Abstract
Selective oxidation of alcohols to aldehydes and acids is an important reaction in organic synthesis and will likely play a significant role in the development of a sustainable route to value-added chemicals from biomass. Under environmentally-friendly conditions (<400 K, dioxygen as oxidant, aqueous solution), platinum can efficiently catalyze the oxidation of biomass-derived alcohols, such as 5-hydroxymethylfurfural, glycerol and 1,6-hexanediol to the corresponding carbonyl compounds. However, the high cost of Pt as well as the severe deactivation during reaction limits the application of this process on large scale and distributed systems. Therefore, the motivation of this project is to rationally develop an efficient, stable and affordable catalyst based on the mechanistic understanding of the reaction and deactivation path.
During Pt-catalyzed 1,6-hexanediol oxidation, the initial rate decreases significantly with reaction time because of competitive adsorption of products and irreversible adsorption of unknown strongly-bonded species. To identify the poisoning species, in situ surface-enhanced Raman spectroscopy (SERS) and solid-state 13C nuclear magnetic resonance (NMR) spectroscopy were applied in this work. In situ SERS during 1,6-hexanediol oxidation revealed an accumulation of di-σ-bonded olefinic species with features at ~1150 cm-1 and ~1460 cm-1 on the poisoned Pt surface. Consistent with SERS, results from 13C NMR spectroscopy of a Pt catalyst deactivated from oxidation of 13C-labeled 1,4-butanediol revealed a C=C feature associated with ethylene. Molecules containing olefinic groups are two orders of magnitude more effective at competing for Pt surface sites compared to the aldehyde and acid products from alcohol oxidation. The poisoning olefinic species were generated by decarbonylation of product aldehyde (as revealed by head space analysis) and could be easily removed from the deactivated catalyst by mild treatment in H2.
In an attempt to suppress the deactivation and increase the rate of alcohol oxidation, a series of carbon-supported bimetallic Bi-Pt catalysts with various Bi/Pt atomic ratios was prepared by selectively depositing Bi on Pt nanoparticles. The catalysts were evaluated for 1,6-hexanediol oxidation activity under different dioxygen pressures. The rate of diol oxidation based on Pt loading over a Bi-promoted catalyst was three times faster than an unpromoted Pt catalyst in 0.02 MPa O2, whereas the unpromoted catalyst was more active than the promoted catalyst in 1 MPa O2. After liquid-phase catalyst pretreatment and 1,6-hexanediol oxidation, migration of Bi on the carbon support was observed. The reaction order in O2 was zero over Bi-promoted Pt/C compared to 0.75 over unpromoted Pt/C in the range of 0.02 – 0.2 MPa O2. Under low O2 pressure, rate measurements in D2O instead of H2O solvent revealed a moderate kinetic isotope effect (〖rate〗_(H_2 O)/〖rate〗_(D_2 O)) on 1,6-hexanediol oxidation over Pt/C (KIE = 1.4) whereas a negligible effect was observed on Bi-Pt/C (KIE = 0.9), indicating the promotional effect of Bi could be related to the formation of surface hydroxyl groups from reaction of dioxygen and water. However, no significant change of product distribution or catalyst stability was observed with Bi promotion, regardless of the dioxygen pressure, which is likely related to the high mobility of Bi.
From the perspective of replacing the rare and expensive Pt catalyst, an atomically-dispersed Fe catalyst on nitrogen-doped carbon (Fe-N-C) containing bio-mimic nitrogen-coordinated Fe sites (FeNx) sites was provided by collaborators who used a sacrificial support method using inexpensive ferrous iron salt and nicarbazin as precursors, which exhibited modest activity in the oxidation of benzyl alcohol and 5-hydroxymethylfurfural by O2 in the aqueous phase. Whereas deactivation was observed, the activity of the catalyst can be regenerated by a mild treatment in H2. An observed kinetic isotope effect indicates β-H elimination from the alcohol is the kinetically-relevant step in the mechanism, which can be accelerated by substituting Fe with Cu. Dispersed Cr, Co and Ni also convert alcohols, demonstrating the general utility of metal-nitrogen-carbon materials for alcohol oxidation catalysis. Oxidation of aliphatic alcohols was substantially slower than that of aromatic alcohols, but addition of 2,2,6,6-tetramethyl-1-piperidinyloxy as a co-catalyst with Fe can significantly improve the reaction rate.
Among the series of M-N-C catalysts that we tested, highest reaction rates of benzyl alcohol oxidation were observed over Co-N-C and Cu-N-C. The selective poisoning of highly active Co sites allowed for the estimation of a turnover frequency, which was determined to be nearly the same as that over Pt nanoparticles under identical conditions. Whereas highly-active CuNx sites also catalyze the reaction, they are about of an order of magnitude less active than the CoNx moieties. Results from X-ray absorption spectroscopy suggest the active CoNx sites are also coordinated to oxygen, which can be removed by H2 at 523 K or higher. Since spectroscopy results showed that the CoNx sites do not reduce up to 750 K, they were tested and shown to be active catalysts for propane dehydrogenation at 773 K. Characterization of CuNx sites revealed that reduction of the Cu cations begins above 473 K, which indicates the atomically-dispersed Cu catalysts can only be effective at low reaction temperature. The quantification and identification of active sites provides insights for the optimization of M-N-C catalysts in the future.
