MOLECULAR STRUCTURE ELSEVIER
Journal of Molecular Structure 410-411 (1997) 547-550
Cationic mixed pillared layer clays: infrared and Miissbauer characteristics of the pillaring agents and pillared structures in Fe,Al and Cr,Al pillared montmorillonites I. PBlinkba3*,
K. LBz&rb, I. Kiricsi”
“Department of Organic Chemistry, Jdzsef Attila University, Ddm te’r 8, H-6720 Szeged, Hungary ‘Institute of Isotopes, Hungarian Academy of Sciences, PO Box 77, H-1525 Budapest, Hungary ‘Applied Chemistry Department, Jdzsef Attila University, Rerrich B. t& I, H-6720 Szeged, Hungary Received 26 August 1996; accepted 6 September
Abstract Pillared layer clays containing Fe,AI or Cr,AI mixed pillars were synthesized and characterized with 57Fe Mossbauer spectroscopies. Spectra of the pillaring ions and the pillared materials indicated that pillaring occurred. Isomorphous substitution of either tetrahedral or octahedral Al in the Al ,3-Keggin The layers of montmorillonite were propped apart by a mixture of [AIOIAI &OH)r,(H20) ir]” Keggin oxide or hydrous chromium oxide, alternately. 0 1997 Elsevier Science B.V.
Pillared layer clay; Fe,AI or Cr,AI mixed pillars; Mid and far IT-IR Co-pillaring
1. Introduction Recent years have seen tremendous activity in the synthesis and catalytic applications of pillared layer structures (for a current book and a review, see [ 1,2], respectively). A class of pillared materials consists of various layered clays, capable of swelling in, e.g., aqueous suspension, as hosts and oligomeric polyhydroxy cations as guests. After proper heat treatment, these substances become good catalysts, since they contain molecular sized openings, a relatively uniform quasi-two-dimensional channel network and acidic sites of varying strengths, due to the framework and the pillars. Modifications of the pillaring agent * Corresponding 0022-2860/97/$17.00
and 57Fe Mossbauer
(mostly [AlOdAl 12(OH)24(H20) iz]‘+ ion of the Keggin type) may result in structures of peculiar properties. When iron or chromium is the modifier, redox properties are introduced; thus the range of reactions to be catalysed can be widened appreciably. The properties of the pillared clays depend largely on the position of the modifier. This can be well assessed by various methods such as IR and Mossbauer spectroscopies (when relevant). Results of such characterization are reported in this work.
2. Experimental Polyoxometalate ion solutions were prepared from mixtures of AlCls and FeC12 (for reasons why Fe(I1)
author. 0 1997 Elsevier
mid and far FT-IR and co-hydrolysis and coion did not take place. ions and hydrous iron
B.V. All rights reserved.
salt was used instead of Fe(III), see ) or CrCl3 where the metal/Al ratios were varied (O/13, l/12, 2/ 11, 3/10 and 4/9). The solutions were partially hydrolysed (OH/metal = 2 using NaOH solution). After aging (overnight reflux at 330 K), parts of them were used for crystallizing Keggin ion sulfates of various compositions (by adding excess Na2S04 solution to the Keggin ion-containing solution at pH 4.5), while the other parts were used for pillaring an aqueous suspension of swollen Na-montmorillonite. Both types of materials were studied by spectroscopic methods. For comparison, Fe3+- and Cr3+-exchanged montmorillonite samples were also prepared (in the following they will be denoted Fe-ex or Cr-ex). IR spectroscopic measurements were performed in the mid (Matson Genesis or BIO-RAD FTS-65A/896 spectrometers, KBr technique: 2 mg sample in 200 mg KBr, 32 scans) and far infrared (BIO-RAD ITS-40 spectrometer, high density polyethylene as matrix: 2 mg sample in 200 mg HDPE, 128 scans) regions. “Fe Mossbauer spectra were taken in an in situ cell at 300 K. A 57Co/Cr source was used in constant acceleration mode. The spectra of the as-prepared as well as the pretreated samples were registered.
3. Results and discussion When AlC13 is hydrolysed under pH control the [A104Al 12(OH)24(H20) i2]‘+ ion forms. In this structure one Al is in the tetrahedral position (in the very Table 1 Location of absorption Metal/Al &i/AI o/13 l/12 Fe/AI l/12 2/l 1 3/10 419 WA1 l/12 2/l 1 3/10 419
bands of co-hydrolysed
Location of absorption
I (1997) 547-550
centre of the complex ion); the other twelve are in the centre of edge-sharing octahedra. It is very often claimed that transition metal ions like Fe+3 [4-81 or Cr3+ [4,8,9] are able to occupy both positions; i.e., isomorphous substitution of iron or chromium for aluminium takes place. Actually, these statements were based on measurements of pillared layer clays, i.e. of the intercalated material after calcination and not of the pillaring moieties. Fortunately, the pillaring species can be crystallized in the form of sulfate (or selenate) and can be scrutinized by various methods such as MAS NMR [ 10,l l] and, as in the present work, by IR and “‘Fe Mossbauer spectroscopies. For assigning the IR absorption bands the results of Bradley et al. were used . The positions of the absorption bands for the A1i3-Keggin sulfate, the co-hydrolysed and co-crystallized Fe,Al and Cr,Al sulfates of various metal/Al ratios and the GaAl r2Keggin sulfate are listed in Table 1. The bands of the Al r3-Keggin sulfate form the basis of comparison and those of the GaAl ,2-Keggin sulfate (where Ga for Al substitution in the tetrahedral position certainly takes place [ 131) are also included in order to have a reference in assessing band shifts thought to mean isomorphous substitution. In the IR spectra of the co-hydrolysed Fe,Al ions, bands due to (Al-Or& (Al-OH)oh, (A1-O)oi,, and (Al-0H2)oh only were found. The bands retained the positions observed in the A1i3-Keggin sulfate. At low stoichiometric ratios (Fe/Al = 1:12 and 2:ll) no band typical of either tetrahedrally or octahedrally coordinated iron could be detected. When the
ions with various metal/Al ratios in the 800-400
bands in the 800-400
720 (Al-Oj~d 698
611 (AI-OH),, 610
548 (Al-O)oh 543
494 (Al-OH2)o,, 491
720 723 720 722
614 616 609 615
551 551 545 552
494 492 465 465
61 I 610
I. Pdinkd et al./Journal of Molecular Structure 410-411
Table 2 Isomer shift and quadrupole
splitting data in co-crystallized
sulfate and iron oxyhydroxide
after various treatments
IS” (mm s-‘)
QS (mm se’)
IS” (mm ss’)
QS (mm se’)
lSa (mm s-’
QS (mm s-‘)
a Relative to the centre of the o-iron spectrum
proportion of iron was increased at the low frequency end of the spectrum (near 465 cm-‘) a new band started to develop. It is believed to belong to octahedrally coordinated iron in the coprecipitated hydrous iron oxide. Octahedrally coordinated iron (actually in three different octahedral positions in the as-prepared sample) was detected by 57Fe Mijssbauer spectroscopy even at a low stoichiometric ratio (Fe/Al = 2: 11) when IR spectroscopy was not sensitive enough to indicate even its very presence (Table 2). The IR characteristics of the Cr,Al sulfates were different. Even at the lowest Cr/Al ratio a band at 764 cm-’ and one at 525 cm-’ started to develop. The latter was so intense (and its spectral area increased with chromium content) that it suppressed every other weaker band. Only the band typical for (Al-OH)o,, remained unchanged as far as its position and spectral area were concerned. This observation indicates that octahedral Al was not substituted by chromium. Although the (Al-O)rd band was not detectable in any of the Cr,Al ions, tetrahedral substitution did not seem to be probable since the (CrO)rd band was not found and it is known that Cr3+ tends to adopt octahedral coordination instead of tetrahedral [ 141. As is expected from measurements on the ions, the corresponding spectra of the pillared as well as the ion-exchanged materials were also nearly identical (for the iron-modified clays see Fig. 1).
4. Conclusion Vibrational spectroscopic and 57Fe Miissbauer measurements on co-hydrolysed ions have provided further evidence that isomorphous substitution of iron(III) or chromium(II1) for aluminium(II1) did not
occur. Instead, intercalation of hydrous chromium or iron oxides and exchange of the Al 13-Keggin ion took place separately, forming a material with mixed pillars.
Acknowledgements This work was supported by the National Science Foundation of Hungary through grants TO14275 and TO21 131.
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