Showing papers in "Advances in Catalysis in 1993"

Selectivity Control and Catalyst Design in the Fischer-Tropsch Synthesis: Sites, Pellets, and Reactors

Abstract: Publisher Summary This chapter focuses on selectivity control and catalyst design in the Fischer-Tropsch (FT) synthesis. Chain growth during the FT synthesis is controlled by surface polymerization kinetics that place severe restrictions on our ability to alter the resulting carbon number distribution. Intrinsic chain growth kinetics are not influenced strongly by the identity of the support or by the size of the metal crystallites in supported Co and Ru catalysts. Transport-limited reactant arival and product removal, however, depend on support and metal site density and affect the relative rates of primary and secondary reactions and the FT synthesis selectivity. Diffusion-limited removal of products from catalyst pellets leads to enhanced readsorption and chain initiation by reactive α-olefins. Diffusive and convective transport processes introduce flexibility in the design of catalyst pellets and in the control of FT synthesis selectivity. The model is proposed in the chapter that describes the catalytic behavior of more complex Fe based materials, where several chain termination steps and highly non-uniform and dynamic surfaces introduce additional details into the models required to describe FT synthesis selectivity models.

Zeolite-Supported Transition Metal Catalysts

Abstract: Publisher Summary This chapter discusses zeolite-supported transit ion metal catalysts. In catalytic applications, zeolites are predominantly used in their acidic form. The most important process in this category is fluidized catalytic cracking, based on rare-earth-exchanged zeolites, mainly X and Y of the faujasite structure with small admixtures of ZSM-5. Another industrial process in this group is catalytic dewaxing using mordenite and ZSM-5. Within the vast group of zeolite catalysts, the focus in the chapter is on one subgroup: materials that contain reduced particles of a transition metal or several transition metals dispersed inside zeolite cavities. A majority of transition metal/zeolite catalysts are bifunctional, i.e., strong acid sites are present in the same zeolite. A process based on a zeolite-encaged metal in the absence of acid sites is the dehydrocyclization of small linear alkanes (such as n -hexane) to aromatics. NO x abatement by zeolite-supported Cu is mentioned briefly in the chapter that illustrates the potential for environmental catalysis; it also opens prospectives for stabilizing elements in unusual valence states, in addition to unusual states of aggregation and complexation.

Oscillatory Reactions in Heterogeneous Catalysis

Abstract: Publisher Summary This chapter focuses on the oscillatory reactions in heterogeneous catalysis. Oscillations are possible on all levels in a catalytic reactor, from the single-crystal plane to the crystallite to the catalyst pellet to the packed-bed reactor, and each level adds another degree of complexity. So, it is necessary to isolate the major influences at each level and to separate the characteristics of the oscillations on one level from the effects caused by coupling with other levels. Only when each level is well understood is it possible to fully understand the overall oscillatory behavior. Oscillations in heterogeneous catalysis, therefore, remain an intriguing and demanding problem. There are four primary reasons for studying oscillatory catalytic reactions. (1) Many surface reactions oscillate; probably all bimolecular oxidation reactions on transition metals will exhibit oscillations under some conditions. (2) They are intriguing and poorly understood phenomena, with no universally acknowledged mechanism. (3) Oscillatory states of reactors are potentially dangerous in chemical plants. In order to avoid these states, it is important to learn about the conditions favoring the occurrence of oscillations. (4) In spite of the potential hazards of oscillatory states, there are in some cases possible benefits to operating reactors in unstable regimes.

Catalytic synthesis of chlorofluorocarbon alternatives

Abstract: Publisher Summary This chapter describes the synthetic routes available to several of the hydrofluorocarbon (HFC) and hydrochlorofluorocarbon (HCFC) alternatives to chlorofluorocarbons (CFCs) under consideration for commercial development, as well as some that have already been commercialized The need for retention of hydrogen in the molecule has led to the development of a wide variety of catalysts and processes that are selective in this regard Starting materials for such processes have included hydrocarbons, halohydrocarbons, olefins and haloolefins The general processes of HF addition to olefins, halogen exchange, isomerization, disproportionation, chlorofluorination, and hydrogenolysis constitute the majority of the approaches The chapter discusses the catalytic synthesis of key CFC alternatives An ideal CFC substitute is one that offers identical performance properties of the CFC it replaces with the added stipulation that it does not destroy the ozone layer and has low global warming potential Because CFCs are in such widespread use and realistic HFCs for every need is not been identified, HCFCs, when available, is suggested as bridging molecules to fulfill a short-term need

Molecular mobility measurement of hydrocarbons in zeolites by NMR techniques

Abstract: Publisher Summary This chapter reviews the most relevant experimental methods to follow molecular translations and/or reorientations of guest molecules in zeolite pores. The benefit of combining these techniques is illustrated by a series of diffusion studies of hydrocarbons adsorbed in zeolite catalysts. Because of the key role of structure-related diffusion processes in shape-selective catalysis and the unique possibilities of the nuclear magnetic resonance (NMR) self-diffusion techniques to investigate such processes, the fundamentals of the NMR self-diffusion techniques are presented in the chapter and examples are given of their application to characterize zeolite catalysts. The NMR results (self-diffusion coefficients, anisotropic diffusivities, jump lengths, and residence times) can be correlated with corresponding neutron scattering data and sorption kinetics as well as molecular dynamics calculations, thus giving a comprehensive picture of molecular motions in porous solids. Analyzing the self-diffusion behavior of guest molecules in a microporous catalyst by the combined application of pulsed-field gradient NMR self-diffusion techniques reveals the spatial distribution of transport resistances over the catalyst particles.

Application of Percolation Theory to Describing Kinetic Processes in Porous Solids

Abstract: Publisher Summary This chapter focuses on the application of percolation theory to describing kinetic processes in porous solids, including desorption of condensate, mercury penetration, and catalytic deactivation by site coverage and pore blockage. All these processes are of fundamental importance for heterogeneous catalysis. In the framework of this theory, the medium is defined as an infinite set of sites interconnected by bonds. Percolation theory can be applied to porous solids via identification of network sites with voids, and bonds with necks. Thus, the theory is applicable primarily to spongy porous structures but in some cases also to corpuscular structures.Percolation theory has been successfully used to analyze condensate desorption from porous solids, mercury penetration into porous solids, and the kinetics of catalytic deactivation by site coverage and pore blockage. The chapter briefly outlines the main ideas and results of percolation theory, which are of interest from the point of view of describing the kinetic processes in porous solids.

Catalysis by metal ions intercalated in layer lattice silicates

Abstract: Publisher Summary This chapter discusses the catalysis by metal ions intercalated in layer lattice silicates. Fluorotetrasilisic mica is a synthetic swelling layer lattice silicate. Because of the cation-exchange properties and the lack of acid sites on the sili cate sheet, fluorotetrasilicic mica (TSM) is a particularly appropriate carrier for use in the study of the intrinsic activities of metal ions in heterogeneous catalysis. The catalytic activities of metal ion-exchanged TSMs for the methanol conversion are individually different, depending on the metal ion. The interlayer spaces of layer lattice silicates are able to be utilized as a microvessel of catalyst solution, and all of the interlayer metal ions are effectively incorporated into the reaction when the interlayer spacing is expanded to a width great enough for the reactants to diffuse into the interlayers. Recently, the need for new catalytic materials has revived special interest in the layer lattice silicates because of their ion-exchange properties and their expandable layer structures. It is expected that the material shows the characteristic activity of the metal ion when the silicate sheet is catalytically inactive.