Gluconic acid from biomass fast pyrolysis oils: Specialty chemicals from the thermochemical conversion of biomass

Article abstract of DOI:10.1002/cssc.201402431

Fast pyrolysis of biomass to produce a bio-oil followed by catalytic upgrading is a widely studied approach for the potential production of fuels from biomass. Because of the complexity of the bio-oil, most upgrading strategies focus on removing oxygen from the entire mixture to produce fuels. Here we report a novel method for the production of the specialty chemical, gluconic acid, from the pyrolysis of biomass. Through a combination of sequential condensation of pyrolysis vapors and water extraction, a solution rich in levoglucosan is obtained that accounts for over 30% of the carbon in the biooil produced from red oak. A simple filtration step yields a stream of high-purity levoglucosan. This stream of levoglucosan is then hydrolyzed and partially oxidized to yield gluconic acid with high purity and selectivity. This combination of costeffective pyrolysis coupled with simple separation and upgrading could enable a variety of new product markets for chemicals from biomass.

Full text of DOI:10.1002/cssc.201402431

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DOI: 10.1002/cssc.201402431
Gluconic Acid from Biomass Fast Pyrolysis Oils: Specialty
Chemicals from the Thermochemical Conversion of
Biomass
Daniel Santhanaraj,[a] Marjorie R. Rover,[b] Daniel E. Resasco,[a] Robert C. Brown,[b, c] and
Steven Crossley*[a]
Fast pyrolysis of biomass to produce a bio-oil followed by cata-
lytic upgrading is a widely studied approach for the potential
production of fuels from biomass. Because of the complexity
of the bio-oil, most upgrading strategies focus on removing
oxygen from the entire mixture to produce fuels. Here we
report a novel method for the production of the specialty
chemical, gluconic acid, from the pyrolysis of biomass.
Through a combination of sequential condensation of pyrolysis
vapors and water extraction, a solution rich in levoglucosan is
obtained that accounts for over 30% of the carbon in the bio-
oil produced from red oak. A simple filtration step yields
a stream of high-purity levoglucosan. This stream of levogluco-
san is then hydrolyzed and partially oxidized to yield gluconic
acid with high purity and selectivity. This combination of cost-
effective pyrolysis coupled with simple separation and upgrad-
ing could enable a variety of new product markets for chemi-
cals from biomass.
Introduction
Fast pyrolysis of biomass is not typically regarded as an effec-
tive primary conversion path for the production of specialty
chemicals, which require high purity while pyrolysis of biomass
produces a liquid containing over 400 compounds[1–4] that
cannot be easily separated. Owing to the complexity and ther-
mal instability of the compounds present in the whole mixture,
approaches to produce specialty chemicals usually require ex-
cessive amounts of catalyst to produce a family of aromatic
products, for example, benzene, toluene, and xylene.[5]
ration steps that limit potential for commercial application.
Fast pyrolysis is an appealing process for the large-scale pro-
duction of liquids from biomass due to the relatively low foot-
print required when compared to other alternatives.[4,16] The
main challenge associated with the upgrading lies in the sepa-
ration of the numerous components present in the bio-oil.
Recent advancements in biomass pretreatment and optimiza-
tion of pyrolysis conditions have led to increases in the
amount of the anhydrosugar, levoglucosan, from the degrada-
tion of cellulose in the biomass.[17] While these advancements
alone do not yield great potential for specialty chemical pro-
duction of biomass, pure streams can result when combined
with advancements in separation. Recent results from Brown
et al. demonstrate a bio-oil recovery system based on sequen-
tial condensation and separation vapors and aerosols in the
pyrolysis product stream to yield bio-oil fractions with distinc-
tive compositions.[18] The heaviest fraction consists of water-
soluble anhydrosugars and water-insoluble phenolic oligomers.
A simple water washing procedure is able to separate these
into a concentrated anhydrosugar solution consisting mostly
of levoglucosan and a tarry phenolic oligomer fraction.[19]
Herein, we describe a catalytic strategy to convert this levo-
glucosan-rich stream into glucose via hydrolysis with solid
resins, followed by subsequent partial oxidation to gluconic
acid with supported metal catalysts. While selective oxidation
via the use of heterogeneous catalysts has been an area of ex-
tensive study in an effort to improve rates and ease catalyst
separation,[20–24] to the best of our knowledge, this is the first
report on the production of high-purity gluconic acid from bio-
mass through a combination of thermochemical conversion
and catalytic partial oxidation.
Sugar acids such as gluconic acid are attractive intermedi-
ates for numerous applications ranging from the food to paper
industries.[6–12] d-Gluconic acid has the potential as a co-mono-
mer for the production of a variety of biocompatible, biode-
gradable polymers.[13] The estimated market of d-gluconic acid
is 60000 tons per year.[12,14] Currently, gluconic acid is produced
via fermentation of glucose,[6] which is limited by a narrow
range of operating conditions and separation challenges.[15]
While glucose may be produced via the acid pretreatment
and enzymatic hydrolysis of cellulose, this is a slow process
when compared to fast pyrolysis and requires expensive sepa-
[a] Dr. D. Santhanaraj, Prof. D. E. Resasco, Prof. S. Crossley
School of Chemical, Biological and Materials Engineering
University of Oklahoma
100 E. Boyd Street, Room T301 Norman, OK 73019 (USA)
E-mail: stevencrossley@ou.edu
[b] Dr. M. R. Rover, Prof. R. C. Brown
Bioeconomy Institute
Iowa State University
Ames, IA 50011-3270 (USA)
[c] Prof. R. C. Brown
Department of Mechanical Engineering
Iowa State University
2025 Black Engineering, Ames, IA 50011 (USA)
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Upgrading strategy
Table 1. Weight and carbon yields in each biomass fraction obtained after pyrolysis and extraction.
Fraction
Amount Normalized yield
[kg] (wet bio-oil basis) [%] content [wt%] content [kg] (wet bio-oil basis) [%]
Carbon
Carbon
Carbon yield
Because levoglucosan was identi-
fied as the pyrolysis product with
the highest purity, a strategy for
the conversion of levoglucosan
to gluconic acid has been devel-
oped, as shown in Scheme 1.
This strategy contemplates an in-
itial hydrolysis step followed by
partial oxidation as described in
Clean Phenolic Oligomers 7.04
Anhydrosugar
Middle
LightOxygenates
Total
24.4
22.6
9
54.3
47.5
47.6
16.1
3.82
3.11
1.24
2.04
10.21
37.4
30.5
12.1
20
6.54
2.6
12.7
28.88
44
100
100
Results and Discussion
the following sections. While Table 1 indicates that levogluco-
san is the most-abundant monomer present in the anhydrosu-
gar solution, in order to improve the purity of this compound
a simple purification step was employed, as will be described
in the next section.
Analysis of bio-oil fractions
The carbon content of the various fractions obtained from the
pyrolysis oil is shown in Table 1. It is important to note that
the anhydrosugar fraction, obtained from the separation of the
heavy ends as described in the experimental section followed
by water extraction, contains over 30% of the carbon in the
bio oil.
Purification of the anhydrosugar solution
Composition of the anhydrosugar solution
The anhydrosugar solution obtained from the pyrolysis of bio-
mass was caramel-colored, indicating the presence of polymeric
species. The solution was diluted with distilled water at a ratio
of 1:20 to facilitate analysis by GC-MS and HPLC. The pH of the
diluted oil was measured as 3.23. The GC-MS results revealed
that the most abundant compound boiling low enough to
elute from the column was levoglucosan, along with low con-
centrations of furanic compounds. In general, the anhydrosugar
solution may also contain some larger soluble carbohydrates
that could not be identified by GC-MS. In an effort to quantify
these compounds, the anhydrosugar solution was analyzed by
HPLC. The HPLC results also confirmed that levoglucosan was
the main component in this fraction, with a low concentration
of furanics, mannose, and sorbose. The concentration of the
identified compounds was quantified by a normalization curve
of standard compounds as represented in Table 2. The carbon
weight of each compound was then calculated and compared
to the total carbon in the fraction as measured by elemental
analysis to determine the weight percentage of unidentified
compounds too heavy to elute from GC or HPLC. These would
include humin compounds and suspended char.
Scheme 1. Upgrading strategy of anhydrosugar solution.
It is anticipated that the presence of furanics and larger poly-
meric species in the anhydrosugar solution results in genera-
tion of humins (polymers) under the acidic conditions required
for hydrolysis, and the basic conditions necessary for partial ox-
idation. These polymeric species are expected to accelerate
rates of catalyst deactivation. Therefore, it is essential to
remove the furanic compounds and humins prior to the hy-
drolysis step. To accomplish this task, silica and activated
carbon were chosen as adsorbent materials to filter the anhy-
drosugar solution prior to hydrolysis. Results after stirring in
the presence of the adsorbent material are shown in Figure 1.
While a moderate improvement in feed quality was observed
when silica was used as a filtering material, activated carbon
was much more effective for the removal of polymeric species
as well as furanic compounds in
the solution. It is remarkable
that while the furanic and poly-
Table 2. The initial molar composition and carbon concentrations of compounds in the anhydrosugar solution.
meric species were removed
from the solution below the de-
tection limits, the levoglucosan
concentration was virtually un-
changed, as confirmed by HPLC
analysis. In order to demonstrate
the removal of heavy polymeric
species, it is necessary to quanti-
fy the carbon content before
and after filtration.
Concentrations were determined with HPLC, with the total carbon content estimated via elemental analysis.
Compound
type
Concentration
[m]
Carbon in anhydrosugar
Carbon in
fraction [wt%]
solution [gcarbon Lꢀ1
]
Furanics
HMF
0.46
0.06
2.2
0.26
0.15
–
28
4
158
19
11
250
470
6
1
34
4
2
53
100
Levoglucosan
Mannose
Sorbose
Unidentified Heavies
Total
–
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cosan to glucose after 24 h of reaction with 100% selectivity.
While sulfuric acid demonstrates high conversion, the major
problem associated with sulfuric acid is the separation chal-
lenges and the resulting increase in acidity of the anhydrosu-
gar solution. In addition, this low pH enhances the rate of
polymerization of any furanic compounds present in the anhy-
drosugar solution. These polymerized compounds may cause
a serious inhibition of the catalyst active sites in the subse-
quent step, decreasing the rate of selective oxidation of glu-
cose to gluconic acid. Moreover, the highly acidic mixture
would increase corrosion rates in industrial scale systems, and
catalyst separation would be problematic. In comparison, solid
acid catalysts have a distinct advantage. For example, Amber-
lyst-15 is a strong polymer-based solid acid with high SO3H
density and very high activity for a range of reactions.[25–28]
Likewise, HY zeolites (Si/Al=2.6) are potential acidic catalysts
that can potentially be used for hydrolysis. To compare these
solid acid catalysts, the reaction was carried out using the
same weight of catalyst in all cases (0.3 g). The activity of each
catalyst is shown in Figure 3. The results demonstrate that Am-
Figure 1. Diluted anhydrosugar solution (A) before, and (B) after filtration
with silica gel and (C) activated carbon.
Figure 2. Carbon content in the diluted anhydrosugar solution before and
after filtration with activated carbon. Note: the corresponding carbon con-
tent in the undiluted stream will be 20ꢁ the values reported here.
The carbon weight percentages of the corresponding liquids
prior to and after filtration are shown in Figure 2. It should be
noted that the carbon wt% values reported here are for the
fraction that has been diluted 1:20 with water to facilitate anal-
ysis. Thus, the corresponding fraction of carbon in the raw ma-
terial will be 20-fold the values reported in Figure 2. The results
confirm that almost 50% of carbon is still retained after the fil-
tration process, and the great majority of that carbon is now in
the form of levoglucosan. Notably, acid wash pretreatment of
biomass has been proven to increase yields of levoglucosan.[17]
The amount of levoglucosan and its subsequent products re-
ported herein are likely to be increased even further by apply-
ing such pretreatments.
Figure 3. Hydrolytic conversion of diluted anhydrosugar solution (rich in lev-
oglucosan) over different acid catalysts. Reaction conditions: initial conc.
0.11m levoglucosan; conc. H2SO4 0.5m; catalyst 0.3 g, reaction time 24 h;
temperature 383 K.
berlyst-15 is a very efficient and promising catalyst for the hy-
drolysis of levoglucosan to glucose when compared with an
equal weight of HY, which demonstrated no measurable con-
version of levoglucosan under these conditions. This conclu-
sion is in qualitative agreement with the results of Van de
Vyver et al., where Amberlyst-15 was found to have dramatical-
ly greater activity than HY for the hydrolysis of cellulose when
compared on an equal catalyst weight basis.[29] Also, Amber-
lyst-15 was found to yield 100% selectivity to glucose, which is
in agreement with results obtained in the presence of a 0.5m
solution of H2SO4 at comparable conversions.
Hydrolysis of anhydrosugar solution (rich in levoglucosan)
Prior to hydrolysis the anhydrosugar solution was diluted in
water at a volume ratio of 1:20. Hydrolysis of the levoglucosan
present in the activated-carbon purified and diluted anhydro-
sugar solution was conducted at 1108C in the presence of
0.5m H2SO4. These conditions yield 88% conversion of levoglu-
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Selective oxidation to gluconic acid
gluconate with 100% selectivity in each case. The reaction pro-
file demonstrates the decrease in reaction rates as the glucose
is consumed.
Several papers and patents demonstrate that the presence of
an alkaline medium (pH 9–10) is essential for selective oxida-
tion of glucose to gluconic acid. Uncontrolled pH in the reac-
tion medium quickly deactivates the catalyst and the oxidation
process is stopped immediately.[30] Benkꢂ et al. reported that
the use of a carbonate buffer is an easier way to control the
solution pH as compared to titration with NaOH.[31] The im-
provement of reaction rates and heterogeneous catalyst stabil-
ity for the oxidation of sugars at lower pH values is a current
area of significant research interest.[32–38]
Deactivation studies
In order to study the sustainability and reusability of the cata-
lysts, a deactivation study was carried out over 5% Pd/C cata-
lyst. After 24 h of reaction in a batch reactor, the reactant con-
centration was adjusted back to the original feed concentra-
tion (0.02m glucose) by adding more hydrolyzed anhydrosugar
solution (Figure 5). After this, the reaction was allowed to run
The oxidation of the diluted anhydrosugar solution after hy-
drolysis with Amberlyst-15 was conducted in a carbonate
buffer to maintain the pH at 9.3 during the course of the reac-
tion. It is important to note that when the reaction was con-
ducted with the diluted hydrolyzed anhydrosugar solution
without the activated carbon purification step, rapid polymeri-
zation took place, as indicated by the black color of the solu-
tion, which is likely due to the polymerization of furanic com-
pounds in strong alkaline medium.[39] Because polymerized
compounds may strongly deactivate the noble metal catalysts
necessary for the oxidation reaction, additional purification is
necessary. For this reason, the reactions described below were
all conducted with the diluted anhydrosugar solution after pu-
rification with the activated carbon filter and subsequent hy-
drolysis.
The selective oxidation of the purified glucose post-purifica-
tion and hydrolysis stream was conducted in alkaline medium
using air as the oxidant and 5% Pd/C as catalyst. Air was bub-
bled into the solution with a flow rate of 60 mLminꢀ1 and, as
mentioned above, the carbonate buffer was used to maintain
the pH at 9.3. Figure 4 depicts the conversion of glucose pres-
ent in the anhydrosugar solution to sodium gluconate as
a function of time. Notably, glucose was converted to sodium
Figure 5. Deactivation studies on conversion of glucose (rich in hydrolyzed
anhydrosugar solution) to sodium gluconate; Reaction conditions: tempera-
ture 508C; flow rate of air 60 mLminꢀ1: initial conc. of glucose 0.02m; cata-
lyst 5% Pd/C; pH 9.3.
for another 3 h. We found that the catalytic activity was practi-
cally the same as the original after the reactants were re-intro-
duced over the spent catalyst. That is, this experiment confirms
that no measurable loss in catalyst activity occurs after 24 h of
reaction with this purified feedstock. This result suggests that
the filtration procedure used here is adequate to remove any
heavy species that might otherwise rapidly deactivate the cata-
lyst.
Conclusion
A thermochemical route for the production of a high-purity
gluconic acid stream from bio-oil is presented. Sequential con-
densation followed by washing was used to separate anhydro-
sugars from the bio-oil to create a stream consisting of primari-
ly levoglucosan with some furanics and humins. Simple con-
tact with activated carbon selectively removes the heavy char,
humins, and furanics while preserving the levoglucosan in so-
lution. This is the first report of the production of gluconic acid
at high purity from pyrolysis products. Because a relatively
clean anhydrosugar solution is obtained, a simple contact with
Figure 4. The effect of reaction time on the conversion of glucose present in
the diluted and purified anhydrosugar solution to sodium gluconate. Reac-
tion conditions: temperature 508C; flow rate of air 60 mLminꢀ1; initial conc.
of glucose 0.02m; catalyst 5% Pd/C; pH 9.3.
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