Catalytic conversion of biogenic substrates into valuable building blocks

Nowadays, terpenes represent an interesting class of biomass constituents as potential precursors for key building blocks in fine chemical industry. More in details, the production of a terpene such as limonene is exponentially increasing, already in the 2013, its yearly production was estimated over 70000 tons year-1 from citrus waste. The hydrogenation of this terpene is a suitable reaction to prepare selectively monohydrogenated molecules, like p-menthene, used in pharmaceuticals, agrochemical, perfume industries and in polymers field, while, the full hydrogenated products, cis- and trans-p-menthane, cover a relevant role as green additives into fuels. As reported in recent literature, the catalytic reactions involving limonene were conducted under high H2 pressure (from 5 to 50 bar) and high-temperature (from 80 to 140°C) conditions using heterogeneous metal based catalysts (Pt, Ni and Pd), with low selectivity towards (+)-p-1-menthene. Nevertheless, whenever mild conditions were used, a good selectivity in (+)-p-1-menthene was reached. However, a comparative study of different catalysts under mild reaction conditions for the limonene hydrogenation has not been reported in literature. Therefore, the goal of this PhD project was in an early stage to study the activities of several commercial metal nanoparticles based catalysts for the mild hydrogenation of limonene. The hydrogenation of limonene has been performed in neat limonene and under mild conditions, e.g. low temperature (30°C) and low molecular hydrogen pressure (3 bar), aiming at a sustainable production route for (+)-p-1-menthene. In our study, the active metal nanoparticles (Pt, Pd and Ru) and supports (carbon, silica and alumina) were systematically varied and tested. It was found that the heterogeneous catalyst Pt/C alongside Pt/Al2O3 under mild reaction conditions (room temperature and 3 bar H2) was highly active and selective in the reduction of R-(+)-limonene to the partial hydrogenation product (+)-p-1-menthene. Moreover, the catalytic activity and stability of Pt/C were maintained during recycling tests under batch conditions and thus allowed the implementation of this catalytic system into continuous flow operation. The selective hydrogenation of terminal C=C bond over the internal one in limonene was rationalized by detailed kinetic studies which revealed an 8-fold difference in reaction rate between the two reactions. This previous study with commercial catalysts gave the possibility to tune the synthesis of heterogeneous metal-based catalysts for the next step of the study, where different heterogeneous metal based catalysts (Pt, Ru, Pt3Sn, and Ni), developed from a colloidal-based approach were tested in the hydrogenation of limonene. Those catalysts contain the same metal loading and similar particle sizes (ca. 2 nm) homogeneously dispersed onto non structured oxides (silica and alumina), carbon, or embedded into the walls or at the pore surface of a mesostructured silica materials (SBA-15). All the catalysts from the Pt series were particularly active in the selective hydrogenation of limonene towards p-menthene with further conversion into p-menthane, showing a very high reaction rate. Among of all those catalysts, the one containing Pt nanoparticles embedded in the walls of the silica showed the highest TOF, of ca. 2200 h-1 after two hours of reaction and a maximum yield in p-menthene of ca. 85 % was obtained after 10 hours of reaction. The same catalyst was tested in a continuous flow system and a stable yield of ca. 80% during 6 hours of reaction was reached. No products of isomerization were detected in the crude mixture during the reaction. We could therefore conclude that, using either a heterogeneous commercial catalyst like Pt/C or using a heterogeneous metal based catalyst developed from a colloidal-based approach like SBA-15{walls}, it was possible to achievea selective conversion of limonene into p-menthene in batch and in continuous flow conditions