Effects of Alkali Metal (Li, Na, and K) Incorporation in NH2–MIL125(Ti) on the Performance of CO2 Adsorption

A series of titanium-based, metal–organic framework (MOF) materials, xM@NH2-MIL125(Ti) (x is the alkali metal loading percentage during the synthesis; M = Li, Na, K), have been synthesized solvothermally. Alkali metal doping in the NH2–MIL125(Ti) in situ solvothermal process demonstrated a vital modification of the material structure and surface morphology for the CO2 adsorption capacity at ambient conditions. By changing the reactants’ precursor, including different kinds of alkali metal, the morphology of xM@NH2–MIL125(Ti) can be adjusted from a tetragonal plate through a circular plate to a truncated octahedron. The variation of the alkali metal loading results in substantial differences in the CO2 adsorption. The properties of xM@NH2–MIL125(Ti) were evaluated via functional group coordination using FT-IR, phase identification based on X-ray diffraction (XRD), surface morphology through scanning electron microscopy (SEM), as well as N2 and CO2 adsorption by physical gas adsorption analysis. This work reveals a new pathway to the modification of MOF materials for high-efficiency CO2 adsorption.


Introduction
With rapid economic growth, excessive carbon consumption correlated with enhanced CO 2 emission into the atmosphere has caused involved environmental problems, such as global warming and climate change [1]. Carbon capture and storage (CCS) has been considered a prospective technological strategy to slow down gas emissions and alleviate the climate [2]. Up to now, there are multifarious approaches to CO 2 storage and separation, such as solvent absorption, physicochemical adsorption, membrane separation, cryogenic distillation, and so on [3]. Among the technologies applied for this purpose, adsorption of CO 2 into porous solid materials, such as zeolites, mesoporous silicas, porous carbon, and metal-organic frameworks (MOFs), has been gaining increasing attention due to its low energy requirements, cost-effectiveness, high adsorption capacity, and regeneration [4][5][6][7].
MOFs, exhibiting large internal surface area, controllable porosity and pore walls, and affinity for particular valuable gases, have been regarded as potential CO 2 adsorption and separation materials under mild conditions. Usually, MOFs are constructed through metal ions or clusters as connected centers, and polyfunctional organic ligands as connected linkers. As a matter of fact, the extended framework can be controlled by selecting the appropriate metal centers and organic linkers to obtain the desired structural features and physicochemical properties. In order to promote the CO 2 adsorption capacity and the separation selectivity over other gases, various strategies have been reported, such as metal cation incorporation [8,9], pore size and shape tuning [10], and ligand functionality [11][12][13]. provided by Tianjin Fuchen Chemicals Co. Ltd. (Tianjin, China). All the chemicals were used as received, without further purification.

Synthesis of NH 2 -MIL125(Ti)
The synthesis of NH 2 -MIL125(Ti) was performed by dissolving H 2 BDC-NH 2 (1.449 g, 8 mmol) in 30 mL DMF/methanol (1:1 V/V) and stirred for 10 min at room temperature; then, titanium isopropoxide (1.308 g, 1.39 mL, 4.5 mmol) was added to the above solution and stirred for another 30 min. The mixture was transferred into a Teflon liner inserted in a stainless-steel autoclave. Then the autoclave was sealed and the mixture was heated for 16 h at 150 • C. After cooling slowly to ambient temperature, the obtained yellow powder was filtered and washed with DMF and methanol, and the resultant product was finally vacuum-dried at 60 • C overnight.

Synthesis of xM@NH 2 -MIL125(Ti)
The alkali metal-doped NH 2 -MIL125(Ti) samples were prepared through the in situ synthesis method. MCl (M = Li + , Na + , K + ) was milled into fine particles and dissolved in DMF. The above solution containing M + was then added to the solution containing H 2 BDC-NH 2 and titanium isopropoxide. The synthesis and activation of the alkali metal cation incorporated NH 2 -MIL125(Ti) followed the same route of NH 2 -MIL125(Ti). The yellow powder product was denoted as xM@NH 2 -MIL125(Ti), where M is related to the alkali metal cation (Li + , Na + , K + ), and x represents the mass ratio (1% and 2%) of alkali metal chloride to the pure NH 2 -MIL125(Ti). For the 1% case, as the sum mass of H 2 BDC-NH 2 and titanium isopropoxide were 2.757 g, so 0.028 g of MCl (0.65 mmol for LiCl, 0.47 mmol for NaCl, and 0.37 mmol for KCl) was added. All the obtained samples after final filtration were vacuum-dried at 60 • C overnight. The yield, 90% of the product, is nearly the same as the pristine NH 2 -MIL125(Ti).

Sample Characterization
FT-IR spectra were obtained on AXS TENSOR-27 FT-IR spectrometer (Bruker, Karlsruhe, Germany), with KBr pellets at room temperature in the range of 4000-500 cm −1 , with a resolution of 4 cm −1 . Powder X-ray diffraction (PXRD) data were performed on a Advance-D8 (Bruker, Karlsruhe, Germany) with Cu K α radiation operating at 40 kV and 40 mA, in the range of 5 • < 2θ < 50 • , with a step length of 0.02 • (2θ). X-ray photoelectron spectrometer (XPS) data were collected on an Axis Ultra DLD (Kratos Analytical Ltd. of Shimadzu Corporation, Manchester, UK) with Al K α (hν = 1486.7 eV), operating at 15 kV and 10 mA. All curves have been charge-corrected to the main line of the carbon C1's spectral component (C/C, C/H), set to 284.80 eV. Thermal stability was characterized via TA-SDT Q600 (TA Instruments, NewCastle, DE, USA) from room temperature to 900 • C, under a heating rate of 5 • C·min −1 in N 2 atmosphere and with a flowing rate of 10 mL·min −1 . The contents of the metal ions were analyzed by ICP-OES730 (Agilent, Santa Clara, CA, USA). The scanning electron microscope (SEM) and energy-dispersive spectroscopy (EDS) were carried out using a Hitachi SU8020 (Hitachi High-Technologies Corporation, Tokyo, Japan). The powder surface was gold that had been metalized previously. N 2 and CO 2 adsorption isotherms were obtained using Autosorb-1 (Quantachrome Instruments, Boynton Beach, FL, USA). The samples were pre-treated to remove excess water molecules and impurities at 150 • C for 8 h under vacuum, and the CO 2 adsorption isotherm was tested at 273 and 293 K.

FT-IR Analysis
The FT-IR spectra of xM@NH 2 -MIL125(Ti), as shown in Figure 1, shows the characteristic vibration peaks before and after the alkali metal doping. There is no obvious change in the region of the peaks, but the intensity is slightly changed. To be specific, 3420-3452 cm −1 shows the contribution of −NH 2 group in the structure. The vibration bands located at about 1637 and 1500 cm −1 belonged to carbonyl asymmetric stretching vibrations, and vibration bands located at 1298 cm −1 could be assigned to the C-H symmetric stretching vibrations of the benzene ring. The peak around 760 cm −1 is the Ti-O stretching for non-bound oxygen atoms, and 500-755 cm −1 is the Ti-O-Ti stretching vibration [27]. carbonyl asymmetric stretching vibrations, and vibration bands located at 1298 cm −1 could be assigned to the C-H symmetric stretching vibrations of the benzene ring. The peak around 760 cm −1 is the Ti-O stretching for non-bound oxygen atoms, and 500-755 cm −1 is the Ti-O-Ti stretching vibration [27].

X-ray Diffraction Analysis
XRD patterns of NH2-MIL125(Ti) synthesized with different loadings of Li, Na, and K are shown in Figure 2. It can be seen that all samples still maintained the crystal structure of NH2-MIL125(Ti). It shows that xM@NH2-MIL125(Ti) exhibited the distinct characteristic diffraction peaks at 2θ = 6.8°, 9.8°, 12.1°, 17.3°, and 18.8°, which were consistent with the experimental and calculated patterns of NH2-MIL125(Ti) [27], and no additional peaks were observed; this means that no other crystalline impurity phases formed after the modification, indicating that the crystal structure of NH2-MIL125(Ti) was well maintained. Therefore, we speculate that the doping of alkali metal ions during the solvothermal synthesis would not affect the crystalline structures of NH2-MIL125(Ti). However, the characteristic diffraction peak positions around 6.8° of xK@NH2-MIL125(Ti) have been shifted slightly for the case of potassium ion doping, as shown in Figure 2d, which is owing to the larger ionic radius of K + than of Li + and Na + [28]. The larger ionic radius may cause differences in the construction of the framework during the solvothermal synthesis.

X-ray Diffraction Analysis
XRD patterns of NH 2 -MIL125(Ti) synthesized with different loadings of Li, Na, and K are shown in Figure 2. It can be seen that all samples still maintained the crystal structure of NH 2 -MIL125(Ti). It shows that xM@NH 2 -MIL125(Ti) exhibited the distinct characteristic diffraction peaks at 2θ = 6.8 • , 9.8 • , 12.1 • , 17.3 • , and 18.8 • , which were consistent with the experimental and calculated patterns of NH 2 -MIL125(Ti) [27], and no additional peaks were observed; this means that no other crystalline impurity phases formed after the modification, indicating that the crystal structure of NH 2 -MIL125(Ti) was well maintained. Therefore, we speculate that the doping of alkali metal ions during the solvothermal synthesis would not affect the crystalline structures of NH 2 -MIL125(Ti). However, the characteristic diffraction peak positions around 6.8 • of xK@NH 2 -MIL125(Ti) have been shifted slightly for the case of potassium ion doping, as shown in Figure 2d, which is owing to the larger ionic radius of K + than of Li + and Na + [28]. The larger ionic radius may cause differences in the construction of the framework during the solvothermal synthesis.

X-ray Photoelectron Spectrometer
XPS measurements were carried out to determine the chemical composition and electronic structure of xM@NH 2 -MIL125(Ti). As shown in Figure 3, the wide-scan XPS spectra of all the samples show four peaks. The peak of 288.7 eV, corresponding to C 1s, indicates the existence of a C element. Similarly, the peaks of 399.6 eV for N at 1s, 458.9 eV for Ti at 2p, and 288.7 eV for O at 1s, indicate the existence of N, Ti, and O elements. In the 1s spectrum for N, the peak at 399.6 eV belonged to the N of the amine group protruding or stretching out into the cavities. The peak at 403.1 eV can be assigned to the positively charged -N= + and -NH-+ [29]. The symmetric peaks in the Ti 2p spectrum located at 458.9 eV and 464.6 eV are attributed for Ti 2p 3/2 and Ti 2p 1/2 , respectively. This means that the oxidation state Ti in the titanium-oxo cluster remains in IV. Combining with the experimental results of XRD and N 2 adsorption, it can be inferred that the amine groups are not coordinated with metal ions, but may be protruding into the empty space of the internal porous [29]. The XPS peaks of xM@NH 2 -MIL125(Ti) are nearly the same as those of NH 2 -MIL125(Ti), which indicates that the alkali metal doping procedure has little influence on the electronic structures of NH 2 -MIL125(Ti). MIL125(Ti) was well maintained. Therefore, we speculate that the doping of alkali metal ions during the solvothermal synthesis would not affect the crystalline structures of NH2-MIL125(Ti). However, the characteristic diffraction peak positions around 6.8° of xK@NH2-MIL125(Ti) have been shifted slightly for the case of potassium ion doping, as shown in Figure 2d, which is owing to the larger ionic radius of K + than of Li + and Na + [28]. The larger ionic radius may cause differences in the construction of the framework during the solvothermal synthesis.

X-ray Photoelectron Spectrometer
XPS measurements were carried out to determine the chemical composition and electronic structure of xM@NH2-MIL125(Ti). As shown in Figure 3, the wide-scan XPS spectra of all the samples show four peaks. The peak of 288.7 eV, corresponding to C 1s, indicates the existence of a C element. Similarly, the peaks of 399.6 eV for N at 1s, 458.9 eV for Ti at 2p, and 288.7 eV for O at 1s, indicate the existence of N, Ti, and O elements. In the 1s spectrum for N, the peak at 399.6 eV belonged to the N of the amine group protruding or stretching out into the cavities. The peak at 403.1 eV can be assigned to the positively charged -N= + and -NH-+ [29]. The symmetric peaks in the Ti 2p spectrum located at 458.9 eV and 464.6 eV are attributed for Ti 2p3/2 and Ti 2p1/2, respectively. This means that the oxidation state Ti in the titanium-oxo cluster remains in IV. Combining with the experimental results of XRD and N2 adsorption, it can be inferred that the amine groups are not coordinated with metal ions, but may be protruding into the empty space of the internal porous [29]. The XPS peaks of xM@NH2-MIL125(Ti) are nearly the same as those of NH2-MIL125(Ti), which indicates that the alkali metal doping procedure has little influence on the electronic structures of NH2-MIL125(Ti).

Thermal Stability
The thermal stability and decomposition temperatures of NH2-MIL125 and xM@NH2-MIL125 were investigated via thermogravimetric analysis (TGA). It can be seen from Figure 4 that there are three stages of weight losses in the TGA curves. NH2-MIL125, xLi@NH2-MIL125, xNa@NH2-MIL125, and xK@NH2-MIL125 showed similar weight-loss behaviors. The first weight loss, between 30 and 130 °C, was caused by the removal of physically adsorbed water, bound water, and free solvent molecules (such as methanol) [27]. At the region of 130 to 300 °C, the weight loss curves of all the samples gradually decreased, which was due to the removal of the DMF and non-coordinated organic linkers of H2BDC-NH2 trapped in the material. When the temperature reached around 300 °C, a sudden weight loss occurred, which corresponds to the degradation of the framework to TiO2 anatase [11,23]. The thermogravimetric results show that xM@NH2-MIL125 can remain thermally stable at 300 °C.

Thermal Stability
The thermal stability and decomposition temperatures of NH 2 -MIL125 and xM@NH 2 -MIL125 were investigated via thermogravimetric analysis (TGA). It can be seen from Figure 4 that there are three stages of weight losses in the TGA curves. NH 2 -MIL125, xLi@NH 2 -MIL125, xNa@NH 2 -MIL125, and xK@NH 2 -MIL125 showed similar weight-loss behaviors. The first weight loss, between 30 and 130 • C, was caused by the removal of physically adsorbed water, bound water, and free solvent molecules (such as methanol) [27]. At the region of 130 to 300 • C, the weight loss curves of all the samples gradually decreased, which was due to the removal of the DMF and non-coordinated organic linkers of H 2 BDC-NH 2 trapped in the material. When the temperature reached around 300 • C, a sudden weight loss occurred, which corresponds to the degradation of the framework to TiO 2 Materials 2019, 12, 844 6 of 13 anatase [11,23]. The thermogravimetric results show that xM@NH 2 -MIL125 can remain thermally stable at 300 • C.

Scanning Electron Microscopy
The morphology of pristine NH2-MIL125 and xM@NH2-MIL125 are shown in Figure 5. NH2-MIL125 ( Figure 5a) and xNa@NH2-MIL125 ( Figure 5c) exert thin and circular plate shapes. The NH2-MIL125 involves an average particle size of 6 μm, which is similar to the conventional synthetic method of NH2-MIL125 (5 μm) [23], while xLi@NH2-MIL125 has a tetragonal plate shape with a particle size of 5 μm (Figure 5b). The average particle size of xNa@NH2-MIL125 is 8 μm, with a circular plate shape ( Figure 5c). However, xK@NH2-MIL125 has a truncated octahedron shape with the particle size of 15 μm (Figure 5d). The crystal size increases in the sequence of Li, Na, and K, reflecting the different growth rates of the xM@NH2-MIL125. The titanium precursor contains different amount of alkali metal ions during the solvothermal synthesis. As for the case of 1M@NH2-MIL125(Ti), there was 0.65 mmol of LiCl, 0.47 mmol of NaCl, and 0.37 mmol of KCl in the solvothermal precursor, respectively. The titanium precursor containing K ions involves the lowest doping ions molar concentration, which may have less influence on the moving of Ti and the growing rate of Ti-O in the titanium precursor, resulting in the highest particle size. The alkali metal incorporation has an influence on the kinetics of hydrolysis and condensation reactions. The morphology evolution of the crystal has affected the changes of NH2-MIL125. Thus, special polyhedrons of NH2-MIL125 from the tetragonal plate through the circular plate to a truncated octahedron were obtained. The crystal morphology of NH2-MIL125(Ti) changing in our work is a little different from what Hu et al. previously reported [27]. They controlled the crystal morphology from the circular plate to the octahedron, by altering the concentration of reactants from

Scanning Electron Microscopy
The morphology of pristine NH 2 -MIL125 and xM@NH 2 -MIL125 are shown in Figure 5. The NH 2-MIL125 involves an average particle size of 6 µm, which is similar to the conventional synthetic method of NH 2 -MIL125 (5 µm) [23], while xLi@NH 2-MIL125 has a tetragonal plate shape with a particle size of 5 µm (Figure 5b). The average particle size of xNa@NH 2 -MIL125 is 8 µm, with a circular plate shape (Figure 5c). However, xK@NH 2 -MIL125 has a truncated octahedron shape with the particle size of 15 µm (Figure 5d). The crystal size increases in the sequence of Li, Na, and K, reflecting the different growth rates of the xM@NH 2 -MIL125. The titanium precursor contains different amount of alkali metal ions during the solvothermal synthesis. As for the case of 1M@NH 2 -MIL125(Ti), there was 0.65 mmol of LiCl, 0.47 mmol of NaCl, and 0.37 mmol of KCl in the solvothermal precursor, respectively. The titanium precursor containing K ions involves the lowest doping ions molar concentration, which may have less influence on the moving of Ti and the growing rate of Ti-O in the titanium precursor, resulting in the highest particle size.

Scanning Electron Microscopy
The morphology of pristine NH2-MIL125 and xM@NH2-MIL125 are shown in Figure 5. NH2-MIL125 ( Figure 5a) and xNa@NH2-MIL125 (Figure 5c) exert thin and circular plate shapes. The NH2-MIL125 involves an average particle size of 6 μm, which is similar to the conventional synthetic method of NH2-MIL125 (5 μm) [23], while xLi@NH2-MIL125 has a tetragonal plate shape with a particle size of 5 μm (Figure 5b). The average particle size of xNa@NH2-MIL125 is 8 μm, with a circular plate shape (Figure 5c). However, xK@NH2-MIL125 has a truncated octahedron shape with the particle size of 15 μm (Figure 5d). The crystal size increases in the sequence of Li, Na, and K, reflecting the different growth rates of the xM@NH2-MIL125. The titanium precursor contains different amount of alkali metal ions during the solvothermal synthesis. As for the case of 1M@NH2-MIL125(Ti), there was 0.65 mmol of LiCl, 0.47 mmol of NaCl, and 0.37 mmol of KCl in the solvothermal precursor, respectively. The titanium precursor containing K ions involves the lowest doping ions molar concentration, which may have less influence on the moving of Ti and the growing rate of Ti-O in the titanium precursor, resulting in the highest particle size. The alkali metal incorporation has an influence on the kinetics of hydrolysis and condensation reactions. The morphology evolution of the crystal has affected the changes of NH2-MIL125. Thus, special polyhedrons of NH2-MIL125 from the tetragonal plate through the circular plate to a truncated octahedron were obtained. The crystal morphology of NH2-MIL125(Ti) changing in our work is a little different from what Hu et al. previously reported [27]. They controlled the crystal morphology from the circular plate to the octahedron, by altering the concentration of reactants from  [27]. They controlled the crystal morphology from the circular plate to the octahedron, by altering the concentration of reactants from low to high and by changing the total solvent volume. In our work, the concentration of the reactant was the same when adding different kind of alkali metal salts, but the total ion concentration was decreased in the sequence of Li, Na, and K, which caused the different growth rates. Hu et al. reported that the perfect NH 2 -MIL125 crystal should be rhombic dodecahedron morphology by Bravais-Friedel-Donnay-Harker (BFDH) theory [30]. It can be concluded that these morphology changes were affected by the total concentration of the ions in the solvothermal precursors.
To verify that the alkali metals were doped into NH 2 -MIL125 homogeneously, the mapping images of the 1K@NH 2 -MIL125 through SEM-EDS were performed. As can be seen in Figure 6, the C, N, Ti, K, and Cl elements are uniformly dispersed in the 1K@NH 2 -MIL125, indicating that the K ions have been well distributed into the framework of the 1K@NH 2 -MIL125. The same results were found in the 1Na@NH 2 -MIL125(Ti). The doped alkali metal ions might be well incorporated into the structure, or just dispersed on the surface of the Ti 8 O 8 (OH) 4 -(O 2 C-C 6 H 5 -CO 2 -NH 2 ) 6 . The alkali metals presumably exist in the form of chlorides, because the alkali oxide needs a higher temperature to be obtained [31]. The signal intensity of lithium is too weak to determine its dispersion in xLi@NH 2 -MIL125 by SEM-EDS imaging, due to the high background noises. low to high and by changing the total solvent volume. In our work, the concentration of the reactant was the same when adding different kind of alkali metal salts, but the total ion concentration was decreased in the sequence of Li, Na, and K, which caused the different growth rates. Hu et al. reported that the perfect NH2-MIL125 crystal should be rhombic dodecahedron morphology by Bravais-Friedel-Donnay-Harker (BFDH) theory [30]. It can be concluded that these morphology changes were affected by the total concentration of the ions in the solvothermal precursors.
To verify that the alkali metals were doped into NH2-MIL125 homogeneously, the mapping images of the 1K@NH2-MIL125 through SEM-EDS were performed. As can be seen in Figure 6, the C, N, Ti, K, and Cl elements are uniformly dispersed in the 1K@NH2-MIL125, indicating that the K ions have been well distributed into the framework of the 1K@NH2-MIL125. The same results were found in the 1Na@NH2-MIL125(Ti). The doped alkali metal ions might be well incorporated into the structure, or just dispersed on the surface of the Ti8O8(OH)4-(O2C-C6H5-CO2-NH2)6. The alkali metals presumably exist in the form of chlorides, because the alkali oxide needs a higher temperature to be obtained [31]. The signal intensity of lithium is too weak to determine its dispersion in xLi@NH2-MIL125 by SEM-EDS imaging, due to the high background noises.

Alkali Metal Content
Inductively Coupled Plasma-Optical Eemission Spectroscopy (ICP-OES) analyses were performed to examine the presence and quantity of the alkali metal for xLi@NH2-MIL125(Ti), and the results are listed in Table 1. The experimental titanium content for both doped and original NH2-MIL125(Ti) are nearly the same, which is between 15.7~16.9 wt %. This is close to the theoretical calculating value 23 wt % for NH2-MIL125(Ti) and Ti8O8(OH)4-(O2C-C6H3-CO2-NH2)6 [11]. The higher lithium content can be observed by increasing the lithium loading in the precursor. The amount of Li and Na in the framework drops greatly after a series of solvent exchanging treatment, because the alkali metal does not coordinate with the organic ligands and its small atomic radius. For the K case, maybe there are more K ions trapped in the pores or on the surface of the framework, so the ICP results of K are higher than those for Li and Na.

Alkali Metal Content
Inductively Coupled Plasma-Optical Eemission Spectroscopy (ICP-OES) analyses were performed to examine the presence and quantity of the alkali metal for xLi@NH 2 -MIL125(Ti), and the results are listed in Table 1. The experimental titanium content for both doped and original NH 2 -MIL125(Ti) are nearly the same, which is between 15.7~16.9 wt %. This is close to the theoretical calculating value 23 wt % for NH 2 -MIL125(Ti) and Ti 8 O 8 (OH) 4 -(O 2 C-C 6 H 3 -CO 2 -NH 2 ) 6 [11]. The higher lithium content can be observed by increasing the lithium loading in the precursor. The amount of Li and Na in the framework drops greatly after a series of solvent exchanging treatment, because the alkali metal does not coordinate with the organic ligands and its small atomic radius. For the K case, maybe there are more K ions trapped in the pores or on the surface of the framework, so the ICP results of K are higher than those for Li and Na.

N 2 Asorption Isotherms
Nitrogen adsorption isotherms were measured to obtain the porosity of xM@NH 2 -MIL125, and the results are shown in Figure 7. All obtained N 2 adsorption isotherms of the xM@NH 2 -MIL125 series exhibited a type I isotherm curve. The Brunauere Emmette Teller (BET) surface area was calculated in the relative pressure (P/P 0 ) range of 0.05 to 0.3, and the detailed adsorption data were summarized in Table 1. The NH 2 -MIL125(Ti) structure involves octahedral cages of 12.5 Å and tetrahedral cages of 6.1 Å, and are accessible through triangular windows of 5-7 Å. At low relative pressure, the significant adsorption amount indicates that the materials involve microporosity [22,28].

N2 Asorption Isotherms
Nitrogen adsorption isotherms were measured to obtain the porosity of xM@NH2-MIL125, and the results are shown in Figure 7. All obtained N2 adsorption isotherms of the xM@NH2-MIL125 series exhibited a type I isotherm curve. The Brunauere Emmette Teller (BET) surface area was calculated in the relative pressure (P/P0) range of 0.05 to 0.3, and the detailed adsorption data were summarized in Table 1. The NH2-MIL125(Ti) structure involves octahedral cages of 12.5 Å and tetrahedral cages of 6.1 Å, and are accessible through triangular windows of 5-7 Å. At low relative pressure, the significant adsorption amount indicates that the materials involve microporosity. [22,28]. It was found that the surface areas of xM@NH2-MIL125 increased observably after alkali cation doping, in the sequence of 1Li + > 1Na + > 1K + > 2Li + > 2Na + > 2K + . The surface areas are 1470, 1451, and 1226 m 2 ·g −1 after 1 wt % Li, Na, K incorporation, respectively, which is a significant improvement compared with 1038 m 2 ·g −1 for the pristine NH2-MIL125(Ti). It is worth pointing out that the BET specific surface area of the pristine NH2-MIL125(Ti) in our work is lower than those reported in the reference, including 862~1469 m 2 g −1 [23], 1203 m 2 g −1 [21], 1215 m 2 g −1 [32], and 1041~1268 m 2 g −1 [27]. The pristine NH2-MIL125(Ti) and the xM@NH2-MIL125 are using the same synthetic procedure and calcination temperatures in this work, to investigate the tendency of the alkali metal doping effects.
The enhancement of 1Li@NH2-MIL125 was greater than that observed for 1Na@NH2-MIL125 and 1K@NH2-MIL125. The increasing trend of surface area correlates excellently with the conclusions from the SEM and XRD investigations, as described previously. However, the specific surface area dropped rapidly when the alkali metal doping amount increased to 2 wt %. Perhaps a doping amount 2 wt % of the dopants themselves (Li, Na, and K) is too much, and the channels or pores of NH2-MIL125 might be blocked [8]. Therefore, the alkali metal doping can promote the specific surface area and pore volumes of the NH2-MIL125(Ti), but excessive doping will block the pores of metal organic It was found that the surface areas of xM@NH 2 -MIL125 increased observably after alkali cation doping, in the sequence of 1Li + > 1Na + > 1K + > 2Li + > 2Na + > 2K + . The surface areas are 1470, 1451, and 1226 m 2 ·g −1 after 1 wt % Li, Na, K incorporation, respectively, which is a significant improvement compared with 1038 m 2 ·g −1 for the pristine NH 2 -MIL125(Ti). It is worth pointing out that the BET specific surface area of the pristine NH 2 -MIL125(Ti) in our work is lower than those reported in the reference, including 862~1469 m 2 g −1 [23], 1203 m 2 g −1 [21], 1215 m 2 g −1 [32], and 1041~1268 m 2 g −1 [27]. The pristine NH 2 -MIL125(Ti) and the xM@NH 2 -MIL125 are using the same synthetic procedure and calcination temperatures in this work, to investigate the tendency of the alkali metal doping effects.
The enhancement of 1Li@NH 2 -MIL125 was greater than that observed for 1Na@NH 2 -MIL125 and 1K@NH 2 -MIL125. The increasing trend of surface area correlates excellently with the conclusions from the SEM and XRD investigations, as described previously. However, the specific surface area dropped rapidly when the alkali metal doping amount increased to 2 wt %. Perhaps a doping amount 2 wt % of the dopants themselves (Li, Na, and K) is too much, and the channels or pores of NH 2 -MIL125 might be blocked [8]. Therefore, the alkali metal doping can promote the specific surface area and pore volumes of the NH 2 -MIL125(Ti), but excessive doping will block the pores of metal organic framework, consequently reducing its performance. It is very important to select suitable alkali metal type and doping dosage for the purpose of improving the gas adsorption performance of MOFs.

CO 2 Adsorption Isotherms Measured
CO 2 adsorption isotherms of NH 2 -MIL125(Ti) and xM@NH 2 -MIL125(Ti) at 293 K and 1 atm are shown in Figure 8. All the samples show a steep initial increase at low pressures, which is characteristic of a high-CO 2 -adsorption material with microporsity. In the research system for carbon dioxide adsorption, the presence of amino functionalization provides more active sites, NH 2 -MIL125 showed a higher quantity of CO 2 adsorption, as summarized in Table 2.
Materials 2019, 12, x FOR PEER REVIEW 9 of 14 framework, consequently reducing its performance. It is very important to select suitable alkali metal type and doping dosage for the purpose of improving the gas adsorption performance of MOFs.

CO2 Adsorption Isotherms Measured
CO2 adsorption isotherms of NH2-MIL125(Ti) and xM@NH2-MIL125(Ti) at 293 K and 1 atm are shown in Figure 8. All the samples show a steep initial increase at low pressures, which is characteristic of a high-CO2-adsorption material with microporsity. In the research system for carbon dioxide adsorption, the presence of amino functionalization provides more active sites, NH2-MIL125 showed a higher quantity of CO2 adsorption, as summarized in Table 2.  It can be seen from the CO2 adsorption isotherms that the CO2 adsorption amount of xM@NH2-MIL125(Ti) increased when the 1 wt % alkali metal was introduced during the solvothermal synthesis. The adsorption amounts are 4.60, 4.57, and 3.55 mmol g −1 for 1Li@NH2-MIL125(Ti), 1Na@NH2-MIL125(Ti), and 1K@NH2-MIL125(Ti), respectively. The CO2 adsorption capacity of 1M@NH2-MIL125 is in the sequence of Li > Na > K, which is consistent with the order of the decreasing ionic radius, and in agreement with the N2 adsorption performance. Of these compounds, 1Li@NH2-MIL125(Ti) showed the highest CO2 adsorption capacity, but the amount is still much lower than that of Mg-MOF-74 [36].
One reason why alkali metal doping increases the carbon dioxide adsorption capacity is the influence of the framework structure. The enhancement is maybe due to the defect site of the frameworks caused by the alkali metal doping tuning the crystallite framework (interpenetrated level), surface (surface area, pore etc.), and adsorptive site (dipole interaction) [28]. As we discussed before, the alkali metal doped xM@NH2-MIL125(Ti) has a larger specific surface area, and the morphology of the MOFs changed. The specific surface area of NH2-MIL125(Ti) increases due to the  It can be seen from the CO 2 adsorption isotherms that the CO 2 adsorption amount of xM@NH 2 -MIL125(Ti) increased when the 1 wt % alkali metal was introduced during the solvothermal synthesis. The adsorption amounts are 4.60, 4.57, and 3.55 mmol g −1 for 1Li@NH 2 -MIL125(Ti), 1Na@NH 2 -MIL125(Ti), and 1K@NH 2 -MIL125(Ti), respectively. The CO 2 adsorption capacity of 1M@NH 2 -MIL125 is in the sequence of Li > Na > K, which is consistent with the order of the decreasing ionic radius, and in agreement with the N 2 adsorption performance. Of these compounds, 1Li@NH 2 -MIL125(Ti) showed the highest CO 2 adsorption capacity, but the amount is still much lower than that of Mg-MOF-74 [36].
One reason why alkali metal doping increases the carbon dioxide adsorption capacity is the influence of the framework structure. The enhancement is maybe due to the defect site of the frameworks caused by the alkali metal doping tuning the crystallite framework (interpenetrated level), surface (surface area, pore etc.), and adsorptive site (dipole interaction) [28]. As we discussed before, the alkali metal doped xM@NH 2 -MIL125(Ti) has a larger specific surface area, and the morphology of the MOFs changed. The specific surface area of NH 2 -MIL125(Ti) increases due to the decrease of the particle size, which leads to a large amount of CO 2 adsorption. The CO 2 adsorption capacity has a positive correlation with the S BET , as shown in Figure 9. The surface area of the 1Li@NH 2 -MIL125(Ti) is much higher than that of the NH 2 -MIL125(Ti) and 2Li@NH 2 -MIL125(Ti), so the higher CO 2 adsorption of 1Li@NH 2 -MIL125(Ti) is understandable. The approximate CO 2 adsorption capacity for 1Li@NH 2 -MIL125(Ti) and 1Na@NH 2 -MIL125(Ti) may because the surface area of them are closed.
Materials 2019, 12, x FOR PEER REVIEW 10 of 14 decrease of the particle size, which leads to a large amount of CO2 adsorption. The CO2 adsorption capacity has a positive correlation with the SBET, as shown in Figure 9. The surface area of the 1Li@NH2-MIL125(Ti) is much higher than that of the NH2-MIL125(Ti) and 2Li@NH2-MIL125(Ti), so the higher CO2 adsorption of 1Li@NH2-MIL125(Ti) is understandable. The approximate CO2 adsorption capacity for 1Li@NH2-MIL125(Ti) and 1Na@NH2-MIL125(Ti) may because the surface area of them are closed. The results of the CO2 adsorption on xM@NH2-MIL125(Ti) is not only relevant to the specific surface area, but also to the alkali metal dispersed in the framework of the xM@NH2-MIL125(Ti). Another reason why alkali metal doping increases the carbon dioxide adsorption capacity is that carbon dioxide preferentially adsorbs around the alkali metal ions, and computer simulations by Mu et al. confirm this view [16]. They found that the first ionization energies and the electron donating ability of the alkali metal atoms are much lower than the boron group metal and alkaline earth metal atoms, thereby causing more ion partial charges. The first ionization energy is the minimal energy required to remove an electron from the atom or molecule isolated in free space and in its ground electronic state. The electrostatic interaction between CO2 and MOFs increases with increasingly induced charges. The 2Na@NH2-MIL125(Ti) has a bit of a smaller surface area than NH2-MIL125(Ti), but a larger CO2 adsorption, which may contribute to its smaller pore size and the doping of alkali metals. Therefore, the doping of alkali metal helps to improve the CO2 adsorption properties of NH2-MIL125(Ti).
The heat of CO2 adsorption for NH2-MIL125 and 1Li@NH2-MIL125 were calculated using the Clausius-Clapeyron equation, based on the CO2 adsorption isotherms at 273 and 293 K, as shown in Figure 10. Of those tested, 1Li@NH2-MIL125 showed a higher heat of CO2 adsorption (42-26 kJ mol −1 ) than NH2-MIL125 (21-17 kJ mol −1 ). However, the value is lower than that of MIL-100 (63-23 kJ mol −1 ) [23]. The heat of CO2 adsorption at a CO2 coverage of 2.0 wt % for 1Li@NH2-MIL125 is estimated to be 28.2 kJ mol −1 , which is much higher than that of NH2-MIL125(17.1 kJ mol −1 ), suggesting a much stronger interaction between adsorbed CO2 and 1Li@NH2-MIL125. The results of the CO 2 adsorption on xM@NH 2-MIL125(Ti) is not only relevant to the specific surface area, but also to the alkali metal dispersed in the framework of the xM@NH 2 -MIL125(Ti). Another reason why alkali metal doping increases the carbon dioxide adsorption capacity is that carbon dioxide preferentially adsorbs around the alkali metal ions, and computer simulations by Mu et al. confirm this view [16]. They found that the first ionization energies and the electron donating ability of the alkali metal atoms are much lower than the boron group metal and alkaline earth metal atoms, thereby causing more ion partial charges. The first ionization energy is the minimal energy required to remove an electron from the atom or molecule isolated in free space and in its ground electronic state. The electrostatic interaction between CO 2 and MOFs increases with increasingly induced charges. The 2Na@NH 2 -MIL125(Ti) has a bit of a smaller surface area than NH 2 -MIL125(Ti), but a larger CO 2 adsorption, which may contribute to its smaller pore size and the doping of alkali metals. Therefore, the doping of alkali metal helps to improve the CO 2 adsorption properties of NH 2 -MIL125(Ti).

CO2 Adsorption Regenerability
The recycle test of 1Li@NH2-MIL125(Ti) on CO2 adsorption was performed to test its

CO 2 Adsorption Regenerability
The recycle test of 1Li@NH 2 -MIL125(Ti) on CO 2 adsorption was performed to test its regenerability for gas capture applications, because of it having the highest capacity for CO 2 adsorption in xM@NH 2 -MIL125(Ti). Figure 11 shows the static CO 2 adsorption capacity of 1Li@NH 2 -MIL125(Ti) at 293 K. The sample was desorpted by vacuum before the new test. The readsorption amount is nearly stable for six cycles, with no obvious loss in activity. This indicates that the adsorption mechanism of CO 2 is mainly based on physical adsorption [8].

CO2 Adsorption Regenerability
The recycle test of 1Li@NH2-MIL125(Ti) on CO2 adsorption was performed to test its regenerability for gas capture applications, because of it having the highest capacity for CO2 adsorption in xM@NH2-MIL125(Ti). Figure 11 shows the static CO2 adsorption capacity of 1Li@NH2-MIL125(Ti) at 293 K. The sample was desorpted by vacuum before the new test. The readsorption amount is nearly stable for six cycles, with no obvious loss in activity. This indicates that the adsorption mechanism of CO2 is mainly based on physical adsorption [8].

Conclusions
A series of xM@NH2-MIL125(Ti) has been synthesized by in situ incorporation of alkali metal, which is straightforward and can tune the morphology of MOFs, compared with post-modification procedures. The effect of in situ alkali metal incorporation into the structure was demonstrated through the experimental data of N2 and CO2 adsorption of MOFs. The CO2 adsorption is mainly affected by the van der Waals forces between the framework and CO2 molecules. The smaller pore size, the Lewis basic active site −NH2 groups, and the alkali metals in our work are all beneficial for CO2 adsorption. The introduction of alkali metal mainly affects the CO2 adsorption in two respects. On the one hand, it impacts the crystal growth rate of MOFs, resulting the different morphology and size of grains, thus leading to the differences in specific surface area and pore size, which has a significant impact on the CO2 adsorption. On the other hand, the alkali metals, existing in the final framework after a series of solvent exchanging treatments, have an influence on the CO2 adsorption.

Conclusions
A series of xM@NH 2 -MIL125(Ti) has been synthesized by in situ incorporation of alkali metal, which is straightforward and can tune the morphology of MOFs, compared with post-modification procedures. The effect of in situ alkali metal incorporation into the structure was demonstrated through the experimental data of N 2 and CO 2 adsorption of MOFs. The CO 2 adsorption is mainly affected by the van der Waals forces between the framework and CO 2 molecules. The smaller pore size, the Lewis basic active site −NH 2 groups, and the alkali metals in our work are all beneficial for CO 2 adsorption. The introduction of alkali metal mainly affects the CO 2 adsorption in two respects. On the one hand, it impacts the crystal growth rate of MOFs, resulting the different morphology and size of grains, thus leading to the differences in specific surface area and pore size, which has a significant impact on the CO 2 adsorption. On the other hand, the alkali metals, existing in the final framework after a series of solvent exchanging treatments, have an influence on the CO 2 adsorption. This enhancement is due to the increased specific surface area and alkali metal coordinated on the defects of the cage construction. It was found that 1Li@NH 2 -MIL125(Ti) is the most efficient CO 2 adsorbent among xM@NH 2 -MIL125(Ti). Although the amount is still lower than the reported MOFs, such as Mg-MOF-74, alkali metal doping has proven to be an effective modification strategy. This preliminary study is a practical attempt to improve the performance of MOF materials on CO 2 adsorption, which will promote related research on CO 2 conversion reactions.