METHOD FOR PREPARING A CAMGAL MIXED OXIDE, A CAMGAL MIXED OXIDE OBTAINABLE THIS WAY AND THE USE THEREOF FOR OLIGOMERIZATION OF GLYCEROL

Abstract

The present invention relates to a method for preparing a CaMgAl mixed oxide comprising the steps: a) providing a modified layered double hydroxide of the formula (I) wherein in formula (I) 0<x<0.9; b is from 0 to 10, preferably 1 to 10; c is from 0 to 10, preferably 1 to 10 and the AMO-solvent is an organic solvent miscible with water; b) calcining the modified layered double hydroxide; c) reacting the calcined modified layered double hydroxide with a calcium salt in the presence of an (a) organic acid; and d) calcining the product obtained in step c) to obtain the CaMgAl mixed oxide; a CaMgAl mixed oxide obtainable this way; and the use thereof.

Claims

1. A method for preparing a CaMgAl mixed oxide comprising the steps: a) providing a modified layered double hydroxide of the formula (I)
[Mg.sub.1xAl.sub.x(OH).sub.2]CO.sub.3.b(H.sub.2O).c (AMO-solvent) (I) wherein in formula (I) 0<x<0.9; b is from 0 to 10; c is from 0 to 10, and the AMO-solvent is an organic solvent miscible with water; b) calcining the modified layered double hydroxide; c) reacting the calcined modified layered double hydroxide with a calcium salt in the presence of an organic acid; and d) calcining the product obtained in step c) to obtain the CaMgAl mixed oxide.

2. The method according to claim 1, wherein the amount of the calcium salt is 5-10 wt. %, with respect to the weight of the calcined modified layered double hydroxide.

3. The method according to claim 1, wherein calcining in step b) is from 300 to 700 C., for 1 to 10 h.

4. The method according to claim 1, wherein reacting of step c) is in the further presence of a weak base.

5. The method according to claim 1, wherein reacting in step c) is at a pH from 5 to 7.

6. The method according to claim 1, further comprising a step of drying the material obtained in step c) before performing step d).

7. The method according to claim 1, wherein calcining in step d) is at a temperature from 450 to 900 C.

8. The method according to claim 1, wherein the organic acid is selected from the group consisting of citric acid, malic acid, tartaric acid, oxalic acid, succinic acid, formic acid, and acetic acid.

9. The method according to claim 1, wherein reacting in step c) is an aqueous solution.

10. A CaMgAl mixed oxide produced by the method according to claim 1.

11-12. (canceled)

13. A method for oligomerization of glycerol comprising a step of contacting the glycerol and the CaMgAl mixed oxide according to claim 10.

14. The method according to claim 13, wherein the amount of CaMgAl mixed oxide is from 2 to 10 wt. % with respect to the weight of glycerol.

15. The method according to claim 13, wherein contacting is carried out at a temperature from 150 to 300 C.

16. The method according to claim 1, wherein, in formula (I), b is from 1 to 10.

17. The method according to claim 1, wherein, in formula (I), c is from 1 to 10.

18. The method according to claim 2, wherein the amount of the calcium salt is 7-8 wt. %, with respect to the weight of the calcined modified layered double hydroxide.

19. The method according to claim 4, wherein the weak base is ammonium hydroxide.

20. The method according to claim 15, wherein the temperature is from 200 to 230 C.

Description

[0030] In the following, the invention will be described in greater detail referring to the specific examples and Figures, without, however, the intention to respectively limit the scope of the invention.

[0031] FIG. 1 XRD patterns of reconstructed 7.5% CaMgAl LDH (a) prepared by method according to the invention and (b) prepared by conventional impregnation.

EXAMPLES

Preparation of CaMgAl Mixed Oxide

Comparative Example A

[0032] Modified layered double hydroxide of the formula (I) as described above (AMO-LDH) was prepared according to WO2014051530. A metal precursor solution was prepared by dissolving 1.575 mol (403.85 g) of Mg(NO.sub.3).sub.2.6H.sub.2O and 0.525 mol (196.94 g) of Al(NO.sub.3).sub.3.9H.sub.2O in 700 mL deionized water. The metal precursor was added dropwise into a 700 mL of Na.sub.2CO.sub.3 (0.315 mol, 33.39 g) solution. The pH of precipitation solution was adjusted to 10 using a NaOH solution in deionized water (4 M). The precipitant was aged at room temperature for about 3 h. LDH was collected by filtration, washing with deionized water until the filtrate pH was neutral, and rinsing with ethanol. The washed wet cake was dispersed in ethanol. After stirring for about 1 h, the sample was filtered. The final AMO-LDH product was dried overnight in the fume hood at room temperature and then in an oven at 110 C. for 12 h.

Example 1

[0033] AMO-LDH, prepared in the same manner as Comparative example A, was calcined, to obtain mixed oxide of Mg and Al, in a muffle furnace at 450-550 C. for 1-5 h. Citric acid solution was prepared by dissolving 2.67 g of citric acid in 2.67 g of deionized water, and adding ammonium hydroxide (30 wt. % ammonia in water) to adjust the solution pH to 5-7. Then, 8.84 g of Ca(NO.sub.3).sub.2.4H.sub.2O were dissolved in the citric acid solution, followed by slowly dropping the Ca solution onto 20 g of the calcined AMO-LDH under stirring. The wet powder was dried in an oven at 100 C. overnight to obtain reconstructed CaMgAl LDH. The sample was analyzed using X-ray diffraction method. Then, it was calcined in a muffle furnace at 650-800 C. for 5 h. The CaMgAl mixed oxide (CaMgAl LDO) with 7.5 wt. % of Ca content was obtained.

[0034] FIG. 1(a) shows XRD pattern of reconstructed CaMgAl LDH with 7.5 wt. % of Ca content. It consists of pure hydrotalcite phase. There was no separated Ca phase and impurities.

Example 2

[0035] Example 2 was prepared in the same manner as Example 1, except 5.89 g of Ca(NO3).sub.2.4H.sub.2O was used to obtain 5 wt. % Ca content.

Conventional impregnation (Comparative Example B)

[0036] AMO-LDH was prepared and calcined following the method of the invention above. Then, it was impregnated with Ca(NO.sub.3).sub.2.4H.sub.2O in water by slowly dropping 8.84 g of Ca(NO.sub.3).sub.2.4H.sub.2O in water on the calcined AMO-LDH under stirring. The wet powder was dried in an oven at 100 C. overnight to obtain the reconstructed CaMgAl LDH. Then, it was calcined in a muffle furnace at 650-800 C. for 5 h. The CaMgAl mixed oxide with 7.5 wt % of Ca content was obtained.

[0037] FIG. 1(b) shows the reconstructed CaMgAl LDH prepared by conventional impregnation. It consists of Ca(OH).sub.2 and CaCO.sub.3 as impurity phases.

Production of Short-Chain Polyglycerols

[0038] 50 g of glycerol were added into a three-necked round-bottom flask equipped with a N.sub.2 gas line and Dean-Stark apparatus to continuously remove water generated as a by-product. The glycerol heated to the reaction temperature under stirring using a heating mantle. Then, the CaMgAl catalyst was added into the flask. The reaction was carried out with 3 wt. % of catalyst loading level with respect to the weight glycerol used at temperature of 220 C. for 24 h.

[0039] The reaction product was diluted with 100 mL of methanol to reduce product viscosity, and then filtered to separate the catalyst. The methanol was removed by rotary evaporator. product was analyzed for the glycerol conversion and the products yield using a gas chromatography. The glycerol conversion, diglycerols yield and diglycerols selectivity were calculated by the following equations.

[00001]$\mathrm{Glycerol}.Math..Math.\mathrm{conversion}.Math..Math.\left(\mathrm{mol}.Math..Math.\%\right)=\frac{\mathrm{Mole}.Math..Math.\mathrm{of}.Math..Math.\mathrm{converted}.Math..Math.\mathrm{glycerol}}{\mathrm{Mole}.Math..Math.\mathrm{of}.Math..Math.\mathrm{initial}.Math..Math.\mathrm{glycerol}}100$$\mathrm{Diglycerols}.Math..Math.\mathrm{selectivity}.Math..Math.\left(\%\right)=\frac{\mathrm{Mole}.Math..Math.\mathrm{of}.Math..Math.\mathrm{diglycerols}}{\mathrm{Mole}.Math..Math.\mathrm{of}.Math..Math.\mathrm{converted}.Math..Math.\mathrm{glycerol}}100$

[0040] Comparative examples A shows 36% glycerol conversion and 51% diglycerols selectivity, while comparative example B shows 55% glycerol conversion and 50% diglycerols selectivity. The inventive examples 1 and 2 show higher diglycerols selectivity of 78% and 66%, respectively because it consists pure phase of CaMgAl LDO resulting in high dispersion of Ca atoms. The interaction between Ca, Mg and Al oxides formed a synergetic mixed oxide complex, which provided an enhanced basicity and contributed to the high catalytic performance during glycerol etherification. While, the conventional CaMgAl LDO consists of less basic oxides, and gives low diglycerols selectivity.

[0041] The features disclosed in the foregoing description and in the claims may, both separately and in any combination be material for realizing the invention in diverse forms thereof.