One-Step Synthesis of Dimethyl Ether from Syngas with Fe-Modified Zeolite ZSM-5 as Dehydration Catalyst
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Catalysis Letters Vol. 98, No. 4, December 2004 (Ó 2004)
One-step synthesis of dimethyl ether from syngas with Fe-modified zeolite ZSM-5 as dehydration catalyst Jianchao Xia1,2, Dongsen Mao2,*, Bin Zhang2, Qingling Chen2 and Yi Tang1 1
Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433, P. R. China 2 Shanghai Research Institute of Petrochemical Technology, SINOPEC, Shanghai 201208, P. R. China
Received 20 July 2004; accepted 09 September 2004
A number of Fe-containing ZSM-5 zeolites, such as HFeZSM-5 and HFeAlZSM-5 prepared by hydrothermal synthesis and Fe-modified ZSM-5 through solid-state ion-exchange, were adopted as methanol dehydration catalysts for syngas to dimethyl ether (STD) process. Their structures, acidic and basic properties were characterized by XRD, ESR, ICP-AES, TPD and FT-IR. Among these Fe-containing zeolites, the Fe-modified ZSM-5 displayed the highest dimethyl ether selectivity, least CO2 production. Some correlations between catalytic performance and acidity and basicity of Fe-containing ZSM-5 zeolite were discussed. KEY WORDS: Fe-containing ZSM-5 zeolites, methanol, dehydration, dimethyl ether, syngas, acidity, basicity, modification.
1. Introduction Dimethyl ether (DME) has received a world-wide attention as a clean alternative fuel for diesel engines since it was disclosed to have better combustion performance than the conventional diesel fuel, e.g. lower NOx emission, lesser smoke and engine noise [1]. Recent analyses have also revealed the approach to apply DME as an economical alternative to LPG. The conventional process of DME production, called two-step method, has become more and more insufficient with the growing demand for DME as fuels for these new applications. Lately, an original technique named STD (synthesis gas to dimethyl ether) process was developed for the direct synthesis of DME from synthesis gas in a single reactor on hybrid catalysts composed of copper-based methanol synthesis catalysts and solid acids [2, 3]. The key steps in the STD process are methanol synthesis, methanol dehydration and the water gas shift reaction (WGSR): 2CO þ 4H2 ¼ 2CH3 OH
DH ¼ 180:58 kJ/mol
2CH3 OH ¼ CH3 OCH3 þH2 O
ð1Þ DH ¼ 23:41 kJ/mol ð2Þ
H2 O þ CO ¼ H2 þ CO2
DH ¼ 40:96 kJ/mol ð3Þ
The combination of these reactions results in a synergistic effect relieving the unfavourable thermo* To whom correspondence should be addressed. E-mail: [email protected]; [email protected]
dynamics for methanol synthesis: methanol, product in the first step, is consumed for reaction to dimethyl ether and water. The water is shifted by the WGSR reaction (3) forming carbon dioxide and hydrogen, the latter being a reactant for the methanol synthesis. Thus, one of the products of each step is a reactant for another. This creates a strong driving force for the overall reaction allowing very high syngas conversion in one single pass. The most common hybrid catalysts reported in the literature for STD process are the physical m
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