Sustainable Utilization of Phoenix Date Palm Biomass via Extraction of Lignin Using Pyridinium-Based Protic Ionic Liquid

Lignin is a renewable biopolymer widely available in industrial crops, yet its efficient separation and valorization remain challenging due to its heterogeneous structure derived from coniferyl, sinapyl, and p-coumaryl alcohol units. Its properties vary with the biomass type and extraction method. This study investigates lignin extraction from four Saudi date palm leaf varieties (Ajwa, Safawi, Amber, and Rabia) using a newly synthesized protic ionic liquid (PIL) composed of a pyridinium cation and formate anion. Extraction was carried out at temperatures ranging from 25 to 125 °C for 1 to 6 h. The PIL achieved high lignin recovery, up to 90%, under mild conditions (75 °C, 3 h), while also enabling subsequent cellulose pulp saccharification. In the second stage, extracted lignin samples were characterized using structural, thermal, and morphological analysis techniques in a comparative approach. It was concluded that PIL-treated lignin resulted in enhanced phenolic and aromatic contents with increased thermal stability, making it a suitable biofuel. The findings highlight date palm leaves as a low-cost lignin source suitable for pharmaceutical applications and the development of bionanocomposites with enhanced thermal performance.