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ADSORPTION OF N2 AND CO2 GAS BY TETRAMETHYLAMMONIUMMONTMORILLONITES
C. Volzone, J. O. Rinaldi and J. Ortiga
C.C.49, Cno. Centenario y 506, (1897) M. B. Gonnet, Prov. Buenos Aires,
ARGENTINA, FAX +54 221 4710075, e-mail:volzcris@netverk.com.ar
CETMIC (Centro de Tecnología de Recursos Minerales y Cerámica)
ABSTRACT
Adsorption gas by tetramethylammonium-montmorillonites were carried out using
a volumetric equipment at 25 C and 1 kg/cm2. N2 and CO2 gas adsorption by
montmorillonites with two different exchange degrees of tetramethylammonium (TMA)
cations were measured. The contacted time between the TMA and the montmorillonite
were 4 and 24 hours. The organo-smectites were characterized by X-ray diffraction and
swelling index. The swelling index and d(001) spacing were correlated with adsorption
gas. The retention of N2 gas by TMA-montmorillonites measured up to 30 min was
small (0.065 mmol/g) whereas the value corresponding to CO 2 retention was higher
(around 0.470 mmol/g).
Key words: clays, tetramethylammonium-montmorillonite, N2 and CO2 gas adsorption.
INTRODUCTION
The smectites are recognize as adsorbent due to their ability to exchange
interlayer cations for charged organic or metal cations in solution.
The strong hydration of the inorganic cations in interlayer position of the smectites
originate surfaces hydrophilic. The smectites can be converted to organo-smectites and
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they are effective for adsorption organic compounds. Several papers are about
retention organic compounds (1-3).
Different papers give information about adsorption for gas separation using
carbon and zeolites. More recently, the studies about retention gas by pillared and acid
smectites indicate that these materials will be a potential for gas separation
(4-6).
Now it
is interesting to analyze the adsorption gas by organo-smectites.
The aim of this paper is analyze the adsorption of N 2 and CO2 gas at 25 C and 1
kg/cm2 by tetramethylammonium-montmorillonites prepared in different conditions.
MATERIALS AND METHODS
Two smectites were the starting clays: SAz-1, Ca montmorillonite (Cheto type),
Apache County, Arizona, U.S.A. and Swa-1, Na montmorillonite (Wyoming type),
Wyoming, U.S.A. The samples were named B1 and B2, respectively.
Tetramethylammonium (TMA) was used to prepare organo-smectites. Aqueous
solution of TMA chloride salt was added to smectite suspension and contacted for 4
and 24 hours. The solids was washed to remove excess salts and stored at room
temperature. The amounts of TMA added to smectite were 1 and 4 times cationic
exchangeable capaticy (CEC) of the smectite. The TMA-B1 was named as: B1(1-4),
B1(1-24), B1(4-4), B1(4-24), where the first number in brackets is the amount of TMA in
time CEC added and the second number is the contacted time in hour. The TMA-B2
was named similary as B1.
The cationic exchange capacity (CEC) was determined using ammonimum
acetate 2N at pH=8.
The swelling index (SI) was measured in water and toluene media. Then, 2 grams
of sample were added to 100 ml distilled water or toluene solution and after 24 hours
the volume of sediment was read.
The d(001) spacing of samples was obtained by means X-ray diffraction (XRD). A
Philips 3020 Goniometer with PW 3710 Controller using Cu K radiation (Erro!
Indicador não definido.=1.5405 Å) and Ni filter at 40 kV and 20 mA was used.
Equilibrium adsorption isotherms were measured using standard volumetric
apparatus. Samples were outgassed at 100 C for 12 hours prior to measurement. The
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gases tested for their adsorption behaviour were N2 and CO2. Adsorption was
measured at 25 C and 1 kg/cm2.
RESULTS AND DISCUSSION
Analysis of TMA-montmorillonites. The CEC of B1 and B2 montmorilllonties were
110 and 97 meq/100g, respectively and the structural composition calculated according
previous paper
(7)
are shown in Table I. The higher CEC of B1 is due to high
isomorphous substitution in the structure mainly when the Mg2+ replaced to Al3+ in
octahedral layer.
Table I. Half-cell unit structural composition of montmorillonites
clay
SiIV4+
AlIV3+
AlVI3+
Fe3+
Mg2+
B1
3.940 0.060
1.360
0.060
0.600
B2
3.910 0.090
1.610
0.130
0.200
Air-dried TMA-montmorillonites tended to give d(001) spacings with13.7-14.87 Å,
similar to that for the air-dried original bentonites (Table II), althought the last contained
Ca and Na cations in interlayer position for B1 and B2 montmorillonites, respectively.
Lee et al.
(8)
found 13.80 Å d(001) spacing value in bentonite treated with similar
amount of TMA added (3-5 times CEC). Nevertheless, our results as shown in the
Table II indicates that the montmorillonites treated with TMA 4 times CEC raised higher
interlayer spacing (13.84-14.87 Å). The interlayer separation of TMA-montmorillonites
obtained by difference between the basal spacing and the height of 2:1 layer (9.5 Å)
were in the range 4.21-5.37 Å (Table II). The higher d(001) spacing of TMA-B1 could be
attributed to higher TMA retention.
The hydrophobic and organophilic characteristics of the TMA-montmorillonite
could be show by means Swellind Index. The Swelling Index of the original bentonites
in water medium decreased after organo treatment. Whereas in toluene medium the
value increased (Table III). This behaviour could be related with hydrofobic behaviour
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becuase less water is retained between montmorillonite particles and more toluene was
uptake. TMA-B1 showed higher hydrophobic behaviour (4.2-4.8 ml) than TMA-B2 (6.07.6 ml), Table III.
Table II. Interlayer dimension of the original montmorillonites and TMA-montmorillonites
Sample
B1
B1(1-4)
B1(1-24)
B1(4-4)
B1(4-24)
B2
B2(1-4)
B2(1-24)
B2(4-4)
B2(4-24)
d(001) spacing
(Å)
14.90
13.71
13.82
14.87
14.74
13.00
13.95
13.89
13.84
14.13
interlayer separation
(Å)
5.40
4.21
4.32
5.37
5.24
3.50
4.45
4.39
4.34
4.63
Table III. Swelling Index (SI) of the original montmorillonites and TMA-montmorillonites
in water (wt) and in toluene (tol) media
Sample
B1
B1(1-4)
B1(1-24)
B1(4-4)
B1(4-24)
B2
B2(1-4)
B2(1-24)
B2(4-4)
B2(4-24)
SI (ml)
wt
5.3
4.6
4.6
4.3
4.8
10
7.4
7.6
6.3
6.0
tol
3.8
4.1
3.6
4.5
5.1
3.6
4.4
4.0
4.7
5.0
In summary, higher d(001) spacing of montmorillonite was obtained after added
TMA 4 times CEC. The results obtained after contacted time of 4 or 24 hours were
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similary. The higher d(001) spacings of TMA-B1 than TMA-B2 could be attributed to
more retention TMA in interlayer position due to more CEC of starting montmorillonite.
Gas adsorption. The adsorption isotherms of N2 and CO2 gas at 25 C and 1
kg/cm2 up to 30 min (1800 s) by starting montmorillonites, TMA-B1 and TMA-B2 are
a d s o r b e d N 2 g a s , m m o l /g
given in Figures 1 and 2.
0,1
0,05
0
0
500
B114T
a d s o r b e d C O 2 g a s , m m o l/g
B1
1000
tim e, s
B1124T
1500
B144T
2000
B1424T
0,5
0,4
0,3
0,2
0,1
0
0
B1
500
B114T
1000
time, s
B1124T
1500
B144T
2000
B1424T
Figure 1: Gas adsorption isotherms by TMA-B1: a) N2; b) CO2.
The montmorillonites treated with TMA adsorbed more gas than original
motmorillonites. The original montmorillonites B1 and B2 have mainly Ca and Na ions
in interlayer, respectively. Probably is strongly hydrated cations inhibit the sorption of
gas. The increased adsorption gas by TMA-montmorillonite could be attributed to more
hydrophobic characteristics of the montmorillonite treated with TMA.
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The TMA-montmorillonites showed different retention gas for N 2 and CO2 . The N2
gas retention capacity was small whereas the value corresponding to CO2 retention was
high for organo-montmorillonites. The reason could be attributed that CO 2 possesses
large quadruple moment compared to N2 (-4.3 x 10
-26
and -1.5 x 10
-26
Erg1/2.cm5/2,
a d s o r b e d N 2 g a s , m m o l/g
respectively).
0,1
0,05
0
0
a d s o r b e d C O 2 g a s , m m o l /g
B2
500
B214T
1000
time, s
B2124T
1500
B244T
2000
B2424T
0,5
0,4
0,3
0,2
0,1
B2
0
0
500
B214T
1000
tim e, s
B2124T
1500
B244T
2000
B2424T
Figure 2: Gas adsorption isotherms by TMA-B2: a) N2 and b)CO2.
The adsorption gas values after 30 min (1800 s) are shown in Table IV. The
retention gases were higher for the TMA-montmorillonite with lower CEC of starting
montmorillonite (B2). Different type of binding interaction between gas molecule and
interlayer organo-montmorillonite could occur. Gas molecule could be interact with TMA
in interlayer position and with structural composition of montmorillonite. Perhaps, the
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montmorillonite with higher CEC (B1) retained more TMA in interlayer position and less
free space for gas adsorption was available (Figures 1 and 2, Table IV).
The best retention CO2 gas by B1 montmorillonite was after TMA added of 1 time
CEC during 24 hours, B1(1-24), as shown in Figure 1 and Table IV (0.445 mmol/g).
Regarding retention N2 gas by TMA-B1 montmorillonite not easy information was
obtained because the values were similary and small (around 0.047 mmol/g). The
added of TMA 4 CEC to B1 montmorillonite decreased the retention gas. This
behaviour could be correlate with d(001) spacing where the smaller retention occur
when the spacing is higher than about 14.5 Å, as shown the samples B1(4-4) and B1(424), 14.87 and 14.74 Å respectively (Table I, Figure 3). Whereas, the best retention
CO2 gas by B2 montmorillonite was after TMA added 4 times CEC (0.461 and 0.477
mmol/g for B2(4-4) and B2(4-24), respectively) that correspond to d(001) spacing
smaller than 14.5 Å (Table I). The retention N2 gas by B2 sample treated with TMA,
also, were small but slightlt higher than those prepared with B1 montmorillonite (around
0.060 mmol/g).
Table IV. Gas adsorption at 25 C and 1 kg/cm2 after 1800 s
sample
B1
B1(1-4)
B1(1-24)
B1(4-4)
B1(4-24)
B2
B2(1-4)
B2(1-24)
B2(4-4)
B2(4-24)
N2
(mmol/g)
0.017
0.055
0.044
0.038
0.049
0.011
0.065
0.060
0.060
0.055
CO2
(mmol/g)
0.210
0.439
0.445
0.353
0.423
0.125
0.445
0.423
0.461
0.477
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r e ta in e d g a s , m m o l/g
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0,5
0,4
0,3
0,2
0,1
0
13,5
14
14,5
d(001) spacing, Å
N2
15
CO2
Figure 3: Correlation between retention gas and d(001) spacing.
The best retention gas by TMA-B2 regarding TMA-B1 could be attributed to
different d(001) spacing originated by TMA retained and more aluminum content in
structural composition (Table I).
The N2 and CO2 gas retentions by the TMA-montmorillonites were higher to those
found for acid and pillared bentonites reported in previous works
(5), (6), (9).
The relative high retention gas found by TMA-montmorillonites may be due to a
combination of the interaction between gas and TMA and the adsorption on the
montmorillonite surface.
CONCLUSIONS
The N2 and CO2 gas adsorption by montmorillonites increased after treatment with
tetramethylammonium.
The retention of N2 gas by TMA-montmorillonites was small (0.065 mmol/g)
whereas the value corresponding to CO2 retention was higher (around 0.470 mmol/g).
The
structural
characteristics
of
the
intercalated
tetramethylammonium
montmorillonites and the physicochemical characteristics of the adsorbate must be
take in account in adsorption gas studies.
REFERENCES
(1) G. Lagaly, Clay Miner., 16 (1981) 1.
(2) G. Lagaly, Phil. Trans. R. Soc. Lond. A 311 (1984) 315.
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(3) M.M. Mortland, S. Shaobai and S.A., Clays and Clay Minerals, 34 (1986) 581.
(4) R.T. Yang and L.S. Cheng, in Äccess in Nanoporous Materials, Ed. T.J.Pinnavaia
and M.F.Thorpe, Plenum Press, N.Y. 1995, p.73.
(5)
C. Volzone and J. Ortiga, Proceedings of International Workshop af Activated
Clays. La Plata Argentina. December 1998, p.45.
(6) C. Volzone and J. Ortiga, 9 Congreso Argentino de Cerámica, Vidrios y
Refractarios y 3 del Mercosur, Olavarría, Argentina, 1998, p.19.
(7) C. Volzone, Mater. Chem. Phys., 47, (1997) 13.
(8) J-F. Lee, M.M. Mortland, C.T. Chiou, D.E. Kile and S. Boyd, Clays and Clay
Minerals, 38,2 (1990) 113.
(9) C. Volzone, J. Ortiga, L.B. Garrido and E. Pereira, Programa Iberoamericano de
Ciencia y Tecnología (CYTED). Subprograma V. Catálisis y Adsorbentes. Proyecto
V.3., Meeting:Caraballeda, Venezuela, Febrero de 1998, 1-7.