Chapter 2
Tetrahedral and Octahedral Extraframework Aluminum in Ultrastable Y Zeolites Implications in the Cracking of Gas Oil 1
2
1
2
A. Corma , V. Fornés , A. Martińez , and J. Sanz Downloaded by TUFTS UNIV on October 15, 2014 | http://pubs.acs.org Publication Date: September 12, 1988 | doi: 10.1021/bk-1988-0375.ch002
1
Institute de Cataliśis y Petroleoquimica, CSIC, Serrano, 119, 28006-Madrid, Spain Instituto de Ciencia de Materiales, CSIC, Serrano, 115 bis, 28008-Madrid, Spain 2
27
I t i s shown by Al MAS-NMR t h a t , b e s i d e s t e t r a h e d r a l framework A l (~60 ppm), extraframework p e n t a c o o r d i n a t e d or t e t r a h e d r a l l y distorted A l (~30-40 ppm), and o c t a h e d r a l extraframework aluminum (~0 ppm), t e t r a h e d r a l A l i n amorphous silica-alumina is formed d u r i n g the dealumination of Y zeolite, e i t h e r by steam o r by SiCl4 treatment. This silica-alumina show characteristic h y d r o x y l groups a t 3600-3610 cm , w i t h a c i d s t r e n g t h higher than that o f zeolite framework h y d r o x y l s . By m i l d acid or b a s i c t r e a t m e n t s it i s p o s s i b l e t o change the p r o p o r t i o n o f the different t y p e s o f aluminum t h a t a f f e c t the c r a c k i n g o f gas-oil. I t has been found t h a t in h i g h l y s t e a m - d e a l u m i n a t e d HY zeolites a radical, c r a c k i n g mechanism i n c r e a s e s the f o r m a t i o n o f C and C on HY zeolites w i t h l e s s than 5 A l p e r u n i t cell. -1
1
2
D e s p i t e the f a c t t h a t many z e o l i t e s have been s y n t h e s i z e d i n the l a s t 20 y e a r s , z e o l i t e Y remains the a c t i v e z e o l i t i c component i n most commercial c r a c k i n g c a t a l y s t s . D i f f e r e n t p r o c e d u r e s can be used i n p r a c t i c e t o a c t i v a t e the zeolite, and the c h o i c e o f a p a r t i c u l a r method w i l l depend on the c a t a l y t i c c h a r a c t e r i s t i c s d e s i r e d . I f the main o b j e c t i v e i s t o p r e p a r e a v e r y a c t i v e c r a c k i n g c a t a l y s t , then a c o n s i d e r a b l e p e r c e n t a g e o f the sodium i s exchanged by r a r e e a r t h c a t i o n s . On the o t h e r hand, i f the main purpose i s t o o b t a i n g a s o l i n e w i t h a h i g h RON, u l t r a s t a b l e Y z e o l i t e s (USY) w i t h v e r y low Na c o n t e n t a r e p r e p a r e d . Then a s m a l l amount o f r a r e e a r t h c a t i o n s i s exchanged, but a c o n t r o l l e d steam d e a c t i v a t i o n s t e p has t o be i n t r o d u c e d i n the a c t i v a t i o n p r o c e d u r e t o o b t a i n a c o n t r o l l e d d e a l u m i n a t i o n o f the zeolite. T h i s p r o c e d u r e a c h i e v e s a h i g h t h e r m a l and h y d r o t h e r m a l s t a b i l i t y o f the z e o l i t e , p r o v i d e d t h a t s i l i c o n i s i n s e r t e d i n the vacancies left by e x t r a c t i o n o f A l from the framework {1) . The commercial c a t a l y s t s so o b t a i n e d have framework S i / A l r a t i o s i n the 0097-6156/88/0375-0017$06.00/0 β
1988
A m e r i c a n C h e m i c a l Society
In Fluid Catalytic Cracking; Occelli, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
Downloaded by TUFTS UNIV on October 15, 2014 | http://pubs.acs.org Publication Date: September 12, 1988 | doi: 10.1021/bk-1988-0375.ch002
18
FLUID CATALYTIC CRACKING: ROLE IN MODERN REFINING
range o f 5 t o 9. When t h e " f r e s h " c a t a l y s t i s i n t r o d u c e d i n a c r a c k i n g u n i t , f u r t h e r dealumination takes place during r e g e n e r a t i o n o f the c a t a l y s t . T h e r e f o r e , the z e o l i t e i n the e q u i l i b r i u m c a t a l y s t can have an average framework S i / A l r a t i o as h i g h as 80. I t i s p o s s i b l e t o e x t r a c t p a r t o f the extraframework aluminum (EFAL) g e n e r a t e d d u r i n g the a c t i v a t i o n p r o c e d u r e by a c h e m i c a l t r e a t m e n t , or even t o dealuminate the z e o l i t e , l e a v i n g p r a c t i c a l l y no EFAL on it ( 2 ) . However, the EFAL g e n e r a t e d d u r i n g c a t a l y s t r e g e n e r a t i o n accumulates i n the z e o l i t e o u t e r s u r f a c e ( 3 , 4 ) . T h i s extraframework aluminum can e i t h e r p l a y a p o s i t i v e r o l e such as c o n v e r t i n g bottoms, or can a c t n e g a t i v e l y , c a t a l y z i n g u n d e s i r e d r e a c t i o n s . T h e r e f o r e the f o r m a t i o n o f EFAL s h o u l d be performed i n such a way t h a t i t s p o s i t i v e e f f e c t s c o u l d be p r e s e r v e d , w h i l e the n e g a t i v e ones a r e avoided or minimized. I t s h o u l d be p o s s i b l e by u s i n g d i f f e r e n t d e a l u m i n a t i o n methods t o produce d i f f e r e n t EFAL l e v e l s as w e l l as c o n t r o l the n a t u r e o f the EFAL g e n e r a t e d . We have c h a r a c t e r i z e d HY z e o l i t e s dealuminated by d i f f e r e n t procedures and at different levels and have shown t h a t the proportion of the different t y p e s o f EFAL can be controlled. Furthermore, a l l the z e o l i t e m o d i f i c a t i o n s c l e a r l y i n f l u e n c e the product d i s t r i b u t i o n during g a s - o i l c r a c k i n g . Experimental Materials The s t a r t i n g NaY z e o l i t e was an SK-40 from Union C a r b i d e w i t h a framework S i / A l r a t i o o f 2.4. U l t r a s t a b l e HY z e o l i t e s (HYUS) were prepared by steam-calcination of partially ammonium exchanged z e o l i t e s a t a t m o s p h e r i c p r e s s u r e and 550-750 °C d u r i n g 3-20 h o u r s . A f t e r d e a l u m i n a t i o n t h e y were exchanged t w i c e w i t h an NH^ solution a t 80 °C f o r one hour and then c a l c i n e d a t 550 °C f o r 3 h o u r s . I n this way dealuminated samples c o n t a i n i n g l e s s t h a n 2% o f the o r i g i n a l Na were o b t a i n e d . One o f these (HYUS-8) was s u b j e c t e d t o d i f f e r e n t t r e a t m e n t s : (1) washed w i t h a s o l u t i o n o f c i t r i c a c i d o r HC1 (pH=3) a t 25 °C f o r one hour (samples HYUSAC and HYUS1, r e s p e c t i v e l y ) ; (2) washed w i t h a s o l u t i o n 0.1 M o f NaOH a t 40 °C for one hour (HYUSN), and (3) washed w i t h a 38% v/v s o l u t i o n o f a c e t y l a c e t o n e i n e t h a n o l a t 20 °C f o r 2 hours (HYUSA). The F Si(NH ) d e a l u m i n a t e d sample (HYF) was o b t a i n e d f o l l o w i n g the procedure d e s c r i b e d i n the patent l i t e r a t u r e ( 2 ) . Samples dealuminated w i t h S i C l ^ were p r e p a r e d f o l l o w i n g Ref. 5, w o r k i n g a t temperatures o f 400, 450 and 500 °C (HYD-T). The c h a r a c t e r i s t i c s of the z e o l i t e samples a r e g i v e n i n T a b l e I . The g a s - o i l was a vacuum g a s - o i l whose p h y s i c o c h e m i c a l p r o p e r t i e s a r e d e s c r i b e d elsewhere ( 6 ) . The u n i t c e l l c o n s t a n t o f t h e z e o l i t e s were determined by X-ray diffraction u s i n g CuKoc r a d i a t i o n and f o l l o w i n g ASTM procedure D-3942-80. The e s t i m a t e d s t a n d a r d d e v i a t i o n was t 0.01 A. The c r y s t a l l i n i t y o f the samples were c a l c u l a t e d by comparing the peak h e i g h t o f the (5,3,3) peak, and c o n s i d e r i n g the r e f e r e n c e NaY SK-40 as r e p r e s e n t i n g 100% c r y s t a l l i n i t y . The A l Magic Angle S p i n n i n g (MAS)-NMR s p e c t r a were o b t a i n e d i n a Bruker MSL-400 s p e c t r o m e t e r (\? =104.25 MHz). A 2 s pulse (30° degree) was used w i t h a r e p e t i t i o n time o f 2 s between p u l s e s i n o r d e r t o a v o i d s a t u r a t i o n e f f e c t s . Z i r c o n a c y l i n d r i c a l sample A1
In Fluid Catalytic Cracking; Occelli, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
CORMA ET AL.
19
Extraframework Al in Y Zeolites
TABLE I
Downloaded by TUFTS UNIV on October 15, 2014 | http://pubs.acs.org Publication Date: September 12, 1988 | doi: 10.1021/bk-1988-0375.ch002
STRUCTURAL AND CHEMICAL CHARACTERISTICS OF DEALUMINATED HY ZEOLITES
Zeolite
Si/Al
a
Si/Al
u.c
b
Al/u.c.
(Â)
Cryst %
HYUS-6
2.9
24. 48
5.9
28.0
85
HYUS-8
2.8
24. 42
8.1
21.1
90
HYUS-14
2.8
24. 34
14.2
12.6
90
HYUS-141
2.3
24. 24
141.0
1.4
70
HYD-400
4.0
24. 31
21.2
8.6
90
HYD-450
10.4
24. 22
—
~0
HYD-500
10.2
24. 20
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