Preparation method of high-stability catalyst for preparing lactide by dehydrating lactic acid

Abstract

The application discloses a preparation method of a high-stability catalyst for preparing lactide by dehydrating lactic acid, which comprises the following steps: drying and calcining ZSM-5 molecular sieve to remove free water, residual ammonium and template impurities, then impregnating the obtained molecular sieve with magnesium organic acid with a larger molecular size, and drying and calcining again to obtain HZSM-5 molecular sieve catalyst with basic MgO modification on the surface. The application uses magnesium organic acid with a larger structural group, can only construct MgO basic sites on the surface of HZSM-5 molecular sieve, so that the magnesium organic acid can be prevented from entering the pore channel of the HZSM-5 molecular sieve and damaging the acid center in the molecular sieve, and the MgO basic sites constructed on the surface of the molecular sieve can help degrade the oligomers generated by self-polymerization of lactic acid, so that the yield of lactide prepared by dehydration and cyclization of lactic acid can be further improved, and the service life of the catalyst can be significantly prolonged.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of preparing lactide from lactic acid, and particularly relates to a preparation method of a high-stability catalyst for preparing lactide from lactic acid dehydration. BACKGROUND

[0002] The annual output of non-degradable polymer materials based on petroleum has exceeded one hundred million tons worldwide. Only a small part of plastic products is recycled after use. The stacking of a large amount of plastic product waste can easily cause soil and water pollution, seriously endangering the health and survival of humans and other organisms. Therefore, renewable biomass resources and biodegradable materials are attracting more and more attention. As a new type of bio-based material, polylactic acid (PLA) can be converted into carbon dioxide and water by microorganisms after use, without generating other harmful substances. PLA has gradually developed into one of the mainstream biodegradable materials and is widely used in disposable household products, packaging materials, clothing, automobiles and medical fields, with a very large market growth space.

[0003] PLA can be prepared by one-step polymerization of lactic acid under self-catalysis. However, under this process condition, it is difficult to efficiently remove water molecules as the polymerization degree of lactic acid increases, which can cause low molecular weight of lactic acid polymer and seriously limit the large-scale application of PLA. At present, the commonly used production method in industry is to first prepare lactide from lactic acid by dehydration, and then to obtain PLA with high molecular weight by ring-opening polymerization of lactide. Therefore, the synthesis of lactide is one of the important links of large-scale industrialization of PLA, and the related technical research has attracted widespread attention from domestic and foreign scholars.

[0004] At present, the production of lactide in industry mainly uses a two-step production process, i.e., first preparing lactic acid polymer with a certain molecular weight range (usually an oligomer with a molecular weight not higher than 15000 g / mol) by dehydration and condensation of lactic acid; and then depolymerizing the lactic acid oligomer under high temperature and reduced pressure to obtain crude lactide. In the two-step process, metal salts of aluminum, zinc or tin are generally used as catalysts, among which the most commonly used are various inorganic salts or organic acid salts of tin. Chinese patent CN1806919A discloses a method for preparing cyclic ester from lactic acid using stannous benzoate as catalyst. The method has the advantages of low catalyst dosage, low cracking temperature and reusability, but the efficiency of the catalyst needs to be further improved. Chinese patent CN114349733A discloses a method for continuously preparing high-purity L-lactide. The method uses tin salt as the main catalyst and hydroxyl organic carboxylic acid as the cocatalyst. The use of the cocatalyst can improve the stability of the main catalyst and effectively alleviate the plugging of the equipment by crude lactide, ensuring long-period operation of the device.

[0005] Although the two-step process for preparing lactide in industry has been relatively mature, there are still a series of problems to be solved. For example, the depolymerization process requires high reaction temperature and vacuum degree, but under the condition of high reaction temperature and catalyst, lactic acid or lactide is easy to isomerize, which ultimately reduces the optical purity of lactide product, and the high vacuum degree requires strict requirements for the material, structure and interface welding point of the equipment such as reactor, pipeline and vacuum pump; at the same time, the mass transfer and heat exchange of high viscosity lactic acid polymer during the reaction process are increased, and coking and other problems are easy to occur during the depolymerization process, which seriously affects the yield and colority of lactide product.

[0006] In recent years, the one-step method of using molecular sieve and other solid acid catalysts to catalyze lactic acid dehydration and cyclization to directly generate lactide has attracted more and more attention from scholars at home and abroad, which has the advantages of low reaction temperature, no vacuum condition and high optical purity of product. Michiel Dusselier et al. (Science 2015, 349, 78-80) reported a method of using H-Beta molecular sieve to catalyze lactic acid to convert to lactide in toluene or xylene and other organic solvents, which used 50% lactic acid at a low temperature of about 150°C, and the single-pass yield of lactide was more than 83%. Chinese patent CN115417851 A discloses a method for directly preparing lactide from lactic acid, which reacts the mixture of lactic acid solution and organic solvent under the action of alumina framework and H-Beta molecular sieve multi-level pore catalyst to obtain lactide, and the multi-level pore structure of the catalyst improves the catalytic efficiency of lactic acid and lactic acid polymer in the micro-pore. Chinese patent CN104981464B discloses a process for preparing cyclic esters or amides, the catalyst used is two or three interconnected and non-parallel channel systems, including 10-membered ring channels, and the Si / Al or Si / B ratio is greater than 12. Chinese patent CN114805284A discloses a method for directly synthesizing lactide from high-concentration lactic acid using Sn-beta-M and H-beta-M molecular sieves with multi-level pore structure, which has the advantages of simple process flow, low energy consumption and low product racemization compared with the traditional two-step process. The mesoporous structure of the catalyst can enhance the contact between the reactants and the active sites, accelerate the diffusion speed of the products, reduce the side reactions of lactide ring opening and polymerization, and increase the yield and reaction rate of lactide by about 35% and 38%, respectively.

[0007] It should be noted that the synthesis of solid acid molecular sieve involves severe hydrothermal conditions and high calcination temperature, which can easily cause the migration of heteroatoms represented by aluminum or boron to the surface, resulting in the surface acid center being much higher than the internal molecular sieve, and even the surface structure being inconsistent with the ordered pore of the bulk phase or the pore being blocked, etc., ultimately leading to more favorable occurrence of side reactions on the surface of the catalyst. When using H-Beta or HZSM-5 molecular sieve to catalyze the dehydration of lactic acid to prepare lactide, lactic acid, lactic acid oligomers or lactide, etc. are prone to occur dehydration or condensation side reactions at the acid center on the surface of the catalyst, and the large size polylactic acid molecules generated are difficult to enter the molecular sieve pore, and are gradually yellowed or even coked on the surface of the molecular sieve, causing the molecular sieve pore to be blocked and seriously affecting the activity and stability of the acidic molecular sieve catalyst.

[0008] Although Na, K and other alkali metal ions and alkaline earth metal CaO can help to degrade lactic acid oligomers, alkali metal related compounds are easily soluble in water and neutralize the acidic sites on the surface and framework of the molecular sieve, and the calcium salt precursor generates CaO after calcination, which reacts with water to generate calcium hydroxide, which is gradually detached from the surface of the molecular sieve by dissolving in lactic acid aqueous solution. The MgO precursor can react with the Al 3+ to generate a small amount of spinel structure magnesium aluminum compound MgAl2O4, which is more difficult to dissolve in lactic acid aqueous solution. At the same time, Fermoso et al. (Catalysis Today 2016, 277, 171-181) impregnated HZSM-5 molecular sieve with flaky and columnar structure with magnesium nitrate ethanol solution for catalyzing fast pyrolysis of eucalyptus wood chips, and found that when using magnesium nitrate, Mg 2+ will ion exchange with Si-OH-Al sites, which will seriously inhibit the Bronsted or Lewis acid sites on the surface of ZSM-5.

[0009] Therefore, in view of the above problems in the process of using HZSM-5 molecular sieve to catalyze the conversion of lactic acid to prepare lactide, it is very necessary to develop a high stability catalyst for the dehydration of lactic acid to prepare lactide. The present application uses an organic acid magnesium with a larger structure group instead of inorganic small molecules such as magnesium nitrate and magnesium chloride as a precursor, which only constructs magnesium oxide basic sites on the surface of HZSM-5 molecular sieve, can avoid the destruction of small molecule magnesium salt to the acid sites inside the 10-ring straight or Zigzag-shaped channels of HZSM-5 molecular sieve, and the MgO basic sites constructed on the surface of the molecular sieve can help to degrade large molecular weight polylactic acid into lactic acid or small molecular lactic acid oligomers, which can help them to enter the HZSM-5 molecular sieve to occur dehydration and cyclization reaction to convert into lactide, and avoid the accumulation, condensation or coking of lactic acid oligomers on the surface of the catalyst to block the micropore of the molecular sieve, thereby improving the service life or cycle of the catalyst. SUMMARY

[0010] In view of the deficiencies of the prior art, the present application provides a preparation method of a high-stability catalyst for preparing lactide by dehydration of lactic acid. The present application constructs a certain amount of highly dispersed magnesium oxide basic sites on the surface of HZSM-5 molecular sieve, and uses the basic centers on the magnesium oxide to assist the degradation of lactic acid oligomers and other by-products generated by the self-catalytic dehydration polymerization of lactic acid in the reaction system into lactic acid or small-molecule lactic acid oligomers, thereby eliminating or alleviating the problem that macromolecular PLA by-products are difficult to contact or enter the molecular sieve channels and be converted into lactide, avoiding the accumulation, condensation or coking of lactic acid oligomers and other by-products on the surface of the catalyst to block the micropores of the molecular sieve, and improving the service life or cycle of the catalyst.

[0011] To achieve the above-mentioned purpose, the present application adopts the following technical solutions:

[0012] A preparation method of a high-stability catalyst for preparing lactide by dehydration of lactic acid, comprising the following steps:

[0013] 1) drying HZSM-5 molecular sieve to remove adsorbed free water;

[0014] 2) calcining the molecular sieve after drying in step 1) to dissociate ammonium and remove residual template impurities, to obtain purified HZSM-5;

[0015] 3) using the purified HZSM-5 obtained in step 2) to impregnate a molecular size larger magnesium organic acid, to obtain a catalyst precursor;

[0016] 4) drying and calcining the catalyst precursor obtained in step 3), to construct a certain amount and highly dispersed MgO basic sites on the surface of the molecular sieve, to obtain a MgO-HZSM-5 catalyst with high stability.

[0017] Further, the HZSM-5 molecular sieve can be a commercially available hydrogen type ZSM-5 molecular sieve with a granular microstructure or an NH4-ZSM-5 molecular sieve product after ammonium exchange, or a HZSM-5 molecular sieve with special morphology such as flaky or layered structure prepared according to the prior art, or a nano HZSM-5 molecular sieve. The specific surface area of the HZSM-5 molecular sieve is 150-600 m 2 / g, and the pore volume is 0.1-1.1 cm 3HZSM-5 has a molar ratio of Si / Al of 10-200. The basic structure of HZSM-5 is 8 five-membered ring units, the basic structure units form five silicon chains by sharing marginal atoms, and the five silicon chains with mirror image relationship are connected together to form a 10-membered ring channel structure. The HZSM-5 framework has two kinds of intersecting channels, wherein the pore size of the curved channel is about 5.5 Åx5.1 Å, and the pore size of the linear channel is about 5.3 Åx5.6 Å. The acid OH content is basically the same as the number of aluminum atoms in the framework, so generally speaking, the lower the Si / Al, the more acid active sites, which is more conducive to catalyzing the dehydration and cyclization of lactic acid.

[0018] Further, the impregnation is equal volume impregnation.

[0019] Further, the magnesium organic acid contains carbon atoms in an amount of 4-36, which can be one or more of water-soluble magnesium salts such as magnesium citrate, magnesium malate, magnesium tartrate, magnesium gluconate, or one or more of oil-soluble magnesium salts such as magnesium myristate, magnesium laurate, magnesium palmitate, magnesium stearate, etc.

[0020] Further, for water-soluble magnesium salts, the magnesium salt is dissolved in a polar solvent such as water, methanol, ethanol, acetonitrile, etc. before use; for oil-soluble magnesium salts, the magnesium salt is dissolved in an organic solvent such as toluene, o-xylene, m-xylene, p-xylene, etc. before use.

[0021] Further, in the operation, the temperature of the two drying is 90-160°C, and the time is 2-8 h.

[0022] Further, in the operation, the temperature of the two calcination is 350-650°C, and the time is 4-10 h.

[0023] Further, the content of MgO in the MgO-HZSM-5 catalyst is 0.1wt%-2.0wt%.

[0024] The high-stability catalyst obtained can be used to catalyze the reaction of lactic acid dehydration to prepare lactide.

[0025] Further, the lactic acid refers to a lactic acid solution with a lactic acid content of 20% or more, preferably a lactic acid solution with a lactic acid content of 60% or more, more preferably a lactic acid solution with a lactic acid content of 80% or more, and more preferably a lactic acid solution with a lactic acid content of 95% or more; wherein the solvent in the lactic acid solution can be one or more of polar solvents such as water, acetone, acetonitrile, etc. The lactic acid can be one of D-lactic acid and L-lactic acid, or a mixture of the two. The lactic acid can be a product fermented from grain crops such as corn, or a lactic acid product obtained by fermenting non-edible biomass waste such as straw.

[0026] The main optical configuration of the lactide is consistent with the configuration of the lactic acid used, which can be L,L-lactide, D,D-lactide, L,D-lactide, or racemic lactide, mainly depending on the stereochemical configuration of the lactic acid used.

[0027] Without using a catalyst, the lactic acid solution can also ionize free hydrogen ions under the reaction conditions to undergo self-catalytic reaction to generate lactic acid self-polymerization, but as the degree of lactic acid polymerization increases, the carboxyl content in the lactic acid material is greatly reduced, and the self-catalytic ability rapidly decreases. When using HZSM-5 molecular sieve for catalyzing lactic acid dehydration and cyclization, the generation of lactide product can be greatly improved due to the high-efficiency acid sites of the catalyst and the confinement effect of the specific size channels, but there are problems such as the catalyst being easily covered or blocked by lactic acid oligomers, causing the catalyst surface to quickly coke and the activity to decrease. In the present application, MgO basic sites are introduced on the surface of HZSM-5, which can assist in degrading the lactic acid oligomers adsorbed on the surface of the catalyst under the reaction conditions, significantly improving the service life of the catalyst and reducing the regeneration frequency of the catalyst per unit time.

[0028] The present application has the following advantages:

[0029] The present application provides a modified HZSM-5 molecular sieve, which is specifically a HZSM-5 molecular sieve surface-modified with MgO basic sites. The magnesium oxide precursor used in the present application is a magnesium organic acid with a large structural group, which mainly constructs MgO basic sites on the surface of the HZSM-5 molecular sieve, can avoid the entry of magnesium ions into the 10 circular straight or Zigzag-shaped channels of the HZSM-5 molecular sieve during catalyst preparation, and can avoid damaging the acid sites inside the HZSM-5 molecular sieve channels, thereby well preserving the acid centers inside the HZSM-5 molecular sieve. The acid centers inside the molecular sieve can form a synergistic effect with the confinement effect of the HZSM-5 channels, and can be better used for catalyzing lactic acid dehydration and cyclization reaction. At the same time, the MgO basic sites constructed on the surface of the HZSM-5 can help to degrade the polylactic acid byproduct into small molecular lactic acid oligomers such as lactic acid or linear lactic acid dimer, linear lactic acid trimer, which can enter the HZSM-5 molecular sieve to undergo dehydration and cyclization reaction to convert into lactide, thereby improving the yield of lactide, and avoiding the accumulation, condensation or coking of lactic acid oligomers on the surface of the catalyst, thereby improving the service life of the catalyst. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 The XRD pattern of the catalyst prepared for Example 1 is shown in the figure.

[0031] Figure 2 The XRD pattern of the catalyst prepared for Example 4 is shown in the figure.

[0032] Figure 3 XRD pattern of the catalyst used in Comparative Example 2.

[0033] Figure 4 Catalyst stability profile for the catalyst used in Example 1 for the dehydration and cyclization of lactic acid to lactide.

[0034] Figure 5 Catalyst stability profile for the catalyst used in Example 2 for the dehydration and cyclization of lactic acid to lactide.

[0035] Figure 6 Catalyst stability profile for the catalyst used in Example 3 for the dehydration and cyclization of lactic acid to lactide.

[0036] Figure 7 Catalyst stability profile for the catalyst used in Example 4 for the dehydration and cyclization of lactic acid to lactide.

[0037] Figure 8 Catalyst stability profile for the catalyst used in Example 1 after 6 runs of catalyzing the dehydration and cyclization of lactic acid to lactide.

[0038] Figure 9 Catalyst stability profile for the catalyst used in Comparative Example 2 for the dehydration and cyclization of lactic acid to lactide.

[0039] Figure 10 Catalyst stability profile for the catalyst used in Comparative Example 2 after 2 runs of catalyzing the dehydration and cyclization of lactic acid to lactide.

[0040] Figure 11 Catalyst stability profile for the catalyst used in Comparative Example 3 for the dehydration and cyclization of lactic acid to lactide. DETAILED DESCRIPTION

[0041] A method for preparing a highly stable catalyst for the dehydration of lactic acid to lactide, comprising the steps of:

[0042] 1) drying the HZSM-5 molecular sieve at 90-160 °C for 2-8 h to remove adsorbed free water;

[0043] 2) calcining the dried molecular sieve of step 1) at 350-650 °C for 4-10 h to dissociate ammonium and remove residual template impurities, to obtain a purified HZSM-5;

[0044] 3) dissolving the magnesium organate in a solvent to form a solution, and then uniformly spraying the solution onto the surface of the purified HZSM-5 obtained in step 2) (with stirring during the spraying process), to obtain a catalyst precursor;

[0045] 4) drying the catalyst precursor obtained in step 3) at 90-160°C for 2-8 h, and calcining at 350-650°C for 4-10 h, thereby constructing a certain amount of highly dispersed MgO basic sites on the surface of the molecular sieve, to obtain a MgO basic site modified HZSM-5 molecular sieve catalyst MgO-HZSM-5, wherein the MgO content is 0.1wt%-2.0wt%.

[0046] The HZSM-5 molecular sieve has a specific surface area of 150-600 m 2 / g, a pore volume of 0.1-1.1 cm 3 / g, and a Si / Al molar ratio of 10-200.

[0047] The preparation method of the high-stability catalyst for preparing lactide by dehydrating lactic acid according to the present application is further illustrated by the following examples. The examples are implemented on the premise of the technical solutions of the present application, and detailed implementation manners, specific operation steps, and technical parameters are given in the examples and comparative examples, but the protection scope of the present application is not limited to the examples. Unless otherwise specified, the experimental methods in the examples are all conventional operations in the field, and the experimental materials or reagents used in the examples can be obtained from the market.

[0048] The one-step method is used for preparing lactide by dehydrating and cyclizing lactic acid, specifically, a certain amount of molecular sieve catalyst, lactic acid aqueous solution, and dimethylbenzene are weighed and sequentially added into a reaction kettle, and then reacted at 130-155°C for 1-4 h. A water separator is installed on the top of the reaction kettle for solvent recycling, and calcium chloride water absorbent is installed in the water separator.

[0049] The mass ratio of the catalyst to the lactic acid aqueous solution is 0.01:1-1:1, the mass ratio of dimethylbenzene to the lactic acid aqueous solution is 1:1-20:1, and the dimethylbenzene can be one or more of o-dimethylbenzene, m-dimethylbenzene, and p-dimethylbenzene. The concentration of the lactic acid aqueous solution is 10wt%-90wt%.

[0050] The analysis of the reaction product is performed by using an Agilent 1260 liquid chromatograph equipped with a C18 column and an SPD10 ultraviolet-visible detector. Two kinds of solvents are used in the elution program: a) water / acetonitrile 95 / 5 (v / v, adding 2 mL of 85% phosphoric acid / L); b) 100% acetonitrile (adding 2 mL of 85% phosphoric acid / L). The elution program is as follows:

[0051] 0-1 min: 100 / 0 (v / v);

[0052] 1-15 min: linearly adjusted to 20 / 80 (v / v);

[0053] 15-20 min: 20 / 80 (v / v);

[0054] 20-30 min: return to 100 / 0 (v / v).

[0055] The water / acetonitrile phase can be used both as sample solvent and as eluent mobile phase, allowing the complete dissolution and separation of the polar hydrophilic lactic acid, lactic acid oligomers (L n A, n<5) and lactide and macromolecular lactic acid polymers (L n A, n≥5).

[0056] The optical purity of lactide was analyzed by gas chromatography. An Agilent 7820A gas chromatograph was used, equipped with a CycloSil-B chiral column (30 m x 0.25 mm x 0.25 pm) and an FID detector. During analysis, the injection port temperature was set to 240 °C, the initial column temperature in the oven was 65 °C, maintained for 5 min, then heated to 220 °C at a rate of 15 °C / min and maintained for 20 min.

[0057] Example 1

[0058] A preparation method of a high-stability catalyst comprises the following steps:

[0059] (1) 100 parts of NH4-ZSM-5 zeolite (atomic molar ratio Si / Al = 15, specific surface area 402 m 2 / g) by weight were weighed in an oven at 130 °C for 4 h to remove adsorbed free water.

[0060] (2) The NH4-ZSM-5 zeolite dried in step (1) was calcined in a muffle furnace at 550 °C for 4.5 h to dissociate ammonium and further remove residual template impurities, to obtain purified HZSM-5;

[0061] (3) The HZSM-5 obtained in step (2) was transferred to a high-speed mixer. 8.29 parts of magnesium gluconate was dissolved in 65.31 parts of deionized water to prepare a magnesium salt solution, and then the magnesium salt solution was sprayed onto the surface of the HZSM-5, and the mixer was set to rotate at 200 r / min. After the spraying solution was finished, the mixer continued to run for 5 min to obtain a catalyst precursor;

[0062] (4) The catalyst precursor obtained in step (3) was transferred to an oven and dried at 130 °C for 4 h, and then transferred to a muffle furnace and calcined at 550 °C for 2 h to obtain MgO-HZSM-5, wherein the content of MgO is about 0.8 wt%.

[0063] Take 5.5 parts of the prepared MgO-HZSM-5 molecular sieve catalyst, 100 parts of 80% lactic acid aqueous solution, and 100 parts of o-xylene, respectively, and add them into the reaction kettle in turn, and react at 148℃ for 2 hours. After the reaction is completed, the catalyst is separated by centrifugation, and the same mass of solvent and lactic acid is added again with the same feeding ratio to repeat the lactic acid dehydration and cyclization experiment, and the yield of lactide product is recorded.

[0064] Example 2

[0065] In example 1 step (3), the amount of magnesium gluconate is changed to 5.18 parts, and the other conditions are the same as in example 1, and MgO-HZSM-5 is prepared, wherein the content of MgO is about 0.5wt%. And according to the same method of example 1, the lactic acid dehydration and cyclization experiment is repeated.

[0066] Example 3

[0067] In example 1 step (3), the amount of magnesium gluconate is changed to 16.58 parts, and the other conditions are the same as in example 1, and MgO-HZSM-5 is prepared, wherein the content of MgO is about 1.6wt%. And according to the same method of example 1, the lactic acid dehydration and cyclization experiment is repeated.

[0068] Example 4

[0069] A method for preparing a high-stability catalyst, comprising the following steps:

[0070] (1) 100 parts of HZSM-5 molecular sieve (atomic molar ratio Si / Al = 40, specific surface area 337 m 2 / g) are weighed by weight parts in an oven, dried at 130℃ for 4 hours to remove adsorbed free water.

[0071] (2) The HZSM-5 dried in step (1) is calcined in a muffle furnace at 550℃ for 4.5 hours to further remove residual template impurities, and a purified HZSM-5 is obtained;

[0072] (3) The HZSM-5 obtained in step (2) is transferred to a high-speed mixer. 14.78 parts of magnesium stearate is dissolved in 42.6 parts of dimethylbenzene to prepare a solution, and then the solution is sprayed onto the surface of the HZSM-5. The speed of the mixer is set to 100 r / min, and after the spraying of the solution is completed, the mixer continues to run for 5 minutes to obtain a catalyst precursor;

[0073] (4) The catalyst precursor obtained in step (3) is transferred to an oven and dried at 150℃ for 4 hours, and then transferred to a muffle furnace and calcined at 550℃ for 4 hours to obtain MgO-HZSM-5, wherein the content of MgO is 1.0wt%.

[0074] 4.0 parts of the prepared MgO-HZSM-5 molecular sieve catalyst, 70 parts of 80% lactic acid aqueous solution, and 120 parts of o-xylene were weighed and added sequentially to a reaction vessel, and reacted at 148℃ for 2 h. After the reaction, the catalyst was separated by centrifugation of the reaction solution. The same mass of solvent and lactic acid were added again in the same feed ratio, and the lactic acid dehydration cyclization experiment was repeated. The yield of lactide product was recorded.

[0075] Comparative Example 1

[0076] Weigh out β molecular sieve (atomic molar ratio Si / Al = 12.5, specific surface area 652 m²). 2 The MgO-β molecular sieve catalyst was prepared by modifying the MgO basic sites according to the method described in Example 1. Then, a lactic acid dehydration cyclization experiment was conducted using the same method as in Example 1. The results showed that the lactide yield in the first reaction was only 65.3%, far less than that achieved using ZSM-5 molecular sieve, proving that ZSM-5 molecular sieve is more suitable as a support.

[0077] Comparative Example 2

[0078] The lactic acid dehydration cyclization experiment was repeated in the same manner as in Example 1 without modification using the same batch of HZSM-5 from Example 1.

[0079] Figures 1-3 The images show the XRD patterns of the catalysts used in Examples 1, 4, and Comparative Example 2, respectively. Figures 1-3 The comparison shows that the XRD pattern of ZSM-5 surface modified with a small amount of MgO does not show diffraction peaks of MgO and its derivatives, indicating that MgO is highly dispersed on the surface of the molecular sieve, without obvious agglomeration, and the bulk structure of the molecular sieve does not change significantly.

[0080] Figures 4-7 The figures show the catalyst stability for the preparation of lactide via lactic acid dehydration cyclization in Examples 1-4, respectively. Figures 4-7 The comparison shows that when using the HZSM-5 catalyst modified with basic MgO sites for lactic acid dehydration cyclization experiments, the lactide yield of the catalyst obtained in Example 1 was 92.4% in the first experiment. Repeating the sixth catalytic lactic acid dehydration cyclization experiment, the lactide yield remained above 87%, demonstrating better stability than the catalysts in Examples 2 and 3, and the catalyst color after the reaction remained close to white (e.g., ...). Figure 8 In Example 4, the use of magnesium stearate, which has a larger molecular structure size, as a precursor made it more difficult for the stearate to enter the molecular sieve channels during impregnation. As a result, the inhibition effect on acidic sites within the molecular sieve channels was lower, leading to a lactide yield of 93.2% in the first experiment. Furthermore, the catalyst exhibited higher stability in use, demonstrating that the size of the magnesium oxide precursor molecular structure directly affects the performance of the resulting catalyst.

[0081] Meanwhile, by Figure 4 The catalyst stability chart for preparing lactide by dehydration and cyclization of lactic acid in Comparative Example 2 Figure 9 As can be seen from the comparison, when the unmodified HZSM-5 catalyst in Comparative Example 2 is used to carry out the dehydration and cyclization experiment of lactic acid, the yield of lactide in the first experiment can reach 84.6%, but the catalyst changes from white to yellow brown after the reaction. When the second dehydration and cyclization experiment of lactic acid is repeated, the yield of lactide decreases to 73.7%, and when the third lactide yield decreases to 53.8%, it is only about 63% of the initial activity, and at this time the catalyst surface has turned dark brown (as shown in Figure 10 ), indicating that the catalyst surface has been significantly adsorbed or wrapped by by-products, which has seriously blocked the catalyst surface pores, seriously affecting the production efficiency of lactic acid dehydration. This shows that the basic sites of MgO can help to degrade the lactic acid oligomers generated by lactic acid self-polymerization, greatly reducing the accumulation, condensation or coking of lactic acid oligomers and other by-products on the catalyst surface, thereby avoiding or greatly inhibiting the blockage of the molecular sieve surface micropores by by-products, and ultimately improving the service life or cycle of the catalyst.

[0082] Comparative Example 3

[0083] HZSM-5 molecular sieve is modified using small molecule magnesium nitrate inorganic salt as precursor, which is specifically that the magnesium gluconate in step (3) of Example 1 is replaced by 5.128 parts of Mg(NO3)2·6H2O, and the content of MgO in the finally obtained catalyst is still 0.8%. And the dehydration and cyclization experiment of lactic acid is repeated according to the same method of Example 1.

[0084] Figure 11 The catalyst stability chart for preparing lactide by dehydration and cyclization of lactic acid in Comparative Example 3. From Figure 11 It can be seen that when the Mg(NO3)2·6H2O small molecule inorganic salt is used as the precursor to prepare the MgO-HZSM-5 molecular sieve for the dehydration and cyclization experiment of lactic acid, the yield of lactide in the first experiment is only 72.3%, which is much lower than the activity of the catalyst using large molecule organic magnesium as the precursor. The possible reason is that the small molecule magnesium nitrate enters the molecular sieve pores during the HZSM-5 impregnation process, and the MgO basic sites generated in the pores after drying and calcination partially destroy the internal acid sites of the HZSM-5 molecular sieve, weaken the synergistic effect of the acid sites and the pore confinement effect of the molecular sieve, and thus the catalytic activity of the catalyst for the dehydration and cyclization of lactic acid is seriously reduced.

[0085] Comparative Example 4

[0086] A preparation method of a catalyst, comprising the following steps:

[0087] (1) 100 parts of HZSM-5 molecular sieve (atomic molar ratio Si / Al = 40, specific surface area 337 m2 / g) by weight was weighed in a high-speed mixer. 14.78 parts of magnesium stearate was dissolved in 42.6 parts of dimethylbenzene to prepare a solution, and then the solution was sprayed on the surface of the HZSM-5, the rotation speed of the mixer was set to 100 r / min, and after the spraying of the solution was completed, the mixer continued to run for 5 min to obtain a catalyst precursor; 2

[0088] (2) The catalyst precursor obtained in step (1) was transferred to an oven and dried at 150°C for 4h, and then transferred to a muffle furnace and calcined at 550°C for 4h. Since the molecular sieve was not pretreated by drying and calcination, the molecular sieve absorbed water seriously, and the dimethylbenzene solution of magnesium stearate was difficult to be effectively adsorbed on the molecular sieve powder due to the influence of surface tension and other factors, so that the MgO-HZSM-5 catalyst with high dispersion of MgO basic sites could not be obtained.

[0089] The above is only the preferred embodiment of the present application, and any equivalent changes and modifications made within the scope of the patent application of the present application shall be included in the scope of the present application.​

Claims

1. Use of a highly stable catalyst in the dehydration of lactic acid to lactide, characterized in that, The high-stability catalyst is prepared by the following steps: drying, calcining HZSM-5 molecular sieve, impregnating the dried and calcined HZSM-5 molecular sieve with magnesium organic acid, and then drying and calcining the impregnated HZSM-5 molecular sieve again to form MgO basic sites on the surface of the HZSM-5 molecular sieve, thereby obtaining the MgO-HZSM-5 catalyst. The magnesium organic acid includes one or more of magnesium citrate, magnesium malate, magnesium tartrate and magnesium gluconate, or one or more of magnesium myristate, magnesium laurate, magnesium palmitate and magnesium stearate.

2. Use according to claim 1, characterized in that, The HZSM-5 molecular sieve has a specific surface area of 150-600 m 2 / g, a pore volume of 0.1-1.1 cm 3 / g, and a Si / Al molar ratio of 10-200.

3. Use according to claim 1, characterized in that, The impregnation is equal-volume impregnation.

4. Use according to claim 1, characterized in that, The temperature of the two drying processes is 90-160 DEG C, and the time of the two drying processes is 2-8 h.

5. Use according to claim 1, characterized in that, The temperature of the two calcining processes is 350-650 DEG C, and the time of the two calcining processes is 4-10 h.

6. Use according to claim 1, characterized in that, The content of MgO in the MgO-HZSM-5 catalyst is 0.1wt%-2.0wt%.

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