# Ultrasonic-Formic Acid Pretreatment Coupled with Metal Ion/Deep Eutectic Synergistic Catalysis: Efficient Conversion of Biomass to 5-Hydroxymethylfurfural

## TL;DR

This study presents a two-step method to efficiently convert bamboo waste into 5-hydroxymethylfurfural (HMF) using ultrasound and a deep eutectic solvent with metal catalysts.

## Contribution

The novel approach combines ultrasonic pretreatment with metal ion/DES synergistic catalysis for high HMF yield from biomass.

## Key findings

- Ultrasonic formic acid pretreatment removed 83.7% hemicellulose and 76.5% lignin with 93.5% cellulose recovery.
- An HMF yield of 51.9% was achieved using a ChCl–citric acid DES and Ca2+ catalyst at 220 °C for 1.5 h.
- The method offers an efficient, mild, and eco-friendly way to valorize waste bamboo.

## Abstract

This study developed a two-step conversion strategy for the efficient conversion of bamboo waste into 5-hydroxymethylfurfural (HMF). First, ultrasonic-assisted formic acid pretreatment was used at 80 °C for 3 h, removing approximately 83.7% of hemicellulose and 76.5% of lignin from the biomass, with a cellulose recovery of 93.5%. The ultrasonic step significantly enhanced the chemical action of formic acid through cavitation, allowing formic acid to penetrate deeper into the biomass, thereby more effectively removing hemicellulose and lignin. Subsequently, glucose was obtained through an enzymatic hydrolysis. In the second step of HMF preparation, citric acid in the hydrolysate was combined with ChCl to form an acidic deep eutectic solvent (DES), and metal chlorides were added as Lewis acid catalysts. Experiments results showed that when the ChCl–citric acid ratio was 2:1, and the Ca2+ concentration was 100 mM, an HMF yield of 51.9% was obtained at 220 °C for 1.5 h. This study provides an efficient, mild, and environmentally friendly method for the high-value valorization of waste bamboo.