PRODUCTION OF UNSATURATED CARBOXYLIC ACIDS OR ACID ESTERS WITH A HALOAPATITE-BASED CATALYST

Abstract

The present invention relates to the use of haloapatites as catalysts of the dehydration reaction of α-hydroxylated carboxylic acids or acid esters, in particular of lactic acid or of methyl lactate, and also to a process for producing unsaturated carboxylic acids or acid esters, in particular acrylic acid or methyl acrylate, in the gas phase in a stainless steel reactor, in the presence of such a catalyst.

Claims

1. A catalyst of the gas-phase dehydration reaction of an α-hydroxylated carboxylic acid or an α-hydroxylated carboxylic acid ester, said catalyst being at least one haloapatite of formula (I) comprising:
Ca.sub.10−x[(PO.sub.4).sub.6−(x+y+z)(HPO.sub.4).sub.x+z(CO.sub.3).sub.y][Y.sub.2−(x+y+z)(CO.sub.3).sub.y+z]  (I) in which: Y represents at least one anion selected from the F.sup.−, Cl.sup.− anions, and the combinations of at least one F.sup.− or Cl.sup.− anion with an OH.sup.− anion; 0≦x≦1; 0≦y≦1; 0≦z≦1; 0≦x+y+z≦1.

2. The catalyst according to claim 1, wherein the dehydration reaction is carried out on an α-hydroxylated carboxylic acid or acid ester of formula (II) below: ##STR00006## in which: R.sup.1 is an alkyl radical having from 1 to 6 carbon atoms, R.sup.2 represents a hydrogen atom or an alkyl radical having from 1 to 6 carbon atoms; and results in an unsaturated carboxylic acid or acid ester of formula (III) below being obtained: ##STR00007## in which R.sup.1 and R.sup.2 have the same meaning as in formula (II).

3. The catalyst according to claim 2, wherein the dehydration reaction is carried out on an α-hydroxylated carboxylic acid or acid ester of formula (II) in which R.sup.1 is an alkyl radical having 1 carbon atom and R.sup.2 represents a hydrogen atom, a methyl radical or an ethyl radical.

4. The catalyst according to claim 1, wherein the dehydration reaction is carried out using lactic acid and results in acrylic acid being obtained.

5. The catalyst according to claim 1, wherein the dehydration reaction is carried out in the presence of a haloapatite of formula (I) in which Y=F.sup.−; x=0; y=0; z=0 and of Ca/P molar ratio=1.67.

6. Process for producing an unsaturated carboxylic acid or an unsaturated carboxylic acid ester in the presence of a catalyst, said process comprising the steps of: dehydrating an α-hydroxylated carboxylic acid or an α-hydroxylated carboxylic acid ester, respectively, said step being carried out in the gas phase, in the presence of a solid catalyst containing at least one halogenated apatite of formula (I) below:
Ca.sub.10−x[(PO.sub.4).sub.6−(x+y+z)(HPO.sub.4).sub.x+z(CO.sub.3).sub.y][Y.sub.2−(x+y+z)(CO.sub.3).sub.y+z]  (I) in which: Y represents at least one anion selected from the F.sup.−, Cl.sup.− anions, and the combinations of at least one F.sup.− or Cl.sup.− anion with an OH.sup.− anion; 0≦x≦1; 0≦y≦1; 0≦z≦b 1; 0≦x+y+z≦1.

7. The process according to claim 6, wherein the dehydration step is carried out on an α-hydroxylated carboxylic acid or acid ester of formula (II) below: ##STR00008## in which: R.sup.1 is an alkyl radical having from 1 to 6 carbon atoms, R.sup.2 represents a hydrogen atom or an alkyl radical having from 1 to 6 carbon atoms; and results in an unsaturated carboxylic acid or acid ester of formula (III) below being obtained: ##STR00009## in which R.sup.1 and R.sup.2 have the same meaning as in formula (II).

8. The process according to claim 6, wherein the dehydration step is carried out using lactic acid and results in acrylic acid being obtained.

9. The process according to claim 6, wherein the catalyst contains at least one haloapatite of formula (I) in which Y=F.sup.−; x=0; y=0; z=0 and of Ca/P molar ratio=1.67.

10. The process according to claim 6, wherein the dehydration reaction is carried out at a temperature greater than or equal to 300° C.

11. The process according to claim 6, wherein the dehydration reaction is carried out at atmospheric pressure.

12. The process according to claim 6, wherein the dehydration reaction is carried out in a stainless steel reactor.

13. The process according to claim 6, wherein the gas phase comprises a carrier gas and/or diluent gas.

14. The process according to claim 14, wherein the carrier gas and/or diluent gas of the gas phase is an inert gas selected from helium and nitrogen.

15. The process according to claim 6, wherein the haloapatite of formula (I) is supported by a solid support.

Description

EXAMPLES

[0052] In the following examples, the following raw materials were used: [0053] diammonium phosphate, [0054] ammonium fluoride, [0055] calcium nitrate, [0056] aqueous ammonia, [0057] lactic acid.

[0058] All these raw materials were obtained from Sigma Aldrich and GPR Rectapure Prolabo, and used as received from the manufacturer, without additional purification.

[0059] From these raw materials, haloapatites of formula (I) with Y═F, x=0 and various fluorine contents were prepared.

[0060] The fluorine content in the final catalyst is determined by the molar amount of ammonium fluoride used during the synthesis relative to the molar amounts of ammonium phosphate and calcium nitrate introduced.

[0061] The synthesis of acrylic acid from lactic acid was carried out in the gas phase in a fixed-bed stainless steel tubular reactor having an internal diameter of 16 mm (external diameter 25 mm) and a length of 178 mm The injection of the lactic acid was carried out using a high performance liquid chromatography (HPLC) pump sold under the trade name 305 Pump by the company GILSON. The temperature of the reactor was regulated in a precise and controlled manner by a type K thermocouple.

Example 1

Synthesis of a Fluoroapatite of Formula Ca.SUB.10.(PO.SUB.4.).SUB.6.F.SUB.2 .in Accordance with the Invention

[0062] Carried out in this example was the synthesis of a haloapatite of formula (I) in accordance with the invention in which Y═F, x=0, y=0 and z=0 and of which the theoretical Ca/P molar ratio in solution is equal to 1.67. The empirical formula of the catalyst in this case is Ca.sub.10(PO.sub.4).sub.6F.sub.2.

[0063] A 0.1M solution of diammonium phosphate in water and containing 33 4 mmol of ammonium fluoride was brought, with stirring, to a temperature of 65° C. in a polytetrafluoroethylene (PTFE) container. The pH of the solution was adjusted to 9 and maintained at this value before and during the precipitation, by adding a 28 wt % solution of aqueous ammonia.

[0064] 200 mL of an aqueous solution containing 0.167 mol of calcium nitrate were then added dropwise to the solution of diammonium phosphate and ammonium fluoride (in 1 to 2 hours) obtained previously, with stirring and at a temperature between 65° C. and 80° C. Throughout the precipitation, the pH of the mixture was maintained at the initial value by suitable additions of a 28 wt % solution of aqueous ammonia.

[0065] At the end of the addition, the mixture was left to cool with stirring for 5 to 6 hours.

[0066] The solid that had precipitated was then filtered on a Büchner funnel then washed and filtered a minimum of three times with demineralized hot water. The precipitated solid cake thus obtained was then kept overnight in the oven at 100° C.

[0067] On leaving the oven, the recovered solid was finely milled then calcined in a muffle furnace at 400° C. for 5 to 6 hours.

[0068] A powder of fluoroapatite of formula Ca.sub.10(PO.sub.4).sub.6F.sub.2 was thus obtained.

Example 2

Synthesis of a Hydroxyfluoroapatite of Formula Ca.SUB.10.(PO.SUB.4.).SUB.6.(OH)F in Accordance with the Invention

[0069] Carried out in this example was the synthesis of a halogenated hydroxyapatite of formula (I) in accordance with the invention in which Y represents a combination of an F.sup.− anion with an OH.sup.− anion, x=0, y=0 and z=0 and of which the theoretical Ca/P molar ratio in solution is equal to 1.67. The empirical formula of the catalyst in this case is Ca.sub.10(PO.sub.4).sub.6(OH)F.

[0070] The synthesis was carried out according to a procedure identical to that of Example 1 above but using a substoichiometric amount of fluorine, namely 16.7 mmol of ammonium fluoride.

[0071] A powder of fluoroapatite of formula Ca.sub.10(PO.sub.4).sub.6(OH)F was thus obtained.

Comparative Example 3

Synthesis of a Non-Halogenated Hydroxyapatite of Formula Ca.SUB.10.(PO.SUB.4.).SUB.6.(OH).SUB.2 .not in Accordance with the Invention

[0072] Carried out in this example was the synthesis of a non-halogenated hydroxyapatite that is not part of the invention and that has the formula Ca.sub.10(PO.sub.4).sub.6(OH).sub.2.

[0073] The synthesis was carried out according to a procedure identical to that of Example 1 above, at a temperature of 85° C. and at pH 10, and by using a diammonium phosphate solution that does not comprise ammonium fluoride, all the other reactants being used in the same molar amounts as in Example 1.

[0074] A powder of hydroxyapatite of formula Ca.sub.10(PO.sub.4).sub.6(OH).sub.2 was thus obtained.

Example 4

Synthesis of Acrylic Acid from Lactic Acid

[0075] Tested in this example were the catalytic properties of a fluoroapatite and of a hydroxyfluoroapatite in accordance with the invention and as respectively prepared according to Examples 1 and 2 above, in comparison with those of the non-halogenated hydroxyapatite not in accordance with the invention and as prepared above according to comparative Example 3.

[0076] The general procedure followed was the following: 1 g of milled catalyst was placed in the stainless steel fixed-bed reactor and held in the middle of the reactor on a quartz wool bed, itself held by stainless steel metal foam. The reactor thus loaded was heated at the dehydration reaction temperature of the lactic acid, that is to say between 350° C. and 375° C.

[0077] A 25 wt % aqueous solution of lactic acid was injected using the HPLC pump, at a flow rate of from 1.5 mL/h to 6 mL/h into an injection chamber heated at 190° C. The evaporation of the lactic acid solution was carried out under a flow of helium with a flow rate of from 15 mL/min to 30 mL/min. The lactic acid supply was stabilized overnight before the start of the catalytic test.

[0078] During the catalytic tests, the liquids flowing out of the reactor were immediately condensed in a water-filled glass trap immersed in a cryostatically controlled bath at 4° C. The condensate was analyzed every 1 h 30 min by gas chromatography equipped with a flame ionization detector.

[0079] The conversion and selectivity calculations were carried out in the following manner:

Conversion C %=[(moles of lactic acid injected)−(moles of lactic acid analyzed at the outlet)/(moles of lactic acid injected)]×100

Acrylic acid selectivity S. AA %=[(moles of acrylic acid analyzed at the outlet)/(moles of lactic acid converted)]×100.

[0080] The results of the catalytic tests carried out with the hydroxyapatite not in accordance with the invention and synthesized according to comparative Example 3, with the fluoroapatite in accordance with the invention and as prepared according to Example 1 and with the hydroxyfluoroapatite in accordance with the invention and as prepared according to Example 2 are presented in Tables I and II below:

TABLE-US-00001 TABLE I Catalyst C % S. AA % Yield % Ca.sub.10(PO.sub.4).sub.6(OH).sub.2 (Ex. 3) 97 30 29.1 Ca.sub.10(PO.sub.4).sub.6F.sub.2 (Ex. 1) 100 37 37.0 Reaction temperature: 350° C.; 25 wt % lactic acid solution; flow rate of the lactic acid solution: 1.5 mL/h, helium flow rate: 15 mL/min.

TABLE-US-00002 TABLE II Catalyst C % S. AA % Yield % Ca.sub.10(PO.sub.4).sub.6(OH).sub.2 (Ex. 3) 93.4 34.3 32.0 Ca.sub.10(PO.sub.4).sub.6(OH)F (Ex. 2) 93.3 36.1 33.7 Ca.sub.10(PO.sub.4).sub.6F.sub.2 (Ex. 1) 99 38.5 38.1 Reaction temperature: 375° C.; 25 wt % lactic acid solution; flow rate of the lactic acid solution: 6 mL/h, helium flow rate: 30 mL/min.

[0081] The results from Tables I and II show that the apatites and hydroxyapatites containing fluorine, and in accordance with the present invention, make it possible to obtain better yields of acrylic acid than the non-halogenated reference hydroxyapatite, under given operating conditions.

[0082] All of the results presented in this invention, under various conditions, demonstrate that haloapatites tested for the dehydration of lactic acid to give acrylic acid are efficient and selective for acrylic acid, and this being under conditions transposable to the industrial scale.