Synthesis, Characterization And Catalytic Properties Of Heteroatoms Substituted Aluminophosphate Sieves

The direct oxidation of benzene by H2O2 into phenol is one of the two challenging problems in the green chemistry field. This reaction is difficult to occur in mild conditions because of the special stability of benzene molecule. So the high active catalyst is required. The heteroatoms substituted aluminophosphate sieves with AFI topology structure (M-AlPO₄-5) show good catalytic activities for the target reaction because of their special channel structure and the various heteroatoms with various functions. In this dissertation, various M-AlPO₄-5 molecular sieves were synthesized and their catalytic activities were tested. At the same time, their physical-chemical properties were characterized by many spectroscopy methods. Some possible catalytic mechanisms were proposed according to the activities and characterization results. For some types of M-AlPO₄-5, the hydrothermal crystallization process and the influence of synthesis condition on the content of framework-site heteroatoms were specially studied. Based on the above-mentioned experiment results, a possible mechanism for the incorporation of heteroatoms into the framework sites of molecular sieve was proposed. From the above research, the following detailed results have been obtained. 1. Various M-AlPO₄-5 molecular sieves were hydrothermally synthesized, here M=Mg,Si,Ti,V,Cr,Mn,Fe,Co,Ni,Cu,Zn,Mo and Sn. Of all the M-AlPO₄-5 molecular sieves, only Fe-AlPO₄-5, V-AlPO₄-5, Cr-AlPO₄-5 and Cu-AlPO₄-5 showed a certain catalytic activities for the target reaction. 2. Compared with pure AlPO₄-5 sieves, the unit cell volume of V-AlPO₄-5 expended and the skeleton vibration peaks in FT-IR spectra of V-AlPO₄-5 shifted to relative low wave numbers, which indicated that V atoms had incorporated into the AlPO₄-5 frameworks. By researching the expansion and shift extent, the influence of preparation conditions such as the V source, the adding sequence, the input ratio of V source and crystallization time on the content of frameworks-site V atoms were qualitatively researched. To obtain V-AlPO₄-5 sample with the highest content of frameworks-site V atoms, the following synthesis conditions were recommended: V2O5 as the V source rather than VOSO4 and NaVO3, first mixing V₂O₅ and H₃PO₄ then adding AlOOH and Et3N subsequently to the mixture, V/(Al+P)=1/80,crystallization time 48h, crystallization temperature 200℃. The catalytic activities of V-AlPO₄-5 samples for the target reaction showed that the phenol yield and the decomposition of H2O2 increased with the content of frameworks-site V atoms, which indicated that the frameworks V species were the active sites. The solvents effect suggested that the reaction goes through the hydroxyl-radicals mechanism. 3. Cr-AlPO4-5 molecular sieves were synthesized hydrothermally and characterized by XRD and FT-IR. The characterization results implied that Cr had been incorporated into the AlPO4-5 frameworks. IR spectroscopy of adsorbed pyridine showed that both the Lewis and Br?nsted acidic sites on the calcined Cr-AlPO4-5 sample were much increased. And the XPS results indicated that Cr atoms in calcined Cr-AlPO4-5 samples were existed in Cr (V) species with a distorted tetrahedral coordination. The catalytic activities of Cr-AlPO4-5 for the target reaction showed the maximum phenol yield 4.7% and selectivity 99%, which were close to the performance of TS-1 catalyst. A possible reaction mechanism containing framework Cr peroxide species was suggested and an evidence for the existence of the key intermediate species was provided by the in-situ IR spectra of the adsorbed reactant on the Cr-AlPO4-5 molecular sieve. The mechanism consists of the following steps. First, the H2O2 molecule adsorbed on the framework Cr(V) species to form the framework Cr peroxide species, then the benzene molecule adsorbed on the Cr peroxide species and an active oxygen atom in the Cr peroxide species transferred to benzene molecule and inserted into the C—H bond to form phenol molecule. Finally the phenol molecule desorbed from the active center and the Cr(V) species returned to the original status for the next catalytic cycle. 4. The hydrothermal crystallization processes of Cr-AlPO4-5 and Fe-AlPO4-5 molecular sieves at various temperatures were studied by XRD. From the crystallization curves determined by the relative crystallinity of each sample, the apparent activation energy for the nucleation and for crystal growth were calculated by Arrhenius equation, which was 66.5 kJ/mol and 51.4 kJ/mol for Cr-AlPO4-5 and 82.7 kJ/mol and 28.2 kJ/mol for Fe-AlPO4-5, respectively. During the Cr-AlPO4-5 crystallization period, the variation of the mother-gel's pH value with crystallization time was similar with that of the relative crystallinity. This implied that the incorporation of Cr3+ into the AlPO4-5 framework goes through the cross-linked dehydroxylation and polycondensation steps between the Cr(OH)4-, Al(OH)4-and P(OH)4+ hydroxides. For Fe-AlPO4-5 molecular sieve, the changes of unit cell volume showed that most Fe3+ ions had already been incorporated into the framework during the beginning period of the crystal growth stage.5. The influences of mother-gel preparing conditions on the content of framework Fe species in Fe-AlPO4-5 molecular sieve were studied by using orthogonal experiments design and H2-TPR. The three investigated factors were listed in the descending order: Fe3+/ Al2O3 >> R/Al2O3 > template types Based on the orthogonal experiment results, an optimum preparing condition was found to produce the Fe-AlPO4-5 molecular sieve with high framework Fe content and without any extra-framework FexOy species. The detailed condition is as follows: Fe3+/ Al2O3=0.10, Et3N as the template and Et3N / Al2O3=1.2. The activity tests of Fe-AlPO4-5 samples indicated that both framework Fe species and extra-framework Fe species could catalyze the target reaction. But the product selectivity caused by the former was high, while the latter resulted in the much lower selectivity because of its undesirable catalytic activity for the decomposition of H2O2. 6. Cu-AlPO4-5 molecular sieve was synthesized hydrothermally and characterized by XRD, FT-IR, H2-TPR and ESR. The characterization results provided the evidences for the existence of framework Cu species in Cu-AlPO4-5 samples. Then the Cu-AlPO4-5 samples were investigated by SEM, IR spectroscopy of adsorbed pyridine and NH3-TPD to get their detailed physico-chemical properties, such as the morphology, the distribution of the Lewis and Br?nsted acidic sites and the distribution of the strong, medium and weak acidic sites. Based on the H2-TPR and ESR characterization results, it can be speculated that there were three kinds of Cu species – framework, extra-framework and bi-nuclear Cu species – existing in the Cu-AlPO4-5 samples. From ESR results we supposed that there is defect structure in some Cu-AlPO4-5 samples. When the samples were used for the target reaction, the yield of phenol was 6.9% with selectivity 96%. It was supposed that the framework Cu species and the defect structure in the Cu-AlPO4-5 samples are two types of possible active sites for the oxidation of benzene into phenol. 7. The influences of Cu2+ concentrations in the original gel, Et3N/Al2O3 molar ratios and different templates on the contents of Cu species in the framework were investigated by H2-TPR. The results showed that purely increasing the Cu2+ concentrations in the gel could not result in the formation of framework Cu species. When tertiary amines compounds that can coordinate with Cu2+ were used as templates, the formation of the framework Cu species was observed. For Et3N, when its mole ratio was in the range of 1.40>Et3N/Al2O3>1.20, the framework Cu speciescould generate. A mechanism called "the polycondensation rates matching mechanism" was supposed to describe that how heteroatoms (including Cu2+) were incorporated into the framework sites of molecular sieves. From a well-designed experiment we found that it was the much faster polycondensation rate of Cu(OH)2 than Al(OH)4-and P(OH)4+ that resulted in the generation of the extra-framework copper oxide. This could be an experimental evidence for the above mechanism. 8. According to the above mechanism, we tried some methods to improve the content of framework Cu species. We found that by increasing the crystallization temperature and adding HF to the original gel could not slow down the polycondensation rate of Cu(OH)2 to match that of Al(OH)4-and P(OH)4+. This may be caused by the great difference between their polycondensation rates. We also found that if an additive that could coordinate with Cu2+ was added to the original gel, the difference between the their polycondensation rates could be greatly reduced and the effect of the additives depended on its cumulative stability constant (i.e. lgKf ) with Cu2+. When the lgKf of the additive was close to the second-level cumulative stability constant (i.e. lgKf2=13.68) of Cu(OH)2, the framework Cu species started to generate. When the lgKf of the additive located between the lgKf2 of Cu(OH)2 and the fourth-level cumulative stability constant (i.e. lgKf4=18.3) of Cu(OH)42-, the content of framework species increased further and the amount of extra-framework copper oxide decreased remarkably. 9. With the aid of Elements Periodic Table, the influences of the radius, the valence and the coordination status of heteroatoms on their possibilities to be incorporated into molecular sieve's framework were studied. For those heteroatoms which can be incorporated into framework, their radius was in the range of 0.55～0.91?, especially in the range of 0.60～0.80?. Their valence could be +2,+3,+4,+5, but +3 and +4 was much more popular. Finally, of all the coordination status, the tetrahedral coordination was dominant with a few special examples of octahedron coordination.