Probing chemistry and kinetics of reactions in heterogeneous catalysts

Using benzene hydrogenation over Pt/SiO2 as an industrially-relevant example, we show that state-of-the-art neutron total scattering methods spanning a wide Q-range now permit relevant time-resolved catalytic chemistry to be probed directly in situ within the pore of the catalyst. The method gives access to the reaction rates on both nanometric and atomic length scales, whilst simultaneously providing an atomistic structural viewpoint on the reaction mechanism itself.

Application of heterogeneous catalysts prepared by mechanochemical synthesis

Mechanochemical synthesis has the potential to provide more sustainable preparative routes to catalysts than the current multistep solvent-based routes. In this review, the mechanochemical synthesis of catalysts is discussed, with emphasis placed on catalysts for environmental, energy and chemical synthesis applications. This includes the formation of mixed-metal oxides as well as the process of dispersing metals onto solid supports. In most cases the process involves no solvent. Encouragingly, there are several examples where the process is advantageous compared with the more normal solvent-based methods. This can be because of process cost or simplicity, or, notably, where it provides more active/selective catalysts than those made by conventional wet chemical methods. The need for greater, and more systematic, exploration of this currently unconventional approach to catalyst synthesis is highlighted.

Some Attempts in the Rational Design of Heterogeneous Catalysts Using Density Functional Theory Calculations

Heterogeneous catalysis is of great importance both industrially and academically. Rational design of heterogeneous catalysts is highly desirable, and the computational screening and design method is one of the most promising approaches for rational design of heterogeneous catalysts. Herein, we review some attempts towards the rational catalyst design using density functional theory from our group. Some general relationships and theories on the activity and selectivity are covered, such as the Brønsted–Evans–Polanyi relation, volcano curves/surfaces, chemical potentials, optimal adsorption energy window and energy descriptor of selectivity. Furthermore, the relations of these relationships and theories to the rational design are discussed, and some examples of computational screening and design method are given.

Highlights from Faraday Discussion: Designing New Heterogeneous Catalysts, London, UK, April 2016

The Faraday Discussion on the design of new heterogeneous catalysts took place from 4-6 April 2016 in London, United Kingdom. It brought together world leading scientists actively involved in the synthesis, characterisation, modelling and testing of solid catalysts, attracting more than one hundred delegates from a broad spectrum of backgrounds and experience levels-academic and industrial researchers, experimentalists and theoreticians, and students. The meeting was a reflection of how big of an impact the ability to control and design catalysts with specific properties for particular processes can potentially have on the chemical industry, environment, economy and society as a whole. In the following, we give an overview of the topics covered during this meeting and briefly highlight the content of each presentation.

The Co-Entrapment of a Homogeneous Catalyst and an Ionic Liquid by a Sol-gel Method: Recyclable Ionogel Hydrogenation Catalysts

Molecular hydrogenation catalysts have been co-entrapped with the ionic liquid [Bmim]NTf(2) inside a silica matrix by a sot-gel method. These catalytic ionogels have been compared to simple catalyst-doped glasses, the parent homogeneous catalysts, commercial heterogeneous catalysts, and Rh-doped mesoporous silica. The most active ionogel has been characterised by transmission electron microscopy, X-ray photoelectron spectroscopy, and solid state NMR before and after catalysis. The ionogel catalysts were found to be remarkably active, recyclable and resistant to chemical change.

Theory of the Kinetics of Chemical Potentials in Heterogeneous Catalysis

Simple and powerful: The reaction kinetics at surfaces of heterogeneous catalysts is reformulated in terms of the involved chemical potentials. Based on this formulism, an approach of searching for good catalysts is proposed without recourse to extensive calculations of reaction barriers and detailed kinetic analyses. (see picture; R=reactant, I=surface intermediate, P=product, and =standard chemical potential).

A more direct way to make catalysts: one-pot ligand-assisted aerobic stripping and electrodeposition of copper on graphite

Electrodeposition of metals onto conductive supports such as graphite potentially provides a lower-waste method to form heterogeneous catalysts than the standard methods such as wet impregnation. Copper electrodeposition onto pressed graphite disc electrodes was investigated from aqueous CuSO4-ethylenediamine solutions by chronoamperometry with scanning electron microscopy used to ascertain the particle sizes obtained by this method. The particle size was studied as a function of pH, CuSO4-ethylenediamine concentration, and electrodeposition time. It was observed that decreasing the pH, copper-ethylenediamine concentration and time each decreased the size of the copper particles observed, with the smallest obtained being around 5-20 nm. Furthermore, electroless aerobic oxidation of copper metal in the presence of ethylenediamine was successfully coupled with the electrodeposition in the same vessel. In this way, deposition was achieved sequentially on up to twenty different graphite discs using the same ethylenediamine solution, demonstrating the recyclability of the ligand. The materials thus prepared were shown to be catalytically active for the mineralisation of phenol by hydrogen peroxide. Overall, the results provide a proof-of-principle that by making use of aerobic oxidation coupled with electrochemical deposition, elemental base metals can be used directly as starting materials to form heterogeneous catalysts without the need to use metal salts as catalyst precursors.

TAP Reactor Development and Application in the Kinetic Characterization of Catalysts for Heterogeneously Catalyzed Gas Phase Reactions

This manuscript describes the application and further development of the TAP technique in kinetic characterization of heterogeneous catalysis. The major application of TAP systems is to study mechanisms, kinetics and transport phenomena in heterogeneous catalysis, all of which is made possible by the sub-millisecond time resolution. Furthermore, the kinetic information obtained can be used to gain an insight into the mechanism occurring over the catalyst system. This is advantageous as heterogeneous catalysts with an improved efficiency can be developed as a result. TAP kinetic studies are carried out at low pressure (~1x10-7 mbar) and TAP pulses are sufficiently small (1013-1015 molecules) so as to maintain this low pressure. The use of a small number of molecules in comparison to the total number of active sites means the state of the catalyst remains relatively unchanged. The use of the low intensity pulses also makes the pressure gradient negligible and so allows the TAP reactor system to operate in the Knudsen Diffusion regime, where gas-gas reactions are eliminated. Hence only gas-catalyst reactions are investigated and, by the use of moment analysis of observed exit flow, rate constants of elementary steps of the reaction can be obtained.

In this manuscript, two attempts to further the TAP technique are reported. Firstly, the work undertaken at QUB to attempt to control the number of molecules of condensable reagents that can be pulsed during a TAP pulse experiment is disclosed. Secondly, a collaborative project with SAI Ltd Manchester is discussed in a separate chapter, where technical details and validation of a customised time of flight mass spectrometer (ToF MS) for the QUB TAP-1 system are reported. A collaborative project with Cardiff Catalysis Institute focusing on the study of CO oxidation over hopcalite catalysts is also reported. The analysis of the experimental results has provided an insight into the possible mechanism of the oxidation of CO over these catalysts. A correction function has also been derived which accounts for the adsorption of reactant molecules over inert materials that are used for the reactor packing in TAP experiments. This function was then applied to the selective reduction of O2 in a H2 rich ethene feed, so that more accurate TAP moment based analysis could be conducted.

The Y-procedure: How to extract the chemical transformation rate from reaction-diffusion data with no assumption on the kinetic model

The paper presents a new method to extract the chemical transformation rate from reaction–diffusion data with no assumption on the kinetic model (“kinetic model-free procedure”). It is a new non-steady-state kinetic characterization procedure for heterogeneous catalysts. The mathematical foundation of the Y-procedure is a Laplace-domain analysis of the two inert zones in a TZTR followed by transposition to the Fourier domain. When combined with time discretization and filtering the Y-procedure leads to an efficient practical method for reconstructing the concentration and reaction rate in the active zone. Using the Y-procedure the concentration and reaction rate of a non-steady state catalytic process can be determined without any pre-assumption regarding the type of kinetic dependence. The Y-procedure is the basis for advanced software for non-steady state kinetic data interpretation. The Y-procedure can be used to relate changes in the catalytic reaction rate and kinetic parameters to changes in the surface composition (storage) of a catalyst.

Synthesis and Reactions of Enantiopure Substituted Benzene cis-Hexahydro-1,2-diols

Enantiopure cis-dihydro-1,2-diol metabolites, obtained from toluene dioxygenase-catalysed cis-dihydroxylation of six monosubstituted benzene substrates, have been converted to their corresponding cis-hexahydro-12-diol derivatives by catalytic hydrogenation via their cis-tetrahydro-1,2-diol intermediates. Optimal reaction conditions for total catalytic hydrogenation of the cis-dihydro-1,2-diols have been established using six heterogeneous catalysts. The relative and absolute configurations of the resulting benzene cis-hexahydro-1,2-diol products have been unequivocally established by X-ray crystallography and NMR spectroscopy. Methods have been developed to obtain enantiopure cis-hexahydro-1,2-diol diastereoisomers, to desymmetrise a meso-cis-hexahydro-1,2-diol and to synthesise 2-substituted cyclohexanols. The potential of these enantiopure cyclohexanols as chiral reagents was briefly evaluated through their application in the synthesis of two enantiomerically enriched phosphine oxides from the corresponding racemic phosphine precursors.

Redispersion of Gold Supported on Oxides

Although many gold heterogeneous catalysts have been shown to exhibit significant activity and high selectivity for a wide range of reactions in both the liquid and gas phases, they are prone to irreversible deactivation. This is often associated with sintering or loss of the interaction of the gold with the support. Herein, we report on the use of methyl iodide as a method of dispersing gold nanoparticles supported on silica, titania, and alumina supports. In the case of titania- and alumina-based catalysts, the gold was transformed from nanometer particles into small clusters and some atomically dispersed gold. In contrast, although there was a drop in the gold particle size on the silica support following CH3I treatment, the size remained in the submicrometer range. The structural changes were correlated with changes in the selectivity and activity for ethanol dehydration and benzyl alcohol oxidation. From these observations, it is clear that this treatment provides a method by which deactivated gold catalysts can be reactivated via redispersion of the gold.

An efficient Cu(II)-bis(oxazoline)-based polymer immobilised ionic liquid phase catalyst for asymmetric carbon–carbon bond formation

The asymmetric Diels-Alder reaction between N-acryloyloxazolidinone and cyclopentadiene and the Mukaiyama-aldol reaction between methylpyruvate and 1-phenyl-1-trimethylsilyloxyethene have been catalysed by heterogeneous copper(II)-bis(oxazoline)-based polymer immobilised ionic liquid phase (PIILP) systems generated from a range of linear and cross linked ionic polymers. In both reactions selectivity and ee were strongly influenced by the choice of polymer. A comparison of the performance of a range of Cu(II)-bis(oxazoline)-PIILP catalyst systems against analogous supported ionic liquid phase (SILP) heterogeneous catalysts as well as their homogeneous counterparts has been undertaken and their relative merits evaluated.

Application of halohydrocarbons for the re-dispersion of gold particles

Methods to control the dispersion of gold in supported heterogeneous catalysts are very valuable due to the strong nanoparticle size dependence on their activity and selectivity towards many reactions. Additionally, the ability to disperse large, inactive gold nanoparticles to smaller nanoparticles provides an opportunity to reactivate, stabilise and increase the lifetime of gold catalysts making them more practical for industrial applications. Previously it has been demonstrated that the use of gas phase iodomethane (J. Am. Chem. Soc., 2009, 131, 6973; Angew. Chem., Int. Ed., 2011, 50, 8912) was able to re-disperse gold from >20 nm particles to dimers and trimers. In the current work, we show that this technique can be applied using less hazardous halohydrocarbons treatments, both in the gas phase and the liquid phase. The ability of these individual halohydrocarbons to re-disperse gold as well as the extent to which leaching occurs is assessed.

Unraveling the synergy between gold nanoparticles and chromium- hydrotalcites in aerobic oxidation of alcohols

The combination of gold nanoparticles (AuNPs) with chromium-substituted hydrotalcite (Cr-HT) supports makes very efficient heterogeneous catalysts (Au/Cr-HT) for aerobic alcohol oxidation under soluble-base-free conditions. The Au-support synergy increases with increasing Cr content of the support and decreasing AuNP size. In situ UV-Raman, X-ray absorption and photoelectron spectroscopic studies firmly establish that the strong Au-Cr synergy is related to a Cr ↔ Cr redox cycle at the Au/Cr-HT interface, where O activation takes place accompanied by electron transfer from Cr-HT to Au. The interfacial Cr species can be reduced by surface Au-H hydride and negative-charged Au species to close the catalytic cycle. A study of kinetic isotope effect indicates that alcohol O-H cleavage is facilitated by the presence of Cr, making a-C-H bond cleavage step more rate-controlling. Accordingly, a dual synergistic effect of Au/Cr-HT catalysts on the activation of O2 and alcohol reactants is proposed.