The Effect of Support Acidity on Olefin Metathesis over Heterogeneous Mo/HBeta Catalyst: A DFT Study

Abstract

The effect of support acidity on the crossmetathesis of ethylene and 2-butylene to propylene over heterogeneous Mo/HBeta catalyst was studied by density functional theory (DFT) calculations. The zeolite acidity was mimicked by changing the terminating Si-H bond lengths. Both of the initiating formation and propagating processes of Mo-carbene were investigated. It is found that there is a correlation between the zeolite acidity and the activation barrier. The influence of acidity on the initial Mo-carbene formation process is greater than that on the next process catalyzed by this active site. Theoretical calculations reveal that the heterogeneous catalyst with more acidic support could have better olefin metathesis activities, which is consistent with the experimental findings in the literature.

In situsolid-state NMR for heterogeneous catalysis: a joint experimental and theoretical approach

“…A detailed DFT study on the crossmetathesis of ethene and 2-butene to propene using the above models demonstrated that the acidity of the support would affect the transition-state structures as well as the activation barriers. 133 The DFT calculations revealed that olefin metathesis would occur more easily over more acidic heterogeneous catalysts, which is consistent with the experimental results from our group and others. 129,[134][135][136] However, too much Brønsted acidity may also cause side reactions such as olefin isomerization and oligomerization.…”

“…129 In order to give a comprehensive understanding of the mechanism, DFT calculations were performed on the Mo/HBeta catalyst with different acidities. 133 The Si-H distance of the cluster model was set between 1.30 and 2.25 Å to represent the supports with different Brønsted acid strengths. 77 Fig .…”

Framework Stability and Brønsted Acidity of Isomorphously Substituted Interlayer‐Expanded Zeolite COE‐4: A Density Functional Theory Study

“…A previous experimental study of COE-4 zeolites confirmed the Si(OH) 2 groups were attached to the bridging unit at the linker position by using solid-state 29 Si and 1 H MAS NMR measurements. [14] Tetravalent Si(OH) 2 could be substituted by the triva-lent T atom at the linker position to result in the Brønsted acidity of the T-COE-4 zeolites.…”

“…Kang et al [28] carried out periodic DFT to study the methanol adsorption in isomorphously substituted M-AlPO-34 (M = Mn, Zn, Mg, Si, Ti, and Zr) zeolites. [29,30] Moreover, acidity characterization may also be achieved by the adsorption of suitable basic probe molecules, such as ammonia, pyridine or trimethylphosphine. [28] From DFT calculations, we revealed the pentavalent Mo-carbene to be the active species in olefin metathesis on Mo/HBeta zeolite, and the greater the acidity was, the higher the catalytic activity.…”

“…[29,30] Moreover, acidity characterization may also be achieved by the adsorption of suitable basic probe molecules, such as ammonia, pyridine or trimethylphosphine. In our previous work, [14] the Rietveld crystal refinement of the powder X-ray diffraction (XRD) pattern as well as solid-state 29 Si and 1 H MAS NMR spectra of COE-4 confirmed interlayer expansion and the unique two-dimensional pore structure, involving the Si(OH) 2 groups attached to the bridging unit at the linker location between the ferrierite (FER)-type layers (see Figure 1). [31,32] Recently, Yang and co-workers [33] predicted the Lewis and Brønsted acidities in different tetravalent metaldoped zeolites by measuring the proton affinity (PA) and NH 3 adsorption.…”