Download thermogravimetric, infrared and mass

Clay Minerals. (1993) 28, 123-137
THERMOGRAVIMETRIC,
INFRARED AND MASSS P E C T R O S C O P I C A N A L Y S I S OF T H E D E S O R P T I O N
OF M E T H A N O L , P R O P A N - 1 - O L , P R O P A N - 2 - O L A N D
2-METHYLPROPAN-2-OL
FROM MONTMORILLONITE
C. B R E E N * ,
J. J. F L Y N N
ANO G. M. B . P A R K E S ?
School of Chemical Sciences, Dublin City Universityr Glasnevin, Dublin 9, Ireland and ~Catalysis Research
Unit, Leeds Polytechnic, Calverley Street, Leeds" LS1 3HE, UK
(Received 22 December 1991; revised 8 April 1992)
ABSTRACT: The desorption of methanol (MeOH), propan-l-ol (n-PrOH), propan-2-ol
(i-PrOH) and 2-methylpropan-2-ol (t-BuOH) from Na +-, Ca2+-, A13+-,Cr3+- and Fe3+-exchanged
montmorillonite has been studied using variable temperature infrared (IR) spectroscopy and
thermogravimetric analysis (TGA). Alcohol-saturated trivalent cation (M3+) exchanged samples
exhibit maxima in the derivative thermograms at 20 and 110~ (MeOH), 30 and 160~ (n-PrOH), 20
and ll0~ (i-PrOH) and 20, 55 and 80~ (t-BuOH). Alcohol-saturated Na§ and Ca2+-exchanged
montmorillonite samples exhibit maxima at higher temperatures in the i-PrOH (20 and 140~ and
t-BuOH (30, 90 and 110~ desorption profilesbut at the same temperatures for MeOH and n-PrOH.
Mass spectroscopic analysis of the vapours desorbed from the alcohol-treated samples show that the
low-temperature maxima in the alcohol desorption from the M3+-exchanged clays are due to
unchanged alcohol, whilst those occurring at 80~ (t-BuOH), ll0~ (i-PrOH) and 160~ (n-PrOH)
are due, in the main, to alkene produced from the intramolecular dehydration of the respective
alcohol. Changes in the IR spectra of the adsorbed alcohols occur at temperatures which are in accord
with the mass spectral data. No mass spectral evidence was found for the formation of dialkylethers
via the competing intermolecular process but dimerisation and oligomerisation of t-BuOH were
observed.
Baltantine et al. (1984) r e p o r t e d that primary aliphatic alcohols, such as ethanol, propan-1ol and b u t a n - l - o l (inter alia) when intercalated in A13+-exchanged montmorillonite react
preferentially via an intermolecular nucleophilic displacement of water to give high yields
(30-70%) of di-(alk-l-yl) ethers, rather than undergo competitive intramolecular
dehydration to alkenes. In contrast, the reaction of secondary aliphatic alcohols gave high
yields (-~80%) of the corresponding alkene, except for propan-2-ol which was converted to
both the di-(alk-2-yl) ether (35%) and alkene (41%). A d a m s et al. (1981) also r e p o r t e d the
formation of di-prop-2-yl ether but at a much lower yield (3%). H o w e v e r , A d a m s and coworkers (1981) carried out their reactions at 60~ using Fe3+-montmorillonite in the
presence of 1,4-dioxan as solvent, whereas Ballantine et al. (1984) e m p l o y e d no solvent and
utilized a reaction t e m p e r a t u r e of 200~ The pertinent results for further discussion are
summarized in Table 1. M o r e o v e r , A d a m s et al. (1981) suggested that the low-temperature
coefficient of the ether forming reaction indicated the possible presence of diffusion
control. This p r o m p t e d a recent investigation into the sorption kinetics of m e t h a n o l
* Current address: Materials Research Institute, Sheffield Hallam University, Pond Street, Sheffield
$1 1WB.
:~ Formerly the National Institute for Higher Education, Dublin.
9 1993 The Mineralogical Society
124
C. B r e e n et al.
TABLE1. Product distributions (wt%) for reactions of alcohols with M3+-montmorillonite.
Reactant
MeOH
n-PrOH
i-PrOH
t-BuOH
Reactant
recovered
a,b
20-7
29
3
Ethers
Alkenes
b(2 )
_
67-2c
34-7d(3)d
--
3
41"4
92e(8)f
Figures in parentheses from Adams et al. (1981) using 1 g Fe3+-montmorillonite,50 mmol of
reactant alcohol and 3 cm3 1,4-dioxan at 60~ Other values from Ballantine et al. (1984) using
0.5 g Al3+-montmorilloniteat 200~ for 4 h.
a Not reported by Adams et al. (1981).
b Not studied by Ballantine et al. (1984).
c Yield of di-prop-l-ylether.
d Yield of di-prop-2-ylether.
e 87% alkene, 5% alkene dimer.
f Alkene dimer.
(MeOH), propan-2-ol (i-PrOH) and 2-methylpropan-2-ol (t-BuOH) on to A13+-, Cr 3+- and
Fe3+-exchanged montmorillonite in the temperature range 18-105~ (Breen et al., 1987c).
However, the diffusion coefficients determined by these workers did not exhibit the
expected temperature dependence which normally follows an Arrhenius formulation.
Consequently, as the alcohols can react within the temperature range encompassed by the
sorption kinetic studies of Breen et al. (1987c), this study was undertaken to elucidate the
nature of the sorbed alcohol in the temperature range 18-800~ and forms part of an
extended study into the dynamic (Breen e t a l . , 1987a,b,c) and steady-state (Breen &
Deane, 1987) behaviour of selected organic species on trivalent cation-exchanged
montmorillonite.
This work complements and extends the derivative thermogravimetric work of A1Oswais e t a l . (1986) on the desorption of ethanoic acid from an A13+-exchanged
montmorillonite, the acidity of which has been investigated by Ballantine et al. (1987), and
addresses more directly the nature of the low-temperature desorption maxima observed in
the thermogram. Thermogravimetry with mass spectroscopic evolved gas analysis (TGE G A ) has recently been used to considerable advantage in the identification of the
desorption products arising from the transformation of butylamine (Shuali et al., 1990) and
pyridine (Shuali et al., 1991) in contact with sepiolite or palygorskite.
EXPERIMENTAL
The parent clay utilized in this study was a Wyoming montmorillonite supplied by Volclay
Ltd., Wallasey, Cheshire. The nominally <2/~m fraction was collected and exchanged
using a 0.3 mol drn 3 solution of the appropriate salt solution. Subsequent to an exchange
period of 24 h, the clay was washed free of excess salt. Chemical analysis (Bennet & Reed,
1971) of the Na+-exchanged form indicated a layer formula of (Si3.9A10.1)(A11.33Fe0.08
Mg0.59)O10(OH)2 and a cation exchange capacity (Adams et al., 1977) of 68 _+ 2 mEq/100 g
clay. Self-supporting clay films (-~2 mg cm -2) for infrared (IR) analysis were prepared by
Desorption of alcohols from clay
125
evaporation of a dilute aqueous slurry upon a polyethylene backing which was subsequently
removed.
Samples for IR spectroscopy, standard thermogravimetric (TG) analysis and T G - E G A
were air-dried (20~ rh ~-60%) prior to exposure to reagent grade MeOH, propan-l-ol
(n-PrOH), i-PrOH and t-BuOH vapour at partial pressures of 100, 20, 40 and 40 mm Hg,
respectively, for periods in excess of 48 h.
The IR spectra were recorded at room temperature, then after 1 h at 50, 100, 150~ and
occasionally at 200~ using an evacuable, variable temperature cell operating under a
dynamic vacuum of 10 2 mm Hg. The spectrometer utilized was a Perkin Elmer model 983
equipped with pre-sample chopping, and with quoted accuracies of 2% (ordinate) and
_+3 cm -1 (abscissa). X-ray diffraction profiles were recorded on a Philips PW1050
diffractometer using Co-KoL radiation ()~ = 1.7707 ~ ) operating at 40 kV and 20 mA.
Derivative thermograms were recorded on a Stanton Redcroft TG750 thermobalance
equipped with a derivative accessory. Samples (-~7 rag) were transferred directly out of the
solvent vapour to the thermobalance and the desorption thermograms were recorded at a
heating rate of 20~ rain i under a flow for dry nitrogen purge gas of 25 cm 3 min -1. All
samples were ground to <45 ~tm prior to exposure to the alcohol vapour.
Mass spectrometric evolved gas analyses, in the temperature range 20-250~ were
obtained using a Stanton Redcroft TG1000 thermobalance interfaced to a VG micromass 12
mass spectrometer. Samples were conditioned in flowing helium for 15 min prior to
initiating the temperature ramp to remove excessive quantities of physisorbed alcohol. The
primary aim in utilizing this technique was to identify the desorbing species and thus no
attempt was made to calibrate the system and quantify the amount of each species
desorbed.
RESULTS
The basal spacing and percentage weight loss (18-800~ data for the water- and alcoholsaturated samples listed in Table 2 confirm that the desorption profiles reported below arise
from intercalated and not surface species.
Figures l a - d show the derivative thermograms for the desorption of MeOH, n-PrOH,
i-PrOH and t-BuOH, respectively, from a range of cation-exchanged forms. Breen et al.
(1986b) have previously reported that the desorption maximum near 600~ can be
attributed to the loss of structural OH. The desorption of M e O H from the trivalent cationexchanged and Na+-forms (Fig. la) was characterized by two peaks at 20 and 110~ In
addition, the A13+-form exhibited a weak, broad desorption near 300~ and the Ca 2+montmorillonite had a sharp peak at -~140~ which partially masked that at ll0~ The
temperatures for the maxima in the profiles for the desorption of n-PrOH from all the
cation-exchanged forms (Fig. lb) were identical at 30 and 160~ although the hightemperature desorption maximum in the Na+-exchanged form was weak. Only the AI 3+form exhibited a weak, broad desorption near 290~ The desorption profiles for i-PrOH
from Na +- and Ca2+-montmorillonite showed two peaks near 20 and 140~ whilst the
higher temperature desorption maximum in the trivalent cation-exchanged forms occurred
at the lower temperature of 110~ (Fig. lc). The derivative thermograms for the desorption
of t-BuOH (Fig. ld) contained the lowest temperature desorption peaks in this study, with
three poorly resolved peaks occurring at, or near, 30, 90 and 110~ in the Na +- and Ca 2+-
126
C. Breen et al.
TA~L~2. Weightloss (%) by 800~ and basal spacingsat 20~ for the M3+montmorillonite/alcohol systems.
Cation
Solvent
Weight loss
(%)
Basal spacing
(A)
A13+
H20
MeOH
n-PrOH
i-PrOH
t-BuOH
HeO
MeOH
n-PrOH
i-PrOH
t-BuOH
H20
MeOH
n-PrOH
i-PrOH
t-BuOH
16
36
26
27
23
16
26
24
24
21
16
25
27
25
24
12.5
14-0
14.0
14.3
15.3
12-5
15.0
14.0
15.8
17.4
12.5
15-5
14.0
16.0
17.7
Cr:~+
Fe3+
montmorillonite, and at 20, 55 and 80~ in the trivalent cation-exchanged clays. Deviations
from the background were observed in the 200-400~ region of the t-BuOH desorption
profiles arising from the trivalent cation-exchanged forms.
The gases evolved from alcohol saturated A13+-exchanged montmorillonite samples were
swept into the mass spectrometer using the helium carrier gas and the characteristic M/Z
peaks for water, alcohol and corresponding alkene were routinely monitored as the
temperature was increased. However, care was taken to identify any other possible
products such as ethers, alkene dimers and higher molecular weight compounds. Figure 2
shows how the total ion count (TIC) and the characteristic mass spectral peaks for water
(M/Z = 18), n-PrOH (M/Z = 31), propene (M/Z = 41), i-PrOH (M/Z = 45), t-BuOH
(M/Z = 59) and 2-methylpropene (M/Z = 41) varied with increasing desorption
temperature. There were definite similarities between the temperatures of the maximum
desorption recorded and in the thermogravimetric studies (Fig. 1). The samples for E G A
were conditioned in the flowing helium carrier gas for 15 min prior to initiating the
desorption ramp. This resulted in loss of physisorbed alcohol and water thus revealing
desorption maxima near 55, 57 and 63~ for the desorption of n-PrOH, i-PrOH and tBuOH, respectively. Similar results were obtained when the samples were preconditioned
in flowing nitrogen carrier gas in the standard thermogravimetric experiment. This form of
pretreatment has been routinely used in the study of cyclohexylamine and pyridine
desorption from variously cation-exchanged montmorillonite (Breen, 1991a,b). Figure 2b
presents a similar picture for the desorption and transformation of i-PrOH, although the
alkene and water desorb at the lower temperatures heralded by the standard thermogravimetric studies. In addition to the contributions from PrOH, propene and water,
evidence of small quantities of oligomeric species was also obtained. However, there was
no evidence of ether formation. Finally, Fig. 2c indicates that the dehydration of t-BuOH to
2-methylpropene and water began at a very low temperature and reached a maximum
Desorption of alcohols from clay
127
Na
dw
dw
dT
dT
i
(1 '
,
,
i
,
1+()0
800
Temperature/*
0
z+00
800
Temperature/*C
Na
I___
Ca
_d_ww
dT
Fe
r~
Cr
A
i
i
,
i
.
t+00'
800
Temperafure/*C
0
400
800
Temperafure/*C
FIG. 1. Derivative thermograms for the desorption of (a) MeOH, (b) n-PrOH, (c) i-PrOH and
(d) t-BuOH fromNa +-, Ca2+ Fe3+ Cr3+- and AI3+-exchanged montmorillonite.
at 63~ M o r e o v e r , there was a very b r o a d , featureless desorption stretching from 100-200~
which m a y be attributed to dimerisation and oligomerisation of the alkene (vide infra).
Figure 3 shows the way in which the I R spectra of n - P r O H a d s o r b e d on Cr3+-exchanged
montmorillonite changed as the sample was degassed and heated. The m a r k e d reduction in
the characteristic I R absorption bands for water, near 3400 cm -1 and 1626 cm -1, showed
that degassing at r o o m t e m p e r a t u r e for one hour r e m o v e d a considerable a m o u n t of water,
whereas the affect on the absorption bands for i - P r O H , centred a r o u n d 1400 cm 1, was not
so m a r k e d . Subsequent degassing for one hour at 50, 100 and 150~ further r e d u c e d the
128
C. Breen
et al.
a
>i-E/)
z
i.iJ
i-z
o
20
50
100
150
200
Temperafure/'C
250
b
t~
Z
I--i,.-...,i
20
50
Z"'t~4
>'
~--
100
150
'~
t--,-4Z
j
'
C
/Z:59
/ /
M/Z=41
...Jr
20
200
250
Temperafure/~
j M/z:18
50
100
150
200
Temperature/~
250
Fro. 2. Mass spectroscopicEGA data for Al3+-exchangedmontmorillonitetreated with (a) n-PrOH,
(b) i-PrOH and (c) t-BuOH. Water (M/Z = 18), n-PrOH (M/Z - 31), propene (M/Z = 41), i-PrOH
(M/Z = 45), t-BuOH (M/Z = 59), 2-methylpropene(M/Z = 41). TIC: total ion count.
Desorption of alcohols from clay
129
oJ
E
L
4000
3000
2500
2000
wavenumberlcm -I
FIG. 3. Effect of outgassing temperature on the 1200-4000 cm 1 region of the IR spectra of Cr 3+montmorillonite/n-PrOH intercalates. Spectra from bottom to top are 20~ and then after one
hour's degassing at 20, 50, 100 and 150~ respectively.
amount of sorbed water and alcohol. The changes in both the intensity and position of the
bands in the 3200-3500 cm -a region shown reflect the general trend for all the samples
studied, although there were subtle differences in detail. In general, the IR spectra of all the
samples studied illustrated that the air-dried films were not substantially dehydrated by
exposure to alcohol vapour, but the co-adsorbed water was less resistant to the room
temperature degassing procedure. Moreover, near the temperature at which desorbed
alkene was observed in the T G A - E G A experiment there was a marked change in the C - H
stretch and C - H bend regions, along with the changes mentioned in the 3200-3500 cm -a
region and the loss of a weak broad absorption near 2500 cm -1. Figures 4 and 5 detail the
changes observed in the 1300-1800 cm a and 2800-3500 cm ~ regions of the IR spectra
obtained from the alcohol treated clays. For clarity the spectra recorded at elevated
temperatures are presented with similar intensities although, as Fig. 3 shows, the actual
intensities of the bands decreased with increasing treatment temperature.
Figure 4 shows the changes in the 1300-1800 cm -1 region of the IR spectrum caused by
outgassing the alcohol-saturated Cr3+-montmorillonite at progressively higher tempera-
130
C. Breen et al.
b
a
~
1463
,26
1457
i347
\f
A
N
o
rrD
162 ~_
-4-..4--
1463
1626
347
\l
E
N
r-rD
1635
1626
~/
1463
1391
I? 7
1397
~,57
f
6 6
347
\1
1626
1626
Z
1457
1800 '
'
1394
'14-'00
i
1800
i
1391
1377
i
1400
i
1800
*
1363
i
1400
w a v e n u m b e r / c m -1
FIG. 4. Effect of outgassing temperature on the IR spectra of Cr3+-montmorillonite/alcohol
intercalates (a) n-PrOH, (b) i-PrOH and (c) t-BuOH. Temperatures from bottom to top are 20, 50,
100 and 150~ respectively.
131
Desorption of alcohols from clay
c
c
rv
I~
2849
32503250
3500
25'o0
3000
wavenumber/em
-1
I
c
a/
o
ea
u
m
E
c
ro
I.-
E
tv
i
3500
3000
wavenumberlcm -I
2500
3500
3000
2500
w a v e n u m b e r l c m -I
FIG. 5. Effect of outgassing temperature on the C-H and O -H stretching regions of the IR spectra of
(a) n-PrOH, (b) i-PrOH and (c) t-BuOH sorbed on Cr3+-exchanged montmorilionite. Spectra from
bottom to top are 20~ and then after one hour's degassing at 20, 50, 100 and 150~ respectively.
132
C. Breen et al.
tures. The IR spectrum of the Cr3+-montmorillonite/n-PrOH intercalate (Fig. 4a) changed
little as the outgassing temperature was increased. The CH3 deformation band at 1457 c m - 1
was maintained up to 150~ whilst the C - H band at 1397 cm 1 and the O H band at
1625 c m - 1 became progressively weaker up to this temperature. In contrast, the IR
spectrum of the Cr3+-montmorillonite/i-PrOH system (Fig. 4b) exhibited a marked change
as the outgassing temperature was increased from 50 to 100~ (Fig. 4b, middle spectra).
The characteristic doublet at 1391 and 1377 cm 1, which has been attributed to iso-propyl
groups (Rochester et al., 1984; Datka, 1980), was lost and the bands at 1626 and 1457 cm -1
became the dominant features of the spectrum. At 150~ poorly resolved bands at 1435 and
1427 cm -1 were observed, together with a weak doublet at 1375 and 1363 cm 1. Increasing
the outgassing temperature to only 50~ caused considerable changes in the IR spectra of
the Cr3+-montmorillonite/t-BuOH intercalate (Fig. 4c). The 1376 cm 1 band was considerably reduced in intensity and the bands at 1626, 1464 and 1363 cm 1 dominated the higher
temperature spectra. Figures 5a-c illustrate the changes that occur in the C - H and O - H
stretching region of the n-PrOH, i-PrOH and t-BuOH saturated Cr3+-montmorillonite as
the outgassing temperature was increased. The important feature to note is that the change
in the characteristic absorption band profile, in both the 2800-3000 cm i and 30003500 cm 1 region, occurred at progressively lower temperatures; 150~ for n-PrOH
(Fig. 5a), 100~ for i-PrOh (Fig. 5b) and 50~ for t-BuOH (Fig. 5c), thus reinforcing the
changes observed in the 1300-1800 cm -1 region of the respective spectra.
DISCUSSION
The low-temperature maximum in the desorption profile of M e O H from the various cationexchanged forms (Fig. la) could be removed by passing dry nitrogen gas through the system
and can thus be attributed to physisorbed alcohol. This was also the case for the peaks in the
region 20-30~ for the desorption of n-PrOH, i-PrOH and t-BuOH from the various cationexchanged forms (Figs. lb-d). The maximum at 110~ in the desorption profile of M e O H
from the various cation-exchanged forms (Fig. la) corresponds closely to the values 117140 and 130~ reported for the desorption of M e O H adsorbed (i) on the surface hydroxyls
of silica-magnesia mixed oxides (Noller & Ritter, 1984), and (ii) in the zeolite
H-ZDM-5 (Ison & Gorte, 1984), respectively. However, Ison & Gorte (1984) observed a
second desorption maximum near 180~ which they attributed to the desorption of a single
M e O H molecule from each cation site in H-ZSM-5, but there was no evidence of such an
interaction in this study. Moreover, the exchange cation present exerted no influence on the
desorption maximum at ll0~ in the derivative thermogram for MeOH. This, together with
the observation that the higher temperature maximum at 160~ in the desorption profile for
n-PrOH (Fig. lb) also showed no cationic dependence, was surprising given that it would be
anticipated that the more polarizing cations would result in stronger sorption of the alcohol
molecules. However, the following discussion will indicate that the observed desorption
thermograms reflect chemical conversion in addition to the desorption of unchanged
alcohol. This conversion is an acid catalysed process involving the acidic protons generated
by the polarization of the primary coordination sphere water molecules by the small, highly
charged trivalent cations.
In contrast to the behaviour of M e O H and n-PrOH, the corresponding maximum for the
desorption of i-PrOH, (Fig. lc) occurred at 140~ in the Na +- and Ca2+-forms but at ll0~
in the trivalent cation-exchanged montmorillonite, and the reason is not immediately
Desorption of alcohols from clay
133
obvious. The results in Table 1 indicate that several processes can occur, which may be in
competition:
ROH(ads) --~ ROH(g)
(1)
ROH(ads) ~ alkene(g) + H20(g)
(2)
2ROH(ads) ~ R-O-R(g) + H20(g)
(3)
where (ads) and (g) indicate adsorbed and vapour phase molecules, respectively. The
problem is further complicated in steps (2) and (3) because the products may not desorb at
the temperature at which they are produced. If eqn. (2) describes the desorption process
then it is necessary to ascertain whether one or both products are desorbed upon formation.
Noller 8: Ritter (1984) found that propene formed by acid-catalysed dehydration of i-PrOH
on silica-magnesia desorbed at least 65~ below that formed from n-PrOH and that the
alkene was desorbed upon formation. This behaviour is repeated here insofar as the
propene formed from the dehydration of n-PrOH desorbed at 165~ and that formed from
i-PrOH desorbed at 115~ (Fig. 2a,b). Moreover, the 2-methylpropene resulting from the
dehydration of t-BuOH exhibited a desorption maximum at the much lower temperature of
63~
This sequence of desorption temperatures reflects the increasing stability of
carbocations formed from primary, secondary and tertiary alcohols and is in accord with the
results of Adams el al. (1983). In all instances the desorption of the water liberated in the
dehydration step occurred at a higher temperature and over a longer temperature interval
than that of the alkene co-product.
Ether formation proceeds via the nucleophilic displacement of water, from a protonated
alcohol molecule, by a neighbouring unprotonated alcohol molecule. The absence of ether
in E G A probably means that the interlayer concentration of alcohol was too low to facilitate
the intermolecular process which gives rise to ethers and thus the intramolecular
dehydration to form alkene and water predominates. This contrasts with the reported
behaviour in the presence of excess alcohol, as used in the catalytic investigations
(Ballantine et al., 1984), where the intermolecular dehydration to form ether molecules is
an important process, particularly with linear alcohols. However, Tennakoon et al. (1983)
reported that dialkylether production was limited to when a two layer pentanol intercalate
was formed and this only occurred when the clay was in direct contact with liquid pentanol.
Adams etal. (1983) and Ballantine etal. (1984) have independently reported the
presence of alkene dimers and oligomers in the product mixture resulting from the
dehydration of t-BuOH by acidic clays. Indeed the speed of the oligomerisation process
results in the failure of t-alkyl ester formation at temperatures above --=-30~ (Ballantine
et al., 1984). It was difficult to attribute the broad, featureless desorption above 100~ in the
T G - E G A study of the desorption of t-BuOH from A13+-exchanged montmorillonite
(Fig. 2c) to a single product although there was some evidence for the alkene dimer (2,4,4trimethylpent-l-ene). Consequently, further identification of the reaction products was
achieved using a Perkin Elmer ATD50 automatic thermal desorption system coupled to a
Hewlett-Packard 5890 capillary gas chromatograph which was interfaced to a VG Trio-1
quadrupole mass spectrometer. The t-BuOH saturated A13+-exchanged montmorillonite
sample was subjected to a flow of helium gas, preheated to 230~ for 15 rain. The vapour
phase products were collected in the cold trap and then swept into the GC-MS. This allowed
the individual components in the reaction mixture to be separated and perhaps identified.
Even though the preliminary studies, of both the alcohol-free clay and the redistilled
t-BuOH used, indicated the presence of no impurities, the TIC chromatogram of the
134
C. Breen et al.
product mixture contained many minor components. However, the mass spectra of
the major peaks in the chromatogram, at retention times of 0.82, 0.97 and 3-07 min,
were readily interpreted (both manually and by computer search) as the expected
2-methylpropene, t-BuOH and 2,4,4-trimethylpent-l-ene, respectively. The more
intense of the remaining peaks occurred at retention times of 1.32, 14.1 and 16.0 min.
Computer matching attributed these compounds to 2- or 4-methyl-2-pentene, the trimer of
2-methylpropene and a C l l hydrocarbon, respectively.
When the desorption was repeated on separate samples using helium gas at 50, 100 and
150~ the number of minor components observed increased with increasing helium
temperature. Moreover, the peaks with retention times of 1.32 and 16.0 min were first
observed after treatment at 100~ and became more intense after treatment at 150~
whereas the peak at 3-07 min, the alkene dimer, was first observed at 150~ Clearly, the
high molecular weight components in the product mixture required higher temepratures for
their production and/or desorption. The exact nature of the C6 and C l l hydrocarbons
remains uncertain but as they appeared at the temperatures when a noticeable darkening of
the sample occurred, their association with the oligomerisation, and hence coking, process
may be indicated. Indeed all the alcohol treated trivalent cation-exchanged clays were dark
grey after heating to 800~ in the thermobalance.
The information provided by the mass spectral analysis of the gases evolved from the
alcohol-treated A13+-montmorillonite samples allows the changes observed in the I R
spectra (Figs. 3, 4 and 5) to be interpreted. Annabi-Bergaya et al. (1980) reported that a
considerable amount of the alcohol in alcohol-saturated montmorillonite resided in
micropores and between the clay particles. Thus the reduction in absorbance noted upon
degassing the samples at room temperature can be attributed, in part, to the removal of
alcohol and water from these environs. The spectra in Figs. 4c and 5c show that some
transformation of the adsorbed t-BuOH occurred after heating at 50~ for one hour whilst
temperatures of 100 and 150~ respectively, were necessary to cause similar changes in the
IR spectrum of adsorbed i-PrOH and n-PrOH. Clearly, these changes result from the
dehydration of alcohol to alkene. However, direct IR evidence for the formation of the
alkene is impossible because the water eliminated during this process would absorb in the
same region as the C--C double bond. Moreover, Adams & Clapp (1985) have shown that
the IR bands due to the double bond in 1-hexene are removed very rapidly and
consequently would not have been observed in this study when spectra were only recorded
once an hour. Indeed, Grady & Gorte (1985) observed very similar changes to those
reported herein (Figs. 4b and 5b) in the IR spectrum of i-PrOH adsorbed on H-ZSM-5 and
attributed a similar marked change in the 2800-3000 cm 1 region, the complete loss of the
C - H band at 1380 c m - 1 together with an increase in intensity of the band at 1630 c m - 1 (due
to the production of water via mechanism (2)) to the formation of propene. They too were
able to confirm this interpretation using mass spectroscopy.
The changes in the O-H-stretching region, 3000-3500 cm 1, of the IR spectra of the
alcohol treated clays should, in principal, provide information concerning the mode of
interaction of the alcohol with the interlayer cation. Infrared spectroscopic studies of the
sorption of ethanol by montmorillonite (Dowdy & Mortland, 1967) have shown that this
alcohol, at least, is capable of displacing water from the primary coordination sphere of
exchange cations such as Ca 2+, Cu 2+ and A13+. The fact that this phenomenon was not
observed here need not necessarily be attributed to the higher alcohols used but may be due
to the parent montmorillonite. Tennakoon etal. (1983) observed that the pentanol
Desorption of alcohols from clay
135
intercalate of A1-Gelwhite was thermally stable up to 200~ whereas pentanol saturated A1bentonite collapsed at 150~ After repeatedly exposing a cae+-exchanged montmorillonite to methanol vapour, in order to remove the initial water, and then degassing it at 170~
Annabi-Bergaya etal. (1980) attributed a band at 3520 cm 1 to methanol directly
coordinated to the exchange cation. Moreover, it is common practice to assign bands near
3450 cm-1, particularly if accompanied by an increase in frequency of the C - H bend region
near 1400 cm -1, to alcohol molecules directly coordinated to Ti 4+ on both anatase (Ross
et al., 1987) and rutile (Suda et al., 1987). This is not, however, to be confused with a much
sharper band observed at 3420 cm-1 which is attributed to a highly coordinated OH group
just below the surface. Nguyen et al. (1987), using corroborative evidence from ethylene
glycol monomethyl ether (EGME)/CaCle solutions, interpreted the IR absorption bands at
3370 and 3250 cm-1, arising from EGME adsorbed on dehydrated Ca2+-montmorillonite,
to OH groups involved in hydrogen bonding and directly bonded to the cation, respectively.
Clearly the bands in this region are open to several interpretations in the presence of alcohol
just as they are in the presence of water.
The IR spectra of all the alcohol treated samples (Fig. 5a-c) exhibited three bands in the
3000-3500 cm -1 region at 3250, 3380 and 3450 c m - 1 . The last of these will be discussed
below. Bands near 3380 and 3250 cm 1 in water containing di- and trivalent cationexchanged clays are attributed to hydroxyl groups involved in hydrogen bonding parallel to
the clay layer and stronger H-bonding in outer spheres of coordination, respectively
(Farmer & Russell, 1971). Clearly, both bands were present in the n-PrOH treated sample
(Fig. 5a), indicating that H-bonding was present in outer spheres of coordination either to
other alcohol or water molecules. The two bulkier alcohols exhibited a strong 3380 c m - 1
band and a weak 3250 cm I band (Fig. 5b,c) indicating that there was H-bonding parallel to
the layers, probably in the form of a water bridge between alcohol and cation, but that it did
not extend very much further. Thus the bulky alcohols may prevent the formation of
extended H-bonded networks, or their greater volatility, which causes them to desorb at
lower temperatures than n-PrOH (Figs. 1 and 2), may mean that there are too few
molecules available to form an extended network.
Figures 5a--c show that the absorption bands in the 3200-3500 cm -a region of each
spectrum underwent a transformation resulting in a band near 3450 cm -1 at the same
temperature that marked spectral changes in the 1300-1800 cm 1 and 2800-3000 cm -1
regions, attributed to the conversion of alcohol to alkene, occurred. The mass spectral data
presented in Fig. 2 showed that the water molecules resulting from the dehydration of the
alcohol continued to desorb after the majority of the alkene had been lost. Farmer &
Russell (1971) attributed absorption bands, in the monohydrate forms of saponite and
vermiculite, at 3490 and 3250 cm 1 to water hydrogen bonded to Si-O-Si and Si-O-A1TM
linkages, respectively. Moreover, Russell & Farmer (1964) observed that heating Ca- and
Mg-exchanged montmorillonite to 150~ resulted in the development of bands at 3533 and
3496 cm 1, respectively, which persisted until dehydroxylation but disappeared upon
rehydration. Given that the AI3+ and Cr 3+ ions studied here are more polarizing due to
their higher charge, then an increase in H-bond strength could move such a bond to
3450 cm 1.
More recently, Schutz et al. (1987) have assigned a band at 3440 c m - 1 in NH4+-beidellite
to an Si-OH group created by the breaking of an Si-O-A1TM bond in the tetrahedral sheet
and attributed its formation to the strong interlayer acidity in the system. Consequently,
given that (i) the band at 1626 cm 1 indicates that water is still present, (ii) these systems
136
C. B r e e n et al.
b e c o m e v e r y acidic as the t e m p e r a t u r e is raised ( B r e e n et al., 1987b), and (iii) the lack of
e v i d e n c e for the existence of a l c o h o l in e i t h e r the I R or the mass spectra, t h e r e is sufficient
p r e c e d e n t in the l i t e r a t u r e to s u p p o r t the supposition that the b a n d at 3450 c m - 1 in t h e s e
systems is unlikely to be associated with an alcohol m o l e c u l e .
T h e use o f mass s p e c t r o s c o p i c E G A has b e e n able to confirm u n e q u i v o c a l l y the m a j o r
species d e s o r b e d f r o m a l c o h o l - t r e a t e d clays and facilitate the i n t e r p r e t a t i o n of changes in
I R spectra of the a d s o r b e d alcohols. H o w e v e r , E G A was less successful w h e n a r a n g e of
similar c o m p o u n d s w e r e f o r m e d p r i o r to or during d e s o r p t i o n . T h e p r e l i m i n a r y
c h r o m a t o g r a p h i c results c o n t a i n e d h e r e i n m a y b e of a d v a n t a g e w h e n v e r y d e t a i l e d analyses
of such m i x t u r e s are r e q u i r e d .
ACKNOWLEDGMENTS
We acknowledge the stimulating discussions and continued interest and committment of Dr Rob Brown of the
Catalysis Research centre at Leeds Polytechnic, and the assistance of Joan Hague, Sheffield City Polytechnic, in
collecting the GC-MS data.
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