Coordination unsaturated structure of titanium sulfate promoting the carbon chain growth for dimethyl ether oxidation

Abstract

Polyoxymethylene dimethyl ethers (DMM_x, CH₃O(CH₂O)_xCH₃, x = 1–8) are appealing oxygenated chemicals with potential applications mainly used as sustainable fuels and solvents. The growth of longer C-O chain for direct oxidation of dimethyl ether (DME) to DMM_x is especially important, but the construction of the acid sites with different strengths and matched redox sites faces great difficulties. Herein, the gradient acidity and enhanced oxidizability have been successfully created by regulating coordination structure of titanium sulfate (Ti(SO₄)_x). The formation of a higher proportion of weak-medium acid sites and appropriate redox sites over a Ti(SO₄)_x/activated carbon (AC) catalyst with nitrogen treatment significantly promotes the synthesis of larger molecules, with DMM_2–3 selectivity up to 42.2%. Combined with the characterizations, designed experiments and theoretical calculations, the configurations of Ti(SO₄)_x and their local chemical environment are revealed. The different distortions of Ti(SO₄)_x structure in nitrogen atmosphere result in the various coordination unsaturated Ti atoms located at the center of polyhedral, which is the main reason for the gradient acidity distribution and enhanced oxidative property.