METHOD OF PREPARING LOW ACIDITY ALKALI METAL CONTAINING ZEOLITES

Abstract: Low acidity alkali metal containing zeolites are prepared by treating zeolites with an alkali metal cation containing medium at a pH range of between about 7 and 12.

Description: LOW ACIDITY ALKALI METAL CONTAINING ZEOLITES BACKGROUND OF THE INVENTION Field of the Invention This invention concerns the preparation of low acidity alkali metal containing zeolites. Description of the Prior Art Since the introduction of zeolite catalysts as exemplified by U.S. Patent No. 3,140,249, a large proportion of the capacity for catalytic cracking and hydrocracking has been converted to use of such highly active catalysts. The high activity catalysts are generally characterized by very low alkali metal content. Sodium, for example, is present as a cation in synthetic faujasites by reason of their manufacture. Expensive ion exchange operations are conducted to replace the sodium or other alkali metal by protons or poly-valent metal cations in the preparation of cracking and hydrocracking catalysts from synthetic faujasite. It has been recognized that such zeolites can function as catalysts when containing a moderate percentage of sodium. Thus the Kimberlin and Gladrow Reissue Patent 26,188 exhibits data showing cracking activity of a faujasite from which only one-third of the sodium has been removed by ion exchange. The extremely high activity of such catalysts as zeolite ZSM-5 has been moderated for specialized purposes by using the zeolite in the partially sodium form. Such moderating is described in U.S. Patents 3,899,544; 4,171,260; and 4,188,282. Conventional preparation of alkali metal exchanged zeolites has heretofore been conducted by using alkali metal salt solutions, such as aqueous solutions of sodium chloride. Techniques for exchanging zeolites are generally well known and describted-in many U.S. Patents such as 3,140,249; 3,140,251; and 3,140,253. Conventionally, alkali metal exchanged zeolites have been prepared by utilizing salts of strong acids, e.g., sodium chloride, sodium sulfate, sodium nitrate, etc. The use of such salts results in high levels of residual acid activity and thus less than complex exchange. It is also known that addition of a strong base to the exchange solution to control acid activity is very difficult to monitor and control. Furthermore, addition of too-much strong base will result in destruction of zeolite crystallinity by virtue of caustic'attack of the silica in the zeolite. It is an object of the present invention to provide a method of preparing low acidity alkali metal containing zeolites with complete alkali metal exchange. It is another object of this invention td provide a method of preparing such low acidity zeolite without undue loss in. zeolite crystallinity. SUMMARY OF THE INVENTION In accordance with the present invention there is now discovered novel methods of preparing low acidity alkali metal containing zeolites with comp.lete alkali metal exchange and without undue loss in crystallinity.- - The zeolites in its pure crystallized'form, or in its final form, e.g., pellets, extrudates, or. beads in combination with a matrix material, are treated with an excess of an alkali metal cation medium, e.g., salt solution, maintained in the pH range of between about 7 and 12 at temperatures ranging from between about room temperature and the boiling point of the salt solution. One method of accomplishing this treatment is to utilize an alkali metal containing salt of a strong acid, e.g., Nazi, in combination with addition of a strong base, e.g., NaOH. Alternatively, the treatment can be attained by use of an alkali metal salt of a weak acid or a mixture of weak acids, e.g., phosphates, borates, oxolates, acetates, aluminates, carbonates, bicarbonates, etc. DESCRIPTION OF PREFERRED EMBODIMENTS Zeolites which can be alkali metal treated according to the novel method of the instant invention include both natural and synthetic zeolites. Non-limiting examples of such natural zeolites include brewsterite, cancrinite, chabazite, clinoptilolite, dachiardite, epistilbite, erionite, faujasite, ferrierite, gmelinite, heulandite, mordenite, offretite, phillipsite, stilbite, just to name a few. Non-limiting examples of synthetic zeolites that can be exchanged according to this invention include ZSM-5 (U.S. Patent No. 3,702,886), ZSM-ll (U.S. Patent No. 3,709,979) , ZSM-12 (U.S. Patent No. 3,832.449), ZSM-35 (U.S. Patent No. 4,016,245), ZSM-38 (U.S. Patent No. 4,046,859) , X (U.S. Patent No. 2,882,244), Y (U.S. Patent No. 3,130,077), ZSM-20 (U.S. Patent No. 3,972,983), ZK-5 (U.S. Patent No. 3,247,195), ZK-4 (U.S. Patent No. 3,314,752), zeolite A (U.S. Patent No. 2,882,243), ZSM-4, Beta, just to name a few. Particularly preferred zeolites as subjects for the alkali exchange method of this invention are the ZSM-5 class of zeolites (ZSM-5, ZSM-ll, ZSM-12, ZSM-23, ZSM-35, ZSM-38 and other similar materials). ZSM-5 is a most particularly preferred zeolite as a candidate for alkali metal exchange according to the method of-the present invention. The novel methods of the instant invention are applicable to zeolites in their pure as crystallized state, i.e., zeolites in powder form. The zeolites can be in the acid form, e.g., hydrogen form, or basic form, e.g., ammonium form. Furthermore, said zeolites can be treated in accordance with this invention when incorporated with another material. Such materi:al is usually resistant to temperature and other processing conditions and is usually a matrix material. Matrix materials are very useful as catalyst binders to impart greater resistance to the zeolite and to allow the catalyst to be formed in various shapes and sizes. Thus zeolite matrix composites in the forms of beads, tablets, extrudates, spheres, etc., can be treated according to the instant invention. Useful matrix materials include both synthetic and naturally occurring substances, as well as inorganic materials such as clay, silica and silica plus metal oxides. The latter may be either naturally occurring or in the form of gelatinous precipitates or gels including mixtures of silica and metal oxides. Naturally occurring clays which can be composited with the zeolite include those of the montmorillonite and kaolin families, which families include the sub-bentonites 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 zeolites employed herein may be composited with a porous matrix material, such a alumina, silica-alumina, silica-magnesia, silica-zirconia, silica-thoria, silica-berylia, and silica-titania, as well as ternary compositions, such as silica-alumina-thoria, silica-alumina-zirconia, silica-alumina-magnesia and silica-magn,esia-zirconia. The matrix'may be in the form of a cogel. The relative proportions of zeolite component and inorganic oxide gel matrix on an anhydrous basis may vary widely with the zeolite content ranging from between about 1 to about 99 percent by weight and more usually in the range of about 5 to about 80 percent by weight of the dry composite. Alkali metals that can be exchanged in accordance with this invention include lithium, sodium, potassium, rubidium, cesium and francium; sodium being the preferred alkali metal. The novel treatment method of the instant invention involves alkali metal exchange of zeolites within the pH range of between about 7 and 12 at temperatures ranging from room temperature up to the boiling point of the particular alkali metal salt utilized. In one particular embodiment of this invention, an alkali metal salt of a strong acid can be used in conjunction with a strong alkali metal base having the same alkali metal as the salt. The addition of the base is carefully controlled so as not to allow the treatment solution to fall outside that prescribed pH range. One method of accomplishing this careful control is to dropwise add base while simultaneously monitoring the solution pH. In accordance with this embodiment, a salt of a strong acid such as a chloride, sulfate, nitrate, etc., would be used in conjunction with a strong base such as an hydroxide. A typical sodium treatment could be conducted using. sodium chloride and dropwise addition of sodium hydroxide. The amount of hydroxide added would depend on the desired particular pH within the above given range. In another embodiment of this invention, an alkali containing salt of a weak acid or a mixture of alkali salts of different weak acids can be employed. Such salts would include such non-limiting examples such as carbonates, phosphates, borates, acetates and aluminates. The particular salt or salts utilized andthe amount of salt solution required would depend on the desired pE within the given range of 7 to 12. Without wishing to be bound by any particular theory of operability, it is believed that conventional exchange methods using solutions of salts derived from strong acids, e.g., nitrates, chlorides, sulfates, etc., which were not modified by bases, contain hydronium ions which compete with the alkali metal ions (these solutions have pH values well below 7). This competition limits exchange to no more than about 98% of the exchangeable sites (M+AlO2- < 0.98 where M represents the alkali metal ion). These hydronium occupied sites become acid sites on calcination. In still another embodiment of this invention alkali metal containing bicarbonate solution can be employed either with or without other alkali salts of weak acids. If used alone, alkali metal bicarbonate can result in a pH of about 8 and when used in combination with other alkali salts of weak acids the pH can be adjusted either above or below 8. The bicarbonate containing solutions are self-buffering and therefore have the advantage of maintaining an almost constant pH during treatment. The utilization of alkali bicarbonate containing solutions with prior pH calibration for proper concentrations allows for automatic pH control without resorting to constant pH monitoring of the exchange solution. Without wishing to be bound by any particular theory of operationality, it is felt that the unique buffering action of bicarbonate is due to the fact that the acid by-product of the exchange, carbonic acid, decomposes to water and carbon dioxide (a non-acid medium). The amount of alkali metal containing bicarbonate solution required is that amount to provide ample alkali metal to attain 100 percent exchange. It is preferred to add an excess of the solution to insure proper exchange and pH control. It ia also preferred to use an aqueous solution of said bicarbonate. The alkali metal exchanged zeolites produced in accordance with this invention are characterized by having low acidity. One procedure to measure the acid activity of zeolites is the alpha test. Such test is defined in JOURNAL OF CATALYSIS, Volume VI, pages 278-287 (1966). To measure low acid activity catalysts with alpha values less than 1, the cyclopropane isomerization test has proved to be particularly useful. This cyclopropane isomerization test is generally conducted at a temperature of about 2500C (480OF) at He/cyclopropane mole ratio of about 4, a flow rate of about 10 to 100 ml/minute over 10-50 mg of catalyst and at conversion levels of about 5 to 60% of the cyclopropane feed. The catalyst is crushed and mixed with 1 ml 30/60 mesh vycor (acid treated and air calcined). The rate constants are determined. A silica alumina of activity i = 1 has a rate constantk, of 63.5 sec. 1 for cyclopropane isomerization. The cyclopropane isomerization index (CPI) for the silica alumina is assumed to be 1000. The CPI for any material tested is 1000k - 63.5. The preferred range for low acidity zeolites is less than about 0.5 alpha or a CPI value of less than about 200. The low acidity alkali exchanged zeolites prepared according to the novel method of the present invention are useful as hydrocarbon conversion catalysts, catalyst supports, and adsorbents. The following examples will serve to illustrate the invention without limiting same. EXAMPLE 1 A sodium silicate solution was prepared by mixing 16 parts water and 27.7 parts sodium silicate (28.7 wt % SiO2, 8.9 wt % Na2O, 62.4 wt % H2Ot followed by addition of 0.08 parts Daxad 27 (W.R. Grace Chemical Division). The solution was cooled to approximately 150C (600F). An acid solution was prepared by adding 1 part aluminum sulfate (17.2 wt % A1203) to 16.4 parts water followed by 2.4 parts sulfuric acid (93'wt % H2S and 1.2 parts NaCl. These solutions were mixed in an agitated vessel while 3.9 parts of NaCl were added. The gel molar ratios expressed as oxides are the following: SiO2/A12O3 = 78.4 Na2O/Al2O3 = 9. 5 The gel was then heated to about 930C (2000F), agitation was reduced and an organic solution containing 0.8 parts n-propyl bromide and 1.5 parts methylethyl ketone was added to the gel. After these organics were added, 2.3 parts of n-propyl amine was added to the organic phase above the gel. This mixture was held at about 930C (2000F) for 6 hours, then severe agitation was resumed. Crystallization was conducted at 93-990C (20,0-2100F) until the gel was 80% crystallized, at which time temperature was increased to 150-f600C (302-3200F). Unreacted organics were removed by flashing and the remaining contents cooled. The zeolite slurry product was diluted with 4-5 parts water per part.slurry and 0.0002 parts of flocculent (Rohm & Haas, Primafloc C-7), per part slurry, allowed to settle and supernatant liquid was drawn off. The settled solids were reslurried to the original volume of the preceeding step with water and 0.00005 parts of flocculant per part slurry. After settling, the aqueous phase was decanted. This was repeated until the sodium level of the zeolite was less than 0.10% wt *. Then 0.1 parts ammonium nitrate per part slurry were added to the settled solids and the water from the previous decantation. This mixture was reslurried and the solids were allowed to settle. The washed zeolite solids were filtered, identified as ZSM-5 by X-ray diffraction, and analyzed as having a SiO2JA1203 mole ratio of 63.4 and a sodium content of 0.02 wt % (dry basis). EXAMPLE 2 A sample of ZSM-5 as prepared according to Example 1 was calcined at 5400C (1000OF) for 2 hours in flowing helium and 1 hour in flowing air. An aliquot was converted to the ammonium form using 1 N (NH42SO4 at reflux temperature. A 4 gram aliquot of the washed filter cake was exchanged with 150 ml of 0.3N NaCl over a six-day period, replacing the exchange medium twice daily. EXAMPLE 3 A 12 gram aliquot of the filter cake as prepared according to Example 2 was exchanged in accordance with Example 2 with the exception that the pH was adjusted to and maintained at a value of 10 by dropwise addition of 0.1 N NaOH solution with frequent pH monitoring. Overnight pH was adjusted to pH-ll to maintain a high pH during the unmonitored period. EXAMPLE 4 A sample of 65% HZSM5/35* A1203 extrudate was converted to the ammonium form by saturating with dry NH3, then treated with NaCl/NaOH as in Example 3 to attain a pH of 10/11. EXAMPLE 5 A sample of ZSM-5 prepared according to the general procedure of Example 1 and calcined at 5400C (10000F) in air to produce HZSM-5. The HZSM-5 was then treated with 0.3 N NaCl solution and with dropwise addition of 0.1 N NaOH solution with frequent pH monitoring to arrive at a pH of 8.0. EXAMPLE 6 A 15 gram of ZSM-5 as prepared according to Example 1 and then calcined to form HZSM-5 was subjected to the same treatment as in Example 2, except 1N NaHC03 aqueous solution was utilized.in place of the NaCl solution No NaOH addition was required to maintain the pH at a constant value of 8. EXAMPLE 7 A sample of Z5M5 as prepared according to Example 1 and calcined to the hydrogen form was ball milled with silica-alumina (as a matrix) in the ratio of 40 parts by weight zeolite to 60 parts by weight silica-alumina. The mixture was pelleted and sized. to 14/30 mesh. It was then treated with 1 N .NaHCO3 as in Example 6. EXAMPLE 8 A 4 gram sample of ZSM-5 as prepared according to Example 1 was calcined at.540 C (1000 F) for 2 hours in flowing helium and 1 hour in flowing air. 'The resultant HZSM-5 was treated with a mixture of equivalent NaHC03 and Na2C03 solutions with a combined pH of 9.2. This pH,, level remained constant throughout the test period. EXAMPLE. 9 A 7 gram sample of ZSM-5 as prepared according to Example 1 was calcined at 5400C (10000F) for 2 hours in flowing helium and 1 hour in flowing air. The HZSM-5 sample was then treated with 1 N Na2HP04 (pH=9.2). The solution pH dropped to 8.2 where it then remained constant. When the exchange solution was replaced with fresh 1 N Na2HPO4, the pH level was raised to 9.2 and remained constant at that level. EXAMPLE 10-17 In these examples 2-9, the alkali exchanged zeolites of Examples 2-9 were subjected to tests to ascertain their degree of crystallinity after treament as compared to before treatment and their resultant residual acid activity. The zeolites were measured for crystallinity before and after treatment by conventional X-ray techniques. Furthermore, the alkali metal exchangd zeolites were tested for residual acid acidity using the cyclopropane isomerization test at the conditions of 2500C (4800F), 4:1 helium/cyclopropane ratio and flow ratio andd flow rates to maintain conversion of about 5 to about 60% of the cyclopropane feed. The results of these tests are given in Table 1 hereinbelow. The results of Table 1 clearly show that the zeolites treated according to the methods of this invention (Examples 11.-17) are well within the required acid activity range (cyclopropane isomerization index of less than 300). The exchanged zeolite prepared according to conventional means (Example 10), however, exhibits a very high residual acid activity. TABLE 1 Catalyst Treatment Cyclopropane According to Catalyst Catalyst or Isomerization Crystallinity Example No. Example No. Treatment Zeolite Type pH Index, CPI (% of (Parent) 10 2 NaCl NH4ZSM-5 6.3 2000* 100 11 3 NaCl/NaOH NH4ZSM-5 10/11 10* 91 12 4 NaCl/NaOH 65% HZSM-5/ 10/11 15 82 35% Al2O3 extrudate 13 5 NaCl/NaOH HZSM-5 8 20* 104 14 6 NaHCO3 HZSM-5 8 12* 102 15 7 NaHCO3 40 wt% of 8 9 H%SM-5 milled with 60 wt% of silica-alumina to form pellets (14/30 mesh) 16 8 NaHCO3/ HZSM-5 9.2 5 Na2CO3 17 9 Na2HPO4 HZSM-5 9.2 1.5 *Estimated values

Claims: WHAT IS CLAIMED IS: 1. A method of preparing low acidity alkali metal containing zeolites which comprises treating a zeolite with an alkali metal cation containing medium at a pH range of between about 7 and 12. 2 The method of claim 1 wherein said alkali metal is selected from the group consisting of lithium, sodium, potassium, rubidium, cesium and francium. 3. The method of claim 2 wherein said alkali metal is sodium. 4. The method of claim 1 wherein said medium is a salt solution and the treatment is conducted at a temperature ranging between about room temperature and the boiling temperature of said salt solution. 5. The method of claim 1 wherein said zeolite is in the hydrogen form. 6. The method of claim 1 wherein said zeolite is in the ammonium form. 7. The method f claim 1 wherein said zeolite is a natural zeolite. 8. The method of claim 1 whetein said zeolite is a synthetic zeolite. 9. The method of claim 8 wherein said zeolite is selected from the group consisting of 75M-5, zsM-ll, ZSM-12, ZSM-23, ZSM-35 and ZSM-38. 10. The method of claim 9 wherein said zeolite is ZSM-5. 11. The method of claim 1 wherein said zeolite is combined with another material and formed into an extrudate. 12. The method of claim 1 wherein said material is alumina. 13. The method of claim 1 wherein said zeolite is combined with a binder material and formed into pellets. 14. The method of claim 4 wherein said. salt is a salt of a strong acid and said pH is maintained between about 7 and 12 by adding a strong alkali metal containing base of the same alkali metal as in the salt and monitoring the pE. 15. The method of claim 14 wherein said salt is NaCl and said base is Na0H. 16. The method of claim 1 wherein said solution is a solution of a salt or mixture of salts of weak acids. 17. The method of claim 1 wherein said salt is selected from the group consisting of phosphates, borates, oxylates, acetates, aluminates, carbonates and bicarbonates. 18. The method of claim 16 wherein said salt is bicarbonate. 19. The method of claim 15 wherein said salt is phosphate. 20. The method of claim 14 wherein said solution is a bicarbonate-carbonate solution.

Inventors: CHEN, NAI YUEN, MIALE, JOSEPH NICHOLAS, SHIHABI, DAVID SAID
Applicants: MOBIL OIL CORPORATION
Application year: 1980
Main classification: C01B33
IPC codes: C01B39, B01J29