Mesoporous SBA-15 zeolite has attracted much attention owing to its interesting mesostructure and wide application as a catalyst and support [1, 2, 3, 4, 5, 6]. However, many catalytic reactions, such as oxidation, hydrogenation, dehydration, hydration and F-T synthesis reactions, are often carried out in the presence of H2O or even in aqueous solution. Moreover, cracking catalysts are usually regenerated in steam at high temperature. Therefore, high hydrothermal stability is one of the most important features required of catalysts and supports [7, 8, 9]. Unfortunately, mesoporous silica has a poor hydrothermal stability. When heated in aqueous conditions or steam for long time, the material suffers from the collapse of its ordered mesostructure and a corresponding loss of surface area. Therefore, much research work has focused on improving the hydrothermal stability of mesoporous silica by various approaches, such as thickening of walls by adding inorganic salts or using some unusual surfactants [10, 11], removing surface Si-OH groups by silylation [12, 13, 14] and F− ions [15, 16, 17] to form a hydrophobic surface, crystallization at high temperature (150-220 °C) using thermally stable fluorine-containing surfactants as templates [18], adjusting the solution pH = 1-3 to avoid the decomposition of the P123 copolymer [19], or employing carbon-propping and high temperature treatment at 800 °C [20] to enhance the silica condensation of the walls [18, 19, 20, 21, 22, 23, 24, 25, 26]. All of these methods have been successful in improving the hydrothermal stability of mesoporous SBA-15 zolite, but some of them are difficult in operation and controlling conditions, and some are time and energy consuming, costly or environmentally unfriendly.
We previously reported a simple and low-energy consumption approach to improve the hydrothermal stability of SBA-15 zeolite via post-treatment with (NH4)2SiF6 [7]. The modified SBA-15 zeolite exhibited good stability. Even after treatment at 800 °C for 12 h in 100% stream, its ordered mesostructure was still well maintained and had a high surface area of 213 m2/g. However, we also found that some F- ions released from (NH4)2SiF6 can etch the framework of SBA-15 zeolite to form new Q3 surface defects, as well as replace surface Si-OH groups to form a hydrophobic surface. Thus, the stabilization effect of the (NH4)2SiF6 treatment on the hydrothermal stability of SBA-15 zeolite, the efficacy of which depends on the combined effect of both positive factors of silicon insertion and the surface hydrophobization by F− ions and the negative factor of F− ion etching, is weakened. Therefore, it is important to reduce the etching of F− ions for enhancing the stabilization effect of (NH4)2SiF6 treatment.
As known, the bond energy of Al-F is 665 kJ/mol, which is larger than that of Si-F (582 kJ/mol) and Al-O (585 kJ/mol); and the bond length of Al-F is 1.63 × 10-10m, which is shorter than that of Si-F (1.71 × 10-10 m) and Al-O (1.75 × 10-10 m). This suggests that Al-F bonds have superior stability to those of Si-F and Al-O bonds. In other words, Al3+ ions have a stronger binding ability with F- ions than Si4+ ions do. Therefore, it is possible that using Al3+ ions to capture F- ions will reduce the amount of etching by F− ions and promote the stabilization effect of (NH4)2SiF6 treatment on the hydrothermal stability of SBA-15 zeolite.
In this research, we demonstrate an improved (NH4)2SiF6 treatment to enhance the hydrothermal stability of mesoporous SBA-15 zeolite by incorporating Al3+ ions into the SBA-15 before the (NH4)2SiF6 treatment, then washing the sample with HCl (2 mol/L) to remove the Al3+ ions. The obtained SBA-15 zeolite displayed higher hydrothermal stability than that without incorporated Al3+, evaluated using hydrothermal treatment at 800 °C for 12 h in 100% steam. The promotion mechanism of Al3+ ions, as well as the influence of the method of Al3+ introduction, is also discussed according to X-ray diffraction (XRD), N2 adsorption, 29Si and 27Al NMR, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) characterizations.
Triblock copolymer EO20PO70EO20 (P123, Aldrich, Milwaukee, WI, USA, Mr = 5800), aluminum isopropoxide (Lingfeng Chemical Corp., Shanghai, China, analytical grade), tetraethyl orthosilicate (TEOS, Aldrich, analytical grade, 98%), ammonium fluorosilicate (NH4SiF6), ammonium acetate (NH4Ac), and hydrochloric acid (Shanghai Chemical Corp., Shanghai, China, analytical grade) were used as received without any further purification.
SBA-15 and AlSBA-15 zeolites were synthesized according to Refs. [27] and [28], respectively. S15-x%SiF and AlS15-x%SiF samples were prepared according to Ref. [7]. In a typical procedure, 3 g of calcined SBA-15 or AlSBA-15 were dispersed in 90 mL of NH4Ac solution (0.3 mol/L). The defined amount of (NH4)2SiF6 solution (5%) was then dropped into the mixture under vigorous stirring at 60 °C. After 1 h, the solution was filtrated, and the solid was washed twice and dried at 120 °C for 4 h. In the sample names, “x” denotes the SiO2 ratio of (NH4)2SiF6 to SBA-15 or AlSBA-15.
S15-(1%SiF+1%Al) was prepared in the same manner as S15-x%SiF, except 1% of AlCl3 (mole ratio of Al to SiO2 in the SBA-15) was dissolved in 90 mL NH4Ac (0.3 mol/L) before the defined amount of (NH4)2SiF6 solution was added.
S15-1%SiF/1%Al was prepared from S15-1%SiF with the impregnation of 1% AlCl3 (pH = 2.0 solution). It was dried at 120 °C for 4 h before use.
AlS15-1%SiF-DAl was prepared by washing 1 g of AlS15-1%SiF twice in 10 mL of HCl (2 mol/L) solution under stirring at 80 °C for 2 h, after which the sample was washed to pH = 7 and dried at 120 °C for 4 h.
All samples were treated hydrothermally in a tube furnace at 800 °C for 12 h under a gas flow (1000 mL/min) of 100% steam.
XRD patterns were recorded on a Bruker D8 X-ray diffractometer (Germany) with Ni-filtered Cu Kα radiation (40 kV, 40 mA). N2 adsorption-desorption isotherms were measured at -196 °C with a Micromeritics Tristar 3000 analyzer. The Brunauer-Emmett-Teller (BET) method was used to calculate the specific surface areas of the samples using adsorption data in a relative pressure range from 0.05 to 0.2. Pore size distributions (PSDs) were derived from the adsorption branches of the isotherms using the Barrett-Joyner-Halenda (BJH) model. The total pore volume Vt was estimated from the amount adsorbed at a relative pressure p/p0 of 0.995. The microporous volume Vmicro was analyzed with V-t plots. TEM and SEM images were obtained using a JEOL 2011 microscope operated at 200 kV and a Philip XL30 instrument, respectively. 27Al and 29Si NMR spectra were measured on a Bruker AVANCE 400MHz spectrometer. 29Si NMR was performed at a frequency of 59.621 MHz, a recycling delay of 600 s, a radiation frequency intensity of 62.5 kHz, and with a Q8M8 reference sample (((CH3)3SiO)8Si8O12).
The XRD patterns (Fig. 1(a)) of SBA-15, S15-1%SiF, and AlS15-1%SiF-DAl display three well-resolved diffraction peaks at 2q = 0.9° and 1.5°-2.0°, corresponding to the (100), (110), and (200) reflections of the 2D hexagonal mesostructure of SiO2. All of the samples exhibited typical type IV isotherms comprising a H1-type hysteresis loop (Fig. 2(a)) and had a fibrous morphology with well-ordered hexagonal mesostructure (Fig. 3). These results suggest that the (NH4)2SiF6 treatment, the incorporation of Al3+ ions and the subsequent dealumination by washing with HCl solution did not destroy the ordered mesostructure. However, Table 1 shows that the surface areas of S15-1%SiF and AlS15-1%SiF-DAl decreased to 490 and 497 m2/g from 878 m2/g of the SBA-15 zeolite, respectively; their pore volumes dropped to 0.81 and 0.72 cm3/g from 1.3 cm3/g, respectively; and their pore diameters were reduced to 6.9 and 5.9 nm from 7.1 nm, respectively. As shown in the XRD patterns, the (100) diffraction peak of the three samples had the same 2θ (= 0.9°), suggesting that they had the same unit cell parameters, so the decrease in pore diameter implies that the mesoporous walls of S15-1%SiF and AlS15-1%SiF-DAl had thickened. As suggested by our previous work [7], during (NH4)2SiF6 treatment, Si atoms can insert into the framework of SBA-15 zeolite to repair surface defects, resulting in the closure of most of the micropores, and SiO2 can deposit on the walls and block mesopores and micropores. These processes may have caused the surface area, pore volume and diameter of S15-1%SiF and AlS15-1%SiF-DAl to decrease drastically from those of SBA-15.
After SBA-15 zeolite was treated hydrothermally at 800 °C for 12 h in 100% steam, its characteristic diffraction peaks at 2q = 0.9° and 1.5°-2.0° (Fig. 1(b-1)) and typical H1-type hysteresis loop (Fig. 2(b-1)) disappeared completely. Its fibrous morphology collapsed, and no ordered mesopores were observed in SEM and TEM images (Fig. 3), while its surface area was reduced to 71 m2/g. These results suggest that the ordered mesostructure of the hydrothermally treated SBA-15 zeolite had been destroyed. However, the S15-1%SiF sample subjected to the same hydrothermal treatment still exhibited a (100) reflection diffraction peak at 2q = 0.9° and a distorted H1-type hysteresis loop (Fig. 2(b-2)). Some ordered mesostructuring and a loose fibrous morphology were also observed in SEM and TEM images. The sample retained a high surface area of 224 m2/g. These results indicate that S15-1%SiF had a higher hydrothermal stability than SBA-15, which is in good agreement with our previous work [7].
Notably, the AlS15-1%SiF-DAl sample treated hydrothermally at 800 °C for 12 h in 100% steam had some significant differences to the S15-1%SiF sample: (1) the (100) reflection peak at 2q = 0.9° had higher intensity, while the diffraction peaks of the (110) and (200) reflections at 2q = 1.5°-2.0° were less distinct; (2) its H1-type hysteresis loop was less distorted; (3) the fibrous morphology and ordered mesostructure were better maintained; (4) a higher surface area of 271 m2/g and bigger pore volume of 0.53 m3/g were maintained. All of these results indicate that AlS15-1%SiF-DAl has a higher hydrothermal stability than S15-1%SiF.
To prevent framework Al from affecting the hydrothermal stability of the SBA-15, the incorporated Al3+ ions in the AlS15-1%SiF-DAl sample were removed by washing with HCl solution after the (NH4)2SiF6 treatment. As shown in Table 1, the Si/Al ratio of the AlS15-1%SiF-DAl sample was 380, which is close to that of SBA-15 (Si/Al = 487). Comparing this with the Si/Al ratio (Si/Al = 78) of the AlS15-1%SiF sample before dealumination by HCl, it is calculated that about 80% of the incorporated Al3+ ions were removed by the acid wash process. Although about 20% of the framework Al still existed in this sample, we believe that these Al atoms had little contribution to the hydrothermal stability, because AlSBA-15, which had more framework Al (Si/Al = 69) than AlS15-1%SiF-DAl, had a surface area of just 177 m2/g and a weak (100) diffraction peak after hydrothermal treatment. Thus, it showed worse hydrothermal stability than AlS15-1%SiF-DAl. Therefore, we propose that the high hydrothermal stability of AlS15-1%SiF-DAl is related to a synergistic effect of the incorporated Al3+ ions that promoted the stabilization effect of (NH4)2SiF6 treatment.
This result poses the question of whether AlS15-1%SiF would have higher hydrothermal stability than AlS15-1%SiF- DAl if the incorporated Al3+ ions were not removed by washing with HCl solution. However, the sample treated hydrothermally at 800 °C for 12 h in 100% steam only retained a surface area of 78 m2/g, and no diffraction peaks or H1-type hysteresis loops were observed in its XRD pattern and N2 adsorption-desorption isotherm, respectively. These results suggest that the hydrothermal stability of this sample was not improved at all by the incorporation of framework Al and (NH4)2SiF6 treatment. On the contrary, the stabilization effect of the (NH4)2SiF6 treatment observed for the S15-1%SiF sample was shielded by the incorporated Al. Unusually, this indicates that the incorporated Al3+ ions and (NH4)2SiF6 treatment have different synergy and influence the hydrothermal stability of SBA-15 zeolite differently. The mechanism will be discussed below.
As shown in Fig. 4, the surface areas of the S15-x%SiF samples were first maintained at around 490 m2/g and then decreased with increasing amount of (NH4)2SiF6 above 4%. The samples displayed a similar variation in surface area after hydrothermal treatment at 800 °C for 12 h in 100% steam. These results indicate that treatment with more (NH4)2SiF6 cannot enhance its stabilization effect further, which is in good agreement with our previous work [7]. This is because more F- ions are produced when an excess of (NH4)2SiF6 is used. These can aggravate the etching into the framework of SBA-15 zeolite and form a greater amount of new surface defects.
However, AlS15-x%SiF-DAl and S15-x%SiF exhibited different relationships between surface area and amount of (NH4)2SiF6. Before they were treated hydrothermally at 800 °C for 12 h in 100% steam, all of the samples had surface areas of about 500 m2/g. The surface areas of the AlS15-x%SiF-DAl samples did not decrease as the amount of (NH4)2SiF6 was increased, unlike the S15-x%SiF samples. The surface areas of the hydrothermally treated AlS15-x%SiF-DAl samples only decreased slightly with increasing amount of (NH4)2SiF6 compared with those of the corresponding S15-x%SiF samples, and remained above 230 m2/g. This result suggests that SBA-15 zeolites containing Al3+ ions can withstand a wide range of conditions of (NH4)2SiF6 treatment. This may be because the incorporated Al3+ ions can prevent the etching of the SBA-15 zeolite framework by F- ions. When treating SBA-15 zeolites without incorporated Al3+ ions, we often need to carefully control conditions such as the temperature, (NH4)2SiF6 concentration, and (NH4)2SiF6 solution dropping speed, to retard or avoid F- ion etching. Now, the (NH4)2SiF6 treatment procedure can be simplified by first incorporating Al3+ ions into the SBA-15.
The S15-(1%SiF+1%Al) sample exhibited a surface area of 745 m2/g and a pore volume of 1.14 cm3/g (Table 1), which were very close to those of SBA-15. After hydrothermal treatment at 800 °C for 12 h in 100% steam, the S15-(1%SiF+1%Al) sample retained a surface area of just 62 m2/g, which is lower than that of S15-1%SiF sample (224 m2/g) and close to that of SBA-15 zeolite subjected to the same hydrothermal treatment. Thus, S15-(1%SiF+1%Al) had worse hydrothermal stability than S15-1%SiF and a similar one to that of SBA-15 zeolite. For this sample, although Al3+ ion incorporation did not promote the (NH4)2SiF6 treatment, (NH4)2SiF6 treatment did not exhibit a stabilizing effect.
In fact, this sample contained almost no aluminum. As shown in Table 1, the Si/Al ratio of the S15-(1%SiF+1%Al) sample was 351, close to that of SBA-15 zeolite (Si/Al = 487). This indicates that the AlCl3, which was dissolved in NH4Ac solution, was not incorporated into SBA-15 effectively. Instead, the Al was washed from the sample completely during (NH4)2SiF6 treatment, so no promotion by Al3+ ions occurred. (NH4)2SiF6 treatment did not render a stabilizing effect in this sample because the method of introducing Al3+ ions has great influence on the synergy with (NH4)2SiF6 treatment. Directly introducing Al3+ into the same solution as the (NH4)2SiF6 treatment is detrimental to the stabilization effect of the (NH4)2SiF6 treatment. The mechanism will be discussed below.
As known from the bond energy data, Al3+ has a stronger binding ability with F- than Si4+. Therefore, when SBA-15 zeolite was first modified with Al3+ ions, then treated with (NH4)2SiF6 solution, the incorporated Al3+ ions acted to “capture” F- ions (Fig. 5 (b-2)) and reduced the F- etching of the SBA-15 zeolite framework (Fig. 5(a-1)). The role of the Al3+ ions is further confirmed by the following two facts.
First, Table 1 shows that the Si/Al ratio of the AlSBA-15 sample was 69. The characteristic peaks of tetrahedral framework Al and octahedral extra-framework Al appeared at d = 53.3 and 0 in the 27Al NMR spectrum of this sample (Fig. 6(1)), respectively. After treatment with (NH4)2SiF6 solution, the Si/Al ratio of the AlS15-1%SiF sample increased to 78. The characteristic peak of octahedral extra-framework Al at d = 0 disappeared (Fig. 6(2)), and the peak of tetrahedron framework Al decreased slightly. This result suggests that some incorporated Al3+ ions, especiallythe extra-framework Al, were removed during the (NH4)2SiF6 treatment because of their reaction with F- ions to form soluble [AlF6]3- ions.
Second, the 29 Si NMR spectra (Fig. 7) show that the Q4/(Q3+Q2) ratio of SBA-15 zeolite was 0.21. After treated with (NH4)2SiF6 solution, the S15-1%SiF sample exhibited a slightly high ratio of Q4/(Q3+Q2) = 0.26. During (NH4)2SiF6 treatment, Si atoms can insert into the framework of SBA-15 zeolite and repair its surface defects to form the Q4 structure from the Q3 and Q2 structures. Meanwhile, F- ions released from (NH4)2SiF6 can etch the SBA-15 zeolite framework to form new Q3 or Q2 surface defects. The combination of these two competitive factors caused the amount of Q4 structure (Si(OSi)4) to increase by about 23.8%. However, when the AlSBA-15 sample was treated with (NH4)2SiF6, the preincorporated Al3+ ions acted as a “trap” for F- ions and reduced the F- etching into the SBA-15 zeolite framework. Combined with a small amount of direct Si insertion which repaired surface defects, the Q4/(Q2+Q3) ratio of AlS15-1%SiF markedly increased from the 0.34 of AlSBA-15 to 0.54.
In the case of the AlS15-1%SiF-DAl sample, most of its framework Al was removed by washing with HCl (2 mol/L) via a process of dealumination. Thus, Si-O-Al and Si-O-Al-O-Si bonds were broken to form new Si-OH and (Si-OH+HO-Si) defects. As a result, the Q4/(Q2+Q3) ratio of this sample decreased to 0.37. Even so, the amount of Q4 structure (Si(OSi)4) in this sample was still about 76.2% higher than that of SBA-15 zeolite, which is also higher than that of the S15-1%SiF sample. Although about 20% of the Al3+ ions still remained in the framework of the sample, they at most contributed about 12% of the amount of Q4, calculated from both the Si/Al and Q4/(Q3+Q2) ratios of the AlS15-1%SiF-DAl and AlSBA-15 samples. Even if this part of the contribution to Q4 is deducted, the amount of Q4 structure was still increased by as much as 64.2% by the (NH4)2SiF6 treatment. Obviously, with the same (NH4)2SiF6 treatment, AlS15-1%SiF-DAl had a greater increase in Q4 structure than S15-1%SiF without incorporated Al. This result indicates that the incorporated Al3+ ions did capture F- ions and slow their etching of the framework of SBA-15, such that the ability of (NH4)2SiF6 to repair the surface defects of the silica appeared to be enhanced.
For the S15-(1%SiF+1%Al) sample, Al3+ ions introduced directly into the NH4Ac solution did not have any promotion effect on the (NH4)2SiF6 treatment. The reason for this is also the strong ability of Al3+ ions to capture F- ions. AlCl3 dissolves well in NH4Ac solution and can quickly react with the dropped (NH4)2SiF6 to form [AlF6]3- ions, causing the (NH4)2SiF6 to quickly decompose. Once the (NH4)2SiF6 decomposed, it no longer had the ability to repair the surface defects of the SBA-15 zeolite. Moreover, there were not enough F- ions in solution to replace the surface Si-OH groups to form a hydrophobic surface because most of them had been captured to form the stable [AlF6]3- ions. Indeed, the dropped (NH4)2SiF6 did not react with the SBA-15, but with Al3+ ions in the solution, resulting in it appearing that the sample had not been treated with ammonium fluorosilicate at all. Therefore, the surface area, pore volume, and Si/Al ratio of the S15-(1%SiF+1%Al) sample, as well as its hydrothermal stability, were very close to those of the original SBA-15 zeolite.
In the cases of the AlS15-1%SiF and AlS15-1%SiF-DAl samples, Al3+ ions were incorporated into the SBA-15 zeolite before the (NH4)2SiF6 treatment. Thus, they existed as framework Al or as amorphous Al2O3 in the channels. Because neither of these aluminum species were free, unlike AlCl3 in the solution, they could not react quickly with the dropped (NH4)2SiF6, but effectively captured nearby excess F- ions which had not reacted with Si-OH groups (Fig. 5(b-2)). Thus, the etching of the SBA-15 zeolite framework by F- ions was retarded effectively. According to the above analysis, both AlS15-1%SiF and AlS15-1%SiF-DAl should have higher hydrothermal stability than S15-1%SiF sample. However, the AlS15-1%SiF sample showed a worse stability when treated hydrothermally at 800 °C. We believe that this is associated with the thermal stability of framework Al.
The AlS15-1%SiF sample was calcined at 600 and 700 °C for 2 h. The resulting samples AlS15-1%SiF-600-2h and AlS15- 1%SiF-700-2h exhibited the characteristic peak of octahedral extra-framework Al at d= 0 in Fig. 6(4) and (5), and the intensity of the peak increased with the calcination temperature. This suggests that some framework Al was removed at high temperature to form amorphous Al. The higher the temperature, the more obvious this trend was. When AlS15-1%SiF was hydrothermally treated at 800 °C, some framework Al was removed to form extra-framework Al. These species strongly interact with F-ions and can break Si-F bonds on the sample surface to form Al and F compounds (Fig. 5 (c-1)). As a result, the hydrophobic effect obtained from Si-F bonds was weakened significantly. Moreover, the compounds of Al and F may have decomposed again and released some HF that further etched the silica framework during the hydrothermal treatment. These two negative effects caused the AlS15-1%SiF sample to show poor hydrothermal stability at 800 °C.
To confirm this conclusion, S15-1%SiF/1%Al, which was prepared from S15-1%SiF by impregnation with 1% AlCl3, was treated hydrothermally at 800 °C for 12 h in 100% steam. Table 1 shows that the sample retained a surface area of only 125 m2/g, lower than that of S15-1%SiF (224 m2/g). This result proves that the presence of extra-framework Al in the channels does weaken the hydrothermal stability of SBA-15 zeolite treated with (NH4)2SiF6.
Interestingly, we found that when AlS15-1%SiF was treated hydrothermally at a low temperature of 190 °C for 5 d in a closed vessel, it retained the highest surface area (result not listed here) compared with those of the S15-1%SiF and AlS15-1%SiF-DAl samples. The hydrothermal stability had good correlation with the ratio Q4/(Q3+Q2). This is because the framework Al was stable at temperatures less than 550 °C, and could not be removed to react with the surface Si-F bonds at 190 °C. This suggests that we must pay attention to the temperature at which SBA-15 materials containing Al3+ ions are used. If they are required undergo high temperature processes above 550 °C, such samples must be dealuminated after they are treated with (NH4)2SiF6.
AlSBA-15 materials only contain weak Lewis acid sites, and cannot completely replace USY zeolite as an acid catalyst. However, AlSBA-15 materials have some ability to crack large molecules, so it can be used as an acid catalyst with microporous zeolites. Thus, some advantages of AlSBA-15 sample become apparent, for example, precracking of heavy molecules to form slightly smaller molecules, enhancement of the diffusion of reactants to the surface of microporous zeolites, and providing free mesoporous channels for the cracking products to quickly escape from the surface of USY zeolite to prevent them from being excessively cracked. It has been reported [5] that a NiW supported catalyst prepared from core-shell USY zeolite and AlSBA-15 material showed higher conversion and better selectivity in the cracking of heavy petroleum feedstocks than commercial catalysts prepared from USY zeolite. The catalyst also displayed a high stability in a pilot scale hydrocracking test of Iran VGO-2 with 200 mL of catalyst. During 800 h running, the reaction temperature was increased by just 1 °C to keep the conversion of the >370 °C fraction of the feedstock at approximately 68.5%. Although the present modified SBA-15 zeolite may still have worse hydrothermal stability than microporous zeolites such as USY and ZSM-5, we think that the obtained improvement is very important to ensure the practical application of SBA-15 zeolite as a catalyst support for the hydrocracking of heavy oil.
Mesoporous SBA-15 zeolite containing preincorporated Al3+ ions exhibited higher hydrothermal stability after treatment with (NH4)2SiF6 solution and washing with HCl solution to remove the incorporated Al3+ than SBA-15 without incorporated Al3+. The incorporated Al3+ ions reduce the etching of the silica framework of SBA-15 by F- ions, and effectively promote the positive effect of (NH4)2SiF6 treatment on the hydrothermal stability of the SBA-15. Moreover, the incorporation of Al3+ ions leads to the (NH4)2SiF6 treatment becoming simpler and easier to control. Therefore, the present approach has potential application in laboratory and industry to prepare ordered mesoporous silicas with high hydrothermal stability.
2015, Vol. 36 Issue (7): 1001-1008
PDF (804 KB)
2015, Vol. 36 Issue (7): 1001-1008
PDF (804 KB)