Citation: Barros, J.M.F.; Fernandes, G.J.T.; Araujo, M.D.S.; Melo, D.M.A.; Gondim, A.D.; Fernandes, V.J., Jr.; Araujo, A.S. Hydrothermal Synthesis and Properties of Nanostructured Silica Containing Lanthanide Type Ln–SiO 2 (Ln = La, Ce, Pr, Nd, Eu, Gd, Dy, Yb, Lu). Nanomaterials 2023, 13, 382. https://doi.org/10.3390/ nano13030382 Academic Editor: Carlos Lodeiro Received: 27 December 2022 Revised: 9 January 2023 Accepted: 13 January 2023 Published: 18 January 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). nanomaterials Article Hydrothermal Synthesis and Properties of Nanostructured Silica Containing Lanthanide Type Ln–SiO 2 (Ln = La, Ce, Pr, Nd, Eu, Gd, Dy, Yb, Lu) Joana M. F. Barros 1, *, Glauber J. T. Fernandes 2 , Marcio D. S. Araujo 2 , Dulce M. A. Melo 3 , Amanda D. Gondim 2 , Valter J. Fernandes, Jr. 4 and Antonio S. Araujo 2, * 1 Center of Education and Health, Academic Unit of Biology and Chemistry, Federal University of Campina Grande, Cuite 58175-000, PB, Brazil 2 Laboratory of Catalysis and Petrochemistry, Institute of Chemistry, Federal University of Rio Grande do Norte, Natal 59078-970, RN, Brazil 3 Institute of Chemistry, Federal University of Rio Grande do Norte, Natal 59078-970, RN, Brazil 4 Laboratory of Fuels and Lubricants, Institute of Chemistry, Federal University of Rio Grande do Norte, Natal 59078-970, RN, Brazil * Correspondence: joana.maria@professor.ufcg.br (J.M.F.B.); antonio.araujo@ufrn.br (A.S.A.) Abstract: The nanostructured lanthanide-silica materials of the Ln–SiO 2 type (Ln = La, Ce, Pr, Nd, Eu, Gd, Dy, Yb, Lu) were synthesized by the hydrothermal method at 100 ◦ C, using cetyltrimethylam- monium as a structural template, silica gel and sodium silicate as a source of silicon, and lanthanide oxides, with Si/Ln molar ratio = 50. The resulting materials were calcined at 500 ◦ C using nitrogen and air, and characterized by X-ray diffraction (XRD), Fourier-Transform infrared absorption spec- troscopy, scanning electron microscopy, thermogravimetry (TG), surface area by the BET method and acidity measurements by n-butylamine adsorption. The XRD and chemical analysis indicated that the SiO 2 presented a hexagonal structure and the incorporation of lanthanides in the structure changes the properties of the Ln–SiO 2 materials. The heavier the lanthanide element, the higher the Si/Ln ratio. The TG curves showed that the decomposition of the structural template occurs in the materials at temperatures below 500 ◦ C. The samples showed variations in specific surface area, mean pore diameter and silica wall thickness, depending on the nature of the lanthanide. The incorporation of different lanthanides in the silica generated acid sites of varied strength. The hydrothermal stability of the Ln–SiO 2 materials evaluated at high temperatures, evidenced that the properties can be controlled for application in adsorption and catalysis processes. Keywords: lanthanides; silica; hydrothermal synthesis; nanostructured materials 1. Introduction The discovery of silica-based nanostructured materials opened new perspectives for the development of new materials with organized structures containing heteroatoms [1,2]. Due to the high surface area and accessibility of their pore systems, these materials have been promising as acid catalysts in petrochemical processes [3–5], supports for heteropoly- acids [6–8], liquid phase catalysis [9–11], and in advanced materials technology [12–17]. Due to the great interest in these materials, it is necessary to develop new methodologies for synthesis, post-synthesis treatments, in addition to new characterization methods. The silica-based Mesoporous Composition of Matter number forty one (MCM-41) is one of the most important materials developed so far with hexagonal arrangement of one-dimensional mesopores with diameters ranging from 2 to 10 nm, good thermal stability, high area val- ues specific and pore volume. These characteristics have made the MCM-41 a promising material for applications in catalysis, adsorption and in the technology of advanced mate- rials based on molecular sieves, such as: electron transfer photosensors, semiconductors, polymers, carbon fibers, clusters, and materials with non-linear optical properties [17–22]. Nanomaterials 2023, 13, 382. https://doi.org/10.3390/nano13030382 https://www.mdpi.com/journal/nanomaterials