Synthesis Of AlPO₄-5 Crystals And The Assembly Of Host-guest Materials

Zeolites and related microporous materials are crystalline porous materials, which are widely used as catalysts, adsorbants, and ion-exchange materials. Recently, the assembly of host-guest nanostructured materials using microporous materials as hosts had attracted much attention. AlPO₄-5 molecular sieve crystal is one kind of microporous materials free of Si species, and is the most famous one of microporous aluminophosphate. Its excellent physical properties, plus their unique pore structure, make it an excellent host to fabricate host-guest nanostructured composites. In this thesis, the synthesis of AlPO₄-5 crystals has been studied, large crystals with high optical quality have been synthesized, and the mechanism of crystals growth is also discussed. Single-walled carbon nanotubes are fabricated by pyrolyzing the organic template in the channels of AlPO₄-5 crystals. DR1, pNA and iodine have been assembled in the channels of AlPO₄-5 crystals respectively, and the study on the main characteristics of these nanostructured host-guest materials are also carried out.In chapter 1, we have reviewed the improvement of porous materials, focusing on the structures, synthesis and main applications of zeolites and related microporous materials. The fabrication of nanostructured materials using templated method is introduced, and the merits of assembly nanostructured host-guest materials using zeolites and related mocroporous materials as host are also illustrated.An AlPO₄-5 crystal consists of one-dimensional 12-ring channels (inner diameter 0.73 nm) which are packed parallel to the c-axis. Host-guest nanostructured materials based on AlPO₄-5 crystals have potential applications in many fields. The main obstacle that restrict these applications is the obtainment of large optically clear AlPO₄-5 crystals. In chapter 2, the influence of various parameters on the synthesis of AlPO₄-5 crystals by using tripropylamine (TPA) as template are studied systematically. The experimental results indicate that, the gel composition, the crystallization temperature, and the crystallization duration are the important parameters that affect the size and quality of as-synthesized AlPO₄-5 crystals. Using gels with excessive tri-isopropylate aluminum and high HF content, under the higher synthesis temperature and shorter crystallization duration, we have successfully synthesized the large optically clear AlPO₄-5 crystals, and the largest crystal have a size of 1.25mm×0.65mm. Observing the surface structure of AlPO₄-5 crystals by SEM and AFM, we discover that the crystal growth mechanism is layer-by-layer. As the AlPO₄-5 crystals are synthesized nonequlibriumly from a heterogeneous system, many defects can be introduced into the framework. The existence of these defects will deteriorate the optical quality of the crystals. Due to the incorporation of large amount of defects, the orientation of the 2-dimensional nuclei will seriously diverse from the crystal facet, this will lead to the bundle growth morphology.In chapter 3, we report firstly the synthesis of AlPO₄-5 crystals using tetrabutylammonium hydroxide (TBAOH) as template, the study on the pyrolysis of organic template in the channels of AlPO₄-5 crystals and the formation of single-walled carbon nanotubes are also carried out. Synthesis of AlPO₄-5 crystals using TBAOH as template is quite difficult comparing with that using TPA as template. Treating the tri-isopropylate aluminum gel solution by ultrasonic, and adding HF solution to adjust the gel pH value are essential for the synthesis of AlPO₄-5 crystals using TBAOH as template. Many centrosymmetric intersecting morphologies were observed in the product. These morphologies were originated from the polar growth of AlPO₄-5 crystals, and were also affected by the properties and content of template. Fourier transform infrared (FTIR) spectra indicate that, the organic template that included in the channels will decomposed into small hydrocarbon molecules when the as-synthesized AlPO₄-5 crystals are heated under the N₂ atmosphere, and with the rising of temperature, the small molecules will carbonize and transform into single-walled carbon nanotubes. We introduced Si species into the AlPO₄-5 crystals framework as substitution atom of P, which can lead to Brφnsted acid sites in the framework, and make the SAPO₄-5 crystals have catalysis effects. The SAPO4-5 crystals framework not only plays an important role as a catalyst in pyrolyzing the hydrocarbon molecules, but also favors the formation of carbon nanotubes in the channels, so that it can dramatically improve the density of carbon nanotubes in the channels.In order to study the SHG effects of host-guest materials based on AlPO₄-5 crystals, in chapter 4, we incorporated the nonlinear chromophores p-nitroaniline (pNA) and Disperse Red 1 (DR1) into the channels of AlPO₄-5 crystals respectively by means of vapor-phase diffusion. Due to the unique absorption mechanism of the polar channels of AlPO₄-5 crystals, the two kinds of polar molecules enter the channels in a same way with the negative nitro group in front, forming the unidirectionally aligned head-to-tail dipole chains in parallel channels. These noncentrosymmetric parallel dipole chains make the host-guest materials have second order nonlinear optical effects. XRD results indicate that, the two kinds of organic molecules were included in the channels of AlPO₄-5 crystals, other than condensed on the surface, and the inclusion of organic molecules has not destroyed the structure of AlPO₄-5 crystals. These two kinds of host-guest materials have a similar optical anisotropy and the absorption band of the molecules in the channels of AlPO₄-5 crystals both red shift comparing with that in methanol. The Raman spectra show that the p-NA molecules within the channels form two phases, one is head-to-tail chains similar to that in the solid crystaline p-NA, and the second is a similar state consists of weakly interacting p-NA molecules as molten p-NA. Second harmonic generation (SHG) experiments reveal that, these two kinds of host-guest materials both have SH effects. By studying the polarization-dependence SH intensity, we discover that different SHG processes occur in the DR1-loaded and pNA-loaded AlPO₄-5 crystals. The SHG of pNA-loaded AlPO₄-5 crystals is linearly polarized with the polarization direction parallel to the c-axis of the crystals, and only the fundamental wave polarized parallel to the c-axis of the crystals can stimulate the SHG The SH intensity (I_p-ex) of the pNA-loaded AlPO₄-5 single crystal versus the polarization angle (α) of the incident fundamental light can be well fitted as: I_p-ex= 3.4cos²Î±. The SHG of DR1-loaded AlPO₄-5 crystal is not linearly polarized. The SHG can be divided into two components: one polarized parallel to the crystal c-axis(p-polarization), another polarized perpendicular to the c-axis(s-polarization). The SH intensities of these two components versus the polarization angle (a) of the incident fundamental light can be respectively fitted as: I_p-ex=0.69cos⁴α+0.37sin⁴α—0.17, and I_s-ex= 0.69cos²Î±sin²Î±+ 0.06. Both the fundamental wave polarized parallel or perpendicular to the c-axis can stimulate the SHG Different SHG processes occurred in pNA-loaded and DR1-loaded AlPO₄-5 crystals, which originate from the different molecular structures of the loaded organics.In chapter 5, iodine molecules have been incorporated into the channels of calcined AlPO₄-5 crystals by using vapor-phase diffusion method. XRD results indicate that the removal of template and the incorporation of iodine have not destroyed the structure of AlPO₄-5 crystals; the iodine is incorporated in the channels other than condensed on the crystal surface. TG results reveal that the desorbing process of adsorbed iodine is quite different from that of adsorbed water. Due to the special capillary condensation sorption mechanism, the quantity of adsorbed water in AlPO₄-5 channels is exceptionally large in comparison with that of other small molecules. Due to the hydrophobic nature of channel wall, the water is easy to escape from the channels, so the mass loss occurs within a narrow temperature range. As an apolar molecule, I₂ molecules can exist in the hydrophobic channels of AlPO₄-5 crystals stably. With the rising of temperature, the I₂ molecules diffuse from the channels gradually, so the mass loss occurs within a wide temperature range. Different loading temperatures lead to different loading density and different iodine structure inside the channels of AlPO₄-5 crystals. With a loading temperature of 393K, TG and XRF results indicate the mass percent of iodine in the iodine-loaded AlPO₄-5 is ca.11.5%, one can calculate that the channels have not compactly filled by I₂ molecules. Under the optical microscope with white linearly polarized light, no obvious optical anisotropy was observed. Polarized Raman scattering indicates that the iodine exist in the channels of AlPO₄-5 crystals as vapor-like I₂ molecules, there are no interactions between the inserted iodine molecules, and the I₂ molecules orientate randomly. Narrow width Raman line and multiple Raman peaks with high intensity that attribute to vapor-like I₂ molecules make the iodine-loaded AlPO₄-5 crystals promising candidates for Raman laser.Finally, I summarize the thesis and propose the future works in chapter 6.