11 Dec 2015
Ion-exchange resins have held an almost unique place in the arsenal of solid catalysts available for chemists and chemical engineers. Over twenty years ago M M Sharma listed over 300 reactions where they show useful activity. And, of course, methyl t-buyl ether synthesis was the largest volume application of acidic ion-exchange resins in catalysis for many years.
Solid catalysts and particularly solid acids have been the focus of rapid development over the past 30 years as the movement to find environmentally acceptable replacements for liquid mineral acid catalysts in liquid phase processes has grown. Much of the work has focussed on rigid inorganic matrices with either intrinsic or grafted acid centres, and porosity in the mesopore range.
Polymer-supported catalysts based on ion-exchange resins differ in being non-rigid. In many cases diffusion of reactants and products to and from active sites is facilitated by their ability to solvate and swell the resins. In addition however, they offer significant advantages over rigid materials for many reactions. Invariably, they have much higher concentrations of active sites, typically for acid catalysts 4-5 mmol g-1 compared to 1-2 mmol g-1 for catalysts on inorganic supports. Well-defined and highly uniform active sites can be designed, allowing catalyst tuning for both activity and selectivity to an extent difficult to achieve with rigid mesoporous catalysts. Resin catalysts are readily available with permanent macropores so that even in reaction solvents that do not swell the resins, rapid diffusion is facilitated. They are relatively inexpensive, simple to manufacture and can be made in highly reproducible and easy-to-handle forms.
Catalytic applications of ion-exchange resins extend far beyond acid catalysis. Functional groups with basic properties can be used. Catalytic metal ions can readily be chelated in functionalised resins. At IEX 2016, Beau Van Vaerenbergh (Ghent University) will describe the stabilisation of palladium and cobalt nanoparticles on ion-exchange resins. Basu Saha (London South Bank University) will report on Mo(VI) supported on polymeric ion exchange resins for alkene epoxidation. So-called “hypercrosslinking” renders the matrix rigid and highly porous and with a very high surface area, and acid catalysts based on these materials have recently been shown to be particularly active in the free fatty acid esterification in biodiesel synthesis. Examples of reactions catalysed by hypercrosslinked resins will be presented at IEX2016 by Sumaiya Zainal Abidin (University of Malaysia Pahang) and Rob Brown (University of Huddersfield). Ion-exchangers are not only those based on functionalised organic polymers; Shogo Shimazu (University of Chiba) will describe some novel inorganic ion exchangers as supports for tin catalysts for Baeyer-Villiger oxidation reactions. Returning to acid catalysis and polymer-supported catalysts Javier Tejero (University of Barcelona) will give several papers, on catalysis of isobutene etherification, on the link between catalyst properties and activities in the synthesis of oxygenates, and on biomass conversion to biofuel. Marta Granollers (University of Huddersfield) will contribute a paper on the characterisation of catalytic properties using adsorption calorimetry. An industrial perspective on new trends in catalytic applications for ion-exchange resins will be provided by Werner Bonrath (DSM Nutritional Products).
We are very pleased to say that, in addition to the presentations mentioned above, the Catalysis session at IEX 2016 will be opened with an introductory lecture on the history of ion-exchange resins and their applications as catalysts by Professor Fidel Cunill of the University of Barcelona.
Most would agree that ion-exchange resin catalysts have enormous potential for further development and remain somewhat unexploited. To illustrate the way catalysis is becoming an increasingly important field in ion-exchange resin chemistry, at IEX 2106 we are devoting two sessions to application in catalysis. This follows a single session in IEX 2012 and in meetings before that, talks on catalysis were incorporated in other sessions. The design of new ion-exchange resins for catalysis is one of the most exciting areas to be covered at IEX 2016.
The SCI IEX 2016 Conference will take place from 6 - 8 July 2016 at the University of Cambridge. For more details on the event, please follow the link below.
Prof Robert Brown
IEX Organising Committee