Microwave Assisted Catalytic Hydrogenation of Aldehydes

Abstract

Nanoparticles were synthesized from simple precursors (metal salts) through modified precipitation method. The various types of synthesized NPs are metal oxide NPs, bimetallic, metal supported over metal oxide and metal promoted metal oxide supported metals. The synthesized catalysts are catalyst I: zirconia (t-ZrO2), catalyst II: palladium supported over zirconia and nickel promoted palladium supported over zirconia (Pd/ZrO2 and Ni-Pd/ZrO2), catalyst III: nickel and nickel iron composite (Ni and Ni/Fe), catalyst IV: platinum supported over zirconia and nickel promoted platinum supported over zirconia (Pt/ZrO2 and Ni- Pt/ZrO2), catalyst V: strontium and cobalt composite (Sr/Co composite) and catalyst VI: cobalt and zinc composite (Co/Zn composite). These NPs were characterized by FTIR, TGA, SEM, EDX, XRD, and surface area analyzer (SAA). The acidic character and nature of acidic sites at the surface of catalysts were investigated using pyridine adsorption method. The hydrogenation reactions of aldehyde with molecular hydrogen (H2 gas) under conventional heating and microwave irradiation systems were conducted. The comparison of conventional and microwave systems showed that microwave system has higher yield and productivity. Moreover, the microwave system is cost friendly, less time consuming and green approach as compared to conventional heating system. Aldehydes like hexanal, octanal and cinnamaldehyde were hydrogenated and % conversion and selectivity were calculated using gas chromatography. The efficient conversion of aldehydes to their respective alcohols were achieved with good productivity. The reaction conditions were optimized in term of time, catalyst amount, intensity of MW irradiation and H2 partial pressure. Tetragonal zirconia was characterized showed production of hot spot under microwave irradiation, which causes a great enhancement in catalytic activity for all the model reactions. Pd/ZrO2 showed a greater activity due to metal support interaction and thermal conductivity. The promotion effect of Ni to Pd/ZrO2 was also observed and the activity was found higher than ZrO2 and Pd/ZrO2. Similarly, the Pt supported zirconia and Ni promoted Pt/ZrO2 was also found to follow the same trend. Ni NPs and Ni/Fe nanocomposite were also used for the hydrogenation of aldehydes which showed good catalytic activity. xix The hexanal and octanal were hydrogenated in solvent free system, while cinnamaldehyde hydrogenation was studied in DMF solvent (green and microwave stable solvent). The solvent free system showed more % conversion as compared to aldehyde in solvents systems. The productivity order was found as hexanal > octanal> cinnamaldehyde. The catalytic activity order was found as Ni- Pt/ZrO2> Ni-Pd/ZrO2> Pt/ZrO2 > Pd/ZrO2 > ZrO2 > Ni/Fe > Ni > Co/Zn > Sr/Co. The temperature of the reaction was monitored and it was found that increase in temperature increases the % conversion while the selectivity to the desired product is compromised up to some extent.