Synthesis of pH indicator from saccharides and agroforestry wastes with Brønsted acids via a thermo-chemical strategy

The utilization of commercial acid-base indicators leads to environmental pollution and CO
2
emissions. A bio-based, unprecedented indicator was synthesized with sugars, cellulose, and agroforestry wastes through a simple, acid-mediated thermo-chemical approach. Its solution was colorless at pH < 4.4 and bright green at pH > 4.4, and the color change took place at pH 6. The mass yield of the indicator reached 3.7% with glucose as the substrate. According to the analysis results of nuclear magnetic resonance (
1
H and
13
C NMR), infrared spectroscopy, ultraviolet-visible spectroscopy (UV-Vis), and liquid chromatography-mass spectrometry (LC–MS), this indicator contained an enol moiety. In an alkaline medium, the OH groups were deprotonated, resulting in the
σ-π
hyperconjugation interaction between C–H bonds and the C=C bond. The absorption wavelength of the conjugated moiety was increased to the visible-light region, leading to the bright green color of this indicator. Hydrolysis of cellulose, isomerization of glucose, dehydration of fructose, hydrodeoxygenation of 5-hydroxymethylfurfural (HMF), opening of furan ring, aldol reaction with formaldehyde, and rearrangement of diketone occurred in the synthesis process of pH indicator. Undesirable colorants were eliminated in the heating-drying procedure, improving the purity of the indicator. Additionally, a carbonaceous byproduct with 0.63 mmol g
-1
of strong acid sites was produced, and this material showed high catalytic activity as a solid acid.