ARTICLE pubs.acs.org/Langmuir
Carbon Dioxide Sorbents with Propylamine Groups-Silica Functionalized with a Fractional Factorial Design Approach Baroz Aziz,†,§ Guoying Zhao,†,‡,§ and Niklas Hedin*,†,‡ †
Department of Materials and Environmental Chemistry and ‡Berzelii Center EXSELENT on Porous Materials, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden
bS Supporting Information ABSTRACT: Mesoporous silica particles (Davisil) were functionalized with aminopropyltriethoxysilane (APTES) in a fractional factorial design with 19 different synthesis and uptake experiments. The number of amino groups and the uptake of CO2 were optimized in a 2V5-1 design. Most important to functionalization was the amount of water present during synthesis, the reaction time, and pretreating the silica with a mineral acid; certain two-way interactions were shown to be statistically significant as well. Modifications performed at 110 or 80 °C showed no significant differences concerning amine content or uptake of CO2. Properly choosing center points for the discrete variables is problematic and is somewhat related to the lack of fit with respect to CO2 uptake; the regression was good. Solid-state 29Si NMR showed that the APTES was mainly fully condensed. Specific surface areas did not correlate with the number of n-propylamine groups on the silica, which is indicative of differential levels of heterogeneity in the coverage of propylamines. The uptake of CO2 and N2 was measured from -20 to 70 °C and from 0 to 1 bar and parametrized by the Freundlich isotherm. Amine-modified silica adsorbed significant amounts of CO2, especially at the low partial pressure, which is important for CO2 capture from flue gas. At such pressures, samples with a high density of amine (4 amines/nm2) showed a much higher uptake of CO2 than did those with densities of ∼2-3 amines/nm2, reflecting differential tendencies to form propylammonium-propylcarbamate ion pairs; these require close proximity among amine groups to form. Water affected the uptake of carbon dioxide in different ways. Certain samples took up more moist CO2 gas than dry CO2, and others took up less moist CO2 than dry CO2, which is indicative of differential tendencies toward water adsorption. We conclude that experimental design is a time-efficient approach to the functionalization of silica with propylamine groups.
1. INTRODUCTION Carbon capture and storage (CCS) could reduce the amount of carbon dioxide released into the atmosphere.1 Capture is the main cost of CCS, whose overall cost is too high for a straightforward implementation in the current energy system. The potential for more cost-effective technologies for postcombustion capture is being actively researched. New solvents, polymer membranes, and nanostructured solids are all shown to be interesting.1,2 Ho et al. examined the economy of using pressure swing adsorption (PSA) to separate CO2 from flue gases of power plants by postcombustion processes.3 They showed that a modern technique for PSA, together with a hypothetical adsorbent (with a working capacity of 4.3 mol/kg and a CO2/N2 selectivity of 150), could reduce the cost of capture significantly. Real adsorbents for CO2 capture need to be inexpensive, have a high CO2 capacity and selectivity, facilitate rapid mass transport, have a moderate heat of adsorption, and be mechanically stable. These oftenconflicting properties need to be optimized for optimal sorbents in either PSA or temperature-swing adsorption processes.4,5 Numerous adsorbents have been studied for CO2 capture, including zeolites, activated carbons, metal organic frameworks, and amine-modified silica. Compilations of the literature on CO2 adsorbents can be found in the reviews by Choi et al. and Hedin et al.6,7 The electric quadruple moment of CO2 is larger than that r 2011 American Chemical Society
for N2.8 This leads to a significant CO2-over-N2 selectivity at pressures relevant in flue gas. Still, the selectivity is far from the level proposed by Ho et al. for hypothetical adsorbents that could significantly reduce the cost of CO2-over-N2 separation.9,10 Amine-functionalized sorbents have been studied for CO2-over-N2 selection in dry and moist gas mixtures6,7 by analogy to the alkanolamines used in CO2 scrubbers in various industrial processes. They are particularly interesting for postcombustion capture from flue gas with mainly N2, 5-15 vol % CO2, and water vapor near atmospheric conditions. Modifications of silica with amines can be made by physical adsorption, filling, and chemical tethering and have been shown to exhibit a highly selective uptake of CO2.6,7 Chemical tethering of silica with aminopropyltriethoxysilane (APTES) has been particularly popular11-30 because of favorable interactions with CO2. The amine groups react with CO2 and enhance the CO2over-N2 selection. The corresponding chemistry in liquids is basically known.31,32 Ammonium-carbamate ion pairs form when amine groups are close enough; these ion pairs are unstable at high temperatures, and the sorbed CO2 can be regenerated at Received: February 6, 2010 Revised: February 7, 2011 Published: March 01, 2011 3822
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Langmuir higher temperatures. With water present, it has been suggested that the carbamate-ammonium ion pairs at the surface react further with CO2 and H2O and form bicarbonate groups.17 Huang et al. studied the uptake of CO2 for dry and moist CO2 and concluded that water doubled the uptake of CO2. They reported that various synthesis conditions affected the rate and capacity for uptake.11 These effects were rationalized by variations in the surface density, activity, and accessibility of the amines. Knowles et al. found a similarly enhanced uptake under moist conditions. They observed a slower rate of uptake than under dry conditions.21 Different uptake behaviors have been observed for the effects of water on the uptake of CO2.11,21-23,33 A doubling of the uptake of CO2 under humid conditions has almost never been reached, and no straightforward spectroscopic evidence of bicarbonates has been presented. Still, the uptake of gas tends to increase in the presence of water. Porous silica loaded physically with amines has shown a large uptake of CO2.38-46 It has been shown that the introduction of n-propylamine functions on silica drastically increases the selectivity for CO2 adsorption.11,17,18,20,23-25,47 Diamine and triamine modifications of silica, and other substrates, have shown a significant uptake of CO2.48-56 On these amines, intermolecular ammoniumcarbamate ion pairs form. Covalently tethered polyethyleneimines capture CO2 effectively and reversibly.7,57 A pure silica surface has various surface states and functionalities, where the most common are OH groups known as silanol groups and Si-O-Si groups known as siloxane bridges.34-36 A large amount of work has been performed on modifying amorphous silica with crystallographically aligned pores (MCM-41, SBA-15, etc.). Data might be easier to rationalize from such ordered materials; however, it has been suggested that much simpler substrates, such as fumed or precipitated silica with high surface areas are actually giving better CO2 sorbents.37 Silica surfaces can be modified by co-condensation and postsynthesis modification.58-62 In co-condensation, functionalities are introduced during synthesis by using mixtures of pure alkoxysilanes and alkoxysilanes carrying a functional group. In postsynthesis routes, functional alkoxysilanes are reacted with the already-formed silica surface. Postsynthesis functionalization avoids the complexities inherent in the multicomponent mixtures needed for co-condensation protocols, but with more heterogeneous distributions of functional groups than in the condensation method. For silica/ APTES, a degree of heterogeneity appears to be advantageous because propylammonium-propylcarbamate ion pairs form more easily on heterogeneously modified silica surfaces.12,13,18 Factorial design allows a set of experimental variables to be studied simultaneously, typically by varying each variable on two levels, and Dabre et al. has used it for the modification of silica with mercaptosilanes.66 It has advantages over more standard methods in those instances where the important variables are not known or are many. In addition, factorial design allows codependencies among experimental variables to be tested statistically. Factorial design lends itself very well to statistical analysis. The experimental response is approximated with a polynomial function, and eq 1 shows a Taylor expansion of such a response: y ¼ β0 þ β1 x1 þ β2 x2 þ :::βk xk þ βij xi xj ::: þ βii x2i þ :::RðxÞ þ e ð1Þ
Here, x denotes variables, β denotes coefficients, and y denotes experimental responses; R(x) and e are small. The coefficients describe the effect of the corresponding variable
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Table 1. Synthesis Conditions Applied to APTES-Modified Davisil parameter values factors
low level (-1)
high level (þ1)
X1: temperature
80 °C
110 °C (reflux)
X2: reaction time
7h
72 h
X3: H2O added X4: acid treatment
0g no
0.45 g yes
X5: heat treatment
no
yes
and combination of variables. Full factorial design needs 2k separate experiments to be conducted, where k is the number of variables. Fractional factorial design systematically reduces the number of experiments needed by compounding various variables in eq 1. It is a time-efficient and rather unbiased approach to optimization with respect to experimental observables as long as the dependencies are not strongly nonlinear (or asymptotic).63-65 Here we use fractional factorial design to study the modification of silica with APTES because several variables were expected to be potentially important, and we identified and quantified the most important experimental variables for the postsynthesis modification of Davisil with APTES.
2. EXPERIMENTAL SECTION Chromatographic particles of porous silica were modified with APTES in a factorial design. Samples were characterized by nuclear magnetic resonance (NMR) and Fourier transform infrared (FT-IR) spectroscopies, thermal gravimetric analysis (TGA), elemental analysis, adsorption measurements, and scanning electron microscopy (SEM). 2.1. Materials and Synthesis. Davisil LC60 (Grace Davison) silica was used as a substrate. It has a particle size of 40-63 μm and a surface area of 550 m2/g. The substrate was used both as received and after acid treatment with 2 mol/dm3 HCl at 60 °C overnight or after degassing at a temperature of 400 °C at very low pressure overnight. 2.1.1. Modifications with Propylamine Groups. Substrates were degassed at temperatures of 150-170 °C before synthesis, and for each synthesis, 3.0 g of substrate and 180 mL of toluene were added to a threenecked flask equipped with a Dean-Stark reflux condenser. Analyticalgrade toluene ([CAS: 108-88-3], Sigma-Aldrich 99.8%) with 0.03 wt % H2O was used. The solutions were heated to 50 °C under stirring for 30 min; 0-0.45 mL water was added slowly (2 monolayers of water), and the mixtures were refluxed for 1-1.5 h. The amount of water was varied among the samples prepared and controlled with a Dean-Stark distillation setup67 and by bubbling N2; see Table 1 for the amounts. We estimated the hydroxyl surface concentration to be 5 OH/nm2.68,69 The temperature was adjusted to either 80 °C or reflux temperature; 15 g of APTES (Sigma-Aldrich 99%, [CAS: 919-30-2]) was added dropwise; reactions proceeded for 7 or 72 h; and the solid was filtered off. The excess amount of APTES was calculated to be 5 times the estimated number of hydroxyl groups on the substrate. Particles were washed with toluene (50 mL 2) and ethanol (50 mL 3) and dried at a temperature of 100 °C overnight. A detailed description of the conditions is presented in Table 1. Three of the samples were prepared as center points with variable values being halfway between the low and high levels. 2.2. Characterization. 2.2.1. NMR Spectroscopy. 29Si and 1H NMR spectra were recorded on a Chemagnetics Infinity 400 spectrometer operating at 79.49 and 400.16 MHz. Magic-angle spinning was used (MAS). A double-resonance 6 mm probe head was used to acquire 29 Si NMR spectra at 8.00 kHz MAS. A double-resonance 3.2 mm probe 3823
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Langmuir head was used to acquire 1H NMR spectra at 18.0 kHz MAS. The 1H NMR chemical shift scale was externally calibrated to adamantine (1.82 ppm), and the 29Si NMR chemical shift scale was externally calibrated to tetrakis(trimethylsilyl)silane (TKS, -9.8 ppm). Small amounts of exponential filtering (as compared to the spectral line widths) were applied to the free induction decays before Fourier transformations. Peak positions and integral intensities were determined with Spinsight software. For the 29Si NMR spectrum, an ∼70° pulse (4.8 μs) was used, 224 transients were added, and a recycling delay of 1000 s (more than 5T1) and continuous 1H decoupling were used during acquisition (21 kHz). For the 1H NMR spectra, an ∼90° pulse (2.5 μs) was used, 64 transients were summed, and a recycling delay of 10 s (larger than 5T1) was used. Before the 1H NMR spectra were recorded, the samples were dried in an oven at 110 °C for 6 h or subjected to water vapor overnight at a humidity level of 37% relative humidity (RH) at 25 °C in a BINDER MKF 115 chamber. The samples were spun for the same amount of time before experimentation to compensate for additional drying. 2.2.2. FT-IR Spectroscopy. FT-IR spectra were recorded with a Varian 610 microscope coupled to a Varian 670-IR spectrometer equipped with a mercury cadmium telluride (MCT) detector cooled with liquid nitrogen. Spectra were obtained from pure powders over the range of 600-4000 cm-1 with a spectral resolution of 4 cm-1. 2.2.3. N2 and CO2 Adsorption. Nitrogen and CO2 adsorption were measured with a Micromeritics ASAP2020 and a Micromeritics Gemini 2375 device, the latter with a room-temperature option installed. CO2 (>99.9%, Linde Gas Company (AGA)) and N2 (>99.9%, Linde Gas Company (AGA)) were used. For the experiments with the Gemini device, temperature was controlled by a circulating a temperaturecontrolled heat-transfer medium to a Dewar flask using a Huber ministat 230. The temperature was measured in the Dewar flask. Temperaturedependent experiments were conducted for dry CO2 and N2. The uptake of moist CO2 and moist N2 occurred at 23 °C. Humid gas containing ∼1.9% water was produced by bubbling pure gas through water in a sealed bottle (at an overpressure of 0.5 bar). About a 50 to 100 mg sample was dried in a stream of N2 for at least 20 h at a temperature of 140 °C using a Micromeritics FlowPrep 60 system. The manufacturer’s specially designed caps were used, allowing samples to be rapidly sealed at an elevated temperature. The uptake of CO2 was also measured at 22 °C with the ASAP2020 device. Nearly isothermal conditions were achieved by immersing samples in a Dewar flask filled with ∼5 dm3 of water equilibrated to room temperature. For these experiments, the samples were treated under near-vacuum conditions for 3 h at 120 °C. Freundlich adsorption isotherms were used to parametrize the uptake of CO2 and N2 at 25 °C by minimizing the sum of the composite meansquare deviations using the nonlinear methods of Origin software. 2.2.4. BET Isotherms. The specific surface areas (SBET) were calculated using the Brunauer-Emmett-Teller theory from the volumetric adsorption of N2 (-196 °C) for a relative pressure of p/p0 = 0.06-0.3 using data aquired with a ASAP2020 device. The total pore volumes (Vtot) were calculated at p/p0 = 0.91, and the pore-size distributions were qualitatively estimated using the density functional theory (DFT) method for a cylindrical pore model. A t-plot analysis allowed the specific micropore area to be determined via the Broekhoff-De Boer thickness equation. Samples were treated under near-vacuum conditions at 120 °C for 3 h. 2.2.5. Quantification of n-Propylamine. Elemental analysis was used to determine the number of C and N atoms. Thermogravimetric analysis (TGA) was used to record the mass loss on heating using a Perkin-Elmer TAG7 instrument in dry air or nitrogen gas, for which samples were heated from 50 to 950 °C at a rate of 10 °C/min in a platinum cup. 2.2.6. Quantification of Water Uptake. Water uptake was determined for the Davisil/APTES samples. Before experimentation, the samples were heat treated under near-vacuum conditions at 120 °C for 3 h, after which they were placed in contact with moist air for 2 days at 25 °C and a
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Table 2. Fractional Factorial Design Matrix for Postsynthesis Modification of Davisil with APTES temperature
time
H2O
acid
heat
sample
(X1)
(X2)
(X3)
(X4)
(X5)
PA5 PA6
-1 1
-1 -1
-1 -1
-1 -1
1 -1
Y1a
Y2b
1.755 0.736 1.393 0.323
PA7
-1
1
-1
-1
-1
1.392 0.589
PA8
1
1
-1
-1
1
2.029 0.925
PA9
-1
-1
1
-1
-1
PA10
1
-1
1
-1
1
2.368 1.233 1.902 0.76
PA11
-1
1
1
-1
1
PA12
1
1
1
-1
-1
2.685 1.2
PA13 PA14
-1 1
-1 -1
-1 -1
1 1
-1 1
1.846 0.916 1.704 0.636
2.938 1.43
PA15
-1
1
-1
1
1
2.084 0.917
PA16
1
1
-1
1
-1
2.223 0.942
PA17
-1
-1
1
1
1
2.145 0.928
PA18
1
-1
1
1
-1
2.196 0.969
PA19
-1
1
1
1
-1
3.014 1.36
PA20
1
1
1
1
1
3.515 1.49
PA21 PA22
0 0
0 0
0 0
0 0
0 0
1.666 0.916 1.570 0.912
PA23
0
0
0
0
0
1.66
0.905
a
Y1 represents the response due to the weight percentage of nitrogen atoms per sample. b Y2 represents the response due to CO2 uptake in mmol g-1 at 1 bar and 298 K. relative humidity of 90%. A BINDER MKF 115 chamber was used to control the temperature and level of relative humidity. 2.2.7. Scanning Electron Microscopy (SEM). The Davisil substrates were prepared for SEM studies by being sprinkled on Oxford aluminum stubs coated with dried colloidal carbon, and excess powder was removed with gentle blowing. Images were recorded with a JEOL JSM-7000F microscope operating at 15 kV with a working distance of 10 mm. 2.2.8. Experimental Design and Analysis of Variance. The MODDE program was used to design and randomize experiments and analyze modeled responses as a function of both continuous and discrete parameters. The 2V5-1 design was chosen, which confounded main effects with four-variable interactions and the two-way interactions with three-variable interactions.
3. RESULTS AND DISCUSSION The results relating to the experimental design are first presented and discussed. Thereafter, the characteristics of selected samples are discussed in detail (e.g., isotherms for the uptake of CO2 and N2). 3.1. Experimental Design. Three different responses—Y1, the number of N atoms, Y2, the uptake of CO2 at 22 °C and 1 bar, and Y3, the uptake of moist air at 25 °C, atmospheric conditions, and 95% RH—were measured for the samples. The number of N atoms (Y1) was determined by elemental analysis; the amount of CO2 uptake (Y2) was determined volumetrically; and the weight increase on contact with moist air (Y3) was determined gravimetrically. The temperature (X1), reaction time (X2), amount of water (X3), acid treatment (X4), and heat treatment of the substrate (X5) were varied. Specific values for X1-X5 are given in Table 1. Even if the partial pressure of CO2 in flue gas is significantly smaller than 1 bar, we choose this to determine Y2 3824
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Table 3. Analysis of Variance for the Nitrogen Content (Y1) and CO2 Adsorption (Y2) for APTES-Modified Davisil DFa
SSb
total
19
96.44
5.076
constant total corrected
1 18
91.215 5.225
91.215 0.29
regression
12
5.078
0.423
residual
6
0.147
0.024
lack of fit
4
0.135
0.034
pure error
2
0.012
0.006
DFa
SSb
MSc
total
19
18.763
0.988
constant total corrected
1 18
17.218 1.545
17.218 0.086
regression
12
1.521
0.127
Y1
Y2
MSc
residual
6
0.024
0.004
lack of fit
4
0.024
0.006
pure error
2
0
0
Fd
pe
SDf
17.289
0.001
0.651
5.794
0.153
0.539 0.156 0.184 0.076 Fd
pe
SDf
32.018
0
0.356
191.074
0.005
0.077
0.293 0.063 0.006
a
Degree of freedom. b Sum of the squares. c Mean square value. d F-distribution value. e p value. f Standard deviation.
Figure 2. Plot of the most significant effects of the variables (X) on the responses to nitrogen content (Y1) and CO2 adsorption (Y2) for APTES-modified Davisil. Interaction effects are double digit.
Figure 1. Correlation between responses Y1 (nitrogen content of the samples in mmol/g) and Y2 (adsorbed CO2 in mmol/g at 1 bar and 298 K) for APTES-modified Davisil.
because it is a standard pressure. Higher pressures of CO2 are relevant for numerous other potential applications, including natural gas upgrading and combustion in pressurized fluidized beds. Nineteen different samples were synthesized in a 2V5-1 design, and the different conditions used and the responses (Y1 and Y2) measured are presented in Table 2. Both discrete and continuous variables were used, which demands certain care when analyzing the responses. The results for the water uptake (Y3) are presented in the Supporting Information (SI Table 1 and ,Figure SI-8a-e). To test the statistical significance of the model for responses Y1 and Y2, p values were determined. For p < 0.05, we rejected the null hypothesis. The sum of the squares (SS), average mean square (MS), critical values for the F distributions, standard deviations (SDs), variance analyses for the number of propylamine groups (Y1), and the uptake of CO2 (Y2) are presented in Table 3. The p values, related to the regression, were very small (0.000 and 0.001) for Y1 and Y2, and in this sense, the model is significant for Y1 and Y2. For the uptake of CO2 (Y2), the model showed a lack of fit for p = 0.005. The data could not be modeled completely with the suggested model, despite a good regression.
Figure 3. Effects on CO2 uptake (Y2) for the modification of Davisil with APTES, plotted as variations on the single variables. (a) Reaction temperature, X1; (b) time, X2; (c) water content, X3; and (d) acid treatment, X4. The effects on responses (Y2) are calculated when variables, on their X axes, are varied from a low (-1) to high (þ1) level. See Table 1 for details.
The curvature in this model could be explained by complications from choosing the so-called center points. In Table 2, the center points are the last three experiments, for which the variables were fixed at levels between the low and high values. How to choose center points is crucial to the validation. The two discrete variables are fundamentally problematic for the definition of center points. The acid and heat treatment directly affected the number of silanols and silica surface properties; therefore, the center points could not be chosen exactly. For Y1, we could reject a lack of fit. Figure 1 correlates the number of n-propylamine (Y1) groups and the uptake of CO2 (Y2). A correlation was expected from the 3825
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Table 4. Coefficients and Statistical Measures for the Uptake of CO2 (Y2) on APTES-Modified Davisil term
variable
constant, βo X1, β1
a
coeffa
stdb
pc
0.01029
2.78 10-6
0.03275
-0.054
0.0112
0.0171
0.0357
0.95195 temperature
confidence interval (()d
X2, β2
time
0.147
0.0112
0.001
0.0357
X3, β3
H2O
0.2116
0.0112
0.0003
0.0357
X4, β4
acid
0.0601
0.0112
0.0127
0.0357
X12, β12
0.0866
0.0112
0.0045
0.0357
X14, β14
0.0435
0.0112
0.0303
0.0357
X23, β23 X25, β25
0.0518 0.0658
0.0112 0.0112
0.0191 0.0099
0.0357 0.0357
X34, β34
-0.0446
0.0112
0.0284
0.0357
X45, β45
-0.0451
0.0112
0.0278
0.0357
Coefficient. b Standard error of the coefficient. c p value. d Confidence interval (95%).
Figure 4. Validation of the model for the content of nitrogen (Y1) on APTES-modified Davisil: (a) cumulative normal probability vs residuals; (b) plot of residuals vs predicted values; (c) residual vs run order, where systematic errors usually show up as a pattern; and (d) histogram of the residuals.
strong influence of CO2 chemisorption via ammonium-carbamate ion pairs. The indicated line does not intercept the origin because physisorbed CO2 also contributed to the uptake. Note that we determined the uptake of CO2 at 1 bar. Mesoporous silica has a high specific surface area and has been shown to physisorb significant amounts of CO2 at high pressure.70 In addition, Kn€ofel et al. and Bacsik et al. have shown significant amounts of CO2 physisorption on APTES-modified sorbents by macroscopic and spectroscopic techniques; at low pressure, chemisorption completely dominates.71,18 The amount of chemisorption of CO2 is expected to depend linearly on the number of accessible n-propylamine groups above a critical local area density (ac) where propylammonium-propylcarbamate ion pairs can form (as long as mass transport is sufficiently rapid). The observed high degree of correlation between Y1 and Y2 supports such a thesis, with the caveat that physisorbed CO2 contributed to Davisil/APTES with high specific surface areas. 3.1.1. Effect of Variables. Calculated contributions (effects) for the variables and interaction effects are presented in Figure 2 with estimated errors, both for the content of N atoms (Y1) and the uptake of CO2 (Y2). The amount of water (X3) and the reaction time (X2) contributed strongly and positively to the number of N atoms (Y1) and the uptake of CO2 (Y2). Acid treatment of the substrate (X4) influenced both responses positively as well. Certain interaction effects (XnXm) were significant for the responses. Positive interaction effects usually involved X2 or X3. (However, for the uptake of water (Y3) interaction effects were more significant; see Supporting Information Figure SI-8e.) Surprisingly, the reaction temperature (X1) had a small negative effect on the uptake of CO2 (Y2) and no significant effect on the quantity of amines (N content, Y1). Heat treatment (X5) had no significant effect. The level of response for each main factor was calculated and found to span not more than about (20%. In Figure 3, the uptake of CO2 (Y2) is plotted as a function of variables X1-X4. Figure 3a displays the small negative effect with rising temperature. Figure 3b displays the significance of the reaction time toward Y2. The kinetics for both the hydrolysis and condensation were time-dependent and influenced the reactions among APTES and with the silica surface. The reaction of APTES with a silica surface proceeded with the hydrolysis of the ethoxy groups of APTES followed by condensation among hydrolyzed APTES and silanol groups on the silica.74 Figure 3c displays the influence of the amount of water. A thicker water layer that adsorbed on the silica surface can rationalize the significant and positive influence of increasing the amount of water.72,73 3826
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Figure 6. Plots of observed values vs the predicted response for the APTES modification of Davisil: (top) N-atom content (Y1) and (bottom) uptake of CO2 (Y2).
Figure 5. Validation of the model for the uptake of CO2 (Y2) on APTES-modified Davisil: (a) cumulative normal probability vs residuals; (b) residuals vs calculated values; (c) residuals vs run order, where systematic errors usually show up as a pattern; and (d) histogram of the residuals.
An increased amount of water expanded the volume for crosscondensation reactions of APTES, and a thicker layer formed on the silica surface. Figure 3d displays the small, positive, significant effect on pretreating silica with acid. The calculated effect (coefficients), standard deviations, p values, and confidence intervals are presented in Table 4. The coefficients (β) in eq 1 show the effect of studied variables on the uptake of CO2 on Davisil modified by APTES. Confidence intervals (95%) are given. For p < 0.005, the null hypothesis could be disregarded. The plots in Figures 4 and 5 support the validity of the fractional factorial design model for N atoms (Y1) and the uptake of CO2 (Y2). The cumulative normal probabilities (N probabilities) in Figures 4a and 5a are roughly normally distributed. For normally distributed residuals, the N probabilities
will be linearly related to the residuals for Y1 and Y2. In Figures 4b and 5b, residuals were plotted versus calculated values of Y1 and Y2. In Figures 4c and 5c, the residuals were plotted versus run order and showed no clear patterns and indicated small levels of systematic errors. In Figures 4d and 5d, histograms of the frequency of residuals were plotted for the measured and predicted Y1 and Y2 and were skewed, in particular for Y2. For an unbiased model, the residuals are expected to follow a normal distribution. This deviation could relate to the lack of fit for the linear model for Y2. Figure 6 plots experimental versus calculated responses, and correlation coefficients R2 are 0.996 and 0.984 for Y1 and Y2, respectively. This R2 = 0.984 for Y2 indicates that, despite the indications of curvature for the uptake of CO2 (Y2), the model could predict the observed data rather well. The strong dependency on time and the amount of water on the uptake of CO2 and the N-atom content were quantified and were consistent with the findings of Harlick and Sayari.56 They showed that the amount of water in toluene during reactions was crucial to the hydrolysis, condensation, and grafting of the trialkoxysilanes on silica surfaces. In contrast to their findings, we surprisingly did not observe that increasing the temperature during the reaction (from 80 to 110 °C) enhanced the N-atom content and increased the uptake of CO2. The pretreatment of silica with HCl before functionalization had a significant positive effect on the amount of APTES and the uptake of CO2. A range of interaction terms (bilinear terms) affected the degree of modification and the CO2 uptake, and these would not easily have been studied by other approaches. The fractional factorial design allows such higher-order effects to be revealed. From the very large variations in the CO2 uptake among seemingly similar syntheses, it is worth the effort to optimize the conditions for the postsynthesis modification of Davisil with APTES, at least the time of reaction and the amount of water added. 3.2. Syntheses of Sorbents and Characterization. Solidstate NMR spectroscopy has been shown to be very useful in studying the molecular details of interfaces in mesoporous 3827
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Figure 7. Magic-angle spinning solid-state 29Si NMR spectra of Davisil modified with APTES (3-aminopropyltriethoxysilane).
materials.58,75,76 Figure 7 displays a direct-polarization 29Si NMR spectrum for Davisil modified by APTES (PA20), the sample with the largest number of n-propylamine groups. This spectrum shows four signatures. The peak at about -112 ppm represents fully condensed silica (Q4). The shoulder at about -100 ppm displays the contributions from partially condensed silica groups having one noncondensed oxygen (Q3) and shows that not all Q3 groups were accessible for reaction with APTES. The peak at about -68 ppm relates to fully condensed APTES groups, T3. The peak/shoulder at about -60 ppm is from T2 groups having one noncondensed oxygen.77-79 The fraction of fully condensed APTES on silica is expressed in the ratio T3/(T2 þ T3) and was about 70% for silica modified by APTES in Figure 7. (Note that these experiments were very time-consuming (several days) and could be applied only to the samples with the largest numbers of APTES groups.) In Figure 8, we present 1H NMR spectra for the Davisil substrate before and after acid treatment at different levels of relative humidity. Acid treatment of the substrates has been shown to be important prior to modification of the silica materials; see, for example, Brust et al. and Yokoi et al.80,62 1H NMR shifts for hydrated silica depend strongly on the humidity, among other factors.81-84 The SEM images recorded before and after acid treatments were very similar; see Supporting Information. The intensity of the spectra were normalized to the intensity of the broad peak at ∼7 ppm in the 1H NMR spectra. It is assigned as strongly hydrogen bonded silanols.83 Adsorbed water is assigned to the peak appearing at ∼2.7 ppm for the dried samples and to that appearing at ∼3.7 ppm for the samples in contact with 37% RH (Figure 8c,d). The additional peak that appeared at a chemical shift at ∼1.7 ppm for the dry samples was assigned to isolated silanol groups. This humidity-dependent shift of the 1H NMR chemical shift for adsorbed water is consistent with the findings of Liu and Maciel and of de la Caillerie et al.83,85 de la Caillerie et al. rationalized the shift in the signal for adsorbed water as being due to the fast exchange of protons between bridged silanols at ∼2.5 ppm and water at 5.8 ( 0.5 ppm. We tentatively assign the peak at ∼4.9 ppm to water molecules not in exchange with bridged silanols. The very intense peak at ∼3.7 ppm in the samples treated at 37% RH (Figure 8c,d) shows the presence of a large amount of adsorbed water as expected, but even in the “dry” samples some remaining water remained (Figure 8a,b). For dry samples, a somewhat larger amount of adsorbed water was observed for HCl-treated than for nontreated Davisil. Compare the higher intensity for the peak observed at ∼2.7 ppm in Figure 8b than in
Figure 8. Magic-angle spinning solid-state 1H NMR spectra of Davisil substrates, recorded at an MAS rate of 18 kHz for (A) Davisil dried at 110 °C for 6 h; (B) HCl-treated Davisil silica dried 110 °C for 6 h; (C) Davisil silica subjected to 37% RH overnight; and (D) HCl-treated Davisil silica subjected to 37% RH overnight.
Figure 8a and, as a corollary, the lower intensity of the peak for isolated silanol groups at ∼1.7 ppm. For the samples in contact with a controlled atmosphere of 25 °C and 37% RH, the opposite trend was observed. Further quantifying such differences in water adsorption tendencies is difficult using MAS NMR spectroscopy because of the rapid spinning conditions. However, very significant differences in water adsorption were not observed by such spectroscopy when comparing spectra for Davisil before and after HCl treatment. No distinct trend was observed for the specific surface areas (BET, N2) as a function of the degree of functionalization; see Figure 9. The absence indicates a varying degree of heterogeneity in the coating among the samples. FT-IR spectroscopy86 was used to study the Davisil silicas and their APTES modifications with a certain focus on the surface silanol groups. The signatures for free and geminal silanol groups were visible for the substrate but vanished for Davisil/APTES. The signatures typical for CH2 and NH2 groups were observed in the infrared spectra.87 Spectra are presented in Supporting Information Figure SI-7. 3.3. Uptake of Dry CO2 and Dry N2. The uptake of CO2 was determined for the 22 samples. Twenty different APTES modifications were studied: PA5-P20 as described above and 3828
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Table 5. Calculated Parameters for the Apparent Selectivity (the Uptake Ratio of CO2 to N2)a sample
Ca (bar-b)
bh
PA1
25
22.8 ( 0.2
-0.177 ( 0.019
PA2
25 50
35.5 ( 0.2 54.5 ( 0.8
-0.567 ( 0.013 -0.517 ( 0.032
70
50.5 ( 1.3
-0.528 ( 0.062
25
34.8 ( 0.2
-0.486 ( 0.011
50
47.4 ( 0.4
-0.687 ( 0.023
70
50.9 ( 1.6
-0.570 ( 0.076
25
102 ( 4
-0.583 ( 0.088
50
27.0 ( 0.3
-0.487 ( 0.024
70 25
47.6 ( 1.6 27.6 ( 0.2
-0.543 ( 0.082 -0.364 ( 0.005
PA3
PA4
Figure 9. Content of nitrogen as a function of the BET surface area for APTES-modified Davisil.
T (°C)
PA9 PA19
25
24.8 ( 0.5
-0.262 ( 0.010
PA20
25
9.72 ( 0.06
-0.220 ( 0.015
BHEAP
25
8.45 ( 0.07
-0.091 ( 0.015
DMAP
25
20.9 ( 0.1
-0.244 ( 0.010
a
Davisil is modified by PA-APTES, BHEAP-bis(2-hydroxyethyl)aminopropyltriethoxysilane, and DMAP-N,N-dimethylacetamidinepropyltriethoxysilane. Errors are in the 95% confidence interval.
Figure 10. Pressure and temperature dependency of the uptake of CO2 on Davisil silica modified by APTES (PA3): 9, -20 °C; \. 0 °C;2, 2 °C 5;1, 50 °C; f, 70 °C; and —, the Freundlich expression.
PA1-PA4. Other data for PA1-PA4, N,N-dimethylacetamidinepropyl-triethoxysilane (DMAP), and 3-[bis(2-hydroxyethyl)amino]propyl-triethoxysilane (BHEAP) have been published elsewhere by Zhao et al.20 The adsorption isotherms for dry CO2 were recorded at five different temperatures for PA2, PA3, PA4, PA9, and BHEAP and at four different temperatures for PA19 and PA20. The data for PA3 are presented in Figure 10 and show a normal temperature dependency for adsorption. None of these samples showed indications of temperature-activated uptake that has been observed for APTES-modified silica by Chaffe et al. and Bacsik et al.88,18 The recorded isotherms were parametrized by a two-parameter isotherm, the Freundlich isotherm, and the corresponding parameters are presented in Supporting Information SI Tables 2 and 3. Data could be well described with the Freundlich isotherm. The amount adsorbed, q, relates to the pressure, p, via the heterogeneity parameter, n, and proportionality constant, Kf, as ð2Þ q ¼ Kf p1=n High Kf and n values were observed for APTES-modified Davisil having a high CO2 uptake. In particular, PA11, PA19, and PA20 had significant surface concentrations of n-propylamine groups: 3.2 -4.2 N atoms/nm2 ; Kf = 1.2-1.5 mmol g-1 bar-1/n; and n = 5.5-7.9. For APTES-modified Davisil having the lowest uptake of CO2, in particular, PA6 and PA7, with a surface concentrations of n-propylamine groups of 1.6 N atoms/nm2, Kf = 0.3-0.6 mmol g-1 bar -1/n, and n = 1.7-3.1, it appeared that the average distance between amine groups was too large for
a significant number of ammonium-carbamate ion pairs to form. We also used the Freundlich model to parametrize CO2 adsorption on silica using the data of Serna-Guerrero et al., which is shown in Supporting Information Figure SI-9.0.89 For BHEAP, uptake was low even though the surface concentration was high. PA1 and DMAP had smaller n values and smaller uptakes of CO2 at low pressure when compared with samples with similar Kf values (e.g., PA19 and PA11). These findings indicated a smaller amount of chemisorption for PA1 and DMAP than for PA11 and PA19-20 and are consistent with the lower surface density of amine groups. This isotherm is formally unphysical in the limit of zero pressure, but it is robust (only two parameters) and provides information on the degree of heterogeneity of the modifications. Next to the Langmuir model, it is the most commonly used adsorption isotherm with two parameters and is an empirical way to represent data,90 but it can also be physically related to the adsorption on heterogeneous surfaces.91 The CO2 adsorption isotherm for the PA1 sample (see Figure 14 in the discussion of moist CO2) showed a significantly reduced differential uptake at a pressure of about 0.6 bar. Such a rather sudden reduction of the derivative of the uptake with respect to pressure could represent a kinetic hindrance. Related amine-modified carbons from biomass have been shown to show similar steps in the isotherms.92 The uptake of dry N2 was studied at 25 °C for amine-modified Davisil: PA1-4, PA9, PA19-20, and BHEAP. The uptake of dry N2 was also studied at 50 and 70 °C for amine modified Davisil: PA2-4. We quantified the adsorption isotherms with the Freundlich equation; the parameters for the adsorption of N2 in the isotherm models are presented in Supporting Information SI Table 4. By dividing the expressions for Freundlich adsorption for the uptake of CO2 and N2, the indicated selectivity S depends on pressure as KfCO2 p1=nCO2 ¼ Ca p1=nCO2 - 1=nN2 ¼ Ca pbh ð3Þ KfN2 p1=nN2 The ideal selectivities were calculated using parameters for the Freundlich isotherms (SI Tables 2 and 4) and eq 3. The resulting Sind ¼
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Table 6. Apparent CO2-over-N2 Selectivities and CO2 Capacities for Adsorbentsa sample
adsorption capacity
selectivity
T (°C)
Zif-100
1.0
25
0
Zif-100 zeolite 13X
0.71 4.5
25 17
25 25
zeolite 13X
3.12
16
50
PA3
0.92
34
25
PA3
0.68
46
50
a
Davisil/APTES (PA3); the other sorbents are reported in the literature.88,89 Figure 12. CO2 adsorption capacity (at 25 °C and 1.0 bar) for aminemodified Davisil correlated to the surface density of the N atom. Samples with letters were studied under dry and moist conditions .O for dry CO2; • for moist CO2. The capacity was measured as the total gas uptake, hence it included water.
Figure 11. (a) Quota of CO2 and N2 adsorption (dry conditions) as a function of pressure and temperature for APTES-modified Davisil (PA3) at 9, 25 °C; b, 50 °C; and 2, 70 °C. (b) Quota of CO2 and N2 adsorption (apparent selectivity) as a function of the pressure and uptake capacity for samples at a temperature of 25 °C. Values close to the data mark the uptake capacities.
Ca and b coefficients of eq 3 are presented in Table 5. The selectivity was higher at low pressure than at high pressure for all studied samples, and bh was negative. The temperature variations for the selectivity (Sind) showed different dependencies for the samples; compare for example with the temperature dependency for parameters Ca and bh for samples PA3 and PA4 in Table 5.
The selectivity of the studied amine-modified Davisil was in general significantly higher than the ideal selectivities of zeolite 13X and zeolitic imidazolate frameworks (ZIFs).93,94 For PA3 modified with n-propylamine groups, the apparent selectivity was improved 2- or 3-fold as compared to that for zeolite 13X; see Table 6. Please note that a high selectivity also lead to additional costs associated with temperature (or pressure) swing adsorption processes.95 The observed ideal selectivity varied among the samples but was always higher at low as compared to high pressure. The ideal CO2-over-N2 selectivities were large for PA3 and are presented in Figure 11a and show that a high uptake of CO2 does not necessary imply a high degree of selectivity. The ideal selectivity for PA solids could not be rationalized only from the amount of captured CO2 (or from the number of amine groups). For example, PA20 showed a lower selectivity than PA3 even though its uptake of CO2 was larger. The higher uptake of N2 on PA20 as compared to that on PA3 is consistent with a higher specific surface area and fewer blocked pores. PA20 had a specific surface area of 200 m2/g. Davisil modified with BHEAP moieties had the smallest amount of adsorbed CO2 (0.35). It had a low CO2-overN2 selectivity of ∼10 (Figure 7), which is typical of regular physisorption and reflect an expected difference in the different boiling and sublimation temperatures of N2(l) and CO2(s). 3.3. Uptake of Moist CO2 and N2. The uptake of moist CO2 and N2 gas was determined. It is important to note that these measures were the combined uptake of CO2 and H2O, and N2 and H2O. As described earlier, the uptake of dry CO2 depended on the number of amine groups. Certain samples increased their uptake in the presence of water, and others decreased their uptake; see Figure 12. For example, PA20 adsorbed a significantly larger amount of gas in the presence of water than in the absence. However, the PA1 sample decreased its capacity for CO2 and H2O uptake, as compared with the capacity for dry CO2 uptake. PA20 had a higher surface area than PA3, and if this effect would have been connected only to water adsorption, then one would have expected this effect to be larger for PA3 than for PA20 because a smaller surface area indicates smaller pores because of its (most probably) narrower pores. However, the increase in uptake was much more dramatic for PA20 than for PA3. The water uptake capacities for all amine-modified Davisil samples were separately determined. We determined the 3830
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Table 7. Parameters for the Freundlich Expression for the Uptake of Moist N2 on Nine Different Amine-Modified Silicas (Davisil) Determined at 25 °Ca sample
a
Kf (mmol g-1 bar -1/n)
n
PA1
0.422 ( 0.003
2.11 ( 0.05
PA2
0.377 ( 0.002
1.28 ( 0.02
PA3
0.263 ( 0.002
1.22 ( 0.02
PA4
0.336 ( 0.001
1.79 ( 0.02
PA9 PA19
0.167 ( 0.002 0.12 ( 0.001
1.42 ( 0.04 1.78 ( 0.03
PA20
0.308 ( 0.002
1.34 ( 0.02
BHEAP
0.105 ( 0.001
1.79 ( 0.05
DMAP
0.143 ( 0.001
1.73 ( 0.06
The errors are given as the estimated 95% confidence interval.
Figure 13. (a) Pressure dependence of the uptake of dry and moist CO2 on Davisil modified with APTES (PA20) at a temperature of 25 °C. O for moist CO2; • for dry CO2. Lines are Freundlich isotherms. (b) Uptake of dry and moist CO2 on Davisil/APTES (PA3) at 25 °C.
Figure 15. Ratio of the CO2 and N2 adsorption (apparent selectivity) presented for sample PA20 as a function of the pressure for dry and moist gases at a temperature of 25 °C. b for dry CO2; O for moist CO2.
Figure 14. Pressure dependency of the uptake of dry and moist CO2 on APTES (PA1) and an N,N-dimethylacetamidine-propyl-modified (DMAP) on Davisil silica at a temperature of 25 °C. 0, 9 for PA1; 4, 2 for DMAP; and — for the associated Freundlich isotherms.
capacities for water adsorption 2 days after the samples have been in contact with moist air (25 °C and 90% RH). Uptake of moist CO2 was studied for samples PA1-PA4, PA20, DMAP, and BHEAP. PA20 had the highest surface density and showed the largest increase as compared with dry CO2. The uptake increased from 1.49 to 2.26 mmol/g (1 bar) in the presence of water for PA20 (Figure 13a). The Freundlich isotherm did not describe the volumetric uptake of moist CO2 well. The gas uptake of moist CO2 was slightly larger than that of dry CO2 on Davisil/PA3 in Figure 13b. For all samples except PA20, the magnitude of the increase was far from the theoretical
value, assuming bicarbonate or carbonate formation. No capillary condensation of water was expected for the studied relative levels of humidity. Davisil/PA1 and Davisil/ DMAP showed a smaller uptake of moist CO2 than of dry CO2; see Figure 14. This was surprising. Short of proof, we speculate that the presence of water hinders the uptake of CO2 kinetically, maybe by releasing surface-bound amine groups. That a 2-fold increase in CO2 sorption was not observed on these amine-modified Davisil samples was more expected. Although it has been presumed that bicarbonates can form in the presence of water, evidence has been mounting that such bicarbonate or carbonate formation is not (generally) the case for amine-modified silica; see the discussion in the review of Choi et al.7 The uptake of moist N2 was studied at 25 °C for nine of the samples. The adsorption isotherms were parametrized by the Freundlich model; see Table 7. The adsorption of moist N2 gas was (significantly) affected by water adsorption as water coadsorbed with N2. Figure 15 presents the uptake ratio for moist CO2 over moist N2 and the ratio for dry CO2 over dry N2. It was low for the full 3831
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Figure 16. Ratio for dry CO2 over dry N2 was high (filled symbols) and moist CO2 over moist N2 (open symbols) for Davisil/APTES (PA3) at 25 °C.
the amount of water, reaction time, and pretreating the silica substrate with acid. Surprisingly, we did not observe that increasing or decreasing the temperature for the reaction (from 80 to 110 °C) enhanced the N-atom content or increased the uptake of CO2. Certain two-way interactions between experimental parameters were shown to be important for successfully modifying silica with APTES. The very large variations in the CO2 uptake among seemingly similar syntheses showed that it is worth the effort to optimize the conditions for the postsynthesis modification of Davisil with APTES. The calculated model showed some lack of fit when it came to CO2 uptake, even though the regression was good and this potentially related to complications in choosing proper center points for discrete variables of the experimental design. The systematic approach that was applied allowed a quantification of the effects in a design model and the achievement of a very high grafting density of 4.0 amine groups/nm2. This density is high as compared with typical values for APTES. A 29Si NMR spectrum showed mainly fully condensed triethoxysilanes. A distinct correlation between the number of amine groups and the uptake of CO2 was observed. However, no significant correlation was found between the specific surface areas and the number of n-propylamine moieties. This indicated differential heterogeneity of the coatings among the samples. The uptake of dry CO2 and dry N2 at temperatures between -20 and 70 °C and pressures between 0 and 1 bar was described well by the Freundlich isotherms. The uptake of dry CO2 increased with the amine density. The high density of amines showed larger heterogeneity parameters in the Freundlich model. The uptake of CO2 was much higher at low pressure as compared with samples with a low density of amines. This difference is related to the fact that propylamine groups need to be close for propylammonium-propylcarbamate ion pairs to form. A certain sample had a higher uptake of moist CO2 gas than of dry CO2 gas, while another had a lower uptake of moist CO2 gas than dry CO2 gas. This could potentially be rationalized by differential water adsorption tendencies. The optimal selectivity for sorbents is very much dependent on the separation processes. Because of chemisorption, the APTES-modified silica had a higher CO2-over-N2 selectivity than do typical physisorbents such as zeolites and activated carbons, especially at low partial pressures of CO2.
Figure 17. Quota of CO2 and N2 adsorption (apparent selectivity) presented for samples PA1 and DMAP as a function of the pressure for dry and moist gases at 25 °C.
’ ASSOCIATED CONTENT
range of studied pressure for PA20. However, the ratio was greater in the dry case than in the moist case, as expected from the additional adsorption of H2O in moist CO2 and moist N2. The effect of coadsorbing water was especially significant for Davisil/ PA3 (Figure 16). The ratio for dry CO2 over dry N2 was high over the full region, and the ratio for moist CO2 over moist N2 was small. The ratios for dry CO2 over dry N2 and moist CO2 over moist N2 were observed for Davisil/DMAP with Davisil/APTES (PA1), from which is was synthesized.20 This trend could relate to the presence of an amidine group in Davisil/DMAP; see Figure 17.
4. CONCLUSIONS A fractional factorial design was used to functionalize mesoporous Davisil with APTES. The most important variables were
bS
Supporting Information. Additional information on water uptake, thermal analysis, SEM micrographs, FT-IR spectra, and parameters for the Freundlich model. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Author Contributions §
B.A. and G.Z. contributed equally to this work.
’ ACKNOWLEDGMENT We thank the Swedish Science Council (VR), the Institute Excellence Centre CODIRECT, and the Berzelii Center EXSELENT on Porous Materials for financial support. We thank 3832
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Langmuir Dr. Zoltan Bacsik, Dr. LiJun Chen, and Professor Aatto Laaksonen for discussions and suggestions.
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