Low-temperature hydroformylation of ethylene by phosphorous stabilized Rh sites in a one-pot synthesized Rh-(O)-P-MFI zeolite

Abstract

Zeolites containing Rh single sites stabilized by phosphorous were prepared through a one-pot synthesis method and are shown to have superior activity and selectivity for ethylene hydroformylation at low temperature (50 °C). Catalytic activity is ascribed to confined Rh₂O₃ clusters in the zeolite which evolve under reaction conditions into single Rh³⁺ sites. These Rh³⁺ sites are effectively stabilized in a Rh-(O)-P structure by using tetraethylphosphonium hydroxide as a template, which generates in situ phosphate species after H₂ activation. In contrast to Rh₂O₃, confined Rh⁰ clusters appear less active in propanal production and ultimately transform into Rh(I)(CO)₂ under similar reaction conditions. As a result, we show that it is possible to reduce the temperature of ethylene hydroformylation with a solid catalyst down to 50 °C, with good activity and high selectivity, by controlling the electronic and morphological properties of Rh species and the reaction conditions.

Fig. 1. Image and spectroscopic characterizations of Rh-MFI-cal zeolite.

Electron microscopic characterization of Rh-MFI-cal sample: a Paired HAADF-STEM and iDPC images and b HAADF-STEM image and particle size distribution. Rh clusters appear as small bright particles with particle sizes ∼0.6–1.0 nm. c, d X-ray adsorption spectroscopic characterization of Rh clusters: c XANES spectra collected at the Rh K-edge (left) and d the Fourier Transform of the k²-weigthed EXAFS function signal for Rh-MFI-cal and references. e IR-CO at −65 °C and at increasing CO coverage (0.01–2 mbar) for the Rh-MFI-cal sample. The contribution of CO coordinated to silanol groups (2156 cm⁻¹) and physisorbed CO (2135 cm⁻¹) to the Rh³⁺-CO band is minimal, as determined from a blank experiment done on pure MFI (see Supplementary Fig. S24).

Fig. 2. Operando XAS studies of Rh-MFI-cal zeolite.

Evolution of the Rh K-edge XAS during operando studies in the Rh-MFI-cal sample. a Time-resolved XANES spectra at 50 °C, 10 bar and 30 mL/min reactant feed over a time interval of 250 min, (red to yellow and green) and after pressure decrease to 1 bar, 10 mL/min reactant feed and over a period of 100 min (blue lines). Spectra are overlaid with Rh(0) (blue dashes) and to Rh₂O₃ (black dashes) references; b Representative Fourier transformed k²-weigthed EXAFS signal along the reaction coordinate. The arrows showing the different contributions are only indicative. The EXAFS fitting results are reported in the Supplementary Information; c Evolution of the normalized spectral intensities at 23221 eV corresponding to Rh-CO/Rh(0) (blue) and 23240 eV corresponding to Rh₂O₃ (black); d Cauchy wavelet transform of spectra in (b) showing evolution of peaks between 1.2 and 2.2 Å indicative of interference from Rh-CO and Rh-O scattering as well as small amounts of Rh⁰ clustering in the final stages of the reaction (2.5 Å).

Fig. 3. Catalytic performance of Rh-MFI-cal zeolite in ethylene hydroformylation with syngas and comparison with state-of-the-art catalysts.

a Propanal (red line) and ethane (violet line) space time yield (STY) with corresponding values at the right axe, at 50, 70 and 90 °C (left, middle and right panels respectively), and selectivity to propanal (green bar), ethane (brown bar) and propanol (violet bar) with corresponding values on the left axe. b 3D map of TOF and propanal selectivity versus temperature of state-of-the-art-phosphine-free solid catalysts. Source data are provided as a Source Data file.

Fig. 4. Comparative catalytic and kinetic results of Rh-MFI-cal and Rh-MFI-calred zeolites.

a Catalytic performance of Rh-MFI-cal and Rh-MFI-calred samples in propanal formation under steady state conditions (left) and the respective apparent activation energy (Ea) to propanal formation (right). b Propanal space time yield with time on stream at 50 °C (left), 70 °C (middle) and 90 °C (right) on calcined (violet) and calcined-reduced (red) samples. Source data are provided as a Source Data file.

Fig. 5. Dynamic behavior of Rh-MFI-cal sample under reaction conditions.

Representative schema showing the disruption of Rh₂O₃ clusters into single Rh³⁺ sites and its stabilization under reaction conditions as Rh(CO)L.

Fig. 6. Dynamic behavior of Rh-MFI-calred sample under reaction conditions.

Representative schema showing the disruption of Rh⁰ clusters into Rh(I)(CO)₂ species under reaction conditions.

Fig. 7. Catalytic performance and spectroscopic characterization of Rh-(O)-P-MFI zeolite.

a Promoting effect of P in the catalytic performance. b IR-CO at −65 °C of samples after exposure to hydroformylation, Rh-(O)-P-MFI-calred (red line), Rh-MFI-calred (violet line), showing stabilization of Rh³⁺ in the presence of P. c–f Evolution of whiteline intensities at 23221 eV corresponding to Rh-CO/Rh(0) (blue) and 23240 eV corresponding to Rh₂O₃ (black) for Rh-MFI-calred and Rh-(O)-P-MFI-calred, as well as Cauchy wavelet, transforms at key time points upon exposure to reaction conditions after H₂ treatment. Source data are provided as a Source Data file.

Fig. 8. Dynamic behavior of Rh-(O)-P-MFI-calred sample under reaction conditions.

Representative schema showing the disruption of Rh⁰ clusters into single Rh sites with the stabilization of Rh³⁺ sites by adjacent phosphate ions.

Fig. 9. Comparative catalytic performance of Rh-MFI-cal, Rh-MFI-calred and Rh-(O)-P-MFI-calred zeolites at 200 °C.

Catalytic performance at 200 °C and 10 bar on the Rh-MFI-cal (blue), Rh-MFI-calred (violet) and Rh-(O)-P-MFI-calred (red) samples. Source data are provided as a Source Data file.