Poison or Promoter? Investigating the Dual-Role of Carbon Monoxide in Pincer-Iridium-Based Alkane Dehydrogenation Systems via Operando Diffuse Reflectance Infrared Fourier Transform Spectroscopy

Abstract

<p>Pincer-ligated iridium complexes of the <a>form [Ir(<sup>R4</sup>PCP)L] (<sup>R4</sup>PCP = κ<sup>3</sup>-C<sub>6</sub>H<sub>3</sub>-2,6-(XPR<sub>2</sub>)<sub>2</sub>; X = CH<sub>2</sub>, O; R = <i>t</i>Bu, <i>i</i>Pr) </a>have previously been shown competent for acceptorless alkane dehydrogenation when supported on silica. It was observed by post-catalysis solid-state NMR that silica-tethered <a>[Ir(C<sub>2</sub>H<sub>4</sub>)(≡SiO-<i><sup>t</sup></i><sup>Bu4</sup>POCOP)] </a>(<b>3-C<sub>2</sub>H<sub>4</sub></b>) was converted fully to [Ir(CO)(≡SiO-<i><sup>t</sup></i><sup>Bu4</sup>POCOP)] (<b>3-CO</b>) at 300 °C. In this work, the characterization of species under dehydrogenation reaction conditions far from equilibrium between butane and butenes (approach to equilibrium <i>Q</i>/<i>K</i><sub>eq</sub> = 0.3 at 300 °C) is performed with <i>operando </i>Diffuse Reflectance Infrared Fourier-Transform Spectroscopy (DRIFTS) to show the kinetics of species conversion from <b>3-C<sub>2</sub>H<sub>4</sub></b> to <b>3-CO</b>. It is further found that [IrClH(≡SiO-<i><sup>t</sup></i><sup>Bu4</sup>POCOP)] (<b>3-HCl</b>), a species considered to be a precatalyst for alkane dehydrogenation, is also fully converted to <b>3-CO</b>. A mechanism of decomposition is proposed that implicates surface silanol groups, while carbon monoxide acts as a “stabilizer” for the catalyst by promoting their reductive elimination and maintaining the complex in the I oxidation state. </p>

In Situ Formation of a Sub-Nanometer Iridium Phosphide Catalyst from Supported Organometallic Species

“…This difference in surface interactions was also observed when comparing diffuse reflectance infrared Fourier-transform spectroscopic (DRIFTS) characterization of 1/Mg(Al)O ( Figure S7) and 1/SiO 2 at 200 °C. 52 1/Mg(Al)O showed a feature at higher wavenumbers corresponding to strong interaction of the iridium center with the surface which was absent in 1/SiO 2 . 52 In addition, multiple features in the spectrum for Flowing butane over a sample of 1/SiO 2 at 440 °C in a quartz tube reactor following a temperature ramp up to this temperature under helium allowed for the analysis of reaction kinetics using in situ generated iridium nanoparticles.…”

“…52 1/Mg(Al)O showed a feature at higher wavenumbers corresponding to strong interaction of the iridium center with the surface which was absent in 1/SiO 2 . 52 In addition, multiple features in the spectrum for Flowing butane over a sample of 1/SiO 2 at 440 °C in a quartz tube reactor following a temperature ramp up to this temperature under helium allowed for the analysis of reaction kinetics using in situ generated iridium nanoparticles. Results indicated an initial rate of butane dehydrogenation of ca.…”

“…2 ) -prepared from molecular precursor complex 1 via the process shown in scheme 1, it was observed via Diffuse Reflectance Infrared Fourier-Transform Spectroscopy (DRIFTS) that loss of the pincer ligand occurs at approximately 420 °C to give an IR-silent species. 52 The resulting decomposition product was found to be active toward alkane dehydrogenation and, in the course of characterization, was found to be a phosphorus-rich iridium phosphide nanophase. Iridium phosphide as a dehydrogenation catalyst may not be without precedent, in spite of its lack of recognition as such in the literature.…”

Regioselective Gas‐Phase n‐Butane Transfer Dehydrogenation via Silica‐Supported Pincer‐Iridium Complexes

“…In all cases, the addition of a hydrogen acceptor depressed the activity in comparison to acceptorless dehydrogenation (Table 2, Figure 3). Given that olefin-bound complexes are not observed during reaction conditions at 300 °C, [5] this reduction in activity was likely a consequence of an increased CO concentration in the system, itself arising from the conversion of acceptor olefin used with trace water present. [2i] Although the rate of CO formation was too small to observe or measure, catalyst activity is has been demonstrated [5] to be very sensitive to CO pressure and, therefore, the rate may be expected to decrease with increasing olefin content in the gas phase.…”

“…[2i] The stability of such catalysts at elevated temperatures up to 300 °C arises from the concurrent generation of CO over these catalysts, which prevents thermal decomposition. [5] However, the selectivity of the system is unremarkable, resulting in an equilibrated mixture of product butenes (ca. 20% 1-butene) even at very low residence times, possibly due to rapid isomerization over the iridium species.…”