Activation of natural mordenite by various acids: Characterization and evaluation in the transformation of limonene into p-cymene

Natural mordenite originated from volcanic soils in Greek islands was treated with aqueous solutions of various acids (CH3COOH, H2SO4, HCl, HNO3). The samples were characterized by the joint use of N-2-physisorption, X-ray Diffraction, Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy, Scanning Electron Microscopy, Energy Dispersive X-ray Spectroscopy, Transmission Electron Microscopy, zeta-Potential, and Equilibrium pH and evaluated for the transformation of limonene into p-cymene under air and N-2 atmosphere. The influence of reaction temperature and time on catalytic performance of the most promising sample (activated by HCl) was also examined. The acid treatment brings about the removal of cations (mainly sodium cations) located inside the framework channels and in the inter-fiber mesopores without disturbing mordenite framework. The emptying of micropores and mesopores increases drastically the BET specific surface area which is mainly due to micropores. The samples studied exhibit fibrous morphology with fibers (10-100 nm) separated by slit-like mesopores. The emptying of micropores and mesopores unmasks negatively charged surface sites. These are transformed into surface acidic sites by adsorbing H+/H3O+ ions. The changes in the texture and acidity are converting the catalytically inactive natural mordenite into very active catalysts for the transformation of limonene into p-cymene. The increase in the conversion of limonene and the yield of p-cymene over the activated samples are following trends similar to that followed by the BET specific surface area. The catalytic performance is maximized over the sample resulted by the treatment of the original mordenite with hydrogen chloride solution. A quite high yield of p-cymene (65%) is obtained over this sample at 140 degrees C, limonene to catalyst ratio equal to 15 and reaction time equal to 7 h. A two-step mechanism established previously over sulfuric acid activated natural mordenite catalysts for transformation of limonene to p-cymene is followed regardless of the kind of acid used for activation of this clay.