HZSM-5 catalyst in supercritical methanol

Energy Conversion and Management 147 (2017) 19–28

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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Upgrading pyrolysis bio-oil to biofuel over bifunctional Co-Zn/HZSM-5 catalyst in supercritical methanol Shouyun Cheng a, Lin Wei a,⇑, James Julson a, Kasiviswanathan Muthukumarappan a, Parashu Ram Kharel b a b

Department of Agricultural & Biosystems Engineering, South Dakota State University, 1400 North Campus Drive, Brookings, SD 57007, USA Department of Physics, South Dakota State University, Brookings, SD 57007, USA

a r t i c l e

i n f o

Article history: Received 17 February 2017 Received in revised form 10 May 2017 Accepted 19 May 2017

Keywords: Biofuel Hydrodeoxygenation Co-Zn/HZSM-5 Supercritical methanol Pyrolysis Hydrocarbons

a b s t r a c t The role of catalyst is essential in processes of upgrading biomass pyrolysis bio-oil into hydrocarbon biofuel. While the majority of heterogeneous catalytic processes are conducted in the presence of gas (nearly ideal) or liquid phase, a growing number of processes are utilizing supercritical fluids (SCFs) as reaction media. Although hydrodeoxygenation (HDO) is proven a promising process for pyrolysis bio-oil upgrading to hydrocarbon biofuel, catalyst efficiency remains a challenge. Integrating heterogeneous catalysts with SCFs in a bio-oil HDO process was investigated in this study. Bifunctional Co-Zn/HZSM-5 catalysts were firstly used to upgrade bio-oil to biofuel in supercritical methanol. The loading of Co and Zn did not change HZSM-5 crystalline structure. Physicochemical properties of biofuel produced by Co and/or Zn loaded HZSM-5 catalysts such as water content, total acid number, viscosity and higher heating value improved. Bimetallic Co-Zn/HZSM-5 catalysts showed enhanced reactions of decarboxylation and decarbonylation that resulted in higher yields of CO and CO2. Bimetallic Co-Zn/HZSM-5 catalysts were more effective for bio-oil HDO than monometallic Co/HZSM-5 or Zn/HZSM-5 catalyst, which was attributed to the synergistic effect of Co and Zn on HZSM-5 support. Bimetallic Co-Zn/HZSM-5 catalysts increased biofuel yields and hydrocarbons contents in biofuels in comparison with monometallic Co/ HZSM-5 and Zn/HZSM-5 catalysts. 5%Co15%Zn/HZSM-5 catalyst generated the highest biofuel yield at 22.13 wt.%, and 15%Co5%Zn/HZSM-5 catalyst produced biofuel with the highest hydrocarbons content at 35.33%. Hydrogenation and esterification are two dominant reactions in bio-oil HDO over Co-Zn/ HZSM-5 catalysts in supercritical methanol. The energy efficiency of biofuel product was 30.99–58.80% for Co-Zn/HZSM-5 catalysts. Co-Zn/HZSM-5 is a promising catalyst to produce biofuel with high quality in bio-oil HDO. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Depleting fossil fuels are the predominant sources of energy supply in the world. However, fossil fuels result in increasing environmental concerns such as global warming and air pollution [1]. Therefore, there is a need to supplement long term energy needs through using renewable resources. Biomass is a renewable and abundantly available resource for biofuel production [2]. Fast pyrolysis is an attractive and prevalent method to convert lignocellulosic biomass to bio-oil, which is a liquid source for the production of transportation fuel [3]. The technique involves thermal decomposition or depolymerisation of large molecules of solid biomass to yield bio-oil, bio-char and gas products at a short residence time (<2 s), high temperature (around 500 °C) and oxygen-absence atmosphere [4]. Fast pyrolysis can be conducted in geographically localized ⇑ Corresponding author. E-mail address: [email protected] (L. Wei). http://dx.doi.org/10.1016/j.enconman.2017.05.044 0196-8904/Ó 2017 Elsevier Ltd. All rights reserved.

units, and it can obtain up to 75 wt.% of bio-oil yield in relatively simple units [5]. However, bio-oil has some adverse properties such as low heating value, high acidity, high water content and high instability derived from oxygenated compounds [6]. Therefore, upgrading bio-oil is necessary before bio-oil is applied as a transportation fuel. Hydrodeoxygenation (HDO) is an effective process for upgrading bio-oil, and it can obtain a biofuel product with enhanced quality [7]. Bio-oil HDO normally occurs with elimination of oxygen from a bio-oil as H2O, CO and CO over heterogeneous catalysts under high hydrogen partial pressure [8,9]. Traditional catalysts such as sulfide CoMo/c-Al2O3 and NiMo/c-Al2O3 catalysts are applied in HDO upgrading of bio-oil [10,11]. However, sulfide catalysts have disadvantages such as final products contamination due to dissolving of sulfur into reactants, water-evoked catalyst deactivation and coke accumulation [12]. Noble metal catalysts including Pd/C, Pt/C and Ru/C exhibit high activities during the HDO of bio-oil [13–17]. However, the high cost and scarcity of noble metals restrict their industrial application in a large scale.

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S. Cheng et al. / Energy Conversion and Management 147 (2017) 19–28

Non-noble transition metals are attracting great attention because of the low cost and high activity for bio-oil HDO. Cobalt is one of such preferred active metals which is a candidate metal for HDO in comparison with noble metals. Cobalt shows higher activity for hydrodeoxygenation reactions in HDO of bio-oil model compounds such as guaiacol, phenol and 2-ethylphenol [13,14]. Zinc is also a cost effective transition metal that exhibits excellent stability and catalytic activity for upgrading oxygenated compounds in vegetable oil to hydrocarbons [15,16]. Support materials with different chemical and physical properties are of great influence to the activities of catalysts. Acidic HZSM-5 zeolite is a thermal stable and effective support for metal loaded catalysts in bio-oil HDO [7,17]. HZSM-5 alone is also an efficient catalyst in bio-oil HDO that converts oxygenated compounds such as acids and aldehydes compounds [18]. The HDO activity of metal-doped HZSM-5 catalysts could be enhanced due to the synergetic effect of the acid sites of the supports and the metallic active phases [8,19]. This is due to that acid centers promote isomerization/ cracking reactions, and metallic centers improve hydrogenation of O-containing compounds over the bifunctional catalysts [20]. To our best knowledge, few studies use non-sulfide bifunctional Co-Zn/HZSM-5 catalysts for HDO of pyrolysis bio-oil. Supercritical fluids (SCFs) as solvents are effective for HDO upgrading of bio-oils [10,21]. SCFs have a number of unique characteristics such as high reactant solubility, high density (liquid-like) for facilitating fast intrinsic reactions and low viscosity (vaporlike) for reducing mass transfer limitations [22,23]. Bio-oil upgrading in supercritical fluids is not a new process. Peng et al. reports upgrading of bio-oil in supercritical ethanol using HZSM-5 catalyst [18]. The study indicates that supercritical ethanol effectively improves the bio-oil quality (acids content decreases to 3.78% from 17.64% in raw bio-oil). Li et al. upgrades bio-oil over PtNi/MgO in supercritical methanol process [24]. The results indicate that the heating value of the upgraded bio-oil increases from 20.4 MJ/kg to 31.7 MJ/kg. The upgraded bio-oil has higher contents of carbon (54.60 wt.%) and hydrogen (8.74 wt.%) than raw bio-oil (46.55 wt. % and 6.69%, respectively). Besides, the use of supercritical methanol solvent in bio-oil upgrading reduces coke deposition on PtNi/ MgO catalyst [24,25]. The contents of acids, ketones and aldehydes in upgraded bio-oil reduces significantly compared to raw bio-oil after upgrading on Pt/Al2(SiO3)3, Pt/C and Pt/MgO catalysts in supercritical methanol [26]. The upgraded bio-oil in supercritical fluid with improved quality is a potential substitute or partial substitute for the fossil transportation fuel in industries [24]. SCFs facilitate the reactants contact, and it promotes hydrogenation and hydrodeoxygenation reactions in bio-oil HDO process [10]. For instance, supercritical methanol promotes hydrodeoxygenation of unsaturated compounds in bio-oil HDO [22]. Therefore, supercritical methanol is used as solvent for bio-oil HDO upgrading in this study. The main goal of this work is to explore the effect of bifunctional Co-Zn/HZSM-5 catalysts on upgrading bio-oil to biofuel in supercritical methanol. This study determines catalytic performance of Co-Zn/HZSM-5 catalysts with different Co and/or Zn loading ratios on biofuel yield and properties (water content, total acid numbers (TAN), higher heating values (HHV), viscosity and chemical compositions).

condenser. The reactor was made of a stainless steel tube with a length of 600 mm and an inner diameter of 90 mm. The feeding rate of feedstock through the screw feeder into the reactor was 1.36 kg h
1, and the residence time of feedstock in the reactor was approximately 1 s. The temperature of the reactor and the condenser were 558 °C and
10 °C, respectively. The bio-oil yield is 49.3 wt.%. The higher heating value of bio-oil was 22.38 MJ kg
1. Similar yield and heating value of raw bio-oil are found in the study of Park and Chen et al. [27,28]. Cobalt nitrate hexahydrate (98 wt.%) was provided by SigmaAldrich. Zinc chloride (97 wt.%) was provided by Fisher Scientific. The HZSM-5 (silica/alumina ratio of 30) was provided by Zeolite International. Co-Zn/HZSM-5 catalysts with different Co and/or Zn mass loading ratios (20%Co/HZSM-5, 15%Co5%Zn/HZSM-5, 10%Co10%Zn/ HZSM-5, 5%Co15%Zn/HZSM-5, 20%Zn/HZSM-5) were prepared through wet impregnation method. Loading of metals on HZSM-5 support was performed by impregnating HZSM-5 in an aqueous cobalt nitrate hexahydrate solution and/or zinc chloride solution at room temperature (20 °C) for 3 h. The catalysts were dried in air at 120 °C for 10 h. The catalysts were then calcined in air at 550 °C for 4 h. The loading of metal was determined based on the assumption of 100% of the impregnated metal loaded on HZSM-5 support. However, in order to determine the real metal loading levels of Co-Zn/HZSM-5 catalysts, EDX (Energy Dispersive X-ray) spectrometer was used to determine the actual metal loading levels of the prepared catalysts.

2.2. Bio-oil HDO upgrading in supercritical methanol Bio-oil HDO experiments were performed in a Parr 4575 autoclave reactor (500 mL) equipped with an stirrer. An electric jacket heater was used to heat the autoclave reactor. 50.0 g bio-oil, 50.0 g methanol and 5.0 g fresh catalyst were loaded in the reactor for each test. In catalyst reusability/stability test, 5.0 g used 15% Co5%Zn/HZSM-5 catalyst (washed by ethanol and dried at 120 °C for 12 h), 50.0 g bio-oil and 50.0 g methanol were loaded in the reactor. Then, the reactor was purged with 100 psig H2 for 6 times to remove the inside air. Subsequently, the reactor was pressurized to 500 psig with H2 at room temperature. The reactor was heated to 300 °C at a heating rate of 5 °C min
1. The reaction temperature was maintained for 5 h at a stirring speed of 1000 rpm. The reaction conditions of 500 psig, 300 °C, 5 h, catalyst/bio-oil ratio and stirring speed for bio-oil HDO were determined according to out preliminary tests and referred to the studies carried out by Ahmadi and Chen et al. [10,29]. Finally, the reactor was rapidly cooled to room temperature by an electric fan for 0.5 h. The gas product was collected in gas sample bags. The liquid bio-oil product left in the reactor was poured out and separated into oil phase (biofuel) and aqueous phase via a separator funnel. The used catalysts with coke deposition were separated from oil phase and aqueous phase by filtration, washed with ethanol and dried at 110 °C for 12 h. The biofuel product was evaporated under reduced pressure at 50 °C to remove the methanol solvent [30]. The yield of products (Y product) including biofuel, aqueous phase and coke were determined based on Eq. (1) and gas yield (Y gas) was calculated following Eq. (2):

2. Materials and methods 2.1. Materials Bio-oil sample was produced from pine sawdust (PSD) pyrolysis at 558 °C using a proprietary pilot reactor in our lab. The reactor is consisted of a screw feeder, a fluidized-bed reactor and a

Y product ¼ M product=ðM bio
oil þ M methanolÞ  100%

ð1Þ

Y gas ¼ ð1
Y biofuel
Y aqueous phase
Y cokeÞ  100%

ð2Þ

where M product represents the mass of biofuel, aqueous phase and coke, respectively.

S. Cheng et al. / Energy Conversion and Management 147 (2017) 19–28

2.3. Catalyst and product characterizations XRD measurements of catalysts were conducted by X-ray Diffractometer (MiniFlex, Rigaku Corporation). 30 kV and 15 mA were used for the X-ray tube operation. The scan range (2 theta) of X-ray pattern was ranging from 5° to 90° using filtered Cu Ka radiation. The scan speed and step size were set as 2° min
1 and 0.02° (2 theta), respectively. The XRD was not used quantitatively in this study. Micrographs of catalysts were analyzed by a transmission electron microscope (JEOL JEM-2100 LaB6) operated at 200 kV. Catalyst were firstly dispersed by isopropyl alcohol. Then the solution was shaked in ultrasonic. The suspension droplets were then loaded and dried on a copper grid (carbon-coated) before test [31]. An Oxford Inca energy-dispersive silicon-drift X-ray (EDX) was used to analyze the elemental compositions of catalyst samples [32]. BET specific surface area and pore texture of fresh and used catalysts were analyzed by an automatic Micromeritics ASAP 2020 apparatus with nitrogen adsorption measurements operated at 77.2 K. Specific surface area was determined by Brunauer–Emmet t–Teller method. The total pore volume was determined at a relative pressure of P/P0 = 0.995. The adsorption average pore diameter was calculated by 4VT/SBET [33]. Acidity of fresh catalysts were determined by a Micrometrics Autochem II Chemisorption Analyzer with a thermal conductivity detector (TCD). The fresh catalyst sample (300 mg) was firstly added to ammonium hydroxide (37.1 wt.%, 4.5 g), and the mixture was kept at room temperature (20 °C) for 3 h. Then, the mixture was dried at 60 °C for 12 h. The dried sample was used for NH3TPD analysis. The helium flow in the chemisorption analyzer was 60 mL min
1. The catalyst sample was heated and maintained at 100 °C for 30 min to remove the physically absorbed ammonia. Then the sample temperature increased from 100 °C to 650 °C at a rate of 10 °C min
1. The final temperature of 650 °C was held for 30 min. During the period of heating from 100 °C to 650 °C, abundant dilute HCl solution (1.0 mol L
1) was used to collect the chemisorbed ammonia from the catalyst sample. Then, NaOH solution (0.1 mol L
1) was employed to titrate the HCl solution to determine the acid sites of the catalyst sample [34]. Water content of bio-oil and biofuel samples were determined by Karl Fischer Titrator V20 (Mettler Toledo) following ASTM E1064 standard [35]. Total acid number was analyzed by an Aquamax Titrator (G.R. Scientific) following ASTM D664 standard. High heating value was measured by a bomb calorimeter (C2000, IKAWorks) following ASTM D4809 standard. Dynamic viscosity was tested by a viscosity analyzer (REOLOGICA Company) at 20 °C [36]. Gas chromatography (AgilentGC-7890A with a DB-5 column: 30 m  0.25 lm  0.25 mm) and mass spectrometry (MSD-5975C with electron ionization of 70 eV, mass range at 50–500 m z
1) were used to determine chemical compositions of bio-oil and biofuel samples [15]. The samples were firstly dissolved in methanol before analysis. Helium was employed as carrier gas at a flow rate of 1 mL min
1. The injection temperature and injection volume were 300 °C and 1 lL respectively. The oven temperature was firstly set at 60 °C and then increased following ramp 1 at 3 °C min
1 to 140 °C, ramp 2 at 10 °C min
1 to 180 °C, ramp 3 at 3 °C min
1 to 260 °C and ramp 4 at 10 °C min
1 to 300 °C. The chemical compositions of bio-oil and biofuel in this study were identified based on National Institute of Standards and Technology (NIST) mass spectral library and closely related literatures [36–43], since many similar studies use NIST Mass Spectral library and related literatures to determine compounds present in bio-oil [36,37,39,41]. The relative contents of compounds in the samples was calculated by the ratio of its peak area to total peak area appeared in the GC–MS spectrogram [35].

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Gas compositions were analyzed by Agilent 7890A GC system (19095P-S25 column: 50 m  15 lm  0.53 mm) [44]. H2, CO2 and CO were identified by thermal conductivity detector (TCD). Light hydrocarbons (C1–5) were analyzed by flame ionization detector (FID). The carrier gas used in GC was argon. GC calibration was conducted through using standard gas mixtures [45]. 3. Results and discussions 3.1. XRD characterization XRD spectra of HZSM-5 based catalysts are displayed in Fig.1. In XRD patterns of all HZSM-5 based catalysts, diffraction peak positions of the HZSM-5 structure at 7.9°, 8.8°, 23.0°, 23.2°, 23.6°, 23.9° and 24.4° were determined [46]. This indicated that the framework of HZSM-5 maintained after loading Co and/or Zn. The diffraction peaks of Co3O4 particles at 36.8°, 55.7°, 59.6° and 65.2° were identified in Co loaded HZSM-5 catalysts, and this is in accordance with JCPDS 80-1541. Similar peak positions of Co3O4 were indicated by Liu et al. [47]. 20%Zn/HZSM-5 showed ZnO phase with peaks at 31.7°, 34.3°, 36.2° and 62.8°, and these peaks are consistent with JCPDS 36-1451. Cheng et al. also identified similar peak positions of ZnO [48]. No diffraction peaks of ZnO species were observed on 15%Co5%Zn/HZSM-5, 10%Co10%Zn/HZSM-5 and 5%Co15%Zn/ HZSM-5 catalysts. This indicated that ZnO might be highly dispersed on the surface of HZSM-5 support in these catalysts. 3.2. BET characterization The textural properties of fresh Co-Zn/HZSM-5 catalysts are listed in Table 1. The BET specific surface area and pore volumes of Co and/or Zn loaded HZSM-5 catalysts decreased significantly compared to HZSM-5. This was due to that parts of the HZSM-5 zeolite pores and external surface of the parent HZSM-5 zeolite were filled up with Co3O4 and/or ZnO particles after the Co and/ or Zn loading [49]. The average pore size of Co and/or Zn loaded HZSM-5 catalysts were lower than that of HZSM-5 support. This was due to the deposition of Co and/or Zn metal oxides in the inside pores and channels of HZSM-5 [15]. The textural properties of used Co-Zn/HZSM-5 catalysts are shown in Table 2. The BET specific surface area, average pore size and pore volumes of Co and/or Zn loaded HZSM-5 catalysts decreased compared to fresh catalysts. This was due to the coke

Fig. 1. XRD spectra of fresh catalysts (a) HZSM-5, (b) 20%Co/HZSM-5, (c) 15%Co5% Zn/HZSM-5, (d) 10%Co10%Zn/HZSM-5, (e) 5%Co15%Zn/HZSM-5, (f) 20%Zn/HZSM-5.

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Table 1 Textural properties of fresh Co-Zn/HZSM-5 catalysts. Catalyst

SBET (m2/g)

Average pore size (nm)

HZSM-5 20%Co/HZSM-5 15%Co5%Zn/HZSM-5 10%Co10%Zn/HZSM-5 5%Co15%Zn/HZSM-5 20%Zn/HZSM-5

416.98 276.62 259.75 261.24 256.29 271.96

3.82 2.32 2.37 2.28 2.25 2.31

Pore volume (cm3/g) Total

Micropores

Mesopores

0.29 0.16 0.17 0.19 0.14 0.17

0.08 0.06 0.04 0.05 0.03 0.04

0.21 0.10 0.13 0.14 0.11 0.13

deposition on pores and channels of the used catalysts. Similar results are found by Wei et al. [50].

3.3. NH3-TPD characterization The NH3-TPD spectra of fresh Co-Zn/HZSM-5 catalysts are shown in Fig. 2. Two desorption peaks are observed for Co-Zn/ HZSM-5 catalysts at around 100–350 °C and 350–550 °C, representing desorption of ammonia from weak acid sites (Lewis acid sites) and strong acid sites (Brönsted acid sites), respectively [18,34]. The acidity of fresh Co-Zn/HZSM-5 catalysts are listed in Table 3. The total acidity of Co/HZSM-5 catalysts (20%Co/ HZSM-5 and 15%Co5%Zn/HZSM-5) decreased compared to HZSM5, which is consistent with the study of Wang et al. [51]. However, the total acidity of Zn/HZSM-5 catalysts increased with the loading of Zn, and this indicated that the loading of Zn2+ on HZSM-5 provided additional acid sites [34]. Fig. 2. NH3-TPD results of fresh Co-Zn/HZSM-5 catalysts (a) HZSM-5, (b) 20%Co/ HZSM-5, (c) 15%Co5%Zn/HZSM-5, (d) 10%Co10%Zn/HZSM-5, (e) 5%Co15%Zn/HZSM5, (f) 20%Zn/HZSM-5.

3.4. TEM and EDS characterizations TEM images of HZSM-5 based catalysts are displayed in Fig. 3. There were no obvious dark spots shown in HZSM-5, since there was no Co3O4 and/or ZnO loading on it. Some dark spots were observed in 20%Co/HZSM-5 catalyst, and this might be due to the existence of Co3O4 particles. Similar dark spots of Co3O4 in the TEM images were found by Zhao et al. [52]. Some dark spots were detected in the 20%Zn/HZSM-5 catalyst, which might be attributed to ZnO particles. Triwahyono et al. detected similar dark spots of ZnO loaded on HZSM-5 catalyst [53]. Dark spots were identified in TEM images of 15%Co5%Zn/HZSM-5, 10%Co10%Zn/HZSM-5 and 5%Co15%Zn/HZSM-5 catalysts. These dark spots might be attributed to the presence of Co3O4 and/or ZnO species. The actual metal contents of fresh Co-Zn/HZSM-5 catalysts are shown in Table 4. The Co and/or Zn elements were detected in Co and/or Zn loaded HZSM-5 catalysts. Co and/or Zn metal contents of Co-Zn/HZSM-5 catalysts were a bit lower than calculated metal contents. This difference might be due to the non-uniform distribution of the metals on the HZSM-5 support [54]. Similar results are found by Thangalazhy-Gopakumar et al. [55].

Table 3 Acidity of fresh Co-Zn/HZSM-5 catalysts. Catalysts

Acidity (mmolNH3/g cat)

HZSM-5 20%Co/HZSM-5 15%Co5%Zn/HZSM-5 10%Co10%Zn/HZSM-5 5%Co15%Zn/HZSM-5 20%Zn/HZSM-5

Weak acid (peak position 100–350 °C)

Strong acid (peak position 350–550 °C)

Total acidity

0.12 0.08 0.09 0.10 0.31 0.47

0.47 0.26 0.16 0.40 0.45 0.31

0.59 0.34 0.25 0.50 0.76 0.78

3.5. Products yields Bio-oil HDO products included gas, liquid (bio-oil) and solid fraction (coke) that deposited on the catalysts. The bio-oil liquid product can be separated into a bottom oil phase (biofuel) and a top aqueous phase. The yields of biofuel, aqueous phase (AP), gas,

Table 2 Structural properties of used Co-Zn/HZSM-5 catalysts. Catalyst

SBET (m2/g)

Average pore size (nm)

HZSM-5 20%Co/HZSM-5 15%Co5%Zn/HZSM-5 10%Co10%Zn/HZSM-5 5%Co15%Zn/HZSM-5 20%Zn/HZSM-5

329.54 203.24 195.12 192.03 207.21 213.23

2.61 2.13 1.94 2.01 1.95 2.20

Pore volume (cm3/g) Total

Micropores

Mesopores

0.26 0.13 0.11 0.14 0.10 0.12

0.07 0.03 0.02 0.03 0.01 0.04

0.19 0.10 0.09 0.11 0.09 0.08

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Fig. 3. TEM images of fresh catalysts (a) HZSM-5, (b) 20%Co/HZSM-5, (c) 15%Co5%Zn/HZSM-5, (d) 10%Co10%Zn/HZSM-5, (e) 5%Co15%Zn/HZSM-5, (f) 20%Zn/HZSM-5.

Table 4 Actual metal contents of fresh Co-Zn/HZSM-5 catalysts. Catalysts

HZSM-5

20%Co/HZSM-5

15%Co5%Zn/HZSM-5

10%Co10%Zn/HZSM-5

5%Co15%Zn /HZSM-5

Co content (wt.%) Zn content (wt.%)

0 0

18.79 0

14.23 4.62

9.75 9.64

4.34 14.71

coke, and methanol consumption (MC) ratio for different catalysts are shown in Fig. 4. The methanol participated in bio-oil upgrading reactions such as esterification and alkylation. The methanol consumption ratio of Co-Zn/HZSM-5 was 21.03–31.15 wt.%. Co and/ or Zn loaded HZSM-5 catalysts improved biofuel yields in comparison with HZSM-5. The promoted hydrodeoxygenation, esterification, hydrogenation, decarboxylation and decarbonylation reactions over Co and/or Zn loaded HZSM-5 catalysts converted more oxygenated compounds to hydrophobic compounds such as hydrocarbons and esters present in biofuel [17,18]. Compared to monometallic Co/HZSM-5 and Zn/HZSM-5 catalysts, bimetallic Co-Zn/HZSM-5 catalysts increased biofuel yields. The synergistic effect of Co and Zn metal active sites might improve HDO performance of bimetallic Co-Zn/HZSM-5 catalysts [56,57]. 5%Co15%Zn/ HZSM-5 catalyst generated the highest biofuel yield at 22.13 wt. %. Coke deposition was responsible for blocking catalysts active sites, preventing active sites access to reactants and deactivating catalysts. The HZSM-5 catalysts can be deactivated easily by coke that is caused by polymerization over zeolites in bio-oil upgrading [58]. However, the coke yield of HZSM-5 catalysts decreased gradually with the loading of Co or Zn, indicating that the stability and

durability against catalyst deactivation was enhanced by the fewer Brönsted acid sites caused by the replacement of H+ with Co or Zn species [59,60]. Therefore, it can be deduced that the incorporation of Co and/or Zn on HZSM-5 might inhibit polymerization reactions that was responsible for coke formation [8]. The coke yield of bimetallic Co-Zn/HZSM-5 catalyst was lower than monometallic Co/HZSM-5 and Zn/HZSM-5 catalysts. Similar results of low coke yield for bimetallic catalysts in HDO were found by Huynh et al. [61]. The biofuel yield of used 15%Co5%Zn/HZSM5 catalyst was lower than fresh 15%Co5%Zn/HZSM-5 catalyst, and this might be due to the higher coke deposition that deactivated the used catalyst quickly. 3.6. Physicochemical properties of biofuels Physicochemical properties of biofuels produced by different catalysts and raw bio-oil are shown in Table 5. The first part of the data in Table 5 was the average value of three repeated measurement results. The second part of the data was the standard deviation of these three measurement results. Water content of biofuel products (0.28–1.09 g) reduced significantly in comparison

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Fig. 4. Product yields and methanol consumption ratio of different treatments.

with raw bio-oil (7.25 g). The demoisturization or dehydration properties of biofuels that contained higher amount of hydrophobic compounds including fatty acid esters and hydrocarbons contributed to the decease of water contents in biofuels [22,62]. The water content of biofuel produced by Co and/or Zn loaded HZSM5 catalysts was lower than that of biofuel produced by HZSM-5. This was due to the enhanced activity of Co and/or Zn loaded HZSM-5 catalysts that produces higher total amount of hydrophobic compounds such as esters and hydrocarbons in biofuels through esterification, hydrogenation and hydrodeoxygenation reactions in supercritical methanol. Low water content was helpful to improve the ignition and higher heating value of biofuels. Acidic compounds were responsible for the high acidity and strong corrosiveness of biofuel products for pipelines and storage tanks. Compared to raw bio-oil, TAN values of biofuels decreased significantly. The conversion of some acidic compounds into other compounds such as esters and hydrocarbons resulted in this decrease. The oxygenated compounds including organic acids and other

acidic components such as phenolic compounds are the main factors causing acidity of bio-oils (TAN value) [63,64]. The total amount of acids and phenols of biofuels produced by bifunctional Co-Zn/HZSM-5 catalysts (especially 15%Co5%Zn/HZSM-5) was higher than 20%Co/HZSM-5, 20%Zn/HZSM-5 and HZSM-5 catalysts. This led to the higher TAN values of biofuels produced by bifunctional Co-Zn/HZSM-5 catalysts such as 15%Co5%Zn/HZSM-5. Similar TAN value of bio-oil is found by Oasmaa et al. [64]. High viscosity of biofuel leads to problems in transportation and injection into an engine [65]. The viscosity of biofuel decreased after bio-oil HDO over HZSM-5 based catalysts. Macromolecule compounds led to the higher viscosity of raw bio-oil. After bio-oil HDO, some macromolecule compounds were converted to smaller molecule compounds through hydrocracking, cracking, decarbonylation and dehydration reactions [66]. The methanol content in biofuel product was ranging from 4.68 to 5.79 wt.%. The higher heating value of biofuel deducting methanol was 23.05–31.98 MJ/kg, and it improved in comparison with

Table 5 Physicochemical properties of different biofuels and raw bio-oil. Treatments

Raw bio-oil

HZSM-5

20%Co / HZSM-5

15%Co5%Zn / HZSM-5

10%Co10%Zn / HZSM-5

5%Co15%Zn / HZSM-5

20%Zn / HZSM-5

Used 15%Co5%Zn / HZSM-5

Water content (wt.%) TAN (mg KOH g
1) Viscosity (Pa s) Water mass in biofuel (g) Methanol content in biofuel (wt.%) HHV deducting methanol (MJ kg-1) Energy efficiency of biofuel (%) Energy recovery of all products (%)

14.50 ± 0.03 320.03 ± 35.96 4.52 ± 0.05 7.25 ± 0.02 0.00 ± 0.00

13.68 ± 0.11 100.40 ± 0.30 0.46 ± 0.06 1.09 ± 0.01 4.91 ± 0.18

3.61 ± 0.32 108.90 ± 19.17 0.16 ± 0.00 0.28 ± 0.02 4.82 ± 0.25

3.77 ± 0.78 140.60 ± 4.50 0.63 ± 0.01 0.31 ± 0.01 5.13 ± 0.13

3.71 ± 0.99 113.75 ± 3.75 1.34 ± 0.03 0.30 ± 0.03 5.79 ± 0.21

9.82 ± 0.51 113.90 ± 6.10 0.97 ± 0.01 1.04 ± 0.01 5.43 ± 0.32

13.26 ± 0.05 72.45 ± 0.25 0.80 ± 0.08 0.39 ± 0.02 4.68 ± 0.15

4.29 ± 0.56 109.51 ± 0.31 0.78 ± 0.13 0.32 ± 0.06 5.21 ± 0.27

22.38 ± 0.23

23.05 ± 0.38

29.74 ± 0.26

30.70 ± 0.51

31.98 ± 0.29

29.73 ± 0.37

23.57 ± 0.48

28.67 ± 0.69

–

30.99 ± 0.79

40.93 ± 1.04

45.16 ± 2.10

45.73 ± 0.76

58.80 ± 0.39

32.69 ± 1.40

38.81 ± 1.39

–

89.17 ± 0.98

92.13 ± 1.31

94.81 ± 2.29

95.27 ± 1.34

96.45 ± 1.58

91.25 ± 1.32

92.24 ± 2.08

25

S. Cheng et al. / Energy Conversion and Management 147 (2017) 19–28

raw bio-oil (22.38 MJ/kg) after bio-oil HDO over Co-Zn/HZSM-5 catalysts. The increase of biofuel heating values was due to the lower contents of water and oxygenated compounds contained in biofuel. The combustion efficiency of biofuel as an engine fuel can be improved due to the increase of heating values. The energy efficiency of biofuel product (Table 5) was 30.99– 58.80% for different Co-Zn/HZSM-5 catalysts. The energy efficiency of biofuels produced by Co and/or Zn loaded HZSM-5 catalysts were higher than that of HZSM-5 due to the higher yield and heating value of biofuel products. The energy recovery of bio-oil upgrading process on Co-Zn/HZSM-5 catalysts (Table 5) ranged from 89.17% to 96.45%. 3.7. Chemical compositions of biofuel Thermochemical degradation of lignin, cellulose and hemicellulose in biomass pyrolysis produced bio-oil that was composed of different categories of chemical compounds including phenols, aldehydes, ketones, esters, alcohols, acids, phenols, furans and hydrocarbons. Bio-oil HDO using different catalysts have a significant effect on the change of chemical components in biofuel products. GC–MS was employed to determine effect of different HZSM5 based catalysts on chemical compositions of biofuel products derived from bio-oil HDO. The main groups of chemical compounds detected in biofuels and bio-oil are listed in Table 6. The stacked GC–MS chromatograms for the biofuels on top of each other and relative differences in specific compounds identified in a per chromatogram area difference were shown Figs. S1–S8 and Tables S1–S8 in supporting information. The standard deviations for the contents of the chemical compounds present in biofuels and bio-oil were used to show the amount of error in Table 6. GC–MS could not identify all compounds precisely due to the limited ability of GC–MS and the chemical complexity of bio-oil [67]. However, the chromatographic peak area of a compound is considered linear with its quantity, and the peak area is linear with its content [38]. During bio-oil HDO experiment, the mass of raw bio-oil in each test was the same. Therefore, the corresponding chromatographic peak area of the compounds such as acids obtained from different treatments can be compared to reveal the changing of its yields, and the peak area can be compared to show the changing of its relative content among the detected compounds [38]. Raw bio-oil contained mainly oxygenated compounds including ketones (27.99%), acids (16.54%), phenols (13.29%), alcohols (6.13%) and aldehydes (5.69%). These compounds resulted in the negative properties of bio-oil including instability and corrosiveness. Acids was mainly responsible for the bio-oil corrosiveness. The existence of aldehydes and ketones resulted in bio-oil storage instability. The contents of desirable esters and hydrocarbons in raw bio-oil were only 18.25% and 11.02%, respectively. Therefore, bio-oil upgrading is required to convert undesirable oxygenated compounds to desirable esters and hydrocarbons.

Chemical compositions of biofuels changed significantly after bio-oil HDO over different HZSM-5 catalysts. Some acidic compounds were probably converted to esters through esterification reactions with alcohols in the supercritical fluid conditions that resulted in the increase of esters contents in biofuels [68]. Decarboxylation, hydrogenation and cracking reactions might also transform some acidic compounds into hydrocarbons [69]. This resulted in the decreased acids content in biofuels produced by catalytic treatments. The decreased content of acid compounds contributed to the relatively lower TAN values of biofuel products compared to raw bio-oil. The contents of ketones and aldehydes in biofuels decreased over the used HZSM-5 catalysts. Ketones might be transformed to hydrocarbons through decarbonylation and decarboxylation reactions [70]. Aldehydes were probably converted to phenols through hydrogenation and decarbonylation reactions [71,72]. The decreased contents of these oxygenated compounds can improve the stability and quality of biofuel products. The content of desirable hydrocarbons in biofuel improved after bio-oil HDO over HZSM-5 based catalysts. Bio-oil HDO reactions including cracking, decarbonylation, decarboxylation, hydrocracking, dehydration and hydrogenation transformed oxygenated organic compounds to hydrocarbons over the used HZSM-5 catalysts [73,74]. Compared to HZSM-5 catalyst, Co and/or Zn modified HZSM-5 catalysts increased hydrocarbons content in biofuel. The hydrodeoxygenation activities induced by Co and/or Zn metal active sites resulted in this increase. The bimetallic Co-Zn/HZSM5 produced biofuel with higher hydrocarbons content than monometallic Co/HZSM-5 or Zn/HZSM-5. This might be due to the synergistic effect and well dispersion of Co and Zn metal active sites that promoted catalytic performance of Co-Zn/HZSM-5 catalysts [61]. 15%Co5%Zn/HZSM-5 catalyst produced biofuel with the highest hydrocarbons content at 35.33%. The hydrocarbons content of biofuel produced by used 15%Co5%Zn/HZSM-5 was lower than fresh 15%Co5%Zn/HZSM-5, and this indicated the reduced catalyst activity for bio-oil HDO reactions in the used catalyst. 3.8. Gas distributions The compositions of gas product in bio-oil HDO is shown in Fig. 5. The main gas collected was hydrogen, since there was a great excess of hydrogen that was added to the reactor to maintain a high partial pressure for bio-oil HDO. The decarboxylation and decarbonylation reactions of oxygenated compounds such as acids, ketones and aldehydes on HZSM-5 based catalysts formed CO2 and CO that was detected in gas products [68,69]. The mass amount of CO2 and CO in gas products is shown in Table 7. Total amount of CO2 and CO produced from fresh Co-Zn/HZSM-5 catalysts was higher than Co/HZSM-5, Zn/HZSM-5 or HZSM-5 catalysts. This indicated the promoted activity of bimetallic Co-Zn/HZSM-5 catalysts, and the bimetallic catalysts enhanced reactions of decarboxylation that produced CO2 respectively. More oxygen and

Table 6 Chemical compositions of different biofuels and raw bio-oil. Relative content (%)

Raw bio-oil

HZSM-5

20%Co / HZSM-5

15%Co5%Zn / HZSM-5

10%Co10%Zn / HZSM-5

5%Co15%Zn / HZSM-5

20%Zn / HZSM-5

Used 15%Co5%Zn / HZSM-5

Phenols Ethers Aldehydes Ketones Esters Alcohols Acids Furans Hydrocarbons

13.29 ± 0.15 0.00 ± 0.00 5.69 ± 0.07 27.99 ± 0.12 18.25 ± 0.89 6.13 ± 0.21 16.54 ± 0.13 1.09 ± 0.07 11.02 ± 0.43

18.31 ± 0.79 0.15 ± 0.02 1.96 ± 0.08 12.92 ± 0.31 37.46 ± 0.42 6.50 ± 0.35 7.84 ± 0.14 0.00 ± 0.00 14.86 ± 0.53

15.04 ± 0.61 0.00 ± 0.00 2.50 ± 0.31 12.25 ± 0.54 35.58 ± 1.35 8.98 ± 0.28 0.32 ± 0.07 0.00 ± 0.00 25.33 ± 1.14

26.60 ± 0.92 0.00 ± 0.00 0.00 ± 0.00 2.95 ± 0.51 33.08 ± 1.26 1.21 ± 0.16 0.83 ± 0.03 0.00 ± 0.00 35.33 ± 1.57

26.67 ± 1.92 0.00 ± 0.00 1.55 ± 0.54 4.82 ± 0.07 35.27 ± 0.57 0.78 ± 0.12 0.99 ± 0.04 0.00 ± 0.00 29.92 ± 1.15

18.89 ± 1.27 0.00 ± 0.00 1.56 ± 0.32 11.58 ± 0.04 35.71 ± 1.21 0.92 ± 0.03 2.01 ± 0.15 0.00 ± 0.00 29.33 ± 1.20

20.73 ± 1.34 0.00 ± 0.00 1.23 ± 0.21 3.62 ± 0.08 36.60 ± 1.13 7.49 ± 0.23 1.11 ± 0.27 1.56 ± 0.62 27.66 ± 1.84

17.53 ± 0.13 0.00 ± 0.00 1.03 ± 0.00 4.98 ± 0.23 30.82 ± 0.97 4.82 ± 0.27 1.56 ± 0.08 0.00 ± 0.00 18.71 ± 0.84

26

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Fig. 5. Gas distributions of different treatments.

carbon were produced in the gas products generated by bimetallic Co-Zn/HZSM-5 catalysts (Table 7). This further indicated that the enhanced bio-oil deoxygenation reactions of bimetallic Co-Zn/ HZSM-5 catalysts that removed more oxygen and carbon in biofuels. However, the content of CO is much lower than CO2 in all treatments. Similar results are found by Dang et al. [23]. This indicated that decarboxylation is the main reaction pathway rather than decarbonylation [22,75]. Cracking and hydrocracking of organic compounds in bio-oil HDO generated light hydrocarbons such as CH4 and C2-5 hydrocarbons [71].

3.9. Synergistic effect of Co and Zn on HZSM-5 support The loading of Zn on HZSM-5 catalyst catalyzed the dissociation of the C–H bond and promoted dehydrogenation and aromatization reactions [76,77]. The Zn/HZSM-5 also promoted oligomerization, dehydration and hydrogenation reactions in bio-oil upgrading process [57,78]. This was due to the large amounts of Lewis acid sites in Zn/HZSM-5, which could facilitate the deoxygenation of reactants [79,80]. In addition to serving as an catalyst, the loading of Zn on HZSM-5 could maintain the catalyst functionality by preventing coke formation over protons [57]. The loading of Co promoted hydrogenolysis and hydrogenation reactions in bio-oil HDO [14,81]. The presence of Co also enhanced the direct CAO bond scission activity, and this promoted hydrodeoxygenation reactions of phenolic compounds [82].

Metallic Co and Zn reduced from their metal oxides are the catalytic active centers for catalytic hydrogenation reactions on CoZn/HZSM-5 catalysts. There are three factors which resulted in the improved catalytic activity of bimetallic Co-Zn/HZSM-5 (synergistic effect of Ni and Zn) than monometallic Co/HZSM-5 or Zn/ HZSM-5 catalyst. Firstly, the incorporation of Zn enhanced the interaction between Co and HZSM-5 and improved the Zn dispersion [83]. Secondly, bimetallic catalysts had higher stability and activity than monometallic catalyst [84]. Finally, the loading of second metal prevented excessive carbon deposition on metal active sites of bimetallic catalysts, and this led to lower coke formation of bimetallic catalyst than monometallic catalyst [85]. Supercritical fluid exhibits better ability of dissolving pyrolytic lignin and resulted in reduced polymerization reactions [86]. The supercritical methanol can protect the catalyst and prevent deactivation, which inhibits the coke formation to some extent [24,86]. This resulted in lower coke yield of catalysts used in bio-oil upgrading in supercritical methanol. However, adding catalysts was more active in reducing coke formation compared to supercritical methanol solvent according to the study of upgrading bio-oil in supercritical methanol with and without catalyst conducted by Li et al. [24].

3.10. Reasons for bio-oil quality improvement Biofuels production from bio-oil HDO test in supercritical methanol at 250 °C under long contact time and below supercriti-

Table 7 CO2 and CO contents of different gases produced by Co-Zn/HZSM-5 catalysts. Catalyst

HZSM-5

20%Co /HZSM-5

15%Co5%Zn /HZSM-5

10%Co10%Zn /HZSM-5

5%Co15%Zn /HZSM-5

20%Zn /HZSM-5

Used 15%Co5%Zn /HZSM-5

CO2 (g) CO (g) CO2 + CO (g) Total O (g) Total C (g)

12.47 ± 0.04 1.49 ± 0.02 13.96 ± 0.01 9.92 ± 0.02 5.68 ± 0.02

12.64 ± 0.04 2.22 ± 0.02 14.86 ± 0.03 10.47 ± 0.03 5.85 ± 0.02

13.97 ± 0.07 1.68 ± 0.05 15.65 ± 0.02 11.12 ± 0.04 5.97 ± 0.01

17.43 ± 0.13 1.69 ± 0.11 19.12 ± 0.03 13.64 ± 0.11 6.81 ± 0.02

14.30 ± 0.06 1.67 ± 0.03 15.97 ± 0.04 11.35 ± 0.05 5.94 ± 0.01

12.57 ± 0.03 1.62 ± 0.05 14.19 ± 0.02 10.07 ± 0.02 5.75 ± 0.04

13.51 ± 0.02 0.34 ± 0.02 13.85 ± 0.04 10.02 ± 0.03 3.83 ± 0.05

S. Cheng et al. / Energy Conversion and Management 147 (2017) 19–28

cal fluid conditions were conducted by Li et al. [26], which is similar to this study. The results indicated that the contents of acids, ketones and aldehydes decreased in the long contact time (6– 9 h) due to esterification and rearrangement reactions. However, adding catalysts could further reduce the contents of ketones and aldehydes through acetalization, etherification, rearrangement, estification and hydrogenation reactions [26]. In subcritical ethanol upgrading processes, HZSM-5 catalyst facilitates cracking reaction of some heavy components of bio-oil that improved bio-oil quality to some content [18]. However, supercritical ethanol upgrading process worked more effectively than subcritical upgrading processes in removing the heavy components in biooil [18]. Therefore, the bio-oil quality improvement was partly due to the long contact time of methanol solvent and bio-oil. However, the supercritical methanol upgrading process over HZSM-5 based catalysts further promoted in the improvement of upgraded bio-oil quality significantly.

[5]

[6]

[7]

[8]

[9]

[10]

[11] [12]

3.11. Main bio-oil upgrading reactions in supercritical methanol [13]

Under experimental conditions with additional methanol as the supercritical medium, hydrogenation and esterification are two dominant reactions in bio-oil HDO process [26]. The two reactions promoted the formation of hydrocarbons and esters in biofuel products. Besides, a series of other transformation reactions such as acetalization, etherification and rearrangement also converted ketones and aldehydes. Similar reactions in bio-oil upgrading in supercritical alchohol are found by Xu et al. [86].

[14]

[15]

[16] [17] [18]

4. Conclusions [19]

Bio-oil HDO upgrading on Co and/or Zn loaded HZSM-5 catalysts were conducted in supercritical methanol solvent. The synergistic effect of Co and Zn on HZSM-5 support improved the activity of bimetallic Co-Zn/HZSM-5 catalysts. Hydrogenation and esterification are two main reactions in bio-oil HDO in supercritical methanol. The integration of bimetallic Co-Zn/HZSM-5 catalysts and supercritical methanol reduced coke yield significantly. Although the bio-oil quality improvement was partly due to the long contact time of liquid methanol solvent and bio-oil, the supercritical methanol upgrading process over HZSM-5 based catalysts further improved biofuel quality significantly.

[20]

[21]

[22]

[23] [24]

Acknowledgements

[25]

This work was supported by USDA NIFA and DOT (Award No. SA0700149) through the North Central Sun Grant Initiative.

[26]

[27]

Appendix A. Supplementary material [28]

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.enconman.2017. 05.044.

[29]

[30]

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