IT9021709A1 - PROCEDURE FOR THE PREPARATION OF ISOBUTENE BY CATALYTIC DEHYDROISOMERIZATION OF N-BUTANE. - Google Patents

Description

DESCRIPTION

The present invention relates to a process for the preparation of isobutene by catalytic dehydroisomerization of n-butane.

Isobutene is a valuable intermediate used in various chemical reactions, for example as a monomer in polymerization and copolymerization reactions, as an alkylating agent, in the production of methyl tert-butyl ether (reaction with methanol) and in the production of isoprene (reaction with formaldehyde ).

Isobutene is traditionally obtained as a by-product in refinery processes such as thermal or catalytic cracking, or by means of a two-stage process in which the n-butane is catalytically isomerized to isobutane and the latter is catalytically dehydrogenated to isobutene.

Catalytic processes have recently been described which allow to carry out the transformation of n-alkanes into iso-olefins in a single reaction stage. In particular, in EP 42.252 isobutene is obtained by dehydroisomerization of n-butane in the presence of a catalyst formed by an element of group IIIa of the periodic table, or relative salt, and by a low acidity support. In US 4,433,190 the dehydroisomerization reaction of an n-alkane in the corresponding iso-olefin is carried out on a catalyst obtained by impregnating a porous crystalline borosilicate called AMS-1B with a noble metal. These dehydroisomerization processes do not produce completely satisfactory results, particularly from the point of view of the limited yield in the desired product. Also known in the art are dehydrogenation catalysts formed by a support on which platinum or other noble metal has been deposited. Such catalysts can contain promoters generally selected from tin, indium, thallium and alkaline earth metals. The latter catalysts are generally used in long-chain paraffin dehydrogenation processes, as described in US 4,486,547 and in Italian patent application IT 23.149 / 87, filed on 22 December 1987.

Finally, BE 877205 describes a porous crystalline boral silicate called Boralite B capable, among other things, of isomerizing n-paraffins. It has now been found that the joint use of a Boralite B and a supported platinum catalyst, especially with promoter, under suitable reaction conditions, allows to dehydroisomerize n-butane to isobutene, in a single step and with high yields in the useful product.

In accordance with this, the present invention relates to a process for the preparation of isobutene by dehydroisomerization of n-butane, characterized in that a gaseous stream of n-butane and hydrogen, possibly diluted with nitrogen, is placed in contact, at high temperature, with a fixed catalytic bed of a first catalyst (a) and a second catalyst (b), where said catalyst (a) is formed by a solid and granular support of porous gamma-alumina on the surface of which catalytic quantities of platinum are deposited , with or without promoters, and said catalyst (b) is Boralite B; the granules of said catalysts (a) and (b) being homogeneously distributed in the catalytic bed, or being arranged in the form of two contiguous layers, the catalyst layer (a) being first contacted by the gas supply stream.

The useful support for the catalyst (a) consists of alumina in the gamma crystallographic form, which has a surface area from 100 to 400 m / g and a global pore volume from 0.5 to 1.2 ml / g, in the form of granules, extrusions or tablets with useful size for use in a fixed catalytic bed and generally variable from 0.4 to 5 mm.

The catalyst (a), useful for the purposes of the present invention, is formed by a gamma alumina having the above characteristics, on which platinum is deposited in an amount from 0.1 to 1% by weight. In the preferred embodiment the catalyst (a) additionally contains, as promoters, tin in a quantity of from 0.1 to 1% by weight and indium in a quantity of from 0.05 to 1%. In this case, the following weight ratios platinum / indium from 0.3 / 1 to 1.5 / 1 and platinum / tin from 0.5 / 1 to 2/1 are conveniently maintained. The catalyst (a) may further contain thallium in an amount of from 0.01 to 1% by weight and an alkali and / or alkaline earth metal, especially magnesium, in an amount of from 0.05 to 3.5% by weight. All the percentages reported above refer to the total weight of the catalyst.

The catalyst (a) can be prepared by impregnating the compounds of the catalytic metals with acid aqueous solution, especially for nitric acid, followed by drying and calcination treatments of the impregnated support. More specifically, for the impregnation, water-soluble and decomposable compounds at high temperature are used of platinum, and possibly of tin, indium, thallium and magnesium, such as for example chloroplatinic acid, stannic chloride and nitrates of indium, thallium and magnesium. The drying of the impregnated support is conveniently carried out at temperatures of the order of 100-130 ° C in an air stream and the calcination is conveniently carried out at 400-600 ° C, in an air stream for a time of the order of 2-8-hours. The catalyst thus obtained is subjected to a reduction treatment before being used in the dehydroisomerization reaction. This treatment, which can conveniently be carried out in the same dehydroisomerization reactor, is generally carried out in a stream of hydrogen, at high temperatures (about 450-650 ° C) for a time of the order of 1-5 hours.

The catalyst (b), useful for the purposes of the present invention, is a Boralite B. This boralite is described in BE 877205. It has the following molar composition in the anhydrous and calcined form with the components expressed as oxides:

where is it

C = metal cation of valence n, H, ΝΗ4, or a mixture of these.

Boralite B is prepared, as described in BE 877205, by reaction under hydrothermal conditions of a silicon derivative, a boron derivative, an alkali metal hydroxide and a tetraethylammonium salt. More specifically, a derivative of silicon selected for example from colloidal silica, silica gel or sodium silicate, a derivative of boron selected for example from boric acid, alkaline borates or trialkylborates, an alkali metal hydroxide and a tetraethylammonium salt, preferably tetraethylammonium hydroxide, are heated in an autoclave at autogenous pressure and at a temperature between 90 ° C and 160 ° C, until complete crystallization. The crystalline product thus obtained is then filtered, dried and calcined.

In particular, crystallization is preferred in the presence of boralite B seeds, in quantities ranging from 1 to 60% by weight, which reduce the time required for synthesis.

These seeds are obtained by placing a silicon derivative, a boron derivative, an alkali metal hydroxide and a tetraalkylammonium salt.

Boralite B is used in the present invention in the form of granules, extrusions or tablets with a size suitable for use in a fixed catalytic bed, generally variable between 0.4 and 5 mm.

The preparation of isobutene, according to the present invention, is carried out by feeding a gaseous stream of n-butane and hydrogen to a fixed bed of catalysts (a) and (b), operating under dehydroisomerization conditions.

A molar ratio of hydrogen to n-butane from 1/2 to 5/1 and preferably from 1/1 to 3/1 is conveniently maintained in the feed gas stream. The gaseous feed stream can also be suitably diluted, for example with nitrogen. In the latter case the molar ratios become from 1/2 to 5/1 between hydrogen and n-butane and from 1/1 to 5/1 between nitrogen and n-butane, preferably from 1/1 to 3/1.

The granules of the catalysts (a) and (b) can be uniformly mixed in the catalytic bed, or the catalysts (a) and (b) can be arranged in the form of contiguous fixed beds. In this second case, the catalyst layer (a) will be arranged in the dehydroisomerization reactor so as to come into contact first with the gaseous feed stream. The catalytic bed will also contain the catalysts (a) and (b) in weight ratios to each other from 30/70 to 70/30, preferably of the order of 50/50.

In the dehydroisomerization reaction one operates at a temperature in the range from 450 to

600 ° C, at a pressure from 200 mm Hg up to 5 Kg / cm and with an hourly space velocity from 0.5 to 4 hours <-1> (n-butane weight / catalyst weight. Hour). Preferably the temperature values vary from 500 to 580 ° C, those of the pressure vary from 400 min Hg to 2 Kg / cm and the space velocity is of the order of 1 hour ^. <-1>

By operating under these conditions, high conversions of the fed n-butane are obtained, with high yields and selectivity in the useful reaction product.

The following experimental examples are reported to better illustrate the invention. Example 1

Preparation of the catalyst fa)

For the preparation of the catalyst (a) a commercial gamma-alumina is used, having a surface area of 196 m / g and a total pore volume of 0.75 ml / g, in the form of granules of size 0.5-0, 8 mm. To 20 g of this gamma-alumina, 30 ml of an aqueous solution obtained starting from 0.25 g of indium nitrate pentahydrate, 0.2 g of stannic chloride, 0.47 g of chloroplatinic acid are slowly added, under stirring. (16% by weight of platinum) and 1.3 g of 65% nitric acid. After one hour of contact at room temperature (about 25 ° C), under continuous stirring, the mass is heated to about 120 ° C, for one hour, under an air flow, to cause substantially complete evaporation of the excess aqueous solvent. The dried solid thus obtained is calcined in a muffle at 500 ° C, for 4 hours in a stream of air. At the end the catalyst (a) is cooled and recovered, which contains 0.37% by weight of platinum, 0.45% by weight of tin and 0.36% by weight of indium.

Preparation of the catalyst (b)

3.0 g of NaOH and 6.4 g of boric acid are dissolved in 28.12 g of an aqueous solution of tetraethylammonium hydroxide at 40% by weight. A clear solution is obtained which is diluted with 30 g of distilled water and added to 51 g of Ludox AS silica at 30% by weight of silica.

The suspension thus obtained, having a pH of 12.2, is left at room temperature under stirring for 4 hours and then left to crystallize in an autoclave, under static conditions, at autogenous pressure, at 150 ° C, for 5 days.

The autoclave is then cooled and the milky suspension of boralite B seeds is recovered. This suspension is added in an amount equal to 15% by weight to a mixture having the following composition, after it has been stirred at room temperature to about 4 hours:

204 g of Ludox AS silica at 30% by weight.

This mixture, with the addition of the seed suspension, is left to crystallize in a steel autoclave under static conditions under autogenous pressure, at a temperature of 150 ° C, for 3 days.

The autoclave is cooled, boralite B is recovered by filtration, washed with distilled water, dried at 120 ° C, calcined for 5 hours at 550 ° C and then exchanged in acid form according to the methods of the known art. The boralite B thus obtained, having crystals of dimensions around 1 u, is tableted into granules of 0.4-0.8 min.

Example 2

0.56 g of catalyst (a) and 0.47 g of catalyst (b), prepared as described in example 1, are introduced separately into a quartz reactor having an internal diameter of 10 mm. The two catalysts are arranged in the form of two contiguous fixed beds with the catalyst (a) arranged above, so that the feed stream, introduced at the top of the reactor, first comes into contact with the catalyst bed (a). catalysts thus prepared are subjected to reduction by feeding a stream of hydrogen for 2 hours, at a temperature of 550 ° C.

After this treatment, the dehydroisomerization test is carried out by feeding the reactor a gaseous mixture containing n-butane, hydrogen and nitrogen, with a hydrogen / n-butane molar ratio equal to 1/1 and with a nitrogen / n-butane molar ratio equal to at 2/1. Furthermore, one operates at a temperature of 553 ° C at atmospheric pressure and with an hourly space velocity, evaluated on the catalyst (a) introduced into the reactor, equal to 2 hours <-1> (n-butane weight / catalyst weight per hour).

The test results are shown in table 1.

Example 3

One operates as in example 2, subjecting the catalysts (a) and (b) to prior reduction in a stream of hydrogen, at 550 ° C, for 2 hours.

After the reduction, the dehydroisomerization test is carried out by feeding the reactor a gaseous mixture containing hydrogen, n-butane and nitrogen, with a hydrogen / n-butane molar ratio equal to 1/1 and with a nitrogen / n-butane molar ratio equal at 2/1. The reaction is also carried out at 570 ° C, at atmospheric pressure and with an hourly space velocity, evaluated on the catalyst (a), equal to 2 hours <-1> (weight of n-butane / weight of catalyst, hour).

The results are reported in table 1.

Example 4

One operates as in example 2, subjecting the catalysts (a) and (b) to prior reduction in a stream of hydrogen, at 550 ° C, for 2 hours.

After the reduction, the dehydroisomerization test is carried out by feeding the reactor a gaseous mixture containing hydrogen and n-butane in a molar ratio of 3/1 to each other. The reaction is also carried out at 572 ° C, at atmospheric pressure and with an hourly space velocity, evaluated on the catalyst (a), equal to 1 hour <-1> (weight of n-butane / weight of catalyst per hour).

The results are reported in table 1.

Example 5

Preparation of the catalyst (a) also containing thallium

For the preparation of the catalyst (a) also containing thallium, a commercial gamma-alumina is used, having a surface area of 196 m <2> / g and a total pore volume of 0.75 ml / g, in the form of granules of size 0.5-0.8 mm. To 90 g of this gamma-alumina, 120 ml of an aqueous solution obtained starting from 0.09 g of thallium nitrate, 1.0 g of indium nitrate pentahydrate, 0.9 g of stannic chloride, 2.25 g of chloroplatinic acid (16% by weight of platinum) and 6 g of 65% nitric acid. After one hour of contact at room temperature (about 25 ° C), under continuous stirring, the mass is heated to about 120 ° C, for one hour, under an air flow, to cause substantially complete evaporation of the excess aqueous solvent. The dried solid thus obtained is calcined in a muffle at 500 ° C, for 4 hours in a stream of air. Finally, the catalyst (a) is cooled and recovered, which contains 0.37% by weight of platinum, 0.40% by weight of tin, 0.08% by weight of thallium, and 0.30% by weight of indium. .

Example 6

0.81 g of catalyst (a), prepared as described in example 5, and 0.32 g of catalyst (b), prepared as described in example 1, are introduced separately into a quartz reactor having an internal diameter of 10 mm. The two catalysts are arranged in the form of two contiguous fixed beds with the catalyst (a) arranged above, so that the feed stream, introduced at the top of the reactor, first comes into contact with the catalyst bed (a). catalysts thus prepared are subjected to reduction by feeding a stream of hydrogen for 2 hours, at a temperature of 550 ° C.

After this treatment, the dehydroisomerization test is carried out by feeding the reactor a gaseous mixture containing n-butane, hydrogen and nitrogen, with a hydrogen / n-butane molar ratio equal to 1/1 and with a nitrogen / n-butane molar ratio equal to at 2/1. The operation is also carried out at a temperature of 558 ° C at atmospheric pressure and with an hourly space velocity, evaluated on the catalyst (a) introduced into the reactor, equal to 2 hours <-1> (n-butane weight / catalyst weight. Hour).

The test results are shown in table 1.

Example 7 (comparative)

A crystalline borosilicate, called AMS-1B, is prepared according to what is described in example 1 of US patent 4,269,813. More particularly, 10.5 g of boric acid and 67.2 g of sodium hydroxide are dissolved in 2.653 g of water under continuous stirring. 394.8 g of tetra-npropylammonium bromide and, after complete dissolution, 400 g of Ludox commercial silica are added to the solution thus obtained. The solution is loaded into an autoclave and left to crystallize at 165 ° C for 7 days. The crystalline solid thus obtained is dried and calcined at 550 ° C for 5 hours, exchanged in acid form and again dried and calcined. A 4 g sample of this solid, in granules with dimensions from 0.5 to 0.8 mm, is impregnated with a chloroplatinic acid solution at 16% by weight, so as to obtain a metallic platinum content equal to 0.55 % by weight. The solid is finally dried and calcined for 12 hours at 350 ° C to obtain the catalyst. 0.34 g of the catalyst prepared as described above are loaded into the quartz microreactor. The reduction is carried out in a stream of hydrogen, at 525 ° C for 2 hours. After this treatment, the dehydroisomerization test is carried out. The feed to the reactor consists of n-butane, hydrogen and nitrogen, with a hydrogen / n-butane molar ratio equal to 1/1 and with a nitrogen / n-butane molar ratio equal to 2/1. The reaction is carried out at the temperature of 542 ° C, at atmospheric pressure and with an hourly space velocity equal to 8 hours <-1> The results are collected in table 1.

Example 8 (comparative!

A sample of crystalline borosilicate AMS-1B prepared as described in example 7, is mixed with 16 g of gamma-alumina. The mixture is granulated to a granule size of 0.5-0.8 mm and the granules are impregnated with a 4.1% by weight chloroplatinic acid solution, according to example 9 of US patent 4,433,190. The impregnated solid is dried and calcined at 350 ° C for 12 hours.

0.3 g of the catalyst thus obtained are charged into the quartz reactor. After the reduction treatment, carried out in a stream of hydrogen, at a temperature of 525 ° C, for two hours, the dehydroisomerization test is carried out. The feed consists of n-butane, hydrogen and nitrogen, with a hydrogen / n-butane molar ratio of 1/1 and a nitrogen / n-butane molar ratio of 2/1. The reaction is carried out at a temperature of 542 ° C, at atmospheric pressure and with an hourly space velocity of 8 hours <-1>.

The results are shown in Table 1.

In this table, conversion means the percent by weight of transformed n-butane with respect to the fed one. In addition, the selectivity and the yield refer respectively to the converted reagent and to the fed one. Finally, C1-C3 and C5 <+> indicate respectively the paraffinic and olefinic products containing from 1 to 3 carbon atoms and those with 5 or more carbon atoms.

Claims (10)

Claims 1. Process for the preparation of isobutene by dehydroisomerization of n-butane, characterized in that a gaseous stream of n-butane and hydrogen is placed in contact, at high temperatures, with a fixed catalytic bed of a first catalyst (a) and of a second catalyst (b), where said catalyst (a) is formed by a solid and granular support of porous gamma-alumina on the surface of which catalytic quantities of platinum are deposited, with or without promoters, and said catalyst (b) is formed by Boralite B having in the anhydrous form the following molar composition with the components expressed as oxides: where is it C = metal cation of valence n, H <+>, NH4 <+> or a mixture of these; said catalysts (a) and (b) being homogeneously distributed in the catalytic bed, or being arranged in the form of two contiguous layers, the catalyst layer (a) being first contacted by the gaseous feed stream. 2. Process according to claim 1, characterized in that the catalyst support (a) is gamma-alumina with surface area from 100 to 400 m / g and with global pore volume from 0.5 to 1.2 ml / g, in the form of granules with a size from 0.4 to 5 mm. 3. Process according to claim 1, characterized in that the catalyst (a) contains platinum in a quantity of from 0.1 to 1% by weight. 4. Process according to claim 1, characterized in that said catalyst (a) additionally contains tin in quantities from 0.1 to 1% and indium in quantities from 0.05 to 1% by weight, with the following platinum / weight ratios indium from 0.3 / 1 to 1.5 / 1 and platinum / tin from 0.5 / 1 to 2/1. 5. Process according to claim 4 characterized in that said catalyst (a) further contains thallium in an amount from 0.01 to 1% by weight and an alkali and / or alkaline earth metal and preferably magnesium, in an amount of 0, 05 to 3.5% by weight. 6. Process according to claim 1 characterized in that the weight ratio between the catalyst (a) and the catalyst (b) in the catalytic bed varies from 70/30 to 30/70 and is preferably of the order of 50/50. 7. Process according to claim 1, characterized in that a molar ratio of hydrogen to n-butane of 1/2 to 5/1 and preferably 1/1 to 3/1 is maintained in the feed gas stream. 8. Process according to claim 1 wherein the gaseous stream of n-butane and hydrogen is diluted with nitrogen before being brought into contact with the fixed catalytic bed. 9. Process according to claim 8, characterized in that a molar ratio of hydrogen to n-butane of 1/2 to 5/1, preferably 1/1 to 3/1, is maintained in the feed gas stream, and a molar ratio between nitrogen and n-butane from 1/1 to 5/1, preferably from 1/1 to 3/1. 9. Process according to claim 1 characterized in that the dehydroisomerization reaction is carried out at a temperature in the range from 450 to 600 ° C, at a pressure from 200 mm Hg up to 5 Kg / cm and with an hourly space velocity from 0.5 to 4 hours-1 (n-butane weight / catalyst weight. Hour). 10. Process according to claim 9 characterized in that the temperature values vary from 500 to 580 ° C, the pressure values vary from 400 mm Hg to 2 Kg / cm and the space velocity is of the order of 1 hour <-1>.