*2.2. HMF Production*

The catalytic performance of different heteropolyacid catalysts prepared according to the experimental design given in Table 1 was evaluated regarding catalytic performance in order to convert glucose into HMF. The catalytic reactions carried out using 100 mL of reaction medium at 160 ◦C using 1% *w*/*v* of catalyst, 100 g/<sup>L</sup> of glucose, 1:1 *w*/*w* acetone-to-water ratio, and 300 rpm over 30 min. The catalyst that achieved optimal HMF production performance was then used in subsequent experiments with the aim of optimizing reaction conditions.

For optimization experiments, different reaction conditions, such as temperature (160 or 200 ◦C), acetone-to-water ratio (1:1 or 3:1 *v*/*v*), glucose concentration (50 or 100 g/L), and catalyst concentration (1% or 5% *w*/*v*) were used based on Taguchi's L16 experimental design. The column "E" had no factor associated to estimate the experimental design error. All catalytic tests were performed in pressurized stainless steel reactors (Parr series 4566). At the end of the runs, the catalyst was recovered from the reaction mixture by centrifugation at 2000 × *g* for 20 min and calcined at 300 ◦C for 3 h.

Catalyst stability was evaluated by reusing it in successive batch runs performed under optimized process conditions.
