TiO₂ Modification Of Mn-W/SiO₂ Catalysts For The Oxidative Coupling Of Methane

It has been considered that nature gas is clean energy source and oxidative coupling of methane to ethylene (OCM) is one of the most effective approaches to methane utilization. It is a promising way to provide ethene, in case of crude petroleum exhausting. Ethene, the fundamental raw material of almost every chemicals, is mainly made from petrochemical industry. The OCM shows substantial potential to break the situation that petroleum is the life line of chemical industry. Up to date, a variety of catalysts for OCM reaction have been explored, but the activity and selectivity is not satisfied yet. Summation of the conversion of CH4 and the selectivity of C2 can hardly exceed over 100%, which limits its industrialization.In this thesis work, modification of the promising Mn-W/SiO2 catalysts via adding TiO2 promotor, signed as Mn-W/SiO2-TiO2, has been investigated for the OCM process. It is found that Mn-W/SiO2-TiO2 catalysts obtained by using titanosilicate zeolites as supports and TiO2 suppliers provided visible promotion on the low-temperature activity and selectivity for the OCM reaction. Besides this advance, such catalysts also exhibit good stability.The effects of catalyst preparation parameters (such as support, calcination temperature and active component content) have been investigated systematically on the catalytic performance of the 1.9%Mn5.0%Na2WO4/SiO2-TiO2 catalysts for the OCM reaction as well as the reaction conditions (such as reaction temperature, gas hourly space velocity (GHSV) and CH4/O2 ratio in the feedgas). Over the 1.9%Mn5.0%Na2WO4/SiO2-TiO2 catalyst, high C2-C3 product selectivity of 72-80% could be achieved at high methane conversion of>30% using a feed of CH4/O2/N2 (3.2/ 1/3.7) and a GHSV of 10,000-35,000 ml-h-1·g-1 (based on CH4 and O2) at reaction temperatures ranged from 750 to 800 ℃. Accordingly, high space time yield of 140 mol·kg-1·h-1 or more could be obtained.Furthermore, the 1.9%Mn5.0%Na2WO4/SiO2-TiO2 catalysts were characterized by means of XPS, XRD and UV-Vis techniques and promotion effect of the TiO2 additives was tentatively discussed.