Formation Of Zeolite Membrane Research Articles

A facile preparation of high flux NaA zeolite membranes by direct secondary growth of wet gel layer

NaA zeolite membranes were first applied in industrial organic solvent dehydration in 1999. However, issues such as poor reproducibility, low flux, and high cost of NaA zeolite membrane preparation have hindered its industrial progress. In this work, a direct secondary growth of wet gel layer method is employed. Specifically, a wet gel layer is coated to the outer surface of the seeded support, which is then placed into the Teflon-lined autoclave without additional hydrogel inside. Under the conditions of conventional secondary growth (100 °C for 4 h), high flux NaA zeolite membranes can be prepared. The pervaporation performance in the 90 wt% ethanol/water solution is tested at 75 °C, showing the flux up to 4.59 kg m−2 h−1 and the separation factor above 10,000. The controlling factors involved in the preparation of high flux NaA zeolite membranes using the direct secondary growth of wet gel layer are investigated. It is found that the introduction method of the seed layer and wet gel layer, the size of the seed, the state of the gel, and the immersion time play significant roles in the formation of zeolite membranes with superior separation performance. This method provides a new idea for the preparation of high performance, low cost and reproducible zeolite membranes with prospects for industrial application.

Hetero-metal solution to construct high permselective zeolite membranes

Zeolite membranes possess great application prospects in purifying chemical substances, however, the current synthetic conditions are insufficient to eliminate defects, restricting the industrial applications of zeolite membranes. In this study, compact X-type zeolite membranes were synthesized in hetero-alkali-metal cation synthesis solutions via in-situ heating method. The obtained X-type zeolite membrane possessed a methanol/methyl tert-butyl ether (MTBE) separation factor greater than 10,000 at a permeation flux of 1.3 kg m−2 h−1. A possible mechanism for the influence of hetero alkali meatal cations on the formation of zeolite membranes was proposed, which involved structural induction and regulation of the rates of crystal nucleation and growth. The hetero-alkali-metal cations entering the zeolite crystal caused distortion of the crystal structure, which promoted transformation of the zeolite crystal structure and accelerated crystal growth. Meanwhile, the rates of crystal nucleation and growth on the surface of the support were regulated by the cooperative action of the hetero-alkali-metal cations and Na+ in the synthesis solution through their solubility for the aluminosilicate gel, leading to the formation of compact zeolite membranes. This method controlled the zeolite crystal structure and compactness of the zeolite membrane by simply adding hetero-alkali-metal cations to the synthesis solution, providing a new idea for designing defect-free membranes with high permselectivity.

Synthesis of least defect NaY membranes: Defining optimum heating and cooling rate of hydrothermal treatment (HT) and application of zeta potential measurement via different seeding methods

Zeolite membrane is one type of microporous inorganic membrane which has gained so much interests among researchers as an alternative especially for gas separation. Among major challenges to produce high quality zeolite membranes with less defects are to have a good seeding and hydrothermal treatment (HT) method. On this matter, a study on the colloidal stability of the zeolite NaY seeds in the seeds solution in order to obtain homogenous seeds layer on top of the alumina support via several seeding methods and pHs of the seeds solutions were investigated. Then, the seeded samples were taken into HT process in order to form zeolite NaY membranes. During this stage, heating and cooling rates ( o C/hr) were varied in order to study their effects towards formation of zeolite membrane with least defects. Characterizations were performed by using X-Ray Diffraction (XRD) in order to identify degree of crystallization and amount of impurities while Scanning Electron Microscopy (SEM) was used in order to study surface morphology of the samples. From this study, it was found that densification effect from different seeding methods played important factor in forming zeolite membranes with least defects. Accordingly, suitable heating and cooling rates were required in order to optimize the growths of zeolite NaY seeds to form zeolite layer on top of the alumina support. In this case, due to thermal mismatch between the substrate and zeolite material, it could form some cracks on top of the alumina support. Thus, by implementing optimum heating/cooling rate, a zeolite NaY membrane with least amount of defects i.e. cracks and pinholes could be achieved with less degree of impurities i.e. zeolite NaP during HT process.

Synthesis of NaA zeolite membrane with high performance

In recent years, zeolite membranes have been hotly investigated due to their potential to be combined in reaction/separation devices such as membrane reactors and chemical sensors, and to achieve gas separation under steady-state operation [1–4]. Molecules with different sizes and shapes can be discriminated or separated by zeolites through their channels. A-type zeolite, due to the pore size (0.3–0.5 nm) of its channel system, can be selective for small molecules, such as H2, O2, N2, CO2 and H2O from other molecules or from each other. Aoki et al. [5] formed zeolite A-type membranes on a porous support tube, and obtained good results for gas separation, of which the permeance of H2 permeance was about 10−7 mol · s−1 · m−2 · Pa−1. Recently, we [6–8] reported the preparation of zeolite NaA membranes by multistage synthesis, the results of pure gas separations were very good, while the permeance of H2 was also about 10−7 mol · s−1 · m−2 · Pa−1. As mentioned above, zeolite NaA membranes usually have a good separation factor, but the permeance is too low for practical applications [9, 10]. The relationship of the permeance and the permselectivity of the membrane is a trade-off, which means to get a high permeance, the permselectivity has to be sacrificed to a certain extent, and reverse also affirms we have to sacrifice the permeance to improve permselectivity. Thus one of the challenges in the field of zeolite NaA membrane is to prepare zeolite membranes with high permeance, while maintaining high separation selectivity. The zeolite membrane is composed of three parts: the zeolite crystal layer, the intermediate layer and the substrate [11]. The intermediate layer is composed of the substrate and the zeolite crystals in the pore of the substrate. The permeance of the membrane is mainly controlled by the thickness of the zeolite layer and the intermediate layer. In order to increase the permeance of the membrane, it’s necessary to reduce the thickness of these two layers. As reported, an active mesoporous layer can assist the formation of zeolite membrane [12], thus possibly forming a continuous but thin zeolite membrane in a shorter time of reaction. And during the formation of the zeolite membrane, the reaction bulk may penetrate into the substrate to form a thick intermediate layer. In this paper, combining these two factors together by adding a more active mesoporous layer on the macroporous substrate before synthesis, we successfully synthesized zeolite A-type membrane with high permeance and gas selectivity (Fig. 1). The initially porous α-Al2O3 substrate used was made by casting technology, with 30 mm in diameter, 3 mm in thickness, 0.3–0.5 μm in pore radius and ca. 50% porosity. One face of the substrate was dipped into a solution of 1.0 M of AlOOH with 1 wt% of PEG400 and 2 wt% of PVA-72000. The dipping time was about 9–12 s. Then the substrate was dried at room temperature for 24 h, heated to 700◦C at a rate of 0.3◦C/min, held at 700◦C for 3 h and cooled down at 0.5◦C/min. The dipping-calcining process was repeated to the same face of the substrate for 3 times to form a continuous thin mesoporous top layer. Then the modified side was coated with NaA zeolite crystals as nucleation seeds. The synthesis mixture was prepared by mixing sodium aluminate, water glass, sodium hydroxide, and water under vigorous stirring for 24 h to form a homogenous gel with molar composition of 3Na2O : 2SiO2 : Al2O3 : 200H2O. After the precursor was poured into a stainless steel autoclave, the substrate was put vertically in the synthesis mixture supported by a Teflon holder, with the unseeded face protected from the reaction bulk. The autoclave was transferred into the oven preheated at 90◦C and reacted for 12 h. After the reaction, the membrane was cooled down, washed in water, and dried at 150◦C for 3 h. Finally, the formation of the zeolite membrane was confirmed by X-ray diffraction (XRD) using a Ragaku D max/b powder diffractometer with Cu Kα (λ = 0.154 nm) radiation and operating at 40 kV and 100 mA. The surface morphologies of the membrane were observed with scanning electron microscopy (SEM) on a JEM-1200E scanning electron microscope. The integrity of the membrane was evaluated by gas permeation tests. The membrane was sealed in a permeation module with the zeolite membrane on the high-pressure side. The gas permeances of the membrane were measured by a soap-film flowmeter under