Three primary kinetic characteristics observed in a pulse-response TAP experiment

The state-by-state transient screening approach based on a pulse-response thin-zone TAP experiment is further developed whereby single-pulse kinetic tests are treated as small perturbations to catalyst compositions and analyzed using integral method of moments. Results on three primary kinetic characteristics, termed basic kinetic coefficients, are presented. These three coefficients were introduced as main observables from experimentally measured TAP-responses in a kinetic-model-free manner. Each was analytically determined from moments of responses with no assumption about the detailed kinetic model. In this paper, the inverse question of how well these coefficients represent the time evolution of the observed responses is addressed. Sets of three basic kinetic coefficients are calculated from model and experimental responses and these calculated values are used to generate 3-coefficient curves in a kinetic-model-free manner. The comparison of these 3-coefficient curves with original responses shows that three basic kinetic coefficients can be sufficient to describe the observed kinetics of exit flow time dependencies with no assumption regarding the detailed kinetic model.     

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.     

A general formula for reactant conversion over a single catalyst particle in TAP pulse experiments

We obtain a general formula for reactant conversion in diffusion-reaction TAP systems over single non-porous catalyst particles as a product of two terms: α=P_H×K.The first term, P_H, is a purely geometric factor dependent only on the shape of the reactor and the shape and position of the catalyst particle, interpreted as the probability that an individual molecule of reactant will hit the catalyst before leaving the reactor. The second term, K=kτ_H/(1+kτ_H), is a geometrical/chemical term involving the kinetic constant k and a transport characteristic (residence time in the catalyst zone, τ_H). Both P_H and τ_H can be effectively calculated for any given reactor-particle configuration. Our formula greatly extends the validity of a formula given in Shekhtman et al. [1999. Thin-zone TAP-reactor—theory and application. Chem. Eng. Sci. 54, 4371-4378] for thin-zone TAP-systems. It is derived by a probabilistic analysis of the residence time of individual molecular trajectories in chemically active regions. Our results are based on the theory previously developed in our paper [Feres, R., Yablonsky, G.S., Mueller, A., Baernstein, A., Zheng, X., Gleaves, J., 2009. Probabilistic analysis of transport-reaction processes over catalytic particles: theory and experimental testing. Chem. Eng. Sci. 64, 568-581].     

Evaluation of different catalyst systems for bulk polymerization through “click” chemistry

“Click” chemistry is a frequently used technique in polymer chemistry for coupling polymer end groups to construct novel copolymer architectures and as a step-growth polymerization technique. In this study, the bulk polymerization of a diyne and a bisazide were achieved through the copper-catalyzed alkyne-azide 1,3-cycloaddition (CuAAC) with different catalysts, including the homogeneous catalyst Cu(PPh₃)₃Br and a heterogeneous Cu/C catalyst. The effects of different catalyst systems on the kinetics of the “click” polymerization were evaluated by differential scanning calorimetry (DSC) and oscillatory rheology. Additionally, the thermomechanical properties of resulting polymers were evaluated by temperature-ramp oscillatory rheology and thermal stabilities by thermogravimetry.     

Mathematical analysis of physicochemical phenomena in the catalyst during hydrogenating depolymerization of coal extract benzene insoluble fraction

Efficiency and selectivity of hydrogenating depolymerization of the coal extract benzene-insoluble part over the heterogeneous Co–Mo/Al₂O₃ catalystwere assessed using a mathematical model. The analytical equations of the mathematical modelwere generated based onmaterial balance incorporating the physico-chemical phenomena (reaction and diffusion) both in the autoclave and the catalyst grain. The equations offer the possibility for predicting changes of the reactants in the autoclave during the process and for determining the distribution of reactant concentrations in the grain as a function of its radius. The analytical equations of the model serve as the basis of the algorithm for assessing the influence of restrictive diffusion on the effectiveness and selectivity of the catalyst, and also for defining the optimal radii of the catalyst's pores to enable free transport of reactants in the grain interior.     

Deactivation behavior of the catalyst in solid acid catalyzed alkylation: effect of pore mouth plugging

The deactivation of solid acid catalysts in liquid-phase alkylation of isobutane with butenes was investigated. The role of pore mouth plugging, in particular, was studied and it was found that it had a significant effect on the deactivation behavior. Simple explicit correlations were developed for catalyst lifetime in a CSTR, both in terms of time-on-stream and butene turnover per catalyst weight. The correlations were tested using available experimental data from the literature and a reasonable agreement was observed. It was shown that for a zeolitic catalyst the butene turnover per catalyst weight was proportional to the square root of catalyst loading, while it was inversely proportional to both the olefin feed concentration and the square root of feed flow rate.     

A kinetic rate expression for the time-dependent coke formation rate during propane dehydrogenation over a platinum alumina monolithic catalyst

Coke formation rates under propane dehydrogenation reaction conditions on a used monolithic Pt/γ-Al₂O₃ catalyst have been experimentally determined in a thermogravimetric analyser (TGA) as a function of time on stream covering wide temperature and concentration ranges. For relatively short times on stream, especially at low temperatures and low propylene concentrations, a remarkable initial quadratic increase has been observed in the coke formation rates versus time with a high apparent propylene reaction order. After longer times on stream the coke formation rate decreases to a constant residual coke growth above approximately 12 wt.% coke content. The experimental data have been successfully described by a kinetic rate expression based on a mechanistic dual coke growth model. In this model it has been assumed that initially coke precursor is formed via a propylene oligomerisation process, explaining the observed auto-catalysis for short times on stream.     

Involvement of catalyst materials in nonthermal plasma chemical processing of hazardous air pollutants

Catalytic effects of metal oxides in nonthermal plasma chemical processing of hazardous air pollutants (HAPs) are discussed, relevant to their activities for the oxidation of HAPs in nonthermal plasma media and their selective control of active oxygen species derived from background O₂. In ferroelectric packed-bed reactors, the oxidation power of barium titanate (BaTiO₃) is not strong enough to oxidize HAPs and their carbon intermediates to CO₂. Only nitrous oxide (N₂O) was formed from background N₂ and lattice oxygen atoms in BaTiO₃. The catalytic effect of BaTiO₃ is negligible under aerated conditions. On the other hand, ozone (O₃) is formed from background O₂ in much higher concentrations in a silent discharge plasma reactor. Manganese dioxide (MnO₂)-catalyzed decomposition of O₃ promotes decomposition of benzene, which is less reactive than trichloroethylene and tetrachloroethylene. The acceleration of benzene consumption rate is ascribed to the promotion of its oxidative decomposition by the triplet oxygen atom. Catalytic control of in situ active oxygen species could be one of the most effective approaches to increase the energy efficiency of the nonthermal plasma reactor and to achieve the complete oxidation of the carbon atoms in HAPs.     

Effect of the diffusion phenomena in the catalyst grain on the selectivity of a system of parallel-consecutive reactions involved in the process of n-heptane dehydrocyclization

Analytical relations were derived for analyzing the selectivity of consecutive-parallel reactions occurring under conditions of the reforming process. With these relations it is also possible to determine how the shape of the catalyst grain, as well as the kinetic and diffusion phenomena that govern the process, affects the efficiency of the desired final products.     

Solid oxide derived from waste shells of Turbonilla striatula as a renewable catalyst for biodiesel production

Biodiesel production via transesterification of mustard oil with methanol using solid oxide catalyst derived from waste shell of Turbonilla striatula was investigated. The shells were calcined at different temperatures for 4 h and catalyst characterizations were carried out by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), Fourier transform infrared spectrometer (FT-IR), thermogravimetric analysis (TGA)/differential scanning calorimetry (DSC) and Brunauer-Emmett-Teller (BET) surface area measurements . Formation of solid oxide i.e. CaO was confirmed at calcination temperature of 800 °C. The effect of the molar ratio of methanol to oil, the reaction temperature, catalyst calcination temperature and catalyst amount used for transesterification were studied to optimize the reaction conditions. Biodiesel yield of 93.3% was achieved when transesterification was carried out at 65 ± 5 °C by employing 3.0 wt.% catalyst and 9:1 methanol to oil molar ratio. BET surface area indicated that the shells calcined in the temperature range of 700 °C-900 °C exhibited enhanced surface area and higher pore volume than the shells calcined at 600 °C. Reusability of the catalysts prepared in different temperatures was also investigated.     

Kinetic study for implementation of a disposable “redox-flexible” glucose biosensor

A “redox-flexible” reagentless amperometric biosensor that can function either in oxidation or reduction and can cover large ranges of substrate concentrations has been developed for glucose measurements. Glucose oxidase (GOx) either uses O₂ as its natural electron acceptor and produces H₂O₂ or uses artificial electron acceptors like ferricinium derivatives. To broaden the range of working potentials, peroxidase enzyme (HRP) was added to GOx, offering the alternative to correlate the glucose concentration to the reduction current of the ferricinium that results from H₂O₂ oxidation of ferrocene. In such a configuration, the same ferrocene/ferricinium couple acts as a redox mediator for both enzymatic reactions involving GOx and HRP. Two ferrocenes were used in this study: Fc(CH₂OH)₂, substituted by electron-donor groups, and FcCOOH, substituted by an electron-withdrawing group. The rates of the reactions involved were determined, and the calibration curves in cathodic and anodic mode were drawn. The comparative study showed that for glucose measurement, the [FcCOOH]/[FcCOOH]⁺ couple is best suited to act as mediator in the construction of a “redox-flexible” glucose biosensor.     

Determination of catechol based on an oscillating chemical reaction involving a macrocyclic complex as catalyst

A novel methodological approach for the determination of catechol based on a Belousov-Zhabotinskii type oscillating system is presented. Such an oscillating reaction involves the oxidation of malic acid in an acidic bromate medium in the presence of a catalyst—macrocyclic complex [CuL] (ClO₄)₂. The unsaturated ligand L in the complex [CuL] (ClO₄)₂ is 5,7,7,12,14,14-hexemethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene. By perturbation of catechol on the oscillating chemical reaction, the increase in the oscillation amplitude is linearly proportional to the logarithm of the concentration of catechol in the range 2.1 × 10⁻⁶–2.1 × 10⁻⁴ M, with a correlation coefficient of 0.9976. The R.S.D obtained for 7.88 × 10⁻⁵ M catechol is 3.8% (n = 5). Cyclic voltammetry was applied to explore the mechanism of catechol perturbation on the oscillating chemical reaction.     

Electron microscopic characterization of Cr/silica catalyst for ethylene polymerization

Ethylene polymerization was carried out over both porous and non-porous 5 wt% Cr/silica catalysts in a slurry reactor. The polymerization was stopped at selected times to obtain samples for SEM and TEM characterization. Despite the different physical characteristics of the two silica-supported catalysts and their different behavior in the early stages of reaction, high resolution SEM micrographs (taken after runs of longer duration) revealed similar, fibrous and very porous polymer layers on both. This accessibility of the ethylene enables transport of monomer to the active sites at the very high reaction rates.     

Characterization and performance of Pd-La/spinel catalyst for preparation of 2,6-diisopropylaniline

The Pd-La/spinel catalyst for the preparation of 2,6-diisopropylaniline (2,6-DIPA) by gas-phase amination of 2,6-diisopropylphenol (2,6-DIPP) has been studied. The catalysts before and after reaction were characterized by BET, XRD, differential thermal analysis (DTA)-thermo gravimetric analysis (TGA), FTIR, and NH₃-TPD techniques. The DTA study results show two kinds of coke deposited on the metal and support of Pd-La/spinel catalyst, and they are combusted at about 242 and 324 °C, respectively. The XRD and FTIR spectra of the Pd-La/spinel catalyst show that the coke contains the aromatic and aliphatic rings, alkyl groups, polynuclear aromatic system, hydroxyl groups, and amine groups. The reason for catalyst deactivation can be expressed as follows: coke formed on palladium metals may move from metal to the interface or boundaries of metal-support and acid sites of the support where further dehydrogenation and polymerization are occurring. The H₂ plays a key role in retarding coke formation, but must be in suitable amount to get relatively high selectivity. The rare earth promoter La not only promotes the activity and selectivity by retaining Pd species in the metallic state, but also decreases the formation of carbon by neutralizing the strong acid sites on the catalyst.     

链行走烯烃聚合催化剂配体的合成与表征

2,6-二异丙基苯胺和丁二酮为原料,通过希夫碱反应合成了一类链行走催化剂配体。采用条件实验确定了希夫碱反应的最佳条件为:n(丁二酮):n(2,6-二异丙基苯胺)=2.5∶1,反应温度25℃,反应时间12h,收率为83.43%。红外光谱和元素分析的结果表明,合成产品的结构与其理论结构相符。     

Periodic solutions in a porous catalyst pellet—homoclinic orbits

The analysis performed as well as extensive numerical simulations have revealed the possibility of the generation of homoclinic orbits as a result of homoclinic bifurcation in the model which describes transport phenomena and chemical reaction in a porous catalyst pellet. A method has been proposed for the development of a special type of diagrams—the so-called bifurcation diagrams. These diagrams comprise the locus of homoclinic orbits together with the lines of limit points bounding the region of multiple steady states as well as the locus of the points of Hopf bifurcation. Thus, they define a set of parameters for which homoclinic bifurcation can take place. They also make it possible to determine conditions under which homoclinic orbits are generated.Two kinds of homoclinic orbits have been observed, namely semistable and unstable orbits. It is found that the character of the homoclinic orbit depends on the stability features of the limit cycle which is linked with the saddle point.Very interesting dynamic phenomena are associated with the two kinds of homoclinic orbits; these phenomena have been illustrated in the solution diagrams and phase diagrams.     

Uniformity in a thin-zone multi-pulse TAP experiment: numerical analysis

The thin-zone TAP reactor (TZTR) model of a multi-pulse experiment is computationally validated based on a more general three-zone reactor model. The analysis is focused on the uniformity of gaseous and surface concentrations in the catalyst zone, which is a key property of TZTR model. It is shown that if the TZTR model is valid for the first pulse in a multi-pulse experiment then it is valid for all subsequent pulses. For a typical reactor packing (the ratio of the thin-zone thickness to the length of reactor is 1/30) and with the first pulse conversion up to 97%, the gaseous and surface concentration profiles can be considered uniform and characterized by their spatial average values only. The reaction rate in the catalyst zone may also be characterized by its spatial average value and directly related to the spatial average gaseous and surface concentrations, in the same way as an elementary rate is related to concentrations. As a result of these unique characteristics, the TZTR may be considered a “perfectly-mixed” reactor even at high conversion.