| Converting CO2 into high value-added chemicals and energy substances and realizing CO2 resource utilization is the demand of sustainable chemical industry and one of the important ways to achieve the goal of carbon neutrality.The direct C-H carboxylation of terminal alkynes with CO2 to form propiolic acids is an atomic economical reaction route.At present,researchers mainly use metal catalysts to catalyze the reaction,but the use of metal catalysts inevitably brings in metal leaching,environmental pollution,high costs and other problems.Obviously,the development of organic catalyst is of great significance.The guanidine-based covalent organic frameworks have a great advantage in CO2 conversion,because the abundant N/NH sites could allow a high affinity for CO2 and form hydrogen bonds with appropriate substrates.On this basis,a series of nitrogen-rich covalent organic frameworks(COFs)were designed and synthesized using trigaminoguanidine as a node to study the catalytic properties for CO2-alkyne carboxylation and chromic response to acid/base.The results show that the trigaminoguanidine nodes and azaheterocyclic linkers cooperate to result in excellent catalytic performance and acid-and base-responsive chromic behaviors.The research content of this thesis mainly includes the following parts:1.Synthesis and characterization of guanidine COFsA two-dimensional imine-linked hexagonal COF with methyl side groups(TG-DMPZ)was synthesized by condensation reaction between triaminoguanidine hydrochloride and 3,6-dimethyl-2,5-pyrazinedicarboxaldehyde under solvothermal conditions.TG-DMPZ was characterized by PXRD,FTIR,N2 adsorption,SEM,TEM and TGA.It was confirmed that TG-DMPZ has good crystallizability,high chemical and thermal stability and large Brunauer-Emmett-Teller(BET)surface area.For comparison,other COFs were synthesized with the same trigaminoguanidine nodes but different ditopic linkers.Small-molecule model compounds corresponding to the COFs were also synthesized for the study of properties and mechanism.2.Catalytic properties of guanidine COFsCatalytic studies reveal that the guanidine COFs as heterogeneous catalysts,in the absence of transition metals,show moderate to excellent catalytic activities in carboxylation of terminal alkynes with CO2.As the number of nitrogen atoms in the linkers gradually increases(from benzene to pyridine and to pyrazine),the catalytic performance of the guanidine COFs is gradually enhanced.The COFs containing pyrazine units show the best catalytic activity.After four catalytic cycles with TG-DMPZ,the activity did not decrease significantly,indicating that the catalyst has good recyclibility.By means of control experiments and spectral analysis,it was proved that the good catalytic activity of the material was due to the cooperation between the trigaminoguanidine node and the pyrazine linker.The triminoguanidine unit plays a decisive role in the catalytic reaction.Under basic conditions,neutral guanidine can be deprotonated into an anion form,which serves to bind and activate CO2.The pyrazine linker plays a promoting role in the catalytic reaction.The pyrazine unit together with the trinaminoguanidine unit serves to activate the C(sp)-H bond through hydrogen bonding.This work provides a novel class of green and high-efficency heterogeneous organic catalysts for carboxylation of terminal alkyne with CO2,providing a new approach for the green high-value conversion and utilization of CO2.3.Acid/base response of guanidine COFsThe guanidine COFs containing the pyrazine unit show obvious chromic response behaviors to acid and base,while the COFs containing pyridine/benzene rings do not.It is confirmed that the COFs show acid-and base-responsive chromic behaviors thanks to the amphoteric nature of the triminoguanidine unit and the auxochromic effect of the pyrazine linker by controlled experiments,XPS,optical spectral analysis.The work opens up new avenues to chromic materials for sensing and switching applications. |
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