Simple Scheme to Predict Transition-State Energies of Dehydration Reactions in Zeolites with Relevance to Biomass Conversion

Abstract

Dehydration of various alcohols over H-ZSM-5 is studied using density functional theory. The activation energies are shown to scale linearly with the van der Waals interaction with the zeolite framework. The van der Waals interaction itself is shown to be a simple function of the number of atoms of the involved alcohol. Consequently, activation barriers for the dehydration of primary alcohols are now easily derived directly from the number of atoms of these alcohols through the obtained scaling relations.

Influence of Confinement on Barriers for Alkoxide Formation in Acidic Zeolites

“…In earlier work employing H-ZSM-5, we identified a difference between MeOH and EtOH of 17, 13, and 18 kJ/mol for the IS, TS and FS, respectively. [19] Similar differences are also observed for the other seven zeotypes as shown in Figure 3, where the transition states of SMS and SES formation are compared. On average SES formation has 20 kJ/mol lower barriers compared to SMS formation, when referenced to the corresponding gas-phase oxygenate, as shown in Figure 3.…”

“…In each case, we focus on the reactivity of one specific choice of T-site, where we choose positions that have been investigated previously. [19,[42][43][44][45][46][47][48][49][50][51] Similar to the MeOH case, we obtain adsorption energies of EtOH ranging from À 113 kJ/mol (H-SSZ-24 and H-BEA) to À 139 kJ/mol (H-ZSM-22) (see also Table 1). Compared to the SMS formation from methanol, the SES formation is on average about 14 kJ/mol lower in energy which is in good agreement with an earlier study for H-ZSM-5.…”

“…Compared to the SMS formation from methanol, the SES formation is on average about 14 kJ/mol lower in energy which is in good agreement with an earlier study for H-ZSM-5. [19] Likewise, the transition state (TS) for SES formation has been calculated to range from 124 kJ/mol (H-MOR) to 144 kJ/ mol (H-FER), when referenced to adsorbed EtOH. The final state (FS) is also lower by 14 kJ/mol on average.…”

“…[11][12][13][14][15] A detailed understanding of how the effect of confinement influences the activity and selectivity of dehydration reactions related to these conversions is therefore highly desirable. Density functional theory (DFT) calculations offer a useful tool to investigate the reaction mechanisms and the intrinsic effect of the confinement in great detail [16][17][18][19][20][21][22] and to deduct trends from one material to another. [23][24][25][26] DFT calculations also help understanding, how electronic and steric effects influence reaction barriers and adsorption energies within zeolite pores.…”

“…Taking the dehydration of a range of alcohols in H-ZSM-5 (MFI framework) as an example, we recently showed how reaction barriers relate to the interaction between the adsorbate and the zeolite through dispersion forces. [19] Furthermore, we were able to invoke a simple scheme based on a straightforward measure of dispersion forces that allowed to predict transition state energies of dehydration reactions quite accurately. Herein, we investigate how the acid catalyzed dehydration of alcohols within a variety of microporous zeotypes depends on the confinement imposed by the porous framework.…”