The present invention relates to a catalytically active si 1 ica-alumina-nickel oxide gel, the process for its preparation and its use in processes of dimerization of isobutene into alpha- and beta-di i sobutene and ol i gomeri zati on of propylene into its relative dimers and trimers .

The conventional processes as used in the dimerization of isobutene are generally based on the use of an acidic catalyst, such as sulphuric acid, polyphosphoric acid (in liquid form or supported by an inorganic solid); heteropolyacids i .e. phosphomolybdic and si 1 icotungstic acid; ion exchange resins of the polystyrenesul phoni c or f1 uorosul phoni c type; boron trifluoride complexed with alcohols; organic acids, esters, ethers, ketones; aluminium trichloride associated to ethyl ether; hydrochloric acid or ni tromethane; simple or complex salts, especially nickel salts activated with an aluminium alkyl , or an aluminium alkyl halide; and metal oxides, as bismuth oxide optionally associated with a phosphor oxide.

Catalysts capable of ol i gomeri zing olefins such as polypropi lene and butenes are, again, al umi nosi 1 icates and zeolites optionally modified with salts or oxides of metals such as nickel, chromium and cobalt, as for instance described in US-A- 3 518 323 and US-A-3 525 456, in EP-A-132 172, EP-A-224 220, EP-A-133 052. Finally, in US-A-3 960 978, US-A-4 150 062, as well as in EP-A-311.675 processes are described for the production of gasoline by means of olefin ol igomerizati on with ZMS-5 type zeolites.

The gasolines so obtained also contain a fraction of aromatic hydrocarbons, mainly benzene, that which is undesirable due to its toxicity towards human beings and the environment. Many of the conventional ol igomerization catalysts have drawbacks due to the corrosive action of the acids that are used, to the requirement of disposing of the exhausted catalysts, their limited life and/or the cost of their preparation. A problem which is common to these conventional catalysts lies in their poor selectivity to the desired reaction products. For instance, when used in the dimerization of isobutene, they cause the formation of relevant amounts of higher oligomers and isomers other than the most appreciable ones, that is, alpha- and beta-diisobutene.

Finally whenever an isobutene stream admixed with other C4 olefins undergoes dimerization, codimers are suaully and concurrently produced.

This is believed to be due to the comparatively high temperatures at which the conventional catalysts unfold their activity, or the exceedingly high acidity of the catalysts themselves, which encourage molecular rearrangements or other undesirable side-reactions. EP-A-340.868 discloses an amorphous and microporous sil ca-alumina gel which is active in processes of isomerfztion, alkylation, dewaxing and dimerization of linear olefins.

According to the present invention it has now been ascertained that the introduction of a limited amount of nickel oxide in a gel of the type suggested in the aforementioned EP-A 340 868 makes it possible to obtain a catalyst having an even and controlled porosity, essentially free from any isomerizing capability and capable of dimerizing isobutene into alpa- and beta-diisobutene with a surprisingly high selectivity. It is therefore possible to obtain high quality products, useful as chemical intermediate or in the preparation of high-octane gasoline. It has furthermore been ascertained that such a catalyst is active in the ol i gomeri zati on of propylene into dimers and trimers, which are useful , as such or after etherification, as high-octane gasolines. According to the foregoing, a first aspect the present invention relates a silica-alumina-nickel oxide gel, amorphous to X-rays, having an Si 0 /A1_0_ molar ratio of from 30/1 to 500/1, an NiO/SiO _? molar ratio of from

2 0,001/1 to 0,01/1, a surface area from 500 m /g to 1.000 /g, a mean pore diameter of about 1 nm (10 A)

(Angstrom), and a porosity from 0,3 ml/g to 0,6 ml/g, devoid of pores having a diameter over 3 nm (30 A).

In the preferred embodiment the silica-alumina-nickel oxide gel of this invention has a molar ratio Si0_/Al„0, of 100/1, a molar ratio NiO/SiO of 0,02/1, a surface

2 area of 800 m /g and a porosity of 0,4-0,5 ml/g.

According to another aspect, the present invention relates to a process for preparing a silica-alumina-nickel oxide gel, having the above mentioned features, which comprises the steps of: _ς (i) preparing an aqueous solution of a water soluble aluminium compound susceptible of being hydrolyzed to A1.0- and of a tetralkyl ammoπiurn hydroxide, wherein the alkyl is selected from ethyl ,n.propyl and n.butyl; _Q ( i) preparing an aqueous solution of a water soluble nickel carboxylate and a water soluble organic amine;

(iii) mixing the two (i) and (ii) solutions and adding thereto a water soluble silicon compound susceptible of being hydrolyzed to SiO , to obtain a homogeneous gel, and

(iv) drying the gel and calcining it initially in an inert atmosphere and finally in an oxidizing atmosphere. The aluminium compounds used are preferably aluminium trialkoxides i.e. aluminium tri -n. -propoxide and aluminium tri-isopropoxide.

The nickel carboxylate is preferably a nickel salt of an aliphatic carboxylic acid and especially nickel acetate. Silicon compounds are preferably the tetraalkyl silicates, such as tetraethylsilicate. These reactants shall profitably be employed in such a mutual ratio as to obtain the composition which is desired for the silica-alumina-nickel oxide gel, on bearing in mind that the reaction yield is virtually quantitati e.

The organic amine is preferably an aliphatic aππ ^' ne, especially ethylenediamine. Advisably, an amount of the amine is employed which is such as to have a molar ratio of the amine to the nickel carboxylate of about 2/1. The preparation of the solutions and their admixture are conveniently carried out at room temperature, or at a temperature close to room temperature. The hydrolysis of step (iii) is carried out at a temperature of from 50°C to 70°C, and preferably of 60 ^C C. Under such conditions, the time required for complete gelling varies from 15 min to 5 hours, as a function of the preselected temperature, and in the preferred embodiment it is of from 25 min to 60 min. The drying step (iv) is conveniently carried out at a temperature below 150°C and preferably of from 90°C to 100°C, for a time sufficient to remove water completely or so. The calcination of step (iv) is carried out in an atmosphere which is initially inert (such as nitrogen) and then oxidizing (such as air) at a temperature of from 500°C to 700°C, preferred being of from 550°C to 600°C, for times of from 4 hours to 20 hours, as a function of the preselected temperature, and which, in the preferred embodiment are of from 6 hours to 16 hours. The silica-alumina-nickel oxide gel thus obtained can be granulated into particles of the desired size, or can be supported by an appropriate inert solid subtrate material, or it can also be admixed with an inert solid material. More particularly, the gel can be admixed with appropriate metal oxides, which essentially act as binders.

Oxides which are suitable to this purpose are aluminas, silicas and the oxides of titanium, magnesium and zirconium. The gel and the binder can be blended in weight ratios of from 50:50 to 95:5, preferred being from 30:90 to 90:10. The two components can be blended with the conventional means and the blend is conveniently compacted in the desired final shape, such as in the forms of extrudates and granulates. By so doing, it is possible to impart improved mechanical properties to the catalyst.

According to a further embodiment, the present invention relates to an olefin ol gomerization process which uses as the catalyst the sil ca-alumina-nickel oxide gel referred to above. By ol igomerization, to the ends of the present disclosure, the conversion of isobutene is intended into mixtures which essentially consist of alpha- and beta-dimers of diisobutene and by oligomerization the conversion of propylene into mixtures which are essentially comprised of propylene dimers and trimers.

The ol igomerization reaction can be carried out as a continuous or a semicontinuous process or as a batch-process, by working in liquid, gaseous or mixed form (liquid-vapour) When working di scontinuousl , the conveniently employed catalyst amount is from 1 % to 50% relative to the olefin, at a temperature of from 50°C to 150°C, preferred being from 55°C to 80°C, under a pressure of from 1 ata to 200 ata, preferred being from 1 ata to 6 ata.

For the semiconti nuous or continuous runs, it is convenient to work at a temperature of from 50°C to 200°C, preferred being from 50°C to 150°C, at a spatial velocity (WHSV) for the olefin of from 0,5 h ^" to 8h~ . At any rate, it is possible to use pure olefins or olefins admixed with other olefins and/or paraffins. The ol i gomerization reaction is exothermic, so that the reactor temperature should be checked to prevent that an exceedingly high rise may be conducive to the formation of higher oligomers and/or undesirable i someri zation phenomena.

As outlined above, the olefins to be ol i gomeri zed according to the process of this invention are isobutene and propylene. More particual rly, the ol i gomeri zati on of isobutene selectively proceeds towards the formation of the two isomers of the 2,4,4-trimethyl -1 -pentene (al pha-diisobutene) and 2 ,4,4-trimethyl -2-pentene (beta-diisobutene) dimer.

These dimers can be used as chemical intermediates for preparing non-ionic surfactants, cross-linking agents for elastomers, pi asticizers , and other chemicals, by alkylation, amination or carbonyl ation reactions. In addition, the alpha- and the beta-diisobutene are useful when preparing high-octane gasolines since they exhibit RON and MON values (leadless): R0N=106 and MON-86,5 (alpha isomer) and R0N=103,5 and M0N=86,2 (beta isomer) and can be hydrogenated to isooctane (2,2,4-trimethylpentane) having R0N=100 and M0N=100.

When ol igomerizing propylene, one predominantly obtains an oligomer mixture which prevailingly contains a high-octane gasoline fraction (boiling point from C at 175°C) together with a small amount of a hydrocarbon fraction having a higher molecular weight (boiling point from 175° to 370°C).

The ensuing experimental examples are reported to better illustrate the invention.

EXAMPLE 1 Preparation of the catalyst.

(A) Solution: 2 g of aluminium isopropylate are dissolved, at room temperature, in 68,5 g of a 13,35% by weight-aqueous solution of tetrapropyl am oπi urn hydroxide (TPA-OH).

(B) Solution:

1,85 g of nickel acetate are dissolved in 75,6 of demi neral i zed water and 1,25 g of ethylenediamine (EDA) are added thereto.

The solutions (A) and (B) are combined, and the mixture is heated to 60°C: to the heated mixture there are added 104,1 g of tetraethylsilicate (TES). The resultant mixture has the following molar ratios: Si0 ₂ /A1.0 ₃ = 102

TPA-OH/SiO- = 0,09

NiO/SiO = 0,021

EDA/NiO = 2 This mixture is kept stirred at 60°C for 40 min, until obtaining a homogeneous gel, which is dried in an air stream at 90°C and then calcined at 550°C, first in a nitrogen stream for 3 hours, and then in an air stream for 10 hours. A silica-alumina-nickel oxide gel is obtained, with a quantitative yield relative to the initially charged materials, said gel being granulated into lmm-2mm particles.

The catalyst so prepared has the following specifications :

-X-ray amorphous (powder analysis by a Philips vertical goniometer, using the Cu a radiation; -SiO /Al 0 molar ratio = 102;

-NiO/SiO. molar ratio = 0,021;

2 -Superficial area = 800 m /g

(measured with the B.E.T.method in a Carlo Erba

Sorptomatic 1880 apparatus); -Porosity = 0,4 ml/g

(mean pore diameter 1 nm (10 Angstrom), and absence of pores having a diameter over 3 nm

(30 Angstrom): (values determined by the Carlo

Erba Sorptomatic 1880 apparatus). EXAMPLE 2

A glass vial, which has been flame-sealed in a nitrogen atmosphere and containing 2,13 g of the catalyst prepared according to Example 1 and the finely ground in a mortar and heated to 550°C for 6 hours, is introduced into a 200-ml autoclave. The autoclave is evacuated, charged at a nitrogen pressure of 1,01325 bar (1 ata) and 9,4 g of isobutene are condensed in the cold therein.

The reaction of isobutene dimerization is started by rupturing the catalyst-containing vial by the mechanical stirrer with which the autocalve is equipped.

Dimerization is a strongly exothermic reaction and the actual reaction temperature is monitorized by a thermocouple inserted into the autoclave interior.

By varying the temperature of the bath, is is possible to check the temperature oscillations within ±2°C. During the entire reaction run, the pressure within the autoclave is essentially due to the vapour pressure of the unreacted isobutene. Consequently, while the initial pressure was about 6,195 bar (6 ata), correspoding to the vapour pressure of the isobutene in equilibrium with its liquid phase, as dimerization proceeds, the pressure decreases due to the disappearance of the isobutene liquid phase, followed by the gradual disappearance of the relevant vapour phase.

The reaction mixture is sampled at different times from the autoclave, by a dip tube, and the samples are condensed in test tubes fitted with an appendix for the subsequent analyses. During the enti e reaction run a vigorous mechanical stirring is maintained to ensure both a satisfactory contact between the liquid and the solid (catalyst) phases, and a good homogeneousness of the mixture during sampling. The values of the conversion and the quantitative analysis of the mixed products corresponding to the several samplings is effected gaschromatographi cal l , with a wide-bore-column Hewlett Packard chromatographe , containing an apolar stationary phase RSL300, at a temperature which is programmed for 30°C to 280°C.

The quantitative determination of the several isomers of the isobutene dimer, that is, 2,2,4-trimethyl -1 -pentene (alpha diisobutene) and 2,4,4-trimethyl -2-pentene (beta diisobutene) is carried out gaschromatographical ly again, by using a capillary column with stationary apolar phase SPB1 which permits the separation of the isomers, whereas their structural identification is carried out on a mixture which contains 96% of the dimer, and is obtained by fractional distillation of the first fraction sampled during the reaction, by means of a mass spectrometer matched to a capillary-column gaschromatographer, on the basis of the and ¹³ C NMR. The results are tabulated on Table 1.

EXAMPLE 3 The procedure is similar to that of Example 2, with the same catalyst as in Example 2, which has been reactivated by heating to 550°C for 6 hours, the reaction temperature being the same as in Example 2, but with a catalyst weight percentage halved relative to that of Example 2. More particularly, the autoclave is charged with 2 g of activated catalyst and 18 g of isobutene. In this case, the starting temperature is 55°C, so that the thermal contribution afforded by the dimerization reaction brings about a rise to 60°C of the internal temperature of the autoclave (temperature of the oil bath) during the first 60 s, whereafter the reaction temperature is fairly stabilized. The results of this example are tabulated on Table 1.

T A B L E 1

EX. 2 EX.3

C12: after 2 min do. 6 min do. 15 min do. 60 min

C16 and higher: after 2 min do. 6 min do. 15 min do. 60 min

T A B L 1 (continued and concluded)

Dimer Composition: alpha 89

86 beta n

14

Others after 2 min traces traces do. 6 min 2 do 15 min 2 traces

EXAMPLE 4 A steel flux-reactor having an inside diameter of 12 mm, heated by an electric oven, is charged with 3,0 g (5 ml) of the catalyst prepared according to Example 1. The reactor is fed with propylene under the following working conditions:

-Inlet temperature 80°C

-Hot spot temperature 94°C -Pressure 30,8 bar (30 ata)

-Spatial velocity 2,87 h

Under the conditions reported above, the following results are obtained:

-Propylene conversion 79% -Hexenes 45% -Nonenes 40%

-Oligomers higher than C9 15% The fraction of the product which contains hydrocarbons having up to 9 carbon atoms is a high-grade gasoline exhibiting values of R0M=97 and M0N ^~ 84 (leadless). EXAMPLE 5

To the flux-reactor of Example 4, charged with the same catalyst and the same amounts of reactants, propylene is fed under the following working conditions:

-Inlet temperature 120°C -Hot spot temperature 135°C

-Pressure 30,8 bar (30 ata)

-Spatial velocity 4,59 h

Under the conditions reported above, the following results are obtained: -Propylene conversion 73%

-Hexenes 25%

-Nonenes 35%

-Oligomers higher than C9 40%

The fraction of the product which contains hydrocarbons having up to 9 carbon atoms is a high-grade gasoline exhibiting values of R0M=97 and M0N=84 (leadless).