Tailoring the product distribution with batch and continuous process options in catalytic hydrogenation of furfural

Article abstract of DOI:10.1021/op500196x

Various noble metal catalysts were screened in a batch operation for a furfural (FFR) single-step decarbonylation and hydrogenation reaction to obtain THF in high selectivity. Among these, the 3% Pd/C showed complete FFR conversion with a total of 80% selectivity to ring hydrogenated products including tetrahydrofuran (THF). The order of activity exhibited by other noble metals was Pt/C > Re/C > Ru/C. Although Pt/C exhibited the highest activity, its decarbonylation and ring hydrogenation ability were the least (24%) with a major product selectivity of 66% to furfuryl alcohol (FAL). Similarly, the Cu catalyst gave almost complete selectivity to FAL. In a continuous operation (23 g catalyst bed), the 3% Pd/C catalyst showed higher selectivity of >40% compared to THF alone with complete FFR conversion and on-stream activity of ～100 h. The reaction pathway elucidated from some control experiments revealed that the decarbonylation of FFR to furan over the Pd/C catalyst is a prerequisite for THF formation.

Full text of DOI:10.1021/op500196x

Article
pubs.acs.org/OPRD
Tailoring the Product Distribution with Batch and Continuous
Process Options in Catalytic Hydrogenation of Furfural
Narayan S. Biradar, Amol A. Hengne, Shobha N. Birajdar, Rameshwar Swami,
and Chandrashekhar V. Rode*
Chemical Engineering & Process Development Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pashan Pune
411008, India
ABSTRACT: Various noble metal catalysts were screened in a batch operation for a furfural (FFR) single-step decarbonylation
and hydrogenation reaction to obtain THF in high selectivity. Among these, the 3% Pd/C showed complete FFR conversion
with a total of 80% selectivity to ring hydrogenated products including tetrahydrofuran (THF). The order of activity exhibited by
other noble metals was Pt/C > Re/C > Ru/C. Although Pt/C exhibited the highest activity, its decarbonylation and ring
hydrogenation ability were the least (24%) with a major product selectivity of 66% to furfuryl alcohol (FAL). Similarly, the Cu
catalyst gave almost complete selectivity to FAL. In a continuous operation (23 g catalyst bed), the 3% Pd/C catalyst showed
higher selectivity of >40% compared to THF alone with complete FFR conversion and on-stream activity of ∼100 h. The
reaction pathway elucidated from some control experiments revealed that the decarbonylation of FFR to furan over the Pd/C
catalyst is a prerequisite for THF formation.
as ring hydrogenation steps in a tandem way, an appropriate
combination of a catalyst system and reaction conditions needs
to be developed in order to achieve the highest selectivity to
THF.
1. INTRODUCTION
Utilization of biomass has received a lot of attention for the
development of a sustainable society. Paradigm change from
fossil to biomass resources for the manufacture of commodity
and fine chemicals will become inevitable in the near future
because biomass is the only renewable source of carbon.1
Lignocellulosic biomass is a more promising feedstock for
downstream applications as it is more abundant, cheaper, and
potentially more sustainable.2 The acid hydrolysis of pentose
sugars followed by dehydration evolving three water molecules
gives furfural (C5 molecule).3 Furfural (FFR) is one of the
most appropriate platform molecules for the production of a
variety of value added chemicals and fuels such as furan, furfuryl
alcohol (FAL), tetrahydrofurfuryl alcohol (THFAL), 2-methyl
furan (MF), 2-methyl tetrahydrofuran (MTHF), and tetrahy-
drofuran (THF).4 All these products are industrially important;
however, their distribution in FFR hydrogenation primarily
depends on reaction conditions and the catalyst, both of which
can be suitably tailored to suit the requirement.5 Among several
products of FFR hydrogenation, THF and other ring
hydrogenated products are of seminal interest commercially
and more so if obtained in a single-step conversion of FFR.
These products have versatile applications as industrial solvents,
as monomers, etc. One of these products, THF, can be
produced by decarbonylation of furfural to furan under
reductive conditions and then subsequently hydrogenated to
THF. Conventionally, THF can be produced by a number of
different routes, the major ones include dehydration of 1,4-
butanediol (BDO) and hydrogenation of maleic anhydride,
both of which are dependent on fossil feedstock and multistep
processes involving use of reagents, heavy metal complexes, and
harsh reaction conditions.6 These drawbacks can be easily
overcome by a single-step selective hydrogenation of FFR
derived from biorenewable feedstock. Since the single-step
formation of THF from FFR involves decarbonylation as well
Although several supported noble (Pt, Pd, Ru, Rh, Pd) and
non-noble (Cu, Ni) metals have been reported for the
hydrogenation of FFR, the major product of hydrogenation is
furfuryl alcohol. A 5% Pt/C catalyst has been found to give 58%
FFR conversion and >98.5% selectivity to FAL at 403−448 K
and 10−20 bar H2 pressure.5c Non-noble metal catalysts
included Raney nickel and Cu, mainly used for FFR
hydrogenation to FAL showing the highest selectivity of
>98% to FAL with conversion in the range of 65 to 99%.7
Poliakoff and his group reported a twin catalyst system (Cu and
Pd/C) for the continuous conversion of FFR to a variety of
products at different reaction temperatures, which included
FAL, THFAL, MeTHF, MeTHF, and furan at 393−523 K
reaction temperature.8 Sethissa and Resasco studied different
silica-supported metal catalysts for the hydrogenation of FFR
among which 1% Pd/SiO2 gave 20% THF with 69% conversion
in a continuous fixed-bed reactor.9 Using microreactor system,
80% (mol %) selectivity to THF was achieved with 90% (mol
%) conversion of FFR over carbon-supported Pd NPs.10 In the
case of the Pd/Al2O3 catalyst, K-doping was effective for
decarbonylation of FFR with complete conversion, but almost
complete selectivity (>99%) was obtained with an unsaturated
product, furan.11
In our work on FFR hydrogenation, while screening several
catalysts, total selectivity of 70% was achieved for ring
hydrogenated products including THF over the 3% Pd/C
catalyst. THF selectivity enhanced 2-fold at complete
Special Issue: Continuous Processes 14
Received: June 18, 2014
Published: August 7, 2014
© 2014 American Chemical Society
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conversion of FFR when the hydrogenation was performed in a
continuous mode. THF selectivity could be tuned by
manipulating the process parameters in both batch and
continuous operations.
below the catalyst bed was packed with carborandum as an
inert packing, and the remaining reactor was filled with catalyst
powder in four sections which were separated by carborandom.
Before the actual experiment was started, the reactor was
flushed thoroughly, first with N2 and then with H2 at room
temperature. Then the reactor was pressurized with H2 after
attaining the desired temperature. The liquid feed was
“switched on” after the reactor reached the operating pressure
and was kept at that value for 1 h to obtain the constant liquid
flow rate. Liquid samples withdrawn time to time during the
reaction were analyzed with a Trace GC 700 series GC System
(Thermo SCIENTIFIC) coupled with an FID detector and a
capillary column (HP-5 capillary column, 30 m length × 0.32
mm i.d.). The following temperature programme was used for
GC analysis: 40 °C (3 min), 1 °C/min, 45 °C (1 min), 10 °C/
min, 60 °C (0 min), 20 °C/min, 25 °C (1 min). Several
experiments were carried out at different inlet conditions of
liquid and gas flow rates and in the temperature and pressure
ranges of 423−513 K and 20−50 bar, respectively. Steady-state
performance of the reactor was observed by analysis of the
reactant and products in the exit stream.
2. EXPERIMENTAL SECTION
2.1. Material. Furfural (99%) and activated charcoal were
purchased from Sigma-Aldrich, Bangalore, India, while sodium
hydroxide (NaOH), sodium borohydride (NaBH4), and
isopropyl alcohol were purchased from Thomas Baker, India.
Palladium chloride (PdCl2) was purchased from Lab-India.
Hydrogen gas of high purity (99.99%) was obtained from Inox,
India.
2.2. Catalyst Preparation. Supported Pd, Pt, Ru, and Re
catalysts were prepared by a wet-impregnation method. In a
typical preparation of the 3% Pd/C catalyst, 0.159 g of PdCl2
was dissolved in a minimum amount of dilute hydrochloric
acid, ensuring the complete dissolution of the precursor. Under
stirring, 2 g of a slurry of carbon support prepared in water was
added to the above solution, and solution was stirred for 2 h at
room temperature. After 1 h, 10 M sodium hydroxide solution
was added under stirring to make the resultant pH = 7−8, and
the stirring was continued for another 30 min. After that, 3 mL
formaldehyde was added under stirring as a reducing agent.
The reaction mixture was further stirred for 30 min and was
then cooled and filtered to obtain the catalyst, which was dried
at 373 K. Similarly, other catalysts screened for hydrogenation
of furfural were prepared using the same procedure.
2.3. Catalyst Characterization. BET surface area and N2
adsorption isotherms of all Pd-supported catalysts were
obtained along with the pore size distribution calculated
using the DFT method on a Quantachrome QuadraWi (5.02
version). Particle size was studied by transmission electron
microscopy (HR-TEM) on a JEOL 1200 EX model. For this
purpose, a small amount of the solid sample was sonicated in 2-
propanol for 1 min. A drop of the prepared suspension was
deposited on a Cu grid coated with a carbon layer, and the grid
was then dried at room temperature before analysis.
The conversion and selectivity were calculated as follows:
(%) conversion
initial moles of furfural − final moles of furfural
=
× 100
initial moles of furfural
(1)
moles of product formed
moles of furfural consumed
(%) selectivity =
× 100
(2)
3. RESULTS AND DISCUSSION
This study was mainly focused on (i) identifying a catalyst for
single-step decarbonylation followed by ring hydrogenation of
furfural and studying the product distribution in a batch
reactor; (ii) evaluating time on stream (TOS) activity and
product distribution of the best catalyst for continuous furfural
hydrogenation; (iii) optimizing the reaction conditions and
approach for batch and continuous reaction to achieve the
highest selectivity to intermediate furan and THF via
decarbonylation and hydrogenation.
2.4. Catalyst Activity. The performance of the prepared
catalyst was tested in both batch as well as continuous
operations.
Batch reactions were carried out in a 300 mL capacity
autoclave supplied by Parr Instruments Co. USA. The typical
hydrogenation conditions were as follows: temperature, 473−
513 K; hydrogen pressure, 20−50 bar; furfural concentration, 5
wt %; solvent, 95 mL; agitation speed, 1000 rpm; and catalyst
loading, 0.5 g.
3.1. Catalyst Characterization. Textural properties of all
the Pd-supported catalysts are shown in Table 1 from which it
Table 1. Characterization of Pd-Supported Catalysts
catalyst
Continuous hydrogenation of furfural was carried out in a
bench scale, high-pressure, fixed-bed reactor supplied by M/s
Geomechanique, France. This reactor setup consisted of a
characterization
3% Pd/C
3% Pd/SiO2
135.3 m2/g
3% Pd/CaCO3
231.4 m2/g
BET surface area
pore size
639.8 m2/g
́
́
́
6 Å
65−85 Å
10−15 nm
35 Å
stainless steel single tube of 0.34 m length and 1.5 × 10−2
m
particle size
4−5 nm
8−9 nm
inner diameter. The reactor was heated by two tubular furnaces
whose zones (TIC1 and TIC2) were independently controlled
at the desired bed temperature. The reactor was provided with
mass flow controllers, pressure indicator, and controller (PIC)
devices and two thermocouples to measure the temperature at
two different points. A storage tank was connected to the
HPLC pump through a volumetric burette to measure the
liquid flow rate. The pump had a maximum capacity of 3 ×
10−4 m3/h under a pressure of 100 bar. The gas−liquid
separator was connected to the other end of the reactor
through a condenser. Ten grams of catalyst was charged into
was clear that the surface area of the carbon-supported catalyst
was the highest (613 m2/g), and the CaCO3- and silica-
supported catalysts showed lower surface areas of 231.4 and
135.3 m2/g, respectively. N2 physisorption studies showed type-
I and type-IV adsorption isotherms for all the supported Pd
catalysts. The 3% Pd/C catalyst showed microporous nature,
while the 3% Pd/SiO2 and 3% Pd/CaCO3 showed meso/
macroporous nature. HR-TEM characterization of the 3% Pd/
C catalyst also showed an excellent dispersion of palladium
particles on the carbon support without any agglomeration as
the reactor. The section of 7 × 10−2 m above and 7 × 10−2
m
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a
Table 2. Catalyst Screening for Furfural Hydrogenation
selectivity %
MF
run
catalyst
conversion %
FAL
THF
THFAL
MTHF
pentanediol
furan
1
2
3
4
5
6
7
3% Pd/C
3% Pd/SiO2
3% Pd/CaCO3
3% Pt/C
100
75.5
91.5
89.3
57.3
71.78
100
16
52
20
3
32
37
14
<1
15
10
17
45
0
18
<1
16
1
0
6
21
0
49
66
0
23
3% Ru/C
3% Re/C
CuAl
31
0
40
12
10
0
23
0
23
>99.5
0
<0.5
a
Reaction conditions: furfural, 5% (w/w); solvent, isopropyl alcohol (95 mL); temperature, 493 K; H2 pressure, 35 bar; agitation speed, 1000 rpm;
catalyst (3% Pd/C), 0.5 g; reaction time, 5 h.
the average particle size was found to be 4−5 nm, determined
by counting about 50 individual particles.
other supports. In case of Pd/SiO2 catalyst, the major product
formed was FAL (entry 2, Table 2) with ∼40% of ring
hydrogenated products. Although Pd/CaCO3 showed ∼92%
FFR conversion, surprisingly, no formation of any THF was
observed (entry 3, Table 2). As carbon was found to be the best
support for the Pd catalyst, other noble metals such as Pt, Ru,
and Re with the same loading of 3% on carbon were also
evaluated for FFR hydrogenation. However, all three metal
functions showed performance inferior to that of Pd, and the
order of activity among them was found to be Pt/C > Re/C >
Ru/C. Although Pt/C exhibited the highest activity, its ring
hydrogenation ability was the least (24%), with a major product
selectivity of 66% to FAL (entry 4, Table 2). Ru/C showed
significant formation of ring hydrogenated products (52%,
entry 5, Table 2), while the Re/C catalyst gave a total selectivity
of 34% to ring hydrogenation (entry 6, Table 2). Contrary to
our expectation, none of the noble metals other than Pd
showed any selectivity to THF. A non-noble metal catalyst
tested was CuAl that showed complete conversion of FFR with
the highest selectivity to a single product, that is, furfuryl
alcohol >99.5% (entry 7, Table 2) which was in accordance
with Cu−Cr catalysts.13
3.2. Catalyst Screening. Batch Operation. The results of
screening various catalysts for the decarbonylation−hydro-
genation of furfural are summarized in Table 2. In our previous
studies, Pd/MFI was found to be the best catalyst for FFR
hydrogenation, however, with the highest selectivity to THFAL
formed by ring hydrogenation without decarbonylation.12
Hence, initially, Pd catalysts on supports other than MFI
were explored, and interestingly, it was found that the 3% Pd/C
gave complete FFR conversion with 20% selectivity to THF,
which has been the highest reported so far. The total selectivity
to ring hydrogenated products (THF, THFAL, and MTHF)
achieved was as high as 80% (entry 1, Table 2). Figure 1 shows
As only the Pd/C catalyst showed the highest activity for
FFR conversion with the highest selectivity to ring hydro-
genated products (>70%) and to THF as high as 20% in a
single step, the reaction pathway for FFR hydrogenation was
also studied by some control experiments. The experiment with
FFR was conducted by replacing H2 with N2 under the same
reaction conditions that gave complete conversion of FFR with
>99% selectivity to furan, confirming that the decarbonylation
of FFR over the Pd/C catalyst is a prerequisite for THF
formation. The results of hydrogenation experiments with
various FFR hydrogenation products as substrates over the 3%
Pd/C catalyst are shown in Table 3. With FAL as a substrate,
complete selectivity to ring hydrogenation product THFAL was
obtained, indicating that the side chain hydrogenation/
decarbonylation did not proceed via THFAL (entry 2, Table
3). This was also confirmed when THFAL itself was used as a
starting material (entry 3, Table 3). Similarly, hydrogenation of
2-methyl furan and furan gave complete selectivity to MTHF
and THF, respectively, thus establishing efficient ring hydro-
genation over the Pd/C catalyst under the conditions of the
present work. Based on these results, the proposed reaction
pathway is shown in Scheme 1. In summary, furfuryl alcohol
can be selectively obtained by the use of Cu-based catalysts
under mild reduction conditions, while with noble metal
catalysts, Pd/C was particularly effective for decarbonylation to
give furan that can be subsequently hydrogenated to THF.
Figure 1. CT profile of FFR hydrogenation over 3% Pd/C catalyst.
Reaction conditions: furfural, 5% (w/w); solvent, isopropyl alcohol
(95 mL); H2 pressure, 35 bar; agitation speed, 1000 rpm; catalyst (3%
Pd/C), 0.5 g; reaction time, 5 h.
the reaction profile in terms of FFR conversion and
simultaneous formation of THF along with other products
(FAL, MF, THFAL, and MTHF) as a function of reaction time,
over the 3% Pd/C catalyst. This indicates that the decarbon-
ylation and hydrogenation of FFR took place from the start of
the reaction. However, initially, THF selectivity was 12% which
after 1 h increased to 20% in 5 h along with an increase in
selectivity to other ring hydrogenated products such as THFAL
(34%) and MTHF (20%) at the cost of FAL and MF. The
order of activity for three supports for the obtained Pd catalyst
was Pd/C > Pd/CaCO3> Pd/SiO2. The selectivity to THF was
the highest for carbon support due to higher surface area and
metal dispersion and smaller particle size compared to those of
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a
Table 3. Hydrogenation of Different Substrates
a
Reaction conditions: substrate, 5% (w/w); solvent, isopropyl alcohol (95 mL); temperature, 493 K; H2 pressure, 35 bar; agitation speed, 1000 rpm;
catalyst (3% Pd/C), 0.5 g; reaction time, 5 h.
Scheme 1. General Reaction Pathway for Furfural
Hydrogenation
Figure 2. Effect of agitation speed. Reaction conditions: furfural, 5%
(w/w); solvent, isopropyl alcohol (95 mL); temperature, 493 K;
catalyst (3% Pd/C), 0.5 g; reaction time, 5 h.
As 3% Pd/C catalyst showed the excellent performance for
hydrogenation of FFR with the highest selectivity to ring
hydrogenated products along with THF, further optimization
of reaction conditions was done over the same catalyst in batch
as well as continuous operations, and the results are discussed
below.
3.2.1. Effect of Agitation Speed. In order to check the
external mass transfer resistances, the effect of agitation speed
was studied in the range of 500 to 1000 rpm, keeping all other
reaction parameters the same, and the results are shown in
Figure 2. The complete conversion of FFR was observed
irrespective of the agitation speed variation from 500 to 1000
rpm, indicating clearly that the external mass transfer did not
play any significant role under the conditions of the present
work. Interestingly, selectivity to THF slowly increased from 15
to 20% with an increase in the agitation speed from 500 to
1000 rpm. Along with THF, other ring hydrogenated products
formed were about 52−55%. All of the studies were then
carried out at the agitation speed of 1000 rpm.
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3.2.2. Effect of Hydrogen Pressure. Figure 3 shows the effect
of hydrogen pressure on FFR hydrogenation at 493 K. The
3.2.4. Effect of Catalyst Concentration. The effect of
catalyst concentration on the conversion of furfural and
selectivity to THF was also studied in the range of 0.250−
0.750 g at 493 K and 35 bar H2 pressure, and the results are
shown in Figure 5. It was found that as the catalyst loading
Figure 3. Effect of temperature. Reaction conditions: furfural, 5% (w/
w); solvent, isopropyl alcohol (95 mL); H2 pressure, 35 bar; agitation
speed, 1000 rpm; catalyst (3% Pd/C), 0.5 g; reaction time, 5 h.
Figure 5. Effect of catalyst concentration. Reaction conditions:
furfural, 5% (w/w); solvent, isopropyl alcohol (95 mL); temperature,
493 K; H2 pressure, 35 bar; agitation speed, 1000 rpm; reaction time, 5
h.
selectivity to THF increased from 13 to 20% with increasing H2
pressure from 20 to 35 bar with almost complete conversion of
FFR. While increasing H2 pressure beyond 35 bar, selectivity to
pentanediols increased up to 6% and that to other ring
hydrogenated products was found to be in the range of 50−
55%. The increase in selectivity to THF (22%) at high H2
pressure and other ring hydrogenated products was due to
higher surface concentration of hydrogen.
3.2.3. Effect of Temperature. The effect of temperature on
FFR conversion and product distribution was studied in the
range of 473−513 K by keeping all other parameters the same.
Figure 4 shows that the selectivity to THF increased from 15 to
increased from 0.25 to 0.75 g, selectivity to THF increased
from 14 to 22%. The increase in THF selectivity was mainly
due to the increase in number of catalytic active sites with
increasing catalyst concentration at constant FFR loading.
3.2.5. Effect of Pd Metal Loading. As Pd was the active
metal function for both hydrogenation and decarbonylation in
FFR conversion to THF, the effect of its loading on carbon
support on selectivity of THF was studied, and the results are
shown in Figure 6. For this purpose, Pd loading was varied in
the range of 1 to 5%, and it was found that the conversion of
FFR remained constant (100%) for all the metal loadings, while
the selectivity to THF increased from 10 to 20% as the metal
loading increased from 1 to 3%. At a higher metal loading of
5%, the selectivity to THFAL and pentanediols increased at the
cost of THF, which decreased to 14%. At higher Pd loading,
Figure 4. Effect of hydrogen pressure. Reaction conditions: furfural,
5% (w/w); solvent, isopropyl alcohol (95 mL); agitation speed, 1000
rpm; catalyst (3% Pd/C), 0.5 g; reaction time, 5 h.
20% as the temperature increased from 473 to 493 K; however,
further increase in temperature from 493 to 513 K marginally
influenced the THF selectivity (21%), but the further
hydrogenation product, pentanediol, was formed to the extent
of 6%. Along with THF, other ring hydrogenated products
formed were 53−54%. Thus, both decarbonylation and ring
hydrogenation reactions were favored at higher temperature.
Figure 6. Effect of Pd metal loading. Reaction conditions: furfural, 5%
(w/w); solvent, isopropyl alcohol (95 mL); temperature, 493 K; H2
pressure, 35 bar; agitation speed, 1000 rpm; catalyst, 0.5 g; reaction
time, 5 h.
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hydrogenation reactions were apparently faster than the
decarbonylation reaction, resulting in lowering the THF
selectivity.
3.2.6. Effect of Furfural Concentration. In order to achieve
maximum productivity for FFR hydrogenation, the effect of
substrate concentration on conversion and selectivity was
studied and the results are shown in Figure 7. With an increase
Figure 8. Catalyst recycle study. Reaction conditions: furfural, 5% (w/
w); solvent, isopropyl alcohol (95 mL); temperature, 493 K; H2
pressure, 35 bar; agitation speed, 1000 rpm; catalyst (3% Pd/C), 0.5 g;
reaction time, 5 h.
Figure 7. Effect of furfural concentration. Reaction conditions: solvent,
isopropyl alcohol (95 mL); temperature, 493 K; H2 pressure, 35 bar;
agitation speed, 1000 rpm; catalyst (3% Pd/C), 0.5 g; reaction time, 5
h.
in concentration of FFR from 5 to 20 wt %, its conversion
drastically decreased from 100 to 70% accompanied by huge
decrease of 10-fold in THF selectivity (2%). This was mainly
due to the unconverted intermediates, furan (35%) and FAL
(35%). Interestingly, furan and FAL are the intermediates of
two different reactions, decarbonylation and side chain
hydrogenation. This means that the ring hydrogenation was
drastically affected by higher substrate loading that was
responsible for the higher extent of coverage of active sites
required for ring hydrogenation. This result also suggests that
the hydrogenation of the FFR to THF pathway is via
decarbonylation of FFR to furan followed by its ring
hydrogenation to THF (Scheme 1).
Figure 9. Decarbonylation followed by hydrogenation. Reaction
conditions: furfural, 5% (w/w); solvent, isopropyl alcohol (95 mL);
temperature, 493 K; H2 pressure, 35 bar; agitation speed, 1000 rpm;
catalyst (3% Pd/C), 0.5 g; reaction time, 5 h.
3.2.7. Catalyst Recycle Study. To check reproducibility and
stability of the 3% Pd/C catalyst after the first hydrogenation
experiment, the reaction crude was allowed to settle down and
the clear supernatant product mixture was separated out. To
the catalyst remaining in the reactor was added fresh charge,
and the subsequent hydrogenation run was continued. This
procedure was followed for four subsequent runs, and the
results are shown in Figure 8. Our 3% Pd/C catalyst showed
almost complete conversion of FFR with consistent selectivity
to THF and all other remaining products as that of the fresh
catalyst even after the fourth reuse.
3.2.8. Decarbonylation Followed by Hydrogenation. It was
observed from Table 3 that the ring hydrogenation of furan
directly gives THF. Hence, FFR to THF via decarbonylation
(Scheme 1) followed by the ring hydrogenation pathway could
be more facile. This was evidenced by a control experiment in
which concentration versus time profile for decarbonylation of
FFR to furan in N2 atmosphere was studied. In this case, almost
complete conversion of FFR with selective formation of furan
was achieved (Figure 9). In the same experiment after complete
conversion of furfural, nitrogen was replaced by hydrogen and
the reaction was continued. Selective formation of THF was
observed due to ring hydrogenation of furan; however, furan
conversion was restricted to only 40%, due to poisoning of
active Pd sites by CO formed during the decarbonylation
reaction.
Continuous Operation. As discussed above, the 3% Pd/C
was established as the best catalyst for direct FFR conversion to
THF and other ring hydrogenated products. The same catalyst
was therefore evaluated for its time on stream activity in a
continuous operation over a bed of catalyst packed in a single
tube reactor, and the results are shown in Figure 10. Complete
conversion of FFR was achieved consistently over a period of
100 h, and very interestingly, the selectivity to THF enhanced
by 2-fold (20 to 41%) as compared to the batch operation,
while other ring hydrogenated products comprised 36%
THFAL and 7% MTHF. As TOS increased, catalyst was
exposed to hydrogen at the reaction temperature for a longer
duration of time, resulting in its continuous in situ activation.
This might have significantly contributed to the higher extent of
ring hydrogenation, resulting in enhanced THF selectivity. This
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3.2.10. Effect of Temperature. Effect of reaction temper-
ature on conversion and selectivity in FFR hydrogenation was
studied at 35 bar H2 pressure, and the results are summarized in
Figure 12. Complete conversion of FFR was obtained for the
Figure 10. Continuous reaction. Reaction conditions: catalyst (3%
Pd/C), 10 g; solvent, isopropyl alcohol; 5 wt % furfural; feed flow rate,
30 mL/h; H2 flow rate, 10 Nl/h; H2 pressure, 35 bar; temperature, 493
K.
TOS performance of the 3% Pd/C catalyst giving THF with as
much as >40% selectivity becomes an excellent process option
for direct conversion of biorenewable FFR to value added THF
and other ring hydrogenation products from furfural. From a
scale-up point of view, the effects of various reaction parameters
on conversion and selectivity patterns are discussed below.
These studies were conducted in a single continuous run after
establishing the TOS of the catalyst for more than 100 h. Each
parameter effect was continued for 24 h under steady-state
conditions, and a standard run was conducted after every 24 h
to ensure the original activity of the catalyst.
Figure 12. Effect of temperature. Reaction conditions: catalyst (3%
Pd/C), 10 g; solvent, isopropyl alcohol; 5 wt % furfural; feed flow rate,
30 mL/h; H2 flow rate, 10 Nl/h; pressure, 35 bar.
temperatures in a range of 473 to 493 K. Similar to the H2
pressure effect, the extent of enhancement in selectivity to THF
as well as to THFAL and MTHF was observed. There was no
appreciable change observed in THF selectivity with further
increase in temperature to 513 K.
3.2.11. Effect of Liquid Flow Rate. To achieve the optimum
productivity in FFR hydrogenation, the effect of liquid flow rate
on conversion was studied in the range of 30−60 mL/h, under
constant temperature, H2 pressure, and gas flow rate
conditions, and the results are shown in Figure 13. Although
3.2.9. Effect of Hydrogen Pressure. Figure 11 shows the
effect of hydrogen pressure on continuous hydrogenation of
Figure 11. Effect of hydrogen pressure. Reaction conditions: catalyst
(3% Pd/C), 10 g; solvent, isopropyl alcohol; 5 wt % furfural; feed flow
rate, 30 mL/h; H2 flow rate, 10 Nl/h; temperature, 493 K.
Figure 13. Effect of liquid flow rate. Reaction conditions: catalyst (3%
Pd/C), 10 g; solvent, isopropyl alcohol; 5 wt % furfural; pressure, 35
bar; H2 flow rate, 10 Nl/h; temperature, 493 K.
FFR at 493 K. FFR conversion was found to be consistent for
the H2 pressure range of 20−50 bar studied in this work. The
selectivity to THF increased appreciably from 31 to 41% with
other major ring hydrogenated products such as THFAL and
MTHF up to 52%, with an initial increase in the H2 pressure
from 20 to 35 bar. Thus, total selectivity to ring hydrogenated
products was >90%. Selectivity to THF remained constant at
41% with further increase in H2 pressure to 50 bar.
conversion of FFR remained constant (>99%), the selectivity to
THF dramatically decreased from 41 to 17%, with an increase
in feed flow rate from 30 to 60 mL/h. The decrease in THF
selectivity was mainly due to accumulation of unconverted
furan (24%). Furan formed in the first step of decarbonylation
could not be hydrogenated in the following step due to
decreasing contact time at higher liquid flow rate. At a lower
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liquid flow rate, the intermediate furan readily converted to
THF, which is consistent with the results obtained in a batch
operation.
conversion of biorenewable FFR to value added THF and other
ring hydrogenation products from furfural.
3.2.12. Effect of Furfural Concentration. The effect of
furfural concentration on continuous hydrogenation of FFR
was also studied in the range of 5−20 wt % furfural, and the
results are shown in Figure 14. Similar to the results observed
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: cv.rode@ncl.res.in. Tel: 020-25902349.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
One of the authors (N.S.B.) acknowledges the Council of
Scientific and Industrial Research, New Delhi, for its financial
support of this work under the NMITLI program (TLP
002926).
ABBREVIATIONS
■
FFR = furfural
THF = tetrahydrofuran
FAL = furfuryl alcohol
THFAL = tetrahydrofurfuryl alcohol
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Figure 14. Effect of furfural concentration. Reaction conditions:
catalyst (3% Pd/C), 10 g; solvent, isopropyl alcohol; feed flow rate, 30
mL/h; H2 flow rate, 10 Nl/h; H2 pressure, 35 bar; temperature, 493 K.
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