Nuno Batalha
email: nuno.rocha-batalha@ircelyon.univ-lyon1.fr

Ruben Checa

Chantal Lorentz

Pavel Afanasiev

Krzysztof Stańczyk

Krzysztof Kapusta

Dorothée Laurenti

Christophe Geantet

Hydrocarbon fuels from sequential hydrothermal liquefaction (HTL) of lignite and catalytic upgrading of crude oil

Keywords: Lignite, hydrotreatment, Crude oil, liquid fuels, coal valorization

Lignite from Belchatów in Poland was converted to hydrocarbon fuels, particularly in the kerosene and diesel range, through sequential hydrothermal liquefaction (HTL) and hydrotreatment (HDT) of the obtained crude oil. Different HDT temperatures were investigated (from 350°C up to 390°C).

Despite the highly aromatic nature and high heteroatom content, the HTL lignite crude oil could undergo deep hydrodesulfurization (HDS = 99 %) and hydrodeoxygenation (HDO =98 %) in a single HDT treatment stage, yielding a liquid product rich in aliphatic and aromatic, particularly monoaromatic, hydrocarbons. The hydrotreatment and hydrogenating capacity of the lignite crude oil was linked to the low presence of refractory compounds in lignite and, consequently, lignite's crude oil, permitting the easy transformation of the crude oil into hydrocarbon fuel.

In addition to the crude oil, the HTL of lignite yielded a stream of char, which displayed a higher calorific value and lower heteroatom content than the original lignite feedstock, thus having a high potential for electricity production, the common use of lignite. Thus, introducing an HTL stage to extract crude oil before lignite use in electricity production could provide an alternative fuel source used as a strategy to increase multiple countries' energetic independence before the transition to renewable fuels.

INTRODUCTION

The Russian invasion of Ukraine in February 2022 has redistributed the energetic policy in Europe (and in the world) and deeply impacted the food and energetic markets and climate issues [START_REF] Tollefson | What the War in Ukraine Means for Energy, Climate and Food[END_REF] . For instance, the European Union (E.U.) is the first market for diesel fuel and must import 20% of its consumption. Half of this imported diesel was coming from Russia. Despite E.U. policy attempts to eliminate fossil resources, with a new target of 45% of renewables in 2030, energetic transition and evolution of energetic policies are challenging to manage, and a portfolio of options must be investigated. Biofuels and biodiesel are slowly increasing their production. However, these are only expected to cover 4.6% of transport fuel demand in 2024 2 . Therefore, there might be a need for a local diesel resource in the E.U. before vehicles electrification and green fuels are fully implemented.

Despite its relatively low heat content, almost 80% of lignite is used for electricity generation [START_REF]Uses and Benefits of Lignite[END_REF] .

Even if its production has strongly decreased since the 90's in Europe, the inland consumption of brown coal by E.U. member states was estimated, in 2021, at 300 Mt 4 . An alternative pathway to valorize lignite, pure or mixed with waste biomass, is hydrothermal liquefaction. In the absence of oxygen, subcritical or supercritical water as a reaction medium converts organic matter, independently of the origin, into a liquid of high heating value (HHV). In early works, Deshpande et al., Kershaw et al., and Hu et al. extracted brown coals or Dayan lignites, respectively, with sub and supercritical water and claimed that the extraction residue would be a good feedstock for combustion or gazification [START_REF] Kershaw | Extraction of Victorian Brown Coals with Supercritical Water[END_REF][START_REF] Deshpande | Extraction of Coal Using Supercritical Water[END_REF][START_REF] Hu | Extraction of Lignite with Water in Sub-and Supercritical States[END_REF] . These studies led to the establishment of the hydrothermal dewatering (HTD) process, a pretreatment (developed at pilot scale) of lignite with no particular address regarding the crude oil formed [START_REF] Nikolopoulos | Report on Comparison among Current Industrial Scale Lignite Drying Technologies (A Critical Review of Current Technologies)[END_REF] . This was only tackled more recently by Hartman et al., which investigated the HTL of selected coal feedstocks providing a high liquid yield (21%) and concluded that high-value petroleum-like products could be obtained [START_REF] Hartman | Valuable Crude Oil from Hydrothermal Liquefaction of an Aliphatic Coal[END_REF] . Liu et al. used HTL as a pretreatment before pyrolysis of Xiaolongtan lignite and suggested three reaction steps discussed hereafter [START_REF] Liu | Effect of Hydrothermal Treatment on the Structure and Pyrolysis Product Distribution of Xiaolongtan Lignite[END_REF] . Like HTL of biomass, numerous reactions occurring during HTL are favorable for deoxygenation and monomer formation, even if condensation reactions may co-occur. Due to the complexity of the HTL process, the optimization of operating conditions, i.e., the impact of lignite nature and the understanding of the formation of the product, have yet to be deeply investigated. Recently, Li et al. attempted to optimize HTL process parameters for the transformation of demineralized and pretreated lignite, observing these can impact the liquid yield and composition [START_REF] Li | Effects of Hydrothermal Treatment on Physico-Chemcial Structures and Liquefaction Behaviors of Lignite[END_REF] . They also suggested that HTL promotes the transfer from polycyclic aromatic hydrocarbons to monocyclic aromatic hydrocarbons. All these studies indicate that HTL of lignite can potentially provide some fuel. However, no one attempted to upgrade lignite HTL oil like it was done with the HTL biocrudes of biomass or wastes.

In the present work, we investigated the HTL conversion of a Polish lignite and the catalytic upgrading of the HTL lignite oil obtained. A detailed study of the upgrading operating conditions and their impact on the nature of the products was performed. After upgrading, a diesel fraction was obtained, showing that a preliminary HTL stage can provide a fraction of a diesel source and increase the HHV of the fraction used for electricity generation.

EXPERIMENTAL

Hydrothermal liquefaction of lignite

Lignite from the coal mine of Bełchatów in Poland was used as feedstock. The physicochemical characteristics of the feedstock are presented in Table 1. The HTL experiments were conducted in a batch type 1000 ml stainless steel autoclave reactor (Autoclave Engineers, USA), equipped with a magnetically coupled mechanical stirrer. In each of the HTL experiments, 600 g of 10 wt% of lignite in water solution was used. After loading the reactor, the residual air was removed by nitrogen purging and vacuuming four times. Subsequently, the vessel was pressurized with N2 (~2 bar) and then heated to 360 °C (reaction temperature) with a constant heating rate 10 ⁰C/min and maintained at this temperature for 30 min (HTL reaction time). The pressure in the reactor vessel was self-generated.

After completion of the reaction, the autoclave was cooled to ambient temperature, and the gas samples were taken for analysis by micro-GC SRA instruments equipped with the following three modules: MolSieve 5 Å with backflush, PoraPLOT U, and Plot Al2O3/KCl. The HTL products mixture (crude oil, aqueous phase, and char) were then recovered from the reaction vessel. The separation of the products mixture obtained after the HTL processes was performed using a vacuum filtration technique. The post-reaction mixture was filtered on glass microfiber filters. The aqueous phase obtained was separated from the char. The reactor vessel and the char were washed with acetone, and the solid fraction and the liquid organic fraction (crude oil) solution in acetone were separated by vacuum filtration. Next, the solid fraction was dried to constant weight in the laboratory drying oven at 105 °C. The filtered crude oil solution was concentrated using a rotary vacuum evaporator. The scheme of the process is presented in Figure S1. 𝐴𝑞𝑢𝑒𝑜𝑢𝑠 𝑝ℎ𝑎𝑠𝑒 𝑠𝑜𝑙𝑢𝑏𝑙𝑒𝑠 𝑦𝑖𝑒𝑙𝑑 (𝑤𝑡. %) = 100 -𝑔𝑎𝑠 𝑦𝑖𝑒𝑙𝑑 -𝑐ℎ𝑎𝑟 𝑦𝑖𝑒𝑙𝑑 -𝑐𝑟𝑢𝑑𝑒 𝑜𝑖𝑙 𝑦𝑖𝑒𝑙𝑑 Equation 4 Where: mcrude, mchar, and mlignite are the mass of recovered crude oil, recovered char, and initial lignite feedstock on a dry, ash-free (daf) basis; Mwi and yi are the molecular weight and gas composition of compound i, respectively. P is the residual reactor pressure at room temperature V is the reactor volume occupied by gas (400 mL) R, is the ideal gas constant T, is the room temperature

Hydrotreatment of lignite crude oil

The hydrotreatment of the crude oil obtained from the hydrothermal liquefaction of lignite was performed under batch conditions using a 300 mL reactor from Parr (Model 4566). The hydrothermal conversion of 3 g of crude oil was performed in the presence of 25 g of n-heptane (solvent) and 1 g of sulfided NiMo/Al2O3 catalyst (See main properties in Table S1). Heptane was considered inert during the reaction and is used to provide enough volume inside the reactor to enable continuous stirring (≈ 700 rpm) and, consequently, facilitate the contact between HTL oil, hydrogen, and catalyst. Before reaction, the catalyst was activated through sulfidation carried at 400 °C for 3 h, under a continuous flow (4 L/h) of H2S (15%) in H2. After being sulfided, the catalyst is kept under an inert atmosphere (N2) until being transferred into the reactor.

At room temperature and after flushing with H2, the reactor was pressurized with 10 bar of H2 to prevent catalyst deactivation during heating. After, the reactor was heated up to the reaction temperature, i.e., 350 °C, 375 °C, or 390 °C . Once the reaction temperature is attained (20 min), an additional pressure of 50 bar of H2 was introduced into the reactor, reaching a reaction pressure of ≈ 100 bar. After the second H2 introduction in the reactor, the crude oil hydrotreatment was performed for 1 h or 3 h leading to a pseudo-WHSV (Equation 5) of 3 h -1 and 1 h -1 , respectively. Additionally, a test where the reaction was stopped after introducing the 50 bar of H2 was also performed, denominated t = 0 h, as a reference.

(𝑃𝑠𝑒𝑢𝑑𝑜 -𝑊𝐻𝑆𝑉(ℎ -1 ) = 𝑚 𝑜𝑖𝑙 𝑚 𝑐𝑎𝑡 𝑡 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 Equation 5Where: moil and mcat are the mass of crude oil and catalyst, respectively, treaction is the reaction time.

After the reaction, the reactor was cooled to room temperature, and the residual gas was collected in a gas bag and analyzed with a micro-GC (SRA instruments) equipped with three modules: Molsieve 5Å (H2, CO, CH4), Hayesep A (CO2, C2 hydrocarbons), PLOT Al2O3/KCl (C3-C4 hydrocarbons).

The liquid and spent catalyst were separated through filtration, and the heptane in the liquid was evaporated, using a rotovapor apparatus, for 2 h at a pressure of 30 mBar and temperature of 40°C.

The liquid remaining after evaporation was considered to be the liquid product from hydrotreatment.

Elemental analysis was used to determine the coke deposited in the catalyst, i.e., coke equivalent to carbon in the spent catalyst.

The yields of the products obtained from the HDT reaction, i.e., liquid, solid, gas, H2O, and H2S were calculated through the following equations: Equation 10 Where: mcrude oil, msolid, and mliquid are, respectively, the mass of crude oil to be hydrotreated, the excess of mass found in the catalyst after reaction (mspent catalyst-mfresh catalyst), and the mass liquid obtained after evaporation of n-heptane; Mwi and yi are the molecular weight and gas composition of compound i, respectively. P is the residual reactor pressure at room temperature V is the reactor volume occupied by gas (≈ 263 mL) R, is the ideal gas constant T, is the room temperature O, is the weight fraction of oxygen in the crude oil (Ocrude oil) and liquid (Oliquid) determined by elemental analysis. %CO and %CO2, are the weight fraction of CO and CO2, respectively, in the gas S, is the weight fraction of Sulfur in the crude oil (Scrude oil) and liquid (Sliquid).

Characterization methods

Elemental compositions (CHNS-O) of lignite, crude oil, hydrotreatment liquid, and spent catalyst were analyzed on a Thermo Scientific FLASH 2000 Organic Elemental Analyzer (accuracy of ± 0.1 wt%).

Additionally, the S composition of the crude oil and hydrotreatment liquid samples were also quantified using ANTEK 9000NS apparatus, which is more precise (from few ppm until percent levels) and accurate. Prior to analysis, the samples were diluted in THF ( ≈ 5 wt.%).

The boiling point of the sulfur-containing compounds in the crude oil was estimated by using a Agilent 7890A GC coupled with an FPD+ detector and fitted with a CP-SimDist GC column (10 m × 0.53mm ×

μm).

The molecular weight distribution of the compounds in the lignite crude oil and the hydrotreatment liquid was obtained by size exclusion chromatography (SEC) performed using an Agilent 1200 series HPLC equipped with two PLgel Columns (50 and 500 Å), a refractive index detector (RID), and a diode array detector (DAD). The analysis was carried out at 35 °C using tetrahydrofuran (THF -1 mL/min) as the eluent. Before analysis, the samples were diluted to a concentration of ≈1 wt.% in THF and filtered (0.45 μm). A mixture composed of hydrocarbons (Mw from 86 to 1000 g/mol) was used as standard to convert elution time into molecular weight (hydrocarbons equivalent) leading to an average MwHC data.

The [START_REF] Scanlon | Calculation of Flame Ionization Detector Relative Response Factors Using the Effective Carbon Number Concept[END_REF] C-NMR analysis of the liquid samples was performed in a Bruker AVANCE 400 MHZ spectrometer and analyzed using the software TopSpin 3.0. The measurements were performed at room temperature with an accumulation of 4700 scans for approximately 15 h. The samples (≈100 mg) were diluted with deuterated chloroform (CDCl3) (0.6 mL). Table S2 includes the chemical shifts attributed to each functional group.

Comprehensive GC×GC-TOF MS and GC×GC-FID were used to identify and quantify the compounds in the lignite crude oil and HDT liquid. The GC×GC/TOF system was equipped with a cryogenic modulator from Zoex Corporation (USA) and a BenchTOF 2 TI spectrometer from Sepsolve. The chromatograph and TOF parameters are described in Tables S3 andS4. The GC×GC/FID was equipped with a switch valve modulator (Agilent G3486A CFT modulator), and modulation parameters were optimized according to ref [START_REF] Lelevic | Advanced Data Preprocessing for Comprehensive Two-Dimensional Gas Chromatography with Vacuum Ultraviolet Spectroscopy Detection[END_REF] . The parameters used for product separation are described in Table S3. The quantification of the compounds detected by GC×GC-FID was performed using the relative responsive factor method (RRF) [START_REF] Scanlon | Calculation of Flame Ionization Detector Relative Response Factors Using the Effective Carbon Number Concept[END_REF] with cyclohexanol as the reference. The data obtained from comprehensive GCxGC-FID was also used to provide the distillation curve of the HDT liquid products by using a mixture of n-alkanes (C7 to C40) as a reference.

RESULTS AND DISCUSSION

Hydrothermal liquefaction of lignite

Crude oil

While the primary target product for liquid fuel production is crude oil, only 12.3 wt.%daf of lignite was transformed into this product with char (48.8 wt.%daf), gas (23wt.% daf), and aqueous phase organics (15.9 wt.%daf) representing the main fractions. Liu et al. [START_REF] Liu | Optimization Study of Sub/Supercritical Water Liquefication of Lignite: Fast Liquefaction for High Bio-Oil Yield[END_REF] studied the optimal conditions for maximizing crude oil yield from lignite HTL and found that short residence times, i.e., 5 min, and high temperatures, i.e., 400 °C, are necessary. In this study [START_REF] Liu | Optimization Study of Sub/Supercritical Water Liquefication of Lignite: Fast Liquefaction for High Bio-Oil Yield[END_REF] , the authors found a maximum crude yield of 34 wt.%. Hence, the high residence time (30 min) and low temperature (360 °C) presently used can explain the low crude oil yield. Another important parameter known to affect the yield and quality of crude oil in HTL is the amount of ash [START_REF] Audu | Ash-Pretreatment and Hydrothermal Liquefaction of Filamentous Algae Grown on Dairy Wastewater[END_REF][START_REF] Chen | Effect of Ash on Hydrothermal Liquefaction of High-Ash Content Algal Biomass[END_REF][START_REF] Liu | Conversion of High-Ash Microalgae through Hydrothermal Liquefaction[END_REF][START_REF] Liu | Effects of Inorganic and Organic Acid Pretreatments on the Hydrothermal Liquefaction of Municipal Secondary Sludge[END_REF][START_REF] Shafizadeh | Machine Learning Predicts and Optimizes Hydrothermal Liquefaction of Biomass[END_REF] . On biogenic feedstocks, such as algae, high ash contents have been reported to decrease the yield and quality of the crude product [START_REF] Audu | Ash-Pretreatment and Hydrothermal Liquefaction of Filamentous Algae Grown on Dairy Wastewater[END_REF][START_REF] Chen | Effect of Ash on Hydrothermal Liquefaction of High-Ash Content Algal Biomass[END_REF][START_REF] Liu | Conversion of High-Ash Microalgae through Hydrothermal Liquefaction[END_REF] . Although no reference applied to the conversion of lignite under HTL conditions was found, a direct analogy with biomass feedstocks suggests that high-ash in lignite might have a similar effect. Thus, a high concentration of ash found in lignite (25.94 wt.%) could also be responsible for the low crude oil yield (12.3 wt.%). Indeed, Liu et al. [START_REF] Liu | Optimization Study of Sub/Supercritical Water Liquefication of Lignite: Fast Liquefaction for High Bio-Oil Yield[END_REF] used lignite with 7.14 wt.% of ash. It should be mentioned that Oner et al. demonstrated that ash positively enhances the crude oil product in lignite liquefaction under organic solvents, like tetraline, and under H2 pressure [START_REF] Oner | The Effect of Ash and Ash Constituents on the Liquefaction Yield of Turkish Lignites and Asphaltites[END_REF] . However, the positive effects were attributed to the capacity of certain elements in ash to promote hydrogenation by H2 or hydrogen transfer from the solvent, a reaction not likely to occur under HTL conditions. While ash is bound to affect lignite HTL conversion, the chemical composition of lignite and lignite's ash is substantially different from that of biomass feedstocks like algae. Thus, further studies, out of the scope of this work, should be performed to infer the impact of this component.

When compared to the lignite feedstock, the crude oil stream contains a higher concentration of carbon (77.7 wt.% vs. 50.5 wt.%dry) and hydrogen (8.9 wt.% vs.4.6 wt.%dry) (Tables 1 and2).

Additionally, the crude oil displayed a lower concentration of oxygen (10.8 wt.% vs. 16.0 wt.%dry) and sulfur (2.4 wt.% vs. 3.78 wt.%dry) than the parent lignite sample. However, it is essential to mention that the sulfur content in crude oil is still well above the specifications for liquid hydrocarbon fuels, e.g., diesel specification < 10 ppm. Thus, significant desulfurization must take place during upgrading.

In the case of nitrogen, no specification states a maximum N concentration in hydrocarbon fossil fuels [START_REF] Prado | Nitrogen Removal from Oil: A Review[END_REF] . Still, nitrogen compounds impact other regulated properties, such as gum content, storage stability, and thermal stability [START_REF] Prado | Nitrogen Removal from Oil: A Review[END_REF] . However, synthetic aviation fuels (SAF) have a nitrogen concentration inferior to 2 ppm [START_REF]ASTM D7566-22 -Standard Specification for Aviation Turbine Fuel Containing Synthesized Hydrocarbons[END_REF] . It is worth mentioning that there is still no regulation concerning HTL-derived fuels.

The functional group's composition, determined by [START_REF] Scanlon | Calculation of Flame Ionization Detector Relative Response Factors Using the Effective Carbon Number Concept[END_REF] C NMR, of the crude oil is shown in Table 2. The crude oil is mainly composed of aliphatic (53.6%) and aromatic (40.3%) carbon. Among the aromatic carbon, 24.0% corresponded to C-H bonds, 9.6% to C-C bonds, and 6.8% to C-O bonds (chemical shift and NMR Spectrum in Table S2 and Figure S2, respectively). Alkyl groups or polyaromatics compounds in crude oil can explain the occurrence of C-C bonds in aromatic fraction. Additionally, the high incidence of aromatic C-O and carbonyl C=O , compared to the methoxy, aliphatic, carboxylic acid, and esters, indicates that phenolic compounds and ketones represent a significant portion of the crude oil oxygen. Indeed, the crude oil composition analysis by comprehensive GC×GC-TOFMS (Figure 1)

showed the presence of ketones, furans, phenols, and naphthols. A small amount of mono-aromatic and di-aromatic hydrocarbons and non-aromatic cyclic hydrocarbons, possibly derived from biomarkers typically present in lignites [START_REF] Stefanova | Aliphatic Biomarkers from Miocene Lignites Desulphurization[END_REF] , were also found. The quantification performed by GCxGC-FID is shown in Table 3. Only 16.7 wt.% of the crude oil sample was detectable by GCxGC-FID analysis.

The non-detected compounds were too heavy to be analyzed by gas chromatography. Indeed, Liu et al. analyzed the heteroatom-containing compounds obtained from the ethanolysis of lignite via Fourier transform ion cyclotron resonance mass spectroscopy (FTICR-MS), observing a significant number of oxygenated compounds with up to 6 aromatic rings with the particular occurrence of oxygenated molecules with carbon number between 17 and 26 carbons and two oxygen atoms, like organic acids and diols [START_REF] Liu | Characterization of Heteroatom-Containing Species in the Soluble Portion from the Ethanolysis of the Extraction Residue from Xinghe Lignite by Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry[END_REF] . In the same study, authors characterized the sulfur-containing compounds, mostly ranging above C20 and associated in the same molecule with another S or several O atoms. [START_REF] Liu | Characterization of Heteroatom-Containing Species in the Soluble Portion from the Ethanolysis of the Extraction Residue from Xinghe Lignite by Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry[END_REF] .

The relatively large carbon number of the sulfur-containing compounds in crude oil can explain the few S compounds detected in the products by comprehensive GCxGC-TOFMS (Figure 1). Indeed, the analysis of the sulfur content of the crude oil sample by GC-FPD + using a Simdis chromatography method equivalent to ASTM D2887 reveals that such molecules have a retention time higher than hexatriacontane (C36 -Bp = 497 °C) (Figure S3), which defines the upper detection limit of the GCxGC analysis. The same explanation can be given for the few nitrogen-containing compounds among the GC×GC detected compounds in the crude oil (Figure 1). Table 3. Quantification of the lignite HTL oil compounds detected by GCxGC-FID using the relative response factor method (supporting information). The chromatogram and the areas attributed to each compound family are represented in Figure 1. A better insight into heavy molecules is given by SEC. The molecular weight distribution of the crude oil components (Figure 2-A) reveals a large range of compounds, with a molecular weight of up to 1,800 g/mol in HC eq. and an average molecular weight of 297 g/mol in HC eq. The significant presence of large Mw molecules, which are expected to have a high boiling point, agrees with the small detection level observed in the GCxGC analysis. The evolution of the DAD spectra as a function of molecular weight (Figure 2-B) reveals an intense absorption between 240 and 400 nm, reflecting the high aromatic nature of the compounds in the crude oil. The presence of two absorption maximums, 245 nm and 293, and a shoulder at 316 nm indicates that polyaromatic compounds are abundant in the crude oil sample. Indeed, as exemplified in Figure S4, monoaromatic compounds like benzene generate a DAD band at 250 nm. In opposition, polyaromatic compounds with two or more conjugated aromatic rings present a second absorption band at  > 290 nm (Figures S5, S6, and S7). In the particular case of naphthalene, a second sharp peak was observed at 290 nm, with identical intensity to the peak centered at 250 nm (Figure S5), while in anthracene and pyrene, this band is significantly broader, extending to significantly higher  (Figures S6 andS7). Therefore, in the crude oil (Figure 2-A), the wide absorption band at 250 >  > 400 nm indicates the presence of compounds with at least 3 and 4 conjugated aromatic rings.

Compounds

The comparison of the Mw distribution and the UV absorbance at =240 nm and 290 nm as a function of the Mw reveals that both RI and UV curves have a maximum at Mw  230 g/mol (Figure 2-A).

However, the UV spectra absorption distribution is shifted to higher Mw, evidencing heavier molecules with high aromaticity presence.

Char, gas, and aqueous phase

The hydrothermal liquefaction of lignite (soft Miocene lignite) obtained in the Bełchatów coal mine in Poland led to four product streams, i.e., gas, char, crude oil, and an aqueous phase rich in organic matter. Char was the most abundant among the product streams, representing 48.8 wt.% of the lignite feedstock yield. Even though char and lignite are both solids, the former displayed a higher carbon content with lower oxygen, nitrogen, hydrogen, and sulfur (Table S4). Hence, char displayed O/C and H/C ratios of 0.1 and 0.4, respectively, while lignite O/C was 0.2 and H/C was 1.1. The different solid composition explains char's slightly higher heating value compared with the lignite feedstock, i.e., 22.6 MJ/kgDry vs. 19.0 MJ/kgDry, respectively. P. Liu and D. Zhang 14 studied the transformation of lignite under hydrothermal conditions, observing that, at T=260 °C, significant removals of oxygen and hydrogen occur. Similar observations were made by Z. Shi et al. [START_REF] Shi | Effect of Hydrothermal Treatment on Structure and Liquefaction Behavior of Baiyinhua Coal[END_REF] , who, like P. Liu and D. Zhang, observed a significant reduction of carboxyl, carbonyl, and oxygenated aliphatic carbon in the solid after HTL treatment. These authors also observed a reduction of the aliphatic CH2 and CH3 groups after hydrothermal treatment, which justifies the decrease in the H/C ratio. The authors linked the elimination of oxygen and hydrogen from the lignite structure to the production of light gases, such as CO, CO2, and light hydrocarbons [START_REF] Liu | Effect of Hydrothermal Treatment on the Carbon Structure of Inner Mongolia Lignite[END_REF][START_REF] Shi | Effect of Hydrothermal Treatment on Structure and Liquefaction Behavior of Baiyinhua Coal[END_REF] . Like O and H, the sulfur in char was significantly smaller than in the lignite feedstock, i.e., 1.1 wt.% vs. 3.78 wt.%, respectively, indicating only 14.2 % of sulfur remained in the solid phase after hydrothermal liquefaction. The significant extent of desulfurization is directly related to the nature of the sulfur compounds in lignite. As low-ranking coals, lignites contain more substantial amounts of sulfides and thiols than other types of coal [START_REF] Maes | Rank Dependence of Organic Sulfur Functionalities in Coal[END_REF] , which are more easily transformed under hydrothermal conditions [START_REF] Patwardhan | Supercritical Water Desulfurization of Organic Sulfides Is Consistent with Free-Radical Kinetics[END_REF][START_REF] Meng | Sulfur Transformation in Coal during Supercritical Water Gasification[END_REF] .

The lower impurities content, i.e., O, N, and S, and increased calorific value indicate that char is a higher quality coal than the original lignite. Indeed, the treatment of coal samples through hydrothermal liquefaction has been demonstrated to increase the quality of coal samples, both by eliminating impurities and improving hydrophobicity [START_REF] Li | Effects of Hydrothermal Treatment on Physico-Chemcial Structures and Liquefaction Behaviors of Lignite[END_REF][START_REF] Shi | Effect of Hydrothermal Treatment on Structure and Liquefaction Behavior of Baiyinhua Coal[END_REF][START_REF] Liao | Hydrothermal Dewatering of a Chinese Lignite and Properties of the Solid Products[END_REF][START_REF] Liu | Investigation on Chemical Structure and Hydrocarbon Generation Potential of Lignite in the Different Pretreatment Process[END_REF] .

The composition of the gas stream formed during the hydrothermal liquefaction of lignite is shown in Table S5. CO2 was the majority product in the gas phase, corresponding to 92.6 wt.% of all the produced gases. The high CO2 selectivity in the gas products obtained from lignite HTL had already been reported previously [START_REF] Shi | Effect of Hydrothermal Treatment on Structure and Liquefaction Behavior of Baiyinhua Coal[END_REF][START_REF] Liu | Optimization Study of Sub/Supercritical Water Liquefication of Lignite: Fast Liquefaction for High Bio-Oil Yield[END_REF] , indicating that decarboxylation is one of the primary processes responsible for oxygen removal during the reaction. Indeed, CO only represented 1.4 wt.% of the gases, suggesting decarbonylation is significantly less favored. H2S represents the second most abundant compound in the gas stream, representing 3.4 wt.% of all gases. Furthermore, 19.4% of the sulfur in the lignite feedstock was transformed into H2S. The significant formation of H2S reflects the transformation of organic, i.e., thiol and sulfide, and inorganic sulfur, i.e., pyrite, compounds in lignite promoted by H radicals under hydrothermal conditions [START_REF] Patwardhan | Supercritical Water Desulfurization of Organic Sulfides Is Consistent with Free-Radical Kinetics[END_REF][START_REF] Meng | Sulfur Transformation in Coal during Supercritical Water Gasification[END_REF][START_REF] Calkins | The Chemical Forms of Sulfur in Coal: A Review[END_REF] . In addition to CO2, CO, and H2S, small amounts of light hydrocarbons and H2 (Table S5) were also observed. Nonetheless, the amount of such compounds is not enough to justify alone the small H/C observed in the char, thus suggesting that light hydrocarbons and H2 formation are not the main ones responsible for the elimination of hydrogen in the lignite as proposed by Z. Shi et al., and P. Liu and D. Zhang [START_REF] Liu | Effect of Hydrothermal Treatment on the Carbon Structure of Inner Mongolia Lignite[END_REF][START_REF] Shi | Effect of Hydrothermal Treatment on Structure and Liquefaction Behavior of Baiyinhua Coal[END_REF] . Instead, the transformation of lighter fractions of lignite into aqueous and crude organic compounds is more likely to cause the low H/C of char compared to lignite.

Finally, the organic matter solubilized in the aqueous reaction media, i.e., the aqueous phase, represented 15.9 wt.% of feedstock lignite. In hydrothermal liquefaction processes, the aqueous phase is typically composed of organic acids, e.g., acetic acid, oxygenated aromatics, aromatic, like phenols, nitrogenated compounds, and oxidized sulfur compounds [START_REF] Meng | Sulfur Transformation in Coal during Supercritical Water Gasification[END_REF][START_REF] Fraga | Chapter 5 -Advances in Liquefaction for the Production of Hydrocarbon Biofuels[END_REF][START_REF] Yan | Beneficial Migration of Sulfur Element during Scrap Tire Depolymerization with Supercritical Water: A Molecular Dynamics and DFT Study[END_REF] , thus justifying this stream's acidic pH of 5.2.

Hydrotreatment of lignite HTL crude oil

The significant concentration of heteroatoms and polyaromatic compounds in the crude oil stream clearly demonstrates the necessity of further upgrading for the production of hydrocarbon fuels.

Therefore, this section will focus on the hydrotreatment of the crude oil stream obtained from the hydrothermal liquefaction of lignite. The hydrotreatment of the lignite crude oil was performed at three different temperatures, i.e., 350 °C, 375 °C, and 390°C, and at two different reaction times, 1 h and 3 h. An additional residence time, where the reaction was stopped after the reaction temperature was attained (theating = 20 min ), designated t = 0 h, was also performed at each temperature. The crude oil conversion occurring before t = 0 h are caused primarily by thermal processes. For each operating condition, five product streams (Figure 3-A) were obtained, i.e., gas, liquid, solid, and H2S and H2O estimated using equations 9 and 10.

Solid product

The solid product, which was determined through the difference in the catalyst weight before and after the reaction (equation 7), was found to decrease as a function of the reaction time. Indeed, for the t = 0 h experiments, the solid fraction yield was ≈ 16.7 -19.6wt.%, while the solid yield decreased to ≈ 9 -12 wt.% for longer residence time. It should be mentioned that no solid matter other than the spent catalyst was found in the reactor. Consequently, at least part of the solid product is represented by heavy asphaltene-like products that can be converted under the reaction conditions but are not soluble in heptane and stay with the catalyst after the reaction. Table S6 contains the carbon loading of the spent catalyst after the reaction, showing that higher reaction time and temperatures lead to lower carbon deposition on the catalyst's surface. Furthermore, at the temperature of 375 °C and 390 °C after 3 h, the carbon deposition on the spent catalyst was ≈ 5 wt.%, which is compatible with the typical coke loadings observed in the early stages of operating HDT catalysts [START_REF] Saturnino | Compréhension de la relation entre la structure physico-chimique et l'activité des catalyseurs d'HDS vieillis[END_REF] . Hence, the crude oil feedstock did not lead to enhanced deactivation by coke deposition of the sulfided catalyst, as observed for HTL crude oils from other sources, like sewage sludge [START_REF] Zhu | Impact of Coprocessing Biocrude with Petroleum Stream on Hydrotreating Catalyst Stability[END_REF] .

Gas product

The yield and composition of the gas products are shown in Figure 3-B. In general, the yield of gas increases with the reaction temperature and reaction time, achieving a maximum of 4.7 wt.% for the reaction conditions of 390 °C and 3 h. Still, the yield of gas products like CO and CO2, which result from decarboxylation and decarbonylation reactions, was found to stay constant, between 0.4-0.6 wt.% and 0.6 -0.8 wt.%, respectively, after 1 h of reaction independently of the temperature (Figure 3-B).

The low yield of CO and CO2 is attributed to the minor contribution of aliphatic C-O, methoxy, and acid groups in the crude oil (Table 2). Indeed, the oxygen in the crude oil was primarily eliminated as H2O, whose yield is between 7.5 wt.% and 10.0 wt.%. Light hydrocarbons, like CH4, C2H6 (C2), C3H8 (C3), and C4H10 (C4), were the main compounds responsible for the gas yield increase observed at higher temperature and reaction time (Figure 3-B). While the formation of light hydrocarbons can be explained by C-C bound cleavage caused by hydrogenolysis, which is favored at higher temperatures, heptane hydrogenolysis does not seem to contribute significantly to the gas products since the 𝐶 3 𝐶 4 ratio is significantly higher than 1 (Figure 3-B). Instead, light hydrocarbons most likely result from the cleavage of larger molecules from the crude oil during the hydrotreatment reaction. However, it is worth mentioning that, after 3 h of reaction, the O content in the liquid product is quite low for the catalytic tests performed at 375 °C and 390 °C (Figure 4), and the contribution of light hydrocarbons to the gas yield is significantly higher at 390°C. Hence, the occurrence of C-C hydrogenolysis and its contribution to the gas yield cannot be discarded entirely.

Liquid product

After upgrading, the liquid product yield varied between 65 wt.% and 77 wt.% depending on the reaction temperature and time (Figure 3-A). In all cases, the liquid yield was higher after 1 h of reaction as compared to treaction = 0 h, which agrees with the hypothesis previously stated on the conversion of solid products existing at the early stages of the reaction (section 3.2.1). After 3h of reaction, the liquid produced stabilized at 375 °C or slightly decreased at 350 °C and 390 °C due to the lower amount of solid converted in this time (Figure 3-A and Table S6) and the HDT reactions.

The elemental composition of the HDT liquid product is shown in Figure 4. When compared with the crude oil feedstock (Table 2), the HDT liquid products contained a higher concentration of carbon and hydrogen (Figure 4) even at treaction = 0 h. Similarly, the concentration of heteroatoms in the liquid product was also significantly smaller at treaction = 0 h than on the parent crude oil sample, with deoxygenation, denitrogenation, and desulfurization in the range of 60%, 30% and 50% respectively (Tables S7 to S9). It should be mentioned during heating, H2 pressure is reduced (10 bar, at room temperature) and that heating up to the reaction temperature takes 20 min. Thus, the significant elimination of heteroatoms during heating suggests these be easily removable under relatively mild conditions or by thermal degradation. Furthermore, HDO, HDS, and, to a lesser extent, HDN (Tables S7 to S9) significantly increased with the reaction time, reaching above 94 %, 96 %, and 78%, respectively, after 3 h of reaction. High denitrogenation levels requires a higher temperature 390 °C, HDN rate was significantly favored at higher temperatures. The efficient removal of heteroatoms suggests that the hetero-compounds in the crude oil feedstock are not in the form of refractory compounds, i.e., polyaromatic sulfur and nitrogen compounds [START_REF] Girgis | Reaction Networks, and Kinetics in High-Pressure Catalytic Hydroprocessing[END_REF] , as indicated by Liu et al. [START_REF] Liu | Characterization of Heteroatom-Containing Species in the Soluble Portion from the Ethanolysis of the Extraction Residue from Xinghe Lignite by Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry[END_REF] . It is relevant to mention that most HTL crude oils from bio-sourced feedstocks, like algae, food waste, and sewage sludge, require two stages of upgrading to remove heteroatoms effectively [START_REF] Fraga | Chapter 5 -Advances in Liquefaction for the Production of Hydrocarbon Biofuels[END_REF][START_REF] Cronin | Sustainable Aviation Fuel from Hydrothermal Liquefaction of Wet Wastes[END_REF][START_REF] Xu | Catalytic Hydrothermal Liquefaction of Algae and Upgrading of Biocrude: A Critical Review[END_REF] . Yet, nearly all lignite crude oil's sulfur, nitrogen, and oxygen were removed after a single hydrotreatment step on NiMoS/P-Al2O3 catalyst. Still, in the best-case scenario, 30 ppm of sulfur was found in the liquid, indicating further desulfurization via a second HDT stage, harsher conditions, or blending is necessary to meet the minimum requirements of a drop-in fuel from lignite crude oil. The evolution of the functional groups upon HDT, determined by [START_REF] Scanlon | Calculation of Flame Ionization Detector Relative Response Factors Using the Effective Carbon Number Concept[END_REF] C NMR, is shown in Figure 5. In comparison with the crude oil, the distribution of the functional groups of the liquid product clearly shows a significant increase in the aliphatic carbon, i.e., from 40.1%, in crude oil, to 54.8 -74.2% in the liquid, accompanied by a reduction of the aromatic carbon, i.e., 54.3% to 42.3 -24.0%. When comparing the HDT liquids composition obtained at treaction = 0 h, it can be observed that the concentration of aromatic C-H and C-C increases with temperature. In contrast, the occurrence of functional groups containing oxygen and aliphatic C follows the opposite trend (Figure 5). The increase of aromatic carbon in the liquid with the reaction temperature for treaction = 0 h suggests that thermal phenomena cause transformations during reactor heating. Indeed, the UV absorption at λ=290 nm (Figure 6-B), characteristic of polyaromatic compounds, which can be formed by unstable products in the crude oil during heating, clearly increases as a function of temperature for T = 0 h. As the reaction time increases, the concentration of aliphatic C increases together with a reduction of aromatic C as a consequence of hydrogenation, as confirmed by the H/C ratio increase from ≈1.5 at 0 h to >1.6 after 3 h for reaction (Figure 4). Additionally, the 𝐶 𝐴𝑙𝑖𝑝ℎ𝑎𝑡𝑖𝑐 𝐶 𝐴𝑟𝑜𝑚𝑎𝑡𝑖𝑐 ratio increases more with the reaction time for higher temperatures due to the faster kinetics of the HDT reactions promoted by the catalyst. While HDT significantly reduced the aromatic C-H bonds concentration, the concentration of aromatic C-C only decreased slightly at 375 °C and 390°C and even increased at 350 °C (Figure 5). Additionally, the 𝐶-𝐶 𝑎𝑟𝑜𝑚𝑎𝑡𝑖𝑐 𝐶 𝐴𝑟𝑜𝑚𝑎𝑡𝑖𝑐 . 6 (where 6 refers to the number of carbon atoms in one aromatic ring), which accounts for the average number of C-CAromatic bonds per aromatic ring, increased from 1.5 at 0 h to 2.0 after 3 h of reaction. It should be mentioned that aromatic C-C bounds can result from polyaromatic hydrocarbons, partially hydrogenated polycyclic-aromatics, like tetraline, and alkylated aromatics, such as toluene. Aromatic ring hydrogenation in polyaromatic compounds decreases for each ring already hydrogenated in the molecule [START_REF] Girgis | Reaction Networks, and Kinetics in High-Pressure Catalytic Hydroprocessing[END_REF][START_REF] Cooper | Aromatic Saturation of Distillates: An Overview[END_REF][START_REF] Stanislaus | Aromatic Hydrogenation Catalysis: A Review[END_REF] . Indeed, our previous work screening the upgrading of various HTL oils has already demonstrated that lignite crude oils were particularly rich in aromatic compounds, which results in a significant concentration of naphthenes and cyclic monoaromatic compounds after HDT [START_REF] Batalha | Lignite and Biomass Waste Hydrothermal Liquefaction Crude Upgrading by Hydrotreatment[END_REF] . The molecular weight (MwHC) distribution of the HDT liquid products is shown in Figure 6A. At low reaction time, the MwHC distribution presented a maximum centered at ≈200 g/mol and a maximum molecular weight of 1 000 g/mol. As the reaction time increased, the compounds centered at 200 g/mol progressively decreased as a new group of compounds centered at ≈120 g/mol appeared.

Similarly, the higher molecular weight compounds progressively disappeared, and at 390 °C after 3 h of reaction, no compounds above 800 g/mol were detected. Indeed, the degree of MwHC reduction observed in the liquid products after HDT highly depended on the reaction time and temperature, with the average MwHC (Table S10) ranging from 243 g/mol to 160 g/mol. The average molecular weight of the liquid products is reported in Table S10.

The removal of the heteroatom elements, i.e., O, N, and S, is considered to be the main responsible for the significant MwHC reduction since the cleavage of C-C bonds via hydrogenolysis is not predominant under the reaction conditions used in this study.

Figure 6B shows the absorbance at λ=290 nm, characteristic of conjugated polyaromatic compounds, as a function MwHC. The absorbance decreases as a function of the reaction time due to the full or partial hydrogenation of the polyaromatic compounds. Still, Figure 6B clearly shows that polyaromatics undergo faster conversion in the first hour of a reaction than in the subsequent 2 h, a phenomenon explained by the more straightforward conversion of certain compounds in the oil. Also, at all temperatures, the UV absorption for MwHC > 300 g/mol decreases fast during HDT, with nealy all polyaromatics in this range being converted at T = 390 °C after 3 h of reaction. However, the refraction index detector (RID -Figure 6A) clearly shows that the amount of compounds with a MwHC > 300 g/mol are only reduced slightly during HDT. The difference between the two detectors suggests that higher MwHC polyaromatic compounds undergo hydrogenation faster. Indeed, Korre et al. studied the hydrogenation kinetic of multiple polyaromatic compounds identifying that the higher the number of conjugated rings, the faster the hydrogenation reaction rate [START_REF] Korre | Polynuclear Aromatic Hydrocarbons Hydrogenation. 1. Experimental Reaction Pathways and Kinetics[END_REF] .

The composition of the liquid product, quantified by comprehensive GC×GC/FID (Figure 7), shows a progressive increase in the concentration of aliphatic and mono-aromatic hydrocarbon under harsher reaction conditions, as already suggested by [START_REF] Scanlon | Calculation of Flame Ionization Detector Relative Response Factors Using the Effective Carbon Number Concept[END_REF] C NMR (Figure 5), SEC-DAD (Figure 6B), and elemental analysis (Figure 4) results. At the same time, the oxygenated compounds, such as phenols, ketones, furans, and other high molecular weight compounds, are progressively transformed. Furthermore, the concentration of compounds in the comprehensive GC×GC-FID analysis detection range, i.e., up to ≈C40, progressively increased with HDT reaction time and temperature, which is a consequence of the reduction of the Mw of the samples (Figure 6A). At 390 °C and 3 h of reaction, 88.4 wt.% of the HDT liquid product composition was quantified, with aliphatic and monoaromatic hydrocarbon, mainly composed of naphthenes and partially hydrogenated aromatics, respectively, contributing to ≈75% of the sample. Additionally, 9 wt.% of the liquid was composed of polyaromatic hydrocarbons with two or more aromatic rings. The high concentration of cyclic hydrocarbons in the liquid is a direct consequence of the aromatic nature of the products in the crude oil and, by extension, in lignite.

Indeed, in our previous work, we demonstrated that the nature of the feedstock was crucial in determining the composition of the HDT liquid product and that lignite led to an HDT liquid rich in cyclic hydrocarbons [START_REF] Batalha | Lignite and Biomass Waste Hydrothermal Liquefaction Crude Upgrading by Hydrotreatment[END_REF] .

As mentioned before, lignite crude oil was already transformed during the 20 min heating stage of the reactor, as easily observable by the comprehensive GCxGC's significant compound detection boost, i.e., 16.7 wt.% to >56.7 wt.%. While the increase in aromatic content in the liquid showed that thermal phenomena caused at least part of the transformation, the rise in detectable aliphatic and monoaromatic hydrocarbon compounds suggests that catalytic HDT was also responsible for the transformation of the feedstock during the heating stage, indicating that lignite crude oil can undergo significant heteroatoms removal, even under relatively mild conditions. The distillation curves for the HDT liquid products are shown in Figure 8. As expected SEC results, the number of products in the fuel range increases with the reaction temperature and time. While at 0h, the fuel range products concentration is quite similar at all temperatures (17-20 wt.%), fuel yield quickly rises with the reaction time (Table S11). It is worth mentioning that oxygenated products, like phenols, are present in the fuel product in all cases, except for the experiments performed at 375°C and 390 °C after 3 h of reaction, which are exclusively composed of hydrocarbon molecules.

Additionally, diaromatic hydrocarbons are present in all cases and makeup ≈ 5 wt.% of the fuel range products obtained at 375°C and 390 °C after 3 h of reaction. Additionally, the maximum fuel yield obtained at 375°C and 390°C was 47.0 wt.% and 47.6 wt.%, respectively, showing that increasing the HDT temperature has little effect on the overall fuel yield after the elimination of most heteroatoms (Figure 4). Still, a slight variation in the composition of the fuel range products was observed (Tables S12 andS13), with the test at 375°C yielding more products, i.e., 33.5 wt.% Vs 30..8 wt.%, in the middle distillates range (Bp between 250°C and 350°C) and the test at test at 390°C being more selective for kerosene (Bp between 175°C and 250 °C), i.e., 14.7 wt.% Vs. 11.9 wt.%. The increase in lower Bp products observed at 390 °C might be associated with C-C hydrogenolysis, favored by a higher temperature.

CONCLUSION

In several European countries, gross electricity is partly generated by the combustion of lignite. Here, we propose diverting one fraction of the lignite to produce kerosene and diesel through a HTL and single-stage HDT sequence. The overall mass balance of the process, shown in Figure 9, indicates that the HDT liquid yield obtained from lignite under the HTL and HDT conditions used in this study is 8-9

wt.% and 47 wt.% of HDT liquid products have a boiling point in the range of standard fuels, i.e., Bp < 350°C (Figure 8). The unsatisfactory production of crude oil during HTL, i.e., 12.3%, is responsible for the low overall conversion of lignite into fuel. Indeed, the HDT liquid yield obtained in this study, i.e., 65-73 wt.%, was comparable to that in the literature when upgrading HTL biocrudes produced from different feedstocks [START_REF] Castello | Catalytic Upgrading of Hydrothermal Liquefaction Biocrudes: Different Challenges for Different Feedstocks[END_REF][START_REF] Sharma | Continuous Co-Processing of HTL Bio-Oil with Renewable Feed for Drop-in Biofuels Production for Sustainable Refinery Processes[END_REF][START_REF] Subramaniam | Extended Catalyst Lifetime Testing for HTL Biocrude Hydrotreating to Produce Fuel Blendstocks from Wet Wastes[END_REF][START_REF] Da Costa Magalhães | Catalytic Hydroconversion of HTL Micro-Algal Bio-Oil into Biofuel over NiWS/Al2O3[END_REF][START_REF] Magalhães | Catalytic Hydrotreatment of Algal HTL Bio-Oil over Phosphide, Nitride, and Sulfide Catalysts[END_REF] . The low crude oil yield can be explained by the experimental conditions used in the HTL process, as Liu et al. were able to increase the crude oil production to 34 wt.% by using a lower HTL reaction time (5 min) and higher temperature (440 °C). Additionally, the use of catalysts [START_REF] Nagappan | Catalytic Hydrothermal Liquefaction of Biomass into Bio-Oils and Other Value-Added Products -A Review[END_REF] and recycling the aqueous phase [START_REF] Watson | Valorization of Hydrothermal Liquefaction Aqueous Phase: Pathways towards Commercial Viability[END_REF] are known to increase crude yield during HTL. Therefore, adequate process optimization can orient a more significant fraction of lignite towards fuel generation. The values in brackets refer to the calorific value of the respective streams and were determined by using the expression from [START_REF] Channiwala | A Unified Correlation for Estimating HHV of Solid, Liquid and Gaseous Fuels[END_REF] . *values determined using the data from HDT performed during 3 h at temperatures of 375°C and 390°C (Figures 3-A and4).

The hydrotreatment experiments showed lignite's crude oil to be quite easily hydrotreated. Near complete heteroatom removal was attained with a single HDT stage and using a milder pseudo-WHSV (equation 5) than those reported in the literature for pilot-scale bio-based crude oil HDT [START_REF] Subramaniam | Extended Catalyst Lifetime Testing for HTL Biocrude Hydrotreating to Produce Fuel Blendstocks from Wet Wastes[END_REF][START_REF] Verdier | Pilot-Scale Hydrotreating of Catalytic Fast Pyrolysis Biocrudes: Process Performance and Product Analysis[END_REF] . After HDT, despite a significant improvement in fuel quality, i.e., heteroatom removal and hydrogenation, substantial amounts of aromatic compounds, particularly monoaromatic and diaromatic compounds (≈45% of fuel range products), are still present in all products. This suggests that further upgrading or harsher conditions, i.e., higher contact time and H2 pressure, are required. Indeed, aromatics have low cetane numbers, which can be detrimental for using the middle distillate in diesel fuels [START_REF] Song | New Design Approaches to Ultra-Clean Diesel Fuels by Deep Desulfurization and Deep Dearomatization[END_REF] . In addition to decreasing the aromatic content in the liquid product, our results showed that harshening HDT conditions did not significantly augment the fuel range products, suggesting that subsequent hydrocracking or using an HDT catalyst with acid properties might be necessary. Nevertheless, the crude oil obtained from the hydrothermal liquefaction of lignite has an excellent potential for producing hydrocarbon fuels, particularly when compared with other feedstocks typically used in the HTL process, like sewage sludge and algae [START_REF] Cronin | Sustainable Aviation Fuel from Hydrothermal Liquefaction of Wet Wastes[END_REF][START_REF] Hao | Chemical Reactions in the Hydrothermal Liquefaction of Biomass and in the Catalytic Hydrogenation Upgrading of Biocrude[END_REF] . Finally, it is important to mention that the presence of this aromatic fraction can be a benefit, acting as a polar solvent in the case of co-processing between HTL of lignite and less proper biogenic feedstocks, such as biomass or sewage sludge.

While lignite is not a green feedstock, its easily upgradable products obtained during HTL can be beneficial under mixed hydrothermal liquefaction with greener feedstocks, like lignocellulosic biomass, sewage sludge or algae, which yield crude oils more difficult to upgrade as proposed, for instance, by Wang et al. [START_REF] Wang | Hydrothermal Liquefaction of Lignite, Wheat Straw and Plastic Waste in Sub-Critical Water for Oil: Product Distribution[END_REF] . Indeed, it was shown that the hydrothermal liquefaction of a mixture of Jingou lignite, wheat straw, and plastic waste could provide a synergetic effect on oil yields. Synergetic effects were also observed in the co-liquefaction of coal and polymers (polypropylene and polystyrene) by Y. Shen et al. [START_REF] Shen | Co-Liquefaction of Coal and Polypropylene or Polystyrene in Hot Compressed Water at 360-430[END_REF] . Finally, it is worth mentioning that the char from lignite HTL can be further used for electricity production, the original purpose of lignite.

Figure 1 .

 1 Figure 1. GCxGC-TOF MS chromatogram of the crude oil obtained from the HTL of lignite. BHT (butylated hydroxytoluene) originates from THF solvent, where it serves as a stabilizer.

Figure 2 .

 2 Figure 2. A) Compounds in the liquid product molecular weight (MwHC) distribution determined by refraction index (RI) (blue curve) and UV absorbance ( = 240 nm and 290 nm) as a function of the compounds Mw. Average Mw = 297 g/mol. B) DAD spectrum (( = 190 -400 nm) of crude oil as a function of molecular weight (MwHC).

Figure 3 .

 3 Figure 3. A) Product yields, determined according to equations 6 to 10, obtained from the HDT of the lignite HTL crude oil. B) Gas products yield as a function of reaction temperature and time, obtained from the HDT of the lignite HTL crude oil.

Figure 4 .

 4 Figure 4. Elemental analysis of the liquid product obtained from HDT of lignite crude oil at different reaction temperatures (350 °C, 375 °C, and 390 °C) and reaction times (treaction).

Figure 5 .

 5 Figure 5. 13 C functional group composition of the liquid product obtained from HDT of lignite crude oil at different reaction temperatures (350 °C, 375 °C, and 390 °C) and reaction times (treaction).

Figure 6 .

 6 Figure 6. A) Liquid product molecular weight (MwHC) distribution determined by refraction index (RI), and B) UV absorbance ( = 290 nm) as a function of the compounds Mw. A hydrocarbon mixture with Mw comprised between 86 g/mol and 1000 g/mol was used as a standard to determine MwHC.

Figure 7 .

 7 Figure 7. Composition of HDT liquid as a function of the reaction temperature (350 °C, 375 °C, and 390

Figure 8 .

 8 Figure 8. Distillation curve of the hydrotreatment liquid product as a function of reaction time (treaction). Fuel range: Bp < 350°C.

Figure 9 .

 9 Figure 9. Block flow diagram of the transformation of lignite through hydrothermal liquefaction (HDT) and subsequent hydrotreatment of the HTL crude oil. Mass basis = 100 kg of dry ash-free lignite (kgdaf).The values in brackets refer to the calorific value of the respective streams and were determined by using the expression from[START_REF] Channiwala | A Unified Correlation for Estimating HHV of Solid, Liquid and Gaseous Fuels[END_REF] . *values determined using the data from HDT performed during 3 h at temperatures of 375°C and 390°C (Figures3-A and 4).

Table 1 .

 1 Physicochemical properties of lignite feedstock.

					Moisture total (wt.%)	50.59
	Proximate Analysis	Organic matter (wt.% dry) a Volatiles (wt.% dry) b	72.48 41.99
					Ash (wt.% dry)	25.94
					C (wt.% dry)	50.53
					H (wt.% dry)	4.55
	Ultimate analysis		N (wt.% dry)	0.56
					O (wt.% dry)	15.99
					S total (wt.% dry)	3.78
			Calorific valueas received (kJ/kg)	8 130
			Calorific valuedry (kJ/mol)	18 960
			𝑚 𝑐𝑟𝑢𝑑𝑒 𝑚 𝑙𝑖𝑔𝑛𝑖𝑡𝑒	100	Equation 1
	𝐶ℎ𝑎𝑟 𝑦𝑖𝑒𝑙𝑑 (𝑤𝑡. %) =	𝑚 𝑐ℎ𝑎𝑟 𝑚 𝑙𝑖𝑔𝑛𝑖𝑡𝑒	100	Equation 2
	𝐺𝑎𝑠 𝑦𝑖𝑒𝑙𝑑 (𝑤𝑡. %) =	𝑖 ∑ 𝑀𝑤 𝑖 𝑦 𝑖 𝑃𝑉 𝑅𝑇𝑚 𝐿𝑖𝑔𝑛𝑖𝑡𝑒	100	Equation 3

a Weight loss of dry sample upon combustion under air atmosphere (T = 575 °C). b Weight loss of dry sample under inert atmosphere (T=950 °C) The yields of the products were calculated based on dry, ash-free (daf) initial lignite using the following formulas:

𝐶𝑟𝑢𝑑𝑒 𝑜𝑖𝑙 𝑦𝑖𝑒𝑙𝑑 (𝑤𝑡. %) =

Table 2 .

 2 Composition of crude oil obtained from the HTL of Bełchatów lignite

		Composition	Carbon chemical composition ( 13 C NMR) a
	C (wt.%)	77.7± 0.1	CAliphatic	53.6
	H (wt.)	8.9 ± 0.3	CMethoxy (-O-CH3)	0.1
	O (wt.%)	10.8 ± 1.0	CAliphatic-O (-O-(CH2)n)	0.8
	S (ppm)	2.4 ± 0.1	Caromatic	40.3
	N (ppm)	0.5 ± 0.1	CCarboxylic/Amide	0.8
	TAN (mgKOH/g)	29.2 ± 0.2	CAldehyde/Ketone	4.4
	HHV (MJ/kg)	31.8
	a Chemical shift range in Table S1 and NMR spectra in Figure S2

Acknowledgements

The authors gratefully acknowledge the financial support of the European Commission Research Programme of the Research Fund for Coal and Steel for the project HyCon (Grant agreement: 899471).

In the case of Polish authors, this scientific work is published as part of an international project cofinanced by the program of the Ministry of Science and Higher Education entitled "PMW" in the years 2020 -2023 agreement No. 5157/FBWiS/2020/2021/2


domain: chim.mate chim phys

text:
Khedidja Korichi

Bendouma Doumi

Allel Mokaddem

Adlane Sayede

Abdelkader Tadjer

Ferromagnetism, half-metallicity and spin-polarised electronic structures characterisation insights in Ca 1-x Ti x O

HAL is

Introduction

A modern electronics technology called spintronics has attracted considerable interest in data processing for magnetic memory devices applications, ultrahigh density information storage and longer-term design of new electronic components [START_REF] Boeck | Technology and materials issues in semiconductor-based magnetoelectronics[END_REF]. Spin (or spintronic) electronics [START_REF] Wolf | Spintronics: A spin-based electronics vision for the future[END_REF] exploits the magnetic spin of electron for the technology of electronics devices; it is originated from the discovery of giant magnetoresistance [START_REF] Baibich | Giant magnetoresistance of (001) Fe/(001) Cr magnetic superlattices[END_REF][START_REF] Binasch | Enhanced magnetoresistance in layered magnetic structures with antiferromagnetic interlayer exchange[END_REF]. Spintronics technology is focused on the simultaneous control and manipulation of the electron charge as well as its magnetic moment (spin) in order to realise substantial applications in memory devices and sensors to improve the performance of new logical and information storage devices [START_REF] Wolf | Spintronics: A spin-based electronics vision for the future[END_REF][START_REF] Žutić | Spintronics: Fundamentals and applications[END_REF]. The expected benefit of spintronics over conventional electronics would be non-volatility, increased data processing speed, increased transistor density and reduced power consumption [START_REF] Kaminska | Mn and other magnetic impurities in GaN and other III-V semiconductors-perspective for spintronic applications[END_REF].

A revolution has been marked by the integration of spintronics into spintronics devices based on magnetic semiconductors by the possibility of combining the storage and manipulation functions through both magnetic and semiconductor elements [START_REF] Wolf | Spintronics: A spin-based electronics vision for the future[END_REF][START_REF] Žutić | Spintronics: Fundamentals and applications[END_REF][START_REF] Ohno | Making nonmagnetic semiconductors ferromagnetic[END_REF]. The doped magnetic semiconductors called diluted magnetic semiconductors (DMS) based on II-VI semiconductors are considered as innovative candidates for the development of new spintronics devices. Many researchers have extensively studied these compounds because they exhibit a half-metallic feature [START_REF] Doumi | First-principles investigation of half-metallic ferromagnetism in V-doped BeS, BeSe, and BeTe[END_REF][START_REF] Mokaddem | Investigations of electronic structure and half-metallic ferromagnets in Cr-doped zincblende BeS semiconductor[END_REF] and owing to their ferromagnetic stability at temperatures above room temperature [START_REF] Sato | Material design of GaN-based ferromagnetic diluted magnetic semiconductors[END_REF][START_REF] Wu | Synthesis, characterization, and modeling of high quality ferromagnetic Cr-doped AlN thin films[END_REF]. The calcium chalcogenides CaX (X = S, Se, Te) belong to the II-VI type alkaline earth chalcogenides, which are important compounds because of their extensive applications ranging from catalysis to microelectronics as well as in the field of electro-and photo-luminescent devices [START_REF] Rao | The preparation and thermoluminescence of alkaline earth sulphide phosphors[END_REF][START_REF] Versluys | Photoluminescent and structural properties of CaS: Pb electron beam deposited thin films[END_REF][START_REF] Hakamata | Preparation of CaS: Cu, F thin-film electroluminescent devices with an emission including purple region[END_REF][START_REF] Kravtsova | Electronic structure of M S (M = Ca, Mg, Fe, Mn): X-ray absorption analysis[END_REF][START_REF] Jeanloz | Equations of state of FeO and CaO[END_REF][START_REF] Zimmer | High-pressure phase transitions in CaTe and SrTe[END_REF][START_REF] Richet | Static compression and equation of state of CaO to 1.35 Mbar[END_REF][START_REF] Luo | Structural phase transformations and the equations of state of calcium chalcogenides at high pressure[END_REF][START_REF] Yamanaka | B1-B2 transition in CaO and possibility of CaSiO3-perovskite decomposition under high pressure[END_REF][START_REF] Speziale | High-pressure elasticity of calcium oxide: A comparison between Brillouin spectroscopy and radial X-ray diffraction[END_REF][START_REF] Murtaza | Shift of band gap from indirect to direct and optical response of CaO by doping S, Se, Te[END_REF]. The CaO calcium oxide wide-bandgap semiconductor is one of the families of calcium chalcogenides, which crystallised in the NaCl-type structure (B1) and undergoes a firstorder phase transition from NaCl (B1) phase to the CsCl (B2) structure under high pressure [START_REF] Jeanloz | Equations of state of FeO and CaO[END_REF][START_REF] Zimmer | High-pressure phase transitions in CaTe and SrTe[END_REF][START_REF] Richet | Static compression and equation of state of CaO to 1.35 Mbar[END_REF][START_REF] Luo | Structural phase transformations and the equations of state of calcium chalcogenides at high pressure[END_REF][START_REF] Yamanaka | B1-B2 transition in CaO and possibility of CaSiO3-perovskite decomposition under high pressure[END_REF][START_REF] Speziale | High-pressure elasticity of calcium oxide: A comparison between Brillouin spectroscopy and radial X-ray diffraction[END_REF]. The CaO is considered a potential DMS material for spintronics according to first-principle theoretical studies of Kenmochi et al. [START_REF] Kenmochi | New class of diluted ferromagnetic semiconductors based on CaO without transition metal elements[END_REF][START_REF] Kemmochi | New class of high-TC diluted ferromagnetic semiconductors based on CaO without transition metal elements[END_REF] and An Dinh et al. [START_REF] Dinh | Exchange interaction and TC in alkaline-earth-metal-oxide-based DMS without magnetic impurities: First principle pseudo-SIC and Monte Carlo calculation[END_REF], which investigated the origin of magnetism and room-temperature ferromagnetism in CaO induced by the substituting effect of non-magnetic carbon (C) and nitrogen (N) impurities. Recently, Jun et al., [START_REF] Jun | Research on new rare-earth half-metallic ferromagnets X0. 75Eu0. 25O (X = Ca, Sr and Ba) based on the first-principles calculations[END_REF] have predicted new rareearth half-metallic ferromagnets based on CaO doped with europium (Eu).

The objective of this study is to elucidate the structural, electronic and magnetic properties of CaO under the substituting effect of titanium (Ti) magnetic impurities such as the Ca 1-x Ti x O compounds at concentrations x = 0.125, 0.25, 0.5 and 0.75. The calculations of these properties have been made by the first-principle concepts of density functional theory (DFT) [START_REF] Hohenberg | Inhomogeneous electron gas[END_REF][START_REF] Kohn | Self-consistent equations including exchange and correlation effects[END_REF] within the full-potential linearised augmented plane-wave (FP-LAPW) method [START_REF] Singh | Planewaves, Pseudopotentials, and the LAPW Method[END_REF], the generalised gradient approximation of Wu-Cohen (GGA-WC) [START_REF] Wu | More accurate generalized gradient approximation for solids[END_REF] and the Tran-Blaha-modified Becke-Johnson (TB-mBJ) exchange potential [START_REF] Tran | Accurate band gaps of semiconductors and insulators with a semilocal exchange-correlation potential[END_REF][START_REF] Koller | Merits and limits of the modified Becke-Johnson exchange potential[END_REF] as implemented in WIEN2k package [START_REF] Blaha | WIEN2k, An Augmented Plane Wave + Local Orbitals Program for Calculating Crystal Properties[END_REF].

Computational method and details of calculations

We have performed the calculations of structural, electronic and magnetic properties of Ca 1-x Ti x O compounds at concentrations x = 0.125, 0.25, 0.5 and 0.75 by the use of WIEN2k code [START_REF] Blaha | WIEN2k, An Augmented Plane Wave + Local Orbitals Program for Calculating Crystal Properties[END_REF] within the first-principle computations of DFT [START_REF] Hohenberg | Inhomogeneous electron gas[END_REF][START_REF] Kohn | Self-consistent equations including exchange and correlation effects[END_REF] theory and FP-LAPW method [START_REF] Singh | Planewaves, Pseudopotentials, and the LAPW Method[END_REF]. The structural parameters are computed by Wu-Cohen gradient generalised gradient approximation (GGA-WC) [START_REF] Wu | More accurate generalized gradient approximation for solids[END_REF], whereas the appropriate Tran-Blaha-modified Becke-Johnson (TB-mBJ) exchange potential [START_REF] Tran | Accurate band gaps of semiconductors and insulators with a semilocal exchange-correlation potential[END_REF][START_REF] Koller | Merits and limits of the modified Becke-Johnson exchange potential[END_REF] is employed to characterise the magnetic properties, electronic structure and accurate band-gaps.

The wave functions are extended to plane waves in the interstitial zone with K max = 8.0/R MT , where the K max presents the largest K vector of plane wave and R MT defines the average radius of muffin-tin sphere. Inside the atomic sphere, we have chosen the value of l max = 10 for maximum partial waves. The Fourier charge density is extended up to G max = 14 (a.u.) -1 , where G max is the largest vector in the Fourier expansion. In the Brillouin-zone integration, we have employed the Monkhorst-Pack mesh [START_REF] Monkhorst | Special points for Brillouin-zone integrations[END_REF] of (4 × 4 × 4) k-points for CaO, Ca 0.75 Ti 0.25 O and Ca 0.25 Ti 0.75 O, (4 × 4 × 2) k-points for Ca 0.875 Ti 0.125 O and (4 × 4 × 3) k-points for Ca 0.5 Ti 0.5 O.

Results and discussions

Structural properties

The CaO calcium oxide belongs to the family of II-VI type semiconductors, which crystallises in the rocksalt NaCl (B1) structure with space group of Fm 3m No. 225, where the (Ca) calcium atom occupies the (0, 0, 0) position and the oxygen (O) at (0.5, 0.5, 0.5) site. The Ca 1-x Ti x O compounds at concentrations (x = 0.125) and (x = 0.25, 0.5 and 0.75) are generated respectively by using the supercells of 16 and 8 atoms. The Ca 7 TiO 8 (1 × 1 × 2) supercell of 16 atoms is created from Ca 8 O 8 structure by substituting one Ca cationic site with Ti impurity. We get the Ca 0.875 Ti 0.125 O tetragonal structure with concentration x = 0.125 and space group of P4/mmm No. 123. The Ca3TiO4 (1 × 1 × 1), Ca 2 Ti 2 O 4 (1 × 1 × 1) and CaTi 3 O 4 (1 × 1 × 1) supercells of 8 atoms are obtained by substituting Ca cationic sites respectively with one, two and three Ti impurities. We get the cubic structures of Ca 0.75 Ti 0.25 O for x = 0.25 and Ca 0.25 Ti 0.75 O for x = 0.75 with space group of Pm 3m No 221, and tetragonal structure of Ca 0.5 Ti 0.5 O with space group of P4/mmm No. 123.

The optimised structural parameters are determined by fitting the variations of energies as a function of equilibrium volumes using the empirical Murnaghan equation of state [START_REF] Murnaghan | The compressibility of media under extreme pressures[END_REF]. The computed structural properties such as the lattice constants (a), bulk modules (B) and their pressure derivatives (B') of CaO and Ca 1-x Ti x O at concentrations x = 0.125, 0.25, 0.5 and 0.75 with other theoretical [START_REF] Tran | Performance on molecules, surfaces, and solids of the Wu-Cohen GGA exchange-correlation energy functional[END_REF][START_REF] Cinthia | Structural, electronic and mechanical properties of alkaline earth metal oxides MO (M = Be, Mg, Ca, Sr, Ba)[END_REF][START_REF] Salam | Theoretical study of CaO, CaS and CaSe via first-principles calculations[END_REF][START_REF] Santana | Cohesive energy and structural parameters of binary oxides of groups IIA and IIIB from diffusion quantum Monte Carlo[END_REF] and experimental [START_REF] Mammone | Equations of state of CaO under static pressure conditions[END_REF] results are given in Table 1. The lattice constant and the bulk modulus of CaO are close to theoretical values [START_REF] Tran | Performance on molecules, surfaces, and solids of the Wu-Cohen GGA exchange-correlation energy functional[END_REF] of GGA-WC [START_REF] Wu | More accurate generalized gradient approximation for solids[END_REF] and appeared to be consistent with the experimental ones [START_REF] Mammone | Equations of state of CaO under static pressure conditions[END_REF]. Also, these parameters are improved than that of the results of Refs. [START_REF] Tran | Performance on molecules, surfaces, and solids of the Wu-Cohen GGA exchange-correlation energy functional[END_REF][START_REF] Cinthia | Structural, electronic and mechanical properties of alkaline earth metal oxides MO (M = Be, Mg, Ca, Sr, Ba)[END_REF][START_REF] Salam | Theoretical study of CaO, CaS and CaSe via first-principles calculations[END_REF] and Ref. [START_REF] Santana | Cohesive energy and structural parameters of binary oxides of groups IIA and IIIB from diffusion quantum Monte Carlo[END_REF] calculated respectively by the generalised gradient approximation of Perdew-Burke-Ernzerhof (GGA-PBE) [START_REF] Perdew | Generalized gradient approximation made simple[END_REF] and the local density approximation (LDA) [START_REF] Perdew | Self-interaction correction to density-functional approximations for many-electron systems[END_REF]. The better performance of GGA-WC approximation for optimising structural parameters is due to its fourth-order gradient expansion of exchange and correlation functional [START_REF] Wu | More accurate generalized gradient approximation for solids[END_REF][START_REF] Doumi | First-principle investigation of half-metallic ferromagnetism in octahedrally bonded Cr-doped rock-salt SrS, SrSe, and SrTe[END_REF][START_REF] Sajjad | The half-metallic ferromagnetism character in Be1-xVxY (Y = Se and Te) alloys: An abinitio study[END_REF][START_REF] Bourega | Electronic structures and magnetic performance related to spintronics of Sr 0[END_REF]. In addition, Table 1 shows that the lattice parameter decreases with increasing concentration (x) of substituted Ti impurity owing to smaller size of the Ti ionic radius with respect to the Ca. Consequently, the Ca 1-x Ti x O doping compound becomes harder due to the increase in bulk modulus with increasing Ti concentration.

Spin-polarised electronic structures and half-metallic properties

The results of the lattice parameters found by GGA-WC [START_REF] Wu | More accurate generalized gradient approximation for solids[END_REF] were used to determine the spin-polarised electronic structures such as the densities of states, the band structures as well as the band-gaps of the compounds under study. Figures 12345show the plots of calculated band structures with TB-mBJ [START_REF] Tran | Accurate band gaps of semiconductors and insulators with a semilocal exchange-correlation potential[END_REF][START_REF] Koller | Merits and limits of the modified Becke-Johnson exchange potential[END_REF], respectively for CaO, Ca 0.875 Ti 0.125 O, Ca 0.75 Ti 0.25 O, Ca 0.5 Ti 0.5 O and Ca 0.25 Ti 0.75 O. It is clear that Figure 1 exhibits the identical band structure with bandgap for two spins channels, implying that CaO is a semiconductor with indirect band-gap positioned between valence bands maximum and the conduction bands minimum respectively at Γ and Χ high symmetry points. For high concentration x = 0.75, the Fermi level is crossed by overlapping bands, leading to metallic nature for Ca 0.25- Ti 0.75 O compound. For concentrations x = 0.125, 0.25 and 0.5, the Ca 0.875- Ti 0.125 O, Ca 0.75 Ti 0.25 O and Ca 0.5 Ti 0.5 O are semiconductors for spins down, while the spins up are metallic as 3d-Ti bands intersect Fermi level. Thus, these compounds have a half-metallic ferromagnetic character.

The half-metallic ferromagnetic materials are described by two important band-gaps; the first is called the half-metallic ferromagnetic gap localised [START_REF] Tran | Performance on molecules, surfaces, and solids of the Wu-Cohen GGA exchange-correlation energy functional[END_REF] 116 [START_REF] Tran | Performance on molecules, surfaces, and solids of the Wu-Cohen GGA exchange-correlation energy functional[END_REF] GGA-WC 4.841 [START_REF] Tran | Performance on molecules, surfaces, and solids of the Wu-Cohen GGA exchange-correlation energy functional[END_REF] 105 [START_REF] Tran | Performance on molecules, surfaces, and solids of the Wu-Cohen GGA exchange-correlation energy functional[END_REF] GGA-PBE 4.843 [START_REF] Cinthia | Structural, electronic and mechanical properties of alkaline earth metal oxides MO (M = Be, Mg, Ca, Sr, Ba)[END_REF] 107 [START_REF] Cinthia | Structural, electronic and mechanical properties of alkaline earth metal oxides MO (M = Be, Mg, Ca, Sr, Ba)[END_REF] 4.2 [START_REF] Cinthia | Structural, electronic and mechanical properties of alkaline earth metal oxides MO (M = Be, Mg, Ca, Sr, Ba)[END_REF] GGA-PBE 4.836 [START_REF] Salam | Theoretical study of CaO, CaS and CaSe via first-principles calculations[END_REF] 102.3 [START_REF] Salam | Theoretical study of CaO, CaS and CaSe via first-principles calculations[END_REF] 4.17 [START_REF] Salam | Theoretical study of CaO, CaS and CaSe via first-principles calculations[END_REF] GGA-PBE 4.734 [START_REF] Santana | Cohesive energy and structural parameters of binary oxides of groups IIA and IIIB from diffusion quantum Monte Carlo[END_REF] 119.54 [START_REF] Santana | Cohesive energy and structural parameters of binary oxides of groups IIA and IIIB from diffusion quantum Monte Carlo[END_REF] 4.23 [START_REF] Santana | Cohesive energy and structural parameters of binary oxides of groups IIA and IIIB from diffusion quantum Monte Carlo[END_REF] LDA 4.8105 [START_REF] Mammone | Equations of state of CaO under static pressure conditions[END_REF] 115 [START_REF] Mammone | Equations of state of CaO under static pressure conditions[END_REF] 4.1 [START_REF] Mammone | Equations of state of CaO under static pressure conditions[END_REF] Experimental around Fermi level between the valence bands maximum and the conduction bands minimum, and the second is known as the half-metallic gap, which is defined as the absolute value of the maximum energy of valence bands of majority (minority)-spin or the minimum between the minimum energy of conduction bands of majority (minority)-spin with respect to the Fermi level [START_REF] Yao | Half-metallic ferromagnetism of zinc-blende CrS and CrP: A first-principles pseudopotential study[END_REF][START_REF] Gao | Half-metallic ferromagnetism in zinc-blende CaC, SrC, and BaC from first principles[END_REF]. Table 2 summarises the computed values of half-metallic ferromagnetic gaps (G HMF ) and half-metallic gaps (G HM ) for Ca 1-x Ti x O at concentrations x = 0.125, 0.25 and 0.5, and indirect gap (E GX ) of CaO with other theoretical [START_REF] Santana | Cohesive energy and structural parameters of binary oxides of groups IIA and IIIB from diffusion quantum Monte Carlo[END_REF][START_REF] Tran | Importance of the kinetic energy density for band gap calculations in solids with density functional theory[END_REF] and experimental [START_REF] Whited | Exciton thermoreflectance of MgO and CaO[END_REF] values. Due to the accurate TB-mBJ potential for computing electronic structures, our calculated indirect gap of 5.38 eV for CaO is in good agreement with the value of 5.35 eV [START_REF] Tran | Importance of the kinetic energy density for band gap calculations in solids with density functional theory[END_REF] found by the same potential and it is better than that of the results of Refs. [START_REF] Santana | Cohesive energy and structural parameters of binary oxides of groups IIA and IIIB from diffusion quantum Monte Carlo[END_REF][START_REF] Tran | Importance of the kinetic energy density for band gap calculations in solids with density functional theory[END_REF] and Ref. [START_REF] Tran | Importance of the kinetic energy density for band gap calculations in solids with density functional theory[END_REF] using GGA-PBE [START_REF] Perdew | Generalized gradient approximation made simple[END_REF] and LDA [START_REF] Perdew | Self-interaction correction to density-functional approximations for many-electron systems[END_REF] approximations, GGA-PBE 3.67 [START_REF] Tran | Importance of the kinetic energy density for band gap calculations in solids with density functional theory[END_REF] GGA-PBE 3.49 [START_REF] Tran | Importance of the kinetic energy density for band gap calculations in solids with density functional theory[END_REF] LDA 5.35 [START_REF] Tran | Importance of the kinetic energy density for band gap calculations in solids with density functional theory[END_REF] TB-mBJ 7.00 [START_REF] Whited | Exciton thermoreflectance of MgO and CaO[END_REF] Experimental band [START_REF] Bourega | Electronic structures and magnetic performance related to spintronics of Sr 0[END_REF]. It is an important property that characterises the half-metallic materials for spintronics applications [START_REF] Yao | Half-metallic ferromagnetism of zinc-blende CrS and CrP: A first-principles pseudopotential study[END_REF][START_REF] Gao | Half-metallic ferromagnetism in zinc-blende CaC, SrC, and BaC from first principles[END_REF]. The Ca 0.875 Ti 0.125 O, Ca 0.75 Ti 0.25 O and Ca 0.5 Ti 0.5 O compounds have half-metallic (HM) gaps of 1.345, 1.134 and 0.094 eV, respectively. We have noticed that the HM gap decreases with increasing Ti concentration because conduction bands minimum shifted towards the Fermi level. Besides, the Ca 0.875 Ti 0.125 O at concentration x = 0.125 has higher HM gap of 1.345. Consequently, the Ca 1-x Ti x O material doped with a lower Ti concentration is a perfect half-metallic ferromagnetic with a spin polarisation of 100%, making it a suitable candidate for spintronics. Furthermore, the total and partial densities of states (DOS) of Ca 0.875 Ti 0.125- O, Ca 0.75 Ti 0.25 O, Ca 0.5 Ti 0.5 O and Ca 0.25 Ti 0.75 O are given by Figures 6789. The plots of DOS show that the Ca 0.875 Ti 0.125 O, Ca 0.75 Ti 0.25 O, Ca 0.5 Ti 0.5 O materials have a half-metallic ferromagnetic character described by the metallic nature of spins up and the semiconductor feature of spins down, where the metallic property of majority-spin states results from large p-d hybridisation between p states of oxygen (O) and 3d states of Ti atom. From Figure 9, we have depicted that the Ca 0.25 Ti 0.75 O at concentration x = 0.75 is metallic because the DOS of both majority and minority-spin states dominate the Fermi level. The partial DOS of Ca 0.875 Ti 0.125 O, Ca 0.75 Ti 0.25 O, Ca 0.5 Ti 0.5 O show that the 3d states of Ti impurity are separated into two levels such as the three low-lying t 2g (d xy , d xz , d yz ) partially filled states and two high-lying e g (d z 2 , d x 2 -y 2 ) unfilled states. This is due to crystal field produced by the surroundings oxygen (O) ions. We have noticed that the e g states occur above t 2g states, meaning that the Ti atom is sited in an octahedral crystal field environment [START_REF] Doumi | First-principle investigation of half-metallic ferromagnetism in octahedrally bonded Cr-doped rock-salt SrS, SrSe, and SrTe[END_REF][START_REF] Doumi | A novel theoretical design of electronic structure and half-metallic ferromagnetism in the 3d (V)-doped rock-salts SrS, SrSe, and SrTe for spintronics[END_REF].

The metallic nature and ferromagnetism in Ca 1-x Ti x O doping materials are originated from the itinerant electrons of localised 3d-Ti majority-spin states. For minority spin, the bonding states of valence band are originated principally from DOS of p-O states, while the empty 3d-Ti states are located at the bottom of conduction bands. Therefore, the density of states are absent around Fermi level and a band gap is formed between the maximum and minimum of valence and conduction bands, respectively. In addition, the spin polarisation (P) of a half-metallic material is measured from the contributions of total DOS of spins up N (E F ) and spins down N (E F ) at Fermi level (E F ); it is given by the following expression [START_REF] Soulen | Measuring the spin polarization of a metal with a superconducting point contact[END_REF]:

P = N (E F ) -N (E F ) N (E F ) + N (E F ) (1

Magnetic properties

Magnetic moments

In the diluted magnetic semiconductors, the ferromagnetic state stability is originated from the double exchange mechanism when the 3d states are partially filled [START_REF] Sato | Ab initio materials design and Curie temperature of GaN-based ferromagnetic semiconductors[END_REF][START_REF] Sato | Curie temperatures of III-V diluted magnetic semiconductors calculated from first-principles in mean field approximation[END_REF]. According to this rule, the 3d unfilled states of Ti transition metal in the Ca 1-x Ti x O, suggest the stability of ferromagnetic state arrangement related to the double exchange mechanism [START_REF] Akai | Ferromagnetism and its stability in the diluted magnetic semiconductor (In, Mn) As[END_REF]. The origin of ferromagnetism in Ca 1-x Ti x O materials is due to the itinerant electrons of 3d-Ti partially occupied levels. For the Ca 1-x Ti x O systems at concentrations x = 0.125, 0.25 and 0.5, the Ti (4s 2 3d 2 ) impurity occupies the Ca site and provides two electrons to host valence bands, resulting in Ti +2 (4s 0 3d 2 ) ion. Therefore, the 3d states keep two unpaired electrons that generate a total magnetic moment of 2 μ B per Ti ion, where the μ B is the Bohr magneton. However, the strong p-d exchange interactions arise between p-O and 3d-Ti states, which reduce the magnetic moment of Ti less than 2 μ B and induce less important partial magnetic moments for Ca and O atoms. The calculated total and partial magnetic moments of Ca 1-x Ti x O compounds at concentrations x = 0.125, 0.25, 0.5 and 0.75 are given in Table 3. The total magnetic moments equal to 2 μ B per Ti atom for concentrations x = 0.125, 0.25, 0.5, confirming that the Ca 0.875 Ti 0.125 O, Ca 0.75 Ti 0.25 O, Ca 0.5 Ti 0.5 O are half-metallic, but the Ca 0.75 Ti 0.25 O compound at higher concentration x = 0.75 is metallic ferromagnetic with total magnetic moment of 1.33 μ B per Ti atom. On the other hand, the ferromagnetic interactions occur between the positive magnetic moments of Ti, Ca and O for Ca 1-x Ti x O at concentrations x = 0.125, 0.5 and 0.75, and between positive Ca and Ti magnetic spins for Ca 1-x Ti x O at x = 0.25. In contrast, the anti-ferromagnetic interaction is formed between positive Ti magnetic spin and negative magnetic moment of O atom for Ca 1-x Ti x O at x = 0.25.

Exchange constants and exchange splittings

In this section, we have computed the exchange constants to investigate the exchange couplings between valence and conduction bands with the 3d-Ti magnetic atoms. The exchange constants are determined by the use of the mean-field theory with the following expressions [START_REF] Sanvito | First-principles study of the origin and nature of ferromagnetism in Ga 1-x Mn x As[END_REF][START_REF] Raebiger | Intrinsic hole localization mechanism in magnetic semiconductors[END_REF]:

N 0 a = DE c x〈s〉 (2)
N 0 b = DE v x〈s〉 (3)
where the DE c = E c -E c and DE v = E v -E v are the valence and conduction band-edge spin-splittings at Γ high symmetry point for Ca 1-x Ti x O at concentrations x = 0.125, 0.25 and 0.5, respectively. The 〈s〉 represents the half total magnetic moment per Ti atom [START_REF] Sanvito | First-principles study of the origin and nature of ferromagnetism in Ga 1-x Mn x As[END_REF] and the x correspond to the concentration of Ti (Table 4).

The N 0 α constant defines the exchange coupling between the 3d-Ti levels and the s-type conduction bands, whereas the exchange coupling between the 3d-Ti levels and the p-type valence bands is explained by the N 0 β parameter. For Ca 1 -x Ti x O compounds at concentrations x = 0.125, 0.25 and 0.5, the positive values of N 0 β reflect the p-d anti-ferromagnetic couplings between valence bands and 3d levels as shown in Table 4. The N 0 α parameter is positive for concentration x = 0.125 and it is negative for x = 0.25 and 0.5, indicating that the interaction between 3d-Ti levels and valence bands is ferromagnetic for Ca 0.875 Ti 0. peaks of t 2g and 3d states of spins up are situated at the same level of energy for the concentrations x = 0.25 and 0.5. In contrast, for the d-Ti states of spins down with respect to the e g states of spins up, the d-Ti states are more dominated than that of the e g states for the concentrations x = 0.25 and 0.5, but for the smaller concentration x = 0.125, the pick of the e g majority-spin states have a higher energy compared to the d-Ti minority-spin states.

Conclusion

The structural, spin-polarised electronic structures and magnetic properties of Ti-doped CaO semiconductor such as the Ca 1-x Ti x O compounds at various concentrations x = 0.125, 0.25, 0.5 and 0.75 were studied by using the WIEN2k code based on the GGA-WC approximation and the TB-mBJ potential.

The structural parameters and indirect band gap of CaO computed respectively by GGA-WC and TB-mBJ potentials show a good concordance with other recent theoretical calculations found by the same potentials. The lattice parameter of Ca 1-x Ti x O is reduced than that of the CaO, which decreases with increasing concentration of Ti due to the difference between the sizes of ionic radii of Ca and Ti atoms. The Ca 0.25 Ti 0.75 O at high concentration x = 0.75 has a metallic ferromagnetic nature. For other concentrations, the Ca 0.875 Ti 0.125 O, Ca 0.75 Ti 0.25 O, Ca 0.5 Ti 0.5 O are perfect half-metallic ferromagnets with a spin polarisation of 100% and a large half-metallic gap of 1.345 eV for x = 0.125. The ferromagnetism is explained by both crystal field and direct exchange splittings, where it is more appropriate by the contribution of crystal field splitting mechanism for Ca 1-x Ti x O at low concentration x = 0.125. Therefore, Ca 1 -x Ti x O material doped with lower concentration of titanium is considered as potential candidate for exploration in the future spintronics applications.

Figure 1 .

 1 Figure 1. Band structures of CaO. (a) Majority spins (up) and (b) Minority spins (dn).

Figure 2 .

 2 Figure 2. Band structures of Ca 0.875 Ti 0.125 O. (a) Majority spins (up) and (b) Minority spins (dn).

Figure 3 .

 3 Figure 3. Band structures of Ca 0.75 Ti 0.25 O. (a) Majority spins (up) and (b) Minority spins (dn).

Figure 4 .

 4 Figure 4. Band structures of Ca 0.5 Ti 0.5 O. (a) Majority spins (up) and (b) Minority spins (dn).

Figure 5 .

 5 Figure 5. Band structures of Ca 0.25 Ti 0.75 O. (a) Majority spins (up) and (b) Minority spins (dn).

Table 2 .

 2 Calculated indirect band gap (E GX ) of CaO, half-metallic ferromagnetic (HMF) (G HMF ) and half-metallic gap (G HM ) of minority-spin bands for Ca 1-x Ti x O at concentrations x = 0.125, 0

  ) the spin polarisation P = 1 of the Ca 1-x Ti x O at concentrations x = 0.125, 0.25 and 0.5 because the total DOS of minority spins N (E F ) equal zero. Therefore, the Ca 0.875 Ti 0.125 O, Ca 0.75 Ti 0.25 O, Ca 0.5 Ti 0.5 O compounds are half-metallic ferromagnets with spin polarisation of 100%.

Figure 6 .

 6 Figure 6. Spin-polarised total and partial densities of states (DOS) of Ca 0.875 Ti 0.125 O.

Figure 7 .

 7 Figure 7. Spin-polarised total and partial densities of states (DOS) of Ca 0.75 Ti 0.25 O.

Figure 8 .

 8 Figure 8. Spin-polarised total and partial densities of states (DOS) of Ca 0.5 Ti 0.5 O.

Figure 9 .

 9 Figure 9. Spin-polarised total and partial densities of states (DOS) of Ca 0.25 Ti 0.75 O.

  125 O and it is anti-ferromagnetic for Ca 0.75 Ti 0.25 O and Ca 0.5 Ti 0.5 O compounds. Besides, the typical attraction in the half-metallic ferromagnetic Ca 1-x Ti x O compounds is determined from the indirect D x (pd) = E v -E v exchange splitting that represents the difference between E v and E v energies of valence band maximums respectively for spins down and spins up. The indirect exchange splittings of Ca 0.875 Ti 0.125 O, Ca 0.75 Ti 0.25 O, Ca 0.5 Ti 0.5 O are -3.40, -3.91 and -3.93 eV, respectively, indicating that spin-polarised Ca 1-x Ti x O materials exhibit an important feature for the spins down, which have a effective nature of potential compared to spins up [57-59]. Furthermore, we have characterised the ferromagnetism in the Ca 0.875 Ti 0.125 O, Ca 0.75 Ti 0.25 O and Ca 0.5 Ti 0.5 O compounds by computing the magnitudes of the crystal field and direct exchange splitting. The crystal field energy DE CF = E eg -E t2g is calculated from the difference energies between of e g and t 2g levels. The direct exchange splitting D x (d) = d -d corresponds to the energy separation between the peaks of d-Ti (d ) and d-Ti (d ) respectively for spins down and spins up. From the partial DOS, we have depicted that the

Table 4 .

 4 Computed conduction band-edge DE c (eV) and valence band-edge DE v (eV) spinsplittings, sp-d exchange constants N 0 a and N 0 b, crystal field DE CF (eV) and direct exchange D x (d) (eV) splittings for Ca 1-x Ti x O at concentrations x = 0.125, 0.25 and 0.5.

  These behaviours are reflected by the energies of crystal field and direct exchange splittings (DE CF and D x (d)) of (4.46 and 4.41), (2.48 and 3.24) and (2.29 and 3.24 eV) respectively for the Ca 0.875- Ti 0.125 O, Ca 0.75 Ti 0.25 O and Ca 0.5 Ti 0.5 O compounds. Consequently, the crystal field splitting is more dominated than that of direct exchange splitting for Ca 0.875 Ti 0.125- O at low concentration x = 0.125, while for the Ca 0.75 Ti 0.25 O and Ca 0.5 Ti 0.5 O compounds the crystal field is confined in the direct exchange splitting. We understand that the ferromagnetism is more favourable by the crystal field splitting in the Ca 0.875 Ti 0.125 O compound at the lower concentration x = 0.125.

Table 1 .

 1 Calculated lattice constants (a), bulk modules (B) and their pressure derivatives (B') for CaO and Ca 1-x Ti x O compounds at concentrations x = 0.125, 0.25, 0.5 and 0.75.

	Material	a (Å)	B (GPa)	B'	Method
	This work				GGA-WC
	CaO	4.775	115.23	4.36
	Ca 0.875 Ti 0.125 O	4.706	119.81	4.27
	Ca 0.75 Ti 0.25 O	4.640	125.22	4.39
	Ca 0.5 Ti 0.5 O	4.478	132.56	3.90
	Ca 0.25 Ti 0.75 O	4.311	166.41	4.30
	Other calculations
	CaO
		4.777

Table 3 .

 3 Calculated total and partial magnetic moments at Ti, Ca and O sites and in the interstitial site of Ca 1-x Ti x O compounds at concentrations x = 0.125, 0.25, 0.5 and 0.75.

	Material	Total (μ B )	T i( μ B )	C a( μ B )	O( μ B )	Interstitial (μ B )
	Ca 0.875 Ti 0.125 O	2	1.568	0.016	0.041	0.375
	Ca 0.75 Ti 0.25 O	2	1.498	0.037	-0.072	0.537
	Ca 0.5 Ti 0.5 O	4	3.036	0.037	0.044	0.883
	Ca 0.25 Ti 0.75 O	3.988	2.274	0.017	0.15	1.547