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Methods of Saturation with Alkali Ions. Influence of the Properties of Oxides
595
METHODS OF SATURATION WITH ALKALI IONS.
INFLUENCE OF THE PROPERTIES OF OXIDES
R. HOMBEK, J. KIJENSKI and S. MALINOWSKI Institute of Organic Chemistry and Technology Technical University (Politechnika), Warsaw (Poland)
INTRODUCTION
Many industrial catalysts, especially for dehydrogenation and cracking processes, are modified by the introduction of alkali metal ions.
The problem
of the influence of alkali ions on the properties of oxide catalysts has been investigated by us (ref. 1-5) since several years.
Numerous papers concerning
this subject have been published by other authors (ref. 6-15). Alumina is the most thoroughly investigated catalyst from this point of view.
Parera and
Figoli (ref. 6 ) and Scharme (ref. 7) reported that the addition of sodium to alumina involved a decrease of surface acidity.
Chuang and cowork. (ref. 8 )
and Bremer and cowork.(ref. 9) have found using IR spectroscopy that sodium ions react with the most acidic surface OH groups only.
According to Pines
(ref. 10) it is possible that sodium hydroxide undergoes a reaction with Lewis acid centres :
- A1 + NaOH
+
-
A1-OH
-
+
Na
On the other hand our recent results (ref. 2) and those obtained by other authors (ref. 12) support the supposition that after impregnation with small amounts of alkali hydroxide the acidity of the surface increases, so the studied process is much more complicated than one might assume.
The common method of
introducing alkali ions onto oxide surfaces lies in the saturation of the solid oxide with an alkali hydroxide aqueous solution. In the present work the influence of various alkali ions, introduced from ethanolic solutions of corresponding alkoxides, on the physicochemical properties of alumina was investigated. Our new impregnation procedure permits avoiding the reaction of water with dehydrated alumina surface.
Water being a
source of considerable changes in the properties of catalysts prepared by the hydroxide addition method.
We have taken into consideration the higher reacti-
vity of the alkoxides than that of the respective hydroxides, assuming that the reaction with the A1203 surface will be more effective and cover a greater number of surface sites.
596
EXPERIMENTAL Alumina has been obtained by precipitation of A1(0Hl3 with water from a benzene solution Al(iso-C3H7)3. The hydroxide was then washed with distilled water and dried at 12OoC for 24 hr.
The resulting preparation was calcined
at 550 or 75OoC for 24 hr in a stream of dry and oxygen-free nitrogen. Lithium, sodium, potassium and caesium ethoxides were obtained by dissolving the corresponding alkali metals in absolute alcohol at room temperature. Alkali cations were introduced onto the A1203 surface during adsorption of alkoxides from absolute alcohol solution. Impregnation procedure was as follows
:
alumina calcined at 550 or 750°C was cooled down to room temperature
and was soaked in a known volume of corresponding alkali metal alkoxide. After 20 hr the catalyst was filtered-off, washed with absolute alcohol and then
calcined at 100, 150, 2 0 0 , 220, 250, 350 and 55OOC in a stream of dry and deoxidized nitrogen for 3 hr. The number of alkali cations deposited on the A1 0 2 3 surface was determined by titration of the filtered-off alkoxide excess with a hydrochloric acid solution in the presence of phenolphtalein.
The titer of
the alkoxide used for the impregnation was determined by the same method. Surface basicity and acidity of the catalysts were determined by the titration method with benzoic acid or n-butylamine using a series of Harnmett indicators (ref. 14,151.
The specific surfaces areas of the catalysts were mea-
sured by the BET method. Hydroxyl group concentrations were determined by titration with sodium naphthenide (ref. 16) and by chromatography using the reaction with Zn(CH3)2. 2THF (ref. 17). One-electron donor and one-electron acceptor properties were determined on the basis of adsorption on the catalyst surface of appropriate acceptor and donor and by recording the signals of newly-formed anion- or cation-radicals using ESR spectroscopy (ref. 18,19).
Perylene was used as the electron donor
and tetracyanoethylene was used as the electron acceptor. Quantitative results were obtained by comparing the signal intensity of the investigated sample with that of the standard
- DPPH solution in NaC1.
Adsorption of perylene was
carried out in the presence of molecular oxygen. ESR measurements were performed on an X-band spectrometer (modulation 100 kHz) at room temperature. RESULTS Basic and acidic properties The values of maximum basic strength (H of catalysts under study are -max For comparison the values of basic strength of sodium
given in Table 1.
hydroxide doped alumina were investigated in the same conditions.
597 TABLE 1. Basic and acidic strength (H and H ) of alkali alkoxides doped alumina -max omax
Alkali oation
Na+ a
Caloination em*~rature H--HemaxH-( c)
18.4 18.4 % ig.4 a t8.4 v 22.3 22.3 -3.0 24.6 -3.0 24.6 -3.0
20
100 150
200 220
250 350 550
Li+
Na'
Ti+
CS+
I3omarI-max HomaxH-max HomaxH-maxHomax
17.2 18.4 17.2 18.4 "0. $8.4 22.3 3 18.4 V 26.5 V 22.3 26.5 2.8 22.3 -3.0 26.5 -5.6 24.6 -5.6 26.5 -5.6 26.5 -5.6 26.5 -5.6
18.4 22.3 26.5 26.5 26.4 26.5 26.5 26.5
18.4 22.3 v 26.5 2.8 26.5 -3.0 26.5 -5.6 26.5 -5.6 26.5 -5.6 26.5
2v
2.8 -3.0 -5.6 -5.6 -5.6
The basic strength of catalysts doped with alkoxides and sodium hydroxide increases with the rise of calcination temperature after alumina impregnation. H-max of preparations containing alkali alkoxide is in all cases smaller or equal to H- of pure alumina. Caesium ethoxide doped alumina reaches the basic strength 26.5
Generally catalysts obtained from alumina pretreated at 75OOC
have a smaller concentration of basic centres than those with at 550'C.
precalcined 2 3 The highest concentration of basic sites is achieved on the alumina A1 0
pretreated at 550OC and then doped with potassium ethoxide. On the surface of this catalyst two types of basic centres (at strengths of 26.5
TABLE 2
Basic and aoidic sites concentrations on investigated oatalysts surfaces (calcination after
i:
Basic centres concentration at various (=ol/g) strength ( H - ) 12.2 15.0 17.2 18.4 22.3 24.6 26.5
atalyst
a
1.17
0.99
0.99
0.99
0.91
--- Li Li - NaNa
1.02
1.02
1.02
1.02
1.02
1.60 0.93 1.82 1.42
1.60 0.93 1.82 1.42
1.60 0.93 1.82
1.31 0.93 1.19
2.22
2.03
2.03
1.69
1.69
- cs
1.69 2.42 1.86
1.31 0.93 1.82 1.42 2.03 1.69
2.22
2.22
2.02
1.86
1.86
1.86
--
- K IZ cs
~
aAl
0
2 3
1.42
~
~
~
~
1.42 2.03
1.69 1.81 1.86
0.91 0.93
1 .oo
0.91 0.81 1.00
0.93 1.19
0.93 1.19
1.42
1.42 2-03
2.03 1.69 1.81 1.86
1.69 1.81
1.86
~
(550) = pretreatment temperature of alumina 55OoC
33.0 0 0 0 0 0 0 0 0 0 0
1 Aoidio centres conoentration at various strength (Ho)(mmol/g) +4.8 +1.5 -3.0 -5.6 -8.2
2.73 2.79 2.77 0.59 7.64 2.89 7.00 3.k9 4.50 3.97
1.92 2.79
0.96 0.81
2.11 0.42
2.11 0.21
7.64 2.89 7.00 3.49 4.50 3.97
7.64 2.89
0.63 0.40 0.23 0.09 6.47 2.02
6065 2.99 2.52 4.19 3.27 3-09 3.09 7.00
0.56 0.40 0 0 0 0 0 0
0 0
centres were observed for systems obtained from alumina calcined at lower temperatures, i.e. at 550°C. Specific surface area Specific surface areas of alumina doped with lithium, sodium, potassium and caesium ethoxides are presented in Table 3.
Introduction of alkali alkoxi
des on A1 0 surface resulted in an increase of specific surface area (about 2 3 lo%), the differences between preparations doped with various ethoxides are insignificant. Hydroxyl group concentration Surface hydroxyl group concentrations found for the investigated catalysts are given in Table 3.
Introduction of alkali ethoxides on alumina surface cau-
ses a decrease of the amount of OH groups.
Only alumina doped with caesium
alkoxide has a concentration of surface hydroxyl groups similar to that of the pure alumina. Alkali ion content The quantity of alkali cations deposited on alumina surface was measured by titration with HC1 (Table 3). In the series of catalysts investigated the number of lithium ions deposited during the reaction of the alkoxide with A1 0 2 3 was the greatest among alkali cations. The decrease of hydroxyl group concentration, as compared with pure alumina, was the greatest in the case of the lithium ions doped alumina.
The number of Cs'
ions deposited on alumina surface
was the smallest in the series and simultaneously after reaction of caesium alkoxide the minimal change in OH groups concentration was observed. All alumina preparations have a y-Al2O3 structure, which has been checked Assuming that 100 faces dominate on the surface,the ratio 02 of the number of alkali cations per 100 A of catalyst to the number of surface by X-ray analysis.
oxygen anions on the same area of surface was calculated. One-electron donor and one-electron acceptor properties The concentrations of one-electron donor centres on the surfaces increase with an increase of the calcination temperature after impregnation, reaching a maximum in the case of preparations calcined at 250°C.
Calcination at higher
temperatures causes a rapid fall of the one-electron donor centre concentration. The greatest number of centres of this type occurs on the surface of A1 0 2 3 doped with caesium alkoxide (Fig. 1 ) . Catalysts which have not been calcined after impregnation do not exhibit any one-electron acceptor properties at all. Among the preparations calcined at 550°C only the catalyst
doped with lithium ions had one-electron acceptor pro-
perties comparable to pure alumina. Minute concentrations of one-electron
600
TABLE 3
Specific surface area, alkali ion content and hpdroxyl group concentration on investigated systems surfaces (oaloination after impregnation at 55OoC ) Specific OH group, Aur. ion surface ooncentration content
Catalyst
(=ol/g
area (m2//g )
a
1
(=ol/g)
b
d
0
a = alkali naphthenide titration b Ichromatographic method c = nlknli ion number per 100H2 of catalyst surface '2 d = oxygen ion6 number to W i ions number ratio en 1OOA of catalyret surface
& l
9
0 20
400
450
100
2%
359
5%
oc
Fig. 1. One-electron donor aites ooncentration as a funotion of the oaloination temperature after alumina impregnation
601 acceptor centres occurred on the surfaces of all the other catalysts (Table 4).
TABLE 4 One-electron donor and one-eleotron aooeptor oentree ooncentration on investigated oatalysts surfaces (catcfnatfon after impregnation at 55OoC ) he-eleotron aooeptor One-eleotron donor oentrea ooncentration mntres oonoentration atalyet
-
(S P W g 1
(a p w g ) 2.1 x 4.4 x 1.1 x 2.2 I 1.2 x
550 750 550 750 550
10;: 10,-
1o;i 10 15 lol5
750
4.5 x lol4
550 750 550
5.2 X 1015 2.7 x 1014
<5 x 1.7 x 10
750 I
DISCUSSION
The addition of alkali metal ethoxides to alumina causes considerable changes in the physicochemical properties of the oxide. The type of apparent change depends on the pretreatment temperature of alumina and on the calcination temperature of the catalyst after impregnation. It is quite clear that the properties of the catalytic systems obtained depend mainly on the pathway of the reaction between alkoxide and surface active sites on A1203. At room temperature it is possible that alkali metal ethoxides undergo reactions with Surface hydroxyl groups and with Lewis type acidic centres, presumably with the naked surface aluminum cations. A s a result of the reaction with hydroxyl groups the new bond between alkali metal and surface oxygen would be formed z ~ i -+ 0RO-M+ ~
-+
-AL-O-M+
:
+ ROH
In the case of reaction with Lewis sites the equation may be envisaged as follows
:
Reaction (1) causes a decrease of the surface OH group concentration. As a result of reaction (2) the blocking of most acidic surface centres (Lewis type centres with strength Ho < -8.2)
occurs.
The basic strength of catalysts not calcined after the introduction of
alkoxide is much smaller than for pure alumina. This fact seems to be a result 2- . of suppressing the surface oxygen anions 0 with alkali cations (Eq. 2). The surface alkoxide anions may act as electron pair donors and are probably responsible for the basicity in the range of H- = 17.2 - 18.2.
According to
McEwen (ref. 20) the ethoxide anions in solution have the same basic strength. The relatively low concentration of one-electron donor sites on the surface of catalytic systems not calcined after impregnation may be a result of the destruction of part of the surface OH groups and the suppresion of surface oxygen anions.
It is well known that both types of active sites can act as
one-electron donors.
In this same reaction pathway the one-electron acceptor
sites (aluminum cations) (ref. 21) are blocked by alkoxide anions. The calcination of the impregnated catalysts causes considerable changes in the properties and structure of the surfaces.
As
a result of reaction (3) the formation of new hydroxyl groups and new
naked oxygen anions may take place.
The two surface sites formed are respon-
sible for the increase of the concentration and strength of the basic sites. An increase in the concentration of basic and one-electron donor centres is probably due to the formation of new naked oxygen anions after the evolution of ethylene.
Secondary hydroxyl groups formed in reaction ( 3 ) may also act as
new one-electron donor centres. Differences in the properties of alumina doped with alkoxides of different alkali metals point to the role played by the properties of the cation introduced (electron affinity and ionic radius).
The electron affinity and ionic
radius of the cation in the respective alkali metal alkoxide determine the basicity and the reactivity of the anion.
It seems likely that the reaction
of the alkoxide molecule with the surface hydroxyl group of alumina begins with the attack of the anion at the acidic proton of the hydroxyl group as in analogous homophase reactions.
The second step is the addition of the cation to
2the oxygen anion 0 released. Such a mechanism is supported by the observed
differences in the reaction of alkoxides and sodium hydroxide with the A 1 0 2 3 surface. Obviously, the structure of the anion is important so far as it determines the type of product formed during the calcination of the impregnated material. Larger cations such as potassium and caesium possess greater or comparable radii in relation to oxygen anions. It cannot be ruled out that their coordination number is higher than that of Li+ or N a ' .
The presumption is supported 02 by the ratio of the alkali metal ions to the oxygen anions per 100 A of
603 alumina surface (Table 3 ) .
One must presume that
the ratios close to the value
of the coordination number in the sequence of the cations under discussion may
be an explanation of the fact that various quantities of different cations are retained on the same alumina surface
under unaltered conditions.
Moreover the
ionic radius determine the hardness of the cation. The small lithium cation is much harder than the other alkali metal cations. As the oxygen anion is one of the hardest anions, its affinity for a cation will grow with an increase of
cation hardness and should be highest for the lithium cation. It is interesting to note that catalysts doped with alkali metal alkoxides (calcination after impregnation at 550°C) possess higher overall concentrations
of acidic centres than the initial alumina preparations.
We think that the
alkali cations are responsible for the formation of new acidic sites on the surface.
It is possible that the titration with n-butylamine leads to side
reactions in which complexes of the amine with surface alkali cations are formed.
The number of coordinated titre molecules may be greater than one. A
similar phenomenon is observed during the formation of stable complexes of polycyclic amines with alkali metal cations in solutions (ref. 2 2 ) .
Thus,
Benesi's method would provide results allowing a qualitative comparison of acidity rather than the obtention of true, absolute values of the acid sites concentration.
REFERENCES
1. S. Malinowski, W. Grabowski,W.J. Palion and B. Zielinski, Bull. Acad. Polon. Sci., Ser. Sci. Chim., 2 1 (1973) 737. 2. M. Marczewski, S. Malinowski, Bull. Acad. Polon. Sci., Ser. Sci. Chim., 24 (1976) 1. 3 . M. Marczewski, S. Malinowski, Bull. Acad. Polon. Sci., Ser. Sci. Chim., 24 (1976) 187. 4. J. Kijenski, S. Malinowski, Bull. Acad. Polon., Sci., Ser. Sci. Chim., 25 (1977) 329. 5. J. Kijenski, S. Malinowski, J . C . S . , Faraday I, 74 (1978) 250. 6. N.S. Figoli, S.A. Hillar, M. Parera, J. Catal., 20 (1971) 230. 7. L.D. Scharme, J. Phys. Chem., 20 (1974) 2070. 8 . T.T. Chuang, I.G. Dalla Lona, J.C.S., Faraday I, 68 (1972) 777. 9 . H. Bremer, K.H. Steinberg, K.D. Wendlant, 2. Anorg. Allgem. Chem., 366 (1969) 130. 10. H. Pines, W.O. Haag, J. Am. Chem. SOC., 82 (1960) 2471. 11. K.V. Topchieva, K. Yun Pin, I.V. Smirnova, Adv. Catal., 9 ( 1 9 5 7 ) 799. 12. S. Santhangopalan, C.N. Pillai, Indian J. Chem., 11 (1973) 957. 13. M. Iato, T. Kanbayashi, N. Kobayashi, Y. Shima, J. Catal., 7 (1967) 342. 14. H.A. Benesi, J. Phys. Chem., 6 1 (1957) 970. 15. K. Tanabe, Solid Acids and Bases, Academic Press, New York, 1970. 16. J. Kijenski, R. Hombek, S . Malinowski, J. Catal., 50 (1977) 186. 17. L. Nondek, React. Kinet. Catal. Lett., 2 (1976) 283. 18. M. Che, C. Naccache, B. Imelik, Trans. Faraday SOC., 63 (19671 2254. 19. B.D. Flockhart, J.A.N. Scott, R.C. Pink, Trans. Faraday SOC., 62 ( 1 9 6 6 ) 730. 20. W.K. McEwen, J. Am.Chem.Soc., 58 (1936) 1124. 21. J.B. Peri, J. Phys. Chem., 69 (1965) 220. 22. J.J. Christensen, D.J. Eatough, R.M. Izatt, Chem.Rev. 74 ( 1 9 7 4 ) 351.
METHODS OF SATURATION WITH ALKALI IONS.
INFLUENCE OF THE PROPERTIES OF OXIDES
R. HOMBEK, J. KIJENSKI and S. MALINOWSKI Institute of Organic Chemistry and Technology Technical University (Politechnika), Warsaw (Poland)
INTRODUCTION
Many industrial catalysts, especially for dehydrogenation and cracking processes, are modified by the introduction of alkali metal ions.
The problem
of the influence of alkali ions on the properties of oxide catalysts has been investigated by us (ref. 1-5) since several years.
Numerous papers concerning
this subject have been published by other authors (ref. 6-15). Alumina is the most thoroughly investigated catalyst from this point of view.
Parera and
Figoli (ref. 6 ) and Scharme (ref. 7) reported that the addition of sodium to alumina involved a decrease of surface acidity.
Chuang and cowork. (ref. 8 )
and Bremer and cowork.(ref. 9) have found using IR spectroscopy that sodium ions react with the most acidic surface OH groups only.
According to Pines
(ref. 10) it is possible that sodium hydroxide undergoes a reaction with Lewis acid centres :
- A1 + NaOH
+
-
A1-OH
-
+
Na
On the other hand our recent results (ref. 2) and those obtained by other authors (ref. 12) support the supposition that after impregnation with small amounts of alkali hydroxide the acidity of the surface increases, so the studied process is much more complicated than one might assume.
The common method of
introducing alkali ions onto oxide surfaces lies in the saturation of the solid oxide with an alkali hydroxide aqueous solution. In the present work the influence of various alkali ions, introduced from ethanolic solutions of corresponding alkoxides, on the physicochemical properties of alumina was investigated. Our new impregnation procedure permits avoiding the reaction of water with dehydrated alumina surface.
Water being a
source of considerable changes in the properties of catalysts prepared by the hydroxide addition method.
We have taken into consideration the higher reacti-
vity of the alkoxides than that of the respective hydroxides, assuming that the reaction with the A1203 surface will be more effective and cover a greater number of surface sites.
596
EXPERIMENTAL Alumina has been obtained by precipitation of A1(0Hl3 with water from a benzene solution Al(iso-C3H7)3. The hydroxide was then washed with distilled water and dried at 12OoC for 24 hr.
The resulting preparation was calcined
at 550 or 75OoC for 24 hr in a stream of dry and oxygen-free nitrogen. Lithium, sodium, potassium and caesium ethoxides were obtained by dissolving the corresponding alkali metals in absolute alcohol at room temperature. Alkali cations were introduced onto the A1203 surface during adsorption of alkoxides from absolute alcohol solution. Impregnation procedure was as follows
:
alumina calcined at 550 or 750°C was cooled down to room temperature
and was soaked in a known volume of corresponding alkali metal alkoxide. After 20 hr the catalyst was filtered-off, washed with absolute alcohol and then
calcined at 100, 150, 2 0 0 , 220, 250, 350 and 55OOC in a stream of dry and deoxidized nitrogen for 3 hr. The number of alkali cations deposited on the A1 0 2 3 surface was determined by titration of the filtered-off alkoxide excess with a hydrochloric acid solution in the presence of phenolphtalein.
The titer of
the alkoxide used for the impregnation was determined by the same method. Surface basicity and acidity of the catalysts were determined by the titration method with benzoic acid or n-butylamine using a series of Harnmett indicators (ref. 14,151.
The specific surfaces areas of the catalysts were mea-
sured by the BET method. Hydroxyl group concentrations were determined by titration with sodium naphthenide (ref. 16) and by chromatography using the reaction with Zn(CH3)2. 2THF (ref. 17). One-electron donor and one-electron acceptor properties were determined on the basis of adsorption on the catalyst surface of appropriate acceptor and donor and by recording the signals of newly-formed anion- or cation-radicals using ESR spectroscopy (ref. 18,19).
Perylene was used as the electron donor
and tetracyanoethylene was used as the electron acceptor. Quantitative results were obtained by comparing the signal intensity of the investigated sample with that of the standard
- DPPH solution in NaC1.
Adsorption of perylene was
carried out in the presence of molecular oxygen. ESR measurements were performed on an X-band spectrometer (modulation 100 kHz) at room temperature. RESULTS Basic and acidic properties The values of maximum basic strength (H of catalysts under study are -max For comparison the values of basic strength of sodium
given in Table 1.
hydroxide doped alumina were investigated in the same conditions.
597 TABLE 1. Basic and acidic strength (H and H ) of alkali alkoxides doped alumina -max omax
Alkali oation
Na+ a
Caloination em*~rature H--HemaxH-( c)
18.4 18.4 % ig.4 a t8.4 v 22.3 22.3 -3.0 24.6 -3.0 24.6 -3.0
20
100 150
200 220
250 350 550
Li+
Na'
Ti+
CS+
I3omarI-max HomaxH-max HomaxH-maxHomax
17.2 18.4 17.2 18.4 "0. $8.4 22.3 3 18.4 V 26.5 V 22.3 26.5 2.8 22.3 -3.0 26.5 -5.6 24.6 -5.6 26.5 -5.6 26.5 -5.6 26.5 -5.6
18.4 22.3 26.5 26.5 26.4 26.5 26.5 26.5
18.4 22.3 v 26.5 2.8 26.5 -3.0 26.5 -5.6 26.5 -5.6 26.5 -5.6 26.5
2v
2.8 -3.0 -5.6 -5.6 -5.6
The basic strength of catalysts doped with alkoxides and sodium hydroxide increases with the rise of calcination temperature after alumina impregnation. H-max of preparations containing alkali alkoxide is in all cases smaller or equal to H- of pure alumina. Caesium ethoxide doped alumina reaches the basic strength 26.5
Generally catalysts obtained from alumina pretreated at 75OOC
have a smaller concentration of basic centres than those with at 550'C.
precalcined 2 3 The highest concentration of basic sites is achieved on the alumina A1 0
pretreated at 550OC and then doped with potassium ethoxide. On the surface of this catalyst two types of basic centres (at strengths of 26.5
TABLE 2
Basic and aoidic sites concentrations on investigated oatalysts surfaces (calcination after
i:
Basic centres concentration at various (=ol/g) strength ( H - ) 12.2 15.0 17.2 18.4 22.3 24.6 26.5
atalyst
a
1.17
0.99
0.99
0.99
0.91
--- Li Li - NaNa
1.02
1.02
1.02
1.02
1.02
1.60 0.93 1.82 1.42
1.60 0.93 1.82 1.42
1.60 0.93 1.82
1.31 0.93 1.19
2.22
2.03
2.03
1.69
1.69
- cs
1.69 2.42 1.86
1.31 0.93 1.82 1.42 2.03 1.69
2.22
2.22
2.02
1.86
1.86
1.86
--
- K IZ cs
~
aAl
0
2 3
1.42
~
~
~
~
1.42 2.03
1.69 1.81 1.86
0.91 0.93
1 .oo
0.91 0.81 1.00
0.93 1.19
0.93 1.19
1.42
1.42 2-03
2.03 1.69 1.81 1.86
1.69 1.81
1.86
~
(550) = pretreatment temperature of alumina 55OoC
33.0 0 0 0 0 0 0 0 0 0 0
1 Aoidio centres conoentration at various strength (Ho)(mmol/g) +4.8 +1.5 -3.0 -5.6 -8.2
2.73 2.79 2.77 0.59 7.64 2.89 7.00 3.k9 4.50 3.97
1.92 2.79
0.96 0.81
2.11 0.42
2.11 0.21
7.64 2.89 7.00 3.49 4.50 3.97
7.64 2.89
0.63 0.40 0.23 0.09 6.47 2.02
6065 2.99 2.52 4.19 3.27 3-09 3.09 7.00
0.56 0.40 0 0 0 0 0 0
0 0
centres were observed for systems obtained from alumina calcined at lower temperatures, i.e. at 550°C. Specific surface area Specific surface areas of alumina doped with lithium, sodium, potassium and caesium ethoxides are presented in Table 3.
Introduction of alkali alkoxi
des on A1 0 surface resulted in an increase of specific surface area (about 2 3 lo%), the differences between preparations doped with various ethoxides are insignificant. Hydroxyl group concentration Surface hydroxyl group concentrations found for the investigated catalysts are given in Table 3.
Introduction of alkali ethoxides on alumina surface cau-
ses a decrease of the amount of OH groups.
Only alumina doped with caesium
alkoxide has a concentration of surface hydroxyl groups similar to that of the pure alumina. Alkali ion content The quantity of alkali cations deposited on alumina surface was measured by titration with HC1 (Table 3). In the series of catalysts investigated the number of lithium ions deposited during the reaction of the alkoxide with A1 0 2 3 was the greatest among alkali cations. The decrease of hydroxyl group concentration, as compared with pure alumina, was the greatest in the case of the lithium ions doped alumina.
The number of Cs'
ions deposited on alumina surface
was the smallest in the series and simultaneously after reaction of caesium alkoxide the minimal change in OH groups concentration was observed. All alumina preparations have a y-Al2O3 structure, which has been checked Assuming that 100 faces dominate on the surface,the ratio 02 of the number of alkali cations per 100 A of catalyst to the number of surface by X-ray analysis.
oxygen anions on the same area of surface was calculated. One-electron donor and one-electron acceptor properties The concentrations of one-electron donor centres on the surfaces increase with an increase of the calcination temperature after impregnation, reaching a maximum in the case of preparations calcined at 250°C.
Calcination at higher
temperatures causes a rapid fall of the one-electron donor centre concentration. The greatest number of centres of this type occurs on the surface of A1 0 2 3 doped with caesium alkoxide (Fig. 1 ) . Catalysts which have not been calcined after impregnation do not exhibit any one-electron acceptor properties at all. Among the preparations calcined at 550°C only the catalyst
doped with lithium ions had one-electron acceptor pro-
perties comparable to pure alumina. Minute concentrations of one-electron
600
TABLE 3
Specific surface area, alkali ion content and hpdroxyl group concentration on investigated systems surfaces (oaloination after impregnation at 55OoC ) Specific OH group, Aur. ion surface ooncentration content
Catalyst
(=ol/g
area (m2//g )
a
1
(=ol/g)
b
d
0
a = alkali naphthenide titration b Ichromatographic method c = nlknli ion number per 100H2 of catalyst surface '2 d = oxygen ion6 number to W i ions number ratio en 1OOA of catalyret surface
& l
9
0 20
400
450
100
2%
359
5%
oc
Fig. 1. One-electron donor aites ooncentration as a funotion of the oaloination temperature after alumina impregnation
601 acceptor centres occurred on the surfaces of all the other catalysts (Table 4).
TABLE 4 One-electron donor and one-eleotron aooeptor oentree ooncentration on investigated oatalysts surfaces (catcfnatfon after impregnation at 55OoC ) he-eleotron aooeptor One-eleotron donor oentrea ooncentration mntres oonoentration atalyet
-
(S P W g 1
(a p w g ) 2.1 x 4.4 x 1.1 x 2.2 I 1.2 x
550 750 550 750 550
10;: 10,-
1o;i 10 15 lol5
750
4.5 x lol4
550 750 550
5.2 X 1015 2.7 x 1014
<5 x 1.7 x 10
750 I
DISCUSSION
The addition of alkali metal ethoxides to alumina causes considerable changes in the physicochemical properties of the oxide. The type of apparent change depends on the pretreatment temperature of alumina and on the calcination temperature of the catalyst after impregnation. It is quite clear that the properties of the catalytic systems obtained depend mainly on the pathway of the reaction between alkoxide and surface active sites on A1203. At room temperature it is possible that alkali metal ethoxides undergo reactions with Surface hydroxyl groups and with Lewis type acidic centres, presumably with the naked surface aluminum cations. A s a result of the reaction with hydroxyl groups the new bond between alkali metal and surface oxygen would be formed z ~ i -+ 0RO-M+ ~
-+
-AL-O-M+
:
+ ROH
In the case of reaction with Lewis sites the equation may be envisaged as follows
:
Reaction (1) causes a decrease of the surface OH group concentration. As a result of reaction (2) the blocking of most acidic surface centres (Lewis type centres with strength Ho < -8.2)
occurs.
The basic strength of catalysts not calcined after the introduction of
alkoxide is much smaller than for pure alumina. This fact seems to be a result 2- . of suppressing the surface oxygen anions 0 with alkali cations (Eq. 2). The surface alkoxide anions may act as electron pair donors and are probably responsible for the basicity in the range of H- = 17.2 - 18.2.
According to
McEwen (ref. 20) the ethoxide anions in solution have the same basic strength. The relatively low concentration of one-electron donor sites on the surface of catalytic systems not calcined after impregnation may be a result of the destruction of part of the surface OH groups and the suppresion of surface oxygen anions.
It is well known that both types of active sites can act as
one-electron donors.
In this same reaction pathway the one-electron acceptor
sites (aluminum cations) (ref. 21) are blocked by alkoxide anions. The calcination of the impregnated catalysts causes considerable changes in the properties and structure of the surfaces.
As
a result of reaction (3) the formation of new hydroxyl groups and new
naked oxygen anions may take place.
The two surface sites formed are respon-
sible for the increase of the concentration and strength of the basic sites. An increase in the concentration of basic and one-electron donor centres is probably due to the formation of new naked oxygen anions after the evolution of ethylene.
Secondary hydroxyl groups formed in reaction ( 3 ) may also act as
new one-electron donor centres. Differences in the properties of alumina doped with alkoxides of different alkali metals point to the role played by the properties of the cation introduced (electron affinity and ionic radius).
The electron affinity and ionic
radius of the cation in the respective alkali metal alkoxide determine the basicity and the reactivity of the anion.
It seems likely that the reaction
of the alkoxide molecule with the surface hydroxyl group of alumina begins with the attack of the anion at the acidic proton of the hydroxyl group as in analogous homophase reactions.
The second step is the addition of the cation to
2the oxygen anion 0 released. Such a mechanism is supported by the observed
differences in the reaction of alkoxides and sodium hydroxide with the A 1 0 2 3 surface. Obviously, the structure of the anion is important so far as it determines the type of product formed during the calcination of the impregnated material. Larger cations such as potassium and caesium possess greater or comparable radii in relation to oxygen anions. It cannot be ruled out that their coordination number is higher than that of Li+ or N a ' .
The presumption is supported 02 by the ratio of the alkali metal ions to the oxygen anions per 100 A of
603 alumina surface (Table 3 ) .
One must presume that
the ratios close to the value
of the coordination number in the sequence of the cations under discussion may
be an explanation of the fact that various quantities of different cations are retained on the same alumina surface
under unaltered conditions.
Moreover the
ionic radius determine the hardness of the cation. The small lithium cation is much harder than the other alkali metal cations. As the oxygen anion is one of the hardest anions, its affinity for a cation will grow with an increase of
cation hardness and should be highest for the lithium cation. It is interesting to note that catalysts doped with alkali metal alkoxides (calcination after impregnation at 550°C) possess higher overall concentrations
of acidic centres than the initial alumina preparations.
We think that the
alkali cations are responsible for the formation of new acidic sites on the surface.
It is possible that the titration with n-butylamine leads to side
reactions in which complexes of the amine with surface alkali cations are formed.
The number of coordinated titre molecules may be greater than one. A
similar phenomenon is observed during the formation of stable complexes of polycyclic amines with alkali metal cations in solutions (ref. 2 2 ) .
Thus,
Benesi's method would provide results allowing a qualitative comparison of acidity rather than the obtention of true, absolute values of the acid sites concentration.
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