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Catalytic Fast Pyrolysis of Bagasse Using Activated Carbon Catalyst to Selectively Produce 4‑Ethyl Phenol Qiang Lu,*,† Xiao-ning Ye,† Zhi-bo Zhang,† Min-Shu Cui,† Hao-qiang Guo,† Wei Qi,*,‡ Chang-qing Dong,† and Yong-ping Yang† †

National Engineering Laboratory for Biomass Power Generation Equipment, North China Electric Power University, Beijing 102206, China ‡ Key Laboratory of Renewable Energy, and Guangdong Key Laboratory of New and Renewable Energy Research and Development, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China S Supporting Information *

ABSTRACT: Fast pyrolysis of bagasse catalyzed by activated carbon (AC) at low temperatures offered a new and promising way to produce the valuable 4-ethyl phenol (4-EP) compound in high selectivity. In this study, the technique of pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) was first applied to investigate several factors on the 4-EP production, including biomass type, fast pyrolysis temperature, AC-to-bagasse ratio, and catalytic pattern (in situ or ex situ catalysis). Moreover, fast pyrolysis experiments by using AC catalyst were conducted in a lab-scale setup to quantitatively determine the pyrolytic products. The experimental results indicated that, among these five herbaceous biomass materials, bagasse was the best material to produce 4-vinylphenol (4-VP) from the noncatalytic process and 4-EP from the catalytic fast pyrolysis process, respectively. 4-VP and its precursors could be catalytically hydrogenated into 4-EP with high selectivity under the catalysis of AC catalyst. Both catalytic pyrolysis temperature and AC-to-bagasse ratio affected the 4-EP selectivity significantly, whereas the catalytic pattern had minor effects on the 4-EP production. The highest yield of 4-EP from bagasse in Py-GC/MS experiments was 2.13 wt %, obtained at the pyrolysis temperature of 300 °C and AC-to-bagasse ratio of 4 from the in situ catalytic pattern. Moreover, lab-scale fast pyrolysis of bagasse catalyzed by AC catalyst obtained the maximal 4-EP yield of 2.49 wt %, with the selectivity of 10.71%.

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INTRODUCTION

materials, especially lignins of herbaceous and woody materials, differ in compositions, pyrolysis behaviors, and product distributions.26−29 Currently, the fast pyrolysis behaviors and characteristics of different herbaceous biomass materials have been investigated, such as cornstalk,30 bagasse,31,32 rice husk,33 and wheat straw.34 On the basis of the particular pyrolysis properties of certain herbaceous biomass, Qu et al.35 first reported a protocol for selectively preparing 4-VP through fast pyrolysis of bagasse, bamboo, and corn stalk at low temperatures. As a valuable compound, 4-VP can be used as a flavor agent as well as for production of poly(4-vinylphenol). Among these herbaceous materials, bagasse could achieve the best performance for selective 4-VP production.35 On the basis of this result, our research group developed a new process for the selective preparation of another high-valued phenolic compound, 4-EP, via ex situ catalytic pyrolysis of bagasse over Pd/ SBA-15 catalyst.36 4-EP is a valuable aroma compound for flavoring and can be widely utilized for synthesis of synthetic resins and antioxidants.37,38 Current industrial production of 4EP is complex and environmentally unfriendly, mainly consisting of three steps, i.e., sulphonation of ethylbenzene to produce an ethylbenzenesulfonic acid mixture, separation of para-ethylbenzenesulfonic acid, and finally alkali fusion to obtain 4-EP.38 In our previous study, promising hydrogenating

Pyrolysis of biomass is a thermal decomposition process to convert solid biomass into various liquid, solid, and gaseous products which can be used as either fuels or chemicals.1,2 Among different pyrolysis techniques, selective fast pyrolysis of biomass for high-valued compounds production, which offers a promising way for value-added usage of biomass, has been widely investigated during the past decade.3 During the biomass pyrolysis process, hundreds of parallel or successive pyrolytic pathways will occur.1,4 Hence, the key issue for the selective pyrolysis process is to control the pyrolytic pathways of biomass, so as to prepare specific bio-oils with abundant target compounds. Catalytic pyrolysis is typically the most effective way to selectively produce target compounds.5 Currently, various techniques for selective pyrolysis of biomass have been proposed. Most of the techniques were aimed at production of holocellulose-derived valuable compounds, such as levoglucosenone,6−11 furfural,12−16 1-hydroxy-3,6-dioxabicyclo[3.2.1]octan-2-one,17−20 etc. However, although various lignin-derived phenolic compounds are also valuable chemicals, only selective preparation of phenolic-rich bio-oil (mixed phenolic compounds) has been widely investigated at present.21−25 Very limited researches were reported on the selective preparation of individual valuable phenolic compounds. To obtain specific lignin-derived phenolic compounds, biomass types, which greatly affect the lignin composition, will be the essential factor for the development of biomass selective pyrolysis technology. Lignins of different biomass © XXXX American Chemical Society

Received: October 10, 2016 Revised: November 24, 2016 Published: November 25, 2016 A

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catalyst together, while the ex situ catalytic pattern was achieved by placing the AC catalyst at two sides of biomass (without direct contact). The detailed preparation method of experimental samples can be seen in the Supporting Information (Figure S1) and our previous study.36 For each experimental sample, bagasse quantity (or other biomass materials) was strictly fixed to be 0.30 mg, while AC catalyst quantity varied from 0 to 2.40 mg calculated by AC-to-bagasse ratios of 0, 1, 2, 4, 6, and 8, respectively. The selection of this AC-tobagasse ratio range was achieved by exploratory experiments prior to detailed experiments. The weighing used an analytical balance with readability of 0.01 mg. The pyrolysis experiments were carried out for 20 s under temperatures of 250, 300, 350, 400, and 450 °C with the heating rate of 20 000 °C/s (the maximal heating rate of the pyrolyser). Each experiment was performed for at least three times to ensure its reproducibility. The vapors from the pyrolysis process were transferred into GC/MS for direct analysis. The GC injector temperature was kept at 300 °C. The GC oven temperature was set at 40 °C (stay for 2 min), then heated to 160 °C with a heating rate of 10 °C/min, followed by maintaining at 280 °C for 3 min with a heating rate of 15 °C/min. An Elite-35MS capillary column was used for GC separation, whose specification was 30 m × 0.25 mm i.d., 0.25 μm film thickness. Additional detailed parameters for GC/MS can be found in a previous study.10 The detected products were determined based on the NIST library, Wiley library, and the literature data of previous studies.35,42,43 Peak area and peak area percentage (%) values of all identified pyrolytic products were recorded. Quantitative Determination of Important Products in PyGC/MS Experiments. Quantitative results could not be obtained directly from Py-GC/MS experiments. The actual yield of each pyrolytic product could be determined individually by the external standard method (external calibration). In this study, the yields of four important phenolic products (4-VP, 4-EP, 4-VG, and 4-EG) were quantitatively determined. The calibration experiments were conducted via the Py-GC/MS instrument by using the pure 4-VP, 4-EP, 4VG, and 4-EG, to obtain the calibration line of each product based on the product quantity and corresponding chromatographic peak area. The details can be found in our previous studies.10,25 On the basis of the calibration line, each product peak area, and the biomass quantity (0.30 mg), we are able to calculate the four products actual yields under different pyrolysis conditions. The other pyrolytic products were not quantitatively analyzed due to their low yields in the catalytic pyrolysis process or the lack of commercial standard chemicals. Lab-Scale in Situ Catalytic Fast Pyrolysis Experiments. With the purpose of further confirming the catalytic effects of AC catalyst and to collect pyrolytic products, catalytic fast pyrolysis experiments were conducted using a lab-scale experimental setup, as shown in Figure 1. The biomass raw material (bagasse) and the catalyst (AC) were mechanically mixed together and stored in the feedstock container at the top of the reactor, then fed into the downflow pyrolysis reactor continuously. The continuous feeding of bagasse was achieved by controlled slow rotation of the feedstock supporter in the container, and a diagram of the feeding process is shown in the Supporting Information (Figure S2). N2 as carrier gas was fed through the feedstock container, with a constant flow rate of 100 mL/min. The quartz reactor was vertically placed in a tubular heating furnace which was capable of heating the quartz reactor to the required temperature. A certain amount of quartz wool was placed in the quartz reactor to support the materials as well as prevent the formed char from entering into the condensing unit. Passing through the quartz wool, the pyrolysis vapor entered the condenser which was cooled by a mixture of ice and water to collect the liquid product. The noncondensable gas was collected by a gas bag for Micro GC analysis. During the experiments, the bagasse quantity was 3.0 g in each sample. The corresponding AC quantity varied (0, 1.0, 1.5, 3.0, 4.5, 6.0, or 9.0 g) to ensure the AC-to-bagasse ratio of 0, 0.33, 0.5, 1, 1.5, 2, and 3, respectively. This AC-to-bagasse ratio range was also determined via prior exploratory experiments. When the quartz reactor reached the desired temperature, the well-mixed bagasse and AC feedstock was injected slowly at a strictly controlled feeding rate to

capability of Pd/SBA-15 catalyst to catalytically convert 4-VP and its precursors into 4-EP has been confirmed. Some pyrolytic products could act as the H-donors for 4-EP formation. The highest 4-EP yield was around 2.0 wt %.36 However, Pd/SBA-15 is an expensive noble metal-based catalyst, which will significantly limit the development of this selective pyrolysis technique in commercial scale. In this study, AC was employed as another efficient catalyst to obtain 4-EP from catalytic fast pyrolysis of herbaceous biomass at low temperatures. Compared with Pd/SBA-15 catalyst, the cheap AC catalyst will offer significant economic advantages. In recent years, the AC catalyst has already been utilized for catalytic pyrolysis of biomass.23,39−41 However, no studies on selective 4-EP production have been reported since the employed biomass materials and pyrolysis conditions could not allow efficient 4-EP production. In this study, five herbaceous biomass materials were selected as the feedstock. The Py-GC/MS technique was first employed to investigate several effects on product distribution, including biomass type, pyrolysis temperature, AC-to-bagasse ratio, and catalytic pattern (ex situ or in situ catalytic patterns). The yields of important pyrolytic products under different pyrolysis conditions were quantitatively determined by using an external standard method. Moreover, catalytic fast pyrolysis experiments were conducted with a lab-scale setup to quantitatively determine the pyrolytic products, especially the yields and selectivity of 4-EP. Finally, the mechanism for the formation of 4-EP was investigated.

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EXPERIMENTAL SECTION

Materials. Five biomass materials including sugarcane bagasse, bamboo, cornstalk, rice husk, and wheat straw were utilized in this study. Prior to experiments, biomass materials were ground in a roller mill, then sieved to obtain particles with the size of 0.1−0.2 mm and dried under 100 °C for 4 h. The elemental composition results of the five materials are listed in Table 1.

Table 1. Elemental Composition of the Five Herbaceous Biomass Materials (Dry Basis, wt %) bagasse bamboo cornstalk rice husk wheat straw a

C

H

N

S

Oa

49.6 47.3 44.5 39.8 41.6

6.0 6.2 6.0 5.5 5.8

0.3 0.5 1.0 0.5 1.1

0.2 0.1 0.2 0.2 0.3

40.7 43.5 45.6 40.0 48.7

Calculated by difference based on the CHNS and ash contents.

The AC catalyst (DARCO, 100 mesh) and cellulose (Avicel PH101) were purchased from Sigma-Aldrich. The textural properties of the AC catalyst were analyzed by the Autosorb-iQ-MP physisorption analyzer. The BET surface area (Brunauer−Emmett−Teller method) was detected as 843 m2/g. The pore volume and average pore diameter (Barrett−Joyner−Halenda method) were detected as 0.39 cm3/g and 1.92 nm, respectively. The 4-EP (99%), p-coumaric acid (98%), 4-VP (10% in propylene glycol solution), 4-vinyl guaiacol (4-VG, 98%), and 4-ethyl guaiacol (4EG, 98%) were purchased from Aladdin company, Alfa Aesar company, J&K Scientific company, and Adamas Reagent company, respectively. Py-GC/MS Experiments. The analytical Py-GC/MS instrument consisted of a pyroprobe pyrolyser (CDS, 5200HP) and GC/MS (PerkinElmer, Clarus 560). The catalytic experiments with both in situ and ex situ patterns were conducted for analysis. The in situ catalytic pattern was performed by mechanically mixing the biomass and AC B

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Figure 1. Lab-scale biomass fast pyrolysis set. ensure the temperature varying within ±3 °C. The feeding process typically lasted for 30 min, and the feeding rate was around 0.1 g/min. The pyrolysis temperature range was 270−420 °C, selected via exploratory experiments and our previous study.36 After experiments, the condenser was warmed at room temperature, and the quartz reactor was cooled to room temperature under N2 flow. Then, liquid and solid products were collected and weighed. Gas product could be determined by difference. Analysis of the Products from Lab-Scale Catalytic Fast Pyrolysis Experiments. The liquid product was collected in the quartz condenser. Its yield was determined based on the mass difference of the condenser before and after the experiment. Besides, considering that the liquid product was not always homogeneous, a certain amount of ethanol was added to the condenser and mixed up with the liquid product to form a homogeneous solution. The pretreated homogeneous liquid was analyzed by GC/MS (Clarus 560) for its chemical composition. 4-VP and 4-EP contents in the liquid were quantitatively determined by external calibration. The water content in the pretreated homogeneous liquid was determined by the Karl Fischer method, and the titration solvent was a mixed solution of methanol and methylene chloride at a mass ratio of 3:1. The water content in the original liquid product could be calculated based on the amount of ethanol addition. The gas product from the quartz condenser was collected by a gas bag, and then analyzed by a Micro GC analyzer (INFICON 3000 Micro GC; INFICON, East Syracuse, NY, USA).

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Figure 2. Typical ion chromatograms of bagasse in noncatalytic and in situ catalytic fast pyrolysis at 300 °C. 1: acetic acid; 2: 4-ethyl phenol (4-EP); 3: 4-vinylphenol (4-VP); 4: 4-ethyl guaiacol (4-EG); 5: 4-vinyl guaiacol (4-VG).

for 4-VP, which agreed well with the previous study.35 During the noncatalytic process, 4-EP (peak 2) could rarely be detected from all the five herbaceous biomass materials. As shown in Figure 2, pyrolytic product distribution altered significantly, when AC catalyst was introduced into the system by mechanically mixing with bagasse. 4-VP was decreased significantly in the presence of AC catalyst, while 4-EP was newly formed. The 4-EP should be derived from hydrogenation of 4-VP or its precursors, which will be revealed in the later section. Moreover, 4-VG (peak 5) and 4-EG (peak 4) exhibited similar changes as 4-VP and 4-EP. The results indicated that AC catalyst possessed promising hydrogenation capabilities during the catalytic fast pyrolysis process. Among the five biomass materials, bagasse showed the highest 4-EP selectivity. The above results distinctly indicated that AC catalyst was able to obtain 4-EP from herbaceous biomass with high selectivity. Bagasse was the best material among the five herbaceous biomass materials. Notably, 4-EP was the hydrogenated product of 4-VP. Compared with our previous study, AC catalyst exhibited

RESULTS AND DISCUSSION

Biomass Screening for 4-EP Production by Py-GC/MS Experiments. Five herbaceous biomass materials were employed for experiments. Figure 2 shows the typical ion chromatograms of bagasse in noncatalytic and in situ catalytic fast pyrolysis at 300 °C. Corresponding peak area percentage (%) results are given in the Supporting Information (Table S1). Results from other biomass materials are also shown in the Supporting Information (Figure S3). Product distributions from the noncatalytic process were similar to those reported for bagasse,36 bamboo,44 cornstalk,45 wheat straw,34 and rice husk.46 A large amount of 4-VP (Peak 3) was formed from bagasse, bamboo, corn stalk, and wheat straw during the noncatalytic process, which should be derived from the decarboxylation of p-coumaric acids (the special chemical composition of certain herbaceous biomass) and decomposition of the p-hydroxyphenol subunits of lignin.43,47 Among the five biomass materials, bagasse showed the highest selectivity C

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Figure 3. Effects of pyrolysis temperature on the yields and peak area percentage (%) of 4-VP and 4-EP.

Effects of AC-to-Bagasse Ratio. Figure 4 shows the yields and peak area percentage (%) of four important phenolic

similar catalytic hydrogenation capabilities to Pd/SBA-15 catalyst to produce 4-EP.36 Pd/SBA-15 has been widely used as a hydrogenation catalyst, but AC catalyst has not been wellstudied on its catalytic hydrogenation ability. The reason for its capability will be revealed in the later section. Preparation of 4-EP from in Situ Catalytic Pyrolysis of Bagasse via Py-GC/MS Experiments. Effects of Pyrolysis Temperature. Figure 3 shows the effects of pyrolysis temperature on the actual yields and peak area percentage (%) of 4-VP and 4-EP from bagasse in both noncatalytic and in situ catalytic (AC-to-bagasse ratio of 4) processes. From the noncatalytic process, the pyrolytic products mainly consisted of 4-VP, while 4-EP could hardly be formed. As the pyrolysis temperature increased, the yield of 4-VP first increased to reach 55.2 mg/g (5.52 wt %) at 350 °C, and then showed a slight decrease. Its peak area percentage (%), which corresponded to its concentration, decreased significantly from 76.6% at 250 °C to only 16.6% at 450 °C. These results confirmed that selective production of 4-VP should be performed under low temperatures.35 In the case of 4-EP, its yield and peak area percentage (%) were very low during the noncatalytic process, with the maximal values of only 1.5 mg/g and 0.6% at the temperature of 450 °C. The results also agreed with previous studies that little 4-EP could be formed in traditional fast pyrolysis of both nonwoody biomass48,49 and woody biomass.50−52 During the in situ catalytic process, 4-EP was increased greatly, accompanied by the significant decrease of 4-VP. According to Figure 3, both the yield and the peak area percentage (%) of 4-VP were low, with the maximum values of 10.5 mg/g and 4.6% at the temperature of 450 °C. The 4-EP yield, whose value was only 1.5 mg/g in the noncatalytic process, reached as high as 21.3 mg/g (2.13 wt %) at 300 °C. Moreover, as the temperature was raised from 250 to 450 °C, the peak area percentages (%) of 4-EP were 20.8, 28.8, 12.6, 9.1, and 9.0%, respectively. The results obviously indicated that the selective production of 4-EP was favorable at low pyrolysis temperatures around 300 °C. High pyrolysis temperature would result in the significant decomposition of holocellulose to generate various pyrolytic products,53,54 which greatly reduced the selectivity of 4-EP.

Figure 4. Effects of AC-to-bagasse ratio on the yields and peak area percentage (%) of 4-VP, 4-EP, 4-VG, and 4-EG at 300 °C.

compounds (4-VP, 4-EP, 4-VG, and 4-EG) under different ACto-bagasse ratios from the in situ catalytic process. All results were obtained at the pyrolysis temperature of 300 °C. Along with the increasing of AC-to-bagasse ratio, both the yield and the concentration of 4-VP decreased monotonously, while the yield and concentration of 4-EP first increased and then decreased. The maximal 4-EP yield (2.13 wt %) was obtained at the AC-to-bagasse of 4. The peak area percentage (%) of 4-EP was 0, 10.0, 21.1, 28.8, 28.7, and 26.8% with the AC-to-bagasse ratio from 0 to 8. The above results clearly indicated that AC catalyst was capable to decrease 4-VP formation and promote 4-EP production at the proper catalyst amount. In addition, yields and peak area percentage (%) of 4-VG and 4-EG shared similar trends as 4-VP and 4-EP, while their values were much lower than those of 4-VP and 4-EP. Such changes should also be attributed to the catalytic hydrogenation effect of AC catalyst, over which the unsaturated 4-VG or its precursors could be converted into the saturated 4-EG during the catalytic pyrolysis process. D

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Figure 5. Yields and peak area percentage (%) of 4-EP from in situ and ex situ catalytic patterns.

Table 2. Pyrolytic Products Yields from Catalytic Fast Pyrolysis of Bagasse under Different Temperatures at the AC-to-Bagasse Ratio of 1.5 temperature (°C)

char (wt %)

liquid (wt %)

gasa (wt %)

water contentb (wt %)

4-VP yield (wt %)

4-EP yield (wt %)

4-EP selectivityc (wt %)

270 300 330 360 390 420

76.82 56.67 50.42 44.23 36.82 31.73

19.62 36.07 38.42 41.19 47.58 51.56

3.56 7.26 11.16 14.58 15.60 16.71

61.42 35.54 34.37 32.12 30.94 29.86

0 0 0 0 0 0

1.48 2.49 2.24 2.01 1.63 1.22

19.55 10.71 8.88 7.19 4.96 3.37

a

Calculated by difference. bWater content in liquid. cOn water-free basis, calculated by 4-EP yield divided by organic liquid yield.

Table 3. Pyrolytic Products Yields from Catalytic Fast Pyrolysis of Bagasse under Different AC-to-Bagasse Ratios at 300 °C AC-to-bagasse ratio

char (wt %)

liquid (wt %)

gasa (wt %)

water contentb (wt %)

4-VP yield (wt %)

4-EP yield (wt %)

4-EP selectivityc (wt %)

0 0.33 0.5 1 1.5 2 3

53.06 64.21 63.93 59.53 56.67 59.49 74.08

41.9 27.32 28.26 33.15 36.07 34.83 21.44

5.04 8.47 7.81 7.32 7.26 5.68 4.48

20.00 40.96 39.59 36.75 35.54 42.12 69.42

5.26 0.22 0.18 0.10 0 0 0

0 1.44 1.73 2.10 2.49 2.26 0.72

0 8.93 10.13 10.02 10.71 11.21 10.98

a

Calculated by difference. bWater content in liquid. cOn water-free basis, calculated by 4-EP yield divided by organic liquid yield.

Effects of Catalytic Pattern. Catalytic pattern is another essential factor to affect the distribution of pyrolytic products.55 The above experimental results were all obtained from the in situ catalytic pyrolysis pattern. Accordingly, bagasse ex situ catalytic fast pyrolysis was also conducted. Similar results were observed from both patterns. Figure 5 gives the optimal results of 4-EP production from the ex situ catalytic pattern, as well as results from the in situ catalytic pattern for comparison. According to Figure 5, the formation characteristics of 4-EP were similar in two catalytic patterns. The highest 4-EP yield of 21.2 mg/g from the ex situ catalytic pattern was obtained at the AC-to-bagasse ratio of 4 and temperature of 350 °C. This value was almost the same with the maximal 4-EP yield in the in situ catalytic pattern (21.3 mg/g). The corresponding peak area percentage (%) of 4-EP in the ex situ catalytic pattern was 25.1%, slightly lower than that from the in situ catalytic pattern (28.8%). The results clearly indicated that 4-EP could be prepared from both patterns. The in situ catalytic pattern was slightly better than the ex situ catalytic pattern. Notably, during our previous study of selectively producing 4-EP over Pd/SBA15 catalyst, the ex situ catalytic pattern performed better than the in situ catalytic pattern. The difference between the two

studies should be attributed to the different catalytic capabilities of Pd/SBA-15 and AC catalysts. Preparation of 4-EP from Lab-Scale in Situ Catalytic Fast Pyrolysis of Bagasse. To further verify the feasibility of AC catalyzed pyrolysis of bagasse for selective 4-EP production, lab-scale experiments were performed to quantitatively determine the pyrolytic products under different pyrolysis temperatures and AC-to-bagasse ratios. Major results are given in Tables 2 and 3, including the yields of pyrolytic char, liquid, and gas, as well as 4-VP and 4-EP. In addition, other results from the catalytic fast pyrolysis experiments are listed in the Supporting Information (Figure S4, Tables S2 and S3), including typical ion chromatograms of the pyrolytic liquid products from noncatalytic and catalytic processes, as well as the composition of the pyrolytic gas products. The distributions of pyrolytic products changed remarkably under different temperatures and AC-to-bagasse ratios, suggesting that the pyrolytic reactions of bagasse would be greatly affected by the AC catalyst under different conditions. As shown in Table 2, during catalytic fast pyrolysis of bagasse in 270−420 °C, char yield decreased monotonically, while liquid and gas yields increased continuously along with the rise E

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Figure 6. Typical ion chromatograms from noncatalytic and catalytic fast pyrolysis of p-coumaric acid and 4-VP.

of temperature. Such yield changes should be attributed to the temperature improved decomposition of bagasse to increase gas and liquid products on the expense of char product. The highest 4-EP yield of 2.49 wt % was obtained at 300 °C, which matched well with the Py-GC/MS results. Notably, no 4-VP was formed in the catalytic condition. According to Table 3, catalytic fast pyrolysis of bagasse under different AC-to-bagasse ratios led to significant changes of pyrolytic products yields. As the hydrogenation product of 4VP, 4-EP appeared when AC catalyst was introduced to the pyrolysis system. The yield of 4-EP showed a continuous increase along with the rise of AC content until it reached the maximum of 2.49 wt % under the AC-to-bagasse ratio of 1.5. Overloaded AC catalyst would result in a drop of 4-EP yield. It is noted that the maximal 4-EP yield from the lab-scale experiment (2.49 wt %) was a little higher than that of the PyGC/MS experiment (2.13 wt %). They were obtained at the same pyrolysis temperature (300 °C), but under different ACto-bagasse ratios (4 in Py-GC/MS vs 1.5 in lab-scale experiments). The difference in the maximal 4-EP yields and the optimal AC-to-bagasse ratios in the two experiments should be due to the following reasons. In Py-GC/MS experiments, the bagasse/AC mixture was kept motionless. The contact between the bagasse and AC was deficient, which would inhibit the catalytic conversion of 4-VP and its precursors into 4-EP, since 4-VP was still formed in the catalytic pyrolysis process (Figures 3 and 4). Hence, sufficient catalytic pyrolysis required high catalyst quantity, but high overloaded AC catalyst would decrease 4-EP formation (Figure 4). During the lab-scale fast

pyrolysis process, the contact between bagasse and AC was improved, which allowed sufficient catalysis to reduce catalyst quantity and increase 4-EP formation, since no 4-VP was generated in the catalytic pyrolysis process. Moreover, the two pyrolysis systems (Py-GC/MS instrument vs lab-scale setup) also differed in the scale (biomass pyrolysis capacity), heating rate, vapor residence time, vapor treating method (condensation or noncondensation), and so on. All of these differences would affect the pyrolytic products distributions.56 Furthermore, it is essential to indicate that, for the large-scale application of this technique to selectively produce 4-EP, many problems should be considered, such as the fast pyrolysis reactor type, heating supply for the pyrolysis reactor, bagasse and catalyst physical properties, catalytic fast pyrolysis conditions, catalyst deactivation, recovery and regeneration, 4EP separation, etc.57 Many of the above problems would be possible challenges for the successful industrial application process. For example, the ash in bagasse would bring negative effects to the catalyst deactivation, recovery, and regeneration. Hence, further studies are required to investigate and solve these problems. Mechanism of 4-EP Formation. As indicated in the above results, noncatalytic fast pyrolysis of bagasse at low temperatures could generate abundant 4-VP, while the catalytic pyrolysis process could produce 4-EP with high selectivity. Herbaceous biomass especially bagasse contained abundant pcoumaric acid which could be selectively converted into 4-VP by decarboxylation reaction during the pyrolysis process.36,43 Hence, p-coumaric acid should be an important origin of 4-VP. F

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Energy & Fuels Both of p-coumaric acid and 4-VP could then be selected as the model precursors to study the mechanism of 4-EP formation. Experiments on p-coumaric acid and 4-VP were performed via the Py-GC/MS technique. Typical ion chromatograms from the noncatalytic process are shown in Figure 6a, b and Figure 6e, f, respectively. As shown in Figure 6a, 4-VP was selectively generated from noncatalytic fast pyrolysis of p-coumaric acid, which was consistent with a previous result.35 However, during catalytic fast pyrolysis of pure p-coumaric acid (Figure 6b) and 4-VP (Figure 6f), 4-VP was still the major product, accompanied by a certain amount of 4-EP. Considering that 4-EP was a hydrogenated product, external H-donors were required for its formation. Hence the lack of H-donors during catalytic fast pyrolysis of pure p-coumaric acid and 4-VP should be responsible for the low yield of 4-EP. Certain functional groups (such as carboxyl groups) in AC catalyst, which could act as the H-donors, might lead to the formation of 4-EP.50 However, due to the limited amount of H-donors from AC catalyst,51 significant improvement on 4-EP yield could hardly be achieved. Previously, Bu et al.23 pointed out that, during the pyrolysis process, water from dehydration reactions and the carboxylic anhydride groups in AC catalyst could react to generate carboxylic acids. The formed carboxylic acids, which could act as H-donors, would facilitate the generation of 4-EP. Inspired by this point, a small amount of water was introduced into the catalytic fast pyrolysis of p-coumaric acid and 4-VP. As shown in Figure 6c,g, 4-EP in high selectivity was produced from pcoumaric acid and 4-VP, respectively. To further confirm this point, cellulose was employed to mix with p-coumaric acid or 4VP for catalytic fast co-pyrolysis experiments, since a lot of water would be formed from pyrolysis of cellulose due to the dehydration reactions. The results are shown in Figure 6d,h. 4EP with a high selectivity was also obtained. Therefore, the above results clearly indicated that, during the bagasse catalytic fast pyrolysis process, AC catalyst could act as a promising hydrogenation catalyst in the presence of water for the selective 4-EP formation. A possible mechanism of 4-EP formation could be concluded from the above results. During the catalytic fast pyrolysis process, various pyrolytic decomposition reactions including the dehydration reactions generating abundant water would occur. Water then reacted with carboxylic anhydrides of AC catalyst to form carboxylic acids. The carboxylic acids later served as H-donors for the hydrogenation and cracking of unsaturated 4-VP precursor, to directly produce saturated 4-EP. In addition, even if a certain amount of 4-VP was formed, it would easily be hydrogenated into 4-EP by AC catalyst to achieve high selectivity for 4-EP during the catalytic fast pyrolysis process. The existence of AC catalyst would allow the hydrogen transfer reactions through H-donors and H-acceptors. In this way, the 4-EP could be obtained in high selectivity from bagasse without any extra hydrogen.

catalytic pattern was slightly better than the ex situ catalytic pattern for 4-EP production. Maximal 4-EP yield of 2.13 wt % in the Py-GC/MS experiments was obtained in the in situ catalytic pattern under an AC-to-bagasse ratio of 4 at 300 °C. The corresponding peak area percentage (%) at this condition was 28.8%. Lab-scale catalytic fast pyrolysis experiments obtained the highest 4-EP yield of 2.49 wt % at 300 °C under an AC-to-bagasse ratio of 1.5, with the 4-EP selectivity of 10.71%.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b02628. Figures of preparation of the samples for pyrolysis experiments, diagram of the feeding process, ion chromatograms from fast pyrolysis, and ion chromatograms from GC/MS analysis, and tables of peak area percentage of major pyrolytic products, composition of noncondensable gas products from catalytic fast pyrolysis of bagasse under different temperatures, and composition of noncondensable gas products from catalytic fast pyrolysis of bagasse under different AC-to-bagasse ratios (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +86 10 61772063 (Q.L.). *E-mail: [email protected]. Tel.: +86 20 87057727 (W.Q.). ORCID

Qiang Lu: 0000-0002-4340-1803 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (51576064), Guangdong Key Laboratory of New and Renewable Energy Research, Development and Application (Y607s91001), 111 Project (B12034), Science & Technology Planning Project of Hebei (15273706D), and Fundamental Research Funds for the Central Universities (2016YQ05, 2016MS55) for financial support.

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REFERENCES

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CONCLUSION A new technique was developed to obtain 4-EP with high selectivity via catalytic fast pyrolysis of bagasse by AC catalyst at low temperatures. During the noncatalytic process, abundant 4VP would be produced. During the catalytic process, 4-VP and its precursors could be catalytically hydrogenated into 4-EP in high selectivity by AC catalyst which could provide H-donors in the presence of water. Low pyrolysis temperature and proper AC-to-bagasse ratio favored 4-EP formation. The in situ G

DOI: 10.1021/acs.energyfuels.6b02628 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

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DOI: 10.1021/acs.energyfuels.6b02628 Energy Fuels XXXX, XXX, XXX−XXX