A DFT Study of Hydrogen–deuterium Exchange over Oxidized and Reduced Gallium Species in Ga/HZSM-5 Zeolite

Abstract

Quantum-chemical calculations give insight in the experimentally observed higher rate of hydrogen-deuterium exchange for oxidized Ga/HZSM-5 over reduced Ga/HZSM-5. The reaction is computed to be more facile over reduced (Ga + ) than over oxidized (GaO + ) cations. The difference lies in the difficult formation of active GaH 2 + cations from Ga + compared to facile hydrogen dissociation over GaO + to give active GaHOH + cations. Neutral gallium oxide clusters are shown to have a lower intrinsic activity than GaO + cations.

ONIOM study of Ga/SAPO-11 catalyst: Species formation and reactivity

“…Even though the formation of [HGaOH] + from [GaO]+ is favored ( G = −71.7 kJ/mol, reaction 5), further reduction of [HGaOH] + to form the dihydride species does not occur ( G = +244.4 kJ/mol, reaction 7). These results are coherent with those reported by Kuzmin et al [37] and Chakraborty and co-workers [38] for Ga/ZSM-5 catalysts; that is, G = −347.7 kJ/mol [37] and G = −119.7 kJ/mol [38] To determine whether Ga behaves as a hard or a soft acid in the Ga/SAPO-11 system, adsorption energies for NH 3 , CH 3 NH 2 and CH 3 SH were calculated. The first two adsorbents are hard bases while the last one is a soft base.…”

H/D exchange of molecular hydrogen with Brønsted acid sites of Zn- and Ga-modified zeolite BEA

“…[11][12][13] Recombinative desorption of hydrogen is assumed to occur only with involvement of metal species; Brønsted acid sites of metal-modified zeolite are not involved in molecular hydrogen desorption. [14][15][16][17] Since the recombination of hydrogen atoms has been proposed as the rate-limiting step in the dehydrocyclodimerization of C 3+ alkanes over Ga/H-ZSM-5, 2 and both metal and Brønsted acid sites are indeed involved in alkane activation, 8,11 it is of considerable interest to understand the concurrent influence of both metal species and acidic OH groups on the molecular hydrogen activation by these catalysts. The hypothesis that hydrogen recombinative desorption can also occur with involvement of acidic OH groups requires verification.…”

Swift hopping gallium over [AlO4]− tetrahedra in Ga/ZSM‐5: A DFT study

“…On the other hand, this motion is activated and the system must get over barrier of 11.4 kcal/mol. Still this energy is very low compared to activation energy of catalyzed processes on gallium‐exchanged ZSM‐5 like hydrogen–deuterium exchange 12 or ethane dehydrogenation 16. Thus frequency of gallium hops must be times greater than that of catalyzed reactions, giving us new means of catalyzed reactions pathway modeling.…”

“…Most computational models include AlO 2 Ga (Fig. 1) cycle fragment representing the core of catalytic site 11–16. Some reaction pathways have stable intermediates with weak‐bonded neutral gallium containing molecules with one of [AlO 4 ] oxygen atom 11, 12, 16.…”

“…1) cycle fragment representing the core of catalytic site 11–16. Some reaction pathways have stable intermediates with weak‐bonded neutral gallium containing molecules with one of [AlO 4 ] oxygen atom 11, 12, 16. However, a structure with neutral GaH bonded to both oxygen atoms of [AlO 4 ] was reported in mordenite 15 with charge compensating proton on third oxygen of aluminum–oxygen tetrahedron.…”