15 research outputs found
Mechanistic Study of a Ru-Xantphos Catalyst for Tandem Alcohol Dehydrogenation and Reductive Aryl-Ether Cleavage
We employ density functional theory
(DFT) calculations and kinetics measurements to understand the mechanism
of a xantphos-containing molecular ruthenium catalyst acting on an
alkyl aryl ether linkage similar to that found in lignin to produce
acetophenone and phenol. The most favorable reaction pathway suggested
from DFT is compared to kinetics measurements, and good agreement
is found between the predicted and the measured activation barriers.
The DFT calculations reveal several interesting features, including
an unusual 5-membered transition state structure for oxidative insertion
in contrast to the typically proposed 3-membered transition state,
a preference for an O-bound over a C-bound Ru–enolate, and
a significant kinetic preference for the order of product release
from the catalyst. The experimental measurements confirm that the
reaction proceeds via a free ketone intermediate, but also suggest
that the conversion of the intermediate ketone to acetophenone and
phenol does not necessarily require ketone dissociation from the catalyst.
Overall, this work elucidates the kinetically and thermodynamically
preferred reaction pathways for tandem alcohol dehydrogenation and
reductive ether bond cleavage by the ruthenium-xantphos catalyst
Computational Study of Bond Dissociation Enthalpies for a Large Range of Native and Modified Lignins
Lignin is a major component of plant cell walls that is typically underutilized in selective conversion strategies for renewable fuels and chemicals. The mechanisms by which thermal and catalytic treatments deconstruct lignin remain elusive, which is where quantum mechanical calculations can offer fundamental insights. Here, we compute homolytic bond dissociation enthalpies (BDEs) for four prevalent linkages in 69 lignin model compounds, including β-O-4, α-O-4, β-5, and biphenyl bonds, with a large range of natural and oxidized substituents. These calculations include ab initio benchmark values extrapolated to the complete basis set limit and full conformational searches for each compound. The results quantify both the relative BDEs among common lignin bonds and the effect of native and oxidized substituents on the functional groups in lignin. These data yield insights into thermal lignin deconstruction for a large range of prevalent linkages and aid in the identification of targets for catalytic cleavage
A Mechanistic Investigation of Acid-Catalyzed Cleavage of Aryl-Ether Linkages: Implications for Lignin Depolymerization in Acidic Environments
Acid catalysis has
long been used to depolymerize plant cell wall
polysaccharides, and the mechanisms by which acid affects carbohydrates
have been extensively studied. Lignin depolymerization, however, is
not as well understood, primarily due to the heterogeneity and reactivity
of lignin. We present an experimental and theoretical study of acid-catalyzed
cleavage of two non-phenolic and two phenolic dimers that exhibit
the β-O-4 ether linkage, the most common intermonomer bond in
lignin. This work demonstrates that the rate of acid-catalyzed β-O-4
cleavage in dimers exhibiting a phenolic hydroxyl group is 2 orders
of magnitude faster than in non-phenolic dimers. The experiments suggest
that the major product distribution is similar for all model compounds,
but a stable phenyl-dihydrobenzofuran species is observed in the acidolysis
of two of the γ-carbinol containing model compounds. The presence
of a methoxy substituent, commonly found in native lignin, prevents
the formation of this intermediate. Reaction pathways were examined
with quantum mechanical calculations, which aid in explaining the
substantial differences in reactivity. Moreover, we use a radical
scavenger to show that the commonly proposed homolytic cleavage pathway
of phenolic β-O-4 linkages is unlikely in acidolysis conditions.
Overall, this study explains the disparity between rates of β-O-4
cleavage seen in model compound experiments and acid pretreatment
of biomass, and implies that depolymerization of lignin during acid-catalyzed
pretreatment or fractionation will proceed via a heterolytic, unzipping
mechanism wherein β-O-4 linkages are cleaved from the phenolic
ends of branched, polymer chains inward toward the core of the polymer