Clara López-Aguado

The effects of the nature and strength of surface acid sites on the properties of the catalyst used for the one-pot conversion of levulinic acid to valeric biofuels have been investigated. The acid supports were prepared from a commercial beta zeolite (HBEA) modified by partial substitution of Al by Zr atoms. This procedure enables changes in the nature and strength of the acidic sites of the starting zeolite. A constant amount of Pd (2 wt%) was incorporated on these acid supports by the incipient wetness impregnation method. These catalysts were tested in liquid phase reactions and the Pd/HBEA catalyst showed the best catalytic performance, achieving an 80% yield of valeric biofuel after 2 h of the reaction. The fresh catalysts were characterized by DRIFT spectroscopy using pyridine, CD₃CN, and CO as probe molecules and by XRD to explain the differences in catalytic performance. The presence of strong Brønsted acid sites (BAS) was shown to be critical for reaching high ethyl valerate yields since the differences in catalytic behavior and the concentration of these acid sites parallel each other. Up to three types of Lewis acid sites (LAS) of different strengths were also identified. These LAS have also been shown to be active, although their intrinsic activities decrease as they are weaker. Reutilization tests of the Pd/HBEA catalyst were also carried out and mild deactivation was observed. The spent catalyst underwent a simple calcination treatment at 773 K and this was adequate for the recovery of the surface acidity, but the reducibility of the Pd species was affected. This prevented complete recovery of catalytic activity.

The one-pot conversion of biomass-derived platform molecules such as levulinic acid (LA) and furfural (FAL) into γ-valerolactone (GVL) is challenging because of the need for adequate multi-functional catalysts and high-pressure gaseous hydrogen. As a more sustainable alternative, here we describe the transfer hydrogenation of LA to GVL using isopropanol as a hydrogen donor over a Zr-modified beta zeolite catalyst in a continuous fixed-bed reactor. A stable sustained production of GVL was achieved from the levulinic acid, with both high LA conversion (ca. 95%) and GVL yield (ca. 90%), for over at least 20 days in continuous operation at 170 °C. Importantly, the small decay in activity can be advantageously overcome by the means of a simple in situ thermal regeneration in the air atmosphere, leading to a complete recovery of the catalyst activity. Key to this outstanding result is the use of a Zr-modified dealuminated beta zeolite with a tailored Lewis/Brønsted acid sites ratio, which can synergistically catalyze the tandem steps of hydrogen transfer and acid-catalyzed transformations, leading to such a successful and stable production of GVL from LA.

The integration of GVL production in a lignocellulosic biorefinery where furfural and levulinic acid are co-produced is investigated in a one-pot process over a Zr-Al-Beta bi-functional catalyst. Under the synthesis and reaction conditions previously optimized for the transformation of furfural, the yield of GVL for different mixtures of levulinic acid–furfural over this catalyst is very high (60–90%). Complete conversion of the reagents (both furfural and levulinic acid) is achieved after 2 hours of reaction. As the percentage of levulinic acid in the mixture increases, higher yields of GVL are obtained. The results indicate the presence of a synergistic effect due to an improvement of the catalyst stability against furfural-deactivation phenomena. In addition, the mass balances obtained are high, evidencing the efficient use of both platform molecules. An optimization of the transformation of levulinic acid into GVL has shown that it is possible to work with highly concentrated reaction media, allowing excellent productivities of GVL per gram of catalyst (levulinic acid concentration of 300 g L⁻¹, catalyst loading = 12.5 g L⁻¹) leading to a GVL productivity of up to 8 g_GVL g_CAT ⁻¹ after 6 h at 190 °C. Under these conditions, a pseudohomogeneous kinetic model has been fitted to the experimental data. Also remarkable is that the Zr-Al-Beta catalyst is stable and reusable in this transformation, since no activity loss was evidenced after repeated reaction cycles without any kind of regeneration treatment.

The one-pot conversion of xylose into γ-gammavalerolactone in 2-propanol over bifunctional Zr-Al-Beta zeolites, prepared via a post-synthetic route, was optimized in terms of both catalyst synthesis and reaction conditions. In the catalyst preparation, the use of Zr(NO₃)₄ as zirconium source as well as the tuning of the amount of water used during the impregnation had a strong impact on the activity of the Zr species due to an improved dispersion of Zr species. As for the aluminium to zirconium exchange, an optimal Al/Zr ratio of 0.20 was identified to provide a catalyst with better activity. The modelization of the catalytic system through experimental design methodology allowed to identify the optimal values of the most influential reaction conditions: temperature 190 °C, catalyst loading 15 g L⁻¹, and starting xylose concentration 30.5 g L⁻¹. Under these optimized reaction conditions, Zr-Al-Beta catalyst provides a GVL yield from xylose (ca. 34%) after only 10 h. The catalysts are stable and reusable after thermal regeneration at 550 °C.

The one-pot conversion of xylose into GVL in 2-propanol has been achieved over bifunctional Zr- and Al-containing beta zeolite catalysts, prepared via a post-synthetic route, possessing both Brønsted and Lewis functionalities. A GVL yield of 35 mol% was obtained at 190 °C after 48 h.