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Yoshimi Kawamoto, Jinrui Li, [Yusuke Ide](https://orcid.org/0000-0002-6901-6954), Tsuneji Sano, Masahiro Sadakane

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This is a pre-copyedited, author-produced version of an article accepted for publication in Chemistry Letters following peer review. The version of record Yoshimi Kawamoto, Jinrui Li, Yusuke Ide, Tsuneji Sano, Masahiro Sadakane, Selective insertion of La oxides into micropores of SBA-15 and preparation of highly ordered mesoporous La–Si oxides with high stability against alkaline hydrolysis, Chemistry Letters, Volume 54, Issue 9, September 2025, upaf156 is available online at: https://doi.org/10.1093/chemle/upaf156.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[Selective insertion of La oxides into micropores of SBA-15 and preparation of highly ordered mesoporous La–Si oxides with high stability against alkaline hydrolysis](https://mdr.nims.go.jp/datasets/aad98ef1-4428-4a6a-bfb0-ab2dc6484337)

## Fulltext

1  Yoshimi Kawamoto,1 Jinri Li,1 Yusuke Ide,2 Tsuneji Sano,1 and Masahiro Sadakane*1  1Applied Chemistry Program, Graduate School of Advanced Science and Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan. 2Research Center for Materials Nanoarchitectronics (MANA) National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan. *Corresponding author: Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Email: sadakane09@hiroshima-u.ac.jpMesoporous silica with well-ordered pores (pore 1 diameters of 2 – 50 nm) have attracted considerable 2 attention, because these materials have large surface 3 area and their pore sizes are large enough to entrap 4 large organic molecules which do not enter in 5 microporous materials such as zeolites. Therefore, they 6 have been utilized or investigated in variety of field such 7 as catalysis,1, 2 separation,3 drug delivery,4 and CO2 8 adsorption materials.5 One serious disadvantage of 9 mesoporous silica materials is instability under alkaline 10 hydrolysis condition.6  11 Among the various mesoporous silicas, SBA-15 12 with hexagonally ordered mesopore channel connected 13 by micropores (Scheme S1) have attracted great 14 interest due to their larger pores, tunability of pore size, 15 thicker pore walls, and significant amounts of 16 micropores in the walls.2 Due to the thick walls, SBA-17 15 has higher stability than other mesoporous silicas, 18 however, further improvement of stability is needed.  19 It is known that there is strong interaction 20 between lanthanoid metal with silicas. Rodriguez-21 Izquierdo group reported that calcination of La(NO3)3 22 with silica (Cabosil M-5) produced La-silicate.7 Schüth 23 group reported that lanthanum oxides coat the surface 24 of mesopores of SBA-15 due to strong interaction 25 between the lanthanoid metal and silica.8 Xue group 26 also reported La(NO3)3 is a suitable reagent to coat the 27 surface of mesopores with lanthanum oxide,9 and Fang 28 group reported that lanthanum reacted with silanol 29 moiety.10 30 In this study, we present that lanthanum oxide 31 selectively inserts into the micropores of SBA-15 by a 32 simple impregnation-calcination method. By filling 33 micropores of SBA-15 with La oxides, well-ordered 34 mesoporous La-Si oxides (SBA-La) are produced. When 35 Selective Insertion of La-Oxides into Micropores of SBA-15 and Preparation of highly Ordered Mesoporous La-Si Oxides with High Stability against Alkaline Hydrolysis Graphical abstract  Calcinationat 500 °CLa(NO3)3 ImpregnationSBA-15Micropore filling Mesopore fillingLow La2O3 loading High La2O3 loadingSBA-15 with La2O3MesoporeMicroporeHigh stability against alkaline hydrolysisAbstract Lanthanum oxides were selectively inserted into the micropores of SBA-15 via a simple impregnation and subsequent calcination method, forming mesoporous La-Si oxides (SBA-La). Characterization by XRD, N₂ adsorption, and TEM confirmed that La oxides preferentially fill the micropores without damaging the mesopore structure. Upon excess loading, La oxides occupy mesopores while retaining cylindrical mesoporous morphology. SBA-La exhibits significantly enhanced stability against alkaline hydrothermal conditions compared to unmodified SBA-15. The improved durability is attributed to the formation of La oxides in micropores, which suppresses degradation pathways. This strategy provides a simple and effective route to improve the chemical robustness of mesoporous silica materials. Keywords: SBA-15, mesoporous La-Si oxide, High stability 2   the volume of La oxide exceeds the micropore volume, 1 La oxide begins to occupy the mesopores. These SBA-2 La show high stability against alkaline hydrolysis, 3 whereas parent SBA-15 collapses.  4 La(NO3)3-6H2O was impregnated in SBA-15 by 5 stirring SBA-15 in ethanol solution of La(NO3)3-6H2O at 6 room temperature until the solvent was evaporated. 7 They were calcined in a muffle oven in air at 500 ºC for 8 3 hours. TG-DTA curves of SBA-15-La(NO3)3 composite  9 before the calcination, indicated that calcination at 500 10 ºC is enough to decompose La(NO3)3 (Fig. S1), which 11 was also confirmed by the decrease of a IR band at ca. 12 1385 cm−1 for NO3− (Fig. S2 (f)). The prepared samples 13 were denoted as SBA-La(X), where X represented 14 amount (mmol) of La(NO3)3 with 1g of SBA-15. 15 Small angle XRD pattern of SBA-15 used in this 16 work exhibited the three characteristic peaks of the 17 (100), (110), and (200) lattice planes of the hexagonal 18 (P6mm) pore structure (Fig. 1(a)). Nitrogen adsorption-19 desorption isotherm of the SBA-15 revealed type IV 20 behavior typical for mesoporous materials with type H1 21 hysteresis loop (adsorption and desorption branches 22 are parallel) (Fig. 2 (a) and Table S1). The type H1 23 hysteresis is characteristic for cylindrical pores.11 IR 24 spectra exhibits bands at 1095 and 807 cm − 1 25 correspond to the vibrations of Si-O-Si, and 965 cm−1 26 correspond to the vibrations of Si-O of silanol (Fig. S2 27 (b)).10  28  29 Fig. 1. XRD patterns of (a) SBA-15 and SBA-La(X) 30 prepared using (b) 0.43, (c) 0.86, (d) 1.72, (e) 3.44, (f) 31 5.16, (g) 6.87, (h) 13.75, and (i) 20.62 mmol of La(NO3)3. 32 (inset) Peak intensities of 100 peak against amount of 33 La(NO3)3. 34  35 Three XRD peaks characteristic for hexagonal 36 porous structure remained until SBA-15-La(6.87) (Fig. 37 1(g)) indicating presence of mesopores. Wide angle 38 XRD did not show any distinct peaks (Fig. S3 (d)), 39 indicating the obtained compound consists of both 40 amorphous silica and amorphous Lanthanum species. 41 Morphologies of SBA-La were similar to the SBA-42 15 and no plate-like particle of La oxide was observed 43 even in the SBA-15-La(20.62) (Fig. S4), indicating all the 44 La oxides exist in the pores of SBA-15. 45 The obtained solids were analyzed using nitrogen 46 adsorption-desorption isotherm (Fig. 2). The type IV 47 isotherms were obtained until SBA-15-La(6.87) (Fig. 48 2(g)) suggesting presence of mesopores similar to XRD 49 results. Obtained porous properties were summarized 50 in Table S1 and the estimated microporous volumes 51 and mesoporous volumes per 1g of SBA-15, 52 respectively, were plotted against amount of La(NO3)3 53 (Fig. 3). Addition of a small amount (until 1.72 mmol) of 54 La(NO3)3 decreased adsorbed N2 at very low P/P0 55 corresponding to the micropore volumes whereas 56 amount of uptake at P/P0 of ca. 0.7 corresponds to the 57 mesopore volumes kept almost constant (Table S1 and 58 Fig. 3). This result indicates that La oxides selectively 59 filled into the micropores until all micropores are filled 60 with La oxides. The amount of 1.72 mmol is smaller but 61 in the same range to the calculated value of 4.0 mmol 62 assuming that the micropore (volume: 0.10 cm3 g−1) 63 was filled with crystalline La2O3 (MW: 325.81 g mol−1; 64 density: 6.51 g cm−3). We believe that this difference is 65 due to amorphous nature of La oxide in the micropore. 66  67 Fig. 2. Nitrogen adsorption-desorption isotherms of (a) 68 SBA-15 and SBA-La(X) prepared using (b) 0.43, (c) 0.86, 69 (d) 1.72, (e) 3.44, (f) 5.16, (g) 6.87, (h) 13.75, and (i) 70 20.62 mmol of La(NO3)3. 71  72 Interestingly, sharp H1-type hysteresis loops were 73 retained even if large amount of La(NO3)3 was loaded, 74 indicating that cylinder-shape mesopores was 75 maintained. It is reported that the hysteresis loop was 76 affected by loading of metal oxides in the mesopore. If 77 the mesopores were homogeneously coated by metal 78 oxide, H1-type hysteresis was observed with 79 decreased mesopore diameter.12 If the metal oxide 80 particles were attached on the mesopore walls, 81 desorption branch of hysteresis became stepwise.13 In 82 our case, the relative pressure of adsorption branch of 83 hysteresis and therefore BJH mesopore diameter do 84 not change until large amount of La oxides was loaded 85 (Fig. 2). After the micropores are filled, La oxides fill the 86 mesopores without changing mesopore diameters 87 (Scheme S1). The selective insertion of La(NO3)3 was 88 3   also confirmed by powder XRD. Until 1.72 mmol, the 1 intensities of characteristic peak for hexagonal walls did 2 not decrease, but the peak intensity gradually 3 decreased when mesopore was filled by La oxides (Fig. 4 1, inset). Filling of mesopores decreased X-ray 5 scattering difference between the hexagonal wall and 6 mesopores. 7  8 Fig. 3. (open circles) mesopore volumes and (open 9 squares) micropore volumes per 1.0 g of SBA-15 10 against amount of La(NO3)3. An arrow indicates amount 11 (4.0 mmol) of La(NO3)3 to fill only micropore of SBA-15 12 (1.0 g) with La2O3 crystal. 13  14 Filling of La oxide in the mesopores were also 15 confirmed by TEM. Channel-like pores were observed 16 until the micropores were filled, but pore and wall 17 contrast became small when mesopores were started 18 to be filled (Fig. S5). 19 In order to ensure this filling model, we have 20 performed IR measurement (Fig. S2 (c)), where IR peak 21 corresponding to Si-OH at 965 cm−1 disappeared by 22 filling of micropores by La loading. SBA-15 was 23 prepared using a block copolymer of poly(ethylene 24 oxide) and poly(propylene oxide) as a template, and 25 micropores were formed around hydrophilic 26 polyethylene oxide moiety. Therefore, it is expected 27 that large amount of Si-OH group exists in the 28 micropores.14-16 Disappearance of Si-OH group agrees 29 with our model where micropore is first filled by La 30 oxides.9, 10 31 To understand transformation behavior of La(NO3)3 32 in SBA-15, SBA-15-La(NO3)3(6.87) heated at different 33 temperatures were characterized by powder XRD (Fig. 34 S6) and N2 isotherm (Fig. S7). We selected sample with 35 6.87 mmol of La(NO3)3 because the estimated volume 36 of 1.29 cm3 with density of 2.3 g cm−3 is larger than sum 37 of micropore and mesopore volume of SBA-15. 38 Characteristic powder XRD pattern for SBA-15 was not 39 observed until the sample was heated at 200 ºC, but 40 started to appear when heating temperature was more 41 than 300 ºC (Fig. S6), indicating that the mesopores 42 were filled with mixture of La nitrate, La oxide and La 43 hydroxide (La-oxo-nitrate) at temperatures until at 300 44 ºC and the mesopores were opened at 400 ºC. This 45 phenomenon was also confirmed by N2 isotherm. 46 Adsorption uptake into mesopores at ca. 0.6-0.7 of P/P0 47 gradually increased and the desorption at ca. 0.7-0.4 of 48 P/P0 occurred stepwise after heating at 200 and 300 ºC 49 indicating presence of La-oxo-nitrate particles in the 50 mesopores (Scheme S2). Adsorption and desorption 51 became sharp and H1-type hysteresis loops was 52 obtained after heating at 400 ºC. The characteristic 53 powder XRD pattern and the sharp H1-type hysteresis 54 was kept after heating at 800 ºC. Further heating 55 decomposed mesoporous structure. 56 We also checked the possibility of insertion of Co, 57 Fe, and Ce oxides under same conditions in the SBA-58 15. N2 isotherms and XRD pattern with different metal 59 amount were shown in Fig. S8, and micropore volumes 60 and mesopore volumes after calcination at 500 ºC were 61 summarized in Fig. S9. In the cases of Fe, Co, and Ce, 62 micropores were still open when enough metal was 63 impregnated to fill the micropores indicative of non-64 selective insertion into the micropores. 65 Enhancement of stability against alkaline 66 hydrolysis is one of the most important modifications 67 of SBA-15 related materials, and doping of Al has been 68 reported to be effective. We checked hydrolysis 69 stability by heating samples at 60 ºC in the 2M NaOH 70 solution for 2 hours where bare SBA-15 dissolved 71 completely and SBA-15 hard template dissolved from 72 SBA-15-metal oxide composites (Metal: Co, Fe, Ce, Ni, 73 etc).6, 17 The SBA-La samples, on the other hand, did not 74 dissolve under the NaOH treatments, and the remained 75 amount and the mesopore volumes were summarized 76 in Table S2. As confirmed by XRD (Fig. S10 (a)-(b)), only 77 some SBA-La samples retained mesoporous structure. 78 The remaining amount increased by increasing La 79 loading amount, indicated that La enhances the stability 80 against alkaline hydrolysis. When loading of La was 81 higher than 3.44 mmol/g-SBA, in which micropore were 82 fully filled with La oxide, the XRD peaks corresponding 83 to the mesopore was kept after NaOH treatment (Fig. 84 S10(b)). When the amount of La was lower than 3.44 85 mmol/g-SBA, the characteristic XRD peaks disappeared. 86 Well-defined H1-type hysteresis loop was obtained in 87 the case of SBA-La(5.16) and SBA-La(6.87) (Fig. S10(c)) 88 where XRD peaks were observed. These results 89 indicate that micropore filling of La increased stabilities 90 against alkaline hydrolysis. TEM images of samples 91 after NaOH treatment clearly indicated that SBA-92 La(1.72) contained less ordered mesopore whereas 93 SBA-La(5.16) retained well-ordered mesopores (Fig. 4). 94 We also checked stability by heating the samples 95 in the presence of NH3 and H2O vapor (Fig. S11). As 96 shown in Fig. S12, XRD pattern of SBA-15 and SBA-97 La(1.72) disappeared after hydration treatment for 3 98 hours, whereas one of SBA-15-La (5.16) remained after 99 7 day. N2 isotherm indicated that mesopores of SBA-15 100 was decomposed after 3 hours NH3 hydrothermal 101 treatment, although one of SBA-La(5.16) remained 102 after 7 days (Fig. 5 and S12). On the other hand, SBA-103 M(5.16) (M = Co, Fe, and Ce), did not kept the 104 4   mesoporous structure after 3 hours treatment. It is 1 reported that degradation of SBA-15 starts in the 2 micropores where reactive Si-OH is present.18 Filling of 3 the micropore enhanced hydrolysis stability. 4  5   6 Fig. 4 TEM images of (a) SBA-La(1.72) and (b) SBA-7 La(5.16) after NaOH treatments. 8  9 Fig. 5 Nitrogen adsorption-desorption isotherms of SBA-10 La(5.16) (a) before and (b) after NH3 hydrothermal 11 treatment (3 hours), and (c) SBA-15 after NH3 12 hydrothermal treatment (3 hours). 13  14 Selective insertion of lanthanum oxides into SBA-15 15 micropores produced mesoporous La-Si oxides with 16 preserved structure and significantly enhanced stability. 17 The micropore filling suppressed degradation under 18 alkaline and hydrothermal conditions. This simple 19 strategy provides an effective route to improve the 20 durability of mesoporous silica materials for advanced 21 catalytic and adsorption applications. 22  23 Supplementary data 24 Supplementary material is available at Chemistry 25 Letters 26  27 Funding 28 This work was supported by SUZUKI foundation, JSPS 29 KAKENHI Grant-in-Aid for transformative Research 30 Area (A) “Supra-ceramics” (JP22H05144), and JSPS 31 Core-to-Core Program. 32  33 Conflict of interest statement. None declared. 34  35 References 36 1 A. Velty, A. Corma, Chem. Soc. Rev. 2023, 52, 1773, 37 http://doi.org/10.1039/d2cs00456a.  38 2 P. Verma, Y. Kuwahara, K. Mori, R. Raja, H. 39 Yamashita, Nanoscale 2020, 12, 11333, 40 http://doi.org/10.1039/d0nr00732c.  41 3 E. Da'na, Microporous Mesoporous Mater. 2017, 247, 42 145, http://doi.org/10.1016/j.micromeso.2017.03.050.  43 4 V. F. Vavsari, G. M. Ziarani, A. Badieic, RSC Adv. 44 2015, 5, 91686, http://doi.org/10.1039/c5ra17780d.  45 5 J. Li, N. 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