Renewable High-Purity Mono-Phenol Production from Catalytic

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Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 5349−5357

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Renewable High-Purity Mono-Phenol Production from Catalytic Microwave-Induced Pyrolysis of Cellulose over Biomass-Derived Activated Carbon Catalyst Yayun Zhang, Hanwu Lei,* Zixu Yang, Kezhen Qian, and Elmar Villota Department of Biological Systems Engineering, Washington State University, 2710 Crimson Way, Richland, Washington 99354-1671, United States

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ABSTRACT: The activated carbons (ACs) enriched in P-containing functional groups were obtained through one-pot microwave-induced pyrolysis from corn stover activated with phosphoric acid and were further tested as the catalyst for selective monophenol production from cellulose pyrolysis for the first time. Maximum AC yield (44.3 wt %) was obtained with an acid to biomass ratio of 0.85. Increasing phosphoric acid to corn stover ratios could enhance the porosity and peak intensities of Pcontaining functional groups in obtained ACs. Attained ACs had an excellent catalytic performance in phenol production with the highest selectivity of phenol (99.02 % based on peak area) in the obtained organic compounds at the catalytic temperature of 450 °C. The catalytic performance of ACs remained highly selective for phenol after using two times. The experimental results indicated that P-containing groups such as −O−P, OP, and −O−P−O− were the active reaction sites and more mespores promoted phenol production. The phenol can be generated from reforming of levoglucosenone and furfural over AC catalysts. The present work provides an efficient route to produce high selective monophenol from cellulose pyrolysis by using activated carbons as the catalyst, which further advanced the utilization of biomass to produce high-value chemicals. KEYWORDS: Phenol production, Catalytic reactions, Cellulose microwave pyrolysis, Activated carbon, Phosphoric acid

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INTRODUCTION

efficient, and cost-effective technology, has been regarded as a promising method of converting lignocellulosic biomass into liquid fuels.3 Fortunately, phenol can be obtained directly from biomass catalytic pyrolysis with active catalysts, such as basic, acidic, and carbonates (e.g., NaOH, KOH, and NaCO3).4−7 Phosphates (e.g., K3PO4, K2HPO4, and KH2PO4) also exhibited some active performance in production of phenol-rich bio-oils with the phenols selectivity up to 64.2 %.8 Transition metal catalysts, such as Co, Mn, Cu, and Fe, were also used for the oxidative degradation of lignin to phenols, but the phenol selectivity was still relatively low, leaving further extraction and purification necessarily being acquired.9−12

Phenol, an important industrial chemical intermediate, is widely applied in the manufacturing of various chemicals such as bisphenol A, phenol resins, caprolactam, alkylphenols, and adipic acid, etc. The current majority of phenol is produced industrially by the three-step cumene process from benzene, which requires abundant consumption of fossil fuels and also accompanies with the production of byproducts and environment pollution.1 Hence, obtaining phenol from earth-abundant and inexpensive renewable feedstock instead of benzene, such as lignocellulosic biomass, is of critical importance and in high demand to alleviate continuous increase of fossil fuel consumption and associated environment pollution. Biomass, the most abundant and inexpensive renewable feedstock, has a great potential for sustainable production of fuels, chemicals, and carbon-based materials, which can be used in various industrial processes.2 Catalytic pyrolysis, a simple, © 2018 American Chemical Society

Received: January 9, 2018 Revised: February 28, 2018 Published: March 7, 2018 5349

DOI: 10.1021/acssuschemeng.8b00129 ACS Sustainable Chem. Eng. 2018, 6, 5349−5357

Research Article

ACS Sustainable Chemistry & Engineering

AC3, and AC4, respectively. Later, the mixtures were dried at room temperature for 12 h, then transferred into an oven at 100 °C overnight until a constant weight, during which a partial biomass carbonization occurred. The obtained samples were further processed by microwave-induced pyrolysis to generate porous carbons. Specifically, nitrogen gas was inflated for 15 min to create an oxygen-free pyrolytic condition before activation reaction. The pyrolytic activation process was conducted with the input power of 700 W in a Sineo MAS-II microwave pyrolysis system. Details of this reaction system were described elsewhere.19,20 The produced activated carbon was subsequently washed with distilled water at room temperature until the pH was neutral in order to remove all residual phosphoric acids, where the imbedded P-containing functional groups were left in obtained ACs. Finally, ACs were dried in an oven at 100 °C until a constant weight. Surface area and pore structures of corn stover derived ACs were evaluated by N2 adsorption/desorption isotherms at 77 K using a physisorption analyzer (MicromeriticsRTristarII 3020). Prior to analysis, the AC samples were degassed at 250 °C overnight. The specific surface area was calculated according to Brunauer−Emmett− Teller (BET) theory. Micropore volume, micropore, and external surface area were determined using the t-plot method.21 Fourier Transform Infrared (FTIR) spectra of AC samples were obtained with an IR Prestige 21 spectrometer in an attenuated total reflection (ATR) mode (Shimadzu, Ge crystal). The spectra were recorded at a range of 500−4500 cm−1 with a resolution of 8 cm−1 using a combined 64 scans. The surface morphology of ACs were identified using scanning electronic microscope/energy dispersive X-ray (SEM/EDX, FEI Quanta 200 F). Catalytic Microwave Pyrolysis and Analysis. Microwaveinduced pyrolysis is a promising alternative technique to conventional pyrolysis due to the dielectric heating used. Usually, a heating rate of 10−200 °C s−1 can be achieved by using microwave absorbers, favoring the liquid and gaseous products.13,18,19 The catalytic pyrolysis of cellulose over activated carbon was conducted with the microwaveassisted pyrolysis system integrated with the tandem catalytic process. Detailed procedures of the microwave pyrolysis process were presented in our previous studies.20,22,23 The cellulose was air-dried at 100 °C for 24 h to remove the physically bound moisture prior to conducting the experiments. A fixed amount of cellulose (15 g) was loaded in a 250 mL quartz bottle, which was placed inside the microwave oven. One gram of commercial activated carbon powder was used as the microwave absorber for pyrolysis of cellulose. The added microwave absorber has little effect on the chemical distribution in obtained bio-oil due to its quite small doze and negligible in situ catalytic performance.20 All reactions of microwave pyrolysis were conducted at the setting temperature of 480 °C, and the whole reaction time lasted for 15 min with an initial input microwave power of 700 W. The microwave heating system also equipped with an automatic temperature/power control module for real-time reaction temperature control so that after approaching the setting reaction temperature (normally within 5 min of irradiation to reach 480 °C),24 a minimum power input (0−100 W) was then used to maintain the setting temperature. Biomass samples were rapidly pyrolyzed within the initial 12 min of microwave irradiation; however, to ensure the complete pyrolysis, the total reaction time was set to 15 min. Theoretical energy consumption through the process was estimated as 4.67−5.55 kWh/kg biomass, which is not energy efficient. However, previous study25 shows that the specific energy consumption in microwave heating decreases by 90−95 % when the sample amount increases from 5 to 100 g and remains minimum when the sample amount exceeds 200 g. Therefore, substantial energy savings can be expected using microwave heating at a larger scale, from which the energy consumption is about 0.58−0.65 kWh/kg biomass.26 It should also be noticed that the heating rate here was around 100 °C min−1, which therefore cannot be regarded as a fast pyrolysis process. The pyrolysis volatile vapors from the quartz bottle passed through a tandem packed-bed catalytic reactor that was filled with the catalyst. The mass ratio of AC to cellulose was set to 1:5, 1:3, 2:3, and 3:3,

Recently, increasing attention has been contributed to investigating activated carbons (ACs) as the catalyst in the bio-oil refinery. Pioneer work conducted in our group unveiled a fresh strategy of using the commercial activated carbon as the catalyst to convert biomass into phenol-rich bio-oils via microwave catalytic pyrolysis.13−15 Subsequently, other raw biomass, such as palm kernel shell, peanut shell, and pine sawdust, were also investigated by a similar method to obtain high phenol selective bio-oil.16,17 Compared with traditional fixed-bed pyrolysis, microwave-induced pyrolysis could obtain a high phenol selectivity, where the highest phenol selectivity in bio-oils was achieved at 500 °C with a biomass to AC mass ratio of 10:2.16 Similarly, other experimental results also exhibited active catalytic performance, making this process a promising route to produce phenols from renewable biomass.18 However, previous studies were based on the commercial activated carbon, whose production were at the expense of coal and other high-value agroforestry resources such as wood, bamboo, and coconut as the feedstock. The high loading ratio of catalyst to biomass and the selectivity of monophenol production (highest selectivity based on peak area was 64.58 %) was not high enough to meet industrial production, which requires cost separation and purification, and further impeded the development of producing phenols from the renewable biomass.14−18 In the present work, a facile process with the assistance of microwave irradiation was employed to synthesize activated carbons enriched in P-containing functional groups from the pyrolysis of corn stover activated with phosphoric acid. The activated carbons obtained here had excellent catalytic performance in converting cellulose into phenol-rich pyrolytic oils, and the selectivity of phenols can be achieved as high as 100 % (peak area) among obtained organic compounds by GC/MS analysis with the highest phenol concentration up to 7.2 mg mL−1. To the best of our knowledge, this is the first time to achieve converting cellulose into pure phenol with the activated carbon as the catalyst, which opens a new window of producing phenols from catalytic pyrolysis of cellulose.

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EXPERIMENTAL SECTION

Materials. Corn stover was used as biomass feedstock, which was collected from Brookings, South Dakota. The as-received corn stover was milled and sieved to a particle size of less than 2 mm prior to use. Microcrystalline cellulose (CAS number 9004-34-6) was purchased from Sigma-Aldrich Corporation (St. Louis, MO), which is in the form of microcrystalline powder with an average particle size of 50 μm. Phosphoric acid (85 wt %) was purchased from Alfa-Aeser corporation, which was used without further treatment. Activated Carbon Preparation and Characterization. The porous carbons were derived from corn stover via phosphoric acid activation. Our previous study has systematically investigated the effect induced by using phosphoric acid of various concentration on heating rate and carbonization time. Results indicated that the textural characteristics of obtained activated carbon was determined by the acid concentration to a large extent. Therefore, different concentration of phosphoric acid was used in the present work, and a longer carbonization time was set (1 h) here due to the fact that a large mass of feedstock would be activated here.19 The corn stover was washed with tap water several times before using in order to remove dust impurities. The washed feedstock was dried in an oven at 100 °C for 24 h until a constant weight. Phosphoric acids (85 wt %) of 25, 50, 100, and 200 mL were added into 700 mL distilled water to generate four batches with various acid concentrations. Then corn stover (with a fixed mass of 100 g) was impregnated into four acid solutions for 12 h at room temperature. The final weight ratios of phosphorous acid to biomass were 0.21, 0.43, 0.85, and 1.70, which marked as AC1, AC2, 5350

DOI: 10.1021/acssuschemeng.8b00129 ACS Sustainable Chem. Eng. 2018, 6, 5349−5357

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ACS Sustainable Chemistry & Engineering respectively. The packed-bed reactor was constructed with quartz tube and externally heated by a heating tape. A thermocouple was introduced between the reactor and the heating tape to measure the catalytic temperature. The separate heating regimes ensured a different catalytic temperature from that in microwave pyrolysis. The condensable vapors were recovered as the pyrolytic bio-oil, and the char residue was collected and weighed after the end of pyrolysis. The gaseous product yield was determined by mass balance. The chemical compositions of the bio-oils were characterized and qualified by Agilent 7890A GC/MS (GC/MS; GC, Agilent7890A; MS, Agilent 5975C) with a DB-5 capillary column. Prior to analysis, the bio-oil components were extracted by dichloromethane (HPLC grade, 99.7 %, Alfa Aesar) with the volume ratio of dichloromethane to bio-oil being 4:1. After phase separation, the bottom layer (solvent phase) was sampled for subsequent analysis. The GC was first heated to 40 °C for 5 min followed by heating to 280 °C for 5 min at a rate of 10 °C min−1. The injection sample volume was 1 μL. The flow rate of the carrier gas (helium) was 0.6 mL min−1. The ion source temperature was 230 °C for the mass selective detector. Compounds were identified by comparing the spectral data with those in the NIST Mass Spectral library. The area percent of compounds obtained from GC/MS results was utilized to predict product selectivity. In order to determine the concentration of phenol, different standard solutions with various concentrations of phenol were also injected to GC/MS, and the obtained data was used to quantify the concentration of phenol in the obtained bio-oils.

AC yield increased with the rising acid ratios and reached a maximum (44.2%) at the acid to biomass ratio of 0.85. This can be contributed to the increase of impregnation phosphoric acid ratios because the carbonization temperature could be dramatically decreased by adding an increased amounts of phosphoric acid into biomass. In addition, increasing mass of phosphoric acid resulted in a decrease of the final carbonization temperature, which in return boosted the solid residue as obtained ACs due to less volatiles were released,29 as shown in Figure 1 (right Y axle). Differences of ±1 wt % for obtained AC yields and ±5 °C for final activation temperatures were observed in repeated experiments of AC production, which revealed that activated carbon generated by phosphoric acid activation are repeatable. It was worth noticing that the corn stove impregnated with phosphoric acid had encountered a partial low-temperature carbonization when it was dried in the oven at 100 °C. The more phosphoric acid was added, the more proportion of biomass would be carbonized according to the color difference of partially carbonized corn stover after drying (Figure S1). In addition to the moisture loss, some phosphoric acid also escaped during the drying process, resulting in less than 10 wt % weight loss of the mixture. The vapor of phosphoric acid can be recovered in a larger scale by a vapor-scrubbing operation. The result indicated that a high acid ratio (1.7) had no further contribution to enhancing the yield of AC due to the weight loss at the low-temperature carbonization compared to the similar final carbonization temperature with the ratio of 0.85. Catalyst Characterization. It was well-known that the acid functional groups, such as −OH, −OOH, and O exist extensively in the activated carbon.29,30 The FTIR spectra were employed to detect the surface functional groups in corn stover derived ACs as shown in Figure 2. One can see that each

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RESUTS AND DISCUSSION Activated Carbon Production. The yield and the final activation temperature were important factors in describing attained activated carbons from phosphoric acid activation in the present work. Here, we focused on the catalytic performance of obtained ACs activated with different mass ratio of phosphoric acid to corn stover. Thus, more details of heating rate and activation time for generating activated carbon via phosphoric acid activation can be referred to our previous study.19 Four different activated carbons were synthesized based on different mass ratios of phosphoric acid to corn stover, namely, 0.21, 0.43, 0.85, and 1.70, which were marked as AC1, AC2, AC3, and AC4, respectively. The obtained AC yields were calculated by final dried AC mass divided by original mass of corn stover (100 g). Figure 1 describes the AC yields and the final carbonization temperature with various ratios of acid to biomass. It can be seen that the corn stover-derived AC yields ranged from 39.3 to 44.2 wt %, which were comparable to those obtained from other straw-based biomass, such as rice straw (34.7−44.4 wt %)27 and Oreganum stalk (35−42 wt %).28 The

Figure 2. FTIR spectra of corn stover derived activated carbons via phosphoric acid activation.

spectrum presents a wide absorption band at 3300−3600 cm−1 with a maximum located at 3400 cm−1, which can be assigned to the −OH stretching mode of hydroxyl groups and absorbed moisture. The absorption band around 1550 and 700 cm−1 is ascribed to be −CC and aromatic ring −C−H stretching, respectively. The absorption band near 880 cm−1 can also be assigned to the alkene −CC group. Our previous study evidenced aliphatic −C−H stretching (around 2920 cm−1) and

Figure 1. Obtained activated carbon yield (left Y axle) and final carbonization temperature (right Y axle) varied along with different mass ratios of phosphoric acid to biomass. 5351

DOI: 10.1021/acssuschemeng.8b00129 ACS Sustainable Chem. Eng. 2018, 6, 5349−5357

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ACS Sustainable Chemistry & Engineering −CO stretching vibrations (around 1750 cm−1) on corn stover derived biochar which were however not observed in biomass derived ACs here, indicating a deep degree of carbonization with aromatic structures dominating inner ACs.31 Phosphoric groups were successfully incorporated into ACs, evidenced by appearance of absorption peaks at 1050 cm−1 (−P−O), 1140 cm−1 (−PO), and 1150 cm−1 (−C− O−P).32,33 During the activation processes, corn stover first reacted with H3PO4 to form a corn stover-H3PO4 complex and water at relative low temperatures, which can be evidenced by the initial low-temperature carbonization with some mass lose. When the complex was further heated in a microwave oven, evaporation of bonded water occurred and the P2O5 was generated. With elevated temperature applied, the P2O5 volatilized and the corn stover-H3PO4 complex was converted into char and volatiles. Finally, some gaseous molecules were released from further pyrolysis of char and activated carbon was left as the residue with P-containing functional groups.34 High phosphoric acid impregnation ratios could result in high absorption peak intensities of phosphorus-containing functional groups, especially for −P−O and −PO groups. Therefore, the activated carbons with P-containing functional groups can be achieved via facile activation processes presented in the current work. In order to understand surface morphology of produced ACs, SEM was used to image AC 1−4 as shown in Figure 3 (panels

with the development of porous structures due to phosphoric acid activation. It can be seen that all the selected ACs exhibited straight fiber bundles with a tunnel-like structure. No major morphological changes were observed among the ACs derived from different combination of treatments. The particles shown in Figure 3d exhibited an intact cell wall structure, whereas the particles shown in other images presented breakage of the cell wall in various degrees. Activation temperature played an important role in variation of AC morphology. A high activation temperature (993 °C) gave rise to significant melting and fusion, thus leading to the collapse of the well-developed porous structure that was visible in the low temperature activation (450 °C).32 Additionally, the EDX surface analysis indicated that corn stover derived ACs prepared by phosphoric acid activation mainly contained C, O, and P (Figure S2−S5). With increased acid ratios, P peaks in EDX were enhanced, which further confirmed the incorporation of phosphorus into the carbonaceous structures of ACs. The results indicated that more P-containing functional groups could be chelated by using more acids during activation processes, which was in accordance with the FTIR analysis. It should be noted that there was still few vestigial inorganic element (e.g., Si and Ca) in corn stover derived ACs, which was due to no acid washing after the carbonization. The porous structure of the corn stover derived ACs was considered as one of the important factors of activated carbons. The porous structure parameters of AC2 and AC4 were shown in Table 1. The BET surface area of AC4 was 1283.10 m2/g, where the micropore surface area only accounted for 73.23 m2/ g. AC2 had a much lower BET surface area (710.09 m2/g) but a higher micropore surface area (438.38 m2/g) compared to AC4. The pore distribution had a similar trend of which the activation process with more acid added resulted in increased total pore volumes with less micropore. Figure 4 compared the

Figure 3. SEM images of corn stover derived activated carbons, AC1(a), AC2(b), AC3(c), and AC4(d).

a−d, respectively). Previous studies have revealed that chemical activation by phosphoric acid would promote the release of volatiles, shrinkage, fusion, and cracking to create the porosity.29,30 The surface morphology of corn stover derived ACs was maintained for most of the cell wall structures along

Figure 4. Nitrogen adsorption/desorption isotherms of ACs at 77 K.

N2 adsorption/desorption isotherms performed at 77 K. AC2 showed a typical microporous structure (type I) where the

Table 1. Porous Structure of the Corn Stover Derived AC2 and AC4 sample ID

SBET,total (m2/g)

Smicro (m2/g)

Sext (m2/g)

Vtotal (cm3/g)

Vmicro (cm3/g)

Vext (cm3/g)

AC2 AC4

710.09 1283.10

438.39 73.24

271.71 1209.86

0.38 1.20

0.22 0.023

0.17 1.18

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ACS Sustainable Chemistry & Engineering Table 2. Experimental Design and Product Yield Distribution yield (wt %) run no.

catalytic temperature (°C)

AC/cellulose mass ratio

AC type

oil yield

char yield

gas yield

coke yield

R-1 R-2 R3̅ R-4 R-5 R-6 R-7 R-8 R-9 R-10 R-11 R-12 R-13 R-14b R-15b R-16c R-17d

450 350 400 450 500 450 450 450 450 450 450 450 450 450 450 450 −

1/3 1/3 1/3 1/3 1/3 1/5 1/3 2/3 3/3 1/3 2/3 2/3 2/3 1/3 1/3 3/3 −

AC1 AC2 AC2 AC2 AC2 AC3 AC3 AC3−1a AC3 AC4 AC3−2a AC3−3a AC3−4a AC3 AC3 AC3 −

38.1 33.3 34.2 31.7 28.3 31.4 28.6 28.7 29.3 28.9 28.2 30.7 33.1 28.8 28.4 28.8 40.3

24.7 24.1 22.3 23.8 23.1 23.5 21.3 22.6 24.6 21.8 22.1 22.2 25.1 21.7 21.1 24.1 20.2

35.9 39.5 40.9 43.8 47.8 40.5 46.8 45.8 43.1 47.2 48.1 45.8 40.6 46.0 47.3 43.8 39.5

1.3 3.1 2.6 0.7 0.8 4.6 3.3 2.9 3.0 2.1 1.6 1.3 1.2 3.5 3.2 3.3 −

The AC3 was reused four times. bRepeated experiment at 450 °C with AC3 to cellulose ratio of 1/3. cRepeated experiment at 450 °C with AC3 to cellulose ratio of 3/3. dPyrolysis of cellulose without catalytic process.

a

filling up of micropores occurred at relatively low pressures. For AC4, the isotherm was a mixture of type I and type IV with a hysteresis loop, suggesting the coexistence of micropores and mesopores. The shapes of isotherms and results of porous structure parameters indicated that higher impregnation ratio conditions gave rise to a broadening porous structure of the obtained ACs. Mesoporosity and the development transition from microporosity to mesoporosity was enhanced with more activating agent was incorporated into the AC precursors. Catalytic Pyrolysis Product Yields. The catalytic pyrolysis product yield distributions were summarized as a function of catalyst type, catalytic temperature, and mass ratio of catalyst to cellulose as shown in Table 2. One can see that the yield of bio-oils and gaseous products were in the range of 28.3−38.1 wt % and 35.9−48.1 wt %, respectively, showing inverse trends because of different catalytic conditions. The yields of pyrolytic bio-oil were lower than that of pyrolysis without the catalyst, revealing that gaseous products were enhanced as a consequence of volatile reforming steps during catalytic processes.14,16,20 Results indicated that catalytic temperature had a significant effect on the yield of bio-oil. Specifically, compared with catalytic pyrolysis at 400 °C, a lower bio-oil yield was obtained at 350 °C, which was caused by partial carbonization of volatile on the activated carbon. This was evidenced by the fact that more coke was deposited at 350 °C than at 400 °C. When temperature was further increased, the oil yield decreased owning to enhanced catalytic reactions occurring inside of ACs, which in return boosted the yield of gaseous products. Loading ratio of catalyst to reactant also had an effect on the oil yield. Adding more catalyst (with loading ratios of ACs to cellulose less than 1/3) resulted in lower oil yields. This is due to the fact that catalytic reactions were enhanced with more catalytic sites, which further demonstrated by gaseous molecules releasing during catalytic reforming processes. However, with ACs loading ratio more than 1/3, the oil yields changed slightly, indicating few changes of components in bio-oil occurred due to catalytic conversion reaching the maximum, which can be further evidenced by the following oil component analysis. Similarly, the effect induced

by different types of catalysts also had a similar trend in oil yield variation. Under the same operating conditions, oil yield decreased from using AC1 to AC4 as a consequence of more functional groups involved which were formed with increased acid ratios during ACs generation. Solid carbonaceous residues (char and coke) from pyrolysis and catalytic processes can be differentiated due to the ex situ catalysis. The char yield ranged from 21.3 to 25.1 wt %. It was found that catalytic temperature had a less effect on the char formation due to separated cellulose pyrolysis and catalytic processes. The type of catalyst, on the other hand, had influence on the yield of char, from which more char was generated when AC1 and AC2 were applied compared to AC3 and AC4. This can be attributed to the fact that the volatile flow resistance passing through the catalyst was greater in AC1 and AC2, due to less porosity and more micropores in AC1 and AC2, which resulted in longer residence time of volatile inside of the pyrolysis reactor. More porosity with more mesopores in AC3 and AC4 allowed the volatiles passing through catalytic channel smoothly, which decreased the char yield. The same phenomenon were also observed during the processes of catalyst deactivation, where with longer time used, more char was produced due to losing porosity during repeated uses of catalysts. In addition, the mass ratio of ACs to cellulose was found as one of the important factors during char formation processes. Specifically, with more AC added, the flow resistance increased dramatically. As a result, more char was generated due to long residence time of volatiles in the pyrolysis reactor. Coke deposition on catalysts was notorious during catalytic processes. It was observed that elevating catalytic temperatures contributed to reduced coke formation because the reforming reactions were promoted and volatiles were therefore converted to products rather than turned to cokes at high catalytic temperatures. The ratio of AC to cellulose ratio had less effect on the coke formation at the same temperature as minor decrease of coke occurred when more AC was applied. It can be seen that the coke tended to be deposited on ACs activated with more phosphoric acid. Repeated experiments of cellulose catalytic pyrolysis (R-14 and R-15) with AC3 to cellulose of 1/ 5353

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ACS Sustainable Chemistry & Engineering 3 at 450 °C presented a similar result to that in R-7, which revealed that the presented microwave-induced catalytic pyrolysis of cellulose was repeatable. Chemical Composition of Bio-Oils. In order to further understand the chemical reaction of catalytic microwave pyrolysis of cellulose to maximize the phenols content, the chemical composition of bio-oils was identified and the concentration of phenol was quantified by GC/MS. It was observed that the chemical compounds of the upgraded bio-oils from catalytic microwave pyrolysis of cellulose varied dramatically along with different catalytic conditions. The major compounds were phenols, furans, and ketones, which were accounted for more than 80 % of catalytic bio-oils. Other compounds containing an hydrosugar and hydrocarbon with small concentrations were also identified, and a detailed chemical compound distribution in bio-oils was shown in Table S1. It can be seen that the upgraded bio-oil after catalytic reactions mainly contained the C5 and C6 compounds with retention time before 12 min. The generation of these main chemicals were discussed in detail so as to understand the mechanism of phenols evolution from catalytic pyrolysis of cellulose. The selectivity (area percentage) of phenols in these compounds was in a large range from 2.34 to 100 %, according to different catalytic conditions, in which phenol was the major compound (>90%). The largest selectivity (100%) of phenols was achieved, where phenol accounted for 99.02 % when the AC3 was applied and the loading ratio of AC3 to cellulose was 1:1 under the catalytic temperature of 450 °C from the experiment of run R-9. The selectivity of furfural also varied according to various catalytic conditions from 0 to 39.28 %, and the maximum was obtained by using AC3 catalyst under temperature of 450 °C with ratio of AC3 to cellulose being 1:5 from the experiment of run R-6. Effect of Catalytic Temperature on the Chemical Composition of Bio-Oils. Catalytic temperature is one of the most important factors that determine the catalytic performance. The catalytic activity of AC2 was tested with various temperatures from 350 to 500 °C, and the mass ratio of catalyst to cellulose was fixed at 1:3 in experiment runs from R-2 to R-5. Figure 5 shows the selectivity of major compounds [i.e., phenols, furfural, 5-methyl-furfural (HMF), levoglucosenone (LGO), and 2-acetyl-furan], which were varied along with

temperatures. One can see that the selectivity of phenols, furfural, HMF, and 2-acetyl-furan were enhanced compared with those without catalyst. The selectivity of LGO from catalytic pyrolysis was less than that without catalyst, which indicated that cracking reactions of LGO occurred during biooil catalytic reactions. With the temperature increased, HMF and 2-acetyl-furan experienced a small decrease although a slight increase from 450 to 500 °C for the selectivity of HMF, indicating that catalytic temperature had less effect on the reforming of these compounds within ACs. On the other hand, the temperature had a vital effect on the catalytic reactions of phenols, furfural, and LGO. Specifically, increasing temperature from 350 to 400 °C, a slight increase of selectivity occurred in producing furfural. While the selectivity of phenols and LGO decreased slightly due to the fact that part of LGO was converted to furfural. Compared with furfural, phenols witnessed a sharp increase when temperature was further elevated from 400 to 450 °C. At the same time, the concentration of LGO, on the other hand, decreased to zero percent when the temperature was increased to 450 °C, indicating that at 450 °C all LGO from cellulose pyrolysis was further reformed during catalytic processes and most of the consumed LGO was converted into furfural and phenols. Further temperature increase to 500 °C caused the decrease of selectivity of phenols due to the fact that less LGO was converted, which proved that the optimized catalytic temperature of phenols generation was 450 °C. In contrast, a slight increase of selectivity occurred in furfural generation. In accordance with the former discussion, it can be concluded that the optimized catalytic temperature for generating maximum phenols production occurred at the temperature of 450 °C. The formation of HMF and 2-acetyl-furan varied a little with the temperature increase. The LGO was further cracked during catalytic reaction to form furfural and phenols. The concentration of phenol was enhanced from 1.6 mg mL−1 without catalyst to 3.6 mg mL−1 with AC2 at 350 °C with loading catalyst to cellulose ratio of 1/3. Increasing the catalytic temperature promoted the phenol concentration in upgraded bio-oil, which were 3.2, 3.8, and 4.3 mgmL−1 at 400, 450, 500 °C, respectively, although a slight decrease occurred at temperature of 400 °C. Therefore, the quantified concentrations of phenol were in accordance with changes in the selectivity of phenol. In addition, repeated experiments of R-14 and R-15 also presented similar chemical composition distributions to those in R-7, which further confirmed that current catalytic pyrolysis of cellulose was repeatable. Effect of Catalyst Type on the Chemical Composition of Bio-Oils. In order to understand the effect of various catalysts on the product distribution, AC1−AC4 obtained from different ratios of H3PO4 to corn stover were investigated in the catalytic processes at the same operating temperature (450 °C) and the catalyst to cellulose ratio (1:3). It can be seen from Figure 6 that using ACs as the catalyst promoted the generation of major compounds except for the LGO. LGO was generated from microwave-induced pyrolysis of cellulose and was one of feeding compounds passing through the tandem catalytic process, which can be converted into furfural and phenols with the AC catalyst. The more mass ratios of phosphoric acid to corn stover were applied, the more porosity and P-containing functional groups would be introduced to ACs according to FTIR and BET investigations. When AC1 was used as the catalyst, the production of phenols, furfural, HMF, and 2-acetylfuran was enhanced as a result of the consumption of LGO

Figure 5. Selectivity of major compounds varied along with temperatureat the fixed AC2 to cellulose ratio of 1:3. 5354

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ACS Sustainable Chemistry & Engineering

7. It can be seen that when AC3 catalyst to cellulose ratio was 1/5, the production of target compounds were promoted

Figure 6. Selectivity of major compounds varied by using different catalysts at catalytic temperature of 450 °C with catalyst to cellulose ratio of 1:3. Figure 7. Selectivity of major compounds varied by using AC3 with different loading ratios to cellulose at catalytic temperature of 450 °C.

during reforming reactions. The concentration of LGO was therefore decreased dramatically. Increasing acid ratio during the corn stover activation enhanced the catalytic performance of obtained ACs for selectively producing phenols. The selectivity of phenols obtained from catalytic reactions leaped to 19.77 % when the AC2 (acid to biomass ratio: 0.425:1) was applied as the catalyst. Moreover, the selectivity almost doubled from 19.77 % by using AC2 to 34.86 % by using AC3 which was activated with doubled acid input (acid to biomass ratio, 0.85:1). Nevertheless, further increase of acid to corn stover ratio in AC4 production only slightly changed the phenols generation. The overall selectivity of furfural increased gradually, although a little fluctuation occurred. The concentration of HMF and 2-acethyl-furan remained at a relatively low level. It should be noticed that no LGO was detected in upgraded bio-oil by using AC2−4, indicating that LGO reforming reactions was enhanced by increasing porosity and P-containing function groups that promoted the formation of active reaction sites in the activated carbons. Hence, it can be revealed that during processes of corn stover activation by phosphoric acid, the ratio of acid to feedstock had a vital influence on the catalytic activity of obtained activated carbons for selective phenol production due to high porosity and Pcontaining functional groups imbedded. Similarly, the concentration of phenol was enhanced by using the catalyst with more P-containing functional groups and was 2.9, 3.6, 4.2, and 4.7 mg mL−1 in the upgraded bio-oil catalyzed by AC1, AC2, AC3, and AC4, respectively. However, more acid input increased the cost of producing activated carbons, from which the acid accounted for the major cost. Therefore, the optimized phosphoric acid to corn stover mass ratio was 0.85:1 for activated carbon generation with excellent catalytic performance in phenols production. Effect of Mass Ratio of Catalyst to Cellulose on the Chemical Composition of Bio-Oils. According to former studies on catalytic temperature and catalyst type, bio-oil reforming reaction with AC3 at 450 °C was regarded as the optimized catalytic condition. Thus, the effect of catalyst loading ratio on the obtained upgrading bio-oils was explored in this section. The loading ratio of AC3 to cellulose varied from 0:1 to 1:1 (mass weight varied from 0 to 15 g), and the selectivity of obtained major compounds was shown in Figure

except for LGO, from which the largest increase occurred for furfural from 25.0 to 39.79 %, and the LGO totally disappeared. As expected, the chemicals distribution changed dramatically with rising mass loadings of the catalysts. Specifically, phenol generation experienced a sharp increase of selectivity, meanwhile, a slight enhancement took place in producing methylphenol and 2-acetyl-furan when the loaded AC3 mass to cellulose ratio was increased to 1/3. On the contrary, the selectivity of furfural and HMF started to decrease when AC3 mass to cellulose ratio was increased from 1/5 to 1/3. In addition, with further increase of the catalyst mass to 10 g, the selectivity of phenol maintained the highest augment, which reached to 80.94 %. The production of methyl-phenol was also stimulated, while the generated furfural, HMF, and 2-acethylfuran were further converted into phenols or uncondensed gases with their concentration being zero percent. Finally, when the loading ratio of AC3 to cellulose was 1:1, most of methylphenol was further converted to phenol or uncondensed gases, resulting in the 99.02 % selectivity for phenol and the total selectivity of phenols was 100 %. The concentration of phenol in catalytic bio-oil was 2.8, 4.9, 7.2, and 5.6 mg mL−1, with the loading ratio of catalyst to cellulose being 1/5, 1/3, 2/3, and 3/ 3, respectively. The high selectivity of phenol was also achieved in the repeated experiment that conducted with AC3 to cellulose ratio of 3/3 at 450 °C as shown in run-16, which further confirmed the repeatability of the current experiment. It should be noticed that the selectivity of phenol increased through rising catalyst ratios from 2/3 to 3/3, but the concentration, on the other hand, experienced some decrease from 7.2 to 5.6 mg mL−1. The concentration drop can be ascribed to the second cracking reaction of generated phenol to form other uncondensed gases with the assistance of the catalyst. It is well-known that high water content is always accompanied by pyrolytic bio-oil, which also varies with different starting feedstocks, type of catalysts, and heating methods. Water production is favored during cellulose pyrolysis due to high hydroxyl groups, and therefore the water content was high (around 90 wt %) in the current study. A low concentration of phenol was observed in previous studies, 5355

DOI: 10.1021/acssuschemeng.8b00129 ACS Sustainable Chem. Eng. 2018, 6, 5349−5357

ACS Sustainable Chemistry & Engineering where the selectivity of phenol was much lower than those in the current work.16,17 It was worth noting that different loading ratios of catalyst to cellulose resulted in different residence time of volatiles passing through the packed-bed that filled with the catalyst. In other words, the more catalyst was loaded, the longer reaction time of volatiles contacting with the catalyst would be during bio-oil upgrading processes. Therefore, increasing catalyst loadings could be regarded as rising the catalytic reaction time during the phenols evolution processes. It was found that the increase of phenol from 3 to 10 g AC3 was a consequence of consumption of furfural and HMF, indicating that during the catalytic processes, furfural and HMF could be converted to phenols. Methyl-phenol was also converted into phenol though it had low concentrations during the whole process. Therefore, we proposed that phenol can be obtained through two routes from LGO and furans, respectively. Besides, furans could also be obtained from LGO via catalytic reactions over the activated carbon catalyst. Catalyst Deactivation Test. In order to explore the catalytic activity after repeated uses of catalysts, AC3 was selected by cyclic utilization for four times with a mass ratio to cellulose of 2:3 at the catalytic temperature of 450 °C. Figure 8

Research Article

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CONCLUSIONS

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ASSOCIATED CONTENT

In the present work, activated carbons (ACs), which were rich in P-containing functional groups, were obtained and tested as the catalyst for monophenol production from cellulose catalytic pyrolysis. The optimized AC yield (44.3 wt %) was obtained with a phosphoric acid to biomass ratio of 0.85:1. BET/EDX and FTIR analysis showed that increasing phosphoric acid to corn stover mass ratio could enhance the porosity and peak intensities of P-containing functional groups in obtained ACs. Catalyst testing results revealed that the obtained ACs had the excellent catalytic performance for phenol production. The optimized catalytic temperature for phenol production was at 450 °C. Additionally, activated with more phosphoric acid could introduce more P-containing functional groups and porosity in the obtained activated carbons, resulting in better performance in selective phenol generation. Taking into consideration for catalytic performance and phosphoric acid consumption, the acid to biomass ratio of 0.85:1 was regarded as the optimized ratio. Increasing loadings of catalyst mass dramatically enhanced the selectivity (peak area) of phenol in the obtained organic compounds with the highest selectivity of monophenol being 99.02 % and the maximum mass concentration was achieved as high as 7.2 mg mL−1. The catalytic performance remained high for phenol selectivity after using two times. The experimental results indicated that Pcontaining groups such as −O−P, OP, and −O−P−O− were successfully chelated into ACs, which were the most catalytically active sites during the bio-oil reforming reactions. The more mesopores and P-containing groups were generated in ACs, the more volatile would be converted into phenols. According to current studies, the phenol was proposed to be converted from reforming of levoglucosenone and furfural that came from cellulose pyrolysis. Future work will be conducted to investigate the mechanism of phenol generation from cellulose pyrolysis and to seek the methods that can efficiently regenerate the activated carbons.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b00129. Appearance of corn stover impregnated with phosphoric acid after drying at 100 °C (Figure S1); EDX surface analysis of AC1−AC4 (Figure S2−S5); FTIR spectra of AC3 after using 0 to 4 times (Figure S6); and the compounds distribution in obtained bio-oil for each run (Table S1) (PDF)

Figure 8. Selectivity of major compounds varied by reusing AC3 with a fixed loading ratio to cellulose of 2:3 at catalytic temperature of 450 °C.

describes the selectivity of major compounds along with reusing times. It can be seen that the catalytic performance of selective phenols generation still kept at a high level when AC3 was used for two times, while few phenols were formed when the catalyst was further used for 3 or more times. The deactivation of catalyst in converting furfural into phenols should be ascribed to the coke deposition on the catalyst resulting in decreased reaction active sites (phosphoric groups) that determined the phenols production during catalytic processes. This can be evidenced by the continuous coke accumulation on the catalyst and FTIR analysis of used AC3 from which the peak area shrinkage was noticed with catalyst reusing (Figure S6). The concentration of phenol was also decreased with increased catalyst reusing times from 7.2 to 1.6 mg mL−1 at the first and fourth time, respectively. However, the selectivity of furfural increased and remained at a relatively high selectivity of about 38 % after the catalyst was used for four times.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +1 509 372 7628. Fax: +1 509 372 7690. ORCID

Yayun Zhang: 0000-0002-1714-3126 Elmar Villota: 0000-0002-6585-2533 Notes

The authors declare no competing financial interest. 5356

DOI: 10.1021/acssuschemeng.8b00129 ACS Sustainable Chem. Eng. 2018, 6, 5349−5357

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ACS Sustainable Chemistry & Engineering

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ACKNOWLEDGMENTS This study was partially supported by The Agriculture and Food Research Initiative of National Institute of Food and Agriculture, United States Department of Agriculture (Awards 2016-67021-24533 and 2018-67009-27904). We are grateful to Dr. Aftab Ahamed for helping with the GC/MS analysis, and Dr. Valerie Lynch-Holm from Franceschi Microscopy & Imaging Center (FMIC), Washington State University, for the help with SEM training.

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ABBREVIATIONS ACs, activated carbons; HMF, 5-methyl-furfural; LGO, levoglucosenone

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REFERENCES

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DOI: 10.1021/acssuschemeng.8b00129 ACS Sustainable Chem. Eng. 2018, 6, 5349−5357