Catalytic Composition for the Isomerization of Paraffinic Hydrocarbons

The present invention is related to an improved catalytic composition and a hydroisomerization process employing that catalytic composition. More particularly, this invention involves an isomerization catalyst composition comprising a hydrogenation function selected from the Group VIII metals, a hydrogen form crystalline alumino-silicate zeolite, and a refractory inorganic oxide.

The isomerization of low molecular weight normal paraffins is well established in the art. This reaction is of considerable importance in the petroleum industry because of the substantially higher octane numbers of isoparaffins compared to their normal paraffin counterparts. Since gasoline blends require a distribution of boiling range materials, the isoparaffins in the C.sub.4 -C.sub.7 range are valuable blending components. It has been the practice up until this time to isomerize paraffins to equilibrium mixtures of their branched chain isomers with a variety of catalysts. Friedel-Crafts catalysts, such as aluminum chloride, are known to be effective isomerization catalysts. Noble metals, such as platinum supported on halogenated alumina or silica alumina have also been used effectively to isomerize hydrocarbons. More recently, crystalline alumino-silicate zeolites which have shown catalytic activity have been effectively used in the isomerization of hydrocarbons. Both natural and synthetic crystalline alumino-silicates have been employed. Included among these are the Type X and Type Y zeolites as well as synthetic mordenite.

Specifically, the zeolites known as mordenites have received great attention. Mordenites are crystalline natural or synthetic zeolites of the alumino-silicate type: generally, they have a composition expressed in moles of oxide of

the quantity of SiO.sub.2 may also be larger. Instead of all or part of the sodium, other alkali metals and/or alkaline earth metals may be present.

In general, it has been found that the sodium form of mordenite is not particularly effective for isomerization of hydrocarbons and that replacing all or, for the greater part, the sodium cations with hydrogen ions yields the more advantageous hydrogen form mordenite. Conversion of the sodium form to the hydrogen form can be accomplished by a number of means. One method is the direct replacement of sodium ions with hydrogen ions using an acidified aqueous solution where the process of ion exchange is employed. Another method involves substitution of the sodium ions with ammonium ions followed by decomposition of the ammonium form using a high temperature oxidative treatment.

The activity and selectivity of hydroisomerization catalysts depend on a variety of factors, such as the mode of catalyst preparation, the presence or absence of promotors, quality of raw materials, feedstock quality, process conditions, and the like. Suitable catalysts can be conventionally prepared by combining commercially available crystalline zeolites, such as, a hydrogen form mordenite, with a suitable matrix material followed by the addition of a Group VIII metal, and thereafter activating by conventional means. A new catalyst has been discovered which exhibits greatly improved isomerization performance when compared to conventionally prepared catalysts.

Accordingly, there is provided a catalytic composite for the isomerization of isomerizable hydrocarbons. This catalytic composite comprises a Group VIII metal hydrogen form alumino-silicate and refractory inorganic oxide, with the essential feature that the composite have a surface area of at least 580 m.sup.2 /g. Platinum is the preferred Group VIII metal, and is preferably present in the amount of from 0.15 to 0.5 wt. % of said composite. The preferred hydrogen-form alumino-silicate has a silica to alumina ratio of at least 16 and is present in amount from 75 to 95 wt. % of said composite and it most preferably has a mordenite structure. The preferred refractory inorganic oxide is alumina, selected from the group consisting of gamma-alumina, eta-alumina, and mixtures thereof.

In another aspect the invention is a method of manufacturing the aforementioned catalytic composition and a hydroisomerization process employing that catalytic composition. Manufacturing of the catalyst comprises forming a composite comprising hydrogen form mordenite and refractory oxide, thereafter contacting formed composite with an acidic aqueous solution containing ammonium ions, followed by incorporation of a Group VIII metal into the formed composite.

The preferred isomerization process conditions employing the instant catalytic composite comprise a temperature range from about 200.degree. F. to 800.degree. F., a pressure of from about 100 to 1,000 psia and a liquid hourly space velocity in the range of 0.25 to 5 hr.sup.-1. Hydrogen is also required in the amount ranging from 0.5 to 5 mole H.sub.2 /mole hydrocarbon feed.

These, as well as other embodiments of the present invention, will become evident from the following, more detailed description.

The prior art recognizes a myriad of catalyst formulations for the isomerization of hydrocarbons. It is well known that acids, such as, strong mineral acids, can be used to modify crystalline alumino-silicate powders through decationization and dealumination. Ammonium compounds have also been successfully employed to convert crystalline alumino-silicates from alkali and/or alkaline metal cation form to the hydrogen form.

Combinations of the acid and ammonium treatments have been disclosed for use on alumino-silicate powders. U.S. Pat. No. 3,475,345 (Benesi) discloses a method of converting alumino-silicate zeolites, particularly a sodium form synthetic mordenite, to the hydrogen form utilizing a three-step pretreatment performed on the powdered zeolite. These pretreatment steps consist of (1) a hot acid treatment, (2) a cold acid treatment, and (3) treatment with an ammonium compound. U.S. Pat. No. 3,442,794 (Van Helden et al.) also discloses a method for the pretreatment of alumino-silicate zeolites to the hydrogen form. Again, the preferred zeolite is the synthetic sodium form of mordenite. The method disclosed is very similar to U.S. Pat. No. 3,475,345 mentioned above, with the distinguishing feature being a separately performed two-step pretreatment with (1) an acid compound, and (2) an ammonium compound in arbitrary order. An important feature of both references is that the treatments are performed solely on the alumino-silicate zeolite with the express intention of modifying said zeolite before being utilized in a catalyst formulation and that no mention of the importance of the surface area of the catalytic composite is disclosed. This is distinguished from the present invention in that any treatment performed is subsequent to the zeolite being incorporated into a formed catalyst composite and more importantly without any apparent modification of the zeolite itself.

Treatment of the alumino-silicates with acid have not only been effective for conversion to the hydrogen form, but also have been used as a means for increasing the silica to alumina ratio. Typically, a silica to alumina ratio of about 10:1 is observed for a sodium form synthetic mordenite and is substantially unchanged if an ammonium treatment is used to convert the mordenite to the hydrogen form. If a mordenite powder is subjected to an acid treatment as taught in U.S. Pat. No. 3,597,155 (Flanigen) an increase in the silica to alumina ratio is effected. The acid treatment is believed to cause a reduction of the framework tetrahedra aluminum atoms, thus increasing the proportion of silicon atoms present in the zeolitic structure. Isomerization performance is enhanced when the silica to alumina ratio of a mordenite powder is increased. As taught in U.S. Pat. No. 3,507,931 (Morris et al), a silica to alumina ratio above about 20:1 significantly improves the isomerization of light hydrocarbons. U.S. Pat. No. 4,018,711 (Bertolacini) also teaches that isomerization performance is improved if a pretreated mordenite powder having a silica to alumina ratio of at least 19:1 is incorporated in a catalytic composition. Again, these references specifically teach the use of acid treatment on the zeolite powder alone for the purpose of increasing the silica to alumina ratio, whereas the subject invention incorporates an already high silica to alumina ratio crystalline alumino-silicate into the catalytic composite. These references also do not teach the importance of the surface area of the catalytic composite or its relationship to isomerization performance.

A common attribute of the above mentioned prior art is that, in all cases, the crystalline alumino-silicate alone, in particular the synthetic sodium form of mordenite, is subjected to an acid and/or an ammonium pretreatment step(s) to modify the alumino-silicate before its incorporation into the catalyst composition. Although the pretreatment of mordenite as described in the above references enhances the isomerization performance of catalytic composites comprising such pretreated mordenite, further improvements are still obtainable.

Reference to the accompanying drawing will facilitate understanding of the present invention. The graph shown in the drawing illustrates that the isomerization performance is a function of the Langmuir surface area of the catalytic composite. The isomerization performance is characterized as selectivity to desired products, namely, 2,2 dimethylbutane (2,2 DMB) and isopentane (i-C.sub.5).

While previous work dealt exclusively with pretreatment of the alumino-silicate component of an isomerization catalyst, it is one of the objects of the present invention to provide a novel catalytic composition which exhibits improved isomerization performance.

According to the present invention, there is provided a catalytic composition for the isomerization of isomerizable hydrocarbons into their branched chain equivalents, a method of manufacturing said catalytic composition, and a process which uses said catalytic composition to produce high octane blending components.

The catalyst composition of the present invention comprises a Group VIII noble metal, hydrogen form crystalline alumino-silicate, and a refractory inorganic oxide with said catalyst composition having a surface area of at least 580 m.sup.2 /g. We have found that significant improvements in isomerization performance are realized when the surface area of the catalytic composition is at or above 580 m.sup.2 /g. Obtaining such a surface area is the object of one of the embodiments of the subject invention and is further illustrated in subsequent examples.

The metal that is incorporated into the catalytic composite to supply the hydrogenation-dehydrogenation function is a Group VIII noble metal. The Group VIII noble metals include the metals of the "Platinum Series" and the metals of the "Palladium Series", i.e., platinum, iridium, osmium, palladium, rhodium, and ruthenium. The preferred Group VIII noble metal is platinum. The Group VIII noble metal of the catalytic composition of the present invention will be utilized in an amount from about 0.1 to about 5% by weight of the composite. It is particularly preferred that the metal component be at least about 0.15% by weight and not over 0.5% by weight.

Of course, it is not beyond the scope of the instant invention that the catalytic composition contain a catalytically effective amount of a promoter metal. Examples of such promoter metals include tin, lead, germanium, cobalt, nickel, iron, tungsten, chromium, molybdenum, bismuth, indium, gallium, cadmium, zinc, uranium, copper, silver, gold, tantalum, one or more of the rare earth metals and mixtures thereof.

The crystalline alumino-silicate of the present invention is a hydrogen form silica-alumina having either a three-dimensional or channel-pore-structure crystal lattice framework. The three-dimensional alumino-silicates include both synthetic and naturally occurring silica aluminas, such as, the faujasites which include X-type, Y-type, ultrastable-Y and the like. L-type, omega-type, and mordenite are examples of the channel-pore-structure crystalline alumino-silicates.

The alumino-silicate material that is preferred in the catalytic composition of this invention is the particular form of alumino-silicate material known as mordenite. While mordenite is naturally occurring, a variety of synthetic mordenites are available commercially, usually in a powder form. These synthetic mordenites can be obtained in both the sodium form and hydrogen form and at varied silica to alumina ratios. It is a preferred embodiment of the present invention that the mordenite be of the hydrogen form and that the silica to alumina ratio be at least 16:1, more specifically, in the range from 16:1 to 60:1. The pretreatment steps taught in the aforementioned references are routinely and typically employed in the manufacture of commercial available mordenite powders which meet the requirements as a starting material as set forth in the present invention. These pretreatment steps are used to increase the silica to alumina ratio of the mordenite zeolite and to convert the sodium form to the more desirable hydrogen form.

The hydrogen form alumino-silicate is incorporated with a refractory inorganic oxide and formed into a catalyst composite. The formed catalyst composite may be prepared by any known method in the art including the well-known oil drop and extrusion methods. The hydrogen form alumino-silicate may be present in an amount within the range of 50 to about 99.5 weight percent, preferably within the range of 75 to about 95 weight percent, and the refractory inorganic oxide may be present in an amount within the range of from 0.5 to about 50 percent.

The preferred inorganic oxide for use in the present invention is alumina. The alumina is preferably selected from the group consisting of gamma-alumina, eta-alumina and mixtures thereof. Other refractory inorganic oxides which are contemplated include, for example, silica gel, silica-alumina, magnesia-alumina, zirconia alumina, and the like.

Surprisingly and unexpectedly, it has been found that a strong correlation exists between the isomerization performance of a catalyst composite of the subject invention and the surface area of said composite. One preferred method to achieve maximum isomerization performance is by utilizing formed catalytic composites with surface areas measuring at least 580 m.sup.2 /g. Surface area, as referred to herein, is determined by employing the Langmuir method of correlating adsorption/desorption isotherm data. The Langmuir method is especially suitable for catalytic composites containing high percentages of crystalline alumino-silicates. The data needed for the Langmuir method is typically obtained by well known adsorption/desorption apparatuses, preferably a nitrogen adsorption/desorption apparatus. Any method may be employed which results in a final catalyst composite having at least a surface area of 580 m.sup.2 /g. Catalytic composites with high surface areas can be arrived at in a number of ways, such as, using a crystalline alumino-silicate powder which inherently has a very high surface area, or by having one component of the composite, which has a high surface area, in great proportion to other components. A preferred method of achieving a surface area of at least 580 m.sup.2 /g is to contact the formed catalyst composite with an acidic aqueous solution containing ammonium ions. The formed catalyst composite may be dried and/or calcined prior to its contact with the aqueous solution.

The acidic nature of the aqueous solution is attained by employing an acid. Particularly suitable are strong mineral acids such as H.sub.3 PO.sub.4, H.sub.2 SO.sub.4, HNO.sub.3 and HCl. HCl is the preferred acid of the present invention. Of course, it is contemplated that mixtures of various acids may also be employed. The preferred source of the ammonium ion incorporated in the acidic aqueous solution is NH.sub.4 Cl, but any ammonium compound which can form ammonium ions, such as NH.sub.4 OH, NH.sub.4 NO.sub.3, NH.sub.4 sulfates, NH.sub.4 phosphates and the like, should be suitable.

Concentrations of the acid and ammonium ions in the aqueous solution are not critical and can vary from 0.5M to 6M for the acid concentration and 0.5M to 4M for the ammonium ion concentration. Particularly good results are obtained using a solution containing acid and ammonium ion concentrations within the range of 2 to 5M for the acid and 1 to 3 for the ammonium ion.

A plurality of methods for contacting the formed catalytic composition and the acidic aqueous solution containing ammonium ions is envisioned with no one method of particular advantage. Such contacting methods may include, for example, a stationary catalyst bed in a static solution, a stationary catalyst bed in an agitated solution, a stationary catalyst bed in a continuously flowing solution or any other means which efficiently contacts the catalyst composition with the acidic aqueous solution.

The temperature of the contacting solution should be within the range of 25.degree. C. (77.degree. F.) to about 100.degree. C. (212.degree. F.), preferably within the range of from about 50.degree. C. (122.degree. F.) to about 98.degree. C. (208.degree. F.). The time required for the contacting step will depend upon concentrations, temperature and contacting efficiency. In general, the contacting time should be at least 0.5 hour, but not more 4 hours, preferably between 1 and 3 hours in duration.

As a result of contacting the formed catalytic composition with the acidic aqueous solution containing ammonium ions, an increase in the measured surface area is observed. Surprisingly and unexpectedly, this increase in surface area, to 580 m.sup.2 /g or higher, is not accompanied by an increase in the silica to alumina ratio of the hydrogen form crystalline alumino-silicate as measured by Magic Angle Spinning NMR (MASNMR). The MASNMR technique, which is a well known analytical method of the art, indicates no reduction in the framework tetrahedral aluminum atoms of catalyst compositions of the present invention. As will be demonstrated hereinafter, catalysts of the instant invention with 580 m.sup.2 /g surface area surprisingly and unexpectingly exhibit improved isomerization performance.

The catalyst of the invention has particular utility for the isomerization of isomerizable hydrocarbons. Included in the group of isomerizable hydrocarbons are saturated hydrocarbons including paraffin hydrocarbons and is still more particularly suitable for the hydroisomerization of straight chain or slightly branched chain paraffins containing four or more carbon atoms per molecule. The isomerization reaction can be conducted over a wide range of temperatures, but, in general, in the range from about 93.degree. C. (200.degree. F.) to about 427.degree. C. (800.degree. F.) Space velocities from about 0.25 to about 5 liquid volumes per hour of said isomerizable hydrocarbons per volume of said catalytic composite are preferred with reaction zone pressures preferably within the range from about 6.9 bar (100 psi) to about 69 bar (1000 psi). It is particularly desirable to carry out the isomerization reaction in the presence of hydrogen preferably in the range from about 0.5 to about 5 moles of H.sub.2 per mole of isomerizable hydrocarbon. The function of the hydrogen is primarily to improve catalyst life, apparently by preventing polymerization of intermediate reaction products which would otherwise polymerize and deposit on the catalytic composite. It is not necessary to employ pure hydrogen since hydrogen containing gases, e.g., hydrogen-rich gas from the catalytic reforming of naphthas are suitable.

The following examples are presented for purposes of illustration only and are not intended to limit the scope of the present invention.

A number of experiments were conducted to study how changes in the surface area of isomerization catalyst composites affect isomerization performance. Five catalysts were prepared for evaluation. In all the catalyst preparations, described in the following examples, the starting material was the hydrogen form, low sodium, partially dealuminated synthetic mordenite powder (marketed by Union Carbide under the name LZ-M-8), hereinafter referred to as the as received mordenite.

In this example, the method of formulating the catalyst, designated as Catalyst A, was not in accordance with the subject invention. A 9:1 weight ratio mixture of as received mordenite and alumina was admixed with an acidified peptization solution and extruded by means known in the art. The extruded composite was dried, calcined in an oxidative atmosphere, impregnated with platinum, and calcined again. The platinum was added to a level of 0.324 wt. %, based on the weight of the finished catalyst. A surface area of 567 m.sup.2 /g was measured for Catalyst A and correlated against its isomerization performance. This correlation is shown graphically in the attached drawing.

The catalyst composite of this example was designated at Catalyst B and was also not made in accordance with the subject invention. The as received mordenite powder was contacted with an acidic aqueous solution containing ammonium ions comprising 10 wt. % HCl and 10 wt. % NH.sub.4 Cl at 140.degree. F. (60.degree. C.) for 150 minutes at a solution to zeolite weight ratio of 5:1. The resulting mordenite powder was washed with H.sub.2 O and calcined prior to admixture with alumina and peptization solution. Extrusion and platinum addition were performed in an equivalent manner for Catalyst B as for Catalyst A of Example 1. The platinum level of Catalyst B was 0.321 wt. %. A surface area of 534 m.sup.2 /g was measured for Catalyst B and correlated against isomerization performance as shown in the attached drawing.

Catalyst C made in accordance with the invention was formulated substantially in the same manner as Catalyst A of Example 1. However, in accordance with the subject invention, the dried extrudate prior to calcining and platinum addition was contacted with an acidic aqueous solution containing ammonium ions. This solution contained 10 wt. % HCl and 10 wt. % NH.sub.4 Cl. Contacting of the solution and the extrudate was performed at 140.degree. F.(60.degree. C.) for 120 minutes at a solution to zeolite weight ratio of 25:1. The extrudate was subsequently dried, calcined and treated with platinum following the same procedures used for Catalysts A and B. Catalyst C had a platinum level of 0.396 wt. %. A surface area of 622 m.sup.2 /g was measured for Catalyst C and correlated against its isomerization performance as shown in the attached drawing.

The calcined catalyst composite of Example II, prior to platinum addition, was subjected to a second contact with an acidic aqueous solution in the manner as was Catalyst C in Example III. This second contact of the formed composite with the acidic aqueous solution is in accordance with the subject invention. Platinum was then added to the composite to a level of 0.308 wt. % and designated as Catalyst D. A surface area of 630 m.sup.2 /g was measured for Catalyst D and correlated against its isomerization. This correlation is illustrated in the accompanying drawing.

Catalyst E was formulated in the same manner as Catalyst A except that the acidity of the peptization solution was reduced by 90%. As in Example I, Catalyst E was not formulated in accordance with the present invention. Catalyst E had a platinum level of 0.313 wt. %. A surface area of 542 m.sup.2 /g was measured for Catalyst E was measured and correlated against its isomerization performance test results. This correlation is shown in the attached drawing.

The five catalyst formulations of Examples I through V consisting of platinum supported on a hydrogen form mordenite gamma-alumina composite were evaluated for isomerization performance in a flow reactor processing a feed comprising a mixture of 46 wt. % n-pentane, 47.0 wt. % n-hexane, 5.5 wt. % methyl cyclopentane and 1.5 wt. % benzene.

The operating conditions used to test the isomerization performance of Catalysts A through E comprised a reactor pressure of 21.7 bar (315 psia), a liquid hourly space velocity of 1.0 hr.sup.-1, a H.sub.2 to feed hydrocarbon molar ratio of 1.0 and temperatures ranging between 254.degree. C. (490.degree. F.) and 277.degree. C. (530.degree. F.) Test run data, specifically the selectivities to iso-pentane (i-C.sub.5 and to 2,2-dimethyl butane (2,2 DMB) at 97 wt. % C.sub.5.sup.+ yield, were used as measures of isomerization performance. The selectivity value for i-C.sub.5 presented in the accompanying drawing is defined as the weight fraction of the i-C.sub.5 in the liquid isomerization product divided by the weight fraction of the total amount of C.sub.5 hydrocarbon in the product. To obtain the 2,2 selectivity value shown in the drawing, the weight fraction of 2,2 DMB in the liquid isomerization product is divided by the total, non-cyclic, C.sub.6 hydrocarbon in the product.

At the isomerization test run conditions utilized in the above examples, the selectivities to i-C.sub.5 and 2,2 DMB are a direct function of the measured surface area of the finished catalyst. As graphically illustrated in the attached drawing, the selectivities to i-C.sub.5 and 2,2 DMB increase as the measured surface area increases. Catalyst C of Example III and Catalyst D of Example IV demonstrate the highest selectivities to i-C.sub.5 and 2,2 DMB in accordance with the invention, both having measured surface areas of at least 580 m.sup.2 /g. Comparison of the platinum levels of Catalysts C and D to each other and to the other three catalysts shows that the platinum level does not correlate with isomerization performance further substantiating the benefit of having catalyst composites with surface areas of at least 580 m.sup.2 /g. Thus, utilizing the catalytic composition of the present invention clearly yields catalysts with superior isomerization performance.