Research Note pubs.acs.org/IECR
One-Pot Conversion of Bio-oil to Diesel- and Jet-Fuel-Range Hydrocarbons in Supercritical Cyclohexane Wen Shi,† Yahui Gao,† Shaodi Song,‡ and Yaping Zhao*,† †
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, China School of Chemistry and Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China
‡
S Supporting Information *
ABSTRACT: This study demonstrates a new route for converting bio-oil, prepared from the hydrothermal liquefaction of cornstalks, to diesel- and jet-fuel-range hydrocarbons over Ni/ZrO2 in supercritical cyclohexane. Under relatively mild conditions (573 K, 5 MPa H2), we obtained a high yield (81.6 C%) of hydrocarbons with an excellent quality (90% of diesel- and jet-fuelrange hydrocarbons and 7% of gasoline-range hydrocarbons). Ni/ZrO2 efficiently and stably catalyzed all types of compounds in the bio-oil to the corresponding alkanes via hydrogenation, dehydration, hydrogenolysis, decarbonylation, and isomerization, without polymerizations among the different reactive compounds in bio-oil. The activity and selectivity for diesel- and jet-fuelrange hydrocarbons of Ni/ZrO2 showed no obvious changes after three cycles. Ni/ZrO2 was fairly stable in supercritical cyclohexane after 72 h of reaction time. This strategy provides a novel high-efficiency pathway for the preparation of high-quality hydrocarbons from bio-oil.
1. INTRODUCTION The depletion of stored fossil fuels, as well as the warming global climate, have contributed to a desire to increase the use of sustainable and clean biomass-derived biofuels.1 Bio-oil, derived from the liquefaction or pyrolysis of biomass, is considered to be a promising second-generation energy-laden fuel.2 However, because of its high oxygen content, corrosion, high viscosity, and instability, bio-oil cannot be directly used currently as a transportation fuel.3−6 Thus, it is imperative to upgrade bio-oil to a nonoxygenated liquid fuel that possesses desirable properties for combustion. Hydrocracking and hydrodeoxygenation (HDO) are the two main approaches for upgrade of bio-oil to hydrocarbons. Hydrocracking has been extensively researched in the past decades, and it was typically performed at a temperature of >623 K on zeolite catalysts (such as ZSM-5, H-Y-zeolite, Hmordenite, silicalite, and silica−alumina).7,8 The low yield (17− 34 wt %) and poor quality (containing 20 wt % phenols) of the hydrocarbon product, as well as the high coking (8−25 wt %), are the most obvious shortcomings for hydrocracking.9−11 HDO, on the other hand, is considered to be one of most promising methods for bio-oil upgrading, because of the advantage that the required infrastructures are directly available in traditional petrochemical plants.12,13 Bio-oil was typically upgraded via a one-step or two-step HDO process with CoMoS2, NiMoS2, or noble-metal catalysts (such as Ru, Rh, and Pd).14−19 A one-step HDO method frequently obtains either low yields (26−38 wt %) or a poor quality (73−81% of the oxygen-removal ratio) of the hydrocarbon fuels.20,21 The two-step HDO method can upgrade bio-oil to deeply deoxidized fuels. However, this process also produces a relatively low hydrocarbon fuel yield (35−44 wt %) and requires a high partial pressure of hydrogen (13−18 MPa) to enhance the solubility of H2 in the reaction medium.22,23 The © 2014 American Chemical Society
solubility of H2 in the liquid phase extremely limits both the yield and quality of the hydrocarbon fuel. In addition, CoMo, NiMo sulfides, and noble-metal catalysts are undesirable, because of either the contamination of the hydrocarbon fuel or the high cost.24,25 Most recently, Ni/ZrO2 was found to be the best performing catalyst out of 23 selected different catalysts for the HDO of phenol to cyclohexane at 548 K, with 100 bar H2 and a water solvent.26 The low metal−oxygen bond energy of ZrO2 and the excellent activity of the Ni nanocluster result in the best balanced rates of both hydrogenation and deoxygenation. However, bio-oil that contains a complex chemical composition was not tested in their work, perhaps because the poor solubility of H2 in water limits the conversion rate of bio-oil, which results in deactivation of the catalyst, because of a serious carbon deposit produced from the polymerization of the reactive compounds.27 Here, we report a novel, stable, and highly efficient upgrading strategy via the combination of excellent catalytic activity of Ni/ZrO2 and the enhanced H2 solubility in supercritical cyclohexane. This new strategy converts bio-oil directly to diesel- and jet-fuel-range hydrocarbons (C8−C22) with a high yield under relatively mild reaction conditions (573 K, 5 MPa H2).
2. EXPERIMENTAL SECTION 2.1. Preparation of Bio-oil. Bio-oil was obtained from the hydrothermal liquefaction of the cornstalks using our reported method.28 The process can be succinctly described through three steps: Received: Revised: Accepted: Published: 11557
April 24, 2014 June 21, 2014 June 24, 2014 June 24, 2014 dx.doi.org/10.1021/ie501682r | Ind. Eng. Chem. Res. 2014, 53, 11557−11565
Industrial & Engineering Chemistry Research
Research Note
model compounds (4-ethyl-2-methoxy-phenol, 5-ethyl-2-furaldehyde, 2,3-dihydro-1H-inden-1-one, and palmitic acid) were carried out in a 10-mL stainless steel reactor (Nantong Huaxing Petroleum Instrument Co., Ltd., China). In a typical run, 0.05 g of bio-oil or 1 mmol of model compounds (if it is a mixture of model compounds, each component contains 0.5 mmol), 0.05 g of Ni/ZrO2 (10 wt %), and 4.0 mL of cyclohexane were loaded into the reactor. After displacing the air with H2, the reactor was pressurized to 5 MPa with H2 (99.999%) at ambient temperature and then heated to 300 °C (10 °C/min) for a desired time. After the reaction, the reactor was cooled immediately to an ambient temperature by tap water. The gas was collected using a gas sample bag in order to be analyzed using a gas chromatography (GC) system (Shimadzu, Model 14B) with a thermal conductivity detector (TCD) and two capillary columns (Plot Q and TDX-02). The liquid product was collected and analyzed using gas chromatography−mass spectrometry (GC-MS). For model compounds, an internal standard (2-isopropylphenol) was used to calculate the conversion, selectivity, and carbon balance. For bio-oil, the liquid product was concentrated and purified by a nitrogen blowing method at ambient temperature. Afterward, the mass yield of the liquid product was calculated, and then the carbon content was determined using an Elementar Vario EL III analyzer. 2.5. Analysis Method. Bio-oil and liquid products were analyzed by an Agilent Model 7890A/5975C system equipped with a HP-5MS column (5% Phenyl Methyl Siloxane, 30 m × 0.25 mm × 0.25 μm). The oven temperature was set programmatically: isothermal at 60 °C for 4 min, then a temperature increase at a rate of 5 °C/min to 300 °C and a hold for 8 min at the final temperature. An internal standard (2isopropylphenol) was used to determine the amount of the compounds, which was calculated by eq 1:
(1) The cornstalk powder underwent a process of ultrasonic pretreatment to improve the physicochemical properties of the cornstalks with the objective of enhancing the yields of bio-oil. The ultrasonic pretreatment was carried out in an ultrasound generation system. (2) The sonicated cornstalks slurry was directly loaded into the reactor (volume of 450 mL stainless steel material, Nantong Huaxing Petroleum Instrument Co., Ltd., China) and then liquefaction was executed at 573 K. (3) Liquid and solid products were separated by a flow path in Figure S1 in the Supporting Information (see details in the literature28). Liquid products obtained include heavy oil (HO, acetone-soluble) and water-soluble oil (WSO), with yields of ∼25 wt % and ∼27 wt %, respectively. In this paper, the term “bio-oil” refers to the fresh acetonesoluble HO, which is a viscous black-brown liquid containing a high oxygen content (26.79%). The properties and the chemical composition of the bio-oil are shown in Tables S1 and S2 in the Supporting Information. The bio-oil consists of very complex components and primarily consists of C6−C22 substituted phenols, ketones, aldehydes, and acids, such as 4ethylphenol, 4-ethyl-2-methoxyphenol, 2,6-dimethoxyphenol, 5-ethyl-2-furaldehyde, 2,3-dihydro-1H-inden-1-one, palmitic acid, etc. 2.2. Preparation of the Ni/ZrO2. ZrO2 support was synthesized according to the reported method.29,30 Ten grams (10 g) of ZrOCl2·8H2O was dissolved in 50 mL deionized water, and then the solution was heated and maintained at 100 °C with vigorous stirring. Ammonium hydroxide was slowly added dropwise into the solution and obtained a final pH of 9− 10. The white precipitate was filtered and washed with deionized water and absolute ethyl alcohol at least three times, and then it was dried at 90 °C for 12 h and calcined at 400 °C for 4 h to form ZrO2. The Ni/ZrO2 catalyst was prepared by the precipitation-impregnation method. Ni(NO3)2· 6H2O (2.33 g) was dissolved in H2O (4 mL) and then the solution was added dropwise onto ZrO 2 with stirring constantly. After metal incorporation for 2 h, the catalyst was dried in an oven at 110 °C overnight, and then it was calcined at 400 °C for 4 h and reduced in H2 atmosphere at 500 °C for 4 h. 2.3. Characterization of Catalyst. The content of metal elements in Ni/ZrO2 catalyst was determined using an inductively coupled plasma analyzer (Model ICAP 6000 Radial, Thermo). The BET surface area and pore volume of Ni/ZrO2 catalyst were determined on Model NOVA2200e surface area analyzer (Quantachrome, USA). The structure of Ni/ZrO2 catalyst was measured on a Model D/max-2200/PC X-ray diffractometer (XRD, Rigaku Co., Japan). The samples were irradiated at 10°−70° with a scan rate of 1°/min. Nickel dispersion was measured by H2 chemisorption on a Micromeritics ASAP 2010 unit. The procedure was applied as the following: evacuation and heating to 573 K in a helium flow for 1 h, reduction under H2 flow for 1 h at 573 K, and then cooling to 308 K for 1 h to determine the adsorption isotherm (including physisorption and chemisorptions). The following step is outgassing under He atmosphere at 308 K for 1 h, and then determination of the physisorption isotherm. Nickel dispersion was estimated by assuming adsorption stoichiometry of H/Ni = 1. 2.4. Hydrodeoxygenations of Bio-oil and Model Compounds. The HDO experiments for the bio-oil and
ni = n i.s. ×
Ai fi A i.s.
(1)
where ni is the number of moles of compound i, ni.s. is the number of moles of the internal standard, Ai is the peak area of compound i, Ai.s. is the peak area of the internal standard, and f i represents the ratios of response factors of compound i to the internal standard. A series of f i values (such as 5-ethyl-2furaldehyde, palmitic acid, 2,3-dihydro-1H-inden-1-one, ntetradecane, and ethylcyclohexane) were determined to calculate the conversion, selectivity, and carbon balance. 2.6. Conversion and Selectivity. The conversion of the model compounds (4-ethyl-2-methoxy-phenol, 5-ethyl-2-furaldehyde, 2,3-dihydro-1H-inden-1-one, and palmitic acid) is calculated by eq 2: conversion (%) =
amount of compound comsumed × 100 total amount of model compound (2)
The selectivity of the product is calculated based on the carbon mole basis, and it is defined as the ratio of the number of moles of C atom in each product to the total number of moles of C atom in products (see eq 3). selectivity (%) =
moles of carbon atom in each component total moles of carbon in products × 100
11558
(3)
dx.doi.org/10.1021/ie501682r | Ind. Eng. Chem. Res. 2014, 53, 11557−11565
Industrial & Engineering Chemistry Research
Research Note
2.7. Carbon Balance. For model compounds, the carbon balances were calculated based on the quantitative analysis results from the GC-MS and GC equipment. For liquid hydrocarbons upgraded from bio-oil, carbon balance was calculated base on the weight yield and elemental analysis (see eq 4). carbon balance (%) =
M p × Cp M f × Cf
× 100
Table 1. Comparison of the Hydrocarbons (HC) Produced from Bio-oil at Different Temperatures and Solventsa HC Content (area %)d
(4)
where Mp is the mass of the product (upgraded liquid oil), Cp the carbon content of the product, Mf the mass of the feedstock (bio-oil), and Cf the carbon content of the feedstock. Note that cyclohexane, as a solvent, was removed in the process of purification of liquid hydrocarbons, so the carbon balance of cyclohexane (derived from phenol, 2-methoxyphenol, and 2,6dimethoxyphenol in bio-oil) is estimated using the following steps: (1) Estimation of the peak area of cyclohexane produced (by the peak area of ethylcyclohexane derived from 4ethylphenol, 4-ethyl-2-methoxyphenol, and 2-ethylphenol in bio-oil). The peak area of cyclohexane is equal to the peak area of ethylcyclohexane multiplied by the sum of peak areas of phenol, 2-methoxyphenol, and 2,6dimethoxyphenol in bio-oil/the sum of peak areas of 4ethylphenol, 4-ethyl-2-methoxyphenol, and 2-ethylphenol in bio-oil. (2) Quantitative analysis of cyclohexane, with 2-isopropylphenol being selected as an internal standard. (3) Calculation of the carbon balance of cyclohexane, where the carbon balance of cyclohexane is equal to the amount of carbon of cyclohexane divided by total carbon content of the bio-oil. So, the carbon balance of liquid hydrocarbons (including cyclohexane produced) was calculated using eq 5. carbon balance (%) M p × Cp + 0.8571Mcyclohexane × 100 = M f × Cf
solvent
T (K)
yield of HC (C%)
cyclohexane cyclohexaneb cyclohexaneb cyclohexaneb cyclohexane water
573 573 573 573 533 573
81.6 81.0 80.2 80.0 53.3 48.6
c
C8−C22
C6−C7
C>22
90 91 90 90 44 73
7 6 7 7 7 6
3 3 3 3 2 4
a
Reaction conditions: bio-oil (0.05 g), solvent (4 mL), 10 wt % Ni/ ZrO2 (0.05 g), reaction time (4 h), and 5 MPa H2. bThe three runs were performed to test the recyclability of the Ni/ZrO2. After each run, the catalyst underwent the processes of recovery, calcination, and reduction. cYield of liquid hydrocarbons is calculated based on C. d Hydrocarbon content refers to the peak area percent of hydrocarbon in the cyclohexane phase (detected by GC-MS). For the cyclohexane solvent at 533 K, other compounds (including phenols, alcohols, acids, and ketones) account for 47% of the total peak area. For water as the solvent, phenols and acids account for 17% of the total peak area.
Table 2. Comparison of Properties of Bio-oil and Upgraded Oil Produced on Ni/ZrO2 at 573 K for 4 h in Supercritical Cyclohexane bio-oil appearance elemental content (wt %)a C H O N S HHV (MJ/kg)b water content (wt %)
upgraded oil
viscous black-brown liquid
yellowish transparent liquid
65.45 6.74 26.79 1.02 0 26.96
