Plastic pollution is one of the most important environmental issues being faced today. In this work, the hydrothermal liquefaction performance of polycarbonate (PC) in sub-/supercritical water was investigated using a quartz tube reactor. Response surface methodology (RSM) was employed to demonstrate the correlation between reaction conditions and liquefaction efficiency of PC. The conversion selectivity and product recovery efficiency of PC were also analyzed. Moreover, according to the experimental results, the liquefaction reaction pathways of PC were speculated. In addition, based on the established reaction pathways, the lumped parameter method was also used to calculate the liquefaction kinetics of PC. The results showed that phenol was the largest liquefied product of PC, followed by 4-isopropylphenol (IPrP) and 4-isopropenylphenol (IPP), and the recovery efficiency of these three components determined the level of liquefaction efficiency. The PC liquefaction favored a mild reaction temperature, long residence time, and low feedstock concentration. Finally, for identified products, a 57.70 wt % carbon liquefaction efficiency was obtained with a 5 wt % feedstock concentration at 400 °C for 60 min. The liquefaction pathways showed that in the initial stage of depolymerization, 4-tert-butylphenol was first formed as a structural regulator, and phenol and IPP were the primary liquefied products. The IPrP, 4-ethylphenol, and p-cresol were the secondary products. The kinetic results indicated that the liquefied intermediates were easily converted to form phenol and IPP, and the IPP and IPrP were not directly converted to phenol, but there was a clear conversion relationship between them.
Lignin
and cellulose were gasified at 500 and 600 °C in supercritical
water over co-precipitated CeO2–ZrO2 catalyst.
The addition of CeO2–ZrO2 improved the
gasification efficiency and hydrogen production, and the catalytic
effect was more significant at lower temperatures. The H2 yield from cellulose gasification at 500 °C increased by over
2.5 times to 8.50 mol/kg with the presence of CeO2–ZrO2. The gas chromatography-mass spectrometry (GC-MS) analysis
showed that the catalyst reduced the content of cyclopentenone and
furan derivatives in the aqueous product but increased the content
of refractory phenols. We characterized the fresh and the used catalysts
with H2-temperature-programmed reduction (H2-TPR), X-ray diffraction (XRD), and scanning electron microscopy-energy-dispersive
X-ray spectroscopy (SEM-EDS) analysis, showing that CeO2 was the main active component, which catalyzed gasification through
redox reactions. ZrO2 enhanced the catalytic activity of
CeO2 by lowering the reduction temperature in hydrogen,
increasing the dispersion of CeO2, and facilitating H2 adsorption on the catalyst surface.
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