In situ preparation of Pd/Al2O3–SiO2 composite microspheres by combining a sol–gel process and precipitation process in a microchannel

• A novel method for the preparation of Pd/Al2O3–SiO2 microspheres in a microchannel.
• The specific surface area of the spheres was >600 m2/g.
• The smallest Pd particles had an average size of 3.79 nm after calcination at 550 °C.
• The sphere with the diameter of 400 μm was suitable for packing into a fixed bed.

Pd/Al2O3–SiO2 composite microspheres with 4 nm Pd nanoparticles were prepared in a microchannel by combining a sol–gel process with the precipitation of Pd ion. The sol of silica and alumina with Pd ion was used as the dispersed phase, and liquid paraffin-containing soluble organic amines were used as the continuous phase. The mass transfer of organic amines from the organic phase to the aqueous phase in microchannels changed the pH of the aqueous phase, which resulted in the precipitation of PdCl2 in acidic solution and the faster sol–gel process of SiO2–Al2O3. The resulting catalyst was characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Brunauer–Emmett–Teller analyses. Experimental results showed that the prepared microspheres with a diameter about 400 μm had a special structure that was externally compact and internally, and they were suitable for packing into a fixed bed. Each microsphere had a microporous–mesoporous–macroporous structure and the specific surface area was larger than 600 m2/g. After calcination at 550 °C, the smallest Pd particles still had an average size of 3.79 nm and were highly dispersed. These microspheres in a fixed bed also showed a high performance for the hydrogenation of cyclohexene. When the retention time was 2.2 min in the liquid phase, a conversion rate reached 0.447. This one-step in situ synthesis had the advantages of high utilization of Pd, small Pd nanoparticles with high monodispersity, and adjustability of resulting microspheres at the micron level.