A First Principles Analysis of Methanol Dehydration Over Tungstated Zirconia

Methanol dehydration to dimethyl ether (DME) is the first step of forming olefins, aliphatics, and aromatics from methanol^1,2. The key factor for each of these elementary steps involves stabilizing carbenium ion in transition states³. Recent experimental efforts reveal that specific tungstated zirconia can be quite active in converting methanol to DME^4,5. However, the reaction mechanism, as well as relationship between dehydration reactivity and catalyst structure, most importantly surface WOx domain size effect^6,7,8,9, has not been developed. Methanol dehydration is used herein as probe to study the structure-property relationship of tungstated zirconia. Density functional theory (DFT) calculations were carried out to systematically explore the mechanisms that can control the dehydration of methanol over tungstated zirconia. Different size surface tungstate species were modeled to establish fundamental structure-acidity-reactivity relationship which describes the surface chemistry.

The calculated proton affinity on supported tungstate clusters showed a decreasing trend in proton affinity with increasing the tungstate size, which means larger size tungstate clusters tend to have stronger solid acidity. Various gas phase molecules were chosen as adsorbates to calculate adsorption energies on supported tungstates. Generally, adsorption becomes more exothermic to tungstates with stronger acidity. However, the unpredictable interaction energy between protonated adsorbate and deprotonated tungstate anion makes the adsorption energy sometimes not linearly correlate to surface tungstate acidity.

Our calculation shows there are two plausible pathways for methanol dehydration forming dimethyl ether (DME) on zirconia supported tungstates. In first path, two methanol molecules involved react sequentially in two seperate steps. The first methanol is dehydrated with formation of surface methoxy group, then the second methanol react with methoxy group to produce DME. In second path, two methanol molecules are adsorbed on the surface simultaneously, and reactions proceed in similar two steps as first path. Path 2 is energetically more favored than path 1, since the second methanol molecule in first elementary step has significant contribution to stabilize protonated methanol ion in transition state. A Bader charge analysis was used to establish the charge associated with specific fragments for a transition state. Relating activation barriers with reference to gas phase, Bader partial charge analysis results, and proton affinity of supported tungstate domains, we can summarize that larger size tungstate domains tend to have stronger acidity and more capable to disperse extra charge density created during transition states, which stabilize protonated methanol and lower activation barriers of methanol dehydration.

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