Base-acid relay catalytic upgrading of coal pyrolysis volatiles over CaO and HZSM-5 catalysts

摘要

The relay catalytic upgrading of coal pyrolysis volatiles employing different layouts of basic CaO and acidic HZSM-5 zeolite was proposed to improve the tar quality as well as alleviate catalyst deactivation. The synergistic effect is found in relay catalytic upgrading of coal pyrolysis volatiles over dual CaO and HZSM-5 catalysts system. The relay CaO-HZSM-5 layout is better compared to layouts of relay HZSM-5-CaO, mixed CaO/HZSM-5, and their independent employments. The relay CaO-HZSM-5 layout ensures the highest light fraction in tar with 70.1 and the minimized carbon deposition over HZSM-5 while remaining a relatively high tar yield (7.8 wt ). Meanwhile, the contents of aromatics and phenols in tar increase by 221 and 164 compared with the blank test (without catalyst), respectively. In this scenario, CaO promotes the cracking of large molecules in tar to form more olefins and small molecules firstly, which subsequently diffuses into micropores of HZSM-5 zeolite to produce more aromatics by dehydrocyclization. In addition, the roles of pre-cracking and pre-deoxidation of volatiles over CaO reduce the possibility of the polycondensation reactions of large molecules over acidity sites of HZSM-5 zeolite, thus lowering the carbon deposition over zeolite. These findings suggest that the base-acid relay catalytic upgrading of volatiles during coal pyrolysis will be a promising strategy to increase the tar quality and slow down catalyst deactivation.