Formaldehyde Encapsulated in Lithium-Decorated Metal-Organic Frameworks: A Density Functional Theory Study

Electronic Structure Modeling of Metal-Organic Frameworks

Owing to their molecular building blocks, yet highly crystalline nature, metal-organic frameworks (MOFs) sit at the interface between molecule and material. Their diverse structures and compositions enable them to be useful materials as catalysts in heterogeneous reactions, electrical conductors in energy storage and transfer applications, chromophores in photoenabled chemical transformations, and beyond. In all cases, density functional theory (DFT) and higher-level methods for electronic structure determination provide valuable quantitative information about the electronic properties that underpin the functions of these frameworks. However, there are only two general modeling approaches in conventional electronic structure software packages: those that treat materials as extended, periodic solids, and those that treat materials as discrete molecules. Each approach has features and benefits; both have been widely employed to understand the emergent chemistry that arises from the formation of the metal-organic interface. This Review canvases these approaches to date, with emphasis placed on the application of electronic structure theory to explore reactivity and electron transfer using periodic, molecular, and embedded models. This includes (i) computational chemistry considerations such as how functional, k-grid, and other model variables are selected to enable insights into MOF properties, (ii) extended solid models that treat MOFs as materials rather than molecules, (iii) the mechanics of cluster extraction and subsequent chemistry enabled by these molecular models, (iv) catalytic studies using both solids and clusters thereof, and (v) embedded, mixed-method approaches, which simulate a fraction of the material using one level of theory and the remainder of the material using another dissimilar theoretical implementation.

Phenol Tautomerization Catalyzed by Acid-Base Pairs in Lewis Acidic Beta Zeolites: A Computational Study

We investigate the tautomerization of phenol catalyzed by acid-base active pair sites in Lewis acidic Beta zeolites by means of density functional calculations using the M06-L functional. An analysis of the catalytic mechanism shows that hafnium on the Beta zeolite causes the strongest absorption of phenol compared to zirconium, tin, and germanium. This can be rationalized by the highest delocalization of electron density between the Lewis site and the oxygen of phenol which can in turn be quantified by the perturbative E⁽²⁾ stabilization energy. The reaction is assumed to proceed in two steps, the phenol O-H bond dissociation and the protonation of the intermediate to form the cyclohexa-2,4-dien-1-one product. The rate determining step is the first one with a free activation energy of 26.3, 25.0, 22.1 and 22.7 kcal mol^-1 for Ge-Beta, Sn-Beta, Zr-Beta, and Hf-Beta zeolites, respectively. The turnover frequencies follow these reaction barriers. Hence, the intrinsic catalytic activity of the Lewis acidic Beta zeolites studied here is in the order of Hf-Beta≈Zr-Beta>Sn-Beta> Ge-Beta for the tautomerization of phenol.

Insights into the reaction mechanism of n-hexane dehydroaromatization to benzene over gallium embedded HZSM-5: effect of H2 incorporated on active sites

The catalytic dehydroaromatization of alkanes to aromatics has attracted considerable attention from the scientific community, because it can be used for the upgrading of low-cost alkanes into high added-value aromatics, such as benzene, toluene, and xylene (BTX). In this context, we report the reaction mechanism of n-hexane dehydroaromatization to benzene over two different reduced gallium species embedded in HZSM-5, including univalent Ga+ embedded in HZSM-5 (Ga/HZSM-5) and dihydrido gallium complex (GaH2+) embedded in HZSM-5 (GaH2/HZSM-5) by using the M06-2X/6-31G(d,p) level of calculation. The reaction proceeds by following two main steps: (i) the dehydrogenation of hexane to haxa-1,3,5-triene; (ii) the dehydroaromatization of haxa-1,3,5-triene to benzene. For the univalent Ga+ embedded in HZSM-5, the first step of the hexane dehydrogenation is considered to be the rate-determining step, which requires a high activation energy of 76.6 kcal mol-1. In strong contrast to this, in the case of the GaH2/HZSM-5 catalyst the rate determining step is found to be the second hydrogen abstraction from n-hexane with a lower activation barrier of 11.1 kcal mol-1. The reaction is therefore preferentially taking place over the GaH2/HZSM-5 catalyst. These observations clearly confirm the existence of a dihydrido gallium complex (GaH2+) as one of the most active species for the dehydroaromatization of alkanes and it is obtained in the presence of hydrogen in the catalytic system. This example opens up perspectives for a better understanding of the effect of active species on the catalytic reaction.

A mechanistic study of ethanol transformation into ethene and acetaldehyde on an oxygenated Au-exchanged ZSM-5 zeolite

Ethanol transformation to ethene and acetaldehyde over low- and high-spin state oxygenated Au-exchanged ZSM-5 zeolite have been investigated by means of density functional calculations with the M06-L functional.

Theoretical study on the reaction mechanism of hydrogenation of furfural to furfuryl alcohol on Lewis acidic BEA zeolites: effects of defect structure and tetravalent metals substitution

We examined the catalytic roles of the defect structure and tetravalent-metal substitution on Lewis acidic BEA Zeolites for the furfural hydrogenation reaction using the DFT approach.

Ethylene Epoxidation with Nitrous Oxide over Fe-BTC Metal-Organic Frameworks: A DFT Study

The epoxidation of ethylene with N₂ O over the metal-organic framework Fe-BTC (BTC=1,3,5-benzentricarboxylate) is investigated by means of density functional calculations. Two reaction paths for the production of ethylene oxide or acetaldehyde are systematically considered in order to assess the efficiency of Fe-BTC for the selective formation of ethylene oxide. The reaction starts with the decomposition of N₂ O to form an active surface oxygen atom on the Fe site of Fe-BTC, which subsequently reacts with an ethylene molecule to form an ethyleneoxy intermediate. This intermediate can then be selectively transformed either by 1,2-hydride shift into the undesired product acetaldehyde or into the desired product ethylene oxide by way of ring closure of the intermediate. The production of ethylene oxide requires an activation energy of 5.1 kcal mol^-1 , which is only about one-third of the activation energy of acetaldehyde formation (14.3 kcal mol^-1 ). The predicted reaction rate constants for the formation of ethylene oxide in the relevant temperature range are approximately 2-4 orders of magnitude higher than those for acetaldehyde. Altogether, the results suggest that Fe-BTC is a good candidate catalyst for the epoxidation of ethylene by molecular N₂ O.

Adsorption and decarbonylation of furfural over H-ZSM-5 zeolite: a DFT study

The biomass-derived furfural adsorption and decarbonylation to furan over H-ZSM-5 (see picture) have been unraveled by means of density functional calculations with the M06-2X functional.