(263e) ZnO@ZSM-5 Nanostructured Core-Shell Catalysts for Methane Dehydroaromatization to Benzene

2013 AIChE Annual Meeting

November 3rd - November 8th, 2013

Hilton San Francisco Union Square

California, CA

Abstract Submission

PROGRAM TOPIC:

TE002 Natural Gag rection
engineering

Exact Title of Paper:

ZnO@ZSM-5 Nanostructured Core-Shell Catalysts for
Methane Dehydroaromatization to Benzene

Presenting Author: Yungchieh Lai

Department of Chemical engineering, University of
Pittsburgh, Pittsburgh, PA

Contact Information: yul69@pitt.edu

Co-Authors: Götz Veser

Department of Chemical Engineering, University of
Pittsburgh, Pittsburgh, PA

Contact Information: gveser@pitt.edu

Abstract:

Benzene is one of
the most important organic intermediates in
the US, with an annual volume over 18 billion gallons in 2010 [1]. Currently, most benzene is produced from crude oil. However,
increasing crude oil prices and the low cost of
natural gas due to the vast amount of accessible shale gas reserves in the US makes natural gas a potential alternative.

The most promising path for methane conversion to benzene is the
catalytic dehydroaromatization (DHA) according to:                                        6CH₄ à 9H₂ + C₆H₆

The reaction proceeds over zeolite-based catalysts at ~700-800C with good selectivities (>60%) but low conversion, limited by
thermodynamic equilibrium (X_CH4~11.8% at 700C, and 21% at 800C). The most widely investigated catalysts for this reaction are bi-functional metal/ZSM-5
catalysts in which the
metal site activates methane, followed by oligomerization of the methyl to
benzene on the Bronsted
Acid Site (BAS) of the zeolite. The pore
size of ZSM-5 which is almost identical to the benzene diameter (~ 6 angstrom) results in good selectivity for the desired
product benzene.

Among the metals investigated, the most intensively studied
one is Mo/ZSM-5 due to its comparatively
high reactivity. Membrane
reactors used to remove hydrogen in the product was shown to have a further
enhancement of methane conversion [2-4]. However, similar to other metal/ZSM-5
systems, this catalyst tends to deactivate due to coke formation especially as
produced hydrogen is removed. While this coke formation is reversible (via
oxidative regeneration of the catalyst), it results in an oxidized form 
of the metal which then has to undergo a lengthy activation period to (re)form
the active carbide phase. These issues motivate the search for more coking resistant and/or more
easily regenerable catalysts for this reaction.

ZnO/ZSM-5 is a possible candidate for this reaction which is active in
the oxide form and would hence not require further activation after oxidative
regeneration of a spent catalyst. We therefore targeted the synthesis of a
nanostructured ZnO@ZSM-5 catalyst with the following desired properties: (1)
small ZnO particle size to increase the reactivity of the catalyst, (2) small
ZSM-5 particle size and hierarchical pore structure to facilitate removal of
the reaction products and hence reduce secondary coke formation.

Here, we report the first successful synthesis of a nanostructured
ZnO@ZSM-5 core-shell catalyst system which encapsulates small (~5-10 nm) ZnO
nanoparticles in a highly crystalline H-ZSM-5 shell. We will discuss the
synthesis path, which is based on a systematic and well-controlled bottom-up
approach, and present a detailed characterization of the material. Reactivity and
stability tests of the catalyst are planned for the coming future, and will
also be included. 

References:

[1] B. Balboa. ICIS NEWS 2011.

[2] O. Rival, B. P. A. Grandjean, C. Guy, A. Sayari and F.
Larachi. Ind. Eng. Chem. Res. 40(2001) 2212-2219.

[3] M. C. Iliuta, F. Larachi, B. P. A.
Grandjean, I. Iliuta and A. Sayari. Ind. Eng. Chem. Res. 41(2002) 2371-2378.

[4] M. C. Iliuta, B. P. A. Grandjean and
F. Larachi. Ind. Eng. Chem. Res. 42( 2003) 323-330.