Role of Hydrogen Transfer during Catalytic Copyrolysis of Lignin and

Research Article pubs.acs.org/journal/ascecg

Role of Hydrogen Transfer during Catalytic Copyrolysis of Lignin and Tetralin over HZSM‑5 and HY Zeolite Catalysts Yuan Xue,§,† Shuai Zhou,§,‡ and Xianglan Bai*,† †

Department of Mechanical Engineering, Iowa State University, Ames, Iowa 50011, United States Bioeconomy Institute, Iowa State University, Ames, Iowa 50011, United States

‡

ABSTRACT: In the present study, catalytic pyrolysis of lignin and tetralin was explored in a tandem micropyrolyzer using HY and HZSM-5 as the catalysts. Tetralin is an effective hydrogen donor at atmospheric pressure and its molecular size is bigger than the static pore size of HZSM-5 but smaller than that of HY zeolite. Therefore, copyrolysis of lignin and tetralin is also useful in understanding the role of hydrogen transfer in the zeolites related to shape selectivity. A strong synergistic effect between lignin and tetralin was found with HY zeolite as catalytic coke decreased from an additive yield of 12.72 C% to an experimental yield of 3.8 C%, whereas the aromatic hydrocarbon yield increased from 48.79 C% to 66.23 C% at catalyst temperature of 600 °C. Carbon balance of the measurable pyrolysis products was high because lignin-derived phenols were effectively deoxygenated by HY zeolite in the presence of tetralin. Hydrogen transfer from tetralin to phenols within HY zeolite pores changed the mode of oxygen removal by zeolite to promote hydrodeoxygenation and suppressed decarboxylation or decarbonylation. Bimolecular reactions between tetralin and lignin also produced alkylated aromatics. It was also found that the extent of coke reduction and aromatic increase by copyrolysis linearly correlate with the amount of hydrogen transferred from tetralin to lignin at HY zeolite pores. In comparison, hydrogen transfer at the catalyst surface of HZSM-5 was less effective as nearly no synergistic effect between lignin and tetralin was observed at low catalyst temperatures. Pore enlargement and stable acid sites better promote the synergistic effects between lignin and tetralin. KEYWORDS: Lignin, Tetralin, Catalytic pyrolysis, HZSM-5 zeolite, HY zeolite, Hydrogen

■

INTRODUCTION Lignin is the second most abundant natural polymer on the earth, accounting for up to 30% of lignocellulosic biomass.1 In addition to being present in lignocellulosic biomass, extracted lignin is also available from the pulp and paper industry and emerging cellulosic biorefineries as a byproduct.2 Lignin is a phenyl-propane-based polymer biosynthesized from random polymerization of three precursor monomers.3 Because of its unique chemical structure and abundance, lignin can be a potential source of renewable aromatic hydrocarbons.4,5 However, the complex structure of lignin and its recalcitrance for chemical or biological deconstruction have hampered its effective utilization.6 Fast pyrolysis is a robust liquefaction technology commonly used to depolymerize biomass.7 Typically, dry feedstock is rapidly heated to moderate temperatures (450−600 °C) under oxygen-free environment and the pyrolysis vapor is condensed to become liquid products. In general, fast pyrolysis is conducted at atmospheric pressure and the whole process completes within seconds. Lignin is thermally depolymerized to phenols during pyrolysis, which can later be catalytically deoxygenated to become aromatic hydrocarbons. Catalytic pyrolysis is a method to deoxygenate pyrolysis vapors of biomass before the vapor condenses.8 Compared to © XXXX American Chemical Society

upgrading condensed bio-oil, catalytic pyrolysis can avoid secondary reactions of unstable bio-oil during bio-oil condensation and revaporization.9 However, the contact time of the vapor products and catalyst is usually short during catalytic pyrolysis, requiring a powerful deoxygenation catalyst. Zeolite catalyst is the most frequently used catalyst in biomass conversion due to its excellent deoxygenation ability. Zeolite is a porous, acidic catalyst and its active sites can promote a number of reactions, such as cracking, dehydration, decarboxylation, decarbonylation, oligomerization, isomerization and aromatization. Although zeolite catalyst is shape-selective, HZSM-5 zeolite with medium size of pores size was found to be the most effective catalyst in terms of deoxygenating biomass. When it was pyrolyzed using HZSM-5 zeolite, about 30% of cellulose was converted to aromatic hydrocarbons without needing external hydrogen.10−13 Compared to carbohydrates, lignin or lignin-derived phenols are much more difficult to deoxygenate. Not only is the hydrocarbon yield low, the high yield of solid residues also rapidly deactivates catalysts. Despite the need for improving its conversion efficiency, the detailed reaction Received: April 10, 2016 Revised: June 6, 2016

A

DOI: 10.1021/acssuschemeng.6b00733 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

washed using 0.1 N nitric acid prior to pyrolysis tests to remove the inorganic impurities. After purification, the lignin contained 61.34% of carbon, 4.67% of hydrogen, 2% of nitrogen and 0.18% of sulfur. The oxygen content, 31.81%, was determined by the difference. Tetralin and other chemicals used in this study were purchased from SigmaAldrich, USA. HY zeolite catalyst (CBV400, SiO2/Al2O3 = 5.1) and HZSM-5 zeolite catalyst (CVB2314, SiO2/Al2O3 = 23) were purchased from Zeolyst International. HZSM-5 and HY zeolites were activated in a muffle furnace at 450 °C for 4 h. Pyrolysis. A Frontier Tandem micropyrolyzer (Frontier laboratory, Japan) coupled with an Agilent GC-MS/FID-TCD (Agilent 6890) system was used to perform both catalytic pyrolysis and online analysis of pyrolysis products. The description of the system configuration can be found elsewhere.36 Briefly, the tandem micropyrolyzer consists of two microfurnace ovens. The first oven is a pyrolysis reactor and the second oven hosts a catalytic bed made of quartz tube. The temperature of each oven is controlled independently and can reach up to 900 °C. The two furnaces were 5 cm apart and connected by a needle with heat insulation. Helium was used as the carrier gas with the flow rate of 156 mL/min. The pyrolysis vapor leaving the first reactor is converted at the catalytic bed. The identification and quantification of the vapor products were conducted online by GC/MS-FID-TCD. The GC oven was kept at 40 °C initially and then ramped up to 280 °C at a heating rate of 6 °C/min. Two capillary columns separately connected to a mass spectrometer (MS, 5975C, Agilent, USA) and flame ionization detector (FID) were ZB-1701 (60 m × 250 μm × 0.25 μm). The compounds identified by MS were quantified using the FID by injecting authentic chemicals. Carbon oxides and light hydrocarbon gases (CH4, C2H4, C2H6, C3H6, C3H8 and C4H8) were calibrated by TCD. A porous layer open tubular column (60 m × 0.320 mm) (GS-GasPro, Agilent, USA) was connected to a thermal conductivity detector (TCD), which is used for noncondensable gas separation. Five different concentrations of each compound were prepared to generate a calibration curve. Approximately 500 μg of lignin was used for each pyrolysis test. First, the lignin sample was placed at the bottom of a sample cup made of deactivated stainless steel, followed by the addition of the same amount of tetralin into the cup. The sample cup was then dropped into the preheated reactor where the sample was pyrolyzed within a second. Approximately 10 mg of the zeolites particles with the size between 50 and 70 mesh was packed into the catalytic bed, representing the catalyst to lignin ratio of 20:1. The pyrolysis temperature was kept at 500 °C and the catalytic bed temperatures changed between 400, 500 and 600 °C. Lignin or tetralin alone was also pyrolyzed at the same conditions. The amount of unconverted tetralin vapor was quantified by GC/FID. Tetralin conversion was then determined by subtracting this amount of unconverted tetralin from the initial amount of tetralin. The carbon yields of pyrolysis products of lignin, tetralin or the mixture of lignin and tetralin were calculated using the equations listed below. For catalytic pyrolysis of lignin:

mechanisms of lignin over zeolite catalysts have been studied infrequently compared to carbohydrates.10,14−16 Because of their large molecular sizes compared to the pore sizes of zeolite, it is questionable if lignin-derived phenols can gain access to the inner-pore active sites. Although phenolic monomers or dimers have been used as model compounds to investigate deoxygenation mechanisms over zeolite catalyst,17 the conversion of lignin polymer or the complex mixture of lignin-derived phenols cannot be easily represented with the simple phenols. Thus, it has been generally described that the strong adsorption of phenols on catalyst and low reactivity of phenols for deoxygenation result in the low conversion and high coke yield.18,19 On the other hand, it has been reported that catalytic hydropyrolysis of biomass using high partial pressure of hydrogen and/or bifunctional zeolite catalysts could increase the yields of hydrocarbons.20−22 Although hydropyrolysis of lignin over zeolite was less frequently studied,20 previous results suggest that external hydrogen may play an important role in promoting catalytic deoxygenation of lignin by zeolite. Pyrolysis of lignin produces reactive free radicals and phenolic functionalities.23,24 External hydrogen can act as a capping agent to prevent undesired polymerization and promote hydrocracking. In this study, lignin and tetralin were catalytically copyrolyzed by zeolite catalysts in a tandem reactor maintained at atmospheric pressure. In addition to HZSM-5, HY zeolite was also studied because it is the major composition of fluid catalytic cracking (FCC) catalyst. Fogassy et al.25 previously studied coprocessing of bio-oil and petroleum fuel using the FCC catalyst to explore the potential for upgrading bio-oil in the existing petroleum infrastructure. Unmodified or modified HY zeolite is also frequently used to study biomass conversion.10,26−28 Tetralin, instead of hydrogen gas, was chosen for following reasons: first, tetralin is an effective hydrogen donor, frequently used in coal liquefaction as well as biomass conversion.29−31 Tetralin was also used to convert lignin in the absence or presence of catalyst to produce liquid products with increased amounts of monomers.32−34 As the hydrogen donor solvent, tetralin is able to rapidly stabilize reactive free radicals generated from lignin decomposition and prevent undesired repolymerization reactions that can lead to char formation. Nevertheless, the conversion of lignin in tetralin mostly occurred in sealed reactors for extended reaction times, which are significantly different than pyrolysis conditions. During pyrolysis, tetralin can be easily dehydrogenated to naphthalene and provide hydrogen radicals without needing elevated pressures. Second, the molecular size of tetralin is 7.389 Å, which is smaller than the mesopores in HY zeolite (7.4 Å) yet much larger than the micropores of HZSM-5 zeolite (5.6 Å). Thus, it may be possible to use tetralin to probe how hydrogen is transferred within the zeolites with different shape selectivities. Third, tetralin itself is also one kind of hydrocarbon produced from petroleum oil and coal tar.35 Although our reaction conditions and feedstocks do not exactly resemble to FCC cracking, understanding the reactions among tetralin and lignin derived phenols in the presence of zeolite catalyst could be useful in exploring cocracking of bio-oil and petroleum heavy oil in existing petroleum infrastructure. To evaluate catalytic cracking of lignin in the presence of the hydrogen donor, lignin and tetralin were converted independently or by a mixture over the two zeolite catalysts and the results were compared.

■

carbon yield of a product (C%) mole of carbon in product = × 100% total mole of carbon in lignin

(1)

For catalytic pyrolysis of tetralin:

carbon yield of a product (C%) mole of carbon in product = × 100% total mole of carbon in converted tetralin

(2)

For catalytic copyrolysis of lignin and tetralin:

carbon yield of a product (C%) mole of carbon in product = total mole of carbon in (lignin plus converted tetralin)

MATERIALS AND METHODS

Materials. Corn stover-derived acetosolv lignin was provided by the Archer Daniels Midland (ADM) Company. As-received lignin was

× 100% B

(3) DOI: 10.1021/acssuschemeng.6b00733 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering Table 1. Catalytic Pyrolysis of Lignin with HY and HZSM-5 Zeolites HY temperature (°C) overall yield (C%) pyrolysis char catalytic coke CO CO2

400

HZSM-5

500

600

400

500

600

33.73 33.93 1.15 4.60

± ± ± ±

0.78 0.41 0.01 0.02

33.73 28.22 1.49 4.78

± ± ± ±

0.78 0.56 0.02 0.09

33.73 25.81 2.36 4.86

± ± ± ±

0.78 0.69 0.21 0.05

33.73 24.96 1.45 4.43

± ± ± ±

0.78 1.99 0.03 0.10

33.73 19.87 1.98 5.28

± ± ± ±

0.78 1.71 0.13 0.10

33.73 15.35 3.36 5.64

± ± ± ±

0.78 1.68 0.22 0.38

benzene toluene ethyl-benzene p-xylene o-xylene benzene, 1-ethyl-3-methyl benzene, 1,2,3-trimethylindane indene naphthalene naphthalene, 2-methyl1-ethyl-napthalene fluorene anthracene total aromatics

0.05 0.23 0.03 0.15 0.08 0.10 0.13 0.08 0.03 0.14 0.34 0.34 0.01 0.11 1.82

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.00 0.03 0.01 0.02 0.00 0.02 0.05 0.01 0.01 0.03 0.03 0.04 0.01 0.03 0.30

0.17 0.66 0.07 0.47 0.18 0.16 0.26 0.09 0.06 0.53 0.82 0.52 0.10 0.24 4.32

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.01 0.06 0.00 0.01 0.01 0.01 0.01 0.01 0.04 0.00 0.02 0.03 0.00 0.09 0.30

0.61 1.45 0.07 0.68 0.26 0.11 0.18 0.09 0.04 0.91 0.86 0.38 0.16 0.33 6.14

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.08 0.01 0.00 0.03 0.01 0.01 0.02 0.01 0.00 0.07 0.02 0.01 0.05 0.08 0.39

0.18 0.57 0.07 0.59 0.13 0.09 0.15 0.07 0.07 0.22 0.38 0.23 0.05 0.15 2.95

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.01 0.01 0.01 0.05 0.00 0.00 0.01 0.01 0.00 0.04 0.03 0.03 0.02 0.01 0.24

0.58 1.43 0.05 0.80 0.22 0.03 0.12 0.12 0.36 1.03 0.91 0.36 0.07 0.17 6.25

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.06 0.21 0.01 0.15 0.02 0.00 0.02 0.02 0.06 0.13 0.02 0.01 0.01 0.04 0.75

2.27 2.59 0.01 0.68 0.35 0.02 0.05 0.07 0.84 1.57 1.05 0.47 0.08 0.24 10.28

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.10 0.12 0.00 0.05 0.00 0.00 0.00 0.00 0.08 0.02 0.02 0.22 0.00 0.01 0.63

methane ethane propane total alkane (C ≤ 5)

1.74 0.48 0.18 2.41

± ± ± ±

0.07 0.02 0.01 0.10

1.75 0.53 0.45 2.73

± ± ± ±

0.09 0.03 0.02 0.14

2.26 0.57 0.47 3.30

± ± ± ±

0.16 0.04 0.03 0.23

1.13 0.95 0.55 2.63

± ± ± ±

0.12 0.10 0.06 0.28

1.39 0.66 0.60 2.65

± ± ± ±

0.08 0.04 0.04 0.16

2.54 0.87 0.24 3.65

± ± ± ±

0.34 0.12 0.03 0.49

ethylene propene butene total alkene (C ≤ 5)

1.03 1.15 0.22 2.40

± ± ± ±

0.16 0.18 0.03 0.37

2.05 1.72 0.19 3.96

± ± ± ±

0.11 0.09 0.01 0.21

3.20 1.70 0.18 5.07

± ± ± ±

0.27 0.14 0.01 0.42

0.74 2.42 0.41 3.56

± ± ± ±

0.03 0.09 0.02 0.13

2.25 3.16 0.65 6.06

± ± ± ±

0.25 0.35 0.07 0.66

4.29 4.01 0.28 8.58

± ± ± ±

0.23 0.22 0.02 0.47

sum

80.04 ± 1.72

79.23 ± 1.80

81.28 ± 2.44

The selectivity of a product is calculated as

(4) The additive yield of products is calculated as YL × C L + YT × C T × 100% CL + CT

75.82 ± 4.26

78.61 ± 4.16

The yields of CO and CO2 both increased at higher catalyst temperatures. The yield of CO increased faster than that of CO2, indicating that decarbonylation is favored at higher catalyst temperatures. Because of the significant loss of carbon through solid residues (char and coke), the yield of aromatic hydrocarbons was low and its maximum yield was only 6.14 C% at 600 °C. Among aromatics, the increasing catalyst temperature leads to an increased selectivity of benzene, toluene and naphthalene, but a decreased selectivity of alkylbenzene, alkylnaphthalene and polyaromatic hydrocarbons. Polyaromatics are known as the major coke precursors; thus the decreasing polyaromatics corresponds to the lower coke yields at higher temperatures. The yields of both alkanes and alkenes increased at higher catalyst temperatures, as they are likely the products of aromatic dealkylation. Methane, ethylene and propylene were the major hydrocarbon gases, accounting for about 80% of total aliphatic hydrocarbons. Converting lignin over HZSM-5 produced fewer amounts of coke compared to HY zeolite. The coke yield was 24.96 C% at 400 °C and then further decreased to 15.35 C% at 600 °C. The yields of CO and CO2 were slightly higher, and increased faster at higher temperatures with HZSM-5 than with HY zeolites. Although the yield of alkane produced by HZSM-5 was similar to that from HY zeolite, the yield of total alkenes was higher with HZSM-5 zeolite, especially at a higher catalyst temperature

product selectivity (%) mole of carbon in aromatic/aliphatic product = × 100% total mole of carbon in aromatic/aliphatic group

=

73.71 ± 3.49

(5)

where YL/T is product yield when lignin or tetralin was pyrolyzed and CL/T is carbon content of lignin or tetralin in the feedstock (CL + CT = 1). The coke deposition on the used catalysts was analyzed by a Vario Micro Cube elemental analyzer (Elementar, Germany) to determine the carbon content in the coke. Each pyrolysis condition was triplicated and the standard error was reported.

■

RESULTS AND DISCUSSION Catalytic Pyrolysis of Lignin. The distribution and selectivity of pyrolysis products of lignin at different catalytic temperatures are summarized in Table 1 for both HY zeolite and HZSM-5 zeolite. Pyrolysis char recovered in the first reactor was 33.73 C%, due to the thermal decomposition of lignin. The yield of catalytic coke was high with HY zeolite, although it decreased from 33.93 C% at 400 °C to 25.81 C% at 600 °C. C


Research Article

ACS Sustainable Chemistry & Engineering Table 2. Catalytic Pyrolysis of Tetralin with HY and HZSM-5 Zeolite Catalysts HY temperature (°C) overall yield (C%) pyrolysis char catalytic coke CO CO2 benzene toluene ethyl-benzene p-xylene o-xylene benzene, 1-ethyl-3-methyl benzene, 1,2,3-trimethylindane indene benzene, n-butyl methyl Indane 2H tetralin naphthalene naphthalene, 2-methylnaphthalene, 1-methyl1,2-dimethyl-napthalene fluorene anthracene total aromatics

400

600

400

500

600

0.00 0.00 0.00 0.00

0.00 1.37 0.00 0.00

± ± ± ±

0.00 0.51 0.00 0.00

0.00 3.12 0.00 0.00

± ± ± ±

0.00 0.14 0.00 0.00

0.00 9.73 0.00 0.00

± ± ± ±

0.00 1.28 0.00 0.00

0.00 5.51 0.00 0.00

± ± ± ±

0.00 0.22 0.00 0.00

0.00 4.10 0.00 0.00

± ± ± ±

0.00 1.72 0.00 0.00

4.11 ± 0.13 1.24 ± 0.02 0.52 ± 0.01

17.82 10.99 1.34 1.82 0.56 0.70

± ± ± ± ± ±

1.43 0.68 0.13 0.14 0.06 0.05

19.11 13.28 1.36 2.26 0.75 0.42

± ± ± ± ± ±

0.44 0.51 0.06 0.09 0.02 0.03

22.45 3.70 1.29 0.55 0.32 0.37 0.96 1.95

± ± ± ± ± ± ± ±

0.70 0.02 0.04 0.03 0.04 0.00 0.10 0.10

29.43 4.34 0.72 1.01 0.71 0.24 0.60 1.40

± ± ± ± ± ± ± ±

1.36 0.01 0.07 0.10 0.04 0.01 0.05 0.12

31.33 6.71 0.20 0.97 0.90 0.10 0.31 0.89

± ± ± ± ± ± ± ±

0.27 0.42 0.01 0.09 0.07 0.01 0.03 0.03

± ± ± ± ±

1.52 0.30 0.35 0.19 0.16

9.01 5.31 5.77 4.89 5.97

± ± ± ± ±

0.51 0.46 0.49 0.01 0.51

6.15 4.34 7.12 3.00 5.36

± ± ± ± ±

0.19 0.16 0.63 0.31 0.11

0.00 2.54 0.00 0.00

± ± ± ±

HZSM-5

500

2.93 ± 0.12 6.79 46.73 2.14 25.48 4.04 1.10

± ± ± ± ± ±

0.14 0.07 0.11 0.94 0.25 0.30

92.15 ± 1.98

0.39 11.63 1.37 26.50 0.79 7.23

± ± ± ± ± ±

0.66 ± 0.11 0.31 ± 0.01

0.01 0.02 0.09 0.55 0.03 0.34

± ± ± ± ± ± ± ± ±

0.03 0.01 0.32 0.04 0.01 0.04 0.00 0.01 1.73

21.82 6.65 7.41 5.48 4.01

0.21 ± 0.03 0.12 ± 0.00 84.40 ± 3.68

0.40 0.13 29.14 0.05 9.69 2.01 0.14 0.39 80.09

76.96 ± 3.54

69.40 ± 3.73

67.38 ± 2.34


0.25 0.68 0.40 1.33

± ± ± ±

0.02 0.07 0.04 0.13

0.28 0.85 0.74 1.88

± ± ± ±

0.03 0.09 0.08 0.20

0.00 1.11 0.51 1.62

± ± ± ±

0.00 0.06 0.03 0.09

0.00 1.64 0.87 2.51

± ± ± ±

0.00 0.31 0.16 0.47

0.87 3.12 1.36 5.35

± ± ± ±

0.13 0.47 0.20 0.81

1.26 1.90 0.91 4.06

± ± ± ±

0.03 0.05 0.02 0.11


1.48 3.52 0.54 5.54

± ± ± ±

0.13 0.32 0.05 0.50

0.71 13.52 1.03 15.26

± ± ± ±

0.03 0.55 0.04 0.62

1.30 16.98 1.00 19.28

± ± ± ±

0.02 0.26 0.02 0.30

2.76 9.64 0.79 13.19

± ± ± ±

0.23 0.82 0.07 1.12

4.86 13.71 0.73 19.30

± ± ± ±

0.07 0.19 0.01 0.27

5.34 14.20 0.73 20.27

± ± ± ±

0.11 0.30 0.02 0.43

sum

101.56 ± 1.62

102.91 ± 4.78

104.12 ± 2.13

(8.58 C% for HZSM-5 versus 5.07 C% for HY at 600 °C). The aromatic yields were also higher with HZSM-5, reaching a maximum of 8.28 C% at 600 °C. This result agrees with previous findings that HZSM-5 is a more efficient catalyst than other zeolite catalysts in converting whole biomass as well as lignin.11 Compared to HY zeolite, HZSM-5 more selectively produced one ring aromatics. Dealkylation of alkylated benzenes and alkylated naphthalene was also observed with HZSM-5 when temperature increased. It should be noted in the table that total carbon balance of the measurable pyrolysis products was about 70−80 C% of lignin with both the catalysts. The mass balance of lignin pyrolysis products is usually low,37 because lignin-derived phenolic oligomers were not detected by GC/MS due to their low-volatility. Previously, Carlson et al.13 catalytically pyrolyzed isotope glucose and suggested that the oxygenated compounds are converted through a hydrocarbon pool existing in the zeolite’s cages. However, the mechanism for catalytic conversion of lignin-derived phenols is not well understood because the ligninderived phenolic monomers and oligomers are too large to enter the pores and unable to contribute directly to the hydrocarbon

102.39 ± 3.34

99.57 ± 2.00

95.82 ± 2.55

pool. For example, the molecular size of phenol is 6.728 Å, larger than the static pore size of HZSM-5.17 Thus, To et al.38 proposed that a phenolic pool exists on the catalyst’s surface. Mullen et al.39 previously suggested that the catalyst surface promotes thermal cracking of lignin and only aliphatic linkers among aromatic units enter the zeolite pores and further aromatize. They also suggested that simple phenols are difficult to be deoxygenated using zeolite and eventually polymerize to become coke precursors. On the contrary, Ma et al.40 suggested that all phenol alkoxy can be converted into aromatics. Other researchers previously suggested that phenols are selectively deoxygenated inside the pores and the narrow pore structure can stabilize phenols by preventing bimolecular polymerization reactions.41 Although it can be expected that the large pores of HY zeolite are beneficial in converting phenols with large molecule sizes, the results do not support it as a better catalyst for converting lignin than HZSM-5. Besides the pore size, HY zeolite is different from HZSM-5 in terms of acidity and acid strength, and they can also influence the pathway through which lignin is deoxygenated.17,42 Although the HY zeolite has a much larger surface area and higher acid density than the HZSM-5 D


Research Article

ACS Sustainable Chemistry & Engineering zeolite, its surface area is mostly located inside of the pores17 and the average acid strength is lower than that of HZSM-5.42 Thus, the deoxygenation ability of the external surface of the catalyst is less important for HY zeolite than for HZSM-5 zeolite.43,44 During lignin pyrolysis, the reactivity of the external surface of HZSM-5 zeolite could play an important role in addition to the reaction within the zeolite framework. Catalytic Pyrolysis of Tetralin. Tetralin was also catalytically converted by HY and HZSM-5 catalysts, and the results are summarized in Table 2. Tetralin can undergo dehydrogenation, hydrogenation, ring opening and isomerization when it is catalytically converted by zeolite.45 Tetralin was dehydrogenated primarily to naphthalene. Because the primary conversion of tetralin does not produce decalin, decalin found among pyrolysis products is the result of partial hydrogenation of naphthalene. Tetralin also produced alkylated benzene and aliphatic hydrocarbons through alicyclic ring-opening, and isomerization of tetralin produced methyl-indanes.45 The yield of aromatics was 92.15 C% when tetralin was converted by HY zeolite at 400 °C and it decreased at higher catalyst temperatures, accompanied by increasing aliphatic yields. Methyl-indanes were the major aromatics at lower catalysts temperature, but they rapidly decreased at higher temperatures; in turn, the selectivity of naphthalene, benzene, toluene and xylene (BTX) increased. This suggests that isomerization of tetralin was favored only at low temperatures and a higher temperature promotes dehydrogenation, alicyclic ring opening and dealkylation. This was also evident by the increasing yield of alkene from 5.54 C% at 400 °C to 19 C% at 600 °C. Nevertheless, the yield of alkylated naphthalene and other polyaromatics also increased with temperature. This is because HY zeolite easily promotes nondissociative bimolecular reactions among aromatics and alkenes due to high reactivity of the aromatic ring.38 The yields of light hydrocarbon gases were distributed among the gases with different numbers of carbon atoms at a low temperature, whereas propylene became the predominant gas at higher temperature. The light gases with lower carbon numbers (i.e., C1, C2) could be consumed by the alkylation of naphthalene at high temperature. Catalytic coke ranged from 1.37 to 3.17 C%, likely due to the overaromatization of the hydrocarbons. Contrary to lignin, tetralin had lower conversion with HZSM-5 than HY zeolite. The products also consisted of fewer amounts of aromatic hydrocarbons and more aliphatic hydrocarbons when HZSM-5 was the catalyst. Among the aromatics, the selectivity of naphthalene was low compared to it with HY zeolite, indicating that dehydrogenation of tetralin is not as strong as it with HY zeolite. The yield of aliphatics also increased with temperature, and the light gases were mostly alkenes as it was seen with HY zeolite. The selectivity of the light gases with lower carbon numbers increased at higher temperature, which is contradictory to when it was observed with HY zeolite. The yields of catalytic coke were higher than they were when tetralin was converted using HY zeolite. These results are slightly different from previous results shown by Townsend et al.45,46 as they reported that the coke yields are negligible for both HY zeolite and HZSM-5 when tetralin was converted in a fixed-bed flow reactor at 400 °C. Coke is carbonized solid or high-molecular-weight product absorbed onto the catalyst.17 The differences in coke yields and the product distributions with HY and HZSM-5 catalysts are due to the shape selectivity of the catalysts. The pore size of HZSM-5 is smaller than the molecular size of tetralin; thus tetralin mostly converts on the catalytic surface where the number of

Table 3. Copyrolysis of Lignin and Tetralin Using HY Zeolite temperature (°C)

400

500

600

lignin to tetralin ratio

1:0.41

1:0.68

1:0.92

overall yield (C%) pyrolysis char catalytic coke CO CO2 benzene toluene ethyl-benzene p-xylene o-xylene styrene benzene, 1-ethyl-3-methyl benzene, 1,2,3-trimethylindane indene methyl indane 2H tetralin naphthalene naphthalene, 2-methylnaphthalene, 1-methyl1-ethyl-napthalene 1,2-dimethyl-napthalene fluorene anthracene total aromatics

26.14 15.63 0.45 2.88

± ± ± ±

1.03 2.70 0.05 0.20

19.89 5.91 0.43 1.86

± ± ± ±

0.79 0.58 0.02 0.25

17.72 3.80 0.44 1.36

± ± ± ±

0.70 0.30 0.01 0.03

0.96 0.84 0.49 0.29 0.16

± ± ± ± ±

0.11 0.04 0.03 0.04 0.01

7.87 5.03 1.20 0.88 0.32

± ± ± ±

0.28 0.20 0.00 0.02

12.87 8.48 0.99 1.70 0.62

± ± ± ±

0.17 0.56 0.01 0.07

0.48 ± 0.03

0.68 ± 0.01

1.86 ± 0.26

2.45 ± 0.09

0.54 0.22 0.47 0.22 29.12 0.09 8.34 1.70

± ± ± ± ± ± ± ±

0.04 0.01 0.06 0.04 0.61 0.04 0.16 0.05

9.11 0.73 17.32 3.35 2.74

± ± ± ± ±

0.21 0.08 0.95 0.19 0.03

1.23 0.17 0.11 39.83

± ± ± ±

0.03 0.02 0.01 2.07


0.59 0.52 0.71 1.82

± ± ± ±

0.07 0.06 0.08 0.20

1.61 0.74 0.35 2.71

± ± ± ±

0.08 0.04 0.02 0.13

1.20 0.55 0.54 2.29

± ± ± ±

0.05 0.02 0.02 0.10


0.45 2.31 0.33 3.10

± ± ± ±

0.02 0.10 0.02 0.14

2.16 6.26 0.85 9.27

± ± ± ±

0.06 0.16 0.02 0.24

2.23 11.27 1.29 14.79

± ± ± ±

0.09 0.43 0.05 0.57

sum

89.86 ± 4.42

9.96 8.99 10.49 0.80 5.25 1.20

± ± ± ± ± ±

0.37 ± 0.02

0.36 0.60 0.51 0.10 0.04 0.04

0.20 ± 0.02 0.13 ± 0.00 55.45 ± 2.28

95.52 ± 2.93

0.25 ± 0.02 0.27 ± 0.01 66.23 ± 1.87

106.63 ± 1.89

active site is limited. As a result, adsorption and polymerization of tetralin-derived products on the catalyst surface could occur at low temperatures for HZSM-5. The coke yield decreased at higher temperatures because desorption and cracking on the surface increased. The coke formation within the pore is also related to the shape and structure of zeolite, especially pore size. HZSM-5 has narrow pores and confined channel structure and it does not support bimolecular reactions of bulky aromatic molecules. However, benzene and the light alkenes could undergo alkylation and polymerization reaction. Once the size of the molecule exceeds the pore size of HZSM-5, it can only stay within the pores. As for tetralin conversion with HY zeolite, dehydrogenation and isomerization were the two main reactions occurred. In HY zeolite, bimolecular reactions within the zeolite supercages could further proceed to form condensed polyaromatic coke. For example, methyl indanes from isomerization and naphthalene could react to form polyaromatic coke. The coke yield with HY zeolite is low because the open-end pore structure also allows some polyaromatics to leave the pores before forming coke.47 E


Research Article


Figure 1. Comparison of experimental yields and additive yields of products obtained from catalytic copyrolysis of lignin and tetralin over HY zeolite.

Catalytic Copyrolysis Lignin and Tetralin. HY Zeolite. The product distribution and selectivity for catalytic copyrolysis of lignin and tetralin over HY zeolite are given in Table 3. Tetralin conversion was varied at different catalyst temperatures and the ratios of lignin to converted tetralin are given in the table. Carbon balance is calculated based on the sum of lignin and converted tetralin because both lignin and tetralin produce hydrocarbons and coke. As shown in the table, more tetralin was converted at higher catalyst temperatures as the ratio of lignin to tetralin changed from 1:0.41 to 1:0.92. The yields of carbon oxides in the mixture decreased at higher temperatures although their absolute values increased, due to the increasing conversion of tetralin along with temperature. The major products from tetralin pyrolysis are aromatics. Because the amount of converted tetralin increases with increased temperature while the lignin amount remains the same, the aromatic yield in the mixture increased. Naphthalene was the major aromatic product at all temperatures and its selectivity among aromatics did not change significantly. Partially dehydrogenated tetralin or decalin were no longer found because lignin abstracts the hydrogen radicals from tetralin. Increasing temperature also

increased the selectivity of BTX at the expense of decreasing selectivity for alkylated benzene, alkylated naphthalene and polyaromatics. On the other hand, the yield of alkenes increased rapidly with temperature and propylene was the major gas as its selectivity reached 64.51% at 600 °C. In comparison, the yield of alkanes was low and did not change noticeably with increasing temperature. The yield of catalytic coke decreased, which corresponds to increased hydrocarbon yields at higher temperatures. The synergetic effect among lignin and tetralin during catalytic copyrolysis was evaluated and the results are given in Figure 1. The product yields measured during catalytic copyrolysis of lignin and tetralin are labeled as “experimental”. The “additive” yields are the products yields if there is no interaction between lignin and tetralin, calculated based on the respective product yields when lignin or tetralin was pyrolyzed independently. Copyrolysis had negligible effect on pyrolysis char, though it reduced catalytic coke significantly. The extent of coke reduction by copyrolysis became stronger at higher catalysis temperature as the coke yield was only 3.8 C% compared to 12.87 C% of the additive yield at 600 °C. This change was also observed F


Research Article


Figure 2. GC/MS chromatograms of the pyrolysis vapor produced from catalytic pyrolysis of lignin, tetralin and the mixture of ligin and tetralin over HY zeolite (pyrolysis temperature, 500 °C; catalytic temperature, 400 °C).

light gases. The aliphatics were possibly consumed during the above-mentioned bimolecular alkylation reactions. Despite increasing aromatic yields, the yields of carbon oxides decreased by copyrolyzing lignin with tetralin. This suggests that not only was the extra oxygen removed by hydrodeoxygenation, but hydrogen transfer also changed the deoxygenation mode of the phenolic compounds at zeolite catalyst by suppressing decarboxylation or decarbonylation. It was also noted in Table 3 that the overall carbon balance of the measurable pyrolysis products greatly improved compared to pyrolyzing lignin alone, especially at higher catalyst temperatures. This suggests that heavy phenolic oligomers that were previously undetected by GC/MS were also effectively deoxygenated by the zeolite in the presence of tetralin. HZSM-5 Zeolite. The product distribution and selectivity for catalytic copyrolysis of lignin and tetralin using HZSM-5 zeolite are given in Table 4. Although a higher temperature converted more tetralin, conversion of tetralin was overall lower with HZSM-5 zeolite compared to HY zeolite. Despite the fact that naphthalene was still the major conversion product at all temperatures, its yield was much lower than what that was produced with HY zeolite because fewer amounts of tetralin were dehydrogenated. The yield of catalytic coke decreased to 8.87 C% at 600 °C, whereas the yields of aromatic and alkene both increased with increasing temperature. Similar to HY zeolite, higher temperature decreased the selectivity of alkylated benzene and increased the selectivity of benzene and toluene. Propylene was the main light aliphatic hydrocarbon, followed by ethylene and methane. The synergetic effects between lignin and tetralin for producing different groups of the pyrolysis products are shown

from the used catalytic beds retrieved from the reactor after pyrolyzing lignin alone or pyrolyzing the mixture of lignin and tetralin. Fresh catalyst had a white color, whereas the used catalytic bed after pyrolyzing lignin was entirely covered by black coke. When lignin was copyrolyzed with tetralin, only a very thin layer of the catalyst bed at the top where the vapor enters was dark black, while the rest of catalyst bed immediately became light gray. This observation clearly indicates that the interaction between phenols and tetralin at the catalyst site strongly suppresses coke formation. The synergistic reduction in coke was accompanied by a dramatic increase in the yield of aromatic hydrocarbon and the extent of synergy became stronger with increasing temperatures. Figure 2 compares the GC/MS chromatograms of the products derived from catalytic pyrolysis of lignin, tetralin and copyrolysis of lignin and tetralin over HY zeolite. As shown, copyrolysis of lignin and tetralin over the zeolite strongly promoted the formation of alkylated naphthalene. This is due to bimolecular reactions among tetralin and phenols. It was suggested that trans-alkylation reactions among tetralin, naphthalene and methoxyl groups could eliminate methoxyl group.48 Tetralin-derived aromatics could also be alkylated by the side chain fragments of lignin. Similarly, an aliphatic chain fragment produced from the cleavage of an alicyclic ring of tetralin could combine with a lignin-originated aromatic ring to form alkylated aromatics.48 The yields of alkanes and alkenes shown in Figure 1e,f did not change noticeably with copyrolysis, although the extra hydrogen supplied by tetralin could have promoted hydrocracking of the phenols to increase the yields of G


Research Article


catalyst affects the hydrodeoxygenation mechanism of ligninderived phenols. Most lignin-derived phenols are inaccessible to the micropores of HZSM-5. Thus, cracking and deoxygenation of the phenolic oligomers and monomers (i.e., phenolic pool) on the external surface of the zeolite are the primary reactions. In HZSM-5, the side chain fragments of the phenols that resulted from the cracking could sequentially enter the zeolite pores and be deoxygenated through the hydrocarbon pool mechanism. An interesting observation was made by pyrolyzing lignin samples over the same catalyst bed sequentially and monitoring the changes in the product distribution. When the HZSM-5 catalyst was fresh, the GC/MS detectable pyrolysis products of lignin were mostly hydrocarbons (and carbon oxides). As the catalyst deactivates at subsequence test runs, a number of phenols with various side chain functionalities including phenol, methoxylphenols, methylphenols and propenyl phenol all appeared around the same time (Figure 5). Catalyst deactivation reduces the effectiveness of the catalyst and prevents the deoxygenation reactions to be fully proceeded. Thus, the change in the product distribution of lignin as catalyst deactivates could shed a light into the order of reactions of phenols occurring on the zeolite. The above observation supports that the simultaneous cracking of various phenolic compounds on the external catalyst surface is followed by the formation of aromatic hydrocarbons inside the pores from their side chain fractions. Nevertheless, the number of the external active sites is limited; thus the adsorbed phenols could also thermally polymerize and dehydrate to form coke on the catalyst surface. The polymerized phenols or dehydrated coke would deactivate the surface active sites and also block the entrance of catalyst pores. At a higher catalyst temperature, thermal cracking and desorption of the phenols on the catalyst surface are improved and more molecules are converted inside the pores. A higher temperature is also able to expand the catalyst pore17 and as a result, simple phenols could enter the thermally enlarged pores and be converted. Because of its molecular size, tetralin mostly reacts on the external surface of HZSM-5 catalyst when it was copyrolyzed with lignin at low temperatures (below 500 °C). For HZSM-5, the hydrogen radicals on the catalyst surface can stabilize and/or crack the adsorbed phenols to reduce polymerization and enhance surface desorption. However, dehydrogenation of tetralin is weak not only due to the limited acid sites on the external surface, but also because the adsorption of lignin-derived phenols on the catalyst surface49 deprives the accessibility of tetralin molecules on the external active sites for further reactions. Moreover, the hydrogen atoms could become diluted by surrounding and sweep gas and cannot be effectively utilized by lignin. As the catalyst temperature increases, the surface desorption and thermal cracking of the phenols and tetralin are promoted. As a result, an increased amount of side chain fragments of phenols and tetralin-derived alkenes enter the catalyst pores to be deoxygenated and aromatized through the hydrocarbon pool mechanism. The synergistic effect at higher temperatures could also be caused by the enlargement of the catalyst pores. Yu et al.17 previously suggested that the effective pore size of HZSM-5 can increase to 8.1 Å at 650 °C. Nondissociative bimolecular reaction within the pores is discouraged due to its smaller pore sizes and structural hindrance. However, dissociative bimolecular reaction could still occur within the confined HZSM-5 pores. It was reported that methyl group dissociated from anisole remains on the catalyst site and then further reacts with tetralin to form alkylated aromatics.48 Because

Table 4. Copyrolysis of Lignin and Tetralin Using HZSM-5 Zeolite temperature (°C)

400

500

600

lignin to tetralin ratio

1:0.28

1:0.30

1:0.40

overall yield (C%) pyrolysis char catalytic coke CO CO2

29.65 19.21 1.23 3.56

± ± ± ±

1.17 1.17 0.20 0.31

28.98 14.02 1.59 3.85

± ± ± ±

1.15 2.31 0.13 0.00

26.41 8.87 1.58 2.85

± ± ± ±

1.04 1.94 0.15 0.11

4.83 1.66 0.35 0.60 0.15

± ± ± ± ±

0.02 0.00 0.01 0.02 0.01

14.73 3.46 0.26 1.15 0.46

± ± ± ± ±

0.60 0.09 0.00 0.01 0.04

15.57 4.07 0.11 0.81 0.60

± ± ± ± ±

1.51 0.11 0.04 0.05 0.06

0.18 0.23 0.45 6.93 1.76 2.23 1.97 1.85 23.19

± ± ± ± ± ± ± ± ±

0.00 0.00 0.13 0.03 0.31 0.21 0.01 0.04 0.80

0.13 0.23 0.63 3.74 2.48 3.53 1.53 2.73 28.69

± ± ± ± ± ± ± ± ±

0.01 0.02 0.02 0.08 0.10 0.02 0.16 0.06 1.22

0.06 0.16 0.46 3.16 2.41 4.77 1.26 2.49 35.93

± ± ± ± ± ± ± ± ±

0.01 0.02 0.08 0.23 0.22 0.13 0.24 0.38 3.09


0.92 0.61 0.60 2.13

± ± ± ±

0.15 0.10 0.10 0.35

1.79 0.31 0.21 2.31

± ± ± ±

0.09 0.02 0.01 0.12

1.65 0.19 0.08 1.92

± ± ± ±

0.07 0.01 0.00 0.08


0.91 3.52 0.30 4.73

± ± ± ±

0.06 0.25 0.02 0.33

4.18 5.83 1.88 11.90

± ± ± ±

0.10 0.13 0.04 0.28

4.82 6.08 1.48 12.39

± ± ± ±

0.20 0.26 0.06 0.52

benzene toluene ethyl-benzene p-xylene o-xylene styrene benzene, 1-ethyl-3-methyl benzene, 1,2,3-trimethylindane methyl indane 2H tetralin naphthalene naphthalene, 2-methylnaphthalene, 1-methyltotal aromatics

sum

83.69 ± 3.77

97.73 ± 4.15

89.96 ± 6.18

in Figure 3. Unlike HY zeolite, for which the synergistic effect was observed at all the tested catalyst temperatures, there was no noticeable synergy between lignin and tetralin until the catalyst temperature reached 600 °C. At this temperature, the aromatic yield increased to 35.93 C% from its additive yield of 30.14 C% for HZSM-5 zeolite. In comparison, the yield increased to 66.23 C% from 48.79 C% for HY catalyst at the given temperature (Figure 2d). The used catalyst bed after copyrolysis of lignin and tetralin over HZSM-5 was covered entirely with black coke, similar to the used catalytic bed recovered after lignin was pyrolyzed alone. Compared to HY zeolite, the alkylation reaction is not significant with HZSM-5 (Figure 4). As shown in Table 4, the overall carbon balance of measurable copyrolysis products was lower with HZSM-5 compared to it with HY zeolite, suggesting that phenolic oligomers could still be present due to ineffective deoxygenation. Deoxygenation Mechanisms of Lignin-Derived Phenols and Role of Hydrogen Transfer at Different Zeolites. HZSM-5 was a more effective catalyst when lignin was converted alone, whereas HY zeolite was better at deoxygenating lignin when lignin and tetralin were copyrolyzed. The variation in synergistic effects among lignin and tetralin during catalytic copyrolysis over HY and HZSM-5 suggests that the shape selectivity of the H


Research Article


Figure 3. Comparison of experimental yields and the additive yields of products obtained from catalytic copyrolysis of lignin and tetralin over HZSM-5 zeolite.

of the cracking on the external surface and the aromatization in the confined catalyst pores, HZSM-5 promotes higher selectivity of simple aromatics rather than naphthalenes. With the pore enlargement, the effect of the pore size of HZSM-5 zeolite at a higher temperature was expected to be similar to that of HY zeolite at a lower temperature, because tetralin and simpler phenolic monomers could enter the enlarged pores and interact more effectively at the inner active sites. The product distribution at 600 °C for HZSM-5 and at 400 °C for HY present in Tables 3 and 4 shows good agreement in terms of CO2, aromatics and alkane yields. However, alkenes and coke yields were different for HZSM-5 and HY at these temperatures, suggesting that other factors could also play roles. For example, a higher catalytic temperature tends to promote the olefin formation and coke reduction.50 The acidity difference in the two catalysts also affects the conversion outcomes. Nondissociative bimolecular reaction within the pores is discouraged due to its smaller pore sizes and structural hindrance. However, dissociative bimolecular reaction could still occur within the confined HZSM-5 pores. It was reported that methyl

group dissociated from anisole remains on the catalyst site and then further reacts with tetralin to form alkylated aromatics.48 Because of the cracking on the external surface and the aromatization in the confined catalyst pores, HZSM-5 promotes higher selectivity of simple aromatics rather than naphthalenes. On the other hand, the static pore size of HY zeolite is 7.4 Å and it could further expand an additional 2.5−3.4 Å at an elevated temperature.17 Previously, it has been shown that lignin primary depolymerizes to phenolic monomers rather than oligomers.23,51 Although the adsorption and cracking of phenolic pool could still occur on the external surface of the catalyst, an increased amount of phenolic monomers directly enter the pores of HY zeolite17 and be deoxygenated within the zeolite framework through sequential cleavage of phenolic side chains. The different pattern that was produced for phenolic formation due to HY catalyst deactivation (Figure 6) compared to that with HZSM-5 zeolite supports the theory. As deactivation of the catalyst progressed, the amount of phenol increased quickly and anisole and cresols were also found to increase at later runs (Figure 6). Methoxylphenols and other phenols with I


Research Article


Figure 4. GC/MS of the pyrolysis vapor produced from catalytic pyrolysis of lignin, tetralin and the mixture of ligin and tetralin over HZSM-5 zeolite (pyrolysis temperature, 500 °C; catalytic temperature, 400 °C).

Figure 5. Formation of phenols due to catalyst deactivation when lignin samples were converted over the same HZSM-5 catalyst bed sequentially (pyrolysis temperature, 500 °C; catalyst temperature, 500 °C).

Figure 6. Formation of phenols due to catalyst deactivation when lignin samples were converted over the same HY catalyst bed sequentially (pyrolysis temperature, 500 °C; catalyst temperature, 500 °C).

longer side chains that were previously seen when HZSM-5 deactivated were not found, which indicates that deoxygenation of lignin-derived phenols proceeds through the cleavage of the longer side chain of phenols, followed by demethoxylation, and then finally dehydroxylation of the phenolic OH. Anisole was formed when phenolic radicals (PhO·) reacted with methyl side chain fragments. Deoxygenation of phenol is supposedly to produce benzene as the main aromatic hydrocarbon. However, this was not the case because bimolecular reactions and alkylation of a reactive benzene ring could easily occur inside the large pores. Nevertheless, deoxygenating phenolic rings through cracking and subsequence demethoxylation and dehydroxylation

is not an efficient pathway when lignin is converted alone. Phenolic hydroxyl group is difficult to be cleaved as its dissociation active energy is 463.6 kJ/mol, much higher than that of a methoxyl group, which is 268.6 kJ/mol.2 Phenolic hydroxyl is known for its reactivity for polymerization and cross-linking, and the phenolic methoxyl group also promotes polymerization and coke formation.52,53 When phenols are converted by zeolite, the acidity of the catalyst can promote polymerization and dehydration of the phenols within and outside of the pores. Polymerization and overaromatization through bimolecular reactions in the zeolite supercages also occur, thus the selectivity of polyaromatic hydrocarbons is higher in HY zeolite than with J


Research Article


theory, the relationship between the amount of hydrogen atoms provided by tetralin and the extent of synergistic change in aromatic hydrocarbon yield and coke yield by copyrolysis of lignin and tetralin was investigated in the present study and the results are plotted in Figure 7. In the figure, the amount of hydrogen atoms provided by tetralin during copyrolysis was calculated based on the dehydrogenation products of tetralin. Although lignin pyrolysis also produced naphthalenes, the amount is minimal when compared to that produced from tetralin. The synergistic factors of coke reduction and the aromatic increase were calculated by dividing the experimental yields of aromatics or coke by their respective additive yields at their respective temperatures. As shown, the extent of aromatic increase or coke reduction was nearly linearly correlated to the amount of hydrogen atoms donated by tetralin. Although the interactions between lignin and tetralin include hydrogen transfer and alkylation by bimolecular reactions, the amount of hydrogen atoms can be transferred from tetralin to lignin is directly related to the synergistic effects. It is known that the catalytic coke yield can be correlated to the effective hydrogen to carbon ratio, (H/C)eff, of the feedstock55−57 Because the (H/C)eff of tetralin is higher than that of lignin, the mixture of tetralin and lignin has a more improved (H/C)eff than lignin, which may also explain the decreased coke yield during copyrolysis. The conversion mechanisms of lignin-derived phenols at HY or HZSM-5 catalysts in the presence of tetralin are illustrated in Figure 8. Hydrogen transfer and bimolecular reaction account for the major reactions in the catalytic copyrolysis of lignin and tetralin over HY zeolite. For HZSM-5 zeolite, the products from hydrogen-promoted cracking and deoxygenation on the surface of the catalyst become the feedstock for the hydrocarbon pool. Deoxygenation and cracking ability of the catalyst are also correlated with the acidity of the catalyst. During catalytic pyrolysis of biomass, coke formation and dehydration are two reasons for reversible and irreversible deactivation of acid sites.58 Although HY zeolite has higher acidity than HZSM-5 due to lower SiO2/Al2O3 ratio, water formed by dehydration of lignin both on the surface and inside the pores could hydrolyze the Al−O bonds and break the framework.59 Together with the heavy coking due to the large pore size, the acid sites of HY are deactivated rapidly. For HZSM-5, most dehydration reactions could occur at the catalyst surface, where water can be swept away by the carrier gas instantly after formation. The coke formation is also limited because of the small pore size. Thus, the acid sites of HZSM-5 could be more stable, leading a better performance than the HY zeolite during lignin pyrolysis. However, HY zeolite outperformed HZSM-5 in tetralin conversion because the acid sites are expected to be stable as there is no water formation and also less coke is formed. Because the addition of tetralin in lignin pyrolysis favors hydrodeoxygenation rather than decarbonylation and decarboxylation, initially formed water could deactivate the acid sites at low temperatures, therefore inhibit the synergistic effects. As temperature increased, enhanced mass transfer at higher temperature within enlarged pores encourages the removal of water and the reaction products. This, in turn, can help maintain the acid sites and favor the reactions between lignin and tetralin.

Figure 7. Relationship between the amount of hydrogen atoms provided and synergistic factors: (a) aromatic hydrocarbons; (b) coke.

HZSM-5 zeolite. HZSM-5 was more effective than HY zeolite in converting lignin when tetralin was absent, suggesting that deoxygenation of lignin through the combination of the surface cracking of the phenols and aromatization of the side chain fragments through the hydrocarbon pool mechanism within the pores in HZSM-5 zeolite was more effective than directly deoxygenating phenols on active sites in HY zeolite. When lignin was copyrolyzed with tetralin over HY zeolite, tetralin provides hydrogen radicals at the inner pores where the density of active site is high. This external hydrogen can directly influence the deoxygenation pathway of the phenols at the active sites to remove oxygen effectively and reduce coke formation. As described previously, decreased amounts of carbon oxides were formed because hydrodeoxygenation competes with decarboxylation and decabonylation at the catalyst site. Thilakaratne et al.19 suggested that phenol is initially converted to aryl, phenoxyl and hydrogen radicals when it is converted by zeolite catalyst. Although the hydrogen radicals then react with aryl or hydroxyl radicals to form benzene or water, phenolic hydroxyl radicals react with the hydroxyl radical to form 1,4benzoquinone and cyclopenta-2,4-dien-1-one and eventually to polyaromatics through series of decarbonylation reactions. In fact, decarbonylation is thought to be the main deoxygenation pathway of biomass by zeolite.37 When hydrogen presents, hydrodeoxygenation of phenol or alkylated phenols (methyl or ethyl phenols) directly remove the phenolic hydroxyl group as water to form benzene and alkylated benzene without involving decarbonylation.54 Thus, hydrodeoxygenation can avoid unnecessary carbon loss from aromatic rings. The hydrogen also suppresses polymerization and dehydration of the polymers, therefore reducing coke formation. Accordingly, the higher concentration of hydrogen available at the active sites of zeolite should be more beneficial to HY zeolite. To confirm this K


Research Article


Figure 8. Reaction pathways for copyrolysis of lignin and tetralin: (a) HY zeolite (R is allylic side chain); (b) HZSM-5 zeolite.

■

CONCLUSIONS Catalytic copyrolysis of lignin and tetralin was conducted in atmospheric pressure using HZSM-5 and HY zeolites. Strong synergistic effects were observed when lignin and tetralin were copyrolyzed using HY zeolite, including a dramatic decrease in coke formation and a significant increase in aromatic yields compared to converting lignin and tetralin independently. Deoxygenation of lignin-derived phenols mostly occurs within the large pores of HY zeolite through sequential cleavage of phenolic side chain, whereas this pathway was ineffective due to low reactivity of phenols for deoxygenation. When tetralin was copyrolyzed, hydrogen transfer inside the pores changed the deoxygenation mode of the phenols to promote hydrodeoxygenation and suppress decarbonylation and decarboxylation. The amount of hydrogen atoms transferred at HY catalyst was linearly correlated to the extent of synergistic decrease in coke formation and increase in aromatic yield, suggesting that hydrogen transfer at active sites is highly effective for HY catalyst. Copyrolysis also increased the yields of alkylated aromatics due to bimolecular reactions between lignin-derived phenols and tetralin within the pores. On the other hand,

synergistic effects between lignin and tetralin were insignificant with HZSM-5 zeolite, especially at lower catalyst temperatures. Because of its micropores, cracking on the external surface of HZSM-5 is the primary reaction of lignin-derived phenols, and the side chain fragments subsequently enter the zeolite pores to be deoxygenated and aromatized through the hydrocarbon pool mechanism. The external hydrogen mostly interacts with the phenols on the external surface of the catalyst to prevent polymerization reaction and promote desorption of phenols. However, the effect was not significant due to the low local concentration of hydrogen on the catalyst surface.

■

AUTHOR INFORMATION

Corresponding Author

*Tel: +1 515 294 6886. Fax: +1 515 294 3261. E-mail: [email protected] (X. Bai). Author Contributions §

Both authors contributed equally to this work.

Notes

The authors declare no competing financial interest. L


Research Article


■

(20) Jan, O.; Marchand, R.; Anjos, L. C.; Seufitelli, G. V.; Nikolla, E.; Resende, F. L. Hydropyrolysis of Lignin Using Pd/HZSM-5. Energy Fuels 2015, 29 (3), 1793. (21) Thangalazhy-Gopakumar, S.; Adhikari, S.; Gupta, R. B.; Tu, M.; Taylor, S. Production of hydrocarbon fuels from biomass using catalytic pyrolysis under helium and hydrogen environments. Bioresour. Technol. 2011, 102 (12), 6742. (22) Melligan, F.; Hayes, M.; Kwapinski, W.; Leahy, J. Hydropyrolysis of biomass and online catalytic vapor upgrading with NiZSM-5 and Ni-MCM-41. Energy Fuels 2012, 26 (10), 6080. (23) Patwardhan, P. R.; Brown, R. C.; Shanks, B. H. Understanding the fast pyrolysis of lignin. ChemSusChem 2011, 4 (11), 1629. (24) Kim, K. H.; Bai, X.; Cady, S.; Gable, P.; Brown, R. C. Quantitative Investigation of Free Radicals in Bio-Oil and their Potential Role in Condensed-Phase Polymerization. ChemSusChem 2015, 8 (5), 894. (25) Fogassy, G.; Thegarid, N.; Schuurman, Y.; Mirodatos, C. From biomass to bio-gasoline by FCC co-processing: effect of feed composition and catalyst structure on product quality. Energy Environ. Sci. 2011, 4 (12), 5068. (26) Ma, Z.; van Bokhoven, J. A. Deactivation and Regeneration of H-USY Zeolite during Lignin Catalytic Fast Pyrolysis. ChemCatChem 2012, 4 (12), 2036. (27) Jeon, M.-J.; Jeon, J.-K.; Suh, D. J.; Park, S. H.; Sa, Y. J.; Joo, S. H.; Park, Y.-K. Catalytic pyrolysis of biomass components over mesoporous catalysts using Py-GC/MS. Catal. Today 2013, 204, 170. (28) Nimmanwudipong, T.; Runnebaum, R. C.; Ebeler, S. E.; Block, D. E.; Gates, B. C. Upgrading of Lignin-Derived Compounds: Reactions of Eugenol Catalyzed by HY Zeolite and by Pt/γ-Al2O3. Catal. Lett. 2012, 142 (2), 151. (29) Neavel, R. C. Liquefaction of coal in hydrogen-donor and nondonor vehicles. Fuel 1976, 55 (3), 237. (30) Kim, K. H.; Brown, R. C.; Kieffer, M.; Bai, X. Hydrogen-donorassisted solvent liquefaction of lignin to short-chain alkylphenols using a micro reactor/gas chromatography system. Energy Fuels 2014, 28 (10), 6429. (31) Vasilakos, N. P.; Austgen, D. M. Hydrogen-donor solvents in biomass liquefaction. Ind. Eng. Chem. Process Des. Dev. 1985, 24 (2), 304. (32) Thring, R. W.; Breau, J. Hydrocracking of solvolysis lignin in a batch reactor. Fuel 1996, 75 (7), 795. (33) Connors, W.; Johanson, L.; Sarkanen, K.; Winslow, P. Thermal degradation of kraft lignin in tetralin. Holzforschung 1980, 34 (1), 29. (34) Thring, R. W.; Chornet, E.; Overend, R. P. Thermolysis of glycol lignin in the presence of tetralin. Can. J. Chem. Eng. 1993, 71 (1), 107. (35) Krichko, A.; Skvortsov, D.; Titova, T.; Filippov, B.; Dogadkina, N. Production of tetralin by the hydrogenation of naphthalenecontaining fractions. Chem. Technol. Fuels Oils 1969, 5 (1), 18. (36) Xue, Y.; Kelkar, A.; Bai, X. Catalytic co-pyrolysis of biomass and polyethylene in a tandem micropyrolyzer. Fuel 2015, 166, 227. (37) Wang, K.; Kim, K. H.; Brown, R. C. Catalytic pyrolysis of individual components of lignocellulosic biomass. Green Chem. 2014, 16 (2), 727. (38) To, A. T.; Resasco, D. E. Role of a phenolic pool in the conversion of m-cresol to aromatics over HY and HZSM-5 zeolites. Appl. Catal., A 2014, 487, 62. (39) Mullen, C. A.; Boateng, A. A. Catalytic pyrolysis-GC/MS of lignin from several sources. Fuel Process. Technol. 2010, 91 (11), 1446. (40) Ma, Z.; Troussard, E.; van Bokhoven, J. A. Controlling the selectivity to chemicals from lignin via catalytic fast pyrolysis. Appl. Catal., A 2012, 423, 130. (41) Li, C.; Zhao, X.; Wang, A.; Huber, G. W.; Zhang, T. Chem. Rev. 2015, 115, 11559. (42) Elordi, G.; Olazar, M.; Lopez, G.; Castaño, P.; Bilbao, J. Role of pore structure in the deactivation of zeolites (HZSM-5, Hβ and HY) by coke in the pyrolysis of polyethylene in a conical spouted bed reactor. Appl. Catal., B 2011, 102 (1), 224.

ACKNOWLEDGMENTS The authors greatly acknowledge the research funding supported by ExxonMobil Co. The authors thank Wenqi Li and Dakota Even for assisting the experimental studies and acknowledge Dr. Robert Brown, Mr. Ryan Smith, Mr. Patrick Johnston and Dr. Marjorie Rover at Bioeconomy Institute of Iowa State University for useful discussions and technical support.

■

REFERENCES

(1) Nowakowski, D. J.; Bridgwater, A. V.; Elliott, D. C.; Meier, D.; de Wild, P. Lignin fast pyrolysis: results from an international collaboration. J. Anal. Appl. Pyrolysis 2010, 88 (1), 53. (2) Jiang, G.; Nowakowski, D. J.; Bridgwater, A. V. Effect of the temperature on the composition of lignin pyrolysis products. Energy Fuels 2010, 24 (8), 4470. (3) Azadi, P.; Inderwildi, O. R.; Farnood, R.; King, D. A. Liquid fuels, hydrogen and chemicals from lignin: a critical review. Renewable Sustainable Energy Rev. 2013, 21, 506. (4) Pandey, M. P.; Kim, C. S. Lignin depolymerization and conversion: a review of thermochemical methods. Chem. Eng. Technol. 2011, 34 (1), 29. (5) Holladay, J.; Bozell, J.; White, J.; Johnson, D. Top value-added chemicals from biomass; DOE Report PNNL-16983; PNNL: Richland, WA, 2007; DOI: 10.2172/921839. (6) Kleinert, M.; Barth, T. Phenols from lignin. Chem. Eng. Technol. 2008, 31 (5), 736. (7) Bridgwater, A.; Meier, D.; Radlein, D. An overview of fast pyrolysis of biomass. Org. Geochem. 1999, 30 (12), 1479. (8) De, S.; Saha, B.; Luque, R. Hydrodeoxygenation processes: Advances on catalytic transformations of biomass-derived platform chemicals into hydrocarbon fuels. Bioresour. Technol. 2015, 178, 108. (9) Goyal, H.; Seal, D.; Saxena, R. Bio-fuels from thermochemical conversion of renewable resources: a review. Renewable Sustainable Energy Rev. 2008, 12 (2), 504. (10) Carlson, T. R.; Vispute, T. P.; Huber, G. W. Green gasoline by catalytic fast pyrolysis of solid biomass derived compounds. ChemSusChem 2008, 1 (5), 397. (11) Jae, J.; Tompsett, G. A.; Foster, A. J.; Hammond, K. D.; Auerbach, S. M.; Lobo, R. F.; Huber, G. W. Investigation into the shape selectivity of zeolite catalysts for biomass conversion. J. Catal. 2011, 279 (2), 257. (12) Vispute, T. P.; Zhang, H.; Sanna, A.; Xiao, R.; Huber, G. W. Renewable chemical commodity feedstocks from integrated catalytic processing of pyrolysis oils. Science 2010, 330 (6008), 1222. (13) Zhang, H.; Cheng, Y.-T.; Vispute, T. P.; Xiao, R.; Huber, G. W. Catalytic conversion of biomass-derived feedstocks into olefins and aromatics with ZSM-5: the hydrogen to carbon effective ratio. Energy Environ. Sci. 2011, 4 (6), 2297. (14) Carlson, T. R.; Jae, J.; Lin, Y.-C.; Tompsett, G. A.; Huber, G. W. Catalytic fast pyrolysis of glucose with HZSM-5: the combined homogeneous and heterogeneous reactions. J. Catal. 2010, 270 (1), 110. (15) Carlson, T. R.; Tompsett, G. A.; Conner, W. C.; Huber, G. W. Aromatic production from catalytic fast pyrolysis of biomass-derived feedstocks. Top. Catal. 2009, 52 (3), 241. (16) Huber, G. W.; Iborra, S.; Corma, A. Synthesis of transportation fuels from biomass: chemistry, catalysts, and engineering. Chem. Rev. 2006, 106 (9), 4044. (17) Yu, Y.; Li, X.; Su, L.; Zhang, Y.; Wang, Y.; Zhang, H. The role of shape selectivity in catalytic fast pyrolysis of lignin with zeolite catalysts. Appl. Catal., A 2012, 447, 115. (18) Ausavasukhi, A.; Huang, Y.; To, A. T.; Sooknoi, T.; Resasco, D. E. Hydrodeoxygenation of m-cresol over gallium-modified beta zeolite catalysts. J. Catal. 2012, 290, 90. (19) Thilakaratne, R.; Tessonnier, J.-P.; Brown, R. C. Conversion of methoxy and hydroxyl functionalities of phenolic monomers over zeolites. Green Chem. 2016, 18, 2231. M


Research Article

ACS Sustainable Chemistry & Engineering (43) Adjaye, J. D.; Bakhshi, N. Production of hydrocarbons by catalytic upgrading of a fast pyrolysis bio-oil. Part I: Conversion over various catalysts. Fuel Process. Technol. 1995, 45 (3), 161. (44) Fu, Y.; Hong, T.; Chen, J.; Auroux, A.; Shen, J. Surface acidity and the dehydration of methanol to dimethyl ether. Thermochim. Acta 2005, 434 (1), 22. (45) Townsend, A.; Abbot, J. Catalytic cracking of tetralin on HY zeolite. Appl. Catal., A 1992, 90 (2), 97. (46) Townsend, A.; Abbot, J. Catalytic reactions of tetralin on HZSM-5 zeolite. Appl. Catal., A 1993, 95 (2), 221. (47) Zheng, Y.; Li, X.; Dutta, P. K. Exploitation of unique properties of zeolites in the development of gas sensors. Sensors 2012, 12 (4), 5170. (48) Prasomsri, T.; To, A. T.; Crossley, S.; Alvarez, W. E.; Resasco, D. E. Catalytic conversion of anisole over HY and HZSM-5 zeolites in the presence of different hydrocarbon mixtures. Appl. Catal., B 2011, 106 (1), 204. (49) Zhou, G.; Jensen, P. A.; Le, D. M.; Knudsen, N. O.; Jensen, A. D. Direct upgrading of fast pyrolysis lignin vapor over the HZSM-5 catalyst. Green Chem. 2016, 18, 1965. (50) Wang, K.; Johnston, P. A.; Brown, R. C. Comparison of in-situ and ex-situ catalytic pyrolysis in a micro-reactor system. Bioresour. Technol. 2014, 173, 124. (51) Bai, X.; Kim, K. H.; Brown, R. C.; Dalluge, E.; Hutchinson, C.; Lee, Y. J.; Dalluge, D. Formation of phenolic oligomers during fast pyrolysis of lignin. Fuel 2014, 128, 170. (52) Hosoya, T.; Kawamoto, H.; Saka, S. Secondary reactions of lignin-derived primary tar components. J. Anal. Appl. Pyrolysis 2008, 83 (1), 78. (53) Hosoya, T.; Kawamoto, H.; Saka, S. Role of methoxyl group in char formation from lignin-related compounds. J. Anal. Appl. Pyrolysis 2009, 84 (1), 79. (54) Graça, I.; Lopes, J.; Ribeiro, M.; Ribeiro, F. R.; Cerqueira, H.; De Almeida, M. Catalytic cracking in the presence of guaiacol. Appl. Catal., B 2011, 101 (3), 613. (55) Wang, S.; Cai, Q.; Chen, J.; Zhang, L.; Zhu, L.; Luo, Z. Cocracking of bio-oil model compound mixtures and ethanol over different metal oxide-modified HZSM-5 catalysts. Fuel 2015, 160, 534. (56) Wang, S.; Cai, Q.; Wang, X.; Zhang, L.; Wang, Y.; Luo, Z. Biogasoline production from the co-cracking of the distilled fraction of bio-oil and ethanol. Energy Fuels 2014, 28 (1), 115. (57) Dang, Q.; Luo, Z.; Zhang, J.; Wang, J.; Chen, W.; Yang, Y. Experimental study on bio-oil upgrading over catalyst in supercritical ethanol. Fuel 2013, 103, 683. (58) Gayubo, A. G.; Aguayo, A. T.; Atutxa, A.; Prieto, R.; Bilbao, J. Role of reaction-medium water on the acidity deterioration of a HZSM-5 zeolite. Ind. Eng. Chem. Res. 2004, 43 (17), 5042. (59) Mihalcik, D. J.; Mullen, C. A.; Boateng, A. A. Screening acidic zeolites for catalytic fast pyrolysis of biomass and its components. J. Anal. Appl. Pyrolysis 2011, 92 (1), 224.

N


Role of Hydrogen Transfer during Catalytic Copyrolysis of Lignin and

Recommend Documents