SBA-15 with Crystalline Walls Produced via Thermal Treatment with the Alkali and Alkali Earth Metal Ions

Crystalline walled SBA-15 with large pore size were prepared using alkali and alkali earth metal ions (Na+, Li+, K+ and Ca2+). For this work, the ratios of alkali metal ions (Si/metal ion) ranged from 2.1 to 80, while the temperatures tested ranged from 500 to 700 °C. The SBA-15 prepared with Si/Na+ ratios ranging from 2.1 to 40 at 700 °C exhibited both cristobalite and quartz SiO2 structures in pore walls. When the Na+ amount increased (i.e., Si/Na increased from 80 to 40), the pore size was increased remarkably but the surface area and pore volume of the metal ion-based SBA-15 were decreased. When the SBA-15 prepared with Li+, K+ and Ca2+ ions (Si/metal ion = 40) was thermally treated at 700 °C, the crystalline SiO2 of quartz structure with large pore diameter (i.e., 802.5 Å) was observed for Ca+2 ion-based SBA-15, while no crystalline SiO2 structures were observed in pore walls for both the K+ and Li+ ions treated SBA-15. The crystalline SiO2 structures may be formed by the rearrangement of silica matrix when alkali or alkali earth metal ions are inserted into silica matrix at elevated temperature.

Mesoporous silica materials with metal (i.e., Al, Ga, In, Ti, Zr and Zn, etc.) oxide incorporated frameworks have greatly expanded and advanced their applications such as catalysis and magnetics [6]. The incorporated nanoparticles can provide framework with crystalline structures. Meanwhile, periodic mesoporous organosilicas (PMOs) with crystal-like layered structures in frameworks were synthesized using bridged organosilica precursors including aromatic groups such as benzene and biphenyl groups as silica sources [26,39,40]. Mesoporous materials with inorganic crystalline frameworks were also synthesized with zeolite, which was well-known as microporous material with well-defined structure [41][42][43]. The resulting zeolite materials with hierarchical pore size distributions exhibit significantly improved catalytic properties for various reactions with respect to microporous zeolite materials [41][42][43].

Synthesis of SBA-15
SBA-15 was synthesized by the similar method as reported elsewhere [2,60]; in a typical synthesis, 4 g of the triblock copolymer was dissolved in 125 g of DI water and 20 g of con. HCl was added with stirring at 35 • C for 1 h. 8.5 g of TEOS was added to the solution, followed by stirring the mixture at 35 • C for 24 h. Then, it was kept at 100 • C for 24 h in a Teflon bottle. The crystallized product was filtered, washed with water, dried, followed by calcinating at 550 • C for 5 h in air to remove occluded template.

Synthesis of SBA-15 with Crystalline Walls
A total of 5 mL of sodium chloride aqueous solutions were added to 0.5 g of SBA-15. Sodium chloride aqueous solutions were prepared with different concentrations (Si/Na = 2.1-80). The slurries were stirred at 25 • C for 1 h. After that, water was removed by evaporation method under vacuum at 80 • C and then the samples were heated in the range of 500-700 • C for 3 h in air. Mesoporous silica with crystalline walls were also synthesized using other metal ions (K + , Li + and Ca 2+ ) aqueous solutions. The other process was the same as that for mesoporous silica with crystalline walls synthesized using sodium chloride solution. The materials synthesized were called as SBA15-A-C-T, where A represents metal ion species, C represents the ratio of Si/A, T is the thermal treatment temperature. Figure 1A shows SAXS patterns of (a) SBA-15, (b) SBA15-Na-80-700, (c) SBA15-Na-60-700, (d) SBA15-Na-50-700, (e) SBA15-Na-40-700, (f) SBA15-Na-30-700, (g) SBA15-Na-20-700 and (h) SBA15-Na-2.1-700. Inset shows the magnified SAXS patterns in the q = 0.10-0.20 ranges for samples. The SAXS pattern of mesoporous silica, SBA-15 ( Figure 1A(a)) showed three scattering peaks of the 100, 110 and 200 reflections of the hexagonal symmetry lattice of the SBA-15 [2,60]. Though the intensities of the scattering peaks were decreased after thermal treatment at 700 °C when the Si/Na ratio was decreased to 50 ( Figure 1A(d)), well-resolved three scattering peaks (100, 110, 200) of the hexagonal symmetry were still observed. On the other hand, the characteristic peaks in SAXS patterns disappeared after thermal treatment at 700 °C when the Si/Na ratios were above 40 ( Figure 1A(e)). Scheme 1. Illustration for the synthesis of SBA-15 (I) and SBA-15 with mesoporous silica with crystalline pore walls (II). Figure 1A shows SAXS patterns of (a) SBA-15, (b) SBA15-Na-80-700, (c) SBA15-Na-60-700, (d) SBA15-Na-50-700, (e) SBA15-Na-40-700, (f) SBA15-Na-30-700, (g) SBA15-Na-20-700 and (h) SBA15-Na-2.1-700. Inset shows the magnified SAXS patterns in the q = 0.10-0.20 ranges for samples. The SAXS pattern of mesoporous silica, SBA-15 ( Figure 1A(a)) showed three scattering peaks of the 100, 110 and 200 reflections of the hexagonal symmetry lattice of the SBA-15 [2,60]. Though the intensities of the scattering peaks were decreased after thermal treatment at 700 • C when the Si/Na ratio was decreased to 50 ( Figure 1A(d)), well-resolved three scattering peaks (100, 110, 200) of the hexagonal symmetry were still observed. On the other hand, the characteristic peaks in SAXS patterns disappeared after thermal treatment at 700 • C when the Si/Na ratios were above 40 ( Figure 1A(e)).

Discussion
In this work, we synthesized mesoporous silica with crystalline walls in the presence of alkali and alkali earth metal ions (Li + , Na + , K + , Ca 2+ ) via thermal treatment. The crystalline walled SBA-15 has also large mesopore size when silica matrix was rearranged in pore walls. We characterized the crystalline wall structures of SBA-15 prepared with different thermal treatment conditions and different ratios of Si to alkali or alkali earth metal ions (Na + , Li + , K + and Ca 2+ ) by XRD patterns, since the XRD patterns clearly evidenced different crystalline structures of the walls, though other sophisticated techniques including electron diffraction on a transmission electron microscopy may provide more accurate nature of the crystalline wall structures. Figure 1A shows SAXS patterns of samples synthesized with various content (Si/metal ion = ∞-2.1) of Na + at 700 • C. Three peaks (100, 110, 200) in SAXS patterns were retained after thermal treatment at 700 • C with the content of Na + in the range of Si/Na = ∞~30. In particular, the well-reserved 100 scattering peak indicates the two-dimensional (2D) hexagonal mesostructure. On the other hand, the characteristic peaks in SAXS patterns disappeared with the content of Na + in the range of Si/Na = 40-2.1. The result indicates that the arrangement of mesochannels was collapsed at high temperature of 700 • C with the high amount of Na + . The collapse is due to the rearrangement of the silica matrix by Na + inserted into silica.
As shown in XRD patterns of Figure 1B, when the Na + amount was increased to Si/Na = 40, interestingly, a weak peak appeared at 2θ = 21.9 • (Figure 1B(e)). The peak can be attributed to the cristobalite, one of silicas with crystalline structure [50][51][52][53][54]. When the Na + amount was increased up to Si/Na = 2.1 ( Figure 1B(h)), the intensity of cristobalite peaks increased due to the increased amount of cristobalite species.
It was reported that at low pressure such as atmospheric pressure, quartz, tridymite and cristobalite are stable up to 870 • C, 870-1470 • C and 1470-1700 • C, respectively [44,52]. Even traces of impurities inhibit kinetically and influence all phase transformations [44,52]. For instance, any additives such as alkali metal ions play a noteworthy role in forming a specific silica phase [44,[50][51][52][53][54]. In a certain case, a type of doping ion affects the transition from amorphous silica to crystalline phases [52]. In particular, the size of the ion exhibits a certain relationship with the cell volume of the crystalline phase [52]. In that way, Na + ion having the ionic radius of 1.02 Å favors the transition to cristobalite (cell volume = 171 Å 3 , density = 2.32 g cm −3 , tetragonal) [52,61,62].
Mesoporous silica materials, SBA-15 were treated with different temperatures (500-650 • C) using Si/Na = 40. Although the peak intensity of the sample (SBA15-Na-40-600) treated at 600 • C was decreased, the mesostructure of both samples (SBA15-Na-40-500, SBA15-Na-40-600) treated at 500 and 600 • C were retained as shown in SAXS patterns of Figure 2A(a,b). With higher temperature of 650 • C, the mesostructure of SBA15-Na-40-650 was collapsed as shown in SXAS pattern of Figure 2A(c). The result can be due to the insertion of Na + into silica matrix of pore walls. It is clearly supported by the XRD pattern with a small peak at 2θ = 21.9 • indicating the production of cristobalite by the rearrangement of silica matrix. SBA15-Na-40-500 and SBA15-Na-40-600 did not produce crystalline silica species. The result means that it depends not only on the amount of alkali metal ion (Na + ) but also on the temperature.
The morphologies of samples synthesized with different content (Si/metal ion = ∞-2.1) of Na + at 700 • C were observed directly by SEM technique as shown in Figure 3. With the increase of Na + content, the morphologies of the particles changed to spherical shapes with large pores in each particle. The collapse of the mesostructure in samples was observed directly by TEM technique (Figure 4). When the Na + content was increased up to Si/Na = 2.1, the large pores were produced while the arrangement of mesochannels were collapsed. The results accord well with the XRD results in Figure 1A. These results can be explained by the rearrangement of silica matrix when the alkali metal ions are incorporated into silica by thermal treatment at elevated temperature.
The morphologies and the mesostructures of samples synthesized after thermal treatment with different temperatures (500-650 • C) using Si/Na = 40 were also observed directly by SEM and TEM techniques as displayed in Figure 5. Unlike SBA15-Na-40-500 and SBA15-Na-40-500 with the ordered mesostructures, the sample synthesized at 650 • C (SBA15-Na-40-650) has the collapsed mesostructure with the expanded mesopores. The result can be due to the insertion of Na + into silica matrix of pore walls to produce crystalline silica (cristobalite). The results also correspond to the SAXS and XRD results in Figure 2. Figure 6 shows the N 2 adsorption/desorption isotherm curves (A) and the pore size distributions (B) of samples prepared with different content (Si/metal ion = ∞-2.1) of Na + at 700 • C. As the increase up to Si/Na = 2.1, Pore diameter was increased 66.7 to 1779.9 Å while BET surface area and pore volume were decreased 713 to 11 m 2 g −1 and 0.92 to 0.03 cm 3 ·g −1 , respectively ( Table 1). The increase in the pore diameter is due to the pore size expansion with the collapse of the mesostructures by Na + incorporation into the silica matrix. The result accords well with the SEM and TEM results in Figure 4. Meanwhile, the decrease in surface area and pore volume was caused by the collapse of the ordered mesostructures with high surface area and large pore volume.
In our work, thermally treated SBA-15 at 700 • C using Ca 2+ ion with higher valence and similar ion size (1.00 Å) compared to the Na + (ionic radius of 1.02 Å) favored the transition to quartz, while thermally treated SBA-15 with Na + exhibited the transition to cristobalite and quartz. The result can be due to the higher field strength of Ca 2+ than that of Na + , based on the following Equation: Field strength of cation = Z/r2 (1) where Z is the valence and r is the ionic radius [26,63].
However, SBA15-Li-40-700 synthesized with Li + (ionic radius of 0.76 Å) ( Figure 8a) and SBA15-K-40-700 synthesized with K + (ionic radius of 1.38 Å) (Figure 8b) did not produce crystalline silica species under the condition in this work. It may be assumed that the SBA-15 with crystalline structure may be prepared even in the presence of Li + and K + if the thermal treatment temperature is higher than 700 • C according to previous works [45,51,52]. As described in the Introduction, a few research groups obtained silica materials with crystalline structure when they used such high thermal treatment temperature as >800 • C. In the present work, we intended to study the effect of Li + and K + on the wall matrix structure of mesoporous silicas under various reaction conditions (different concentration of Li + and K + , different annealing temperature and annealing time, etc.) through in-depth studies, even though the SBA-15 with crystalline walls was not obtained.

Conclusions
In this work, we synthesized mesoporous silica with the crystalline wall via thermal treatment with SBA-15 in the presence of alkali or alkali earth metal ions (Na + , Li + , K + and Ca 2+ ) as catalyst. With Na + (ionic radius of 1.02 Å) as source of alkali metal ion, mesoporous silica SBA-15 showed clearly the production of crystalline silica walls (i.e., quartz and cristobalite) when the Na + content of Si/Na = 20 or more was used at the temperature of 650 • C or higher. When the Na + amount was increased up to Si/Na = 2.1 at 700 • C (SBA15-Na-2.1-700), the crystalline silica was dominated by cristobalite. Meanwhile, pore diameter was increased with dual size (18.3 and 972.7 Å) compared to the pristine SBA-15, but the surface area and pore volume were decreased to 11 m 2 ·g −1 and 0.03 cm 3 ·g −1 , respectively. After thermal treatment SBA-15 with Si/Ca = 40 at 700 • C (SBA15-Ca-40-700), the mesoporous silica showed clearly the production of crystalline silica wall dominated by quartz. However, SBA15-Li-40-700 prepared with Li + (ionic radius of 0.76 Å) and SBA15-K-40-700 prepared with K + (ionic radius of 1.38 Å) did not produce crystalline silica species under the same condition. Pore size, surface area and pore volume of SBA15-Ca-40-700 were 802.5 Å, 35 m 2 ·g −1 and 0.27 cm 3 ·g −1 , respectively. Transition of amorphous silica to crystalline structure can be explained by the rearranging silica matrix when alkali or alkali earth metal ions are inserted into silica matrix at elevated temperature. Na + ion in silica was favorable for the formation of cristobalite structure with large cell volume (cell volume = 171 Å 3 ). On the other hand, Ca 2+ ion with the lager valence (i.e., the higher field strength) than Na + ion in silica was favorable for the formation of quartz structure with smaller cell volume (cell volume = 113 Å 3 ) than cristobalite.