High zirconium loads in Zr-SBA-15 mesoporous materials prepared by direct-synthesis and pH-adjusting approaches

SBA-15 is a material with interesting characteristics associated with the size, order, uniformity, and pores distribution. However, the absence of acidity in SBA-15 restricts its applications as a potential acid catalyst in the industry for the large-scale production of fuels and chemicals. Mesoporous SBA-15 modified by the incorporation of heteroatomic species can be a good strategy to include acidity and prepare an efficient catalyst. In this work, we present a comparative study in the preparation of SBA-15 and Zr-SBA-15 mesoporous materials with high zirconium loads (Si/Zr ¿= ¿10, 5, and 2) using direct-synthesis and pH-adjusting approaches with a low crystallization temperature of 60 ¿°C. The materials were characterized using XRPD, HRTEM, ICP, SEM-EDS, XPS, DRS UV-VIS, FT-IR, NH3-TPD, in situ FT-IR with pyridine probe, and N2-physisorption. Characterization by ICP and SEM-EDS indicated that the Si/Zr molar ratio of Zr-materials prepared by the pH adjustment better matched the nominal values. N2-physisorption showed that the mesoporosity of the materials was preserved even at the high Zr load. XRPD and HRTEM pointed out that Zr-SBA-15 materials prepared by the pH adjustment showed a higher level of ordering in the porous network. Regardless of the synthesis approach, the presence of a higher Zr load caused a loss in the structural ordering. It must be highlighted that the high-resolution XPS spectra of Zr 3d suggested the formation of Si–O–Zr bonds giving some indication of Zr incorporation into the SBA-15 framework. The formation of Si–O–Zr linkage in the Zr-materials was also corroborated by DRS UV-VIS due to the presence of a blue shift in the absorption edge for both series of the Zr-SBA-15 materials. NH3-TPD and in-situ FT-IR spectroscopy with pyridine probe showed that the presence of Zr species introduces acidity in the SBA-15 and at higher Zr load the Brønsted acidity increased particularly for the Zr-materials prepared by pH adjustment. In conclusion, this study evidences that the pH-adjusting approach improves structural, textural, morphological, and acidic properties in SBA-15 and Zr-SBA-15 materials compared to the direct-synthesis approach.Peer ReviewedPostprint (author's final draft

Revealing the effects of high Al loading incorporation in the SBA-15 silica mesoporous material

High aluminum loading incorporation in the SBA-15 silica structure was investigated. Different Si/Al molar ratios (15, 10, and 2) were evaluated. The SBA-15 and the aluminum-containing materials (Al-SBA-15) were prepared by the “pH adjusting” method with modifications. The mesoporous structure of the materials was demonstrated by the type IV isotherms. The SBA-15 pore changed from a cylindrical to a slit-like structure in the presence of higher aluminum content. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) pointed out that the structural order is compromised in the presence of a higher aluminum load in the Al-SBA-15 materials, although the mesoporous structure was preserved. Higher Al loading increases the total quantity of Lewis acid sites as well as generates Brönsted acid sites. CO adsorption FTIR spectroscopy suggests aluminum incorporation into the SBA-15 and generation of acid sites. The Si–O–Al linkage in the aluminum-containing materials was corroborated by UV–Vis DRS due to the presence of a peak centered at 241 nm related to the Al-O bond, which is ascribed to four-coordinated framework aluminum in the SBA-15 structure. XPS spectra of Al 2p suggested that the Al species are less oxidized than the Al2O3 phase giving some indication of Al incorporation into the SBA-15 framework. 27Al MAS NMR results revealed that the aluminum species are in a tetrahedral oxygen coordination environment for Al-SBA-15 with Si/Al molar ratios of 15 and 10. Aluminum species in both tetrahedral and octahedral environments were evidenced for Al-SBA-15 with a Si/Al molar ratio of 2.Peer ReviewedPostprint (author's final draft