The research in our group stays at the interface of materials synthesis and catalytic science, with an emphasis on precisely controlling composition and constitution of nanostructured materials for membrane and catalyst technologies to address the challenging issues in renewable energy and chemical production, sustainable water supply, and environmental remedy. In particular, we are focusing on four avenues: (i) Studying membrane reactor systems to promote fossil fuel conversion efficiency and energy sustainability; (ii) Innovatively synthesizing high-performance catalysts for efficient chemical/fuel conversions; (iii) Understanding the basic physicochemical properties of catalysts for performance optimization; and (iv) Broadening research impacts to improve the water and environment sustainability. Our overall objective is nano-engineering of advanced catalyst and membrane materials with structural elucidation, growth mechanism perception, and industrial application exploration to significantly improve renewable energy and chemical production, water quality and the environment. Below, you will find brief descriptions of our ongoing research projects.
Design of Membrane Reactor Systems to Promote Methane Conversion
The depletion of crude oil is shifting the market attention to methane (CH4), the main constituent of natural gas, shale gas and oil-associated gases, which is of great abundance and synthetic importance as the substituent for liquid petroleum in petrochemical and fine chemical industries. Oxidative coupling of methane (OCM) and non-oxidative methane aromatization (DMA) have been practiced to directly convert methane into useful hydrocarbons including C2 (ethylene and ethane) and benzene. Membrane reactors hold the promise to circumvent kinetic constraints and thermodynamic equilibrium limitations in these chemistries. We are developing efficient catalyst and membrane materials, and fundamental insight into methane activation kinetics in the “unusual” membrane reactor environment in methane conversion chemistry. Our work will pave the way to the development of high-performance, energy-efficient, low-cost pathways for methane valorization. The coupling of chemical reaction and the separation process in the same membrane reactor device fulfills the criteria of process intensification, and thus minimizes environmental, economical and social impacts.
Synthesis of Hierarchical Lamellar Zeolites as High-Performance Catalysts
Crystalline microporous zeolites are the most widely used catalysts in petrochemical and fine chemical synthesis, mainly due to their well-defined structures and angstrom-size micropores that are responsible for shape selectivity. The processing of heavy feedstocks over zeolites, however, is limited by mass transport due to the restricted access and slow transport to or from the active sites in micropores. Hierarchical unit-cell thick lamellar zeolites, which expose all or most of the pores to surface adsorption to eliminate the need for diffusion to the internal pores, have been shown exceptional activity in processing bulky molecules in the catalytic reactions. The shape-selectivity of zeolites is, however, diminished as the environment of active sites emerges from well-defined micropores to mesopores or exterior surfaces in hierarchical lamellar zeolites. Enabling hierarchical lamellar zeolites for efficient catalysis requires rational design schemes capable of selectively tailoring their micro-/mesopore structures and their consequences on the catalytic performances.
Our research lab is taking innovative synthetic strategies to address this critical issue. For instance, by employing hydrothermal crystallization of zeolites under assistance of dual molecular ammonium cation and polyquaternary ammonium surfactant templates, we aims to achieve precisely design of hierarchical zeolites at the micro- and mesoporosity levels. It is highly promising that the hierarchical lamellar zeolites achieve tunable textural properties, active acid site locations, and catalytic performances by simply dialing the dual template ratios in the synthesis. The tunable textural, acidity, and the consequences on catalysis allows for bringing the hierarchical zeolites further close to practical commercial uses since they are important potential catalysts for cracking of paraffins or vacuum gas oil in oil refinery and alkylation of aromatics in the organic syntheses.
Understanding of Fundamental Catalytic Properties of Hierarchical Lamellar Zeolites
Molybdenum/zeolite (Mo/zeolite) has been an efficient catalyst for direct methane aromatization (DMA) reaction. The zeolite topology, porosity (meso- and microporosity), and active site location influence its performance in DMA reactions. Enabling hierarchical lamellar zeolites for efficient catalysis requires mechanistic understanding and rational design schemes capable of selectively tailoring their micro-/mesopore structures and their consequences on the catalytic performances. Our research group has made a critical discovery regarding the simple method for practical deigned synthesis of hierarchical lamellar zeolite catalysts with tunable textural properties. Implications of the tunable meso-/microporosity on the spatial distribution and catalytic performance of metal-acid sites in Mo/zeolite catalysts in DMA reactions have been studied innovatively by the combination of organic chemical titration, x-ray photoelectron spectroscopy and kinetic measurements. Volcano-type dependence between the distribution of Brønsted acid and Mo sites, as well as aromatics production rate with the meso- and microporosity ratios suggest that the textural and composition properties and thus the catalytic performance of the zeolite catalysts are dominated by the structural hierarchy. The internal and external coke formed on the catalysts in DMA linearly depended on the relative external surface area of the hierarchical lamellar zeolite catalysts. This research work advanced the mechanistic understanding of local environments of hierarchical lamellar zeolites in methane activation. Also, the research created correlations among the synthesis, textural and acidity properties, and catalytic functions of the hierarchical lamellar zeolite catalysts in DMA chemistry.
Broadening Research to Impact on the Water and Environment Sustainability
Our research lab aims to expand the innovative research in catalysis and material synthesis into other exciting applications such as desalination, waste water treatment, and environmental remedies. As the core component of a membrane reactor technology, catalysts and membranes function as chemical and physical filters to remove toxic compounds and wastes while retaining clean water and environment. In order to drastically increase the water flux and fouling resistance of the water purification membranes, we are applying the novel two dimensional (2D) zeolite materials into polymers to develop the composite membranes. To improve the efficiency in adsorption and photocatalytic degradation of toxic industrial chemicals and disposal of chemical warfare agents, we are synthesizing photocatalytic inorganic films supported on a light absorber. These research activities will bring significant impacts on the water and environmental sustainability in our society.