Preparation And Properties Of Metal-Organic Framework Menbranes And Zeolite Membranes

Metal-Organic Framework (MOF) is a novel kind of crystalline porous materials, which has regular channels with controllable size, internal surface and connectivity. The pores in MOFs could always be designed or tailored to adsorb or exclude some gases, and overcome the limits of pore size such as zeolites. Due to its excellent sorption properties and potential applications in gas storage, optics, catalysis and sensors, MOF has attracted much interests and attention all over the world. Because of the interactions between the internal surface of MOFs and guest molecules, MOFs can be used to sense ions, amines, alcohols, and other organic molecules by its fluorescence properties. Usually, sensors should be made films for practical applications because of its good mechanical playability and some properties like single crystals, which causes the huge interests to preparation of MOF films. Appropriate assembly of MOF crystals into films can impart the selective mass transport and storage behaviors, like inorganic zeolite films which have been widely explored for sensor applications. However, for inorganic zeolite films, their small pore diameters and low porosities together with difficulties in functionalizing the pore walls have largely limited their applications as efficient sensors. In contrast, the versatility of fine-tuned dimensions and compositions of MOFs may overcome those limitations and pave a way for the preparation of supported MOF films as a new type of sensors. Nonetheless, the synthesis of large-scale uniform and continuous thin MOF films remains a big challenge.We have successfully prepared the large-scale continuous thin Cu3(BTC)2 film supported on a copper slice. The film has high quality continuation and homogeneity through the characterization of XRD and SEM, and is also very thin with a thickness of about 1μm. To achieve a large-scale continuous high quality thin Cu3(BTC)2 film on the copper slice, three key factors should be considered. Firstly, the surface of the substrate must be polished with a-Al2O3 powder to remove the oxide to ensure a very smooth surface, since the surface roughness plays an important role in the formation of continuous films. Secondly, the copper source should be carefully selected to avoid the oxidation of the polished copper surface. To this point, copper nitrate which has been mostly used for the preparation of Cu3(BTC)2, can not be employed, and we did observe some spots on the MOF film due to gas generation in the redox reaction when using copper nitrate as the copper source. Therefore, we selected cupric acetate as the copper source. Thirdly, since water could also oxidize the polished copper surface, the water content in the solvent for MOF film growth should be minimized.After successfully constructing the MOF-based thin film, we investigated its response to various humidity concentrations including response time, sensitivity, hysteresis loop, measuring range, repeatability, stability, temperature constant and selectivity etc. after activation. The film shows excellent humidity sensing properties by measuring capacitance. The capacitance changed in different humidity concentrations, showing high sensitivity. The resulting sensitivity is with a good linearity coefficient of 0.992. Meanwhile, the film shows quick response and good repeatability and stability. At present, most research about MOF film aims at gas separation. The presented study paves a way to explore MOF-based thin films as a new type of sensor for practical applications.The physical and chemical properties of film-based materials depend not only on their chemical composition, but also on their homogeneity, continuity and thickness which is mostly due to the morphology and orientation of crystals in the films. Therefore, the elaborate control of the crystal morphology and orientation would be very indispensable for microporous material films. Some attempts have been made to prepare oriented films or membranes on modified supports being used in various applications mediums. However, the progress is still very limited. To synthesize continuous MOF membranes with unique morphology and orientation, several strategies are reported including in-situ growth, microwave irradiation, secondary growth and chemical solution deposition process. Among these strategies, the secondary growth was commonly used approach, because the nucleation and crystal growth processes could be separated and controlled. It has already been demonstrated to be a simple, practical and efficient process for the fabrication of thin films. With these merits of this approach, herein, we find a general and facile synthetic route to the syntheses of continuous MIL-68(In) films with controllable morphology and orientation.We prepared continuous MIL-68(In) films on the substrate single crystalline silica via secondary growth. The morphology of MIL-68(In) seeds was investigated by SEM, TEM and AFM. It showed that the average size of MIL-68(In) seeds is about 80 nm. The crystal morphologies in the film were controlled by the compositions of initial reaction solutions and the time of crystallization. Additionally, different factors on the crystal morphologies of the obtained MOF films were studied in details. When the concentration of mother liquor was decreased, crystals with hexagonal faces were obtained. Based on the changes of size and morphology, it is suggested a fairly linear growth rate of crystals during the crystallization of films relative to solvent contents (namely the concentration of mother solution). It is assumed that when the concentration of the solution for the secondary growth of films was low, the nutrition of the film growth was reduced, leading to slow nucleation and crystallization rates. The MIL-68(In) film could be prepared between 85℃to 160℃, indicating that the film can be facilely synthesized in a large range of temperatures. Once pyridine was introduced, the growth along c-axis was suppressed, and meanwhile the growth rates along other directions were accelerated. The crystal size becomes much smaller with more addition of pyridine. In this way, pyridine is much efficient agent for modulating the crystal growth. Infrared spectroscopy was monitored to shed light on the interaction between the crystals and pyridine molecules and verified this hypothesis. We have also studied the effect of organic amines like TEA and piperidine. Thin pillar-like crystals with the lengths of 1-2μm were formed on the silicon slice when TEA was used. When the organic amine was switched to piperidine, it was found that the crystals appeared more or less the same as the seeds, even if solvothermal treatment was prolonged. The evolution of crystal orientations has been briefly discussed and a possible growth mechanism was proposed based on the observations of SEM and XRD. Furthermore, the pore structure of the oriented crystals in the film was probed by loaded organic dye (rhodamine 6G) molecules, followed by the fluorescence emission spectroscopy.The preparation and applications of LTA film is a hot research area of zeolite films. So far, there are a lot of research reported. LTA has regular micro pores with 4A, which is similar to many common molecules of raw materials in industry including nitrogen, carbon dioxide, methane, methanol, ethanol and acetone etc.. Moreover, LTA consists with low Si/Al ratio (equal to 1), so it is with strong hydrophilia and can be used to separate molecules with polar and non-polar like water and other organic solvents. In this thesis, we prepared LTA membrane on stainless steel net after pretreatment under two hydro thermal reactions. The pretreatment included the activation of substrate and the dip-coating of seeds. The LTA membrane was characterized by XRD and SEM. We discussed the effect of different pretreatment method of substrate to the prepared LTA membrane in detail. The permeance of various gases through the prepared LTA membrane were measured. The results shows that the permeance of single gases through the LTA membrane increase as H2>O2>N2, which is in accordance with their kinetic diameters (H2 (0.29nm), O2 (0.35nm), N2 (0.36nm)). The ideal permselectivities of H2/N2 and N2/O2 are 6.8 and 4.67 respectively. Their mixture selectivities are 5.94 and 4.14, much high than the Knudsen selectivity (3.74 and 0.96), which demonstrates that the separation process is the diffusion on the internal pore surface and the prepared LTA membrane has high compactness. We have discussed the separation mechanism by the permeation of H2/N2 and N2/O2 mixtures under different temperatures and pressures. The premeance would increase and the mixture selectivities would be decrease when the pressures increase, as well as when the temperatures increase, which demonstrates that there are some defects between crystals.This kind of LTA membrane on stainless steel net has advantages as following:1. Compared to ceramic tubes or plates, stainless steel net as substrate can avoid the nuclear of zeolite in the pores of substrate, which could increase the premeance through the membrane. Meanwhile, it could avoid the dissolution of substrate to effect the ratio of mother solution.2. The stainless steel net is with high mechanical strength and smooth surface, which is helpful to obtain uniform seed layer via dip-coating method. Otherwise, the high mechanical strength of stainless steel net is hardly distorted and can bear a certain degree of pressure.