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...

Catalysing sustainable fuel and chemical synthesis

“…support charge transfer [183]. These Pd/meso-Al 2 O 3 catalysts exhibited similar TOFs to their silica counterparts (7,080 and 4,400 h -1 for crotyl and cinnamyl alcohol selox, respectively) [137], consistent with a common active site and reaction mechanism (Fig. 17).…”

“…For Pd/NaX and Pd/SiO 2 -Al 2 O 3 , benzyl alcohol selox was fastest over particles between 3 and 5 nm, whereas geraniol and 2-octanol were structure-insensitive. Systematic studies of particle size effects in cinnamyl and crotyl alcohol selox over amorphous and mesostructured alumina and silica supports have likewise uncovered pronounced size effects in both initial selox rates and TOFs [133][134][135][136], which increase monotonically with shrinking nanoparticle diameters (even down to single atoms) [137]. HAADF-STEM analysis reveals atomically dispersed palladium exhibits maximal rates towards benzyl, cinnamyl and crotyl alcohols, with selectivities to their corresponding aldehydes [70 %.…”

“…In a parallel research programme, the author's group systematically characterised the physicochemical properties of palladium nanoparticles as a function of size over non-reducible supports to quantify structure-function relations in allylic alcohol selox [133-137, 158, 159]. The combination of XPS and XAS measurements revealed that freshly prepared alumina [134,137] and silica [135,158] supported nanoparticles are prone to oxidation as their diameter falls below *4 nm, with the fraction of PdO proportional to the support surface area and interconnectivity. Complementary kinetic analyses uncovered a direct correlation between the surface PdO content and activity/ TOFs towards cinnamyl and crotyl alcohol selox [134,137].…”

“…The combination of XPS and XAS measurements revealed that freshly prepared alumina [134,137] and silica [135,158] supported nanoparticles are prone to oxidation as their diameter falls below *4 nm, with the fraction of PdO proportional to the support surface area and interconnectivity. Complementary kinetic analyses uncovered a direct correlation between the surface PdO content and activity/ TOFs towards cinnamyl and crotyl alcohol selox [134,137]. Operando liquid phase XAS of Pd/C and Pd/Al 2 O 3 -SBA-15 catalysts during cinnamyl alcohol selox evidenced in situ reduction of PdO (Fig.…”

“…centres [137]. Such single-site catalysts were 10 times more active in crotonaldehyde and cinnamaldehyde production than comparable materials employing conventional (100 m 2 g -1 ) c-alumina, owing to the preferential genesis of higher concentrations of electron-deficient palladium [134,137], due to either pinning at cation vacancies or metal ? support charge transfer [183].…”