3.2 Effect of temperature and residence time
The CB conversion is an important parameter that can be used to characterize the reaction process and measure the reaction rate; it also affects the selectivity. The CB conversion was investigated as a function of temperature and residence time, as shown in Figure 2 . The mass fraction of sulfuric acid was 95%. The results showed that under the condition of incomplete CB conversion, the conversion increased with increasing residence time, and at the same residence time, the conversion increased with increasing temperature, indicating that higher temperatures significantly increased the reaction rate. It is worth noting that the reaction was carried out at a low temperature and with a short residence time. When the conversion was very low, the concentration of nitric acid did not obviously decrease and was approximated to be constant. Therefore, the conversion of CB increased linearly and in accordance with pseudo-first-order reaction kinetics.