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Study of Carbon Dioxide Adsorption on Mesoporous Aminopropylsilane-Functionalized Silica and Titania Combining Microcalorimetry and in Situ Infrared Spectroscopy Christina Kno¨fel,*,† Ce´line Martin,‡ Virginie Hornebecq,† and Philip L. Llewellyn† Laboratoire Chimie ProVence, UniVersite´s d’Aix-Marseille I, II, et III, CNRS, UMR 6264, Centre de Saint Je´roˆme, 13397 Marseille, France, and Laboratoire de Physique des Interactions Ioniques et Mole´culaires, UniVersite´ de ProVence, CNRS, UMR 6633, Centre de Saint Je´roˆme, 13397 Marseille, France ReceiVed: July 24, 2009; ReVised Manuscript ReceiVed: NoVember 18, 2009
Two calcined mesoporous supports, silica and titania, were functionalized with aminopropylsilane (APS). The samples were characterized using ATR (attenuated total reflectance), nitrogen sorption at 77 K, and thermogravimetric analysis. The functionalized silica and titania are mesoporous and were grafted with 1.4 and 1.6 molecules of APS per nm2, respectively. Infrared measurements propose that the properties of the amine sites are affected by the chemical properties of the support. For example, the NH2 bending vibration δ(NH) was shifted to lower wavenumbers from 1597 to 1575 cm-1 from the silica to the titania grafted sample, respectively. This could be explained by different interactions with the surface hydroxyl groups of silica and titania. The grafted samples were investigated for carbon dioxide adsorption by combining microcalorimetry and in situ FTIR spectroscopy. Their CO2 adsorption properties are presented in comparison to the nongrafted support materials. Microcalorimetric measurements show important enthalpies of adsorption at low CO2 coverage (more than -80 kJ mol-1) for the APS-grafted materials, indicating a strong reactivity between carbon dioxide and the amine sites. In situ infrared spectroscopy was used to study this reactivity. The formation of three products (carbamate, carbamic acid, and bidentate carbonate) is proposed. Introduction Carbon dioxide is the most important greenhouse gas implicated in the causes of global warming. It is a major combustion product in many industries, ranging from energy production in power stations (fossil fuel and natural gas) to glass and cement fabrication. Furthermore, carbon dioxide is used as reactant in industrially relevant processes. For example, CO2 is a component in the Solvay process for the production of NaHCO3, next to ammonia and NaCl.1 Therefore, the recovery of carbon dioxide is an important issue, on one side, to decrease the carbon dioxide emission into the atmosphere and, on the other side, to reuse the captured carbon dioxide for industrial applications. One of the main processes applied to recover industrially produced CO2 involves the use of amine baths. This carbon dioxide scrubbing is also used in life support systems in confined spaces such as submarines, space vehicles, and other inhabited vessels for space exploration platforms.2 However, these industrial processes of carbon dioxide capture using amines are characterized by several disadvantages, such as solvent degradation, corrosion, and foaming.3 A fraction of the amine and its decomposition products is also lost by evaporation, posing severe operational problems.4 Therefore, a transfer from carbon dioxide absorption in liquid amine baths to adsorption on aminefunctionalized solids may be an interesting way to overcome these mentioned disadvantages. Leal et al.5 were the first to functionalize the surface of silica with an aminosilane for its use in the adsorption of carbon dioxide. After that, further work was carried out to investigate * To whom correspondence should be addressed. E-mail: christina.knofel@ etu.univ-provence.fr. Tel: +33 (0) 491637116. Fax: +33 (0) 491637111. † Universite´s d’Aix-Marseille I, II, et III. ‡ Universite´ de Provence.
the adsorption of carbon dioxide on amine-functionalized materials using different amine molecules and different techniques. For example, in situ infrared investigations,5-11 where flow conditions have often been applied, gave insights into the reaction mechanisms between carbon dioxide and amine site functionalized silica materials. The formation of carbamate and carbonate species was discussed. However, the obtained infrared bands were interpreted in different ways. In this work, we are interested in studying the carbon dioxide interaction with amine-functionalized materials in more detail. We have used a combination of microcalorimetric and in situ infrared spectroscopic measurements to investigate the carbon dioxide adsorption on aminopropylsilane-functionalized mesoporous materials. The use of the two complementary methods allows correlating the values of enthalpies of adsorption to the interaction of carbon dioxide with specific energetic surface sites. The in situ infrared measurements are carried out in a home-built cell, allowing a successive introduction of carbon dioxide quantities with increasing pressures, presenting comparable conditions as in microcalorimetric measurements. The carbon dioxide adsorption results are discussed with respect to the characteristics of the materials. Two different mesoporous supports are used for the functionalization, a silica and a titania matrix. To our knowledge, we show for the first time that the different structural and chemical properties of the supports can affect the characteristics of the amine groups and thus the carbon dioxide adsorption. This is reflected in the different behavior of the aminosilane-functionalized materials toward carbon dioxide adsorption. We discuss that in case of the titaniasupported functionalized material, the carbon dioxide can also interact with the titania surface, promoting the formation of carbonate species, whereas for the silica-supported analogue, the adsorption of carbon dioxide in anhydrous conditions does not lead to adsorbed carbonate species. However, we observe
10.1021/jp907054h 2009 American Chemical Society Published on Web 12/04/2009
CO2 Adsorption on Functionalized Silica and Titania the formation of a bidentate carbonate species during the desorption of carbon dioxide from the functionalized silica under heating. The latter could be identified due to the comparison with the in situ infrared study carried out on the titania materials. This is new and interesting information, suggesting that different interactions may take place with increasing temperature on aminosilane-functionalized silica materials. Experimental Methods Used Chemicals. Toluene was purchased from Riedel-deHaen and dried over 5A zeolite. All other chemicals were procured from Aldrich and used as received without further purification. The 3-(trimethoxysilyl)propylamine [(CH3O)3Si(CH2)3NH2, APS] was used as grafting agent. Hexane (g95%) was applied for the deposition of the samples prior to the in situ infrared studies. Carbon dioxide, argon, and helium were obtained from Air Liquide and were of 99.9999% minimum purity (N60). For the in situ infrared studies, a CO2 4.5 minican from Linde was used. Mesoporous Oxides Used as Supports. Mesoporous silica and titania were used as supports for the postsynthesis grafting procedure. The general synthesis procedures applied for silica and titania were already presented elsewhere.12,13 Both oxides were prepared using the triblock copolymer F127 [PEO106PPO70PEO106; with PEO ) poly(ethylene oxide), PPO ) poly(propylene oxide)] as structure-directing agent. The silica (Si) was calcined at 823 K and the titania (Ti) was calcined at 623 K for 5 h using a heating rate of 1 K min-1. Both oxide supports were free of organic matter, which could have remained from the surfactant template F127 as determined from thermogravimetric measurements (not shown). Functionalization with APS. For the postsynthesis grafting procedure, 1 g of silica (or titania) was pretreated at 393 K to remove physisorbed water. Then it was cooled down to room temperature under argon flow, and toluene was added. After that, the grafting agent APS (∼3.3 mmol) was added under vigorous stirring while the solid/liquid mass ratio was kept constant at 0.05. The reaction mixture continued to stir for 2 h. Then the solution was decanted and the remaining solid was washed four times with 20 mL of toluene. Finally, the solid was dried under argon flow at 353 K for approximately 30 min. The functionalized samples were named Si-APS for silica and Ti-APS for titania. Characterization of the Materials. Thermogravimetric (TGA) measurements were carried out with a TGA Q500 apparatus (TA Instruments) under dynamic air atmosphere (sample flow rate 40 mL min-1). After 60 min under isothermal conditions, the high resolution heating mode was used with an initial heating rate of 10 K min-1. The samples (∼5-20 mg) were treated up to a maximum temperature of 1073 K. The TGA curves were used to calculate the quantity of aminopropylsilane molecules grafted on the oxide surfaces. The ATR (attenuated total reflectance) infrared spectra of the different samples were taken with a Bruker IFS 66/S spectrometer, which was purged with dry air. The samples were used without further purification and they have been ground before measurement. The ATR spectra were recorded using a diamond or a germanium ATR crystal and a DTGS (deuterated triglycine sulfate) detector. For each spectrum, 500 scans were taken with a resolution of 2 cm-1 from 4000 to 600 cm-1 at ambient temperature. Nitrogen adsorption/desorption experiments were carried out with an ASAP 2010 Micromeritics apparatus, at 77 K. Prior to adsorption, samples (∼40-100 mg) were outgassed under
J. Phys. Chem. C, Vol. 113, No. 52, 2009 21727 vacuum for the removal of all physisorbed species during several hours and heating to 393 and 323 K for the supports and for the functionalized samples, respectively. The BET surface area and the total pore volume (t-plot method) and BJHads pore size distributions were determined.14 Adsorption of CO2 Using Microcalorimetry. Prior to each adsorption experiment, the sample was outgassed at 323 and 393 K for the functionalized materials and the supports, respectively, ensuring a constant residual vacuum pressure of 0.02 mbar. The adsorption at 303 K and up to 1 bar was carried out by means of a Tian-Calvet type isothermal microcalorimeter coupled with a manometric device built in house.15 Pure carbon dioxide was introduced by a point-by-point procedure. This complete apparatus allowed us to obtain both the isotherms and the pseudodifferential enthalpies of adsorption as a function of the coverage of gas for each system. Adsorption of CO2 Using in Situ Infrared Spectroscopy. The carbon dioxide adsorption was investigated using in situ infrared spectroscopy equipment which was coupled to a manometric device built in-house. It is composed of a stainless steel adsorption cell16,17 equipped with two CaF2 windows allowing measurements in transmission mode. The sample was ground and placed on a metal grid in the middle of the cell using hexane as solvent. The cell with the sample was outgassed at 343 K under vacuum overnight down to a pressure of ∼1.3 × 10-7 mbar. A coupled manometric device allowed successive point-by-point introduction of carbon dioxide at ambient temperature. Thus, comparable conditions were applied as for the microcalorimetric investigations. Infrared spectra were recorded with a Bruker Vertex 70 FTIR spectrometer equipped with a DTGS detector. Spectra were recorded with 2000 scans and a resolution of 4 cm-1 from 3900 to 1000 cm-1. Results and Discussion At first, the characterization of the four prepared materials will be presented. After that, the carbon dioxide adsorption will be discussed using microcalorimetry followed by in situ infrared spectroscopy. Two different oxides, silica and titania, were used as support materials for the postsynthesis functionalization. The different properties of these supports may influence the characteristics of the resulting APS-modified materials. Indeed, the silica is amorphous, having a high surface area, while titania is crystalline and has a significantly smaller surface area. Furthermore, the chemical nature of the oxides influences the reactivity of the surface sites. It was reported that the basicity of OH groups depends on the character of the metal-oxygen bond of the oxide.18 In our case, the OH groups on the titania surface have a more basic character compared to the silica one. The latter is an important point, since the hydroxyl groups present on the oxide surfaces (i) are used as linkage for the APS molecules during the grafting process, (ii) can also form hydrogen bonds with the grafted APS molecules, and (iii) can interact with carbon dioxide, leading to adsorption. Characterization of the Silica and Titania Materials. The nitrogen sorption results are summarized in Table 1 for all samples and are shown in Figure 1 for the APS-functionalized titania Ti-APS in comparison to the titania support Ti. The isotherm for the APS-grafted titania sample in comparison to the titania support shows that the mesoporous structure (indicated by the type IV shape of the isotherm and the hysteresis loop14) is retained after the functionalization. The mesoporous structure is also retained for the APS-functionalized silica. Thus, the functionalized samples are mesoporous; only the silica
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TABLE 1: Summary of the Nitrogen Sorption (at 77 K) and TGA Results nitrogen sorption BET surface area (m g )
total pore volume (cm g )
BJH average pore diameter (nm)
mmol g-1
molecules per nm2 a
Si Si-APS Ti Ti-APS
672 409 193 115
0.65 0.41 0.22 0.13
4.2 4.1 4.6 4.3
1.6 0.5
1.4 1.6
a
-1
3
-1
grafted amount of APS
samples
2
Molecules per nm2 were calculated due to the total surface area of the supports.
Figure 1. Nitrogen sorption isotherms at 77 K for the pure (Ti) and the functionalized titania sample (Ti-APS).
support contained about 16% of microporosity with respect to the total surface area. The surface areas, pore volumes, and average pore size distributions decrease due to postsynthesis functionalization, showing that the grafting was carried out on the internal surface, thus inside the pores of the titania and silica supports. Indeed, the surface area and pore volumes decrease by about 40% for both supports. Table 1 also summarizes the amounts of APS grafted on the oxide surfaces. These quantities were calculated with respect to the loss of mass that the samples undergo with increasing temperature during thermogravimetric analysis (not shown). Regarding the mmol g-1 values, one can suppose that significantly less APS was grafted on Ti-APS (0.5 mmol g-1) in comparison to the Si-APS sample (1.6 mmol g-1). However, this can be explained by the about 3-4 times smaller surface area of the titania support with respect to the silica support. Therefore, the grafted amounts of APS were also calculated as molecules per surface area (nm2), resulting in similar quantities of grafted APS, 1.4 and 1.6 molecules per nm2 for the Si-APS and Ti-APS sample, respectively. Thus, the materials are wellcomparable for this feature presented in dependence of surface area. The four samples were investigated with ATR infrared spectroscopy and the spectra are shown in figure 2. The calcined silica and titania supports are characterized by broad infrared bands between 3700 and 3000 cm-1, which can be attributed to the vibration of internal and hydrogen-bonded hydroxyl groups and adsorbed water species (also evidenced by a band at ∼1630 cm-1).19 For the titania, it was shown that two isolated hydroxyl species exist on the surface;17,20 however, different wavenumbers for the OH stretching vibrations were reported. In our case, bands attributed to free OH groups may be present in the spectrum of the Ti sample, but a clear attribution is difficult, as their intensities are weak. In case of
the silica support (Si), an O-H stretching vibration characteristic of free surface silanol groups was clearly detected at 3748 cm-1.6,10 For the APS-functionalized samples, Si-APS and Ti-APS, additional bands were detected, others disappeared, with respect to the support materials, and a red shift of existing vibration bands was observed. These observations will now be discussed in more detail. Several additional bands, corresponding to N-H and C-H stretching and bending vibrations (underlined in Figure 2), were present after the grafting procedure, indicating the presence of APS molecules. This evidences the successful introduction of amine sites into both oxide supports. Furthermore, differences were found for the N-H vibrations comparing Ti-APS and Si-APS. The stretching vibrations of the NH2 groups were not well distinguishable for the APSgrafted titania. On one side, this may be related to a smaller amount of grafted molecules per gram (due to 3-4 times smaller surface area of the original titania with respect to silica), which may be reflected in a smaller intensity of the bands. On the other side, an enlargement or shift of the bands may be related to the different chemical properties of the silica and titania support. In fact, a red-shift (∼20 cm-1) to lower wavenumbers was observed for the N-H bending vibration (1575 cm-1) of the Ti-APS sample. This shift may be related to an interaction of the amine sites with the titania surface. Indeed, amine sites can interact via hydrogen bonding not only with each other but also with surface hydroxyl groups.21,22 Furthermore, an interaction with Ti atoms present on the titania surface may influence the characteristics of the amine sites. The particularity of the spectra for the functionalized titania Ti-APS is that additional bands at wavelengths 3748 3750-3600 3700-3000 3643 3000-2800 1630 1597/1575 1563/1528 1467 and 1445 1381 1250-1200 1148 1060 with broad shoulder at 1200
assignment
correspond to
OH stretching of free surface silanol groups OH stretching of free surface OH groups OH stretching of hydrogen-bonded OH groups OH stretching of hydrogen-bonded OH groups C-H stretching H-O-H bending vibration N-H bending of NH2 group O-C-O asymmetric stretching of COO- in alkylcarbamate anion C-H bending O-C-O symmetric stretching of COO- in ammonium carbamate and/or silent symmetric stretching mode of adsorbed CO2 Si-CH2-R stretch C-N stretching of alkyl carbamate (SiO)n siloxane stretching vibrations
support Si support Ti adsorbed water supports APS adsorbed water APS adsorbed CO2 APS adsorbed CO2
6, 10 17, 20 19 6 23 19 21 25 6 24
APS adsorbed CO2 support Si
21, 23, 24 25 27
dioxide. Therefore, the resulting adsorbed amounts na are presented in µmol m-2 (Figure 3a). First, the adsorption of carbon dioxide will be shortly discussed for the support materials because the adsorption of CO2 on mesoporous titania materials in comparison to silica was already presented elsewhere.13 The Ti support adsorbs significantly more carbon dioxide per surface area (∼3 times more) then the Si support (Figure 3a). As the comparison is done per surface area, it can be concluded that significantly more attractive adsorption sites exist on the titania surface. However, this difference may be also explained from an energetic point
ref
of view, thus in correlation with the enthalpies of adsorption (Figure 3b). Carbon dioxide only physisorbs on silica materials, which is reflected in a low enthalpy of adsorption (about -25 kJ mol-1) at low coverage. On the other hand, the interaction between carbon dioxide and titania surfaces is much stronger (around -100 kJ mol-1) and corresponds to chemisorption at low coverage. These different strengths of interaction with carbon dioxide can be related to the different chemical nature of the oxides. First, the Ti atoms on the surface need to be taken into account, because they can interact with the oxygen atoms of the CO2 molecules. Second, the hydroxyl groups on the titania
Figure 3. Microcalorimetric results for the adsorption of CO2 at 303 K: (a) adsorbed amounts and (b) enthalpies of adsorption.
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Figure 4. Comparison of infrared spectra at 66.7 mbar and ambient temperature for the systems CO2 + Si-APS, CO2 + Si, and He + Si-APS: (a) region 3800-2700 cm-1 and (b) region 1800-1200 cm-1.
surface have a more basic character than silanol groups and thus stronger interact with carbon dioxide, which can be presented as an acid molecule. The CO2 adsorption results for the APS-functionalized samples are also presented in Figure 3. The adsorption isotherms in Figure 3a indicate that the APS-functionalized samples, Si-APS and Ti-APS, adsorbed more carbon dioxide then the parent support materials at low carbon dioxide coverage. The increasing amount of adsorbed carbon dioxide is related to the acid-base interaction between the CO2 molecules and the amine sites, which favor the adsorption. It is very interesting that the Ti-APS sample adsorbs significantly more carbon dioxide then the Si-APS sample, although a similar quantity of amine sites per surface area was present on both materials. As discussed before, the interaction of carbon dioxide with titania is much stronger then with silica. Therefore, in case of the Ti-APS sample, the carbon dioxide can interact not only with the amine sites but also with other surface sites, such as Ti cations or hydroxyl groups from the support. The enthalpies of adsorption of the APS functionalized oxides are presented in Figure 3b. The enthalpy values for the APSgrafted materials start at very high values (higher than -80 kJ mol-1) and decrease with increasing carbon dioxide loading to about -25 to -30 kJ mol-1. This indicates an energetically heterogeneous distribution of adsorption sites for carbon dioxide. The initial enthalpy values at low coverage of carbon dioxide correlate well with published data12 and can be attributed to the interaction of carbon dioxide with present amine sites. For example, Satyapal et al.28 measured the energy of adsorption for carbon dioxide on solid amine beads (polyethyleneimine bonded to polymethyl methacrylate) by isothermal flow microcalorimetry. The energy of adsorption was calculated to be -94 kJ mol-1, which is consistent with results anticipated for amine and CO2 reactions. Arcis et al.29 reported an enthalpy of solution of carbon dioxide in aqueous solution of 2-amino-2-methylpropanol to be about -70 to -80 kJ mol-1. Knowles et al.30 calculated the energies of adsorption of carbon dioxide for aminosilane-functionalized silicas between -50 and -60 kJ mol-1 under anhydrous conditions, and under wet conditions the enthalpies were between -54 and -52 kJ mol-1. Zukal et al.31 reported an isosteric heat of CO2 adsorption of -65 kJ mol-1 on APS-functionalized SBA-15. However, these reported results originate from calculations and indirect measurements, whereas here the enthalpies of adsorption have been measured directly using microcalorimetry with increasing carbon dioxide
coverage, which can give information toward carbon dioxide-aminosilane reactivity as a function of coverage. However, the enthalpy values for the APS-functionalized samples are not identical at low coverage. In the case of the amine-functionalized oxides, the high enthalpies of adsorption at low coverage correlate with a strong reactivity between the carbon dioxide and the amine sites. For the amine-grafted titania sample, additional interactions of carbon dioxide with the titania surface can play a role, since strong interactions occur between CO2 molecules and titania surfaces, as was already discussed above. In conclusion, the microcalorimetric measurements indicate that a different adsorption mechanism may take place when carbon dioxide interacts with the amine-functionalized titania sample in comparison to the silica analogue. This is reflected by a different amount of attractive adsorption sites leading to higher adsorbed amounts of CO2 and a modification of the enthalpies of adsorption at low coverage, thus the interaction strengths. Therefore, further investigations using in situ infrared measurements were carried out to get more insight into the reactivity of carbon dioxide toward the APS-functionalized materials. Adsorption of CO2 Studied Using in Situ Infrared Spectroscopy. In the following paragraphs, the results will be presented in several parts. First, the carbon dioxide adsorption will be discussed for the APS-grafted silica sample Si-APS. After that, a closer look will be taken on Ti-APS followed by a comparison to the Si-APS sample. Finally, the regeneration of Si-APS, i.e., the desorption of carbon dioxide from this sample, will be presented. The results for the adsorption of CO2 are presented in the form of difference spectra. This means that the spectrum of the bare sample was subtracted from the spectrum of the sample interacting with carbon dioxide. Thus, negative bands are obtained when a present vibration was modified, and positive bands occur when a vibration is shifted to higher or lower wavenumbers and/or a new bond is formed. Adsorption of CO2 on Si-APS. Preliminarily, different gas-solid systems were investigated with in situ infrared spectroscopy. For example, the carbon dioxide adsorption was performed on the silica support Si and helium was introduced on the APS-grafted silica. The corresponding results are compared to the system CO2 + Si-APS in Figure 4 for difference spectra taken at 66.7 mbar. The comparison of the infrared spectra for the gas-solid combinations leads to the conclusion that the various bands obtained for the CO2 +
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Figure 5. Infrared spectra of the adsorption of carbon dioxide with increasing pressure at ambient temperature on the amine-functionalized silica Si-APS: (a) region between 3550 and 3000 cm-1 and (b) region between 1900 and 1200 cm-1 (microcalorimetric results for Si-APS added).
Si-APS system are only due to carbon dioxide adsorption on amine sites, as these bands were not detected for the two other systems (CO2 + Si and He + Si-APS). Thus, other effects, such as an increasing pressure in the sample cell, can be neglected. For the adsorption of carbon dioxide on pure silica materials, only physisorption takes place, which is consistent with the low enthalpies of adsorption detected with microcalorimetry, shown above. Carbon dioxide is only weakly adsorbed, which was related to electrostatic interactions,32 whereas no bond or carbonate formation with the silica surface takes place.33,34 Therefore, no significant bands were observed in the 1800-1000 cm-1 region of the corresponding infrared spectrum (CO2 + Si) presented in Figure 4. Last, but not least, the difference spectra for the CO2 adsorption systems show several bands in the 3760-3520 cm-1 region. These are combination vibrations of the CO2 gas phase within the cell.35 Additionally, a broad band at about 2340 cm-1 was observed in the spectra (not shown here), which is assigned to a stretching vibration of gaseous carbon dioxide, too.35 In order to investigate the interaction between carbon dioxide and Si-APS in more detail, in situ infrared spectra were taken with increasing pressures of CO2, and corresponding results are presented in Figure 5. Negative bands (3370 and 3307 cm-1) are observed in the region of the N-H stretching vibrations (Figure 5a) corresponding to the symmetric and asymmetric stretching vibration of the amine site in the grafted APS molecules. Their intensities decrease with increasing CO2 pressure (Figure 5a), indicating a significant modification of the amine sites in the presence of carbon dioxide. Furthermore, a new NH-stretching vibration band shifted to higher wavenumbers (3435 cm-1) is formed. The latter correspond to a formed molecular structure that will be discussed below. In the 1900-1200 cm-1 region, several other infrared bands appeared whose intensity increased with increasing carbon dioxide pressure (Figure 5). They can be used to identify the molecular species that are formed between the amine groups and the carbon dioxide. Here, three different band combinations (noted A, B, and C) have been identified in the spectra. This assignment was done studying both the different evolution of the band intensities with increasing pressure and the published literature. A comparison with literature is complicated, as measurements were often performed in different conditions regarding humidity,
SCHEME 1: Molecular Structures of the Reactants and Determined Products for the Reaction CO2 + Si-APS: (A) Carbamate and (B) Carbamic Acid
gas flow, etc. and no consensus is found on band assignments. For example, several papers report that the adsorption of carbon dioxide on aminosilane-functionalized silica materials leads to the formation of carbamate but also carbonate species.5-11 The latter can be formed in the presence of water or silanol groups. As the in situ infrared study here was carried out under anhydrous conditions in a closed cell, the formation of carbonate species due to the presence of water can thus be neglected. Furthermore, the interaction between the carbon dioxide and the silica is weak (as shown and discussed before in the case of Figure 4); thus, the silanol groups which are present on the surface do not tend to form carbonate species. A clear assignment can be done for the band combination A, which is correlated with the presence of a carbamate structure (scheme 1) after carbon dioxide adsorption on the APS-grafted silica. The carbamate structure is formed when one carbon dioxide is interacting with two amine sites. In the infrared spectra, the carbamate is evidenced by bands corresponding to the N-H stretching (3435 cm-1),6,36,37 the N-H bending (1626 cm-1),37-39 the COO- asymmetric stretching vibration (∼1545 cm-1),25,36,37,40 and the NH3+ bending (∼1487 cm-1).37,39 The band combination B can be attributed to the presence of a carbamic acid structure (Scheme 1). It was reported that the
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Figure 6. Infrared spectra for the adsorption of carbon dioxide at 66.7 mbar and ambient temperature, in the region between 2000 and 1100 cm-1: (a) comparison of Ti and Ti-APS and (b) comparison of Ti-APS and Si-APS.
Figure 7. Infrared spectra for the desorption of CO2 with increasing temperature (vacuum ∼10-7 mbar) on the APS-functionalized silica Si-APS.
formation of carbamic acid is difficult to observe because of the instability of this molecule.38 However, carbamic acid was reported to be stabilized when attached to long organic chains38 or by the formation of dimer or tetramer structures.25 In this work, the infrared band attributed to the CdO stretching (1680 cm-1)25,37 of the carboxyl group was observed with increasing intensity in correlation with increasing carbon dioxide pressure. A clear observation of the O-H stretching and O-H bending vibration corresponding to an acidic structure is not possible in this work as the O-H stretching vibration at 3140 cm-1 was reported to appear as a broad band.37 The corresponding O-H bending may be related with the peak at 1378 cm-1 overlaying with the symmetric stretching vibration of COO- from the carbamate structure. The assignment of the band combination C (at 1430 and 1330 cm-1) is difficult. First, the presence of organic chains in the grafted aminopropylsilane molecules has to be considered. In the ATR spectra (Figure 2) C-H bending vibrations were detected at 1470 and 1445 cm-1. If a modification induced by the interaction with carbon dioxide would take place, a shift to lower wavenumbers may be possible. On the other side, it may be possible to attribute these bands to the presence of a socalled zwitterionic structure, which is known to be an intermediate step in the formation of carbamic acid and carbamates.41 Finally, the band combination C may be also discussed in connection with weakly adsorbed carbon dioxide molecules.10
However, a clear assignment of the two bands is not possible here, and it would be interesting to further investigate this problem. In conclusion, for the in situ infrared investigations for carbon dioxide adsorption on the APS-functionalized silica Si-APS, it can be stated that three different band combinations were observed, resulting from the carbon dioxide-amine site reactivity. The three combinations represent the carbamate, the carbamate acid, and a third component, C, not clearly identified up to this point. With respect to the microcalorimetry results (see small enthalpy graph in Figure 5b), it can be summarized that the high enthalpies of adsorption at low coverage may be related to the formation of a chemical bond between the nitrogen of the amine site and the carbon of the carbon dioxide, resulting in carbamate and carbamic acid structures. However, all three band combinations are present for the different carbon dioxide pressures applied, indicating a coexistence of the adsorption modes. In case of the carbamic acid, it can be noted that the corresponding peak at 1680 cm-1 seems to increase faster with increasing pressure in comparison to the band at 1626 cm-1 corresponding to the carbamate. This correlates well with published data25 in which it was discussed that under amine excess (thus low pressure of CO2) the carbamate formation is dominant, whereas at carbon dioxide excess (high CO2 pressure), the formation of carbamic acid is increased. Adsorption of CO2 on Ti-APS. The APS-functionalized titania sample (Ti-APS) was also investigated for the adsorption of carbon dioxide with in situ infrared spectroscopy. However, the adsorption of carbon dioxide on the pure Ti sample will be discussed first. The corresponding infrared difference spectrum recorded at 66.7 mbar is presented in Figure 6a, showing that several infrared bands are already observed when carbon dioxide is adsorbed on titania surfaces. In fact, the carbon dioxide adsorption on titania surfaces was studied both by experimental18,19,42-45 and theoretical46-49 methods. It was shown that various adsorption modes can be formed, ranging from linearly physisorbed carbon dioxide to the formation of carbonate CO3and bicarbonate HCO3- structures. Two adsorption modes were often reported in the literature in relation with infrared measurements for carbon dioxide adsorption on titania surfaces: the bicarbonate19,42 and the bidentate carbonate.19,44 The latter, for example, is characterized by bands at 1673 and 1245 cm-1.19 Though a strong interaction between carbon dioxide and titania surfaces exists, they have to be taken into account when
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TABLE 3: Infrared Bands Observed for the Adsorption of Carbon Dioxide on Si-APS wavenumber (cm-1)
assignment
band combination
ref
3435 1680 1626 1545 1487 1430 1378
N-H stretching vibration of carbamate ν(CdO) for carbamic acid ν(N-H) of NH in carbamate νas(COO-) of carbamate δ(N-H) of NH3+ in carbamate not identified νs(COO-) of adsorbed carbon dioxide and/or δ(O-H) of carbamic acid not identified
A B A,C A A C A,B
6, 36, 37 25, 37 37-39 25, 36, 37, 40 37, 39 25, 37
C
-
1330
SCHEME 2: Proposition for Molecular Structure of Bidentate Carbonate Adsorption Modes on APS-Functionalized Materials Si-APS and Ti-APS and on Titania Ti
interpreting the in situ infrared results for the adsorption of carbon dioxide on the amine-functionalized Ti-APS sample. In Figure 6a, the infrared difference spectra of the Ti-APS sample is also presented. A comparison of the detected infrared bands for Ti-APS and Ti suggests that it is possible that some carbonate species are also formed on the Ti-APS sample. For example, the two bands at 1691 and 1265 cm-1 can be correlated with the bidentate carbonate structure observed for the Ti sample at 1673 and 1245 cm-1 in our experiments. If carbonate species are formed, an interaction of carbon dioxide with surface hydroxyl groups needs to be assumed, since the in situ infrared investigations were carried out under anhydrous conditions. Thus, it may be proposed, in the case of the Ti-APS sample, that the carbon dioxide can adsorb on surface hydroxyl groups, forming a bidentate carbonate structure. The shift of the bands to higher wavenumbers for the Ti-APS sample may be promoted by the presence of the APS molecules creating a different molecular environment for the carbonate species. The difference spectra obtained after carbon dioxide adsorption are presented in Figure 6b for the aminopropylsilanefunctionalized silica and titania. In comparison of the two spectra, a similar distribution of vibration bands exists, except the appearance of the bands at 1691 and 1265 cm-1 in case of the aminosilane-grafted titania, which were assigned to a bidentate carbonate before. Additionally, the distribution of the intensity ratios of the various bands is different. The notations A, B, and C correspond to the determined band combinations in the case of carbon dioxide adsorption on the APS-grafted silica, the carbamate structure, the carbamic acid, and the third component (which was not clearly identified), respectively. In case of the Ti-APS sample, the presence of the carbamate species A is not clearly observed. However, the formation of carbamic acid (B) and the species C is more evident when carbon dioxide is adsorbed on this sample. In conclusion, it can be summarized that carbon dioxide adsorbs on Ti-APS with a formation of carbamic acid and bidentate carbonate with the participation of present surface hydroxyl groups from the titania support. Thus, two reactivities toward carbon dioxide exist next to each other: one induced by the interaction with the titania surface and one induced by the interaction with the amine sites. This is interesting, since this behavior was not observed for the
Si-APS sample discussed above. Remembering the microcalorimetric results (Figure 3), a different reactivity was already indicated by comparing both APS-functionalized materials, and this difference is thus confirmed by this infrared study. Desorption of CO2 from Si-APS. One important investigation concerning the properties of an adsorbent is the possibility of its regeneration. In the literature it was already shown that carbon dioxide can be desorbed from aminosilane-functionalized silicas by a slight heat treatment up to 373-423 K.5,6 Carbon dioxide was adsorbed on the sample Si-APS. After that, desorption was started at ambient temperature by pumping. The corresponding spectrum can be seen in Figure 7. The spectrum at ambient temperature indicates that carbon dioxide was not completely desorbed, as vibration bands assigned to adsorbed carbon dioxide were still present. Then, the temperature was increased in several steps, and corresponding spectra were recorded. As can be seen in Figure 7, with increasing temperature, the intensities of the detected bands decreased, showing that more carbon dioxide molecules were desorbed. The temperature was only increased to about 323 K due to experimental restrictions. Thus, under these conditions the carbon dioxide was not completely desorbed. Furthermore, two bands appeared in the spectra (1733 and 1278 cm-1), indicating that a new species is formed during the desorption process. They can be attributed to the bidentate carbonate species, as determined for the adsorption of carbon dioxide on the Ti-APS and Ti samples. This indicates that the surface chemistry of Si-APS was modified and surface sites (surface silanol groups or amine sites) were activated due to the increasing temperature influencing and modifying the interaction with carbon dioxide. However, to our knowledge, this species was not reported before for the adsorption/desorption of carbon dioxide on aminosilane-grafted samples. It is interesting that in case of the adsorption of CO2 on Ti-APS and desorption on Si-APS the formation of a bidentate carbonate species was detected. The corresponding wavenumbers of the bands are listed in Table 4 in comparison to the titania sample. The occurring blue-shift in the wavenumbers from the sample Ti to Ti-APS and then to Si-APS should be due to the different surface chemistry. The proposed molecular structures for the three bidentate adsorption modes are presented
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Kno¨fel et al.
TABLE 4: Stretching Vibrations of the Bidentate Carbonate Structure Formed on Different Samples -1
ν(CdO)/cm ν(COO)/cm-1
Si-APS
Ti-APS
Ti
1733 1278
1691 1265
1673 1245
in Scheme 2. In the adsorption modes proposed for the aminopropylsilane-grafted materials, the carbonate structure may be stabilized by interaction with a NH3+ cation. Indeed, the bending vibration δ(NH3+) was reported before to appear at 1487 cm-1 for the APS-grafted silica and titania sample, respectively. Conclusions The modification of mesoporous silica and titania materials with aminopropylsilane was presented. The resulting samples were characterized with ATR-infrared spectroscopy, thermal gravimetric analysis, and nitrogen sorption. Similar amounts of aminopropylsilane molecules, and thus a similar number of amine sites, were grafted per nm2 on the silica and titania support, simplifying the comparison of the materials for their carbon dioxide adsorption properties. The adsorption of carbon dioxide was investigated using microcalorimetry combined with in situ infrared spectroscopy. In case of the silica support, weak interactions (-25 kJ mol-1) in the physisorption range are obtained, correlating well with the infrared results, where no significant adsorption bands were observed. The titania support is characterized with a much stronger interaction with carbon dioxide. High enthalpies of adsorption (-100 kJ mol-1) at low coverage indicate chemisorption. In situ infrared investigations show the formation of various carbonate species. It is discussed that CO2 can interact with Ti cations but also with OH groups of the titania surface, while silica does not show such kind of specific interactions. The functionalization with APS led to an increase of the adsorbed amounts of carbon dioxide at low coverage in comparison to the support materials. This is related to the strong interaction of carbon dioxide with the introduced amine sites, which is also reflected in high enthalpies of adsorption (higher than -80 kJ mol-1) at low coverage. A significantly greater amount of carbon dioxide was adsorbed on the APS-grafted titania sample. We have shown that this is promoted by the additional interaction of CO2 with the titania surface next to the interaction with the amine sites. This effect was not observed for the APS-grafted silica sample. In situ infrared investigations confirm these observations. On the APS-functionalized silica, carbamate and carbamate acid species are formed due to carbon dioxide adsorption. For the APS-grafted titania sample, a bidentate carbonate species was observed, indicating the interaction with surface hydroxyl groups on the titania surface. The desorption of carbon dioxide from the APS-grafted silica using in situ infrared spectroscopy was also investigated. We have seen the formation of two new bands. The latter are proposed to correspond to a bidentate carbonate species in comparison to the adsorption results obtained for the two titania materials. Acknowledgment. C.K. thanks the INSIDE-PORES Network of Excellence (FP6-2004-NOE-500895-2) for financial support. References and Notes (1) Kurz, F.; Rumpf, B.; Sing, R.; Maurer, G. Ind. Eng. Chem. Res. 1996, 35, 3795. (2) Harlick, P. J. E.; Sayari, A. Ind. Eng. Chem. Res. 2006, 45, 3248.
(3) Kohl, A.; Nielsen, R. Gas Purification; Gulf Publishing Co.: Houston, TX, 1997. (4) (a) Veawab, A.; Tontiwachwuthikul, P.; Chakma, A. Ind. Eng. Chem. Res. 1999, 38, 3917. (b) Hook, R. J. Ind. Eng. Chem. Res. 1997, 36, 1779. (5) Leal, O.; Bolı´var, C.; Ovalles, C.; Jose´ Garcı´a, J.; Espidel, Y. Inorg. Chim. Acta 1995, 240, 183. (6) Hiyoshi, N.; Yogo, K.; Yashima, T. Microporous Mesoporous Mater. 2005, 84, 357. (7) Chang, A. C. C.; Chuang, S. S. C.; Gray, M.; Soong, Y. Energy Fuels 2003, 17, 468. (8) Srivastava, R.; Srinivas, D.; Ratnasamy, P. Microporous Mesoporous Mater. 2006, 90, 314. (9) Srivastava, R.; Srinivas, D.; Ratnasamy, P. J. Catal. 2005, 233, 1. (10) Huang, H. Y.; Yang, R. T. Ind. Eng. Chem. Res. 2003, 42, 2427. (11) Khatri, R. A.; Chuang, S. S. C.; Soong, Y.; Gray, M. Energy Fuels 2006, 20, 1514. (12) Kno¨fel, C.; Descarpentries, J.; Benzaouia, A.; Zelenˇa´k, V.; Mornet, S.; Llewellyn, P. L.; Hornebecq, V. Microporous Mesoporous Mater. 2007, 99, 79. (13) Kno¨fel, C.; Hornebecq, V.; Llewellyn, P. L. Langmuir 2008, 24 (15), 7963. (14) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouque´rol, J.; Siemieniewska, T. IUPAC 1985, 57 (4), 603. (15) Llewellyn, P. L.; Maurin, G. C. R. Chim. 2005, 8, 283. (16) Manca, C.; Roubin, P.; Martin, C. Chem. Phys. Lett. 2000, 330, 21. (17) Manca, C.; Martin, C.; Allouche, A.; Roubin, P. J. Phys. Chem. B 2001, 105, 12861. (18) Auroux, A.; Gervasini, A. J. Phys. Chem. 1990, 94, 6371. (19) Tanaka, K.; White, J. M. J. Phys. Chem. 1982, 86, 4708. (20) Mohamed, M. M.; Bayoumy, W. A.; Khairy, M.; Mousa, M. A. Microporous Mesoporous Mater. 2007, 103, 174. (21) White, L. D.; Tripp, C. P. J. Colloid Interface Sci. 2000, 232, 400. (22) Chaffee, A. L. Fuel Process. Technol. 2005, 86, 1473. (23) Jiu, J.; Kurumada, K.; Wang, F.; Pei, L. Mater. Chem. Phys. 2004, 86, 435. (24) Yiu, H. H. P.; Wright, P. A.; Botting, N. P. J. Mol. Catal. B: Enzym. 2001, 15, 81. (25) Bossa, J.-B.; Borget, F.; Duvernay, F.; Theule´, P.; Chiavassa, T. J. Phys. Chem. A 2008, 112, 5113. (26) Jamro´z, M. H.; Dobrowolski, J.Cz.; Borowiak, M. A. Vib. Spectrosc. 2000, 22, 157. (27) Ojeda, F.; Montero, I.; Abel, F.; Albella, J. M. Chem. Mater. 2001, 13, 3986. (28) Satyapal, S.; Filburn, T.; Trela, J.; Strange, J. Energy Fuels 2001, 15, 250. (29) Arcis, H.; Rodier, L.; Coxam, J.-Y. J. Chem. Thermodyn. 2007, 39, 878. (30) Knowles, G. P.; Graham, J. V.; Delaney, S. W.; Chaffee, A. L. Fuel Process. Technol. 2005, 86, 1435. (31) Zukal, A.; Dominguez, I.; Mayerova´, J.; E`ejka, J. Langmuir 2009, 25 (17), 10314. (32) (a) He, Y.; Seaton, N. A. Langmuir 2003, 19, 10132. (b) He, Y.; Seaton, N. A. Langmuir 2006, 22, 1150. (33) Goodman, A. L. Energy Fuels 2009, 23, 1101. (34) Ueno, A.; Bennett, C. O. J. Catal. 1978, 54, 31. (35) http://webbook.nist.gov/chemistry, site accessed 14 Sep, 2007. (36) Aresta, M.; Quaranta, E. Tedrahedron 1992, 48, 1515. (37) Bossa, J.-B.; Theule´, P.; Duvernay, F.; Borget, F.; Chiavassa, T. Astron. Astrophys. 2008, 492, 719. (38) Belli Dell Amico, D.; Calderazzo, F.; Labella, L.; Marchetti, F.; Pampaloni, G. Chem. ReV. 2003, 103, 3857. (39) Frasco, D. L. J. Chem. Phys. 1964, 41 (7), 2134. (40) Aresta, M.; Ballivet-Tkatchenko, D.; Belli Dell Amico, D.; Bonnet, M. C.; Boschi, D.; Calderazzo, F.; Faure, R.; Labella, L.; Marchetti, F. Chem. Commun. 2000, 1099. (41) Caplow, M. J. Am. Chem. Soc. 1968, 90, 6795. (42) Rethwisch, D. G.; Dumesic, J. A. Langmuir 1986, 2, 73. (43) Raupp, G. B.; Dumesic, J. A. J. Phys. Chem. 1985, 89, 5240. (44) Liao, L.-F.; Lien, C.-F.; Shieh, D.-L.; Chen, M.-T.; Lin, J.-L. J. Phys. Chem. B 2002, 106, 11240. (45) Morterra, C.; Chiorizo, A.; Boccuzzi, F. Z. Phys. Chem. 1981, 124, 211. (46) Markovits, A.; Fahmi, A.; Minot, C. J. Mol. Struct. (THEOCHEM) 1996, 371, 219. (47) Takahashi, H.; Yuki, K.; Nitta, T. Fluid Phase Equilib. 2002, 194197, 153. (48) Funk, S.; Burghaus, U. Phys. Chem. Chem. Phys. 2006, 8, 4805. (49) Takaba, H.; Katagiri, M.; Kubo, M.; Vetriveli, R.; Broclawik, E.; Miyamoto, A. Energy ConVers. Manage. 1995, 36 (6-9), 439.
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