Abstract
In this study, three raw Brazilian clays from Joao Pessoa, Paraiba State (Aco AP, AVL and Verde Lodo) were characterized before and after purification and acid activation. Several instrumental techniques were used, and the raw clays were classified as dioctahedral smectites with different compositions, where the AVL sample had the highest content of montmorillonite. In addition, analyses showed modification in clay mineral compositions, mainly due to removal of impurities and octahedral sheet cation leaching during acid activation, which resulted in specific areas ranging from 42 to 93 m2 g-1 and increase of the Bronsted/Lewis acid site ratio, until 1.52 (AVL). Then, methyl esterification reactions were performed using the acid-activated clays as catalysts. The maximum acid conversions obtained were of 92 and 64%, for acetic acid using activated AVL and for lauric acid using activated Verde Lodo, respectively, and these data were correlated mainly to the Bronsted-Lowry acidity.
Highlights
Clay minerals are hydrated layered aluminum silicates containing different amounts of iron, magnesium and alkaline metal cations, found abundantly in nature, which makes them inexpensive
Clay purification procedure adopted in this study is not sufficient for obtaining pure and maximum enrichment of montmorillonite clay mineral, it was used to facilitate clay characterization and use by the diminishing of quartz and organic matter
The three clays were different in composition, so the acid activation resulted in distinct changes in their structure and acidity
Summary
Clay minerals are hydrated layered aluminum silicates containing different amounts of iron, magnesium and alkaline metal cations, found abundantly in nature, which makes them inexpensive. In clay minerals’ structure, tetrahedral sheets are composed of SiO4 units, and octahedral sheets of M(OH)[6] can be formed by two different structural arrangements depending on the central cation. The tetrahedral (T) and octahedral (O) sheets are linked together forming layers that are packed along the basal direction, resulting in a layered structure, by which clay minerals can be classified mainly in two groups, 1:1 (T:O) and 2:1 (T:O:T). Raw clays are not composed of only one type of mineral. Instead, they are mixed with associated minerals, such as
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