Thermoregulated Phase-Transfer Property Of Noble Metal Nanoparticle Catalysts And Their Catalytic Applications

Noble metal nanoparticles in catalysis have drawn much attention due to their high catalytic activity and selectivity.However,very similar to traditional homogeneous catalysts,one of the main disadvantages of noble metal nanoparticle catalysts is the problem of separating the catalyst from the products.In this paper,noble metal（Pt,Ru,Ir,Pd and Au）nanoparticles stabilized by thermoregulated ligand Ph2P（CH₂CH₂O）nCH3（n = 22）were prepared by hydrogenation reduction.The as-prepared noble metal nanoparticle catalysts exhibited the continuously reversible thermoregulated phase-transfer property in the aqueous/1-pentanol biphasic system.That is to say,noble metal nanoparticle catalysts were in the lower water phase at room temperature,afterwards,when the temperature reached up to the cloud point,they could transfer from the water phase into the upper 1-pentanol phase.After cooling to room temperature,noble metal nanoparticle catalysts could go back to the lower water phase from the upper 1-pentanol phase.Based on the thermoregulated phase-transfer property of noble metal nanoparticle catalysts,we also investigated their catalytic effect on the selective hydrogenation of diphenyl acetylene（DP A）,the selective hydrogenation of α,β-unsaturated aldehydes and ketones,and the alkoxycarbonylation reaction of iodobenzene and its derivatives.Thermoregulated phase-transfer noble metal nanoparticle catalysts,especially Pt,were used to catalyze the selective hydrogenation of diphenylacetylene.Under the conditions of DPA/Pt=500（molar ratio）,T= 70℃,PH₂= 1 MPa and t = 20 min,the conversion of DPA reached up to 99%and the selectivity of cis-stilbene was 86%.The catalytic activity remained unchanged within fourteen successive cycles.In general,the as-prepared Pt nanoparticle catalyst exhibited a very high turnover number（TON）of 7,400,which was the highest number among relevant Pt nanoparticle catalysts,for the selective hydrogenation of DPA.Thermoregulated phase-transfer Ir nanoparticle catalyst was investigated to catalyze the selective hydrogenation of α,β-unsaturated aldehydes and ketones.In the same catalytic system（substrate/Ir = 100（molar ratio）,T= 70 ℃,PH₂= 1 MPa）,the catalyst exhibited a totally different orientation for the hydrogenation of α,β-unsaturated aldehydes and ketones,namely,highly selective hydrogenation of the C=0 bond for α,β-unsaturated aldehydes,such as cinnamaldehyde,the conversion was 99%along with a selectivity of 99%to the C=O group within 1 h,while the C=C bond for α,β-unsaturated ketones,such as chalcone,the conversion was 98%along with a selectivity of 99%to the C=C group within 9 h.To our knowledge,the results for the selective hydrogenation of α,β-unsaturated aldehydes and ketones catalyzed by Ir nanoparticle catalyst have not been reported in the literature.Thermoregulated phase-transfer Pd nanoparticle catalyst was investigated to catalyze the selective hydrogenation of α,β-unsaturated ketones.In terms of the selective hydrogenation of chalcone,as a template reaction,under the conditions of room pressure（H₂）,chalcone/Pd =500（molar ratio）,T = 90 ℃ and t = 90 min,the conversion was up to 99%and the selectivity to C=C bond was not less than 98%.Moreover,the catalyst could be easily used for four cycles without evident loss in activity.Thermoregulated phase-transfer Pd nanoparticle catalyst was also investigated to catalyze the alkoxycarbonylation reaction of iodobenzene and its derivatives.Under the conditions of thermoregulated ligand/Pd =20（molar ratio）,iodobenzene/Pd = 100（molar ratio）,tri-n-hexylamine（3 equiv.）,T= 120℃,PCO= 1 MPa and t=3 h,the conversion of iodobenzene was 99%.The catalyst could be also used for four cycles and the leaching of Pd was below the lower detection limit of instrument（3 μg/L）in the cycle experiments.In summary,thermoregulated phase-transfer property of Pt,Ir and Pd nanoparticle catalysts not only showed good dispersion stability and high catalytic activity,but also owned the feature of easy separation and recycling.It opened up a new avenue to address the recovery and recycling of noble metal nanoparticle catalysts that was desired in catalysis community.