Henry C. Foley | Research
Click on each thumbnail image below to view the enlargment.
Nanoporous carbons (NPC) are solids materials with void spaces that have dimensions which are similar in size to many small molecules. Using Methyl Chloride (CH3Cl) adsorption, a technique developed in our group, we find that the mean pore size resides at about 0.5 nm within a narrow distribution of pores sizes. This makes the materials ideal suited for separations of small molecule mixtures and for shape selective catalysis, a process which combines the steric effects on diffusion with reaction to improve selectivity. At the upper left is a simple conceptualization of a slice through a material with nanopores consisting of tightly restricted gates between open galleries. Transport through such a network is activated and is extremely sensitive to small variations in molecular size and shape. This is the source of shape selectivity in separation and catalysis within NPC (and zeolites). However, this is a highly idealized and over simplified view of the NPC structure and the transport process.
The NPC are easily synthesized by polymer pyrolysis; the nanopore structure literally emerges as the solid forms with evolution of gas molecules. As a result of this kinetic process of formation, the material is not crystalline and is best considered to be a complex solid with a narrow pore size distribution on the one hand and yet no long range order on the other. Due to its complexity relatively little effort has been made to develop structural simulations and models of NPC with high similitude. With such models it becomes hard to understand the behavior of the materials and of small molecules diffusing and adsorbing within them. Hence a fraction of our efforts (~15-20%) of late has been directed toward deriving and validating structural simulations of the NPC. The figure at the middle of the slide is the graphical output of one of our latest simulations which includes 75,000 carbons atoms in a 100 cubic angstrom box.
The greater portion of our research is devoted to experimental studies of the NPC materials particularly in the form of membranes and, especially, for separations and catalysis. We can prepare supported nanoporous carbon membranes either in planar or tubular form. At the upper right-hand corner is pictured one of the tubular membranes units and the membranes tube itself which is placed in the unit. In both geometries, we use porous stainless steel as the medium on which to support the NPC, which is too fragile to be self-supporting.
By placing catalytic active sites within the NPC membrane or on its outer surface, we can do both catalysis and separation simultaneously. The middle right picture is a highly magnified (>1x 106 X) view of platinum in nanoporous carbon. The dark spheresare platinum nanoparticles with diameters of approximately 7.5 nm on average. To reach the catalytic surface of these particles, molecules must diffuse through the nanopores of the carbon (red pathway). Once they reach the platinum particle only a small portion of its surface, that which projects into the nanopore, is accessible. This creates the opportunity for exerting very real and very powerful steric effects on the catalysis taking place at platinum, which we observe in experiment. For example we find that the rates of hydrogenation of propene are literally one to two orders of magnitude larger than that of isobutene.
When we adsorb an acid catalyst (12-tungstophosphroc acid) at the surface of an NPC membrane, we can again exert an influence on the chemistry through coupled separation. The conversion of methyl tertiary butyl ether (M T B E) into isobutene and methanol is a case in point. The reaction (pictures at the lower left side) is limited to low equilibrium conversions at temperatures low enough (25-50oC) to keep the rates of sequential reactions slow. When the temperature is raised (>100oC), the equilibrium conversion to primary products goes up, but the rates of the deleterious secondary reactions (hydration and etherification) also rise dramatically, diminishing the yields of i-butene and methanol.
We can, however, engineer a nanomaterial into a reactor which will operate at low temperatures for clean chemistry and overcome the low equilibrium conversion by coupling the catalysis to membrane. When we do this reaction over an acid functionalized N P C membrane in unit like the one shown at the lower middle, then we find that methanol is selectively transported through the membrane while M T B E and i-butene are only slowly transported through it. This is shown in the date for breakthrough the membrane of each of these molecules in the vapor phase as a function of time. At the end of the experiment the initial charge of MTBE on the top side of the membrane is almost completely converted, but there is mostly i-butene left and methanol has been transported away and to the bottom side.
Other work is underway with nanoporous carbon which is not pictured, including the catalytic nanographitization (at left) of N P C by the action of alkali metals and the preparation of so-called templated carbons for biological separations, especially of proteins. We are also interested in using these materials as components of hydrogen fuel cells.
This work is funded by two separate N S F Grants and partly by a third. It has also been funded in the recent past by the Department of Energy and this is likely to be continued in the future.