Preparation and characterization of cross-linked polymer supports for catalysts immobilization

Catalysis underpins the chemical/petrochemical industry, being fundamental to the chemical production, e.g. fuels, plastics, etc. Indeed, around 90 percent of chemical manufacturing processes use catalysis to enhance production efficiency and reduce energy use, thereby reducing, in turn, greenhouse gas emissions. [1] Currently, the general trend in catalysis is the engineering of heterogeneous systems that should allow the recovery and reuse of the catalyst [2-5], therefore solving the crucial economic issues as well as environmental concerns. The usually high catalyst cost can be affordable in commercial applications only when the productivity of the catalyst, measured as total kg of products produced per kg of catalyst, is high enough to make the chemical process economically viable.

Besides, the principles of Green Chemistry (or Sustainable Chemistry) [6] express the need of industry to make maximum efforts to minimize waste, particularly if containing toxic/exhaustive metals such as those typically present in transition metal catalytic systems [7]. In this regard, over the past decades a large number of catalysts have been supported on a variety of materials ranging from inorganic/organic polymers to mesoporous silica [1-5].

Catalyst supports based on organic insoluble (cross-linked) polymers are arguably among the most versatile supports in the literature [1-5].In particular, insoluble polymers supports are advantageous since, for instance, several catalysts immobilized on the insoluble matrix show enhanced stability to hydrolysis and oxidation, they are easier to handle, and generally give purer products in chemical processes. Further, the development of highly functional group tolerant controlled and living polymerization methods [8-14] has allowed for the tuning of the structure and density of catalyst sites along polymers and the easy incorporation of catalyst into polymer structures. Nevertheless, the use of insoluble polymers presents some drawbacks, such as lower activities and selectivities when compared with their non-supported analogues. Diffusion effects, accessibility of the catalytic sites by the reagents in solution, and site heterogeneity might be considered in part responsible for these results [1-5, 15-16]. In this regards, the availability of analytical tools enabling the determination of functional groups distribution and accessibility of reagents, which can be of simple and general use, is rather limited. 

We have contributed [17-21] to the development of sustainable and efficient procedures based on the use of metal-free catalysts supported on macroporous cross-linked copolymers of 4-vinylbenzylchloride with divinylbenzene [22]. While often successful in the recycling as well as the easy separation from the reaction mixtures, the use of such supported catalytic systems led, in some cases, to the occurrence of slow reactions and poor yields.