BACKGROUND: Acrylic acid commercially produced by the successively catalytic oxidation of propylene has a high cost. The synthesis of acrylic acid by aldol condensation between coal derivatives, acetic acid and formaldehyde, is an economic alternative to the propylene oxidation process. However, the development of an environmentally friendly and effective catalyst has remained a challenge. RESULTS: Diboron trioxide [B 2 O 3 (6-20%)]/silica (SiO 2 ) nanocomposites with B 2 O 3 particle sizes of 1-2 nm prepared by the wetness impregnation of boric acid into silica aerogel and subsequent calcination at 500 °C effectively catalyzed the gas-phase aldol condensation reaction between acetic acid and formaldehyde (trioxymethylene) to acrylic acid with a selectivity of ≈87% at 340-400 °C. Bismuth (Bi-), tungsten (W-) and caesium (Cs)-doped B 2 O 3 /SiO 2 nanocomposites had higher catalytic activities in the gas-phase aldol condensation reaction to acrylic acid than the undoped B 2 O 3 /SiO 2 nanocomposite. CONCLUSION: Weak-strength Lewis acid and alkali sites of the B 2 O 3 (6-20%)/SiO 2 nanocomposites co-catalyzed the aldol condensation reaction to form acrylic acid. The doping of B 2 O 3 /SiO 2 nanocomposites with Bi, W and Cs components results in the formation of BiBO 3 , WO 3 and CsBO 2 phases, which lead to an increase in acidity and basicity resulting in higher catalytic activity in the formation of acrylic acid.
Series active power filter (SAPF) improves power quality as a key technology, which has been highly valued and widely used at present. NPC (neutral point clamped) three-level SAPF enables to compensate harmonic voltage in the field of high voltage, but traditional NPC three-level SAPF has a problem of neutral-point voltage excessive fluctuation which will cause switch devices damage. In order to solve this problem, a NPC three-level SAPF with neutral-point voltage self-restraining function is proposed. A controllable voltage source will be put in series between the neutral point of the DC-side capacitors and point O in traditional NPC three-level SAPF. To obtain better control effects, sliding mode control method is used to control the compensation voltage generated. Neutral-point voltage will be compensated in the real-time. Simulation results show that the proposed SAPF has good performances.
Titania nanotube films were produced by anodization of titanium foil. The titania nanotube films were annealed at different temperatures. Morphology evolution, phase transformation and electrical conductivity of the titania nanotubes were studied. Results showed that the nanotube walls became rough, porous and even collapsed after annealed at 400, 500 and 600 °C respectively. Titania anatase phase formed after annealed at 400 °C; the amount of anatase phase increased as the annealing temperature increased. The conductivity of the nanotube film annealed at 400 °C was improved greatly compared with the conductivity of the as-anodized nanotube film. However, the conductivity of the nanotube films annealed at higher temperatures decreased. The effect of the morphology on the electronic conductivity of the titania nanotube films was discussed.
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