A catalyst for catalyzing dehydration of lactic acid to prepare propylene lactone and a preparation method thereof

Abstract

The present application relates to a kind of catalysts for catalyzing lactic acid dehydration to prepare lactide, which is used for catalyzing lactic acid dehydration to prepare lactide, and has higher lactic acid conversion rate, lactide selectivity and stability.The present application also relates to a kind of preparation method of the above-mentioned catalyst for catalyzing lactic acid dehydration to prepare lactide, which has simple preparation process, good performance of the prepared catalyst and lower operating cost during use.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of catalyst preparation and relates to a catalyst for catalyzing lactic acid dehydration to prepare lactide and a preparation method thereof. BACKGROUND

[0002] Polylactic acid is an important degradable plastic and is mainly prepared by directly polymerizing lactide. At present, lactide is mainly produced by two-step heating of lactic acid under low vacuum degree, and the energy consumption is high. In the past, a liquid-phase one-step method for converting lactic acid into lactide is proposed, and the development of a catalyst is the key. Therefore, it is of important research significance and practical value to develop an efficient catalyst for catalyzing lactic acid to convert into lactide.

[0003] Traditionally, a Beta zeolite is generally used to catalyze the lactic acid conversion reaction in a toluene system. In the reaction process, lactic acid is first formed into lactic acid oligomers under the action of acid sites on the outer surface of the Beta zeolite, and the oligomers are then generated into lactide under the catalysis of the Beta zeolite. The traditional method has problems of low catalytic efficiency and high operating cost in the reaction process.

[0004] Therefore, the existing problem is that it is urgent to develop a catalyst with high catalytic efficiency and low operating cost in the reaction process. SUMMARY

[0005] One of the purposes of the application is to overcome the deficiencies in the prior art and provide a catalyst for catalyzing lactic acid dehydration to prepare lactide. The catalyst is used to catalyze lactic acid dehydration to prepare lactide and has high lactic acid conversion rate, lactide selectivity and stability.

[0006] The second purpose of the application is to provide a preparation method of the above-mentioned catalyst for catalyzing lactic acid dehydration to prepare lactide. The method has low raw material price and simple preparation process.

[0007] Therefore, the first aspect of the application provides a catalyst for catalyzing lactic acid dehydration to prepare lactide, which is a full-silicon Beta zeolite covering mesoporous Beta zeolite catalyst.

[0008] In some embodiments of the application, the lactic acid conversion rate of the zeolite catalyst is ≥95%; and the lactide selectivity of the zeolite catalyst is ≥70%.

[0009] The second aspect of the application provides a preparation method of the catalyst according to the first aspect of the application, which comprises:

[0010] Step A, mixing a tetraethylammonium hydroxide aqueous solution and a silicon source and stirring, then adding hydrofluoric acid, stirring uniformly, and volatilizing the solvent to obtain a full-silicon Beta synthesis gel;

[0011] Step B, mixing the mesoporous Beta and the full-silica Beta synthesis gel uniformly, treating the obtained mixture, and then performing filtration, water washing, and drying to obtain a catalyst precursor;

[0012] Step C, after calcining the catalyst precursor, a catalyst for catalyzing the dehydration of lactic acid to prepare lactide is obtained.

[0013] In the present application, the aqueous solution of tetraethylammonium hydroxide is formed by dissolving tetraethylammonium hydroxide in water.

[0014] In some embodiments of the present application, the concentration of the aqueous solution of tetraethylammonium hydroxide is 10wt%-40wt%.

[0015] According to the present application, the molar ratio of tetraethylammonium hydroxide, silicon source calculated based on silicon dioxide, hydrofluoric acid, and water in the full-silica Beta synthesis gel is 0.1-0.5:1.0:0-0.5:3.0-20.0.

[0016] Preferably, the silicon source is selected from fine silica gel, coarse silica gel, white carbon black, tetraethyl silicate, and tetramethyl silicate.

[0017] In some embodiments of the present application, in step B, the mass ratio of mesoporous Beta to full-silica Beta synthesis gel is 1:0.01-0.2.

[0018] In some embodiments of the present application, in step B, the temperature of the heat treatment is 100-180℃; and / or, the time of the heat treatment is 1-24 hours.

[0019] In some embodiments of the present application, the temperature of the calcination is 550℃, and the time of the calcination is 5 hours.

[0020] The third aspect of the present application provides the use of the catalyst as described in the first aspect of the present application or prepared by the preparation method as described in the second aspect of the present application in catalyzing the dehydration of lactic acid to prepare lactide.

[0021] Compared with the prior art, the present application has the following beneficial effects:

[0022] 1. The catalyst synthesized by the method of the present application has the following characteristics: (1) The synthesis method of the present application covers the acid sites on the outer surface of the zeolite and introduces mesopores, which reduces the molecular weight of lactic acid oligomers, the intermediate in the reaction, and improves the diffusion capacity thereof, so that the activity in the catalytic lactic acid dehydrogenation reaction is higher. (2) The catalyst is relatively stable, and the special structure endows the catalyst with higher stability. Even if the activity is reduced due to carbon deposition during the reaction, the catalyst can be regenerated by a simple calcination process.

[0023] 2. The catalyst preparation process of the present application is simple, and the prepared catalyst has a lower operating cost during use. Attached Figure Description

[0024] The present invention will now be described in further detail with reference to the accompanying drawings:

[0025] Figure 1 This is a schematic diagram of the catalyst used in the present invention for the dehydration of lactic acid to produce lactide. Detailed Implementation

[0026] To facilitate understanding of the present invention, it will be described in detail below with reference to the accompanying drawings and embodiments. However, before describing the present invention in detail, it should be understood that the present invention is not limited to the specific embodiments described. It should also be understood that the terminology used herein is for describing specific embodiments only and is not intended to be restrictive.

[0027] Where numerical ranges are provided, it should be understood that every intermediate value between the upper and lower limits of the range and any other specified or intermediate value within the specified range is covered by this invention. The upper and lower limits of these smaller ranges may be independently included in the smaller range and are also covered by this invention, subject to any explicitly excluded limits within the specified range. Where a specified range includes one or two limits, the range excluding any or both of those included limits is also included by this invention.

[0028] Unless otherwise defined, all terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. While any methods and materials similar to or equivalent to those described herein may also be used in the practice or testing of this invention, preferred methods and materials are now described.

[0029] I. Terminology

[0030] In this invention, the term "water" refers to deionized water, distilled water, or ultrapure water unless otherwise specified or limited.

[0031] II. Implementation Plan

[0032] As mentioned earlier, Beta zeolite is traditionally used to catalyze the lactic acid conversion reaction in a toluene system. During the reaction, lactic acid first forms lactic acid oligomers at the acid sites on the outer surface of the Beta zeolite, and these oligomers then generate lactide under the catalysis of the Beta zeolite. Traditional methods suffer from low catalytic efficiency and high operating costs. Therefore, the inventors have conducted extensive research on "using Beta zeolite to catalyze the lactic acid conversion reaction in a toluene system."

[0033] The inventors have found that if the molecular weight of lactic acid oligomer is high, the large size will hinder its access to the acid sites of the Beta zeolite, thereby affecting the smooth progress of the reaction. In addition, the traditional zeolite only has micropores, which also affects the smooth progress of the reaction to some extent; and the traditional method has a higher reaction temperature and needs to be vacuumed, so the equipment operation cost is high.

[0034] The inventors have further found that the mesoporous Beta zeolite coated with the all-silicon Beta zeolite can be used to prepare a novel high-efficiency mesoporous zeolite-based catalyst material. The catalyst preparation method is simple, the reaction temperature is low during use, the operation is carried out at normal pressure, the operation cost is low, and the lactic acid can be converted into lactide in a liquid-phase one-step method, thereby providing a new option for preparing lactide from lactic acid.

[0035] Therefore, the preparation method of the catalyst for efficiently catalyzing lactic acid dehydration to prepare lactide provided by the present application comprises the following steps:

[0036] (1) mixing and stirring a tetraethylammonium hydroxide aqueous solution and a silicon source, then adding hydrofluoric acid, stirring uniformly, and volatilizing the solvent to obtain an all-silicon Beta synthesis gel;

[0037] (2) uniformly mixing the mesoporous Beta and the all-silicon Beta synthesis gel, heat-treating the obtained mixture at 100-180 ℃ for 1-24 hours, then performing suction filtration, water washing, and drying to obtain a catalyst precursor;

[0038] (3) calcining the catalyst precursor at 550 ℃ for 5 hours to obtain the catalyst.

[0039] In the present application, the tetraethylammonium hydroxide aqueous solution is formed by dissolving tetraethylammonium hydroxide in water.

[0040] In the present application, the concentration of the tetraethylammonium hydroxide aqueous solution is 10wt%-40wt%.

[0041] In the present application, the silicon source is any one of fine silica gel, coarse silica gel, white carbon black, tetraethyl silicate, or tetramethyl silicate; and the composition (molar ratio) of the all-silicon Beta synthesis gel is 0.1-0.5 tetraethylammonium hydroxide: 1.0 silicon source (calculated based on silicon dioxide): 0-0.5 hydrofluoric acid: 3.0-20.0 water.

[0042] In the present application, the mesoporous Beta zeolite used can be various mesoporous Beta zeolites synthesized by a top-down strategy or a bottom-up strategy.

[0043] According to the present application, in the above step (2), the mass ratio of the mesoporous Beta to the all-silicon Beta synthesis gel is 1:0.01-0.2.

[0044] The skilled in the art should understand that the water in the above-mentioned full-silicon Beta synthesis gel refers to the water contained in the aqueous solution of tetraethylammonium hydroxide.

[0045] The skilled in the art should also understand that the volatile solvent in the above-mentioned step (1) refers to the liquid component (i.e. solvent) in the mixture formed by volatilizing the aqueous solution of tetraethylammonium hydroxide and the silicon source and hydrofluoric acid to form the full-silicon Beta synthesis gel, wherein the solvent may contain ethanol (produced by hydrolysis of tetraethyl silicate), methanol (produced by hydrolysis of tetramethyl silicate) and water (water contained in the aqueous solution of tetraethylammonium hydroxide). In this process, according to the molar ratio of the raw materials and the mass of the silicon source, it can be calculated what mass needs to be volatilized; during the volatilization process, the mass is weighed at intervals, if the volatilization amount is insufficient, the volatilization is continued, if the volatilization amount is excessive, the water is appropriately supplemented until the full-silicon Beta synthesis gel is formed.

[0046] In the present application, the structure diagram of the catalyst for efficiently catalyzing lactic acid to dehydrate to prepare propylene lactone prepared by the above preparation method is as shown in Figure 1 It can be seen from Figure 1 that the catalyst for catalyzing lactic acid to dehydrate to prepare propylene lactone prepared by the above preparation method is composed of a full-silicon Beta zeolite shell layer covering a mesoporous Beta zeolite.

[0047] The research results show that the lactic acid conversion rate of the above-mentioned zeolite catalyst with the above-mentioned specific structure is ≥95%; the selectivity of the zeolite catalyst for propylene lactone is ≥70%.

[0048] The principle of the present application at the micro level: catalyzing lactic acid to prepare propylene lactone needs to control the molecular weight of the lactic acid oligomer intermediate and reduce the diffusion resistance of the lactic acid oligomer in the zeolite catalyst. The former needs to cover the acid sites on the outer surface of the zeolite, and the latter needs to introduce mesopores into the zeolite catalyst. The synergistic effect of the two can efficiently catalyze the conversion of lactic acid to propylene lactone. In order to achieve the above two points, the present patent first uniformly mixes the mesoporous Beta zeolite and the full-silicon Beta zeolite synthesis gel, and then heat-treats and crystallizes to obtain a full-silicon Beta zeolite covering mesoporous Beta zeolite catalyst, that is, covering the acid sites on the outer surface of the mesoporous zeolite and introducing mesopores. Therefore, the full-silicon Beta zeolite covering mesoporous Beta zeolite catalyst can greatly reduce the molecular weight of the lactic acid oligomer, reduce side reactions and ultimately improve the yield of propylene lactone during the reaction process.

[0049] The above-mentioned zeolite catalyst or the zeolite catalyst prepared by the above-mentioned preparation method in the application of catalyzing lactic acid to dehydrate to prepare propylene lactone can be understood as a method of using the above-mentioned zeolite catalyst or the zeolite catalyst prepared by the above-mentioned preparation method to catalyze lactic acid to dehydrate to prepare propylene lactone.

[0050] Examples

[0051] The present application is specifically described below by specific examples. The experimental methods described below are all conventional laboratory methods unless otherwise specified. The experimental materials described below are all available from commercial channels unless otherwise specified.

[0052] The catalyst is prepared as follows:

[0053] The tetraethylammonium hydroxide aqueous solution is mixed with the silicon source and stirred, then hydrofluoric acid is added and stirred until uniform, and the solvent is volatilized to obtain a full-silicon Beta synthesis gel. Then the mesoporous Beta is mixed with the full-silicon Beta synthesis gel, and the mixture is treated at 100-180°C for 1-170 hours, then filtered, washed with water, and dried. The catalyst is obtained by calcining at 550°C for 5 hours.

[0054] The catalyst prepared by the present application is used to catalyze the dehydration of lactic acid to lactide, and the specific steps are as follows:

[0055] The activity of the catalyst is evaluated by the following reaction device: the catalyst, toluene, and lactic acid are placed in a round-bottom flask, a water separator is connected to the round-bottom flask, toluene is loaded in the water separator, and a spherical condenser is connected to the water separator. During the reaction, the round-bottom flask is placed in a preheated oil bath at 140°C for 3 hours. After the reaction is completed, the flask is removed from the oil bath and placed in cold water to stop the reaction. The catalyst is removed by filtration, and the obtained product is analyzed by GC. 1 H NMR (400 meganuclear magnetic resonance spectrometer, AVANCE IIIHD 400, Bruker) quantitative analysis.

[0056] The catalyst is successfully prepared in 10 examples, and the experimental data in each example is shown in Table 1 below.

[0057] Table 1. Material ratio and reaction conditions of Examples 1-10

[0058]

[0059]

[0060] It should be noted that the above examples are only preferred embodiments of the present application, which are used for illustration and understanding, and do not constitute any limitation on the present application. The present application is described by referring to typical examples, but it should be understood that the words used therein are descriptive and explanatory words, rather than limiting words. The present application can be modified as specified within the scope of the claims of the present application, and the present application can be revised without departing from the scope and spirit of the present application. Although the present application described therein relates to specific methods, materials and examples, it does not mean that the present application is limited to the specific examples disclosed therein, on the contrary, the present application can be extended to all other methods and applications with the same function.

Claims

1. A catalyst for the dehydration of lactic acid to lactide, wherein the catalyst is a mesoporous Beta zeolite covered by all-silica Beta zeolite. The method for preparing the catalyst includes: Step A: Mix tetraethylammonium hydroxide aqueous solution with silicon source and stir, then add hydrofluoric acid, stir evenly, and evaporate the solvent to obtain all-silicon Beta synthetic gel; Step B: The mesoporous Beta and the all-silica Beta are mixed evenly to form a gel. The resulting mixture is then subjected to heat treatment to crystallize it. After filtration, washing with water, and drying, the catalyst precursor is obtained. Step C involves calcining the catalyst precursor to obtain a catalyst for the dehydration of lactic acid to lactide.

2. The catalyst according to claim 1, characterized in that, The catalyst has a lactic acid conversion rate of ≥95% and a lactide selectivity of ≥70%.

3. The catalyst according to claim 1, characterized in that, The tetraethylammonium hydroxide aqueous solution is formed by dissolving tetraethylammonium hydroxide in water; the concentration of the tetraethylammonium hydroxide aqueous solution is 10wt%-40wt%.

4. The catalyst according to claim 3, characterized in that, The molar ratio of tetraethylammonium hydroxide, silicon source (calculated as silica), hydrofluoric acid, and water in the all-silicon Beta synthetic gel is 0.1~0.5:1.0:0~0.5:3.0~20.0, and the hydrofluoric acid content is not zero; the silicon source is selected from fine silica gel, crude silica gel, silica fume, tetraethyl silicate, and tetramethyl silicate.

5. The catalyst according to claim 1, characterized in that, In step B, the mass ratio of mesoporous Beta to all-silica Beta to synthesize the gel is 1:0.01~0.

2.

6. The catalyst according to any one of claims 1-5, characterized in that, In step B, the temperature of the heat treatment is 100–180°C; the time of the heat treatment is 1–24 hours.

7. The catalyst according to any one of claims 1-5, characterized in that, The roasting temperature is 550°C, and the roasting time is 5 hours.

8. The catalyst according to claim 6, characterized in that, The roasting temperature is 550°C, and the roasting time is 5 hours.

9. The use of the catalyst according to any one of claims 1-8 in the catalytic dehydration of lactic acid to lactide.

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