Catalytic Conversion of Biomass by Natural Gas for Oil Quality

Article pubs.acs.org/IECR

Catalytic Conversion of Biomass by Natural Gas for Oil Quality Upgrading Peng He and Hua Song* Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, Alberta T2N 1N4, Canada ABSTRACT: The development of an economically attractive process with abundant and readily available raw feedstocks to achieve the upgrading of biomass is highly desirable. Unlike conventional fast pyrolysis followed by hydrotreating for upgraded bio-oil production under high pressure in which expensive and naturally unavailable hydrogen is heavily engaged, this work clearly demonstrates the feasibility of upgrading biomass by directly using cheap natural gas on zeolite-supported catalyst at atmospheric pressure. The introduction of methane during biomass pyrolysis not only increased the yield of the collected oil from 6.79 to 7.48 wt % but also improved its quality in terms of higher H/C ratio, from 1.21 to 1.71, under the facilitation of Ag/ ZSM-5 catalyst. A synergistic effect was clearly observed among methane, biomass pyrolysis, and catalyst, which contributed to the exciting performance. This novel process can be extended to the conversion of coals, other biomass, and heavy oil to more valuable products. alcohols, which improve the quality of bio-oil.20−23 But these cofed materials are not naturally available on a large scale. Accordingly, it is greatly desirable to develop an economically attractive process with abundant and readily available raw materials to achieve the upgrading of bio-oil. The recent discoveries of large reserves of natural gas (i.e., shale gas) in North America have motivated the development of viable methods to convert this cheap energy sources into higher value products. The major component of natural gas, methane, has the highest H/C ratio, but is also the most stable and symmetric organic molecule consisting of four C−H covalence bonds with bond energy of 435 kJ/mol. Correspondingly, the effective activation and direct conversion of methane into higher hydrocarbons is a great challenge for the entire catalysis field.24,25 In 1993, Wang et al. demonstrated for the first time the feasibility of dehydrogenation and aromatization of methane with benzene selectivity of 100% under nonoxidizing conditions on ZSM-5 loaded with Mo and/or Zn.26 Later, Choudhary et al. showed that methane can be highly converted into higher hydrocarbons and aromatics in the presence of alkenes and/or higher alkanes at low temperatures (400−600 °C) over H-galloaluminosilicate ZSM-5 type zeolite under nonoxidizing conditions at atmospheric pressure.27 Similar results have been widely reported in later literature on other catalysts such as Ag/ZSM-5 and Zn/ ZSM-5 when methane was co-fed with ethane, ethylene, propane, propylene, pentane, hexane, light gasoline, liquefied petroleum gas, and even oxygenated hydrocarbons like methanol.28−35 These results shed light on the development of processes for biomass upgrading; that is, it is possible to upgrade biomass with methane for high-quality bio-oil production because various alkanes, alkenes, and oxygenated hydrocarbons are generated during pyrolysis, which can act as natural

1. INTRODUCTION Biomass is receiving increasing attention worldwide as a feedstock, as the only renewable source of carbon that can be converted into liquid fuel1 and used for chemical production2−6 owing to its low cost, ready availability, and carbon-neutral nature. Nevertheless, it is still not widely used as a raw material in those areas because of technological and economic concerns. Pyrolysis of biomass is an effective way to produce crude bio-oils in addition to gases and biochars.1,2,7−9 The bio-oil obtained from direct pyrolysis, however, gradually ages because of low H/ C ratio, and it ages faster when exposed to light, oxygen gas, or heat above 80 °C, leading to storage and stability issues.10 In addition, because of its high oxygen content, the produced biocrude has lower heating value, making it unsatisfactory for being employed as substituent for traditional liquid fuel to power our world, not to mention the copresent contaminants such as sulfur, chlorine, and trace metals. To overcome these issues, various processes have been developed to upgrade the bio-oil by removing or chemically modifying the undesired compounds.11−15 The most widely employed process is hydrodeoxygenation.11−13 It can produce a quality, energy dense, and noncorrosive product, which can be further upgraded. However, the process consumes large quantities of hydrogen and operates under high-pressure conditions (typically 70−140 atm and even higher than 200 atm).16,17 The requirement of expensive hydrogen source which is not naturally available will inevitably result in significant cost increase of this upgrading process. Moreover, such high-pressure operation will definitely lead to further increased capital and operation cost.16 An alternative way is catalytic cracking on zeolite,14,15,17,18 which can produce aromatics at atmospheric pressure without the hydrogen requirement. This process is still in its infancy and suffers from a low H/C ratio of the product due to the absence of an external hydrogen resource. The problems due to low H/C, such as low oil yield and high coke deposition, should be addressed by cofeeding with feedstocks that have high a hydrogen-to-carbon ratio in biomass pyrolysis.19 The concept has been explored by cocatalytic pyrolysis of biomass and waste oil, plastics, and © 2014 American Chemical Society

Received: Revised: Accepted: Published: 15862

June 5, 2014 September 17, 2014 September 22, 2014 September 22, 2014 dx.doi.org/10.1021/ie502272j | Ind. Eng. Chem. Res. 2014, 53, 15862−15870

Industrial & Engineering Chemistry Research

Article

Table 1. Proximate and Ultimate Analysis of Biomass Feedstock proximate analysis (wt %)

a

ultimate analysis (wt %)

feedstock

moisture

volatile matter

fixed carbon

ash

C

H

N

S

Oa

sawdust flex straw

4.98 5.4

79.03 75.3

15.25 16.8

0.74 2.5

47.7 45.0

6.1 6.5

1.1 2.1

1.1 1.3

44.0 45.0

By difference.

AgNO3 was the same as described above. The Ag/P-Ce-HZSM-5 catalyst was finally obtained by calcining at 800 °C for 3 h. For comparison, the pure HZSM-5 catalyst was also calcined at 800 °C for 3 h. 2.2. Sample Characterization. The proximate analysis of samples was carried out according to ASTM Standard D 7582-12 with a thermogravimetric analysis instrument (TGA; PerkinElmer STA6000). Ceramic crucibles were used in order to minimize any thermal lag and to optimize heat transfer between thermocouples and crucibles. TGA of this type was also employed to examine coke deposition of spent catalysts. The samples were heated from room temperature to 850 °C at a heating rate of 10 °C min−1 in air flow (70 mL min−1). The ultimate analysis of feedstock and CHNS/O analysis were carried out using Elemental Analyzer (PerkinElmer 2400 Series) to determine the C, H, N, and S compositions of the samples. Because of the unavailability of oxygen sensor, the reported oxygen contents of feedstock were calculated as the remainder after subtracting the ash and CHNS content of biomass. The oxygen contents of liquid samples were calculated as the remainder after subtracting the CHNS content. The composition of the as-received feedstock is identified in Table 1. According to the resolution of the engaged analyzer, all the reported elemental analysis results in this paper have errors of ±0.01−0.02. The water content of a liquid sample produced from each run was determined using Karl Fischer titration (Metrohm 870 Titrino Plus) through averaging the results collected from at least three independent measurements. The crystalline phase compositions of prepared catalysts were examined by X-ray diffraction on a Rigaku Multiflex diffractometer with Cu Kα irradiation at a voltage of 20 kV and current of 40 mA in the 2θ range of 5−50°. The texture properties of fresh and spent catalysts such as pore volume, Brunauer−Emmett−Teller (BET) surface area, and average pore diameter were determined by N2 adsorption− desorption isotherms at −196 °C on an automated porosimeter (Micromeritic ASAP 2020). The samples were degassed at 150 °C and 100 μmmHg vacuum overnight before analysis. The transmission electron microscopy (TEM) experiments were performed by using a Philips Tecnai TF-20 TEM instrument operated at 200 kV. An X-ray analyzer for energydispersive spectrometry (EDS) is incorporated into the instrument for elemental analysis under scanning transmission electron microscopy mode for improving image contrast between C and Ag phases. The sample was first dispersed in ethanol and supported on lacey-Formvar carbon on a 200 mesh Cu grid before the TEM images were recorded. The volumetric measurement of H2 chemisorption was conducted using a Micromeritics ASAP 2020 Chemisorption system. Prior to adsorption measurements, calcined samples were reduced in situ under 5% H2/He at the desired reduction temperature for various times followed by evacuation to 1.33− 0.67 KPa and cooling to 100 °C to maximize activated chemisorption while minimizing H2 spillover. The adsorption

promoters for methane activation and its further engagement into the upgrading process under the facilitation of specially tailored catalysts. There is no report on the upgrading of biomass with this novel process in the open publications to the authors’ best knowledge. In this work, the feasibility of upgrading biomass with methane is demonstrated on Ag/ZSM-5 catalyst. Ag/ZSM-5 was selected because it is reported to show promising performance in aromatization of methane in nonoxidizing conditions.29,35,36 Enhanced upgrading performance was witnessed when Ag/ ZSM-5 catalyst was properly modified with P and Ce additions. This novel process will not only upgrade the quality of formed bio-oil but also produce more oil because of the introduction of cheap methane while emitting a significantly reduced amount of CO2 during upgrading compared to its conventional hydrodeoxygenation counterpart, making it more economically favorable and environmentally friendly. Furthermore, this process also features atmospheric and integrated operation (no separate steam reformer and upgrader needed when hydrodeoxygenation process is referred) as well as simultaneous formation of valuable byproduct hydrogen, making it even more economically attractive and thus creating even greater commercialization potential for its wide implementation.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The ammonium ZSM-5 zeolite with Si/Al = 23 and specific surface area of 425 m2 g−1 was purchased from Alfa Aesar and calcined at 600 °C for 5 h in air to attain the H-type ZSM-5 for further use. The 1 wt % Ag/HZSM5 was prepared by incipient wetness impregnation of HZSM-5 with AgNO3 (99.9+%, Alfa Aesar) solution at different concentrations, dried in the oven at 92 °C overnight, followed by being fired at various temperatures for 3 h in ambient air. In a similar manner, gallium nitrate hydrate (Ga(NO3)3·xH2O, 99.9%, Alfa Aesar), iridium chloride hydrate (IrCl3·xH2O, 99.9%, Alfa Aesar), zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 99%, Alfa Aesar), cerium nitrate hexahydrate (Ce(NO3)3·6H2O, 99.5%, 99.5%, Alfa Aesar), ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O, 99+%, Sigma), and indium nitrate hydrate (In(NO3)3·xH2O, 99.99%, Alfa Aesar) were used as the active metal precursor for Ga-, Ir-, Zn-, Ce-, Mo-, and Inimpregnated catalyst preparations, respectively. P- and Ce-modified catalyst 1%Ag/P-Ce-HZSM-5 was also prepared. In a typical experiment, 5 g of HZSM-5 was added into 50 mL of aqueous solution containing 0.4 g of Ce(NO3)·6H2O (99.5%, Alfa Aesar) while mixing. After the pH was adjusted to 3.5−4.0 with diluted hydrochloric acid (0.35 mol l−1), the mixture was heated at 95 °C for 2 h under agitation (300 rpm, Corning PC-420D). Subsequently, 0.3 g of (NH4)3PO4 (derived from (NH4)2HPO4, 98%, Amresco) was added and continued to react for another 1.5 h. After cooling to room temperature, the solution was filtered and the sample was washed with deionized water 5 times. The collected cake was dried at 92 °C overnight and then calcined at 600 °C for 2.5 h in air. The impregnation of 15863

dx.doi.org/10.1021/ie502272j | Ind. Eng. Chem. Res. 2014, 53, 15862−15870


Article

Figure 1. Process flow diagram of a house-made multifunctional reactor system.

sieved to the particle size of 150−206 μm. The feed gases, Ar (PP grade), CH4 (CP grade), and N2 (PP grade), were provided by Praxair and used as received. The gas mixtures were introduced into the reactor with a total flow rate of 200 sccm controlled by calibrated mass flow meters (Cole-Parmer). The 5% N2 was used as internal standard. The inert gas is 95% Ar + 5% N2, and the reactant gases are 65% Ar + 30% CH4/H2 + 5% N2. The introduction of nitrogen here was used as internal standard to account for the gas expansion occurring during the pyrolysis of involved solid feedstock. In a typical run, 1 g of catalyst bed and solid feedstock (4 g biomass, sawdust or flex straw) were sandwiched between three layers of quartz wool in the vertically oriented reactor. The gas was introduced into the reactor by following a down-flow direction. After being exposed to the top layer of quartz wool, the gas came into contact with solid feedstock bed where pyrolysis took place upon heating. The generated volatile matter was then carried out by the feed gas stream to the catalyst bed after passing through the middle separation quartz wool layer. Under the facilitation of the charged catalyst, the reactive feed gas interacted with the formed volatile matter and exited from the bottom of the reactor after passing through the bottom quartz wool layer which is exclusively used to prevent particulate matter from diffusing into the lower porous frit and thus resulting in its blockage. The reactor was heated by an electric furnace (MTI OTF-1200X-SVT) at a heating rate of 50 °C min−1 and held at desired temperature until almost no product composition could be detected by Microgas chromatography (Micro-GC 3000, Agilent Tech.). The temperature was held at 300, 400, and 500 °C for 30 min each. The gas products were analyzed by an online fourchannel micro-GC equipped with thermal conductivity detectors, which can precisely analyze H2, O2, N2, CH4, and CO in the first channel equipped with a 3 m U-Plot precolumn and a 10 m molecular sieve column; CO2, C2H4, and C2H6 in the second channel installed with a 1 m Q-Plot precolumn and a 8 m U-Plot column; and C3−C6 and C3=−C5= in the third channel charged with a 10 m alumina column. Ar and He are the carrier gases for the first and other three channels, respectively. The

isotherms were measured at equilibrium pressures between 6.67 and 66.7 KPa. The first adsorption isotherm was established by measuring the amount of H2 adsorbed as a function of pressure. After the first adsorption isotherm was completed, the system was evacuated for 1 h at 1.33−0.67 KPa. Then a second adsorption isotherm was obtained. The amount of probe molecule chemisorbed was calculated by taking the difference between the two isothermal adsorption amounts. The metal dispersion was determined by assuming a one-to-one stoichiometry between active metal and atomic hydrogen. The GC-MS analysis of the selected liquid product was performed on an Agilent GC-MS 6890N/HP5973N equipped with a F05 column manufactured by SGE Analytical Science. The length, inner diameter, and film thickness of the column were 50 m, 0.22 mm, and 0.25 μm, respectively. The GC program was as follows: initial temperature of 50 °C, held for 3 min, ramped to 200 °C at 15 °C per min, and held for 20 min. The char yield, liquid yield, oil yield, and water formed reported in this paper are calculated by the equations given below with reported error of ±0.1 wt %, ± 0.1 wt %, ± 0.1 wt %, and ±0.1 mg/g biomass, respectively. char yield % wt of collected solid reside in solid feedstock bed after each run = initial wt of charged biomass × 100 wt of collected liquid liquid yield % = × 100 initial wt of charged biomass oil yield % =

wt of collected liquid − wt of water in liquid × 100 initial wt of charged biomass

⎛ mg ⎞ wt of water in liquid − wt of water in biomass water formed⎜ ⎟ = initial wt of charged biomass ⎝ g ⎠

2.3. Catalyst Activity Evaluation. The catalytic test was performed in a fixed-bed continuous-flow stainless steel reactor (i.d., 2.54 cm; length, 61 cm) at atmospheric pressure using a house-made multifunctional reactor system with configuration illustrated in Figure 1. The carbon feedstock was ground and 15864



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liquid products were collected in a cold trap at −20 °C powered by a 50% ethylene glycol-50% DI water cooled chiller (Polyscience LS5) and weighted after each run. The mass and carbon balances were conducted after each run with measured closures of 0.92−0.96 and 0.89−0.94, respectively.

Table 2. Sawdust Pyrolysis Performance under Various Environments oil quality

trial

3. RESULTS AND DISCUSSION The XRD patterns of fresh catalysts are presented in Figure 2. 1% Ag loading results in no additional peaks besides those of HZSM-

biomass, inert biomass, inert, Ag/ZSM-5 biomass, 30% H2 biomass, 30% H2, Ag/ZSM-5 biomass, 30% CH4 biomass, 30% CH4, Ag/ZSM-5 biomass, 30% CH4, Ag/P-CeZSM-5 30% CH4, Ag/ZSM-5 a

oila yield (wt %)

water formed (mg/g biomass)

H/C atomic ratio

O content (wt %)

O/C atomic ratio

5.47 4.07

97.0 135.6

1.62 1.29

5.25 0.18

0.226 0.009

4.17 3.42

73.4 100.2

1.46 1.45

3.41 0.45

0.145 0.024

4.68

119.0

1.38

0.22

0.009

4.85

128.3

1.76

0.07

0.003

6.89

110.9

2.26

7.35

0.356

−

−

−

0

0

Moisture-free liquid collections with boiling point less than 150 °C.

Table 3. Flex Straw Pyrolysis Performance under Various Environments

Figure 2. XRD patterns of fresh catalysts of HZSM-5 and Ag/ZSM-5.

5, which indicates that the silver species is highly distributed at the surface as evidenced in the corresponding TEM image (i.e., Figure 3a) and/or in the channel of zeolite support. Further analysis reveals that the relative crystallinity of the Ag-loaded sample decreases as evidenced by the lower diffraction intensities. Furthermore, the Ag-loading results in lattice distortion of HZSM-5 support because of the change of relative intensity of peaks at 8−10° and 23−25°. Considering the fact that the SiO2/Al2O3 ratio of HZSM-5 used in this work is 23, the maximum loading of Ag cations is 7.5%. According to the report of Miao et al.,36 the isolated Ag+ ions are the main silver species within the HZSM-5 zeolite when the impregnated silver is less than the maximum value. Higher-temperature calcination will facilitate the migration of Ag+ species into the channel of zeolite25 and thus result in structural change. Correspondingly, the 800 °C calcined 1% Ag in this work probably mainly exists in the channel of HZSM-5 support, which will change the acidity of catalysts and exert an influence on the catalytic performance. The morphologies of the synthesized catalysts are also analyzed by employing transmission electron microscopy. As shown in Figure 3a, silver particles (the near spherical particle in

trial biomass, inert biomass, inert, Ag/ ZSM-5 biomass, 30% H2 biomass, 30% H2, Ag/ ZSM-5 biomass, 30% CH4 biomass, 30% CH4, Ag/ZSM-5 biomass, 30% CH4, Ag/P-Ce-ZSM-5 a

char yield (wt %)

oila yield (wt %)

water formed (mg/g biomass)

H/C atomic ratio

23.5 22.5

8.12 6.79

140.0 182.6

1.57 1.21

20.0 20.2

6.54 5.98

108.0 144.3

1.42 1.39

21.8 22.5

6.47 7.48

175.6 181.5

1.32 1.71

23.0

9.56

185.1

2.24


the image whose composition is confirmed by performing an EDS analysis at several selected spots) are widely dispersed throughout the whole surface of the ZSM-5 support with averaged diameter of ∼13 nm. After introduction of P and Ce into the support matrix, many needle-shaped rods are self-

Figure 3. TEM images of the fresh catalysts of Ag/ZSM-5 (a) and Ag/P-Ce-ZSM-5 (b). 15865



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Table 4. Gas Product Analyses Collected over Flew Straw Pyrolysis at 400 °C and 1 atm gas species generation rate (mL (g min)−1) methane conversion (%)

trials

H2

CH4

CO

CO2

C2+

biomass, inert biomass, inert, Ag/ ZSM-5 biomass, 30% H2 biomass, 30% H2, Ag/ZSM-5 biomass, 30% CH4 biomass, 30% CH4, Ag/ZSM-5 biomass, 30% CH4, Ag/P-Ce-ZSM-5

0.45 0.75

1.05 1.38

1.43 1.58

4.28 3.89

1.04 0.07

− −

− −

1.15 1.19

1.52 1.83

4.78 5.70

0.97 0.86

− −

0.65 0.73

− −

1.32 1.91

3.64 5.92

1.53 2.60

2.96 8.01

0.48

−

2.38

6.24

3.25

10.56

Figure 5. Ag dispersion and H/C atomic ratio as a function of calcination temperature used for Ag/ZSM-5 synthesis.

Table 5. Flex Straw Pyrolysis Performance Collected during Methanolysis over Ag/ZSM-5 Prepared with Various Precursor Concentrations concentration (mol L )

liquid yield (wt %)

oila yield (wt %)

H/C atomic ratio

0.321 0.154 0.093 0.058 0.039 0.029 0.018

15.63 21.24 31.03 25.83 23.78 21.57 19.88

5.13 6.34 7.48 6.14 5.67 5.43 5.32

1.48 1.61 1.71 1.65 1.58 1.51 1.49

−1

a

Table 7. Flex Straw Pyrolysis Performance Collected during Methanolysis over Ag/ZSM-5 at Different GHSVs GHSV (h−1)

liquid yield (wt %)

H/C atomic ratio

25 500 12 750 10 200 6 375 3 188

26.98 29.13 31.03 34.62 38.41

1.42 1.59 1.71 1.89 2.14


Figure 6. Effect of GHSV on biomass methanolysis performance over Ag/ZMS-5.

agglomerated into small clusters surrounding the irregularly shaped zeolite support, as evidenced in Figure 3b. Through further elemental examination, the formed needle-shaped material is mainly composed of cerium oxide with small decoration of phosphorus oxide on the outer surface. Instead of concentrating on the zeolite support surface, silver particles now spread over the entire surface, including the newly formed P-doped CeO2 support. Additional information such as the porous structure of the ZSM-5 support and silver distribution within such structures will become available upon the engagement of a TEM instrument with even higher resolution. Upon heating to certain temperature in the atmosphere of inert gas, biomass pyrolysis takes place and yields liquid product because of the condensation of volatile matter, denoted as platform chemicals,1,37 contained in its matrix. As is listed in Table 2, when sawdust is used as the carbon feedstock, the H/C atomic ratio of the formed light oil is 1.62 with yield of 5.47 wt %

Figure 4. Ag dispersion and H/C atomic ratio as a function of AgNO3 concentration used for 600 °C calcined Ag/ZSM-5 synthesis.

Table 6. Flex Straw Pyrolysis Performance Collected during Methanolysis over Ag/ZSM-5 Prepared with Various Calcination Temperatures calcination temperature (°C)

liquid yield (wt %)

H/C atomic ratio

400 500 600 700 800

23.87 26.01 31.03 28.23 24.09

1.54 1.62 1.71 1.63 1.49

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Table 8. Flex Straw Pyrolysis Performance Collected during Methanolysis over ZSM-5 Supported Catalysts catalyst liquid yield (%) water formed (mg/g biomass) H/C atomic ratio

Ag/ZSM-5

Ir/ZSM-5

In/ZSM-5

Zn/ZSM-5

Ce/ZSM-5

Mo/ZSM-5

Ga/ZSM-5

31.03 181.5 1.71

25.69 152.9 1.66

24.72 148.3 1.63

20.67 91.2 1.58

16.55 71.3 1.44

15.27 68.4 1.42

11.88 53.2 1.38

Table 9. BET Surface Area, Pore Volume, and Pore Size of Fresh and Spent Catalysts Run in Different Conditions catalyst

spent conditions

BET surface area (m2 g−1)

total pore volume (cm3 g−1)

average pore width (Å)

Ag/HZSM-5 Ag/HZSM-5 Ag/HZSM-5

fresh flex straw-inert flex straw-30% CH4

269.6 251.1 286.5

0.19 0.17 0.19

28.5 26.7 26.8

improved water removal. The collected liquid products underwent further GC/MS analysis for composition determination. Because of its inherent complexity coming from the presences of hundreds of organic compounds, only qualitative measurement was performed at current stage. According to the results, abundant C6−C8 unsaturated hydrocarbons like ethylcyclohexene and dimethylcyclohexene are produced along with a small amount of C2−C5 oxygenated hydrocarbons such as acetone and acetic acid (data not shown) when the upgrading process is taking place under methane with Ag/ZSM-5 charged in the reactor. During this so-called methanolysis (i.e., biomass pyrolysis under methane environment) process, Ag/ZSM-5 plays two roles. One is the activation and oligomerization or incorporation of CH4 into the carbon chain of the formed oil mainly because of the existence of active metal such as Ag in this case, leading to higher H/C atomic ratio and oil yield, which is favorable for practical applications. The other role, the overcracking of organic molecules resulting in a lower H/C ratio and liquid yield because of the coexisting zeolite support, however, should be alleviated. Surface modification with various ions such as lanthanide series,42 phosphorus,43,44 or silicon45 has been proven effective at undermining the cracking activity by modifying Brønsted acidity. In this work, P and Ce were coemployed to modify the performance of upgrading biomass. The catalytic results revealed that a significantly higher H/C atomic ratio and oil yield were achieved, implying the reduced cracking capability of the catalyst after surface modification. This result indicated that when the catalyst is modified, the biomass can be further upgraded with this novel process. Nevertheless, the authors also noticed the formidably high oxygen content and O/C atomic ratio of the formed bio-oil with the phosphor and cerium-doped Ag/ZSM-5 catalyst, even higher than the crude bio-oil obtained without the engagement of reducing agent in the gas phase. Such unexpected results might be attributed to the presence of cerium oxide which is well-known in the catalysis field for its high oxygen storage capacity (OSC). The organic molecules generated from biomass pyrolysis might get oxidized by the surface oxygen present in the Ag/P-Ce-ZSM-5, probably following a Mars−van Krevelen-like mechanism, resulting in increased oxygen content in the upgraded oil, which has been evidenced by the considerable amount of oxygenated C2−C6 hydrocarbons such as ethylhexanol, dimethylhexandiol, acetic acid, and acetone present in the formed liquid product using qualitative GC/MS analysis (data not shown). Therefore, the Ag/ZSM-5 after only phosphor modification will be synthesized and evaluated in future work to verify our aforementioned hypothesis regarding the effect of cerium addition.

Figure 7. TGA plot of Ag/ZSM-5 catalysts run in inert and 30% CH4 environment during flew straw pyrolysis.

on the basis of biomass and oxygen content as high as 5.25 wt %. When there is Ag/ZSM-5 catalyst charged in the reactor, both the light oil yield and H/C atomic ratio are lowered, accompanied by increased water formation, which might be due to the enhanced cracking of the formed volatile matter during pyrolysis under the facilitation of the engaged catalyst, particularly the ZSM-5 support.38−41 It is also worth noting that the amount of produced water is higher with catalyst, indicating the removal of oxygen-containing groups during catalytic cracking, which is further evidenced by the significantly reduced oxygen content and decreased O/C atomic ratio. The reported oxygen content is the weight percentage on the basis of oil without water. Similar phenomena are also observed when H2 is introduced to the gas flow (Table 2). When CH4 is present in the feed gas flow, however, the H/C atomic ratio and the yield of the formed light oil as well as the produced water increase along with prominent reductions in oxygen content and decreasing O/C atomic ratio in the presence of Ag/ZSM-5 catalyst. Methane exhibits better bio-oil upgrading performance as reducing agent by itself in terms of higher yield, less oxygen content, and O/C atomic ratio as well as more water formed when compared to its H2 counterpart. With the help of charged catalyst, upgraded biooil can be produced with better quality and enhanced yield. The reaction is also performed without biomass, where no liquid product is collected. Such observations indicate that the higher H/C ratio and liquid yield is due to the synergistic effect among CH4, platform chemicals, and catalyst. In the presence of Ag/ ZSM-5, CH4 is activated and added into the platform chemicals while removing oxygen in the form of water, resulting in the increased H/C atomic ratio and oil yield accompanied by the 15867



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Similar experiments have also been performed by using flex straw as a low-cost carbon source, and the results are tabulated in Table 3. When compared to the results collected over sawdust, similar trends have been clearly observed, further evidencing that methane can act as a better reducing agent than H2 for upgraded bio-oil production during biomass pyrolysis. Moreover, flex straw is a better source of biomass than sawdust for oil production with increased yield. The simultaneously formed solid and gas products from each run were also investigated, resulting in the char yield and gas composition analyses reported in Tables 3 and 4, respectively. No notable variation of char yield is witnessed between trials, which is probably attributed to no direct involvement of catalyst in the solid feedstock bed originating from our special reactor configuration as detailed in Catalyst Activity Evaluation. The gas product is mainly composed of hydrogen and single-carbon-containing compounds including CH4, CO, and CO2 as well as a small amount of C2+ species (i.e., saturated and unsaturated C2−C6 hydrocarbons). It is worth noting that gas species, especially hydrogen and oxygen containing gases (e.g., CO and CO2), are generated at a higher flow rate accompanied by the notably enhanced methane conversion when the developed catalyst is charged under the methane environment, clearly indicating the participation of methane in the upgrading reaction. Furthermore, this result also delivers the valuable message that the oxygen contained in the biomass is removed from the upgraded bio-oil product not only in the form of water but also in the form of gaseous oxygenated hydrocarbons such as CO and CO2 during the catalytic methanolysis process. The effect of AgNO3 aqueous solution concentration on its catalytic performance during methanolysis has also been evaluated and the results are included in Table 5. The highest liquid yield and H/C atomic ratio are identified when the Ag/ ZSM-5 catalyst is prepared using AgNO3 aqueous concentration of ∼0.1 mol L−1. To establish the relationship between the catalytic performance and its properties, the H/C atomic ratio of the produced light oil has been correlated with the Ag dispersion on the catalyst’s surface. As shown in Figure 4, both curves exhibit the same general trend and peak at ∼0.1 mol L−1, implying that better active metal dispersion on the catalyst surface benefits the achievement of better upgrading performance. Calcination temperature used during catalyst synthesis will also impact the associated catalytic performance. Therefore, we have also evaluated the influence of calcination temperature on the biomass methanolysis performance in this work when the prepared Ag/ZSM-5 is engaged. The results are listed in Table 6, from which we can conclude that 600 °C is the optimized calcination temperature for producing upgraded bio-oil with highest yield and H/C atomic ratio. The similar correlation profile is also drawn in Figure 5 to identify the relationship between H/C atomic ratio and Ag dispersion on the ZSM-5 surface. The initial temperature increase facilitates the migration of Ag ions across the zeolite surface, leading to better dispersion. Nevertheless, further temperature elevation results in the agglomeration of nearby Ag atoms and formation of large Ag particles, thus causing lowered Ag dispersion. In addition, a higher calcination temperature also favors the transformation of Brønsted acid sites to Lewis ones on the surface of the charged catalyst, which might also contribute to the variation of the C/H atomic ratio of the formed bio-oil. Therefore, the suitable calcination temperature (i.e., 600 °C) has to be chosen for

enhanced active metal dispersion and thus improved upgrading performance. According to reaction knowledge, lower gas hourly space velocity (GHSV, i.e., longer residence time) will benefit the achievement of better reaction performance. Therefore, the influence of GHSV has also been investigated for the biomass methanolysis reaction when Ag/ZSM-5 is charged, which is demonstrated in Table 7 and Figure 6. As expected, lower GHSV is desirable in order to obtain more bio-oil product with better quality (i.e., higher H/C atomic ratio). Nevertheless, higher GHSV operation is beneficial for its industrial application because of reduced capital and operation cost originating from the reduced reactor size and improved productivity. Therefore, future work must identify a better catalyst with higher performance even under higher GHSV. In addition to Ag, other transition and precious metals (Mo, Ir, In, Ce, Zn, and Ga) have also been employed as the active metal for methane activation and following reaction. Their performances toward biomass methanolysis are summarized in Table 8. It is obviously concluded that Ag ranks as the most active metal among all the aforementioned metals for triggering methane activation and the following incorporation of methane molecule into the upgraded bio-oil or self-oligmerization for achievement of increased liquid yield and H/C atomic ratio. These results are fairly in alignment with those reported by Baba et al.,46 which might be mainly attributed to the lower activation energy of C− H breakage when Ag is charged.36,47 The spent catalysts collected after flex straw runs have been further characterized to investigate its evolution during the upgrading reaction. The Ag/HZSM-5 catalysts turned from white to black after biomass run and then were characterized by BET and TGA. The results are shown in Table 9 and Figure 7, respectively. When compared with that of the fresh catalyst, the BET specific surface area of Ag/HZSM-5 run in inert atmosphere decreased slightly and the total pore volume and average pore size also decreased obviously. However, the specific surface area of Ag/HZSM-5 run in 30% CH4 even slightly increased; the average pore size also decreased, but the pore volume remained constant. The decrease in specific surface area, pore size, and pore volume must originate from the coke deposition,25 which blocked the channel, as discussed below. Three weight loss regions were found from TGA results (Figure 7). The first (350 °C) is derived from the combustion of pregraphitic type carbon.48 The first two weight regions are almost the same for both catalysts, whereas the catalyst run in 30% CH4 exhibited less pregraphitic type carbon deposition, which is the carbon type that should be responsible for the deactivation of the catalysts.25,48 Less coke deposition of the catalyst run in 30% CH4 may be correlated to a sufficient supply of hydrogen source from the feed gas25 and the product, which will enhance the oligmerization of methane and upgrading of biomass by methane, leading to higher liquid yield and H/C ratio.

4. CONCLUSION The present work demonstrates the feasibility of upgrading lowvalued biomass with cheap methane on Ag/ZSM-5 catalyst at atmospheric pressure to yield high-valued liquid with a higher H/ C ratio. With methane introduction during biomass pyrolysis, the liquid yield and the H/C ratio are increased. The performance of this novel process is comparable with the state-of-the-art 15868



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hydrodeoxygenation process but is much more cost-effective. The performance variation with feed gases, biomass pyrolysis, and catalysts demonstrated the existence of a synergistic effect among them. Moreover, the biomass can be further upgraded by modifying the catalysts, which is a promising future work.

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AUTHOR INFORMATION

Corresponding Author

*E-Mail: [email protected]. Tel: 403-220-3792. Fax: 403-2844852. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the funding from Alberta Innovates− Energy and Environment Solutions through Grant AI-EES 2105 and experimental contributions from Xueting Lyu and Cuijuan Zhang.

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REFERENCES

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