Natural Mg silicates with different structures and morphologies: Reaction with K to produce K 2 MgSiO 4 catalyst for bio
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Natural Mg silicates with different structures and morphologies: Reaction with K to produce K2MgSiO4 catalyst for biodiesel production Fabiane Carvalho Ballotin1), Mayra Nascimento1), Sara Silveira Vieira1), Alexandre Carvalho Bertoli1), Ottávio Carmignano2), Ana Paula de Carvalho Teixeira1), and Rochel Montero Lago1) 1) Departamento de Química, Universidade Federal de Minas Gerais, Belo Horizonte, Brasil 2) Doutorado Inovação/Mineradora Pedras Congonhas, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte-MG, Brazil (Received: 28 April 2019; revised: 1 August 2019; accepted: 8 August 2019)
Abstract: In this work, different magnesium silicate mineral samples based on antigorite, lizardite, chrysotile (which have the same general formula Mg3Si2O5(OH)4), and talc (Mg3Si4O10(OH)2) were reacted with KOH to prepare catalysts for biodiesel production. Simple impregnation with 20wt% K and treatment at 700–900°C led to a solid-state reaction to mainly form the K2MgSiO4 phase in all samples. These results indicate that the K ion can diffuse into the different Mg silicate structures and textures, likely through intercalation in the interlayer space of the different mineral samples followed by dehydroxylation and K2MgSiO4 formation. All the materials showed catalytic activity for the transesterification of soybean oil (1:6 of oil : methanol molar ratio, 5wt% of catalyst, 60°C). However, the best results were obtained for the antigorite and chrysotile precursors, which are discussed in terms of mineral structure and the more efficient formation of the active phase K2MgSiO4. Keywords: silicate; serpentinite; lizardite; antigorite; talc; chrysotile; biodiesel
1. Introduction Magnesium-based materials can be found in nature as silicates, for example, serpentinites (Mg3Si2O5(OH)4) and talc (Mg3Si4O10(OH)2). Serpentinite is a family of hydrated magnesium silicates having an octahedral magnesium layer linked with a tetrahedral silicon sheet [1]. They are constituted mainly by lizardite, antigorite, and chrysotile minerals, which have the same composition, Mg3Si2O5(OH)4, but different structures [2] due to the balance of the octahedral and tetrahedral layers [3]. Generally, the misfit of ions leads to a curved or flat structure. Lizardite has a flat crystal structure, which is favored by Al3+ and Fe3+ substitution with Mg and Si [3]. Chrysotile has Al3+ and Fe3+ deficiencies and a tubular/fibrous texture, which has been suggested to present some toxicity [4]. Antigorite has a curved structure and is composed of octahedral continuous layers linked with alternated tetrahedral layers [5] by strong covalent Si–O bonds [3]. Talc, Mg3Si4O10(OH)2, also common in nature, is constituted Corresponding author: Rochel Montero Lago
of three layers, whereby an Mg(OH)6 layer is located between two SiO4− 4 layers [6]. The antigorite mineral has been demonstrated to be an excellent precursor to produce Mg silicates containing alkaline metals Li+ [7], Na+ [8], and K+ [9]. These alkaline metal Mg silicates have many potential technolog
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