NANO-HOST Workshop SINGLE SITES in HETEROGENEOUS CATALYSIS location:Milan (Italy) date:28-29 September 2009
Finding the optimum catalyst for a chosen reaction is a challenging task: catalysts are generally complex systems and their synthesis may involve a multi-step mechanism influenced by many parameters. This implies that studying a catalytic system usually requires the investigation of large sets of experimental parameters, for which purpose High-Throughput Experimentation (HTE) techniques are a powerful and versatile tool. Combinatorial and High Throughput Experimentation techniques are among the newest and most promising experimental methods in many fields of chemical research, including materials science and catalysis [ , ]. By allowing the rapid and efficient synthesis, characterisation and testing of a vast number of samples, these techniques offer a large potential for increasing the rate of scientific and technological progress. The high throughput of these techniques is made possible by the use of tailored automated workstations and fast-analysis equipments for the preparation and screening of the samples.
HTE techniques represent a complement and not an alternative to rational methods for the design of catalysts: a multi-dimensional and scientific approach – based, e.g., on literature data, computational modelling, chemical knowledge and intuition – is essential to determine which parameter space has to be investigated to gain the desired information from each set of experiments. When studying a system by means of HTE, the choice of the most appropriate experimental strategy depends on the features of the system, on the size of the parameter space to be screened, on the kind of information that is wanted and on the experimental equipment available [ ]. HTE can prove useful in all phases of the development of a catalyst: the synthesis of novel catalytic materials, their testing in chosen reactions and the optimisation of the conditions in which the catalytic reaction takes place can all be conducted with much greater speed and efficiency employing HTE procedures than by the use of conventional methods.
A successful application of High-Throughput Experimentation in the study of single-site heterogeneous catalysts will be presented in this lecture. Starting from the reported activity of partially hydroxylated aluminium oxides as heterogeneous catalysts for the epoxidation of alkenes with H2O2, HTE techniques were employed to explore the synthesis of transition metal-free oxides as catalysts for sustainable epoxidation processes [ , ]. The proposed catalytically active sites in this type of catalysts are surface hydroxyl groups of mild Brønsted acidity [ ]. Using a full factorial High-Throughput approach, the chemical composition of the catalysts was screened, leading to the identification of gallium oxide as a novel, active epoxidation catalyst. The catalytic performances of the ‘leads’ from this initial screening were further optimised by the systematic variation of the most relevant synthesis parameters. This allowed tuning the physicochemical features of the materials towards the desired catalytic properties, which implies finding the most suitable balance between the population of surface hydroxyl groups acting as catalytic sites and the hydrophilicity of the surface: a too high concentration of these sites is detrimental because it makes the catalyst surface very hydrophilic, thus preventing the approach of the apolar alkene to the active site. The range of synthesis conditions in which gallium oxides with high catalytic activity can be prepared is quite narrow and could have been overlooked with a conventional research approach: this exemplifies the value of using HTE to investigate this type of catalytic systems.