PdNP@Titanate Nanotubes as Effective Catalyst for Continuous‐Flow Partial Hydrogenation Reactions

Abstract

Pd nanoparticles were easily immobilized onto titanate nanotubes by a straightforward procedure. The material (0.50 wt % Pd) was used as catalyst in the continuous‐flow, liquid‐phase hydrogenation reaction of unsaturated C−C bonds and it showed excellent performance and durability under very mild conditions (room temperature, 1–2 bar H2, residence time 13–36 s). In particular, very high productivity was obtained in the synthesis of the perfumery component cis‐3‐hexen‐1‐ol (40.6 mol gPd−1 h−1) without additives or metal contamination, with clear benefits in terms of process economy and environmental impact compared with conventional catalysts. The catalyst performance is discussed in the light of comparable systems.

Unconventional Pd@Sulfonated Silica Monoliths Catalysts for Selective Partial Hydrogenation Reactions under Continuous Flow

“…In the recent years, many catalysts have been reported for the synthesis of 2 a under continuous‐flow conditions (Table ). These include packed‐bed systems based onto commercial (Pd@C, Lindlar, entries 3, 4) and laboratory catalysts (Pd@ion‐exchange resins, Pd@SiO 2 ,TiO 2 mesoporous xerogels, Pd@titanate nanotubes, entries 5–8), as well as monolithic systems (Pd@MonoSil, Pd@TiO 2 monolith, Pd@MonoBor, entries 9–11). In terms of product yield, the Pd@MonoSil‐ArSO 3 system ranks just below the best catalyst so far reported for this reaction (Pd@MonoBor organic monolith, entry 11), and its selectivity is the only one comparable to that of the industrial batch process.…”

“…The above results point out to hydrogenation pathways involving a fast interaction of the substrate with easily available Pd sites onto the solid catalyst. As previously reported for comparable heterogeneous systems, the high substrate flow rates and the short residence times allowed by the monolithic catalyst may result in a fast interaction of the active sites with the intermediate semi‐hydrogenation products, which quickly leave the metal after formation with no possibility to react further, thus to be beneficial in terms of both selectivity and productivity of partial hydrogenation. If the substrate would have a prolonged adsorption with the metal sites inside the pores, the intermediate formed cannot diffuse away fast enough and it is hydrogenated before leaving the catalyst, resulting in a lower selectivity at the same conversion level.…”

Continuous-flow processes for the catalytic partial hydrogenation reaction of alkynes

“…Best yields (94–100%) were achieved using either 16% CeO 2 @TiO 2 (100%) [119] or Pd@MonoBor catalyst [136], under 90 bar H 2 and 413 K or 1.3 bar H 2 and 294 K, respectively (Table 1, entries 28, 29). As above reported for other substrates, best results in terms of productivity were provided by the monolithic and the titanate nanotubes-supported Pd catalysts (2.9–6.5 mol g Pd −1 h −1 , Table 1, entries 29, 30) [137]. Lower performances were observed using packed-bed catalysts and conventional support materials [123], an amorphous Pd 81 Si 19 alloy catalyst in supercritical CO 2 (76% sel.…”

“…7), that was justified in terms of both accessibility of Pd sites and high permeability of the packed 1D tubular material (weight hourly space velocity, g substrate g catalyst −1 h −1 ca. 11088 h −1 ) [137–138]. In fact, much lower efficiency was observed under analogous flow conditions using Pd immobilized into the pores of conventional mesoporous powder titania.…”

“…12) [136–137 161]. Good yield (68.2%) and productivity (11.4 mol g Pd −1 h −1 ) of 11a were obtained by packing pellets of a hybrid zirconia/polyvinylalcohol matrix with embedded PdNP (Table 2, entry 16) [142].…”