Hydrodeoxygenation of Lignocellulosic Fast Pyrolysis Bio-Oil

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Biofuels and Biomass

Hydrodeoxygenation of lignocellulosic fast pyrolysis bio-oil. Characterization of the products and effect of the catalyst loading ratio. Mario Benés, Rafael Bilbao, Jandyson Machado Santos, Josué Alves Melo, Alberto Wisniewski Jr, and Isabel Fonts Amador Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00265 • Publication Date (Web): 30 Mar 2019 Downloaded from http://pubs.acs.org on April 1, 2019

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Hydrodeoxygenation of lignocellulosic fast pyrolysis bio-oil. Characterization of the products and effect of the catalyst loading ratio. Mario Benés[a], Rafael Bilbao[a], Jandyson Machado Santos[c,d], Josué Alves Melo[d], Alberto Wisniewski Jr[d], Isabel Fonts*[a,b] aThermochemical

Processes Group, Aragon Institute for Engineering Research (I3A), Chemical and Environmental Engineering Department, University of Zaragoza, Zaragoza, Spain bChemical

and Environmental Department, Centro Universitario de la Defensa, Zaragoza, Spain

cChemistry

Department, Federal Rural University of Pernambuco, Recife, Pernambuco 52171-900,

Brazil dPetroleum

and Energy from Biomass Research Group – PEB, Chemistry Department, Federal University of Sergipe, São Cristóvão, Sergipe 49100-000, Brazil * Corresponding author: [email protected]

Abstract The hydrodeoxygenation (HDO) of bio-oil at 350 °C and 200 bar in a batch reactor over Ru/C catalyst has been studied experimentally with the aim of contributing to the understanding of the HDO reaction and its effect on the physicochemical properties of the organic liquid fraction obtained. Moreover, the effect of the catalyst loading ratio used in the HDO treatment and a previous stabilization stage carried out at 250 °C have also been assessed. Under the studied operational conditions, reactions of decarboxylation, hydrodeoxygenation, polymerization, decarbonylation, methanation, demethylation and pyrolytic lignin depolymerization took place during the HDO process. In these experiments, O was removed from the bio-oil mainly in the form of CO2 (15 – 26 g of CO2·100 g-1 of dry bio-oil) and also as H2O (1.8 – 5.8 g of H2O·100 g-1 of dry bio-oil). The consumption of H2 was between 0.75 and 1.0 g·100 g-1 of dry bio-oil. A comparison of the physicochemical properties of the raw bio-oil and the HDO organic phases shows that the major effects of the HDO were: a reduction in the O content from 34 to 13 wt %, an increase in the HHV (dry basis) from 24.3 to 35.5 MJ·kg-1, a lower polarity of the organic compounds determined by the significant increase in the hexane solubility, a lower corrosiveness evidenced by the smaller TAN and acid concentrations, and a marked change in the GC-MS detectable compounds, increasing the presence of mono-phenols and cyclic ketones and decreasing the presence of levoglucosan, methoxyphenols and furans. ESI(±)-FT-MS analyses of the raw bio-oil and the HDO liquid fractions show a widespread reduction of the O/C molar ratio of the compounds, an efficient deoxygenation and depolymerization of pyrolytic lignin and a non-desirable increase in the range of molecular weights of the organic molecules after the HDO treatment.

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1. Introduction The route of producing liquid transportation biofuels by processing lignocellulosic biomass via fast pyrolysis is an interesting option, since it involves lower capital and operating costs than gasificationand biochemical-based routes, and presents a higher processing capacity if compared with the biochemical one [1-3]. Three products are obtained from the fast pyrolysis of lignocellulosic biomass: a solid product or char, a gas, and a liquid commonly known as pyrolysis liquid or bio-oil. The latter has potential applications as a fuel. However, its use as a liquid transportation fuel is limited by several factors that derive from its high concentration of oxygen-containing compounds. Oxygen contents (wet basis) exceed 40 wt % in the case of pyrolysis liquids from woody biomass. The high presence of oxygen-containing compounds in bio-oil means that it has low thermal and storage stability, high corrosive power, and low calorific value. In addition, there are other properties that differentiate it from petroleum-based fuels and hinder its co-feeding in conventional refineries, such as its higher viscosity and density, the difficulty of distillation at atmospheric pressure, and the presence of polar compounds and water, the latter representing usually between 25 and 30 wt %. Bio-oil composition, rich in polar compounds, causes its low miscibility with petroleum-based transportation fuels. Two catalytic post-treatments of bio-oil have been studied in order to reduce its oxygen content and consequently improve its properties as a liquid transportation fuel: catalytic cracking and hydrodeoxygenation (HDO) [4]. The HDO process seems to have some advantages over catalytic cracking due to the higher organic phase yield and the lower coke yield obtained [4, 5] and also due to the higher degree of deoxygenation achieved by the HDO post treatment. Bio-oil HDO is a process carried out at elevated pressure (15 – 352 bar), moderate temperature (80 – 500 °C), using H2 or hydrogen-donor compounds and in the presence of a catalyst. Three products are obtained: a gas mixture, a carbonaceous solid, and a liquid which is usually formed by an aqueous phase and one or two organic phases. Oxygen removed from the organic compounds of the bio-oil is mainly in the form of H2O, CO2 and CO. It is interesting that the O is removed as H2O so that the C is available to finish in the organic phase. One of the most significant costs of this process is the H2 used as a reagent. Therefore, it is interesting that H2 was consumed in O removal reactions in the form of H2O, in hydrocracking reactions of the heaviest compounds or in hydrogenation reactions of unsaturated bonds of the liquid organic compounds, instead of in hydrogenation reactions of gaseous compounds. HDO has been intensively studied during the last decade [5, 6]. Most of these studies have focused on the effect of the temperature [1, 7-10], the origin of the pyrolysis liquid [11, 12], the pretreatment, for example fractionation [9] or esterification [13], and the kind of catalyst used [7, 11, 14-17].


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Moreover, in the case of continuous reactors, it is usual to analyze the effect of one or two previous hydrotreating stages carried out at lower temperatures (80 – 300 °C) and high pressures, and also in the presence of catalysts, with the main aim of stabilizing the carbonyl groups [8, 18-21]. HDO reactor temperatures between 325 and 400 °C are the most commonly studied, according to the review by Mortensen et al. [5]. The most frequently studied catalysts are those based on noble metals Ru, Pt and Pd [7, 11] and catalysts similar to those used in the hydrodesulphuration process in petroleum refineries, such as Co-MoS2/Al2O3 and Ni-MoS2/Al2O3 [5, 22]. It can be concluded from these studies that one of the catalysts offering the best results as regards deoxygenation, selectivity to the organic phase and H2 consumption is composed of Ru over C (Ru/C). This catalyst has been chosen in many works analyzing the effect of other operational conditions [8, 9, 11, 20]. Although the effect of different catalysts and supports have been studied, to the best of our knowledge there are no studies about the effect of the ratio between the mass of catalyst and the mass of bio-oil treated, or the catalyst loading ratio, when a Ru/C catalyst is used. Most studies limit the catalyst loading ratio to 5 wt % [1, 9, 11, 13, 21, 23, 24]. An in depth analysis and understanding of the HDO process is not easy due to the multiple reactions that can take place during this process (hydrogenation, dehydration, decarbonylation, decarboxylation, polymerization, cracking or hydrocracking, among others). This difficulty is even greater considering that oxygen in the bio-oil can be present in different functional groups and in compounds of very different size and molecular structure. One approach to understanding this process is through a characterization of both the raw bio-oil and the HDO products obtained, especially the organic phase. Apart from the most conventional analyses used, such as elemental analysis, heating value, water content, solvent fractionation or gas chromatography, some authors have foregrounded an interest in knowing the change suffered by the functional groups [1, 18] and the heavy fractions [1, 25, 26] in the HDO process. A petroleomics approach involves a wide range of tools to characterize pyrolysis products, including the aqueous phase [26, 27]. Fourier transform-Mass spectrometry (FT-MS) techniques combined with electrospray ionization (ESI) mode have an important function in the chemical elucidation of the composition of complex mixtures like lignocellulosic bio-oil, which contains a high number of nonvolatile and high-molecular-weight oxygenated compounds. The ESI-FT-MS technique makes it possible to obtain the exact masses of the compounds present in a sample. Each mass is converted into a molecular formula such as CxHyNwOzSk, and the compounds can be classified into chemical classes considering, per example, the number of oxygen atoms or the number of carbon atoms in the molecular formula. Other authors using this technique have found that the distribution of oxygen


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between the raw and HDO bio-oil changed from O2-O18 class compounds, of which O7 class compounds were the most abundant, to O1-O10 with O4 dominating [28]. In this context, the main objectives of this work are to contribute to the knowledge of the chemical modifications occurring in the liquid organic fraction after catalytic HDO treatment of lignocellulosic bio-oil and to the understanding of the ways of this process. Special attention is paid to the physicochemical properties of the HDO organic phases and the raw bio-oil, with an in depth characterization of both using the following techniques: elemental analysis, Karl-Fischer, bomb calorimetry, hexane solubility, Gas Chromatography–Mass Spectrometry (GC-MS), total acid number (TAN), carboxylic acids quantification by GC-MS, Folin Ciocalteu, Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR), simulated distillation, and, lastly, ElectrosprayFourier Transform Mass Spectrometry (ESI-FT-MS) for a comprehensive analysis in the field of bio-oilomics. Total and atomic mass balances of the HDO process are calculated in order to know how the C, H and O are distributed before and after the HDO process, which sheds valuable light on the reactions taking place during bio-oil HDO. The effect of the catalyst loading ratio and the use of a previous stabilization stage in the HDO process are also assessed. 2. Materials and methods 2.1. Materials BTG-BTL bio-oil supplied by the Biomass Technology Group was used as raw material [29]. This liquid was produced from pine wood with an average particle size of 3 mm, using a rotating cone reactor at a pyrolysis temperature of 510 °C and with a gas residence time lower than 2 s. Condensation of the vapors was carried out in one-step at 40 °C. When bio-oil was received from the company, it was stored between one and two months at -24 °C previously to the performance of the experiments. The elemental analysis of the raw bio-oil showed that, on a moisture basis, it contained C (43.4 wt %), H (7.4 wt %), N (0.15 wt %) and O (49.1 wt %) (O calculated by difference). The water content of the raw bio-oil determined by Karl–Fisher titration was 27.3 wt %. The raw bio-oil was characterized using the solvent fractionation scheme proposed by Oasmaa et al. [30]. According to this procedure, the high molecular weight lignin (HMWL), corresponding to the fraction insoluble in water and in dichloromethane (Wins-DCMins), accounted for 16.9 wt % of the bio-oil on a moisture basis. 13.9 wt % of the bio-oil did not dissolve in water but was dissolved in dichloromethane (Wins-DCMsol); this would correspond to the low molecular weight pyrolytic lignin (LMWL). 27.2 wt % of the bio-oil (excluding the water of the raw bio-oil itself) was soluble in water and insoluble in ether. This corresponds to a fraction called sugars, containing mainly sugars and oligomers derived from cellulose and hemicellulose pyrolysis. Lastly, 14.7 wt % was soluble in both


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water and ether (ES) and contained aldehydes, ketones, furans and monophenols. The raw bio-oil was also characterized by the same methods used for the organic phases obtained from the HDO, explained in the following sections. The results will be shown as a comparison with the values determined for the organic phases obtained in the HDO experiments. H2 of 99.99% purity supplied by Air Liquide was used as a reagent in the HDO experiments. The catalyst used in the experiments was a commercial Ru/C with 5 wt % of ruthenium (Sigma-Aldrich). 2.2. Experimental procedure in the HDO experiments The HDO experiments were carried out in a stirred batch autoclave of 500 mL volume (PARR HP/HT, series 4570 and model 4575). The maximum temperature and pressure of the set-up are 450 °C and 350 bar. In each experiment, 150 g of bio-oil and a varying amount of Ru/C catalyst, depending on the catalyst loading ratio, were placed in the autoclave reactor. The reactor content was stirred at 1000 rpm. The reactor was closed and purged with N2, after which the N2 was removed by means of a vacuum pump. Then, in all the experiments, H2 was introduced until a pressure of 40 bar was reached. Next, the reactor was heated to 350 °C with a heating rate of 5 °C ·min-1 in the experiments carried out only with one stage. The temperature was maintained for a reaction time of 4 h. When two stages were used in the HDO experiments, the reactor was firstly heated until 250 °C with a heating rate of 5 °C min-1 and then the temperature was maintained for 2 h. The temperature was then increased until 350 °C with the same heating rate and also maintained for 2 h. Temperature and pressure were measured and recorded continuously during the experiment. When the experimental final temperature was reached, the pressure value was raised to values around 150 bar. During the isothermal period, it continued increasing until approximately 200 bar. After the reaction time, the reactor was cooled until ambient temperature using an embedded spiral cooling system. Once the ambient temperature was reached, the pressure was recorded. It was between 30.8 and 44.2 bar, depending on the experiment. Once the experiment was finished, the gas phase formed during the HDO treatment was allowed to exit the reactor at the same time as its volume was measured at a specific temperature and pressure using a volumetric gas meter. Part of this gas phase was collected in a Tedlar 3 L sampling bag for posterior analysis by a gas chromatograph on an Agilent 7890A GC-TCD/FID. This gas chromatograph was equipped with both a HP Plot Molecular Sieve column and a HP Plot Q column for the TCD channel, and a HP-PONA column for the FID channel. Calibration of the equipment was carried out for the following gas compounds: CO, CO2, H2, CH4, C2H2, C2H4, C2H6, C3H8, C3H6, 1,3-butadiene, iC4H10, n-C4H10, C6H6, C7H8 and C8H10.


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Once the gas was evacuated, the autoclave reactor was opened and the mixture of solid and liquid products was poured without the aid of a solvent and weighed. Two phases were observed: a waterlike phase and an organic phase that contained the solids. These two phases were separated by gravity and weighed. The organic phase was dissolved in methylene dichloride and then filtered in order to separate the solids, which contained the catalyst and the possible solid product formed during the reaction. The aqueous phase and the filtered organic phase dissolved in methylene dichloride were stored in the fridge for further characterization. Previously to each characterization analysis, methylene dichloride was removed in the rotary evaporator at 34 °C and 0.4 bar. In all the experiments, a small portion of solids and liquid was unable to be poured from the autoclave reactor without the aid of a solvent. To recover this fraction, the reactor was washed with acetone and the solids present in the mixture were filtered and weighed. The acetone was removed in a rotary evaporator with the aim of including the liquid compounds in the mass balance. For all the experiments, these liquid compounds accounted for less than 5 wt % of the organic phase obtained from the HDO. The weights of the solids recovered by filtration from both the acetone and the methylene dichloride fraction were summed. This total amount of solids minus the original catalyst intake was considered as the amount of carbonaceous solids formed during the HDO process, because it was not possible to separate the solid product formed during the reaction and the catalyst by any physical method. The operational conditions studied were the catalyst loading ratio, defined as the mass of catalyst per 100 g of bio-oil on a wet basis and which had values of 2 wt %, 3.33 wt %, 5 wt % and 6.5 wt %, and the use of one (1S) or two (2S) stages during the HDO treatment. The names given to the experiments combine the value of the two operational conditions studied. Next, the name given to the experiments and the closure of the mass balance of each one, taking into account the two reagents of the reaction (bio-oil and H2) are provided: 2.0%Cat_1S (84.9% without taking into account the gas species), 3.3%Cat_1S (87.7%), 5.0%Cat_1S (102.1%), 5.0%Cat_2S (98.8%) and 6.5%Cat_1S (95.9%). 2.3. Characterization of the raw bio-oil and the HDO oils All the characterization analyses presented in this section, except the total organic carbon analysis, were applied to the raw bio-oil and to the HDO organic phases. The analyses carried out on the aqueous phases were water content, total organic carbon (TOC), total acid number (TAN) and gas chromatography–mass spectrometry (GC-MS). 2.3.1.Water content, elemental analysis and higher heating value


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The water content (wt %) of the raw bio-oil and of the two liquid phases (organic and aqueous) obtained from the HDO was quantified by the Karl-Fischer titration method using a Mettler Toledo V-20 analyzer and appropriate Karl-Fisher reagents for samples that contained aldehydes and ketones (Hydranal composite 5K one-component titrating solution and Hydranal medium K). The higher heating values (HHV) of the raw bio-oil and of the HDO organic phases were analyzed using a C2000 IKA Bomb Calorimeter. Elemental analyses were carried out in a Leco CHN628 - 628S. 2.3.2.Total organic carbon Total organic carbon of the aqueous phases obtained from the HDO treatment was measured using a total organic carbon analyzer (TOC-L CSH/CSN Shimadzu analyzer). Previously, the aqueous phases were filtered using a 0.45 µm syringe filter in order to remove catalyst fines. 2.3.3. Polarity by hexane solubility Hexane solubility analyses were carried out in order to know the fraction of non-polar compounds in the organic samples, which is related with the removal of O from the organic compounds. Approximately 0.2 g of each organic phase was weighed in a vial, to which 4 mL of hexane was added. It was then placed in the ultrasound equipment for 30 minutes. Later, the hexane soluble fraction was poured into a previously weighed evaporation vessel and placed in a sand bath at 80 °C. After one hour, the evaporation vessel was weighed, and the soluble fraction in hexane was calculated by the mass difference. 2.3.4.Phenols by Folin Ciocalteu Phenols produced in lignocellulosic fast pyrolysis bio-oils come from the pyrolysis of the lignin fraction, and the bio-oil contains a significant amount of them. Moreover, they may have very different applications [31], so it is very interesting to know their concentration before and after the HDO treatment. The Folin Ciocalteu method used to calculate the phenol concentration as a mass percentage of gallic acid equivalent (GAE %) is based on that described elsewhere [1]. These authors used a mixture of 5 mL of ethanol and 5 mL of distilled water. In this work ethanol was substituted by methanol in order to achieve complete dissolution of the HDO organic phases. This change resulted in the solutions obtained after the reaction with the Folin-Ciocalteu reagent being green instead of blue, causing a change in the results if they are compared with those obtained with the mixture of ethanol and water. In this work, we analyzed the raw bio-oil using also a mixture of ethanol and water in a similar way as Stankovikj et al. [1] and obtained a similar concentration (17.2 GAE %) than the value shown by these authors for BTG bio-oil (16.8 GAE %). However, this result was different when we used a mixture of methanol and water (10 GAE %), for this reason, the values reported in this work should not be taken as absolute values, but can be useful to compare the phenols concentration of the raw bio-oil and the HDO organic phases obtained.


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2.3.5.Volatile compounds by Gas Chromatography – Mass Spectrometry (GC-MS) The raw bio-oil and the HDO organic phases were analyzed by GC-MS in an Agilent 7890A. About 0.1 g of raw bio-oil and HDO organic phases were dissolved in 1.8 mL of solvent. Acetonitrile was used for the raw bio-oil and methanol for the HDO organic phases in order to get complete dissolution. A DB-17 MS column (60 m x 0.25 mm x 0.25 µm) was used for these analyses. 1 mL·min-1 of helium of 99.999 % purity was used as carrier gas. The initial temperature was set at 45 °C and maintained during 10 min. The temperature was then increased with a heating rate of 3 °C·min-1 until reaching 250 °C, which was maintained for 3 min. After this isothermal period, another ramp was implemented, again with a heating rate of 3 °C·min-1 until reaching 310 °C. The injector had a temperature of 280 °C and the MS detector 230 °C. MS full-scan mode between m/z values of 50 and 550 was used. Calibration was not carried out. This method, used also by other authors [32, 33], considers the response factors of all the compounds to be similar and therefore gives semiquantitative results, which are suitable for comparing relative percentages of compounds or families of compounds in different samples. The chemical composition of the aqueous phases was also analyzed qualitatively by GC-MS (Agilent 7890A). A polar capillary column for aqueous solution, HP-FFAP (50 m x 0.20 mm x 0.3 µm), was used. The carrier gas was 1 mL min-1 of helium of 99.999 % purity. The injector and MS detector temperatures were set at 300 °C and 320 °C, respectively. The following temperature program was adopted: oven starting temperature 60 °C held for 4 min, increased to 80 °C at a heating rate of 1.5 °C min-1 and held for 5 min, increased to 100 °C at a rate of 3.5 °C min-1 and held for 10 min, and lastly increased to 240 °C at a rate of 1.8 °C min-1 and held for 5 min. The MS detector operated in full-scan mode between m/z 10 and 500. 2.3.6.Acids by total acid number and GC-MS ASTM D664 standard test method for determining the acid content of petroleum products was used for obtaining the total acid number (TAN) of the samples. The TAN value includes carboxylic acids as well as weaker acidic compounds such as phenolics. Approximately 0.15 g of the bio-oil or HDO organic phases were weighed in a beaker. After that, 20 mL of a mixture of solvents, toluene: isopropanol:water (71 mL:24 mL:4 mL) were added to the beaker and the solution was titrated against 0.10 mol L-1 KOH. Titrations were stopped when the pH was higher than 7. A sample with the volume of solvent used was also titrated in order to correct the measure. Aqueous phases were titrated using distilled water as solvent. The concentration of acids of the raw bio-oil and of the HDO organic phases were determined by the following procedure. Approximately 0.15 g of sample was weighed and placed in a vial to which 1 mL of 1-butanol and a drop of concentrated sulfuric acid were then added as reagents for the


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esterification of the acids. Next, the vial was inserted during 5 min in the ultrasound bath. Once this time had elapsed, it was kept in a sand bath at 60 °C for 30 min. Then, 3 mL of NaCl solution (10 wt.%) and 1.5 mL of heptane were added. The heptane fraction was separated and reserved for analysis by GC-MS. The column and the heating ramp used were the same as in the method described above and used for the characterization of the organic phases. The MS detector worked in full scan mode, but the quantitative results have only been analyzed taking into account the ion (m/z 57), which corresponds to the butyl loss of the butyl esters obtained from the esterification of the acids. The peaks corresponding to m/z 57 were integrated, and their areas summed, obtaining the total chromatographic area of the acids present in the sample. The total chromatographic area obtained for each sample was divided by the exact mass of sample that was subjected to esterification, in order to obtain the total acid area normalized to the mass of the esterified sample. 2.3.7.Functional groups by ATR-FTIR ATR-FTIR analyses carried out in this work enabled us to know the evolution of some bio-oil functional groups (mainly alkanes, aromatics, carboxylic and carbonyls) during the HDO treatment. The analyses were performed using a Bruker Alpha spectrometer equipped with a platinum ATR single reflection diamond reflector. Total reflection spectra were collected in-situ in the range of 4000-400 cm-1 at a resolution of 4 cm-1 over 24 scans. The spectra of the region 1850-1400 cm-1 were analyzed by a method adapted from one developed by other authors [23, 34]. The spectra of this region were baseline corrected. An iterative procedure based on nonlinear least squares fitting was used to fit the experimental data of the infrared spectra zone analyzed (1850 – 1400 cm-1) to Gaussian peaks with the center wavenumbers of the peaks shown in Table 3. As the chemical nature and the water content (hydrogen bonding) of the raw bio-oil and the HDO organic phases are significantly different, three of the six center wavenumbers used have shifted slightly for spectra of the raw bio-oil and the HDO organic phases (see Table 3). During the iterative fitting procedure of the spectra of each sample, center wavenumbers of the peaks were fixed whilst bandwidths were restrained to different maximum limits. The adjusted R-squared values of the fit of the different spectra were greater than 0.99 for all cases, which verified the goodness of fit. Absorbance areas of the deconvoluted peaks were calculated as percentage of total area of the FTIR spectra region analyzed. An example of the whole procedure used to fit and calculate the area percentage of each functional group/structure in the HDO organic phase from the experiment 5%Cat_1S has been included in the Supporting Information. 2.3.8.Distillable fractions by simulated distillation


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Simulated distillation analyses are usually used to analyze petroleum products in order to know the mass fraction of the main distillable fractions (gasoline, light cycle oil and heavy cycle oil). The analyses in this study were performed by the Catalytic Processes & Waste Valorization (CPWV) research group of the University of the Basque Country (Spain) following ASTM 2887. 2.3.9.ESI-FT-MS analysis ESI-FT-MS analyses are able to give the exact masses of hundreds or thousands of molecules that

can be present in a pyrolysis bio-oil sample. These masses are converted into a molecular formula such as CxHyNzOwSk and can even be converted to a structural formula using information from the literature and the Double Bound Equivalent (DBE). The raw bio-oil and the aqueous and the organic phases obtained from the experiment 5.0%Cat_1S were analyzed by electrospray ionization Fourier transform mass spectrometry in both positive and negative ion modes (ESI(±)-FTMS). These analyses were performed by dissolving the sample in a toluene/methanol mixture (1:1 v/v) to produce a final solution of 20 ppm. The ESI-FT-MS data were collected in an HCD Exactive Plus mass spectrometer (Thermo Scientific, Bremen, Germany) with the following conditions: capillary voltages at +4.4 and S3 B kV, S-lens RF level at 80 and capillary temperature at 320 °C. Nitrogen was used as nebulization gas. The MS acquisition was operated in Full Scan MS mode at a resolution of 140,000 FWHM (full width at half maximum) at m/z 200 along the m/z range of 100–1000 Da using the Xcalibur 3.0 software, and a total of 100 U

was accumulated in each run. The final mass

spectra for each sample were obtained from the blank mass spectra subtraction. Identification of the ions was achieved by comparing the m/z values in the mass spectra obtained by ESI(±)-FT-MS with a library of compounds using the software PetroMS. The PetroMS processing is described elsewhere [35]. A molecular formula match was considered when the mass error between the experimental m/z and the theoretical m/z from the library value was less than 3 ppm.


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3. Results and discussion 3.1. Product distribution and gas yields obtained in the HDO experiments Four different kinds of products were obtained in the HDO reaction: 1) a gas comprising the noncondensable gases generated during the reaction and the unreacted H2, 2) a carbonaceous solid or coke, 3) an aqueous liquid phase and 4) an organic liquid phase. When a 2 wt % catalyst loading ratio was used in the HDO experiments, the main product obtained was a carbonaceous solid that gave the liquid product a sludge-like appearance, which impeded its characterization. For this reason, the results obtained using this catalyst loading ratio will not be shown. 3.1.1. HDO product distribution The mass yield of each one of these four products (Yproduct i) was calculated as the mass of product obtained per 100 g of wet bio-oil fed. For the gas product, the unreacted H2 was not taken into account. These results are presented in Figure 1.

Y (g product i · 100 g -1 wet bio-oil)

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50 45 40 35 30 25 20 15 10 5 0

43.8 39.2

37.8 33.7

38.1 32.9

31.5

32.6

24.1

22.6

22.3

13.2 5.3

3.1

1.7 3.3%Cat_1S Organic phase

5.0%Cat_1S Aqueous phase

6.5%Cat_1S Gas

2.8 5.0%Cat_2S

Carbonaceous solid

Figure 1. Yield of each product type obtained in the HDO experiments. As can be observed, the majority product obtained in all the HDO experiments was the organic phase, whose yield varied between 37.8 and 43.8 wt %. As regards the effect of the catalyst loading ratio, it seems that the yield of gas increased with the ratio of catalyst used, while the yield of the organic phase and the yield of solid were at a maximum and a minimum for the experiment 5.0%Cat_1S, respectively. The decrease in the yield of the organic phase observed when moving from 5.0 to 6.5 wt % of catalyst ratio could be associated with the increase in the gas yield and with the increase in the solid yield. In fact, growing amounts of catalyst seem to favor further cracking of the organic phase compounds to gas phase compounds. The increase in the yield of coke generated when


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

increasing the catalyst ratio from 5.0 to 6.5 wt % could also be related with those cracking reactions that transformed the organic liquid fraction compounds into gas compounds. The formation of light compounds, such as those contained in the gas, must be accompanied by the formation of heavy compounds, as could be coke, which would be accounted for the carbonaceous solid. The highest yield of carbonaceous solid (5.3 wt %) was formed in the experiment carried out with the lowest catalyst loading ratio (3.3%Cat_1S), which could suggest that low catalyst loading ratios promote the formation of solid, as was observed in the experiment using only a 2 wt % catalyst loading ratio. On the other hand, the yield of aqueous phase hardly varied with the different operational conditions used in the experiments, fluctuating between 31.5 and 33.7 wt %.

The yields of organic phase obtained are equal to or higher than those achieved by other authors (26 – 37 wt %) in HDO experiments carried out with bio-oil from the same origin and also the same catalyst, but at lower temperatures (100 – 200 °C) [1]. However, the yields of aqueous phase obtained by these authors [1] were significantly higher (60 – 70 wt %) than those presented in this work. This was probably because at those lower temperatures there was a greater proportion of water soluble compounds, whose polarities were still too high to become part of the organic phase. This would explain the lower water contents (36 – 50 wt %) of the aqueous phases obtained by these authors [1], in comparison with those obtained in the present work (87 – 93 wt %). 3.1.2.Yields of gas products The mixture of gas compounds obtained once the experiment finished contained between 9 and 26 vol % of H2, so the experiments were not performed under H2 starvation conditions. The H2 consumption and H distribution among the reaction products are discussed in section 3.2.3. The mass yields of each of the gaseous compounds produced in the HDO experiments are shown in Figure 2.


Page 12 of 36

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Y (g gas compound i · 100 g -1 wet bio-oil)

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20 18 16 14 12 10 8 6 4 2 0

19.2

19.1

19.0

11.0

1.0 0.70 0.41 3.3%Cat_1S

2.0 2.6 0.44

1.9 1.6 0.16 5.0%Cat_1S

CO2 CO2

6.5%Cat_1S

HC's (C2-C4)

CH4 CH4

1.5 1.5 0.11 5.0%Cat_2S CO

Figure 2. Yields of gas products obtained in the HDO experiments. The mass yield of CO2 was significantly higher than the mass yields of any of the other gaseous compounds (see Figure 2). A marked rise in the mass yield of CO2 was observed when the catalyst loading ratio used in the HDO experiment was increased from 3.3 to 5.0 wt %. However, when the ratio of catalyst was increased from 5.0 to 6.5 wt %, or one or two stages were used in the HDO (5.0%Cat_1S and 5.0%Cat_2S), the yield of CO2 maintained constant under the HDO operational conditions selected in this work. The mass yields of CO were significantly lower than those of CO2 and no clear trend was observed either with the amount of catalyst or by the use of one or two stages. As regards the production of light hydrocarbons (C1-C4), the compound with the highest mass yield was CH4, followed by C2H6, C3H8 and C4H6. The mass yield of C2-C4 hydrocarbons increased when the catalyst loading ratio was augmented from 3.3 to 5.0 wt %, while the yield of CH4 suffered an increase throughout the range of catalyst loading ratios studied (3.3 – 6.5 wt %). The high yields of CO2 indicate a significant occurrence of decarboxylation reactions of carboxylic acids. The decarboxylation of acetic acid (CH3COOH

CO2 + CH4), a compound highly abundant in

the raw bio-oil but not in the HDO organic phases, could be the origin of both CO2 and CH4. CH4 could also be formed in methanation reactions of CO and CO2 (CO + 3H2

CH4 + H2O and CO2 + 4H2

CH4

+ 2H2O), and in hydrogenation reactions of the methoxy groups of the methoxy phenols via demethylation [1] (-OCH3 + H2 W -OH + CH4). Ru based catalysts are known to favor methanation reactions of CO and CO2 in the range of temperatures and pressures used in this work [7, 36, 37]. In addition, in CO/CO2 mixtures, the presence of 30% water enhances CO hydrogenation over Ru based catalysts [37], which could explain the low concentration of CO obtained. Lastly, at the temperature used in this study in the isothermal period (350 °C), the equilibrium of the water gas shift reaction


Energy & Fuels

(CO + H2O

CO2 + H2) is shifted towards products (Kp ~ 21), so this reaction could be another origin

for CO2 and a sink for CO. Other authors [1, 18, 24] have also reported CO2 and CH4 as being the gas compounds generated in a greatest proportion. However, the yields of CO2 obtained in the present work, 11.0 – 19.2 g· 100 g-1 of wet bio-oil or 15 – 26 g·100 g-1 of dry bio-oil, were significantly higher than those obtained by Stankovikj et al. [1], 0.5 – 1.6 g·100 g-1 of wet bio-oil, who performed the hydrotreatment at lower temperatures (100 – 200 °C). 3.2. Atomic mass balances of the HDO experiments Mass balances of C, O and H have been carried out. The experimental data used to calculate these mass balances are the yields of products, the water content and TOC of the HDO aqueous phase (see Table 1) and the water content and elemental analysis of the raw bio-oil and the HDO organic phases (see Table 2). Tables 1 and 2 are shown in sections 3.3.1 and 3.3.2, respectively. 3.2.1.Yield of C The yield of C to each one of the products (YC) is defined as the mass percentage of the C of the raw bio-oil that ended up in each of the C-containing products of the HDO reaction (organic compounds of the organic phase, organic compounds of the aqueous phase, carbonaceous solid, CO, CO2, CH4 and other light hydrocarbons). It has been assumed that carbonaceous solid only contained C. The C mass balance ranged between 95 and 105 %. Figure 3 shows the values obtained, and as can be observed, the organic compounds in the organic phase accounted for the highest YC.

YC (g C to product i · 100 g -1 C in dry bio-oil)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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90 78.0

80 70

69.0

66.7

66.0

60 50 40 30 20 10

20.7

18.5

10.6 12.3 5.9

5.2

3.8

4.1

7.1

17.6 5.3

6.4

0 3.3%Cat_1S

5.0%Cat_1S

6.5%Cat_1S

5.0%Cat_2S

C to organic compounds in org. ph. C to organic compounds in aq. ph. C to gaseous compounds

C to carbonaceous solid

Figure 3. Yield of C to each product (YC) obtained in the HDO experiments.


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The maximum YC to the organic phase (78.0 wt %) and the minimum YC to carbonaceous solid (3.8 wt %) were obtained in the experiment 5.0%Cat_1S. This maximum in the YC to organic compounds in the organic phase could be a consequence of the decrease in the YC to carbonaceous solid from 12.3 wt% to 3.8 wt % when the catalyst loading ratio was increased from 3.3 wt % to 5.0 wt %, and to the increase of the YC to gas (20.7 wt %) and the YC to carbonaceous solid (7.1 wt %) when the catalyst loading ratio was increased from 5.0 wt % to 6.5 wt %. As already mentioned, the lowest ratios of catalyst studied (2.0 and 3.3 wt %) promoted the formation of carbonaceous solid and the highest ratio (6.5 wt %) favored the transformation of the organic liquid compounds in lighter compounds, which, in this case, was also accompanied by a greater Yc to carbonaceous solid. The YC values of the organic phase obtained in this work are slightly lower than those reported by Wildschut et al. (82 wt %) [24], who studied the influence of the reaction time under very similar operational conditions but using beech bio-oil. These authors also observed a significant increase of C in the gaseous fraction within the interval of reaction time studied (2 – 6 h). This could have had an effect on the conversion to gas compounds similar to the effect caused by the increase in the catalyst loading ratio. 3.2.2.Yield of O The yield of O to each of the products (YO) was defined as the mass percentage of O in the organic compounds of the raw bio-oil that ended up in each of the O-containing compounds of the HDO reaction (organic compounds of the organic phase, organic compounds of the aqueous phase, CO, CO2 and H2O formed during HDO). Many different reactions take place during the HDO treatment carried out, being water sometimes a product, as in hydrodeoxygenation reactions, or a reagent, as in the water gas shift reaction. In our experimental conditions, some extra water, apart from the water of the own raw bio-oil, was always obtained after the HDO. Therefore, the balance among the production and the consumption of water during the process was always positive and, for this reason, the water contained in the raw bio-oil was not taken into account for the O atomic balances. Although it was not possible to determine the O contained in the organic compounds of the aqueous phase, O atomic balances closed over 90%, except for the experiment carried out with the lowest ratio of catalyst (3.3 wt %). The O atomic balance of this experiment closed only to 57%, possibly because this aqueous phase contained more organic compounds accumulating some of the missing O. The YO to each product obtained in the HDO experiments is shown in Figure 4.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

YO (g O to product i · 100 g -1 O in dry biooil)

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56.3

55.9

55.5

60

Page 16 of 36

50 40 30 20

32.3

15.1 24.3

4.7

15.1 19.8

19.0

14.5 20.3

10 0.9

1.0

0.3

6.5%Cat_1S

5.0%Cat_2S

0.4

0 3.3%Cat_1S

5.0%Cat_1S

org. comp. of org. ph. OOtotoorganic compounds of org. Ph. (%)

H2Oproduct formed(%) OOtotoH2O

CO2(%) OOtotoCO2

CO(%) OOtotoCO

Figure 4. Yield of O to each product (YO) obtained in the HDO experiments. Most of the O that took part in the reaction was transformed into CO2, a significant percentage ended up as water and around 20 wt % remained in the organic compounds of the organic phase. The highest percentage of O that ended up in the organic compounds of the organic phase was obtained for the experiment 5.0%Cat_1S, possibly because the organic phase of this experiment had the maximum yield (43.8 wt %) (see Figure 1). The O that ended up as both CO2 and as H2O had minimum values for the experiment (3.3%Cat_1S). However, catalyst loading ratios equal to or higher than 5 wt % (5.0%Cat_1S and 6.5%Cat_1S) did not affect the amount of O that ended up as CO2 and as H2O. Similarly, these values were not modified by the use of one or two stages in the HDO treatment. The percentage of O that ended up as CO was very low in all the experiments and did not show any noticeable trend with the different operational conditions studied.

3.2.3.H and water formed yields and H2 consumption The yield of H (YH) was defined as the mass percentage of the H of the reagents (organic compounds of the raw bio-oil and H2) that ended up in each one of the H-containing products of the HDO reaction (organic compounds of the organic phase, organic compounds of the aqueous phase, H2O formed during HDO, light hydrocarbons and unreacted H2). As in the case of the O yield, the H coming from the water contained in the raw bio-oil was not taken into account because for the H atomic balance, because some extra water, apart from the water of the own raw bio-oil, was always obtained after the HDO process. Although it was not possible to determine the H contained in the organic compounds of the aqueous phase, the H mass balances of the HDO experiments were closed between 71 and 94%. Figure 5 shows the distribution of the H in the reagents (H in bio-oil organic


Page 17 of 36

compounds and H2) and in the H-containing HDO products. H2 consumption values are also presented in Figure 5. 1.05 1,05

100 90

84.3

1.0 1

80 70

0.95 0,95

66.2

62.0

56.7

60

54.0

0.9 0,9

50

0.85 0,85

40 30 20

0.8 0,8 19.2

15.7

10

4.5

2.9

13.5 9.0

6.9

3.1

2.3

8.9 12.2

9.3

H2 consumption (g H2 · 100 g -1 dry bio-oil)

YH (g H to product i·100 g -1 H in dry bio-oil and H2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.75 0,75

2.4

0.7 0,7

0 Reagents

3.3%Cat_1S

5.0%Cat_1S

6.5%Cat_1S

5.0%Cat_2S

H HasasH2H2(%)

organic compounds HHininliquid organic compounds (%)

H2O(%) formed HHastoH2O H consumption H2 2cosume (g H2/100 g dry bio-oil)

C1 - C4 hydrocarbons HHintoC1-C4 HC's (%)

Figure 5. Yield of H (YH) to each product and H2 consumption obtained in the HDO experiments. Part of the H2 introduced as reagent was consumed under all the operational conditions studied, although the highest consumption was observed in the experiment 5%Cat_1S. Moreover, in this experiment the highest YH in the form of organic liquid compounds in the organic phases was obtained (66.2 wt %), possibly because this experiment presented the highest yield of organic phase. The experiment carried out with the highest ratio of catalyst 6.5%Cat_1S showed the maximum YH that ended up in light hydrocarbons (C1-C4). This fact points to a non-desirable consumption of H2 in methanation reactions of CO and/or CO2 to produce CH4, as was already suggested based on the yield of CH4 obtained. The H2 consumption obtained in this work is comparable to that obtained by Stankovikj et al. [1], which varied between 0.95 and 2.2 g of H2 per 100 g of dry bio-oil, and by Ardiyanti et al. [38], whose H2 consumption varied between 0.7 and 2.2 g of H2 per 100 g of dry bio-oil. H2 consumption in works addressing hydrotreating of fast pyrolysis oils shows a tremendous variability, finding values from less than 10 to over 400 m3(STP) of H2 per ton of bio-oil (0.1 – 4.7 g of H2 per 100 g of dry bio-oil), depending on the feedstock [39]. To assess the production of water during the HDO treatment, the yield of water formed (Ywater formed) was calculated as the difference between the mass of water in the products and in the wet bio-oil fed divided by the mass of dry bio-oil fed, in mass percentage. The Ywater


formed

varied between

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Page 18 of 36

1.8 and 5.8 g per 100 g of dry bio-oil used in the HDO experiments. As can be seen by the H that went to water formed in the reaction (see Figure 3), a significantly lower Ywater formed was obtained in the experiment 3.3%Cat_1S (1.8 g per 100 g of dry bio-oil). However, higher and similar Ywater formed, between 5.6 and 5.8 g per 100 g of dry bio-oil, were obtained when the experiments were carried out with higher catalyst loading ratios (5.0 – 6.5 wt %) or when two stages were used in the HDO experiments. The water formed during the HDO process probably comes from dehydration and hydrodeoxygenation

reactions

[24,

39].

Dehydration,

probably

caused

by

condensation/polymerization reactions, could take place for the lowest catalyst loading ratio (3.3%Cat_1S), which would also explain the highest yield of solid obtained in this experiment. On the other hand, hydrodeoxygenation reactions could need a greater proportion of active catalyst, which could explain the higher yield of water obtained in experiments carried out with catalyst loading ratios equal to or greater than 5 wt % (5%Cat_1S, 6.5%Cat_1S, 5%Cat_2S). 3.3. Physicochemical characterization of the HDO liquid product The bio-oil used in the HDO experiments as raw material was not phase separated. And, as previously mentioned, two liquid phases, aqueous and organic, were obtained from the HDO process. Although the separation of the organic and aqueous phases by decantation is selective, the aqueous phases still contained organic compounds (water content between 87 and 93 wt % and TOC between 5 and 8 wt %) and the organic phases had low water contents (< 5 wt %). After the separation of the aqueous and the organic phases, it was noticed that the HDO organic phases had lower viscosity as the ratio of catalyst used in the HDO experiments increased, especially in the case of the experiment carried out with the greatest catalyst loading ratio (6.5%Cat_1S). Lastly, the organic phase obtained in each experiment was completely soluble in methylene dichloride, which could point to the disappearance of the high molecular weight lignin fraction present in the raw bio-oil (16.9 wt %) (see section 2.1). In this section, the physicochemical characterization of the aqueous and organic phases obtained will be shown and discussed, paying special attention to a comparison of the physicochemical properties of the HDO organic phases and the raw bio-oil. 3.3.1.Aqueous phase Water content, TOC and TAN. The water content, the total organic carbon content (TOC) and the total acid number (TAN) of the aqueous phases obtained in the different experiments are shown in Table 1.


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Table 1. Water content, TOC and TAN of the aqueous phases obtained in the HDO.

3.3%Cat_1S 5.0%Cat_1S 6.5%Cat_1S 5.0%Cat_2S

Water content (wt %) 87.20 92.35 92.52 91.45

TOC (wt %) 8.10 6.69 5.46 7.10

TAN (mg KOH g-1 aqueous phase) 105 87 69 91

The water content of the aqueous phases was very high, which could explain their water-like appearance. The lowest water content, and therefore the highest content of organic compounds, was obtained for the minimum ratio of catalyst used (3.3%Cat_1S). The highest content of organic compounds in the aqueous phase from the experiment 3.3%Cat_1S indicates a lesser occurrence of the HDO reactions in this experiment that would cause the presence of organic compounds with a greater proportion of O. Because of this greater proportion of O, these compounds would be polar and would then be solubilized in this aqueous phase. This supposition is also supported by the maximum TAN value (105 mg KOH·g-1) of this aqueous phase (3.3%Cat_1S). The progressive reduction of both the TOC and the TAN values of the aqueous phase when the ratio of catalyst increased indicates that the compounds present in this phase reacted in the HDO when greater ratios of catalyst were used. Quite similar levels of TOC and TAN have been reported in the literature [8, 11, 24], although higher TOC values were obtained in aqueous phases hydrotreated at lower temperatures [1]. Volatiles by GC-MS. The GC-MS analyses of these aqueous phases revealed the majority content of short chain carboxylic acids ( 343 °C) (< 212 °C) (%)* 343 °C) (%) (%) Raw 51.93 35.95 12.12 3.3%Cat_1S 32.73 42.87 24.40 5.0%Cat_1S 27.53 49.35 23.12 6.5%Cat_1S 24.88 41.91 33.21 5.0%Cat_2S 25.87 38.87 35.26 * Water present in the samples is not included in this fraction.


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converted these O7 to O10 species into less oxygenated species with different functional groups, since species O3-O4-O5 that were not present in the ESI(+) spectrum of the raw bio-oil appeared after the HDO process. As known in literature [28], HDO promotes the partial conversion of highly oxygenated compounds into less oxygenated compounds, or the breaking of large oxygenated structures into two or more small oxygenated compounds. The representation of the ESI-FT-MS results on a van Krevelen plot is one way of identifying possible reaction pathways [47]. The slopes (and intercepts) of the lines in the van Krevelen plots can be visually associated to cleavage reactions or the addition of certain molecules or functional groups (H2O, CO2, CO, H, O, CH4, CH3O, etc). A summary of the main reactions and the molecules or fragments associated with them, together with the slopes and intercepts of the lines can be found elsewhere [42]. Figure 10 shows the van Krevelen plots built from the ESI(±)-FT-MS results for the CxHyOz molecules of the raw bio-oil and the HDO organic and aqueous phases obtained from the experiment 5.0%Cat_1S. In the van Krevelen plot of the raw bio-oil built from the ESI(+)-FT-MS, it can be observed that most of the molecules have H/C ratios from 0.5 to 1.0 and O/C ratios from 0 to 0.35. However, after the HDO treatment these molecules did not show such significant abundance. Clear patterns can be distinguished in the van Krevelen plot of the raw bio-oil and the HDO organic phase built from the ESI(-)-FT-MS. In the case of the raw bio-oil, the place corresponding to pyrolytic lignin (H/C=0.86-0.91 and O/C=0.26-0.29, structures from [48]), cellulose (H/C=1.67 and O/C=0.83), hemicellulose (H/C~1.6 and O/C~0.8), and humins (H/C=1.11 and O/C=0.30, structure from [1]) are highlighted. As can be observed, during pyrolysis the cellulose and hemicellulose structures disappeared almost completely via dehydration reactions, humins being one of their products (Figure 10a, ESI(-)). The vertical line down denotes the occurrence of dehydrogenation. Moreover, the dehydrated products (H/C~1.6 and O/C~0.8) and the pyrolytic lignin seem to suffer decarboxylation reactions during pyrolysis. It can be observed in figure 10(b) that after HDO the organic phase did not contain any compound with a O/C ratio higher than 0.5, and the pyrolytic lignin seems to have disappeared. The O/C ratio of pyrolytic lignin (~0.3) in the raw bio-oil moved to 0.1 without changes in the H/C ratio (~0.8) of the HDO organic phase. The line patterns indicate that, during HDO, O was removed from these molecules, probably in decarboxylation and decarbonylation reactions. On the other hand, the ESI(-)-FT-MS plots of the HDO organic phase clearly evidences the decrease in the presence of the pyrolytic lignin compounds (H/C~0.9 and O/C~0.3) that were initially present in the raw bio-oil (Figure 10a).


Page 28 of 36

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Figure 11 shows the double bond equivalent (DBE) versus carbon number for the O1 to O10 molecules. The formula for DBE calculation can be found elsewhere [26]. The data shown in these plots corroborate the identification of lignin degradation products in the raw bio-oil, presenting oxygenated compounds mainly comprising between 10-20 carbon with DBE

Hydrodeoxygenation of Lignocellulosic Fast Pyrolysis Bio-Oil

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