Acid-Treated, Size-Fractionated Smectites
J. Phys. Chem. B, Vol. 101, No. 27, 1997 5325
TABLE 1: Structural Formulas for the Fine Fractions of
the Samples Used
SCHEME 1
sample
M+a
Sib
Alb
Alc
Fec
Mgc
Lic
SAz-119
JP19
1.11
0.91
0.95
0.81
0.89
8.00
7.71
7.22
7.33
7.99
0.00
0.29
0.78
0.67
0.01
2.67
3.00
1.96
0.91
0.01
0.15
0.38
1.60
2.86
0.01
1.20
0.63
0.58
0.28
4.76
0.00
0.00
0.00
0.00
0.62
ST26
PW2400 XRF spectrometer using calibration graphs prepared
from certified reference materials.
SWa-130
HC21
Thermogravimetric traces were obtained using a Mettler
TG50 thermobalance equipped with a TC10A processor.
Samples (7 mg) were exposed to cyclohexylamine vapor for
24 h and then transferred directly out of the vapor into the
balance and heated from 35 to 800 °C at 20 °C min-1 under a
flow of 20 cm3 min-1 of dry nitrogen carrier gas.
Infrared spectroscopy was performed using a Nicolet Magna
750 FTIR spectrometer. Each spectrum consisted of 256 scans
at a resolution of 4 cm-1. Samples were prepared as KBr disks
using standard procedures.
a interlayer cations. b tetrahedral cations. c octahedral cations.
of differing octahedral magnesium content. The five parent
minerals, which were purified using standard procedures, were
treated using different combinations of acid concentration,
temperature, and time which allowed for the different dissolution
rates associated with the differing octahedral compositions
(Table 1). Using this protocol we were able to prepare samples
in which the octahedral ion composition decreased in a
controlled, stepwise manner thus allowing us to study the
evolution of catalytic activity as the octahedral layer was
progressively depopulated and relate this to information derived
from a wide range of materials characterization techniques
including X-ray diffraction analysis, X-ray fluorescence (XRF),
29Si MAS NMR, FTIR, and thermal analysis.
Diffuse reflectance infrared Fourier transform spectroscopy
(DRIFTS) data were recorded using a Specac DRIFTS cell and
a Mattson Polaris FTIR spectrometer at a resolution of 4 cm-1
.
Samples were transferred directly out of the pyridine vapor and
placed in the DRIFTS cell. The sample was then heated to
200 °C at 50 °C intervals.
High-resolution solid-state 29Si MAS NMR spectra of acid-
treated hectorite were recorded on a Bruker MSL 400 spec-
trometer at 79.49 MHz and at a sample spinning frequency in
the range 3.5-4 kHz. A 30 s recycle delay between successive
accumulations was used and 2000-3000 acquisitions were
collected for each 29Si spectrum. Reported chemical shifts for
29Si are referenced to tetramethylsilane, and they correspond to
peak maxima in the spectrum and are not corrected for
quadrupolar shift contributions.
Experimental Section
Materials. Four dioctahedral smectites were used in this
study. Two were montmorillonites (SAz-1 and JP), the third
was a ferruginous smectite (SWa-1) and the fourth (ST) was a
beidellite. SAz-1 (Cheto, AZ), a magnesium rich montmoril-
lonite, and SWa-1, a ferruginous smectite, were obtained from
The Clay Mineral Repository of the Clay Minerals Society,
Columbia, MO. JP is a hydrothermal, aluminum-rich mont-
morillonite from the Kremnica mountains in central Slova-
kia,24,25 and Stebno (ST) is an iron-rich beidellite from the Czech
Republic, which contains about 21% of total iron bound in
goethite.26 The goethite is present as an admixture in the <2
µm fraction and hence could not be removed. The fifth mineral
used was a trioctahedral hectorite (HC) from Hector, CA, and
was obtained from Ward’s Natural Science Establishment, Inc.
Coarse samples of JP, SAz-1, ST, SWa-1, and HC were
suspended in deionized water, treated five times with 1.0 M
aqueous calcium chloride, washed until free of chloride, and
centrifuged, and the nominally <2 µm sample was collected,
dried at 60 °C, and ground to <0.2 mm prior to storage. The
structural formulas of the purified smectites used are given in
Table 1.
Equipment and Methods. 3g portions of the Ca form of
each mineral were treated with 300 cm3 of x M HCl for periods
of y minutes. The HCl concentrations and treatment temper-
atures were 6 M and 95 °C for JP, 6 M and 80 °C for SAz-1,
6 M and 60 °C for ST, 1M and 95 °C for SWa-1, and 0.25 M
and 30 °C for HC. After acid treatment, the samples were
filtered, then washed with 1.5 dm3 deionized water, and
centrifuged before being dried and ground to pass a 0.2 mm
sieve. The treatment time is identified in the sample name.
Thus, JP30 indicates that JP was treated in 6 M HCl for 30
min at 95 °C and ST120 means that ST was treated with 6 M
HCl for 120 min at 60 °C.
The catalytic activity was determined by reacting 2,3-
dihydropyran with methanol to yield the tetrahydropyranyl ether
(Scheme 1).13
This reaction does not produce water as a byproduct thus
making it more attractive than other ether-forming reactions
where the water evolved may weaken or remove active Brønsted
sites. Fifty milligrams of catalyst was activated at 120 °C in
air for 2 h and then cooled to 0 °C before addition to a mixture
of 0.01 mol of dihydropyran and 0.3 mol of methanol at 0 °C.
The percentage conversion to the tetrahydropyranyl ether after
a contact time of 30 min was determined using gas chroma-
tography. The identity of the product was confirmed using gas
chromatography-mass spectrometry.
Results and Discussion
The X-ray diffraction profiles (not shown) of the purified
starting materials showed little evidence of any crystalline
impurities. The strong characteristic 001 peak for the purified
material became weaker and weaker as the treatments used
became more severe, indicating that the samples became
increasingly disordered. Moreover, a broad hump in the region
15-30° (2Θ), indicative of amorphus silica, grew as the severity
of the treatment increased, as observed earlier in less extensive
studies on SAz-1 and JP in which only three acid treatments
were investigated.19
The elemental analyzes, determined using XRF (Table 2),
are reported in full for the SAz-1 samples, whereas only selected
values are presented for the other samples. The values were
calculated on an ignited (0% H2O) basis and confirmed that
the chosen treatment times and conditions caused substantial
changes in the clays. The almost complete reduction in the Ca
content for all the acid-treated samples indicated that these
cations, which originally occupied the exchange sites, were
Sample Characterization. X-ray diffraction profiles of
pressed powder samples were obtained using a Philips PW1830
X-ray diffractometer using a copper tube (λ ) 1.5418 Å)
operating at 40 kV and 35 mA. Profiles were recorded at 2°
(2Θ) min-1
.
Samples for XRF analyzis were prepared using the Li2B4O7
fusion method. The resulting beads were analyzed on a Philips