AmericanChemicalSociety.com

Although most researchers in heterogeneous catalysis focus on the reaction chemistry, there is a large push to develop better molecular-level understanding of the synthesis steps by which complex catalysts are synthesized.  In other words, the goal is to transform the art of catalyst preparation into a science- the economic impact is large as more than 80% of today's large-scale chemical processes depend on heterogeneous catalysts.  Heterogeneous catalysts are used throughout petroleum refining, pollution abatement, and production of fuels and chemicals. This symposium focused on the scientific basis and advances in heterogeneous catalyst preparation.

During this symposium, we heard from Professor Ferdi Schüth, director of the Max Planck Institute for Coal Research, in Mülheim, Germany of efforts to control catalyst composition on the nanometer scale using many of the newer approaches of nanotechnolgy to allow greater activity and selectivity and make products with less energy input than the ones used today.  For example, Schüth's group developed a way to protect gold particles from agglomeration by encapsulating them in a hollow oxide zirconia shell.  First, the team synthesized monodisperse gold particles roughly 16 nm in diameter and coated them with silica. That step yielded uniformly sized composite particles, 95% of which contained exactly one gold nanoparticle in the center. Then, the group formed a porous zirconia shell around the intermediate product and finally removed the silica by treating the material with sodium hydroxide. Displaying micrographs reminiscent of frog eggs, Schüth noted that the procedure resulted in hollow zirconia spheres containing a single off-center gold nanoparticle.  The encapsulated gold particles formed in this way are effectively separated from other particles but are highly accessible to gas molecules, which is crucial for heterogeneous catalysis, Schüth said. Indeed, he reported that these particles are remarkably active CO-oxidation catalysts and that they resist sintering even when exposed to high temperatures (800 ºC) for an extended period.

In the group of Krijn P. de Jong, a chemistry professor at Utrecht University, studies were ongoing involving one of the most common approaches to making catalysts, namely impregnating a porous support with a metal-nitrate or other catalyst precursor solution, followed by drying and heat-treatment steps. One of de Jong's objectives is to develop synthetic tools that can fine-tune catalysts for Fischer-Tropsch chemistry, which is a carbon-carbon coupling process for making synthetic fuels and chemicals.

Previously, the Utrecht group found that roughly 6 nm is an ideal size for supported cobalt Fischer-Tropsch catalyst particles.  The particles need to be uniform in size, evenly spaced to avoid agglomeration (clumping) and undesirable sintering (fusing into larger particles), and loaded onto a support at high concentrations—on the order of 20% by weight.  In general, Fischer-Tropsch catalysts described in research papers and symposia do not meet those criteria.  The group found from X-ray and microscopy studies that air calcination can reduce a catalyst's activity by driving the particles to agglomerate and cluster. To sidestep those harmful effects, the group searched for alternative calcination conditions and found that a dilute mixture of NO in helium does the job well by reducing the decomposition rate of metal nitrate precursors.

 Professor Mizuki Tada of the Institute of Molecular Science, Okazaki,  Japan presented exciting results on  including unprecedented selectivities of benzene to phenol oxidation obtained with alloy complexes by controlling the location of these complexes in discrete sites in zeolites.  This may provide a new synthetic route to this useful chemical without the accompanying side products that occur with current technology.

Other highlights included: Professor John R. Regalbuto,  University of Illinois at Chicago: who presented a seminar on a simple, rational method to prepare supported metal catalysts;  Professor Chris W. Jones, Georgia Institute of Technology, who spoke on designing cooperative catalysts: Co-salen catalysts for epoxide ring-opening; Dr. Stacey I. Zones, Chevron Research and Technology Center, Richmond, Ca: who described molecular sieve catalysts and the challenges to bringing forward new materials; and Professor  Susannah L. Scott, University of California at Santa Barbara who described her studies on the synthesis of chromates in chromasiloxane ring structures as active site models for the Phillips' catalyst.

It became clear that heterogeneous catalysis is a complex field that is a hybrid of several chemistry disciplines, the field draws from coordination chemistry, solid-state chemistry, inorganic and organic chemistry, surface chemistry, and chemical engineering.  Catalysis researchers would like to be able to specify and control the position of every atom in a catalyst, they would like to design, a priori, molecular bonds, particle sizes, morphology, and other catalyst properties.  Although there has been a lot of progress in that direction in recent years, there is still a long way to go.