(394c) 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.

Reference

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