Alfred Clark

Olefin Polymerization on Supported Chromium Oxide Catalysts

Abstract There are at least three heterogeneous catalyst systems of interest for low-pressure polymerization of olefins to high polymers:

Reduction studies on supported metal oxide catalysts

Abstract A procedure is described for determining the reduction characteristics of metal oxidepromoted catalysts. The method involves circulating a measured volume of hydrogen in a closed system that includes a suitable tube for the catalyst, an absorption tube for removing the water formed, and a manometer for measuring the consumption of hydrogen. Reductions may be conducted either at a definite temperature or the temperature may be increased on a fixed time schedule. In the latter case, regular temperature and pressure readings provide information necessary to plot “reduction profiles” in which the reduction rate is plotted versus temperature (or time). Characteristic curves are obtained for various promoter-support combinations and the areas below the “reduction profile” curves are proportional to the amount of reduction. However, the latter is obtained more precisely by accurate measurement of the hydrogen consumption. The procedure was used for studying the reduction characteristics of supported nickel oxide and chromium oxide catalysts as affected by methods of preparation, kinds of support, and the effects of heat treatment, temperature, and promoter concentrations. The results showed that promoter oxide-support oxide interaction increased the difficulty of reducing the promoter metal oxide. This effect was most pronounced for nickel oxide when supported on alumina and for chromium oxide supported on silica. Coprecipitated catalysts were more difficult to reduce than those prepared by impregnation, apparently because of better distribution and therefore better opportunity for interaction. For the same reason, higher temperatures of catalyst heat treatment resulted in increased difficulty of reduction. The interaction between the metal oxide promoter and the support oxide appears to have an important effect on the catalytic properties of metal oxidepromoted catalysts. This is illustrated by results obtained with chromium oxide—silicaalumina catalysts.

Formation of high polymers on solid surfaces I. Theoretical study of mechanisms

Abstract Four models are set up for polymerization on solid surfaces. In two of the models, it is assumed that all adsorption sites have the same adsorption energy. The first of these (Rideal mechanism) assumes that polymerization occurs by reaction of monomer in the gas phase with adsorbed monomer or adsorbed, growing polymer chains. The second model (Langmuir-Hinshelwood mechanism) assumes that polymerization occurs by reaction of an adsorbed monomer molecule with an adjacently adsorbed monomer molecule or growing polymer chain. The other two models are the counterparts of the first two with a variation of adsorption energies among the sites. With all adsorption sites of equal energy, it is shown that in both mechanisms there is a unique weight average molecular weight for each number average molecular weight. Weight average-number average molecular weight ratio, ( W N ), a measure of the broadness of molecular weight distribution, is shown to be in the range, 1 ⩽ W N ⩽ 2, for all values of velocity constants and pressure. In the Rideal mechanism, number and weight average molecular weights and rates of reaction increase without limit as pressure is increased. In the Langmuir-Hinshelwood mechanism, molecular weights and rates approach a maximum value asympotically as pressure is increased. With a distribution of adsorption energies, weight average-number average molecular weight ratios ( W N ) can have values far in excess of 2 for both mechanisms. Molecular weights and rates of reaction again level out with increasing pressure for the Langmuir-Hinshelwood mechanism, and increase without restriction in the Rideal mechanism.

The nature of silica-alumina surfaces: I. Thermodynamics of adsorption of ammonia

Abstract The adsorption of ammonia on eleven silica-alumina gels ranging in composition from pure alumina to pure silica has been studied. Isosteric heats and differential surface entropies have been determined from adsorption isotherms at 50-degree intervals in the range 100–400 °C. The adsorption data are correlated satisfactorily by means of the Freundlich equation. Isosteric heats drop off sharply with increasing coverage for all the gels. Compositions in the range of 55–90% silica have the largest fraction of weak adsorption sites. This is the region which exhibits high catalytic activity for various acid-type reactions. The magnitude of the differential surface entropies indicates that in this range most of the adsorbed molecules are in a highly mobile state.

Rate temperature maxima for the olefin disproportionation reaction

Abstract Olefin disproportionation on cobalt molybdate-alumina shows a rate-temperature maximum which is caused by the reversible deactivation of sites superimposed on the irreversible poisoning of sites. A Langmuir-Hinshelwood model applied to a heterogeneous surface can be used to describe both the general kinetics and the reversible rate-temperature maxima which are observed.

Mass transfer effects in the olefin disproportionation reaction: I. Promoter concentration and temperature effects for propylene on WO3-silica catalysts

Abstract Interphase mass transfer effects have been observed in propylene disproportionation with WO 3 -silica catalysts, although calculations based on the external catalyst area predict the mass transfer rate to be several orders of magnitude greater than the observed rate of product formation. These diffusional effects were noted at all levels of conversion obtained by changes in WO 3 promoter concentration and reactor temperature. A reduction in the WO 3 concentration from 5.0 to 0.2 wt% reduced catalyst activity by a factor of ~120; however, the systematic dependence of conversion on linear velocity persisted. The reaction rate varied exponentially with temperature having an apparent activation energy of from 37–59 kcal/mole. These unexpected mass transfer limitations are, of course, difficult to resolve in the light of existing theory which predicts a weak temperature dependency and a constant rate of reaction with changing promoter concentration (on catalyst particles of the same size). These observed mass transfer effects may be accounted for by assuming that reaction occurs on a small number of very active sites which are widely separated on the catalyst surface. Hence, the reaction may be limited by site-localized diffusion effects which are a function of Reynolds number, WO 3 concentration, and temperature.

The nature of silica-alumina surfaces II. Cracking of n-octane, polymerization of propylene, o-xylene isomerization, hydrogen transfer, and HD exchange

Abstract In Part I the thermodynamics of ammonia adsorption on eleven silica-alumina gels ranging from pure alumina to pure silica was presented. In this paper, thermodynamic quantities are shown to correlate qualitatively with catalytic reactions occurring on these materials. The catalyst compositions showing highest activity for acid-type reactions are those possessing the highest differential surface entropies, indicating mobile adsorption.

The mechanism of propylene disproportionation

Abstract Disproportionation of [1- 14 C] propene and [2- 14 C] propene has shown that the postulated four-center mechanism is correct over a cobalt-molybdate-alumina catalyst. At temperatures above 60 °C, the isomerization activity of the catalyst becomes a factor, so that at 160 °C, nearly one-half the [1- 14 C] propene has isomerized to [3- 14 C] propene prior to disproportionation. From experimental values of the rate constant for disproportionation and the degree of isomerization, rate constants for isomerization are calculated. The apparent energy of activation of the disproportionation reaction was found to be 7.7 kcal/mole and for the isomerization reaction 17.3 kcal/mole.

Nature of the sites on fluorided alumina

Abstract The reactions of 1-butene with fluorided alumina containing 1.0, 7.5, and 12.1% have been investigated using ammonia-blocking and deuterium-tracer techniques. On all catalysts there was initial formation of a polymeric complex. Isomerization then continued under steady-state conditions. Ammonia-blocking studies indicate that complex formation is confined more to the higher energy sites than is the same reaction on silica-alumina. These data also suggest that the isomerization activity is associated with the presence of the complex. Products obtained from passing pulses of 1-butene over perdeuterobutene-treated catalyst contain significantly more deuterium than does the starting material. This is evidence that the complex participates in the isomerization reaction. The complex probably furnishes the protons to form carbonium ions necessary for reaction. The possibility that water present in low concentrations on fluorided alumina may participate in the reaction cannot be excluded. The evidence indicates that complex formation depends at least in part on fluoride content and that isomerization depends on both fluoride content and the presence of the complex.

Formation of high polymers on solid surfaces II. Polymerization of ethylene on chromium oxide-silica-alumina catalysts

Abstract The four models of the preceding paper (1) (Part I) are compared with the experimental results of polymerizing ethylene over chromium oxide-silica-alumina catalyst. Experimental curves are shown for number average molecular weights, weight average molecular weights, their ratios, and rates of polymerization as functions of pressure. The effects of temperature variations on these quantities are discussed qualitatively. The conclusion is reached that the Langmuir-Hinshelwood mechanism on an energetically heterogeneous surface duplicates the general character of the experimental curves most closely.