The
catalytic conversion of methyl and ethyl levulinates into γ-valerolactone
(GVL) by using methanol, ethanol, and 2-propanol as the H-donor/solvent,
promoted by the ZrO2 catalyst, is described as carried
out under both batch and gas-flow conditions. Under batch conditions,
2-propanol was found to be the best H-donor molecule, with ethyl levulinate
giving the highest yield in GVL. The reactions occurring under continuous
gas-flow conditions were found to be much more efficient, also showing
excellent yields in GVL when EtOH was used as the reducing agent.
These experiments clearly show that the ability to release hydrogen
from the alcoholic H-donor/solvent is the main factor driving CTH
processes, while the tendency to attack the esteric group is the key
step in the formation of transesterification products.
This
paper reports about the gas-phase reduction of methyl levulinate
to γ-valerolactone (GVL) via catalytic transfer hydrogenation
using ethanol as the H-donor. In particular, high-surface-area, tetragonal
zirconia has proven to be a suitable catalyst for the reaction. Under
optimized conditions, the reaction is selective toward the formation
of GVL (yield 70%). However, both the deposition of heavy oligomeric
compounds over the catalytic surface and the progressive conversion
from Lewis to Brønsted acidity, due to the reaction with the
water formed in situ, led to a progressive change
in the chemo-selectivity, promoting side reactions, e.g. the alcoholysis
of angelica lactones to ethyl levulinate. However, the in
situ regeneration of the catalyst performed by feeding air
at 400 °C for 2 h permitted an almost total recovery of the initial
catalytic behavior, proving that the deactivation is reversible. The
reaction has been tested also using a true bioethanol, derived from
agricultural waste.
The relative dehydrogenation/dehydration reactivity imparted by nanostructured gallium(iii) oxide on alcohols was investigated via electronic structure calculations, reactivity tests and DRIFT-IR spectroscopy.
Levulinic acid (LA) and its esters (alkyl levulinates) are polyfunctional molecules that can be obtained from lignocellulosic biomass. Herein, the catalytic conversion of methyl and ethyl levulinates into γ-valerolactone (GVL) via catalytic transfer hydrogenation (CTH) by using methanol, ethanol, and 2-propanol as the H-donor/solvent, was investigated under both batch and gas-flow conditions. In particular, high-surface-area, tetragonal zirconia has proven to be a suitable catalyst for this reaction. 2-propanol was found to be the best H-donor under batch conditions, with ethyl levulinate giving the highest yield in GVL. However, long reaction times and high autogenic pressures are needed in order to work in the liquid phase at high temperature with light alcohols. The reactions occurring under continuous gas-flow conditions, at atmospheric pressure and relatively low contact time (1 s), were found to be much more efficient, also showing excellent GVL yields when EtOH was used as the reducing agent (GVL yield of around 70% under optimized conditions). The reaction has been tested also using a true bio-ethanol, derived from agricultural waste. These results represent the very first examples of the CTH of LE under continuous gas-flow conditions reported in the literature.
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