Mina Arshad*a,
Iram Mahmoodb,
Ali Sarosha,
Asim Umera,
Muhammad Athar
a and
Mahboob Ahmed Aadila
aDepartment of Chemical Engineering, Muhammad Nawaz Sharif University of Engineering and Technology, Multan, Pakistan. E-mail: minaarshad@mnsuet.edu.pk; me.mina34@yahoo.com
bUniversity of Bolton, Islamabad, Pakistan
First published on 30th June 2025
An emerging alternative energy source is formic acid, which has low toxicity and high hydrogen-carrying capacity. Metal-containing nanoparticles are very attractive for many applications, allowing large-scale and environmentally friendly production. This study proposes liquid-state synthesis for clean and facile formic acid production via methanol oxidation over metal oxide nanoparticles. MoO3, Fe2O3, TiO2 and V2O5 nanocatalysts were prepared through sol–gel, solvothermal, reflux condensation and ball milling techniques, respectively, and their efficacy in formic acid production via methanol oxidation was assessed. The synthesized nanoparticles were further characterized through scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. The performance of laboratory-prepared nanoscale metal oxide catalysts for formic acid production was evaluated through batch reactions under ambient temperature and pressure conditions to enhance energy efficiency and maximize conversion. Formic acid was quantitatively analyzed using high-performance liquid chromatography (HPLC). Results revealed that the nanocatalysts considerably promoted the generation of formic acid, especially MoO3, which provided a 91% product acid yield, which was the greatest among the other nanocatalysts under the employed reaction conditions.
Most interestingly, formic acid is undergoing rapid advancements to overcome the energy crisis caused by conventional fossil fuels and is projected to be used as direct formic acid fuel cells for future portable equipment and automobile applications.5 Moreover, formic acid is industrially utilized as a detergent and cleanser for powerful descaling. Formic acid is an immensely versatile chemical with extensive application. Formic acid is widely used in agriculture for crop protection and as an industrial chemical. It is also used as a coagulant in many rubber manufacturing industries and latex coagulation.6 Formic acid is also used in the pharmaceutical industry as an active ingredient in the production of different pharmaceutical products. Medically, it is used for the treatment of warts.7
Through the oxidation of wet biomass, formic acid yields of 20–75% can be achieved.8 This process involves aqueous catalytic oxidation to convert biomass such as wood, waste paper, and sugar into formic acid with CO2 as a byproduct. However, because of the low yield of formic acid, this process is not usually preferred, and biomass oxidation does not fulfill the industrial and domestic requirements of formic acid.
The catalytic hydrogenation of carbon dioxide is also used to synthesize formic acid. Homogeneous catalysts have been used in this process.9 However, the main drawback of this process is that the reaction moves in the backward direction, and the reaction is at equilibrium with reactants, which reduces the efficiency of this process. Therefore, to improve this process, additional separation steps for formic acid are required, which increases the cost of this process. Moreover, a large amount of CO2 is not easy to transport because of environmental issues.
Industrially, the production of formic acid is carried out through methyl formate. Initially, when methanol reacts with carbon monoxide (CO) in the presence of a base, methyl formate is obtained.10 The reaction occurs at elevated pressure. The obtained methyl formate is treated further in two ways to produce formic acid on an industrial scale. First, during the direct hydrolysis of methyl formate, the reaction occurs in the presence of excess water to produce formic acid. Consequently, methanol and formic acid are obtained. In another method to synthesize formic acid, the obtained methyl formate is first reacted with ammonia to give formamide. The formamide is then hydrolyzed in the presence of sulfuric acid to produce formic acid. However, during the reaction, ammonium sulfate is also produced as a by-product, which is hazardous and not easy to handle. Therefore, these two routes for formic acid production are not generally favourable because direct hydrolysis of methyl formate requires an excess amount of water, which is not cost-effective, and the hydrolysis of formamide produces hazardous by-products whose handling is a major issue. Recently, the formation of methyl formate has also been practised through the electron irradiation process of methanol and carbon monoxide,11 which can be converted to formic acid. Efforts to increase CO2 to HCOOH electrochemical reduction have been given high focus in research over the past several decades. Formic acid produced from CO2 via electrochemical reduction using electrocatalysts has great potential.12 A very effective technique employing bimetallic alloy catalysts for CO2 electro-reduction to formic acid is also proposed.13
Recently, a new and environmentally acceptable gas phase method for formic acid production was introduced. In their work, commercially annulated iron and molybdenum catalysts were used for the oxidation of methanol to formaldehyde. For further conversion of formaldehyde to formic acid, vanadium and titanium catalysts were used in the shape of the pellets. Because this reaction is highly exothermic, a tubular reactor was used for this purpose. The resulting gas product from the reactor was condensed, and formic acid, water, and residual formaldehyde were obtained as the final product. The condensate showed a formic acid content of up to 55–62 wt%.14 The yield of the formic acid produced is low. This process is not very economical and efficient because of the use of larger-sized catalysts in the mm range. An additional cost is required for evaporation and then cooling the system.
In general, the process of producing formic acid from CO2 has detrimental effects on the environment, primarily in the form of greenhouse gas emissions. An effective approach for reducing greenhouse gas (GHG) emissions and the depletion of fossil fuel resources is carbon capture and utilization (CCU) technology. A potential CCU method is the CO2-based synthesis of formic acid (FA) using H2. Additionally, the impact on global warming can be lowered by 95.01% by synthesizing FA through CO2 hydrogenation.15 Recently, a novel approach utilizing a magnesia catalyst through direct hydration to produce formic acid has been introduced.16
In recent research findings, formic acid synthesis using electrocatalysis,17 heterogeneous catalysis and its feasibility for energy applications are presented.18 Many approaches, including physical, chemical, and biological methods, have been employed in nanoparticle production. For the past ten years, the “clean” production of metal and metal oxide nanoparticles has been a very interesting research area. To synthesize uniformly sized nanoparticles with long-term stability, physical and chemical methods are generally believed to be the best.19
Transition metal oxide nanoparticles are considered efficient for methanol oxidation catalysis owing to their higher selectivity towards desired products and multiple chemical intermediates.20 The present work is an economical and safe route for formic acid production through the photocatalytic oxidation of methanol in the liquid phase using metal oxide nanoparticles. Photocatalytic activity is enhanced owing to the greater surface-to-volume ratio of nanoparticles compared with their respective counterparts in larger dimensions.21 The photocatalytic oxidation reaction is stimulated by UV radiation, thereby providing energy and facilitating the synthesis of formic acid. In this regard, metal oxide nanoparticles serve as efficient photocatalysts by enhancing the activity of UV radiation in oxidation reactions. Nanoparticles, such as titania, enhance charge transfer mechanisms and boost reaction rates when exposed to UV light, which makes it easier to convert methanol to formic acid. In this work, four metal oxide catalysts, namely molybdenum oxide, iron oxide, titanium oxide, and vanadium oxide, are used in the nanometric range. Nanocatalysts can enhance the reaction rate while having at least one nanoscale dimension. Moreover, nano-structured catalysts are expected to have more chemical, optical, and electronic properties. For nanocatalysts, the increasing surface-to-volume ratio with decreasing particle size strongly increases the particular catalytic activity, which also helps in the case of methanol oxidation to formic acid. This process is energy efficient and more reactive and modifies the previously used methods of manufacturing in a safe and green way, with a good yield of formic acid. This work is limited in considering the common part of Fe2O3 using the solvothermal method, while MoO3, V2O5, and TiO2 were synthesized using the sol–gel method for comparison. This study observed that the production of formic acid via methanol oxidation is heavily influenced by the choice of catalyst, as different catalysts exhibit different performances owing to their varied sizes and crystalline structures on a specific reaction path. Therefore, these factors should be considered in future studies.
Batch reactions are performed to check the catalytic activity of nanocatalysts towards formic acid production, as shown in Fig. 2. The wavelength of the UV lamp used to conduct the batch reaction is 357 nm. Table 1 shows the values of various reaction parameters, such as the quantity of catalysts, solvent methanol, oxidizing agent H2O2, time for UV degradation, and time for stirring. For any catalytic reaction, the sizes, materials and crystalline structures of catalysts directly influence the overall reaction performance in terms of selectivity, catalytic activity and stability. The comparison of the four metal oxide nanoparticles is made for the main catalytic methanol oxidation reaction. In this regard, various reaction parameters, such as the amount of catalyst, reaction phase, time for reaction under UV-radiation and the amount of alcohol and oxidizing agent, as depicted in Table 1, are kept constant to serve as a control for better catalytic comparison under ambient temperature and pressure conditions. The basis for parametric amounts in Table 1 is by weight for solid catalysts and by volume for liquid alcohol and oxidizing agents.
Catalyst | Catalyst quantity (mg) | Methanol (ml) | H2O2 (ml) | Time under UV lamp (h) | Time for stirring (h) |
---|---|---|---|---|---|
MoO3 | 200 | 05 | 0.20 | 02 | 01 |
Fe2O3 | 200 | 05 | 0.20 | 02 | 01 |
TiO2 | 200 | 05 | 0.20 | 02 | 01 |
V2O5 | 200 | 05 | 0.20 | 02 | 01 |
It is an important perspective that catalyst composition significantly impacts the reaction kinetics of methanol oxidation to formic acid, affecting both the selectivity and activity of the process. A thorough comprehension of the reaction mechanism and surface customization is necessary for the development of a selective catalyst.22 The catalytic oxidation of methanol under UV light using prepared nano-catalysts (molybdenum trioxide, titania, iron oxide and vanadium pentoxide) has been explored to optimize this reaction. The reason is that the choice of catalyst can alter the reaction pathway by enhancing the reaction rate and improving the formic acid yield.
The catalytic oxidation of methanol employing a molybdenum trioxide (MoO3) catalyst has high activity in producing formic acid. Molybdenum metal possesses five oxidation states that are readily accessible and a rich coordination chemistry, along with its refractory nature, making molybdenum an excellent candidate for use as a heterogeneous catalyst. The mechanism for catalytic oxidation involves the formation of surface alkoxides over a molybdenum trioxide catalyst. Moreover, titania (TiO2), a well-known photocatalyst, assists in the alcoholic oxidation pathway by first causing methanol to dehydrogenate into formaldehyde and then continuously oxidizing it to formic acid.23 In comparison, titanium dioxide catalysts and iron oxide (Fe2O3) catalysts exhibit higher activity and a higher yield of formic acid. The formaldehyde formation involves the activation of methanol through acidic sites; the possibility of this activation primarily determines oxidation activity. The combination of metal oxides contributes to the acidic property's enhancement or modification. Selective oxidation is determined by an acid–base-type interaction between the catalyst surface and the organic material to be oxidized; the results of methanol and Fe2O3 catalysts have shown good acid–base properties. Similarly, the vanadium pentoxide (V2O5) nanoparticles have also significantly converted the methanol by multiple electron transfer and high solubility during the catalytic oxidation process, resulting in considerable conversion and a tendency to increase the product yield.
The SEM images showed that the iron oxide nanoparticles were spherical, as shown in Fig. 3b. The average particle size of iron oxide nanoparticles was 18 nm, varying from 10 nm to 26 nm.
SEM images of titanium dioxide nanoparticles showed spherical morphology. Agglomerated particles were also observed in addition to individual nanoparticles owing to the high-temperature treatment applied during the synthesis and drying process and the conversion from gel to powder. The particle size was detected from 16 nm to 23 nm. Thus, the average size observed through SEM images was 20 nm, as shown in Fig. 3c. Titania, as a photocatalyst, is affected by the presence of agglomerates and aggregates in terms of a decrease in the fraction of light absorbed. Photoactivity is affected by the cluster formation of photocatalysts. Moreover, at the particle-to-particle level, agglomeration or aggregation may result in the creation of new surface states that alter the quantum yield and photon absorption in ways unrelated to the main characteristics of the nanoparticles.
The vanadium oxide nanoparticles presented by the SEM image in Fig. 3d were well dispersed and spherical. These nanoparticles ranged from 13 nm to 23 nm. The average particle size of vanadium oxide nanoparticles was observed as 18 nm.
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Fig. 5 XRD patterns of (a) molybdenum trioxide (MoO3) nanoparticles, (b) iron oxide (Fe2O3) nanoparticles, (c) titanium oxide (TiO2) nanoparticles and (d) vanadium pentoxide (V2O5) nanoparticles. |
The XRD pattern of Fe2O3 revealed a rhombohedral crystal with nanoparticles that were hematite with distinct peaks hkl positions (104), (110), (116), (012) and (024) represented under reference code 00-024-0072, as shown in Fig. 5b.
Similarly, the distinctive peaks of anatase TiO2 are depicted in Fig. 5c at respective 2θ positions and are represented by the hkl planes (101), (004), and (200), confirming the anatase phase of titanium oxide nanoparticles (XRD JCPDS card no. 78-2486).26
The recorded XRD pattern in Fig. 5d depicted a well-crystalline sample and showed the presence of V2O5, with characteristic (001), (101), (110), (301), and (020) planes at corresponding 2θ positions confirmed by JCPDS card no. 41-1426, confirming the presence of V2O5, as also prepared by Saqib Rafique and Shahino Mah Abdullah et al.27 The calculated density (g cm−3), volume of cell (106 pm3), and crystallite size through Scherrer's equation are presented in Table 2.
Catalyst | Volume of cell (106 pm3) | Density (g cm−3) | Crystallite size (nm) |
---|---|---|---|
MoO3 | 202.99 | 4.71 | 43.46 |
Fe2O3 | 302.72 | 5.26 | 58.12 |
TiO2 | 136.92 | 3.82 | 63.85 |
V2O5 | 180.13 | 3.34 | 69.21 |
The XRD technique was used to measure the crystallite size by neglecting the amorphous part. As an outcome, XRD's three-dimensional structure showed larger crystallite size due to selected domains, specifically for Fe2O3, TiO2 and V2O5 nanoparticles, as depicted in Table 2. Similarly, the crystalline size of a catalyst can exceed its particle size owing to the structural characteristics and synthesis methods employed during the preparation of Fe2O3, TiO2 and V2O5. This phenomenon is particularly found in heterogeneous catalysis; consequently, the relationship between crystallite and particle size can significantly influence catalytic performance. For example, in some cases, catalysts are designed as single crystals, where the entire particle is a single crystal. Here, the crystalline size equals the particle size. In heterogeneous catalysts, both the selectivity and activity are strongly correlated with the geometric structure and electronic configuration of nano-sized particles, which directly depend on the crystallographic phase, particle size and morphology of the catalysts.28 The present research highlights a limited scope and suggests that further research potential should be explored. Agglomeration can lead to particles composed of smaller crystallites. However, in some synthesis methods, such as hydrothermal synthesis, crystals may grow larger than the agglomerated particle size, especially under specific conditions. An understanding of the effect of crystal size can further be studied by developing a strategy for synthesizing different-sized catalysts.29 The current work has limitations in this respect. An attempt has been made not solely on particle size but also on critical aspects of crystallite structure that influence reactivity and selectivity in catalytic processes.
HPLC chromatographs showed a better intensity absorbed peak of formic acid using methanol as a solvent system at 210 nm wavelength and 1 ml min−1 flow rate at a time ranging from 2.5 to 3.0 minutes. The peak of standard formic acid (purity > 99%) showed a sharp peak, as shown in Fig. 6a. Sample A containing the V2O5 nanocatalyst also showed a formic acid peak at the same retention time as the standard formic acid, confirming the formation of formic acid, as shown in Fig. 6b. Sample B containing the MoO3 nanocatalyst also showed the formic acid peak at the same retention time as the standard formic acid, confirming the formation of formic acid, as shown in Fig. 6c. Sample C containing the Fe2O3 nanocatalyst displayed a formic acid peak at the same retention time as the standard formic acid, confirming the formation of formic acid, as shown in Fig. 6d. Sample D containing the TiO2 nanocatalyst showed the formic acid peak at the same retention time as the standard formic acid, confirming the formation of formic acid, as shown in Fig. 6e.
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Fig. 6 HPLC chromatographs of (a) standard formic acid (200 ppm), (b) sample A (200 ppm), (c) sample B (200 ppm), (d) sample C (200 ppm) and (e) sample D (200 ppm). |
Results given in Table 3 revealed that sample B containing MoO3 nanocatalyst gave 91% yield of formic acid as suggested by J. Thrane and U. V. Mentzel et al.30 which is the highest among all other nanocatalysts used because molybdenum trioxide is highly specific and catalytically active for maximum conversion of methanol to formic acid via oxidation. Table 4 shows a comparison of the results in terms of percentage product yield with those of bench-mark catalysts available for the conversion of methanol into formaldehyde and formic acid.
Sample | Solvent | Oxidizing agent | Catalyst | Concentration (ppm) | Retention time (min) | Compound | Area (μV s) | Yield (%) |
---|---|---|---|---|---|---|---|---|
Pure FA | Pure FA | — | — | 200 | 2.55 | FA | 2.22 × 107 | 100 |
Sample A | Methanol | H2O2 | V2O5 | 200 | 2.54 | FA | 5.33 × 106 | 24 |
Sample B | Methanol | H2O2 | MoO3 | 200 | 2.54 | FA | 2.02 × 107 | 91 |
Sample C | Methanol | H2O2 | Fe2O3 | 200 | 2.59 | FA | 1.69 × 107 | 76 |
Sample D | Methanol | H2O2 | TiO2 | 200 | 2.56 | FA | 1.51 × 107 | 68 |
Comparison | Catalyst | Reaction process | % age product yield | |
---|---|---|---|---|
Prepared catalysts | MoO3 | Oxidation process under UV light | 91% | |
Fe2O3 | 76% | |||
TiO2 | 68% | |||
V2O5 | 24% | |||
Bench-mark catalyst | Methanol conversion into formaldehyde | Ag catalyst | BASF process | 86.5–90.5%31 |
Iron-molybdate catalyst | Formox process30 | 88% and 92%31,32 | ||
Formaldehyde conversion into formic acid | Titania–vanadia catalyst | Oxidation process | 87–88%14 |
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