Method for synthesising glycolic acid

Abstract

The invention relates to a method for synthesizing glycolic acid or one of the salts thereof, including the following step: placing glycerol and an oxidizing agent in contact with one another in a reaction medium in the presence of a silver-based catalyst on a substrate, said substrate including a material selected from the group consisting of CeO.sub.2, basic Al.sub.2O.sub.3 optionally doped with a calcium or cerium oxide, an amphoteric resin, ZrO.sub.2, and a mixture of said materials. The invention also relates to the use of a silver-based catalyst on a substrate that can be used in said method.

Claims

1. A process for the synthesis of glycolic acid or one of its salts comprising the following step: bringing glycerol and an oxidizing agent into contact in a reaction medium in the presence of a catalyst based on silver on a support, said support comprising a material chosen from the group consisting of CeO.sub.2, basic Al.sub.2O.sub.3, optionally doped with a calcium or cerium oxide, an amphoteric resin, ZrO.sub.2 and a mixture of these materials; wherein a base which produces hydroxide ions is present in the reaction medium; and wherein the molar ratio of hydroxide ions to glycerol [OH]/[gly] brought together is from 0.5 to 1.2.

2. The process as claimed in claim 1, wherein said catalyst is a catalyst on a support of formula Ag/CeO.sub.2, basic Ag/Al.sub.2O.sub.3, optionally doped with calcium oxide or with cerium oxide, or Ag/IR45.

3. The process as claimed in claim 1, wherein the catalyst comprises, as metal entity, only silver.

4. The process as claimed in claim 1, wherein the pH of the reaction medium is greater than or equal to 8.

5. The process as claimed in claim 1, wherein which said oxidizing agent is oxygen.

6. The process as claimed in claim 1, wherein which the temperature of the reaction medium is chosen within a range extending from 50 to 150 C.

7. The process as claimed in claim 1, wherein the catalyst based on silver on a support is a catalyst, the proportion by weight of silver with respect to the support of which varies from 10 to 0.2%.

8. The process as claimed in claim 1, wherein the glycerol/catalyst ratio by weight in the reaction medium is chosen within a range extending from 2 to 100.

9. The process as claimed in claim 1, wherein the glycerol is of vegetable origin.

10. The process as claimed in claim 1, wherein the amounts of calcium or cerium oxides optionally used are less than 3 mmol of the atom used with respect to 1 g of support.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) A better understanding of the invention will be obtained on reading the appended figures, which are provided by way of examples and do not exhibit a limiting nature, in which:

(2) FIG. 1 represents the amount of products present during the reaction for the conversion of glycerol into different synthesis products described in example 3 as a function of the reaction time, this reaction taking place in the presence of a 1.486 wt % Ag/Al.sub.2O.sub.3 catalyst according to the invention and as described in example 1.

(3) FIG. 2 represents the amount of products present during the reaction for the conversion of glycerol into different synthesis products described in example 4 as a function of the reaction time, this reaction taking place in the presence of a 1.486 wt % Ag/IR45 catalyst according to the invention and as described in example 4.

(4) FIG. 3 represents the amount of products present during the reaction for the conversion of glycerol into different synthesis products of example 5.

(5) FIG. 4 represents the amount of products present during the reaction for the conversion of glycerol into different synthesis products of example 6.

DETAILED DESCRIPTION OF THE INVENTION

EXAMPLE 1

Synthesis of a Catalyst on Alumina According to the Invention: 1.46 wt % Ag/Al2O3

(6) 9.9643 g of basic Al.sub.2O.sub.3 (MerckBET120 m.sup.2/g) and 75 ml of MeOH were heated with stirring at 67 C. for one hour.

(7) After this period of time, 0.2332 g of AgNO.sub.3 (eq. 0.1481 g of Ag), dissolved in 25 ml of MeOH, was slowly added to the reaction mixture. The beaker and the funnel were washed with an additional 30 ml of distilled water, which were also added to the reaction mixture.

(8) Heating with stirring was continued for 1 h and then 26.4 ml (0.018 mol) of formaldehyde (HCHO) were added. Up to this stage, the color of the suspension is off white.

(9) After 1 h, 13.2 ml of an NaOH solution (0.3M) were introduced into the solution, and the latter became dark brown, Heating under stirring is continued for 1.5 h.

(10) After this period of time, the solution is slowly cooled down to ambient temperature and filtered. The filtrate obtained is washed with 50 ml of distilled water and then dried at 110 C. for 12 h. The color of the powder collected is dark grey.

(11) The catalyst obtained is named according to the convention in force according to its nominal weight of charge, that is to say the nominal percentage by weight of silver used, with respect to the total weight of the catalyst.

EXAMPLE 2

Synthesis of a Catalyst on Resin According to the Invention: 1.486 Ag/IR45 (Calculated for the Dry Weight of the Resin)

(12) The support chosen is IR45(OH), a resin formed of polystyrene polymers highly connected to one another and which are functionalized by amine groups, sold by Rohm & Haas. This resin is a neutral or amphoteric resin, H.sub.0=7.2-7.8. 9.1485 g of IR45(OH) resin (6.7535 g by dry weight as 26.2% of the weight is lost during the initial drying) are dried and then added to 75 ml of MeOH: this mixture is subsequently heated with stirring at 67 C. for one hour.

(13) After this period of time, 0.1581 g of AgNO.sub.3 (eq. 0.1004 g of Ag), dissolved in 25 ml of MeOH, was slowly added to the reaction mixture. The beaker and the dropping funnel were washed with an additional 30 ml of distilled water, which were also added to the reaction mixture.

(14) Heating under stirring was continued for one hour and then 0.012 mol of acid HCHO (9 ml of solution) was added. After one hour, 9 ml of NaOH solution (0.3M) were introduced into the solution. Heating under stirring was continued for 1.5 hours.

(15) After this period, the solution was slowly cooled to ambient temperature and filtered. The filtrate is washed with 50 ml of distilled water and then dried at 110 C. for 12 h. The color of the powder collected is nonuniform and exhibits two colors: orange and green.

EXAMPLE 3

Use of the Catalyst of Example 1 in the Synthesis of Glycolic Acid from Glycerol

(16) The oxidation of glycerol in the liquid phase is carried out in a 300 ml stainless steel reactor equipped with a gas entrainment impeller, with four baffles, with a thermocouple and with a system for feeding with thermally regulated oxygen. 200 ml of an aqueous glycerol solution ([gly]=0.3M) are heated to the desired temperature of 60 C.

(17) The sodium hydroxide (molar ratio NaOH/Gly=4) and the catalyst (ratio by weight gly/cat=11 (g/g)) are introduced into the reactor (t.sub.0) and the system is pressurized with oxygen (5 bar) with continuous stirring (1500 rpm). The temperature and the O.sub.2 partial pressure are continuously monitored while the sampling is carried out periodically. The products are analyzed by high performance liquid chromatography (HPLC) using a device of Agilent 1200 type equipped with a Rezex ROA-Organic Acid H+ column (3007.8 mm) and with a refractive index detector (RID). A solution of H.sub.2SO.sub.4 (0.0025M) in demineralized water (0.5 ml.Math.min.sup.1) is used as eluent. The identification and the quantification of the products obtained are carried out by comparison with the corresponding calibration curves.

(18) The catalyst composed of approximately 1% of silver supported on alumina makes it possible to obtain a selectivity for glycolic acid (that is to say, the molar amount of glycolic acid formed with respect to the glycerol consumed), stable over the whole of the experiment, or approximately 50% (cf. FIG. 1). The two other products formed, in a similar amount, are glyceric acid and formic acid (each 25%). The degree of conversion is greater than 30%.

EXAMPLE 4

Use of the Catalyst of Example 2 in the Synthesis of Glycolic Acid from Glyercol

(19) The oxidation of glycerol in the liquid phase is carried out in a 300 ml stainless steel reactor equipped with a gas entrainment impeller, with four baffles, with a thermocouple and with a system for feeding with thermally regulated oxygen. 200 ml of an aqueous glycerol solution ([gly]=0.3M) are heated to the desired temperature of 100 C.

(20) The sodium hydroxide (molar ratio NaOH/Gly=1) and the catalyst (ratio by weight gly/cat=5.5 (g/g)) are introduced into the reactor (t.sub.0) and the system is pressurized with oxygen (5 bar) with continuous stirring (1500 rpm). The temperature and the O.sub.2 partial pressure are continuously monitored while the sampling is carried out periodically. The products are analyzed with an Agilent 1200 HPLC device equipped with a Rezex ROA-Organic Acid H+ column (3007.8 mm) and with a refractive index detector (RID). A solution of H.sub.2SO.sub.4 (0.0025M) in demineralized water (0.5 ml.Math.min.sup.1) is used as eluent. The identification and the quantification of the products obtained are carried out by comparison with the corresponding calibration curves.

(21) In this example, the use of a low amount of base (NaOH/Gly=1) makes it possible to obtain very good results after one hour of reaction (35% conversion and 55% selectivity for glycolic acid, cf. FIG. 2). The use of an amphoteric and/or neutral support is thus preferred.

(22) The use of purely acidic resin (IRA120, HA=0.8) results in the conversion of the glycerol into glyceric acid. The use of a highly basic resin (IRA400, Fluka, HA>14) converts the glycerol into glyceric acid and into glycolic acid in the some proportions (selectivity=30%).

EXAMPLE 5

Use of the Catalyst of Example 1 in the Synthesis of Glycolic Acid from Glycerol Using a Reduced Concentration of Base

(23) Example 4 was repeated with the catalyst of example 1 (1.486 wt % Ag/Al.sub.2O.sub.3) and the results obtained, presented in FIG. 3, are similar to those of example 4.

EXAMPLE 6

Method for the Synthesis of Glycolic Acid Using a Catalyst of Silver on a CeO2 Support

(24) Preparation of the CeO.sub.2 Support

(25) An aqueous cerium nitrate solution (0.5M) was prepared from cerium(III) nitrate hexahydrate (puriss p.a., 99.0%, Fluka). The solution obtained was added dropwise to a solution (in excess) of triethylamine (1.5M) diluted in methanol. The hydroxide precipitate obtained was recovered by filtration. It was subsequently washed and rinsed with water and methanol several times. It was dried in an oven at 100 C. The solid was subsequently ground into the form of a fine powder and then calcined under air at 500 C. for 4 h.

(26) The synthesis of the catalyst was carried out according to the procedure of example 1. The synthesis of glycolic acid was carried out according to the procedure of example 3, that is to say at 60 C., 5 bar of O.sub.2, 0.5 g of catalyst, 1500 rpm, 200 ml of a pure glycerol solution (0.3M) and a molar ratio NaOH/glycerol=4.

(27) This catalyst makes it possible to obtain a selectivity for glycolic acid (that is to say, the molar amount of glycolic acid formed with respect to the glycerol consumed), stable over the whole of the experiment, of approximately 50% (cf. FIG. 4). The degree of conversion is greater and approximately 60%.

(28) The invention is not limited to the embodiments presented and other embodiments will be clearly apparent to a person skilled in the art.