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Reversible adsorption of carbon dioxide on amine surface-bonded silica gel
e,y~7-.-v~
ELSEVIER
lnorganica Chimica Acta 240 (1995) 183-189
Reversible adsorption of carbon dioxide on amine surface-bonded silica gel Orlando Leal a.., Carmelo Bolivar a, C6sar Ovalles b, Juan Jos6 Garcia b, Youssef Espidel b a Escuela de Qufmica, Facultad de Ciencias, Universidad Central de Venezuela, Apartado 47102, Caracas 1040, Venezuela b INTEVEP S.A, Los Teques, Venezuela Received 7 January 1995; revised 31 March 1995
Abstract
Carbon dioxide adsorbs reversibly on a silica gel containing 3-aminopropyl groups bonded to surface atoms of silicon. These act as the active sites for the chemisorption of CO2 at room temperature which is liberated by temperature programmed desorption at about 100 *(2.The material is capable of adsorbing about 10 STP cm3 of dry CO2 per gram and can be regenerated upon heating. It might be used as scrubber for carbon dioxide from industrial gaseous streams. Adsorption of humid CO2 produces a small amount of formaldehyde which suggests activation of the carbon dioxide molecule. Keywords: Chemisorption; Carbon dioxide; Reversible adsorption; Surface modification; Alkoxysilane; Silica gel
1. Introduction
Carbon dioxide, being a normal component of the atmosphere, is involved in the respiratory cycles of plants and animals. The results of research on the greenhouse effect have shown that the levels of trace gases naturally occurring in the atmosphere, such as carbon dioxide, methane and nitrous oxide, have increased sharply above preindustrial era levels. Carbon dioxide has gone from 315 to 350 ppm in the last 35 years and the rate of increase has speeded up in recent years [ 1 ]. This increase has been mainly linked to the intense use of fossil fuels in internal combustion engines and other human activities. Therefore, current concern about an anthropogenic impact on global climate has made it of interest to compare the potential effect of various human activities [2]. It may be that human-generated emissions of carbon dioxide, mainly from fossil fuels and the burning of forests, will have to be cut by as much as 50-80% to avoid major climate changes. Such a reduction in the CO2 emission rate probably cannot be accomplished without a massive switch to non-fossil energy sources. Nevertheless, emissions from fossil fuels can be moderated somewhat by three strategies: exploiting the fuels more efficiently, replacing coal by natural gas, and recovering and sequestering CO2 emissions [3]. A rough analysis, based on the use of currently accepted values, shows This paper is dedicated to Professor F. Basolo. * Corresponding author. Elsevier Science S.A. SSD10020-1693 ( 95 ) 0 4 5 3 4 - G
that natural gas is preferable to other fossil fuels in consideration of the greenhouse effect [2]. The most futuristic strategy for reducing CO2 emissions from fossil fuels is to capture the emissions and sequester them for later use in afforestation or transport them to a storage site. Storage options include piping the gas deep into the ocean or into depleted natural gas reservoirs. Removal of carbon dioxide from gaseous streams has been a current procedure in the chemical industry. The presence of tiny amounts of carbon dioxide can act as poison in catalytic chemical processes such as ethylene polymerization or ammonia synthesis [4]. In ammonia plants, carbon dioxide cannot be tolerated in the catalyst under the high temperature and pressure prevailing in the converter. Therefore, several procedures have been devised to eliminate carbon dioxide from synthesis gas in ammonia plants as well as in natural gas plants and fuel gas power plants. They can be summarized in three categories: absorption on liquid amines [5]; adsorption on solid materials [6]; membrane technology [7]. The former makes extensive use of alkanol-amines such as monoethanolamine (MEA), diethanolamine (DEA) or the like to be used in plants at low temperature and high pressures (900-1000 psi). This method suffers a corrosion problem which arises from the formation of carbamates, then requiring the use of inhibitors. A process, commercially known as Selexol, uses a mixture of polypropyleneglycoldimethyl ethers and removal of carbon dioxide
! 84
O. Leal et al. /Inorganica Chimica Acta 240 (1995) 183-189
pyleneglycoldimethyl ethers and removal of carbon dioxide is based on physical absorption in much heavier molecules [8]. On the other hand Bhide and Stern [7] have recently assessed the economics of some separation processes for the removal of acid gases from crude natural gases by using membranes. Aromatic polyimide separation membranes are particularly useful for carbon dioxide enrichment [9], because they achieve high flux rates with good selectivity at low temperatures. This has proved effective to remove CO2 in concentrations ranging between 5 and 40 mol%. This procedure is bounded to gas losses due to membrane saturation. In this paper, we describe the chemisorption of carbon dioxide on a chemically modified surface of silica gel, which due to its specificity has found applications in the separations of this gas from closed ambient and very likely could lead to chemical activation of the carbon dioxide molecule.
2. Experimental 2.1. Surface modification The aminopropyl gel (APG) was prepared as described elsewhere [ 10]. Ten grams of 60-80 mesh, Davison grade 62 silica gel (a wide-pore gel with a nominal pore diameter of 12 nm, a specific area of 340 m2/g, and a pore volume of 1.4 cm3/g) was dried by azeotropic distillation in m-xylene for 1 h and then refluxed in m-xylene with 4.5 g of 3-aminopropyltriethoxysilane. Different coverage was obtained by varying the time of reflux from 1.5 to 30 h. The product was washed consecutively in the absence of moisture with mxylene and n-pentane to produce a modified aminopropyl gel. Coverages calculated from C, H and N micro-analysis ranged between 1.18 and 2.80 molecules of aminopropyl groups/ nm 2 (0.57-1.27 mmol of nitrogen/g).
2.2. Characterization The IR spectra of the solids were obtained by two procedures. (i) In the near-infrared region (NIR) (4500-7600 c m - 1) they were recorded on a Cary 17 D spectrometer using the technique described by Burwell et al. [ 11 ]. The sample was dried at 160 °C and then located in a fused silica cell of 1 cm light path embedded in carbon tetrachloride which had previously been dried on anhydrous K2CO3. Samples were allowed to stand until the air had been displaced from the pores of the gels. A cuvette with silica gel in carbon tetrachloride was used as a reference in the case of the surface modified materials. (ii) In the fundamental region (10004000 c m - ~) spectra were recorded on an FT-IR Perkin-Elmer spectrometer. Samples were pelletized by applying pressure of 10 ton per square inch to approximately 20 mg of the solid material. Pellets were placed in the sample holder of a sealed cell built of fused silica and equipped with calcium fluoride
windows. This cell could be coupled to a glass vacuum system which allowed evacuation of the sample to pressures better than 10 -5 torr, heating, and exposure to the carbon dioxide adsorptive. NMR spectra of the solids were performed in a Bruker MSL-300 at a frequency of 75468 MHz under high power, magic angle spinning (HP/MS) with relaxation time of 2.5 s. A 4 mn seconun rotor was used rotating at 9.7 kHz. The chemical shifts were measured against TMS.
2.3. Apparatus The solid adsorption capacity test for carbon dioxide was carried out in a continuous flow experimental system, using ultra pure helium as carrier gas [ 12]. Successive volumes of CO2 were introduced onto the adsorbent by means of a gas injection valve with a 0.65 cm 3 volume loop which was injected every injection lap. The adsorbent was placed in a Pyrex U tube of 6 mm outer diameter and connected to the carrier flow in the oven of a Hewlett-Packard model 5710 A gas chromatograph. One gram of adsorbent was employed and successive volumes of CO2 were injected at room temperature (27 °C) until saturation of the adsorbent. Then by programmed temperature desorption (PTD) between 27 and 150 °C, at a heating rate of 10 °C m i n - 1 and with helium flow rate of 30 cm 3 min-~ the adsorbed carbon dioxide was released and measured to determine the adsorptive capacity of the material. An additional test for capacity was carried out by determining the adsorption isotherm at different temperatures by means of a volumetric system shown in Fig. 1. It consists basically of a high vacuum system built out of Pyrex glass and equipped with greaseless stopcocks. A vacuum system, Leybold-Heraus model PD 30L, was used to reach pressures as l o w a s 10 - 6 torr. The gases used for adsorption studies were introduced from different inlets. To study the adsorption, the gel was placed in bulb 1 and evacuated overnight at 100 °C to remove any residual water and then heated at 160 °C for 1 h to activate it. Temperatures were measured with a chromel-alumel ther-"-- VIICCUID
--,.- vent air
VT
! Fig. 1. Diagram of volumetric glass apparatus for measuring the adsorption isotherms of CO2 on APG. 1: Sample, 2: He reservoir, 3:CO2 reservoir, 4: reference cell, 5: Klister servo pressure sensor, V: greaseless stopcocks.
O. Leal et al. / lnorganica Chimica Acta 240 (1995) 183-189
185
injected from a tank by means of valve V5. Nitrogen was used occasionally as carrier. Natural gas having a composition of 83.1% methane, 7.6% ethane, 0.7% propane, 0.2% butanes, 0.1% pentanes and 8.3% carbon dioxide was used in the scrubbing tests which were carried out at pressures ranging between 1 and 4 atm and space velocities between 28 and 170 cubic foot per hour ( 1000-6000 l/h). ?"i:: i
3. Results and discussion
G~I8
Cnroma~r~
Reaction of the surface of silica gel with 3-aminopropyltriethoxysilane leads to the condensation of the hydroxyl groups producing organic residues bound to the surface, and might involve the reaction of one, two or three hydroxyl groups of the surface per molecule of alkoxysilane. Therefore, the nature of the bonded species will depend upon the number and distribution of the hydroxyl groups at the surface of the silica gel. Previous studies indicate that annealed and rehydroxylated silica gel has a surface hydroxyl concentration of 4.6 - O H groups per square nanometer [ 13], of which 1.4 hydroxyl groups/nm 2 exist as single non-hydrogen bonded species and the remaining 3.2 hydroxyl/nm 2 are interacting groups arranged in pairs: - -
Sf Fig. 2. Schematic diagram of flow system, D: flow splitters, R: reactors, S: gas outlet, T: thermocouple, V: three way valves, SH: water saturator, BP: back pressure valve, MI: mass flow controller, M2: flowmeter, M3: manometer.
mocouple (Omega Inc.), by means of a Hewlett-Packard recorder model 713 A. A temperature programmer was employed to control the temperature of the sample. Purified helium, used to establish a blank, was kept in a permanent bulb 2 and carbon dioxide from a high purity tank was kept in bulb 3. To perform measurements, dry air at atmospheric pressure was admitted into reference cell 4. Any pressure in Vo or VT was measured with respect to the pressure of the reference cell. The difference between the reference and the working cells was measured by a Kistler servo pressure sensor 5, which measured + 1 atm relative to the reference cell. A digital display 6 was connected to the sensor 5. Testing of the performance of the adsorbent as scrubber for carbon dioxide when scaling up the process was carried out in the apparatus shown in Fig. 2. It consists of a flow system built out of stainless steel tubing equipped with temperature, pressure and mass flow controllers. The adsorbent is confined in two parallel cylinders which allow its activation and use in an alternate fashion. Each cylinder is loaded with 250 g of adsorbent. A Varian 3400 gas chromatograph equipped with a 6 foot stainless steel column of 10% Carbowax 20 M on Chromosorb W-HP 80/100 mesh was used to monitor the samples
- -
I
Si
\ /
OH 0
S i -
OH
I All these groups can be involved in the reaction of the surface of the silica with polyalkoxysilanes. Burwell [ 14] indicated that this reaction gives a product with an average composition corresponding to the diattachment of the silane to the surface, according to reaction ( 1) o-o. +
Si
I~
/
Si
+ C2H50H
(1) where o- stands for a silicon atom at the surface of silica and X = NH2 if the silane is 3-aminopropyltriethoxysilane. This structure was estimated from the carbon-nitrogen ratios of the products. When the reaction is carried out by refluxing in m-xylene for 24 h or more we obtained a gel which carries an average of 2 . 5 + 0 . 1 molecules (1.2 mmol of N / g ) of --CH2CH2CH2NH2 per square nanometer of the surface. The kinetics of the surface reaction are shown in Fig. 3 which indicates that the nitrogen content of aminopropyl surface modified gel (APG) varied between 0.57 and 1.26 mmol of N per gram of gel when reaction was carried out between 1.5 and 30 h. After approximately 10 h the reaction has reached 80% completion. The NIR spectrum in the overtone region of silica gel Davison grade 62, dried at 160 °C for 15 min, is given in
O. Leal et al. / lnorganica Chimica Acta 240 (1995) 183-189
186
1.25 Z m ,.. 1.00
r, "-
|
0.75
0.50
0.26
l 10
0
I
I
20
30
llnuo, hours
Fig. 3. Effect of time of reaction on degree of modification of surface of silica gel Davison grade 62. Reaction temperature: 136 °C.
I
4752
I
t
5263
I
o
OH
S!b o/
\/ /\
Si a
H%S,c//OH
\o \ / /\
o/ Si a
\o\ /
I 61 I 6 67 ilVlnumller cllf ~
I
Fig. 4. IR spectrain the overtoneregion ( 1.3-2.1/~m): (a) silicagel Davison 62; (b) aminopropylgel (APG); (c) 3-aminopropyltriethoxysilanein CC14. ~
'tJ
Fig. 4(a). Absorption bands are observed at 4545, 5268 and 7220 c m - n. The latter has been assigned to the first overtone of the - O - H stretching vibration [ 15]. The band at 5268 c m - n disappears almost completely upon heating at 160 °C for 1 h, indicating this band derives from molecular water, and that at 4545 c m - 1 arises from combination modes of - O - H and lattice groups [ 16]. Surface modification resulted in a decrease of the intensity of the bands. Thus the decline in intensity of the band at 7220 c m - 1 was of 75%. New bands appeared at 6560 c m - ~ (first overtone of symmetrical N - H stretching), at 5010 c m - l (combination band of N - H group) and a series of bands between 5480 and 5970 c m - ~ associated with the first overtone of the methyl and methylene groups (Fig. 4(c) ). Furthermore the 295i C P / M A S NMR spectra shown in Fig. 5 indicate that upon modification of the surface of silica a band centered at - 103.4 ppm with respect to TMS almost disappeared (spectrum a) becoming a shoulder, and a broad small band is developed around - 70 ppm (spectrum b). The latter band can be attributed to extra network silicon atoms coming from the alkoxysilane and attached to the silica framework. These results are consistent with the occurrence of a surface reaction between the amino alkoxysilane and t h e - O H groups of the silica gel, which act as anchorage points to the solid surface, as was previously postulated in reaction ( 1 ). Schematically the surface can be depicted as:
I
5882
~;
b
8
I -40
I -60
f -80
f , -100
I -120
I.. -140
PPM
Fig. 5. 295i CP/MA NMR spectra: (a) silica gel Davison 62; (b) aminopropyl gel (APG).
atoms 'd' are responsible for the NMR signal at 40 ppm [ 17 ]. This assigment has been reported elsewhere [ 18]. The material obtained was a solid having a surface area of 200 m2/g, a total pore volume 0.7 cm3/g and average pore diameter of 11 nm, and exhibits between 2 and 3 molecules of amino modifying residue per square nanometer. Adsorption experiments on different preparations of the material on the flow apparatus previously described resulted in an adsorptive capacity of 9.10-t-0.05 cm3/g of dry CO2,
H2~ H2
1"12l~H2~
/o Si a
\
and the - O H groups bound to the outer silicon atom signaled as 'b' and 'c' would be those involved in the surface reaction, leading to a surface species like the one of Fig. 6 where silicon
F i g . 6. S c h e m e o f p r e d o m i n a n t s u r f a c e s p e c i e s o n A P G .
O. Leal et al. / lnorganica Chimica Acta 240 (1995) 183-189
which was doubled to 20.80+0.05 cm3/g when the carbon dioxide was saturated with water. The addition of cupric sulfate aqueous solution to the material rendered a material with a surface complex between Cu 2 ÷ ions and the surface amino groups of the adsorbent. This yielded a material unable to adsorb carbon dioxide. This is a clear demonstration that the amino groups are the sites responsible for the carbon dioxide adsorption. Isotherms determined at different temperatures in the apparatus shown in Fig. 1 are presented in Fig. 7. At higher temperatures a greater pressure was required for adsorption of a particular volume of carbon dioxide. The adsorbent displays an adsorptive capacity of 12.0+0.5 STP cm 3 of CO 2 per gram at 0 °C and a heat of adsorption A H of 14 + 1 Kcal mo1-1 as determined from In P2/Pt = - A H / R ( 1 / T 2 - 1 / T1). The adsorption of carbon dioxide is reversed when the adsorbent is heated at 100 °C. All the evidence suggests that the adsorption process is due to the formation of carbamate species on the modified surface of the material, where two amino groups from it are used up per carbon dioxide molecule adsorbed, as indicated in Fig. 8. In the presence of water, or when the carbon dioxide is wet, the capacity of the material towards the adsorption of carbon dioxide is duplicated due to a surface chemical transformation of the carbamate into two adsorbed bicarbonate molecules. To illustrate the behavior of this material as a scrubber for carbon dioxide, a study of the adsorption was carried out using the apparatus of Fig. 2. The experiments performed at different space velocities show that the adsorbed carbon dioxide started to be liberated by temperature programmed desorption (TPD) above 40 °C and the process remains so until above 100 °C. The center of the peak is shifted to lower temperatures as the spatial velocity increases. At a space velocity of 2.4 h - ~ ( 1.2 l/h) the adsorption band is centered at 70 °C and desorption is completed at 110 °C whereas at a space velocity of 0.6 h - ~ (0.3 l/h) the desorption band is centered at 90 °C and desorption is completed at 120 °C. On the other hand if the desorption temperature was held at 140 °C the adsorbent is regenerated in 2 h.
~'16
o~ 12
0 8 4
0
t 0
I 2
,
I 4
I
1 I I t I I 6 8 10 PRESSURE OF CO 2 , Itel
I 1.2
Fig. 7. Adsorption isotherms of carbon dioxide on APG run at different temperatures: *, 0; O, 23; A, 50 °C.
187
C H2-C H2-C H2 NH2 C H2-CH2-CH 2 NI.I2
~- ~ +
CH2"~I'~"CH2 N H + O ~
CO 2 ..~.~C.2_C.2_C.2 N.3
"
+
co2 +
2H20 2
CH2-CH2-CH2 NH ~ HCO3
Fig. 8. Schemeof the surfacereactionof carbon dioxide with APG. 100, o41-,~I'.: ,~-- -
t,
._*=..=
*, ¢
"
.o
4O
20
~
0
I
I
I
I
i
4
8
12
16
20
lime, hours
Fig. 9. Effectof spacevelocityon adsorption of CO2 on APG. Table 1 Effect of space velocity and pressure on CO2 adsorption
Experimenta
Pressure (atm)
Space velocity (1 h -t)
1
1
3
2 3 4 5 6
1 1 1 4 4
6 12 18 12 18
Adsorptionb (I kg -1 ) 13.3 13.7 15.5 13.8 16.3 14.0
a At ambient temperature. Activation of adsorbents done at 100 °C for 4 h under nitrogen stream. Experiments are average of two runs. b Liters of carbon dioxide adsorbed per kilogram of adsorbent at ambient temperature and pressure.
The influence of space velocity on the adsorption of carbon dioxide is presented in Fig. 9 where 100% adsorption corresponds to complete stripping of CO2 from natural gas containing 8.3% CO2 and activation was undertaken under nitrogen as carrier. It is clear from this that as space velocity increases from 6 to 36 h - ~ (3-18 l/h) the breakthrough time is reduced from 14 to 4 h, however the elution front remains clearcut confirming the chemical nature of the surface adduct. The process also quantitatively subtracts carbon dioxide from the natural gas streams. Table 1 shows the influence of pressure and space velocity on the adsorption of carbon dioxide. The capacity of the adsorbent before immediate breakthrough when the system is operated at atmospheric pressure is 14.0 + 0.7 1 of dry carbon dioxide per kilogram of adsorbent. At a pressure of 4 atm the obtained capacity is 15 + 2 1 CO2/ kg taken as average value at the different space velocities tested.
188
O. Leal et al. / lnorganica Chimica Acta 240 (1995) 183-189 Table 2 FT-IR frequencies of the surfaces species
c
a
~
'
~
'
~bo
'
~
WAVENUMBER, cg1-1
Fig. 10. F['-IR spectra in the region 3400-2800 cm-I:'(a) APG treated at 150 °C in vacuo; (b) after adsorption of dry CO2 on APG; (c) after adsorption of moist CO2 on APG.
C
Ikl
o z,<
b
m n-
O
(n
m ,<
+
I
1670
i
I
!
I
I
!
1550 1450 --1 1350 WAVENUMBER, ¢11
Fig. 11. FT-IR spectra in the region 1670-1350 c m - i: (a) APG treated at 150 °C in vacuo; (b) after adsorption of dry CO2 on APG; (c) after adsorption of moist CO2 on APG.
Figs. 10 and 11 show the FT-IR spectra in the regions 3400-2800 and 1670-1350 c m - l , respectively. The lower spectrum 'a' corresponds to a modified gel after being heated in vacuo at 150 °C for 2 h. When the gel is exposed to a pressure of 10 torr of dry carbon dioxide spectrum 'b' is generated. Spectrum 'c' corresponds to exposure to humid carbon dioxide. Bands observed are summarized in Table 2. Doublets are present between 3000 and 3100 cm-~ which would correspond to the presence of hydrogen bonded N - H and O--H, respectively. In the low frequency region the spectra show a band at 1596 c m - ~ assigned to the formation of NH 3 ÷ as the carbon dioxide, dry or humid, is adsorbed onto the gel; a band at 1430 c m - ] due to the presence of adsorbed molecular water; and bands at 1411 and 1385 c m - 1 which have been assigned to the carbamate and bicarbonate species [ 19]. The relative intensities of these two bands vary as moisture is introduced
Frequency ( c m - ~)
Assignment"
3380 3300 3071 3055 3026 3005 2962 2930 2870 1650 1590 1510 1430 1411 1384
asymmetrical stretching N-H symmetrical stretching N-H interactive stretching O-H asymmetrical bicarbonate interactive stretching N-H ammonium stretching N-H stretching C-H b asymmetrical stretching C-H symmetrical stretching C-H stretching C=O ammonium bending N-H doublet carbamate-bicarbonate water bending O-H carbamate C--O bending bicarbonate C ~ ) bending
Tentative assignment. b Corresponds to stretching C-H in formaldehyde. a
into the system. When carbon dioxide is dry, the band at 1411 c m - 1 is intense, the band at 1385 c m - ~ increasing gradually as the adsorptive become humid. Likewise, there is a gradual shift in intensity in the small doublet which is observed at about 1510 c m - 1, changing its relative intensities from spectrum 'a' to 'c'. All the evidence confirms that each molecule of carbon dioxide uses two surface amino groups to form an ammonium carbamate species (Fig. 12(a)) when water is absent and this becomes an ammonium bicarbonate surface species (Fig. 12(b) ) when water is incorporated into the system. The most striking feature is the appearance of an intense band at 2962 c m - 1 when moist carbon dioxide is adsorbed. This has been identified as a band associated with formaldehyde [ 20], suggesting that the adsorption of carbon dioxide on these modified gels may be a conceivable pathway for activation of this gaseous molecule. The regenerability and
a
.~, L~
0 b ..
Fig. 12. Surface reaction mechanism of C02 in the presence of water.
O. Leal et al. /lnorganica Chimica Acta 240 (1995) 183-189
selectivity of this adsorption process make it an attractive system for analytical and industrial purposes
References [ 1] [2] [3] [4] [5] [6] [7] [8] [9] [ 10 ]
V. Ramanathan et ai., Rev. Geophys., 25 (1987) 1441. H. Rodhe, Science, 248 (1990) 1217. W. Fulkerson, R. Judkins and M. Sanghvi, Sci. Am., (1990) 83. M. Buckthorp, Nitrogen, 113 (1978) 34. K. Voikamer and U. Wagner, Fertilizer, (1983) 139. D.A. Boyta, US Patent No. 3 847837 (1974). B.D. Bhide and S.A. Stem, J. Membr. Sci., 81 (1993) 209. V.A. Shah, Energy Prog., 8 (1988) 67. M. Weinberg, US Patent No. 5 234 471 (1993). R.L. B urwell, Jr. and O. Leai, J. Chem. Soc., Chem. Commun., (1974) 342.
189
[ 11 ] R.L. Burwell, Jr., R.G. Pearson, G.L. Hailer, P.B. Tjok and S.P. Chock, Inorg. Chem., 4 (1965) 1123. [12] O. Leai, D.L. Anderson, R.G. Bowman, F. Basolo and R.L. Burwell, Jr., J. Am. Chem. Soc., 97 (1975) 5125. [ 13] J.H. de Boer, M.E. Hermans and J.M. Vleeskins, K. Ned. Akad. Wet., B60 (1957) 44. [ 14] R.L. Burwell, Jr., Chem. Technol., (1974) 370. [ 15] J.H. Anderson, Jr., and K.A. Wickersheim, Surf. Sci., 2 (1964) 252. [16] J. Peri, J. Phys. Chem., 70 (1966) 2937. [17] G.E. Maciel, D.W. Sindorf and V.J. Bartuska, J. Chromatogr., 205 (1981) 438. [18] G.E. Maciel, in B.L. Shapiro (ed.), Heterogeneous Catalysis, Texas A&M University Press, College Station, TX, 1984, p. 364. [ 19] S. Naviroj, S.R. Culler, J.L. Koening and H. Ishida, J. CoUoidlnterface Sci., 97 (1984) 311. [20] M. Avram and G.H. Mateescu, Infrared Spectroscopy, WileyInterscience, New York, 1972, p. 342.
ELSEVIER
lnorganica Chimica Acta 240 (1995) 183-189
Reversible adsorption of carbon dioxide on amine surface-bonded silica gel Orlando Leal a.., Carmelo Bolivar a, C6sar Ovalles b, Juan Jos6 Garcia b, Youssef Espidel b a Escuela de Qufmica, Facultad de Ciencias, Universidad Central de Venezuela, Apartado 47102, Caracas 1040, Venezuela b INTEVEP S.A, Los Teques, Venezuela Received 7 January 1995; revised 31 March 1995
Abstract
Carbon dioxide adsorbs reversibly on a silica gel containing 3-aminopropyl groups bonded to surface atoms of silicon. These act as the active sites for the chemisorption of CO2 at room temperature which is liberated by temperature programmed desorption at about 100 *(2.The material is capable of adsorbing about 10 STP cm3 of dry CO2 per gram and can be regenerated upon heating. It might be used as scrubber for carbon dioxide from industrial gaseous streams. Adsorption of humid CO2 produces a small amount of formaldehyde which suggests activation of the carbon dioxide molecule. Keywords: Chemisorption; Carbon dioxide; Reversible adsorption; Surface modification; Alkoxysilane; Silica gel
1. Introduction
Carbon dioxide, being a normal component of the atmosphere, is involved in the respiratory cycles of plants and animals. The results of research on the greenhouse effect have shown that the levels of trace gases naturally occurring in the atmosphere, such as carbon dioxide, methane and nitrous oxide, have increased sharply above preindustrial era levels. Carbon dioxide has gone from 315 to 350 ppm in the last 35 years and the rate of increase has speeded up in recent years [ 1 ]. This increase has been mainly linked to the intense use of fossil fuels in internal combustion engines and other human activities. Therefore, current concern about an anthropogenic impact on global climate has made it of interest to compare the potential effect of various human activities [2]. It may be that human-generated emissions of carbon dioxide, mainly from fossil fuels and the burning of forests, will have to be cut by as much as 50-80% to avoid major climate changes. Such a reduction in the CO2 emission rate probably cannot be accomplished without a massive switch to non-fossil energy sources. Nevertheless, emissions from fossil fuels can be moderated somewhat by three strategies: exploiting the fuels more efficiently, replacing coal by natural gas, and recovering and sequestering CO2 emissions [3]. A rough analysis, based on the use of currently accepted values, shows This paper is dedicated to Professor F. Basolo. * Corresponding author. Elsevier Science S.A. SSD10020-1693 ( 95 ) 0 4 5 3 4 - G
that natural gas is preferable to other fossil fuels in consideration of the greenhouse effect [2]. The most futuristic strategy for reducing CO2 emissions from fossil fuels is to capture the emissions and sequester them for later use in afforestation or transport them to a storage site. Storage options include piping the gas deep into the ocean or into depleted natural gas reservoirs. Removal of carbon dioxide from gaseous streams has been a current procedure in the chemical industry. The presence of tiny amounts of carbon dioxide can act as poison in catalytic chemical processes such as ethylene polymerization or ammonia synthesis [4]. In ammonia plants, carbon dioxide cannot be tolerated in the catalyst under the high temperature and pressure prevailing in the converter. Therefore, several procedures have been devised to eliminate carbon dioxide from synthesis gas in ammonia plants as well as in natural gas plants and fuel gas power plants. They can be summarized in three categories: absorption on liquid amines [5]; adsorption on solid materials [6]; membrane technology [7]. The former makes extensive use of alkanol-amines such as monoethanolamine (MEA), diethanolamine (DEA) or the like to be used in plants at low temperature and high pressures (900-1000 psi). This method suffers a corrosion problem which arises from the formation of carbamates, then requiring the use of inhibitors. A process, commercially known as Selexol, uses a mixture of polypropyleneglycoldimethyl ethers and removal of carbon dioxide
! 84
O. Leal et al. /Inorganica Chimica Acta 240 (1995) 183-189
pyleneglycoldimethyl ethers and removal of carbon dioxide is based on physical absorption in much heavier molecules [8]. On the other hand Bhide and Stern [7] have recently assessed the economics of some separation processes for the removal of acid gases from crude natural gases by using membranes. Aromatic polyimide separation membranes are particularly useful for carbon dioxide enrichment [9], because they achieve high flux rates with good selectivity at low temperatures. This has proved effective to remove CO2 in concentrations ranging between 5 and 40 mol%. This procedure is bounded to gas losses due to membrane saturation. In this paper, we describe the chemisorption of carbon dioxide on a chemically modified surface of silica gel, which due to its specificity has found applications in the separations of this gas from closed ambient and very likely could lead to chemical activation of the carbon dioxide molecule.
2. Experimental 2.1. Surface modification The aminopropyl gel (APG) was prepared as described elsewhere [ 10]. Ten grams of 60-80 mesh, Davison grade 62 silica gel (a wide-pore gel with a nominal pore diameter of 12 nm, a specific area of 340 m2/g, and a pore volume of 1.4 cm3/g) was dried by azeotropic distillation in m-xylene for 1 h and then refluxed in m-xylene with 4.5 g of 3-aminopropyltriethoxysilane. Different coverage was obtained by varying the time of reflux from 1.5 to 30 h. The product was washed consecutively in the absence of moisture with mxylene and n-pentane to produce a modified aminopropyl gel. Coverages calculated from C, H and N micro-analysis ranged between 1.18 and 2.80 molecules of aminopropyl groups/ nm 2 (0.57-1.27 mmol of nitrogen/g).
2.2. Characterization The IR spectra of the solids were obtained by two procedures. (i) In the near-infrared region (NIR) (4500-7600 c m - 1) they were recorded on a Cary 17 D spectrometer using the technique described by Burwell et al. [ 11 ]. The sample was dried at 160 °C and then located in a fused silica cell of 1 cm light path embedded in carbon tetrachloride which had previously been dried on anhydrous K2CO3. Samples were allowed to stand until the air had been displaced from the pores of the gels. A cuvette with silica gel in carbon tetrachloride was used as a reference in the case of the surface modified materials. (ii) In the fundamental region (10004000 c m - ~) spectra were recorded on an FT-IR Perkin-Elmer spectrometer. Samples were pelletized by applying pressure of 10 ton per square inch to approximately 20 mg of the solid material. Pellets were placed in the sample holder of a sealed cell built of fused silica and equipped with calcium fluoride
windows. This cell could be coupled to a glass vacuum system which allowed evacuation of the sample to pressures better than 10 -5 torr, heating, and exposure to the carbon dioxide adsorptive. NMR spectra of the solids were performed in a Bruker MSL-300 at a frequency of 75468 MHz under high power, magic angle spinning (HP/MS) with relaxation time of 2.5 s. A 4 mn seconun rotor was used rotating at 9.7 kHz. The chemical shifts were measured against TMS.
2.3. Apparatus The solid adsorption capacity test for carbon dioxide was carried out in a continuous flow experimental system, using ultra pure helium as carrier gas [ 12]. Successive volumes of CO2 were introduced onto the adsorbent by means of a gas injection valve with a 0.65 cm 3 volume loop which was injected every injection lap. The adsorbent was placed in a Pyrex U tube of 6 mm outer diameter and connected to the carrier flow in the oven of a Hewlett-Packard model 5710 A gas chromatograph. One gram of adsorbent was employed and successive volumes of CO2 were injected at room temperature (27 °C) until saturation of the adsorbent. Then by programmed temperature desorption (PTD) between 27 and 150 °C, at a heating rate of 10 °C m i n - 1 and with helium flow rate of 30 cm 3 min-~ the adsorbed carbon dioxide was released and measured to determine the adsorptive capacity of the material. An additional test for capacity was carried out by determining the adsorption isotherm at different temperatures by means of a volumetric system shown in Fig. 1. It consists basically of a high vacuum system built out of Pyrex glass and equipped with greaseless stopcocks. A vacuum system, Leybold-Heraus model PD 30L, was used to reach pressures as l o w a s 10 - 6 torr. The gases used for adsorption studies were introduced from different inlets. To study the adsorption, the gel was placed in bulb 1 and evacuated overnight at 100 °C to remove any residual water and then heated at 160 °C for 1 h to activate it. Temperatures were measured with a chromel-alumel ther-"-- VIICCUID
--,.- vent air
VT
! Fig. 1. Diagram of volumetric glass apparatus for measuring the adsorption isotherms of CO2 on APG. 1: Sample, 2: He reservoir, 3:CO2 reservoir, 4: reference cell, 5: Klister servo pressure sensor, V: greaseless stopcocks.
O. Leal et al. / lnorganica Chimica Acta 240 (1995) 183-189
185
injected from a tank by means of valve V5. Nitrogen was used occasionally as carrier. Natural gas having a composition of 83.1% methane, 7.6% ethane, 0.7% propane, 0.2% butanes, 0.1% pentanes and 8.3% carbon dioxide was used in the scrubbing tests which were carried out at pressures ranging between 1 and 4 atm and space velocities between 28 and 170 cubic foot per hour ( 1000-6000 l/h). ?"i:: i
3. Results and discussion
G~I8
Cnroma~r~
Reaction of the surface of silica gel with 3-aminopropyltriethoxysilane leads to the condensation of the hydroxyl groups producing organic residues bound to the surface, and might involve the reaction of one, two or three hydroxyl groups of the surface per molecule of alkoxysilane. Therefore, the nature of the bonded species will depend upon the number and distribution of the hydroxyl groups at the surface of the silica gel. Previous studies indicate that annealed and rehydroxylated silica gel has a surface hydroxyl concentration of 4.6 - O H groups per square nanometer [ 13], of which 1.4 hydroxyl groups/nm 2 exist as single non-hydrogen bonded species and the remaining 3.2 hydroxyl/nm 2 are interacting groups arranged in pairs: - -
Sf Fig. 2. Schematic diagram of flow system, D: flow splitters, R: reactors, S: gas outlet, T: thermocouple, V: three way valves, SH: water saturator, BP: back pressure valve, MI: mass flow controller, M2: flowmeter, M3: manometer.
mocouple (Omega Inc.), by means of a Hewlett-Packard recorder model 713 A. A temperature programmer was employed to control the temperature of the sample. Purified helium, used to establish a blank, was kept in a permanent bulb 2 and carbon dioxide from a high purity tank was kept in bulb 3. To perform measurements, dry air at atmospheric pressure was admitted into reference cell 4. Any pressure in Vo or VT was measured with respect to the pressure of the reference cell. The difference between the reference and the working cells was measured by a Kistler servo pressure sensor 5, which measured + 1 atm relative to the reference cell. A digital display 6 was connected to the sensor 5. Testing of the performance of the adsorbent as scrubber for carbon dioxide when scaling up the process was carried out in the apparatus shown in Fig. 2. It consists of a flow system built out of stainless steel tubing equipped with temperature, pressure and mass flow controllers. The adsorbent is confined in two parallel cylinders which allow its activation and use in an alternate fashion. Each cylinder is loaded with 250 g of adsorbent. A Varian 3400 gas chromatograph equipped with a 6 foot stainless steel column of 10% Carbowax 20 M on Chromosorb W-HP 80/100 mesh was used to monitor the samples
- -
I
Si
\ /
OH 0
S i -
OH
I All these groups can be involved in the reaction of the surface of the silica with polyalkoxysilanes. Burwell [ 14] indicated that this reaction gives a product with an average composition corresponding to the diattachment of the silane to the surface, according to reaction ( 1) o-o. +
Si
I~
/
Si
+ C2H50H
(1) where o- stands for a silicon atom at the surface of silica and X = NH2 if the silane is 3-aminopropyltriethoxysilane. This structure was estimated from the carbon-nitrogen ratios of the products. When the reaction is carried out by refluxing in m-xylene for 24 h or more we obtained a gel which carries an average of 2 . 5 + 0 . 1 molecules (1.2 mmol of N / g ) of --CH2CH2CH2NH2 per square nanometer of the surface. The kinetics of the surface reaction are shown in Fig. 3 which indicates that the nitrogen content of aminopropyl surface modified gel (APG) varied between 0.57 and 1.26 mmol of N per gram of gel when reaction was carried out between 1.5 and 30 h. After approximately 10 h the reaction has reached 80% completion. The NIR spectrum in the overtone region of silica gel Davison grade 62, dried at 160 °C for 15 min, is given in
O. Leal et al. / lnorganica Chimica Acta 240 (1995) 183-189
186
1.25 Z m ,.. 1.00
r, "-
|
0.75
0.50
0.26
l 10
0
I
I
20
30
llnuo, hours
Fig. 3. Effect of time of reaction on degree of modification of surface of silica gel Davison grade 62. Reaction temperature: 136 °C.
I
4752
I
t
5263
I
o
OH
S!b o/
\/ /\
Si a
H%S,c//OH
\o \ / /\
o/ Si a
\o\ /
I 61 I 6 67 ilVlnumller cllf ~
I
Fig. 4. IR spectrain the overtoneregion ( 1.3-2.1/~m): (a) silicagel Davison 62; (b) aminopropylgel (APG); (c) 3-aminopropyltriethoxysilanein CC14. ~
'tJ
Fig. 4(a). Absorption bands are observed at 4545, 5268 and 7220 c m - n. The latter has been assigned to the first overtone of the - O - H stretching vibration [ 15]. The band at 5268 c m - n disappears almost completely upon heating at 160 °C for 1 h, indicating this band derives from molecular water, and that at 4545 c m - 1 arises from combination modes of - O - H and lattice groups [ 16]. Surface modification resulted in a decrease of the intensity of the bands. Thus the decline in intensity of the band at 7220 c m - 1 was of 75%. New bands appeared at 6560 c m - ~ (first overtone of symmetrical N - H stretching), at 5010 c m - l (combination band of N - H group) and a series of bands between 5480 and 5970 c m - ~ associated with the first overtone of the methyl and methylene groups (Fig. 4(c) ). Furthermore the 295i C P / M A S NMR spectra shown in Fig. 5 indicate that upon modification of the surface of silica a band centered at - 103.4 ppm with respect to TMS almost disappeared (spectrum a) becoming a shoulder, and a broad small band is developed around - 70 ppm (spectrum b). The latter band can be attributed to extra network silicon atoms coming from the alkoxysilane and attached to the silica framework. These results are consistent with the occurrence of a surface reaction between the amino alkoxysilane and t h e - O H groups of the silica gel, which act as anchorage points to the solid surface, as was previously postulated in reaction ( 1 ). Schematically the surface can be depicted as:
I
5882
~;
b
8
I -40
I -60
f -80
f , -100
I -120
I.. -140
PPM
Fig. 5. 295i CP/MA NMR spectra: (a) silica gel Davison 62; (b) aminopropyl gel (APG).
atoms 'd' are responsible for the NMR signal at 40 ppm [ 17 ]. This assigment has been reported elsewhere [ 18]. The material obtained was a solid having a surface area of 200 m2/g, a total pore volume 0.7 cm3/g and average pore diameter of 11 nm, and exhibits between 2 and 3 molecules of amino modifying residue per square nanometer. Adsorption experiments on different preparations of the material on the flow apparatus previously described resulted in an adsorptive capacity of 9.10-t-0.05 cm3/g of dry CO2,
H2~ H2
1"12l~H2~
/o Si a
\
and the - O H groups bound to the outer silicon atom signaled as 'b' and 'c' would be those involved in the surface reaction, leading to a surface species like the one of Fig. 6 where silicon
F i g . 6. S c h e m e o f p r e d o m i n a n t s u r f a c e s p e c i e s o n A P G .
O. Leal et al. / lnorganica Chimica Acta 240 (1995) 183-189
which was doubled to 20.80+0.05 cm3/g when the carbon dioxide was saturated with water. The addition of cupric sulfate aqueous solution to the material rendered a material with a surface complex between Cu 2 ÷ ions and the surface amino groups of the adsorbent. This yielded a material unable to adsorb carbon dioxide. This is a clear demonstration that the amino groups are the sites responsible for the carbon dioxide adsorption. Isotherms determined at different temperatures in the apparatus shown in Fig. 1 are presented in Fig. 7. At higher temperatures a greater pressure was required for adsorption of a particular volume of carbon dioxide. The adsorbent displays an adsorptive capacity of 12.0+0.5 STP cm 3 of CO 2 per gram at 0 °C and a heat of adsorption A H of 14 + 1 Kcal mo1-1 as determined from In P2/Pt = - A H / R ( 1 / T 2 - 1 / T1). The adsorption of carbon dioxide is reversed when the adsorbent is heated at 100 °C. All the evidence suggests that the adsorption process is due to the formation of carbamate species on the modified surface of the material, where two amino groups from it are used up per carbon dioxide molecule adsorbed, as indicated in Fig. 8. In the presence of water, or when the carbon dioxide is wet, the capacity of the material towards the adsorption of carbon dioxide is duplicated due to a surface chemical transformation of the carbamate into two adsorbed bicarbonate molecules. To illustrate the behavior of this material as a scrubber for carbon dioxide, a study of the adsorption was carried out using the apparatus of Fig. 2. The experiments performed at different space velocities show that the adsorbed carbon dioxide started to be liberated by temperature programmed desorption (TPD) above 40 °C and the process remains so until above 100 °C. The center of the peak is shifted to lower temperatures as the spatial velocity increases. At a space velocity of 2.4 h - ~ ( 1.2 l/h) the adsorption band is centered at 70 °C and desorption is completed at 110 °C whereas at a space velocity of 0.6 h - ~ (0.3 l/h) the desorption band is centered at 90 °C and desorption is completed at 120 °C. On the other hand if the desorption temperature was held at 140 °C the adsorbent is regenerated in 2 h.
~'16
o~ 12
0 8 4
0
t 0
I 2
,
I 4
I
1 I I t I I 6 8 10 PRESSURE OF CO 2 , Itel
I 1.2
Fig. 7. Adsorption isotherms of carbon dioxide on APG run at different temperatures: *, 0; O, 23; A, 50 °C.
187
C H2-C H2-C H2 NH2 C H2-CH2-CH 2 NI.I2
~- ~ +
CH2"~I'~"CH2 N H + O ~
CO 2 ..~.~C.2_C.2_C.2 N.3
"
+
co2 +
2H20 2
CH2-CH2-CH2 NH ~ HCO3
Fig. 8. Schemeof the surfacereactionof carbon dioxide with APG. 100, o41-,~I'.: ,~-- -
t,
._*=..=
*, ¢
"
.o
4O
20
~
0
I
I
I
I
i
4
8
12
16
20
lime, hours
Fig. 9. Effectof spacevelocityon adsorption of CO2 on APG. Table 1 Effect of space velocity and pressure on CO2 adsorption
Experimenta
Pressure (atm)
Space velocity (1 h -t)
1
1
3
2 3 4 5 6
1 1 1 4 4
6 12 18 12 18
Adsorptionb (I kg -1 ) 13.3 13.7 15.5 13.8 16.3 14.0
a At ambient temperature. Activation of adsorbents done at 100 °C for 4 h under nitrogen stream. Experiments are average of two runs. b Liters of carbon dioxide adsorbed per kilogram of adsorbent at ambient temperature and pressure.
The influence of space velocity on the adsorption of carbon dioxide is presented in Fig. 9 where 100% adsorption corresponds to complete stripping of CO2 from natural gas containing 8.3% CO2 and activation was undertaken under nitrogen as carrier. It is clear from this that as space velocity increases from 6 to 36 h - ~ (3-18 l/h) the breakthrough time is reduced from 14 to 4 h, however the elution front remains clearcut confirming the chemical nature of the surface adduct. The process also quantitatively subtracts carbon dioxide from the natural gas streams. Table 1 shows the influence of pressure and space velocity on the adsorption of carbon dioxide. The capacity of the adsorbent before immediate breakthrough when the system is operated at atmospheric pressure is 14.0 + 0.7 1 of dry carbon dioxide per kilogram of adsorbent. At a pressure of 4 atm the obtained capacity is 15 + 2 1 CO2/ kg taken as average value at the different space velocities tested.
188
O. Leal et al. / lnorganica Chimica Acta 240 (1995) 183-189 Table 2 FT-IR frequencies of the surfaces species
c
a
~
'
~
'
~bo
'
~
WAVENUMBER, cg1-1
Fig. 10. F['-IR spectra in the region 3400-2800 cm-I:'(a) APG treated at 150 °C in vacuo; (b) after adsorption of dry CO2 on APG; (c) after adsorption of moist CO2 on APG.
C
Ikl
o z,<
b
m n-
O
(n
m ,<
+
I
1670
i
I
!
I
I
!
1550 1450 --1 1350 WAVENUMBER, ¢11
Fig. 11. FT-IR spectra in the region 1670-1350 c m - i: (a) APG treated at 150 °C in vacuo; (b) after adsorption of dry CO2 on APG; (c) after adsorption of moist CO2 on APG.
Figs. 10 and 11 show the FT-IR spectra in the regions 3400-2800 and 1670-1350 c m - l , respectively. The lower spectrum 'a' corresponds to a modified gel after being heated in vacuo at 150 °C for 2 h. When the gel is exposed to a pressure of 10 torr of dry carbon dioxide spectrum 'b' is generated. Spectrum 'c' corresponds to exposure to humid carbon dioxide. Bands observed are summarized in Table 2. Doublets are present between 3000 and 3100 cm-~ which would correspond to the presence of hydrogen bonded N - H and O--H, respectively. In the low frequency region the spectra show a band at 1596 c m - ~ assigned to the formation of NH 3 ÷ as the carbon dioxide, dry or humid, is adsorbed onto the gel; a band at 1430 c m - ] due to the presence of adsorbed molecular water; and bands at 1411 and 1385 c m - 1 which have been assigned to the carbamate and bicarbonate species [ 19]. The relative intensities of these two bands vary as moisture is introduced
Frequency ( c m - ~)
Assignment"
3380 3300 3071 3055 3026 3005 2962 2930 2870 1650 1590 1510 1430 1411 1384
asymmetrical stretching N-H symmetrical stretching N-H interactive stretching O-H asymmetrical bicarbonate interactive stretching N-H ammonium stretching N-H stretching C-H b asymmetrical stretching C-H symmetrical stretching C-H stretching C=O ammonium bending N-H doublet carbamate-bicarbonate water bending O-H carbamate C--O bending bicarbonate C ~ ) bending
Tentative assignment. b Corresponds to stretching C-H in formaldehyde. a
into the system. When carbon dioxide is dry, the band at 1411 c m - 1 is intense, the band at 1385 c m - ~ increasing gradually as the adsorptive become humid. Likewise, there is a gradual shift in intensity in the small doublet which is observed at about 1510 c m - 1, changing its relative intensities from spectrum 'a' to 'c'. All the evidence confirms that each molecule of carbon dioxide uses two surface amino groups to form an ammonium carbamate species (Fig. 12(a)) when water is absent and this becomes an ammonium bicarbonate surface species (Fig. 12(b) ) when water is incorporated into the system. The most striking feature is the appearance of an intense band at 2962 c m - 1 when moist carbon dioxide is adsorbed. This has been identified as a band associated with formaldehyde [ 20], suggesting that the adsorption of carbon dioxide on these modified gels may be a conceivable pathway for activation of this gaseous molecule. The regenerability and
a
.~, L~
0 b ..
Fig. 12. Surface reaction mechanism of C02 in the presence of water.
O. Leal et al. /lnorganica Chimica Acta 240 (1995) 183-189
selectivity of this adsorption process make it an attractive system for analytical and industrial purposes
References [ 1] [2] [3] [4] [5] [6] [7] [8] [9] [ 10 ]
V. Ramanathan et ai., Rev. Geophys., 25 (1987) 1441. H. Rodhe, Science, 248 (1990) 1217. W. Fulkerson, R. Judkins and M. Sanghvi, Sci. Am., (1990) 83. M. Buckthorp, Nitrogen, 113 (1978) 34. K. Voikamer and U. Wagner, Fertilizer, (1983) 139. D.A. Boyta, US Patent No. 3 847837 (1974). B.D. Bhide and S.A. Stem, J. Membr. Sci., 81 (1993) 209. V.A. Shah, Energy Prog., 8 (1988) 67. M. Weinberg, US Patent No. 5 234 471 (1993). R.L. B urwell, Jr. and O. Leai, J. Chem. Soc., Chem. Commun., (1974) 342.
189
[ 11 ] R.L. Burwell, Jr., R.G. Pearson, G.L. Hailer, P.B. Tjok and S.P. Chock, Inorg. Chem., 4 (1965) 1123. [12] O. Leai, D.L. Anderson, R.G. Bowman, F. Basolo and R.L. Burwell, Jr., J. Am. Chem. Soc., 97 (1975) 5125. [ 13] J.H. de Boer, M.E. Hermans and J.M. Vleeskins, K. Ned. Akad. Wet., B60 (1957) 44. [ 14] R.L. Burwell, Jr., Chem. Technol., (1974) 370. [ 15] J.H. Anderson, Jr., and K.A. Wickersheim, Surf. Sci., 2 (1964) 252. [16] J. Peri, J. Phys. Chem., 70 (1966) 2937. [17] G.E. Maciel, D.W. Sindorf and V.J. Bartuska, J. Chromatogr., 205 (1981) 438. [18] G.E. Maciel, in B.L. Shapiro (ed.), Heterogeneous Catalysis, Texas A&M University Press, College Station, TX, 1984, p. 364. [ 19] S. Naviroj, S.R. Culler, J.L. Koening and H. Ishida, J. CoUoidlnterface Sci., 97 (1984) 311. [20] M. Avram and G.H. Mateescu, Infrared Spectroscopy, WileyInterscience, New York, 1972, p. 342.