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.