Aromatization Process Utilizing a Pillared Layered Silicate Plus a Base Metal or Noble Metal

The present invention relates to aromatization of non-aromatic C.sub.6 to C.sub.12 hydrocarbons using a catalyst which comprises a layered silicate and a base metal or noble metal.

The production of aromatic hydrocarbons from non-aromatic hydrocarbons using certain shape-selective catalysts materials is well-known. U.S. Pat. No. 3,756,942 to Cattanach teaches such a method using a zinc-exchanged ZSM-5. U.S. Pat. No. 4,180,689 to Davies et al teaches conversion of C.sub.3 to C.sub.12 hydrocarbons to aromatics using gallium-activated zeolite such as ZSM-5, ZSM-11, ZSM-12, or ZSM-35 which is gallium-exchanged or gallium-impregnated. Although the incorporation of a metal of mild dehydrogenation function such as zinc or gallium activates the catalyst for aromatization reactions, loss of the metal, for example by elution, commonly occurs under the high temperature reducing conditions encountered in aromatization. U.S. Pat. No. 4,490,569 to Chu et al teaches a method to reduce such elution by incorporating gallium as well as zinc into a zeolite aromatization catalyst. All of these U.S. Patents are incorporated herein by reference.

There is provided a process for producing aromatic hydrocarbons which comprises contacting a feed containing non-aromatic C.sub.6 to C.sub.12 hydrocarbons with a catalyst at a pressure of about atmospheric to 1,000 psig, a weight hourly space velocity of about 0.05 to 300 and a temperature of about 204.degree. to 675.degree. C., wherein said catalyst comprises a layered silicate and pillars of an oxide of at least one element selected from Groups IB, IIB, IIIA, IIIB, IVA, IVB, VA, VB, VIA, VIIA and VIIIA of the Periodic Table of Elements separating the layers of the silicate, said catalyst comprising at least one element selected from the group consisting of Cr, Mo, W, Fe, Co, Ni, Ru, Rn, Pd, Os, Ir, and Pt.

Many layered materials are known which have three-dimensional structures which exhibit their strongest chemical bonding in only two dimensions. In such materials, the stronger chemical bonds are formed in two-dimensional planes and a three-dimensional solid is formed by staking such planes on top of each other. However, the interactions between the planes are weaker than the chemical bonds holding an individual place together. The weaker bonds generally arise from interlayer attractions such as Van der Waals forces, electrostatic interactions, and hydrogen bonding. In those situations where the layered structure has electronically neutral sheets interacting with each other solely through Van der Waals forces, a high degree of lubricity is manifested as the planes slide across each other without encountering the energy barriers that arise with strong interlayer bonding. Graphite is an example of such a material. The silicate layers of a number of clay materials are held together by electrostatic attraction mediated by ions located between the layers. In addition, hydrogen bonding interactions can occur directly between complementary sites on adjacent layers, or can be mediated by interlamellar bridging molecules.

Laminated materials such as clays may be modified to increase their surface area. In particular, the distance between the interlamellar layers can be increased substantially by absorption of various swelling agents such as water, ethylene glycol, amines, ketones, etc., which enter the interlamellar space and push the layers apart. However, the interlamellar spaces of such layered materials tend to collapse when the molecules occupying the space are removed by, for example, exposing the clays to high temperatures. Accordingly, such layered materials having enhanced surface area are not suited for use in chemical processes involving even moderately severe conditions.

The extent of interlayer separation can be estimated by using standard techniques such as X-ray diffraction to determine the basal spacing, also known as "repeat distance" or "d-spacing." These values indicate the distance between, for example, the uppermost margin of one layer with the uppermost margin of its adjoining layer. If the layer thickness is known, the interlayer spacing can be determined by subtracting the layer thickness from the basal spacing.

The aromatization catalyst employed in the present invention comprises a layered silicate containing interspathic polymeric oxide such as silica. The interlayer distance of the silicate may be such that polycyclic hydrocarbons can pass between adjacent layers of the silicate, preferably a distance greater than about 10 A or even 15 A, perhaps about 15 to 20 A. Aromatization activity can be enhanced by incorporating a suitable dehydrogenation component into the catalyst, such dehydrogenation components include one or more base metals or noble metals which include metals from Groups VIA (Cr, Mo, and W) and VIII (Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, and Pt) of the Periodic Table. Base metals are Cr, Mo, W, Fe, Co, and Ni. The noble metals are Ru, Rh, Pd, Os, Ir, and Pt.

The process of the present invention utilizes an aromatization catalyst which comprises a layered silicate which contains interspathic polymeric silica. The layered silicate may also comprise an interspathic polymeric oxide of an element selected from the group consisting of Al, B, Cr, Ga, In, Mo, Nb, Ni, Ti, Tl, W, and Zr, e.g., polymeric silica-alumina between the layers of the silicate material. Preferably, such "pillared" materials are thermally stable, i.e., capable of withstanding calcining at a temperature of about 450.degree. C. for at least 2 hours without significant reduction, e.g., not greater than 10 or 20 percent, in the spacing between the silicate layers. Preferably, such materials can withstand prolonged exposure to the conditions encountered during aromatization. Polymeric interspathic silicas displaced between silicate layers are considered to include oxides of two or more reporting units, e.g., four or more or even five or more repeating units. The extent of polymerization of the interspathic polymeric silica is believed to affect the ultimate interlayer separation of the layered product; that is to say, the greater the extent of polymerization occuring, the greater the interlayer distance resulting in the pillared layered silicate. A layered material suited for use in the present aromatization process, having a desired interlayer spacing can be prepared according to the method set out in U.S. Pat. No. 4,859,648, incorporated herein by reference. In this method, the interlayer spacing of the layered material can be tailored by careful selection of the "propping" agent used to separate the layers during treatment with interspathic polymeric silica precursors which are eventually converted to the thermally stable polymeric silica "pillars." Indeed, a wide range of interlayer spacings can be achieved by this method. Interlayer distances can range anywhere from 2 to 30 A or more, e.g., greater than 5, 10, 15, or 20 A, depending largely on the type of "propping" agent used as well as the layered silicate being treated.

The pillared layered silicates employed herein can be prepared by treating a layered silicate which contains ion exchange sites having interspathic cations associated therewith, with an organic compound which is a cationic species or capable of forming a cationic species to effect exchange with the interspathic cations. An electrically neutral compound capable of conversion to the interspathic polymeric metal or non-metal oxide is provided between the layers of the treated layered silicate. The compound is then converted to the interspathic polymeric silica to form the layered material.

The pillared layered silicate employed in the present invention can be prepared by treating a layered silicate, e.g., a high silica alkali silicate such as synthetic magadiite, or synthetic kenyaite. These pillared layered silicate materials possess a framework composed essentially of only tetrahedral sheets, i.e., silicon is coordinated with four oxygen atoms, condensed on each other. These materials lack octahedral sheets, such as those found in clays, wherein an element such as aluminum is coordinated with six oxygen atoms. Besides the interspathic polymeric silica, interspathic polymeric oxides of one or more elements selected from the group consisting of B, Al, Ga, In, and Tl can also be incorporated between the layers of the silicate either separate from or incorporated into the interspathic polymeric silica pillars. Interspathic polymeric alumina is particularly useful in imparting acidic activity to the layered silicate. Interspathic polymeric oxides containing silica-alumina are a preferred pillar for these layered silicates.

Pillared silicates containing from about 5 to 50 wt % silica-alumina incorporated as the pillar material are desirable. Particularly preferred are silicates containing from about 10 to 20 wt % silica-alumina as the pillared material. The silica/alumina molar ratio (SiO.sub.2 /Al.sub.2 O.sub.3) of the pillared material may vary between about 5 to 1,000 or even greater.

Layered silicate materials of relatively high interplanar distance (d-spacing), e.g., greater than about 10, 15, 18, 20, 25, or even 30 or more A, can be prepared using the above-discussed techniques. These materials are capable of being exposed to severe conditions such as those encountered in calcining, e.g., at temperatures of about 450.degree. C. for about two or more hours, e.g., four hours, in nitrogen or air, without significant decrease, perhaps, e.g., less than about 10 percent, in interlayer distance. Charge density should be taken into consideration in determining the suitability of the cationic species introduced between the layers in the procedure used to prop open the layers prior to pillaring. The use of an electrically neutral polymeric oxide precursor allows the formation of materials in which the interlayer spacing can be widely varied.

The layered silicate starting material can contain ion exchange sites having interspathic cations associated therewith. Such interspathic cations may include hydrogen ion, hydronium ion or alkali metal cation. The starting material is treated with a "propping" agent comprising a source of organic cation, which source may include the cation itself, in order to effect an exchange of the interspathic cations resulting in the layers of the starting material being propped apart. In particular, alkylammonium cations have been found useful. The C.sub.3 and large alxylammonium cations, e.g., n-octylammonium, can be readily incorporated within the interlayer species of the layered silicates, serving to prop open the layers in such a way as to allow incorporation of the polymeric oxide precursor. The extent of the interlayer spacing can be controlled by the size of the organoammonium ion employed so that use of the n-propylammonium cation can achieve a d-spacing of about 2 to 5 A or an opening of about 2-3 A, whereas to achieve an interlayer opening of 10 to 20 A, an n-octylammonium cation or a cation of equivalent length is required. The organic ammonium cations separating the silicate layers may also be formed in situ by reaction of the neutral amine species with interlayer hydrogen or hydronium cations of the layered silicate starting material.

The polymeric oxide pillars are formed from a precursor material which is preferably introduced between the layers of the organic "propped" species as a cationic, or more preferably, electrically neutral, hydrolyzable compound of the desired elements. The precursor material is preferably and organic compound containing said desired elements which is a liquid under ambient conditions. In particular, hydrolyzable compounds, e.g., alkoxides, of the desired elements of the pillars are utilized as the precursors. Suitable polymeric silica precursor materials include tetraalkylsilicates, e.g., tetrapropylorthosilicate, tetramethylorthosilicate, and, most preferably, tetraethylorthosilicate. Introduction of interspathic polymeric oxide of an element selected from the group consisting of Al, B, Cr, Ga, In, Mo, Nb, Ni, Ti, Tl, W, and Zr to the pillar system can be achieved by contacting a hydrolyzable compound of the desired element with the organic "propped" species before, after or simultaneously with the contacting of the layered chalcogenide with the silicon compound. The hydrolyzable aluminum compound employed may be an aluminum alkoxide, e.g., aluminum isopropoxide.

After hydrolysis to produce the polymeric oxide pillars and calcination to remove the organic propping agent, the final pillared product may contain residual exchangeable cations. Such residual cations in the layered material can be ion exchanged by known methods with other cationic species to provide or alter the catalytic activity of the pillared product. In particular, base metal or noble components containing at least one element selected from the group consisting of Cr, Mo, W, Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, and Pt, can be introduced by ion-exchange or impregnation techniques known in the art. Representative ion exchange techniques are disclosed in a wide variety of patents including U.S. Pat. Nos. 3,140,249; 3,140,251; and 3,140,253, all of which are incorporated herein by reference. Generally, the aromatization catalyst of the present invention can contain about 0.1 to 20 wt %, preferably about 0.5 to 15 wt % of the base metal or noble metal.

The polymeric oxide precursor-containing product can be exposed to suitable conversion conditions, such as hydrolysis and/or calcination to form the layered material employed in the present invention. The hydrolysis step may be carried out by any method, for example, by interspathic water already present in the organic-"propped" layered silicate material. Because of the effect of interspathic water on hydrolysis, the extent of hydrolysis may be modified by varying the extent to which the organic-"propped" species is dried prior to addition of the polymeric oxide precursor. As noted earlier, the product after conversion to the polymeric oxide form may be exposed to conditions which remove organic compounds such as the organic cation propping agents, e.g., exposure to elevated temperatures such as those encountered by calcining in air or nitrogen. Such products, especially when calcined, exhibit high surface area, e.g. greater than 200, 300, 400 or even 600 m.sup.2 /g, and thermal and hydrothermal stability.

The pillared silicates can be composited with porous inorganic oxide matrix materials such as silica-alumina, silica-magnesia, silica-zirconia, silica-thoria, silica-beryllia, and silica-titania, as well as ternary compositions, such as silica-alumina-thoria, silica-alumin-zirconia, silica-alumina-magnesia, and silica-magnesia-zirconia. The matrix may be in the form of a cogel. The relative proportions of pillared silicate component and inorganic matrix, on an anhydrous basis, may vary widely with the silicate content ranging from about 1 to about 99 wt % and more usually in the range of from about 5 to about 80 wt % of the dry composite.

Layered silicates, e.g., high silica alkali silicates such as magadiite, natrosilite, kenyaite, makatite, nekoite, kanemite, okenite, dehayelite, macdonaldite and rhodesite, unlike swellable clays, lack octahedral sheets, i.e., sheets composed of atoms which are octahedrally coordinated with oxygen atoms. Such high silica alkali silicates, as well as their synthetic analogues, are well-suited as starting materials used in preparing the pillared layered silicates employed in the process of the present invention. Without stable intercalated pillars, these starting materials tend to undergo collapse of their layers at elevated temperatures, which results in low porosity and low surface area.

The layered silicate starting materials known as high silica alkali silicates, whose layers lack octahedral sheets, can be prepared hydrothermally from an aqueous reaction mixture containing silica and caustic at relatively moderate temperatures and pressures. These layered silicates may contain tetracoordinate framework atoms other than Si. Such layered silicates can be prepared by co-crystallizing in the presence of non-silicon tetravalent elements, e.g., those selected from the group consisting of B, Al, Ga, In, and Tl as well as any other such elements which are catalytically useful when incorporated in the silicate structure. Alternatively, non-silicon framework elements already in a layered silicate may be substituted by a tetracoordinate element. Both co-crystallized and substituted layered high silica alkali silicates may be treated by the procedure described above to provide layered materials containing interspathic polymeric oxide pillars.

Synthetic magadiite materials which contain interspathic polymeric oxides may be particularly suited to use in the aromatization process of the present invention. Synthetic magadiite is readily synthesized hydrothermally from a reaction mixture containing expensive sources of silica and caustic. Tetracoordinate elements other than silicon, e.g., those selected from the group consisting of B, Al, Ga, In, Tl and other catalytically useful metals, may be added to the reaction mixture to produce synthetic magadiite layered silicates. Preferably, such elements are selected from the group consisting of Al and Ga. An organic directing agent may also be added to the reaction mixture. The reaction mixture for synthetic layered silicate materials can be described in molar ratios as follows:

where R can be an organic such as benzyltriethylammonium chloride, benzyltrimethylammonium chloride, dibenzyldimethylammonium chloride, N, N-dimethylpiperazine, triethylamine, or other quaternary compounds or heterocyclic amines.

The reaction mixture can be maintained at a temperature of about 100.degree. to 200.degree. C. for anywhere from about 1 to 150 days in order to form a product having the following composition:

The synthetic layered silicate materials thus prepared have a low surface area. Introduction of interspathic polymeric oxides according to the above-described procedure can increase the surface area of these materials. Generally, the synthetic magadiite (or layered silicate) material is acidified by any suitable means, e.g., treatment with aqueous 0.1 N HCl, and thereafter treated with a "propping" agent. A suitable compound capable of conversion to a polymeric oxide is combined with the "propped" layered silicate and the resulting material can then be calcined to remove residual organics.

Another embodiment of the present invention involves aromatization reactions using synthetic kenyaite materials which contain interspathic polymeric oxides. Kenyaite, a layered silicic acid which is known to exist in nature as a sodium salt (Na.sub.2 Si.sub.22 O.sub.45 H.sub.2 O), can be prepared in the potassium form K.sub.2 Si.sub.22 O.sub.45 10H.sub.2 O in the laboratory. Synthetic kenyaite is readily synthesized hydrothermally from a reaction mixture containing inexpensive sources of silica and caustic, preferably KOH. Tetracoordinate elements other than silicon, e.g., those selected from the group consisting of Al, B, Cr, Fe, Ga, In, Ni, Zr and other catalytically useful metals, may be added to the reaction mixture to produce synthetic kenyaite layered silicates. Al(NO.sub.3).sup..multidot..sub.3 9H.sub.2 O and aluminum-tri-sec-butoxide are suitable reagents for introduction of non-silicon tetracoordinate elements in the kenyaite framework. Co-crystallizing with B, Al, and/or Zr is particularly preferred. The reaction mixture may also be seeded with kenyaite.

The present aromatization reaction involves contacting a suitable feedstock with the present catalyst under sufficient aromatization conditions. Such conditions may include a pressure of about atmospheric to 1,000 psig, preferably about atmospheric to 200 psig; a weight hourly space velocity of about 0.05 to 300, preferably about 0.2 to 10; and a temperature of about 204.degree. to 675.degree. C., preferably about 315.degree. to 593.degree. C.

The feedstock may include one or more C.sub.6 to C.sub.12 non-aromatic hydrocarbons, especially C.sub.6 to C.sub.12 paraffins, most especially n-paraffins.

A tetraethylorthosilicate-impregnated kenyaite (10 g, 60% ash) was added to an aqueous Cr(NO.sub.3).sub.3 solution (120 g, 0.5 M). The slurry was stirred for 2 hours, filtered, air-dried, and calcined at 540.degree. C. for 6 hours in air. The composition and surface area are listed in Table 1.

A silica-pillared (Si) kenyaite (2 g) was impregnated with an aqueous Cr(NO.sub.3).sub.3 solution (0.223 g in 2 g H.sub.2 O). The material was calcined at 540.degree. C. for 3 hours in air. The composition and surface area are listed in Table 1.

A silica-pillared (Si) kenyaite (5 g) was impregnated with an aqueous Cr(NO.sub.3).sub.3 solution (1.2 g in 5 g H.sub.2 O). The material was rolled for 1 hour, then calcined at 540.degree. C. for 3 hours in air. The composition and surface area are listed in Table 1.

An amorphous silica gel (Ultrasil, 5 g) was impregnated with an aqueous Cr(NO.sub.3).sub.3 solution (1.2 g in 7 g H.sub.2 O). The material was rolled for 2 hours, then calcined at 540.degree. C. for 3 hours. The composition and surface area are listed in Table 1.

An amorphous silica gel (Hi-Sil, 5 g) was treated the same as described in Example 4. The composition and surface area are listed in Table 1.

Kaiser alumina (15 g) was impregnated with an aqueous Cr(NO.sub.3).sub.3 solution (3.6 g in 15 g H.sub.2 O). The material was rolled for 1 hour, then calcined at 540.degree. C. for 3 hours. The composition and surface area are listed in Table 1.

A sample similar to that described in Example 6 was prepared. Only the alumina was calcined at 540.degree. C. for 3 hours prior to the treatment. The composition and surface area are listed in Table 1.

Catalyst samples prepared in the above Examples were tested for dehydrogenation of light alkanes. Fixed weights of the catalysts were charged into a small Pyrex reactor. Normal hexane or other specified feed such as n-heptane or cyclohexane was charged over the catalyst bed at 2 WHSV. The operating conditions were set at 900.degree. to 1,000.degree. F. under atmospheric pressure. After being on stream for a specified time, a product sample was analyzed by online gas chromatography using a fused silica capillary column. The results are listed in Tables 2 and 3.

The foregoing Examples demonstrate the use of novel high-silica catalyst supports, silica-pillared silicates. It displays conversion and selectivity that is not observed with amorphous silica. Dehydrocyclization over the pillared silicates produces a narrow product distribution. The low-acidity silicate supports would be preferred over the high-acidity alumina supports to eliminate unfavorable, non-selective, side reactions, such as cracking, hydrolysis, etc.