Study of Fe/C and FeMo/C hydrodesulfurization catalysts :
preparational aspects and metal support interaction
Citation for published version (APA):
Ramselaar, W. L. T. M., Kraan, van der, A. M., & Beer, de, V. H. J. (1986). Study of Fe/C and FeMo/C
hydrodesulfurization catalysts : preparational aspects and metal support interaction. Hyperfine Interactions,
28(1-4), 895-898. https://doi.org/10.1007/BF02061588
DOI:
10.1007/BF02061588
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Published: 01/01/1986
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Hyperfine Interactions 28 (1986) 895-898
895
STUDY OF Fe/C AND FeMo/C
HYDRODESULFURIZATION CATALYSTS:
PREPARATIONAL ASPECTS A N D METAL SUPPORT
INTERACTION
W.L,T.M. RAMSELAAR and A.M, VAN DER KRAAN
Interuniversitair Reactor Instituut, 2629 JB Delft, the Netherlands
V.H,J,
DE BEER
Laboratory of Inorganic Chemistry and Catalysis, Eindhoven;
University of Technology, 5600 MB Eindhoven, the Netherlands
From the Mossbauer spectra of various oxidic catalysts precursors obtained at 4.2 K, 77 K and 300 K, the sequences in particle size and thermal motions relative to the support are established.
I. I N T R O D U C T I O N
H y d r o t r e a t i n g c a t a l y s t s are u s e d for the u p g r a d i n g of f o s s i l f u e l f r a c t i o n s . S e v e r a l p r o c e s s e s take p l a c e s i m u l t a n e o u s l y , s u c h as h y d r o c r a c k i n g , r e m o v a l of sulfur, n i t r o g e n , o x y g e n a n d m e t a l s , a n d h y d r o g e n a t i o n of o l e f i n s and a r o m a t i c h y d r o c a r b o n s . A t p r e s e n t , in- d u s t r i a l l y e m p l o y e d h y d r o t r e a t i n g c a t a l y s t s c o n s i s t of M o and ~ p r o - m o t e d w i t h Co or Ni d i s p e r s e d on a h i g h - s u r f a c e a r e a a l u m i n a s u p p o r t . T h e c a t a l y s t s are a c t i v e in the s u l f i d e d state. M ~ s s b a u e r e m i s s i o n s p e c t r o s c o p y has b e e n v e r y s u c c e s f u l in the c h a r a c t e r i z a t i o n of in- d u s t r i a l h y d r o t r e a t i n g c a t a l y s t s /1,2/. R e c e n t l y , c a r b o n s u p p o r t e d m e t a l s u l f i d e s h a v e a t t r a c t e d a t t e n - tion as f a v o u r a b l e h y d r o t r e a t i n g c a t a l y s t s /3/. In p a r t i c u l a r , i r o n s u l f i d e on c a r b o n w o u l d be a p o t e n t i a l l y a t t r a c t i v e c a t a l y s t , as iron is i n e x p e n s i v e a n d a b u n d a n t l y p r e s e n t . F o r F e / C catal~,sts p r e p - a r a t i o n a l a s p e c t s s u c h as the k i n d of c a r b o n - s u p p o r t a n d i r o n s a l t s o l u t i o n for i m p r e g n a t i o n , in r e l a t i o n to t h e i r c a t a l y t i c p r o p e r t i e s , are d e s c r i b e d by G r o o t /4/. In his s t u d y M ~ s s b a u e r s p e c t r o s c o p y has p l a y e d an i m p o r t a n t r o l e in c h a r a c t e r i z i n g the o x i d i c c a t a l y s t p r e - c u r s o r s . In a d d i t i o n , n o w w e p r e s e n t the i n f l u e n c e of the d r y i n g p r o c e d u r e a n d the s o l v e n t on the d i s p e r s i o n a n d r e l a t e d p a r t i c l e - s u p p o r t i n t e r a c t i o n s t u d i e d by M ~ s s b a u e r s p e c t r o s c o p y . 2. E X P E R I M E N T A N D R E S U L T S T h e c a r b o n - s u p p o r t e d c a t a l y s t p r e c u r s o r s are p r e p a r e d by p o r e v o l u m e i m p r e g n a t i o n of N o r i t R X 3 - e x t r a a c t i v e c a r b o n ( a u r f a c e a r e a I190 m 2 / g r ) . A n a q u e o u s s o l u t i o n c o n t a i n i n g the d e s i r e d a m o u n t of iron or m o l y b d e n u m is a d d e d to the c a r b o n s u p p o r t u n t i l the i n c i p i e n t w e t n e s s p o i n t is r e a c h e d . As s t a r t i n g m a t e r i a l s are u s e d F e ( N O 3 ) 3 . 9 H 2 0 (Merck, P . A , ) a n d ( N H 4 ) 6 M o 7 0 2 4 . 4 H 2 0 (Merck, > 99%). T h e i m p r e g n a t e d s a m p l e s are d r i e d at r o o m t e m p e r a t u r e in d i f f e r e n t w a y s , in o r d e r to s t u d y the i n f l u e n c e of the d r y i n g p r o c e d u r e . O p t i m u m F e - M o / c c a t a l y s t s a r e o b t a i n e d by a s e q u e n t i a l p o r e v o l u m e i m p r e g n a - t i o n in w h i c h M o is i n t r o d u c e d p r i o r to iron. A l l c a t a l y s t s c o n t a i n 2.7 wt% Fe, the F e - M o / C c a t a l y s t s in a d d i t i o n 15 w t % Mo.
In o r d e r to o b t a i n a h i g h l y a c t i v e c a t a l y s t , a h i g h d i s p e r s i o n of the s u p p o r t e d i r o n p a r t i c l e s is d e s i r a b l e . M ~ s s b a u e r s p e c t r o s c o p y can in p r i n c i p l e y i e l d i n d i r e c t l y the i m p o r t a n t i n f o r m a t i o n on p a r t i c l e s i z e in two w a y s : b y s t u d y i n g the s u p e r p a r a m a g n e t i c t r a n s i - t i o n t e m p e r a t u r e of the p a r t i c l e s /5/ a n d by a n a l y z i n g the r e c o i l l e s s f r a c t i o n as a f u n c t i o n of t e m p e r a t u r e .
896
W.L.T.M. Ramselaar, et al., Study o f Fe/C and F e M o / C catalysts
Z O 0 to 0 I-- (13 Z IJJ I-- ZT
i , i J , I 1 ! ! i i i J Fe Mo/C RX3 exfroa
N "
2,45; IAIi
3 . 3 0 - T=4.2 K I I I I I I I I l I I - 1 0 - 8 - 6 - 4 - 2 0 2 4 6 8 I0---*- DOPPLER VELOCITY (mm.s -I) f
Fig.2. M~ssbauer spectra of various FeMo/C catalyst presursors, a: airflow 100 ml/min; b: evacuated descicat~r; c: descicator at -I0-8-6-4-2 0 2 4 6 8 I0 atmospheric pressure. - -
DOPPLER VELOCITY (mm.s -I)
+
Fig. I. M~ssbauer spectra of various Fe/C catalyst precursors, a: airflow 50 ml/min <@D=I75K); b: airflow I00 ml/min (@D=225K); c: airflow 500 ml/min (@D=i50K); d: descicat~r (@D=iSOK); i: solvent ethanol,--airflow ]00 ml/min (@D=I25K).
F r o m the l a t ~ e r an e f f e c t i v e D e b y e t e m p e r a t u r e (gD) c a n be d e t e r m i n e d /6/. H o w e v e r , D e b y e t e m p e r a t u r e s d e r i v e d f r o m the t e m p e r - a t u r e d e p e n d e n c e of the r e s o n a n t a b s o r p t i o n a r e a s w i l l be t o o low, w h e n the s u p p o r t e d c a t a l y s t p a r t i c l e s in t h e i r t o t a l i t y p e r f o r m t h e r m a l m o t i o n s r e l a t i v e to the s u p p o r t /7/. T h e r e f o r e w e d e t e r m i n e the s e q u e n c e in the m e a n p a r t i c l e s i z e of the F e ( I I I ) o x i d e p a r t i c l e s f r o m the s p e c t r a l c o n t r i b u t i o n of the m a g n e t i c h y p e r f i n e s e x t u p l e t at a c e r t a i n t e m p e r a t u r e . T h e t h e r m a l m o t i o n s of the p a r t i c l e s
d e p e n d on the i n t e r a c t i o n s t r e n g t h b e t w e e n t h e p a r t i c l e s a n d s u p p o r t . T h e l o w e r the i n t e r a c t i o n s t r e n g t h , the l a r g e r the m o b i l i t y of the p a r t i c l e s o v e r the s u p p o r t , l e a d i n g m o r e e a s i l y to s i n t e r i n g . So, by d e t e r m i n i n g the e f f e c t i v e D e b y e t e m p e r a t u r e 8 D f r o m the t e m p e r a t u r e d e p e n d e n c e of the r e s o n a n t a b s o r p t i o n a r e a of the s p e c t r a , i n f o r m a - t i o n is o b t a i n e d a b o u t the i n t e r a c t i o n s t r e n g t h b e t w e e n the p a r - t i c l e s a n d s u p p o r t . In F i g . l M ~ s s b a u e r s p e c t r a r e c o r d e d at 4.2 K are p r e s e n t e d of the o x i d i c F e / C c a t a l y s t p r e c u r s o r a f t e r d i f f e r e n t d r y i n g t r e a t m e n t s . T h e e f f e c t i v e D e b y e t e m p e r a t u r e s for t h e s e s a m p l e s d e t e r m i n e d f r o m the t o t a l r e s o n a n t a b s o r p t i o n a r e a s of the s p e c t r a at 4.2K, 77K a n d 300K r e s p e c t i v e l y are g i v e n in the c a p t i o n s of F i g . l . T h e i m p r e g n a t e d s a m p l e is d r i e d in a d e s c i c a t o r a b o v e P 2 0 5 for 7 days, or in an c o n t i n u o u s a i r f l o w of 50 m l / m i n , 100 m l / m l n or 500 m l / m i n d u r i n g 18 h. D u e to the p h e n o m e n o n of s u p e r p a r a m a g n e t i s m , an i n c r e a s e in s p e c t r a l c o n t r i b u t i o n of the m a g n e t i c h y p e r f i n e s e x t u p l e t m e a n s an i n c r e a s e
W.L.T.M. Ramselaar, et al., Study o f FelC and F e M o / C catalysts
897 '~2,0~
o l ~ m o ~ b O o ~ oI
I
I
100
200
300
t e m p e r a t u r e (K)Fig.3. Temperature dependence of the resonant absorption area of Fe/C catalyst precursor (dried in 50 ml/min airflow); a: paraffin embedded; b: non embedded. in m e a n p a r t i c l e size. A l t h o u g h o n l y a l i t t l e d i f f e r e n c e in m e a n p a r - t i c l e s i z e is o b s e r v e d for the s a m p l e s d r i e d in an a i r f l o w of 50 m l / m i n a n d i00 m l / m i n , it f o l l o w s f r o m F i g . l t h a t by i n c r e a s i n g as w e l l as d e c r e a s i n g the e v a p o r a t i o n r a t e of w a t e r , an i n c r e a s e in m e a n p a r t i c l e s i z e is found. T h e h i g h e s t e f f e c t i v e D e b y e t e m p e r a t u r e is f o u n d for the s a m p l e w i t h the s m a l l e s t m e a n p a r t i c l e size. So this s a m p l e w i l l be the
l e a s t s e n s i t i v e to s i n t e r i n g .
M ~ s s b a u e r s p e c t r a h a v e b e e n t a k e n at r o o m t e m p e r a t u r e of the s a m p l e d r i e d in the d e s c i c a t o r , a f t e r d i f f e r e n t p e r i o d s of d r y i n g . T h e r e l a t i v e t o t a l r e s o n a n t a b s o r p t i o n a r e a is d i r e c t l y a f t e r i m p r e g n a - t i o n 0.27, a f t e r one d a y in the d e s c i c a t o r 0.51, a f t e r two d a y s 0.98 a n d a f t e r s e v e n d a y s 1.0. A f t e r two d a y s in the d e s c i c a t o r it is h a r d l y p o s s i b l e to r e m o v e m o r e w a t e r of the c a t a l y s t p r e c u r s o r . T h e t h e r m a l m o t i o n s of the p a r t i c l e s r e l a t i v e to the s u p p o r t are s t i l l large as f o l l o w s f r o m the e f f e c t i v e D e b y e t e m p e r a t u r e (in the c a p t i o n of F i g . l ) .
In F i g . 2 the M ~ s s b a u e r s p e c t r a of the o x i d i c F e M o / C c a t a l y s t p r e c u r s o r a f t e r d r y i n g in a i r f l o w (i00 m l / m i n ) for 18 h, or in a d e s c i c a t o r a b o v e P 2 0 5 or an e v a c u a t e d (3.103 Pa) d e s c i c a t o r a b o v e P 2 0 5 a r e shown. H a r d l y any s p e c t r a l c o n t r i b u t i o n of a m a g n e t i c h y p e r f i n e s e x t u p l e t is f o u n d for the d i f f e r e n t s a m p l e s , so the m e a n p a r t i c l e s i z e of the i r o n ( I I I ) o x i d e p a r t i c l e s w i l l be the s a m e and a p p e a r s to b e s m a l l e r in the F e M o / C t h a n in t h e F e / C c a t a l y s t p r e c u r - sors. A c o m p a r i s o n w i t h d a t a o b t a i n e d b y J u n g et al /8/ and N i e m a n t s - v e r d r i e t et al /9/ for c a r b o n - s u p p o r t e d i r o n F i s c h e r - T r o p s c h c a t - a l y s t s s h o w s t h a t the m e a n p a r t i c l e s i z e of the i r o n ( I I I ) o x i d e p a r - t i c l e s in the F e M o / C s a m p l e s is b e l o w 20 ~. F o r the t h r e e F e M o / C s a m p l e s a s a m e t e m p e r a t u r e d e p e n d e n c e of the t o t a l r e s o n a n t a b s o r p - t i o n a r e a is found, w h i c h leads to a 8 D of 150K.
T o o v e r c o m e the t h e r m a l m o t i o n of the p a r t i c l e s r e l a t i v e to the s u p p o r t a n d t h e e n h a n c e d v i b r a t i o n s of the s u r f a c e atoms, w e h a v e e m b e d d e d a F e / C c a t a l y s t p r e c u r s o r in a p a r a f f i n m a t r i x f o l l o w i n g H a y a s h i et a l . / 1 0 / . As is s h o w n in F i g . 3 the m e a s u r e d t o t a l r e s o n a n t a b s o r p t i o n a r e a is i n c r e a s e d a n d the t e m p e r a t u r e d e p e n d e n c e of A is d r a s t i c a l l y c h a n g e d u p o n e m b e d d i n g the c a t a l y s t p r e c u r s o r . B e t w e e n 77K a n d 300K, the t e m p e r a t u r e d e p e n d e n c e of A in the e m b e d d e d s a m p l e c o r r e s p o n d s to the b e h a v i o u r of f i x e d i r o n ( I I I ) o x i d e p a r t i c l e s of this s i z e (=30-40 ~). H o w e v e r , b e t w e e n 77K a n d 4.2K A of the e m b e d d e d s a m p l e i n c r e a s e s m u c h m o r e w i t h d e c r e a s i n g t e m p e r a t u r e t h a n A of the n o n - e m b e d d e d s a m p l e . T h i s i n d i c a t e s t h a t d u e to t h e p a r a f f i n m a t r i x a l s o a f r a c t i o n of the o r i g i n a l i r o n ( I I I ) o x i d e p a r t i c l e s , m i s s i n g in
898
W.L.T.M. Ramselaar, et al., Study o f F e / C and F e M o / C catalysts
the s p e c t r u m d u e to t h e l a r g e t h e r m a l m o t i o n s of the p a r t i c l e s , b e - c o m e s s t r o n g e r b o u n d to t h e s u p p o r t . H o w e v e r s t i l l n o t c o m p l e t e l y f i x e d . B y u s i n g e t h a n o l as a s o l v e n t , the p o r e v o l u m e of t h e c a r b o n - s u p p o r t t u r n s o u t to b e h i g h e r ( l . 2 x l 0 - 3 m 3 / k g ) t h a n f o r w a t e r ( l . 0 x l 0 - 3 m J / k g ) . In o r d e r to a c h i e v e a h i g h e r d i s p e r s i o n of t h e i r o n ( I I I ) o x i d e p a r t i c l e s w e h a v e m e a s u r e d a F e / C c a t a l y s t p r e c u r s o r o b t a i n e d by i m p r e g n a t i o n w i t h a s o l u t i o n of F e ( N O 3 ) 3 . 9 H 2 0 in e t h a n o l (see F i g . l e ) . T h e s p e c t r u m is m o r e c o m p l i c a t e d t h a n w h e n a n a q u e o u s s o l u t i o n is used. A l a r g e r f r a c t i o n of t h e s p e c t r a l a r e a is d u e to p a r a m a g n e t i c r e l a x a t i o n of m o n e a t o m i c a l l y d i s p e r s e d Fe, i n f l u e n c e d b y a n i o n i c s p e c i e s p r e s e n t o n t h e s u p p o r t . So it t u r n s o u t t h a t i n d e e d a h i g h e r d i s p e r s i o n c a n be a c h i e v e d by u s i n g e t h a n o l as a s o l v e n t . I n v e s t i g a t i o n s of the e f f e c t on the c a t a l y t i c a c t i v i t y a r e in p r o g r e s s . 3. C O N C L U S I O N S T h e r e e x i s t s an o p t i m u m e v a p o r a t i o n r a t e of t h e i m p r e g n a t e d s o l v e n t in r e l a t i o n to t h e h i g h e s t d i s p e r s i o n of t h e c a t a l y s t p r e c u r - sor. By u s i n g t h a t d r y i n g p r o d e d u r e t h e p a r t i c l e s a r e t h e n m o s t s t r o n g l y b o u n d to t h e s u p p o r t a n d so l e s s s i n t e r i n g s e n s i t i v e . It s e e m s to b e w o r t h w h i l e to i n v e s t i g a t e t h e c a t a l y t i c a c t i v i t y of F e / C c a t a l y s t s o b t a i n e d b y i m p r e g n a t i o n w i t h a s o l u t i o n of F e ( N O 3 ) 3 . 9 H 2 0 in e t h a n o l . F o r t h e b i m e t a l l i c F e M o / C c a t a l y s t p r e c u r s o r a n i n f l u e n c e of the d r y i n g p r o c e d u r e o n t h e d i s p e r s i o n of the c a t a l y s t h a s n o t b e e n o b s e r v e d . A p o s s i b l e e x p l a n a t i o n f o r t h i s p h e n o m e n o n is t h a t in F e M o / C t h e F e is s u p p o r t e d b y t h e m o l y b d e n u m r a t h e r t h a n b y the c a r b o n . T h i s r e s u l t w o u l d be in a g r e e m e n t w i t h m e a s u r e m e n t s o n C o M o / A l 2 0 3 c a t a l y s t s /2/. R E F E R E N C E S/ I/ H. Tops~e, J.A. Dumesic and S. M~rup in "Application of MSssbauer Spectroscopy' (R.L. Cohen, Ed.) Vol.ll, p.55, Academic Press, New York 1980.
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