Application of modified CNTs with Ti(SO4)2in selective oxidation of dimethyl etherElectronic supplementary information (ESI) available. See DOI: 10.1039/c6cy01367h

A new and efficient Ti(SO4)2-modified carbon nano-tube (CNTs) catalyst was designed and prepared for the selective oxidation of dimethyl ether (DME). The modification effects of Ti(SO4)2on the catalytic performance of CNTs were examined in a continuous flow-type fixed-bed reactor. The influences of SO42−content, calcination temperature and different feedstocks on DME selective oxidation were further investigated. It is found that dimethoxy dimethyl ether (DMM2) selectivity in the product was significantly improved up to 89.4% from 6.1% when 40% SO42−was used to modify the CNTs, and more importantly, there was no COxproduction in the reaction. The SO42−content and calcination temperature obviously affected the distribution and existing form of sulfate species and further influenced the acid and redox functions of the Ti(SO4)2/CNTs catalyst. The 40% SO42−/CNTs catalyst calcined at 240 °C exhibited the highest selectivity to DMM2. The structure, surface properties and morphology of the catalysts were deeply characterized by XRD, NH3-TPD, BET, XPS, H2-TPR-MS, TEM, TEM-EDS, O2-TPD-MS and DME-TPSR-MS. The modification of Ti(SO4)2not only increased the amount of weak acid sites, but also enhanced the redox properties of CNTs to activate O2over the defect sites of CNTs. The formation pathway of DMM2from DME oxidation was proposed and dimethoxymethane (DMM) could be the intermediate for DME oxidation to DMM2. The present CNTs modified by Ti(SO4)2with excellent catalytic performance can replace noble metal oxide catalysts and may be potential alternatives to conventional catalysts for DME selective oxidation to diesel oxygenate additives.