Renewable N-Heterocycles Production by Thermocatalytic

Research Article pubs.acs.org/journal/ascecg

Renewable N‑Heterocycles Production by Thermocatalytic Conversion and Ammonization of Biomass over ZSM‑5 Lujiang Xu,† Qian Yao,† Jin Deng,† Zheng Han,† Ying Zhang,*,† Yao Fu,*,† George W. Huber,*,‡ and Qingxiang Guo† †

Collaborative Innovation Center of Chemistry for Energy Materials (2011-iChEM), Anhui Province Key Laboratory of Biomass Clean Energy, Department of Chemistry, University of Science and Technology of China, No. 96, Jinzhai Road, Hefei, Anhui 230026, P. R. China ‡ Department of Chemical and Biological Engineering, University of Wisconsin−Madison, 21415 Engineering Drive, Madison, Wisconsin 53706, United States S Supporting Information *

ABSTRACT: Chemical conversion of biomass to value-added products provides a sustainable alternative to the current chemical industry that is predominantly dependent on fossil fuels. N-Heterocycles, including pyrroles, pyridines, and indoles, etc., are the most abundant and important classes of heterocycles in nature and widely applied as pharmaceuticals, agrochemicals, dyes, and other functional materials. However, all starting materials for the synthesis of N-heterocycles currently are derived from crude oil through complex multistep-processes and sometimes result in environmental problems. In this study, we show that N-heterocycles can be directly produced from biomass (including cellulose, lignocelluloses, sugars, starch, and chitosan) over commercial zeolites via a thermocatalytic conversion and ammonization process (TCC-A). All desired reactions occur in one single-step reactor within seconds. The production of pyrroles, pyridines, or indoles can be simply tuned by changing the reaction conditions. Meanwhile, N-containing biochar can be obtained as a valuable coproduct. We also outline the chemistry for the conversion of biomass into heterocycle molecules by the addition of ammonia into pyrolysis reactors demonstrating how industrial chemicals could be produced from renewable biomass resources. Only minimal biomass pretreatment is required for the TCC-A approach. KEYWORDS: N-Heterocycles, Thermocatalytic conversion and ammonization, Biomass, Zeolites, Reaction pathway

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INTRODUCTION

N-Containing chemicals, especially N-containing heterocycles (N-heterocycles) such as pyrroles, pyridines, and indoles, are widely used in the pharmaceutical, agrochemical and dye industry. Due to their special properties, they are also used in new molecular materials for electronic, optic or transmission applications.6,7 More than one billion dollars of N-heterocycles are produced every year including 10,000 tons of pyrroles/y ($25,000/ton); 200,000 tons of pyridines/y ($5,000/ton), and 10,000 tons of indoles/y ($20,000/ton).8−12 Industrially, pyrroles are prepared by reaction of petroleum derived furan with ammonia in the presence of solid acid catalysts.12 Pyridines are synthesized through condensation of aldehydes, ketones, or α, β-unsaturated carbonyl compounds with ammonia or ammonia derivatives over zeolites catalysts.13,14 Indoles are produced from aniline and ethylene glycol.15 As shown in Figure 1, all starting compounds for the synthesis of

With the dwindling reserves of crude oil and environmental concerns, biomass, the most abundant sustainable source of carbon on the planet, has been regarded as an important alternative source for the production of renewable fuels and commodity chemicals.1,2 Renewable fuels including bioethanol, biodiesel, and bio-oil have been successfully converted from biomass.3 Meanwhile, olefins, aromatic hydrocarbons, and a variety of oxygenated hydrocarbon chemicals have been prepared from abundant biomass resources by a host of different technologies including pyrolysis, gasification, liquefaction, hydrogenation, and fermentation.4 However, the oxygenated hydrocarbons derived from the oxygenated biomass are drastically different in their chemical nature than the products obtained from crude oil.5 In addition to deep hydrodeoxygenation to produce liquid fuel, the oxygenated functional groups obtained from biomass could be utilized as feedstocks for value-added chemicals which are traditionally produced from functionalization and oxidation of petroleum molecules.2 © XXXX American Chemical Society

Received: August 8, 2015 Revised: September 17, 2015

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DOI: 10.1021/acssuschemeng.5b00841 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 1. From biomass or crude oil to versatile N-heterocycles. methylpyridine (AR), 3-methylpyridine (AR), glycerol (AR), ethylene glycerol (AR), and 1,3-propylene glycerol (AR) were purchased from Sinopharm Chemical Reagent Co. Ltd. without further purification. Glucose (AR), xylose (AR), chitosan (AR), 2-methylfuran (AR), 2,5dimethylfuran (AR), hydroxyacetone (AR), bicyclohexane (AR), benzofuran (AR), pyrazine (AR), pyrrole (AR), aniline (AR), indole (AR), 2-methylindole (AR), 3-methylindole (AR), and 4-methylindole (AR) were purchased from Aladin Chemical Reagent Co. Ltd. without further purification. NH3 (AR), N2 (99.999%), Ar (99.999%), He (99.999%), and standard gases were purchased from Nanjing Special Gases Factory. Bagasse was purchased from Guangxi Guigang Ganhua Inc., China. The bagasse was washed with distilled water, dried at 60 °C, ground in a high speed rotary cutting mill, and then sieved to obtain particles less than 0.6 mm. Catalysts and Characterizations. HZSM-5 with different Si/Al ratios (Si/Al = 25, 50, 80) were purchased from the Catalyst Plant of Nankai University. The typical properties of these catalysts are shown in Table 1. The particle size of the catalysts was about 40 meshes.

N-heterocycles currently are derived from crude oil through complex multistep-processes.16−20 Recently, we found that indoles could be produced from bioderived furans (furan and furfural) via a thermocatalytic conversion and ammonization process (TCC-A process).21,22 Meanwhile, we also found that bioderived polyols (glycerol) also could be converted to N-heterocycles (pyridines) via TCC-A process.23 However, the furans (furan and furfural) and glycerol are the derived products from carbohydrates or oil, are not the real biomass. Thermal decomposition or pyrolysis, a method involves the rapid heating of biomass (∼500 °C s−1) in an inert atmosphere to intermediate temperatures (300−600 °C), can be used to convert the solid biomass toward liquid products, gases, and biochar, which is an efficient way about the biomass conversion.4,24 The oxygenated compounds such as furan, aldehydes, and ketones, are formed during biomass thermal decomposition or pyrolysis process. This leads us to conclude that these oxygenates could probably react with NH3 to produce N-heterocycles via TCC-A process directly. Accordingly, in this paper we report on a thermo-catalytic conversion and ammonization (TCC-A) approach to directly convert biomass to N-heterocycles and N-containg biochar. With this method, N-heterocycles including pyrroles, pyridines, and indoles can be produced directly from various lignocellulosic biomass over HZSM-5 catalysts. The effect of the reaction temperature, feedstock-catalyst contacting pattern, reaction residence time of the feedstock and the Si/Al ratio of HZSM-5 on the cellulose conversion via TCC-A process were investigated systematically. Besides, other realistic biomass based feedstocks including sugars, starch, chitosan and bagasse were also used to produce N-heterocycles under selected conditions. Meanwhile, N-containing biochar, could be obtained as a valuable coproduct during the TCC-A process. Based on the experimental investigation, we proposed the potential reaction pathway from biomass to N-containing chemicals.

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Table 1. Typical Properties of Catalysts Used in This Study catalyst

BET surface area (m2/g)

pore diameter (nm)

Si/ Al

total acid (μmol/g)

HZSM-5-1 HZSM-5-2 HZSM-5-3

370 350 375

0.5 0.5 0.5

25 50 80

580.6 293.6 92.4

The N2 adsorption/desorption isotherms of the catalysts were measured at −196 °C using the COULTER SA 3100 analyzer. The surface area of the catalyst was calculated by the Brunauer−Emmett− Teller (BET) method. The Si/Al ratio of zeolites was measured by XRF (Shimadzu Corporation, Japan). The data was analyzed using the semiquantitative program UniQuant. For the NH3-TPD tests, about 200 mg of sample were put in a reactor and pretreated, in situ, during 1 h at 500 °C in a flow of argon. After cooling to 90 °C, ammonia adsorption was performed by feeding pulses of reactant grade ammonia (>99.995%) to the reactor using a flow of dry argon (>99%) of 60 mL/min. After the catalyst surface became saturated the sample was kept at 90 °C for 2 h to remove the base excess. Ammonia was thermally desorbed by rising the temperature with a linear heating rate of approximately 8 °C/min from 90 to 700 °C. The amount of NH3 desorbed was measured by a gas chromatograph (GC-SP6890, Shandong Lu-nan Ruihong Chem-

EXPERIMENTAL SECTION

Materials. Cellulose (AR), starch (AR), furan (AR), ethanol (AR), benzene (AR), toluene (AR), xylene(AR), pyridine (AR), 2B


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heated to 180 °C at 5 °C/min, finally heated to 280 °C at 10 °C/min, and holding at 280 °C for 5 min. The amounts of the chemicals in liquid product were determined by the GC analysis. A fraction of the liquid product mixed with bicyclohexane as the internal standard was diluted by ethanol for its measurement by gas chromatography (GC 1690, Kexiao, China) employing a 30 m × 0.25 mm × 0.25 μm fused-silica capillary column (OV 1701, China). The operating conditions were as follows: carrier gas nitrogen; injection port 250 °C in a split mode; detector (FID) temperature 250 °C; column temperature 40 °C; oven temperature program heating from 40 to 250 °C at a rate of 10 °C/min and holding at the final temperature for 5.0 min. For gas product analysis, the total gas in each run was collected with air bags. The collected gases passed through two washing bottles containing phosphoric acid to remove the excess ammonia and, then, was collected and weighed. Finally, the gases was analyzed using a gas chromatograph (GC-SP6890, Shandong Lunan Ruihong Chemical Instrument Co., Ltd., Tengzhou, China) with two detectors, a thermal conductivity detector for analysis of H2, CO, CH4, and CO2 separated on TDX-01 column, and a flame ionization detector for gas hydrocarbons separated on Porapak Q column. The moles of each gas product were determined by the normalization method with standard gas. The WHSV, residence time of the reactant, the carbon yield and selectivity of biochar, gases, N-heterocycles, and aromatic hydrocarbons were calculated from eqs 1−10 in Table S1 in the Supporting Information.

ical Instrument Co., Ltd., Tengzhou China) with a thermal conductivity detector (TCD). Apparatus for the Thermocatalytic Conversion and Ammonization Process. TCC-A Process Experiments in the Benchtop Units. The benchtop unit consisted of a gravity feed type feeder, a quartz tube reactor heated by furnace, and a condensation tube bathed in liquid nitrogen. In each TCC-A run, the reactor was heated to the preset temperature (673−873 K) and the ammonia flow rate was adjusted to 40−100 mL/min. In the thermocatalytic conversion process, the catalyst could be either directly mixed with biomass feedstock or only mixed with the thermal decomposition vapors. The process where the catalyst is mixed directly with the feedstock in the reactor is referred to as in situ thermocatalytic conversion (in situ TCC); while the process where the catalysts are only contacted with the vapors is referred to as ex situ thermocatalytic conversion (ex situ TCC). According to the experimental design, three feeding modes were employed and the corresponding units were designated as TCCA/1, 2, or 3. In situ TCC-A/1 Unit. Solid biomass was mixed with certain amount of catalyst uniformly and cofed into the reactor. The biomass underwent catalytic decomposition and conversion with ammonia. The scheme of TCC-A/1 unit was shown in Supplementary Figure S1. Considering that in our case the mixture of the HZSM-5 catalyst and cellulose may accumulate on the bottom of the reactor which might change their interaction time, determined by the length of the reaction zone and the speed of the carrier gas, the total mixture amount was kept constant and the diameter of the reactor was big enough to eliminate the effect of the interaction time difference. Ex situ TCC-A/2 unit. The catalyst bed was built up before feeding the feedstock. Solid biomass was fed into the reactor. The biomass decomposed first and the volatile intermediates together with ammonia passed through the catalyst bed for further conversion. The scheme of TCC-A/2 was shown in Supplementary Figure S2. Ex situ TCC-A/3 unit: . The catalyst bed was built up before feeding the feedstock. The liquid compounds derived from biomass were fed into the heating zone by Peristaltic pump. The volatile intermediates together with ammonia passed through the catalyst bed for further conversion. The scheme of TCC-A/3 was shown in Supplementary Figure S3. Whichever unit was employed, 10 min after all the feedstock was fed into the reactor, the product collection was terminated. The volatile products were trapped in the condensation tube. The amount was determined by the weight difference of the condensation tube before and after the experiment. The gases were collected with gas bag for further analysis. The solid was weighed and the residue amount was determined by deducting the weight of the catalyst. Biochar and Catalyst Characterization. The elemental contents of the biochar were measured by elemental analyzer (Atomscan Advantage, Thermo Jarrell Ash Corporat ion, USA). The biochar and catalysts were also investigated by BET analyses. The catalysts were investigated by NH3-TPD (temperatureprogrammed desorption of ammonia) to study the acidity of HZSM-5 with different Si/Al ratios. For the NH3-TPD tests, the catalysts were treated at 500 °C under helium flow (ultrahigh purity, 40 mL/min) for 2 h, and the adsorption of ammonia was carried out at 90 °C for 1 h. After that, the catalysts were flushed with helium at 90 °C for 2 h, and the programmed-desorption of NH3 was run from 90 to 700 °C with a heating rate of 8 °C/min. The desorbed ammonia was measured by a gas chromatograph (GC-SP6890, Shandong Lunan Ruihong Chemical Instrument Co. Ltd., Tengzhou China) with a thermal conductivity detector (TCD). The N2 adsorption/desorption isotherms of the catalysts were measured at −196 °C using the COULTER SA 3100 analyzer. Product Analysis and Quantitative Determination. The content of carbon in the solid residue was determined by elemental analysis. The liquid product distribution was analyzed by a GC-MS (Thermo Trace GC Ultra with an ISQ i mass spectrometer) equipped with a TR-35MS capillary column (30 m × 0.25 mm × 25 um). Split injection was performed at a split ratio of 50 using helium (99.999%) as carrier gas. The oven temperature was held at 40 °C for 3 min, then

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RESULTS AND DISCUSSION In situ TCC-A of Cellulose to Pyrroles. During the in situ TCC-A process, the catalyst is directly mixed with feedstock and usually their interaction time is very short. As shown in Figure 2 (The detailed information was shown in Table S2),

Figure 2. Effect of reaction temperature, catalyst to cellulose feed ratio and Si/Al of catalyst on the product distribution for converting cellulose + NH3 via in situ TCC-A process. (a) Effect of reaction temperature: 1 g of cellulose, catalysts to cellulose feed ratio = 2:1, HZSM-5 with Si/Al = 25; WHSV = 0.5 h−1. (b) Effect of catalyst to cellulose feed ratio: T = 773 K, HZSM-5 with Si/Al = 25, total mass of cellulose and catalyst 3 g. (c) Effect of the Si/Al of the catalyst: T = 773 K, catalyst to cellulose feed ratio = 2:1, WHSV = 0.5 h−1. In all the cases, NH3 flow rate = 40 mL/min.

pyrroles with high selectivity were produced from cofeeding of cellulose with ammonia using acidic HZSM-5 catalysts in the in situ TCC-A process under different conditions. Small fraction of pyrazines was also produced, which might derive from the condensation of light N-containing chemicals such as acetamide, propylamine and hydroxyethylamine over the acidic catalyst, as suggested by previous studies.25,26 With temperature rising from 573 to 873 K, the selectivity of pyrroles in the C


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ACS Sustainable Chemistry & Engineering organic liquid products moderately decreased, while that of indoles and aromatic hydrocarbons increased. Considering the effect of temperature on the carbon yield, the optimal temperature for the production of total organic liquid products, pyrroles, and pyrazines was around 773 K. When cellulose was converted during in situ TCC-A process, the particles either interacted with or without catalyst, which depended on the catalyst to feed ratios (cat:feed). Furthermore, once the pyrolysis vapor was formed, it would react with the surrounding catalyst immediately and the extent of reaction was also determined by the cat:feed. Therefore, cat:feed should be an important parameter to determine the product distribution. Figure 2 showed that if the cat:feed was low (1:1), the carbon yield of the organic liquid product would be limited by the deficiency of the catalyst. On the contrary, biochar production is promoted by the excess catalysts if the ratio was high (5:1). Thus, the yield of pyrroles was the highest when the cat:feed was 2:1. The yield of aromatic hydrocarbons has been reported to be a function of the Brønsted acid in the catalytic fast pyrolysis process.27−30However, based on our study shown in Table S3, both catalysts with Lewis acid such as γ-Al2O3 and Brønsted acid such as SO42−/ZrO2 could catalyze cellulose to pyrroles and pyrazines via the in situ TCC-A process. Therefore, the acid type for preparing pyrroles may not be as important as preparing aromatic hydrocarbons. HZSM-5 catalysts (Si/Al ratios = 25, 63, and 80) were selected to investigate the acid amounts and acidity influence on the product distribution. The typical characteristics of the HZSM-5 catalysts are given in Table 1. It can be seen from Figure 2 that both the organic liquid product and the pyrroles yields decrease with increasing Si/Al ratio. According to the NH3-TPD measurement shown in Figure S4, the acid amounts of HZSM-5 is a function of the Si/ Al ratio of the catalyst, with the acid amounts and acidity increasing as the Si/Al ratio decreases. It demonstrated that higher acid amounts could promote the organic liquid products and pyrroles production from cellulose, which could be due to the necessity of acid on the furan formation from cellulose, ring opening of furans and ring closing of pyrroles. If the cat:feed ratio increased from 2:1 to 5:1, the carbon yield of the liquid product and pyrroles decrease, while the yield of biochar increased significantly. It may be caused by higher acid amounts of more catalyst usage, which promoted cellulose dehydration reaction and pyrroles polymerization reaction to increase the biocahr formation during the in situ TCC-A process. Ex situ TCC-A of Cellulose to Pyridines and Indoles. Recently, we found that indoles can be thermo-catalytically converted from bioderived furans with ammonia, and pyridines could also be converted from glycerol with ammonia via thermocatalytic conversion and ammonization process in a trickle bed reactor.21−23 Due to bioderived furans (furan, 2methylfuran) and some light oxygenated chemicals (acetol, acetaldehyde, hydroxyacetaldehyde, acetic acid) could be produced in the cellulose pyrolysis process, which could be used to produce indoles or pyridines via TCC-A process. Herein, we tried to investigate whether indoles and pyridines could be selectively produced from cellulose directly. It has been proven that indoles and pyridines could not be selectively obtained from cellulose via in situ TCC-A process, we employed an ex situ TCC-A approach. This involves first thermal decomposition and then passing the vapor with ammonia over a fixed catalyst bed. As shown in Figure 3, in this approach we were able to maximize the indole selectivity at

Figure 3. Effect of reaction temperature, residence time, and Si/Al ratio of catalyst on the product distribution via ex situ TCC-A process. (a) Effect of reaction temperature: 1 g of cellulose; 1 g of HZSM-5 with Si/Al = 25; WHSV = 1 h−1; residence time = 2.6 s. (b) Effect of residence time: T = 823 K; 1 g of cellulose; HZSM-5 with Si/Al = 25. (c) Effect of Si/Al ratio of catalyst: T = 873 K, 1 g of cellulose; 1 g of H-ZSM-5. In all the cases, NH3 flow rate = 40 mL/min.

a 5.2 s residence time and 823 K (The detailed information is shown in Table S4 in the Supporting Information). Meanwhile, different from that during in situ TCC-A, the feedstock decomposed first without catalyst during ex situ TCC-A, which might result in the formation of light N-containing chemicals. When these mixtures passed through the catalyst bed for deep conversion, the vapor tended to form pyridines instead of pyrazines in the pores of the HZSM-5 catalyst. Increasing temperature increased the pyridines yield. Aromatic hydrocarbons were also positively correlated with temperature, indicating that they could be the downstream products formed at higher temperature. The production of pyrroles was preferred at lower temperature. The yield of indoles increased with temperature, while aniline formed at temperature higher than 773 K. Residence time is another important factor which determines the product distribution. Long residence time promoted pyrroles converting to downstream products. The production of indoles, aniline, and aromatic hydrocarbons increased with the residence time extending. High Si/Al ratio of the HZSM-5 catalyst facilitated the production of pyridines. When temperature was high and acidity was low, more light oxygenated chemicals (such as aldehydes, ketones, alcohols) produced, which are the major intermediates toformpyridines.31 Low acidity also could increase the yield of pyridines, because the catalyst with higher acidity could promote the furans’ production and thus to pyrroles and indoles. Moreover, more acid amounts would also promote to produce more biochar. Based on the above results, cellulose was selectively converted to different N-heterocycles via the TCC-A process by changing the reaction conditions. For pyrroles, the desired conditions are cofeeding the feedstock and HZSM-5 catalyst with low Si/Al ratio at around 773 K (in situ TCC-A) and the mass ratio of catalyst to feedstock (cat:feed) is about 2:1. For pyridines and indoles, the ex situ TCC-A process is preferred. Pyridines production requires high temperature (873 K) and high Si/Al ratio of the catalyst, while indoles production requires long residence time. The N-containing chemicals contain up to 79% pyrroles, 63% pyridines, and 57% indoles, respectively. D


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Figure 4. Selective production of renewable N-heterocycles from cellulose and NH3 over HZSM-5 via the TCC-A process. Reaction conditions 1 for maximal pyrrole production: in situ TCC-A in the TCC-A-1 unit; 1 g of cellulose; 2 g of HZSM-5 catalyst with Si/Al = 25; WHSV = 0.5 h−1; T = 773 K. Reaction condition 2 for maximal indole production: ex situ TCC-A in the TCC-A-2 unit; 1 g of cellulose; 2 g of HZSM-5 catalyst with Si/Al = 25; WHSV = 1.5 h−1; T = 823 K; residence time = 5.2 s. Reaction condition 3 for maximal pyridine production: ex situ TCC-A in the TCC-A-2 unit; 1 g of cellulose; 1 g of HZSM-5 catalyst with Si/Al = 80; WHSV= 3 h−1; T = 873 K; fixed bed reactor; residence time = 2.6 s. In all cases, NH3 flow rate = 40 mL/min.

of different biomass for producing N-heterocycles), which could be used to provide process heat for the TCC-A process. Coproduct Characterization. It is worth noting that biochar, produced as coproduct in the TCC-A of cellulose process (about 30−70%carbon yield) has a high nitrogen content. As shown in Table S8 in the Supporting Information, the biochar produced in the TCC-A, has an elemental content of C, H, O, N as 78.77%, 3.51%, 9.36%, and 8.36%, respectively. This high nitrogen content makes the biochar produced during the TCC-A process different from traditional pyrolysis biochar, whose nitrogen content was very low or no nitrogen in it. X-ray photoelectron spectroscopy (XPS) was used to better understand the chemical state of the carbon, nitrogen, and oxygen present in the N-containing biochar. As shown in Figure 6, from C 1s, peak 1 (284.7 eV) is corresponding to pure graphitic or amorphous C−C; peak 2 (286.3 eV) is corresponding to an sp2 type carbon, such as CN groups, carbon in phenolic, alcohol, or ether; peak 3 (288.2 eV) is corresponding to sp3 type carbon, such as C−N bonding states, carbon in carbonyl, or quinone groups. From O 1s, the only peak (534.0 eV) is corresponding to hydroxyl groups. From N 1s, peak 1 (398.6 eV) could be attributed to pyridinic-like (398.5 ± 0.2 eV) nitrogen atoms incorporated into graphitic sheets (pyridinicN); peak 2 (400.7 eV) can be mainly assigned to pyrrolic/ pyridone-N (401.0 ± 0.5 eV); peak 3 (404.5) could be mainly assigned to pyridione-N-oxide (404.0 ± 0.5 eV).32,33 It is evident that the N atoms are boned within the carbon lattice instead of dangling on the carbon surface. Doping with nitrogen has a significant effect on the structure of the carbon material. It can enhance the donor properties and electrical

Based on the above results, cellulose could be selectively converted to target N-heterocycles via the TCC-A process by changing the reaction conditions (Figure 4). The selectivity of pyrroles, pyridines and indoles in N-containing products was up to 79%, 63%, and 57%, respectively, depending on the reaction conditions and reactor configurations. We further tested the TCC-A process with other realistic biomass based feedstocks including sugars, starch, chitosan, and bagasse (see details in Table S5−7 in the Supporting Information). The reaction conditions were adopted from the cellulose conversion without further optimization. The primary results in Figure 5 demonstrated that for all of these different feedstocks, N-containing chemicals were the major products in the organic liquid phase. The selectivity from different feedstocks to pyrroles, pyridines, and indoles in the Ncontaining products was 75.2−88.8%, 44.9−59.8%, and 44.8− 63.6%, respectively. The difference could be related to the special composition and molecular structure of the feedstocks. It is worth mentioning that although lignin inbagasse, it produced comparable amount of N-heterocycles as some other carbohydrates and a little bit more aromatic hydrocarbons in some conditions. Anyhow, the results demonstrate that the TCC-A process is a robust method to produce N-hereocycles from a wide variety of different types of biomass. Besides the detected organic compounds in the liquid during the TCC-A process, the amount of gases and biochar is also considerable. The carbon yield of gas from each TCC-A process was about 20−30%. The light gas products from the TCC-A process were mainly CO, CH4, C2H4, and C3H6 (as shown in Table S5−7 in the Supporting Information of TCC-A E


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Figure 5. Production of the N-heterocycles from different feedstocks through TCC-A process. (1) Production of pyrroles: (reaction conditions) in situ TCC-A process, using the TCC-A/1 unit, T = 773 K, feedstocks = 1 g, HZSM-5 = 2 g, Si/Al = 25, flow rate of NH3 = 40 mL/min. (2) Production of pyridines: (reaction conditions) ex situ TCC-A process, using the TCC-A/2 unit, T = 873 K, feedstocks = 1 g, HZSM-5 = 1 g, Si/Al = 80, residence time = 2.6 s, flow rate of NH3 = 40 mL/min. (3) Production of indoles: (reaction conditions) ex situ TCC-A process, using the TCCA/2 unit, T = 823 K, feedstocks = 1 g, HZSM-5 = 2 g, Si/Al = 25, residence time = 5.2 s, NH3 flow rate = 40 mL/min.

Figure 6. XPS spectra of biochar of TCC-A process. (a) Survey XPS spectra of C, N, and O of biochar. (b) X-ray photoelectron C 1s spectra of N 1s of biochar. (c) X-ray photoelectron N 1s spectra of N 1s of biochar. (d) X-ray photoelectron O 1s spectra of N 1s of biochar.

conductivity of carbon materials. It can also increase the activity of the carbon material in electron-transfer reactions and electrocatalytic reduction of oxygen.34 Thus, there are more and more studies on the application of N-doped carbonaceous

materials in electronics, catalysis, CO2 adsorption, batteries, and electrical double-layer capacitors (EDLCs).35−43 Therefore, the biochar obtained by TCC-A could be used in a range of applications including fertilizer, CO2 sequestration, removals of F


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produced. Addition of HZSM-5 led N-heterocycles to be the major products and the oxygenated compounds selectivity decreased both in situ and ex situ TCC-A processes. Pyrroles were the main products in the in situ TCC-A process, and the peak area reached 73.1%. Meanwhile, except for pyrroles, pyridines and indoles also became the main products in the ex situ TCC-A process. This confirmed that the replacing nitrogen by ammonia as carrier gas to the biomass pyrolysis process can promote the N-containing chemicals production. Furthermore, we tested the TCC-A process with furan (FA), acetol (HA), and acetaldehyde (AA) as feedstocks to investigate whether theoxygenated compounds produced from biomass pyrolysis served as the intermediates for producing Nheterocycles. As shown in Table 3, pyrroles and indoles were the only products produced from furanic feedstocks under different TCC-A conditions with yields up to 80% and 30%, respectively. Furthermore, it showed that the production of pyrroles and indoles were negatively correlated. At high reaction temperature and long residence time, pyrroles decreased while indoles increased in the reaction of furan to indole. In addition, pyrrole was also used as raw material to investigate the indole production via TCC-A process. When at higher reaction temperature (873 K) and longer residence time (5.2 s), the conversion of pyrrole increased to 95.0%, and the product selectivity is similar to TCC-A of furan under the same reaction condition, suggesting that pyrrole is a main intermediate for indoles production. Small amount of pyridines also produced from furan and pyrrole, becausethey could undergo a cracking reaction to form aldehydes or light Ncontaining chemicals, which could be the intermediate for pyridines production. To verify the main intermediates from cellulose to pyridines during TCC-A process, HA and AA, which could be formed from cellulose thermal decomposition process, were used as starting materials in the TCC-A process. Pyridines were the main products and the yield was up to 30%. Therefore, as expected, light oxygenated chemicals (such as HA and AA), which formed in the cellulose thermal decomposition process, are the main intermediates for producing pyridines from cellulose via TCC-A process. In addition to the Nheterocycles, small amounts of aniline and aromatic hydrocarbons were also produced from furan and pyrrole during TCC-A. Considering the molecular structures, aniline cannot form directly from either furan or pyrrole. When indole was used as the feedstock, aniline was the only detected Ncontaining compound in the organic liquid products. Certain amount of aromatic hydrocarbons (mainly benzene) were also produced in this TCC-A of indole process. In addition, benzene and pyridines were used as feedstocks in the TCC-A process, and they were stable in the TCC-A process because no conversion of benzene and pyridines were observed during TCC-A process. It suggested that aniline was mainly produced from indole. Based on the results of Tables 2 and 3 and our previous studies on production of indoles from furans and production of pyridines from glycerol, we demonstrate how N-heterocycles, including pyrroles, indoles, and pyridines, can be produced from biomass (Figure 7). During the TCC-A process, cellulose and hemicellulose first thermally decompose (or pyrolyze) and undergo rearrangement reactions to form furans and other light oxygenated chemicals (glycolaldehyde, hydroxyacetone and aldehydes, etc.), which are the intermediate to N-heterocycles from cellulose. For producing pyrroles, moderate temperature and acidic catalysts promote furans production from biomass,

contaminants from gas and liquid phases, environmental protection, catalysts and catalyst supports, or in electrochemistry as supercapacitors, cells, and batteries after further treatment.44−47 Possible Reaction Pathway from Biomass to NHeterocycles. Understanding the chemistry of the TCC-A process is important to further improve the N-heterocycles selectivity. The systematical experiments were carried out to investigate the reaction pathway from biomass to N-heterocycles using different biomass and model compounds. Table 2 Table 2. Peak Area (%) of the Product Distributions of Cellulose TCC and TCC-A under Different Conditions carrier gas

under N2 atmosphere

reaction style

TDa

in situ TCCb

ex situ TCCc

under NH3 atmosphere TDd

in situ TCC-Ae

ex situ TCC-Af

catalyst

N

Y

Y

N

Y

Y

Peak Area (%) light oxygenated chemicals furans anhydro sugars aromatic hydrocarbons light Ncontaining chemicals N-heterocycles pyrazines pyridines pyrroles indoles other

6.4

6.7

3.2

2.7

3.1

11.3 78 ND

26.7 59.7 ND

ND ND 91.3

14.3 46.5 ND

ND ND 1.0

ND ND 8.2

ND

ND

ND

12.3

0.8

ND

ND ND ND ND ND 4.3

ND ND ND ND ND 6.9

ND ND ND ND ND 4.2

20.6 2.7 ND 17.9 ND 4.7

89.0 11.5 ND 73.1 4.4 6.1

86.9 ND 39.9 31.7 15.3 4.9

a No catalyst. bIn situ TCC process, the mass ratio of catalysts to cellulose = 2:1, HZSM-5 with Si/Al = 25; WHSV = 0.5 h−1. cEx situ TCC process, 1 g of cellulose; 1 g of HZSM-5 with Si/Al = 25; WHSV = 1 h−1; residence time = 2.6 s. dNo catalyst, the flow rate of NH3 was 40 mL/min. eIn situ TCC-A process, the mass ratio of catalysts to cellulose = 2:1, HZSM-5 with Si/Al = 25; WHSV= 0.5 h−1; d. fEx situ TCC-A process, 1 g of cellulose; 1 g of HZSM-5 with Si/Al = 25; WHSV = 1 h−1; residence time = 2.6 s. All the pyrolysis temperature was at 773 K, all the flow rate of carrier gases was 40 mL/min. ND means not detected. The definition of peak area (%) was shown eq 10 in the Supporting Information.

shows the product distributions of cellulose TCC and TCC-A under different conditions (The detailed peak areas (%) of each chemical were shown in the Table S9 in the Supporting Information). Consistent with previous findings of thermal conversion of cellulose under N2 atmosphere,48−52 anhydrosugars, furans, and light oxygenated chemicals (such as glycolaldehyde, hydroxyacetone, and aldehydes, etc.) were the major products from cellulosic thermal or in situ thermocatalytic conversion (under N2 atmosphere), and the peak area (%) of anhydro-sugars in liquid product was 78% and 59.7%, respectively. While aromatic hydrocarbons were dominant in ex situ TCC of cellulose under N2 atmosphere, the peak area (%) of aromatic hydrocarbons in liquid product could reach 91.3%. When ammonia was introduced to the thermal or thermocatalytic conversion process as source cofeed, the product distributions changed dramatically. If the catalyst was absent, anhydro-sugars were also the major product (peak area 46.5%), while some N-containing chemicals such as light N-containing chemicals (12.3%) and N-heterocycles (20.6%) were also G


Research Article


Table 3. Summary of TCC-A of the Cellulose Thermal Decomposition Intermediates for Investigating the Possible Reaction Pathways from Cellulose to N-Heterocycles feedstock

furana

furanb

conversion biochar liquid product yield (C %) gases

100 3.4 88.2 7.5

100 14.0 40.2 32.3

N-contained chemicals aromatic hydrocarbons

97.9 2.1

97.9 2.1

pyridines pyrroles aniline indoles

9.9 90.1 ND ND

4.3 13.8 9.5 72.4

pyrroleb

acetolc

acetaldehydec

95.0 100 100 17.2 18.5 15.8 43.3 28.4 34.0 28.5 40.1 38.9 Product Selectivity in the Liquid Part (%) 96.8 79.8 73.0 3.2 20.2 27.0 Selectivity of N-Containing Chemicals (%) 16.9 100 100 ND ND ND 7.8 ND ND 75.3 ND ND

indoleb

benzeneb

pyridinec

35.7 3.1 27.7 4.4

ND ND ND ND

ND ND ND ND

8.3 91.7

ND ND

ND ND

Renewable N-Heterocycles Production by Thermocatalytic

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