Maximizing the production of aromatic hydrocarbons from lignin conversion by coupling methane activation

Accepted Manuscript Maximizing the Production of Aromatic Hydrocarbons from Lignin Conversion by Coupling Methane Activation Aiguo Wang, Hua Song PII: DOI: Reference:

S0960-8524(18)31120-9 https://doi.org/10.1016/j.biortech.2018.08.026 BITE 20309

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Bioresource Technology

Received Date: Revised Date: Accepted Date:

7 July 2018 7 August 2018 9 August 2018

Please cite this article as: Wang, A., Song, H., Maximizing the Production of Aromatic Hydrocarbons from Lignin Conversion by Coupling Methane Activation, Bioresource Technology (2018), doi: https://doi.org/10.1016/ j.biortech.2018.08.026

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Maximizing the Production of Aromatic Hydrocarbons from Lignin Conversion by Coupling Methane Activation

Aiguo Wang1, Hua Song1*

1Department

of Chemical and Petroleum Engineering, University of Calgary

2500 University Drive, NW, Calgary, Alberta T2N 1N4, Canada

*Corresponding author. Fax: +1 (403) 284-4852; Tel: +1 (403) 220-3792; E-mail: [email protected]

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Abstract Maximizing the production of aromatic hydrocarbons from lignin conversion by coupling methane activation without solvent was investigated over Zn-Ga modified zeolite catalyst. The co-loading of Zn and Ga greatly improves lignin conversion, arene yield along with BTEX (i.e., benzene, toluene, ethylbenzene, and xylene) selectivity, which gives 37.4 wt.% yield of aromatic hydrocarbons with 62.2 wt.% selectivity towards BTEX at 400 ºC and 3.0 MPa. Methane presence has a negligible impact on lignin conversion, but improves arene yield and BTEX selectivity. Liquid 13C, 1H and 2H NMR investigations confirm the incorporation of methane into the final arene products. The NMR results reveal that methane might be incorporated into both the methyl group and aromatic ring, possibly via methylation of aryl moieties and coaromatization of alkyl moieties. Pyridine and ammonia as probes for surface acidity analysis of the developed catalysts demonstrate that HZSM5 modification with Zn and Ga species could redistribute BAS (Brønsted acid sites) and LAS (Lewis acid sites) and significantly increase the fraction of weak acidic sites on the catalyst, rendering better catalytic performance on the depolymerization of lignin to arene products with higher BTEX selectivity. The results reported in this work may open a novel route to a promising technique for lignin valorization into aromatics and cost-efficient utilizations of biomass and natural gas resources.

Keywords: Zeolite, Methane, Lignin, Catalyst, Aromatics

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1. Introduction Lignin is a complex aromatic-based macromolecule and accounts for up to 30 % of biomass. Highly abundant aromatic monomers make lignin as a promising resource to produce industrially relevant aromatic bulk and fine chemicals. (Deuss, Scott et al. 2015) In addition to extensive work to isolate lignin from biomass, there has been increased interest in developing technologies to harness the aromaticity of lignin for the production of value-added chemicals. Pyrolysis, solvolysis, hydrothermal processing and hydro-conversion are the major thermochemical routes for the catalytic lignin transformations. (Azadi, Inderwildi et al. 2013, Joffres, Laurenti et al. 2013) Fast pyrolysis is generally performed at high temperature (>500 ºC) over zeolite catalysts to obtain aromatics-rich or gasoline range liquid products. (Galadima and Muraza 2015, Kim, Lee et al. 2015, Shen, Zhao et al. 2016) However, high affinity to form high content of small molecular oxygenated compounds and large amount of low-value char (20-40 wt.%) results in the low yield of aromatics. Catalytic pyrolysis or hydropyrolysis is proposed to enhance deoxygenation and further improve the yield and quality of the targeting products. Few attempts are developed to convert lignin to monomeric aromatic compounds via an aerobic oxidationhydrolysis sequence. (Das, Rahimi et al. 2018) High yield (>60 wt.%) of aromatic monomers is obtained by chemo-selective aerobic oxidation of the secondary benzylic alcohol in the β-ether unit of lignin and following hydrolytic depolymerization. However, the main disadvantages of this process are low yield and poor selectivity of low-molecular-weight aromatics. Reductive depolymerization of lignin with hydrogen or a hydrogen donor, such as hydro-conversion and hydrodeoxygenation (HDO), has emerged as an attractive approach to maximize the production of low-molecular-weight compounds such as phenolics (Parsell, Owen et al. 2013, Kumar, Anand et al. 2015, Chen, Zhang et al. 2017), aromatic hydrocarbons (Laskar, Tucker et al. 2014), and cycloalkanes (Kong, He et al. 2015) at low temperature. High pressure of H2 required for the 3

cleavage and HDO of C-O bonds makes this process unpractical and economically unfeasible in a large scale. Many research efforts have been devoted to lignin degradation into aromatic monomers in the solvent. These solvent-based approaches usually couple lignin depolymerization with hydrogenolysis/hydrogenation. Polar solvents, such as water (Laskar, Tucker et al. 2014, Zhang, Asakura et al. 2014), alcohols (Song, Wang et al. 2013, Chen, Zhang et al. 2017), and acid (Rahimi, Ulbrich et al. 2014, Deuss, Scott et al. 2015), could effectively prevent re-polymerization by stabilizing carbocationic centers during the fragmentation of lignin. Co-solvent (water-formic acid (Oregui Bengoechea, Miletic et al. 2017), ethanol-formic acid (Oregui-Bengoechea, Gandarias et al. 2017), methanol-ethanol (Jongerius, Bruijnincx et al. 2013, Singh, Prakash et al. 2014), water-methanol (Deepa and Dhepe 2014)) and supercritical fluid (Gosselink, Teunissen et al. 2012, Kristianto, Limarta et al. 2017) are also explored for lignin depolymerization and conversion to valuable chemicals. Although solvolysis methods show the potential to reduce the formation of more condensed products like char, they face the risk of corrosion, loss of activity, difficulty in catalyst recovery, and sophisticated separation and purification of products. (Deepa and Dhepe 2014) From a techno-economic perspective, it is advantageous to conduct large-scale lignin valorization without extensive solvent recovery and complicated product separation. Therefore, methods to selectively convert lignin to commercially useful chemicals without solvent or expensive reagents are highly desired. (Upton and Kasko 2016) Compared with hydrogen, methane is cheaper and more naturally available, which could be a good alternative to hydrogen for lignin conversion. Methane (CH4) with the highest H/Ceff ratio (equals to 4) could benefit the formation of hydrocarbon products and reduce the coke. (Zhang, Cheng et al. 2011) The activation of methane could potentially donate the proton or carbon 4

moieties for the deoxygenation and capping the fragments, thus enhance lignin conversion and the yield of products. It would be industrially and economically attractive to develop an upgrading process that can integrate methane activation and lignin depolymerization to maximize the yield of the products with higher energy density and value, such as petrochemicals. However, methane participation in a chemical reaction is quite challenging due to the absence of low-energy empty orbitals and high-energy filled orbitals. (Tang, Zhu et al. 2014) Methane activity can be controlled by the catalyst and reaction environment. For instance, methane activation could be improved when employing higher hydrocarbons (Luzgin, Rogov et al. 2009, He, Gatip et al. 2017) or oxygenated (Choudhary, Mondal et al. 2005, Wang, Austin et al. 2017) as a co-reactant. By using 13C solid-state NMR spectroscopy, it is observed that the presence of ZnO or Ga2O3 species on the zeolite could promote the hemolytic cleavage of C-H bond in methane. (Luzgin, Rogov et al. 2008, Mikhail V. Luzgin 2010, Xu, Zheng et al. 2012) Lignin consists of various aryl ethers, irregularly connected by a variety of interunit linkages (such as β-O-4, α-O-4), (Parthasarathi, Romero et al. 2011) which typically leads to heterogeneous streams of aromatic compounds like phenolics, aromatic ethers, arene, other monomers. Arene, particularly, benzene, toluene, ethylbenzene and xylene (BTEX), are more valuable compared with others. BTEX are the most desirable aromatic starting materials to produce a multitude of commercially important chemicals. The abundant aromaticity of lignin makes it as an ideal alternative to petroleum to yield the arene products. Catalytic pyrolysis of lignin over solid-acid catalysts (HZSM5, H-β, H-Y) at high temperature is a one-step strategy commonly used for producing aromatic hydrocarbons. (Li, Su et al. 2012, Kim, Lee et al. 2015, Shen, Zhao et al. 2016) 2.0-5.2 wt.% yield of arene was obtained during catalytic fast pyrolysis of Kraft lignin with HZSM-5 zeolite at 600 ºC by Li, et al., (Li, Su et al. 2012). A maximum 3.57 5

wt.% yield of arene from catalytic pyrolysis of lignin over zeolite catalyst was reported by Kim, et al (Kim, Lee et al. 2015). Low yield (~5 wt.%) is the main drawback of this process. The twostep route for the production of arene from lignin involves liquid-phase depolymerization of lignin into aromatic monomers followed by selective HDO to remove covalently bonded oxygen. (Fan, Jiang et al. 2013, Jongerius, Bruijnincx et al. 2013, Laskar, Tucker et al. 2014) Laskar. et al., (Laskar, Tucker et al. 2014) developed an aqueous phase catalytic process for lignin conversion, and 35-60% conversion of lignin with 65-70% selectivity of aromatic hydrocarbons was achieved over supported noble metals catalyst and solid acid zeolites. Despite high lignin conversion and high yield of aromatics products, sophisticated isolation of aromatic monomer from solvent and utilization of high pressure of H2 make this approach challenging. In this work, the technical feasibility of coupling methane activation and lignin depolymerization to maximize the arene production without solvent was investigated. Three types of zeolites, HZSM5, H-β, and H-Y, were examined. Among them, HZSM5 showed the best performance in terms of lignin conversion and arene yield at 400 ºC under a methane environment. Zn (Gabrienko, Arzumanov et al. 2017) and Ga (Mikhail V. Luzgin 2010) species have been reported to promote methane activation and aromatization. The oxophilic metal Ru is known to catalyze hydrodeoxygenation, which could selectively cleave C-O bonds in a variety of monomeric, dimeric, and polymeric lignin molecules. (Parsell, Owen et al. 2013, Kristianto, Limarta et al. 2017, Oregui Bengoechea, Miletic et al. 2017) HZSM5 zeolite was modified with Zn, Ga, and Ru species to improve the conversion of lignin and methane as well as the yield of aromatic hydrocarbons. Isotope labeled methane molecules (13CH4 and CD4) were employed to evidence methane incorporation into the final arene products. Powder X-ray diffraction (XRD) analysis, diffuse reflectance infrared transform spectroscopy (DRIFT), and temperature 6

programmed desorption (NH3-TPD) were conducted to link the physicochemical properties of catalyst and its catalytic performance on lignin conversion. This coupling approach may open a novel route to a promising technique for lignin valorization into aromatics and cost-efficient utilization of biomass and natural gas resources.

2. Experimental 2.1 Catalyst preparation The ammonium ZSM-5 zeolite with various SiO2/Al2O3 ratios (25, 30, 50, and 80), Beta zeolite (H-β, CP814E*) and zeolite Y (H-Y, CBV720) were purchased from Zeolyst and calcined at 600 ºC for 5 h in air to attain H-type zeolites. 1%Zn/ZSM5, 5%Zn/ZSM5, 0.5%Ga/ZSM5, 1%Ga/ZSM5, 1%Ru/ZSM5, and 1%Zn-0.5%Ga/ZSM5 were prepared by incipient wetness impregnation of HZSM5 with SiO2/Al2O3 ratio of 30. (Oregui-Bengoechea, Gandarias et al. 2017) The amount of metal supported on the zeolite is indicated in each catalyst. The precursors for acquiring active metal species, Zn, Ga, Ru, were Zn(NO3)2·6H2O (99%, Alfa Aesar), Ga(NO3)3·xH2O (99.9%, Alfa Aesar), and RuCl3 (Ru content 45-55%, Sigma-Aldrich). Typically, incipient wetness impregnation was made by initially dissolving the corresponding amount of precursors in 10.0 g deionized water, then dropwise impregnated on 8.0 g HZSM5 support until pore saturation. The obtained wet powder was dried in the oven at 92 ºC overnight, followed by calcination at 550 ºC with a heating rate of 10 °C/min and a holding time of 3 h in ambient air. The preparation of 1%Zn-0.5%Ga/SiO2-Al2O3: the SiO2-Al2O3 support was prepared using tetraethyl orthosilicate (TEOS, 98%, Sigma-Aldrich) as Si source and Al(NO3)3·9H2O (99.9%, Alfa Aesar) as Al source through co-precipitation using NH3.H2O as alkaline agent. The molar 7

ratio of SiO2/Al2O3 is 30. 1%Zn-0.5%Ga/SiO2-Al2O3 was then prepared by incipient wetness impregnation as described above. The preparation of 1%Zn-0.5%Ga/Si-MFI: pure Si-MFI structure is synthesized by hydrothermal synthesis method. Typically, the mixture of 22.66 g TEOS (98%, Sigma Aldrich), 26.97 g TPAOH (0.1M, Sigma Aldrich) and 22.65 g DI H2O was stirring for 2 h with 300 rpm and then transferred in an autoclave and held at 170 ˚C for 3 days. The solid was washed and recovered by centrifuge, dried and calcined at 550 ºC for 3 h in the air. 1%Zn-0.5%Ga/Si-MFI was then prepared using incipient wetness impregnation as described above. 2.2 Catalyst characterization The 1H, 2H and 13C liquid nuclear magnetic resonance experiments were conducted at 9.4 T (ν0 (1H) = 400.1 MHz; ν0 (2H) = 61.4 MHz and ν0 (13C) = 100.6 MHz) on a BRUKER AVANCE Ⅲ 400 NMR spectrometer with a BBFO probe. 1H NMR chemical shifts were referenced to CDCl3 at 7.24 ppm. A spectral width of 12 kHz and a pulse delay of 2 s were used to acquire 64 scans per spectrum. A spectral width of 2.5 kHz and a pulse delay of 7 s were used to acquire 256 scans per 2H spectrum. 13C NMR chemical shifts were referenced to CDCl3 at 77.3 ppm. A spectral width of 26 kHz and a pulse delay of 2 s were used to acquire 10,000 scans per spectrum. Thermographic Analysis (TGA) profiles were used to determine the mass of solid residue after reaction. TGA measurement was performed with a simultaneous thermal analyzer (PerkinElmer STA 6000). The samples were held at 30 ºC for 2 min, then heated to 800 ºC at a rate of 20 ºC/min under the air flow with 30 mL/min.

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The X-ray Diffraction (XRD, Rigaku ULTIMA III X-ray diffractometer) analysis was conducted scanning from 3-60º at 3º step/min with a Cu Kα irradiation generated at a voltage of 40 kV and current of 44 mA. The surface acidity was examined using Diffuse Reflectance Infrared Fourier Transform spectroscopy upon pyridine adsorption at room temperature. The experiment was conducted on a Thermo-Scientific-Nicolet iS50 equipped with an environmental chamber and mercury– cadmium–telluride (MCT) detector. A small amount of sample (fresh catalyst) was loaded in the environmental chamber. The background spectra was collected using 512 scans with a resolution of 4 cm-1. Then, N2 flow was switched to go through the bubbler filled with pyridine and carried the pyridine vapor into the environmental chamber. After the adsorption of pyridine for 20 min, the gas flow was switched back to bypass the bubbler for another 20 min. The spectrum of the sample was then recorded in an absorbance mode using 512 scans with a resolution of 4 cm-1. The characterization of acidity was also performed on a Chemisorption Analyzer (Finesorb3010) using ammonia as the probe. Around 200 mg of fresh catalyst was loaded into U-type quartz tube. The sample was heated up to 600 °C with a ramp rate of 20 °C/min and held for 30 min in 5%O2/He flow with 30 sccm. Then, it was cooled down to 120 °C and conducted the adsorption of ammonia for 30 min under 10% NH3/He flow with 25 sccm. After flushing with pure He for 10 min, the sample subsequently heated to 600 °C with a heating rate of 30 °C/min and a hold of 30 min. A thermal conductivity detector (TCD) determined the amount of desorbed NH3. 2.3 Catalytic conversion of lignin The conversion of lignin (Alkali, 370959, Sigma-Aldrich) to arene was carried out in a 300 mL Parr® reactor with a digital pressure indicator under a methane environment. Typically, ~1.0 9

g catalyst with ~0.25 g lignin (61.0 wt.% C, 5.8 wt.% H, 0.9 wt.% N, 32.3 wt.% O by difference, 5.6 wt.% moisture, see E-supplementary data) were well mixed and then loaded into the reactor. After passing a leak test and completely purging out the air from the reactor, it was pressurized to 1 bar with N2 as the external standard, and then pressurized to 15 bar with a reactive gas (i.e., methane or nitrogen). The reactor temperature was then ramped up with a rate of 15-20 ºC/min to the target temperature (400 ºC) and held for 60 min. Upon reaction completion, the reactor was cooled down to room temperature before product collection. The formed condensable products embedded into the charged solid catalyst were extracted out using ~10 mL CS2 (GC grade, EMD Chemicals) as the solvent. In a similar manner, isotopic labeled reactions between cellulose and 13CH

4/CD4 (99.9%

13C

and 99% 2H, respectively, Cambridge Isotope Laboratories Inc.) were

conducted in a 100 mL Parr® reactor to reduce the usage of 13CH4/CD4, and the initial pressure of labeled gas was 5 bar. The composition of liquid products was determined by pre-calibrated Gas Chromatography-Mass Spectrometer (GC-MS: PerkinElmer GC Claus 680 and MS Clarus SQ 8T) equipped with a Paraffins-Olefins-Naphthenes-Aromatics (PONA) column (Agilent HPPONA). The oven temperature of the GC was programmed to hold at 35 ºC for 15 min, ramp to 70 ºC at 1.5 ºC/min, rise to 150 ºC at 3 ºC /min and 30 min hold time, then ramp to 250 ºC at 3 ºC /min and hold for 2 min. The yield of each component in liquid products were quantified with calibration curves produced using internal standards of the condensable products, including benzene, toluene, xylene, naphthalene, and 2-methyl naphthalene. The incondensable gas products were analyzed by a four-channel micro-GC (490, Agilent). After the reactor was cool down to room temperature before product collection, it was connected to the micro GC. The gas product was analyzed by micro-GC equipped with thermal 10

conductivity detectors, which can precisely analyze H2, O2, N2, CH4, and CO in the first channel equipped with a 10 m molecular sieve 5A column; CO2, C2H2, C2H4, and C2H6 in the second channel installed with a 10 m PPU column; and C3- C6 and C3= - C5= (“=” denotes alkenes) in the third and fourth channels charged with a 10 m alumina column and one 8 m CP-Sil 5CB column, respectively. Ar and He were the carrier gases for the first and other three channels, respectively. Nitrogen with known moles was used as the external standard for each run to calculate the moles of each gas component based on the composition of gas products determined by micro-GC. The mass of solid residue after reaction was calculated by weight loss from TGA profiles in the range of 300 to 700 °C. All yields are reported in terms of mass yield (mass of the products relative to the dry mass of lignin). Selectivity is determined as the mass of one component relative to the total mass of all identified liquid hydrocarbons by GC-MS. The lignin conversion, the yields of gas and arene products, selectivity as well as methane conversion are given by the following equations:

(

Lignin conversion (wt.%) = 1 -

)

The mass of residue after reaction, g × 100% The dry mass of lignin,g

The mass of gas products, g × 100% The dry mass of lignin,g The mass of arene products, g × 100% The yield of arene products (wt.%) = The dry mass of lignin,g The mass of one component Selectivity (wt.%) = × 100% The mass of all identified condensable liquid products mass of methane after reaction ) × 100% Methane conversion (%) = (1 mass of methane before reaction The yield of gas products (wt.%) =

All experiments were performed in triplicate for each condition. The data reported are the mean values of three trials, and the standard devastation is given. 3. Results and Discussion 11

3.1 Catalytic performance evaluation Zeolite catalysts play a crucial part in the conversion of lignin to aromatic hydrocarbons as the final products. Three zeolites with different pore structure are selected to evaluate the catalytic performance on lignin conversion at 400 ºC under a methane environment. The physicochemical properties of zeolites are listed in Table 1, and their catalytic performance on lignin conversion is shown in Fig. 1a. The arene yield and lignin conversion over three zeolites follow the order of: HZSM5>H-β>H-Y. This may be related with the shape selectivity of zeolite. Zeolite with the moderate internal pore size could sterically hinder the formation of larger compounds in the pores, thus favor lignin conversion and increase the arene yield. The further increase in pore diameter would allow the fragments from lignin depolymerization to repolymerize into macromolecular compounds or coke. Thence, it is observed that the conversion of lignin and the yield of arene products are higher over HZSM5 with moderate pore size (5.5 Å) than that over both H-β (6.6 Å) and H-Y (7.4 Å) zeolites with larger pore space. Similar phenomenon is observed during the catalytic fast pyrolysis of softwood by Yu et al. (Yu, Li et al. 2012) The acidity is another important factor in the conversion of lignin to aromatic hydrocarbons. HZSM5 with various SiO2/Al2O3 ratios (23, 30, 50, and 80) on lignin conversion are further examined under similar conditions as shown in Fig. 1b. The lignin conversion and arene yield show the similar trend. Both increase first, then decrease with SiO2/Al2O3 ratio increasing from 23 to 80. The higher the SiO2/Al2O3 ratio is, the lower the number of acidic sites. HZSM5 with higher acidity could result in over-cracking of lignin to form gas and coke products, thus lower yield of desired arene products is obtained over HZSM5 with SiO2/Al2O3 ratio of 23. While HZSM5 with less acidic sites have the limited acidity for deoxygenation of oxygenate

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intermediates from lignin depolymerization to form aromatic hydrocarbons, therefore, lower yields of arene and gas products are observed over HZSM5 with higher SiO2/Al2O3 ratio of 50 or 80. These intermediates repolymerize into macromolecular compounds as the solid residue deposited on the zeolite surface, thereby decreasing the lignin conversion. Therefore, HZSM5 with SiO2/Al2O3 ratio of 30 is the optimal zeolite for the conversion of lignin to aromatic hydrocarbons in this study since higher lignin conversion and more arene products are simultaneously achieved. The reaction conditions, mass ratio of lignin to HZSM5 catalyst, reaction temperature and time, are also optimized to maximize lignin conversion and arene yield as shown in Fig. 2. With mass ratio increasing from 0.1 to 0.25, lignin conversion and arene yield are improved by 12.1% and 15.1%, respectively. Further increase in mass ratio has little effect on lignin conversion, but gradually reduces the yield of arene products. The trend in Fig. 2a is quite similar with that in Fig. 1b, because the variation in mass ratio could change the concentration of acidic sites with respect to substrate. In case of lower mass ratio (lignin/HZSM5) of 0.1 in Fig. 2a, more acidic sites available would result in higher concentration of acidic sites with respect to lignin, which could produce more gas and coke with less arene products due to excessive deoxygenation and cracking. This observation is the same as that over HZSM5 with higher acidity (SiO2/Al2O3=23) in Fig. 1b. The effect of reaction temperature (Fig. 2b) and time (Fig. 2c) on lignin conversion are very similar. Lower temperature or shorter reaction time is not advantageous for lignin conversion and arene yield. Therefore, the reaction temperature and time are optimized to be 400 ºC and 60 min. The total ion chromatogram (TIC) of liquid products collected from lignin conversion over HZSM5 under the optimized conditions (400 ºC, 60 min) shows that the reaction generates 13

benzene (1), toluene (2), naphthalene (10), and methyl naphthalene (11, 12) as primary products, together with a small amount of ethylbenzene (3), xylene (4, 5), C9 (6-8) and C10 (9) monoaromatics, alkyl naphthalene (14, 15), fluorene (16) and anthracene (17). (see Esupplementary data) A total of 17 compounds are identified by GC-MS. These identified compounds are aromatic hydrocarbons, and no oxygenated aromatic product is detected. Among them, BTEX (1-5) is more valuable products than polyaromatics (PA, 10-17). However, BTEX selectivity over HZSM5 is low, probably due to the fact that bare zeolite catalyst has the limited ability of methane activation to offer proton or carbon moieties for the formation of desired monoaromatics. A negligible methane conversion (0.2%) is observed over HZSM5 (entry 1 in Table 2). HZSM5 zeolite is further modified by various metal species to improve the selective conversion of lignin to BTEX products. The catalytic performances on lignin conversion are summarized in Table 2. The loading of Zn has insignificant effect on lignin conversion and arene yield, but produces more gas products. More importantly, Zn loading enhances BTEX selectivity by ~10% (entry 2 and 3). High gas yield may be owing to the deoxygenation of oxygenated fragments from lignin depolymerization promoted by Zn species to form CO and CO2, which is witnessed by more CO and CO2 products formed over 1%Zn/ZSM5 (see Esupplementary data). This promotive effect is more pronounced when introducing more Zn (5%) on zeolite matrix, and higher yield of CO and CO2 is observed (see E-supplementary data). Ga addition into the HZSM5 framework improves lignin conversion and arene yield, but has a limited contribution to BTEX selectivity improvement (entry 4 and 5). Higher methane conversion observed over supported Ga catalysts implies that Ga species on zeolite may effectively facilitate methane activation and provide more proton or methyl moiety for the formation of aromatic hydrocarbons, leading to higher yield of arene products. The addition of

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Ru slightly improves the yield of arene products and the selectivity of BTEX, while leading to a decrease in lignin conversion (entry 6). Lower lignin conversion is probably because Ru could effectively catalyze hydrodeoxygenation (under a H2 environment), but has limited deoxygenation activity under a methane environment. The co-loading of Zn (1%) and Ga (0.5%) on HZSM5, albeit leading to slight drop in methane conversion, could greatly improve lignin conversion, arene yield and BTEX selectivity (entry 7). Such good catalytic performance over 1%Zn-0.5%Ga/ZSM5 may be attributed to the synergistic effect from metal species and acidic sites, which could couple methane activation with lignin depolymerization to maximize the arene yield. The morphology and acidic sites of zeolite catalysts are thought to be quite crucial for the selective conversion of lignin to aromatic hydrocarbons. Therefore, two catalysts, 1%Zn0.5%Ga/Si-MFI and 1%Zn-0.5%Ga/SiO2-Al2O3 are prepared to explore the role of MFI structure and acidic sites played in the reaction. As described in section 2.1, 1%Zn-0.5%Ga/Si-MFI catalyst has the MFI structure without acidic sites, while 1%Zn-0.5%Ga/SiO2-Al2O3 catalyst is amorphous and has the same SiO2/Al2O3 ratio of HZSM5 but without its ordered porous structure. 1%Zn-0.5%Ga/ZSM5 catalyst has both MFI structure and acidic sites and thereby included as reference. Their correlated catalytic performances on lignin conversion are listed in Table 3. Compared with the performance over 1%Zn-0.5%Ga/ZSM5, lower lignin conversion over 1%Zn-0.5%Ga/Si-MFI (entry 8) and 1%Zn-0.5%Ga/SiO2-Al2O3 (entry 9) suggests that both MFI structure and acidic sites play a critical part in lignin conversion. It seems that MFI structure of zeolite may benefit the formation of mono-aromatics such as BTEX and phenols, which leads to higher selectivity of mono-aromatics over 1%Zn-0.5%Ga/ZSM5 (62.2 % BTEX) and 1%Zn0.5%Ga/Si-MFI (13.0 % BTEX and 45.8 % phenols). Acidic sites favor the deoxygenation, 15

resulting in less oxygenated compounds in liquid products over the catalysts with abundant acidic sites such as 1%Zn-0.5%Ga/ZSM5 (no oxygenated compounds) and 1%Zn-0.5%Ga/SiO2Al2O3 (9 % phenols). Due to low conversion of lignin, less aryl moieties are available. Alkyl moieties derived from methane activation catalyzed by metal species or the removal of alkoxy groups in lignin would recombine to form light hydrocarbons (CxHy) in the gas phase, (Gosselink, Teunissen et al. 2012, Guo 2014, Joffres, Nguyen et al. 2016) thus leading to much higher yield of CxHy over 1%Zn-0.5%Ga/Si-MFI (9.2 %) and 1%Zn-0.5%Ga/SiO2-Al2O3 (10.4 %). 3.2 Effect of methane The effect of methane on lignin conversion is investigated by performing equivalent reactions over HZSM5 and 1%Zn-0.5%Ga/ZSM5 under methane and nitrogen environments. As shown in Fig. 3, methane has a negligible impact on the lignin conversion, however, methane could improve the yield of arene products and the selectivity of BTEX. BTEX selectivity increases by 1.6% and 4.0 % over HZSM5 and 1%Zn-0.5%Ga/ZSM5, respectively. The yield of arene products increase by 1.9% and 9.3% over HZSM5 and 1%Zn-0.5%Ga/ZSM5, respectively. The yield of arene reaches up to 37.4 wt.% over 1%Zn-0.5%Ga/ZSM5 under a CH4 environment, which is much higher than that obtained during catalytic fast pyrolysis (2.0-5.2 wt.%) (Li, Su et al. 2012, Kim, Lee et al. 2015) and catalytic hydrotreatment (3.0-5.9 wt.%) (Kumar, Anand et al. 2015) in the absence of a solvent. It is also higher than the yield (31.8 wt.%) of aromatic hydrocarbons obtained during the ethanolysis of lignin (Yan, Ma et al. 2017). The promotion effect on arene yield and BTEX selectivity might be due to that methane is activated by Zn and Ga species to generate a proton or methyl group for capping the aryl moieties from lignin depolymerization, thus reduce the repolymerization of aryl moieties to form

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polyaromatics and maximize the yield of BTEX. (Mikhail V. Luzgin 2010, Gabrienko, Arzumanov et al. 2017) Higher yield of arene and better selectivity of BTEX achieved under a methane environment indicate methane may facilitate the formation of arene, or participate in the reaction to form aromatic hydrocarbons possibly through aromatization with alkyl moieties or reaction with aryl moieties. Methane incorporation into arene products is evidenced using liquid 13C, 1H and 2H NMR. Isotope labeled methane molecules (13CH4 and CD4) are employed to conduct equivalent reactions over 1%Zn-0.5%Ga/ZSM5. GC-MS analysis of the extracted products shows that a mixture of BTEX and naphthalene is formed in the reaction. Mass-spectra of benzene (1), toluene (2), xylene (4, 5), naphthalene (10), and methyl naphthalene (11, 12) collected from 13Clabeled and regular methane are analyzed and compared in Fig. 4. Preliminary mass spectrometry analysis confirms that singly (13C1) and doubly (13C2) labeled molecules are present in final arene products, which provides a complement to the proof for methane incorporation into aromatic hydrocarbons during lignin conversion. Liquid 13C NMR spectrum of arene products further renders more solid evidence. The 13C signals of arene products collected from lignin and 13CH

4

run are enhanced unevenly (see E-supplementary data), compared to that from non-

isotopic labeled counterparts, strongly supporting the incorporation of methane into aromatic hydrocarbons (i.e., co-aromatization). Several peaks in the region of 120-140 ppm are the resonances of carbons within the aromatic ring. The signals at 137.5, 129.0, 128.3, and 125.4 ppm are attributed to the phenyl carbon, ortho carbon, meta carbon (or carbons of benzene), and para carbon of toluene, respectively. Some small peaks near 128.3 and 125.4 ppm are associated with the phenyl carbons of naphthalene and alkyl naphthalene. One aliphatic resonance at 21.7 ppm originates from the benzylic carbon. (Pretsch, Buhlmann et al. 2009, Fulmer, Miller et al. 17

2010) The significant increase in intensity of peaks at 21.7 (~1.4 times), 129.0 (~1.3 times) and 125.4 (~1.3 times) ppm implies that methane prefers to be incorporated into both the methyl group and aromatic ring. The activated methane reacts with aryl moieties to form alkyl aromatics (compounds 2-9) via methylation, which contributes to methane incorporation into aromatic side chains. The enrichment of 13C atoms within aromatic ring from methane (13CH4) might go through the co-aromatization of methane and alkyl moieties generated from lignin depolymerization. The evolution of hydrogen atoms in methane during the reaction is traced by employing deuterium-enriched methane(CD4). The resonances in the regions of 6.5-8.0 ppm and 2.0-3.0 ppm of 1H NMR spectra are attributed to the aromatic and benzylic hydrogens, respectively. The signals below 2.0 ppm are assigned to the alkyl hydrogens. When using deuterated methane (CD4) as the reactive gas, the intensities of peaks originating from aromatic and benzylic protons are significantly reduced by approximately 60%, (see E-supplementary data) while the signal due to alkyl hydrogen changes insignificantly. This matches with the observations on the 2H NMR spectra that two large peaks appear in the regions of aromatic (6.5-8.0 ppm) and benzylic (2.03.0 ppm) hydrogens, and very small peaks are located in the region of alkyl (0.0-2.0 ppm) hydrogen. (see E-supplementary data) The results of 1H and 2H NMR demonstrate that the hydrogen atoms of methane are incorporated into the aromatic and benzylic hydrogen sites of the final arene products, which coincides with the observation of 13C NMR that methane prefers to be incorporated into the aromatic ring and benzylic carbon sites. 3.3 Characterizations The crystal structure of three catalysts, 1%Zn-0.5%Ga/SiO2-Al2O3 1%Zn-0.5%Ga/Si-MFI and 1%Zn-0.5%Ga/ZSM5, is confirmed. (see E-supplementary data) As expected, no XRD 18

pattern being observed on 1%Zn-0.5%Ga/SiO2-Al2O3 catalyst indicates that this is an amorphous catalyst with no ordered porous structure embedded, while XRD patterns of 1%Zn-0.5%Ga/SiMFI catalyst, which is prepared by hydrothermal synthesis method, shows a typical pattern of MFI structure (2θ ≈ 8.3°, 9.2°, 23.4°, and 24.2° corresponding to (101), (200), (501) and (303) crystal planes). Meanwhile, the typical pattern of MFI structure is clearly detected on other modified HZSM5 catalysts. There is no additional peak appearing in the XRD patterns of these catalysts, suggesting that the loading of metal species leads to insignificant distortion of zeolite framework and loaded metals are well dispersed on the catalyst surface. It is true that there are varying degrees of change in the intensity of diffraction peaks originating from (101) and (200) crystal planes of MFI, which might be because of the deposition of metal species on these two crystal surfaces when introducing metal species on the zeolite framework. The characterization of catalyst acidity is conducted using pyridine and ammonia as the probes and provided in Supplementary Material. The bands located at 1442-1456 cm-1, 15811599 cm-1 and 1615 cm-1 are attributed to pyridine adsorption on Lewis acid sites (LAS). The peaks at 1540-1550 cm-1 and 1641-1644 cm--1, are due to the pyridine adsorption on Bronsted acid sites (BAS). The peaks at 1484-1493 cm-1 originate from coordinately bonded pyridine (pyridine and pyridinium ion) absorbed on both LAS and BAS. (PARRY 1963, Tommy Barzetti 1996, Woolery G L 1997, Topaloǧlu Yazıcı and Bilgiç 2010) It is noticed that large amounts of LAS (1445, 1489, 1596 cm-1) with a small number of BAS (1489, 1540 cm-1) are present on 1%Zn-0.5%Ga/SiO2-Al2O3 catalyst, (see E-supplementary data) while only LAS are detected on 1%Zn-0.5%Ga/Si-MFI catalyst (see E-supplementary data) because of aluminum deficiency. The typical signals due to pyridine protonated on BAS (1490, 1540, 1640 cm-1) and pyridine coordinated to LAS (1445, 1492, 1600 cm-1) are observed on other supported metal catalysts and 19

HZSM5. More loading of Zn leads to the disappearance of BAS at 1640 cm-1, instead strong LAS appear at 1615 cm-1. (see E-supplementary data) It is speculated that HZSM5 modification with Zn and Ga redistributes BAS and LAS on the catalyst, which renders better catalytic performance in the depolymerization of lignin to aromatic hydrocarbons. Based on the area integral of entire NH3-TPD curves, the corresponding quantitative analysis of acid sites are tabulated in Table 4. The peaks in the region of 100-300 ºC are attributed to the desorption of NH3 molecules on weak acidic sites (WAS), while the peaks at higher temperature (300-500 ºC) originate from NH3 molecules absorbed on strong acidic sites (SAS). (Lónyi F 2001) 1%Zn-0.5%Ga/Si-MFI catalyst has a negligible number of acidic sites since there is no obvious signal detected in the NH3-TPD profiles. (see E-supplementary data) The addition of metal species on HZSM5 reduces the total acid sites (TAS) as evidenced by smaller relative ratio (<1) of total acid sites (TAS). Interestingly, 1% Zn loading slightly decreases the TAS, but WAS fraction is greatly increased by 25.5 %. More Zn loading (5%) would further reduce the TAS and lower the fraction of WAS, compared to that of 1%Zn/ZSM5. A similar trend is observed on Gamodified HZSM5 catalysts. The introduction of Ru into HZSM5 leads to a significant decrease in the TAS, despite that WAS fraction is higher. The co-loading of Zn and Ga could significantly increase the proportion of WAS (from 30.1 % to 77.7 %). Referring to the catalytic performance in Table 2 and 3, it’s speculated that abundant acidic sites with higher fraction of WAS might be beneficial for the conversion of lignin to arene products with better BTEX selectivity.

4. Conclusion The addition of Zn and Ga promotes methane activation and lignin conversion, leading to higher yield of arene products with better BTEX selectivity. MFI structure of zeolite benefits the 20

formation of mono-aromatics, while acidic sites favor the deoxygenation. Methane could facilitate the formation of aromatic hydrocarbons, particularly, BTEX. Methane incorporation into aromatic hydrocarbons is evidenced by 13C, 1H and 2H NMR investigations. The NMR results show that methane is incorporated into aromatic side chains via methylation of aryl moieties, whereas the embedding of methane into the aromatic ring might go through coaromatization of methane and alkyl moieties. E-supplementary data for this work can be found in e-version of this paper online.

Acknowledgements We gratefully acknowledge the financial support from Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant Program (RGPIN/04385-2014).

21

Table 1 Physicochemical properties of zeolites used in the study Zeolites

HZSM5

H-β

H-Y

Structure

MFI orthorhombic 10-10

BEA tetragonal 12-12

FAU cubic 12-12

Pore size a (Å)

5.6×5.3 5.5×5.1 5.5

7.6×6.4 5.6×5.6 6.6

SiO2/Al2O3 ratio

30

25

Channel system Channel size (Å)

a:

obtained from the paper (Ma, Troussard et al. 2012).

22

7.4×7.4 7.4 30

Table 2. The catalytic conversion of alkali lignin to aromatic hydrocarbons over various catalysts under a methane environment Entry

1

2

3

4

5

6

Catalysts

HZSM5

1%Zn /ZSM5

5%Zn /ZSM5

0.5%Ga /ZSM5

1%Ga /ZSM5

1%Ru /ZSM5

43.8 ±1.8 18.0 ±0.7 31.0 ±1.4 61.4 ±2.1

43.2 ±2.3 23.1 ±1.4 24.2 ±1.2 62.5 ±2.8

44.8 ±2.1 15.2 ±0.5 34.1 ±2.3 56.0 ±2.0

39.7 ±2.4 16.3 ±0.9 28.8 ±1.9 54.2 ±1.7

Lignin conversion (wt.%)

42.1 ±2.1 15.1 Gas products (wt.%) ±0.4 Arene products 27.6 (wt.%) ±0.9 52.9 BTEX ±1.6 Selectivity 4.3 Others (wt.%) ±0.3 42.8 PA ±1.5 Methane conversion 0.2 (%) ±0.1 Mass balance 100.6 (wt.%) ±4.1

2.8±0.1 2.7±0.1 35.8 ±1.3 0.6 ±0.1 105.2 ±5.1

34.8 ±2.1 0.5 ±0.1 104.1 ±4.8

45.2±2.2 15.3±0.6 33.5±2.0 55.9±1.8 2.1±0.1 42.0±1.9 1.3±0.1 104.6 ±4.4

7 1%Zn0.5%Ga /ZSM5 48.2 ±2.9 16.9 ±1.1 37.4 ±2.5 62.2 ±2.3

2.6±0.1 2.8±0.2

3.9±0.2

41.4 ±2.3 1.4 ±0.1 103.5 ±3.8

42.9 ±2.1

33.8 ±1.7

0.8±0.1

1.2±0.1

105.4 ±4.9

106.1 ±5.4

Reaction conditions: ~0.25 g of lignin with ~1.0 g catalyst, initial 15 bar of CH4, 400 ºC, 60 min. BTEX: benzene (1), toluene (2), with small proportion of ethylbenzene (3) and xylene (4, 5); Other: mono aromatics (6-9); PA: naphthalene (10), alkyl naphthalene (11-15), and a tiny fraction of fluorene (16) and anthracene (17).

23

Table 3. Catalytic performance on the conversion of lignin to aromatic hydrocarbons under a methane environment Entry

7 1%Zn-0.5%Ga Catalysts /ZSM5 Lignin conversion (wt.%) 47.2±2.9 The yield of gas products (wt.%) H2 0.8±0.1 CO 2.9±0.2 CO2 12.6±1.7 CxHy 0.6±0.1 Total 16.9±1.1

8 1%Zn-0.5%Ga /Si-MFI 35.5±2.2

9 1%Zn-0.5%Ga /SiO2-Al2O3 31.9±2.3

0.5±0.1 1.7±0.1 9.0±0.5 9.2±0.7 20.4±1.2

0.4±0.1 2.3±0.2 5.9±0.2 10.4±0.6 19.0±1.1

Liquid yield (wt.%) BTEX Selectivity Others (wt.%) PA Phenols

13.1±1.3 13.0±1.4 2.5±0.1 38.7±2.3 45.8±3.2

11.2±0.9 26.3±1.8 5.5±0.3 59.2±2.9 9.0±0.7

37.4±2.5 62.2±2.3 3.9±0.2 33.8±1.7 0.0

Reaction conditions: ~0.25 g of lignin with ~1.0 g catalyst, initial 15 bar of CH4, 400 ºC, 60 min. Phenols: phenol, cresol, and xylenol.

24

Table 4. Quantitative analysis of acid sites over various catalysts by NH3-TPD Catalysts HZSM5 1%Zn/ZSM5 5%Zn/ZSM5 0.5%Ga/ZSM5 1%Ga/ZSM5 1%Ru/ZSM5 1%Zn-0.5%Ga/ ZSM5 1%Zn-0.5%Ga/ Si-MFI 1%Zn-0.5%Ga/ SiO2-Al2O3

WA a SA a WA SA WA SA WA SA WA SA WA SA WA SA

Peak position (ºC) 263 470 256 403 229, 290 383 255 442 233 406 203 368 229, 287 443

Acid site fraction (%) 30.1 69.9 55.6 44.4 50.3 49.7 70.6 29.4 54.1 45.9 51.7 48.3 77.7 22.3

-

-

-

0

WA SA

249 339

30.1 69.9

0.59

Acid site type

a

TAS ratio b 1.00 0.93 0.85 0.94 0.72 0.48 0.87

WA and SA refer to weak acid sites and strong acid sites, respectively. TAS refers to total sites ratio, which is the ratio of NH3-TPD peak area with respect to that of HZSM5. b

25

Figure 1. Lignin conversion (wt.%), the yields (wt.%) of gas and arene products collected from (a) different zeolite supports and (b) HZSM5 with various SiO2/Al2O3 ratios under a methane environment. Reaction conditions: ~0.25 g lignin mixed with ~1.0 g zeolite catalyst, initial 15 bar of CH4, 400 °C, 60 min.

26

Figure 2. The effect of (a) mass ratio of lignin to HZSM5, (b) reaction temperature and (c) time on lignin conversion to aromatic hydrocarbons under a methane environment.

27

Figure 3. Lignin conversion, the yields of gas and arene products, and selectivity collected over (a) 1%Zn-0.5%Ga/ZSM5 and (b) HZSM5 catalysts under CH4 and N2 environments. Reaction conditions: ~0.25 g of lignin with ~1.0 g catalyst, initial 15 bar of CH4 or N2, 400 ºC, 60min.

28

Figure 4. Mass spectra of products: (a) C6H6, (b) C7H8, (c) C8H10, (d) C10H8, and (e) C11H10, collected from lignin conversion under methane with the natural abundance of 13C (black) and 13CH

4

(red), and the estimated isotopic composition (mol%) is indicated.

29

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