Synthesis, Structure And Catalysis Of Mesoporous Catalytic Materials With High Oxidative Activity

As an important part of inorganic crystal materials, microporous zeolitemolecular sieves gradually become the starting point of research for novel inorganicmaterials because of their unique pore structures and exchanged cations. They havebeen widely used in many fields such as ion-exchange, adsorption and separation,and industrial catalysis. However, microporous zeolite molecular sieves cannoteffectively deal with large molecules due to the limitation of the pore size. Forexample, when microporous zeolite molecular sieves are used as catalysts in thefields of catalysis, the size of the reactants is limited to 12 ?, which greatly restrainstheir application in catalysis of large molecules. Furthermore, various modificationsof pore channel cannot be realized for the limitation of pore size. The discovery ofmesoporous molecular sieves overcomes the difficulty of microporous zeolitemolecular sieves. They show great potential applications in catalysis of largermolecules and attract much attention because of their uniform and larger mesopores.However, compared with microporous zeolite, mesoporous materials show poorhydrothermal stability and weaker catalytic activities, which seriously limit theirextensive uses.Recently, Porf. Xiao and his coworkers have prepared successfully a novelmesoporous titanosilicates (MTS-9) by self-assembly of zeolite nanoclusters. Theirhydrothermal stability and catalytic activity have been improved greatly, comparedwith conventional mesoporous titanosilicates. It is proposed that the improvement ofhydrothermal stability and catalytic activity should be attributed to the presence ofprimary building units of TS-1 zeolite in MTS-9. Notably, MTS-9 shows highercatalytic activity in catalysis of large morlecule than TS-1 zeolite, which is needful inindustrial production. Unfortunately, calcination of as-synthesized MTS-9 at 500°Cfor several hours results in significant reduction of catalytic activities, which isassigned to relatively low stability of titanium species in mesostructuredtitanosilicates. It is well known that calcination is a very important process ofregeneration of catalysts. Therefore, improving thermal stability of titanium speciesin the mesoporous walls is still a great task for preparation of ordered mesostructuredtitanosilicates. Here we use zeolite TS-1 "seed solution"which contains primary building unitsof TS-1 zeolite as inorganic precursor and CTAB as template to prepared MTS-5 andMTS-8 in alkaline and acidic media, respectively. MTS-5 and MTS-8 are proved tocontain the primary building units of TS-1 zeolite in their mesoporous walls and theyshow greatly improved hydrothermal stability compared to conventional mesoporoustitanosilicate Ti-MCM-41. Characterization results indicate that the titanium speciesin MTS-5 and MTS-8 are similar to those of TS-1 zeolite because titanium specieshave been fixed in the framework of TS-1 zeolite primary building units when theseed solution is prepared and titanium species are directly introduced into theframework of MTS-5 and MTS-8 along with the self-assembly of TS-1 zeoliteprimary building units surrounding the surfactant. In phenol hydroxylation, MTS-5and MTS-8 show high activities similar to that of TS-1. In 2,3,6-trimethylphenolhydroxylation, MTS-5 and MTS-8 exhibit their specially high ability for largemolecule oxidation than TS-1 due to their large pore sizes. More importantly, MTS-5assembled from preformed titanosilicate precursors in alkaline media has highersilica condensation degree, compared with MTS-8 synthesized in strongly acidicmedia. After heating at 550°C for 4 h, MTS-5 basically remains its catalytic activitiesin hydroxylation of phenol and 2,3,6-trimethylphenol, indicating its higher stabilityof Ti species for calcination. In contrast, calcined MTS-8 shows much lowerconversion than as-synthesized MTS-8. Additionally, we also prepare another novelmesoporous titanosilicate (Sn-Ti-MS-1) with highly thermal stability by introducingthe other heteroatom Sn into the silica framework of MTS-9 in strongly acidic media.Characterization results indicate that incorporation of Sn makes no change oftitanium species in Sn-Ti-MS-1 after calcination at 550 °C. Furthermore, Sn-Ti-MS-1shows different catalytic kinetics in hydroxylation of phenol and2,3,6-trimethylphenol after Sn is introduced into the framework of MTS-9. Since the discovery of mesoporous aluminosilicates and silicates such asMCM-41 materials by Mobil scientists, a large number of mesoporousaluminophosphates have been successfully prepared. Notably, compared withmesoporous silicates, most of mesoporous aluminophosphates prepared throughsurfactant synthetic strategies exhibit relatively low thermal stability, which limitstheir wide applications in catalysis and adsorption. It has been reported that thermaland hydrothermal stabilities of ordered mesoporous aluminosilicates could beimproved significantly by the assembly of preformed aluminosilicate zeolitenanoclusters. However, the syntheses of ordered mesoporous materials assembledfrom preformed zeolite nanoclusters are only limited in silica-based mesoporousmaterials. We show here that the assembly of preformed aluminophosphate zeoliteAlPO4-18 nanoclusters with the surfactant of cetyltrimethylammonium bromide(CTAB) can synthesize an ordered hexagonal mesoporous aluminophosphate(JLU-50) that is thermally stable. Similar to other mesoporous materials preparedfrom zeolite nanoclusters, JLU-50 contains AlPO4-18 primary building units in itsmesoporous walls. Furthermore, we synthesize mesoporous aluminophosphatescontaining Fe (Fe-JLU-50) and Cu (Cu-JLU-50) by using zeolite Fe-AlPO4-18 andCu-AlPO4-18 seed solutions as inorganic precursors. Catalytic results ofhydroxylations of phenol and 2,3,6-trimethylphenol show that Fe-JLU-50 andCu-JLU-50 are both active in oxidations of small and large molecules aftercalcination, which indicates the advantages of preparation of heteroatom-containingmesoporous materials by using zeolite seed solution. All the mesoporous materials prepared from zeolite "seed solution"have somecommon structural characters including the thicker wall, disorder in some extent,zeolite-like connectivities in the wall. We also propose another method of combing the advantages of microporous andmesoporous molecular sieves except for preparation of mesoporous materials byusing zeolite seed solution, which is formation of mesopores in zeolite crystals. Here,we synthesize TS-1 crystals of uniform mesoporous channels by usingPoly(tetraalkyl ammonium)(M-100), designated as MTS-1(M). On the other hand,we prepare a novel mesoporous titanosilicate by using anion exchange resins (D201)as shape-directing macro-templates and TS-1 zeolite seed solution, which is namedas MTS-1(D). Although XRD of MTS-1(D) does not give characterized peaks ofTS-1 crystals, the primary building units of TS-1 zeolite are present in MTS-1(D).Furthermore, N2 adsorption-desorption isotherms of MTS-1(D) show the presence ofmesopores of 24 nm. In hydroxylation of phenol, MTS-1(M) and MTS-1(D) bothshow similar activity to that of TS-1. And in hydroxylation of 2,3,6-trimethylphenolshows that mesoporous channels makes it easy that MTS-1(M) and MTS-1(D)catalyses oxidation of large molecule, indicating MTS-1(M) and MTS-1(D) combinethe advantages of microporous and mesoporous molecular sieves. Notably,...