高选择性催化甲醇制芳烃双壳层空心结构纳米ZSM-5分子筛构建及催化活性精准调控

High value-added aromatic hydrocarbons, particularly benzene, toluene and xylene (BTX) can be synthesized from the catalytic conversion of methanol. This reaction process is complex and contains two catalytic cycles, one is olefins cycle and the other is aromatics cycle. Many different kinds of hydrocarbons are produced in this reaction and aromatics selectivity is difficult to be precisely adjusted on a single structured ZSM-5 catalyst. So aromatics selectivity is low and BTX aromatics is much lower for the difficulty on the distribution controllation of aromatics production, which limited the industrial applications for methanol to aromatics technology. In order to promote the conversion of methanol into BTX aromatics with high selectivity, the hollow structured ZSM-5 bifunctional catalyst with double shells will be fabricated in this project to efficiently connect and coordinate the reaction of methanol to light olefins and the aromatization of light olefins. The self-assembly process of the hollow structured ZSM-5 with double shells during the desilication and recrystallization under the protection of quaternary ammonium hydroxide will be focused on. The influence of the crystal size and silica-alumina ratios of nano ZSM-5, alkali processing conditions and additives on the thickness of two shells, mesoporous structure, surface acid strength and density as well as the B/L acid value will be discussed. Shell structure and surface acidic property will be adjusted combining the characteristics of the catalytic step reaction to study the coupling rules and influencing factors for these two reactions. Through the precise regulation of catalyst structure and the optimization of the combination of catalytic active centers, these two reactions will occur efficiently and sequentially and the methanol can be directionally converted into BTX aromatics with high selectivity. This project will provide the theory basis for the industrial application of directional and precise conversion of methanol into aromatic hydrocarbons.

甲醇直接催化转化为高附加值芳烃特别是苯、甲苯和二甲苯（BTX），反应过程复杂，存在烯烃和芳烃两个循环过程。单一结构ZSM-5催化剂难以精准调节芳烃选择性，合成产物品种多，芳烃选择性低，特别是BTX选择性更低，应用受到限制。本项目拟构筑双壳层空心ZSM-5双功能催化剂，将甲醇制低碳烯烃和低碳烯烃制芳烃两个高选择性催化过程贯序进行，高效协同，促使甲醇高选择性转化为BTX为主的芳烃。重点研究季铵碱溶液保护性脱硅重结晶形成双壳层空心ZSM-5的自组装过程，探讨纳米分子筛基体尺寸、硅铝比，碱处理条件和助剂等对催化剂两壳层厚度、介孔结构、表面酸的强弱和密度以及B/L酸值影响；针对分步催化反应的特征，调节内外壳层结构和表面酸性质，探讨两步催化反应的耦合规律和影响因素；通过精准调控催化剂结构，优化催化活性中心组合，促使两个反应高效贯序进行，实现甲醇高选择性定向转化为BTX芳烃，为工业应用提供理论基础。

煤经甲醇转化为高附加值芳烃产品是煤资源清洁高效转化利用的重要技术，但传统单一ZSM-5上难以有效控制反应过程，催化剂易失活且芳烃选择性低。本项目首先采用传统水热及蒸汽辅助固相转化方法制备具有特定酸梯度分布的核壳ZSM-5，构建甲醇制低碳烯烃和低碳烯烃芳构化功能区，促进了甲醇经低碳烯烃到芳烃的分步转化。发现调控壳层硅铝比对核壳结构形成及酸性质影响显著，高硅铝比下易形成规整核壳结构，同时有利于形成强酸位点推动低碳烯烃芳构化。采用TPAOH溶液脱硅重结晶处理核壳ZSM-5，构建了双壳层空心纳米ZSM-5，克服了芳构化功能区外包覆制低碳烯烃壳层功能区引发的分子传输限制，保障了外壳层生成的低碳烯烃在高酸密度内壳层上继续转化及产物的快速移出，有效提升了催化稳定性和芳烃选择性。在TPA+重结晶溶液中引入Na+，获得了体相介孔可控构建的关键机制，通过控制两种离子浓度及处理溶液碱性，可以有效诱导壳层介孔的形成，进一步促进内部大分子物种向外扩散，减缓积炭进而延长催化寿命。催化剂表面酸修饰研究发现，应紧密结合催化剂尺寸开展酸碱处理，且Zn助剂引入前可对催化剂进行水汽处理。基于分段装填双ZSM-5体系中下段芳构化ZSM-5的酸性质影响研究，发现较低密度酸位即可催化低碳烯烃芳构化。双壳层功能耦合过程进一步在双ZSM-5催化体系被验证和发展应用。物理混合两催化剂能催化甲醇两步转化制芳烃，调节高硅铝催化剂的引入量和酸密度可改变芳构化ZSM-5周围的甲醇浓度，积炭的有效抑制使催化寿命达原来的10倍，同时提升了轻质芳烃选择性。鉴于高亲密度下大分子芳烃等积炭前驱体的返混会加重积炭失活，分层装填两催化剂明显抑制了大分子芳烃物种的返混，但受限的床层间的二次转化过程降低了芳烃选择性。采用粘结法将高硅铝比ZSM-5粘结在低硅铝比ZSM-5外围，结合硅铝比和粘结比等要素制备了毫米尺度的核壳ZSM-5，核壳间活性中心的高亲密度及有序性有效促进了甲醇分步转化过程。本项目研究为高性能甲醇制芳烃催化剂研制提供了科学理论基础。