Small crystal ferrierite and method of making the same

Abstract

There is disclosed a highly crystalline, small crystal, ferrierite zeolite prepared from a gel containing a source of silica, alumina, alkali metal and a combination of two templating agents. The resulting material includes ferrierite crystals having a particle size of about or less than about 200 nm. The desired crystal size can be achieved by using a specific composition of the gel. The purity of the material and the crystal size was determined by using X-ray powder diffraction and scanning electron microscopy. The material has excellent surface area and micropore volume as determined by nitrogen adsorption.

Claims

1. A crystalline microporous material having a ferrierite (FER) framework type, a molar ratio of greater than about 10 of a first tetravalent metal oxide to a second tetravalent or trivalent metal oxide, and a mean crystal size of about 200 nm or less, wherein the crystalline microporous material has a surface area ranging from about 300 m.sup.2/g to about 450 m.sup.2/g and a micropore volume ranging from about 0.10 cc/g to about 0.20 cc/g.

2. The crystalline microporous material of claim 1, wherein said material has a crystal size of about 200 nm or less in all crystal dimensions.

3. The crystalline microporous material of claim 1, wherein said first tetravalent metal oxide comprises silica, said second tetravalent metal oxide comprises alumina, and wherein said molar silica to alumina ratio (SAR) ranges from about 10 to about 60.

4. The crystalline microporous material of claim 1, wherein said material has a mean crystal size of about 100 nm or less in all crystal dimensions.

5. The crystalline microporous material of claim 1, further comprising at least one hydrogenation-function metal.

6. The crystalline microporous material of claim 5, wherein said at least one hydrogenation-function metal is chosen from Pt, Pd, Rh, Ru, Ni, Re, and mixtures thereof.

7. The crystalline microporous material of claim 1, further comprising at least one transition metal for selective catalytic reduction of nitrogen oxides and/or catalytic decomposition of nitrous oxide.

8. The crystalline microporous material of claim 7, wherein said at least one transition metal is chosen from Cu and Fe.

9. A method of preparing a crystalline microporous material having a ferrierite (FER) framework type, a molar silica to alumina ratio (SAR) of greater than about 10, and a mean crystal size of about 200 nm or less, said method comprising forming a synthesis mixture comprising a silica source, an alumina source, an alkali metal source, and organic structure directing agents comprising: a first organic structure directing agent (OSDA-1), and a second organic structure directing agent (OSDA-2) that promotes ferrierite structure formation, wherein a molar ratio of OSDA-1 to OSDA-2 ranges from about 0.2 to about 1.0; and, heating the synthesis mixture to a temperature ranging from about 120 to about 200° C. for a period of time sufficient to crystallize the material, to form a crystalline microporous material having a surface area ranging from about 300 m.sup.2/g to about 450 m.sup.2/g and a micropore volume ranging from about 0.10 cc/g to about 0.20 cc/g.

10. The method of claim 9, wherein said OSDA-1 comprises a tetramethylammonium cation.

11. The method of claim 9, wherein said OSDA-2 is chosen from pyrrolidine, ethylenediamine 1,3-diaminopropane, 1-methylpyrrolidine, piperidine, pyridine and 1,4-diaminobutane.

12. The method of claim 9, wherein said organic structure directing agents comprise a tetramethylammonium cation and 1,3-diaminopropane.

13. The method of claim 9, wherein said organic structure directing agents comprise a tetramethylammonium cation and pyrrolidine.

14. A catalyst comprising a crystalline microporous material having a ferrierite (FER) framework type, a molar silica to alumina ratio (SAR) of greater than about 10, a surface area ranging from about 300 m.sup.2/g to about 450 m.sup.2/g, a micropore volume ranging from about 0.10 cc/g to about 0.20 cc/g, and a mean crystal size of about 200 nm or less in all dimensions, wherein said catalyst is catalytically active for at least one process chosen from hydrocarbon conversions, selective catalytic reduction of nitrogen oxides, and catalytic decomposition of nitrous oxide.

15. The catalyst of claim 14, further comprising at least one hydrogenation-function metal for hydrocarbon conversions.

16. The catalyst of claim 15, wherein said at least one hydrogenation-function metal is chosen from Pt, Pd, Rh, Ru, Ni, Re, and mixtures thereof.

17. The catalyst of claim 14, further comprising at least one transition metal for selective catalytic reduction of nitrogen oxides and/or catalytic decomposition of nitrous oxide.

18. The catalyst of claim 17, wherein said at least one transition metal is chosen from Cu and Fe.

Description

BRIEF DESCRIPTION OF FIGURES

(1) The accompanying figures are incorporated in, and constitute a part of this specification.

(2) FIG. 1 is an XRD pattern of ferrierite material described in Example 1.

(3) FIG. 2 is an SEM of ferrierite material described in Example 1.

(4) FIG. 3 is an XRD pattern of ferrierite material described in Example 2.

(5) FIG. 4 is an SEM of ferrierite material described in Example 2.

(6) FIG. 5 is an XRD pattern of ferrierite material described in Example 3.

(7) FIG. 6 is an SEM of ferrierite material described in Example 3.

(8) FIG. 7 is an XRD pattern of ferrierite material described in Example 4.

(9) FIG. 8 is an SEM of ferrierite material described in Example 4.

(10) FIG. 9 is an XRD pattern of ferrierite material described in Comparable Example 5.

(11) FIG. 10 is an SEM of ferrierite material described in Comparable Example 5.

(12) FIG. 11 is an XRD pattern of ferrierite material described in Comparable Example 6.

(13) FIG. 12 is an SEM of ferrierite material described in Comparable Example 6.

(14) FIG. 13 is an XRD pattern of ferrierite material described in Comparable Example 7.

(15) FIG. 14 is an SEM of ferrierite material described in Comparable Example 7.

(16) FIG. 15 is an XRD pattern of ferrierite material described in Comparable Example 8.

(17) FIG. 16 is an SEM of ferrierite material described in Comparable Example 8.

(18) FIG. 17 is an XRD pattern of ferrierite material described in Comparable Example 9.

(19) FIG. 18 is an SEM of ferrierite material described in Comparable Example 9.

(20) FIG. 19 is an XRD pattern of ferrierite material described in Comparable Example 10.

(21) FIG. 20 is an SEM of ferrierite material described in Comparable Example 10.

(22) FIG. 21 is an XRD pattern of ferrierite material described in Comparable Example 11.

(23) FIG. 22 is an SEM of ferrierite material described in Comparable Example 11.

DETAILED DESCRIPTION OF THE INVENTION

(24) Definitions

(25) The following terms or phrases used in the present disclosure have the meanings outlined below:

(26) “Defined by the Structure Commission of the International Zeolite Association,” is intended to mean those structures included in, but not limited to, the structures described in “Atlas of Zeolite Framework Types,” ed. Baerlocher et al., Sixth Revised Edition (Elsevier 2007), which is herein incorporated by reference in its entirety.

(27) “Having the FER framework type” is intended to mean zeolite minerals belonging to the ferrierite group, as defined by the Structure Commission of the International Zeolite Association.

(28) “Hydrogenation-function metal for hydrocarbon conversions” is intended to mean a metal that can catalyze hydrogenation reaction in hydrocarbon conversion processes.

(29) “Selective Catalytic Reduction” or “SCR” refers to the reduction of NO.sub.x (typically with ammonia, ammonia generating compound such as urea, or hydrocarbon) in the presence of oxygen to form nitrogen and H.sub.2O. The reduction is catalyzed to preferentially promote the reduction of NO over the oxidation of ammonia by the oxygen, hence “selective catalytic reduction.”

(30) The phrases “chosen from” or “selected from” as used herein refer to selection of individual components or the combination of two (or more) components. For example, the phrase “the transition metal may be chosen from copper and iron” means that the metal may comprise copper, or iron, or a combination of copper and iron.

(31) Preparation of a small crystal ferrierite according to the present disclosure is largely but not solely dependent on the precursor gel composition. In one embodiment, the presence of two organic structure directing agents, or templates, is an important condition for controlling the ferrierite crystal size. Examples of silica sources include, but are not limited to, sodium silicate, colloidal silica and precipitated silica. Examples of alumina sources include, but are not limited to, aluminum isopropoxide, sodium aluminate, and a combination of alumina and sodium hydroxide. Tetramethylammonium (“TMA”) cation is used in the form of hydroxide or salt, such as chloride, bromide or iodide.

(32) In one embodiment, the second template can be pyrrolidine, 1,3-diaminopropane, piperidine, pyridine, 1-methylpyrrolidine, ethylene diamine, 1,4-diaminobutane or any organic structure directing agent (“OSDA”) that promotes crystallization of ferrierite zeolite. Ferrierite seed crystals are optionally added to the mixture to further assist with the rate of crystallization.

(33) In one embodiment, the sources of silica and alumina, water, sodium hydroxide, templates, and optionally seeds are combined together and the resulting gel is thoroughly mixed.

(34) The overall composition of the gel is described as:
(10-60)SiO.sub.2:Al.sub.2O.sub.3:(0.5-5.6) Na.sub.2O:(0.5-25)TMA:(1-25)SDA:(80-1000)H.sub.2O where TMA denotes tetramethylammonium, and SDA denotes the second template molecule, such as pyrrolidine, 1,3-diaminopropane, piperidine, pyridine, 1-methylpyrrolidine, ethylene diamine, 1,4-diaminobutane or any OSDA suitable for synthesis of ferrierite zeolite.

(35) The morphology of ferrierite crystals may change depending on the ratio between the two templates. The molar ratio between TMA and the second template can range from about 0.20 to about 1, such as from about 0.25 to about 0.80, or even from about 0.30 to about 0.70. The molar ratio of the second template relative to silica can be chosen from a reasonably wide range, such as from about 0.02 to about 0.80, or even from about 0.10 to about 0.40. The ratio of hydroxide (“OH”) to SiO.sub.2 is also an important factor for controlling the size and shape of the crystals. In one embodiment, the ratio of OH to SiO.sub.2 can range from about 0.15 to about 0.30. Lower OH/SiO.sub.2 ratios may result in undesired crystal size, while higher ratios may lead to the formation of impurities, such as ZSM-5, quartz or tridymite.

(36) The OH/SiO.sub.2 ratio may be controlled by using sodium hydroxide or its combination with tetramethylammonium hydroxide. When sodium hydroxide alone is used for controlling the OH/SiO.sub.2 ratio, the Na.sub.2O/SiO.sub.2 ratio typically ranges from about 0.05 to about 0.14, such as from about 0.07 to about 0.13. This ratio can be somewhat decreased if sodium hydroxide is used in combination with tetramethylammonium hydroxide.

(37) The gels are heated under autogenous conditions at temperatures ranging from 100 to 250° C., preferably from 120 to 200° C. The ferrierite phase typically crystallizes within 48 hours of heating.

(38) Ferrierite seeds can be added to the gel to decrease the crystallization time. In one embodiment, a ferrierite zeolite, for example CP 914C (Zeolyst International), can be used for seeding.

(39) In one embodiment, the resulting ferrierite product is pure ferrierite and does not contain any other zeolitic or condensed silica impurities. The ferrierite has a surface area of at least about 350 m.sup.2/g, or even at least about 380 m.sup.2/g. The ferrierite crystals have an irregular shape. The average crystal size along all crystal dimensions is about or less than 200 nm as determined by Scanning Electron Microscopy (“SEM”).

(40) The resulting ferrierite can be converted to NH.sub.4-form, H-form, or loaded with metals either by impregnation or ion-exchange, for catalytic conversions of hydrocarbons, selective catalytic reduction of nitrogen oxides, or catalytic decomposition of nitrous oxide.

(41) In one embodiment, the ferrierite may be used as an inventive catalyst in a method for SCR of exhaust gases, such as by contacting the gas mixture with the ferrierite composition described herein.

(42) Embodiments of the invention will be further clarified by the following non-limiting examples, which are intended to be purely exemplary of the invention.

EXAMPLES

Example 1

(43) This example illustrates the synthesis of a small crystal ferrierite according to one embodiment of the present invention. The molar composition of the gel was as follows:
23.4SiO.sub.2:1.0Al.sub.2O.sub.3:2.71Na.sub.2O:1.8TMA:3.5 1,3-DAP:304H.sub.2O

(44) 485 grams of water was combined with 28.9 grams of sodium hydroxide (50% w/w solution), 74.1 grams of sodium aluminate solution (23.5% wt. Al.sub.2O.sub.3, 19.6% wt. Na.sub.2O) to form an aqueous solution. 65.6 grams of tetramethylammonium chloride solution (50% w/w, Sachem) and 44.8 grams of 1,3-diaminopropane (“1,3-DAP”) (Sigma Aldrich) were mixed in to the aqueous solution. Finally, 600 grams of silica sol (40% wt. SiO.sub.2, Nyacol) and 2.7 grams of CP 914C seeds (Zeolyst International) were added, and the resulting gel was stirred until it became homogenous.

(45) The gel was heated at 180° C. for 36 hours. The resulting crystalline product was filtered, washed with deionized water and dried in air at 105° C. The resulting zeolite powder was calcined at 550° C. for 6 hours to remove the organic molecules from the pores. The calcined sample had a surface area of 395 m.sup.2/g and micropore volume of 0.14 cc/g, as determined by nitrogen adsorption. The sample was found to be pure phase ferrierite. The X-ray diffraction pattern of the sample is shown in FIG. 1. The crystal size of the sample was about 100 nm or less, as shown in the SEM image of FIG. 2.

Example 2

(46) This example describes the synthesis of a small crystal ferrierite material from a gel having a lower H.sub.2O/SiO.sub.2 ratio than that of Example 1. The molar composition of the gel was as follows:
23SiO.sub.2:1.0Al.sub.2O.sub.3:2.07Na.sub.2O:0.96TMA:2.88 1,3-DAP:230H.sub.2O

(47) 320 grams of water was combined with 12.0 grams of sodium hydroxide aqueous solution (50% w/w), 94.5 grams of sodium aluminate solution (23.5% wt. Al.sub.2O.sub.3, 19.6% wt. Na.sub.2O) to form an aqueous solution. 41.9 grams of tetramethylammonium chloride solution (50% w/w, Sachem) and 42.9 grams of 1,3-diaminopropane (“1,3-DAP”) (SigmaAldrich) were mixed in to the aqueous solution. Finally, 700 grams of silica sol (39.4% wt. SiO.sub.2, Nyacol 1440) and 3.1 grams of CP 914C seeds (Zeolyst International) were added, and the resulting gel was stirred until it became homogenous.

(48) The gel was heated at 180° C. for 36 hours. The resulting crystalline product was filtered, washed with deionized water and dried in air at 105° C. The resulting zeolite powder was calcined at 550° C. for 6 hours to remove the organic molecules from the pores. The calcined sample had a surface area of 406 m.sup.2/g and micropore volume of 0.14 cc/g, as determined by nitrogen adsorption. The sample was found to be pure phase ferrierite. The X-ray diffraction pattern of the sample is shown in FIG. 3. The crystal size of the sample was about or less than about 100 nm, as shown in the SEM image of FIG. 4.

Example 3

(49) This example describes the synthesis of a small crystal ferrierite material having a higher SAR than that of Example 1. The molar composition of the gel was as follows:
35SiO.sub.2:1.0Al.sub.2O.sub.3:3.50Na.sub.2O:1.75TMA:5.25 1,3-DAP:350H.sub.2O

(50) 391 grams of water was combined with 42.3 grams of sodium hydroxide (50% w/w solution), 64.3 grams of sodium aluminate solution (23.5% wt. Al.sub.2O.sub.3, 19.6% wt. Na.sub.2O) to form an aqueous solution. 58.3 grams of tetramethylammonium chloride solution (50% w/w, Sachem) and 59.8 grams of 1,3-diaminopropane (“1,3-DAP”) (SigmaAldrich) were mixed in to the aqueous solution. Finally, 800 grams of silica sol (40% wt. SiO.sub.2, Nyacol) and 3.4 grams of CP 914C seeds (Zeolyst International) were added, and the resulting gel was stirred until it became homogenous.

(51) The gel was heated at 160° C. for 48 hours. The resulting crystalline product was filtered, washed with deionized water and dried in air at 105° C. The resulting zeolite powder was calcined at 550° C. for 6 hours to remove the organic molecules from the pores. The calcined sample had a surface area of 382 m.sup.2/g and micropore volume of 0.14 cc/g, as determined by nitrogen adsorption. The sample was found to be pure phase ferrierite. The X-ray diffraction pattern of the sample is shown in FIG. 5. The crystal size of the sample was about or less than about 100 nm, as shown in the SEM image of FIG. 6.

Example 4

(52) This example describes the synthesis of a small crystal ferrierite material having a higher SAR and a different source of silica than those of Example 1. The molar composition of the gel was as follows:
35SiO.sub.2:1.0Al.sub.2O.sub.3:3.50Na.sub.2O:1.75TMA:5.25 1,3-DAP:350H.sub.2O

(53) 724 grams of water was combined with 45.3 grams of sodium hydroxide (50% w/w solution), 56.7 grams of sodium aluminate solution (23.5% wt. Al.sub.2O.sub.3, 19.6% wt. Na.sub.2O) to form an aqueous solution. 50.6 grams of tetramethylammonium chloride solution (50% w/w, Sachem) and 51.9 grams of 1,3-diaminopropane (1,3-DAP) (SigmaAldrich) were mixed in to the aqueous solution. Finally, 300 grams of precipitated silica (HiSil 233, PPG Industries) and 3.0 grams of CP 914C seeds (Zeolyst International) were added, and the resulting gel was stirred until it became homogenous.

(54) The gel was heated at 160° C. for 48 hours. The resulting crystalline product was filtered, washed with deionized water and dried in air at 105° C. The resulting zeolite powder was calcined at 550° C. for 6 hours to remove the organic molecules from the pores. The calcined sample had a surface area of 405 m.sup.2/g and micropore volume of 0.14 cc/g, as determined by nitrogen adsorption. The sample was found to be pure phase ferrierite. The X-ray diffraction pattern of the sample is shown in FIG. 7. The crystal size of the sample was about or less than about 100 nm, as shown in the SEM image of FIG. 8.

Comparable Example 5

(55) This example illustrates a comparable method of ferrierite zeolite synthesis in the absence of tetramethylammonium cations. The molar composition of the gel was as follows:
23.4SiO.sub.2:1.0Al.sub.2O.sub.3:1.65Na.sub.2O:2.34 1,3-DAP:304H.sub.2O

(56) 535 grams of water, 74.1 grams of sodium aluminate solution (23.5% wt. Al.sub.2O.sub.3, 19.6% wt. Na.sub.2O), and 29.9 grams of 1,3-diaminopropane were combined together to form an aqueous solution. Finally, 600 grams of silica sol (40% wt. SiO.sub.2, Nyacol) and 2.7 grams of CP 914C seeds (Zeolyst International) were added, and the resulting gel was stirred until it became homogenous. The gel was heated at 180° C. for 36 hours. The sample was found to be pure phase ferrierite. The X-ray diffraction pattern of the sample is shown in FIG. 9. Plate-like ferrierite crystals having a particle size of about 300 nm to about 1000 nm were formed, as shown in the SEM image of FIG. 10.

Comparable Example 6

(57) This example illustrates a comparable ferrierite zeolite synthesis. The molar composition of the gel was as follows:
23.4SiO.sub.2:1.0Al.sub.2O.sub.3:1.65Na.sub.2O:1.8TMA:3.5 1,3-DAP:304H.sub.2O

(58) 503 grams of water was combined with 74.1 grams of sodium aluminate solution (23.5% wt. Al.sub.2O.sub.3, 19.6% wt. Na.sub.2O) to form an aqueous solution. 65.6 grams of tetramethylammonium chloride solution (50% w/w, Sachem) and 44.8 grams of 1,3-diaminopropane (“1,3-DAP”) (SigmaAldrich) were mixed in to the aqueous solution. Finally, 600 grams of silica sol (40% wt. SiO.sub.2, Nyacol) and 2.7 grams of CP 914C seeds (Zeolyst International) were added, and the resulting gel was stirred until it became homogenous.

(59) The gel was heated at 180° C. for 36 hours. The resulting crystalline product was filtered, washed with deionized water and dried in air at 105° C. The resulting zeolite powder was calcined at 550° C. for 6 hours to remove the organic molecules from the pores. The calcined sample had a surface area of 392 m.sup.2/g and micropore volume of 0.14 cc/g, as determined by nitrogen adsorption. The sample was found to be pure phase ferrierite. The X-ray diffraction pattern of the sample is shown in FIG. 11. The crystal size of the sample was between about 300 nm and about 1000 nm, as shown in the SEM image of FIG. 12.

Comparable Example 7

(60) This example illustrates a comparable method of ferrierite zeolite synthesis. The molar composition of the gel was as follows:
25SiO.sub.2:1.0Al.sub.2O.sub.3:3.35Na.sub.2O:3.75Pyrrolidine:325H.sub.2O

(61) 544 grams of water, 69.4 grams of sodium aluminate solution (23.5% wt. Al.sub.2O.sub.3, 19.6% wt. Na.sub.2O) and 42.6 grams of pyrrolidine were combined together. Finally, 600 grams of silica sol (40% wt. SiO.sub.2, Nyacol) and 2.6 grams of CP914C seeds (Zeolyst International) were added, and the resulting gel was stirred until it became homogenous. The gel was heated at 170° C. for 36 hours. The sample was found to be pure phase ferrierite. The X-ray diffraction pattern of the sample is shown in FIG. 13. Ferrierite crystals having a particle size of about 300 nm to about 1500 nm size were formed, as shown in the SEM image of FIG. 14.

Comparable Example 8

(62) This example describes the synthesis of ferrierite zeolite in the absence of tetramethylammonium cations and having a higher Na.sub.2O/SiO.sub.2 ratio than that of Example 5. The molar composition of the gel was as follows:
23.4SiO.sub.2:1.0Al.sub.2O.sub.3:2.71Na.sub.2O:3.51 1,3-diaminopropane:304H.sub.2O

(63) 604 grams of water was combined with 33.7 grams of sodium hydroxide (50% w/w solution), 86.4 grams of sodium aluminate solution (23.5% wt. Al.sub.2O.sub.3, 19.6% wt. Na.sub.2O) to form an aqueous solution. 52.3 grams of 1,3-diaminopropane (“1,3-DAP”) (SigmaAldrich) was mixed in to the aqueous solution. Finally, 700 grams of silica sol (40% wt. SiO.sub.2, Nyacol) and 3.0 grams of CP 914C seeds (Zeolyst International) were added, and the resulting gel was stirred until it became homogenous. The gel was heated at 180° C. for 36 hours. The resulting sample was found to be pure phase ferrierite. The X-ray diffraction pattern of the sample is shown in FIG. 15. Ferrierite having plate-like crystals of about 300 nm to about 500 nm in size were formed, as shown in the SEM image of FIG. 16.

Comparable Example 9

(64) This example illustrates the synthesis of a ferrierite material from a gel having a composition similar to that used for small crystal ferrierite synthesis, when the gel is allowed to crystallize in the absence of stirring:
23SiO.sub.2:1.0Al.sub.2O.sub.3:2.1Na.sub.2O:0.97TMA:2.89 1,3-DAP:230H.sub.2O

(65) 342 grams of water was combined with 13.6 grams of sodium hydroxide (50% w/w solution), 87.9 grams of sodium aluminate solution (23.5% wt.

(66) Al.sub.2O.sub.3, 19.6% wt. Na.sub.2O) to form an aqueous solution. 42.9 grams of tetramethylammonium chloride solution (50% w/w, Sachem) and 43.6 grams of 1,3-diaminopropane (“1,3-DAP”) (SigmaAldrich) were mixed in to the aqueous solution. Finally, 700 grams of silica sol (40% wt. SiO.sub.2, Nyacol) and 3.1 grams of CP 914C seeds (Zeolyst International) were added, and the resulting gel was stirred until it became homogenous.

(67) The gel was heated at 180° C. for 36 hours. The gel was stirred with an anchor blade at 150 rpm during the heat up. Then the crystallization proceeded in static mode. The resulting crystalline product was filtered, washed with deionized water and dried in air at 105° C. The resulting zeolite powder was calcined at 550° C. for 6 hours to remove the organic molecules from the pores. The sample was found to be pure phase ferrierite. The X-ray diffraction pattern of the sample is shown in FIG. 17. The sample had a crystal size between about 1 micron and about 2 microns, as shown in the SEM image of FIG. 18.

Comparable Example 10

(68) This example describes CP 914C commercial small crystal zeolite, which is a pure phase ferrierite (Zeolyst International). The X-ray diffraction pattern of the sample is shown in FIG. 19. The sample consisted of irregularly shaped crystals having a particle size of about 200 nm to about 300 nm, as shown in the SEM image of FIG. 20.

Comparable Example 11

(69) This example describes CP 914 commercial zeolite, which is a pure phase ferrierite (Zeolyst International). The X-ray diffraction pattern of the sample is shown in FIG. 21. The sample consisted of plate-like crystals having a particle size ranging from about 0.5 microns to about 2 microns, as shown in the SEM image of FIG. 22.

(70) Unless otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the present invention.

(71) Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with the true scope of the invention being indicated by the following claims.