Formaldehyde production via hydrogenation of carbon monoxide in the aqueous phase

Article abstract of DOI:10.1039/c5gc00599j

Formaldehyde (HCHO) is an essential building block in many industries for producing value-added chemicals like resins, polymers and adhesives. Industrially, formaldehyde is produced via partial oxidation and/or dehydrogenation of methanol. Methanol is produced from natural gas in a series of processes, with synthesis gas as an intermediate. This study presents for the first time, formaldehyde production via hydrogenation of carbon monoxide in the aqueous phase, which eliminates the need for methanol synthesis, which may potentially save capital costs and reduce energy consumption. Gas phase hydrogenation of CO into formaldehyde is thermodynamically limited and therefore, resulted in a low CO conversion of only 1.02 × 10^-4%. However, the aqueous phase hydrogenation of CO into formaldehyde was found to be thermodynamically favourable and kinetically limited. The highest CO conversion of 19.14% and selectivity of 100% were achieved by using a Ru-Ni/Al₂O₃ catalyst at 353 K and 100 bar. The rapid hydration of formaldehyde in the aqueous phase to form methylene glycol shifts the CO hydrogenation reaction equilibrium towards formaldehyde formation. Increasing the pressure and stirring speed increased the yield of formaldehyde, whereas increasing the temperature above 353 K resulted in a lower yield.

Full text of DOI:10.1039/c5gc00599j

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Green Chemistry
RSC
PAPER
Formaldehyde Production via Hydrogenation of
Carbon Monoxide in Aqueous Phase
Ali Mohammad Bahmanpour,a Andrew Hoadleya and Akshat Tanksale*a,
Formaldehyde (HCHO) is an essential building block in many industries for producing value-
added chemicals like resins, polymers and adhesives. Industrially formaldehyde is produced
via partial oxidation and/or dehydrogenation of methanol. Methanol is produced from natural
gas in a series of processes, with synthesis gas as an intermediate. This study presents for the
first time formaldehyde production via hydrogenation of carbon monoxide in the aqueous
phase, which eliminates the need for methanol synthesis, which may potentially save capital
costs and reduce energy consumption. Gas phase hydrogenation of CO into formaldehyde is
thermodynamically limited and therefore, resulted in low CO conversion of only 1.02×10-4 %.
However, the aqueous phase hydrogenation of CO into formaldehyde was found to be
thermodynamically favourable and kinetically limited. Highest CO conversion of 19.14% and
selectivity of 100% was achieved by using Ru-Ni/Al2O3 catalyst at 353 K and 100 bar. The
rapid hydration of formaldehyde in the aqueous phase to form methylene glycol shifts the CO
hydrogenation reaction equilibrium towards formaldehyde formation. Increasing the pressure
and stirring speed increased the yield of formaldehyde, whereas increasing the temperature
above 353 K resulted in a lower yield.
Eq. 5 to a very small value. Moreover, if we consider that
natural gas is a valuable energy resource, it becomes evident
Introduction
that an alternative feedstock for HCHO production is much
needed. An alternative is direct conversion of synthesis gas into
HCHO. Syngas can be produced from a range of sources
including biomass and allows the mole fractions of CO to H2 to
be controlled more easily through the use of H2O and CO2
which is helpful in climate change abatement.9 Gas phase
hydrogenation of CO to produce HCHO (Equation 6) is not
feasible because of positive Gibbs free energy change of the
reaction.10 Only trace amount of HCHO in the product has been
reported with the highest CO conversion of 0.2%.11 Therefore,
direct conversion of synthesis gas into HCHO has not been
studied extensively. In this report, hydrogenation of CO into
HCHO in a slurry reactor is presented as a viable alternative.
By comparing with gas phase conversion in a fixed bed reactor,
it is demonstrated that the thermodynamic limitation can be
overcome in the slurry reactor. A low temperature active
catalyst is desirable for the slurry phase reaction as the reaction
was found to be favourable below 373 K. Generally, Ni Pd and
Ru are considered as active hydrogenation catalysts in the
literature for many reactions.12-21 Based on the density
functional theory (DFT) studies done in the literature, Pd and
Industrially, formaldehyde (HCHO) is produced in three stages
– (a) Steam reforming of natural gas to produce syngas (Table
1, Eq.1), (b) Methanol (CH3OH) Synthesis (Eq. 2) and (c)
partial oxidation of CH3OH to produce HCHO (Eq. 3).
Alternatively, HCHO is industrially produced via
dehydrogenation of CH3OH (Eq. 4).1, 2 However, these are all
high temperature reactions which require combustion,
compression and large process units for purification, which are
the root cause of energy losses.3, 4 We have recently shown that
this series of processes from natural gas to HCHO production
suffers from ~57% losses in exergy (i.e. energy quality).5 Given
the large quantity of HCHO produced in the world, when
combined with the high losses in exergy, leads to high energy
losses and also high CO2 emissions, globally. Many researchers
have tried to overcome this issue by finding ways to produce
HCHO directly from natural gas by partial oxidation of CH4
(Eq. 5).6-8 However, there has been no significant progress to
date due to low CH4 conversion and poor selectivity.6 The rate
of HCHO decomposition into CO and H2 is much greater than
the rate of partial oxidation of CH4, especially at temperatures
in excess of 373 K, which means that in order to produce
HCHO selectively, one must limit the conversion of CH4 in
Ni have been shown to produce HCHO as an intermediate of
23
CO hydrogenation to produce CH3OH.22,
Previous studies
have also shown that bi-metallic Pd-Ni catalyst has better
reducibility and higher metal surface area.24 Therefore, Pd-Ni
and Ru-Ni supported on γ-Al2O3 were used as the catalysts in
this study.
a
Catalysis for Green Chemicals Group
Department of Chemical Engineering
Monash University, Clayton, VIC 3800, Australia
* Email: akshat.tanksale@monash.edu
Ph.: 61 3 99024388 and Fax : 61 3 99055686
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Table 1 Chemical reactions used for the production of HCHO in the gas phase
Reaction Name
Reaction Stoichiometry
Equation
No.
(1)
표
−1
̂
Steam Reforming
퐶퐻4 + 퐻2푂 ⇌ 퐶푂 + 3퐻2
∆퐻푟 = +206 kJ. 푚표푙
표
−1
̂
Methanol Synthesis
(2)
퐶푂 + 2퐻2 ⇌ 퐶퐻3푂퐻
∆퐻푟 = −91 kJ. 푚표푙
퐶퐻3푂퐻 + 1 푂2 → 퐻퐶퐻푂 + 퐻2푂
퐶퐻3푂퐻 ⇌2퐻퐶퐻푂 + 퐻2
표
−1
̂
Methanol Partial Oxidation
Methanol Dehydrogenation
Methane Partial Oxidation
(3)
(4)
(5)
∆퐻푟 = −159 kJ. 푚표푙
표
−1
̂
∆퐻푟 = +84 kJ. 푚표푙
표
−1
̂
퐶퐻4 + 푂2 → 퐻퐶퐻푂 + 퐻2푂
퐶푂 + 퐻2 ⇌ 퐻퐶퐻푂
∆퐻푟 = −319 kJ. 푚표푙
표
−1
̂
Syngas to Formaldehyde in Gas
Phase
(6)
∆퐻푟 = −5.4 kJ. 푚표푙
,
∆퐺표 = +34.6 kJ. 푚표푙−1
Results and Discussion
Catalyst Characterization
BET surface area of the support and CO-chemisorption results
are shown in Table 2. BET surface area is given for the
commercial γ-Al2O3 before metal impregnation. It is expected
that the surface area reduced to some extent after
impregnation.24
The nominal and actual metal content of the catalysts are
presented in Table 2, which shows good agreement. The
promoter content was calculated based on the mass balance as
the actual content of the NiO and Al2O3 was measured by X-ray
Florescence (XRF) spectroscopy. Based on the amount of CO
adsorption on the catalyst surface, metal dispersion was
calculated according to the following formula:
푀퐶푂
퐷푖푠푝푒푟푠푖표푛(%) =
×퐴푊 × 100
(7)
푊퐹
where 푀퐶푂 is the amount of adsorbed CO (µmol.g-1), AW is the
atomic weight of the metals (g.µmol-1), and WF is the weight
fraction of the metals in the catalysts. Table 2 shows that CO
uptake and metal dispersion of Ru-Ni/Al2O3 was significantly
higher than Pd-Ni/Al2O3, even though similar metal loading
was used. This suggests that the supported Ru-Ni nanoparticles
are much smaller than the Pd-Ni nanoparticles. This is
confirmed from the Transmission Electron Microscopy (TEM)
results shown in Figure 1. The nanoparticles sizes were
estimated using ImageJ software (NIH) and shown in the in-set.
The mean particle size of Pd-Ni nanoparticles was 3.69 nm
compared to the mean particle size of Ru-Ni nanoparticles of
2.14 nm. Lower particle size and hence higher dispersion of
the catalyst nanoparticles is favourable for HCHO formation
because it would provide higher surface area for CO and H2
adsorption which is expected to be a necessary step for
hydrogenation reaction.
The X-ray Diffraction (XRD) patterns of the calcined and
reduced Ru-Ni/Al2O3 and Pd-Ni/Al2O3 catalysts are presented
in Figure 2. The peaks representing PdO (34.06°) and RuO2
(28.19° and 34.06°) disappeared after reduction and peaks
representing Pd0 (40.15°) and Ru0 (43.98°) were observed,
which confirmed complete reduction of the catalysts.25, 26 Broad
NiO and Ni0 peaks were also observed at 62.88° and 51.83° 2θ
angles, which also confirm that the Ni nanoparticles are finely
dispersed.27
Figure 1 TEM image and particle size distribution of a) Pd-
Ni/Al2O3. Scale bar= 50nm and b) Ru-Ni/Al2O3. Scale bar= 20nm
Thermodynamic Investigation
Hydrogenation of CO into HCHO in the gas phase and aqueous
phase was thermodynamically investigated using the HSC
Chemistry® version 7.11 (Outotec, Finland) software and
published data28 for a wide range of temperatures (298-623 K)
and pressures (50-500 bar). The results (Table 3) show that the
Gibbs free energy (∆퐺) of the reaction is positive in the gas
phase and it increases with increasing temperature. Therefore
the reaction is non-spontaneous at all temperatures above
298 K. The equilibrium constant of the gas phase reaction is
very low (8.76×10-7 mol-1 at 298 K) which, along with the
positive ∆퐺, suggests that the forward reaction is not
favourable. In the aqueous phase however, ∆퐺 of the reaction is
negative at low temperatures and the equilibrium constant is
relatively high (17.33 mol-1 at 298 K), which suggests that
HCHO formation is favourable in the aqueous phase. This is
because the heat of solution of HCHO is −62 kJ.mol-1,1 which
is also relatively high while the heat of solution of H2 and CO is
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Table 2: BET surface area, metal loading, and CO chemisorption data
Catalyst
Nominal Metal
Loading
(%w/w)
Actual
Metal
Loading*
(%w/w)
Ni-9.3
Ru-1.4
Ni-9.5
Pd-1.3
Total CO uptake Metal
BET Surface
(µmol.g-1)
Dispersion (%) area of the
Support
(m2.g-1)
Ru-Ni/Al2O3
Pd-Ni/Al2O3
Ni-10
Ru-1
Ni-10
Pd-1
41.96
26.50
2.33
1.47
108.51
insignificant in comparison. This results in the improvement of
the equilibrium conversion. The effect of pressure on the Gibbs
free energy of the reaction in the aqueous phase is shown in
Figure 3. The data presented in this Figure are calculated based
on the interpolation of experimental data presented by Oelkers
et al.28 It can be seen that as the operating pressure increases,
the Gibbs free energy of the reaction decreases. Although it is
concluded that higher pressures are favourable for this reaction,
the operating pressure in this study was limited to 100 bar by
the maximum available gas cylinder pressure.
Figure 3 Effect of pressure on the Gibbs free energy of the
reaction in the aqueous phase, T= 298 K28
As two moles of reactants are combining to form one mole of
the product, higher pressures favour the forward reaction,
according to Le Chatelier’s Principle and the previous studies.2,
10
The highest yield of HCHO (8.2×10-3 mmol.L-1.gcat-1) in
fixed bed gas phase reactor was obtained at 117 bar pressure at
293 K, which equates to conversion of only 1.02×10-4 %, which
is well below the equilibrium conversion. It was also confirmed
from the experiments, as shown in Figure 4b, higher
temperatures are unfavourable for the reaction in the gas phase
because the overall yield of HCHO reduced from 4.84×10-3
Figure 2 XRD patterns of calcined and reduced Pd-Ni/Al2O3 and
Ru-Ni/Al2O3
-1
-1
mmol.L-1.gcat to less than 2.12×10-3 mmol.L-1.gcat after 180
min reaction time.
It can be seen in Table 3, the equilibrium conversion of the
reaction is significantly higher in the aqueous phase, which is
due to low ∆퐺 of the components in the aqueous media.
Therefore, the reaction is thermodynamically feasible in the
liquid phase and can achieve sufficiently high equilibrium
conversion to make this process viable. The reaction is
therefore expected to be kinetically limited, which can be
improved by the use of an appropriate catalyst.
Slurry Reactor
Figure 5 presents the Arrhenius plot of the reaction in the
aqueous phase after the first two hours of operation. The
highest rate of reaction in the first two hours was 0.0378
µmol.L-1.s-1 at 403 K. The activation energy was calculated to
be 27.58 kJ.mol-1 for the Ru-Ni/Al2O3 as the catalyst. Figure 6
and Figure 7 show the effect of temperature on the yield of
HCHO as a function of time. Although the equilibrium constant
decreased with increasing temperature in the aqueous phase,
HCHO production rate increased which proves that the process
was kinetically limited, unlike the gas phase which was
thermodynamically limited. After 48 hours, it was observed that
HCHO yield was higher at 353 K and 373 K compared to 403
K.
Comparison of HCHO Production in Fixed Bed and Slurry
Reactors
Fixed Bed Reactor
Figure 4 shows the effect of pressure (Figure 4a) and temperature
(Figure 4b) on the molar yield of HCHO using Pd-Ni/ Al2O3 as the
catalyst in the gas phase reaction.
As shown in Figure 7, the highest level of the HCHO yield was
4.55 mmol.L-1.gcat-1 at 72 hours of operation at 353 K, which is
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Table 3 CO hydrogenation thermodynamic data
푪푶 + 푯ퟐ ↔ 푪푯ퟐ푶
Gas Phase
Aqueous Phase
ΔG
T
Xe
ΔG
K
Xe
K
K
%
kJ.mol-1
34.565
37.297
42.933
48.705
54.586
60.554
66.592
72.685
mol-1
%
kJ.mol-1
mol-1
298 4.38×10-3
323 4.67×10-3
373 4.88×10-3
423 4.86×10-3
473 4.70×10-3
523 4.49×10-3
573 4.26×10-3
623 4.03×10-3
8.760×10-7
9.348×10-7
9.764×10-7
9.711×10-7
9.405×10-7
8.983×10-7
8.523×10-7
8.068×10-7
31.40
-7.071
17.332
18.42
-5.648
8.186
3.86
-1.297
1.519
5.84×10-1
8.88×10-2
1.50×10-2
1.72×10-3
3.95×10-4
4.853
2.52×10-1
4.30×10-2
8.00×10-3
1.00×10-3
2.46×10-4
12.343
21.129
31.171
43.053
11
hydrogenation.10,
This conversion is also higher than the
theoretical equilibrium conversion because the equilibrium is
shifted when HCHO is dissolved in water. HCHO reacts with
water rapidly to form methylene glycol (CH2(OH)2).29 In the
solution, HCHO and CH2(OH)2 co-exists in dynamic
equilibrium with a HCHO/CH2(OH)2 ratio of 1:2499 at STP
and pH = 7,30 which means 99.96% of HCHO is converted to
CH2(OH)2. Therefore, HCHO produced in this method is
instantly absorbed in water, shifting the reaction equilibrium in
the forward direction, as shown below.
(a)
Formaldehyde
Methylene Glycol
Conversion of HCHO into CH2(OH)2 has been well studied in
31-35
the literature.29,
CH2(OH)2 also polymerizes to form
polyoxymethylene ((CH2O)n).33 However, the polymerization
reaction rate is much lower compared with the rate of hydration
and dehydration reactions and it can be inhibited by CH3OH.29
Based on the studies done by Whinkelman et al, the HCHO
hydration rate (kh) and the equilibrium constant for hydration
(Kh) are calculated to be as follows:34, 35
(b)
푘ℎ = 2.04 × 105 × 푒−2936
(8)
(9)
푇
3769−5.494
푇
퐾ℎ = 푒
It is demonstrated in Figure 7 that the HCHO yield peaked in
all cases, and that peak shifted towards lower time as the
temperature was increased. Therefore, higher temperatures
promoted the side reaction in which HCHO (and CH2(OH)2) is
consumed. The final product was checked for the presence of
other possible compounds such as ethylene glycol. No other
compound was detected in the liquid phase which indicates loss
of HCHO into the gas phase.
Figure 4: Effect of (a) pressure and (b) temperature on the
molar yield of HCHO in the fixed bed reactor using Pd-Ni/Al2O3
equal to turn over frequency (TOF) of 0.0602 h-1. The highest
conversion of soluble CO was 19.14% which is significantly
higher than the published works on gas phase CO
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Figure 5: Reaction constant based on the first 2 hour of the
reaction, P=100 bar, Catalyst: Ru-Ni/Al2O3
Figure 7: Effect of temperature on molar yield of HCHO in the
slurry reactor after 120 hours using Ru-Ni/Al2O3
Effect of the Stirring Speed on the Aqueous Phase Process
Effect of stirring speed was tested by varying RPM= 0, 400,
800, 1200 at 298 K at 100 bar using Ru-Ni/Al2O3. Figure 9
shows that the yield of HCHO increased from 0.26 mmol.L-
1.gcat-1 to 0.4 mmol.L-1.gcat-1 as the stirring rate increased from 0
RPM to 800 RPM. But further increasing the RPM did not
increase the yield of HCHO. There are many factors which
affects catalytic conversion in a slurry reaction. In a non-stirred
reactor the catalyst particles may settle at the bottom of the
reactor which makes the process diffusion limited. Stirring can
decrease the mass transfer limitation by increasing the
convective mass transfer coefficient and exposing the catalyst
surface to the dissolved gases. Increasing the mass transfer
coefficient increases the apparent global rate of reaction and
this was observed in Figure 9. However, once the rate of mass
transfer is sufficiently high, further increasing the stirring rate
Figure 6: Effect of temperature on the molar yield of HCHO in
the slurry reactor after 48 hours using Ru-Ni/Al2O3
has no impact on the global rate of reaction because the
39
reaction is kinetically controlled.38,
Therefore it can be
concluded that at 800 RPM the test was conducted in a
kinetically controlled regime.
At high temperatures and low pH values, the HCHO/CH2(OH)2
equilibrium shifts towards HCHO which may vaporise into the
gas phase.29 CH2(OH)2 is known to be very unstable in the gas
phase because it tends to dehydrate rapidly to HCHO and
water.36 Therefore, heating a solution of HCHO and CH2(OH)2
may lead to HCHO emission.29
Comparison of the HCHO yield in the fixed bed reactor and the
slurry reactor at identical operating conditions are presented in
Figure 8. The yield of HCHO in the slurry reactor at room
temperature was more than an order of magnitude higher
compared to the fixed bed reactor. The TOF of HCHO
production was higher for the Ru-Ni/Al2O3 than Pd-Ni/Al2O3
catalyst in the slurry reactor, which suggests that the former
catalyst is more active in the aqueous conditions.37 At room
temperature in the aqueous phase, TOF for Ru-Ni/Al2O3 was
0.0475 h-1 compared to 0.0319 h-1 for Pd-Ni/Al2O3.
Figure 8: Comparison between the results of the fixed bed
reactor and the slurry reactor at 293 K and 100 bar for a) Pd-
Ni/Al2O3 and b) Ru-Ni/Al2O3
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Fixed Bed Reactor
A schematic diagram of the fixed bed reactor setup is illustrated
in Figure 10. CO and H2 gases were mixed in 1:1 mole ratio (30
ml.min-1 each) using mass flow controllers in a gas manifold
prior to feeding it in the fixed bed reactor. The reactor was
Swagelok ¼” OD seamless tube in which 1 g of catalyst was
fixed using quartz wool. The reactor was heated by a tube
furnace which was used for in-situ reduction of the catalyst
prior to the test. A back pressure regulator controlled the
desired pressure upstream in the system. The gas stream leaving
the back pressure regulator was passed through a scrubber
containing 40 ml of 5 vol% CH3OH in water to recover HCHO.
CH3OH was used in order to prevent hydrated HCHO
molecules from polymerization. These runs were conducted at a
range of pressures (20bar, 40bar, 85bar, and 117bar) and
temperatures (293 K and 313 K). At the end of the run, the
HCHO concentration was measured using the photometric cell
test kit (Merck Millipore) in
a
DR 5000TM UV/VIS
Figure 9: Effect of the stirrer rotation speed on the molar yield
of HCHO using Ru-Ni/Al2O3
spectrophotometer (HACH Company, USA) at 575nm
wavelength using a chromotropic acid method.40 In this
method, 4.5 ml of 75% sulphuric acid (H2SO4) was measured in
a plastic tube. 0.1 g of chromotropic acid disodium salt
dihydrate (C10H10Na2O10S2) was added to the tube and was
shaken vigorously. 3 ml of the sample (diluted if required) was
added to the mixture. The intensity of the violet colour resulting
from the reaction was checked in the spectrophotometer and
quantified based on the pre-prepared calibration curve.
Experiments and Methods
Catalyst Preparation and Characterization
The catalysts used in this process were produced by the wet
impregnation method followed by calcination. Nickel nitrate
((NO3)2.6H2O,
Sigma-Aldrich),
ruthenium
chloride
(RuCl3.xH2O, Sigma-Aldrich) and palladium nitrate (10 wt%
Pd(NO3)2 in 10wt% nitric acid, Sigma-Aldrich) were used as
Ni, Ru and Pd precursors, respectively. The desirable amounts
of Ni and noble metal precursors were added simultaneously to
commercial γ-Al2O3 suspended in 20 ml water. The mixture
was stirred at 333 K for 6 h. The suspension was then dried at
373 K overnight. The dried catalyst was calcined at 873 K for 6
h.
Fixed bed reactor tests were started by reducing the catalyst in-
situ prior to the experiment by flowing 50 ml.min-1 of H2
through the catalyst bed at 673 K for 5 h followed by purging
with Ar at 673 K for 1 h. The catalyst bed was cooled to room
temperature under Ar flow overnight. For the slurry reactor
tests, the same procedure of catalyst reduction was carried out
ex-situ.
Figure 10: Schematic diagram of the fixed bed reactor setup
Slurry Reactor
BET surface area of the support was measured by the N2
physisorption method in Micrometric ASAP2020 at 77 K. CO
chemisorption was used to measure the amount of active sites
of each catalyst and the metal dispersion percentage using
ASAP2020 (Micrometrics). XRF was used to determine the
actual mass percentage of the metals using an Ametek Spectro
iQ II XRF. XRD patterns of fresh calcined and reduced
catalysts were recorded using a REGAKU MiniFlex 600 X-ray
diffraction instrument equipped with a Ni-filtered Cu Kα
radiation in order to study the situation of the catalyst before
and after reduction. XRD patterns were gained for 2θ between
20° to 80° using step size of 0.01. TEM was used to evaluate
the particle size of each promoter (Pd and Ru) in the fresh
calcined catalysts and ImageJ 1.48 (National Institutes of
Health) was used to generate the particle size distribution.
The slurry rector, illustrated in Figure 11, was charged with 40
ml of 5 vol% CH3OH in water. 1 g of the desired catalyst was
reduced ex-situ before adding it to the reactor. The reactor was
subsequently pressurized up to the desired pressure with
equimolar mixture of CO and H2. Subsequently the reactor was
heated to the desired temperature (293 K, 333 K, 353 K, 373 K,
or 403 K). During the run, liquid samples were collected at
regular intervals through the dip tube. HCHO concentration in
the liquid samples was measured using a photometric cell test
kit (Merck Millipore) in
a
DR 5000TM UV/VIS
spectrophotometer (HACH Company, USA) at 575nm
wavelength. Alternatively, the concentration was measured
using
a
FluoroQuik fluorimeter version 4.3.A using
360nm/490nm as the excitation/emission wavelengths. In this
method, a working reagent was prepared by addition of
acetoacetanilide (C10H11NO2) solution and ammonium acetate
(C2H3O2NH4) solution (Amiscience Corporation), as described
elsewhere.41 50 µl of the working reagent was mixed with 50 µl
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of each sample and the mixtures were incubated in dark for 30
min before measuring the fluorescence intensity. The HCHO
concentration was quantified based on the calibration curve
provided.
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Conclusions
In this study, the direct formaldehyde (HCHO) production from
synthesis gas in a slurry reactor is reported for the first time.
Thermodynamic investigation showed that CO hydrogenation
in the gas phase is limited by positive Gibbs free energy (∆퐺) at
all temperatures above 298 K, whereas in the aqueous phase the
reaction is thermodymically favourable because the ∆퐺 is
negative below 383 K. This resulted in low yield of HCHO in
the fixed bed reactor, and significantly higher yield in the slurry
reactor (8.25×10-3 and 8.48×10-2 mmol.L-1.gcat-1, respectively, at
298 K in 4 h using Pd-Ni/Al2O3). The HCHO yield reduced
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The highest yield of the HCHO was 4.55 mmol.L-1.gcat at 353
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methylene glycol which shifts the equilibrium of CO
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as solvent in a low temperature reaction, this method is greener
than the current HCHO production methods. Synthesis gas and
methanol may both be produced from biomass conversion
technologies, which will offer environmentally friendly route
for HCHO production. Currently, low solubility of CO and H2
in water is one of limitations of this method; however,
solubility of these reactants may be improved with the use of
other solvents in future studies.
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Discovery of a low temperature route to produce formaldehyde via catalytic hydrogenation of
carbon monoxide in aqueous phase

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