Methyl formate as a hydrogen energy carrier

The study addresses the challenge of sustainable energy storage and transport by proposing methyl formate (MF) as an efficient chemical hydrogen carrier. Current hydrogen carriers (e.g., methanol, ammonia, liquid organic hydrogen carriers, and formic acid) have limitations in toxicity, corrosivity, availability, or energy density. Hydrogen itself is difficult to store and transport due to low volumetric energy content and safety issues. The authors hypothesize that MF—an industrially available, non-toxic, non-irritating, non-corrosive liquid with favorable thermodynamics and suitable energy density—can serve as a practical hydrogen carrier and enable rapid, selective hydrogen release under mild conditions using appropriate catalysts, supporting a carbon-neutral hydrogen energy cycle.

The paper situates MF among established hydrogen carriers: methanol and ammonia have high hydrogen content but are toxic and flammable; arene-based liquid organic hydrogen carriers have medium hydrogen densities and handling advantages but face availability and toxicity issues; formic acid has favorable dehydrogenation thermodynamics and can be dehydrogenated under ambient conditions but has low hydrogen content (<5 wt%) and is corrosive. MF offers intermediate hydrogen storage capacity (8.4 wt%), favorable dehydrogenation thermodynamics (HCO2CH3 + 2H2O → 4H2 + 2CO2, ΔG° = −16.6 kJ mol−1), and benign GHS classification. MF is produced industrially at multi-million-tonne scale (from methanol carbonylation) and can also be synthesized sustainably via CO2 hydrogenation in methanol, with multiple literature precedents spanning catalytic and photocatalytic routes. Prior research on dehydrogenation largely focused on formic acid and methanol using Ru-pincer catalysts; MF dehydrogenation had not been reported before this study.

Catalytic screening and optimization: MF dehydrogenation was evaluated with a series of Ru pincer catalysts (C1–C8) known from FA and MeOH dehydrogenation. Standard reaction conditions for screening (Table 1): 4 µmol Ru catalyst (0.005 mol%, 48 ppm), 10 mmol KOH (0.561 g), 10.0 ml triglyme, 84 mmol MF (5.2 ml), 168 mmol H2O (3.0 ml), 90 °C, 20 h in a 100-ml autoclave under Ar. After reaction, the autoclave was cooled, pressure released to a burette, and gas composition analyzed by GC; CO2 additionally quantified by trapping with aqueous HCl. Each experiment was performed at least twice (standard deviations <5%). Water content, solvent, base identity/amount, and temperature were optimized: optimal water 2–4 equiv to MF; insufficient water increased CO. Co-solvents (THF, dioxane, DMOA, DMF) gave similar results; acetonitrile was unsuitable. Bases KOH, NaOH, CsOH, K2HPO4, and K3PO4 were effective, with KOH and K3PO4 best; optimal KOH 10–40 mmol, base necessary for maximal H2 evolution. Temperature range 23–110 °C was probed; 90–100 °C gave best gas evolution with CO <10 ppm; dehydrogenation possible even at room temperature. Comparative kinetics: Under identical mild conditions (C2, 90 °C), initial gas evolution was compared for MF, FA, and MeOH; slopes over 0–15 h showed MF was 5× faster than FA and 20× faster than MeOH. Mechanistic studies: Reaction rates were measured between 60–90 °C to construct an Arrhenius plot (Ea = 65 kJ mol−1). Kinetic isotope effects were measured by substituting D2O and/or DCOOCH3, yielding secondary KIEs (kH/kD = 1.40, 1.59, 1.80), indicating facile activation of the MF formyl C–H. DFT calculations (PNP–Ru system with KOH) mapped two pathways from [KRu–OH]: (1) direct MF dehydrogenation via [KRu–OCOOCH3] with a key H2-release barrier of 80 kJ mol−1 and subsequent CO2 release; (2) MF hydrolysis to form [KRu–OOCH] and CH3OH (46 kJ mol−1), followed by higher-barrier H2/CO2 release (124 kJ mol−1). The direct pathway exhibited a lower effective barrier and calculated Ea (61 kJ mol−1) in agreement with experiment; base-free calculations also favored direct dehydrogenation but with a higher barrier (95 vs 129 kJ mol−1). Isolation and characterization of intermediates: The thermodynamically stable [H–Ru–OCOOCH3] intermediate was crystallized and its use as a precatalyst reproduced dehydrogenation profiles similar to C1. NMR studies with [H–Ru–OH] and 13C-labeled MF detected [H–Ru–13OOCH3] and [H–Ru–13COOCH3] as major species, with observation of H2, 13CO2, methanol (and labeled variants), and formate/formic acid products; [H–Ru–OCH3] was not observed due to rapid conversion upon MF addition, consistent with DFT. Time-resolved analysis: During initial heating to 90 °C, ~23% MF was consumed, generating H2, CO2, MeOH, and FA/formate; pH shifted from 10.9 to ~7.4, then stabilized (6.8–7.6) due to buffering. FA/formate peaked at 1–5 h while H2, CO2, and MeOH continued to increase as MF was consumed. Performance and durability tests: Consecutive batch dehydrogenation with 84 mmol MF/168 mmol H2O and periodic gas release demonstrated catalyst stability over >25 runs, producing >4.3 L total gas with 2.5 L (103 mmol) H2 (TON >25,000). A long-term experiment released >9.4 L gas overall with 60% H2 yield (241 mmol) based on MF (100 mmol), H2:CO2 ≈ 1.7:1 (near the 2:1 theoretical ratio). High-pressure generation: Using 310 mmol MF with 6.5 ppm of C5 afforded 70 bar (2 h) and 128 bar (10 h), with TON(H2) >107,000 and TOF(H2)max >44,000 h−1; CO was undetectable (GC limit 10 ppm). Solvent-free dehydrogenation: A mixture of MF and H2O (closed autoclave) with KOH and 25-ppm Ru catalyst achieved >75 bar pressure and TON(H2) >16,871. Analytical methods: Reactions under inert gas; reagents degassed; NMR (1H, 13C, 31P) for liquid/solid products; GC with CO quantification limits of 78 ppm (Ar carrier) and 10 ppm (He carrier); pH measured at 24 °C. Hydrogen and CO2 amounts calculated from calibrated GC; TON and TOF computed from evolved moles and catalyst loading.

• MF is identified as a practical, non-toxic, non-corrosive hydrogen carrier with 8.4 wt% H2 storage and favorable dehydrogenation thermodynamics (ΔG° = −16.6 kJ mol−1 for MF + 2 H2O → 4 H2 + 2 CO2). - Under screening conditions (90 °C, KOH, triglyme), Ru-pincer catalysts C1–C5 achieved high productivity with TON(H2) up to ~21,590 and TOF(H2)max up to 8,376 h−1; CO in product gas was <10 ppm (undetectable for C5). Less optimal catalysts (C6–C8) showed lower performance and higher CO (up to 2,672 ppm). - MF dehydrogenation exhibited much faster initial gas evolution than formic acid or methanol under identical conditions: rate ratio MF:FA:MeOH = 20:4:1 (MF 5× FA; 20× MeOH). - Optimized conditions included 2–4 equiv water to MF, strong base (KOH or K3PO4; 10–40 mmol range), and 90–100 °C; acetonitrile was unsuitable as co-solvent; dehydrogenation was possible even at room temperature. - Mechanistic and kinetic data: experimental Ea = 65 kJ mol−1; secondary KIEs kH/kD = 1.40 (D2O), 1.59 (DCOOCH3), and 1.80 (DCOOCH3 + D2O). DFT supports a direct MF dehydrogenation pathway via [KRu–OCOOCH3] with lower barriers (80 kJ mol−1 for H2 release; calculated Ea 61 kJ mol−1) than the MF hydrolysis route (124 kJ mol−1; Ea 121 kJ mol−1). The [H–Ru–OCOOCH3] intermediate was crystallographically confirmed. - Durability: >25 consecutive runs produced >4.3 L gas with 2.5 L H2 (103 mmol), TON >25,000. A long-term run released >9.4 L gas with 60% H2 yield (241 mmol) from 100 mmol MF; H2:CO2 ≈ 1.7:1, close to the 2:1 theoretical value. - High performance demonstration: With 310 mmol MF and only 6.5 ppm C5, pressures of 70 bar (2 h) and 128 bar (10 h) were achieved; TON(H2) >107,000; TOF(H2)max >44,000 h−1; CO was undetectable (≤10 ppm detection limit). - Solvent-free operation: MF/H2O dehydrogenation yielded >75 bar pressure with KOH and 25-ppm Ru catalyst; TON(H2) >16,871.

The findings validate the hypothesis that methyl formate is a highly effective hydrogen carrier. Compared with established carriers (formic acid, methanol), MF dehydrogenates substantially faster under mild, practical conditions using Ru-pincer catalysts while maintaining high selectivity (CO undetectable) and activity (TOFmax >44,000 h−1; TON >100,000 in high-pressure runs). Mechanistic experiments (KIE), DFT, isolation of a key intermediate, and NMR support a direct MF dehydrogenation pathway that rationalizes the observed low activation energy and fast rates. Operationally, MF enables both continuous low-pressure hydrogen release and rapid high-pressure generation, and it performs in solvent-free systems, highlighting applicability for mobile or distributed hydrogen supply. Given MF’s benign handling properties, industrial availability, and potential synthesis from CO2 and methanol, the results underscore MF’s relevance for a carbon-neutral hydrogen energy cycle and complement existing chemical hydrogen storage strategies.

This work introduces methyl formate as a previously overlooked, industrially available hydrogen energy carrier that combines favorable safety, handling, and storage properties with fast, selective hydrogen release under mild conditions. The authors develop Ru-pincer catalytic systems that achieve undetectable CO, TOF(H2)max >44,000 h−1, and TON(H2) >100,000, outperforming formic acid and methanol in initial dehydrogenation rates. Mechanistic insights (KIE, DFT, crystallography, NMR) establish a direct dehydrogenation pathway. The study demonstrates durability over many cycles, high-pressure H2 generation, and solvent-free operation, suggesting practical potential. Future research could focus on integrating sustainable MF synthesis from CO2 at scale, exploring earth-abundant catalysts, optimizing lower-temperature operation, continuous-flow reactor implementation, and system-level assessments for real-world deployment.

• In closed autoclave systems the dehydrogenation reaches equilibrium, requiring periodic gas release to continue hydrogen evolution. - Performance depends on basic conditions and water content; insufficient water increases CO formation and base-free conditions yield lower H2 and higher calculated barriers. - Optimal temperatures were 90–100 °C, with lower rates at room temperature despite feasibility. - Certain solvents (e.g., acetonitrile) are unsuitable. - CO was reported as undetectable within the GC method’s quantification limit (as low as 10 ppm with He carrier), so trace CO below detection may not be excluded. - Under less optimal catalyst systems (C6–C8), higher CO levels (up to 2,672 ppm) were observed, indicating catalyst-dependent selectivity. - The observed H2:CO2 ratio in extended runs (~1.7:1) was slightly below the 2:1 theoretical value for complete MF aqueous reforming, indicating incomplete conversion under those conditions.