Sonochemical preparation of alumina-spheres loaded with Pd nanoparticles for 2-butyne-1,4-diol semi-hydrogenation in a continuous flow microwave reactor

Abstract

A novel protocol for microwave-assisted alkyne semi-hydrogenation under heterogeneous catalysis in a continuous flow reactor is reported herein. This challenging task has been accomplished using a multifaceted strategy which includes the ultrasound-assisted preparation of Pd nanoparticles (average Ø 3.0 ± 0.5 nm) that were synthesized on the μ-metric pores of sintered alumina spheres (Ø 0.8 mm) and a continuous flow reaction under H₂ (flow rate 7.5 mL min^-1) in a microwave reactor (counter-pressure 4.5 bar). The semi-hydrogenation of 2-butyne-1,4-diol in ethanol was chosen as a model reaction for the purposes of optimization. The high catalyst efficiency of the process, in spite of the low Pd loading (Pd content 111.15 mg kg^-1 from ICP-MS), is due to the pivotal role of ultrasound in generating a regular distribution of Pd nanoparticles across the entire support surface. Ultrasound promotes the nucleation, rather than the growth, of crystalline Pd nanoparticles and does so within a particularly narrow Gaussian size distribution. High conversion (>90.5%) and selectivity to (Z)-2-butene-1,4-diol (95.20%) have been achieved at an alkyne solution flow rate of 10 mL min^-1. The lead-free, alumina-stabilized Pd catalyst was fully characterized by TEM, HR-TEM, EDX, IR, XRPD and AAS. Highly dispersed Pd nanoparticles have proven themselves to be stable under the reaction conditions employed. The application of the method is subject to the dielectric properties of substrates and solvents, and is therefore hardly applicable to apolar alkynes. Considering the small volume of the reaction chamber, microwave-assisted flow hydrogenation has proven itself to be a safe procedure and one that is suitable for further scaling up to industrial application.

Scheme 1. Synthetic US-assisted procedure for the preparation of highly dispersed, alumina-sphere stabilized Pd nanoparticles.

Scheme 2. MW flow ByD semi-hydrogenation to (Z)-BeD on Pd/Al₂O₃ catalyst.

Fig. 1. A schematic representation of the arrangement of the alumina-sphere stabilised Pd catalyst inside MW reaction chamber.

Fig. 2. (a) MW flow reactor for alkyne semi-hydrogenation. (b) Scheme of MW continuous flow reactor setup for liquid-phase catalytic alkyne hydrogenation reaction.

Fig. 3. ByD flow semi-hydrogenation (10 mL min⁻¹) at 65 °C and 7.5 (mL min⁻¹) H₂ flow and 4.5 bar total pressure.

Fig. 4. Influence of ByD flow rate on (Z)-BeD conversion and selectivity at 65 °C, 7.5 (mL min⁻¹) H₂ flow and 4.5 bar total pressure.

Fig. 5. Influence of H₂ total pressure (at a H₂ flow rate of 7.5 mL min⁻¹) (a) and influence of H₂ flow rate (at 4.5 bar) (b) on ByD conversion and selectivity to (Z)-BeD at 65 °C, over a total ByD flow rate of 10 mL min⁻¹.

Fig. 6. Influence of temperature on ByD conversion and selectivity to (Z)-BeD under MW irradiation (ByD (10 mL min⁻¹) and H₂ (7.5 mL min⁻¹)) and 4.5 bar of total pressure.

Scheme 3

Fig. 7. HRTEM image of the fresh Pd catalyst (a), FFT of the image reported in section a (b) and Pd particle size distribution (c). Instrumental magnification 300 000×.

Fig. 8. HRTEM image of used Pd catalyst (a), FFT of the image reported in section a (b) and Pd particle size distribution (c). Instrumental magnification 300 000×.