Preparation Method of Hydrotalcite

As a new type of material, hydrotalcite has some special physicochemical properties, such as adsorption, catalytic, interlayer anion exchangeability, weak alkalinity, etc., due to its special layered structure. Talc is now widely used in chemical, materials, medicine and other industries. At present, the research on the synthesis, preparation and application of hydrotalcite has become one of the hotspots of new materials, and with the deepening of the research on hydrotalcite materials in the future, the hydrotalcite materials will better serve the human beings. Our company's magnesium hydroxide ZH-H2-2 has been maturely used in the manufacturing field of hydrotalcite, welcome to inquire this product.

Crystal structure of hydrotalcite

The molecular composition of talc is Mg6Al12(OH)16CO3.4H2O, which is an anionic layered compound. Talc in the Mg2 +, Al3 + by M2 +, M3 + homocrystalline substitution to get the structure of a similar class of compounds, known as talc-like compounds. Molecular formula M2 +1-xM3+x(OH)2(An-)x/n.mH20, where M2+=Mg2+, Ni2+, Fe2+, Cu2+; M3+=Al3+, Fe3+, Cr3+, Sc3+, An- for the stabilization of the anions in alkaline solutions, such as: CO32-, NO3-, CL-, OH-. X represents M3+. fraction of the total number of cations accounted for x = 0.2 to 0.33 and m denotes the amount of water of crystallization in the molecule. Different M2+ and M3+, different gap-filling anions An-, can form different hydrotalcites. Its structure is very similar to that of magnesite Mg(OH)2, which consists of MgO6 octahedra sharing prisms to form a unit layer on which the Mg2+,Al3+, and OH- layers are positively charged. The interlayer Mg2+ can be replaced by Al3+ homocrystalline to a certain extent, so that the exchanged anion CO32+ is balanced with the positive charge on the laminate, making this structure electrically neutral. In addition, there are some water molecules present in the hydroxide layer at the same time, and these water molecules can be removed without destroying the lamellar structure, and the typical structure of hydrotalcite can be derived from the above.

Talc in nature is the hydroxyl carbonate of magnesium and aluminum, and later people synthesized a variety of hydrotalcite-like compounds (hydrotalcite-likecompounds, HTLcs for short), which are compounds generated by replacing Mg2+ and Al3+ in hydrotalcite by other cations with the same valence, and it is structurally the same as hydrotalcite, and because hydrotalcite-like compounds have ion-exchangeability and internal ion-exchangeability, the compounds can have the typical structure of hydrotalcite. Due to the catalytic properties of ion exchange and internal ions can be interspersed with other ions, hydrotalcite and hydrotalcite-like compounds occupy an important position in the field of adsorption and catalysis because of their special layered structure and physicochemical properties, and more and more researches are being conducted on them.

Methods of Preparing Talc and Their Advantages and Disadvantages

Natural talc is scarce and difficult to mine. Most of the natural talc occurs in the form of variants such as serpentine or spinel, and it is not pure; it contains other substances such as chlorite and muscovite. Practice has shown that it is impractical to separate and remove these impurities from the talc, so that synthetic synthesis to obtain high-purity talc has become the choice for various applications.

Talc is generally produced by reacting oxides with salts, reacting salts with bases, and reducing by roasting. In order to produce ideal talc-like materials, domestic and foreign scholars and experts have done a lot of exploration work, with the change of the synthesis route coupled with the improvement of the synthesis conditions, and constantly derived from a number of new preparation methods.

A. Co-precipitation method

Co-precipitation method is currently one of the most commonly used preparation methods, it is through the mixed solution of metal salts and alkali metal oxides reacted, that is, salt and alkali co-precipitation reaction, and then the precipitate under certain conditions into crystals that is talc. Metal salts mainly include nitrates, sulfates, chlorides and carbonates, etc., and alkalis mainly include sodium hydroxide, ammonia, potassium hydroxide, etc. Theoretically, the method has the following advantages:

    (1) Wide range of application. Almost all hydrotalcite substances can be produced by this method.

    (2) A larger number of products can be obtained. Since the feedstock ratio of co-precipitated M2+ and M3+ can be adjusted, a range of hydrotalcites with different ratios of force n(M2+)/n(M3+) can be prepared. It is also possible to obtain hydrotalcites with the same cations and different anions by selecting different synthetic raw material types and interspersing different anions between the layers.

B. Template method

Template method is to a certain organic body or its polymer as a template, with the template and solution reactants in the liquid phase interface reaction, thus forming a talc template complex, and then baking, etching method of separation of the template, because the template style is artificially designed, so that you can get the corresponding shape, size, orientation of the talc. Organic matter template can be used by the surfactant formation of vesicles, embedded copolymers and so on.

The template method is a relatively new method for the preparation of hydrotalcite, and its advantage lies in the ability to produce hydrotalcite materials of artificially set shapes and sizes on a macroscopic scale, which in turn facilitates the next step of processing and utilization. However, compared with the general preparation method of hydrotalcite, the product of template method has a large specific surface area, and the microscopic morphology is relatively rough.

C. Anion exchange method

An important characteristic of hydrotalcite is its interlayer anion "with exchangeability" using this property, different anions can be inserted into the hydrotalcite layer, the formation of structurally and functionally different hydrotalcite materials, anion exchange method is easy to synthesize hydrotalcite as a precursor, the target anion through ion exchange will be inserted into the interlayer to replace the original anion, you can get the target anion. By inserting the target anion into the interlayer through ion exchange to replace the original anion, the target ionized hydrotalcite can be obtained.

The advantage of this method is that by controlling the reaction conditions of ion exchange, not only can it maintain the inherent layer structure of the original precursor hydrotalcite, but also can re-select the type and number of new interlayer anions, so as to obtain the corresponding ion-inserted hydrotalcite materials with different structures and functions. The ion-exchange method is suitable for the preparation of metal ions that are unstable in alkaline media, the anion An- does not have soluble M2+ and M3+ salts, and hydrotalcites that cannot be prepared by co-precipitation. However, this method requires strict reaction conditions and is easy to introduce impurity ions.

D. Roasting reduction method  

Roasted hydrotalcite at a certain temperature in a liquid medium containing a certain anion, due to the thermal stability of hydrotalcite, the reconstruction of hydrotalcite layer column structure occurs at high temperatures, so that the anion enters the interlayer of hydrotalcite, and after the temperature is lowered, the formation of a new anionic hydrotalcite.

The advantage of the roasting reduction method is that the desired anionic hydrotalcite can be obtained in a targeted manner and the principle is simple. However, the operation of this method is complicated, and there are many conditions to consider in order to generate the desired anionic hydrotalcite, such as too high a roasting temperature may destroy the layer structure of the hydrotalcite, and too low a roasting temperature may not yield the corresponding anionic hydrotalcite product, and the concentration of the anion in the liquid medium has to be taken into consideration, and so on.