The aerobic oxidation of 5-hydroxymethylfurfural (HMF), a key platform compound in cellulose transformation, into 2,5-furandicarboxylic acid (FDCA), a promising renewable alternative to petroleum-derived terephthalic acid, is one of the most attractive reactions for establishing biomass-based sustainable chemical processes. Supported Au catalysts have shown encouraging performance for this reaction, but the need of an excess amount of base additives makes the process less green and less cost-effective. Here, we report a stable and efficient carbon nanotube (CNT)-supported Au−Pd alloy catalyst for the aerobic oxidation of HMF to FDCA in water without any bases. The functionalization of CNT surfaces is crucial for FDCA formation. We have clarified that the CNT containing more carbonyl/ quinone and less carboxyl groups favors FDCA formation by enhancing the adsorption of the reactant and reaction intermediates. Significant synergistic effects exist between Au and Pd in the alloy for the base-free oxidation of HMF to FDCA through three tandem steps. The present work provides understanding of the support-enhanced adsorption effect and the alloying effect for supported Au-based bimetallic catalysts, and this knowledge may help develop efficient catalysts for the aerobic oxidation of relatively complicated organic compounds with different functional groups in water.
Gold nanoparticles with uniform mean sizes (≈3 nm) loaded onto various supports have been prepared and studied for the oxidant-free dehydrogenation of benzyl alcohol to benzaldehyde and hydrogen. The use of hydrotalcite (HT), which possesses both strong acidity and strong basicity, provides the best catalytic performance. Au/HT catalysts with various mean Au particle sizes (2.1-21 nm) have been successfully prepared by a deposition-precipitation method under controlled conditions. Detailed catalytic reaction studies with these catalysts demonstrate that the Au-catalyzed dehydrogenation of benzyl alcohol is a structure-sensitive reaction. The turnover frequency (TOF) increases with decreasing Au mean particle size (from 12 to 2.1 nm). A steep rise in TOF occurs when the mean Au particle size becomes smaller than 4 nm. Our present work suggests that the acid-base properties of the support and the size of Au nanoparticles are two key factors controlling the alcohol dehydrogenation catalysis. A reaction mechanism is proposed to rationalize these results. It is assumed that the activation of the β-C-H bond of alcohol, which requires the coordinatively unsaturated Au atoms, is the rate-determining step.
This paper describes the key technology and circuit design issues facing the design of an efficient linear RF CMOS power amplifier for modern communication standards incorporating high peak-to-average ratio signals. We show that most important limitations arise from the limited breakdown voltage of nanoscale CMOS devices and the large back-off requirements to achieve the required linearity, both of which result in poor average efficiency. Two fundamentally different approaches to tackle these problems are presented along with silicon prototype measurements. In the first approach, transformer power combining and bias-point optimization are used to increase the output power and linearity of the "analog" amplifier. In the second approach, a mixed-signal "digital" polar architecture is employed, wherein the amplitude modulation is formed through an RF DAC structure.Index Terms-CMOS power amplifier (PA), CMOS RF PA, digital PA, digital RF.
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