FR2748021A1 - Oxidative dehydrogenation of paraffinic hydrocarbon(s) to mono-olefin(s) - Google Patents

Abstract

Use of a CrO3- based catalyst with opt. an alkali metal oxide on a porous support , for the oxidative de-hydrogenation of paraffinic hydrocarbons to the corresponding monoolefins is claimed. Also claimed is the process itself using the catalyst system.

Description

APPLICATION OF A SUPPORTED CATALYST BASED ON CHROME OXIDE
OXIDIZING DEHYDROGENATION OF PARAFFINIED HYDROCARBONS
IN THE CORRESPONDING MONOOLEFINS
The present invention relates to the application of a supported catalyst, based on chromium oxide and, if appropriate, at least one alkali metal oxide, to the oxidative dehydrogenation of paraffinic hydrocarbons to the corresponding monoolefins. .

 The dehydrogenation of isobutane to isobutene and that of propane to propene is known from the use of direct dehydrogenation at reduced space velocity (VVH = 100-1000 hl) and at high temperatures (550 to 750 ° C).

This conventional industrial process has several inherent disadvantages (1) it is an endothermic reaction that requires
significant amount of energy (2) the isobutene yield is balanced
thermodynamics; and (3) at temperatures favoring high yields of
isobutene, catalyst deactivation rate
is also important because of a coke deposit
on the surface of the catalyst.

 Catalysts used in the industry for this direct dehydrogenation include platinum on alumina, platinum on tin oxide, as well as chromium oxide catalysts. It is thus possible to cite British patent application GB-A-2 162 082 which describes the dehydrogenation of C3-C5 hydrocarbons on a catalyst obtained by spraying a solution of a soluble chromium compound on a support of alumina, followed by drying and calcination.

 For the oxidative dehydrogenation of paraffinic hydrocarbons to form the corresponding monoolefins by contacting the hydrocarbon with molecular oxygen, the literature has already reported many catalysts.

One can quote the use of a catalyst of the type
NiMoO4 (French patent FR-B-2 642 669) for the oxidative dehydrogenation of paraffinic hydrocarbons containing from 3 to 6 carbon atoms. However, in the examples, only reactions relating to the conversion of propane to propene at temperatures above 550 ° C are reported.

 US Pat. No. 4,777,319 discloses a process for the oxidative dehydrogenation of propane, n-butane and isobutane on mixed vanadium magnesium oxide catalysts (V-Mg-O). The conversion of isobutane is 35% and the isobutene selectivity is 22.1% at 540 C.

We can mention the European patent application
EP-A-0 638 534 which describes a catalyst comprising platinum deposited on a mixture of tin oxide and zirconium oxide, for the dehydrogenation of an alkane to alkene, carried out in a mixture with oxygen and in the absence of added water vapor.

 US-A-5,306,585 claims a process for oxidizing paraffinic hydrocarbons containing 2 to 5 carbon atoms by reacting said paraffinic hydrocarbon with molecular oxygen and a microporous molecular sieve of the VAPO-5 type as a catalyst. . The conversion of isobutane is 15.6% and the isobutene selectivity is 40.6% at 480 ° C.

 European patent application EP-A-0 661 254 describes the use of super-acid solid catalysts, such as sulphated zirconia, in the conversion of isobutane to isobutene in the presence of oxygen-containing oxidizing agents under reaction including temperatures ranging from 260 to 540 C and high pressures (above 25 bar).

 European Patent Application EP-A-0 557 790 describes a process for the production of isobutene with high selectivity by oxidative dehydrogenation of isobutane in the gas phase by molecular oxygen. The catalyst used consists of a phosphorus oxide and an oxide of at least one of the elements selected from the group consisting of zinc, nickel, iron, chromium, vanadium, manganese, cobalt, silver, copper, magnesium, lithium, scandium, titanium, gallium, yttrium, zirconium, antimony, lanthanum, cerium, samarium, ytterbium, hafnium and lead.

Example 16 of this patent application EP-A-0 557 790 describes a catalyst in which the element chosen is iron (III).

This catalyst used for the oxidative dehydrogenation of isobutane at 500 ° C. makes it possible to reach a conversion of 12.1% per pass with an isobutene selectivity of 65.9%.

This type of catalyst has the disadvantage of giving cracking products since a selectivity of 20.6% in propene is also obtained.

 As described above, the catalytic systems reported so far are capable of selectively activating isobutane only at high temperatures. No examples have been reported in which olefin conversion and selectivity levels are acceptable for industrial development at reaction temperatures below 450C.

 The present invention aims to provide catalytic systems for the oxidative dehydrogenation in the gas phase of hydrocarbons with a high yield at moderate temperatures.

 The Applicant Company has discovered that this object can be achieved by using a chromium oxide catalyst and, if appropriate, at least one alkali metal oxide, where these oxides are deposited on a porous support. As indicated above, this type of catalyst is used industrially in the production of olefins by conventional catalytic dehydrogenation of the corresponding paraffins and, more specifically, in the dehydrogenation of paraffins having from 3 to 5 carbon atoms. However, there is no suggestion in the literature to use such a system to catalyze the oxidation of a paraffin to olefin in the presence of molecular oxygen as an oxidant.

 The present invention firstly relates to the use of a CrO 3 chromium oxide catalyst and, where appropriate, at least one alkali metal oxide, in the state supported on a porous support, for the oxidative dehydrogenation of paraffinic hydrocarbons to form the corresponding monoolefins.

 The alkali metal oxide, when present, is preferably potassium oxide.

 The porous support is especially chosen from alumina, silica, zirconia, silicon carbide, titanium oxide and mixtures thereof. Alumina is the preferred carrier.

 There is no particular limitation as to the shape of the support. It may be spherical, in pellets or rings, with a diameter for example of 20 mm to 10 mm.

In particular, granules of 20 μm may be mentioned in the case of a fluidized bed application at 10 mm for a fixed bed application. The specific surface of the support may be variable, for example from 20 m 2 / g to 250 m 2 / g. The pore volume of the support may be of the order of 0.1 to 1.0 cm3 / g.

The layers of catalytically active substance are formed by impregnation of the support with an aqueous solution containing Cr03 and K2Cr2o7 (it is also possible to use
Cr203 or any other type of product liable to decompose to calcination oxide, in particular nitrates, carbonates, sulphates and chlorides), drying of the support for 2 to 16 hours at 90-150 ° C., and calcination for 2 - 24 hours at 500 - 800 ° C.

 In the catalyst according to the invention, the chromium oxide is generally present in a proportion of 5 to 30%, in particular 5 to 20%, and the alkali metal oxide (s), in a percentage of up to 3%. %, especially from 0.01 to 3%, in particular from 0.2 to 2%, these percentages being given by weight relative to the total weight of the catalyst itself and of the support.

 The subject of the present invention is also a process for vapor phase oxidative dehydrogenation of paraffinic hydrocarbons to form the corresponding monoolefins, in particular for the oxidative dehydrogenation of isobutane to isobutene, said process consisting in bringing the paraffinic hydrocarbon into contact with molecular oxygen or an oxygen-containing gas, and, if appropriate, an inert diluent gas, said reaction being moreover capable of being conducted in the presence of water, characterized in that it operates in the presence of a catalyst as defined above, based on chromium oxide CrO 3 and, where appropriate, at least one alkali metal oxide, in particular potassium oxide, in the state supported on a porous support.

The conditions of this process are generally as follows
- the overall load volume per system volume
global catalytic (catalyst proper +
support) per hour is between 360 and 36000 hl
preferably between 720 and 7200 hl
the reaction is carried out at a temperature of 200 ° C.
450 C, preferably from 250 C to 400 ° C, under a
pressure of 1 to 5 bar absolute, preferably 1 to 5 bar
3 bars
- the proportion of paraffinic hydrocarbon in the
reaction mixture (paraffinic hydrocarbon +
oxygen + any inert diluent + possible water) is
between 5 and 95 mol%, preferably between
10 and 50 mol%
the proportion of water introduced into said mixture can
go up to 30% in moles, in particular up to 15%
in moles
the proportion of inert diluent gas, up to 15% by
moles, being in particular 20 to 60 mol% in
said mixture; and
the molar ratio of the paraffinic hydrocarbon of
Oxygen start is usually between 1: 5
and 1: 0.05, preferably between 1: 2 and 1: 0.2.

 This type of reaction generally takes place in a fixed-bed tubular reactor whose overall operation can be described by the theoretical model of the piston reactor. The operating conditions of such a circuit are particularly as follows: on the supported catalyst, which has preferably been mixed with diluent grains of the same diameter as the catalyst grains, such as steatite or any other inert material of low specific surface area, to reduce the formation of hot spots along the catalytic bed (volume ratio supported catalyst - diluent: 10: 90 to 100: 0), a charge is sent consisting of a preheated gas mixture consisting of paraffinic hydrocarbon , molecular oxygen, water and an inert gas diluent, such as nitrogen or helium.

 The following Examples illustrate the present invention without however limiting its scope.

EXAMPLE 1
The catalyst is prepared by simultaneous impregnation of a commercial alumina with chromium and potassium from a solution containing CrO 3 and
K2Cr207; the support is an γ-alumina (Harshaw 3916) of 170 m 2 / g.

 10.94 g of CrO3 and 1.56 g of K2Cr207 are dissolved in distilled water (the amount of water and calculated so as to fill 100% of the pore volume of the support). This solution is added dropwise to 100 g of the alumina.

 The alumina thus impregnated is dried overnight at 120 ° C. and then calcined at 600 ° C. for 6 hours. The catalyst obtained contains 12 g of CrO 3 and 0.5 g of K 2 O per 100 g of alumina.

 For the oxidative dehydrogenation of isobutane, a tubular reactor operating at atmospheric pressure is used. It consists of a stainless steel tube with an inside diameter of 11.5 mm and 42 cm in length. The heating is provided by electrical resistances.

 In the catalyst bed is introduced a stainless steel sheath 2 mm in diameter inside which is inserted a sliding thermocouple to raise the reaction temperature along the catalyst bed.

 1.5 ml of catalyst is used in the form of grains with a diameter of between 0.3 and 0.6 mm. The catalyst (in a volume ratio of 1/1) is diluted with an inert compound: steatite, which is a magnesium silico-aluminate calcined at high temperature and with a low specific surface area.

 The reaction mixture consists of 26% of isobutane, 13% of oxygen, 12% of water vapor and the remainder of an inert diluent gas, in this case helium (these percentages being in moles). . The components of the reaction mixture are passed at a rate of 1.5 normal liters per hour so as to have a contact time in relation to the catalyst volume of 3.6 seconds and at the temperature T indicated in the table below. . The products of the reaction are analyzed by gas chromatography to determine their concentration and in particular the concentration of isobutene. The molar conversion indicated in the table below, and expressed in%, represents the converted total isobutane; the selectivity to isobutene represents the percentage of isobutene obtained relative to the converted isobutane.

EXAMPLE 2
The procedure for the preparation of the catalyst of Example 1 is repeated except for the following: only 12 g of CrO 3 are dissolved to obtain the impregnating solution. The final catalyst obtained contains only 12 g of CrO 3 per 100 g of alumina.

 This catalyst is used under the same conditions as in Example 1 except that the rate of the reaction mixture is increased to 2.7 normal liters per hour so as to have a contact time with respect to the catalyst volume of 2 seconds. The results obtained are shown in the Table below.

EXAMPLE 3
The catalyst prepared according to Example 1 is used, but it is used under the conditions described in Example 2. The results obtained are shown in FIG.
Table below.

EXAMPLE 4
The procedure for the preparation of the catalyst of Example 1 is repeated, except that 8.82 g of CrO 3 and 4.68 g of K 2 Cr 2 O 7 are dissolved to obtain the impregnating solution. The final catalyst obtained contains only 12 g of CrO 3 and 1.5 g of K 2 O per 100 g of alumina.

 This catalyst is used under the same conditions as in Example 2 and the results obtained are shown in the Table below.

BOARD

<tb> Example <SEP> Temperature <SEP> Conversion <SEP> Selectivity
<tb><SEP>(<SEP> C) <SEP> isobutane <SEP> isobutene
<tb><SEP> (%) <SEP> (%)
<tb><SEP> 1 <SEP> 260 <SEP> 4.2 <SEP> 83.1
<tb><SEP> 300 <SEP> 10.0 <SEP> 67.3
<tb><SEP> 340 <SEP> 16.1 <SEP> 58.2
<tb><SEP> 2 <SEP> 325 <SEP> 7.0 <SEP> 63.4
<tb><SEP> 3 <SEP> 325 <SEP> 11.8 <SEP> 59.9
<tb><SEP> 4 <SEP> 325 <SEP> 14.8 <SEP> 60.9
<Tb>

Claims (14)

 1 - Use of a ChrO3 chromium oxide catalyst and, where appropriate, at least one alkali metal oxide, in the state supported on a porous support, for the oxidative dehydrogenation of paraffinic hydrocarbons for form the corresponding monoolefins.  2 - Use according to claim 1, characterized in that the alkali metal oxide is potassium oxide.  3 - Use according to one of claims 1 and 2, characterized in that the porous support is selected from alumina, silica, zirconia, silicon carbide, titanium oxide and mixtures thereof.  4 - Use according to claim 3, characterized in that the support is an alumina.  5 - Use according to one of claims 1 to 4, characterized in that the chromium oxide is present in a proportion of 5 to 30%, and the alkali metal oxide or oxides, in a percentage up to 3%, especially from 0.01 to 3%, these percentages being by weight relative to the total weight of the catalyst itself and of the support.  6 - Use according to claim 5, characterized in that the chromium oxide is present in a proportion of 5 to 20%, and the alkali metal oxide or oxides, when (s) is (or are) present ( s), at 0.2 to 2%, these percentages being by weight relative to the total weight of the catalyst itself and the support.  A process for vapor phase oxidative dehydrogenation of paraffinic hydrocarbons to form the corresponding monoolefins, said process comprising contacting the paraffinic hydrocarbon with molecular oxygen or an oxygen-containing gas, and, if appropriate , an inert diluent gas, said reaction being able to be carried out in the presence of water, characterized in that it is carried out in the presence of a CrO 3 chromium oxide catalyst and, where appropriate, from minus an alkali metal oxide, especially potassium oxide, in the state supported on a porous support.  8 - Process according to claim 7, characterized in that the paraffinic hydrocarbon is isobutane.  9 - Process according to one of claims 7 and 8, characterized in that the porous support of the catalyst is selected from alumina, silica, zirconia, silicon carbide, titanium oxide and mixtures thereof.  10 - Process according to one of claims 7 to 9, characterized in that the chromium oxide is present in a proportion of 5 to 30% by weight, preferably 5 to 20%, and, when (s) is (or are) present, the alkali metal oxide (s), in particular potassium oxide (s), in a proportion of 0.01 to 3%, preferably 0.2 to 2%, these percentages being by weight relative to total weight of the catalyst itself and the support.  11 - Process according to one of claims 7 to 10, characterized in that the supported catalyst is in the form of grains with a diameter of about 20 ssm to 10 mm, these supported catalyst grains being mixed with pellets or grains of at least one diluent, such as steatite or any other inert material of low specific surface area, the supported catalyst / diluent ratio by volume being between 10: 90 and 100: 0.  12 - Process according to one of claims 7 to 11, characterized in that the overall load volume per volume of overall catalyst system (catalyst proper + support) per hour is between 360 and 36000 hl, preferably between 720 and 7200 hl.  13 - Method according to one of claims 7 to 12, characterized in that the reaction is conducted at a temperature of 200 C to 450 C, preferably 250'C to 400 C.  14 - Process according to one of claims 7 to 13, characterized in that the reaction is carried out under a pressure of 1 to 5 bar absolute, preferably 1 to 3 bar.  15 - Method according to one of claims 7 to 14, characterized in that  - the proportion of paraffinic hydrocarbon in the  reaction mixture (paraffinic hydrocarbon +  oxygen + any inert diluent + possible water) is  between 5 and 95 mol%, preferably between  10 and 50 mol%  the proportion of water introduced into said mixture can  go up to 30% in moles, in particular up to 15%  in moles  the proportion of inert diluent gas, up to 15% by  moles, being in particular 20 to 60 mol% in  said mixture; and  the molar ratio of the paraffinic hydrocarbon of  Oxygen departure is between 1: 5 and 1: 0.05,  preferably between 1: 2 and 1: 0.2.