Catalytic hydropyrolysis of lignin: Suppression of coke formation in mild hydrodeoxygenation of lignin-derived phenolics

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Catalytic hydropyrolysis of lignin: Suppression of coke formation in mild hydrodeoxygenation of lignin-derived phenolics Pouya Sirous-Rezaei, Young-Kwon Park

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School of Environmental Engineering, University of Seoul, Seoul 02504, Republic of Korea

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

catalytic hydropyrolysis of • In-situ lignin using HY catalyst. catalytic hydrodeoxygenation • Ex-situ of lignin-derived phenolics at mild conditions.

conversion of lignin into • Selective aromatic hydrocarbons. FeReO /ZrO : a potential catalyst for • mild HDO of lignin-derived phenolics. x

2

A R T I C LE I N FO

A B S T R A C T

Keywords: Hydropyrolysis Mild hydrodeoxygenation Lignin-derived phenolics FeReOx/ZrO2 Aromatic hydrocarbon Moderate acid strength

Lignin, with its polyaromatic structure and as a main component of lignocellulosic biomass, is considered as an important renewable source of aromatics which are currently obtained from fossil fuels. Lignin pyrolysis gives a liquid product with a high content of phenolic compounds which can be further upgraded to aromatic hydrocarbons through catalytic approaches. In this work, in-situ catalytic hydropyrolysis combined with a subsequent ex-situ catalytic hydrodeoxygenation step was implemented to achieve an enhanced conversion of kraft lignin into aromatic hydrocarbons. The main point is that the ex-situ catalytic upgrading was conducted at mild conditions (temperature: 350 °C; pressure: 1 atm). HY was used as in-situ catalyst for enhanced decomposition of lignin. Fe/HBeta, FeReOx/MCM-41, Fe/ZrO2 and FeReOx/ZrO2 were used as ex-situ catalyst, among which the oxophilic, mesoporous and mild-acidic catalyst of FeReOx/ZrO2 revealed the highest HDO efficiency. Importantly, FeReOx/MCM-41, Fe/ZrO2 and FeReOx/ZrO2 led to significantly lower yield of coke compared to a zeolite-supported catalyst like Fe/HBeta. This suppression of coke formation is a result of reduced phenolic trapping inside catalyst mainly due to the mesoporosity and moderate acid strength of catalyst.

1. Introduction The rapid depletion of fossil resources and the growing environmental concerns caused by extensive utilization of fossil fuels necessitate a serious search for renewable and sustainable resources to be replaced for fossil reserves [1–3]. Biomass is a potential option to be used as such alternative due to its abundant availability [4,5]. Among the various techniques for exploitation of biomass, pyrolysis is being

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considered as an efficient approach through which a high yield of liquid oil product is achieved [6]. Pyrolysis, as a thermal decomposition process for degradation of organic compounds in the absence of oxygen, produces liquid bio-oil, biogas and solid biochar [7]. These products have potential multi-applications, and can be used as fuel source, chemical feedstock, soil amendment and catalyst [8,9]. The yield of biooil produced from pyrolysis process can be maximized by optimizing process parameters such as temperature, heating rate and reactant

Corresponding author. E-mail address: [email protected] (Y.-K. Park).

https://doi.org/10.1016/j.cej.2019.03.224

1385-8947/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Pouya Sirous-Rezaei and Young-Kwon Park, Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2019.03.224

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nanoparticles were collected by filter, and dried at 100 °C to obtain the final product. Mesostructured silica MCM-41 was prepared by the following method [32]. 24.29 g cetyltrimethylammonium bromide (CTAB, 99+%, ACROS) was dissolved in 280 g distilled water. Then, 100 g sodium silicate solution (20 wt% SiO2, Na/Si = 0.5) was added dropwise to the CTAB solution at room temperature. After being vigorously stirred at room temperature for 1 h, the mixture was aged in an oven at 100 °C for 24 h. Afterward, the sample was cooled down to room temperature, and its pH was adjusted to 10 by a 50 wt% aqueous acetic acid solution. Then, it was kept at 100 °C for 48 h. After repeating the cooling-pH adjustment-aging process for two more times, the sample was filtered, washed with distilled water, and dried at 80 °C for 24 h. Subsequently, the dried powder was washed by HCl (36 wt%, SAMCHUN)-dissolved ethanol solution, and dried at 80 °C for 24 h. The sample was finally calcined at 550 °C for 3 h. Incipient wetness method using the aqueous solutions of Fe(NO3)3·9H2O, Pd(NO3)2·2H2O, Ni (NO3)2·6H2O and NH4ReO4 was implemented for impregnation of iron, palladium, nickel and rhenium species on the supports of zirconia, silica and HBeta. The catalyst samples were dried (60 and 110 °C) and calcined (3 °C min−1/550 °C/12 h) after impregnation.

residence time [10,11]. However, the bio-oil obtained by biomass pyrolysis lacks the high quality to be used as a transportation fuel due to its high oxygen content [12–14]. Consequently, a catalytic pyrolysis process, consisting biomass pyrolysis and subsequent catalytic upgrading of pyrolyzates, has been developed for the production of high quality bio-oil [4]. The exploration on this process is in its initial stages, and there are still several technical problems remained making its commercialization a challenging issue. Among the main components of lignocellulosic biomass (lignin, cellulose and hemicellulose), conversion of lignin fraction is a key problem which limits the efficiency of the catalytic pyrolysis process. Lignin constitutes 30 wt% of biomass, and is the major renewable aromatic source due to its polyaromatic structure [15–17]. Meanwhile, lignin is the waste product of several processes (e.g., pulp and paper refinery and lignocellulosics-to-ethanol) [18–20]. There are two major challenges in the conversion of lignin: (i) high formation of char, remained from pyrolysis of lignin, which depends on the type of lignin, reactor design, catalyst characteristics and operational parameters; (ii) low reactivity of lignin-derived phenolics and high coke formation over the zeolite-supported catalysts which are the commonly used catalysts in biomass conversion. The high formation of char results in low carbon efficiency of the process. Besides, the large agglomeration of char can cause the reactor blockage which prevents from operating the pyrolysis process in a continuous condition. Furthermore, the lignin-derived phenolics have low reactivity over the commonly used acidic catalysts as a result of the strong Caromatic-OH bond leading to a high energy required for its direct cleavage [21]. Alternatively, the oxygen removal from lignin-derived phenolics can be significantly enhanced through hydrodeoxygenation (HDO) reaction by the use of metal–acid bifunctional catalysts [22]. HDO includes the reactions of hydrogenation and dehydration which can be promoted by metal and acid active sites, respectively. Meanwhile, lignin-derived phenolics has high potential to be adsorbed on the acid sites of zeolites and trapped inside zeolite channels, causing high formation of coke, and in turn, low carbon efficiency and rapid catalyst deactivation [23–29]. Therefore, the aim of this study was to develop a catalytic system to suppress the formation of both char and coke, and to enhance the HDO of phenolics. A reaction system including an in-situ catalytic hydropyrolysis combined with a subsequent ex-situ catalytic hydrodeoxygenation was used in this work in order to achieve an enhanced conversion of kraft lignin into aromatic hydrocarbons. The key point in this process is that the ex-situ catalytic upgrading was performed at mild conditions. In this study, kraft lignin was used as the feedstock, and aromatic hydrocarbons were the target products. HY zeolite was used for the insitu catalytic hydropyrolysis, and the catalytic performance of Fe/ HBeta, FeReOx/MCM-41, Fe/ZrO2, ReOx/ZrO2, FeReOx/ZrO2, NiReOx/ ZrO2 and PdReOx/ZrO2 was examined for the ex-situ catalytic HDO of phenolic compounds derived from a real lignin sample. Rhenium oxide was implemented to increase catalyst acidity for enhanced dehydration efficiency.

2.2. Catalyst characterization The crystalline structure of the catalyst samples was characterized using X-ray diffraction (XRD) on a Rigaku Miniflex diffractometer. Xray fluorescence (XRF) device (ZSX Primus II, Rigaku) was employed to measure the metal content of the catalysts. The catalyst acidity was characterized by temperature-programmed desorption of ammonia (NH3-TPD). Before TPD analysis, the sample was reduced in a stream of 5% H2/95% He (50 ml/min) at 350 °C for 60 min. After being cooled to 100 °C, a 50 ml/min flow of 5% ammonia/95% helium was introduced into the sample cell for 30 min. Then, helium gas was passed through the sample for 30 min to eliminate the physisorbed ammonia. NH3 desorption was performed by heating the sample (10 °C/min) in a 50 ml/min flow of helium. Isothermal (−196 °C) nitrogen adsorption–desorption was implemented to measure the textural properties of catalyst samples such as surface area, pore volume and pore size distribution. Before analysis, the catalysts were degassed under vacuum at 180 °C for 4 h. 2.3. Measurement of the amount of coke deposition on catalyst Thermogravimetric analysis was performed to determine the coke content of the spent catalysts. In an air flow (100 ml/min), the temperature of the sample was increased (30–750 °C: 10 °C/min) and kept at 750 °C for 30 min. The weight loss observed in the range of 350–750 °C was considered as the combustion of coke. 2.4. Catalytic hydropyrolysis A tandem micro-reactor (Rx-3050TR) with two zones in series was used for the catalytic hydropyrolysis experiments. The first zone was used for hydropyrolysis and the second zone for ex-situ catalytic HDO upgrading. Kraft lignin (product number: 370959, Sigma-Aldrich) with H:G:S (p-hydroxyphenyl:guaiacyl:syringyl) ratio of 7:93:traces was used as the feedstock [33]. The elemental and proximate compositions (wt%) of the kraft lignin sample are C, 62.45; H, 5.68; O, 30.61; N, 0.56; S, 0.7 and moisture, 1.8; volatiles, 62.9; fixed carbon, 32.6; ash, 2.7, respectively. Kraft lignin (4 ± 0.01 mg) or a mixture of lignin (4 ± 0.01 mg)/HY catalyst (8 ± 0.01 mg) were the samples introduced to the reaction system. Based on our preliminary experiments, the HY/lignin ratio of 2 ensures that the sufficient amount of zeolite acid sites is provided for an effective acid-catalyzed pyrolysis of lignin. The sample was loaded in a stainless steel cup which was dropped into the first preheated zone (600 °C) for pyrolysis. The pyrolysis vapors were conducted to the upgrading zone (second zone set at 350, 450 or

2. Experimental 2.1. Catalyst preparation HY, HBeta, Fe/HBeta, FeReOx/MCM-41, Fe/ZrO2, ReOx/ZrO2, FeReOx/ZrO2, NiReOx/ZrO2 and PdReOx/ZrO2 were the catalysts used in this study. HY (CBV 720, SiO2/Al2O3: 30) was obtained from Zeolyst, and HBeta was prepared by the calcination (550 °C/12 h) of Beta zeolite in ammonium form (Zeolyst, CP814C, SiO2/Al2O3: 38). Zirconia was synthesized through the following synthesis method [30,31]. The solution of ZrO(NO3)2 and ammonia water (pH control agent) were pumped into the mixing point (400 °C) located above the reactor (NH3/ NO3 molar ratio: 1.5), and mixed with distilled water (preheated at 550 °C). Supercritical condition (250 bar) was implemented to form zirconia nanoparticles using a residence time of 6 s. The synthesized 2

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550 °C) containing a catalyst bed with the weight of 40 ± 0.01 mg which was fixed by the plugs of quartz wool. Based on some preliminary experiments, 40 mg was selected as the optimum amount of ex-situ catalyst for an efficient HDO reaction. Pure hydrogen gas (100 ml/min) was injected to the reactor for hydropyrolysis and HDO upgrading. The experiments were carried out at atmospheric pressure. In-situ reduction of the ex-situ catalysts was conducted at 350 °C for 1 h before the reactions. The products from the second zone were conducted through an interface (320 °C) to a gas chromatograph (7890A, Agilent Technologies) (split ratio, 100:1) for separation in a UA-5 capillary column (0.25 mm i.d. × 30 m length × 0.25 μm film thickness) and detection by MSD (qualitative analysis) and FID (quantitative analysis). 1 ml/min flow of pure helium was used as carrier gas. Prior to GC separation, the products were cryo-focused (at −196 °C) for 3 min at the front part of the column by liquid nitrogen supplied from a MicroJet Cryo Trap (MJT-1030E). The GC oven program for the separation of the products was as follows: from 40 °C (4 min hold) to 320 °C (10 min hold) at 5 °C/min. The product yields were determined by the external standard calibration procedure, with calibration curves for a number of individual compounds including benzene, toluene, m-xylene, naphthalene, m-cresol, guaiacol, catechol and 2-naphthalenol. Each experiment was performed in triplicate, and the average values are reported. The standard deviation of the obtained data was below 2% which confirms the reproducibility of data. The solid residue remained from pyrolysis of lignin in the first zone was considered as char, and its amount was measured by subtraction of the weights of cup and fresh catalyst from the weight of cup and its content after reaction.

Fig. 2. NH3-TPD profiles of catalysts (Except HY, other catalysts were reduced at 350 °C before TPD analysis). Table 1 Acid site density and strength of catalysts determined by NH3-TPD. Except HY, other catalysts were reduced at 350 °C before TPD analysis. Sample

HY FeReOx/MCM-41 Fe/ZrO2 FeReOx/ZrO2 Fe/HBeta

3. Results and discussion 3.1. Catalyst characterization As illustrated by the XRD patterns shown in Fig. 1, zirconia prepared in this study contains monoclinic structure, and MCM-41 has amorphous structure (as depicted by the broad XRD peak). The amount of each metal (Fe, Pd, Ni and Re) loaded on all the prepared catalysts was 4 ± 0.1 wt% (measured by XRF). The acidic properties of the in-situ catalyst of HY and ex-situ catalysts of FeReOx/MCM-41, Fe/ZrO2, FeReOx/ZrO2 and Fe/HBeta are shown in Fig. 2 and Table 1. The total acidity of ex-situ catalysts is in the order: Fe/HBeta > FeReOx/ ZrO2 > Fe/ZrO2 > FeReOx/MCM-41. Fe/HBeta, containing both Brønsted and Lewis acidity, has a high percentage of strong acid sites (32%), while the weak and medium strength acid sites are dominant in MCM-41 and ZrO2 supported catalysts. A comparison of the densities and distributions of the acid sites of Fe/ZrO2 and FeReOx/ZrO2 reveals that rhenium oxide addition caused a significant increase in the medium strength acidity. The low acid site density of FeReOx/MCM-41

Acidity (mmol g−1) Total

Weak (< 300 °C)

Medium (300–600 °C)

Strong (> 600 °C)

1.083 0.093 0.242 0.276 1.188

0.294 0.039 0.128 0.101 0.404

0.386 0.050 0.088 0.133 0.408

0.403 0.004 0.026 0.042 0.376

(27%) (42%) (53%) (37%) (34%)

(36%) (54%) (36%) (48%) (34%)

(37%) (4%) (11%) (15%) (32%)

is due to the low acid amount of MCM-41 support. HY zeolite, used as in-situ catalyst, also has high acid site density with a high proportion of strong acidity (37%). Based on the textural analysis data shown in Fig. 3 and Table 2, ZrO2 and MCM-41 are mesoporous with BJH adsorption average pore diameters of 18.9 and 2.8 nm, respectively, while HBeta zeolite is a microporous support with channels of 0.56 × 0.56 and 0.66 × 0.67 nm [34]. Among the catalyst supports, ZrO2 and MCM41 have the lowest (56 m2 g−1) and highest (1038 m2 g−1) surface areas, respectively. No microporous volume was detected for ZrO2 and MCM-41, while HBeta and HY zeolites mostly contain microporous structure. The textural properties of zirconia supported catalysts are almost similar to those of zirconia support. However, surface area and pore volume were considerably affected by the impregnation of iron and rhenium species on HBeta and MCM-41 supports. 3.2. In-situ and ex-situ catalytic hydropyrolysis of lignin The yields of pyrolyzates produced from non-catalytic/catalytic hydropyrolysis of kraft lignin are presented in Table 3. A comparison between the two first columns of this table clearly shows the significant effect of using HY as in-situ catalyst; the total yield of detected liquid products in the case HY was physically mixed with lignin is considerably higher than that when pure lignin was introduced to the system. This is due to the reduced formation of char, remained from lignin pyrolysis, by addition of HY; the char yields obtained from thermal and HY-catalyzed hydropyrolysis of lignin were 43.2 and 32.6 wt%, respectively. The suppressed char formation in the pyrolysis of lignin mixed with HY catalyst is a result of the enhanced decomposition of lignin polymer over the acid sites of HY. This results in increased production of pyrolysis vapors which can be further upgraded through hydrodeoxygenation reaction over an ex-situ catalyst. Furthermore, using HY as in-situ catalyst resulted in different composition of liquid products with a shift towards monomeric phenolics due to the

Fig. 1. X-ray diffraction patterns of the synthesized supports. 3

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Table 3 Yields of liquid products and char (wt% on feed) obtained by the non-catalytic/ catalytic hydropyrolysis of kraft lignin. Reaction conditions: pyrolysis temperature, 600 °C; pressure, 1 atm; lignin: 4 mg; HY, 8 mg; HBeta, 40 mg; H2 flow, 100 ml min−1. Feed In-situ catalyst Ex-situ catalyst

Lignin – –

Lignin HY –

Lignin HY HBeta

Aromatic hydrocarbons BTX

0.4 0.2

0.8 0.5

0.7 0.5

Oxygenates Phenol type Guaiacol type Catechol type Phenolic oligomers

12.1 3.7 3.2 1.6 3.6

15.5 12.3 1.3 0.8 1.1

3.3 1.1 0.7 0.5 1.0

Char

43.2

32.6

32.4

% Yield

Fig. 4. Selectivity of phenolic compounds obtained by the non-catalytic and insitu catalytic hydropyrolysis of kraft lignin.

acid-catalyzed cracking over HY zeolite (shown in Fig. 4). HY, with strong Brønsted acid sites, can be effectively used for an enhanced cracking of lignin-derived oligomers. Meanwhile, the relatively large pore size of HY (0.74 × 0.74 nm) compared to other commonly used zeolites such as HZSM-5 and HBeta allows large oligomers to easily diffuse into catalyst and be cracked over zeolite acid sites [35]. Moreover, HY efficiently converted the guaiacol and catechol type phenolics into phenol type compounds, indicating the significant role of this catalyst in promoting the dehydroxylation and demethoxylation reactions. However, it should be mentioned that in-situ catalytic pyrolysis has some disadvantages which make its application difficult. One main problem is that the recovery and reusability of catalyst is difficult since catalyst and feedstock are physically mixed in in-situ catalytic pyrolysis. Moreover, the contact of metals and minerals in feedstock with catalyst can cause catalyst poisoning. The catalytic activities of Fe/HBeta, FeReOx/MCM-41, Fe/ZrO2 and FeReOx/ZrO2 as ex-situ catalyst for HDO of kraft lignin pyrolysis-derived vapors at 350 °C are shown in Table 4. No saturated cyclic compound was detected in the liquid products obtained over the catalysts used in this work. This shows that phenolic ring hydrogenation did not occur, and lignin-derived phenolics were selectively converted into aromatic hydrocarbons through carbonyl group hydrogenation. In the HDO of phenolic compounds to aromatic hydrocarbons, it is supposed that phenolics undergo an initial tautomerization to produce highly unstable keto-tautomer intermediates (cyclohexadienones) which are subsequently converted to cyclohexadienols via carbonyl group hydrogenation. Then, the produced cyclohexadienols are dehydrated to produce aromatic hydrocarbons over catalyst acid sites [36]. The selective hydrogenation of carbonyl group over the tested catalysts is

Fig. 3. Pore size distributions (obtained by BJH method using the adsorption branch) and N2 adsorption–desorption isotherms of (a) HBeta, (b) MCM-41 and (c) ZrO2. Table 2 Textural properties of catalysts. Sample

SBET (m2 g−1)

Smeso (m2 g−1)

SBET/ Smeso

Vtotal (cm3 g−1)

Vmeso (cm3 g−1)

HY HBeta Fe/HBeta ZrO2 Fe/ZrO2 FeReOx/ZrO2 MCM-41 FeReOx/MCM41

645 698 623 56 55 55 1038 892

158 79 71 48 45 43 1021 868

4.08 8.84 8.77 1.17 1.22 1.28 1.02 1.03

0.429 0.344 0.309 0.249 0.228 0.215 0.818 0.627

0.191 0.098 0.094 0.245 0.223 0.210 0.818 0.627

4

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Table 4 Yields of liquid products and coke (wt% on feed) obtained by the catalytic hydropyrolysis of kraft lignin using HY as in-situ catalyst and Fe/HBeta, FeReOx/MCM-41, Fe/ZrO2, FeReOx/ZrO2, NiReOx/ZrO2 and PdReOx/ZrO2 as ex-situ catalyst. Reaction conditions: pyrolysis temperature, 600 °C; HDO temperature, 350 °C; pressure, 1 atm; lignin: 4 mg; in-situ catalyst (HY), 8 mg; ex-situ catalyst, 40 mg; H2 flow, 100 ml min−1. Ex-situ catalyst

Fe/HBeta

FeReOx/MCM-41

Fe/ZrO2

FeReOx/ZrO2

NiReOx/ZrO2

PdReOx/ZrO2

Aromatic hydrocarbons BTX

1.4 1.1

5.0 3.5

1.8 1.3

7.1 4.8

5.2 3.4

7.6 5.4

Oxygenates Phenol type Guaiacol type Catechol type Phenolic oligomers

5.5 3.5 0.8 0.5 0.7

5.6 3.3 0.8 0.6 0.9

11.4 8.7 1.0 0.7 1.0

3.2 1.1 0.7 0.5 0.9

3.3 1.6 0.6 0.4 0.7

2.5 0.9 0.3 0.4 0.9

Coke

8.2

0.9

0.7

1.0

1.1

1.0

% Yield

catalyst for effective HDO of phenolic compounds by promoting the acid-catalyzed dehydration reaction. However, the zeolite-supported catalyst of Fe/HBeta, with much higher acid site density, resulted in considerably lower hydrocarbon yield compared to FeReOx/ZrO2. This is caused by the strong acid sites of Fe/HBeta resulting in a high trapping of phenolic compounds inside the channels of this catalyst. According to the acid strength distribution measured by NH3-TPD analysis, the percentages of low, medium and high strength acid sites of Fe/ HBeta and FeReOx/ZrO2 are 34, 34, 32% and 37, 48, 15%, respectively (Table 1). As a result of the strong acidity of Fe/HBeta, this catalyst has a high phenolic trapping potential since its strong acid sites can retain the phenolic compounds adsorbed on catalyst surface at the mild temperature of 350 °C. The adsorbed phenolics can act as coke precursors and cause a high coke formation. This is evidenced by the significantly higher yield of coke deposited on Fe/HBeta compared to FeReOx/ZrO2; as shown in Table 4, the coke yields obtained over the exsitu catalysts of Fe/HBeta and FeReOx/ZrO2 were 8.2 and 1.0 wt%, respectively. The presence of phenolic compounds in the coke deposited on HBeta used in catalytic pyrolysis of lignin was reported in a research held by Rezaei et al. [44]. Therefore, it can be inferred that although Fe/HBeta contains high acid site density to proceed the dehydration step in the HDO reaction, but its high acid strength causes high phenolic trapping, and consequently, low number of phenolic molecules can undergo HDO reaction and pass the catalyst bed. In contrast, FeReOx/ ZrO2 with sufficient acid site density and mild acid strength has a wellbalanced acidity which leads to an efficient dehydration activity and a low phenolic trapping potential as well. It should also be mentioned that the severe phenolic trapping inside Fe/HBeta observed in this work is due to the low catalyst temperature of 350 °C since adsorption is exothermic, and is increased at low temperatures. In order to clearly show the high phenolic trapping occurred inside zeolite channels at 350 °C, HBeta with no supported phase (no hydrogenation promoter) was examined as ex-situ catalyst. The product yields are shown in Table 3, and can be compared with the data obtained from the experiment with no ex-situ catalyst. Aromatic hydrocarbon yield almost remained constant by the addition of HBeta as exsitu catalyst, but the yield of phenolic compounds dramatically decreased from 15.5 to 3.3 wt% due to the high trapping of phenolics inside HBeta structure. As a result of this trapping, the yield of coke deposited on HBeta was 11.8 wt%. Fig. 5 summarizes the acidic properties of the ex-situ catalysts tested in this study. As illustrated by this figure, the HDO activity of catalyst depends on the both density and strength of acid sites. High acid density and medium acid strength are the key factors to achieve enhanced HDO efficiency. Although Fe/HBeta has a high acid density, but its high acid strength (low M/S (medium to strong acid sites) ratio) leads to high phenolic trapping and coke formation. In contrast, FeReOx/ZrO2, with the both sufficiently high acid density and mild acid strength, gave the highest aromatic hydrocarbon yield and a low coke formation.

attributed to the high oxophilicity of iron metal; the strong interaction between the oxophilic iron metal and carbonyl group leads to its selective hydrogenation. The HDO activity of catalysts reduced in the order: FeReOx/ZrO2 > FeReOx/MCM-41 > Fe/ZrO2 > Fe/HBeta. For comparison, ReOx/ZrO2 was also tested as ex-situ catalyst, but no increased aromatic hydrocarbon production was observed using this catalyst, indicating that the presence of an effective hydrogenation promoter like iron is needed to promote the hydrogenation step of HDO reaction. Toluene, benzene, xylenes, trimethylbenzene, pentamethylbenzene and naphthalenes were the major aromatic hydrocarbons produced from FeReOx/ZrO2 catalyzed HDO of lignin-derived pyrolyzates. The yields of BTX (benzene, toluene and xylenes) and total aromatic hydrocarbons obtained over FeReOx/ZrO2 were 4.8 and 7.1 wt %, respectively. Compared to the hydrocarbon yields and reaction temperatures previously reported in the literature, FeReOx/ZrO2 seems to be a potential catalyst for aromatic hydrocarbon production from atmosphericpressure HDO of lignin pyrolysis vapors at the low temperature of 350 °C. To the best of our knowledge, this is the lowest temperature used for atmospheric upgrading of lignin pyrolysis-derived oxygenates. Catalytic pyrolysis of lignin has been mostly carried out at temperatures higher than 500 °C using different zeolites as catalysts, with the reports of enhanced hydrocarbon production at catalytic reaction temperatures above 600 °C [37–43]. Zhou et al. [28] reported that high catalyst temperature of 600 °C is required to produce aromatic hydrocarbons in catalytic pyrolysis of lignin using HZSM-5 catalyst. The yield of total aromatic hydrocarbons produced over HZSM-5 at 600 °C was 4.0 wt%. In a research performed by Rezaei et al. [44], the yield of oxygen free aromatics produced from catalytic hydropyrolysis of lignin over Fe/ HBeta at 500 °C was 5.13 wt% which is lower than the yield achieved over FeReOx/ZrO2 at the low temperature of 350 °C. Also, in our present work, the aromatic hydrocarbon yield of FeReOx/ZrO2 was 5.1 times more than that of Fe/HBeta at 350 °C. Considering that HBeta is a commonly used catalyst support for lignin conversion, it can be inferred that FeReOx/ZrO2 is a potential catalyst for mild HDO of lignin-derived phenolics.

3.3. Catalytic performance of FeReOx/ZrO2 One main reason for the high catalytic activity of FeReOx/ZrO2 is its well-balanced acidity which results in both effective dehydration (sufficient acidity) and low phenolic trapping potential (not too high acid strength). As shown by the NH3-TPD data presented in Fig. 2 and Table 1, FeReOx/ZrO2 has more suitable acidity than the other examined catalysts. The acid site densities of catalysts decrease in the order: Fe/HBeta > FeReOx/ZrO2 > Fe/ZrO2 > FeReOx/MCM-41, which are 1.188, 0.276, 0.242 and 0.093 mmol g−1, respectively. Compared to FeReOx/MCM-41 and Fe/ZrO2 catalysts, FeReOx/ZrO2 contains higher acid site density which makes it a more potential 5

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Fig. 5. Acid site density and strength of Fe/HBeta, FeReOx/MCM-41, Fe/ZrO2 and FeReOx/ZrO2.

Fig. 6. Effect of reaction temperature on catalytic performance of Fe/HBeta as ex-situ catalyst in the catalytic hydropyrolysis of kraft lignin. Reaction conditions: pyrolysis temperature, 600 °C; pressure, 1 atm; lignin: 4 mg; in-situ catalyst (HY), 8 mg; ex-situ catalyst (Fe/HBeta), 40 mg; H2 flow, 100 ml min−1.

Although the strengths and densities of the acid sites of Fe/ZrO2 and FeReOx/ZrO2 are almost equal, Fe/ZrO2 resulted in considerably lower aromatic hydrocarbon yield. This is attributed to the absence of rhenium oxide which, as shown in our previous work, has high efficiency to promote the dehydration reaction [36]. Meanwhile, a comparison between the acid site densities of Fe/ZrO2 and FeReOx/ZrO2 indicates that the addition of rhenium oxide to zirconia support resulted in an increase in the total acidity of the catalyst; the densities of the acid sites of FeReOx/ZrO2 and Fe/ZrO2 are 0.276 and 0.242 mmol g−1, respectively. The acid strength distribution of these catalysts reveals that the addition of rhenium oxide mostly affected the medium strength acidity, and increased it from 0.088 to 0.133 mmol g−1. This illustrates that the loaded rhenium oxide species induce mild acidity which significantly causes an enhanced catalytic activity for the HDO of phenolics; aromatic hydrocarbon yields obtained over FeReOx/ZrO2 and Fe/ZrO2 were 7.1 and 1.8 wt%, respectively. FeReOx/MCM-41 has a high M/S ratio of 12.5, but its low acid amount (0.093 mmol g−1) results in lower HDO activity compared to FeReOx/ZrO2. The better performance of FeReOx/ZrO2 compared to FeReOx/MCM-41 is also due to the high oxophilicity of zirconia support; the activation of oxygenates on the surface of FeReOx/ZrO2 is enhanced by the zirconia oxophilic sites, giving a high reactivity of lignin-derived phenolics [45,46]. Moreover, another key property of MCM-41 and ZrO2 supported catalysts which makes them more efficient than Fe/HBeta is their mesoporosity. The diffusion rate of phenolics in the mesoporous structures of MCM-41 and ZrO2 is supposed to be higher than that in the microporous channels of HBeta, causing a lower possibility of phenolic adsorption and trapping inside MCM-41 and ZrO2 supported catalysts. This is especially important in the process studied in this work since the temperature implemented for HDO reaction is low (350 °C) which causes a difficult and slow diffusion. The effect of reaction temperature on HDO activity of Fe/HBeta is shown in Fig. 6. The yield of coke deposited on Fe/HBeta was significantly reduced from 8.2 to 3.3 wt% by the increase of reaction temperature from 350 to 550 °C. This indicates that the adsorption of phenolics on zeolite acid sites, which is the major cause of coke formation, is remarkably attenuated at elevated temperature due to the exothermicity of adsorption. As a result of the reduced phenolic trapping by temperature increase, the yield of aromatic hydrocarbons produced by the use of Fe/HBeta as ex-situ catalyst was increased from 1.4 to 4.9 wt% by an increase of the HDO upgrading temperature from 350 to 550 °C. It should also be mentioned that the suppression of phenolic adsorption at high temperatures allows the use of HY as in-situ catalyst in this process. Although HY is a zeolite with a high acid strength comparable to that of HBeta, but its use as an in-situ catalyst did not cause considerable phenolic trapping due to the high

temperature of this catalyst (600 °C) implemented for lignin decomposition. In short, it can be inferred that zeolite-supported catalysts are not a proper option for atmospheric upgrading of lignin pyrolyzates at mild temperatures. Alternatively, a catalyst such as FeReOx/ZrO2, with wellbalanced acidity (caused by both zirconia and rhenium oxide), high mesoporosity and oxophilicity, is a potential catalyst to effectively proceed the hydrodeoxygenation of lignin-derived phenolics at mild reaction conditions. The combination of hydrogenation promoter and acid sites in FeReOx/ZrO2 catalyst enables the cleavage of the strong Caromatic-OH bond. The high strength of this bond is due to the delocalization effect between the phenolic ring π bond orbital and the outof-plane lone pair electron orbital of oxygen [47,48]. In the HDO of phenolics over FeReOx/ZrO2, it is supposed that keto-tautomer intermediates undergo carbonyl group hydrogenation over iron metals to form cyclohexadienols. The C-O bond strength in cyclohexadienols is weakened due to the elimination of the electron delocalization effect. Consequently, oxygen can be easily removed through dehydration of cyclohexadienols over the mild acid sites of rhenium oxide and zirconia support. Rhenium oxide, in particular, revealed remarkable dehydration activity at mild reaction conditions. A process including an in-situ catalytic pyrolysis of lignin followed by an ex-situ catalytic hydrotreatment of lignin pyrolyzates over a catalyst like FeReOx/ZrO2 can be used to achieve the low formation of char remained from pyrolysis (increased production of pyrolysis vapors) and the enhanced deoxygenation of lignin pyrolysis-derived phenolics as well. Such process with the upgrading conditions of atmospheric pressure and mild temperature of 350 °C can be indeed of high potential to be commercialized as an economically feasible lignin to aromatic hydrocarbon process. 3.4. Hydrogenation efficiency of iron, palladium and nickel Table 4 compares the effects of three different metals (Fe, Ni and Pd) as hydrogenation promoter in the ex-situ catalytic HDO of kraft lignin pyrolysis-derived vapors at 350 °C. The hydrogenation efficiency of the metals reduced in the order palladium > iron > nickel, with the aromatic hydrocarbon yields of 7.6, 7.1 and 5.2 wt%, respectively. Palladium, with the potential for high hydrogen uptake and high dissociation of hydrogen molecules, is an effective metal as hydrogenation promoter giving the highest aromatic hydrocarbon yield. However, the high price of this metal is a negative point which makes the use of palladium not suitable from economic point of view. On the other hand, iron metal with its considerably lower price and its hydrogenation activity comparable with that of palladium (the hydrocarbon yield of 6

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FeReOx/ZrO2 is slightly lower than that of PdReOx/ZrO2) can be considered as a potential hydrogenation metal for the catalytic conversion of lignin into aromatic hydrocarbons. The lower hydrocarbon yield of NiReOx/ZrO2 is explained by the low yield of liquid phase produced over this catalyst which can be attributed to the high formation of gaseous compounds in the presence nickel. Similar to iron, the metals of palladium and nickel also did not produce saturated cyclic compounds, indicating that ring hydrogenation did not occur over PdReOx/ZrO2 and NiReOx/ZrO2 at 350 °C.

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4. Conclusions In the hydropyrolysis of kraft lignin at 600 °C, the use of YH as insitu catalyst facilitated the decomposition of lignin resulting in reduced formation of char and increased conversion of oligomeric phenolics into monomeric phenolics. Moreover, FeReOx/ZrO2, used as ex-situ catalyst under mild reaction conditions, led to considerably enhanced HDO of lignin-derived phenolics to aromatic hydrocarbons in comparison with the zeolite supported catalyst of Fe/HBeta. The aromatic hydrocarbon yield achieved over Fe/HBeta was 1.4 wt% which was increased to 7.1 wt% by the use of FeReOx/ZrO2. This was due to the low phenolic trapping occurred over FeReOx/ZrO2 causing significantly reduced coke formation using this catalyst; the coke yields obtained over Fe/HBeta and FeReOx/ZrO2 were 8.2 and 1.0 wt%, respectively. The low phenolic adsorption and trapping inside FeReOx/ZrO2 is mainly due to its high mesoporosity and mild acidity. These properties along with the high oxophilicity make FeReOx/ZrO2 a potential catalyst to be used for hydrodeoxygenation of lignin-derived oxygenates at mild conditions which is highly important to achieve a cost-effective process. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) funded by the Korea government (MSIT) (No. 2018R1A2B2001121). References [1] E.E. Kwon, Y.T. Kim, H.J. Kim, K.-Y.A. Lin, K.-H. Kim, J. Lee, G.W. Huber, Production of high-octane gasoline via hydrodeoxygenation of sorbitol over palladium-based bimetallic catalysts, J. Environ. Manage. 227 (2018) 329–334. [2] C. Yao, H. Tian, Z. Hu, Y. Yin, D. Chen, X. Yan, Characteristics and kinetics analyses of different genus biomass pyrolysis, Korean J. Chem. Eng. 35 (2018) 511–517. [3] C. Perego, A. Bosetti, Biomass to fuels: the role of zeolite and mesoporous materials, Microporous Mesoporous Mater. 144 (2011) 28–39. [4] P.S. Rezaei, H. Shafaghat, W.M.A.W. Daud, Production of green aromatics and olefins by catalytic cracking of oxygenate compounds derived from biomass pyrolysis: a review, Appl. Catal. A 469 (2014) 490–511. [5] Q. Zhang, J. Chang, Wang, Y. Xu, Upgrading bio-oil over different solid catalysts, Energy Fuels 20 (2006) 2717–2720. [6] Y. Zhang, T.R. Brown, G. Hu, R.C. Brown, Techno-economic analysis of two bio-oil upgrading pathways, Chem. Eng. J. 225 (2013) 895–904. [7] J.Y. Park, J.-K. Kim, C.-H. Oh, J.-W. Park, E.E. Kwon, Production of bio-oil from fast pyrolysis of biomass using a pilot-scale circulating fluidized bed reactor and its characterization, J. Environ. Manage. 234 (2019) 138–144. [8] R.K. Liew, W.L. Nam, M.Y. Chong, X.Y. Phang, M.H. Su, P.N.Y. Yek, N.L. Ma, C.K. Cheng, C.T. Chong, S.S. Lam, Oil palm waste: an abundant and promising feedstock for microwave pyrolysis conversion into good quality biochar with potential multi-applications, Process Saf. Environ. Prot. 115 (2018) 57–69. [9] S.S. Lam, R.K. Liew, X.Y. Lim, F.N. Ani, A. Jusoh, Fruit waste as feedstock for recovery by pyrolysis technique, Int. Biodeterior. Biodegrad. 113 (2016) 325–333. [10] S.S. Lam, R.K. Liew, C.K. Cheng, N. Rasit, C.K. Ooi, N.L. Ma, J.H. Ng, W.H. Lam, C.T. Chong, H.A. Chase, Pyrolysis production of fruit peel biochar for potential use in treatment of palm oil mill effluent, J. Environ. Manage. 213 (2018) 400–408. [11] S.S. Lam, R.K. Liew, A. Jusoh, C.T. Chong, F.N. Ani, H.A. Chase, Progress in waste oil to sustainable energy, with emphasis on pyrolysis techniques, Renew. Sustain. Energy Rev. 53 (2016) 741–753. [12] J. Lee, I. Ro, H.J. Kim, Y.T. Kim, E.E. Kwon, G.W. Huber, Production of renewable C4–C6 monoalcohols from waste biomass-derived carbohydrate via aqueous-phase hydrodeoxygenation over Pt-ReOx/Zr-P, Process Saf. Environ. Prot. 115 (2018) 2–7. [13] H. Kim, H. Shafaghat, J. Kim, B.S. Kang, J.-K. Jeon, S.-C. Jung, I.-G. Lee, Y.-K. Park, Stabilization of bio-oil over a low cost dolomite catalyst, Korean J. Chem. Eng. 35 (2018) 922–925.

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