(166d) Bio-Butanol Dehydration in Zeolites: Mechanistic Insights from DFT-Based Microkinetic Modeling

Besides being a
renewable energy source, the catalytic conversion of bio-alcohols can serve as
a sustainable means for the production of high value chemicals. Butenes
produced by dehydration of butanol could serve as a building block for several
essential compounds such as fuels and polymers. Nevertheless, the
selective conversion of the feed is the key to cost effectiveness and success
of these processes.
An in-depth understanding of the underlying reaction mechanism is necessary for
the selection and design of an appropriate catalyst. Alcohol dehydration on
Brønsted acid sites can occur via intramolecular and intermolecular reaction
pathways [1,2,3]. Moreover, 1-butene produced by dehydration of 1-butanol can
undergo double bond and skeletal isomerization to form cis/trans-2-butene and
isobutene respectively.

In this study,
we present a first principles based microkinetic model to study the conversion
of 1-butanol to di-1-butyl ether and butene isomers in H-ZSM-5 and H-ZSM-22
zeolite (Figure 1). Dispersion-corrected periodic density functional theory
(DFT-D2) is used to elucidate the underlying reaction mechanism and to
construct the microkinetic model, which in turn allows to gain insights into the
dominant reaction pathway and the effect of reaction conditions on reaction
rates and product selectivity. The adsorbed 1-butanol molecule can undergo a
direct dehydration reaction producing 1-butene via several mechanisms (i.e. E1,
syn-elimination, anti-elimination, butoxide-mediated dehydration, butanol-assisted
syn-elimination) or react in a sequential manner to yield di-1-butyl ether (via
SN1 or SN2 substitution reactions) which can further decompose to 1-butene and
1-butanol (via syn- or anti-elimination). The double bond isomerization (via butoxide-mediated
stepwise or concerted mechanisms) and the skeletal isomerization (via a cyclic transition
state) of 1-butene produced from the dehydration reaction is also investigated.

The
reaction energetics clearly favor the intermolecular dehydration of butanol to
ether via the SN2 reaction mechanism followed by ether decomposition via syn-elimination.
The calculated activation barriers of 92 and 140 kJ/mol for ether formation and
 decomposition in H-ZSM-5  are in close agreement with the literature-reported values
[4]. A comprehensive investigation of the effect of reaction conditions, viz.
reaction temperature, site time,1-butanol and water partial pressure, on the
reaction rates and product selectivity is performed using reaction path
analysis. The microkinetic  simulation  results were able to capture the
experimentally observed trends for the dehydration of 1-butanol in H-ZSM-5 [4].
They also reveal  the crucial role of reaction conditions in determining the key
surface species, dominant reaction mechanism and pathway. These insights on the
change in dominant mechanisms allow us to reconcile the conflicting
observations reported at different operating conditions. Finally, the
difference in performance of the investigated zeolites in butene skeletal isomerization
reaction  is attributed to a better stabilization of the transition state
structures in H-ZSM-22 over H-ZSM-5. Such insights on the effect of zeolite
topology on the reaction rates and product selectivity can guide us towards a rational
catalyst design.

References:

[1]
 H.
Chiang and A. Bhan, J. Catal. 2010, 271,
251.

[2]
 M.F. Reyniers, G.B. Marin, Annu. Rev. Chem. Biomol. 2014, 5,
563.                            

[3]
 M. John, K. Alexopoulos, M.F. Reyniers, G.B. Marin, J. Catal. 2015 submitted.

[4]
 M.A. Makarova, E.A. Paukshtis, J.M. Thomas, C. Williams, K.I. Zamaraev, J. Catal. 1994, 149, 36.

Figure 1: Reaction scheme for conversion
of 1-butanol to di-1-butyl ether and butene isomers