This project deals with the study of advanced membranes and membrane reactors for enhancing the performance of catalytic systems by coupling of transport and reaction processes. The specific reaction to be examined will be the oxygen-assisted, autothermal reforming of methane for producing hydrogen. This is an attractive reaction because the use of oxygen allows the simultaneous operation of the exothermic combustion reaction and the endothermic reforming reaction which leads to efficient heat utilization. In addition oxygen prevents catalyst deactivation by coke formation. The studies are made possible by our recent development of inorganic membranes that combine high hydrogen permeance (5 x 10-7 mol m-2s-1Pa-1), selectivity (> 1500), stability (> 500 h) and inertness at high pressure and temperature. The membranes are tubular composites consisting of a base support made of porous a -alumina with graded layers of a alumina substrate, and a topmost thin layer (30 nm) of silica. They are prepared by a combination of sol processing and chemical vapor deposition (CVD). The current permeance of the membranes exceeds that of pure Pd, but we will undertake research to improve the permeance significantly. This will be done in two ways. First, by optimizing the pore size of the intermediate graded layer derived from the boehmite sols through control of the particle size and thickness of the layers. Second, by engineering the composition of the topmost layer by using mixed-element CVD. This latter work will be guided by ab initio DFT calculations. We have successfully obtained activation energies for passage of molecules through model Si ring structures that match those of actual permeation data. We plan to carry out a theoretical combinatorial screening of a wide variety of compositions that include metals (Y, Zr, Ti, etc.), nonmetals (B, Al, etc.), and lanthanides (La, Ce, etc.) to guide the experimental work. The membrane reactor studies on the autothermal reforming will be carried out at high pressure (20-30 atm) and will include measurement of the kinetics of the reaction and the description of the reactor with 1-d (longitudinal) and 2-d (longitudinal and radial) mathematical models. A major goal is to investigate the transition regime between 1-d and 2-d descriptions and to develop criteria for the applicability of the models that can be used for general reactions. The reactor studies will also explore the feasibility of overcoming pressure drop losses across the membrane in a novel manner: by harnessing the increase in moles of the reaction itself.
The broader impacts of the project are substantial. The topic of the research, autothermal reforming is important in the energy and chemicals area, addressing the efficient utilization of natural gas. The developments in the silica membrane preparation will also find applicability in other areas such as zeolite or oxygen conduction membranes. The research will provide advanced training for graduate students in a broad discipline that includes experimental and theoretical aspects so as to stimulate creative thinking. Importantly, great emphasis will be placed on the involvement of women, minority and undergraduate students by active recruitment. A substantive collaboration will be initiated with a faculty member from a local undergraduate school (Radford Univ.) to promote personal growth of the individual, as well as to stimulate the participation of students from that school in higher education.
