Hexane Isomerization

42 Deactivation of Pd-H-Mordenite Catalyst during n-Hexane Isomerization D. J. CHICK, J. R. KATZER, and B. C. GATES

Downloaded by FUDAN UNIV on April 11, 2017 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0040.ch042

Department of Chemical Engineering, University of Delaware, Newark, Del. 19711

ABSTRACT Rates of reaction of n-hexane catalyzed by Pd-H-mordenite were measured at H partial pressures near 1 atm and temperatures near 250°C. The rate of cracking decreased sharply as the Pd content increased to about 0.7 wt%, whereas the rate of isomerization increased gradually with increasing Pd content. The rate of catalyst deactivation increased with the rate of cracking and the associated depostion of coke in the pore mouths. 2

Introduction Processes f o r p a r a f f i n i s o m e r i z a t i o n to g i v e g a s o l i n e components with h i g h octane numbers r e q u i r e a c a t a l y s t with h i g h a c t i v i t y and good s t a b i l i t y to take advantage o f the r e l a t i v e l y h i g h e q u i l i b r i u m conversions a t low temperatures (1). Such a process using a noble-metal-containing H-mordenite c a t a l y s t has been introduced commercially (2,_3,4_,5). The l i t e r a t u r e i n d i c a t e s that much i n d u s t r i a l r e s e a r c h has been devoted to c h a r a c t e r i z a t i o n of the a c t i v i t i e s of metalcontaining z e o l i t e catalysts i n this class. The n-C^ i s o m e r i z a t i o n a c t i v i t y (at 24-26 hr on stream) of RE-NH^-Y, f o r example, has been found to increase l i n e a r l y w i t h Pt (or Pd) content up to 0.4 wt% and then become constant (6). The Pd-H-Y c a t a l y s t has been reported to be more a c t i v e than Pt-H-Y (7); Pd-H-mordenite i s t y p i c a l l y found to be more a c t i v e than Pd-H-Y (18,^,10,11), but these two c a t a l y s t s have s i m i l a r a c t i v i t i e s i f t h e i r Na^ contents are extremely low (12). P a r a f f i n i s o m e r i z a t i o n a c t i v i t y of Hmordenite appears to be independent of noble metal content (9,13) or to i n c r e a s e somewhat w i t h metal i n c o r p o r a t i o n (8). Metal i n c o r p o r a t i o n reduces c r a c k i n g a c t i v i t y (8,9,12). C a t a l y s t s t a b i l i t y i s no l e s s important than a c t i v i t y , but the l i t e r a t u r e provides l i t t l e q u a n t i t a t i v e i n f o r m a t i o n about d e a c t i v a t i o n r a t e s . Q u a l i t a t i v e r e s u l t s show that m e t a l - f r e e Hmordenite d e a c t i v a t e s very r a p i d l y , and a p p l i c a t i o n of h i g h hydro515

Katzer; Molecular Sieves—II ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

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gen p a r t i a l pressures or a d d i t i o n of metal s t a b i l i z e s the c a t a l y s t (2,3,4,5,12,14). Slow d e a c t i v a t i o n has been reported f o r Pd-Y and f o r Pt-Y i n p a r a f f i n i s o m e r i z a t i o n (6,^7). The o b j e c t i v e of t h i s work was to p r o v i d e a q u a n t i t a t i v e c h a r a c t e r i z a t i o n of the Pd-H-mordenite i s o m e r i z a t i o n c a t a l y s t . The d e s i r e d data were r a t e s of n-hexane conversion to i s o m e r i z a t i o n and c r a c k i n g products and r a t e s of c a t a l y s t d e a c t i v a t i o n as a f u n c t i o n of Pd content, r e a c t i o n temperature, and r e a c t a n t compo sition. R e l a t i v e l y severe r e a c t i o n c o n d i t i o n s were chosen to g i v e high r a t e s of d e a c t i v a t i o n and allow experiments to be completed rapidly.


Experimental Methods Apparatus. C a t a l y s t pretreatment and r e a c t i o n were c a r r i e d out i n a c o n v e n t i o n a l continuous-flow, packed-bed m i c r o r e a c t o r (15). An aluminum c y l i n d e r surrounding the r e a c t o r and h e l d i n a thermostatted Lindberg e l e c t r i c furnace provided r e a c t i o n tempera tures which v a r i e d only ±0.2°C. Hydrogen and helium feeds flowed to the r e a c t o r through columns c o n t a i n i n g reduced copper c a t a l y s t and molecular s i e v e to remove Oo and ^ 0 . Normal-hexane flowed from a Sage s y r i n g e pump through a v a p o r i z e r to a tee where i t was mixed with hydrogen. The well-mixed vapor stream passed through a heated l i n e to the r e a c t o r . Oxygen f o r c a t a l y s t c a l c i n a t i o n flowed through a separate l i n e . Product vapors from the r e a c t o r passed through a heated l i n e to a heated gas sampling v a l v e , which allowed p e r i o d i c i n j e c t i o n of samples i n t o a gas chromatograph. A 2-m by 3.2-mm η-octane on P o r a s i l C column separated a l l the hydrocarbon products except CH^, which appeared w i t h H , and C j , which appeared with Q^. 2

Procedure. Z e o l i t e powder (0.1 to 0.7g) was weighed onto a 5-cm by 10-cm l a y e r of g l a s s wool, which was r o l l e d i n t o a 10-cm long c y l i n d e r and i n s e r t e d lengthwise i n t o the r e a c t o r . Glass wool f i l l e d the space downstream of the c a t a l y s t s e c t i o n , and 3-mm diameter g l a s s beads f i l l e d a 16-cm long preheater s e c t i o n up stream of the c a t a l y s t . The z e o l i t e was c a l c i n e d i n f l o w i n g 0 (^ 100 cm /min) as i t was heated from 25 to 346°C at 2°C/min then h e l d f o r 3.0 hr a t 346°C (16,17). The c a t a l y s t was cooled i n flowing O 2 , then i t was purged with He and reduced i n f l o w i n g hydrogen 60 cm /min) by h e a t i n g a t 2°C/min to 397°C and h o l d i n g f o r 3.1 h r . T y p i c a l run c o n d i t i o n s were as f o l l o w s : temperature, 247°C; pressure, 1.35 atm; hydrogen flow r a t e 0.039 moles/hr; and n-hexane feed r a t e , 0.0063 moles/hr ( g i v i n g a ^ i n - C ^ molar r a t i o of 6.1). The f i r s t sample was taken 10 min a f t e r i n i t i a t i o n of hexane flow; subsequent samples were taken at 35-min i n t e r v a l s , the time r e q u i r e d f o r a n a l y s i s . T o t a l run time was t y p i c a l l y 6 hr. 2

3

3

Materials.

A l l chemicals were at l e a s t of reagent

grade


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p u r i t y (15). C a t a l y s t s were prepared from H-mordenite (Zeolon 100 H powder f r o m N o r t o n Company; 0.35 w t % Na^O; S i 0 : A l 0 = 12.5). T h e m o r d e n i t e w a s c o n v e r t e d t o t h e NHT f o r m b y s l o w a d d i t i o n o f a q u e o u s 1 . 0 Ν NH^OH u n t i l t h e p H w a s 7 . 8 ( 1 8 ) . The s l u r r y was f i l t e r e d , a n d t h e z e o l i t e was w a s h e d t h e n d r i e d a t 1 0 2 ° C f o r 12 h r . The r e s u l t i n g b a s e m a t e r i a l was u s e d f o r p r e p a r a t i o n o f c a t a l y s t s c o n t a i n i n g 0 . 1 0 6 , 0 . 2 4 9 , 0 . 5 0 5 , 0.711 and 1.47 w t % P d . The m o r d e n i t e was s l u r r i e d i n a I N NH4NO3 s o l u t i o n h a v i n g a pH of 10. The d e s i r e d amount o f P d ( N H ) ^ C l (Matthey Bishop), d i s s o l v e d i n 1 Ν NH^NO^ s o l u t i o n , w a s a d d e d d r o p w i s e o v e r a 1 - h r p e r i o d t o t h e s t i r r e d s l u r r y ( 2 , 8 ) , t h e n t h e s l u r r y was h e l d f o r 96 h r b e f o r e f i l t e r i n g , t o a l l o w d i s t r i b u t i o n o f m e t a l t h r o u g h o u t the mordenite pore s t r u c t u r e . (Pd r e m o v a l f r o m t h e s o l u t i o n was c o m p l e t e w i t h i n a f e w m i n u t e s o f Pd a d d i t i o n , a s i n d i c a t e d b y atomic absorption analysis.) The P d - c o n t a i n i n g m o r d e n i t e was f i l t e r e d and washed u n t i l no C l ~ c o u l d be d e t e c t e d ; no Pd l o s s e s were observed. 2


3

a

2

3

2

D e t a i l s of the e x p e r i m e n t a l methods a r e complete compilation of the r e s u l t s (15).

given

elsewhere

with

Results I n t h e a b s e n c e o f c a t a l y s t , t h e r e was no h e x a n e c o n v e r s i o n at temperatures as h i g h as 350°C. When c a t a l y s t w a s p r e s e n t , a v a r i e t y o f c r a c k i n g and i s o m e r i z a t i o n p r o d u c t s were o b s e r v e d . For each product, a p l o t of conversion against inverse space v e l o c i t y gave a s t r a i g h t l i n e p a s s i n g through t h e o r i g i n , demon s t r a t i n g d i f f e r e n t i a l r e a c t o r behavior f o r t o t a l conversions of at l e a s t 15%. Consequently, the conversion data provided r e a c t i o n rates directly. F i g u r e 1 s h o w s how t h e t o t a l i s o m e r i z a t i o n r a t e depended on t i m e on s t r e a m and g i v e s an i n d i c a t i o n of r u n - t o - r u n r e p r o d u c i b i l i t y of the data. These r e s u l t s a r e s i m i l a r to those o b t a i n e d by p l o t t i n g r a t e s o f i s o m e r i z a t i o n and c r a c k i n g t o g i v e each of the i n d i v i d u a l products (15). Since the r a t e of each r e a c t i o n evidenced a n e a r l y exponen t i a l decrease w i t h t i m e , a l l the d a t a have been summarized i n a set of equations of the form t_ R

1 = R 1,0 e

1

[1]

This e x p r e s s i o n has been f r e q u e n t l y observed f o r d e a c t i v a t i n g c a t a l y s t s ( e . g . , 1 9 , 2 0 ) , and i t suggests t h a t the r a t e of d e a c t i v a t i o n was d i r e c t l y p r o p o r t i o n a l t o t h e number o f c a t a l y t i c s i t e s (21). E q . [1] was f i t t e d t o t h e r a t e d a t a f o r e a c h p r o d u c t w i t h l e a s t s q u a r e s l i n e a r r e g r e s s i o n , t h e n C h a u v e n e t ' s c r i t e r i o n (22) was u s e d t o r e j e c t d a t a p o i n t s d e v i a t i n g t o o f a r f r o m t h e mean, a n d e x p r e s s i o n [1] was r e f i t t e d t o t h e r e m a i n i n g p o i n t s . The f u l l s e t o f r e s u l t s h a s b e e n s u m m a r i z e d i n t e r m s o f i n i t i a l r a t e (R^ ) , t h e t i m e c o n s t a n t f o r d e a c t i v a t i o n ( Y ^ ) , and d e v i a t i o n a t t h e 90% c o n f i d e n c e l i m i t s ; some o f t h e s e d a t a a p p e a r i n t h e a c c o m p a n y i n g Q



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f i g u r e s and t a b l e s , and a l l of them are given i n a t h e s i s (15). The r a t e of c r a c k i n g i n the presence of m e t a l - f r e e H-morden i t e was i n i t i a l l y h i g h and decreased r a p i d l y with time; f o r example, conversion of n-hexane at 247°C a f t e r 10 min was 60.6% (54.1% cracked and 5.7% isomerized); a f t e r 43 min, the t o t a l conversion had f a l l e n to 15.7% (11.6% cracked and 4.2% isomerized). In the presence of 0.106 wt% Pd-H-mordenite under the same c o n d i t i o n s , the t o t a l conversion of n-hexane a f t e r 10 min was 15.7% (9.9% cracked and 5.8% isomerized), and that a f t e r 40 min was 8.3%. For each of these c a t a l y s t s , the two e a r l i e s t p o i n t s f e l l s i g n i f i c a n t l y above the l i n e f i t t e d to the remaining p o i n t s , and the e a r l y p o i n t s were excluded from the f i n a l f i t t i n g , so that the e x t r a p o l a t e d i n i t i a l r a t e s are low f o r these two c a t a l y s t s . For c a t a l y s t s with higher Pd contents, no such systematic d e v i a t i o n from the general p a t t e r n was observed, and the e x t r a p o l a t e d i n i t i a l r a t e s are considered to be good estimates of the true values (Figure 1 ) . The data summarizing the i n i t i a l r a t e s of formation of each s a t u r a t e d c r a c k i n g product are shown i n F i g u r e 2. Each r a t e decreased markedly with i n c r e a s i n g c a t a l y s t Pd content up to 0.7 wt%, but i t d i d not decrease f u r t h e r with f u r t h e r i n c r e a s e s i n Pd content. In the presence of m e t a l - f r e e H-mordenite, a s i g n i f i c a n t amount of propylene was produced; with 0.106 wt% Pd-H-mordenite, a small amount was produced; and with higher Pd contents, no o l e f i n i c products were detected. The i n i t i a l r a t e data r e p r e s e n t i n g formation of each iso m e r i z a t i o n product are summarized i n F i g u r e 3. The r a t e of f o r mation of each product except 2,2-dimethylbutane g e n e r a l l y i n creased with i n c r e a s i n g Pd content, i n c o n t r a s t to the c r a c k i n g results. An i n c r e a s e i n Pd content exerted a g r e a t e r r e l a t i v e i n f l u e n c e on production of dimethylbutanes than methylpentanes, l e a d i n g to a marked s h i f t i n s e l e c t i v i t y toward the methylpentanes-dimethylbutanes e q u i l i b r i u m . The r a t i o of the t o t a l i s o m e r i z a t i o n r a t e to the t o t a l crack ing r a t e increased with i n c r e a s i n g Pd content (Figure 4 ) . Only f o r Pd contents greater than about 0.4 wt% was the t o t a l i s o m e r i z a t i o n r a t e g r e a t e r than the t o t a l c r a c k i n g r a t e . The r a t e s of d e a c t i v a t i o n represented by 1/Y^ f o r each of the i s o m e r i z a t i o n products are c o l l e c t e d i n F i g u r e 5, and the comparable data f o r the c r a c k i n g products are shown i n F i g u r e 6. The r a t e of d e a c t i v a t i o n f o r each c r a c k i n g and i s o m e r i z a t i o n r e a c t i o n decreased markedly w i t h i n c r e a s i n g c a t a l y s t Pd content up to 0.5-0.6 wt%, beyond which v a l u e l i t t l e f u r t h e r decrease and even a small i n c r e a s e occurred. The i n i t i a l r a t e s of d e a c t i v a t i o n observed with the m e t a l - f r e e H-mordenite and w i t h the 0.106 wt% Pd-H-mordenite were higher than i n d i c a t e d because the r a p i d i n i t i a l d e a c t i v a t i o n was not represented by the f i t t i n g technique; the d e v i a t i o n i s e s p e c i a l l y s i g n i f i c a n t f o r c r a c k i n g . The impor tant r e s u l t i s the s i m i l a r i t y i n d e a c t i v a t i o n behavior f o r crack ing and i s o m e r i z a t i o n r e a c t i o n s .


42.

CHICK ET A L .

Deactivation

of Pd-H-Mordenite

Catalyst

519

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CO CO LU


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Hexane Isomerization

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