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Adsorption of octamethylcyclotetrasiloxane (D4) on silica gel (SG): Retention mechanism
Microporous and Mesoporous Materials 213 (2015) 118e124
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Adsorption of octamethylcyclotetrasiloxane (D4) on silica gel (SG): Retention mechanism a Sigot, Gae €lle Ducom*, Patrick Germain Le Universit e de Lyon, INSA-Lyon, LGCIE-DEEP, 20 Avenue Albert Einstein, F-69621 Villeurbanne, France
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 February 2015 Received in revised form 31 March 2015 Accepted 10 April 2015 Available online 18 April 2015
Silica gel (SG) is an efficient adsorbent for the removal of octamethylcyclotetrasiloxane (D4) e a volatile organic silicon compound present in biogas. Complementary physicochemical characterizations of SG before and after D4 adsorption were performed: specific surface area, porosity, pH measurements, infrared spectroscopy, thermodesorption, thermogravimetry and differential scanning calorimetry. A retention mechanism, based on a combination of hydrogen bonding (at low uptake) and polymerization (at higher uptake) is proposed. This mechanism is able to explain the difficulties of siloxane thermal desorption. © 2015 Elsevier Inc. All rights reserved.
Keywords: Silica gel Siloxane Biogas Adsorption mechanism Polymerization
1. Introduction Biogas is a promising source of energy. However, its use in energy conversion devices is hampered by the presence of trace compounds such as volatile organic silicon compounds (VOSiCs) including siloxanes [1e5]. In biogas, linear siloxanes hexamethyldisiloxane (L2), octamethyltrisiloxane (L3), decamethyltetrasiloxane (L4), dodecamethylpentasiloxane (L5), cyclic siloxanes hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), dodecamethylcyclohexasiloxane (D6), and trimethylsilanol (TMSol) are mainly found. Adsorption is a widely used technology for the removal of these contaminants [6e13]. In a previous study [14], D4 was chosen as a representative molecule of VOSiCs to perform lab-scale dynamic adsorption experiments. Three adsorbents were compared for the removal of D4: an activated carbon, a zeolite and a silica gel (SG). SG proved to be the most efficient adsorbent for D4 removal, with an adsorption capacity in the order of 250 mgD4/gSG at saturation. However, it was shown that poor thermal regeneration was achieved by heating at 300 C: adsorption capacity was reduced by
* Corresponding author. Tel.: þ33 4 72 43 81 94. E-mail addresses: [email protected] (L. Sigot), [email protected] (G. Ducom), [email protected] (P. Germain). http://dx.doi.org/10.1016/j.micromeso.2015.04.016 1387-1811/© 2015 Elsevier Inc. All rights reserved.
90%. The present paper focuses on understanding the main reasons for the very limited desorption. Similar observations were reported for D3 on different adsorbents [8,10] while desorption efficiencies greater than 95% were obtained by Schweigkofler and Niessner [7] for both L2 and D5 adsorbed on silica gel. One way to explain the difficulty of regeneration is to identify the chemical nature of the adsorbate retained in SG and the related retention mechanism. This study is mainly based on complementary physicochemical characterizations of the SG solid matrix before and after D4 adsorption. Using this approach, obtained results help to converge to a conclusion. 2. Material and methods: SG characterization 2.1. Materials: SG and D4 Silica gel is an inorganic polymer (SiO2, n H2O) made up of a 3D tetrahedral structure with silanol (SieOH) groups at the surface [15,16]. The silica gel used in this study was 1e3 mm grain size Chameleon® from VWR BDH Prolabo. It was characterized before and after D4 adsorption. The dynamic adsorption tests were described before [14]. They were performed with a synthetic polluted gas e nitrogen containing 30 ppmv D4 e prepared using liquid D4 with a purity of 98% (Chimie Plus). D4 molar mass is 296.6 g/mol. The average quantity of adsorbed D4 in the
L. Sigot et al. / Microporous and Mesoporous Materials 213 (2015) 118e124
characterized sample was 222 ± 12 mgD4/gSG, corresponding to 182 ± 10 mgD4/gSGþD4 (if expressed relatively to the mass of adsorbent after adsorption). In the following, the notation “SG” refers to silica gel before adsorption (fresh SG) and the notation “SG þ D4” refers to silica gel after D4 adsorption (spent SG).
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3. Results: adsorption mechanism As mentioned in the introduction, the objective was to understand the interactions between SG and D4 and identify the physicochemical changes of the adsorbent and the adsorbate. 3.1. BET surface
2.2. Specific surface area and porosity Surface area, micropore volume and pore size measurements were performed by nitrogen adsorption (at 77 K) in an ASAP 2010 (Micromeritics) instrument. SG samples were first outgassed under vacuum at 300 C. Specific surface area and porosity were determined before and after D4 adsorption. The micro- and mesopore size distribution was determined by the density functional theory (DFT). 2.3. pH of adsorbent leachate The pH of SG leachate was determined according to the EN 12457-2 standard for leaching tests of granular waste material [17]. A liquid to solid ratio of 10 L/kg was used. 200 mL of demineralized water were added to a sample of about 20 g of adsorbent and the mixture was stirred for 24 h to reach equilibrium. The suspension was filtered (<0.45 mm) under vacuum and the pH of the filtrate was measured. 2.4. Fourier transform infrared spectroscopy (FTIR) SG ground samples (<1 mm) before and after D4 adsorption were analyzed by FTIR in attenuated total reflectance (ATR) mode. FTIR analyses allow the characterization of the chemical bonds present in the material thanks to the measurement of their typical vibration frequency. The spectrum of liquid D4 was also obtained. Absorbance spectra were acquired between 500 and 4000 cm1 with a resolution of 4 cm1. OMNIC software (Nicolet instrument) was used for data acquisition and post-treatment. A qualitative and comparative approach was preferred instead of a precise identification of the absorption bands. 2.5. Thermodesorption Desorption tests in inert flow (He) were conducted at 350 C for 30 min after D4 adsorption with approximately 90 mg of sample (He flow rate: 38 mL/min). The desorbed gas was collected in a gas sampling bag. The gas sample compounds were then preconcentrated on a sorbent tube. VOSiC analysis was finally performed by flash thermal desorption followed by gas chromatography coupled with mass spectrometry (TD-GC-MS) according to the NF EN ISO 16017 standard [18]. Searched VOSiCs were: L2, L3, L4, L5, D3, D4, D5, D6 and TMSol. 2.6. Thermogravimetry and differential scanning calorimetry (TGDSC) Using a Labsys 1600 Setaram thermal analyzer, thermogravimetry (TG) and differential scanning calorimetry (DSC) were performed on SG before and after D4 adsorption in either nitrogen or air atmosphere. This apparatus, equipped with a thermobalance, recorded, as a function of temperature, both the weight sample evolution (TG) and the heat flow variation between the sample and a reference material (DSC). Samples of approximately 20 mg were introduced in an alumina crucible and heated from 25 C to 850 C at a rate of 10 C/min.
The BET surface area of the studied SG is 690 m2/g and its micropore volume is 0.39 cm3/g (Table 1, sample “SG”). These values are in the range of literature data [15,16]. The pore network consists of two ranges of porosity: a first peak at about 8 Å and a second larger peak from 13 to 50 Å. The approximate size of D4 molecule is 10 Å (molecular cross-sectional size of 1.08 1.03 nm [19]). Due to the presence of pores with diameters larger than 10 Å, the adsorption of D4 into SG pores is physically possible. The DFT method indicates that the specific surface area for pore diameters over 10 Å is about 540 m2/g. After D4 adsorption (sample “SG þ D4”), a substantial reduction (>95%) of the BET surface area was noticed (Table 1). The BET surface area of SG þ D4 was approximately 30 m2/g. The pore diameter distribution could not be determined with the methods used. The pore network initially accessible to D4 seemed to be totally blocked by the adsorbate. This blocking of all accessible adsorption sites suggests the formation of a layer at the SG surface. Assuming that D4 is a spherical molecule with a diameter of 10 Å, the minimal area occupied by a D4 molecule is 78 Å2 (projected area). The D4 adsorption capacity (222 mgD4/gSG at saturation) corresponds then to an occupied area of about 350 m2/gSG. But there are dead areas between the projected discs due to the repulsion forces between the adsorbed molecules. Suppose the specific surface area for pore diameters over 10 Å (540 m2/g) is entirely covered, it corresponds to a 1e2 Å distance between two molecules. This order of magnitude seems reasonable. These calculations are compatible with the hypothesis of a uniform layer at the SG surface. 3.2. pH The pH of SG leachate is 2.5. In the presence of water, these acidic conditions can promote D4 ring opening and polymerization by condensation of the formed silanols [20]. 3.3. FTIR Fig. 1 and Fig. 2 show the spectra of SG before and after D4 adsorption in the range 2500e3900 cm1 and 500e1400 cm1 respectively. The spectrum of liquid D4 is also given for comparison. The global SG spectrum is similar to the ones presented by Montanari et al. [10] and Gun'ko et al. [21]. 3.3.1. OH vibrations After D4 adsorption, the large bands at 2700e3700 cm1 (Fig. 1) and 1570e1700 cm1 (not shown) assigned to OH bond
Table 1 Specific surface area, micropore volume and pore size of SG before and after D4 adsorption. Sample
BET surface area (m2/g)
Micropore volume (cm3/g)
Main pore diameter (Å)
SG SG þ D4
690 30
0.39 0.01a
8 and 13e50 N/A
N/A: Not applicable. a Data close to measurement limit.
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Fig. 1. FTIR spectra of SG, SG þ D4 and D4 (liquid) between 2500 and 3900 cm1.
vibrations (stretching and bending respectively) at SG surface are less intense. This indicates that free OH bonds (SieOH) at SG surface are less numerous. This suggests the formation of hydrogen bonds between the adsorbate and the silanol groups at SG surface. Similar conclusions have been drawn by Montanari et al. [10]. 3.3.2. Adsorbate signature The comparison of SG and SG þ D4 spectra shows the appearance, modification or intensification of various peaks, especially at 2960 cm1, 2900 cm1 (Fig. 1), 1260 cm1, 1130 cm1 (shoulder), 1050 cm1, 980 cm1 (shoulder) and 800 cm1 (Fig. 2). A literature review allowed the identification of the main vibrations observed on each spectrum (Table 2). SG þ D4 spectrum shows the fingerprint of the siloxane backbone (SieOeSi). Moreover, new vibrations are observed (Table 2) and are characteristic of methyl group bonds originating from D4. However, this signature is at the same time characteristic of molecular D4 and of any linear or cyclic oligomeric siloxane and polydimethylsiloxane (PDMS). Nevertheless, the comparison of the subtraction spectrum (SG þ D4) e SG with D4 spectrum suggests that the adsorbate retained in SG is not molecular D4 (Fig. 3). The bands in the range
850e1100 cm1 are significantly different. The subtraction spectrum shape is more similar to the one reported by Finocchio et al. [8] for a PDMS obtained by D3 ring opening polymerization. Indeed, both spectra present two peaks around 1000 and 1100 cm1 and a peak at about 850 cm1 which are not visible on D4 spectrum. Thus, FTIR results lend weight to the hypothesis of D4 alteration and polymerization. 3.4. Thermodesorption Thermodesorption was performed at 350 C in helium after D4 adsorption. Results of the VOSiC analysis are given in Table 3. Only cyclic siloxanes (D3, D4 and D5) were desorbed, and desorbed quantities were very small compared to the quantity of D4 adsorbed in the sample (182 mgD4/gSGþD4): about one thousandth. The desorption temperature (350 C) seems to be insufficient to enable the adsorbate desorption. However, D4 boiling temperature is 175 C and, at 350 C, D4 should have evaporated and then been removed. As a consequence, the adsorbate is probably not retained as molecular D4, but possibly as higher molecular weight species with higher boiling points or degradation temperatures. This suggests that polymerization occurred. This is in accordance with literature which indicates that PDMS thermally decompose to cyclic
Fig. 2. FTIR spectra of SG, SG þ D4 and D4 (liquid) between 500 and 1400 cm1.
L. Sigot et al. / Microporous and Mesoporous Materials 213 (2015) 118e124
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Table 2 Characteristic bonds of SG, SG þ D4 and D4 (liquid) observed in experimental FTIR spectra. Wavenumber (cm1)
2700e3700 2960 2900 1570e1700 1260
Related spectrum
OH stretching CH stretching CH bending OH bending SieC bending CH3eSieO SieOeSi asymmetric stretching CH3 bending SieC2 asymmetric stretchinga SieOeSi symmetric stretching SieC2 symmetric stretching SieOeSi symmetric stretching
1050 800
690 560 and 660 a
Vibration
Reference
SG
SG þ D4
✓
✓ ✓ ✓ ✓ ✓
D4
✓
✓ ✓
✓ ✓
[21,22] [10,23] [10,23] [21,22] [23] [10] [24] [24]
✓
✓
✓ ✓ ✓
[23] [24] [24]
✓
✓ ✓ ✓
Shoulder at 784 cm1 on D4 spectrum.
Fig. 3. Comparison of FTIR subtraction spectrum (SG þ D4) e SG and D4 (liquid) spectrum.
Table 3 VOSiC quantities thermodesorbed from SG þ D4 at 350 C in helium. Compound
Desorbed quantity from SG þ D4 (mgcompound/gSGþD4)
Adsorbed quantity (mgcompound/gSGþD4)
D3 D4 D5
0.13 0.17 0.02
0 182 0
oligomers (especially D3) through intra and inter-chain mechanisms and SieO bond scission over 340e400 C in inert atmosphere [25e27]. Note that among the quantified siloxanes, D4 is actually found (0.17 mgD4/gSGþD4) but D3 is also analyzed at the same concentration level (0.13 mgD3/gSGþD4). 3.5. TG-DSC TG-DSC was performed on SG before and after D4 adsorption in nitrogen and air atmospheres. Fig. 4 shows the TG and DSC profiles obtained in air. Derivative thermogravimetric curve (dTG) is also presented for sample SG þ D4. As concerns SG, in both air and nitrogen atmospheres, TG curve shows a first weight loss associated with an endothermic peak centered at 125 C assigned to water vaporization. From the measured weight loss, the calculated water content of SG is about 50 mg/gSG that matches Ruthven's data [16]. A second weight loss
(constant slope), without any associated thermal effect, is observed between 250 C and 850 C. Costa et al. [28] and Yang [15] assigned it to SG dehydroxylation due to surface silanol groups bonding and condensation. After D4 adsorption (SG þ D4), no additional weight loss or endothermic peak at 175 C (D4 boiling point) is observed whatever the atmosphere (air or nitrogen). Indeed, in the case of adsorption, not only D4eD4 interactions but also D4eSG interactions occur. Consequently, D4 vaporization could be shifted toward higher temperatures. However, even at higher temperature, no endothermic effect is observed. Vaporization seems not to be the main phenomenon. A similar argument can be proposed for other oligomeric siloxanes, which boiling points are in the range 107e245 C. In nitrogen, SG and SG þ D4 profiles are very similar (not shown). But in air (Fig. 4), the DSC profile of SG þ D4 sample shows a broad exothermic peak between 250 C and 750 C (maximum around 620 C) while it is not visible in nitrogen or for SG sample. This peak is associated with a weight loss. So this heat release is attributed to the adsorbate oxidation. Nevertheless, it can be noticed that the maximum of the exothermic peak does not correspond to the inflexion point of the weight loss and that the thermal effect is small at the beginning of the peak. According to the dTG, it is possible that several phenomena are superimposed: SG dehydroxylation, possible vaporization of oligomeric siloxanes or small PDMS (endothermic) and PDMS oxidation (exothermic).
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Fig. 4. Comparison of TG-DSC profiles for SG and SG þ D4 in air.
Supposing the adsorbate is retained as a linear PDMS, equation (1) is the associated oxidation reaction.
C2nþ6 H6nþ18 Onþ1 Sinþ2ðgÞ þ ð4n þ 12ÞO2ðgÞ /ðn þ 2ÞSiO2ðsÞ þ ð2n þ 6ÞCO2ðgÞ þ ð3n þ 9ÞH2 OðgÞ
(1)
Subscripts (g) and (s) correspond to compounds in gas and solid phase respectively. The weight loss associated with the exothermic peak should then be the resultant of the combustion of organic methyl groups (weight loss) and of the oxidation to silica of the inorganic skeleton (SieO) coming from the adsorbate (weight gain). A mass balance can be performed. The calculated weight loss is 35 mg/gSGþD4. The experimental weight loss of SG þ D4 (DmSG þ D4 e DmSG) measured between 250 C and 750 C is 53 mg/gSGþD4. The order of magnitude is similar. The experimental weight loss is higher than expected. This confirms that several phenomena are superimposed. The difference may be due to an incomplete oxidation of the skeleton or to direct vaporization of some oligomeric siloxanes and small PDMS. An energy balance can also be performed. The integration of the SG þ D4 exothermic peak (Fig. 4) indicates a heat release of 2150 J/gSGþD4, that is to say, by dividing by the D4 adsorbed quantity, a heat of combustion of 11.8 kJ/gD4. In the literature, heats of combustion (at 298 K) of several oligomeric siloxanes were found and are reported in Table 4. As a first approximation, the influence of the temperature is neglected. For linear siloxanes, the heat of combustion decreases when the chain length increases, but for linear PDMS, predicted values tend toward a constant value: 26.4e26.8 kJ/g [29,30]. This can be explained by the constant Si/C ratio for long chain PDMS. The experimental heat of combustion, determined by DSC, is inferior to the ones found in the literature. Several explanations can be proposed. The proposed reaction (equation (1)) is a model reaction assuming a complete oxidation of the PDMS. Several endothermic phenomena can superimpose such as bond scissions and volatilization. Moreover, an incomplete oxidation of the SieO skeleton is also possible. 4. Discussion During previous dynamic adsorption tests, D4 was removed by SG from a synthetic polluted gas. Therefore, molecular D4 could be expected in the adsorbent. But the chemistry of silicones (including
Table 4 Standard heats of combustion for siloxanes (298 K). Compound
Heat of combustion (kJ/g)
L2 L3 L4 L5 D4
36.1a 32.9a 30.8a 30.0a 26.8a
a b
e e e e
34.21b 30.74b 29.71b 27.97b
[29]. [30].
oligomeric siloxanes, silanols and PDMS) is very complex. Condensation and hydrolysis reactions, ring opening and polymerization have been reported [20,31e33]. Moreover, the silanol groups on the SG surface are very reactive. Consequently, low molecular weight (linear or cyclic) siloxanes or higher molecular weight PDMS could also occur. A mixture of cyclic oligomers plus a distribution of polymers is another possibility. All the characterizations converged to the conclusion that the adsorbate is not retained as molecular D4 or any other oligomeric siloxane. A simplified retention mechanism is proposed in Fig. 5. At low uptake, hydrogen bonding between SieOeSi bonds of the siloxane skeleton and silanol groups at SG surface is supposed. This interaction is about 10 kJ/mol, namely lower than covalent bonds but stronger than Van der Waals interactions. It is then possible that ring opening occurs and linear siloxanes are formed. The cleavage of the SieO bonds is favored in acidic aqueous conditions which are gathered in SG. At higher uptake, polymerization of short linear chains into PDMS is promoted by the proximity of the molecules. It follows a SG pore obstruction: its whole specific surface area and micropore volume is lost. The formation of non-volatile compounds such as silicone polymers (PDMS) could explain the limited thermal regeneration of SG after D4 adsorption. Consequently, after thermal desorption, the sample cannot be used in another D4 adsorption test. This proposed mechanism matches the observations of the literature. Schweigkofler and Niessner [7] reported an almost complete desorption (>95%) for L2 or D5 at 250 C in nitrogen circulation. This can be explained by the low SG uptake: only 10 mg/gSG versus 222 mg/gSG in our experiment. Montanari et al. [10] also proposed a polymerization phenomenon and revealed that after adsorption of D3 on SG, desorption up to 200 C by purging with nitrogen as well as by vacuum treatment was incomplete. Note that whatever the adsorbent type (activated
L. Sigot et al. / Microporous and Mesoporous Materials 213 (2015) 118e124
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Fig. 5. Schematic representation of D4 adsorption/polymerization on SG.
carbon, impregnated activated carbon, activated carbon fiber cloth), the desorption of siloxanes seems difficult [8,34,35]. 5. Conclusion Characterizations of SG after D4 adsorption led to several key observations: pore blocking, acidic aqueous conditions promoting ring opening, diminution of the quantity of free silanol groups at the SG surface, D4 alteration, non-desorption of the adsorbate in the range 25e800 C in inert atmosphere, oxidation of the adsorbate over 500 C. A retention mechanism based on hydrogen bonding, D4 ring opening and polymerization into PDMS was then proposed. Further experiments could be conducted using other SG samples with different properties (specific surface area for pore diameter > 10 Å or eOH density for instance) to corroborate the conclusions. In the context of biogas purification, this mechanism may be affected by the presence of other kinds of pollutants and humidity. From an operational point of view, it results that thermal regeneration of SG is restricted, but the understanding of this mechanism should facilitate the implementation of efficient regeneration processes [36]. Acknowledgments This study, part of the PILE-EAU-BIOGAZ project, was supported by the French Agence Nationale de la Recherche (ANR), program HPAC 2010.
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Adsorption of octamethylcyclotetrasiloxane (D4) on silica gel (SG): Retention mechanism a Sigot, Gae €lle Ducom*, Patrick Germain Le Universit e de Lyon, INSA-Lyon, LGCIE-DEEP, 20 Avenue Albert Einstein, F-69621 Villeurbanne, France
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 February 2015 Received in revised form 31 March 2015 Accepted 10 April 2015 Available online 18 April 2015
Silica gel (SG) is an efficient adsorbent for the removal of octamethylcyclotetrasiloxane (D4) e a volatile organic silicon compound present in biogas. Complementary physicochemical characterizations of SG before and after D4 adsorption were performed: specific surface area, porosity, pH measurements, infrared spectroscopy, thermodesorption, thermogravimetry and differential scanning calorimetry. A retention mechanism, based on a combination of hydrogen bonding (at low uptake) and polymerization (at higher uptake) is proposed. This mechanism is able to explain the difficulties of siloxane thermal desorption. © 2015 Elsevier Inc. All rights reserved.
Keywords: Silica gel Siloxane Biogas Adsorption mechanism Polymerization
1. Introduction Biogas is a promising source of energy. However, its use in energy conversion devices is hampered by the presence of trace compounds such as volatile organic silicon compounds (VOSiCs) including siloxanes [1e5]. In biogas, linear siloxanes hexamethyldisiloxane (L2), octamethyltrisiloxane (L3), decamethyltetrasiloxane (L4), dodecamethylpentasiloxane (L5), cyclic siloxanes hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), dodecamethylcyclohexasiloxane (D6), and trimethylsilanol (TMSol) are mainly found. Adsorption is a widely used technology for the removal of these contaminants [6e13]. In a previous study [14], D4 was chosen as a representative molecule of VOSiCs to perform lab-scale dynamic adsorption experiments. Three adsorbents were compared for the removal of D4: an activated carbon, a zeolite and a silica gel (SG). SG proved to be the most efficient adsorbent for D4 removal, with an adsorption capacity in the order of 250 mgD4/gSG at saturation. However, it was shown that poor thermal regeneration was achieved by heating at 300 C: adsorption capacity was reduced by
* Corresponding author. Tel.: þ33 4 72 43 81 94. E-mail addresses: [email protected] (L. Sigot), [email protected] (G. Ducom), [email protected] (P. Germain). http://dx.doi.org/10.1016/j.micromeso.2015.04.016 1387-1811/© 2015 Elsevier Inc. All rights reserved.
90%. The present paper focuses on understanding the main reasons for the very limited desorption. Similar observations were reported for D3 on different adsorbents [8,10] while desorption efficiencies greater than 95% were obtained by Schweigkofler and Niessner [7] for both L2 and D5 adsorbed on silica gel. One way to explain the difficulty of regeneration is to identify the chemical nature of the adsorbate retained in SG and the related retention mechanism. This study is mainly based on complementary physicochemical characterizations of the SG solid matrix before and after D4 adsorption. Using this approach, obtained results help to converge to a conclusion. 2. Material and methods: SG characterization 2.1. Materials: SG and D4 Silica gel is an inorganic polymer (SiO2, n H2O) made up of a 3D tetrahedral structure with silanol (SieOH) groups at the surface [15,16]. The silica gel used in this study was 1e3 mm grain size Chameleon® from VWR BDH Prolabo. It was characterized before and after D4 adsorption. The dynamic adsorption tests were described before [14]. They were performed with a synthetic polluted gas e nitrogen containing 30 ppmv D4 e prepared using liquid D4 with a purity of 98% (Chimie Plus). D4 molar mass is 296.6 g/mol. The average quantity of adsorbed D4 in the
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characterized sample was 222 ± 12 mgD4/gSG, corresponding to 182 ± 10 mgD4/gSGþD4 (if expressed relatively to the mass of adsorbent after adsorption). In the following, the notation “SG” refers to silica gel before adsorption (fresh SG) and the notation “SG þ D4” refers to silica gel after D4 adsorption (spent SG).
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3. Results: adsorption mechanism As mentioned in the introduction, the objective was to understand the interactions between SG and D4 and identify the physicochemical changes of the adsorbent and the adsorbate. 3.1. BET surface
2.2. Specific surface area and porosity Surface area, micropore volume and pore size measurements were performed by nitrogen adsorption (at 77 K) in an ASAP 2010 (Micromeritics) instrument. SG samples were first outgassed under vacuum at 300 C. Specific surface area and porosity were determined before and after D4 adsorption. The micro- and mesopore size distribution was determined by the density functional theory (DFT). 2.3. pH of adsorbent leachate The pH of SG leachate was determined according to the EN 12457-2 standard for leaching tests of granular waste material [17]. A liquid to solid ratio of 10 L/kg was used. 200 mL of demineralized water were added to a sample of about 20 g of adsorbent and the mixture was stirred for 24 h to reach equilibrium. The suspension was filtered (<0.45 mm) under vacuum and the pH of the filtrate was measured. 2.4. Fourier transform infrared spectroscopy (FTIR) SG ground samples (<1 mm) before and after D4 adsorption were analyzed by FTIR in attenuated total reflectance (ATR) mode. FTIR analyses allow the characterization of the chemical bonds present in the material thanks to the measurement of their typical vibration frequency. The spectrum of liquid D4 was also obtained. Absorbance spectra were acquired between 500 and 4000 cm1 with a resolution of 4 cm1. OMNIC software (Nicolet instrument) was used for data acquisition and post-treatment. A qualitative and comparative approach was preferred instead of a precise identification of the absorption bands. 2.5. Thermodesorption Desorption tests in inert flow (He) were conducted at 350 C for 30 min after D4 adsorption with approximately 90 mg of sample (He flow rate: 38 mL/min). The desorbed gas was collected in a gas sampling bag. The gas sample compounds were then preconcentrated on a sorbent tube. VOSiC analysis was finally performed by flash thermal desorption followed by gas chromatography coupled with mass spectrometry (TD-GC-MS) according to the NF EN ISO 16017 standard [18]. Searched VOSiCs were: L2, L3, L4, L5, D3, D4, D5, D6 and TMSol. 2.6. Thermogravimetry and differential scanning calorimetry (TGDSC) Using a Labsys 1600 Setaram thermal analyzer, thermogravimetry (TG) and differential scanning calorimetry (DSC) were performed on SG before and after D4 adsorption in either nitrogen or air atmosphere. This apparatus, equipped with a thermobalance, recorded, as a function of temperature, both the weight sample evolution (TG) and the heat flow variation between the sample and a reference material (DSC). Samples of approximately 20 mg were introduced in an alumina crucible and heated from 25 C to 850 C at a rate of 10 C/min.
The BET surface area of the studied SG is 690 m2/g and its micropore volume is 0.39 cm3/g (Table 1, sample “SG”). These values are in the range of literature data [15,16]. The pore network consists of two ranges of porosity: a first peak at about 8 Å and a second larger peak from 13 to 50 Å. The approximate size of D4 molecule is 10 Å (molecular cross-sectional size of 1.08 1.03 nm [19]). Due to the presence of pores with diameters larger than 10 Å, the adsorption of D4 into SG pores is physically possible. The DFT method indicates that the specific surface area for pore diameters over 10 Å is about 540 m2/g. After D4 adsorption (sample “SG þ D4”), a substantial reduction (>95%) of the BET surface area was noticed (Table 1). The BET surface area of SG þ D4 was approximately 30 m2/g. The pore diameter distribution could not be determined with the methods used. The pore network initially accessible to D4 seemed to be totally blocked by the adsorbate. This blocking of all accessible adsorption sites suggests the formation of a layer at the SG surface. Assuming that D4 is a spherical molecule with a diameter of 10 Å, the minimal area occupied by a D4 molecule is 78 Å2 (projected area). The D4 adsorption capacity (222 mgD4/gSG at saturation) corresponds then to an occupied area of about 350 m2/gSG. But there are dead areas between the projected discs due to the repulsion forces between the adsorbed molecules. Suppose the specific surface area for pore diameters over 10 Å (540 m2/g) is entirely covered, it corresponds to a 1e2 Å distance between two molecules. This order of magnitude seems reasonable. These calculations are compatible with the hypothesis of a uniform layer at the SG surface. 3.2. pH The pH of SG leachate is 2.5. In the presence of water, these acidic conditions can promote D4 ring opening and polymerization by condensation of the formed silanols [20]. 3.3. FTIR Fig. 1 and Fig. 2 show the spectra of SG before and after D4 adsorption in the range 2500e3900 cm1 and 500e1400 cm1 respectively. The spectrum of liquid D4 is also given for comparison. The global SG spectrum is similar to the ones presented by Montanari et al. [10] and Gun'ko et al. [21]. 3.3.1. OH vibrations After D4 adsorption, the large bands at 2700e3700 cm1 (Fig. 1) and 1570e1700 cm1 (not shown) assigned to OH bond
Table 1 Specific surface area, micropore volume and pore size of SG before and after D4 adsorption. Sample
BET surface area (m2/g)
Micropore volume (cm3/g)
Main pore diameter (Å)
SG SG þ D4
690 30
0.39 0.01a
8 and 13e50 N/A
N/A: Not applicable. a Data close to measurement limit.
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Fig. 1. FTIR spectra of SG, SG þ D4 and D4 (liquid) between 2500 and 3900 cm1.
vibrations (stretching and bending respectively) at SG surface are less intense. This indicates that free OH bonds (SieOH) at SG surface are less numerous. This suggests the formation of hydrogen bonds between the adsorbate and the silanol groups at SG surface. Similar conclusions have been drawn by Montanari et al. [10]. 3.3.2. Adsorbate signature The comparison of SG and SG þ D4 spectra shows the appearance, modification or intensification of various peaks, especially at 2960 cm1, 2900 cm1 (Fig. 1), 1260 cm1, 1130 cm1 (shoulder), 1050 cm1, 980 cm1 (shoulder) and 800 cm1 (Fig. 2). A literature review allowed the identification of the main vibrations observed on each spectrum (Table 2). SG þ D4 spectrum shows the fingerprint of the siloxane backbone (SieOeSi). Moreover, new vibrations are observed (Table 2) and are characteristic of methyl group bonds originating from D4. However, this signature is at the same time characteristic of molecular D4 and of any linear or cyclic oligomeric siloxane and polydimethylsiloxane (PDMS). Nevertheless, the comparison of the subtraction spectrum (SG þ D4) e SG with D4 spectrum suggests that the adsorbate retained in SG is not molecular D4 (Fig. 3). The bands in the range
850e1100 cm1 are significantly different. The subtraction spectrum shape is more similar to the one reported by Finocchio et al. [8] for a PDMS obtained by D3 ring opening polymerization. Indeed, both spectra present two peaks around 1000 and 1100 cm1 and a peak at about 850 cm1 which are not visible on D4 spectrum. Thus, FTIR results lend weight to the hypothesis of D4 alteration and polymerization. 3.4. Thermodesorption Thermodesorption was performed at 350 C in helium after D4 adsorption. Results of the VOSiC analysis are given in Table 3. Only cyclic siloxanes (D3, D4 and D5) were desorbed, and desorbed quantities were very small compared to the quantity of D4 adsorbed in the sample (182 mgD4/gSGþD4): about one thousandth. The desorption temperature (350 C) seems to be insufficient to enable the adsorbate desorption. However, D4 boiling temperature is 175 C and, at 350 C, D4 should have evaporated and then been removed. As a consequence, the adsorbate is probably not retained as molecular D4, but possibly as higher molecular weight species with higher boiling points or degradation temperatures. This suggests that polymerization occurred. This is in accordance with literature which indicates that PDMS thermally decompose to cyclic
Fig. 2. FTIR spectra of SG, SG þ D4 and D4 (liquid) between 500 and 1400 cm1.
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Table 2 Characteristic bonds of SG, SG þ D4 and D4 (liquid) observed in experimental FTIR spectra. Wavenumber (cm1)
2700e3700 2960 2900 1570e1700 1260
Related spectrum
OH stretching CH stretching CH bending OH bending SieC bending CH3eSieO SieOeSi asymmetric stretching CH3 bending SieC2 asymmetric stretchinga SieOeSi symmetric stretching SieC2 symmetric stretching SieOeSi symmetric stretching
1050 800
690 560 and 660 a
Vibration
Reference
SG
SG þ D4
✓
✓ ✓ ✓ ✓ ✓
D4
✓
✓ ✓
✓ ✓
[21,22] [10,23] [10,23] [21,22] [23] [10] [24] [24]
✓
✓
✓ ✓ ✓
[23] [24] [24]
✓
✓ ✓ ✓
Shoulder at 784 cm1 on D4 spectrum.
Fig. 3. Comparison of FTIR subtraction spectrum (SG þ D4) e SG and D4 (liquid) spectrum.
Table 3 VOSiC quantities thermodesorbed from SG þ D4 at 350 C in helium. Compound
Desorbed quantity from SG þ D4 (mgcompound/gSGþD4)
Adsorbed quantity (mgcompound/gSGþD4)
D3 D4 D5
0.13 0.17 0.02
0 182 0
oligomers (especially D3) through intra and inter-chain mechanisms and SieO bond scission over 340e400 C in inert atmosphere [25e27]. Note that among the quantified siloxanes, D4 is actually found (0.17 mgD4/gSGþD4) but D3 is also analyzed at the same concentration level (0.13 mgD3/gSGþD4). 3.5. TG-DSC TG-DSC was performed on SG before and after D4 adsorption in nitrogen and air atmospheres. Fig. 4 shows the TG and DSC profiles obtained in air. Derivative thermogravimetric curve (dTG) is also presented for sample SG þ D4. As concerns SG, in both air and nitrogen atmospheres, TG curve shows a first weight loss associated with an endothermic peak centered at 125 C assigned to water vaporization. From the measured weight loss, the calculated water content of SG is about 50 mg/gSG that matches Ruthven's data [16]. A second weight loss
(constant slope), without any associated thermal effect, is observed between 250 C and 850 C. Costa et al. [28] and Yang [15] assigned it to SG dehydroxylation due to surface silanol groups bonding and condensation. After D4 adsorption (SG þ D4), no additional weight loss or endothermic peak at 175 C (D4 boiling point) is observed whatever the atmosphere (air or nitrogen). Indeed, in the case of adsorption, not only D4eD4 interactions but also D4eSG interactions occur. Consequently, D4 vaporization could be shifted toward higher temperatures. However, even at higher temperature, no endothermic effect is observed. Vaporization seems not to be the main phenomenon. A similar argument can be proposed for other oligomeric siloxanes, which boiling points are in the range 107e245 C. In nitrogen, SG and SG þ D4 profiles are very similar (not shown). But in air (Fig. 4), the DSC profile of SG þ D4 sample shows a broad exothermic peak between 250 C and 750 C (maximum around 620 C) while it is not visible in nitrogen or for SG sample. This peak is associated with a weight loss. So this heat release is attributed to the adsorbate oxidation. Nevertheless, it can be noticed that the maximum of the exothermic peak does not correspond to the inflexion point of the weight loss and that the thermal effect is small at the beginning of the peak. According to the dTG, it is possible that several phenomena are superimposed: SG dehydroxylation, possible vaporization of oligomeric siloxanes or small PDMS (endothermic) and PDMS oxidation (exothermic).
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Fig. 4. Comparison of TG-DSC profiles for SG and SG þ D4 in air.
Supposing the adsorbate is retained as a linear PDMS, equation (1) is the associated oxidation reaction.
C2nþ6 H6nþ18 Onþ1 Sinþ2ðgÞ þ ð4n þ 12ÞO2ðgÞ /ðn þ 2ÞSiO2ðsÞ þ ð2n þ 6ÞCO2ðgÞ þ ð3n þ 9ÞH2 OðgÞ
(1)
Subscripts (g) and (s) correspond to compounds in gas and solid phase respectively. The weight loss associated with the exothermic peak should then be the resultant of the combustion of organic methyl groups (weight loss) and of the oxidation to silica of the inorganic skeleton (SieO) coming from the adsorbate (weight gain). A mass balance can be performed. The calculated weight loss is 35 mg/gSGþD4. The experimental weight loss of SG þ D4 (DmSG þ D4 e DmSG) measured between 250 C and 750 C is 53 mg/gSGþD4. The order of magnitude is similar. The experimental weight loss is higher than expected. This confirms that several phenomena are superimposed. The difference may be due to an incomplete oxidation of the skeleton or to direct vaporization of some oligomeric siloxanes and small PDMS. An energy balance can also be performed. The integration of the SG þ D4 exothermic peak (Fig. 4) indicates a heat release of 2150 J/gSGþD4, that is to say, by dividing by the D4 adsorbed quantity, a heat of combustion of 11.8 kJ/gD4. In the literature, heats of combustion (at 298 K) of several oligomeric siloxanes were found and are reported in Table 4. As a first approximation, the influence of the temperature is neglected. For linear siloxanes, the heat of combustion decreases when the chain length increases, but for linear PDMS, predicted values tend toward a constant value: 26.4e26.8 kJ/g [29,30]. This can be explained by the constant Si/C ratio for long chain PDMS. The experimental heat of combustion, determined by DSC, is inferior to the ones found in the literature. Several explanations can be proposed. The proposed reaction (equation (1)) is a model reaction assuming a complete oxidation of the PDMS. Several endothermic phenomena can superimpose such as bond scissions and volatilization. Moreover, an incomplete oxidation of the SieO skeleton is also possible. 4. Discussion During previous dynamic adsorption tests, D4 was removed by SG from a synthetic polluted gas. Therefore, molecular D4 could be expected in the adsorbent. But the chemistry of silicones (including
Table 4 Standard heats of combustion for siloxanes (298 K). Compound
Heat of combustion (kJ/g)
L2 L3 L4 L5 D4
36.1a 32.9a 30.8a 30.0a 26.8a
a b
e e e e
34.21b 30.74b 29.71b 27.97b
[29]. [30].
oligomeric siloxanes, silanols and PDMS) is very complex. Condensation and hydrolysis reactions, ring opening and polymerization have been reported [20,31e33]. Moreover, the silanol groups on the SG surface are very reactive. Consequently, low molecular weight (linear or cyclic) siloxanes or higher molecular weight PDMS could also occur. A mixture of cyclic oligomers plus a distribution of polymers is another possibility. All the characterizations converged to the conclusion that the adsorbate is not retained as molecular D4 or any other oligomeric siloxane. A simplified retention mechanism is proposed in Fig. 5. At low uptake, hydrogen bonding between SieOeSi bonds of the siloxane skeleton and silanol groups at SG surface is supposed. This interaction is about 10 kJ/mol, namely lower than covalent bonds but stronger than Van der Waals interactions. It is then possible that ring opening occurs and linear siloxanes are formed. The cleavage of the SieO bonds is favored in acidic aqueous conditions which are gathered in SG. At higher uptake, polymerization of short linear chains into PDMS is promoted by the proximity of the molecules. It follows a SG pore obstruction: its whole specific surface area and micropore volume is lost. The formation of non-volatile compounds such as silicone polymers (PDMS) could explain the limited thermal regeneration of SG after D4 adsorption. Consequently, after thermal desorption, the sample cannot be used in another D4 adsorption test. This proposed mechanism matches the observations of the literature. Schweigkofler and Niessner [7] reported an almost complete desorption (>95%) for L2 or D5 at 250 C in nitrogen circulation. This can be explained by the low SG uptake: only 10 mg/gSG versus 222 mg/gSG in our experiment. Montanari et al. [10] also proposed a polymerization phenomenon and revealed that after adsorption of D3 on SG, desorption up to 200 C by purging with nitrogen as well as by vacuum treatment was incomplete. Note that whatever the adsorbent type (activated
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Fig. 5. Schematic representation of D4 adsorption/polymerization on SG.
carbon, impregnated activated carbon, activated carbon fiber cloth), the desorption of siloxanes seems difficult [8,34,35]. 5. Conclusion Characterizations of SG after D4 adsorption led to several key observations: pore blocking, acidic aqueous conditions promoting ring opening, diminution of the quantity of free silanol groups at the SG surface, D4 alteration, non-desorption of the adsorbate in the range 25e800 C in inert atmosphere, oxidation of the adsorbate over 500 C. A retention mechanism based on hydrogen bonding, D4 ring opening and polymerization into PDMS was then proposed. Further experiments could be conducted using other SG samples with different properties (specific surface area for pore diameter > 10 Å or eOH density for instance) to corroborate the conclusions. In the context of biogas purification, this mechanism may be affected by the presence of other kinds of pollutants and humidity. From an operational point of view, it results that thermal regeneration of SG is restricted, but the understanding of this mechanism should facilitate the implementation of efficient regeneration processes [36]. Acknowledgments This study, part of the PILE-EAU-BIOGAZ project, was supported by the French Agence Nationale de la Recherche (ANR), program HPAC 2010.
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