Preparation of Phosphorus-Modified HZSM-5 Zeolite and Its Application in the One-Step Synthesis of Dimethyl Ether from Syngas

A series of HZSM-5 zeolite samples modified with various contents of phosphorus were prepared by impregnation with an aqueous solution of phosphoric acid and characterized by N_2-adsorption, X-ray diffraction, ~ 27 Al magic angle spinning nuclear magnetic resonance spectroscopy, temperature-programmed desorption of NH_3, and pyridine adsorption Fourier-transform infrared reflectance spectroscopy. The one-step synthesis of dimethyl ether (DME) from syngas was carried out over the admixed catalysts of the industrial methanol synthesis catalyst (Cu-ZnO-Al_2O_3) and the prepared HZSM-5 zeolites under pressurized fixed-bed continuous flow conditions. The modification of HZSM-5 with a suitable amount of phosphorus (10%P_2O_5) significantly enhanced the selectivity for DME from 49.7% to higher than 63%, while the selectivity for the undesired by products of hydrocarbons and CO_2 decreased from 3% and 43.3% to less than 0.1% and 33%, respectively. The decrease in the formation of hydrocarbons and CO_2 can be attributed to the significant decline in the number of strong acid sites induced by the phosphorus modification. However, when the phosphorus content was too high (15%P_2O_5), both the conversion of CO and the selectivity for DME decreased remarkably because the produced methanol could not be efficiently converted to DME on account of the significant decline in the number of acid sites and surface area resulted from the phosphorus modification.