- Email: [email protected]
Nanoscale redox catalysts: Cr- and Cr,Al-pillared layer clays: Characterization and catalytic activity
ELSETVIER
Solid State Ionics
Nanoscale
101-103
(1997) 793-797
redox catalysts: Cr- and Cr,Al-pillared layer clays: Characterization and catalytic activity
Imre Kirksi”‘“,
h-pad
Molnh-‘,
Istvh
Phlinkbb, Agnes Fudala”, Jhos
B. Nagy”
“Applied Chemistry Department, Jdzsef Attilu University, Rerrich B. t&r I, Szeged, H-6720 Hungaq bDepartment of Organic Chemistry, Jdzsef Attila lJniversi@, Ddm tt? 8, Szeged, H-6720 Hungary ‘Laboratoire de Rbonance Magrktique Nucleaire, Facultk Universitaires Notre-Dame de la Paix, 61 rue Bnuelles, Belgium
Namur B-5&@
Abstract Via the co-hydrolysis of CrCl, and AlCl, in the presence of swollen Na-montmorillonite, pillared layer clays were prepared. The location of chromium in the pillars and in the primary pillaring agent, the Al,,-Keggin ion, was investigated by various methods (mid and far IR and “Al NMR spectroscopies). We found that isomorphous substitution of chromium for aluminium did not occur, rather co-hydrolysis and co-pillaring took place. Upon heat treatment the loss of outer-sphere water and dehydroxylation at higher temperatures resulted in the formation of bulky alumina pillars decorated with chromia species. The resulting material was used in the oxidation of I-phenyl-1-propanol in the presence of tert-butylhydroperoxide as co-oxidant. Keywords: Cr- and Cr,Al-pillared liquid phase Materials: 1-Phenyl-1-propanol;
layer clays: Viability
tert-Butylhydroperoxide;
of isomorphous
CrCl,;
Recent years have seen tremendous activity in the synthesis, characterization, and applications of nanoscale species of various types. A number of methods are known to produce species of this size, however, preserving this state is very often a serious problem. Using suitable matrix materials may slow down diffusion and coalescence of these metastable species. One of the possible matrices may be Namontmorillonite and a way of introducing nanoscale E-mail:
IR and 27A1 NMR spectroscopies;
Oxidation
in the
AU,
1. Introduction
*Corresponding author. fax: + 36 62 321523.
substitution;
[email protected];
0167-2738/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO167-2738(97)00295-6
species may be ion exchange of appropriate cations. If these cations are of complex structure, that is if they are polyhydroxy cations obtained from the controlled hydrolysis or cohydrolysis of metallic salts or their mixture in preset composition, nanostale species may be formed after ion exchange and heat treatment and the system remains stable under a wide range of conditions. The most often examined polyhydroxy cation is the Al ,,-Keggin ion: [A10,A1,,(0H),,(H,0),,]‘~ which can be crystallized as sulphate or selenate. The structure of these salts has been solved long time ago [ 11. The Al,,Keggin ion is the most frequently used modifier in preparing pillared layer clays (PILCs). These materi-
194
I. Kiricsi
et al. I Solid State Ionics 101-103
als have a substantial number of Bronsted and Lewis acid sites and a quasi two-dimensional channel system, therefore they may be used as shape selective acid-base catalysts. Their value as catalyst may be further increased if other functionalities such as redox properties are introduced. Therefore, pillared clays containing mixed polyhydroxy cations were prepared in large number (for a very recent review partially covering this topic, see Ref. [2]). The industrial importance of such clays is expressed by the large number of patents covering the various ways of preparing and using such systems (for representative examples, see Refs. [3,4]). In this work we report on the synthesis, characterization and catalytic application of such systems: pillared layer clays, containing nanoscale chromia species on the alumina pillars.
2. Experimental
The polyoxometallate ion solutions were prepared from mixtures of AlCl, and CrCl, where the Cr/Al ratios were O/13, l/12, 2/11, 3110, 419, and 13/O. These solutions were hydrolyzed until OH/metal = 2 using NaOH solution or crystalline Na,CO,. The solutions were aged overnight at 330 K and crystallized in the form of sulphate salts. After spectroscopic investigations they were redissolved and used for pillaring the preswollen Na-montmorillonite. A set of Na,Cr-montmorillonite was prepared as well by simple ion exchange of Cr3+ for comparison. For characteristic data, see Table 1. Substances, so prepared and heat treated in the 298-773 K temperature range, were characterized by X-ray diffraction (DRON-3), solution phase and M(agic) A(ngle) S(pinning) NMR, FTIR spectroscopies and BET measurements. The NMR spectra were recorded on a Bruker MSL400 spectrometer at 104.2 MHz. The spinning rate was 3.8 KHz. The pulse length corresponded to 7~112 (1.0 ps). In the initial measurements variable waiting time was applied and finally 10 s was chosen. IR spectroscopic measurements were performed in the mid (Mateson Genesis or BIO-RAD FI’S-65A/896 spectrometers, the KBr technique: 2 mg sample in 200 mg KBr, 32 scans) and far infrared (BIO-RAD FTS-40 spectrometer, high density polyethylene as matrix: 2 mg sample in 200 mg HDPE, 128 scans) regions.
(1997) 793-797
Table 1 Composition
and characteristic
Designation Ion-exchanged Na-Mont Cr, Na-Mont Cr,Na-Mont Cr,Na-Mont Cr,Na-Mont
lCr1 (a) materials 5.4 10.2 10.3 9.2
data on the samples d(OO1) (nm)
BET (m2g-‘)
1.46 1.57 1.57 1.57 1.57
90.0 97.8 91.1 97.8 142.8
1.84 1.90 1.91 1.90 1.90 1.88
265.5 225.1 224.3 174.6 171.2 152.3
Pillared materials Al-PILC Cr, Al, ,-PILC Cr,Al,,-PILC Cr,Al,,-PILC Cr,Al,-PILC Cr-PILC
1.7 3.2 7.2 6.1 16.2
The redox properties of the materials were tested in the oxidation of 1-phenyl- 1-propanol in a wellstirred batch reactor under N, atmosphere, at 298 K using teti-butyl-hydroperoxide in CH,Cl,.
3. Results Solution phase *‘Al NMR spectra revealed the formation of the Keggin unit when either AlCl, or the salt mixture was forced to hydrolyse (Fig. 1).
/m 70
60
60 bpm
10
Fig. 1. 27A1 NMR spectra of solutions: (B) AlCl, partially hydrolyzed, (C) 1 I 12) partially hydrolyzed.
0 (A) AlCl, unhydrolyzed, CrCl, + AlCl, (Cr/Al=
7%
I. Kiricsi et al. I Solid State Ionics IOI- 103 (1997) 793-797
The intensity of the line near 0 ppm, i.e. the signal of octahedrally coordinated monomeric A13+ decreased and a line near 63 ppm grew out. This line is the signal of tetrahedral aluminium in the Al,,-Keggin ion. Further evidences were provided by the 27A1 2MAS NMR spectra of the crystallized sulphate salts (Fig. 2). Quantitative evaluation of the spectra, i.e. by calculating the ratio of octahedral to tetrahedral aluminium in the salt, resulted in AlO/AIT close to 12, irrespective of the presence of chromium. The presence of paramagnetic ion did not decrease the quality of the spectrum except splitting of the broad octahedral resonance in the Al, ,-Keggin sulphate centred at 4.8 ppm. Tetrahedral aluminium continued to give sharp lines between 61 and 62 ppm. Figs. 1 and 2. For assigning the IR absorption bands of the polyhydroxy-ion salts, the results of Bradley et al. were used [5]. The positions of the absorption bands for the Al, ,-Keggin sulphate, the co-hydrolyzed and
80
40
0
-40
-80
100
pm
Fig. 2. *‘Al MAS NMR spectra of sulfate salts: (A) AlCl, partially hydrolyzed, (B) CrCl, + AlCl, (Cr/Al = I/ 12) partially hydrolyzed.
Table 2 Location of absorption
bands of co-hydrolysed
Cr/AI
Location
0113 l/12 2111 3/10 419
720 (Al-O)’ 764 764 764 764
co-crystallized Cr,Al sulphates of various Cr/Al ratios are listed in Table 2. The characteristics of the mixed metal sulphates differed substantially from those of the Al,,-Keggin sulphate. Even at the lowest CrlAl ratio, a band at 764 cm-’ and 525 cm-’ started to develop. The latter was so intense (and its spectral area increased with chromium content) that it overlapped every other weaker bands. Only the band typical for (Al-OH)O remained unchanged as far as its position and spectral area are concerned. Although the (A1-O)T band was not detectable in any of the Cr,Al salts, tetrahedral substitution is still not probable, since (Cr-0)’ band was not found. XRD and BET measurements on the heat-treated samples revealed that ion-exchanged polyhydroxyions propped the layers of the clay open and the pillared structure remained stable up to 773 K. 27A1 MAS NMR (Fig. 3) were very similar irrespective of
of absorption
Keggin-type
0
6 11 (Al-OH)O 610 610 611 610
ppm
Fig. 3. “Al MAS NMR spectra of (A) Na-montmorillonite and pillared layer clays: (B) Al-PILC and (C) Cr,Al-PILC (Cr/Al = 1112).
ions with various Cr/Al ratios in the 800-400
bands in the 800-400
-100
cm-’
cm -’ range
region 548 (Al-O)O 525 526 525 525
494 (Al-OH,)’
I. Kiricsi et al. I Solid State tonics
Fig. 4. FTIR spectra of Na-montmorillonite,
Cr(III) ion-exchanged
whether the Al,,-Keggin ion ‘alone’ or a mixture of polyoxometallate ions were the pillaring agents. The corresponding IR spectra on the pillared as well as the ion-exchanged materials also were nearly identical (Fig. 4). The oxidation of 1-phenyl- 1-propanol to propiophenone did proceed on the co-pillared layer clays, however, allylic oxidation did not take place. The specific rate of the reaction increased with increasing chromium content (Fig. 5).
-CH&H3 -0-c 2 mmol akohol, 0.05 mmol Cr catalyst, CN+12, 288 K, 24 h
5.4
10.2 10.3 ionaxchanged
9.2
1.7 3.2 7.2 Cr,AlPILCa
Fig. 5. Specific activity of Cr-containing I-phenyl-1-propanol.
6
6.1 ‘6.2 cr-PILC
101-103
xc
clays in the oxidation
of
(1997) 793-797
montmorillonite,
and the pillared
layer clays.
4. Discussion Contrary to received wisdom [4], 27A1 MAS NMR and FTIR spectra on the polyhydroxy-ion salts as well as the pillared structures convincingly showed that isomorphous substitution of chromium for aluminum did not occur, rather co-hydrolysis and co-pillaring took place. Actually, substitution of the tetrahedral aluminium should not be even expected since Cr(II1) overwhelmingly prefers octahedral to tetrahedral coordination in cations [6]. The lack of substitution in any site may be explained on one hand, by the widely different conditions needed for the partial hydrolysis of the two salts. Whilst AlCl, polymerizes around pH = 6, CrCl, precipitates as hydroxide at this pH. On the other hand, there is a significant difference in size between the two ions and the Keggin unit is too small to release the strain, which would arise due to the substitution. Upon heat treatment the loss of outer-sphere water and dehydroxylation at higher temperatures resulted in the formation of bulky alumina pillars decorated with chromia species. These species are on the alumina pillars and are getting more and more accessible to the reactant since in the oxidation increased with reaction the specific activity chromium content. It is known, however, that the oxidizing agent is not the Cr(II1) ion. The role of the co-oxidant tert-butylhydroperoxide is to turn the inactive Cr(II1) ions active by oxidizing them and keeping the high oxidation state throughout the
I. Kiricsi et al. I Solid State tonics
reaction. Nevertheless, Cr-PILC was not the most active catalyst, since the large amount of bulk Cr(II1) in the pillar is not available for the reactant.
101-103
(1997) 793-797
TO14275 / 1994. preciated.
The
797
financial
help
is highly
ap-
References 5. Conclusion The low amounts of chromium of the pillaring solution assured the formation of chromium oligohydroxides of small size. These oligohydroxides, which are the known pillaring agents when Cr-pillared layer clays are prepared 171, together with (and on) the primary pillaring agent (the Al ,3-Keggin ion) turned into nanoscale oligomeric mixed oxides upon heat treatment.
Acknowledgements This research was sponsored by the National Science Foundation of Hungary through grant
I11 G. Johansson, Acta Chem. Stand. 14 (1960) 769-771. r21 R. Szostak, C. Ingram, Stud. Surf. Sci. Catal. 94 (1995) 13. [31 F.P. Gortsema, J.R. McCauley, R.J. Pellet, J.G. Miller, J.A. Rabo, US Patent No. 4,995,964 (1991). I41 D.E.W. Vaughan, US Patent No. 4,666,877 (1987). ISI S.M. Bradley, R.A. Kydd, R. Yamdagni, J. Chem. Sot., Dalton Trans. (1990) 2653. 161 F.A. Cotton, G. Wilkinson, in: Advanced Inorganic Chemistry 5th edn., John Wiley, New York, Chichester, Brisbane, Toronto, Singapore, 1988, p. 680. r71 T.J. Pinnavaia, M.-S. Tzou, S.D. Landau, J. Amer. Chem. Sot. 107 (1985) 4783.
Solid State Ionics
Nanoscale
101-103
(1997) 793-797
redox catalysts: Cr- and Cr,Al-pillared layer clays: Characterization and catalytic activity
Imre Kirksi”‘“,
h-pad
Molnh-‘,
Istvh
Phlinkbb, Agnes Fudala”, Jhos
B. Nagy”
“Applied Chemistry Department, Jdzsef Attilu University, Rerrich B. t&r I, Szeged, H-6720 Hungaq bDepartment of Organic Chemistry, Jdzsef Attila lJniversi@, Ddm tt? 8, Szeged, H-6720 Hungary ‘Laboratoire de Rbonance Magrktique Nucleaire, Facultk Universitaires Notre-Dame de la Paix, 61 rue Bnuelles, Belgium
Namur B-5&@
Abstract Via the co-hydrolysis of CrCl, and AlCl, in the presence of swollen Na-montmorillonite, pillared layer clays were prepared. The location of chromium in the pillars and in the primary pillaring agent, the Al,,-Keggin ion, was investigated by various methods (mid and far IR and “Al NMR spectroscopies). We found that isomorphous substitution of chromium for aluminium did not occur, rather co-hydrolysis and co-pillaring took place. Upon heat treatment the loss of outer-sphere water and dehydroxylation at higher temperatures resulted in the formation of bulky alumina pillars decorated with chromia species. The resulting material was used in the oxidation of I-phenyl-1-propanol in the presence of tert-butylhydroperoxide as co-oxidant. Keywords: Cr- and Cr,Al-pillared liquid phase Materials: 1-Phenyl-1-propanol;
layer clays: Viability
tert-Butylhydroperoxide;
of isomorphous
CrCl,;
Recent years have seen tremendous activity in the synthesis, characterization, and applications of nanoscale species of various types. A number of methods are known to produce species of this size, however, preserving this state is very often a serious problem. Using suitable matrix materials may slow down diffusion and coalescence of these metastable species. One of the possible matrices may be Namontmorillonite and a way of introducing nanoscale E-mail:
IR and 27A1 NMR spectroscopies;
Oxidation
in the
AU,
1. Introduction
*Corresponding author. fax: + 36 62 321523.
substitution;
[email protected];
0167-2738/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO167-2738(97)00295-6
species may be ion exchange of appropriate cations. If these cations are of complex structure, that is if they are polyhydroxy cations obtained from the controlled hydrolysis or cohydrolysis of metallic salts or their mixture in preset composition, nanostale species may be formed after ion exchange and heat treatment and the system remains stable under a wide range of conditions. The most often examined polyhydroxy cation is the Al ,,-Keggin ion: [A10,A1,,(0H),,(H,0),,]‘~ which can be crystallized as sulphate or selenate. The structure of these salts has been solved long time ago [ 11. The Al,,Keggin ion is the most frequently used modifier in preparing pillared layer clays (PILCs). These materi-
194
I. Kiricsi
et al. I Solid State Ionics 101-103
als have a substantial number of Bronsted and Lewis acid sites and a quasi two-dimensional channel system, therefore they may be used as shape selective acid-base catalysts. Their value as catalyst may be further increased if other functionalities such as redox properties are introduced. Therefore, pillared clays containing mixed polyhydroxy cations were prepared in large number (for a very recent review partially covering this topic, see Ref. [2]). The industrial importance of such clays is expressed by the large number of patents covering the various ways of preparing and using such systems (for representative examples, see Refs. [3,4]). In this work we report on the synthesis, characterization and catalytic application of such systems: pillared layer clays, containing nanoscale chromia species on the alumina pillars.
2. Experimental
The polyoxometallate ion solutions were prepared from mixtures of AlCl, and CrCl, where the Cr/Al ratios were O/13, l/12, 2/11, 3110, 419, and 13/O. These solutions were hydrolyzed until OH/metal = 2 using NaOH solution or crystalline Na,CO,. The solutions were aged overnight at 330 K and crystallized in the form of sulphate salts. After spectroscopic investigations they were redissolved and used for pillaring the preswollen Na-montmorillonite. A set of Na,Cr-montmorillonite was prepared as well by simple ion exchange of Cr3+ for comparison. For characteristic data, see Table 1. Substances, so prepared and heat treated in the 298-773 K temperature range, were characterized by X-ray diffraction (DRON-3), solution phase and M(agic) A(ngle) S(pinning) NMR, FTIR spectroscopies and BET measurements. The NMR spectra were recorded on a Bruker MSL400 spectrometer at 104.2 MHz. The spinning rate was 3.8 KHz. The pulse length corresponded to 7~112 (1.0 ps). In the initial measurements variable waiting time was applied and finally 10 s was chosen. IR spectroscopic measurements were performed in the mid (Mateson Genesis or BIO-RAD FI’S-65A/896 spectrometers, the KBr technique: 2 mg sample in 200 mg KBr, 32 scans) and far infrared (BIO-RAD FTS-40 spectrometer, high density polyethylene as matrix: 2 mg sample in 200 mg HDPE, 128 scans) regions.
(1997) 793-797
Table 1 Composition
and characteristic
Designation Ion-exchanged Na-Mont Cr, Na-Mont Cr,Na-Mont Cr,Na-Mont Cr,Na-Mont
lCr1 (a) materials 5.4 10.2 10.3 9.2
data on the samples d(OO1) (nm)
BET (m2g-‘)
1.46 1.57 1.57 1.57 1.57
90.0 97.8 91.1 97.8 142.8
1.84 1.90 1.91 1.90 1.90 1.88
265.5 225.1 224.3 174.6 171.2 152.3
Pillared materials Al-PILC Cr, Al, ,-PILC Cr,Al,,-PILC Cr,Al,,-PILC Cr,Al,-PILC Cr-PILC
1.7 3.2 7.2 6.1 16.2
The redox properties of the materials were tested in the oxidation of 1-phenyl- 1-propanol in a wellstirred batch reactor under N, atmosphere, at 298 K using teti-butyl-hydroperoxide in CH,Cl,.
3. Results Solution phase *‘Al NMR spectra revealed the formation of the Keggin unit when either AlCl, or the salt mixture was forced to hydrolyse (Fig. 1).
/m 70
60
60 bpm
10
Fig. 1. 27A1 NMR spectra of solutions: (B) AlCl, partially hydrolyzed, (C) 1 I 12) partially hydrolyzed.
0 (A) AlCl, unhydrolyzed, CrCl, + AlCl, (Cr/Al=
7%
I. Kiricsi et al. I Solid State Ionics IOI- 103 (1997) 793-797
The intensity of the line near 0 ppm, i.e. the signal of octahedrally coordinated monomeric A13+ decreased and a line near 63 ppm grew out. This line is the signal of tetrahedral aluminium in the Al,,-Keggin ion. Further evidences were provided by the 27A1 2MAS NMR spectra of the crystallized sulphate salts (Fig. 2). Quantitative evaluation of the spectra, i.e. by calculating the ratio of octahedral to tetrahedral aluminium in the salt, resulted in AlO/AIT close to 12, irrespective of the presence of chromium. The presence of paramagnetic ion did not decrease the quality of the spectrum except splitting of the broad octahedral resonance in the Al, ,-Keggin sulphate centred at 4.8 ppm. Tetrahedral aluminium continued to give sharp lines between 61 and 62 ppm. Figs. 1 and 2. For assigning the IR absorption bands of the polyhydroxy-ion salts, the results of Bradley et al. were used [5]. The positions of the absorption bands for the Al, ,-Keggin sulphate, the co-hydrolyzed and
80
40
0
-40
-80
100
pm
Fig. 2. *‘Al MAS NMR spectra of sulfate salts: (A) AlCl, partially hydrolyzed, (B) CrCl, + AlCl, (Cr/Al = I/ 12) partially hydrolyzed.
Table 2 Location of absorption
bands of co-hydrolysed
Cr/AI
Location
0113 l/12 2111 3/10 419
720 (Al-O)’ 764 764 764 764
co-crystallized Cr,Al sulphates of various Cr/Al ratios are listed in Table 2. The characteristics of the mixed metal sulphates differed substantially from those of the Al,,-Keggin sulphate. Even at the lowest CrlAl ratio, a band at 764 cm-’ and 525 cm-’ started to develop. The latter was so intense (and its spectral area increased with chromium content) that it overlapped every other weaker bands. Only the band typical for (Al-OH)O remained unchanged as far as its position and spectral area are concerned. Although the (A1-O)T band was not detectable in any of the Cr,Al salts, tetrahedral substitution is still not probable, since (Cr-0)’ band was not found. XRD and BET measurements on the heat-treated samples revealed that ion-exchanged polyhydroxyions propped the layers of the clay open and the pillared structure remained stable up to 773 K. 27A1 MAS NMR (Fig. 3) were very similar irrespective of
of absorption
Keggin-type
0
6 11 (Al-OH)O 610 610 611 610
ppm
Fig. 3. “Al MAS NMR spectra of (A) Na-montmorillonite and pillared layer clays: (B) Al-PILC and (C) Cr,Al-PILC (Cr/Al = 1112).
ions with various Cr/Al ratios in the 800-400
bands in the 800-400
-100
cm-’
cm -’ range
region 548 (Al-O)O 525 526 525 525
494 (Al-OH,)’
I. Kiricsi et al. I Solid State tonics
Fig. 4. FTIR spectra of Na-montmorillonite,
Cr(III) ion-exchanged
whether the Al,,-Keggin ion ‘alone’ or a mixture of polyoxometallate ions were the pillaring agents. The corresponding IR spectra on the pillared as well as the ion-exchanged materials also were nearly identical (Fig. 4). The oxidation of 1-phenyl- 1-propanol to propiophenone did proceed on the co-pillared layer clays, however, allylic oxidation did not take place. The specific rate of the reaction increased with increasing chromium content (Fig. 5).
-CH&H3 -0-c 2 mmol akohol, 0.05 mmol Cr catalyst, CN+12, 288 K, 24 h
5.4
10.2 10.3 ionaxchanged
9.2
1.7 3.2 7.2 Cr,AlPILCa
Fig. 5. Specific activity of Cr-containing I-phenyl-1-propanol.
6
6.1 ‘6.2 cr-PILC
101-103
xc
clays in the oxidation
of
(1997) 793-797
montmorillonite,
and the pillared
layer clays.
4. Discussion Contrary to received wisdom [4], 27A1 MAS NMR and FTIR spectra on the polyhydroxy-ion salts as well as the pillared structures convincingly showed that isomorphous substitution of chromium for aluminum did not occur, rather co-hydrolysis and co-pillaring took place. Actually, substitution of the tetrahedral aluminium should not be even expected since Cr(II1) overwhelmingly prefers octahedral to tetrahedral coordination in cations [6]. The lack of substitution in any site may be explained on one hand, by the widely different conditions needed for the partial hydrolysis of the two salts. Whilst AlCl, polymerizes around pH = 6, CrCl, precipitates as hydroxide at this pH. On the other hand, there is a significant difference in size between the two ions and the Keggin unit is too small to release the strain, which would arise due to the substitution. Upon heat treatment the loss of outer-sphere water and dehydroxylation at higher temperatures resulted in the formation of bulky alumina pillars decorated with chromia species. These species are on the alumina pillars and are getting more and more accessible to the reactant since in the oxidation increased with reaction the specific activity chromium content. It is known, however, that the oxidizing agent is not the Cr(II1) ion. The role of the co-oxidant tert-butylhydroperoxide is to turn the inactive Cr(II1) ions active by oxidizing them and keeping the high oxidation state throughout the
I. Kiricsi et al. I Solid State tonics
reaction. Nevertheless, Cr-PILC was not the most active catalyst, since the large amount of bulk Cr(II1) in the pillar is not available for the reactant.
101-103
(1997) 793-797
TO14275 / 1994. preciated.
The
797
financial
help
is highly
ap-
References 5. Conclusion The low amounts of chromium of the pillaring solution assured the formation of chromium oligohydroxides of small size. These oligohydroxides, which are the known pillaring agents when Cr-pillared layer clays are prepared 171, together with (and on) the primary pillaring agent (the Al ,3-Keggin ion) turned into nanoscale oligomeric mixed oxides upon heat treatment.
Acknowledgements This research was sponsored by the National Science Foundation of Hungary through grant
I11 G. Johansson, Acta Chem. Stand. 14 (1960) 769-771. r21 R. Szostak, C. Ingram, Stud. Surf. Sci. Catal. 94 (1995) 13. [31 F.P. Gortsema, J.R. McCauley, R.J. Pellet, J.G. Miller, J.A. Rabo, US Patent No. 4,995,964 (1991). I41 D.E.W. Vaughan, US Patent No. 4,666,877 (1987). ISI S.M. Bradley, R.A. Kydd, R. Yamdagni, J. Chem. Sot., Dalton Trans. (1990) 2653. 161 F.A. Cotton, G. Wilkinson, in: Advanced Inorganic Chemistry 5th edn., John Wiley, New York, Chichester, Brisbane, Toronto, Singapore, 1988, p. 680. r71 T.J. Pinnavaia, M.-S. Tzou, S.D. Landau, J. Amer. Chem. Sot. 107 (1985) 4783.