Overcoming Limitation in the Design of Selective Solid Catalysts by Manipulating Shape and Size of Catalytic Particles: Epoxidation Reactions on Silver

Conventional solid metal catalysts, usually synthesized using wet impregnation strategies, are almost always quasi-spherical particles mainly terminated by low-energy surface facets, such as the (111) surface for fcc crystals and the (0001) surface for hcp crystals, and supported on high surface area materials. The diameter of these metal particles is from a few tens to a few hundreds of nanometers. The particle shapes and sizes represent a reasonable compromise between simple and scalable synthesis, relatively high surface to volume ratio, and high resistance to sintering and degradation under reaction conditions. A major negative consequence of the lack of diversity in the size and shape of metallic catalytic particles is that it has been difficult to identify and optimize selective catalysts for the production of reactive, commodity chemicals. These difficulties stem from large differences in the chemical activity of different metal elements. For example, identical surface sites of neighboring metals in the periodic table of elements bind various adsorbates with differences in adsorption energies of up to about 1 eV (ca. 100 kJmol ), which means that whereas one metal might be chemically inert for a particular chemical transformation, its first neighbor in the periodic table might be overly reactive and equally inefficient. The above-discussed obstacles have prevented the design of selective solid catalysts for direct partial oxidation of propylene to propylene oxide (PO; C3H6+ =2 O2!C3H6O). PO is used as a chemical intermediate in the production of foams, coatings, adhesives, propylene glycol, and glycol ethers. It has a market on the order of several billion dollars per year. PO is currently produced in costly homogeneous processes which require expensive post-reaction separation and use environmentally harmful oxidation reagents, mainly chlorohydrins or hydrogen peroxide, instead of air or oxygen. Direct heterogeneous processes are attractive because of the easy separation, long lifetime, and regenerability of solid catalysts, and the environmentally benign and cheap oxidizing agents (molecular O2). Conventional epoxidation catalysts containing silver particles dispersed on a-alumina support, used in the analogous catalytic epoxidation of ethylene to form ethylene oxide (C2H4+ =2 O2!C2H4O), are not selective since they activate allylic hydrogen in propylene, leading to complete combustion. In contrast, less chemically reactive gold-based catalysts do not activate O2 efficiently without sacrificial agents such as H2 and are therefore limited by low activity. Development of a robust heterogeneous catalyst for direct epoxidation of propylene is one of the most important problems in heterogeneous catalysis and surface chemistry, and it is often referred to as the holy grail of heterogeneous catalysis. Lei et al. recently showed that size-selected clusters of Ag (Ag3 trimers; Figure 1), and small aggregates of the Ag trimers (effectively nanoparticles with a diameter of ca. 3.5 nm) dis-