High‐Activity, Single‐Site Mesoporous Pd/Al₂O₃ Catalysts for Selective Aerobic Oxidation of Allylic Alcohols

Abstract

The heterogeneously catalyzed aerobic selective oxidation of hydrocarbons offers new, environmentally benign routes to a diverse range of valuable intermediates for the pharmaceutical, fine chemical, and agrochemical sectors.[1] Such powerful catalytic technologies circumvent the use of stoichiometric reagents and expensive homogeneous complexes along with the associated process disadvantages and safety issues. [2] While there has been recent interest in the potential of gold as a partial oxidation catalyst, [3] such systems typically offer low oxygenate yields and require either radical initiators, [4] high temperatures, or high O 2 partial pressures.[5] Supported ruthenium, [6] palladium, and platinum clusters are also promising candidates to catalyze the selective oxidation (selox) of primary alcohols to their corresponding aldehydes under mild conditions. [7] These reactions are highly regioselective in the presence of diverse functionalities, including allylic groups. Most studies in this area employing Pd and Pt have utilized commercial formulations based upon amorphous carbon or oxide supports. However, the poor intrinsic performance of these materials relative to their homogeneous counterparts has often necessitated ad hoc promotion of the reaction by non-noble metals to achieve even moderate yields.[8] Two factors presently constrain the wider adoption of heterogeneous Pd selox catalysts amongst both academic and industrial communities: first, uncertainty over the active site responsible for the rate-limiting oxidative dehydrogenation step, [9] and second, the use of conventional supports with restrictive pore dimensions that inhibit efficient mass and heat transfer to and from reaction sites and limit the range of viable solvents and substrates. [10] Recent studies have implicated surface palladium oxide as the active center in allylic alcohol selox.[11] Since the energetics of metal clusters increasingly favor oxide-terminated surfaces with decreasing cluster size, [12] we hypothesized that a mesoporous high-area support would serve to both stabilize highly dispersed palladium oxide nanoparticles and would facilitate efficient alcohol and aldehyde diffusion. Herein, we report the successful synthesis of tailored PdAl 2 O 3 catalysts and demonstrate that extremely low palladium loadings generate atomically dispersed Pd II surface species that confer exceptional selox activity of allylic alcohols.Mesoporous alumina-supported palladium catalysts (Pd/ meso-Al 2 O 3 ) were prepared by a modified surfactant-templated route through hydrolysis of aluminum sec-butoxide and subsequent aging in the presence of lauric acid.[13] The organic template was removed by calcination prior to incipient wetness impregnation with tetraammine palladium(II) nitrate solution (see the Supporting Information). Samples were then calcined, reduced, and stored in air. Porosimetry and powder X-ray diffraction confirmed that the final processed materials possessed well-defined, hexagonal pore structures, with surface areas of 35...

An oxidized magnetic Au single atom on doped TiO2(110) becomes a high performance CO oxidation catalyst due to the charge effect

“…Here, we note that a previous investigation also predicted a similar phenomenon on a defective oxide surface, 29 where the healing of the oxygen vacancy is believed to promote the adsorbed metal SAC clustering, and hence to reduce the selectivity and performance of the SAC. 34 On the contrary, in the present case, the remaining O atom healing the V O renders the Au atom to form a very stable bow-shaped surface O-Au-O species, with an exothermic energy of 300.069 kJ mol À1 . Therefore, the Au atom is now highly stabilized and oxidized, i.e., Q(Au) ¼ +0.64 e. Note that, recently, Wang et al 53 found that single Au cation active sites can be generated from adsorbed Au 20 clusters on TiO 2 (110), exhibiting excellent catalysis for CO oxidation.…”

Single Isolated Pd2+ Cations Supported on N-Doped Carbon as Active Sites for Hydrogen Production from Formic Acid Decomposition

“…Single-site catalysis with Pd for formic acid decomposition. As we reported above, single isolated Pd atoms supported mainly on oxide supports were shown to be active in reactions of oxidation [13][14][15] and CO 2 reduction. 16 However, for some of these reactions a deactivation of the catalyst was observed.…”

Catalysis by gold: New insights into the support effect

“…Since then, the SMSI has been intensively studied on PGMs by applying various characterizations and it has been found in many other supported catalysts including Rh/TiO 2 [160], Ru/TiO 2 [161], Pd/TiO 2 [162], Pd/Al 2 O 3 [163], and so on. Taking advantage of the SMSI, it is very easy to obtain highly dispersed nanoparticles, even single atom dispersion of Pt or Pd [164][165][166].…”

“…Isolated Pd supported on Al 2 O 3 was reported to be efficient for the aerobic oxidation of alcohols [165]. Recently, Pt single atom supported on FeOx was reported to show excellent catalytic activity in both CO oxidation and PROX reactions [164,166].…”