MXPA99006604A - Catalyst based on molibden and its use in the isomerization of n-paraffins - Google Patents

Abstract

Catalyst based on molibden and silicon having a surface area ranging from 20 to 400 m<2>/g and a molar ratio Mo/Si>0.2 obtainable with a process which comprises:a) dissolving a soluble molibden salt in an aqueous solution containing at least one basic compound selected from ammonium hydroxides having general formula (I):R1R2R3R4N<+>OH<->b) adding to the solution of step (a) at least one compound of silicon capable of hydrolyzing to SiO2;c) gelifying and calcining the gel obtained.

Description

CATALYST BASED ON MOLYBDENUM AND ITS USE IN THE ISOMERIZATION OF N-PARAFFINS DESCRIPTION OF THE INVENTION The present invention relates to a catalyst based on molybdenum and its use in the isomerization of n-paraffins. More specifically, the present invention relates to a catalyst based on molybdenum and silicon and its use in the reaction of isomerization of n-paraffins with a number of carbon atoms greater than or equal to 5, preferably between 5 and 40. The use of molybdenum-based catalysts for the isomerization of n-paraffins is known in the literature. In particular, as described in "Proceedings of the 10th International Congress on Catalysis, Budapest, 1992, 955, the catalytic activity of these materials is linked to the formation of molybdenum carbides or oxycarbides which, with respect to the starting oxide , has a larger surface area which ranges from approximately 4 m2 / g (Mo03 to 99.95% purity) at values which can reach approximately 200 m2 / g.

The transition from molybdenum oxide to the catalyst is laborious and can be carried out in various ways, as illustrated in the following. The "Journal of Solid State Chemistry", 59, 1985, 332 and 348, describes tests of molybdenum oxide with a mixture of ammonia / hydrogen at 880 ° C to obtain the corresponding nitride which is then transformed into carbide by treatment with me. pressure / hydrogen at 900 ° C. The carbides obtained with method have a surface area of 140-180 m2 / g. Another method is described in "Journal of Catalysis ", 106, 1987, 125. According to method, the molybdenum oxide is treated with a methane / hydrogen stream at an increased temperature.In" Journal of Catalysis "112, 1988, 44, the oxide can be impregnated previously with 0.25% by weight of platinum that acts as a carburization catalyst, which takes place at an increased temperature above 700 ° C. The final solids have an area of approximately 200 m2 / g.Alternatively, according to what described in "Journal of Catalysis" 117, 1989, 371, molybdenum oxide can be reduced with hydrogen to the metal phase which is then carburized with CO at 100 ° C. O, the carburization reaction can be carried out using M0O3 vapors in activated carbon that obtain materials with a surface area of 100-200 m2 / g, as described in European patent 396 475. The synthesis of molybdenum oxycarbonate itself has recently been described, starting from M0O3 treated or at a low temperature (350 ° C) in a hydrogen / n-tear stream for 24 hours ("Catalysis Today", 35, 1997, 51). The Applicant has found a new catalytic structure based on molybdenum oxide and silica which can be used as such in the reaction of the n-paraffins without requiring any particular reactions. The present invention therefore relates to a catalyst based on molybdenum and silicon having a surface area in the range of 20 to 400 m2 / g and a molar ratio Mo / Si > 0.2. A further object of the present invention relates to a catalyst based on molybdenum and silicon obtainable with a process comprising: a) dissolving a soluble molybdenum salt in an aqueous solution containing at least one basic compound selected from ammonium hydroxides having the general formula (I): wherein the groups R1.-R4, the same or different, represent aliphatic groups containing from 1 to 7 carbon atoms; b) adding to the solution of step (a) at least one silicon compound capable of hydrolyzing SiO2 in such amounts as to give a Mo / Si molar ratio greater than 0.2 and, optionally, an aliphatic alcohol; c) gelling the mixture thus obtained and calcining the obtained gel in air at a temperature in the range of 500 to 600 ° C. Any salt of molybdenum soluble in water or in a basic environment can be used in the process of preparation of the catalyst of the present invention. Practical examples are halogenated molybdenum derivatives, for example, having the formula M0O2X2 wherein X represents a halogen such as chlorine, bromine or fluorine, or has the formula M0OX4, wherein X represents a chlorine or fluorine atom, molybdic anhydride , olibdic acid and hept amol ibdato of harmonium tetrahydrate.

The molybdenum salt is dissolved in an aqueous solution of the basic compound having the general formula (I). Of these basic products, t-propiopropyl ammonium hydroxide is preferred. When the molybdenum salt has dissolved, the hydroxylic silicon compound, optionally diluted with an alcohol, is added to the solution. Preferred silicon compounds according to the present invention are tetraalkyloxy silicon orthosilicate in which the alkyl group contains from 1 to 4 carbon atoms such as, for example, tetra-ethyl orthosilicate. The alcohol is preferably selected from aliphatic alcohols, in particular monoalkoxide is C2_C6 alkyl. The preparation of the molybdenum-based gelifiable solution, steps (a) and (b), substantially takes place at room temperature, the reaction ingredients being dosed in this respect with the following molar proportions: Mo / Si greater than 0.2; OH / (Si + Mo) greater than 0.1; H20 / (Si + Mo) greater than 5; Alcohol / H20 between 0 and 20.

More specifically, the ingredients are preferably dosed to obtain the following molar proportions: Mo / Si between 1 and 100; OH "/ (Si + Mo) between 0.2 and 5; H20 / (Si + Mo) between 10 and 100; Alcohol / H20 between 0.5 and 2. When the reaction mixture has been prepared, the gelation phase begins. carried out at room temperature or at a temperature in the range of ambient values to 100 ° C. The gelation may require times in the range of a few minutes to several hours (still above 100) and may take place both under agitation and under conditions This leads to the formation of a homogeneous gel which may be transparent or opaque.The supernatant phase formation has never been observed.At the end of the gelation phase, the produced gel was dried at 100 ° C for a few hours and then calcined in air at 500-600 ° C. The catalyst of the present invention appears as a solid having a surface area in the range of 20 to 400 m2 / g, a pore volume in the range of 0.5 to 1 cm3 / g, with distribution centered in the mesoporous region. The invention is useful in the reaction of n-paraffins, especially n-paraffins with a number of carbon atoms greater than or equal to 5, preferably between 5 and 40. A further object of the present invention therefore, it is related to a process for the synthesis of n-paraffins characterized in that the isomer reaction is carried out in the presence of a catalyst obtainable with a process comprising: a) dissolving a soluble molybdenum salt in an aqueous solution containing at least one basic compound selected from ammonium hydroxides having the general formula (I): R? R2R3R4N + OH "(I) Wherein the R? -R4 groups, the same or different, represent aliphatic groups containing 1 to 7 carbon atoms; b) adding to the solution of step (a) at least one silicon compound capable of hydrolyzing SiO2 and, optionally, an aliphatic alcohol; c) gelling the mixture thus obtained and calcining the obtained gel in air at a temperature in the range of 500 to 600 ° C. The preferred catalyst for the isomerization reaction is the molybdenum-silicon-based catalyst having an area, area in the range of 20 to 400 m / g and a Mo / Si molar ratio > 0.2. The somerification of n-paraffins can be carried out in any type of reactor. This is preferable, however, to operate with fixed bed or fluid bed reactors, both continuously or intermittently. The isomer reaction is carried out in the presence of hydrogen, at a temperature in the range of 200 to 550 ° C, preferably between 250 and 450 ° C, and at a pressure of hydrogen in the range of atmospheric pressure a 10 MPa, preferably from 2 to 6 MPa. Some illustrative but non-limiting examples are provided for a better understanding of the present invention and for its modality.EXAMPLE 1 20 g of hep t amo 1 ibad t or ammonium tetrahydrate (EMA) were dissolved in 200 g of an aqueous solution of tetropropyl ammonium hydroxide (TPAOH) at 10% by weight. A solution consisting of 53 g of tetra-ethyl orthosilicate (TES) and 160 g of ethanol is then added. After about 7 minutes the presence of a homogeneous opaque gel is observed, without phase separation of supernatant. It is allowed to stand at room temperature overnight and then dried at 100 ° C for 15 hours and calcined at 550 ° C for 6 hours in air. The solid obtained has the following composition (% by weight): Mo03 = 50%; Si02 = 50%.

It has a surface area of 161 m2 / g, a pore volume of 0.53 cm3 / g, an average pore diameter of 126 nm, calculated from the isotherm of s abso r c ion.

EXAMPLE 2 20 g of EMA were dissolved in 200 g of an aqueous solution of 15% TPAOH by weight. A solution consisting of 53 g of TES and 160 g of ethanol is then added.

After about 7 minutes the presence of a homogeneous opaque gel is observed, without phase separation of supernatant. It is allowed to stand at room temperature overnight and is then dried at 100 ° C for 15 hours and calcined at 550 ° C for 6 hours in air. The solid obtained has the following composition (% by weight): Mo03 = 50%; Si02 = 50%. It has a surface area of 116 m2 / g, a pore volume of 0.45 cm3 / g, an average pore diameter of 204 nm, calculated from the isotherm of desorption.

EXAMPLE 3 20 g of EMA were dissolved in 200 g of an aqueous solution of TPAOH at 30% by weight. A solution consisting of 53 g of TES and 160 g of ethanol is then added. After approximately 15 hours at room temperature the formation of a clear gel is observed which was dried at 100 ° C for 22 hours and calcined at 550 ° C for 6 hours in air. The solid obtained has the following composition (% by weight): Mo03 = 50%; Si02 = 50%.

It has a surface area of 48 m2 / g, a pore volume of 0.11 cm3 / g, an average pore diameter of 72 nm, calculated from the desorption isotherm.

EXAMPLE 4 The synthesis described in Example 3 is repeated without coho 1. The solid obtained has the following composition (% by weight) Mo03 = 50%; Si02 It has a surface area of 53 m2 / g, a pore volume of 0.08 cm3 / g - EXAMPLE 5 20 g of EMA were dissolved in 200 g of an aqueous solution of 35% TPAOH by weight. A solution consisting of 53 g of TES and 160 g of ethanol is then added. After approximately 60 hours at room temperature the formation of a clear gel is observed which was dried at 100 ° C for 15 hours and calcined at 550 ° C for 6 hours in air e. The solid obtained has the following composition (% by weight): M0O3 = 50%; Si02 = 50%. It has a surface area of 54 m2 / g, a pore volume of 0.09 cm3 / g, an average pore diameter of 38 nm, calculated from the isotherm of the absorption.

EXAMPLE 6 20 g of ENA were dissolved in 150 g of an aqueous solution of 40% TPAOH by weight. A solution consisting of 53 g of TES and 160 g of ethanol is then added. After about 7 hours at room temperature the formation of a clear gel is observed which was dried at 100 ° C for 15 hours and calcined at 550 ° C for 6 hours in air e. The solid obtained has the following composition (% by weight) Mo03 = 50%; Si02 = 50%. It has a surface area of 50 m2 / g, a pore volume of 0.08 cm3 / g.

EXAMPLE 7 20 g of AME were dissolved in 150 g of an aqueous solution of TPAOH at 40% by weight. A solution consisting of 53 g of TES and 230 g of ethanol is then added. After approximately 60 hours at room temperature the formation of a clear gel is observed which was dried at 100 ° C for 15 hours and calcined at 550 ° C for 6 hours in air. The solid obtained has the following composition (% by weight): M0O3 = 50%; Si02 = 50%.

EXAMPLE 8 20 g of EMA were dissolved in 150 g of a 40% by weight aqueous solution of TPAOH. A solution consisting of 24 g of TES and 160 g of ethanol is then added. After approximately 24 hours at room temperature the formation of a clear gel was observed which was dried at 100 ° C for 15 hours and calcined at 550 ° C for 6 hours in air. The solid obtained has the following composition; (% by weight): Mo03 = 68.8%; SiO_ 31.2%. _ _ EXAMPLE 9 20 g of EMA were dissolved in 150 g of a 40% by weight aqueous solution of TPAOH. A solution consisting of 5 g of TES and 160 g of ethanol is then added.

After approximately 24 hours at room temperature the formation of a clear gel is observed which was dried at 100 ° C for 15 hours and calcined at 550 ° C for 6 hours in air e. The solid obtained has the following composition (% by weight): M0O3 = 91.4%; Si02 = 8.6%.

EXAMPLE 10 (Comparative) 20 g of EMA were dissolved in 230 g of an aqueous solution of 23% NH 4 OH by weight. A solution consisting of 50 g of TES and 160 g of ethanol is then added. A lactescent suspension is formed, which after approximately 16 hours at room temperature gives a white solid, without separation of a supernatant solution. The product obtained was dried at 100 ° C for 15 hours and calcined at 550 ° C for 6 hours in air. The solid obtained has the following composition (% by weight): M0O3 = 51.5%; Si02 = 48.5%.

The surface area is 4 m2 / g.

EXAMPLE 11 (Comparative) Example 9 is repeated without TES. A final solid consisting of 100% molybdenum oxide is obtained. The surface area is 2 m2 / g. As can be seen from comparative examples 10 and 11, the morphological characteristics of materials according to the present invention are linked to the contemporary presence of tetraalkylammonium hydroxide and the silicon compound in the reactive mixture.

EXAMPLE 12 The catalyst described in Example 3 was evaluated in the hydro isomer reaction of n-heptane. The reactor is a tubular reactor, with a fixed bed, having an internal diameter of 1 cm and a length of 35 c. 5g of catalyst sieve at 20-40 meshes where it was loaded in the reactor. The reaction of the reactor was started by bringing the reactor to a temperature of about 100 ° C in a stream of nitrogen, a mixture of hydrogen / n-heptane was then fed in a molar ratio of 30/1 and the temperature rose to 350 ° C for a period of about 1 hour.

The reaction conditions are the following: - T = 350 ° C; - p = 2 MPa; - H2 / n C7 = 30 mol / mol - WHSV = 1 h "1. WHSV (Daily Heavy Duty Velocity) refers to n-heptane and is expressed as grams of n-heptane per gram of catalyst per hour. The isomerization was carried out for 50 hours, noting that the conversion and selectivity remain constant within this period of time.In particular, the conversion of n-heptane was 79.5% with an iso-C? selectivity of 97.4. %.

EXAMPLE 13 The catalyst described in Example 2 was evaluated in the reaction of hydro is omer iz ac ion 'of n-hexadecane. The reactor is a tubular, fixed-bed reactor, having an internal diameter of 1.2 cm and a length of 45 cm. 9g sieving at 20-40 meshes where it was loaded in the reactor.

The reaction was initiated by feeding a mixture of hydrogen / n-hexadecane in a molar ratio of 32/1 and the temperature was raised to 350 ° C for a period of about 2 hours. The reaction conditions are the following: - T = 350 ° C; - P = 5 MPa; - H2 / n-C16 = 32 mol / mol; - WHSV = 1 h "1. The isomer reaction was carried out for 60 hours, observing that the conversion and the selectivity remain constant within this period of time, in particular, the conversion of n-hexadecane was 94%. with an iso-C16 selectivity of 75%.

EXAMPLE 14 (Comparative) The isomerization reaction described in Example 12 was repeated using the catalyst of Comparative Example 11. The following table indicates the test results in terms of conversion and selectivity.

TABLE TIME CONVERSION SELECTIVITY SELECTIVITY (h) N-HEPTANO ISO-C7 DE CRAQUEO 2 17 96 4 20 50 95.7 4.3 40 43 91.3 8.7 68 44 82.2 17.8

Claims (11)

. CLAIMS

1. A catalyst based on molybdenum and silicon having a surface in the range of 20 to 400 m2 / g and a molar ratio of Mo / Si > 0.2.

2. The catalyst according to claim 1, obtainable with a process comprising: a) dissolving a soluble molybdenum salt in an aqueous solution containing at least one basic compound selected from ammonium hydroxides having the general formula (I): R 1 R 2 R 3 R 4 N + OH ~ (I) wherein the groups R? ~ R4, the same or different, represent aliphatic groups containing from 1 to 7 carbon atoms; b) adding to the solution of step (a) at least one silicon compound capable of hydrolyzing Si02 in such amounts as to give a molar ratio of Mo / Si greater than 0.2 and, optionally, an aliphatic alcohol; c) gelifying the mixture thus obtained and calcining the obtained gel in air at a temperature in the range of 500 to 600 ° C.

3. The catalyst according to claim 1 or 2, wherein the soluble molybdenum salt is selected from halogenated molybdenum derivatives, for example, having the formula Mo02X2 wherein X represents a halogen such as chlorine, bromine or fluorine, or it has the formula MoOX4, in which X represents a chlorine or fluorine atom, molybdic anhydride, molybdic acid and heptyl amino acid of ammonium tetrahydrate.

4. The catalyst according to claim 1, 2 or 3, wherein the silicon compound is tetra-alkyl silicon orthosilicate in which the alkyl group contains 1 to 4 1-carbon atoms.

5. The catalyst according to any of the preceding claims, wherein the gelifiable solution prepared according to steps (a) and (b) has the following molar composition: Mo / Si greater than 0.2; OH7 (Si + Mo) greater than 0.1; H20 / (Si + Mo) greater than 5;

Alcohol / H20 between 0 and 20. The catalyst according to any of the preceding claims, wherein the gelation phase is carried out at room temperature or at a temperature between the ambient values and 100 ° C.

7. The catalyst according to any of the preceding claims, having a surface area in a range of 20 to 400 m2 / g, a pore volume in the range of 0.05 to 1 cm3 / g, with distribution centered in the region of the mesoporous

8. A process for the ionization of n-paraffins characterized in that the isocy ation reaction is carried out in the presence of a catalyst based on molybdenum and silicon obtainable with a process comprising: a) dissolving a molybdenum salt soluble in an aqueous solution containing at least one basic compound selected from ammonium hydroxides having the general formula (I): R 1 R 2 R 3 R 4 N + OH "(I) wherein the R 1 R groups, the same or different, represent aliphatic groups they contain from 1 to 7 carbon atoms; b) adding to the solution of step (a) at least one silicon compound capable of hydrolyzing Si02 and optionally, an aliphatic alcohol; c) gelling the mixture thus obtained and calcining the obtained gel in air at a temperature in the range of 500 to 600 ° C.

9. The process according to claim 8, wherein the catalyst has a surface area in the range of 20 to 400 m2 / g and a molar ratio of Mo / Si > 0.2.

10. The process according to claim 8 6 9, wherein the reaction is carried out in the presence of hydrogen, at a temperature in the range of 200 to 550 ° C and at a pressure of hydrogen in the range of atmospheric pressure to 10 MPa .

11. The process according to claim 8, 9 or 10, wherein the n-paraffins have a number of carbon atoms equal to or greater than 5.