Transformation of limonene into high added value products over acid activated natural montmorillonite

Abstract

Abstract Eight samples of activated montmorillonite were prepared by treating natural montmorillonite with HCl and HNO3 aqueous solutions in order to investigate the influence of several treatment parameters on its physicochemical characteristics and its catalytic activity for limonene transformation to high added value products (isomers and p-cymene). The samples were characterized using various techniques (SEM-EDS, TEM/HRTEM, XRD, ATR-FTIR, nitrogen adsorption-desorption isotherms, microelectrophoresis and determination of equilibrium pH) and evaluated in the transformation of limonene to isomers and p-cymene utilizing atmospheric oxygen as a green oxidant. The acid treatment, even under mild conditions, caused the removal of the sodium and calcium ions from the interlayer regions of the triple layers without disturbing the crystal structure of the triple layers, the plate-like morphology of the mineral at nanoscale and the pore morphology as well. Only the treatment under the hardest conditions (6 M HCl for 2 h) caused the transformation of plate-like particle morphology into granular one and the pore morphology from slit like-pores developed between plate-like particles into internal voids of irregular shape and broad pore size distributions. In contrast, the acid treatment brings about the reorganization/dislocation of the triple layers inside a plate-shaped particle causing the development of small mesopores, mainly in the range of 3–3.5 nm, and consequently the increase in the specific surface from 62 to 155 m2/g. In parallel, the removal of the sodium and calcium ions/phases brings about the development of negatively charged surface sites. These are transformed into Bronsted acid sites by adsorbing hydronium ions (H3O+). The above sites are catalytically active in the transformation of the limonene into intermediate isomers and undesired “polymers”. The intermediate isomers are transforming into p-cymene and additional “polymers” through a catalyst-free mechanism, in the presence of atmospheric oxygen acting as a green oxidant. The drastic decrease of the amount of the undesired “polymers” ( The low reaction temperature (100 °C), the use of limonene as a renewable reactant, the use of natural montmorillonite as a natural catalyst and the use of atmospheric oxygen as a green oxidant render the proposed process actually green.