However, these catalysts are sensitive to the presence of water and exhibit the best performance for water contents below 1000 ppm [5].

Leclercq et al. [6] have found that magnesium oxide and mixed magnesium-aluminum oxides are active in the transesterification of oils at the reflux temperature of methanol, but the conditions used (methanol/oil molar ratio of 275 for mixed magnesium and aluminum oxides and 75 for magnesium oxide) and the reaction time (22 hours) required in order to have high conversions do not allow industrial use of this process.

Suppes et al. [7] have found that catalysts based on zeolite NaX and ETS-10 are active in the transesterification reaction at temperatures below 125 ⁰ C. The activity of these catalysts, however, is compromised by the presence of free acidity. Disclosure of the Invention

The aim of the present invention is to provide a process for producing esters from vegetable oils or animal fats which overcomes the drawbacks of the known processes mentioned above, has a lower cost and can be performed continuously, using low ratios between the quantities of alcohol and oil or fat and low ratios between the quantities of catalyst and oil or fat. An object of the present invention is to provide a process which is adapted for producing esters, particularly biodiesel, from vegetable oils or animal fats and also allows to produce the glycerin co-product with high purity and therefore with a higher market price.

Another object of the present invention is to provide a process for preparing esters, particularly biodiesel, from vegetable oils or animal fats even in the presence of large amounts of water and/or free fatty acid.

Another object of the present invention is to provide a process for preparing esters, particularly biodiesel, from vegetable oils or animal fats with limited formation of surfactant species in solution which facilitate the forming of emulsions and slow the step of separation of glycerin and ester, particularly of biodiesel, avoiding the need to neutralize the products and

3 the need for an operation for separating the glycerin from the residues of homogeneous catalyst.

This aim and these and other objects, which will become better apparent by reading the following description of the present invention, are achieved by a process according to the present invention as defined in the appended claims, which comprises the stages of:

- reaction of vegetable oils or animal fats with an aliphatic monoalcohol, at a temperature ranging from 100 to 250 ⁰ C, in the presence of a catalyst which comprises magnesium oxide or mixed oxides of magnesium and aluminum, obtained by calcination of hydrotalcite-like compounds which contain Al and Mg with an atomic ratio of Mg/Al > 1, forming esters of fatty acids and glycerin;

- separation of the unreacted monoalcohol; and - separation of the fatty acid esters and of the glycerin.

The aliphatic monoalcohol contains for example 1 to 5 carbon atoms. Brief description of the drawings

Figures 1 and 2 are X-ray diffractograms of some catalysts given in Example 1. Ways to carrying out the Invention

In the reaction stage, a molar ratio of monoalcohol/oil or fat ranging from 4 to 30 is preferably used.

The oil or fat used in the process according to the present invention can contain free acid, even at high concentrations, particularly even higher than 1% by weight of free acid.

The reaction stage can also occur in the presence of water, particularly even in the presence of more than 10,000 ppm of water.

An example of monoalcohol which can be used in the process of the present invention is bioethanol, optionally partially rectified, containing more than 90% ethanol.

4

The reaction stage of the process according to the present invention can be performed continuously or discontinuously.

The catalyst used in the process according to the present invention can be obtained by calcination of a hydrotalcite-like compound, preferably performed at a temperature ranging from 300 to 700 ⁰ C.

An example of a compound such as hydrotalcite adapted to obtain the catalyst used in the present invention is a hydrotalcite-like compound which comprises carbonate anions.

The catalysts used in the process according to the present invention can be obtained by calcination of the hydrotalcite-like compound, preferably performed for a time ranging from 2 to 20 hours and/or preferably performed with a heating rate ranging from 1 °C/min to 10 °C/min, and/or preferably performed in an atmosphere of air or inert gas.

The crystalline phase obtained after calcination is markedly different from that of the original hydrotalcite-like solids.

The catalyst used in the process according to the present invention can comprise for example a phase such as magnesium oxide or mixed aluminum and magnesium oxide with the characteristic positions of the 2- theta angles in X-ray diffraction spectra, as shown in Figures 1 and 2. In particular, the catalyst can comprise magnesium oxide as periclase phase.

Optionally, the catalyst can be heated to a temperature ranging from 150 to 200 ⁰ C, for example 200 ⁰ C, before it is used in the reaction stage.

The hydrotalcite-like compounds suitable to obtain the catalysts used in the process according to the present invention can be obtained for example by means of a method which comprises the stages of coprecipitation of an aqueous solution of magnesium and aluminum salts with a solution of a salt of an alkaline metal, for example potassium or sodium carbonate, rendered strongly basic for example by means of potassium or sodium hydroxide respectively. Precipitation is performed by

5 adding its solution, in drops, heated for example to 60 ⁰ C, followed by keeping it under agitation and heated for example to 60 ⁰ C overnight. The resulting precipitate is filtered and washed with water and then dried with hot air, for example at 100 ⁰ C. The expression "hydrotalcite-like compound" as used here references a double hydroxide with a layered structure, particularly a compound having the formula [M(II)i. _x M(III) _x (OH) ₂ ] ^x+ (A ⁿ -)x/n.mH ₂ O, in which M(II) is a cation (SI) of a divalent metal; M(III) is a cation of a trivalent metal; A is an anion with a charge n, x is the atomic ratio M(II)/(M(II)+M(III)), m is the number of water molecules present in the crystalline structure or reticular water.

In the hydrotalcite-like compounds used to prepare the catalysts used in the process according to the present invention, M(II) is Mg, M(III) is Al, and preferably A is carbonate and x is < 0.5. The expression "hydrotalcite- like compound" as used here also refers to synthetic compounds but also to natural hy drotalcite .

The expressions "hydrotalcite-like compounds" and "hydrotalcites" are used here interchangeably.

Methods for preparing hydrotalcite-like compounds such as those used in the present invention are described in [8], pages 201-211, a document which is included here in by reference.

It has been found unexpectedly that catalysts constituted by magnesium oxide or mixed oxides of magnesium and aluminum obtained by calcination of hydrotalcite-like Al-Mg compounds with a ratio of Mg/Al > 1 allow transesterifϊcation of oils and esterification of fats, especially for producing biodiesel, with high conversion rates. The reaction can be performed even by using oils which contain high concentrations of water and/or free fatty acid.

Moreover, 95% bioethanol can be used directly as alcohol without additional purifications. The term "bioethanol" as used here means 95% ethyl alcohol by

6 volume (with 5% water by volume), obtained from partial rectification of biomass fermentation products.

All the reagents used were supplied by Fluka, except for the soybean oil, supplied by Casa Olearia Italiana S. p. A. (Monopoli, BA). The process according to the invention comprises mixing the vegetable oils or animal fats with an aliphatic alcohol, preferably methanol and ethanol. The reaction mixture is then heated to the reaction temperature and placed in contact with the catalyst.

It has been found unexpectedly that catalysts constituted by magnesium oxide or magnesium and aluminum oxides obtained by calcination of Al-Mg hydrotalcites with a ratio of Mg/Al > 1 are active in the reaction conditions adopted in the reaction for transesterification of oils and esterification of fats.

The reaction conditions used are: reaction temperature ranging from 100 to 250° C, alcohol/oil molar ratio ranging from 4 to 30. These catalysts can be used in the process according to the present invention even in the presence of high concentrations of water, achieving high conversions.

After the reaction, the catalyst is separated, the excess methanol is distilled, and the glyceric phase is separated from the ester phase. If the conversion of the ester phase is o be increased, said phase can be subjected to an additional transesterification stage.

The transesterification reaction can be performed in batch mode or in continuous reactors, both of the agitator-equipped type and of the fixed-bed type. The examples that follow are given as illustration of the invention and must not be considered as limiting its scope.

Examples

Example 1

7

Synthesis of the catalysts

The catalysts were prepared by following the method described by McKenzie et al. [8].

Two solutions were mixed: A, containing Mg(NO ₃ )2 and A1(NO ₃ )2 1.0 molar in Mg + Al and different Mg/Al atomic ratios (0, 3, 4, 10, ∞); B, prepared by dissolving NaOH and Na ₂ CO ₃ as indicated in [8]. Solution A was fed at the rate of 1 cmVmin for 4 hours under vigorous agitation, while solution B was fed, when necessary, in order to keep the pH constant at 10.

The resulting gels were kept at 65 ⁰ C for 24 hours and then filtered and washed to pH 7. They were dried at 85 ⁰ C for 14 hours and the resulting solids were then calcined in air at 500 ⁰ C for 14 hours.

Table 1 lists the theoretical compositions of the various prepared catalysts and the compositions determined by atomic absorption. CHT3 designates the catalyst obtained for an Mg/Al atomic ratio of approximately 3; CHT4 designates the catalyst obtained for an Mg/Al atomic ratio of approximately 4; CHTlO designates the catalyst obtained for an Mg/Al atomic ratio of approximately 10.

Figures 1 and 2 illustrate X-ray diffractograms of the synthesized catalysts. The crystalline phase obtained after calcination is markedly different from that of the original hydrotalcite-like solids: the signs of a crystalline phase such as periclase magnesium oxide are in fact evident. The reflections have positions which are close to those of periclase magnesium oxide, and the precise position and the presence of the lower reflections depend on the composition of the catalyst.

Table 1 Example 1 - S nthesis of the catalysts

Example 2

Tests for transesterification of oil without catalyst Since homogeneous reactions or reactions catalyzed by the steel walls of the reactor are possible [9] at the high temperatures that are used, a reaction test was performed by loading into a small steel reactor 2 g of soybean oil and 0.9 g of methanol.

The reactor was placed in an oven with forced ventilation and subjected to the following temperature program: 14 minutes at 50 ⁰ C, heating at 20 °C/min up to the set reaction temperature. The reactors were held at this temperature for 60 minutes. The reactor was then cooled rapidly down to ambient temperature.

The resulting conversion was determined by using the H-NMR technique [7]. Tests were conducted at 2 temperatures: 180 and 200 ⁰ C.

Conversions of 7% and 8% were recorded at 180 and at 200 ⁰ C, respectively.

Example 3 Tests for transesterification of acid oil at 180 ⁰ C without catalyst

9

Since the presence of free acidity also can have an effect on the transesterification reaction, a reaction test was performed by loading into a small steel reactor 1.9 g of soybean oil, 0.1 g of stearic acid (acid oil 10% by weight) and 0.9 g of methanol. The reactor was placed in a forced-ventilation oven and subjected to the following temperature program: 14 minutes at 50 ⁰ C, heating at 20 ° C/min up to 180 ⁰ C; the reactors were kept at this temperature for 60 minutes. The reactor was then cooled rapidly down to ambient temperature.

The resulting conversion was determined by using the H-NMR technique [10]. The resulting value of the conversion, equal to 25%, highlights the effect of free acidity on the transesterification reaction. The final acidity was also measured and was found to be equal to 6.5% by weight.

Example 4

Tests for transesterification of oil at 180 ⁰ C (activity comparison).

For the various catalysts listed in Table 1, reaction tests were conducted by loading into small steel reactors 2 g of soybean oil, 0.9 g of methanol, and 0.1 g of catalyst. Before use, the catalysts were kept at 200 ⁰ C for 2 hours.

The reactors were placed in a forced-ventilation oven and subjected to the following temperature program: 14 minutes at 50 ⁰ C, heating to 20 °

C/min up to 180 ⁰ C; the reactors were kept for 60 minutes at this temperature. The reactors were then cooled rapidly down to ambient temperature.

The resulting conversions were determined by using H-NMR [10]. Table 2 lists the results obtained for the various tests.

As can be seen by the results given in the table, the alumina is not active, whereas there is a maximum of activity for an Mg/Al ratio ranging from 3 to 8. Pure magnesium oxide also exhibits good activity in

10 transesterification.

Table 2 - Example 4 - Catalytic tests 180 ⁰ C

Example 5

Tests for transesterification of oil containing a high concentration of water at 180 ⁰ C (activity comparison).

The catalyst CHT4, which gave the best performance in the tests of Example 2 was tested in the presence of large amounts of water. A reaction test was conducted by loading into a small steel reactor 2 g of soybean oil, 0.9 g of methanol, 0.1 g of catalyst, and 10,000 ppm of water.

The reactor was placed in a forced- ventilation oven and subjected to the following temperature program: 14 minutes at 50° C, heating at 20 ° C/min up to 180 ⁰ C; the reactor was held at this temperature for 60 minutes. The reactor was then cooled rapidly down to ambient temperature.

The conversion value obtained by using H-NMR [10] was equal to 92%. This value, which is identical to the conversion obtained in the preceding example with the same catalyst, highlights that the catalyst is not affected by the presence of water.

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Example 6

Test for esterification of oil with MgO in an autoclave at 225 ⁰ C

A reaction test was conducted by loading into a 1 -liter autoclave with agitator 250 g of soybean oil, 114 g of methanol, and 5 g of catalyst (MgO). The autoclave was heated to 225 ⁰ C. After 120 minutes, a sample was taken and H-NMR [10] analysis yielded a conversion of 88.4%.

The result of this test can be compared with a similar test performed by Stern et al. [4] with zinc oxide. The comparison of the results given in

Table 3 shows that the resulting conversions are comparable in the two cases despite using, in the test with MgO, a lower methanol/oil ratio than in the test performed by Stern et al. with ZnO.

The autoclave was then cooled to ambient temperature. The product unloaded from the autoclave was filtered. The methanol was distilled and the glyceric phase was separated from the ester phase by means of a separation funnel.

The ester phase was analyzed by gas chromatography [11] and the composition shown in Table 3 was obtained.

Example 7 Test for esterification of oil with HT4 in an autoclave at 225 ⁰ C

A reaction test was conducted by loading into a 1 -liter autoclave, equipped with an agitator, 250 g of soybean oil, 114 g of methanol, and 2.5 g of catalyst (HT4).

The autoclave was heated to 225 ⁰ C. After 55 minutes, a sample was taken and H-NMR [10] analysis yielded a 96% conversion.

Examination of the data given in Table 3 confirms that the activity of the HT4 catalyst is higher than the activity of MgO. With a lower concentration of catalyst, higher conversions for a shorter reaction time are achieved. The data also shows that HT4 is a more active catalyst with respect to the zinc oxide catalyst used by Stern et al. [4],

12

The autoclave was then cooled to ambient temperature. The product discharged from the autoclave was filtered. The methanol was distilled and the glyceric phase was separated from the ester phase by means of a separation funnel.

The ester phase is analyzed by gas chromatography [11] and the composition listed in Table 3 is obtained.

Table 3 - Tests in autoclave

Example 8

Tests for trans esterification of oil with HT4 at 200 ⁰ C with methanol, ethanol and bioethanol (activity comparison)

13

Reaction tests at 200 ⁰ C were performed in the presence of methanol, ethanol (99%) and bioethanol.

The reaction tests were performed by loading small steel reactors. The test with methanol was performed by loading 2 g of soybean oil, 0.9 g of alcohol and 0.1 g of catalyst; the tests with ethanol and bioethanol were instead performed by loading 2 g of oil, 2 g of alcohol and 0.1 g of catalyst.

The catalysts were kept at 200 ⁰ C for 2 hours before use.

The reactors were placed in a forced- ventilation oven and subjected to the following temperature program: 2 minutes at 50 ⁰ C, heating at 15°C/min up to 200 ⁰ C; the reactors were held at this temperature for 100 minutes. The reactors were then cooled rapidly down to ambient temperature.

The compositions of the ester phase were determined by gas chromatography [11] and are listed in Table 4.

The ester content obtained by using methanol was equal to 95.5%; this value is slightly higher than the value obtained in Example 2, with the same catalyst at 18O ⁰ C, and this clearly points out that catalytic activity is influenced by temperature.

The ester content obtained by using 99% ethanol by weight was equal to 95.8%; this value points out that the catalyst is not influenced by the different chain length of the two alcohols.

The ester content obtained by using bioethanol was equal to 95.9%; this value confirms that the catalyst is not affected by the presence of water.

14

Example 9

Test for transesterification of oil with a high content of free acid (27%) at low temperature (120 ⁰ C).

A test was performed in order to compare the performance of the HT4 catalyst with the catalyst proposed by Suppes et al. [7].

Suppes et al. [7] have found that catalysts based on zeolite NaX and

ETS-10 are active in the transesterification reaction at temperatures below 125 ⁰ C. However, the activity of these catalysts is compromised by the presence of free acidity and for an acid oil containing 27% by weight of free acidity the final conversion obtained after 4 hours of reaction remains fixed at 13.7%. Moreover, there is no effect on the free acidity, which remains very close to the initial value. The comparison test was performed by using the following conditions:

Oil acidity = 27% by weight; methanol/oil ratio = 6/1; percentage of catalyst/oil by weight: 10%

The catalyst was kept at 200 ⁰ C for 2 hours before use. The reactors were placed in a forced- ventilation oven and subjected to

15 the following temperature program: 14 minutes at 50 ⁰ C, heating at 20 ° C/minute up to 180 ⁰ C; the reactors were kept at this temperature for various reaction times. The reactors were then cooled rapidly down to ambient temperature. The resulting conversions were determined by using H-NMR[IO].

Table 5 lists the results obtained for the various tests.

As can be seen from the values of Table 5, the HT4 catalyst allows to obtain higher conversions to methyl esters than what has been reported by Suppes, is not deactivated by the high concentration of free acid, and also has a catalytic effect on the esterification reaction, since a significant decrease in the acidity of the oil is observed.

The disclosures in Italian Patent Application No. MI2004A002163 from which this application claims priority are incorporated herein by reference.

References [I] US 4,164,506 (1979) [2] US 5,730,029 (1998)

[3] S. Gryglewicz Bioresource Technology 70 (1999) 249-253 [4] US 5,908,946 (1999)

16

[5] EP 1352893 Al (2003)

[6] E. Leclercq, A. Finiels, C. Moreau, JAOCS 78 (2001) 1161

[7] GJ. Suppes, M.A. Dasari, EJ. Doskocil, PJ. Mankidy, MJ. Goff, App.

Catalysis A: General 257 (2004) 213

[8] A.L. McKenzie, CT. Fishel, RJ. Davis, J. Catal. 138 (1992) 547

[9] Dasari, M.A., Goff, MJ., Suppes, GJ. JAOCS 80 (2003) 189-192

[10] G. Gelbard, O. Bres, R.M. Vargas, F. Vielfaure, U.F. Schuchardt,

JAOCS, 1995, 72, 1239

[11] UNI 10946: 2001