59th Annual Report on Research 2014

The use of heterogeneous catalysis is at the center of most processes in the chemical industry, including the manufacturing of fine chemicals, petrochemicals, and agrochemicals; however, traditional preparation methods for heterogeneous catalysts lead to the formation of ill-defined active sites and make the rational design of highly active, stable, and selective catalysts very challenging. In this project, we propose a new type of heterogeneous catalyst with a well-defined nanoparticle core and crystalline nanoporous shell. A heterogeneous catalytic process typically involves three important steps: the adsorption of reactant molecules on the active metal surface, reaction between molecules on the metal surface, and, finally, desorption of products. The proposed catalyst design focuses on controlling these critical steps. In the design, the functional metal-organic frameworks (MOFs) are coated on the surface of colloidal metal nanoparticles to form a core-shell structure. The size, shape, and composition of the metal nanoparticles, and the shell thickness, pore structure, and functional group of the MOFs will be simultaneously controlled. By combining the well-defined metal surfaces with MOFs’ controllable nanopore structures, we intend to engineer well-defined active cavities on the metal surface, which could concurrently manipulate the adsorption geometries and sorption energies of organic reactants on the catalyst surface.

During this funding period, we have extensively focused on developing new colloidal synthetic strategies for coating nanoparticles with different MOFs. We mainly use nanoparticles with capping agents of cetyltrimethylammonium bromide (CTAB) and polyvinylpyrrolidone (PVP). We have synthesized numerous CTAB and PVP capped single metal, alloy, and core-shell metal nanoparticles with size, shape, and composition control. Then we developed strategies to coat these nanoparticles with MOFs. We used SEM and TEM images to demonstrate that metal nanoparticles are incorporated into crystalline MOF crystals. In all samples, nearly every MOF crystal contains few metal nanocrystals, and the MOF crystals are of a narrow size distribution. Single core-shell structure, multiple cores in one shell structure, and yolk-shell structure of metal in MOFs were all synthesized (Figure 1) The PXRD patterns reveal the formation of crystalline MOF structures. The thermal stability of these catalysts was tested by temperature-programmed CO oxidation. We are also applying the same synthetic principle to zeolite systems, which are of desirably higher stability. The gas-phase hydrogenation of ethylene and cis-cyclooctene were carried out to study the molecular-size-selective catalytic behavior of the. The catalyst consisting of metal nanocrystals directly deposited on MOF crystal surfaces and metal nanocrystals coated by a MOF shell were prepared for comparison. For ethylene hydrogenation, both of the catalysts show high activity and similar activation energies. For cyclooctene hydrogenation, the core-shell catalyst shows no detectable activity but the metal on MOF catalyst shows high activity. This result clearly exhibits the molecular-size-selective property of the MOF shells.

The budget during this funding period was restricted to partial PI summer salary, student salary, meeting travel, and necessary laboratory materials. ACS-PRF funds made it possible to partially support two of my first Ph.D. students, Brian Sneed and Maggie Sheehan. Both of them are expected to obtain their Ph.D. degrees in the next academic year. The ACS-PRF grant also has a great impact to my own career. It helped me to establish this new research direction and create a productive research program. All the results to date are the fundamental data of my NSF CAREER proposal. Figure 1. Various types of core-shell structures: (a) single core-shell structure; (b) multiple cores in one shell structure; (c) yolk-shell structure.