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[Display omitted] • Autohydrolysis enhanced the delignification of p -TsOH post-treatment of poplar. • Unexpectedly, the glucose yields of combined pretreated poplar decreased. • The intensified surface lignin coverage hindered the enzymatic hydrolysis. • Tween 80 enabled the excellent enzymatic hydrolysis of combined pretreated poplars. In this study, a combined pretreatment involving autohydrolysis and p -toluenesulfonic acid (p -TsOH) was performed on poplar to coproduce xylooligosaccharides (XOSs) and monosaccharides. The autohydrolysis (180 °C, 30 min) yielded 53.2 % XOS and enhanced the delignification efficiency in the subsequent p -TsOH treatment. Furthermore, considerably high glucan contents (64.1 %∼83.1 %) were achieved in the combined pretreated substrates. However, their enzymatic digestibilities were found to be extremely poor (9.6 %∼14.2 %), which were even lower than the single p -TsOH pretreated substrates (10.2 %∼35.8 %). The underlying reasons were revealed by systematically investigating the effects of the single and combined pretreatment strategies on substrate properties. Moreover, the Tween 80 addition successfully reversed the adverse effects of combined pretreatment on the enzymatic hydrolysis, achieving a high glucose yield of 99.3 % at an enzyme loading of 10 filter paper units/g (FPU/g) glucan. These results deepen the understanding of the synergy of combined pretreatment on biomass fractionation and enzymatic saccharification. [ABSTRACT FROM AUTHOR]

To elucidate the effects of lignin repolymerization on the combined pretreatment, the sequential acid and deep eutectic solvent (DES) pretreatment was implemented on Populus (poplar) in this study. Effects of acid pretreatment with or without phenolic additives (2-naphthol (2 N) and resorcinol (RS)) on the DES post-treatment efficiency were examined. The results showed that 2 N inhibited the lignin repolymerization in the first-step pretreatment, and slightly enhanced the lignin removal during DES post-treatment. However, RS intensified the lignin repolymerization, significantly decreasing the delignification of DES post-treatment from 73.6%∼79.4% to 39.0∼43.0%. Moreover, the poorer enzymatic hydrolysis was observed in the combined pretreated substrates as compared to the single DES pretreated ones, due to the higher surface lignin coverage. The addition of 2 N counteracted the adverse effects of acid pretreatment on the hydrolysis yields of combined pretreated substrates, and facilitated the yields from 25.8%∼36.0% to 26.2∼67.3%, while the RS addition further decreased the hydrolysis yields to 8.6∼9.6%. The mechanism for the changes on enzymatic digestibilities of the combined pretreated substrates was elucidated. [Display omitted] • 2-Naphthol inhibited lignin polymerization, while resorcinol intensified it. • Lignin repolymerization hindered the delignification of the DES post-treatment. • 2-Naphthol promoted the hydrolysis of pretreated poplar, but resorcinol impeded it. • 2-Naphthol addition benefited the lignin fractionation and valorization. [ABSTRACT FROM AUTHOR]