Final Report Summary - POLYCAT (Polymeric catalysts and supports: A new paradigm for biomass processing)
The POLYCAT grant we designed for the exploration of the potential of porous polymeric materials in the catalytic conversion of biomass and biomass derived molecules. The rationale behind the project was the fact that typical petrochemical conversions require high temperature/gas phase processing, for which oxidic catalysts are optimally suited. In contrast, biomass processing typically proceeds in the liquid, often an aqueous phase at relatively low temperature, for which polymers seemed to be an attractive alternative.
The most versatile method for the synthesis of polymers with adjustable porosity was hard templating (also called nanocasting). A template with suitable porosity is impregnated with a polymer precursor, then polymerization is induced in the pore system, and finally the template, mostly inorganic oxides, such as silica gel, is leached. If the polymers are sufficiently highly cross-linked, a true negative replica of the template is obtained. BET surface areas around 500 m2/g and pore volumes of around 0.5 ml/g could routinely be achieved following this pathway. Porous systems synthesized via hard templating included crosslinked polystyrenes, different types of polyacrylates, polyvinylpyridinum-co-polydivinylbenzene, polyacrylonitrile, and polymethylmetacrylates. The porous polymers were in most cases not used as catalysts in the as-synthesized form, but modified by the introduction of acidic groups (mostly by sulfonation), groups mimicking ionic liquids, metal nanoparticles, or metal oxides. An alternative pathway are organic sol-gel processes. The prototypical system are resorcinol-formaldehyde gels, which can be synthesized with surface areas around 500 m2/g and adjustable pore sizes. A very interesting system was produced by replacing the resorcinol by 2,4-dihydroxybenzoic acid. If gelation is carried out in the presence of surfactants, hollow polymeric spheres of several hundred µm diameter could be synthesized, into which noble metal particles can be incorporated during synthesis, and into which additionally transition metal ions could be exchanged. This material formed the basis for highly active and highly selective catalysts for the almost quantitative conversion of 5-hydroxymethylfurfural (5-HMF) to 2,5-dimethylfuran, a very attractive fuel molecule. Finally, using a Suzuki coupling reaction, a high porosity Pd supported on polyphenylene could be synthesized, which is an interesting catalyst for C-C coupling reactions.
Various catalytic biomass conversion reactions were explored, and while it cannot be stated that polymeric catalysts were found to be generally superior to other catalyst systems, there were various reactions, in which polymeric systems outperformed more conventional solid catalysts. These reactions included the oxidation of ethanol to acetic acid, where a Pt/divinylbenzene catalysts was better than Pt/C or Pt/Al2O3, the synthesis of solketal from aceton and glycerol, where a sulfonated RF gel was by far the best catalyst, or the dehydration of fructose to 5-HMF over a polymer modified with ionic liquid type functional groups. However, it was not generally found that permanent porosity is required for good performance; also highly swellable polymers can be effective catalysts, if the polarity matches the reagents and products. Clear advantages of porous resins were only observed in the case of very bulky substrates, for instance the fructose-polymer inulin. In some cases, not the polymer itself, but carbons derived from designed polymers were found to be the best catalysts for a given conversion. Au/C-aerogels were excellent for almost quantitative oxidation of glucose to gluconic acid, which could be carried out in a continuous manner in a trickle bed reactor. The most exciting catalytic result was the quantitative hydrogenolysis/hydrogenation of 5-HMF to the fuel molecule 2,5-dimethlyfuran over a PtCo/C catalyst. Together with two other reaction steps, this allows the synthesis of a fuel in an overall yield of 75 % from cellulose.
An important serendipitous discovery with possibly far-reaching consequences was the discovery of the mechanocatalytic depolymerization of cellulose or even raw biomass to fully water soluble products. If the hydrolysate from lignocellulose was heated, lignin precipitated and could be separated, so that this pathway is also a novel method for biomass fractionation. This could provide a completely new entry point into biorefinery schemes. Further processing of the oligomers from the mechanocatalytic process resulted, for instance, in ethanol, sugar alcohols, or 5-HMF.
Overall, polymeric catalysts were established as a class of catalyst which should receive the same level of interest for biomass conversion reactions as oxides or carbon-based catalysts.
The most versatile method for the synthesis of polymers with adjustable porosity was hard templating (also called nanocasting). A template with suitable porosity is impregnated with a polymer precursor, then polymerization is induced in the pore system, and finally the template, mostly inorganic oxides, such as silica gel, is leached. If the polymers are sufficiently highly cross-linked, a true negative replica of the template is obtained. BET surface areas around 500 m2/g and pore volumes of around 0.5 ml/g could routinely be achieved following this pathway. Porous systems synthesized via hard templating included crosslinked polystyrenes, different types of polyacrylates, polyvinylpyridinum-co-polydivinylbenzene, polyacrylonitrile, and polymethylmetacrylates. The porous polymers were in most cases not used as catalysts in the as-synthesized form, but modified by the introduction of acidic groups (mostly by sulfonation), groups mimicking ionic liquids, metal nanoparticles, or metal oxides. An alternative pathway are organic sol-gel processes. The prototypical system are resorcinol-formaldehyde gels, which can be synthesized with surface areas around 500 m2/g and adjustable pore sizes. A very interesting system was produced by replacing the resorcinol by 2,4-dihydroxybenzoic acid. If gelation is carried out in the presence of surfactants, hollow polymeric spheres of several hundred µm diameter could be synthesized, into which noble metal particles can be incorporated during synthesis, and into which additionally transition metal ions could be exchanged. This material formed the basis for highly active and highly selective catalysts for the almost quantitative conversion of 5-hydroxymethylfurfural (5-HMF) to 2,5-dimethylfuran, a very attractive fuel molecule. Finally, using a Suzuki coupling reaction, a high porosity Pd supported on polyphenylene could be synthesized, which is an interesting catalyst for C-C coupling reactions.
Various catalytic biomass conversion reactions were explored, and while it cannot be stated that polymeric catalysts were found to be generally superior to other catalyst systems, there were various reactions, in which polymeric systems outperformed more conventional solid catalysts. These reactions included the oxidation of ethanol to acetic acid, where a Pt/divinylbenzene catalysts was better than Pt/C or Pt/Al2O3, the synthesis of solketal from aceton and glycerol, where a sulfonated RF gel was by far the best catalyst, or the dehydration of fructose to 5-HMF over a polymer modified with ionic liquid type functional groups. However, it was not generally found that permanent porosity is required for good performance; also highly swellable polymers can be effective catalysts, if the polarity matches the reagents and products. Clear advantages of porous resins were only observed in the case of very bulky substrates, for instance the fructose-polymer inulin. In some cases, not the polymer itself, but carbons derived from designed polymers were found to be the best catalysts for a given conversion. Au/C-aerogels were excellent for almost quantitative oxidation of glucose to gluconic acid, which could be carried out in a continuous manner in a trickle bed reactor. The most exciting catalytic result was the quantitative hydrogenolysis/hydrogenation of 5-HMF to the fuel molecule 2,5-dimethlyfuran over a PtCo/C catalyst. Together with two other reaction steps, this allows the synthesis of a fuel in an overall yield of 75 % from cellulose.
An important serendipitous discovery with possibly far-reaching consequences was the discovery of the mechanocatalytic depolymerization of cellulose or even raw biomass to fully water soluble products. If the hydrolysate from lignocellulose was heated, lignin precipitated and could be separated, so that this pathway is also a novel method for biomass fractionation. This could provide a completely new entry point into biorefinery schemes. Further processing of the oligomers from the mechanocatalytic process resulted, for instance, in ethanol, sugar alcohols, or 5-HMF.
Overall, polymeric catalysts were established as a class of catalyst which should receive the same level of interest for biomass conversion reactions as oxides or carbon-based catalysts.