Preparation Of TEMPO-IL-PF₆ And Its Catalytic Performance And Mechanism For Alcohol Oxidation

The selective oxidation of alcohols to the corresponding aldehydes,ketones and nitriles has important applications in industrial development,while 2,2,6,6-tetramethylpiperidine-nitroxyl radicals(TEMPO)is an important class of catalysts for these reactions.At present,there are some shortcomings in the TEMPO application,such as the difficulty in recycling the catalyst.If it can overcome its existing defects,TEMPO catalytic system will have greater application prospects in the field of oxidation.In this paper,an imidazole hexafluorophosphate ionic liquid(TEMPO-IL-PF6)immobilized with TEMPO structural unit was prepared by 4-OH-2,2,6,6-tetramethylpiperidine-nitroxyl radical(4-OH-TEMPO)acylated with chloroacetyl chloride firstly,and further reacted with N-methylimidazole,then carried out anion exchange with KPF6.The synthesized catalyst and its intermediates were characterized by GC-MS,LC-MS.Furthermore,TEMPO-IL-PF6/TBN and TEMPO-IL-PF6/DPA/CuCl2 catalytic systems,etc.were respectively composed of TEMPO-IL-PF6 and TBN,DPA/CuCl2,etc.These catalytic systems have highly efficient and selective for catalytic oxidation of benzyl alcohol under normal temperature and atomspheric pressure and using air as oxidant.In addition,TEMPO-IL-PF6 can be recovered by simple liquid separation,and the recovered TEMPO-IL-PF6 can be reused many times.The main findings of this paper are:Firstly,the catalytic performance of TEMPO-IL-PF6/TBN catalytic system for the oxidation of benzyl alcohol to benzaldehyde was investigated,and the reaction conditions were optimized.When 2.1 g(5 mol%)of TEMPO-IL-PF6,40 mol%TBN,2.0 mL of deionized water and 20 mL of acetonitrile solvent were added to 0.10 mol of benzyl alcohol,using air as an oxidant,the system exhibits excellent catalytic performance and good stability for selective oxidation of benzyl alcohol under normal temperature and atmospheric pressure.During the five repeated use of the catalytic system,the conversion of benzyl alcohol was maintained above 95%,and the selectivity of benzaldehyde was nearly 100%.In addition,the catalytic oxidation performance of these catalysts on different substrates:p-benzyl alcohol,p-methoxybenzyl alcohol and p-nitrobenzyl alcohol was investigated in detail.The results showed that the catalytic effects were:p-methoxybenzyl alcohol>benzene Methanol>p-nitrobenzyl alcohol.Secondly,TEMPO-IL-NO3 ionic liquid was prepared by reacting TEMPO-IL-Cl with silver nitrate.TEMPO-IL-NO3/diluted hydrochloric acid catalyst system has excellent catalytic performance and stability for the catalytic oxidation of benzyl alcohol.During the five repeated use of the catalytic system,the conversion rate was maintained above 95%and the selectivity was greater than 96%.Next,the catalytic performance of the oxidation of benzyl alcohol to benzaldehyde by TEMPO-IL-PF6 and transition metal complexes was investigated.Cu2+,Ni2+,Fe3+,and ligands DPA,BBP,BMBP are inspected.The results showed that the TEMPO-IL-PF6/DPA/CuCl2 system has the best catalytic performance.Furthermore,the reaction conditions were optimized.The results showed that 5 mol%TEMPO-IL-PF6,5 mol%DPA and 5 mol%CuCl2 were added to 0.10 mol of benzyl alcohol,and using air as an oxidant,the system exhibits excellent catalytic performance and good stability for selective oxidation of benzyl alcohol under normal temperature and atmospheric pressure.During the five repeated use of the catalytic system,the conversion of benzyl alcohol has been maintained above 80%and the selectivity is above 96%.In addition,the catalytic oxidation performance of this catalyst system for different substrates:p-benzyl alcohol,p-methoxybenzyl alcohol and p-nitrobenzyl alcohol was also investigated.The results showed that the catalytic effects were:p-methoxybenzyl alcohol>benzene methanol>p-nitrobenzyl alcohol.Finally,the catalytic performance of TEMPO-IL-PF6/DPA/CuI catalyst system for the ammoxidation of benzyl alcohol to benzonitrile was investigated.During the five repeated use of the catalytic system,with ammonia as nitrogen source and air as oxidant at 65?,the conversion of benzyl alcohol is more than 70%and the selectivity is more than 65%.