Sulfuric acid modified clinoptilolite as a solid green catalyst for solvent-free α-pinene isomerization process

The isomerization of α-pinene – a renewable bioresource – was investigated as a relatively simple method for the syntheses of camphene and limonene, industrially important products. The catalytic activity of H2SO4-modified clinoptilolite was evaluated without any solvent and the results show its applicability as a novel, green, reusable and promising catalyst in organic syntheses. The method is cost-effective and energy efficient because of the use of relatively low temperatures: at 30 °C and after 1 h, conversion was equal to 18%. In addition, this process has a short-time performance: 100% conversion after only 4 min at 70 °C. Camphene and limonene were the products that were formed with the highest selectivities of 50% and 30%, respectively. Clinoptilolite modified by H2SO4 solutions (0.01–2 M) as a catalyst for α-pinene isomerization has not been described up to now. The catalyst samples were characterized using various instrumental methods (XRD, FT-IR, UV–Vis, SEM, and XRF). The nitrogen sorption method was used to determine the textural parameters, and the acid-sites concentration was established with the help of the titration method. The mixtures of compounds were analyzed via gas chromatography (GC). The comprehensive kinetic modeling of α-pinene isomerization over the most active catalyst (clinoptilolite modified by 0.1 M H2SO4 solution) was performed. The first order kinetics of α-pinene conversion was found, and the value of the reaction rate constant at 70 °C was equal to 8.19 h−1. It was concluded that the high activity of the prepared modified clinoptilolite in α-pinene isomerization is a multifaceted function of textural properties, crystallinity, chemical composition, and acid-sites concentration.