Structural Construction Of Octahedral Molecular Sieves And Their Photocatalytic Performance

Traditional porous materials are comprised mainly by the tetrahedral framework or a variety of mixed coordination polyhedra. The framework is often coordinated with tetrahedron of P, Si or Al, but porous materials with octahedral framework structure are rare. To find high-efficient redox catalysts, considerable attempts have been focused on incorporation of multivalent transition metal ions into microporous framework materials. However, as far as the structural properties are concerned, such as ionic radius, ionic charge, and coordination preference, transition metal ions are quite different from those main group elements in zeolite materials. Therefore, the isomorphous incorporation of transition metal ions into zeolite materials is quite difficult.Octahedral molecular sieve materials provide a way of accommodating the metal ions that can only be six-coordinate into microporous materials. These metal ions are transition metal element with unpaired electrons and variable valence, so such materials may show characteristics of unique oxidation and reduction, ion exchange and electromagnetic properties of light. Meanwhile, octahedral molecular sieve can make up the weakness of conventional Si-Al zeolite and aluminophosphates'photocatalytic performance. With the outstanding characteristics of transition metal on the induction of visible light, octahedral molecular sieve can be applied to solar power to achieve the effective coupling of shape-selective molecular sieve effect and the photocatalytic efficiency.We have synthesized metal incorporated PKU-1 molecular sieves and their photocatalytic performance was investigated for the first time. PKU-1 is a three-dimensional pore structure of the octahedral molecular sieve compounds, boric acid in the root structure of the balance has played a negative charge and act as the important role of template. Son of some transition metals can not form the structure of PKU-1, but can be replaced with aluminum to form a product. Fe-PKU-1 was synthesized by using boric acid as the flux without using any organic template as described in previous reports, and characterized by XRD, SEM, IR, UV-vis, XPS. Their catalytic activity was performed by photocatalytic reduction of nitrate ions. The as-synthesized Fe-PKU-1 samples have a needlelike hexagonal prismatic morphology of about 1μm in diameter with a deep brown color a. The XRD spectrum show that the lattice parameters such as unit cell volume, a-axis and c-axis calculated by Terror 90 program increase with the incorporated Fe content. The expansion of the unit cell parameters is consistent with the larger crystal radius of Fe3+ (0.785 ?) than Al3+ (0.675 ?). Under the given conditions, non-incorporated PKU-1 and Cr-PKU-1 did not show any photocatalytic activity for nitrate reduction. But for Fe-PKU-1, the nitrate conversion was increased with the incorporated Fe content. These results are interesting and can be well interpreted by experimental proofs of UV-vis spectra and XRD patterns.PKU-1 is an inorganic aluminoborate compound and belongs to nonconductor, its forbidden band is so wide that d-d transition can not occur under the irradiation of ultraviolet radiation and visible light, so it is not strange undoubtedly that unloaded PKU-1 is no response to UV-Vis light. The synthesis and physiochemical properties of Cr-PKU-1 have been reported in our previous paper, and it proves that Cr-PKU-1 is an excellent redox catalyst for selective oxidation of styrene. However, Cr-PKU-1 sample didn't show any photocatalytic activity for catalytic reduction of nitrate although it has two strong characteristic absorption peaks in the region of visible light. The reason for lack of photocatalytic activity for Cr-incorporated PKU-1 probably originates from wide bandgap between valence band (VB) and conduction band (CB) of chromium oxide (about 3.50 eV for Cr2O3), so Cr-PKU-1 is a poor conductor and photogenerated electrons have no enough energy to jump into conduction band from valence band. Different from the Cr-PKU-1, Fe-PKU-1 series samples show excellent photocatalytic performance for nitrate reduction, which may stem from narrow bandgap between valence band (VB) and conduction band (CB) of iron oxide (about 2.20 eV for Fe2O3 and 2.40 eV for FeO). It is more important that Fe2+ and Fe3+ may coexist in Fe-PKU-1 samples so that metal Fe with mixed valence state can act as active centre of redox reaction. Moreover, photocatalytic activity for Fe-PKU-1 is variable with the increasing of Fe content in the PKU-1 sample, and the variety rule is well accordance with the content of loaded Fe in PKU-1 shown in the XRD pattern, we can infer that Al atom substituted isomorphously by Fe atom in PKU-1 is restricted and the up-limit boundary reaches in the sample Fe-PKU-1 (D), which has maximum value for nitrate conversion. When the Fe content put in the reaction system exceeds the maximum amount incorporated in the PKU-1 framework (Fe-PKU-1 (D) ), the impurity phases appear and therefore result into the decline of catalytic activity.In this article two different phases (pyrochlore type and perovskite type) of lead titanates were synthesized by hydrothermal method. The studies show that the type of product depends on the pH of the water heat system. There exists mounts of lead space in pyrochlore type lead titanates, in which lead is present in bivalent and tetravalent state, while without lead absence in perovskite type ead titanates. Studies on the Photocatalytic activity of PbTiO3 show that, under suitable condition, such catalyst has some catalytic activity for photocatalytic degradation of phenol. When the amount of catalyst is 25 mg/100 mL and phenol concentration is 100 mg / L, obtained pyrochlore typ PbTiO3 has good photocatalytic activity under the experimental conditions.