John T. Gleaves

TAP-2: An interrogative kinetics approach

The TAP-2 reactor system and the theoretical basis of TAP pulse response experiments are discussed. On the basis of the TAP system, an alternative to the traditional kinetic approach in heterogeneous catalysis is developed. This new approach, termed ‘interrogative kinetics’ involves a combination of two types of kinetic experiments called ‘state-defining’ (friendly) and ‘state-altering’ (typical) experiments. The state defining kinetic parameter, or ‘kinetic parameter of the catalyst’ (KPC) is proposed and compared with the kinetic characteristic ‘turnover number’. The theory of TAP pulse response experiments is developed. Two deterministic models based on partial-differential equations are analyzed for the cases of diffusion, irreversible adsorption/reaction and reversible adsorption. The ‘standard diffusion curve’ that can be used for distinguishing the Knudsen flow regime is described, and simple criteria of the Knudsen regime are proposed. The concept of relative flow is described, and different fingerprints of TAP kinetics for irreversible adsorption/reaction and reversible adsorption are presented. TAP-2 experimental results on the selective oxidation of n-butane are used to illustrate the methodology of interrogative kinetics.

Thin-zone TAP-reactor – theory and application

Abstract A new reactor model for TAP pulse response experiments called a ‘thin-zone reactor’ is developed and applied to irreversible adsorption/reaction and reversible adsorption. In a thin-zone reactor, concentration gradients across the catalyst bed can be neglected, and diffusion and chemical reaction can be separated. The expressions for kinetic parameters for a thin-zone reactor can be calculated using moments, and are much simpler than the expressions for a one-zone or a three-zone reactor. The thin-zone model is particularly useful for investigating fast chemical reactions, since the extent of reaction can be controlled by the thickness and position of the catalyst zone. The expression for conversion in the case of irreversible adsorption/reaction in a thin-zone reactor is governed by a relationship that is analogous to the expression for first-order reaction in a CSTR. Moment based ‘fingerprints’ are defined for irreversible adsorption, reversible adsorption, and diffusion. The thin-zone model is used to determine kinetic parameters for the oxidation of propene over a VPO catalyst.

Ethylene oxidation on silver powder: A tap reactor study

Abstract The oxidation of ethylene over silver powder catalysts was studied between 475 and 575 K at pressures of circa 10 Torr under transient conditions that allowed reactions of atomic and molecular oxygen to be distinguished. Dioxygen and d 4 -ethylene were pulsed separately over silver powder in a microreactor with pulse durations of 200 μs, and the products were detected by a multiplexed mass spectrometer. This method, denoted as temporal analysis of products, allows either simultaneous pulsing of the reactants or pulse delays ranging from a few milliseconds to minutes. In these experiments both ethylene oxide and carbon dioxide were detected as products. Ethylene oxide formed instantaneously on the time scale of the reactant pulse, but carbon dioxide formed with a much slower time constant, characteristic of the decomposition of surface carbonate, indicating the importance of secondary interactions of CO 2 with surface oxygen in the kinetics of CO 2 formation. Pulse-probe experiments, in which the catalyst was first loaded with adsorbed atomic oxygen and then reacted with anaerobic ethylene pulses or ethylene-oxygen mixtures, showed that the adsorbed species giving rise to ethylene oxide is atomic, not molecular, oxygen.

Nature of active species of (VO)2P2O7 for selective oxidation of n-butane to maleic anhydride

TEM, EXAFS, FTIR, temporal analysis of products (TAP), stopped-flow desorption (SFD) and catalytic measurements of (VO)2P2O7 are reported. The reduced interaction between (020) planes of (VO)2P2O7 in samples prepared in an organic medium induces a charge localization on the V atoms of the coupled trans-vanadyl present in this plane, enhancing their catalytic reactivity in butane oxidation. Contiguous surface Bronsted sites (P—OH) also participate in the mechanism of selective oxidation. C-containing residues are present in relevant amount on the surface during catalytic experiments and give rise to a specific fouling of the active sites, but their possible role as co-catalysts in the transfer mechanisms of single activated species is also discussed.

A comparison of steady-state and unsteady-state reaction kinetics of n-butane oxidation over VPO catalysts using a TAP-2 reactor system

Abstract The reaction of n-butane with ‘reactor-equilibrated’ (VO)2P207 based catalysts has been investigated using a combination of low-pressure transient response experiments and atmospheric-pressure steady-flow reaction experiments. The activation energy for n-butane conversion in both steady-flow and transient response experiments was a function of the oxidation state of the catalyst. Under steady-flow conditions an activation energy of 17 ± 2 kcal/mol was obtained for ‘reactor-equilibrated’ catalysts, and 13 ± 2 kcal/mol for an oxygen-treated catalyst. The activation energy for n-butane conversion in transient-response experiments varied between 12 ± 1 and 23 ± 2 kcal/mol. An analysis of the transient response curves of the reaction products shows that the rates of formation and desorption of products are strong functions of the catalyst oxidation state. Analysis of the kinetic results indicated that the same sites are involved in the steady-state and unsteady-state conversion of n-butane.

Multi-zone TAP-reactors theory and application : I. The global transfer matrix equation

Abstract A general theory of single-pulse state-defining experiments for a multi-zone TAP (temporal analysis of products) reactor, is developed using the Laplace transform formalism; the theory gives explicit expressions for the moments of the outlet flux, series expansions for the transient values of this flux, and offers an efficient means to compute the actual profiles of gas concentration in the reactor and the values of the outlet flux numerically, using e.g. Fast Fourier Transform. The central concept of the theory is the global transfer matrix equation, which determines completely the dynamic behavior of the reactor. Using efficient computer algebra methods, the theory generates previous theoretical results reported in the literature for all the known TAP–reactor configurations, and yields new results related to the reversible adsorption/reaction–diffusion case and the thin-zone case. It can be used for further theoretical studies in the area of diffusion/reaction dynamics.

General expression for primary characterization of catalyst activity using TAP pulse response experiment

A general expression for primary catalyst characterization using TAP pulse response data has been obtained for porous and non-porous catalysts, and for one- and two-step irreversible catalytic reactions. Using this expression or the corresponding nomogram, the apparent kinetic parameter can be obtained.

Tetrahydrothiophene desulfurization on Co-Mo/γ-Al2O3: A temporal analysis of products (TAP) investigation

The catalytic reactions of tetrahydrothiophene, thiophene, 1-butene, 1,3-butadiene, and n-butane with hydrogen were studied at low pressure over a commercial cobalt molybdate catalyst. The formation sequence of tetrahydrothiophene desulfurization products was monitored with submilli-second time resolution using the temporal analysis of products (TAP) transient microreactor technique. The TAP experiments showed that butene and butadiene were the only hydrocarbon desulfurization products formed, although rapid dehydrogenation to thiophene was also observed. The exceptional time resolution of the TAP spectrometer provided evidence that the butene formed could not be accounted for by a mechanism involving butadiene hydrogenation. The results suggested a desulfurization mechanism for tetrahydrothiophene wherein C4 hydrocarbon formation proceeds via a surface butene thiolate intermediate produced by a single β-hydride elimination. It is proposed that 1,3-butadiene is formed by a slow subsequent β-hydride elimination of the intermediate, while rapid C-S bond hydrogenolysis involving surface hydrogen is responsible for butene formation.

Problems and Outlook for the Selective Heterogeneous Oxidation of C5 Alkanes

Abstract Economic, kinetic, catalytic and mechanistic aspects of the n- pentane and cyclopentane oxidation to maleic and phthalic anhydrides on vanadyl pyrophosphate are discussed and analyzed in order to illustrate the main problems and prospects in the selective heterogeneous oxidation of 5 alkanes, with specific reference to (i) the possibility of developing a new process for the synthesis of phthalic anhydride from these alkanes, (ii) the surface chemistry and dynamics of surface adsorbed species which characterize the reactions of C-C bond formation in the presence of gaseous oxygen and thus the synthesis of phthalic anhydride from 5 alkanes, and (iii) the possibility of tuning the surface properties of the catalyst in order to increase the relative formation of phthalic anhydride as compared to maleic anhydride.

Selective oxidation of butan-2-one to diacetyl over vanadium pentoxide: An investigation by temporal analysis of products

Abstract The selective oxidation of butan-2-one to diacetyl over vanadium pentoxide in the temperature range 250–350° C has been studied using the technique of temporal analysis of products. Evidence is presented that butan-2-one and molecular oxygen compete for the same adsorption sites on vanadium pentoxide. When 18 O 2 was used as oxidizing agent only 16 O appeared in the product diacetyl indicating that the reaction of butan-2-one to diacetyl proceeds by interaction between the substrate and lattice oxygen. Acetoin (CH 3 COCHOHCH 3 ) could be detected as an intermediate in the reaction network. In the experimental conditions of this study acetic acid was the major side product observed, and it was produced by decomposition of diacetyl. Smaller amounts of acetic acid were produced via oxidative cleavage of the enol form of butan-2-one.