Enhancement of Recyclable pH-Responsive Lignin-Grafted Phosphobetaine on Enzymatic Hydrolysis of Lignocelluloses

Enzymatic conversion of lignocelluloses into fermentable sugar is a key step in the production of cellulosic ethanol. In this work, enzymatic hydrolysis lignin (EHL)-grafted phosphobetaine (EHLPB) was prepared, and the phosphobetaine intermediate, 3-chloro-2-hydroxypropyl(2-(trimethylammonio)ethyl)phosphate, was synthesized from phosphocholine chloride calcium salt and epichlorohydrin. EHLPB showed a pH-sensitive response. When pH ≥ 5.0, it was completely dissolved in the buffer solution, whereas 95.5% of EHLPPB was precipitated when pH ≤ 3.0. Adding 1.2 wt % EHLPB-210 can increase the high-solid enzymatic digestibility of Eu-SPORL (sulfite pretreatment to overcome recalcitrance of eucalyptus) and CCR (corncob residue) from 33.6% and 52.6% to 71.5% and 73.6%, respectively. After the enzymatic hydrolysis of Eu-SPORL, 95% of EHLPB-210 was recovered by adjusting the slurry pH from 5 to 3. The recovered EHLPB-210 still kept the ability to enhance enzymatic hydrolysis of lignocelluloses. Adding 1.2 wt % recovered EHLPB-210 can increase the enzymatic digestibility of Eu-SPORL from 33.6% to 75.1%. Investigation of the adsorption of cellulase on lignin by sodium dodecyl sulfate–polyacylamide gel electrophoresis (SDS-PAGE) showed that EHLPB can significantly reduce the nonproductive absorption of cellulase on lignin and therefore enhance the enzymatic hydrolysis of lignocelluloses. By adding recoverable pH-responsive EHLPB, the enzymatic hydrolysis efficiency of lignocelluloses improved and the value-added utilization of enzymatic hydrolysis lignin was realized.