Organic-inorganic Hybrid Materials With Metal Complexes Of Schiff Bases Or 8-quinolinol: Synthesis, Characterization And Catalytic Properties In Olefin Epoxidations

The epoxidation of alkenes to epoxides, which are highly useful synthetic intermediates and can be easily converted to other valuable compounds through selective ring opening and functional group transformation, is a subject of considerably academic interest. Styrene oxide and cyclooctene oxide are two of the most important products produced through olefin epoxidations, and are widely used for plasticizer, flavor and epoxy resin production. So far epoxides are mainly produced through Halcon, halogenohydrin and superacid processes. However, many side-products are produced in this complicated Halcon process, which suffers from the costly material consumption and equipment cauterization. For the halogenohydrin process, a great deal of chlorine-containing wastewater produced leads to severely environmental pollution. Though the superacid process is dependable and highly efficient, the peroxide is expensive and bears potential safety hazard. It will bring about a revolutionarily technical progress if new and environmental benign epoxidations are carried out. Therefore it is of great academic and industrial importance to explore new epoxidation methods.There are many reports about the catalytic performances of Cu, Co, Fe, V, Mo, W, Ti and Zr complexes of Schiff bases, multi-dentate amine, cyclopentadienyl, bipyridyl, porphyrin and 3-(2-pyridyl)-1-pyrazolyl catalyzed olefin epoxidations with H2O2, air or TBHP as the oxidant. However, these homogeneous catalysts suffer from severe deactivation due to easy formation of dimeric peroxo- andμ-oxo species. The combined merits of organometallic groups and solid supports can be obtained by heterogenization of neat metal complexes onto silica matrix. The formed organometallic functionalized hybrid materials possess excellent activity, selectivity and allow for catalyst recycling and product separation. Generally, four general methods have been employed to heterogenize homogeneous catalysts: (i) by encapsulation of metal complex into zeolite, (ii) by covalent attachment of homogeneous counterparts to inert supports, (iii) by a coordination bond, and (iv) by ion interaction. Covalent attachment is commonly used and most effective for anchoring metal complexes. However, the main problem for the heterogenized metal complex is that metal species are easily leached from the support to solution, leading to activity decline and even deactivation. Therefore, exploring proper heterogenization method and improving the catalytic epoxidation performances is still a challenge.Compared with macrocyclic ligands, such as porphyrin and phthalocyanine, Schiff base ligands are various and can be easily prepared, and are widely used for luminescent materials and dyes. Schiff base ligands can coordinate with most metal ions in periodic table to form stable metal complexes with a variety of geometries, viz. planar, umbrella-type and stepped conformations, to generate various active site environments for the different oxidation reactions. However, most metal Schiff base complexes are recently encapsulated into the supercages of zeolite and this noncovalent attachment leads to the metal leaching. Covalent immobilization of metal Schiff base complexes onto inorganic support may overcome this problem. On the other hand, 8-quinolinol is a strong bidentate ligand and can coordinate with more than sixty metal ions in periodic table to form stable metal complexes, which are widely studied as luminescent materials. However, covalent anchoring of metal complexes of 8-quinolinol is scarcely reported, let alone reports of controlling their structure on silica matrix and their catalytic performances. Different from Schiff base ligands, the compositions and structures of metal complexes of 8-quinolinol are varied as prepared via different methods, which plays a role on catalytic performances. Therefore, grafting site isolated organometallic molecules onto porous materials and controlling their concentration and distribution is crucial.In this thesis, we focused on the immobilization of neat metal complexes and controlling the composition, distribution and structure of heterogenized active species. From the molecular engineering aspect, we try to design and prepare novel organometallic functionalized organic-inorganic hybrid materials, and to study their structure, composition and even catalytic performances.Besed on the background mentioned above, work involving two main aspects is carried out. The one is heterogenization of Cu, Co, Fe, V complexes of Schiff bases onto SBA-15 matrix, and study of the effect of reaction conditions, immobilization routs, ligand hydrogenization, ligand substitution and anchorage status of active species on catalytic performances through analysis of various characterization data. The other is relative to anchoring, characterization and catalytic application of metal complexes of 8-quinolinol. 8-quinolinol complexes of Fe, Co, Cu are encapsulated into zeolite-Y by a flexible ligand technique; and its V, Mo, W complexes are introduced into the SBA-15 by a metal template/metal exchange method; and Mo complexes of 8-quinolinol are introduced into the framework of PMOs by a convenient one-pot method. The different performances of these metal complexes of 8-quinolinol in olefin epoxidations are studied, and the relationship between the distribution, composition of active species and their catalytic performances is also discussed. Combining these results, we can draw the conclusion that successful immobilization of metal complexes and controlling the composition and structure of active species can improve catalytic activity, selectivity and stability.1. Immobilization, characterization of copper salicylaldimine complex and its application in styrene oxidation by H₂O₂Copper salicylaldimine functionalized SBA-15 hybrid mesoporous material is synthesized by a post-grafting rout. The modified material retains its structure, which is verified by XRD, N₂ adsorption/desorption and SEM. FT-IR and UV-vis spectroscopic studies demonstrate that metal complexes are successfully immobilized. The amount of metal and ligand is quantified by ICP-AES and elemental analysis measurements. Best catalytic results are obtained by optimizing reaction conditions and solvent effect. Alkaline additive can improve the catalytic activity, selectivity and stability. Furthermore, copper salicylaldimine modified SBA-15 is silylated via post treatment of amino-modified SBA-15 by HMDS, and was confirmed by XRD, N₂ adsorption/desorption, FT-IR and UV-vis spectroscopy and TG/DTA. Both silylated and non-silylated samples are comparatively examined as catalysts in epoxidation of styrene with 30 wt% aqueous hydrogen peroxide as oxidant. It is found that silylated material is more active and selective to styrene oxide than the non-silylated one. The epoxide selectivity and catalyst stability were further improved by the combination of surface silylation and alkaline tunation. These superior effects are attributed to the high hydrophobic character of the solid surface produced by the silanol neutralization.2. Preparation, characterization of copper(II) and oxovanadium(IV) Schiff base modified SBA-15 and their application in styrene oxidationTwo functionalized copper(II) and oxovanadium (IV) Schiff base complexes of type [M(N₂O2), M = Cu, VO] bearing chloromethyl groups are directly anchored onto amino-modified SBA-15 and examined as catalysts for styrene oxidation. XRD, N₂ adsorption/desorption and TEM results indicate that the mesoporous structure of SBA-15 remains intact throughout the grafting procedure. FT-IR, UV-vis spectroscopy plus TG/DTA data demonstrate the incorporation of copper(II) and oxovanadium(IV) complexes on amino-modified SBA-15. ICP-AES, SEM-EDX combined with XPS data further show the different anchorage status of copper(II) and oxovanadium(IV) species on amino-modified SBA-15. The copper(II) Schiff base complex is anchored through the coordination of copper atom with the nitrogen atom of the amino group modified on the SBA-15 external surface. However, the oxovanadium(IV) Schiff base complex is covalently anchored on SBA-15 interal surface via the condensation reaction of the chloromethyl group of the Schiff base with the amino group from modified SBA-15 matrix.All copper and oxovanadium catalysts are active for styrene oxidation. It is found that both heterogeneous copper(II) and oxovanadium(IV) catalysts are more active than their homogeneous analogues and the product selectivity varies in cases of different oxidants, due to site-isolation and cooperative effect between the SBA-15 support and the anchored metal complexes. The supported oxovanadium catalyst shows high selectivity to styrene oxide with air as the oxidant and better selectivity to benzaldehyde when using H₂O₂ as the oxidant. Moreover, the heterogenized oxovanadium catalyst is significantly stable and can be recycled many times. The excellent performance of heterogenized oxovanadium catalyst can be attributed to the homogeneity of active species and its covalent bonding to the interior mesoporous channels.3. Tethering of Cu(II), Co(II) and Fe(III) tetrahydro-salen and salen complexes onto amino-functionalized SBA-15: Effects of salen ligand hydrogenation on catalytic performances for aerobic epoxidation of styreneCu(II), Co(II) and Fe(III) tetrahydro-salen and the corresponding salen complexes have been successfully covalently tethered onto amino-functionalized SBA-15, as evidenced by FT-IR, UV-vis spectroscopy, N₂ adsorption/desorption and TG/DSC techniques. XRD, SEM and N₂ adsorption/desorption data show the characteristic channel structures of the support SBA-15 remain intact and accessible after tethering of metal complexes. ICP-AES, TG/DSC and XPS data demonstrate the location, anchorage status, and loadings of active species.Both tethered and untethered copper or cobalt tetrahydrosalen complexes show much higher activity in aerobic epoxidation of styrene than their corresponding copper or cobalt salen complexes due to the improved electronic environment around metal ions and the intriguing oxidation ability. However, tethered iron catalysts possesse an opposite trend due to the easily formed dinuclear iron species resulting in activity decline. The catalytic performances of the M-[H4]Salen-SBA are also closely related to the type of transition metals because of their different redox nature. Moreover, the heterogenized catalysts, e.g., Co-Salen-SBA, Co-[H4]Salen-SBA and Fe-Salen-SBA, can be recycled without significant loss of catalytic activity and selectivity. However, only Co-Salen-SBA and Co-[H4]Salen-SBA show good recoverability and relatively high stability against leaching of active species.4. Preparation, characterization of heterogenized Fe(III) Schiff base complexes: the relationship between electronic effect and catalytic performancesA series of hybrid mesoporous SBA-15 materials containing four iron(III) Schiff base complexes is synthesized by a post-grafting route. The XRD, N₂ adsorption/desorption, TEM, FT-IR, UV-Vis spectroscopy, ICP-AES and TG/DSC data demonstrate the successful anchoring of iron(III) Schiff base complexes over the modified mesoporous support. It is found that the catalytic activity increases by the decrease of the electro-donating ability of Schiff bases and the selectivity varies according to the types of substituents in the ligands.5. Iron(III), cobalt(II) and copper(II) complexes bearing 8-quinolinol encapsulated in zeolite-Y for the aerobic oxidation of styreneFe(III), Co(II) and Cu(II) complexes of 8-quinolinol are successfully encapsulated in the supercages of zeolite-Y by a flexible ligand method, as evidenced by FT-IR, UV-vis spectroscopy, N₂ adsorption/desorption techniques. The loadings of metal and ligand are quantified by ICP-AES, TG/DSC and CHN elemental analysis, and the internal location of active species is also demonstrated. The structural integrity of zeolite throughout the encapsulation procedure is confirmed by XRD and SEM studies.These encapsulated metal complexes display better activity than their corresponding homogeneous analogues due to the site-isolation as well as the synergism effect between the exchangeable metal ions and encapsulated metal complexes. Leaching tests confirm only FeQ3-Y is stable against leaching of active species into solution and can be recycled without significant loss of activity and selectivity within successive runs. Moreover, the selectivity to benzaldehyde is higher than to styrene oxide in all cases due to the acidic nature of zeolite matrix.6. Improved olefin epoxidation performances of discrete bis(8-quinolinol)oxovanadium(IV), dioxomolybdenum(VI), dioxotungsten(VI) covalently attached on SBA-15 by a metal-template/metal-exchange methodThe incorporation of oxovanadium(IV), dioxomolybdenum(VI) and dioxotungsten(VI) centers into an immobilized, site-isolated, bis(8-quinolinol) coordination sphere on SBA-15 is fulfilled by a newly-developed metal-template/ metal-exchange method to control the distribution of covalently attached independent ligands. All templated metal complexes possess fixed composition and structure, which is confirmed by ESR or elemental analysis. These templated heterogeneous catalysts are more reactive, selective and efficient for olefin epoxidation with TBHP than either their homogeneous or randomly grafted analogues. Furthermore, the templated oxovanadium(IV) catalyst has a greater substrate scope than tempated dioxomolybdenum(VI) and dioxotungsten(VI) catalysts.7. Preparation and characterization of periodic mesoporous organosilicas with bis(8-quinolinolato)dioxomolybdenum(VI) inside the channel wallsNovel periodic mesostructured organometallic silicas of MCM-41 type bearing homogeneously distributed bis(8-quinolinolato)dioxomolybdenum(VI) inside the channel walls are synthesized via a convenient one-pot method. These PMOs materials are with well-ordered hexagonal arrays of 2D mesoporous pores, evidenced by XRD, TEM, N₂ adsorption and SEM. Various spectroscopic characterization data demonstrate that organometallic groups have been introduced into the framework. The active species are quantified by XPS, ICP-AES and CHN elemental analysis. The hybrid material MoO2Q2@PMO-6% obtained when the molar ratio of TEOS to the bis-silylated bis(8-quinolinolato)dioxomolybdenum(VI) complex is 1: 3 in the synthetic gel possesses excellent catalytic performance in the epoxidation of cyclooctene, which may be ascribed to its uniform spherical morphology, well- defined order, and massive mesopores embedded in the framework facilitating the mass transfer.