Catalytic Conversion of Ethylene to Propylene and Butenes over H-ZSM-5 Baomin Lin, Qinghong Zhang, and Ye Wang* State Key Laboratory of Physical Chemistry of Solid Surfaces, National Engineering Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen UniVersity, Xiamen 361005, People’s Republic of China Among 11 kinds of molecular sieves examined, H-ZSM-5 exhibited the highest activity for the direct conversion of ethylene to propylene. The conversion of ethylene was 58% and the selectivities to propylene and butenes were 42% and 21%, respectively, over H-ZSM-5 at 723 K. Our studies demonstrate that the conversion of ethylene increases with the degree of H + exchange in the H-Na-ZSM-5 series of samples with different H + exchange degrees and the Al content in the H-ZSM-5 samples with different Si/Al ratios. The strong Brønsted acid sites are proposed to account for the conversion of ethylene. The modification of H-ZSM-5 with phosphorus or boron could enhance the selectivity of propylene but decreased the conversion of ethylene due to the decreased acidity. In situ FT-IR studies confirm the reaction of ethylene molecules with the Brønsted acid sites associated with -Si-(OH)-Al- groups. In situ FT-IR results further suggest the occurrence of oligomerization of ethylene on the surface of H-ZSM-5. We speculate that the cracking of the oligomeric intermediates may lead to the formation of propylene. 1. Introduction Propylene is one of the most important chemicals for the production of polypropylene, acrylonitrile, and propylene oxide. In the current chemical industry, propylene is mainly produced as a coproduct of ethylene via steam cracking of naphtha. The ratio of propylene to ethylene can vary from 0.4:1 to 0.75:1 depending on cracking conditions. However, the demand for propylene is growing much faster than that for ethylene because of higher needs for propylene derivatives such as polypropylene and propylene oxide. Therefore, the development of other routes for propylene production has become urgent in recent years. Several strategies such as the dehydrogenation of propane and the metathesis of ethylene and butenes have been proposed for propylene production. 1,2 The dehydrogenation of propane is an energy-intensive process and the capital cost of this process is relatively high. The metathesis process requires 2-butene and ethylene in equal quantities. Lunsford et al. 3 once reported a conversion of methane and ethylene to propylene, but high temperatures (>873 K) were required and the activity was also not sufficient. The direct conversion of ethylene to propylene would be a more desirable route. Moreover, ethylene may also be obtained readily from resources other than crude oil such as coal and natural gas via the syngas to methanol and methanol to olefin processes, 4 and biomass via bioethanol. 5 Thus, the development of a process for the direct conversion of ethylene to propylene would also be helpful for the utilization of nonpetroleum resources for propylene production. The phenomenon of propylene formation from ethylene was observed in a few early papers on olefin metathesis catalysis over supported molybdenum or tungsten catalysts, but the activity was quite low. 6,7 Recently, three types of catalysts have been reported for the transformation of ethylene into propylene with higher efficiency. Iwamoto and co-workers 8-10 found that Ni ions loaded on MCM-41 prepared by a template ion exchange method catalyzed the conversion of ethylene to propylene in the presence of water vapor. At 673 K, the Ni-MCM-41 (Si/ Ni ) 43) catalyst could afford an ethylene conversion of 55% with selectivities to propylene and butenes of 54% and 35%, respectively. 8,10 The main problems of this catalyst are the low reactant feed rate (11 cm 3 min -1 at a catalyst weight of 0.5 g) and the requirement of the presence of steam. Because MCM- 41 is not adequately stable in the presence of steam, 11 the long- term stability of Ni-MCM-41 is questionable. Basset and co- workers 12 reported a direct transformation of ethylene to propylene over a tungsten hydride supported on γ-alumina, i.e., W(H) 3 /γ-Al 2 O 3 , which was an efficient metathesis catalyst. 13 Ethylene conversion at the initial stage was ∼40% and it decreased to ∼7% after ∼120 h of reaction, while propylene selectivity increased rapidly up to 95% and was then kept almost unchanged. Oikawa et al. 14 disclosed that a silicoaluminophos- phate microporous molecular sieve, SAPO-34, catalyzed the propylene formation from ethylene efficiently. Propylene with a selectivity of 73% was attained at an ethylene conversion of 71% at 723 K. Liu et al. 15 confirmed that SAPO-34 was effective for the conversion of ethylene to propylene, but only obtained lower performances under similar reaction conditions, e.g., propylene selectivity of 64% at ethylene conversion of ∼40% at 723 K. Furthermore, Liu et al. 15 also found a rapid deactivation of SAPO-34 with prolonging time on stream. The conversion of ethylene over the Ni-MCM-41 or the W(H) 3 / γ-Al 2 O 3 catalysts was believed to proceed over Ni or W centers via a metathesis mechanism, i.e., the dimerization of ethylene to butenes in the first step and then the metathesis of butenes and ethylene to form propylene in the second step. 8-10,12 On the other hand, in the case of SAPO-34, the acidity and the porous structure were proposed to be key controlling factors. However, insights into the effect of acidity and the active sites are still scarce. Moreover, studies on other types of molecular sieves for the conversion of ethylene to propylene will also be helpful for further understanding this reaction. In the present paper, we first examine the catalytic perfor- mances of several types of molecular sieves for the conversion of ethylene to propylene. Then, we focus our studies on H-ZSM-5 catalyst, which demonstrates the highest activity, with an aim to clarify the effects of acidity on the transformation of ethylene. In situ Fourier transform infrared (FT-IR) spectro- * To whom correspondence should be addressed. Tel.: +86-592- 2186156. Fax: + 86-592-2183047. E-mail: wangye@xmu.edu.cn. Ind. Eng. Chem. Res. 2009, 48, 10788–10795 10788 10.1021/ie901227p CCC: $40.75  2009 American Chemical Society Published on Web 10/16/2009