Developing rational strategies for a priori tuning the self-assembly of ordered materials with predictable structural outcomes is a grand challenge in materials design, wherein few synthetic schemes of solid state materials are amenable to systematic and precise manipulation of crystal habit. In zeolite synthesis, engineering facile routes to precisely control crystal size and morphology is a benchmark for addressing systemic design limitations, which can marginalize their performance and economic viability in commercial applications. The objective of this proposed research plan is to develop a rational design strategy for manipulating the growth of ZSM-5, a ubiquitous zeolite catalyst, which is synthesized empirically with little fundamental understanding of crystallization. This BRIGE proposal will leverage the PI's expertise in crystal engineering and surface science to investigate ZSM-5 crystallization at an interfacial level, using atomic force microscopy to perform the following tasks: (i) pioneer in situ measurements of anisotropic growth kinetics; (ii) develop a design strategy using tailored modifiers with molecular recognition for binding to specific crystal faces and mediating growth; and (iii) monitor growth dynamics in real time to uncover the underlying mechanisms of self-assembly, which will facilitate the development of predictive models for tuning crystal habit. ZSM-5 is a promising catalyst for greenhouse gas emissions technologies due to its high activity for NOx reduction. The judicious modification of ZSM-5 crystal habit can alter porous surface area and internal diffusion pathlength, which regulate catalytic activity. Indeed, recent studies reveal that ultrathin ZSM-5 platelets, which are difficult to achieve by conventional syntheses, exhibit notably higher yield, selectivity, and lifetime. The successful completion of objectives in this research plan will provide a transformative approach to zeolite synthesis, and heuristic guidelines for design with potentially broader applicability to inorganic materials for viable applications in areas of energy and selective separations. The long-term trajectory of this research program aims to establish a comprehensive platform to design, model, and test zeolites for selective catalytic reduction (SCR) of NOx, using methane for on-board vehicle SCR technology development. Intellectual merit of the proposed activities: This proposed research will advance our fundamental understanding of zeolite crystallization, capitalizing on the PI's expertise in crystal engineering to apply AFM in ways that have not been utilized in zeolite science - namely in situ growth measurements to systematically quantify anisotropic kinetics, and force measurements to probe molecular recognition and binding at crystal interfaces. Molecular design principles of ZSM-5 will become a platform for addressing a broader range of zeolite structures, offering unprecedented control of crystal properties, which are unattainable by conventional methods. Long-term initiatives will institute synergistic collaborations with faculty at UH's Texas Diesel Testing and Research Center (TDTRC) to design and optimize zeolites for NOx CH4-SCR. Broader impact of the proposed activities: This BRIGE grant will help establish an outreach program at the K-12, undergraduate, and Ph.D. levels to promote engineering education and research, with emphasis on minority and female students through the PROMES and LSAMP programs at UH (whose minority enrollment ranks 2nd among national research universities). This plan will foster active learning through hands-on experience and classroom lectures, using concepts in crystallization to engage student interest in the sciences and increase awareness of interdisciplinary opportunities in engineering careers. The PI will partner with KIPP Houston High School (a minority institution ranked 16th in national college readiness) to establish a dynamic program for student and teacher (NSF-RET) research in the PI's lab and periodic guest lectures in KIPP's AP chemistry class. The PI will mentor NSF-REU, UH undergraduate and graduate research, using results of these studies as integrated topics in a colloids elective course.
This BRIGE award focused on designing methods capable of controlling zeolite crystallization to achieve tailored properties for commercial applications. The primary research thrusts were focused on two areas: (i) Validation and testing of zeolite growth modifiers (ZGMs) as site-specific adsorbates capable of altering the anisotropic rates of crystal growth, and hence bulk crystal properties such as size and shape; and (ii) Developing methods of high temperature in situ atomic force microscopy (AFM) imaging that permit visualization of zeolite surface growth under realistic synthesis conditions. Here, the key outcomes from this project with respect to intellectual merit and broader impact are briefly summarized. Outcomes of Intellectual Merit: The results of this project led to advancements in ZGM design for the synthesis of zeolite MFI. We identified effective molecules with site specificity for each principal surface of MIF crystals, and showed that the judicious selection of modifiers results in an order of magnitude reduction in crystal thickness (i.e. diffusion path length) while preserving large porous surface area (i.e. molecule accessibility to internal pores). Moreover, we used AFM to assess the influence of ZGMs at the microscopic level. One of the key benefits of this approach is its commercial viability given that ZGMs are inexpensive, commercially available molecules that can be recovered and recycled. By extending our studies to other zeolite framework types during this project, we demonstrated the broader applicability of this design approach. Lastly, we used this award to design and test a retrofitted AFM liquid sample cell for imaging silicalite-1 surface growth. These studies revealed valuable information regarding the mechanism of crystallization, and quantitative data on the kinetics of anisotropic zeolite growth. In the future, we will use in situ AFM measurements to assess the effects of ZGMs on zeolite surface growth. Outcomes of Broader Impact: Improvements in zeolite properties can lead to enhanced performance in catalytic applications with socioeconomic impact in areas ranging from emissions abatement and biofuels to methanol-to-hydrocarbon processes. To this end, the results of this project will be useful in the continued development of structure-performance relationships in zeolite catalysis. In terms of training, the PI has used this award to mentor one graduate student, two undergraduate students, one high school student and one high school teacher (RET program). The PI was involved in many activities to promote STEM awareness at local high schools, and served as a research mentor to a team of high school students selected as a MIT-Lemelson InvenTeam (a design project that involved the use of zeolites for pollution abatement). The results of this BRIGE project have led to 8 student awards, four publications in peer-reviewed journals, one patent, and numerous presentations at both national and international conferences and meetings.
