Clay Minerals. (1993) 28, 123-137

T H E R M O G R A V I M E T R I C , I N F R A R E D A N D M A S S - S 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 - M E T H Y L P R O P A N - 2 - O L F R O M M O N T M O R I L L O N I T E

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 +-, Ca 2+-, A13+-, Cr 3+- and Fe3+-exchanged montmorillonite has been studied using variable temperature infrared (IR) spectroscopy and thermogravimetric analysis (TGA). Alcohol-saturated trivalent cation (M 3+) 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 profiles but 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) repor ted that pr imary aliphatic alcohols, such as ethanol , propan-1- ol and bu tan- l -o l (inter alia) when intercalated in A13+-exchanged montmori l loni te react preferent ial ly via an intermolecular nucleophilic displacement of water to give high yields (30-70%) of di - (a lk- l -yl ) ethers, ra ther than undergo competi t ive in t ramolecular dehydrat ion to alkenes. In contrast , the react ion of secondary aliphatic alcohols gave high yields (-~80%) of the corresponding alkene, except for propan-2-ol which was conver ted to both the di-(alk-2-yl) e ther (35%) and alkene (41%). A d a m s et al. (1981) also repor ted the format ion of di-prop-2-yl e ther but at a much lower yield (3%). However , A d a m s and co- workers (1981) carried out their reactions at 60~ using Fe3+-montmori l loni te in the presence of 1,4-dioxan as solvent, whereas Ballantine et al. (1984) employed no solvent and utilized a react ion tempera ture of 200~ The per t inent results for further discussion are summarized in Table 1. Moreover , A d a m s et al. (1981) suggested that the low-temperature coefficient of the e ther forming react ion indicated the possible presence of diffusion control. This p rompted a recent investigation into the sorpt ion kinetics of methanol

* 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.

TABLE 1. Product distributions (wt%) for reactions of alcohols with M3+-montmorillonite.

Reactant Reactant recovered Ethers Alkenes

MeOH a , b b ( 2 ) _

n-PrOH 20-7 67-2 c 3 i-PrOH 29 34-7d(3) d 41 "4 t-BuOH 3 -- 92e(8) f

Figures in parentheses from Adams et al. (1981) using 1 g Fe3+-montmorillonite, 50 mmol of reactant alcohol and 3 cm 3 1,4-dioxan at 60~ Other values from Ballantine et al. (1984) using 0.5 g Al3+-montmorillonite at 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 a l . , 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 A1- Oswais 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 (TG- E G A ) has recently been used to considerable advantage in the identification of the desorption products arising from the transformation of butylamine (Shuali et a l . , 1990) and pyridine (Shuali et a l . , 1991) in contact with sepiolite or palygorskite.

E X P E R I M E N T A L

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 a l . , 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 TG-EGA 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.

R E S U L T S

The basal spacing and percentage weight loss (18-800~ data for the water- and alcohol- saturated 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 MeOH from the trivalent cation- exchanged 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 high- temperature 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. Weight loss (%) by 800~ and basal spacings at 20~ for the M 3+- montmorillonite/alcohol systems.

Weight loss Basal spacing Cation Solvent (%) (A)

A13+ H20 16 12.5 MeOH 36 14-0 n-PrOH 26 14.0 i-PrOH 27 14.3 t-BuOH 23 15.3

Cr :~+ He O 16 12-5 MeOH 26 15.0 n-PrOH 24 14.0 i-PrOH 24 15.8 t-BuOH 21 17.4

Fe 3+ H20 16 12.5 MeOH 25 15-5 n-PrOH 27 14.0 i-PrOH 25 16.0 t-BuOH 24 17.7

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 EGA 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 t- BuOH, 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 thermo- gravimetric 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

dw

dT

_d_ww dT

Na

dw

dT

i , , i ,

(1 ' 1+()0 800 0 z+00 800 Temperature/* Temperature/*C

Na

Fe

Cr

i i , i .

t+00' 800 Temperafure/*C

r~

A

I___

Ca

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 +-, Ca 2+ Fe 3+ Cr 3+- and 3+ AI -exchanged montmorillonite.

127

at 63~ Moreover , there was a very broad, featureless desorpt ion stretching from 100-200~ which may be at t r ibuted to dimerisat ion and ol igomerisat ion of the alkene (vide infra).

Figure 3 shows the way in which the IR spectra of n -P rOH adsorbed on Cr3+-exchanged montmori l loni te changed as the sample was degassed and heated. The marked reduct ion in the characterist ic IR absorpt ion bands for water, near 3400 cm -1 and 1626 cm -1, showed that degassing at room tempera ture for one hour removed a considerable amount of water , whereas the affect on the absorpt ion bands for i -PrOH, centred around 1400 cm 1, was not so marked. Subsequent degassing for one hour at 50, 100 and 150~ further reduced the

128 C. Breen et al.

a

>- i - -

E/) z i.iJ i - - z

o

20 50 100 150 200 250 Temperafure/'C

b

t~ Z

I--- i , . - . . . , i

20 50 100 150 200 250 Temperafure/~

Z"'4 ~-- >'t~ ' ~ ' C

t--- j /Z :59 ,-4Z / / M/Z=41

j M/z:18 . . .Jr

20 50 100 150 200 250 Temperature/~

Fro. 2. Mass spectroscopic EGA data for Al3+-exchanged montmorillonite treated 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 TGA-EGA 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

o r- rD

-4-- ..4--

E

r-- rD

a

1457 ~ i347 \f N

347 \l N

1635 ~/

1397

~,57

f 347 \1

Z 1394

1457

' ' '14-'00 1800

C. Breen et al.

b

,26

162 ~ _

1626

1391 I? 7

1626

1391 1377

i i i

1800 1400

w a v e n u m b e r / c m -1

1 6 2 6

1463

A 1463

1463

6 6

1626

1363 i * i

1800 1400

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.

c

c r v

I~ 2849 32503250

3500 3000

w a v e n u m b e r / e m - 1

25'o0

I c

131

ea u

m

E

c ro I.-

Desorption of alcohols from clay

a / o

E

tv

i

3500 3000 2500 3500 3000 2500

w a v e n u m b e r l c m - I 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 cm- 1 was maintained up to 150~ whilst the C-H band at 1397 cm 1 and the OH 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 consider- ably 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 3000- 3500 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.

D I S C U S S I O N

The low-temperature maximum in the desorption profile of MeOH from the various cation- exchanged 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 cation- exchanged forms (Figs. lb-d) . The maximum at 110~ in the desorption profile of MeOH from the various cation-exchanged forms (Fig. la) corresponds closely to the values 117- 140 and 130~ reported for the desorption of MeOH 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 MeOH 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 MeOH 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 EGA 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 TG-EGA 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 (2,4,4- trimethylpent-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 IR 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 cm- 1 together with an increase in intensity of the band at 1630 cm- 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 o f alcohols from clay 135

intercalate of A1-Gelwhite was thermally stable up to 200~ whereas pentanol saturated A1- bentonite collapsed at 150~ After repeatedly exposing a cae+-exchanged montmorillo- nite 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 cation- exchanged 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 AI 3+ 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. Breen et al.

b e c o m e ve ry acidic as the t e m p e r a t u r e is raised (Breen et al . , 1987b), and (iii) the lack of

ev idence for the exis tence of a lcohol in e i ther the I R or the mass spectra , the re is sufficient

p r e c e d e n t in the l i t e ra ture to suppor t the supposi t ion that the band at 3450 c m - 1 in these

systems is unl ikely to be associa ted with an a lcohol molecu le .

The use o f mass spec t roscopic E G A has b e e n able to conf i rm unequ ivoca l ly the ma jo r

species de so rbed f rom a lcoho l - t r ea ted clays and faci l i tate the in t e rp re t a t ion of changes in

I R spectra of the adso rbed alcohols . H o w e v e r , E G A was less successful w h e n a range of

s imilar c o m p o u n d s w e r e f o r m e d pr ior to or dur ing desorp t ion . The p re l iminary

c h r o m a t o g r a p h i c resul ts con ta ined he re in may be of advan tage when very de ta i led analyses

of such mixtures are requ i red .

A C K N O W L E D G M E N T S

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.

R E F E R E N C E S

ADAMS J.M. & CLAPP T.V. (1985) Infrared studies of 1-hexene adsorbed onto Cr3+-exchanged montmorillonite. Clays Clay Miner. 33, 15-20.

ADAMS J.M., CLAPP T.V. & CLEMENT D.E. (1983) Catalysis by montmorillonites. Clay Miner. 18, 411-421. ADAMS J.M., CLEMENT D.E. & GRAHAM S.H. (1981) Low temperature reaction of alcohols to form t-butyl ethers

using clay catalysts. J. Chem. Res. 254-255. ADAMS J.M., REID P.I. & WALTERS M.J. (1977) The cation exchange capacity of clays. School Sci. Rev. 59, 722-724. AL-OSwAIS A.A., BALLANTINE J.A., PURNELL J.H. & THOMAS J.M. (1987) Thermogravimetric study of the

intercalation of acetic acid and water by Al3+-exchanged montmorillonite. J. Mol. Catal. 35, 201-212. ANNAaI-BERGAYA F., CP,uz M.I., GATINEAU L. & FRIPIAT J.J. (1980) Adsorption of alcohols by smectites. II Nature

of the bonds. Clay Miner. 15, 225-237. BALLANTINE J.A., DAV1ES M., PATEL I., PUP, NELL J.H., RAYANAKORN M., WILLIAMS K.J. & THOMAS J.M. (1984)

Organic reactions catalysed by sheet silicates: ether formation by the intermolecular dehydration of alcohols and by addition of alcohols to alkenes. J. Mol. Catal. 26, 37-56.

BALLANTINE J.A., GRAHAM, P., PATEL I,, PURNELL J.H. & WILLIAMS K.J. (1987) New differential thermogravimetric method using cyclohexylamine for measuring the concentration of interlamellar protons in clay catalysts. Proc. Int. Clay Conf. Denver, 311-318.

BENNET H. & REED R.A. (1971) Chemical Methods For Silicate Analysis, pp. 71-79. Academic Press, London. BREEN C. (1991a) Thermogravimetric study of the desorption of cyclohexylamine and pyridine from an acid-treated

Wyoming bentonite. Clay Miner. 26, 473-486. BREEN C. (1991b) Thermogravimetric and infrared study of the desorption of butylamine, cyclohexylamine and

pyridine from Ni and Co exchanged montmorillonite. Clay Miner. 26, 487-496. BREEN C. & DEANE A.T. (1987) The adsorption of 1,4-dioxan, tetrahydrofuran and tetrahydropyran from binary

solution with methanol on Al 3+- and Cr3+-exchanged montmorillonite. Clay Miner. 22, 199-205. BREEN C., DEANE A.T. & FLYNN J.J. (1987a) Vapour phase sorption kinetics for tetrahydrofuran, tetrahydropyran

and 1,4-dioxan by A13+-, CF 3+- and Fe3+-exchanged montmorillonite. Clays Clay Miner. 35, 343-346. BREEN C., DEANE A.T. & FEYNN J.J. (1987b) The acidity of trivalent cation-exchanged montmorillonite.

Temperature programmed desorption and infrared studies of pyridine and n-butylamine. Clay Miner. 22, 169-178.

BREEN C., DEANE A.T., FLYNN J.J. & REYNOLDS D. (1987c) Vapour phase sorption kinetics for methanol, propan-2- ol and 2-methylpropan-2-ol on AI 3+-, Cr 3+- and Fe3+-exchanged montmorillonite. Clays Clay Miner. 35, 336-342.

DATKA J. (1980) Transformations of but-l-ene molecules adsorbed in NaHY zeolites studied by infrared spectroscopy. J. Chem. Soc. Faraday Trans. 1. 76, 2437-2447.

Desorption o f alcohols f r o m clay 137

DowDY R.H. & MORELAND M.M. (1967) Alcohol water interactions on montmorillonite surfaces. I. Ethanol. Clays Clay Miner. 15, 25%271.

FARMER V.C. & RUSSELL J.D. (1971) Interlayer complexes in layer silicates. The structure of water in lamellar solids. Trans. Faraday Soc. 67, 2737-2749.

GRADY M.C. & GORTE R.J. (1985) Adsorption of 2-propanol and propene on H-ZSM-5: evidence for stable carbenium ion formation. J. Phys. Chem. 89, 1305-1308.

ISON A. & GORTE R.J. (1984) The adsorption of methanol and water on H-ZSM-5. J. Catal. 89, 150-158. NGUYEN T.T., RAUPACH M. & JANI~ L.J. (1987) Fourier transform infrared study of ethylene glycol monoethyl ether

adsorbed on montmorillonite: implications for surface area measurements of clays. Clays Clay Miner. 35, 53-59.

NOLLER H. & RITrER G. (1984) Temperature programmed desorption of methanol, ethanol, propan-l-ol and propan-2-ol on silica-magnesia mixed oxides. J. Chem. Soc. Faraday Trans. 1. 80, 275-283.

ROCHESTER C.H., GRAHAM J. & RUDHAM R. (1984) Infrared study of the adsorption of propan-2-ol on rutile at the solid/vapour and solid/heptane interface. J. Chem. Soc. Faraday Trans. 1. 80, 2459-2466.

Ross P.F., BUSCA G., LORENZELLI V., SAUR O. & LAVALLEV L.-C. (1987) Microcalorimetric and FT-IR spectroscopic study of the adsorption of isopropyl alcohol and hexafluoroisopropyl alcohol on titanium dioxide. Langmuir, 3, 52-58.

RVSSELL J.D. & FARMER V.C. (1964) Infra-red spectroscopic study of the dehydration of montmorillonite and saponite. Clay Miner. Bull. 5,443-464.

SCHUTZ A., STONE W.E.E., PONCELET G. & FRIPIAT J.J. (1987) Preparation and characterisation of bidimensional zeolitic structures obtained from synthetic beidellite and hydroxy-aluminium solutions. Clays Clay Miner. 35, 251-261.

SHUAH U., S~EINBERG M., YARIV S., MULLER-VONMOOS M., KAHR G. & RUB A. (1990) Thermal analysis of sepiolite and palygorskite treated with butylamine. Clay Miner. 25, 107-119.

SHUALI U., YARJV S., SXElr~BER6 M., MULLER-VONMOOS M., KAHR G. & RUB A. (1991) Thermal analysis of pyridine- treated sepiolite and palygorskite. Clay Miner. 26, 497-506.

SUDA Y., MORIMOTO T. & NA6AO M. (1987) Adsorption of alcohols on titanium dioxide (rutile) surface. Langmuir 3, 99-104.

TENNAKOON D.T.B., SCHLOGL R., RAYMENT T., KLINOWSKI J., JONES W. • THOMAS J.M. (1983) The characterisation of clay-organic systems. Clay Miner. 18, 357-371.