A process for preparing an amorphous silica-alumina GEL-based catalyst

Abstract

A process for preparing a silica-alumina gel-based catalyst produced in extruded form and usable in the field of hydrocarbon catalytic conversion processes, e.g., alkylation, isomerization and oligomerization, wherein the flammable alcohols resulting from the hydrolysis of metal precursors and gelling are removed by evaporation and replaced, in whole or in part in terms of weight, with demineralized water added to the mixture in an amount of 30 to 100 weight-% of the amount of formed alcohols, so as to maintain the gel with a rheology comparable to that of the original gel containing the alcohols, thus allowing the step of mixing the mixture obtained by gelling with a binder precursor (boehmite or pseudo-boehmite) and a subsequent extrusion of the paste to also be carried out in non-explosion-proof electrical equipment and systems.

Description

[0001] "A PROCESS FOR PREPARING AN AMORPHOUS SILICA-ALUMINA GEL- BASED CATALYST"

[0002]

[0003] Summary of the invention

[0004] [1] The present invention relates to a method for preparing a catalytically active silica and alumina gel in extruded form, which includes mixing a fluid gel, resulting from the hydrolysis and polycondensation of silicon and aluminum alkoxides, with demineralized water and an inert binder, in which the sequence of processing steps allows that the mixing step and the subsequent extrusion of the paste can also be carried out in non-explosion-proof electrical equipment and systems.

[0005] [2] The invention further relates to the use of such a silica and alumina gel as a catalyst in hydrocarbon conversion processes.

[0006] Field of the invention

[0007] [3] Therefore, the present invention describes a method for producing a catalytically active silica-alumina gel, and a related product for use in catalytic hydrocarbon conversion processes, such as: oligomerization of olefins, alkylation of aromatics, isomerization of olefins and / or paraffins .

[0008] [4] Furthermore, the catalyst can conveniently be used as a support for metals or mixtures of metals, in the production of bifunctional hydroprocessing catalysts (hydrocracking, hydroisomerization, etc . ) .

[0009] Background art

[0010] [5] Some silica-alumina gels of amorphous naturehaving catalytic activity are known.

[0011] [6] For example, the article "OLigomerization of Fischer-Tropsch OLefins to DistiLLates over Amorphous SiLica-ALumina" to Arno de Klerk, published in Energy & FueLs 2006, VoLume 20, Issue 5, 2006, pp. 1799-1805, reports the catalytic activity of an amorphous silica-alumina in the oligomerization of olefins produced by Fischer-Tropsch synthesis .

[0012] [7] The article "Control. Led pore sizes and active site spacings determining seLectivity in amorphous siLica-aLumina cataLysts" ro Mark R.S Manton and John C. Davidtz published in JournaL of CataLysis, VoLume 60, Issue 1, 1979, pp. 156-166 describes a method for the synthesis of amorphous silica-alumina catalysts having a controlled pore volume, typically with a diameter between 3.7 and 15 nm.

[0013] [8] It is also known that silica-aluminas are used as catalysts for reactions which require acid catalysis.

[0014] [9] For example, in patent EP 0340862 Bl a method for alkylating aromatic hydrocarbons is described, which uses a silica-alumina gel catalyst, being X-ray amorphous, having a surface area between 500 / g and 1000 m2 / g, and a pore volume between 0.3 ml / g and 0.6 ml / g.

[0015]

[0010] Patent EP 0340868 Bl describes the preparation of a silica-alumina gel according to the following steps: a) preparing an aqueous solution containing a tetraalkylammonium hydroxide (TAA-OH), a soluble aluminum compound, capable of hydrolyzing to Al203, and a silicon compound capable of hydrolyzing to Si02;

[0016] b) heating the solution thus obtained to obtain the hydrolysis and gelling;

[0017] c) drying the obtained gel;d) calcinating the dried gel, first in an inert atmosphere and then in an oxidizing atmosphere.

[0018]

[0011] The silica-alumina gel thus obtained is catalytically active in hydrocarbon conversion methods.

[0019]

[0012] Further methods for preparing this type of catalysts and catalytic applications have been reported in the article "Amorphous mesoporous siLica-aLumina with control. Led pore size as acid cataLysts" to Bellussi Giuseppe, Perego Carlo et al. , published in Studies in Surface Science and Catalysis, Volume 84, 1994, Elsevier Science B.V., pp. 85-92, in which the synthesis, characterization, and catalytic activity of an amorphous, mesoporous silica-alumina are described; the material is the precursor of a ZSM-5 zeolite, prepared in the presence of tetrapropylammonium hydroxide from an alkali-free reaction mixture, and in which the presence of Brbnsted and Lewis sites, the high surface area, and the controlled porosity in the mesopore domain give this material interesting catalytic properties with respect to amorphous silica-aluminas and conventional zeolites .

[0020]

[0013] Patent EP 0691305 Bl describes that, by introducing an alcohol solvent into the preparation of the silica-alumina gel and changing the amount of tetraalkylammonium hydroxide (TAA-0H) with respect to the Si02source, it is possible to obtain silica-alumina gels with different structural and morphological features.

[0021]

[0014] A more detailed study on the role of the ratio between tetrapropylammonium hydroxide (TPA-OH) and Si02is described in the article "InfLuence of pH in mesoporous siLica aLuminas (MSA) synthesis" to Rizzo et al. published in Studies in Surface Science and Catalysis, Volume 144,2002, pp. 625-632, where it is observed that with increasing TPA-OH, the obtained gels change from opalescent white, opalescent, up to clear transparent.

[0022]

[0015] Obviously, there existed the problem of making the above-described silica-alumina gels optimally industrially usable, imparting adequate mechanical resistance properties thereto without affecting the high catalytic performance.

[0023]

[0016] In this respect, some possible procedures for preparing extrudates having sufficiently high mechanical resistance are known to those skilled in the art .

[0024]

[0017] For example, the gel can be ground so as to obtain a powder of appropriate size to be then mixed with a thickener .

[0025]

[0018] Alternatively, powders of silica-alumina gel can be mixed with a second powder of a metal oxide in the presence of a thickener.

[0026]

[0019] Both of the cited techniques allow obtaining extrudates having good mechanical resistance and unaltered catalytic performance, if performed on the gel after drying and calcination.

[0027]

[0020] The same techniques can also be carried out on the simply dried gel: in this case, the presence of TPA-OH and / or its degradation products therein can introduce the formation of a further porosity in the extruded catalyst during calcination, which reduces the mechanical resistance thereof.

[0028]

[0021] Patent EP 0550922 Bl describes a catalyst in extruded form, prepared according to known techniques, comprising a catalytically active part consisting of the silica-alumina gel, described in patent EP 340868 Al, and aninert binder consisting of aluminas belonging to the class of boehmite or pseudo-boehmite; this catalyst generally has good mechanical resistance and is more active than the silica-alumina gel as such.

[0029]

[0022] It is prepared by mixing the catalytically active part consisting of the silica-alumina gel, conveniently ground to obtain a powder with an average diameter of less than 50 microns, with the inert binder and in the presence of a thickener containing a mineral or organic acid, until a homogeneous paste is obtained, which is extruded to give rise to cylindrical catalyst extrudates that are subjected to aging, drying at 100-120°C, and calcination in air at a temperature between 500 and 600°C.

[0030]

[0023] During calcination, the pseudo-boehmite turns into y-alumina (Al203), which contributes to imparting mechanical resistance to the catalyst.

[0031]

[0024] Patent EP 0665055 Bl describes a different method for producing an extruded catalyst similar to that described in EP 0550922 Bl: boehmite, or pseudo-boehmite, is added when preparing the silica-alumina gel, which precedes the drying and calcination thereof, thus obtaining a more active catalyst than both the gel as such and the catalyst in extruded form described in EP 550922 Bl.

[0032]

[0025] In more detail, the preparation of this catalyst in extruded form includes the following steps:

[0033] (1) preparing an aqueous solution of a tetraalkylammonium hydroxide (TAA-OH), an aluminum alkoxide capable of hydrolyzing to Al203, and a silicon alkoxide capable of hydrolyzing to Si02,

[0034] (2) heating the solution thus obtained to trigger hydrolysis and gelling, thus obtaining a mixture (A)with a viscosity between 0.01 and 100 Pa*sec;

[0035] (3) adding to mixture (A) , respectively:

[0036] (i) a binder belonging to the class of boehmites or pseudo-boehmites, and then

[0037] (ii) a mineral or organic acid;

[0038] (4) mixing and heating the mixture obtained in (3) at a temperature between 40 and 90°C until a homogeneous paste is obtained, which is subjected to extrusion;

[0039] (5) drying the extrudate obtained;

[0040] (6) calcinating the dried extrudate in an oxidizing atmosphere.

[0041]

[0026] It is also known indeed that the gels prepared as described in (1) and (2) described above are precursors for the synthesis of zeolites, crystalline microporous materials .

[0042]

[0027] In the literature, these gels have been extensively studied so as to determine when they can be defined as proto-zeolites or embryonic zeolites, for example in the article "Synthesis of Embryonic ZeoLites with Control. Led PhysicochemicaL Properties" to Mariame Akouche et al. , published in Chemistry of Materials, Volume 32, Issue 5, 2020, pp. 2123-2132.

[0043]

[0028] However, during stage (2) the alkoxides hydrolyze, releasing silica and alumina in the desired ratios, generating free alcohols as by-products .

[0044]

[0029] Furthermore, the use of tetraethyl orthosilicate, as a source of silica, releases a large amount of ethyl alcohol that is much higher than that due to the hydrolysis of aluminum isopropoxide (isopropyl alcohol) present in lower amounts .

[0045]

[0030] The presence of alcohols, e.g. , ethyl alcohol,represents a complication in mixing and extrusion step (4) .

[0046]

[0031] In fact, as is known, alcohols are flammable and volatile; therefore for the processing referred to in (4) it is necessary to provide explosion-proof equipment and systems, with associated electric systems suitable for operating in hazardous areas where risks of fire or explosion can exist.

[0047]

[0032] At present, this often is a limitation since the operations of mixing and extruding inorganic materials, such as catalysts, do not require equipment and systems of this type.

[0048] Task of the finding

[0049]

[0033] It is the task of the finding to overcome the limitations of the prior art by providing a method for preparing a silica-alumina gel-based catalyst in extruded form, in which the mixing step and the subsequent extrusion can also be carried out in non-explosion-proof electrical equipment and systems, characterized by investment and operating costs less than those which are explosion-proof .

[0050] Suggested solution

[0051]

[0034] The suggested solution is a method for preparing a silica-alumina gel-based catalyst in extruded form, obtained with an Al203binder, in which the alcohols resulting from the reactions of hydrolysis and gelling are removed and replaced, in whole or in part in terms of weight, with demineralized water, so as to maintain the gel with a rheology comparable to that of the original gel in the presence of alcohols, thus allowing the step of mixing the mixture obtained by gelling with the binder precursor (boehmite or pseudo-boehmite) and the subsequent extrusion of the paste to also be carried out in non-explosion-proofelectrical equipment and systems.

[0052] List of figures

[0053]

[0035] A better understanding of the finding will be achieved with the following detailed description and with reference to the accompanying figure, showing a preferred embodiment by way of non-limiting example.

[0054] In the drawings:

[0055]

[0036] Figure 1 shows a diagram of the steps of the process according to the finding.

[0056] Detailed description of the invention

[0057]

[0037] As already stated, the present invention relates to an innovative method for preparing a silica-alumina gelbased catalyst, processable in non-explosion-proof equipment in the mixing and extrusion steps.

[0058] Process

[0059]

[0038] According to the finding, the method for preparing the silica-alumina gel-based catalyst in extruded form comprises the following steps:

[0060] I. preparing an aqueous solution comprising a tetraalkylammonium hydroxide (TAA-OH), an aluminum alkoxide capable of hydrolyzing to Al203, and a silicon alkoxide capable of hydrolyzing to Si02;

[0061] II. heating the previously obtained solution to initiate hydrolysis and gelling, thus obtaining a mixture (A) with a viscosity of less than 100 Pa*sec (viscosity measured in a dynamic mode, at room temperature, with a rheometer equipped with a coaxial cylinder geometry (couette), with shear rate control);

[0062] III. adding to mixture (A) an amount of demineralized water between 30 and 100% in terms of weight, of the amount of alcohols formed after the hydrolysis of thealkoxides in II, i.e. , the amount of alcohols present in (A); the amount of alcohols present in (A) can be easily calculated from the amount of silicon alkoxide and aluminum alkoxide introduced in step (I), as reported below in the illustrative examples. For example, if the alkoxides used in step I are tetraethyl orthosilicate Si(C2H50)4and aluminum triisopropoxide Al(iC3H70)3, respectively, the overall weight amount Z (g) of ethyl alcohol and isopropyl alcohol is obtainable as follows: .- 0.88461*X + 0.88235*Y, where X is the amount (g) of Si(C2H50)4and Y is the amount (g) of Al(iC3H70)3added in step I . As a result, the amount of demineralized water (g) to be added to the mixture (A) should be between 0.3*Z and Z.

[0063] IV. evaporating the mixture (A) with the addition of demineralized water, referred to in the previous point, in an evaporator capable of operating under vacuum; this operation allows obtaining a mixture with a viscosity, measured by the same methods described for step II, comparable to that of the mixture (A) of step II, i.e. , less than 100 Pa*sec . V. adding to the alcohol-free mixture exiting the previous evaporation step (IV), respectively (in the order) :

[0064] a. a binder belonging to the class of boehmites or pseudo-boehmites, and then

[0065] b. a mineral or organic acid acting as a peptizer; VI. mixing and heating the mixture obtained in (V) at a temperature between room temperature and 90°C until a homogeneous paste is obtained, which is subjected toextrusion;

[0066] VII . drying the extrudate obtained;

[0067] VIII. calcinating the dried extrudate in an oxidizing atmosphere (e.g. , in air) . In this step the binder, boehmite or pseudo-boehmite, loses water this turning into y-alumina . In this sense, it is to be understood as a precursor thereof .

[0068] Step I

[0069]

[0039] According to a preferred non-limiting embodiment, the alkyl group present in the tetraalkylammonium hydroxide (TAA-OH) used for preparing the solution and which serves the function of tempering agent can be ethyl or propyl or butyl or pentyl or hexyl.

[0070]

[0040] Even more preferably, said alkyl group is tetra-n-propylammonium hydroxide (CH3CH2CH2)4N(0H), (TPA-OH) .

[0071]

[0041] Furthermore, according to the preferred nonlimiting embodiment described, the aluminum alkoxide added to the solution is aluminum triisopropoxide Al(i-C3H70)3or aluminum tri-sec-butoxide Al(s-C4H9O)3, while the silicon alkoxide is a tetraalkyl silicate selected from tetramethyl-, tetraethyl-, tetrapropyl- , tetrabutyl- .

[0072]

[0042] Even more preferably, said silicon alkoxide is tetraethyl silicate Si(C2H50)4.

[0073]

[0043] Therefore, the composition of the aqueous solution in stage (I), and the nature of the reagents used, is in accordance with the description in EP 0340868 Bl and EP 0665055 Bl.

[0074] Step II

[0075]

[0044] In the preferred non-limiting embodiment described, the heating stage (II) is carried out at a temperature between 60 and 100°C for a time between 15minutes and 2 hours; in such a stage, as already specified, a mixture (A) is obtained, which is characterized by a viscosity less than 100 Pa*sec.

[0076]

[0045] Viscosity is a critical parameter for both this step and the evaporation step (IV), since too viscous products are difficult to process in the mixing and extrusion step.

[0077]

[0046] It is further necessary to avoid maintaining the mixture (A), obtained in steps (I) and (II), for prolonged periods at high temperatures, such as to promote the hydrothermal synthesis of the zeolite.

[0078]

[0047] According to the finding, the gels prepared as described in steps (I) and (II) are precursors for the synthesis of zeolites, microporous crystalline materials.

[0079]

[0048] As already stated, these gels have been extensively studied in the literature so as to determine when they can be defined as proto-zeolites or embryonic zeolites, as described in the article "Synthesis of Embryonic ZeoLites with Control. Led PhysicochemicaL Properties" to Mariame Akouche et al. , published in Chemistry of Materials, Volume 32, Issue 5, 2020, pp. 2123-2132.

[0080]

[0049] In this specific case, the gel is a precursor of the zeolite ZSM-5, as can be inferred from the article "Amorphous mesoporous siLica-aLumina with controLLed pore size as acid cataLysts" to Bellussi Giuseppe, Perego Carlo et al. , published in Studies in Surface Science and Catalysis, Volume 84, 1994, Elsevier Science B.V., pp. 85-92.

[0081] Step III

[0082]

[0050] The amount of water to be introduced into themixture should not generally exceed the amount of alcohols to be removed corresponding to the alcohols formed upon hydrolysis of the alkoxides in step (II) .

[0083]

[0051] Such an amount of water will preferably be between 30% and 100% of the alcohols obtained during the hydrolysis referred to in (II) in terms of weight.

[0084] Step IV

[0085]

[0052] Both the amount of demineralized water added in the previous step (III) and the operating conditions in the step of evaporating the alcohols referred to in (IV) are conveniently selected to control viscosity and avoid the formation of precipitates .

[0086]

[0053] It is known that the boiling temperature of the ethanol-water azeotrope, at atmospheric pressure, is equal to 78.1 °C; therefore, it is possible to evaporate an ethanol / water mixture (about 95.6 / 4.4 by weight) at atmospheric pressure, heating the mixture (A) to 78.1 °C.

[0087]

[0054] For this reason, the evaporation described in (IV) is preferably carried out in an evaporator under vacuum, by conveniently adjusting the pressure so as to maintain the temperature of the boiler at values generally less than 100°C, more preferably of less than 80 °C.

[0088]

[0055] Therefore, a pressure lower than 760 torr (1 bar) will be applied to the evaporator, down to 100 torr (133 mbar) .

[0089]

[0056] The temperature of the heating fluid in the jacket or coil of the evaporator will be less than or, at most, equal to 80 °C, while the temperature of the mixture (A) in the boiler is a function of the boiling temperature of ethanol, strictly correlated to the degree of vacuum applied .

[0057] As known, there are different types of evaporators, depending on the viscosity of the fluid to be treated and on its possible instability to temperature.

[0090]

[0058] According to the finding it is preferable to use dynamic evaporators, where the treated liquid is moved either by rotation of the evaporator itself (rotary evaporator) or with the aid of a stirrer.

[0091]

[0059] The possibility of operating under vacuum allows the evaporation to be carried out at low temperatures so as to avoid structural modifications or decompositions of the gel components.

[0092] Step V

[0093]

[0060] According to the finding, in stage (V) the y-alumina precursor is preferably a high dispersibility pseudo-boehmite which is added in powder form, preferably with an average particle size of less than 50-60 pm.

[0094]

[0061] As is known, boehmites have the chemical formula A100H and are precursors of y-alumina (Al203), which forms during the step of calcinating the boehmite by loss of water .

[0095]

[0062] Optionally, in stage (V), a plasticizer can also be added in order to promote the extrudability of the paste.

[0096]

[0063] Furthermore, according to the present invention, the introduction of said plasticizer occurs in stage (V) between the addition of the binder and the addition of the mineral or organic acid .

[0097]

[0064] According to the finding, the plasticizer can be a cellulose derivative, e.g. , methyl cellulose or carboxymethyl cellulose, stearin or glycerin.

[0098]

[0065] The methyl cellulose or carboxymethyl cellulose can be added directly in stage (V) in powder form, after thepseudo-boehmite, or by separately preparing an aqueous solution thereof, to which the acid acting as a peptizing agent can be added; the resulting solution is then inserted into the mixer at stage (V) .

[0099]

[0066] The use of an acid to promote the peptization of the pseudo-boehmite is a function of the quality thereof; in fact, there exist commercially available pseudo-boehmites already having a high degree of dispersibility which do not require further peptization.

[0100]

[0067] A peculiarity of the present invention is the use of carboxylic acids, e.g. , formic, acetic, propionic, lactic, oxalic acid, as peptizing agents; in fact, these are eliminated by combustion during the step (VIII) of calcinating the extrudates in air, unlike mineral acids such as HC1, H2SO4, H3PO4, which are not eliminated during calcination and of which, therefore, it is necessary to preliminarily assess whether the presence thereof is detrimental to the catalyst itself or not.

[0101]

[0068] By way of example, chlorine (Cl) binds to aluminum (Al), conferring acidity that could further interfere with the acidity of the final catalyst; therefore, the use of HC1 is thus discouraged.

[0102]

[0069] As for nitric acid (HNO3), on the other hand, it is removed during calcination and therefore its use as a peptizing agent can be considered; however, it should be considered that it can generate nitrogen oxides by decomposition, and that such products add to those produced by the combustion of TPA-OH.

[0103]

[0070] Therefore, nitric acid (HNO3) is also not among the preferred acids for this application.

[0104]

[0071] In the preferred non-limiting embodimentdescribed, the carboxylic acid used is preferably acetic acid .

[0105] Step VI

[0106]

[0072] Advantageously, the replacement of the alcohols with demineralized water, which alcohols are removed during the evaporation step (IV), allows the mixture to which a binder and a carboxylic acid exiting from stage (V) have been added to be easily treated in stage (VI) of mechanical mixing and heating; in fact, such mixing and heating operations promote the evaporation of the solvent which, being water, does not represent a critical issue, unlike the case occurring in the presence of ethanol and isopropanol or other alcohols depending on the alkoxides used in step I, for both issues concerning safety, flammability, and explosiveness, and workers* health, as the vapors of such alcohols can cause severe eye injuries, eye irritation, and irritation of the respiratory tract.

[0107]

[0073] According to the finding, the mixing is carried out until reaching a homogeneous paste with a texture adapted for extrusion.

[0108]

[0074] In a preferred non-limiting embodiment of the finding, small catalyst cylinders are obtained by extrusion, the dimensions of which can vary depending on application requirements; alternative shapes other than cylindrical (e.g. , trilobal, perforated tubes) are obtainable with suitable extrusion heads.

[0109]

[0075] It is good practice to age the extrudates produced at a temperature between 20 and 40°C before drying them.

[0110] Step VII

[0111]

[0076] In stage (VII) the catalyst is subjected to ovendrying at 100-120°C.

[0112] Step VIII

[0113]

[0077] In stage (VIII), the dried catalyst is subjected to calcination in air at a temperature between 500 and 600°C.

[0114]

[0078] Advantageously the catalyst prepared according to the process described in the present invention can be conveniently used in the usual acid-catalyzed petrochemical reactions, e.g. , alkylation, isomerization, and oligomerization .

[0115]

[0079] Furthermore, it can be used as a carrier for transition metal or metals, so as to obtain bifunctional catalysts to be used in hydro-treatment reactions such as hydrocracking and hydroisomerization of hydrocarbon fractions .

[0116] Examples, experimental tests, tables

[0117]

[0080] Production tests of the catalyst according to the finding are given by way of example.

[0118] 1. GeL preparation

[0119]

[0081] A 5 1 stirred reactor is loaded with 615 g of tetra-n-propylammonium hydroxide (CH3CH2CH2)4N(0H), also known as TPA-OH, at 13.35% by weight, 18 g of aluminum triisopropoxide Al(iC3H70)3, and 800 g of demineralized water .

[0120]

[0082] The solution thus obtained is heated at 60°C until complete solubilization of the aluminum compound.

[0121]

[0083] 918 g of tetraethyl silicate Si(C2H50)4are then added under stirring.

[0122]

[0084] Upon addition, two distinct and immiscible liquid phases are observed .

[0123]

[0085] The reactor is closed and thermostated so as tomaintain the temperature between 60°C and 70°C.

[0124]

[0086] Initially, an increase in temperature is observed due to the exothermicity of the hydrolysis reaction.

[0125]

[0087] The temperature then stabilizes at the value previously set on the thermostat and is maintained under these conditions for 60 minutes .

[0126]

[0088] After this time, it is cooled to room temperature and a homogeneous, viscous, opalescent liquid having the following composition is unloaded :

[0127] Composition % by weight

[0128] Si0-Al20311.5

[0129] Ethyl Alcohol 34.5

[0130] Isopropyl Alcohol 0.7

[0131] TPA-OH 3.5

[0132] Water 49.8

[0133] Ratios

[0134] Si02 / Al203100

[0135] TPA- 0H / Si020.092

[0136] H20 / Si0216.8

[0137]

[0089] The liquid thus obtained corresponds to the mixture (A) obtained in steps (I) and (II) of the process according to the present invention.

[0138] 2. ALcohoL evaporation

[0139]

[0090] Some examples of alcohol evaporation, starting from the previously obtained mixture (A), for obtaining a mixture adapted to be then transformed into catalyst, are reported below.2a. Production of mixture B

[0140]

[0091] 400 g of the mixture (A) produced in 1 above are loaded into a rotary evaporator. This mixture contains 140.8 g of alcohols (ethyl and isopropyl) . 130 g of demineralized water, i.e. , 92% of the alcohols present, is added thereto, which is an amount between the previously indicated lower (30%) and upper (100%) limits .

[0141]

[0092] The system is brought under vacuum to a pressure equal to 133 mbar (corresponding to 100 torr) and the thermostatic bath is set at 80°C.

[0142]

[0093] Furthermore, mains water is recirculated in the refrigerant to reflux.

[0143]

[0094] When about 140 g of distillate accumulates in the collecting flask, the evaporation is stopped, returning the system to atmospheric pressure and cooling to room temperature .

[0144]

[0095] The liquid residue remaining in the boiler has a texture, a viscosity, and an opalescence comparable to the starting mixture (A) .

[0145]

[0096] No phase separation nor precipitate formation is observed .

[0146]

[0097] For simplicity reasons, the liquid obtained under the conditions described in the present example is referred to as the mixture (B) .

[0147] 2b. Production of mixture C

[0148]

[0098] 400 g of the mixture (A) produced in 1 above are loaded into a rotary evaporator. This mixture contains 140.8 g of alcohols (ethyl and isopropyl) . 80 g of demineralized water, i.e. , 57% of the alcohols present, is added thereto, which is an amount between the previously indicated lower (30%) and upper (100%) limits .

[0099] The system is brought under vacuum to a pressure equal to 133 mbar (corresponding to 100 torr) and the thermostatic bath is set at 50°C.

[0149]

[0100] Furthermore, mains water is recirculated in the refrigerant to reflux.

[0150]

[0101] When about 140 g of distillate accumulates in the collecting flask, the evaporation is stopped, returning the system to atmospheric pressure and cooling to room temperature .

[0151]

[0102] The liquid residue remaining in the boiler has a texture, a viscosity, and an opalescence comparable to the starting mixture (A) .

[0152]

[0103] No phase separation nor precipitate formation is observed .

[0153]

[0104] For simplicity reasons, the liquid obtained under the conditions described in the present example is referred to as the mixture (C) .

[0154] 2c. Production of mixture D

[0155]

[0105] 200 g of the mixture (A) produced in 1 above are loaded into a three-neck flask, provided with a Claisen head, and stirred with a magnetic stir bar. This mixture contains 70.4 g of alcohols (ethyl and isopropyl) .

[0156]

[0106] The flask is placed in a thermostatic bath.

[0157]

[0107] 100 g of demineralized water (i.e. , 142% of the amount of alcohols present) is added to such a mixture (A), while maintaining the flask under stirring; furthermore, the temperature of the thermostat is set so as to reach 90°C, so as to obtain boiling of the alcohol-water mixture, in the absence of a significant reflux.

[0158]

[0108] Ethanol-propanol and water evaporate under these conditions .

[0109] When an amount of distillate equal to the amount of alcohols initially present, i.e. , equal to about 70 g, has been collected, the formation of a whitish solid precipitate is noted in the viscous liquid remaining in the flask.

[0159]

[0110] Such a solid precipitate does not mix well compared to to the liquid gel, and this results in the product exiting the extrusion step being less homogeneous, due to the presence of domains with the composition of the precipitate and to the lower dispersion than the liquid gel, the extrusion products of which, on the contrary, have very good dispersion and very small silica-alumina domains in the alumina matrix (derived from boehmite) .

[0160]

[0111] Furthermore, the solid precipitate could have a different composition from the starting gel due to segregation phenomena, for example caused by a silica-alumina with a different ratio of the two components from the starting one, or could contain an undesired crystalline phase (zeolite); in particular this latter occurrence is particularly deleterious.

[0161]

[0112] For simplicity reasons, the liquid obtained under the conditions described in the present example is referred to as the mixture (D) .

[0162] 3. Catalyst preparation

[0163]

[0113] Two examples of catalyst preparation starting from the mixtures (B) or (C) previously obtained are reported below.

[0164] 3a. Catalyst obtained from mixture B

[0165]

[0114] An amount equal to 300 g of liquid (B) produced in 2a above is loaded into a double-z type mixer.

[0166]

[0115] 70 g of pseudo-boehmite VERSAL 250 (UOPHoneywell) and 11 g of methylcellulose (Methocel Sigma Aldrich 64625) are added thereto while keeping the mixture under stirring.

[0167]

[0116] After about one hour of mixing, 1.2 g of 30% acetic acid in water are added, and the temperature of the mixer jacket is brought to about 50-60°C.

[0168]

[0117] Hot mixing is continued until a homogeneous paste of a texture adapted for extrusion is obtained.

[0169]

[0118] After extrusion, aging for a few hours at ambient temperature, drying at 100°C for 5 hours and calcinating at 550°C for 8 hours in air.

[0170]

[0119] A catalyst is obtained in the form of cylindrical extrudates adapted to be loaded into fixed-bed reactors, with an apparent density of 0.92 g / cm3.

[0171] 3b. CataLyst obtained from mixture C

[0172]

[0120] An amount equal to 300 g of liquid (C) produced in 2b above is loaded into a double-z type mixer.

[0173]

[0121] 70 g of pseudoboehmite CATAPAL B (Sasol) and 11 g of methylcellulose (Methocel Sigma Aldrich 64625) are added thereto while keeping the mixture under stirring.

[0174]

[0122] After about one hour of mixing, 1.2 g of 30% acetic acid in water are added, and the temperature of the mixer jacket is brought to about 50-60°C.

[0175]

[0123] Hot mixing is continued until a homogeneous paste of a texture adapted for extrusion is obtained.

[0176]

[0124] After extrusion, aging for a few hours at ambient temperature, drying at 100°C for 5 hours and calcinating at 550°C for 8 hours in air.

[0177]

[0125] A catalyst is obtained in the form of cylindrical extrudates adapted to be loaded into fixed-bed reactors, with an apparent density of 1.06 g / cm3.4. ConcLusions

[0178]

[0126] The tests carried out allow determining the optimal amount of demineralized water to be added in order to avoid obtaining a solid precipitate in the residual liquid obtained by the evaporation of alcohols. This amount should be between 30 and 100% of the alcohols originally present .

[0179]

[0127] It is apparent that several variations can be made by those skilled in the art without however departing from the scope of protection of the present invention.

Claims

Claims1. A process for preparing a silica-alumina gel-based catalyst produced in extruded form and usable in processes of catalytic conversion of hydrocarbons, for example alkylation, isomerization and oligomerization, characterized in that it comprises the following steps: I . preparing an aqueous solution comprising a tetraalkylammonium hydroxide (TAA-OH), an aluminum alkoxide capable of hydrolyzing to Al203, and a silicon alkoxide capable of hydrolyzing to Si02;II. heating the previously obtained solution to initiate hydrolysis and gelling, thus obtaining a mixture (A) with a viscosity of less than 100 Pa*sec; such a viscosity being measured in a dynamic mode, at room temperature, with a rheometer equipped with a coaxial cylinder geometry (couette), with shear rate control;III . adding to mixture (A) an amount of demineralized water between 30 and 100% in terms of weight of the amount of alcohols formed after the hydrolysis of the alkoxides in (II), therefore such as to equal, at most, the amount of alcohols present in (A);IV. evaporating the mixture (A) with the addition of demineralized water, referred to the previous point, in an evaporator capable of operating under vacuum, with collection of the alcohol byproducts in (II) in the cold vessel;V. adding to the alcohol-free mixture exiting the previous evaporation step (IV) and having aviscosity, measured as already described for step II, of less than 100 Pa*sec, respectively, in the order :a. a binder belonging to the class of boehmites or pseudo-boehmites, and then, b. a mineral or organic acid acting as a peptizer;VI. mixing and heating the mixture obtained in point (V) to a temperature between 40 and 90°C until a homogeneous paste is obtained, which is subjected to extrusion;VII . drying the extrudate obtained;VIII. calcinating the dried extrudate in an oxidizing atmosphere,whereinthe sequence of said processing steps allows carrying out the mixing step and the subsequent extrusion in non-explosion-proof electrical equipment and systems.

2. A process according to the preceding claim, characterized in that the alkyl group of tetraalkylammonium hydroxide (TAA-OH) used in step (I) serves the function of a tempering agent and is selected from ethyl or propyl or butyl or pentyl or hexyl, preferably it is tetra n-propylammonium hydroxide .

3. A process according to one or more of the preceding claims, characterized in that the aluminum alkoxide capable of hydrolyzing to Al203used in step (I) is selected from aluminum tri-isopropoxide Al(iC3H70)3or aluminum trisec-butoxide Al(s-C4H9O)3, preferably aluminum tri-isopropoxide Al(iC3H70)3.

4. A process according to one or more of the preceding claims, characterized in that the silicon alkoxide used in step (I) is a tetraalkyl silicate selected from tetramethyl-, tetraethyl-, tetrapropyl- , tetrabutyl-, preferably it is tetraethyl silicate Si(C2H50)4.

5. A process according to one or more of the preceding claims, characterized in that the heating step (II) is conducted at a temperature between 60 and 100°C and for a time between 15 minutes and 2 hours .

6. A process according to one or more of the preceding claims, characterized in that the amount of water to be added to the mixed phase during step (III), which does not exceed, in terms of weight, the amount of alcohols obtained during the hydrolysis of point (II), is between 30% to 100% by weight of said alcohols.

7. A process according to one or more of the preceding claims, characterized in that the evaporation step (IV) is preferably conducted in a rotary vacuum evaporator, suitably adjusting the pressure so as to maintain the temperature of the boiler at values generally lower than 100°C, more preferably of less than 80°C.

8. A process according to one or more of the preceding claims, characterized in that the y-alumina precursor added in step (V) is preferably a high dispersibility pseudo-boehmite which is added in powder form, preferably with an average particle size of less than 50-60 pm.

9. A process according to one or more of the preceding claims, characterized in that in step (V) a plasticizing agent is added, such as a cellulose derivative, for example methyl cellulose orcarboxymethyl cellulose, stearin, or glycerin.

10. A process according to one or more of the preceding claims, characterized in that the acid used in step (V) is a carboxylic acid selected from formic, acetic, propionic, lactic, oxalic acid, preferably acetic acid.

11. A process according to one or more of the preceding claims, characterized in that the extruded catalyst exiting step (VI) is subjected to aging at a temperature between 20 and 40°C.

12. A process according to one or more of the preceding claims, characterized in that the drying step (VII) carried out on the aged catalyst exiting step (VI) occurs in an oven at a temperature equal to 100-120°C.

13. A process according to one or more of the preceding claims, characterized in that the calcination step (VIII) is carried out in air at a temperature between 500 and 600°C.

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