Pseudo-boehmite (PB powder for short, chemical formula AlOOH·nH2O, n=0.08~0.62) has an unfixed composition and a typical structure of extremely thin, wrinkled lamellae. Compared with boehmite, pseudo-boehmite has a smaller crystal size and higher water content. It is a kind of boehmite with incomplete crystallinity. It has the characteristics of high specific surface area, good colloidal solubility, strong adhesion, and large pore volume. It is a widely used chemical raw material. Pseudo-boehmite products are tasteless and odorless, usually appearing in the form of white colloid (wet product) or powder (dry product).
Pseudo-boehmite can be converted into γ-Al2O3 after roasting under specific conditions, and plays an important role in many industrial fields. It is not only a catalyst carrier widely used in the fields of Claus reaction, catalytic reforming, hydrorefining, hydrocracking and automobile exhaust purification, but also can be used as a catalyst for alcohol dehydration to olefins. When pseudo-boehmite comes into contact with acid, a reversible reaction occurs, resulting in a sol phenomenon. This property makes it a high-quality adhesive, for example, used as a binder in the production of aluminum silicate refractory fibers. In addition, pseudo-boehmite and its derivatives can also be used to prepare inorganic membranes, bioceramics, or as highly efficient and non-toxic flame retardants.
一.Pseudo-boehmite is a good match for catalysts
1. Controllable structure and high porosity. Pseudo-boehmite has a high specific surface area and adjustable pore size, and can be converted into γ-Al2O3 with a high specific surface area after roasting. In addition, its surface is rich in hydroxyl groups, which can effectively promote the loading and dispersion of active components (such as metals, metal oxides and metal sulfides) in the form of nanoparticles, thereby ensuring high dispersion and excellent activity of the catalyst.
2. Good peptization and strong adhesion. As a catalyst binder, pseudo-boehmite not only ensures the mechanical strength of the catalyst molded body, but also helps improve the activity of the catalyst. It is suitable for various molding processes such as extrusion, spray granulation, tableting, and pelletizing, and can prepare catalysts in various shapes such as spherical, columnar, and honeycomb shapes.
二.Application of pseudo-boehmite in catalyst industry
1.Catalyst carrier
In the petroleum industry, γ-Al2O3 is used as the carrier of catalytic cracking catalysts and fixed-bed residual oil hydrogenation catalysts. γ-Al2O3 is usually produced by high-temperature calcination and dehydration of pseudo-boehmite or boehmite. Compared with boehmite, pseudo-boehmite has a higher specific surface area and larger pore volume, thereby reducing the diffusion resistance of reactants in the catalyst pores, improving the dispersion of active components, and thus significantly enhancing the catalytic performance of the catalyst.In addition, pseudo-boehmite has strong peptizing properties and can also be used as a binder to enhance the wear resistance of the catalyst. Therefore, in the petrochemical industry, pseudo-boehmite is often used as a precursor to prepare γ-Al2O3.
2.catalyst binder
In the composition of industrial catalysts, in addition to molecular sieves and active metals, acidified pseudo-boehmite is usually added as a binder during the molding process (i.e., peptized pseudo-boehmite). This binder can not only enhance the mechanical strength and wear resistance of the catalyst, but also serve as a carrier to adjust the pore structure and acidity of the catalyst. During the preparation process of the catalyst, the binder will interact with the molecular sieve after high-pressure extrusion and high-temperature roasting, thereby affecting the activity of the catalyst.
3.Preparation of gamma alumina coatings
Coating a layer of active coating with high specific surface area on the surface of the carrier is a common method to improve the performance of the carrier (such as cordierite honeycomb ceramic carrier used for automobile exhaust catalysts). Gamma alumina is currently the most widely used coating material because of its high specific surface area, which promotes uniform distribution of active components during the impregnation process. Usually, the precursor of gamma alumina coating is AlOOH sol, which is prepared by acid peptization method using pseudo-boehmite as raw material. This method is simple in process and easy to control, and is very suitable for industrial production.
From the above content, we can see that pseudo-boehmite is the core carrier of various industrial catalysts and an indispensable key component in the catalyst industry. Compared with other catalytic materials, pseudo-boehmite is not well-known, but it is widely used and irreplaceable, so it is called the "hidden champion" of the catalyst industry.