Preparation Of BTP Ligand-assisted Palladium-Nickel Bimetallic Catalyst And Its Application In Hydrogenation And C-C Coupling Reactions

Palladium(Pd)catalysts play a leading role in many chemical reactions and it is a relatively expensive noble metal.Therefore,their higher efficiency,selectivity,reusability,and stability are vital for the economic feasibility of the reaction.In this scenario,heterogeneous catalysis appears as a promising alternative,because heterogeneous catalysts are recoverable,efficient,stable,and facile during application.However,it requires continuous improvements to sustain in the catalysis sector especially to be ahead of homogeneous catalysts.Therefore,as an effort to improve the heterogeneous catalysts,we developed the following four catalysts systems.Pd nanoparticles widely involve in catalysis,however,in monometallic version,a considerable fraction of metal in the core is wasted due to without proactively being used.Therefore,the synthesis of a Pd-rich shell with a cost-efficient metal Nickel(Ni)in the core is highly sought as a solution.As such an attempt,we synthesized two catalyst systems of PdNi bimetallic nanoparticles supported on Si02 and rGO.During the synthesis of the catalysts,we used a Pd-specific small molecule,BTP,as a ligand or surfactant,which as reported can form a strong bond with the active metal and may aid the metal to be pulled out towards the shell during the co-reduction.The as-prepared catalysts were labeled as PdNi-BTP/SiO2 or PdNi-BTP/rGO.For comparison,a series of mono and bimetallic catalysts with PVP or without ligands were also prepared.Furthermore,to investigate the impact of the Pd:Ni ratios on the catalytic activity,we synthesized the catalysts with Pd:Ni ratios of 3:7,1:1 and 7:3 and labeled them as Pd3Ni7-BTP/SiO2,Pd5Ni5-BTP/SiO2 and Pd7Ni3-BTP/SiO2,respectively.In addition to the bimetallic nanoparticles on Si02 and rGO,we focused on alternative reduction methods and a novel support material.Nanoparticles are frequently synthesized under high-temperature and high-pressure H2 environment.This classical approach entails an environment that will boost the aggregation of nanoparticles or inactivity during the reaction cycle.Therefore,we used gaseous reductant ethanol,methanol,formaldehyde,and hydrazine to reduce Pd metal precursors on silica and labeled the catalysts Pd-e/SiO2,Pd-m/SiO2,Pd-f/Si02,and Pd-h/SiO2,respectively.Furthermore,there is a broad range of catalyst supports in the catalysis science.However,it is still interesting to find out potential for a novel support system.Therefore,we evaluated Pd monometallic catalysts supported on silicon,which was labeled as Pd/Si.The characterization of the catalysts provides impressive results.The primary hypothesis of Pd-rich surface was proven with XPS analysis as it confirms that the BTP ligand aids the surface Pd enrichment while ligand-less route failed to do so.According to XPS analysis,PdNi-BTP/Si02 shows positive value towards Pd-richness of the shell while the ligand-less counterpart indicates a negative value.The Pd3Ni7-BTP/SiO2 demonstrated the smallest particle size among the series(average 2.14 nm).The rest of PdNi mono and bimetallic nanoparticles on SiO2 and rGO catalysts exhibited well-dispersed and small-sized particles(2-4 nm in size range)on the surface according to the TEM analysis.The Pd-f/Si02 consists finely distributed&small-size nanoparticles(average 3 nm)which confirmed the efficacy of milder reductant.The monometallic nanoparticles on Pd/Si catalysts were found to be aggregated which can be assumed due to the minimal surface area of as-prepared silicon.The prepared catalysts were tested for the hydrogenation of nitro/phthalate compounds and Heck/Suzuki C-C cross-coupling reactions.PdNi-BTP/Si02 and PdNi-BTP/rGO exhibited excellent performance for all the reactions involved.Furthermore,we found out that Pd3Ni7-BTP/SiO2 provided the best performance among the PdNi-BTP/SiO2 catalysts with different Pd:Ni ratios.The small size nanoparticle,Pd enriched surface,and synergistic effects between catalyst compositions seemed as the driven factors of these excellent results.The Pd-f/SiO2 showed the best catalytic performance for the hydrogenation of phthalic acid and DOP.The catalysts reduced by using other milder gaseous reductants also showed good performance.However,the novel support,Silicon supported Pd catalyst was inactive for the hydrogenation reactions as it was consisted with highly aggregated Pd nanoparticles due to low surface areas of the support.