Layered double metal hydroxide (Layered
Double Hydroxide (LDH) is a combination of hydrotalcite (HT) and hydrotalcite-like compounds (Hydrotalcite-Like
Compounds, HTLCs), a series of supramolecular materials assembled by intercalation of these compounds are called hydrotalcite-like intercalated materials (LDHs). In 1842, Hochstetter first discovered natural hydrotalcite ore from schist deposits in Sweden. In the early 20th century, people began to study its structure because they discovered that LDH had a catalytic effect on hydrogen addition reactions. In 1969, Allmann et al. The single crystal structure confirmed the layered structure of LDH for the first time; after the 1990s, with the widespread application of modern analytical techniques and testing methods, research on the structure and properties of LDHs continued to deepen.
The synthesis method and structural characteristics of hydrotalcite
Hydrotalcite materials are anionic Type layered compound bromopyridine. Layered compounds refer to a type of compound with a layered structure, interlayer ions, and exchangeability. The main body of the layered compound is used to intercalate and exchange the interlayer ions under the action of strong polar molecules. , introducing some functional guest substances into the interlayer gaps and spreading the distance between the layers to form layer pillar compounds. Hydrotalcites (LDHs)
It is a new type of inorganic functional material with layered structure.
The chemical composition of the main laminate of LDHs is closely related to factors such as the cationic properties of the laminate, the charge density of the laminate or anion exchange capacity, and the supramolecular intercalation structure. Generally speaking, as long as the metal cation has a suitable ionic radius (similar to Mg2
+ ionic radius 0.072 nm (not much different) and charge number, both can form LDHs layers.
LDHs is a compound assembled from positively charged main layers and interlayer anions through non-covalent interactions. Its structure is similar to brucite Mg(OH)2, composed of MgO6 Octahedrons share edges to form unit layers. It has the following outstanding features:
(1) The chemical composition of the main layer can be adjusted;
(2) The type and number of interlayer guest anions can be adjusted ;
(3) The particle size and distribution of the intercalated assembly can be controlled
A typical LDHs compound is magnesium aluminum carbonate hydrotalcite: Mg6Al2(OH)16CO3·4H2O [4]. The structure of LDHs is very similar to brucite [Mg(OH)2]. The MgO6 octahedrons share edges to form a unit layer. The Mg2+ located on the layer can be replaced by Al3+ isomorphously within a certain range, making the layer positively charged. , the exchangeable CO32- between the layers balances with the positive charges on the laminate, making the overall structure of LDHs electrically neutral. Since the layer plates and interlayer anions are connected through hydrogen bonds, the interlayer anions of LDHs are exchangeable. In addition, interlayer water exists in LDHs and these water molecules can be removed without destroying the layered structure.
Synthesis method of hydrotalcite Synthesis method
Synthesis method of hydrotalcite Low saturation co-precipitation method
Low saturation co-precipitation method, according to a certain proportion, the metal nitrate solution is prepared into a mixed salt solution (SolS) of a certain concentration, and NaOH and Na2CO3 are prepared into a mixed alkali solution (SolB) according to a certain proportion, in a large beaker Pre-fill a certain amount of distilled water into the beaker, heat it to a certain temperature, drop SolS and SolB simultaneously into the large beaker at a certain dropping speed, maintain the pH of the reaction system at a constant value, and stir vigorously. After the titration is completed, continue stirring and aging, and finally filter, wash, and dry to obtain the product. This synthesis method is a common method in hydrotalcite synthesis. Among them, magnesium salts and aluminum salts can be nitrates, sulfates, chlorides, etc., alkali can be sodium hydroxide, potassium hydroxide, ammonia, etc., carbonates can be sodium carbonate, potassium carbonate, etc., and urea can also be used instead of alkali. and carbonates.
The synthesis method of hydrotalcite is the high supersaturation co-precipitation method
The high supersaturation co-precipitation method is to pre-heat SolS and SolB to the reaction temperature. Quickly pour the two solutions into a large beaker containing twice-distilled water that has been preheated to the same temperature as the solution, and stir vigorously
Hydrotalcite synthesis method Hydrothermal synthesis
The hydrothermal synthesis method first slowly drops SolS and SolB together and mixes them quickly, and then immediately transfers the resulting slurry to an autoclave, where it is heated at a certain temperature (usually is 100
°C) for a long time, and finally filtered, washed, dried, and ground to obtain the product. The characteristic of this method is to isolate the nucleation and crystallization processes of hydrotalcite and promote the crystallization process by increasing the aging temperature and pressure. In the hydrothermal synthesis method, no other impurities are introduced because the reaction occurs in a closed system. The prepared nanometal oxide has excellent properties such as fine powder (nanoscale), high purity, good dispersion, uniform particles, complete grain development, and controllable shape. In addition, the hydrothermal method can also avoid shortcomings such as volatilization of reactants at high temperatures, stress-induced defects, and phase interactions. More importantly, the hydrothermal method can control the morphology, size, and viscosity distribution of the product by adjusting the reaction conditions.
The synthesis method of hydrotalcite is the nucleation and crystallization isolation method
The nucleation/crystallization isolation method is to rapidly mix SolS and SolB in a complete backmix Mix in a rotating liquid film nucleation reactor, circulate vigorously for a few minutes, and then crystallize the slurry at a certain temperature. This reactor is used to realize the co-precipitation reaction of salt liquid and alkali liquid. By controlling the linear speed of the reactor rotor (5
m·s-1) can make the reactionThe objects fully contact and collide instantly, the nucleation reaction is completed instantly, and the synchronous growth of crystal nuclei ensures the uniformity of crystal size during the crystallization process.
The synthesis method of hydrotalcite is the ion exchange method
When metal ions are unstable in alkaline media, or when the anion An- has no soluble M2+ and For M3+ salts, when co-precipitation method is not possible, ion exchange method can be used. This method starts from a given hydrotalcite and forms a new phase through the exchange of certain anions in the solution for the original anions. However, it is difficult to directly use large-volume inorganic anions to prepare layered double-metal hydroxide materials through ion exchange. Generally, large-volume organic anions are first used to open the layers, and then inorganic anions are used to exchange the samples.
Hydrotalcite synthesis method: roasting and restoration method
This method is a preparation method based on the structural memory effect of HTLcs. That is to say, after heat treatment of HTLcs under certain conditions, its roasted product, layered bimetallic composite oxide (LDO), is added to a solution containing certain anions to reabsorb various anions or simply placed in the air, so that it can recover From the original layered structure, we get new HTLcs. The selectivity of the pillaring process is related to the spatial structure and electronic structure of the layer composition elements, reaction medium, and pillared organic anions. This method is mostly used to insert larger guest molecules. The outstanding advantage of this method is that it eliminates the metal salt inorganic anions that compete with organic anions for pillar support, but the disadvantage is that amorphous phase substances are easily generated and the preparation process is cumbersome. HTLcs prepared by this method is easily affected by drying conditions, calcination temperature, calcination time, pH value and other factors. In particular, the calcination temperature has a great impact on the alkalinity and specific surface area of the catalyst.
The synthesis method of hydrotalcite is urea decomposition-uniform co-precipitation method
This method uses urea to be neutral at low temperature and can form a uniform form with metal ions. solution, and the solution temperature exceeds 90
°C, the decomposition of urea causes the pH value of the solution to increase evenly and gradually. Urea is used instead of the mixed alkali solution. The advantage of this method is that the pH value inside the solution is always consistent, so Mg- with high crystallinity can be synthesized. Al, Zn-Al, and Ni-Al hydrotalcites are used, but it is difficult to synthesize Co-Al, Mn-Al, and Co-Cr hydrotalcites. On the other hand, using urea as the precipitant, NH2COO- intercalation is formed between the layers during the reaction, which is converted into CO32- after hydrothermal treatment, while the [Ni(NH3)6]2+ formed in the solution is released under hydrothermal conditions. NH3 is produced, so urea can replace the strong alkali mixture to prepare carbonated hydrotalcite, and a hydrotalcite sample with good crystallization and uniform particle size can be prepared.
The synthesis method of hydrotalcite N2 protection synthesis method
Continuously pass N2 into the reaction system during synthesis. The use of N2 protection is usually due to the following two considerations: one is to prevent some easily oxidized substances from being oxidized by oxygen in the air during synthesis; the other is to prevent the interference of CO2 in the air when preparing non-carbonate hydrotalcite . This method is suitable for more sophisticated synthesis.
The synthesis method of hydrotalcite is microwave crystallization method
This method uses microwave radiation during the synthesis to promote the rapid formation of hydrotalcite with good crystal form. CurrentP.
Benito et al. have used the microwave irradiation zinc borate method to study a series of hydrotalcite compounds. They have used the special reaction environment of microwaves (uniform rapid temperature rise and heating) to obtain ideal experimental results (rapid and uniform growth of product particles), and also combined with Materials with a certain pore structure were obtained by treating the materials under microwave and hydrothermal conditions. On the basis of traditional methods, certain innovations were made in the preparation of hydrotalcite materials.
