Surachai Karnjanakoma,
Asep Bayua,
Pairuzha Xiaoketia,
Xiaogang Haoc,
Suwadee Kongparakuld,
Chanatip Samartd,
Abuliti Abudulaa and
Guoqing Guan*ab
aGraduate School of Science and Technology, Hirosaki University, 1-Bunkyocho, Hirosaki 036-8560, Japan
bNorth Japan Research Institute for Sustainable Energy (NJRISE), Hirosaki University, 2-1-3, Matsubara, Aomori 030-0813, Japan. E-mail: guan@hirosaki-u.ac.jp; Fax: +81-17-735-5411; Tel: +81-17-762-7756
cDepartment of Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China
dDepartment of Chemistry, Faculty of Science and Technology, Thammasat University, Pathumtani 12120, Thailand
First published on 13th May 2016
The selective production of aromatic hydrocarbons from bio-oil derived from the fast pyrolysis of sunflower stalks over Cu or Fe-modified mesoporous rod-like alumina catalysts was investigated. Uniform mesoporous rod-like alumina with different pore sizes were successfully synthesized using a hydrothermal method with the assistance of Pluronic P123 surfactant. A high relative total hydrocarbon amount of about 59% in the upgraded bio-oil was obtained when pure mesoporous Al2O3 with a uniform pore size of 5.81 nm was used. Mesoporous Al2O3 with a larger pore size resulted in more polycyclic aromatic hydrocarbons (PAHs) such as indenes and naphthalenes being generated. Cu or Fe loaded Al2O3 with a loading amount in the range of 1–2.5 wt% showed a high selectivity towards monocyclic aromatic hydrocarbons (MAHs) such as benzene, toluene and xylenes (BTXs) over 80%. By using 2.5 wt% Cu/Al2O3-0.01, the highest relative total hydrocarbon amount reached 89%, which consisted of about 84% aromatic hydrocarbons and 4.9% aliphatic hydrocarbons. Both catalysts showed good catalytic stability and regeneration properties. A catalytic system with high effectiveness and long-term stability was expected to be obtained to convert the oxygenated compounds in bio-oil to high value-added hydrocarbons.
To solve the reactant diffusion problem in the pores, mesoporous catalysts such as Al-MCM-41 and Al-MSU-S have been developed.9–11 Yu et al.12 found that the use of La-Al-MCM-41 can reduce the amount of oxygenated compounds in bio-oil by up to 43.1%. Park et al.13 upgraded bio-oil over mesoporous MFI zeolite with a pore size of about 5 nm, and found that this kind of catalyst had a high selectivity for BTXs production. Mesoporous alumina should be another suitable catalyst for deoxygenation reactions due to its high acidity and large surface area.14 However, only a few studies have investigated the effect of pore size of Al2O3 or metal modified Al2O3 for the selective catalytic upgrading of bio-oil to aromatic hydrocarbons. On the other hand, Al2O3 prepared using a conventional hydrothermal synthesis without template assistance always has a low surface area and broad pore size distribution, which will affect the selectivity and activity of the obtained Al2O3. In contrast, by using a surfactant-templating method, it is possible to achieve a high surface area and uniform pores.15
Pluronic P123 (PEO)20(PPO)70(PEO)20, a commercially available triblock copolymer is an interesting surfactant template for mesoporous alumina synthesis as it is inexpensive and biodegradable.16 The textural properties of alumina such as its surface area and morphology can be controlled by using it in a synthesis process. For aromatic hydrocarbon production from the upgrading of bio-oil, a pore size range of about 3–9 nm is generally required, which could be realized by adjusting the P123/Al molar ratio. On the other hand, it should be noted that severe coking usually occurs on pure Al2O3 due to excessive dehydrogenation reactions at the Lewis acid sites.17 Modification by loading a transition metal on Al2O3 or other porous materials can improve the acidity and generate new acid sites which are beneficial for the selective production of hydrocarbons from the oxygenated compounds in bio-oil and the resistance to coke deposition of the catalyst.18–21
In this study, mesoporous alumina with a controllable pore size distribution and high surface area was synthesized firstly using a P123-assisted hydrothermal method and then, copper (Cu) and iron (Fe) were separately doped on it with different loading amounts. The prepared catalysts were characterized using BET surface area measurements, X-ray diffraction (XRD), scanning electron microscopy coupled with an energy dispersive X-ray detector (SEM-EDX), H2-temperature programmed reduction (H2-TPR) and NH3-temperature programmed desorption (NH3-TPD) analysis. The Cu or Fe loaded mesoporous alumina with optimum loading amount was determined by using the prepared catalysts for the in situ deoxygenation of bio-oil derived from the fast pyrolysis of sunflower stalks. The reusability of the catalyst with the best performance was also investigated both with and without regeneration. A catalytic system with high effectiveness and long-term stability was expected to be obtained to convert the oxygenated compounds in bio-oil to high value-added hydrocarbons.
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Fig. 1 (A) N2 adsorption–desorption isotherms and (B) pore-size distributions of Al2O3, Al2O3-0.005, Al2O3-0.01, Al2O3-0.025 and Al2O3-0.05. |
Catalyst | Surface area (m2 g−1) | Pore volume (cm3 g−1) | Pore size (nm) |
---|---|---|---|
Al2O3 | 195 | 0.22 | 2.29 |
Al2O3-0.005 | 285 | 0.51 | 4.19 |
Al2O3-0.01 | 345 | 0.79 | 5.81 |
Al2O3-0.025 | 327 | 0.94 | 6.86 |
Al2O3-0.05 | 311 | 0.96 | 8.42 |
1 wt% Cu/Al2O3-0.01 | 332 | 0.78 | 5.80 |
2.5 wt% Cu/Al2O3-0.01 | 315 | 0.75 | 5.77 |
5 wt% Cu/Al2O3-0.01 | 302 | 0.71 | 5.72 |
10 wt% Cu/Al2O3-0.01 | 309 | 0.69 | 5.70 |
1 wt% Fe/Al2O3-0.01 | 339 | 0.78 | 5.80 |
2.5 wt% Fe/Al2O3-0.01 | 333 | 0.76 | 5.79 |
5 wt% Fe/Al2O3-0.01 | 326 | 0.75 | 5.76 |
10 wt% Fe/Al2O3-0.01 | 317 | 0.70 | 5.71 |
Fig. 2 shows the XRD patterns of pure Al2O3-0.01 without metal loading and of various metal-loaded Al2O3-0.01 catalysts. One can see that in the cases of low metal loading (1–5 wt%), no diffraction peaks corresponding to metal loaded Al2O3 can be observed compared with pure Al2O3-0.01 without metal loading, indicating that the metal particles with a small size have been well dispersed on the mesoporous Al2O3 without accumulation during the preparation process.26 In contrast, at high metal loading (e.g., 10 wt% of Cu or Fe), peaks corresponding to copper oxide or iron oxide phases can be clearly observed, indicating the accumulation and/or bad dispersion of large metal particles. These XRD analysis results are also in good agreement with the BET analysis.
Fig. 3 shows the SEM images of the various catalysts. As shown in Fig. 3A and B, the mesoporous Al2O3 prepared with P123 assistance has a rod-like morphology with a uniform larger particle size compared to Al2O3 without P123 assistance, which shows a broken-sheep-wool-like morphology.
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Fig. 3 SEM images of (A) Al2O3, (B) Al2O3-0.01, (C) 2.5 wt% Cu/Al2O3-0.01, (D) 10 wt% Cu/Al2O3-0.01, (E) 2.5 wt% Fe/Al2O3-0.01 and (F) 10 wt% Fe/Al2O3-0.01. |
During the mesoporous Al2O3 synthesis, numerous ammonium aluminum carbonate hydroxide (AACH) crystals can be formed gradually with the increase in pH via urea decomposition. As such, –OH groups of AACH crystals should be further adsorbed on the oxide groups of P123 surfactant micelles via hydrogen bonding, leading to the systematic formation of mesoporous rod-like particles. As shown in Fig. 3C–F, when 2.5 wt% Cu or Fe metal is loaded on Al2O3-0.01, no metal bulk can be observed and the surface of the rods remains smooth. However, in the case of high Cu or Fe loading (10 wt%), the surface becomes hairy, especially in the case of Cu loading.
Fig. 4 shows the H2-TPR profiles of the various catalysts. For 1–10 wt% Cu/Al2O3-0.01, shown in Fig. 4A, at the low temperature range, the main reduction peak of the CuO species corresponding to the conjunct reductions from Cu2+ to Cu+ and from Cu+ to Cu0 can be clearly observed. Here, the small reduction peak appearing at high temperature is ascribed to the reduction of the CuAl2O4 phase.27 Furthermore, it should be noted that the reduction peaks shift towards the lower temperature range when the Cu loading amount is increased. This could be a result of the interaction between the Al2O3 support and Cu species with different particle sizes. In general, at higher Cu loadings (e.g. 10 wt% Cu loading), larger particles would be formed, resulting in a weaker interaction due to the poorer dispersion of Cu species. As a result, a lower temperature is needed for the reduction by hydrogen.28 On the other hand, for 1–10 wt% Fe/Al2O3-0.01, as shown in Fig. 4B, the reduction peaks are present, especially for 5 and 10 wt% Fe loading, which reveals the isolated reduction step of Fe2O3 at low temperature and of FeAl2O4 at high temperatures as follows:27
Fe2O3 + H2 → Fe3O4 + H2O | (1) |
Fe3O4 + H2 → Fe0 + H2O | (2) |
FeAl2O4 + H2 → Al2O3 + Fex2+O | (3) |
Fex2+O + H2 → Fe0 + H2O | (4) |
Here, Fe/Al2O3 has the opposite trend in the H2-TPR profiles compared to Cu/Al2O3 with increasing metal loading. It is possible that they have different ionic radii, different interactions with Al2O3 and different microstructure on mesoporous Al2O3.
Fig. 5 shows the NH3-TPD profiles of various catalysts. Here, a single NH3-desorption peak in a wide temperature range of 150–500 °C is observed in the pure Al2O3 without metal loading. After Cu or Fe metal is loaded on Al2O3, the desorption peak area increases to a wider temperature range, indicating that the substitution of proton sites with the doping metal species occurs, resulting in the generation of new proton sites on the catalyst.29 Moreover, two NH3 desorption peaks corresponding to the weak and strong acid sites appear for the 2.5 to 10 wt% Cu or Fe loaded Al2O3, indicating the formation of new strong acid sites due to the metal loading. It should be noted that the amount of weak acid sites also decreases with the increase in strong acid sites when the metal loading amount is increased. Table 2 shows the acidities quantified from the peak area for these catalysts. One can see that the acidity is increased after metal loading. However, the acidity gradually decreases with increasing metal loading, which could result from the covering of some proton sites with larger metal particles.30 Here, the high acidity should be beneficial for deoxygenation and aromatization, leading to the reduction of oxygenated compounds in bio-oil with the increase in aromatic hydrocarbons; on the other hand, if the acidity is too high, coke is more easily formed due to the cracking of hydrocarbons on the catalyst, resulting in rapid deactivation during the deoxygenation process.
Catalyst | Acidity (mmol g−1, low temp.) | Acidity (mmol g−1, high temp.) | Total acidity (mmol g−1) |
---|---|---|---|
Al2O3-0.01 | 0.271 | — | 0.271 |
1 wt% Cu/Al2O3-0.01 | 0.602 | — | 0.602 |
2.5 wt% Cu/Al2O3-0.01 | 0.530 | 0.054 | 0.584 |
5 wt% Cu/Al2O3-0.01 | 0.357 | 0.074 | 0.431 |
10 wt% Cu/Al2O3-0.01 | 0.295 | 0.102 | 0.397 |
1 wt% Fe/Al2O3-0.01 | 0.630 | — | 0.630 |
2.5 wt% Fe/Al2O3-0.01 | 0.500 | 0.007 | 0.507 |
5 wt% Fe/Al2O3-0.01 | 0.415 | 0.010 | 0.425 |
10 wt% Fe/Al2O3-0.01 | 0.438 | 0.009 | 0.447 |
Fig. 6B shows the distribution of aromatic hydrocarbons in the upgraded bio-oils using various Al2O3-X. Here, the aromatic hydrocarbons are categorized as MAHs, including benzene, toluene, xylenes and ethylbenzene, and PAHs including indenes and naphthalenes. It is found that different pore sizes of the Al2O3 catalysts lead to different selectivity for aromatic hydrocarbon production. As shown in Fig. 6B, the highest BTXs selectivity of 59% is obtained using Al2O3 prepared without P123 assistance. With increasing pore size of the catalyst, the MAHs amount in the upgraded bio-oil decreases while the PAHs amount clearly increases. This indicates that the pore size of Al2O3 plays a significant role in the aromatic selectivity of the upgraded bio-oil. However, even though the Al2O3-0.01 catalyst has the highest catalytic activity compared with the others, PAHs as undesirable products are still high in the case when pure mesoporous Al2O3 is used. It has been reported that although mesoporous catalysts with large pores are beneficial for the diffusion of compounds with large molecular sizes, it also promotes PAH formation, more easily leading to coking on the catalyst via condensation and polymerization reactions.9
To minimize the amount of PAHs and promote MAH formation, Cu or Fe loaded Al2O3-0.01 with various loadings were tested for the deoxygenation of bio-oil. As shown in Fig. 7A, the relative total hydrocarbon amount increased from 59 to 71.3% using 1 wt% Cu/Al2O3-0.01 instead of Al2O3-0.01, and further increased to 89% using 2.5 wt% Cu/Al2O3-0.01. Here, oxygenated compounds such as phenols and ketones in the upgraded bio-oil are significantly reduced. This confirms that the loading of Cu on Al2O3 can greatly promote dehydroxylation, dehydration and decarbonylation reactions. Interestingly, the main hydrocarbons in the upgraded bio-oil using Cu/Al2O3-0.01 are aromatic hydrocarbons, indicating that this catalyst favors promoting the conversion of aliphatic to aromatic. It is possible that the existence of Cu on Al2O3 can increase the Lewis acid sites or electron pair acceptors and promote hydride ion release, which is beneficial for the transformation of olefins to carbenium ions through intermediate dienes, and substantially enhances aromatic formation.32 In addition, a possible route for aromatic hydrocarbon formation in the in situ catalytic upgrading of bio-oil derived from biomass pyrolysis is shown in Fig. 8 and can be described as follows: firstly, cellulose and hemicellulose are decomposed to anhydrosugars such as levoglucosan (LGA), 1,6-anhydro-β-D-glucofuranose (AGF) and levoglucosenone (LGO) via a pyrolysis process. These sugars then undergo dehydration and re-arrangement reactions to form furans. The furans are carried by the carrier gas to pass through the catalyst layer, where they undergo decarbonylation to form C2–C4 alkenes or alkynes in the mesopores. Finally, the formed olefins are converted to cyclic aliphatic hydrocarbons via combination and further transform to aromatic hydrocarbons via Diels–Alder reactions and cyclization aromatization.2
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Fig. 8 The possible reaction mechanism for aromatic hydrocarbon formation obtained from the in situ catalytic deoxygenation of bio-oil derived from the pyrolysis of biomass. |
The lignin in biomass is decomposed to phenols and phenol alkoxy species via depolymerization at first and then the aromatics can be formed by decarbonylation, dehydration and dehydroxylation. Meanwhile, char is generally formed via re-polymerization and secondary pyrolysis of biomass. However, when the loading amount of Cu is further increased from 5 to 10 wt%, the hydrocarbon amount decreases while oxygenated compounds increase to some extent. It is possible that the occurrence of metal sintering results in the reduction of acidity. Moreover, the decrease in surface area and pore size also influences the mass transfer. For Fe loaded Al2O3-0.01, as shown in Fig. 7C, the same trend as Cu/Al2O3-0.01 for deoxygenation is found. However, Fe loaded catalysts show a lower catalytic activity. As indicated by the NH3-TPR results, more proton sites and new strong acid sites are available in the Cu/Al2O3-0.01 catalysts than Fe/Al2O3-0.01 for aromatization, resulting in more aromatic hydrocarbon formation.
Fig. 7B and D show the mass balance of the products and gas yields derived from the in situ catalytic upgrading of bio-oil using 1–10 wt% Cu/Al2O3-0.01 and 1–10 wt% Fe/Al2O3-0.01. One can see that the yield of bio-oil clearly decreases while the gas yield increases compared to the case without catalyst. Here, deoxygenation reactions such as dehydration, decarboxylation, decarbonylation, oligomerization, cracking, aromatization and dehydrogenation during the catalytic upgrading process result in an increase of the yields of CO, CO2, H2O and coke. Considering the coke formation, when 1 wt% Cu or Fe loaded Al2O3-0.01 is used to replace Al2O3-0.01 to upgrade bio-oil, the yield of coke clearly decreases. This should be attributed to the promotion of hydrogen atom migration through C–H activation on the catalytically active sites.33,34 However, higher metal loading leads to an increase in coke deposition due to the catalytic decomposition of the gas phase and polycondensation of furans, phenols and aromatics on the metal species.
Fig. 9 shows the distribution of aromatic hydrocarbons in the upgraded bio-oil using 1–10 wt% Cu/Al2O3-0.01 and 1–10 wt% Fe/Al2O3-0.01. It can be seen that a high selectivity towards MAHs over 80% can be obtained in the presence of 1–2.5 wt% Cu or Fe/Al2O3-0.01. In contrast, when the loading amount is over 2.5 wt%, more PAHs are generated. This indicates that overloading of Cu or Fe on Al2O3 promotes secondary reactions, favoring the formation of aromatics with larger molecular sizes such as naphthalenes via further aromatization and polymerization. Moreover, overloading of Cu or Fe on Al2O3 may also favor alkylation with MAHs to form PAHs.
Fig. 10 shows the effect of the catalyst amount on the deoxygenation efficiency based on the relative total hydrocarbon amount in the upgraded bio-oil using 2.5 wt% Cu/Al2O3-0.01 and 2.5 wt% Fe/Al2O3-0.01. Here, the biomass amount in the reactor is fixed but the catalyst amount is adjusted to change the residence time of the reactants in the catalyst layer. One can see that the yield of bio-oil decreases when the catalyst/biomass weight ratio increases since the deoxygenation reactions could occur on more active sites, which results in the generation of more gas, water, coke and others. In this study, an optimum catalyst/biomass weight ratio of 8 is obtained using 2.5 wt% Cu/Al2O3-0.01 as well as 2.5 wt% Fe/Al2O3-0.01. However, when the weight ratio is increased over 8, a little reduction in the relative total hydrocarbon amount is observed. This is probably due to the promotion of secondary reactions, where hydrocarbon molecules are further cracked into gases, coke and others during the long residence time.
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Fig. 10 Effect of catalyst amount on the relative total hydrocarbon amount in the upgraded bio-oil obtained using (A) 2.5 wt% Cu/Al2O3-0.01 and (B) 2.5 wt% Fe/Al2O3-0.01. |
The effect of alkali metal or alkaline earth metal (AAEM) species such as potassium from the biomass deposited on the spent catalyst should also have a contribution.35 In the presence of the in situ produced H2O and AAEM, more H2 can be generated during biomass pyrolysis and the deoxygenation process via a redox cycle reaction (Fig. 12). Here, the in situ produced H2 may enhance the dehydrogenation pathway on the catalyst, resulting in the increase of alkanes as a source of MAHs. Also, when additional H2 is present during the deoxygenation process, reactive H˙ radicals can be generated, which can simultaneously promote oxygen removal via capping free radicals.18 In order to confirm the presence of AAEM species, element mapping of the spent catalysts was performed and the results are shown in Fig. 13. It can be clearly seen that K element exists on the surface of the catalyst. Here, K should be the main AAEM species because its content in the ash of sunflower stalks is found to be up to 72%. However, the relative total hydrocarbon amount gradually reduces after the first reuse cycle. This should be a result of the increased coke deposition on the catalyst with increasing test cycle (Fig. 10). Interestingly, one can see that 2.5 wt% Fe/Al2O3-0.01 without any regeneration has better long-term stability with a reduction in the relative total hydrocarbon amount of 19% after the fourth reuse cycle compared to 2.5 wt% Cu/Al2O3-0.01 (26%). This difference can be attributed to the coke amount formed on the catalyst.
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Fig. 13 SEM images, EDX mapping images and EDX spectra of (A–D) 2.5 wt% Cu/Al2O3-0.01 and (E–H) 2.5 wt% Fe/Al2O3-0.01 after reaction (4th reuse). |
Fig. 14 shows the TPO signals of the spent catalysts (after 4th reuse), which represent the thermal decomposition of deposited coke on the catalyst. One can see that Cu/Al2O3 and Fe/Al2O3 have different thermal decomposition ranges. In the case of Cu/Al2O3, two thermal decomposition ranges can be observed for coke burning.
In contrast, for Fe/Al2O3, only one low thermal decomposition range is observed. This indicates that the formed coke has different types and/or particle sizes. It has been reported that coke with a lower removal temperature range belongs to oxygenated coke while that with a higher removal temperature range is graphite-like coke.34 Furthermore, the coke amount on 2.5 wt% Cu/Al2O3-0.01 after the 4th reuse is 9.2%, which is higher than that on 2.5 wt% Fe/Al2O3-0.01 (7.6%). It is also possible that 2.5 wt% Cu/Al2O3-0.01 has a higher acidity than 2.5 wt% Fe/Al2O3-0.01. In addition, the new strong acid sites of 2.5 wt% Cu/Al2O3-0.01 might promote the generation of stable graphite-like coke on the catalyst. On the other hand, after both catalysts are regenerated, i.e., the fifth reuse cycle, no serious reduction in the relative total hydrocarbon amount was found when compared with the fresh catalysts, indicating that the performance of the spent catalyst can be perfectly recovered using this regeneration method.
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