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Al2O3 Catalysts
G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
37
INFLUENCE O F THE PREPARATION PROCEDURE ON THE PHYSICAL PROPERTIES, SURFACE A C I D I T Y AND D I S P E R S I O N OF MoP/A1,0,
CATALYSTS
R. PRADA S I L V Y , Y. ROMERO, J . GUAREGUA, R. GALIASSO
I N T E V E P S.A, Seccion de C a t a l i s i s ' A p l icada, Apdo. 76343, Caracas 1070-A, VENEZUELA.
SUMMARY The i n f l u e n c e o f t h e p r e p a r a t i o n procedure on t h e p h y s i c a l p r o p e r t i e s , s u r f a c e a c i d i t y , d i s p e r s i o n and d i s t r i b u t i o n o f t h e supported phases i n MoP/A1,0, c a t a l y s t s i s here investigated. The r e s u l t s i n d i c a t e t h a t t h e b e h a v i o u r o f t h e molybdenum-phosphorus c a t a l y s t s depend s t r o n g l y on t h e i m p r e g n a t i o n sequence used ( c o i m p r e g n a t i o n o r d o u b l e i m p r e g n a t i o n ) . Coimpregn a t i o n procedure (Mo+P) a l l o w s t h e o b t e n t i o n o f c a t a l y s t s w i t h l a r g e r C o n c e n t r a t i o n o f a c i d s i t e s and s u r f a c e d i s p e r s i o n ! when compared t o consecuti v e i m p r e g n a t i o n procedures (P->Mo, Mo->P). T h i s sample a l s o .showed h i g h a c t i v i t y f o r t h e HDS, HDN and MHC r e a c t i o n s . The d i f f e r e n c e s i n b e h a v i o u r observed f o r t h e d i f f e r e n t samples c o u l d be m a i n l y a t t r i b u t e d t o t h e e x i s t e n c e o f d i f f e r e n t molybdenum d i s t r i b u t i o n phases, p r o b a b l y Mo-P . h e t e r o p o l compounds a r e formed f o r t h e coimpregnated sample, . w h i l e t h e f o r m a t i o n o f b u l t MOO, species c o u l d be f a v o r i z e d f o r t h e sample preimpregnated wJth phosphorus. I t was a l s o observed t h a t phosphorus produces t h e f o l l o w i n g e f f e c t s on alumina: i ) i t improves t h e . s u r f a c e a c i d i t y , ii) i t i n c r e a s e t h e mechanical s t r e n g t h o f t h e e x t r u d a t e s , iii) i t decreases t h e s u r f a c e a r e a o f t h e s u p p o r t . We e x p l a i n o u r r e s u l t s on t h e b a s i s o f t h e mechanism proposed f o r t h e a d s o r p t i o n o f molybdate. phosphate and phosphomolybdate compounds on alumina. INTRODUCTION Molybdenum supported alumina c a t a l y s t s promoted by m e t a l s o f t h e group
V I I I ( N i , Co) a r e w i d e l y used f o r h y d r o p r o c e s s i n g heavy o i l s o r c o a l d e r i v e d fuels.
E f f o r t s have been made t o improve c a t a l y t i c a c t i v i t y by f i n d i n g appro-
p r i a t e a d d i t i v e s (such as P,
B,
S i , T i , F , Ga, e t c . ) . , Phosphorus can be c o n s i -
dered one o f t h e most e f f e c t i v e a d d i t i v e s o f t h e molybdenum supported alumina c a t a l y s t s . I n f a c t , i t appears as a component i n a number o f f o r m u l a t i o n s o f commercial h y d r o t r e a t i n g c a t a l y s t s . The p a t e n t l i t e r a t u r e c l a i m s t h a t i t s use p r o v i d e s b e t t e r c a t a l y t i c performance, i n c r e a s i n g h y d r o d e s u l f u r i z a t i o n (HDS), h y d r o d e n i t r o g e n a t i o n (HDN), hydrodemetall i z a t i o n (HDM) , and i m p r o v i n g t h e m i l d hydrocracking
(MHC)
activity
(ref.
1-41.
phosphorus produces s e v e r a l e f f e c t s , such as:
I t has been demonstrated t h a t
i) i t p r o v i d e s a more s t a b l e
i m p r e g n a t i n g s o l u t i o n and t h u s b e t t e r d i s p e r s i o n o f t h e metal ( r e f . 1 , 4 ) , ii) i t i n h i b i t s t h e f o r m a t i o n o f Ni(Co)Al,O,-like
on
t h e support species and
enhances t h e f o r m a t i o n o f N i z t ' i o n s w h i c h a r e p r i m a r l y r e s p o n s i b l e f o r t h e f o r m a t i o n o f c a t a 1 . y t i c a l l y a c t i v e n i c k e l s u l f i d e o r NiMoS-like phase ( r e f . 3 ) ,
iii) i t a l t e r s t h e a c i d s t r e n g t h d i s t r i b u t i o n on alumina, t h e c o n c e n t r a t i o n o f medium a c i d s i t e s i n c r e a s e s p r o g r e s s i v e l y w i t h i n c r e a s i n g phosphorus l e v e l
38
5) and i v ) i t improves t h e thermal s t a b i l i t y o f gamma alumina w i t h t o s i n t e r i n g and and phase t r a n s i t i o n t o alpha alumina ( r e f . 5).
(ref.
respect
The b e n e f i c a l e f f e c t s o f phosphorus has s t i m u l a t e d research on i t s i n f l u e n c e on molybdena based c a t a l y s t s . However, most o f t h e above mentioned
1 i t e r a t u r e e s s e n t i a l l y focused on t h e i n f l u e n c e o f phosphorus on t h e c a t a l y t i c p r o p e r t i e s o f t h e m o d i f i e d system and i t s e f f e c t on t h e d i s p e r s i o n on the a c t i v e phase deposited on t h e alumina surface has n o t y e t been i n v e s t i g a t e d . Also,
very few works deal w i t h t h e i n f l u e n c e o f t h e sequence o f phosphorus c a t a l y s t s on t h e
i n c o r p o r a t i o n d u r i n g t h e p r e p a r a t i o n step o f the MoP/A1 ,O,
surface a c i d i t y , d i s p e r s i o n and d i s t r i b u t i o n o f t h e supported phases.
As the
phosphate i o n s s t r o n g l y i n t e r a c t w i t h alumina, competing w i t h molybdate ions, a f a c t o r o f p o s s i b l e importance i s t h e p r e p a r a t i o n procedure. Three
different
procedures
can
be
followed .to
prepare a MoP/A1 ,03
catalyst.
(1) P A1,0,
-->
MO
(2) MO + P ( 3 ) Mo --> P
->
MOP/ A1,0,
-->
This c o m u n i c a t i o n i s p a r t o f a research program aimed a t a systematic i n v e s t i g a t i o n o f t h e p r e p a r a t i o n procedure o f MoP/A1,0, catalysts.
Essentially,
we study
mild-hydrocracking
the e f f e c t o f phosphorus i n c o r p o r a t i o n
sequence on t h e s t a t e o f d i s p e r s i o n o f t h e a c t i v e phase, surface a c i d i t y and physical p r o p e r t i e s . the
following
For t h i s purpose, t h e samples were c h a r a c t e r i z e d using
physico-chemical
techniques:
BET
strength, X-ray p h o t o e l e c t r o n spectroscopy (XPS).
(SEM),
surface
area,
mechanical
scanning e l e c t r o n microscopy
surface a c i d i t y determined by p y r i d i n e adsorption.
EXPERIMENTAL Catalyst preparation Three MoP/A1,0, c a t a l y s t s having constant molybdenum and phosphorus conwere prepared by e i t h e r consecutive t e n t s (10 w t % MOO, and 4.5 w t % P,O,) (P->Mo
o r Mo-->P) o r simultaneous (Mo+P) impregnation o f alumina extrudates
(1/20 i n c h l e n g t h ) w i t h aqueous s o l u t i o n s o f ammonium heptamolybdate and/or orthophosphoric acid.
The alumina has a s p e c i f i c surface area o f 269 m 2 / g and
a pore volume of 0.69 cc/g.
The impregnating s o l u t i o n s were adjusted t o pH
=
1.5-2.0 by adding n i t r i c a c i d b e f o r e c o n t a c t i n g w i t h t h e support. The samples were d r i e d a t 373 K f o r 2 h and then c a l c i n e d i n two steps a t 623 K f o r 2 h and a t 7 7 3 K f o r 3 h.
39
Characterization Surface areas o f c a t a l y s t s were determined by t h e BET method from N, analyzer. An Erweka apparatus was used t o measure t h e s i z e c r u s h i n g s t r e n g t h o f c a t a l y s t extrudates.
The method determines p a r t i c l e c r u s h i n g s t r e n g t h by
measuring t h e f o r c e i n kilograms ( k ) r e q u i r e d t o crush an e x t r u d a t e o f measured s i z e .
A l a r g e number o f extrudates (about f o r t y o f each sample) were t e s t -
ed and t h e average value was e s t a b l i s h e d . X-ray
p h o t o e l e c t r o n spectra
(XPS)
o f c a t a l y s t s were recorded u s i n g a
Leybold Heraeus LHS-11 apparatus equipped w i t h a computer system, which a1 lowed
the determination o f
Alka l i n e ( E = 1486 eV).
peak areas.
The e x c i t a t i o n r a d i a t i o n was
the
A l l t h e samples were grounded and then pressed i n t o
Cis.
Alpp, A l Z s . M o ~ ~M , o ~ ~P2p , and P 2 s e n e r g y l e v e l s were recorded. The CIS energy l e v e l (284.5 eV) was taken as a reference. Atomic surface c o n c e n t r a t i o n o f t h e sample h o l d e r s b e f o r e t h e a n a l y s i s .
Signals corresponding t o
supported elements was evaluated form t h e peak i n t e g r a t e d areas and t h e sensit i v i t y f a c t o r s provided by t h e equipment manufacturer. The elemental p r o f i l e d i s t r i b u t i o n o f b o t h molybdenum and phosphorus, across t h e t r a n s v e r s a l s e c t i o n o f t h e alumina extrudates, was obtained u s i n g the
scanning
e l e c t r o n microscopy
technique
(SEM).
An
ISI-60 apparatus
equipped w i t h an energy d i s p e r s i v e X-ray analyzer (Kevex 5-7000) was used f o r these measurements.
C a t a l y s t extrudates were mounted on an epoxy s l i d e and
then p o l i s h e d b e f o r e scanned under t h e e l e c t r o n beam. The a c i d i t y measurements were c a r r i e d o u t i n a Cahn 1000 e l e c t r o b a l a n c e u s i n g p y r i d i n e as probe molecule adsorbed on t h e c a t a l y s t
surface.
The
i r r e v e r s i b l y adsorbed p y r i d i n e amounts were determined a t 273 K, 473 K and 573 K.
Results a r e expressed as m o l o f p y r i d i n e i r r e v e r s i b l y adsorbed p e r
surface area o f c a t a l y s t . RESULTS Surface areas and mechanical s t r e n g t h Specific
surface areas corresponding t o MoP/A1 209 c a t a l y s t s prepared
f o l l o w i n g d i f f e r e n t procedures a r e given i n Table 1. t h e same t a b l e t h e values obtained f o r A1,0, Mo/A1,0,
samples.
I t i s also reported i n
support and f o r P/A1,0,
and
40
TABLE 1 S p e c i f i c s u r f a c e areas and mechanical s t r e n g t h corresponding t o d i f f e r e n t p r e p a r a t i o n procedures Sample
Surface Area (m'/g)
A1 Z 0 3 P / A 1 ,03 Mo/A1 ,O,
Crushing S t r e n g t h ( k g / p e s t l e )
269 241 248 214 222 234
P->Mo Mo->P MotP
For b o t h Mo/A1,0,
5.2 6.0 5.3 6.1 5.5 6.1
and P/A1,0,
samples,
t h e surface area decreased i n
8% and 11%, r e s p e c t i v e l y , a f t e r impregnation w i t h ammonium -heptamolybdate o r phosphoric acid, whereas f o r MoP/A1 ,03 c a t a l y s t s t h e l o s s
approximatively,
i n surface areas was more pronounced.
One can observe t h a t t h e s u r f a c e area
value o b t a i n e d f o r t h e sample prepared by coimpregnation i s s l i g h t l y higher than t h a t observed f o r those samples prepared by consecutive impregnation.
A l l MoP/A1,0,
c a t a l y s t s presented a pore volume i n t h e 0.52 - 0.55 c c / g range.
Concerning t h e mechanical s t r e n g t h measurements,
i t i s observed i n Table
1 t h a t phosphorus s l i g h t l y improves t h e mechanical s t r e n g t h o f alumina e x t r u dates, w h i l e molybdenum seems t o have no i n f l u e n c e on these p r o p e r t i e s .
The
p r e p a r a t i o n procedures (P->Mo) and (Mo+P) produce c a t a l y s t s w i t h simi 1 a r c r u s h i n g s t r e n g t h values. X-ray p h o t o e l e c t r o n spectroscopy (XPS) The XPS r e s u l t s obtained f o r P/A1 ,O,,
Mo/A1 ,O,
and MoP/A1 ,O,
catalysts,
a r e presented i n Table 2. The percentage o f surface d i s p e r s i o n o f both Mo and P elements ( I e / I A l x 100) i s r e p o r t e d as a f u n c t i o n o f c a t a l y s t p r e p a r a t i o n
procedure. Mo/A1,0,
When comparing t h e (Mo/A1) o r ( P / A l )
o r P/A1,0,
i n t e n s i t y r a t i o obtained f o r
s w p l e s w i t h t h a t o f MoP/A1,0,
c a t a l y s t s , we can observe
s t r i k i n g d i f f e r e n c e s i n t h e d i s p e r s i o n s t a t e o f t h e supported species.
Molyb-
denum d i s p e r s i o n o f samples v a r i e s as f o l l o w s :
while,
i n t h e case o f t h e phosphorus d i s p e r s i o n , t h e observed sequence i s as
follows: Mo+P
>
P/A1,0,
>
Mo->P
>
P->Mo
41
MO
MO
-070
0
070
Radial Position (mm) Fig. 1. Electron Microprobe p r o f i l e of Mo and P corresponding t o MoP/A1,0, c a t a l y s t s prepared following d i f f e r e n t procedures.
A t 373 K (weak acid s i t e s ) , t h e pyridine adsorbed amounts per surface area v a r i e s as follows: Mo+P > P->Mo > Mo->P = A1,0,
Whereas, f o r desorption temperatures of 473 K and 573 K (medium and strong acid s i t e s , r e s p e c t i v e l y ) , the observed sequence i s :
42
TABLE 2 XPS r e s u l t s c o r r e s p o n d i n g t o t h e c a t a l y s t s prepared u s i n g d i f f e r e n t
procedures. IMojp/ I A 1
Sample
2P
I P 2 p / I A 1 2P
__
Mo/A1 ,O,
4.5
2.7 __
P->Mo
4.1
2.3
Mo->P
4.4
2.5
Mo+P
4.8
3.3
P / A 1 *03
Scanning e l e c t r o n microscopy (SEM) F i g u r e 1 r e p r e s e n t s t h e p r o f i l e d i s t r i b u t i o n , across t h e t r a n s v e r s a l sect i o n o f t h e alumina e x t r u d a t e s , b o t h f o r phosphorus and molybdenum elements, obtained
through
procedures
(Mo+P)
SEM
and
technique. (Mo->P),
For
catalysts
b o t h Mo and
prepared
P elements
following are
the
distributed
homogeneously i n t h e s u p p o r t , whereas sample prepared a c c o r d i n g t o procedure (P->Mo),
c l e a r l y shows some h e t e r o g e n e i t i e s .
Strength o f a c i d i t y S u r f a c e a c i d i t y r e s u l t s , o b t a i n e d t h r o u g h p y r i d i n e a d s o r p t i o n , correspond i n g t o A1,0,
s u p p o r t and d i f f e r e n t MoP/A1,0,
catalysts,
Table 3 as a f u n c t i o n o f t h e d e s o r p t i o n temperature.
are presented i n
Differences i n pyridine
i r r e v e r s i b l y adsorbed amounts p e r s u r f a c e a r e a u n i t can be observed i n t h e 373 -573 K temperature range, depending on t h e p r e p a r a t i o n method used. TABLE 3 I r r e v e r s i b l y a c i d i t y o f MoP/A1,0,
prepared c a t a l y s t s ( m o l p y r i d i n e l m ' )
Sample 373
TEMPERATURE ( K ) 473
573
1.41
0.67
0.11
Mo->P
1.78 1.44
0.83 0.76
0.42 0.32
Mo+P
2.06
0.85
0.45
3'2
P->Mo
x lo3
43
DISCUSSION The above r e s u l t s show how a c i d i t y and s u r f a c e d i s p e r s i o n o f supported phases found i n MoP/A1,0,
c a t a l y s t s can be s t r o n g l y i n f l u e n c e d by t h e prepara-
Tables 2 and 3 i n d i c a t e t h a t c o i m p r e g n a t i o n o f Mo and P
t i o n procedure.
a l l o w s t h e o b t e n t i o n o f a c a t a l y s t w i t h l a r g e r c o n c e n t r a t i o n o f a c i d s i t e s and surface dispersion,
(P->Mo,
Mo->P).
t h e mechanical
i n comparison w i t h c o n s e c u t i v e i m p r e g n a t i o n procedures
Phosphorus seems t o improve t h e a c i d i t y s t r e n g t h as w e l l as p r o p e r t i e s o f alumina e x t r u d a t e s .
However,
this additive
s t r o n g l y a f f e c t s t h e s p e c i f i c surface area o f t h e support. L e t u s d i v i d e t h e d i s c u s s i o n o f o u r r e s u l t s i n two p a r t s :
we s h a l l f i r s t
d i s c u s s t h e r e s u l t s o b t a i n e d on t h e sample prepared by c o i m p r e g n a t i o n (MotP) and subsequently, examine t h e b e h a v i o u r o f those c a t a l y s t s p r e p a r e d by consec u t i v e impregnations (P->Mo and Mo->P). P r e p a r a t i o n Procedure (Mo+P) Table 2 shows t h a t c o i m p r e g n a t i o n method improves t h e s u r f a c e d i s p e r s i o n
o f b o t h molybdenum and phosphorus.
T h i s agrees w i t h r e c e n t r e s u l t s o b t a i n e d
by Atanasova e t a l . ( r e f . 6), who s t u d y i n g a s e r i e s o f NiMoP/Al,O, XPS
and c a t a l y t i c
measurements,
stated
that
c a t a l y s t by
c o i m p r e g n a t i o n method and a
s u i t a b l e alumina c a r r i e r can l e a d t o a b e t t e r d i s p e r s i o n o f a c t i v e components and consequently, t o an i n c r e a s e i n
HDS a c t i v i t y .
We s h a l l a t t e m p t t o e x p l a i n now o u r r e s u l t s supported on fundamental s t u d i e s d e a l i n g w i t h a d s o r p t i o n o f Mo and/or P
on
alumina.
L e t us r e f e r t o t h e
more i m p o r t a n t r e s u l t s o f t h e s e s t u d i e s . L i t e r a t u r e shows t h a t phosphorus i n c r e a s e s t h e s o l u b i l i t y and s t a b i l i t y o f molybdenum s o l u t i o n s
(ref.
1,4). A d s o r p t i o n s t u d i e s proposed t h a t when
alumina i s coimpregnated w i t h s o l u t i o n s c o n t a i n i n g molybdate and phosphate, t h e r e i s a c o m p e t i t i o n between b o t h i o n s f o r t h e same a d s o r p t i o n s i t e s ( b a s i c hydroxyl
groups
of
alumina),
thus,
a d s o r p t i o n o f rnolybdates (3,6-9).
the
adsorbed
phosphate
inhibits
the
However, most o f t h e s e s t u d i e s deal w i t h
molybdate and phosphate a d s o r p t i o n s e p a r a t e l y w i t h o u t c o n s i d e r i n g f o r m a t i o n o f phosphomolybdate compounds i n t h e i m p r e g n a t i n g s o l u t i o n .
the
R e c e n t l y , Cheng and L u t h r a ( r e f . 8 ) , u s i n g t h e NMR t e c h n i q u e , s t u d i e d t h e a d s o r p t i o n o f v a r i o u s phosphomolybdate compounds on alumina spheres.
Authors
observed t h a t when phosphoric a c i d i s added t o a s o l u t i o n c o n t a i n i n g amonium hep tam0 1 y b d a t e , p e t amol ybdodi phosphate compounds ( P ,Mo s o l u t i o n s c o n t a i n i n g P/Mo m o l a r r a t i o h i g h e r t h a n 0.4, remained i n f o r m o f phosphates.
1 a r e formed.
For
amounts o f phosphorus
T h i s suggests t h e e x i s t e n c e o f a chemical
e q u i l i b r i u m between b o t h phosphate and molybdate i o n s i n s o l u t i o n .
44
8 H * + 5 MOO:'
+ 2 HP0:-
=
P,Mo,O,,
6-
+
(2)
5H20
A c c o r d i n g t o t h e above e q u a t i o n , decomposition o f phosphomolybdate i n t o s i m p l e molybdate and phosphate c o u l d be f a v o u r e d by a r i s e o f s o l u t i o n pH, which would s h i f t t h e chemical e q u i l i b r i u m t o l e f t . ,
Indeed, t h i s behaviour
was observed d u r i n g phosphomolybdate a d s o r p t i o n on alumina.
The i n c r e a s e i n
pH o f t h e i m p r e g n a t i n g s o l u t i o n was a t t r i b u t e d t o w a t e r f o r m a t i o n d u r i n g i o n exchange r e a c t i o n . L u t h r a and Cheng ( r e f . 10) observed t h a t e q u i l i b r i u m b e t w e e n heptamolybdate Mo,O:i rise Mo,O,,
and molybdate MOO:-
i o n s i s a l s o a f f e c t e d by a
i n pH. 6-
+
4 H,O
I n short,
=
2-
7 MOO,
+
(3)
8 H
r e s e a r c h o f Cheng and L u t h r a c l e a r l y i l l u s t r a t e s t h a t t h e h i g h
s o l u b i l i t y and s t a b i l i t y observed when p h o s p h o r i c a c i d i s added t o molybdenum i s m a i n l y due t o t h e f o r m a t i o n o f 'phosphomolybdate compounds.
These compounds
a r e v e r y s e n s i t i v e t o changes i n t h e i m p r e g n a t i n g s o l u t i o n pH. The f a c t t h a t t h e r e a r e d i f f e r e n c e s i n a c i d s i t e s d i s t r i b u t i o n and s u r f a c e d i s p e r s i o n when u s i n g d i f f e r e n t procedures t o p r e p a r e a MoP/A1 ,O,
catalyst,
suggests t h a t n a t u r e and c o n c e n t r a t i o n o f t h e o x i d i c supported phases p r e s e n t i n these c a t a l y s t s are d i f f e r e n t . Our h y p o t h e s i s i s c o n s i s t e n t w i t h t h e r e s u l t s o b t a i n e d by o t h e r researche r s ( r e f . 3, 6, 9, 111, who combining v a r i o u s c h a r a c t e r i z a t i o n techniques, s t u d i e d t h e s t r u c t u r a l changes t h a t o c c u r r e d when phosphorus i s used as an a d d i t i v e o f Mo/A1 ,O,
Atanasova and Halachev ( r e f . l l ) , s t u d y i n g
catalysts.
t h r o u g h I R spectroscopy t h e phases p r e s e n t i n NiMoP/Al ,O, by coimpregnation, observed bands c o r r e s p o n d i n g t o AlPO, and Ni-Mo-P
heteropoly
compounds.
IR-bands
c a t a l y s t s , prepared and t o a mixed A1-Mo
c o r r e s p o n d i n g t o b u l k MOO,,
Al,(MoO,), and NiMoO, were n o t observed i n t h o s e samples. Authors observed t h a t h i g h phosphorus c o n t e n t l e a d s t o an i n c r e a s e i n degree o f molybdenum p o l y m e r i z a t i o n and t o changes
i n t h e r a t i o between t h e d i f f e r e n t t y p e s o f
h e t e r o p o l y compounds, Ni-Mo-P/Al-Mo loadings.
Lopez Corder0 e t a l .
r a t i o i n c r e a s e s w i t h i n c r e a s i n g phosphorus ( r e f . 9) s t u d i e d by TPR and DRS t h e s u r f a c e
d i s t r i b u t i o n o f molybdenum on two s e r i e s o f MoP/A1 ,O,
c a t a l y s t s which were
prepared u s i n g simul taneous (P+Mo) o r double i m p r e g n a t i o n (P->Mo) methods. I n c o n t r a s t w i t h t h e r e s u l t s o b t a i n e d by Atanasova and Halachev ( r e f . 111, t h e a u t h o r s observed t h e presence o f b u l k MOO,
and a l s o small c l u s t e r s o f p o l y -
molybdate m u l t i l a y e r s f o r b o t h c a t a l y s t s e r i e s . l a t t e r specieswas
The c o n c e n t r a t i o n o f t h e
more i m p o r t a n t f o r t h e preimpregnated samples.
45
C o n s i d e r i n g o u r r e s u l t s t o g e t h e r w i t h t h o s e found i n l i t e r a t u r e ,
two
p o s s i b l e e x p l a n a t i o n s can be proposed: The f i r s t e x p l a n a t i o n i s based on t h e mechanism proposed f o r molybdate o r phosphomolybdate phosphate a d s o r p t i o n on alumina ( r e f . 121, because compounds were observed t o decompose i n t o these two species d u r i n g a d s o r p t i o n . These s t u d i e s suggest t h a t phosphate as w e l l as molybdate i n t e r a c t f i r s t w i t h b a s i c h y d r o x y l groups o f alumina, g e n e r a t i n g a w a t e r molecule. Competition between phosphorus and molybdenum t a k e s p l a c e . Phosphate i s adsorbed on alumina more r a p i d l y t h a n molybdate i s adsorbed.However, t h e r a t e o f a d s o r p t i o n o f b o t h compounds depends on s e v e r a l f a c t o r s , such as: i ) n a t u r e and c o n c e n t r a t i o n o f t h e i o n s i n t h e i m p r e g n a t i n g s o l u t i o n , ii) pH o f s o l u t i o n , iii) t y p e o f alumina and i v ) a d s o r p t i o n temperature. For each exchanged h y d r o x y l group by phosphoric a c i d molecule, two new a c i d s i t e s a r e c r e a t e d . T h i s would e x p l a i n t h e i n c r e a s e s i n a c i d s i t e s c o n c e n t r a t i o n observed a f t e r phosphorus i n c o r p o r a t i o n i n t o alumina. Morales e t a l . ( r e f . 12) suggested t h a t when a l l b a s i c h y d r o x y l groups a r e t i t r a t e d , t h e a c i d h y d r o x y l groups b e g i n t o be t i t r a t e d and t h e n a monolayer o f phosphate i s formed by f u r t h e r a d d i t i o n o f p h o s p h o r i c a c i d . I n t e r a c t i o n s between n e i g h b o r i n g adsorbed phosphates c o u l d occurs l e a d i n g t o t h e f o r m a t i o n o f p o l y m e r i c phosphate c h a i n s . I n t h i s p a r t i c u l a r case, t h e a u t h o r s mentioned t h a t t h e number o f a c i d s i t e s remains almost c o n s t a n t because t h e s u b s t i t u t i o n o f two a c i d h y d r o x y l groups o f alumina would y i e l d two a c i d s i t e s a s s o c i a t e d t o phosphorus. I n t h e c o m p e t i t i v e system, we can propose t h a t b o t h molybdate and phosp h a t e i o n s c o u l d be adsorbed i n n e i g h b o r i n g s i t e s . T h i s would i m p l y t h a t adsorbed molybdates impede t h e p o l y m e r i z a t i o n o f adsorbed phosphate and phosphates would produce t h e same e f f e c t on molybdates. The l a t t e r may e x p l a i n t h e h i g h e s t s u r f a c e d i s p e r s i o n o f b o t h P and Mo observed f o r t h e coimpregnated sample. Since p o l y m e r i z a t i o n o f phosphorus as we1 1 as molybdenum was f a v o u r e d by h i g h c o n t e n t s o f t h e s e elements, we c o u l d suggest t h a t i n t e r a c t i o n s between n e i g h b o r i n g molybdate and phosphate adsorbed species may occur c a u s i n g t h e f o r m a t i o n o f Mo-P h e t e r o p o l y compounds a f t e r c a l c i n a t i o n . One may s p e c u l a t e t h a t a c i d i t y produced f o r t h e l a t t e r compounds should be d i f f e r e n t f r o m t h a t produced by phosphates on alumina. The second e x p l a n a t i o n would be t o c o n s i d e r t h a t phosphomolybdate remains stable during impregnation. In t h i s case, t h e b e h a v i o u r o f these compounds towards a d s o r p t i o n would be d i f f e r e n t f r o m t h a t o f p h o s p h a t e a n d molybdate. One may s p e c u l a t e t h a t Mo-P h e t e r o p o l y compounds may be formed f r o m t h e adsorbed phosphomolybdate a f t e r c a l c i n a t i o n . T h e r e f o r e , t h e h i g h e s t d i s p e r s i o n observed f o r t h e coimpregnated sample may be a t t r i b u t e d t o t h e f o r m a t i o n o f Mo-P h e t e r o p o l y compounds, which were observed on coimpregnated samples i n a r e c e n t study ( r e f . 11). L e t us now complement o u r d i s c u s s i o n showing some i m p o r t a n t e f f e c t s
46
observed through scanning e l e c t r o n microscopy technique i n t h e a n a l y s i s o f t h e samples.
Figure 1 shows t h a t b o t h phosphorus and molybdenum a r e homogeneously
d i s t r i b u t e d i n t h e coimpregnated sample. and L u t h r a ( r e f .
8)
during adsorption o f
However, r e s u l t s obtained by Cheng
showed an i n t e r e s t i n g chromatographic e f f e c t appearing phosphomolybdate on alumina spheres,
p r e f e r e n t i a l l y l o c a t e d a t t h e edge, w h i l e molybdenum c e n t e r o f t h e spheres.
phosphorus was
was concentrated a t t h e
The l a t t e r o b s e r v a t i o n makes e v i d e n t t h e decomposition
o f phosphomolybdate d u r i n g a d s o r p t i o n and t h e c o m p e t i t i o n o f b o t h phosphate and molybdate
ions
for
the
same adsorption
sites.
I n our o p i n i o n ,
we
a t t r i b u t e t h e d i f f e r e n t behaviour observed i n b o t h s t u d i e s t o d i f f e r e n c e s i n p r e p a r a t i o n c o n d i t i o n s o f t h e samples, as w e l l as t h e type o f alumina employed. Preparation Procedures (P->Mo) and (Mo->P) As shown i n Tables 1 and 3, phosphorus seems t o improve t h e mechanical
p r o p e r t i e s as w e l l as t h e a c i d i t y s t r e n g t h o f alumina extrudates,
However,
t h i s a d d i t i v e s t r o n g l y a f f e c t s t h e s p e c i f i c surface area o f support. Our r e s u l t s a r e i n agreement w i t h those obtained by several i n v e s t i g a t o r s ( r e f . 7,9,12).
Lopez Cordero e t a l . ( r e f . 9)
suggested t h a t l o s s i n surface
area o f alumina a f t e r phosphorus i n c o r p o r a t i o n i s probably due t o a c o r r o s i v e e f f e c t o f surface caused by phosphoric a c i d molecules o r t o a pore blockage by phosphate species. t h e 30-60 explanation
A'
We have observed t h a t pores having an average diameter i n
range were t h e most a f f e c t e d by phosphorus d e p o s i t i o n .
An
o f these r e s u l t s c o u l d be t h e f a c t t h a t these pores c o u l d be
s e l e c t i v e l y plugged by polymeric phosphate adsorbed species,
However, we can
n o t d i s c a r d a p o s s i b l e c o r r o s i v e e f f e c t produced by t h e phosphoric a c i d molecules on t h e alumina surface.
An e l e c t r o n microscopy study o f t h e P/A1,0,
sample c o u l d reveal p o s s i b l e morphological changes due t o phosphorus. Table 2 i n d i c a t e s t h a t f o r t h e sample prepared f o l l o w i n g procedure ( P - > M o ) . t h e Mo d i s p e r s i o n ( I M o / I A 1 ) was a f f e c t e d b y phosphorus 3P 2P i n c o r p a t i o n . The I P 2 p / I A 1 2 p i n t e n s i t y r a t i o a l s o decreased w i t h respect t o t h e value obtained f o r P/A1,0,
sample,
a f t e r molybdenum d e p o s i t i o n .
These
r e s u l t s can be explained by t h e f a c t t h a t preimpregnation o f alumina w i t h phosphorus reduces t h e number o f s i t e s a v a i l a b l e f o r molybdate adsorption. Therefore, changes i n d i s p e r s i o n and d i s t r i b u t i o n o f molybdenum species should be expected. I n t h i s case, one c o u l d suggest t h a t phosphorus promotes t h e formdtion o f b u l k MOO, species, which i s i n agreement w i t h t h e r e s u l t s o b t a i n ed by Lopez Cordero e t a l . deposited on t h e AlPO,
( r e f . 9).
monolayer,
A p a r t o f t h e l a t t e r species may be
which would e x p l a i n t h e decreasing i n
47
phosphorus
intensity
e l e c t r o n microscopy
r a t i o observed f o r study
confirms
the
this
sample.
Additionally,
heterogeneous d i s t r i b u t i o n
molybdenum species when alumina i s preimpregnated w i t h phosphorus.
the
o f the
A similar
s i t u a t i o n m i g h t be expected when t h e c a r r i e r i s preimpregnated w i t h molybdenum.
I n this
particular
case,
mechanical
strength
as w e l l
as a c i d i t y
p r o p e r t i e s s h o u l d n o t be improved by phosphorus. A c t i v i t y o f t h e (MotP) and (P->Mo) samples I n t h i s work we have e v a l u a t e d t h e c a t a l y t i c p r o p e r t i e s o f t h e molybdenum-phosphorus
catalysts.
For
this
purpose,
we
have
prepared
two
Ni-Mo-P/Al,O,
samples f o l l o w i n g t h e i m p r e g n a t i o n sequences (Mo+P->Ni) and
(P->Mo->Ni).
These samples p r e s e n t t h e same chemical
MOO,,
7 . 5 w t % P,O,
and 5.0 w t % N i O ) .
c o m p o s i t i o n (15 w t %
The c a t a l y t i c r e a c t i o n was c a r r i e d o u t
i n a h i g h p r e s s u r e f i x e d bed r e a c t o r u s i n g a vacuum g a s o i l under t y p i c a l mild-hydrocracking conditions
(T = 653K, P
= 5 MPa, LHSV = 0.65 l / h , H,/Hc
=
600). We p r e s e n t i n Table 4 t h e a c t i v i t y r e s u l t s o f t h e NiMoP/Al,O, Both samples
show comparable a c t i v i t y
i n MHC.
However,
r e a c t i o n s a r e h i g h e r f o r t h e coimpregnated sample.
catalysts.
t h e HDS and HDN
The same b e h a v i o u r was
observed by o t h e r r e s e a r c h e r s ( r e f . 13 and 1 4 ) . TABLE 4 Activity o f the
NiMoP/Al,O,
samples
% CONVERSION
HDS
HDN
MHC
(Mo+P-> N i l
82
59
13
(P-> Mo-> N i )
77
50
11
SAMPLE
I n o r d e r t o analyze t h e p o s s i b l e changes i n t h e molybdenum d i s t r i b u t i o n phases induced by phosphorus, we have c a r r i e d o u t I R measurements MoP/A1 ,O, samples b e f o r e n i c k e l i m p r e g n a t i o n . published
elsewhere
(ref.
molybdenum d i s t r i b u t i o n ,
15),
depending
confirm
the
on
both
The r e s u l t s , which w i l l be fact
modify
the
on t h e p r e p a r a t i o n procedure f o l l o w e d .
phosphorus
No
evidences about t h e presence o f phosphomolybdate compoundswere observed f o r t h e coimpregnated sample.
48
To summarize. we may conclude t h a t The d i f f e r e n c e s i n a c t i v i t y observed f o r t h e (MotP-,
N i l and (P->Mo->
N i ) samples c o u l d be associated t o changes
induced by phosphorus on t h e s u r f a c e molybdenum d i s t r i b u t i o n phases. CONCLUSIONS In
the
frame
of
the
present
work
we
conclude
that
coimpregnation
p r o c e d u r e i s t h e more a p p r o p r i a t e method o f p r e p a r i n g MoP/Al *03c a t a l y s t s . This procedure a l l o w s t h e o b t e n t i o n o f samples w i t h h i g h e r d i s p e r s i o n , surface a c i d i t y and c a t a l y t i c p r o p e r t i e s . Changes observed i n molybdenum d i s t r i b u t i o n phases caused by phosphorus depend on t h e impregnation sequence employed.
Coimpregnation procedure would
l e a d t o t h e Mo-P heteropoly compound formation,
w h i l e preimpregnation w i t h
phosphorus c o u l d induce t h e b u l k MOO, formation. I t was a l s o observed t h a t phosphorus produces t h e f o l l o w i n g e f f e c t s on
alumina:
i ) i t improves t h e surface a c i d i t y , ii) i t increases t h e mechanical
s t r e n g t h o f extrudates, iii) i t decreases t h e surface area o f support.
REFERENCES
L. Hilfman, U.S.
A. Morales, M.M. K. G i s h t i , A.
Patent 3.617.528
(1971).
Ramirez de Agudelo, Appl. Catal. 23 (1986) 23. i a n n i b e l l o , S. Marengo, G. M o r e l l i , Appl. Catal. 1 2 (1984)
381. G.A.
Mickelson, U.S.
Patent 3.749.663
S. S t a n i s l a u s , A. Asi-Habi,
(1973).
C. Dolana, Appl. Catal.,
39 (1988) 239.
P. Atanasova, T. Halachev, J. U c h y t i l , M. Kraus, Appl. Catal., 38 (1988), 235.
D. Chadwick, D.W.
Atchison, R.
C a t a l y s t s 111, (G. Poncelet, Amsterdan, p 323, 1983. W.C.
R.
B a d i l l a , L. Josefsson, P.
Grange,
P.
" I n preparation o f
Jacobs
eds).
Elsevier,
Cheng, N.P. Luthra, J. Catal., 109, (1988), 163. Lopez Cordero. N. Esquivel, J . Lazaro, J.L.G. F i e r r o , A. Lopez Agudo.,
Appl
. Catal . 48
(1989), 341.
Luthra, W.C., Cheng, J . Catal., 107, (19871, 154. 11 P. Atanasova, T. Halachev, App. Catal ., 48, (19891, 295.
10 N.P.
12 A. Morales, M.M. Ramirez de Agudelo, F. Hernandez, Appl. Catal., 41 (1988), 261. 13 J.L.G. F i e r r o , A. LBpez Agudo, N. Esquivel, R. L6pez Cordero, Appl. Catal.
48 (1989), 353. 14 P. Atanasova, T. Halachev, J. U c h y t i l , M. Kraus, Appl. C a t a l . 38 (1988) 235. 15 R . Prada S i l v y , Y . Romero, M. GonzBlez, t o be published.
37
INFLUENCE O F THE PREPARATION PROCEDURE ON THE PHYSICAL PROPERTIES, SURFACE A C I D I T Y AND D I S P E R S I O N OF MoP/A1,0,
CATALYSTS
R. PRADA S I L V Y , Y. ROMERO, J . GUAREGUA, R. GALIASSO
I N T E V E P S.A, Seccion de C a t a l i s i s ' A p l icada, Apdo. 76343, Caracas 1070-A, VENEZUELA.
SUMMARY The i n f l u e n c e o f t h e p r e p a r a t i o n procedure on t h e p h y s i c a l p r o p e r t i e s , s u r f a c e a c i d i t y , d i s p e r s i o n and d i s t r i b u t i o n o f t h e supported phases i n MoP/A1,0, c a t a l y s t s i s here investigated. The r e s u l t s i n d i c a t e t h a t t h e b e h a v i o u r o f t h e molybdenum-phosphorus c a t a l y s t s depend s t r o n g l y on t h e i m p r e g n a t i o n sequence used ( c o i m p r e g n a t i o n o r d o u b l e i m p r e g n a t i o n ) . Coimpregn a t i o n procedure (Mo+P) a l l o w s t h e o b t e n t i o n o f c a t a l y s t s w i t h l a r g e r C o n c e n t r a t i o n o f a c i d s i t e s and s u r f a c e d i s p e r s i o n ! when compared t o consecuti v e i m p r e g n a t i o n procedures (P->Mo, Mo->P). T h i s sample a l s o .showed h i g h a c t i v i t y f o r t h e HDS, HDN and MHC r e a c t i o n s . The d i f f e r e n c e s i n b e h a v i o u r observed f o r t h e d i f f e r e n t samples c o u l d be m a i n l y a t t r i b u t e d t o t h e e x i s t e n c e o f d i f f e r e n t molybdenum d i s t r i b u t i o n phases, p r o b a b l y Mo-P . h e t e r o p o l compounds a r e formed f o r t h e coimpregnated sample, . w h i l e t h e f o r m a t i o n o f b u l t MOO, species c o u l d be f a v o r i z e d f o r t h e sample preimpregnated wJth phosphorus. I t was a l s o observed t h a t phosphorus produces t h e f o l l o w i n g e f f e c t s on alumina: i ) i t improves t h e . s u r f a c e a c i d i t y , ii) i t i n c r e a s e t h e mechanical s t r e n g t h o f t h e e x t r u d a t e s , iii) i t decreases t h e s u r f a c e a r e a o f t h e s u p p o r t . We e x p l a i n o u r r e s u l t s on t h e b a s i s o f t h e mechanism proposed f o r t h e a d s o r p t i o n o f molybdate. phosphate and phosphomolybdate compounds on alumina. INTRODUCTION Molybdenum supported alumina c a t a l y s t s promoted by m e t a l s o f t h e group
V I I I ( N i , Co) a r e w i d e l y used f o r h y d r o p r o c e s s i n g heavy o i l s o r c o a l d e r i v e d fuels.
E f f o r t s have been made t o improve c a t a l y t i c a c t i v i t y by f i n d i n g appro-
p r i a t e a d d i t i v e s (such as P,
B,
S i , T i , F , Ga, e t c . ) . , Phosphorus can be c o n s i -
dered one o f t h e most e f f e c t i v e a d d i t i v e s o f t h e molybdenum supported alumina c a t a l y s t s . I n f a c t , i t appears as a component i n a number o f f o r m u l a t i o n s o f commercial h y d r o t r e a t i n g c a t a l y s t s . The p a t e n t l i t e r a t u r e c l a i m s t h a t i t s use p r o v i d e s b e t t e r c a t a l y t i c performance, i n c r e a s i n g h y d r o d e s u l f u r i z a t i o n (HDS), h y d r o d e n i t r o g e n a t i o n (HDN), hydrodemetall i z a t i o n (HDM) , and i m p r o v i n g t h e m i l d hydrocracking
(MHC)
activity
(ref.
1-41.
phosphorus produces s e v e r a l e f f e c t s , such as:
I t has been demonstrated t h a t
i) i t p r o v i d e s a more s t a b l e
i m p r e g n a t i n g s o l u t i o n and t h u s b e t t e r d i s p e r s i o n o f t h e metal ( r e f . 1 , 4 ) , ii) i t i n h i b i t s t h e f o r m a t i o n o f Ni(Co)Al,O,-like
on
t h e support species and
enhances t h e f o r m a t i o n o f N i z t ' i o n s w h i c h a r e p r i m a r l y r e s p o n s i b l e f o r t h e f o r m a t i o n o f c a t a 1 . y t i c a l l y a c t i v e n i c k e l s u l f i d e o r NiMoS-like phase ( r e f . 3 ) ,
iii) i t a l t e r s t h e a c i d s t r e n g t h d i s t r i b u t i o n on alumina, t h e c o n c e n t r a t i o n o f medium a c i d s i t e s i n c r e a s e s p r o g r e s s i v e l y w i t h i n c r e a s i n g phosphorus l e v e l
38
5) and i v ) i t improves t h e thermal s t a b i l i t y o f gamma alumina w i t h t o s i n t e r i n g and and phase t r a n s i t i o n t o alpha alumina ( r e f . 5).
(ref.
respect
The b e n e f i c a l e f f e c t s o f phosphorus has s t i m u l a t e d research on i t s i n f l u e n c e on molybdena based c a t a l y s t s . However, most o f t h e above mentioned
1 i t e r a t u r e e s s e n t i a l l y focused on t h e i n f l u e n c e o f phosphorus on t h e c a t a l y t i c p r o p e r t i e s o f t h e m o d i f i e d system and i t s e f f e c t on t h e d i s p e r s i o n on the a c t i v e phase deposited on t h e alumina surface has n o t y e t been i n v e s t i g a t e d . Also,
very few works deal w i t h t h e i n f l u e n c e o f t h e sequence o f phosphorus c a t a l y s t s on t h e
i n c o r p o r a t i o n d u r i n g t h e p r e p a r a t i o n step o f the MoP/A1 ,O,
surface a c i d i t y , d i s p e r s i o n and d i s t r i b u t i o n o f t h e supported phases.
As the
phosphate i o n s s t r o n g l y i n t e r a c t w i t h alumina, competing w i t h molybdate ions, a f a c t o r o f p o s s i b l e importance i s t h e p r e p a r a t i o n procedure. Three
different
procedures
can
be
followed .to
prepare a MoP/A1 ,03
catalyst.
(1) P A1,0,
-->
MO
(2) MO + P ( 3 ) Mo --> P
->
MOP/ A1,0,
-->
This c o m u n i c a t i o n i s p a r t o f a research program aimed a t a systematic i n v e s t i g a t i o n o f t h e p r e p a r a t i o n procedure o f MoP/A1,0, catalysts.
Essentially,
we study
mild-hydrocracking
the e f f e c t o f phosphorus i n c o r p o r a t i o n
sequence on t h e s t a t e o f d i s p e r s i o n o f t h e a c t i v e phase, surface a c i d i t y and physical p r o p e r t i e s . the
following
For t h i s purpose, t h e samples were c h a r a c t e r i z e d using
physico-chemical
techniques:
BET
strength, X-ray p h o t o e l e c t r o n spectroscopy (XPS).
(SEM),
surface
area,
mechanical
scanning e l e c t r o n microscopy
surface a c i d i t y determined by p y r i d i n e adsorption.
EXPERIMENTAL Catalyst preparation Three MoP/A1,0, c a t a l y s t s having constant molybdenum and phosphorus conwere prepared by e i t h e r consecutive t e n t s (10 w t % MOO, and 4.5 w t % P,O,) (P->Mo
o r Mo-->P) o r simultaneous (Mo+P) impregnation o f alumina extrudates
(1/20 i n c h l e n g t h ) w i t h aqueous s o l u t i o n s o f ammonium heptamolybdate and/or orthophosphoric acid.
The alumina has a s p e c i f i c surface area o f 269 m 2 / g and
a pore volume of 0.69 cc/g.
The impregnating s o l u t i o n s were adjusted t o pH
=
1.5-2.0 by adding n i t r i c a c i d b e f o r e c o n t a c t i n g w i t h t h e support. The samples were d r i e d a t 373 K f o r 2 h and then c a l c i n e d i n two steps a t 623 K f o r 2 h and a t 7 7 3 K f o r 3 h.
39
Characterization Surface areas o f c a t a l y s t s were determined by t h e BET method from N, analyzer. An Erweka apparatus was used t o measure t h e s i z e c r u s h i n g s t r e n g t h o f c a t a l y s t extrudates.
The method determines p a r t i c l e c r u s h i n g s t r e n g t h by
measuring t h e f o r c e i n kilograms ( k ) r e q u i r e d t o crush an e x t r u d a t e o f measured s i z e .
A l a r g e number o f extrudates (about f o r t y o f each sample) were t e s t -
ed and t h e average value was e s t a b l i s h e d . X-ray
p h o t o e l e c t r o n spectra
(XPS)
o f c a t a l y s t s were recorded u s i n g a
Leybold Heraeus LHS-11 apparatus equipped w i t h a computer system, which a1 lowed
the determination o f
Alka l i n e ( E = 1486 eV).
peak areas.
The e x c i t a t i o n r a d i a t i o n was
the
A l l t h e samples were grounded and then pressed i n t o
Cis.
Alpp, A l Z s . M o ~ ~M , o ~ ~P2p , and P 2 s e n e r g y l e v e l s were recorded. The CIS energy l e v e l (284.5 eV) was taken as a reference. Atomic surface c o n c e n t r a t i o n o f t h e sample h o l d e r s b e f o r e t h e a n a l y s i s .
Signals corresponding t o
supported elements was evaluated form t h e peak i n t e g r a t e d areas and t h e sensit i v i t y f a c t o r s provided by t h e equipment manufacturer. The elemental p r o f i l e d i s t r i b u t i o n o f b o t h molybdenum and phosphorus, across t h e t r a n s v e r s a l s e c t i o n o f t h e alumina extrudates, was obtained u s i n g the
scanning
e l e c t r o n microscopy
technique
(SEM).
An
ISI-60 apparatus
equipped w i t h an energy d i s p e r s i v e X-ray analyzer (Kevex 5-7000) was used f o r these measurements.
C a t a l y s t extrudates were mounted on an epoxy s l i d e and
then p o l i s h e d b e f o r e scanned under t h e e l e c t r o n beam. The a c i d i t y measurements were c a r r i e d o u t i n a Cahn 1000 e l e c t r o b a l a n c e u s i n g p y r i d i n e as probe molecule adsorbed on t h e c a t a l y s t
surface.
The
i r r e v e r s i b l y adsorbed p y r i d i n e amounts were determined a t 273 K, 473 K and 573 K.
Results a r e expressed as m o l o f p y r i d i n e i r r e v e r s i b l y adsorbed p e r
surface area o f c a t a l y s t . RESULTS Surface areas and mechanical s t r e n g t h Specific
surface areas corresponding t o MoP/A1 209 c a t a l y s t s prepared
f o l l o w i n g d i f f e r e n t procedures a r e given i n Table 1. t h e same t a b l e t h e values obtained f o r A1,0, Mo/A1,0,
samples.
I t i s also reported i n
support and f o r P/A1,0,
and
40
TABLE 1 S p e c i f i c s u r f a c e areas and mechanical s t r e n g t h corresponding t o d i f f e r e n t p r e p a r a t i o n procedures Sample
Surface Area (m'/g)
A1 Z 0 3 P / A 1 ,03 Mo/A1 ,O,
Crushing S t r e n g t h ( k g / p e s t l e )
269 241 248 214 222 234
P->Mo Mo->P MotP
For b o t h Mo/A1,0,
5.2 6.0 5.3 6.1 5.5 6.1
and P/A1,0,
samples,
t h e surface area decreased i n
8% and 11%, r e s p e c t i v e l y , a f t e r impregnation w i t h ammonium -heptamolybdate o r phosphoric acid, whereas f o r MoP/A1 ,03 c a t a l y s t s t h e l o s s
approximatively,
i n surface areas was more pronounced.
One can observe t h a t t h e s u r f a c e area
value o b t a i n e d f o r t h e sample prepared by coimpregnation i s s l i g h t l y higher than t h a t observed f o r those samples prepared by consecutive impregnation.
A l l MoP/A1,0,
c a t a l y s t s presented a pore volume i n t h e 0.52 - 0.55 c c / g range.
Concerning t h e mechanical s t r e n g t h measurements,
i t i s observed i n Table
1 t h a t phosphorus s l i g h t l y improves t h e mechanical s t r e n g t h o f alumina e x t r u dates, w h i l e molybdenum seems t o have no i n f l u e n c e on these p r o p e r t i e s .
The
p r e p a r a t i o n procedures (P->Mo) and (Mo+P) produce c a t a l y s t s w i t h simi 1 a r c r u s h i n g s t r e n g t h values. X-ray p h o t o e l e c t r o n spectroscopy (XPS) The XPS r e s u l t s obtained f o r P/A1 ,O,,
Mo/A1 ,O,
and MoP/A1 ,O,
catalysts,
a r e presented i n Table 2. The percentage o f surface d i s p e r s i o n o f both Mo and P elements ( I e / I A l x 100) i s r e p o r t e d as a f u n c t i o n o f c a t a l y s t p r e p a r a t i o n
procedure. Mo/A1,0,
When comparing t h e (Mo/A1) o r ( P / A l )
o r P/A1,0,
i n t e n s i t y r a t i o obtained f o r
s w p l e s w i t h t h a t o f MoP/A1,0,
c a t a l y s t s , we can observe
s t r i k i n g d i f f e r e n c e s i n t h e d i s p e r s i o n s t a t e o f t h e supported species.
Molyb-
denum d i s p e r s i o n o f samples v a r i e s as f o l l o w s :
while,
i n t h e case o f t h e phosphorus d i s p e r s i o n , t h e observed sequence i s as
follows: Mo+P
>
P/A1,0,
>
Mo->P
>
P->Mo
41
MO
MO
-070
0
070
Radial Position (mm) Fig. 1. Electron Microprobe p r o f i l e of Mo and P corresponding t o MoP/A1,0, c a t a l y s t s prepared following d i f f e r e n t procedures.
A t 373 K (weak acid s i t e s ) , t h e pyridine adsorbed amounts per surface area v a r i e s as follows: Mo+P > P->Mo > Mo->P = A1,0,
Whereas, f o r desorption temperatures of 473 K and 573 K (medium and strong acid s i t e s , r e s p e c t i v e l y ) , the observed sequence i s :
42
TABLE 2 XPS r e s u l t s c o r r e s p o n d i n g t o t h e c a t a l y s t s prepared u s i n g d i f f e r e n t
procedures. IMojp/ I A 1
Sample
2P
I P 2 p / I A 1 2P
__
Mo/A1 ,O,
4.5
2.7 __
P->Mo
4.1
2.3
Mo->P
4.4
2.5
Mo+P
4.8
3.3
P / A 1 *03
Scanning e l e c t r o n microscopy (SEM) F i g u r e 1 r e p r e s e n t s t h e p r o f i l e d i s t r i b u t i o n , across t h e t r a n s v e r s a l sect i o n o f t h e alumina e x t r u d a t e s , b o t h f o r phosphorus and molybdenum elements, obtained
through
procedures
(Mo+P)
SEM
and
technique. (Mo->P),
For
catalysts
b o t h Mo and
prepared
P elements
following are
the
distributed
homogeneously i n t h e s u p p o r t , whereas sample prepared a c c o r d i n g t o procedure (P->Mo),
c l e a r l y shows some h e t e r o g e n e i t i e s .
Strength o f a c i d i t y S u r f a c e a c i d i t y r e s u l t s , o b t a i n e d t h r o u g h p y r i d i n e a d s o r p t i o n , correspond i n g t o A1,0,
s u p p o r t and d i f f e r e n t MoP/A1,0,
catalysts,
Table 3 as a f u n c t i o n o f t h e d e s o r p t i o n temperature.
are presented i n
Differences i n pyridine
i r r e v e r s i b l y adsorbed amounts p e r s u r f a c e a r e a u n i t can be observed i n t h e 373 -573 K temperature range, depending on t h e p r e p a r a t i o n method used. TABLE 3 I r r e v e r s i b l y a c i d i t y o f MoP/A1,0,
prepared c a t a l y s t s ( m o l p y r i d i n e l m ' )
Sample 373
TEMPERATURE ( K ) 473
573
1.41
0.67
0.11
Mo->P
1.78 1.44
0.83 0.76
0.42 0.32
Mo+P
2.06
0.85
0.45
3'2
P->Mo
x lo3
43
DISCUSSION The above r e s u l t s show how a c i d i t y and s u r f a c e d i s p e r s i o n o f supported phases found i n MoP/A1,0,
c a t a l y s t s can be s t r o n g l y i n f l u e n c e d by t h e prepara-
Tables 2 and 3 i n d i c a t e t h a t c o i m p r e g n a t i o n o f Mo and P
t i o n procedure.
a l l o w s t h e o b t e n t i o n o f a c a t a l y s t w i t h l a r g e r c o n c e n t r a t i o n o f a c i d s i t e s and surface dispersion,
(P->Mo,
Mo->P).
t h e mechanical
i n comparison w i t h c o n s e c u t i v e i m p r e g n a t i o n procedures
Phosphorus seems t o improve t h e a c i d i t y s t r e n g t h as w e l l as p r o p e r t i e s o f alumina e x t r u d a t e s .
However,
this additive
s t r o n g l y a f f e c t s t h e s p e c i f i c surface area o f t h e support. L e t u s d i v i d e t h e d i s c u s s i o n o f o u r r e s u l t s i n two p a r t s :
we s h a l l f i r s t
d i s c u s s t h e r e s u l t s o b t a i n e d on t h e sample prepared by c o i m p r e g n a t i o n (MotP) and subsequently, examine t h e b e h a v i o u r o f those c a t a l y s t s p r e p a r e d by consec u t i v e impregnations (P->Mo and Mo->P). P r e p a r a t i o n Procedure (Mo+P) Table 2 shows t h a t c o i m p r e g n a t i o n method improves t h e s u r f a c e d i s p e r s i o n
o f b o t h molybdenum and phosphorus.
T h i s agrees w i t h r e c e n t r e s u l t s o b t a i n e d
by Atanasova e t a l . ( r e f . 6), who s t u d y i n g a s e r i e s o f NiMoP/Al,O, XPS
and c a t a l y t i c
measurements,
stated
that
c a t a l y s t by
c o i m p r e g n a t i o n method and a
s u i t a b l e alumina c a r r i e r can l e a d t o a b e t t e r d i s p e r s i o n o f a c t i v e components and consequently, t o an i n c r e a s e i n
HDS a c t i v i t y .
We s h a l l a t t e m p t t o e x p l a i n now o u r r e s u l t s supported on fundamental s t u d i e s d e a l i n g w i t h a d s o r p t i o n o f Mo and/or P
on
alumina.
L e t us r e f e r t o t h e
more i m p o r t a n t r e s u l t s o f t h e s e s t u d i e s . L i t e r a t u r e shows t h a t phosphorus i n c r e a s e s t h e s o l u b i l i t y and s t a b i l i t y o f molybdenum s o l u t i o n s
(ref.
1,4). A d s o r p t i o n s t u d i e s proposed t h a t when
alumina i s coimpregnated w i t h s o l u t i o n s c o n t a i n i n g molybdate and phosphate, t h e r e i s a c o m p e t i t i o n between b o t h i o n s f o r t h e same a d s o r p t i o n s i t e s ( b a s i c hydroxyl
groups
of
alumina),
thus,
a d s o r p t i o n o f rnolybdates (3,6-9).
the
adsorbed
phosphate
inhibits
the
However, most o f t h e s e s t u d i e s deal w i t h
molybdate and phosphate a d s o r p t i o n s e p a r a t e l y w i t h o u t c o n s i d e r i n g f o r m a t i o n o f phosphomolybdate compounds i n t h e i m p r e g n a t i n g s o l u t i o n .
the
R e c e n t l y , Cheng and L u t h r a ( r e f . 8 ) , u s i n g t h e NMR t e c h n i q u e , s t u d i e d t h e a d s o r p t i o n o f v a r i o u s phosphomolybdate compounds on alumina spheres.
Authors
observed t h a t when phosphoric a c i d i s added t o a s o l u t i o n c o n t a i n i n g amonium hep tam0 1 y b d a t e , p e t amol ybdodi phosphate compounds ( P ,Mo s o l u t i o n s c o n t a i n i n g P/Mo m o l a r r a t i o h i g h e r t h a n 0.4, remained i n f o r m o f phosphates.
1 a r e formed.
For
amounts o f phosphorus
T h i s suggests t h e e x i s t e n c e o f a chemical
e q u i l i b r i u m between b o t h phosphate and molybdate i o n s i n s o l u t i o n .
44
8 H * + 5 MOO:'
+ 2 HP0:-
=
P,Mo,O,,
6-
+
(2)
5H20
A c c o r d i n g t o t h e above e q u a t i o n , decomposition o f phosphomolybdate i n t o s i m p l e molybdate and phosphate c o u l d be f a v o u r e d by a r i s e o f s o l u t i o n pH, which would s h i f t t h e chemical e q u i l i b r i u m t o l e f t . ,
Indeed, t h i s behaviour
was observed d u r i n g phosphomolybdate a d s o r p t i o n on alumina.
The i n c r e a s e i n
pH o f t h e i m p r e g n a t i n g s o l u t i o n was a t t r i b u t e d t o w a t e r f o r m a t i o n d u r i n g i o n exchange r e a c t i o n . L u t h r a and Cheng ( r e f . 10) observed t h a t e q u i l i b r i u m b e t w e e n heptamolybdate Mo,O:i rise Mo,O,,
and molybdate MOO:-
i o n s i s a l s o a f f e c t e d by a
i n pH. 6-
+
4 H,O
I n short,
=
2-
7 MOO,
+
(3)
8 H
r e s e a r c h o f Cheng and L u t h r a c l e a r l y i l l u s t r a t e s t h a t t h e h i g h
s o l u b i l i t y and s t a b i l i t y observed when p h o s p h o r i c a c i d i s added t o molybdenum i s m a i n l y due t o t h e f o r m a t i o n o f 'phosphomolybdate compounds.
These compounds
a r e v e r y s e n s i t i v e t o changes i n t h e i m p r e g n a t i n g s o l u t i o n pH. The f a c t t h a t t h e r e a r e d i f f e r e n c e s i n a c i d s i t e s d i s t r i b u t i o n and s u r f a c e d i s p e r s i o n when u s i n g d i f f e r e n t procedures t o p r e p a r e a MoP/A1 ,O,
catalyst,
suggests t h a t n a t u r e and c o n c e n t r a t i o n o f t h e o x i d i c supported phases p r e s e n t i n these c a t a l y s t s are d i f f e r e n t . Our h y p o t h e s i s i s c o n s i s t e n t w i t h t h e r e s u l t s o b t a i n e d by o t h e r researche r s ( r e f . 3, 6, 9, 111, who combining v a r i o u s c h a r a c t e r i z a t i o n techniques, s t u d i e d t h e s t r u c t u r a l changes t h a t o c c u r r e d when phosphorus i s used as an a d d i t i v e o f Mo/A1 ,O,
Atanasova and Halachev ( r e f . l l ) , s t u d y i n g
catalysts.
t h r o u g h I R spectroscopy t h e phases p r e s e n t i n NiMoP/Al ,O, by coimpregnation, observed bands c o r r e s p o n d i n g t o AlPO, and Ni-Mo-P
heteropoly
compounds.
IR-bands
c a t a l y s t s , prepared and t o a mixed A1-Mo
c o r r e s p o n d i n g t o b u l k MOO,,
Al,(MoO,), and NiMoO, were n o t observed i n t h o s e samples. Authors observed t h a t h i g h phosphorus c o n t e n t l e a d s t o an i n c r e a s e i n degree o f molybdenum p o l y m e r i z a t i o n and t o changes
i n t h e r a t i o between t h e d i f f e r e n t t y p e s o f
h e t e r o p o l y compounds, Ni-Mo-P/Al-Mo loadings.
Lopez Corder0 e t a l .
r a t i o i n c r e a s e s w i t h i n c r e a s i n g phosphorus ( r e f . 9) s t u d i e d by TPR and DRS t h e s u r f a c e
d i s t r i b u t i o n o f molybdenum on two s e r i e s o f MoP/A1 ,O,
c a t a l y s t s which were
prepared u s i n g simul taneous (P+Mo) o r double i m p r e g n a t i o n (P->Mo) methods. I n c o n t r a s t w i t h t h e r e s u l t s o b t a i n e d by Atanasova and Halachev ( r e f . 111, t h e a u t h o r s observed t h e presence o f b u l k MOO,
and a l s o small c l u s t e r s o f p o l y -
molybdate m u l t i l a y e r s f o r b o t h c a t a l y s t s e r i e s . l a t t e r specieswas
The c o n c e n t r a t i o n o f t h e
more i m p o r t a n t f o r t h e preimpregnated samples.
45
C o n s i d e r i n g o u r r e s u l t s t o g e t h e r w i t h t h o s e found i n l i t e r a t u r e ,
two
p o s s i b l e e x p l a n a t i o n s can be proposed: The f i r s t e x p l a n a t i o n i s based on t h e mechanism proposed f o r molybdate o r phosphomolybdate phosphate a d s o r p t i o n on alumina ( r e f . 121, because compounds were observed t o decompose i n t o these two species d u r i n g a d s o r p t i o n . These s t u d i e s suggest t h a t phosphate as w e l l as molybdate i n t e r a c t f i r s t w i t h b a s i c h y d r o x y l groups o f alumina, g e n e r a t i n g a w a t e r molecule. Competition between phosphorus and molybdenum t a k e s p l a c e . Phosphate i s adsorbed on alumina more r a p i d l y t h a n molybdate i s adsorbed.However, t h e r a t e o f a d s o r p t i o n o f b o t h compounds depends on s e v e r a l f a c t o r s , such as: i ) n a t u r e and c o n c e n t r a t i o n o f t h e i o n s i n t h e i m p r e g n a t i n g s o l u t i o n , ii) pH o f s o l u t i o n , iii) t y p e o f alumina and i v ) a d s o r p t i o n temperature. For each exchanged h y d r o x y l group by phosphoric a c i d molecule, two new a c i d s i t e s a r e c r e a t e d . T h i s would e x p l a i n t h e i n c r e a s e s i n a c i d s i t e s c o n c e n t r a t i o n observed a f t e r phosphorus i n c o r p o r a t i o n i n t o alumina. Morales e t a l . ( r e f . 12) suggested t h a t when a l l b a s i c h y d r o x y l groups a r e t i t r a t e d , t h e a c i d h y d r o x y l groups b e g i n t o be t i t r a t e d and t h e n a monolayer o f phosphate i s formed by f u r t h e r a d d i t i o n o f p h o s p h o r i c a c i d . I n t e r a c t i o n s between n e i g h b o r i n g adsorbed phosphates c o u l d occurs l e a d i n g t o t h e f o r m a t i o n o f p o l y m e r i c phosphate c h a i n s . I n t h i s p a r t i c u l a r case, t h e a u t h o r s mentioned t h a t t h e number o f a c i d s i t e s remains almost c o n s t a n t because t h e s u b s t i t u t i o n o f two a c i d h y d r o x y l groups o f alumina would y i e l d two a c i d s i t e s a s s o c i a t e d t o phosphorus. I n t h e c o m p e t i t i v e system, we can propose t h a t b o t h molybdate and phosp h a t e i o n s c o u l d be adsorbed i n n e i g h b o r i n g s i t e s . T h i s would i m p l y t h a t adsorbed molybdates impede t h e p o l y m e r i z a t i o n o f adsorbed phosphate and phosphates would produce t h e same e f f e c t on molybdates. The l a t t e r may e x p l a i n t h e h i g h e s t s u r f a c e d i s p e r s i o n o f b o t h P and Mo observed f o r t h e coimpregnated sample. Since p o l y m e r i z a t i o n o f phosphorus as we1 1 as molybdenum was f a v o u r e d by h i g h c o n t e n t s o f t h e s e elements, we c o u l d suggest t h a t i n t e r a c t i o n s between n e i g h b o r i n g molybdate and phosphate adsorbed species may occur c a u s i n g t h e f o r m a t i o n o f Mo-P h e t e r o p o l y compounds a f t e r c a l c i n a t i o n . One may s p e c u l a t e t h a t a c i d i t y produced f o r t h e l a t t e r compounds should be d i f f e r e n t f r o m t h a t produced by phosphates on alumina. The second e x p l a n a t i o n would be t o c o n s i d e r t h a t phosphomolybdate remains stable during impregnation. In t h i s case, t h e b e h a v i o u r o f these compounds towards a d s o r p t i o n would be d i f f e r e n t f r o m t h a t o f p h o s p h a t e a n d molybdate. One may s p e c u l a t e t h a t Mo-P h e t e r o p o l y compounds may be formed f r o m t h e adsorbed phosphomolybdate a f t e r c a l c i n a t i o n . T h e r e f o r e , t h e h i g h e s t d i s p e r s i o n observed f o r t h e coimpregnated sample may be a t t r i b u t e d t o t h e f o r m a t i o n o f Mo-P h e t e r o p o l y compounds, which were observed on coimpregnated samples i n a r e c e n t study ( r e f . 11). L e t us now complement o u r d i s c u s s i o n showing some i m p o r t a n t e f f e c t s
46
observed through scanning e l e c t r o n microscopy technique i n t h e a n a l y s i s o f t h e samples.
Figure 1 shows t h a t b o t h phosphorus and molybdenum a r e homogeneously
d i s t r i b u t e d i n t h e coimpregnated sample. and L u t h r a ( r e f .
8)
during adsorption o f
However, r e s u l t s obtained by Cheng
showed an i n t e r e s t i n g chromatographic e f f e c t appearing phosphomolybdate on alumina spheres,
p r e f e r e n t i a l l y l o c a t e d a t t h e edge, w h i l e molybdenum c e n t e r o f t h e spheres.
phosphorus was
was concentrated a t t h e
The l a t t e r o b s e r v a t i o n makes e v i d e n t t h e decomposition
o f phosphomolybdate d u r i n g a d s o r p t i o n and t h e c o m p e t i t i o n o f b o t h phosphate and molybdate
ions
for
the
same adsorption
sites.
I n our o p i n i o n ,
we
a t t r i b u t e t h e d i f f e r e n t behaviour observed i n b o t h s t u d i e s t o d i f f e r e n c e s i n p r e p a r a t i o n c o n d i t i o n s o f t h e samples, as w e l l as t h e type o f alumina employed. Preparation Procedures (P->Mo) and (Mo->P) As shown i n Tables 1 and 3, phosphorus seems t o improve t h e mechanical
p r o p e r t i e s as w e l l as t h e a c i d i t y s t r e n g t h o f alumina extrudates,
However,
t h i s a d d i t i v e s t r o n g l y a f f e c t s t h e s p e c i f i c surface area o f support. Our r e s u l t s a r e i n agreement w i t h those obtained by several i n v e s t i g a t o r s ( r e f . 7,9,12).
Lopez Cordero e t a l . ( r e f . 9)
suggested t h a t l o s s i n surface
area o f alumina a f t e r phosphorus i n c o r p o r a t i o n i s probably due t o a c o r r o s i v e e f f e c t o f surface caused by phosphoric a c i d molecules o r t o a pore blockage by phosphate species. t h e 30-60 explanation
A'
We have observed t h a t pores having an average diameter i n
range were t h e most a f f e c t e d by phosphorus d e p o s i t i o n .
An
o f these r e s u l t s c o u l d be t h e f a c t t h a t these pores c o u l d be
s e l e c t i v e l y plugged by polymeric phosphate adsorbed species,
However, we can
n o t d i s c a r d a p o s s i b l e c o r r o s i v e e f f e c t produced by t h e phosphoric a c i d molecules on t h e alumina surface.
An e l e c t r o n microscopy study o f t h e P/A1,0,
sample c o u l d reveal p o s s i b l e morphological changes due t o phosphorus. Table 2 i n d i c a t e s t h a t f o r t h e sample prepared f o l l o w i n g procedure ( P - > M o ) . t h e Mo d i s p e r s i o n ( I M o / I A 1 ) was a f f e c t e d b y phosphorus 3P 2P i n c o r p a t i o n . The I P 2 p / I A 1 2 p i n t e n s i t y r a t i o a l s o decreased w i t h respect t o t h e value obtained f o r P/A1,0,
sample,
a f t e r molybdenum d e p o s i t i o n .
These
r e s u l t s can be explained by t h e f a c t t h a t preimpregnation o f alumina w i t h phosphorus reduces t h e number o f s i t e s a v a i l a b l e f o r molybdate adsorption. Therefore, changes i n d i s p e r s i o n and d i s t r i b u t i o n o f molybdenum species should be expected. I n t h i s case, one c o u l d suggest t h a t phosphorus promotes t h e formdtion o f b u l k MOO, species, which i s i n agreement w i t h t h e r e s u l t s o b t a i n ed by Lopez Cordero e t a l . deposited on t h e AlPO,
( r e f . 9).
monolayer,
A p a r t o f t h e l a t t e r species may be
which would e x p l a i n t h e decreasing i n
47
phosphorus
intensity
e l e c t r o n microscopy
r a t i o observed f o r study
confirms
the
this
sample.
Additionally,
heterogeneous d i s t r i b u t i o n
molybdenum species when alumina i s preimpregnated w i t h phosphorus.
the
o f the
A similar
s i t u a t i o n m i g h t be expected when t h e c a r r i e r i s preimpregnated w i t h molybdenum.
I n this
particular
case,
mechanical
strength
as w e l l
as a c i d i t y
p r o p e r t i e s s h o u l d n o t be improved by phosphorus. A c t i v i t y o f t h e (MotP) and (P->Mo) samples I n t h i s work we have e v a l u a t e d t h e c a t a l y t i c p r o p e r t i e s o f t h e molybdenum-phosphorus
catalysts.
For
this
purpose,
we
have
prepared
two
Ni-Mo-P/Al,O,
samples f o l l o w i n g t h e i m p r e g n a t i o n sequences (Mo+P->Ni) and
(P->Mo->Ni).
These samples p r e s e n t t h e same chemical
MOO,,
7 . 5 w t % P,O,
and 5.0 w t % N i O ) .
c o m p o s i t i o n (15 w t %
The c a t a l y t i c r e a c t i o n was c a r r i e d o u t
i n a h i g h p r e s s u r e f i x e d bed r e a c t o r u s i n g a vacuum g a s o i l under t y p i c a l mild-hydrocracking conditions
(T = 653K, P
= 5 MPa, LHSV = 0.65 l / h , H,/Hc
=
600). We p r e s e n t i n Table 4 t h e a c t i v i t y r e s u l t s o f t h e NiMoP/Al,O, Both samples
show comparable a c t i v i t y
i n MHC.
However,
r e a c t i o n s a r e h i g h e r f o r t h e coimpregnated sample.
catalysts.
t h e HDS and HDN
The same b e h a v i o u r was
observed by o t h e r r e s e a r c h e r s ( r e f . 13 and 1 4 ) . TABLE 4 Activity o f the
NiMoP/Al,O,
samples
% CONVERSION
HDS
HDN
MHC
(Mo+P-> N i l
82
59
13
(P-> Mo-> N i )
77
50
11
SAMPLE
I n o r d e r t o analyze t h e p o s s i b l e changes i n t h e molybdenum d i s t r i b u t i o n phases induced by phosphorus, we have c a r r i e d o u t I R measurements MoP/A1 ,O, samples b e f o r e n i c k e l i m p r e g n a t i o n . published
elsewhere
(ref.
molybdenum d i s t r i b u t i o n ,
15),
depending
confirm
the
on
both
The r e s u l t s , which w i l l be fact
modify
the
on t h e p r e p a r a t i o n procedure f o l l o w e d .
phosphorus
No
evidences about t h e presence o f phosphomolybdate compoundswere observed f o r t h e coimpregnated sample.
48
To summarize. we may conclude t h a t The d i f f e r e n c e s i n a c t i v i t y observed f o r t h e (MotP-,
N i l and (P->Mo->
N i ) samples c o u l d be associated t o changes
induced by phosphorus on t h e s u r f a c e molybdenum d i s t r i b u t i o n phases. CONCLUSIONS In
the
frame
of
the
present
work
we
conclude
that
coimpregnation
p r o c e d u r e i s t h e more a p p r o p r i a t e method o f p r e p a r i n g MoP/Al *03c a t a l y s t s . This procedure a l l o w s t h e o b t e n t i o n o f samples w i t h h i g h e r d i s p e r s i o n , surface a c i d i t y and c a t a l y t i c p r o p e r t i e s . Changes observed i n molybdenum d i s t r i b u t i o n phases caused by phosphorus depend on t h e impregnation sequence employed.
Coimpregnation procedure would
l e a d t o t h e Mo-P heteropoly compound formation,
w h i l e preimpregnation w i t h
phosphorus c o u l d induce t h e b u l k MOO, formation. I t was a l s o observed t h a t phosphorus produces t h e f o l l o w i n g e f f e c t s on
alumina:
i ) i t improves t h e surface a c i d i t y , ii) i t increases t h e mechanical
s t r e n g t h o f extrudates, iii) i t decreases t h e surface area o f support.
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