Hydrogenation of Carbon Monoxide into Formaldehyde in Liquid Media

Research Article pubs.acs.org/journal/ascecg

Hydrogenation of Carbon Monoxide into Formaldehyde in Liquid Media Ali Mohammad Bahmanpour,† Andrew Hoadley,† Samir H. Mushrif,‡ and Akshat Tanksale*,† †

Department of Chemical Engineering, Monash University, 18 Alliance Lane, Clayton, VIC 3800, Australia School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, Singapore 637459, Singapore

‡

S Supporting Information *

ABSTRACT: Formaldehyde is a bulk chemical which is produced in excess of 30 million tons per annum and is growing in demand. However, the current production process requires methanol production, which is oxidized in air to produce formaldehyde, which must then be absorbed into water. Our recent work introduced a novel method to produce formaldehyde through CO hydrogenation in the aqueous phase. However, the aqueous phase has certain limitations which must be overcome to make it commercially viable. By applying a deuterium labeling technique and investigating the potential intermediates, the reaction mechanism was established which showed that solvents play a vital role in determining the yield. Various solvents were used for formaldehyde production, and the highest formaldehyde yield was achieved by using pure methanol followed by methanol−water mixtures. Formaldehyde reacts with methanol and water to produce hemiacetal and methylene glycol, respectively, thereby shifting the equilibrium of CO hydrogenation toward formaldehyde production. Methanol and water stabilize the hemiacetal and methylene glycol molecules, respectively, via hydrogen bonding. The highest yield of formaldehyde in methanol solvent was found to be 15.58 mmol L−1 gcat−1 at 363 K and 100 bar, which is four times higher than our previous report. The liquid phase method shown here has the potential to be greener and more sustainable than the commercial processes because it operates at low temperatures and results in 100% selectivity toward formaldehyde with no CO2 generation. KEYWORDS: Formaldehyde production, Solvent effects, CO hydrogenation, Reaction mechanism

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INTRODUCTION Due to the increasing global energy demand, many researchers have focused on reduction of energy consumption in the chemical industry. It is essential to search for more efficient, energy saving processes for the production of valuable chemicals. Many aspects can be considered and evaluated during process optimization. Increasing the conversion of reactants and product yield are often considered; however, process alternatives to save energy and capital costs are sometimes overlooked by researchers. Formaldehyde (HCHO) industrial production through methanol (CH3OH) partial oxidation dates back to 1882.1 Since then, other methods of production such as methane (CH4) partial oxidation2−8 or CO2 hydrogenation9,10 were considered, but none were able to compete with the high conversion achieved by the methanol oxidation process. However, despite the high conversion of CH3OH and high selectivity toward HCHO, the industrial methods of HCHO production suffer from CO2 generation as a byproduct1 and a high energy consumption rate which leads to low exergy efficiency (43.2%).11 A novel HCHO production method through direct CO hydrogenation in the aqueous phase using Ru−Ni/Al2O3 was recently introduced, which could © XXXX American Chemical Society

potentially increase the exergy efficiency with no CO 2 production,12 as a result of bypassing the production of methanol and also the much lower reaction temperatures. However, it was not clear how the reaction proceeds in the aqueous phase. Although the most likely pathway is that the dissolved synthesis gas is adsorbed on the surface of the catalyst and reacts to form HCHO, there are other possible reaction pathways. This study investigates the reaction pathway and the effect of solvents on the HCHO yield. Based on the source of hydrogen atoms forming the HCHO molecule, different reaction pathways are possible. Four possible reaction pathways were considered for the CO hydrogenation reaction: CO + H 2 ↔ HCHO

(1)

These reaction pathways are shown in Figure 1. Reaction pathway 1 has been shown to be feasible in the gas phase according to the DFT studies on CO hydrogenation to CH3OH.13,14 It has been shown that HCHO can form as an Received: April 21, 2016 Revised: May 20, 2016

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DOI: 10.1021/acssuschemeng.6b00837 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

soluble in nonpolar solvents, it is known that polar solvents may enhance the adsorption of nonpolar reactants on the catalyst surface.17,23 Interaction between catalyst supports and solvents can also affect the reaction. An interesting example of this case is the effect of hydrophobic and hydrophilic catalyst supports on hydrogenation reaction in two phase liquid mixtures. Hydrogenation of cyclohexene (C6H10) and cyclohexanone (C6H10O) in water/cyclohexane mixture showed that using hydrophobic catalyst support (such as activated carbon) favors the hydrogenation of C6H10 whereas by using hydrophilic catalyst support (such as SiO2) favors the hydrogenation of C6H10O.24 Effect of cosolvents on the molecular structure of the solvation medium is found to have an impact on the selectivity toward the products. Molecular dynamic simulation has shown that glucose selective conversion into 5-hydroxymethyl furfural (HMF) is enhanced by using cosolvents (dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), and N,N-dimethylformamide (DMF)) due to the arrangement of cosolvent molecules around the H atom of glucose hydroxyl groups.25 It is therefore concluded from literature review that solvents may have complex effects on the conversion and selectivity of the hydrogenation reactions. Different types of solvents including polar protic (water, methanol, and ethanol), polar aprotic (DMSO), and nonpolar (C7H8) solvents were used in the present study.

Figure 1. Potential reaction mechanisms of CO hydrogenation reaction to CH2(OH)2.

intermediate on the surface of the catalyst but is further hydrogenated into CH3OH. Hydrogenation of HCOOH into HCHO (pathway 2) has also been shown to be feasible in previous studies.15 It is shown that CO is adsorbed on the catalyst surface and hydrates to form COOH*. The carbon atom in COOH* can receive a proton before separation of the hydroxyl group from the carbonyl side. Pathways 3 and 4 were not discussed before, but they are also considered in this study to cover various possible ways of producing HCHO (or methylene glycol, CH2(OH)2). CH3OH dehydrogenation or partial oxidation to HCHO is highly unlikely due to the low temperature range used and the presence of H2 in the reactor. To identify the actual pathway, the source of H atoms forming the HCHO molecules may be studied by deuterium labeling, and the various HPLC detectable intermediates may be studied. Table 1 presents the possible products of each experiment based on the four possible mechanisms. The effect of solvents on hydrogenation reactions have been studied in detail.16−22 The solvent effects have been shown to be significant in a similar study of CO2 hydrogenation into formic acid (HCOOH) in aqueous phase.20 It was shown that the HCOOH yield increased by approximately an order of magnitude by using a dimethyl sulfoxide (DMSO)−water solution compared to pure water due to the higher activity of the catalyst in DMSO. Liquid phase hydrogenation reaction of toluene (C7H8) on Ni/Al2O3 showed higher rate using isooctane ((CH3)3CCH2CH(CH3)2) and n-heptane (C7H16) compared with cyclohexane (C6H12) due to 40% lower hydrogen solubility in C6H12 compared with other solvents.22 However, solvent effects on hydrogenation reactions are not always dependent on solubility only. Other parameters such as solvent polarity, catalyst−solvent interactions, and solvent molecular structure may affect the reaction. Although nonpolar reactants (such as H2 in the hydrogenation reactions) are more

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EXPERIMENTAL SECTION

The methods of catalyst synthesis, reduction, and characterization have been described before.12 Detection of HCHO derivatives and other potential intermediate products were done in HPLC (Agilent) by using RHM-Monosaccharide column (Phenomenex), using a refractive index detector (RID). HCHO solution (formalin solution, SigmaAldrich) was used as standard by diluting it to 0.8, 1.6, 2.5, and 3.3 mmol L−1 to generate a calibration curve for the detection of methylene glycol (CH2(OH)2), which is the hydrated form of HCHO. A 10 μL portion of each solution was injected into the column using milli-Q water as the carrier phase. Then, 0.5 mL min−1 of carrier phase was used, and the column was kept at 338 K. The RID temperature was kept constant at 318 K. The chromatograms (Figure S1) and standard calibration curve (Figure S2) are presented in the Supporting Information. D2 (Sigma-Aldrich), D2O, and CD3OD (Merck Millipore) were used in the batch reactor for the detection of the source of hydrogen transfer in HCHO production. Liquid samples before and during the experiment were collected from the batch reactor in a sealed vial and were heated to 338 K for 10 min. The headspace of each vial was injected into an injection port situated in the Ar carrier gas line of a custom-made rig. Figure 2 shows the schematic diagram of the rig equipped with a Residual Gas Analyzer (RGA-300, Stanford Research Systems), which is a quadrupole mass spectrometer. The carrier gas flow rate (70 mL min−1) was controlled via a mass flow controller and after the injection port the gas line was connected to a vacuum chamber via a capillary tube. The gas samples were checked on stream by RGA-300 which measured the partial pressures of the gas components passing through the vacuum chamber, which was plotted against time. The RGA was programmed to monitor up to 10 values of

Table 1. Possible Intermediate and Final Products Based on Feedstock and Possible Mechanisms productsa

a

feed

solvent

pathway 1

pathway 2

pathway 3

pathway 4

H2 and CO D2 and CO H2 and CO

5% CH3OH in H2O 5% CH3OH in H2O 5% CD3OD in D2O

HCHO (Nil) DCDO (Nil) HCHO (Nil)

HCHO (HCOOH) HCHO (HCOOH) DCDO (DCOOD)

CH2(OH)2 (Nil) CHDO2HD (Nil) CHDO2HD (Nil)

HCHO (CH3OCOH) HCDO (CH3OCOH or CH3OCOD) HCDO (CD3OCOH or CD3OCOD)

Molecules in the parentheses are HPLC detectable intermediates. B


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in the reaction with 5% v/v CH3OH in H2O as the solvent at T = 353 K and P = 100 bar (equimolar CO:D2). Figure 3 shows the result of samples collected at 24, 30, and 48 h. The m/z ratios of 29, 30, and 31 were selected to monitor

Figure 2. Schematic diagram of the custom-made setup equipped with quadrupole mass spectrometer (RGA 300). mass to charge ratios (m/z). After studying the standard spectra of each gas component in the mixture, one m/z value with high intensity and no overlap with other gas components was chosen. To reduce the random noise in the raw data, a smoothing function was applied. Figure S3 in the Supporting Information shows an example of the raw data superimposed with smoothed curve which shows that the signalto-noise ratio was acceptable to eliminate errors. The HCHO concentration in the liquid phase was measured using a FluoroQuik fluorometer version 4.3.A using 360 nm/490 nm as the excitation/emission wavelengths. In this method, a working reagent was prepared by addition of 22 μL of acetoacetanilide (C10H11NO2) and DMSO solution and 33 μL of ammonium acetate (C2H3O2NH4) solution (Amiscience Corporation). A 50 μL portion of the working reagent was mixed with 50 μL of each sample, and the mixtures were incubated in the dark for 30 min before measuring the fluorescence intensity. The accuracy of the HCHO yield measurement is in the range of ±4%. This method of HCHO detection and quantification is used based on previous studies in the literature.26 The condensed phase simulation of hemiacetal in methanol molecules at room temperature was performed using CPMD package which provides an implementation of the density functional theory (DFT) using Car−Parrinello molecular dynamics scheme.27 The DFT calculations were performed using the planewave−pseudopotential implementation of the Kohn−Sham formulation of DFT.28 The Troullier−Martins pseudopotential29 with the Perdew−Burke− Ernzerhof generalized gradient approximation30 was used. Only the Γ-point was used for integration over the Brillouin zone in reciprocal space. An energy cutoff of 125 ryd proved sufficient to achieve energy convergence. Temperature control was achieved using the Nosé− Hoover thermostat. The frequency for the ionic thermostat was set to 1800 cm−1 (characteristic of a C−C bond vibration frequency) and for the electron thermostat to 10000 cm−1. The fictitious electron mass was set equal to 600 au. The system was first equilibrated for 1 ps, and then, a production run was performed. Energies, including the fictitious electronic kinetic energy, were monitored to ascertain that the system did not deviate from the Born−Oppenheimer surface during the molecular dynamics simulation. Molecular dynamics trajectories were visualized using the VMD software.31

Figure 3. Mass to charge ratio (m/z) profiles for HCHO (29), DCDO (30), and CH3OH (31) using CO and D2 as reactants in 5% v/v CH3OH in H2O as the solvent. “Control” is defined as the sample at t = 0 h.

the presence of HCHO, DCDO (or DCHO), and CH3OH, respectively. No peak was observed in the control solution (sample at t = 0 h) which is expected since it did not contain any formaldehyde (HCHO, DCHO, or DCDO). Samples taken at 24, 30, and 48 h showed peaks at m/z = 30, corresponding to DCDO or DCHO, whereas no peak was observed at m/z = 29 (HCHO). This confirms the hypothesis of reaction pathway 1. The absence of a peak at m/z = 29 rules out the possibility of formation of HCHO, which meant that the source of hydrogen was not from water (Pathways 2 and 3) or methanol (Pathway 4), further confirming the HPLC results. Therefore, the hydrogenation of CO proceeded via transfer of adsorbed D atoms only, which was in the gas phase. Formation of side products such as D2O (m/z = 20) and CD3OD (m/z = 34) were also observed (Figure 4). No peaks were observed at these m/z ratios which shows that neither D2O nor CD3OD were produced in this reaction. This suggests that in the presence of CH3OH in the reaction mixture HCHO would not hydrogenate further to form CH3OH since the equilibrium is not favorable. To confirm the results of the deuterium labeling in the gas phase, 5% v/v CD3OD in D2O was used as the solvent with H2 gas in the reactor. In this experiment, the headspace of the control solution was injected at 93 s, and the samples were injected at 332, 650, and 936 s. The m/z ratios of 29, 30, and 31 were selected to monitor the presence of HCHO, DCDO (or DCHO), and CH3OH, respectively. As seen in Figure 5, HCHO peak at m/z = 29 is observed for the samples taken after 48, 30 and 24 h reaction time. This is in agreement with the results achieved in the D2 labeled experiment. DCDO (or DCHO) peak at m/z = 30 was not observed in these samples which indicate that hydrogenation of CO did not take place via deuterium transfer from D2O or CD3OD. Figure 5 also shows that CH3OH peak at m/z = 31 is not observed which indicate that HCHO did not go through further hydrogenation to form

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RESULTS AND DISCUSSION Investigation of the Reaction Pathway. Figure S4 in the Supporting Information shows the chromatograms of samples taken from the reaction at regular interval, and it can be seen that no intermediate was detected. Therefore, it may be concluded that pathways 2 and 4 are not probable. To study the hypothesized reaction pathway 1, H2 gas was replaced with D2 C


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Figure 6. Effect of the presence of CH3OH in the reaction mixture on the molar yield of HCHO. Figure 4. Mass to charge ratio (m/z) profiles for D2O (20) and CD3OD (34) using CO and D2 as reactants in 5% v/v CH3OH in H2O as the solvent. “Control” is defined as the sample at t = 0 h.

HCHO into CH3OH in the absence of CH3OH. The liquid samples collected at 6, 48, and 72 h of the run were analyzed in the HPLC which confirmed CH3OH production. The CH3OH yield in these samples is shown on the right y-axis in Figure 6. In the absence of CH3OH as a solvent, the molar yield of CH3OH is three times higher than the molar yield of HCHO in the presence of 5% v/v CH3OH/H2O solution. The source of HCHO hydrogenation was confirmed by D2 labeling in the gas phase. The samples collected at 5, 48, and 72 h reaction times were tested in the custom-made rig. Figure 7 shows that the control sample does not have any peak; whereas, peaks at m/z = 34, corresponding to CD3OD, were observed for all the other samples.

Figure 5. Mass to charge ratio (m/z) profiles for HCHO (29), DCDO (30), and CH3OH (31) using CO and H2 as reactants 5% v/v CD3OD in D2O as the solvent. “Control” is defined as the sample at t = 0 h.

CH3OH. The results presented in Figure 5 rule out the possibility of HCHO formation through CD3OD dehydrogenation since DCDO is not formed. On the basis of the above results, it is concluded that no intermediate product is formed during CO hydrogenation to produce CH2(OH)2 and the source of H atoms for hydrogenation is gaseous H2 in the reactor. Therefore, the hypothesized reaction pathway 1 is the most likely pathway. It is proposed that CO and H2 gas molecules are dissolved in the solvent. The dissolved gases are adsorbed on the surface of the catalyst. While CO is adsorbed in its molecular form, H2 adsorbs dissociatively.14 In the next step, adsorbed H* attaches to the adsorbed CO forming HCHO which desorbs from the surface of the catalyst and hydrates to form CH2(OH)2. Reaction Pathway in the Absence of CH3OH as Solvent. The comparison of the HCHO yield in the presence and absence of CH3OH as a solvent can be seen in Figure 6. The HCHO yield in the absence of CH3OH is significantly lower than the yield in the presence of 5% v/v CH3OH/water solution, which may be due to hydrogenation of the formed

Figure 7. Mass to charge ratio (m/z) profiles for DCDO (30) and CD3OD (34) using CO and D2 as reactants in H2O as the solvent. “Control” is defined as the sample at t = 0 h.

This confirms the hypothesis that in the absence of CH3OH as a solvent, HCHO is hydrogenated further into CH3OH. The peaks at m/z = 30, corresponding to DCDO (or DCHO) shows that at lower reaction time there is no heavy formaldehyde produced but heavy methanol is produce. Once sufficient heavy methanol is produced in the reaction, the yield of heavy formaldehyde is achieved. D


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ACS Sustainable Chemistry & Engineering Therefore, the reaction pathway in this process depends on the presence of CH3OH in the initial reaction mixture. The pathway for this reaction is presented in Figure 8.

Figure 8. Reaction pathway for CO hydrogenation reaction into HCHO in aqueous phase. Figure 10. Effect of temperature on the molar yield of HCHO, solvent: CH3OH, P = 100 bar.

Effect of Solvents on HCHO Yield. Effect of solvents on the yield of HCHO is shown in Figure 9. Highest HCHO yield

increased with temperature, which is in agreement with the results reported previously.12 Figure 11 shows the Arrhenius plot based on the rate of reaction calculated after 5 h, assuming first order batch reactor

Figure 9. Effect of using different solvents on HCHO yield, T = 353 K, P = 100 bar. Figure 11. Arrhenius plot based on the rate of reaction calculated at t = 5 h, solvent: CH3OH, P = 100 bar.

is achieved by using pure CH3OH, followed by 5%v/v CH3OH/water solution. The rapid reaction of HCHO with CH3OH and water to form hemiacetal (also known as methoxymethanol, C 2 H 6 O 2 ) and methylene glycol (CH2(OH)2), respectively, shifts the CO hydrogenation reaction equilibrium toward HCHO side which results in higher HCHO yield compared with using other solvents.12 Using pure CH3OH as opposed to 5%v/v CH3OH/water solution potentially simplifies the downstream separation process since reactive distillation will not be needed to separate CH3OH from water. Higher yield of HCHO in pure CH3OH solvent may be due to the higher solubility of CO and H2 in CH3OH compared to water. However, the CO and H2 solubility is also very high in C2H5OH, but the yield of HCHO in this solvent was the lowest.32,33 Therefore, it may be concluded that the HCHO yield is not only a function of reactant solubility but also the HCHO reactivity and stability in the solvent. Figure 10 shows the result of HCHO yield at 353, 363, 373, and 403 K using pure CH3OH as a solvent. From the initial slope of the curves it can be seen that the rate of reaction

kinetics. The rate of formation is calculated based on the yield of HCHO per unit time. The apparent activation energy (Ea) is calculated to be 8.17 kJ/mol. The highest yield was 15.58 mmol L−1 gcat−1, which was achieved at 363 K and 100 h. However, as it can be seen in Figure 10, the yield approaches an equilibrium value at each temperature at long residence times. Moreover, the equilibrium yield reduces as the temperature increases, due to the lower solubility of HCHO, CO, and H2 in the solvent as the temperature increases. Figure 12 shows the effect of the composition of water− CH3OH mixture on the yield of HCHO. It is evident that using pure CH3OH as a solvent results in the highest HCHO yield and the addition of water to CH3OH in any composition decreases the HCHO yield. Moreover, from Figure 12 an interesting trend is observed which shows that the yield of HCHO at t = 48 h is higher at mole fractions of CH3OH approaching 0 or 1, but the yield is lower for 0 < x < 1.0. The lowest value of x in Figure 12 was set to 4.5 × 10−4 (equal to E


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condensed phase environment. The initial configuration did not have a systematic arrangement of methanol molecules around the hemiacetal molecule; however, after equilibration and running the simulation for 2 ps, it was observed that methanol molecules formed a circular chainlike hydrogen bonded network around the hemiacetal molecule and also formed hydrogen bonds with the hemiacetal molecule, as shown in Figure 13 inset. This circular chainlike structure was found to be stable when the simulation was run for longer time scales. Figure 13 shows the complete theorized process of formation, desorption, and stabilization of the HCHO molecules in CH3OH cyclic oligomers when pure CH3OH was used as a solvent. It is known from literature that water molecules form three-dimensional network that dynamically change to form cages and chains35 and CH3OH molecules form cyclic oligomers of 6 and/or 8 molecules which provide strong hydrogen bonding (191 kJ mol−1).36,37 It is also known that the interspace of the CH3OH cyclic oligomers can be filled with water molecules in water−CH3OH mixtures.36 According to Guo et al.,36 although CH3OH is fully soluble in water in all proportions, the mixture is not homogeneously mixed at the molecular level. A chain of 8 CH3OH molecules bond with 1−2 water molecules to form a ring, and a chain 6 CH3OH molecules bonds with 2−4 water molecules to form a ring. Their findings also suggest that “free swimming” water molecules (without hydrogen bonding) do not exist in any appreciable amount. Therefore, from the trend observed in Figure 12 it can be theorized that the tendency of the CH3OH cyclic oligomers to accommodate the C2H6O2 molecules may decrease in the presence of water molecules, since water molecules may preferentially fill the interspace of the CH3OH cyclic oligomers, resulting in reduced stability of C2H6O2 in water−CH3OH solutions. The highest mole fraction of CH3OH at which water would fill the interspace of the CH3OH cyclic oligomers is 0.89 (configuration of 8 CH3OH molecules with 1 water molecule). Above this mole fraction, “water free” cyclic oligomers of CH3OH may be found. Figure 12 shows that the yield of HCHO increased slightly when the mole fraction of CH3OH increased from 0.8 to 0.9; however,

Figure 12. Effect of the CH3OH mole fraction in the solvent mixture on the HCHO yield at t = 48 h. Note, the lowest CH3OH mole fraction tested was x = 4.5 × 10−4.

0.1 v/v% CH3OH) which was added to favor the equilibrium toward HCHO. It is known that CH2(OH)2 and C2H6O2 which are formed through HCHO reacting with water and CH3OH, respectively, are unstable molecules. They are stabilized by the hydrogen bonding with the surrounding water or methanol molecules.34 The trend that is observed in Figure 12 may be explained by understanding this stabilization effect of water and methanol via hydrogen bonding. In this study, it is theorized that the CH2(OH)2 molecules are stabilized by hydrogen bonding with water molecules which form a cage structure;35 and the C2H6O2 molecules are stabilized by hydrogen bonding with methanol molecules which form cyclic oligomer structure.36 The latter was studied using ab initio molecular dynamics simulations, performed in the Car−Parrinello molecular dynamics scheme (Figure 13, inset).27 Thirty methanol molecules were randomly placed around the hemiacetal molecule in a simulation cell, with periodic boundary conditions from all sides to simulate the

Figure 13. Simplified schematic of HCHO formation, desorption, and stabilization in CH3OH (inspired by ideas explained in ref 36). (inset) Intermolecular hydrogen bonding network among methanol and hemiacetal molecules, as captured from condensed phase CPMD simulation of hemiacetal in methanol after 2 ps of simulation time. Cyan, red, and white colors indicate carbon, oxygen, and hydrogen atoms, respectively. Only those methanol molecules that form the hydrogen bonded network around the hemiacetal molecule are shown for clarity. F


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increased by a factor of 4 compared to the previous study in aqueous phase. It was observed that the reaction rate increased as the temperature increased. However, lower HCHO yield was achieved at higher temperatures and long residence times, since the solubility of the gases as well as the thermodynamic equilibrium of the reaction was less favorable at higher temperatures. The long reaction times at the lower temperatures indicate that catalyst activity is important, and this will be the subject of further investigations.

the yield increased significantly when the mole fraction of CH3OH was increased to 1.0. This theory may therefore be able to explain the effect of water on the HCHO yield in concentrated CH3OH solutions. Figure 12 closely resembles the excess molar volume of water−CH3OH mixture which shows nonideal mixing of water and CH3OH at the molecular level.38 However, to provide strong evidence to support this theory, further experimental and molecular modeling work similar to the X-ray absorption and X-ray emission spectroscopy, and density functional theory work of Guo et al.36 is necessary. The molecular structure of C2H5OH molecules may not provide the required molecular space and hydrogen bonding for the produced HCHO molecules since they form winding chain-like structures instead of cyclic oligomers or cages39 which may explain the poor yield of HCHO in this solvent.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b00837. Figures S1−S4 as mentioned in the text (PDF)

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CONCLUSIONS The reaction pathway for the novel production method of formaldehyde (HCHO) via hydrogenation of CO was studied by an ex-situ investigation of the potential intermediate products in the samples collected during the experiments, which included deuterium labeling of the various reactants and solvents. On the basis of this study, it was shown that both CO and H2 gases are dissolved into the solvent and adsorbed on the surface of the catalyst. It was proposed that while CO is adsorbed as a molecule, H2 adsorbs dissociatively. The adsorbed H atoms then attach to the adsorbed CO to form HCHO molecule. It was shown that in the absence of methanol (CH3OH), the adsorbed HCHO molecules are further hydrogenated to form CH3OH. The reaction is favored at lower temperatures in the proposed HCHO production method (353−403 K) as opposed to the industrial method (673−973 K). Furthermore, there is no production of CO2 as a byproduct, and 100% selectivity toward HCHO is achieved. This makes the proposed process a potentially more sustainable method of HCHO production. On the basis of the proposed reaction pathway, the HCHO yield is limited by the solubility of CO and H2. Therefore, the effects of solvents on the hydrogenation of CO to HCHO in the slurry reactor were studied. Many types of solvents including polar protic (water, methanol, and ethanol), polar aprotic (DMSO), and nonpolar (C7H8) solvents were used in the present study. It was shown that the highest HCHO yield was achieved by using pure CH3OH as a solvent followed by CH3OH−water mixtures. Using pure CH3OH as the solvent may potentially ease the downstream process of separation since formaldehyde may be concentrated via stripping or a membrane separation process rather than the more complex distillation process currently used for water−methanol−formaldehyde ternary mixtures. HCHO reacts with CH3OH and water to produce C2H6O2 and CH2(OH)2, respectively, thereby shifting the equilibrium of hydrogenation toward HCHO production. The CH3OH and water solvents stabilize the C2H6O2 and CH2(OH)2 molecules via hydrogen bonding. The yield of HCHO was reduced by using water-CH3OH mixtures compared to pure CH3OH since water molecules may occupy vacancies within the CH3OH cyclic oligomer rings, reducing the stability of HCHO molecules in the solvent. The effect of temperature on HCHO yield using CH3OH as the solvent was studied and the highest HCHO yield was found to be 15.58 mmol L−1 gcat−1 at 363 K and 100 bar which

AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]. Ph: +61 3 99024388. Fax: +61 3 99055686. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the financial support provided by the Monash Engineering Postgraduate Publication Award.

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REFERENCES

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Hydrogenation of Carbon Monoxide into Formaldehyde in Liquid Media

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