CATALYSIS AND MAGNETIC RESONANCE GROUP

Heterogeneous catalysis is a technology of major importance in global industry, but it can be ineffective and is poorly understood. We use new methods to examine it more closely than ever before.

The challengeHeterogeneous catalysis accelerates and enables a vast number of industrial chemical reactions and is involved in 35% of the world’s GDP.

Heterogeneous catalysis often involves liquids or gases reacting on solid catalysts, whereas in homogeneous catalysis, catalyst and reactive species are in the same physical state.

Heterogeneous catalysis has the key advantage that the solid catalyst can be separated from the reacting mixture and reused. While the technology has been in use for a long time the catalyst activity (i.e. effectiveness in converting) and selectivity (i.e. producing the desired species) are poor, unlike in homogeneous catalysis. This is wasteful and costly. A better understanding is needed.

Our work in tackling thisWe are improving our understanding of this technology focusing on catalysts relevant to the chemical industry. Several physico-chemical aspects are involved: the diffusion of molecules within the catalyst and the adsorption of the same molecules over the active catalyst sites. In many cases, such phenomena can significantly affect the overall performance of the process, particularly for industrial catalysts in the form of highly porous pellets.

The nature of the pore structure (very small micropores and larger mesopores) and catalytic surface are particularly important to understand. Catalyst performance can be limited when molecules are too slow to reach the reactive sites (diffusion limitations), or when competitive adsorption occurs between reagents and other species, such as solvents or products. Molecules can also get blocked from reaching the reaction site inside the pellets, like getting stuck in a maze (see diagram). This slows the rate of

Heterogeneous catalysis involves solid catalysts, like these pellets,

working on reactants in a different state (often liquid).

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The process: the catalyst is the solid material (grey) with the pore space inside. The reactant A travels into the catalyst pore to reach a reactive site (purple) where the reaction happens and is converted into product B.The product B then travels backout of the pore space.

Blockages: in the bottom ‘pore’ the fish (the reactant) cannot reach the gold and platinum reactive sites (orange) as these are blocked by unwanted deposits formed during reactions. We are working to understand how and why this happens and how to prevent this.

production and decreases productivity. Pore structures also change as catalysts age.

MethodologyWe use magnetic resonance (NMR/MRI) and other methods to examine catalysts in real working conditions. This is an advance on current research as it is non-invasive, so does not perturb the reaction, and it is able to look inside the catalyst, none of which are possible using most traditional techniques. We look at how reactant and product species interact with the catalyst surface and how they move within its pore structure. We also study the deactivation of catalysts, a crucial issue in the chemical industry.

Applications we are currently working on1. Industrial applications a) Catalyst deactivation. We are studying how to lengthen the

lifespan of catalysts which is important as they can be very expensive (often made with gold and platinum).

c) Automotive catalysts. We are working on understanding the behaviour and ageing of automotive exhaust catalysts.

d) Fine chemicals, such as those used in perfume and food – eg for producing benzaldehyde, the almond flavour in pastries.

2. Green chemistryMany of our studies focus on sustainable or ‘green’ catalytic processes – using renewable resources, benign solvents and lower energy use. Chemicals that can be produced in this way are lactic acid (to make biodegradable plastic), propylene glycol (an anti-freezing agent) and allyl alcohol (to produce resins and polymers).

Our techniques and skills• Nuclear Magnetic Resonance spectroscopy, microscopy and relaxation• Mass spectrometry; infrared spectrometry• Gas and liquid phase reactors• Micro-reactor for surface analysis (TPD, TPR, TPO, Chemisorption)

What is new about our work? We are the first group to develop new protocols to use NMR relaxation to probe adsorption in porous catalysts and gain insights into catalytic reactions. Unlike in conventional reaction studies that rely on analysing fluids around the catalyst particles, we look directly inside the catalyst pores, and simultaneously study reactivity, diffusion and adsorption. This can be very challenging, if not impossible, for other methods.

Current industrial partnerships• With Johnson Matthey on automotive catalysts and catalysts for

producing chemicals• With Scionix on a new class of ionic liquids (IL), deep eutectic

solvents (DES)

Other research interests• Structuring and dynamics in liquid mixtures• Diffusion fundamentals in non-ideal, multi-component mixtures• Ionic liquids• Functionalized porous materials for catalysis

Dr Carmine D’Agostino, University of Cambridge, Department of Chemical Engineering and Biotechnology, Pembroke Street, Cambridge, CB2 3RA Tel: +44 1223 761628 Email: cd419@cam.ac.uk Web: www.ceb.cam.ac.uk/directory/carmine-dagostino

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Ageing catalysts: this shows how older catalysts (blue) are less efficient than fresh ones (black). Catalysts tend to age with use and will eventually wear out. By improving our understanding of catalyst structures we are hoping to extend the lifespan of these expensive materials.

Evolution of NMR signalsrepresenting formation of products (benzaldehyde) and consumption of reactants (benzyl alcohol) during aerobic oxidation reaction in porous catalysts. The method allows us to see directly what is happening in the pore space of the catalyst.

Old and new: accessibility to catalytic sites (gold and palladium) in fresh and spent porous catalyst samples. The formation of deposits during reaction (brown bits) hinders the access of molecules to the catalytic sites.