Amine-Based Nanofibrillated Cellulose As Adsorbent for CO2

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Amine-Based Nanofibrillated Cellulose As Adsorbent for CO2 Capture from Air Christoph Gebald,†,‡,§ Jan Andre Wurzbacher,†,§ Philippe Tingaut,‡ Tanja Zimmermann,‡ and Aldo Steinfeld†,||,* †

Department of Mechanical and Process Engineering, ETH Z€urich, 8092 Zurich, Switzerland EMPA Material Science and Technology, 8600 D€ubendorf, Switzerland § Climeworks LLC, 8044 Z€urich, Switzerland Solar Technology Laboratory, Paul Scherrer Institute, 5232 Villigen, Switzerland

)

‡

bS Supporting Information ABSTRACT: A novel amine-based adsorbent for CO2 capture from air was developed, which uses biogenic raw materials and an environmentally benign synthesis route without organic solvents. The adsorbent was synthesized through freeze-drying an aqueous suspension of nanofibrillated cellulose (NFC) and N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane (AEAPDMS). At a CO2 concentration of 506 ppm in air and a relative humidity of 40% at 25 °C, 1.39 mmol CO2/g was absorbed after 12 h. Stability was examined for over 20 consecutive 2-hadsorption/1-h-desorption cycles, yielding a cyclic capacity of 0.695 mmol CO2/g.

1. INTRODUCTION CO2 capture from atmospheric air—usually referred to as “air capture”—can be coupled with CO2 sequestration in geological formations to reduce the atmosphere’s CO2 content, or with the conversion into CO2-neutral liquid hydrocarbon fuels using renewable energy.1
3 A recent report of the American Physical Society (APS)4 questioned the economic viability of air capture, but assumes old designs and outdated values. Recent studies, including this, introduced amine-based adsorbents feasible for air capture featuring CO2 capacities comparable to those obtained under flue gas conditions.5
7 Chemical systems investigated for air capture include those based on calcium oxide and hydroxide,8
11 sodium hydroxide,12
17 potassium hydroxide,18 amines immobilized on silica nanoparticles,19 amine modified mesoporous silicas,6,7,20 amine impregnated natural fibers21 and basic ion exchange resins.22
25 Because of the need to process 2600 moles of air per mole of CO2 captured, the feasibility of the chemical system will be strongly dependent on its ability to tolerate moisture without the need to compress, heat, or cool the stream of ambient air. Aminebased adsorbents are especially suitable for this purpose, as amines react selectively with atmospheric CO2 in the presence of moisture at ambient temperature and pressure while pure CO2 can subsequently be released upon heating to about 100 °C.6 In previous studies, the synthesis of amine-based solid adsorbents was carried out either via covalent bonding or physisorption onto the support,5,26 often using mesoporous silicas as support.27 r 2011 American Chemical Society

Covalent bonding further reduces the risk of amine volatilization during the adsorption/desorption cycles.27 The reaction mechanism of CO2 with immobilized amines has been analyzed in detail for silica supports modified with aminosilanes. For dry CO2 adsorption, the formation of silylpropylcarbamate, ammonium carbamate and carbamic acid was observed.28
31 Danon et al. did not confirm the presence of carbamic acid after dry CO2 adsortion.30 For humid CO2 adsorption, ammonium carbamate formation was observed as the dominant mechanism (Reaction Scheme 1) and carbamic acid was formed to a lesser extent as compared to dry CO2 adsorption.28 CO2 þ RNH2 þ RNH2 T RNHCOO
þ RNH3 þ

ð1Þ

For the enthalpy change of reaction for dry CO2 adsorption, values of
50 to
118 kJ/mol CO2 were reported.29,31
39 For humid CO2 adsorption the enthalpy change of reaction was reported to be
56 kJ/mol CO2 and additionally
47 to
53 kJ/mol for H2O adsorption.32,33 Amine-based adsorbents proved to reversibly adsorb 0.9 mmol/g ( 0.09 mmol CO2/g (TRI-PE-MCM-41, a triaminesilane modified pore expanded mesoporous silica),20 1.72 mmol CO2/g (HAS6, a hyperbranched aminosilica),6 2.36 mmol Received: July 1, 2011 Accepted: September 13, 2011 Revised: September 13, 2011 Published: September 14, 2011 9101

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Environmental Science & Technology CO2/g (PEI/silica, a polyethylenimine modified porous silica), 2.26 mmol CO2/g (A-PEI/silica, a polyethylenimine modified porous silica stabilized with 3-aminopropyltriethoxysilane) and 2.19 mmol CO2/g (T-PEI/silica, a polyethylenimine modified porous silica stabilized with tetrapropyl orthotitanate)7 from air. Previous synthesis of amine-based adsorbent materials employed mainly inorganic materials as support.5 To promote the application of renewable materials, we suggest the use of nanofibrillated cellulose (NFC) as a solid support. NFC is an abundant natural material composed of cellulose fibril aggregates, 10
100 nm in diameter and several micrometers length.40,41 Furthermore, NFC is rich in surface hydroxyl groups, which can be used to anchor chemical functionalities covalently to the cellulose backbone. Amine functionality has been introduced on NFC through grafting of aminosilanes, which has been the compound of choice to bond amines covalently to mesoporous silicas,42 cellulose fibers43
49 and NFC.50 However, the hydrolysis and self-condensation of aminosilanes with three alkoxy groups is fast and leads to the build-up of three-dimensional siloxane networks.51 This characteristic is unfavorable for air capture, as functional amine sites and pores are likely to be blocked by siloxane networks. This problem may be overcome by the use of anhydrous organic solvents.27 We decided to use aminosilanes having two alkoxy functions instead of three alkoxy functions, as dialkoxy silanes build up only two-dimensional linear structures or ring structures upon self-condensation,52 hence, avoiding the undesired blocking of functional amine sites and enabling the use of water as solvent. Recently, it has been reported that the use of pure water is superior to water/alcohol mixtures for aminosilane grafting on cellulose.53 The overall objective of this paper is to propose a novel aminebased adsorbent suitable for air capture. The new adsorbent was characterized by BET, SEM, FTIR, elemental analysis and its CO2 capacity as well as CO2 uptake rate were evaluated through adsorption of humid air at 40% relative humidity and desorption at 90 °C in Ar.

2. EXPERIMENTAL SECTION Materials. N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane (AEAPDMS, 97% purity) was obtained from ABCR (Germany) and used as received. As starting material for the isolation of NFC commercially available refined fibrous beech wood pulp suspension was used (Arbocel P 10111, 13.5% w/w aqueous suspension, Rettenmeier & S€ ohne GmbH & Co. KG, Germany). Synthesis. A 0.5% w/w NFC hydrogel was obtained through a two-step mechanical isolation process from the refined fibrous beech wood pulp suspension.54 AEAPDMS was added to this NFC hydrogel until the total silane concentration was 4% w/w. After sittring for 24 h, the suspension was centrifuged (Hettich Rotanta/AP) at 3600 rpm for 20 min and the remaining water was removed by freeze-drying. For this last processing step, the retentate was poured in a copper cylinder that was then immersed in liquid N2 and the frozen sample was dried in a freezedryer (Leybold Lyovac GT2). Once dried, the material was removed from the copper cylinder and heated in an inert Ar atmosphere to 120 °C for 2 h. The resulting material is in the following referred to as AEAPDMS-NFC-FD. For comparison, the synthesis procedure above was repeated without silane addition and the produced sample is referred to as NFC-FD. Material Characterization. The morphology was examined by scanning electron microscopy (SEM, FEI Nova NanoSEM

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230). Specific surface areas were determined by N2 adsorption (Micrometritics TriStar) by Brunauer
Emmett
Teller (BET) method with samples degassed (Micrometritics FlowPrep 060) at 105 °C for 24 h in dry N2 flow. The density was calculated from weight and volume, which was determined by measuring the diameter and height of the dried cylindrical product. The composition was determined through elemental analysis and Fourier-transform-infrared-spectroscopy (FTIR, Digilab BioRad FTS 6000 spectrometer, ATR mode, 32 scans, 4 cm
1 resolution). CO2 Adsorption/Desorption Measurements. CO2 adsorption/ desorption measurements were performed in a fully automated system, schematically shown in Supporting Information Figure S1. It consists of a 40 mm-i.d 30 mm-height cylindrical packed bed filled with 1.2 g of adsorbent material. AEAPDMS-NFC-FD was broken in irregularly shaped pieces of mean size approximately 10 mm. The temperature of the packed bed was measured by a K-type thermocouple. The gas flow was controlled by two electronic mass flow controllers (Bronkhorst EL-FLOW). One of the gas streams was bubbled through a water bath kept at 25 °C. The relative humidity before and after the packed bed arrangement was monitored by two electronic humidity sensors (Vaisala HMP110). After passing the off-gas through a filter and a condenser, the CO2 content was analyzed by an IR analyzer (Siemens Ultramat 23) equipped with two detectors for the CO2 concentration ranges of 0
1000 ppm and 0
5%, at 1 Hz sampling rate and 0.2% range detection limit. Pressurized air with a CO2 concentration of 506 ppm was used for CO2 adsorption. Argon was used for CO2 desorption. All CO2 adsorption experiments were carried out at an air flow rate of 1 L/min at 25 °C and 40% relative humidity. All CO2 desorption experiments were performed at an Ar flow rate of 0.8 L/ min containing a level of moisture which corresponds to a relative humidity of 40% at 25 °C. In this study Ar was used as a purge gas during desorption to enable precise measurement of the amount of desorbed CO2 in the off-gas. In an industrial application, concentrated CO2 is obtained by a temperature-vacuum-swing process.55 With the onset of CO2 desorption, the packed bed arrangement was heated externally by a water bath at 95 °C until the packed bed temperature reached 90 °C ( 1 °C. After CO2 desorption, the packed bed was cooled by a water bath at 20 °C until the packed bed temperature reached 25 °C. Prior to all CO2 adsorption measurements, preadsorbed CO2 was removed through CO2 desorption at the defined conditions for 1 h. To determine the equilibrium CO2 capacity of the sample, CO2 adsorption was performed until the CO2 outlet concentration was higher than 97% of the CO2 inlet concentration, which occurred after 12 h of CO2 adsorption in this study. To determine the cyclic CO2 capacity of the sample and to examine the stability of the AEAPDMS grafting, consecutive 2 h-adsorption/1 h-desorption cycles were carried out. For all adsorption and desorption experiments, the CO2 uptake and release, Δq, was calculated by integrating the signal of the IR gas analyzer, Z t ¼ tads=des n_ gas ðc0
c1 Þ dt Δq ¼ ms t¼0 where tads/des is the adsorption and desorption time, respectively, n_ gas the molar flow rate of the respective gas stream, c0 and c1 the CO2 concentrations at the inlet and outlet of the packed bed, respectively, and ms the mass of the adsorbent material contained in the packed bed. FTIR spectra after CO2 adsorption and desorption were recorded from a sample which was removed 9102

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Table 1. BET Surface Area, Density, And Amine Loading of NFC-FD and AEAPDMS-NFC-FD BET surface

density

amine loading

sample

area [m2/g]

[kg/m3]

[mmol N/g]

NFC-FD AEAPDMS-NFC-FD

26.8 7.1

26 61

4.9

Figure 2. FTIR spectra of NFC-FD and AEAPDMS-NFC-FD.

Figure 1. (a) SEM of NFC hydrogel, (b) NFC-FD, (c) AEAPDMSNFC-FD.

from the packed bed arrangement after 12 h of humid CO2 adsorption and 1 h of CO2 desorption, respectively.

3. RESULTS AND DISCUSSION Material Characterization. SEMs of the NFC before and after freeze-drying are shown in Figure 1a and b, respectively. Before freeze-drying, the NFC hydrogel morphology was composed of entangled cellulose nanofibrils of submicrometer diameter. After freeze-drying, the NFC-FD morphology was composed of cellulose sheet structures with single irregularly distributed cellulose nanofibrils attached to the cellulose sheets. Aggregation of cellulose nanofibrils to form sheets upon freezing and drying was previously observed.56
58 The cellulose sheet forming was explained through ice crystal growth during freezing, which squeezed cellulose nanofibrils around the ice crystal in the form of cellulose sheets.58 The presence of AEAPDMS in the NFC suspension further increased the sheet-forming tendency and eliminated single cellulose nanofibrils, as shown in Figure 1c. Ice crystal growth presumably caused cellulose nanofibril aggregation while the hydrolyzed silane molecules in solution are pushed into the spaces between the ice crystals, forming dense cellulose-silane aggregates. Comparing Figure 1a and Figure 1c indicates the potential of AEAPDMS-NFC-FD: by finding ways to keep the nanofibrillar structure intact after amine modification and drying, more amine groups can be accessible for surface reaction with CO2, which will enhance the CO2 uptake rate of the adsorbent.

The BET surface area of NFC-FD was 26.8 m2/g and the density was 26 kg/m3 (Table 1). These values are comparable to those of cellulose aerogels prepared from NFC.56,58,57 The BET surface area of AEAPDMS-NFC-FD was 7.1 m2/g and the density 61 kg/m3, indicating that the surface modification reduced the available surface area but increased the density of the sample. For comparison, the BET surface area was 367 m2/g for TRI-PE-MCM-4120 and 45 m2/g for HAS6.6 Nevertheless, previous studies indicated that, for CO2 adsorption from gas streams with CO2 partial pressures of less than 0.01 atm, the equilibrium CO2 capacity of the adsorbent is mainly influenced by the amine loading and less dependent on the surface area.59,6 The amine loading of AEAPDMS-NFC-FD was obtained through elemental analysis and determined to be 4.9 mmol N/g. For comparison, the amine loading was 7.95 mmol N/g for TRI-PE-MCM-41,42 9.9 mmol N/g for HAS66 and 10.5 mmol N/g for T-PEI/silica.7 The FTIR spectra of NFC-FD and AEAPDMS-NFC-FD are shown in Figure 2. The spectrum of NFC-FD exhibited typical bands for cellulose, such as OH stretching at 3345 cm
1, CH stretching at 2900 cm
1, CH2 symmetric bending at 1430 cm
1, OH and CH bending as well as C—C and C—O stretching at 1380, 1317, and 1256 cm
1, C
O
C skeletal vibrations around 1055 cm
1, C1
H deformation with ring vibration contribution and OH bending at 898 cm
1, and OH out of plane bending at 666 cm
1.50,60,61 The spectrum of AEAPDMS-NFC-FD after thermal treatment showed successful grafting of AEAPDMS on NFC through NH2 bending at 1600 cm
1.29,62
65 The band at 1662 cm
1, similar to the one for mesoporous silica modified with N-(2-aminoethyl)-3-aminopropyltrimethoxysilane,62 was assigned to aminosilane employed herein and is associated with asymmetric NH3+ deformation, formed through the interaction of primary amines with water or silanol groups.28,62,64
66 The 9103

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Figure 3. CO2 outlet concentration during humid CO2 adsorption on AEAPDMS-NFC-FD (solid line) and fractional uptake of the CO2 capacity (dashed line).

bands at 2917 cm
1, 2877 cm
1, and 2810 cm
1 were assigned to CH stretching and the band at 1446 cm
1 was assigned to CH2 bending of the silane propyl chain.29,62,65 The grafting conditions employed herein were sufficient for covalent bonding of silane molecules, as corroborated by the appearance of signals associated with vibrations from silicon-based linkages at 1256 cm
1 (νSi
C), and between 1180 and 700 cm
1 (Si—OH, Si—O—C, νSi
C, νas Si
C, νas Si
O
Si, νs Si
O
Si).29,44,50 CO2 Adsorption/Desorption Measurements. CO2 Adsorption. The CO2 outlet concentration as a function of the adsorption time of AEAPDMS-NFC-FD is shown in Figure 3. CO2 adsorption on AEAPDMS-NFC-FD was stopped after 12 h of experiment, when the CO2 uptake rate became small (0.67 μmol/g/min) and the CO2 outlet concentration reached 97% of the inlet concentration. By integration, the CO2 loading of AEAPDMS-NFC-FD was calculated to be 1.39 mmol CO2/g. The amine efficiency of AEAPDMS-NFC-FD was calculated to be 28%. The highest CO2 loading reported to date was 2.36 mmol CO2/g obtained by Choi et al.7 for CO2 adsorption from dry Ar with 400 ppm CO2 concentration on PEI/silica having an amine density of 10.5 mmol N/g, resulting in an amine efficiency of 22%. Note that the amine efficiency of Choi et al.7 is given for CO2 adsorption from a dry gas, which usually increases when a wet gas stream is used. Moreover, Choi et al.6 achieved 1.72 mmol CO2/g for CO2 adsorption from humid air with 400 ppm CO2 concentration on HAS6 having an amine loading of 9.9 mmol N/g, corresponding to an amine efficiency of 17% in the fully loaded state. Belmabkhout et al.20 reported an equilibrium CO2 capacity of 0.98 mmol CO2/g for CO2 adsorption from dry air with 400 ppm CO2 concentration on TRI-PEMCM-41, where the amine efficiency was 12%. Its capacity increased from 1.9 mmol CO2/g to 2.04 mmol CO2/g when moisture was introduced in a gas stream with 5% CO2 concentration. Lackner22 reported that a commercially available strong base ion-exchange resin having a charge density of 1.7 mmol/g can theoretically absorb 0.85 mmol CO2/g. In the patent literature, Olah et al.19 reported a CO2 loading of 0.61 mmol CO2/g sorbent on amine modified fumed silica nanoparticles for dry air containing 380 ppm of CO2, and Gebald et al.21 reported a CO2 loading of 1.87 mmol CO2/g on amine modified air oxidized carbon fibers for humid CO2 adsorption from air having a CO2 concentration of 500 ppm. To fully characterize amine-based adsorbents, both the CO2 capacity and CO2 adsorption rate as a function of time are reported, as shown in Figure 4. Choi et al.6,7 reported the kinetic

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Figure 4. CO2 adsorption rate of AEAPDMS-NFC-FD (solid line) and adsorbed amount CO2 (dashed line).

Figure 5. Comparison of adsorption capacity and adsorption half time of AEAPDMS-NFC-FD (star), HAS6 (sphere), PEI/silica (rhombus), A-PEI/silica (triangle), T-PEI/silica (square).6,7.

data in the form of adsorption half time (time to 50% of final CO2 uptake), but this indicator is only meaningful in combination with the CO2 capacity as a small adsorption half time does not necessarily correspond to a high CO2 adsorption rate. Nevertheless, the adsorption half time is also given here for comparison (Figure 5). Note that particle size, residence time, linear velocity of the incoming gas and length over diameter of the packed bed investigated here differ from those used by Choi et al.6,7 The adsorption half time of AEAPDMS-NFC-FD was 92 min, corresponding to an average CO2 adsorption rate of 7.6 μmol/g/min during the adsorption half time. The adsorption half time and corresponding average CO2 adsorption rate was 309 min and 3.8 μmol/g/min for PEI/silica, 196 min and 5.8 μmol/g/min for A-PEI/silica, 210 min and 5.2 μmol/g/min for T-PEI/silica and 167 min and 5.2 μmol/g/min for HAS6.6,7 The amine-based solid adsorbent TRI-PE-MCM-41 synthesized by Belmabkhout et al.20 needed 15 min to reach 90% of its equilibrium CO2 capacity for CO2 adsorption from a dry gas mixture with 1000 ppm CO2 concentration. Lackner22 reported that 2500 kg of strong base ion-exchange resin are needed to capture 0.25 mol CO2/s from air, corresponding to an uptake rate of 6 μmol/g/min, which is comparable to that of AEAPDMS-NFC-FD during the first 120 min (6.7 μmol/g/min). In spite of the relatively small BET surface area and a particle diameter of 10 mm, the CO2 9104

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Figure 6. Comparison of CO2 capacity as a function of time of AEAPDMS-NFC-FD (dashed line), HAS6 (spheres), PEI/silica (rhombuses), A-PEI/silica (triangles), and T-PEI/silica (squares)6,7.

Figure 7. FTIR spectra of AEAPDMS-NFC-FD after humid CO2 adsorption.

adsorption rate of AEAPDMS-NFC-FD is comparable to that of HAS6, PEI/silica, A-PEI/silica and T-PEI/silica in powder form,6,7 as shown in Figure 6, where the CO2 adsorption rate of the literature adsorbents was assumed as constant. From this result we conclude that the bulk CO2 uptake rate of 10 mm AEAPDMS-NFC-FD particles is feasible for air capture. The favorable CO2 uptake rate can be explained by the highly porous structure of AEAPDMS-NFC-FD, which is composed of cellulose sheets separated by pores in the micrometer range (see Figure 1). For comparison purposes, NFC-FD was also tested for CO2 adsorption. The CO2 outlet concentration reached the value of the CO2 inlet concentration immediately after gas introduction, confirming that no physisorption of CO2 occurs on the solid support without modification. The FTIR spectrum of AEAPDMS-NFC-FD after 12 h of humid CO2 adsorption is shown in Figure 7. FTIR spectra of CO2 adsorption on aminosilane modified NFC have not been reported yet. The FTIR spectrum obtained after CO2 adsorption on AEAPDMS-NFC-FD is tentatively compared to FTIR spectra obtained from CO2 adsorption on amine-modified silicas. The band at 1568 cm
1 was ascribed to (N)COO
asymmetric stretching,28
30,62,64
66 the band at 1466 cm
1 to symmetric NH3+ deformation and the bands at 1410 cm
1, 1369 cm
1 and 1310 cm
1 to symmetric stretching of (N)COO
.28,30 From CO2 absorption in amine solutions it is known that CO2 reacts

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Figure 8. Multicycle adsorption (squares) and desorption (spheres) experiments with AEAPDMS-NFC-FD.

with primary and secondary amines to form carbamic acid, carbamate and bicarbonate species.28,67,68 By comparing the FTIR spectra obtained in this work to the FTIR spectra obtained from CO2 absorption in amine solution, the band at 1369 cm
1 could be further assigned to bicarbonate.67 However, this assignment remains questionable. First the formation of carbamate species, through deprotonation of carbamic acid, is kinetically and thermodynamically favored over bicarbonate formation for primary and secondary amines.67,68 Second aminosilane modification of NFC rendered the NFC surface less hydrophilic when compared to unmodified NFC,44 questioning the feasibility of comparing the FTIR spectra obtained in this work to FTIR spectra obtained from CO2 scrubbing in amine solutions. CO2 Desorption. CO2 desorption from AEAPDMS-NFC-FD was fast and completed after 30 min, as shown in Supporting Information Figure S2. By integration, the amount of desorbed CO2 was calculated to be 1.41 mmol CO2/g, which confirmed the amount of adsorbed CO2 within 1.5% error. More than 85% of the CO2 is desorbed within 19 min at a packed bed temperature below 80 °C. The FTIR spectra of AEAPDMS-NFC-FD after CO2 desorption is shown in Supporting Information Figure S3. The peaks that evolved after CO2 adsorption disappeared and the FTIR spectrum of the pristine AEAPDMS-NFC-FD was re-established. Cyclic CO2 Capacity. Figure 8 shows 20 consecutive CO2 adsorption/desorption cycles. No decrease in cyclic capacity was observed, which confirmed the structural stability of AEAPDMSNFC-FD under the conditions used. During 2 h of CO2 adsorption at 25 °C and 40% relative humidity and 1 h of CO2 desorption in Ar at 90 °C and 40% relative humidity, the average cyclic CO2 capacity was 0.695 mmol CO2/g. During CO2 desorption, humidity was added to avoid urea formation and a related loss of amine functionality.69 In an industrial application, the cycle duration might deviate from the one used in this study. Short cycle times maximize the amount of CO2 captured per mass of adsorbent per time and therefore reduce the capital cost. Long cycle times favor high CO2 loading, which minimizes the energy requirement for desorption. The optimal duration results from a trade-off between these two aspects.

’ ASSOCIATED CONTENT

bS

Supporting Information. Figures S1
S3. This material is available free of charge via the Internet at http://pubs.acs.org.

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’ AUTHOR INFORMATION Corresponding Author

Phone: +41-44-6327929; fax: +41-44-6321065; e-mail: aldo.steinfeld@ ethz.ch.

’ ACKNOWLEDGMENT € F-STIFTUNG We thank the Swiss foundation GEBERT-RU for financial support. ’ NOMENCLATURE c0 CO2 concentration at the inlet of the packed bed CO2 concentrations at the outlet of the c1 packed bed mass of the adsorbent material contained ms in the packed molar flow rate of gas stream (compressed n_ gas air or Ar) Δq adsorption/desorption CO2 capacity adsorption time tads desorption time tdes Acronyms

ABCR AEAPDMS

Supplier of specialty chemicals N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane AEAPDMS-NFC-FD aminosilane modified nanofibrillated cellulose aerogel A-PEI/silica aminosilane stabilized polyethylenimine modified porous silica7 BET Brunauer
Emmett
Teller method FTIR Fourier-Transform-Infrared-Spectroscopy HAS6 hyperbranched aminosilica6 MEA monoethanolamine NFC nanofibrillated cellulose NFC-FD nanofibrillated cellulose aerogel PEI polyethylenimine PEI/silica polyethylenimine modified porous silica7 SEM scanning electron microscopy T-PEI/silica tetrapropyl orthotitanate stabilized polyethylenimine modified porous silica7 TRI-PE-MCM-41 aminosilane modified pore expanded mesoporous silica20

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