CAREER: Elucidating Mechanisms and the Effects of Zeolite Framework, Acid Site Location and Strength in Methanol-to-Hydrocarbon Reactions

沸石是一种晶体材料，多年来在商业上一直用作分离气体混合物的助剂和将原油精炼成燃料和化学品的催化剂。或者，沸石可以用来加工甲醇，甲醇可以从天然气、生物质或煤炭精炼中形成，转化为各种各样的燃料和化学品。这些过程统称为甲醇制烃反应（MTH）。本研究项目将探索MTH的反应机理，以及沸石结构和酸强度如何影响MTH催化剂的效率、选择性和稳定性。这项研究项目的成果将改善我们如何利用我国的天然气、页岩气和生物质资源来生产液体运输燃料和化学品。在这个项目中开发的计算方法将向公众发布，从而为全球的研究人员提供访问协议的机会。该项目的研究人员将为中学科学教师开展为期三周的校园实习，帮助他们在课堂上实施基于计算科学的课程计划。项目研究人员将为高中生和本科生提供参与计算催化研究的机会。甲醇在沸石上与一组烃类助催化剂发生两个循环：烯烃和芳烃循环。在这两种情况下，甲醇与烯烃和芳烃发生反应，在异构化和碳-碳键断裂反应之前形成碳-碳键，形成烯烃和芳香产物的混合物。氢化物转移反应可生成烷烃和二烯烃，二烯烃可生成芳香物质和多芳香焦前体，使催化剂失活。对这些途径的原子理解将通过色散校正密度泛函理论（DFT）计算来建立，该计算估计了吸附、反应和活化的自由能。沸石可以包含多个独特的酸位点，反应物质和过渡态可以在每个位点周围采取许多不同的方向。实验技术目前无法区分这些取向，但新的计算技术的发展可以区分酸位及其在整个反应方案中的作用。通过对比不同t位点位置的行为，将使用H-ZSM-5沸石（MFI框架）模型来研究酸位点位置的作用。分子筛框架的作用将通过对比H-ZSM-5和H-SSZ-13 （CHA框架）来确定，H-SSZ-13具有不同的孔隙和连通性，应该显示出动力学和输运行为的差异。将H-SSZ-13与H-SAPO-34（均为CHA）进行比较，研究其酸强度效应。两者结构相同，但组成和酸强度不同。总之，这些结果将描述酸位点位置（例如，MFI中的通道与交叉点）、沸石框架（MFI与CHA）和酸位点强度（H-SSZ-13与H-SAPO-34）的明确影响。dft计算的自由能将用于参数化动力学蒙特卡罗（KMC）模拟，该模拟将识别反应性和非反应性途径，并考虑输运效应，以便在结合dft计算的输运受限物种的扩散屏障时了解晶粒尺寸效应。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Zeolites are crystalline materials that have been used commercially for many years as agents for separating gas mixtures and as catalysts for refining crude oil to fuels and chemicals. Alternatively, zeolites can be used to process methanol, which can be formed from natural gas, biomass, or coal refining, into a wide range of fuels and chemicals. Such processes are collectively known as methanol-to-hydrocarbons, or MTH, reactions. This research project will explore reaction mechanisms for MTH and how zeolite structure and acid strength influence the efficiency, selectivity, and stability of MTH catalysts. The outcomes of this research project will improve how we utilize our nation's natural gas, shale gas, and biomass resources for the production of liquid transportation fuels and chemicals. The computational methods developed during this project will be released to the public, thereby providing researchers around the globe access to the protocols. The project researchers will develop a three-week on-campus internship for secondary school science teachers to help them implement computational science-based lesson plans for their classes. The project researchers will provide opportunities for high school and undergraduate students to participate in computational catalysis research.Methanol reacts over zeolites with a pool of hydrocarbon co-catalysts in two cycles: the alkene and aromatic cycles. In both cases, methanol reacts with alkenes and arenes to form carbon-carbon bonds prior to isomerization and carbon-carbon bond scission reactions which form a mixture of alkene and aromatic products. Hydride transfer reactions can form alkanes and dienes, the latter of which can form aromatic species and polyaromatic coke precursors that deactivate catalysts. An atomistic understanding of these pathways will be built by dispersion corrected density functional theory (DFT) calculations that estimate the free energies of adsorption, reaction, and activation. Zeolites can contain multiple unique acid site locations, and reacting species and transition states can take many distinct orientations around each site. Experimental techniques currently cannot distinguish between those orientations, but the development of novel calculation techniques allows discrimination between acid sites and their roles in the overall reaction scheme. The role of acid site location will be investigated using models of the H-ZSM-5 zeolite (MFI framework) by contrasting the behavior of various T-site locations. The role of zeolite framework will be determined by contrasting H-ZSM-5 with H-SSZ-13 (CHA framework), which has distinct pores and connectivity, which should show differences in kinetic and transport behavior. Acid strength effects will be examined by comparing H-SSZ-13 with the zeotype H-SAPO-34 (both CHA) which have the same structure but differ in their composition and acid strength. Together, these results will describe the unconfounded effects of acid site location (e.g., channels vs. intersections in MFI), zeolite framework (MFI vs. CHA), and acid site strength (H-SSZ-13 vs. H-SAPO-34). DFT-calculated free energies will be used to parameterize kinetic Monte Carlo (KMC) simulations which will identify reactive and unreactive pathways and account for transport effects to understand crystallite size effects when combined with DFT-calculated diffusion barriers for transport-limited species.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.