Synthesis, Characterization and Evaluation of Pd–Nanoparticles Supported On Titania Nanofibers as Catalyst for Heck Reactions
Omutoko, Leah Nyangasi
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Selective organic transformations for most sought after aromatic carbon-carbon coupling reactions have been conventionally accomplished using homogeneous catalysts in the presence of ligands. These catalysts are known to be unstable, not easy to separate and recover, expensive and works over a narrow range of applicable reaction conditions resulting in relatively low catalytic ability. In the recent past, nanocatalysts that underpin the principles of the emerging era of nanotechnology have attracted huge attention given their superior properties compared to the bulk ones. Synthesis of heterogeneous nanocatalysts can be done in such a way that the morphology of the catalyst is controlled making the catalyst more efficient. The objective of this study was to synthesize Pd nanoparticles supported on TiO2 nanofibers followed by efficiency test of the catalyst in Heck coupling reaction. TiO2 nanofibers (NFs) were prepared through electrospinning of a composite mixture of poly (methyl methacrylate) (PMMA)/Titanium isopropoxide (TIP) and the fibers obtained calcined in air at 500 ºC. Pd nanoparticles (PdNPs) were prepared starting with Pd(acac)2 through deposition-reduction method and agglomeration minimized using Cetrylmethylammonium Bromide (CTAB). The present study demonstrated that electrospinning can produce anatase TiO2 and Pd nanoparticles dispersed on TiO2 fibers (Pd/TiO2) nanocatalyst. Complementary studies of X-ray diffraction (XRD), Scanning electron microscope (SEM) and Transmission electron microscope (TEM) indicated that the anatase TiO2 fiber diameter was 180±30 nm. PdNPs of diameter range 10–40 nm were successfully adsorbed on the surface and in the pores of TiO2 nanofibers. Brunauer−Emmett−Teller (BET) analysis revealed that the synthesized TiO2 and Pd/TiO2 had a surface area of 53.5 and 43.4 m2/g respectively, confirming adsorption of Pd NPs on TiO2 NFs. Aryl bromide (ArBr) was coupled with styrene at optimum conditions of 140 ºC using 0.2 mol% Pd/TiO2 in the presence of triethylamine (Et3N) and NaOAc base; giving 89.7% conversion, with a turn over number (TON) of 1993.4 and turnover frequency (TOF) value of 332.2 hr-1. 1H and 13C NMR indicated 100% selectivity for trans – stilbene. Similarly, coupling of methyl acrylate (an olefin with electron withdrawing group) and ArBr under the same reaction conditions yielded 80.6% conversion of ArBr. The Pd/TiO2 catalyst was observed to be recyclable but experienced 1.6% leaching in the fourth cycle. From this study; nanosized Pd/TiO2 catalyst was prepared by the electrospinning, deposition-reduction wet chemical method. The type of catalyst was found to be highly active in Heck coupling reaction due to their high disparity, and large surface area to volume ratio, thereby providing more active sites for the catalytic reaction. The most favorable conditions for the coupling of ArBr-STY was obtained with Pd/TiO2 as a catalyst, DMF as the reaction medium and reaction temperature maintained at 140 ˚C for 6 h in the presence of NaOAc base. The effect of type and particle size of Pd based catalysts on the catalytic activity and product selectivity was observed to favor trans-products. Based on the findings in this study, it is recommended that application of the synthesized catalyst Heck coupling reaction be carried out a temperature of 140 ˚C, in the 0.2 mol of Pd/TiO2 catalyst, DMF solvent and NaOAc The catalyst is recyclable thrice with a leaching of 0.1% after which the leaching increases to 1.6%.The efficiency of the catalyst can be investigated in Suzuki coupling reaction.
- PHD-Chemistry