(350c) Monolithic Catalysts Coated with Hierarchical ZSM-5 for Distributed Fischer-Tropsch Synthesis

Converting remote shale gas to liquids to monetize shale wells and reduce greenhouse gas emissions from flaring is a feasible near-term objective for sustainable production of domestic transportation fuel. Shale gas flaring is a substantial, albeit geographically distributed, contributor to greenhouse gas emissions. A practical solution to this problem is the conversion of shale gas to liquid (GTL) products, which can be more easily transported and utilized. Commonly, GTL refers to technologies for the conversion of natural gas to liquids and can be extended to mixtures of C1-C4 components, commonly found in shale gas. A GTL modular system could significantly reduce the carbon footprint of shale oil extraction, while creating revenue from the currently wasted shale gas. For a dispersed system like this, with uncertainty in the shale gas composition, indirect GTL processes (which convert hydrocarbons to synthesis gas and then upgrade them to liquid fuels) is a promising approach.

Fischer-Tropsch synthesis (FTS) is a known and tested solution to this type of conversion, and has been shown to be economically feasible at larger scale. According to a recent techno-economical assessment,¹ the oil upgrading processes comprise 30% of the total capital investment in a FTS plant. At the same time, the U.S. transportation fuels market is in demand for gasoline-range hydrocarbons, instead of the long linear alkanes and alkenes, produced by conventional FTS. Therefore, the subject of this work is the intensification of FTS, in three respects: (a) reactor volume decrease; (b) process equipment reduction; (c) gasoline-range selectivity of the products. There are two methods for the selective production of gasoline-range hydrocarbons from FTS: (a) the production of heavy wax followed by ex-situ upgrading to produce fuel in the middle distillate range, or (b) the conversion of FTS products into high octane gasoline with use of in-situ zeolite co-catalysts or bifunctional catalysts with upgrading activity.² For intensified and modular processes, there is a need to develop a novel, one pot catalyst with superior activity and high gasoline selectivity. For FTS intensification, microreactors have been shown promising in prior work. Here, we present a structured reactor comprising a Co co-catalyst combined with a mesoporous ZSM-5 coating, supported on a high-surface-area Al₂O₃ coated monolith. The monolithic structure serves as a catalyst support to provide high surface area, stability, and relaxation of mass and heat transfer limitations, while the ZSM-5 facilitates mass transfer control of the FTS products, while providing in-situ acid-catalyzed hydrocarbon cracking and isomerization. Introduction of the zeolite layer introduces desirable final product-selective transport limitations, but also some undesirable reactant and intermediate products transport resistance. The latter is a consequence of the relatively small ZSM-5 pore size (5.5 â„«). Mesoporous ZSM-5 with hierarchical pore structure enables the upgrading of synthesis gas to gasoline-range hydrocarbons, without introducing additional diffusion barriers, which reduces overall CO conversion. Structured FTS catalysts with mesoporous ZSM-5 coating were seen to improve CO conversion by 10%, with selectivity to gasoline-range hydrocarbons on the order of 90 wt.% of the liquid product. These structured catalysts exhibited increased catalyst stability and lifetime compared to FTS structured catalysts coated with microporous ZSM-5. Introduction of mesoporosity improved CO transport from the bulk to the Co phase, and reduced deposition of >C12 hydrocarbons, which is known to deactivate the catalyst. The effects of ZSM-5 layer thickness and reactor pressure, both parameters related to mass transport through the zeolite layer, were systematically varied in order to determine the optimal catalyst configuration and reaction conditions. Bench-scale results show that it is feasible to operate FTS at lower pressures, small reactor footprint and high conversion and selectivity to desirable C5-C12 liquid products, which can be leveraged in the upgrading of stranded shale gas.

Literature

[1] R.M. Swanson, A. Platon, J.A. Satrio, R.C. Brown, Techno-economic analysis of biomass-to-liquids production based on gasification, Fuel. 89 (2010) S11–S19.

[2] F.G. Botes, W. Böhringer, The addition of HZSM-5 to the Fischer-Tropsch process for improved gasoline production, Appl. Catal. A Gen. 267 (2004) 217–225.