分级分子筛优化木质纤维素与聚丙烯共催化热解及其协同机制研究

Catalytic fast pyrolysis (CFP) is a promising biorefinery technology that can convert lignocellulosic biomass directly to valuable petrochemicals. However, because lignocellulosic biomass contains high contents of oxygen but insufficient hydrogen, it also produces large amounts of coke in CFP. The coke deposits would result in rapid catalyst deactivation, which has severely limited the development of CFP technology. To minimize the coke formation, we have proposed co-CFP of biomass with hydrogen-rich plastic wastes. The results suggested that co-CFP of biomass with polypropylene could decrease the coke formation and increase the yields of valuable chemicals, which may offer a viable way to overcome the main barrier to CFP of biomass. Our preliminary study showed that in the presence of conventional micropore ZSM-5, co-feeding polypropylene with biomass in CFP have a less synergy than co-feeding polypropylene with biomass. This may because the steric hindrance and electronic effects of branched polypropylene in co-CFP. Polyethylene and polypropylene are both main components of plastic wastes. Thus, co-CFP of biomass and polypropylene synergy optimization is quite essential to further application of co-CFP of biomass and plastic wastes. In this light, this study planned to adjust pore structure and acid sites of convention micropore ZSM-5 by post-synthetic alkaline desilication modification. This hierarchical ZSM-5 zeolite with mesopores may optimize the synergy in co-CFP of polypropylene and biomass. Based on the catalytic performance of hierarchical ZSM-5 zeolite and our preliminary study, our main objective is to investigate the fundamental mechanisms of the synergy between biomass and plastic. This work will advance the development of biorefinery technology significantly, and may provide a promising way to valorize solid wastes such as municipal wastes, which contain considerable amounts of biomass and plastic wastes.

木质纤维素生物质催化快速热解制备高附加值化学品是一项极具潜力的生物炼制新技术。但是木质纤维素生物质氧含量过高、氢含量不足，催化热解时易积碳引起催化剂失活的问题严重制约该技术发展。我们在前期研究中提出富含氢的废弃塑料与生物质共催化热解的思路，发现使用常规ZSM-5分子筛催化剂时，聚乙烯和木质纤维素生物质共催化热解具有抑制积碳、提高高附加值化学品产率的协同效果；虽然聚丙烯和聚乙烯结构和性质相似，但是聚丙烯在共催化热解时协同效果不明显。聚乙烯和聚丙烯均为废弃塑料的主要组分，优化聚丙烯的协同作用对于生物质和废弃塑料共催化热解的应用十分必要。本课题拟通过碱处理脱硅技术调整ZSM-5的孔道和酸性分布，优化生物质与聚丙烯的协同作用，结合前期研究揭示生物质和塑料共催化热解协同作用的主要机制。研究结果将推动生物冶炼技术的发展，为生物质和塑料废弃物的资源化提供一条极具潜力的技术路线。

使用富含氢的废塑料和含有纤维素的木质纤维素生物质共同催化热解，能够有效优化产物分布，解决木质纤维素生物质单独催化热解易积碳、高附加值芳烃产率低的问题。然而，聚丙烯（PP）和PE两者结构相似，氢含量一致，在共催化热解时未表现出与PE相似的协同作用。本研究合成了含有框架镓的硅镓铝分子筛GaMFI，并使用碱处理的方法获得了具有介孔结构的分级分子筛DS-ZSM-5和DS-GaMFI，改善了生物质和PP共催化热解时PP含有支链裂解产物的扩散问题，催化剂中氧化镓类物质进一步将低附加值的烷烃通过脱氢、芳构化转化为高附加值的芳烃化合物。因此，使用碱处理的分级分子筛显著提升了生物质和PP共催化热解时芳烃产率，减少了积碳。结果表明，在生物质和PP共催化热解时，使用具有介孔结构Ga-MFI分级分子筛能够有效改善产品品质，提升生物质催化热解转化技术经济效益。