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Al2O3 Catalysts
Guni, L et al. (Editors), New Frontiers in Catalysis
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1W, Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rights rcserved
ENHANCED HDS ACTIVITY VIA MULTIPLE IMPREGNATION OF SULFIDED Mo/A1203 CATALYSTS
C.-S.Kima, F. E. Massotho, C. Geantefi andM. Breysseb aDepartment of Fuels Engineering, University of Utah, Salt Lake City, Utah 841 12, USA bInstitut de Recherches sur la Catalyse, CNRS, 2 avenue Albert Einstein, 69626 Villeurbanne Cedex, France
Abstract Catalysts, prepared by mu1 tiple impregnations of previously sulfided catalysts (SIP), together with conventionally prepared catalysts (CON), were characterized by high resolution electron microscopy, NO chemisorption and HDS activity. The SIP catalysts displayed similar morphology as the CON catalysts, but exhibited lower NO adsorptions and higher HDS activites. It is proposed that the higher activities are due to the creation of more catalytically active sites from the repeated exposure to air during their preparation. 1. INTRODUCTION
One of the 1 imitations of hydrotreating catalysts for hydrodesulfurization
(HDS) of sulfur-containing feeds is that the HDS activity reaches a maximum
at about 10-12% Mo, with no further increase in activity with additional amounts of Mo. The object of the current research was to improve the HDS activity by sequential additions of Mo to presulfided catalysts. The rationale for this approach came from prior studies, where it was found that the MoS, coverage of the alumina support remained low, even at high levels of Mo [ l ] . Thus, it was thought that addition of Mo to a presulfided catalyst would allow additional coverage the uncovered alumina with MoS,, giving a superior dispersion and catalytic activity. 2. EXPERIMENTAL
Sulfide-impregnated catalysts (SIP) were prepared starting with a 7.7% Mo/Al,O,, which was presulfided at 400' C. Additional Mo was added by an incipient wetness impregnation, followed by drying in He at 100' C and stream up to 400' C, holding at this temperature sulfiding with a 10% H,S/H, for 2 h. Additional incremental amounts of amounts of Mo were again added and the catalysts sulfided as before. Another series o f catalysts (CON) were prepared by a conventional method, consisting of multiple impregnations with
oven drying between additions, followed by air calcination at 500' C for overnight. Catalysts (CON) were sulfided at 400' C prior to testing. The sulfided catalysts were examined by high-resolution electron microscopy, as described previously [ 2 ] , and were characterized by NO chemisorption at '0 C, using a pulse technique [3]. Catalytic activities for thiophene HDS were measured in a fixed-bed reactor under vapor-phase conditions at atmospheric pressure and 350' C [4]. Pseudo first-order rate constants were calculated from conversions obtained after 18 h on stream. 3. RESULTS AND DISCUSSION
Electron microscope pictures of a sulfide CON and SIP catalyst are given in Figure 1. No morphological differences were observed between the two sets
Figure 1. TEM micrographs of 14.8% Mo CON (a) and 15.5% SIP (b) catalysts. o f catalysts. With increasing loading, the stacking of CON catalysts remained
low, but a growing number of bulk crystallites was observed. These particles of bulk MoS, probably did not represent a large fraction of the Mo, as XRD failed to detect MoS,. For SIP catalysts, stacking was slightly promoted, but few bulk crystallites were visible in this series. Measurements o f the average size and number of layers of MoS, slabs, given in Table 1, show an increase in both with increasing Mo content. The SIP catalysts exhibited slightly larger slab sizes and number of layers than the CON catalysts at a given Mo content. Evidently, the SIP procedure did not give a better dispersion with respect to monolayer slabs as hoped. The NO adsorption for the SIP catalysts was significantly less than for the CON catalysts (Table 1); the former were independent o f Mo content, while the latter showed a slight maximum. Previous literature has suggested that edge sites of the MoS, slabs are responsible for adsorption and catalytic reaction, and these are believed to be associated with sulfur anion vacancies
1937
Table 1 Characterization results of sulfided catalysts %lo
Prep'
Lb
nc
NOd
kC
7.7 11.3 14.8 18.2 12.9 15.5 19.5
CON CON CON CON SIP
3.0 3.5 3.0 3.5 3.2 3.7 4.0
1.4 1.5 1.8 2.0 2.1 2.2 2.6
0.252 0.305 0.276 0.263 0.182 0.180 0.186
14.7 17.3 16.5 16.1 20.7 24.7 31.4
SIP SIP
'CON-conventi onal preparation; SIP-sul f ide impregnated preparation bSlab length, nm 'No. layers in cluster dNO chemisorption, mmol/g ePseudo first-order rate constant, cc/min-g cat
1 2
I
%MOIL
I
4
I
% Mo/L
Figure 2. NO adsorption ( a ) and rate constant (b) vs. %Mo/L. [5-71. The total edge area per gram of catalyst is proportional to %Mo/L. A plot of NO/g vs. Y/dlo/L is given in Figure 2a. The NO adsorption for the first two CON catalysts is proportional to their relative edge area (%lo/L is not the true area but is proportional to it), but then drops off with increase in relative edge area for the higher Mo-loaded CON catalysts; whereas it is constant, for the SIP catalysts. This shows that NO adsorption is not proportional to edge area for either set of catalysts, and suggests that the number of adsorption centers do not increase proportionally with increase in edge area. As seen in Table 1, catalytic activities (as first-order rate constants) increased and then decreased with Mo content for the CON catalyts. These
1938 r e s u l t s a r e s i m i l a r t o t h o s e found i n t h e l i t e r a t u r e . On t h e o t h e r hand, q u i t e s u r p r i s i n g l y , t h e c a t a l y t i c a c t i v i t e s f o r t h e SIP c a t a l y s t s c o n t i n u e d t o i n c r e a s e w i t h Mo l e v e l . The r e l a t i o n s h i p between c a t a l y t i c a c t i v i t y and r e l a t i v e edge area i s shown i n F i g u r e 2b. While k m i r r o r e d t h a t o f NO ( F i g u r e 2a) f o r t h e CON c a t a l y s t s , k showed a r a p i d r i s e w i t h edge area f o r t h e S I P c a t a l y s t s , and showed no r e l a t i o n t o NO. The r e l a t i o n s h i p between a c t i v i t y and NO c h e m i s o r p t i o n i s shown i n F i g u r e 3, i n terms o f i n t r i n s i c a c t i v i t y , k/NO ( s i m i l a r t o a t u r n o v e r number) vs. independent o f edge area, i n d i c a t e s a d i r e c t c o r r e l a t i o n between NO a d s o r p t i o n s i t e s and c a t a l y t i c sites, t h a t i s , regardless o f the change i n s i t e c o n c e n t r a t i o n w i t h edge area. T h i s r e l a t i o n s h i p i s n o t observed f o r t h e SIP c a t a l y s t s , where the intrinsic activity i n c r e a s e d w i t h edge area ( F i g . 3). The unusual increase in i n t r i n s i c a c t i v i t y n o t e d above f o r
200
150
-
0
5 100 50 -
J
1
I
I
I
F i g u r e 3. I n t r i n s i c a c t i v i t y vs. s u l f i d e d Mo/A1,0, c a t a l y s t s exposed %Mo/L . t o a i r and t h e n r e s u l f i d e d , develop more a c t i v e r e a c t i o n s i t e s due t o i n c o r p o r a t i o n o f oxygen, f o r m i n g more a c t i v e O-vacancy s i t e s as compared w i t h S-vacancy s i t e s . It i s suggested t h a t t h i s may be t h e reason f o r t h e h i g h a c t i v i t y o f t h e SIP c a t a l y s t s , s i n c e a f t e r each a d d i t i o n o f Mo, t h e p r e v i o u s l y s u l f i d e d c a t a l y s t was exposed t o air.
4. REFERENCES
1 W. Zmierczak, Q. Qader and F.E. Massoth, J. C a t a l . 106 (1987) 65. 2 C. Mauchausse, H. Mozzanega, P. T u r l i e r and J-A. Dalmon, 9 t h I n t . Cong. C a t a l . 2 (1988) 775. 3 J. M i c i u k i e w i c z , W. Zmierczak and F.E. Massoth, B u l l . SOC. Chim. B e l g . 96 (1987) 915. 4 F.E. Massoth, C.-S. K i m and J-W. Cui, Appl. C a t a l . 58 (1990) 199. 5 H. Topsoe and B.S. Clausen, Appl . C a t a l . 25 (1986) 273. 6 S.J. T a u s t e r , T.A. Pecararo and R.R. C h i a n e l l i , J. C a t a l . 63 (1983) 265. 7 C.B. Roxlo, M. Daage, A.F. Ruppert and R.R. C h a n e l l i , J. C a t a l . 100 (1986) 176.
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1W, Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rights rcserved
ENHANCED HDS ACTIVITY VIA MULTIPLE IMPREGNATION OF SULFIDED Mo/A1203 CATALYSTS
C.-S.Kima, F. E. Massotho, C. Geantefi andM. Breysseb aDepartment of Fuels Engineering, University of Utah, Salt Lake City, Utah 841 12, USA bInstitut de Recherches sur la Catalyse, CNRS, 2 avenue Albert Einstein, 69626 Villeurbanne Cedex, France
Abstract Catalysts, prepared by mu1 tiple impregnations of previously sulfided catalysts (SIP), together with conventionally prepared catalysts (CON), were characterized by high resolution electron microscopy, NO chemisorption and HDS activity. The SIP catalysts displayed similar morphology as the CON catalysts, but exhibited lower NO adsorptions and higher HDS activites. It is proposed that the higher activities are due to the creation of more catalytically active sites from the repeated exposure to air during their preparation. 1. INTRODUCTION
One of the 1 imitations of hydrotreating catalysts for hydrodesulfurization
(HDS) of sulfur-containing feeds is that the HDS activity reaches a maximum
at about 10-12% Mo, with no further increase in activity with additional amounts of Mo. The object of the current research was to improve the HDS activity by sequential additions of Mo to presulfided catalysts. The rationale for this approach came from prior studies, where it was found that the MoS, coverage of the alumina support remained low, even at high levels of Mo [ l ] . Thus, it was thought that addition of Mo to a presulfided catalyst would allow additional coverage the uncovered alumina with MoS,, giving a superior dispersion and catalytic activity. 2. EXPERIMENTAL
Sulfide-impregnated catalysts (SIP) were prepared starting with a 7.7% Mo/Al,O,, which was presulfided at 400' C. Additional Mo was added by an incipient wetness impregnation, followed by drying in He at 100' C and stream up to 400' C, holding at this temperature sulfiding with a 10% H,S/H, for 2 h. Additional incremental amounts of amounts of Mo were again added and the catalysts sulfided as before. Another series o f catalysts (CON) were prepared by a conventional method, consisting of multiple impregnations with
oven drying between additions, followed by air calcination at 500' C for overnight. Catalysts (CON) were sulfided at 400' C prior to testing. The sulfided catalysts were examined by high-resolution electron microscopy, as described previously [ 2 ] , and were characterized by NO chemisorption at '0 C, using a pulse technique [3]. Catalytic activities for thiophene HDS were measured in a fixed-bed reactor under vapor-phase conditions at atmospheric pressure and 350' C [4]. Pseudo first-order rate constants were calculated from conversions obtained after 18 h on stream. 3. RESULTS AND DISCUSSION
Electron microscope pictures of a sulfide CON and SIP catalyst are given in Figure 1. No morphological differences were observed between the two sets
Figure 1. TEM micrographs of 14.8% Mo CON (a) and 15.5% SIP (b) catalysts. o f catalysts. With increasing loading, the stacking of CON catalysts remained
low, but a growing number of bulk crystallites was observed. These particles of bulk MoS, probably did not represent a large fraction of the Mo, as XRD failed to detect MoS,. For SIP catalysts, stacking was slightly promoted, but few bulk crystallites were visible in this series. Measurements o f the average size and number of layers of MoS, slabs, given in Table 1, show an increase in both with increasing Mo content. The SIP catalysts exhibited slightly larger slab sizes and number of layers than the CON catalysts at a given Mo content. Evidently, the SIP procedure did not give a better dispersion with respect to monolayer slabs as hoped. The NO adsorption for the SIP catalysts was significantly less than for the CON catalysts (Table 1); the former were independent o f Mo content, while the latter showed a slight maximum. Previous literature has suggested that edge sites of the MoS, slabs are responsible for adsorption and catalytic reaction, and these are believed to be associated with sulfur anion vacancies
1937
Table 1 Characterization results of sulfided catalysts %lo
Prep'
Lb
nc
NOd
kC
7.7 11.3 14.8 18.2 12.9 15.5 19.5
CON CON CON CON SIP
3.0 3.5 3.0 3.5 3.2 3.7 4.0
1.4 1.5 1.8 2.0 2.1 2.2 2.6
0.252 0.305 0.276 0.263 0.182 0.180 0.186
14.7 17.3 16.5 16.1 20.7 24.7 31.4
SIP SIP
'CON-conventi onal preparation; SIP-sul f ide impregnated preparation bSlab length, nm 'No. layers in cluster dNO chemisorption, mmol/g ePseudo first-order rate constant, cc/min-g cat
1 2
I
%MOIL
I
4
I
% Mo/L
Figure 2. NO adsorption ( a ) and rate constant (b) vs. %Mo/L. [5-71. The total edge area per gram of catalyst is proportional to %Mo/L. A plot of NO/g vs. Y/dlo/L is given in Figure 2a. The NO adsorption for the first two CON catalysts is proportional to their relative edge area (%lo/L is not the true area but is proportional to it), but then drops off with increase in relative edge area for the higher Mo-loaded CON catalysts; whereas it is constant, for the SIP catalysts. This shows that NO adsorption is not proportional to edge area for either set of catalysts, and suggests that the number of adsorption centers do not increase proportionally with increase in edge area. As seen in Table 1, catalytic activities (as first-order rate constants) increased and then decreased with Mo content for the CON catalyts. These
1938 r e s u l t s a r e s i m i l a r t o t h o s e found i n t h e l i t e r a t u r e . On t h e o t h e r hand, q u i t e s u r p r i s i n g l y , t h e c a t a l y t i c a c t i v i t e s f o r t h e SIP c a t a l y s t s c o n t i n u e d t o i n c r e a s e w i t h Mo l e v e l . The r e l a t i o n s h i p between c a t a l y t i c a c t i v i t y and r e l a t i v e edge area i s shown i n F i g u r e 2b. While k m i r r o r e d t h a t o f NO ( F i g u r e 2a) f o r t h e CON c a t a l y s t s , k showed a r a p i d r i s e w i t h edge area f o r t h e S I P c a t a l y s t s , and showed no r e l a t i o n t o NO. The r e l a t i o n s h i p between a c t i v i t y and NO c h e m i s o r p t i o n i s shown i n F i g u r e 3, i n terms o f i n t r i n s i c a c t i v i t y , k/NO ( s i m i l a r t o a t u r n o v e r number) vs. independent o f edge area, i n d i c a t e s a d i r e c t c o r r e l a t i o n between NO a d s o r p t i o n s i t e s and c a t a l y t i c sites, t h a t i s , regardless o f the change i n s i t e c o n c e n t r a t i o n w i t h edge area. T h i s r e l a t i o n s h i p i s n o t observed f o r t h e SIP c a t a l y s t s , where the intrinsic activity i n c r e a s e d w i t h edge area ( F i g . 3). The unusual increase in i n t r i n s i c a c t i v i t y n o t e d above f o r
200
150
-
0
5 100 50 -
J
1
I
I
I
F i g u r e 3. I n t r i n s i c a c t i v i t y vs. s u l f i d e d Mo/A1,0, c a t a l y s t s exposed %Mo/L . t o a i r and t h e n r e s u l f i d e d , develop more a c t i v e r e a c t i o n s i t e s due t o i n c o r p o r a t i o n o f oxygen, f o r m i n g more a c t i v e O-vacancy s i t e s as compared w i t h S-vacancy s i t e s . It i s suggested t h a t t h i s may be t h e reason f o r t h e h i g h a c t i v i t y o f t h e SIP c a t a l y s t s , s i n c e a f t e r each a d d i t i o n o f Mo, t h e p r e v i o u s l y s u l f i d e d c a t a l y s t was exposed t o air.
4. REFERENCES
1 W. Zmierczak, Q. Qader and F.E. Massoth, J. C a t a l . 106 (1987) 65. 2 C. Mauchausse, H. Mozzanega, P. T u r l i e r and J-A. Dalmon, 9 t h I n t . Cong. C a t a l . 2 (1988) 775. 3 J. M i c i u k i e w i c z , W. Zmierczak and F.E. Massoth, B u l l . SOC. Chim. B e l g . 96 (1987) 915. 4 F.E. Massoth, C.-S. K i m and J-W. Cui, Appl. C a t a l . 58 (1990) 199. 5 H. Topsoe and B.S. Clausen, Appl . C a t a l . 25 (1986) 273. 6 S.J. T a u s t e r , T.A. Pecararo and R.R. C h i a n e l l i , J. C a t a l . 63 (1983) 265. 7 C.B. Roxlo, M. Daage, A.F. Ruppert and R.R. C h a n e l l i , J. C a t a l . 100 (1986) 176.