473261 Gasoline-Selective Fischer-Tropsch Synthesis with Hierarchical ZSM-5 Coated Monolith Catalysts

Wednesday, November 16, 2016: 4:27 PM
Franciscan B (Hilton San Francisco Union Square)
David P. Gamliel, Chunxiang Zhu, Julia A. Valla and George M. Bollas, Chemical & Biomolecular Engineering, University of Connecticut, Storrs Mansfield, CT

Energy independence is important for economic stability and security. Despite significant incentives and developments for the decrease of crude oil imports, the U.S. still imports about 24% of its oil (2015 EIA data) [1]. Moreover, the infrastructure of the U.S. transportation sector imposes requirements for high-octane, gasoline-range hydrocarbons. Gasoline consumption accounts for about 50% of the total U.S. oil consumption [2]. Fischer-Tropsch Synthesis (FTS) is a well-known process for the conversion of (U.S. abundant) natural and shale gas to hydrocarbons [3,4]. However, FTS is historically conceived as a diesel-producing process, with long-chain hydrocarbon products favored due to the polymerization nature of the reaction network [3,5,6]. Also, FTS produces mainly linear olefins and paraffins [7]; and therefore, low-octane gasoline. These limitations can be overcome by the addition of an acidic co-catalyst to enhance cracking, oligomerization, isomerization and aromatization. In-situ upgrading of primary FTS products is possible by use of bifunctional or hybrid catalysts comprising the FTS active phase and an acid catalyst. Acid catalysts enhance oligomerization, cyclization, aromatization, cracking, and isomerization reactions and provide shape selectivity towards gasoline-range hydrocarbons. In this work, multilayered structured catalysts were synthesized, composed of a supported FTS active phase (Co catalyst), and coated with a layer of ZSM-5 catalyst. Both layers were formed on the internal surface of a monolith support. The structure of these multilayered catalysts is conceptually presented in Figure 1. The synthesized multilayered catalysts enable: a) relaxation of heat and mass transfer limitations; b) high activity with low water-gas shift potential; c) control of product size through manipulation of the catalyst outer film thickness; and d) enhanced selectivity to gasoline-range hydrocarbons. Early work has shown excellent selectivity and performance of these catalysts. The bi-functional, multi-layered catalysts with Co dispersed on an Al2O3 layer over the monolith and coated with ZSM-5 doubled the selectivity to gasoline range compounds, without affecting the overall conversion of the process.

Figure 1: Bi-functional, multi-layered, structured catalyst.

The production of gasoline-range hydrocarbons depends on the extent of cracking and isomerization reactions, which in turn depend on the ZSM-5 to intermediate hydrocarbons ratio in the local neighborhoods of the catalyst acid sites. Increasing the ZSM-5 loading (layer thickness) can possibly improve the gasoline selectivity of the process. However, increase of the ZSM-5 membrane thickness also contributes to diffusion limitations. In essence, a thick layer of ZSM-5 has the potential to enhance selectivity, but negatively impacts conversion because the process becomes diffusion-limited. For that reason, we explored mesoporous ZSM-5 as an acidic catalyst layer capable of relaxing diffusion limitations. Hierarchical zeolitic materials have demonstrated improved mass transfer properties at high catalytic activity for a number of applications [8,9]. Inclusion of mesopores in the ZSM-5 structure eases transport, specifically of molecules in the C5-C12 range [10]. Thus, it can enable higher loadings of ZSM-5 on the FTS monolith catalysts without affecting conversion and diffusional characteristics; i.e. the transport of syngas to the catalyst and hydrocarbons out of it. We will present the synthesis methods, process configuration and results of the effect of hierarchical ZSM-5 catalytic films, as selective and reactive membranes for FTS. The hierarchical ZSM-5 was prepared using a finely controlled desilication technique, with mesopore diameters between 100 and 300 Å. They were then coated over FTS monoliths, using wash-coating methods and were tested for their reactivity, selectivity and stability as gasoline-producing FTS catalysts. The effect of pressure, temperature and gas residence time will be presented. The effect of thickness and the improvement of diffusion tolerances of the mesoporous catalysts will be presented and explained on the basis of materials characterization results. FTS with these catalysts maintains CO conversion, but may also shift selectivity from diesel-range compounds to high-octane gasoline.        

Acknowledgment

Support from the ACS PRF 53648-DNI5 is gratefully acknowledged.

 

References

[1]      U.S. Energy Information Administration, U.S. Imports by Country of Origin. (2016). http://www.eia.gov (accessed 09 May 2016).

[2]      U.S. Energy Information Administration, Short-Term Energy and Summer Fuels Outlook, 2016.

[3]      M.E. Dry, J. Chem. Technol. Biotechnol. 77 (2002) 43–50.

[4]      A.Y. Khodakov, W. Chu, P. Fongarland, ChemInform 107 (2007) 1692–1744.

[5]      M.E. Dry, Catal. Today 71 (2002) 227–241.

[6]      S. A. Kondrat, R.P. Marin, J.R. Gallagher, D.I. Enache, T.E. Davies, J.K. Bartley, S.H. Taylor, M.J. Rosseinsky, G.J. Hutchings, J. Am. Chem. Soc. 3 (2013) 764–772.

[7]      M.E. Dry, J.R. Boudart, M. Anderson (Eds.), Catalsis Science and Technology, vol. I, Springer, New York, 1981.

[8]      K. Li, J. Valla, J. Garcia-Martinez, ChemCatChem 6 (2014) 46–66.

[9]      D.P. Gamliel, H.J. Cho, W. Fan, J.A. Valla, Appl. Catal. A Gen. (2016).

[10]    J.C. Groen, W. Zhu, S. Brouwer, S.J. Huynink, F. Kapteijn, J.A. Moulijn, J. Pérez-Ramírez, J. Am. Chem. Soc. 129 (2007) 355–60.

 


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