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Synthesis, Characterization and Catalytic Performance of Heteroatom-substituted Mesoporous Aluminophosphate Molecular Sieves

Author: LiXinZhu
Tutor: YangShuLin
School: Nanjing University of Technology and Engineering
Course: Chemical Engineering and Technology
Keywords: hydrothermal synthesis mesoporous aluminophosphate molecular sieve heteroatom framework structure characterization catalytic properties
CLC: O643.36
Type: PhD thesis
Year: 2013
Downloads: 143
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Abstract

The mesoporous aluminophosphates represent a family of molecular sieves that are constituted of PO4(+) and AlO4(-) tetrahedral having an electronically neutral framework. Upon incorporation of heteroatoms, for example silicon or transition metal elements, into the framework of an aluminophosphate molecular sieve, silicoaluminophosphate or metalaluminophosphate which have acid sites or redox-active sites due to the replacement of phosphorus or both phosphorus and aluminums by silicon or transition metals can be obtained. The substitution of phosphorus or both phosphorus and aluminium also provides these molecular sieves with potential catalytic applications. Compared with microporous materials, the mesoporous aluminophosphates have higher specific surface area, larger pore size and narrower pore size distribution. These materials have showed potential application prospect in dealing with even large bulky molecules, such as hydrogenation, dehydrogenation, reformation, crack, isomerization, cyclization and oxidation, etc. The mesoporous aluminophosphates are thermally unstable compared with microporous aluminophosphates and mesoporous silicon-based materials. Due to many influence factors in such kind of synthetic method, the synthesis process is empirical and uncontrollable. This dissertation is devoted to improving the thermal stability and investigating the application of mesoporous aluminophosphates.The mesoporous aluminophosphates were synthesized by hydrothermal crystallization. Based on the experiments of crystallization time, crystallization temperature, amount of tetramethylammonium hydroxide (TMAOH), aluminium/phosphorus molar ratio, different templates and template removal methods etc., the effects of these methods on property and structure of mesoporous aluminophosphate molecular sieves were carefully researched. Moreover, the optimum synthesis conditions were determined by X-ray powder diffraction (XRD), fourier transform infrared spectrophotometery (FT-IR), thermogravimetric and differential thermal analysis (TG-DTA), Brunner-Emmett-Teller method (BET), etc. Based on the above mentioned experiments, the influences of transition metal elements (iron, cobalt, chromium) on structure of mesoporous aluminophosphates and existent form of transition metal elements were investigated. The catalytic performances of mesoporous aluminophosphates were also studied.From the obtained characterization results, it could also be concluded that both the framework and extraframework iron species (iron oxide) were together present in the iron -substituted mesoporous aluminophosphates (Fe-MAPs), and most iron ions (Fe3+) in tetrahedral coordination were incorporated into the Fe-MAPs. The catalytic performances of Fe-MAPs for hydroxylation of phenol were studied. The results indicated that the framework Fe3+species were active centers, and the catalytic performance of framework Fe3+species was higher than that of extraframework Fe3+species. The extraframework Fe3+species (mainly existing in oxide aggregates or clusters) could lead to the invalid decomposition of hydrogen peroxide (H2O2). Under the optimized conditions, the conversion of phenol, the selectivity to catechol and the selectivity to hydroquinone reached21.0%,66.8%and31.4%respectively.The catalytic properties of Fe-MAPs were also studied in the wet oxidation of phenol. Fe-MAPs as heterogeneous catalysts were effective for catalytic oxidation of phenol in aqueous solutions with H2O2as oxidant. Under the optimized conditions, the removal of phenol and chemical oxygen demand (COD) reached99.5%,87.9%respectively. Compared with Fenton systems, the concentration of Fe3+in the treated solution was very lower by Fe-MAP, and the possibility of induced pollution caused by the metal ions in the solution was avoided. The catalytic oxidation mechanism of phenol was proposed and verified by the designed experiments. The phenol oxidation reaction proceeds through the radical chain mechanism. The mechanism consists of the following steps.First, H2O2is adsorbed on the surface of the Fe-MAP and reacts with Fe3+to form hydroxyl radical (OH). Then, the benzene ring is attacked by OH to form intermediates, such as benzoquinone, catechol, hydroquinone and resorcinol. These intermediates can react with OH to form fatty acids, such as oxalic acid, acetic acid and fumaric acid. Finally, these fatty acids further react to form carbon dioxide (CO2) and water (H2O).The cobalt-substituted mesoporous aluminophosphate (Co-MAP) was also prepared by hydrothermal synthesis. FT-IR spectrum showed that the peak around700cm-1of Co-MAP was strengthened observably than that of mesoporous aluminophosphate (MAP). Co-MAP was effective for oxidation of styrene with H2O2to benzaldehyde. Under the optimized conditions, the conversion of styrene, the selectivity to benzaldehyde and the yield of benzaldehyde reached42.2%,82.0%and34.6%, respectively.The hydrothermal crystallization processes of chromium-substituted mesoporous aluminophosphate (Cr-MAP) at various temperatures were studied by XRD. From the crystallization curves determined by the relative cyrstallinity of each sample, the apparent activation energy for the nucleation and for crystal growth were calculated by Arrhenius equation, which were63.7kJ·mol-1and14.7kJ·mol-1for Cr-MAP. The catalytic performance of Cr-MAP for selective oxidation of ethylbenzene was studied. The results indicated that the framework chromium ions (Cr5+) were active centers. Under the optimized conditions, the conversion of ethylbenzene, the selectivity to acetophenone and the yield of acetophenone reached72.8%,85.4%, and62.2%, respectively.

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