Catalytic Fast Pyrolysis of Cellulose and Biomass to Selectively

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10815-10825

Catalytic Fast Pyrolysis of Cellulose and Biomass to Selectively Produce Levoglucosenone Using Activated Carbon Catalyst Xiao-ning Ye, Qiang Lu,* Xin Wang, Hao-qiang Guo, Min-shu Cui, Chang-qing Dong,* and Yong-ping Yang National Engineering Laboratory for Biomass Power Generation Equipment, North China Electric Power University, No. 2 Beinong Road, Changping District, Beijing 102206, China S Supporting Information *

ABSTRACT: Activated carbon (AC) prepared by chemical activation with H3PO4 (named AC-P) was employed for catalytic fast pyrolysis of cellulose and biomass to selectively produce levoglucosenone (LGO). The catalytic pyrolysis behaviors and product distributions were revealed via both analytical pyrolysis-gas chromatography/mass spectrometry (PyGC/MS) and lab-scale experiments. Moreover, the performance of AC-P catalyst was compared with previously reported strong acid catalysts and other ACs prepared by different activation methods. In addition, reusability and stability of the AC-P catalyst have been examined via recycling experiments. The results indicated that the AC-P catalyst was effective for selectively preparing LGO from both cellulose and biomass and performed better than other catalysts. Among the three biomass materials (pine wood, poplar wood, and bagasse), pine wood showed the best selectivity for producing LGO. The maximal LGO yields of 18.1 and 9.1 wt % were obtained from cellulose and pine wood, respectively, in Py-GC/MS experiments under a catalyst-to-feedstock ratio of 1:3 at 300 °C, whereas the lab-scale setup obtained the highest LGO yields of 14.7 and 7.8 wt % from cellulose and pine wood with selectivities of 76.3 and 43.0%, respectively, based on organic liquid products. Furthermore, granular AC-P catalyst exhibited good reusability and stability in the recycling experiments. Stable yields of LGO above 12.5 wt % from cellulose were obtained in six consecutive runs without any regeneration of the recycled granular AC-P catalyst. KEYWORDS: Levoglucosenone, Catalytic fast pyrolysis, Activated carbon, Py-GC/MS, Cellulose, Biomass

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INTRODUCTION Fast pyrolysis is one of the most promising biomass utilization technologies, which converts solid biomass mainly into bio-oil together with char and noncondensable gases. This technology to prepare bio-oil has grown in popularity due to the advantages of liquid bio-oil in universality, renewability, easy transportation, and high energy density compared with those of solid biomass.1,2 However, crude bio-oil obtained from traditional pyrolysis is only a low-grade liquid product due to its complex composition and poor properties, which significantly inhibits its application in current thermal and chemical industries.3−5 For this shortcoming to be overcome, selective pyrolysis of biomass toward specific high-grade liquid fuels or value-added chemicals has become increasingly attractive in recent years.6−8 As one of the important biomass pyrolytic products, levoglucosenone (LGO) was first confirmed in 1973. It is a highly dehydrated sugar retaining one of the cellulose natural chiral centers that can be used to prepare chiral synthons and pharmaceuticals.9−11 Furthermore, LGO can be used to synthesize biologically active compounds because it has α,βunsaturated ketone and protected aldehyde functional groups.12 Traditional noncatalytic pyrolysis of cellulose or biomass can obtain little or only a small amount of LGO.1,13 It was found © 2017 American Chemical Society

that, with the aid of certain acid catalysts, catalytic fast pyrolysis of cellulose or biomass at low temperatures could significantly promote the yield of LGO.9,14 Various catalysts were found to be effective as impregnated on or mechanically mixed with cellulose/biomass materials to improve LGO formation, including liquid strong acids (H3PO4, H2SO4), solid super acids (SO42−/TiO2, SO42−/ZrO2), solid phosphoric acid, and ionic liquids. On the basis of these catalysts, a series of technical solutions were developed to selectively prepare LGO from cellulose/biomass.15−19 Dobele et al.20,21 first reported H3PO4 as a promising catalyst to enhance LGO production and found that catalytic pyrolysis of microcrystalline cellulose impregnated with 2% H3PO4 at 500 °C could generate LGO in high yield. Sui et al.22 employed H2SO4 as the catalyst to examine the LGO production from low-temperature pyrolysis of biomass impregnated with H2SO4, and the highest LGO yield reached 7.58 wt % by using 3 wt % H2SO4 loading. For avoiding complex impregnation pretreatment for the utilization of liquid acid catalysts, solid acid catalysts were used to replace strong liquid acids in preparing LGO because they had only to be Received: August 11, 2017 Revised: September 9, 2017 Published: September 27, 2017 10815

DOI: 10.1021/acssuschemeng.7b02762 ACS Sustainable Chem. Eng. 2017, 5, 10815−10825

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ACS Sustainable Chemistry & Engineering mechanically mixed with cellulose/biomass. Wang et al.23 reported LGO preparation in high selectivity with a yield of 8.1 wt % from catalytic fast pyrolysis of mechanically mixed cellulose and solid superacid SO42−/ZrO2. Similar results were obtained by using other solid super acid catalysts.24,25 Our research group further confirmed that solid phosphoric acid was also promising to selectively prepare LGO, and the LGO yields could reach 16.1 and 8.2 wt % from catalytic fast pyrolysis of microcrystalline cellulose and poplar wood.26 In addition to the above strong liquid or solid acid catalysts, ionic liquids also have catalytic selectivity on the formation of LGO. Shinji Kudo et al.16,27 employed ionic liquid (1-butyl-2,3-dimethylimidazolium triflate) for catalytic pyrolysis of cellulose to obtain the highest LGO yield over 30% on a cellulose carbon basis. Previous studies clearly indicated that proper acid catalysts could significantly increase LGO yield. However, there are many problems for the utilization of these acid catalysts. The liquid acid catalysts (H3PO4, H2SO4, ionic liquid) should be used via complex impregnation pretreatment. Moreover, they are all thermally unstable, and thus make their recycling impossible. The presence of liquid acid catalyst will also inhibit the utilization of the char product. Compared with the liquid acids, solid acids are easy to utilize and recycle, and thus are promising for the catalytic pyrolysis process. However, both solid and liquid acid catalysts are easy to leach in the pyrolysis process; the leached acid will remain in the pyrolytic liquid product to catalyze the secondary conversion of LGO, resulting in the decrease of LGO yield and difficulty in the storage of pyrolytic liquid. Herein, we developed a novel pyrolysis technique to selectively prepare LGO from cellulose/biomass by using a simple AC-P catalyst. ACs are common carbon materials and are also widely used as the catalysts or catalyst carriers in the biomass pyrolysis field to show different catalytic properties.28,29 They can be directly used as catalysts for the selective production of phenolic compounds via microwave-assisted pyrolysis,30 or they can be carries for the preparation of various AC-based catalysts for different catalytic purposes.31 ACs can be produced from various carbonaceous materials via physical activation (steam, CO2, etc.), chemical activation (H3PO4, ZnCl2, etc.), and combined physical/chemical activation methods. Compared with previously reported acid catalysts for LGO production, the AC catalyst is cheap, stable, environmental friendly, and easy to be utilized and recycled after shaping. In this study, catalytic pyrolysis experiments were performed with both an analytical pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) instrument and a labscale experimental setup to investigate the effects of various factors including pyrolysis temperature, catalyst-to-feedstock ratio, and pyrolysis system (Py-GC/MS instrument vs lab-scale setup) on the product distribution. In addition, other catalysts including ACs prepared by activation with steam, CO2, and ZnCl2 as well as previously reported acid catalysts (H3PO4, H2SO4, SO42−/TiO2-Fe3O4, and solid phosphoric acid) were adopted for comparison. Moreover, LGO yields from different pyrolysis systems were quantitatively determined and compared.

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materials of sugar cane bagasse, poplar wood, and pine wood were typical herbaceous, hardwood, and softwood materials. The microcrystalline cellulose was purchased from Sigma. Glucose, mannose, and xylose were purchased from Aladdin. Pure LGO and pure LG were purchased from TCI. Prior to experiments, the three biomass feedstocks were ground and sieved to obtain particles within the size range of 0.20−0.30 mm. After drying at 105 °C for 3 h in air, the selected particles were stored in a vacuum desiccator at room temperature. Preparation of ACs via Different Activation Methods. All ACs were prepared from sugar cane bagasse via different activation methods, including physical activation with H2O and CO2, as well as chemical activation with H3PO4 and ZnCl2. These ACs are named ACP, AC-H2O, AC-CO2, and AC-Zn from activation with H3PO4, steam, CO2, and ZnCl2, respectively. Sugar cane bagasse was chosen as the precursor to prepare ACs because it is a common agricultural residue and one of the three lignocellulosic biomass materials employed in this study, and it is also a good source of AC. Compared with woody biomass materials, sugar cane bagasse is low in density, and thus the AC prepared from it also has a low density. During the catalytic pyrolysis process, sufficient catalysis requires sufficient physical contact between biomass and AC catalyst. Hence, compared with ACs in high density, a relative small quantity of the AC prepared from bagasse will be required to achieve sufficient catalysis, which will save the catalyst quantity for the catalytic pyrolysis process. AC-P was prepared by one-step activation with H3PO4. A certain amount of bagasse was impregnated with H3PO4 solution (85 wt % concentration) using an incipient wetness impregnation method at room temperature and dried for 24 h at 100 °C. The mass ratio of bagasse and H3PO4 solution was 1:0.65. The impregnated bagasse was activated at 500 °C for 2 h with heating from room temperature at a rate of 10 °C/min under continuous N2 flow (120 mL/min). After cooling to room temperature under a N2 flow, the activated sample was washed with distilled water at 60 °C until neutral pH and negative phosphate analysis in the eluate. For the preparation of AC-H2O, a certain amount of bagasse was first pyrolyzed under N2 flow (150 mL/min) at 850 °C for 1 h with heating from room temperature at a rate of 14 °C/min; bagasse char was then obtained after cooling. Steam activation was conducted with first heating of bagasse char to 850 °C in N2 flow (100 mL/min) and then activated for 2 h with the addition of H2O (0.1 mL/min at liquid phase). AC-H2O was obtained after cooling in a N2 atmosphere. AC-CO2 was prepared in similar way as AC-H2O. The bagasse char was first heated to 800 °C under a N2 gas flow (100 mL/min) and then activated for 2 h in a CO2 atmosphere (100 mL/min). AC-CO2 was obtained after cooling in a N2 atmosphere. For the preparation of AC-Zn, a certain amount of bagasse was impregnated with a zinc chloride solution (9.1 wt % concentration) with stirring at 80 °C for 6 h. The mass ratio of bagasse and ZnCl2 solution was 1:33. The impregnated bagasse was activated at 700 °C for 90 min with a heating rate of 5 °C/min under N2 flow (100 mL/ min). After cooling to room temperature under N2 flow, the activated sample was washed with 0.5 N HCl solution followed by hot distilled water several times until the solution reached pH 6. All prepared ACs were powders and screened to obtain particles with sizes of 0.10−0.20 mm for experiments. The preparation and properties of SO42−/TiO2-Fe3O4 and solid phosphoric acid catalysts can be found in our previous studies.26,32 Catalyst Characterization. The catalysts were characterized by Fourier transform infrared (FTIR) analysis, textural property analysis, and X-ray photoelectron spectroscopy (XPS). A PerkinElmer spectrophotometer was used for FTIR analysis. An Autosorb-iQ-MP physisorption analyzer was adopted for nitrogen adsorption/ desorption isotherms at 77 K. The surface areas were measured using the Brunauer−Emmett−Teller (BET) method, and the pore volumes were determined by the Barrett−Joyner−Halenda (BJH) method. XPS analysis was performed using a 5700C model Physical Electronics apparatus with Mg Kα radiation (1253.6 eV), and C 1s peak position was set at 284.6 eV for reference.

EXPERIMENTAL SECTION

Materials. The feedstock used in this study included sugar cane bagasse, poplar wood, pine wood, microcrystalline cellulose (Avicel PH-101), glucose (>99.5%), mannose (99%), xylose (>99%), pure LGO (>96%), and pure levoglucosan (LG, 99%). The three biomass 10816


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Figure 1. Lab-scale biomass fast pyrolysis set. Py-GC/MS Experiments. Analytical Py-GC/MS experiments were conducted on the connected CDS Pyroprobe 5200HP pyrolyzer (Chemical Data Systems) and PerkinElmer GC/MS (Clarus 560). The details of the experimental sample preparation can be found in Figure S1. For the catalytic performance of the ACs and other solid catalysts to be evaluated, biomass and solid catalyst were mechanically mixed together. The cellulose or biomass quantity was strictly controlled to be 0.20 mg in each Py-GC/MS experiment. The solid catalyst quantity varied from 0 to 0.40 mg to achieve catalyst-tobiomass ratios of 0, 1:10, 1:5, 1:3, 1:2, 1:1, and 2:1, respectively. For the liquid catalysts (H3PO4 and H2SO4) to be evaluated, they were first impregnated on cellulose using the incipient wetness impregnation method to obtain impregnated cellulose with different H3PO4 or H2SO4 loadings. Then, certain quantities of the pretreated cellulose samples were filled in the quartz tube. The quantities were strictly weighed to be 0.21, 0.21, 0.22, 0.22, 0.24, and 0.25 mg for the samples impregnated with 2.5, 5.0, 7.5, 10.0, 15.0, and 20.0 wt % H3PO4 or H2SO4, respectively, to ensure the pure cellulose quantity of each sample to strictly be 0.20 mg. The pyrolysis experiments were carried out for 20 s at different temperatures ranging from 250 to 500 °C. The heating rate was fixed to 20 °C/ms. Pyrolysis vapors were directly transferred into GC/MS for analysis. The temperature procedure for GC started from 40 to 280 °C with a heating rate of 15 °C/min and was maintained for 2 min at 280 °C. Each experiment was repeated at least three times to ensure its reproducibility. Additional detailed parameters for GC/MS can be found elsewhere.33,34 The detected products were determined according to the NIST library, Wiley library, and the literature data of previous studies.26,32 Peak area and peak area % values of each determined product were recorded. Quantitative Determination of LGO in Py-GC/MS Experiments. Because Py-GC/MS experiments could not directly provide quantitative results, an external calibration method was utilized to quantitatively determine the actual yield of LGO. The calibration curve was constructed based on the pure LGO quantities and corresponding chromatographic peak area values. The details of the calibration process and the calibration curve are given in Figure S2.35,36 According to the calibration curve, the peak area values of LGO, and the feedstock quantity (0.20 mg), the actual yields of LGO under different reaction conditions could be calculated. Lab-Scale Catalytic Fast Pyrolysis Experiments. Catalytic fast pyrolysis experiments were also conducted using a lab-scale experimental setup, as shown in Figure 1. The well-mixed cellulose (or biomass) and catalyst were fed into the pyrolysis reactor through the feedstock container. The carrier gas N2 was fed in with a constant flow rate of 100 mL/min. The vertical quartz reactor in a tubular heating furnace could be heated to required temperatures. For support of the solid materials and chars, a certain amount of quartz wool was

placed in the quartz reactor. A condenser, which was cooled by a mixture of ice and water to collect the liquid product, was placed below the quartz reactor. The noncondensable gas was collected by a gas bag. The feedstock quantity in each experiment was 2.0 g. Corresponding catalyst quantity differed (0, 0.2, 0.4, 0.5, 1.0, or 2.0 g) to ensure the catalyst-to-biomass ratio of 0, 1:10, 1:5, 1:4, 1:3, 1:2, and 1:1, respectively. The mixture was injected when the quartz reactor reached the desired temperature. Typically, the pyrolysis process continued for 5 min under pyrolysis temperatures of 270−420 °C. After the experiments, as the condenser and quartz reactor recovered to room temperature, liquid and solid products could be collected and weighed. Gas products could be calculated by the difference. Analysis of the Pyrolytic Products from Lab-Scale Catalytic Fast Pyrolysis Experiments. The yield of the liquid product was determined by the mass difference of the condenser before and after the experiment. After weighing, ethanol was added into the liquid product to form homogeneous solution for following GC/MS (Clarus 560) analysis. LGO content in the liquid products was quantitatively determined using external calibration. The water content in the homogeneous liquid was determined using the Karl Fischer method with methanol and methylene chloride at a mass ratio of 3:1 as the titration solvent. 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). Catalyst Recycling Experiments. Recycling experiments were carried out to examine the reusability and stability of the AC-P catalyst using the lab-scale setup. For conducting the recycling experiments, shaped granular AC-P catalyst (named AC-PR) was prepared. Powder AC-P was mechanically mixed with 40% (w/w) carboxymethyl cellulose (CMC) followed by the addition of a certain amount of deionized water with stirring for 30 min. The mixture was dried at 120 °C for 12 h. The obtained solid was placed in a tube furnace and calcined at 400 °C for 2 h at a rate of 15 °C/min from room temperature under continuous N2 flow (100 mL/min). After cooling to room temperature, the sample was ground to sieve particles within the size of 0.35−0.45 mm for recycling experiments, and these pellets were named as AC-PR. Because AC-PR had larger particle sizes than those of cellulose and biomass raw materials, it was also larger in size than the char particles. As a result, the AC-PR catalyst could be easily separated and recovered from the pyrolytic solid residues via simple sieving. Catalytic fast pyrolysis of cellulose was conducted using the AC-PR catalyst in the recycling experiments. After each test, the used AC-PR catalyst was separated and collected without any regeneration treatment and then subjected to the next recycling test. Six consecutive 10817


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ACS Sustainable Chemistry & Engineering runs were performed, and the pyrolytic products in each test were collected and analyzed.

O−C linkage, or POOH bond. This suggests the presence of phosphorus-containing groups in AC-P. The band at 1085 cm−1 can be identified as P+−O− in acid phosphate esters42 and the symmetrical vibration in polyphosphate chain P−O−P. XPS is useful to analyze the surface chemical characteristics of the ACs. The survey XPS spectra are shown in Figure 3(a). It can be seen that the spectrum of AC-P contains distinct peaks for C, O, and P, whereas the other three ACs only contain distinct peaks for C and O. The relative contents of these elements are given in Figure 3(a). The high-resolution P2p spectrum is given for further analysis using a curve-fitting procedure, and the results are shown in Figure 3(b). The first peak with a value close to 134.4 eV represents ∼63% of the total surface phosphorus. Wu and Radovic43 assigned this band to C−O−PO3 groups. The second peak at a binding energy of 133.7 eV, characteristic of P atom bonded to one C atom and three O atoms, as in C−PO3 groups,43 represents ∼37% of the total surface phosphorus of AC-P. The XPS P2p results indicate the presence of C−O−PO3 and C−PO3 groups on the surface of the AC-P catalyst. Preparation of LGO from Catalytic Fast Pyrolysis of Cellulose and Biomass via Py-GC/MS Experiments. The noncatalytic analytical fast pyrolysis of cellulose has been reported in our previous study. 44 Under noncatalytic conditions, volatile organic pyrolytic products would be largely formed at temperatures over 400 °C. The typical ion chromatogram at 500 °C is shown in Figure 4(a). LG was the predominant product, mainly derived from pyrolytic depolymerization of cellulose through scission of the glycosidic bond followed by the formation of 1,6-anhydride. LGO was a minor product from combined depolymerization and dehydration of cellulose with the peak area % less than 3%.45 During the catalytic pyrolysis of cellulose, pyrolytic products could be detected at as low as 250 °C. Moreover, LGO became the only predominant product, together with minor furans (furfural (FF) and 5-hydromethyl-furfural (5-HMF)) and other anhydrosugars (1-hydroxy-3,6-dioxabicyclo[3.2.1]octan-2-one (LAC) and 1,5-Anhydro-4-deoxy-D-glycero-hex-1-en-3-ulose (APP)) as byproducts. The typical ion chromatogram of cellulose in catalytic fast pyrolysis at 300 °C is shown in Figure 4(b). At the catalytic pyrolysis temperature of 300 °C, the peak

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RESULTS AND DISCUSSION Catalyst Properties. The textural properties of the four AC catalysts (AC-P, AC-H2O, AC-CO2, and AC-Zn) are given in Table S1. Figure 2 shows the FTIR transmission spectra of AC-

Figure 2. FTIR spectra of the AC catalysts.

P, AC-H2O, AC-CO2, and AC-Zn to characterize surface groups on the four ACs. The band at 1570 cm−1 is indicative of the carboxylate anion (-COO−) salt.37 The medium intensity band at 920 cm−1 conforms to γ(O−H), and this band is regarded as fair evidence of the presence of the COOH dimer.38 The band at around 1180 cm−1 is usually found in oxidized carbons and has been assigned to C−O stretching in acids, alcohols, phenols, ethers, and esters.39−41 This band is also characteristic of phosphorus and phosphocarbonaceous compounds present in AC-P. According to Puziy et al.,39 the band around 1220−1180 cm−1 can be attributed to the stretching of PO bond in phosphate ester, O−C bond in P−

Figure 3. XPS spectra of the four ACs: (a) survey spectra; (b) high-resolution P2p spectrum of AC-P. 10818


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4(b)) in the order of AC-P > AC-CO2 > AC-Zn > AC-H2O. Such results confirmed that phosphorus-containing groups on AC-P could effectively improve the formation of LGO. During noncatalytic fast pyrolysis of pine wood, poplar wood, and bagasse, various pyrolytic products were formed, including linear acids, linear aldehydes, linear ketones, furans, anhydrosugars, phenolics, cyclopentanones, and so forth.7,46,47 The typical ion chromatograms of the three biomass materials at 500 °C are shown in Figure 5(a, c, and e). As the AC-P catalyst was added, the distribution of pyrolytic products changed significantly. Corresponding typical ion chromatograms are shown in Figure 5(b, d, and f), revealing that LGO became the predominant product accompanied by small amounts of AA, FF, 5-HMF, and other byproducts. According to Figure 5, LGO selectivity of the three biomass materials were different from each other, which was due to the difference in chemical structures and component contents of the three biomass materials. Corresponding effects will be fully discussed in the following sections. Effect of Pyrolysis Temperature on Cellulose. Pyrolysis temperature plays an important role in the product distribution. Figure 6 shows the effect of temperature (250−500 °C) on the actual yield and peak area % of LGO from the noncatalytic and catalytic processes at an AC-P-to-cellulose ratio of 1:3. As can be clearly seen in Figure 6, the yield of LGO in noncatalytic pyrolysis was very low. LGO was barely detected at 300 °C only with a yield of 0.3 wt %, and decreased to 0.2 wt % at 500 °C, in agreement with previous results.26,32 During the catalytic fast pyrolysis process, the formation of LGO could be detected at 250 °C with a high yield of 8.9 wt %. This phenomenon indicated that the AC-P catalyst not only could greatly promote the formation of LGO but also played a role in reducing the temperature of cellulose pyrolysis. As the pyrolysis temperature increased, the yield of LGO reached a maximum of 18.1 wt % at

Figure 4. Typical ion chromatograms of cellulose pyrolysis: (a) noncatalytic pyrolysis at 500 °C; (b) catalytic pyrolysis at 300 °C (ACP-to-cellulose of 1:3) (1) hydroxyacetaldehyde (HAA), (2) furfural (FF), (3) levoglucosenone (LGO), (4) 1-hydroxy-3,6dioxabicyclo[3.2.1]octan-2-one (LAC), (5) 5-hydromethyl-furfural (5-HMF), (6) 1,5-anhydro-4-deoxy-D -glycero-hex-1-en-3-ulose (APP), and (7) levoglucosan (LG).

area % of LGO reached as high as 64.8%. In addition, the typical ion chromatograms of cellulose noncatalytic pyrolysis at 300 °C and catalytic pyrolysis at 500 °C are given in Figure S3 for comparison. These results clearly indicate that the AC-P catalyst could reduce the pyrolysis temperature of cellulose while significantly inhibiting the formation of LG and promoting the generation of LGO. In addition to AC-P catalyst, three other ACs (AC-H2O, ACCO2, and AC-Zn) were also applied for catalytic pyrolysis of cellulose. As shown in Figure S4, the selectivity of the three ACs on LGO was significantly lower than that of AC-P (Figure

Figure 5. Typical ion chromatograms of biomass pyrolysis: (a) pure pine wood in noncatalytic pyrolysis at 500 °C, (b) pine wood in catalytic pyrolysis at 300 °C, (c) pure poplar wood in noncatalytic pyrolysis at 500 °C, (d) poplar wood in catalytic pyrolysis at 300 °C, (e) pure bagasse in noncatalytic pyrolysis at 500 °C, and (f) bagasse in catalytic pyrolysis at 300 °C. (1) HAA, (2) acetic acid (AA), (3) FF, (4) LGO, (5) 5-HMF, and (6) LG. 10819


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increase in the catalyst amount, yield and peak area % of LGO first increased and then decreased, and the highest values of 18.1 wt % and 64.8% were obtained at the AC-P-to-cellulose ratio of 1:3. It is worth noting that in the previous analytical PyGC/MS studies using solid phosphoric acid and solid super acid for selective preparation of LGO, the optimal results were obtained around a catalyst-to-feedstock ratio of 1.26,32 The decrease in catalyst amount might be mainly due to the larger surface area and smaller density of AC-P as compared with the solid acid catalysts described above. Hence, a smaller catalyst quantity would be required to efficiently produce LGO. When the AC-P-to-cellulose ratio was lower than 1:3, sufficient contact between catalyst and cellulose could not be realized, and thus, no obvious catalytic effect would be observed. When the AC-P-to-cellulose ratio was higher than 1:3, overloaded catalyst would accelerate the dehydration and carbonization of cellulose, hence decreasing the overall yield of volatile organic products. Furthermore, a high AC-P-to-cellulose ratio would catalyze the secondary decomposition of LGO to decrease the LGO yield. Comparison with Previously Reported Acid Catalysts and Formation Mechanism of LGO. For the catalytic capability of the AC-P catalyst to be investigated further, four acid catalysts reported in previous studies were selected for catalytic fast pyrolysis of cellulose to compare with the AC-P catalyst. The four catalysts included two liquid acid catalysts (H3PO4 and H2SO4) and two solid acid catalysts (solid phosphoric acid and SO42−/TiO2-Fe3O4). The highest LGO yield from each catalyst in the study is given in Table 1. The

Figure 6. Effect of temperature on the yield and peak area % of LGO from noncatalytic and catalytic pyrolysis of cellulose at an AC-P-tocellulose ratio of 1:3.

300 °C, which was over 60-times that under noncatalytic conditions (0.3 vs 18.1 wt %). When the temperature continued to increase, the yield of LGO decreased continuously to 8.1 wt % at 500 °C, which might be due to other competing pyrolytic reactions and the secondary cracking of LGO at relatively high pyrolysis temperatures, especially under the catalytic conditions.48 The peak area % of LGO decreased monotonously along with the increase in temperature in sequence of 77.8% at 250 °C, 64.8% at 300 °C, 50.2% at 350 °C, and only 19.1% at 500 °C. This was because, with the increase in pyrolysis temperature, other competitive reactions such as the ring scission reaction of cellulose would be promoted, and the content of other products would increase relatively, thereby reducing the peak area % of LGO. These results clearly indicated that the low pyrolysis temperature would promote the selectivity of LGO production in agreement with previous results.24,49 Though LGO had the highest peak area % at 250 °C, the yield of LGO at 300 °C was the highest. Hence, the optimal pyrolysis temperature for LGO should be 300 °C. Effect of AC-P-to-Cellulose Ratio. The AC-P-to-cellulose ratio was another important factor of pyrolytic product distribution. Figure 7 shows the yield and peak area % of LGO under different AC-P-to-cellulose ratios. Along with the

Table 1. Highest LGO Yields from Cellulose by Different Catalysts (wt %) H2SO4

H3PO4

SO42−/TiO2-Fe3O4

solid phosphoric acid

AC-P

8.3

14.8

15.4

16.0

18.1

results indicated that the selectivity of the five catalysts on LGO was in the order of AC-P > solid phosphoric acid > SO42−/ TiO2-Fe3O4 > H3PO4 > H2SO4. The different catalytic capabilities might be attributed to the different catalytic sites in the five catalysts. Compared with the other four catalysts, during the preparation process of AC-P catalyst, phosphoric acid and carbon formed chemical bonds including C−O−PO3 in C−O−P combination form and C−PO 3 in C−P combination form.50,51 These functional groups enhanced the acidity of AC-P, hydrogen supply capacity, and dehydration ability. On the basis of these results, LG was the predominant product in the noncatalytic fast pyrolysis process, whereas LGO was only significantly generated in the catalytic process, suggesting the AC-P catalyst possessed dehydration capability of cellulose. For the catalytic effect of the AC-P catalyst to be verified, noncatalytic (350 °C) and catalytic (300 °C) pyrolysis experiments were carried out with LG as the raw material. Typical ion chromatograms are shown in Figure 8. As can be seen from Figure 8, under the noncatalytic pyrolysis conditions, LG was stable and did not have remarkable secondary cracking, although it could be easily converted into LGO with high selectivity by the AC-P catalyst. In addition, it is worth noting that, as revealed by previous studies, LGO could be further catalyzed into different products such as FF under strong acid catalysts. According to Figure 8, only a little FF was detected in the catalytic process, which might be due to relatively weak

Figure 7. Effect of AC-P-to-cellulose ratio on the yield and peak area % of LGO at 300 °C. 10820


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from catalytic fast pyrolysis of both mannose and glucose, but no LGO could be generated from xylan. Such results confirmed the importance of glucomannan in hemicellulose for LGO formation. In addition, other different properties of the three biomass materials would also affect the LGO yields, such as the lignin and ash in biomass. Previous studies have confirmed that both lignin and ash would inhibit the depolymerization of holocellulose while promoting the ring opening reactions, thus inhibiting the formation of anhydrosugar products.54,55 Among the three biomass materials, bagasse has the highest ash content, which should be partly responsible for its lowest LGO yield. Preparation of LGO from Lab-Scale Catalytic Fast Pyrolysis of Cellulose and Biomass. Production of LGO from Cellulose. For the capability of the AC-P catalyst to be further verified, lab-scale catalytic fast pyrolysis of cellulose experiments were performed to quantitatively determine the distribution of pyrolytic products under different pyrolysis temperatures and AC-P-to-cellulose ratios. Figure 9 gives the yields of char, liquid, and gas products. The composition results of the gas products are listed in Table S2. According to Figure 9(a), in the temperature range of 270 to 420 °C, as the temperature rises, char yield decreases while liquid and gas yields increase. Notably, as the temperature rose from 270 to 300 °C, liquid and gas yields increased by 1.84-times (49.3/ 26.8) and 1.74-times (7.5/4.3), respectively. This indicated that the promoted temperature brought sufficient pyrolysis of biomass to increase the yields of liquid and gas products. According to Figure 9(b), with AC-P-to-cellulose ratios increasing from 1:10 to 1:1, the yield of char first decreased and then increased. The lowest char yield was obtained at an AC-P-to-cellulose ratio of 1:5. The yields of liquid and gas products exhibited the opposite trend, which indicated that the increased catalyst quantity in a certain range would facilitate the formation of liquid and gas whereas overloaded catalyst would promote the carbonization process. The distributions of char, liquid, and gas products changed remarkably at different temperatures and AC-P-to-cellulose ratios, suggesting that the pyrolytic reactions of cellulose would be greatly affected by the AC-P catalyst under different conditions. Properties and Composition of the Liquid Products from Catalytic Pyrolysis of Cellulose. The liquid products consisted of water and organic compounds. The chemical composition was analyzed by GC/MS, and the water was measured by Karl Fischer titration. Table 3 shows the water content, LGO yield, and selectivity in the liquids from different pyrolysis temperatures and AC-P-to-cellulose ratios. According to Table 3, the water content in the liquid product was very high at low pyrolysis temperatures and gradually decreased along with the temperature. This phenomenon was due to the fact that dehydration was the major reaction during low-temperature pyrolysis of cellulose, whereas significant decomposition of cellulose to form organic compounds would take place at relatively high temperatures. In addition, along with the increase in the AC-P amount, water content in

Figure 8. Typical ion chromatograms of LG in noncatalytic and catalytic pyrolysis.

acidity of the AC-P catalyst compared with those of the other four acid catalysts. This fact would be beneficial for LGO production with high selectivity and also might be partly responsible for the high LGO yield from the AC-P as compared with other strong acid catalysts (Table 1). Effect of Biomass Raw Materials. An obvious difference in LGO selectivity has also been detected from catalytic pyrolysis of different biomass materials. In this work, three typical softwood, hardwood, and herbaceous materials were selected for experiments. Yield and peak area % of LGO from the three biomass materials are shown in Table 2. The chemical component results of the three materials are also given in Table 2. Under optimal conditions of pyrolysis temperature of 300 °C and AC-P-to-biomass ratio of 1:3, pine wood had the highest LGO yield of 9.1 wt %, and LGO yields from poplar wood and bagasse were 8.3 and 6.2 wt %, respectively. Notably, as LGO was mainly derived from cellulose, this result was not consistent with the cellulose content in each biomass material. The reasons might be attributed to the different chemical compositions of the three biomass materials, and the difference in hemicellulose should be mainly responsible for the result. The hemicellulose of pine wood mainly consists of glucomannan and xylan.52 Glucomannan is similar in structure to cellulose and can be converted to either LG during the fast pyrolysis process or LGO under acidic conditions.53 However, hemicellulose of poplar wood and bagasse are only rich in xylan and are thus unable to be converted into LGO. As a result, pine wood resulted in better selectivity for LGO than poplar wood and bagasse. For this fact to be verified, catalytic fast pyrolysis experiments on mannose, glucose, and xylan were conducted, and the results are shown in Figure S5. It is notable that glucomannan was not commercially available; thus, experiments were performed on mannose and glucose, which are the two basic units of glucomannan, instead of glucomannan itself. According to Figure S5, LGO could be selectively produced

Table 2. Chemical Composition of the Three Biomass Materials as Well as LGO Yield and Peak Area % biomass

LGO yield wt %

LGO peak area %

cellulose wt %

hemicellulose wt %

lignin wt %

extractive wt %

ash wt %

Pine wood Poplar wood Bagasse

9.1 8.3 6.2

35.6 31.0 26.5

45.8 49.8 44.0

19.1 24.4 28.9

30.7 23.3 19.9

4.1 2.2 4.0

0.3 0.3 3.2

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Figure 9. Char, liquid, and gas yields of cellulose at (a) an AC-P-to-cellulose ratio of 1:5 and (b) under the pyrolysis temperature of 300 °C.

Furthermore, the two highest LGO yields were obtained at the same pyrolysis temperature (300 °C) but under different AC-Pto-cellulose ratios (1:3 in Py-GC/MS vs 1:5 in the lab-scale setup). The difference in the highest LGO yields and the optimal AC-P-to-cellulose ratios in the two experiments should be due to the following reasons. In Py-GC/MS experiments, the AC-P and cellulose were fixed in the quartz tube and kept motionless, and thus, more AC-P amount was needed to ensure that the catalyst could fully contact with cellulose for sufficient catalysis. Meanwhile, after a short pyrolysis time (20 s), the pyrolysis vapor was quickly blown into GC/MS by carrier gas for detection, whereas during the lab-scale fast pyrolysis process, the contact between AC-P and cellulose was improved due to the flow condition; therefore, a smaller amount of AC-P was required to ensure sufficient catalysis. As a result, the catalyst amount in the lab-scale pyrolysis process was less than that of the Py-GC/MS process. However, during the lab-scale experiments, the pyrolysis time was much longer than that of Py-GC/MS experiments, leading to possible secondary reactions of pyrolysis vapors catalyzed by pyrolytic solid residues (char and AC-P catalyst), causing partial decomposition of LGO to reduce its yield. Production of LGO from Catalytic Pyrolysis of Biomass. In addition to cellulose, raw biomass materials were also applied for lab-scale experiments. Table 4 gives the LGO yield and selectivity in liquid products of cellulose and biomass under the optimum operating conditions. According to Table 4, pine wood was still the best biomass for the production of LGO. The highest yield of LGO from pine wood was 53.1% of that from cellulose (7.8 vs 14.7 wt %), whereas the cellulose content of pine wood was only 40.2%, which further confirmed that LGO could be formed from hemicellulose of pine wood during the catalytic pyrolysis process as discussed above. However, LGO selectivity in liquid products of biomass raw materials was lower than that of cellulose, which was because, under the pyrolysis temperature of 300 °C, the hemicellulose and lignin in biomass would also undergo thermal decomposition to form

Table 3. LGO Yield and Selectivity in Liquid Products from Catalytic Fast Pyrolysis of Cellulose at Different Temperatures and AC-P-to-Cellulose Ratios

a

AC-P-tocellulose ratio

temperature (°C)

liquid (wt %)

water content (wt %)

LGO yield (wt %)

LGO selectivitya (wt %)

1:5 1:5 1:5 1:5 1:5 1:5 0 1:10 1:5 1:4 1:3 1:2 1:1

270 300 330 360 390 420 300 300 300 300 300 300 300

26.8 49.3 52.1 54.2 56.2 59.2 26.3 43.3 49.3 48.7 45.3 44.0 39.5

63.0 60.9 60.0 57.7 53.2 51.0 48.6 57.9 60.9 64.2 64.8 66.6 73.2

8.1 14.7 10.3 7.3 6.1 4.5 0.3 8.0 14.7 13.0 10.5 6.8 4.1

81.7 76.3 49.4 31.8 23.2 15.5 2.2 43.8 76.3 74.6 65.8 46.3 38.7

Calculated by LGO yield divided by organic liquid yield.

the liquid product increased continuously to higher than that in noncatalytic pyrolysis. It should be attributed to the enhanced dehydration reactions by the AC-P catalyst because LGO was formed as a dehydrated product of cellulose. This also proved that the AC-P catalyst had a strong effect on the dehydration of cellulose, promoting the formation of LGO from cellulose. Changes in LGO yield and selectivity in the liquid products in lab-scale experiments were similar to those in Py-GC/MS experiments. The yield of LGO first increased then decreased with the increase in pyrolysis temperature and AC-P content; the maximum LGO yield was 14.7 wt % under an AC-P-tocellulose ratio of 1:5 at 300 °C. The corresponding LGO selectivity was as high as 76.3%, whereas the highest LGO selectivity was 81.7% at 250 °C. Of note is that the highest LGO yield from the lab-scale setup (14.7 wt %) was a little lower than that of the Py-GC/MS experiment (18.1 wt %).

Table 4. LGO Yield and Selectivity in Liquid Products of Cellulose and Biomass under Optimal Conditions

a

feedstock

AC-P-to-feedstock ratio

temperature (°C)

liquid (wt %)


LGO yield (wt %)


cellulose Pine wood Poplar wood Bagasse

1:5 1:5 1:5 1:5

300 300 300 300

49.3 40.2 42.9 42.3

60.9 54.9 54.2 56.7

14.7 7.8 7.0 5.8

76.3 43.0 35.6 31.7

Calculated by LGO yield divided by organic liquid yield. 10822


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the AC-PR-to-cellulose ratio of 1:3 at 300 °C. In addition, the AC catalyst prepared from H3PO4 activation exhibited better performance on LGO production than previously reported strong acid catalysts and other ACs prepared by different activation methods.

liquid products, resulting in a remarkable decline in the selectivity of LGO. Reusability and Stability of AC-P Catalyst. For the potential and feasibility of commercial usage of AC-P catalyst for LGO production to be explored, recycling experiments were performed using granular AC-P catalyst (AC-PR) with six consecutive runs without any regeneration of the recycled ACPR catalyst. Results are shown in Table 5, which were obtained

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02762. Textural properties of the AC catalysts, composition of noncondensable gas products from lab-scale catalytic fast pyrolysis of cellulose at different temperatures and AC-Pto-cellulose ratios, experimental sample preparation, calibration process and calibration curve of LGO, typical ion chromatograms of cellulose in noncatalytic and catalytic pyrolysis, typical ion chromatograms of cellulose catalytic pyrolysis with different ACs at 300 °C, typical ion chromatograms of catalytic fast pyrolysis of mannose, glucose, and xylan with AC-P at 300 °C, and typical ion chromatograms from GC/MS analysis of the pyrolytic liquid products obtained in the recycling experiments (PDF)

Table 5. LGO Yield and Selectivity in Liquid Products of Cellulose in Each Run of Recycling Experiments

a

run time

liquid (wt %)


LGO yield (wt %)


1 2 3 4 5 6

49.5 47.7 47.4 46.9 46.6 46.4

59.8 58.7 58.2 57.5 56.9 56.2

14.5 13.4 12.9 12.6 12.5 12.5

72.9 68.0 65.1 63.2 62.2 61.5

ASSOCIATED CONTENT

Calculated by LGO yield divided by organic liquid yield.

at the AC-PR-to-cellulose of 1:3 and 300 °C. In addition, the typical ion chromatograms from GC/MS analysis of the liquid products are given in Figure S6. The textural properties of fresh AC-PR and AC-PR after the sixth run are also listed in Table S1. According to Table 5, a slight decrease in LGO yield was observed after the first recovery of the AC-PR catalyst. After the fifth run of AC-PR, LGO yield could be maintained at ∼12.5 wt %, indicating promising reusability and stability of the AC-PR catalyst for the selective production of LGO. The BET surface area of the AC-PR catalyst decreased considerably after the sixth run (Table S1), and its weight increased by around 3 wt % (determined by weighing), which should be due to the coke deposition on the AC-PR catalyst. Further studies will be conducted to investigate the detailed properties and mechanism of coke deposition on AC-PR catalyst and subsequent effects on LGO production in the recycling process to optimize this new technique for low-cost LGO production.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86 10 61772030; E-mail: [email protected]. *Tel.: +86 10 61772063; E-mail: [email protected]. 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, 51676193), Beijing Nova Program (Z171100001117064), Beijing Natural Science Foundation (3172030), Foundation of Stake Key Laboratory of Coal Combustion (FSKLCCA1706), and Fundamental Research Funds for the Central Universities (2016YQ05, 2015ZZD02) for financial support.

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CONCLUSIONS A novel method was developed for catalytic fast pyrolysis of cellulose/biomass to selectively prepare LGO using a common AC catalyst prepared from the H3PO4 activation method. The results indicated that biomass type, catalytic temperature, catalyst-to-feedstock ratio and pyrolysis system played important roles in selectively producing LGO. Pine wood was the best biomass type of the three biomass materials to prepare LGO. Low pyrolysis temperature favored LGO formation. In Py-GC/MS experiments, the maximal LGO yields of 18.1 and 9.1 wt % from cellulose and pine wood, respectively, could be achieved under a catalyst-to-feedstock ratio of 1:3 at 300 °C. Corresponding peak area % at this condition were 64.8 and 35.6%, respectively, whereas the lab-scale experiments obtained the highest LGO yields of 14.7 and 7.8 wt % from cellulose and pine wood under the catalyst-to-feedstock of 1:5 at 300 °C with the LGO selectivity based on organic liquid products of 76.3 and 43.0%, respectively. In addition, recycling experiments confirmed that the AC-PR catalyst possessed good catalytic capability, reusability, and stability on the preparation of LGO in six consecutive runs without any regeneration of the recycled AC-PR catalyst. The yields of LGO from cellulose tended to be stable at ∼12.5 wt % after 3−5 times of catalyst recovery under

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Catalytic Fast Pyrolysis of Cellulose and Biomass to Selectively

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