Abstract
In an attempt to synthesize silicas with respect to green chemistry, the synthesis of MCM-41 and SBA-15 silicas by a low-cost approach through the recycling of waste mother liquors was investigated. The addition of the mother liquor in consecutive syntheses was carried out in order to evaluate the different effects on the physical and chemical properties of the silicas. MCM-41 silicas were characterized by X-ray diffraction, which confirmed the formation of the hexagonal phase in the mother liquor used. According to the results of XRD, BET and SEM, SBA-15 silica has a structure that includes mesopores of various levels, crystallites and grains within the original particles. From the analysis of the textural characteristics of the various samples obtained through nitrogen adsorption/desorption studies carried out at -196 °C, the following conclusions can be highlighted: (i) For the synthesis of MCM-41, there was a trend of increase in specific surface areas, reduction in pore diameter and increase in wall thickness; (ii) For the synthesis of SBA-15 there was an increase in the specific surface area, an increase in the pore diameter and a reduction in the wall thickness. It was found that, under the synthesis conditions, the mother liquor user had an influence on the final characteristics of these materials.
Highlights
Mesoporous materials are a class of solids that have uniform pore sizes (1.5 to 40 nm), large surface areas, large pore volumes, excellent thermal stability, and structures that can be modified in different ways [1]
The typical synthesis process for MCM-41 involves reagents such as: the silicon source, which serves to build the structure of the walls of the mesoporous material, the most commonly-used being pyrolyzed silica (Aerosil), tetraethyl orthosilicate (TEOS), and sodium silicate; surfactants, which are molecules with hydrophilic and hydrophobic characteristics that serve to guide the formation of the mesoporous structure, with cetyltrimethylammonium (CTABr) in chloride, bromide, or hydroxide form being the most commonly-used [7]; the hydroxyl source, which provide the organic cation that assists in the formation of basic silica construction units, with the most commonly-used being tetramethylammonium hydroxide (TMAOH) and ammonium hydroxide (NH4OH) [8]
This study aims to reuse the mother liquor in three subsequent syntheses of the SBA-15 and MCM-41 molecular sieves to assess any possible changes in the physical, chemical, and morphological properties of each material
Summary
Mesoporous materials are a class of solids that have uniform pore sizes (1.5 to 40 nm), large surface areas (up to 2500 m2.g-1), large pore volumes, excellent thermal stability, and structures that can be modified in different ways [1]. These materials are of great interest, mainly as catalysts [2] and adsorbents [3]. Several models have been proposed to explain the formation of mesoporous materials that provide a rational basis for the various synthesis routes These models are based on the presence of surfactants in a solution that guide the formation of the inorganic mesostructure [5, 6]. The typical synthesis process for MCM-41 involves reagents such as: the silicon source, which serves to build the structure of the walls of the mesoporous material, the most commonly-used being pyrolyzed silica (Aerosil), tetraethyl orthosilicate (TEOS), and sodium silicate; surfactants, which are molecules with hydrophilic and hydrophobic characteristics that serve to guide the formation of the mesoporous structure, with cetyltrimethylammonium (CTABr) in chloride, bromide, or hydroxide form being the most commonly-used [7]; the hydroxyl source, which provide the organic cation that assists in the formation of basic silica construction units, with the most commonly-used being tetramethylammonium hydroxide (TMAOH) and ammonium hydroxide (NH4OH) [8]
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