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Study of the adsorption properties of binary oxide catalysts prepared on a γ-alumina base
Surface Technology, 18 (1983) 349 - 358
349
STUDY OF THE ADSORPTION PROPERTIES OF BINARY OXIDE CATALYSTS PREPARED ON A 7-ALUMINA BASE W. KANIA, K. J U R C Z Y K and Z. FOLTYNOWICZ
Chemistry Department, A. Michiewicz University, 60-780 Poznah (Poland) (Received November 8, 1982)
Summary The porous structure of 7-alumina-based catalysts for the oxidative dehydrogenation of ethylbenzene was studied. The effect of the nature and the a m o u n t of the metal oxide added to alumina on the porosity of the catalysts investigated is described.
1. Introduction The shape of the pores as well as their volume and surface area distributions as a function of pore radius play an important role in the determination of the catalytic and adsorption properties of solids. Catalysts based on alumina are widely used in the chemical industry [1 - 3]. Recently, a number of papers have been published on the application of alumina [ 4 - 6] and alumina-based binary oxide systems [7, 8] as catalysts for the oxidative dehydrogenation of ethylbenzene. The porous structure of some aluminasupported oxide catalysts has been studied by several workers, e.g. Rubinshtein and coworkers [9 - 13] and Simonova et al. [14]. A clear effect of an oxide added to alumina on the surface area and the pore distribution of the alumina was reported in these papers. The present study concerns catalysts formed by the combination of 7alumina with another metal (M) oxide (M - Ni, Cr, Fe, Zr, Mg, Mo) prepared under conditions different from those used in the above-mentioned papers. In order to obtain samples free from nitrate anions, all the catalysts were prepared by precipitation to pH 10.5. Only for the nickel oxide-alumina catalysts was the pH value lower because of the solubility of nickelous hydroxide in excess ammonia. The influence on the porous structure of the catalysts obtained by the addition of the oxides to alumina is described in this paper. 2. Experimental procedure
2.1. Preparation o f the catalysts Alumina was prepared by precipitation with 20 wt.% ammonia solution from a 20 wt.% solution of aluminium nitrate until pH 8.0 or pH 10.5 was reached. 0376-4583/83/0000-0000/$03.00
© Elsevier Sequoia/Printed in The Netherlands
350 Magnesia-alumina (1, 3 and 5 wt.% MgO), ferric oxide-alumina (1, 3 and 5 wt.% Fe203), chromia-alumina (1, 3 and 5 wt.% Cr203) and zirconiaalumina (1 and 3 wt.% ZrO2) catalysts were obtained by coprecipitation with 20 wt.% ammonia from a 20 wt.% solution of aluminium nitrate and the respective metal nitrate to pH 10.5. Molybdena-aiumina catalysts containing 3 wt.% MoO 3 were prepared by coprecipitation with 20 wt.% ammonia from a 20 wt.% solution of aluminium nitrate and a m m o n i u m molybdate until pH 10.5 was obtained. Nickel oxide-alumina catalysts containing 1, 3 and 5 wt.% NiO were obtained by coprecipitation with 20 wt.% ammonia from a 20 wt.% solution of nickel nitrate and aluminium nitrate until pH 8.0 was reached. The hydroxide precipitates obtained by the above methods were aged in the m o t h e r liquor for 24 h and then washed with distilled water until the filtrate was free from nitrate anions. Then, the precipitates were dried at 105 °C for 24 h, calcined at 550 °C for 6 h and screened in order to obtain particles 0 . 2 5 - 0 . 5 0 mm in size, which were used in adsorption measurements. X-ray analysis, which was carried out for the catalysts studied, showed the presence of a 7-alumina phase only. 2.2. D e t e r m i n a t i o n o f the p o r e v o l u m e d i s t r i b u t i o n versus p o r e radius
The microdistribution was determined by the Cranston-Inkley m e t h o d [15] on the basis of the results of low temperature (77 K) nitrogen adsorption measurements carried out on a Gravimat type 4133 Sartorius microbalance. Before the adsorption measurements were made, the samples were outgassed at 350 °C and 10 -6 Torr until constant weight was established. The calculations, based on the adsorption branch of the sorption isotherm, were performed on an ODRA 1204 computer using a program elaborated by Foltynowicz and coworkers [16]. As a result of the calculations, data on the cumulative pore volume Vc~-,, the cumulative surface area Sc~-, and the average pore radius ray as well as plots of the pore volume and the pore surface area distributions as functions of the pore radius r were obtained directly from the computer.
3. Results and discussion Adsorption measurements show a clear change in the pore structure of the binary oxide catalysts in comparison with that of 7-alumina. Data on the pore structure of the 7-alumina sample are shown in Figs. 1 - 6, curves 1, and in Table 1. They are in good agreement with the literature data [1, 17, 18]. Nitrogen adsorption isotherms and pore distributions versus pore radius for the binary oxide catalysts are also presented in Figs. 1 - 6. The range of pore radii, as obtained from nitrogen adsorption measurements, is from 10 to 150 A. The adsorption isotherm for alumina is very similar to a type IV isotherm, according to the Brunauer-Deming-Deming-Teller classification [19], as is particularly clear for the sample precipitated at pH 10.5. A similar isotherm
351 TABLE 1 Cumulative pore volume Vcurmcumulative surface area Scum and average pore radius ray of alumina and alumina-based binary oxide catalysts
Catalyst A1203 A1203 + 1 wt.% A1203 + 3 wt.% A1203 + 5 wt.% A1203 + 1 wt.% A1203 + 3 wt.% A1203 + 5 wt.% A1203 + 1 wt.% A1203 + 3 wt.% A1203 + 5 wt.% A1203 + 1 wt.% A1203 + 3 wt.% A1203 + 3 wt.% A1203 A1203 + 1 wt.% A1203 + 3 wt.% A1203 + 5 wt.%
Cr203 Cr203 Cr203 MgO MgO MgO Fe203 Fe203 Fe203 ZrO2 ZrO2 MoO3 NiO NiO NiO
p H a t the end of precipitation
Vcum (cm 3 g-l)
Scuma (m 2 g-l)
ray (A)
10.5 10.5 10.5 10.5 10.5 10.5 10.5 10.5 10.5 10.5 10.5 10.5 10.5 8.0 8.0 8.0 8.0
0.272 0.415 0.425 0.445 0.303 0.290 0.323 0.341 0.331 0.314 0.406 0.380 0.343 0.323 0.382 0.305 0.287
223 221 247 259 233 235 250 227 222 224 222 229 215 243 223 182 203
23.7 36.3 33.4 35.4 25.9 26.1 29.3 30.5 29.9 28.5 34.9 34.8 29.2 25.6 34.4 33.7 28.2
aThe cumulative surface area overlaps the value of the surface area calculated according to the Brunauer--Emmett-Teller equation.
for n i t r o g e n a d s o r p t i o n on a l u m i n a has been r e p o r t e d b y Lubarskii and Ermakova [20]. However, w h e n oxides o f iron(III}, m o l y b d e n u m ( V I ) , m a g n e s i u m ( I I ) , c h r o m i u m ( I I I ) , z i r c o n i u m ( I V ) a n d nickel(II} are a d d e d , t h e shape o f t h e i s o t h e r m changes. F o r the ferric o x i d e - a l u m i n a a n d m o l y b d e n a - a l u m i n a samples (Figs. l ( a ) a n d 2(a)) t h e a d s o r p t i o n isotherms resemble t h o s e o f t y p e II [ 1 9 ] , particularly w h e n t h e c o n c e n t r a t i o n o f the a d d i t i o n a l metal o x i d e is increased. A nearly p e r f e c t t y p e II i s o t h e r m is observed for the m a g n e s i a - a l u m i n a catalysts (Fig. 3(a)), whereas a t y p e IV i s o t h e r m is characteristic o f c h r o m i a - a l u m i n a (Fig. 4(a)), z i r c o n i a - a l u m i n a (Fig. 5(a}) a n d nickel o x i d e - a l u m i n a (Fig. 6(a)). T h e hysteresis l o o p s resemble t h o s e o f E and B t y p e s a c c o r d i n g t o t h e de Boer classification, s o m e t i m e s s h o w i n g a m i x e d n a t u r e . A n e x c e p t i o n o c c u r s f o r m a g n e s i a - a l u m i n a samples, f o r w h i c h the hysteresis l o o p s are similar t o t h o s e o f t y p e A [21, 22]. T h e d i f f e r e n t shapes o f the hysteresis l o o p s p r o v e t h e existence o f differences in t h e p o r e s t r u c t u r e s ; this is p a r t i c u l a r l y e m p h a s i z e d b y t h e a b s e n c e o f steep sections in the respective f r a g m e n t s o f the i s o t h e r m s f o r s o m e catalysts. T h e above considerations correlate well with t h e p o r e v o l u m e d i s t r i b u t i o n as a f u n c t i o n o f p o r e radius (Figs. l ( b } - 6{b}). A m o n o d i s p e r s i v e p o r e v o l u m e d i s t r i b u t i o n was observed f o r the a l u m i n a samples in w h i c h pores smaller t h a n 50 • are p r e d o m i n a n t . F o r the c h r o m i a - a l u m i n a (Fig. 4(b)), ferric o x i d e - a l u m i n a
352
30C
250 2_00 150 10{]
,3,,f 2
0,2
13,6 0,8 P/Po
(a) 6,0
7.0c
--1
i
YO~#,5
--2
,--..., 525 o~
-3
'.~
u
L
) <3 1,5
I.~5
so
(b)
,~o
15or f l ]
~oo
,i,r,orC'U
(c)
Fig. 1. (a) N i t r o g e n a d s o r p t i o n i s o t h e r m s , (b) p o r e v o l u m e d i s t r i b u t i o n as a f u n c t i o n of p o r e radius a n d (c) p o r e surface area d i s t r i b u t i o n as a f u n c t i o n o f p o r e radius for a l u m i n a a n d ferric o x i d e - a l u m i n a catalysts: curves 1, Al:O3, pH at t h e e n d of p r e c i p i t a t i o n 10.5; curves 2, A1203 + 1 wt.% Fe203; curves 3, A1203 + 3 wt.% Fe203; curves 4, A1203 + 5 wt.% Fe203.
(Fig. l ( b ) ) and z i r c o n i a - a l u m i n a (Fig. 5(b)) catalysts as well as for the m a g n e s i a - a l u m i n a catalyst c o n t a i n i n g 5 wt.% MgO (Fig. 3(b), curve 4), local m a x i m a a p p e a r in the range r = 50 - 100 A o n the d i s t r i b u t i o n curve Vp = f(r). Thus the a d d i t i o n o f a foreign o x i d e t o 7-alumina changes the m o n o d i s p e r sive c h a r a c t e r o f t h e a l u m i n a p o r o s i t y and a polydispersive n a t u r e results in the binary o x i d e catalysts. T h e i n t r o d u c t i o n o f a foreign oxide, however, causes o n l y a small change in t h e surface area in c o m p a r i s o n with t h a t o f the initial 7-alumina. F o r the c h r o m i a - a l u m i n a catalysts an increase in surface area o c c u r s with an increase in c h r o m i a c o n t e n t ; this is in a g r e e m e n t with the results o f R u b i n s h t e i n e t al. [9]. A c c o r d i n g t o R u b i n s h t e i n , the observed increase in surface area, in spite o f t h e fact t h a t t h e surface area o f p u r e c h r o m i a is very small [ 8 ] , results f r o m t h e f o r m a t i o n o f a solid s o l u t i o n o f
353
30C 2.51] '2_OE "lSO
v [cm3/s]
0,2
(a)
0,6 0,8 P/ o
6,0 ~"
-2
~-3P ,i
(b)
(c)
Fig. 2. (a) Nitrogen adsorption isotherms, (b) pore v o l u m e distribution as a f u n c t i o n o f pore radius and (c) pore surface area distribution as a f u n c t i o n of p o r e radius for alumina and the m o l y b d e n a - a l u m i n a catalysts: curves 1, A1203, p H at the end o f precipitation 10.5; curves 2, A1203 + 3 wt.% MOO3.
chromia in v-alumina. A similar change in surface area was found for magnesia-alumina samples {Table 1). For the other samples, the surface areas are relatively similar to the surface area observed for v-alumina. A slight decrease in surface area in comparison with v-alumina occurs for m o l y b d e n a - a l u m i n a (in agreement with the results obtained by Giordano e t al. [23]) and for nickel oxide-alumina. However, it should be mentioned that for the nickel oxide-alumina catalysts the reproducibility of the surface area distribution is poor on repeating the catalyst preparation [12, 14]. As the results from Figs. l ( b ) - 6(b) and from Table 1 show, the addition of another oxide to v-alumina only slightly influences pore structure parameters such as the cumulative volume and the average pore radius. The curves of the pore surface area distribution plotted as a function of the radius have the appearance of curves appropriate to micropores for 7-A120 a prepared at pH 10.5. In contrast, the introduction of a foreign oxide causes the formation of a local m a x i m u m in the distribution curve {Figs. l ( c ) 5(c)), except for samples admixed with higher amounts of chromia and magnesia. A surface area m a x i m u m occurs for pores of about 30 )k for the
354
SO0 250 200 150 100 I
I
I
I
0,20,~ 0,60,BP/Po (a)
'~,00
T
"~. ~,5
i
5.25 ~ - 2 e ~"
~, s,o
~t
"c 3,60 1
<~1.5
~o
(b)
~oo
;oo
,so "[~]
~o ~[~]
(c)
Fig. 3. (a) N i t r o g e n a d s o r p t i o n i s o t h e r m s (b) p o r e v o l u m e d i s t r i b u t i o n as a f u n c t i o n of p o r e radius a n d (e) p o r e surface area d i s t r i b u t i o n as a f u n c t i o n o f p o r e radius for a l u m i n a a n d m a g n e s i a - a l u m i n a catalysts: curves 1, A12Os, p H at t h e e n d o f p r e c i p i t a t i o n 10.5; curves 2, Ai203 + 1 wt.% MgO; curves 3, A1203 + 3 wt.% MgO; curves 4, A1203 + 5 wt.% MgO.
binary oxide catalysts. When catalysts are prepared at a lower pH, namely 8.0, the pore distribution does n o t undergo marked changes compared with the 7-alumina curves (Fig. 6(c)). Small increases in cumulative volume and average pore radius are observed for catalysts containing oxides of chromium(III), zirconium(iV) and iron(III) (Table 1). The differences in pore shape, however, are more pronounced, as shown by changes in the hysteresis loops of the isotherms compared with those of 7-alumina. The hysteresis loop of T-alumina (Figs. l(a) - 6(a), curves 1) belongs to type E (according to the de Boer classification), which suggests that the alumina is distinguished by pores of different diameters with narrow necks of relatively uniform diameters. The pores can be cylindrical with narrowings from both
355
3oo va[:m'/9]
'*
2
250 200 150 "10~
1
Q20,'t O,6 0,8 P/po (a) ~.00
.'~'-'5.2S
"'E 3~ i,,,..,., <1 "1,t ~.
1~o (b)
1dor i l l
5o
~oo
~so rD]
(c)
Fig. 4. (a) Nitrogen adsorption isotherms, (b) pore volume distribution as a function of pore radius and (c) pore surface area distribution as a function of pore radius for alumina and chromia-alumina catalysts: curves 1, A1203, pH at the end of precipitation 10.5; curves 2, A1203 + 1 wt.% Cr203; curves 3, A1203 + 3 wt.% Cr203; curves 4, AI203 + 5 wt.% Cr203. sides [24] or closed from one side only (so-called "ink bot t l e" pores). Hysteresis loops for samples containing foreign oxides are significantly different f r o m the characteristic hysteresis loop for 7-alumina, which is of t y p e E [18, 24]. The shape of the hysteresis loop of the isotherms o f aluminabased binary oxide catalysts tends towards t hat of t y p e A as the a m o u n t of added oxide increases. This occurs for the catalysts containing oxides of m o l y b d e n u m ( V I ) (Fig. 2(a)), iron(III) (Fig. l (a)), chrom i um (III) (Fig. 4(a)) and zirconium(IV) (Fig. 5(a)). The greatest t e n d e n c y to approach the t y p e A shape is shown by t he hysteresis loop of the magnesia-alumina samples (Fig. 3(a)). If the hysteresis loop of t y p e A originates from the presence o f tubular pores whose diameters change significantly (such pores belong to group XV according t o the de Boer classification), t hen the trans-
356
30C w[cm3/9] 25E 20E 15C dOE] |
!
i
0,2 0,4. o,6 0,8 P/po (a) 6,O
~,oo
T ,
~i
s, z 5
~E ~_%
~
3
,
"T,o~ 0
t. L.
<1
~
50
(b)
~oo
p
~o
~6o [11
~o
~so fiX]
(c)
Fig. 5. (a) Nitrogen adsorption isotherms, (b) pore volume distribution as a function of pore radius and (c) pore surface area distribution as a function of pore radius for alumina and zirconia-alumina catalysts: curves 1, A1203, pH at the end of precipitation 10.5; curves 2, A1203 + 1 wt.% NiO; curves 3, A1203 + 3 wt.% NiO; curves 4, Al203 + 5 wt.% NiO. f o r m a t i o n o f a t y p e E l o o p into a t y p e A l o o p can be explained as a result o f p o r e w i d t h e q u a l i z a t i o n and o f t h e shift f r o m g r o u p XV t o g r o u p II [22, 2 4 ] . When d i f f e r e n t a m o u n t s o f nickel o x i d e are i n t r o d u c e d t o 7-alumina, t h e r e is n o c h a n g e in t h e t y p e o f hysteresis l o o p ; o n l y t h e d e v e l o p m e n t of its area is observed.
4. C o n c l u s i o n s T h e m e t a l oxides, a d d e d t o T-alumina in o r d e r t o o x i d e s y s t e m s u n d e r s t u d y , significantly c h a n g e t h e p o r e c o m p a r i s o n with t h a t o f ~/-alumina alone, b u t their e f f e c t face area, c u m u l a t i v e p o r e v o l u m e a n d average p o r e radius
o b t a i n the b i n a r y size d i s t r i b u t i o n in o n c u m u l a t i v e suris quite small. T h e
357
300 2S(] 20C IS(] 100
q2 0,4 o,6 0,8 p/po
(a) 6,0
I
i
1
,._5,25
,.,E ~.5
t
(o
~.3.0
1.5
toe
(b)
~eo r [ l ]
(c)
Fig. 6. (a) N i t r o g e n a d s o r p t i o n i s o t h e r m s , ( b ) p o r e v o l u m e d i s t r i b u t i o n as a f u n c t i o n o f p o r e radius a n d (c) p o r e surface area d i s t r i b u t i o n as a f u n c t i o n o f p o r e radius for a l u m i n a a n d nickel o x i d e - a l u m i n a catalysts: curves 1, A1203, p H a t the end o f p r e c i p i t a t i o n 8.0; curves 2, A1203 + 1 wt.% NiO ;curves 3, A1203 + 3 wt.% NiO ;curves 4, A l 2 0 3 + 5 wt.% NiO.
change in the adsorption isotherm shape from the type IV observed for ~,-alumina to type II, f o u n d for most of the binary oxide samples, as well as the change in the hysteresis loop from type E to type A reflect changes in the pore structure caused by the introduction of the various metal oxides to alumina. Generally, the oxides added transform the pore structure of 7alumina from monodispersive to polydispersive, which may be useful from a catalytic point of view. A study of the dependence of the catalytic activity of the binary oxide systems for oxidative dehydrogenation of ethylbenzene on the pore structure of the catalysts is under way.
References 1 C. P. Poole a n d O. S. McIver, Adv. Catal., 1 7 ( 1 9 6 7 ) 223. 2 R. C h o j n a c k i , W. T~cza, M. S~awecki, J. K w i a t k o w s k i , S. Mafika a n d J. Zi~bicki, Przem. Chem., 58 ( 1 9 7 9 ) 38.
358 3 4 5 6 7 8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
G. D. Lubarskii, Usp. Khim., 27 (1958) 316. A. E. Lisovskii, T. G. Alkhazov and M. G. Safarov, Neflekhimiya, 12 (1972) 166. P. Ciambelli, S. Crescitelli, V. De Simone and G. Russo, J. Chim. Ind., 55 (1973) 634. R. Fiedorow, W. Przystajko, M. Sopa and J. G. Dalla Lana, J. Catal., 68 (1981) 33. B. Malinowska and E. Stec, Chemik, 27 (1974) 50. W. Kania, Physical Chemistry o f Chromia-Alumina Catalysts, A. Mickiewicz University Press, Poznafi, 1981 (in Polish). A. M. Rubinshtein, A. L. Klyachko-Gurvich arid V. M. Akimov, Izv. Akad. Nauk S.S.S.R., Otd. Khim. Nauk, 5 (1961) 780. A. M. Rubinshtein, V. M. Akimov and A. A. Slinkin, Izv. Akad. Nauk S.S.S.R., Otd. Khim. Nauk, 2 (1960) 163. A. M. Rubinshtein, V. A. Afanasev, V. M. Akimov, N. A. Pribytkova and K. Ya. Slovetskaya, Dokl, Akad. Nauk S.S.S.R., 5 (1959) 1076. A. M. Rubinshtein, A. A. Slinkin and N. A. Pribytkova, Izv. Akad. Nauk S.S.S.R., Otd. Khim. Nauk, 7 (1958) 814. F. Yosht, A. L. Klyachko-Gurvich and A. M. Rubinshtein, Izv. Akad. Nauk S.S.S.R., Set. Khim., 12 (1963) 2105. L. G. Simonova, V. A. D~isko, M. S. Borisova and L. G. Karakchiev, Kinet. Katal., 14 (1973) 1566. R. W. Cranston and F. A. Inkley, Adv. Catal., 9 (1957) 143. S. Zielifiski, Z. Foltynowicz and J. Foltynowicz, Zh. Fiz. Khim., in the press. T. K. Boreskov, V. A. D~isko, M. S. Borisova and V. N. Krasnopolskaya, Zh. Fiz. Khim., 26 (1952) 492. R. Fiedorow, Surface Chemistry o f Some Alumina Modifications, A. Mickiewicz University Press, Poznafi, 1972 (in Polish). S. Brunauer, L. S. Deming, W. E. Deming and E. Teller, J. Am. Chem. Soc., 62 (1940) 1723. G. D. Lubarskii and S. K. Ermakova, Zh. Fiz. Khim., 33 (1957) 2052. S. J. Gregg and K. S. W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1967, p. 172. J. H. de Boer, in D. H. Everett and F. S. Stone (eds.), The Structure and Properties o f Porous Materials, Butterworths, London, 1958, p. 68. N. Giordano, J. C. J. Bart, A. Vaghi, A. Castellan and G. Martinotti, J. Catal., 36 (1975)81. B. G. Linsen (ed.), Physical and Chemical Aspects o f Adsorbents and Catalysts, Academic Press, London, 1970, p. 299.
349
STUDY OF THE ADSORPTION PROPERTIES OF BINARY OXIDE CATALYSTS PREPARED ON A 7-ALUMINA BASE W. KANIA, K. J U R C Z Y K and Z. FOLTYNOWICZ
Chemistry Department, A. Michiewicz University, 60-780 Poznah (Poland) (Received November 8, 1982)
Summary The porous structure of 7-alumina-based catalysts for the oxidative dehydrogenation of ethylbenzene was studied. The effect of the nature and the a m o u n t of the metal oxide added to alumina on the porosity of the catalysts investigated is described.
1. Introduction The shape of the pores as well as their volume and surface area distributions as a function of pore radius play an important role in the determination of the catalytic and adsorption properties of solids. Catalysts based on alumina are widely used in the chemical industry [1 - 3]. Recently, a number of papers have been published on the application of alumina [ 4 - 6] and alumina-based binary oxide systems [7, 8] as catalysts for the oxidative dehydrogenation of ethylbenzene. The porous structure of some aluminasupported oxide catalysts has been studied by several workers, e.g. Rubinshtein and coworkers [9 - 13] and Simonova et al. [14]. A clear effect of an oxide added to alumina on the surface area and the pore distribution of the alumina was reported in these papers. The present study concerns catalysts formed by the combination of 7alumina with another metal (M) oxide (M - Ni, Cr, Fe, Zr, Mg, Mo) prepared under conditions different from those used in the above-mentioned papers. In order to obtain samples free from nitrate anions, all the catalysts were prepared by precipitation to pH 10.5. Only for the nickel oxide-alumina catalysts was the pH value lower because of the solubility of nickelous hydroxide in excess ammonia. The influence on the porous structure of the catalysts obtained by the addition of the oxides to alumina is described in this paper. 2. Experimental procedure
2.1. Preparation o f the catalysts Alumina was prepared by precipitation with 20 wt.% ammonia solution from a 20 wt.% solution of aluminium nitrate until pH 8.0 or pH 10.5 was reached. 0376-4583/83/0000-0000/$03.00
© Elsevier Sequoia/Printed in The Netherlands
350 Magnesia-alumina (1, 3 and 5 wt.% MgO), ferric oxide-alumina (1, 3 and 5 wt.% Fe203), chromia-alumina (1, 3 and 5 wt.% Cr203) and zirconiaalumina (1 and 3 wt.% ZrO2) catalysts were obtained by coprecipitation with 20 wt.% ammonia from a 20 wt.% solution of aluminium nitrate and the respective metal nitrate to pH 10.5. Molybdena-aiumina catalysts containing 3 wt.% MoO 3 were prepared by coprecipitation with 20 wt.% ammonia from a 20 wt.% solution of aluminium nitrate and a m m o n i u m molybdate until pH 10.5 was obtained. Nickel oxide-alumina catalysts containing 1, 3 and 5 wt.% NiO were obtained by coprecipitation with 20 wt.% ammonia from a 20 wt.% solution of nickel nitrate and aluminium nitrate until pH 8.0 was reached. The hydroxide precipitates obtained by the above methods were aged in the m o t h e r liquor for 24 h and then washed with distilled water until the filtrate was free from nitrate anions. Then, the precipitates were dried at 105 °C for 24 h, calcined at 550 °C for 6 h and screened in order to obtain particles 0 . 2 5 - 0 . 5 0 mm in size, which were used in adsorption measurements. X-ray analysis, which was carried out for the catalysts studied, showed the presence of a 7-alumina phase only. 2.2. D e t e r m i n a t i o n o f the p o r e v o l u m e d i s t r i b u t i o n versus p o r e radius
The microdistribution was determined by the Cranston-Inkley m e t h o d [15] on the basis of the results of low temperature (77 K) nitrogen adsorption measurements carried out on a Gravimat type 4133 Sartorius microbalance. Before the adsorption measurements were made, the samples were outgassed at 350 °C and 10 -6 Torr until constant weight was established. The calculations, based on the adsorption branch of the sorption isotherm, were performed on an ODRA 1204 computer using a program elaborated by Foltynowicz and coworkers [16]. As a result of the calculations, data on the cumulative pore volume Vc~-,, the cumulative surface area Sc~-, and the average pore radius ray as well as plots of the pore volume and the pore surface area distributions as functions of the pore radius r were obtained directly from the computer.
3. Results and discussion Adsorption measurements show a clear change in the pore structure of the binary oxide catalysts in comparison with that of 7-alumina. Data on the pore structure of the 7-alumina sample are shown in Figs. 1 - 6, curves 1, and in Table 1. They are in good agreement with the literature data [1, 17, 18]. Nitrogen adsorption isotherms and pore distributions versus pore radius for the binary oxide catalysts are also presented in Figs. 1 - 6. The range of pore radii, as obtained from nitrogen adsorption measurements, is from 10 to 150 A. The adsorption isotherm for alumina is very similar to a type IV isotherm, according to the Brunauer-Deming-Deming-Teller classification [19], as is particularly clear for the sample precipitated at pH 10.5. A similar isotherm
351 TABLE 1 Cumulative pore volume Vcurmcumulative surface area Scum and average pore radius ray of alumina and alumina-based binary oxide catalysts
Catalyst A1203 A1203 + 1 wt.% A1203 + 3 wt.% A1203 + 5 wt.% A1203 + 1 wt.% A1203 + 3 wt.% A1203 + 5 wt.% A1203 + 1 wt.% A1203 + 3 wt.% A1203 + 5 wt.% A1203 + 1 wt.% A1203 + 3 wt.% A1203 + 3 wt.% A1203 A1203 + 1 wt.% A1203 + 3 wt.% A1203 + 5 wt.%
Cr203 Cr203 Cr203 MgO MgO MgO Fe203 Fe203 Fe203 ZrO2 ZrO2 MoO3 NiO NiO NiO
p H a t the end of precipitation
Vcum (cm 3 g-l)
Scuma (m 2 g-l)
ray (A)
10.5 10.5 10.5 10.5 10.5 10.5 10.5 10.5 10.5 10.5 10.5 10.5 10.5 8.0 8.0 8.0 8.0
0.272 0.415 0.425 0.445 0.303 0.290 0.323 0.341 0.331 0.314 0.406 0.380 0.343 0.323 0.382 0.305 0.287
223 221 247 259 233 235 250 227 222 224 222 229 215 243 223 182 203
23.7 36.3 33.4 35.4 25.9 26.1 29.3 30.5 29.9 28.5 34.9 34.8 29.2 25.6 34.4 33.7 28.2
aThe cumulative surface area overlaps the value of the surface area calculated according to the Brunauer--Emmett-Teller equation.
for n i t r o g e n a d s o r p t i o n on a l u m i n a has been r e p o r t e d b y Lubarskii and Ermakova [20]. However, w h e n oxides o f iron(III}, m o l y b d e n u m ( V I ) , m a g n e s i u m ( I I ) , c h r o m i u m ( I I I ) , z i r c o n i u m ( I V ) a n d nickel(II} are a d d e d , t h e shape o f t h e i s o t h e r m changes. F o r the ferric o x i d e - a l u m i n a a n d m o l y b d e n a - a l u m i n a samples (Figs. l ( a ) a n d 2(a)) t h e a d s o r p t i o n isotherms resemble t h o s e o f t y p e II [ 1 9 ] , particularly w h e n t h e c o n c e n t r a t i o n o f the a d d i t i o n a l metal o x i d e is increased. A nearly p e r f e c t t y p e II i s o t h e r m is observed for the m a g n e s i a - a l u m i n a catalysts (Fig. 3(a)), whereas a t y p e IV i s o t h e r m is characteristic o f c h r o m i a - a l u m i n a (Fig. 4(a)), z i r c o n i a - a l u m i n a (Fig. 5(a}) a n d nickel o x i d e - a l u m i n a (Fig. 6(a)). T h e hysteresis l o o p s resemble t h o s e o f E and B t y p e s a c c o r d i n g t o t h e de Boer classification, s o m e t i m e s s h o w i n g a m i x e d n a t u r e . A n e x c e p t i o n o c c u r s f o r m a g n e s i a - a l u m i n a samples, f o r w h i c h the hysteresis l o o p s are similar t o t h o s e o f t y p e A [21, 22]. T h e d i f f e r e n t shapes o f the hysteresis l o o p s p r o v e t h e existence o f differences in t h e p o r e s t r u c t u r e s ; this is p a r t i c u l a r l y e m p h a s i z e d b y t h e a b s e n c e o f steep sections in the respective f r a g m e n t s o f the i s o t h e r m s f o r s o m e catalysts. T h e above considerations correlate well with t h e p o r e v o l u m e d i s t r i b u t i o n as a f u n c t i o n o f p o r e radius (Figs. l ( b } - 6{b}). A m o n o d i s p e r s i v e p o r e v o l u m e d i s t r i b u t i o n was observed f o r the a l u m i n a samples in w h i c h pores smaller t h a n 50 • are p r e d o m i n a n t . F o r the c h r o m i a - a l u m i n a (Fig. 4(b)), ferric o x i d e - a l u m i n a
352
30C
250 2_00 150 10{]
,3,,f 2
0,2
13,6 0,8 P/Po
(a) 6,0
7.0c
--1
i
YO~#,5
--2
,--..., 525 o~
-3
'.~
u
L
) <3 1,5
I.~5
so
(b)
,~o
15or f l ]
~oo
,i,r,orC'U
(c)
Fig. 1. (a) N i t r o g e n a d s o r p t i o n i s o t h e r m s , (b) p o r e v o l u m e d i s t r i b u t i o n as a f u n c t i o n of p o r e radius a n d (c) p o r e surface area d i s t r i b u t i o n as a f u n c t i o n o f p o r e radius for a l u m i n a a n d ferric o x i d e - a l u m i n a catalysts: curves 1, Al:O3, pH at t h e e n d of p r e c i p i t a t i o n 10.5; curves 2, A1203 + 1 wt.% Fe203; curves 3, A1203 + 3 wt.% Fe203; curves 4, A1203 + 5 wt.% Fe203.
(Fig. l ( b ) ) and z i r c o n i a - a l u m i n a (Fig. 5(b)) catalysts as well as for the m a g n e s i a - a l u m i n a catalyst c o n t a i n i n g 5 wt.% MgO (Fig. 3(b), curve 4), local m a x i m a a p p e a r in the range r = 50 - 100 A o n the d i s t r i b u t i o n curve Vp = f(r). Thus the a d d i t i o n o f a foreign o x i d e t o 7-alumina changes the m o n o d i s p e r sive c h a r a c t e r o f t h e a l u m i n a p o r o s i t y and a polydispersive n a t u r e results in the binary o x i d e catalysts. T h e i n t r o d u c t i o n o f a foreign oxide, however, causes o n l y a small change in t h e surface area in c o m p a r i s o n with t h a t o f the initial 7-alumina. F o r the c h r o m i a - a l u m i n a catalysts an increase in surface area o c c u r s with an increase in c h r o m i a c o n t e n t ; this is in a g r e e m e n t with the results o f R u b i n s h t e i n e t al. [9]. A c c o r d i n g t o R u b i n s h t e i n , the observed increase in surface area, in spite o f t h e fact t h a t t h e surface area o f p u r e c h r o m i a is very small [ 8 ] , results f r o m t h e f o r m a t i o n o f a solid s o l u t i o n o f
353
30C 2.51] '2_OE "lSO
v [cm3/s]
0,2
(a)
0,6 0,8 P/ o
6,0 ~"
-2
~-3P ,i
(b)
(c)
Fig. 2. (a) Nitrogen adsorption isotherms, (b) pore v o l u m e distribution as a f u n c t i o n o f pore radius and (c) pore surface area distribution as a f u n c t i o n of p o r e radius for alumina and the m o l y b d e n a - a l u m i n a catalysts: curves 1, A1203, p H at the end o f precipitation 10.5; curves 2, A1203 + 3 wt.% MOO3.
chromia in v-alumina. A similar change in surface area was found for magnesia-alumina samples {Table 1). For the other samples, the surface areas are relatively similar to the surface area observed for v-alumina. A slight decrease in surface area in comparison with v-alumina occurs for m o l y b d e n a - a l u m i n a (in agreement with the results obtained by Giordano e t al. [23]) and for nickel oxide-alumina. However, it should be mentioned that for the nickel oxide-alumina catalysts the reproducibility of the surface area distribution is poor on repeating the catalyst preparation [12, 14]. As the results from Figs. l ( b ) - 6(b) and from Table 1 show, the addition of another oxide to v-alumina only slightly influences pore structure parameters such as the cumulative volume and the average pore radius. The curves of the pore surface area distribution plotted as a function of the radius have the appearance of curves appropriate to micropores for 7-A120 a prepared at pH 10.5. In contrast, the introduction of a foreign oxide causes the formation of a local m a x i m u m in the distribution curve {Figs. l ( c ) 5(c)), except for samples admixed with higher amounts of chromia and magnesia. A surface area m a x i m u m occurs for pores of about 30 )k for the
354
SO0 250 200 150 100 I
I
I
I
0,20,~ 0,60,BP/Po (a)
'~,00
T
"~. ~,5
i
5.25 ~ - 2 e ~"
~, s,o
~t
"c 3,60 1
<~1.5
~o
(b)
~oo
;oo
,so "[~]
~o ~[~]
(c)
Fig. 3. (a) N i t r o g e n a d s o r p t i o n i s o t h e r m s (b) p o r e v o l u m e d i s t r i b u t i o n as a f u n c t i o n of p o r e radius a n d (e) p o r e surface area d i s t r i b u t i o n as a f u n c t i o n o f p o r e radius for a l u m i n a a n d m a g n e s i a - a l u m i n a catalysts: curves 1, A12Os, p H at t h e e n d o f p r e c i p i t a t i o n 10.5; curves 2, Ai203 + 1 wt.% MgO; curves 3, A1203 + 3 wt.% MgO; curves 4, A1203 + 5 wt.% MgO.
binary oxide catalysts. When catalysts are prepared at a lower pH, namely 8.0, the pore distribution does n o t undergo marked changes compared with the 7-alumina curves (Fig. 6(c)). Small increases in cumulative volume and average pore radius are observed for catalysts containing oxides of chromium(III), zirconium(iV) and iron(III) (Table 1). The differences in pore shape, however, are more pronounced, as shown by changes in the hysteresis loops of the isotherms compared with those of 7-alumina. The hysteresis loop of T-alumina (Figs. l(a) - 6(a), curves 1) belongs to type E (according to the de Boer classification), which suggests that the alumina is distinguished by pores of different diameters with narrow necks of relatively uniform diameters. The pores can be cylindrical with narrowings from both
355
3oo va[:m'/9]
'*
2
250 200 150 "10~
1
Q20,'t O,6 0,8 P/po (a) ~.00
.'~'-'5.2S
"'E 3~ i,,,..,., <1 "1,t ~.
1~o (b)
1dor i l l
5o
~oo
~so rD]
(c)
Fig. 4. (a) Nitrogen adsorption isotherms, (b) pore volume distribution as a function of pore radius and (c) pore surface area distribution as a function of pore radius for alumina and chromia-alumina catalysts: curves 1, A1203, pH at the end of precipitation 10.5; curves 2, A1203 + 1 wt.% Cr203; curves 3, A1203 + 3 wt.% Cr203; curves 4, AI203 + 5 wt.% Cr203. sides [24] or closed from one side only (so-called "ink bot t l e" pores). Hysteresis loops for samples containing foreign oxides are significantly different f r o m the characteristic hysteresis loop for 7-alumina, which is of t y p e E [18, 24]. The shape of the hysteresis loop of the isotherms o f aluminabased binary oxide catalysts tends towards t hat of t y p e A as the a m o u n t of added oxide increases. This occurs for the catalysts containing oxides of m o l y b d e n u m ( V I ) (Fig. 2(a)), iron(III) (Fig. l (a)), chrom i um (III) (Fig. 4(a)) and zirconium(IV) (Fig. 5(a)). The greatest t e n d e n c y to approach the t y p e A shape is shown by t he hysteresis loop of the magnesia-alumina samples (Fig. 3(a)). If the hysteresis loop of t y p e A originates from the presence o f tubular pores whose diameters change significantly (such pores belong to group XV according t o the de Boer classification), t hen the trans-
356
30C w[cm3/9] 25E 20E 15C dOE] |
!
i
0,2 0,4. o,6 0,8 P/po (a) 6,O
~,oo
T ,
~i
s, z 5
~E ~_%
~
3
,
"T,o~ 0
t. L.
<1
~
50
(b)
~oo
p
~o
~6o [11
~o
~so fiX]
(c)
Fig. 5. (a) Nitrogen adsorption isotherms, (b) pore volume distribution as a function of pore radius and (c) pore surface area distribution as a function of pore radius for alumina and zirconia-alumina catalysts: curves 1, A1203, pH at the end of precipitation 10.5; curves 2, A1203 + 1 wt.% NiO; curves 3, A1203 + 3 wt.% NiO; curves 4, Al203 + 5 wt.% NiO. f o r m a t i o n o f a t y p e E l o o p into a t y p e A l o o p can be explained as a result o f p o r e w i d t h e q u a l i z a t i o n and o f t h e shift f r o m g r o u p XV t o g r o u p II [22, 2 4 ] . When d i f f e r e n t a m o u n t s o f nickel o x i d e are i n t r o d u c e d t o 7-alumina, t h e r e is n o c h a n g e in t h e t y p e o f hysteresis l o o p ; o n l y t h e d e v e l o p m e n t of its area is observed.
4. C o n c l u s i o n s T h e m e t a l oxides, a d d e d t o T-alumina in o r d e r t o o x i d e s y s t e m s u n d e r s t u d y , significantly c h a n g e t h e p o r e c o m p a r i s o n with t h a t o f ~/-alumina alone, b u t their e f f e c t face area, c u m u l a t i v e p o r e v o l u m e a n d average p o r e radius
o b t a i n the b i n a r y size d i s t r i b u t i o n in o n c u m u l a t i v e suris quite small. T h e
357
300 2S(] 20C IS(] 100
q2 0,4 o,6 0,8 p/po
(a) 6,0
I
i
1
,._5,25
,.,E ~.5
t
(o
~.3.0
1.5
toe
(b)
~eo r [ l ]
(c)
Fig. 6. (a) N i t r o g e n a d s o r p t i o n i s o t h e r m s , ( b ) p o r e v o l u m e d i s t r i b u t i o n as a f u n c t i o n o f p o r e radius a n d (c) p o r e surface area d i s t r i b u t i o n as a f u n c t i o n o f p o r e radius for a l u m i n a a n d nickel o x i d e - a l u m i n a catalysts: curves 1, A1203, p H a t the end o f p r e c i p i t a t i o n 8.0; curves 2, A1203 + 1 wt.% NiO ;curves 3, A1203 + 3 wt.% NiO ;curves 4, A l 2 0 3 + 5 wt.% NiO.
change in the adsorption isotherm shape from the type IV observed for ~,-alumina to type II, f o u n d for most of the binary oxide samples, as well as the change in the hysteresis loop from type E to type A reflect changes in the pore structure caused by the introduction of the various metal oxides to alumina. Generally, the oxides added transform the pore structure of 7alumina from monodispersive to polydispersive, which may be useful from a catalytic point of view. A study of the dependence of the catalytic activity of the binary oxide systems for oxidative dehydrogenation of ethylbenzene on the pore structure of the catalysts is under way.
References 1 C. P. Poole a n d O. S. McIver, Adv. Catal., 1 7 ( 1 9 6 7 ) 223. 2 R. C h o j n a c k i , W. T~cza, M. S~awecki, J. K w i a t k o w s k i , S. Mafika a n d J. Zi~bicki, Przem. Chem., 58 ( 1 9 7 9 ) 38.
358 3 4 5 6 7 8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
G. D. Lubarskii, Usp. Khim., 27 (1958) 316. A. E. Lisovskii, T. G. Alkhazov and M. G. Safarov, Neflekhimiya, 12 (1972) 166. P. Ciambelli, S. Crescitelli, V. De Simone and G. Russo, J. Chim. Ind., 55 (1973) 634. R. Fiedorow, W. Przystajko, M. Sopa and J. G. Dalla Lana, J. Catal., 68 (1981) 33. B. Malinowska and E. Stec, Chemik, 27 (1974) 50. W. Kania, Physical Chemistry o f Chromia-Alumina Catalysts, A. Mickiewicz University Press, Poznafi, 1981 (in Polish). A. M. Rubinshtein, A. L. Klyachko-Gurvich arid V. M. Akimov, Izv. Akad. Nauk S.S.S.R., Otd. Khim. Nauk, 5 (1961) 780. A. M. Rubinshtein, V. M. Akimov and A. A. Slinkin, Izv. Akad. Nauk S.S.S.R., Otd. Khim. Nauk, 2 (1960) 163. A. M. Rubinshtein, V. A. Afanasev, V. M. Akimov, N. A. Pribytkova and K. Ya. Slovetskaya, Dokl, Akad. Nauk S.S.S.R., 5 (1959) 1076. A. M. Rubinshtein, A. A. Slinkin and N. A. Pribytkova, Izv. Akad. Nauk S.S.S.R., Otd. Khim. Nauk, 7 (1958) 814. F. Yosht, A. L. Klyachko-Gurvich and A. M. Rubinshtein, Izv. Akad. Nauk S.S.S.R., Set. Khim., 12 (1963) 2105. L. G. Simonova, V. A. D~isko, M. S. Borisova and L. G. Karakchiev, Kinet. Katal., 14 (1973) 1566. R. W. Cranston and F. A. Inkley, Adv. Catal., 9 (1957) 143. S. Zielifiski, Z. Foltynowicz and J. Foltynowicz, Zh. Fiz. Khim., in the press. T. K. Boreskov, V. A. D~isko, M. S. Borisova and V. N. Krasnopolskaya, Zh. Fiz. Khim., 26 (1952) 492. R. Fiedorow, Surface Chemistry o f Some Alumina Modifications, A. Mickiewicz University Press, Poznafi, 1972 (in Polish). S. Brunauer, L. S. Deming, W. E. Deming and E. Teller, J. Am. Chem. Soc., 62 (1940) 1723. G. D. Lubarskii and S. K. Ermakova, Zh. Fiz. Khim., 33 (1957) 2052. S. J. Gregg and K. S. W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1967, p. 172. J. H. de Boer, in D. H. Everett and F. S. Stone (eds.), The Structure and Properties o f Porous Materials, Butterworths, London, 1958, p. 68. N. Giordano, J. C. J. Bart, A. Vaghi, A. Castellan and G. Martinotti, J. Catal., 36 (1975)81. B. G. Linsen (ed.), Physical and Chemical Aspects o f Adsorbents and Catalysts, Academic Press, London, 1970, p. 299.