Preparation, Structural Properties, and Hydrogenation Activity of Highly Porous Palladium-Titania Aerogels

Abstract

Mesoporous to macroporous palladium-titania aerogels with high surface area have been synthesized by the sol-gel-aerogel route. A titania gel was prepared by the addition of an acidic hydrolysant to tetrabutoxytitanium(IV) in methanol. The palladium precursor solutions, added after the redispersion of the titania gel, were either Na2PdCl4, (NH4)2PdCl4, Pd(acac)2, or Pd(OAc)2 dissolved in protic or aprotic solvents. The palladium-titania aerogels have a BET surface area of 170-190 m2 g−1 after a thermal treatment up to 673 K and contain well-developed anatase crystallites of about 7-8 nm mean size. Depending on the palladium precursor used, the volume-weighted-mean particle size, determined by TEM, varies significantly in the range 21-224 nm, this being independently consistent with XRD line-broadening results. All aerogel samples showed pronounced structural stability of both the titania matrix and the palladium particles towards the pretreatment media used (air or hydrogen) at temperatures up to 773 K. Thermal analysis, combined with mass spectrometry, revealed that the untreated catalysts contain a considerable amount of entrapped organic impurities after high-temperature supercritical drying. Liquid-phase hydrogenations of trans-stilbene and benzophenone were used as test reactions for characterizing the activity and accessibility of the palladium particles. A comparison of the best dispersed Pd(OAc)2-derived aerogel catalysts with conventionally impregnated titania-supported palladium catalysts in the liquid-phase hydrogenation of 4-methylbenzaldehyde reveals superior activity and selectivity for the aerogel catalysts.