Nanotechnology: applications and potentials for heterogeneous catalysis

Nanotechnology offers the potential to design, synthesize, and control at nanometer length scale. While the catalysis community has many techniques at their disposal to synthesize catalytic materials at such a scale, the ability to fully design and control is still lacking. Examples are presented to illustrate what can nanotechnology do for heterogeneous catalysis to help achieve the goal of designing catalysts for perfect selectivity in a chemical reaction. Some current state-of-the-art approaches and potential limitations are discussed. Some examples of what can catalysis do for nanotechnology are also presented. However, this aspect is much less studied, although it offers rich opportunities for the catalysis community.     

Selectivity enhancement in heterogeneous catalysis induced by reaction modifiers

The application range of solid catalysts can be greatly extended by reaction or process modifiers, that is by simple addition of an inorganic or organic compound to the reaction mixture. The modifier, used in catalytic amounts, ideally interacts strongly with the active sites in a fashion which induces favorable changes in the outcome of the reaction. Evolution of the actual modified metal catalyst during reaction and the importance of in situ characterization in understanding these processes are illustrated using the examples of promotion by metal ions and nitrogen-containing bases. The major part of the review describes the advantages and limitations of employing N-base modifiers for tuning the performance of solid catalysts. Reactions discussed include chemo-, stereo-, enantio- and diastereoselective hydrogenations over metal catalysts, aerobic oxidation of alcohols with Pt and Pd, and epoxidation of allylic alcohols with titania–silica mixed oxides and alkyl hydroperoxides.     

Discrimination of rival kinetic models in heterogeneous catalysis by the dynamic behaviour of products

The transient response method has been applied to discriminate between rival kinetic models derived from the conventional Hougen-Watson procedure in heterogeneous catalysis, by using some old data for the NO-CO reaction, dehydrogenation of sec-butanol, hydrogenation of phenol, and dehydrogenation of tert-butanol. A computer technique is commonly used for the simulation of the transient response curves of products caused by the concentration jump of reactants in a flow type reactor. The shapes of the calculated response curves are of various characteristic types (depending on the given model) which allow for easy discrimination between rival kinetic models. The difficulty of the discrimination in some particular models is also discussed.     

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.     

Bubble-size distributions and interfacial areas in a jetloop reactor for multiphase catalysis

Jetloop reactors show specific properties, such as intensive mixing and effective gas dispersion, which make them an interesting alternative for conventional reactor concepts in homogeneous multiphase catalysis. This contribution investigates the effect of specific power input and common additives in multiphase catalysis on bubble-size distributions in the draft tube and riser section of a miniplant-scale jetloop reactor and elucidates the coalescence during circulation. Finally interfacial areas are calculated and compared to standard reactors.     

Use of stirred batch reactors for the assessment of adsorption constants in porous solid catalysts with simultaneous diffusion and reaction. Theoretical analysis

A simple, pseudo-equilibrium model was derived for a catalytic system with a first order chemical reaction and simultaneous diffusive and adsorptive processes, in order to assess the corresponding kinetics and Henry law's-type adsorption parameters. Solutions from this model were compared to exact solutions from a more detailed, general model. It was shown that under most of the experimental conditions used in stirred batch reactors and the usual model considerations, it is only possible to assess apparent adsorption parameters. Also, we observed that a stable relationship between the concentrations in the gas and solid phases is reached. The error produced in assuming that the apparent adsorption constant is the real one was calculated to be very important. The value of the apparent adsorption constant depends on various system properties and experimental conditions, such as the Thiele modulus, the amount of catalyst and the contact time. The ratio between the apparent and real adsorption constants was shown to be the transient effectiveness factor at any moment. This ratio reaches a maximum value for the pseudo-equilibrium state, that is always larger than the steady-state effectiveness factor, becoming closer as long as the system's adsorption capacity decreases. The analysis determines the operative conditions to reduce the parametric correlation. Also a criterion for the applicability of usual approximations in the assessment of kinetics and equilibrium adsorption parameters in porous solid catalysts by means of pulse injection methods is established.     

Measurement and enhancement of gas-liquid mass transfer in milliliter-scale slurry reactors

Due to the limited availability of chemical reactants in the early process development of pharmaceuticals and fine chemicals, and sometimes the high-cost of catalyst, it is increasingly popular to use milliliter-scale slurry reactors with reaction volumes of 20 ml or less to screen catalyst candidates for three-phase reactions. To ensure the success of catalyst screening, it is advantageous to run reactions under kinetically controlled conditions so that the activities of different catalysts can be compared. Because catalysts with small particle sizes are used in slurry reactors, the reactions are susceptible to gas-liquid mass transfer limitations. This work presents an efficient way of enhancing gas-liquid mass transfer in milliliter-scale reactors through the use of magnetically driven agitation with complex motion. In the reactor described here, gas-liquid mass transfer coefficients can be doubled over those obtained with the agitation technique used in commercial milliliter-scale units. In addition, the reactor can achieve the top range of mass transfer coefficients obtained in a full-scale reactor. This work also presents the first measurements of gas-liquid mass transfer coefficients in milliliter-scale reactors, which are two orders-of-magnitude smaller than systems for which mass transfer coefficients have been reported earlier. Both physical and chemical absorption techniques are used.     

Transesterification cure for coatings: catalysis by epoxy and nucleophiles

Transesterification cure in coatings can be catalyzed by an alkoxide which is formed during bake by the combination of an epoxide with a nucleophile. Support for transesterification cure in the presence of epoxides and nucleophiles has been provided by FTIR evolved gas analysis (EGA/FTIR) and thermal gravimetric analysis (TGA). Effective nucleophiles include tertiary amines, certain inorganic nucleophiles like cyanide, hydroxide and azide, and quaternary ammonium salts. Quaternary ammonium carboxylates are particularly effective nucleophile sources for this cure chemistry, especially those that have better thermal stability. EGA/FTIR was used to demonstrate differences in the thermal stability of quaternary ammonium salts, and these differences were correlated with differences in cure at higher bake temperatures. Transesterification cure of hydroxyl functional acrylics can provide greatly improved impact resistance if the acrylics are blended with polyesters. Dynamic mechanical analysis (DMA) has shown differences between acrylic and acrylic-polyester blends which may help to explain the differences in impact resistance.     

胶束催化作用及其在动力学分析中的应用

研究表明，适量的胶束可催化化学反应，在动力学分析中大有潜力，本文介绍了胶束催化作用，并就其在动力学分析中的最新研究进展及时予以评述。     

Spherical polyelectrolyte brushes as nanoreactors for the generation of metallic and oxidic nanoparticles: Synthesis and application in catalysis

If long chains of a polyelectrolyte are densely affixed to colloidal spheres, a spherical polyelectrolyte brush (SPB) results. It has been demonstrated that most of the counterions that balance the charges of the polyelectrolyte chains in aqueous solution are confined within the brush layer. Some years ago it has been shown that metal ions confined within a SPB can be reduced to yield well-defined metal nanoparticles with high catalytic activity. In a similar way, metal oxides as TiO₂ and MnO_x can be generated within the brush layer. Here we review this work with special emphasis on catalysis. We first discuss recent papers that study the confinement of the counterions within SPB and the consequences thereof for the solution properties of the SPB. Then a survey of synthetic studies on these composite particles is presented. Finally, a comprehensive review of the investigations on the catalytic activity of metallic and oxidic nanoparticles generated on SPB will be given. All results obtained on these systems demonstrate SPB act as true “nanoreactors” that can be used for an efficient and safe handling of metallic and oxidic nanoparticles in catalysis without impeding their catalytic activity.     

A new approach to diagnostics of ideal and non-ideal flow patterns: I. The concept of reactive-mixing index (REMI) analysis

A new approach is proposed allowing to characterize the hydrodynamic regime in the presence of chemical reactions, through small pulse-response experiments using a reacting component as a tracer instead of inert components which are used in the traditional Danckwerts-type experiment.Theoretically, our approach is based on the escape time defined as the mean exit age of molecules that escaped all reactions, and uses the original concept of the reactive mixing index (REMI), which is the ratio of the normalized difference between the escape time (ET(X)=M₁(X)/M₀(X)) in the presence and in the absence of reaction, to conversion X: REMI(X)=(1-ET(X)/ET(0))/X.Experimentally, the observed values are the numbers of additional exiting molecules per second as a function of time, F(t) (mol/s). Based on these values, the moments M₀(X) and M₁(X) can be computed and REMI calculated.This REMI is a global, macroscopic, external, model-free characteristic and to our knowledge has not been introduced or studied before. This value is identical to 1 for perfectly mixed continuous-flow reactors, and identical to zero for ideal plug flow reactors (PFR). For non-ideal PFR, the REMI depends on two independent characteristic dimensionless numbers, the Péclet number and the conversion or, equivalently, the Damköhler number. Expressions for these dependencies are developed.The smallness of the perturbations allows to use a linearized kinetic model. The theory of this new index is developed; application to experimental data will be the subject of future articles.     

Chemistry, spectroscopy and the role of supported vanadium oxides in heterogeneous catalysis

Supported vanadium oxide catalysts are active in a wide range of applications. In this review, an overview is given of the current knowledge available about vanadium oxide-based catalysts. The review starts with the importance of vanadium in heterogeneous catalysis, a discussion of the molecular structure of vanadium in water and in the solid state and an overview of the spectroscopic techniques enabling to study the chemistry of supported vanadium oxides. In the second part, it will be shown that advanced spectroscopic tools can be used to obtain detailed information about the coordination environment and oxidation state of vanadium oxides during each stage of the life-span of a heterogeneous catalyst. Three topics will be discussed: (1) the molecular structure of supported vanadium oxide catalysts under hydrated, dehydrated and reduced conditions, including the parameters, which influence the molecular structures formed at the surface of the support oxide; (2) elucidation of the active surface vanadium oxide during the oxidation of methanol to formaldehyde, the reaction mechanism and the vanadium oxide---support effect; and (3) deactivation of fluid catalytic cracking (FCC) catalysts by migration of vanadium oxides and the development of a method preventing the structural breakdown of zeolites by trapping the mobile vanadium oxides in an aluminum oxide coating.     

化学吸收增强因子的计算和应用

本文介绍化学吸收增强因子的计算方法和应用     

Structure and size effects in catalysis by immobilized nanoclusters of iron oxides

The activities of catalysts containing -Fe₂O₃ or γ-Fe₂O₃ nanoparticles supported on two types of silica (silica gel and layer silica) are tested in 3,4-dichlorobutene-1 isomerization and benzene alkylation by allyl chloride. γ-Fe₂O₃ particles supported on layer silica are the most active catalysts for both reactions. The high activity is associated with features of inverse spinel structure of γ-ferric oxide. The particle size and their location on the surface or interplane cavity of layer silica affect the catalytic activity.     

Phenomenological approach to interpret the effect of liquid flow modulation in trickle bed reactors at the particle scale

This work analyzes the influence of liquid flow modulation on the behavior of a reaction occurring in a spherical porous particle within a trickle bed reactor. A single first-order reaction between a gaseous reactant and a non-volatile liquid reactant is considered. Non-steady-state mass balances for gas and liquid reactants are formulated and solved under isothermal conditions in order to focus the analysis on the mass transport effects. Dynamic reactant profiles inside the catalytic particle are obtained for different cycling and system conditions. The enhancement factor (ε) due to periodic operation is defined to evaluate the impact of induced liquid flow modulation on reaction rate. Influence of cycling and system parameters on the enhancement factor is also reported for a wide range of conditions. Experimental trends observed by several authors can be explained with this approach.     

New frontiers in X-ray spectroscopy in heterogeneous catalysis: Using Fe/ZSM-5 as test-system

We have applied a number of novel X-ray spectroscopic tools to Fe/ZSM-5 systems. Fe/ZSM-5 can be considered as an ideal test-system for the characterization techniques in heterogeneous catalysis. The existence of a large range of sites and structures creates a good testing ground to determine which experimental tools are able to resolve such complex system. In situ soft X-ray absorption provides important information on the valence and electronic structure of iron during treatments, with a time scale down to 30 s. Kβ-detected XANES yields unprecedented resolution for pre-edge structures and using hard X-rays can be used under any condition and treatment, including high-pressures. It can be expected that both in situ soft X-ray absorption and Kβ-detected XANES become ‘standard’ tools for catalysis research, similar to traditional XANES and EXAFS today. The X-MCD is also used in this paper but it will probably remain a rather specialized technique in the field of heterogeneous catalysis. Further new developments for catalysis characterization are for all to be expected from X-ray spectro-microscopy, where one will have the possibility to perform the in situ soft X-ray absorption and Kβ-detected XANES experiments with nanometer size spatial resolution.     

Modelling of organic liquid-phase decomposition reactions through gas-phase product analysis: Model systems and peracetic acid

A new method for modelling of liquid-phase decomposition reactions was developed. The method is based on rapid on-line mass spectrometric analysis of liberated products in gas phase. The experimental set-up and the generalized modelling principles were introduced. The approach was demonstrated with first and zero order as well as for complex decomposition kinetics (decomposition of peracetic acid). The interaction of decomposition kinetics and mass-transfer effects was illustrated with theoretical and real reaction conditions. The approach is generally applicable to any liquid-phase decomposition, provided that gaseous products can be quantitatively analyzed.     

Nonlinear model reconstruction by frequency and amplitude response for a heterogeneous binary reaction in a chemostat

Here we present an analysis of a binary heterogeneous reaction in a chemostat for the particular case of reagents with unequal mass transfer coefficients. For fast irreversible kinetics, there are two potential steady states—the dispersed phase is preferentially populated by one reagent and essentially depleted of the other. The selection of which steady state occurs depends on the operating parameter which is the concentration ratio σ of the slower (B) to the faster (A) transferring reagent in the feed stream. The demarcation between these operating regimes is the critical value of this ratio: