Physical and chemical properties【1】
Aluminum alkoxide is produced by the direct reaction of metallic aluminum and the corresponding alcohol under the action of a catalyst. For the same kind of metal, the longer the carbon chain and the more branches of the alcohol reacting with it, the slower the reaction rate. Comparing the reaction speeds of aluminum and various alcohols, the reaction time of aluminum with butanol and alcohols above is longer. The reaction with methanol and ethanol produces solid substances that coat the surface of the aluminum flakes, hindering the further interaction between alcohol and aluminum. reaction.
If another solvent is added to this type of reaction to dissolve the product and promote the progress of these two reactions, the process will be complicated. The reaction product of aluminum with n-propyl alcohol and isopropyl alcohol is liquid, and the reaction proceeds faster and easier. It can be seen from Table 2-1 that under the same conditions, aluminum isopropoxide has the lowest boiling point among several aluminum alkoxides and is easy to purify by vacuum distillation. Aluminum isopropoxide has more advantages than aluminum n-propoxide, so Its application is more widespread.
Aluminum isopropoxide (AI(OPr); abbreviation for aluminum triisopropoxide), molecular formula C, H2AI03, relative molecular weight 204.33, density 1. 0346g/cm3(25C). Aluminum isopropoxide was first mentioned by Alexander Tysenko in a paper at the Russian Annual Meeting of Physical Chemistry in 189815. This substance is a white solid that easily absorbs moisture at room temperature. It is soluble in organic solvents such as ethanol, isopropyl alcohol, benzene, toluene, chloroform, carbon tetroxide and petroleum ether, and decomposes when exposed to water’6. The melting point is 119C. The corresponding values of its boiling point and pressure are shown in Table 2-1.
Purification method
There are three conventional purification methods for metal alkoxides,
(1) Vacuum distillation method. This method is mainly based on the low boiling point of metal alkoxides, but its boiling point is higher than that of conventional distilled organic compounds. Therefore, vacuum distillation can be used for purification and skin reduction
(2) How to prepare meat. This method is mainly based on the characteristics of metal alkoxides being easily soluble in organic solvents, allowing insoluble impurities to settle at the bottom of the container, and then using filtration to separate and purify them under the protection of inert gas. This method makes up for the shortcomings of some substances that cannot be distilled under reduced pressure. It is also a more common method, but this method takes a long time. Not conducive to mass production:
(3) Recrystallization method. This method mainly dissolves the metal alkoxide with high impurity content in the above-mentioned organic solvent (such as benzene), separates the insoluble impurities, and then crystallizes the alkoxide through solvent volatilization or evaporation and cooling, and the impurities or soluble organic The impurities remain in the saturated liquid because they have not reached saturation, thereby achieving the purpose of separation and purification. The purification method of aluminum isopropoxide is usually achieved by vacuum distillation among the above methods. For the two special impurities of iron and silicon in aluminum isopropoxide, special methods can be used to remove them when their content is high.
Ju Mingli et al. reported that adding a complexing agent that is soluble in isopropyl alcohol to the aluminum alkoxide system can form a complex with the iron impurities in the alkoxide. The use of aluminum isopropoxide in isopropyl alcohol The difference in solubility causes aluminum isopropoxide to crystallize from propyl alcohol, while the iron complex can be more distributed in isopropyl alcohol, thereby separating from aluminum isopropoxide. To achieve the purpose of iron removal.
Yang Yonglai et al. reported that the crude aluminum alkoxide product was dissolved in an extraction solution such as benzene or toluene, and then a complexing agent that can form an insoluble complex with impurities was added to the aluminum alkoxide. The mixture reacts with impurities by stirring, letting it stand, settling (or filtering), and then distilling the extract and complex liquid to obtain the final high-purity aluminum alkoxide product. This method has a better effect of removing iron impurities in the alkoxide, but the process It requires a long time and uses maximum solvent. Not easy for industrial production.
Wang Mingyan uses the vacuum distillation additive method to purify aluminum alkoxides by traditional vacuum distillation. She directly adds silicon-removing reagents to the reaction between aluminum flakes and alcohol. Other silicon substances are generated during the reaction, and the vacuum distillation When remaining in the substrate, high purity aluminum alkoxide is obtained. This method: the process is simple, environmentally friendly, and easy for industrial production.
Synthesis process of aluminum isopropoxide
The raw materials for the synthesis of aluminum isopropoxide are high-purity aluminum, isopropyl alcohol and catalysts to prepare high-purity oxidation, and the aluminum raw materials used are different. To prepare alumina with a purity of more than 99.995%, aluminum flakes with a purity of 99.996% must be used when synthesizing aluminum isopropoxide. The composition is shown in��2—2.
The key to selecting isopropyl alcohol is its water content. Generally, isopropyl alcohol of grade 3 or above should be selected. Trace amounts of water often cause the synthesis time of aluminum isopropoxide to be prolonged or even impossible. There are reports in the literature that when the water content reaches 0.11%, the initiation time of the reaction between aluminum and isopropyl alcohol takes 125 minutes; when the water content reaches 0.5% Above, aluminum does not react with iso-cation alcohol.
To determine whether there is water in isopropyl alcohol and to remove trace amounts of water in isopropyl alcohol, you can add aluminum isobutyrate to isopropyl alcohol. When aluminum isopropoxide is added to isopropyl alcohol, if the isopropyl alcohol contains trace amounts of water, a hydrolysis reaction will occur between the azyl alcohol and water, and a white floc will appear in the mixed solution, which is aluminum hydroxide, and will be consumed at the same time. If the mixed solution is transparent and unchanged after adding aluminum isopropoxide, it means that there is no water in isopropyl alcohol.
The catalyst used in the synthesis of aluminum isopropoxide should be anhydrous aluminum chloride or mercury chloride. When anhydrous aluminum chloride is selected as the catalyst, HCI gas is generated, which will corrode stainless steel equipment, so the reaction must be carried out in an enamel kettle. If mercury chloride is used as a catalyst, it can be carried out in stainless steel equipment.
In addition to considering the material when designing the aluminum isopropoxide synthesis reactor, it is also necessary to consider the heating area on the synthesis reactor body and the heat exchange area of the condensation tower.
Figure 2-3 shows the schematic diagram of the equipment used in the synthesis of aluminum isopropoxide. It is mainly divided into two parts: kettle body and condenser. The kettle body has two jackets. The bottom jacket is for heating and can pass through thermal oil or high-pressure steam. The side wall jacket and the inner tube in the kettle body can pass through cooling water in an emergency and are used to process the synthesis of aluminum isopropoxide. Due to bumping caused by overheating, safety factors are taken into consideration in the process of synthesizing aluminum isopropoxide, and the feeding sequence also has certain requirements. Generally, aluminum flakes are added first, and then isopropyl alcohol and catalyst are added.
Conversely, if isopropyl alcohol is added first in a hot pot, a large amount of isobutanol steam will be generated. When the steam contacts the house gas and reaches the explosion limit (volume fraction) of 2% to 8%, aluminum will be added later. If static electricity is generated during the process, it will cause an explosion. The synthesis of aluminum isopropoxide is a butadiene chemical synthesis process, which involves a large amount of organic solvents and produces a large amount of hydrogen. Therefore, the generation of static electricity must be prevented in the synthesis workshop. The factory building must have measures to protect against lightning and fireworks. Electrical equipment in the factory building must use explosion-proof devices. Open flames are prohibited in the factory building.
The synthetic ingredient ratio of aluminum isopropyl alcohol can be adopted in two ways, one is the excess isopropyl alcohol method, and the other is the excess aluminum method. Both methods have their pros and cons. , adopt the isopropyl alcohol excess method. After the reaction is completed, the aluminum is completely consumed, and there is no aluminum residue in the kettle. It does not need to be cleaned regularly. Manual operation is more convenient, but after the synthesis is completed, a step of steaming out the isopropyl alcohol needs to be added, and at the same time the reaction is completed. It’s hard to judge. If the aluminum excess method is used, there will be residual aluminum after the reaction, which needs to be cleaned regularly. However, the end point should be easy to judge and the synthesis process can be controlled.
References
[1]Wang Jing, Preparation process and performance of high-purity alumina by metal alkoxide method, Metallurgical Industry Press, 2015.11, page 27
��Literature
[1]Wang Jing, Preparation process and performance of high-purity alumina by metal alkoxide method, Metallurgical Industry Press, 2015.11, page 27
