Upgrading of Bio-Oil Aqueous Fraction by Dual-Stage Hydrotreating

Research Article pubs.acs.org/journal/ascecg

Upgrading of Bio-Oil Aqueous Fraction by Dual-Stage Hydrotreating−Cocracking with Methanol Qinjie Cai, Jia Xu, and Suping Zhang* Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China S Supporting Information *

ABSTRACT: Upgrading of the bio-oil aqueous fraction (BAF) for liquid hydrocarbon production was studied in this work. Hydrotreating pretreatment and methanol cocracking were combined as a dual-stage hydrotreating−cocracking process to overcome the severe coking problem caused by the hydrogen-lacking property of BAF. The influences of hydrotreating temperature, reaction pressure, and BAF/ methanol ratio in feedstock were investigated. It was found that the hydrogenation efficiencies of phenols in BAF were elevated with increases in hydrotreating temperature and reaction pressure, which were important for maintaining cracking catalyst activity. However, a too high hydrotreating temperature significantly enhanced the gasification reaction and led to reduction in the final liquid hydrocarbon yield. Meanwhile, an excessively high BAF/methanol ratio obviously accelerated the deactivation of the cracking catalyst. The comparison of different cracking processes showed that dual-stage hydrotreating−cocracking was the most superior in stable liquid hydrocarbon generation. Finally, the reaction mechanism was proposed based on experimental results. KEYWORDS: Bio-oil aqueous fraction, Upgrading, Liquid hydrocarbon, Hydrotreating−cocracking, Methanol, Hydrogen supply

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INTRODUCTION Bio-oil production from biomass fast pyrolysis is considered to be a promising way to generate renewable liquid fuel. However, some inferior properties, such as high oxygen content, strong acidity, high viscosity, and low heating value, limit the application of bio-oil.1 Therefore, upgrading is necessary to realize its high-grade utilization.2 The common bio-oil upgrading technologies for high-grade liquid fuel production include catalytic hydrotreatment3−5 and catalytic cracking.6,7 Catalytic cracking over zeolite catalysts can partially achieve the removal of oxygen and the generation of liquid hydrocarbons. However, some problems like low yield of desired products and serious coking should be solved.8,9 The coke formation is found to be strongly related to the hydrogenlacking properties of bio-oil components.10 These components with high unsaturation degrees tend to undergo condensation reactions to form carbonaceous deposits. In addition, some large-molecular-weight compounds in bio-oil, such as phenolic oligomers (pyrolytic lignin) and sugars, have high boiling points and therefore easily deposit in the catalytic bed.11 To overcome the coking problem, removing large-molecularweight compounds, and improving the hydrogen-lacking property by proper hydrogen supply are required. Vispute et al. proposed an integrated process combining hydrotreating and cracking to upgrade bio-oil aqueous fraction (BAF).8 Due to the removal of some lignin-derived phenols and their © 2017 American Chemical Society

oligomers, BAF exhibited better cracking performance than crude bio-oil. To improve its hydrogen-lacking property, they conducted a hydrotreating pretreatment to convert some unsaturated compounds like ketone and aldehydes into alcohols, which successfully reduced the coke yield in the following cracking process. Nevertheless, to raise hydrogenation efficiency, a relatively high reaction pressure of 10 MPa was used in that work; meanwhile, it was difficult to saturate all unsaturated compounds in BAF, and consequently, there was still a certain amount of coke in the cracking stage. In a similar bio-oil hydrotreating−cracking study using a high reaction pressure of 32.5 MPa and a high hydrotreating temperature of 500 °C, Samolada et al. still obtained a coke selectivity of greater than 15%.12 Besides hydrotreating pretreatment, coprocessing with hydrogen-rich chemicals is considered another way to improve the cracking performance of bio-oil components.13,14 The common cocracking chemicals include aliphatic hydrocarbons and alcohols. Studies showed that there was hydrogen transfer from the hydrogen-rich chemicals to bio-oil components, which could promote the latter ones’ conversion and reduce coke formation.15,16 Moreover, because some alcohols like methanol can be generated from biomass,17 the cocracking of bio-oil and Received: May 14, 2017 Published: June 14, 2017 6329

DOI: 10.1021/acssuschemeng.7b01505 ACS Sustainable Chem. Eng. 2017, 5, 6329−6342

Research Article

ACS Sustainable Chemistry & Engineering

acid, and formic acid are homologues, the most abundant acetic acid was chosen as the representative acid. Hydroxyketones were the dominant ketones; hence, hydroxypropanone was chosen as the representative ketone. Moreover, since ketones and aldehydes show similar hydrogenation activities,25 study of representative ketones also provides reference value for aldehydes. Because phenols have significant effects on cracking catalyst lifetime,26 and phenols in BAF were mainly catechol, phenol, and their derivatives and isomers, catechol and phenol were both selected as the representative phenols. Gas chromatography (GC) was employed to determine the contents of these representative compounds. The contents of acetic acid, hydroxypropanone, catechol, and phenol in BAF were 6.2%, 4.5%, 2.2%, and 1.3%, respectively (wet basis). Catalyst Preparation and Characterization. The cracking catalyst was commercial HZSM-5 (Si/Al = 25) purchased from the Catalyst Plant of Nankai University, and it was pretreated at 550 °C for 2 h before reaction. The hydrotreating catalyst was 15Ni-5Cu/SiO2 prepared by the incipient impregnation method, and details can be found in our previous work.20 Here, a relatively high Ni/Cu ratio was used based on the conclusion that it was beneficial for the hydrogenation of oxygenated aromatics.27 15Ni-5Cu/SiO2 was reduced at 500 °C for 2 h with a H2 flow rate of 50 mL/min before reaction. In addition, the catalyst used for single-stage hydrococracking, 15Ni-5Cu/HZSM-5, was also prepared by incipient impregnation and reduced under the same condition as that for 15Ni-5Cu/SiO2. Textural properties of 15Ni-5Cu/SiO2 and HZSM-5 catalysts were measured by N2 adsorption−desorption at 77 K using an Autosorb-1 Quantachrom apparatus. The specific surface area was determined by the multipoint Brunauer−Emmett−Teller (BET) method. The micropore volume was determined by the t-plot method, while the mesopore volume was determined by the Barrett−Joyner−Halenda (BJH) method using the desorption branch. The acidity of HZSM-5 was measured by NH3-temperatureprogrammed desorption (NH3-TPD) using an AutoChem II 2920 instrument. Here, 0.05 g of sample was first preheated at 550 °C for 2 h in a N2 flow. After cooling to 100 °C, sufficient ammonia was injected until adsorption saturation. Each sample was flushed in a He flow for 1 h to remove the remaining NH3. Then, the temperature was increased from 100 to 600 °C at a rate of 10 °C/min. The desorbed species were measured by a thermal conductivity detector (TCD) H2-temperature-programmed reduction (H2-TPR) of 15Ni-5Cu/ SiO2 was performed on an AutoChem II 2920 instrument. The sample was first pretreated at 200 °C for 1 h in an Ar flow and then cooled to room temperature. Afterward, the sample was heated to 800 °C at a ramp of 5 °C/min in a mixture flow of 10% H2/Ar. The consumed H2 for reduction was measured by TCD. Catalytic Run. Catalytic reactions were performed in a fixed-bed reaction system, which has been introduced in detail in our previous study.20 This reaction system contained two connected tubular reactors. Therefore, it could implement not only single-stage hydrotreating, cocracking, and hydro-cocracking processes but also the continuous dual-stage hydrotreating−cocracking process. In addition, in view of the thermosensitivity of bio-oil components, a nozzle was added at the entrance of the reactor (hydrotreating reactor for dual-stage hydrotreating−cocracking) to realize the atomization of liquid feedstock; furthermore, the distance between reactor entrance and catalytic bed was also shortened as far as possible. Hydrotreating lasted for 6 h, and the products were collected and analyzed after reaction. In the experiments of dual-stage hydrotreating−cocracking, single-stage cocracking, and single-stage hydro-cocracking, liquid and gas products were collected and analyzed every hour in 6-h runs to monitor reaction stability. The weights of hydrotreating and cracking catalysts were both 2 g. BAF and methanol were blended as the liquid feedstock, and the weight hourly space velocity (WHSV) of feedstock was consistently 2 h−1, in terms of the weight of cracking catalyst. The cracking temperature was set at 400 °C, which has been proved to favor aromatic hydrocarbon formation over zeolite catalyst.28,29 The influences of hydrotreating temperature, reaction pressure, and weight ratio of BAF to methanol in feedstock were studied: Hydrotreating

methanol can implement renewable biobased liquid fuel production. However, due to the serious hydrogen-lacking properties of bio-oil components, a single-stage cocracking process usually needs large blending amounts of cocracking chemicals.18,19 In consideration of the operation cost and stability, the combination of hydrotreating pretreatment and cocracking with hydrogen-rich chemicals can be a practical way for bio-oil upgrading.20,21 The hydrogen supply by hydrotreating pretreatment can decrease the requisite amount of cocracking chemicals; the secondary hydrogen supply by hydrogen-rich chemicals can ensure the efficient conversion of the compounds which are not saturated in hydrotreating stage and also can allow milder hydrotreating condition such as a lower reaction pressure. Therefore, in this work, BAF upgrading by continuous dualstage hydrotreating−cocracking with methanol was studied. The influences of hydrotreating temperature, reaction pressure, and weight ratio of BAF to methanol in feedstock were investigated to optimize the reaction condition and understand the functions of hydrogen supply in hydrotreating and cracking stages. In particular, the hydrotreating behaviors under different temperatures and pressures were also studied to realize the integral hydrotreating−cocracking behaviors. Furthermore, three cracking processes were compared, namely, dual-stage hydrotreating−cocracking, single-stage cocracking, and singlestage hydro-cocracking. Finally, a hydrotreating−cocracking mechanism was proposed based on experimental results.

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EXPERIMENTAL SECTION

Feedstock. Bio-oil was produced from rice husk by fast pyrolysis in a fluidized-bed reactor. Then, BAF was obtained by water extraction using a water/oil weight ratio of 0.8. The water content in BAF was 70.6%, determined by the Karl Fischer titration method, and correspondingly the organic compound content was 29.4%. This water content was close to that obtained by Vitasari et al. using the same water/oil weight ratio.22 The chemical composition of BAF was analyzed by gas chromatography−mass spectrometry (GC-MS). As calculated based on the peak area normalization method, the main chemical families in BAF were acids, ketones, phenols, sugars, and aldehydes, with the total relative contents of 26.7%, 27.5%, 16.9%, 13.2%, and 7.9%, respectively. Acids included acetic acid, propionic acid, formic acid, etc., among which acetic acid predominated. Hydroxyketones like hydroxypropanone and hydroxybutanone were the main ketones in BAF, which was also observed by other researchers,8,23 and there were also small amounts of other ketones such as cyclopentenones and furanones. It was noteworthy that some phenols were found in BAF, typically catechol and phenol, as well as some their derivatives and isomers such as 4-methyl-1,2-benzenediol, 4-methyl-phenol, and hydroquinone. The BAF obtained by Valle et al. and Vispute et al. also contained certain amounts of phenols especially catechol.8,23 Aldehydes included hydroxyacetaldehyde, furfural, etc. In the detected sugars by GC-MS, levoglucosan predominated with a relative content up to 10.9%, and other sugars like 1,4:3,6-dianhydroα-D-glucopyranose were also found. In addition, BAF still contained some large molecular compounds that cannot be detected by GC-MS such phenolic oligomers (pyrolytic lignin), but their amounts in BAF were much lower than those in crude bio-oil due to their hydrophobicity.24 According to the composition of BAF, the main unsaturated components were acids, ketones, and phenols. Their hydrogenation efficiency in the hydrotreating stage were crucial for their subsequent cracking performances. Therefore, abundant and representative compounds in these three chemical families were selected and monitored to understand the integral conversion behaviors of compounds in these chemical families. Because acetic acid, propionic 6330


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ACS Sustainable Chemistry & Engineering temperatures were 150, 200, 250, and 300 °C; reaction pressures were 1, 2, 3, and 4 MPa; BAF/methanol ratios were 2:1, 1:1, 1:2, and 0:1 (pure methanol). In the comparison of different cracking processes, the BAF/methanol ratio and reaction pressure were set as 1:1 and 4 MPa, respectively. For dual-stage hydrotreating−cocracking, the hydrotreating and cracking temperatures were 250 and 400 °C respectively. The hydrotreating and cracking catalysts were 15Ni-5Cu/ SiO2 and HZSM-5, respectively. For single-stage cocracking, the reaction temperature and catalyst were 400 °C and HZSM-5, respectively. For single-stage hydro-cocracking, the reaction temperature and catalyst were 400 °C and 15Ni-5Cu/HZSM-5, respectively. Product Analysis. The liquid products from hydrotreating appeared homogeneous, while those from dual-stage hydrotreating− cocracking and single-stage cocracking consisted of two phases. The upper phase was oil phase, and the bottom phase was aqueous phase. The amounts of monitored representative compounds (acetic acid, hydroxypropanone, catechol, and phenol) and their typical hydrogenation products (ethanol, 1,2-propanediol, 1,2-cyclohexanediol, and cyclohexanol) in liquid products are quantified by an external standard method using GC (Agilent 6820, Agilent Technologies) equipped with a DB-WAX capillary column. The GC program involved a hold at 40 °C for 5 min, followed by a rise to 240 °C at a rate of 8 °C/min and a hold at 240 °C for 10 min. Meanwhile, the liquid products from hydrotreating and oil phase products from hydrotreating−cocracking and cocracking reactions were also analyzed by GC-MS (Clarus 500, PerkinElmer) using a DB-WAX UI capillary column. The oven temperature was maintained at 40 °C for 5 min and then increased to 240 °C at a ramp of 8 °C/min and maintained at 240 °C for 20 min. The identified chemicals in hydrotreating-derived liquids and oil phases were further quantified by a peak area normalization method to calculate their relative contents. In addition, BAF was also analyzed by GC and GC-MS, and the results have been discussed in the Feedstock section. Gas products were analyzed by GC (Agilent 6820, Agilent Technologies). Light olefins and paraffins were separated on an HPPlot Q capillary column and measured by a flame ionization detector (FID). CO and CO2 were separated on Porapak N, Porapak Q, and Carbon Sieve-11 columns and measured by TCD. The GC oven temperature was held at 50 °C for 1 min and then increased to 180 °C at a rate of 10 °C/min. For all catalytic runs, solid products were collected after reaction. In the dual-stage hydrotreating−cocracking process, although the liquid feedstock was atomized, some carbonaceous solids inevitably deposited on the inner surface of hydrotreating reactor above the catalytic bed, and they were defined as “char”.7 No obvious carbonaceous deposits were formed on the hydrotreating catalysts, but some carbonaceous deposits were found on the spent cracking catalysts. They were defined as “coke”. In the processes of single-stage cocracking and single-stage hydro-cocracking, char and coke were also found. Char was directly collected and weighed, while the amount of coke on a spent cracking catalyst was measured by thermogravimetry analysis (TGA) in an air atmosphere. The yields of products were calculated based on eq 1, in which “m” represents the corresponding mass, and the masses of liquid feedstock and aqueous phase excluded the original water in BAF.

Yi =

mi × 100% (mLiquid feedstock )in

respectively. The BET specific surface area and mesopore volume of 15Ni-5Cu/SiO2 were 111.7 m2/g and 0.57 cm3/g, respectively. The NH3-TPD pattern of HZSM-5 is shown in Figure S1. Three peaks at 180, 300, and 400 °C were recognized, which corresponded to the weak, medium, and strong acid sites. On the basis of the integrated areas of peaks, the distributions of weak, medium, and strong acid sites were calculated,30 which were 44%, 37%, and 19%, respectively. The H2-TPR pattern of 15Ni-5Cu/SiO2 is shown in Figure S2. Two main reduction peaks were distinguished: The lowtemperature reduction peak centered at about 210 °C corresponded to the reduction of Cu2+ to Cu; the shoulder reduction peak between 250 and 300 °C corresponded to the reduction of bulk NiO (Ni2+ to Ni0), whose reducibility was enhanced by the presence of Cu.27,31 Influence of Hydrotreating Temperature on Hydrotreating−Cocracking. Hydrotreating temperature can affect the hydrogenation efficiencies of BAF components and thus the integral hydrotreating-cracking performance. Therefore, the hydrotreating behaviors at different temperatures were first studied, and then, the hydrotreating−cocracking behaviors using different hydrotreating temperatures were investigated. Hydrotreating Behavior. Product yields at different hydrotreating temperatures are shown in Figure 1, under reaction

Figure 1. Product yields in hydrotreating at different reaction temperatures.

pressure of 4 MPa and BAF/methanol ratio of 1:1. At 150 and 200 °C, the gas yields were low, and the liquid yields were above 92%. When the temperature was 250 °C, more gaseous products were produced, including COx and light hydrocarbons generated from decomposition and hydrodeoxygenation reactions,27 and the liquid yield decreased to 89.7%. As the reaction temperature increased to 300 °C, the gas yield reached 11.8% and the liquid yield further decreased to 80.6%. The char yield rose as the temperature increased, reaching 2.8% at 300 °C. They were produced from homogeneous condensation of BAF components.32 Through GC analysis, the amounts of representative compounds and their typical hydrogenation products in feedstock and liquids from hydrotreating at different temperatures were quantified and are presented in Table 1. At hydrotreating temperatures above 200 °C, the residual amounts of hydroxypropanone were small and the corresponding productions of 1,2-propanediol were significant. The liquids were also analyzed by GC-MS analysis, and the total relative

(1)

where i = liquid/oil phase/aqueous phase, gas/gaseous hydrocarbons/ COx, Char, Coke. For hydrotreating reactions, the mass balances were all above 95%; for dual-stage hydrotreating−cocracking reactions, the mass balances were above 90%. The experimental errors of product yields determined by repeated experiments were within 5%.

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RESULTS AND DISCUSSION Characterization of Catalysts. As determined by N2 adsorption−desorption characterization, the BET specific surface area, micropore volume, and mesopore volume of HZSM-5 were 340.0 m2/g, 0.12 cm3/g, and 0.16 cm3/g, 6331


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ACS Sustainable Chemistry & Engineering Table 1. Amounts of Representative Compounds and Their Typical Hydrogenation Products in Feedstock and Liquid Products from Hydrotreating at Different Temperaturesa

Table 2. Solid Yields in Hydrotreating−Cocracking of BAF and Methanol Using Different Hydrotreating Temperaturesa

temperatures (°C)

a

compounds (mmol)

feedstock

150

200

250

300

acetic acid (mmol) hydroxypropanone (mmol) phenol (mmol) catechol (mmol) ethanol (mmol) 1,2-propanediol (mmol) cyclohexanol (mmol) 1,2-cyclohexanediol (mmol)

12.40 7.30 1.66 2.40 0.02 0 0 0

4.87 0.69 1.46 1.84 0.18 6.20 0.01 0.05

3.29 0.26 1.19 1.38 0.24 6.42 0.05 0.14

2.05 0.16 0.81 0.56 0.47 6.10 0.33 1.09

1.26 0.23 0.61 0.29 0.63 5.06 0.55 1.55

condition

char (%)

coke (%)

1:1-150-4 1:1-200-4 1:1-250-4 1:1-300-4

1.1 1.4 2.0 2.5

3.7 3.4 2.5 2.4

a

1:1-250-4 represents hydrotreating−cocracking with BAF/methanol ratio of 1:1, hydrotreating temperature of 250 °C, and reaction pressure of 4 MPa.

In general, as the representative of ketones and aldehydes, hydroxypropanone exhibited high hydrogenation activity, which could improve its cracking performance; meanwhile, high hydrotreating temperature enhanced their secondary gasification reactions like hydrodeoxygenation and decomposition, which would reduce the available feedstock for subsequent cracking. The representative acid, acetic acid, was difficult to hydrogenate. Instead, it underwent esterification with alcohols, and it was also decomposed at high temperatures. Ramasamy et al. found that during cracking over HZSM-5 ester first underwent a hydrolysis reaction to regenerate acid and alcohol, and then, they were converted according to their own cracking mechanisms.38 Therefore, the formation of esters in the hydrotreating stage would not affect the cracking efficiency. Furthermore, the conversion of acids in the hydrotreating stage can relieve the corrosion of the cracking reactor with higher reaction temperature. Nevertheless, esterification did not contribute to improvement of the hydrogen-lacking property. The hydrogenation efficiencies of two representative phenols increased notably with the increase in reaction temperature. Because the deactivation of the cracking catalyst could be accelerated in the presence of phenols,26 their hydrogenation was important for the stability of the cracking catalyst. In particular, at 150 and 200 °C, although some catechol and phenol were converted, the amounts of 1,2-cyclohexanediol and cyclohexanol were very limited. This indicated the occurrences of side reactions instead of hydrogenation, which would not improve the hydrogen-lacking property. Additionally, sugars like levoglucosan were efficiently converted in the hydrotreating stage. Hydrotreating−Cocracking Behavior. On the basis of hydrotreating research, the hydrotreating−cocracking behavior was further studied. In view of the relationship between cracking stability and the improvement of the hydrogen-lacking property in the hydrotreating stage, the liquid yields, oil phase compositions, and gas yields were monitored continuously. Liquid yields in hydrotreating−cocracking using different hydrotreating temperatures are shown in Figure 2. Condition 1:1-250-4 represents dual-stage hydrotreating−cocracking with a BAF/methanol ratio of 1:1, hydrotreating temperature of 250 °C, and reaction pressure of 4 MPa. In the conditions of 1:1250-4 and 1:1-300-4, the oil phase yields were steadily around 22.0% and 18.5%, respectively. However, in the conditions of 1:1-200-4 and 1:1-150-4, obvious decreases in oil phase yields were observed after 2 h; at 6 h, the oil phase yields dropped from initial 23.3% and 23.5% to 10.1% and 7.6%, respectively, indicating decreasing catalyst activities. Figure 3 presents the compositions of oil phases. When the hydrotreating temperatures were low (150 and 200 °C), the relative contents of oxygenated compounds increased gradually as the reaction proceeded. The detected oxygenated com-

For 6-h run.

contents of different chemical families are summarized in Table S1. Remarkable reductions of ketones and aldehydes were observed after hydrotreating accompanied by the generation of alcohols. The high hydrogenation activities of ketones and aldehydes were confirmed by other researchers.25,32 When the temperature increased to 250 and 300 °C, the amounts of 1,2propanediol decreased. This could be attributed to further hydrodeoxygenation and even gasification at high temperatures as the variation tendency of gas yield shows. A certain amount of acetic acid was converted even at 150 °C. However, the amounts of produced ethanol were consistently quite small. As Table S1 shows, certain amounts of esters were found in the liquid products. Therefore, the high conversion efficiency of acetic acid was attributed to the esterification reactions by its self-catalysis.33 Meanwhile, the reduction of its residual amount with the elevation of temperature could be due to the higher intensities of esterification and decomposition reactions.34,35 The residual amounts of catechol and phenol decreased as the temperature increased, and the amounts of 1,2-cyclohexanediol and cyclohexanol increased correspondingly. This variation trend was particularly remarkable from 200 to 250 °C. Elliott et al. observed the elevation of guaiacol hydrogenation efficiency at higher temperatures.35 It is noteworthy that even at 300 °C these two phenols were not completely converted, suggesting their lower hydrogenation activities than ketones.25 In addition, according to Table S1, sugars exhibited high conversion efficiency. A small amount of sugar (entirely levoglucosan with the relative content of 1.7%) was only found at 150 °C; when the hydrotreating temperature was higher than 200 °C, no levoglucosan was detected by GC-MS. Possible conversion routes for levoglucosan might involve homogeneous degradation, catalytic hydroconversion, and some other catalytic degradation reactions. As indicated by Sanna et al., homogeneous degradation of levoglucosan could produce char,32 and this product was also observed above the catalytic bed in this work. Meanwhile, levoglucosan could also undergo hydrogenolysis (cleavage of C−O and C−C bonds) and hydrogenation reactions to produce alcohols,32,36 such as 1,2propanediol presented in Table 2, and alcohols could further undergo hydrodeoxygenation to generate gaseous alkanes as Figure 1 shows. Moreover, a few γ-valerolactone and γbutyrolactone were also detected (classified into esters in Table S1), and they could be produced by hydrogenation− dehydration and acid-catalyzed reactions because of the presence of acids.32 Mentzel et al. found that the cracking performance of γ-valerolactone could be improved by cocracking with methanol.37 6332


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Figure 2. Liquid yields in hydrotreating−cocracking using different hydrotreating temperatures: (a) oil phase and (b) aqueous phase. (1:1-250-4 = hydrotreating−cocracking with a BAF/methanol ratio of 1:1, hydrotreating temperature of 250 °C, and reaction pressure of 4 MPa.)

Figure 3. Oil phase compositions in hydrotreating−cocracking using different hydrotreating temperatures: (a) hydrocarbon relative content and (b) oxygenated compound relative content. (1:1-250-4 = hydrotreating−cocracking with a BAF/methanol ratio of 1:1, hydrotreating temperature of 250 °C, and reaction pressure of 4 MPa.)

Figure 4. Gas yields in hydrotreating−cocracking using different hydrotreating temperatures: (a) gaseous hydrocarbon and (b) COx. (1:1-250-4 = hydrotreating−cocracking with a BAF/methanol ratio of 1:1, hydrotreating temperature of 250 °C, and reaction pressure of 4 MPa.)

olefins like C2H4 and C3H6 were the important intermediates after deoxygenation of oxygenated reactants, and the light olefins could further undergo aromatization reactions and polymerization reactions to produce liquid aromatic and aliphatic hydrocarbons.39,40 Our previous cracking study showed that the decrease in catalyst activity would lower its capability for aromatization of these light olefins and thus lead to their massive release.18,41 Hence, the increases in gaseous hydrocarbon yields here were attributed to the deactivation of cracking catalysts. In the conditions of 1:1-250-4 and 1:1-300-4, the yields of gaseous hydrocarbons and COx were generally stable. Meanwhile, the gas yield in the condition of 1:1-300-4 was higher than that in 1:1-250-4.

pounds included phenols, alcohols, and esters. When the hydrotreating temperatures were 250 and 300 °C, the hydrocarbon relative contents in oil phases were consistently above 98%. In particular, aromatic hydrocarbons accounted for more than 95%, mainly monoaromatics such as toluene, xylenes, trimethylbenzenes, methylethylbenzenes, and tetramethylbenzenes. The typical aliphatic hydrocarbons included 2methylbutane, 2-methyl-pentane, and 3-methyl-pentane. Gas yields are presented in Figure 4. In the conditions of 1:1200-4 and 1:1-150-4, gaseous hydrocarbon yields (particularly olefins) started to increase obviously after 2 h, corresponding to the variation tendencies of oil phase yields. Researches on cracking of bio-oil components and methanol showed that light 6333


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feedstock. Therefore, the suitable hydrotreating temperature for hydrotreating−cocracking was 250 °C. Influence of Reaction Pressure on Hydrotreating− Cocracking. For the continuous hydrotreating−cocracking process, reaction pressure can affect both hydrotreating and cracking. Our previous research on cocracking of bio-oil components and alcohols showed that when the reaction pressure was above 1 MPa the influence of reaction pressure was not very significant.14 Therefore, the hydrotreating and hydrotreating−cocracking behaviors at different reaction pressures were studied. Hydrotreating Behavior. Product yields in hydrotreating at different reaction pressures are shown in Figure 5, under a

Table 2 shows the solid yields. The char yield increased slightly as the hydrotreating temperature increased. Since it was not formed in the catalytic bed and its yield was not high, the influence on reaction stability was quite limited. As determined by TGA, the coke yields in the conditions of 1:1-200-4 and 1:1150-4 were 3.4% and 3.7%, respectively, higher than those of 2.5% and 2.4% in the conditions of 1:1-250-4 and 1:1-300-4. In the cocracking study of bio-oil/methanol mixture by Valle et al.,11 two types of coke on spent catalysts were identified through the temperature-programmed oxidation (TPO) analysis in the temperature range of 300−550 °C: Thermal coke was generated from thermal polymerization of phenolic compounds, with the combustion temperature of about 450 °C; catalytic coke was generated from catalytic cyclization, aromatization, and condensation reactions, with higher combustion temperatures of 500−520 °C. The coke types in this work were identified in the same way, namely, using the DTG curves of spent cracking catalysts. As Figure S3 shows, weight loss peaks in the DTG curves of spent HZSM-5 from conditions of 1:1-200-4. 1:1-250-4, and 1:1-300-4 are all centered at the temperatures above 500 °C, corresponding to the combustion of catalytic coke; however, the DTG curve of spent HZSM-5 from the condition of 1:1-150-4 exhibits a main weight loss peak centered at about 500 °C for the combustion of catalytic coke, as well as a slight shoulder peak at about 400 °C for the combustion of thermal coke, possibly due to more unhydrogenated phenolic compounds after hydrotreating. Because coke was formed on the surface (inner surface and outer surface) of the HZSM-5 catalyst, larger coke formation in the conditions of 1:1-200-4 and 1:1-150-4 would cover more active sites and enhance blockage of inner pores, which resulted in more severe deactivation of catalyst. The above hydrotreating−cocracking results showed that when the hydrotreating temperatures were low (150 and 200 °C) the cracking catalysts were deactivated faster, leading to decreases in yields and qualities of oil phases; in contrast, hydrotreating−cocracking using hydrotreating temperatures of 250 and 300 °C were relatively stable. According to the research on hydrotreating at different temperatures, the main difference was the hydrogenation efficiencies of phenols, which increased with the elevation of temperature. Graça et al. found that the fast and strong adsorption of phenol (even a very small amount) on the acid sites of HZSM-5 and difficulty in its conversion could cause quick deactivation of the catalyst.26 Therefore, the faster deactivations of catalysts in the conditions of 1:1-200-4 and 1:1-150-4 resulted from the entrance of more unconverted phenols and their nonhydrogenated byproducts to the cracking stage. For the conditions of 1:1-250-4 and 1:1-3004, although a few phenols and their nonhydrogenated byproducts were also introduced in the cracking stage, the activities of cracking catalysts were better maintained, suggesting that the cocracking methanol could play a positive role in the efficient conversion of small amounts of phenols. Meanwhile, for the unsaturated acids and esters entering the cracking stage, which exhibited poor cracking characteristics during their individual conversions,42,43 methanol also promoted their conversions according to the hydrotreating− cocracking results. In addition, the comparison of conditions of 1:1-250-4 and 1:1-300-4 showed that the latter one produced more gaseous products but less desired liquid hydrocarbons. This was mainly attributed to the remarkable enhancement of gasification reactions which reduced the available cracking

Figure 5. Product yields in hydrotreating at different reaction pressures.

reaction temperature of 250 °C and BAF/methanol ratio of 1:1. The influence of reaction pressure on product yields was not obvious. The yields of liquid, gas, and char were around 89.5%, 4.9%, and 1.7%, respectively. The amounts of representative compounds and their typical hydrogenation products in feedstock and liquids from hydrotreating at different pressures are presented in Table 3. Similar Table 3. Amounts of Representative Compounds and Their Typical Hydrogenation Products in Feedstock and Liquid Products from Hydrotreating at Different Pressuresa pressure (MPa)

a

compounds

feedstock

1

2

3

4

acetic acid (mmol) hydroxypropanone (mmol) phenol (mmol) catechol (mmol) ethanol (mmol) 1,2-propanediol (mmol) cyclohexanol (mmol) 1,2-cyclohexanediol (mmol)

12.4 7.30 1.66 2.40 0.02 0 0 0

3.72 0.34 1.19 1.30 0.23 6.08 0.10 0.32

3.32 0.28 1.10 1.15 0.27 6.36 0.14 0.49

2.49 0.14 0.99 0.95 0.34 6.39 0.21 0.66

2.05 0.16 0.81 0.56 0.47 6.10 0.33 1.09

For 6-h run.

to the influence of hydrotreating temperature, hydroxypropanone exhibited high hydrogenation activity to produce 1,2propanediol at different pressures, while acetic acid was difficult to be hydrogenated, and instead preferred to undergo esterification according to relative contents of chemical families presented in Table S2. The hydrogenation reactions of catechol and phenol to 1,2-cyclohexanediol and cyclohexanol were both 6334


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Figure 6. Liquid yields in hydrotreating−cocracking at different reaction pressures: (a) oil phase and (b) aqueous phase. (1:1-250-4 = hydrotreating−cocracking with a BAF/methanol ratio of 1:1, hydrotreating temperature of 250 °C, and reaction pressure of 4 MPa.)

Figure 7. Oil phase compositions in hydrotreating−cocracking at different reaction pressures: (a) hydrocarbon relative content and (b) oxygenated compound relative content. (1:1-250-4 = hydrotreating−cocracking with a BAF/methanol ratio of 1:1, hydrotreating temperature of 250 °C, and reaction pressure of 4 MPa.)

Figure 8. Gas yields in hydrotreating−cocracking at different reaction pressures: (a) gaseous hydrocarbon and (b) COx. (1:1-250-4 = hydrotreating− cocracking with a BAF/methanol ratio of 1:1, hydrotreating temperature of 250 °C, and reaction pressure of 4 MPa.)

conversion, and other catalytic degradation reactions as discussed previously. The hydrotreating behaviors of representative compounds demonstrated that as the reaction pressure rose from 1 to 4 MPa ketones and aldehydes showed high hydrogenation activities, whereas the hydrogenation efficiencies of acids stayed low, and the hydrogenation efficiencies of phenols increased gradually, which was important for the improvement of cracking performance. Hydrotreating−Cocracking Behavior. Hydrotreating−cocracking of BAF and methanol at different pressures was then studied. Figure 6 presents liquid yields in hydrotreating−

promoted at higher reaction pressures. Besides catechol and phenol, their derivatives in BAF, such as 4-methyl-1,2benzenediol and 4-methyl-phenol, were expected to exhibit similar hydrogenation performances. Therefore, as shown in Table S2, when the reaction pressure increased from 1 to 4 MPa, the total relative content of phenols decreased from 10.3% to 5.2%. As stated before, even a small amount of phenol could accelerate catalyst deactivation.26 Therefore, this difference in amount of phenols could possibly affect catalyst stability. No sugars were detected by GC-MS at different pressures, indicating their efficient conversions through the reaction routes of homogeneous degradation, catalytic hydro6335


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cocracking study of the bio-oil component and methanol, it was found that blending methanol could promote the conversion of bio-oil components, and this promotion mainly worked in the cracking stage and was enhanced by raising the methanol blending ratio in a certain range.21 Therefore, the hydrotreating−cocracking behaviors with different BAF/methanol ratios were directly studied and compared with pure methanol cracking (0:1-4). Figure 9 shows liquid yields in hydrotreating−cocracking with different BAF/methanol ratios. In the conditions of 1:1250-4, 1:2-250-4, and 0:1-4, the oil phase yields were steadily around 22.0%. When the BAF/methanol ratio was 2:1, the initial oil phase yield was lower (20.4%), and it quickly decreased to only 8.3% at 5 h, indicating fast deactivation of the catalyst. The compositions of oil phases are presented in Figure 10. The analysis of the oil phase from the condition of 2:1-2504 showed the presence of oxygenated compounds at the initial time, and their total relative content increased gradually to 9.2% at 6 h. The relative contents of hydrocarbons in the oil phases from conditions of 1:1-250-4, 1:2-250-4, and 0:1-4 stayed above 98% in 6-h runs. In addition, the relative content of aliphatic hydrocarbon increased as the methanol blending ratio rose due to the elevation of the integral saturation degree of feedstock. Gas yields in hydrotreating−cocracking with different BAF/ methanol ratios are presented in Figure 11. The gaseous hydrocarbon yield in the condition of 2:1-250-4 kept increasing, while those in conditions of 1:1-250-4, 1:2-250-4, and 0:1-4 were relatively stable. It was also observed that with the increase in the methanol blending ratio less COx was produced, also due to the elevation of the integral saturation degree of feedstock. Yields of solid products are shown in Table 5. Decreasing the blending ratio of BAF resulted in the reduction of char yield, while the char formation in pure methanol cracking was negligible. The coke yields in conditions of 1:1-250-4, 1:2-2504, and 0:1-4 were 2.5%, 2.1%, and 1.4%, respectively, whereas that in the condition of 2:1-250-4 reached 3.9%, proving the more severe deactivation of the cracking catalyst. Moreover, the DTG curves of spent cracking catalysts in Figure S5 suggest that in the condition of 2:1-250-4 a small amount of thermal coke burning at about 400 °C and a large amount of catalytic coke with the combustion temperature above 500 °C were both generated, while hydrotreating−cocracking with other BAF/ methanol blending ratios and pure methanol cracking mainly

cocracking at different pressures. When the reaction pressures were 1, 2, and 3 MPa, the oil phase yields dropped from initial 21.5%, 22.2%, and 22.8% to 16.6%, 13.8%, and 11.9% at 6 h, respectively. The oil phase yield in the condition of 1:1-250-4 was maintained at around 22.0%. Figure 7 shows the oil phase compositions. The relative contents of oxygenated compounds in oil phases from hydrotreating−cocracking at low pressures increased faster, while the condition of 1:1-250-4 produced oil phase with steady good quality. The above results indicated that under lower reaction pressure the activity of the catalyst decreased more quickly. Gas yields are shown in Figure 8. Under lower reaction pressure, the increasing tendency of gaseous hydrocarbons was more obvious. As discussed previously, this reflected the more serious catalyst deactivation, which was in accordance with the higher coke yield as shown in Table 4. Additionally, according Table 4. Solid Yields in Hydrotreating−Cocracking of BAF and Methanol at Different Pressuresa condition

char (%)

coke (%)

1:1-250-1 1:1-250-2 1:1-250-3 1:1-250-4

1.6 1.8 2.1 2.0

3.4 3.2 2.8 2.5

a

1:1-250-4 represents hydrotreating−cocracking with BAF/methanol ratio of 1:1, hydrotreating temperature of 250 °C, and reaction pressure of 4 MPa.

to the DTG curves of spent cracking catalysts in Figure S4, the coke formed in these conditions was mainly catalytic coke, with the combustion temperatures above 500 °C. Combining with the hydrotreating behavior, it could be deduced that the different stabilities of hydrotreating− cocracking at different pressures were mainly attributed to the difference in hydrogenation efficiencies of phenols. At low reaction pressures, phenols were more difficult to hydrogenate, and thus, more phenols and their nonhydrogenated byproducts went into the cracking stage, which accelerated catalyst deactivation and led to the decreases in oil phase yield and quality. When reaction pressure was 4 MPa, the hydrotreating− cocracking process was generally stable, and therefore, 4 MPa was the suitable reaction pressure. Influence of BAF/Methanol Ratio in Feedstock on Hydrotreating−Cocracking. In our previous hydrotreating−

Figure 9. Liquid yields in hydrotreating−cocracking with different BAF/methanol ratios: (a) oil phase and (b) aqueous phase. (1:1-250-4 = hydrotreating−cocracking with a BAF/methanol ratio of 1:1, hydrotreating temperature of 250 °C, and reaction pressure of 4 MPa; 0:1-4 = cracking of pure methanol with reaction pressure of 4 MPa.) 6336


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Figure 10. Oil phase compositions in hydrotreating−cocracking with different BAF/methanol ratios: (a) hydrocarbon relative content and (b) oxygenated compound relative content. (1:1-250-4 = hydrotreating−cocracking with a BAF/methanol ratio of 1:1, hydrotreating temperature of 250 °C, and reaction pressure of 4 MPa; 0:1-4 = cracking of pure methanol with reaction pressure of 4 MPa.)

Figure 11. Gas yields in hydrotreating−cocracking with different BAF/methanol ratios: (a) gaseous hydrocarbon and (b) COx. (1:1-250-4 = hydrotreating−cocracking with BAF/methanol ratio of 1:1, hydrotreating temperature of 250 °C, and reaction pressure of 4 MPa; 0:1-4 = cracking of pure methanol with reaction pressure of 4 MPa.)

produced catalytic coke with burning temperatures above 500 °C. The above results demonstrated that the hydrotreated BAF could produce liquid hydrocarbon with the yield comparable to that of methanol, as shown in the conditions of 1:1-250-4, 1:2250-4, and 0:1-4. However, this capability for liquid hydrocarbon generation was on the premise of sufficient assistance from methanol. Otherwise, like the condition of 2:1-250-4, the integral deoxygenation efficiency was lowered and more coke was formed on the surface of the catalyst which accelerated its deactivation. Because the liquid hydrocarbon yields in the

Table 5. Solid Yields in Hydrotreating−Cocracking of BAF and Methanol with Different Blending Ratios condition

char (%)

coke (%)

2:1-250-4 1:1-250-4 1:2-250-4 0:1-4

2.2 2.0 1.3 0

3.9 2.5 2.1 1.4

a

1:1-250-4 represents hydrotreating−cocracking with BAF/methanol ratio of 1:1, hydrotreating temperature of 250 °C, and reaction pressure of 4 MPa; 0:1-4 represents cracking of pure methanol with reaction pressure of 4 MPa.

Figure 12. Liquid yields in different cracking processes: (a) oil phase and (b) aqueous phase. 6337


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Figure 13. Oil phase compositions in different cracking processes: (a) hydrocarbon relative content and (b) oxygenated compound relative content.

Figure 14. Gas yields in different cracking processes: (a) gaseous hydrocarbon and (b) COx.

Table 6 shows the solid yields. The char yields in single-stage cocracking and hydro-cocracking were higher than that in dual-

conditions of 1:1-250-4 and 1:2-250-4 were close, in consideration of cost, the suitable BAF/methanol ratio was 1:1. Comparison of Different Cracking Processes. The study on the influence of the BAF/methanol ratio has confirmed the function and necessity of methanol blending. In this section, three cracking processes were compared, including the optimized dual-stage hydrotreating−cocracking, single-stage cocracking, and single-stage hydro-cocracking similar to the hydrocracking in petroleum refining. Detailed reaction parameters can be found in the Experimental Section. Liquid product yields in different cracking processes are shown in Figure 12. Compared with dual-stage hydrotreating− cocracking which steadily produced high-quality oil phase products, the single-stage cocracking process was unstable, in which the oil phase yield was only 7.1% at 4 h and then further decreased to 5.8% at 6 h. In single-stage hydro-cocracking, no obvious oil phase product was generated. Figure 13 presents the oil phase compositions. The oxygenated compound relative content in the oil phase from single-stage cocracking rose fast from the initial 3.3% to 40.9% at 6 h. The oxygenated compounds included anisole and its derivatives, phenols, ketones, and esters, indicating the dramatic decrease in integral deoxygenation efficiency. Gas yields are presented in Figure 14. The gaseous hydrocarbon yield in dual-stage hydrotreating−cocracking was generally stable, whereas that in single-stage cocracking increased continuously. It was noteworthy that in single-stage hydro-cocracking the yield of gaseous hydrocarbon (mainly C2H6 and C3H8) stayed much higher than that in dual-stage hydrotreating−cocracking, corresponding to its few oil phase product generation.

Table 6. Solid Yields in Different Cracking Processes cocracking hydro-cocracking hydrotreating−cocracking

char (%)

coke (%)

2.9 2.8 2.0

4.7 2.1 2.5

stage hydrotreating−cocracking. This was attributed to the higher reaction temperature in single-stage reactions (400 °C) than the hydrotreating temperature (250 °C) in hydrotreating− cocracking, which strengthened homogeneous condensation of BAF components. Meanwhile, the coke yield of 4.7% in singlestage cocracking was obviously higher than that of 2.5% in dualstage hydrotreating−cocracking, confirming the more serious catalyst deactivation. The DTG curves of spent cracking catalysts in Figure S6 indicates that during single-stage cocracking some thermal coke with burning temperatures at 350−450 °C was formed, but the majority was still catalytic coke burning at about 520 °C, while the coke generated in dualstage hydrotreating−cocracking was almost catalytic coke with the combustion temperature above 500 °C. The comparison of different cracking processes showed that the speed of catalyst deactivation in single-stage cocracking was much higher than that in dual-stage hydrotreating−cocracking. From the study of hydrotreating behaviors, it was found that mild hydrotreating could achieve the saturation of ketones, aldehydes, and partial phenols, which improved the hydrogenlacking property of BAF. Particularly, the hydrogenation of phenols were very important for maintaining the cracking 6338


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Figure 15. Reaction mechanism of dual-stage hydrotreating−cocracking of BAF and methanol.

comparison of dual-stage hydrotreating−cocracking and singlestage cocracking. In addition, some gas and solid products were also generated in the hydrotreating stage. Due to the thermosensitivity of some BAF components such as sugars and phenols, homogeneous gas phase reactions happened above the catalytic bed to produce char and gases such as COx and light hydrocarbons.24,32 Moreover, some primary products in the hydrotreating stage, such as alcohols and esters, might undergo secondary gasification reactions like hydrodeoxygenation and decomposition to generate COx and light hydrocarbons. Homogeneous gas phase reactions and secondary gasification reactions were more intensive at higher temperatures. Hydrotreating−cocracking with a hydrotreating temperature of 300 °C has shown that the excessive high intensities of secondary gasification reactions would remarkably decrease the available feedstock for subsequent cracking and thus lower the ultimate liquid hydrocarbon yield. According to the hydrotreating mechanism, some unsaturated compounds were introduced into the cracking stage, such as unconverted acids and phenols as well as some esters. Their hydrogen-lacking property still required improvement. The individual cracking studies of acids and esters showed that their conversions to liquid hydrocarbons were accompanied by certain amounts of oxygenated byproducts and coke;42,43 furthermore, phenols showed very low cracking activity and tended coking.39,44 From the result of optimized hydrotreating−cocracking (1:1-250-4), the cracking performances of acids, esters, and phenols were improved in the presence of methanol. In the coprocessing studies of biomass or bio-oil components with hydrogen-rich chemicals (alcohols, aliphatic hydrocarbons, and tetralin), internal hydrogen transfers were believed to improve the conversions of biomass or bio-oil components.15,16,24,45,46 Combining with the cracking mechanism of methanol, the main cocracking mechanism involving hydrogen transfer was proposed, and some simplified reactions corresponding to the pathways in Figure 15 are listed as eqs 2−6.

catalyst activity. Therefore, for single-stage cocracking with the studied BAF/methanol ratio, the hydrogen supply only from methanol cocracking was not sufficient. As a result, the coking and catalyst deactivation problems were not effectively overcome. Single-stage hydro-cocracking produced a large amount of gaseous hydrocarbons but few liquid hydrocarbons. As mentioned above, light olefins were the important intermediates after primary deoxygenation,39,40 and their subsequent aromatization and polymerization produced liquid hydrocarbons. Combining with the characteristics of the 15Ni5Cu/HZSM-5 catalyst, it was inferred that the massive production of light alkanes were due to the in situ hydrogenation of light olefins on metallic Ni−Cu. As a result, aromatization and polymerization reactions hardly happened and few liquid hydrocarbons were generated. Therefore, dualstage hydrotreating−cocracking was the most superior for BAF upgrading. Reaction Mechanism of Dual-Stage Hydrotreating− Cocracking of BAF and Methanol. On the basis of experimental results, the reaction mechanism of hydrotreating−cocracking of BAF and methanol was proposed as shown in Figure 15. In the hydrotreating stage, ketones and aldehydes in BAF showed high hydrogenation activities and were readily converted into alcohols such as 1,2-propanediol, 1,2-butanediol, and ethylene glycol. Sugars like levoglucosan could also undergo hydrogenolysis and hydrogenation reactions to produce these alcohols. Phenols like catechol and phenol could undergo benzene-ring hydrogenation to produce cyclic alcohols such as 1,2-cyclohexanediol and cyclohexanol. The hydrogenation of phenols was enhanced by raising reaction temperature and pressure, which was important for the stability of subsequent cracking as indicated by the results of hydrotreating−cocracking under different hydrotreating temperatures and pressures. Acids were difficult to hydrogenate and preferred to undergo esterification reactions with alcohols to produce esters.33 Although mild hydrotreating did not implement the complete saturation of components in BAF, this hydrogen supply for the improvement of BAF’s hydrogenlacking property was of great significance, as can shown in the

CH4O → CnH 2n + H 2O 6339

(2) DOI: 10.1021/acssuschemeng.7b01505 ACS Sustainable Chem. Eng. 2017, 5, 6329−6342

Research Article

ACS Sustainable Chemistry & Engineering CnH 2n → Cx H y (y < 2x) + H

(3)

H + CnH 2n → CmH 2m + 2

(4)

H + CiH jOk → CnH 2n + COx + H 2O

(5)

surplus hydrogen so as to promote deoxygenation to form light olefins and lower coking tendency.41 The olefins produced from unsaturated compounds with the assistance of methanol, as well as those from deoxygenation of the alcohols except methanol, underwent aromatization reactions to produce liquid aromatic hydrocarbons. The surplus hydrogen formed in this step could also be used in deoxygenation of unsaturated compounds and the formation of aliphatic hydrocarbons. In summary, methanol could act as a hydrogen donor because dehydrogenation reactions happened during the aromatization of light olefins, which was the primary deoxygenation products of methanol. The surface hydrogen produced from dehydrogenation reactions improved the deoxygenation of unsaturated compounds. The amount of hydrogen supply was determined by the blending ratio of methanol in feedstock. Therefore, when the BAF/methanol ratio was 2:1, because of the insufficient hydrogen supply by methanol, oxygenated byproducts were formed, and the catalyst was quickly deactivated due to coke formation.

CiH jOk → CnH 2n + COx + H 2O + CpHqOr + Coke (6)

After primary deoxygenation (mainly dehydration), methanol was transformed into light olefins such as C2H4 and C3H640,47 (eq 2). For the aromatization of light olefins, it was proposed that light olefins first underwent oligomerization and dehydrogenation reactions to form larger dienes, which further underwent cyclization to produce C6−C10 cyclic olefins, and then, dehydrogenation reactions happened to generate aromatic hydrocarbons.48,49 This integral process can be simplified as eq 3, in which CxHy was the generated aromatic hydrocarbon molecule, and H was the surface-adsorbed hydrogen atom produced from dehydrogenation reactions.49 As proposed by Dejaifve et al., this surplus hydrogen could participate in the saturation of olefins to produce paraffins (eq 4).50 The olefins here could be the primary light olefins and also the larger olefins produced by subsequent polymerization and cracking.49,50 Therefore, the corresponding paraffins could be gaseous-saturated hydrocarbons, as well as the aliphatic hydrocarbons observed in the oil phases such as methylbutane and methylpentane, whose formation might involve further alkylation reactions. Moreover, during cocracking, some surplus hydrogen was also supposed to take part in the deoxygenation of unsaturated oxygenated compounds (eq 5), whose individual deoxygenation was often incomplete and accompanied by coke (eq 6). In the copyrolysis study of lignin and tetralin over zeolites, Xue et al. confirmed the hydrogen transfer from tetralin to lignin-derived phenols, and they proposed that the mechanism involved the dehydrogenation of tetralin to produce naphthalene and surface hydrogen, which could saturate the adsorbed phenols and their intermediates, and thereby enhance oxygen removal in the form of water and suppress coke formation.46 This mechanism supports the reaction pathways proposed in this work (eqs 3 and 5) that surface-adsorbed hydrogen generated from dehydrogenation reactions was utilized for the deoxygenation of unsaturated compounds, and in particular, the dehydrogenations of C6−C10 cyclic olefins to aromatics were similar to the dehydrogenation of tetralin to naphthalene. More detailed mechanisms for the participation of surplus hydrogen in the conversions of unsaturated compounds (acids, esters, and phenols) are presented in the following discussions. References showed that in the conversion of acetic acid over zeolites, active and highly unsaturated adsorbed acetyl or acylium ion and acetone could be the possible intermediates.38,51 The surplus hydrogen supplied by methanol could saturate them and form compounds with hydroxyl groups, which enhanced the dehydration reaction to produce olefins and suppressed the condensation reaction to generate coke. As for esters, researchers proposed that they first underwent hydrolysis to generate acids and alcohols, which were then converted according to their own cracking mechanisms.38 In this mechanism, the acids generated from esters could receive the surplus hydrogen from methanol as previously mentioned. The study on phenol cracking over zeolite showed that it first underwent protonation to form highly unsaturated cyclic ketene,52 and this intermediate could also be saturated by

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CONCLUSIONS Upgrading of BAF by continuous hydrotreating−cocracking with methanol was carried out on a fixed-bed reaction system. It was found that in the hydrotreating stage ketones and aldehydes showed high hydrogenation activities, while acids were difficult to be hydrogenated; the hydrogenation efficiencies of phenols increased with the elevations of hydrotreating temperature and reaction pressure. Under low hydrotreating temperatures (150−200 °C) and low reaction pressures (1−3 MPa), because more phenols and their nonhydrogenated byproducts were introduced in the cracking stage, the deactivation of a cracking catalyst was accelerated. Meanwhile, it was observed that an excessive high hydrotreating temperature (300 °C) which significantly enhanced gasification reactions reduced the ultimate liquid hydrocarbon yield. The study of hydrotreating−cocracking with different BAF/ methanol ratios showed that when the BAF/methanol ratio was too high (2:1), the activity of the catalyst decreased fast due to the insufficient hydrogen supply from methanol. Therefore, the optimum reaction parameters were a BAF/ methanol ratio of 1:1, hydrotreating temperature of 250 °C, and reaction pressure of 4 MPa. Under these conditions, the oil phase yield was around 22.0% in a 6-h run and the relative content of hydrocarbon in it was consistently above 98% according to GC-MS analysis. In addition, the comparison of dual-stage hydrotreating−cocracking with single-stage cocracking and single-stage hydro-cocracking showed that hydrotreating−cocracking was the most favorable for liquid hydrocarbon production.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01505. Table S1: Relative contents of chemical families in feedstock and liquid products from hydrotreating at different temperatures. Table S2: Relative contents of chemical families in feedstock and liquid products from hydrotreating at different pressures. Figure S1: NH3TPD pattern of HZSM-5. Figure S2: H2-TPR pattern of 6340


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(14) Wang, S.; Cai, Q.; Wang, X.; Zhang, L.; Wang, Y.; Luo, Z. Biogasoline production from the co-cracking of the distilled fraction of bio-oil and ethanol. Energy Fuels 2014, 28, 115−122. (15) Graça, I.; Lopes, J. M.; Ribeiro, M. F.; Ribeiro, F. R.; Cerqueira, H. S.; de Almeida, M. B. B. Catalytic cracking in the presence of guaiacol. Appl. Catal., B 2011, 101, 613−621. (16) Wang, S.; Cai, Q.; Chen, J.; Zhang, L.; Wang, X.; Yu, C. Green aromatic hydrocarbon production from cocracking of a bio-oil model compound mixture and ethanol over Ga2O3/HZSM-5. Ind. Eng. Chem. Res. 2014, 53, 13935−13944. (17) Kumabe, K.; Fujimoto, S.; Yanagida, T.; Ogata, M.; Fukuda, T.; Yabe, A.; Minowa, T. Environmental and economic analysis of methanol production process via biomass gasification. Fuel 2008, 87, 1422−1427. (18) Wang, S.; Cai, Q.; Wang, X.; Guo, Z.; Luo, Z. Bio-gasoline production from co-cracking of hydroxypropanone and ethanol. Fuel Process. Technol. 2013, 111, 86−93. (19) Graça, I.; Lopes, J. M.; Cerqueira, H. S.; Ribeiro, M. F. Bio-oils Upgrading for Second Generation Biofuels. Ind. Eng. Chem. Res. 2013, 52, 275−287. (20) Cai, Q.; Xu, J.; Zhang, S.; Wang, S. Aromatic hydrocarbon production from bio-oil by a dual-stage hydrogenation-cocracking process: furfural as a model compound. Ind. Eng. Chem. Res. 2016, 55, 10839−10849. (21) Zhang, S.; Xu, J.; Cai, Q.; Cui, Y. Production of aromatic hydrocarbons by hydrogenation-cocracking of bio-oil and methanol. Fuel Process. Technol. 2017, 161, 232−239. (22) Vitasari, C. R.; Meindersma, G. W.; de Haan, A. B. Water extraction of pyrolysis oil: the first step for the recovery of renewable chemicals. Bioresour. Technol. 2011, 102, 7204−7210. (23) Valle, B.; Remiro, A.; Aguayo, A. T.; Bilbao, J.; Gayubo, A. G. Catalysts of Ni/alpha-Al2O3 and Ni/La2O3-alpha Al2O3 for hydrogen;production by steam reforming of bio-oil aqueous fraction with pyrolytic;lignin retention. Int. J. Hydrogen Energy 2013, 38, 1307− 1318. (24) Remiro, A.; Valle, B.; Oar-Arteta, L.; Aguayo, A. T.; Bilbao, J.; Gayubo, A. G. Hydrogen production by steam reforming of bio-oil/ bio-ethanol mixtures in a continuous thermal-catalytic process. Int. J. Hydrogen Energy 2014, 39, 6889−6898. (25) Elliott, D. C. Historical Developments in Hydroprocessing Biooils. Energy Fuels 2007, 21, 1792−1815. (26) Graça, I.; Comparot, J. D.; Laforge, S.; Magnoux, P.; Lopes, J. M.; Ribeiro, M. F.; Ribeiro, F. R. Influence of Phenol Addition on the H-ZSM-5 Zeolite Catalytic Properties during Methylcyclohexane Transformation. Energy Fuels 2009, 23, 4224−4230. (27) Ardiyanti, A. R.; Khromova, S. A.; Venderbosch, R. H.; Yakovlev, V. A.; Heeres, H. J. Catalytic hydrotreatment of fastpyrolysis oil using non-sulfided bimetallic Ni-Cu catalysts on a δ-Al2O3 support. Appl. Catal., B 2012, 117-118, 105−117. (28) Wang, S.; Cai, Q.; Wang, X.; Zhang, L.; Wang, Y.; Luo, Z. Biogasoline production by co-cracking of model compound mixture of bio-oil and ethanol over HSZM-5. Chin. J. Catal. 2014, 35, 709−722. (29) Wang, S.; Cai, Q.; Guo, Z.; Wang, Y.; Wang, X. Renewable gasoline produced by co-cracking of methanol and ketones in bio-oil. BioResources 2012, 7, 5019−5031. (30) Li, Y.; Liu, S.; Xie, S.; Xu, L. Promoted metal utilization capacity of alkali-treated zeolite: Preparation of Zn/ZSM-5 and its application in 1-hexene aromatization. Appl. Catal., A 2009, 360, 8−16. (31) Ashok, J.; Reddy, P. S.; Raju, G.; Subrahmanyam, M.; Venugopal, A. Catalytic Decomposition of Methane to Hydrogen and Carbon Nanofibers over Ni−Cu−SiO2 Catalysts. Energy Fuels 2009, 23, 5−13. (32) Sanna, A.; Vispute, T. P.; Huber, G. W. Hydrodeoxygenation of the aqueous fraction of bio-oil with Ru/C and Pt/C catalysts. Appl. Catal., B 2015, 165, 446−456. (33) Chen, J.; Cai, Q.; Lu, L.; Leng, F.; Wang, S. Upgrading of the acid-rich fraction of bio-oil by catalytic hydrogenation-esterification. ACS Sustainable Chem. Eng. 2017, 5, 1073−1081.

15Ni-5Cu/SiO2. Figure S3: DTG curves of spent cracking catalysts from hydrotreating−cocracking of BAF and methanol using different hydrotreating temperatures. Figure S4: DTG curves of spent cracking catalysts from hydrotreating−cocracking of BAF and methanol at different pressures. Figure S5: DTG curves of spent cracking catalysts from hydrotreating−cocracking with different BAF/methanol ratios. Figure S6: DTG curves of spent cracking catalysts from different cracking processes. (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel: +86 21 64253283. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 51606069) and China Postdoctoral Science Foundation (Grant No. 2016M591616).

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REFERENCES

(1) Oasmaa, A.; van de Beld, B.; Saari, P.; Elliott, D. C.; Solantausta, Y. Norms, Standards, and Legislation for Fast Pyrolysis Bio-oils from Lignocellulosic Biomass. Energy Fuels 2015, 29, 2471−2484. (2) Bridgwater, A. V. Review of fast pyrolysis of biomass and product upgrading. Biomass Bioenergy 2012, 38, 68−94. (3) Zhang, X.; Wang, T.; Ma, L.; Zhang, Q.; Jiang, T. Hydrotreatment of bio-oil over Ni-based catalyst. Bioresour. Technol. 2013, 127, 306−311. (4) Zhang, X.; Zhang, Q.; Wang, T.; Li, B.; Xu, Y.; Ma, L. Efficient upgrading process for production of low quality fuel from bio-oil. Fuel 2016, 179, 312−321. (5) Zhang, X.; Chen, L.; Kong, W.; Wang, T.; Zhang, Q.; Long, J.; Xu, Y.; Ma, L. Upgrading of bio-oil to boiler fuel by catalytic hydrotreatment and esterification in an efficient process. Energy 2015, 84, 83−90. (6) Mortensen, P. M.; Grunwaldt, J. D.; Jensen, P. A.; Knudsen, K. G.; Jensen, A. D. A review of catalytic upgrading of bio-oil to engine fuels. Appl. Catal., A 2011, 407, 1−19. (7) Adjaye, J. D.; Bakhshi, N. N. Production of hydrocarbons by catalytic upgrading of a fast pyrolysis bio-oil. Part I: Conversion over various catalysts. Fuel Process. Technol. 1995, 45, 161−183. (8) Vispute, T. P.; Zhang, H. Y.; Sanna, A.; Xiao, R.; Huber, G. W. Renewable Chemical Commodity Feedstocks from Integrated Catalytic Processing of Pyrolysis Oils. Science 2010, 330, 1222−1227. (9) Veses, A.; Puértolas, B.; López, J. M.; Callén, M. S.; Solsona, B.; García, T. Promoting deoxygenation of bio-oil by metal-loaded hierarchical ZSM-5 zeolites. ACS Sustainable Chem. Eng. 2016, 4, 1653−1660. (10) Zhang, H.; Cheng, Y.-T.; Vispute, T. P.; Xiao, R.; Huber, G. W. Catalytic conversion of biomass-derived feedstocks into olefins and aromatics with ZSM-5: the hydrogen to carbon effective ratio. Energy Environ. Sci. 2011, 4, 2297−2307. (11) Valle, B.; Gayubo, A. G.; Aguayo, A. T.; Olazar, M.; Bilbao, J. Selective Production of Aromatics by Crude Bio-oil Valorization with a Nickel-Modified HZSM-5 Zeolite Catalyst. Energy Fuels 2010, 24, 2060−2070. (12) Samolada, M. C.; Baldauf, W.; Vasalos, I. A. Production of a biogasoline by upgrading biomass flash pyrolysis liquids via hydrogen processing and catalytic cracking. Fuel 1998, 77, 1667−1675. (13) Ibarra, Á .; Rodríguez, E.; Sedran, U.; Arandes, J. M.; Bilbao, J. Synergy in the Cracking of a Blend of Bio-oil and Vacuum Gasoil under Fluid Catalytic Cracking Conditions. Ind. Eng. Chem. Res. 2016, 55, 1872−1880. 6341


Research Article

ACS Sustainable Chemistry & Engineering (34) Ye, J.; Liu, C.; Fu, Y.; Peng, S.; Chang, J. Upgrading Bio-oil: Simultaneous Catalytic Esterification of Acetic Acid and Alkylation of Acetaldehyde. Energy Fuels 2014, 28, 4267−4272. (35) Elliott, D. C.; Hart, T. R. Catalytic Hydroprocessing of Chemical Models for Bio-oil. Energy Fuels 2009, 23, 631−637. (36) Bindwal, A. B.; Vaidya, P. D. Kinetics of Aqueous-Phase Hydrogenation of Levoglucosan over Ru/C Catalyst. Ind. Eng. Chem. Res. 2013, 52, 17781−17789. (37) Mentzel, U. V.; Holm, M. S. Utilization of biomass: Conversion of model compounds to hydrocarbons over zeolite H-ZSM-5. Appl. Catal., A 2011, 396, 59−67. (38) Ramasamy, K. K.; Gerber, M. A.; Flake, M.; Zhang, H.; Wang, Y. Conversion of biomass-derived small oxygenates over HZSM-5 and its deactivation mechanism. Green Chem. 2014, 16, 748−760. (39) Adjaye, J. D.; Bakhshi, N. N. Catalytic conversion of a biomassderived oil to fuels and chemicals I: Model compound studies and reaction pathways. Biomass Bioenergy 1995, 8, 131−149. (40) Haw, J. F.; Song, W.; Marcus, D. M.; Nicholas, J. B. The mechanism of methanol to hydrocarbon catalysis. Acc. Chem. Res. 2003, 36, 317−326. (41) Wang, S.; Cai, Q.; Chen, J.; Zhang, L.; Zhu, L.; Luo, Z. Cocracking of bio-oil model compound mixtures and ethanol over different metal oxide-modified HZSM-5 catalysts. Fuel 2015, 160, 534−543. (42) Wang, S.; Guo, Z.; Cai, Q.; Guo, L. Catalytic conversion of carboxylic acids in bio-oil for liquid hydrocarbons production. Biomass Bioenergy 2012, 45, 138−143. (43) Gayubo, A. G.; Aguayo, A. T.; Atutxa, A.; Aguado, R.; Olazar, M.; Bilbao, J. Transformation of oxygenate components of biomass pyrolysis oil on a HZSM-5 zeolite. II. Aldehydes, ketones, and acids. Ind. Eng. Chem. Res. 2004, 43, 2619−2626. (44) Gayubo, A. G.; Aguayo, A. T.; Atutxa, A.; Aguado, R.; Bilbao, J. Transformation of oxygenate components of biomass pyrolysis oil on a HZSM-5 zeolite. I. Alcohols and phenols. Ind. Eng. Chem. Res. 2004, 43, 2610−2618. (45) Hassan, H.; Lim, J. K.; Hameed, B. H. Recent progress on biomass co-pyrolysis conversion into high-quality bio-oil. Bioresour. Technol. 2016, 221, 645−655. (46) Xue, Y.; Zhou, S.; Bai, X. L. Role of Hydrogen Transfer during Catalytic Copyrolysis of Lignin and Tetralin over HZSM-5 and HY Zeolite Catalysts. ACS Sustainable Chem. Eng. 2016, 4, 4237−4250. (47) Olsbye, U.; Bjørgen, M.; Svelle, S.; Lillerud, K.-P.; Kolboe, S. Mechanistic insight into the methanol-to-hydrocarbons reaction. Catal. Today 2005, 106, 108−111. (48) Lukyanov, D. B.; Gnep, N. S.; Guisnet, M. R. Kinetic modeling of ethene and propene aromatization over HZSM-5 and GaHZSM-5. Ind. Eng. Chem. Res. 1994, 33, 223−234. (49) Choudhary, V. R.; Devadas, P.; Banerjee, S.; Kinage, A. K. Aromatization of dilute ethylene over Ga-modified ZSM-5 type zeolite catalysts. Microporous Mesoporous Mater. 2001, 47, 253−267. (50) Dejaifve, P.; Védrine, J. C.; Bolis, V.; Derouane, E. G. Reaction pathways for the conversion of methanol and olefins on H-ZSM-5 zeolite. J. Catal. 1980, 63, 331−345. (51) Pham, T. N.; Sooknoi, T.; Crossley, S. P.; Resasco, D. E. Ketonization of Carboxylic Acids: Mechanisms, Catalysts, and Implications for Biomass Conversion. ACS Catal. 2013, 3, 2456−2473. (52) Chantal, P. D.; Kaliaguine, S.; Grandmaison, J. L. Reactions of phenolic compounds over HZSM-5. Appl. Catal. 1985, 18, 133−145.

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Upgrading of Bio-Oil Aqueous Fraction by Dual-Stage Hydrotreating

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