High Quality Bio-Oil Obtained from Catalyzed Pyrolysis of Olive Mill Solid Wastes in a Bi-Functional Reactor

Olive Mill Solid Wastes (OMSW) released in nature without any treatment is a major environmental problem in the Mediterranean region. In this work, the catalyzed pyrolysis of OMSW has been investigated. A catalyst based on SBA-15 mesoporous silica doped with chromium ferrite nanoparticles was prepared by the double solvent technique (DS). The prepared catalyst was cha-racterized by scanning electron microscopy (SEM), Wide and Small Angle X-ray Scattering (WAXS, SAXS), Energy Dispersive X-ray (EDX) and FT-IR spectroscopies. Reverse spinel chromium ferrite nanoparticles were located inside the SBA-15 pores as confirmed by SEM images. The obtained catalyst was tested for pyrolysis reactions of OMSW. Several parameters were studied to optimize the conditions of the pyrolysis reaction in order to increase the bio-oil conversion yield. The GC-MS results demonstrated that the quality of the obtained bio-oil was improved by decreasing the quantity of phenolic and oxygenated components as well as the size of the obtained molecules. The produced bio-oil from pyrolysis of OMSW is identical to that obtained from the pyrolysis of commercial cellulose under the same conditions. A 37% conversion yield of bio-oil was obtained for the best conditions.


Introduction
Due to the rapid population growth, urbanization and industrial development, the demand for energy resources is constantly increasing [1]. The current resources used to meet this demand are mainly conventional energies like charcoal, oil and natural gas [2]. The expenditures for energy supply were 10% of the word gross domestic product in 2011 [3]. Between 1990 and 2008, the world population increased by 27% where the global energy usage increased by 39% [4]. According to the International Energy Agency (IEA), a remarkable increase of energy demand by 2.3% was observed in 2018. The IEA estimated that the energy demand would rise by 1.3% each year until 2040 [5]. Furthermore, the use of conventional energy is the main cause of atmospheric pollution with irreversible impacts on the environment such as global warming [1] [6] [7] [8] [9] [10]. Taking into consideration the increasing demand and environmental impacts, the use of alternative resources like renewable energy sources is essential to improve the security of energy supply [11] [12] [13]. In addition, increasing the use of renewable energy instead of fossil fuel, reduces the emission of greenhouse gases and therefore global warming [14] [15]. Indeed there are several energy resources with minimal impacts on the ecosystem such as wind, solar, water and biomass [16].
Biomass is the third largest energy resource after the primary coal and oil [17]. For developing countries, the use of biomass is of great importance because their economies are predominantly based on agriculture and the industrial sector is mainly food related. Various thermochemical and biochemical conversion processes such as carbonization, gasification, liquefaction, hydrogenation, pyrolysis, and anaerobic and methanol fermentation were used to convert biomass into energetic products [18] [19] [20] [21].
The production of olive oil is accompanied by a large amount of water (liquid) waste (OMWW) and solid waste (OMSW) [22]. The total annual world production of olive oil is 2.9 million tones, where 97% of this production is concentrated in the Mediterranean [23]. The amount of solid and liquid residues is a function of olive oil extraction technology [24] [25] [26]. However, for the Mediterranean countries, the removal of residues of olive mills (OMW) is considered a major environmental problem. The uncontrolled disposal of OMW in nature provokes powerful phytotoxic and antimicrobial effects because of its high chemical oxygen demand and the phenolic components present in the olive oil [27] [28] [29] [30] [31]. Several methods were performed to manage solid wastes. The impregnation of olive mill waste water (OMWW) on dry biomasses and (OMSW) is one of the strategies [32]. Researchers found that the mineral contents present in the OMWW accelerate the thermal degradation of dry biomasses. Pellets production used for domestic boilers have been investigated by Kraeim et al. [33] and Lajili et al. [34] [35]. Yet, an increase in particulates emission and residual ash content may limit the application of these pellets in domestic and industrial boilers. Other techniques are used for such purpose. Ag-glomeration, roasting and the use of solid wastes to ameliorate the fertility of soil are examples of these techniques [36] [37] [38]. Cellulose, hemicellulose and lignin are the main components of the olive mill solid wastes. The composition ratio of each component is affected by various parameters such as olive varieties and cultivation conditions [23]. Nevertheless, pyrolysis is an important technique for OMSW disposal. In this technique, biomass are heated to moderate temperatures, followed by a fast vapors quenching. This gives three main products, solid char, non-condensable gases, and a liquid phase which is known as bio-oil. The latter may represent up to 70 wt% of the raw biomass, which renders the process highly efficient [23] [39]. Many parameters affect the ratio and the quality of each product namely moisture removal, the temperature, the residence time in the reactor, the reaction atmosphere and the biomass compositions [40]- [45]. The bio-oil issued from pyrolysis has many important industrial applications. It can be used as combustible fuel, in the production of chemical products and as oil additive [43] [46] [47]. The use of a catalyst increases the conversion of lignocellulose into bio-oil and improves its quality. Such advance is obtained by increasing small size aromatic molecules formation, by cracking.
In addition, reducing oxygen content while the products are still in the vapor phase (prior to condensation) enhances their quality by internal catalytic pyrolysis depending on the placement of the catalyst bed with respect to the pyrolysis reactor. Consequently, the use of catalyst increases the energy content of bio-oil, decreases its density and increases its solubility when used as a fuel additive. The catalysts selected for such reactions should have an activity, selectivity and thermal stability to maximize the yield and improve the quality of the bio-oil [48].  [53]. Acid catalysts, especially the microporous materials, are confronted with a rapid deactivation by the deposition of coke due to the large molecules in the pyrolysis vapor. The molecules cannot penetrate into the micropores of the zeolite, but eventually deposit in the form of coke on the surfaces of the catalyst.
Then, the inhibition of the catalyst by coke accumulation leads to a low bio-oil yield during pyrolysis. Mesoporous acid catalysts, such as MCM-41, have the potential to break down large molecular oligomers into bio-oil but their low hydrothermal stability and high production cost limit their use for organic reactions [49].
In this work, a special pyrolysis reactor has been designated for two purposes. The first is to accelerate the pyrolysis of the OMSW. The second is to catalyze the cracking reactions before the condensation step where the pyrolysis products are in vapor phase to ameliorate the bio-oil quality. Different conditions have been studied such as final temperature, temperature ramp, and the use of catalyst (nature, mass and ratio). SBA-15 mesoporous silica modified by chromium ferrite nanoparticles have been used as catalyst. SBA-15 has a high thermal stability due to its thick silica framework and a large mesoporous diameter up to 30 nm. In this study, SBA-15 was produced by a special method allowing to increase its specific surface area, its total pores volume, the connectivity between the mesopores, the main pores diameter and the number of functional groups present at the surface of the particles (mainly hydroxyl groups) was used as support [54]. The double solvent technique, which is a special replication technique, has been used to introduce the chromium ferrite nanoparticles inside the pores of the SBA-15. The special geometric form of the nanoparticles present in the porosity of SBA-15 mesopores enables our material to be micro-reactors that ameliorate the biomass cracking into bio-oil, accelerate products de-oxygenation and improve the bio-oil quality.

Synthesis of SBA-15 in a Low-Acidic Medium
Mesoporous SBA-15 was prepared by the sol-gel method described by Tabaja et al. [54] In this method, 329.75 ml of distilled water were acidified with 8.395 ml HCl (37%) and then 17.9025 g of P123 were dissolved in the obtained acid aqueous solution. The thermostat connected to the reactor is set at 35˚C and the solution is maintained under mechanical stirring. After the complete dissociation of P123, a clear solution was obtained. Then, 42 ml of TEOS were added drop-wise to the solution with a burette. The stirring was stopped immediately after the addition of the TEOS and the temperature was maintained at 35˚C for 63 h in static conditions (open beaker to evaporate ethanol obtained from the TEOS hydrolysis). Then, the obtained solution was suspended with its synthesis liquor in a 500 ml autoclave and subjected to a hydrothermal treatment at 130˚C in an oven for 33 h. The solid phase was collected, filtered by Buchner and washed with distilled water several times (1.5 L). A white powder was obtained after drying in air for 24 h. Finally, the pores of the powder were liberated by calcination in a furnace at a temperature of 500˚C with a heating rate of 2˚C min −1 and maintained at this temperature for 6 h. Then the SBA-15 was recovered and stored away from moisture.

Preparation of Chromium Ferrite Nanoparticles Supported by SBA-15
The catalyst was prepared from calcined SBA-15 using the double solvent me- The solution was mixed well to homogenize and the n-hexane was removed then by sample decantation, and the obtained catalyst was left on a filter paper to dry for 24 h to evacuate organic solvent traces, before being stored uncalcined and named Fe(6)/Cr(6)-SBA_(U). The prepared catalyst was thermally activated in air without flux in a muffle oven, at 2˚C•min −1 up to 700˚C (for 5 h) and then removed from the oven to cool down at room temperature and named Fe(6)/Cr(6)-SBA_(C). The quenching was used to avoid the formation of big crystals, and to introduce gaps inside the statures of the crystalline lattice.

Catalytic Pyrolysis Reactions
The pyrolysis reactions were carried out in a special designed reactor represented in Figure 1. First, the OMSW were thermally treated at 80˚C for 48 h in an oven, and then the dried solid was ground into fine powder in a mortar. The thermal treatment of the OMSW is common for all pyrolysis reactions and it was used to remove moisture from the biomass. A desired mass of OMSW was introduced into the reactor and a desired mass of the catalyst was added into the catalytic bed inside the reactor. After installation of the catalytic bed, the reactor was closed and the pyrolysis conditions (temperature, temperature rate and residence time) were determined using digital programming system for temperature control. This is followed by degassing using nitrogen flow for 5 min with a gas tube related from the side of the thermocouple, and then the thermocouple is reinstalled. Consequently, the pyrolysis reactions were performed under nitrogen with and without catalyst at different temperatures (200˚C, 250˚C, 350˚C, 450˚C and 550˚C). Table 1 summarizes the conditions of the performed pyrolysis reactions.

Characterization of the Catalyst and the Analytical Techniques
Phase identification of SBA-15, Fe(6)/Cr (6) Elementary study was performed on SBA-15 and Fe(6)/Cr(6)-SBA (with and without calcination) to determine wt% of elements in each of these materials especially the percentage of introduced metals (Fe and Cr using AMETEK EDAX device to compare the results with the desired percentages. Qualitative analysis of bio-oils obtained from pyrolysis reactions was carried out using GCMS-QP2010 Plus. Approximately 200 mg of bio-oil were dissolved in 5 ml of acetone. Prior to injection, the diluted samples were first dried with 1 g of anhydrous sodium sulfate in a beaker and filtered through a 0.22 μm syringe filter. Chromatographic separation was performed using a HP-INNO Wax capillary column (30 m × 0.25 mm × 0.25 μm). Helium (99.9995%) was used as carrier gas at a flow rate of 1 ml•min −1 . The injector (7683B ALS) and the GC-MS interface were maintained at a constant temperature of 250˚C. A sample volume of 1 μl was injected in split mode with a ratio of split of 20:1. The temperature ramps of the CPG oven were as follows: start heating to 50˚C maintained for 2 min, then at 90˚C at a speed of 10˚C•min −1 , then at 120˚C at a speed of 4˚C•min −1 , and then brought to 230˚C at a speed of 8˚C•min −1 , and this final temperature was maintained for 10 min [55].

Solid Physico-Chemical Properties
The average weight percent of the elements Si, O, C, Cr and Fe in the SBA-15 before (U) and after (C) calcination and in the samples Fe(6)/Cr(6)-SBA_(U) and Fe(6)/Cr(6)-SBA_(C) before and after pyrolysis reaction are presented in Table 2. The percentage of carbon decreases after calcination due to the elimination of the organic surfactant at 500˚C for 6 h which influences the percentages of the elements Si and O. Chromium and iron were absent in the calcined and the uncalcined SBA-15 material. After impregnation, the percentages of Fe and Cr in the catalyst before calcination were very close to the desired percentage for those elements (~6 wt%). The average weight percentages of Fe and Cr increased up to ~8 wt% after calcination. The experimental measurements were performed in the same sample at different locations. This increase from the desired percentage of both metals, Fe and Cr, is related to further decrease in carbon contents from the SBA-15, after calcination at higher temperature (700˚C), from 2.8 to 2.1 wt%. The average percentages of Fe and Cr in the catalyst after a pyrolysis reaction decreases to the half. This decrease corresponds to an increase in the ratio of carbon and not to the metallic elements loss during the catalytic reaction. The carbon weight percentage increased from 2 wt% before the reaction to 19 wt% after reaction. The observed carbon was assigned to the pyrolysis products which were adsorbed on the surface of the catalyst during the reaction.
SEM micrographs displaying the grain morphology of SBA-15 and SBA-15 modified with chromium ferrite prepared under the same conditions (without metallization) are shown in Figure 2. The SBA-15 particles presented in Figure  2(A) had uniform smooth hexagonal morphology with very short length. The particles have a uniform average size of average diameter of 2 μm, and an average thickness of 0.5 μm. Figure 2(B) is a micrograph of the Fe(6)/Cr(6)-SBA_(C). Fe (6)  was confirmed in this case by the absence of the indexed diffraction peak (111) which is expected at low angles. Therefore, CrFe 2 O 4 is an inverse spinel structure in our preparation conditions. In this case, Fe 2+ cations occupy the tetrahedral sites whereas half of the trivalent Fe 3+ and Cr 3+ occupy octahedral and tetrahedral sites in equal ratio. The diffraction peaks at 24˚, 34˚, 36˚, 50˚, 55˚, 63˚ and 65˚ 2θ are attributed to the presence of hematite rhombohedra structure. The intensity inversion of the two peaks at 34˚ and 36˚ (in pure hematite the peak at 34 is more intense), is related to the presence of the inverse spinel structure that have a diffraction peak at 35˚ 2θ.
The optical properties of the white silica SBA-15 before and after calcination and the prepared catalyst Fe(6)/Cr (6)

The Results of Pyrolysis Reactions
To study the activity of the prepared catalyst in pyrolysis reactions, blank pyrolysis reactions on commercial cellulose were performed at 350˚C with a temperature rate of 10˚C min −1 . These reactions were carried out on 15 g of cellulose with different catalyst masses. The conditions and the results are summarized in Table 3. The results showed that the use of 100 mg of catalyst increased the conversion yield of bio-oil from 25% to 37%. This yield decreased to 23% as the mass of the catalyst increased to 1 g. The decrease of bio-oil conversion yield was accompanied with an increase in the conversion yield of the bio-gas which increased from 44% to 61% when the catalyst mass was multiplied by 10. According Pyrolysis reactions were performed to study the activity and the effect of the prepared catalyst on the yield of bio-oil conversation and its quality. Several conditions that can affect the yields of pyrolysis reaction were investigated. The effect of the mass of the OMSW, the weight ratio OMSW/catalyst and the temperature were studied.
Pyrolysis reactions, without catalysts, and with different OMSW masses, different temperatures and different temperature ramps were performed. The reaction conditions and the obtained yields of bio-char, bio-oil and bio-gas are summarized in Table 4. An increase of bio-oil conversion from 8% to 24% was observed, whereas the bio-char yield is reduced when the mass of OMSW increase from 5 g to 15 g. More bio-char and less bio-gas conversion yields were obtained compared to pure cellulose. This observation can be explained by the presence of lignin in OMSW that reduces bio-oil and bio-gas conversion and increase bio-char yield [62]. The increase of maximum temperature from 350 to Pyrolysis reactions were carried out at 350˚C with a temperature rate of 10˚C•min −1 with different OMSW/catalyst weight ratio. In such reactions, the catalyst mass was fixed at 300 mg where the mass of OMSW was changed. The results are summarized in Table 5. The experimental results showed an increase of Table 4. The effect of the OMSW mass, the temperature and the temperature rate on the yield of pyrolysis products in the absence of catalyst.  the bio-oil conversion yield from 24% to 33% and a decrease of bio-gas conversion yield from 47% to 40% when 300 mg of catalyst were used (OMSW/catalyst wt. ratio = 50 wt%). The most interesting result was the significant increase in biooil conversion yield with the increasing weight ratio OMSW/catalyst. The conversion yield of bio-oil increased from 27.5% to 33% with the increase of OMSW/ catalyst weight ratio from 27 to 50. This observation proves the effect of the catalyst in the secondary reactions (cracking recombination, de-oxygenation…), after depolimerization and fragmentation which exist in the two cases. Therefore, more depolemerization and fragmentation products lead to more bio-oil conversion. The same reactions were performed but with a constant mass of OMSW (15 g) but by changing the catalyst mass to change the OMSW/Catalyst weight ratio. The results are summarized in Table 6. The same observation was obtained. The bio-oil conversion ratio decreased with the decrease of OMSW/catalyst weight ratio (increase of the catalyst weight). An important conversion yield of bio-oil of 35% was obtained with a weight ratio OMSW/catalyst of 150 (100 mg of catalyst). This conversion yield is significant compared to 27% obtained with activated alumina by Sensoz et al. under the same conditions [63]. Otherwise, 37 wt% liquid yields were obtained Sensoz et al. at 500˚C. A decrease in bio-oil conversion is observed at smallest ratios accompanied by an increase in bio-gas conversion ratio. This decrease can be explained by increasing cracking rate, with the increase in the number of the active sites and proved by the significant increase of bio-gas conversion yield. The same results were observed by Encinar et al. but using dolomite activated as catalysts. An increase in catalyst mass decrease liquid conversion and increase gas yield [64]. Demiral et al. obtained 37 wt% and 31 wt% by using activated alumina and sodium fold spar respectively at 500˚C/10˚C•min −1 but with a ratio 40% catalyst/solid [65]. By comparing our best results obtained over our catalyst with 0.6% ratio of catalyst/solid, the Lewis and Bronsted acid sites created at the surface and in the mesopores of the SBA-15 show high activity in the conversion into liquid products either at lower temperature.
The same effect of the maximum temperature was obtained with and without catalyst. The conversion yield of bio-oil decreased from 35% at 350˚C to 24% at 450˚C. The results are summarized in Table 7. The results showed that the presence of the catalyst at 450˚C with a rate of 10˚ min −1 increased the conversion yield of bio-oil from 16% (Table 4) to 24%. Bio-oil conversion decreased significantly at the same temperature (450˚C) to 19% when the mass of the catalyst increased to 1 g. A significant increase to 57% of bio-gas conversion yield was obtained in these conditions. Pyrolysis reactions at lower temperatures (200˚C and 250˚C) were performed.
The results are summarized in Table 8. The low conversion yield into bio-oil and bio-gas observed at 200˚C is attributed to the effect that the decomposition (primary reactions) of the biomass was very low at this temperature. Valenzuela-Calahorro et al. find the same results where an increase in bio-char yield was observed at low temperatures [66]. The conversion yield of bio-oil increased with temperature to reach its maximum at 350˚C and then decreased to 24% at 450˚C where the flash point was reached.
Pyrolysis reactions at 350˚C and 450˚C but at two different temperature rates, 10 and 50˚C•min −1 were performed. The conditions and the results are summarized in Table 9. At 450˚C, the bio-oil conversion yield increased from 24% to 27% with the increase of temperature rate from 10˚C•min −1 to 50˚C•min −1 . This effect was confirmed by the decrease of the residence time at flash point. A reverse effect was observed at 350˚C, where a significant decrease from 35% to 30% was obtained.

Bio-Oil Compositions by GC-MS
GC-MS analysis was carried out for four samples to identify the components of the obtained bio-oil from different pyrolysis reactions. The reactions were carried out with a temperature rate of 10˚C•min −1 . The retention times obtained from the GC and the molecular structures obtained from Mass Spectroscopy are summarized in Tables 10-12 and Table 13. In all chromatograms the set of peaks appeared between 18 and 21 min were very close and difficult to identify, so it is suggested that they might be alcohols and ketones (Example 1.2-cyclopentanedione, 3-methyl-1,2-cyclopentanedione, 2-methoxyphenol, 2-methoxy-4-methyl phenol) [67].
The main peaks and the corresponding products of the bio-oil components obtained from pyrolysis of 15 g of OMSW at 350˚C in the presence of 200 mg catalyst are summarized in Table 11. It was noticed a total elimination of the phenolic products in the bio-oil in the presence of catalyst. More cyclic and less oxygenated products of small molecules were obtained. Alkane products appear      Table 12.
The phenolic products reappeared at low pyrolysis temperature despite the presence of 200 mg of catalyst. The results are summarized in Table 13. Other oxygenated products (ketones and alcohols) were also detected. The cracking, reforming, de-oxygenation and other secondary reactions that occur when the products are in gas phase needs higher temperature. Therefore, our catalyst is not active at low temperature and so the same products as without catalysts were obtained.

Conclusion and Perspectives
SBA-15 mesoporous silica was successfully synthesized by the sol-gel method with 63 h maturation step at 35˚C followed by 33 h hydrothermal treatment step at 130˚C. An inverse spinel variety of chromium ferrite nanocrystals were obtained perfectly inside the silica pores after replication by the double solvent technique. The obtained catalyst was successfully used in pyrolysis reactions of OMSW and presented an important activity where the bio-oil conversion was improved and a yield up from 24% to 37% was obtained. Mainly Bronsted acid sites located at the surface and in the mesopores of the silica were obtained with small concentration of isolated silanol groups. The surface acid sites are of medium and strong Bronsted acidity. Lewis acid sites were created by the insertion of metal oxydes (MO). The weight ratio of OMSW/catalyst, the maximum temperature and the temperature rate were optimized to obtain the highest bio-oil conversion yield. Under the optimal conditions, the bio-oil quality was improved and was demonstrated by the GC-MS results. By using the catalyst, the toxic phenolic components, present in the bio-oil obtained without catalyst, were totally eliminated and deoxygenated; decarboxylated and cyclic components with small molecular size were obtained. The results were close to those obtained under the same conditions in bio-oil derived from the pyrolysis reactions of commercial cellulose. Shorter residence time at high temperature improved the bio-oil conversion yield. Bio-oil yield was increased from 24% to 27% when the temperature rate was increased from 10˚C•min −1 to 50˚C•min −1 . In prospects, fast pyrolysis reactions (at very high temperature ramp) at a higher temperature will be performed. Another catalyst bed at the outside of the reactor will be added which may promote the de-oxygenation and the decarbonylation of the pyrolysis products and improves the quality of the resulting bio-oil.