(783b) Furfural Production By Continuous Reactive Extraction

1.     
Introduction

The
bio-based economy grows rapidly during the 21^st century. So far, oil
and natural gas dominate the industrial world and are widely considered as raw
materials. During the last few years, the use of biomass as raw material has increased
significantly (1).

One
important renewable bio-based chemical is furfural. It is widely used as
solvent for saturated hydrocarbons because of its chemical structure. Moreover,
it has the ability to dissolve aromatics and other unsaturated olefins (2)
and is extensively used in oil and gas industry. Finally, furfural has a high
potential as a platform chemical.

The
objective of this study is to investigate the reaction mechanism and kinetics
of xylose conversion towards furfural production by a continuous reactive
extraction process. Moreover, the reaction pathway will be further elucidated
and the kinetic rates of the intermediate steps are determined. Finally, the
kinetic model for xylose dehydration to furfural is used to maximize furfural
yield.

2.     
Experimental section

2.1.Reaction
mechanism

The
conversion of xylose to furfural is quite complex. Several studies
(2), (3) have been performed in order to analyze and interpret the
mechanism of the xylose dehydration. The scheme below shows the suggested
mechanisms.

Scheme
1 Reaction mechanisms of xylose dehydration (2), (3)

According
to Scheme 2 (blue), xylose is in equilibrium with intermediate I1. I1 undergoes
three dehydration reactions to furfural. Byproducts are formed directly from
either xylose or furfural (orange). The furfural degradation to byproduct 2 is
significant and influences the selectivity. Increasing the temperature and
acidity will further catalyze the degradation.  An additional reaction is
mentioned in (3), where xylose and furfural react and form side products (green).

The
formation of byproducts from furfural can be reduced by in situ removal of
furfural from the aqueous solution, by evaporation (4) or by extraction.
Various studies (3), (4) have been performed to investigate which solvent
performs best. Toluene and MIBK seem the most promising candidates with high
affinity to furfural. Therefore, in this study, toluene is selected.

2.2.Experimental
Setup

The
experiments are performed in a biphasic continuous reactor as shown in Scheme
2. The configuration includes 3 pumps for sulfuric acid, xylose solution, and
toluene. The maximum total flow is 15 mL/min and the minimum flow is 4 mL/min,
with the flow ratio organic/aqueous of 2:1.

Scheme 2
Experiment lay out

The
three streams flow separately in stainless steel tubes of 1.5 m length and 0.5
mm inner diameter through a heat exchanger to acquire the desired temperature.
The preheat system is followed by two consecutive T-joints that allow the
capillaries to be inserted in each other. Therefore the three streams are
injected and mixed at the reactor’s entrance (Scheme 3).

Scheme 3 Reactor inlet flow
development

The
reactor of 1.5 m in length and an inner diameter of 3.1 mm consists of a PTFE
wall and a stainless steel mesh jacket to provide structural integrity. The
PTFE wall is hydrophobic and wetted by toluene only, which prevents solid
deposition at the reactor wall. The mixture is then quenched followed by a
pressure safety relief and a filter with 2 μm pores that protects the
back-pressure regulator (BPR) from blocking by solids formed during the
reaction. Samples are taken after the BPR at atmospheric pressure and ambient
temperature and analyzed with HPLC (aqueous phase) and GC (toluene phase).

The
temperature range of the experiment is 100-170^oC and the pressure is
30 bar so the presence of any vapor or gas at the working temperature is
avoided. The selected catalyst is sulfuric acid (98%, Emsure)
at 0.1 M concentration. The xylose inlet concentration is 12 wt %.

3.     
Results

3.1.Product
characterization

In
Figures 1 and 2 examples of the water phase HPLC are presented at τ=100s and 500s, respectively. Two
signals are presented; (a) from the RI detector and (b) from the UV-vis
detector. The compounds known from the calibration are indicated in Figures 1
and 2. In the RI signal, two unknown peaks are present: I1 and I2. In the UV
signal more peaks are present. The most significant are termed A, B, C and D.
Peaks B_a and B_b have a similar retention time as peak I1,
who appears to have a shoulder and probably consists of two components.
Component C is close to the retention time of I2.  However, its
concentration increases while I2 decreases in concentration.

Regarding
UV chromatograms (Figures 1(b) and 2(b)), peaks corresponding to compounds B_a,
B_b increase from 2 and 4 mAu to 10 and 8 mAu respectively with
residence time. Moreover, peak of compound C rises from 10 mAu to 50 at the end
of the residence time. Finally, peak of D has a raise from 52 to 180 mAu at
500s.

3.2.
Reactor performance

In
Figure 3 the reactor outlet concentration of furfural and xylose are presented.
The highest xylose conversion is 44.52 % and the overall furfural yield is
14.45 %. The selectivity towards furfural appears to be relatively constant at
35% with increasing residence time.

In
Figure 4, the concentrations of the unknown compounds are shown as a function
of residence time, with the concentration of furfural. The intermediates’
concentrations are based on the RI chromatograms (Figures 1(a) and 2(a)).
However, the intermediates have not been isolated, characterized, and
calibrated in the HPLC. Their concentrations are based on an estimate from
their RI peak areas and overall mole balance. The RI sensitivity of I1, I2, and
I3 are assumed equal and is estimated from the mole balance (Eq. (1)) for the
highest conversion. The balance accuracy is more than 99.9%.

Byproducts
that are not detected by the HPLC are neglected, although a very small amount
of solid residue is noticed in the samples with higher residence times.

4.     
Discussion 

It
can be observed from Figures 1(a), 2(a) and 4 that component I1 shows a similar
trend as furfural at higher concentration. This cannot be explained by the
irreversible consecutive reaction scheme 1. It is very likely that I1
corresponds to intermediate 3 in Scheme 1 and is in equilibrium with furfural.
Alternatively, all dehydration reactions (Scheme 1) are equilibrium reactions
and I1 corresponds to any of the three intermediates. In Figures 1 and 2 the RI
does not respond to components C and D and they are presented only in UV. Thus,
it can be assumed that their concentration is low. Possibly, C and D correspond
to the intermediates 2 and 3 (Scheme 1).

The
concentration of component I2 increases before the lowest residence time and
decreases to zero after 250s. I2 is probably a precursor to the solid residue formed
and reacts with itself and the solid residue. Thus, the declining concentration
with residence time can be explained. The concentration of I3 remains low and
follows the same trend as furfural, which would indicate that it is a byproduct
in equilibrium with I1.

5.     
Conclusions

By
the continuous reactive extraction at 150^oC, 44.52 % xylose
conversion is achieved and 14.45 % total furfural yield. Compounds in
equilibrium with furfural have been detected validating the theory about
reaction mechanism presented earlier (Scheme 1). Moreover, additional compounds
are presented in the system potentially in equilibrium with furfural
intermediates. Finally, solids are formed from furfural degradation in aqueous
phase even in short residence times.

6.     
Outlook

Future
work includes additional experiments in the temperature range of 100°C to 170°C
and shorter and higher residence times. Moreover, the influence of the catalyst
will be investigated. Regarding the extraction part, variations will also be
made on the nature and the ratio of the solvent. Finally, a kinetic model will
be developed to describe the reaction rates as a function of temperature and
concentrations.

7.     
References

1. B. Kamm, P. R. Gruber, M. Kamm. Biorefineries-Industrial processes
and Products. s.l. :
Wiley, 2006.

2. The
Chemistry and technology of furfural production in modern
lignocellulose-feedstock biorefineries. Marcotullio, Gianluca. s.l. :
Delft University of technology, 2011.

3. Kinetics
of furfural production by dehydration of xylose in a biphasic reactor with
microwave heating. R. Weingarten, J. Cho, Wm. C. Conner, Jr. and G.W.
Huber. 1423-1429, s.l. : Green Chemistry, 2010, Vol. 12.

4. Extraction
of furfural from aqueous solution uding Alcohols. J.
L. Cabezas, L. A. Barcena,
J. Coca, M, Cockrem. 435-437, s.l. : J. Chem. Eng. Data,
1983, Vol. 33.

5. The
chemistry and technology of furfural and its many by-products. Zeitch, K. J. s.l. : Elsevier, 2000.