Research paper 4-Hydroxybenzoic acid from hydrothermal pretreatment of oil palm empty fruit bunches e Its origin and influence on biomass conversion Helena Rasmussen a , Kit H. Mogensen a , Martin D. Jeppesen a , Hanne R. Sørensen a , Anne S. Meyer b, * a DONG Energy, Kraftværksvej 53, DK-7000 Fredericia, Denmark b Center for BioProcess Engineering, Dept. of Chemical and Biochemical Engineering, Technical University of Denmark, Søltofts Plads 229, DK-2800 Lyngby, Denmark article info Article history: Received 11 July 2016 Accepted 14 July 2016 Keywords: Lignin 5-Methylfuran-2-carbaldehyde Reaction mechanism Degradation compound Pectin abstract An unknown major compound, characteristically occurring during processing of oil palm empty fruit bunches was identified with LC-DAD-ESI-MS/MS to be 4-hydroxybenzoic acid. Lignin from oil palm empty fruit bunches contains 4-hydroxybenzoic acid so a tempting conclusion was that the 4- hydroxybenzoic acid originated from lignin. However, another hypothesis to its origin was also tested. The route considered involves degradation of rhamnose to 5-methylfuran-2-carbaldehyde followed by reaction with formic acid. Experimental hydrothermal pretreatment of pure rhamnose in the presence of formic acid revealed that 5-methylfuran-2-carbaldehyde is in fact a degradation product from rhamnose, analogous to glucose degradation to 5-(hydroxymethyl)-2-furaldehyde. However, the subsequent step of carboxylation with formic acid to form 4-hydroxybenzoic acid was found not to take place in practice at realistic biomass hydrothermal pretreatment conditions. 5-methylfuran-2-carbaldehyde only differs from furfural by having an extra methyl group and the degradation route indicates that it may be a new important degradation compound to consider in other biomass feedstocks rich in deoxysugars such as rhamnose or fucose, e.g. pectin rich biomasses. Assessment of the influence of 4-hydroxybenzoic acid in the enzymatic hydrolysis of pretreated oil palm empty fruit bunches as well as its presence during fermentation showed that 4-hydroxybenzoic acid is not inhibiting or mediating neither on the enzymatic hydrolysis or fermentation in the quantified range from 0.1 g/L to 1 g/L, indicating an option for reaping the 4-hydroxybenzoic acid from the biomass liquor directly after hydrothermal pretreatment for bio- refinery value-addition. © 2016 Elsevier Ltd. All rights reserved. 1. Introduction A wealth of biomasses as well as processing methods are available for biorefinery solutions e whether the goal is to supply environmentally friendly energy or valuable compounds available for further synthesis [1e5]. In order to avoid formation of unde- sirable products during processing and thus to attain optimal product yields it is a key prerequisite to know not only the struc- tural composition of the biomass, but also to understand the reactions of the structural elements during processing. e Especially seen in the light of that undesired compounds that inhibit the enzymes and yeast can be formed during pretreatment in the production of 2nd generation bioethanol [6]. The empty fruit bunches left over from palm oil processing contain ~30e50% by weight of cellulose and have been prospected as an abundantly available lignocellulosic feedstock for cellulosic ethanol production, notably in the South East Asia and South America [7e10] with about 40  10 6 ton/year produced annually in Malaysia and Indonesia alone [11]. In order to implement oil palm empty fruit bunches (EFB) as a successful feedstock in 2nd gener- ation bioethanol production much work is going on to obtain effi- cient conversion processes [12e14]. In the present work, the appearance of an unknown compound after hydrothermal EFB biomass pretreatment was consistently observed in HPLC chro- matograms. The unknown compound was characteristic for EFB Abbreviations: EFB, oil palm empty fruit bunches; 4-HBA, 4-hydroxybenzoic acid; LfHP, Liquid from Hydrothermal Pretreatment; HMF, 5-(hydroxymethyl)-2- furaldehyde. * Corresponding author. E-mail address: am@kt.dtu.dk (A.S. Meyer). Contents lists available at ScienceDirect Biomass and Bioenergy journal homepage: http://www.elsevier.com/locate/biombioe http://dx.doi.org/10.1016/j.biombioe.2016.07.024 0961-9534/© 2016 Elsevier Ltd. All rights reserved. Biomass and Bioenergy 93 (2016) 209e216