UA141885U - METHOD OF OBTAINING LACTID FROM ALKYLLACTATES - Google Patents

Abstract

A process for the preparation of a lactide from alkyl lactates involves carrying out a transesterification reaction to a cyclic lactic acid ester in the presence of an acid catalyst. The reaction is carried out at elevated temperature in a flow reactor by passing vapors of alkyl lactates without carrier gas under reduced pressure over a heterogeneous oxide catalyst, and as catalysts using TiO2 / SiO2 and / or SnO2 / SiO2.

Description

A useful model belongs to the chemical technology of organic substances, namely to the synthesis of lactide - a cyclic dimer of lactic acid from its alkyl esters - methyl lactate and ethyl lactate. More specifically, it concerns the improvement of the method of obtaining lactide by vapor phase condensation of alkyl lactates into lactide on a solid acid catalyst in a flow reactor at reduced pressure.

One of the ways to solve the problem of catastrophic pollution of our planet with plastic waste is the use of environmentally safe biodegradable polymers.

Polylactide is able to decompose in the natural environment to carbon dioxide and water in a few months, unlike polyethylene, polypropylene or polystyrene, which do not decompose for tens of years. This determines its wide use as an ecological packaging material. In addition, due to its non-toxicity, biocompatibility and ability to bioresorb, polylactide is used in medicine, in particular, in dentistry, surgery, orthopedics and pharmacy | 1-3|. The growing demand for this polymer (according to forecasts in 2020, 1.2 million tons |4Y) requires the improvement of lactide synthesis processes, as a starting compound for its production.

Usually, the raw material for the synthesis of lactide is lactic acid, which is mainly obtained by fermentation of sugary substances such as molasses, starch, etc. A two-stage method of obtaining lactide from lactic acid (5-7) is known, where in the first stage, heating (140-250 C) in the presence of a catalyst for several hours produces an oligomer, which is further processed in the second reactor under reduced pressure (mostly 10 mm Hg .) and a temperature above 200 C is depolymerized to lactide. In this process, zp (II)-containing catalysts are usually used, in particular tin octanoate, which cannot be recycled. Disadvantages include the two-stage process, the formation of a large number of by-products (dimers and trimers of lactic acid, water, etc.), the long duration and energy consumption of the process.

A one-stage vapor-phase method of obtaining lactide from lactic acid is also known

I8B, 9|. Lactic acid vapors in the carrier gas stream are passed over a stationary layer of a solid catalyst at a temperature of 180-260 "C. Various mixed oxides are used as catalysts.

In this way, on the AI2Oz-51O2 catalyst (13 95 aluminum oxide and 87 95 silicon oxide) at 220 "C and contact time 3 sec, the yield of lactide was 25-44 95 |8I, and in work I9|I on the 5pO-51O2 catalyst ( 80 95 of tin oxide and 20 95 of silicon oxide) - up to 80 95. A significant disadvantage of such

One of the methods is the use of a very low concentration of lactic acid vapors relative to the inert carrier gas (for example, in I9|, nitrogen with a lactic acid content of only 1.6 vol. 95 was passed over the catalyst), which leads to a low volumetric productivity of the target product - 0.06

of Glactide/Lreactor/hOD. (8| and up to 1 Glactide/Lreactor/HOUR. (9). The need to use significant volumes of carrier gas (which determines the energy consumption of the process) is due to the fact that lactic acid is a weakly volatile substance that easily forms oligomers at a temperature above 150 " WITH.

Alternative raw materials for the production of lactide can be its esters (for example, methyl and ethyl), which are more volatile and stable compared to lactic acid.

A two-stage method of obtaining lactide from alkyl lactates (10) is known, which includes the stage of oligomerization of methyl or ethyl lactate (145-165 C, 3-5 hours) using mostly organic tin compounds as catalysts (toporshuyip ohide, bshuikip (IM) ygiz-2 -eIpuInehoaieye, airshutsip ohide) and the stage of depolymerization (200 "C, 10 mm Hg) of oligomers with the production of lactide. That is, the process is carried out under practically the same conditions as in the case of lactic acid I5-7| and, accordingly, has the same Disadvantages: However, in this case, the lactide purification process is simplified, which only includes the stages of removing alkyl lactate, its dimer, and meso-lactide.

A one-step method of obtaining lactide from solutions of lactic acid esters, mainly from ethyl lactate, is known (11). The process is carried out for 6 hours. in the liquid phase with the use of a batch reactor at 120 "C in an inert gas atmosphere and reduced pressure for the distillation of vapors of the formed alcohol. High-boiling dimethyl ether of triethylene glycol and homogeneous catalysts, namely titanium alkoxides, halides, and acetylacetonates, are used as a solvent for esters. The maximum conversion of ethyl lactate and the selectivity for lactide was 98 and 89 95, respectively (the closest analogue).

The main disadvantage of this method is the use of homogeneous catalysts, which must be separated from the reaction products, which leads to the complexity of the technical equipment, periodicity and long duration of the process.

The task of useful morel is the development of an effective one-stage method of obtaining lactide from alkyl lactates on a heterogeneous catalyst in a flow reactor and reducing the energy consumption of the process.

The problem is solved by the fact that in the method of obtaining lactide from alkyl lactates, which includes carrying out a transesterification reaction into a cyclic ester of lactic acid in the presence of an acid catalyst, according to a useful model, the reaction is carried out at an elevated temperature in a flow reactor by passing alkyl lactate vapors without a carrier gas at reduced pressure over a heterogeneous oxide catalyst, and TiOg/5iOg" and/or 5pO"g/5iO2 are used as catalysts. Catalysts are used with the content of deposited tin and/or titanium dioxide in the range of 2.5-10 wt. 95, the reaction is carried out at a temperature of 240-270 C, preferably at 260 "C, with a contact time of 1-2 seconds, both methyl lactate and ethyl lactate are used as alkyl lactates, and the reaction is carried out at a pressure of 100-150 mbar, which is created in the reactor according to using a vacuum pump.

The developed method of vapor-phase production of lactide makes it possible to obtain a technical result, which consists in reducing the energy consumption of the process due to the removal from the technological process of the use of a carrier gas, which must be heated to the reaction temperature, as well as experimentally determined technological modes. The essence of the proposed method is also that the contact time of 1-2 seconds, which is necessary for the effective formation of lactide, is provided not by the flow of the carrier gas, but by the flow of vapors of pure alkyl lactate at reduced pressure (100 mbar), which is created using a vacuum pump

The main difference between the useful model and the closest analogue is that no carrier gas is used for the flow process, and the flow of reagents is ensured by reduced pressure in the reactor (100 mbar), which is created by a vacuum pump.

To implement the claimed method, alkyl lactates were used - methyl lactate (ML) and ethyl lactate (EL) and oxide catalysts - TiO"/5iOg and ZpO"/51iO", which contain 2.5 and 5 and 5 and 1Omas. 95 titanium dioxide and tin, respectively. Ti- and 5p-containing samples were obtained by impregnation of KSKG silica gel with appropriate solutions. When obtaining TiOg/5iO2, an alcoholic solution of tetrabutoxytitanium (98 95, AlYidgisp) was used, and in the case of obtaining

ZpOg/5iOg". - an aqueous solution of tin tetrachloride. The obtained samples were dried at 110" followed by calcination in air at 500"C for 4 hours.

The textural parameters of the catalysts were obtained by the method of low-temperature adsorption - desorption of nitrogen on the device Ochapiaspgote Moma 2200e Z!ipase Agea apa Roge 5i7e Apaiuseg. The strength (No) and concentration of acid centers (C acid centers) of the synthesized samples were determined, respectively, using Hammett indicators and the method of reverse titration of n-butylamine adsorbed on the surface of the samples with a 0.05 M solution of hydrochloric acid in the presence

From bromothymol blue. Structural characteristics and acidic properties of the synthesized catalysts are given in Table. 1.

Table 1

Structural characteristics and acidity of the used catalysts

Catalyst' Opyt, M? /G Mpor, MmZ/g Dporr, NM C acid. centers; But tah m mol/h at 25TLOUVO» | 350 | 088 | 98 | 14 30 x- The number before the marking of the catalyst corresponds to the content of 5pO" or Tio" by mass. 95.

Catalytic experiments were performed as follows. From the burette, the alkyl lactate entered the evaporator under the influence of vacuum, where the lactate turned into steam and was heated to the reaction temperature. Next, the heated vapor of alkyl lactate entered a vertical flow reactor with a fixed catalyst layer, in which its transformation into lactide took place. A stainless steel metal tube with a length of 165 mm and an internal diameter of 11 mm served as the reactor. The mass of the catalyst in the reactor was 0.8 g, the volume was 2 cm3, and the particle size was 0.5-2 mm.

The catalyst was placed in the middle part of the reactor. The free volume above and below the catalyst was filled with an inert filler (quartz). The reaction was carried out at a temperature of 250-310 "C in a vacuum and a pressure (P) of 100-150 mbar, a contact time of 1-2 seconds.

The load on the catalyst was 25-55 mmol of alkyl lactate/(Gkat h.). After the reaction, the steam mixture was condensed in a receiver that was cooled with ice. Analysis of reaction products after three hours. catalyst work was carried out by the methods of gas chromatography (chromatograph "Spgot-5" with a 50-meter capillary column) and C NMR spectroscopy (spectrometer "VgyKeg

Auapse-400"). The reaction products were identified by "C NMR spectra using the database of organic compounds (ZOV5, Maiyopa! Ipbiyshe oi Admapsei Ipayvigia!

Zsiepse apa Tesppoiodu, Udarap, ulum/.aivi dor). Quantitative analysis of the reaction products was performed on a chromatograph using o-xylene as an internal standard. The conversion of alkyl lactates and the selectivity of the reaction products (wt. 90) were calculated from the chromatographic analysis data. When calculating the selectivity, the formed alcohol was not taken into account as a product of the target reaction.

Below are examples of the conversion of ethyl lactate and methyl lactate on oxide catalysts, which confirm the possibility of implementing the claimed method.

Example 1.

The reaction was carried out at 220 "C on a TiO"g/5iO" catalyst containing 5 95 TiO", the load on the catalyst was 25.3 mmol EL/Hcat h.) and a pressure of 100 mbar. The size of the catalyst particles was 0.5-2 mm. The conversion of ethyl lactate is 35 95, selectivity for lactide 60 95, yield of lactide - 21 Ob, productivity of the catalyst for lactide - 4.3 mmol L/(Gcat HOUR).

Experiments corresponding to examples 2-6 were conducted similarly to example 1, but at other temperatures (Table 2).

Table 2

Conversion of ethyl lactate at different temperatures on the TiOg/51O" catalyst with lactide, mmol of the reaction example, "C | ethyl lactate, 90 lactide, 95 | lactide, 90 p/ (Gkat gOD.) 22111135 | 1111601111711121 11111430 1-5T1O2/51O" catalyst ;2-2, TIO»g/51О» - catalyst.

From the examples given, it can be seen that the best results are within the range of temperatures from 240 to 2707C. The conducted experiments showed that a decrease in temperature below 240 "С leads to a decrease in the conversion of ethyl lactate and the yield of the target product. An increase in temperature above 270 "С leads to a decrease in the selectivity for lactide.

It should be noted that halving the content of TiOg in the catalyst does not significantly affect the conversion of ethyl lactate and the selectivity and productivity of lactide (examples 7, 8, Table 2)

Examples 9-11 show the conversion of ethyl lactate to TiOg/5iOg depending on the load on the catalyst from 14.7 to 34.7 mmol EL/(Hcat h) (Table 3).

Example 9. The reaction was carried out on a 5TiO»2/5iO» catalyst at a loading of 14.7 mmol

EL/(gcat h), which corresponded to 0.7 g EL//mLcat h) at a temperature of 260 "C and a pressure of 100 mbar.

The size of the catalyst particles was 0.5-2 mm. The contact time was 2 seconds. The conversion of ethyl lactate is 61 95, the selectivity for lactide is 3195, the yield of lactide is 1995, the productivity is

With lactide - 2.3 mmol L/(Gcat hour) (Table 3).

Example 10. The experiment was carried out similarly to example 9, but with a catalyst load of 25.2 mmol EL/(gcat h), which corresponded to 1.2 g EL/(mlcat h) (Table 3).

Example 11. The experiment was carried out similarly to example 9, but with a load on the catalyst of 34.7 mmol EL/(Hcat hour), which corresponded to 1.7 g EL//mlLcat hour). The contact time was 1 second (Table 3).

Examples 9-11 show that the highest value of ethyl lactate conversion (61 95) is observed at the lowest load on the catalyst (14.7 mmol EL/(Ggcat h) (Table 3). But the highest selectivity for lactide (59 95), yield ( 26 95) and lactide productivity (7.4 mmol L/(Gcat h)) is at the highest load on the catalyst (34.7 mmol EL/(Gcat h) (Table 3).

Table C

Conversion of ethyl lactate to TiO»g/51О:» under different catalyst loads

Mo Loading Conversion Selectivity per Output Productivity by example on catalyst, ethyl lactate, All! lactide, 9o | lactide, 90 lactide, mmol L/

MMOL/Gkat/HOUR. " " " (Gkat HOURS) 911477 1771117611111717711111311 11719 | 2g3 0

The following are examples 12-18 (Table 4), which show the effect of the working time of the TiOg/51O2 catalyst containing 5 wt. 9o titanium dioxide, to convert ethyl lactate into lactide.

Example 12. The reaction was carried out at 260 "C, a pressure of 100 mbar and a load on the catalyst of 25.2 mmol EL/(Hcat h.), which corresponded to 1.2 g EL/(MLcat h.). The size of the catalyst particles was 0.5-2 mm (Table. 3).Ethyl lactate conversion after 5 hours of catalyst operation is 5495, lactide selectivity 41 95, lactide yield - 22 95, lactide productivity - 4.5 mmol L/(Gcat h) (Table 4).

Further experiments, corresponding to examples 13-19, were carried out similarly to example 12, but with longer operation of the catalyst (Table 4).

From these examples, it can be seen that the conversion of ethyl lactate, the selectivity to lactide, the yield of lactide, and the productivity of the catalyst to lactide after 8, 11, 14, 17, and 20 h. performance does not change significantly (Table 4), which indicates the stability of the catalyst.

Table 4

Conversion of ethyl lactate into lactide on TiO»/5iOg depending on the reaction time zhtnnnn chekh et MEKEYA | nodia MIemYa

For example Time, hours oh oh su! lactide, mmol L/ ethyl lactate, 90 lactide, 90 lactide, 90 (Gkat gOD.) 7141178 11114117 6о0171125111111111522sh 716. | 74 | 45 | 66 | 30 | eh

The following are examples that show the conversion of methyl lactate to lactide on oxide catalysts.

Example 19. The reaction was carried out at 260 "C on a ZpO"/5iO" catalyst containing 5 95 (wt.) of tin oxide, a pressure of 100 mbar and a load on the catalyst of 25.0 mmol ML/(gcat h), which corresponded to 1.0 g ML Contact time was 2 seconds. Methyl lactate conversion is 28 95, lactide selectivity 15 95, lactide yield - 4 95, catalyst productivity according to lactide - 0.8 mmol L/Hcat gHOD.).

Example 20. The reaction was carried out at 260 "C on a TiO"/5iO"2 catalyst containing 10 wt. 95 of tin oxide, a loading on the catalyst of 25.0 mmol ML/(Hukagt h.) and a pressure of 100 mbar. The conversion of ethyl lactate is 29 95, selectivity according to lactide 15 95, yield of lactide - 4 95, productivity of the catalyst according to lactide - 0.8 mmol L/(Gcat HOUR).

As can be seen from the results of experiments illustrating examples 19 and 20, the increase in content

Zp" in the catalyst does not significantly affect the yield of lactide.

From Example 21. The experiment was carried out similarly to example 19, but 5TiO2/51O2 was used as a catalyst. Methyl lactate conversion, selectivity and lactide yield are 50, 58 95 and 29 95, respectively. The productivity of the catalyst based on lactide is 5.2 mmol L/(Gcat h) (Table 5).

Examples 19-21 show that a higher yield of lactide is observed on the titanium-containing catalyst.

The following examples show the conversion of methyl lactate to 5TiO25/5iO" depending on the load on the catalyst (Table 5).

Example 22. The reaction was carried out similarly to example 19, but with a catalyst loading of 35.0 mmol ML/(Gcat h), which corresponded to 1.5 g ML//mlcat h). The contact time was 1 second. (Table 5).

Example 23. The reaction was carried out similarly to example 19, but with a catalyst load of 54.5 mmol ML/(Gcat h), which corresponded to 2.3 g ML/(mlcat h) and a pressure of 150 mbar. The contact time was 1 second. (Table 5).

From these examples, it can be seen that methyl lactate conversion, selectivity, and lactide yield do not differ much from each other with increasing catalyst loading from 25 to 54.5 mmol

ML/(gkat h.). But at the same time, the productivity of the catalyst according to lactide increases almost twice - from 5.2 to 10.3 mmol L/(Gcat h.) due to a greater load on the catalyst.

Table 5

Conversion of ethyl lactate to 5TiO"2/51O" under different catalyst loads

Conversion Output Performance by

For example) on the catalyst, Selectivity of methyl lactate, o lactide, | lactide, mmol L/ o/; by lactide, 90 o/; (et HOURS)

Examples 24-28 (Table 6) show the effect of the working time of the 5TiO2/5iO2 catalyst on the conversion of methyl lactate into lactide at 260 "С, pressure 100 mbar, and the load on the catalyst 35 mmol ML/(Gcat h.), which corresponded to 1.5 g ML/ (mlkat h). The contact time was 1 sec. From these examples, it can be seen that the conversion of methyl lactate, selectivity to lactide, yield of lactide and productivity of the catalyst to lactide (Table b) do not change significantly during 24 h, which indicates the stability of operation catalyst.

Table 6

Conversion of methyl lactate to lactide by 5TiO"/51O" depending on the time of catalyst operation and ""| methyl lactate, 90 by lactide, 90 by lactide, 90 '

Lu (gkat h.) 724116 | юЮюЮюмжя49.4ЮюЮ.| 2 ЮЙКБ(5Б5 (| 27 | 70 725 17113 | 745 2 | 5 | 2г3 | 60 261.15 | ш(Б48 | .--ЮЙ60 2 4юЮюЮЮЮІ1ї729 | щ (BG75

It can be seen from the given data that the technical result is optimally achieved on the TiOg/51O2 catalyst at 240-270 "С, the load on the catalyst is 25-55 mmol/(Gcat h), the pressure is 100-150 mbar and the contact time is 1 sec. At the same time, the selectivity according to lactide is 47-59 95 with 45-54 95 conversion of ethyl lactate or methyl lactate, yield of lactide - 23-31 95, productivity of the catalyst according to lactide - 4.9-10.3 mmol L/Hgcat H.).

Thus, we propose a vapor-phase method of obtaining lactide from alkyl lactates by passing them through ZpO»2/5iO2. or TiO»/5iOg2 catalysts with a contact time of

With the catalyst within 1 second under the action of a vacuum created by a vacuum pump. Also, the claimed method shows the possibility of obtaining high productivity of lactide

TiO»/5iO» catalyst (10.0 mmol L/gkg/h), which is of practical interest.

Compared to the closest analogue, the proposed method does not involve the use of carrier gas (nitrogen), which significantly reduces energy costs when obtaining the target product.

Sources of information: 1. Syria AR, Kitag M. Mem etegdipu igepaz ip zupiniiys Biodedgadabie roiutegv-rouasiide: a sptiidne //Yeshg. Roiuteg u., 2007, M. 43, I6. 10, R. 4053-4074.

2. V. Atsga5, I.-T. (it, 5.E.M. bZeiKe, N. Tvcii (ed5). NoroKep Roiu(Iasiis asii): zupipezviv, zygisiigez, rgorepiev, rgosezvipu apa arriisayopv., Mem degveu: dop U/yeu in 5opvv5, Ips., 2010 . 9, Mo. 5, R. 435-455. 4. I asiis asia apa roiu Iasiis asia (RI A) tagkKei apaiuziv Bu arriisayop (rasKadipdu, adgisinige, igapzrogi, eIesigopisv, yehshShev) apa zedtepi yuogesaviv 10 2020. Stapa miem/ gezvagsp , 2014. 5. 05 Rai. 5247058 Stpyreg R.V., Nai!E.5., Koiviai .., Imep M.I.., Vepzop V.O., VogsNagai V.I.

Sopiiipysyv rgosevz5 Tog tapiiasiige oi Iasiide roiuteg5 m/y sopiigoiIey oriisa! snapper //Fish. Oaie: 21.09.1993 6. 005 Rai. 6326458 VI Stbreg R.V., Nai E.5., Koiviai 99., IMm"ep M.I., Vepzop V.O., VogsNagai

V.G. Sopiipysiv rgosez5 og She tapiiasiige oi Iasiide apa Iasiide roiutetgv //RybI. Date: 04.12.2001. 7. 0О5 Rai.7488,783 В2 Sovlasy RY., Vodaepy 4.-Sp., Map Saperegdpe E. MeiShtoi og She rgodisiop oi roiuiasiide iot a 5oIshiop oi Iasiis asii og ope oi Pe deguaiime5 ISHPegeoi// Rybri. Date: 10.02. 2009 8. 05 Rai. 5332839 Vepeske N.R., MagkiIe V.A., Bipsiaiyi V.S. Saitamis rgodisiup oi Iasiide digesPu ytot Iasiis asia //Pisces. Date: 26.07. 1994. 9. Rai. 05 9539561 В2 Nuapd YULMU., Spapd 9.-5., Orage R.R., Yeee O-N., Nymapd m.-K. Meitoa og rgodisipa Iasiide dikgesPu ijeot Iasiiis asia apa a saiaiuvi izey IPegeip /Pisces. Date: 10.01.2017. 10. 05 Paradise. 5247059 Stpreg R.V., Nai E.5., Koiviai 4., Imep M.G., Vepzop V.O., VogsNagai V.I.

Sopiipysyv rgose55 og She tapiiasishhyge oi a rigiiei Iasiide iot evzievgv5 oi Iasiis asii //Ribi. Date: 21.09.1993. 11. ER 2264020 AI Spapa .-5., Nyapd M.-K., I ee 9.-N., I ee U.-M., dUypa M.-N. Saiaiyuvi yog aigesi sopmegviop oi evivg5 oi Iasiiis asii o Iasiide apa She teinosy og rhodisipa Iasiide Shte zate //Rybi.

Date: 22.12.2010.

Claims (5)

USEFUL MODEL FORMULA 1. The method of obtaining lactide from alkyl lactates, which includes carrying out the transesterification reaction of Zo into the cyclic ester of lactic acid in the presence of an acid catalyst, which is characterized by the fact that the reaction is carried out at an elevated temperature in a flow reactor by passing alkyl lactate vapors without a carrier gas at reduced pressure over a heterogeneous an oxide catalyst, and TiOg/51O" and/or 5pO"2/510" are used as catalysts. 2. The method of obtaining lactide according to claim 1, which differs in that the catalysts are used with a content of deposited tin and/or titanium dioxide in the range of 2.5-10 wt. 9. C. The method of obtaining lactide according to claim 1, which differs in that the reaction is carried out at temperatures of 240-270 "C, preferably at 260 "C, with a contact time of 1-2 seconds. 4. The method of obtaining lactide according to claim 1, which differs in that methyl lactate and/or ethyl lactate are used as alkyl lactates. 5. The method of obtaining lactide according to claim 1, which differs in that the reaction is carried out at a pressure of 100-150 mbar, which is created in the reactor with the help of a vacuum pump. 000 Komputernaverstka!, Skvortsova.dG (00000000 777 Ministry of Economic Development, Trade and Agriculture of Ukraine, (0000000 12/2 M. Hrushevsky St., Kyiv, 01008, Ukraine SE "Ukrainian Institute of Intellectual Property", Glazunova St., 1, Kyiv - 42, 01601