Process for the obtainment of a light paraffin isomerization catalyst

Abstract: The invention refers to a process for the obtainment of a catalyst useful in the isomerization of light para­ ffins, fundamentally based on acid treatment of a synthe­ tic mordenite at temperatures between 40 and 100º C follow­ ed by treatment with steam-air at temperatures between 300 and 600º C which makes it possible to optimize the SiO₂/­ Al₂O₃ framework ratio and the amount of extraframework aluminum of the zeolite. The mordenite thus obtained is mixed with an inorganic refractory oxide such as Al₂O₃ and a metal of group VIII, basically Pt, is added in an amount between 0.1 and 0.5 % by weight referred to the final cata­ lyst. The catalyst has been tested in the isomerization of light refinery fractions (C₅ and C₆) and has a high activity and selectivity of branched isomers with a low production of gases and a high octane number for the light gasoline ob­ tained.

Description: The present invention refers to a process for the obtainment of a new catalyst for isomerization of light paraffins based on a modified synthetic mordenite in which the SiO₂/Al₂O₃ framework ratio and the amount of extraframework Al have been optimized by specific acid and steam treatments. The zeolite mixed with a refractory material oxide and, at least, with one metal of group VIII is calcined giving rise to the final catalyst. The isomerization reaction of light paraffins is of considerable importance in the petroleum industry due to the present demand for unleaded gasoline which imposes modification of legislation concerning the environment in industrialized countries. Since light straight run is an important ingredient of the blending of gasolines, the increase of the octane number of this fraction by means of catalystic isomerization processes is of vital importance. Of the catalysts used the most up to now for the isomerization of light straight run, Friedel-Crafts type, nobel metal on flourinated or chlorinated aluminas and nobel metal on zeolitic supports, these last ones have created the most interest in the last few years. The advantages that the zeolitic catalysts have over the above cited ones are simplicity of operation, the absence of additional problems of corrosion and a greater resistance to sulfur. Most of the catalysts described up until now consist of modified zeolites, fundamentally mordenite, mixed with an inorganic refractory oxide such as Al₂O₃ and with a noble metal impregnated or exchanged on the zeolite, on the alumina or on a mixture of both of them. Mordenite is a zeolite with a structure in monodimensional channels. The diameter and type of channels, as well as the structure, is well established in the prior art. Numerous processes have been described to convert natural or synthetic mordenite into the active acid form in isomerization. Treatment with ammonium ions with subsequent calcination (US patent 3190939) and acid treatment prior to exchange with NH₄⁺ and calcination (US patent 3442794 SHELL) make it possible to pass from the Na⁺ form of the mordenite to the protonic form. On the other hand, numerous processes to increase the acidity of the mordenite, based on the partial dealuminization of the crystalline structure, have also been described. By means of acid treatments it is possible to increase the SiO₂/Al₂O₃ ratio of the mordenite and its activity in isomerization. Thus, US patents 3507931 (MORRIS) and 4018711 (BERTOLACINI) describe an improvement in the capacity of isomerization of mordenite with a silica-alumina ratio of at least 20:1 and 19:1, respectively, obtained by acid treatments. However, although said ratio increases the effective lifetime of the so obtained catalyzers, the acidity thereof is not sufficient as to provide a really satisfactory selectivity and efficiency. The acid attack of zeolite with solutions that contain Na⁺ and K⁺ ions (US patent 4400576 SHELL) and with very severe acid treatments: for ex. 12N HCl, 100º C (US patent 3480539 ESSO) to produce its dealuminization, has also been described. By thermal treatments with steaming and by self-steaming followed by acid attack (US 3506400 ESSO), (US 3551353 mobil) higher levels of dealuminization in the mordenite are obtained, thus SiO₂/Al₂O₃ ≧ 35 and up to 100 in the patent filed by MOBIL. Other dealuminization processes with Cl₄Si which replaces aluminiums by silicons in the framework of the mordenite causing stability not to be lost have also been described in the prior art (US 4273753 MOBIL.) Finally, other methods to increase the acidity of the mordenite such as treatment with halogenated compounds have been previously patented (US 3413370 UPO), (US 3932554 NIPPON OIL.) Acid treatments of mordenite alone cannot attain a very thorough dealuminization without the risk of breaking the structure and washing alone in the washing the extra-framework aluminum produced. The same thing happens for acid treatment with Na⁺ and K⁺ competing ions. Thermal treatments with steaming give rise to higher SiO₂/Al₂O₃ framework ratios but the extraframework aluminum produced can clog up the channels and prevent free access of the reagents to the acid centers. Washing with acid the zeolite subjected to steaming can "clean" the channels washing away the extraframework Al produced in the previous process. Several authors have described maximums of activity for isomerization in terms of the SiO₂/Al₂O₃ framework ratio. Voorhies and Bryan (Aiche. J. 852 (1968)) have found a maximum of activity for isomerization of C₅ at a SiO₂/Al₂O₃ ratio of 17:1; said value has been subsequently confirmed by the studies of Koradia et al. (J. Cat, 66, 290 1980.) Bremer et al. (Procd. of the symp. of zeolites, Szeged, Hungary, September 1978) put the maximum of activity at a ratio of 23:1 for hydrocracking of n and isooctane. These studies concluded that an increase of the number of strong Bronsted centers increased the activity of the catalyst. Regarding the deposition of noble metal to carry out the hydrogenating function of the catalyst, the preferred metal is Pt in almost all patents and the preferred method is the impregnation method, either on mordenite or on alumina and subsequent mixing with the zeolitic material (US 4400576 SHELL.) Processes for a better dispersion of Pt introducing competitors in the impregnation process and treating the final catalyst in a stream of air with chlorinated compounds have also been described (E.P. 0256945 IFP.) The deposition of another metal (Zr) along with Pt has also been described to improved the isomerizing activity of the catalyst (E.P. 0253743 TOTAL.) The synergic effect that the extraframework Al can have on the Bronsted acid centers of zeolite has been described in literature (c. Mirodatos, D. Barthomeft "J. Chem. Comun 39, 1981"; A. Corma, V. Fornes, O. Pallota and F. Melo "Innovation in Zeolites, New Port 1987"; R.M. Lago, W.O. Haag, R.J. Mikousky, D.H. Olson, F.D. Hellring, K.D. Schmitt, G.T. Kerr "7th. Int. Zeolite Conference 1986,") and it can be attributed to the electronegativity of the polymerized Al0⁺ species that contribute to a greater polarization of the O-H bond. In the present invention as claimed a mordenite with an optimum Al framework:Al extraframework ratio is attained to the extent that no obstruction is produced in the channels and the effect of improvement of the activity of the above mentioned extraframework species is taken advantage of. Thereby a catalyzer combining a high level of selectivity and efficiency combined with a long lifetime is obtained in a simple and unexpensive manner. For this purpose a mordenite with a low Na⁺ content is treated with an acid solution to obtain a SiO₂/Al₂O₃ ratio between 15 and 22 (framework) and subsequently it is heated in a controlled steam-air atmosphere to obtain a dealuminized mordenite with a SiO₂/Al₂O₃ ratio = 15-40 (network) and with a controlled amount of extraframework Al. Due to the effect of this extraframework Al a maximum of activity for isomerization of light parraffins is attained at a higher SiO₂/Al₂O₃ framework ratio than in the above cited studies in which very low activities were obtained for this ratio (30:1.) Besides higher SiO₂/Al₂O₃ ratios produce a smaller amount of coke thus lengthening the life of the catalyst. These catalysts can be used in the isomerization of C₄ to C₇ light paraffins and particularly in the refinery LSR fraction and have a high level of conversion to isoparaffins with a low production of gases coming from cracking reactions. The present invention refers to a process for the obtainment of a new light paraffin isomerization catalyst with an optimum Al (framework):extraframework Al ratio that has a high activity and selectivity of branched isomers. The catalyst consists of 50 to 97 % by weight of a modified mordenite with a SiO₂/Al₂O₃ framework ratio between 15 and 40, between 50 and 1 % of a refractory metal oxide (preferably Al₂O₃) and a metal of group VIII and particularly platinum, palladium or Ni , in an amount of 0.1 to 0.5 by weight referred to the total of the catalyst. The preferred metal of group VIII is platinum. The starting mordenite can be natural or synthetic and it will preferably be found in acid form. The methods to convert the sodium form into the acid form of mordenite are well known in the prior art. In this case exchange with an ammonium salt followed by calcination to remove the NH₃ and to elave the protonic form, or else direct treatment of the sodium mordenite with a mineral acid is preferred. Acid mordenites obtained this way have a SiO₂/Al₂O₃ ratio between 13 and 18 and are treated with an acid solution preferably of strong mineral acids: HCl, H₂SO₄, HNO₃, H₃PO₄. Hydrochloric and sulfuric acids are the ones preferred in this invention. The treatment temperature must be between 40 and 100 º C, better between 60and 90 º C, and the contact time of the catalyst with the acid solution will be between 1 and 8 hours, preferably between 2 and 6 hours. The liquid/solid (volume: weight) ratio will be between 5 and 20,; preferably between 8 and 15. The acid concentration must be between 1 and 6 N and preferably between 2 and 5 N. This acid treatment permits obtainment of mordenites with a SiO₂/Al₂O₃ ratio between 15 and 22 and with the minium extraframework Al. There is also the possibility of effecting initial acid treatment of the sodium mordenite and a subsequent exchange with NH₄⁺ to obtain the protonic form, or to treat the sodium form of the two above cases a SiO₂/Al₂O₃ ratio similar to the one obtained in the acid treatment described in the first place can be attained. The mordenite obtained in the previous treatment, either in the acid or ammonium form, is heated in the presence of a stream of gas that contains oxygen and steam, preferably air/steam. The percentage of steam in the stream of gas must be between 5 and 100 %, preferably between 15 and 80, (by volume.) The treatment temperature will be between 300 and 600 º C, preferably between 350 º and 500 º C and the treatment time between 2 and 8 hours, preferably between 2 and 6 hours. The zeolite obtained by this treatment has a SiO₂/Al₂O₃ ratio between 15 and 40, preferably between 22 and 35, and a framework Al/extraframework Al ratio between 1 and 10, preferably between 1 and 5, and it must be mixed with an inorganic refractory oxide, preferably alumina in a zeolite/oxide proportion between 1:1 and 9:1. The metal that furnishes the hydrogenating function to the catalyst must be of group VIII, preferably Pt, and it must be found in an amount from 0.1 to 0.5 % by weight, referred to the total of the catalyst. The metal can deposit on the mordenite, on the alumina or on a mixture of the two. In our case it is preferred to add platinum on the zeolite/alumina mixture. The Pt deposition method can be by impregnation or by exchange with a competing ion or without one and with chloroplatinic acid or any amined Pt complex (for ex. Keller salt.) In the present invention it is preferred to impregnate the mordenite with a solution of chloroplatinic acid/HCl where the concentration of HCl is between 0.05 and 1 N, preferably between 0.1 and 0.5 N. The liquid/solid (1/kg) ratio must be between 1 and 10, preferably between 2 and 5. The dry product is normally calcined at temperatures between 300 and 600 º C in an air atmosphere to obtain the final catalyst. Before it is used as an isomerization catalyst, the catalyst must be reduced in a stream of hydrogen at a temperature between 300 and 500 º C. The catalyst that the invention refers to is useful in catalytic isomerization of short chain (C₅ to C₇) light straight run and very especially in the refinery LSR stream. The isomerization reaction can be carried out at a temperature range of 150 to 400 º C and space velocities of 0.5 to 4 h⁻¹ and under a total hydrogen pressure that will be between 10 and 50 bars. The molar hydrogen/hydrocarbon ratio must be between 0.5 and 4. EXAMPLES A series of examples in which the catalysts prepared by our method are compared with others prepared by different methods claimed up to now and covering a broad spectrum of SiO₂/Al₂O₃ framework ratios from 14 to 62 and framework Al/extraframework Al ratios from 0 to 10 are given in this part. Example 1 This example is carried out in accordance with the process of the invention. A Norton commercial acid mordenite with a SiO₂/Al₂O₃ ratio = 14 was treated with 3 N hydrochloric acid for 6 hours at 80 º C and with a liquid/solid (liters/kilo) ratio of 10. The mordenite obtained this way had a SiO₂/Al₂O₃ ratio of 22 and it was subsequently treated with air/steam mixtures for 3.5 hours. The treatment temperature was 400 º C. In the following tables one can see the treatment conditions with steam-air and the SiO₂/Al₂O₃ ratios attained for the catalysts given in this example. Catal. N º % in vol. of H₂O t (hr.) Trat. t. º C (SiO₂/Al₂O₃) framework Framework Al/Extraframework Al 1A 100 3.5 400 20 9 1B 3 " " 26 5.25 1C 20 " " 30 2.97 1D 84 " " 34 1.84 The zeolites treated this way were mixed with γ Al₂O₃ in a ratio of 8:2 and they were impregnated with a solution of H₂PtCl₄ in 0.2 N HCl such that the Pt content in the final catalysts was 0.35 % by weight. The catalysts were calcined at 300 º C for 3 hours and at 450 º C for 1 hour and were subjected to a reduction process at 300 º C with H₂ before evaluating their activity. Example 2 (Comparative) It has been carried out by a conventional process different from the one claimed here and it is given for comparison purposes. As of the same commercial acid mordenite of example 1 four catalysts were prepared by different acid treatments of the same. The reaction conditions as well as the SiO₂/Al₂O₃ framework ratios attained can be seen in the following table: Catal. N º (HCl) N T ( º C) t (h) Liq/Sol SiO₂/Al₂O₃ (framework) 2A 3 80 6 10 20 2B 3.5 90 6 10 22 2C 3 80 3 4 18 2D - - - - 16 The treated mordenites were mixed with γ Al₂O₃, Pt was added and they were calcined as in example 1. Example 3 (Comparative) It has been carried out by a conventional process different from the one claimed here and it is given for comparison purposes. As of the commercial acid mordenite of the previous examples. 3 catalysts were prepared by calcination treatments in 100 % H₂O atmosphere at different temperatures. The treatment conditions and the SiO₂/Al₂O₃ ratios obtained can be seen in the following table: Catal. N º T ( º C) t (h) SiO₂/Al₂O₃ (framework) Framework Al/extra framework Al 3A 450 3 26 1.16 3B 500 3 34 0.7 3C 600 3 64 0.34 The mordenites treated with steam were mixed with Al₂O₃ and Pt was added as in example 1. Example 4 (Comparative) It has been carried out by a conventional process different from the one claimed here and it is given for comparison purposes. From the commercial acid mordenite of the above examples several catalysts, 4A, 4B and 4C were prepared by treatments with moisture saturated air at room temperature for 3.5 hours. The treatment temperature was 400 º C and this was repeated twice for catalyst 4C. Treated mordenite 4B was subjected afterwards to an exchange process with calcination twice. The exchange was done with 2.5 N NH₄Cl at a temperature of 80 º C for 1 hour with a liquid:solid (volume/weight) ratio of 10. The calcination was done at 350 º C for 2 hours and at 500 º C for 3 hours. The SiO₂/Al₂O₃ reached can be seen in the following table: Catal. N º SiO₂/Al₂O₃ (framework) Al (framework) / Al (extraframework) 4A 17 4.67 4B 23 1.55 4C 26 1.16 The mordenites treated by this method were mixed with γ Al₂O₃ and platinum was added thereto as in example 1 Example 5 The catalysts prepared with and without the process of the invention were tested by the isomerization reaction of nC₅ and nC₆ in a flux microreactor. The reaction conditions tested were the following: Feed: 60 % nC₅ and 40 % nC₆ Pressure: 30 Atm. Temperature: 250 º C LHSV: 35 h⁻¹ H₂/hydrocarbon: 1 (molar) The results of conversion from pentane to isopentane as contrasted with the SiO₂/Al₂O₃ ratio are shown in Table 1 and in Figure 1. One can observe that with other conventional methods of preparation of the catalysts which are not in accordance with the invention and as pointed out in the prior art, the maximum activity for isomerization is found at a SiO₂/Al₂O₃ ratio between 17 and 23. In the catalysts prepared by our process, the maximum activity reaches higher SiO₂/Al₂O₃ ratios by virtue of the synergic effect upon the acidity which the extraframework Al has. The activity for isomerization of the catalyst which has the maximum prepared by our method is even higher than the maximum of the catalysts prepared by other methods. Besides, since the catalysts prepared by our method have a higher SiO₂/Al₂O₃ framework ratio, a greater coking resistance is to be expected. The framework Al/extraframework Al ratio in contrast with the activity for isomerization of the catalysts presented in the figures is represented in figure 2. One can see how a maximum activity at certain framework Al/extraframework Al ratios which illustrates the above mentioned fact, the increase of the Bronsted acidity by the extraframework species produced upon preparing the catalyst by our process, is produced. Example 6 In this example the results obtained in a pilot plant with actual feed (refinery LSR stream) for the catalysts that had activity maximum in accordance with example 5 are shown. The experiments were carried out at a pressure of 30 atm. , a temperature of 250 º C, a LSHV of 1 h⁻¹ and a H₂/hydrocarbon ratio of 1 molar. The composition of the added material and the results are shown hereinafter: Composition % weight C₁-C₄ 0.05 iC₅ 42 nC₅ 51.7 2.2 MDI C₄ 0.4 2 Me C₅ 3.9 3 Me C₅ 0.14 nC₆ 0.03 Benzene - Sulfur (ppm) 10 Water (ppm) - Catalyst iC₅/Total C₅ Total gases/hydrocarbons ( ) RON (calculated) 1-C 68.2 2.0 82.0 2-A 67.5 1.89 80.5 It can be seen in the above table how isoparaffin/paraffin ratios close to balance with high octane numbers and low production of gases were attained for the catalyst prepared by our process. Catalyst SiO₂/Al₂O₃ (framework) Framework Al/extraframework Al iC₅/C₅total (%) 1A 20 9 7.7 1B 26 5.25 9 1C 30 2.97 12 1D 34 1.84 10 2A 20 - 11 2B 22 - 10.7 2C 28 - 7 2D 16 - 4 3A 26 1.16 4.5 3B 34 0.7 3 3C 64 0.34 1 4A 17 4.67 6 4B 23 1.55 9 4C 26 1.16 10 Figure 1 is a graphical representation of the results obtained by the conversion/isomerization reaction of pentane to isopentane in view of the silica / alumina (framework) ratio, when the catalysts used in examples 1, 2, 3 and 4 are used. In the abscissa the silica / alumina (framework) ratio is represented whilst the ordinate represents the isopentane / total pentane ratio of the conversion process under the conditions set forth in example 5. Figure 2 is a graphical representation of the results obtained by the conversion/isomerization reaction of pentane to isopentane in view of the framework aluminium / extraframework aluminium ratio when the catalysts of examples 1 , 3 and 4 are used. In the abscissa the framework aluminium / extraframework aluminium ratio is represented whilst the ordinate represents the isopentane / total pentane ratio obtained by the conversion process under the conditions set forth in example 5. In both figures, the catalysts of the examples are identified as follows:  Example 1 + Example 2 * Example 3 □ Example 4

Claims: A process for obtaining a light paraffin isomerization catalyst comprising treating mordenite with an acid solution and subjecting it to a heating treatment; mixing said mordenite with 50% to 1% of an inorganic refractory oxide, preferredly alumina; depositing a metal of group VIII, selected preferredly among Pd, Pt, Ni on said mordenite, on said refractory oxide or on the mixture of said mordenite and said refractory oxide,, in an amount of 0.1% to 0.5% by weight referred to the total of the catalyst; calcining said catalyst to which said metal has been added at temperatures between 300 º C and 600 º C; reducing said catalyst in a hydrogen stream at temperatures between 300 º C and 500 º C; characterized in that said treatment with an acid solution is carried out at temperatures between 40 º C and 100 º C until a silica / alumina framework ratio between 15 and 22 is obtained in said mordenite, and said heating treatment is subsequent to said treatment with an acid solution and comprises treating said mordenite having said silica / alumina framework ratio with a gas stream containing oxygen and steam at temperatures between 300 º C and 600 º C until a silica / alumina framework ratio between 15 and 40 and a framework aluminium / extraframework aluminium ratio between 1 and 10 is obtained. Process according to claim 1 characterized by the use of mordenite in sodium or acid form. Process according to claim 2 characterized by the conversion of the sodium mordenite into acid mordenite by means of exchange treatments with NH₄⁺ followed by calcination, or directly with acid treatments. Process according to claim 1 characterized in that the treatment of said mordenite with an acid solution is performed at temperatures between 60 and 90 º C. Process according to claim 1 characterized in that the steaming of said mordenite previously treated with an acid solution is performed at temperatures between 350 º C and 500 º C. Process according to claim 1, characterized in that the acid treatment of the mordenite is carried out with a strong mineral acid at a liquid to solid ratio of 5 to 20 (liters/kilo) at a concentration between 1 and 6N and a treatment time between 1 and 8 hours. Process according to claim 6, characterized in that said mineral acid is H₂SO₄ . Process according to claim 6, characterized in that said mineral acid is H₃PO₄ . Process according to claim 6, characterized in that said mineral acid is HCl. Process according to claim 6, characterized in that the concentration of said mineral acid is between 2 and 5N. Porcess according to claim 1, characterized in that the steam-air treatment lasts between 2 and 8 hours and the steam content of H₂O of the air is from 5 to 100% by volume. Process according to claim 1, characterized in that the deposition of the metal is done by impregnation or exchange as of chloroplatinic acid or any other amined or aqueous platinum complex. Process according to claim 1, characterized in that the impregnation is done with chloroplatinic acid dissolved in hydrochloric acid at a concentration between 0.05 and 1N. Use of the catalyst according to any of the preceding claims in an isomerization reaction of C₄ to C₇ light straight chain paraffins carried out between 150 º C and 400 º C, at a total hydrogen pressure of 10 to 50 bars, a LHSV between 0.5 and 4 h⁻¹ and a H₂ /HC ratio between 0.5 and 4. Use of the catalyst according to claim 14 wherein the paraffins comprise pure hydrocarbons or mixtures thereof.

Inventors: CORMA CANOS AVELINO, FRONTELA DELGADO JUANA MARIA, LAZARO MUNOZ JESUS JAVIER, CORMA CANOS, AVELINO, FRONTELA DELGADO, JUANA MARIA, LAZARO MUNOZ, JESUS JAVIER, Lazaro Munoz, Jesús Javier
Applicants: ESPANOLA PETROL, COMPANIA ESPANOLA DE PETROLEOS S.A. -CEPSA
Application year: 1990
Main classification: C07C5
IPC codes: C07C5, B01J29