Research Article pubs.acs.org/journal/ascecg
Catalytic Copyrolysis of Cellulose and Thermoplastics over HZSM‑5 and HY Beom-Sik Kim,† Young-Min Kim,†,‡ Hyung Won Lee,† Jungho Jae,§,∥ Do Heui Kim,⊥ Sang-Chul Jung,# Chuichi Watanabe,‡ and Young-Kwon Park*,† †
School of Environmental Engineering, University of Seoul, Seoul 02504, Republic of Korea Frontier Laboratories Ltd., 1-8-14, Saikon, Koriyama, Fukushima 963-8862, Japan § Clean Energy Research Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea ∥ Department of Clean Energy and Chemical Engineering, Korea University of Science and Technology, Daejeon 34113, Republic of Korea ⊥ School of Chemical and Biological Engineering, Seoul National University, Seoul 08826, Republic of Korea # Department of Environmental Engineering, Sunchon National University, Suncheon 57922, Republic of Korea ‡
S Supporting Information *
ABSTRACT: Catalytic pyrolysis with HZSM-5 is a promising method for the production of renewable aromatic hydrocarbons directly from biomass, even though the aromatic yields are still very low. Recent studies have shown that cofeeding of biomass with plastic significantly improves the aromatic yield due to high hydrogen content in plastic. In an effort to determine the influence of the zeolite pore size and the molecular diameter of cofeeding plastic on the aromatic production, catalytic copyrolysis of cellulose and thermoplastics, including random polypropylene (PP) and linear low density polyethylene (LLDPE) was conducted over HZSM-5 and HY catalysts. Thermogravimetric (TG) results showed that maximum decomposition temperature of PP was shifted to the higher temperature when PP was copyrolyzed with cellulose over HZSM-5 because the diffusion of PP molecules was hindered by the cellulose-derived coke and char. This hindering effect was attenuated by employing LLDPE as the cofeeding plastic due to its smaller molecular diameter than PP, and/or applying HY due to its larger pore size than HZSM-5. Heart-cut-evolved gas analysis (EGA)-GC/MS and flash pyrolysis-GC/FID were used to monitor the detailed product distribution and yields. The synergistic aromatic formation was easily achieved over HY catalyst for both PP and LLDPE, demonstrating the effectiveness of the larger pore zeolite for the catalytic copyrolysis. In contrast, HZSM-5 was very effective for the enhancement of aromatic production under severe reaction conditions, such as high catalyst to feed ratio (i.e., 10:1) or high pyrolysis temperature (i.e., 600 °C). KEYWORDS: Catalytic copyrolysis, Cellulose, PP, LLDPE, HZSM-5, HY
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INTRODUCTION
from biomass, it cannot be used directly as a fuel because of its low quality due to thermal instability, high oxygen content, high viscosity, and high acidity.5 Therefore, many researchers have investigated the catalytic pyrolysis of biomass using various catalysts to produce a high quality bio-oil, mainly composed of aromatic hydrocarbons, such as benzene, toluene, and xylenes, which can be used for petrochemical feedstocks.6−12 An effective hydrogen to carbon (H/Ceff) ratio of feedstock was introduced as an important parameter for an evaluation of
The value of biomass as a renewable energy source is growing rapidly together with worldwide concerns regarding the limited fossil fuel reserves and global warming. Recently, the Intergovernmental Panel on Climate Change (IPCC) also reported that a large amount of greenhouse gas emission can be reduced by applying biomass to coal-fired power stations.1 The recent target of using biomass as a renewable energy source is to produce transportation fuels and/or petrochemical feedstocks, such as aromatics and olefins, via thermochemical conversion processes, such as pyrolysis.2,3 The pyrolysis of biomass, i.e., the thermal degradation of biomass in the absence of oxygen, can produce a liquid fuel, called a bio-oil.4 Although bio-oil is being considered as the most promising fuel derived © XXXX American Chemical Society
Received: October 27, 2015 Revised: January 12, 2016
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DOI: 10.1021/acssuschemeng.5b01381 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering aromatic hydrocarbon production via catalytic pyrolysis.9,13−18 The H/Ceff ratio is defined as H/Ceff = (H − 2O − 3N − 2S)/C
copyrolysis. Cellulose was selected as the model biomass, and two kinds of thermoplastic, random PP and LLDPE, were applied as the cofeeding polymers because PP and PE occupy quite a large portion of waste plastics and have different molecular diameters. For catalytic copyrolysis, HZSM-5 and HY were selected as catalysts because these two catalysts are the representative medium (HZSM-5) and large pore (HY) zeolites, having different pore sizes. In addition, the effects of the catalyst to reactant ratio and the reaction temperature were investigated to determine the optimal reaction conditions for maximizing the aromatic yield from the isothermal fast catalytic copyrolysis of cellulose and thermoplastics.
(1)
where C, H, O, N, and S are the moles of carbon, hydrogen, oxygen, nitrogen, and sulfur, respectively.19 According to experimental reports, a pyrolysis sample with a high H/Ceff ratio produces larger amounts of olefins and aromatics, which indicates that biomass is not an efficient feedstock for aromatic hydrocarbon production because of its high oxygen content and low hydrogen content. To enhance the production of aromatic hydrocarbons from biomass, cofeeding hydrogen-rich feedstock, such as alcohols or plastics, which have high H/Ceff ratios, has been suggested for the catalytic pyrolysis of biomass.15−18,20 Zhang et al.15 reported that cofeeding alcohols could enhance the yield of petrochemicals from the catalytic pyrolysis of pine wood over HZSM-5 zeolite. Li et al.16,17 found that the production of aromatics from the catalytic pyrolysis of biomass was enhanced when a mixture of biomass and plastics was used as the reactants. Other researchers also reported that cofeeding plastics has a positive effect on the catalytic pyrolysis of biomass and provided a higher yield of aromatics compared to the biomass or plastic pyrolysis alone.18,20 They attributed the higher aromatic yield to the synergistic effects for the reaction between furanic compounds from biomass pyrolysis and olefins from plastic pyrolysis over zeolites during the catalytic copyrolysis of biomass and plastics. Some studies examined the mutual interactions between biomass and plastics during noncatalytic copyrolysis.21−24 Ç epelioğullar el al.21 pyrolyzed a mixture sample of various biomass and plastics, and reported that the copyrolysis of biomass with plastics caused significant changes not only in the thermal behaviors but also in the kinetic properties. Zhou et al.22 performed the copyrolysis of Chinese pine with LDPE, HDPE, and PP, and reported an interaction at high reaction temperatures. Although many studies have examined the thermal and catalytic copyrolysis of biomass−plastic mixtures, HZSM-5 has mainly been applied as a catalyst. The influence of zeolite pore size and molecular diameter of cofeeding plastic on the pyrolysis characteristic and aromatic production is not well understood. Recent studies on the catalytic pyrolysis of biomass reported that large pore zeolites such as HY and H-Beta are also effective for aromatic formation from bulky reactants such as lignin.25,26 Therefore, the relationship of the plastic molecular dimensions to zeolite pore size during catalytic copyrolysis must be investigated if improved zeolites are to be developed. In addition, most previous catalytic copyrolysis research has been conducted at high catalyst to reactant ratios (e.g., 10:1) to maximize aromatic production, even though a relatively high catalyst cost is required. Hence, it is necessary to employ the reaction condition of low catalyst to reactant ratios to reduce the cost of the catalyst. The objective of this paper is to study the role of zeolite pore size, molecular diameter of cofeeding plastic, reaction temperature, and catalyst to reactant ratio on pyrolysis behaviors and aromatic production during catalytic copyrolysis. For this purpose, thermogravimetric analysis and analytical pyrolysis system were employed to compare the detailed pyrolysis behaviors of the thermal and catalytic copyrolysis. Especially, heart-cut-evolved gas analysis-GC/MS (EGA-GC/MS) mode, which can provide detailed product distributions at each temperature region, was carried out for the first time in catalytic
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EXPERIMENTAL SECTION
Materials. Commercial cellulose powder (430 °C), which were not detected during the pyrolysis of LLDPE alone over HZSM-5 and HY. Table 3 lists the solid residues in the residual char and cokedeposited catalyst after TGA of each sample. The thermal and catalytic pyrolysis of cellulose with a low H/Ceff ratio produced larger amounts of solid residue, whereas those of plastics with a high H/Ceff ratio produced very low amounts of solid residue. By the cofeeding of hydrogen-rich plastics into the catalytic pyrolysis of cellulose, the formation of solid residues was reduced significantly to amounts smaller than the average solid residue amounts of the individual polymers and cellulose. This suggests that smaller amounts of coke were deposited during the catalytic copyrolysis of MCP and MCL. Some studies also reported that coke formation can be decreased by increasing the H/Ceff ratio of the pyrolysis feedstock.13−19 Between the two catalysts, HY produced larger amounts of solid residue than those over HZSM-5 during the catalytic pyrolysis reactions. As shown in the DTG results (Figure 2) for the catalytic copyrolysis of cellulose and plastic mixture sample, the cellulose decomposed at a lower temperature than the plastics and the peak temperatures of the plastics were changed. It can be considered that the temperature change of the PP DTG curve is caused by the remained char and deposited coke on the E
DOI: 10.1021/acssuschemeng.5b01381 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
HY, suggesting that HZSM-5 is more effective than HY on the aromatization reactions. During the catalytic pyrolysis of cellulose, levoglucosan was initially converted to smaller molecules, such as furans, aldehydes, and other acids via dehydration, decarbonylation, and decarboxylation on the external surface acid sites of the catalysts. These small molecules could then diffuse into the pores of the catalysts for further reactions. In this aspect, HY can be a more effective catalyst than HZSM-5 due to the larger pore size, in which the reaction intermediates can easily diffuse into the pores of the catalyst. On the other hand, it was reported that the formation of aromatics from cellulose-derived oxygenates over a zeolite catalyst proceeds through a “hydrocarbon pool”, in which the active organic species formed inside the zeolite pores act as a cocatalyst and the pore size of HZSM-5 is ideal for the production of active hydrocarbon pool species.5,35,39 Therefore, HZSM-5 had better activity than HY for aromatic production in the catalytic pyrolysis of cellulose. HZSM-5 was reported to show the highest catalytic activity for the production of aromatic hydrocarbons from biomass.40−43 Several studies have examined the reaction pathway for the conversion of furans to aromatics over HZSM-5. Diels−Alder condensation between furans and olefins formed through a hydrocarbon pool inside the catalyst pore was reported to be an important route for aromatic formation.15−17,35,44 Cheng and Huber44,45 reported that furan initially formed oligomers on HZSM-5 at low temperatures and began to form aromatics via dehydration, decarbonylation, Diels−Alder condensation, etc. at temperatures higher than 400 °C according to temperatureprogrammed analysis. Therefore, as shown in Table S1, aromatic hydrocarbons were detected mainly in the second temperature zone when the catalysts were applied. The formation of aromatic hydrocarbons was promoted at temperatures higher than 355 °C over both HZSM-5 and HY catalysts, which is consistent with previous reports.11,15,46 Thermal and Catalytic Pyrolysis of PP and LLDPE. In the thermal pyrolysis of PP and LLDPE, large amounts of aliphatic hydrocarbons were produced (Tables S2 and S3) by random radical reactions consisting of initiation, propagation and termination steps.47 These aliphatic hydrocarbons were cracked to light hydrocarbons (440 °C). This suggests that the formation of aromatics is restricted at low temperatures and occurs at a higher temperature zone than that of PP pyrolysis alone. This is in accordance with the temperature shift of the PP DTG curve for the copyrolysis of MCP over HZSM-5 (Figure 2d). Aromatic formation from MCP copyrolysis over HY occurred mainly at the second temperature zone (376−490 °C), which is consistent with the DTG result of MCP copyrolysis over HY. The yield of total aromatic hydrocarbons from MCP copyrolysis over HY was significantly higher than that over HZSM-5 because of the higher diffusion efficiency of reactants in the HY with a larger pore size and cavities. On the other hand, larger amounts of benzene, toluene and small olefins (C3−C4) were produced from the catalytic copyrolysis of MCP over HZSM-5 because of the strong acidity of HZSM-5. The catalytic copyrolysis of MCL produced large amounts of aliphatic hydrocarbons (
