Kinetics of hydrogenolysis of methylamine on a rhodium catalyst

The kinetics of hydrogenolysis of methylamine to methane and ammonia were investigated over a catalyst consisting of small clusters of rhodium dispersed on silica. Data obtained in the temperature range 353–408 K exhibit a characteristic pattern in which the rate passes through a maximum as the hydrogen partial pressure is increased by two orders of magnitude from 0.01 to 1.0 atm. At a given temperature, the position of the maximum shifts slightly in the direction of higher hydrogen partial pressure when the methylamine partial pressure increases by one to two orders of magnitude. Of particular interest is the finding that the rate increases with decreasing methylamine partial pressure over a broad range of hydrogen partial pressures covered in the investigation. As the hydrogen pressure increases, the inverse dependence of the rate on methylamine pressure becomes less pronounced and eventually disappears at a sufficiently high hydrogen pressure. At hydrogen partial pressures somewhat higher than those at which the rate maxima are observed, there is some indication that the inverse dependence changes to a positive dependence, especially at the lowest temperatures investigated. It seems likely that the rate limiting step of the reaction changes when the hydrogen pressure varies over a wide range. At the highest hydrogen pressures studied, it is suggested that the limiting step is one in which the scission of the carbon-nitrogen bond occurs in a hydrogen deficient surface intermediate formed in the chemisorption of methylamine, with no direct participation of hydrogen as a reactant in the step. On the other hand, at the lowest hydrogen pressures investigated, it is proposed that the rate is limited by a step in which chemisorbed hydrogen does participate directly as a reactant.     

Methylamine hydrogenolysis on a rhodium catalyst: kinetics at high hydrogen partial pressures

The kinetics of hydrogenolysis of methylamine to methane and ammonia on a rhodium catalyst were investigated at hydrogen partial pressures in the range of 2–10 atm at temperatures of 368, 383, and 408 K. At a fixed methylamine partial pressure, the rate decreased with increasing hydrogen partial pressure. When the hydrogen pressure was held constant, the rate increased with increasing methylamine pressure. Results of a previous investigation by our group at lower hydrogen partial pressures (0.01–1 atm) indicated that the hydrogenolysis rate passed through a maximum with increasing hydrogen pressure. Moreover, at the lower hydrogen pressures, there was an inverse rather than positive dependence of the rate on methylamine partial pressure. With the aid of the present results, there is a much clearer definition of the maximum in the experimental data relating the reaction rate to hydrogen partial pressure. The inversion of the effect of methylamine pressure on the rate as the hydrogen pressure is varied over a sufficiently wide range is also firmly established. With regard to the interpretation of the many interesting features of the kinetics, we retain the suggestion from our earlier work that the rate limiting step at the highest hydrogen pressures is the scission of the carbon-nitrogen bond in a partially dehydrogenated methylamine intermediate chemisorbed on the rhodium, with no direct participation of hydrogen as a reactant in this step. At the lowest hydrogen pressures, however, there is a different rate limiting step in which hydrogen does participate directly as a reactant.     

Comparison of cracking and hydrocraking of n-heptane on H-ZSM-5 catalysts

Acid catalyzed cracking and bifunctional cracking of n-heptane were investigated on HZSM-5 catalysts. At a reaction temperature of 543 K, the cracking on metal-free zeolite was found to be directly proportional to hydrogen partial pressure. Hydrogen influences the hydrogenation of product olefins and carbon deposits and therefore enhances the overall activity. Under the same conditions, in the presence of platinum, the hydrocracking rate reaches a maximum with increasing hydrogen partial pressure. The reaction can be formally described by a Langmuir Hinshewood mechanism: hydrogen is adsorbed on Pt in competition with hydrocarbons. The maximum reaction rate depends on a favourable ratio of the two adsorbed reactants. The energy of activation of hydrocracking was over 100 kJ/mol higher than that of acid cracking.     

Microkinetic analysis of isopropanol dehydrogenation over Cu/SiO2 catalyst

The reaction of isopropanol dehydrogenation to acetone over the Cu/SiO₂ catalyst was studied using the method of microkinetic analysis. This analysis provided a picture about the coverage of different surface species and their influences on the reaction rate. Specifically, the analysis reinforced the previously proposed mechanism and added some new findings for the reaction. It was found that under the normal reaction conditions, the catalyst surface is mostly covered by the isopropoxyl groups and the overall reaction rate is controlled by the elimination of α-H of the isopropoxyl groups. In addition, the elimination of the α-H is significantly affected by the availability of surface Cu sites. At the low hydrogen partial pressure, the number of bare surface Cu sites increases with the increasing of hydrogen partial pressure, leading to the increased overall reaction rate and thus the positive reaction order with respect to hydrogen. Thus, hydrogen can enhance the rate of this reaction via some kinetic factors under the specific coverage regime, although it is thermodynamically unfavorable for the reaction.     

Rapid devolatilization and hydrogasification of bituminous coal

Rapid devolatilization and hydrogasification of a Pittsburgh Seam bituminous coal were studied and an appropriate coal conversion (weight loss) model was developed that accounts for thermal decomposition of the coal, secondary char-forming reactions of volatiles, and homogeneous and heterogeneous reactions involving hydrogen. Approximately monolayer samples of coal particles supported on wire mesh heating elements were electrically heated in hydrogen, helium, and mixtures thereof. Coal weight loss (volatiles yield) was measured as a function of residence time (0–20 s), heating rate (65–10000 °C/s), final temperature (400–1100 °C), total pressure (0.0001–7 MPa), hydrogen partial pressure (0–7 MPa), and particle size (70–1000 μm). Volatiles yield under these conditions increases significantly with decreasing pressure, decreasing particle size, increasing hydrogen partial pressure and increasing final temperature, but only slightly with increasing heating rate. The data support the view that coal conversion under these conditions involves numerous parallel thermal decomposition reactions forming primary volatiles and initiating a sequence of secondary reactions leading to char. Intermediates in this char-forming sequence can escape as tar if residence time in the presence of hot coal surfaces is sufficiently short (e.g. low pressures and small particles well dispersed). Hydrogen at sufficiently high partial pressure can interrupt the char-forming sequence thereby increasing volatile yield. Rate of total product generation is largely controlled by coal pyrolysis while competition between mass transfer, secondary reactions, and rapid hydrogenation affects only the relative proportions of volatile and solid products formed.     

On the effect of hydrogen on the palladium-catalyzed formation of benzene from acetylene

An infrared spectrum of a Pd(111) surface collected in the presence of 5 Torr of acetylene as a function of hydrogen pressure reveals that the ethylidyne coverage increases with hydrogen pressure (P(H₂) between zero and 20 Torr). The amount of CO that can be accommodated onto the surface at a pressure of 5 Torr, measured after evacuating the acetylene and hydrogen, increases linearly with hydrogen pressure, and this effect is ascribed to the presence of a more open surface produced by the formation of ethylidyne. It is found that acetylene adsorbs in ultrahigh vacuum on ethylidyne-covered Pd(111) and reacts to form benzene, where the benzene desorbs at 280 K. This effect is mirrored in the catalytic chemistry where the rate of benzene formation from acetylene in the presence of hydrogen increases linearly with hydrogen pressure.     

Cyclohexane dehydrogenation on a nickel catalyst: Kinetics and catalyst fouling

The main reaction and deactivation kinetics of cyclohexane dehydrogenation in the presence of hydrogen has been investigated at atmospheric pressure over a nickel kieselguhr catalyst in the temperature range 583–623 K. The rate of reaction for the fresh catalyst increased with increasing temperature, cyclohexane and hydrogen partial pressures whereas it decreased with an increase in the benzene partial pressure. The experimental data could be adequately modelled by a power law rate expression. The catalyst activity decreased with run time due to catalyst fouling by coke deposition. The rate of deactivation was independent of cyclohexane partial pressure, increased with increasing benzene concentration and decreased with increasing hydrogen partial pressure. It is postulated that coke is most likely formed by the successive dehydrogenation of benzene.     

Bismuth‐Doped Ceria,Ce0.90Bi0.10O2: A Selective and Stable Catalyst for Clean Hydrogen Combustion

Bismuth‐doped cerias are successfully applied as solid “oxygen reservoirs” in the oxidative dehydrogenation of propane. The lattice oxygen of the ceria is used to selectively combust hydrogen from the dehydrogenation mixture at 550 °C. This process has three key advantages: it shifts the dehydrogenation equilibrium to the desired products side, generates heat, aiding the endothermic dehydrogenation, and simplifies product separation (water vs. hydrogen). Furthermore, the process is safer, since it uses the catalyst’s lattice oxygen instead of gaseous oxygen. We show here that bismuth‐doped cerias are highly active and stable towards hydrogen combustion, and explore four different approaches for optimising their application in the oxidative dehydrogenation of propane: first, the addition of extra hydrogen which lowers hydrocarbon conversion by suppressing both combustion and coking; second, the addition of tin which completely inhibits coking; third, the addition of platinum which increases selectivity, but at the expense of lower activity. The best results are obtained through tuning the reaction temperature. At 400 °C, high activity and selectivity were obtained for the bismuth‐doped ceria Ce_0.90Bi_0.10O₂. Here, 90% of the hydrogen feed is converted at 98% selectivity. This optimal reaction temperature can be rationalised from the hydrogen and propene temperature‐programmed reduction (TPR) profiles: 400 °C lies above the reduction maximum of hydrogen, yet below that of propene. That is, this temperature is sufficiently high to facilitate rapid hydrogen combustion, but low enough to prevent hydrocarbon conversion.     

Investigation of the reaction between single aerosol acid droplets and ammonia gas

An experimental technique has been developed for the continuous measurement of the reaction dynamics between single liquid droplets and reactive gases. The technique is based on the suspension of electrically charged droplets in an electrodynamic field where droplet masses are determined from weight-balancing direct-current voltages. From the droplet mass history, the extent of the reaction is determined as a function of time, particle size and ammonia gas partial pressure. Droplet reactions are conducted under continuously mixed fluid-dynamical conditions.

It is found that surface phase reaction, gas phase diffusion and internal particle diffusion sequentially control the reaction dynamics of the particles. At constant external ammonia gas partial pressure, the maximum extent of reaction achieved during the surface phase reaction is insensitive to particle size while the maximum extent of reaction achieved during the gas phase diffusion-controlled process increases with increasing particle size. In fact for sufficiently small particles the gas phase diffusion process is completely inhibited and subsequent internal particle diffusion does not occur. Correspondingly, the final extent of reaction is determined by the duration of the gas phase diffusion-controlled process. Furthermore, increasing the external ammonia gas partial pressure decreases the maximum extent of reaction achieved during the gas phase diffusion-controlled process and consequently the final extent of reaction.     

Nitridation Kinetics and Thermodynamics of Silicon Powder Compacts

The nitridation kinetics of Si powder compacts were studied by measuring the flow rate dependence of N₂ and N₂–H₂ gas mixtures during slow heating of Si compacts while simultaneously monitoring the oxygen partial pressure of the egress gas. The reaction was found to occur in two distinct stages. In pure nitrogen the initial stage was interpreted in terms of devitrification of the native silica layer, catalyzed by Fe impurities, and the exposure of the underlying Si. The reaction sequence at that point has been unequivocally shown to be followed by the formation of oxynitride according to as evidenced by a large increase in the oxygen partial pressure simultaneously with the onset of the initial stage. In the absence of hydrogen, both reactions are rapidly suppressed as the oxygen liberated competes with the nitrogen for the exposed silicon surface and reoxidizes it. However, in the presence of hydrogen, the oxygen liberated reacts with the hydrogen and prevents the reoxidation of the Si. The net reaction in this case is and/or with the second reaction being thermodynamically favored. The main nitridation reaction occurring between 1300° and 1400°C appears to be diffusion controlled and depends on the nature of the passivating layer that forms during the initial stage.     

A selectivity method for modelling the kinetics of pentene-2 hydrogenation

A maximum likelihood ratio was used as a statistical criterion for discriminating among twenty-two kinetic models posited to describe the hydrogenation of mixed pentenes (cis- and trans-2-pentenes) with a Raney nickel catalyst. Selectivity ratio, the ratio of reaction rates of two different species, was used and found to be a suitable extension of conventional techniques for identifying rate controlling steps owing in this case to its reproducibility. Selectivity ratio models were of the Langmuir-Hinshelwood form. Hydrogenation was performed isothermally at 20°C in benzene at five constant pressures corresponding to partial pressures of hydrogen of 22.5, 42.3, 62.3, 112.3 and 122.1 psia. Discrimination was attempted at each pressure, but found acceptable only at the three intermediate pressures. At each of these pressures, a model of similar functional form was found to represent the selectivity data; however, this model appears to be empirical. The likelihood ratio approach appears to be a powerful one for discriminating among a large number of models even with only a moderate number (? 20) of experimental observations.     

氢气在无催化煤液化中的反应机理

采用平衡液相取样法气体溶解度测定装置测定了氢气在萘中的溶解规律,并采用间歇式微型反应釜研究了氢气在无催化煤液化中的反应机理.结果表明:1)氢气在萘中的溶解随着温度和压力的升高而增加,溶解速率先快后慢,在5min时达到最大溶解量的76.21%左右,直到30min达到平衡;2)在萘溶剂的无催化煤液化反应中,氢气的溶解不是控制步骤,溶解氢参与液化反应的速度才是控制步骤;3)在较短时间的萘溶剂无催化煤液化时,氢气在萘溶剂中的预溶解提高了无催化煤液化的总转化率,其主要原因是部分预溶氢提前活化,使得煤液化反应初期活性氢增加;4)在较长时间的萘溶剂无催化煤液化时,预溶氢对总转化率的提高很小,但促进了液化产物的进一步裂解加氢轻质化.     

Study of coal conversion in polycondensed aromatic compounds

Coal conversions of up to 90 wt% can be obtained in solvents such as pyrene in an inert gas atmosphere. The extent of conversion is strongly dependent on the carbon content of the coal, those coals with 82–88 wt% maf carbon being the most easily converted. Under these conditions, conversion is sensitive to coal preparation and even mild air exposure can be greatly detrimental. In a hydrogen atmosphere, enhanced coal conversions are obtained in pyrene. The degree of enhancement is related to the interaction of pyrene, hydrogen and the coal mineral matter. Analyses of pyrene-coal reaction products showed the presence of hydro- and alkylated pyrenes. Alkylation can be very extensive, accounting for the transfer to pyrene of as many as one in 60 of the carbon atoms in the coal. It appears that dihydropyrene formation is catalysed by coal mineral matter and tends to increase with increasing convertibility of the coal. A mechanism is proposed by which the hydroderivative, formed in-situ by reaction with hydrogen gas, increases conversion. If the catalytic activity for hydrogenation is high and the demand for hydrogen by the coal is sufficiently low the hydroderivative will be fromed at a rate in excess of that required to liquefy coal through a hydrogen-donor mechanism. The properties of pyrene and similar solvents are discussed in relation to the beneficial effects of recycling high-boiling coal liquefaction products in process units.     

Kinetics and mechanism of N-substituted amide hydrolysis in high-temperature water

N-methylacetamide (NMA) served as a model to investigate the hydrolysis kinetics and mechanism of N-substituted amides in high-temperature water. The major products are acetic acid and methylamine, and the reaction is reversible. The hydrolysis reaction is first order in water and first order in NMA at both subcritical and supercritical conditions. The hydrolysis rate is also pH dependent, and three distinct regions of pH dependence exist. At low and high pH, the conversion increased rapidly with added acid and base, respectively. At near-neutral pH, however, the rate was essentially insensitive to changes in pH. Further investigation revealed that the hydrolysis rate constant was very sensitive to the size of the substituent on the carbonyl carbon atom. An S_N2 mechanism with water as the nucleophile appears to be a likely candidate for the hydrolysis mechanism in high-temperature water at near-neutral conditions.     

Reactivities of carbon to steam and hydrogen and applications to technical gasification processes—A review

Experimental results of reactivity measurements on coal and chars to steam, hydrogen and steamhydrogen mixtures are compared and their application to technical processes is discussed. The change of surface area as a function of burn-off has a minor significance for chars made from coal with high initial accessible surface areas. In this case the reaction rate under constant steam pressure is first order with respect to the amount of carbon being gasified. The reaction rate of hydrogasification markedly decreases with burn-off, since the activation energy increases with burn-off due to the consumption of reactive carbon during the reaction. The reaction of carbon with steam hydrogen mixtures can be described as a superposition of carbon steam reaction and hydrogasification. The overall rate relative to that with hydrogen or steam alone can increase if a high amount of reactive carbon is present and immediately reacts with hydrogen. It can decrease if the inhibition of the steam carbon reaction by hydrogen predominates. Some consequences for the technical performance of coal gasification are: The overall reaction rate in a fluid bed decreases with increasing bed height, since hydrogen-steam ratios and therefore the inhibition by hydrogen increase. The overall reaction rate in a moving bed with countercurrent flow of carbon and reacting gases increases with increasing overall pressure since highly reactive carbon, formed in the pyrolysis zone, can immediately react with high partial pressures of hydrogen.     

Increasing the burning velocity of a low-pressure hydrogen-oxygen flame by the addition of trimethyl phosphate in terms of Zel’dovich’s chain mechanism of flame propagation

The chain reaction mechanism and theoretical approach proposed by Zel’dovich for hydrogen flame propagation are used to describe the effect of increasing the burning velocity of a low-pressure hydrogen-oxygen flame by the addition of trimethyl phosphate (TMP), which induce a catalytic recombination of hydrogen atoms. The promotion of a stoichiometric hydrogen-oxygen flame at subatmospheric pressure by the addition of TMP at a low concentration (0.1–0.5%) is described using a model of the catalytic recombination of hydrogen atoms. The results of calculation using Zel’dovich’s theory with the proposed simplified kinetic model are in good agreement with simulation results for the complete kinetic mechanism. It is shown that increasing the recombination rate of hydrogen atoms in a catalytic reaction involving phosphorus-containing species increases the heat release rate and, hence, the flame burning velocity. A kinetic analysis was performed of the dependence of the ratio of the recombination and branching rates, the temperature at the maximum reaction rate, and the maximum mole fraction of hydrogen atoms on the pressure and additive concentration. The study confirmed Zel’dovich’s prediction that the recombination not only has the harmful effect of terminating chains, but it also has the beneficial effect of releasing heat.     

The influence of hydrogen pressure on the heterogeneous hydrogenation of β-myrcene in a CO2-expanded liquid

This paper presents the second part of work on the effect of hydrogen partial pressure on the hydrogenation of a terpene in a CO₂-expanded liquid. The effect of hydrogen partial pressure on the hydrogenation of β-myrcene possessing three CC bonds catalysed by alumina-supported ruthenium and rhodium was studied. Experiments were performed at various hydrogen pressures in the range from 2.0 up to 4.5 MPa at a fixed total pressure of 12.5 MPa. In all the conditions the reaction proceeded in two phases (liquid + gas), that is, the total pressure was below the critical pressure of the CO₂ + β-myrcene + H₂ system. The liquid phase volume is expanded in relation to the initial volume of β-myrcene in a fashion that is strongly dependent on the hydrogen and carbon dioxide pressures. An increase of H₂ pressure concomitantly diminishes carbon dioxide pressure, which leads to the enhancement of the liquid phase in hydrogen and a terpene. It does not direct to straightforward higher reaction rate, but surprisingly the effect of higher concentrations either hydrogen or β-myrcene is opposite. It is attributed to the fact that the hydrogenation of β-myrcene rate-controlling factor turns out to be the hydrogen to β-myrcene ratio which decreases as the hydrogen pressure increases. These unexpected appealing results present that lower pressures of hydrogen guide to higher hydrogen/β-myrcene ratios in the liquid phase, but on the other hand they also amplify the initial reaction rate constant. The obtained results are opposite to the results achieved for effect of hydrogen pressure on the Pd-catalysed hydrogenation of limonene consisting of two CC double bonds.     

Fixed-bed pyrolysis of coal under hydrogen pressure at low heating rates

Fixed-bed hydropyrolysis has been investigated by treating 100 g coal up to 900°C and 10 MPa. The devolatilization rate of Beringen coal (32.8 wt% volatile matter) treated on a fixed bed approximates to that obtained by flash hydropyrolysis. However, the oil yield is smaller because of the slower heating of the coal and the rather longer residence time of the primary volatile matter in the reaction space. The product gas is mainly methane. The oil composition depends on the temperature of pyrolysis. The benzene content of the oil rises with temperature. At constant temperature, the influence of hydrogen partial pressure is important between 0–1 MPa. At higher pressure, the yields and compositions vary only slightly with pressure. It has also been shown that from 580°C pyrolysis under hydrogen yields an additional quantity of water, when compared with pyrolysis under inert atmospheres or under atmospheric pressure. This additional water comes from the hydrogenation reactions of the hydroxyl functions of heavy phenols and xylenols. This implies a hydrogen consumption (from 0.2–0.3 wt% of the coal), varying with the pyrolysis temperature.     

甲醇水溶液超声波制氢实验研究

介绍了一种新的制氢方法-超声波分解甲醇水溶液制氢.详细阐述了该方法的制氢反应机理,并在低频超声波(40 kHz,500 W)辐射下对该方法进行了系统的实验研究,得到了环境温度、溶液浓度、超声波振幅等因素对产氢量/速率的影响规律.实验结果表明:随着甲醇浓度的增加,产氢量呈现先增加后下降的趋势,并且浓度为1000(体积)时...