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A study of the catalytic reaction of olefins with methane Liu, Qingdong


The catalytic reaction of CH₄ with C₃H₆, referred to as CH₄/C₃H₆ homologation or coupling, was investigated in an attempt to selectively produce C₄ hydrocarbons. The CH₄/C₃H₆ coupling reaction was conducted over various Ni catalysts at temperatures in the range 300 °C to 375 °C and 101 kPa. The effects of process variables, catalyst promoters and different supports on the CH₄/C₃H₆ coupling reaction were examined. In addition, the role of the carbonaceous species deposited on the catalyst surface during catalyst reduction in CH₄ and during the CH₄/C₃H₆ coupling reaction, was investigated. In preliminary experiments, Ni catalysts supported on AI₂O₃ were modified by K and P, and used for the CH₄/C₃H₆ coupling reaction. The calcined catalysts were reduced in CH₄ at 600 °C for 1 hour before reaction. Compared with Ni/Al₂0₃ catalyst, the Ni/K/Al₂0₃ and Ni/P/Al₂0₃ catalysts showed higher C₃H₆ conversion and C₄ selectivity. The maximum selectivity of 10.0 mole % to the desired C₄ product was achieved over the Ni/K/Al₂0₃ catalyst at 350 °C and 101 kPa with a feed gas composition of 90 mol % CH₄/IO mol % C₃H₆. The C₃H₆ conversion decreased significantly with time-on-stream. During catalyst reduction in CH₄, decomposition of CH₄ occurred and a large amount of carbonaceous species deposited on the catalyst surface. Subsequently, during the CH₄/C₃H₆ coupling reaction, additional carbonaceous species were deposited on the catalyst surface. The presence of two types of carbonaceous species were identified using temperature programmed surface reaction (TPSR) in H₂ . The first type of carbonaceous species was relatively active, and reacted with H₂ at low temperature, in the range 192 °C to 237 °C. A second type of carbonaceous species was relatively inactive, and reacted with H₂ at high temperature, in the range 547 °C to 667 °C. The amount of the low temperature carbonaceous species present on the catalyst surface after the CH₄/C₃H₆ coupling reaction, was shown to correlate with the C₄ yield. The significance of the treatment of catalyst in CH₄ before reaction was shown to be less important than what was claimed in previous studies. The CH₄ reduced catalyst surface had higher C₃H₆ conversion than H₂ reduced catalysts but the C₄ selectivity was unchanged. A model of the carbonaceous species deposition process on the catalyst surface was proposed. The effect of supports on the CH₂/C₃H₆ coupling reaction was also investigated. Different reaction paths were proposed. The major reactions over non-zeolite supported Ni catalysts were C₃H₆ decomposition and C₃H₅ hydrogenation with a low activity for homologation. The C₃H₅ conversion was high and C₃H₆ metathesis and olefin dimerization were the dominant reactions over the Na-Y supported Ni catalyst. The high activity of the Ni/Na-Y catalyst was a consequence of the acidic property of the Na-Y support. After the acidic property of the Ni/Na-Y catalyst was neutralized, the catalyst activity decreased dramatically and evidence for the C₃H₆ homologation was apparent. Results of the present study have shown that C₄ selectivity, particularly 1-butene, was formed by the homologation reaction: CHX + C₃H₆ -> 1-C₄H₈, where CHX is a surface carbon species generated either from catalyst reduction in CH₄ or from C₃H₆ decomposition reactions that occur during reaction. Most importantly, the present study has shown that gas phase CH₄ does not homologate C₃H₆ directly at the conditions studied. The maximum C₄ selectivity of 10 %, obtained using Ni/K/Al₂0₃ catalyst operated at 350 °C and 1O1kPa, was also very low which together with the fact that direct CH₄/C₃H₆ homologation did not occur, point to the need for a multi-step reaction if this approach is to be utilized for direct CH₄ upgrading.

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