Single-Site Iridium Picolinamide Catalyst Immobilized onto Silica for the Hydrogenation of CO2 and the Dehydrogenation of Formic Acid

Abstract

The development of an efficient heterogeneous catalyst for storing H₂ into CO₂ and releasing it from the produced formic acid, when needed, is a crucial target for overcoming some intrinsic criticalities of green hydrogen exploitation, such as high flammability, low density, and handling. Herein, we report an efficient heterogeneous catalyst for both reactions prepared by immobilizing a molecular iridium organometallic catalyst onto a high-surface mesoporous silica, through a sol-gel methodology. The presence of tailored single-metal catalytic sites, derived by a suitable choice of ligands with desired steric and electronic characteristics, in combination with optimized support features, makes the immobilized catalyst highly active. Furthermore, the information derived from multinuclear DNP-enhanced NMR spectroscopy, elemental analysis, and Ir L₃-edge XAS indicates the formation of cationic iridium sites. It is quite remarkable to note that the immobilized catalyst shows essentially the same catalytic activity as its molecular analogue in the hydrogenation of CO₂. In the reverse reaction of HCOOH dehydrogenation, it is approximately twice less active but has no induction period.

Scheme 1. Relevant Organometallic Catalysts for FA Dehydrogenation and CO₂ Hydrogenation

Scheme 2. Steps of the Synthesis of Ir_PicaSi_SiO₂

Figure 1

DNP-enhanced MAS NMR spectra of PicaSi_SiO₂ and Ir_PicaSi_SiO₂ materials. a) ¹³C CPMAS NMR spectrum of PicaSi_SiO₂ (MAS 10 kHz, 100 K), b) ¹³C CPMAS NMR spectrum of Ir_PicaSi_SiO₂ (MAS 10 kHz, 100 K), c) {¹H}¹³C HETCOR NMR spectrum of Ir_PicaSi_SiO₂ (MAS 10 kHz, 100 K, black = positive, red = negative), d) ¹⁵N CPMAS NMR spectrum of PicaSi_SiO₂ (MAS 10 kHz, 100 K), e) ¹⁵N CPMAS NMR spectrum of Ir_PicaSi_SiO₂ (MAS 10 kHz, 100 K), and f) ²⁹Si CPMAS NMR spectrum of PicaSi_SiO₂ (MAS 10 kHz, 100 K). Asterisks mark spinning sidebands.

Figure 2

XAS data of Ir_PicaSi_SiO₂ and selected reference compounds. a) Normalized Ir L₃ edge XANES and b) the first derivative of Ir L₃ edge XANES. From bottom to top: Ir(COD)(acac) (dark gray, dash), Ir(acac)₃ (gray, dot), Na₂IrCl₆ (light gray, short dash), 2 (dark blue), IrCl₃ (red), 1 (purple), and Ir_PicaSi_SiO₂ (turquoise). c) k²-weighted R-space EXAFS data (turquoise) and fitting (gray) with the corresponding fitting parameters summarized in the table (for full details of fit see Table S5, SI).

Figure 3

Kinetic trends of the performances of Ir_PicaSi_SiO₂ in the FA dehydrogenation reaction. a) TOF (h^–1) vs pH trend for formic acid dehydrogenation catalyzed by Ir_PicaSi_SiO₂ ([cat] = 250 μM, [HCOOH]+[HCOO^–] = 3 M, 298 K). b) log(d(nO₂)/dt) vs log[cat] for formic acid dehydrogenation catalyzed by Ir_PicaSi_SiO₂ ([HCOOH]+[HCOO^–] = 1 M, pH = 3.7, 298 K). c) log[d(nO₂)/dt] vs log([HCOOH]+[HCOO^–]) for formic acid dehydrogenation catalyzed by Ir_PicaSi_SiO₂ ([cat] = 250 μM, pH = 3.7, 298 K). d) ln(TOF) vs 1/T for formic acid dehydrogenation catalyzed by Ir_PicaSi_SiO₂ in a temperature range 288–353 K ([cat] = 50 μM, [HCOOH]+[HCOO^–] = 1 M, pH = 3.7).

Scheme 3. Simplified Reaction Mechanism of FA Dehydrogenation Mediated by Ir-Based Catalysts

L = ancillary ligand.

Figure 4

Kinetic trends of the performances of Ir_PicaSi_SiO₂ and 3 in the FA dehydrogenation reaction. a) TON vs t (h) ([HCOOH]+[HCOO^–] = 1 M, pH = 3.7, [Cat] = 25 μM, T = 298 K) for 3 and Ir_PicaSi_SiO₂. b) TOF (h^–1) vs t (h) ([HCOOH]+[HCOO^–] = 1 M, pH = 3.7, [Cat] = 25 μM, T = 298 K) for 3 and Ir_PicaSi_SiO₂.

Figure 5

Trend of the n_Formate vs Ir_PicaSi_SiO₂ content. Experimental conditions: t = 24 h, T = 423 K, P = 50 atm with CO₂:H₂ = 1:1 ratio, [DABCO] = 1 M.