GB2211756A - Process for the preparation of paraffinic hydrocarbons and of a catalyst for said process - Google Patents

Abstract

Process for the preparation of paraffins by conversion of naphthenes having the same number of carbon atoms, by contacting the naphthenes in the presence of H2 with a catalyst comprising metallic Pt on an alumina-containing carrier, from which catalyst, prior to said contacting, ionic Pt has been extracted with a solvent which is selective for ionic platinum. A process for the preparation of a catalyst for said process is also claimed. The solvent may be acetylacetone.

Description

PROCESS FOR THE PREPARATION OF PARAFFINIC HYDROCARBONS AND OF A CATALYST FOR SAID PROCESS The invention relates to a process for the preparation of paraffinic hydrocarbons by conversion of saturated cyclic hydrocarbons, said paraffinic and cyclic hydrocarbons having the same number of carbon atoms per molecule. The invention also relates to a process for the preparation of a catalyst for said process.

The process to which the invention relates is also referred to as "hydrodecyclization" and the saturated cyclic hydrocarbons are also referred to as "naphthenes".

Paraffinic hydrocarbons are present in hydrocarbon fractions such as gasoline, kerosine and gas oil and the content thereof is of much importance for the quality of each of these three fractions.

Improving the quality of hydrocarbon fractions, for example kerosine and gas oil fractions, usually takes place by means of catalytic hydrogenation in the presence of a catalyst comprising one or more metals supported on a carrier.

Processes of this type are already known. The purpose of said hydrogenation is to saturate unsaturated hydrocarbons, which may be aromatic or olefinic, branched or unbranched and may also contain substituents. In this context the word "hydrocarbons" is meant to include heterocyclic aromatics as well as polynuclear aromatics of which one or more nuclei are heterocyclic. Examples of kerosine fractions and gas oil fractions are petroleum fractions, both straight run fractions such as petroleum distillates, and fractions obtained by means of petroleum oil conversion processes.

For kerosine fractions (boiling range usually 150-300 "C) in which full consideration is given to burning properties, such as lamp kerosine and kerosine for jet fuel. the smoke point, i.e. the height in mm of the flame just before it becomes smoky, determined according to ASTM 1322, is important. The smoke point, as is known is i.a. dependent on the contents of aromatics and naphthenes of the kerosine; the smoke point can be raised and the specific gravity can be decreased by hydrogenating said aromatics to naphthenes and by hydrodecyclizing of naphthenes into paraffins.

Usually, the smoke point should be at least 20 mm.

For gas oil fractions (boiling range usually 180-370 C) in which full consideration is given to ignition quality, such as gas oil for use in diesel engines, the cetane number is important, i.e.

the percentage by volume of cetane in a mixture with alpha-methylnaphthalene which matches the ignition performance of the gas oil in a test engine, determined according to ASTM D 976. The cetane number, therefore, is i.a. dependent on the paraffin content of the gas oil and can be raised by converting naphthenes into paraffins.

Usually, the cetane number should be at least 50.

Catalytic hydrogenation may be carried out in the presence of a catalyst comprising platinum supported on an alumina carrier.

Such catalysts may be prepared by impregnating alumina with â solution containing a dissolved platinum compound drying the impregnated alumina, calcining the dried material and reducing the calcined material at elevated temperature in a stream of hydrogen.

The reduced catalyst thus obtained can be used for the preparation of paraffinic hydrocarbons by conversion of saturated cyclic hydrocarbons in the presence of hydrogen; a disadvantage thereof is that the selectivity to paraffinic hydrocarbons having the same number of carbon atoms per molecule as the starting saturated cyclic hydrocarbons is relatively low. The "selectivity to certain products", expressed in a percentage, is defined herein as 100xa:b in which "a" is the amount of saturated cyclic hydrocarbons that has been converted into these certain products and "b" is the total amount of saturated cyclic hydrocarbons that has been converted.

It has now, surprisingly, been found that the selectivity to paraffinic hydrocarbons having the same number of carbon atoms per molecule as the starting saturated cyclic hydrocarbons is considerably improved by carrying out the conversion in question in the presence of a catalyst which, prior to use, has been extracted with a special solvent.

Accordingly, the invention provides a process for the preparation of paraffinic hydrocarbons by conversion of saturated cyclic hydrocarbons, said paraffinic and cyclic hydrocarbons having the same number of carbon atoms per molecule, which process comprises contacting said saturated cyclic hydrocarbons in the presence of hydrogen and at elevated temperature and pressure with a catalyst comprising metallic platinum incorporated with an alumina-containing carrier, from which catalyst, prior to said contacting, ionic platinum has been extracted with the aid of a solvent which is selective for ionic platinum.

Catalysts prepared by impregnating an alumina-containing carrier with a solution containing a dissolved platinum compound, drying and calcining the impregnated material and reducing the calcined material in a stream of hydrogen usually have metallic platinum incorporated with an alumina-containing carrier which also contains ionic platinum. It has been found that a decreased content of ionic platinum enhances the selectivity to paraffinic hydrocarbons having the same number of carbon atoms per molecule as the starting saturated cyclic hydrocarbons, the selectivity to paraffinic hydrocarbons having a lower number of carbon atoms per molecule than the starting saturated cyclic hydrocarbons being decreased.

Any solvent which selectively extracts ionic platinum from an alumina-containing carrier in the presence of metallic platinum ma be used. The extraction is selective for ionic platinum, that is to say vis-a-vis metallic platinum and the alumina-containing carrier and may be physical or may involve a chemical rection. Preference is given to beta-diketones, in particular to alkanediones having 5 or 6 carbon atoms per molecule. Most preferred is 2,4-pentanedione, also referred to as "acetylacetone".

The extraction may be carried out in a wide temperature range, suitably at a temperature in the range of from 20 C to the boiling point of the solvent at atmospheric pressure. Preferably, the extraction is carried out at a temperature in the range of from 100 C to said boiling point. After extraction, the catalyst may be washed with water to remove excess of solvent and then dried in a stream of inert gas, for example a noble gas, to remove water.

Drying may take place at a temperature in the range of, for example, 50 "C to 400 C.

The alumina-containing carrier to be used preferably contains more than 50% by weight and, more preferably, 90% by weight of alumina; alumina's are particularly preferred. If desired, the alumina may contain silica. Among the alumina's preference is given to gamma-alumina. The carrier suitably has a specific surface area 2 in the range of from 5 to 500 m /g and preferably more.than 25 2 m2/g.

The catalyst, prior to the extraction in question, may have been prepared in any desired manner, provided it contains platinum in metallic and platinum in ionic form. Usually, such preparation involves impregnating an alumina-containing carrier with a solution containing a dissolved platinum compound, drying the impregnated carrier, calcining the dried carrier and reducing the calcined material at elevated temperature in the presence of a suitable reducing gas. The solution may contain, for example, H2PtCl6 or Pt(SH3)4(OH)2, and may be, for example, aqueous or alcoholic.

Furthermore, a solution of platinum acetylacetonate may be used, for example in a hydrocarbon solvent, which may be aliphatic, cycloaliphatic or aromatic. The solution preferably contains so much of the platinum compound that the catalyst to be used in the process according to the present invention contains in the range of from 0.2 to 5e by weight of platinum. Impregnation may take place in any desired manner, for example by immersing the aluminacontaining carrier in the solution and separating the impregnated carrier from excess solution by decantation, filtration or centrifugation. The impregnation may also be according to the "dry method", the volume of solution being about equal to the pore volume of the carrier. Impregnation is usually conducted at a temperature in the range of from 10 C to 50 "C; ambient temperature is usually most suitable. Drying of the impregnated carrier is carried out to remove solvent present in the pores and on the outer surface and suitably takes place at a temperature in the range of from 50 "C to 200 "C, usually in a stream of air but; if desired, an inert gas such as nitrogen or a noble gas may be used. Drying may take place at atmospheric or, if desired, sub-atmospheric pressure.

Calcination of the dried material usually takes place at a temperature in the range of from 300 "C to 500 "C in an oxygen-containing gas, for example a stream of air. Drying and calcination suitably last for a period in the range of from 0.5 to 5 hours. The calcined material is reduced at a temperature which is usually in the range of from 200 "C to 600 OC. As reducing gas hydrogen or a hydrogencontaining gas may be used.

The process according to the present invention can be carried out at relatively mild conditions so as to promote ring opening and obtain a high selectivity to paraffinic hydrocarbons having the same number of carbon atoms per molecule as the starting saturated cyclic hydrocarbons. This object is obtained by selection of the operating conditions which are used. The process according to the present invention is preferably carried out at a temperature in the range of from 200 C 450 C, a pressure in the range of from 1 bar to 100 bar, a space velocity in the range of from 0.3 to 30 kg of saturated cyclic hydrocarbons per litre of catalyst per hour and a molar ratio hydrogen to hydrocarbons in the range of from 1:1 to 100:1. Within these ranges conditions can be chosen depending on the quality of the kerosine required.As a hydrogen source use can be made of pure hydrogen or of hydrogen-containing mixtures, for instance the gases produced in catalytic reforming processes.

The process according to the present invention has given very good results for the preparation of paraffins from cyclopentane derivatives, particularly from dimethyl-substituted cyclopentanes such as 1,2-dimethylcyclopentane.

The process according to the present invention can be used for improving the quality of naphthenes-containing hydrocarbon fractions having a final boiling point at atmospheric pressure of up to, for example, 390 "C, such as gasoline fractions, kerosine fractions and gas oil fractions. The improvement in quality is ascribed to the conversion of naphthenes to paraffins by ring opening, without a substantial change in boiling range. Furthermore, a substantial part of aromatics, if present, is converted into naphthenes. Therefore, the process is very suitable for improving the smoke point of kerosine fractions and for improving the cetane number of gas oil fractions.The process is much preferred for the removal of 1,2,4,5-tetramethylbenzene (also referred to as "durene") from gasoline fractions, via hydrogenation to 1,2,4,5,-tetramethylcyclohexane and subsequent ring opening of the latter compound.

Hydrocarbon fractions containing sulphur and/or nitrogen compounds, such as light cycle oil resulting from catalytic cracking of hydrocarbons may advantageously be hydrotreated before they are subjected to the process according to the present invention, thus removing at least a portion of such compounds and increasing the life of the catalyst being used in the present process. Therefore, according to a preferred embodiment of the present invention the present process is the second stage of a two-step scheme in which the first step is a hydrotreatment to remove nitrogen and sulphur from the hydrocarbon fraction. So, the process according to the present invention has the important advantage that kerosine fractions of low quality can now be converted to give high quality kerosine in high yield, which kerosine may be used as, for example, jet fuel.

Hydrotreating processes are well known in the art. Any suitable catalyst can be employed in the hydrotreating process.

Such catalysts may comprise at least one of the transition metals, or their oxides or sulphides, and especially at least one of Cr, Mo, W, Fe, Co, Ni, Ru, Rh, Pd, Os, Ir and/or Pt, or their oxides or sulphides. It is preferred to use an iron group metal together with Cr, Mo or W, such as tungsten with nickel, or molybdenum with cobalt or nickel. The catalyst is usually supported on a carrier in proportions ranging from 2 to 25% by weight. Suitable carriers are refractory inorganic oxides such as alumina, silica, zirconia or titania, and clays such as bauxite or bentonite. Catalysts having high selectivity for nitrogen removal usually comprise an acidic support, such as silica-alumina or alumina-boria. Such catalysts usually also promote the isomerization of cycloalkyl rings having six carbon atoms in the ring to methyl-substituted cycloalkyl rings having five carbon atoms in the ring.The latter compounds are then converted into paraffinic hydrocarbons by means of the process according to the present invention.

Hydrotreating is preferably conducted at a temperature in the range 280 "C to 450 "C, a pressure of 35 to 210 bar, a space velocity of 0.1 to 5 volumes of oil per volume of catalyst per 3 3 hour and a hydrogen rate of 180 to 1780 standard m per m of oil.

The hydrocarbon fraction for the second stage preferably contains less than 10 parts per million by weight of sulphur compounds, calculated as sulphur and less than 10 parts per million by weight of nitrogen compounds, calculated as nitrogen.

The process according to the present invention may be carried out in any suitable equipment, a fixed bed reactor system wherein the saturated cyclic hydrocarbons are passed over a stationary bed of catalyst, being preferred. The reactor effluent is passed to a separation zone where hydrogen-rich gas is separated off and suitably recycled to the reaction zone together with fresh hydrogen.

The invention further provides a process for the preparation of a catalyst for the process according to the present invention which process comprises extracting ionic platinum from a catalyst comprising metallic platinum and ionic platinum, both incorporated with an alumina-containing carrier, with the aid of a solvent which is selective for ionic platinum.

The following Example further illustrates the invention.

Example Preparation of catalyst platinum on alumina Cylindrical extrudates having a diameter of 0.16 cm and a 2 length of 1 cm of gamma-alumina having a surface area of 183 m and a pore volume of 0.51 ml/g (measured using nitrogen) were calcined in air for 1 h at a temperature of 300 "C. The calcined material (25 g) was impregnated with 15 ml of an aqueous ammoniacal solution of Pt(NH3)4(OH)2 having a pH of 12.6 and containing 248.9 mg of Pt. The material thus impregnated was separated from liquid by decantation and dried for 1 hour in a stream of air at a temperature of 60 "C. The dried material was heated up at a rate of 50 "C per h until a temperature of 450 "C was reached and then kept at this temperature for 1 h.The catalyst thus obtained had a platinum content of 1% and was reduced in a stream of hydrogen at a temperature of 500 "C. The platinum crystals on the alumina had a size below 0.5 nm. This catalyst contained metallic and ionic platinum and is referred to hereinafter as "catalyst N". The catalyst was sieved to a size of 30-80 mesh.

Preparation of catalyst platinum on alumina extracted with acetylacetone Catalyst N (2 g) was suspended in acetylacetone (20 ml) and the suspension obtained was heated and kept for 6 h under reflux at atmospheric pressure in a nitrogen atmosphere. Then, the suspension was separated by decantation and the solid material obtained was washed three times with water (100 ml each time) under a nitrogen atmosphere. The washed material was heated up at a rate of 50 C per h under a stream of helium until a temperature of 300 C was reached and then kept at this temperature for 3 h. This catalyst is referred to hereinafter as "catalyst P" and contained 6% less platinum than catalyst N. Ionic platinum could not be detected in catalyst P. The catalyst was sieved to a size of 30-80 mesh.

Preparation of paraffinic hydrocarbons A vertically positioned pipe of stainless steel type 316 surrounded by a heating element and having an internal diameter of 0.95 cm and a height of 15 cm was, consecutively, charged with (a) 30-80 mesh silica gel particles over a height of 7.5 cm, (b) a mixture of catalyst P (2 g) and 30-80 mesh silica gel particles (2 g) over a height of 1 cm, and (c) 30-80 mesh silica gel particles over a height of 2 cm. A gaseous mixture having a total pressure of 30 bar and consisting of l,2-dimethylcyclopentane and hydrogen in a molar ratio of 1:30, respectively, was introduced into the pipe at the top with a weight hourly Space velocity of 3 g of 1,2-dimethyl cyclopentane per g of catalyst P per hour. The reaction mixture withdrawn from the bottom of the pipe was analysed by means of gas chromatography.

Six experiments were carried out in this manner, each at another temperature. Table 1 hereinafter states which temperatures were used and presents the conversion of 1,2-dimethylcyclopentane and the selectivity to paraffins having seven carbon atoms per molecule.

Table 1 Experiment Temperature, Conversion, Selectivity No. "C % % % 1 217 2.7 96.1 2 239 7.2 95.4 3 251 11.7 95.5 4 276 33.3 95.0 5 291 69.0 93.0 6 305 92.0 86.3 Comparative Experiments A-E The experiments described hereinbefore were repeated with the difference that catalyst P (2 g) was replaced with catalyst N (2 g). Five experiments were carried out in this manner, each at another temperature. Table 2 hereinafter states which temperatures were used and gives the conversion of l,2-dimethylcyclopentane and the selectivity to paraffins having seven carbon atoms per molecule.

Table 2 Experiment Temperature, Conversion, Selectivity No. "C % A 206 0.8 63 B 230 4.5 73 C 256 20.1 77 D 276 43.6 75 E 301 95.6 73 A comparison between the Comparative Experiments and the Example shows that in the Example, which is according to the present invention, much higher selectivities have been observed than in the Comparative Experiments, which are not according to the invention.

Claims (14)

1. A process for the preparation of paraffinic hydrocarbons by conversion of saturated cyclic hydrocarbons, said paraffinic and cyclic hydrocarbons having the same number of carbon atoms per molecule, which process comprises contacting said saturated cyclic hydrocarbons in the presence of hydrogen and at elevated temperature and pressure with a catalyst comprising metallic platinum incorporated with an alumina-containing carrier, from which catalyst, prior to said contacting, ionic platinum has been extracted with the aid of a solvent which is selective for ionic platinum.

2. A process as claimed in claim 1 in which said solvent is a beta-diketone.

3. A process as claimed in claim 2 in which said beta-diketone is acetylacetone.

4. A process as claimed in any one of the preceding claims in which the alumina-containing carrier is gamma-alumina.

5. A process as. claimed in any one of the preceding claims in which use is made of a catalyst containing in the range of from 0.2 to 58 by weight of platinum, calculated on the total of platinum and alumina-containing carrier, of a temperature in the range of from 200 "C to 450 "C, a pressure in the range of from 1 bar to 100 bar, a space velocity in the range of from 0.3 to 30 kg of saturated cyclic hydrocarbons per litre of catalyst per hour, and a molar ratio hydrogen to hydrocarbons in the range of from 1:1 to 100:1.

6. A process as claimed in any one of the preceding claims in which said saturated cyclic hydrocarbons are present in a gasoline fraction, a kerosine fraction or a gas oil fraction.

7. A process as claimed in any one of the preceding claims which is the second stage of a two-step scheme in which the first stage is a hydrotreatment to remove nitrogen and sulphur from the hydrocarbon fraction.

8. A process as claimed in claim 7 in which the hydrocarbon fraction for the second stage contains less than 10 parts per million by weight of sulphur compounds, calculated as sulphur and less than 10 parts per million by weight of nitrogen compounds, calculated as nitrogen.

9. A process as claimed in claim 1 substantially as hereinbefore described with reference to the Example.

10. Paraffinic hydrocarbons whenever obtained by a process as claimed in any one of the preceding claims.

11. Process for the preparation of a catalyst for a process as claimed in any one of claims 1 to 9 which process comprises extracting ionic platinum from a catalyst comprising metallic platinum and ionic platinum, both incorporated with an alumina-containing carrier, with the aid of a solvent which is selective for ionic platinum.

12. Process as claimed in claim 11 in which the catalyst comprising metallic platinum and ionic platinum has been prepared by impregnating an alumina-containing carrier with a solution containing a platinum compound, drying and calcining the impregnated carrier and reducing the calcined material at elevated temperature with the aid of a reducing gas.

13. Process as claimed in claim 11 substantially as hereinbefore described with reference to the Example.

14. A catalyst whenever prepared by a process as claimed in any one of claims 11 to 13.