Method for preparing cumene

Abstract: Cumene is prepared by alkylation of benzene with propylene, or by transalkylation of diisopropylbenzenes in the presence of benzene, on a silica and alumina gel catalyst which is amorphous to X-rays and has an SiO₂/Al₂O₃ ratio of between 50/1 and 300/1, a surface area of between 500 and 1000 m²/g, an overall pore volume of between 0.3 nad 0.6 ml/g and mean pore diameter of the order of 10 Å, and which is free or substantially free of pores having a diameter exceeding 30 Å.

Description: This invention relates to a catalytic process for preparing cumene by alkylating benzene with propylene, or by transalkylating diisopropylbenzenes in the presence of benzene, and, also, to a process for preparing an alkylation/transalkylation catalyst. GN-A-769 383, US-A-2 887 520 and US-A-4 912 279 disclose several hydrocarbon modification processes, including alkylation runs, but are impaired by drawbacks inherent in corrosion problems and disposal of the spent catalyst. To overcome such shortcomings, US-A-4 459 425 and US-A-4 395 372 have suggested zeolites, more particularly zeolite Y, as the catalyst for the reactions of the kind referred to above. Zeolite catalysts, however, have a low selectivity towards the useful reaction product, due to a large production of polyalkylation products and oligomerization of the alkylating agent. EP-A-0 160 145 discloses an aromatics alkylation process catalyzed by an amorphous silica and alumina gel with a pore diameter in the order of 5 nm to 50 nm (50 A to 500 A), having a silica to alumina ratio of from 1:1 to 10:1. Catalysts of this kind are fairly active with C₄-C₂₀ alkylating agents. Finally, US-A1-B 366 790 discloses a cumene preparation process which is carried out by reacting benzene with propylene in the liquid phase in the presence of an amorphous aluminium silicate at a comparatively high temperature, that is, a temperature of from 249°C to 316°C. The technical problem was thus to prepare cumene by the reactions referred to above with an unexpectedly high selectivity towards the useful reaction product and adopting lower reaction temperatures. The present invention, therefore, provides a catalytic process for preparing cumene by alkylating benzene with propylene, or by transalkylating diisopropylbenzenes in the presence of benzene, either reaction being catalyzed by an X-ray-amorphous silica-alumina gel having an SiO₂/Al₂O₃ of from 50:1 to 300:1, comprising the step of reacting benzene with propylene at a temperature of from 100°C to 250°C under a pressure of from 2 MPa to 5 Mpa (20 to 50 ata), the molar ratio of benzene to propylene being from 2:1 to 30:1, or transalkylating diisopropylbenzenes in the presence of benzene at a temperature of from 150°C to 250°C under a pressure of from 1 Mpa to 4 MPa (10 to 40 ata), the molar ratio of benzene to the diisopropylbenzenes being from 10:1 to 40:1, in the presence of an X-ray-amorphous silica-alumina gel having a surface area of from 500 m²/g to 1000 m²/g, an overall pore volume of from 0,3 ml/g to 0, 6 ml/g, and a mean pore diameter of from 1 nm (10 A), said silica-alumina gel being free of pores having a diameter greater than 3 nm (30 A). According to another aspect, the present invention also provides a process for preparing an X-ray-amorphous silica-alumina gel having a surface area of from 500 m²/g to 1000 m²/g, an overall pore volume of from 0,3 ml/g to 0, 6 ml/g, and a mean pore diameter of from 1 nm (10 A), said silica-alumina gel being free of pores having a diameter greater than 3 nm (30 A) for use as an alkylation or transalkylation catalyst, comprising the steps of: (a) Preparing an aqueous solution of a tetraalkyl hydroxide (TAA-OH), a water soluble Al compound capable of being hydrolyzed to Al₂O₃ and a soluble Si compound capable of being hydrolyzed to SiO₂, the molar ratios of the solutes being: SiO₂ to Al₂O₃: from 30:1 to 500:1 TAA-OH to SiO₂: from 0,005:1 to 0,2:1 H₂O to SiO₂ from 5:1 to 40:1; (b) Heating said aqueous solution to gel it; (c) Drying the as-obtained gel, and (d) Calcining the dried gel initially in an inert atmosphere and finally in an oxidizing atmosphere. Preferred aluminium trialkyl hydroxides (trialkoxides) for step (a) are Al tri-n.propoxide or Al tri-isopropoxide. A preferred tetraalkyl silicate for stage (a) is tetraethylsilicate. A convenient temperature for step (a) is from 20°C to 25°C. According to a preferred embodiment of the catalyst preparation process, the molar ratios of the solutes are: SiO₂ to Al₂O₃: from 50:1 to 300:1 TAA-OH to SiO₂: from 0,005:1 to 0,2:1 H₂O to SiO₂ from 10:1 to 25:1. Step (b) is preferably conducted at a temperature of from 50°C to 70°C, and a still preferred temperature is 60°C. The drying step (c) is carried out at a temperature not above 150°C and stage (c) can be conveniently started by spray-drying. The calcining step (d) is conveniently conducted at a temperature of from 550°C to 700°C and is carried out initially in a nitrogen environment and completed in the presence of air. The silica-alumina gel thus obtained, having the properties reported in the foregoing, is extremely active in the alkylation of aromatics with olefins (or their precursors), and furthermore it is particularly selective in the alkylation of benzene with propylene. Consequently such silica and alumina gel has the inherent advantages of zeolite Y such as the high activity and compared with these is more selective towards the monoalkylation product cumene (isopropylbenzene) for equal reactant ratios. The silica and alumina gel used as catalyst can be mixed with suitable metal oxides acting as binders. Suitable oxides for this purpose are aluminas, silicas, or titanium, magnesium or zirconium oxides. The silica and alumina gel and the binder can be mixed in weight ratios of between 50/50 and 95/5, and preferably between 70/30 and 90/10. The two components can be mixed by conventional methods and the mixture is conveniently consolidated into the final desired form, such as extrusions or granules. By operating in this manner the catalyst can be given improved mechanical properties. The alkylation reaction can be conducted batchwise, semicontinuously or preferably continuously The reaction is preferably carried out continuously in a flow reactor at a temperature of between 100°C and 250°C, preferably between 110°C and 200°C, at a pressure of between 2 MPa and 5 MPa (20 and 50 ata), preferably of 3 MPa (30 ata), and with the reactants (benzene and propylene) being fed at a WHSV of between 0.1 and 50 hour⁻¹. The reaction can be conducted in the gaseous phase, however it is preferable to operate in the liquid or mixed (liquid-vapour) phase. This minimizes carbon formation, so extending the catalyst life and giving improved selectivity. The molar feed ratio of benzene to propylene can generally vary from 2/1 to 30/1 but it is preferable to operate within the range of 4/1 to 15/1. Within this range the catalyst life (time before regeneration) can be kept long, and the formation of higher molecular weight compounds (polyalkylates and propylene oligomers) can be minimized. The alkylation reaction is exothermic and therefore under industrial conditions the temperature is controlled preferably by feeding cold benzene or inert paraffins (such as propane) at various levels of the catalyst bed. The described silica and alumina gel is also active in the transalkylation of diisopropylbenzenes in the presence of benzene. These diisopropylbenzenes can be the by-products of the alkylation of benzene with propylene. The molar ratio of benzene to diisopropylbenzenes can conveniently vary from 10/1 to 40/1 and the transalkylation reaction can be conducted at a temperature of between 150 and 250°C at a pressure of between 10 and 40 ata and, in the case of a continuous process, with a reactant feed rate in terms of WHSV of between 1 and 30 hour⁻¹. The transalkylation reaction can be conveniently conducted in the liquid phase. In the experimental examples given hereinafter a silica and alumina gel is used having the following characteristics: amorphous to X-rays (analysis conducted on powder by Philips vertical goniometer using Cu K α radiation); molar SiO₂/Al₂O₃ ratio 100/1 surface area = 800 m²/g (measured by Carlo Erba Sorptomatic 1800 apparatus); porosity = 0.44 ml/g, mean pore diameter 1 nm (10 Å), absence of pores with diameter exceeding 3 nm (30 Å) (values determined by Carlo Erba Sorptomatic 1800 apparatus). This silica and alumina gel was prepared by the following procedure. 2.0 g of aluminium tri-isopropylate are dissolved in 34 g of 30.6 weight% tetrapropyl ammonium hydroxide (TPA-OH), and 162 g of demineralized water are added. These operations are conducted at ambient temperature (about 20°C). The solution obtained is heated to 60°C and 104 g of tetraethyl silicate (TES) are added while stirring. The mixture obtained has the following molar ratios: SiO₂/Al₂O₃ 100/1 TAA-OH/SiO₂ 0.10/1 H₂O/SiO₂ 21/1. The mixture is kept stirred at 60°C and after 30 minutes a homogeneous gel is obtained and dried in a Rotavapor flask in an air stream with a bath temperature-controlled at 90°C, followed by drying in an oven at 100°C. The dried gel is calcined at 550°C for 3 hours in a nitrogen stream and then for 10 hours in an air stream. In this manner 30 g of silica and alumina gel are obtained with a quantitative yield with respect to the initially fed silicon and aluminium, and having the said characteristics. The experimental examples given hereinafter further illustrate the invention. In these examples alkylation tests on benzene with propylene and transalkylation tests on diisopropylbenzenes in the presence of benzene are described using the aforesaid silica and alumina gel catalyst. For comparison purposes alkylation tests on benzene with propylene are described using zeolite Y of the known art. EXAMPLE 1 20.0 g of silica and alumina gel having the characteristics given in the description are fed into an autoclave together with 300 ml of benzene and 9.8 g of propylene. The molar ratio of feed benzene to propylene is therefore 14.5/1. The autoclave is pressurised with nitrogen to obtain a total pressure of 4 MPa (40 ata) at the reaction temperature, which is fixed at 160°C. The mass is kept stirred for one hour after which the reactor contents are discharged and analysed chromatographically by means of a Hewlett Packard gas chromatograph with an QV 101 wide bore column and an 80°C-240°C programmed temperature. The results of this analysis are given in Table 1, in which the headings have the following meanings: C₆ % conv. = percentage conversion or benzene C₃ % conv. = percentage conversion of propylene C₆ % sel. = molar percentage selectivity of benzene converted into cumene C₃ % sel. = molar percentage selectivity of propylene converted into cumene. EXAMPLE 2 (comparison) Example 1 is repeated using as catalyst a protonic zeolite Y produced commercially by Union Carbide (Linde LZ-Y 62; 1,6 mm (1/16") extrusions). The results of this test are given in Table 1. EXAMPLE 3 The procedure of Example 1 is followed using the same silica and alumina gel catalyst, but with the following conditions: temperature 150°C, pressure 3 MPa (30 ata) molar ratio of feed benzene to propylene 8.0/1 (300 ml benzene fed) and reaction time 1.5 hours. The results of this test are given in Table 1. EXAMPLE 4 (comparison) The procedure of Example 3 is followed using as catalyst a commercial zeolite Y of Toyo Soda (TSZ 360 HUD, in extrusion form). The results of this test are given in Table 1. EXAMPLE 5 The procedure of Example 1 is followed using the same silica and alumina gel catalyst, but with the following conditions: temperature 150°C, pressure 1,05 MPa (10.5 ata) (autogenous system pressure at the stated temperature), molar ratio of feed benzene to propylene 8.0/1 and reaction time 2 hours. The results of this test are given in Table 1. EXAMPLE 6 (comparison) The procedure of Example 5 is followed using the zeolite Y of Example 2 as catalyst. The results are given in Table 1. EXAMPLE 7 (comparison) The procedure of Example 5 is followed using the zeolite Y of Example 4. The results are given in Table 1. EXAMPLE C₆ % CONV. C₃ % CONV. C₆ % SEL. C₃ % SEL. 1 4.8 72.1 95.7 91.7 2 5.3 85.2 93.2 87.2 3 8.5 73.8 91.3 83.4 4 9.0 82.2 88.3 78.7 5 8.3 70.4 92.7 86.3 6 8.1 71.6 90.0 81.4 7 8.0 71.5 89.3 80.3 Besides a high activity, better selectivity will be noted in the case of the examples conducted according to the invention than in those conducted with zeolite Y catalysts. These better selectivity values derive from the smaller quantity of diisopropylbenzene by-products and the absence or virtual absence of butylbenzenes and pentylbenzenes. The diisopropylbenzenes form by consecutive alkylation reactions between benzene and propylene. The butylbenzenes and pentylbenzenes present as by-products in the examples conducted using zeolite Y as catalyst presumably form following oligomerization of the propylene to nonene, cracking of said nonene to C₅ and C₄ olefins and benzene alkylation by these olefins. EXAMPLE 8 The autoclave of Example 1 is used for a transalkylation test using silica and alumina gel with the characteristics given in the description. Specifically, 20 g of said catalyst are used, feeding into the reactor 256 g of benzene and 17 g of diisopropylbenzenes (molar ratio of the two reactants 31/1). The reaction conditions are: pressure 3 MPa (30 ata), temperature 250°C, reaction time 2 hours. Analysis of the reaction products indicates a diisopropylbenzene conversion of 86% with a selectivity towards cumene of 90%. EXAMPLE 9 The procedure of Example 8 is repeated except that the reaction temperature is fixed at 210°C. In this case the diisopropylbenzene conversion is 76% with a cumene selectivity of 90%. EXAMPLE 10 The alkylation reaction in the presence of silica and alumina gel having the characteristics given in the description is conducted in a vertical flow reactor. The reactor is formed from a 36 mm diameter tube of the height of 900 mm. A multiple thermocouple is inserted longitudinally to measure the catalyst bed temperature at various levels. The reactor is immersed in a bath of fluidized alumina to remove the heat of reaction. In this example the silica and alumina gel catalyst is mixed with γ-alumina in a weight ratio of 100:25 and formed into 1/8" diameter extrusions of average length 8 mm. The reaction conditions are: temperature 160°C, pressure 4 MPa (40 ata), molar ratio of feed benzene to propylene 14.5/1, feed rate of organic phase (benzene + propylene) in WHSV 9.1 hour⁻¹. The operation proceeds continuously for about 300 hours without observing appreciable variations in the catalyst behaviour. Reactant conversions and selectivity towards the useful reaction product are totally similar to those of Example 1.

Claims: A catalytic process for preparing cumene by alkylating benzene with propylene, or by transalkylating diisopropylbenzenes in the presence of benzene, either reaction being catalyzed by an X-ray-amorphous silica-alumina gel having an SiO₂/Al₂O₃ of from 50:1 to 300:1, comprising the step of reacting benzene with propylene at a temperature of from 100°C to 250°C under a pressure of from 2 MPa to 5 Mpa (20 to 50 ata), the molar ratio of benzene to propylene being from 2:1 to 30:1 , or transalkylating diisopropylbenzenes in the presence of benzene at a temperature of from 150°C to 250°C under a pressure of from 1 Mpa to 4 MPa (10 to 40 ata), the molar ratio of benzene to the diisopropylbenzenes being from 10:1 to 40:1, in the presence of an X-ray-amorphous silica-alumina gel having a surface area of from 500 m²/g to 1000 m²/g, an overall pore volume of from 0,3 ml/g to 0, 6 ml/g, and a mean pore diameter of from 1 nm (10 A), said silica-alumina gel being free of pores having a diameter greater than 3 nm (30 A). Process according to Claim 1, wherein the alkylation of benzene with propylene is carried out at a temperature of from 110°C to 200°C under a pressure of 3 MPa (30 ata), the molar ratio of benzene to propylene being from 4:1 to 15:1. Process for preparing an X-ray-amorphous silica-alumina gel having a surface area of from 500 m²/g to 1000 m²/g, an overall pore volume of from 0,3 ml/g to 0, 6 ml/g, and a mean pore diameter of from 1 nm (10 A), said silica-alumina gel being free of pores having a diameter greater than 3 nm (30 A) for use as an alkylation or transalkylation catalyst, comprising the steps of: (a) Preparing an aqueous solution of a tetraalkyl hydroxide (TAA-OH), a water soluble Al compound capable of being hydrolyzed to Al₂O₃ and a soluble Si compound capable of being hydrolyzed to SiO₂, the molar ratios of the solutes being: SiO₂ to Al₂O₃: from 30:1 to 500:1 TAA-OH to SiO₂: from 0,005:1 to 0,2:1 H₂O to SiO₂ from 5:1 to 40:1; (b) Heating said aqueous solution to gel it; (c) Drying the as-obtained gel, and (d) Calcining the dried gel initially in an inert atmosphere and finally in an oxidizing atmosphere. Process according to Claim 3, wherein the molar ratios of the solutes are: SiO₂ to Al₂O₃: from 50:1 to 300:1 TAA-OH to SiO₂: from 0,005:1 to 0,2:1 H₂O to SiO₂ from 10:1 to 25:1. Process according to Claim 3, wherein the alkyl in said tetraalkyl hydroxide (TAA-OH) is selected from ethyl, n.propyl and n.butyl.

Inventors: ARRIGONI VIRGINIO, BELLUSSI GIUSEPPE, CAVANI FABRIZIO, ARRIGONI, VIRGINIO, BELLUSSI, GIUSEPPE, CAVANI, FABRIZIO
Applicants: ENICHEM SINTESI, ENIRICERCHE SPA, ENICHEM SYNTHESIS S.P.A., ENIRICERCHE S.P.A.
Application year: 1989
Main classification: C07C15
IPC codes: B01J35, B01J21, C07C15, C07C2, C07C67, C07C1, C07C6