Clay Minerals (1987) 22, 169-178
T H E A C I D I T Y OF T R I V A L E N T
CATION-EXCHANGED
MONTMORILLONITE.
TEMPERATURE-PROGRAMMED
DESORPTION
A N D I N F R A R E D S T U D I E S OF P Y R I D I N E A N D
N-BUTYLAMINE
C. B R E E N * ,
A. T. D E A N E
AND J. J. F L Y N N
School of Chemical Sciences, National Institute for Higher Education, Glasnevin, Dublin 9, Eire
(Received 17 January 1986; revised 12 February 1987)
ABSTRACT : Temperature-programmed desorption (TPD) and IR spectroscopy were used to
characterize the number and strength of acid sites in A13§ Cr3§ and Fe3§
montmorillonite. The bases pyridine and n-butylamine occupied three different sites in the
interlamellar space: (i) physisorbed base, (ii) base bound to Lewis acid sites, and (iii) protonated
base. TPD profiles for pyridine were characterized by maxima at 40~ 150~ and 340~ whilst
those for n-butylamine occurred at 30~ 200~ and 410~ The AI3§ and Cr3§
forms
were stable up to pretreatment temperatures of 300~ but the Fe3+-form required > 3 day
exposure to base vapour to re-establish the high-temperature desorption peak. Variabletemperature IR studies showed that the number of Br/Snsted-bound pyridine molecules
increased with increased outgassing temperature.
In a recent review, Thomas (1982) has shown that clay minerals act as efficient catalysts for a
variety of organic reactions. Suitably exchanged A13+-, Cr 3+-, Fe 3+- or Cu 2+- montmorillonites have been used successfully as catalysts for the formation of di-alkylethers via the high
temperature ( > 100~ dehydration of alcohols (Ballantine et al., 1981a) or low-temperature
( < 70~ conversion of hex-l-ene (Adams et al., 1982a), the hydration of ethene (Gregory et
al., 1983), the esterification of organic acids by alkenes (BaUantine et al., 1981b), the lowtemperature synthesis of tert-methybutylether (MTBE) from 2-methylpropene and methanol
(Adams et al., 1982b) and the reaction of alcohols to form tert-butylethers (Adams et al.,
1981). It is currently accepted that the Br6nsted acidity which gives rise to the catalytic
activity of clays is derived from the polarization of solvent water molecules by the small,
highly charged interlayer cations (Thomas, 1982). Consequently, the high catalytic activity of
A13§ compared to Fe 3+- and Cr3+-exchanged montmorillonites (Tennakoon et al., 1983;
Atkins et al., 1983 ; Gregory et al., 1983) has been attributed to the enhanced polarization of
water molecules in the primary coordination sphere of the A13+ cation. However, this
suggested order of catalyst acidity is the reverse of that observed in aqueous solutions of these
ions. Burgess (1978) reported that the p K a values of aqueous A13+, Cr 3+ and Fe 3+ ions are
4.95, 3.89 and 2.2, respectively. In addition, Mortland (1968) presented semi-quantitative
data concerning the protonation of urea by AI 3+-, Fe 3+- and H+-exchanged montmorillonite,
in which the observed sequence of protonating ability was A13+ < Fe 3+ < H +. N o
* Present address: Chemistry Department, Sheffield City Polytechnic, Pond Street, Sheffield S1 1WB, UK
9 1987 The Mineralogical Society
170
C. Breen et al.
protonation was observed for the alkali, and alkaline-earth cation-exchanged forms and no
data were presented for Cr3§
Consequently, an investigation into the
acidity of those cation-exchanged forms of montmorillonite which exhibit catalytic activity
appeared warranted.
Since its inception, the technique of temperature-programmed desorption (TPD) has been
used to considerable advantage for elucidation of the number and type of adsorption sites on
a wide range of solid surfaces which exhibit catalytic properties. For example, Morishige and
co-workers studied the thermal desorption of organic bases from chromic oxides (1982) and
methylamines from dehydrated alkaline-earth metal zeolites (1984), and the thermal
desorption of acetonitrile, methanol, ethanol, propan-l-ol and propan-2-ol from silicamagnesia mixed oxides was examined by Ritter et al. (1982) and Noller & Ritter (1984).
However, as with many physical techniques, the results from the TPD technique cannot be
interpreted in isolation and corroborative evidence is generally sought from infrared (IR)
spectra.
Until very recently when Plee et al. (1986) studied the desorption of pyridine from pillared
clays, the technique of temperature-programmed desorption per se had not been applied t 9
clay--organic systems, although the closely related technique of thermogravimetry has been
routinely utilized to determine the total weight of adsorbed species. In this study the TPD
technique has been used to help clarify the number and type of acid sites in the trivalent
cation-exchanged montmorillonites which have been shown to be efficient catalysts for a
number of novel and industrially significant reactions (Thomas, 1982). The particular probe
molecules utilized were chosen because Tanabe (1970) has shown that pyridine (PKb = 8.8)
will react with relatively strong Lewis and Br6nsted centres, whilst n-butylamine, by virtue of
its stronger basicity (pKb = 3.2), interacts with both weak and strong acid sites.
EXPERIMENTAL
Homoionic samples of the < 2 #m fraction of Wyoming montmorillonite (supplied by
Volclay Ltd., Wallasey, Cheshire) were prepared by immersing the clay in 0.3 M solutions of
the appropriate salt solution for 24 h. Excess salt was removed by successive washing and
centrifugation steps. Chemical analysis (Bennet & Reed, 1971) of the Na+-exchanged form
produced results consistent with a layer formula of (Si3.9A10.1)(A1L33Fe0.ssMg0.s9)O1o(OH)2
and a CEC (Adams et al., 1977) of 68 + 2 mEq/100 g clay. Self-supporting clay films ( ~ 2 mg
cm -2) for IR analysis were prepared by evaporation of a dilute aqueous slurry on a
polyethylene backing, which was subsequently removed.
The samples for both TPD and IR spectroscopy were air-dried (20~ r.h. ~ 60%) prior to
exposure to reagent grade pyridine or n-butylamine vapour at partial pressures of 20 and
~ 100 mm Hg, respectively, for periods of 48 h unless otherwise specified.
IR spectra were recorded at room temperature, then after one hour at 50~ 100~ 150~ and
200~ using an evacuable variable-temperature cell with a maximum operating temperature
of 200~ The spectrometer utilized was a Perkin Elmer model 983 equipped with pre-sample
chopping, and with quoted accuracies at 1600 cm -1 of 2% (ordinate) and + 3 cm -t (abscissa).
Following the suggestion of Russell (1965) and the example of Mortland & Raman (1968),
variations in clay film thickness were compensated for by using IR bands arising from the
clay matrix itself as internal standards. X-ray diffraction profiles were recorded on a Jeol JDS
8X diffractometer using Cu-K~ radiation at 40 kV and 20 mA.
TPD spectra were recorded on a Stanton Redcroft TG 750 thermobalance equipped with a
TPD and IR of pyridine and n-butylamine on montmorillonite
171
derivative accessory. Samples ( ~ 7.0 mg) were transferred directly out of the solvent vapour
to the thermobalance and the desorption thermograms were generally recorded at a heating
rate of 20~ min -1 under a flow rate for dry N2 purge gas of 25 cm 3 min -1. All samples were
ground to < 45 #m prior to exposure to the vapour.
RESULTS
A striking feature of most of the T P D profiles (Figs 1-4) was the excellent resolution between
different thermal events. This indicated that the different adsorption sites had well
characterized desorption energies which greatly facilitated the interpretation of the
experimental data.
Table 1 gives the basal spacings and relative intensities (most intense peak in a profile
given an intensity equal to ten) of the various samples studied. These data are included
because van Olphen & Deeds (1961) demonstrated that the Na+-montmoriUonite/pyridine/
water system can form four different intercalates, exhibiting basal spacings of 29.3, 23.3, 19.4
and 14.8 A depending on the pyridine "water ratio. In general the greater the amount of
physisorbed pyridine associated with the metal/base complex, the larger the basal spacing.
TABLE1. Summary of basal spacing data of samples described in
text.
d-spacing
A
Sample
no.*
Relativet
intensities
Fe/pyridine
23.0
19.2
15.0
ld
10
5
7
A1/pyridine
15-0
lb
10
Cr/pyridine
15-0
1c
10
Fe/n-butylamine
21.0
13.0
3d
10
9
Al/n-butylamine
21.0
13.0
3b
10
4
Cr/n-butylamine
21.0
13.0
3c
10
Cation/solvent
* = sample number is the same as the number of the figure in
which the TPD profile is presented.
]" = most intense peak given intensity equal to 10.
172
C. Breen et al.
For example, Na+-montmorillonite saturated with pyridine, under the conditions used in this
study, contained 34~ by weight of pyridine and exhibited a rational series of (001) reflections
commensurate with a 23.3/~ intercalate. This high-spacing form converted to the 14.8/~
intercalate in 2-3 h at room temperature (Adams & Breen, 1982). In contrast, a Greensplatt
kaolin sample exposed to pyridine vapour contained < 1~ of pyridine and did not swell. In
general, the desorption profiles for pyridine and n-butylamine represent a weight loss in
excess of 20~ which, in conjunction with the basal spacing data summarized in Table 1,
confirms that the bases are intercalated and not surface moieties.
Pyridine desorption
Fig. 1 shows the desorption profiles for several cation-exchanged forms. The pyridiniumexchanged montmorillonite (Fig. l a) exhibits four desorption peaks with maxima at 30~
190~ 340~ and 620~ The peaks at 30~ and 620~ occur in hydrated Na-montmorillonite and
may be attributed to hydration water and lattice dehydroxylation, respectively. The
desorption profiles for the A13§ Cr 3+- and Fe3+-exchanged forms (Figs lb-d, respectively)
are very similar, with three pre-dehydroxylation peaks at 40~ 150~ and 340~ although the
third peak occurs at 360~ in the A13+-form.
The desorption profiles for the Ca 2+- and Na+-exchanged montmorillonite (Figs 1e and 1f,
respectively) show four pre-dehydroxylation peaks. However, only the peak at 150~ in the
CaZ+-exchanged form is common with the trivalent cation-exchanged clays, although it does
have a very weak peak at 300~
Fig. 2 illustrates how the desorption profiles for the A13+- and Cr3+-exchanged forms vary
with pretreatment temperature. It is evident that temperatures >300~ are required to
modify the type of sorption sites in these exchange forms (Figs 2d, e, i, j).
Pretreatment at 300~ shifted peaks II and III in both samples to lower temperatures but
the effect was more marked in the A13+-samples (Figs 2d, e). This trend continued on
pretreatment at 450~ prior to exposure to pyridine vapour.
The behaviour of the Fe3+-exchanged form following thermal pretreatment was similar to
that of the other two trivalent cation-exchanged forms except that it required considerably
longer to re-expand even after heating at temperatures as low as 40~ Contact times in excess
of several days were necessary to re-establish peak III whereas peaks I and II quickly
reappeared and increased in intensity.
n-butylamine desorption
Fig. 3 shows the desorption profiles of butylamine from the various cation-exchanged
forms. The n-butylammonium exchanged-montmorillonite (Fig. 3a) exhibits two desorption
maxima, prior to the dehydroxylation peak, at 40~ and 410~ The peak at 410~ is very
asymmetric on the low-temperature side. The A13+-, Cr 3+-, Fe 3+- and Ca2+-exchanged forms
(Figs 3b-d and 3f, respectively) all exhibit three pre-dehydroxylation maxima at, or near, 30~
200 ~ and 410~ The Na+-exchanged form shows only two peaks at 30~ and 90~
Fig. 4 illustrates that even rigorous thermal pretreatment at 450~ did not severely effect
the number or position of thermal events in the desorption profile of n-butylamine from the
A13+-exchanged montmorillonite. The Cr 3+- and Fe3+-forms behaved identically following
thermal pretreatment at these elevated temperatures.
TPD and IR o f pyridine and n-butylamine on montmorillonite
173
~
o
o ~
9
o
....
~
o
2~
6~
...~
o
e...~
~z
o~
~+
,..t:Z ~
I
t~
i~
+
....; ~_.,
,<
C. Breen et al.
174
"~o ~
O,A.
I--
e~
~1 ~I
=.
~_
~1
~
o
,,
|
~
. ~ -
~~
".~
I
0
TPD and IR of pyridine and n-butylamine on montmorillonite
175
Infrared studies
Figs 5a-d present representative IR spectra of the air-dried Na § AI 3+-, Fe 3+-, and Cr 3+montmorillonite, respectively, following exposure to pyridine and subsequent evacuation at
increasing temperatures. The samples fresh from pyridine vapour exhibited absorption
bands which, following Ward (1968), were assigned to physisorbed pyridine (1434 and 1485
cm-1), hydrogen-bonded pyridine (753 and 1590 cm-Z), Lewis-bound pyridine (1445, 1485,
1578, 1590 and 1613 cm -1) and the pyridinium cation (1485, 1540, 1606 and 1635 cm-~).
Subsequent to degassing at 50~ 100~ 150~ and 200~ it was apparent that: (i) the Na+-sample
(Fig. 5a) contained no Br6nsted sites, only coordinately bound pyridine and that this was
virtually removed by 200~ (ii) the A13§ and Fe3+-exchanged forms (Figs 5b and c,
respectively) contained predominantly Br6nsted sites, and (iii) the Cr3§
(Fig. 5d) contained both Br6nsted and Lewis sites that were stable to 200~ Fig. 6 illustrates
the effect of temperature on the corrected intensity of the IR bands attributed to the Lewis
and Br6nsted acid species up to 200~ The number of Br6nsted acid sites increased with
increased degassing temperature in all the trivalent cation-exchanged clays but this IR
evidence suggests that the number of Br6nsted sites in the Fe3§
form was only half
that in the A13§ and Cr3+-exchanged forms.
H
1+o6
l!l
1BOO
1600
1~00
=
,+,+
18~
1600
1~00
I&A~
I
Frequency/cm "1
FIo. 5. The effectof evacuationtemperature on the IR spectra of pyridine sorbed on (a) Na§
(b) A13+-,(c) Fe3§ (d) Cr3+-exchangedmontmorillonite.Temperaturesfrom bottom to top are
50~ 100~ 150~ and 200~ respectively.
176
C. Breen et al.
15
15
OJ
f.)
.o ~ 10"
15.
I
I
I
10" t~
'I
Fe
I
s- ~ .
r
A[
.
100
2 0
I 0
200
Temperafur e / ~
FIG. 6. Corrected transmittance of chosen IR bands as a function of outgassing temperature.
(A) 1434, (9 1445, (1--1)1485 and (O) 1540 cm-1. Broken lines represent the decrease in
intensity as a result of outgassing at room temperature.
DISCUSSION
The low desorption temperature of peak I (40~ in the pyridine desorption profiles (Fig. 1)
and the fact that it was reduced in a flow of dry nitrogen at room temperature indicated that it
arises from physisorbed pyridine. This interpretation was corroborated by the rapid loss of
the absorption band for physisorbed pyridine (1434 cm -1) for all samples in Fig. 6.
The desorption maximum in the pyridine desorption thermograms at ~ 150~ had the
strongest intensity in the Ca2+-form, was approximately equal in the trivalent cationexchanged forms, and did not occur in the Na +- or pyridinium forms. Moreover, it was
present in all the trivalent cation-exchange forms irrespective of the pretreatment
temperature. Furthermore, it was not removed in a dry nitrogen gas flow (as peak I was) but
was eradicated following exposure to atmospheric moisture. This evidence, together with the
almost complete removal of the Lewis band at 1445 cm -1 by 200~ in the Al 3+- and Fe 3+exchanged forms, implies that peak II represented pyridine coordinated to Lewis sites.
The IR spectroscopic evidence presented in Figs 5 and 6 show conclusively that
pyridinium ion was the predominant species in the A13+- and Fe3+-exchanged forms
subsequent to heating at 200~ This together with the desorption profile for pyridiniummontmorillonite (Fig. la) proved that the desorption maximum near 340~ was due to the
pyridinium ion. Furthermore, this desorption temperature agrees closely with the value of
360~ found for the desorption of pyridine from silica-alumina by Lercher et al. (1985). The
existence of Lewis sites on Cr3+-montmorillonite at 200~ had no observable effect on the
pyridine desorption profile, except perhaps that peaks II and III were not as well resolved as
they were in the A13+- and Fe3+-forms.
The strong acid sites, exemplified by the pyridine desorption peak near 340~ were quite
stable to thermal pretreatment, requiring temperatures >300~ to modify them. This
temperature agrees well with the dehydration temperature of these cations. Occelli et al.
(1984) have shown that heating pillared clays to these temperatures converts the Br6nsted
sites to Lewis sites and that appears to be the case here. The broad peaks centred on 300~ in
Figs 2d, e, i and j are thus considered to be the result of pyridine desorption from Lewis sites
created by the--essentially complete--dehydration of the interlamellar cation.
Therefore, by analogy with the pyridine desorption profiles in Fig. 1 and the desorption
thermogram of the n-butylammonium-exchanged montmorillonite (Fig. 3a), peaks I', II' and
T P D and I R o f pyridine and n-butylamine on montmorillonite
177
III' in the n-butylamine desorption profiles are attributed to physisorbed, Lewis-bound and
protonated amine, respectively. The higher desorption temperatures of peaks II' and I I r in
Fig. 3 compared to the corresponding values for pyridine reflect the increased interaction
between the stronger base butylamine and the acid centres. The low intensity of peak II' (Fig.
4) may be due to the enhanced interaction, with these Lewis sites shifting their desorption
maxima to coincide with the desorption of protonated butylamine. However, Ballantine et al.
(1985) have reported that ~ 5 ~ of n-butylamine is converted to di-n-butylamine on heating at
215~ for 14 h over A13§
in a sealed reaction vessel. The corresponding
yields using the other exchange forms investigated here were not reported. However, the
yields of dicyclohexylamine from cyclohexylamine over A13+-, Cr 3+-, Ca 2+- and Na §
exchanged bentonite were 91, 72, 35 and 22~, respectively, indicating that all these exchange
forms are capable of producing di-n-butylamine. Moreover, Fig. 3e shows that the derivative
thermogram of di-n-butylamine from A13§
had an intense maximum which
coincides exactly with the peak assigned to protonated butylamine. Consequently, peak II'
may be weak because most of the butylamine has been converted to the dimer. Further
evidence to support this interpretation was found, in that rigorous thermal pretreatment,
resulting in dehydration of the cations did not remove the peak tentatively assigned to
protonated amine. However, Tennakoon et al. (1986) recently reported that treatment of
calcined pillared clays with strong bases, such as ammonia, can abstract protons which have
migrated into the clay layer during the calcination process at 500~ It is therefore suggested
that peak I I I ' in the n-butylammonium desorption profile is due to protonated amine but that
some of its intensity may result from formation of the dimer.
In summary, the desorption profiles indicate that there is little difference in the number of
acid sites on the three cation-exchanged forms, although the desorption temperature of the
protonated base from A13§
does occur at a higher temperature (~20~
indicating stronger acidity. However, the IR study shows that the number of Br6nsted sites
increase up to 200~ for all the trivalent cation-exchanged forms, and that in contrast the
Cr3+-montmorillonite maintains a large number of Lewis sites in addition to the Br6nsted
sites. Heating the trivalent cation-exchanged forms above 300~ results in dehydration of the
cation and the formation of Lewis acid centres of a strength intermediate to that of the
naturally occurring Lewis sites and the Br6nsted sites destroyed by the high pretreatment
temperatures. However, this dehydration also results in the migration of protons into the clay
layer, and strong bases such as butylamine can abstract them but weaker bases such as
pyridine cannot.
ACKNOWLEDGMENTS
We gratefully acknowledge maintenance grants from the Research and Postgraduate Students Committee
(AD) and the School of Chemical Sciences (JJF) NIHE, Dublin and the technical assistance of Mr P. Byrne
and Ms M. Bannon of the Institute for Industrial Research and Standards in collecting the X-ray data.
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