Progress Toward Design of Solid Catalysts

Progress toward Design of Solid Catalysts Bruce C. Gates Abstract The 2016 Faraday Discussion on the topic “Designing new Heterogeneous Catalysts” brought together a group of scientists and engineers to address forefront topics in catalysis and the challenge of catalyst design—which is daunting because of the intrinsic nonuniformity of the surfaces of catalytic materials. “Catalyst design” has taken on a pragmatic meaning which implies the discovery of new and better catalysts on the basis of fundamental understanding of catalyst structure and performance. The presentations and discussion at the meeting illustrate rapid progress in this understanding linked with improvements in spectroscopy, microscopy, theory, and catalyst performance testing. The following essay includes a statement of recurrent themes in the discussion and examples of forefront science that evidences progress toward catalyst design. Catalysis and catalyst design Catalysis is the key to control of chemical change, in processes ranging from the biological to the technological. It is used to make products including chemicals, fuels, materials, food, beverages, and personal care products, and together these have a value of roughly 5–10 trillion dollars (US) per year worldwide. Catalysis is also essential for the removal of environmental pollutants such as those generated in motor vehicles and fossil fuel-fired power plants. Thus, the science underlying catalytic technology is essential. Catalysis science is also challenging, because almost all large-scale industrial catalysts are solids. These work at their surfaces—and these surfaces are notoriously nonuniform in both composition and structure, often being substantially different from simple terminations of the bulk material—and they undergo changes when exposed reactants. The 2016 Faraday Discussion “Designing new Heterogeneous Catalysts” attracted a full house of researchers and practitioners, eliciting novel contributions and stimulating discussion that brought forth a diversity of ideas about what the terms “catalyst design” and “heterogeneous” mean; we address these points below. Avelino Corma opened the Discussion, introducing the subject with a few history lessons and a linked set of case studies recounting a family of new catalysts based on insights guided by the emerging understanding of supported metal catalysts—this introductory lecture provided a basis for assessing the premise of catalyst design and the Discussants’ efforts to home in on the meaning of this term. The history of catalyst discovery reflects the complexity of the typical catalyst surface and of the reactions that occur on it as well as the enormous array of materials that can be considered candidate catalysts. The original approach to catalyst discovery was trial and error experimentation, and it is linked to the alchemists’ search for the philosopher’s stone and their early discoveries of “contacts” which today we would call solid catalysts. The work of Fritz Haber, Carl Bosch, and Alwin Mittasch more than a hundred years ago stands as the monument to the trial-and-error approach to catalyst discovery; their work at BASF involved 1 the systematic testing of some 20,000 materials as catalysts for the synthesis of ammonia from H2 and N2, with the resulting iron-containing catalyst being only little different from today’s technological ammonia synthesis catalysts. These pioneers worked from the emerging principles of chemical equilibrium, chemical kinetics, and process engineering, guiding them to optimal testing. They realized the need to evaluate candidate catalysts in reactors and under conditions that mimic a potential process—thus, they used high pressures (to favor the equilibrium for ammonia formation), recycle of unconverted H2 and N2 in a continuous process for efficient use of these raw materials, and long-term testing of catalysts in operation to evaluate their lifetimes. They found a highly active and stable catalyst, a porous iron- containing mineral with a high internal surface area, and the high catalytic activity allowed operation at relatively low temperatures, favoring the equilibrium of the exothermic ammonia synthesis reaction, maximizing the conversion, minimizing the recycle of unconverted reactants, and pointing toward optimization of the process. The trial-and-error approach is still widely practiced in technology, and it will not go away soon, because the catalyst design challenge is enormous. But catalyst testing technology has advanced markedly since the Haber-Bosch-Mittasch discoveries, and advances in catalyst testing methodology keep improving, with many highly automated rapid-throughput test systems now in operation worldwide, many of them operating on quite a small scale. Most important for this essay, and reflecting the give and take of the Faraday Discussion, the science of surface catalysis has advanced markedly since Haber’s day. An historically important example illustrating how fundamental understanding guided catalyst discovery was introduced by Corma to start the Discussion. The prototypical process is petroleum naphtha reforming, which emerged more than 50 years ago in work by Vladimir Haensel and coworkers at UOP.1 Major goals were to increase the octane numbers of hydrocarbons in fuels such as petrol (gasoline). We can describe the reasoning that led to a breakthrough catalyst and a novel process as emerging from the inventors’ recognition of catalyst functions —they realized that hydrogenation and dehydrogenation reactions are essential in the naphtha reforming process and are catalyzed by the surfaces of metals (e.g., nickel, palladium, platinum) and that reactions that lead to octane number increases result from increased branching of hydrocarbons such as olefins—which are known to be catalyzed by strong acids, which protonate the olefins to give reactive intermediates (carbocations) that readily undergo skeletal isomerization. A practical goal was thus to increase the branching of paraffins (principal components of naphtha) to increase their octane numbers, and a route to this branching involves (1) a catalyst that dehydrogenates the paraffin on a metal surface, to give olefins, which are (2) isomerized by the acid catalyst and then (3) rehydrogenated on the metal surface. Seeing the value of having a high catalyst surface area per unit volume for high reaction rates per unit volume, Haensel and coworkers saw the value of having their metal dispersed finely as small particles (nanoparticles) to give a high metal surface area and to place these particles directly on a porous acidic support, so that the products of the initial metal-catalyzed dehydrogenation immediately migrate to the nearby acidic sites, where isomerization occurs, allowing the ensuing branched product in turn to move quickly to the metal for the final step of rehydrogenation. Sterba and Haensel1 selected transition aluminas as their catalyst supports, because these are inexpensive and can be prepared as porous materials with high internal surface areas. Knowing that the acid had to be strong and that aluminum chloride (in the presence of water) 2 is a strong Brønsted acid, they treated their support to convert it to a surface chloride to increase its acid strength and its catalytic activity for isomerization. The resulting catalyst consisted of metal nanoparticles on the support, and it is called a bifunctional catalyst, because the metal function gives the olefin from the paraffin, the acid function gives the branched olefin from the unbranched olefin, and the metal function gives the branched (high- octane-number) paraffin from the branched olefin. Because the equilibrium limitation allows the formation of only little olefin under the processing conditions, the olefin has to be transported efficiently to the metal to drain it away and speed up the overall reaction, and so the two functions have to be located close to each other on the catalyst surface and the metal particles need to be small. The optimal metal was found to be platinum, and catalyst performance testing showed it to offer not only the expected hydrogenation/dehydrogenation activity but also high activity for dehydrocyclization reactions to give aromatic compounds such as benzene and toluene formed from n-hexane and n-heptane, respectively. These aromatic compounds have high octane numbers and were considered good fuel components (but they are minimized in applications today because they are carcinogenic). The bifunctional catalyst was discovered substantially on the basis of early catalyst design principles—understanding of what combination of catalyst functions was needed and what kinds of materials provided them. Subsequent work in various industrial organizations, again mostly done by trial-and-error, led to better catalysts that contain not just platinum in the supported particles, but a combination of platinum with another metal, rhenium or tin. Today, researchers seeking better catalysts have far better tools for characterization of catalysts and surface reactions than were available half a century ago. Corma illustrated the thought processes that led his research group to new, markedly improved catalysts for selective reduction of nitroaromatic compounds, as described in the following section. His methodology leads us toward an understanding of how to approach the idea of catalyst design in a realistic and nuanced way. Corma’s case studies of supported metal catalysts for selective reduction of nitroaromatic compounds: toward a working definition

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