In the present study, we demonstrate the utility of “admixture screening” for the discovery of new multicomponent heterogeneous Pd catalyst compositions that are highly effective for aerobic oxidative methyl esterification of primary alcohols. The identification of possible catalysts for this reaction was initiated by the screening of simple binary and ternary admixtures of Pd/charcoal in combination with one or two metal and/or metalloid components as the catalyst. This approach permitted rapid evaluation of over 400 admixture combinations for the oxidative methyl esterification of 1-octanol at 60 °C in methanol. Product yields from these reactions varied widely, ranging from 2% to 88%. The highest yields were observed with Bi-, Te-, and Pb-based additives, and particularly from those containing both Bi and Te. Validation of the results was achieved by preparing specific PdBiTe catalyst formulations via a wet-impregnation method, followed by application of response surface methodology to identify the optimal Pd-Bi-Te catalyst stoichiometry. This approach revealed two very effective catalyst compositions: PdBi0.47Te0.09/C (PBT-1) and PdBi0.35Te0.23/C (PBT-2). The former catalyst was used in batch aerobic oxidation reactions with different primary alcohols and shown to be compatible with substrates bearing heterocycle and halide substituents. The methyl ester products were obtained in >90% yield in nearly all cases. Implementation of the PBT-2 catalyst in a continuous-flow packed-bed reactor achieved nearly 60 000 turnovers with no apparent loss of catalytic activity.

•Suspensions of fibrous biomass are readily made to flow with polymer additives, such as carboxymethyl cellulose.•The mechanism for this rheological modification is poorly understood.•Increasing the effectiveness of such additives requires an understanding of the mechanism through which they function.•We have found that fiber surface chemistry and polymer type are important variables.•This work reveals that adsorbed quantity of polymer on fiber surfaces is critical to their action as rheological modifiers.The mechanisms governing the ability of water-soluble polymers (WSP) to alter the rheological properties of lignocellulosic biomass to achieve processing advantages are investigated. Lignocellulosic fiber surface chemistry is found to be an important factor in the efficacy of WSPs as rheological modifiers, and a strong correlation between the amount of adsorbed polymer at fiber surfaces and the yield stress of the fiber suspension indicate that adsorption of polymer is important for rheological modification. Three fiber suspensions of varying physical chemistry were produced from Aspen wood chips, and a number of additional fiber suspensions with chemically functionalized surfaces were generated from cellulose pulp. Polymer adsorption and suspension rheology are found to correlate in every case examined, but the amount of adsorbed polymer alone cannot be used to predict the efficacy of a WSP as a rheological modifier, suggesting that there are contributions from additional variables such as adsorbed-polymer conformation.Download high-res image (352KB)Download full-size image

An improved Cu/nitroxyl catalyst system for aerobic alcohol oxidation has been developed for the oxidation of functionalized primary and secondary alcohols to aldehydes and ketones, suitable for implementation in batch and flow processes. This catalyst, which has been demonstrated in a >50 g scale batch reaction, addresses a number of process limitations associated with a previously reported (MeObpy)CuI/ABNO/NMI catalyst system (MeObpy = 4,4′-dimethoxy-2,2′-bipyridine, ABNO = 9-azabicyclo[3.3.1]nonane N-oxyl, NMI = N-methylimidazole). Important catalyst modifications include the replacement of [Cu(MeCN)4]OTf with a lower-cost Cu source, CuI, reduction of the ABNO loading to 0.05–0.3 mol%, and use of NMI as the only ligand/additive (i.e., without a need for MeObpy). Use of a high flash point solvent, N-methylpyrrolidone, enables safe operation in batch reactions with air as the oxidant. For continuous-flow applications compatible with elevated gas pressures, better performance is observed with acetonitrile as the solvent.

Ru(OH)x/Al2O3 is among the more versatile catalysts for aerobic alcohol oxidation and dehydrogenation of nitrogen heterocycles. Here, we describe the translation of batch reactions to a continuous-flow method that enables high steady-state conversion and single-pass yields in the oxidation of benzylic alcohols and dehydrogenation of indoline. A dilute source of O2 (8% in N2) was used to ensure that the reaction mixture, which employs toluene as the solvent, is nonflammable throughout the process. A packed bed reactor was operated isothermally in an up-flow orientation, allowing good liquid–solid contact. Deactivation of the catalyst during the reaction was modeled empirically, and this model was used to achieve high conversion and yield during extended operation in the aerobic oxidation of 2-thiophene methanol (99+% continuous yield over 72 h).

A scalable, continuous-flow process has been developed to implement a homogeneous CuI/TEMPO catalyst system for aerobic oxidation of primary alcohols to aldehydes. This catalyst system is compatible with a wide range of alcohols bearing diverse functional groups. A dilute oxygen source (9% O2 in N2) is used to avoid flammable oxygen/organic mixtures. Residence times in the heated reaction zone can be as low as 5 min with activated (e.g., benzylic) alcohols. The method has been demonstrated with nine different alcohols, including one up to 100 g scale. This flow-based catalytic method exhibits significant advantages for aerobic oxidation of alcohols, including substantially shorter residence times and broader substrate scope relative to a Pd-catalyzed method that we reported recently.

MCM-41 mesoporous molecular sieves have been silylated with trimethylchlorosilane (TMCS) and trimethylethoxysilane (TMES) under anhydrous, vapor-phase reaction conditions. The silylation reactions were performed at 473 and 623 K following pre-treatment at either 523 or 973 K. At the pre-treatment temperature of 973 K, most hydrogen-bonded surface silanols in MCM-41 are dehydroxylated to form siloxane bridges. The reactivity of surface silanols and siloxane bridges with the silylation agents was monitored using in-situ infrared spectroscopy. More extensive silylation by TMCS and TMES is achieved at the higher reaction temperature. At 623 K, TMCS reacts with most lone and hydrogen-bonded silanols in non-dehydroxylated MCM-41 to yield a trimethylsilyl (TMS) surface coverage of approximately 1.2 TMS/nm2. Silylation protocols incorporating the dehydroxylation pre-treatment step yield lower organic loadings, because the siloxane bridges do not completely react with the silylation agents or their co-products. Silylation of titanium-containing MCM-41 catalysts with TMCS enhances their hydrophobicity and improves their activity and selectivity for the epoxidation of cyclohexene with aqueous hydrogen peroxide.