Kinetically induced unstable behaviour in heterogeneously catalyzed reactions

Evidence has appeared in the literature that ignition-extinction phenomena in heterogeneously catalyzed reactions can be induced by pure kinetic effects. This paper presents an elegant and simple analytical reasoning based upon elementary properties of polynomials to derive explicit necessary and sufficient conditions for this behaviour for various reaction mechanisms, including Langmuir as well as non-Langmuir-type of adsorption. It is inspired on geometric arguments on the steady state equation, involving tangent adsorption and surface reaction rate curves as a function of surface coverage, which represent the limiting cases of rate multiplicity. The approach is particularly attractive for Langmuir-type adsorption as the adsorption rate equations are linear in this case. The technique is also applicable with the same ease in most situations of nonlinear adsorption characteristics and is to be preferred over a number of more intricate alternatives, because of its simplicity and its ability of defining explicit maximal bounds on the fundamental parameters.     

Examination of a model for oscillating heterogeneously catalyzed reactions

Numerous cases of self-sustained oscillations have been reported for heterogeneously catalyzed oxidation reactions. For these self-sustained oscillations, the conversion of reactants varies with time in the absence of external perturbations. Several models have been proposed for these systems. One of these models is based on Langmuir-Hinshelwood mechanisms involving an adsorbed species which acts as a buffer. Previous predictions based on this model have involved simplifying assumptions. Model predictions without these assumptions are presented in this paper. These results are compared with previously reported simulations.     

Selective heterogeneously catalyzed hydrogenations

A number of catalytic hydrogenations have been discussed with emphasis on reaction selectivity. These reactions were examples of chemoselectivity, regioselectivity, stereoselectivity, diastereoselectivity and enantioselectivity. Factors such as catalyst selection, reaction conditions and the steric nature of the substrate were discussed in terms of their contribution to the selectivity of particular hydrogenations.     

Kinetic instabilities in heterogeneously catalyzed reactions—II: Oscillatory instabilities with langmuir-type kinetics

An analysis of the kinetic model studied in Part I of this contribution gave no indication for the occurrance of periodic solutions. However if an additional slow chemisorption (“buffer”) step is added to the model, relaxation oscillations can develop in regions where the original model exhibited multiple solutions. The behaviour of the extended model is analysed via the singular perturbation approach and by simulation. It is shown that a broad variety of slow chemisorption steps can serve as buffer with the restriction that the buffer step should not, or at least only slightly, contribute to the surface reaction.     

Kinetic instabilities in heterogeneously catalyzed reactions—I: Rate multiplicity with langmuir-type kinetics

The question under which conditions kinetic instabilities can be described by Langmuir type kinetics is studied using a fairly general model for the surface reactionpA + qB → C. This paper is concerned with the occurrence of rate multiplicities (ignition-extinction phenomena). It is shown that rate multiplicity can be caused by the competing chemisorption of A and B upon the same active sites of the catalyst. Rate multiplicity can occur if at least one of the chemisorption or reaction steps is of second order. No rate multiplicity can be expected, if either A or B reacts via an Eley-Rideal type mechanism.     

Chain transfer reactions in metallocene catalyzed polymerization of allylbenzene

Summary Allylbenzene was polymerized in the presence of two types of homogeneous zirconocene catalysts (co-catalyst methylaluiminoxane). Selective chain termination through β-hydride elimination or chain transfer to aluminum was observed depending upon the catalysts employed. The rac-Et(Ind)₂ZrCl₂, and rac-Me₂Si(Ind)₂ZrCl₂, catalysts gave the polyallylbenzenes with saturated end groups due to chain transfer to aluminum, while the Cp₂ZrCl₂ catalyst gave the polyallylbenzenes with vinylidene end groups due to β-hydride elimination. Received: 7 December 1998/Revised version: 20 January 1999/Accepted: 29 January 1999     

活性炭固载三氯化铁非均相催化合成乙酸环己酯

以活性炭固载三氯化铁为非均相催化剂,对环己醇与乙酸酐之间的酰化反应进行了研究,考察了催化剂用量、醇酐物质的量的比、反应时间对乙酸环己酯收率的影响。结果表明,活性炭固载三氯化铁有着较好的催化活性,当乙酸酐的加入量为0.1mol,醇酐物质的量比为1.1∶1,催化剂质量为0.30g时,反应30min,乙酸环己酯收率可达到96.8%,且催化剂重复使用5次仍保持较高活性。所得产品无色透明,具有很高的纯度。     

有机-无机杂化黏土催化四氢呋喃聚合

采用后合成方法,将γ-巯基丙基三乙氧基硅烷嫁接到黏土表面,获得含磺酸有机功能团的黏土固体酸催化剂。经XRD、N₂ 吸附-脱附、FTIR、吡啶吸附和酸量滴定等物性结构表征及催化四氢呋喃聚合的反应活性评价,结果表明,改性后的黏土保持了原有黏土介孔结构,孔径分布集中在3 ～4.3 nm,酸量增加了35％,在催化四氢呋喃聚合反应中显现出较高的催化活性,在反应温度为35℃、反应时间为8 h的条件下,聚合产物收率达30％,得到数均分子量为2000左右、可用作氨纶生产原料的聚合物。     

Competitive reactions catalyzed by NiMo-containing zeolites

The rates of benzene hydrogenation and of n-heptane hydroconversion were measured simultaneously over a series of bifunctional catalysts. The catalysts were Y zeolites containing a Ni or Mo sulfide. At 280 °C, and for the more acidic catalysts, both rates appeared linearly correlated, suggesting that the hydrogenation function was limiting.     

钨锗酸的催化脱水反应

报道了钨锗酸对醇脱水成醚或成烯的催化反应。     

Development of a microstructured reactor for heterogeneously catalyzed gas phase reactions: Part I. Reactor fabrication and catalytic coatings

Due to their superior heat transfer properties, microstructured reactors are well suited for performing strongly exothermic heterogeneously catalyzed gas phase reactions. In order to utilize the full potential of this reaction technology, a new low-cost manufacturing concept was developed, using a Ni–Ag–Sn solder system for bonding the individual structured steel platelets. Three different methods for depositing a VO_x/γ-Al₂O₃ material on the micro-channels were investigated with respect to morphology, mechanical stability and catalytic behavior of the obtained coatings. Especially the influence of different binder materials (Al-tri-sec-butylate, tetraethoxysilane, hydroxypropyl cellulose and polyvinyl pyrrolidone) was analyzed. For evaluating the performance of the coatings, the oxidative dehydrogenation of propane (ODP) served as a sensitive test reaction. The modules and the catalytic coatings withstood the applied reaction conditions (400–600 °C at ambient pressure), which makes them safe and flexible tools for research activities and small scale production processes.     

Development of a microstructured reactor for heterogeneously catalyzed gas phase reactions: Part II. Reactor characterization and kinetic investigations

A new type of microstructured reactor was characterized and tested in the oxidative dehydrogenation of propane (ODP) with respect to reaction engineering aspects. Residence time behavior was measured using O₂/N₂ step injection experiments and theoretically analyzed by applying the axial dispersion model, resulting in Bodenstein numbers in the range of 70. Catalytic performance in the ODP was predicted on the basis of a kinetic model from the literature, showing good agreement for low degrees of propane conversion. In order to elucidate discrepancies between experimental and forecasted model data at higher degrees of propane conversion (i.e., >12.5%), possible sources of error were systematically investigated. Specifically, heat and mass transport limitations were excluded as well as possible inaccuracies of the applied kinetic model were examined. It could be shown that microstructured reactors are well suited to be applied for strongly exothermic heterogeneously catalyzed gas phase reactions since they allow isothermal reaction conditions over a wide range of concentrations and temperatures.     

Ring-opening polymerization of lactones catalyzed by decamolybdate anion

An efficient process for the ring-opening polymerization (ROP) of lactones by ammonium decamolybdate (NH₄)₈[Mo₁₀O₃₄] (obtained in situ from thermal decomposition of ammonium heptamolybdate (Hep) (NH₄)₆[Mo₇O₂₄]), is reported for the first time. Polymerization of the ε-caprolactone (CL) and δ-valerolactone (VL) proceeds with quantitative conversions, and high molecular weight polyesters (M_n in the order of 10⁴) with moderate polydispersity (M_w/M_n≤1.81) can be obtained at 150 °C in two hours using a CL/Hep molar ratio of 20,000. Hydroxylic compounds can be efficiently used to control the final molecular weight. Hydroxylic compounds act as both co-initiators and chain transfer agents. Incorporation of the hydroxylic compounds as end groups in the polymeric backbone was corroborated by ¹H and ¹³C-NMR. The molecular weight of poly(ε-caprolactone) (PCL) measured by gel permeation chromatography (GPC) shows a linear dependence on the CL/OctOH ratio. Decamolybdate anion (isolated as a product from the thermal decomposition of heptamolybdate) shows a similar catalytic activity toward polymerization compared to that observed for ammonium decamolybdate obtained in situ. Catalysts recovered after a polymerization batch maintain most of their initial properties, and can be reused to polymerize lactones in an efficient manner.     

Syndiospecific polymerization of styrene catalyzed by half-titanocene catalysts

Those effective catalyst precursors for syndiotactic styrene polymerization, Cp*Ti(OCH₂-CHCH₂)₃ (I), Cp*Ti(OCH₂-CHCHC₆H₄)₃ (II), Cp*Ti(OCH₂C₆H₅)₃ (III), Cp*Ti(OCH₂C₆H₄OCH₃)₃ (IV) were synthesized, and the influence of catalyst ligands on the catalytic activity and properties of polymer were investigated. The polymer thus obtained coupled with higher molecular weight and higher syndiotacticity determined by GPC and ¹³C NMR as well as solvent extraction manners, respectively. Those catalysts promoted by methyaluminoxane (MAO) as cocatalyst exhibited higher catalytic activity. Of all catalysts mentioned foregoing, Cp*Ti(OCH₂-CHCHC₆H₄)₃ (II), Cp*Ti(OCH₂C₆H₅)₃/MAO (III) and Cp*Ti(OCH₂C₆H₄OCH₃)₃ (IV) catalysts showed higher activity and stability even at fairly low Al/Ti ratio of 600, and possessed excellent control of the stereoregular insertion of monomer, exhibited a significant increase of the ratio of the propagation rates to chain transfer termination. The kinetic and titration results also indicated that those metallocene catalysts (II), (III), and (IV) showed higher catalytic activity and produced polymer with higher molecular weight, because of a great number of active species, and lower ratio of Kβtr/Kp, higher ratio of Kβtr/Ktrs which indicate that β-H elimination was predominant.     

Experimental study and model of reaction kinetics of heterogeneously catalyzed methylal synthesis

Reaction kinetics of the heterogeneously catalyzed formation of methylal from aqueous methanolic formaldehyde solutions are studied in a plug flow reactor at 323, 333 and 343 K using the acidic ion exchange resin Amberlyst 15 (Rohm and Haas) as catalyst. Parameters of an activity-based pseudo-homogeneous reaction kinetic model are fitted to the experimental results. The model is based on the true speciation in the reacting solution and explicitly includes the oligomerization reactions of formaldehyde in aqueous methanolic solutions. The reaction kinetic model describes the experimental data well and is suited for process simulations in which both chemical reactions and phase equilibria have to be described simultaneously.     

Ethylene polymerization catalyzed by metallocene supported on mesoporous materials

Summary  In this work, different mesoporous materials were employed for the preparation of supported metallocene catalysts to be evaluated in ethylene polymerization and their performance was related to the chemistry of the materials surface used as support. The supports employed were MCM-41, SBA-15 and mesoporous TiO₂. The performance of the prepared catalysts was compared with the homogeneous catalyst precursor system. Those mesoporous materials, as well as the prepared metallocene catalysts, were analyzed by infrared absorption spectrometry (FTIR). Polymers were also characterized by FTIR, for the determination of the number-average molecular weight, and by differential scanning calorimetry (DSC) to determine thermal characteristics of the produced polyethylene. Among the studied metallocene supported catalysts, the one based on the mesoporous support SBA-15 achieved the highest activities, almost as high as that observed for the homogeneous system.