Fuel Processing Technology 238 (2022) 107509 Contents lists available at ScienceDirect Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc Production and separation of value-added compounds from pine wood using pyrolysis and biorefinery techniques Mireia Mora a, b, *, Esteve Fàbregas b, Francisco Céspedes b, Jordi Bartrolí b, Neus Puy a, b a Forest Science and Technology Centre of Catalonia (CTFC), Crta. Sant Llorenç de Morunys, km 2, 25280 Solsona, Lleida, Spain b Department of Chemistry, Universitat Autònoma de Barcelona (UAB), Edifici Cn, Campus de la UAB, 08193 Cerdanyola del Vallès, Barcelona, Spain A R T I C L E I N F O A B S T R A C T Keywords: Value-added compounds were obtained from pine wood bio-oil, converting it into a renewable source for several Alkaline extraction chemicals: acids (acetic acid), sugars (levoglucosan), furan derivatives (furfural) and phenolics (catechol, phenol, Bio-oil 4-propylguaiacol, vanillin). To achieve this, firstly a comparison of a direct pyrolysis at 500 ◦ C versus a two- Biorefinery staged pyrolysis (300 ◦ C and 500 ◦ C) was made. It was determined that a two-staged pyrolysis ensue in the Two-Staged Pyrolysis Vacuum distillation obtainment of less complex liquid fractions since it concentrates families of compounds in different phases of bio- Value-added compounds oil, facilitating their subsequent separation. Secondly, two methodologies (A and B) were designed to establish a procedure for separating the various chemical groups present in bio-oil. Methodology A consisted of a first vacuum distillation followed by an extraction process that combined an alkaline extraction and a simple extraction with ethyl acetate. Methodology B consisted of a first extraction with trioctylamine and ethyl acetate followed by a vacuum distillation and an extraction with hot water. The results showed that methodology A was better for the separation of chemical compounds from bio-oil, obtaining one fraction with the carboxylic acid content and furan derivatives, a second fraction containing the phenolic compounds and a third fraction enriched with the sugar content. 1. Introduction presents phase separation (aqueous phase (AP) and non-aqueous phase (NAP)) whereas for fast pyrolysis usually only one phase is formed [5,6]. Given the need in our actual society to substitute the use of fossil fuels, This is important because it facilitates the later separation of compounds. new renewable feedstock and energy sources are being investigated. One These different pyrolysis modes can be performed in several reactors like a promising alternative is bio-oil [1] (BO), a liquid fuel and a chemical raw fluidized bed reactor, fixed-bed reactor, ablative, cyclone reactor, etc., material derived from the pyrolysis of biomass, which not only poses a being the Auger reactor the best suited one for intermediate pyrolysis [7]. green alternative to non-renewable energies, but also raises the possibilities Some authors have studied the influence of performing a two-staged py­ to find organic waste management solutions, making a step forward within rolysis of lignocellulosic biomass instead of direct pyrolysis at only one the context of circular economy and bioeconomy [2]. Pyrolysis consists of temperature [8,9]. This operation consists of performing a first torrefaction the thermal decomposition of lignocellulosic matter in the absence of ox­ at lower temperatures, usually at 200–350 ◦ C, followed by a pyrolysis of the ygen at elevated temperatures (usually between 250 and 700 ◦ C) giving char obtained at a higher temperature, usually 500–600 ◦ C. The idea behind place to three products: BO (liquid phase), char (solid phase) and gas [3]. It this procedure is to concentrate the different families of chemicals in can be classified into slow, intermediate or fast pyrolysis, depending on the different liquid fractions. In spite of the attractive potential of two-staged operating conditions, particularly heating rate and vapor residence time. pyrolysis (TSP), there are not so many studies related to it. Some authors The heating rate increases from slow to fast pyrolysis, while the vapor found that the composition of the bio-oils obtained in this type of procedure residence time decreases, going from seconds for fast pyrolysis to minutes were not different enough to consider the process successful [10], on the for intermediate and days for slow pyrolysis [4]. Each type offers different other side, other authors found that it was possible to obtain wood bio-oils product distributions, in decreasing order the BO yield obtained for each with very different chemical compositions, where the products coming one is: fast, intermediate, slow. Intermediate pyrolysis is the one which was from cellulose and hemicellulose where concentrated in the aqueous frac­ used in this study because as opposed to fast pyrolysis, the BO obtained tion, while products coming from lignin degradation were concentrated in * Corresponding author at: Forest Science and Technology Centre of Catalonia (CTFC), Crta. Sant Llorenç de Morunys, km 2, 25280 Solsona, Lleida, Spain. E-mail address: Mireia.Mora@uab.cat (M. Mora). https://doi.org/10.1016/j.fuproc.2022.107509 Received 21 July 2022; Received in revised form 13 September 2022; Accepted 15 September 2022 Available online 22 September 2022 0378-3820/© 2022 The Authors. Published by Elsevier B.V. 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. Fuel Processing Technology 238 (2022) 107509 the organic fraction [8]. of one family of compounds but from several chemical families of The composition of BO depends on the type of biomass used and the compounds from BO. Some authors that have attempted to do it, have pyrolysis conditions (type of reactor, temperature, use of catalysts, etc.), made use of sequential organic solvent extraction with water, chloro­ but there are some main organic compounds that are always present in form and ethyl acetate to separate into different fractions the acidic this complex mixture, due to the decomposition of the large polymeric content, the sugar content and the rest of the compounds (phenolics, structures of cellulose, hemicellulose and lignin. During the pyrolytic alcohols, furan and ketones) [39]. Methods combining organic solvent process, the first biopolymers to break into smaller fragments at tem­ extraction and multi-step pH adjustments have also been developed to peratures around 200–300 ◦ C are cellulose and hemicellulose giving separate the chemical components of BO, obtaining a phenolic fraction place to anhydrosugars, furans, carboxylic acids and other light and an acid enriched fraction. oxygenated compounds [11,12]. Lignin exhibits a broader degradation Another common technique in the refining industry is distillation, range, between 250 and 500 ◦ C, giving place to aromatic aldehydes, which is one of the most promising separation technologies for BO due phenols and polyphenols [13–15]. to its complex composition, however the thermal instability of BO poses In Table 1, some of the main families of chemicals present in BO are some problems, like coking or polymerization reactions during the shown, together with the most relevant compounds of each one and their distillation process [40,41]. For this reason it is important to perform the potential application in industry. distillation in vacuum conditions, to be able to operate at lower tem­ BO has a very complex composition. To be able to use it as a source peratures avoiding overheating problems and obtaining higher separa­ for the obtaining of value-added chemicals the development of a sepa­ tion efficiencies [42]. Other studies have also made use of other more ration system that could be integrated into a biorefinery is needed complex separation techniques like column chromatography [38], mo­ [26,27]. Several authors have studied the separation of specific com­ lecular distillation [43], supercritical carbon dioxide extraction [44], pounds from BO with different techniques. One of the most common polymeric nanofiltration and reverse osmosis membranes [45], etc. separation strategies to separate the compounds of BO is liquid-liquid Nevertheless, they have found that techniques involving the use of resins extraction. This comprises different types of extractions, like simple and membranes are not suitable for the separation of BO due to the extractions with organic solvents, reactive liquid-liquid extraction, blocking of the pores. On the other side, techniques involving the use of switchable hydrophilicty solvent extraction, etc. [28] Organic solvent supercritical fluids and molecular distillation are not easily accessible extraction is one of the most common strategies. This technique is based due to their expensiveness. on the higher affinity of some compounds towards some solvents, which Contrary to current approaches for bio-oil refining, where usually results in a phase transfer that allows the selective separation of those. only one separation technique is used and only one chemical family is The BO compounds that have attracted most of the interest are the separated, the present study presents two new different methodologies phenolics. that integrate for the first time different techniques: vacuum distillation, To separate the phenolic fraction of BO, several authors have studied liquid-liquid extraction, reactive liquid-liquid extraction with sodium liquid-liquid extractions with different organic solvents like toluene hydroxide and with TOA, allowing the separation of different families of [29,30], ethyl acetate [30–32] and dichloromethane [30,33]. Other compounds into different fractions, facilitating its refining and use. organic solvents have also been studied like hexane [33], octane and The present work was divided in two different parts to achieve the hexadecane [34]. It has been found that the best extraction yields of goal of separating value-added compounds from bio-oil. In the first part, phenolics are obtained when using polar organic solvents, and the best two production methodologies were compared: a direct pyrolysis at 500 suited one is ethyl acetate, which is also an environmentally safe solvent ◦ C versus a two-staged pyrolysis, where a first pyrolysis at 300 ◦ C was compatible for agri-food and nutraceutical uses [35]. performed followed by a successive pyrolysis at 500 ◦ C of the char ob­ To separate phenolic compounds, the use of sodium hydroxide has tained. With this staged configuration, it was expected to separate at also been employed by several authors, to make a reactive extraction 300 ◦ C the compounds coming from the degradation of cellulose and and obtain phenolates that can be transferred into an aqueous phase hemicellulose and at 500 ◦ C the ones coming from the degradation of [30,36]. lignin [46], achieving this way a first separation step during the pro­ Studies to recover the acidic fraction of BO are also present in the duction of BO and facilitating the later separation of compounds. In the current literature. They mainly focus on the recovery of acetic acid, second part, two methodologies for separating the value-added com­ which is the main acid found in BO. Trioctylamine and tridodecylamine pounds listed in Table 1 from BO into different fractions were designed, have been employed to recover the acidic content from BO via a reactive considering the principles of green chemistry and the economic feasi­ extraction between the long-chain tertiary amine and the acids [37]. bility of the procedures, making the future scaling-up possible. To Other strategies have made use of calcium oxide and quaternary separate valuable fractions from crude BO several techniques were used, ammonium anion-exchange resins to react with the organic acids and including vacuum distillation [42,47], reactive liquid extractions with separate them from the BO [38]. sodium hydroxide [30,36] and trioctylamine [37], and simple extrac­ However, there are not many articles yet in the current literature that tions with water or organic solvents like ethyl acetate [32]. describe an integrated methodology that allows the separation not only Table 1 Value-added compounds of bio-oil and their importance for industry. Compound of interest Application in industry Carboxylic acids (acetic acid) Found in great amounts in BO and used in the fabrication of numerous industrial and pharmaceutical products (for example, in the synthesis of vinyl acetate [16], cellulose acetate [17], as a food additive [18], as an antiseptic for medical applications [19], etc.). This renewable source of acetic acid could pose an alternative to the nowadays most common process for its production, the carbonylation of methanol, a procedure that requires the use of fossil fuels, such as natural gas, coal and light products of petroleum [20]. Sugars (levoglucosan) Levoglucosan is an anhydrosugar present in great amounts with high economic value and with an interest in the chemical and food industries due to their promising applications as a chemical platform [21]. Furan derivatives (furfural) Furfural is an aromatic aldehyde derived from furan that has a lot of applications for petroleum refining, synthetic resins, furan derivatives production, etc.[22]. Phenolic compounds (4-propylguaiacol, Phenolic compounds comprise a wide variety of chemicals: phenol, polyphenols like catechol, methoxyphenols like 4-propylguaiacol, vanillin, catechol) aromatic aldehydes like vanillin, etc. These compounds have a lot of interest in the cosmetic, pharmaceutical, nutraceutical, chemical and food industries due to their use as antioxidants [23], flavouring and fragrance agents (vanillin) [24], reagents for the synthesis of phenolic resins (phenol) [25], etc. 2 M. Mora et al. Fuel Processing Technology 238 (2022) 107509 2. Materials and methods 2.1. Materials The feedstock used to produce BO was Aleppo Pine wood from Catalan Mediterranean forests. The biomass was ground to 1–2 cm long and 2–3 mm wide to obtain a splinter size that could be fed into the reactor. All chemicals used in the experiments were commercially available and were used without further purification: Ethyl acetate (≥99.8%) and diethyl ether (≥99.5%) from Panreac, sodium hydroxide pellets (≥98%), hydrochloric acid (37 wt% in H2O) trifluoroacetic acid (≥99.0%), methanol (≥99.8%) and trioctylamine (≥92.5%) from Sigma Aldrich. Seven external standards, all purchased from Sigma Aldrich, were used for the GC–MS quantification: Acetic Acid (≥99.7%), Furfural (≥99%), Levoglucosan (≥99%), Vanillin (≥99.0%), Phenol (≥99.0%), Catechol (≥99%), and 4-Propylguaiacol(≥99.0%). 2.2. Bio-oil production Two different types of pyrolysis reactors were used to produce BO: a lab-scale fixed bed reactor with a capacity of 30 g of biomass [48] and a pilot plant with an auger reactor [49] with a capacity of 3 Kg/h for woody Fig. 1. Separation Methodology A for the aqueous phase (AP) of bio-oil and the biomass [50,51]. With the fixed bed reactor, two different intermediate non-aqueous phase (NAP) of bio-oil. pyrolysis were carried out, a direct at 500 ◦ C and a TSP (1st step at 300 ◦ C followed by a 2nd step at 500 ◦ C), to compare the chemical composition of maintain the organic acids deprotonated in the aqueous phase (pKa the bio-oils obtained. With the auger reactor, only the TSP was performed, acetic acid = 4.7). Both organic solvents used in the procedure (diethyl because according to previous studies, this is the best configuration for ether and ethyl acetate) can be recovered by distillation. separating the chemical compounds in different fractions [8]. For this reason, the separation procedures explained in the following sections were 2.3.2. Methodology B only performed with the bio-oils obtained from the TSP with the auger Methodology B was designed to propose a separation that avoids reactor. These BOs, because they were produced with an intermediate possible degradation problems of thermally sensitive compounds, for e. pyrolysis, showed different phases: one aqueous phase (AP) for the BO g. sugars, that could appear in Methodology A because of the distillation. obtained at 300 ◦ C and one aqueous phase and a non-aqueous phase (NAP) In this separation process (see Fig. 2) an extraction with trioctylamine for the BO obtained at 500 ◦ C. (TOA) and ethyl acetate (EtOAc) was carried out, transferring the acid content into the ethyl acetate phase thanks to the formation of an ionic 2.3. Separation procedures pair [37]. The use of TOA during this extraction is necessary to Two different separation procedures were designed for the obtain­ ment of value-added compounds from BO, combining different separa­ tion techniques like liquid-liquid extractions, reactive liquid extractions and vacuum distillation. The instrumentation used to perform the ex­ tractions was a rotary mixer Model LD 79 from Labinco to shake the solvents and a separating funnel to separate the immiscible phases. The analytical balance used was from Mettler Toledo and the pH meter used was a 716 DMS Titrino with an electrode of LiCl in ethanol. For the vacuum distillation, a fractionating column and a Polytetrafluoro­ ethylene Diaphragm Vacuum Pump from Fisherbrand™ was used and the necessary laboratory glassware. 2.3.1. Methodology A Separation methodology A can be seen in Fig. 1, where the process followed and the expected compounds to be separated from BO are shown. It involved on one side the distillation of the aqueous phases (AP) in vacuum conditions, with the intention to distillate the carboxylic acids found in bio-oil, which is mainly acetic acid. On the other side the non-aqueous phase of bio-oil (NAP) and the undistilled fraction from the distillation of the bio-oil AP were subjected to an alkaline extraction with an aqueous solution of NaOH 4 M and diethyl ether, followed by an acidification and extraction with ethyl acetate. On the reactive extrac­ tion with sodium hydroxide the medium was basic enough (pH > 12) to deprotonate the polyphenols which have a pKa around 10–12 and transfer them to the aqueous phase, separating them from other com­ pounds without acid-base properties, for e.g. naphthalene and benzene derivatives. The following step involved a liquid-liquid extraction with ethyl acetate, followed by an acidification with HCl 4 M to pH 7.0 to protonate the polyphenols and transfer them to the organic phase and Fig. 2. Separation methodology B for the aqueous phase (AP) of bio-oil. 3 M. Mora et al. Fuel Processing Technology 238 (2022) 107509 concentrate the acids, mostly acetic acid, into the organic phase since it necessary, in order to have all the species protonated (including acetic is soluble in both water and ethyl acetate. The next step of the procedure acid). The acidification of the aqueous phase was carried out with a was a vacuum distillation to remove the solvent (EtOAc: bp 77 ◦ C at 1 solution of HCl 4 M and the acidification of the organic phases of ethyl atm; bp 0 ◦ C at 0.07 atm), leaving the remaining fraction still liquid acetate and diethyl ether was performed with trifluoroacetic acid (TFA). thanks to the presence of TOA which acts as a solvent. Finally, to The aqueous phases of bio-oil were injected without previous dilution, separate the acids from the phenolic compounds in the undistilled but the non-aqueous phases were diluted 1:10 with methanol. fraction, an extraction with water at 90 ◦ C was performed, breaking the bond between the acid and TOA. This methodology was only conducted 2.4.2.3. HPLC-UV/Vis analysis. Bio-oils were analysed by HPLC with the aqueous phase of the BO obtained in the pyrolysis at 300 ◦ C coupled to an UV/Vis detector because the heavier compounds present because it is the fraction with the highest acid content [8,52], therefore in BO are not detectable with GC–MS [58]. The instrument used was an it is ideal to evaluate the efficiency of the separation with TOA. 1200RR Chromatograph with a DAD detector from Agilent Technolo­ gies. The column used was a Luna Omega C18 100A (150 m × 4.6 mm, 5 μm) from Phenomenex, using two mobile phases (0.1% HCOOH in water 2.4. Analytical methods and CH3OH). The aqueous phases of bio-oil were injected without pre­ vious dilution, but the non-aqueous phases were diluted 1:10 with 2.4.1. Biomass characterisation methanol. The water content of the biomass was analysed with the Moisture Analyzer HB43-S from Mettler Toledo. The elemental composition of the 3. Results and discussion biomass was determined with a Carlo Erba-1108 analyzer. This tech­ nique provides the content of C, H, N and S of the sample and it is based 3.1. Biomass characterisation on the complete oxidation of the sample at 1000 ◦ C in an atmosphere of pure oxygen and the analysis of the different products resulting from the The water content of the biomass was 7.2 ± 0.3 wt% and the combustion. elemental analysis showed that it was composed of 46.3 wt% of carbon, 6.3 wt% of hydrogen (the hydrogen content includes the hydrogen from 2.4.2. Bio-oil characterisation the water content), 0.2 wt% of nitrogen and 47.2 wt% of oxygen (calculated by difference). Therefore, the results were coherent with the 2.4.2.1. pH and density. The pH of BO was measured with the pH meter composition of lignocellulosic biomass and the number of elements that 716 DMS Titrino using an electrode of LiCl in ethanol, which can be used could form potentially hazardous compounds during the pyrolysis, like for a wide range of samples including non-aqueous solutions. The den­ nitrogen [59] or sulphur [60] were not significant. sity of bio-oil was measured using an analytical balance from Mettler Toledo and a micro pipette from Labnet. 3.2. Bio-oil characterisation 2.4.2.2. GC–MS analysis. The composition of BO was analysed with GC/MS using standards of acetic acid, furfural, phenol, vanillin, cate­ Bio-oil resulting from intermediate pyrolysis conditions is usually chol, 4-propylguaiacol and furfural for a quantitative determination. composed of two phases [61], an aqueous phase (AP), that concentrates The chromatographic method was adapted from previous experience most of the hydrophilic thermochemical products that mainly come [53–57]. The instrument used was an HP 6890 Series II GC System from the degradation of cellulose and hemicellulose, and a non-aqueous Chromatograph with a mass spectrometry electron impact ionization phase (NAP), that concentrates most of the hydrophobic compounds detector HP5973 from Agilent Technologies. The separation was made derived from the degradation of lignin. The bio-oil obtained, in the first in a Zebron ZB-5 capillary column (30 m × 0.25 mm × 0.25 μm) from step of the two-staged pyrolysis, at 300 ◦ C only showed one aqueous Phenomenex, using Helium as carrier gas, an injector temperature of phase and bio-oils produced at 500 ◦ C exhibited two phases, one 300 ◦ C and a maximum column temperature of 360 ◦ C. To analyse with aqueous phase (AP) and a non-aqueous phase (NAP), that were easily GC–MS the different fractions obtained an acidification to pH 3 was separated via decantation (see Fig. 3). Fig. 3. Bio-oils and phases obtained in the two-staged pyrolysis and in the direct pyrolysis. 4 M. Mora et al. Fuel Processing Technology 238 (2022) 107509 3.3. Comparison of bio-oil production methodologies One of the objectives of this work was to compare two different methodologies for the production of bio-oil regarding the yield and the chemical composition of the fractions obtained. A direct pyrolysis at 500 ◦ C was compared to a more complex procedure, a TSP (first py­ rolysis at 300 ◦ C followed by a successive pyrolysis at 500 ◦ C of the char obtained). Fig. 4. From left to right: BO TSP AP 300, BO TSP AP 500 and BO TSP 3.3.1. Yields NAP 500. In Table 2 the yields obtained with the different reactors and con­ figurations are depicted. The conversion to BO is quite similar for the three experiments, around 20%, having higher conversion yields in the first step of the TSP and lower yields in the second step. All the produced bio-oils were acidic due to the presence of organic acids, especially acetic acid, exhibiting a pH range of 3.2–3.6 and a 3.3.2. HPLC-UV/Vis density range of 1.05–1.06 g/mL for the aqueous phases of BOs and 1.1 In Fig. 5 the chromatograms obtained with HPLC-UV/Vis obtained g/mL for the non-aqueous phases of BOs. for the bio-oils of the TSP with the auger reactor are depicted. It was seen As it can be observed in Fig. 4, aqueous phases had a brownish colour how BO TSP AP 300 presents most of the peaks in the range of 10–15 while non-aqueous phases exhibited a black colour and higher density. min, which belongs to light oxygenated compounds coming from the Also, the brown colour was lighter for the BO obtained in the TSP at 300 degradation of cellulose and hemicellulose and some small phenolic ◦ C and darker for the BOs obtained in the TSP at 500 ◦ C. compounds coming from the degradation of lignin. On the other side BO The chemical composition of the bio-oils obtained was determined TSP NAP 500 mainly presented peaks in the range of 20–25 min, which with GC–MS. It was seen that for all the bio-oils, the aqueous phases had were not seen in BO TSP AP 300 and correspond to the zone of lignin a relatively similar composition, exhibiting a high content of acetic acid derived heavier aromatic compounds that are not GC–MS detectable. and levoglucosan. Other compounds that were also found were ketones, like propanone and cyclopentenone derivatives, furan derivatives and 3.3.3. GC–MS aldehydes. Also, phenolic compounds were found in the aqueous phases The results obtained with GC–MS can be seen on Fig. 6. It was due to the early degradation of lignin. Nevertheless, these compounds observed that when performing a TSP, the compounds that come from were especially concentrated in the non-aqueous phases, where several the decomposition of cellulose and hemicellulose (acetic acid, levoglu­ benzene, naphthalene and phenol derivatives were found. This coincides cosan and furfural) are mostly concentrated in the aqueous phases (BO with previous literature where the procedures of intermediate pyrolysis TSP AP 300 and BO TSP AP 500), especially acetic acid and levogluco­ are described resulting in the obtainment of an aqueous phase of bio-oil san, which are almost not found in BO TSP NAP 500. On the other side (also called light fraction) that concentrates the light oxygenated com­ compounds that come from the degradation of lignin, like phenol, 4-pro­ pounds and a non-aqueous phase (also called heavy fraction) where pylguaiacol, catechol are mostly concentrated in the fractions obtained phenolic and heavier compounds are concentrated [62]. in the pyrolysis at 500 ◦ C, especially in the non-aqueous phase (BO TSP Table 2 Yields of the bio-oil production. The yields are always referred to initial biomass. Yield (wt%) Fixed bed reactor Auger reactor Direct pyrolysis 500 ◦ C Two-Staged pyrolysis Two-Staged pyrolysis 1st step 300 C ◦ 2nd step 500 C ◦ 1st step 300 ◦ C 2nd step 500 ◦ C BO AP 24.2 24.0 17.0 22.5 11.5 BO NAP 3.2 – 4.0 – 2.8 Char 25.2 64.0 25.7 55.2 29.3 Gas 17.8 15.2 17.5 23.2 20.0 Fig. 5. HPLC-UV/Vis Chromatogram at 190–950 nm of BO TSP AP 300 (orange), BO TSP AP 500 (green) and BO TSP NAP 500 (yellow). All samples were injected directly except for BO TSP NAP 500 which was diluted 1:10. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 5 M. Mora et al. Fuel Processing Technology 238 (2022) 107509 Fixed Bed Reactor Auger Reactor Two-Staged Two-Staged Direct Pyrolysis Pyrolysis Pyrolysis 30 mg of compound / g of biomass Fixed Bed Reactor Auger Reactor 25 Two-Staged Two-Staged Direct Pyrolysis Pyrolysis Pyrolysis 20 4 15 mg of compound / g of biomass 10 3 5 0 2 cid san cid san cid san c A co c A co c A co ceti oglu ceti oglu ceti oglu A Lev A Lev A Lev 1 Fixed Bed Reactor Auger Reactor Two-Staged Two-Staged Direct Pyrolysis Pyrolysis Pyrolysis 0.7 mg of compound / g of biomass 0 0.6 ral nol hol r a l nol hol l l l 0.5 rfu Phe atec rfu Phe atec rf ura heno echo u u P Ca t 0.4 F C F C Fu 0.15 BO AP 500 BO TSP AP 300 0.10 BO NAP 500 BO TSP AP 500 BO TSP NAP 500 0.05 0.00 l ol illin ol illin co llin iac iac u aia Vani ua Van ua Van ylg yl g y lg rop rop rop 4-P 4-P 4- P Fig. 6. Mass of compounds obtained with GC–MS for the different bio-oils and phases in the direct and two-staged pyrolysis with the fixed bed and the auger reactor. NAP 500) except for catechol which is found in a greater amount in the parts: First, a distillation of the aqueous phases of bio-oil (BO TSP AP aqueous phase (BO TSP AP 500). Finally, vanillin is only found in the 300 and BO TSP AP 500) and second, an alkaline extraction, followed by aqueous fraction of the 300 ◦ C pyrolysis since it comes from the an acidification to pH 7.0 with a final extraction using ethyl acetate. The degradation of lignin at low temperatures. second part was performed on the non-aqueous phase of bio-oil (BO TSP When performing a direct pyrolysis at 500 ◦ C, the compounds that NAP 500) and on the undistilled fractions of the two aqueous phases. It come from the degradation of cellulose and hemicellulose (acetic acid, was expected to distillate the acids (mostly acetic acid) and the water levoglucosan and furfural) even though they are mainly concentrated in from the BO, since they have the lowest boiling temperatures. Because the aqueous phase (BO 500 AP) they are also found in smaller amounts the chemical composition of the BO TSP AP 300 and BO TSP AP 500 was in the non-aqueous phase (BO 500 NAP). Compounds that come from very similar, the results are only shown for the first one and the results the degradation of lignin, like phenol, catechol and vanillin are for the second one can be found in the Supporting Information. concentrated in the AP and 4-propylguaiacol in the NAP. The results shown in Fig. 7 demonstrated that when performing a Comparing the results obtained for the two different processes, it was vacuum distillation of the aqueous phases of bio-oil it was possible to inferred that a two-staged pyrolysis offers significant advantages in front obtain almost all the acetic acid in the distillate. Furthermore, it was of a direct pyrolysis regarding the chemical composition of the fractions possible to distil most of the furfural and part of the phenol content. It obtained, despite having lower product yields. Firstly, TSP concentrated should be pointed out, that only one distillate fraction was obtained almost all the acetic acid, levoglucosan, furfural and vanillin generated even though a fractionating column was used, so the different com­ during the degradation process in the aqueous phases, making their pounds of lower boiling temperatures distilled together at 24 ◦ C. It subsequent separation easier. Secondly, it concentrated most of the should be also noted that small amounts of 4-propylguaiacol were also phenolic and polyphenolic content in the non-aqueous phase, except for found in the distillate fraction, even though no catechol was found, catechol. On the other side, when performing a direct pyrolysis all the which has a very similar boiling point. This phenomenon is probably due compounds were found in both phases (AP and NAP), so no previous to the higher interactions of catechol with the matrix because of the separation was achieved. presence of one additional OH group in the chemical structure in rela­ tion to 4-propylguaiacol. This could provoke an increase of catechol 3.4. Separation methodologies boiling temperature. After the distillation of the BO TSP AP 300, the undistilled fraction A 3.4.1. Separation methodology A was subjected to the extraction procedure as well as the BO TSP NAP The main objective of the present work was to develop a separation 500. It was seen that in the first step of the extraction procedure, where process that allowed the obtainment of value-added compounds from an alkaline extraction with diethyl ether was performed, only the 4-pro­ bio-oil (see Table 1). For this reason, two different separation method­ pylguaiacol was found in small amounts in the Organic Phase A-1 (ether ologies were designed and evaluated. Methodology A was divided in two phase). This was probably due to the non-complete deprotonation of the 6 M. Mora et al. Fuel Processing Technology 238 (2022) 107509 15 BO TSP AP 300 BO TSP AP 300 BO TSP NAP 500 mg of compound / g of biomass 0.6 mg of compound / g of biomass 10 0.4 5 0 0.2 cid o sa n e t ic A g luc Ac Le vo BO TSP AP 300 BO TSP NAP 500 0.20 0.0 mg of compound / g of biomass 0.15 al no l ol al ol ol fu r ch rfur hen tech 0.10 r he t e Fu P Ca Fu P Ca 0.05 0.020 BO TSP Distillate A Organic Phase A-1 0.015 AP 300 0.010 Organic Phase A-2 0.005 BO TSP Undistilled Aqueous Phase A-2 0.000 NAP 500 Fraction A co l lin co l aia nil aia u Va u ylg ylg rop rop 4-P 4-P Fig. 7. Distribution of compounds in Methodology A for BO TSP AP 300 and BO TSP NAP 500 (obtained with GC–MS). 15 mg of compound / g of biomass 0.6 10 mg of compound / g of biomass 0.4 5 0 0.2 cid sa n cA co eti glu Ac Le vo 0.20 mg of compound / g of biomass 0.15 0.0 0.10 ral en ol ch ol urfu Ph t e 0.05 F Ca 0.03 BO TSP AP 300 Organic Phase B-1 0.02 Aqueous Phase B-1 Undistilled Fraction B 0.01 Aqueous Phase B-2 0.00 l n ia co n illi ua Va ylg rop 4-P Fig. 8. Distribution of compounds during the extraction process of methodology B for the BO TSP AP 300 (obtained with GC–MS). 7 M. Mora et al. Fuel Processing Technology 238 (2022) 107509 phenolic compound and their higher affinity for the organic phase. In levoglucosan in Aqueous Phase B-1 successfully. The rest of the com­ the ether phase, the main extracted compounds were benzene, naph­ ponents, mainly phenolics, were found mostly in Organic Phase B-1 as it thalene, cyclopentene and quinoline derivatives. The results also was expected, and only small amounts of furfural and vanillin were showed that the phenolic compounds (phenol, catechol, 4-propylguaia­ found in the Aqueous Phase B-1. The second part of Methodology B col and vanillin) were mostly extracted into the Organic Phase A-2 (ethyl involved a vacuum distillation to remove ethyl acetate, which did not acetate phase) as it was expected. Levoglucosan and furfural remained distil together with acetic acid as it happened in Methodology A due to always in the aqueous phases, this was totally expected for the sugar, the chemical interactions with the TOA. For this reason, a second since the solubility of sugars in organic solvents is very limited. On the extraction with hot water was needed to transfer the acetic acid into other side this was not so predictable for furfural since it is soluble in Aqueous phase B-2. It was seen that most of the acetic acid was extracted both ethyl acetate and water. into Aqueous phase B-2, but the extraction efficiency could probably be improved with successive extractions. Furfural and vanillin did also 3.4.2. Separation methodology B transfer to Aqueous phase B-2, leaving phenol, catechol, 4-propylguaia­ The second separation methodology that was carried out (Method­ col and TOA in Organic phase B-2. ology B) was mainly focused on the separation of acetic acid and sugars, for this reason it was only applied to the BO TSP 300 AP since it was the 3.4.3. Comparison between separation methodology a and B bio-oil with a higher content of these compounds. The final fractions obtained with each methodology for the different This methodology offered a contrary approach to the previous one samples (BOs AP and NAP) are summarized in Fig. 9, where the quan­ because it involved a first extraction followed by a distillation. The idea tities obtained in the different fractions for BO TSP AP 300 and BO TSP behind this procedure was to remove the sugar content from the bio-oil NAP 500 are shown. The chemical compounds showed on each fraction prior to the distillation to avoid the possible degradation of the sugars are the ones found in greater amounts, but separation processes are during the distillation process due to their thermal sensitivity. Since never completely efficient, so small amounts of some compounds can be acetic acid is soluble in both water and ethyl acetate, the use of tri­ also found in other fractions. octylamine (TOA) was needed to extract the acetic acid in the organic Considering the two studied separation methodologies it could be phase thanks to the formation of an ionic pair. The results showed in determined that both exhibited some advantages and disadvantages. On Fig. 8 demonstrated that a first extraction with ethyl acetate and TOA one side, in Methodology A, the distillation in the first step of the pro­ concentrated almost all the acetic acid in Organic Phase B-1 and all the cedure could cause the degradation of some compounds of bio-oil due to Fig. 9. Final fractions obtained for Methodology A and B. Quantities obtained are shown in mg compound//g biomass for the BO TSP AP 300 and BO TSP NAP 500. 8 M. Mora et al. Fuel Processing Technology 238 (2022) 107509 their thermal instability, especially sugars. Also, the need to have to use 117216RB-100 and the Group of Sensors and Biosensors of the Uni­ NaOH and HCl in the procedure introduced an additional number of versitat Autònoma de Barcelona. inorganic ions in the bio-oil that in some cases could pose a problem in its further commercialization as a bioproduct. On the other side Meth­ Appendix A. 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