Paul B. Weisz

Interpretation of Measurements in Experimental Catalysis

Publisher Summary This chapter discusses the interpretation of measurements in experimental catalysis. It focuses on the most frequent problem in the study of heterogeneous catalysis—that is, the determination of a quantity called “catalyst activity,” the experimental procedures, and methods of interpretation leading to a proper description of this quantity. Catalyst activity manifests itself in terms of a chemical reaction rate characteristic of the presence of the catalyst. It is always such a reaction velocity that is the subject of primary experimental data from which a description of the catalysts activity must be derived. The reaction velocity under any particular operating condition may well be affected by a variety of more or less independent variables. For example, in many fundamental researches, relationships are sought with the composition, crystal lattice, and electronic structure of the catalyst solid, yet measured reaction velocities are often influenced by gas transport phenomena that are unrelated to any of these fundamental properties. Gas transport phenomena and diffusion effects have been described occasionally in the literature. The powerful role played by these in determining the nature of experimental data cannot be overemphasized. The chapter discusses the behavior of catalysts related to the design and the interpretation of catalytic experiments in the course of fundamental research.

Molecular shape selective catalysis

Molecular shape selective catalysis in intracrystalline space of siliceous zeolites has opened up a spectrum of new opportunities in catalytic science and in industry. The mechanisms involved include: (a) selective, diffusional mass transport dynamics, and (b) steric modification of the intrinsic reaction rates.

Transport and reactivity of hydrocarbon molecules in a shape-selective zeolite

For the catalytic cracking of C6 to C9 hydrocarbons on ZSM-5, we demonstrate quantitatively the contributions of each of two mechanisms for molecular shape selectivity. Using crystallites of different sizes and activities, and classical methods for evaluating diffusion inhibition of the reaction rate, we separate the effects of mass-transport-induced selectivity from that created by steric inhibition by the size of a reaction complex. The selective cracking of n-paraffins compared to monomethyl paraffins (from C6 to C9) is due to a higher intrinsic rate constant of the n-paraffin, with diffusional mass transport playing no appreciable role. In contrast, dimethyl paraffin cracking is strongly diffusion-inhibited. The methyl paraffin/n-paraffin discrimination is a result of steric constraint on the sizeable methyl paraffin/carbonium ion reaction complex. This structural selectivity is shown to be absent for the corresponding olefins where such complexes do not arise. The diffusivities at reaction conditions have been determined. For the linear hydrocarbon, diffusivity notably exceeds that expected from the Knudsen model. This reminds us to review assumptions of conventional concepts of mass transport. The availability of zeolites now allows us to probe many basic phenomena in catalysis, molecular configuration and dynamics, including mass transport.

Catalysis by crystalline aluminosilicates: IV. Attainable catalytic cracking rate constants, and superactivity

Abstract Crystalline aluminosilicate zeolite catalysts possessing a number of different crystal structures and ionic forms are characterized by rate constants for the cracking of n -hexane of from one to more than four orders of magnitude greater than the activity of conventional amorphous siliceous oxide cracking catalysts. Some are highly molecular shape selective at similarly high “superactivities.” Product spectra change with the temperature at which activity is achieved. Apparent activation energies are remarkably uniform. Catalysts can now be made that span a wide range of activities, from ones that crack at conventionally high temperatures to those that approach the low-temperature performance of acid catalysts of the aluminum-hydrogen halide type.

Combustion of carbonaceous deposits within porous catalyst particles: II. Intrinsic burning rate

Abstract All observations concerning intrinsic burning rates of carbonaceous deposits on various porous oxide supports are consistent within a relatively simple mechanistic framework of behavior. 1. (1) For a given support, the rate of carbon burnoff is largely independent of the source and structure of the coke. 2. (2) The kinetics of coke burning in oxides of silica, alumina, and magnesia show both quantitative and qualitative resemblances to the combustion of pure graphite, and suggest the existence of a basic, uncatalyzed burning rate on such oxides. 3. (3) The presence of oxides of transition metals results in coke burning rates up to several orders of magnitude higher than the aforementioned uncatalyzed rate at any given temperature. 4. (4) An examination of the Arrhenius plots of catalyzed and uncatalyzed coke burning rates presents a challenge to mechanistic interpretation, particularly of the catalytic effect of metal additions.

Effective diffusivities in zeolites. 1. Aromatics in ZSM-5 crystals

Abstract Real or apparent inconsistenceis of up to several orders of magnitude exist in reported diffusivities of zeolites. Some are caused by differences in definition, usage, and methodologies in their determination. Some result from operating conditions involving transport mechanisms deviating from a strictly stochastic process. Diffusivities effective in non-steady-state methods of determination (e.g., uptake diffusivities) can be translated into diffusivities applicable to characterization of catalytic (i.e., to steady-state) behavior. For ortho -xylene and other aromatic molecules in ZSM-5, the magnitude of these diffusivities follows the expectations of shape selectivity; i.e., they decrease systematically with increasing effective minimum dimension of the molecules. These diffusivities are found to be invariant in both temperature and for a 360-fold variation in applied concentration. A mechanistic model for activated zeolite diffusion, involving inernalized molecules of more than one type (energy level), is suggested to be useful in future considerations of the relationship of intrinsic and effective diffusivities and their activation energies to shape, structure, and molecular mechanism.

Affinity of fibroblast growth factors for β-cyclodextrin tetradecasulfate

Abstract β-Cyclodextrin tetradecasulfate was found to have a very strong affinity for fibroblast growth factor (FGF) and could substitute for heparin in FGF purification. Basic FGF was purified about 200,000-fold from a rat chondrosarcoma using a method of biaffinity chromatography in which the β-cyclodextrin tetradecasulfate polymer was mixed with copper-Sepharose. This method takes advantage of the strong affinity of FGF for both β-cyclodextrin tetradecasulfate and copper.

Hydrocarbon conversion over crystalline aluminosilicates containing S, Se, and Te

Contact with H2S, or S followed by oxygen (air), or with SO2 alone, raises the catalytic activity of NaX aluminosilicate for n-hexane cracking by a factor of five to ten. In each case, the same sulfur zeolite complex appears to be formed. The sulfur contents correspond to one sulfur atom for two of the mobile sodium atoms of the aluminosilicate lattice. Combination of NaX with Se, and with Te, leads also to active catalytic solids; however, the catalytic characteristics shift from carbon-carbon cracking to dehydrocyclization.

Combustion of carbonaceous deposits within porous catalyst particles: III. The CO2CO product ratio

In the combustion of carbonaceous deposits (“coke”) on porous catalysts, a large spectrum of CO2CO product ratios is observed. It results from a definite initial ratio at the burning site dependent on temperature only, and subsequent oxidation of CO to CO2 determined by a catalytic rate constant and the rate of diffusional escape of the carbon monoxide from the particle.

Inhibition of smooth muscle cell proliferation and experimental angioplasty restenosis by beta-cyclodextrin tetradecasulfate.

Heparin inhibits smooth muscle cell proliferation in vitro, a property that makes it potentially useful in preventing restenosis after angioplasty. Its utility in this setting is limited by the inability to use high doses (secondary to anticoagulant effects) and the need for subcutaneous administration. We tested the ability of beta-cyclodextrin tetradecasulfate (CDT), a nonanticoagulant synthetic heparin mimic, to inhibit smooth muscle cell proliferation in vitro and tested its efficacy when orally administered for the prevention of angioplasty restenosis in a rabbit atherosclerosis model. Vascular smooth muscle cells were cultured from rabbit aortas by the explant technique. Passaged cells were plated at low density in microtiter plates in the presence or absence of varying concentrations of heparin or CDT in culture medium containing 10% fetal calf serum. Using both 3H-thymidine incorporation and total protein assays, both heparin and CDT caused a similar dose-dependent inhibition of proliferation. We next tested the effect of orally administered CDT in the prevention of restenosis in focal femoral artery arteriosclerotic lesions created in hypercholesterolemic New Zealand White rabbits by air-dessication endothelial injury and subsequent peripheral angioplasty. Animals were followed up for 1 month and were fed normal chow supplemented by tap water with or without CDT. In animals receiving the highest concentration of CDT (2 mg/mL drinking water), the percentage of arterial cross-sectional area with intimal hyperplasia decreased from 50.5 +/- 1.7% (control) to 26.9 +/- 2.2% (p < 0.001), with the intimal/medial ratio being decreased from 1.4 +/- 0.4 to 0.5 +/- 0.2 (p = 0.056).(ABSTRACT TRUNCATED AT 250 WORDS)