Applications of ceramic membranes in the preparation of nanocatalysts and nanopowders

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This post was last edited by goldliyang on 2021-11-2 at 10:43. A membrane is an inorganic or polymer material with special selective and separation capabilities; it can divide a fluid into two separate parts, allowing one or several substances to pass through while separating out the others. Ceramic membranes used in membrane separation technology are made from high-purity alumina and zirconia, and have been sintered at high temperatures of over a thousand degrees, enabling them to withstand harsh acidic, alkaline, or highly oxidizing environments. They offer advantages such as high separation precision, a narrow pore size distribution, high mechanical strength, good chemical stability, reliable operation, and ease of regeneration. The applications of membrane ceramics membranes mainly involve liquid-phase separation and purification, gas-phase separation and purification, as well as reactors. Applications in liquid phase separation mainly involve two areas: microfiltration with pore sizes ranging from 100 nm to 1000 nm, and ultrafiltration with pore sizes ranging from a few nm to 100 nm. Thanks to the excellent properties of ceramic membranes, they play an important role in the separation of micron, sub-micron, and nanoparticle-sized particles. Coupled with the excellent properties of ceramic membranes, such as high-temperature resistance and solvent resistance, their applications extend beyond fields like environmental protection, food processing, chemicals, and bioengineering. In particular, in demanding environments such as chemical synthesis and petrochemical industries, where high temperatures and pressures exist along with strong organic solvents, acids, and bases, ceramic membranes exhibit functions that organic membranes do not possess. The field of ceramic membrane treatment for ultra-fine particles mainly includes the production of nanocatalysts, the preparation of ultra-fine powders, membrane reactors, the collection and separation of micron and sub-micron particles in wastewater, as well as the removal and collection of particles used for decolorization in biomedical activated carbon. Thanks to the use of ceramic membranes, the reaction process was optimized, cleaning efficiency was improved, and reaction energy consumption was reduced, allowing the reaction to proceed to completion. This increased the purity and yield of the product, resulting in significant economic, social, and environmental benefits. 1. Applications of ceramic membranes in the preparation of nanocatalysts: 1.1 Properties of nanomaterials: Nanomaterials refer to materials whose structural units have sizes ranging from 1 nm to 100 nm, including various material particles such as metals, non-metals, organics, inorganics, and biomaterials. Since its size is close to the coherence length of electrons and its scale is near the wavelength of light, coupled with the special effects of its large surface area, the properties it exhibits—such as melting point, magnetism, optical properties, thermal conductivity, electrical conductivity, and so on—are often different from those of the substance in its bulk state. Due to this series of excellent properties, nanomaterials are widely used in many industries. Introduction to the production process of 1.2-nanometer catalysts: Nanocatalysts mainly include nanoscale rare metals, nanoscale titanium-silicon molecular sieves, nanoscale perovskite-type composite oxides, and nanoscale NiO catalysts, with wet chemical methods being commonly used for their preparation. In the wet chemical method for producing nanocatalysts, it is necessary to repeatedly wash the product slurry, which is rich in impurity ions, in order to remove these ions, thereby increasing the purity and concentration of the product ； To prevent the introduction of other impurities, high standards are required for water quality; generally, deionized water is used, and a large amount of it is consumed during the washing process. It is of great practical value to know how to use appropriate methods to reduce the amount of washing water used. (2) 1.3 Process and effects of using ceramic membranes to clean nanocatalysts: 1.3.1 Preparation and cleaning process of nanocatalysts. Figure 1: Ceramic membrane washing and concentration process for powder slurries. Taking the production of ZSM-5 molecular sieve catalysts as an example, water, silicon source, aluminum source, template agent, and alkali solution are mixed in certain proportions and added to a crystallization tank where crystallization occurs, resulting in molecular sieve crystals with particle sizes of around 200 to 400 nm. The mixture contains sodium ions as well as other impurity ions, which can cause catalyst poisoning; therefore, deionized water is required to remove these ions. The cleaning process of the powder slurry is shown in Figure 1. The powder slurry undergoes several rounds of concentration and cleaning using ceramic membranes, so that the impurity content in the resulting product slurry meets the specified requirements. The concentrated slurry then proceeds to the next drying stage, where it is processed into the final product for sale ； The template agent in the clear liquid enters the reaction system for reuse, while the excess washing water goes into the wastewater system and is discharged after treatment. The purity of the final product obtained after cleaning with ceramic membranes fully meets the customer’s requirements, and can even reach electronic-grade standards. 1.3.2 Comparison of cleaning effects For some powder systems with excellent dispersibility and nanoscale particle sizes, conventional washing and concentration methods such as plate-and-frame filtration and high-speed centrifugation are unable to effectively separate the solids from these nanopaste solutions. These methods present drawbacks including easy leakage of powder, turbid filtrate, large amounts of water required for washing, high labor intensity, and low concentration ratios. Some processes involve the addition of flocculants to increase the particle size, thereby achieving a desired particle size, after which traditional treatment methods are used. However, the introduction of other compounds leads to additional post-treatment steps, increased costs, and reduced product quality. Figure 2: Comparison of the effects after treating the nano-paste using plate-and-frame or centrifugal methods, as well as ceramic membranes. As can be seen from the photographs in Figure 2, the separation efficiency of the paste after treatment with ceramic membranes is much better than that achieved through plate-and-frame or centrifugal filtration. This allows for the maximum retention of the active components in the paste, thereby increasing the yield of the product, reducing costs, protecting the environment, as well as decreasing the amount of water needed for washing and thus easing the burden on wastewater treatment. 2. Applications of ceramic membranes in the preparation of nanopowders: In addition to being used for cleaning and concentrating slurries in nanocatalysts, ceramic membranes are now widely applied in various other aspects related to ultra-fine and nanopowders. These applications include the following: (1) Purification and concentration of oxides such as nano-titanium oxide, zinc oxide, and aluminum oxide; (2) Purification and concentration of materials such as nano-barium titanate and barium carbonate; (3) Purification and concentration of materials such as nano-silver and nano-diamond; (4) Concentration of nano-silica sols; (5) Purification and concentration of nano-antimicrobial materials; (6) Purification and concentration of minerals such as nano-clay and montmorillonite; (7) Purification and concentration of nano-graphene. Thanks to their high filtration precision, narrow pore size distribution, high mechanical strength, and good chemical stability, ceramic membranes have replaced traditional processes such as plate-and-frame filters and centrifugation in the separation, collection, and purification of ultra-fine particles. This leads to improved product purity and yield, reduced operating costs, and lower energy consumption. With the continuous development of new processes, it is believed that ceramic membranes will be able to find wider applications in the petrochemical industry.