Developing structure-function relationships for CO2-assisted ethane dehydrogenation through zeolite-supported chromium sites

国内天然气是美国能源和化学制造业的重要组成部分，对经济和国家安全有着重大影响。在化工行业的背景下，利用天然气的轻烷烃成分（如乙烷）作为生产增值化学品的原料存在着独特的机会。值得注意的是，乙烷可以催化脱氢生产乙烯，乙烯是制造许多类型聚合物的重要原料。目前乙烷脱氢工艺的经济效益受副反应和催化剂失活的影响，收率低。使用二氧化碳（CO2）作为助反应物可提高催化剂活性，促进高乙烯选择性，并延长催化剂寿命。此外，二氧化碳（一种工业废物和温室气体）被转化为一氧化碳（CO），一氧化碳本身就是化学工业中使用的增值产品。然而，催化剂技术的进步需要确保二氧化碳辅助乙烷脱氢的商业可行性。该项目通过催化剂结构的新工程以及对化学反应的系统研究，对这一转化过程进行了基本的研究。总的来说，这项研究将产生关于高单道烯烃产量的机制起源的关键基础信息，这将转化为改进烷烃-二氧化碳联合转化系统的过程经济。该项目得到了更广泛的努力的支持，以增加妇女和代表性不足的群体的教育和研究机会，特别是调查人员所在大学所服务的大量西班牙裔人口。该项目侧重于沸石负载的铬（Cr）催化剂，以深入了解整个催化过程的多个方面：乙烷活化，CO2活化，共发生反应之间的竞争和合作，以及金属支撑相互作用和活性位点结构的影响。具体来说，杂原子取代的沸石载体（包括MFI沸石中的框架Al， B和Ga）用于生成具有一致配位几何形状但具有不同金属-载体相互作用的Cr活性位点（这将由目标杂原子元素组成决定）。通过一套全面的表征工具，包括探针分子吸附红外光谱、非原位和operando同步加速器x射线吸收光谱、化学吸附和质谱热重分析以及拉曼光谱，分散的Cr位点的结构、化学和电子性质将与催化性能相关。使用定义明确的催化剂，CO2对反应动力学的影响将在平行乙烷脱氢和逆向水气转换（RWGS）反应的背景下分离和解释。反应网络将通过反应器研究进行分析，采用不同的进料组成、探针反应和同位素标记实验。在低乙烷转化条件下得到的催化剂结构-功能关系和优化反应条件，将在与商业单道反应器操作相关的高转化条件下进一步研究。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。

Domestic natural gas is an important contributor to the U.S. energy and chemical manufacturing sectors, with strong effects on the economy and national security. In the context of the chemicals industry, a unique opportunity exists to utilize light alkane components of natural gas, such as ethane, as feedstocks for production of value-added chemicals. Notably, ethane can be catalytically dehydrogenated to produce ethylene, which is a critically important building block feedstock for the manufacture of many types of polymers. Current ethane dehydrogenation process economics are limited by low yields, which result from the prevalence of side reactions and catalyst deactivation. The use of carbon dioxide (CO2) as a co-reactant enhances catalyst activity, facilitates high ethylene selectivity, and extends catalyst lifetimes. Additionally, the CO2 (an industrial waste product and greenhouse gas) is converted to carbon monoxide (CO), which is itself a value-added product used in the chemicals industry. Nevertheless, advances are needed in catalyst technology to assure commercial viability of carbon dioxide-assisted ethane dehydrogenation. The project develops fundamental insights into this conversion process through novel engineering of catalyst structures coupled with systematic investigation of the chemical reactions. Overall, the study will generate critical fundamental information on the mechanistic origins of high single-pass olefin yields, which will translate to improved process economics for combined alkane-CO2 conversion systems. The project is supported by broader efforts to enhance educational and research opportunities for women and underrepresented groups, especially the large Hispanic population served by the investigators’ university.The project focuses on zeolite-supported chromium (Cr) catalysts to develop insights into multiple aspects of the overall catalytic process: ethane activation, CO2 activation, competition and cooperation between co-occurring reactions, and the influence of both metal-support interactions and active site structure. Specifically, heteroatom-substituted zeotype supports (including framework Al, B, and Ga in MFI zeolites) are used to generate Cr active sites with consistent coordination geometry but with varied metal-support interaction (which will be determined by the targeted heteroatom elemental composition). Structural, chemical, and electronic properties of the dispersed Cr sites will be related to catalytic performance via a comprehensive suite of characterization tools, including probe molecule adsorption infrared spectroscopy, ex situ and operando synchrotron-based X-ray absorption spectroscopy, chemisorption and thermogravimetric analysis with mass spectrometry, and Raman spectroscopy. Using well-defined catalysts, the impact of CO2 on reaction kinetics will be isolated and interpreted in the context of the parallel ethane dehydrogenation and reverse water-gas shift (RWGS) reactions. The reaction network will be analyzed through reactor studies, employing varying feed compositions, probe reactions, and isotope labeling experiments. Catalyst structure-function relationships and optimized reaction conditions, obtained at low ethane conversion, will be further investigated in the high conversion regime, relevant to commercial single-pass reactor operation.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.