MXPA00009752A - Superacid catalyst for the hydroisomerization of n-paraffins - Google Patents

Abstract

A superacid catalyst is described, based on zirconium sulfate, optionally containing a noble metal, having a porosity ranging from 0.1 to 0.30 cm2/g, consisting of at least 70%of pores with a diameter ranging from 1 to 4 nm.

Description

SUPERACTIVE CATALYST FOR THE HYDROISOMERIZATION OF N- PARAFFINS DESCRIPTION OF THE INVENTION The present invention relates to a superacid catalyst based on sulfated zirconium oxide, optionally containing a noble metal, prepared in the presence of acetylacetone. This catalyst is useful in acid catalyzed processes in the hydroisomerization of n-paraffins. Catalysts based on sulfated oxides of zirconium, titanium, iron, have superacidic characteristics, are known in the art, according to the definition of Gillespie, as described, for example, by K. Arata, Adv. Catal. , 37., 165, 1990. These superacid catalysts are usually prepared by means of an articulated synthesis comprising numerous steps. For example, sulfated zirconia (Zr02 / S04 ~ 2) is generally prepared as follows: 1) precipitation of fresh zirconia hydroxide; 2) drying; 3) impregnation with a sulfating agent; 4) calcination. Step (3) can be carried out in various ways: via wet inhibition point, as described in EP 520 543, by treatment with a gaseous stream of H2S or S02, __ as described by J.R. Shon, H.W.Kim, in J. Mol. Catal. , 52, (1989), 361, or by treatment in solution (R. Le Van Mao, S. Xiao and T. Si Le, Catal.Lett., 35, (1995), 107; D. Farcasiu, J. Qi Li, in Appl. Cat. A, 128, (1995), 97). All stages of this preparation are critical: the zirconium precursor, the drying temperature, the sulfating agent, the concentration of the sulfating agent solution used, the temperature and the calcination time. As a result, the control and reproduction capacity of the synthesis is complicated. Recently, a simplified synthesis of Zr02 / S042 ~ comprising a single synthesis step has been carried out. For example, H. Arata et al., Use Zr (S04) 2 as a precursor (Bull, Chem. Soc. Jpn., 63, (1990), 244): however, this method does not allow control of the sulfur content. and its dispersion. U. Ciesla et al., Precipitated zirconium hydroxide in the presence of alkyl sulfonates or sulfonates (EUROPACAT II Congress 3-8 September 1995). The crystallization of the amorphous phase begins at very high temperatures, greater than 650 ° C. Another method of synthesis in a single step is based on the gelation of Zr (OC3H7) 4 dispersed in propanol, in an acidic environment by HN03 / H2SO4. The material, before being calcined, must be dried under supercritical conditions (D. A. Ward, E. I. Ko, J. Cat. 150, (1994), 18). In D. Tichit et al., Catal. Let. , 38 (1996) 109-113, Zr (OC3H7) 4 is dispersed in propanol in an acidic environment by H2SO4. The materials obtained after calcination at 650 ° C consist of the tetragonal phase associated with small amounts of monoclinic phase. Patent application MI 97A00358 describes a sulfated zirconia catalyst with particular porosity characteristics and with high acid properties, prepared by means of a process in a single reaction step. This superacid catalyst comprises zirconium oxide on the surface of which sulfate groups are present in an amount corresponding to a total coverage of the zirconium oxide surface by means of a monolayer of these sulfate groups, which is characterized by a varying porosity. from 0.1 to 0.3 cm3 / g consisting of at least 70% of pores having a diameter ranging from 1 to 4 nm. According to a preferred aspect, this material can additionally contain a noble metal, preferably platinum, in an amount ranging from 0.1 to 3% by weight. These materials are prepared by means of a process which comprises: (a) hydrolysis in an alkaline environment of a zirconium hydrolyzable compound in the presence of tetra-alkylammonium hydroxide (TAA) and sulfuric acid (b) drying of the resulting product and its calcination at a temperature that varies from 250 to 650 ° C. The materials obtained in step (b) can be impregnated with a solution of a noble metal compound to obtain a sulfated zirconia on the surface of which a noble metal is deposited in an amount ranging from 0.1 to 3% by weight . The presence of superacid sites in these materials is verified by pyridine absorption and by FT-IR spectrum analysis. In fact it is specified by K. Tanabe et al., Successful Design of Catalysts, T. Inui Ed., (1988), 616, that sulfated zirconia has an intense IR band at approximately 1370 cm "1, attributed to the asymmetric tension of the group S = 0. Absorption of pyridine causes a consistent displacement of this signal and the entity of this displacement correlates with the superacid strength of the material and its catalytic properties.The material described in MI 97A00358 shows a displacement ranging from 50 to 60 cm "1 against a maximum value that is These catalysts based on sulfated zirconia are superacid solids and therefore can be used in acid-catalyzed reactions. When they additionally contain a noble metal, these are bifunctional catalysts that can be used in processes. hydroisomerization of n-paraffins, to convert these hydrocarbons with a linear chain to hydrocarbons with a branched chain. However, the light n-paraffins can be subjected particularly to hydroisomerization to obtain hydrocarbons with a branched chain having a higher octane number, for use as fuels.

Unexpectedly we have now fothat by carrying out the synthesis of sulfated zirconia in the presence of acetylacetone, catalysts based on zirconium oxide are obtained, on whose surface sulfate groups with improved superacid characteristics are foand which are therefore more active in reactions catalyzed by acid. Therefore, the present invention relates to a superacid catalyst comprising zirconium oxide on whose surface sulphate groups are present, in an amount corresponding to the total coverage of the zirconium oxide surface by means of a monolayer of these groups sulfate, which has a porosity ranging from 0.1 to 0.30 cm3 / g, consisting of at least 70% pores with a diameter ranging from 1 to 4 nm, and optionally containing a noble metal in an amount varying from 0.1 to 3% by weight, obtained by means of: (a) hydrolysis in an alkaline environment of a hydrolyzable zirconium compoin the presence of tetraalkylammonium hydroxide (TAA) and sulfuric acid and acetylacetone; (b) drying the resulting product and calcination at a temperature ranging from 250 to 650 ° C; (c) optional treatment of the product resulting from step (b) with an aqueous solution of a noble metal compo drying and calcination.

The presence in step (a) of the synthesis of acetylacetone (AcAc) allows the preparation of sulfated zirconia with improved superacid characteristics, and therefore more active in acid catalyzed reactions. In step (a) of the process for preparing the material of the present invention, the tetraalkylammonium hydroxide is selected from hydroxide of the R type: R 2 R 3 R 4 NOH, wherein R 2 R 2, R 3 and R 4 are the same or different and are alkyl groups , preferably comprising 1 to 6 carbon atoms, the hydrolysable zirconium derivative is selected from alkoxy derivatives, nitrate, sulfate and preferably tetrapopilortozirconate ester. The sulfuric acid is used in aqueous solution in a concentration ranging from 0.01 to 10 M. The molar ratios of the mixture of step (a) are as follows: AcAc / Zr = 0.001 - 0.5 TAA / Zr = 0.05 - 0.25 ROH / Zr = 10-100 H2S04 / Zr = 0.1-0.5 H20 / Zr = 2-100 According to a preferred aspect, the zirconium compois dispersed in an alcohol ROH, where R is an alkyl with 1 to 6 carbon atoms and preferably is propanol; then acetylacetone is added to this mixture followed by tetraalkylammonium hydroxide in an aqueous solution, preferably tetrapropylammonium hydroxide, and the resulting solution is left r stirring for a few hours before adding the sulfuric acid solution. The resulting dense suspension, which should have a basic pH, is left r agitation for a period of time ranging from 2 to 20 hours. In step (b) of the resulting product, after possible concentration, it is dried at a temperature ranging from 80 to 150 ° C and then calcined at a temperature ranging from 250 to 650 ° C, preferably from 400 to 600 ° C C. According to a preferred aspect, the catalytic material of the present invention can additionally contain a noble metal, preferably platinum, in an amount ranging from 0.1 to 3% by weight. These catalysts are prepared by treating the material obtained in step (b) above with an aqueous solution of a noble metal compo drying and calcination. Preferably, the material obtained in step (b) is impregnated by means of the "wet inhibition" method which is well known in the art, with an aqueous solution of a noble metal compo preferably platinum, to deposit an amount of noble metal ranging from 0.1 to 3% by weight. Hexachloroplatinic acid and tetravalent platinum ammonia complexes are preferred for the purposes. This is followed by drying at a temperature ranging from 80 ° to 150 ° C and calcination at a temperature ranging from 400 ° to 600 ° C. The materials of the present invention are completely X-ray crystallized and consist of tetragonal zirconia crystallites having dimensions less than 300A generally between 50 and 150 A. The surface area of these materials is greater than 30 m2 / g, preferably between 60 and 120 m2 / g. The sulphate content can be determined by chemical analysis and corresponds to the theoretical value which can be calculated for the total coverage of the zirconia surface by means of a monolayer of sulphate groups, as described by P. Nascimiento et al., In "New Frontiers in Catalysis", Proceedings of the Third International Congress on Ctalytics (L. Guczi et al., EDS.) P. 1185, Elsevier (1993). For a surface area between the preferred values indicated above, the sulfur content in the catalyst, which corresponds to a monolayer of sulfate groups, is between 1 and 3% by weight. The sulphate groups are bound to zirconia by means of the hydroxyls present on their surface, and therefore the possibility of obtaining a monolayer of sulphates, which corresponds to the maximum acidity of the material, is related to the presence of a sufficient number of hydroxyls on the surface of ziconia. The sulfated zicone based catalysts of the present invention are superacid solids which are more active than the sulfated zirconia prepared as described in MI 97A00358, ie, without the presence of acetylacetone in the synthesis mixture. They are materials which can be used in reactions catalyzed by acid and in particular, when they additionally contain a noble metal, they are difunctional catalysts with an improved activity in the hydroisomerization process of n-paraffins, to convert these hydrocarbons with a linear chain into hydrocarbons with a branched chain, that is, they provide much greater conversions and comparable selectivities with respect to the sulfated zirconia prepared without acetylacetone. According to a preferred aspect, the best results can be obtained particularly when the light n-paraffins containing from 4 to 10 carbon atoms are subjected to hydroisomerization to obtain hydrocarbons with a branched chain having a higher octane number, for use as fuels. By subjecting C5-C6 n-paraffins, or a mixture thereof, to hydroisomerization, as the light straight running fraction is derived from the upper part, light gasolines are obtained with a high octane number. On the other hand, by subjecting C6-C8 n-paraffins, or their mixtures, to hydroisomerization, it is possible to obtain heavy gasolines with a high octane number. The operation is carried out in the presence of hydrogen, at a temperature ranging from 25 ° to 300 ° C, preferably between 50 ° and 130 ° C, and at a pressure ranging from 5 to 80 bars, preferably between 20 and 50 bars . The noble metal is preferably platinum. The space velocity WHSV (hours "1), is expressed as g of paraffin / g of catalyst, hours, is between 0.01 and 1, and the molar ratio of hydrogen / paraffins 5 and 30. According to another aspect, these catalysts Bifunctional can be used in wax hydroisimerization processes (n-paraffin with a number of carbon atoms equal to or greater than 12) to improve the pour point and the viscosity index, to obtain bases for lubricating oils.

EXAMPLE 1 66 g of Zr (OC3H7) 4 are added to 70% by weight in propanol, 0.14 g of acetylacetone and 10 g of tetrapropylammonium hydroxide at 40% by weight, in aqueous solution, to 364 g of n-C3H7OH. After two hours of stabilization under stirring, 50 g of an aqueous solution of H2SO4 0.44 M are added. The mixture is left under stirring for 4 hours at room temperature, and then an additional 4 hours at 60 ° C. The mixture is dried for 8 hours at 100 ° C and then calcined for 5 hours at 550 ° C. The material obtained after calcination consists of a pure tetragonal phase, with crystallites having an average diameter of 90 Á, a surface area of 74 m2 / g, a pore volume of 0.1 cm 3 / g. Porosity less than 10 Á or greater than 40 Á is not present. The final sulfur content, determined by chemical analysis, is 1.5%, which corresponds to a total monolayer surface coverage of sulphate groups.

EXAMPLE 2 g of the material prepared in the example are impregnated 1 with the wet-point inhibition technique, with 1.6 ml of an aqueous solution of H2PtCl6 containing 0.031 g of Pt per ml. The resulting product is dried at 100 ° C and calcined at 550 ° C. A catalyst with a total Pt content of 0. 5% by weight.

EXAMPLE 3 g of the material prepared in the example are impregnated 1 with the point of inhibition technique, with 1.6 ml of an aqueous solution of H2PtCl6 containing 0.063 g of Pt per ml. The resulting product is dried at 100 ° C and calcined at 550 ° C.

A catalyst with a total Pt content of 1% by weight is obtained.

EXAMPLE 4 (comparative) A sulphated zirconia is prepared as described in MI97A0038: 33 g of Zr (OC3H7) 4 are added to 70% by weight in propanol and 5 g of tetrapropylammonium hydroxide at 40% by weight in an aqueous solution, at 182 g of n -C3H7OH. After two hours of stabilization under stirring, 25 g of an aqueous solution of H2SO4 0.44 M are added. The mixture is left under stirring for 4 hours at room temperature and then an additional 4 hours at 60 ° C. The sample is dried for 8 hours at 100 ° C and then calcined for 5 hours at 550 ° C. The material obtained after calcination consists of a pure tetragonal phase, with crystallites having a diameter of 85 Á, a surface area of 103 m2 / g, a pore volume of 0.162 cm3 / g with a diameter distribution of pore centered on 35 Á. The final sulfur content determined by chemical analysis is 1.7%, which corresponds to the total monolayer surface coverage of sulphate groups.

EXAMPLE 5 (comparative) 10 g of the material prepared in Example 4 are impregnated with the technique of the point of inhibition, with 1.6 ml of an aqueous solution of H2PtCl6 containing 0.031 g of Pt per ml. The resulting product is dried at 100 ° C and calcined at 550 ° C. A catalyst with a total Pt content of 0.5% by weight is obtained.

Example 6 (comparative) 0 g of the material prepared in the example are impregnated 4 with the wet-point inhibition technique, with 1.6 ml of an aqueous solution of H2PtCl5 containing 0.063 g of Pt per ml. 5 The resulting product is dried at 100 ° C and calcined at 550 ° C. A catalyst with a total Pt content of 1% by weight is obtained. 0 EXAMPLE 7 (catalytic test) A sample of the catalyst synthesized as described in Example 2 is loaded into a fixed bed tubular reactor and tested by the hydroisomerization reaction of n-heptane, under the following operating conditions: T = 100 ° C P H2 = 50 bars H2 / n "-C7 = 18 moles / moles l / WHSV = 1-15 hours Figure 1 indicates the conversion of n-heptane in relation to the contact time expressed as l / WHSV The WHSV parameter is calculated as (g of n-C7) / (catalyst hours) The obtained hydroisomerization products are: methylhexanes, dimethylpentanes and trimethylbutanes The selectivity of hydroisomerization is greater than 90%.

Claims (19)

1. CLAIMS 1. A superacid catalyst, comprising zirconium oxide, on whose surface sulfate groups are present in an amount corresponding to the total coverage of the zirconium oxide surface by means of a monolayer of these sulfate groups, which optionally contains a noble metal in an amount ranging from 0.1 to 3% by weight, with a porosity ranging from 0.1 to 0.30 cm3 / g, consisting of at least 70% pores with a diameter ranging from 1 to 4 nm, obtained by means of a process, which comprises: (a) hydrolysis in a basic environment of a hydrolyzable zirconium derivative in the presence of tetraalkylammonium hydroxide, sulfuric acid and acetylacetone; (b) drying the product resulting in its calcination at a temperature ranging from 250 to 650 ° C; (c) optional treatment of the product resulting from step (b) with an aqueous solution of a noble metal compound, drying and calcination.
2. 2. The catalyst as described in claim 1, having a surface area ranging from 60 to 120 m2 / g and a sulfur content by weight, based on the weight of the catalyst, ranging from 1 to 3%.
3. 3. The catalyst as described in claim 1, wherein the noble metal is platinum.
4. 4. The catalyst as described in claim 1, wherein tetraalkylammonium hydroxide of type R1 # R2, R3 and R4NOH is selected, wherein R1 # R2, R3 and R4 are the same or different alkyl groups. 5. The catalyst as described in claim 4, wherein the alkyl groups contain 1 to 6 carbon atoms. 6. The catalyst as described in claim 5, wherein the tetraalkylammonium hydroxide is tetrapropylammonium hydroxide. The catalyst as described in claim 1, wherein the hydrolysable zirconium derivative is selected from alkoxy derivatives, nitrate and sulfate. The catalyst as described in claim 7, wherein the zirconium hydrolyzable derivative is tetrapropylortozirconate. 9. The catalyst as described in claim 1, wherein sulfuric acid is used in aqueous solution with a concentration ranging from 0.01 to 10 M. The catalyst as described in claim 1, wherein the molar ratios in the mixture in stage (a) are as follows: AcAc / Zr = 0.001 - 0.
5. 5 TAA / Zr = 0.05 - 0.25 ROH / Zr = 10-100 H2S04 / Zr = 0. 1 - 0 5 H2? / Zr = 2 - 100 11. The catalyst as described in claim 1, wherein, in step (c) the material obtained in step (b) is impregnated with the "wet inhibition" method with an aqueous solution of a noble metal compound, to deposit a noble metal amount of 0.1 to 3% by weight. 12. The use of the catalyst, as described in claim 1, in acid catalyzed processes. 13. A n-paraffin hydroisomerization process, characterized in that an n-paraffin, or a mixture of n-paraffins, is contacted, under hydroisomerization conditions, with a superacid catalyst containing a noble metal, as described in any of claims 1 to 11. The process as described in claim 13, wherein the n-paraffin is a paraffin having from 4 to 10 carbon atoms. 15. The process as described in claim 13, characterized in that it is carried out in the presence of hydrogen, at a temperature ranging from 25 ° to 300 ° C, at a pressure ranging from 5 to 80 bars, at a speed of space WHSV, at hours "1, which is expressed as g of paraffin / g of catalyst hours, which varies from 0.01 to 1, and a molar ratio of hydrogen / paraffin that varies from 5 to 30. 16. The process as it is described in claim 15, wherein the temperature varies from 50 to 130 ° C and the pressure varies from 20 to 50 bars. 17. The process as described in claim 13, wherein the n-paraffin contains from 5 to 6 carbon atoms. 18. The process as described in claim 13, wherein the n-paraffin contains from 7 to 9 carbon atoms. 19. The process as described in claim 13, wherein the n-paraffin is a wax.