A Research On Efficient And Selective Acetylene Hydrogenation In N,N-Dimethylformamide Under Mild Conditions

Ethylene,known as the building brocks in the production of polymers,is one of the most produced chemicals in the world.Ethylene is industrially produced by the steam cracking method and always contains traces of acetylene formation,which acts as a poison for the downstream polymerization catalyst.Therefore,the removal of acetylene from ethylene streams is a key requirement in polymer industry.Two approaches,based on either catalytic selective hydrogenation or isolation using suitable solvents,have currently driven considerable scientific research in academia and industry.These two approaches exhibit their own specific advantages,but an acetylene-removal strategy combining all these distinct features was still missing.Moreover,the acetylene hydrogenation is a paradigmatic reaction normally catalyzed in gas phase and quite severe temperature and pressure conditions.We carried outa research on selective acetylene hydrogenation in liquid phase under mild conditions,due to the significantly difference solubility between acetylene and ethylene in N,N-Dimethylformamide（DMF）.Zero-valent metal nanocatalysts,involving bimetal particles,have been applied in a variety of heterogeneous catalytic reactions such ashydrodechlorination processes.In this study,zero-valent metal reductionwas used as a reduction of the selective acetylene hydrogenation.This report also investigates the effects of a variety of bimetal（X-Fe）catalysts（0.2 mol%,n/n）,the ratio of DMF and water,and the dosage of bimetal catalysts on the acetylene conversion and ethylene selectivity.The result clearly shows that high ethylene selectivity was obtained over 0.2 g Ag（99%）,Pt（93%）and Au（80%）bimetal catalysts in the processes of selective acetylene hydrogenation,but with relative lower hydrogenation activities（about 20%）;Despite remarkable acetylene conversion rates achieved on Ni-Fe and Pd-Fenanoparticle（89%and 57%respectively）,the ethylene selectivity over them were a little lower,particularly the latter（48%）after 6 h;besides high selectivity（>91%）,Cu behaves aremarkable catalytic activity.Therefore,we examined the effect of triple catalysts like Ag-Pd-Fe and Cu-Pd-Fe,on the selective acetylene hydrogenation.As opposed to the benchmark catalyst Pd₂₀Ag₈₀（acetylene conversion activity is set as 1.00）,which has been highly optimized in industry for application to gaseous acetylene hydrogenation,Pd₂₀Cu₈₀-Febehaves both high acetylene conversion activity（3.90）and high ethylene selectivity（79%）.When NaBH₄ was used as hydrogen source,Pd-Fe NPs behave much better hydrogenation activity（higher by a factor of 2.2 orders of magnitude）and selectivity（>87%）.At the same time,the study uses H₂ or NaBH₄ as the hydrogen source,to evaluate the impacts of the ratio between DMF and water,the dosage of Pd nanoparticle,and the amount ofreductants（H₂ or NaBH₄）on selective acetylene hydrogenation.The result shows that in the presence of 1.0 mol%Pd and 1.0 equiv NaBH₄ in 10 ml DMF-water solution（7:3,V:V）,acetylene hydrogenationto ethylene occurs promptly and proceeds with high acetylene conversion（>80%）and an ethylene selectivity of ca.90%in 10 min.When pure H₂（2.86equiv）was used as hydrogen source,this approach also leads to high acetylene conversion（81%）and remarkable ethylene selectivity（94%）in 10 min.Under the optimal conditions tested,we attained an ethylene selectivity of over 90%and a remarkable activity higher by 2.7orders of magnitude compared to current industrial catalytic method.This excellent catalytic performance is significantly attributed to surface properties of PdNPs and the isolation function of DMF.In case of NaBH₄ used as the reductant,this process was repeated 5 times:an evident decline of acetylene conversion is detectable for the first two cycles;afterwards conversion tends to stabilize at ca.30%.If the reaction time is extended to 2 h,the conversion can still reach ca.80%in the last cycle,and agglomeration phenomena for the PdNPsappear.During the recycle tests,deposition of inorganic sodium salts onto the PdNPs is the most plausible cause for the deactivation of the catalyst when NaBH₄ is used as a hydrogen source.Whereas the outcome of the recycle experiments shows that acetylene conversion slightly decreases in the first cycle and then stabilizes at ca.60%during the following cycles,while ethylene selectivity remains consistently at 90%,with H₂used as hydrogen source.No obvious agglomeration phenomena were observed for the Pd catalyst after recycling,which indicates deactivation of Pd is not associated with the coupling reactions.No matter whether hydrogenation is carried out using molecular H₂ or NaBH₄ as the hydrogen source,PdNPs exhibit a excellent catalytic performance of acetylene hydrogenation,good stability and high reactivity.The potential applicability of the developed approach was demonstrated using a mixture of acetylene and ethylene（1:9,n:n）in the presence of 1.0 mol%Pd and 2.86 equiv H₂in a 250 ml flask.The selectivity in acetylene hydrogenation（78%acetylene conversion with more than 90%ethylene selectivity in 10 min）was in excellent agreement with the aforementioned results.Further optimization of this system and catalyst proposed in this study may lead to an even better catalytic performance,and higher potential value in practical applications.