Highly Ordered Covalent Triazine Frameworks:Preparations And Applications

In 2005,Omar M.Yaghi used the dynamic reversible covalent bond strategy to realize the self-correcting ability of molecular connection,and extended the molecular structural units into a crystalline covalent organic network(COFs).In the field of organic polymer materials,COFs have many outstanding advantages,such as long-distance ordered nature(high crystallinity),high specific surface area,low density,functionability,and rich monomer selection.In particular,the characteristics of highly ordered nature make the structure of COFs material can be clearly determined,so that the relationship between structure and performance can be one-to-one correspondence,and then the structure can be adjusted from the atomic point of view to solve the problems in application.However,for most COFs materials,they are unstable due to their reversible covalent bond properties.In practical production applications,a material with poor durability often brings huge costs and unknown risks(such as toxicity,by-products and other hazards).As a branch of COFs,covalent triazine frameworks(CTFs)constructed by the strong aromatic C=N bond have high stability in physical and chemical properties.The stable CTF structure is not easy to aging,long service life and good economic benefits.Therefore,the stable CTFs has incomparable advantages in large-scale application.However,the long-distance ordered nature of CTFs has become very poor.And only a few ordered CTF structures have been prepared in harsh conditions,such as:high temperature(?400?),super strong acid corrosion,etc.On the other hand,the high nitrogen content of CTFs has heteroatom effect(HAE),which greatly improves the performance of materials in many application fields,such as gas adsorption and separation,heterogeneous catalysis,energy conversion and storage,and photoelectric technology.Therefore,it is of great significance to develop a synthesis strategy for CTFs with high crystallinity under mild conditions.In this paper,based on monomer concentration control,a new method of preparing highly crystalline CTFs is proposed.The nucleation rate is the key factor affecting the crystallization of CTFs.The highly odered CTFs were synthesized through the precursor oxidation and monomer drop addition methods,respectively,which were used to control the concentration of aldehyde monomer to regulate the nucleation rate.Highly crystalline CTFs has special advantages in photoelectricity,such as improving the separation ability of photogenerated electrons and holes and the electron migration ability,which is beneficial to the improvement of photocatalytic performance.However,since CTFs is an organic semiconductor in nature,the photogenic electron-hole recombination rate is high.Usually,the precious metal platinum is needed as a co-catalyst to further improve the electron migration ability and photogenic electron-hole separation ability of CTFs.Due to the high cost of Pt,the photocatalytic system of Pt@CTFs has a very high cost,which limits its practical application potential.In situ growth of metal Pd on the structure of CTFs and composition of Pd-@CTFs composite catalyst can not only effectively reduce the cost,but also have a very good performance in photocatalytic splitting water product hydrogen.The main contents of the paper are as follows:1)Chapter i:this chapter is the introduction part of the whole paper,which mainly summarizes the research background,structural characteristics and development history of triazine framework structure(CTFs),and introduces the research progress of triazine framework structure in gas adsorption and separation,heterogeneous catalysis,energy storage and transformation,photoelectric and other fields.2)Chapter ii:covalent triazine framework prepared via controlling monomer concentration by precursor oxidation.Using benzyl alcohol as the precursor,aldehyde monomer is slowly oxidized to form aldehyde monomer in the polymerization system.So the aldehyde monomer could maintain a relatively low concentration in the reaction system.The relatively low concentration of aldehyde monomer could control the nucleation rate of CTFs,reduce the forward reaction rate,and improve the self-healing ability of the polymerization reaction.Therefore,a series of highly crystalline CTFs with good thermal stability,namely,CTF-HUST-C1,CTF-HUST-C5 and CTF-HUST-C6,were synthesized by this precursor oxidation method.The specific surface areas of CTF-HUST-C1,CTF-HUST-C5 and CTF-HUST-C6 were 599 m² g^-1,520 m² g^-1 and 598m² g^-1,respectively.It was found that the crystalline CTF-HUST-C1 had higher photocatalytic performance than the amorphous CTF-HUST-1 one due to its easier electron transfer ability and better photoelectron-hole pair separation performance.3)Chapter iii:synthesis of covalent triazine framework structure with high crystallinity via controlling monomer concentration by drop addition.By controlling the monomer dropping rate,the progress of polymerization reaction was controlled,so as to allow sufficient time to realize the self-correction process of bond opening and reconnecting,which improves the self-healing ability.Thus a series of highly crystalline CTFs were synthesized successfully,which are comparable to COFs with weak bonds,such as boron esters,imine bonds,etc.This highly crystalline CTF-HUST-HC1,characterized by HR–TEM,is found with the exposed crystal face is{001},which has better ability of charge transfer and photoelectron-hole pair separation.It was found that the NO removal rate of CTF-HUST-HC1 was better than that of amorphous CTF-HUST-1,and even far better than the conventional g-C₃N₄.4)Chapter iv:based on CTFs,a low cost catalytic system for photocatalytic hydrogen production was developed.Because of the stronger interaction between Pd and pyridine nitrogen in CTFs,the smaller Pd nanoparticles are formed,which is more favorable for the separation of photogenic electron-hole pairs and electron transfer.Under the same conditions,the hydrogen production rate of 1 wt%Pd@CTF-HC2 and 1 wt%Pd@CTF-HC6 was 2 and 11 times higher than that of 1wt%Pt@CTF-HC2 and1wt%Pt@CTF-HC6,respectively.The hydrogen production rate of 1wt%Pd@CTF-N is10556 mol h^-1 g^-1,which is 5 times that of 1wt%Pt@CTF-N.Owing to its low cost and high efficiency,Pd@CTFs has a great advantage for large-scale photocatalytic hydrogen production by splitting water.