Extruded catalyst based on silica/alumina gel

Abstract: A catalyst in the form of extruded bodies is disclosed, which consists of an inert binding agent, boehmite or pseudo-boehmite and a catalytically active phase of a silica/alumina gel. The catalyst is particularly active in acid-catalysed reactions.

Description: The present invention relates to a catalyst in extruded form which is obtained by mixing a high-viscosity sol obtained from the hydrolysis and polycondensation of silicates and aluminates, with an inert binding agent. The invention relates as well to the use of such a catalyst in olefin oligomerization processes. Some silica/alumina gels, of amorphous character, displaying catalytic activity, are known in the art. So, e.g., in EP 160,145 a process for alkylating aromatic hydrocarbons is disclosed, which uses a catalyst consisting of a silica/alumina gel, of amorphous character, having pores with a diameter typically comprised within the range of from 5 to 50 nm (50 to 500 Ängstroms), and with a ratio of silica to alumina typically comprised within the range of from 1 : 1 to 10 : 1. M.R.S. Manton and J. Davidtz in Journal of Catalysis, 60 , 156-166 (1979) describe a process for the synthesis of amorphous silica/alumina catalysts having a controlled pore volume, i.e., typically comprised within the range of from 3.7 to 15 nm. European patent application EP 340,868 discloses a silica/alumina gel, amorphous when examined by X-ray analysis, having a molar ratio of SiO 2 /Al 2 O 3 of from 30 : 1 to 500 : 1, with a specific surface area comprised within the range of from 500 to 1000 m 2 /g, a total pore volume of from 0.3 to 0.6 ml/g, and substantially free from pores with larger diameter than 3 nm (30 Ångstroms). This silica-alumina gel is prepared as follows: (a) an aqueous solution is prepared of a tetraalkyl ammonium hydroxide (TAA-OH), a soluble aluminum compound capable of yielding Al 2 O 3 by hydrolysis and a silicon compound capable of yielding SiO 2 by hydrolysis, in the following mutual molar ratios: -- SiO 2 :Al 2 O 3 = from 30:1 to 500:1 -- TAA-OH:SiO 2 = from 0.05:1 to 0.2:1 -- H 2 O:SiO 2 = from 5:1 to 40:1; (b) the resulting solution is heated in order to cause it to undergo hydrolysis and gelation; (c) the resulting gel is dried; (d) the dried gel is calcined, firstly under an inert atmosphere and then under an oxidizing atmosphere. The resulting silica/alumina gel is catalytically active in hydrocarbon conversion processes. Of course, the problem existed of rendering the silica/alumina gel disclosed in the above said patent application best suitable for industrial use, by endowing it with adequate properties of mechanical strength, without endangering the high catalytic performance thereof. Those skilled in the art are aware of the possible procedures for preparing extruded bodies having high enough mechanical strength values, without altering their catalytic performance. Thus, for example, the catalyst can be ground, so as to obtain powders consisting of particles with a suitable size, which are subsequently blended with a thickener. A further preparation method consists in blending silica/alumina gel powders with a second powder of a metal oxide in the presence of a thickening agent. All these techniques yield extrudates endowed with good mechanical strength and unchanged catalytic performance. In EP 550,922 an extruded catalyst is disclosed which is prepared according to known techniques, and consists of: -- a catalytically active portion constituted by the silica/alumina gel disclosed in EP 340,868; -- an inert binding agent constituted by aluminas belonging to the class of bohemite or pseudobohemite. This catalyst displays a good mechanical strength, and results to be more active than pristine silica/ alumina gel. Said catalyst is prepared by blending the catalytically active portion constituted by silica/alumina gel, suitably ground in order to obtain a powder with a smaller average diameter than 50 microns, with the inert binding agent, in the presence of a thickener containing a mineral or organic acid, until a homogeneous paste is obtained which is then extruded yielding small cylindrical bodies of catalyst which are submitted to ageing, drying at 100-120°C and calcination in air at a temperature comprised within the range of from 500 to 600°C. The present Applicants found now unexpectedly, that by adding boehmite or pseudo-boehmite during that silica/alumina gel preparation step which precedes the drying and calcination steps, a catalyst is obtained which is more active than pristine gel, and the extruded catalyst disclosed in EP 550,922, based on a silica/alumina gel and a binding agent selected from bohemite or pseudo-bohemite. Furthermore, the catalyst according to the present invention requires a lower number of synthesis steps than as required in order to prepare the extruded catalyst of EP 550,922: in particular, the step of grinding the dried/calcined gel is no longer necessary, and the drying and calcination steps are carried out once only, on the extruded catalyst. Therefore, the object of the present invention is a catalyst in extruded form, consisting of an inert binder agent and a catalytically active portion of silica/alumina gel, which is obtained by: (a) preparing an aqueous solution of a tetraalkyl ammonium hydroxide (TAA-OH), a soluble aluminum compound capable of yielding Al 2 O 3 by hydrolysis and a silicon compound capable of yielding SiO 2 by hydrolysis, in the following mutual molar ratios: -- SiO 2 :Al 2 O 3 = from 30:1 to 500:1 -- TAA-OH:SiO 2 = from 0.05:1 to 0.2:1 -- H 2 O:SiO 2 = from 5:1 to 40:1; (b) heating the resulting solution in order to cause the reaction mixture to undergo hydrolysis and gelation, and obtain an (A) mixture having a viscosity comprised within the range of from 0.01 to 100 Pa.sec; (c) adding to said (A) mixture, firstly a binding agent selected from the class of boehmites or of pseudo-boehmites, in a ratio, by weight, to said (A) mixture comprised within the range of from 0.05 to 0.5; and then a mineral or organic acid, in an amount comprised within the range of from 0.5 to 8 g per 100 g of binding agent; (d) mixing and heating up to a temperature comprised within the range of from 40 to 90°C, the resulting mixture from above (c) step, until a homogeneous paste is obtained, which is submitted to extrusion; (e) the resulting extrudate is dried; (f) the dried extrudate is calcined under an oxidizing atmosphere. The composition of the mixture from the (a) step and the nature of the reactants used are in accordance with the teachings of EP 340,868. The (b) step is carried out at a temperature comprised within the range of from 60 to 100°C, during a time comprised within the range of from 15 minutes to 2 hours. Reaching a viscosity of from 0.01 to 100 Pa.sec is critical in this step. More viscous, or less viscous products are unsuitable for those treatments according to the subsequent processing steps. In the (c) step, the binding agent is preferably used in powder form, with an average particle diameter of less than 50 µm. According to a preferred aspect of the present invention, in the (c) step also a plasticizer is added. The plasticizer can be, e.g., methyl cellulose, stearin, glycerol. The plasticizer is added at an intermediate time between the addition of the binding agent and the addition of mineral or organic acid. In the (d) step, the mechanical mixing and heating cause the solvent to evaporate until a homogeneous paste is obtained which has such a consistency as normally regarded as suitable for the extrusion. Cylindical catalyst bodies are obtained with sizes which may be varied as a function of application requirements, and are then submitted to ageing at a temperature comprised within the range of from 20 to 40°C. In the (e) step, the catalyst is submitted to an oven-drying at 100-120°C, and then, in the (f) step, to a calcination in air, at a temperature comprised within the range of from 500 to 600°C. The resulting catalyst has a higher catalytic activity than pristine silica/alumina gel, as well as than silica/alumina gel bound, according to the well-known methods, with boehmite or pseudo-boehmite. This catalyst is furthermore well useable at an industrial level, because it displays an axial breaking strength comprised within the range of from 25 to 280 kg/cm 2 , a bimodal porosity distribution and a specific surface area comprised within the range of from 400 to 600 m 2 /g. The catalyst of the present invention can be suitably used in the usual acid-catalysed petrochemical reactions, as alkylation, isomerization and oligomerization. In particular, it is very effective in the reaction of oligomerization of light olefins, in particular propylene, to yield hydrocarbon cuts displaying extremely good properties as gasoline and jet fuel. Said oligomerization is suitably carried out at a temperature comprised within the range of from 100 to 250°C and under a pressure of from 10 to 70 bars. The following experimental examples are reported in order to better illustrate the invention. Example 1 (Catalyst preparation) An amount of 12 g of aluminum tripropoxide is added to 205 g of tetra-n-propylammonium hydroxide (TPA-OH) at 13.35% by weight; 389 g of demineralized water is then added. The resulting solution is heated up to 60°C until aluminum compound is completely dissolved; 306 g of tetraethyl silicate is then added with stirring. The resulting mixture displays the following molar ratios: -- SiO 2 :Al 2 O 3 = 50 -- TPA-OH:SiO 2 = 0.09 -- H 2 O:SiO 2 = 21. The temperature is kept at 60-65°C until an (A) mixture is obtained with a viscosity of 0.011 Pa.sec. Eight hundred g of this mixture, after a 20-hour ageing at room temperature, is charged to a mixer and then 80 g of pseudo-boehmite (VERSAL 150, tradename, ex La Roche) and 13 g of methyl cellulose (Methocel 64625, tradename, ex Fluka) are added. After about a 1-hour mixing, 1.6 g of glacial acetic acid is added and the temperature of the jacket of the kneader is increased up to about 50-60°C. Kneading is continued of the mixture while hot, until a homogeneous paste is obtained with a suitable consistency for extrusion. After the extrusion, the resulting extrudate is aged overnight at room temperature, the aged extrudate is dried at 100°C during 5 hours and the dried extrudate is submitted to a 8-hour calcination at 550°C in air. A catalyst is obtained which displays a mechanical axial strength of 249 kg/cm 2 and a specific surface area of 608 m 2 /g. Example 2 (Catalyst preparation) An (A) mixture is prepared as disclosed in above Example 1. An amount of 500 g of this mixture is charged to a mixer and then 115 g of pseudo-boehmite (VERSAL 150, tradename, ex La Roche) and 19 g of methyl cellulose (Methocel 64625, tradename, ex Fluka) are added to the mixer. After an about 1-hour mixing, 0.6 g of glacial acetic acid is added and the temperature of the jacket of the kneader is increased up to about 50-60°C. Kneading is continued of the mixture while hot, until a homogeneous paste is obtained with a suitable consistency for extrusion. After the extrusion, the resulting extrudate is aged overnight at room temperature, the aged extrudate is dried at 100°C during 5 hours and the dried extrudate is submitted to a 8-hour calcination at 550°C in air. A catalyst is obtained which displays a mechanical axial strength of 278 kg/cm 2 and a specific surface area of 500 m 2 /g. Example 3 (Catalyst preparation) The process is carried out as in Example 2, without adding methyl cellulose and using 2.3 g of glacial acetic acid. A catalyst is obtained which displays a mechanical axial strength of 25 kg/cm 2 and a specific surface area of 485 m 2 /g. Example 4 (Catalyst preparation) The process is carried out as in Example 3, using 5 g of glacial acetic acid dissolved in 10 ml of water. A catalyst is obtained which displays a mechanical axial strength of 99 kg/cm 2 and a specific surface area of 500 m 2 /g. Example 5 (Reference catalyst preparation) An amount of 12 g of aluminum tripropoxide is added to 205 g of tetra-n-propylammonium hydroxide (TPA-OH) at 13.35% by weight; 389 g of demineralized water is then added. The resulting solution is heated up to 60°C until aluminum compound is completely dissolved; 306 g of tetraethyl silicate is then added with stirring. The resulting mixture displays the following molar ratios: -- SiO 2 :Al 2 O 3 = 50 -- TPA-OH:SiO 2 = 0.09 -- H 2 O:SiO 2 = 21. The temperature is kept at 60-65°C during a 60-minute time. The resulting gel is caused to age for 10 hours at room temperature, is then dried for 3 hours on the Rotavapor Rotary Evaporator, under a flowing air stream, then in an oven at 100°C. After an 8-hour calcination at 550°C, a silica/alumina gel is obtained with a ratio of SiO 2 :Al 2 O 3 of 50, a surface area of 672 m 2 /g and a porosity of 0.454 ml/g. 100 g of this silica/alumina gel is ground within a Rotary Drum Mill until a powder with an average distribution of particle size comprised within the range of from 10 to 200 microns is obtained. To such a powder, 100 g of a commercial pseudo-boehmite (CATAPAL B, tradename, VISTA CHEMICAL COMPANY) is added. Both powders are mechanical mixed on the kneader, during 10 minutes. Separately, an aqueous solution of methylcellulose at 1% by weight is prepared and 130 g of this solution is then acidified with 2 g of glacial acetic acid (at 99.8% by weight). The acidified aqueous methylcellulose solution is then added to the powder blend and mixing is continued until a homogeneous paste is obtained. The so obtained paste is extruded and the resulting extrudate is then submitted to an overnight ageing at room temperature. The aged extrudate is dried at 100°C for 5 hours, and is calcined at 550°C for 8 hours in air. At the end of this process, a catalyst is obtained which shows a mechanical axial strength of 87 kg/cm 2 and of a specific surface area of 482 m 2 /g. Example 6 Propylene oligomerization The extruded catalyst obtained as disclosed in Example 1 was tested in the reaction of propylene oligomerization under the following operating conditions: -- catalyst shape: cylindrical extruded body; -- catalyst size: average diameter of approximately 2.7 mm, average length of approximately 5 mm; -- reactor type: fixed-bed reactor; -- reactor size: inner diameter of 25.4 mm, length of 700 mm; -- feed: 70:30 propylene/propane mixture; -- reactor temperature: 120°C; -- reactor pressure: 35 bars; -- space velocity WHSV: 1 and 1.76 g of propylene per gram of active phase per hour. The results obtained from these tests are reported in Table 1, in which T.O.S. (time on stream) is the total test time: WHSV (h -1 ) Pressure (bars) Temp. (°C) T.O.S. Convers. rate (%) 1 35 120 17 96 1 35 120 28 96 1 35 120 45 96 1.76 35 120 51 91 1.76 35 120 69 90 Example 7 The catalyst obtained according to Example 3 was tested in the reaction of propylene oligomerization under similar operating conditions as of Example 6, except for temperature and WHSV: -- reactor temperature: comprised within the range of from 120°C to 160°C; -- space velocity WHSV: 1.85 g of propylene per gram of active phase per hour. The results obtained from these tests are reported in Table 2: WHSV (h -1 ) Pressure (bars Temp. (°C) T.O.S. Convers. rate (%) 1.85 35 120 15 82 1.85 35 130 38 80 1.85 35 140 62 82 1.85 35 160 133 86 1.85 35 160 210 84 Example 8 The catalyst obtained according to Example 4 was tested in the reaction of propylene oligomerization under similar operating conditions as of Example 6, except for temperature: -- reactor temperature: comprised within the range of from 120°C to 200°C. The results obtained from these tests are reported in Table 3: WHSV (h -1 ) Pressure (bars Temp. (°C) T.O.S. Convers. rate (%) 1.85 35 120 22 91 1.85 35 130 44 92 1.85 35 140 67 93 1.85 35 160 138 92 1.85 35 170 163 92 1.85 35 180 185 92 1.85 35 190 208 92 1.85 35 200 230 93 The product obtained by operating under such conditions as summarized in Table 3, first line, contains a suitable fraction for use as gasolines (boiling temperature comprised within the range of from 60 to 175°C), and a suitable fraction for use as jet fuel (boiling temperature comprised within the range of from 175 to 300°C). The distillation curve of said product is reported in the single accompanying Figure. Example 9 (Comparison Example) The catalyst obtained as disclosed in Example 5 was tested in the reaction of propylene oligomerization under the following operating conditions: -- catalyst shape: cylindrical extruded body; -- catalyst size: average diameter of approximately 2.7 mm, average length of approximately 5 mm; -- reactor type: fixed-bed reactor; -- reactor size: inner diameter of 25.4 mm, length of 700 mm; -- feed: 70:30 propylene/propane mixture; -- reactor temperature: comprised within the range of from 120°C to 200°C; -- inner reactor pressure: 38 bars; -- space velocity WHSV: from 1.8 to 3.1 g of propylene per gram of active phase per hour. The results obtained from these tests are reported in following Table 4: WHSV (h -1 ) Pressure (bars Temp. (°C) T.O.S. Convers. rate (%) 1.8 38 120 24 73 2.7 38 160 139 81 3.1 38 160 204 58 2.2 38 200 480 55

Claims: Catalyst in extruded form, consisting of an inert binder agent and a catalytically active portion of silica/alumina gel, which is obtained by: (a) preparing an aqueous solution of a tetraalkyl ammonium hydroxide (TAA-OH), a soluble aluminum compound capable of yielding Al 2 O 3 by hydrolysis and a silicon compound capable of yielding SiO 2 by hydrolysis, in the following mutual molar ratios: -- SiO 2 :Al 2 O 3 = from 30:1 to 500:1 -- TAA-OH:SiO 2 = from 0.05:1 to 0.2:1 -- H 2 O:SiO 2 = from 5:1 to 40:1; (b) heating the resulting solution in order to cause the reaction mixture to undergo hydrolysis and gelation, and obtain an (A) mixture having a viscosity comprised within the range of from 0.01 to 100 Pa sec; (c) adding to said (A) mixture, firstly a binding agent selected from the class of boehmites or of pseudo-boehmites, in a ratio, by weight, to said (A) mixture comprised within the range of from 0.05 to 0.5; and then a mineral or organic acid, in an amount comprised within the range of from 0.5 to 8 g per 100 g of binding agent; (d) mixing and heating up to a temperature comprised within the range of from 40 to 90°C, the resulting mixture from above (c) step, until a homogeneous paste is obtained, which is submitted to extrusion; (e) the resulting extrudate is dried; (f) the dried extrudate is calcined under an oxidizing atmosphere. Catalyst according to claim 1, characterized in that the (b) step is carried out at a temperature comprised within the range of from 60 to 100°C, during a time comprised within the range of from 15 minutes to 2 hours. Catalyst according to claim 1, characterized in that in the (c) step the binding agent is used in powder form with a lower average diameter of than 50 micrometers. Catalyst according to claim 1, characterized in that in the (c) step, at an intermediate time between the addition of the binder agent and the addition of mineral or organic acid, a plasticizer is added. Catalyst according to claim 4, characterized in that said plasticizer is selected from methyl cellulose, stearin and glycerol. Catalyst according to claim 1, characterized in that the drying in the (e) step is carried out at a temperature comprised within the range of from 100 to 120°C. Catalyst according to claim 1, characterized in that the calcination in the (f) step is carried out at a temperature comprised within the range of from 500 to 600°C. Use of catalyst of claim 1 in acid-catalysed reactions. Use of the catalyst of claim 1 in the oligomerization of light olefins carried out at a temperature comprised within the range of from 100 to 250°C, and under a pressure comprised within the range of from 10 to 70 bars. Use of the catalyst of claim 1 in propylene oligomerization.

Inventors: BASSI GIANLUCA, GIROTTI GIANNI, PEREGO CARLO, BASSI, GIANLUCA, GIROTTI, GIANNI, PEREGO, CARLO
Applicants: AGIP PETROLI, ENICHEM SPA, ENIRICERCHE SPA, ENIRICERCHE S.P.A.
Application year: 1995
Main classification: B01J21
IPC codes: B01J37, C07C2, B01J21