Catalysis Looks to the Future (1992) / Chapter Skim
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3 RESEARCH OPPORTUNITIES IN CATALYTIC SCIENCE
3 RESEARCH OPPORTUNITIES IN CATALYTIC SCIENCE
Pages 43-64
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From page 43... ...
, and this too can result in savings in capital and operating costs. The amount of reactant required to produce a unit of product, the properties of the product, and the amount of energy required to separate the desired product from reactants and by-products are all governed by catalyst selectivity.
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From page 44... ...
Knowledge derived from scientific studies provides a basis for conceiving new catalysts and catalytic reactions, and for interpreting the results of experimental observation. Moreover, many of the analytical techniques developed in the pursuit of catalytic science can be used effectively to elucidate cause-effect relationships and, thereby, to accelerate the process of catalyst development.
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From page 45... ...
Based on laboratory-scale tests, the resulting material exhibits a catalytic activity 50% higher than that previously available and promises improved poison resistance due to its bimodal pore structure. This illustration shows the manner in which knowledge of material properties can be combined with an analysis of reaction dynamics and mass transfer to design a catalyst with optimal performance characteristics for a targeted .
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From page 46... ...
The discovery of a great variety of structures and chemical compositions in the aluminophosphate molecular sieve family of crystals opens opportunities to prepare molecular sieves with transition metal cations in the crystal framework. This unique, predictable positioning of transition metal ions in microporous crystal lattices offers opportunities in oxidation, redox, or other transition-metal-related catalysis.
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From page 48... ...
Recent studies have demonstrated rate accelerations for a number of antibody-catalyzed reactions of up to 106 times the uncatalyzed rate. A development in the design of catalytic antibodies involves the introduction of catalytic activity into antibodies via either molecular biological , , ~ or chemical means.
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From page 49... ...
49 Table 3.1 Experimental Techniques for Characterizing Catalysts and Adsorbed Species Technique Acronym Type of Information Low-energy electron LEED Two-dimensional structure and registry diffraction with metal surface Auger electron spectroscopy AES Elemental analysis X-ray photoelectron XPS Elemental analysis and valence state spectroscopy Ion scattering ISS Elemental analysis spectroscopy Ultraviolet photoelectron UPS Electronic structure spectroscopy Electron energy loss EELS Molecular structure spectroscopy Infrared spectroscopy IRS Molecular structure Laser Raman spectroscopy LRS Molecular structure X-ray diffraction XRD Bulk crystal structure Extended x-ray absorption EXAFS Bond distance and coordination fine structure number Transmission electron TEM Crystal size, shape, morphology, microscopy and structure Scanning transmission STEM Microstructure and composition electron microscopy Scanning tunneling microscopy Ultraviolet spectroscopy Mossbauer spectroscopy Nuclear magnetic resonance NMR spectroscopy STM Microstructure Electronic state Ionic state Molecular structure and motion
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From page 50... ...
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From page 51... ...
In homogeneous solution the understanding of reaction mechanisms is aided greatly by the broad knowledge of the chemistry of homogeneous acid solutions. Understanding the chemistry over solid acids is hampered by a lack of absolute acidity measurements or of sound comparisons between liquid and solid acids.
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From page 52... ...
Significant advances have also been made in the characterization of enzymes, catalytic antibodies, and homogeneous catalysts. Where such catalysts can be obtained in the form of single crystals, the analysis of x-ray diffraction patterns can provide a complete atomic structure.
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From page 53... ...
, NMR spectroscopy, and EXAFS. MECHANISM AND DYNAMICS OF CATALYTIC REACTIONS Knowledge of catalytic reaction mechanisms in particular the structure, dynamics, and energetics of reaction intermediates formed along the catalytic reaction path can provide insights for developing new catalysts and improving existing ones.
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From page 54... ...
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From page 55... ...
Vibrational spectroscopies, such as electron energy loss spectroscopy (EELS) and infrared spectroscopy, and 13C NMR spectroscopy have revealed that paraffins are only weakly adsorbed but undergo a loss of hydrogen on heating.
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From page 57... ...
This type of information has proved useful in understanding how zeolite acidity and pore size influence the rate at which deactivation occurs. The selective catalytic reduction of nitric oxide by ammonia over titaniasupported vanadia catalysts provides an effective means for reducing NOx emissions from stationary sources.
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From page 58... ...
In summary, investigations of reaction mechanisms and kinetics and, especially, in situ observations of catalytic reaction intermediates are essential for advancing the science of catalysis, inasmuch as the results of such studies provide an overall view of catalysis and help elucidate the relationships between catalyst structure and function. The current interest in developing catalysts for the production of environmentally benign gasoline, the abatement of air pollution from mobile and stationary sources, the synthesis of enantiomerically pure drugs, and the synthesis of novel polymers all benefit from studies of the relevant reaction mechanisms and kinetics.
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From page 59... ...
There are an increasing number of examples in the literature indicating the feasibility of computing the potential energy curves for elementary steps in catalyzed reactions of small molecules. Figure 3.2 illustrates a potential energy diagram obtained from an ab initio quantum chemical study of ethylene hydrogenation by using a model Wilkinson catalyst, RhCl(PH3~3.
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From page 60... ...
. mser~lon ACHE CH2 O/efin coordination Figure 3.2 Potential energy diagram obtained from an ab initio quantum chemical study of ethylene hydrogenation by using a model Wilkinson catalyst.
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From page 61... ...
Promising progress has also been made recently in the use of ah initio quantum chemical techniques to characterize the interaction of reactant molecules with acid sites within a zeolite. Such calculations can provide a description of the reactant-zeolite potential surface, from which it will then be possible to define the reaction intermediates and to determine the rate coefficients for chemical transformations.
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From page 62... ...
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From page 63... ...
In summary, the application of theoretical methods to describe and predict the properties of catalysts is a rapidly developing area of research that can contribute much to the scientific understanding of catalysts and the chemical reactions occurring under their influence. Critical areas for future research include the computation of accurate catalyst-substrate potential surfaces by using ab initio quantum chemical techniques, the evaluation of rate and diffusion coefficients via molecular dynamics and transition state theory, the application of Monte Carlo techniques for describing the dynamics of
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From page 64... ...
There is also a need to link heterogeneous catalytic phenomena to the broader knowledge base in solutions and in well-defined metal complexes. Substantial progress and scientific breakthroughs have been made in recent years in several fields including atomic resolution of metal surfaces, in situ observation of an olefin complexed to zeolite acid sites by NMR spectroscopy, and in situ characterization of several reaction intermediates by a variety of spectroscopic techniques.
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