SYNTHESIS OF CRYSTALLINE SILICATE ZSM-11

Abstract: A crystalline silicate having the structure of ZSM-11 is synthesized from a reaction mixture comprising a source of alkali metal cations (M), a source of silicon, octylamine (R), seed crystals of ZSM-11 and water, and having a composition, in terms of mole ratios, within the following ranges: SiO₂/Al₂O₃ = 40- > 2000 H₂O/SiO₂ = 5-50 OH⁻/SiO₂ = 0.05-0.5 M/SiO₂ = 0.05-0.5 R/SiO₂ = 0.03-1.0

Description: This invention relates to a method for synthesizing a crystalline silicate having the structure of ZSM-11. Crystalline silicate ZSM-11 and its conventional preparation, e.g. from a reaction mixture containing a quaternary compound directing agent, such as a tetrabutylammonium compound, are taught by U.S. Patent 3,709,979. The present invention seeks to provide an improved, economical and reproducible method for preparing zeolite ZSM-11 exhibiting high purity, catalytic activity and other valuable properties. Accordingly, the invention resides in a method for synthesizing a crystalline silicate having the structure of ZSM-11 exhibiting a characteristic X-ray diffraction pattern as shown in Table 1 of the specification, which comprises (i) preparing a mixture capable of forming said silicate, said mixture comprising a source of alkali metal cations (M), a source of silicon, octylamine (R), seed crystals of ZSM-11 structure and water, and having a composition, in terms of mole ratios, within the following ranges: SiO₂/Al₂O₃ = 40- >2000 H₂O/SiO₂ = 5-50 OH⁻/SiO₂ = 0.05-0.5 M/SiO₂ = 0.05-0.5 R/SiO₂ = 0.03-1.0 (ii) maintaining said mixture under sufficient conditions until crystals of said silicate are formed and (iii) recovering said crystalline silicate having the structure of ZSM-11 from step (ii), said recovered crystalline silicate containing said R and M. Zeolite ZSM-11 synthesized by conventional procedure is proven to have catalytic application. When the crystalline silicate is synthesized in accordance with the present method, it exhibits excellent catalytic activity for certain conversions of interest, including hydrocracking and dewaxing. Further, the directing agent for the present method is readily commercially available and of significantly lower cost than quarternary ammonium or phosphonium and diamine compounds used for that purpose in the prior art. The use of cost effective silica precursor, hereinafter more particularly defined, allows for high solids loading which produces even higher product yield. The present method provides low cost, pure ZSM-11 crystals with low alkali metal levels. The product can be ion exchanged without a pre-calcination step, giving further economic advantage over prior art ZSM-11 synthesis methods. The ZSM-11 as prepared by the present method has a characteristic X-ray diffraction pattern, the values of which are set forth in Table 1, hereinafter. The parenthesis around lines 3.07 and 3.00 indicate that they are separate and distinct lines, but are often superimposed. These X-ray diffraction data were collected with a Philips diffraction system, equipped with a graphite diffracted beam monochromator and scintillation counter, using copper K-alpha radiation. The diffraction data were recorded by step-scanning at 0.04 degrees of two-theta, where theta is the Bragg angle, and a counting time of 4 seconds for each step. The interplanar spacings, d's, were calculated in Angstrom units (A), and the relative intensities of the lines, I/I o , where I o is one-hundredth of the intensity of the strongest line, above background, were derived with the use of a profile fitting routine (or second derivative algorithm). The intensities are uncorrected for Lorentz and polarization effects. The relative intensities are given in terms of the symbols vs = very strong (60-100), s = strong (40-60), m = medium (20-40) and w = weak (0-20). It should be understood that diffraction data listed for this sample as single lines may consist of multiple overlapping lines which under certain conditions, such as differences in crystallite sizes or very high experimental resolution or crystallographic changes, may appear as resolved or partially resolved lines. Typically, crystallographic changes can include minor changes in unit cell parameters and/or a change in crystal symmetry, without a change in topology of the structure. These minor effects, including changes in relative intensities, can also occur as a result of differences in cation content, framework composition, nature and degree of pore filling, and thermal and/or hydrothermal history. The ZSM-11 composition as prepared hereby can also be identified, in terms of mole ratios of oxides and in the anhydrous state, as-synthesized, as follows: (0.5 - 10.0)R:(0 - 0.5)M₂O:Al₂O₃: x SiO₂ wherein M is an alkali metal, R is octylamine and x is from 40 to greater than 2000. The original "M" cations, e.g. alkali metal, can be replaced, at least in part, by ion exchange with other cations. Thus, the original cations are exchanged into a hydrogen or hydrogen ion precursor form or a form in which the original cation has been replaced by a metal of Groups IIA, IIIA, IVA, IB, IIB, IIIB, IVB, VIB or VIII of the Periodic Table. Thus, for example, it is contemplated to exchange the original cations with ammonium ions or with hydronium ions. Catalytically active forms of these would include, in particular, hydrogen, rare earth metals, aluminum, metals of Groups II and VIII of the Periodic Table and manganese. The synthesis method of the present invention comprises forming a reaction mixture containing a source of alkali metal cations, octylamine directing agent, a source of silicon, seeds of ZSM-11 structure and water and having a composition, in terms of mole ratios, within the following ranges: wherein R is octylamine and M is an alkali metal ion, and maintaining the mixture until crystals of the desired crystalline silicate ZSM-11 are formed. The quantity of OH⁻ is calculated only from the inorganic sources of alkali without any organic base contribution. Reaction conditions for crystallizing ZSM-11 from the above mixture include heating the reaction mixture to a temperature of from 100°C to 350°C for a period of time of from 12 hours to 10 days. A more preferred temperature range is from 120°C to 150°C with the amount of time at a temperature in such range being from 16 hours to 5 days. The reaction is carried out until a fully crystalline product is formed. The solid product comprising ZSM-11 is recovered from the reaction medium, as by cooling the whole to room temperature, filtering and water washing. The synthesis of the present invention is facilitated when the reaction mixture comprises seed crystals having the structure of ZSM-11. The use of at least 0.01%, preferably at least about 0.10%, and even more preferably from 1% to 5% seed crystals (based on total weight of silica in the reaction mixture) will facilitate crystallization in the present method. The reaction mixture composition for the present process can be prepared utilizing any materials which can supply the appropriate oxides. Such materials include, when a separate source of aluminum is desired, aluminates or alumina; and silicates, silica hydrosol, silica gel, silicic acid and hydroxides. It will be understood that each oxide component utilized in the reaction mixture for preparing zeolite ZSM-11 can be supplied by one or more essential reactants and they can be mixed together in any order. For example, any oxide can be supplied by an aqueous solution, sodium hydroxide or by an aqueous solution of a suitable silicate. The reaction mixture can be prepared either batchwise or continuously. Crystal size and crystallization time for the product composition comprising ZSM-11 will vary with the exact nature of the reaction mixture employed. Preferred sources of silicon for the reaction mixture of the present invention are solid silicas or silica precursors. Such sources are cost effective and allow high solids loading of the reaction mixture. The use of a solid silica, e.g. Ultrasil (a precipitated, spray dried silica) or HiSil (a precipitated hydrated SiO₂ containing about 6 weight percent free H₂O and about 4.5 weight percent bound H₂O of hydration and having a particle size of about 0.02 micron) as the oxide of silicon source favors economic synthesis of ZSM-11 from the above reaction mixture. Such solid silica sources are commercially available. The silica precursor source of silicon for the present reaction mixture is an amorphous silica precipitate made from a solution of a soluble silica source, preferably sodium silicate. Conveniently, the solution is an aqueous solution of a pH ranging from 9 to 12. The silica precursor is formed by its continuous precipitation from the solution phase so that precipitation comprises initiating precipitation and maintaining said precipitation. Alteration of the composition of the solution of soluble silica source is undertaken by introducing a precipitating reagent. In one embodiment, the precipitating reagent is a source of said, conveniently a solution of a mineral acid, such as H₂SO₄, HCl and HNO₃ with a pH ranging from essentially 0 to 6. Thus, precipitation of the silica precursor can be effected by acid neutralization of a basic solution of a silicate. The silica can be precipitated alone in the absence of sources of other zeolitic framework elements, e.g. aluminum. In this fashion, both the precipitating reagent and the solution of silica source can be free of intentionally added alumina or alumina source. However, aluminum is ubiquitous and may be present in minor amounts due to impurities in the precursors of the reactants or impurities extracted from the reaction vessel. When no source of alumina is added, the amount of alumina in the silica precursor precipitate will be less than about 0.5 weight percent, and generally less than 0.2 weight percent. When a source of alumina is added, the amount of alumina in the silica precursor precipitate will be up to about 5 weight percent. Silicate precipitation can be coprecipitation in the presence of soluble sources of other zeolite framework elements including gallium, indium, boron, iron and chromium. The soluble source of these other zeolitic framework components can be, for example, nitrates. The coprecipitation product would be amorphous, for example an amorphous gallosilicate, borosilicate or ferrosilicate. Continuous precipitation of the amorphous silica precursor may comprise introducing the solution of silica source and the precipitating reagent to a reaction zone while maintaining the molar ratio of silica source to precipitating reagent substantially constant. The continuous precipitation of silica precursor has two results. Firstly, silica gel formation is substantially eliminated and secondly, precipitated silica precursor particle size exceeds the size at which silica gel formation is possible. The precipitated silica precursor comprises agglomerated solids in the shape of microspheres. Suspensions of these particles exhibit low viscosities at high solids loading in the subsequent zeolite synthesis reaction mixture of the present invention, even at solids loading equal to or greater than 20-40%. The particle size of the precipitated silica precursor ranges between 1-500 microns, but the average size is 50-100 microns. Other conditions affecting precipitation of silica precursor include time, pH and temperature. The temperature of the precipitation mixture can range from 80°F to 300°F (27°C to 150°C). The time of contact of the solution of silica source and the precipitating reagent can range from about 10 minutes to several hours at pH maintained from 6 to 11. Generally, the silica precursor is processed by isolating it, for example by filtration, and removing soluble contaminants therefrom by washing and/or ion exchange. This stage can be considered a solids consolidation step. Synthetic ZSM-11 prepared in accordance herewith can be used either in the as-synthesized form, the alkali metal form and hydrogen form or another univalent or multivalent cationic form. It can also be used in intimate combination with a hydrogenating component such as tungsten, vanadium, molybdenum, rhenium, nickel, cobalt, chromium, manganese, or a noble metal such as platinum or palladium where a hydrogenation-dehydrogenation function is to be performed. Such components can be exchanged into the composition, impregnated therein or physically intimately admixed therewith. Such components can be impregnated in or on to zeolite ZSM-11 such as, for example, by, in the case of platinum, treating the zeolite with a platinum metal-containing ion. Suitable platinum compounds for this purpose include chloroplatinic acid, platinous chloride and various compounds containing the platinum amine complex. Combinations of metals and methods for their introduction can also be used. Synthetic ZSM-11, when employed either as an adsorbent or as a catalyst in a hydrocarbon conversion process, should be dehydrated at least partially. This can be done by heating to a temperature in the range of from 65°C to 550°C in an inert atmosphere, such as air, nitrogen, etc. and at atmospheric or subatmospheric pressures for between 1 and 48 hours. Dehydration can be performed at lower temperature merely by placing the zeolite in a vacuum, but a longer time is required to obtain a particular degree of dehydration. The thermal decomposition product of the newly synthesized ZSM-11 can be prepared by heating same at a temperature of from 200°C to 550°C for from 1 hour to 48 hours. As above mentioned, synthetic ZSM-11 prepared in accordance herewith can be ion exchanged without a pre-calcination step to have original cations associated therewith replaced by a wide variety of other cations according to techniques well known in the art. Typical replacing cations include hydrogen, ammonium and metal cations including mixtures thereof. Of the replacing metallic cations, particular preference is given to cations of metals such as rare earths, Mn, Ca, Mg, Zn, Cd, Pd, Ni, Cu, Ti, Al, Sn, Fe and Co. Typical ion exchange technique would be to contact the synthetic 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. Patents 3,140,249; 3,140,251; and 3,140,253. As in the case of many catalysts, it may be desirable to incorporate the ZSM-11 prepared by the present method with another material resistant to the temperatures and other conditions employed in certain organic conversion processes. Such matrix materials include active and inactive materials and synthetic or naturally occurring zeolites as well as incorganic materials such as clays, silica and/or metal oxides, e.g. alumina. The latter may be either naturally occurring or in the form of gelatinous precipitates, sols or gels including mixtures of silica and metal oxides. Use of a material in conjunction with the ZSM-11, i.e. combined therewith, which is active, may enhance the conversion and/or selectivity of the catalyst in certain organic conversion processes. Inactive materials suitably serve as diluents to control the amount of conversion in a given process so that products can be obtained economically and orderly without employing other means for controlling the rate or reaction. Frequently, crystalline silicate materials have been incorporated into naturally occurring clays, e.g. bentonite and kaolin. These materials, i.e. clays, oxides, etc., function, in part, as binders for the catalyst. It is desirable to provide a catalyst having good crush strength, because in a petroleum refinery the catalyst is often subjected to rough handling, which tends to break the catalyst down into powder-like materials which cause problems in processing. Naturally occurring clays which can be composited with the hereby synthesized ZSM-11 include the montmorillonite and kaolin families which include the subbentonites, and the kaolins commonly known as Dixie, McNamee, Georgia and Florida clays, or others in which the main mineral constituent is halloysite, kaolinite, dickite, nacrite or anauxite. Such clays can be used in the raw state as originally mined or initially subjected to calcination, acid treatment or chemical modification. In addition to the foregoing materials, the ZSM-11 crystals hereby synthesized can be composited with a porous matrix material such as silica-alumina, silica-magnesia, silica-zirconia, silica-thoria, silica-beryllia, silica-titania, as well as ternary compositions such as silica-alumina-thoria, silica-alumina-zirconia, silica-alumina-magnesia and silica-magnesia-zirconia. The matrix can be in the form of a cogel. A mixture of these components could also be used. The relative proportions of finely divided crystalline silicate and matrix vary widely with the crystalline silicate content ranging from 1 to 90 percent by weight, and more usually in the range of 2 to 50 percent by weight of the composite. The present invention provides a low cost ZSM-11 catalyst component for hydrocarbon conversion reactions, particularly cracking, hydrocracking and dewaxing. In general, conditions for such hydrocarbon conversion reactions include a temperature of from 100°C to 800°C, a pressure of from 10 to 20000 kPa (0.1 atmosphere to 200 atmospheres), a weight hourly space velocity of from 0.08 hr ⁻¹ to 2000 hr ⁻¹ or a liquid hourly space velocity of from 0.5 hr ⁻¹ to 100 hr⁻¹, and a hydrogen/feedstock hydrocarbon compound mole ratio of from 0 (no added hydrogen) to 100. More specifically, when the conversion process involves cracking hydrocarbons to lower molecular weight hydrocarbons, the conditions preferably include a temperature of from 230°C to 800°C, a pressure of from 10 to 3500 kPa (0.1 to 35 atmospheres), a weight hourly space velocity of from 0.1hr⁻¹ to 200 hr⁻¹ or a liquid hourly space velocity of from 0.6 hr ⁻¹ to 10 hr⁻¹, and a hydrogen/feedstock hydrocarbon compound mole ratio of from 0 to 100. In order to more fully illustrate the nature of the invention and the manner of practicing same, the following examples are presented. In the examples, whenever sorption data are set forth for comparison of sorptive capacities for vapors of water, cyclohexane and/or n-hexane, they were determined as follows: A weighed sample of the calcined adsorbant was contacted with the desired pure adsorbate vapor in an adsorption chamber, evacuated to less than 1 mm and contacted with 40 mm Hg of n-hexane or cyclohexane vapor, or 12 mm Hg of water vapor, pressures less than the vapor-liquid equilibrium pressure of the respective adsorbate at 90°C. The pressure was kept constant (within about ± 0.5 mm) by addition of adsorbate vapor controlled by a manostat during the adsorption period, which did not exceed about 8 hours. As adsorbate was adsorbed by the crystal, the decrease in pressure caused the manostat to open a valve which admitted more adsorbate vapor to the chamber to restore the above control pressures. Sorption was complete when the pressure change was not sufficient to activate the manostat. The increase in weight was calculated as the adsorption capacity of the sample in g/100 g of calcined adsorbant. When Alpha Value is examined, it is noted that the Alpha Value is an approximate indication of the catalytic cracking activity of the catalyst compared to a standard catalyst and it gives the relative rate constant (rate of normal hexane conversion per volume of catalyst per unit time). It is based on the activity of the highly active silica-alumina cracking catalyst taken as an Alpha of 1 (Rate Constant = 0.016 sec⁻¹). The Alpha Test is described in U.S. Patent 3,354,078 and in The Journal of Catalysis, Vol. IV, pp. 522-529 (August 1965). It is noted that intrinsic rate constants for many acid-catalyzed reactions are proportional to the Alpha Value for a particular crystalline silicate catalyst, i.e. the rates for toluene disproportionation, xylene isomerization, alkene conversion and methanol conversion (see "The Active Site of Acidic Aluminosilicate Catalysts," Nature , Vol. 309, No. 5969, pp. 589-591, 14 June 1984). Example 1 An 85.0 g quantity of octylamine was added to a solution containing 39.5 g Al₂(SO₄)₃ x H₂O, 133.0 g 50% NaOH solution and 602 g water. The resulting solution was added to 717 g of ammonium form of amorphous silica precursor (47.5% solids) prepared by neutralizing a sodium silicate solution in a continuous manner under carefully controlled conditions, collecting the precipitated solids, washing them with water to be anion free, and washing the anion-free solids with ammonium nitrate. More particularly with regard to the silica precursor synthesis, an acid solution was prepared by adding concentrated sulfuric acid to demineralized water to achieve a concentration of about 24 weight percent acid. A silicate solution was prepared separately by dissolving 100 parts of sodium silicate (29% SiO₂, 9% Na₂O and 50% NaOH) and 1.03 parts of 50% NaOH in 98.9 parts of demineralized water. The silicate solution and acid solution were pumped into a stirred reaction vessel containing water in a continuous manner at a silicate/acid mole ratio of about 1/1. The reaction was carried out in the vessel at a pH of about 8, a temperature within the range of 27°C to 150°C, and an average residence time of about 30 minutes. Product amorphous precipitate was discharged continuously from the vessel by an overflow tube, collected, filtered, washed with demineralized water to be sulfate free and then washed with ammonium nitrate. A 4.23 g quantity of ZSM-11 seed crystals was then added to the mixture of ocytlamine, aluminum sulfate, sodium hydroxide, water and silica precursor to form the reaction mixture for the present synthesis. The reaction mixture having a composition in mole ratios of SiO₂/Al₂O₃ = 85.4 H₂O/SiO₂ = 10.2 OH⁻/SiO₂ = 0.22 Na⁺/SiO₂ = 0.3 Octylamine/SiO₂ = 0.11 and a solids content of 22 wt.%, was thoroughly mixed. The mixture was then crystallized in a stainless steel reactor with agitation for 2 days at 120°C, followed by 3 days at 143°C. The crystalline product was separated from unreacted material by filtration, washed with water and dried at 120°C. The dried crystalline product was submitted for chemical and X-ray diffraction analysis. Table 2 lists the analytical composition of the product. X-ray diffraction analysis established the product of this example to be ZSM-11 (crystallinity = 170% based on standard sample). Example 2 A portion of the crystalline product of Example 1 was calcined at 538°C for 3 hours in air and tested for sorption capacities. The sorption capacities were found to be as follows: The calcined sample was also found to have a surface area of 326 m²g. Example 3 A portion of the dried Example 1 product was ammonium exchanged without prior calcination to a sodium level of 0.01 wt.% and tested for Alpha Value, which proved to be 410. Example 4 For comparative purposes, the synthesis procedure of Example 1 was repeated, except without the ZSM-11 seed crystals in the reaction mixture. The crystalline product was recovered, washed and dried as in Example 1. The dried product was submitted for X-ray analysis, the results of which proved it to be a mixture of ZSM-11 with ZSM-5. Example 5 For comparative purposes, the synthesis procedure of Example 1 was again repeated, except without the octylamine component in the reaction mixture. X-ray analysis of the dried product of this example proved it to be a mixture of ZSM-5 and Magadiite. There was no ZSM-11 product. Examples 6-11 Experiments listed in Table 3 further demonstrate the present improved method for synthesis of crystalline ZSM-11. Amorphous silica precipitate precursor, prepared as in Example 1, was used as the source of silicon in Examples 6-9. Solid silica, i.e. Ultrasil, was used as the silicon source in Examples 10 and 11. Products from each example were recovered by filtration, washed with water and dried at 120°C. Samples of the dried products were analyzed by X-ray diffraction and for chemical composition. Samples of each product were calcined at 538°C as in Example 2 and tested for sorption capacities and surface area. The results are also listed in Table 3.

Claims: 1. A method for synthesizing a crystalline silicate having the structure of ZSM-11 exhibiting the following characteristic X-ray diffraction pattern: and, comprises (i) preparing a mixture capable of forming said silicate, said mixture comprising a source of alkali metal cations (M), a source of silicon, octylamine (R), seed crystals of ZSM-11 structure and water, and having a composition, in terms of mole ratios, within the following ranges: SiO₂/Al₂O₃ = 40-> 2000 H₂O/SiO₂ = 5-50 OH⁻/SiO₂ = 0.05-0.5 M/SiO₂ = 0.05-0.5 R/SiO₂ = 0.03-1.0 (ii) maintaining said mixture under sufficient conditions until crystals of said silicate are formed and (iii) recovering said crystalline silicate having the structure of ZSM-11 from step (ii), said recovered crystalline silicate containing said R and M. 2. The method of claim 1 wherein said mixture comprises at least 0.01 wt.% see crystals of ZSM-11 structure. 3. The method of claim 1 wherein said mixture comprises at least 0.1 wt.% seed crystals of ZSM-11 structure. 4. The method of claim 1 wherein said mixture comprises from 1 wt.% of 5 wt.% seed crystals of ZSM-11 structure. 5. The method of claim 1 wherein said source of silicon is a solid silica or an amorphous silica precipitate. 6. The method of any preceding claim wherein the mixture has the following composition, in terms of mole ratios: SiO₂/Al₂O₃ 50-1000 H₂O/SiO₂ 8-30 OH⁻/SiO₂ 0.1-0.3 M/SiO₂ 0.15-0.4 R/SiO₂ 0.05-0.4 7. The method of any preceding claim wherein the mixture has a composition, in terms of mole ratios, within the following ranges: SiO₂/Al₂O₃ 60-200 H₂O/SiO₂ 8-20 OH⁻/SiO₂ 0.15-0.25 M/SiO₂ 0.2-0.35 R/SiO₂ 0.05-0.3 8. The method of any preceding claim wherein said conditions include a temperature of 100-350°C for 12 hours to 10 days.

Inventors: MCWILLIAMS, JOHN PAUL, RUBIN, MAE KOENIG
Applicants: MOBIL OIL CORPORATION
Application year: 1988
Main classification: C01B33
IPC codes: C01B39, C01B37