Highly Efficient Additive-Free Dehydrogenation of Neat Formic Acid

Abstract

Formic acid (FA) is a promising hydrogen carrier which can play an instrumental role in the overall implementation of a hydrogen economy. In this regard, it is important to generate H₂ gas from neat FA without any solvent/additive, for which existing systems are scarce. Here we report the remarkable catalytic activity of a ruthenium 9H-acridine pincer complex for this process. The catalyst is unusually stable and robust in FA even at high temperatures and can catalyse neat FA dehydrogenation for over a month, with a total turnover number of 1,701,150, while also generating high H₂/CO₂ gas pressures (tested up to 100 bars). Mechanistic investigations and DFT studies are conducted to fully understand the molecular mechanism to the process. Overall, the high activity, stability, selectivity, simplicity and versatility of the system to generate a CO-free H₂/CO₂ gas stream and high pressure from neat FA makes it promising for large-scale implementation.

Figure 1. Different aspects of the catalytic activity of 1 in the dehydrogenation of neat FA.

(A) FA decomposition cata-lysed by 1. (B) Gas chromatogram of the generated gas mixture during the dehydro-genation of neat FA at 95 °C; (C) Effect of temperature on the catalytic activity (from 65 to 95 °C; reaction conditions: FA (1g, 21.7 mmol), 1 (10 µmol), T (as specified). (D) Reaction setup. (E) Gas evolution with time at a reaction temperature of 95 °C (F) Changes in TON with reaction time (at a catalyst loading of 0.2 µmol) (G) Plot of flowrate vs time in a continuous scaled up experiment; reaction conditions: FA (5 mL; 0.13 mol), 1 (40 µmol), 95 °C, fresh FA injected at a rate of 2-5 mL/h via syringe pump to replenish the reacting FA, flowrate measurement error: ±5 mL/min. (H) Pressure generation with time during the dehydrogenation of neat FA in a closed 30 mL autoclave; reaction conditions: FA ( 5 mL, 0.13 mol), 1 (40 µmol, 23 mg), 103 °C. The reported gas evolution volumes (except for Figure 1G), TON and TOFs are averaged over two runs. Maximum error is ±10%.

Figure 2. Dehydrogenation of different grades of FA.

Gas evolution vs time profile is shown during the de-hydrogenation of 98-100%, 97% and 85% formic acid. Reaction conditions: FA (1 g), [Ru] 0.01 mmol, 95 °C. Average of two / four (85%) runs. Maximum error in values ±10%.

Figure 3. Mechanistic studies.

a) reactivity of com-plex 1 with formic acid, CO₂ and H₂ at RT. b) reac-tivity of complex 1 with acetic acid at RT. c) Crystal structure of the acetate complex 1c’. Atoms are drawn with a probability level of 50%. Colour code: ruthenium (magenta), carbon (light grey), oxygen (red), phosphorus (yellow) and nitrogen (blue). Isopropyl groups are in wireframe and hydrogen atoms are not displayed for clarity. Selected bond lengths (Å) and angles (degree) Ru(1)-P(1) 2.2868(7), Ru(1)-P(2) 2.2918(6), Ru(1)-O(1) 2.1952(16), Ru(1)-O(2) 2.1863(15), Ru(1)-N(1) 2.1583(18), Ru(1)-C(30) 1.862(2), C(30)-Ru(1)-N(1) 179.71(8), C(30)-Ru(1)-O(2) 93.64(7), N(1)-Ru(1)-O(2) 86.64(6), C(30)-Ru(1)-O(1) 94.43(8), N(1)-Ru(1)-O(1) 85.78(6), O(2)-Ru(1)-O(1) 60.23(6), C(30)-Ru(1)-P(1) 92.18(7), N(1)-Ru(1)-P(1) 87.59(5), O(2)-Ru(1)-P(1) 157.56(4), O(1)-Ru(1)-P(1) 97.73(5), C(30)-Ru(1)-P(2) 92.54(7), N(1)-Ru(1)-P(2) 87.35(5), O(2)-Ru(1)-P(2) 95.64(5), O(1)-Ru(1)-P(2) 155.23(4), P(1)-Ru(1)-P(2) 105.73(3), O(2)-C(28)-O(1) 119.9(2).

Figure 4. Mechanistic cycle.

A Plausible scheme for the de-hydrogenation of neat formic acid.

Figure 5. DFT-calculated energy profile for FA dehydrogenation catalysed by 1.

a) elemental steps of the dehydrogenation b) energy profile as calculated by DFT studies. Energy values correspond to Gibbs free energies in kcal/mol with respect to 1-mer+FA at 298 K at atmospheric pressure.

Figure 6. DFT-optimized structure of 1a-fac.

The close H^δ-…H^δ+ bond dis-tance is shown. This interaction likely provides stability to the molecular structure.

Figure 7. Comparison of the catalytic activity of 1, 1e and 1-Cl towards neat FA dehydrogenation.

Reaction conditions: FA (1 g, 21.7 mmol), catalyst (10 µmol), 95 °C.

Figure 8