Producing Pyridines via Thermocatalytic Conversion and

Dec 24, 2015 - When ammonia was introduced as the carrier gas during the thermal decomposition of PLA, the product distributions changed dramatically...
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Research Article pubs.acs.org/journal/ascecg

Producing Pyridines via Thermocatalytic Conversion and Ammonization of Waste Polylactic Acid over Zeolites Lujiang Xu, Qian Yao, Zheng Han, Ying Zhang,* and Yao Fu* iChEM, CAS of Key Laboratory of Urban Pollutant Conversion, Anhui Province Key Laboratory of Biomass Clean Energy, Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, China S Supporting Information *

ABSTRACT: In this study, polylactic acid served as raw material to produce fine chemicals (pyridines) via a thermocatalytic conversion and ammonization (TCC-A) process. Ammonia was employed as not only carrier gas but also a reactant in this process. The thermal decomposition behavior of PLA under N2 or NH3 atmosphere was investigated. Different catalysts, including MCM-41, β-zeolite, ZSM-5 (Si/Al = 50) and HZSM-5 with different Si/Al ratios (Si/Al = 25, 50, 80) were also screened. Reaction temperature and residence time, which may affect the pyridines production, were investigated systematically. It was verified that all the investigated factors, including catalyst structure, catalyst acid amounts, reaction temperature, and residence time, influenced the PLA conversion and the pyridines production. The highest pyridines yield, 24.8%, was achieved by using HZSM-5 (Si/Al = 25) at around 500 °C. The catalyst regeneration tests were carried out. It demonstrated that the catalyst was stable after five regenerations and the catalytic activity did not change significantly. A possible reaction pathway from PLA to pyridines was also proposed. PLA initially thermally decomposed to form lactic acid and some byproducts such as acetaldehyde, acetone, etc., and then lactic acid, the mixture of acetaldehyde and acetone, or other byproducts reacted with ammonia to form imines and finally underwent complicated reactions to form pyridines. KEYWORDS: Pyridines, Thermocatalytic conversion and ammonization, Polylactic acid, Ammonia, Zeolites

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INTRODUCTION Currently, more and more petroleum-based polymers are disposed into the ecosystem as industrial waste products.1,2 They are resistant to microbial attack and cannot be degraded in short time. They also lead to serious environmental issues and have seriously adverse effects on human health.3−5 Therefore, the petroleum based plastics are called as “white pollution” around the world. The development of degradable plastics is beneficial for both environment and human society. Polylactic acid (PLA) is a biodegradable polymer derived from renewable sources such as corn starch (in the United States and Canada), tapioca roots, chips or starch (mostly in Asia), or sugarcane (in the rest of the world).6,7 Combined with its low toxicity, environmentally benign characteristics, desired mechanical property, thermoplastic processability, biocompatibility and biodegradability, PLA becomes the most commonly applied biopolymer and has the great potential for widespread usage. The annual production/consumption of PLA is about 200 000 tons in industry, and the annual demand of PLA will be over 1 million tons by 2020.1,8 Meanwhile, most of PLA plastics are discarded as industrial wastes and then decomposed to carbon dioxide via microbiological degradation process in the ecosystem. However, a considerable content of carbon and hydrogen in waste PLA could be used to produce hydrocarbon petrochemicals. Dozens of studies on pyrolysis and catalytic fast © XXXX American Chemical Society

pyrolysis of waste plastics have been carried out to produce liquid fuels and demonstrate that plastics pyrolysis is a promising technology for producing liquid fuels.9−18 Sharma and co-workers produced alternative diesel fuel via pyrolysis of waste plastic grocery bags. Ojha and co-worker selectively produced aromatics via catalytic fast pyrolysis of polystyrene (PS) over different zeolites. Muhammad and co-workers conducted catalytic pyrolysis of waste plastic from electrical and electronic equipment for producing aromatics. There were also some studies on pyrolysis or copyrolysis of wasted PLA plastic for producing fuel and chemicals.19−21 Fan and coworkers used TGA and Py-GC/MS to investigate the pyrolysis kinetics and mechanism of PLA. Wang et al. and Cornelissen et al. copyrolyzed PLA and biomass to produce high-quality biooil and studied the thermal decomposition behaviors of the PLA and biomass mixture. These studies also showed that acetaldehyde, acetone and some other oxygenated compounds were the products of PLA pyrolysis at 500 °C, which could be served as the raw materials to produce pyridines. Pyridines (pyridine, 2-methlypyridine and 3-methylpyridine) are widely used as building blocks for the synthesis of Received: September 29, 2015 Revised: December 22, 2015

A

DOI: 10.1021/acssuschemeng.5b01178 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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TPD tests, the catalysts were treated at 500 °C under helium flow (ultrahigh purity, 40 mL min−1) 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 at a heating rate of 9 °C min−1. The desorbed ammonia was measured by a gas chromatograph (GCSP6890, Shandong Lunan Ruihong Chemical Instrument Co. Ltd., Tengzhou China) with a thermal conductivity detector (TCD). The N2 adsorption and desorption isotherms of the catalysts were measured at −196 °C using the COULTER SA 3100 analyzer. TCC-A Tests. The benchtop devices for TCC-A of solid PLA or TCC-A of liquid raw materials are shown in Figures S1 and S2, respectively. Both devices consisted of a gravity feed type feeder, a quartz tube reaction reactor heated by a furnace and a condensation tube bathed in liquid nitrogen. The catalyst was fixed in the reactor as the catalyst bed. Under a certain rate and purged with ammonia, the solid PLA was fed into the reactor manually, while the liquid compounds were fed into the heating zone by a peristaltic pump. Volatile products were trapped in the condensation tube. The gas product was collected with a gasbag. For catalyst regeneration, air (100 mL/min) was used to remove the coke at 550 °C for 3 h. Product Analysis. The liquid samples were analyzed by a GC−MS instrument (Thermo Trace GC Ultra with an ISQ i mass spectrometer) equipped with a TR-35MS capillary column (30 m × 0.25 mm × 0.25 mm). Split injection was performed at a split ratio of 50 using helium (99.999%) as a carrier gas. The oven temperature was held at 40 °C for 3 min, then heated to 180 °C at 5 °C/min, and finally heated to 280 °C at 10 °C/min, and held at 280 °C for 5 min. The total amount of liquid product was determined by the weight difference of the condensation tube before and after the TCC-A test. The coke in each test was weighed, and the carbon yield of coke was further determined by the elemental analysis. Pyridines and other major liquid products were quantitatively determined by gas chromatography (GC 1690, Kexiao, China) employing a 30 m × 0.25 mm × 0.25 μm fused-silica capillary column (HP-Innowax, Agilent). The liquid sample was mixed with bicyclohexane as the internal standard and diluted by ethanol. The GC operating conditions were as follows: carrier gas, nitrogen; injection port, 250 °C in a split mode; detector (FID), 250 °C; column temperature, 40 °C; oven temperature program, heating up to 250 °C at a rate of 10 °C/min, and holding at a final temperature for 5.0 min. For gas product analysis, the entire gas of each run was collected with air bags, weighed, and analyzed using a gas chromatograph (GCSP6890, Shandong Lu-nan Ruihong Chemical Instrument Co., Ltd., Tengzhou, China) with two detectors, a TCD (thermal conductivity detector) for analysis of H2, CO, CH4 and CO2 separated on TDX-01 column, and a FID (flame ionization detector) for gas hydrocarbons separated on Porapak Q column. The moles of gas products were determined by the normalization method with standard gases. The carbon yields of coke, gas, pyridines and aromatics, and the selectivity of pyridines and gases were calculated from eqs 1−8:

agrochemicals and pharmaceuticals due to their high chemical activity and biological activity.22,23 They have also been used as solvents as well as catalysts in the chemical industry.24,25 Traditionally, pyridines were produced from coal tar as a byproduct of the coal gasification. While, pyridines produced by this method could not meet the increasing demand of the market, which resulted in the development of more synthetic methods by heterogeneous converting the acyclic molecules (carbonyls, dicarbonyls, dicyano alkanes, alkenes, and alkynes) over zeolites.26−31 Formaldehyde, acetaldehyde and acrolein currently are used to produce pyridines in industry. However, these feedstocks come from nonrenewable fossil based resources.32 Recently, we reported that pyridines could be produced from glycerol via a thermocatalytic conversion and ammonization (TCC-A) process over zeolites under ammonia atmosphere.33 In this study, polylactic acid was served as raw material to produce fine pyridines via a thermocatalytic conversion and ammonization (TCC-A) process. Ammonia was employed as not only a carrier gas but also a reactant in this process. The pyrolysis behavior of PLA under N2 or NH3 atmosphere was investigated. Different catalysts, MCM-41, β-zeolite, ZSM-5 (Si/Al = 50) and HZSM-5 with different Si/Al ratios (Si/Al = 25, 50, 80) were screened. Reaction temperature and residence time, which may affect the pyridines production, were studied systematically. The catalyst regeneration tests were carried out to investigate the stability of the catalyst. A possible reaction pathway from PLA to pyridines was also proposed.

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EXPERIMENTAL SECTION

Materials. Ethanol (AR), acetone (AR), methanol (AR), benzene (AR), toluene (AR), xylene (AR), lactic acid (AR), acetonitrile (AR) and naphthalene (AR), were purchased from Sinopharm Chemical Reagent Co. Ltd. Bicyclohexane (AR), pyridine (AR), 2-methylpyridine (AR), 3-methylpyridine (AR), 4-methylpyridine (AR), 2,3pentanedione (AR), propanoic acid (AR) and acrylic acid (AR) were purchased from Aladdin Chemical Reagent Co. Ltd. Acetamide (AR), lactamide (AR) and propionitrile (AR) were purchased from Energy Chemical Reagent Co. Ltd. Acrylonitrile was purchased from TCI Chemical Reagent Co. Ltd. PLA powder (∼100 meshes) were purchased from Changzhou RongSheng New Material Co. Ltd. All these chemicals were used without further purification. NH 3 (≥99.995), N2 (99.999%), Ar (99.999%), He (99.999%) and standard gases were purchased from Nanjing Special Gases Factory. Catalyst and Characterization. MCM-41, β-zeolite, ZSM-5 (Si/ Al = 50) and HZSM-5 with different Si/Al ratios (Si/Al = 25, 50, 80) were purchased from the catalyst plant of Nankai University. The particle size of the catalysts was about 40 meshes. The typical properties of zeolites are shown in Table 1. The elemental contents of the catalysts were measured by inductively coupled plasma and atomic emission spectroscopy (ICP/ AES, Atomscan Advantage, Thermo Jarrell Ash Corporation, USA). The catalysts were investigated by temperature-programmed desorption of ammonia (NH3-TPD) and N2 physisorption. For the NH3-

coke yield (C%) =

gas yield (C%) =

Table 1. Typical Property of the Catalysts catalyst

BET surface area (m2/g)

pore diameter (nm)

Si/Al

total acid (μmol g−1)

β-zeolite MCM-41 ZSM-5 HZSM-5-1 HZSM-5-2 HZSM-5-3

640 1000 420 350 370 375

0.7 3.8 0.5 0.5 0.5 0.5

50 50 50 50 25 80

683.3 260.0 255.5 293.6 580.6 92.4

carbon moles in solid residue × 100% carbon moles in PLA fed

carbon moles in gases × 100% carbon moles in PLA fed

pyridines yield (C%) =

carbon moles of pyridines × 100% carbon Moles in PLA fed

aromatics yield (C%) =

carbon moles of aromatics × 100% carbon moles in PLA fed

(1)

(2)

(3)

(4)

oxygenates yield (C%) =

carbon moles of oxygenates × 100% carbon moles in PLA fed (5)

B

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gas, non N-containing compounds and N-containing compounds. When PLA thermally decomposed under N2, lactic acid dimer was the main product, and some other light oxygenated compounds, such as acetone, propanoic acid and acrylic acid, were also produced. With temperature increasing from 400 to 600 °C, the carbon yield of coke decreased from 35.4% to 11.5%, whereas the gas yield increased from 19.3% to 37.2%. The carbon yield of lactic acid dimer increased from 400 to 500 °C and then decreased. Meanwhile, the carbon yield of light oxygenated compounds, such as acetone, propanoic acid and acrylic acid, also increased with temperature increasing, which could be produced from lactic acid dimer cracking at higher temperature. When ammonia was introduced as the carrier gas during the thermal decomposition of PLA, the product distributions changed dramatically. No lactic acid dimer was detected. The main detected products in the liquid became N-containing compounds, such as lactamide, acetamide, acetonitrile and acrylonitrile, etc., which could result from the products in the N2 atmosphere reacted with ammonia. With temperature increasing from 400 to 600 °C, the carbon yield of coke decreased from 36.0% to 12.7%, the carbon yield of gas increased from 17.7% to 35.9%, and the carbon yield of lactamide decreased from 33.1% to 19.2%, respectively. Meanwhile, the carbon yield of other N-containing compounds (such as acetamide, acetonitrile and acrylonitrile, etc.) increased with temperature increasing, which could be produced from lactamide cracking at higher temperature. Effect of Catalyst. Zeolites were chosen as the catalysts in this study due to their higher thermostability and better shapeselectivity than other amorphous solid acid catalysts, which are essential for pyridines production. Moreover, according to our previous studies on TCC-A of bioderived compounds to Nheterocycles (glycerol to pyridines, furan/furfural to indoles), the pore structure and acidity of the catalyst were essential to selectively producing the target chemicals.33−35 Six kinds of different zeolites were chosen for catalytic conversion of PLA in this study. The typical properties of the catalysts are shown in Table 1, and the product distribution of TCC-A of PLA over different zeolites are shown in Table 3. To investigate the effect of pore structure on the pyridines production, three kinds of zeolites with different pore structure (MCM-41, β-zeolite, and ZSM-5 with Si/Al ratio as 50) were chosen. Compared with ZSM-5, MCM-41 and β-zeolite tended to form coke and oxygen compounds. No pyridine was detected in the liquid

pyridines selectivity (%) carbon moles in specific pyridine = × 100% total moles of carbon in all pyridines identified (6)

gases selectivity (%) =

carbon moles of one gas total moles of carbon in all gases identified (7)

× 100% residence time =

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volume of catalyst volume flow rate of carrier gas

(8)

RESULTS AND DISCUSSION Thermal Decomposition Behaviors of PLA under N2 or NH3 Atmosphere. The thermal decomposition behavior of PLA under N2 or NH3 was investigated by a thermal decomposition process in the range from 400 to 600 °C, respectively. Table 2 shows the overall carbon yields of coke, Table 2. Product Distributions of Thermal Decomposition of PLA under N2 and NH3a carrier gas temperature (°C) overall carbon selectivity (C coke gas non N-containing compounds acetone 2,3-pentanedione propanoic acid acrylic acid lactic acid dimer N-containing compounds acetamide acetonitrile propionitrile acrylonitrile lactamide unidentified a

under N2 atmosphere

under NH3 atmosphere

400

500

600

400

500

600

%) 35.4 19.3

18.3 34.9

11.5 37.2

36.0 17.7

20.4 33.6

12.7 35.9

0.7 0 0.5 0.9 34.8

1.5 0.5 1.6 2.0 38.0

3.4 1.3 2.6 2.4 33.9 0.6 0.2 3.0 0.3 33.1 9.1

1.5 1.1 3.6 0.5 25.0 14.3

3.2 2.9 6.4 1.6 19.2 18.1

8.4

3.2

7.7

The overall carbon selectivity, carbon yield (C %).

Table 3. Product Distributions as a Factor of Catalyst in the TCC-A of PLA Process catalyst

MCM-41

β-zeolite

ZSM-5

HZSM-5/1

HZSM-5/2

HZSM-5/3

Si/Al

50

50

50

50

25

80

34.2 34.4 N.D. 2.5 15.7

28.7 32.5 5.6 7.0 10.3

25.0 33.8 17.9 5.3 7.6

24.3 32.6 22.3 5.2 4.8

22.6 31.9 24.8 6.9 2.1

24.1 35.3 17.4 4.7 6.9

37.9 24.2 6.4 13.7 17.8

43.7 23.7 5.2 15.0 12.4

44.5 23.8 4.6 15.9 11.2

40.4 26.1 4.9 15.4 13.2

47.1 21.7 3.7 16.9 10.6

overall carbon selectivity (C %) coke gas pyridines aromatics oxygenated compounds pyridines selectivity (%) pyridine 2-methylpyridines 3-methylpyridines 4-methylpyridine other alkylpyridines

Reaction conditions: reaction temperature, 500 °C; NH3 flow rate, 200 mL/min; residence time, 5.9 s; PLA fed in, 2 g. C

DOI: 10.1021/acssuschemeng.5b01178 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering products catalyzed by MCM-41, and the carbon yield of pyridines catalyzed by β-zeolite was only 5.6%. The carbon yield of pyridines catalyzed by ZSM-5 was 17.9%, which was much higher than those of MCM-41 and β-zeolite. MCM-41 possesses hexagonally packed arrays of channels with pore diameter around 3.8 nm.36−38 β-Zeolite has intersecting channels similar to ZSM-5, and larger pore diameter (0.7 nm) than that of ZSM-5(0.5 nm).39 ZSM-5 consists of two perpendicularly intersecting channels. The larger ones have a near circular pore structure with dimensions of 0.54 × 0.56 nm, and the smaller ones have geometry of 0.51 × 0.54 nm. The intersection of these channels that contains the proposed active site is approximately a 0.9 nm cavity. The kinetic diameter of pyridines is about 0.585 nm, which is similar to the pore diameter of ZSM-5. The results suggested that the shapeselective catalysis played an essential role in the TCC-A of PLA to produce pyridines. In addition, acid amount of zeolite was also an important factor to influence the product distribution. HZSM-5 with different Si/Al ratios (25, 50, 80), which are the protonated series of ZSM-5 and have higher acid amounts and better shape selectivity, were also tested in this study. The acid densities of these catalysts determined by NH3-TPD are shown in Table 1, and the NH3-TPD spectra are shown in Figure S3 in the Supporting Information. When the Si/Al ratio was 25, the total acid amount of HZSM-5/2 was up to 580.6 μmol g−1, which was much more than those of HZSM-5/1 (293.6 μmol g−1) and HZSM-5/3 (92.4 μmol g−1). It demonstrated that for HZSM-5, the lower Si/Al ratio, the higher the acid amounts and acid strength.40 As shown in Table 3, the yields of pyridines were increased as the Si/Al ratio decreased from 80 to 25. The carbon yields of pyridines over HZSM-5 (Si/Al = 80, 50 and 25) were 17.4%, 22.3% and 24.8%, respectively. Therefore, pyridines production from PLA was a function of catalyst pore structure, acid amount and acidity. Meanwhile, the carbon yield of aromatics was about 6.9%. The selectivities of pyridine, 2methylpyridine, 3-methylpyridine and 4-methylpyridines were similar by using different catalysts. On the basis of the above studies, HZSM-5 (Si/Al = 25) was the desired catalyst for producing pyridines in the TCC-A of PLA process. It was employed for further investigation. Effect of Reaction Temperature. Numbers of reactions, such as thermal decomposition, decarboxylation, dehydration, ammonization, condensation, coking, and so on, occurred during the TCC-A process.33 Therefore, the reaction temperature is an important factor that may affect the product distribution of TCC-A of PLA process. In this study, the temperature effect on PLA conversion was investigated by using HZSM-5 as catalyst at the temperature range of 300 to 650 °C. Table 4 shows the overall carbon yield of coke, gas, Ncontaining chemicals, aromatic and oxygenated compounds in the TCC-A of PLA process. The effect of reaction temperature on the pyridines selectivity and gases selectivity is shown in Figure 1. On the basis of Table 4, the yield of coke decreased and the yield of gas increased with reaction temperature increasing. The yield of oxygenated compounds in the liquid products also decreased with reaction temperature increasing. When the reaction temperature reached 550 °C, no oxygenated compound was detected. The highest yield of pyridines (23.0%) was obtained at about 500 °C. If the reaction temperature further increased to 600 °C, the yield of pyridines decreased to 16.8%. Meanwhile, aromatics reached the maximum yield (14.9%). If the temperature continued

Table 4. Product Distribution as a Factor of Reaction Temperaturea yield (%) entry temperature 1 2 3 4 5 6

300 400 500 550 600 650

coke

gas

pyridines

aromatics

oxygenated compounds

46.8 37.1 23.2 17 12.3 7.9

11.1 21.2 33.6 37.6 40.8 46.7

12.3 16.4 23.0 20.3 16.8 14.4

2.5 4.8 7.1 11.1 13.9 10.5

12.4 7.3 1.4 N.D. N.D. N.D.

a

Reaction conditions: catalyst, HZSM-5, Si/Al = 25; residence time, 7.4 s; NH3 flow rate, 200 mL/min, PLA fed in, 2 g.

Figure 1. Effect of reaction temperature on the selectivity of pyridines and gas; (a) pyridines selectivity; (b) gas selectivity (reaction conditions: catalyst, HZSM-5, Si/Al = 25; catalyst usage, 10 g; NH3 flow rate, 200 mL/min, PLA fed in, 2 g).

increasing to 650 °C, the yield of pyridines and aromatics decreased to 14.4% and 10.5%, respectively. It indicated that temperature at around 500 °C was in favor of pyridines production in the TCC-A of PLA process. Figure 1a shows the effect of reaction temperature on the pyridines selectivity. It displayed that from 300 to 650 °C, the selectivity of 3-methylpyridine kept at about 5%, whereas the selectivity of pyridine, 2-methylpyridine, 4-methylpyridine and other alkylpyridines changed a lot. When the reaction temperature was 300 °C, the selectivity of 4-methylpyridine was highest (26.0%), and then decreased at higher temperature. D

DOI: 10.1021/acssuschemeng.5b01178 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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regeneration cycles of catalyst were conducted. For each cycle, the reaction was performed at 500 °C. After reaction, coke was formed on the surface of the catalyst. The spent catalyst was regenerated in an air stream at 550 °C for 3 h to remove the coke. As shown in Figure 2, the catalyst did not deactivate

The optimal selectivity of 2-methylpyridine, 26.1%, was obtained at 500 °C. With reaction temperature increasing from 300 to 650 °C, the selectivity of pyridines increased from 38.9% to 60.4%, whereas the selectivity of other alkylpyridines decreased from 20.0% to 10.4%. Figure 1b shows the effect of reaction temperature on the gases selectivity from TCC-A of PLA. During the TCC-A process, after a trace amount of CO2 produced since when the gas passed through the Ca(OH)2 solution, a very small amount of white precipitant could be observed. However, because CO2 could react with ammonia to form ammonium carbonate, we did not report the yield and selectivity of CO2 in the gas products. Besides CO2, the main products detected in the gas phase were CO, CH4 and some olefins. With reaction temperature increasing, the selectivity of CO decreased dramatically, but the selectivity of CH4 and C2H4 increased a lot. When the reaction temperature was lower than 450 °C, no C3H6 was detected. At higher temperature, the selectivity of C3H6 was kept at about 3%. Effect of Residence Time. In this study, the catalyst was fixed in the reactor as a catalyst bed. The effect of residence time on the production of pyridines was also investigated in the range of 2.2 to 8.8 s by fixing the ammonia flow rate at about 200 mL/min and changing the catalyst amount from 3 to 12 g. The PLA feeding was 2 g for each test. Table 5 shows the effect

Figure 2. Products distributions of at 500 °C in recycle catalytic runs. (reaction conditions: reaction temperature, 500 °C, catalyst, HZSM-5, Si/Al = 25; NH3 flow rate, 200 mL/min, residence time, 5.9 s; PLA fed in, 2 g).

Table 5. Product Distribution As a Factor of Residence Time

significantly. After six runs, the yield of pyridines was about 21.1%, which slightly decreased compared with the first run. The carbon yield of coke decreased from 22.6% to 17.5%. Meanwhile, the yield of gas and oxygenated compounds increased from 31.9% and 2.1% to 39.7%, and 4.5%, respectively. The variation in the yield of aromatics was not obvious, and the yield of aromatics kept at around 5%. These results suggested that HZSM-5 (Si/Al = 25) was stable for the production of pyridines in the TCC-A of PLA process. Possible Reaction Pathway From PLA to Pyridines. In our previous studies, it was found that pyrroles and indoles could be selectively produced from furan, methylfuran, and furfural, while pyridines could be produced from glycerol via the TCC-A process. However, no furans, pyrroles or indoles was produced during the TCC-A of PLA process. It indicated that PLA conversion did not go through the furan to indole pathway. Pyridines were the main products from TCC-A of PLA, which was similar to TCC-A of glycerol. Thus, the pathway from PLA to pyridines might be overlapped with that from glycerol to pyridines. On the basis of the previous study from glycerol to pyridines, glycerol initially underwent a dehydration process to form acrolein. At the same time, glycerol also underwent a cracking process to form some other oxygenated compounds as byproducts, including acetaldehyde, acetol, acetone, etc. Then, these oxygenated compounds reacted with ammonia to form imine compounds, which were the intermediates in the pyridines production process. Finally, the imine compounds could be further converted to pyridines via the catalytic condensation reaction. To study the possible reaction pathway from PLA to pyridines in the TCC-A process, a series of experiments, such as TCC-A of different oxygenated compounds that could form in the PLA thermal decomposition process, were carried out in this study. As shown in Table 2 in the section of thermal decomposition behavior of PLA under N2 atmosphere, lactic acid, lactimide, acetone, acetaldehyde, acrylic acid and 2,3-pentanedione were the main products in the PLA thermal decomposition process, which might be the

yield (%) entry

residence time (s)

coke

gas

pyridines

aromatics

oxygenated compounds

1 2 3 4 5

2.2 3.6 5.9 7.3 8.8

19.3 21.5 22.6 23.2 24.7

31.4 31.7 31.9 33.6 36.1

15.3 17.5 24.8 23.2 19.2

3.7 4.3 6.9 7.1 7.4

9.8 6.5 2.1 1.4 N.D.

Reaction conditions: catalyst, HZSM-5, Si/Al = 25; reaction temperature, 500 °C; NH3 flow rate, 200 mL/min, PLA fed in, 2 g.

of residence time on the product distribution of TCC-A of PLA. When the residence time was 2.2 s, the carbon yields of pyridines and aromatics were only about 15.3% and 3.7%, respectively, whereas the carbon yield of oxygenated compounds was up to 9.8%. The shorter residence time could cause the reaction time of feedstock stream over catalyst decreased, which made the reaction from polylactic acid to pyridines insufficient. With the residence time increasing, the carbon yield of oxygenated compounds decreased, while the carbon yield of aromatics, coke and gas increased. When the residence time was 5.9 s, the carbon yield of pyridines was highest, reached about 24.8%. When the residence time was up to 8.8 s, oxygenated compounds could not be detected in the liquid products. Meanwhile, the carbon yields of aromatics, coke and gas were 7.4%, 24.7% and 36.1%, respectively. The carbon yield of pyridines was lower than that when the residence time was 5.9 s, which was about 19.2%. Therefore, the optimal residence time for producing pyridines from PLA was about 5.9 s, and the yield of pyridines was about 24.8%. The conversion of NH3 was about 5% in this process, which was estimated based on the flow rate of NH3 feed in and the N content in the Nheterocycles, biochar and ammonium carbonate. Catalyst Regeneration. To study further the stability of the catalyst during the TCC-A of PLA process, five reaction− E

DOI: 10.1021/acssuschemeng.5b01178 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Table 6. Summary of TCC-A of Different Oxygenated Compounds for the Mechanism Studya feedstocks overall carbon yield (C %) coke gas pyridines aromatics other pyridines selectivity (%) pyridine 2-methylpyridine 3-methylpyridine 4-methylpyridine other alkylpyridinesd

entry 1b

entry 2c

entry 3c

entry 4c

entry 5c

entry 6c

entry 7c

PLA

LA

lactimide

acetaldehyde

acetone

2,3-pentanedione

acrylic acid

22.6 31.9 24.8 6.9 2.1

13.2 50.5 24.8 4.9

15.6 51.6 23.7 6.4

9.4 54.1 32.6 3.8

10.8 55.3 26.4 4.4

11.5 49.7 1.7 22.5

10.1 58.3 2.2 20.5

40.4 26.1 4.9 15.4 13.2

47.3 20.9 7.4 11.3 13.1

41.2 18.6 13.2 15.6 11.4

7.6 65.3 1.4 25.7