Patent Description:
Biodiesel is a biodegradable fuel that is produced from plant- or animal- derived oils or fats. Biodiesel can be used as a component of diesel fuel or as a replacement for diesel fuel. Biodiesel is biodegradable, non-toxic and is a cleaner-burning fuel than diesel. Therefore its use can result in substantial environmental benefits.

Biodiesel is comprised of fatty acid methyl esters (FAME) (and also fatty acid ethyl esters) and is obtained from vegetable oils and animal fats. It is produced via the transesterification of triglycerides (TGs) and esterification of free fatty acids (FAAs), the two main components of oil, with alcohols of low molecular weight in the presence of an alkaline catalyst (<NPL>). Homogeneous alkaline catalysts have been widely used for this purpose. However, the use of homogeneous catalysts leads to contamination of the biodiesel and requires separation and purification processes that are very energy intensive, increase the cost of the process and can produce large amounts of wastewater. The use of solid base heterogeneous catalysts for biodiesel production can overcome these issues and improve the process efficiency. More specifically, the catalyst can be easily separated and recycled, making the process more economically feasible and more environmentally friendly. In addition, the separation of the glycerol from the biodiesel is much simpler and no purification step is required (<NPL>). Some of the most promising heterogeneous catalysts for biodiesel production from vegetable oils or animal fats are CaO-based materials (<NPL>). These catalysts are low-cost materials, with high basicity and demonstrate high activity in moderate reaction conditions, producing high quality biodiesel. However, one of the main limitations of these catalysts is their low stability and deactivation during repeated cycles, due to leaching of CaO (<NPL>)(Oueda, Bonzi-Coulibaly, & Ouédraogo, <NUM>). The effect of some support materials on the transesterification activity of CaO-La<NUM>O<NUM> and CaO-CeO<NUM> has been investigated (<NPL>).

<NPL>, <NUM>, describe the synthesis of calcium aluminium oxide (Ca<NUM>Al<NUM>O<NUM>) by calcining calcium carbonate and alumina at <NUM>. The catalyst is reported to have been used for the production of biodiesel using waste vegetable oil as feedstock and methanol through transesterification.

<NPL>, <NUM>, <NUM>, describe the development of a calcium oxide-calcium aluminate solid base catalyst for biodiesel synthesis from mustard oil. Optimal parametric values are reported to include a CaO loading of <NUM> wt%.

In <CIT>), a particulate, heterogeneous solid composition is described comprising decomposition products of Ca<NUM>Al<NUM>O<NUM>, after heating to a temperature of between <NUM> and <NUM> in the presence of H<NUM>O and CO<NUM>. The composition is reported to be useful in separating CO<NUM> from a process gas.

<NPL>, describe the preparation of KOH/Ca<NUM>Al<NUM>O<NUM> nanocatalysts using a microwave heating system. The nanaocatalysts are reported as useful in the production of biodiesel from canola oil.

The aquaculture industry produces large amounts of waste, which has no significant commercial value. A typical example is the farming and processing of tilapia, which is the second most cultivated freshwater fish worldwide. During the food processing, the main product is the fillet of the fish, which represents only <NUM>% of the wet fish weight with the rest discarded. This waste has been typically used in animal feed. However, there is a significant opportunity to utilise this fish waste for renewable fuel. More specifically, as the waste from tilapia processing has high oil content, it is possible to use it as feedstock for biodiesel. Fish oil extracted from waste not only reduces the amount of waste that is generated, but also reduces the total cost of biodiesel synthesis. Moreover, this biodiesel can be used by the local farmers in diesel generators, allowing them to be energy independent, while reducing their waste disposal burden. Therefore, there is a need to provide a low cost, efficient way to produce biodiesel from fish waste.

It is an object of the invention to provide a new catalyst for use in the transesterification of fatty acid glycerol esters and in producing biodiesel. It is an object of the invention to provide a new method for transesterifying fatty acid glycerol esters and for producing biodiesel. In particular, it is an object of the invention to provide such a catalyst and/or a method that alleviates or mitigates at least one of the above-mentioned problems.

According to a first aspect, the invention provides a method for producing fatty acid alkyl esters comprising reacting a feedstock comprising fatty acid monoglycerides, diglycerides or triglycerides with a C1 to C4 alcohol in the presence of a mixed oxide composite. The mixed oxide composite comprises CaO and Ca<NUM>Al<NUM>O<NUM>, wherein the mixed oxide composite is in the form of particles. The composite comprises two oxide phases, namely a calcium oxide phase (CaO) and a tricalcium aluminate or dialuminium tricaclcium hexaoxide phase (Ca<NUM>Al<NUM>O<NUM>). The composite is a solid and can act as a heterogeneous catalyst for transesterification of glycerides (mono-, di- and triglycerides). It has been found that, for such a reaction: the mixed oxide composite can be reused; its use results in reduction in the amount of impurities in downstream products when compared to a homogeneous catalyst; it can be further activated in situ (by a side reaction that takes place when CaO reacts with glycerol produced by the transesterification reaction; and it is relatively inexpensive to produce. Furthermore, it has advantages over known heterogeneous transesterification catalysts in that it is more stable (more resistant to CaO leaching and thus has a longer lifetime) and also has a high conversion rate. In particular, it has been found that the Ca<NUM>Al<NUM>O<NUM> used with the CaO in the composite enhances the stability of the CaO catalyst during repeated cycling of the transesterification of triglycerides reaction. It is believed that the Ca<NUM>Al<NUM>O<NUM> enhances the stability of the CaO catalyst in an activated form (calcium diglyceroxide), which activated form is very active for transesterification of glycerides and is formed during the transesterification of glycerides reaction.

The mixed oxide composite comprising CaO and Ca<NUM>Al<NUM>O<NUM>, wherein the mixed oxide composite is in the form of particles, may be prepared by a method comprising:.

This is a combustion method by which the aqueous solution is heated until it ignites and burns leaving a solid powder residue. The method results in a composite material which comprises a mixture of a CaO phase and a Ca<NUM>Al<NUM>O<NUM> phase and is in the form of particles. This method can be used to prepare the mixed oxide composite as defined inthe first aspect of the invention.

According to a second aspect, the invention provides a method of activating the mixed oxide composite defined in the first aspect, comprising heating a C1 to C4 alcohol with glycerol in the presence of the catalyst so as to form calcium diglyceroxide, i.e. so that the CaO reacts with the glycerol to form calcium diglyceroxide. Calcium diglyceroxide has been found to be particularly effective in catalysing the transesterification of fatty acid glycerides with a C1 to C4 alcohol. In this aspect of the invention the mixed oxide composite may be obtainable/obtained by one of the methods described above. It is believed that the presence of the Ca<NUM>Al<NUM>O<NUM> in the composite results in an enhancement of the stability of the CaO in this activated form, i.e. calcium diglyceroxide. In a third aspect, the invention extends to an activated CaO catalyst, e.g. an activated mixed oxide composite obtainable/obtained by a method according to the second aspect of the invention. Such an activated catalyst is formed during the transesterification of fatty acid monoglycerides, diglycerides or triglycerides with a C1 to C4 alcohol in the presence of a mixed oxide composite as defined in the first aspect of the invention (glycerol is a byproduct of this reaction). CaO in the composite is activated during the transesterification reaction by the formation of an intermediate stable phase of calcium diglycerol oxide.

As described above, the invention provides a method for producing fatty acid alkyl esters comprising reacting a feedstock comprising fatty acid monoglycerides, diglycerides or triglycerides with a C1 to C4 alcohol in the presence of a mixed oxide composite comprising CaO and Ca<NUM>Al<NUM>O<NUM>, wherein the mixed oxide composite is in the form of particles. The method of the invention involves transesterifying the fatty acid monoglycerides, diglycerides or triglycerides with the C1 to C4 alcohol. The transesterification produces fatty acid alkyl esters, including fatty acid methyl esters and fatty acid ethyl esters which are useful as biodiesel. The mixed oxide composite may be obtainable/obtained by one of the methods described above or may be an activated mixed oxide composite according to the third aspect of the invention.

According to a fourth aspect, the invention provides for the use of a mixed oxide composite comprising CaO and Ca<NUM>Al<NUM>O<NUM>, wherein the mixed oxide composite is in the form of particles, as a catalyst for the transesterification of monoglycerides, diglycerides or triglycerides with a C1 to C4 alcohol. The mixed oxide composite is the mixed oxide composite as defined in the first aspect of the invention, which may be obtainable/obtained by one of the methods described above, or is an activated mixed oxide composite according to the third aspect of the invention. The transesterification of monoglycerides, diglycerides or triglycerides with a C1-C4 alcohol can be according to the method of the first aspect of the invention.

In a first aspect, the invention provides a method for producing fatty acid alkyl esters comprising reacting a feedstock comprising fatty acid monoglycerides, diglycerides or triglycerides with a C1 to C4 alcohol in the presence of a mixed oxide composite. The mixed oxide composite comprises CaO and Ca<NUM>Al<NUM>O<NUM>, wherein the composite is in the form of particles. The composite comprises a mixture of two oxide phases, namely a calcium oxide phase (CaO) and a tricalcium aluminate phase (Ca<NUM>Al<NUM>O<NUM>). A particle of the composite comprises a mixture of two oxide phases, namely a calcium oxide phase (CaO) and a tricalcium aluminate phase (Ca<NUM>Al<NUM>O<NUM>). The mixed oxide composite is also referred to herein as a composite, a particulate composite, a mixed oxide composite or a mixed oxide particulate composite.

The mixed oxide composite comprises CaO and Ca<NUM>Al<NUM>O<NUM> but may also comprise other oxides such as silicates, strontium oxide, magnesium oxide, and may also comprise calcium sulphate. Preferably, the mixed oxide composite comprises CaO and Ca<NUM>Al<NUM>O<NUM> as a major component, i.e. the amount of CaO and Ca<NUM>Al<NUM>O<NUM> in the composite represents greater than <NUM> wt % based on the total weight of the composite and the amount of any other component is less than <NUM> wt %. By the amount of CaO and Ca<NUM>Al<NUM>O<NUM> is meant the total of the amount of CaO and the amount of Ca<NUM>Al<NUM>O<NUM>. The amount of CaO and Ca<NUM>Al<NUM>O<NUM> in the composite can represent greater than <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> or <NUM> wt % based on the total weight of the composite. In a preferred embodiment, the amount of CaO and Ca<NUM>Al<NUM>O<NUM> in the composite represents <NUM> wt % based on the total weight of the composite. In a preferred embodiment, the composite is biphasic with respect to oxide phases, i.e. the mixed oxide composite contains no oxide phases other than a CaO phase and a Ca<NUM>Al<NUM>O<NUM> phase. This composite is referred to herein as a CaO-Ca<NUM>Al<NUM>O<NUM> composite. This embodiment includes a composite where the amount of CaO and Ca<NUM>Al<NUM>O<NUM> in the composite represents <NUM> wt % of the composite. In another embodiment, the CaO-Ca<NUM>Al<NUM>O<NUM> composite can consist essentially of CaO and Ca<NUM>Al<NUM>O<NUM>.

The amount of CaO and Ca<NUM>Al<NUM>O<NUM> in the composite based on the total weight of the composite can be determined by means known in the art, for example X-ray diffraction analysis using standards and chemical analysis.

The relative amounts of calcium oxide and tricalcium aluminate in the composite can vary. The percentage weight of CaO based on the total weight of CaO and Ca<NUM>Al<NUM>O<NUM> ranges from <NUM> to <NUM> wt % (thus the percentage weight of Ca<NUM>Al<NUM>O<NUM> based on the total weight of CaO and Ca<NUM>Al<NUM>O<NUM> ranges from <NUM> to <NUM> wt %). Thus the mixed oxide composite comprises CaO and Ca<NUM>Al<NUM>O<NUM>, wherein CaO is present in an amount of <NUM> to <NUM> wt % based on the total weight of CaO and Ca<NUM>Al<NUM>O<NUM> and, wherein the composite is in the form of particles. The percentage weight of CaO based on the total weight of CaO and Ca<NUM>Al<NUM>O<NUM> can range from <NUM> to <NUM> wt %. The percentage weight of CaO based on the total weight of CaO and Ca<NUM>Al<NUM>O<NUM> can range from <NUM> to <NUM> wt %. The percentage weight of CaO based on the total weight of CaO and Ca<NUM>Al<NUM>O<NUM> can be <NUM> wt %, i.e. the molar ratio of Ca to Al in the composite may be <NUM>. The percentage weight of CaO based on the total weight of CaO and Ca<NUM>Al<NUM>O<NUM> can be <NUM> wt %, i.e. the molar ratio of Ca to Al in the composite may be <NUM>. Where the amount of CaO and Ca<NUM>Al<NUM>O<NUM> in the composite represents <NUM> wt % of the composite and the percentage weight of CaO is greater than about <NUM> wt %, composite stability issues may arise.

The relative amounts of calcium oxide and tricalcium aluminate in the composite can vary as described above for each embodiment of the composite as described herein. This includes embodiments where the composite comprises other components, for example, where the amount of CaO and Ca<NUM>Al<NUM>O<NUM> in the composite represents greater than <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> or <NUM> wt % based on the total weight of the composite.

The percentage weights of CaO and Ca<NUM>Al<NUM>O<NUM> relative to the total amount of CaO and Ca<NUM>Al<NUM>O<NUM> present can be determined using techniques known in the art such as inductively coupled plasma optical emission spectroscopy (ICP-OES).

The composite particles can have an average size of from <NUM> to <NUM>, and preferably they have a size of <NUM> to <NUM>, <NUM> to <NUM> or <NUM> to <NUM>. By size of a particle is meant the size of the longest dimension (also referred to herein as the longest diameter) of the particle as measured from an SEM micrograph.

Preferably the particles of the mixed oxide composite comprise a uniformly dispersed mixture of CaO and Ca<NUM>Al<NUM>O<NUM>. This is indicated by a more uniform and smaller particle size in SEM images of the composite and is more typical of composite particles that have high weight percentages (e.g. greater than <NUM> wt %, when the amount of CaO and Ca<NUM>Al<NUM>O<NUM> in the composite represents <NUM> wt % of the composite) of the CaO. The particles can be partially coated with CaO coated on Ca<NUM>Al<NUM>O<NUM> support. This is more typical of composite particles that have lower weight percentage of CaO.

The presence of the CaO and Ca<NUM>Al<NUM>O<NUM> phases can be determined by routine XRD analysis and EDX (Energy Dispersive X-Ray Spectroscopy) analysis, for example.

The mixed oxide composite has been found to be particularly effective in catalysing the transesterification of triglycerides with lower alcohols in the formation of acid alkyl esters which are useful in biodiesel production. One of the advantages is that as the composite is solid, it is a heterogeneous catalyst for this reaction and, as such, easier to separate from a reaction mixture than a homogeneous catalyst.

The mixed oxide composite, which is in the form of particles, can be supported on a support, such as a monolith. Suitable monoliths are well known in the art and include corderite and alumina. The mixed oxide composite can be coated on a porous support such as a monolith using deep coating techniques known in the art. The mixed oxide composite can also be self-supporting, i.e. the composite particles can be moulded into the form of a self-supporting structure. For example, the composite particles can be pressed into a mould to form a self-supporting structure. Ceramic injection moulding techniques known in the art can be used. The self-supporting structure can be a honeycomb structure, for example. In both cases, the catalyst is rendered easier to remove from the reaction mixture once the reaction has completed.

As described above, the mixed oxide composite comprising CaO and Ca<NUM>Al<NUM>O<NUM>, wherein the composite is in the form of particles, may be prepared by a method comprising:.

The aqueous solution can also contain nitrates or nitrate hydrates of other elements such as silica, magnesium or strontium. These other nitrates or nitrate hydrates are optional. This method can be used to prepare the mixed metal oxide composite as defined in each of the embodiments of the first aspect of the invention.

The aqueous solution contains calcium nitrate or calcium nitrate hydrate and aluminium nitrate or aluminium nitrate hydrate, i.e. it contains calcium in the form of a nitrate or a nitrate hydrate and aluminium in the form of a nitrate or a nitrate hydrate. For example, the aqueous solution can contain Ca(NO<NUM>)<NUM> or Ca(NO<NUM>)<NUM>. <NUM><NUM>O and Al(NO<NUM>)<NUM> or Al(NO<NUM>)<NUM>. <NUM><NUM>O. Preferably, the aqueous solution contains Ca(NO<NUM>)<NUM>. <NUM><NUM>O and Al(NO<NUM>)<NUM>. <NUM><NUM>O. By varying the relative amounts of calcium and aluminium nitrates (or nitrate hydrates) used in the reaction mixture, the relative quantities of CaO and Ca<NUM>Al<NUM>O<NUM> in the resultant composite particles can be altered. The relative amounts of calcium and aluminium nitrates (or nitrate hydrates) used can be such that so that a composite having a percentage weight of CaO based on the total weight of CaO and Ca<NUM>Al<NUM>O<NUM> in the range from <NUM> to <NUM> wt %, <NUM> to <NUM> wt %, <NUM> to <NUM> wt %, <NUM> to <NUM> wt %, <NUM> to <NUM> wt % or of <NUM> wt % or of <NUM> wt % is obtained.

The aqueous solution also contains an organic fuel to aid combustion. The organic fuel can be, for example, ethylene glycol, citric acid, urea, glycine, sucrose or mixtures thereof. A preferred organic fuel is a mixture of ethylene glycol and citric acid.

The aqueous solution contains water and, preferably, contains deionised water.

The combustion involves an exothermic reaction of the metal nitrates (or metal nitrate hydrates) and the organic fuel. Upon sufficient heating, the mixture foams and ignites with the evolution of gases and the resultant product crumbles into a powder. Preferably the solution is heated to temperatures of from <NUM> to <NUM> to cause combustion.

Step (a) of the method can involve heating the aqueous solution containing calcium and aluminium nitrates or nitrate hydrates, optional other nitrates and nitrate hydrates, and an organic fuel to evaporate water so as to form a gel, followed by further heating of the gel until it combusts to form a powder. For example, step (a) can involve heating the aqueous solution containing calcium and aluminium nitrates or nitrate hydrates, optional other nitrates and nitrate hydrates, and an organic fuel to a temperature of <NUM> to evaporate water so as to form a gel, followed by further heating of the gel to a temperature of from <NUM> to <NUM> or of about <NUM> until it combusts to form a powder. Combustion involves the gel igniting and burning, leaving a powder residue.

Step (a) of the method can involve heating the aqueous solution containing calcium and aluminium nitrates or nitrate hydrates, and an organic fuel to evaporate water so as to form a gel, followed by further heating of the gel until it combusts to form a powder. For example, step (a) can involve heating the aqueous solution containing calcium and aluminium nitrates or nitrate hydrates, optional other nitrates and nitrate hydrates, and an organic fuel to a temperature of <NUM> to evaporate water so as to form a gel, followed by further heating of the gel to a temperature of from <NUM> to <NUM> or of about <NUM> until it combusts to form a powder. Combustion involves the gel igniting and burning, leaving a powder residue.

In step (b) of the method, the powder resulting from the combustion is calcined at a temperature of <NUM> or higher, or from <NUM> to <NUM> or from <NUM> to <NUM> or <NUM>. The object of the calcination step is to decompose the nitrates to oxides and, e.g. form the Ca<NUM>Al<NUM>O<NUM> structure via a solid state reaction between CaO and Al<NUM>O<NUM>. Typically, the powder is calcined for <NUM> to <NUM> hours, and can be calcined for about <NUM> hours, for example. The product of the calcination step (b) is a mixed oxide composite comprising CaO and Ca<NUM>Al<NUM>O<NUM>, wherein the composite is in the form of particles.

In this embodiment, the method results in a composite material which is a mixture of a CaO phase and a Ca<NUM>Al<NUM>O<NUM> phase and is in the form of particles. That is, the composite is biphasic with respect to oxide phases, i.e. the mixed oxide composite contains no oxide phases other than a CaO phase and a Ca<NUM>Al<NUM>O<NUM> phase. This embodiment can be used to prepare a composite in which the amount of CaO and Ca<NUM>Al<NUM>O<NUM> in the composite represents <NUM> wt % of the composite.

The method of preparing the mixed oxide composite can comprise:.

According to a second aspect, the invention provides a method of activating a CaO catalyst comprising heating a C1 to C4 alcohol, preferably methanol, with glycerol in the presence of the catalyst so as to form calcium diglyceroxide. The calcium oxide reacts with the glycerol to form calcium diglyceroxide. Calcium diglyceroxide has been found to be particularly effective in catalysing the transesterification of fatty acid glycerides with a C1 to C4 alcohol. In this aspect of the invention the CaO catalyst is a mixed oxide composite as defined in the first aspect of the invention, which may be obtainable/obtained by one of the methods described above. The invention extends, in a third aspect, to an activated CaO catalyst, e.g. an activated mixed oxide composite obtainable/obtained by a method according to the second aspect of the invention. The activated CaO catalyst (i.e. calcium diglyceroxide-containing catalyst) is formed during the transesterification of fatty acid monoglycerides, diglycerides or triglycerides with a C1 to C4 alcohol in the presence of the mixed oxide composite defined in the first aspect of the invention (glycerol is a byproduct of this reaction). This reaction is discussed below, and similar reaction conditions apply. For example, the reaction mixture is heated to a temperature that does not exceed the boiling point of the alcohol. The reaction is usually carried out at atmospheric pressure and at a temperature of below <NUM>, preferably at from <NUM> to <NUM>. <NUM> is the boiling point of methanol at atmospheric pressure. The reaction can also be performed at higher temperatures under pressure higher that atmospheric pressure, provided the alcohol remains in the liquid state. Further, preferably the alcohol is methanol or ethanol. More preferably the alcohol is methanol.

The invention provides a method for producing fatty acid alkyl esters comprising reacting a feedstock comprising fatty acid monoglycerides, diglycerides or triglycerides with a C1 to C4 alcohol in the presence of a mixed oxide composite comprising CaO and Ca<NUM>Al<NUM>O<NUM>, wherein the mixed composite is in the form of particles. The method of the invention involves transesterifying the fatty acid monoglycerides, diglycerides or triglycerides with the C1 to C4 alcohol. The transesterification produces fatty acid alkyl esters, including fatty acid methyl esters and fatty acid ethyl esters which are particularly useful as biodiesel. The mixed oxide composite may be obtainable/obtained by one of the methods described above, or may be an activated mixed oxide composite according to the third aspect of the invention.

The feedstock can be a plant oil and/or an animal oil or fat. For example, the feedstock can be a vegetable oil (e.g. rape seed oil or palm oil), tallow or an oil derived from an animal (e.g. a fish). Suitable fish oils include cod liver oil and oil derived from tilapia. Preferably the oil is high in triglycerides. The oil or fat can treated with glycerol to convert free fatty acids to triglycerides thus lowering the free fatty acid content of the plant oil/animal oil/animal fat feedstock.

The method of the invention involves transesterifying the fatty acid monoglycerides, diglycerides or triglycerides with the C1 to C4 alcohol. Transesterification of triglycerides with an alcohol proceeds in a reversible equilibrium reaction according to the scheme:
<CHM>.

In the above formula, R' represent the hydrocarbyl moieties of fatty acid constituents of the vegetable oils and R is a C1-C4 alkyl group. As shown in the above scheme, glycerol is formed as a byproduct in addition to the fatty acid alkyl esters usable as fuel. The equilibrium can be shifted towards the formation of the required fatty acid esters by increasing the amount of alcohol reactant and/or by removing the glycerol byproduct.

Typically the feedstock is reacted with the alcohol in the presence of the catalyst at a temperature below the boiling point of the alcohol. The reaction mixture comprises feedstock (fatty acid glycerides), alcohol and catalyst. These components are added to a reaction vessel and the reaction mixture is heated to a temperature that does not exceed the boiling point of the alcohol. The reaction is usually carried out at atmospheric pressure and at a temperature of below <NUM>, preferably at from <NUM> to <NUM>. <NUM> is the boiling point of methanol at atmospheric pressure. The reaction can also be performed at higher temperatures under pressure higher that atmospheric pressure, provided the alcohol remains in the liquid state. The use of higher temperature increases the reaction rate. As the alcohol and the feedstock have limited miscibility in each other, preferably the reaction mixture is stirred. Glycerol, which is generated as a by-product, accumulates in the polar (alcohol) phase of the reaction mixture and, in accordance with the equilibrium nature of the reaction, is prone to reconvert the produced fatty acid alkyl esters into glyceride esters. Thus full conversion of the vegetable oil cannot be attained. After a period, the reaction mixture is taken off heat and the catalyst is recovered from the reaction mixture. The reaction mixture can be filtered to remove (separate out) the catalyst. Prior to filtering, the reaction mixture can be centrifuged, after which the liquid (containing the desired fatty acid alkyl esters) is decanted off leaving a portion of the reaction mixture containing the catalyst. This remaining portion of the reaction mixture containing the catalyst is filtered, preferably under vacuum, to remove the catalyst. If the catalyst is still active, e.g. if it has not been deactivated due to extended use in transesterification, it can be reused. Preferably it is washed and dried before reuse. It can be washed in methanol and dried, for example in an oven at <NUM> for about <NUM> hours, prior to reuse. The decanted liquid containing the fatty acid alkyl esters is allowed to stand for a period of time without stirring to allow it to separate into two phases. The lower polar phase which contains alcohol and glycerol is removed, leaving the upper apolar phase (fuel phase) which contains the fatty acid alkyl esters. This upper apolar phase can be further refined, for example, by being subjected to distillation to remove any methanol that may be present.

The amount of alcohol used is the amount effective to undergo the transesterification with the feedstock oil in the appropriate stoichiometric ratios. Usually an excess of alcohol is used. The alcohol can be methanol, ethanol, propanol, butanol or mixtures thereof. When the alcohol is methanol, the method produces fatty acid methyl esters, i,e, it is a method for producing biodiesel. When the alcohol is ethanol, the method produces fatty acid ethyl esters, i,e, it is a method for producing biodiesel. Preferably the alcohol is methanol.

Typically the composite is present in an amount of <NUM> to <NUM> weight percent based on the weight of the feedstock.

The reaction can take place in a continuous flow reactor in which the feedstock and the alcohol form a feedstream that is continuously flowed over a fixed catalyst bed. This method attracts economies associated with efficiency of process and is particularly suited to the catalyst due to its high stability. As the catalyst has high stability (i.e. it takes longer to deactivate than known CaO catalysts), it does not have to be replaced so often and therefore is more suited to use in a continuous flow, fixed bed reactor.

According to a fifth aspect, the invention provides for the use of a mixed oxide composite comprising CaO and Ca<NUM>Al<NUM>O<NUM>, wherein the mixed oxide composite is in the form of particles, as a catalyst for the transesterification of monoglycerides, diglycerides or triglycerides with a C<NUM>-C alcohol. The mixed oxide composite is the mixed oxide composite as defined in the first aspect of the invention, and may be obtainable/obtained by one of the methods described above, or is an activated mixed oxide composite according to the third aspect of the invention. The transesterification of monoglycerides, diglycerides or triglycerides with a C<NUM>-C alcohol can be according to the method of the first aspect of the invention.

As used herein, the term "comprising", which is inclusive or open-ended and does not exclude additional unrecited elements or method steps, is intended to encompass as alternative embodiments, the phrases "consisting essentially of" and "consisting of" where "consisting of" excludes any element or step not specified and "consisting essentially of" permits the inclusion of additional unrecited elements or steps that do not materially affect the essential or basic and novel characteristics of the composition or method under consideration.

The advantages of the method of the invention are discussed below in relation to the following non-limiting examples.

The first catalyst (used as a reference) was synthesized by incipient wetness impregnation of CaO on Al<NUM>O<NUM>. The resultant CaO impregnated Al<NUM>O<NUM> catalyst is also referred to herein as CaO-Al<NUM>O<NUM>. For the synthesis of this catalyst, Al<NUM>O<NUM> powder was added in ethanol at room temperature under stirring with the stoichiometric amount of Ca(NO<NUM>)<NUM>·<NUM><NUM>O, required to obtain <NUM> wt% of CaO loading on the Al<NUM>O<NUM> support, based on the total weight of CaO and Al<NUM>O<NUM>. The solution was left under stirring at <NUM>, until the ethanol was evaporated. After calcination at <NUM>, the CaO impregnated Al<NUM>O<NUM> (CaO-Al<NUM>O<NUM>) catalyst was obtained.

The second catalyst (used as a reference) is a mixed oxide phase of CaO and Ca<NUM>Al<NUM>O<NUM> (Ca<NUM>Al<NUM>O<NUM> is also referred to as C3A herein) that was synthesized via combustion. Ca(NO<NUM>)<NUM>·<NUM><NUM>O and Al(NO<NUM>)<NUM>·<NUM><NUM>O were diluted in deionized water, with ethylene glycol and citric acid. The amounts of Ca(NO<NUM>)<NUM>·<NUM><NUM>O and Al(NO<NUM>)<NUM>·<NUM><NUM>O used were so that the molar ratio of Ca:Al was <NUM>. The solution was heated under stirring at <NUM> in order to evaporate the water and form a gel. The gel was then combusted at <NUM>. The resulting powder was calcined at <NUM> for <NUM> (hours) and a particulate composite comprising a mixture of CaO and Ca<NUM>Al<NUM>O<NUM> (C3A) phases was obtained. The calculated weight percents of each of the CaO and the C3A phases in the composite based on the total weight of CaO and C3A were <NUM> wt % of CaO and <NUM> wt% of C3A. The resultant CaO-Ca<NUM>Al<NUM>O<NUM> composite catalyst is also referred to herein as 2Ca/Al.

The third catalyst was prepared according to the same procedure as the second catalyst except that the amounts of Ca(NO<NUM>)<NUM>·<NUM><NUM>O and Al(NO<NUM>)<NUM>·<NUM><NUM>O used were so that the molar ratio of Ca:Al was <NUM>. The calculated weight percents of each of the CaO and the C3A phases in the composite based on the total weight of CaO and C3A were <NUM> wt % of CaO and <NUM> wt% of C3A. The resultant CaO-Ca<NUM>Al<NUM>O<NUM> composite catalyst is also referred to herein as 3Ca/Al.

The fourth catalyst was prepared according to the same procedure as the second catalyst except that the amounts of Ca(NO<NUM>)<NUM>·<NUM><NUM>O and Al(NO<NUM>)<NUM>·<NUM><NUM>O used were so that the molar ratio of Ca:Al was <NUM>. The calculated weight percents of each of the CaO and the C3A phases in the composite based on the total weight of CaO and C3A were <NUM> wt % of CaO and <NUM> wt% of C3A. The resultant CaO-Ca<NUM>Al<NUM>O<NUM> composite catalyst is also referred to herein as 6Ca/Al.

The fifth catalyst (used as a reference) was prepared according to the same procedure as the second catalyst except that the amounts of Ca(NO<NUM>)<NUM>·<NUM><NUM>O and Al(NO<NUM>)<NUM>·<NUM><NUM>O used were so that the molar ratio of Ca:Al was <NUM>. The calculated weight percents of CaO and C3A phase in the composite based on the total weight of CaO and C3A were <NUM> wt % of CaO and <NUM> wt% of C3A. The resultant Ca<NUM>Al<NUM>O<NUM> catalyst is also referred to herein as C3A.

The sixth catalyst (used as a reference) was a commercially available CaO powder that was calcined at <NUM> C for <NUM> hours to remove any impurities such as Ca(OH)<NUM> of CaCO<NUM>.

Room temperature powder X-ray diffraction (XRD) was performed with a PANalytical Empyrean diffractometer operated in reflection mode using Cu-Kα1 radiation. The obtained XRD patterns were analysed with STOE WinXPOW software to determine the crystal structure of the catalysts and the evolution of different phases during testing.

The microstructure of the samples was analysed with a JEOL JSM-<NUM> scanning electron microscope (SEM). Elemental analysis was performed with an Oxford Inca EDX system.

The Ca/Al ratios, and the weight percents of the CaO and C3A, of the catalysts was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) on a Thermo-iCAP <NUM> spectrometer. The samples were treated in hydrochloric acid and compared to standards.

The total basicity of the prepared catalysts was measured based on their temperature programmed CO<NUM> desorption profiles. The catalysts were pretreated at <NUM> under an Ar flow rate of <NUM>/min to remove any adsorbed CO<NUM> and water from their surface and then cooled down to <NUM>. At this temperature, the CO<NUM> chemisorption was carried out by a CO<NUM> flow rate of <NUM>/min for <NUM>. The excess of CO<NUM> was then desorbed at the temperature of the adsorption in an Ar flow (<NUM>/min) for <NUM>. Finally, desorption of CO<NUM> took place with Ar from <NUM> to <NUM>. The evolution of the mass of the catalysts during these treatments was measured by Thermogravimetric analysis (TGA) in a Netzsch STA 449C instrument. The TGA was equipped with a Pfeiffer mass spectrometer (MS), which analysed the CO<NUM> evolution during the different steps.

Transesterification of cod liver oil was performed in a <NUM> three-neck round bottom flask equipped with a water-cooled reflux condenser and a magnetic stirrer. The temperature was controlled at <NUM> with an oil bath and it was monitored during the reaction with a thermocouple probe that was placed in the reaction mixture. The reaction mixture was stirred at <NUM> rpm in order to achieve uniform temperature distribution and suspension of the catalyst in the fish oil and methanol mixture. Samples from the reaction mixture were collected at different time intervals, for monitoring the evolution of the transesterification reaction. After running the reaction for the desired duration, the mixture was centrifuged at <NUM> rpm, the liquid was decanted, and the remaining catalyst was filtered under vacuum and washed thoroughly with methanol. Then, the recovered catalyst was dried in an oven (<NUM>) overnight and was used for analysis and stability tests. No fresh catalyst was added during the stability tests. The reaction was carried out with an oil to methanol ratio of <NUM> to <NUM>, catalyst loading of <NUM> wt % based on the fish oil weight and reaction time of maximum <NUM>.

The conversion of the fish oil triglycerides to the methyl esters of the biodiesel was determined by <NUM>H Nuclear Magnetic Resonance (NMR) u a Bruker AVII <NUM> NMR spectrometer. The biodiesel yield was calculated based on the integration of the signal at <NUM> ppm of the hydrogen of the methoxy groups in the methyl esters and the signal at <NUM> ppm of the hydrogen of the methylene groups of the fatty acid derivatives (<NPL>).

The catalytic activity of each of the first two catalysts (CaO-Al2O<NUM> and 2Ca/Al) was evaluated based on the biodiesel yield during the transesterification reaction of fish oil triglycerides, operating at the reaction conditions described above. <FIG> shows the evolution of the biodiesel yield in hourly time intervals, until the reaction reached over <NUM>% conversion, and the recyclability of the two different catalysts. Comparing the activity of the two materials during the first test, CaO-Al<NUM>O<NUM> demonstrated faster reaction rates than 2Ca/Al. The reaction reached over <NUM>% conversion in <NUM> hours, while for 2Ca/Al <NUM> hours reaction time was necessary. However, both catalysts showed faster reaction kinetics when they were reused for test <NUM> and demonstrated a biodiesel yield of over <NUM>% in <NUM> hours. During the first hour of test <NUM>, the biodiesel yield was less than <NUM>% for both catalysts. However, in test <NUM> the biodiesel yield was over <NUM>% during the first hour of reaction for both catalytic systems. These changes in the catalysts activity are attributed to changes in their structure, and this is discussed later.

The recyclability of the catalyst is another important parameter to be considered when designing heterogeneous catalysts for biodiesel production. <FIG> presents the recyclability of the two catalysts, when operating at the same conditions. CaO-Al<NUM>O<NUM> catalyst was successfully recycled for <NUM> tests, but it was fully deactivated by test <NUM>. On the other hand, 2Ca/Al performed with over <NUM>% biodiesel yield for <NUM> tests, in <NUM> hours reaction time. This catalyst was fully deactivated by test <NUM>. These results suggest that the 2Ca/Al system was more stable than CaO-Al<NUM>O<NUM> and that the C3A phase improved the stability of the CaO phase and increased the catalyst lifetime.

In order to explain the changes in the catalysts activity and their deactivation process, the phase evolution of the materials after each test was investigated. According to <FIG>, the XRD pattern of the as-prepared CaO-Al<NUM>O<NUM> catalyst shows the peaks that correspond to Al<NUM>O<NUM>, which was the support, and CaO, that was impregnated on it. After test <NUM>, the Al<NUM>O<NUM> peaks were still present, but the intensity of the CaO peaks decreased significantly. Moreover, the formation of an extra phase took place, which was calcium diglyceroxide (CaDG). This phase gradually disappeared during test <NUM> and test <NUM>. Finally, when the catalyst was deactivated, the phases that were mainly present were the Al<NUM>O<NUM> and CaO with decreased intensity, compared to the fresh catalyst.

<FIG> illustrates the evolution of the XRD patterns of the 2Ca/Al catalyst. For the as-prepared catalyst the phases of CaO and C3A are evident with no additional phases. After the first test, the C3A phase was retained, but the CaO phase decreased, while the formation of CaDG took place. The CaDG phase gradually disappeared during the recyclability tests and when the catalyst was deactivated the C3A phase was present with a lower content of CaO.

The formation of the CaDG phase after test <NUM>, that took place for both catalysts, can explain the enhanced catalytic activity of the materials in test <NUM> onwards. According to the literature, this phase can be formed when CaO reacts with the glycerol by-product of the transesterification reaction. This phase has proved to be more active, due to the presence of a basic non-protonated O- anion on the surface of CaDG (<NPL>). Finally, the decreased intensity of the CaO peaks and the gradual disappearance of the CaDG phase suggest possible leaching of Ca ions, which led to the deactivation of the catalyst.

The microstructure of the catalysts was studied by SEM and the elemental analysis was performed with EDX in different areas of the samples. The Ca/Al molar ratios were calculated for the as-prepared and the deactivated catalysts, in order to estimate the extent of Ca leaching for each catalyst.

<FIG> shows the SEM micrographs of the CaO-Al<NUM>O<NUM> catalyst before and after the recyclability test. No significant changes were observed to the catalysts microstructure and CaO was uniformly coated on the surface of the Al<NUM>O<NUM> support. However, the EDX analyses suggested a decrease of the Ca content, of approximately <NUM> %. According to the EDX results, the CaO weight % was calculated and from the initial <NUM>%, only <NUM>% of CaO remained on the surface of the catalyst. These results are in good agreement with the XRD analysis that showed the decrease of the CaO phase and suggest that Ca leaching took place and led to the deactivation of the catalyst after <NUM> successful tests.

<FIG> presents the microstructure of the CaO-C3A catalyst before and after the recyclability tests. The morphology of the particles of this catalyst was different than the CaO-Al<NUM>O<NUM>. The particles have larger particle size and they are more crystalline. Moreover, the CaO phase appeared to be mixed with the C3A phase and not deposited as a layer on the top of it. After the recyclability testing, the morphology of the particles was similar, but the particle size decreased probably due to the agitation that took place during the transesterification reaction that helped to break down the agglomerates. Moreover, the EDX results suggested a decrease of the Ca content of approximately <NUM>%. Based on these results the CaO weight % was calculated and the CaO content decreased from <NUM> to <NUM> wt%. Therefore, the extent of calcium leaching from this catalyst was lower than that of CaO-Al<NUM>O<NUM>, even though it was used successfully for <NUM> tests instead of <NUM>.

Table <NUM> summarises the results from the EDX analyses and the Ca/Al molar ratio was calculated before and after the catalysts deactivation. For the CaO-Al<NUM>O<NUM> catalyst the Ca/Al molar ratio dropped by approximately <NUM>%. The 2Ca/Al catalyst demonstrated higher Ca/Al molar ratios due to the Ca present in the C3A phase. After the recyclability tests, the ratio dropped from <NUM> to <NUM>, which was a <NUM>% decrease. For both catalysts the Ca/Al ratio decreased due to Ca leaching and led to the catalysts deactivation. The Ca leaching was also confirmed by the XRD analyses that showed the decrease of the CaO peaks.

This deactivation process was slower for the 2Ca/Al catalyst than the CaO-Al<NUM>O<NUM>. The first catalyst was successfully recycled for <NUM> times, while the second for just <NUM>. This difference in the catalyst recyclability was due to the samples microstructure and preparation. The CaO-C3A catalyst was more stable, because CaO and C3A phases were homogeneously mixed. On the other hand, for the sample prepared by incipient wetness impregnation, the CaO phase formed a layer on the surface of the Al<NUM>O<NUM> support. This catalyst was less stable and deactivated faster due to Ca leaching, because of the weaker interaction between CaO and Al<NUM>O<NUM>.

In summary, the XRD and EDX analyses suggest that Ca leaching is hindered by the presence of the Ca<NUM>Al<NUM>O<NUM> phase and that the catalyst lifetime is increased (i.e. the catalyst is more stable), as the CaO-Ca<NUM>Al<NUM>O<NUM> composite performed successfully with over <NUM>% biodiesel yield for <NUM> cycles.

The catalytic activity of each of the second to sixth catalysts was evaluated based on the biodiesel yield during the transesterification reaction of fish oil triglycerides, operating at the reaction conditions described above. <FIG> shows the evolution of the biodiesel yield in hourly time intervals, until the reaction reached over <NUM>% conversion, and the recyclability of the four different catalysts. Comparing the activity of the four catalysts, the 6Ca/Al composite performs best in terms of performance and recyclability. The 6Ca/Al composite was not fully deactivated until test <NUM>.

<FIG> presents the XRD patterns of the as-prepared catalysts. From this graph, it can be seen how the catalysts crystal structure changes when the Ca/Al ratio is altered. More specifically, by decreasing the Ca/Al molar ratio, the formation of the C3A phase is increased and the CaO phase is decreased. Finally, there is no evidence of any other phases formed between the two oxides.

<FIG> shows the morphology of the as-prepared catalysts with the different Ca/Al ratios. By increasing the Al content, the formation of the C3A phase is more evident, as it can be seen by the larger particles that appear on the sample. Consequently, the formation of the CaO phase is decreased and the CaO particles can form a layer on the top of the C3A particles. Therefore, for high Ca/Al ratios, the two phases are mixed uniformly, but for lower ratios, (2Ca/Al), the CaO is coated on the surface of the C3A particles.

<FIG> shows presents the CO<NUM> gas evolution when the CO<NUM> desorption took place between <NUM> and <NUM>, as it was recorded by the MS. The strongest CO<NUM> signal was detected for CaO at <NUM>. By decreasing the Ca/Al ratio from <NUM> to <NUM>, the CO<NUM> signal deceases and a slight shift to lower temperatures takes place. Finally, no CO<NUM> signal was detected for C3A. Therefore, the higher the Ca/Al ratio is the more basic the catalyst will be.

<FIG> shows the catalytic activity of the different catalysts for the transesterification reaction of cod liver oil to biodiesel. The samples were tested at the same reaction conditions in order to compare their catalytic activity. The reaction was carried out at <NUM>, with <NUM>:<NUM> oil to methanol molar ratio and <NUM> rpm stirring. The amount of catalyst used was fixed at <NUM> wt% based on the oil used. <FIG> presents the evolution of the transesterification reaction with time for the synthesized catalysts. The conversion of the cod liver oil triglycerides to methyl esters in different time intervals was calculated by H<NUM> NMR. According to <FIG>, all the catalysts demonstrated conversion higher than <NUM>% after maximum <NUM> hr of reaction time, except C3A. Notably, the reaction rates differ and they are proportional to the Ca/Al ratio. More specifically, no conversion was observed for C3A. The slowest reaction rate was demonstrated by 2Ca/Al and full conversion was achieved at <NUM> hr. Following that, 3Ca/Al showed full conversion after <NUM> hr and then 6Ca/Al at <NUM> hr. Finally, pure CaO demonstrated full conversion at <NUM> hr, which was similar with 6Ca/Al.

Claim 1:
A method for producing fatty acid alkyl esters comprising reacting a feedstock comprising fatty acid monoglycerides, diglycerides or triglycerides with a C1 to C4 alcohol in the presence of a mixed oxide composite comprising CaO and Ca<NUM>Al<NUM>O<NUM>, wherein the composite is in the form of particles and wherein the CaO is in an amount of from <NUM> to <NUM> wt % based on the total weight of CaO and Ca<NUM>Al<NUM>O<NUM>, determined using inductively coupled plasma optical emission spectrometry.