AEROGEL CATALYST MODIFICATION TO IMPROVE CATALYTIC STABILITY DURING THE NITROXIDATION OF PROPYLENE

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AEROGEL CATALYST MODIFICATION TO IMPROVE CATALYTIC STABILITY DURING THE

NITROXIDATION OF PROPYLENE

M. Rahman, R. Willey, S. Teichner

To cite this version:

M. Rahman, R. Willey, S. Teichner. AEROGEL CATALYST MODIFICATION TO IMPROVE CAT-

ALYTIC STABILITY DURING THE NITROXIDATION OF PROPYLENE. Journal de Physique

Colloques, 1989, 50 (C4), pp.C4-7-C4-11. �10.1051/jphyscol:1989402�. �jpa-00229477�

REVUE DE PHYSIQUE APPLIQUÉE

Colloque C4, Supplément au n'4, Tome 24, Avril 1989 C4-7

AEROGEL CATALYST MODIFICATION TO IMPROVE CATALYTIC STABILITY DURING THE NITROXIDATION OF PROPYLENE

M. RAHMAN, R.J. WILLEY* and S.J. TEICHNER**

K.S.E., Amherst, Mass., U.S.A.

'Department of Chemical Engineering, Northeastern University, Boston, Mass., U.S.A.

**University Claude Bernard, Lyon I, F-69622 Vllleurbanne, France

Résumé - L'introduction de MgO ou de Fe3Û^ dans l'aérogel binaire NiO/Al2Û3 augmente très notablement la stabilité du catalyseur dans la nitroxydation du propylène.

Abstract - By introducing MgO or Fe3C>4 into a binary NiO/Al2C>3 aerogel the catalytic stability in the nitroxidation of propylene may be dramatically improved.

INTRODUCTION

The nitroxidation of propylene over nickel oxide alumina aerogel catalysts, the combination of nitric oxide and propylene to form acrylonitrile, was first reported by Zidan, et al. in 1977 /l/. Recently, during an investigation of the mechanism of this reaction /2/, attempts were made to understand the nature of deactivation which occurred and to understand how it may be controlled by aerogel catalyst modifications /3/. Work has continued to model the rate of deactivation and to quantify the deactivation process which will be discussed below.

The nitroxidation reaction is :

(1)

The product, acrylonitrile, is an important monomer used in further processing for the manufacturing of acrylic fibers, nylon-66, and several other polymeric compounds.

EXPERIMENTAL

Aerogel catalysts synthesis

The reference catalyst was a 1:1 atomic ratio of Ni to Al aerogel. The aerogel was formed by autoclaving a mixture of 5 % nickel acetate tetrahydrate in methanol and 12.5 % aluminum-tri-sec-butoxide in sec-butanol. The procedure was as follows : a solution of 12.5

% aluminum-tri-sec-butoxide in sec-butanol was carefully made up and capped. A second solution of 5 % nickel acetate tetrahydrate in methanol was made up. Additional water, required for the hydrolysis of the alumina, was added to the second solution.

Then the proper amount of the second solution was rapidly .added to first solution which would give a resultant atomic ratio of Ni to Al of 1:1. An aluminum hydroxide precipitate formed immediately while the Ni ions remained in solution (the alcogel). This mixture was then placed into an autoclave and supercritically dried. The Ni ions deposited onto the alumina during the drying process. The final product was greyish black. X-ray and Raman analysis showed the composition to be amorphous alumina, nickel oxide and a surface nickel ions.

Other atomic ratios of Ni to Al investigated were 0.05, 0.2, 0.4, 0.6, and 0.86. Another modification examined in the preparation of the 0.86 Ni to Al aerogel was the addition of ammonia to solution 2 before addition to solution 1. The color turned from green to blue for this solution, and the resultant aerogel had higher surface area of 426 m^/g versus 360 m^/g for the 0.86 material without ammonia. Further, evaluation of this aerogel showed tremendous activity for the cracking of propylene to methane with no activity for nitroxidation. Electron micrographs of this catalyst after testing showed a "peppered"

surface indicating the agglomeration of Ni atoms into clusters. These agglomerations were not seen on the reference catalyst.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989402

F u r t h e r a e r o g e l modiErcations i n v e s t i g a t e d were t h e s u b s t i t u t i o n of magnesium o r i r o n f o r a p o r t i o n of t h e n i c k e l c o n t e n t . S p e c i f i c compositions made were 0.2 Mg:0.8 N i : l . O A 1 and 0.2 Fe:0.8 N i : l . O .

The magnesium and i r o n s o l u t i o n s used i n t h e a l c o g e l s s o l u t i o n s were 5 % magnesium a c e t a t e t e t r a h y d r a t e i n methanol and 5 % ' f e r r i c a c e t y l a c e t o n a t e i n methanol.

Experimental t e s t u n i t

The experimental t e s t u n i t c o n s i s t e d of a q u a r t z r e a c t o r , a r e a c t a n t mixture panel, and an a n a l y t i c a l s e c t i o n . The q u a r t z r e a c t o r was designed a s an up flow r e a c t o r c a t a l y s t bed. An a e r o g e l c a t a l y s t (0.2 g ) was placed between two q u a r t z f r i t t e d d i s k s (volume approximately 10 cm3) which kept t h e c a t a l y s t i n s i d e t h e q u a r t z r e a c t o r . The r e a c t o r was heated by a q u a r t z tube oven. A l l r e a c t i o n measurements f o r t h i s work were conducted a t a c a t a l y s t temperature of 410°C (+_ 2°C).

The r e a c t a n t gas feed mixture was 2 cclmin n i t r i c oxide, 18 cc/min propylene (CP g r a d e ) , and 10 cc/min helium (1:9:5 NO:C3H6:He). These flow r a t e s were c o n s t a n t f o r a l l e v a l u a t i o n s except i n t h e e v a l u a t i o n s where moisture was added when (0.2 Fe:0.8 N i : l . O A 1 and 1:l Ni:Al) helium was passed through a packed column w i t h water. The s a t u r a t e d helium was t h e n blended w i t h t h e feed mixture a t a r a t e of 10 cclmin. A l l a e r o g e l c a t a l y s t s r e c e i v e d s i m i l a r p r e t r e a t m e n t s of pure oxygen (30 cc/min) a t 410°C f o r 24 hours. T h i r t y minutes p r i o r t o a run, the oxygen flow was switched t o pure helium t o prevent any e x p l o s i o n s before s w i t c h i n g t o t h e propylene r e a c t a n t mixture.

Gas a n a l y s i s of t h e e f f l u e n t gas stream was done by two g a s chromatographs and a quadrupole mass spectrometer. F o r a n a l y s i s of hydrocarbons, a gas chromatograph equipped with a FID was used. I n i t i a l l y , t h e FID chromatograph was a P e r k i n Elmer Model 3920 w i t h a 118" O.D. 2 m column packed with Porapak QS. L a t e r a Hewlett-Packard Model 5890 g a s chromatograph was used. Oven temperature f o r t h e gas chromatographs was s e t i s o t h e r m a l l y a t 120°C. Sampling was done w i t h 1 cc g a s chromatograph sample loop placed i n l i n e with t h e e f f l u e n t r e a c t o r l i n e s . Attached t o t h e Hewlett-Packard chromatograph was a Hewlett Packard Model 3393-A i n t e g r a t o r .

For a n a l y s i s of n i t r i c oxide, oxygen, carbon d i o x i d e , and o t h e r i n o r g a n i c gases, an Antek Model 300 gas chromatograph w i t h TCD was used. This chromatograph had a 114'' O.D. 2.5 m column packed w i t h Porapak QS. I t was operated i s o t h e r m a l l y a t 12CPC. F u r t h e r d e t a i l s about t h e gas chromatograph o p e r a t i n g c o n d i t i o n s a r e presented i n 121.

D e a c t i v a t i o n a n a l y s i s

-

Relevant e q u a t i o n s

The m o d e l l i n g of t h e d e a c t i v a t i o n r e p o r t e d below can be d e s c r i b e d by a c o n c e n t r a t i o n independent d e a c t i v a t i o n s i t e balance : i. e.,

where S i s t h e c o n c e n t r a t i o n of a c t i v e s i t e s p e r gram of c a t a l y s t and kd i s t h e d e a c t i v a t i o n r a t e c o n s t a n t . The lower kd, t h e more s t a b l e t h e c a t a l y s t . Defining a c t i v i t y , a , a s t h e r a t i o of t h e r a t e a t any time ( t - t p ) t o t h e peak r a t e observed a t time t p and assuming t h a t r a t e i s p r o p o r t i o n a l t o t h e c o n c e n t r a t i o n of a c t i v e s i t e s t h e n t h e f o l l o w i n g a c t i v i t y e q u a t i o n r e s u l t s :

which i n t e g r a t e s t o :

where r i s t h e r a t e of a c r y l o n i t r i l e formation i n moles/g/sec and t d = ( t - t p ) i s t h e time a f t e r t h e peak a c t i v i t y was observed. t d = 0 i s t h e time when t h e r a t e of formation t o a c r y l o n i t r f l e e q u a l s t h e maximum. Thus, kd f o r v a r i o u s a e r o g e l s and run c o n d i t i o n s can be determined by l e a s t s q u a r e s a n a l y s i s f o r t h e r/rpeak vs. t d d a t a and q u a l i t a t i v e comparison

can be made.

R e s u l t s and d i s c u s s i o n

The e v a l u a t i o n of 1:l N i t o A 1 a e r o g e l c a t a l y s t f o r n i t r o x i d a t i o n of p r o p y l e n e is a s shown i n F i g . 1. F o c u s i n g on t h e t o p c u r v e i n F i g . 1 which r e p r e s e n t s r e s u l t s f o r t h e r e f e r e n c e c a t a l y s t , 1 : l N i t o A l , t h e f i r s t 20 m i n u t e s of t h e run shows a s u r f a c e o x i d a t i o n a d j u s t m e n t p e r i o d .

Because t h e a e r o g e l i s o x i d i z e d w i t h pure oxygen f o r a p e r i o d of 24 h o u r s . p r i o r t o t e s t i n g , t h e s u r f a c e c o n t a i n s a s u r p l u s of oxygen which o x i d i z e s p r o p y l e n e t o c a r b o n d i o x i d e . A s t h e r u n c o n t i n u e s , t h e c a t a l y s t i s r e o x i d i z e d by n i t r i c o x i d e which i n t r o d u c e s n i t r o g e n t o t h e s u r f a c e and p r o p y l e n e becomes more p r e f e r a b l y o x i d i z e d t o a c r y l o n l t r i l e . F u r t h e r , p a r t i a l o x i d i a t i o n i s p r e f e r r e d o v e r t o t a l o x i d a t i o n b e c a u s e t h e m i x t u r e i s s e v e r e l y oxygen l e a n ( 1 t o 9 f o r NO t o C3H6). P r o d u c t d i s t r i b u t i o n a f t e r 20 m i n u t e s was a p p r o x i m a t e l y 74 % a c r y l o n i t r i l e , 26 X a c e t o n i t r i l e , and 8 X c a r b o n d i o x i d e . P r o d u c t d i s t r i b u t i o n c o n t i n u e d t o improve t o v a l u e s of a p p r o x i m a t e l y 90 X a c r y l o n i t r i l e , 5 X a c e t o n i t r i l e , and 5 % c a r b o n d i o x i d e by t h e end of a 3 hour e v a l u a t i o n .

The d i s c o u r a g i n g r e s u l t was a 4 5 X d e c l i n e i n a c r y l o n i t r i l e p r o d u c t i o n o v e r t h e t h r e e h o u r t e s t p e r i o d from i t s peak v a l u e .

N i c k e l i s a known h y d r o c r a c k i n g c a t a l y s t and a p r o d u c t of c r a c k i n g i s s u r f a c e c a r b o n and c a r b o n f i l a m e n t s which grow o n t h e s u r f a c e . Carbon d e p o s i t i o n was confirmed i n p o s t t e s t i n g by p a s s i n g a 5 X oxygen i n h e l i u m m i x t u r e o v e r t h e c a t a l y s t and d e t e c t i n g c a r b o n d i o x i d e . F u r t h e r , c a t a l y s t e x a m i n a t i o n by e l e c t r o n m i c r o g r a p h s confirmed t h e p r e s e n c e of c a r b o n f i l a m e n t s w i t h m e t a l l i c n i c k e l c e n t e r s . T h e r e f o r e , d e c r e a s i n g n i c k e l c o n t e n t was t h e i n i t i a l a p p r o a c h i n v e s t i g a t e d f o r s t a b i l i t y improvement.

Rate X 10-8 g-mole/g sec

loo 0

0

^{I I I}

I

0 50 100 150 200

Time (mins.)

F i g u r e 1. R a t e o f F o r m a t i o n of A c r y l o n i t r i l e a s a F u n c t i o n of Time o v e r NiO/A1203 A e r o g e l C a t a l y s t s w i t h D i f f e r e n t N i c k e l t o Alumjnum Atomic R a t i o s .

The i n f l u e n c e of N i t o A 1 r a t i o o n c a t a l y s t s t a b i l i t y i s shown i n T a b l e 1 and F i g u r e 1 f o r 4 l e v e l s of N i t o A1 r a t i o s . F o r t h e r a t i o s of 0.2, 0.4, and 1.0 N i t o A l , p e r c e n t a c t i v i t y r e t a i n e d o v e r t h r e e h o u r s i s s i m i l a r a t 56 % , 59 %, and 58 X , r e s p e c t i v e l y and Q ranged from 0.142 t o 0.191 h r - l . F u r t h e r , t h e peak a c t i v i t y p e r gram of n i c k e l was s i m i l a r

f o r t h e t h r e e r a t i o s a t 2.52, 2.82, and 3.27 u m o l e s / g N i s e c . These r e s u l t s s u g g e s t t h e a c t i v e s i t e t h a t i s r e l a t e d t o N i O . However, e v a l u a t i o n of t h e 0.05 N i t o A1 r a t i o gave a d i f f e r e n t t r e n d . F i r s t i t s o v e r a l l a c t i v i t y was s u p e r i o r t o t h e 0.2 N i t o A 1 a e r o g e l and f u r t h e r i t n e a r l y matched t h e a c t i v i t y a c h i e v e d w i t h t h e 0.4 N i t o A1 a e r o g e l . I t s peak a c t i v i t y p e r gram of n i c k e l was 16.27 m o l e s / g N i s e c . The 0.05 N i t o A1 a e r o g e l had p o o r e r s e l e c t i v i t y , however. I t was n e v e r t h e l e s s a more s t a b l e m a t e r i a l r e t a i n i n g 69 % of i t s a c t i v i t y o v e r a 3 h o u r run. The r e s u l t s s u g g e s t t h a t t h e a c t i v e s i t e f o r n i t r o x i d a t i o n i s more t h a n j u s t NiO and t h a t t h e s u p p o r t , a l u m i n a , p l a y s a r o l e .

Influence of Mg and F e on c a t a l y s t s t a b i l i t y

An a e r o g e l c a t a l y s t was made w i t h 0.2 Mg s u b s t i t u t e d f o r 0.2 N i t o g i v e a r e s u l t a n t a e r o g e l composed of 0.2 Mg:0.8 N i : l . O A l . T h i s c a t a l y s t gave t h e b e s t s t a b i l i t y o v e r a t h r e e h o u r p e r i o d w i t h no l o s s i n a c t i v i t y o v e r t h r e e h o u r s .

O v e r a l l , t h e Mg c o n t a i n i n g a e r o g e l had t h e l o w e s t d e a c t i v a t i o n r a t e c o n s t a n t of 0.058 h r - l . The e x p l a n a t i o n of why magnesium improved t h e s t a b i l i t y i s a s f o l l o w s . C a t a l y t i c a c t i v i t y

f o r t h e c r a c k i n g of h y d r o c a r b o n s o c c u r s o v e r a l u m i n a - s i l i c a c a t a l y s t s , and i t h a s been shown t h a t t h i s a c t i v i t y i s d i r e c t l y r e l a t e d t o a c i d i t y l e v e l 141.

T h e a d d i t i o n o f b a s i c magnesium (MgO) n e u t r a l i z e s a c i d i t y due t o alumina and l o w e r s c r a c k i n g a c t i v i t y , t h e r f o r e i n c r e a s i n g s t a b i l i t y . T h u s , t h e d e a c t i v a t i o n p r o c e s s i s p a r t i a l l y r e l a t e d t o t h e a c i d i t y of t h e a e r o g e l .

A n o t h e r a e r o g e l c a t a l y s t was made w i t h 0.2 F e s u b s t i t u t e d f o r 0.2 N i t o g i v e a r e s u l t a n t a e r o g e l composed of 0.2 Fe:0.8 N i : r . O Al. T h i s c a t a l y s t ' s s t a b i l i t y was p o o r e r t h a n t h e Ni/Al a e r o g e l ( k d = 0.23 h r - l ) . The m o t i v e t o i n c l u d e i r o n i n t h e a e r o g e l c a t a l y s t was t o p r o v i d e a n a l t e r n a t i v e t o t h e Boudouard r e a c t i o n ( 2 CO + C ( s )

+

C 0 2 ) , a s o u r c e of s u r f a c e c a r b o n , by promoting a n a l t e r n a t i v e r e a c t i o n p a t h f o r CO v i a t h e w a t e r g a s s h i f t r e a c t i o n , CO

+

Hz0 = C02

+

Hz. F u r t h e r t e s t i n g i n which m o i s t u r e was added t o t h e f e e d showed a d r a m a t i c improvement i n k t a b i l i t y . kd d e c r e a s e d by a f a c t o r 4 t o 0.062 h r - l and t h e c a t a l y s t r i v a l s t h e s t a b i l i l i t y of t h e Mg c o n t a i n i n g a e r o g e l . However, i n comparing t h e peak a c t i v i t y between t h e magnesium and t h e i r o n c o n t a i n i n g a e r o g e l s , t h e magnesium ' a e r o g e l i s a b o u t a f a c t o r 2 b e t t e r .

T a b l e 1 : D e a c t i v a t i o n C o n s t a n t s f o r V a r i o u s A e r o g e l C a t a l y s t s I n v e s t i g a t e d f o r t h e N i t r o x i d a t i o n of P r o p y l e n e t o A c r y l o n i t r i l e .

C a t a l y s t Atomic M o i s t u r e R a t i o C o n t e n t o f t h e i n t h e M e t a l l i c R e a c t a n t Components ~ e e d l )

P e a k

~ c t i v i t ~ ~ ) x10-8

A c t i v i t y A f t e r 3 Hours x10-8

1 ) k pa

2) g mole/cm3/g/sec

3, t h e a c t i v i t y was o b s e r v e d f o r 48 h

S i n c e t h e a d d i t i o n of m o i s t u r e t o t h e f e e d s t r e a m improved t h e s t a b i l i t y o f i r o n c o n t a i n i n g a e r o g e l a n e v a l u a t i o n w i t h m o i s t u r e added was performed o n t h e r e f e r e n c e c a t a l y s t , 1 : l N i / A l . The r e s u l t s of t h i s m o d i f i c a t i o n was e n c o u r a g i n g w i t h t h e h i g h e s t peak r a t e o b s e r v e d f o r any of t h e a e r o g e l s i n v e s t i g a t e d and kd d e c r e a s i n g from 0.191 h r - l ( t a b l e 1) t o 0.105 h r - l .

Carbon monoxide u s e d a s a m o l e c u l a r probe

F u r t h e r e v i d e n c e t h a t t h e Boudouard r e a c t i o n i s t h e m a j o r s o u r c e o f d e a c t i v a t i o n was e s t a b l i s h e d by f l o w i n g a 5 % c a r b o n monoxide i n h e l i u m s t r e a m a c r o s s t h e 1 : l N i / A 1 a e r o g e l a t 410°C. The c a t a l y s t d e a c t i v a t e d and kd was 0.23 h r - l . T h i s v a l u e of t h e d e a c t i v a t i o n r a t e c o n s t a n t compares w i t h t h e v a l u e f o r i r o n c o n t a i n i n g a e r o g e l shown i n T a b l e 1. When a m i x t u r e of 5 % c a r b o n monoxide and 5 % n i t r i c o x i d e i n h e l i u m was flowed a c r o s s t h e c a t a l y s t t h e d e a c t i v a t i o n r a t e c o n s t a n t d e c r e a s e d by a f a c t o r of 2 t o 0.12 h r - l . The p r e s e n c e of n i t r i c o x i d e p r o v i d e d a n a l t e r n a t i v e p a t h f o r c a r b o n monoxide o x i d a t i o n i n which no c a r b o n was d e p o s i t e d on t h e s u r f a c e .

CONCLUSIONS

1. S t a b i l i t y of n i c k e l o x i d e alumina a e r o g e l c a t a l y s t s i s n o t d i r e c t 1 r e l a t e d t o N i c o n t e n t . The d e a c t i v a t i o n r a t e c o n s t a n t s , k d , r a n g e from 0.160 t o 0.191 hr-T.

2. The i n t r o d u c t i o n of magnesia i n t o t h e n i c k e l - o x i d e - a l u m i n a a e r o g e l c a t a l y s t improved t h e s t a b i l i t y , kd = 0.058 h r - l , by l o w e r i n g t h e a e r o g e l ' s s u r f a c e a c i d i t y .

3. The i n t r o d u c t i o n of i r o n i n t o t h e n i c k e l oxide-alumina a e r o g e l c a t a l y s t d e c r e a s e d s t a b i l i t y f o r a d r y f e e d m i x t u r e , kd = 0.230 hr-1, and improved s t a b i l i t y f o r a m o i s t f e e d m i x t u r e , k d = 0 . 0 6 1 hr-l. T h i s i s b e c a u s e a s u s p e c t e d p r o d u c t i n t e r m e d i a t e , c a r b o n monoxide, r e a c t s v i a t h e Boudouard r e a c t i o n i n t h e f i r s t c a s e d e p o s i t i n g s u r f a c e c a r b o n and r e a c t s v i a t h e w a t e r g a s s h i f t r e a c t i o n i n t h e second c a s e .

4. D e a c t i v a t i o n v i a t h e Boudouard r e a c t i o n was f u r t h e r s u b s t a n t i a t e d by f l o w i n g c a r b o n monoxide a c r o s s t h e c a t a l y s t and m e a s u r i n g a d e a c t i v a t i o n r a t e of 0.23 h r - l .

T h i s m a t e r i a l i s based upon work s u p p o r t e d by t h e N a t i o n a l S c i e n c e F o u n d a t i o n u n d e r G r a n t CPE 84-04340. The a u t h o r s a l s o w i s h t o acknowledge E . I . DuPont d e Nemours and Company and t h e Cabot F o u n d a t i o n f o r p a r t i a l f i n a n c i a l a s s i s t a n c e i n c a r r y i n g o u t t h i s p r o j e c t and IBM company f o r t h e IBM s c h o l a r fund.

REFERENCES

/ 1 / Z i d a n , F., P a j o n k , G.M., Germain, J.E. and T e i c h n e r , S.J., B u l l . Soc. Chim. F r . , ( 1 9 7 7 ) , 603.

/ 2 / Rahman, M., Ph.D. D i s s e r t a t i o n , N o r t h e a s t e r n U n i v e r s i t y , B o s t o n , MA 1988.

/ 3 / Rahman, M., W i l l e y , R . J . and T e i c h n e r , S.J., A p p l i e d C a t a l y s i s ,

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( 1 9 8 8 ) , 209.

/ 4 / S a t t e r f i e l d , C.N., H e t e r o g e n e o u s C a t a l y s i s i n P r a c t i c e , McGraw-Hill, 1980, 159.