Application of modified CNTs with Ti(SO4)2 in selective oxidation of dimethyl ether

Abstract

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)₂ on 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 CO_x production 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)₂ not only increased the amount of weak acid sites, but also enhanced the redox properties of CNTs to activate O₂ over the defect sites of CNTs. The formation pathway of DMM₂ from DME oxidation was proposed and dimethoxymethane (DMM) could be the intermediate for DME oxidation to DMM₂. The present CNTs modified by Ti(SO₄)₂ with 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.