Study on NiPMo/TiO2 Catalyst for Direct Synthesis MAA from Propylene and Carbon Dioxide

The NiPMo/TiO2catalystswere prepared fromhydrolysis tetrabutyl titanate by aqueous solutions ofNi salts of the 12-molybdophosphoric acid via the sol-gel method. Chemical composition, thermal stability, chemisorption character and reaction behaviours have been studied byvarious techniques, such as, X-ray diffraction, thermogravimetric analysis, infrared spectrum analysis, inductively coupled plasma atomic emission spectroscopy, temperature-programmed desorption, and microreactor. The experimental results showed that inorganic precursor polyoxomolybdate was introduced to the surface of titanium dioxide and formed the NiPMo/TiO2hybrid materials. Therefore, a coordination structure of the NiPMo/Ti catalysts was proposed. At the temperature of 623 K, the catalyst still remained the original Keggin structure of polyoxomolybdate. The chemisorbed IR showed that CO2chemisorbs at bridging oxygen of Ni-O-Mo(Lewis base site) and Ni2+(Lewis acid site) forming bridged absorption states Ni(Ⅱ)←O-(CO)←(O--Ni), and propylene chemisorbs on the surface of catalystwith three absorption states. Underthe reaction conditions of 563 Kand 1MPa, the main reaction products of CO2and propylene on the surface of catalysts were MAA, the conversion of propylene was 3.2%, and the selectivity ofMAAwas 95%