Sol–gel preparation of NiO/ZrO2(x)–MgO(100−x) nanocatalyst used in CO2/O2 oxidative dehydrogenation of ethane to ethylene: influence of Mg/Zr ratio on catalytic performance

Abstract

A series of NiO/MgO–ZrO2 mixed oxide catalysts were prepared using the sol–gel method in which the content of MgO and ZrO2 was varied and the NiO amount was maintained around 5 wt%. The synthesized samples were characterized by XRD, FESEM, PSD, TEM, EDX dot-mapping, FTIR and the BET techniques and tested for the dehydrogenation of ethane in the presence of CO2 with limited amount of O2. Irrespective of the various Mg/Zr ratios, the uniform dispersion of NiO as a common feature of all synthesized samples was evident from the XRD and EDX results. As indicated by XRD and FESEM analyses, a decrease in the size of nanocrystallites and particles and also an increase in the specific surface area were found when MgO phase is present in the sample. The catalytic evaluations revealed that the existence of basic MgO leads to more efficient performance of zirconia especially at higher temperatures. Hence, using both ZrO2 and MgO increased the ethane conversion and the ethylene yield. This could be attributed to the effect of high reducibility and low surface acidity. It was observed that among the synthesized catalysts, NiO/ZrO2(75)–MgO(25) was the most active catalyst with an ethane conversion of 66.35 %. However, NiO/ZrO2(25)–MgO(75) exhibited the best catalytic performance in the oxidative dehydrogenation of ethane with an ethylene yield of 56.9 % at 650 °C. The smaller particle and crystallite size, the narrower particle size distribution, uniform dispersion of the active species and higher surface area with efficient base–acid characteristics seem to be responsible for the superior performance of NiO/ZrO2(25)–MgO(75) catalyst. CO2/O2 oxidative dehydrogenation of ethane was investigated over a series of sol–gel synthesized Ni/ZrO2–MgO catalysts differing in Mg/Zr ratio. The presence of MgO alongside zirconia contributed to obtain better textural properties and satisfying ethylene yield. The variation of Mg/Zr ratio indicates that there is optimum MgO content in Ni/ZrO2(25)–MgO(75) for the best catalytic activity. It is found that Ni/ZrO2(25)–MgO(75) nanocatalyst, with weight ratio of MgO/ZrO2:3/1, effectively dehydrogenated ethane to ethylene in the presence of CO2/O2 at 650 °C, giving 56.9 % ethylene yield. This is due to higher surface area, smaller particles and narrow distribution of particle size.