Structural mimicking of inorganic catalyst supports with polydivinylbenzene to improve performance in the selective aerobic oxidation of ethanol and glycerol in water
Graphical abstract
Introduction
Biomass has been identified as a promising resource for the sustainable production of fuels and chemicals [1]. One of the challenges in the conversion of various biorenewable feedstocks is the development of catalysts adapted to the processing of each individual feedstock [2]. In contrast to the functionalization of fossil feedstocks, which usually takes place in the gas phase at high temperatures, the processing of biomass largely proceeds in aqueous phase at comparatively lower temperatures (<200 °C). Many organic polymers are stable under these conditions and should be considered as catalysts and catalyst supports, in addition to inorganic materials. The oxidation of hydroxyl groups with molecular oxygen in water is an important reaction for the conversion of biomass into value-added chemicals [3]. Prominent examples amongst these are the oxidation of ethanol to acetic acid [4] and glycerol to glyceric acid [5].
The potential of polymers lies in the principally infinite variety of their chemical and functional nature. The variables include the nature of the polymer backbone, the type and density of the cross-linker, grafted functional groups, morphology, porosity, and macroscopic dimension [6]. The complexity is further increased by the fact that the polymer itself seldom shows catalytic activity. Instead, it is necessary to graft, incorporate, solubilize, or deposit active sites on the polymer [7]. The span of polymer-supported catalysts ranges from soluble non-cross-linked polymers to highly cross-linked polymers that retain porosity in the dry state, the so-called macroreticular polymers. Intermediate cross-linking densities lead to gel-type polymers that are swellable in an appropriate solvent, but are non-porous in the dry state. In polymer catalysis with bead-shaped commercial polymers, these concepts have been explored in great detail [8].
Several studies have proven the effectiveness of polymer-supported noble metal nanoparticles as catalysts in the oxidation of a large variety of compounds [7], [9]. Kobayashi and coworkers have prepared and tested a series of polymer-incarcerated noble metal nanoparticles. They demonstrated the efficiency and versatility of the catalysts in the base-free aerobic oxidation of alcohols at ambient conditions mainly in organic solvents [10]. Uozumi and coworkers achieved the uniform deposition of platinum and palladium nanoparticles on amphiphilic polymer beads and showed the activity of the catalyst for aerobic oxidation of alcohols in water [11]. Similar results were obtained with microgel- and gel-type polymer-stabilized noble metal nanoparticles [12]. Platinum-containing hypercross-linked polystyrene was prepared and used for the oxidation of L-sorbose in water by Spontak and coworkers [13]. Chang and coworkers compared styrene–divinylbenzene copolymer-supported palladium catalysts to γ-alumina-supported palladium catalysts in the aerobic oxidation of ethanol to ethyl acetate in the presence of water in a fixed-bed reactor. They found the polymer-supported catalyst to be more active, which they ascribed to the hydrophobicity of the polymeric support [14]. The large preparative, chemical, and structural variety of organic polymers simultaneously mean that the fundamental aspects that influence catalysis with polymeric polymers are challenging to identify.
For the direct comparison of polymer-supported catalysts with classical inorganic catalysts, it is ideal if particle size, porosity, and structure of the materials are identical, and the materials differ exclusively in their chemical composition. This is a challenge when comparing carbon, metal oxide, and polymer supports due to the different synthesis conditions for each material. Since carbons and metal oxides are known to be non-swellable and rigid in structure, the polymers that are comparatively closer to them must be highly cross-linked. Liquid-phase syntheses of highly cross-linked porous polymers depend on the use of a solvent as porogen, and typically monoliths are formed. These often contain pores on the order of tens of nanometers, which are larger than those in most inorganic catalysts [15].
Via nanocasting, it is possible to influence the porosity in a direct and shape-selective way, generating pore structures that are difficult to prepare otherwise [16]. The use of an appropriate silica template allows the synthesis of polymer powders that compare closely in size, shape, and morphology to inorganic supports. Therefore, we expect the highly cross-linked mesoporous polydivinylbenzene polymers made by nanocasting to be ideal for the comparison of polymer-, carbon-, and alumina-supported platinum catalysts. The catalytic properties of these catalysts are compared in the aerobic oxidation of ethanol to acetic acid and of glycerol to glyceric acid where the polymer-based catalysts are found to combine the advantages of carbon- and alumina-supported platinum.
Section snippets
Chemicals
Silica gel was acquired from Sigma–Aldrich as TLC high purity grade (5–25 μm particle diameter; 0.75 cm3/g total pore volume; product number 288519) and dried by heating in a vacuum prior to use. Technical divinylbenzene (DVB, Aldrich, approximately 80% divinylbenzene, 20% ethylstyrene) was used after the removal of the inhibitor by vacuum distillation. 2,2’-azodi(2-methylbutyronitrile) (AMBN, Fluka, 98%) was used as received. K2PtCl4 (99.99%) and 30% oleum were acquired from Aldrich and used as
Synthesis and characterization of mesoporous PDVB
The overall synthesis scheme in Fig. 1 illustrates the principal steps used for the preparation of mesoporous PDVB-supported catalysts. Nanocasting comprises a three-step process that involves impregnation, bulk free radical polymerization within the silica pore system, and selective leaching of the silica template in basic solution. Silica gel is selectively and completely removed as confirmed by TGA in air. The residual weight after combustion of all organics is below 0.3%.
Conclusion
This study compares polymer-supported platinum catalysts to structurally similar commercial carbon- and alumina-supported platinum catalysts. Structural similarity of the catalysts is obtained using nanocasting in the synthesis of the polymer. Platinum nanoparticles are deposited in defined size, content, and distribution on the polymer. Bifunctional catalysts containing acidic and metal functionality are obtained by sulfonation of Pt/PDVB, thereby also changing the polarity of the support from
Acknowledgments
This work was funded by the European Research Council (POLYCAT, Grant No. 247081). We are also very grateful to H. Bongard for the sample preparation and recording of SEM and STEM images, to B. Spliethoff for the recording of TEM images, to Dr. C. Weidenthaler and U. Holle for the XPS measurements, to H. Hinrichs, A. Deege and G. Breitenbruch of the HPLC department of the institute, and the microanalytical laboratory Kolbe for the analysis of the samples.
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