Hornbeam pretreatment with protic ionic liquids: Cation, particle size, biomass loading and recycling effects

Abstract

This work evaluated the use of protic ionic liquids (PILs) for the valorization of biomass at cost effective conditions; use of larger particle sizes, elevated biomass loadings and the reuse of PILs for pretreatment. PILs, triethylammonium hydrogen sulfate (TEAHSO4), 1-butylimidazolium hydrogen sulfate (HBIMHSO4) and 4-methylmorpholinium hydrogen sulfate (HMMorpHSO4) were used to pretreat the hardwood species, hornbeam. Hornbeam (Carpinus betulus L.) is regarded as an important forest residue since it is found among common hardwoods in Turkey and possess great potential to be valorized in the biorefinery context. Each PIL showed similar impacts on biomass structure; they enriched the cellulose content by lignin removal and increased the crystallinity of biomass. Notably, HBIMHSO4 removed 91 % lignin and the cellulose content of biomass increased to 81 % which was attributed to the lower hydrogen bond donor capacity of imidazolium cation in HBIMHSO4. The cellulose in the untreated biomass was hydrolyzed into glucose with 95 % yield. The results were also verified with FTIR and XRD findings; the intensities of the peaks associated with the presence of lignin got lower and higher crystallinity indices were obtained upon HBIMSO4 pretreatment. The best results in terms of biomass composition were achieved when hornbeam with particle size, 0.15–1.18 mm was used. Pretreatment of biomass correlated inversely with an increase in biomass loading. Nevertheless, TEAHSO4 and HBIMHSO4 were able to extract lignin as high as 70 % at 30 % biomass loading. Recycled HBIMHSO4 at 20 % biomass loading performed very stably regarding the changes in the biomass structure and saccharification. Besides, no changes were observed between the chemical shifts of pristine PILs and recycled PILs analyzed with 1H-NMR after their 6th reuse in pretreatment. The findings addressed the potential of HBIMHSO4, for its use at larger volumes of biomass-PIL interactions.