Diamine functionalized gel-type resin as a support for palladium catalysts: Preparation, characterization and catalytic properties in hydrogenation of alkynes

Gel-type resin (FCN) obtained as a result of copolymerization of glycidyl methacrylate (20 mol%), styrene (77 mol%) and ethylene glycol dimethacrylate (3 mol%) and functionalized by amine groups is studied as the support for Pd-catalysts (0.5-2 wt% Pd). Pd/FCN catalysts are characterized by number of techniques (FTIR, UV-Vis, XPS, SEM and HRTEM) and tested in the hydrogenation of alkynes, 2-butyne-1,4-diol (B3-D) and phenylacetylene (Phc). Catalysts are prepared by incorporation of palladium to the swollen particles of FCN resin using THF solution of Pd(OAc)2. In these conditions N and carbonyl groups of FCN polymer are involved in the bonding of Pd2+ ions. This leads to decrease in swelling ability of polymer mass due to “crosslinking” of the polymer via Pd2+ ions. Since the polymer-bound Pd species are only partially reduced under hydrazine treatment the reduced Pd/FCN samples swell less than pure FCN. In the reduced Pd/FCN samples, only at the lowest Pd-loading amounted to 0.5 wt% Pd, very well dispersed Pd-nanoclusters appear. At higher Pd content within the range 1-4 wt% Pd, the particles of Pd of size within the range 4-28 nm are formed and in all the samples the particles 8-16 nm in size predominate. In these catalysts, partial aggregation of Pd-nanoparticles proceeds, especially when the loading of Pd is higher than 1 wt%. The activity/selectivity behaviour of Pd/FCN catalysts is determined by the size of Pd-particles and swelling ability of polymer matrix under the catalytic run. In the presence of highly expanded catalysts with low Pd content, 0.5-1 wt% Pd, the alkynes substrates, B3-D or Phc are hydrogenated to alkene with selectivity 91-92% up to high alkynes conversion (90%). The suppression of alkene to alkane hydrogenation in the stage of alkynes is ascribed to high ability of Pd-centers to strong adsorption of alkyne substrate. It may also be related to steric hindrances of polymer in the vicinity of active Pd-centers caused by the swollen polymer network.