The role of defect-modulated HKUST-1 MOF nodes in non-oxidative ethanol dehydrogenation: an observed phenomenon of catalyst transfiguration

Bioethanol production from agricultural residues has emerged as an important process of biomass valorization. The production of acetaldehyde from bioethanol has also started gaining ground. Since Cu-based catalysts are well-known for their ability to catalyse ethanol dehydrogenation, we have used a defect-modulated Cu-based metal-organic framework (MOF), HKUST-1, for obtaining mechanistic insights into the process. Defect-modulation in the form of a missing linker creates an easily accessible dual-atom site which can simultaneously participate in catalysing the reaction. Although ethanol dehydrogenation to ethylene competes with acetaldehyde production over both the defective HKUST-1(H) and HKUST-1(OH) MOF nodes, acetaldehyde formation occurs selectively. However, HKUST-1(OH) could not be regenerated at the end of the acetaldehyde formation pathway; HKUST-1(OH) ultimately transformed to HKUST-1(H) at the end of the cycle. This led to the introduction of the term 'catalyst transfiguration' where the catalyst, although transfigured, retains its ability to catalyse the reaction. Since, the HKUST-1(H) MOF node has the ability to selectively transform ethanol to acetaldehyde, we can safely conclude that the use of HKUST-1(OH) will not cause acetaldehyde formation to come to a halt and the reaction can go on beyond the first catalytic cycle. Thus, both the defective MOF nodes can selectively transform ethanol to acetaldehyde.

Adsorption and dehydration of ethanol on isomorphously B, Al, and Ga substituted H-ZSM-5 zeolite: an embedded ONIOM study

Dehydration reactions are important in the petroleum and petrochemical industries, especially for the feedstock production. In this work, the catalytic activity of zeolites with different acidities for the dehydration of ethanol to ethylene and diethylether is investigated by density functional calculations on cluster models of three isomorphous B, Al, and Ga substituted H-ZSM-5 zeolites. Both unimolecular and bimolecular mechanisms are investigated. Detailed reaction profiles for the dehydration reaction, assuming either a stepwise or a concerted mechanism, were calculated by using the ONIOM(MP2:M06-2X) + SCREEP method. The adsorption energies of ethanol are -21.6, -28.1, and -27.7 kcal mol^-1 on H-[B]-ZSM-5, H-[Al]-ZSM-5, and H-[Ga]-ZSM-5 zeolites, respectively. The activation energies for the rate-determining step of the unimolecular concerted mechanism for the ethylene formation are 48.5, 42.6, and 43.6 kcal mol^-1 on H-[B]-ZSM-5, H-[Al]-ZSM-5, and H-[Ga]-ZSM-5 zeolites, respectively. The activation energies for the ethoxy formation as the rate-determining step for the bimolecular formation of diethylether are 42.3, 40.0, and 41.1 kcal mol^-1 on H-[B]-ZSM-5, H-[Al]-ZSM-5, and H-[Ga]-ZSM-5 zeolites, respectively. The results indicate that the catalytic activities for the dehydration of ethanol decrease in the order H-[Al]-ZSM-5 ~ H-[Ga]-ZSM-5 > H-[B]-ZSM-5. Besides the acid strength, the zeolite framework affects the reaction by stabilizing the reaction intermediates, leading to more stable adsorption complexes and lower activation barriers.

Theoretical study of CO2 hydrogenation into formic acid on Lewis acid zeolites

The hydrogenation of carbon dioxide (CO₂) to formic acid over Lewis acidic zeolites as catalyst has been investigated by means of density functional theory (DFT) with the M06-L functional.