QM/MM study of the stability of dimethyl ether in zeolites H-ZSM-5 and H-Y

Abstract

Computational techniques are used to study the adsorption of dimethyl ether in zeolite frameworks. Binding strength is shown to increase for more open acid sites where proton transfer, from the framework to dimethyl ether, occurs more readily.

A quantitative multiscale perspective on primary olefin formation from methanol

“…Ab initio static simulations offer a clear description of the potential energy surface, which can be used to determine molecular structures, charge distribution, and vibrational modes (with corresponding frequencies), together with kinetic and thermodynamic observables that can indicate reaction intermediates and pathways used to decipher experimental data. [38][39][40] Molecular dynamics techniques have the ability to simulate real-time dynamics of atoms in zeolite pores, and has been used to investigate surface adsorption, proton transfers and diffusion effects, 41,42 whilst positive bias metadynamic simulations can uncover rare-events and map out the free energy surface for pathways leading to meta-or un-stable intermediates. 43,44 All of these techniques have been successfully applied to complement experimental analysis and improve the development of heterogeneous catalysts.…”

“…The chemistry of steady-state MTH conversion have been briefly described by Ilias and Bhan. 4 The process includes olefin 94.4 34 100-126 38 110 34 107-132 39 94-116 116 115 117 100 120 H-Y -96-106 38 -105 39 89-104 116 SAPO-34 64 118 86 119 45 120 -80 120 ZSM-22 -107 116 --AlOH-(SiOH) -75 121 -a Energies were obtained only for medium temperature site. E des -activation energies of desorption (kJ mol À1 ).mpuc -molecules per unit cell.The activation energy of desorption is the sum of the heat of adsorption and the activation energy of adsorption: E des = ÀDH ads + E ads .…”

“…E des -activation energies of desorption (kJ mol À1 ).mpuc -molecules per unit cell.The activation energy of desorption is the sum of the heat of adsorption and the activation energy of adsorption: E des = ÀDH ads + E ads . 122 Citations are given in brackets.Level of theory: 38,39 cluster: QM/MM TZVP, DFT, B-97-D, Hill and Sauer forcefield; 123,124 As various mechanisms can account for the same kinetic data, the verification of reaction mechanisms through spectroscopy is very important (Table 2). Recently, Oyama et al 126 developed a method for scrutinizing adsorbed intermediates by in situ spectroscopy through analysis of coverage transients (ACT).…”

“…139 Theoretical analysis indicated that hydrogen bonding networks can stabilise these oxonium salts with clusters of methanol, methanol and water, or dimethyl ether and water. 39 Clusters of methanol and dimethyl ether adsorbed at the acid site are very stable, with 27 kJ mol À1 necessary to substitute dimethyl ether and methanol within the cluster. 44 The investigation of which oxonium salt is more stable, and how this contributes to the initial C-C bond, is important for future understanding.…”

“…35 Theoretical studies confirm that H-ZSM-5 catalysts have Brønsted acid sites with a difference of B20 kJ mol À1 in adsorption strength for both methanol and dimethyl ether. 39 Previous QM/MM studies indicate that the co-adsorption of the second methanol on the acid sites will lower the average adsorption energy per methanol molecule by B15 kJ mol À1 . A key aspect to consider when accounting for adsorption ensembles are adsorbate clusters connected through hydrogen bonds.…”