Canadian Journal of Chemistry
Efficient Reduction of Formic Acid to Formaldehyde by Zinc
Journal: Canadian Journal of Chemistry
Manuscript ID cjc-2018-0284.R1
Manuscript Type: Article
Date Submitted by the 29-Aug-2018 Author:
Complete List of Authors: Alderman, Nicholas; University of Ottawa, Department of Chemistry Peneau, Virginie; University of Ottawa, Department of Chemistry Viasus, Camilo; University of Ottawa, Department of Chemistry Korobkov, Ilia; SABIC CDR Centre at KAUST, Advanced Catalysis Vidjayacoumar, Balamurugan; SABIC CDR Centre at KAUST, Advanced Catalysis Draft Albahily, Khalid; SABIC CDR Centre at KAUST, Advanced Catalysis Gambarotta, Sandro; University of Ottawa, Department of Chemistry
Is the invited manuscript for consideration in a Special Not applicable (regular submission) Issue?:
Keyword: Formic Acid Reduction, Formaldehyde, Methanol
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Efficient Reduction of Formic Acid to Formaldehyde by Zinc
Nicholas P. Alderman,a Virginie Peneau,a Camilo J. Viasus,a Ilia Korobkov,b Balamurugan Vidjayacoumar,b Khalid Albahily b and Sandro Gambarottaa,*
a Department of Chemistry, University of Ottawa, Ottawa, Ontario, K1N 6N5, Canada b Advanced Catalysis SABIC CDR Centre at KAUST, Thuwal, 23955, Saudi Arabia * Corresponding author: [email protected], 613-562-5800 ext. 2849
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Abstract
The possibility of thermally reducing formic acid to formaldehyde selectively has been probed using metallic zinc. Good selectivity (over 80%) was obtained with low concentrations of formic acid, with methanol and methyl formate as secondary products. The selectivity can be tuned by changing the carrier gas flow, temperature and zinc amount. Zinc was oxidised to zinc oxide during this process.
Keywords: Formic Acid Reduction, Formaldehyde, Methanol
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1. Introduction
Formaldehyde is a commonly used industrial reagent for numerous materials such as plastics and resins,
with a production capacity of over 30 million tons per year.1 We have previously shown that formaldehyde
can be also used as a hydrogen storage molecule, efficiently generating formate/formic acid upon
dehydrogenation (Figure 1).2 We have recently reported a photochemical process for the reduction of
formate back to formaldehyde using Bi2WO6 and water as hydrogen source. However, the yields in this
process are very low, in the order of a few micromoles. For the formaldehyde/formate shuttle to be a viable
hydrogen storage cycle, a faster, highly selective process with good yields needs to be found for the
reduction of formate/formic acid to formaldehyde.
The synthesis of formaldehyde industrially is achieved via the Formox process, where methanol is
reacted with oxygen at 250 to 400oC using a silver molybdenum/vanadium catalyst on iron oxide.[3–6]
Methanol is in turn produced from methane via syngas.7–10
Currently very few methods to produceDraft formaldehyde from formic acid or carbon dioxide exist,
likely due to the similar reduction potential of formic acid/carbon dioxide to methanol.11–16 This leads to a
further reduction of formaldehyde to methanol with consequent loss of reaction selectivity. Methanol can be
produced in good yields from both formic acid and carbon dioxide using a hydrothermal reactor with zinc
as the sacrificial agent and metallic copper catalysts.17 In this reaction, however, formaldehyde is not
produced in any significant amount. A likely reason for this is due to the long contact time of the catalyst
with any intermediate products that could form, as well as the decomposition of any formaldehyde
produced inside the batch reactor over long reaction durations.
Formic acid can also be produced from carbon dioxide and water with low overpotentials and high
faradaic efficiencies electrochemically, with current efficiencies that exceed 95% being reported.16,18–24 The
electrochemically produced formic acid can then be used to produce methanol, dimethyl ether and other
products.25 Photochemically, carbon dioxide has been reduced to formic acid using ruthenium based
catalysts and visible light, with over 500 turn overs being reported.26 Finally, formaldehyde has been
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disproportionated into methanol and formic acid, methanol and carbonic acid or glycolic acid
hydrothermally.27
We have now investigated a system to produce formaldehyde in high selectivity from formic acid,
using zinc as a sacrificial agent (forming zinc oxide).
CHOOH + Zn ZnO + CH2O
Metallic zinc is a convenient co-reactant since it can be reformed from zinc oxide via a number of
different methods, such as electrochemical, reacting thermally with hydrogen, carbon/carbon dioxide or
methane, as well as the direct solar thermal dissociation of zinc oxide.25–30 Zinc has also been show to act as
a reducing agent for many common organic functional groups.34
For these studies, we decided upon using flow chemistry for the reaction of zinc with formic acid,
due to previous literature work suggesting that no formaldehyde would be produced in batch reactors. This
is could be due to the batch reactor being above the decomposition temperature of formaldehyde (>150oC),
which results in the formation of CO2, CO, MethanolDraft and H2.
2. Experimental
Zinc (30 or 325 mesh) was purchased for Strem Chemicals. Formic acid, acetyl acetone, ammonium acetate and glacial acetic acid were purchased from Sigma-Aldrich. All chemicals were used as-supplied with no
further purification.
Analytical equipment. Powder XRD diffractograms were obtained on a Rigaku Ultima IV diffractometer
set to 2 2θ/min from 10-70 2θ.
Product analysis. Methanol and formic acid determination was performed with an Agilent 7820A GC with
a flame ionization detector (FID), using a Restek Rt-U-Bond column and helium carrier gas. Gas products
were analysed by an Agilent 7820A GC with a thermal conductivity detector (TCD), using a CO2-
HR/Select gasses column.
Reduction of formic acid. In a typical experiment, Zn (30/325 mesh, 30-153 mmoles) was placed in a 1cm
diameter, 50 cm long Pyrex tube and purged with argon and heated to the desired temperature (250-400oC).
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Formic acid (10-100%, v/v) was placed in a syringe, and injected at a rate of 10 mL/hr into the tube with
argon as a carrier gas (5-100 mL/minute). The liquid products collected in a cooled water trap (10 mL).
Determination of formaldehyde concentration. Formaldehyde concentrations were determined through a
colorimetric reaction with acetyl acetone.35 To a solution of ammonium acetate (15.4 g) in water (50 mL),
acetyl acetone (0.2 mL) and glacial acetic acid (0.3 mL) were added whilst stirring. This was further diluted
with water (49.5 mL) and stored in the fridge for up to 3 days. To determine the formaldehyde
concentration, 2 mL of the sample were mixed with 2 mL of the acetyl acetone solution and heated to 60oC
for 10 minutes. After cooling for 10 minutes, the absorbance of the solution was measured at 412 nm and
compared to a calibration curve.
3. Results and Discussion
Firstly, the effect of temperature on formic acidDraft reduction with zinc was examined (Figure 2). Temperatures between 250 and 400oC were used, with 350oC being found to be the optimum temperature. At
temperatures below 250oC, no significant amount of organic products were detected. At higher
temperatures, the yield is lowered due to the decomposition of the reactants and products into hydrogen and
carbon dioxide. The selectivity for formaldehyde increases between 250 and 300oC, with methanol being
preferentially formed at lower temperature. However, at the optimum temperature the ratio of formaldehyde
to methanol remains constant with around 80% formaldehyde production.
In all of these cases, the zinc is converted into zinc oxide (determined by X-ray diffraction,
Supplementary Figure S1). We speculate that this could be a two-step process to generate formaldehyde and
zinc oxide. Firstly, zinc reacts with formic acid to generate zinc formate. This zinc formate decomposes in-
situ to generate formaldehyde and zinc oxide. To try to confirm this hypothesis, decomposition experiments
with zinc formate were attempted at 325oC. Upon heating the zinc formate, both formaldehyde and zinc
oxide were formed (Supplementary Table S1), confirming that this could be a plausible reaction pathway.
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As the amount of zinc is augmented (Figure 3), both the yield of products and selectivity towards formaldehyde increases. Although the yield increases with the amount of zinc, the increment was not linear.
Using larger amounts of zinc ensures full consumption of formic acid through an increased contact time between reagents, with less formic acid simply passing through the tube. In all of the reactions shown, zinc is in excess when compared to formic acid so that none of the reactions are limited by the amount of zinc used. In all cases, the selectivity for formaldehyde remains high, at approximately 80% (with methanol as the remaining product).
When the carrier gas flow is increased, both the product yield and formaldehyde selectivity decreases (Figure 4).
For maximum conversion of formic acid to formaldehyde, a low carrier gas rate is required to maximize the
interaction between zinc and formic acid. At higher flow rates, methanol is the dominant product. It is interesting to
note that by changing the contact time by changing the amount of zinc, the selectivity towards formaldehyde did not
change. When changing the contact time by changing the argon flow rate, the formaldehyde selectivity changes from over 95% to under 40%. This could be dueDraft to numerous reasons, including lower gas temperature or lower time of the reagents in the furnace. The mass balance of this reaction is shown in Table 1. When the argon flow rate
is increased, a larger portion of the formic acid passes through the tube without reacting, resulting in a lower
formaldehyde yield. However, less decomposition of formic acid to hydrogen and carbon dioxide is observed at
higher flow rates. To maximise the yield, both of these factors should be considered. In all reactions, no carbon
monoxide was observed.
Increasing the formic acid concentration also increases the product yield, as expected. At formic acid
concentrations above 10%, methyl formate starts to be produced reducing the yield of methanol/formaldehyde
(Figure 5). This is likely due to the reaction of the formed methanol with formic acid at high concentrations and
temperatures. At 100% formic acid, methanol formation is substantially increased, which shows that water might
play a part in the formaldehyde production. The highest production and selectivity for formaldehyde is reached with
a 10% concentration of formic acid, where approximately 80% selectivity is observed. A summary of the best results
are shown in Table 2.
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4. Conclusions
Formic acid has been successfully reduced to formaldehyde, methanol and methyl formate. Zinc has been
successfully used in a flow reactor, where it is oxidized to zinc (II) oxide. Good selectivity of formaldehyde
was achieved using a low carrier gas flow rate with a large amount of zinc and a 10% solution of formic
acid in water. For this system, the optimal temperature is 350oC. At temperatures below this, the reduction
of formic acid is slow, whilst above this temperature the decomposition of formic acid, formaldehyde and
methanol reduces the yield.
Acknowledgements
We wish to thank the Saudi Arabia Basic Industries Corporation (SABIC) for funding this project.
References Draft
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Tables
Table 1 - Products formed for the reaction of formic acid with zinc with different argon flow rates
In Out (mmoles)
Ar flow Formic Formic Methanol Formaldehyde Hydrogen Carbon Carbon rate Acid in Acid Dioxide Monoxide (mL/min) (mmoles)
5 79 23 0.021 1.44 53.5 52.1 0
25 79 35 0.082 0.41 34.3 41.3 0
50 79 47 0.104 0.16 22.3 28.5 0
100 79 65 0.079 0.04 15.4 15.2 0
Table 2 - Summary of the best conversion efficiencies for formic acid to formaldehyde using zinc. Formic acid was used as a 10% solution in water, with a Draftflow rate of 10 mL/hr for 100 minutes Ar Flow Zn pFA MeOH (mL/min) (mmoles) produced produced (mmoles) (mmoles) 5 31 0.57 0.13 5 76 0.47 0.13 5 153 3.30 0.14 10 153 2.71 0.40 25 153 1.44 0.22
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Figure Captions
Figure 1 – Our previous and current work on hydrogen storage using formaldehyde
Figure 2 - The conversion of formic acid to formaldehyde and methanol under different reaction temperatures using Zn (31 mmoles), Formic Acid (79 mmoles), Argon (25 mL/min) for 3 hrs
Figure 3 - The role of changing zinc mass using Formic Acid (79 mmoles), Argon (25 mL/min) at 350oC for 3 hours
Figure 4 - The role of changing the argon flow rate using Zn (155 mmoles), Formic Acid (79 mmoles), at 350oC for 3 hours
Figure 5 - Optimisation of the formic acid concentration using Zn (31 mmoles), Argon flow of 5mL/ minute at 350oC for 3 hours
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H H2O, NaOH O Na4Fe(CN)6.10H2O H Previous work
H2
ZnO
This work O
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ONa Zn
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Figure 2 - The conversion of formic acid to formaldehyde and methanol under different reaction temperatures using Zn (31 mmoles), Formic Acid (79 mmoles), Argon (25 mL/min) for 3 hrs
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Figure 3 - The role of changing zinc mass using Formic Acid (79 mmoles), Argon (25 mL/min) at 350oC for 3 hours
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Figure 4 - The role of changing the argon flow rate using Zn (155 mmoles), Formic Acid (79 mmoles), at 350oC for 3 hours
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Figure 5 - Optimisation of the formic acid concentration using Zn (31 mmoles), Argon flow of 5mL/ minute at 350oC for 3 hours
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Graphic Abstract
Zn ZnO O O
250-400oC H OH H H
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