Waste Management 168 (2023) 311–320 Contents lists available at ScienceDirect Waste Management journal homepage: www.elsevier.com/locate/wasman Research Paper Dialysis and column chromatography for biomass pyrolysis liquids separation Mireia Mora a, *, Esteve Fàbregas a, Francisco Céspedes a, Pere Rovira b, Neus Puy a, b a Department of Chemistry, Universitat Autònoma de Barcelona (UAB), Edifici Cn, Campus de la UAB, 08193 Cerdanyola del Vallès, Barcelona, Spain b Forest Science and Technology Centre of Catalonia (CTFC). Crta. Sant Llorenç de Morunys, km2, 25280, Solsona, Lleida, Spain A R T I C L E I N F O A B S T R A C T Keywords: In the current study, a novel approach for separating value-added chemicals from pine wood residues’ pyrolysis Column Chromatography liquids (bio-oil) was effectively carried out. It combined two separation techniques used for the first time in this Biomass field: dialysis with water, methanol and acetone, and column chromatography with Amberlite™ XAD7 resin. Bio-oil This strategy made it possible to separate bio-oil into four fractions: (1) pyrolytic lignin, which can be utilized in Dialysis the synthesis of resins, foams, electrodes, asphalt, etc. (2) acid-rich fraction, with particular relevance to the Pyrolytic Lignin chemical industry, (3) antioxidant fraction, containing phenolic compounds, with a lot of interest for pharma­ ceutical and nutraceutical industry, and (4) a final fraction containing the most non-polar chemicals from bio-oil. Thus, it was possible to develop a process that allows the obtention of bioproducts from woody biomass, a residue obtained in significant quantities in the management of non-profitable forests, making a step forward within the context of circular economy and bioeconomy. 1. Introduction and at lower temperatures, around 200–300 ◦ C it is called torrefaction (Moldoveanu, 2021). Three products are obtained during the pyrolysis Lignocellulosic biomass residues are obtained in big amounts in the of wood: bio-oil (BO) (liquid fraction), char (solid fraction) and gas, management of forests (Sultana and Kumar, 2011). They offer a prom­ mainly composed of CO2, CO, H2 and CH4 (Costa et al., 2022). The ratio ising platform for the obtention of fine chemicals but its intrinsic of BO:char:gas obtained, depends on the feedstock, type of reactor and recalcitrant nature and resistance to degradation make it a difficult the conditions used, like temperature, heating rate, pressure, etc.(Bédué, feedstock to work with. In this context, different thermochemical 2013). Char, also called biochar, is a highly carbonized solid residue treatments have been developed for processing lignocellulosic biomass with a porous structure obtained (Wang et al., 2022). It can be used as a like combustion, gasification, pyrolysis and liquefaction (Bédué, 2013). fuel for energy production or a functional material for soil amendments, One of the most efficient and promising treatments is pyrolysis, a highly carbon fertilizers to improve the water retention of the soil, CO2 capture, efficient process with good environmental performance characteristics water purification, etc. (Güleç et al., 2022) BO is formed by the con­ (Tang et al., 2022; Xia et al., 2023). There are different types of pyrol­ densable gases produced during the pyrolysis process and it contains ysis, depending on the operation conditions, which can be classified water (usually between 15 and 30 wt%), oxygenated aliphatic and ar­ into: slow, intermediate and fast. In Spain, pyrolysis is used in industries omatic compounds (Yin et al., 2013). Its appearance is a dark brown, mainly for tyre recycling and urban waste management, but not much viscous liquid with a barbecue smell. It is the chemically richest fraction attention has been given to woody biomass pyrolysis (Anthony, 2022; obtained and it contains a great variety of value-added chemicals that Bioenergy International, 2020). Some of the main advantages of pyrol­ can be isolated with biorefinery techniques and integrated into the ysis are its capacity to enhance energy density and chemical variety of economy, promoting a bio and circular economy. The main applications the initial feedstock reducing transport and handling costs at a relatively of pyrolysis liquids have been as a liquid fuel to substitute fuel oil or to low cost and with the possibility of performing it in remote locations produce commodity chemicals (Bridgwater et al., 1999). However, bio- (Puy et al., 2011; Zafar, 2021). It consists of the thermal decomposition oil cannot be used directly as a fuel due to its chemical instability and of matter in the absence of oxygen at high temperatures, around 500 ◦ C, acidity and hydrotreatment is necessary (Han et al., 2019). BO contains * Corresponding author. E-mail address: Mireia.Mora@uab.cat (M. Mora). https://doi.org/10.1016/j.wasman.2023.06.005 Received 21 February 2023; Received in revised form 22 May 2023; Accepted 5 June 2023 Available online 16 June 2023 0956-053X/© 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by- nc-nd/4.0/). M. Mora et al. Waste Management 168 (2023) 311–320 between hundreds to thousands of compounds coming from the depo­ depolymerization reactions like levoglucosan, levoglucosenone, cello­ lymerization of woody biomass building blocks: hemicellulose, cellulose biosan, oligomeric hybrids from the combination of cellulose and lignin and lignin (see Fig. 1) (Han et al., 2019; Vispute, 2011). The main degradation products and other species difficult to identify (Han et al., monomeric products obtained from the degradation of the first two 2019; Stankovikj et al., 2017). The degradation of lignin gives place to biopolymers are water and light-oxygenated compounds such as car­ aromatic compounds such as phenols, polyphenols, phenoxy species, boxylic acids, ketones, alcohols, furan derivatives, cyclopentenones, benzene, benzene derivatives, etc. (Anca-Couce, 2016; Wang et al., simple sugars, etc. These chemicals can be used as platform molecules in 2021a). These chemicals have attracted great interest in pharmaceutical the chemical industry (Itabaiana Junior et al., 2020; Shrotri et al., 2017), and nutraceutical industries thanks to their antioxidant properties, as food additives for food industries and in pharmaceutical industries which can act against free radicals, protecting the body against oxidative (Ryssel et al., 2009). stress (Rice-Evans et al., 1997). Some of them can also be used for fla­ Also, oligomeric products are obtained, forming a water-soluble vouring and fragrances (Fache et al., 2016) and for the synthesis of heavy fraction of bio-oil difficult to characterise (Rover et al., 2014; phenol–formaldehyde resins (Gardziella and Mueller, 1990). Apart from Wang et al., 2015). It mainly contains anhydrosugars from cellulose this monomeric species, an important oligomeric fraction is obtained, Fig. 1. Major typical components of BO from the pyrolysis of lignocellulosic biomass. 312 M. Mora et al. Waste Management 168 (2023) 311–320 also called pyrolytic lignin (PL) (Charon et al., 2015). PL is formed by and PL. The second part consists of a separation with column chroma­ lignin oligomers with relatively high molecular weight, and it can make tography using a different strategy than the common ones, where the up to 40 wt% of BO (Xu and Ferdosian, 2017). It is insoluble in water, stationary phase is usually silica gel or alumina. In this work, Amber­ viscous and reactive, causing instability problems in bio-oil (Matos et al., lite™ XAD7 resin is used. This resin has been typically used for the 2021). Thus, it is necessary to remove it from BO. Nevertheless, PL fractionation of fulvic acids (Bellera et al., 2015; Lopez-Sangil and should not be considered as a residue, because it has a lot of potential Rovira, 2013), which are organic acids with a phenolic structure that applications in diverse fields: as a fuel, in the synthesis of resins, such as come from the humic fraction of soil. Amberlite™ XAD7 resin has a phenol–formaldehyde resins, epoxy resins, in the synthesis of poly­ macroreticular structure and is composed of a non-ionic aliphatic acrylic urethane foams (Figueirêdo et al., 2022), as a pitch for asphalt or elec­ polymer (Zimehl et al., 1996). When protonated it has the capability of trodes, etc. (Zhang et al., 2021). adsorbing non-polar and slightly polar compounds which can be later Therefore, BO coming from wood pyrolysis is a complex liquid eluted when a basic solution of NaOH is passed through de resin. fraction in kinetic equilibrium formed by a continuous aqueous phase Therefore, this resin has the ability to separate bio-oil into different containing monomeric species and water-soluble oligomers and a fractions with different chemical compositions, allowing the separation discontinuous phase containing the water-insoluble oligomers (PL) not only of one type of chemical component but of chemical families. (Mohan et al., 2006). With this methodology, it was expected to separate several families of It is of vital importance to create a methodology integrating refining value-added chemicals from bio-oil with the minimum use of reagents techniques to separate high-value chemicals from BO, achieving the and the minimum alterations to the resin thanks to the previous dialysis obtention of bioproducts from a renewable source: woody biomass, and step. diminishing the current dependence on fossil fuels (Cherubini, 2010). With this purpose in mind, diverse authors have attempted to create a 2. Materials and methods separation process of bio-oil, focusing usually on one type of chemical family. The most common technique used is liquid–liquid extraction 2.1. Materials with organic solvents or basic aqueous solutions to extract phenolic compounds from BO (del Pozo et al., 2018; Mantilla et al., 2015; Wei 2.1.1. Bio-oils et al., 2014). The use of supercritical solvents like CO2, ionic liquids and The bio-oils used were produced elsewhere from Aleppo Pine wood deep eutectic solvents has also been described by several authors to from Catalan Mediterranean forests (Mora et al., 2022). Two-staged extract phenolics. Nevertheless, the economic costs of these techniques pyrolysis (TSP) was used to produce bio-oil because it was established difficult their implementation (Cesari et al., 2019; Maqbool et al., 2017; in previous articles that it results in the obtainment of less chemically Yao et al., 2018). Distillation is another technique that has been pro­ complex bio-oils (Hernando et al., 2021; Mora et al., 2022). The two posed in different articles, but the thermal instability of BO limits its temperatures used in the TSP for the BO production were 300 and application and forces the use of reduced pressure conditions either with 500 ◦ C, resulting in the obtention of three different fractions of bio-oil vacuum distillation or with molecular distillation (Mora et al., 2022; (Mora et al., 2022): Nam et al., 2016; Wang et al., 2021b; Wang et al., 2009; Zheng and Wei, (1) Aqueous phase (AP) bio-oil obtained at 300 ◦ C, also called BO 2011). Distillation procedures usually focus on the obtention of low TSP AP 300. (2) Aqueous phase bio-oil obtained at 500 ◦ C, also called BO molecular weight compounds from BO in the distilled fraction, such as TSP AP 500. These two bio-oils concentrated most of the compounds small carboxylic acids, ketones, furans and some phenols (Wang et al., coming from the degradation of cellulose and hemicellulose, like acetic 2009). Column chromatography is another technique that some authors acid, furan derivatives, sugars, alcohols, ketones, etc. (3) Non-aqueous have used to tackle the problem. They have usually made use of silica gel phase (NAP) bio-oil obtained at 500 ◦ C, also called BO TSP NAP 500. or alumina as stationary phases and organic solvents like ethyl acetate, This bio-oil was composed mainly of lignin decomposition products such cyclohexane or ethanol for the mobile phase (Han et al., 2021; Zeng as phenols, polyphenols, benzene derivatives, naphthalene derivatives, et al., 2011). It has proved useful for relatively simple samples where etc. These three bio-oils were the ones used for the elaboration of this compounds are separated according to their polarity. Nevertheless, it is article. not well suited for complex bio-oils, because it cannot offer an efficient separation due to the high number of different compounds present in the 2.1.2. Reagents sample and the resin can be altered. There exist other techniques to To perform the dialysis of BO, the dialysis membrane used was separate complex mixtures that have not been described yet for the Spectra/PorTM 3 with a MWCO of 3500 Da from the brand SpectrumTM. separation of BO. One of them is dialysis, which is based on the sepa­ The resin used in the column chromatography was Amberlite® XAD7 ration of molecules according to their molecular weight through a from Sigma Aldrich. All chemicals used in the experiments were semipermeable membrane, made of cellulose, modified cellulose or a commercially available from Sigma Aldrich and were used without synthetic polymer, with a particular porous size (Ninfa et al., 2009). The further purification: Methanol (≥99.8 %), Acetone (≥99.5 %), Sodium compounds diffuse from the original sample (concentrated solution) to a hydroxide pellets (≥98%), Hydrochloric acid (37 wt% in H2O) and lower-concentrated solution, the dialysate. It is typically used in H3PO4 (≥85 %). Seven external standards, all purchased from Sigma biochemistry to separate large macromolecules like proteins, DNA and Aldrich, were used for the GC–MS quantification: Acetic Acid (≥99.7 %), polysaccharides from other small molecules such as salts, dyes, etc. Furfural (≥99 %), Levoglucosan (≥99 %), Vanillin (≥99.0 %), Phenol (Berg, 2007) It can also be used for separating colloidal solutions, where (≥99.0 %), Catechol (≥99 %), and 4-Propylguaiacol(≥99.0 %). particles of colloidal size are dispersed in a continuous phase of different composition, allowing the obtention of a colloid-free solution (Everett, 2.2. Dialysis and column chromatography 1972). This is the case of BO, which can be conceived as a colloidal solution. In this way, dialysis offers a promising and unique type of The strategy for the separation of chemical compounds from BO separation in BO, that cannot be achieved with other techniques. consisted of first dialysis followed by column chromatography of the In this work the use of separation techniques like dialysis and column dialyzed liquid. chromatography are proposed to separate high-value compounds from BO was placed inside a dialysis bag of 13 cm × 5 cm and it was BO, focusing especially on monomeric compounds, coming either from submerged in the solvent for 24 h, which was agitated with a stirring the degradation of lignin, cellulose and hemicellulose, and PL. The magnet (see Fig. 2). BO AP 300 and BO AP 500 20 were placed directly process consists of first dialysis of BO, where small chemicals are sepa­ into the bag and BO NAP 500 was diluted 1:4 with acetone and placed rated from the larger oligomeric fractions like water-soluble oligomers inside the bag. 313 M. Mora et al. Waste Management 168 (2023) 311–320 To perform the dialysis on BO different solvents were used: water and methanol (MeOH) for BO AP 300 and BO AP 500, and water and acetone for BO NAP 500. In Fig. 3a, the dialysis strategy used for the sample of BO AP 300 is shown. The first dialysis with water was per­ formed, where the water was changed thrice. The first two aqueous solutions (AQS) obtained were mixed, resulting in the solution called AQS1_AP300, and the third one, AQS2_AP300, was analysed separately. Then the solvent for dialysis was changed to methanol and the solution obtained was called MES_AP300. For BO AP 500 the dialysis strategy was very similar to BO AP 300 (see Fig. 3b) but the dialysis water was only changed twice, because it was observed that it was enough, and the Fig. 2. Dialysis bag technique: Small molecules diffuse outside the dialysis bag AQS were mixed to form AQS1_AP500. Then the solvent for dialysis was through the semi-permeable membrane while colloids and macromolecules changed to methanol and the solution obtained was called MES_AP500. remain inside. Finally for BO NAP 500 the procedure performed was a bit different (see Fig. 3. Separation methodologies using dialysis and column chromatography for (a) BO AP 300 (b) BO AP 500 (c) BO NAP 500. The colours used for labelling the solutions represent their real colour. 314 M. Mora et al. Waste Management 168 (2023) 311–320 Fig. 3c). Because of the high viscosity of BO NAP 500 it was necessary to the first fraction. It was observed that almost no colouration was ob­ perform a dilution 1:4 with acetone before placing it inside the dialysis tained in the third dialysis with water for BO AP 300 (AQS2_AP300), for bag. The first solvent used to dialyze was acetone, and the resulting this reason when dialysis of the BO AP 500 was performed, the third solution was called ACS_NAP 500. Finally, two more dialysis steps with change of the dialysis water was eliminated. water were performed and the AQS were mixed to form AQS1_NAP500. Dialysis of BO NAP 500 was performed under a slightly different AQS1_AP300 was further separated with Amberlite™ XAD7 resin approach since the solvent used was first acetone and second water. To using column chromatography separation (see Fig. 3a). A 2 cm diameter the naked eye, no apparent diffusion of BO was observed with acetone, column was filled to a height of 6 cm with the hydrated resin, so the because the solution remained colourless. On the other side, in the volume of XAD7 used was 19 cm3. The procedure for separating into following dialysis with water, the aqueous phase obtained did have a different fractions the dialyzed liquid with the resin XAD7 was the brown colour, coming from the transfer of phenolic compounds. following: (1) Cleaning of the resin with NaOH 0,1 M, water and HCl 0,1 After dialysis was finished a solid fraction remained inside the dial­ M; (2) Sample aliquot was passed through the column and distilled ysis bag, which is mainly constituted of PL. The solid obtained was water was added to ensure all the compounds that were not retained in brown, and the amounts obtained were the following ones: 2.15 mg the column were eluted, the collected liquid was called the non-retained solid/g BO AP 300, 20.40 mg solid/g BO AP 500 and 323.56 mg solid/g fraction (NR fraction); (3) NaOH 0,1 M was added to elute part of the BO NAP 500. It was observed that a higher amount of PL was obtained at retained compounds in the resin and in the end distilled water was 500 ◦ C than at 300 ◦ C, that is because the lignin polymer decomposes at added to make sure the column was free from NaOH, the collected liquid higher temperatures than cellulose (Wang et al., 2021a). was called the NaOH fraction; (4) Methanol was added to elute com­ pounds that were retained in the column but not eluted with NaOH, the 3.1.2. Gc–MS collected liquid was called the MeOH fraction. 3.1.2.1. BO AP 300 and BO AP 500. In Fig. 4a) the comparison of the 2.3. Analytical methods GC–MS spectra of AQS1_AP300 and AQS2_AP300 is shown. It is observed that almost no compounds are obtained in the second solution, 2.3.1. pH for this reason, in the following dialysis procedures with BO AP 500 and The pH of the bio-oils was measured with the pH meter HI9124N BO NAP 500 the third water addition which gives place to AQS2_AP300 using the electrode HI1049B for non-aqueous solutions from Hanna is not performed. The principal compounds that are found in AQS1_AP Instruments. 300 are carboxylic acids (acetic acid, formic acid, etc.), ketones (mainly hydroxyacetone), alcohols (e.g. 1-propanol), furan derivatives (furfural, 2.3.2. GC–MS analysis 5-hydroxymethylfurfural, etc.), phenolic compounds (phenol, methox­ The composition of BO was analysed with GC/MS using standards of yphenols, methyl and ethyl phenols, etc.), quinolines (e.g. 2,4-dimethyl­ acetic acid, furfural, phenol, vanillin, catechol, 4-propylguaiacol and quinoline) and sugars (mainly levoglucosan, anhydro-mannosan). furfural for a quantitative determination. The chromatographic method In Fig. 4b) the spectra of MES_AP300 can be seen. The abundance of was adapted from previous experience (Artigues et al., 2014; del Pozo the peaks is much lower than for AQS1_AP300 but different compounds et al., 2021a, 2021b, 2020, 2018; Mora et al., 2022). The instrument non-polar compounds are observed because they have a greater affinity used was an HP 6890 Series II GC System Chromatograph with a mass for methanol than for water. Some of these chemicals are benzene, ethyl spectrometry electron impact ionization detector HP5973 from Agilent benzenes, methyl benzenes, phenolics like some ethyl methyl phenols, Technologies. The separation was made in a Zebron ZB-5 capillary naphthalenes, anthracenes, long-chain alkanes like eicosane, etc. column (30 m × 0.25 mm × 0.25 μm) from Phenomenex, using Helium A semi-quantitative analysis was also performed with seven external as the carrier gas, an injector temperature of 300 ◦ C and a maximum standards (see Fig. 5). These standard chemicals used are some of the column temperature of 360 ◦ C. To analyse with GC–MS the different most interesting of BO due to the high amounts present in bio-oil and fractions obtained acidification to pH 3 was necessary, in order to have their relevance for chemical, pharmaceutical and nutraceutical all the species protonated (including acetic acid). The acidification of industries. the aqueous phase was carried out with H3PO4. The aqueous phases of For BO AP 300, the results showed that six out of seven compounds bio-oil were injected without previous dilution, but the non-aqueous analysed, acetic acid, furfural, phenol, catechol, 4-propylguaiacol and phases were diluted 1:18 with acetone. vanillin, were mostly found in AQS1_AP300, therefore it would only be necessary to perform a two-step dialysis with water since almost no 2.3.3. Ft-IR amount was found neither in AQS2_AP300 or MES_AP300. Levoglucosan The solid obtained in the separation with dialysis was characterized was not found in great amounts in AQS1_AP300 nor in the other frac­ with the FT-IR spectrometer Bruker Tensor 27 IR. It has a spectral range tions. Since its size is small enough to cross the dialysis membrane, this of 7,500 to 370 cm− 1 with standard KBr beam splitter and a resolution indicates that it probably forms interactions with water-soluble oligo­ better than 1 cm− 1. Spectra were recorded with the Attenuated Total mers preventing it from diffusing outside of the dialysis bag (Han et al., Reflectance (ATR) mode. 2019). The results obtained for BO AP 500 were very similar to BO AP 300. 3. Results and discussion In Fig. 5, the results from the semi-quantitative analysis with GC–MS are depicted. All compounds were mostly obtained in AQS1_AP500, even 3.1. Dialysis though 4-propylguaiacol, which is the less polar compound of the studied ones, was found in important amounts in MES_AP500. 3.1.1. General characterisation When dialysis was performed on the three samples of BO, the 3.1.2.2. Bo NAP 500. On the GC–MS spectra of the acetone fraction aqueous solutions obtained had an acid pH of around 3. This was due to ACS_NAP 500 it was seen that some compounds did cross the dialysis the presence of acetic acid in BO, a small molecule that diffuses outside membrane outside of the bag, especially compounds like benzene, the dialysis membrane. For BO AP 300 and BO AP 500 the first aqueous methyl benzenes, ethyl benzenes, styrene, naphthalene derivatives, solution obtained had a brownish colour due to the presence of phenolic phenanthrene derivatives, etc. Nevertheless, the most interesting com­ compounds that diffuse through the semipermeable membrane outside pounds from bio-oil were found in the aqueous phase AQS1_NAP500. of the dialysis bag (see Fig. 7a). In both cases, the second solution had a This was also observed in Fig. 5 where it was seen that the target lighter colour because most of the compounds were already obtained in 315 M. Mora et al. Waste Management 168 (2023) 311–320 Fig. 4. (A) comparison of the gc–MS spectra of AQS1_AP300 (brown color) and AQS2_AP300 (blue color). (b) GC–MS spectra of MES_AP300. Fig. 5. Semi-quantitative results for GC–MS analysis for the dialysis of BO AP 300, BO AP 500 and BO NAP 500. compounds were found in the AQS1 not in the ACS, only small amounts in AQS1_NAP500, allowing the separation of these compounds from of 4-propylguaiacol were found in ACS_NAP 500. Most of the acetic acid, initial BO inside the dialysis bag. As it was observed with the other furfural, phenol, catechol, 4-propylguaiacol and vanillin were obtained samples levoglucosan was not found in considerable amounts in either 316 M. Mora et al. Waste Management 168 (2023) 311–320 fraction. also found in the solids obtained, even though sometimes with lower It can be concluded that dialysis can separate light chemicals of bio- intensities than in organosolv lignin (e.g. peak at 1100 cm− 1). This in­ oil from heavy ones. This is very important because it allows on one side dicates that even though these groups are still found in the solids ob­ the obtention of a simpler liquid fraction containing value-added com­ tained, their structure might be a bit affected by the pyrolysis process, a pounds and on the other side the obtention of PL, a solid which can be phenomenon already described by several authors (Asmadi et al., 2011; used directly as a product itself. Moreover, dialysis can be considered a Guizani et al., 2017; Saiz-Jimenez and De Leeuw, 1986). It can be green technique if the solvents used are considered green solvents or if concluded that the solid separated with the dialysis procedure is PL, they can be reused in the procedure. which has a lignin-like structure, conserving most of the basic structural units of lignin and incorporating an increase in reactive sites. Thus, as it 3.1.3. Ft-IR has been described previously in the literature, PL can be used for the When dialysis was performed, a solid was obtained which remained synthesis of resins (Effendi et al., 2008), foams (Figueirêdo et al., 2022), inside the bag. This solid is mainly composed of the oligomeric fraction asphalts (Zhang et al., 2021), carbon fibres (Qin and Kadla, 2012), etc. coming from the degradation of lignin, also called Pyrolytic Lignin (PL). Its high hydrophobicity and elevated molecular weight prevent it from 3.2. Column chromatography separation escaping the dialysis bag into the dialysate. PL has already been well characterized by other authors (Bai et al., 2014; Jiang et al., 2010; Li 3.2.1. General characterisation et al., 2020; Matos et al., 2021; Sammons et al., 2013; Wang et al., A column chromatography separation was performed on the dia­ 2015). In this study, a FT-IR analysis was carried out to characterise the lyzed liquid to further separate the compounds and be able to obtain solid obtained and confirm its PL structure. In Fig. 6, the FT-IR spectra of simpler fractions. Amberlite™ XAD7 was used, a resin that retains non- the solids obtained in the dialysis of bio-oils are depicted and compared polar or slightly non-polar compounds. For this reason, it is important to the FT-IR of organosolv lignin. It was appreciated that there are a lot that the pH of the sample before passing it through the column is acidic, of similarities in the region of typical FT-IR lignin bands: O–H stretching in order to have the phenolic species protonated and that they can (3421 cm− 1), C–H stretching (2937 cm− 1 and 2840 cm− 1), C = O consequently be retained in the resin. To elute the retained compounds, stretching (1700 cm− 1), aromatic skeletal vibration + C = O stretching a sodium hydroxide solution was passed through the column, and the (1600 cm− 1), aromatic skeletal vibration (1515 cm− 1), C–H de­ slightly non-polar compounds were eluted. To elute the non-polar formations methyl and methylene (1460 cm− 1), C-O stretching of the compounds, it was necessary to use a less polar solvent, like methanol. guaiacyl ring (1270 cm− 1), C–C, C-O, C = O stretching in guaiacyl ring Sample AQS1_AP300 was the chosen one to further refine using (1215 cm− 1), aromatic C–H deformation in the syringil ring (1100 column chromatography with the resin Amberlite™ XAD7. In Fig. 7b the cm− 1) and aromatic C–H in-plane deformation (1033 cm− 1) (Aljerf and colouration of each fraction obtained is shown: yellow–brown for the Nadra, 2019). In the FT-IR spectra it was observed that the peak at 1366 AQS1_AP300, colourless for the NR fraction, yellow–brown for the cm− 1 had higher intensity in the solids obtained in the dialysis (PL) than NaOH fraction due to the presence of phenolic species and yellowish for in organosolv lignin. This phenomenon was already observed by (Wang the MeOH fraction, also due to the presence of some phenolics. et al., 2015). They noticed that PL, besides conserving the basic struc­ tural units of lignin, it also had abundant side chains as indicated by the 3.2.2. Gc–MS peak at 1366 cm− 1, originated by the aliphatic C–H and phenolic OH The three fractions obtained (NR fraction, NaOH fraction and MeOH stretching. This peak suggested that PL has more aliphatic side chains, fraction) were characterized with GC–MS. The detailed identification of carbonyls and unsaturated double bonds than lignin which increases its compounds from GC–MS chromatograms (see Supporting Information) reactivity. The peaks associated with syringyl and guaiacyl units were showed that the chemicals observed in the NR fraction are carboxylic Fig. 6. FT-IR of the solids obtained in the dialysis of BO AP 300, BO AP 500, BO NAP 500 and organosolv lignin. 317 M. Mora et al. Waste Management 168 (2023) 311–320 Fig. 7. (A) first dialysis of bo ap 300 with water after 24 h. (b) From left to right: AQS1_AP300, NR fraction, NaOH fraction, MeOH fraction. acids like acetic acid and formic acid, alcohols and ketones like interest for bioproducts obtention and can be reutilized in the column hydroxyacetone, furan derivatives like furfural and 5-methylfurfural, chromatography procedure, making the process greener. and sugars like anhydro-mannosan. In the NaOH fraction phenolic compounds were identified, such as guaiacol, vanillin, phenol, etc. 4. Conclusions Finally, in the MeOH fraction a mixture of non-polar compounds was observed, composed of a wide variety of benzene derivatives (toluene, The novel separation strategy combined dialysis and column chro­ xylene, cumene), quinolines, long-chain alkanes and also some highly matography with Amberlite™ XAD7 resin, techniques never used before non-polar phenolic species. in this field. It resulted in the obtention of pyrolytic lignin, which can be In Fig. 8, the semi-quantitative results using seven external standards used for resin synthesis, and acid-rich fraction, which can be further are shown. It was seen that acetic acid was mostly found in the NR refined and used for chemical and food industries, a fraction rich in fraction as was expected. Furfural was obtained in both the NR fraction antioxidants, key products for nutraceutical and pharmaceutical in­ and MeOH fraction. Phenol and vanillin were found in the NaOH frac­ dustries and a fraction with highly non-polar compounds. In this way, it tion as it was expected and 4-propylguaiacol was eluted in the MeOH was possible to separate value-added chemicals from the light and heavy fraction since it is a less polar compound than the others. Catechol did fractions of pine wood pyrolysis liquids. Thus, it was possible to trans­ not quantitatively elute in any of the three fractions, meaning that most form lignocellulosic biomass, a by-product of the management of un­ of it remained in the resin. Levoglucosan did not appear quantitatively profitable forests like Mediterranean woodlands, into a renewable on the NR fraction, which was the expected one, because it was not feedstock to produce fine chemicals. obtained in an appreciable amount in the AQS1_AP300. Therefore, the separation with the resin Amberlite™ XAD7 was CRediT authorship contribution statement capable of separating different chemical families of BO in different fractions according to their polarity. Firstly, carboxylic acids, alcohols Mireia Mora: Conceptualization, Methodology, Validation, Formal and ketones were obtained in the NR fraction, secondly, phenolic analysis, Investigation, Writing – original draft. Esteve Fàbregas: compounds were obtained in the NaOH fraction and finally, other non- Writing – review & editing, Supervision. Francisco Céspedes: Writing – polar compounds such as benzene derivatives and quinolines were ob­ review & editing, Supervision. Pere Rovira: Supervision. Neus Puy: tained in the MeOH fraction. This MeOH fraction is the one with less Writing – review & editing, Supervision, Project administration, Fig. 8. Column chromatography separation with Amberlite™ XAD7 resin of the fraction AQS1_AP300. 318 M. Mora et al. Waste Management 168 (2023) 311–320 Funding acquisition. biorefinery approach. Waste Manag. 109, 19–27. https://doi.org/10.1016/j. wasman.2020.04.044. del Pozo, C., Bartrolí, J., Alier, S., Puy, N., Fàbregas, E., 2021a. Production, identification, and quantification of antioxidants from torrefaction and pyrolysis of Declaration of Competing Interest grape pomace. Fuel Process. Technol. 211, 106602 https://doi.org/10.1016/j. fuproc.2020.106602. The authors declare that they have no known competing financial del Pozo, C., Rego, F., Yang, Y., Puy, N., Bartrolí, J., Fàbregas, E., Bridgwater, A.V., 2021b. Converting coffee silverskin to value-added products by a slow pyrolysis- interests or personal relationships that could have appeared to influence based biorefinery process. Fuel Process. Technol. 214, 106708 https://doi.org/ the work reported in this paper. 10.1016/j.fuproc.2020.106708. Effendi, A., Gerhauser, H., Bridgwater, A.V., 2008. 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