CN116060088A - Catalyst for preparing lactide by lactic acid dehydration and synthesis method and application thereof - Google Patents
Catalyst for preparing lactide by lactic acid dehydration and synthesis method and application thereof Download PDFInfo
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- CN116060088A CN116060088A CN202111279104.7A CN202111279104A CN116060088A CN 116060088 A CN116060088 A CN 116060088A CN 202111279104 A CN202111279104 A CN 202111279104A CN 116060088 A CN116060088 A CN 116060088A
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- JVTAAEKCZFNVCJ-UHFFFAOYSA-N lactic acid Chemical compound CC(O)C(O)=O JVTAAEKCZFNVCJ-UHFFFAOYSA-N 0.000 title claims abstract description 120
- 239000003054 catalyst Substances 0.000 title claims abstract description 101
- JJTUDXZGHPGLLC-UHFFFAOYSA-N lactide Chemical compound CC1OC(=O)C(C)OC1=O JJTUDXZGHPGLLC-UHFFFAOYSA-N 0.000 title claims abstract description 77
- 239000004310 lactic acid Substances 0.000 title claims abstract description 60
- 235000014655 lactic acid Nutrition 0.000 title claims abstract description 60
- 230000018044 dehydration Effects 0.000 title claims abstract description 12
- 238000006297 dehydration reaction Methods 0.000 title claims abstract description 12
- 238000001308 synthesis method Methods 0.000 title claims description 19
- 229910004298 SiO 2 Inorganic materials 0.000 claims abstract description 120
- 239000002245 particle Substances 0.000 claims abstract description 84
- XEEYBQQBJWHFJM-UHFFFAOYSA-N iron Substances [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 claims abstract description 83
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 claims abstract description 65
- HEMHJVSKTPXQMS-UHFFFAOYSA-M Sodium hydroxide Chemical compound [OH-].[Na+] HEMHJVSKTPXQMS-UHFFFAOYSA-M 0.000 claims abstract description 48
- 238000006243 chemical reaction Methods 0.000 claims abstract description 43
- 239000011258 core-shell material Substances 0.000 claims abstract description 41
- 238000002156 mixing Methods 0.000 claims abstract description 27
- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 claims abstract description 22
- 229910052710 silicon Inorganic materials 0.000 claims abstract description 22
- 239000010703 silicon Substances 0.000 claims abstract description 22
- LFQSCWFLJHTTHZ-UHFFFAOYSA-N Ethanol Chemical compound CCO LFQSCWFLJHTTHZ-UHFFFAOYSA-N 0.000 claims abstract description 18
- 239000002105 nanoparticle Substances 0.000 claims abstract description 13
- LZZYPRNAOMGNLH-UHFFFAOYSA-M Cetrimonium bromide Chemical compound [Br-].CCCCCCCCCCCCCCCC[N+](C)(C)C LZZYPRNAOMGNLH-UHFFFAOYSA-M 0.000 claims abstract description 9
- AOTOMVRCWUMWDC-UHFFFAOYSA-N CCCCP(CCCC)(CCCC)CCCC.O Chemical compound CCCCP(CCCC)(CCCC)CCCC.O AOTOMVRCWUMWDC-UHFFFAOYSA-N 0.000 claims abstract description 8
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 claims abstract description 7
- 238000000034 method Methods 0.000 claims description 47
- YXFVVABEGXRONW-UHFFFAOYSA-N Toluene Chemical compound CC1=CC=CC=C1 YXFVVABEGXRONW-UHFFFAOYSA-N 0.000 claims description 30
- UHOVQNZJYSORNB-UHFFFAOYSA-N Benzene Chemical compound C1=CC=CC=C1 UHOVQNZJYSORNB-UHFFFAOYSA-N 0.000 claims description 27
- BOTDANWDWHJENH-UHFFFAOYSA-N Tetraethyl orthosilicate Chemical compound CCO[Si](OCC)(OCC)OCC BOTDANWDWHJENH-UHFFFAOYSA-N 0.000 claims description 19
- 239000002808 molecular sieve Substances 0.000 claims description 17
- URGAHOPLAPQHLN-UHFFFAOYSA-N sodium aluminosilicate Chemical compound [Na+].[Al+3].[O-][Si]([O-])=O.[O-][Si]([O-])=O URGAHOPLAPQHLN-UHFFFAOYSA-N 0.000 claims description 17
- 238000003756 stirring Methods 0.000 claims description 16
- 238000001035 drying Methods 0.000 claims description 15
- 239000003960 organic solvent Substances 0.000 claims description 15
- 238000005406 washing Methods 0.000 claims description 15
- YNQLUTRBYVCPMQ-UHFFFAOYSA-N Ethylbenzene Chemical compound CCC1=CC=CC=C1 YNQLUTRBYVCPMQ-UHFFFAOYSA-N 0.000 claims description 14
- 239000012071 phase Substances 0.000 claims description 13
- 239000007771 core particle Substances 0.000 claims description 12
- JVTAAEKCZFNVCJ-REOHCLBHSA-N L-lactic acid Chemical compound C[C@H](O)C(O)=O JVTAAEKCZFNVCJ-REOHCLBHSA-N 0.000 claims description 10
- 230000003287 optical effect Effects 0.000 claims description 10
- 238000002425 crystallisation Methods 0.000 claims description 9
- 230000008025 crystallization Effects 0.000 claims description 9
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 claims description 8
- 238000005191 phase separation Methods 0.000 claims description 8
- KHMOASUYFVRATF-UHFFFAOYSA-J tin(4+);tetrachloride;pentahydrate Chemical compound O.O.O.O.O.Cl[Sn](Cl)(Cl)Cl KHMOASUYFVRATF-UHFFFAOYSA-J 0.000 claims description 8
- 239000012074 organic phase Substances 0.000 claims description 6
- 229930182843 D-Lactic acid Natural products 0.000 claims description 5
- JVTAAEKCZFNVCJ-UWTATZPHSA-N D-lactic acid Chemical compound C[C@@H](O)C(O)=O JVTAAEKCZFNVCJ-UWTATZPHSA-N 0.000 claims description 5
- 239000008346 aqueous phase Substances 0.000 claims description 5
- 238000001354 calcination Methods 0.000 claims description 5
- 229940022769 d- lactic acid Drugs 0.000 claims description 5
- 239000007788 liquid Substances 0.000 claims description 5
- 238000010992 reflux Methods 0.000 claims description 5
- 239000000377 silicon dioxide Substances 0.000 claims description 4
- 238000006482 condensation reaction Methods 0.000 claims description 3
- 238000004821 distillation Methods 0.000 claims description 3
- 238000000926 separation method Methods 0.000 claims description 3
- LFQCEHFDDXELDD-UHFFFAOYSA-N tetramethyl orthosilicate Chemical compound CO[Si](OC)(OC)OC LFQCEHFDDXELDD-UHFFFAOYSA-N 0.000 claims description 3
- KSBAEPSJVUENNK-UHFFFAOYSA-L tin(ii) 2-ethylhexanoate Chemical compound [Sn+2].CCCCC(CC)C([O-])=O.CCCCC(CC)C([O-])=O KSBAEPSJVUENNK-UHFFFAOYSA-L 0.000 claims description 3
- CTQNGGLPUBDAKN-UHFFFAOYSA-N O-Xylene Chemical compound CC1=CC=CC=C1C CTQNGGLPUBDAKN-UHFFFAOYSA-N 0.000 claims description 2
- 229910006404 SnO 2 Inorganic materials 0.000 claims description 2
- 230000007935 neutral effect Effects 0.000 claims description 2
- 230000035484 reaction time Effects 0.000 claims description 2
- 238000004064 recycling Methods 0.000 claims description 2
- 239000008096 xylene Substances 0.000 claims description 2
- 150000001555 benzenes Chemical class 0.000 claims 1
- 239000013043 chemical agent Substances 0.000 claims 1
- 238000011084 recovery Methods 0.000 abstract description 7
- 238000007363 ring formation reaction Methods 0.000 abstract description 4
- QGZKDVFQNNGYKY-UHFFFAOYSA-N Ammonia Chemical compound N QGZKDVFQNNGYKY-UHFFFAOYSA-N 0.000 abstract 1
- 238000010189 synthetic method Methods 0.000 abstract 1
- 239000000047 product Substances 0.000 description 32
- 230000015572 biosynthetic process Effects 0.000 description 10
- 239000000243 solution Substances 0.000 description 10
- 238000003786 synthesis reaction Methods 0.000 description 10
- 230000000052 comparative effect Effects 0.000 description 9
- 230000000694 effects Effects 0.000 description 9
- 238000012360 testing method Methods 0.000 description 9
- 239000000463 material Substances 0.000 description 6
- 239000011259 mixed solution Substances 0.000 description 6
- 239000002994 raw material Substances 0.000 description 6
- 230000007797 corrosion Effects 0.000 description 5
- 238000005260 corrosion Methods 0.000 description 5
- CSDREXVUYHZDNP-UHFFFAOYSA-N alumanylidynesilicon Chemical compound [Al].[Si] CSDREXVUYHZDNP-UHFFFAOYSA-N 0.000 description 4
- 238000009833 condensation Methods 0.000 description 4
- 230000005494 condensation Effects 0.000 description 4
- 239000000203 mixture Substances 0.000 description 4
- 230000006340 racemization Effects 0.000 description 4
- 230000002194 synthesizing effect Effects 0.000 description 4
- JJTUDXZGHPGLLC-IMJSIDKUSA-N 4511-42-6 Chemical compound C[C@@H]1OC(=O)[C@H](C)OC1=O JJTUDXZGHPGLLC-IMJSIDKUSA-N 0.000 description 3
- NIPNSKYNPDTRPC-UHFFFAOYSA-N N-[2-oxo-2-(2,4,6,7-tetrahydrotriazolo[4,5-c]pyridin-5-yl)ethyl]-2-[[3-(trifluoromethoxy)phenyl]methylamino]pyrimidine-5-carboxamide Chemical compound O=C(CNC(=O)C=1C=NC(=NC=1)NCC1=CC(=CC=C1)OC(F)(F)F)N1CC2=C(CC1)NN=N2 NIPNSKYNPDTRPC-UHFFFAOYSA-N 0.000 description 3
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 3
- 238000004587 chromatography analysis Methods 0.000 description 3
- 229910052742 iron Inorganic materials 0.000 description 3
- -1 iron ions Chemical class 0.000 description 3
- 239000006249 magnetic particle Substances 0.000 description 3
- 229920000747 poly(lactic acid) Polymers 0.000 description 3
- 239000004626 polylactic acid Substances 0.000 description 3
- 238000001179 sorption measurement Methods 0.000 description 3
- KFZMGEQAYNKOFK-UHFFFAOYSA-N Isopropanol Chemical compound CC(C)O KFZMGEQAYNKOFK-UHFFFAOYSA-N 0.000 description 2
- 238000002474 experimental method Methods 0.000 description 2
- 230000007062 hydrolysis Effects 0.000 description 2
- 238000006460 hydrolysis reaction Methods 0.000 description 2
- 238000005470 impregnation Methods 0.000 description 2
- 238000007086 side reaction Methods 0.000 description 2
- JIAARYAFYJHUJI-UHFFFAOYSA-L zinc dichloride Chemical compound [Cl-].[Cl-].[Zn+2] JIAARYAFYJHUJI-UHFFFAOYSA-L 0.000 description 2
- 229910018072 Al 2 O 3 Inorganic materials 0.000 description 1
- XDTMQSROBMDMFD-UHFFFAOYSA-N Cyclohexane Chemical compound C1CCCCC1 XDTMQSROBMDMFD-UHFFFAOYSA-N 0.000 description 1
- 229910021627 Tin(IV) chloride Inorganic materials 0.000 description 1
- 239000002253 acid Substances 0.000 description 1
- 229910052782 aluminium Inorganic materials 0.000 description 1
- 230000009286 beneficial effect Effects 0.000 description 1
- VEFXTGTZJOWDOF-UHFFFAOYSA-N benzene;hydrate Chemical compound O.C1=CC=CC=C1 VEFXTGTZJOWDOF-UHFFFAOYSA-N 0.000 description 1
- 229920000704 biodegradable plastic Polymers 0.000 description 1
- 230000003197 catalytic effect Effects 0.000 description 1
- 239000007795 chemical reaction product Substances 0.000 description 1
- 239000003153 chemical reaction reagent Substances 0.000 description 1
- 239000011248 coating agent Substances 0.000 description 1
- 238000000576 coating method Methods 0.000 description 1
- 230000007547 defect Effects 0.000 description 1
- 238000011161 development Methods 0.000 description 1
- 238000010812 external standard method Methods 0.000 description 1
- 238000005216 hydrothermal crystallization Methods 0.000 description 1
- 238000011065 in-situ storage Methods 0.000 description 1
- 238000004811 liquid chromatography Methods 0.000 description 1
- 239000007791 liquid phase Substances 0.000 description 1
- 238000004519 manufacturing process Methods 0.000 description 1
- 239000002923 metal particle Substances 0.000 description 1
- 239000012452 mother liquor Substances 0.000 description 1
- 239000013502 plastic waste Substances 0.000 description 1
- 230000004043 responsiveness Effects 0.000 description 1
- 230000002441 reversible effect Effects 0.000 description 1
- 239000002904 solvent Substances 0.000 description 1
- 239000000126 substance Substances 0.000 description 1
- 230000002195 synergetic effect Effects 0.000 description 1
- 239000013638 trimer Substances 0.000 description 1
- 239000011592 zinc chloride Substances 0.000 description 1
- 235000005074 zinc chloride Nutrition 0.000 description 1
Classifications
-
- B01J35/33—
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
- B01J29/00—Catalysts comprising molecular sieves
- B01J29/04—Catalysts comprising molecular sieves having base-exchange properties, e.g. crystalline zeolites
-
- B01J35/396—
-
- B01J35/40—
-
- C—CHEMISTRY; METALLURGY
- C07—ORGANIC CHEMISTRY
- C07D—HETEROCYCLIC COMPOUNDS
- C07D319/00—Heterocyclic compounds containing six-membered rings having two oxygen atoms as the only ring hetero atoms
- C07D319/10—1,4-Dioxanes; Hydrogenated 1,4-dioxanes
- C07D319/12—1,4-Dioxanes; Hydrogenated 1,4-dioxanes not condensed with other rings
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
- B01J2229/00—Aspects of molecular sieve catalysts not covered by B01J29/00
- B01J2229/10—After treatment, characterised by the effect to be obtained
- B01J2229/18—After treatment, characterised by the effect to be obtained to introduce other elements into or onto the molecular sieve itself
- B01J2229/183—After treatment, characterised by the effect to be obtained to introduce other elements into or onto the molecular sieve itself in framework positions
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
- B01J2229/00—Aspects of molecular sieve catalysts not covered by B01J29/00
- B01J2229/10—After treatment, characterised by the effect to be obtained
- B01J2229/24—After treatment, characterised by the effect to be obtained to stabilize the molecular sieve structure
-
- C—CHEMISTRY; METALLURGY
- C07—ORGANIC CHEMISTRY
- C07B—GENERAL METHODS OF ORGANIC CHEMISTRY; APPARATUS THEREFOR
- C07B2200/00—Indexing scheme relating to specific properties of organic compounds
- C07B2200/07—Optical isomers
Abstract
The invention relates to a catalyst for preparing lactide by lactic acid dehydration, a synthetic method and application thereof, which comprises (1) preparing silicon source, sodium hydroxide, water and Fe 3 O 4 Mixing nano particles in proportion to prepare Fe 3 O 4 @SiO 2 Particles; (2) Mixing a silicon source, tetrabutylphosphine hydroxide, ethanol, water and a tin source in proportion, and adding Fe 3 O 4 @SiO 2 Standing and crystallizing the particles to obtain Fe 3 O 4 @SiO 2 An @ Sn-SPP particle; (3) Mixing silicon source, sodium hydroxide, hexadecyl trimethyl ammonium bromide and water, adding Fe 3 O 4 @SiO 2 The @ Sn-SPP particles produce Fe 3 O 4 @SiO 2 @Sn‑SPP@meso‑SiO 2 A core-shell catalyst. The catalyst synthesized by the invention is used for preparing lactide by lactic acid dehydration cyclization, and has the advantages of mild reaction conditions, high lactide yield and purity, easy and rapid recovery of the catalyst, good repeated use stability and the like.
Description
Technical Field
The invention belongs to the field of synthesis of biodegradable materials, and particularly relates to a catalyst for preparing lactide by lactic acid dehydration, a synthesis method and application thereof.
Background
In recent years, plastic waste pollution has become one of the important pollution sources for human ecological environment, and biodegradable plastics are considered as the most effective way to solve the above problems. Polylactic acid is a novel green environment-friendly material integrating resource reproducibility, environment and biological complete degradability, biocompatibility and good processability, and is praised as the most development prospect. Lactide is used as a key intermediate of biodegradable polylactic acid material, and has important significance for producing high-quality polylactic acid.
In recent years, the process for preparing lactide by directly dehydrating and cyclizing lactic acid under the action of a catalyst has been widely paid attention to by researchers. CN108610323a discloses a method for preparing lactide: lactic acid is heated and reacted under the condition of reduced pressure in the presence of an H-Beta molecular sieve to obtain lactide; the reaction pressure is 50-1500Pa, and the reaction temperature is 120-150 ℃. The invention directly completes the synthesis of lactic acid to lactide to obtain lactide with high optical purity (96%). But also has the problems that high vacuum condition is needed in the reaction process, aluminum element in the framework of the silicon-aluminum H-Beta molecular sieve is easy to be corroded and removed by the lactic acid raw material with stronger acidity, and the like. In addition, in the reaction of preparing lactide by direct dehydration cyclization of lactic acid, lactic acid is easily deeply polycondensed into trimer, tetramer and even multimer under the action of the surface acid activity of a molecular sieve, so that the primary yield of lactide is low.
In recent years with Fe 3 O 4 The typical magnetic particles are widely applied due to higher magnetic responsiveness, particularly, the magnetic particles and the catalytic material are compounded to prepare the magnetic catalyst, and then the magnetic catalyst is quickly recovered by using an external magnetic field, so that a new thought is provided for recovering the catalyst from a reaction system. However, in the reaction process of preparing lactide by dehydration of lactic acid, lactic acid is also presentRaw material pair Fe 3 O 4 The corrosion of the magnetic particles and the generated iron ions can increase racemization of lactide, resulting in the decrease of optical purity of lactide products.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides a catalyst for preparing lactide by lactic acid dehydration and a synthesis method and application thereof. The catalyst synthesized by the invention is used for preparing lactide by lactic acid dehydration cyclization, and has the advantages of mild reaction conditions, high lactide yield and purity, easy and rapid recovery of the catalyst, good repeated use stability and the like.
The invention provides a method for synthesizing a catalyst for preparing lactide by lactic acid dehydration, which comprises the following steps:
(1) Silicon source, sodium hydroxide, water and Fe 3 O 4 Mixing the nano particles in proportion, stirring at a certain temperature, separating out product particles by using a magnet, washing and drying to obtain Fe 3 O 4 @SiO 2 Particles;
(2) Mixing a silicon source, tetrabutylphosphine hydroxide, ethanol, water and a tin source in proportion, and adding Fe in the step (1) 3 O 4 @SiO 2 Standing for crystallization, separating product particles with magnet, washing, and drying to obtain Fe 3 O 4 @SiO 2 An @ Sn-SPP particle;
(3) Mixing a silicon source, sodium hydroxide, cetyltrimethylammonium bromide and water in proportion, and adding Fe in the step (2) 3 O 4 @SiO 2 Stirring at a certain temperature to obtain particles of Sn-SPP, separating the particles with magnet, washing, drying, and calcining to obtain Fe 3 O 4 @SiO 2 @Sn-SPP@meso-SiO 2 A core-shell catalyst.
In the synthesis method of the present invention, the silicon source in the steps (1) to (3) is at least one of ethyl orthosilicate, methyl orthosilicate, etc., preferably ethyl orthosilicate.
In the synthesis method of the invention, fe in the step (1) 3 O 4 The particle size of the nano particles is 5-15nm.
The inventionIn the synthesis method, the silicon source, sodium hydroxide, water and Fe in the step (1) 3 O 4 The molar ratio of the nano particles is 1:0.2-0.4:80-120:3-5.
In the synthesis method, the treatment temperature in the step (1) is 30-60 ℃, the stirring speed is 500-1000r/min, and the treatment time is 3-6h.
In the synthesis method of the invention, the tin source in the step (2) is at least one of tin tetrachloride pentahydrate, stannous octoate and the like, and preferably the tin tetrachloride pentahydrate.
In the synthesis method, the molar ratio of the silicon source, the tetrabutylphosphine hydroxide, the ethanol, the water and the pentahydrate stannic chloride in the step (2) is 1:0.2-0.4:3-5:10-30:0.004-0.024.
In the synthesis method of the invention, fe in the step (2) 3 O 4 @SiO 2 Particles and a silicon source (in SiO 2 Content) is 1:10-20.
In the synthesis method, the crystallization temperature in the step (2) is 130-150 ℃ and the crystallization time is 2-5 days.
In the synthesis method, the molar ratio of the silicon source to the sodium hydroxide to the hexadecyl trimethyl ammonium bromide to the water in the step (3) is 1:0.2-0.4:0.25-0.4:80-120.
In the synthesis method of the invention, fe in the step (3) 3 O 4 @SiO 2 The @ Sn-SPP particles were mixed with a silicon source (in SiO 2 Content) is 10-20:1.
In the synthesis method, the treatment temperature in the step (3) is 30-60 ℃, the stirring speed is 500-1000r/min, and the treatment time is 3-9h.
In the synthesis method of the invention, the products obtained in the step (1) to the step (3) are separated into product particles by a magnet, washed to be neutral by water, and dried for 2 to 12 hours at the temperature of 80 to 100 ℃. Roasting the mixture for 3 to 5 hours at the temperature of 450 to 500 ℃ to finally obtain Fe 3 O 4 @SiO 2 @Sn-SPP@meso-SiO 2 A core-shell catalyst.
Fe according to the invention 3 O 4 @SiO 2 @Sn-SPP@meso-SiO 2 The core-shell catalyst adopts the method of the inventionPrepared by the method. The prepared core-shell catalyst is prepared by Fe 3 O 4 The nano particles are cores and SiO 2 And Sn-SPP molecular sieve are respectively two intermediate layers, mesoporous silica is used as an outer shell layer, the particle size of the core-shell catalyst is 300-700nm, and Fe is contained in the core-shell catalyst 3 O 4 The particle diameter of the core particles is 5-15nm, and the intermediate SiO 2 The thickness of the layer is 10-20nm, and the thickness of the shell layer is meso-SiO 2 Is 25-50nm thick. Based on the mass of the catalyst, siO 2 The content of the Fe is 87.8 to 95.6 percent 3 O 4 4.0 to 8.6 percent of SnO 2 The content is 0.4% -3.6%.
The invention also provides a method for preparing lactide by lactic acid, which is realized by adopting the core-shell catalyst prepared by the method, and the specific synthesis method comprises the following steps:
mixing lactic acid solution with organic solvent, adding Fe 3 O 4 @SiO 2 @Sn-SPP@meso-SiO 2 And (3) carrying out stirring reflux reaction on the core-shell catalyst at normal pressure and a certain temperature, adding water after the reaction is finished, carrying out two-phase separation, carrying out reduced pressure distillation on an organic phase to obtain a lactide product, and separating and recycling the catalyst from the water phase by an externally added magnet.
In the method for preparing lactide, the lactic acid is at least one of L-lactic acid, D-lactic acid and the like, the optical purity of the lactic acid is more than or equal to 99.5%, and the mass concentration of the lactic acid solution is 20% -80%.
In the above method for preparing lactide, the benzene-based organic solvent may be at least one of benzene, toluene, ethylbenzene, xylene, etc., preferably toluene.
In the above method for preparing lactide, the mass ratio of the catalyst to lactic acid is 1:0.5-5, preferably 1:2-3.
In the above method for preparing lactide, the solid-to-liquid ratio of the catalyst to the organic solvent is 1g:5-100mL, preferably 1g:10-30mL.
In the method for preparing lactide, the reaction temperature is 100-170 ℃, the stirring speed is 100-1000r/min, and the reaction time is 2-6h. And continuously removing free water of the lactic acid solution in the reaction system during the reflux reaction, and generating water molecules by the condensation reaction of lactic acid molecules.
In the method for preparing lactide, after the reaction is finished, water is added into a reaction system to carry out two-phase separation of an aqueous phase and an organic phase, wherein the volume ratio of the water to the organic solvent is 1-2:1.
In the method for preparing lactide, the lactide product is obtained from the organic phase obtained after the two-phase separation under the absolute pressure condition of 1-5kPa at the temperature of 100-120 ℃, and the organic solvent is recovered for repeated use.
In the method for preparing lactide, the water phase obtained after the separation of the two phases is stirred and reacts for 3 to 6 hours at the temperature of 100 to 150 ℃ and the stirring speed is 100 to 1000r/min. Fe recovery by magnet separation 3 O 4 @SiO 2 @Sn-SPP@meso-SiO 2 The core-shell catalyst is washed by water for multiple times, dried for 2-12 hours at 80-100 ℃ and reused.
Compared with the prior art, the invention has the following beneficial effects:
(1) The invention firstly comprises Fe 3 O 4 Growing a layer of SiO on the surface of the nano particle 2 The shell layer and the Sn-SPP molecular sieve are used for Fe 3 O 4 @SiO 2 Coating the particles to obtain Fe 3 O 4 @SiO 2 The @ Sn-SPP particles are finally coated with a layer of mesoporous silica to obtain Fe 3 O 4 @SiO 2 @Sn-SPP@meso-SiO 2 The core-shell catalyst is used as a catalyst for preparing lactide by directly dehydrating and cyclizing lactic acid, so that lactide can be synthesized in one step under the conditions of normal pressure and low temperature, racemization of the lactide at a higher reaction temperature is effectively avoided, and the optical purity of a lactide product is improved; and high vacuum operation conditions are avoided, and the operation cost is reduced. Has the advantages of mild reaction conditions, high yield and purity of lactide products, easy and quick recovery of catalyst, good repeated use stability and the like.
(2) By Fe 3 O 4 @SiO 2 @Sn-SPP@meso-SiO 2 The core-shell catalyst is used as a catalyst for synthesizing lactide from lactic acid, so that corrosion removal of lactic acid raw materials to aluminum element in a silicon-aluminum type molecular sieve framework at a higher reaction temperature is avoided, the structural stability of the molecular sieve framework is improved, and simultaneously, the lactic acid raw materials can be prevented from corroding the silicon-aluminum type molecular sieve frameworkFe 3 O 4 The corrosion of particles avoids the generated iron ions from aggravating racemization of lactide, and improves the optical purity of lactide products.
(3) By Fe 3 O 4 @SiO 2 @Sn-SPP@meso-SiO 2 The core-shell catalyst is used as a catalyst for preparing lactide by directly dehydrating and cyclizing lactic acid, so that deep condensation side reaction of lactic acid molecules can be avoided, and the yield of lactide products is effectively improved.
(4) Synthesizing Fe by adopting in-situ hydrothermal crystallization 3 O 4 @SiO 2 @Sn-SPP@meso-SiO 2 The core-shell catalyst enables Sn to be a firm molecular sieve framework component, so that the conditions that metal particles are easy to fall off and the like in the use process of the metal-supported molecular sieve catalyst prepared by the traditional impregnation method are avoided, and the reusability and high efficiency of the catalyst are ensured.
(5) By utilizing the solubility difference of lactic acid, lactic acid oligomer and lactide in an organic solvent, a benzene solvent is selected to dissolve a lactide product generated in the reaction process and then separate the lactide product from a reaction system, so that on one hand, side reactions such as hydrolysis, deep condensation and the like of the lactide product in the reaction system can be effectively avoided, on the other hand, the chemical balance of lactic acid condensation and lactide hydrolysis on reversible reaction can be effectively broken through by removing the lactide product from the reaction system, the occurrence of lactic acid condensation reaction is promoted, and the problem of thermodynamic balance limitation is effectively solved.
(6) The organic solvent, the core-shell catalyst and the like in the invention are easy to recycle and reuse, the utilization rate of raw materials is high, the equipment is simple and convenient, the cost is low, and the method has good industrial application prospect.
Detailed Description
The method and effects of the present invention will be described in further detail with reference to examples. The embodiments and specific operation procedures are given on the premise of the technical scheme of the invention, but the protection scope of the invention is not limited to the following embodiments.
The experimental methods in the following examples, unless otherwise specified, are all conventional in the art. The experimental materials used in the examples described below, unless otherwise specified, were purchased from conventional biochemical reagent stores.
The composition of the catalyst is detected by an ICPS-7510 type plasma emission spectrometer, the content of lactic acid and lactide in the reaction product is detected by an Agilent 1260 type liquid chromatography external standard method, and the optical purity of the lactide is determined by an Autopol VI polarimeter.
Example 1
(1) Ethyl orthosilicate, sodium hydroxide, water and Fe 3 O 4 Mixing nanoparticles (particle size of 10 nm) at a molar ratio of 1:0.3:100:3, treating the mixed solution at 40deg.C and 800r/min for 4 hr, separating the product particles by magnet adsorption, washing with water to neutrality, and drying at 100deg.C for 12 hr to obtain Fe 3 O 4 @SiO 2 And (3) particles.
(2) Mixing ethyl orthosilicate, tetrabutyl phosphine hydroxide, ethanol, water and tin tetrachloride pentahydrate according to the mol ratio of 1:0.3:4:20:0.015, and then mixing according to Fe 3 O 4 @SiO 2 Particles and ethyl orthosilicate (SiO) 2 Content meter) mass ratio of 1:15, adding Fe into the mixed solution 3 O 4 @SiO 2 Standing at 140deg.C for crystallization for 3 days, separating the product particles with magnet, washing with water to neutrality, and drying at 100deg.C for 12 hr to obtain Fe 3 O 4 @SiO 2 @ Sn-SPP particles.
(3) Mixing ethyl orthosilicate, sodium hydroxide, cetyltrimethylammonium bromide and water according to the mol ratio of 1:0.3:0.3:100, and then mixing according to Fe 3 O 4 @SiO 2 Sn-SPP particles and Ethyl orthosilicate (in SiO) 2 Content) in a mass ratio of 15:1, fe is added 3 O 4 @SiO 2 Treating Sn-SPP particle at 40deg.C and 800r/min for 6 hr, separating the product particles with magnet, washing with water to neutrality, drying at 100deg.C for 12 hr, and calcining at 500deg.C for 3 hr to obtain Fe 3 O 4 @SiO 2 @Sn-SPP@meso-SiO 2 A core-shell catalyst.
The particle size of the prepared core-shell catalyst is about 430nm, fe 3 O 4 The particle size of the core particles is about 10nm, and the intermediate SiO 2 Layer thickness of about15nm, outermost meso-SiO 2 The thickness is about 40nm. SnO based on the mass of the catalyst 2 The content is 2.12 percent, fe 3 O 4 Content 5.80%, siO 2 The content was 92.08%.
Example 2
(1) Ethyl orthosilicate, sodium hydroxide, water and Fe 3 O 4 Mixing nanoparticles (particle diameter of 5 nm) at a molar ratio of 1:0.2:120:5, treating the mixed solution at 30deg.C and 500r/min for 3 hr, separating the product particles by magnet adsorption, washing with water to neutrality, and drying at 100deg.C for 12 hr to obtain Fe 3 O 4 @SiO 2 And (3) particles.
(2) Mixing ethyl orthosilicate, tetrabutyl phosphine hydroxide, ethanol, water and tin tetrachloride pentahydrate according to the mol ratio of 1:0.2:3:10:0.006, and then mixing according to Fe 3 O 4 @SiO 2 Particles and ethyl orthosilicate (SiO) 2 Content meter) mass ratio of 1:20, adding Fe into the mixed solution 3 O 4 @SiO 2 Standing at 130deg.C for crystallization for 2 days, separating the product particles with magnet, washing with water to neutrality, and drying at 100deg.C for 12 hr to obtain Fe 3 O 4 @SiO 2 @ Sn-SPP particles.
(3) Mixing ethyl orthosilicate, sodium hydroxide, cetyltrimethylammonium bromide and water in a molar ratio of 1:0.2:0.25:120, and then mixing according to Fe 3 O 4 @SiO 2 Sn-SPP particles and Ethyl orthosilicate (in SiO) 2 Content) of 20:1, fe is added 3 O 4 @SiO 2 Treating Sn-SPP particles at 30deg.C and 500r/min for 3 hr, separating the product particles with magnet, washing with water to neutrality, drying at 100deg.C for 12 hr, and calcining at 500deg.C for 3 hr to obtain Fe 3 O 4 @SiO 2 @Sn-SPP@meso-SiO 2 A core-shell catalyst.
The particle size of the prepared core-shell catalyst is about 300nm, fe 3 O 4 The particle size of the core particles is about 5nm, and the intermediate SiO 2 The thickness of the layer is about 11nm, and the outermost layer is meso-SiO 2 The thickness is about 25nm. SnO based on the mass of the catalyst 2 The content is 0.69 percent, fe 3 O 4 Content 4.16%, siO 2 The content is 95.15 percent.
Example 3
(1) Ethyl orthosilicate, sodium hydroxide, water and Fe 3 O 4 Mixing nanoparticles (particle diameter of 15 nm) at a molar ratio of 1:0.4:80:3, treating the mixed solution at 60deg.C and 1000r/min for 6 hr, separating the product particles by magnet adsorption, washing with water to neutrality, and drying at 100deg.C for 12 hr to obtain Fe 3 O 4 @SiO 2 And (3) particles.
(2) Mixing ethyl orthosilicate, tetrabutyl phosphine hydroxide, ethanol, water and tin tetrachloride pentahydrate according to the mol ratio of 1:0.4:5:30:0.02, and then mixing according to Fe 3 O 4 @SiO 2 Particles and ethyl orthosilicate (SiO) 2 Content meter) mass ratio of 1:10, adding Fe into the mixed solution 3 O 4 @SiO 2 Standing at 150deg.C for crystallization for 5 days, separating the product particles with magnet, washing with water to neutrality, and drying at 100deg.C for 12 hr to obtain Fe 3 O 4 @SiO 2 @ Sn-SPP particles.
(3) Mixing ethyl orthosilicate, sodium hydroxide, cetyltrimethylammonium bromide and water in a molar ratio of 1:0.4:0.4:80, and then mixing according to Fe 3 O 4 @SiO 2 Sn-SPP particles and Ethyl orthosilicate (in SiO) 2 Content meter) mass ratio of Fe 3 O 4 @SiO 2 Treating Sn-SPP particles at 60deg.C and 1000r/min for 9 hr, separating the product particles with magnet, washing with water to neutrality, drying at 100deg.C for 12 hr, and calcining at 500deg.C for 3 hr to obtain Fe 3 O 4 @SiO 2 @Sn-SPP@meso-SiO 2 A core-shell catalyst.
The particle size of the prepared core-shell catalyst is about 660nm, fe 3 O 4 The particle size of the core particles is about 10nm, and the intermediate SiO 2 The thickness of the layer is about 21nm, and the outermost layer is meso-SiO 2 The thickness was about 48nm. SnO based on the mass of the catalyst 2 The content is 3.10 percent, fe 3 O 4 Content 7.40%, siO 2 The content is 89.5%.
Example 4
The difference from example 1 is that: the silicon source is methyl orthosilicate. Prepared byFe 3 O 4 @SiO 2 @Sn-SPP@meso-SiO 2 The particle size of the core-shell catalyst is about 400nm, fe 3 O 4 The particle size of the core particles is about 10nm, and the intermediate SiO 2 The thickness of the shell layer is about 12nm, and the outermost layer is meso-SiO 2 The thickness was about 31nm. SnO based on the mass of the catalyst 2 The content is 2.20 percent, fe 3 O 4 Content 5.90%, siO 2 The content was 91.9%.
Example 5
The difference from example 1 is that: the tin source is stannous octoate. Prepared Fe 3 O 4 @SiO 2 @Sn-SPP@meso-SiO 2 The particle size of the core-shell catalyst is about 440nm, fe 3 O 4 The particle size of the core particles is about 10nm, and the intermediate SiO 2 The thickness of the layer is about 15nm, and the outermost layer is meso-SiO 2 The thickness was about 39nm. SnO based on the mass of the catalyst 2 The content is 2.18 percent, fe 3 O 4 Content 5.63%, siO 2 The content is 92.19 percent.
Comparative example 1
The difference from example 1 is that: omitting step (3) and using Fe 3 O 4 @SiO 2 Replacing Fe with @ Sn-SPP catalyst 3 O 4 @SiO 2 @Sn-SPP@meso-SiO 2 A core-shell catalyst. Fe prepared 3 O 4 @SiO 2 Particle size of @ Sn-SPP is about 350nm, fe 3 O 4 The particle size of the core particles is about 10nm, and the intermediate SiO 2 The layer thickness was approximately 14nm. SnO based on the mass of the catalyst 2 The content is 2.25 percent, fe 3 O 4 Content 6.20%, siO 2 The content was 91.55%.
Comparative example 2
The difference from example 1 is that: step (1) is omitted, and Fe is directly added 3 O 4 Adding the Sn-SPP molecular sieve mother liquor to prepare Fe 3 O 4 The @ Sn-SPP particles are coated by mesoporous silica shells to obtain Fe 3 O 4 @Sn-SPP@meso-SiO 2 A core-shell catalyst. Fe prepared 3 O 4 @Sn-SPP@meso-SiO 2 The particle size of the core-shell catalyst is about 380nm, fe 3 O 4 The particle size of the core particle is about 10nm, and the outer layermeso-SiO 2 The thickness was about 41nm. SnO based on the mass of the catalyst 2 The content is 2.20 percent, fe 3 O 4 Content 6.00%, siO 2 The content was 91.8%.
Comparative example 3
The difference from example 1 is that: in the step (2), ethanol is replaced by isopropanol, and Fe cannot be obtained 3 O 4 @SiO 2 @ Sn-SPP particles.
Comparative example 4
The difference from example 1 is that: cetyl trimethylammonium bromide in step (3) is omitted. Fe prepared 3 O 4 @SiO 2 @Sn-SPP@SiO 2 The particle size of the core-shell particles is about 410nm, fe 3 O 4 The particle size of the core particles is about 10nm, and the intermediate SiO 2 The thickness of the layer is about 14nm, the outermost SiO layer 2 The thickness was about 29nm. SnO based on the mass of the catalyst 2 The content is 2.11 percent, fe 3 O 4 Content 5.78%, siO 2 The content is 92.11%.
Comparative example 5
The difference from example 1 is that: step (2) replacing tin tetrachloride pentahydrate with zinc chloride to synthesize Fe 3 O 4 @SiO 2 @Zn-SPP@meso-SiO 2 A core-shell catalyst. The particle size of the prepared core-shell particles is about 430nm, fe 3 O 4 The particle size of the core particles is about 10nm, and the intermediate SiO 2 The thickness of the layer is about 16nm, and the outermost layer is meso-SiO 2 The thickness was about 38nm. Based on the mass of the catalyst, the ZnO content is 1.58 percent, fe 3 O 4 Content 5.77%, siO 2 The content was 92.65%.
Comparative example 6
The difference from example 1 is that: sn-SPP molecular sieve prepared by traditional liquid phase impregnation method is firstly adopted, and then Fe is prepared by the method 3 O 4 @SiO 2 @Sn-SPP@meso-SiO 2 A core-shell catalyst. The particle size of the prepared core-shell catalyst is about 420nm, fe 3 O 4 The particle size of the core particles is about 10nm, and the intermediate SiO 2 The thickness of the layer is about 16nm, and the outermost layer is meso-SiO 2 The thickness is about 35nm. SnO based on the mass of the catalyst 2 Content 2.00%，Fe 3 O 4 Content 5.72%, siO 2 The content is 92.28 percent.
Test example 1
The molecular sieve prepared in the examples and the comparative examples is used as a catalyst for synthesizing lactide by lactic acid dehydration cyclization, an L-lactic acid solution with the mass concentration of 50% and toluene are mixed, the catalyst is added, the mass ratio of the catalyst to the lactic acid is 1:2, the solid-to-liquid ratio of the catalyst to the toluene is 1g:20mL, the mixture is stirred and refluxed for 3 hours at 140 ℃ and 500r/min, and free water and reaction generated water in the lactic acid solution are removed during the reaction. After the reaction was completed, water was added at a volume ratio of water to organic solvent of 1:1 to conduct water/toluene two-phase separation, and the two phases were separated by a separating funnel and subjected to chromatography, respectively. The toluene phase is distilled under reduced pressure at 100 ℃ and 3kPa absolute pressure to obtain an L-lactide product, and toluene is recovered; the aqueous phase was treated at 120℃with 300r/min stirring for 4 hours, and then the catalyst and the lactic acid recovery were separated by a magnet. The synthesis effect of lactide is shown in table 1.
Table 1 results of lactide synthesis by the catalysts of examples and comparative examples
Test example 2
Fe prepared by example 1 3 O 4 @SiO 2 @Sn-SPP@meso-SiO 2 The method comprises the steps of (1) mixing an L-lactic acid solution with the mass concentration of 80% and ethylbenzene, wherein the mass ratio of the catalyst to the lactic acid is 1:1, the solid-liquid ratio of the catalyst to the ethylbenzene is 1g:10mL, stirring and refluxing at 110 ℃ for 6h at 100r/min, and removing free water and reaction generated water in the lactic acid solution during the reaction. After the reaction, adding corresponding amount of water into the reaction system according to the volume ratio of water to organic solvent of 1:1 to separate water/ethylbenzene two phases, and separating the two phases by using a separating funnel to perform chromatographic analysis respectively. The ethylbenzene phase is distilled under reduced pressure at 100 ℃ and 3kPa absolute pressure to obtain an L-lactide product, and ethylbenzene is recovered; the aqueous phase was treated at 100℃for 3 hours at 100r/min, and then the catalyst and the lactic acid recovery were separated by a magnet. The synthesis effect of lactide is shown in table 2.
Test example 3
Fe prepared by example 1 3 O 4 @SiO 2 @Sn-SPP@meso-SiO 2 The core-shell particles are used as a catalyst, an L-lactic acid solution with the mass concentration of 20% and benzene are mixed, the mass ratio of the catalyst to the lactic acid is 1:5, the solid-to-liquid ratio of the catalyst to the benzene is 1g:50mL, the mixture is stirred and refluxed at 160 ℃ and 1000r/min for 2 hours, and free water and reaction generated water in the lactic acid solution are removed during the reaction. After the reaction was completed, a corresponding amount of water was added to the reaction system in a volume ratio of water to organic solvent of 2:1 to conduct water/benzene two-phase separation, and the two phases were separated by a separating funnel and subjected to chromatography, respectively. The benzene phase is subjected to reduced pressure distillation to obtain an L-lactide product, and benzene is recovered at the same time; the aqueous phase was treated at 150℃and 1000r/min for 6 hours, and then the catalyst and the lactic acid recovery were separated by a magnet. The synthesis effect of lactide is shown in table 2.
Test example 4
The same test example 1 and using the catalyst prepared in example 1 was used, except that: the lactic acid is D-lactic acid, the concentration of the D-lactic acid solution is 80%, the optical purity of the D-lactic acid is 99.5%, and the lactide synthesis effect is shown in Table 2.
Test example 5
The same test example 1 and using the catalyst prepared in example 1 was used, except that: the lactide synthesis effect using cyclohexane instead of toluene is shown in table 2.
Test example 6
The difference from example 1 is that: sn-Beta molecular sieves (Al 2 O 3 Content 6.20%, snO 2 Content 2.14%) instead of Fe in example 1 3 O 4 @SiO 2 @Sn-SPP@meso-SiO 2 A core-shell catalyst. The lactide synthesis effect is shown in table 2.
TABLE 2 test of the effects of lactide Synthesis in examples 2 to 6
As is apparent from the results of the experiments in the above examples and comparative examples, fe prepared by the method of the present invention 3 O 4 @SiO 2 @Sn-SPP@meso-SiO 2 The core-shell catalyst is used as a catalyst for the reaction of preparing lactide by directly dehydrating and cyclizing lactic acid, so that corrosion of a lactic acid raw material to framework aluminum in a traditional silicon-aluminum molecular sieve catalyst at a high temperature is avoided, and the stability of the molecular sieve catalyst is improved; avoid the lactic acid raw material to produce Fe 3 O 4 The optical purity of the lactide product can be effectively improved due to the corrosion of particles; avoiding the deep condensation of lactic acid molecules on the surface of the catalyst and improving the yield of lactide products. The lower reaction temperature of the invention effectively relieves the racemization problem of lactide products. In a word, through the synergistic effect of each technical characteristic of the invention, the catalyst of the invention is used as a catalyst for preparing lactide by directly dehydrating and cyclizing lactic acid, and has the advantages of high one-time yield of lactide products, high optical purity of the products and the like.
Claims (26)
1. The synthesis method of the catalyst for preparing lactide by lactic acid dehydration is characterized by comprising the following steps:
(1) Silicon source, sodium hydroxide, water and Fe 3 O 4 Mixing the nano particles in proportion, stirring at a certain temperature, separating out product particles by using a magnet, washing and drying to obtain Fe 3 O 4 @SiO 2 Particles;
(2) Mixing a silicon source, tetrabutylphosphine hydroxide, ethanol, water and a tin source in proportion, and adding Fe in the step (1) 3 O 4 @SiO 2 Standing for crystallization, separating product particles with magnet, washing, and drying to obtain Fe 3 O 4 @SiO 2 An @ Sn-SPP particle;
(3) Mixing a silicon source, sodium hydroxide, cetyltrimethylammonium bromide and water in proportion, and adding Fe in the step (2) 3 O 4 @SiO 2 Stirring at a certain temperature to obtain particles of Sn-SPP, separating the particles with magnet, washing, drying, and calcining to obtain Fe 3 O 4 @SiO 2 @Sn-SPP@meso-SiO 2 A core-shell catalyst.
2. The method according to claim 1, characterized in that: the silicon source in the step (1) to the step (3) is at least one of ethyl orthosilicate and methyl orthosilicate, and preferably ethyl orthosilicate.
3. The method according to claim 1, characterized in that: fe as described in the step (1) 3 O 4 The particle size of the nano particles is 5-15nm.
4. A method according to claim 1 or 2 or 3, characterized in that: the silicon source, sodium hydroxide, water and Fe in the step (1) 3 O 4 The molar ratio of the nano particles is 1:0.2-0.4:80-120:3-5.
5. The method according to claim 1, characterized in that: the treatment temperature in the step (1) is 30-60 ℃, the stirring speed is 500-1000r/min, and the treatment time is 3-6h.
6. The method according to claim 1, characterized in that: the tin source in the step (2) is at least one of tin tetrachloride pentahydrate and stannous octoate, and preferably the tin tetrachloride pentahydrate.
7. The method according to claim 1 or 6, characterized in that: in the step (2), the molar ratio of the silicon source to the tetrabutyl phosphine hydroxide to the ethanol to the water to the tin source is 1:0.2-0.4:3-5:10-30:0.004-0.024.
8. The method according to claim 1, characterized in that: fe in step (2) 3 O 4 @SiO 2 Particles and a silicon source (in SiO 2 Content) is 1:10-20.
9. The method according to claim 1, characterized in that: the crystallization temperature in the step (2) is 130-150 ℃ and the crystallization time is 2-5 days.
10. The method according to claim 1, characterized in that: in the step (3), the molar ratio of the silicon source, sodium hydroxide, cetyltrimethylammonium bromide and water is 1:0.2-0.4:0.25-0.4:80-120.
11. The method according to claim 1, characterized in that: fe in step (3) 3 O 4 @SiO 2 The @ Sn-SPP particles were mixed with a silicon source (in SiO 2 Content) is 10-20:1.
12. The method according to claim 1, characterized in that: the treatment temperature in the step (3) is 30-60 ℃, the stirring speed is 500-1000r/min, and the treatment time is 3-9h.
13. The method according to claim 1, characterized in that: the product obtained in the step (1) -step (3) is separated from product particles by a magnet, washed to be neutral by water and dried for 2-12h at 80-100 ℃.
14. The method according to claim 1, characterized in that: and (3) roasting at 450-500 ℃ for 3-5h.
15. Fe (Fe) 3 O 4 @SiO 2 @Sn-SPP@meso-SiO 2 A core-shell catalyst prepared by the method of any one of claims 1-14.
16. The catalyst of claim 15, wherein: the prepared core-shell catalyst is prepared by Fe 3 O 4 The nano particles are cores and SiO 2 And Sn-SPP molecular sieve are respectively two intermediate layers, mesoporous silica is used as an outer shell layer, the particle size of the core-shell catalyst is 300-700nm, and Fe is contained in the core-shell catalyst 3 O 4 The particle diameter of the core particles is 5-15nm, and the intermediate SiO 2 The thickness of the layer is 10-20nm, and the thickness of the shell layer is meso-SiO 2 Is 25-50nm thick.
17. The catalyst according to claim 15 or 16, characterized in that: to promoteMass of chemical agent, siO 2 The content of the Fe is 87.8 to 95.6 percent 3 O 4 4.0 to 8.6 percent of SnO 2 The content is 0.4% -3.6%.
18. A method for preparing lactide from lactic acid, which is characterized in that the method is realized by adopting the core-shell catalyst as claimed in any one of claims 15-17, and the specific synthesis method is as follows: mixing lactic acid solution with organic solvent, adding Fe 3 O 4 @SiO 2 @Sn-SPP@meso-SiO 2 And (3) carrying out stirring reflux reaction on the core-shell catalyst at normal pressure and a certain temperature, adding water after the reaction is finished, carrying out two-phase separation, carrying out reduced pressure distillation on an organic phase to obtain a lactide product, and separating and recycling the catalyst from the water phase by an externally added magnet.
19. The method of preparing lactide according to claim 18, wherein: the lactic acid is at least one of L-lactic acid and D-lactic acid, the optical purity of the lactic acid is more than or equal to 99.5%, and the mass concentration of the lactic acid solution is 20% -80%.
20. The method of preparing lactide according to claim 18, wherein: the benzene series organic solvent is at least one of benzene, toluene, ethylbenzene and xylene, preferably toluene.
21. The method of preparing lactide according to claim 18, wherein: the mass ratio of the catalyst to the lactic acid is 1:0.5-5, preferably 1:2-3.
22. The method of preparing lactide according to claim 18, wherein: the solid-to-liquid ratio of the catalyst to the organic solvent is 1g:5-100mL, preferably 1g:10-30mL.
23. The method of preparing lactide according to claim 18, wherein: the reaction temperature is 100-170 ℃, the stirring speed is 100-1000r/min, and the reaction time is 2-6h; and continuously removing free water of the lactic acid solution in the reaction system during the reflux reaction, and generating water molecules by the condensation reaction of lactic acid molecules.
24. The method of preparing lactide according to claim 18, wherein: after the reaction is completed, water is added into the reaction system to carry out two-phase separation of an aqueous phase and an organic phase, wherein the volume ratio of the water to the organic solvent is 1-2:1.
25. The method of preparing lactide according to claim 18, wherein: the organic phase obtained after the two-phase separation is subjected to the absolute pressure of 1-5kPa at the temperature of 100-120 ℃ to obtain the lactide product, and the organic solvent is recovered for repeated use.
26. The method of preparing lactide according to claim 18, wherein: the water phase obtained after the separation of the two phases is stirred and reacts for 3 to 6 hours at the temperature of 100 to 150 ℃ with the stirring speed of 100 to 1000r/min.
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