More specifically, the invention relates to the partial oxidation field of light paraffin hydrocarbons to obtain olefins.

The partial oxidation of light hydrocarbons has been studied, particularly during the last ten years, as an alternative to thermal cracking for the production of olefins (Schmidt, L. D. et al. , Aiche J. 46: 1492 (2000)).

Thermal cracking processes do in fact have numerous drawbacks. In the case of the production of ethylene through the dehydrogenation of ethane, for example, the process, which is effected in a homogeneous phase, is en- dothermic, and temperatures of around 850°C are required to allow it to take place at a reasonable rate.

For this reason, the external surface of the reac- tion tubes must be heated by exposure to flames, which not only means that the structures must be manufactured using materials which are resistant to very high tempera- tures, but also that there is the formation of thermal NO,. Moreover, this kind of reactor has the drawback of the formation of coke, due to the long exposure times of the ethane to high temperatures (about 1 s).

If the process is carried out by adding oxygen in the feedstock (oxidative dehydrogenation) and in the presence of an oxidation catalyst, the whole reaction be- comes exothermic and the process is thermally self- sustaining.

The advantages in terms of materials considerably exceed the disadvantage linked to the separation of the oxygen. With the addition of oxygen, in fact, there is no longer the necessity of exposing the reaction tubes to flames, as the heat is produced in situ inside the reac- tor. A further advantage lies in the fact that the forma- tion of thermal NO,, is reduced by the comparatively low temperatures (below 1000°C) and by the absence of flames.

Finally, the formation of coke is considerably reduced by the presence of the catalyst and the reduced residence time of the reagents in the high temperature zone (about 0.005 s).

Oxidative dehydrogenation processes can be divided into purely catalytic processes or processes with a mixed hetero-homogeneous mechanism. The latter are certainly more interesting on an industrial scale, whereas the for- mer are limited, by the low operating temperature, with respect to the olefin yield that can be obtained (Cavani F. and Trifiro F. , Catal. Today 24: 307 (1995)).

US patent 4,940, 826 was the first to describe a catalyst based on platinum laid on a support, preferably an alumina, in the form of spheres or granules, prefera- bly in a monolithic form, for the production of mono- olefins through the catalytic oxidative dehydrogenation of light hydrocarbons. Said patent also suggested the ad- dition of hydrogen to the feedstock to the catalyst, in order to enhance the olefin yield. Platinum is a catalyst capable of tolerating the combustion of hydrocarbon- oxygen mixtures above the upper flammability limit. As demonstrated by recent studies, the feeding of large amounts of reagents (mixtures of hydrocarbons and oxygen) to this catalyst generates an autothermal process which is carried out through the catalytic oxidation of a frac- tion of hydrocarbon on the platinum surface, whereas the remaining hydrocarbon fraction generates oxidative dehy- drogenation reactions in gas phase, for the production of olefins (Beretta A. et al., Chem. Eng. ci. 56: 779 (2001)).

After the disclosure of the patent content, the oxidative dehydrogenation of ethane in catalysts based on noble metals was studied in depth by the research team of Professor L. Schmidt of the University of Minnesota, arousing interest in the partial oxidation processes of light hydrocarbons. It has been demonstrated, since 1993, that olefins can be produced by means of the autothermal oxidative dehydrogenation of light hydrocarbons in struc- tured reactors containing platinum, operating at high space velocities (in the order of milliseconds) and at temperatures ranging from 900 to 1000°C (Huff M. C., e Schmidt L. D. , J. Phis. Chem. 97: 11815 (1993)). Under these conditions, and starting from an ethane-oxygen mix- ture having a molar ratio close to the stoichiometric value, for the oxidative dehydrogenation of ethane (i. e. = C2H6/02 = 2), the ethylene yield is so high that it can be compared to the existing cracking processes. The selec- tion of catalysts based on platinum for this process, is the result of studies which have compared the activity of several noble metals, in high space velocity reactors for partial oxidation processes, reaching the conclusion that platinum is the optimal active phase for oxidative dehy- drogenation, whereas rhodium is the most suitable in par- tial oxidation to synthesis gas (Schmidt et al., Chem.

Eng. Sci. 49: 3981 (1994)).

An improvement in the catalyst based on platinum was obtained by the same research team, who describes, in US patent 6,072, 097, an innovative catalyst for the pro- duction of mono-olefins from gaseous paraffinic hydrocar- bons. The catalyst described in the patent consists of a ceramic monolith support covered with a platinum active phase to which a layer of tin or cupper is added. The ad- dition of tin or copper, as mentioned by the authors, causes an improvement in the performances with respect to platinum alone, with an increase in the ethylene yield (Yokoyama C. , et al., Catal. Lett. 38: 181 (1996)).

Moreover, the selectivity to olefins is further increased by the addition of molecular hydrogen to the feedstock, causing the substitution of the hydrocarbon, which acts as a sacrificial fuel, with hydrogen. Catalysts based on Sn/Pt or Cu/Pt, however, have the disadvantage of a high volatility of tin and copper, which causes serious limits to the duration of the catalyst.

Some studies have also considered the use of alter- native materials to noble metals. In particular, it has been proved that the use of ceramic monoliths based on Cr203, in the oxidative dehydrogenation of ethane, allows very promising performances to be reached, but which is limited however by the rapid deactivation of the catalyst (Flick, D. W. , Huff, M. C. , Appl. Catal. A 187: 13 (1999)).

A structured catalyst has also been proposed, based on hexa-aluminates (BaMnAl1lOl9) (Beretta, A. , and For- zatti, P. , J. Catal. 200: 45 (2001) ). This material is ex- tremely thermally stable, but less active compared to platinum, and causes the formation of coke.

Finally, a work published by the same research team who developed the solution, object of the present inven- tion, proposes a structured catalyst based on oxides of the perovskitic type (LaMnO3), for the oxidative dehydro- genation of ethane to ethylene (Donsi et al. , J. Catal.

209: 51 (2002) ). The high thermal stability of the cata- lyst, in addition to the good oxidation activity, allows a satisfactory running of the autothermal process, with- out problems relating to instability and the formation of coke. Furthermore, said perovskites show a certain in- trinsic activity in the oxidative dehydrogenation of eth- ane, giving interesting performances as far as the yield to ethylene is concerned.

From a study of the limits of the solution of the known art, it emerges that it would be desiderable to prepare a catalyst which can combine the high activities of noble metals with a high thermal stability. Research in this direction has identified several materials, in which the noble metals are deposited on oxides, in turn with or without catalytic activity, or in which said no-

ble metals are included in the structure of mixed oxides, used for oxidative dehydrogenation and other oxidation or non-oxidation processes. In addition to the stabilization of the noble metal, the metals proposed have the further advantage of using a lower quantity of said noble metals, due to the improved dispersion of the same, and of influ- encing the catalytic properties of the material through a suitable selection of the support.

U. S. patent 4,919, 902 describes, among other solu- tions, a catalyst based on noble metals dispersed on an oxide of rare earth metals supported on a refractory in- organic oxide selected from alumina, silica, titanium ox- ide, zirconium oxide, alumino-silicates and mixtures thereof. The use of this composite material was contem- plated for the oxidation of exhaust gas from combustion engines.

Furthermore, U. S. patent 5,877, 377 relates to a catalyst consisting of a metal oxide incorporating palla- dium particles for the low temperature oxidative dehydro- genation iso-1-butene to butadiene. Palladium is incorpo- rated into the catalyst to enhance its activity and carry out the reaction at a lower temperature.

The patent EP 0937697 describes a catalyst compris- ing a noble metal deposited on tin and zirconium oxide and a process, making use of this catalyst, for the

oxidative dehydrogenation of an alkane (mainly iso- butane) to obtain an alkene, the reaction mix comprising oxygen. Also in this case, the presence of tin and zirco- nium oxide enhances the performances of the catalyst, particularly if hydrogen is added to the feedstock.

U. S. patent 3,897, 367 describes a catalyst with an AB03 structure, of the perovskitic type, wherein 1 to 20% . of the cations in B position are substituted by Ru or Pt.

This catalyst can be used in the oxidation reactions of exhaust gas. Furthermore, by substitution in the perov- skitic structure, the noble metal is stabilized to resist high temperatures, through the reduction of the volatil- ity of the pure noble metal. The activity of the perov- skites obtained by the substitution, is significantly in- creased with respect to non-substituted perovskites.

Chinese patent 1,208, 669, relating to a catalyst and oxidation process of a mix of methane and ethane for the production of ethylene, also describes oxides of the perovskitic type, based on titanium, partially substi- tuted with Pt and Pd. A further example is disclosed in the article"Selective oxidation of methane and ethane to ethylene over AB03 perovskite catalysts"in the Journal Shiyou Huagong (1999), 28 (10), pages 653-656.

Finally, U. S. patent 5, 508, 257 describes a supercon- ductor composite material wherein a metallic phase (Cu,

Ag, Au, Pt, Ni, Zn) is deposited on a perovskitic or al- most-perovskitic oxide.

None of the above-mentioned patents suggests the use of a catalyst consisting of a noble metal deposited on an oxide of the perovskitic type for the partial oxidation of light alkanes.

The solution according to the present invention can be inserted within this context, as it proposes a cata- lyst for the partial oxidation of light paraffinic hydro- carbons in gaseous phase to obtain olefins, and has yield and selectivity characteristics which are enhanced with respect to the catalysts of the known art, at the same time, proving to be more stable during use.

A first specific object of the present invention therefore relates to a multilayer catalyst for the par- tial oxidation of hydrocarbons in gaseous phase, compris- ing a monolithic ceramic or metallic substrate having a solid, macroporous structure, consisting of one or more structures on which a first active layer is deposited, having a perovskitic crystalline structure and having the formula AA'i-x By B'i-y 03s wherein: A is a cation of at least one of the rare earth elements,

A'is a cation of at least one element selected from groups Ia, IIa and VIa of the periodic table of elements, B is a cation of at least one element selected from groups IVb, Vb, VIb, VIIb or VIII of the periodic table of elements, B'is a cation of at least one element selected from groups IVb, Vb, VIb, VIIb or VIII of the periodic table of elements, Mg2+ or Al3+, x is a number which is such that 0 # x # 1, y is a number which is such that 0 < y < 1, and 5 is a number which is such that 0 < 5 < 0, 5, a second more external active layer consisting of a dis- persion of a noble metal and a possible supporting layer with a large surface area, positioned between said mono- lithic substrate and said first active layer with a perovskitic structure.

According to the invention, said solid macroporous structure of said ceramic or metal monolithic substrate, has pores with an average hydraulic diameter greater than 0.2 mm and with channels having a linear and parallel, or convoluted form, said average hydraulic diameter of the pores preferably being less than 1 mm.

Furthermore, according to the invention, said ce- ramic or metallic monolithic substrate consists of a non- active material in the reactions of interest and with a

low thermal expansion coefficient, and it preferably con- sists of a material which can resist temperatures close to 1, 000°C, chemically and physically stable, with no oxidation phenomena, or separation, volatility or phase transition at said temperatures.

According to the present invention, said ceramic monolith substrate can consist of an oxide, or a combina- tion of oxides, preferably an oxide selected from A1203 (in the form of a-Al203), SiC, Si02, ZrO2, Y203, CaO, MgO and combinations thereof, in the form of ceramic foam or a ceramic honeycomb monolith.

As an alternative, again according to the invention, said metallic monolith substrate consists of an alloy of metals resistant to high temperature oxidation, prefera- bly a metallic alloy selected from FeCrAlY, Nickel Cr, Nichrome, Hastelloy X, Inconel 600-625, produced in the form of metallic foam, or it consists of a metallic sheet and a corrugated sheet.

Still according to the invention, said first active layer having a crystalline perovskitic structure, can consist of an oxide with a high thermal stability at tem- peratures close to 1, 000°C, a moderate oxidation activ- ity, a good oxidation activity with respect to CO and also has the characteristic of being an atomic oxygen tank.

Furthermore, according to the present invention, in the general formula Ax A'1_x By B'1y O3+st A is a cation of at least one element selected from La, Ce, Pr, Nd, Sm, En, Dy, Ho and Ef ; A'is a cation of at least one element selected from Na, K, Ca, Sr, Ba, Rb ; B is a cation of at least one element selected from Ti, Cr, Fe, Ru, Co, Rh, Ni, Mn, preferably selected from Cr, Fe, Co, Mn, Ni and Ru, and B'is a cation of at least one element selected from Ti, Cr, Fe, Ru, Co, Rh, Ni, Mn, Pt, Nb, Ta, Mo, W, preferably selected from Cr, Fe, Co, Mn, Ni, Mo, W and Ru.

According to the invention, said first active layer with a crystalline perovskitic structure preferably con-- sists of LaMn03.

According to the present invention, said first ac- tive layer with a crystalline perovskitic structure is present in quantities of between 5 and 50% by weight, with respect to the sum of said first active layer and said possible supporting layer, in the presence of said possible supporting layer.

Said more external second active layer, according to the present invention, consists of a dispersion of a no- ble metal selected from Pt, Pd, Rh, Ir, Re, Au or a mix- ture thereof, preferably a dispersion of Pt, said more external second active layer being present in quantities

ranging between 0.1 and 25%, preferably 0.1 and 15%, more preferably 1 and 10%, of the total weight of the cata- lyst, excluding the monolith substrate.

Again according to the invention, said possible sup- porting layer with a large surface area interposed be- tween said monolith substrate and said first active layer with a perovskitic structure, consists of a material hav- ing a good chemical affinity with the material forming said ceramic or metallic monolithic substrate, preferably a non-active oxide in the reactions of interest, with a large surface area, ranging from 75 to 200 m2/g, a low thermal expansion coefficient, compatible with that of said monolithic substrate.

Furthermore, still according to the present inven- tion, said possible supporting layer with a large surface area, interposed between said monolithic layer and said first active layer with a perovskitic structure, consists of MgO, MgAl204, y-Al203 stabilized or not by La203 or an- other stabilizer, or by ZrO2 stabilized or not by La203 or another stabilizer, or by mixtures thereof, said stabili- zation being preferably obtained by means of an amount of La203 or other stabilizer, ranging from 5 to 10% by weight with respect to the total of said possible sup- porting layer having a large surface area.

Again according to the invention, the thickness of

said possible supporting layer with a large surface area, interposed between said monolithic layer and said first active layer with a perovskitic structure, varies from a minimum of 5 pm to a maximum of 50% of the pore average radius of said substrate.

A second specific object of the present invention relates to a process for the preparation of a multilayer catalyst, as defined above, comprising the following phases in succession: -deposition of said first active layer with a perov- skitic crystalline structure on said ceramic or metallic monolithic substrate or on said possible supporting layer having a large surface area, - deposition of said second more external active layer consisting of a dispersion of a noble metal on said first active layer with a crystalline perovskitic structure, - calcination in air at temperatures ranging from 700 to 1000°C.

According to the invention, said deposition of said first active layer having a crystalline perovskitic structure on said ceramic or metallic monolithic sub- strate or on said possible supporting layer a with large surface area, preferably takes place through the follow- ing sub-phases: impregnation with a water solution containing solu-

ble compounds of the cations A, A', B and B'in the pro- portions corresponding to those of the desired final for- mulation, - air calcination at temperatures of between 800 and 1000°C, for a time ranging from 2 hrs to 10 hrs.

Even more preferably, according to the present in- vention, said impregnation in said water solution is ef- fected by means of a"wet impregnation"or"dry impregna- tion"process.

Alternatively, again according to the present inven- tion, said deposition of said first active layer having a crystalline perovskitic structure on said ceramic or me- tallic monolithic substrate or on said possible support- ing layer with a large surface area, is effected through the following sub-phases: - deposition-precipitation in situ starting from the soluble compounds of the cations A, At, B and B'in a wa- ter solution through the slow decomposition of urea at 90°C ; - washing ; -drying ; - air calcination at temperatures of between 800 and 1000°C.

In a further alternative embodiment, according to the present invention, said deposition of said first ac-

tive layer with a crystalline perovskitic structure on said ceramic or metallic monolithic substrate or on said possible supporting layer having a large surface area, is effected by means of a process comprising the sol/gel method or by means of the citrates method.

Still according to the present invention, said depo- sition of said second more external active layer on said first active layer with a crystalline perovskitic struc- ture, can be effected through the following sub-phases: - dry or wet impregnation in a salt solution of said noble metal in proportions corresponding to those of the desired final formulation, air calcination at temperatures of between 600 and 800°C, for a time ranging from 2 hrs to 10 hrs, said im- pregnation in said salt solution of said noble metal be- ing preferably effected through a"wet"or"dry"process.

The process for the preparation of a multilayer catalyst according to the present invention can option- ally include the following additional phases, which pre- cede the deposition phase of said first active layer with a crystalline perovskitic structure: -deposition of a supporting layer with a large sur- face area on said ceramic or metallic monolithic sub- strate, air calcination for about 3 hrs at high tempera-

tures, for example of 800°C.

According to the present invention, said deposition of a supporting layer with a large surface area on a ce- ramic or metallic monolithic substrate preferably in- cludes the following sub-phases, cyclically repeated un- til the desired thickness is reached: - immersion of said substrate in a water solution containing a finely ground powder of supporting material, - elimination from the substrate of the excess solu- tion, said water solution containing a finely ground pow- der of supporting material in which said monolithic sub- strate is immersed, which can further contain a stabi- lizer precursor, a ligand in a weight percentage with re- spect to the total of up to 30% of said supporting mate- rial and nitric acid in a weight percentage of between 10 and 30% of said ligand.

Furthermore, again according to the present inven- tion, said elimination from the substrate of the excess solution is obtained through removal by means of com- pressed air and subsequent air calcination, for example at temperatures close to 550°C, or by flash evaporation of the water, for example by placing the impregnated monolith in an oven, for a few minutes, at a temperature higher than the water evaporation temperature (for exam- ple, between 200 and 500°C).

A third specific object of the present invention re- lates to the use of the multilayer catalyst as defined above in partial oxidation processes of alkanes in gase- ous phase.

A fourth specific object of the present invention also relates to the use of the multilayer catalyst as de- fined above in production processes of olefins starting from a gaseous stream of hydrocarbons, vaporizable within a temperature range of between 25°C and 400°C and pres- sures ranging from 0.1 to 5 atm, and oxygen.

Finally, a fifth specific object of the present in- vention relates to a partial oxidation process, for exam- ple for the production of olefins, starting from a gase- ous stream of hydrocarbons, vaporizable within a tempera- ture range of between 25°C and 400°C and pressures rang- ing from 0.1 to 5 atm, and oxygen, wherein said gaseous stream passes through a catalytic bed consisting of par- ticles of multilayer catalyst, as defined above, said hy- drocarbon comprising alkanes within the range of C2-C20, said gaseous stream also containing a quantity of oxygen which is such that the ratio between hydrocarbons and oxygen is within the range of 0.8 to 3 and in any case is always higher than the upper flammability limit of the mixture and a quantity of nitrogen ranging from 0 to 50% with respect to the total, said process being carried out

at a pressure ranging from 0.1 to 5 MPa, a temperature ranging from 600 to 1200°C, with a space velocity ranging from 1-107 to 1-103 hr~1.

In the partial oxidation process according to the present invention, said alkanes preferably comprise at least one product selected from ethane, propane, butane, or their mixtures.

Even more preferably, still according to the present invention, said ratio between hydrocarbons and oxygen is within the range of 1.25 and 3, said pressure ranges from 101 to 3,000 kPa, said temperature ranges from 800 to 1000°C and said space velocity ranges from 3-106 to 3-104 ho-'.

Furthermore, according to the present invention, said gaseous stream also comprises a quantity of hydro- gen, freshly fed or recycled following the product sepa- ration, so that the ratio between hydrogen and oxygen is within the range of 0 and 3, and a quantity of CO, freshly fed or recycled following the product separation, so that the ratio between CO and oxygen is within the range of 0 and 3.

According to another preferred aspect of the present invention, said gaseous hydrocarbons stream comprises methane in an amount not higher than 20% by weight, pref- erably not higher than 5% by weight, of the total weight

of the hydrocarbons. More preferably methane is substan- tially absent.

Finally, according to the invention, said gaseous stream of vaporizable hydrocarbons can be preheated to between 25 and 400°C and before and after said catalytic bed there is preferably a layer of ceramic monoliths.

The details of the invention are described hereunder for illustrative but non-limiting purposes, with particu- lar reference to various figures, in which: - Figure 1 shows the conversion of ethane in rela- tion to the ethane : oxygen ratio for a catalyst based on Pt/LaMnO3 supported on y-Al203 and deposited on a 45 ppi (pores per linear inch) foam, compared with a catalyst based on Pt and Pt/Sn on the same ceramic substrate under the same experimental conditions, for a total flow-rate equal to 2-105 hr~l, with a dilution of N2 equal to 30% vol ; - Figure 2 shows the selectivity per carbon atom to ethylene in relation to the ethane : oxygen ratio for a catalyst based on Pt/LaMnO3 supported on y-Al203 and de- posited on a 45 ppi (pores per linear inch) foam, com- pared with a catalyst based on Pt and Pt/Sn on the same ceramic substrate under the same experimental conditions specified in Figure 1; - Figure 3 shows the yield to ethylene in relation

to the ethane : oxygen ratio for a catalyst based on Pt/LaMnO3 supported on y-Al203 and deposited on a 45 ppi (pores per linear inch) foam, compared with a catalyst based on Pt and Pt/Sn on the same ceramic substrate under the same experimental conditions specified in Figure 1; Figure 4 shows the selectivity per carbon atom to CO in relation to the ethane : oxygen ratio for a catalyst based on Pt/LaMnO3 supported on y-Al203 and deposited on a 45 ppi (pores per linear inch) foam, compared with a catalyst based on Pt and Pt/Sn on the same ceramic sub- strate under the same experimental conditions specified in Figure 1 ; Figure 5 shows the selectivity per carbon atom to COs in relation to the ethane : oxygen ratio for a cata- lyst based on Pt/LaMnO3 supported on y-Al203 and deposited on a 45 ppi (pores per linear inch) foam, compared with a catalyst based on Pt and Pt/Sn on the same ceramic sub- strate under the same experimental conditions specified in Figure 1; - Figure 6 shows the selectivity per hydrogen atom to H20 in relation to the ethane : oxygen ratio for a catalyst based on Pt/LaMnO3 supported on y-Al203 and de- posited on a 45 ppi (pores per linear inch) foam, com- pared with a catalyst based on Pt and Pt/Sn on the same ceramic substrate under the same experimental conditions

specified in Figure 1; - Figure 7 shows the selectivity per hydrogen atom to H2 in relation to the ethane : oxygen ratio for a catalyst based on Pt/LaMnO3 supported on y-Al203 and de- posited on a 45 ppi (pores per linear inch) foam, com- pared with a catalyst based on Pt and Pt/Sn on the same ceramic substrate under the same experimental conditions specified in Figure 1; -Figure 8 shows the conversion of ethane in rela- tion to the hydrogen : oxygen ratio for a catalyst based on Pt/LaMnO3 supported on y-Al203 and deposited on a 45 ppi (pores per linear inch) foam, compared with a cata- lyst based on Pt and Pt/Sn on the same ceramic substrate under the same experimental conditions for a flow-rate in the absence of H2 equal to 2-105 hr-1, and a C2H6 : 02 : N2 ra- tio equal to 46.7 : 23.3 : 30 ; Figure 9 shows the selectivity per carbon atom to ethylene in relation to the hydrogen : oxygen ratio for a catalyst based on Pt/LaMnO3 supported on y-Al203 and de- posited on a 45 ppi (pores per linear inch) foam, com- pared with a catalyst based on Pt and Pt/Sn on the same ceramic substrate under the same experimental conditions specified in Figure 8; - Figure 10 shows the yield to ethylene in relation to the hydrogen : oxygen ratio for a catalyst based on

Pt/LaMnO3 supported on y-Al203 and deposited on a 45 ppi (pores per linear inch) foam, compared with a catalyst based on Pt and Pt/Sn on the same ceramic substrate under the same experimental conditions specified in Figure 8 ; - Figure 11 shows the selectivity per carbon atom to CH4 in relation to the hydrogen : oxygen ratio for a catalyst based on Pt/LaMnO3 supported on y-Al203 and de- posited on a 45 ppi (pores per linear inch) foam, com- pared with a catalyst based on Pt and Pt/Sn on the same ceramic substrate under the same experimental conditions specified in Figure 8 ; - Figure 12 shows the selectivity per carbon atom to CO in relation to the hydrogen : oxygen ratio for a cata- lyst based on Pt/LaMnO3 supported on y-Al203 and deposited on a 45 ppi (pores per linear inch) foam, compared with a catalyst based on Pt and Pt/Sn on the same ceramic sub- strate under the same experimental conditions specified in Figure 8; - Figure 13 shows the selectivity per carbon atom to C02 in relation to the hydrogen : oxygen ratio for a catalyst based on Pt/LaMnO3 supported on y-Al203 and de- posited on a 45 ppi (pores per linear inch) foam, com- pared with a catalyst based on Pt and Pt/Sn on the same ceramic substrate under the same experimental conditions specified in Figure 8; and

- Figure 14 shows the moles of H2 produced per mole of Os fed in relation to the moles of H2 fed per mole of 02 fed for a catalyst based on Pt/LaMnO3 supported on y- A1203 and deposited on a 45 ppi (pores per linear inch) foam, compared with a catalyst based on Pt and Pt/Sn on the same ceramic substrate under the same experimental conditions specified in Figure 8.

A detailed description of the invention is provided hereunder with the help of an example which forms an il- lustrative and non-limiting aspect of the scope of the present invention.

In this example, the preparation is described of a catalyst according to the present invention, together with the measurement procedures and catalytic activity results of a catalyst consisting of platinum dispersed on an oxide of the perovskitic type having the formula LaMnO3, deposited on a layer of y-Al203 and supported by a ceramic substrate in the form of a foam, in the oxidative dehydrogenation process of ethane for the production of ethylene.

Preparation of the catalyst A 45 ppi (pores per linear inch) ceramic foam, cor- responding to an average pore diameter of 0.42 mm and a vacuum degree of about 80%, was used as substrate. Said ceramic substrate consists of 92% of a-Al203 and the re-

maining percentage of mullite, with a disk-shaped con- figuration having a diameter of 17 mm and a thickness of 10 mm.

Said substrate was coated with a layer of y-Al203 stabilized with lanthanum oxide by means of the following procedure. A slurry was prepared, containing 5% by weight of y-Al203 in the form of particles having a micronic di- mension in water. Lanthanum nitrate was added to this slurry in a quantity corresponding to 12% by weight of the total solids (y-Al203 and lanthanum nitrate), as pre- cursor of a quantity of lanthanum oxide La203 correspond- ing to 5% by weight with respect to the total of y-Al203 and lanthanum oxide.

The ceramic substrate was immersed in this slurry for a few minutes, until the pores were filled, as can be deduced from the lack of air release in the form of bub- bles from the pores, and was then placed in an oven at 300°C for a few minutes, in order to obtain the flash evaporation of the water. This procedure was repeated un- til a charge of stabilized y-Al203 was obtained, equal to 3% by weight of the whole monolith, corresponding to a thickness layer in each pore of about 10 pm. Finally, the whole mixture was calcined at 800°C for 3 hours.

The perovskitic LaMnO3 was deposited on the support by the dry impregnation of the precursor salts, in this

specific case with an equimolar solution of lanthanum ni- trate and manganese acetate, in each salt. The monolith was left to dry in air and calcined at 800"C.

This procedure was repeated until 30% of LaMnO3 was reached, with respect to the total weight of the support (y-Al2O3 and LaMn03), followed by a final calcination at 1000°C.

The platinum was dispersed on the perovskite by the dry impregnation of a diluted solution, platinum hexa chloride (H2PtCl6) and then calcined in air at 800°C, until a charge of about 10% by weight was ob- tained, with respect to the total weight excluding the substrate (Pt + LaMnO3 + y-Al203).

Experimental plant The catalytic activity measurement in the oxidative dehydrogenation of ethane at high flow-rates was effected on the catalyst thus prepared. In order to reduce the temperature drops by irradiation, two inert monoliths were positioned upstream and downstream of the catalyst, and the whole mixture was sealed with a woolen silica- alumina cloth in a quartz tube. The outer part of the quartz tube close to the reaction zone was then isolated with fiberglass.

The flow of gas to the reactor was controlled by means of mass flow controllers and was regulated at 5

slpm, corresponding to a space velocity of 2-105 h-1, at atmospheric pressure and room temperature. The ethane : oxygen ratio was varied from 1.5 to 2 for a fixed nitro- gen dilution equal to 30%, for the calibration of the gas chromatograph. The effect of the addition of hydrogen to the feeding for an ethane : oxygen ratio equal to 2, was also analyzed.

The pressure of the reactor was 1.3 atm in all the tests effected, whereas the reaction temperature remained at about 1000°C, and the contact time varied from 0.2 to 40 ms.

The gaseous products were fed to a gas chromatograph in line with the plant, through heated steel tubes. All the species were analyzed except for the H20, calculated by a balance with respect to the atomic oxygen.

The selectivity data presented in the figures were calculated on the basis of the carbon or hydrogen atoms.

The change in the number of moles due to the chemical re- actions was kept in consideration using the measurement of the N2 concentration. As nitrogen is inert, the ratio between the number of moles of the products and the con- centration of the same products at the outlet, is di- rectly proportional to the ratio between the number of moles of N2 fed and the concentration of N2 at the out- let.

All the experimental measurements were reproducible within a range of 2%.

The temperatures of the system were monitored and registered according to the variation in the feeding con- ditions. The reactor operated under stationary conditions and heat was supplied only for the purpose of obtaining ignition. After ignition, the heat source was removed and stationary conditions were reached over a certain time lapse (about 15 minutes). The quenching was effected by eliminating the oxygen flow followed by that of ethane.

In addition to the products indicated in the fig- ures, traces of acetylene, propane, propene, butane and butene were found in the gaseous stream at the outlet.

All the results were repeatable over numerous hours of reaction.

Experimental results Ethane : Oxygen Figures 1,2 and 3 respectively indicate the ethane conversion, the selectivity and yield to ethylene with a variation in the ethane : oxygen ratio in the feeding for a catalyst based on Pt/LaMnO3 supported on y-Al203 of the type object of the present patent, compared with data provided in literature (Yokoyama C. , et al. , Catal. Lett.

380181 (1996) ) for catalysts based on Pt and Pt/Sn on the same ceramic substrates and under the same experimental

conditions (flow-rate of 5 slpm).

It can generally be said that with a decrease in the quantity of oxygen in the feeding, the ethane conversion decreases, whereas the selectivity to ethylene shows an increasing trend. It can also be seen that in terms of ethylene yield, the catalyst, object of the present pat- ent, has performances comparable with those of the cata- lyst based on Pt/Sn, but it does not have the same deac- tivation problems due to the volatility of tin, and much higher performances with respect to Pt.

Figures 4 and 5 respectively indicate the selectivi- ties to CO and C02 with a variation in the ethane : oxy- gen ratio. Higher quantities of CO and lesser quantities of COs are produced on Pt, whereas the addition of Sn or the dispersion of Pt on the perovskite reduces the forma- tion of CO in favour of C02 with a corresponding decrease in the formation of H2O, as shown in Figure 6.

It appears that the greater tendency to oxidize the hydrogen instead of the oxygen, assumed in the work of Bodke et al (J. Catal. 191: 62 (2000), also extends to the case of Pt dispersed by the perovskite. Figure 7 in fact shows this catalyst and smaller quantities of H2 are formed on Pt/Sn with respect to those formed on Pt.

Ethane : Oxygen : Hydrogen The experimental data relating to the oxidative de-

hydrogenation of ethane in mixtures containing hydrogen on supported Pt/LaMnO3, are compared with the experimen- tal data of Bodke et al. (J. Catal. 191: 62 (2000), for Pt and Pt/Sn. The progressive addition of hydrogen causes a decrease in the ethane conversion, extremely significant on Pt, and less dramatic on Pt/LaMnO3 supported on y-Al203 and on Pt/Sn, as shown in Figure 8. In correspondence with this decrease, the selectivity to ethylene increases with an increase in the quantity of hydrogen present in the feeding, and this increase is less in the case of the catalyst based on Pt (Figure 9). The yields to ethylene are indicated in Figure 10 and show an optimum for hydro- gen : oxygen ratios ranging from 1 to 2.

Figure 11 shows that higher quantities of CH4 are formed on Pt with respect to the other two catalysts. In figures 12 and 13, it can also be seen that higher quan- tities of CO and CO2 are formed on Pt, whereas on Pt/Sn and on Pt/LaMnO3 supported on y-Al203, the selectivities to these species are substantially analogous.

Figure 14 indicates the moles of H2 formed per moles of H2 fed. Only in the case of the catalyst proposed in this patent is the same quantity of H2 obtained as that fed, and consequently the possibility of recycling H2 to the reactor seems feasible.

The catalyst presented in this example, based on

Pt/LaMnO3 supported on y-Al203 and deposited on a ceramic monolithic catalyst, did not show signs of deactivation even after numerous hours of reaction. The dispersion of Pt in very small particles on the perovskite does in fact seem to be effective in preventing cohesion phenomena be- tween the particles themselves. The catalyst based on Pt/Sn, on the other hand, after a few operating hours, begins to show signs of deactivation due to the volatil- ity of the tin.

The present invention has been described for purely illustrative and non-limiting purposes, according to its preferred embodiments. Variations and/or modifications, however, can be applied by experts in the field, all in- cluded in the relative protection scope, as defined by the enclosed claims.