WO2014096870A1

WO2014096870A1 - Process for producing lactic acid

Info

Publication number: WO2014096870A1
Application number: PCT/GB2013/053415
Authority: WO
Inventors: Urvish Rameshchandra PANDYA
Original assignee: Plaxica Limited
Priority date: 2012-12-21
Filing date: 2013-12-23
Publication date: 2014-06-26
Also published as: GB201223319D0

Abstract

A process for producing lactic acid and barium hydroxide from barium lactate is provided. The process comprises (a) reacting barium lactate with an acid selected from the group consisting of hydrochloric acid and nitric acid, to produce lactic acid and inorganic barium salt comprising barium chloride and/or barium nitrate; and(b) reacting the inorganic barium salt with a hydroxide in the presence of water to produce solid barium hydroxide; wherein the hydroxide is selected from the group consisting of sodium hydroxide, lithium hydroxide, potassium hydroxide and ammonium hydroxide. Also provided are processes for the production of alkyl lactate, oligomeric lactic acid, lactide and poly-lactic acid.

Description

Process for producing lactic acid

The invention relates to a process for producing lactic acid from barium lactate, with recovery of barium hydroxide being possible.

Lactic acid is an important industrial chemical typically prepared from microbial fermentation of carbohydrates. A number of chemical processes for preparing lactic acid from carbohydrates are known. For example, GB 400,413, dating from 1933, describes an improved process for preparing lactic acid or lactates comprising reacting a carbohydrate-containing material with a strong alkali at a temperature of at least 200 °C, preferably at a pressure of at least 20 atmospheres, and recovering the lactic acid so produced by adding sulfuric acid or zinc sulfate to the reaction mixture.

According to Boudrant et al, Process Biochem 40 (2005) p. 1642, "In 1987, the world production of lactic acid averaged approximately equal proportions being produced by chemical synthesis and fermentation processes". Such chemical syntheses typically employed the hydrocyanation of acetaldehyde. However, chemical processes of this type have long been regarded as inefficient on an industrial scale, and today virtually all large scale production of the lactic acid available commercially is manufactured by

fermentation processes, see for example Strategic Analysis of the Worldwide Market for Biorenewable Chemicals M2F2-39, Frost and Sullivan, 2009. In a typical fermentation process, biomass is fermented with microorganisms to produce either D- or L-lactic acid. Companies such as Cargill and Purac operate large-scale fermentation processes for the production of optically active lactic acid, and the patent literature is replete with improvements in such processes.

The product of a fermentation process is usually an optically active lactate salt, and efficient recovery of lactic acid from such fermentation processes can be challenging. Many patent documents relate to lactic acid preparation via fermentation and subsequent lactic acid recovery, and a number rely on the preparation of a complex between lactic acid and an amine for the recovery. Such complexes can readily be converted into lactic acid or, if desired, used directly as feedstocks in processes for preparing derivatives of lactic acid. Thus, for example, US 4,444,881 (Urbas, 1984) describes a process for the recovery of an organic acid (which may be lactic acid) from a fermentation reaction, which comprises converting the acid to its calcium salt, and adding a water-soluble tertiary amine carbonate (which may be prepared by addition of carbon dioxide to a solution or suspension of the tertiary amine in water). US 5,510,526 (Baniel, 1994) claims a process, stated to be an improvement over that of Urbas, for the recovery of lactic acid from a lactate feed solution, comprising the use of an extractant comprising at least one water immiscible trialkylamine having a total of at least 18 carbon atoms in the presence of carbon dioxide at a partial pressure of at least 50 psig (about 3½ atmospheres, 3.4 x 10⁵ Pa).

WO 2012/052703 describes an improved process for the production of a complex of lactic acid and either ammonia or an amine, which does not involve production of lactic acid by fermentation. The process comprises reacting one or more saccharides with barium hydroxide to produce a first reaction mixture comprising barium lactate, and contacting at least part of the first reaction mixture with ammonia or an amine and with carbon dioxide, or with the carbonate and/or bicarbonate salt of ammonia or an amine, to produce a second reaction mixture comprising the complex and barium carbonate. This process, which involves preparation of barium salts, has significant advantages over prior art processes. It does, however, have some disadvantages: specifically, if it is required to recycle the barium, an energy intensive calcination step is required.

We have now found an improved process for the preparation of lactic acid, which retains the advantages of using a barium salt, but which does not require the use of ammonia or an amine, and which provides a particularly economic recycle route for the barium.

Accordingly, the present invention provides a process for producing lactic acid and barium hydroxide from barium lactate, comprising

(a) reacting barium lactate with an acid selected from the group consisting of hydrochloric acid and nitric acid, to produce lactic acid and inorganic barium salt comprising barium chloride and/or barium nitrate; and

(b) reacting the inorganic barium salt with a hydroxide in the presence of water to produce solid barium hydroxide; wherein the hydroxide is selected from the group consisting of sodium hydroxide, lithium hydroxide, potassium hydroxide and ammonium hydroxide.

In one preferred embodiment of the process of the invention, once the lactic acid has been formed, it is directly converted into an alkyl ester of lactic acid (also known as alkyl lactate). Most conveniently, this is done after step (a) of the process of the invention, and before step (b) of the process of the invention. Thus, in one preferred embodiment of the process of the invention, there is provided a process for producing an alkyl lactate and barium hydroxide from barium lactate, comprising

(a) reacting barium lactate with an acid selected from the group consisting of hydrochloric acid and nitric acid, to produce lactic acid and inorganic barium salt comprising barium chloride and/or barium nitrate;

(aa) reacting the lactic acid with an alkyl alcohol to produce an alkyl lactate; and subsequently

As an end product of step (b), sparingly soluble barium hydroxide is produced, which precipitates from the reaction mixture and which may be separated by filtration. If desired, it may then be recycled to a process for preparing barium lactate as described below. This recycle route provides significant advantages over the process of

WO 2012/052703, by avoiding the need for energy intensive high temperature calcination which is required if the barium in that process is to be recycled.

Barium lactate, particularly racemic barium lactate, may be prepared by reacting one or more saccharides with barium hydroxide. Anhydrous or hydrated barium hydroxide may be used, e.g. barium hydroxide monohydrate, barium hydroxide octahydrate. One preferred embodiment of the process of the invention comprises a process for producing lactic acid and barium hydroxide, comprising:

a') reacting one or more saccharides with barium hydroxide to produce barium lactate;

a) reacting the barium lactate with an acid selected from the group consisting of hydrochloric acid and nitric acid, to produce lactic acid and inorganic barium salt comprising barium chloride and/or barium nitrate;

b) reacting the inorganic barium salt with a hydroxide in the presence of water to produce solid barium hydroxide; wherein the hydroxide is selected from the group consisting of sodium hydroxide, lithium hydroxide, potassium hydroxide and ammonium hydroxide.

In a preferred embodiment of this process, at least part of the barium hydroxide produced in step (b) is recycled to step (a¹).

In a particularly preferred embodiment of the process of the invention, there is provided a process for producing an alkyl lactate and barium hydroxide, comprising: a') reacting one or more saccharides with barium hydroxide to produce barium lactate;

a) reacting barium lactate with an acid selected from the group consisting of hydrochloric acid and nitric acid, to produce lactic acid and inorganic barium salt comprising barium chloride and/or barium nitrate;

aa) reacting the lactic acid with an alkyl alcohol to produce an alkyl lactate; and subsequently

In step (a), barium lactate is acidified with acid, typically resulting in a solution.

The acid is preferably hydrochloric acid, more preferably concentrated hydrochloric acid. In another embodiment, the acid is nitric acid. In step (a), water will typically be present, for example the barium lactate may be provided as a solution/suspension in water and/or the acid may contain water. Step (a) is carried out at a temperature suitable to effect conversion of barium lactate to lactic acid and inorganic barium salt, for example at a temperature in the range of from 5 to 125 °C. Preferably step (a) is carried out at a temperature in the range of from 50 to 125 °C, for example 65 to 1 10 °C, especially 85 to 100 °C. Conveniently, the reaction is carried out under reflux. Suitably the molar ratio of barium lactate to acid is in the range of from 1 : 1.8 to 1 : 10, preferably from 1 :2 to 1 :5. In one preferred embodiment, concentrated aqueous hydrochloric acid (i.e. about 37% aqueous hydrochloric acid) is used. In another preferred embodiment, concentrated aqueous nitric acid (i.e. about 68% aqueous nitric acid) is used. The reaction mixture resulting from step (a) may be used directly in subsequent steps. Alternatively, the reaction mixture may first be concentrated by the removal of at least some water. This may be achieved by any suitable means, for example by the use of membrane technology or by distillation.

In a preferred embodiment of the process of the invention, step (aa) follows step

(a). If step (aa) of the process is carried out, at least part of the reaction mixture resulting from step (a) is treated with an alkyl alcohol, preferably a Ci to C₆ alkyl alcohol, for example methanol, ethanol, 1-propanol, 2-propanol, preferably n-butanol. This reaction is suitably carried out at a temperature in the range of from 50 to 150°C, preferably 50 to 125 °C, for example 65 to 120 °C, especially 95 to 1 15 °C. Suitably step (aa) is carried out under reflux, using the alkyl alcohol as solvent as well as reactant, with removal of water. Additional solvents may be present if desired. For example, where the alkyl alcohol has a low boiling point (e.g. less than 80 °C) and/or is miscible with water (e.g. an alkyl alcohol such as methanol, ethanol), it may be preferable to use a non-polar hydrocarbon solvent which has a boiling point in the range of from 100 to 200 °C, more preferably 100 to 150 °C, such as toluene. The use of such solvents can assist removal of water from the reaction mixture, and can facilitate the use of higher reaction temperatures and shorter reaction times. Step (aa) is typically catalysed by the presence of acid, for example nitric acid or, more preferably, hydrochloric acid introduced during step (a).

In step (b) of the process, inorganic barium salt is reacted with a hydroxide in the presence of water. Preferably, inorganic barium salt is separated from the lactate species and/or alkyl alcohol prior to reaction with hydroxide. Thus, one preferred embodiment of the process of the invention comprises:

(aa) treating at least part of the reaction mixture from step (a) with an alkyl alcohol to produce an alkyl lactate and inorganic barium salt;

(ab) separating inorganic barium salt from the reaction mixture; and subsequently (b) reacting the separated inorganic barium salt with a hydroxide in the presence of water to produce solid barium hydroxide; wherein the hydroxide is selected from the group consisting of sodium hydroxide, lithium hydroxide, potassium hydroxide and ammonium hydroxide.

Inorganic barium salt is soluble in water but insoluble in some alkyl alcohols, for example n-butanol, so if step (aa) has been carried out, inorganic barium salt may precipitate out from the reaction mixture. It may then be separated from at least some of the reaction mixture from the previous stage of the process, for example by filtration, prior to treatment with hydroxide in the presence of water (i.e. step (ab) may comprise separating solid inorganic barium salt from the reaction mixture, e,g, by filtration , or by decanting or siphoning off liquid products). Where the alkyl alcohol used in step (aa) is one in which inorganic barium salt is soluble (e.g. methanol), the alkyl alcohol may be separated from the inorganic barium salt by other means, such as distillation (i.e. step (ab) may comprise separating solid inorganic barium salt from alkyl alcohol and alkyl lactate, by distilling off alkyl alcohol and alkyl lactate to provide solid inorganic barium salt).

Step (b) requires the use of a hydroxide whose cationic group is capable of forming chloride/nitrate salts that are water soluble, such as sodium hydroxide, lithium hydroxide, potassium hydroxide and ammonium hydroxide. Preferably the hydroxide used in step (b) is sodium hydroxide. Suitably step (b) is carried out at a temperature in the range of from 0 to 125 °C, preferably from 0 to 40 °C. Preferably, following admixing of barium chloride and/or barium nitrate with the hydroxide, the mixture is cooled to /held at a temperature of from 0 to 25 °C, preferably 0 to 10 °C, more preferably 0 to 5 °C, so as to aid precipitation of solid barium hydroxide. The molar ratio of inorganic barium salt to hydroxide in step (b) should be sufficient to effect high conversion of inorganic barium salt to barium hydroxide. Preferably the molar ratio of inorganic barium salt to hydroxide is in the range of from 1 : 1.8 to 1 :3, more preferably from 1 :2 to 1 :2.5. Although less preferred, the present invention also encompasses lower molar ratios of inorganic barium salt to hydroxide, for example 1 : 1. The use of sub- stoicheometric quantities of hydroxide will generally lead to lower conversion of inorganic barium salt to barium hydroxide. It will be appreciated that step (b) may also be carried out using hydroxides other than sodium hydroxide, lithium hydroxide, potassium hydroxide and ammonium hydroxide whose cationic group also forms chloride/nitrate salts that are water soluble. Thus, in another aspect there is provided a process for producing lactic acid and barium hydroxide from barium lactate, comprising (a) reacting barium lactate with an acid selected from the group consisting of

hydrochloric acid and nitric acid, to produce lactic acid and an inorganic barium salt comprising barium chloride and/or barium nitrate; and

(b) reacting the inorganic barium salt with a hydroxide in the presence of water to produce solid barium hydroxide; wherein the hydroxide has a cationic group capable of forming water-soluble chloride and nitrate salts. Examples of such hydroxides include sodium hydroxide, lithium hydroxide, potassium hydroxide and ammonium hydroxide, as well as other alkali metal hydroxides (e.g. caesium hydroxide), and water-miscible amines (e.g. water-miscible alkylamines having 12 or less carbon atoms, such as triethylamine, diisopropylethylamine).

In certain preferred embodiments, the acid used in step (a) is hydrochloric acid (in which case the inorganic barium salt is barium chloride) and the hydroxide is sodium hydroxide. In certain preferred embodiments, the acid used in step (a) is nitric acid (in which case the inorganic barium salt is barium nitrate) and the hydroxide is sodium hydroxide. In certain preferred embodiments, the acid used in step (a) is nitric acid (in which case the inorganic barium salt is barium nitrate) and the hydroxide is ammonium hydroxide. In certain preferred embodiments, the acid used in step (a) is hydrochloric acid (in which case the inorganic barium salt is barium chloride) and the hydroxide is ammonium hydroxide.

The processes of the invention may be carried out under ambient or inert atmospheres. For example, the process may be carried out using equipment that is open to the air, or may be carried out under a nitrogen or argon atmosphere. It may be carried out in a batch, semi-continuous or continuous process, and the various products of the process of the invention may be subject to any desired purification and/or additional processing steps. For example, alkyl lactate may be recovered by distillation.

Preferably the barium lactate starting material is racemic, and as described above, it is preferably prepared by reacting one or more saccharides with barium hydroxide. The saccharide may be a mono-, di-, tri- or poly-saccharide, with disaccharides and, especially, monosaccharides, being preferred. Suitable disaccharides include sucrose, lactose, lactulose, maltose, trehalose and cellobiose. Suitable monosaccharides include for example hexose monosaccharides, for example glucose, fructose, psicose and mannose. Pentoses may also be used, for example arabinose, xylose, ribose, xylulose and ribulose. In one embodiment, the saccharide comprises glucose. In another embodiment, the saccharide comprises fructose. Suitable monosaccharides also include pentose monosaccharides, for example arabinose. Mixtures of saccharides may be used. For example, the saccharide may comprise a mixture of two or more monosaccharides, for example a mixture of glucose and fructose.

Monosaccharides may be obtained from any known monosaccharide source, for example a higher saccharide such as sucrose, starch or cellulose. By way of example, a mixture of glucose and fructose (known as invert sugar) may be obtained from sucrose by enzymatic hydrolysis using a sucrase or invertase, or by heating an aqueous solution of the disaccharide in the presence of an acidic catalyst such as sulfuric acid, citric acid or ascorbic acid. Alternatively, glucose may be obtained by enzymatic hydrolysis (e.g. using an amylase) of starch contained in biomass feedstocks, for example maize, rice or potatoes. Where a saccharide other than a monosaccharide is used as a starting material in the process of the invention and reacted with barium hydroxide, it is possible that monosaccharide is generated in situ and subsequently reacts with barium hydroxide.

The preparation of barium lactate is typically carried out in the presence of one or more solvents. In particular, the reaction between the saccharide and barium hydroxide is normally carried out in the presence of water. Some commercial sources of saccharide, particularly sources of monosaccharide and disaccharide, contain water, and such feedstocks may readily be used in the process of the invention. In certain embodiments, the reaction between the saccharide and barium hydroxide may take place in the presence of additional water (i.e. additional to that present in the starting materials). The reaction between the saccharide and barium hydroxide may also, if desired, take place in the presence of one or more organic solvents, for example an oxygenate such as an alcohol, ester, ether, or ketone; and/or in the presence of one or more reactive extractants such as an amine. However, in a preferred embodiment, the reaction between the saccharide and barium hydroxide does not take place in the presence of an organic solvent.

Barium hydroxide reacts with saccharide to produce barium lactate. Sources of barium hydroxide such as barium oxide may be used to prepare barium lactate, barium oxide being converted into barium hydroxide in the presence of water. The barium hydroxide generated in situ reacts with the saccharide to produce barium lactate.

Preferably, however, barium hydroxide itself is used, and preferably at least part of the barium hydroxide feed to the barium lactide preparation step is a recycle from step (b) of the process of the invention. The ratio of barium hydroxide to saccharide should be sufficient to effect high conversion of saccharide to barium lactate. For example, when the saccharide comprises glucose, for each mole of glucose there is preferably used at least one mole of barium hydroxide (i.e. the molar ratio of barium hydroxide to saccharide (calculated as monosaccharide) is at least 1 : 1). Excess quantities of barium hydroxide may be used, for example the molar ratio of barium hydroxide to saccharide (calculated as monosaccharide) may be up to 10: 1. In a preferred embodiment, the molar ratio of barium hydroxide to saccharide (calculated as monosaccharide) is from 1 : 1 to 5 : 1, more preferably 1 : 1 to 4: 1, especially 1 : 1 to 2: 1. The present invention also encompasses molar ratios of barium hydroxide to saccharide (calculated as monosaccharide) that are lower than 1 : 1, although this is not preferred since use of sub-stoichiometric quantities of barium hydroxide will generally lead to lower conversion of saccharide to barium lactate.

The conversion of saccharide to barium lactate may be carried out at room temperature, although the reaction is preferably carried out at elevated temperature, for example at a temperature of up to 150 °C. Preferably, saccharide is reacted with barium hydroxide at a temperature of from 50 to 120 °C, more preferably from 70 to 1 10 °C, for example from 75 to 100 °C. In one embodiment, saccharide is reacted with barium hydroxide at 80 °C. In another embodiment, saccharide is reacted with barium hydroxide in water at reflux.

In a preferred embodiment, an aqueous solution of at least one saccharide, especially a monosaccharide, is added over a period of time to a mixture of barium hydroxide and water that is at elevated temperature, for example at reflux. Slow addition of the saccharide generally leads to a reduction in the formation of side products during the process of the invention, and leads to an improved conversion of saccharide into barium lactate. Preferably the aqueous solution of saccharide is added over a period of at least 30 minutes, more preferably over at least 1 hour, most preferably over at least 2 hours. The aqueous solution of at least one saccharide preferably has a concentration of less than 4.0 M, more preferably 0.2 - 2.0 M, most preferably 0.5 - 1.5 M.

In a preferred embodiment of the process of the invention, an aqueous solution of at least one saccharide is added to a mixture of barium hydroxide and water that is at elevated temperature, for example at reflux. Upon completion of the addition, concentrated hydrochloric acid or concentrated nitric acid is gradually introduced, and the resulting solution containing lactic acid and barium chloride or barium nitrate is then added over a period of time to an alkyl alcohol, preferably butanol, with heating, for example under reflux, optionally after prior removal of some water. During this phase of the process, water is removed and, in the case of the alkyl alcohol being one in which the inorganic barium salt is insoluble, such as butanol, solid barium chloride precipitates from the reaction mixture. On completion of the reaction, further water is removed if desired, and the solid inorganic barium salt is filtered off. In the case of the alkyl alcohol being one in which the inorganic barium salt is soluble, such as methanol, the alkyl alcohol and alkyl lactate are separated from the inorganic barium salt by distillation. The solid inorganic barium salt is dissolved in water, and treated with hydroxide to produce solid barium hydroxide, which is filtered off and recycled to the first stage in the process.

The produced lactic acid or alkyl lactate may be converted into any other desired lactate-containing species. It may for example be converted into lactide, a cyclic dimer of lactic acid that is itself useful in the production of polylactic acid, into oligomers of lactic acid, or into poly-lactic acid, by known methods involving one or more process steps. An alkyl lactate may also be converted into lactic acid. Therefore, the invention also provides a process for the production of oligomeric lactic acid, lactide or poly-lactic acid, comprising producing lactic acid by a process according to the invention, and converting said lactic acid into lactide, oligomeric lactic acid or poly-lactic acid. The invention also provides a process for the production of lactic acid, oligomeric lactic acid, lactide or polylactic acid, comprising producing alkyl lactate by a process according to the invention, and converting said alkyl lactate into lactic acid, oligomeric lactic acid, lactide or polylactic acid.

In one embodiment, the invention provides a process for the production of oligomeric lactic acid, lactide or poly-lactic acid, which comprises producing lactic acid by a process comprising

b) reacting the inorganic barium salt with a hydroxide in the presence of water to produce solid barium hydroxide; wherein the hydroxide is selected from the group consisting of sodium hydroxide, lithium hydroxide, potassium hydroxide and ammonium hydroxide; and

c) converting the resulting lactic acid into lactide, oligomeric lactic acid or poly- lactic acid.

In another embodiment, the invention provides a process for the production of lactic acid, oligomeric lactic acid, lactide or poly-lactic acid, which comprises producing an alkyl lactate by a process comprising

aa) reacting the lactic acid with an alkyl alcohol to produce an alkyl ester; and subsequently

c) converting the resulting alkyl lactate into lactic acid, lactide, oligomeric lactic acid or poly-lactic acid.

In any of these embodiments, the barium lactate may be prepared by reacting one or more saccharides with barium hydroxide, as described above; the barium hydroxide produced in step (b) may be recycled as described above; and/or solid inorganic barium salt may be separated from the reaction mixture as described above.

Lactide is a particularly useful product which may be directly or indirectly obtained from lactic acid or an alkyl lactate. For example, lactic acid may be heated to produce a pre-polymer or oligomer of lactic acid, which may then be contacted with a transesterification catalyst to produce lactide. There are three forms of lactide, (S,S)- or L-lactide, (R,R)- or D-lactide, and (R,S)- or meso-lactide. Racemic and meso-lactide may be separated by standard separation techniques, for example by distillation, solvent extraction, or crystallisation. It will be understood that references to lactide include (S,S)- lactide, (R,R)-lactide, and (R,S)-lactide, as well as mixtures thereof (e.g. racemic lactide, a mixture containing (R,R)-lactide and (S,S)-lactide in equal amounts). Lactide, and particularly optically pure lactide, may be polymerised to form polylactic acid. The invention therefore further provides a process for the production of polylactic acid, comprising producing lactic acid or an alkyl lactate by a process according to the invention, converting the lactic acid or alkyl lactate directly or indirectly into lactide, and polymerising the lactide to form polylactic acid. The polymerisation may be carried out by contacting lactide with a catalyst at elevated temperature.

References to polylactic acid include poly-R-lactic acid, poly-S-lactic acid, polylactic acid containing both R- and S-lactate subunits, as well as combinations thereof.

Similarly, references to oligomeric lactic acid include oligomeric-R-lactic acid, oligomeric-S-lactic acid, oligomeric lactic acid containing both R- and S-lactate subunits, as well as combinations thereof.

The present invention provides a high-yielding, economic process for the preparation of lactic acid or an alkyl ester of lactic acid from readily available starting materials. It steps away from long-established norms of lactic acid manufacture and provides a chemical process comparing very favourably on economic terms with fermentation processes, by being very selective and providing a good recovery route for barium. Further, it avoids the use of an energy expensive recycle of the barium hydroxide.

The following Examples illustrate the invention.

Example 1

Conversion of C5/C6 monosaccharides into butyl lactate

To a flask was charged either i) barium hydroxide octahydrate (1 to 2.5 molar equivalents based on the monosaccharide), or ii) barium hydroxide anhydrous (1 to 2.5 molar equivalents based on the monosaccharide used) and water (equivalent to make barium hydroxide octahydrate). This was heated to 100 ±3 °C to form a mobile suspension.

A solution of C5 or C6 monosaccharide (50 mL, 0.2 M to 0.4 M in water) was then added to the heated barium hydroxide suspension with stirring over a period of 10 - 30 min using a peristaltic pump. Upon completion of the addition the heating was discontinued and concentrated aqueous HCl (37%, 1.8 to 3 equivalents based on thebarium hydroxide) was introduced over a period of 4 - 7 min (drop-wise) to form an almost clear, light brown to golden yellow solution.

This clear solution was then added into another flask (equipped with a Dean-Stark trap and N₂ bleed) containing refluxing n-BuOH (200 mL, pot temp 114 - 119 °C). The addition period was between 2 - 3 h during which water was continuously removed and the pot temperature varied between 94 and 120 °C. During the addition simultaneous precipitation of BaCl₂ was also observed. Upon completion of the addition, the mixture was continued to reflux until no water was collected in the trap (1-2 h). A total of 50 - 58 mL water was collected. The reaction mixture was then allowed to cool to ambient temperature (25 - 35 °C) and the precipitate was filtered off. The filtrate was analysed quantitatively by GC for content of racemic butyl lactate.

% lactate selectivity is the percentage of lactate ester present in the fully esterified product mixture, as measured by gas chromatography.

Example 2

Recovery of Ba(OH)₂ from BaCl₂

BaCl₂ (0.0228 mol) was dissolved in water (20 mL). To this 9 M aqueous sodium hydroxide (5 mL) was added at room temp. The resulting precipitated solid was filtered, washed with acetone and air dried to afford 2.34 g of barium hydroxide as a white solid. The pH of the suspension of this white solid in water was found to be >12. Note: Out of BaCl₂, NaCl, NaOH and Ba(OH)₂, only Ba(OH)₂ has very limited solubility in water. Example 3

Recovery and recycling of Ba(OH)₂

Barium chloride obtained from experiments described in Example 1 above was dissolved in water (2-4 volumes) and sodium hydroxide (1.1 to 2.1 equivalents) was added (pellets). The mixture for stirred for 30 min at ambient temperature and then cooled to 0 - 5 °C for 1 - 2 hrs. The solid was filtered, washed with cold water and dried in an oven at 70 °C to afford hydrated barium hydroxide as an off white solid (35 - 45%). A further crop of barium hydroxide was observed in the filtrate on standing.

The barium hydroxide produced was found to become molten on heating to about 80 - 100 °C, and was assumed to be barium hydroxide octahydrate. The barium hydroxide produced was used in a typical monosaccharide conversion reaction with 0.2 M fructose, of the type described in Example 1 above. The butyl lactate yields (GC) from the reaction were unaffected.

Claims

1. A process for producing lactic acid and barium hydroxide from barium lactate, comprising

2. A process as claimed in claim 1, wherein the barium lactate has been produced by (a¹) reacting one or more saccharides with barium hydroxide.

3. A process as claimed in claim 2, wherein at last part of the barium hydroxide produced in step (b) is recycled to step (a¹).

4. A process as claimed in any one of the preceding claims, wherein step (a) is carried out at a temperature in the range of from 50 to 125 °C.

5. A process as claimed in any one of the preceding claims, wherein in step (a) the molar ratio of barium lactate to acid is in the range of from 1 :2 to 1 :5.

6. A process as claimed in any one of the preceding claims, wherein the acid in step (a) is hydrochloric acid.

7. A process as claimed in any one of the preceding claims, wherein the hydroxide in step (b) is sodium hydroxide.

8. A process as claimed in any one of the preceding claims, wherein step (b) is carried out at a temperature in the range of from 5 to 40 °C.

9. A process as claimed in any one of the preceding claims, wherein in step (b) the molar ratio of inorganic barium salt to hydroxide is in the range of from 1 :2 to 1 :2.5.

10. A process for producing an alkyl lactate, which comprises producing lactic acid by a process as claimed in any one of the preceding claims, and reacting said lactic acid with an alkyl alcohol to produce an alkyl lactate.

11. A process as claimed in claim 10, which comprises

(a) reacting barium lactate with an acid selected from the group consisting of hydrochloric acid and nitric acid, to produce lactic acid and an inorganic barium salt comprising barium chloride and/or barium nitrate;

12. A process as claimed in claim 11, in which step (aa) is carried out under reflux with removal of water.

13. A process as claimed either claim 11 or claim 12, in which following step (aa), inorganic barium salt is removed from the reaction mixture prior to carrying out step (b).

14. A process as claimed in any one of claims 10 to 13, wherein the alkyl alcohol is a Ci to C₆ alkyl alcohol.

15. A process as claimed in claim 14, wherein the alkyl alcohol is butanol.

16. A process as claimed in any one of claims 10 to 15, wherein the lactic acid is reacted with the alkyl alcohol at a temperature in the range of from 50 to 150 °C.

17. A process as claimed in any one of the preceding claims, wherein the lactic acid or alkyl lactate is recovered by distillation.

18. A process for the production of oligomeric lactic acid, lactide or poly-lactic acid, comprising producing lactic acid by a process according to any one of claims 1 to 9, and converting said lactic acid into lactide, oligomeric lactic acid or poly-lactic acid.

19. A process for the production of lactic acid, oligomeric lactic acid, lactide or poly- lactic acid, comprising producing alkyl lactate by a process according to any one of claims 10 to 17, and converting said alkyl lactate into lactic acid, oligomeric lactic acid, lactide or poly-lactic acid.