountered during scale-up of the technology and discuss several aspects commonly overlooked by academic researchers. Conclusively, we provide a critical outlook and suggest a number of developmental areas currently inhibiting further implementation of the technology.\u3c/p\u3

Catalytic conversion of furanic compounds over Ga-modified ZSM-5 zeolites as a route to biomass-derived aromatics

\u3cp\u3eHerein we report a mechanistic study of aromatization of furanics, as model compounds for cellulosic biomass, over (Ga)HZSM-5 catalysts. Applying combined gas chromatography and mass-spectrometry product analysis we were able to analyse conversion and selectivity reaction profiles with high temporal resolution. The thorough analysis of the product distribution allowed us to resolve the deoxygenation pathways of the furan molecules. We found that depending on the methyl substitution oxygen is removed either as water or CO\u3csub\u3ex\u3c/sub\u3e, effecting the carbon efficiency of the process. While unsubstituted furan undergoes decarbonylation to form CO\u3csub\u3ex\u3c/sub\u3e, methylated furans are deoxygenated by dehydration, resulting in a much higher carbon-efficiency. Furthermore, using in situ IR spectroscopy, we found that promotion of HZMS-5 with Ga in addition to enhanced aromatic selectivity influences the deactivation pathway leading to the preferential formation of proton-deficient polycyclic aromatic compounds.\u3c/p\u3