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The effect of olefin addition to a stream of dimethyl ether on the methanol homologation reaction is investigated using iron-substituted zeolites Fe-beta and Fe-ZSM-5. The reaction was investigated using plug-flow microreactors in the temperature range of 240-400 degrees C, at a total pressure of 0.239 MPa and a WHSV of 6.12 (g DME/ gcat-hr). For Fe-beta (Si/Fe= 9.2) catalysts, isobutene co-feeding almost doubles dimethyl ether (DME) consumption rate and shifts selectivity towards larger olefins with carbon numbers from 5 to 7. Addition of isobutene above 6.3%, however, resulted in a reduction of DME consumption rates, an effect assigned to the replacement of surface methoxy groups for adsorbed olefins in the zeolite pores. Below a temperature of 340 degrees C hydride-transfer rates are negligible; reaction rates are stable for over 5.5 h and the products consist almost exclusively of olefins and a small amount of methane. Above 360 degrees C the onset of catalytic hydride transfer processes is observed leading to fast catalyst deactivation rates and an increase in the concentration of aromatic species. Iron ZSM-5 (Si/Fe = 21.4) catalysts under similar reaction conditions consumes methanol faster than Febeta at approximately three times the TOF (on a per iron basis). The Fe-ZSM-5 catalyst was selective to a distribution of products (C5 to C8) as compared to Fe-beta which was selective to primarily C5 and C7. Co-feeding larger olefins (2-methyl-2-butene, 2,3-dimethyl-2-butene, 2,3,3-trimethyl-1-butene, and 2,4,4-trimethyl-2-pentene) at a 3.9% olefin concentration over Fe-beta changed selectivity towards cracking products (C4 compounds such as isobutene). As the size of the olefin increases, a reduction of DME consumption rate is also observed. These results show that co-feeding olefins with DME over Fe-zeolites is a promising route to increase methylation rates at relatively low temperatures producing larger branched olefins and that the product distribution is highly dependent on the zeolite pore size and structure of the olefin.
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Challenges and Opportunities of Fe‐based Core‐Shell Catalysts for Fischer‐Tropsch Synthesis
Abstract Fe‐based catalysts are an active, selective, and low‐cost option for tuning Fischer‐Tropsch synthesis (FTS) selectivity toward desirable light olefins. By encapsulating Fe within ZSM‐5, the resultant core‐shell catalysts have the potential to control the product distribution via secondary reactions that occur over the acid sites of the zeolite shell. In this paper, Fe is encapsulated within ZSM‐5 via the seed‐directed growth technique and characterized with a suite of analytical techniques including Mössbauer spectroscopy and X‐ray absorption fine structure (XAFS). Characterization of the core‐shell catalysts indicates that some of the Fe‐based active phase is destabilized during seed‐directed growth, demonstrating the challenges associated with encapsulating an Fe‐based active phase within zeolites. However, comparing FTS performance of the core‐shell catalyst with the Fe‐based control synthesized via incipient wetness impregnation demonstrates improved selectivity toward the desired C2‐C4olefins and C5+hydrocarbons.
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- Award ID(s):
- 1954480
- PAR ID:
- 10373405
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- ChemCatChem
- Volume:
- 14
- Issue:
- 19
- ISSN:
- 1867-3880
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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