Abstract
Modification of saponite (Sap) by surface engineering and intercalation chemistry introduces guest species into the structure of Sap and enhances the functionalities of the resultant Sap-based hybrids or composites. This review summarizes and evaluates latest scientific advances in the strategies for surface engineering, intercalation and hybridization of Sap, the insights into the relevant mechanisms, and the properties and applications of the resultant Sap-based materials. Studies have indicated that Sap can be inorganically modified by acid activation, inorganic cation exchange, pillaring, and adsorption. The methods of preparing organo-saponite (OSap) hybrids can be categorized as follows: 1) exchanging the inorganic cations in the interlayer space of Sap with organic cations; 2) covalent grafting of organic moieties or groups onto the surface of Sap; 3) intercalating polymer into the interlayer space of Sap by solution intercalation, and melt mixing or in situ polymerization. Organic-inorganic modified Sap can be made through the reactions between organic species and inorganic-modified Sap, or by the combination of inorganic species with organic-modified Sap. Modified Sap exhibits exceptional thermal stability, surface acidity, optical effects and adsorption. As such, the modified Sap can be used for optical materials, adsorbents, catalysts and clay/polymer nanocomposites (CPN). Literature survey suggests that future studies should place emphasis on optimizing and scaling up the modification of Sap, probing the thermodynamics, kinetics and mechanisms of the modification of Sap, endowing Sap with novel functionalities, and accordingly advancing the practical applications of the resultant Sap-based materials.
Highlights
Saponite (Sap) is a 2:1 trioctahedral clay mineral and belongs to the smectite group
Sap can be firstly intercalated by organic cations, the OSap enables neutral organic molecules to be attracted into the interlayer space by hydrophobic interaction
The soluble polymers (e.g. poly(vinyl alcohol) (PVA), poly(lactic acid) (PLA), poly(amic acid) (PAA), polyethylene glycol (PEG)) or the dispersion of macromolecules in the solution are expected to be introduced into the interlayer space of Sap
Summary
Saponite (Sap) is a 2:1 trioctahedral clay mineral and belongs to the smectite group. A small amount of isomorphous substitution of Si4+ by Al3+ (and possibly other trivalent cations such as Fe3+) in the tetrahedral sheet causes the layer to be negatively charged This charge is compensated by exchangeable interlayer cations including Na+, NH4+, K+, Li+, and Mg2+ (M z+). The frequently-used methods include acid activation (Mz+ exchanges with H+), inorganic cation exchange (exchange with inorganic metal cations), pillaring (Mz+ first exchanged with inorganic metal hydroxycations, followed usually by calcination to form oxide pillars), and the hybridization of metal nanoparticles with Sap to form nanocomposite After such inorganic modifications, the interlayer spacing, the thermal stability and the surface acidity of Sap are usually increased. The resultant Sap-based materials are commonly used as catalysts and adsorbents
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