Kinetics of formaldehyde oxidation on a vanadia-titania catalyst

The heterogeneous catalytic oxidation of formaldehyde in the gas phase may be considered as an alternative to the multistep liquid-phase synthesis of formic acid. Monolayer vanadia-titania catalysts are active and selective in the oxidation of formaldehyde to formic acid. Detailed investigation of kinetics of formaldehyde oxidation over a monolayer vanadia-titania catalyst was carried out. It was established that byproduct form via a consecutive-parallel reaction network. CO₂ results from formaldehyde oxidation via parallel pathway and from formic acid overoxidation via consecutive pathway; CO forms from formic acid via consecutive pathway. It was shown that oxygen and water accelerate formic acid formation and that water retards CO formation. Based on experimental data, a kinetic model of formaldehyde oxidation was developed. The kinetic model was used in the mathematical simulation of the formaldehyde oxidation process and in the determination of dynamic and design parameters of the reactor. Formic acid production by the gasphase oxidation of formaldehyde is unique and does not have any analogue. As opposed to conventional technologies, it is energy-saving, environmentally friendly, and technologically simple. An enlarged-scale pilot plant using this technology is under construction.     

Parallel hydrogenation of 2,2‐dimethylol‐1‐butanal and formaldehyde over supported NiCr and CuCr catalysts

The hydrogenation kinetics of 2,2‐dimethylol‐1‐butanal (TMP‐aldol) and formaldehyde were studied over two commercial supported catalysts; NiCr on silica and CuCr on alumina. Both catalysts hydrogenated the aldol selectively to triol, but the kinetic trends differed widely. With the nickel catalyst, the aldol hydrogenation was not started before almost all of the formaldehyde was consumed from the bulk phase, whereas the copper‐containing catalyst hydrogenated aldol and formaldehyde in parallel. Rate equations for the hydrogenation of aldol and formaldehyde were derived from a competitive surface mechanism. Since formaldehyde retarded significantly the hydrogenation rate of aldol over NiCr, the kinetic model was modified in order to take into account the inhibitory effect of formaldehyde. With the modified model, the experimental data produced over the NiCr catalyst were rather well described. The data from the experiments over the CuCr catalyst followed pseudo‐first order kinetics remarkably well. Furthermore, the kinetic model without the inhibitory effect of formaldehyde was able to describe the governing trends of the data obtained over the CuCr catalyst. © 2002 Society of Chemical Industry     

Modeling of melamine formaldehyde polymerization. II. Development of a simpler model

The kinetic modeling of melamine–formaldehyde polymerization presents a relatively formidable mathematical challenge because of the simultaneous presence of several types of deviations from Flory's equal reactivity hypothesis. A molecule of melamine has six reactive amide hydrogens, which can react with the ? CH₂OH groups of formaldehyde in solution. The reactivity of the secondary hydrogens on the melamine is about 61% of that of the primary hydrogens (induced asymmetry). There is a shielding effect present, i. e., the reactivity of the hydrogens near the outside of a multiringed polymer molecule is higher than that of the hydrogens inside the coiled molecules. Two bound ? CH₂OH groups on the polymer molecules can self-condense to give methylene linkages, the reactivity depending upon the location of the two groups. And, to confound modeling effort still further, all these reactions are reversible. An earlier attempt at molding this system considered the reactions between 36 “basic” species. This model was far too detailed and failed to acount for the reverse reactions. In the present study, a simpler model has been proposed which involves fewer “basic” species. An improved model for intramolecular reactions is also developed. Several important characteristics of the polymerization have been obtained as a function of time. Results from this model have been compared with those obtained from the earlier model, and also compared with the short-time experimental results. The present model can be extended to account for the reverse reactions quite easily.     

甲醛+1,3,5-三聚甲醛+硫酸+水体系汽液相平衡实验和理论研究

工业上利用甲醛水溶液在硫酸催化下的反应精馏工艺生产1,3,5-三聚甲醛,因此,1,3,5-三聚甲醛生产工艺的优化和新型催化剂的开发受到了广泛关注。这需建立反应体系的汽-液相平衡模型,研究1,3,5-三聚甲醛合成过程中催化剂可能发挥的多重作用。为此,测定了(甲醛+1,3,5-三聚甲醛+硫酸+水)体系的汽-液相平衡数据,采用扩展型UNIFAC模型对汽-液相平衡数据进行了关联,确定了模型参数,并对该体系进行了系统的计算,揭示了硫酸催化剂在该反应精馏工艺中的三重作用。上述成果对优化1,3,5-三聚甲醛生产工艺和开发1,3,5-三聚甲醛新型催化剂具有重要意义。     

Comparison of the reactivity of high-surface area, monolayer vanadia/ceria catalysts with vanadia/CeO2(1 1 1) model systems

In this study, temperature programmed desorption (TPD) was used to compare the reactivity of a high-surface area, monolayer vanadia/ceria catalyst with that of a 0.5 ML ceria film supported on the (1 1 1) surface of CeO₂ single crystal. TPD and X-ray photoelectron spectroscopy (XPS) experiments with the vanadia/CeO₂(1 1 1) model system were carried out in an ultra-high-vacuum surface analysis system, while TPD studies for the high-surface area vanadia/ceria catalyst were conducted in a high-vacuum microbalance equipped with a mass spectrometer. The TPD studies showed similar reactivity for both samples. They were both active for the oxidation of methanol to formaldehyde and the temperature at which adsorbed methoxide intermediates underwent dehydrogenation to produce formaldehyde during TPD was found to be a function of the oxidation state of the cations in the supported vanadia layer for both samples. The similarity in the results obtained in this study from the high and low surface area samples indicates that monolayer vanadia films supported on metal oxide single crystals are excellent models of high-surface area, polycrystalline, supported vanadia catalysts.     

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.     

Kinetics of hydroxymethyl phenols formation by in‐line FTIR spectroscopy

The kinetics of phenol–formaldehyde prepolymers catalyzed by sodium hydroxide at various temperatures was studied. Several reactions were conducted with different phenol to formaldehyde as well as phenol to sodium hydroxide molar ratios. The React‐IR system was used to monitor the reaction as well as to determine residual free phenol and formaldehyde. The changes in the concentrations of phenol and formaldehyde with the reaction time were determined. The value of the concentration of the hydroxide ion, [OH^?], was obtained by measuring the pH value of reaction mixture. The concentration of the hydroxide ion, [OH^?], expressed as a function of reaction time, was fitted by the six‐order polynomial to the experimental data. On the basis of the proposed reaction scheme the kinetic model was developed. The kinetic parameters were obtained by adjusting the experimental evolution of phenol and formaldehyde during the synthesis. Using this method the changes in the concentrations of five species of hydroxymethyl phenols with the reaction time was also been calculated. The activation energy and preexponential factor have been calculated for individual reactions. The accuracy of the kinetic model was confirmed by comparing experimental concentration profiles of formaldehyde and phenol with the calculated ones for different molar ratios. The experimental tendencies are in agreement with the results of the model. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

A molecular kinetic model incorporating catalyst acidity for hydrocarbon catalytic cracking

This work built a molecular-level kinetic model for hydrocarbon catalytic cracking, incorporating the catalyst acidity as the parameter to estimate reaction rates. The n-decane and 1-hexene co-conversion catalytic cracking process was chosen as the studying case. The molecular reaction network was automatically generated using a computer-aided algorithm. A modified linear free energy relationship was proposed to estimate the activation energy in a complex reaction system. The kinetic parameters were initially regressed from the experimental data under several reaction conditions. On this basis, the product composition was evaluated for three catalytic cracking catalysts with different Si/Al. The Bronsted acid and Lewis acid as the key catalyst properties were correlated with kinetic parameters. The built model can calculate the product distribution, gasoline composition, and molecular distribution at different reaction conditions for different catalysts. This sensitive study shows that it will facilitate the model-based optimization of catalysts and reaction conditions according to product demands.     

Scientific basis for process and catalyst design in the selective oxidation of methane to formaldehyde

The mechanism and kinetics of the gas-phase selective oxidation of methane to formaldehyde (MPO) are revised in the general context of catalytic oxidations. In agreement with ab initio calculations of the energy barrier for the activation of methane on transition metal oxide complexes, a formal Langmuir-Hinshelwood kinetic model is proposed which accounts for the "steady-state" conditions and activity-selectivity pattern of MPO catalysts, providing an original support to process design and catalyst development.     

Cyclohexene hydrogenation on rhodium catalysts in the gas phase: Kinetics of the reaction and origin,mechanism and kinetics of the deactivation process: Kinetics of the reaction and origin,mechanism and kinetics of the deactivation process

Cyclohexene hydrogenation in the gas phase was studied on bulk or supported rhodium catalysts between 270 and 290 K. The kinetic data for cyclohexene hydrogenation were determined under the initial conditions. The catalyst deactivation during this reaction was shown to obey a second order kinetic law. Temperature-programmed desorption (TPD) showed that benzene was formed during the reaction. The deactivation process was due to the stronger adsorption and to the lower reactivity of benzene produced by disproportionation of cyclohexene. A mechanism is proposed to correlate the deactivation of rhodium catalysts and the hydrogenation of cyclohexene.     

LED可见光下Au/TiO2光催化氧化甲醛表观动力学

甲醛是室内空气中典型的挥发性有机化合物（VOCs）之一。等离子激元型光催化剂可吸收可见光,可在室温常压下利用太阳光驱动光催化氧化脱除室内空气中甲醛的反应,其表观反应动力学研究对设计等离子激元型光催化剂及应用于脱除VOCs等污染物具有重要意义。研究了LED可见光下等离子激元型Au/TiO₂光催化脱除气相中甲醛的表观反应动力学,考察不同波长的可见光源、光强及反应气相对湿度对表观反应动力学的影响,根据对实验数据的分析求得LED可见光催化氧化甲醛表观反应速率常数k（I,H）。结果表明,在13%相对湿度、蓝光光强为38.5 mW·cm^-2的条件下,光反应的甲醛转化率达77%,是暗反应的近5倍。在LED红、绿、蓝、白光照射下,随着光强增加,甲醛转化率快速增加后缓慢增至基本不变。相同光强（低于42 mW·cm^-2）和湿度条件下,红、绿、蓝光源的甲醛转化率相近,白光略低于其他光源。干气氛下,光反应和暗反应的甲醛转化率几乎为0。湿气氛下,光反应和暗反应均有甲醛转化率,光反应的甲醛转化率更高。不同光源条件下甲醛转化率随相对湿度变化规律相似。相对湿度为21.9%时甲醛转化率最高,而后保持不变,基本在80%左右。通过对红、绿、蓝、白光的数据拟合计算,得到表观动力学参数k（I）_H、k（H）_I、k（I,H）以及速率方程。     

Characterization of coals for Lurgi pressure gasification and other moving bed processes: A method for reactivity parameter estimation from bench scale experiments

A method has been developed which yields global reactivity parameters of coals under conditions pertinent to gasification in moving bed reactors, in particular to the Lurgi process. The experimental procedure uses a bench scale pressure apparatus which allows the determination of other coal properties, besides reactivity, significant for gasification in moving bed reactors, like particle disintegration behaviour etc.. Herein, a coal sample of about 2 kg can be exposed subsequently to the conditions in the drying, pyrolysis and gasification zones of the moving bed gasifier. The data analysis procedure for the determination of reactivity parameters comprises a nonlinear fit of carbon gasification rate data by means of a first order kinetic model. As an example, results from a gasification experiment with North Dakota lignite related to the Lurgi process are presented. Pre-exponential factor and activation energy are estimated as global reactivity parameters from the data. They are intended to serve as coal-specific input data for kinetic models of commercial scale gasification reactors, thus widening the range of application of such models in reactor design and optimization.     

Polymerization of melamine and formaldehyde in homogeneous continuous-flow stirred-tank reactors using functional group approach: Part A: Conversion of functional groups

A comprehensive kinetic model using the functional group approach has been proposed for the polymerization of melamine and formaldehyde. The kinetic model is consistent with the basic chemistry of polymerization and involves five rate constants which have been estimated using the experimental data of Tomita. Homogeneous continuous-flow stirred-tank reactors (HCSTRs) have been modelled and the mole balance relations for various functional groups have been written. The performance of HCSTRs is governed by algebraic equations and, for any specified residence time, is found by the method of successive substitution using the Brown's algorithm. The computations show that as long as free formaldehyde is present, the reaction mass would consist predominantly of substituted melamine molecules. However, after formaldehyde is completely reacted, larger oligomers are formed in larger concentrations. On comparison of results with batch reactors, it is found that for the same reaction time HCSTRs yield polymer with higher branching.     

Catalytic properties in deNOx and SO2–SO3 reactions

SCR-deNO_x reaction and SO₂–SO₃ oxidation tests were carried out by different research groups over fresh and used EUROCAT oxide samples in order to characterize the reactivity of the catalysts and to compare data obtained in several laboratories (Politecnico of Milan, Università of Salerno, ENEL of Milan, Boreskov Insitute of Catalysis).

Data are presented which indicate that the used EUROCAT catalyst is slightly more active both in the deNO_x reaction and SO₂–SO₃ oxidation than the fresh sample.

An analyses of data collected over honeycomb catalysts by means of a 2D, single-channel model of the SCR monolith reactor has been performed to evaluate the intrinsic kinetic constant of the deNO_x reaction; a satisfactory comparison has been obtained between estimation of the intrinsic kinetic constant and estimation of the intrinsic catalyst activity from data collected over powdered catalysts. A good agreement has been found in the experimental results collected in the different labs, both for the deNO_x reaction and SO₂–SO₃ oxidation.     

On the link between intrinsic catalytic reactions kinetics and the development of catalytic processes: Catalytic dehydrogenation of ethylbenzene to styrene

A procedure linking kinetic modeling of catalytic reactions to reactor modeling for different configurations is developed and applied to the catalytic dehydrogenation of ethylbenzene to styrene. The procedure is applied to four configurations, namely fixed bed with/without hydrogen selective membranes and bubbling fluidized beds with/without selective membranes. The kinetic data for the industrial catalyst are extracted from industrial fixed bed data using a rigorous heterogeneous model. The kinetic data for the three in-house prepared catalysts are obtained from the laboratory scale experiments using pseudo-homogeneous models.     

Determination of reactivity and ignition behaviour of solid fuels based on combustion experiments under static and continuous flow conditions

The reactivity and ignition behaviour of solid fuels is a major parameter for combustion and gasification processes, but also for a safe transport and storage of pyrophoric solids. In this work, seven non-isothermal methods were compared with respect to characterise the ignition behaviour as well as to calculate kinetic parameters; for comparison also ‘classical’ isothermal measurements were done. Different methods and reactors (fixed and fluidised bed, thermogravimetry, oven heating tests) were used and tested under static and continuous flow conditions, taking charcoal, activated carbon and blast furnace coke as model solid fuels. The accuracy of all tested methods to determine kinetic data is reliable within a range of confidence of about ±50 K (with respect to the temperature needed to reach a certain level of reactivity). For a fast and relative simple determination of kinetic data, the ignition test in a small lab-scale fixed bed reactor can be recommended. Additional calculations show that the critical parameters with respect to ignition during transport and storage can also be calculated quite accurately based on this method, i.e. no elaborated basket heating tests are needed.     

Kinetics and Modeling of Petroleum Residues Hydroprocessing

Several factors must be taken into consideration while interpreting data on kinetics of reactions occurring during hydroprocessing of petroleum residues, i.e., the origin of feed, properties of catalysts and related diffusion phenomena, catalyst deactivation, form of kinetic model, and experimental system employed. Among the operating parameters, temperature, H₂ pressure, H₂S/H₂ ratio, H₂/feed ratio, and contact time have a pronounced effect on kinetic data as well. Moreover, although of the same distillation cut point, residues may vary widely in composition. For example, the content of metals (vanadium+ nickel) may range from few ppm for residues derived from a sweet crude to more than 1000 ppm for one derived from a heavy crude. The content of asphaltenes and CCR may exhibit a similar variability. For the most part, significant discrepancies among reported values of kinetic parameters can be reconciled by closely examining the conditions of experiment used for determination of kinetic data. Kinetic data are incorporat ed in mathematical models used for predicting the life of catalyst during hydroprocessing operation. Good predictions can be made if reliable kinetic data are used for modeling. In this regard, the selection of experimental conditions for determining kinetic results is crucial to ensure their reliability. Thus, properly designed tests for accelerated aging of catalyst may be the source of a valuable information. Otherwise, erroneous conclusions could be reached.     

Catalytic dehydrogenation of methanol to water-free formaldehyde

Catalytic dehydrogenation of methanol is a promising process of producing water-free formaldehyde. The present paper reviews research in this field. As effective catalysts mainly transition metal compounds as well as oxides and salts containing sodium have been reported. Several catalysts exhibit high activity and high selectivity, for formaldehyde at low conversions while further efforts have to be made to improve catalyst stability and selectivity at high conversions. Catalytic dehydrogenation of methanol to formaldehyde is compared with methanol oxidation.     

催化反应与膜分离耦合系统中镍催化剂的使用

在用于对硝基苯酚加氢制备对氨基苯酚的反应-膜分离耦合系统中,催化剂在反应时以悬浮态形式应用,然后用无机陶瓷膜分离回收,催化加氢速率及膜过滤速率是决定该系统运行效率的关键.研究通过综合考察镍粉体催化剂的反应活性与膜分离性能来选择合适的催化剂.实验结果表明,在研究体系中,化学还原法制备的纳米镍粉活性优于物理气相沉积法制备的纳米镍粉和微米镍粉,主要因为化学还原法制备的纳米镍粉具有较大的活性比表面积.膜分离几种镍粉时,活性较高的纳米镍通量较低,而活性较低的微米镍通量较高.通过将具有高活性的纳米镍粉与具有高通量的微米镍粉以适当比例混合使用,不仅可以获得较高的催化活性,同时可以改善膜通量,对反应-膜分离耦合系统的工业化应用具有参考价值.     

Kinetic modeling of polymerization of butadiene using cobalt-based Ziegler–Natta catalyst

The reaction mechanism and subsequent kinetics for polymerization of butadiene using cobalt-based Ziegler-Natta catalysts have been investigated by many researchers. Kinetic models developed from these investigations can be used to predict the monomer conversion quite accurately; however, it is difficult to develop models that accurately predict the molecular weight as a function of time or conversion. In this paper, an attempt is made to model the reaction mechanism for the polymerization of butadiene using the cobalt octoate/diethyl aluminum chloride/water catalyst system with data taken from the literature. A dual active site mechanism is proposed and incorporated in a kinetic model. In this case, all reaction steps except the formation of byproducts step have two rate constants. The simulation results predict the molecular weight as a function of conversion and time better than results from previously published models.