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aldehydes with two-fewer carbon atoms compared to the lactone content commonly encountered in biomass processing can be
reactant (e.g., hexanaldehyde for GOL) are produced as major utilized to positively influence product selectivity. The origin of
by-products, regardless of the nature of the starting lactones. This high 1-butene selectivity over these catalysts, as well as the
behaviour strongly suggests that acetic acid is likely formed via a detailed effect of water on the product distribution, are currently
retro-aldol type mechanism through a common 3-hydroxy acid under investigation and will be reported in a following paper.
intermediate for different lactones, such as the 3-hydroxypentanoic
In summary, we demonstrated that it is possible to produce a
acid in the case of GVL (Scheme 1). Also, a portion of carbon goes stream of highly pure LAOs from lactones or unsaturated carboxylic
to unidentified condensation products, likely formed between acids in a single step using inexpensive, robust heterogeneous acid
butene and GVL or pentenoic acids over g-Al2O3. It appears that catalysts with no precious metal components. We show that
higher lactones and acids have lower reactivity towards decarboxy- analogous decarboxylation chemistry exists for C5 C6, C8 and C11
lation and are more prone to side reactions. Indeed, thermogravi- g-lactones, which indicates the potential of this methodology as a
metric analysis reveals that for the same mass of lactones general approach for the production of LAOs with varying chain
processed, 50% more carbon is deposited on catalysts used to lengths. The feedstock to this process can be derived from biological
process GUL than used for GHL. Thus, further optimization is routes (e.g., polyketide/fatty acid biosynthesis), which thereby gen-
needed to improve olefin yield from these feedstocks.
erates a series of homologous molecules for conversion to LAOs.
Table 1 summarizes basic site densities as titrated by CO2-TPD. Moreover, unlike the present ethylene-based route for production of
The extent of CO2 uptake is negligible for SiO2–Al2O3, whereas the LAOs, this new biological route is capable of targeting molecules
CO2 uptake for g-Al2O3 is 55 mmol CO2 gÀ1. Accordingly, we suggest with specified carbon chain length which after subsequent
that the formation of acetic acid and propionaldehyde on the decarboxylation with the current technology, leads to a single
g-Al2O3 catalyst takes place by a retro-aldol type mechanism, which LAO cut, thus offering additional manufacturing flexibility to
is known to be catalyzed by basic sites.14 To test this hypothesis, we meet market demand. This approach also allows for the produc-
studied the decarboxylation of GVL over magnesium oxide (MgO), tion of both even and odd carbon number LAOs, depending on
a basic oxide. The MgO catalyst produced over seven times more the number of carbons in the starting molecules. Such an
acetic acid than butene from GVL conversion, implicating the integrated approach demonstrates the promise of a synergy
importance of basic sites in this pathway. Importantly, the basic between biocatalysis and chemical catalysis for efficiently con-
site density of g-Al2O3 can be decreased by the progressive addition verting biomass into value-added chemicals.
of WOx (0 to 20 wt%), leading to a corresponding decrease in
This work was supported in part by the U.S. Department of
the yields to acetic acid and condensation products (Table 1). In Energy Office of Basic Energy Sciences and by the DOE Great Lakes
this respect, we suggest that future research should explore the Bioenergy Research Center, which is supported by the U.S. Depart-
synthesis of water-tolerant solid acid catalysts bearing predomi- ment of Energy, Office of Science, Office of Biological and Environ-
nantly Lewis acidity with minimal basicity, which would be ideal mental Research, through Cooperative Agreement between the
for the selective production of LAOs via decarboxylation.
Board of Regents of the University of Wisconsin System and the
It is interesting (and unexpected) that g-Al2O3 and WOx–Al2O3 U.S. Department of Energy. In addition, this material is based upon
can be used to selectively produce LAOs since both catalysts are work supported by the National Science Foundation under Award
known to have good activity for olefin double bond migration and No. EEC-0813570.
skeletal isomerization at temperatures similar to those used in the
present study (673–800 K).15 The key discovery of the present work,
however, is that the presence of water significantly inhibits the
Notes and references
undesirable isomerization of the alpha-olefins over Lewis acid sites,
whereas this isomerization is rapid over Brønsted acid sites, even
in the presence of water. For example, the steady-state rates of
2-pentenoic acid decarboxylation and 1-butene isomerization over
g-Al2O3 are 0.012 (feed: Ptotal = 1 bÀar1, PPEA = 0.07 bar, helium
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5 Alpha Olefins(02/03-4) PERP Report, Nexant Chemical Systems, 2004.
6 G. A. Kraus and S. Riley, Synthesis, 2012, 3003–3005.
balance) and 0.15 mmol minÀ1 gcat (Ptotal = 1 bar, P1-butene
=
0.005 bar, helium balance), respectively. However, when water was
co-fed, the decarboxylation rate increased to 0.051 (Ptotal = 1 bar,
ˆ
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14 R. Mahrwald, Modern Aldol Reactions, Wiley-VCH, 2004.
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PPEA = 0.07 bar, Pwater = 0.93 bar) while the 1-butene isomerization
À1
rate decreased to 0.020 mmol min
g =
À1 (Ptotal = 1 bar, P1-butene
cat
0.005 bar, Pwater = 0.90 bar, helium balance). Under the same
conditions, the rates of decarboxylation and 1-butene isomÀer1izatiÀo1n
over SiO2–Al2O3 without water are 0.48 and 8.78 mmol min gcat
,
respectively. When water was admitted into the system, the
decarboxylation rate increased to 0.52 while the isomerizatiÀo1n
rate decreased only moderately to 4.50 mmol minÀ1 gcat
.
Hence, adding water decreases the 1-butene isomerization rate
by a factor of 7.5 over g-Al2O3 and 1.9 over SiO2–Al2O3. Our
finding thus represents an interesting case where the high water
c
7042 Chem. Commun., 2013, 49, 7040--7042
This journal is The Royal Society of Chemistry 2013