APTES-assisted Preparation Of Ni-based Nanocatalysts And Their Application In The Selective Reduction Of Alkynes

The selective reduction of alkynes is of great significance in the field of polymer and fine chemistry.Due to the advantages of low cost and high activity,nickel-based catalysts,especially those modified by organic compounds,have shown broad application prospects in the selective reduction of alkynes in recent years.However,there are still some deficiencies need to be improved in this field,such as complex preparation process,low selectivity,poor stability and the usage of expensive/highly toxic organics.In this paper,with the assistance of（3-aminopropyl）triethoxysilane（APTES）,a series of nickel-based nanocatalysts with low cost and excellent performance were designed by adjusting the catalyst compositions and preparation methods,then applied to the selective reduction of alkynes.Firstly,nickel was first combined with aminoclay（AC）,which contains APTES,by the impregnation method.Nickel-based catalyts were then prepared by the subsequent calcination-reduction under nitrogen atmosphere.The effects of different ligands on the catalytic performance in the selective reduction of diphenylacetylene（DPA）were investigated.It was found that the nickel-based nanocatalyst prepared with the EDTA ligand（Ni/SiO₂-CN-1）showed the highest activity（99.9%）and selectivity to（Z）-stilbene（88.7%）.The characterization results confirmed that the more available Ni active centers and the electron interaction between nitrogen and nickel are the key factors to improve activity and selectivity of Ni/SiO₂-CN-1 catalyst.Secondly,in order to improve the stability of the catalyst,the nickel-based nanocatalyst using amine-functionalized NiMg-AC as the support（Ni/NiMg-AC）was obtained by a"self-assemble"strategy for the first time.In Ni/NiMg-AC catalyst,Ni²⁺was directly introduced into the skeleton structure of AC to replace parts of Mg²⁺,together with the coordination effect of Ni²⁺with amino groups in APTES.In the selective reduction over the Ni/NiMg-AC catalyst,DPA conversion is 99.9%and（Z）-stilbene selectivity is 93.3%.The catalyst could be recycled for 11 times without significant change in catalytic performance.Experimental and theoretical characterizations show that Ni/NiMg-AC has high dispersability in the reaction systerm,with well dispersed and immobilized Ni nanoparticles.In addition,the aminopropyl groups on NiMg-AC improved the catalytic performance by tuning the adsorption/desorption capacity of DPA/stilbene.Variation of the central metal cation and organo-siloxane could be used to prepare a series of AC-derivated nickel catalysts for highly efficient DPA selective reduction.Therefore this"self-assemble"synthesis strategy provide valuable guidance for the preparation of highly effective nickel-based catalysts.Thirdly,to further explore the application of APTES in catalytic materials,biligand-stabilized nickel-based nanocatalysts（Ni_x-2MI_y-APTES_z）were prepared by using APTES and2-methylimidazole（2-Me Im）as ligands and applied in the selective reduction of DPA.The effects of different ligands and ratios showed significant effects on the catalytic performance of the Ni_x-2MI_y-APTES_z catalysts.Ni_x-2MI_y catalyst had high activity but low（Z）-stilbene selectivity;in contrast,the Ni_x-APTES_z catalyst showed lower DPA conversion but higher（Z）-stilbene selectivity.Optimized catalyst Ni_0.5-2MI₈-APTES₂ enabled the complete DPA conversion with a 92.9%（Z）-stilbene selectivity.The catalyst had good stability and general substrates applicability as well.XRD,FT-IR,TG and XPS characterizations illustrated that the coordination between nickel and ligands improved the dispersion and stability of the Ni active species.In Ni_0.5-2MI₈-APTES₂ catalyst,the competition coordination of 2-Me Im and APTES toward nickel was beneficial to the formation of highly active Ni sites.APTES can reduce the desorption ability of stilbene and improve the selectivity of the reaction.At the same time,high concentration of APTES was conducive to stabilizing the three-dimensional configuration of the catalyst,thereby improving the stability of the catalyst.Therefore,the synergistic effect of the biligands played an important role in the improvement of catalytic performance.Finally,aiming for the improvement of catalyst thermal stability,a series of supported nickel-based nanocatalysts（Ni/NiMg SiO_n）were prepared using nickel-based AC as the precursor.The influences of different catalyst preparation methods and conditions on the performance of were compared.Under the H₂ pressure of 1.0 MPa and 150℃for 2 h,the optimal catalyst Ni/NiMg SiO_n-2 with low nickel loading showed excellent catalytic performance in the selective reduction of DPA,with 99.9%conversion and 93.7%（Z）-stilbene selectivity.Notably,it can be stably used for 12 times.XRD,TEM and H₂-TPR characterizations have proven that Ni/NiMg SiO_n-2 catalyst has moderate Ni content,uniform Ni NPs and strong interaction with the support,which are important reasons for the excellent performance of the catalyst.It is worth mentioning that APTES has a dual role in the catalystic system:i)to in-situ generate the support,which has spatial confining effect with Ni;ii)the coordination of amino groups can enhance the interaction between metal and support,accounting for the highly Ni dispersion and good stability.In conclusion,a series of nickel-based nanocatalysts have been prepared based on the assistance of APTES,and showed high activity,selectivity and stability in the selective reduction of alkynes.This work provides good guidance for the design,preparation and application of nickel-based nanocatalysts with high catalytic performance.