Methane-olefin coupling using supported-nickel and solid superacid catalysts.

Abstract

In this thesis, we have investigated the coupling of methane with olefins, such as ethylene and propylene using silica-supported nickel and sulphate-promoted zirconia-silica catalysts. Control experiments over hydrogen-reduced Ni/SiO$\sb2$ in which propylene was co-fed with helium rather than with methane resulted in increased propylene conversions and in an increase of the yields of propane, C$\sb2$'s and C$\sb4$'s. It was concluded that the production of C$\sb4$'s was due to homologation or metathesis reactions, rather than to methane coupling. Variation of the temperature of the methane preactivation treatment from 350$\sp\circ$C to 600$\sp\circ$C revealed an increased CH$\sb4$ conversion from approximately 1% at 350$\sp\circ$C to 60% at 600$\sp\circ$C. Subsequent reaction of $\rm C\sb3H\sb6/CH\sb4$ mixtures over these catalysts at 350$\sp\circ$C revealed that there was virtually no activity over the former catalyst and very little over the latter. However, catalysts activated between 400$\sp\circ$C and 500$\sp\circ$C showed very similar activities. Experiments in which helium was co-fed with propylene rather than methane over CH$\sb4$-preactivated catalysts suggested that the same processes were responsible for C$\sb4$ production over CH$\sb4$-reduced NiO/SiO$\sb2$ and hydrogen-reduced NiO/SiO$\sb2$ i.e. propylene homologation and/or metathesis. Dissociatively adsorbed methane only served to block metal sites at which reactions could otherwise occur. Zirconia-silica, like many binary oxide mixtures, possesses enhanced surface acidity relative to its parent components. TPD experiments using NH$\sb3$ found that samples of 5, 10, and 20 wt.% ZrO$\sb2$/SiO$\sb2$, previously activated by evacuation at 450$\sp\circ$C, contained a similar amount of surface acid sites, 260 $\mu$mole/g sample. XRD analysis revealed no peaks due to crystalline phases following calcination at 450$\sp\circ$C, suggesting that these materials were amorphous, even with zirconia loadings as high as 20 wt.%. Infrared analysis of 10 wt.% ZrO$\sb2$/SiO$\sb2$ with sulphate loadings ranging from 100 to 1000 $\mu$mole/g revealed that the nature of the surface sulphate species was dependent on the temperature of activation. IR analysis using NH$\sb4$ as a probe revealed that both Lewis and Bronsted acid sites existed after 500$\sp\circ$C activation. The amount of each type of acid site appeared to increase with increased sulphate loading. The acidity of 10 wt.% ZrO$\sb2$/SiO$\sb2$ with various loadings of sulphate was measured by gravimetric ammonia adsorption following evacuation at various temperatures. The amounts of irreversibly adsorbed ammonia were found to increase with increased sulphate loading at 25, 100, and 200$\sp\circ$C. Attempts to measure the amount of irreversible adsorption at temperatures greater than 200$\sp\circ$C were hampered by weight loss due to sulphate. Thus, precise measurement of the number of acid sites at high temperature was not possible. Sulphate-promoted ZrO$\sb2$/SiO$\sb2$, previously activated by evacuation at 500$\sp\circ$C, was tested as a catalyst for the coupling of methane and ethylene (or propylene) in a fixed-bed reactor at atmospheric pressure and at temperatures ranging from 225 to 275$\sp\circ$C. All experiments were hampered by rapid deactivation of the catalyst (within 35 minutes on stream) due to oligomerization. Only minute quantities of propane were detected. Reactions of methane alone showed that it also interacted strongly with the catalyst, leading to deactivation. (Abstract shortened by UMI.)