WO2004080589A2

WO2004080589A2 - Copper-based catalysts, process for preparing same and use thereof

Info

Publication number: WO2004080589A2
Application number: PCT/BR2004/000028
Authority: WO
Inventors: José Bueno; Everaldo Araujo; Augusto Pires
Original assignee: Universidade Federal De São Carlos
Priority date: 2003-03-12
Filing date: 2004-03-12
Publication date: 2004-09-23
Also published as: BR0300729A; WO2004080589A3; BR0300729B1

Abstract

A process for preparing copper-based, zirconia-supported catalysts, useful for obtaining ethyl acetate from ethyl alcohol. The catalysts of the invention show increasing asymptotic conversion of ethyl alcohol into ethyl acetate with increasing content of low-temperature reduced copper oxide species. The process for obtaining ethyl acetate comprises directing a stream 2 of fresh ethyl alcohol through a reactor 7 laden with the inventive catalyst, under reaction conditions of temperature between 180°C and 360°C, pressure between 1 and 2 atmospheres and residence time factor W/F 20 to 600 Kg-catalyst.min/Kg-ethyl alcohol, obtaining a product stream 8 at the outlet of reactor 7 that is cooled in a heat exchanger system 9, where it is separated into a liquid product stream 18 and gaseous product stream 10. Stream 10 yields mainly acetaldehyde, stream 16, and hydrogen, stream 14, both streams being recyclable to reactor 7. Alternatively, acetaldehyde can be stored in tank 17. Liquid stream 18, containing desired end products and by-products, is refined in 19 and the ethyl acetate product is stored in tank 21 while the by-product stream is directed to tank 23.

Description

COPPER-BASED CATALYSTS, PROCESS FOR PREPARING SAME AND

USE THEREOF FIELD OF THE INVENTION

The present invention relates to zirconia-supported copper-based catalysts and to the process for preparing the same, as well as to a one-shot process to obtain ethyl acetate from ethyl alcohol using the said catalysts. More specifically, the inventive process occurs in the presence of catalysts that contain predominantly Cu-species on zirconia which are reduced at low temperature, with a high degree of coverage of the Zr0₂ by Cu, while having lower selectivity for the formation of crotonaldehyde by-product. Catalysts are prepared by depositing copper salts on zirconia, calcining and reducing same to obtain the catalyst end product. The process of preparing ethyl acetate according to the invention comprises contacting ethyl alcohol with the chosen catalyst at temperatures between 180°C and 360°C and at a reaction pressure of preferably between 1 and 2 atmospheres, generally in a tubular fixed bed reactor. The type of reaction that enables ethyl acetate to be obtained by the present process is a dehydrocoupling reaction, hydrogen being obtained as a by-product. BACKGROUND INFORMATION Ethyl acetate is an important commodity, especially suitable as solvent in extraction processes in the food industry, being used as a high-performance degreasing detergent.

It is also used in the cosmetics, glue, lacquer and paint industries and also as a polymeric solution in the paper industry. High-purity ethyl acetate is used as an anhydrous medium and equally as an intermediate in chemical syntheses.

Commercially, ethyl acetate is recovered as a by-product or obtained by chemical synthesis. In the United States in 1988 65% of the ethyl acetate was recovered as a by-product while 35% was synthesized. When it is recovered, ethyl acetate is generally a by-product of the liquid- phase n-butane oxidation. It can also be recovered as a co-product of the polyvinyl butyral production process.

As the demand for ethyl acetate increases, in response to environmental concerns, more ethyl acetate needs to be produced synthetically. There are currently two commercial processes for this synthesis, viz, the Hoechst process based on the Tishchenko reaction and the esterification process based on the direct reaction of acetic acid with ethyl alcohol.

In the Tishchenko reaction, acetaldehyde is dimerized into ethyl acetate in the presence of aluminum ethoxide pellets activated by treatment with caustic soda. However, the pellets generate huge pressure drops in the furnaces and are frequently the source of premature blockage.

In the process of direct esterification, ethyl alcohol is made to react with acetic acid in the presence of an acidic catalyst. Since the pioneering work of the French chemist Paul Sabatier, who won the Nobel Prize in 1912 for his work on catalytic hydrogenation and dehydrogenation, processes for the preparation of primary alcohol esters from alcohols have been the object of numerous studies.

The patent literature also abounds in publications on this same subject. Thus, US patent 1 ,401 ,117 teaches copper catalysts useful for the dehydrogenation of primary alcohols into the corresponding esters, where the catalyst is a copper oxide, fused by heating to the melting temperature of the said oxide, cooled and broken into pieces. It is alleged that in processes of alcohol dehydrogenation in the presence of copper catalysts, whether either powdered copper or copper deposited on an inert carrier from solutions of its salts, there are many drawbacks such as low heat conduction, instability, lack of coherence, problems in preparing the catalysts and in reactivating or recovering the catalyst in commercially viable ways. According to the information in this US patent, the efficiency of the as-prepared, lump catalyst is ten times superior to that verified for powdered copper catalysts or under other forms. US patent 1 ,708,460 teaches that the dehydrogenation of alcohols with more than one carbon atom leads to high yields if carried out at high pressures. According to this proposal, ethyl alcohol is for example pumped into a heated vessel kept above the critical temperature of alcohol. The alcohol vapor is continuously led over a dehydrogenation catalyst such as reduced metallic copper, held in a pressure-resistant tube heated to around 350°C, at a flow rate equalent to four volumes of liquid ethyl alcohol for each volume of catalyst per hour. The effluent gases are conveyed under pressure to a condensing spiral, where unreacted alcohol, containing ethyl acetate, butyl alcohol and minor amounts of other products, is separated. The hydrogen gas produced is bled from the reaction system at such a rate that the pressure in the catalytic chamber is kept at about 275 atmospheres. It is alleged that the process carried out at high pressure allows the usual endothermicity to be avoided and the process thus becomes exothermic. According to US patent 1 ,975,853 the dehydrogenation of alcohols to yield esters in the presence of copper catalysts is heavily influenced by the composition of the catalyst employed. Thus it is proposed that instead of using the pure metal, the catalyst should be composed of copper and certain promoters as small amounts of poorly reducible metal oxides, such as manganese oxide and zinc oxide in combination.

US patent 2,504,497 reports a catalyst for alcohol dehydrogenation based on a spongy catalytic mass consisting essentially of separate hollow pieces the smallest dimension of which is not less than 6 mm, formed from a copper-aluminum alloy, the surface of which is porous and consists mainly of active copper.

US patent 6,399,812B1 reports a two-step process where aliphatic esters R'COOR are first produced by reaction of the corresponding ROH alcohol, where the alkyl groups bear numbers of carbon atoms between 0 and 9 and 1 and 10, respectively, with molecular oxygen in the presence of a double- function catalyst comprising metal on an acidic solid support. The process is utilized to produce ethyl acetate from ethyl alcohol. The process is characterized by high ethanol conversion, high selectivity and high ethyl acetate yield. The preferred catalyst is Pd on zeolites. Other catalysts include Pt and Pd, but Ni, Mo, W, Co, Rh, Ru, Ag, Zn, Cu and Cr are also useful and can be employed as reduced metal, oxide and sulfide. Useful supports for the described process include silica, alumina, zeolites, clays, titania, magnesia and active charcoal from sources such as coal, coke and coconut shell. Besides the requirement of being acidic, the support should be stable under the reaction conditions of oxidation and high reaction temperatures. The metals can be added to the support by impregnation, milling, admixture, co-precipitation or a combination of these techniques. The suitable amount of metal loading on the support is between 0.01 and up to 20 wt %, normally 0.1 to 10 wt %. Ester yields reach around 50 wt % in certain cases, in others, 20 wt % or 30 wt %, relative to the original alcohol feed.

In Brazil, ethyl acetate is produced from ethanol in a three-step process, marketed by "Cloroetil Solventes Aceticos S/A", the process being based on reactions 1.1 to 1.4: a) Dehydrogenation of ethyl alcohol to acetaldehyde

CH₃CH2OH → CH₃CHO + H₂ (1.1) ΔH°_a = 70.6 KJ/mol b) Oxidation of acetaldehyde to acetic acid: CH3CHO + ¹/₂ 0₂ → CH₃COOH (1.2) ΔH°_b = -270.7 KJ/ c) Acetic acid esterification with ethyl alcohol:

CH₃COOH + CH₃CH₂OH -> CH₃COOC₂H₅ + H₂0 (1.3) ΔH°_C=-15.1 KJ/mol Overall process:

2 CH3CH₂OH + ¹/₂ O₂ → CH3COOC2H5 + H₂O + H₂ (1.4) ΔH°o = -215.2 KJ/mol

This process involves three types of catalysts, that are used in distinct reactors. In spite of the low enthalpy change and exothermicity of the overall process, (1.1 ) and (1.2) reactions have strong thermal effects.

None of the cited patents describes or suggests a copper catalyst deposited on a matrix of zirconia or zirconia on silica or alumina, where the preparation conditions yield catalysts with high coverage degree of the zirconia surface by copper.

Thus, in spite of the technical advances in the field, a one-shot process is still not available to produce ethyl acetate from ethyl alcohol, in the presence of copper supported on zirconia or zirconia on silica or alumina containing predominantly Cu species that are reduced at low temperature, with high degree of coverage of the Zrθ₂ surface by copper, while showing reduced selectivity for formation of crotonaldehyde by-product, resulting in high activity and selectivity for ethyl acetate, reduced thermal effects and high hydrogen production, such process being described and claimed in the present application. SUMMARY OF THE INVENTION

Broadly, the copper-based catalysts supported on zirconia, or zirconia on silica or alumina, contain copper species that are reduced at low temperature and show a high degree of coverage of the zirconia surface by copper.

These catalysts are made by preparing a support which is zirconia or zirconia deposited on silica or alumina, depositing copper salts on the prepared support, calcining the supported copper catalysts and reducing the copper oxide to copper metal to obtain finished catalysts. The conditions used in the preparation of the catalysts lead to products where the degree of coverage of the zirconia surface by copper is high, this being a paramount feature for the high selectivity to ethyl acetate.

And the one-shot process for obtaining ethyl acetate from ethyl alcohol in the presence of the inventive catalysts comprises: a) Providing a tubular fixed bed-catalytic reactor laden with a zirconia- or zirconia on silica or alumina-supported copper catalyst; b) With the aid of a pumping device, feeding the said catalytic reactor with a fresh stream of ethyl alcohol into a vaporizer and direct the vaporized reagents to the said catalytic reactor at a temperature between 180°C and 360°C, to effect a vapor-phase reaction and form mixtures of acetaldehyde, ethyl alcohol, crotonaldehyde and ethyl acetate, the reactor being operated at residence time factors W/F between 20 and 600 min, where W= catalyst mass (Kg) and F= ethyl alcohol flow fed (Kg/min); c) In the catalytic reactor, carrying out the ethyl alcohol dehydrocoupling reaction so as to form ethyl acetate, during the appropriate residence time to produce the desired products, the reactor outlet stream being cooled in a first heat exchanger, where it is separated into a gaseous stream and a liquid stream; d) Cooling the said gaseous stream in a second heat exchanger, where the liquid acetaldehyde stream is separated from the hydrogen gas stream, these streams being partially recycled to the catalytic reactor; e) Treating the liquid stream from the first heat exchanger in a refining system, and separating the ethyl acetate product with a selectivity between 20 and 93%.

Thus, the present invention provides zirconia- or zirconia on silica or alumina-supported copper-based catalysts predominantly containing Cu species that are reduced at low temperature, in which the degree of coverage of the ZrO₂ surface by copper is high, showing reduced selectivity for the formation of the crotonaldehyde by-product.

The invention provides further a process for preparing copper-based catalysts on zirconia or zirconia on silica or alumina, that are useful for preparing ethyl acetate from ethyl alcohol.

The invention also provides a one-shot process for production of ethyl acetate from ethyl alcohol, at high yields of ethyl acetate.

In addition, invention provides a one-shot process to produce ethyl acetate from ethyl alcohol in the presence of a copper catalyst deposited on zirconia, or on zirconia on silica or alumina, the catalyst being of high activity and selectivity for the desired end product. BRIEF DESCRIPTION OF THE DRAWINGS

FIGURE 1 attached illustrates Temperature Programmed Reduction (TPR) curves for Cu/Zrθ₂ catalysts with 10 weight % copper calcined at different temperatures. FIGURE 2 attached illustrates the TPR curves for Cu/SiO₂ catalysts, calcined at 500°C, where predominantly high-reduction temperature species called γ(1 , 2, 3 or 4) are formed, these being less selective for ethyl acetate formation than the low-temperature reduction species. FIGURE 3 attached illustrates TPR curves for XCu/Zr0₂ catalysts, where X= weight % of Cu supported on zirconium and calcined at 500°C.

FIGURE 4 attached is a plot of the ethyl alcohol conversion as a function of the residence time factor W/F, where W= catalyst mass in Kg and F= flow- rate oof ethyl alcohol fed to the reactor in Kg/min, for the 20Cu/ZrO₂ catalyst containing a Cu loading of 20 weight % at different reaction temperatures.

FIGURE 5 attached is a plot of the ethyl alcohol conversion as a function of the residence time factor W/F, where W= catalyst mass in Kg and F= flow- rate oof ethyl alcohol fed to the reactor in Kg/min, for the 30Cu/ZrO₂ catalyst containing a Cu loading of 30 weight % at different reaction temperatures. FIGURE 6 attached is a plot of the ethyl alcohol conversion as a function of the residence time factor W/F, where W= catalyst mass in Kg and F= flow- rate of ethyl alcohol fed to the reactor in Kg/min, for the 30Cu/SiO₂ catalyst containing a Cu loading of 30 Cu weight % at different reaction temperatures.

FIGURE 7 attached illustrates a schematic flow sheet of the process of the invention for the production of ethyl acetate from ethyl alcohol. DETAILED DESCRIPTION OF THE PREFERRED MODES

Under a first aspect, the invention comprises supported copper catalysts useful for the preparation of ethyl acetate from ethyl alcohol.

The catalyst for production of ethyl acetate from ethyl alcohol according to the invention is a supported, copper-based catalyst. Useful supports are Zrθ2, or ZrO₂ deposited on the surface of other oxide supports such as AI₂O₃ or Siθ2 , in the form of powder or pellets of various shapes.

The particle size is not restricted in any way, but is preferably between 1 and 5 mm. The pore volume of the zirconium oxide support is also not restricted and it is preferably of a monoclinic structure. The specific surface area of the support is also not restricted and generally is in the range of 50 to 150 m²g^"1. The copper loading will depend on the surface area and on the preparation conditions of the support. Normally the Cu loading is between 0.002 and 0.004 gram Cu per m² of support. The optimum Cu content for a specific support will be determined from the TPR data as a function of the Cu loading, the final catalyst having a desirable ratio (R) between the hydrogen consumed in the reduction of the high-reduction-temperature species (HT) and low reduction temperature species (LT) not higher than R = HT/LT = 3.5.

As illustrated in Figures 1 , 2 and 3, the high-reduction-temperature species are called γ-i, 72, 73 or γ₄ species. As illustrated in Figures 1 , 2 or 3, the low reduction temperature species are called αi, 0C₂ or β species.

The loading of Cu in weight % on the support may vary from 2.5% to 30%. Thus, the types of catalysts prepared go from 2.5Cu.ZrO₂ to 30Cu.ZrO₂ or alternatively 2.5Cu-Zr0₂/Si0₂ or AI₂O₃ to 30Cu Zr0₂/.Si0₂ or AI₂O₃. The selectivity for ethyl acetate formation can be controlled through the dispersion of Cu on the support. When adequately dispersed on the support, CuO can interact with the ZrO surface, yielding CuO species having low reduction temperature. The resulting Cu species are selective for the ethyl alcohol dehydrocoupling reaction. The high CuO dispersion as well as the CuO interaction with the Zr0₂ surface can be controlled through the conditions used to prepare the catalyst precursor, more specifically through the choice of the Cu loading and precursor calcination temperature. The Cu loading on the support should be based on the TPR results as a function of the Cu loading, as described hereinbefore. The calcination temperature should be between 350°C and 600°C, the preferred temperature being around 500°C.

The activity of the catalysts of the invention for ethyl acetate production is determined by using a fixed-bed tubular reactor, continuously fed with ethyl alcohol or a mixture of ethyl alcohol and hydrogen and kept under chosen temperature and pressure conditions. Hydrated ethyl alcohol can also be used as feed, the water content not exceeding 10 weight %, with the preferred feed being anhydrous ethyl alcohol.

Ethyl alcohol can be synthetic or originate from fermentation processes. In order to avoid a rapid catalyst deactivation, it is recommended that the content of sulfur and chloride be not higher than 1 ppm. According to the reaction conditions, the catalytic activity varies between 0.020 and 0.674 mole- ethyl alcohol/h/ g_Cu-

The second aspect of the invention is directed to a process to prepare the copper-based, zirconia- or zirconia on silica or alumina-supported inventive catalysts, such process comprising the following steps: a) Preparation of the catalyst support

For this step, preferably monoclinic zirconia can be used directly, or else zirconia may be prepared by decomposition of zirconium hydroxide.

The decomposition of zirconium oxide should be carried out under a flow of oxygen or air, while the temperature is increased from room temperature to a temperature between 300°C and 600°C. Such is not a limiting temperature, but it is desirable to keep it close to 500°C. The heating rate is also not limiting, but a desirable rate is close to 10 degrees per minute, after which the 500°C temperature is maintained for a period between 1 and 10 hours. This period at the highest temperature is also not limiting, but it is desirably kept close to 6 hours.

The prepared support can be used directly in the impregnation, or else deposited on the surface of other solid supports such as Al₂0₃ or Si0₂, in the form of a powder or pellets of various shapes. Zirconia can be deposited on supports such as AI₂O₃ or SiO₂ by precipitation or impregnation from a zirconium-containing solution. This deposition can be done in several ways, for example using the corresponding nitrate and a hydroxide solution (3 mole dm^"3) as a precipitant. After precipitation the solid is washed with water and air dried at the temperature of 353 K for 3 hours and then calcined in nitrogen at a temperature of 773 K during 3 hours. The zirconium loading is not limiting and will depend on the specific surface area of the support, a loading close to 0.1 gram of zirconium per square meter of support being recommended. b) Copper deposition on the support

At this stage copper is deposited in the +2 oxidation state on the support. For this purpose, useful salts are the copper acetate or copper nitrate, the nitrate being preferred. Aqueous or alcoholic solutions are used at a copper concentration in the range of 0.025 to 0.20 g-Cu/g-Zrθ₂. Impregnation is carried out at room temperature with a solution the volume of which can vary between 2 and 7 liters of solution per Kg of support. c) Calcination of the supported copper salt

In this step, the supported copper salt is calcined under an air-flow while the temperature is increased from ambient to a temperature between 350°C and 600°C, the preferred temperature being close to 500°C. The heating rate is not limiting, but a desirable rate is close to 10 degrees per minute, temperature being held at 500°C for 5 hours.

The supported copper oxide should exhibit predominantly lower reduction temperatures and αi, 0.₂ or β species, while supported copper oxide with higher reduction temperatures, obtained from silica-supported copper oxide, and γ-i, γ₂, Y₃ or γ₄ species as illustrated in Figures 1 and 2, is acceptable but less preferred for the purposes of the invention. These figures show Temperature Programmed Reduction (TPR) curves for zirconia-supported CuO samples calcined at different temperatures and also of silica-supported CuO. TPR curves were obtained at heating rates of 10 degrees per minute and a gas flow containing 5% H₂ and 95% N₂. d) Reduction of the calcined Copper

After calcination, the supported CuO is reduced to metallic copper. This reduction should preferably be carried out with hydrogen, but ethyl alcohol can also be used. The maximum reduction temperature in hydrogen should not be higher than 300°C. It is desirable that the heating from the ambient temperature and up to the maximum temperature be effected at a rate of 1 to 10°C/minute. The period of reduction at 300°C should not exceed 4 hours. A third aspect of the present invention deals with a one-step process to prepare ethyl acetate from ethyl alcohol in the presence of the invented catalysts.

The process for obtaining ethyl acetate from ethyl alcohol according to the invention is not particularly restricted, provided the contact of ethyl alcohol and catalyst are allowed to come into contact at a temperature in the range 180°C to 360°C required to effect a vapor phase reaction.

Different kinds of reactors are useful, with a fixed bed reactor being preferred. The ethyl alcohol normally employed as a reactant is anhydrous, with content of sulfur and chloride not exceeding 1 ppm. Alternatively, hydrated ethyl alcohol may be employed, provided there is not more than 10 weight % of water.

Reaction pressure is not particularly restricted, but the preferred range is between 1 and 2 atmospheres (manometric).

While the reaction can be carried out in the absence of any sweeping gas, the reaction selectivity for ethyl acetate is favored if hydrogen is employed as a sweeping gas. The selectivity in the presence of hydrogen gas is less influenced by the zirconia-supported catalysts, which have a lower reduction temperature, than by silica-supported catalysts, with a higher reduction temperature.

The amount of hydrogen used in the feed is between 0 and 90% of gas volume, relative to the vaporized ethyl alcohol.

Figure 7 shows the schematic process flow-sheet for a catalytic fixed-bed tubular reactor 7. A stream 2 of fresh ethyl alcohol, stored in a tank 1 is piped to a pumping device, such as a centrifuge pump 3, which feeds a vaporizer 5, via line 4. Recycled material, acetaldehyde and hydrogen, supplied respectively by lines 16 and 14, is also fed in vaporizer 5. Via line 6, the vaporized reactants are directed to catalytic reactor 7, where the ethyl alcohol undergoes a dehydrocoupling reaction and forms ethyl acetate. The reaction is endothermic, with a heat of reaction under standard conditions of 26.8 kJ/mole. The reactor 7 outlet stream 8 is cooled in a heat exchanger 9, where the liquid product, line 18, is separated from the gaseous product, line 10.

The gaseous stream, line 10, contains mainly hydrogen and acetaldehyde, while the liquid stream, line 18, includes ethyl alcohol, ethyl acetate and minor amounts of reaction by-products (methyl ethyl ketone, butyl alcohol, acetone, ethene, crotonaldehyde and ethyl ether among others).

The gaseous stream, line 10, is cooled again in heat exchanger 11, where liquid acetaldehyde, line 16, is separated from hydrogen gas, line 14, the latter being stored in tank 12. A portion of hydrogen gas can be recycled to catalytic reactor 7 via line 14, the recycle ratio being controlled by valve 13. The acetaldehyde produced can also be recycled to catalytic reactor 7 via line 16, the recycle ratio being controlled by valve 15, and/or stored in tank 17.

The liquid stream, line 18, from the first heat exchanger 9 should be treated in a refining system 19, so that the product ethyl acetate is obtained in line 20 and stored in tank 21. Residues 22 from the refining can be disposed off or stored in tank 23 to be further processed.

Refining system 19 can be any of the state-of-the-art refining/purification systems and comprises a fractional distillation system.

The fixed-bed reactor operates preferably at a residence time factor (W/F) in the range of 100 to 200 Kg-cat min /.Kg-ethyl alcohol.

The inventive process yields ethyl acetate as product and hydrogen as co-product. The hydrogen produced does not contain contaminants such as CO and the refining/purification system chosen depends on the hydrogen purity and pressure, required. The selectivity towards acetaldehyde and ethyl acetate can be controlled by the reactor feed conditions. For example, an increase in the ethyl alcohol and hydrogen pressure favor the selectivity towards ethyl acetate.

The invention will be illustrated by the following Examples, which should not be construed as limiting same. EXAMPLE 1

The catalyst for production of ethyl acetate, such as for example XCu/ZrO₂, where X= weight % of Cu can be obtained from a Zrθ₂ support, with specific surface area of 125 m²/g, prepared by the decomposition of zirconium hydroxide at 500°C for 8h. The ZrO₂ (50g) support, of particle size 1 mm is impregnated with a Cu(NO₃)₂.3H₂O (99.9%) solution in methyl alcohol. Such methyl alcohol solution is prepared from a certain weight of copper nitrate related to the amount of metal which is intended to be impregnated and a solvent volume equivalent to 500 ml solution per 100 g support. The solution is slowly added on the support so as to wet all the support mass through agitation in an inclined, turning plate. Later, the sample is dried by heating at temperatures of around 50°C. This procedure is repeated until the solution is exhausted. After the impregnation the precursor is calcined in an oven, under a flow of 1000 ml/100g-cat.min of pressurized air, at a heating rate of 10°C/min up to 500°C and keeping such conditions for 5 h.

EXAMPLE 2

Figure 1 shows that the Cu-zirconia interaction depends on the thermal treatment temperature. The supported copper oxide should be predominantly of species reducible at low temperature (200°C-240°C), designated by αι, ₂ or β, relative to the reduction temperature of the silica-supported copper oxide shown in Figure 2, that produces species predominantly reducible at high temperature and designated by γi, γ₂₎ γ₃ or γ . The calcination temperature conditions should be previously chosen in accordance with the features of the zirconia to be used as support. EXAMPLE 3

According to this Example, the distribution of Cu species that are reducible at low and high temperatures strongly depends on the supported Cu loading, as illustrated in Figure 3. The features of the zirconia, such as surface area and crystalline structure, influence the distribution of copper species formed as a function of the copper loading. In order to obtain high yields of ethyl acetate, catalysts should have a high Cu content and predominantly the Cu oxide species of low reduction temperature.

EXAMPLE 4 This Example demonstrates that the ethyl alcohol conversion attained at a given residence time (W/F) is highly dependent on the species of Cu present in the catalyst as well as on the reaction temperature.

Zirconia-supported Cu shows an asymptotic increase in conversion with the increase in the concentration of copper species reducible at low temperature. Figures 4, 5 and 6 illustrate the conversion curves as a function of the residence time (W/F) and reaction temperature, using catalysts of the types Cu/ZrO₂ and Cu/Si0₂ calcined at 500°C, and having different Cu loadings. Data shown in Figures 4, 5 and 6 demonstrate that the Cu/Zrθ₂ catalyst containing 20% by mass of Cu of Figure 4, which has a ratio (R) between the hydrogen consumed in the reduction of the high-reduction-temperature species (HT) and that of low-temperature-reduction species (LT) not higher than R=HT/LT= 3.2, exhibits an asymptotic conversion of ethyl alcohol significantly superior to the conversion found with Cu/Zrθ₂ and Cu/SiO₂ catalysts containing 30% by weight of Cu, which show R values of 12.5 and 52.0, respectively, typical of the predominance of high temperature reducible copper oxide. EXAMPLE 5

This Example demonstrates that the selectivity for the formation of ethyl acetate depends strongly on the hydrogen pressure in the feed, when the process operates at low ethyl alcohol pressure. Table 1 below lists data that demonstrate the influence of the presence of hydrogen in the feed of a tubular fixed bed reactor using ethyl alcohol at low pressure (PEI_OH= 44 mmHg).

Table 1 shows conversion and selectivity data for the 10Cu/ZrO₂ and 10Cu/SiO₂ samples, calcined at 500°C containing 10 % by mass of Cu, at different reaction temperatures, ethyl alcohol pressure PEIO_Γ 44 mmHg and nitrogen or hydrogen pressure PN2 = PH2 = 760 mmHg.

Xe_to_H= = ethyl alcohol conversion; Acet.=Acelaldehyde; Ethyl Ac.=Ethyl Acetate; Crot.=Crotonaldehyde; MEK/But.= ethylethylketone/butyl alcohol

Data from Table 1 lead to the conclusion that the presence of hydrogen in the feed favors the selectivity for ethyl acetate formation. With Cu/Siθ₂ catalysts, having high-temperature reducible species, ethyl acetate formation is more influenced by the presence of hydrogen in the feed than with Cu/Zrθ₂ samples, in which the-low temperature reducible Cu species predominate.

Broadly, it is found that increasing the ethyl alcohol and hydrogen pressures favors the selectivity for ethyl acetate formation.

On the other hand, the presence of nitrogen merely alters the ethyl alcohol partial pressure in the reactor. Thus, by varying the amount of nitrogen in the feed it is possible to observe the effect of the ethyl alcohol pressure, namely, that raising the ethyl alcohol pressure in the feed predominantly favors ethyl acetate formation.

Data from Tables 2 and 3 demonstrate the influence of the hydrogen in the reactor feed on the selectivity for ethyl acetate formation on a Cu/ZrO₂ catalyst. These tables refer to the use of a fixed-bed tubular reactor operated at an ethyl alcohol pressure of 380 mmHg, using solely ethyl alcohol and an ethyl alcohol/hydrogen mixture (at molar ratio ethyl alcohol/H2=1 ) in the feed, respectively.

Data from Tables 1 to 3 show that the effect of the hydrogen in the feed, on the increased selectivity to forming ethyl acetate, diminishes with increased ethyl alcohol pressure in the reactor feed. At high ethyl alcohol pressure, the presence of hydrogen mainly inhibits crotonaldehyde formation.

Table 2 below illustrates data from a fixed-bed reactor using solely ethyl alcohol in the feed at various temperatures, W/F= 38 Kg-cat/Kg-ethyl alcohol/min and PEtoH= 760 mmHg.

TABLE 2

Acet.=Acetaldehyde;. Ethyl Acet.=Ethyl Acetate; Crot.=Crotonaldehyde; Acto=acetone; Ethene=ethene; MEK/But.=Methyl ethyl ketone/butyl alcohol

Table 3 below lists data obtained with ethyl alcohol and H₂ in the feed of a fixed-bed reactor at various temperatures, W/F= 38 Kg-cat/Kg-ethyl alcohol/min, P_eto_H= 380 mmHg and molar fraction (ethanol/ethanol + H ) in the

MEK/But.=Methyl ethyl ketone/butyl alcohol

EXAMPLE 6 Tables 4 to 7 illustrate the product distribution as a function of the residence time (W/Feto_H), for Cu/ZrO₂ catalysts calcined at 500°C and having various Cu loadings, in a fixed-bed reactor at a temperature of 225°C fed with ethyl alcohol at 1 atm pressure.

Data show that although the ethyl alcohol conversions tend to an asymptotic limit when W/F rises above 38 Kg-cat./Kg-ethyl alcohol/min, higher residence times (W/F) favor ethyl acetate and crotonaldehyde formation.

With increasing Cu loading in the Cu/ZrO₂ catalysts, those catalysts that contain predominantly Cu species that are reducible at low temperature show decreased selectivity for crotonaldehyde formation. The lowest crotonaldhyde selectivities are obtained with catalysts exhibiting a high coverage of the ZrO₂ surface by Cu, that is, high Cu loading.

Table 4 below lists data for conversion, activity and selectivity for T = 225°C and W/F = 152 Kg-cat/Kg-ethyl alcohol/min. TABLE 4

a- acetaldehyde; b- ethyl acetate; c- crotonaldehyde; d- acetone; e- ethene; f- ethyl ether; g- methyl-ethyl-ketone and/or butyl alcohol. Table 5 below lists data for conversion, activity and selectivity for T 225°C and W/F = 76 Kg-cat/Kg-ethyl alcohol/min.

TABLE 5

f- ethyl ether; g- methyl-ethyl-ketone and/or butyl alcohol Table 6 below lists data for conversion, activity and selectivity for T = 225°C and W/F = 38 Kg-cat/Kg-ethyl alcohol/min. TABLE 6

ethyl ether; g- methyl-ethyl-ketone and/or butyl alcohol

Table 7 below lists data for conversion, activity and selectivity for T = 225°C and W/F = 19min.

TABLE 7

ethene; f- ethyl ether; g- methyl-ethyl-ketone and/or butyl alcohol

In conclusion, it can be seen that the production of ethyl acetate by the one-step process of the invention, via the ethyl alcohol dehydrocoupling reaction, has numerous advantages in terms of ease of production.

The one-step process specified in the invention is schematically illustrated in equations 1.5 to 1.7 below. a) Ethyl alcohol dehydrogenation to acetaldehyde: CH3-CH₂-OH → CH₃-CHO + H₂ (1.5) ΔH°_a = 70.6 KJ/mol b) Condensation of acetaldehyde and ethyl alcohol:

CH3-CHO + CH₃-CH₂-OH → CH3-COO-CH2-CH3 + H₂ (1.6)

ΔHob = -43.8 KJ/mol Overall reaction:

2 CH3-CH2-OH → CH₃-COO-CH₂-CH₃ + 2 H₂ (1.7)

ΔH°₀ = 26.8 KJ/mol

In this case, ethyl acetate formation from ethyl alcohol is a weakly endothermic process. Besides, this process is advantageous in view of the reduced thermal effects as well as the augmented hydrogen production compared to the three-step state-of-the-art process.

The present process can be adapted and easily extended by any person skilled in the art to alcohols with an increased number of carbon atoms, for example, from three to nine carbon atoms, in order to obtain the corresponding esters.

Claims

1. A process for preparing copper-based, supported catalysts for obtaining ethyl acetate from ethyl alcohol, wherein said process comprises: a) Providing a zirconia support; b) Depositing a Cu⁺² salt on the support provided in a) at ambient temperature and at copper concentrations varying of from 0.025 to 0.20 g-Cu/g-ZrO₂; c) Calcining the support on which the copper salt has been deposited at temperatures between 350°C and 600°C, obtaining a low temperature- reducible supported copper oxide; d) Reducing the supported copper oxide in the presence of a reducing gas at a maximum temperature of 300°C/4 hours, so as to obtain a final catalyst where the recovery degree of the Zrθ₂ surface by copper is high.

2. A process according to claim 1 , wherein the zirconia is a monoclinic zirconia.

3. A process according to claim 1 , wherein the zirconia is prepared by decomposition of zirconium hydroxide.

4. A process according to claim 3, wherein the decomposition of zirconium hydroxide is effected under air flow while the temperature is increased of from 300°C to 600°C, at a heating rate around 10 degrees Celsius by minute, for 1 to 10 hours.

5. A process according to claim 4, wherein the decomposition of the zirconium hydroxide is carried out at a temperature around 500°C.

6. A process according to claims 4 and 5, wherein the heating period is 6 hours.

7. A process according to claim 1 , wherein the zirconia support is used as such in the impregnation step.

8. A process according to claim 1 , wherein the zirconia support is deposited on the surface of AI₂O₃ or Siθ₂ oxide supports as powders or as pellets of various conformations.

9. A process according to claim 8, wherein the zirconia deposition on said oxide supports is carried out by precipitation or impregnation from a zirconium-containing solution.

10. A process according to claim 9, wherein the zirconia charge is around 0.1 g of zirconium per square meter of said oxide support.

11.A process according to claim 1 , wherein said copper salt is copper acetate.

12. A process according to claim 1 , wherein said copper salt is copper nitrate.

13. A process according to claim s 11 and 12, wherein the copper salt solutions are aqueous or alcoholic.

14. A process according to claim 1 , wherein the heating rate of step c) is 10°C/min.

15. A process according to claim 1 , wherein the low reduction temperature is between 200°C and 240°C.

16. Copper-based, supported catalysts wherein said catalysts are prepared according a process comprising the following steps: a) Providing a zirconia support; b) Depositing a Cu⁺² salt on the support provided in a) at ambient temperature and at copper concentrations varying of from 0.025 to 0.20 g-Cu/g-Zr0₂; c) Calcining the support on which the copper salt has been deposited at temperatures between 350°C and 600°C, obtaining a low temperature-reducible supported copper oxide; d) Reducing the supported copper oxide in the presence of a reducing gas at a maximum temperature of 300°C/4 hours, so as to obtain a final catalyst where the recovery degree of the Zr0₂ surface by copper is high.

17. Copper-based, supported catalysts according to claim 16, wherein each specific support requires an optimum copper content, said content being determined from programmed temperature reduction (PTR) data as a function of the Cu charge and wherein the hydrogen consumption for reducing the high reduction temperature (HT) species and low temperature reduction (LT) species is at most 3.5 for the ratio R = HT/LT.

18. Copper-based, supported catalysts according to claim 16, wherein said catalysts comprise between 2.5% and 30% by mass of Cu on said support.

19. Copper-based, supported catalysts according to claim 16, wherein said catalysts comprise 20% by mass of Cu on said support.

20. Copper-based, supported catalysts according to claim 16, wherein said catalysts comprise high reduction temperature species of the kind γi, j₂, 7₃ or

Y4.

21. Copper-based, supported catalysts according to claim 16, wherein said catalysts comprise low reduction temperature species of the kind α-i, ct₂ or β.

22. Copper-based, supported catalysts according to claim 16, wherein said catalysts show an increase in the asymptotic conversion of ethyl alcohol into ethyl acetate with the increase in the concentration of the copper oxide species that is reduced at low temperature.

23. A process for preparing ethyl acetate from ethyl alcohol in the vapor phase, in the presence of the copper-based, supported catalysts according to claim 16, wherein said process comprises: a) From a tank 1, feeding a fresh ethyl alcohol stream 2 with the aid of a pumping device 3 towards an atomizer 5 via line 4, and directing via line

6, the atomized reactants to catalytic reactor 7 laden with said copper catalysts, the residence time factors W/F being in the range of 20 to 600 Kg-cat.min/Kg-ethyl alcohol, at temperatures between 180°C and 360°C, and pressures between 1 and 2 atmospheres; b) In reactor 7, carrying out the dehydrocoupling reaction of ethyl alcohol with ethyl acetate formation, for the desired residence time for forming the products, outlet stream 8 being cooled in a heat exchanger system 9 where the liquid product, stream 18, is separated from the gaseous product, stream 10; c) Cooling the gaseous product stream 10 in a second heat exchanger 11 wherefrom is separated stream 16 of liquid acetaldehyde from hydrogen gas stream 14; d) Treating the liquid stream 18 from the first heat exchanger 9 in a refining system 19 so as to separate the ethyl acetate product to be stored in tank 21.

24. A process according to claim 23, wherein the residence time factor W/F is between 100 and 200 Kg-cat.min/Kg-ethyl alcohol.

25. A process according to claim 23, wherein recycled gaseous hydrogen and acetaldehyde respectively from lines 14 and 16 feeds atomizer 5.

26. A process according to claim 23, wherein alternatively acetaldehyde from line 16 is stored in tank 17.

27. A process according to claim 23, wherein alternatively hydrogen from line 14 is stored in tank 12.

28. A process according to claim 23, wherein liquid stream 18 contains non- reacted ethyl alcohol, acetaldehyde, ethyl acetate and minor amounts of methyl ethyl ketone (MEK), butyl alcohol, acetone, ethene, crotonaldehyde and ethyl ether.

29. A process according to claim 23, wherein the liquid stream 18 refining system 19 separates an upper fraction containing mainly ethyl acetate product, directed via line 20 for storage in tank 21 , while by-products 22 are discarded off or stored in tank 23.

30. A process according to claim 29, wherein refining system 19 comprises fractionate distillation.

31. A process according to claim 23, wherein the selectivity of same for ethyl acetate is higher than 90%.

32. A process according to claim 23, wherein the selectivity of same for acetaldehyde is higher than 90%.

33. A process according to claim 23, wherein hydrogen is used as a sweeping gas in the reaction.

34. A process according to claim 33, wherein the sweeping gas is used in an amount of 0 to 90% by volume of gas based on the atomized ethyl alcohol.

35. A process according to claim 23, wherein the number of Carbon atoms of the alcohol to be submitted to the dehydrocoupling reaction is between 3 and 9.