Ind. Eng. Chem. Res. 1999, 38, 1323-1334
1323
Selective Production of C4 Hydrocarbons from Syngas in a Dual Reactor Using Co-Ni/ZrO2 and SO42-/ZrO2 Catalysts Rajashekharam V. Malyala, Narendra N. Bakhshi, and Ajay K. Dalai* Catalysis and Chemical Reaction Engineering Laboratory, Department of Chemical Engineering, University of Saskatchewan, Saskatoon, SK S7N 5C9 Canada
The product stream from the Fischer-Tropsch (FT) reaction using a Co-Ni/ZrO2 catalyst was modified by SO42-/ZrO2 catalyst (solid acid catalyst) in a dual fixed-bed microreaction system. While the operating conditions for the first reactor containing the FT catalyst was fixed at 523 K and 1 atm, the effects of operating variables such as the second reactor temperature, the Co-Ni/ZrO2:SO42-/ZrO2 catalyst ratio and weight hourly space velocity (WHSV) on the steadystate product distribution were investigated. In this process, high selectivity (33 wt %) toward the total C4 hydrocarbons was achieved when the second reactor was operated at 423 K, with a catalyst ratio of 1:1.5 and at a WHSV of 15 h-1. This selectivity is much higher compared to a maximum selectivity of 14 wt % obtained using a single reactor with a Co-Ni/ZrO2 catalyst. Catalyst characterization using temperature-programmed desorption of NH3 and 1H MAS NMR showed that this high selectivity toward total C4 hydrocarbons was due to the strong acidity of the SO42-/ZrO2 catalyst. However, this catalyst deactivated rapidly which was due to coke deposition on the SO42-/ZrO2 catalyst. Under these experimental conditions, external and intraparticle mass transfer effects were found to be negligible. On the basis of the product distribution in a dual-reaction system, a reaction pathway has been proposed. Introduction On account of stringent environmental regulations on emission control on the automobile exhaust gases, there is considerable interest in the petroleum industry to produce reformulated gasoline. Isooctane is an important constituent in reformulated gasoline as highly branched C8 paraffins used to boost the octane ratings, mandated by the U.S. Clean Air Act of 1990.1,2 Alkylates such as isooctanes are produced from C4 hydrocarbons such as isobutane and isobutylene. Another important application of C4 hydrocarbons (especially isobutylene) is in the manufacture of methyl tert-butyl ether (MTBE) and ethyl tert-butyl ether (ETBE). Both MTBE and ETBE are produced by reacting isobutylene with methanol and ethanol, respectively, and find extensive applications as oxygenated additives in producing reformulated gasoline. In addition, n-butane is used in the manufacture of maleic anhydride which is further hydrogenated to produce chemicals such as γ-butyrolactone (γ-BL), tetrahydrofuran (THF), and 1,4-butanediol (1,4-BD). All these chemicals are extensively used for the synthesis of fine chemicals, plasticizers, specialty products, and pharmaceutical intermediates. Traditionally, C4 hydrocarbons are obtained from petroleum sources such as natural gas and the steam cracking of naphtha and gas oil. These hydrocarbons can also be produced via the Fischer-Tropsch (FT) synthesis process. The FT process converts syngas (a mixture of CO and H2) to a wide range of hydrocarbons and aromatics in the presence of a suitable catalyst. A variety of catalysts based on Ru, Ni, and Co have been used for FT synthesis and extensively reviewed.3-6 Since the product stream consists of a mixture of hydrocarbons, expensive separation and/or upgrading schemes * To whom correspondence should be addressed. Tel.: (306)966-4771. Fax: (306)-966-4777. E-mail:
[email protected].
to improve the selectivity of a particular range of hydrocarbons becomes essential. Considering the present demand for the production of C4 hydrocarbons, a modified FT process using a typical FT catalyst and a solid acid catalyst in various modes of operation (mixed or follow bed in the same reactor or in a dual-reaction system) could be used to enhance the selectivity of C4 hydrocarbons. The solid acid catalysts are known to have strong isomerization, alkylation, and cracking functionalities. Several studies based on the modified FT process using solid acid catalysts such as (i) the production of highoctane gasoline, (ii) production of olefins, and (iii) production of aromatics in the gasoline range have been reported in the literature.7-12 Most of these investigations deal with a FT catalyst and its physical mixture with a solid acid catalyst such as sulfated zirconia, H-ZSM5, and so forth, which produces the desired hydrocarbon selectivity. Recently, Song and Sayari13,14 have studied in detail the selective production of C4 hydrocarbons from syngas using a physical mixture of RuKY and SO42-/ZrO2 catalysts at 523 K and atmospheric pressure. They observed that the performance of SO42-/ZrO2 catalysts was better when the reaction was carried out in mixed mode (a physical mixture of both the catalysts) compared to dual-bed mode. They proposed a reaction mechanism based on the observed product distribution and also recommended the use of a Pt-modified SO42-/ZrO2 catalyst to enhance the catalyst stability. However, it is known that SO42-/ZrO2 catalysts can conveniently carry out isomerization and alkylation reactions at lower temperatures (
