Engineering of Silica Mesoporous Materials for CO2 Adsorption

Adsorption methods for CO2 capture are characterized by high selectivity and low energy consumption. Therefore, the engineering of solid supports for efficient CO2 adsorption attracts research attention. Modification of mesoporous silica materials with tailor-made organic molecules can greatly improve silica’s performance in CO2 capture and separation. In that context, a new derivative of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, possessing an electron-rich condensed aromatic structure and also known for its anti-oxidative properties, was synthesized and applied as a modifying agent of 2D SBA-15, 3D SBA-16, and KIT-6 silicates. The physicochemical properties of the initial and modified materials were studied using nitrogen physisorption and temperature-gravimetric analysis. The adsorption capacity of CO2 was measured in a dynamic CO2 adsorption regime. The three modified materials displayed a higher capacity for CO2 adsorption than the initial ones. Among the studied sorbents, the modified mesoporous SBA-15 silica showed the highest adsorption capacity for CO2 (3.9 mmol/g). In the presence of 1 vol.% water vapor, the adsorption capacities of the modified materials were enhanced. Total CO2 desorption from the modified materials was achieved at 80 °C. The obtained silica materials displayed stable performance in five CO2 adsorption/desorption cycles. The experimental data can be appropriately described by the Yoon–Nelson kinetic model.


Introduction
The increasing volumes of carbon dioxide released into the atmosphere by human activities (greenhouse effect) are considered one of the global concerns of society [1]. Carbon dioxide is among the main constituents of greenhouse gases, being both of natural and anthropogenic origin [2]. Generally, due to the presence of greenhouse gases in the atmosphere, the temperature of the Earth is conducive to living systems. However, over the last decades, a steady rise in the average temperature of the planet has been recorded, and the trend is expected to continue, reaching an increase of 6.4% by the end of this century [3]. The anthropogenic greenhouse effect contributes significantly to climate change and extreme phenomena such as floods and droughts [2]. Therefore, a reduction in the CO 2 emissions into the atmosphere would mitigate the adverse effect of human activity and industrialization in particular [3]. On the other hand, CO 2 is an abundant resource for many chemical industries, and its capture and recycling provide ample opportunities for research [4].

Preparation of KIT-6, SBA-15, and SBA-16
The mesoporous KIT-6, SBA-15, and SBA-16 silicas were prepared through hydrothermal synthesis. The synthesis of KIT-6 [44] includes the following steps: (i) Pluronic 123 (12 g, 2.1 mmol) was added at room temperature to an aqueous medium prepared from 37% HCl (37.1 mL) in 366 mL H 2 O. (ii) The nano-colloidal solution was mixed with butanol (15 mL), and 24 g of tetraethyl orthosilicate (TEOS) was added to the clear solution in 1 h. After that, the mixture was aged at 140 • C for 24 h under static conditions. (iii) The obtained solid product was filtered without further washing and dried under vacuum at 100 • C overnight. (iv) The material was calcined in air at 550 • C for 6 h with a heating rate of 1 K/min.
The synthesis of SBA-15 [45] was performed using the hydrothermal procedure. Pluronic P123 (6 g, 1.0 mmol) was dissolved in 180 g distilled water and 18.5 g 37% HCl at room temperature. Next, 12 g TEOS was added to the solution and stirred at 35 • C for 24 h. The obtained gel was transferred to a Teflon vessel jacketed in a stainless-steel autoclave and heated in an oven at 95 • C for 24 h. The obtained white material was filtered and calcined in an oven in the air at 290 • C for 2 h and at 550 • C for 5 h with a heating rate of 1 K/min. SBA-16 was prepared according to the procedure of Hu et al. [46]. Pluronic F127 triblock copolymer and CTAB were used as templates and tetraethylorthosilicate (TEOS) as the silica source. F127 (4 g, 0.3 mmol) and CTAB (0.48 g, 1.3 mmol) were dissolved entirely into a solution of 130 mL water and 10 mL concentrated HCl. Under continuous stirring, 4.0 mL TEOS was added. The mixture was stirred at 40 • C for 1 h and then the temperature was increased to 80 • C for 24 h under static conditions. The obtained material was filtrated, washed with water three times, and dried in an oven at 50 • C. The white material was calcined in air at 550 • C for 5 h with a heating rate of 1 K/min.

Preparation of DOPO Derivative of APTES
The preparation procedure included firstly the synthesis of a Schiff base (denoted as SAPTES) from the reaction of APTES and furfural as described in our recent publication [40]. The procedure and the NMR and IR data are provided in the Supplementary Material.

Modification of Mesoporous Silicas with DAPTES
SBA-15, SBA-16, or KIT-6 (1.0 g each of samples) was heated in an oven at 120 • C for 1 h. The hot mesoporous silica was dispersed in toluene (50 mL). After that, DAPTES (0.155 g) was added to the dispersion, which was stirred for 24 h at 60 • C on a magnetic stirrer. The obtained material was washed three times with chloroform in order to remove the unreacted modifying agent or residual diglyme. The modified samples were denoted as SBA-15/DAPTES, SBA-16/DAPTES, and KIT-6/DAPTES.  29 Si). The instrument is equipped with a dual direct broadband 1H/109Ag-31 P probe head with a z-gradient. Deuterated chloroform or DMSO was used as the solvent, and the chemical shifts were calibrated to a residual solvent peak. In the solid-state measurements, a 4 mm solid-state CP/MAS dual 1 H/X probe head was used. The samples were loaded in 4 mm zirconia rotors and spun at a magic angle spinning (MAS) rate of 10 kHz for all measurements. The quantitative 29 Si NMR spectra were recorded with a single-pulse sequence, 90 • pulse length of 4.5 µs, 3K time domain data points, spectrum width of 29 kHz, 1024 scans, and a relaxation delay of 60 s. The spectra were processed with an exponential window function (line broadening factor 100) and zero-filled to 16 K data points. The 1 H→X cross-polarization MAS (CP MAS) spectra were acquired with the following experimental parameters: 1 H excitation pulse of 3.6 µs, 5 ms contact time for the 1 H→ 29 Si and 2 ms for the 1 H→ 13 C CP experiments, and 5 s relaxation delay; typically, 6000 scans were accumulated for the 1 H→ 29 Si CPMAS experiments and up to 5000 scans for the 1 H→ 13 C CP MAS spectra. The 1 H SPINAL-64 decoupling scheme was used during the acquisition of CP experiments.
ATR-FTIR spectra were recorded using an IRAffinity-1 "Shimadzu" Fourier-transform infrared (FTIR) spectrophotometer (Shimadzu, Kyoto, Japan) with a MIRacle attenuated total reflectance attachment. For each spectrum, a resolution of 4 cm −1 and 50 scans were applied. IRsolution software was used to process the collected spectral data.
The thermogravimetric measurements were obtained with a STA449F5 Jupiter of NETZSCH Gerätebau GmbH (Netzsch, Selb, Germany) in the temperature interval up to 600 • C, with a 5 • C/min heating rate in air flow, followed by a hold-up of 1 h.
The specific surface by Brunauer, Emmett, and Teller (BET), the diameter, and the pore size distribution of the obtained materials were determined by nitrogen adsorption at −196 • C in the pressure range p/p 0 = 0.001-0.999, using an analyzer "AUTOSORB iQ-MP/AG" (Anton Paar GmbH, Graz, Austria). The samples were degassed at 80 • C with a heating rate of 5 • C/min for 16 h before every measurement.

CO 2 Adsorption
The CO 2 adsorption experiments were carried out in dynamic conditions in a flow system. Prior to the start of the adsorption experiments, the adsorbent sample (0.40 g) was dried at 150 • C for 1 h. The experiments for CO 2 adsorption were performed at 25 • C with 3 vol.% CO 2 /N 2 at a flow rate of 30 mL/min. The samples were pressed and crushed in order to obtain materials with particle sizes of 0.2-0.8 mm. Additionally, the experiments for CO 2 and water vapor adsorption (3 vol.% CO 2 plus 1 vol.% water vapor) were performed at a 30 mL/min flow rate. The adsorption capacities of the materials were calculated based on the amounts of adsorbed CO 2 and water vapor by using online GC analysis (gas chromatograph NEXIS GC-2030 ATF (Shimadzu, Kyoto, Japan) with a 25 m PLOT Q capillary column).
The CO 2 adsorption measurements under static conditions were determined at 0 • C, 25 • C, and 50 • C with an AUTOSORB iQ-MP-AG (Anton Paar GmbH, Graz, Austria) surface area and pore size analyzer (from Quantachrome, Anton Paar GmbH, Graz, Austria). The Quantachrome software was used for the transformation of the primary adsorption data.

Synthesis and Characterization of DAPTES
The synthetic procedure to obtain a DOPO derivative of APTES could include different reaction routes. DOPO possesses a hydrogen phosphonate moiety that is reactive toward amines via the Atherton-Todd reaction [47,48]. However, this reaction proceeds in the presence of a base, i.e., tertiary amines, as scavengers of the liberated HCl. The side product, most often triethyl amine hydrochloride, should be removed from the reaction mixture, which is a laborious process that decreases the yield. Therefore, based on our previous experience, the synthetic path applied included the reaction of DOPO with a Schiff base (denoted as SAPTES) derived from APTES. The preparation of SAPTES has been reported in our previous paper [40]. SAPTES was obtained quantitatively from the reaction of APTES with furfural in toluene. The solvent was removed, and the Schiff base was subjected to a reaction with DOPO. The reagents were taken in an equimolar ratio. The interaction proceeded under mild conditions, i.e., stirring the reaction mixture at 50 • C for 24 h in the presence of cadmium iodide as catalyst (Scheme 1). The formation of the product denoted as DAPTES was proven by NMR analysis including 1 H, 13

CO2 Adsorption
The CO2 adsorption experiments were carried out in dynamic conditions in a flow system. Prior to the start of the adsorption experiments, the adsorbent sample (0.40 g) was dried at 150 °C for 1 h. The experiments for CO2 adsorption were performed at 25 °C with 3 vol.% CO2/N2 at a flow rate of 30 mL/min. The samples were pressed and crushed in order to obtain materials with particle sizes of 0.2-0.8 mm. Additionally, the experiments for CO2 and water vapor adsorption (3 vol.% CO2 plus 1 vol.% water vapor) were performed at a 30 mL/min flow rate. The adsorption capacities of the materials were calculated based on the amounts of adsorbed CO2 and water vapor by using online GC analysis (gas chromatograph NEXIS GC-2030 ATF (Shimadzu, Kyoto, Japan) with a 25 m PLOT Q capillary column).
The CO2 adsorption measurements under static conditions were determined at 0 °C, 25 °C, and 50 °C with an AUTOSORB iQ-MP-AG (Anton Paar GmbH, Graz, Austria) surface area and pore size analyzer (from Quantachrome, Anton Paar GmbH, Graz, Austria). The Quantachrome software was used for the transformation of the primary adsorption data.

Synthesis and Characterization of DAPTES
The synthetic procedure to obtain a DOPO derivative of APTES could include different reaction routes. DOPO possesses a hydrogen phosphonate moiety that is reactive toward amines via the Atherton-Todd reaction [47,48]. However, this reaction proceeds in the presence of a base, i.e., tertiary amines, as scavengers of the liberated HCl. The side product, most often triethyl amine hydrochloride, should be removed from the reaction mixture, which is а laborious process that decreases the yield. Therefore, based on our previous experience, the synthetic path applied included the reaction of DOPO with a Schiff base (denoted as SAPTES) derived from APTES. The preparation of SAPTES has been reported in our previous paper [40]. SAPTES was obtained quantitatively from the reaction of APTES with furfural in toluene. The solvent was removed, and the Schiff base was subjected to a reaction with DOPO. The reagents were taken in an equimolar ratio. The interaction proceeded under mild conditions, i.e., stirring the reaction mixture at 50 °C for 24 h in the presence of cadmium iodide as catalyst (Scheme 1). The formation of the product denoted as DAPTES was proven by NMR analysis including 1 H, 13 C{ 1 H}, and 31 P{ 1 H}, as well as 2D experiments 1 H-1 H COSY, 1 H-13 C HSQC, and 1 H-13 C HMBC spectra (see Supplementary Data).

Scheme 1. Synthesis of DAPTES.
The 1 H NMR spectrum of DAPTES ( Figure S2) shows a complex spectral pattern, particularly in the aromatic region. We suggest that this is due to the structural complex- The 1 H NMR spectrum of DAPTES ( Figure S2) shows a complex spectral pattern, particularly in the aromatic region. We suggest that this is due to the structural complexity of DAPTES including asymmetric carbon atoms and bulky groups, which result in the possible coexistence of at least two isomers in the solution. The 1 H NMR spectrum of DAPTES was compared to that of SAPTES. The assignment of the signals for the two compounds is provided in the Materials and Methods section. A multiplet between 4.50 ppm and 4.23 ppm is seen in the 1 H NMR spectrum of DAPTES as proof of the formation of the -HNCH(Fur)P(O)-motif. The signal for the hydrogen from the imine -N=CH-group in SAPTES appears at 8.00 ppm, i.e., in the spectral region of the protons of DOPO. Therefore, it is not straightforward to conclude that this signal disappeared in the DAPTES spectrum due to its possible overlap with the DOPO resonances. However, it is clearly seen that the signal for the carbon atom from the imine group at about 152 ppm in the 13 C NMR spectrum of SAPTES disappeared, and a doublet at 55.95 ppm with a coupling constant 1 J{PC} = 115.17 Hz appeared in the 13 C NMR spectrum of DAPTES ( Figure S2) due to the newly formed aminophosphonate structure (-HNCH(Fur)P(O)-). Further evidence for the completion of the addition reaction is found in the comparison of the 1 H and 31 P{H} NMR spectra of DOPO and that of DAPTES. The doublet at 8.07 ppm ( 1 J{PH} = 594 Hz) due to the P-H bond in the 1 H NMR spectrum of DOPO ( Figure S3) did not register in the 1 H NMR spectrum of DAPTES as a consequence of the addition of the P-H bond from DOPO to the imine function of SAPTES. Secondly, the signal for the phosphorus atom in DOPO shifted downfield by 14 ppm in the 31 P{H} NMR spectrum of DAPTES, as seen in Figure 1. Additionally, the DEPT-135 experiment ( Figure S4) was performed, and the 2D 1 H-1 H COSY ( Figure S5), 1 H-13 C HSQC ( Figure S6), and 1 H-13 C HMBC ( Figure S7) spectra of DAPTES were registered to further confirm that the product presents one substance (one and the same composition and atoms connectivity), i.e., the complexity of the spectra is due to conformational isomers. ppm and 4.23 ppm is seen in the 1 H NMR spectrum of DAPTES as proof of the formation of the -HNCH(Fur)P(O)-motif. The signal for the hydrogen from the imine -N=CH-group in SAPTES appears at 8.00 ppm, i.e., in the spectral region of the protons of DOPO. Therefore, it is not straightforward to conclude that this signal disappeared in the DAPTES spectrum due to its possible overlap with the DOPO resonances. However, it is clearly seen that the signal for the carbon atom from the imine group at about 152 ppm in the 13 C NMR spectrum of SAPTES disappeared, and a doublet at 55.95 ppm with a coupling constant 1 J{PC} = 115.17 Hz appeared in the 13 C NMR spectrum of DAPTES ( Figure  S2) due to the newly formed aminophosphonate structure (-HNCH(Fur)P(O)-). Further evidence for the completion of the addition reaction is found in the comparison of the 1 H and 31 P{H} NMR spectra of DOPO and that of DAPTES. The doublet at 8.07 ppm ( 1 J{PH} = 594 Hz) due to the P-H bond in the 1 H NMR spectrum of DOPO ( Figure S3) did not register in the 1 H NMR spectrum of DAPTES as a consequence of the addition of the P-H bond from DOPO to the imine function of SAPTES. Secondly, the signal for the phosphorus atom in DOPO shifted downfield by 14 ppm in the 31 P{H} NMR spectrum of DAPTES, as seen in Figure 1. Additionally, the DEPT-135 experiment ( Figure S4) was performed, and the 2D 1 H-1 H COSY ( Figure S5), 1 H-13 C HSQC ( Figure S6), and 1 H-13 C HMBC ( Figure S7) spectra of DAPTES were registered to further confirm that the product presents one substance (one and the same composition and atoms connectivity), i.e., the complexity of the spectra is due to conformational isomers. The IR spectrum of DAPTES is shown in Figure 2. Characteristic bands of the oxa-10-phosphaphenanthrene-10-oxide residue are present: at 1450 cm −1 attributed to the aromatic rings and at 1242 cm −1 due to P=O stretching, and the absorption in the region 960-930 cm −1 can be assigned to the P-O-phenyl vibrations [48,49]. The lack of a band at about 2385 cm −1 , characteristic of the P-H stretching, also confirmed the completion of the addition reaction of the P-H bond from DOPO to the imine group of SAPTES. The broad absorption in the region 3000-2800 cm −1 arises from C-H stretching in the alkyl and aromatic fragments of the DAPTES molecule. The intensive band in the spectral region 1150-1050 cm −1 is attributed to C-O-C stretching in the furanyl ring and Si-OCH2 stretching in the triethoxysilicate moiety [50]. A band due to C-H deformations of the furanyl ring The IR spectrum of DAPTES is shown in Figure 2. Characteristic bands of the oxa-10-phosphaphenanthrene-10-oxide residue are present: at 1450 cm −1 attributed to the aromatic rings and at 1242 cm −1 due to P=O stretching, and the absorption in the region 960-930 cm −1 can be assigned to the P-O-phenyl vibrations [48,49]. The lack of a band at about 2385 cm −1 , characteristic of the P-H stretching, also confirmed the completion of the addition reaction of the P-H bond from DOPO to the imine group of SAPTES. The broad absorption in the region 3000-2800 cm −1 arises from C-H stretching in the alkyl and aromatic fragments of the DAPTES molecule. The intensive band in the spectral region 1150-1050 cm −1 is attributed to C-O-C stretching in the furanyl ring and Si-OCH 2 stretching in the triethoxysilicate moiety [50]. A band due to C-H deformations of the furanyl ring overlapping with Si-C stretching is also seen at 758 cm −1 [51,52]. Both NMR and IR spectral analyses provided evidence for a complete conversion of the reaction of SAPTES with DOPO.

Modification of KIT-6, SBA-15, and SBA-16 Silica Materials
The SBA-15 or SBA-16 and KIT-6 materials were heated at 120 • C for 2 h to remove the adsorbed humidity and then dispersed in dry toluene. After that, DAPTES (15.5% of the silica weight) was added to each of the dispersions and allowed to react with the surface -SiOH groups for 24 h at 60 • C. The modified samples were denoted as SBA-15/DAPTES, SBA-16/DAPTES, and KIT-6/DAPTES. overlapping with Si-C stretching is also seen at 758 cm −1 [51,52]. Both NMR and IR spectral analyses provided evidence for a complete conversion of the reaction of SAPTES with DOPO.

Modification of KIT-6, SBA-15, and SBA-16 Silica Materials
The SBA-15 or SBA-16 and KIT-6 materials were heated at 120 °C for 2 h to remove the adsorbed humidity and then dispersed in dry toluene. After that, DAPTES (15.5% of the silica weight) was added to each of the dispersions and allowed to react with the surface -SiOH groups for 24 h at 60 °C. The modified samples were denoted as SBA-15/DAPTES, SBA-16/DAPTES, and KIT-6/DAPTES.
The amounts of DAPTES grafted onto the KIT-6, SBA-15, and SBA-16 pore surface were calculated by using TGA analysis. The modified samples showed a weight loss between 12.5 and 13.7 wt% at temperatures above 250 °C up to 600 °C ( Figure S8a). The comparison of the initial and the modified sorbents revealed an additional weight lost between 6.3 and 9.4 wt% for the latter, which was attributed to the decomposition of the grafted moieties. These results indicate approximately similar grafting degrees of DAPTES in the modified mesoporous silicas. Moreover, the modification resulted in the hydrophobization of the pore surface of the sorbents as seen by the comparison of the TG curves in the temperature range 50-150 °C for the parent and grafted samples ( Figure  S8b,c).
The IR spectra (Figure 2) of all the sorbent-modified samples show strong absorption in the spectral interval 1100-950 cm −1 attributed to Si-O-Si stretching vibrations of the silica matrix. Besides the dominating absorption of the latter, the presence of shoulders at about 1240 cm −1 , 950-930 cm −1 , and at 758 cm −1 evidenced the presence of DOPTES residues on the pore surface of the silica matrix.
The successful modification of the mesoporous silicas with DAPTES was also evidenced by 1 H→ 29 Si CP MAS ( Figure 3) and 1 H→ 13 C cross polarization magic angle spinning (CP-MAS) spectra. The CP technique is based on the transfer of magnetization from abundant spins ( 1 H) to low-sensitivity nuclei ( 29 Si or 13 C) via through-space dipole-dipole interactions. In the 1 H→ 29 Si CP MAS spectra, this method gives selective enhancement of the resonances from 29 Si units with 1 H in their vicinity, such as Si-OH groups or Si-sites with attached organic functional groups. In the 1 H→ 29 Si CP MAS spectra of the modified samples, a broad, low-intensity spectral pattern centered at around −57 ppm characteristic for T 2 type ((SiO)2SiOH-R; R: DAPTES residue) organosiloxane moieties was detected, The amounts of DAPTES grafted onto the KIT-6, SBA-15, and SBA-16 pore surface were calculated by using TGA analysis. The modified samples showed a weight loss between 12.5 and 13.7 wt% at temperatures above 250 • C up to 600 • C ( Figure S8a). The comparison of the initial and the modified sorbents revealed an additional weight lost between 6.3 and 9.4 wt% for the latter, which was attributed to the decomposition of the grafted moieties. These results indicate approximately similar grafting degrees of DAPTES in the modified mesoporous silicas. Moreover, the modification resulted in the hydrophobization of the pore surface of the sorbents as seen by the comparison of the TG curves in the temperature range 50-150 • C for the parent and grafted samples ( Figure S8b,c).
The IR spectra (Figure 2) of all the sorbent-modified samples show strong absorption in the spectral interval 1100-950 cm −1 attributed to Si-O-Si stretching vibrations of the silica matrix. Besides the dominating absorption of the latter, the presence of shoulders at about 1240 cm −1 , 950-930 cm −1 , and at 758 cm −1 evidenced the presence of DOPTES residues on the pore surface of the silica matrix.
The successful modification of the mesoporous silicas with DAPTES was also evidenced by 1 H→ 29 Si CP MAS ( Figure 3) and 1 H→ 13 C cross polarization magic angle spinning (CP-MAS) spectra. The CP technique is based on the transfer of magnetization from abundant spins ( 1 H) to low-sensitivity nuclei ( 29 Si or 13 C) via through-space dipole-dipole interactions. In the 1 H→ 29 Si CP MAS spectra, this method gives selective enhancement of the resonances from 29 Si units with 1 H in their vicinity, such as Si-OH groups or Si-sites with attached organic functional groups. In the 1 H→ 29 Si CP MAS spectra of the modified samples, a broad, low-intensity spectral pattern centered at around −57 ppm characteristic for T 2 type ((SiO) 2 SiOH-R; R: DAPTES residue) organosiloxane moieties was detected, in addition to the Q 4 (−110 ppm), Q 3 (−100 ppm), and Q 2 (−90 ppm) resonances that are typically observed in the spectra of non-modified silicas ( Figure S9).   The 1 H→ 13 C CP MAS spectra ( Figure S10) of SBA-16/DAPTES, KIT-6/DAPTES, and SBA-15/DAPTES show a typical spectral pattern in the region 155-110 ppm characteristic of the aromatic and furan moieties as well as the resonances for carbon atoms from the Si-CH 2 CH 2 CH 2 -NH-CH-P(O) structural motif (Si-CH 2 -at 9 ppm, -CH 2 -at 22 ppm, -CH 2 -NH-CH-from 50 to 60 ppm). Resonances from the residual -Si(OCH 2 CH 3 ) 3 structural fragments of DAPTES were also detected at around 62 ppm (-OCH 2 ) and 18 ppm (-CH 3 ), indicating that not all three OCH 2 CH 3 groups have reacted with the silanol groups from the mesoporous silicas. As an example, Figure S10 shows the 1 H→ 13 C CP MAS spectra of the KIT-6/DAPTES and SBA-15/DAPTES samples.
The textural properties of the initial and modified materials were determined using N 2 adsorption/desorption measurements. The isotherms displayed type-IV isotherms according to the IUPAC classification, which is typical for mesoporous silicas [53]. They are presented in Figure 4, and the calculated parameters are listed in Table 1. pores of the modified material are blocked with the grafted bulk molecules. The specific surface area of the modified KIT-6/DAPTES decreased less than that of modified SBA-16/DAPTES. This might be due to the interpenetrating cylindrical pores, which favors the deposition of DAPTES in the pores of the SBA-16. The results from the N2 physisorption correspond very well with the TG and IR data, indicating grafting of DAPTES into the pores of the silica materials. The preservation of the mesoporous structure of all the used mesoporous silicas during the modification procedure was observed, which is in agreement with the XRD patterns in the low-angle range ( Figure S11).

CO2 Adsorption Measurements
The mesoporous silicas (SBA-15, SBA-16, and KIT-6) modified with DAPTES were tested in dynamic conditions at atmospheric pressure. The results are presented in Table 2 and Figure 5. A higher amount of adsorbed CO2 was detected on the modified mesoporous silicas than on the initial ones. This effect is predetermined by the type of the adsorption sites, which, in the case of the initial silica materials, are the silanol groups. Quantitative 29 Si NMR measurements were performed to evaluate the amount of silanol groups in the three silica samples. The quantitative single-pulse 29 Si spectra ( Figure S12) demonstrated the presence of a higher number of silanol groups in SBA-16 as compared to KIT-6 and SBA-15 (Table S1). A correlation between the adsorption capacity and the number of SiOH groups was observed. Among the sorbents studied, SBA-16 displayed the highest value of the CO2 adsorption capacity, while the formation of a smaller number of SiOH groups in the SBA-15 and KIT-6 silicas led to lower adsorption of CO2 on them. In addition, the time needed for achieving the total adsorption capacity for the SBA-15 silica (T = 8 min) was shorter in comparison to the time determined for CO2 adsorption on to the SBA-16 and KIT-6 silica (T = 17 min) due to the different pore structures. Thus, the structure peculiarities of the three-dimensional pores of KIT-6, cage-like SBA-16, and two-dimensional pores in the hexagonal SBA-15 also influenced the performance of the sorbents in the CO2 adsorption experiments.   The calculated values for the specific surface area of the initial materials decreased in the following order: KIT-6>SBA-16>SBA-15, while those for the modified materials followed the order: SBA-15/DAPTES>KIT-6/DAPTES>SBA-16/DAPTES. Among the prepared initial samples, KIT-6 possessed the highest specific surface area. The nitrogen physisorption measurements of SBA-15 and SBA-16 showed a surface area of 880 m 2 /g and 890 m 2 /g and a pore volume of 1.00 cm 3 /g and 0.53 cm 3 /g, respectively. The significant decrease in the textural parameters, such as specific surface area, total pore volume, and pore diameter of the modified materials, indicate pore filling by DAPTES. This effect is more pronounced for SBA-16/DAPTES than for the SBA-15/DAPTES and KIT-6/DAPTES samples. Probably, this is due to the three-dimensional channel system and uniform-sized pores of the super large, cage-like structure of the SBA-16 with small pore sizes around 4.9 nm. Because of the structural peculiarities of the SBA-16, some pores of the modified material are blocked with the grafted bulk molecules. The specific surface area of the modified KIT-6/DAPTES decreased less than that of modified SBA-16/DAPTES. This might be due to the interpenetrating cylindrical pores, which favors the deposition of DAPTES in the pores of the SBA-16. The results from the N 2 physisorption correspond very well with the TG and IR data, indicating grafting of DAPTES into the pores of the silica materials. The preservation of the mesoporous structure of all the used mesoporous silicas during the modification procedure was observed, which is in agreement with the XRD patterns in the low-angle range ( Figure S11).

CO 2 Adsorption Measurements
The mesoporous silicas (SBA-15, SBA-16, and KIT-6) modified with DAPTES were tested in dynamic conditions at atmospheric pressure. The results are presented in Table 2 and Figure 5. A higher amount of adsorbed CO 2 was detected on the modified mesoporous silicas than on the initial ones. This effect is predetermined by the type of the adsorption sites, which, in the case of the initial silica materials, are the silanol groups. Quantitative 29 Si NMR measurements were performed to evaluate the amount of silanol groups in the three silica samples. The quantitative single-pulse 29 Si spectra ( Figure S12) demonstrated the presence of a higher number of silanol groups in SBA-16 as compared to KIT-6 and SBA-15 (Table S1). A correlation between the adsorption capacity and the number of SiOH groups was observed. Among the sorbents studied, SBA-16 displayed the highest value of the CO 2 adsorption capacity, while the formation of a smaller number of SiOH groups in the SBA-15 and KIT-6 silicas led to lower adsorption of CO 2 on them. In addition, the time needed for achieving the total adsorption capacity for the SBA-15 silica (T = 8 min) was shorter in comparison to the time determined for CO 2 adsorption on to the SBA-16 and KIT-6 silica (T = 17 min) due to the different pore structures. Thus, the structure peculiarities of the three-dimensional pores of KIT-6, cage-like SBA-16, and two-dimensional pores in the hexagonal SBA-15 also influenced the performance of the sorbents in the CO 2 adsorption experiments.  The modification with DAPTES led to an increase in CO2 adsorption capacity for the three silica materials ( Table 2). The highest value for the CO2 adsorption capacity in dy- The modification with DAPTES led to an increase in CO 2 adsorption capacity for the three silica materials ( Table 2). The highest value for the CO 2 adsorption capacity in dynamic conditions among the studied materials was detected for the modified SBA-15/DAPTES sample (3.9 mmol/g). The CO 2 adsorption capacities of the samples decreased in the following order: SBA-15/DAPTES>SBA-16/DAPTES>KIT-6/DAPTES. The results showed that structural peculiarities of the mesoporous silicas significantly influenced the formation and localization of the adsorption sites. We assumed that the structural characteristics of SBA-15 led to the excellent CO 2 uptake of 3.9 mmol/g due to its open structure that favors the modification with DAPTES (Scheme 2). The total CO 2 desorption was reached at 80 • C for the DAPTES-modified materials and at 40 • C for the parent ones. That implies strong interaction of the modified pore surface of the material, probably accompanied with chemisorption of CO 2, leading to the formation of bicarbonates that are electrostatically attracted to the positively charged ammonium groups (Scheme 3). That was confirmed by the adsorption experiments performed with the addition of 1 vol.% water vapor to the CO 2 /N 2 flow at a rate of 30 mL/min to determine the selectivity of the prepared materials for CO 2 adsorption in the presence of water vapor, i.e., in a flow of CO 2 /H 2 O/N 2 . Interestingly, the modified samples showed higher adsorption capacities for CO 2 in the presence of water vapor compared to that determined in a CO 2 /N 2 flow, which is the opposite for the parent silicas. The higher selectivity for CO 2 adsorption in the presence of water on the modified materials is most probably due to the chemisorption of CO 2 on the adsorption sites (Table 2). Probably, the effect is enhanced by the presence of the highly polar P=O groups and the electron-rich aromatic structures. The modification with DAPTES led to an increase in CO2 adsorption capacity for th three silica materials ( Table 2). The highest value for the CO2 adsorption capacity in dy namic conditions among the studied materials was detected for the modified SBA-15/DAPTES sample (3.9 mmol/g). The CO2 adsorption capacities of the samples de creased in the following order: SBA-15/DAPTES>SBA-16/DAPTES>KIT-6/DAPTES. Th results showed that structural peculiarities of the mesoporous silicas significantly influ enced the formation and localization of the adsorption sites. We assumed that the struc tural characteristics of SBA-15 led to the excellent CO2 uptake of 3.9 mmol/g due to it open structure that favors the modification with DAPTES (Scheme 2). The total CO2 de sorption was reached at 80 °C for the DAPTES-modified materials and at 40 °C for th parent ones. That implies strong interaction of the modified pore surface of the material probably accompanied with chemisorption of CO2, leading to the formation of bicar bonates that are electrostatically attracted to the positively charged ammonium group (Scheme 3). That was confirmed by the adsorption experiments performed with the ad dition of 1 vol.% water vapor to the CO2/N2 flow at a rate of 30 mL/min to determine th selectivity of the prepared materials for CO2 adsorption in the presence of water vapor i.e., in a flow of CO2/H2O/N2. Interestingly, the modified samples showed higher adsorp tion capacities for CO2 in the presence of water vapor compared to that determined in CO2/N2 flow, which is the opposite for the parent silicas. The higher selectivity for CO adsorption in the presence of water on the modified materials is most probably due to th chemisorption of CO2 on the adsorption sites (Table 2). Probably, the effect is enhanced by the presence of the highly polar P=O groups and the electron-rich aromatic structures. Results on the CO2 capture on functionalized SBA-type adsorbents have been r ported by other research groups. An adsorption capacity of 0.72 mmol/g under 101. kPa at 60 °C was determined for SBA-16 modified wi N-(2-aminoethyl)-3-aminopropyltrimethoxysilane. The adsorbent displayed a high h drothermal stability, and the combustion of the amino-groups in air was observed temperatures above 200 °C [33,54]. Hiyoshi et al. [31] reported that the adsorption c pacity of amino-silica (SBA-15) reached 1.8 mmol/g under 15 kPa CO2 and at 60 °C. It w found that the efficiency of adsorption increased on increasing the surface density amine. Triamine-functionalized SBA-15 had a greater CO2 retention capacity value (3. mmol/g) than the one grafted with mono-and di-amine matrices. The presence of mo ture enhanced the adsorbent performance, suggesting the participation of water mol cules in the CO2 retention [55]. The modified materials studied in the present investig tion show comparable or higher and more stable adsorption capacities than that for sim ilar sorbents as described above.
The adsorption capacities of the synthesized nanomaterials were determined using laboratory-scale fixed-bed reactor. The Yoon-Nelson model [56] has been applied for t investigation of adsorption kinetics in a fixed-bed column, and the linear form of t model is represented by Equation (1): where kYN is the Yoon-Nelson rate constant (min −1 ), τ is the time required for 50% of a sorbate breakthrough (min), t is the sampling time (min), C0 is the initial concentration CO2, and C is the concentration of CO2 at any time during evaluation. We found that t model and experimental data had a strong correlation (R 2 > 0.99) ( Figure S13). The initial and the modified silicas were studied in five adsorption/desorption cycl to check their adsorption potential and selectivity for CO2 adsorption in the presence water, which is very important for their practical application. The results show stab performance with preserved high adsorption capacity and selectivity for CO2 in five a sorption/desorption cycles for all the studied samples ( Table 2).
The calculated heats of adsorption using the Clausius-Clapeyron equation for samples based on the CO2 adsorption at 0 °C, 25 °C, and 50 °C are presented in Figure  The  Results on the CO 2 capture on functionalized SBA-type adsorbents have been reported by other research groups. An adsorption capacity of 0.72 mmol/g under 101.32 kPa at 60 • C was determined for SBA-16 modified with N-(2-aminoethyl)-3-aminopropyltrimethoxysilane. The adsorbent displayed a high hydrothermal stability, and the combustion of the aminogroups in air was observed at temperatures above 200 • C [33,54]. Hiyoshi et al. [31] reported that the adsorption capacity of amino-silica (SBA-15) reached 1.8 mmol/g under 15 kPa CO 2 and at 60 • C. It was found that the efficiency of adsorption increased on increasing the surface density of amine. Triamine-functionalized SBA-15 had a greater CO 2 retention capacity value (3.12 mmol/g) than the one grafted with mono-and di-amine matrices. The presence of moisture enhanced the adsorbent performance, suggesting the participation of water molecules in the CO 2 retention [55]. The modified materials studied in the present investigation show comparable or higher and more stable adsorption capacities than that for similar sorbents as described above.
The adsorption capacities of the synthesized nanomaterials were determined using a laboratory-scale fixed-bed reactor. The Yoon-Nelson model [56] has been applied for the investigation of adsorption kinetics in a fixed-bed column, and the linear form of the model is represented by Equation (1): where κ YN is the Yoon-Nelson rate constant (min −1 ), τ is the time required for 50% of adsorbate breakthrough (min), t is the sampling time (min), C 0 is the initial concentration of CO 2 , and C is the concentration of CO 2 at any time during evaluation. We found that the model and experimental data had a strong correlation (R 2 > 0.99) ( Figure S13). The initial and the modified silicas were studied in five adsorption/desorption cycles to check their adsorption potential and selectivity for CO 2 adsorption in the presence of water, which is very important for their practical application. The results show stable performance with preserved high adsorption capacity and selectivity for CO 2 in five adsorption/desorption cycles for all the studied samples ( Table 2).
The calculated heats of adsorption using the Clausius-Clapeyron equation for all samples based on the CO 2 adsorption at 0 • C, 25 • C, and 50 • C are presented in Figure 6. The calculated heat of adsorption values for all samples were in the interval of 20-52 kJ/mol. Higher values were determined for the DAPTES-functionalized silicas in comparison to their parent analogs. The highest heat of adsorption (around 50 kJ/mol) was displayed by DAPTES-modified SBA-15 and SBA-16, which was two times higher than that of the initial SBA-15 and SBA-16 (25-27 kJ/mol). DAPTES-functionalized SBA-15 and SBA-16 silicas also exhibited a steep decrease in the heats of adsorption as a function of the adsorbed amount of CO 2 as compared to the initial silicas, which indicates the presence of different adsorption sites for effective CO 2 adsorption in them. Significantly lower heats of adsorption were detected for the KIT-6/DAPTES material (34 kJ/mol), which can be explained by the lower extent of DAPTES modification and corresponds to the low CO 2 adsorption ( Table 2).
The high CO 2 isosteric heats of adsorption of the DAPTES-modified SBA-15 and SBA-16 materials led to increased CO 2 uptake and selectivity ( Table 2). The obtained results can be related to the appropriate structure of the large-pore mesoporous silicate used, the advantage of which is not only due to the more exposed DAPTES fragments but also due to the provided necessary path for CO 2 diffusion. , x FOR PEER REVIEW 12 of 16 plained by the lower extent of DAPTES modification and corresponds to the low CO2 adsorption (Table 2). The high CO2 isosteric heats of adsorption of the DAPTES-modified SBA-15 and SBA-16 materials led to increased CO2 uptake and selectivity ( Table 2). The obtained results can be related to the appropriate structure of the large-pore mesoporous silicate used, the advantage of which is not only due to the more exposed DAPTES fragments but also due to the provided necessary path for CO2 diffusion.

Conclusions
The DAPTES-modified mesoporous silicas with different pore structures, 2D SBA-15, 3D SBA-16, and KIT-6, were successfully prepared using a simple two-step post-synthesis procedure. The mesoporous structure of the silica materials was preserved during the chemical grafting of the new DOPO derivative. The DAPTES-modified materials showed a higher capacity for CO 2 capture in comparison to the parent ones. The adsorption capacity values depended on the structural peculiarities of the supports used. CO 2 capture is favored by the presence of polar groups and electron-rich fragments in the structure of the grafted agent. Among the studied sorbents, the DAPTES-modified mesoporous SBA-15 silica showed the highest adsorption capacity for CO 2 (3.9 mmol/g). Enhanced CO 2 adsorption capacities were detected for the functionalized silicas in the experiments with 3 vol.% CO 2 plus 1 vol.% water vapor due to the facilitated formation of chemisorbed CO 2 in the form of a bicarbonate ion. The chemisorption of CO 2 for DAPTES-modified silicas was assumed based on the calculated heats of adsorption for the studied materials. Total CO 2 desorption from the modified materials was achieved at 80 • C. The functionalized silica supports displayed good performance parameters such as high adsorption capacity, stability, and low energy consumption for CO 2 desorption, which is worth being considered for further studies and scaling experiments. The experimental data can be appropriately described by the Yoon-Nelson kinetic model.

Supplementary Materials:
The following supporting information can be downloaded at https: //www.mdpi.com/article/10.3390/ma16114179/s1: Figure S1: 1 H NMR spectrum of DAPTES in DMSO-d6; Figure S2: 13 C NMR spectrum of DAPTES in DMSO-d6; Figure S3: 1 H NMR spectrum of DOPO in CDCl 3 ; Figure S4: DEPT-135 NMR spectrum of DAPTES in DMSO-d6; Figure S5: COSY spectrum of DAPTES in DMSO-d6; Figure S6: 1 H-13 C HSQC spectrum of DAPTES in DMSO-d6; Figure S7: 1 H-13 C HMBC spectrum of DAPTES in DMSO-d6. Figure S8. (a) TG curves of the initial and the DAPTES-modified silicas in the temperature range 150 • C-600 • C used to determine the weight loss due to decomposition of the grafted moieties; (b) TG curves of the initial silicas in the temperature range 50 • C-600 • C. The weight loss up to 120 • C-130 • C is due to adsorbed humidity; (c) TG curves of the modified silicas in the temperature range 50 • C-600 • C. The weight loss at 150 • C is less than 0.5% which is evidence for the hydrophobization of the pore surface as a result of DAPTES grafting and the thermal stability of the DAPTES moieties. Figure S9. 1 H→ 29 Si CP MAS spectra of parent SBA-16, KIT-6 and SBA-15. Figure S10. 1 H→ 13 C CP MAS spectra of KIT-6/DAPTES and SBA-15/DAPTES samples. Figure S11: Low angle XRD patterns of the SBA-15, KIT-6 and their DAPTES-modified analogs. Figure S12. Experimental (black) and simulated (red) single-pulse 29 Si NMR spectra of the (a) SBA-16, (b) KIT-6, (c) SBA-15. The individual contributions of the different Si environments obtained by the deconvolution of the spectral patterns are given with colored lines (Q 4 -blue, Q 3 -green, Q 2 -magenta). Figure S13. Fitting of experimental data on kinetic model at 0 • C. Table S1. Area of the signals of the different Si structural units, obtained by deconvolution of the spectral patterns in the quantitative single pulse 29Si NMR spectra of the parent mesoporous silica materials.

Conflicts of Interest:
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.