Mechanism of Rapid Zeolite Crystallizations and Its Applications to

Chapter 33

Downloaded by UNIV OF MICHIGAN ANN ARBOR on May 18, 2016 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0398.ch033

Mechanism of Rapid Zeolite Crystallizations and Its Applications to Catalyst Synthesis Tomoyuki Inui Department of Hydrocarbon Chemistry, Faculty of Engineering, Kyoto University, Sakyo-ku, Kyoto 606, Japan

Rapid syntheses of zeolites were accomplished. For the hydrogelatinous gel, after nucleation of the crystal growth, the precursor was treated by temperatureprogrammed heating, and the crystallization time was markedly reduced. The seed materials were also effec tive for this type of zeolite synthesis. On the other hand, for the precipitated gel, combined treatments of milling and temperature-programmed heating were highly effective for the synthesis of small and uniform zeo lite materials within very short period of crystal growth. The zeolites prepared by the rapid crystalli zation method had better catalytic performance than that of catalysts prepared by conventional methods. Application of the method to the synthesis of a varie ty of zeolitic materials will be discussed. In the l a s t decade, shape s e l e c t i v e z e o l i t e s such as ZSM-5 (1^ 2) and ZSM-34. (^Vi) have been studied extensively. P a r t i c u l a r interest has been p a i d to these new types of c a t a l y s t s because o f t h e i r e x c e l l e n t s e l e c t i v i t y to g a s o l i n e or lower o l e f i n s y n t h e s i s from methanol. In general, however, these shape selective zeolites need a long c r y s t a l l i z a t i o n period during t h e i r preparation. According to the patent l i t e r a t u r e , preparation of ZSM-34 requires between 25 and 196 days at 100°C, and t h a t of ZSM-5 r e q u i r e s between 20 h and one week at a higher temperature f o r c r y s t a l l i z a t i o n i n hydrothermal conditions. Such a slow c r y s t a l l i z a t i o n method would have the f o l lowing disadvantages: 1) Extensive labor coupled with delay and expense. 2) Low r e p r o d u c i b i l i t y i n the properties of crystals formed. 3) Production of large crystals. The e f f e c t i v e d i f f u s i v i t y of these products would be low and unfavorable f o r t h e i r use i n c a t a l y t i c reactions. 4.) Decrease i n p u r i t y of c r y s t a l structure. Since large single crystals are e a s i l y obtained, the amount impurity i n a c r y s t a l can be reduced. However, i n the case o f c o n s e c u t i v e phase t r a n s f o r m a t i o n s (6), t h e p o s s i b i l i t y o f c o e x i s t i n g c r y s t a l 0097-6156/89/0398-0479$06.00A) o 1989 American Chemical Society Occelli and Robson; Zeolite Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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SYNTHESIS

s t r u c t u r e s i n c r e a s e s , and, t h e r e f o r e , the p u r i t y o f c r y s t a l structure would be lowered. Also, large crystals formed by a slow c r y s t a l l i z a t i o n process give a concentration p r o f i l e of the c r y s t a l components, and t h i s might be unfavorable f o r the use of c a t a l y t i c reactions.


Rapid c r y s t a l l i z a t i o n would overcome the disadvantages of slow c r y s t a l l i z a t i o n and, more s i g n i f i c a n t l y , hetero elements c o u l d be incorporated inside the crystals. Metal-incorporated z e o l i t i c mate r i a l s serve as b i f u n c t i o n a l catalysts, exhibiting properties of both metal catalysts and z e o l i t e catalysts. Experimental ZSM-34 p r e p a r a t i o n . Preparation of ZSM-34 z e o l i t e was based on the patent l i t e r a t u r e (j>). However choline was used instead of choline c h l o r i d e as the o r g a n i c template, on the b a s i s t h a t z e o l i t e s p r e pared by using choline have better performance f o r o l e f i n synthesis from methanol than zeolites prepared by using choline chloride (7). The r e a c t i o n m a t e r i a l s used were 30 wt% s i l i c a s o l s o l u t i o n , sodium aluminate, reagent grade sodium and potassium hydroxides, 50 wt% choline solution, and d i s t i l l e d water. Appropriate amounts of sodium hydroxide, potassium hydroxide, and sodium aluminate were dissolved i n d i s t i l l e d water. After adding the choline solution, the mixed aqueous s o l u t i o n was kept a t 0°C. In another c o n t a i n e r , the s i l i c a s o l s o l u t i o n was a l s o made a t 0°C. The two s o l u t i o n s were mixed quickly and s t i r r e d vigorously f o r 2 min using a Homo-Mixer (Tokushuki Kako Kogyo Co.). An almost t r a n s p a r e n t , h y d r o g e l a t i n o u s mixture was obtained. The g e l solution mixtures were packed i n ampoules and c r y s t a l l i z e d at 100°C f o r various periods from 0.25 to 131 days. The s o l i d products were washed w i t h water to pH = 9 and d r i e d o v e r n i g h t a t 100°C, then c a l c i n e d under passage o f a i r at 5Λ0°0 f o r 3-5 h. The molar r a t i o of S i / A l , (Na + K)/A1, K/(Na + K), and choline/OH" were f i x e d a t 9·3, 7.3, 0.17, and 0.67, r e s p e c t i v e l y . Based on the a n a l y s i s o f the slow c r y s t a l l i z a t i o n r a t e process, the hydrothermal treatment was properly shortened by a temperature-programmed crysta l l i z a t i o n . Furthermore, the e f f e c t s o f seed m a t e r i a l s , type of template, hydrothermal temperature, and S i / A l r a t i o o f the g e l mixture on the c r y s t a l l i z a t i o n were investigated. ZSM-5 p r e p a r a t i o n . P r e p a r a t i o n of ZSM-5 z e o l i t e by Method 1 was based on the method given i n the patent l i t e r a t u r e (2), i n order to give us an authentic sample. However, the temperature condition f o r c r y s t a l l i z a t i o n was m o d i f i e d . A r e a c t i o n mixture was prepared by mixing the f o l l o w i n g three s o l u t i o n s A, B, and C. In case of S i / A l atomic r a t i o 4-0, f o r example, s o l u t i o n A was composed o f 2.70 g o f A l ( S 0 y ) * l 6 * 18H 0, 6.20 g of H?S0y (97.5%), 7.53 g o f t e t r a p r o p y lammonium bromide (TPAB), and 60 g of d i s t i l l e d water. S o l u t i o n Β was prepared by d i s s o l v i n g 6 9 . 0 g of No.3-brand water g l a s s (28.93 wt% S i 0 , 9.28 wt% Na 0, balance H 0) i n Λ5 g of d i s t i l l e d water. Solution C was 130 g of 20 wt% NaCl aqueous solution. Solution A and Β were added dropwise to solution C i n a Pyrex 300 ml beaker while m a i n t a i n i n g a pH o f 9 ^ 11 a t room temperature and v i g o r o u s l y s t i r r i n g with an ultra-disperser. The reaction mixture was heated i n 2

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Occelli and Robson; Zeolite Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


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a one l i t e r autoclave to 160°C with a heating rate of 1°C/min with out s t i r r i n g and was kept at t h i s temperature f o r 2 h. The tempera ture was then r a i s e d to 190°C with a heating rate of 0.33°C/min and held at t h i s temperature f o r 5 h. The product was washed with water u n t i l no CI i o n s were d e t e c t e d , d r i e d at 120°C f o r 3 h, and then calcined i n an a i r stream at 540°C f o r 3.5 h. The material was then converted i n t o the ammonium form by repeated exchanges w i t h 1M ΝΗ,ΝΟ^ solution at 80°C f o r 1 h. The product was washed with water at room temperature, dried overnight at 100°C and then heated i n a i r at 540°C f o r 3.5 h. The r e s u l t i n g powder was compressed i n a tablet machine, and t h i s was crushed into 8 ^ 1 5 mesh pieces to provide the catalyst f o r the reaction. In Method 2, the reacting mixture was c r y s t a l l i z e d by r a i s i n g the temperature from 160 to 210°C at a constant h e a t i n g r a t e of 0.l6°C/min while s t i r r i n g at 60 rpm. In Method 3» a f t e r c e n t r i f u g a l s e p a r a t i o n of the g e l mixture from the mother liquor, the gel was mechanically ground i n a mortar for 15 min. This procedure was repeated twice and the f i n a l mortar i n g time was prolonged f o r another 15 min. The ground g e l mixture was returned to the mother liquor i n an autoclave. The c r y s t a l l i z a t i o n conditions were the same as f o r Method 2. In Method Λ, the change i n c o m p o s i t i o n of the mother l i q u o r during the p r e c i p i t a t i o n of Na20-(TPA)20-Al 0o-Si02-H20 s y n t h e s i s gel mixture was minimized by modification or the procedure. To the s o l u t i o n A, 11.95 g NaCl was added ( s o l u t i o n A )* and to the s o l u t i o n C, 2.16 g TPAB, U.32 g NaCl, 2.39 g NaOH, 1.80 g H S0, and 104 g HpO were added ( s o l u t i o n 0 )· The g e l mixture was prepared as i n Mefliod 1 w i t h the s o l u t i o n s A , B, C . A f t e r s e p a r a t i o n from the mother l i q u o r and g r i n d i n g as i n Method 3» the g e l mixture was returned to a d i f f e r e n t mother liquor which was separately formed by Method 3· The c r y s t a l l i z a t i o n condition were the same as f o r Method 2. When the crystals of d i f f e r e n t S i / A l were synthesized, only the amount of A l was changed. Various m e t a l l o s i l i c a t e s were prepared by Method 4» replacing A l by other t r a d i t i o n metals at the stage of g e l formation (8). 2

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Characterization of catalyst. BET-surface areas of the sample were measured by N2 adsorption at l i q u i d - n i t r o g e n temperature. Thermogravimetric analysis was carried out with a Shimadzu micro TG-DTA 30. X-ray d i f f r a c t i o n (XRD) p a t t e r n s of the z e o l i t e c a l c i n e d a t 540°C were obtained w i t h Cu Κα r a d i a t i o n u s i n g a Rigaku Denki G i g e r f l e x 2013 w i t h a wide source. Morphology of the samples were observed by a scanning electron microscope (SEM), Hitachi-Akashi MSM 102, w i t h a r e s o l u t i o n l i m i t of 70 A. Bulk composition of the sam p l e s was a n a l y z e d by a Shimadzu atomic adsorption/flame emission spectrophotometer (AA) AA-640-01 u s i n g a p p r o p r i a t e hollow cathode lamps f o r r e s p e c t i v e elements. Outer s u r f a c e c o m p o s i t i o n of the crystals was analyzed by a X-raymicroanalyzer (EMAX), Horiba EMAX1800 a t t a c h e d t o the scanning e l e c t r o n microscope. Temperatureprogrammed desorption (TPD) of NHo was measured by a continuous flow method with a Rigaku Denki micro TG-DSC. After adsorption of NH^ at 80°C, e l u t i o n by He was c a r r i e d out at the same temperature; the temperature was then r a i s e d to 750°C a t h e a t i n g r a t e of 20°C/min. TPD p r o f i l e s were obtained from d i f f e r e n t i a l s of the w e i g h t - l o s s curves.


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ZEOLITE SYNTHESIS

Catalytic reaction method. The methanol-conversion r e a c t i o n was carried out i n a ordinary flow reactor under atmospheric pressure. A 0.5 ml p o r t i o n of the c a t a l y s t was packed i n t o a Pyrex t u b u l a r r e a c t o r of 6 mm i n n e r diameter. The r e a c t i o n gas, composed of 20 ^ 100% MeOH balanced w i t h N , was then a l l o w e d to f l o w through the catalyst bed at a temperature i n the range 240 ^ 360°C and a space v e l o c i t y (SV) i n the range A00 ^ A000 l i t e r * l i t e r " h . The o l e f i n conversion reaction was carried out i n a flow reactor of 8 mm inner diameter. The r e a c t i o n gas, composed of an o l e f i n (C H^, C^H^, or C/Hg) and N mixed a t v a r i o u s r a t i o s , was then a l l o w e d to f l o w tnrough the c a t a l y s t bed at a temperature i n the range 260 ^ 360°C and a space v e l o c i t y i n the range 900 ^ 4500 h" . The p a r a f f i n - c o n v e r s i o n r e a c t i o n was c a r r i e d out i n a f l o w reactor of 8 mm inner diameter. The reaction gas composed of paraff i n (C-j n-C Q) and N mixed at v a r i o u s r a t i o s was f e d at a temperature range from 350 to 700°C and a space v e l o c i t y of 1000 ^ 8000 h" . Products were analyzed u s i n g TCD-FID, FID, and TCD type gas chroma to graphs equipped with integrators. 2

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Results and Discussion

Rapid Crystallization from a Hydrogelatinous Precursor of Crystals. Conventional preparation method. The mixed gel f o r c r y s t a l l i zation of ZSM-34 was hydrogelatinous as noted i n the experimental section. The change i n c r y s t a l morphology and BET surface area as a function of the c r y s t a l l i z a t i o n time at 100°C i s shown i n Fig.1 with the r e s u l t s of methanol conversion by these materials. The g e l mixture was converted into spherical p a r t i c l e s within 3 days. The shape and size of spherical p a r t i c l e s were maintained f o r about 3 months, but f o l l o w i n g t h a t the p a r t i c l e s became somewhat larger and irregular. The BET surface area decreased i n the f i r s t 3 days. This p e r i o d corresponds to the p e r i o d of s p h e r i c a l p a r t i c l e formation from the gel mixture. From the comparison of r e l a t i v e i n t e n s i t i e s of XRD p a t t e r n s , the samples at 0.5 and 1 day showed very poor c r y s t a l l i n i t y ; however, the sample at 3 days already showed the p r i n c i p a l XRD l i n e s of ZSM-34, although the r e l a t i v e i n t e n s i t i e s were s t i l l very weak. Of the materials prepared i n the f i r s t 3 days, dimethyl ether (DME) was formed e x c l u s i v e l y and i n c r e a s e d s h a r p l y w i t h an i n c r e a s e of the c r y s t a l l i z a t i o n time. A f t e r 3 days of c r y s t a l l i z a t i o n , the BET s u r f a c e area i n c r e a s e d s h a r p l y and then g r a d u a l l y approached i t s h i g h e s t l e v e l . C o n s i d e r i n g t h i s t r e n d , hydrocarbon f o r m a t i o n i n creased with decreasing DME formation. The XRD patterns of 16 days corresponded to the main c r y s t a l s t r u c t u r e s of ZSM-34 (5). T h i s p e r i o d corresponds to the p e r i o d of i n t e r g r o w t h of the z e o l i t e c r y s t a l i n each i n d i v i d u a l p a r t i c l e . After a very long c r y s t a l l i z a t i o n , such as 131 days, p a r a f f i n s e l e c t i v i t y i n c r e a s e d and o l e f i n s e l e c t i v i t y decreased. The most s i g n i f i c a n t o b s e r v a t i o n of these s e q u e n t i a l experiments i s t h a t d u r i n g the p e r i o d of the f i r s t 3 days, a l l the g e l mixture converted to s p h e r i c a l m a t e r i a l s , p r e c u r s o r s of the z e o l i t e c r y s t a l w i t h very l i t t l e i n t e r n a l s u r f a c e area. The c r y s t a l l i z a t i o n occurs i n each spherical p a r t i c l e , and the change i n BET surface area with an increase i n c r y s t a l l i z a t i o n time, suggests t h a t the r a t e of c r y s t a l l i z a t i o n i s r a p i d i n the e a r l y stage, but slows down with increasing c r y s t a l l i n i t y .


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F i g u r e 1. Change i n p r o p e r t i e s of ZSM-34 z e o l i t e w i t h an i n crease of c r y s t a l l i z a t i o n time. Conditions of MeOH conversion, 12% MeOH, 88% H 400°C, SV 930 h" . * 9 f

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D i r e c t h e a t i n g method. In order to confirm the change i n cryst a l l i z a t i o n process, the g e l mixture was heated d i r e c t l y to 200°C w i t h a constant h e a t i n g r a t e of 2.8°C*min~ , and then i t was maintained f o r 2 h. The r e s u l t i n g product contained sodalite with ZSM3U and the morphology was completely d i f f e r e n t from the c r y s t a l s prepared by the conventional method. As shown i n Fig. 2, the c a t a l y t i c a c t i v i t y of the z e o l i t e prepared by the d i r e c t h e a t i n g method f o r methanol c o n v e r s i o n was higher than that of the zeolite c r y s t a l l i z a t i o n f o r 25 days by the standard preparation method. However, deactivation of the catalyst by carbon deposit occurred early i n the reaction, just as with the catalyst prepared by the standard method. Differences i n c r y s t a l l i t e morphology between those prepared by the standard method and the d i r e c t heating method would be attributed to the stage of the precursor formation. Therefore, a f t e r the precursor formation the rapid heating was adopted as described below. P r e c u r s o r h e a t i n g method. The g e l mixture was maintained a t 100°C f o r 3 days f o r p r e c u r s o r f o r m a t i o n . The p r e c u r s o r w i t h the mother l i q u o r was transferred to autoclaves, and the temperature was r a i s e d at a constant r a t e of 1.7°C*min" to 130, 160, 190, and 220°C. The temperature was maintained at each l e v e l f o r 0.5 h. The synthesized materials were also treated i n the same manner as the standard preparation method. XRD patterns showed that the z e o l i t e s prepared at 190 and 220°C were ZSM-34; however, the z e o l i t e p r e pared a t 220°C c o n t a i n e d some s o d a l i t e s t r u c t u r e . The z e o l i t e s c r y s t a l l i z e d at 130 and 160°C had i n s u f f i c i e n t XRD i n t e n s i t y of ZSM-34 patterns and showed an a c t i v i t y of only DME formation. When the c r y s t a l l i z a t i o n temperature was raised to 190°C, DME decreased to ca. 1/10, and C^-Cy o l e f i n s increased dramatically. However, when the c r y s t a l l i z a t i o n temperature was raised to 220°C, ethylene format i o n decreased markedly and DME increased. As can be seen i n F i g . 2, the c a t a l y t i c a c t i v i t y of the z e o l i t e prepared at 190°C f o r 0.5 h i n the precursor heating method was the largest among those of the zeolites prepared by d i f f e r e n t methods. Furthermore, the s e l e c t i v i t y to valuable ethylene and the catalyst l i f e also increased markedly f o r the z e o l i t e prepared by the precursor heating method. Other f a c t o r s enhancing c r y s t a l l i z a t i o n rate. Kinds of organic template were changed f o r synthesis of ZSM-34. type z e o l i t e , and i t was found that tetramethylammonium hydroxide was the best template f o r the rapid c r y s t a l l i z a t i o n (1^0). The c r y s t a l l i z a t i o n was achieved i n o n l y 2 h a t 187°C, corresponding to 1/1600 of the time i n the standard method. In t h i s case, the amount of necessary template was only 1/8 of c h o l i n e hydroxide, producing the z e o l i t e which had a rather better c a t a l y t i c performance i n methanol conversion Seed c r y s t a l s , prepared by g r i n d i n g the preformed z e o l i t e i n an agate mortar, markedly reduced the c r y s t a l l i z a t i o n time (1^0, 11_). The comparative data f o r the z e o l i t e s with and without seed crystals are shown i n F i g . 3. The s i z e of z e o l i t e s formed was i n v e r s e l y r e l a t e d to the quantity of seed crystals added to the reagents of mixed gel formation. Zeolite c r y s t a l s produced i n the presence of 9 wt% seed crystals with a l l other conditions being i d e n t i c a l , were about 1/70 the volume of z e o l i t e c r y s t a l s formed i n the absence of seed cryst a l s and were more u n i f o r m i n shape. Whereas the change i n BET 1


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C i1-4 . Standard method

ZHC (C-mol/1)

(25 day)

Direct heating method(2 h) Precursor heating method (3 day + 0.5 h)

MeOH conversion , Product distribution (C-mol%)

Figure 2. Comparison of methanol conversion on various ZSM-34 zeolites prepared by d i f f e r e n t methods. Reaction conditions, 12% MeOH, 88% N , 400°C, SV 1000 h " . 1

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0 Seed crystals added (wt%) 100 MeOH conv. (%) 74.9 Selectivity of C2-C4 olefines (C-mol%) 25.9 Integrated amount of hydrocarbon formed (C-mol ) 1.8 Carbon in residual hydrocarbons (mol/I) 5.5 Catalyst l i f e (h) De (ΙΟ- cmVsec) 3.06 5

9.0 100 81.3 56.4 3.9 12.1 6.17

F i g u r e 3. E f f e c t of seed c r y s t a l s on the c r y s t a l l i z a t i o n and the c a t a l y t i c performance of methanol conversion.


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surface area was minor, the e f f e c t i v e d i f f u s i v i t y of n-hexane i n creased i n the z e o l i t e w i t h the seed c r y s t a l s (1J[). The z e o l i t e with the seed c r y s t a l s converted methanol to l i g h t o l e f i n s with higher s e l e c t i v i t y and longer c a t a l y s t l i f e than the z e o l i t e without seed crystal. Metal incorporation into the zeolite using metal loaded seed mate r i a l s . The c o m b i n a t i o n of c a t a l y s t m e t a l w i t h z e o l i t e c a t a l y s t i s one of the most i n t r i g u i n g subjects for b i f u n c t i o n a l c a t a l y s i s . The achievement of prominent effect of the seed c r y s t a l s on the c r y s t a l l i z a t i o n of ZSM-34. type c a t a l y s t induced an i d e a t h a t the seed material on which a c a t a l y s t metal had been supported previously would also be effective f o r rapid c r y s t a l l i z a t i o n . Instead of the seed z e o l i t e c r y s t a l s , a mixture of one part γ a l u m i n a ( 0.2 urn ) and t h r e e p a r t s α - a l u m i n a ( ca. 1 ym ) was used as seed m a t e r i a l . Fortunately, a s i m i l a r effect on the c r y s t a l l i z a t i o n was observed, although about 2 times the quantity of the seed m a t e r i a l s was necessary (2). The m e t a l p r e - l o a d e d γ and α - a l u m i n a mixture was then t r i e d as the seed material and confirmed the same effect on c r y s t a l l i z a t i o n . In the case of 0.4 wt% Ru (2) and 0.7 wt% Rh (1_2) i n the c a t a l y s t product a longer catalyst l i f e was obtained, i.e., 1.32 times and 1.65 times, r e s p e c t i v e l y , without any s i g n i f i cant change i n the a c t i v i t y and s e l e c t i v i t y compared w i t h the non metal-loaded z e o l i t e c a t a l y s t . Carbon dioxide, which corresponded to ca 2% of converted metha n o l , was observed i n the e f f l u e n t gas from the r e a c t o r . T h i s i n d i c a t e s t h a t a p a r t of d e p o s i t e d coke on the z e o l i t e s u r f a c e was burned w i t h the oxygen d e r i v e d from methanol by the a c t i o n of the metal component. The m e t a l component a l s o a c t e d as a combustion catalyst for coke during regeneration treatment with a i r . The effect of m e t a l l o a d i n g on the performance o f methanol c o n v e r s i o n was investigated. Of the four loading methods used [ion-exchange method, p h y s i c a l b l e n d i n g method, impregnation method and c r y s t a l l i z a t i o n n u c l e i method (12)], the c r y s t a l l i z a t i o n n u c l e i method exhibited the best performance. This new method allows the metal component to be h i g h l y d i s p e r s e d upon each z e o l i t e c r y s t a l l i t e , m a i n t a i n i n g the necessary properties of z e o l i t e , because the dispersed metal p a r t i cles are l i m i t e d to the part of seed material located at one of the ends of the rice-shaped c r y s t a l l i t e s . Rapid crystallization from a precipitated-gel precursor of crystals. In the course of p e n t a s i l z e o l i t e ZSM-5 synthesis, the g e l mixture obtained from s t a r t i n g aqueous s o l u t i o n i s a low density p r e c i p i tate, which i s different from the hydrogelatinous state i n ZSM-34 synthesis; i t can be e a s i l y separated from the supernatant f l u i d by centrifuge. Most of the s i l i c o n component feed has been already involved i n t h i s p r e c i p i t a t e , and c r y s t a l l i z a t i o n s occurs i n t e r n a l l y inside the s o l i d phase of the p r e c i p i t a t e . The p r e c i p i t a t e i t s e l f i s the pre cursor of the c r y s t a l . In f a c t , seed materials have no enhancement e f f e c t on the c r y s t a l growth as d e s c r i b e d above. Therefore, the f o l l o w i n g three hypotheses were considered and t r i a l s were made to substantiate them. Uniform crystals could be synthesized by regulating the composition


ZEOLITE SYNTHESIS

488

of starting solutions. In the conventional method of ZSM-5 synthesis (Method 1), the composition of l i q u i d phase markedly changes with an increase of p r e c i p i t a t i o n . In such a case, the composition and the p r o f i l e of components inside the crystals and t h e i r c a t a l y t i c performances c o u l d be a f f e c t e d . Therefore, i n order to s y n t h e s i z e c r y s t a l s having a more uniform composition, compositions of the s t a r t i n g solutions should be regulated to minimize the composition change during the p r e c i p i t a t i o n . A c c o r d i n g l y , the compositions of s o l u t i o n A and C were l a r g e l y r e g u l a t e d as the s o l u t i o n A and C used f o r method 4. However, the mother l i q u o r used during the hydrothermal treatment was the same as i n the patent l i t e r a t u r e (Method 1), because the mother l i q u o r was known to synthesize ZSM-5. Downloaded by UNIV OF MICHIGAN ANN ARBOR on May 18, 2016 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0398.ch033

1

!

Uniform and small c r y s t a l l i t e s could be rapidly synthesized by m i l l i n g of c r y s t a l precursor. The size and concentration p r o f i l e s of p r e c i p i t a t e d p a r t i c l e s a f f e c t e d not only the induction period and the r a t e of c r y s t a l l i z a t i o n , but a l s o the c r y s t a l s t r u c t u r e and d i s t r i b u t i o n of the a c i d sites. Accordingly, i f the c r y s t a l precursor i s as s m a l l and uniform as p o s s i b l e , the s y n t h e s i s of a very small and uniform c r y s t a l l i t e s and very rapid c r y s t a l l i z a t i o n would be expected. Such small p a r t i c l e s would play the role of n u c l e i of the c r y s t a l s . Therefore, i n t h i s study, the p r e c i p i t a t e d g e l was milled by a motor driven mortar. The l i q u i d phase separated from the g e l by the progress of m i l l i n g was removed t w i c e by c e n t r i f u g e (Method 3 ) .

Temperature programmed crystallization considering the rate process could minimize the c r y s t a l l i z a t i o n time. Most of the conventional hydrothermal syntheses are carried out at a constant temperature. As shown i n F i g . 4., c r y s t a l l i z a t i o n has an i n d u c t i o n p e r i o d and then rapid c r y s t a l l i z a t i o n occurs at constant temperature. However, the rate of c r y s t a l growth then slows down and gradually decreases with an i n c r e a s e of c r y s t a l l i z a t i o n time. A c c o r d i n g l y , much time i s consumed i n the l a t t e r stage of c r y s t a l l i z a t i o n i n which the r a t e has decreased. As zeolite crystals are a quasi-stable phase, those which have been formed i n the e a r l y stage are kept i n the hydrot h e r m a l c o n d i t i o n f o r a long time, and a l a c k of u n i f o r m i t y c o u l d result from transformation into other unfavorable phases. Therefore, i f the c r y s t a l l i z a t i o n rate can be keep at a high l e v e l by changing the temperature of hydrothermal synthesis (Fig. 5), i t i s expected that the c r y s t a l l i z a t i o n time w i l l be reduced and the properties of crystals w i l l be more uniform. In t h i s study, the c r y s t a l l i z a t i o n temperature and the heating r a t e were v a r i e d u s i n g the m i l l e d p r e c u r s o r , and ZSM-5 c r y s t a l s could be synthesized. For example, the temperature was elevated from 160 to 210°C with a constant heating rate of 0.2°C/min (Method 2). The c r y s t a l s prepared by Methods 1 ^ 4 . had about same BET-surface area of 385 ± 1 1 m /g and the XRD p a t t e r n s of ZSM-5. The average size of crystals reduced from 8 ym f o r Method 1 to 1 ym f o r Method 4.· The c o n c e n t r a t i o n p r o f i l e s of S i and A l from o u t s i d e to i n s i d e the c r y s t a l s became uniform w i t h r e d u c i n g s i z e . The a c t i v i t y of methanol conversion, the y i e l d of gasoline f r a c t i o n , and the content of aromatics i n the g a s o l i n e c l e a r l y i n c r e a s e d f o r the product of Method U ( F i g . 6 ) . Thus, i t was confirmed that the three control conditions men2


33. INUI

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Mechanism ofRapid Zeolite Crystallizations

100 220

Crystallinity

200 Crystallization rate


50 -

180

160

J 0 1

L 2

140 3

4

5 6

7

8

9

10

11 12

Crystallization time (day)

F i g u r e 4.. C o n c e p t i o n a l i l l u s t r a t i o n f o r the change of c r y s t a l l i z a t i o n r a t e and c r y s t a l l i n i t y with time on hydrothermal treatment a t a constant temperature i n a c o n v e n t i o n a l slowc r y s t a l l i z a t i o n method.

0

1

2 3 4 Crystallization time (h)

5

6

F i g u r e 5· C o n c e p t i o n a l i l l u s t r a t i o n f o r the change of c r y s t a l l i z a t i o n rate and c r y s t a l l i n i t y w i t h time on hydrothermal treatment i n the r a p i d - c r y s t a l l i z a t i o n method with a programmed temperature r i s e .



490

ZEOLITE SYNTHESIS

cr*c

2

C 4 C 5 Aroma. (a) Slow c r y s t a l l i z a t i o n method (b) Rapid c r y s t a l l i z a t i o n method 100

Hydrocarbon distribution (C-mol%) Figure 6· Performance of methanol conversion on ZSM-5 s prepared by the slow and the rapid c r y s t a l l i z a t i o n methods. (a) H-ZSM-5(Si/Al=40) prepared by the conventional slow cryst a l l i z a t i o n method. Reaction conditions, 30% MeOH-70% N , SV 1100 h " , 400°C, MeOH conversion 100% (b) H-ZSM-5(Si/Al=4O) prepared by the r a p i d c r y s t a l l i z a t i o n method (Method 4)· Reaction conditions, 30% MeOH-70% N , SV 1100 h " , 300°C, MeOH conversion 100% 1

1

2

1

2


33. INUI

Mechanism ofRapid Zeolite Crystallizations

491


tioned above were very e f f e c t i v e i n the rapid synthesis of c r y s t a l s and increasing the c a t a l y t i c a c t i v i t y . This indicates that the A l i s more highly dispersed i n the c r y s t a l s and forms of a c i d s i t e s more effectively. A p p l i c a t i o n o f t h e r a p i d c r y s t a l l i z a t i o n method t o syntheses o f various m e t a l l o s i l i c a t e s . V a r i o u s k i n d s o f m e t a l l o s i l i c a t e were synthesized by substituting the A l ingredient of the p e n t a s i l z e o l i t e ZSM-5 with other t r a n s i t i o n elements at the gel-formation stage i n the rapid c r y s t a l l i z a t i o n method. As one of the extreme features of the m e t a l l o s i l i c a t e s , H-Fe-silicates converted methanol exclusively to ethylene and pro pylene a t a low temperature c o n d i t i o n s around 300°C (13). T h i s i s attributed to the properly weaker a c i d i t y than H-ZSM-5, and conse quent conversion i s s u b s t a n t i a l l y eliminated. When the H - F e - s i l i c a t e i s used f o r c o n v e r s i o n of l i g h t o l e f i n s , i t y i e l d s a high octanenumber g a s o l i n e w i t h an e x t r a o r d i n a r y h i g h space-time y i e l d and s e l e c t i v i t y (1_£). Another extreme was H - G a - s i l i c a t e (8, 1J>). T h i s catalyst gave a maximum s e l e c t i v i t y to gasoline f r a c t i o n from metha nol and yielded aromatics from p a r a f f i n s with much higher s e l e c t i v i t y than H-ZSM-5. A combination of Pt with H-Ga-silicate enhanced the a c t i v i t y and markedly moderated the d e a c t i v a t i o n caused by coke f o r m a t i o n ; Pt was e f f e c t i v e f o r combustion of the d e p o s i t e d coke. The marked e f f e c t on a r o m a t i z a t i o n was a l s o confirmed f o r H-Zns i l i c a t e (J^). M e t a l l o s i l i c a t e s incorporating c a t a l y t i c a l l y active metal such as V - s i l i c a t e (V7) and C u - s i l i c a t e (1_8) could a l s o be prepared by the r a p i d c r y s t a l l i z a t i o n method. These novel c a t a l y t i c materials are expected as oxidation catalysts involving shape selective func tion. Conclusion In c o n c l u s i o n , the r a p i d c r y s t a l l i z a t i o n method i s very e f f e c t i v e not only f o r r a p i d s y n t h e s i s , but a l s o f o r s y n t h e s i s o f metalc o n t a i n i n g uniform z e o l i t i c materials which show higher c a t a l y t i c a c t i v i t y and s e l e c t i v i t y .

Literature cited 1. Meisel, S.L.; McCullough, J.P.; Lechthaler,C.H.Chemtech 1976, 86. 2. Plank, C.J.; Rosinski, E.J.; Schwartz, A.B. Brit. Patent 1 402 981, 1974. 3. Mobil Oil U.S. Patent 3 894 107, 1975. 4. Rubin, M.K.; Rosinski, E.J.; Plank, C.J. U.S. Patent 4 086 186, 1978, 5. Mobil Oil Jpn. Patent. Application Disclosure 58 499, 1978. 6. Kostinko, J.A. Preprints of Symposia of Am. Chem. Soc. Las Vegas, 1982, 27, No.2, 487. 7. Inui, T.; Araki, E.; Sezume, T.; Ishihara, T.; Takegami, Y. React. Kinet. Catal. Lett. 1981, 18, 1. 8. Inui, T.; Yamase, O.; Fukuda, K.; Itoh, Α.; Tarumoto, J.; Hagiwara, T.; Takegami, Y. Proc. 8th Intern Congr. Catal., Berlin 1984, DECHEMA, Frankfurt am Main, Vol.III, p.569.



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ZEOLITE SYNTHESIS

9. Inui, T.; Ishihara, T.; Morinaga, N.; Takeuchi, G.; Araki, Α.; Kanie, T.; Takegami, Y. Nippon Kagaku Kaishi 1982, 221. 10. Inui, T.; Morinaga, N.; Ishihara, T.; Kanie, T.; Takegami, I. Catal. 1983, 79, 176. 11. Inui, T.; Ishihara, T., Morinaga, N.; Takeuchi,G.;Matsuda, H.; Takegami, Y. I&EC Res. Dev. 1983, 22, 26. 12. Inui, T.; Takeuchi, G.; Takegami, Y. Appl. Catal. 1982, 4, 211. 13· Inui, T.; Matsuda, H.; Yamase,O.;Nagata, H; Fukuda,K.;Ukawa, T.; Miyamoto, A. J. Catal. 1986, 98, 491. 14. Inui, T. React. Kinet. Catal. Lett. 1987, 35, 227. 15. Inui, T.; Makino,Y.;Okazumi, S.; Nagano, S.; Miyamoto, A. Ind. Eng. Chem. Res. 1987, 26, 647. 16. Inui, T.; Makino, Y.; Okazumi, F.; Miyamoto, A. Stud. Surf. Sci. Catal. 1987, 37, 487. 17. Inui, T.; Medhanavyn, D.; Praserthdam, P.; Fukuda, K.; Ukawa, T.; Sakamoto, Α.; Miyamoto, A. Appl. Catal. 1985,18,311. 18. Inui, T.; Shibata, M; Okukawa, Y. Appl. Catal. to be submitted. RECEIVED

February 23, 1989


Mechanism of Rapid Zeolite Crystallizations and Its Applications to

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