Titanium-aluminum-silicon-oxide molecular sieve compositions

Molecular sieve abstract

Titanium-aluminum-silicon-oxide molecular sieves are disclosed having use as molecular sieves and as catalyst compositions in hydrocarbon conversion and other processes. The molecular sieves have a unit empirical formula on an anhydrous basis of wherein "R" represents at least one organic+ emplating agent; "m" represents the moles of "R" present per mole of (Ti.sub.x Al.sub.y Si.sub.z)O.sub.2 ; and "x", "y" and "z" represent the mole fractions of titanium, aluminum and silicon, respectively, present as tetrahedral oxides. The molecular sieve is useful in separating molecular species having different degrees of polarity and/or kinetic diameters.

Molecular sieve claims

What is claimed is:

1. Process for separating mixtures of molecular species wherein such mixtures contain molecular species having different degrees of polarity and/or kinetic diameters, comprising contacting said mixture with at least one composition selected from crystalline molecular sieves having chemical compositions in the anhydrous form represented by the unit empirical formula:

wherein "R" represents at least one organic templating agent present in the intracrystalline pore system; "m" represents the moles of "R" present per mole of (Ti.sub.x Al.sub.y Si.sub.z)O.sub.2 and has a value of from zero to about 0.3; and "x", "y" and "z" represent the mole fractions of titanium, aluminum and silicon, respectively, present as tetrahedral oxides, said mole fractions being such that they are within the compositional area defined by points A, B, C and D of FIG. 1 and having a characteristic x-ray powder diffraction pattern selected from the group consisting of Tables V, VII, XI and XVIII.

2. Process according to claim 1 wherein said compositional area is defined by points A, B, D, E and F of FIG. 1.

3. Process according to claim 1 wherein said compositional area is defined by points a, b and c of FIG. 2.

4. Process according to claim 1 wherein said composition has pore diameters large enough to adsorb at least one but not all molecular species of said mixture, based on the polarity and/or kinetic diameter of the adsorbed molecular species.

5. Process according to claim 1 wherein substantially all molecular species of said mixture can communicate with the pore system of said composition.

6. Process according to claim 1 wherein said composition is at least partially activated prior to use to remove any molecular species present in the intracrystalline pore system.

7. Process according to claim 1 wherein said at least one composition comprises at least one molecular sieve selected from the group consisting of TASO-20 TASO-38 TASO-48 and TASO-49.

8. Process according to claim 1 wherein said at least one composition comprises TASO-48.

9. Process according to claim 1 wherein said composition is sufficiently hydrophilic to preferentially adsorb the more polar molecular species of a mixture having different degrees of polarity.

10. Process according to claim 1 wherein said composition is sufficiently hydrophobic to preferentially adsorb the less polar molecular species of a mixture having different degrees of polarity.

11. Process according to claim 1 for separating molecular species from admixture with molecular species having a lesser degree of polarity wherein said composition has pore diameters large enough to adsorb at least one of the more polar molecular species, and said molecular sieve is at least partially activated whereby molecules of the more polar molecular species are selectively adsorbed into the intracrystalline pore system thereof.

12. Process according to claim 1 for separating a mixture of molecular species having different kinetic diameters wherein said composition has pore diameters large enough to adsorb at least one but not all molecular species of said mixture, said molecular sieve being at least partially activated whereby at least some molecules whose kinetic diameters are sufficiently small can enter the intracrystalline pore system thereof.

Molecular sieve description

FIELD OF THE INVENTION

The present invention relates to new molecular sieve compositions containing titanium, aluminum and silicon in the form of framework tetrahedral oxide units. These compositions are prepared hydrothermally from reaction mixtures containing reactive sources of titanium, aluminum and silicon and preferably at least one organic templating agent.

DISCUSSION OF MOLECULAR SIEVES

Molecular sieves having crystalline structures and of the aluminosilicate type are well known to those familiar with molecular sieve technology. Both naturally occurring and synthetic aluminosilicates are known to exist and literally hundreds of such have been reported in the literature.

Although hundreds of aluminosilicates (binary molecular sieves) are known, the reports relating to ternary molecular sieves have been relatively few. Further, the reported ternary molecular sieves having titanium as a component have been even fewer and in those instances where titanium has been reported the amount contained in the molecular sieve has been relatively small or present as a deposition or surface modifying agent.

One early report of crystalline titano-silicate zeolites (Of course, these compositions are not zeolites as the term "zeolite" is commonly employed today.) is found in U.S. Pat. No. 3329481. The crystalline titano-silicates are described in U.S. Pat. No. 3329481 by the formula:

The difficulty in obtaining compositions containing titanium is evidenced by the disclosure of U.S. Pat. No. 4358397 which discloses modified aluminosilicates. The aluminosilicates are modified by treating an aluminosilicate with a compound derived from one or more elements of titanium, zirconium or hafnium. The resulting compositions are said to contain a minor proportion of an oxide of such elements. It is clear that in the disclosed compositions the oxides of titanuim, zirconium and hafnium were present as deposited oxides and were present in a minor proportion.

As above mentioned, although there has been an extensive treatment in the patent art and in the published literature of aluminosilicates and recently, aluminophosphates, there has been little information available on the presence of other than such materials. This is particularly true in the area of titanium containing compositions. Molecular sieve compositions wherein titanium is present in the framework of the molecular sieve or is so intimately related as to change the physical and/or chemical characteristics of the molecular sieve have not been extensively reported. This is understandable in the question of aluminosilicates, as indicated by the article, "Can Ti.sup.4+ replace Si.sup.4+ in silicates?", Mineralogical Magazine, September vol 37 No. 287 pages 366-369 (1969). In this article it is concluded that substitution of framework silicon by titanium does not usually occur in aluminosilicates owing to the preference of titanium to be octahedrally bound rather than tetrahedrally bound. Even for the formation of crystalline "titanosilicate zeolites*", as disclosed in U.S. Pat. No. 3329481 and discussed above, wherein a metallo-silicate complex is formed and treated to give the titanosilicate product, the evidence for the claimed titanosilicate is based on the X-ray powder diffraction pattern data which are somewhat suspect as to whether such show substitution of titanium into the silicate framework inasmuch as the same claimed X-ray patterns are also observed for the zirconium silicates. Further, similar X-ray patterns showing similar interplanar distances for the two values in pattern B have been reported for silicalite. (see GB No. 2071071 A).

The incorporation of titanium in a silicalite-type structure is disclosed in GB No. 2071071 A, published Dec. 21 1979. The amount of titanium claimed to be substituted into the silicalite-type structure is very small, being no more than 0.04 mole percent, based on the number of moles of silica, and may be as low as 0.0005. The titanium content was determined by chemical analysis and was not determined to be greater than 0.023 in any of the reported examples. As indicated by a comparison of FIG. 1a and FIG. 1b of GB No. 2071071 A, the amount of titanium present is so small and no significant change in the X-ray diffraction pattern of silicalite was observed and the minor changes observed may simply be due to occluded titanium dioxide. Thus, in the absence of other analytical data the results are not well defined. No comparison data for titanium dioxide are disclosed.

In view of the above, it is clear that the substitution of titanium into a zeolitic-type framework although conceived to be possible wherein titanium substitutes for silicon, has been deemed most difficult to achieve.

The difficulty which is met in preparing titanium-containing molecular sieve compositions is further demonstrated by the failure of European Patent Application No. 82109451.3 (Publication No. 77522 published Apr. 27 1983) entitled "Titanium-containing zeolites and method for their production as well as use of said zeolites.", to actually prepare titanium-containing molecular sieve compositions. Although the applicants claim the preparation of titano-aluminosilicates having the pentasil structure, it is evident from an analysis of the products of the examples that titanium was not present in the form of a framework tetrahedral oxide in a molecular sieve having the pentasil structure. The products of the examples of European patent Application No. 82109451.3 will be discussed in detail in a comparative example hereinafter.

Representative templates which may be employed herein include: tetramethylammonium; tetraethylammonium; tetrapropylammonium; tetrabutylammonium ions; di-n-propylamine; tripropylamine; triethylamine; triethanolamine; piperidine; cyclohexylamine; 2-methylpyridine; N,N-dimethylbenzylamine; N,N-diethylethanolamine; dicyclohexylamine; N,N-dimethylethanolamine; 14-diabicyclo(222)octane; N-methyldiethanolamine, N-methyl-ethanolamine; N-methylcyclohexylamine; 3-methyl-pyridine; 4-methylpyridine; quinuclidine; N,N'-dimethyl-14-diazabicyclo(222)octane ion; di-n-butylamine, neopentylamine; di-n-pentylamine; isopropylamine; t-butylamine; ethylenediamine; pyrrolidine; and 2-imidazolidone. As will be readily apparent from the illustrative examples set forth hereinafter, not every template will produce every TASO composition although a single template can, with proper selection of the reaction conditions, cause the formation of different TASO compositions, and a given TASO composition can be produced using different templates.

In those instances where an alkoxide is the reactive titanium, aluminum and/or silicon source, the corresponding alcohol is necessarily present in the reaction mixture since it is a hydrolysis product of the alkoxide. It has not as yet been determined whether this alcohol participates in the synthesis process as a templating agent, or in some other function and, accordingly, is not reported as a template in the unit formula of the TASOs, although such may be acting as templates.

Alkali metal cations when present in the reaction mixture may facilitate the crystallization of certain TASO phases, although the exact function of such cations in crystallization, if any, is not presently known. Alkali cations present in the reaction mixture generally appear in the formed TASO composition, either as occluded (extraneous) cations and/or as structural cations balancing net negative charges at various sites in the crystal lattice. It should be understood that although the unit formula for the TASOs does not specifically recite the presence of alkali cations they are not excluded in the same sense that hydrogen cations and/or hydroxyl groups are not specifically provided for in the traditional formulae for zeolitic aluminosilicates.

Most any reactive titanium source may be employed herein. The preferred reactive titanium sources include titanium alkoxides, water-soluble titanates, titanate esters and titanium chelates.

Most any reactive source of silicon can be employed herein. The preferred reactive sources of silicon are silica, either as a silica sol or as fumed silica, a reactive solid amorphous precipitated silica, silica gel, alkoxides of silicon, silicic acid or alkali metal silicate and mixtures thereof.

Most any reactive aluminum source may be employed herein. The preferred reactive aluminum sources include sodium aluminate, aluminum alkoxides, such as aluminum isopropoxide, and pseudoboehmite. Crystalline or amorphous aluminosilicates which are a suitable source of silicon are, of course, also suitable sources of aluminum. Other sources of aluminum used in zeolite synthesis, such as gibbsite and aluminum trichloride may be employed but are generally not deemed preferred.

The X-ray patterns carried out herein and all other X-ray patterns appearing herein were obtained using either: (1) standard x-ray powder diffraction techniques; or (2) by use of using copper K-alpha radiation with computer based techniques using Siemens D-500 X-ray powder diffractometers, Siemens Type K-805 X-ray sources, available from Siemens Corporation, Cherry Hill, N.J., with appropriate computer interface. When employing the standard X-ray technique the radiation source is a high-intensity, copper target, X-ray tube operated at 50 Kv and 40 ma. The diffraction pattern from the copper K-alpha radiation and graphite monochromator is suitably recorded by an X-ray spectrometer scintillation counter, pulse height analyzer and strip chart recorder. Flat compressed powder samples are scanned at 2.degree. (2 theta) per minute, using a two second time constant. Interplanar spacings (d) in Angstrom units are obtained from the position of the diffraction peaks expressed as 2.theta. (theta) where theta is the Bragg angle as observed on the strip chart. Intensities are determined from the heights of diffraction peaks after subtracting background, "I.sub.o " being the intensity of the strongest line or peak, and "I" being the intensity of each of the other peaks. When Relative Intensities are reported herein the following abbreviations mean: vs=very strong; s=strong; m=medium, w=weak; and vw=very weak. Other abbreviations include: sh=shoulder and br=broad.

As will be understood by those skilled in the art the determination of the parameter 2 theta is subject to both human and mechanical error, which in combination, can impose an uncertainty of about +0.4.degree. on each reported value of 2 theta. This uncertainty is, of course, also manifested in the reported values of the d-spacings, which are calculated from the 2 theta values. This imprecision is general throughout the art and is not sufficient to preclude the differentiation of the present crystalline materials from each other and from the compositions of the prior art.

PROCESS APPLICATIONS

The TASO compositions of this invention have unique surface characteristics making them useful as molecular sieves and as catalyst or as bases for catalysts in a variety of separation, hydrocarbon conversion and oxidative combustion processes. The TASO composition can be impregnated or otherwise associated with catalytically active metals by the numerous methods known in the art and used, for example, in fabricating catalysts compositions containing alumina or aluminosilicate materials.

TASO's may be employed for separating molecular species in admixture with molecular species of a different degree of polarity or having different kinetic diameters by contacting such mixtures with a TASO(s) having pore diameters large enough to adsorb at least one but not all molecular species of the mixture based on the polarity of the adsorbed molecular species and/or its kinetic diameter. When TASOs are employed for such separation processes the TASOs are at least partially activated whereby some molecular species selectively enter the intracrystalline pore system thereof.

The hydrocarbon conversion reactions catalyzed by TASO compositions include; cracking, hydrocracking; alkylation of both the aromatic and isoparaffin types; isomerization (including xylene isomerization); polymerization; reforming; hydrogenation; dehydrogenation; transalkylation; dealkylation; and hydration.

When a TASO containing catalyst compositions contains a hydrogenation promoter, such promoter may be platinum, palladium, tungsten, nickel or molybdenum and may be used to treat various petroleum stocks including heavy petroleum residual stocks, cyclic stocks and other hydrocrackable charge stocks. These stocks can be hydrocracked at temperatures in the range of between about 400.degree. F. and about 825.degree. F. using molar ratios of hydrogen to hydrocarbon in the range of between about 2 and about 80 pressures between about 10 and about 3500 p.s.i.g., and a liquid hourly space velocity (LHSV) of between about 0.1 and about 20 preferably between about 1.0 and about 10.

TASO containing catalyst compositions may also be employed in reforming processes in which the hydrocarbon feedstocks contact the catalyst at temperatures between about 700.degree. F. and about 1000.degree. F., hydrogen pressures of between about 100 and about 500 p.s.i.g., LHSV values in the range between about 0.1 and about 10 and hydrogen to hydrocarbon molar ratios in the range between about 1 and about 20 preferably between about 4 and about 12.

Further, TASO containing catalysts which contain hydrogenation promoters, are also useful in hydroisomerization processes wherein the feedstock(s), such as normal paraffins, is converted to saturated branched-chain isomers. Hydroisomerization processes are typically carried out at a temperature between about 200.degree. F. and about 600.degree. F., preferably between about 300.degree. F. and about 550.degree. F. with an LHSV value between about 0.2 and about 1.0. Hydrogen is typically supplied to the reactor in admixture with the hydrocarbon feedstock in molar proportions of hydrogen to the feedstock of between about 1 and about 5.

TASO-containing compositions similar to those employed for hydrocracking and hydroisomerization may also be employed at between about 650.degree. F. and about 1000.degree. F., preferably between about 850.degree. F. and about 950.degree. F. and usually at somewhat lower pressures within the range between about 15 and about 50 p.s.i.g. for the hydroisomerization of normal paraffins. Preferably the paraffin feedstock comprises normal paraffins having a carbon number range of C.sub.7 -C.sub.20. The contact time between the feedstock and the TASO containing catalyst is generally relatively short to avoid undersirable side reactions such as olefin polymerization and paraffin cracking. LHSV values in the range between about 0.1 and about 10 preferably between about 1.0 and about 6.0 are suitable.

The low alkali metal content (often not measurable by current analytical techniques) of the instant TASO compositions make them particularly well suited for use in the conversion of alkylaromatic compounds, particularly for use in the catalytic disproportionation of toluene, xylene, trimethylbenzenes, tetramethylbenzenes and the like. In such disproportionation processes it has been observed that isomerization and transalkylation can also occur. The TASO-containing catalysts for such processes will typically include Group VIII noble metal adjuvants alone or in conjunction with Group VI-B metals such as tungsten, molybdenum and chromium which are preferably included in such catalyst compositions in amounts between about 3 and about 15 weight-% of the overall catalyst composition. Extraneous hydrogen can, but need not be present in the reaction zone which is maintained at a temperature between about 400.degree. and about 750.degree. F., pressures in the range between about 100 and about 2000 p.s.i.g. and LHSV values in the range between about 0.1 and about 15.

TASO containing catalysts may be employed in catalytic cracking processes wherein such are preferably employed with feedstocks such as gas oils, heavy naphthas, deasphalted crude oil residues etc. with gasoline being the principal desired product. Temperature conditions are typically between about 850 and about 1100.degree. F., LHSV values between about 0.5 and about 10 pressure conditions are between about 0 p.s.i.g. and about 50 p.s.i.g.

TASO containing catalysts may be employed for dehydrocyclization reactions which employ paraffinic hydrocarbon feedstocks, preferably normal paraffins having more than 6 carbon atoms, to form benzene, xylenes, toluene and the like. Dehydrocyclization processes are typically carried out using reaction conditions similar to those employed for catalytic cracking. For such processes it is preferred to use a Group VIII non-noble metal cation such as cobalt and nickel in conjunction with the TASO composition.

TASO containing catalysts may be employed is catalytic dealkylation where paraffinic side chains are cleaved from aromatic nuclei without substantially hydrogenating the ring structure at relatively high temperatures in the range between about 800.degree. F. and about 1000.degree. F. at moderate hydrogen pressures between about 300 and about 1000 p.s.i.g. with other conditions being similar to those described above for catalytic hydrocracking. TASO containing catalysts for catalytic dealkylation are of the same type described above in connection with catalytic dehydrocyclization. Particularly desirable dealkylation reactions contemplated herein include the conversion of methylnaphthalene to naphthalene and toluene and/or xylenes to benzene.

TASO containing catalysts may be used in catalytic hydrofining wherein the primary objective is to provide for the selective hydrodecomposition of organic sulfur and/or nitrogen compounds without substantially affecting hydrocarbon molecules present therewith. For this purpose it is preferred to employ the same general conditions described above for catalytic hydrocracking. The catalysts are the some typically of the same general nature as described in connection with dehydrocyclization operations. Feedstocks commonly employed for catalytic hydroforming include: gasoline fractions; kerosenes; jet fuel fractions; diesel fractions; light and heavy gas oils; deasphalted crude oil residua; and the like. The feedstock may contain up to about 5 weight-percent of sulfur and up to about 3 weight-percent of nitrogen.

TASO containing catalysts may be employed for isomerization processes under conditions similar to those described above for reforming although isomerization processes tend to require somewhat more acidic catalysts than those employed in reforming processes. Olefins are preferably isomerized at temperatures between about 500.degree. F. and about 900.degree. F., while paraffins, naphthenes and alkyl aromatics are isomerized at temperatures between about 700.degree. F. and about 1000.degree. F. Particularly desirable isomerization reactions contemplated herein include the conversion of n-heptane and/or n-octane to isoheptanes, iso-octanes, butane to iso-butane, methylcyclopentane to cylcohexane, meta-xylene and/or ortho-xylene to para-xylene, 1-butene to 2-butene and/or isobutene, n-hexene to isohexane, cyclohexane to methylcyclopentene etc. The preferred cation form is a combination of a TASO with polyvalent metal compounds (such as sulfides) of metals of Group II-A, Group II-B and rare earth metals. For alkylation and dealkylation processes TASO compositions having pores of at least 5A are preferred. When employed for dealkylation of alkyl aromatics, the temperature is usually at least 350.degree. F. and ranges up to a temperature at which substantial cracking of the feedstock or conversion products occurs, generally up to about 700.degree. F. The temperature is preferably at least 450.degree. F. and not greater than the critical temperature of the compound undergoing dealkylation. Pressure conditions are applied to retain at least the aromatic feed in the liquid state. For alkylation the temperature can be as low as 250.degree. F. but is preferably at least 350.degree. F. In alkylation of benzene, toluene and xylene, the preferred alkylation agents are olefins such as ethylene and propylene.

The TASO compositions of this invention may be employed in conventional molecular sieving processes as heretofore have been carried out using aluminosilicate, aluminophosphate or other commonly employed molecular sieves. TASO compositions are preferably activated prior to their use in a molecular sieve process to remove any molecular species which may be present in the intracrystalline pore system as a result of synthesis or otherwise. For the TASO compositions this is sometimes accomplished by thermally destroying the organic species present in an as-synthesized TASO since such organic species may be too large to be desorbed by conventional means.

The TASO compositions of this invention are also useful as adsorbents and are capable of separating mixtures of molecular species both on the basis of molecular size (kinetic diameters) and based on the degree of polarity of the molecular species. When the separation of molecular species is based upon the selective adsorption based on molecular size, the TASO is chosen in view of the dimensions of its pores such that at least the smallest molecular specie of the mixture can enter the intracrystalline void space while at least the largest specie is excluded. When the separation is based on degree of polarity it is generally the case that the more hydrophilic TASO will preferentially adsorb the more polar molecular species of a mixture having different degrees of polarity even though both molecular species can communicate with the pore system of the TASO.

The instant TASO compositions may be further characterized and distinguished from aluminophosphates by reference to the catalytic properties exhibited by the TASO compositions. When the TASO compositions are tested for n-butane cracking and compared with aluminophosphate compositions having a similar topology it has been observed that the TASO compositions are more active catalysts as indicated by a higher numerical value for n-butane cracking.