More specifically, the present invention relates to a selective cracking process for the production of propylene starting from mixtures of hydrocarbons, prevalently ole- fins, the above hydrocarbons having a boiling point ranging from-15°C to +80°C, preferably from-12°C to +60°C.

A typical, example of these fractions are essentially C4-C6 fractions coming from steam cracking and catalytic cracking, having an olefin content of at least 40% by weight, usually at least 70% by weight.

Propylene is one of the most important chemical prod- ucts from the point of view of demand and production volume and is mainly used in the production of polymers. The main propylene source is the steam cracking process, in which hydrocarbon charges with a high paraffin content are ther-

mally treated in the presence of vapour. The main products of steam cracking are propylene and ethylene, which leave the process in a ratio of about 0.5. As the market request for propylene has become greater in the last few years with respect to that of ethylene and owing to the fact that the propylene/ethylene ratio cannot be significantly varied, it has become necessary to increase the production of propyl- ene using alternative methods. In fact, in 1999 there was a considerable deficit of propylene in Western Europe, with importations of this product amounting to 195,000 tons. The production of large quantities of propylene by means of processes which appropriately treat steam cracking by- products can allow variations in the overall propyl- ene/ethylene ratio, thus meeting market demands. The possi- bility of having flexible processes allowing a certain flexibility in the propylene/ethylene ratio would therefore provide great economic advantages.

An interesting possibility consists in a selective catalytic cracking process which converts C4-C5 fractions to propylene. The fractions can derive from steam cracking but it is also possible to extend the process to other similar streams coming for example from FCC (Fluid Cata- lytic Cracking). The charges can also derive from the above fractions after extraction and/or enrichment in olefins.

The use of solid acid catalysts, among which amorphous

silico-aluminas and in particular zeolites, in the cracking reaction of hydrocarbons, is known in literature (see for example J. Scherzer, Cata. Rev.-Sci. Eng., 31 (3), 215- 354,1989).

The most important application of these materials in cracking reactions, from an industrial point of view, is that called FCC (Fluid Catalytic Cracking) whose purpose, starting from heavy charges such as vacuum gas oils, is to produce lighter hydrocarbon cuts, particularly within the boiling range of gasolines. The catalysts currently used in this process are Y-type zeolites (IUPAC abbreviation: FAU) containing various additives.

A different type of cracking, owing to the charges used and type of products to be obtained, is called"selec- tive cracking". The purpose of"selective cracking"is to produce light olefins, such as ethylene and propylene, starting from C4-C6 hydrocarbon fractions and therefore al- ready light, if compared to a vacuum gas oil. The advantage of this process consists in transforming low quality hydro- carbon fractions, difficult to distribute on the market, to olefins having a higher added value.

Various zeolitic materials active in"selective crack- ing"reactions, are described in literature. For example, EP-A-109,059 and EP-A-109, 060 describe the use of ZSM-5 zeolite (IUPAC abbreviation: MFI) for selective cracking

reactions. These documents demonstrate that the best cata- lytic performances, referring to yields to propylene and ethylene, are obtained when the Si02/Al203 ratio of the zeo- lite is high. More specifically, EP-A-109,059 claims, for MFI-type zeolites (ZSM-5) Si02/Al203 ratios lower than or equal to 300 (mol/mol), preferably between 25 and 220, whereas EP-A-109,060 discloses Si02/Al203 ratios higher than or equal to 350 (mol/mol).

WO 99/57226 describes a method for converting hydro- carbon charges, with a boiling point within the naphtha range, to propylene in the presence of medium-pore zeolites having an SiO2/Al203 ratio greater than 200 (mol/mol). The above document provides two experimental examples: in the first, three medium-pore zeolites ZSM-48 (Si02/Al203 > 1500), ZSM-22 (Si02/Al203 > 1500) and ZSM-5 (Si02/Al203 = 55) are compared. It is shown that the selectivity to propylene for the first two catalysts is higher with respect to ZSM- 5. In the second example, two ZSM-22 zeolites having a dif- ferent Si02/Al203 ratio (>1500 and 120) are compared. It is shown that the one with the greater Si02/Al203 ratio has the higher selectivity to propylene.

Finally, WO 99/29805 describes a process for producing propylene starting from C4 and higher olefinic streams, in the presence of MFI zeolite (ZSM-5) having an Si02/Al203 ra- tio of at least 180 (mol/mol).

Experts in the field, however, still feel the neces- sity for using materials suitable for obtaining greater conversions and at the same time having a higher stability of the catalytic activity over a period of time. An ex- tremely important problem, in fact, which is not taken much into consideration in literature, consists in the poor sta- bility of the catalytic material over a period of time.

A process has now been found, which uses materials ca- pable of improving the yield to propylene and that also have the great advantage of maintaining the catalytic per- formances practically constant over a period of time.

In accordance with this, the present invention relates to a process for the production of propylene starting from mixtures of hydrocarbons, prevalently olefins, the above hydrocarbons having a boiling point ranging from-15°C to +80°C, preferably from-12°C to +60°C, which comprises put- ting the above mixture of hydrocarbons in contact, under cracking conditions, with a large-pore zeolite having a mo- lar ratio Silica/Alumina lower than 200, preferably ranging from 50 to 150.

The hydrocarbon mixtures essentially consist of hydro- carbons, both olefins and paraffins, having a boiling point ranging from-15°C to +80°C, preferably from-12°C to +60°C. Typical examples of hydrocarbons forming the above hydrocarbon mixtures are 1-butene, trans-2-butene, cis-2-

butene, n-butane, isobutane, propane, pentane, isopentane, 1-pentene, 2-pentene, n-hexane, 1-hexene, 2-hexene. The hy- drocarbon mixtures comprise from 30% to 100% by weight of olefins, preferably from 40% to 85% by weight. The paraf- fins contained in the hydrocarbon mixtures range from 5% to 65% by weight, preferably from 10% to 50% by weight, even more preferably from 20% to 45% by weight.

The term"cracking conditions"refers to a temperature at which the contact between the hydrocarbon mixtures and catalyst takes place, ranging from 400°C to 750°C, prefera- bly from 450°C to 700°C, even more preferably from 500°C to 650°C.

The process of the present invention is preferably carried out at a weight hourly space velocity (WHSV) rang- ing from 0.1 h-3-to 1,000 h-1, more preferably from 0. 5 h- to 100 h-1, even more preferably from 0.8 h-1 to 50 h-1.

The pressure in the contact zone between catalyst and hydrocarbon mixtures ranges from 0.1 to 30 absolute atm., preferably from 1 to 3 absolute atm., more preferably about 1 absolute atm.

The process of the present invention can be carried out using any reactor solution, for example, fixed bed, moving bed, a"riser"reactor or a fluid bed, preferably fixed bed.

The catalyst which can be used in the process of the

present invention is a large-pore zeolite having a molar ratio Silica/Alumina lower than 200, preferably ranging from 50 to 150. The term"large-pore zeolite"refers (see N. Y. Chen and T. F. Degnan, Chemcial Engineering Progress, February 1988,32-41) to a zeolite have a lattice consist- ing of 12 tetrahedrons. The above zeolite has a molar ratio Silica/Alumina lower than 200, preferably ranging from 50 to 150. In the preferred embodiment, the zeolite is ZSM-12 (IUPAC abbreviation: MTW), having a molar ratio Sil- ica/Alumina lower than 200, preferably ranging from 50 to 150. The preparation of this zeolite is well known to ex- perts in the field.

The zeolite can be used as such or mixed with inert products, in the form of granules or pellets.

Contrary to what is specified in scientific and patent literature, the ZSM-12 material has the best catalytic per- formances at Si02/Al203 ratios < 200 (mol/mol). The best catalytic performances refer to both yields to propylene and stability (duration) of the catalyst over a period of time.

The following examples are provided for a better un- derstanding of the present invention.

EXAMPLES The catalytic testing experiments were carried out in a continuous laboratory plant, with a fixed bed tubular re-

actor configuration. The reaction products were character- ized with a gaschromatograph model HP 5890 equipped with a "PONA"capillary column.

The synthetic mixture of C4 hydrocarbons indicated in Table 1 was used for the experimental tests. This mixture has a similar composition to the stream called"Refined III"deriving from steam cracking.

Table 1. Mixture used in the catalytic tests Hydrocarbon Feeding (weight %) 1-butene/ Trans-2-butene 52. 35 Cis-2-butene 24. 86 n-butane 22. 61 Iso-butane 0. 18 Sum of olefins 77. 21 The weight quantities of hydrogen, methane, ethane, ethylene, propane, propylene, n-butane, isobutane, 1-butene and isobutene, cis-2-butene, trans-2-butene, butadiene and a fraction of heavier products called C5+, were determined in the gaseous reaction products.

The catalyst was charged in a quantity varying from 2

to 10 g, in granules of 20-40 mesh or in pellets of 2-4 mm, mixed with corindone (inert product), in a weight ratio of 1: 1.

EXAMPLE 1. Synthesis of ZSM-12 (SiO2/Al203 = 100 mol/mol) 2.4 g of sodium aluminate with a content of A1203 equal to 56% are added to an aqueous solution of tet- ramethylammonium hydroxide at 35%. The solution thus ob- tained is poured, under stirring, into 200 g of colloidal silica Ludox HS 40.

A limpid, homogeneous gel is obtained, which is poured into an AISI316 steel autoclave, equipped with an anchor stirrer. The gel is left to crystallize under hydrothermal conditions at 165°C for 90 hours.

After cooling the autoclave, the solid obtained is separated from the mother liquor and washed with demineral- ized water until the washing water has a pH of less than 9.

The solid obtained is calcined at 550°C in a stream of air for 5 hours.

The solid thus obtained is subjected to ionic exchange by means of suspension in an aqueous solution of ammonium acetate. The ammonium ion is present in excess with respect to the nominal aluminum present in the solid. After filtra- tion and washing of the solid, the whole operation (ex- change and washing) is repeated.

The solid obtained is calcined at 550°C in a stream of

air for 5 hours.

The zeolitic solid is thus obtained in its acid form which, upon XRD analysis reveals the presence of the sole crystalline phase of the ZSM-12 type (MTW). Chemical analysis shows a content of residual sodium of less than 50 ppm and a molar ratio SiO2/Al203 = 100.

COMPARATIVE EXAMPLE 2. Synthesis of ZSM-12 (SiO2/Al2O3 = 250 mol/mol) 0.97 g of sodium aluminate with a content of A1203 equal to 56% are added to an aqueous solution of tet- ramethylammonium hydroxide at 35%. The solution thus ob- tained is poured, under stirring, into 200 g of colloidal silica Ludox HS 40.

A limpid, homogeneous gel is obtained, which is poured into an AISI316 steel autoclave, equipped with an anchor stirrer. The gel is left to crystallize under hydrothermal conditions at 165°C for 90 hours.

After cooling the autoclave, the solid obtained is separated from the mother liquor and washed with demineral- ized water until the washing water has a pH of less than 9.

The solid obtained is calcined at 550°C in a stream of air for 5 hours.

The solid thus obtained is subjected to ionic exchange by means of suspension in an aqueous solution of ammonium acetate. The ammonium ion is present in excess with respect

to the nominal aluminum present in the solid. After filtra- tion and washing of the solid, the whole operation (ex- change and washing) is repeated.

The solid obtained is calcined at 550°C in a stream of air for 5 hours.

The zeolitic solid is thus obtained in its acid form which, upon XRD analysis reveals the presence of the sole crystalline phase of the MTW type. Chemical analysis shows a content of residual sodium of less than 50 ppm and a m6- lar ratio Si02/Al203 = 250.

EXAMPLE 3. Catalytic test with ZSM-12 (SiO2/Al203 = 100 mol/mol) The catalytic testing of ZSM-12 zeolite having a molar ratio SiO2/Al203 = 100 (example 1), was carried out using the equipment described above and under the following oper- ating conditions: Reaction T = 500°C ; Total pressure = 1 bar; WHSVtotal = 1 h-l Feeding = see Table 1.

The WHSV is defined as a ratio between the hourly weight flow-rate (g/h) of the mixture in the feeding di- vided by the weight of the catalyst (g). From a dimensional point of view it is h-1.

Figure 1 indicates the two curves relating to total

conversion and selectivity to propylene, obtained with the catalyst ZSM-12 having a molar ratio Si02/Al2O3 = 100, in relation to the time on stream (tos).

The total conversion is defined as follows: Tot. conv. % = [(C4 at the reactor inlet)- (C4 at the re- actor outlet)]/ (C4 at the reactor inlet) * 100.

In this way the C4 fraction is not divided into ole- fins or paraffins but is considered altogether as a poten- tial reagent.

The selectivity to propylene is calculated as: selectivity to propylene % = (yield to propylene)/ (total conversion) * 100.

The yield to propylene is experimentally obtained by gaschromatographic analysis.

In addition to the high conversion and selectivity values, the unexpected stability of this material over a period of time is extremely important. In fact, it can be noted from the graph of Figure 1 that no catalytic deterio- ration phenomena are present until at least 140 h of tos.

This stability over a period time makes the material particularly suitable for use in simple reactor conditions such as fixed beds.

More complicated solutions, however, such as fluid- ized/transported beds can obviously also be used.

Table 2 indicates, for illustrative purposes, the se-

lectivity of the different components forming the product at the outlet of the plant. Among olefins of interest, eth- ylene is also present (7.88%). C5+ refers to the liquid fraction, at atmospheric pressure and room temperature, of the product leaving the plant. Owing to the high number of hydrocarbons present in the C5+ fraction, Table 3 indicates the composition of this fraction subdivided by group of compounds. As the composition of the products depends on the operating conditions, Table 3 specifies two distribu- tions obtained at two different reaction temperatures.

Table 2. Selectivity after 146 hours at a conversion of 55% Product Selectivity (weight%) Hydrogen 0. 38 Methane 0. 8 Ethylene 7. 88 Ethane 0. 78 Propylene 39. 1 Propane 5. 54 C5+ 45. 52 Total 100. 00 Table 3. Composition of the Liquid fraction obtained with ZSM-12 (SiO2/Al203 = 100 mol/mol) in weight % Reaction BTX NAPHTHALENES C5+ OTHERS T % % (non aromatics) % f°r a 500 40 8 17.5 34.5 500 55 7 5.5 32.5 The term BTX refers to benzene, toluene and xylenes. The heading NAPHTHALENES comprises all hydrocarbons, variously substituted, of the naphthaline family. The term C5+ (non aromatics) refers to non aromatic hydrocarbons, saturated and mono-unsaturated, containing 5-8 carbon atoms. The term OTHERS comprises those products for which it was not possi- ble to effect a gaschromatographic characterization It can be seen how among the by-products, there are large quantities of easily exploitable products such as BTX.

COMPARATIVE EXAMPLE 4. ZSM-12 (SiO2/Al203 = 250 mol/mol) The catalytic testing of ZSM-12 zeolite having a molar ratio Si02/Al203 = 250, whose synthesis is described in ex- ample 2, was effected using the equipment described above and under the exact operating conditions specified in exam- ple 3.

Figure 2 indicates the two curves relating to total

conversion and selectivity to propylene, obtained with this zeolite in relation to the time on stream (tos). The con- version and selectivity to propylene are defined as in example 3.

Contrary to what is specified in literature, the cata- lytic performances of ZSM-12 with a ratio SiO2/A12O3 = 250, are lower both in terms of yield (product of selectivity and conversion) and duration, with respect to the zeolite having a greater content of Al203.

It can be observed, in fact, from the graph of Figure 2 how, already after 25 hours of tos, evident catalytic de- terioration phenomena are present.

Table 5 indicates, for illustrative purposes, the se- lectivity of the various components forming the product at the plant outlet. Table 6, on the other hand, indicates the composition of the C5+ liquid fraction subdivided by group of compounds.

Table 5. Selectivity after 24 hours at a conversion of 52% Product Selectivity (weight%) Hydrogen 0. 4 Methane 0. 6 Ethylene 4. 41 Ethane 0. 42 Propylene 37. 0 Propane 3. 82 C5+ 53. 35 Total 100. 00 Table 6. Composition in weight % of the C5+ liquid fraction obtained with ZSM-12 (SiO2/Al2O3 = 250 mol/mol) Reaction BTX NAPHTHALENES C5+ OTHERS T % % (non aromatics) % for 0 500 35 3.5 17.5 44 The term BTX refers to benzene, toluene and xylenes; the heading NAPHTHALENES comprises all hydrocarbons, variously substituted, of the naphthaline family; the term C5+ (non aromatics) refers to non aromatic hydrocarbons, saturated and mono-unsaturated, containing 5-8 carbon atoms. The term

OTHERS comprises those products for which it was not possi- ble to effect a gaschromatographic characterization.

COMPARATIVE EXAMPLE 5. Commercial ZSM-5 The catalytic testing of commercial ZSM-5 zeolite (CBV 3020 E) having a molar ratio SiO2/Al203 = 30, was effected using the equipment described above and under the exact op- erating conditions described in example 3.

Figure 3 indicates the two curves relating to total conversion and selectivity to propylene, obtained with this zeolite in relation to the time on stream (tos).

The conversion and selectivity to propylene are de- fined as in example 3.

The catalytic performances of ZSM-5 are much lower both in terms of yield (product of selectivity and conver- sion) and duration, with respect to the ZSM-12 zeolite.

It can be observed, in fact, from the graph of Figure 3 how, already after 10 hours of tos, evident catalytic de- terioration phenomena are present.

Table 7 indicates, for illustrative purposes, the se- lectivity of the various components forming the product at the plant outlet.

Table 8 specifies the composition of the C5+ fraction subdivided by group of compounds.

Table 7. Selectivity after 27 hours at a conversion of 85% Product Selectivity (weight%) Hydrogen 1. 63 Methane 3. 70 Ethylene 3. 03 Ethane 5. 14 Propylene 3. 84 Propane 34. 15 C5+ 48. 51 Total 100. 00 Table 8. Composition in weight % of the C5+ liquid fraction obtained with commercial ZSM-5 Reaction BTX NAPHTHALENES C5+ OTHERS T % % (non aromatics) % (°C) % 500 80----20