Aromatization Process And Catalyst Therefor

Abstract

Improvements in the aromatization of hydrocarbon streams by contact thereof with a ZSM-5 type of synthetic aluminosilicate zeolite catalyst at elevated temperatures of about 500.degree. to 1500.degree.F, in the absence of added hydrogen, at high severities to convert at least 80 weight percent of the feed to a product, the liquid portion of which comprises aromatics, which improvement is engendered by utilizing a matrix catalyst containing said zeolite in a high silica content binder.

Description

This invention relates to hydrocarbon conversion. It more particularly refers to improvements in the art of converting aliphatic petroleum and/or other chemical fractions to aromatics.

There has recently been developed a commercially feasible process for upgrading aliphatic petroleum feedstocks. According this process a C.sub.2 to 400.degree.F feed, or any given portion thereof is contacted with a ZSM-5 type of synthetic aluminosilicate zeolite at about 650.degree. to 1500.degree.F (depending upon the specific composition of the feed) at a space velocity of up to about 15 WHSV, and in the substantial absence of added hydrogen, under such combination of conditions as to convert substantial portions of the nonaromatic portions of the feed to a mixed gas and liquid product, which liquid product contains new aromatics in a proportion of at least about 30 grams per 100 grams of the non-aromatic portion of the feed. Reference is made to applications Ser. Nos. 153,885 now U.S. Pat. No. 2,756,942 and 253,942 now U.S. Pat. No. 3,756,942 filed June 16, 1971 and May 17, 1972 respectively. The full text of these commonly assigned prior applications is incorporated herein by reference.

Feedstocks for use in this process are illustrated by Udex raffinates, coker gasoline, light cracked gasoline, straight run naphthas, pyrolysis gasoline and the like. These feeds are usually aliphatic in nature being composed of paraffins, olefins and naphthenes. Aromatics may be present in the feedstock to any extent considered to be desirable. Since they are often substantially inert under these processing conditions their proportion may be limited. An oxygen containing gas can be admixed with the feed, such as air, oxygen, oxygen diluted by an inert gas or gases or air enriched with the oxygen. The feed may be or contain oxygenated moieties such as partially oxidized light gases, or even be oxygenated organic chemical compounds such as acetone, acetaldehyde, methanol or the like. It is also within the scope of this invention to utilize organic compound feeds having a lower aliphatic portion and a hetero portion, such as sulfur, halogen or nitrogen.

The catalyst used for this known process has been stated to be a ZSM-5 type of catalyst which includes ZSM-5, ZSM-8, ZSM-11 and other similarly behaving zeolites.

ZSM-5 is disclosed and claimed in copending application Ser. No. 865,462, now U.S. Pat. No. 3,702,886 filed Oct. 10, 1969; ZSM-8 is disclosed and claimed in copending application Ser. No. 865,418, filed Oct. 10, 1969 and ZSM-11 is disclosed and claimed in copending application Ser. No. 31,421 filed Apr. 23, 1970.

The family of ZSM-5 compositions has the characteristic x-ray diffraction pattern set forth in Table 1 hereinbelow. ZSM-5 compositions can also be identified, in terms of mole ratios of oxides, as follows:

0.9 .+-. 0.2 M.sub.2/n O: W.sub.2 O.sub.3 : b YO.sub.2 : z H.sub.2 O

wherein M is a cation, n is the valence of said cation, W is selected from the group consisting of aluminum and gallium, Y is selected from the group consisting of silicon and germanium, z is from 0 to 40 and b is at least 5 and preferably 15-300. In a preferred synthesized form, the zeolite has a formula, in terms of mole ratios of oxides, as follows:

0.9 .+-. 0.2 M.sub.2/n 0 : Al.sub.2 O.sub.3 : 5-100 SiO.sub.2 : z H.sub.2 O

and M is selected from the group consisting of a mixture of alkali metal cations, especially sodium and tetraalkylammonium cations, the alkyl groups of which preferably contain two to five carbon atoms.

In a preferred embodiment of ZSM-5, W is aluminum, Y is silicon and the silica/alumina mole ratio is at least 15, preferably at least 30.

Members of the family of ZSM-5 zeolites which include ZSM-5, ZSM-8 and ZSM-11 possess a definite distinguishing crystalline structure whose x-ray diffraction pattern shows the following significant lines:

Table 1 ______________________________________ Interplanar Spacing d(A) Relative Intensity ______________________________________ 11.1 .+-. 0.3 S 10.0 .+-. 0.25 S 7.4 .+-. 0.2 W 7.1 .+-. 0.15 W 6.3 .+-. 0.1 W 6.04 .+-. 0.1 W 5.97 .+-. 0.1 W 5.56 .+-. 0.1 W 5.01 .+-. 0.1 W 4.60 .+-. 0.08 W 4.25 .+-. 0.08 W 3.85 .+-. 0.07 VS 3.71 .+-. 0.05 S 3.64 .+-. 0.05 M 3.04 .+-. 0.04 W 2.99 .+-. 0.03 W 2.94 .+-. 0.02 W ______________________________________

These values, as well as all other x-ray data were determined by standard techniques. The radiation was the K-alpha doublet of copper, and a scintillation counter spectrometer with a strip chart pen recorder was used. The peak heights, I, and the positions as a function of 2 times theta, where theta is the Bragg angle, were read from the spectrometer chart. From these the relative intensities, 100 I/I.sub.o, where I.sub.o is the intensity of the strongest line or peak, and d(obs.), the interplanar spacing in A, corresponding to the recorded lines, were calculated. In Table 1 the relative intensities are given in terms of the symbols S = strong, M = medium, MS = medium strong, MW = medium weak and VS = very strong. It should be understood that this x-ray diffraction pattern is characteristic of all the species of ZSM-5 compositions. Ion exchange of the sodium ion with cations reveals substantially the same pattern with some minor shifts in interplanar spacing and variation in relative intensity. Other minor variations can occur, depending on the silicon to aluminum ratio of the particular sample, as well as if it has been subjected to thermal treatment.

Zeolite ZSM-5 can be suitably prepared by preparing a solution containing water, tetrapropyl ammonium hydroxide and the elements of sodium oxide, an oxide of aluminum or gallium and an oxide of silica, and having a composition, in terms of mole ratios of oxides, falling within the following ranges:

TABLE 2 ______________________________________ Particularly Broad Preferred Preferred ______________________________________ OH.sup.-/SiO.sub.2 0.07-1.0 0.1-0.8 0.2-0.75 R.sub.4 N+/(R.sub.4 N.sup.++Na.sup.+) 0.2-0.95 0.3-0.9 0.4-0.9 H.sub.2 O/OH.sup.- 10-300 10-300 10-300 YO.sub.2 /w.sub.2 O.sub.3 5-100 10-60 10-40 ______________________________________

Wherein R is propyl, W is aluminum and Y IS silicon. This mixture is maintained at reaction conditions until the crystals of the zeolite are formed. Thereafter the crystals are separated from the liquid and recovered. Typical reaction conditions consist of a temperature of from about 75.degree.C to 175.degree.C for a period of about six hours to 60 days. A more preferred temperature range is from about 90.degree. to 150.degree.C, with the amount of time at a temperature in such range being from about 12 hours to 20 days.

The digestion of the gel particles is carried out until crystals form. The solid product is separated from the reaction medium, as by cooling the whole to room temperature, filtering and water washing.

ZSM-5 is preferably formed as an aluminosilicate. The composition can be prepared utilizing materials which supply the elements of the appropriate oxide. Such compositions include, for an aluminosilicate, sodium aluminate, alumina, sodium silicate, silica hydrosol, silica gel, silicic acid, sodium hydroxide and tetrapropylammonium hydroxide. It will be understood that each oxide component utilized in the reaction mixture for preparing a member of the ZSM-5 family can be supplied by one or more initial reactants and they can be mixed together in any order. For example, sodium oxide can be supplied by an aqueous solution of sodium hydroxide, or by an aqueous solution of sodium silicate; tetrapropylammonium cation can be supplied by the bromide salt. The reaction mixture can be prepared either batchwise or continuously. Crystal size and crystallization time of the ZSM-5 composition will vary with the nature of the reaction mixture employed.

ZSM-8 can also be identified, in terms of mole ratios of oxides, as follows:

0.9 .+-. 0.2 M.sub.2/n O : Al.sub.2 O.sub.3 : 5-300 SiO.sub.2 : z H.sub.2 O

wherein M is at least one cation, n is the valence thereof and z is from 0 to 40. In a preferred synthesized form, the zeolite has a formula, in terms of mole ratios of oxides, as follows:

0.9 .+-. 0.2 M.sub.2/n O : Al.sub.2 O.sub.3 : 15-60 SiO.sub.2 : z H.sub.2 O

and M is selected from the group consisting of a mixture of alkali metal cations, especially sodium, and tetraethylammonium cations.

Zeolite ZSM-8 can be suitably prepared by reacting a water solution containing either tetraethylammonium hydroxide or tetraethylammonium bromide together with the elements of sodium oxide, aluminum oxide, and an oxide of silica.

The operable relative proportions of the various ingredients have not been fully determined and it is to be immediately understood that not any and all proportions of reactants will operate to produce the desired zeolite. In fact, completely different zeolites can be prepared utilizing the same starting materials depending upon their relative concentration and reaction conditions as is set forth in U.S. Pat. No. 3,308,069. In general, however it has been found that when tetraethylammonium hydroxide is employed, ZSM-8 can be prepared from said hydroxide, sodium oxide, aluminum oxide, silica and water by reacting said materials in such proportions that the forming solution has a composition in terms of mole ratios of oxides falling within the following ranges:

SiO.sub.2 /Al.sub.2 O.sub.3 -- from about 10 to about 200

Na.sub.2 O/tetraethylammonium hydroxide -- from about 0.05 to .020

Tetraethylammonium hydroxide /SiO.sub.2 -- from about 0.08 to 1.0

H.sub.2 o/tetraethylammonium hydroxide -- from about 80 to about 200

Thereafter, the crystals are separated from the liquid and recovered. Typical reaction conditions consist of maintaining the foregoing reaction mixture at a temperature of from about 100.degree.C to 175.degree.C for a period of time of from about six hours to 60 days. A more preferred temperature range is from about 150.degree. to 175.degree.C with the amount of time at a temperature in such range being from about 12 hours to 8 days.

ZSM-11 can also be identified, in terms of mole ratios of oxides, as follows:

0.9 .+-. 0.3 M.sub.2/n O : Al.sub.2 O.sub.3 : 20-90 SiO.sub.2 : z H.sub.2 O

wherein M is at least one cation, n is the valence thereof and z is from 6 to 12. In a preferred synthesized form, the zeolite has a formula, in terms of mole ratios of oxides, as follows:

0.9 .+-. 0.3 M.sub.2/n O : Al.sub.2 O.sub.3 : 20-90 SiO.sub.2 : z H.sub.2 O

and M is selected from the group consisting of a mixture of alkali metal cations, especially sodium, and tetrabutylammonium cations.

ZSM-11 can be suitably prepared by preparing a solution containing (R.sub.4 X).sub.2 O, sodium oxide, an oxide of aluminum or gallium, an oxide of silicon or germanium and water and having a composition, in terms of mole ratios of oxides, falling within the following ranges:

Broad Preferred ______________________________________ YO.sub.2 /WO.sub.2 10-150 20-90 Na.sub.2 O/YO.sub.2 .05-0.7 0.05-0-40 (R.sub.4 X).sub.2 O/YO.sub.2 0.02- 0.20 0.02-0.15 H.sub.2 O/Na.sub.2 O 50-800 100-600 ______________________________________

wherein R.sub.4 X is a cation of a quaternary compound of an element of Group 5A of the Periodic Table, W is aluminum or gallium and Y is silicon or germanium maintaining the mixture until crystals of the zerolite are formed. Preferably, crystallization is performed under pressure in an autoclave or static bomb reactor. The temperature ranges from 100.degree.C-200.degree.C generally, but at lower temperatures, e.g. about 100.degree.C crystallization time is longer. Thereafter the crystals are separated from the liquid and recovered. The new zeolite is preferably formed in an aluminosilicate form.

An embodiment of this catalyst resides in the use of a porous matrix together with the ZSM-5 type family of zeolite previously described. The zeolite can be combined, dispersed, or otherwise intimately admixed with the porous matrix in such proportions that resulting products contain from 1 to 95 percent by weight and preferably from 10 to 70 percent by weight of the zeolite in the final composite.

The term "porous matrix" includes non-zeolite inorganic compositions with which the zeolites can be combined, dispersed or otherwise intimately admixed wherein the matrix may be catalytically active or inactive. It is to be understood that the porosity of the composition employed as a matrix can be either inherent in the particular material or it can be introduced by mechanical or chemical means. Representative of matrices which can be employed include metals and alloys thereof, sintered metals, and sintered glass, asbestos, silicon carbide, aggregates, pumice, firebrick, diatomaceous earths, alumina, and inorganic oxides. Inorganic compositions, especially those comprising alumina and those of a siliceous nature are preferred. Of these matrices inorganic oxides such as clay, chemically treated clays, silica, silica alumina, etc. as well as alumina, are particularly preferred because of their superior porosity, attrition resistance and stability.

Techniques for incorporating the ZSM-5 type family of zeolites into a matrix are conventional in the art and are set forth in U.S. No. 3,140,253.

It is to be noted that when a ZSM-5 type zeolite is used in combination with a porous matrix, space velocities which may be set forth as parameters for this process are based on the ZSM-5 type zeolite alone and the porous matrix is ignored. Thus, whether a ZSM-5 type zeolite is used alone or in a porous matrix, the space velocities in all cases refer to the ZSM-5 type component.

It is known that zeolites, particularly synthetic zeolites can have their composition modified by impregnating certain metals thereonto and/or thereinto. The composition can also be modified by exchanging various anions and/or cations into the crystal structure of the zeolite, replacing more or less of the ions originally present upon production of the zeolite.

The ZSM-5 type family of zeolites have been found to be especially active for aromatization if they have at least a portion of the original cations associated therewith replaced by any of a wide variety of other cations according to techniques well known in the art. Typical replacing cations would include hydrogen, ammonium, and metal cations, including mixtures of the same. Of the replacing cations, preference is given to cations of hydrogen, ammonium, rare earth, magnesium, zinc, calcium, nickel, copper and mixtures thereof. Particularly effective members of the ZSM-5 type family of zeolites are those which have been base exchanged with hydrogen ions, ammonium ions, zinc ions or mixtures thereof. Most especially zinc ZSM-5 is the best presently known catalyst for aromatizations as set forth.

Typical ion exchange techniques would be to contact a ZSM-5 type of zeolite with a salt of the desired replacing cation or cations. Although a wide variety of salts can be employed, particular preference is given to chlorides, nitrates and sulfates.

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.

It is also within the scope of the aromatization process to which this application is directed to incorporate a desired metallic component onto the ZSM-5 type family of zeolites by techniques other than ion exchange. Thus, for example, it is possible to impregnate a desired metallic component, such as zinc, platinum or palladium, thereinto by conventional impregnation techniques, as well as merely depositing the elemental metal onto the particular zeolite and in some cases, such as with zinc oxide, to incorporate the metal by physical admixture of the zeolite with an insoluble metal compound.

In any event, following contact with a salt solution of the desired replacing cation, the zeolites are preferably washed with water and dried at a temperature ranging from 150.degree. to about 600.degree.F and thereafter heated in air or inert gas at temperatures ranging from about 500.degree.F to 1500.degree.F for periods of time ranging from 1 to 48 hours or more. It is noted that this heat treatment can be carried out in situ, i.e. while the particular aromatization reaction is taking place, but it is preferred to carry it out as a separate step prior to carrying out the aromatization reaction.

One of the problems presently being coped with in carrying out the process described above stems from the fact that during the hydrocarbon conversion, the solid catalyst has a certain proportion of coke deposited thereon. As the proportion of deposited coke increases, the activity of the catalyst decreases until a point is reached at which it becomes expedient to regenerate the catalyst. This regeneration usually involves burning off the deposited coke with air. The steam generated in this oxidative regeneration step may irreversibly damage the catalyst activity and sometimes results in short ultimate catalyst life. It is obvious that the economics of such a cyclic process will be improved in direct proportion to the length of time the catalyst can be kept on stream between regenerations and ultimate life of the catalyst (i.e. number of regeneration cycles possible).

It is therefore an object of this invention to provide a novel aromatization process utilizing an improved catalyst composition.

Other and additional objects of this invention will become apparent from a consideration of this entire specification including the claims hereof.

In accord with and fulfilling these objects, one aspect of this invention resides in carrying out an aromatization conversion of aliphatic organic compounds in the effective presence of am improved catalyst comprising a matrix of a ZSM-5 type of synthetic aluminosilicate zeolite and a second inorganic material consisting essentially of at least about 80 percent silica.

It is known to provide ZSM-5 type of zeolites as a porous matrix by intimately mixing about 1 to 95 weight percent of the zeolite with an appropriate porous matrix material preferably about 10 to 70 percent ZSM-5. Application Ser. No. 253,942 now U.S. Pat. No. 3,756,942 set forth above reveals representative matrix materials as metals, alloys, sintered metals, sintered glass, asbestos, silicon carbide, aggregates, pumice, firebrick, diatomaceous earths, aluminasilica and/or other inorganic oxides. This prior application indicates that inorganic oxides., especially alumina, clay, silica-alumina and silica are considered to be preferred because of their superior porosity attrition resistance and stability. It was belived that the exact chemical nature of the matrix material was not particularly important but that the porosity was the controlling factor in evaluating suitable matrix materials for admixture with ZSM-5 zeolites. While it was originally thought that silica, alumina and silica-alumina or various proportions were of similar behavior in relation to ZSM-5 catalysts, it has now been discovered that this is not so and that there is indeed a dramatic difference in the coking tendency and steaming stability of these various matrix materials.

Therefore, it is one aspect of this invention to carry out an aromatization conversion utilizing a high silica bonded ZSM-5 type of zeolite catalyst. While silica, silica-alumina and alumina are all relatively inert materials with respect to the feed materials disclosed herein at ordinary temperatures, at aromatization temperatures, alumina and silica-alumina have been found to catalyze conversion of these feeds into coke in relatively large proportions. On the other hand, silica appears to continue to be substantially inert not only at ordinary temperatures but at aromatization temperatures as well.

According to this invention, an aromatizable feedstock having a boiling point at atmospheric pressure of up to about 400.degree.F, which may be hydrocarbon or one or more hetero atom containing organic compounds (e.g., an aliphatic organic compound having an oxygen, nitrogen, halogen or sulfur hetero atom or atoms constituent therein), and which may or may not be admixed with hydrogen or an oxygen containing gas, is converted to a highly aromatic product by effective contact thereof with a matrix cataylst comprising a ZSM-5 type of zeolite and a second, inorganic member consisting essentially of at least about 80 percent silica, and having a zeolite to second member ratio of about 0.01 to 20 to 1, at about 500.degree. to 1500.degree.F, depending upon feedstock composition and a space velocity of up to about 15 WHSV, under such combination of conditions as to convert at least about 80 percent of the feed material to a product which is at least about 35 weight percent liquid and the remainder gas, while the liquid portion of such product consists of at least about 80 weight percent of new aromatics. These new aromatics should represent a yield of at least about 20 grams per 100 grams of aromatizable feed.

A most important aspect of this invention is the fact that less coke is formed on the catalyst when operating the hereindefined process. Therefore a most important operating parameter and a critical measure of the value of this conversion is that the coke deposit on the catalyst be at a rate not to exceed 1 gram per 100 grams of carbon in the feed and 100 grams of coke per 100 grams of the catalyst. For purposes of this coke deposition parameter, feed should be considered only for its carbon content. A preferred deposition rate is less than about 0.6 grams per 100 grams of carbon feed and 60 grams of coke per 100 grams of the catalyst.

According to the process of this invention, the catalyst can be kept on stream for at least about 50 to 100 grams of feed (that is for the product of the feed rate and the total on stream time) per gram of catalyst. The conversion reaction may be carried out with an upflow or downflow reactor. The catalyst bed may be fixed, fluidized or moving as desired. The catalyst matrix particle size will be determined by the type of bed chosen. Generally, catalyst particle sizes will range from about 4 to about 400 mesh, preferably about 60 to 400 mesh for fluid bed operation, and about 4 to about 24 mesh for fixed bed operation.

The high silica matrix catalyst of this invention is made by conventional zeolite catalyst matrix production techniques. In this regard reference is made to the general discussion on ZSM-5 type of catalyst, infra. The catalyst is regenerated by contact with an oxidizing atmosphere at elevated temperatures. Conventional regeneration with steam and/or air is considered to be acceptable.

Since this particular hydrocarbon conversion reaction may be endothermic, exothermic or heat balanced depending upon feed composition, provision should be made for heat transfer within the system. This can be accomplished by indirect heat exchange with a suitable fluid. Heating, if needed, can be accomplished by direct firing as in a furnace. It can also be accomplished by direct heat exchange by means of the heated, regenerated catalyst and/or a preheating of the feed, and/or heating or cooling a recycle stream.

This invention will be illustrated by the following Examples which are in no way to be considered to be limiting on the scope thereof. Parts and percentages are by weight unless expressly stated to be on some other basis.

EXAMPLES 1 - 4

In each of these Examples ZSM-5 catalyst was prepared and matrixed with a particular proportion of a specific binder as set forth in the following Table. Each catalyst matrix was loaded into a fixed bed reactor through which pyrolysis gasoline was passed at atmospheric pressure and 1000.degree.F for 2 hours.

TABLE 1 __________________________________________________________________________ Example No. 1 2 3 4 Wt. % H ZSM-5 65 10 65 10 Binder Alumina Silica/Alumina Silica Silica WHSV 12 10.6 8.3 8.3 Coke Wt. % of 16.9 13.7 6.75 2.0 Catalyst __________________________________________________________________________

These Examples illustrate the fact that even under increasingly severe conditions, the replacement of alumina with silica in the catalyst matrix of this invention reduces the coke make on the catalyst hereof.

EXAMPLES 5 and 6

Zinc and copper promoted ZSM-5 matrixes with silica and alumina binders respectively were used to convert C.sub.6 to 200.degree.F. light virgin naphtha to aromatics. The above defined mid-continent light naphtha was passed through the catalyst bed at 1000.degree.F, 1 LHSV, atmospheric pressure and no added hydrogen for 18 hours after which the naphtha feed was stopped and the catalyst was regenerated with air and then steam at 1000.degree.F and 35 mm Hg water pressure. After each various length of steaming time, the catalyst activity was tested. The ability of the catalyst to convert naphtha to aromatics, reported as aromatics yield, is a measure of the quality of the catalysts. The curves of FIG. 1 show the improvement obtained by using a silica binder as opposed to an alumina binder.

These same tests have been analyzed and the data reported below in Table 2 on the basis of loss of aromatic yield as a function of steaming time:

Table 2 ______________________________________ Aromatics Yield Loss (% of Fresh Activity vs. Steaming ______________________________________ Time) Steaming Time, Hrs. 100 200 300 400 Silica Binder 3.4 6.7 8 8.9 Alumina Binder 7.1 13.6 16.8 19.5 ______________________________________

Claims

What is claimed is:

1. In the process of converting an aliphatic feedstock having an atmospheric boiling point of up to about 400.degree.F to aromatic hydrocarbons by contacting such feedstock with a ZSM-5 type of zeolite at about 500.degree. to 1500.degree.F and a space velocity of up to about 15 WHSV; the improvement, whereby inhibiting the formation of coke on said catalyst and improving the steam stability of the said catalyst, comprising using as said catalyst a matrix of ZSM-5 type of crystalline zeolite and a second, inorganic component consisting essentially of at least about 80 weight percent silica.

2. The improved process claimed in claim 1 wherein said zeolite is ZSM-5.

3. The improved process claimed in claim 1 wherein said second component is silica.

4. The improved process claimed in claim 1 wherein said feedstock comprises olefinic hydrocarbons and said aromatization temperature is at least about 650.degree.F.

5. The improved process claimed in claim 1 wherein said feedstock comprises paraffinic hydrocarbons and said aromatization temperature is at least about 850.degree.F.

6. The improved process claimed in claim 1 wherein said matrix comprises about 1 to 95 weight percent zeolite.

7. The improved process claimed in claim 1 wherein said zeolite is Zn ZSM-5.

8. The improved process claimed in claim 1 wherein said zeolite is Zn Cu ZSM-5.

9. The improved process claimed in claim 1 including depositing coke on said catalyst at a rate of up to about 1 gram per 100 grams of carbon in the feed.

10. The improved process claimed in claim 1 wherein said catalyst has a zeolite to matrix material ratio of about 0.01 to 20 to 1.