甲醇制烃反应大外表面空心ZSM-5催化剂的可控制备及构效关系基础研究

Methanol to hydrocarbon (MTH) technology, one of the hotspots in the development of modern coal chemical industry, can convert methanol into olefins, aromatics, gasoline and other chemical products. However, the ZSM-5 catalyst for this technology is easily inactivated becaused of the carbon deposition caused by the diffusion limitation of micropores, which is the commom question for this technology. The diffusion of the coke precursors from micropores to the external surface of ZSM-5 can be promoted by increasing the external surface area. This can reduce the coke produce in the micropores and is the key to slow down the deactivation rate. Hollow ZSM-5 zeolite with a thin and porous shell has large external surface and is expected to present a long catalytic lifetime. In this project, silicalite-1 zeolite will be hydrothermally treated by tetrapropylammonium hydroxide (TPAOH) solution in which aluminum source has been added. Hollow ZSM-5 zeolite will be obtained after the inner desilicification and followed by rebuilting. To enlarge the external surface, sodium source and silane coupling agent will also be added into TPAOH solution as the pore-forming inducers to form macropores and mesopores in the shell of hollow ZSM-5 zeolite. Based on structure characterization and methanol-to-gasoline reaction, the effect of the key variables during the introduction of the silicalite-1 zeolite, aluminum source, sodium source and silane coupling agent on the crystal size, shell thickness, pore stucture and acidity of the hollow ZSM-5 will be studied. Then the influence mechanism of the microstructure of hollow ZSM-5 on the catalytic performance, and especially the resistance performance for carbon deposition will be deeply discussed. The research on this project will give a new MTH catalyst with long lifetime and also help to grasp the preparation process. Theoretical basis to solve the inactivation of ZSM-5 catalyst caused by carbon deposition will also be provided.

甲醇转化为烯烃、芳烃和汽油等产品的甲醇制烃（MTH）技术是现代煤化工发展的热点之一。但其催化剂ZSM-5受微孔扩散限制易积碳失活，是该技术面临的共性问题。增大ZSM-5外表面能促进微孔内积碳前驱物向外扩散，使微孔内积碳减少，是延缓失活的关键。壳层薄且富孔的空心ZSM-5外表面大，可具有高的催化寿命。本项目拟从silicalite-1全硅分子筛出发，以含铝的四丙基氢氧化铵溶液水热脱除其内部硅并重新组装成空心ZSM-5；在溶液中加入钠源和硅烷偶联剂诱导壳层形成大孔和介孔，进一步增大其外表面。结合表征和甲醇制汽油反应，研究silicalite-1、铝源、钠源和硅烷偶联剂的关键引入变量对空心ZSM-5晶粒尺寸、壳层厚度、孔结构和酸性的影响规律，深入探讨空心微观结构对其催化性能尤其是抗积碳性能的影响机制。本项目将提供一种长寿命MTH催化剂及其制备方法，为解决ZSM-5催化剂的积炭失活提供理论依据。

煤经合成气合成甲醇，再定向转化为烯烃、芳烃和汽油等烃类产品，是实现煤炭清洁高效利用的重要途径。对上述甲醇制烃反应中，ZSM-5催化剂微孔扩散限制引起的积碳失活是当前面临的共性难点问题。项目通过分析引起积碳失活的主要原因，提出构建大外表面ZSM-5，解决催化剂的失活问题。为此，研究从全硅S-1分子筛出发，以添加铝源的TPAOH溶液水热构建壳层薄的酸性空心ZSM-5；并进一步通过控制合成配方诱导壳层成孔，实现了ZSM-5大外表面的构建。结合结构表征，明确了催化剂的关键制备要素对结构的影响机制，获得了空心ZSM-5表面酸性和微观形貌结构对催化性能特别是抗积碳性能的构效关系。本项目研究取得了系列有意义的研究结果，并基于研究结果进行了优化拓展。通过调控母粉S-1粒径及处理的液固比，发现S-1粒径大小决定了重结晶位置，影响了最终ZSM-5的孔结构。纳米S-1尺寸小，内部硅更容易被洗出而容易完成内部脱硅，重结晶之后形成空心ZSM-5。相比较，微米S-1扩散路径更长，需要更高强度的水热处理才能完成重结晶，获得介孔分布均匀的微米ZSM-5。选择60nm的超小纳米S-1，精细控制水热时间，获得了酸性强且外表面大的ZSM-5催化剂，体现出了优异的再生性能。调节脱硅重结晶过程中NaAlO2的引入量，显著调控了壳层介孔和表面酸性。归其原因为添加NaAlO2时，直接调变硅铝比而改变酸性的同时，同步引起的Na+的变化，诱导了壳层介孔的形成。重结晶过程中引入CTAB，明显促进了壳层均匀小介孔的构建。 基于壳层厚度的调控需要，从纳米ZSM-5出发，直接进行脱硅重结晶或者深度脱硅，铝分布限制下获得了壳层厚度和介孔结构更易调控的大外表面ZSM-5，明确了介孔和酸结构演变的机制。催化结果显示，介孔构建过程中外表面和酸性质的变化共同决定了催化稳定性和产品选择性。大的外表面和较弱的酸性，能赋予ZSM-5优异的抗积碳性能，催化寿命更长。项目研究提供了若干用于MTH反应的长寿命催化剂及其制备方法，为解决ZSM-5催化甲醇制烃反应中共性的积碳失活问题提供了相关理论依据。