磷改性HZSM-5分子筛的制备及其在合成气一步法制二甲醚反应中的应用

 以磷酸为前驱体,采用浸渍法制备了一系列不同P2O5含量(分别为5%, 10%和15%)的磷改性HZSM-5分子筛(P2O5/HZSM-5), 并采用氨程序升温脱附和吡啶吸附红外光谱法研究了其酸性. 结果表明,随着P2O5含量的增大, P2O5/HZSM-5样品的酸量尤其是强Brnsted酸位的酸量逐渐减少. 以上述P2O5/HZSM-5分子筛为甲醇脱水催化剂与工业甲醇合成催化剂(Cu-ZnO-Al2O3)组成双功能催化剂,在连续流动加压固定床反应器上考察了其对合成气一步法制二甲醚反应的催化性能. 结果表明,以适量磷(10%P2O5)改性的HZSM-5分子筛作为甲醇脱水催化剂,可使产物中二氧化碳和烃类副产物的选择性分别由改性前的43.3%和3.0%下降至低于33%和0.1%, 从而使二甲醚的选择性由49.7%提高到63%以上. 当P2O5/HZSM-5中的磷含量(15%P2O5)过高时,产物中甲醇的选择性高达30%以上,表明反应所生成的中间产物甲醇无法有效地转化,从而使二甲醚的选择性和一氧化碳的转化率均大大降低.

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.