Production of amino-functionalised condensed tannins via a single step conversion using supercritical fluid processing

Abstract

Amphoteric aminophenol compounds have diverse synthetic and industrial use due to their multi-functional chemical reactivity. In this study, the synthesis of amphoteric flavonoid compounds via chemical modification with sub- and supercritical ammonia was explored. Both product formation and degree of amination of both catechin and condensed tannins were dependent on the processing conditions employed. Applying subcritical ammonia conditions produced ammonium ion formation and polymerization of these flavonoids. Under these conditions, ammonium group hydrogen-bonding was determined at the flavonoid B-ring catechol group and characterized by the δNH₄⁺ bending at 1400 cm⁻¹ in FTIR spectra. In contrast, using supercritical ammonia conditions (150 °C, 115 bar) led to formation of the desired amine-substituted products. Characterization of the aminated catechin and tannin products confirmed hydroxyls of the flavanyl A-ring were substituted to –NH₂. FTIR established C-NH₂ bond formation (1550, 1250 cm⁻¹) with %N and NMR indicating stoichiometric substitutions of hydroxyl groups at the C5 and C7 flavonoid A-ring positions. Moreover, under supercritical ammonia conditions, there was a reduced tendency for the undesired base-catalysed autocondensation observed at lower temperatures and pressures.

Keywords:

• Supercritical fluid
• subcritical
• supercritical
• ammonia
• flavonoids
• catechin
• condensed tannin
• amination
• aminophenols