Synthesis of carbon dots reduced palladium catalysts: application in the hydrogenation of cinnamaldehyde reaction

Abstract

The selective hydrogenation of α,β-unsaturated aldehydes, such as cinnamaldehyde (CALD), continues to gain attention in both academic and industrial fields. The selective hydrogenation of CALD is known for the production of the products (hydrocinnamaldehyde (HCALD), cinnamyl alcohol (CA), and 3-phenyl-1-propanol (3P1P)) that are of importance in the production of fine chemicals, spices, and pharmaceutical intermediates. In this study Pd catalysts supported on carbon dots (CDs), solid carbon spheres (SCSs), and hollow carbon spheres (HCSs) were used in the liquid phase hydrogenation of CALD. The influence of varying the CALD hydrogenation reaction conditions (i.e., catalyst mass, temperature, hydrogen flow rate, CALD concentration) on the activity and selectivity of the catalysts were also investigated. CDs were synthesized via polymerization and carbonization of sucrose and oleic acid, or sucrose and urea under different reaction conditions. CDs with a size range of 0.2 to 16 nm were prepared using a hydrothermal treatment of sucrose and oleic acid. The doping of the CDs using urea resulted in the formation of nitrogen-doped CDs (NCDs) with size < 10 nm, and carbon nanoparticles (size < 100 nm). The as-synthesized CDs (from sucrose and oleic acid) were used to reduce H2PdCl4 to metallic Pd nanoparticles (NPs) with dPd = 9.1 ± 1.6 nm, as confirmed by PXRD and HRTEM studies. Although Pd particles were made to be larger than the CDs, to observe any inverse support effects, the TEM data indicated a formation of carbon layer over the Pd NPs. The synthesized Pd/CDs catalyst (< 1 % loading) and CDs were both tested in the liquid phase hydrogenation of CALD. The CDs gave a CALD conversion (40 %, 4 h) with selectivity towards the reduction of the C=O bond (CA). The Pd/CDs catalyst, by contrast, was only selective in the reduction of the C=C bond (conversion 73 %) indicating the dominance of Pd over the CDs in the reaction. SCSs were prepared via the polymerization of resorcinol and formaldehyde (RF) resins in the presence of a catalyst (ammonia) followed by carbonization of the RF resins at high temperature (900 °C). From a TEM analysis the SCSs were spherical in morphology and had an average diameter of 535 ± 73 nm. The obtained SCSs were functionalized (fSCSs) using a nitric/sulfuric acid treatment. The Pd NPs nanoparticles were deposited on the surface of the CSs support using a liquid phase reduction method. The Pd particle size varied from 6.0 ± 2.3 iv and 12.0 ± 3.4 nm for the SCSs and fSCSs respectively. The XPS data confirmed the presence of PdOx in both Pd/SCSs (8.7 %) and Pd/fSCSs (22.6 %). When the Pd/SCSs and Pd/fSCSs were tested in the hydrogenation of CALD, the complete hydrogenation of CALD was obtained. The Pd/SCSs catalyst had a selectivity of 100 % to 3P1P (i.e., complete reduction of the C=C and C=O bond). The Pd/fSCSs catalyst produced CA as a major product followed by 3P1P, by-products, and HCALD. HCSs (487 ± 28 nm) were prepared using a hard-core templating method, in which polystyrene spheres (PSSs) were used as a hard template followed by coating with an RF polymer followed by the removal of template and annealing of the HCSs using a chemical vapour deposition method under an inert atmosphere. The loading of Pd NPs (6.4 ± 2.9 nm) on the outer surface of the HCSs was shown by TEM and PXRD data. A CALD conversion of 100 % was obtained with the Pd/HCSs catalyst and the catalyst selectivity towards HCALD decreased with an increase in the reaction time. CDs reduced Pd NPs catalyst supported on HCSs (Pd/CDs/HCSs) was also prepared and a Pd particle size of 45.6 ± 26.5 nm was obtained. The catalyst had poor activity in the hydrogenation of CALD, but it was selective to the reduction of the C=C bond, forming HCALD.