The use of functionalized polymers to make hollow carbon spheres to be used as supports in catalysis

Abstract

Metal particle agglomeration is known to be one of the factors responsible for the reduction in catalytic activity (even selectivity) of catalysts when they are supported on carbon. In this study, an approach using functional groups attached to a polymeric support was used to reduce metal catalyst agglomeration. This study reports the functionalization of polystyrene (PS) spheres using poly(acrylic acid) (PAA), poly(ethylene glycol), poly(ethyl cyano acrylate) and poly(methyl methacrylate). CoxOy nanoparticles were supported on both PS and the functionalized PS templates (5%, 10%, and 15% Co loading). The CoxOy/template composites were coated with resorcinol-formaldehyde (RF). Thermal removal of the templates and annealing of the RF yielded CoxOy@HCS materials. The use of functionalized polymer templates allowed small Co particles to be easily transferred from the template to the RF to give CoxOy@HCS. The CoxOy nanoparticles were also supported outside HCSs to give CoxOy/HCSs and their catalytic activity with that of CoxOy@HCSs. The synthesized CoxOy@HCS and CoxOy/HCS materials were evaluated in the oxidation of benzyl alcohol reaction. PXRD studies indicated that the Co oxidation state varied for the different catalysts due to reduction of the Co by the carbon, and a metal oxidation step prior to the benzyl alcohol oxidation enhanced the catalytic activity. The catalytic activity decreased with increasing metal loading, possibly due to pore filling effects. All the catalysts showed similar activity and selectivity towards benzaldehyde formation (70% selectivity at 50% conversion). No poisoning was observed due to product build-up in the HCSs. The catalysts were also evaluated in the hydrogenation of cinnamaldehyde reaction. The hydrogenation of cinnamaldehyde is usually performed in the liquid phase in batch mode. In this study, a vapour phase flow system has been used to evaluate the catalytic activity of both CoxOy@HCS and CoxOy/HCS. The influence of temperature, hydrogen flow rate and catalyst mass on the hydrogenation reaction was investigated. The products produced by this reaction were hydrocinnamaldehyde, 3-phenyl propanol and cinnamyl alcohol. The data revealed that the Co@HCS showed better activity and product selectivity compared to the Co/HCS. The catalysts with smaller particle sizes (ca. 6 nm) were more efficient than big particles (30 – 40 nm). An increase in reaction temperature (200 – 300°C) resulted in a lower cinnamaldehyde conversion and a poor product selectivity. TPR studies revealed that the Co@HCSs had stronger metal-support interaction than the Co/HCSs catalysts. Catalyst recycling studies revealed that only the Co/HCSs could be regenerated (4 cycles) and post-reaction analysis of the catalysts revealed that this was due to HCS pore blockage and not Co sintering