Structures and reaction mechanisms of glycerol dehydration over H-ZSM-5 zeolite: a density functional theory study

Abstract

The initial stage of glycerol conversion over H-ZSM-5 zeolite has been investigated using density functional theory (DFT) calculations on an embedded cluster model consisting of 128 tetrahedrally coordinated atoms. It is found that glycerol dehydration to acrolein and acetol proceeds favourably via a stepwise mechanism. The formation of an alkoxide species upon the first dehydration requires the highest activation energy (42.5 kcal mol(-1)) and can be considered as the rate determining step of the reaction. The intrinsic activation energies for the first dehydration are virtually the same for both acrolein and acetol formation, respectively, suggesting the competitive removal of the primary and secondary OH groups. A high selectivity to acrolein at moderate temperatures can be attributed to the selective activation of the stronger adsorption mode of glycerol through the secondary OH group and the kinetically favoured subsequent consecutive steps. In addition, the less reactive nature of acrolein relative to acetol precludes it from being converted to other products upon conversion to glycerol. In accordance with typical endothermic reactions, the forward rate constant for glycerol dehydration significantly increases with increasing reaction temperature.

Catalytic Dehydration of Glycerol to Acrolein over a Catalyst of Pd/LaY Zeolite and Comparison with the Chemical Equilibrium

“…In the case of acrolein, even though recently values have been reported for ΔH°f by means of computational chemistry methods by Vasiliu et al [73], and later by Lewars and Liebman [84], the subsequent calculations in this work were carried out with data obtained with the Joback method due to its result (ΔH°f = −65.4 kJ·mol −1 ) which is in very good agreement with the value reported by Lewars and Liebman [84] (ΔH°f = −65 kJ·mol −1 ). Although 1,3-DHP, 2,3-DHP, vinyl alcohol, and 3-HP are not final reaction products in glycerol dehydration, they have been proposed as intermediate compounds by other authors [25][26][27]53] but there is a lack of their thermodynamic properties in the literature. From the equations of the Joback method in Table A1 and the values of Table A3, the values for ΔH°f, ΔG°f, and Cp were calculated for each species involved in the reactive system and presented in Table A5.…”

“…The thermodynamic data for water, methanal, and ethanal were obtained from the literature [75]. In the case of acrolein, even though recently values have been reported for ∆H • f by means of computational chemistry methods by Vasiliu et al [73], and later by Lewars and Liebman [84], the subsequent calculations in this work were carried out with data obtained with the Joback method due to its result (∆H [25][26][27]53] but there is a lack of their thermodynamic properties in the literature. From the equations of the Joback method in Table A1 and the values of Table A3, the values for ∆H • f , ∆G • f , and C p were calculated for each species involved in the reactive system and presented in Table A5.…”

“…Independently, Kongpatpanich et al [25], developed Density Functional Theory (DFT) calculations to study the structures and reaction mechanisms of glycerol dehydration over an H-ZSM-5 zeolite. The reaction pathways obtained from their calculations were essentially the same as those of Intermediary compounds in glycerol dehydration: (a) 1,3-dihydroxypropene; (b) 2,3-dihydroxypropene; (c) vinyl alcohol and; (d) 3-hydroxypropanal (keto-isomer of 1,3-DHP) according to [27].…”

“…Independently, Kongpatpanich et al [25], developed Density Functional Theory (DFT) calculations to study the structures and reaction mechanisms of glycerol dehydration over an H-ZSM-5 zeolite. The reaction pathways obtained from their calculations were essentially the same as those of Nimlos et al [27] for neutral glycerol in the sense of the formation of 1,3-DHP and 2,3-DHP, although they consider in the model the adsorption and protonation of the glycerol molecule over an H-ZSM-5 cluster.…”

Effect of Al Content on the Gas-Phase Dehydration of Glycerol over Silica-Alumina-Supported Silicotungstic Acid Catalysts

“…1,2 Several studies have been reported in recent years on solid-acid catalysts such as metal phosphates, 4-7 metal sulfates, 4,8 metal oxides, 4,[9][10][11] supported heteropolyacids (HPAs), 4,[12][13][14][15][16][17][18][19] and zeolites 4,[20][21][22][23] for the dehydration of glycerol to acrolein in the gas phase. Most of these catalysts exhibit superior dehydration activities with higher water/glycerol ratios in the feed at moderate temperatures.…”