Monoliths in Heterogeneous Catalysis

Nitration of aromatics is a key process in the catalytic manufacturing of semiproducts for the synthesis of drugs, pesticides, and dyes. However, the large-scale synthesis, involving the concentrated mineral acids used as catalysts, makes nitration one of the most environmentally harmful processes, producing huge amounts of organic wastes, exhausted acids, and the products of their neutralization. A search for environmentally safer catalysts and nitrating agents appears to be an actuality. Numerous data concerning the aromatics nitration in homogeneous media have been summarized in a set of books and reviews [l-41. Recent papers are of considerable interest, since they reflect the attempts to perform nitration on heterogeneous catalysts, for example, zeolites [5–8]. The data on aromatic nitration on the surface of solid acids [9, 111 and in the gas phase [12–14] allowed us to clarify the role of intermediate Tand a-complexes, estimate the substrate and position selectivity, and elucidate the main approaches of nitration mechanism in various media. In this review the data concerning nitration on solid catalysts are summarized. We hope this information will make it possible to estimate the perspectives of heterogeneous catalyst application on a large scale.     

Monoliths in Heterogeneous Catalysis

The use of structured catalysts in the chemical industry has been considered for years. Conventional fixed-bed reactors have some obvious disadvantages such as maldistributions of various kinds (including a nonuniform access of reactants to the catalytic surface), high pressure drop in the bed, etc. Structured catalysts are promising as far as elimination of these setbacks is concerned. Two basic kinds of structured catalysts can be distinguished:

1. Structural packings covered with catalytically active material, similar in design to those used in distillation and absorption columns and/or static mixers. Good examples of catalysts of this kind are those offered by Sulzer, clearly developed by Sulzer column packings and static mixers. As in packed beds, there is an intensive radial convective mass transport over the entire cross-section of these packings. Structural packing catalysts and the reactors containing them are, however, not within the scope of this review.
2. Monolithic catalysts are continuous unitary structures which contain many small, mostly parallel passages. A ceramic or metallic support is coated with a layer of material in which active ingredients are dispersed. An interaction between these passages can occur if walls are permeable. The catalytically active material is present on or inside the walls of these passages. Radial mass transport can occur only by diffusion through the pores of the permeable walls.

     

Fractals in Heterogeneous Catalysis

In a recent review on the influence of particle size on catalytic properties of supported metals [l], evidently a most important aspect of heterogeneous catalysis by any account, the authors end on a fairly discouraging note since they suggest doing even more work — with ever more sophisticated techniques and methods as well as normalized procedures — on top of an already formidable amount of literature. At such a state of the art it may be appropriate to reflect and to ask whether full pursuit of reductionism as scientific method needs fresh thinking in order to progress more effectively. Perhaps the difficulties are rooted in emphasis on mere data collection, the lack of a good empirical guiding principle, inappropriate models; in brief, the frequent simplification of a vastly complicated object — the real catalyst — into the sum of phenomena of better-understood idealized subsets. It might even be argued that a thorough knowledge of a well-characterized prototype or a particular model catalyst would be actually of little help in seeing the real world where catalysts are of irregular shape, suffer sintering, become poisoned; in other words, lose their ideal state after some time of operation. Also, we realize that a change in “quantity” often engenders a change in “quality”: For instance, a material with a BET area of hundreds of square meters per gram can hardly be considered to have a surface like a huge tennis court or ball field: it is rather “convoluted” in some very irregular fashion, having a shape that is difficult to realistically construct as a linear combination of planar portions.     

Nano-Particles in Heterogeneous Catalysis

The introduction of in situ techniques has had a vast impact on research and development in the area of heterogeneous catalysis as emphasized in many reviews and monographs. Recently, the number of in situ techniques that can give information at the atomic scale has increased significantly and new possibilities exist for making the measurements under industrially relevant conditions. In order to fully exploit the results from the in situ and operando studies, it has also become increasingly gainful to combine the experimental studies with theoretical methodologies based on, for example, Density Functional Theory (DFT). This has allowed one to extract more detailed atomic-scale information from the measurements and it has also allowed the establishment of detailed structure-activity relationships. Furthermore, the interplay between in situ techniques and theory has helped bridging the pressure gap such that in situ information obtained at conditions far from industrial ones may be used in a more relevant manner. Here, we will illustrate how microscopy-, spectroscopy- and X-ray-based techniques in combination with experimental and theoretical surface science methods can aid industrial catalyst developments. We will do this by presenting examples of our current understanding and latest developments in the areas of heterogeneous nano-particle catalysts for methanol synthesis, steam reforming and hydrotreating.     

Fractals in Heterogeneous Catalysis

In a recent review on the influence of particle size on catalytic properties of supported metals [l], evidently a most important aspect of heterogeneous catalysis by any account, the authors end on a fairly discouraging note since they suggest doing even more work — with ever more sophisticated techniques and methods as well as normalized procedures — on top of an already formidable amount of literature. At such a state of the art it may be appropriate to reflect and to ask whether full pursuit of reductionism as scientific method needs fresh thinking in order to progress more effectively. Perhaps the difficulties are rooted in emphasis on mere data collection, the lack of a good empirical guiding principle, inappropriate models; in brief, the frequent simplification of a vastly complicated object — the real catalyst — into the sum of phenomena of better-understood idealized subsets. It might even be argued that a thorough knowledge of a well-characterized prototype or a particular model catalyst would be actually of little help in seeing the real world where catalysts are of irregular shape, suffer sintering, become poisoned; in other words, lose their ideal state after some time of operation. Also, we realize that a change in “quantity” often engenders a change in “quality”: For instance, a material with a BET area of hundreds of square meters per gram can hardly be considered to have a surface like a huge tennis court or ball field: it is rather “convoluted” in some very irregular fashion, having a shape that is difficult to realistically construct as a linear combination of planar portions.     

New Challenges in Heterogeneous Catalysis for the 21st Century

Abstract  

Heterogeneous catalysis has been around for a long time, but has still much room to grow. The empirical trial-and-error mode used to develop catalysts in early times has progressively made way for a more molecularly driven approach to their design. Modern surface-sensitive techniques have opened the way to a better understanding of the mechanisms of catalytic reactions and the demands imposed on catalytic sites. Computational studies have added insights into the structural and energetic details of surface species and the kinetic driving forces for specific surface reactions. Novel nanotechnology and synthetic advances have provided new methods to manufacture better-defined catalysts, with high concentrations of the active sites identified by fundamental mechanistic studies. All combined, these advances have led to the design of new catalysts by taking advantage of the size and shape of the nanoparticles used as active phases and of specific structures and the nature of the support. New research has also been directed to the development of more sophisticated nanostructures, to add new functionalities to simpler catalysts or to combine two or more primary functions into one single catalyst. Much progress has been made in these directions, but the new tools are yet to be fully exploited to resolve present limitations in a myriad of catalytic systems of industrial importance, for energy production and consumption, environmental remediation, and the synthesis of both commodity and fine chemicals.     

Alkali Doping in Heterogeneous Catalysis

It has been estimated that approximately 60% of all commercial chemicals are now obtained using processes involving heterogeneous catalysis. In fact, over 90% of all new large-scale plants employ heterogeneous catalysts. This can be seen very clearly in Table 1, which lists the top 50 chemicals for the year 1980 [1].     

Active Sites in Heterogeneous Catalysis: Development of Molecular Concepts and Future Challenges

The concept that catalytic turnover occurs at a small fraction of the surface sites dates back to the 1920s. The application of modern surface science techniques and model catalysts confirmed the presence of active sites and identified their structures in some cases. Low coordination defect sites on transition metals, steps and kinks, or open rough crystal faces that make high coordination metal sites available have been uniquely active for breaking H–H, C–H, C–C, C=O, O=O and NN bonds. Oxide–metal interfaces provide highly active sites for reactions of C–H and C=O bonds. Electron acceptor and proton donor sites are implicated in hydrocarbon conversion (acid–base catalysis), and sites where metal ion–carbon bonds can form are active for polymerization. The observations of dynamic restructuring of catalytic surfaces upon adsorption of reactants indicate that many catalytic sites are created during the chemical reaction. Similar restructuring is detected for enzyme catalysts. The high mobility of both surface metal atoms and adsorbed molecules during the catalytic process observed recently bring into focus the dynamic nature of active sites that may have a finite lifetime as they form and disassemble. The development of techniques that provide improved time resolution and spatial resolution, and can be employed under catalytic reaction conditions will provide information about the time dependent changes of active site structure and molecular intermediates at these active sites as the reaction products form.     

Alkali Doping in Heterogeneous Catalysis

It has been estimated that approximately 60% of all commercial chemicals are now obtained using processes involving heterogeneous catalysis. In fact, over 90% of all new large-scale plants employ heterogeneous catalysts. This can be seen very clearly in Table 1, which lists the top 50 chemicals for the year 1980 [1].     

Metallic Nanoparticles in Heterogeneous Catalysis

Catalysis Letters - Heterogeneous catalysis is a chemical process achieved at solid–gas or solid–liquid interfaces. Many factors including the particle size, shape and metal-support...     

Silicon Carbide: A Novel Catalyst Support for Heterogeneous Catalysis

Progress in developing a new class of support materials based on silicon carbide (SiC)is reviewed. Silicon carbide has superior mechanical and thermal properties which, coupled to chemical inertness,avoids several of the problems inherent in the use of commercial oxide and carbon based supports and catalysts. High surface area SiC can now be prepared easily in a commercially viable shape,with good mechanical properties,and at reasonable cost.I t can be shaped directly into monolith or honeycomb forms including some catalytically active material, rendering fabrication simple and cost effective. Furthermore, it can be modified for specific catalytic applications through the addition of metals. In many respects, it combines the best properties of oxide and carbon based supports without suffering many of their disadvantages.     

Asymmetric Heterogeneous Catalysis: Science and Engineering

Asymmetric heterogeneous catalysis is a vivid branch of catalysis, remaining, however, largely a domain of organic chemists. The view towards asymmetric heterogeneous catalysis adopted in this review is mainly from catalytic science and engineering. Not only reaction mechanisms, but also catalytic properties, kinetic regularities, as well as chemical engineering aspects, are covered with the main focus on recent developments.     

Asymmetric Heterogeneous Catalysis: Science and Engineering

Asymmetric heterogeneous catalysis is a vivid branch of catalysis, remaining, however, largely a domain of organic chemists. The view towards asymmetric heterogeneous catalysis adopted in this review is mainly from catalytic science and engineering. Not only reaction mechanisms, but also catalytic properties, kinetic regularities, as well as chemical engineering aspects, are covered with the main focus on recent developments.     

Space-Resolved Profiling Relevant in Heterogeneous Catalysis

Knowledge of gradients involved in chemical processes, such as heterogeneously catalyzed reactions, on molecular as well as reactor scale is of paramount importance for understanding and optimizing such processes. This review highlights and discusses recent advances in spatially resolved methods for the detection of chemical (structural) and temperature gradients, with particular focus on in situ methods.     

Hf-based Metal-Organic Frameworks in Heterogeneous Catalysis

Among the large family of metal-organic frameworks (MOFs), Hf-based MOFs are foreseen as promising materials for practical applications due to their exceptional characteristics such as rich structure versatility and outstanding stability. This review discusses catalytic applications of Hf-MOFs since their development in 2012, and are summarized according to the location of the active site: at the hafnium node, at the linker or as a single-site chemical entity located at the hafnium node after post-synthetic functionalization. Special emphasis is made to synthetic preparation methods and catalytic performance.     

New Paradigms and Future Critical Directions in Heterogeneous Catalysis and Multifunctional Reactors

Heterogeneous catalysis is a key pillar of the global industrial chemical and petrochemical sector, and 85% of all chemical products are produced with at least one catalytic step. Indeed, catalysis and catalytic reactors are a critical underpinning science for energy, environmental, and economic security. This paper reviews some future critical directions for research in catalysis science, toward a greener and more sustainable future. We believe that even a relatively mature field as heterogeneous catalysis and nanomaterials can be vitalized and spurred by major discoveries, but an outside-the-box thinking and a focused effort in a large plurality of disciplines is necessary. Thus, critical research needs in several areas, including heterogeneous and homogeneous catalysis, biocatalysis, photocatalysis, electrochemical conversions, and computational catalysis, are reviewed. The research needs of the future lie at the intersection of synthesis of novel nanostructured materials with tunable pore size distribution, controlled porosity, and high surface area; development of new catalytic applications for such materials; and the science of advanced characterization including in situ spatiotemporal analysis. In the area of computational catalysis, we believe that the future lies in the development of hybrid methods (parallel and serial) which can model the typical multiscale phenomena that are typically encountered in protein translocation and signal transduction, charge transport, enzymatic catalysis, surface chemistry, and self-assembly in complex fluids. As we promulgate the new directions to the catalysis fraternity, some prior research areas will unfortunately need to be relegated to obsolescence, to maintain a healthy balance on the research forefront.