Preparation And Catalytic Performance Of Triarylphosphine Porous Aromatic Framework Materials

At present,porous organic polymers（POPs）,as an important research hotspot in the field of porous materials,have undergone rapid developments and are widely used in gas adsorption and separation,catalysis,electrochemistry,energy storage,pollutant treatment and other related fields.Among them,phosphorus-containing porous organic polymers（P-POPs）have attracted intensive attention due to their advantages such as structural diversity and easy functionalization,and show excellent applications in the field of heterogeneous catalysis.There are usually abundant phosphine units in the framework of such polymers,and phosphines with electron-donating properties can coordinate with various transition metals to immobilize metal nanoparticles,so that a variety of heterogeneous metal catalysts can be prepared for organic catalysis,which can improve the yield of product and effectively reduce the cost of catalyst recovery.On this basis,in this dissertation,starting from triarylphosphine and its derivatives as structural units,five P-centered porous aromatic framework materials（P-PAFs）were targeted design and synthesis as heterogeneous catalysts in organic synthesis by adjusting the electronic properties and steric hindrance effect of the P centers.The structures and properties of these materials were systematically investigated.The main research contents are as follows:1.Study on the reductive amination of aldehydes catalyzed by triphenylphosphine porous aromatic framework OTf@PAF-180.Firstly,PAF-180 was successfully synthesized from tris（4-bromophenyl）phosphine monomer by Yamamoto-Ullmann coupling reaction.Then,OTf@PAF-180,I@PAF-180,and BF₄@PAF-180 with Lewis acidic centers were successfully prepared by post-modification method of methylation reaction and ion exchange.The uniform distribution of phosphonium cations in the material framework ensures the uniform dispersion of Lewis acidic catalytic sites.Among them,the OTf@PAF-180 can efficiently catalyze the reductive amination of aldehydes with acetonitrile as the reaction solvent and Ph Me₂Si H as the hydrogen source,and its activity is 37 times higher than that of the corresponding discrete molecule catalyst.The main reasons are:（1）there are electrostatic attractions between the Lewis acidic centers（P⁺）in the OTf@PAF-180 material and the electron-rich atoms of the substrate,which means the polar material has certain adsorption,desorption,and mass transfer effects on the substrate;（2）OTf^–can activate the Si-H bond in Ph Me₂Si H,facilitating the nucleophilic addition of H^–to the imine bond.The synergistic effect of the two factors resulted in higher catalytic activity of OTf@PAF-180.In addition,the catalyst can be recycled and reused by simple filtration without affecting its catalytic activity,which reduces the costs and avoids environmental pollution.2.Study on the efficient catalytic oxidation of benzyl alcohol for the green synthesis of benzoic acid by triarylphosphine porous aromatic framework WO₄@PAF-181.PAF-181 with high specific surface area was firstly synthesized from tris（4-bromobiphenyl）phosphine building block in high yield by Yamamoto-Ullmann coupling reaction.Next,WO₄@PAF-181 was successfully prepared by methylation reaction and ion exchange.On the one hand,organic porous materials can adsorb organic compounds in solution,and WO₄@PAF-181 has a higher specific surface area than that of WO₄@PAF-180,which greatly promotes the enrichment of benzyl alcohol in its framework.On the other hand,the ionized WO₄@PAF-181 has good hydrophilicity,which is beneficial to realize the mass transfer of water-soluble reactants between water and the framework,facilitate the full contact between the substrates and the catalytic active centers,and improve the oxidation efficiency of benzyl alcohol.Subsequent research results showed that using methylated PAF-181 as the carrier,WO₄^2–as the catalytic center,H₂O₂as the oxygen source,and H₂O as the solvent,the green and efficient selective oxidation of benzyl alcohol can be achieved through the synergistic effect of functionalized carriers and active sites,showing WO₄@PAF-181 has high catalytic activity.It is worth noting that the catalyst can be easily recycled and reused by simple filtration and still exhibits high catalytic activity.3.Study on the efficient catalysis of Suzuki coupling reaction by triarylphosphine porous aromatic framework materials with Pd NPs-loading.Two kinds of bisphosphonium-centered structural units were constructed by introducing two-node bridges of different lengths between triphenylphosphine units,then PAF-182 and PAF-183 were synthesized by their Yamamoto-Ullmann coupling reaction,respectively.Such strategy not only increases the specific surface area of the PAFs,but also greatly changes the electronic and steric effects of the PAFs.Followed by ion exchange and Na BH₄reduction,palladium nanoparticles（Pd NPs）were generated in situ,and Pd@PAFs materials were successfully obtained with high Pd loading,high dispersion,and high activity.According to statistics,37.5 wt%（Pd@PAF-183）is the highest value reported so far for POPs-supported Pd NPs.Compared with the similar type of phosphonium-based Pd NPs heterogeneous catalysts,Pd@PAF-183 with high Pd NPs loading can catalyze Suzuki coupling reaction in 99%yield with an order of magnitude lower catalyst dosage,and the turnover number（TON）is as high as 2444.In addition,Pd@PAFs also exhibited excellent recyclability and reusability,and still maintained high catalytic activity after ten catalytic cycles.This study shows that it is more favorable to fix and disperse the nanoparticle precursor(Pd Cl₄^2–)on the PAFs through strong electrostatic interaction,so as to ensure the Pd@PAFs with high loading,high stability,and high activity.4.Study on the efficient catalytic reduction of nitrobenzenes to aromatic amines in water by triarylphosphine porous aromatic framework materials with Au NPs-loading.From tris（1-naphthyl）phosphine and tris（4-biphenyl）phosphine as building blocks,PAF-184 and PAF-185 with high specific surface area and high stability were successfully synthesized by classical Friedel-Crafts alkylation reaction.After the methylation reaction,Au Cl₄^–anions were fixed and dispersed through electrostatic interaction,and Au@PAF-184 and Au@PAF-185 with uniformly dispersed gold nanoparticles（Au NPs）were successfully prepared by subsequent in-situ Na BH₄reduction.Compared with the similar materials Au@PAF-93（Au:2.86wt%）and Au@PAF-94（Au:4.69 wt%）prepared by coordination and reduction methods,the loading of Au NPs in Au@PAF-184（Au:24.2 wt%）and Au@PAF-185（Au:34.9 wt%）increased by nine times.This indicates that the materials with higher loadings of metal nanoparticles can be obtained by electrostatic fixation and in-situ reduction.Later,these two materials were used for the catalytic reduction of nitrobenzene in water,and the K_appof both Au@PAF-184 and Au@PAF-185 is as high as 4.6×10^-1s^-1,which is much higher than that of the former two heterogeneous catalysts,showing very high catalytic activity.After the reaction,the catalysts can be efficiently recovered through simple filtration and still maintain high catalytic activity.