Crystallization in a force field

Abstract

Description

t at about 20.degree. to 400.degree. C. for 1 hour to about 90 days.

The composition of a more specific reaction mixture is:

vNa.sub.2 O--xTPABr--100SiO.sub.2 --zH.sub.2 O, where "v" is about 0.1 to about 10.0, "w" is about 0.1 to about 20.0, "z" is about 100 to about 5,000, and TPABr is tetrapropylammonium bromide.

The reaction and crystallization are carried out at a temperature from about 100.degree. C. to about 200.degree. C.

For a reaction mixture of 2.55Na.sub.2 O-5.0TPABr-100SiO.sub.2 -2800H.sub.2 O, the reaction and crystallization are carried out preferably at about 175.degree. to 185.degree. C. For high yields of small uniform crystals, a gravitational force of about 50 G and a reaction time of about 12 hours to about 60 hours is preferred. For large crystals, a force of about 50 G and a reaction time of about 84 hours to about 144 hours is preferred. For a reaction mixture of 2.78Na.sub.2 O-Al.sub.2 O.sub.3 -2.0SiO.sub.2 -504H.sub.2 O, the reaction and crystallization temperature is about 90.degree. C. with a reaction time greater than about 4 days in a force greater than about 10 G.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic partial cross-sectional elevational view of a centrifuge oven.

FIG. 2 is a schematic partial cross sectional plan view of the centrifuge oven.

FIG. 3 is a photomicrograph at 50 X of the crystals obtained in a 1 G gravitational force after 120 hours according to the method of Example 1.

FIG. 4 is a photomicrograph at 50 X of crystals obtained in a 30 G gravitational force after 120 hours according to the method of Example 1.

DETAILED DESCRIPTION OF THE INVENTION AND BEST MODE FOR CARRYING OUT THE PREFERRED EMBODIMENT

The present invention relates to a new method for producing enhanced crystals of larger size, higher quality, greater yield, and more uniform size using a force greater than 1 G. The method has wide applicability in the crystallization art and may be used for crystal growth from solvent media, amorphous solids, and gels where a gel can be an aqueous solution, a reactive solid, a colloidal sol or a glass. Crystal growth may be proceeded by or concurrent with a chemical reaction. Crystal growth can be initiated by use of seed crystals, a crystal growth template, or by spontaneous nucleation. Crystal growth techniques known in the art are applicable to the crystal growth methodology of this invention, i.e., crystal growth in forces of greater than 1 G.

One area of application is the growth of zeolite-type crystals. Zeolitic crystals are ordered, porous crystalline materials having a definite crystal structure within which there are a number of still smaller channels. These cavities and channels are precisely uniform in size within a specific zeolitic material. And since the dimensions of the pores are such as to accept certain size molecules while rejecting those of larger dimensions, zeolitic materials are known as "molecular sieves" and are used in a variety of ways to take advantage of the adsorbent properties of these compositions. Large size crystals, that is, crystals greater in size than 200 microns, are particularly useful in adsorbent systems. When such large size crystals are used, the zeolitic bed does not pack and channel as quickly as when fine size materials are used and, as a result, the adsorptive properties of the zeolitic material are maintained. Examples of zeolite-type crystals are: Zeolites A, X, Y, ZSM-5, ZSM-11, ZSM-12, ZSM-35, aluminophosphates, metal incorporated alumino-phosphates, pillared inter-layered compounds such as pillared clays and zirconium phosphates, and zeolites in which Si is replaced in whole or in part by Al, Ga, Ge, Be, B, Fe, Cr, P, or Mg or combinations thereof.

One general method for the preparation and crystallization of zeolites may be represented generally in the following fashion: ##STR1##

Zeolites may be prepared from silica sources such as sodium silicate, colloidal silica, silica hydrosol, silica gel, and silicic acid. The silicon may be replaced by one or more elements such as aluminum, gallium, germanium, beryllium, boron, iron, chromium, phosphorus, or magnesium. The preparation of molecular sieves is well-known in the art and is described more fully in the Kirk-Othmer Encyclopedia of Chemical Technoloyy, 3rd Edition, Volume 15, page 638, Hydrothermal Chemistry of Zeolites, R. M. Barrer, Academic Press, 1982; and references cited therein, U.S. Pat. Nos. 3,777,006 to Rundell, 4,375,458 to Dwyer, 4,650,656 to Dwyer, and herein by reference.

As an example, Zeolite ZSM-5 may be prepared from a silica source, an alumina source, a base and an organic template. The silica source may be sodium silicate, silica hydrosol, silica gel, or silicic acid. A colloidal silica such as Ludox-AS40 (DuPont; Wilmington, Del.) is preferably used as the silica source. The alumina source may be an aluminum compound such as sodium aluminate, alumina trihydrate or aluminum chloride with sodium aluminate being preferred.

A relatively high pH is maintained by the use of an alkali metal hydroxide or other strong base such as, but not limited to, tetraalkylammonium hydroxide. Preferably a 50 wt % aqueous sodium hydroxide solution (Mallinckrodt, Inc., Paris, Ky.) is used.

The organic template employed in preparing zeolitic crystals is usually selected from alkylammonium compounds and more particularly from quaternary compounds including tetraalkyl halogenated ammonium compounds such as tetramethyl ammonium or tetrapropyl ammonium bromide. However, any suitable organic compound known to those skilled in the art may be used. Furthermore, instead of the organic compound itself, precursors thereof may also be used including C.sub.1 -C.sub.10 alkylammonium compound precursors. Thus in the case of tetra-n-propyl ammonium bromide, tri-n-propyl amine and n-propyl bromide may be used. Preferably, tetrapropyl ammonium bromide (Aldrich Chemical, Milwaukee, Wis.; reagent grade) is used.

The composition of an illustrative reaction mixture is:

xNa.sub.2 O--yTPABr--100SiO.sub.2 --zH.sub.2 O, where "x" is about 0.1 to about 10.0, "y" is about 0.1 to about 20.0, "z" is about 100 to about 5,000, and TPABr is tetrapropylammonium bromide.

The reaction and crystallization are carried out at a temperature from about 100.degree. C. to about 200.degree. C. in a force of greater than 1 G.

A force is maintained by means of a centrifuge placed in a forced convection oven and driven by a motor located on the outside of the oven. As shown in FIG. 1, this centrifuge oven consists of a forced convection oven 10 (Blue M Electric Company, Blue Island, Ill.) that has been modified by having a hole cut through the top to accommodate a drive shaft 20 that is connected to a motor 30 (Mixing Equipment Co., Inc., Rochester, N.Y.). The motor is secured to the top of the oven by brackets or other securing means (not shown). The rotational speed of the motor 30 is controlled by a speed controller 34 (Reliance Electric Company, Cleveland, Ohio) connected to the motor 30 by wires 36. The drive shaft 20 is rotatably secured to the oven 10 by means of a graphite bearing 40 (International Graphite Corp., Cleveland, Ohio) that is contained in a brass bearing receptacle 42. The bearing receptacle is rigidly attached to a mounting plate 44 that is securely attached to the inside of the oven top by suitable means such as machine screws or nuts and bolts (not shown). The end of the drive shaft opposite the end connected to the motor 30 is rigidly attached to a centrifuge arm plate 50 by suitable means such as a bolt 52. Each of the centrifuge arms 54 of the centrifuge arm plate 50 are equipped with reaction vessel holder 58 that is rotatably held to the centrifuge arms by a swivel 56.

For the specific reaction mixture of 2.55Na.sub.2 O.5.0TPABr.100SiO.sub.2.2800H.sub.2 O, the preferred temperature range is from about 175.degree. to about 185.degree. C. Forces of from greater than 1 G are used. Preferably, a force of about 50 G and a reaction time of about 12 to about 60 hours is used to obtain small uniform, nontwinned crystals in high yield. A force of about 50 G and a reaction time of about 84 to about 144 hours is preferred in order to obtain maximum sized crystals. For a reaction mixture of 2.78Na.sub.2 O-Al.sub.2 O.sub.3 -2.0SiO.sub.2 -504H.sub.2 O, the reaction and crystallization temperature is about 90.degree. C. with a reaction time greater than about 4 days and a force greater than about 10 G.

The following examples are disclosed to further teach the practice of the invention and are not intended to limit the invention as it is delineated in the claims. For a fixed force crystal size and crystallization time will vary with the nature of the reaction mixture employed.

EXAMPLE 1

Preparation of Silicalite (low-alumina ZSM-5)

Ludox AS-40 (25.88 wt. %; an ammonia stabilized 40 wt. % SiO.sub.2 made by DuPont, Wilmington, Del.), was weighed into a plastic beaker. To this, 71.12 wt. % water, 0.702 wt. % sodium hydroxide solution (50 wt % in water, Mallinckrodt, Inc. Paris, Ky.) and 2.298 wt % tetrapropylammonium bromide (TPABr) (Aldrich Chemical Co., Inc., Milwaukee, Wis.) were added to give a reaction mixture of 100SiO.sub.2 -2800H.sub.2 O-2.55Na.sub.2 O-5.0TPABr. The mixture was agitated until a uniform gel was obtained. The gel was rapidly transferred to Teflon-lined Morey-type reaction vessels. The vessels were sealed, weighed, and placed into an eight position centrifuge contained within a preheated forced convection oven. Placement of the weighed vessels was such as to achieve the best possible balance of the centrifuge.

The centrifuge was rotated at a predetermined rotational speed (RPM) and this speed was maintained throughout the experiment. At predetermined intervals, the rotation of the centrifuge was stopped and a vessel was removed from the oven. The vessel was replaced with a vessel of similar weight to preserve the balance of the centrifuge.

The product vessel was quenched using cold tap water. The product mother liquor was tested for pH. The solid products were placed in a Buchner funnel, washed a minimum of three times with 100 ml of distilled water and dried overnight at 110.degree. C. Samples were tested for crystallinity using powder x-ray diffractometry and for size using optical and scanning electron microscopy.

Tables 1 and 2 illustrate the effect of the force field on crystal size. FIGS. 3 and 4 are photomicrographs at 50X magnification showing the size difference attributable to the force. It is clearly evident from Tables 1 and 2 and FIGS. 3 and 4 that the size of the crystals increases significantly with the strength of the force. Moreover, for equal sized crystals, the time of crystallization is considerably decreased with increasing strength of the force.

TABLE 1______________________________________Average ZSM-5 (silicalite) Crystal Size (microns)as a Function of Time in ForceTIME FORCEdays 1 G 30 G 50 G______________________________________1 93 88 1072 60 116 1353 93 152 1214 46 175 1155 50 163 1936 53 192 1987 84 159 176______________________________________ TABLE 2______________________________________Average ZSM-5 (silicalite) Crystal Size (microns)of largest 10% of Crystals as aFunction of Time in ForceTIME FORCEdays 1 G 30 G 50 G______________________________________1 132 135 1402 66 150 1903 98 192 1504 53 254 2215 52 187 2976 54 295 2387 103 166 208______________________________________ TABLE 3______________________________________pH of ZSM-5 (silicalite) as a Function ofTime in ForceTIME FORCEhours 1 G 30 G 50 G______________________________________ 0 11.80 11.83 11.9324 9.95 11.23 11.3348 10.43 10.68 10.0572 10.45 ND 10.5896 10.45 10.64 10.59120 10.42 9.75 10.60144 9.95 10.30 9.55168 9.96 10.48 10.38______________________________________ ND -- Not Determined

EXAMPLE 2

The reaction was carried out as in Example 1. The reaction time was held constant at 7 days while varying the period of time in the force. The results are presented in Table 4.

TABLE 4______________________________________ZSM-5 (silicalite) Crystal Size (microns) as aFunction of Time in ForceTIMEDays in FORCEForce 10 G 30 G 50 G______________________________________1 45 * 235 .+-. 552 50 .+-. 20 150 *3 130 .+-. 10 120 .+-. 10 185 .+-. 54 80 .+-. 10 195 .+-. 5 *5 40 .+-. 10 145 .+-. 5 165 .+-. 256 60 .+-. 20 315 .+-. 15 *7 130 .+-. 30 165 .+-. 5 165 .+-. 15______________________________________ A control sample held at 1 G for 7 days yielded 70 .+-. 10 micron crystals. *No Sample Taken

EXAMPLE 3

Silicalite ZSM-5 Yield Determination

The reaction was carried out as in Example 1. The product yield, as given in TABLE 5, was determined by completely emptying the contents of a reaction vessel into a pyrex beaker. These solids were treated with 500 ml of a 0.1 N sodium hydroxide solution. The slurry was agitated for thirty minutes while boiling at approximately 100.degree. C. to insure complete dissolution of amorphous solids. The remaining purified silicalite crystals were dried at 110.degree. C. and reweighed. The ratio of the weights of the silicate crystals to the calculated amount of SiO.sub.2 contained in the original reaction mixture times one hundred gives the percent yield. As is apparent from TABLE 3, yields increase with the intensity of the force.

TABLE 5______________________________________Yield % of ZSM-5 (silicalite)TIME FORCEdays 1 G 30 G 50 G______________________________________1 1.3 4.3 9.92 2.3 8.9 44.33 3.2 9.3 28.04 4.3 14.8 40.55 2.8 46.7 38.96 1.7 43.5 48.57 1.9 41.3 55.5______________________________________ Yield % = (g ZSM5 (silicalite) crystals/g SiO.sub.2 in reaction mixture) .times. 100

EXAMPLE 4

Preparation of Zeolite A

A thin layer of kaoline (Matheson, Coleman and Bell; Norwood Ohio) is calcined at 600.degree. C. for 13 hrs. to form meta-kaoline, Three grams of meta-kaoline, 6.0 g of 50 wt % NaOH, and 121.8 g of distilled water were mixed for 10 minutes. The mixture of 2.78Na.sub.2 O-Al.sub.2 O.sub.3 -2.0SiO.sub.2 -504H.sub.2 O was transferred to teflon-lined Morey-type reaction vessels. The vessels were sealed, weighed, and placed in the centrifuge oven. The solution was allowed to react and crystalize for 5 and 7 days at a temperature of 90.degree. C. and a force of 1 or 30 G. Each sample was cooled, filtered, and dried overnight at 100.degree. C. As evident in Table 6, crystals of larger size were obtained at 30 G.

TABLE 6______________________________________Zeolite A Crystal Size (microns) as a Function ofTime and ForceTIME FORCEdays 1 G 30 G______________________________________5 16.0 29.07 20.9 24.3______________________________________

Claims

1. A process for preparing enhanced crystals comprising: growing said crystals from a gel in a force greater than the earth's gravitational force.

2. The process claimed in claim 1 wherein the crystal growth takes place after a chemical reaction.

3. The process of claim 2 wherein said chemical reaction takes place in a reaction mixture comprising a base and an oxide source.

4. The process of claim 2 wherein said chemical reaction takes place in a reaction mixture comprising a base and a source oxide of one or more elements selected from the group consisting of silicon, aluminum, gallium, germanium, beryllium, boron, iron, chromium, phosphorous, and magnesium.

5. The process of claim 2 wherein said chemical reaction mixture takes place in a reaction mixture comprising a base, a phosphorous oxide source and an aluminum oxide source.

6. The process claimed in claim 2 wherein said chemical reaction takes place in a reaction mixture comprising a silica source and a base.

7. The process claimed in claim 6 wherein said crystal growth is initiated by a template.

8. The process claimed claim 7 wherein said template is an organic template.

9. The process claimed in claim 6 with said reaction mixture further comprising an alumina source.

10. The process claimed in claim 8 wherein the composition of said reaction mixture is VNa.sub.2 O-w(organic) template-xAl.sub.2 O.sub.3 -ySiO.sub.2 -zH.sub.2 O, where

"v" is about 0.01 to about 20.0,

"w" is about 0 to about 20.0,

"x" is about 0 or 1,

"y" is about 2.0 to 100,000, and

"z" is about 10 to about 100,000.

11. The process as claimed in claim 6 wherein said silica source is selected from the group consisting of colloidal silica, silica gel, silicic acid, sodium silicate, and silica hydrosol.

12. The process as claimed in claim 8 wherein said organic template is a tetralkyl ammonium halide.

13. The process as claimed in claim 12 wherein said tetralkyl ammonium halide is tetrapropyl ammonium bromide.

14. The process as claimed in claim 6 wherein said chemical reaction is carried out at a temperature of about 20.degree. C. to about 400.degree. C.

15. The process as claimed in claim 11 wherein said silica source is colloidal silica.

16. The process as claimed in claim 4 wherein said base is a 50 wt. % sodium hydroxide solution.

17. The process as claimed in claim 10 wherein the composition of said reaction mixture is

vNa.sub.2 O-wTPABr-100SiO.sub.2 -zH.sub.2 O, where

"v" is about 0.1 to about 10.0,

"w" is about 0.1 to about 20.0,

"z" is about 100 to about 5,000, and

TPABr is tetrapropylammonium bromide.

18. The process as claimed in claim 17 wherein the chemical reaction and crystallization are carried out at temperatures of about 100.degree. C. to about 200.degree. C.

19. The process as claimed in claim 18 wherein said force is greater than about 20.degree. G.

20. The process as claimed in claim 17 with the composition of said mixture comprising 2.55Na.sub.2 O-5.0TBABr-100SiO.sub.2 -2800H.sub.2 O.

21. The process as claimed in claim 20 wherein the chemical reaction and crystallization are carried out at temperatures of 175.degree. C. to 185.degree. C.

22. The process as claimed in claim 21 wherein said force is about 50 G.

23. The process as claimed in claim 22 wherein said reaction and crystallization time is about 12 to about 60 hours.

24. The process as claimed in claim 22 wherein said reaction and crystallization time is about 84 to about 168 hrs.

25. The process as claimed in claim 10 with said reaction mixture comprising 2.78Na.sub.2 O-Al.sub.2 O.sub.3 -2.0SiO.sub.2 -504H.sub.2 O.

26. The process as claimed in claim 25 wherein the chemical reaction and crystallization temperatures are about 90.degree. C.

27. The process as claimed in claim 26 wherein the chemical reaction and crystallization time is about 4 days.

28. The process as claimed in claim 27 wherein said force is greater than about 10 G.