Combinatorial Heterogeneous Catalysis: Oxidative Dehydrogenation of Ethane to Ethylene, Selective Oxidation of Ethane to Acetic Acid, and Selective Ammoxidation of Propane to Acrylonitrile

The application of combinatorial methods to three reactions catalyzed by multimetal oxides is described. Catalysts for the oxidative dehydrogenation of ethane to ethylene were tested using a 121- or 144-channel scanning mass spectrometer primary screening reactor and a 48-channel fixed bed secondary screening reactor; catalysts for the selective oxidation of ethane to acetic acid were tested using a 256-channel massively parallel microfluidic reactor primary screen alone, and catalysts for the selective ammoxidation of propane to acrylonitrile were tested using the massively parallel microfluidic reactor and an eight-channel fixed bed secondary/tertiary screening reactor. The details regarding catalyst design, synthesis, and screening are presented. This work has resulted in both the confirmation of published results and the generation of new lead materials for all three chemistries.     

Understanding and optimization of chemical reactor performance for bimodal reaction sequences

The relative contributions of heterogeneously catalyzed and homogeneous bulk phase reactions in bimodal reaction sequences have been assessed via 1D reactor simulations. Starting from a reaction network only comprising two parallel, irreversible heterogeneously catalyzed and homogeneous bulk phase steps, complementary consecutive steps were included with the option of being reversible. The final product formed after a minimum number of homogeneous bulk phase reactions is obtained with high yields in continuous flow fixed bed reactors. The products obtained after a higher number of homogeneous bulk phase reactions generally dominate in slurry reactors. Yields of the latter may exhibit an optimum as a function of the catalyst amount in the reactor. The adsorption enthalpies of the intermediates in the reaction network critically determine the position and shape of this maximum. The reversibility of the homogeneous bulk phase steps provides specific opportunities to tune the product yields in bimodal reaction sequences. © 2016 American Institute of Chemical Engineers AIChE J, 63: 111–119, 2017     

From Mechanistic and Kinetic Understanding of Heterogeneously Catalyzed Reactions to Tuning Catalysts Performance

A number of strategies for tuning performance of heterogeneously catalyzed reactions are being explored. They comprise: (1) optimizing reactor operation based on the knowledge of reaction mechanism and kinetics, (2) application of well-defined nano-sized metal nanoparticles for preparation of supported catalysts, and (3) combining complementary reactions into one process via a proper reactor design. The present mini review demonstrates their potential for conversion of C₃–C₄ alkanes to the corresponding alkenes, oxidative functionalization of methane, Fischer–Tropsch reaction and HCl oxidation to Cl₂ (Deacon process).     

Parallel synthesis and fast screening of heterogeneous catalysts

We are presenting an effective method to prepare and test heterogeneous catalysts much faster than by the conventional way. A catalyst array was prepared via an incipient wetness method by combination of different amounts of Pt, Zr, and V on Al₂O₃ by means of an automatic liquid handler. For catalytic testing for methane oxidation a ceramic monolith reactor module, the channels of which contain the different catalyst compositions, was developed in which up to 250 catalyst compositions can be prepared and tested in parallel. Gas samples from each channel of the monolith were analysed sequentially by a mass spectrometer by moving the QMS inlet capillary into the channels using a three-dimensional positioning system which works at high temperatures. By comparison of the testing results with experiments carried out in flow reactors it is shown that the monolithic reactor is an efficient tool for fast screening of heterogeneous catalysts. This revised version was published online in June 2006 with corrections to the Cover Date.     

Temperature profile in a two-stage fixed bed reactor for catalytic partial oxidation of methane to syngas

Detailed axial temperature distribution has been studied in a two-stage process for catalytic partial oxidation of methane to syngas, which consists of two consecutive fixed bed reactors with oxygen or air separately introduced. The first stage of the reactor, packed with a combustion catalyst, is used for catalytic combustion of methane at low initial temperature. While the second stage, filled with a partial oxidation catalyst, is used for the partial oxidation of methane to syngas. A pilot-scale reactor packed with up to 80 g combustion catalyst and 80 g partial oxidation catalyst was employed. The effects of oxygen distribution in the two sections, and gas hourly space velocity (GHSV) on the catalyst bed temperature profile, as well as conversion of methane and selectivities to syngas were investigated under atmospheric pressure. It is found that both oxygen splitting ratio and GHSV have significant influence on the temperature profile in the reactor, which can be explained by the synergetic effects of the fast exothermic oxidation reactions and the slow endothermic (steam and CO₂) reforming reactions. Almost no change in activity and selectivity was observed after a stability experiment for 300 h.     

An analysis of the Mars–van Krevelen rate expression

An analysis of the “mechanism” and the assumptions incorporated into the derivation of the Mars–van Krevelen rate expression for heterogeneously catalyzed oxidation reactions reveals a number of inconsistencies. The expression as derived is properly applicable only for a reaction involving molecular oxygen adsorbed on a single site; however, the model assumes that lattice O ions (or atoms) react with the substrate being oxidized. Furthermore, the steps describing the model are not elementary steps and, in addition, the competitive adsorption of intermediates and products is not considered. Consequently, the original derivation is incorrect and this rate expression must be viewed only as a mathematical data-fitting function. Alternative Langmuir–Hinshelwood, Hougen–Watson-type catalytic sequences can typically provide similar, and sometimes identical, rate equations which can fit rate data better than, or at least as well as, the Mars–van Krevelen expression. These latter models also provide rate parameters that can be evaluated for thermodynamic consistency.     

A novel reverse flow reactor coupling endothermic and exothermic reactions. Part I: comparison of reactor configurations for irreversible endothermic reactions

A new reactor concept is studied for highly endothermic heterogeneously catalysed gas phase reactions at high temperatures with rapid but reversible catalyst deactivation. The reactor concept aims to achieve an indirect coupling of energy necessary for endothermic reactions and energy released by exothermic reactions, without mixing of the endothermic and exothermic reactants, in closed-loop reverse flow operation. Periodic gas flow reversal incorporates regenerative heat exchange inside the reactor. The reactor concept is studied for the coupling between the non-oxidative propane dehydrogenation and methane combustion over a monolithic catalyst.Two different reactor configurations are considered: the sequential reactor configuration, where the endothermic and exothermic reactants are fed sequentially to the same catalyst bed acting as an energy repository and the simultaneous reactor configuration, where the endothermic and exothermic reactants are fed continuously to two different compartments directly exchanging energy. The dynamic reactor behaviour is studied by detailed simulation for both reactor configurations. Energy constraints, relating the endothermic and exothermic operating conditions, to achieve a cyclic steady state are discussed. Furthermore, it is indicated how the operating conditions should be matched in order to control the maximum temperature. Also, it is shown that for a single first order exothermic reaction the maximum dimensionless temperature in reverse flow reactors depends on a single dimensionless number. Finally, both reactor configurations are compared based on their operating conditions. It is shown that only in the sequential reactor configuration the endothermic inlet concentration can be optimised independently of the gas velocities at high throughput and maximum reaction coupling energy efficiency, by the choice of a proper switching scheme with inherently zero differential creep velocity and using the ratio of the cycle times.In this first part, both the propane dehydrogenation and the methane combustion have been considered as first order irreversible reactions. However, the propane dehydrogenation is an equilibrium reaction and the low exit temperatures resulting from the reverse flow concept entail considerable propane conversion losses. How this ‘back-conversion’ can be counteracted is discussed in part II Chemical Engineering Science, 57, (2002), 855-872.     

Catalysts at work: From integral to spatially resolved X-ray absorption spectroscopy

Spectroscopic studies on heterogeneous catalysts have mostly been done in an integral mode. However, in many cases spatial variations in catalyst structure can occur, e.g. during impregnation of pre-shaped particles, during reaction in a catalytic reactor, or in microstructured reactors as the present overview shows. Therefore, spatially resolved molecular information on a microscale is required for a comprehensive understanding of theses systems, partly in ex situ studies, partly under stationary reaction conditions and in some cases even under dynamic reaction conditions.Among the different available techniques, X-ray absorption spectroscopy (XAS) is a well-suited tool for this purpose as the different selected examples highlight. Two different techniques, scanning and full-field X-ray microscopy/tomography, are described and compared. At first, the tomographic structure of impregnated alumina pellets is presented using full-field transmission microtomography and compared to the results obtained with a scanning X-ray microbeam technique to analyse the catalyst bed inside a catalytic quartz glass reactor. On the other hand, by using XAS in scanning microtomography, the structure and the distribution of Cu(0), Cu(I), Cu(II) species in a Cu/ZnO catalyst loaded in a quartz capillary microreactor could be reconstructed quantitatively on a virtual section through the reactor. An illustrating example for spatially resolved XAS under reaction conditions is the partial oxidation of methane over noble metal-based catalysts. In order to obtain spectroscopic information on the spatial variation of the oxidation state of the catalyst inside the reactor XAS spectra were recorded by scanning with a micro-focussed beam along the catalyst bed. Alternatively, full-field transmission imaging was used to efficiently determine the distribution of the oxidation state of a catalyst inside a reactor under reaction conditions. The new technical approaches together with quantitative data analysis and an appropriate in situ catalytic experiment allowed drawing important conclusions on the reaction mechanism, and the analytical strategy might be similarly applied in other case studies. The corresponding temperature profiles and the catalytic performance were measured by means of an IR-camera and mass spectrometric analysis. In a more advanced experiment the ignition process of the partial oxidation of methane was followed in a spatiotemporal manner which demonstrates that spatially resolved spectroscopic information can even be obtained in the subsecond scale.     

Competition between the gas and surface reactions for the oxidative coupling of methane 1. ‘Non-isothermal’ results in catalytic jet-stirred reactor

A catalytic jet-stirred reactor (CJS reactor) has been developed to investigate the interaction between gas-phase and surface reactions for the oxidative coupling of methane. This reactor allows the modification of the number of catalyst pellets (La₂O₃) for a fixed gas-phase volume. It permits also to set different temperatures for the gas-phase volume and the catalyst. The results of these ‘nonisothermal’ experiments are presented; they suggest that the contribution of the gas-phase reactions is rather significant and that the C₂₊ selectivity is improved by an increase of the gas-phase temperature up to 850°C.     

A New Reactor for Combined Conventional and Microwave Heating

A reactor system was developed to investigate the influence of microwaves on adsorption and desorption processes during heterogeneously catalyzed reactions. The reactor is free of temperature gradients and allows combined conventional and microwave heating. The hydroxylation of benzene with N₂O as an oxidant was used to test the reactor.     

Aspects of kinetic modeling of fixed bed reactors

This paper briefly reviews the most important aspects of catalyst testing in packed-bed catalytic laboratory reactors to properly assess the intrinsic chemical kinetics. Next it discusses approaches to assess the kinetics of fast reactions or those accompanied with strong heat effects that cannot be performed in a packed-bed reactor configuration free from transport limitations. As an example the partial oxidation of methane is presented in a steady-state fixed bed reactor as well as in a TAP (temporal analysis of products) reactor. The continuing increase in computational power leads to more sophisticated reaction and reactor models due to the increasing use of computational chemistry and computational fluid dynamics in reaction engineering.     

Catalytic partial oxidation of methane in a novel heat-integrated wall reactor

A novel reactor has been developed and applied in the reaction of partial oxidation of methane to synthesis gas. The reactor consists of a ceramic tube in the inner and outer surface of which a metal catalyst film is deposited. The CH₄/O₂ feed enters into the tube and a large fraction of the heat generated on the wall by methane combustion is transported across the tube wall towards the outer catalyst film, where the endothermic reforming reactions take place. In this way, the temperature in the combustion zone is controlled and hot spots are significantly reduced in magnitude. Initial results presented in this work demonstrate the feasibility of the concept. This revised version was published online in July 2006 with corrections to the Cover Date.     

Computational optimization and sensitivity analysis of fuel reformer

In this study, the catalytic partial oxidation of methane is numerically investigated using an unstructured, implicit, fully coupled finite volume approach. The nonlinear system of equations is solved by Newton’s method. The catalytic partial oxidation of methane over rhodium catalyst in a coated honeycomb reactor is studied three-dimensionally, and eight gas-phase species (CH₄, CO₂, H₂O, N₂, O₂, CO, OH and H₂) are considered for the simulation. Surface chemistry is modeled by detailed reaction mechanism including 38 heterogeneous reactions with 20 surface-adsorbed species for the Rh catalyst. The numerical results are compared with experimental data and good agreement is observed. Effects of the design variables, which include the inlet velocity, methane/oxygen ratio, catalytic wall temperature, and catalyst loading on the cost functions representing methane conversion and hydrogen production, are numerically investigated. The sensitivity analysis for the reactor is performed using three different approaches: finite difference, direct differentiation and an adjoint method. Two gradient-based design optimization algorithms are utilized to improve the reactor performance.     

A highly active silica(silicon)-supported vanadia catalyst for C1 oxygenates and hydrocarbon production from partial oxidation of methane

A very low surface area silica-silicon substrate has been used as a support for vanadium oxide and has been tested in the partial oxidation of methane. Use of a reactor with variable dead volume ahead of the bed of the catalyst allows determining the relevance of gas phase reactions in initiating methane conversion. Experimental evidence supports that at atmospheric pressure C₁ oxygenates are essentially produced on the catalyst surface rather than in the gas phase. Comparison with a high surface area silica-supported vanadium oxide catalyst clearly highlights the double role of surface area in promoting catalytic activity, but also in promoting non-selective further oxidation of reaction products. It is shown that a reaction system combining dead volume upstream the bed of the catalyst and a very low surface area is very promising to activate methane conversion to C₁ oxygenates and C₂₊ hydrocarbons at remarkable TOF number preventing further non-selective oxidation. In addition, production of C₂₊ hydrocarbons is observed at temperatures as low as 750 K.     

A novel reverse flow reactor coupling endothermic and exothermic reactions: Part II: Sequential reactor configuration for reversible endothermic reactions

The new reactor concept for highly endothermic reactions at elevated temperatures with possible rapid catalyst deactivation based on the indirect coupling of endothermic and exothermic reactions in reverse flow, developed for irreversible reactions in Part I, has been extended to reversible endothermic reactions for the sequential reactor configuration. In the sequential reactor configuration, the endothermic and exothermic reactants are fed discontinuously and sequentially to the same catalyst bed, which acts as an energy repository delivering energy during the endothermic reaction phase and storing energy during the consecutive exothermic reaction phase. The periodic flow reversals to incorporate recuperative heat exchange result in low temperatures at both reactor ends, while high temperatures prevail in the centre of the reactor. For reversible endothermic reactions, these low exit temperatures can shift the equilibrium back towards the reactants side, causing ‘back-conversion’ at the reactor outlet.The extent of back-conversion is investigated for the propane dehydrogenation/methane combustion reaction system, considering a worst case scenario for the kinetics by assuming that the propylene hydrogenation reaction rate at low temperatures is only limited by mass transfer. It is shown for this reaction system that full equilibrium conversion of the endothermic reactants cannot be combined with recuperative heat exchange, if the reactor is filled entirely with active catalyst. Inactive sections installed at the reactor ends can reduce this back-conversion, but cannot completely prevent it. Furthermore, undesired high temperature peaks can be formed at the transition point between the inactive and active sections, exceeding the maximum allowable temperature (at least for the relatively fast combustion reactions).A new solution is introduced to achieve both full equilibrium conversion and recuperative heat exchange while simultaneously avoiding too high temperatures, even for the worst case scenario of very fast propylene hydrogenation and fuel combustion reaction rates. The proposed solution utilises the movement of the temperature fronts in the sequential reactor configuration and employs less active sections installed at either end of the active catalyst bed and completely inactive sections at the reactor ends, whereas propane combustion is used for energy supply. Finally, it is shown that the plateau temperature can be effectively controlled by simultaneous combustion of propane and methane during the exothermic reaction phase.     

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.     

Solid electrolyte membrane reactor for controlled partial oxidation of hydrocarbons: Model and experimental validation

A mathematical model of a solid electrolyte membrane reactor is presented which accounts for the prevailing physical phenomena of the electrochemical partial oxidation of n-butane to maleic anhydride. From an analysis of characteristic dimensionless numbers it was concluded that the reactor behaviour can be described by a one-dimensional pseudo-homogeneous approach with respect to the anodic gas channel and a one-plus-one-dimensional electrochemical model. Beside mass and charge transport processes, electrochemical charge transfer reactions as well as heterogeneously catalysed oxidation reactions are considered. As kinetic model a modified Mars–van Krevelen approach is suggested. Experimental results of oxygen pumping and butane oxidation experiments were used to determine kinetic parameters and to validate the model.     

Kinetic investigation of methacrylic acid synthesis on hetropoly-compounds

The kinetics of heterogeneously catalyzed methacrylic acid synthesis from isobutyric acid has been investigated. Initial catalyst screening pointed to 10-molybdo-2-vanado-phosphoric acid and its Cs-salts as the most promising catalysts. A model describing the reaction behaviour of all the different hetropoly-compounds used in this work was developed. A relationship was found between model parameters and composition of different catalysts. First insight was gained into the catalyst deactivation phenomena. Deactivation appears to be caused mainly by loss of molybdenum with simultaneous collapse of the Keggin structure.     

A sequential approach to modeling catalytic reactions in packed-bed reactors

A sequential modeling approach is proposed to simulate catalytic reactions in packed-bed reactors. The hydrogenation of alpha-methylstyrene and wet oxidation of phenol are selected as studied cases. The modeling scheme combines a reactor scale axial dispersion model with a pellet scale model. Without involving any fitting parameters, such an approach accounts for the non-linear reaction kinetics expression and different types of pellet-liquid wetting contact. To validate the developed modeling scheme and the parallel approach reported in the literature, the experimental observations for hydrogenation of alpha-methylstyrene to cumene have been employed. The predicted results by both approaches agree reasonably with the experimental data for both gas- and liquid-limited reaction. The proposed sequential approach was also used to simulate the dynamic performance of the reactor and pellets for the catalytic wet oxidation of aqueous phenol over a newly developed but rapidly deactivated catalyst (MnO₂/CeO₂). The simulation results for the catalytic wet oxidation process by both approaches were compared. The simulation describes the time evolution of the catalyst stability at different pellet points along the reactor axis. The performance of trickle beds and packed bubble columns over a range of operating conditions were also investigated, and packed bubble columns were found to achieve higher phenol conversion at the cost of more rapid catalyst deactivation.     

Liquid‐Phase Aerobic Oxidation of Benzyl Alcohol Catalyzed by Pt/ZrO2

The oxidation of benzyl alcohol by molecular oxygen in the liquid phase and catalyzed by Pt/ZrO2 using n‐heptane as the solvent was studied. Pt/ZrO₂ was very active and 100 % selective for benzyl alcohol conversion to benzaldehyde. The catalyst can be separated by filtration and reused. No leaching of Pt or Zr into the solution was observed. Typical batch reactor kinetic data were obtained and fitted to the Langmuir‐Hinshelwood, Eley‐Rideal and Mars‐van Krevelen models of heterogeneously catalyzed reactions. The Langmuir‐Hinshelwood model was found to give a better fit. The rate‐determining step was proposed to involve direct interaction of an adsorbed oxidizing species with the adsorbed reactant or an intermediate product of the reactant. H₂O₂ was also proposed to be an intermediate product. n‐Heptane was found to be an appropriate solvent in this reaction system.