Recyclability and reusability are of importance for heterogeneous catalysts. We thus performed the stability study on the ZrO2
catalyst. The same catalyst without regeneration was recycled and
reused for the subsequent runs under identical reaction conditions. The yields of lactic acid and furfural from xylose using the
recycled ZrO2 catalyst are shown in Fig. 4. Over the five consecutive runs, a slight decline in lactic acid yield, from 34.8% to 29.5%,
was found with the re-used ZrO2 catalyst. This suggests that the
catalyst is relatively stable, although a regeneration process that
removes the organic solidresidues wouldbedesirable for the re-use
ofthe ZrO2 catalyst. Itis interesting thatthe furfural yield increased
with increasing catalyst re-use cycles. It is well known that furfural
can be produced through Brønsted acid catalyzed dehydration of
xylose. Weak Brønsted acid sites were found on a ZrO2 surface
based on the infrared spectra of adsorbed pyridine