Amine-Functionalized Titania-based Porous Structures for Carbon

Article pubs.acs.org/JPCC

Amine-Functionalized Titania-based Porous Structures for Carbon Dioxide Postcombustion Capture Cindy C. Aquino,† Gilles Richner,‡ Maryline Chee Kimling,§ Dehong Chen,§ Graeme Puxty,‡ Paul H. M. Feron,‡ and Rachel A. Caruso*,†,§ †

CSIRO Materials Science and Engineering, Private Bag 33, Clayton South, VIC 3169, Australia CSIRO Energy Technology, P.O. Box 330, Newcastle, NSW 2300, Australia § PFPC, School of Chemistry, The University of Melbourne, Melbourne, VIC 3010, Australia ‡

ABSTRACT: An amine-grafted solid sorbent is a promising alternative to aqueous amine scrubbing for removing CO2 from the flue gas of coal power plants. In this study, phosphonic and carboxylic acids have been investigated as alternative anchor groups to trimethoxysilanes to synthesize amine-functionalized TiO2-based sorbents. Several supports, namely, mesoporous TiO2 beads, TiO2 nanoparticles (Degussa P25), and TiO2/ZrO2 composite beads, and a range of amines (1, 2, and 3 carbon-chain primary amine) have been assessed for CO2 adsorption at 30 °C and up to 101 kPa CO2. As a general trend, CO2 adsorption capacity increased with the carbon-chain length of the amine. Finally, materials functionalized with amino acids, L-glutamine or L-arginine, were investigated; the latter showed the highest CO2 adsorption capacity (0.4 mmol/ g at 30 °C and 20 kPa CO2) due to the higher pKa (12.10) of one of the amino groups.

1. INTRODUCTION The capture, reversible release, and storage of carbon dioxide (CO2) from combustion flue gases is now recognized by government and industry as a viable near-term option for greenhouse gas abatement.1,2 It is particularly relevant to electricity generation from fossil fuels (coal, oil, and gas) that accounts for ∼25% of global CO2 emissions, with this figure set to increase drastically in the next 25 years.3 For capturing CO2, postcombustion capture (PCC) is of similar economic cost and has a number of practical advantages over other methods such as oxy-firing to produce a pure CO2 stream and integrated coal gasification combined cycle (IGCC) with precombustion capture.4 In particular, PCC can be retrofitted to existing power stations and integrated into new ones. Additionally, the parasitic energy demand of a PCC plant on a power station can be reduced (at the cost of CO2 removal efficiency) according to electricity output requirements, for example, during times of peak load or optimal electricity pricing. Mature technology to separate CO2 from H2 or CH4 and release it as a pure gas stream already exists in the gas processing and ammonia production industries.5 Traditionally, the CO2 is vented or used for enhanced oil recovery, food manufacturing, or chemicals production.6 PCC requires CO2 separation from a flue gas stream in which it is quite dilute (typically 5−15 kPa). The most mature and applied technology for PCC is thermal swing chemical absorption/desorption using an aqueous amine solution of monoethanolamine (MEA) 30% w/w.7 Aqueous solutions of other amines that have better performance characteristics than MEA are also available as proprietary formulations from a number of commercial © 2013 American Chemical Society

suppliers (e.g., Fluor, BASF, and MHI). In this type of process, CO2 is absorbed into and chemically reacts with the solvent, with regeneration via heating. It is the chemical reactivity that allows effective capture even at low CO2 partial pressure. However, a major drawback to the use of aqueous amine solvents is that water is still the major component. Water has one of the largest heat capacities of any ambient temperature liquid and as such considerable energy is consumed to raise the temperature of water from absorber to desorber conditions (typically from 40 to 120 °C).7 An alternative to liquid absorbents is the use of solid adsorbents. A typical adsorbent is one in which CO2 physically adsorbs to a solid surface with regeneration by pressure swing.8 This requires significantly less energy than using an aqueous solvent with thermal swing,9 avoids corrosion of the equipment, and does not emit potentially harmful chemicals (solvent and/ or decomposition products) to the atmosphere.10 Adsorption processes involving CO2 physisorption on a solid surface are generally more suited for applications at high CO2 partial pressure, such as IGCC and natural gas processing, due to the weaker interactions (van der Waals) occurring between CO2 and the solid compared with chemical interactions with an amine-based solvent.8,9,11 Adsorbents have been functionalized with amines that are chemically reactive toward CO2 to enhance their performance at low pressure. Since the first report by Xu et al.,12 mesoporous silica supports, mainly SBA-15 and MCM-41, functionalized Received: December 10, 2012 Revised: April 8, 2013 Published: May 3, 2013 9747

dx.doi.org/10.1021/jp312118e | J. Phys. Chem. C 2013, 117, 9747−9757

The Journal of Physical Chemistry C

Article

phonic acid). This group was chosen in an effort to eliminate the use of silica in the materials and reduce hydrolysis occurring during application, thereby improving stability. Second, aminecontaining carboxylic acids, structurally analogous to the abovementioned phosphonic acids, were investigated (namely, glycine, β-alanine, and 4-aminobutanoic acid). The carboxylic−TiO2 interaction has been shown to be weaker than the phosphonic−TiO2 interaction,24,25 but carboxylic acids are readily available and offer a cheaper alternative to phosphonic acids that allow trends to be investigated. Finally, larger amino acids such as L-glutamine and L-arginine were studied. Amino acids have been previously reported as potential alternatives to MEA in the liquid phase in the PCC process.30 To the best of our knowledge, phosphonic and carboxylic acid anchors on TiO2-based materials have not been previously reported in the field of CO2 capture. This work also investigates a range of TiO2 supports, including a TiO2/ZrO2 composite material, that possess different porosities, structures, and surface areas. This comparative study could assist in determining supports, other than silica, for application in the capture of CO2.

with amine moieties have been commonly studied for capturing CO2. The result is a solid adsorbent within which adsorption occurs via both physical and chemical processes, making it efficient for capturing CO2 at low partial pressure, as required for PCC. Several extensive reviews have been recently published in this area.8,13,14 The flue gas from a coal power plant may contain up to 20% water vapor. The hydrothermal stability of the silica materials and amino-alkoxysilane-functionalized materials remains a challenge. Silica supports such as SBA-15 and MCM-41 are known to degrade when exposed to boiling water.15 Li et al.16 have also observed a significant loss in porosity and surface area of mesocellular foam supports after treatment with steam between 105 and 180 °C. The chemical stability of silica in aqueous solution is pH-dependent; increasing pH increases silica hydrolysis.17 When silica is functionalized with aminoalkoxysilane groups, the local basicity of the terminal amine groups can promote the destruction of the Si−O−Si bonds, an effect catalyzed by H2O.18,19 Porous TiO2 has been reported as a possible alternative to porous SiO2 for capturing CO2, as TiO2 is robust with respect to temperature and pH in the presence of H2O. Knöfel et al.20 investigated mesoporous TiO2 for CO2 adsorption and concluded that TiO2 solids with high surface areas are potential alternative materials to SiO2 for the capture of CO2 at low pressures. In a later report,21 SiO2 and TiO2 supports were functionalized with 3-aminopropyltrimethoxysilane (3APTMS). Calorimetric and spectroscopic monitoring were used to compare the interactions of CO2 with the different adsorbents. The functionalized TiO2 showed a better performance because of the stronger interaction of CO2 with the surface OH groups, in addition to the interactions with the amine groups from the trimethoxysilane. The anchoring group, which covalently binds the amine to the metal oxide, plays an important role in the hydrothermal stability. Only a limited number of functional groups, including trialkoxysilane, amide, carboxylic acid, and phosphonic acid, have been shown to form stable linkages at ambient temperature under aqueous conditions over various pH ranges.22,23 The carboxylic acid group is the most widely used anchor for binding metal complexes on titania for dyesensitized solar cells (DSSCs). Although phosphonic acid anchors bind to titania more strongly than carboxylic acid24,25 and show a better long-term stability, its use in DSSCs is rarely explored, as these acids may affect the electrical conductivity and consequently the overall cell performance. The modification of nonsiliceous metal oxides with phosphonate groups has been well-documented in the literature.26,27 The attachment of phosphonate groups onto a metal oxide surface can occur through mono-, bi-, or tridentate bonding.27 Ti−O−P bonds are known to be very stable toward hydrolysis, unlike Ti−O−Si bonds. Phosphonates also have the advantage of only interacting with surface −OH groups (as P− O−P bond formation is unlikely under usual conditions), forming monolayers.28,29 Furthermore, the functionalization can be performed in the presence of H2O, as P−O−C bonds are relatively stable toward hydrolysis, contrary to alkoxysilanes.26 In an effort to increase the CO2 uptake and the hydrothermal stability, the functionalization of TiO2 utilizing three classes of organic moieties was examined. The first class includes aminecontaining phosphonic acids (namely, aminomethylphosphonic acid, 2-aminoethylphosphonic acid, and 3-aminopropylphos-

2. EXPERIMENTAL SECTION Alginic acid sodium salt (≥2000 cP viscosity of a 2% solution, mannuronic/guluronic acid ratio of 1.56), titanium(IV) isopropoxide (TIP, 97%), zirconium(IV) propoxide (70% in 1-propanol), hexadecylamine (HDA, 90%), 4-aminobutanoic acid (99%), aminomethylphosphonic acid (99%), 2-aminoethylphosphonic acid (95%), 3-aminopropylphosphonic acid (99%), Pluronic P123, 3-aminopropyltrimethoxysilane (3APTMS), and tetraethylorthosilicate (TEOS) were from Sigma Aldrich. L -Glutamine (>99.5%) and L -arginine (>99.5%) were from Fluka. β-Alanine (>99%) and glycine (>99%) were from Merck. Degussa P25 was provided as a free sample from Degussa-Hüls. Ethanol, isopropanol, toluene, and calcium chloride fused dihydrate (AR) were obtained from Chem-Supply. Toluene was dried overnight over CaH2 and distilled under N2 prior to use. Milli-Q water (Millipore DirectQ3 Water Purification System, resistivity ≥18.2 MΩcm) was used for all experiments. 2.1. Mesoporous TiO2 Bead Preparation. Amorphous precursor beads were first prepared via a sol−gel self-assembly process with a solution composition of HDA/H2O/KCl/ ethanol/TIP = 0.5/3.0/5.5 × 10−3/236.5/1.0 (molar ratio).31 The amorphous precursor beads underwent a solvothermal treatment and then calcination in air to prepare the mesoporous anatase titania beads. In a typical synthesis, 1.6 g amorphous precursor beads was dispersed in a mixture of 20 mL of ethanol and 10 mL of water. This mixture was sealed within a Teflon-lined autoclave (50 mL) and heated to 160 °C for 16 h. The solid products were collected by filtration, washed with ethanol thrice, and dried in air at room temperature. The air-dried powders were heated from room temperature to 500 °C in 5 h and calcined at 500 °C for 2 h in air to remove any remaining organic components. 2.2. Meso-Macroporous TiO2/ZrO2 Composite Sphere Preparation. Meso-macroporous TiO2/ZrO2 composite spheres were synthesized using sol−gel chemistry within preformed calcium alginate template spheres, as reported.32 The template spheres were initially formed by dripping a 1 wt % sodium alginate solution into a Ca2+ bath (0.27 M) using a peristaltic pump (Gilson Minipuls 3) set at a speed of 35 rpm. The aperture size of the tubing was 2 mm. The beads remained 9748



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The amine loading of 3-APTMS-functionalized samples was determined by elemental analysis experiments performed by The Campbell Microanalytical Laboratory, Department of Chemistry, University of Otago, New Zealand. The amine loading of amino-phosphonic and carboxylic-acid-functionalized samples could not be characterized by elemental analysis, as quantities were below the detection limit (∼0.21 mmol amine group/g support). To check the successful grafting of these amine moieties on the surface, we performed FTIR measurements with a PerkinElmer Spectrum System 2000 spectrometer (resolution: 1 cm−1, range 4000−400 cm−1, 256 scans, detector: DTGS). All samples were prepared as pellets mixed with KBr. Then, thermal degradation using thermogravimetric analysis (Setsys Evolution TGA/DSC thermal analyzer, Setaram) was performed to determine the amount of grafted amine. The experimental procedure was adapted from Harlick and Sayari.37 Between 10 and 40 mg of sample was placed in a 100 μL platinum crucible and weighed on a balance (Mettler Toledo, model ML204). The sample was heated in a 40 mL/min N2 flow from 30 to 120 °C at 10 °C/min and held at this temperature for 10 min for drying. The thermal degradation was then performed by heating the samples from 120 to 800 °C at 10 °C/min in a 40 mL/min N2 flow and held at this temperature for 10 min. The amount of amine was determined by the mass loss corresponding to degradation. Under these experimental conditions, phosphonate and trimethoxysilane moieties do not degrade and remain bonded to the titania surface such that only the alkyl chains are thermally degraded.38−40 The molecular weights of the phosphonate and trimethoxysilane moieties were then subtracted from the molecular weight of the amino compounds for converting mass loss into moles. This approach provided qualitative values only because neither residual coke on the solid surface nor hydroxyl surface group (forming H2O between 150 and 400 °C) were taken into account. The method was validated with four amine-grafted sorbents of known amine loads determined by elementary analysis. 2.6. CO2 Uptake. Carbon dioxide adsorption measurements were performed on a Micromeritics TriStar 3020 surface area and porosity analyzer at 30 °C. All samples were degassed at 150 °C for 8 h under vacuum prior to measurement.

in this solution for 24 h prior to washing with water to remove excess Ca2+. The template spheres underwent a solvent exchange from water to ethanol to isopropanol prior to infiltrating with a 70/30% (wt/wt) metal alkoxide precursor/ isopropanol solution under stirring for 24 h to prepare the TiO2/ZrO2 spheres. The precursor-infiltrated template spheres were then transferred to a 1:1 (v/v) water/isopropanol solution for 24 h to instigate hydrolysis and condensation reactions. The hybrid template/inorganic spheres were dried at room temperature for 2 days, then in an oven at 60 °C for 6 h. The template was removed by heating the hybrid spheres to 500 °C for 5 h (air flow, heating ramp of 1.5 °C/min). The mixed metal alkoxide solutions were combined to achieve a concentration of 22 wt % Zr in the Ti/Zr composite. This ratio was chosen because it showed the highest surface area and the best vanadate adsorption rate and capacity over a range of Zr concentrations studied in the Ti/Zr composite.33 2.3. SBA-15 Preparation. The following procedure was used as it appears in Zhao et al.34 Pluronic P123 (4.1 g) was dissolved in a solution of H2O (30.8 g) and HCl (2 M, 120 g) with stirring at 35 °C. Dissolution was complete after 1.5 h. TEOS (8.5 g) was then added to this solution dropwise, and stirring at 35 °C was maintained for 20 h. The mixture was aged overnight without stirring at 80 °C. Once the mixture was cooled to room temperature, the material was washed with copious amounts of water and ethanol and then dried at room temperature overnight. The material was calcined in a muffle furnace at 500 °C under a flow of air (ramp rate 1 °C/min) for 6 h to produce SBA-15 as a fine, white powder. 2.4. Surface Functionalization of Materials. To functionalize the materials, we used 1.56 mmol of the functional molecule for every 1 g of material following Guerrero et al.35 Aminopropyltrimethoxysilane Functionalization. Following Zelenák et al.,36 the material was dried at 110 °C overnight prior to use and stored under N2. The material was placed in toluene (5 mL), then 3-APTMS was added and the solution was heated under reflux for 20 h under a N2 atmosphere. Once cooled to room temperature, the solid sample was washed with toluene (via centrifugation for SBA-15 and P25 and filtration of TiO2/ZrO2 spheres and TiO2 beads) and dried at 45 °C. Phosphonic Acid and Carboxylic Acid Functionalization. The selected phosphonic or carboxylic acid was dissolved in H2O (10 mL). The solution was added to the material, and the mixture was agitated on an orbital shaker overnight (20 h). The samples were collected as above and then dried in air overnight. 2.5. Characterization. Morphological features of the fabricated materials were analyzed using an FEI QUANTA 200F scanning electron microscope (SEM), operating at 10−15 kV. The TiO2/ZrO2 beads were sputter-coated with gold using an Edwards S150B gold sputter coater and where required, silver paint was used to provide additional conductive properties. The TiO2 beads were not coated prior to analysis. Nitrogen sorption isotherms were measured at −196 °C on a Micromeritics TriStar 3020 surface area and porosity analyzer. All samples were degassed at 150 °C under a pressure of no more than 100 mTorr for 9−12 h prior to measurement. The specific surface areas of the samples were determined by a standard multipoint Brunauer−Emmett−Teller (BET) method using the adsorption data in the P/P0 range of 0.05 to 0.20. The pore size distributions of the materials were calculated using the Barrett−Joyner−Halenda (BJH) model based on the adsorption branches of the isotherms.

3. RESULTS AND DISCUSSION 3.1. Characterization of Materials. The stability of TiO2based materials under a wide range of pH values renders them as feasible alternatives to silica supports. Different TiO2-based materials were used in this investigation, namely, mesoporous TiO2 beads, meso-macroporous TiO2/ZrO2 composite spheres, and titania nanoparticles (P25). As a comparison, a silica-based support, SBA-15, was used. The N2 sorption isotherms (Figure 1) were used to calculate the BET surface area and BJH pore size (Table 1). The synthesis and characterization of the exclusively mesoporous TiO2 beads have been previously reported.41 The TiO2 beads used in this investigation have a BET surface area of 110 m2/g and a BJH pore size of 14 nm (Table 1). The TiO2/ZrO2 composite meso-macroporous spheres are much larger in size compared with the TiO2 beads (∼2 mm in diameter)32 and also possess a higher surface area (247 m2/g), as reported in Table 1. Degussa P25 (commercial TiO2) was also used in the present investigation. P25 exists as fine 9749



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blockage (common with mesoporous silica), thus allowing for a higher amount of 3-APTMS to penetrate through the porous system and anchor onto the pore wall. Similarly, with P25 (which is nonporous), pore blockage does not take place and may be the reason behind this material possessing the highest surface coverage of 3-APTMS. 3.2. CO2 Uptake Studies. The CO2 adsorption capacity of the bare TiO2-based materials was measured and compared with that of SBA-15 (Figure 3). The CO2 uptake curve of SBA15 is a relatively straight line, where the amount of CO2 adsorbed is directly proportional to the absolute pressure of CO2. Such a relationship corresponds to physical interactions between the gas and the material, as demonstrated by other groups.21,42 The weak interaction of CO2 with silica (surface OH groups) is due to the covalent structure of silica (strong metal bonds) and its medium to weak Brønsted acidity strength.43,44 At high pressure the weak interaction between CO2 and SiO2 is compensated by the high BET surface area of pure SBA-15. Thus, SBA-15 has the highest adsorption capacity (per mass) at 120 kPa but not at the industrially relevant conditions for PCC (∼15 kPa CO2). Interestingly, the TiO2 beads and TiO2/ZrO2 spheres behave differently to SBA-15. The CO2 uptake curves for both samples have the characteristics of a Langmuir adsorption curve (monolayer adsorption), suggesting a chemical interaction of the acidic gas with the support. At higher CO2 pressures, the curve produced by CO2 adsorption on the TiO2 beads appears to be reaching a plateau, suggesting saturation of sorption sites at higher CO2 pressures. These interactions suggest that significant surface basicity is present in the TiO2-based materials, as has been reported with previous studies of the interaction of acidic gases and metal oxides.43 The TiO2 beads adsorb higher amounts of CO2 compared with the TiO2/ZrO2 spheres over the pressures investigated (Figure 3). Because the composite spheres have a higher surface area compared with the TiO2 beads, the surface area of the materials is not the dominant factor effecting the CO2 uptake of these materials. The presence of Zr (22 wt %) in the TiO2/ZrO2 spheres affects the surface chemistry of the material compared with pure TiO2. The ionic networks formed with titania and zirconia lead to a different surface basicity over Lewis acidity. In general, the more basic sites, such as in ZrO2, would interact more strongly with CO2.43,45,46 However, TiO2rich TiO2/ZrO2 showed a lower hydroxyl surface density than

Figure 1. N2 sorption isotherms of the materials investigated in this study at −196 °C. Black and gray solid lines represent the adsorption and desorption branches, respectively.

nanoparticles of titania in a mixed anatase/rutile form, typically about 80:20. The SEM images of P25, mesoporous TiO2 beads, and composite meso-macroporous TiO2/ZrO2 spheres clearly show the differences between the three samples in terms of porosity and surface morphology (Figure 2). Table 1 summarizes the textural measurements before and after the samples were functionalized with 3-APTMS as well as the amount of grafted amine, as determined by elemental analysis. Upon functionalization, the surface areas of the materials decreased significantly, and some changes in the pore sizes were observed. By comparing the amine loading and surface areas of the materials through a simple calculation, as performed by Knöfel et al.,21 the number of groups on the surface of each material after functionalization can be estimated (in terms of molecules/nm2). The TiO2-based supports have more molecules of amine present per square nanometer of support compared with f-SBA15, even though the latter records the highest amount of 3APTMS (mmol) per gram of material. The differences in the surface chemistry between silica and titania (and titania/ zirconia) could be responsible for this difference, especially the OH surface density. Fully hydroxylated silica typically has 3.5 OH/nm2, which is lower than reported values of mesoporous TiO2 and TiO2/ZrO2, 12.7 and 6 OH/nm2, respectively.33 In addition, the larger pore sizes of TiO2 and TiO2/ZrO2 may offer enhanced diffusion that could reduce the instances of pore

Table 1. Characterization of Adsorbents before and after (f-) Functionalization with 3-Aminopropyltrimethoxysilane (3APTMS) as well as after Hydrothermal Stability Treatment (HT) (i.e., f-SBA-15 is SBA-15 Functionalized with 3-APTMS) SBA-15 f-SBA-15 f-SBA-15 HT TiO2/ZrO2 spheres f-TiO2/ZrO2 spheres TiO2 beads f-TiO2 beads f-TiO2 beads HT P25 f-P25 a

BET surface area (m2/g)

BJH pore diameter (nm)

773 232 5 247 63 110 78 94 55 49

6.2 5.2 n/a 2.2, 5.5, and 86 a

14 12 13 n/a n/a

amine load (mmol/g)

amine surface load (molecules/nm2)

2.2

1.7

1.3

3.2

0.9

4.8

0.6

6.6

f-TiO2/ZrO2 did not fit the BJH model. 9750



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Figure 2. SEM images of the TiO2-based samples: P25 nanoparticles (left), mesoporous TiO2 beads (middle), and TiO2/ZrO2 composite mesomacroporous spheres (right).

Figure 3. CO2 adsorption isotherms of the bare porous supports at 30 °C.

Figure 4. CO2 adsorption isotherms relative to the surface area of the bare porous supports at 30 °C.

the pure oxides.33 This could explain why TiO2/ZrO2 adsorbed less CO2 than TiO2. P25 does not have a similar adsorption trend to the TiO2 porous supports. The expected low CO2 adsorption is due to the nonporous nature of P25. However, due to the medium strength Lewis acidity of TiO2, a chemisorption-type isotherm is observed. 30% less CO2 is adsorbed on P25 than on reported anatase nanosized particles.20 The anatase particles have a higher specific surface area (71 m2/g). Moreover, a higher calcination temperature (600 °C for anatase particles) results in increased CO2 adsorption capacity per surface area.20 Although it is customary to depict the amount of CO2 adsorped per mass of sorbent as a function of CO2 pressure, this does not take into account the surface area available before functionalization for each sample. This surface area is of importance because it influences the extent of functionalization. By comparing the CO2 adsorbed per surface area of the supports before functionalization (i.e., μmol/m2) as a function of CO2 pressure, a variation in the trend was observed in Figure 4. The TiO2 beads adsorb more CO2 per surface area of material over the pressure range studied, followed by P25, TiO2/ZrO2, and SBA-15. This information cannot be seen directly from Figure 3, where the data are plotted as the amount of CO2 adsorbed per mass of solid. The lower CO2 adsorption quantities on TiO2/ZrO2 indicate that the larger surface area of the material (compared with TiO2 beads and P25) does not offer an advantage to adsorb more CO2 − additional evidence that surface chemistry is influential. Finally, SBA-15 performs the worst in terms of CO2 adsorbed per

surface area of material, supporting the argument that only weak interaction occurs between this surface and the gas. A distinct increase in CO2 absorption capacity can be observed in the isotherm of P25 at ∼50 kPa. This behavior was consistent for the isotherms collected in this study, with a high density of data points around this pressure, and is associated with the technique of measurement rather than the sample. Although this results in slight “kinks” in the sorption curves, it does not affect the overall trends seen between samples. 3.3. Aminopropyltrimethoxysilane Functionalization of Porous Supports. To increase the CO2 adsorptivity of the TiO2-based materials studied at low pressure, we functionalized the supports with 3-APTMS according to standard procedures used for silica-based supports.18,21,42 For silica, a functionalization procedure is necessary for CO2 to chemically interact with the material. The high surface areas of mesoporous silicas provide an effective scaffold onto which the 3-APTMS can graft. In general, the higher the surface area of the starting silica, the larger the amount of grafted 3-APTMS and consequently the higher the amount of CO2 adsorbed. Surface modification also induces changes in the shape of the adsorption curves (Figure 5). For example, with the 3-APTMSfunctionalized SBA-15 (f-SBA-15), the sharp increase in the adsorption curve at lower CO2 pressures indicates chemical sorption of CO2 by the amine groups. However, no plateau is reached at higher CO2 pressure. This is assumed to be due to the physical adsorption of CO2 dominating once all the amine sites have been saturated. Adsorption curves utilizing aminotrimethoxysilane-functionalized MCM-41 for CO2 adsorption have been modeled by Serna-Guerrero et al.,42 where the 9751



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Figure 5. CO2 adsorption isotherms of 3-APTMS-functionalized porous supports at 30 °C. Amine load is given in Table 1.

Figure 6. CO2 adsorption isotherms relative to the surface area of 3APTMS-functionalized porous supports at 30 °C. Amine load is given in Table 1.

nature of the adsorption curves is attributed to a combination of physical and chemical interactions. At 15 kPa, f-SBA-15 adsorbs five times more CO2 than the bare support. For f-P25 and f-TiO2/ZrO2, the capacity has almost doubled, whereas for the TiO2 beads, the difference in performance between the bare and functionalized support depends on the pressure. Below 20 kPa, f-TiO2 beads performed better than bare TiO2 beads. For example, at 2.5 kPa, the capacity is two times higher for f-TiO2 beads. This is due to the higher reactivity of amine groups with CO2 than TiO2 at low partial pressure. Over 20 kPa, the bare support surpassed f-TiO2, giving up to 20% higher CO2 capacity at 100 kPa. The trend observed for the adsorption of CO2, following functionalization of the TiO2-based supports, correlates with the amine loading (mmol/g) achieved in the TiO2-based materials (Figure 5 and Table 1). The higher the amine loading per gram, the higher the CO2 adsorptivity. Interestingly, the mole ratio of adsorbed CO2 per grafted amine is ∼0.5 at 100 kPa for all samples. This ratio corresponds to the stoichiometric ratio of the most reported mechanism for sorption of dry CO2 on amines (i.e., forming a carbamate and a protonated amine in a stoichiometric ratio).50 This also suggests that at this pressure only a marginal amount of CO2 is physisorbed on the surface. Comparatively, Serna-Guerrero et al. 4 2 found that with 2-[2-(3trimethoxysilylpropylamino)ethylamino]ethylamine grafted on SiO2, under 101 kPa CO2 the amount of physisorbed CO2 is ten times smaller than the chemisorbed CO2. As for the bare supports (Figure 4), the amount of CO2 adsorbed onto the functionalized supports per surface area (μmol/m2) is presented as a function of CO2 pressure (Figure 6). f-P25 is clearly the best CO2 adsorbent per surface area with 4.2 μmol/m2 at 15 kPa, followed by f-TiO2 beads, f-TiO2/ZrO2 spheres, and finally f-SBA-15. The materials with the highest amount of 3-APTMS per surface area (given in Table 1) show the highest amount of CO2 adsorbed per surface area. A similar trend has also been observed,21 where the 3-APTMS-TiO2 materials outperform those of 3-APTMS-SiO2. The amount of CO2 per surface area adsorbed on f-SBA-15 is in good agreement with Knöfel et al.21 In comparison the f-TiO2 beads in this study show 50% more CO2 adsorbed per surface area because of the higher amount of grafted 3-APTMS per surface area (4.8 and 1.7 molecules/nm2 for f-TiO2 beads and f-SBA15, respectively).

Mesoporous TiO2 beads were selected as the best candidate for hydrothermal stability testing and for investigating alternative anchors to silane, namely, carboxylic and phosphonic acids. 3.4. Hydrothermal Stability Tests. Because one of the reasons to investigate TiO2 as an alternative to silica is the robust nature of the material, tests were conducted to measure the hydrothermal stability of the f-TiO2 beads against f-SBA-15. For PCC application, flue-gas-containing water would pass over these materials; hence extreme conditions were used to test material stability: the functionalized samples of SBA-15 and TiO2 beads were heated in boiling water for 24 h. The resulting materials exhibited a considerable drop in CO2 adsorptivity (as much as 85% for SBA-15), as shown in Figure 7, owing to the

Figure 7. Stability of adsorbent: CO2 adsorption isotherms of 3APTMS-functionalized SiO2 and TiO2 beads at 30 °C before and after hydrothermal treatment (HT).

cleavage of the trimethoxysilane bonds that adhere the functional group to the support and, in the case of silica, of the siloxane bond cleavage of the support itself. Consequently, the surface area of f-SBA-15 dropped to a quasi-nonporous value, supported by the nondetectable pore diameter (Table 1). The textural measurements indicate that the structure of the SBA-15 is also compromised. In contrast, the increase in surface area of the functionalized TiO2 beads after boiling may be due to the removal of the trimethoxysilanes from the surface. The Ti−O−Si bond is reported to be unstable.29,51 Unlike the textural damage observed for the hydrothermally treated f-SBA-15, the SEM analysis of the TiO2 9752



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As the values of the amine loading determined by elemental analysis fell below the detection limit, the successful grafting of amine-phosphonic acids on the surface was confirmed by FTIR measurements. Figure 9 shows the FTIR spectra of amino-

beads shows that the structural integrity of the TiO2 is minimally affected (Figure 8, Table 1). These results clearly

Figure 8. SEM image of 3-APTMS-functionalized TiO2 beads after hydrothermal treatment (f-TiO2 beads HT).

Figure 9. Infrared spectra of aminomethylphosphonic acid before (a) and after (b) grafting on TiO2 beads as well as after hydrothermal treatment (c). The wavenumber regions from 400 to 900 cm−1 and from 1700 to 4000 cm−1 do not provide additional information. Note that spectra (b) is amplified three times because of a low resolution encountered during this particular experiment.

demonstrate the superior stability of the TiO2 beads over porous SiO2 and that the Ti−O−Si bond is too weak, leading to leaching of the amino groups. 3.5. Phosphonic Acid Functionalization. A range of phosphonic acids was used to functionalize the TiO2 beads as an alternative to trimethoxysilane coupling. The phosphonic acid grafting process is facile and does not require an inert atmosphere to prevent undesired condensation. Three aminophosphonic acids with increasing chain length were chosen (Table 2): aminomethylphosphonic acid (AMPA), 2-aminoethylphosphonic acid (2-AEPA), and 3-aminopropylphosphonic acid (3-APPA). The largest phosphonic acid is analogous in chain length to 3-APTMS.

methylphosphonic acid and AMPA-functionalized TiO2 beads. The disappearance of the band at 1220 cm−1 assigned to PO stretching mode,35 suggests coordination of phosphoryl oxygens with TiO2. Moreover, the bands at 1141 and 975 cm−1 after functionalization are assigned to P−C and P−O−Ti stretching modes, respectively.35 P−OH stretching also absorbs in this latter region. These observations support the grafting of the phosphonic acid onto the TiO2 surface.

Table 2. Amount of Amine on Mesoporous TiO2 Beads Determined by TGAa

a Amount of amine on f-SBA-15, f-TiO2/ZrO2 spheres, f-TiO2 beads, and f-P25, with f = 3-aminopropyltrimethoxysilane, was determined by elementary analysis (EA). bSamples not available for analysis. cIonic strength of 0.1 M. dFor NH2 and NH, respectively. pKa values are given at 25 °C.

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The amount of amine was determined by thermal degradation (Table 2). As shown by comparison of these results with the elemental analysis on the 3-APTMS-functionalized samples, the amount of amine determined through thermal degradation is generally overestimated for reasons mentioned in the Experimental Section. The CO2 uptake on these phosphonic-acid-functionalized TiO2 beads is shown in Figure 10 and compared with the trimethoxysilane-functionalized TiO2. An increase in CO2 uptake with an increase in molecular size (chain length) of the phosphonic acids was observed.

Figure 11. CO2 adsorption isotherms on TiO2 beads functionalized with carboxylic acids and 3-aminopropyltrimethoxysilane (f-TiO2 beads) at 30 °C.

In general, the materials functionalized with carboxylic acids performed better than the phosphonic acids but slightly worse than the trimethoxysilane for CO2 adsorption. The results follow a similar trend to the phosphonic acids; increasing chain length of carboxylic acids also increases the CO2 uptake of the samples, confirming the hypothesis related to pKa. This also validates the assumption that cheap and commercially available carboxylic acids can be used for screening and assessing the general behavior of functionalized sorbents. Overall, because the CO2 sorption isotherm on bare TiO2 suggests chemisorption, the functionalization of TiO2 beads using amino-trimethoxysilane, phosphonic acid, or carboxylic acid results in a decrease in the number of functional groups per gram of support that could interact with CO 2 . Theoretically, a bidentate carboxylic TiO2 surface interaction grafting one amine group halves the possible functional groups present for interaction with CO2. For functionalization with phosphonic acid and trimethoxysilane, bidentate or tridentate grafting to the TiO2 surface would similarly decrease the possible functional groups present for CO2 sorption. Because the amine loading was difficult to quantify, in-depth discussion regarding comparisons of the total number of functional groups accessible for CO2 sorption could not be made. The isotherm of 4-aminobutanoic acid-TiO2 beads shows a slight decrease in CO2 adsorption capacity from 0.24 to 0.23 mmol/g around 30 kPa (Figure 11). A decrease in the quantity adsorbed with increasing CO2 pressure cannot be explained by a physical phenomenon. We believe this could be due to instrumental error. 3.7. Amino Acid Functionalization. Functionalization with different types of amino groups per chain was investigated with two amino acids (L-glutamine and L-arginine). The CO2 adsorption of these samples was compared with that of 3APTMS-functionalized materials (Figure 12). The amine content of these samples is given in Table 2. The amino-acid-functionalized TiO2 sample outperforms the amino-trimethoxysilane-functionalized TiO2 sample. The type of amino group (rather than the number) should be considered. The amino group near the carboxylic acid in L-glutamine has a pKa of 9.00.49 The pKa of the other group is much lower. Larginine has pKa values of 12.10 (NH) and 9.00 (−NH2 near the acid group),49 explaining its better performance. Although it may not be a fair comparison with conventionally studied 3APTMS, it is encouraging to see that inexpensive and abundant amino acids could play a role as adsorbers of CO2.

Figure 10. CO2 adsorption isotherms of TiO2 beads functionalized with aminomethylphosphonic acid (AMPA), 2-aminoethylphosphonic acid (2-AEPA), 3-aminopropylphosphonic acid (3-APPA), and 3aminopropyltrimethoxysilane (f-TiO2 beads) at 30 °C.

The difference in CO2 uptake may be a result of the difference in chain length affecting the pKa and the ability of the acids to interact differently with CO2. The trend in adsorption capacity matches the increasing basicity of the nitrogen: at 25 °C and an ionic strength of 0.1 M, the pKa values are 10.05 for AMPA, 10.98 for 2-AEPA, and 11.04 for 3-APPA.48 Similarly, in the liquid phase, CO2 adsorption increases with the chain length (from two carbon chain up to six).52 Moreover, increasing the carbon chain length leads to a more exothermic molar enthalpy of protonation.53 In other words, according to the van’t Hoff equation, pKa change as a function of the temperature is greater for more exothermic enthalpy. This is an important consideration for thermal regeneration of the adsorbents. Regeneration efficiency falls outside the scope of this study and has not been further investigated. Finally, the amino-phosphonic acid was shown to be hydrothermally stable on TiO2 as P−C, P−O−Ti specific bands, as well as NH2 vibrational modes absorbing at 1640 cm−1 (due to N-H rocking), are still clearly visible from the IR spectra after the treatment (Figure 9). Other phosphonic acids could be investigated to obtain an increase in CO2 adsorption. However, as previously mentioned, carboxylic acids offer an inexpensive alternative to phosphonic acids for ascertaining trends in the behavior of the amines and the effect of chain length. 3.6. Carboxylic Acid Functionalization. Carboxylic acids with the same chain length as the above-mentioned phosphonic acids were grafted on the TiO2 beads. The amine content determined by elementary analysis was below the detection limit, and the amine loading was evaluated by TGA, as given in Table 2. The resulting CO2 adsorption isotherms at 30 °C are given in Figure 11. 9754



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TiO2 beads and 1.5 g/cm3 for TiO2/ZrO2 composite sphere) than that of the mesoporous silica (0.9 g/cm3 for SBA-15). Thus, for a hypothetical equal adsorption capacity per mass, a smaller adsorption column would be required for the TiO2based adsorbents to reach the same capture efficiency. Because the adsorption column is a major part of the financial investment of PCC, the cost of CO2 capture could be substantially reduced using a denser adsorbent such as TiO2 or TiO2/ZrO2 composite.

4. CONCLUSIONS TiO2-based adsorbents were investigated as an alternative to SiO2 for CO2 PCC applications. Hydrothermal treatment of 3APTMS-functionalized mesoporous TiO2 and SiO2 showed a drop in the CO2 adsorption capacity. The textural structure of TiO2 was relatively unaffected by the hydrothermal treatment, whereas the SiO2 showed a considerable drop in specific surface area and pore diameter. In both cases, amine leaching was observed. As an alternative to the trimethoxysilane anchor group, phosphonic acid was investigated to functionalize the mesoporous TiO 2 support. The amine loading using phosphonic acid moieties was about five times lower than the corresponding trimethoxysilane. Further investigations are needed to improve the grafting process with a phosphonic acid anchor. Amino-carboxylic acids were used as an alternative to phosphonic acid for screening different amines, as aminocarboxylic acids are readily available compared with aminophosphonic acids. Similar adsorption trends were observed between amino-phosphonic and carboxylic acids when varying the amino-carbon chain length from 1 to 3 carbons. Finally, two amino acids, namely, L-glutamine and L-arginine, have been studied with the L-arginine-functionalized material demonstrating good performance that was due to the higher pKa of the amino group. In this study, three TiO2-based materials (namely, mesoporous TiO2 beads, commercial TiO2 nanoparticles, and TiO2/ ZrO2 composite spheres) were investigated. The synthesis conditions employed to generate the TiO2 materials may significantly affect the surface chemistry and the ability of the supports to adsorb CO2 and bind to anchoring groups. Careful elucidation of the surface acid−base chemistry would be pivotal in optimizing the interactions between the material and the acid used for functionalization.

Figure 12. CO2 adsorption isotherms on TiO2 beads functionalized with amino acids and 3-aminopropyltrimethoxysilane (f-TiO2 beads) at 30 °C.

Finally, it can be noted that hydrothermal treatment was not performed on carboxylic-acid-functionalized beads because carboxylic acid is known to be a weaker anchor than phosphonic acid.24,25 As mentioned before, carboxylic acids have been used here to evaluate trends in CO2 sorption performance because a large number of amino-carboxylic acids are readily available. For this reason as well, isotherms for regeneration at higher temperature were not investigated. 3.8. Final Comments. The specific surface area of SiO2 is commonly reported to be much greater than TiO2 because the samples are generally amorphous and crystalline, respectively. So, more amines can theoretically be grafted on SiO2 supports and, in turn, more CO2 can be adsorbed. This is clearly a drawback for mesoporous TiO2 materials. Recently, TiO2 beads with a specific surface area of 240 m2/g (compared to 110 m2/g for the beads in this study) have been synthesized.54 These will be promising materials for further development of TiO2-based CO2 adsorbents. The amine surface concentration on the nonporous TiO2 material was the highest, whereas for mesoporous materials access into the pores can be limited. Thus, to synthesize high performing sorbents, the pore networks should be optimized to allow the grafting of amine containing molecules on most of the surface while keeping the specific surface area high. Robust porous TiO2 frameworks with structural control over the nanometer-to-micrometer range can be manufactured. Such pore architecture is an important feature for improving the mass transport within the pores and optimizing the overall adsorption rate. During the functionalization procedure, the amount of functional molecule used per gram of sorbent was similar for the trimethoxysilane, phosphonic, and carboxylic acids. Because of the different chemical interactions of the anchor groups with the solid surface and the use of different solvents (water and toluene), there is potential for improving on the grafting efficiency and in turn the CO2 adsorption capacity of aminophosphonic−TiO2 beads. Finally, when comparing different adsorbents, the density of the adsorbent should also be considered together with the specific surface area for designing industrial size adsorption columns. The specific densities of these porous inorganic adsorbents were calculated using the pore volume derived from nitrogen gas sorption measurement (Figure 1) and the inorganic skeleton density from literature.17,55,56 The TiO2based adsorbents possess much higher density (1.7 g/cm3 for

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +61 3 8344 7146. Fax: +61 3 9347 5180. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project is part of the CSIRO Coal Technology Portfolio and received support from the CSIRO OCE Science Leader collaborative fund. R.A.C. acknowledges an Australian Research Council Future Fellowship (FT0990583). The views expressed herein are not necessarily the views of the Commonwealth, and the Commonwealth does not accept responsibility for any information or advice contained herein. 9755



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