The present invention relates to a process for preparing aliphatic aldehydes starting from 1,2-diols. Said aldehydes can be advantageously used for the synthesis of fuel components. In particular, the present invention relates to a process wherein said aldehydes can be produced starting from 1,2-diol intermediates of a biological origin.

In addition, the present invention relates to an apparatus for performing said process for preparing aliphatic aldehydes starting from 1,2-diols.

As is known, compounds belonging to the group of aliphatic aldehydes are widely used in various types of industry, for example as intermediates for the production of plastic materials, in the formulation of paints, cosmetic products, pharmaceutical products.

Some aliphatic aldehydes, in particular, are also used as intermediates for the production of fuel components for motor vehicles. Furthermore, when the aldehydes are obtained from renewable sources (for example, from glycerin obtained from the treatment of triglycerides of a biological origin), they can be advantageously used in the formulation of so-called biofuels.

Biofuels are fuels, or fuel components, that comprise one or more products, in particular oxygenated products, of a natural origin. The European Directive 2009/28/EC which envisages a EU target of 20% for the overall share of energy from renewable sources within 2020, sustains and promotes the production and use of biofuels, among which biodiesel or green diesel.

In particular, the term "biodiesel" refers to a fuel for diesel engines comprising alkyl esters (for example methyl or ethyl) obtained by the transesterification of long-chain fatty acid triglycerides deriving from biological sources (as described, for example, in Ma F.R., Hanna M.A., (1999), "Biodiesel production: a review", Bioresource TechnoL, vol. 70(1), pages 1-15). Said biodiesel preferably falls within the specifications of biodiesel for vehicles according to the standards EN 14214:2008.

The term "green diesel" refers to a fuel for diesel engines comprising hydrogenation or deoxygenation products of lipids containing triglycerides deriving from biological sources and/or free fatty acids (for example products from the hydrolysis of triglycerides) deriving from biological sources in the presence of hydrogen and of at least one catalyst (as described, for example, in Faraci, G., Gosling, C, Holmgren, J., Marinangeli, R., Marker, T., Perego, C, (2007) "New developments in renewable fuels offer more choices", Hydrocarbon Processing, September issue, pages 67-71).

Aldehydes can be used for the preparation of hydrophobic oxygenated compounds which, when added to suitable hydrocarbon mixtures, allow compositions to be obtained which can be advantageously used as fuel, in particular as fuel for both diesel and Otto cycle engines.

International patent application WO2014/125416, for example, describes the use of propionic aldehyde for the synthesis of cyclic acetals obtained from the reaction with an alkoxy-propanediol. Said cyclic acetals are advantageously used as fuel components.

Analogously, Italian patent application MI2014A001529 describes a process in which an aldehyde is reacted with an alcohol in the presence of a solid basic catalyst for producing organic esters that can be used as fuel additives.

It is therefore evident that aldehydes, and in particular aliphatic aldehydes, are extremely important intermediates for the preparation of hydrophobic oxygenated compounds that can be profitably used as fuel components (for example gasoline or gasoil) due to their advantageous characteristics, for example a high octane or cetane number, a high calorific value, complete miscibility with the hydrocarbon phase and an extremely low affinity with the aqueous phase, thus reducing problems linked with miscibility and corrosion of the engine parts due to the presence of traces of water. Their addition to gasoil or gasoline also allows a reduction in particulate emissions deriving from the combustion process, without significantly altering their fuel characteristics (for example, the cloud point (CP), cold filter plugging point (CFPP), the demulsifying characteristics, lubricating properties, or lubricity).

In the state of the art, aliphatic aldehydes are prepared by the hydroformylation of olefins (oxosynthesis), the oxidation of primary alcohols or C ₃ or C ₄ alkenes; as is known, acetic aldehyde can also be produced through the hydration of acetylene or the Wacker oxidation of ethylene (Ulmann's Encyclopedia of Industrial Chemistry, vol. Al, Abrasives to Aluminium Oxide, VCH, 1985).

The methods indicated above require quite severe process conditions and may also require the use of reagents and/or valuable and costly catalysts.

Preparation processes of aliphatic aldehydes starting from the corresponding 1,2-diols of a biological origin have recently become important, above all due to the implications on sustainabi!ity and respect for the regulations on renewable energy sources.

Catalytic processes in gaseous phase for obtaining propanal from 1,2- propanediol are described, for example, in Mori, K., Yamada, Y., Sato, S. "Catalytic dehydratation od 1 ,2-propanediol into propanal" (2009) Applied Catalysis A: general, vol. 366, pages 304-308.

Japanese patent JP-2010-180156A describes the preparation of saturated aldehydes from 1,2-diols in gaseous phase using heteropolyacid catalysts (for example, phosphotungstic acid or tungstosilicic acid supported on silica) activated by treatment at 200°C in a stream of nitrogen. The reaction is carried out at temperatures preferably within the range of 180°C-240°C, at which the 1,2-diol is present in vapour phase.

Under these conditions, the catalysts used are subject to fouling and inactivation in short times due to the formation of carbonaceous products.

Japanese patent JP-2013-56847, on the other hand, describes the preparation of saturated aldehydes by the dehydration of 1,2-diols in gaseous phase using catalysts comprising copper or silver supported on silica-alumina, alumina or zeolite in the presence of molecular hydrogen. Also in this case, the reaction is carried out at high temperatures, ranging from 250°C to 400°C for guaranteeing satisfactory yields and selectivities.

The known solutions however still have room for improvement.

The known processes, in fact, generally have the drawback of having to carry out the reaction in gaseous phase and at very high temperatures. Furthermore, the rapid reduction in the efficiency of the catalysts and/or the necessity of resorting to more valuable catalysts has considerable repercussions on the process costs.

Another drawback that affects the known process costs is associated with the necessity of effecting the conversion in the presence of H ₂ or N ₂ , which however complicates the isolation of the final reaction products.

The Applicant has now developed an innovative process which is aimed at producing aliphatic aldehydes under bland conditions, in the presence of inexpensive catalysts, thus overcoming the limitations of the solutions so far known, that can be carried out either batchwise or in continuous and which allows 1,2-diols of a biological origin to be used as starting products.

In particular, the invention relates to a process for producing aliphatic aldehydes having general formula R-CH ₂ -CHO (I), starting from 1,2-diols having general formula R-CHOH-CH ₂ OH (II), wherein R can be H or a linear or branched alkyl having a number of carbon atoms ranging from 1 to 4, preferably from 1 to 2, as defined in essential terms in the enclosed claim 1.

Additional preferred features of the invention are indicated in the dependent claims.

A further objective of the present invention is to provide a reactor system in which said process for producing aliphatic aldehydes starting from 1,2-diols, can be effected.

For the purposes of the present invention and following claims, the definitions of the numerical ranges always comprise the extremes unless otherwise specified.

For the purposes of the present invention and following claims, unless otherwise specified, all the ratios and percentages are by weight.

For the purposes of the present invention and following claims, the term "comprising" also includes the terms "which essentially consists of or "which consists of.

For the purposes of the present invention, "1,2-diols" indifferently refer to both intermediates of a natural origin, for example deriving from the treatment of triglycerides of a vegetable or animal origin, and also intermediates of a synthetic origin, having various degrees of purity. Consequently, even if reference may be made in the present description to 1,2-diols deriving from renewable sources, the invention is not exclusively limited to these and can be applied to said compounds regardless of their origin.

It should be noted that the process of the present invention can be carried out using 1,2-diols having a high degree of purity or in a mixture with up to 15% of impurities deriving from the previous preparation processes and/or in the presence of up to 13% of water. If the 1,2-diol, for example, is 1,2-propanediol, it can derive from the treatment of glycerin of a biological origin without intermediate extreme purification passages.

Further characteristics and advantages of the present invention will appear evident from the following detailed description and non-limiting embodiment examples, with reference to the enclosed figure 1 which represents an schematic example relating to the process of the present invention.

The present invention relates to a process for producing aliphatic aldehydes having general formula R-CH ₂ -CHO (I), starting from at least one 1,2-diol having formula R-CHOH-CH ₂ OH (II), wherein R can be H or a linear or branched alkyl having a number of carbon atoms ranging from 1 to 4, preferably from 1 to 2, said process comprising the following steps:

a) reacting in liquid phase at least one 1,2-diol having formula (II) at a temperature Ti, in the presence of a suitable solid acid catalyst obtaining at least one cyclic acetal having general formula (III)

(III)

and water; b) separating, by distillation, a minimum-boiling azeotrope composed of said cyclic acetal having general formula (III) and water, from the mixture in liquid phase of step a);

c) hydrolyzing said cyclic acetal having general formula (III), isolated from step b) as azeotrope with water, at a temperature T ₂ in the presence of a solid acid catalyst for obtaining the aliphatic aldehyde having general formula (I) and the 1,2-diol having general formula (II); d) separating, by distillation, said aliphatic aldehyde having general formula (I) from the mixture in liquid phase of step c).

The temperatures Ti and T ₂ at which the thermal treatments of steps a) and c) of said process are carried out, can vary on the basis of the composition of the mixtures and can be selected by skilled persons in the art in relation to the kind of 1,2-diol and the other reaction conditions. Said temperatures are selected so as to keep the reaction mixtures in liquid phase.

In a preferred aspect of the present invention, the temperature Ti of step a) of said process is within the range of 100°C-250°C, preferably within the range of 150°C-200°C, and the temperature T ₂ of step c) of said process can be within the range of 70°C-150°C, preferably within the range of 80°C-120°C.

The process of the present invention can be carried out batchwise or in continuous and is preferably carried out in continuous.

Step c) of said process is preferably preceded by cooling the product isolated in step b) of said process.

In a preferred aspect of the present invention, the process can be carried out at atmospheric pressure, wherein atmospheric pressure refers to a pressure substantially equal to 0.1 MPa (1 atm).

In a preferred aspect of the present invention, the separation by distillation of step b) of said process can be carried out by allowing the vapor phase containing the azeotrope of the cyclic acetal with water to flow through a heat exchanger set at a temperature T ₃ higher than the boiling point of the azeotrope, but lower than the boiling point of water. In a preferred aspect of the present invention, the temperature T ₃ can be higher than or equal to 77°C, and is preferably within the range of 77°C-97°C.

The separation by distillation of step d) of said process can be carried out by allowing the vapor phase containing the aliphatic aldehyde having formula (I) to flow through a heat exchanger set at a temperature T ₄ higher than the boiling point of said aldehyde but lower than the boiling point of the azeotrope of the cyclic acetal (III), produced in step a) of said process, with water.

In a preferred aspect of the present invention, the at least one 1,2-diol having general formula (II) obtained in step d) of said method can be recycled to step a) of said method.

In step a) of the process, object of the present invention, the formation of the cyclic acetal having formula (III) can take place in two consecutive acid- catalyzed reaction sub-steps:

(i) -CHOH-CH ₂ OH→ R-CH ₂ -CHO +

(ii) R-CH ₂ -CHO + R-CHOH-CH ₂ OH→ + H ₂ 0

The aldehyde R-CH ₂ -CHO obtained as intermediate at the end of the sub- step (i) cannot be separated from the reaction mixture and reacts immediately, under the process conditions, with the 1,2-diol having formula (II) still present in the reaction mixture for producing the cyclic acetal having general formula (III).

It is known, in fact, that vicinal diols under acid catalysis conditions can be transformed into the corresponding carbonyl compounds through a pinacol transposition mechanism. When the vicinal diol is a 1,2-diol such as that having general formula (II), the carbonyl compound prevalently obtained is the corresponding aldehyde. The latter cannot be isolated as, in the presence of a Bransted acid, it can react with the starting substrate to form the more stable cyclic acetal having formula (III).

As is known to skilled persons in the art, Bransted acid (according to the Bransted-Lowry acid/base theory) refers to a chemical species capable of donating a H ⁺ ion to another chemical species.

It should be noted that the cyclic acetal having formula (III) is produced as a mixture of various geometrical and optical isomers: for the purposes of the present process, however, the mixture of isomers can be used as such.

According to the present invention, the group R in the 1,2-diol (II) and in the corresponding aldehyde (I) can be H or a linear or branched alkyl, with a number of carbon atoms ranging from 1 to 4, preferably from 1 to 2. In a further preferred aspect, R can be a saturated linear alkyl selected, for example, from methyl, ethyl, ^-propyl, «-butyl.

In a preferred aspect of the present invention, the 1,2-diol having general formula (II) can be obtained starting from glycerin. Said glycerin can be even more preferably obtained as by-product of transesterification or hydrolysis reactions of triglycerides of a biological origin, for example of reactions used in production processes of biodiesel or green diesel.

Preferably, said glycerine can be previously purified of salts and excess water that may be present if it derives from the transesterification of triglycerides. According to a preferred aspect, the raw glycerine deriving from said processes (purity 80-85%), is subjected to a purification pre-treatment to obtain glycerin having a purity of at least 98%. Said purification can be effected as described, for example, in WO2014/125416.

In a preferred aspect of the present invention, the at least one 1,2-diol having general formula (II) that can be used in the present invention can be selected from ethylene glycol, 1,2-propanediol, 1,2 butanediol.

In a preferred aspect of the present invention, the 1,2-diol having general formula (II) is 1,2-propanediol, said 1,2-propanediol preferably being obtained by means of a dehydration and hydrogenation process of glycerin according to conventional methods of the prior art, for example as described in Musolino, M.G., Scarpino, L.A., Mauriello F., Pietropaolo, R. (2009) "Selective transfer hydrogenolysis of glycerol by palladium catalysts in absence of hydrogen". Green Chemistry, vol. 11, pages 1511-1513. It should be noted that the reaction mixture subjected to thermal treatment under acid catalysis conditions can consist of only one diol having general formula (II) or it can comprise one or more diols having general formula (II).

If the reaction mixture comprises only one diol having general formula (II), the corresponding aldehyde having formula (I) can be obtained as product.

If the 1,2-diol used is 1 ,2-propanediol, for example, 1-propanal can be obtained as product from the process of the present invention.

If the reaction mixture comprises two or more diols having general formula (II), a mixture of aldehydes having general formula (I) can be obtained as product. In this case, the above compounds having formula (I) in the mixture can be separated with techniques known to skilled persons in the art (for example by means of distillation) and can be used individually for the preparation of fuel components.

If the reaction mixture comprises both 1 ,2-propanediol and ethylene glycol, for example, a mixture of 1-propanal and acetic aldehyde can be obtained from the process of the present invention.

During step a) of said process, the reactions of the two sub-steps (i) and (ii) indicated above do not remain in equilibrium, but are substantially completed thanks to the particular stability of the cyclic product obtained.

During the reaction of step a), an upper organic phase is obtained in which the cyclic acetal (III) accumulates, and a lower aqueous phase in which the 1,2-diol (II) remains.

Said cyclic acetal (III), in the presence of water, can form a minimum- boiling azeotrope whose boiling point is lower than the boiling point of the starting 1,2-diol (II) and the boiling point of water.

The azeotrope can therefore be isolated by connecting the reactor to a heat exchanger set at a temperature T ₃ lower than Ti and suitably selected so that T ₃ > ailing of the azeotrope and T ₃ < Tboiiing of H ₂ 0. In this way, the condition T ₃ < ailing of the 1,2-diol (II) is always satisfied.

By so doing, the azeotrope is removed from the reaction area flowing in vapour phase through said heat exchanger, whereas the other components of the reaction mixture are re-condensed in the liquid phase.

As is known from chemistry of protective groups, cyclic acetals and ketals can be easily produced by the reaction of a vicinal diol with a carbonyl compound under acid catalysis conditions and can also be easily hydrolyzed to re-form the starting products.

In step d) of the process, the cyclic acetal having general formula (III) in the presence of water, can be subjected to thermal treatment in the presence of a solid acid catalyst, re-producing the starting 1,2-diol (II) and the aldehyde having general formul

When the process is carried out batchwise, step d) of said process can be effected for a time ranging from 1 hour to 10 hours, preferably from 2 to 8 hours.

Under the conditions in which step d) of the process, object of the present invention, is carried out, the aliphatic aldehyde having general formula (I) is formed, which is characterized by a lower boiling point with respect to the other reaction components.

Said aldehyde can therefore be isolated by connecting the reactor to a heat exchanger set at a temperature T ₄ lower than T ₂ and suitably selected so that T ₄ > ailing of the aldehyde and T ₄ < Tailing of the minimum-boiling azeotrope of the cyclic acetal with H ₂ 0.

In a preferred aspect of the present invention, said temperature T ₄ is within the range of 25°C-130°C, and is preferably within the range of 50°C-80°C.

In this way, the aliphatic aldehyde produced is removed from the reaction area flowing in vapour phase through the heat exchanger, whereas the other components of the reaction mixture are re-condensed in the liquid phase.

This also causes a shift of the reaction towards the right and prevents the establishment of equilibrium that would lead again to the synthesis of the cyclic acetal. With time, the whole cyclic acetal having formula (III) is consumed and the starting 1,2-diol accumulates in the liquid phase of the reaction area, which, in a preferred aspect of the present invention, can be recycled again to the first step of the process.

It should be noted that the aldehyde having general formula (I) can, in turn, form azeotropes with water: this means that the aldehyde obtained from said process can contain water in a quantity equal to the percentage of water in the azeotrope.

When 1,2-propanediol is used, for example, the aldehyde (I) produced is 1- propanal, which can form an azeotrope containing 2% of water.

In this case, the process is not influenced by this phenomenon, as the boiling point of the azeotrope is always lower than that of the pure product (for example, in the case of 1-propanal, T ing 1-propanal = 47.9°C whereas T ing azeotrope 1-propanal - water = 47.5°C, as indicated in "Azeotropic Data— III; Horsley, L. - Advances in Chemistry"; American Chemical Society: Washington, DC, 1973).

As already mentioned, steps a) and c) of the process, object of the present invention, are carried out in the presence of at least one solid acid catalyst. The at least one solid acid catalyst used in step a) of said process can be the same or different from the at least one solid acid catalyst used in step c) and is preferably different.

Solid acid catalysts that can be suitably used in steps a) and c) of the process of the present invention can be any of the known catalysts suitable for the purpose, having surface sites with an acid functionality.

In particular, said catalysts can be selected from ion-exchange acid resins, zeolites in acid form, silico-aluminas, supported phosphoric acid and mixtures thereof.

The ion-exchange acid resins can be used directly in the form of microspheres, as normally available on the market. The acid zeolites and silica- alumina are preferably extruded together with a binder.

Acid resins that can be used are those containing sulfonic or carboxylic groups as acid groups.

Commercial resins such as, for example, Amberlyst A-36, Amberlyst A-70, Amberlyst BD-20, Amberlite IR-120, Amberlite IRC-86, Amberlite IRC-50, the Du Pont Nafion series, the Solvay Aquivion series, can be used, for example.

Preferred zeolites are medium- or large-pore zeolites, even more preferably zeolite Y, Beta zeolite or ZSM-5 zeolite.

The zeolites are used in acid form, i.e. in the form in which the cationic sites present in their structure are occupied for at least 50% by hydrogen ions, and it is especially preferable that at least 90% of the cationic sites be occupied by hydrogen ions.

Silico-aluminas that can be used are, for example, those having a silica: alumina molar ratio ranging from 1 : 1 to 1000: 1, and even more preferably ranging from 20: 1 to 200: 1. Silico-aluminas that can be used are described, for example, in Bellussi G., Perego C, Carati A., Peratello S., Previde Massara E. (1994), "Amorphous mesoporous silica-alumina with controlled pore size as acid catalysts", Studies in Surface Science and Catalysis, vol. 84, pages 85-92. Commercial silico-aluminas such as, for example, Siral 1, Siral 5, Siral 20, Siral 30, Siral 40, can also be used.

As already indicated, the process of the present invention can be carried out either batchwise or in continuous.

In an embodiment of the present invention, steps a) and b) of said process can be carried out batchwise in a separate reactor into which the catalyst is introduced (for example, a fixed-bed catalytic reactor), feeding the reagents in liquid phase, said reactor being connected to at least one heat exchanger which, regulated at a suitable temperature, allows the cyclic acetal having formula (III) in the form of an azeotrope with water, to be removed from the reaction area through a vapor phase, and selectively isolated, maintaining the other components of the reaction mixture in the liquid phase. A second heat exchanger having the function of condensing the cyclic acetal (III) is suitably positioned downstream of said heat exchanger.

Similarly, steps c) and d) of said process can be carried out batchwise in a second reactor, separate from the first reactor, into which the catalyst is introduced (for example, a fixed-bed catalytic reactor), said reactor being connected to at least one heat exchanger which, regulated at a suitable temperature, allows the aldehyde (I) formed by hydrolysis of the cyclic acetal (III), to be removed from the reaction area through a vapor phase, and selectively isolated, maintaining the other components of the reaction mixture in the liquid phase. A second heat exchanger having the function of condensing the aldehyde (I) is suitably positioned downstream of said heat exchanger.

In a preferred aspect, the process, object of the present invention, can be carried out batchwise or in continuous in a system comprising at least two reactors used separately or combined with each other, in which each reactor can be connected to at least one heat exchanger that can be regulated at a suitable temperature, in order to remove and selectively isolate one or more products from each reaction area through the vapor phase, maintaining the other components in the respective liquid phases. In a preferred configuration of the system, the two reactors are combined with each other so that the reaction product, characteristic of step a) of the process, passes from the first reactor, through the at least one heat exchanger and suitably condensed, to the second reactor, and is subsequently subjected to the reaction characteristic of step c) of said process.

When the process is carried out in continuous, said process can be effected preferably using a reaction system comprising two reactors arranged in series and which comprises, parallelly to the continuous feeding line of 1,2-diol to the reactor in which step a) of the process is carried out, a partial recycled line of the 1,2-diol produced by the hydrolysis reaction of step c) to the reactor in which step a) of said process is effected.

It should be noted that when the process, object of the present invention, is carried out in continuous in the presence of a solid acid catalyst (1% by weight), said process can continue for up to at least 50 days without observing any drop in productivity or deactivation of the catalyst itself.

In a preferred aspect of the present invention, the system for implementing said process can comprise at least two CSTR reactors ^Continuous Stirred Tank Reactors") used either separately or combined in series with each other.

In a preferred aspect of the present invention, the system for implementing said process can comprise at least two fixed-bed catalytic reactors, used either separately or combined in series with each other.

In a further preferred aspect of the present invention, the system for implementing said process can comprise at least two reactive columns filled with catalyst, used either separately or combined in series with each other.

In another preferred aspect of the present invention, the system for implementing said process can comprise at least two reactors of which at least one consists of a boiling pot reactor in which the reaction mixture recirculates.

For the purposes of the present description and following claims, the term boiling pot reactor refers to a reactor in which a heat exchanger, used in processes in which one or more reaction products pass to the vapour phase, is applied at the head. This characteristic can be exploited either for separating the product, if the reaction is limited by the chemical equilibrium, by removing said product from the reaction area through the exchanger, or for removing part of the heat which develops in the reaction, by completely condensing the product and causing it to flow back again into the reactor. In a further preferred aspect of the present invention, the system for implementing said process can comprise at least two reactors of which at least one consists of a fluid-bed reactor, in which the reaction mixture recirculates.

In a further preferred aspect of the present invention, the system for implementing said process can comprise at least two reactors of which at least one consists of a fixed-bed reactor in which there is a partial recirculating of the reaction mixture and a partial distillation of the most volatile component.

It should be noted that the quantity of catalyst used in steps a) and c) of the process, object of the present invention can vary in relation to the configuration of the reactors and space velocity.

The latter is commonly defined as the ratio between the quantity (e.g. by weight) of reagent that flows in the reactor and the quantity of catalyst present in the reactor per hour and is expressed in h ^"1 . Preferably, when the reactor used is of the CSTR type, the quantity of catalyst used in steps a) and c) of the process is within the range of 0.5-30% by weight with respect to the total mass of reagent, and more preferably within the range of 1-15% by weight.

Figure 1 shows a schematic example relating to the process of the present invention. For illustrative but non-limiting purposes, the scheme shows a preferred configuration in which the two steps of the process are carried out in two generic reactors connected in series to each other.

According to said scheme, the charge of 1,2-diol having general formula (II) is fed to the reactor Ri, through line Li. Step a) of the reaction as described above, takes place in the reactor Ri, kept at the temperature Ti, wherein the cyclic acetal having general formula (III) is obtained from the 1,2-diol by thermal treatment in the presence of a solid acid catalyst. Said thermal treatment is effected maintaining the reaction mixture in liquid phase. The cyclic acetal forms, in the reactor Ri, a minimum-boiling azeotrope with the water produced in the reaction and said azeotrope is removed from Ri, at the temperature T _1; flowing through the heat exchanger Ci, kept at the temperature T ₃ lower than the boiling point of said azeotrope and selected, as previously described, for selectively re-condensing the starting 1,2-diol (II) again in the reactor Ri.

The azeotrope consisting of the cyclic acetal and water is condensed in C ₂ and fed to the reactor R ₂ , through line L ₂ . Step c) of the reaction takes place in the reactor R ₂ , kept at the temperature T _2; wherein the cyclic acetal having general formula (III), in the presence of water and a solid acid catalyst, is hydrolyzed reproducing the starting 1,2-diol (II) (which is collected from the bottom of the reactor, through a line L ₃ , which is joined to line L _1; returning to the area Ri), and the aldehyde (II) which is removed from R ₂ , at the temperature T _2; flowing into the vapour phase through a heat exchanger C ₃ kept at the temperature T ₄ lower than the boiling point of said aldehyde and selected, as previously described, for selectively re-condensing the cyclic acetal (III), the water and 1,2-diol obtained again in the reactor R ₂ .

The aldehyde is finally conveyed through line L ₄ to the condenser C ₄ . When the process according to the present invention is carried out under the conditions described above, it allows conversions of the starting 1,2-diol higher than 97%, to be reached. The completion of the reaction is determined through gas chromatographic analysis.

The process according to the present invention allows to overcome the drawbacks found in the state of the art. In particular, it allows aliphatic aldehydes having formula (I) to be produced starting from 1,2-diols having formula (II) under blander conditions and with high conversion yields and maintained for longer times with respect to what is described in JP-2010-180156.

Preferably, said aliphatic aldehydes obtained with the process according to the present invention can be used for the preparation of hydrophobic oxygenated compounds used as fuel components, in particular diesel fuels.

The present invention also allows the glycerin obtained as by-product of transesterification or hydrolysis reactions of triglycerides contained in lipids of a vegetable or animal origin used in production processes of biofuels, to be exploited in a simple and economically convenient way.

When the 1,2-diols on which the process, object of the present invention, is applied, are obtained starting from glycerin obtained from renewable sources (for example, from the transesterification or hydrolysis of triglycerides contained in lipids of a vegetable or animal origin), compounds intrinsically of a biological, vegetable or animal origin are obtained.

In a preferred aspect therefore, said aliphatic aldehydes obtained with the process according to the present invention can be used for the preparation of hydrophobic oxygenated compounds used as components of a biological origin of biofuels, in particular biodiesel.

Some non-limiting examples are provided hereunder for the practical embodiment of the present invention and for a better understanding thereof.

Example 1 (Batch preparation of 2-ethyl-4-methyl-l,3 dioxolane from pure L2-propanediol with the resin Amberlyst A-36)

This example demonstrates the formation of cyclic acetal 2-ethyl-4-methyl-

1,3-dioxolane starting from 1,2-propanediol. This reaction represents step a) of the process, object of the present invention.

500 g (6.6 moles) of pure 1 ,2-propanediol (99% Sigma- Aldrich) were introduced into a distillation boiler in the presence of 2.5 g (0.5% by weight) of the resin Amberlyst 36 (A-36 Dow Chem. Co.) characterized by a concentration of acid sites greater than or equal to 1.95 eq/L and a humidity retention capacity in the form of H ⁺ within the range of 51-57%.

The reaction mixture was subsequently brought to a temperature of 165°C at 1 atm under stirring at a constant rate.

As a result of the thermal treatment in the presence of the solid acid resin, the 1,2-propanediol generated the cyclic acetal 2-ethyl-4-methyl-l,3 dioxolane (cis and trans isomers).

The reaction mixture slowly decomposed into a lower aqueous phase in which the starting diol remained, and an upper organic phase containing the cyclic acetal.

Table 1 below indicates the results of the gas chromatographic analysis

(values in percentage) of the components recovered in the aqueous phase and in the organic phase after 8 hours of reaction.

Table I

Selectivity

Product Organic phase Aqueous phase

(mol %)

H ₂ 0 ( Karl Fisher) 3.38 83.1

Total 100 100 100.00

The mass recovery of the product is equal to 83%: it should be noted however that the temperature Ti is higher than the operating temperature recommended for the A-36 resin. The molar selectivity values (expressed as moles of the desired product/moles of the limiting product consumed) are normalized on the total quantity of 1 ,2-propanediol.

Example 2 (Batch preparation of 2-ethyl-4-methyl-1.3 dioxolane from pure

1.2- propanediol with the resin Amberlyst A-70)

This example demonstrates the formation of cyclic acetal 2-ethyl-4-methyl-

1.3- dioxolane by thermal treatment of 1,2-propanediol, in the presence of the solid acid resin Amberlyst 70 (A-70, Dow Chem. Co.).

This resin differs from the resin A-36 of Example 1 in the concentration of acid sites (> 0.9 eq/L) and in the humidity retention capacity in the form of H ⁺ (within the range of 53-59%).

The experiment was carried out with the same quantities of reagent (500 g of pure 1,2-propanediol) and catalyst (2.5 g of A-70, 0.5% by weight) and at the same temperatures as Example 1 but for a longer time (14 hours).

Table 2 below indicates the results of the gas chromatographic analysis (values in percentage) of the components recovered in the aqueous phase and in the organic phase at the end of the reaction.

Table II

Selectivity

Product Organic phase Aqueous phase

(mol %) dioxolane (cis+trans

mixture)

1-propanol 0.1 0.1 0.1

Allylic alcohol (2-propenol) 1.0 1.0 1.2

Triethyl-trioxane (TETO) 0.6 0.6 0.8

Acetol (1-hydroxy-acetone) 0.2 0.1 0.2

1,2-propanediol 1.5 2.9

H ₂ 0 ( Karl Fisher) 2.0 87.5

Total 100 100 100

The selectivity values are normalized on the total quantity of reacted 1,2- propanediol.

The resin A-70 is capable of catalyzing the dehydration reaction of 1,2- propanediol and the subsequent formation of the cyclic acetal with extremely high yields (overall yield higher than 90%).

Example 3 (Preparation of 2-ethyl-4-methyl- 1.3 -dioxolane from 1.2- propanediol with 5% by weight of the resin Amberlyst A-70 in continuous in a reactor with a heat exchanger and its isolation by means of azeotropic distillation)

This example demonstrates the preparation of cyclic acetal 2-ethyl-4- methyl- 1,3 -dioxolane starting from 1 ,2-propanediol wherein steps a) and b) of the process of the present invention are carried out in continuous.

500 g of 1,2-propanediol were introduced into a jacketed boiler together with 25 g (5% by weight) of the resin Amberlyst A-70; a heat exchanger was positioned at the head of the boiler, in which a fluid heated to 95°C flows.

The temperature of the reactor was then raised gradually by passing a fluid into the jacket of the boiler at the temperature of 175°C, so that the temperature inside the reactor was maintained between 170°C and 175°C, whereas, at the same time, 1,2-propanediol was fed to the reaction area with a space velocity of 5,2 h ^"1 .

At the reaction temperature the cyclic acetal 2-ethyl-4-methyl- 1,3 -dioxolane distilled as a minimum-boiling azeotrope (boiling point 85-90°C), passing through the heat exchanger positioned at the head of the reaction boiler. Under the experimental conditions adopted, the minimum-boiling azeotrope distilled at a rate equal to 2.16 g/min. The distilled vapour phase that had passed through said heat exchanger was subsequently condensed, obtaining a heterogeneous liquid mixture composed of a lower aqueous phase (26% by weight) and an upper organic phase (74%) by weight). The reaction was interrupted after 7 days, after which a sample of each phase was subjected to gas chromatographic analysis.

Table III below indicates the compositions of the single phases.

Table III

Other components present in traces in the mixture consist of dioxanes. Under these conditions, the conversion of 1 ,2-propanediol is almost quantitative.

Example 4 (Preparation of 2-ethyl-4-methyl- l,3-dioxolane from 1,2- propanediol with 10% by weight of the resin Amberlyst™ A-70 in continuous in a reactor with a heat exchanger and its isolation by means of azeotropic distillation)

The experiment of the present example was carried out under the conditions of the previous Example 3, but a larger quantity of solid acid catalyst was used (50 g of resin, equal to 10%> by weight).

Also in this case, the process was interrupted after 7 days, even if there were no indications of a drop in activity of the catalyst; the two phases obtained after azeotropic distillation were subjected to gas chromatographic analysis.

Table IV below indicates the compositions of the single phases.

Table IV

Further tests in continuous showed that the process can be carried out for long periods of time, even up to 50 days, without there being any drop in the productivity of the same process.

Example 5 (Preparation of 2-ethyl-4-methyl- 1.3-dioxolane from 1.2- propanediol in continuous in a fixed-bed catalytic reactor

Various operating conditions were verified in the present example, in which step a) of the process of the present invention was carried out in a fixed-bed catalytic reactor. The reaction was carried out at temperatures within the range of 145°C-190°C. The catalyst used in all the tests was Amberlyst A-70. 1,2- propanediol was fed to the reactor at different space velocity values. Table V below indicates the various operating conditions, the yields to cyclic acetal and the selectivity of these tests.

Table V

Space Yield to Cyclic

Test TempVPress. velocity Conversion Selectivity Acetal

2 160°C/1 atm l h- ¹ 85.3% 78.7% 67.1%

3 160°C/1 atm l h- ¹ 85.2% 75.8% 64.6%

4 160°C/1 atm 2 11- ¹ 75.5% 83.2% 62.8%

5 160°C/1 atm 2 11- ¹ 75.0% 81.0% 60.8%

6 190°C/1 atm l h ^"1 83.0% 66.4% 55.1%

7 190°C/1 atm l h ^"1 80.1% 74.2% 59.4%

8 190°C/1 atm l h ^"1 76.9% 78.8% 60.6%

9 190°C/1 atm 2 η- ¹ 70.8% 80.7% 57.1%

10 190°C/1 atm 2 η ^"1 71.7% 80.5% 57.7%

11 145°C/1 atm l h ^"1 33.9% 87.0% 29.5%

12 160°C/1 atm 3 11- ¹ 53.1% 74.7% 39.7%

The mixture leaving the reactor is always heterogeneous and consisted of a lower aqueous phase mainly containing 1 ,2-propanediol and an upper organic phase consisting of the cyclic acetal 2-ethyl-4-methyl-l,3-dioxolane. The tests show that in fixed-bed catalytic reactors, the conversion yields of 1,2-propanediol do not exceed 85.3%.

Example 6 (Preparation of 2-ethyl-4-methyl-l ,3 dioxolane from 1,2- propanediol having a purity of 67% with the resin Amberlyst A-70

In order to verify the stability of the resin A-70, the experiment for the production of 2-ethyl-4-methyl- 1 ,3 -dioxolane was repeated under the same conditions as Example 2, but using, as starting substrate, the 1,2-propanediol obtained from the hydrogenation reaction of glycerin (purity 67%, water content equal to 13% by weight) and bringing the quantity of catalyst to 1% by weight with respect to the total weight of the reaction mixture. The results showed that the resin A-70 maintains its specific functionality, allowing a mass recovery of 2-ethyl-4-methyl-l,3-dioxolane equal to 95%.

Example 7 (Batch hydrolysis of 2-ethyl-4-methyl-l,3 dioxolane in liquid phase

This example, which represents steps c) and d) of the process, object of the present invention, demonstrates that the cyclic acetal, produced according to the previous examples, can be effectively hydrolyzed for obtaining 1-propanal aldehyde and the starting 1,2-propanediol.

130.77 g of a mixture containing 2-ethyl-4-methyl-l,3-dioxolane at 95% (1.069 moles) obtained as described in the previous Example 6 were charged into a 250 mL flask together with 1.4 g of solid acid resin Amberlyst BD20 (Dow Chem. Co.) (1%) by weight with respect to the limiting agent).

The flask was placed in a heating mantle and was connected to a heat exchanger set at a temperature of 60°C.

After facilitating the dispersion of the resin, maintaining the reaction mixture under constant stirring for 45 minutes, 24.2 g (1.34 moles) of demineralized water were added.

The reaction mixture was then slowly heated to 85°C. The boiling of the azeotrope composed of 2-ethyl-4-methyl-l,3-dioxolane and water, was immediately observed. Said azeotrope was maintained inside the boiler thanks to the presence of the heat exchanger set at 60°C. The thermal treatment in the presence of a solid acid catalyst promoted the hydrolysis of the cyclic acetal, restoring the starting 1,2-propanediol and 1-propanal which, by passing into the vapor phase, was able to pass through the heat exchanger to be subsequently condensed and collected separately.

The thermal treatment was continued for about 8 hours; the final composition of the reaction mixture, determined by means of gas chromatography analysis is represented for 90% (by weight) by 1,2-propanediol, for 2.5% (by weight) by 2-ethyl-4-methyl- 1,3 -dioxolane and for 4.77% (by weight) by water.

Samples were collected at prefixed times from the reaction mixture and subjected to gas chromatographic analysis. Table VI below indicates the results of said analysis and shows that the hydrolysis reaction can be carried out without an apparent inactivation of the catalyst with a conversion yield higher than 94% and a selectivity equal to 99% to 1-propanal.

Table VI

The gas chromatographic analysis also allowed the reaction kinetics to be evaluated: in particular, under the experimental conditions described, the conversion of 2-ethyl-4-methyl-l ,3-dioxolane is obtained for a percentage of 50% within the first 90 minutes of reaction, and is completed within about 8 hours.