N. Raveendran Shiju

Mesoporous Silica with Site‐Isolated Amine and Phosphotungstic Acid Groups: A Solid Catalyst with Tunable Antagonistic Functions for One‐Pot Tandem Reactions

Coniuga et impera: A bifunctional solid catalyst is prepared by combining acid and base functions on mesoporous silica supports (see picture). The co-existence of these functions is shown by a two-step reaction sequence in one pot. Excellent product yields, which cannot be obtained by separated acid and base functions in one pot, show the validity of our concept.

Catalytic routes towards acrylic acid, adipic acid and ε-caprolactam starting from biorenewables

The majority of bulk chemicals are derived from crude oil, but the move to biorenewable resources is gaining both societal and commercial interest. Reviewing this transition, we first summarise the types of todays biomass sources and their economical relevance. Then, we assess the biobased productions of three important bulk chemicals: acrylic acid, adipic acid and e-caprolactam. These are the key monomers for high-end polymers (polyacrylates, nylon 6.6 and nylon 6, respectively) and are all produced globally in excess of two million metric tons per year. The biobased routes for each target molecule are analysed separately, comparing the conventional processes with their sustainable alternatives. Some processes have already received extensive scientific attention. Other, more novel routes are also being considered. We find several common trends: For all three compounds, there are no commercial methods for direct conversion of biobased feedstocks. However, combinations of biotechnologically produced platform chemicals with subsequent chemical modifications are emerging and showing promising results. We then discuss several distinct strategies for implementing biorenewable processes. For each biotechnological and chemocatalytic route, current efficiencies and limitations are presented, but we urge that these routes should be assessed mainly on their potential and prospects for future application. Today, biorenewable routes cannot yet compete with their petrochemical equivalents. However, given that most of them are still in the early stages of development, we foresee their commercial implementation in the next two decades.

Bimetallic catalysts for the Fischer–Tropsch reaction

This short critical review summarises and analyses the developments in Fischer–Tropsch catalysis using bimetallic alloys. We introduce a simple notation for such catalysts, and monitor the reports of synergistic effects and composition/performance relationships. Special attention is given to CoFe alloys on a variety of supports, and to the effects of catalyst preparation methods and pre-treatment conditions. The key drawbacks in comparing the large amount of data available on Fischer–Tropsch catalysis are the high dimensionality of the problem and the lack of long time-on-stream studies. Based on the new understanding coming from characterisation studies of supported bimetallic particles, we propose a structured approach for effectively studying Fischer–Tropsch catalysis.

Efficient three-component coupling catalysed by mesoporous copper-aluminum based nanocomposites

Traditional synthesis methods for propargylamines have several drawbacks. A recently developed alternative route is the so-called “A3 coupling” in which an alkyne, an aldehyde, and an amine are coupled together. Typically, these reactions are catalysed by homogeneous gold salts, organogold complexes or silver salts. But these homogeneous catalysts are expensive and their separation is difficult. Here we report the discovery that solid Cu/Al/oxide mesoporous “sponges” are excellent A3 coupling catalysts. These materials are robust, inexpensive, and easy to make. They give good to excellent yields (87–97%) for a wide range of substrates. Being heterogeneous, these catalysts are also easy to handle and separate from the reaction mixture, and can be recycled with no loss of activity.

Hemicellulose hydrolysis catalysed by solid acids

Depolymerising hemicellulose into platform sugar molecules is a key step in developing the concept of an integrated biorefinery. This reaction is traditionally catalysed by either enzymes or homogeneous mineral acids. We compared various solid catalysts for hemicellulose hydrolysis, running reactions in water, under neutral pH and relatively mild temperature and pressure (120 °C and 10 bar) conditions. Sulphonated resins are highly active, but they leach out sulphonic groups. Sulphonated silicas are less active, but more stable. They have weakly and strongly bound sites and the strongly bound ones do not leach. Zeolites are moderately active and stable. Among them, H-ferrierite especially, despite its small pores, exhibited high activity as well as good recyclability.

Titania-catalysed oxidative dehydrogenation of ethyl lactate: effective yet selective free-radical oxidation

We research here the catalytic oxidative dehydrogenation of ethyl lactate, as an alternative route to ethyl pyruvate. Testing various solid catalysts (Fe2O3, TiO2, V2O5/MgO–Al2O3, ZrO2, CeO2 and ZnO), we find that simple and inexpensive TiO2 efficiently catalyses this reaction under mild conditions. Furthermore, molecular oxygen was used as the terminal oxidant. Importantly, this reaction runs well also using inexpensive commercial solvent mixtures. Both the desired reaction and the by-products formation follow a free-radical mechanism. Remarkably, adding activated carbon, a solid radical scavenger, hardly affects the catalytic activity, but enhances the product selectivity. This is because this solid radical scavenger hampers the formation of undesired products in solution, without suppressing the oxidation at the catalyst surface.

Understanding the solar-driven reduction of CO2 on doped ceria

With the appropriate materials, one can construct redox cycles that use CO2 as the oxidant, generating CO as the product. Here, we investigate thermochemical cycles using doped ceria compounds as the oxygen exchange medium. Doped samples are prepared using La, Cr, W, Zr, V, Y, and Ti as dopants. Studying the redox kinetics, we show that doping the pure ceria with zirconium strongly increases overall CO production, albeit at lower reaction rates. This is because the CO2 reduction step is second-order with respect to Ce(III). Doping the fluorite lattice with zirconium cations decreases the number of Ce(III) ions at the surface, and consequently slows down the reaction. This result is counter-intuitive, since normally you would think that the more reduction, the better. But the reactivity towards CO2 is actually determined by the surface Ce(III) ions, and so migration of dopant ions on the surface reduces its reactivity, even though the bulk Ce(III) concentration is higher. Our results demonstrate the importance of understanding surface kinetics when designing oxygen exchange materials for solar reactors.

Ru/TiO2-catalysed hydrogenation of xylose: The role of the crystal structure of the support

Effective dispersion of the active species over the support almost always guarantees high catalytic efficiency. To achieve this high dispersion, a favourable interaction of the active species with the support is crucial. We show here that the crystal structure of the titania support determines the interaction and consequently the nature of ruthenium particles deposited on the support. Similar crystal structures of RuO2 and rutile titania result in a good lattice matching and ensure a better interaction during the heating steps of catalyst synthesis. This helps maintain the initial good dispersion of the active species on the support also in the subsequent reduction step, leading to better activity and selectivity. This highlights the importance of understanding the physico-chemical processes during various catalyst preparation steps, because the final catalyst performance often depends on the type of intermediate structures formed during the preparation.

In Situ Fabrication of Cross-Linked Protein Membranes by Using Microfluidics

We report a novel technique for preparing cross‐linked protein membranes within microchannels by using an interfacial cross‐linking reaction. Glass microchannels with a Y input were assembled by using a simple adhesive bonding technique to achieve dual, parallel laminar flows. Membrane formation utilised an interfacial reaction at the liquid–liquid interface, which involved bovine serum albumin (aqueous solution with a flow rate of 300 μL min−1) and terephthaloyl chloride (xylene solution with a flow rate of 700 μL min−1), to form thin (∼25 μm) cross‐linked films along the length of the channel under the continuous pressure‐driven laminar flow. Such microfabricated membranes could extend the separation potential of any microfluidic structure to provide a stable barrier layer. Furthermore, degradation of the membrane was possible by using an alkali sodium dodecyl sulfate solution, which led to the complete disappearance of the membrane. These membranes could facilitate additional modification to allow for different permeability properties by controlled degradation. The one‐step in situ membrane‐fabrication methodology reported here generated precisely localised membranes and avoided the complexities of subcomponent assembly, which require complicated alignment of small, preformed membranes. This methodology could become the basis for sophisticated microseparation systems, biosensors and several “lab‐on‐a‐chip” devices.

Catalytic acetoxylation of lactic acid to 2-acetoxypropionic acid, en route to acrylic acid

We present an alternative synthetic route to acrylic acid, starting from the platform chemical lactic acid and using heterogeneous catalysis. To improve selectivity, we designed an indirect dehydration reaction that proceeds via acetoxylation of lactic acid to 2-acetoxypropionic acid. This intermediate can then be pyrolized to acrylic acid. Acetic acid is used both as a reagent and a solvent in the first step, and may be recovered in the subsequent pyrolysis step. We tested a range of solid acid catalysts for the acetoxylation step (Y zeolites, sulfated zirconia, ion-exchange resins, sulfonated graphene, and various sulfonated silica gels and mixed oxides). Recycling studies were carried out for the most active catalysts. To enable quantitative analysis using gas chromatography we also developed a reliable silylation derivatization method, which is also reported. These results open opportunities for improving the biorenewable production of acrylic acid.