| Metal-organic frameworks(MOFs)are porous hybrid materials built from metal ions or clusters with organic ligands through coordination bonds.In the field of heterogeneous catalysis,MOFs are one of the most promising catalysts due to their high specific surface areas,high density of catalytic active sites,highly designablecatalytic groups,as well as easy recovery.Moreover,the regular pore structures and the diversity of pore cavities of MOFs allow the screening of size,shape and functional groups of the substrates,thus improve the selectivity of catalytic reactions.2,2,6,6-tetramethylpiperidinyl-1-oxy(TEMPO)and its derivatives are a class of small molecular catalysts bearing stable nitrooxide radicals,which have been demonstrated excellent catalysts for the aerobic oxidation of alcohols benefiting from mild reaction conditions,high efficiency and selectivity.In recent years,heterogeneous catalytic systems with the combination of TEMPO and MOFs have been reported.Nevertheless,the synergetic catalytic mechanism between MOFs and TEMPO remains unclear due to the lack of rational design of TEMPO molecules.In this thesis,the incorporation of acetylamino and isonicotinamide groups on the p-position of the TEMPO by amide reaction results in the formation of Acet-TEMPO and iso-NTA-TEMPO,respectively.The catalytic activities of the iso NTA-TEMPO,Acet-TEMPO and TEMPO toward the oxidation of benzyl alcohol to benzaldehyde were studied by the employments of Fe-MOFs,Cu-MOFs and Cr-MOFs,as co-catalysts.The corresponding synergetic catalytic mechanism is proposed.The main research contents and results are as following:1.High specific area and high crystallinity of HKUST-1 was prepared by a solvent-induced crystallization method,and the morphology and defects of HKUST-1 were controlled by modulators(e.g.,hexadecyltrimethylammonium bromide,benzoic acid,acetic acid,polyvinyl pyrrolidone and pivalic acid).The catalytic actvitiy of TEMPO/HKUST-1 for the selective oxidation of alcohols was systematically investigated.The results showed that the catalytic activity of the HKUST-1/iso-NTA-TEMPO system was much higher than that of the HKUST-1/TEMPO system.The high catalytic activity of HKUST-1/iso-NTA-TEMPO can be attributed to(1)the coordination of iso-NTA-TEMPO to the Cu(II)site of HKUST-1 increases the effective concentration of TEMPO radicals in the frameworks,and(2)the HKUST-1 prepared by template-assisted solvent-induced crystallization method contains significant defects,whichfacilitates the diffusion of iso-NTA-TEMPO,substrates and products.HKUST-1/iso-NTA-TEMPO co-catalytst show excellent catalytic activity for para-substituted aromatic alcohols,secondary aromatic alcohols,and heterocyclic alcohols.HKUST-1 prepared by different methods showed significant differences in the selectivity of the products obtained in Knoevenagel condensation reaction of benzaldehyde with malononitrile.When HKUST-1 prepared by hydrothermal method was used as a catalyst,the main product of the above reaction was benzoic acid,while the main product became to 2-benzylidenemalononitrile when HKUST-1prepared by template-assisted solvent-induced crystallization was used.The characterization results of the above two types of HKUST-1 by nitrogen adsorption and desorption experiments,X-ray diffraction,and electron scanning electron microscopy showed that the HKUST-1prepared by the template-assisted solvent-induced crystallization method had more metal node defects,which facilitated the activation of malonononitrile methylene,thus allowing the Knoevenagel condensation reaction to proceed smoothly;while the HKUST-1prepared by the hydrothermal method showhigher activation of benzaldehyde,and led to its oxidation to benzoic acid.For the one-pot tandem reaction of benzyl alcohol with malononitrile catalyzed by HKUST-1/iso-NTA-TEMPO,2-benzylidenemalononitrile was the main product and no benzoic acid was detected,but the conversion of this tandem reaction was low(19%).By stepwise reaction,benzyl alcohol was first completely converted to benzaldehyde catalyzed by HKUST-1/iso-NTA-TEMPO,after which malononitrile was added to continue to participate in the reaction,and the product of benzaldehyde reacting with malononitrile was found to be only 2-benzyl allyl malononitrile with 47% conversion.The above one-pot tandem reactions or stepwise reactions all suffer from low conversion of benzaldehyde,which may be attributed to the fact that iso-NTA-TEMPO occupies part of the Cu(II)vacant coordination site of HKUST-1,resulting in the reduction of the subsequent malonylidene activation site.2.MIL-101(Fe)and MIL-101(Cr)were synthesized by the solvothermal method.The catalytic activitity of MIL-101(Fe),HKUST-1 and MIL-101(Cr)together with iso-NTA-TEMPO,Acet-TEMPO and TEMPO,was investigated by using the conversion of benzyl alcohol to benzyaldehye has a model reaction.Particularly,the effect of functionalities(e.g.,pyridyl group of iso-NTA-TEMPO)of TEMPO molecules for the catalytic activity was discussed.Compared with TEMPO radical,iso-NTA-TEMPO show the highest catalytic activity,followed by Acet-TMEPO.When the above three homogeneous systems were introduced into the co-catalytic system consisting of MIL-101(Fe),HKUST-1 and MIL-101(Cr),it was found that the catalytic activity of the co-catalytic system consisting of MIL-101(Fe),HKUST-1 and MIL-101(Cr)with Iso-NTA-TEMPO was higher than that of the Acet-TEMPO and TEMPO systems.Secondly,the pyridine group of iso-NTA-TEMPO was anchored to the MOFs empty coordination metal sites through coordination,which increased the effective concentration of radicals in the pore of MOFs and improved the catalytic activity.The catalytic kinetic study showed that the order of catalytic activity was MIL-101(Fe)/iso-NTA-TEMPO>HKUST-1/isoNTA-TEMPO>MIL-101(Cr)/iso-NTA-TEMPO,and the MIL-101(Fe)/iso-NTA-TEMPO system was able to convert various para-substituted benzyl alcohols,heterocyclic alcohols and secondary aliphatic alcohols to the corresponding aldehydes or ketones.Finally,the catalytic mechanism of the Fe(II)/Fe(III)redox pair participating in the catalytic cycle to enhance the catalytic performance of the system was proposed using the MIL-101(Fe)/iso-NTA-TEMPO catalytic system as an example.3.MIL-101(Cr)@iso-NTA-TEMPO was prepared by postsynthetic modification(PSM).The open metal site of MIL-101(Cr)allows the grafting of iso-NTA-TEMPO.The structure,morphology and the content of iso-NTA-TEMPO were characterized by XRD,SEM,FT-IR,BET,TGA,EPR,UV-Vis and elemental analysis.The results showed that the morphology and crystallinity of MIL-101(Cr)grafted with iso-NTA-TEMPO did not change,but the specific surface area decreased;UV-Vis spectroscopy and elemental analysis showed that the content of iso-NTA-TEMPO was 0.92 mmol/g.At 80 oC,using trifluorotoluene as solvent,MIL-101(Cr)@iso-NTA-TEMPO was able to completely oxidize benzyl alcohol to benzaldehyde with high selectivity and high efficiency.Interestingly the MIL-101(Cr)@iso-NTA-TEMPO show higher catalytic activity than their simple mixtures(MIL-101(Cr)+iso-NTA-TEMPO).The MIL-101(Cr)@iso-NTA-TEMPO system enable the oxidation of a broad range of alcohols,including para-substituted benzyl alcohols,secondary aromatic alcohols,heterocyclic alcohols and straight-chain alcohols to the corresponding aldehydes or ketones by appropriately extending the reaction time.However,the conversion of MIL-101(Cr)@iso-NTA-TEMPO system reduced to 57% after the second recycling,meaning that this catalyst is poorly recyclable. |
