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Nano-ZSM-5 to produce aromatics: Influence of catalyst properties
Journal Pre-proofs Catalytic pyrolysis of biomass wastes over Org-CaO/Nano-ZSM-5 to produce aromatics: Influence of catalyst properties Linlin Yi, Huan Liu, Sihan Li, Meiyong Li, Geyi Wang, Gaozhi Man, Hong Yao PII: DOI: Reference:
S0960-8524(19)31416-6 https://doi.org/10.1016/j.biortech.2019.122186 BITE 122186
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
5 September 2019 18 September 2019 20 September 2019
Please cite this article as: Yi, L., Liu, H., Li, S., Li, M., Wang, G., Man, G., Yao, H., Catalytic pyrolysis of biomass wastes over Org-CaO/Nano-ZSM-5 to produce aromatics: Influence of catalyst properties, Bioresource Technology (2019), doi: https://doi.org/10.1016/j.biortech.2019.122186
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Catalytic pyrolysis of biomass wastes over OrgCaO/Nano-ZSM-5 to produce aromatics: Influence of catalyst properties Linlin Yia, Huan Liua, b *, Sihan Lia, Meiyong Lia, Geyi Wanga, Gaozhi Mana, Hong Yaoa aState
Key Laboratory of Coal Combustion, School of Energy and Power Engineering,
Huazhong University of Science and Technology, Wuhan, 430074, China bDepartment
of New Energy Science and Engineering, School of Energy and Power
Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
Corresponding author Tel & Fax: +86-27-87542417 E-mail: [email protected]
Abstract Catalytic pyrolysis of biomass wastes is a promising way to produce aromatics. Based on the conventional CaO/ZSM-5 system, this study aims to improve the aromatics yields by developing an upgraded system which combined nanosized ZSM5 with CaO from organic calcium precursors (Org-CaO/NZSM-5). The results showed that the aromatics proportion in bio-oil from pyrolysis of Jatropha de-oiled cake with Org-CaO/NZSM-5 increased to 93%. The corresponding yield of BTX (benzene, toluene and xylene) and naphthalene was around 70% which was almost twice than that with conventional CaO/ZSM-5. Org-CaO showed better thermal cracking ability and deoxygenation with more hydrocarbons intermediates and higher H/Ceff of vapors, contributing to enhanced formation of BTX over NZSM-5. NZSM-5 contained much more mesopores and acid sites on external surface, promoting higher conversion of bulky molecules into naphthalenes. Compared with peanut shell, corncob and bagasse pyrolysis, Jatropha de-oiled cake pyrolysis produced much higher aromatics with OrgCaO/NZSM-5. Keywords: CaO/ZSM-5, Aromatics, Pyrolysis, Bio-oil, Organic calcium precursors
1. Introduction Recently, biomass wastes, as a renewable resource, have been regarded as a potential substitute for fossil fuel since its huge production and high organic content. Catalytic fast pyrolysis (CFP) can achieve effective conversion of biomass waste to high value-added chemicals such as aromatics (Du et al., 2013; Li et al., 2017; Zhang et al., 2015). Aromatics have huge market demand and high economic value, especially for BTX (benzene, toluene and xylene) and naphthalene as the important feedstocks for the production of polycarbonates, polyurethanes and polyethylene terephthalate (Carlson et al., 2009; Cheng et al., 2012). Thus, the conversion of biomass waste to aromatics via CFP technology is considered as a promising and high-efficiency method for the resource utilization. Among various catalysts, ZSM-5 is much suitable for the formation of aromatics because that the pore size of ZSM-5 is about equal to the dynamic diameter of BTX, and ZSM-5 have good shape selectivity for BTX (Carlson et al., 2009; Che et al., 2019; Rahman et al., 2018; Stefanidis et al., 2011). However, the polymerization of large compounds in vapors is easy to cause the formation of coke on external surface of ZSM-5, thus leading to the deactivation and relatively low catalytic efficiency of ZSM5 during catalytic upgrading of bio-oil. Fortunately, it was found that the combined use of basic CaO catalyst and acid ZSM-5 catalyst showed relatively higher catalytic efficiency for hydrocarbons production and lower coke formation during in-situ upgrading of bio-oil than single ZSM-5 catalysis. This is mainly because CaO have excellent ability to deoxygenate bio-
oil and previously promoted the thermal cracking of bulky substances into smallmolecule substances (Che et al., 2019; Ding et al., 2018; Liu et al., 2016; Wang et al., 2017; Yang et al., 2019). Notably, the deoxygenation of primary pyrolytic vapors over CaO has a major influence on the formation of aromatics over ZSM-5 during catalytic pyrolysis. The reduction of oxygen causes the increase of H/Ceff for pyrolytic vapors and bio-oil with higher H/Ceff can produce more aromatics over ZSM-5 catalyst (Che et al., 2019; Vispute et al., 2010; Zhang et al., 2011). Thereby, CaO conducted pretreatment on primary pyrolytic vapors by deoxygenation prior to upgrading of bio-oil over ZSM-5. However, compared with the traditional production industry from petroleum, the aromatics yield obtained from catalytic pyrolysis of biomass over CaO/ZSM-5 was still lower (Manzano et al., 2017; Roma-Torres et al., 2006). Besides, catalysts deactivation during catalytic pyrolysis of biomass over CaO/ZSM-5 existed mainly owing to the coverage of active sites by coke deposition (Ding et al., 2018; Shankar and Jambulingam, 2017; Zhang et al., 2017). Thus, it is still necessary to improve the catalytic performance of CaO/ZSM-5 during catalytic pyrolysis of biomass for the production of aromatics. The first key is to enhance the deoxygenation ability of CaO during catalytic pyrolysis of biomass over CaO/ZSM-5. In our previous study (Yi et al., 2019), we have found that CaO derived from organic calcium precursors exhibited better deoxygenation ability than conventional CaO during upgrading of bio-oil and can be considered as a potential source with longer lifetime. The second key is to improve the catalytic
performance of ZSM-5 existed in CaO/ZSM-5 catalytic system. Previous works claimed that the particle size of ZSM-5 have critical influences on its catalytic performance. Toshihide Baba et al. found that the rate of ethylene conversion increased with decreasing particle size of ZSM-5(Reddy et al., 2012). Nanosized HZSM-5 showed higher activity and stability compared to the micro-sized HZSM-5 on the selectivity of methanol to propylene reaction (Kim et al., 2018; Zheng et al., 2014). However, during in-situ upgrading of bio-oil, there were thousands of organics in pyrolytic vapors to react with ZSM-5 rather than relatively pure reactant mentioned above. And the effects of nanosized ZSM-5 on the production of aromatics during CFP of various biomass waste also have not yet been systematically studied. Simultaneously, it is not sure whether the Org-CaO and proposed Org-CaO/NZSM-5 catalytic system can achieve better selectivity for aromatics like BTX and naphthalene during in-situ upgrading of bio-oil. Therefore, the purpose of this paper is to develop an improved dual catalytic system for increasing the production of aromatics during catalytic pyrolysis of biomass wastes. CFP of some typical biomass wastes over dual catalytic beds of CaO derived from organic precursors (Org-CaO) and nanosized ZSM-5 (NZSM-5) were conducted to verify the feasibility and suitability of the proposed Org-CaO/NZSM-5 system. In order to deeply investigate the influences of catalysts properties, some systematical experiments were employed. On the one hand, different catalysts proportions were applied to explore better catalytic conditions. On the other hand, it is also significant to
make a deeper insight into the roles of Org-CaO and NZSM-5 in effects of OrgCaO/NZSM-5 system on the aromatics formation during in-situ upgrading of bio-oil.
2. Experimental 2.1 Material and methods In this pyrolysis experiments, Jatropha seeds de-oil cake (called JS cake), peanut shell, corncob and bagasse were chosen as the representatives of biomass. Peanut shell, corncob and bagasse were conventional agricultural wastes. The Jatropha seeds de-oil cake is the residue obtained after the mechanical extraction of oil from the Jatropha seeds which are identified as typical energy crop. The ultimate and proximate analysis of these four biomass wastes were listed in Table 1. In our previous study (Yi et al., 2019), it was found that CaO derived from calcium D-gluconate monohydrate showed the best deoxygenation ability among CaOs decomposed from some organic calcium precursors (called Org-CaO). Therefore, in this study, two types of CaO catalysts were used in the upgrading of bio-oil experiments: (a) the conventional CaO was prepared from calcium hydroxide (called for CH-CaO); and (b) CaO derived from calcium D-gluconate monohydrate was chosen as a representative of Org-CaO. All CaO catalysts were prepared by the same method: the calcium precursor (calcium D-gluconat monohydrate) was calcined in muffle furnace at the heating rate of 30 °C/min from room temperature to 900 °C and then at 900 ℃ for 0.5 h. Two commercial ZSM-5 zeolites with different particle size in this experimental
were the nanosized ZSM-5 (NZSM-5) and the micro-sized ZSM-5 (MZSM-5). Both of them were purchased from the catalyst factory of Nankai University. MZSM-5 is generally used as the conventional ZSM-5. The particle size of MZSM-5 is around 5 μm and the particle size of NZSM-5 is around 100 nm. In order to remove the adsorbed water and water of crystallization, MZSM-5 and NZSM-5 were calcined in air at 550 oC for 4 h prior to experiments and the Si/Al ratio of both two ZSM-5 catalysts is 25. Overall, various CaO/ZSM-5 catalytic systems used in this study were introduced in Table 2. CH-CaO/MZSM-5 system was applied as the conventional CaO/ZSM-5 system. CH-CaO/NZSM-5 system and Org-CaO/MZSM-5 system were applied as the control groups to respectively explore the roles of Org-CaO and NZSM-5 in the effects of synergistic catalysis of Org-CaO/NZSM-5. 2.2 Experiment and experimental procedure Catalytic pyrolysis of biowaste was conducted in a dual-catalyst fixed-bed reactor system with a lab-scale continuous feeder. A schematic diagram of this self-designed experimental apparatus was depicted in Fig.1. The effects of catalysts proportions on product distribution and chemical components were investigated. These JS cake/OrgCaO/NZSM-5 ratios (1:0:3, 1:0.5:3, 1:1:3, 1:3:3, and 1:5:3) were to study the effects of Org-CaO proportions, and these JS cake/Org-CaO/NZSM-5 ratios (1:1:0, 1:1:0.5, 1:1:1, 1:1:3, and 1:1:5) were to study effects of NZSM-5 proportions. The quartz reactor was followed by a twin-chamber condenser which condensed bio-oil by liquid nitrogen. Prior to pyrolysis, the CaO and ZSM-5 catalysts were
respectively filled in the upper and lower quartz cradles. Then, the system was flushed with pure N2 (99.99%) at a flow rate of 200 mL/min for 30 min to ensure an inert atmosphere and two reaction zones were heated at desired temperature. The catalytic temperature of CaO layer is 500 ℃ and the catalytic temperature of ZSM-5 layer is 550 ℃. In all pyrolysis runs, the biomass particles were fallen into reactor from the continuous feeding device as 0.1 g/min. 2.3. Products analysis and catalyst characterization Compositions in bio-oil were qualitatively and semi-quantitatively detected by GCMS (Angilent 7890A/5975C, capillary chromatographic HP-5MS). Helium (99.9999%) is used as the carrier gas with 1.0 mL/min. The injector temperature of GC was 250 oC, and the split ratio of the carrier gas was 1:10. The oven temperature program as follow: the temperature was firstly held at 50 oC for 3 min, and the temperature increased from 50 oC to 320 oC at 5 oC/min. Nitrogen adsorption/desorption isotherms at -196 oC were recorded using a Quantachrome Autosorb iQ instrument. Before the measurements, the samples were heated to 200 oC in avacuum for at least 12 h. The Brunauer Emmett Teller (BET) equation was used to calculate the specific surface area. The size distribution was derived from the adsorption branch by using the Barrett Joyner Halenda (BJH) method.
3. Results and discussion 3.1 Effect of Org-CaO/NZSM-5 on aromatics production
To investigate whether the Org-CaO/NZSM-5 system having heightened selectivity toward aromatics compared with conventional CaO/ZSM-5 system, Figure 2(a) exhibited the effect of Org-CaO/NZSM-5 on the distribution of predominant products in bio-oil obtained from the catalytic pyrolysis of JS cake. CH-CaO/MZSM-5 system was applied as the conventional CaO/ZSM-5 system in general works. The mass ratio (biomass/CaO/NZSM-5) in this section were all 1:1:3. For raw bio-oil without catalyst, the total content of aromatic hydrocarbons was lower than 20% and the content of BTX and naphthalene was little. Interestingly, the proportion of aromatic hydrocarbons in bio-oil obtained from pyrolysis of JC cake with Org-CaO/NZSM-5 is up to 93% which is much more than that by CH-CaO/MZSM-5. The relative content of BTX in bio-oil by CH-CaO/MZSM-5 were just around 26%, whereas the relative content of BTX in bio-oil by Org-CaO/NZSM-5 is around 54% which is almost twice as that by CH-CaO/MZSM-5. Besides, Org-CaO/NZSM-5 strongly promoted the formation of naphthalenes mainly as naphthalene and methylnaphthalene, likely because Org-CaO/NZSM-5 catalyzed more conversion of oxygenated compounds especially for acids into aromatics especially for BTX and naphthalenes. According to our previous work, Org-CaO catalysts have stronger basic strength and more basic sites than CHCaO catalyst. The stronger basicity of Org-CaO has strong potential in improving OrgCaO deoxygenation ability and cracking capability compared with CH-CaO (Yi et al., 2019). This probably improved the synergistic catalytic performance of OrgCaO/NZSM-5 and enhanced the selectivity for aromatics and BTX. Comparing with
CH-CaO/MZSM-5, the better catalytic performance of Org-CaO/NZSM-5 system was attributed to both effects of Org-CaO and NZSM-5. However, this section is difficult to separate their respective contributions to this upgraded system. Thus, the specific experiments were designed to ascertain the effects of Org-CaO and NZSM-5 in 3.2 and 3.3 section. Different Org-CaO and NZSM-5 proportions in Org-CaO/NZSM-5 system were respectively applied to explore the effects of two catalysts proportions and find better catalytic conditions. Figure 2(b) shows the products distribution of aromatics from CFP of JC cake with varying Org-CaO proportions or NZSM-5 proportions. As the increase in NZSM-5 catalyst, the proportion of aromatics was promoted obviously from 20.9% with only Org-CaO to 46.7% with biomass/Org-CaO/NZSM-5 at 1:1:1. The increase of NZSM-5 promoted more formation of BTX and naphthalenes when the proportion of NZSM-5 in Org-CaO/NZSM-5 system is relatively low. But there was no significant increase in the relative content of BTX when further increasing the proportion of NZSM-5 to 1:1:3, indicating that the satisfying production should be obtained as ZSM-5 proportion varying from 1:1:1 to 1:1:3. However, the BTX yield decreased with increasing bulky monocyclic aromatic hydrocarbons (MAHs) when the NZSM-5 proportion increased to 1:1:5, probably because an excess of NZSM-5 could prolong the residence time of vapors in NZSM-5 pores and boost the polymerization of organics. According to Fig. 2(b), low proportions of Org-CaO (1:0.5:3) promoted the conversion into naphthalene over NZSM-5. With increasing CaO proportions, more BTX were
formed over NZSM-5. When the ratio of CaO to NZSM-5 was 1:1 (including 3:3), the maximum yield of BTX could be obtained, which might be as a consequence of the optimal catalytic performance of dual catalytic beds system. The synergistic catalysis of Org-CaO/NZSM-5 showed high selectivity for aromatics when the mass ratio of OrgCaO to NZSM-5 is 1:1. These phenomena reflected that the synergistic catalysis of OrgCaO/NZSM-5 could achieve a better selectivity for BTX during the CFP of biomass probably when Org-CaO/NZSM-5 system integrated and balanced the basicity and acidity. 3.2 Role of Org-CaO in aromatics production over Org-CaO/NZSM-5 In this section, the pyrolysis of four kinds of biomass waste with only NZSM-5, as well as two dual catalytic systems (CH-CaO/NZSM-5 and Org-CaO/NZSM-5) were conducted. Figure 3 showed the effects of different catalytic systems on relative content of aromatic hydrocarbons in bio-oil. The pyrolysis with only NZSM-5 and CHCaO/NZSM-5 were applied as experimental control groups to the investigate the role of Org-CaO in effects of Org-CaO/NZSM-5 on aromatics formation during in-situ catalytic pyrolysis. According to the above results in 3.2 section, Org-CaO showed relatively stronger effects during synergistic catalysis of Org-CaO/NZSM-5 when the mass ratio (biomass/CaO/NZSM-5) was 1:3:3. Thereby, the mass ratio in this section were all 1:3:3. As shown from Fig.3, aromatic hydrocarbons were classified into four groups: BTX (benzene, toluene and xylene), naphthalene, methylnaphthalene and other aromatics. The relative content of BTX in bio-oil catalyzed by only NZSM-5 were just
around 27% and these aromatics were mainly as bulky benzene derivatives. The proportion of aromatic hydrocarbons in bio-oil obtained from pyrolysis of JC cake with Org-CaO/NZSM-5 was much more than that by CH-CaO/MZSM-5 and only NZSM-5. The relative content of BTX in bio-oil by CH-CaO/NZSM-5 were just 30%, whereas the relative content of BTX in bio-oil by Org-CaO/NZSM-5 is around 54% which is almost twice as that by CH-CaO/NZSM-5. In contrast, the bio-oil by Org-CaO/NZSM-5 contained less naphthalene and methyl-naphthalene than that by CH-CaO/NZSM-5, indicating that the pre-catalysis of Org-CaO suppressed the formation of polycyclic aromatic hydrocarbons over NZSM-5 compared with CH-CaO. Therefore, OrgCaO/NZSM-5 system has higher selectivity for aromatics and promoted higher conversion of bulky aromatic hydrocarbons and oxygenated compounds into BTX than CH-CaO/MZSM-5 system. Notably, both two CaO/ZSM-5 catalytic systems have not obviously positive effects on the formation of aromatic hydrocarbons compared with only NZSM-5 catalysis. Especially for corncob and bagasse, the relative content of aromatics and BTX in upgraded bio-oil catalyzed by CaO/NZSM-5 were even slightly less than that by only NZSM-5, which indicated that the addition of CaO slightly suppressed the formation of aromatics over NZSM-5 for corncob and bagasse. Apparently, primary pyrolytic vapors firstly underwent the CaO catalyst bed to form some intermediates and then these intermediates further reacted with ZSM-5 were converted to BTX and other aromatics. Thus, it is very important for the effects of CaO pre-catalysis on the formation mechanism of aromatics over NZSM-5 during in-situ
upgrading of bio-oil. In order to further investigate the effects of CaO pre-catalysis on the formation of intermediate products during dual catalytic pyrolysis, the experiments of CFP of biomass with only Org-CaO were conducted. Figure 4 (a) exhibited the relative content of oxygenated compounds and hydrocarbons in bio-oil without and with Org-CaO catalyst. Raw bio-oil mainly contained oxygenated compounds that accounted for 6883%. It is worth to notice that Org-CaO showed excellent deoxygenation activity during JS cake pyrolysis. The addition of Org-CaO effectively facilitated the conversion of oxygenated compounds to hydrocarbons and the amounts of oxygenated compounds decreased from 83% in raw bio-oil to 18% in upgraded bio-oil by Org-CaO. But for pyrolysis of corncob and peanut shell with Org-CaO, the proportion of oxygenated compounds slightly decreased. The proportion of oxygenated compounds in bio-oil from the pyrolysis of bagasse with Org-CaO even increased a little. These phenomena imply that Org-CaO have little effects on the removal of oxygenated compounds from pyrolysis of peanut shell, corncob and bagasse. These differences of deoxygenation process were consistent with the mentioned above differences about BTX yields, which probably was resulted from the different characteristics of bio-oil from four biomass. To make a deeper insight, the proportions of main oxygenated compounds in raw bio-oil and the difference values of the main oxygenated compounds in bio-oil after CaO catalyzing were analyzed as shown in Fig.4(b). It is need to explain that the difference values of the predominant oxygenated compounds were obtained from the
proportion in CaO bio-oil minus the proportion in raw bio-oil. Firstly, there is an obvious difference for characteristic products in raw bio-oil from pyrolysis of four biomass. The characteristic products in raw bio-oil obtained from pyrolysis of JS de-oil cake were mostly fatty acids, like oleic acid, stearic acid and palmitic acid. Whereas, the raw bio-oil of peanut shell, corncob and bagasse contained much furans and phenols which respectively accounted for 4%-20% and 43%-56%. These phenols mainly were composed of methoxy phenols and methoxyphenols, dimethoxyphenol. It was reported that methoxy phenols were the most characteristic products of lignin pyrolysis and furans were the most characteristic products of cellulose/hemicellulose pyrolysis (Mohan et al., 2006; Patwardhan et al., 2011; Vinu and Broadbelt, 2012). Likewise, Org-CaO also showed selective deoxygenation during catalytic pyrolysis of various biomass waste. According to Fig.4(b), Org-CaO enhanced the removal of fatty acids in JC cake bio-oil, whereas it has little effects on the removal of furans and phenols. It was reported that pyrolytic vapors with higher H/Ceff can produce more aromatics over ZSM-5 catalysts (Vispute et al., 2010; Zhang et al., 2011). The effective hydrogen to carbon ratio (H/Ceff) were defined as following Eq.1: H/Ceff =
𝑛(H) ― 2𝑛(o) 𝑛(𝑐)
(1)
Org-CaO showed good deoxygenation via dehydration, decarboxylation, alkylation, and ketonization to remove fatty acids and form more hydrocarbons and low-oxygen compounds like ketones and alcohols (Yi et al., 2019). But Org-CaO hardly promoted the conversion of phenols and furans to hydrocarbons. Therefore, the H/Ceff in
vapors after CaO catalyzing obtained from JS cake pyrolysis were higher than that from pyrolysis of peanut shell, corncob and bagasse, which partly caused that OrgCaO/NZSM-5 exhibited higher selectivity for BTX during CFP of JS cake. Meanwhile, it also proved that the selectivity for aromatics of Org-CaO/NZSM-5 catalytic system is closely related with the biomass characteristics of these feedstocks. However, it is unknown for the in-depth mechanism of effects of biomass characteristics on the formation of aromatics during catalytic pyrolysis of biomass with Org-CaO/NZSM-5. Our next work will further systematically expound the formation pathways of BTX among CFP of biomass with different characteristics over Org-CaO/NZSM-5. 3.3 Role of NZSM-5 in aromatics production over Org-CaO/NZSM-5 The role of NZSM-5 in effects of Org-CaO/NZSM-5 system on the generation of aromatics was studied. Figure 5 illustrates the relative content of aromatics, BTX and naphthalenes (i.e., naphthalene derivatives) during catalytic pyrolysis of JS cake over the nano and MZSM-5 catalysts combined with two types of CaO. According to the above results, NZSM-5 can show relatively strong and obvious effects during synergistic catalysis of Org-CaO/NZSM-5 when the mass ratio (biomass/CaO/NZSM-5) was 1:1:3. Thereby, the mass ratio (biowaste: CaO: ZSM-5) in these experiments was uniform as 1:1:3 in order to preferably investigate the role of NZSM-5 in effects of OrgCaO/NZSM-5 system. As shown from Fig.5, the nanosized ZSM-5 combined with two CaO both showed better performance on the formation of aromatics compared to the MZSM-5. When CaO type is uniform, it is observed that NZSM-5 principally exhibited
higher selectivity to naphthalenes than MZSM-5. It was reported that diffusion resistance in pores increases with increasing the particle size of ZSM-5(Zheng et al., 2014). Notably, the proportion of aromatics and BTX with Org-CaO/NZSM-5 catalytic system is 20% and 10% more than that with the CH-CaO/MZSM-5 system. The effects of particle size on the physical characteristics of ZSM-5 were investigated by N2 adsorption method to further find the reasons of different effects for nanosized and micro-sized ZSM-5. There is a significant difference in the shape of isotherms of both two samples according to N2 adsorption/desorption isotherms for isotherms of NZSM-5 and MZSM-5 catalysts as shown in Figure 6(a). MZSM-5 exhibited type I isotherm which is characteristic of microporous material. Whereas, according to its hysteresis loop of N2 appears appearing at relatively high relative pressure, it can be known that NZSM-5 possesses both microporous and mesoporous because the accumulation of nanosized particles caused a large secondary accumulation mesopore volume (Jia et al., 2017; Su et al., 2016). BJH pore size distribution curves of two ZSM-5 catalysts given in Fig. 6(b) also indicate the presence of higher mesopore volume in NZSM-5. NZSM-5 contained much more mesopores of 20-500 Å diameter range and macropores of >500 Å diameter range than MZSM-5. During in-situ catalysis of pyrolytic vapors, bulky compounds cannot diffuse into the pores of MZSM-5 and even were condensed into coke deposited on the catalyst surface or blocked the pores, which rapidly caused deactivation of ZSM-5 and reduced the catalytic efficiency. In contrast, NZSM-5 with improved porosity firstly facilitated the diffusion of larger
molecules into mesopores/macropores, and subsequently enhanced more conversion of bulky compounds into BTX and naphthalene in pores. In addition, it is beneficial to slow down the polymerization of aromatics and olefins inside the micropores, and then relief catalyst deactivation resulting from the coke clogging the pore. The total pore volume of NZSM-5 and MZSM-5 illustrated in Table 3 were 0.29 cm3/g and 0.18 cm3/g, respectively. Besides, NZSM-5 possesses higher external surface area than MZSM-5, and then NZSM-5 can provide more acid sites. Higher external specific surface area and shorter pore passage of smaller crystallized NZSM-5 can enhance accessibility of active sites and shorten residence time of pyrolytic vapors, which relief the polymerization of organics and loss of acid sites covered by coke deposition. Thus, NZSM-5 catalyst with greater catalytic activity significantly promoted the formation of aromatics over the acid sites located on the external surface.
4. Conclusions During catalytic pyrolysis of biomass, Org-CaO/NZSM-5 promoted that the proportions of aromatics and BTX in bio-oil increased to 93% and 54% which were much more than that by CH-CaO/MZSM-5. The stronger pre-deoxygenation of OrgCaO made volatiles containing more intermediates with high H/Ceff, like hydrocarbons, low-oxygen alcohols and ketones. Besides, NZSM-5 with improved microporous/mesoporous also allowed the entrance of bulky compounds into pores. Thus, more BTX and naphthalene were generated under Org-CaO/NZSM-5 system. The
pyrolysis catalyzed by Org-CaO/NZSM-5 of Jatropha de-oiled cake provided much higher aromatics, especially for BTX, than that of peanut shell, corncob and bagasse.
Supplementary data Supplementary data associated with this article can be found in the online version.
Acknowledgments The authors are grateful to National Natural Science Foundation of China (No. 51661145010) for the financial supports. They also express thanks to the Analytical and Testing Center of Huazhong University of Science and Technology for the testing.
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microwave-assisted catalytic co-pyrolysis of corn stover and scum for bio-oil production with CaO and HZSM-5 as the catalyst. Bioresource Technol. 204, 164170. 11. Manzano, C.A., Marvin, C., Muir, D., Harner, T., Martin, J., Zhang, Y., 2017. Heterocyclic Aromatics in Petroleum Coke, Snow, Lake Sediments, and Air Samples from the Athabasca Oil Sands Region. Environ. Sci. Technol. 51, 54455453. 12. Mohan, D., Pittman, C.U., Steele, P.H., 2006. Pyrolysis of Wood/Biomass for Biooil: A Critical Review. Energ. Fuel. 20, 848-889. 13. Patwardhan, P.R., Dalluge, D.L., Shanks, B.H., Brown, R.C., 2011. Distinguishing primary and secondary reactions of cellulose pyrolysis. Bioresource Technol. 102, 5265-5269. 14. Rahman, M.M., Liu, R., Cai, J., 2018. Catalytic fast pyrolysis of biomass over zeolites for high quality bio-oil-A review. Fuel Process. Technol. 180, 32-46. 15. Reddy, J.K., Motokura, K., Koyama, T., Miyaji, A., Baba, T., 2012. Effect of morphology and particle size of ZSM-5 on catalytic performance for ethylene conversion and heptane cracking. J. Catal. 289, 53-61. 16. Roma-Torres, J., Teixeira, J.P., Silva, S., Laffon, B., Cunha, L.M., Méndez, J., Mayan, O., 2006. Evaluation of genotoxicity in a group of workers from a petroleum refinery aromatics plant. Mutation Research/Genetic Toxicology and Environmental Mutagenesis 604, 19-27.
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273, 77-85. 24. Yi, L., Liu, H., Xiao, K., Wang, G., Zhang, Q., Hu, H., Yao, H., 2019. In situ upgrading of bio-oil via CaO catalyst derived from organic precursors. P. Combust. Inst. 37, 3119-3126. 25. Yi, L., Liu, H., Xiao, K., Wang, G., Zhang, Q., Hu, H., Yao, H., 2019. In situ upgrading of bio-oil via CaO catalyst derived from organic precursors. P. Combust. Inst. 37, 3119-3126. 26. Zhang, B., Zhong, Z., Chen, P., Ruan, R., 2017. Microwave-assisted catalytic fast co-pyrolysis of Ageratina adenophora and kerogen with CaO and ZSM-5. J. Anal. Appl. Pyrol. 127, 246-257. 27. Zhang, B., Zhong, Z., Min, M., Ding, K., Xie, Q., Ruan, R., 2015. Catalytic fast copyrolysis of biomass and food waste to produce aromatics: Analytical Py–GC/MS study. Bioresource Technol. 189, 30-35. 28. Zhang, H., Cheng, Y., Vispute, T.P., Xiao, R., Huber, G.W., 2011. Catalytic conversion of biomass-derived feedstocks into olefins and aromatics with ZSM-5: the hydrogen to carbon effective ratio. Energ Environ Sci 4, 2297-2307. 29. Zheng, A., Zhao, Z., Chang, S., Huang, Z., Wu, H., Wang, X., He, F., Li, H., 2014. Effect of crystal size of ZSM-5 on the aromatic yield and selectivity from catalytic fast pyrolysis of biomass. Journal of Molecular Catalysis A: Chemical 383-384, 2330.
The BTX production by Org-CaO/NZSM-5 is almost twice as that by conventional CaO/ZSM-5.
The proportion of aromatics and BTX with Org-CaO/NZSM-5 were up to 93% and 54%.
Org-CaO contributed to BTX formation during dual catalytic pyrolysis.
NZSM-5 promoted the conversion of bulky molecules into naphthalenes.
Figure captions Fig. 1. Schematic diagram of in-situ upgrading of pyrolytic vapors during catalytic pyrolysis of biomass over various CaO/ZSM-5 systems Fig. 2. (a)Products distribution of main chemical compounds in bio-oil from CFP of JC cake; (b) Products distribution of aromatics from CFP of JC cake with varying Org-CaO proportions or NZSM-5 proportions. Fig. 3. Relative content of various aromatic hydrocarbons during catalytic pyrolysis of JS cake, peanut shell, corncob and bagasse Fig. 4. Relative content of compounds in bio-oil from pyrolysis of JS cake, peanut shell, corncob and bagasse (a) Yields of hydrocarbons (HCs) and oxygenated compounds (OCs);(b) Yields and difference value* of main oxygenated compounds respectively in raw bio-oil and in bio-oil catalyzed by Org-CaO. difference value*=the proportion of X in bio-oil catalyzed by Org-CaO - the proportion of X in raw bio-oil (X means acids, phenols or furans) Fig. 5. Relative content of aromatics, BTX and naphthalenes in bio-oil catalyzed by various dual catalytic system Fig. 6. (a) N2 adsorption/desorption isotherms for isotherms of Nano- and MicroZSM-5 catalysts; (b) BJH pore size distribution of Nano- and Micro- ZSM-5 catalysts.
Fig. 1. Schematic diagram of in-situ upgrading of pyrolytic vapors during catalytic pyrolysis of biomass over various CaO/ZSM-5 systems
( a)
( b)
Fig. 2. (a)Products distribution of main chemical compounds in bio-oil from CFP of JC cake; (b) Products distribution of aromatics from CFP of JC cake with varying Org-CaO proportions or NZSM-5 proportions.
Fig. 3. Relative content of various aromatic hydrocarbons during catalytic pyrolysis of JS cake, peanut shell, corncob and bagasse
( a)
( b)
Fig. 4. Relative content of compounds in bio-oil from pyrolysis of JS cake, peanut shell, corncob and bagasse (a) Yields of hydrocarbons (HCs) and oxygenated compounds (OCs);(b) Yields and difference value* of main oxygenated compounds respectively in raw bio-oil and in bio-oil catalyzed by Org-CaO. difference value*=the proportion of X in bio-oil catalyzed by Org-CaO - the proportion of X in raw bio-oil (X means acids, phenols or furans)
Fig. 5. Relative content of aromatics, BTX and naphthalenes in bio-oil catalyzed by various dual catalytic system
(
180
Nano ZSM-5 Micro ZSM-5
160
Volume adsorbed (cm3/g)
a)
140 120 100 80 60 40 20 0
-0.1 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
Relative pressure (P/Po) (
0.15 Micro ZSM-5 Nano ZSM-5
dV(logd) (cm3/g)
b) 0.10
0.05
0.00
1
10
100
Pore diameter (nm) Fig. 6. (a) N2 adsorption/desorption isotherms for isotherms of Nano- and MicroZSM-5 catalysts; (b) BJH pore size distribution of Nano- and Micro- ZSM-5 catalysts.
Table list Table 1. Ultimate analysis and proximate analysis of biomass wastes (wt.%) Table 2. Various CaO/ZSM-5 catalytic systems Table 3. Characteristics of NZSM-5 and MZSM-5
Table 1. Ultimate analysis and proximate analysis of biomass wastes (wt.%) C
Ultimate analysis H O N
Proximate analysis S
a
JS cake
4 5.3
Peanut shell
.2 4
9.7 Corncob
5
4
Bagasse
4
5
4
0
4
6
.7
sh
18.2
7
84.1
14.5
1 .4
0
86.9
11.8
.2 0
A
.3 0
0
4
73.5
.1
.4
5.8
0
Fixed carbon
.2
.6
7.0
.7
4 .5
3.7
.8
6.4
4 3.8
.8
6.6
a:
6
Volatile matter
1 .3
0
87.4
9.7
.4
2 .9
by difference Table 2. Various CaO/ZSM-5 catalytic systems Definition
Conventional CaO/ZSM-5 Control group Control group Improved CaO/ZSM-5
Label
CaO types
CHCaO/MZSM-5 CHCaO/NZSM-5 OrgCaO/MZSM-5 OrgCaO/NZSM-5
CaO from Ca(OH)2 CaO from Ca(OH)2 CaO from organic calcium precursor CaO from organic calcium precursor
ZSM-5 types Micro ZSM-5 Nano ZSM-5 Micro ZSM-5 Nano ZSM-5
Table 3. Characteristics of NZSM-5 and MZSM-5 Catalysts
BET (m2/g)
External
Total pore
Average
surface area
volume (mL/g)
pore volume
(m2/g)
(nm)
MZSM-5
262
56
0.19
2.08
NZSM-5
385
82
0.27
4.08
S0960-8524(19)31416-6 https://doi.org/10.1016/j.biortech.2019.122186 BITE 122186
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
5 September 2019 18 September 2019 20 September 2019
Please cite this article as: Yi, L., Liu, H., Li, S., Li, M., Wang, G., Man, G., Yao, H., Catalytic pyrolysis of biomass wastes over Org-CaO/Nano-ZSM-5 to produce aromatics: Influence of catalyst properties, Bioresource Technology (2019), doi: https://doi.org/10.1016/j.biortech.2019.122186
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Catalytic pyrolysis of biomass wastes over OrgCaO/Nano-ZSM-5 to produce aromatics: Influence of catalyst properties Linlin Yia, Huan Liua, b *, Sihan Lia, Meiyong Lia, Geyi Wanga, Gaozhi Mana, Hong Yaoa aState
Key Laboratory of Coal Combustion, School of Energy and Power Engineering,
Huazhong University of Science and Technology, Wuhan, 430074, China bDepartment
of New Energy Science and Engineering, School of Energy and Power
Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
Corresponding author Tel & Fax: +86-27-87542417 E-mail: [email protected]
Abstract Catalytic pyrolysis of biomass wastes is a promising way to produce aromatics. Based on the conventional CaO/ZSM-5 system, this study aims to improve the aromatics yields by developing an upgraded system which combined nanosized ZSM5 with CaO from organic calcium precursors (Org-CaO/NZSM-5). The results showed that the aromatics proportion in bio-oil from pyrolysis of Jatropha de-oiled cake with Org-CaO/NZSM-5 increased to 93%. The corresponding yield of BTX (benzene, toluene and xylene) and naphthalene was around 70% which was almost twice than that with conventional CaO/ZSM-5. Org-CaO showed better thermal cracking ability and deoxygenation with more hydrocarbons intermediates and higher H/Ceff of vapors, contributing to enhanced formation of BTX over NZSM-5. NZSM-5 contained much more mesopores and acid sites on external surface, promoting higher conversion of bulky molecules into naphthalenes. Compared with peanut shell, corncob and bagasse pyrolysis, Jatropha de-oiled cake pyrolysis produced much higher aromatics with OrgCaO/NZSM-5. Keywords: CaO/ZSM-5, Aromatics, Pyrolysis, Bio-oil, Organic calcium precursors
1. Introduction Recently, biomass wastes, as a renewable resource, have been regarded as a potential substitute for fossil fuel since its huge production and high organic content. Catalytic fast pyrolysis (CFP) can achieve effective conversion of biomass waste to high value-added chemicals such as aromatics (Du et al., 2013; Li et al., 2017; Zhang et al., 2015). Aromatics have huge market demand and high economic value, especially for BTX (benzene, toluene and xylene) and naphthalene as the important feedstocks for the production of polycarbonates, polyurethanes and polyethylene terephthalate (Carlson et al., 2009; Cheng et al., 2012). Thus, the conversion of biomass waste to aromatics via CFP technology is considered as a promising and high-efficiency method for the resource utilization. Among various catalysts, ZSM-5 is much suitable for the formation of aromatics because that the pore size of ZSM-5 is about equal to the dynamic diameter of BTX, and ZSM-5 have good shape selectivity for BTX (Carlson et al., 2009; Che et al., 2019; Rahman et al., 2018; Stefanidis et al., 2011). However, the polymerization of large compounds in vapors is easy to cause the formation of coke on external surface of ZSM-5, thus leading to the deactivation and relatively low catalytic efficiency of ZSM5 during catalytic upgrading of bio-oil. Fortunately, it was found that the combined use of basic CaO catalyst and acid ZSM-5 catalyst showed relatively higher catalytic efficiency for hydrocarbons production and lower coke formation during in-situ upgrading of bio-oil than single ZSM-5 catalysis. This is mainly because CaO have excellent ability to deoxygenate bio-
oil and previously promoted the thermal cracking of bulky substances into smallmolecule substances (Che et al., 2019; Ding et al., 2018; Liu et al., 2016; Wang et al., 2017; Yang et al., 2019). Notably, the deoxygenation of primary pyrolytic vapors over CaO has a major influence on the formation of aromatics over ZSM-5 during catalytic pyrolysis. The reduction of oxygen causes the increase of H/Ceff for pyrolytic vapors and bio-oil with higher H/Ceff can produce more aromatics over ZSM-5 catalyst (Che et al., 2019; Vispute et al., 2010; Zhang et al., 2011). Thereby, CaO conducted pretreatment on primary pyrolytic vapors by deoxygenation prior to upgrading of bio-oil over ZSM-5. However, compared with the traditional production industry from petroleum, the aromatics yield obtained from catalytic pyrolysis of biomass over CaO/ZSM-5 was still lower (Manzano et al., 2017; Roma-Torres et al., 2006). Besides, catalysts deactivation during catalytic pyrolysis of biomass over CaO/ZSM-5 existed mainly owing to the coverage of active sites by coke deposition (Ding et al., 2018; Shankar and Jambulingam, 2017; Zhang et al., 2017). Thus, it is still necessary to improve the catalytic performance of CaO/ZSM-5 during catalytic pyrolysis of biomass for the production of aromatics. The first key is to enhance the deoxygenation ability of CaO during catalytic pyrolysis of biomass over CaO/ZSM-5. In our previous study (Yi et al., 2019), we have found that CaO derived from organic calcium precursors exhibited better deoxygenation ability than conventional CaO during upgrading of bio-oil and can be considered as a potential source with longer lifetime. The second key is to improve the catalytic
performance of ZSM-5 existed in CaO/ZSM-5 catalytic system. Previous works claimed that the particle size of ZSM-5 have critical influences on its catalytic performance. Toshihide Baba et al. found that the rate of ethylene conversion increased with decreasing particle size of ZSM-5(Reddy et al., 2012). Nanosized HZSM-5 showed higher activity and stability compared to the micro-sized HZSM-5 on the selectivity of methanol to propylene reaction (Kim et al., 2018; Zheng et al., 2014). However, during in-situ upgrading of bio-oil, there were thousands of organics in pyrolytic vapors to react with ZSM-5 rather than relatively pure reactant mentioned above. And the effects of nanosized ZSM-5 on the production of aromatics during CFP of various biomass waste also have not yet been systematically studied. Simultaneously, it is not sure whether the Org-CaO and proposed Org-CaO/NZSM-5 catalytic system can achieve better selectivity for aromatics like BTX and naphthalene during in-situ upgrading of bio-oil. Therefore, the purpose of this paper is to develop an improved dual catalytic system for increasing the production of aromatics during catalytic pyrolysis of biomass wastes. CFP of some typical biomass wastes over dual catalytic beds of CaO derived from organic precursors (Org-CaO) and nanosized ZSM-5 (NZSM-5) were conducted to verify the feasibility and suitability of the proposed Org-CaO/NZSM-5 system. In order to deeply investigate the influences of catalysts properties, some systematical experiments were employed. On the one hand, different catalysts proportions were applied to explore better catalytic conditions. On the other hand, it is also significant to
make a deeper insight into the roles of Org-CaO and NZSM-5 in effects of OrgCaO/NZSM-5 system on the aromatics formation during in-situ upgrading of bio-oil.
2. Experimental 2.1 Material and methods In this pyrolysis experiments, Jatropha seeds de-oil cake (called JS cake), peanut shell, corncob and bagasse were chosen as the representatives of biomass. Peanut shell, corncob and bagasse were conventional agricultural wastes. The Jatropha seeds de-oil cake is the residue obtained after the mechanical extraction of oil from the Jatropha seeds which are identified as typical energy crop. The ultimate and proximate analysis of these four biomass wastes were listed in Table 1. In our previous study (Yi et al., 2019), it was found that CaO derived from calcium D-gluconate monohydrate showed the best deoxygenation ability among CaOs decomposed from some organic calcium precursors (called Org-CaO). Therefore, in this study, two types of CaO catalysts were used in the upgrading of bio-oil experiments: (a) the conventional CaO was prepared from calcium hydroxide (called for CH-CaO); and (b) CaO derived from calcium D-gluconate monohydrate was chosen as a representative of Org-CaO. All CaO catalysts were prepared by the same method: the calcium precursor (calcium D-gluconat monohydrate) was calcined in muffle furnace at the heating rate of 30 °C/min from room temperature to 900 °C and then at 900 ℃ for 0.5 h. Two commercial ZSM-5 zeolites with different particle size in this experimental
were the nanosized ZSM-5 (NZSM-5) and the micro-sized ZSM-5 (MZSM-5). Both of them were purchased from the catalyst factory of Nankai University. MZSM-5 is generally used as the conventional ZSM-5. The particle size of MZSM-5 is around 5 μm and the particle size of NZSM-5 is around 100 nm. In order to remove the adsorbed water and water of crystallization, MZSM-5 and NZSM-5 were calcined in air at 550 oC for 4 h prior to experiments and the Si/Al ratio of both two ZSM-5 catalysts is 25. Overall, various CaO/ZSM-5 catalytic systems used in this study were introduced in Table 2. CH-CaO/MZSM-5 system was applied as the conventional CaO/ZSM-5 system. CH-CaO/NZSM-5 system and Org-CaO/MZSM-5 system were applied as the control groups to respectively explore the roles of Org-CaO and NZSM-5 in the effects of synergistic catalysis of Org-CaO/NZSM-5. 2.2 Experiment and experimental procedure Catalytic pyrolysis of biowaste was conducted in a dual-catalyst fixed-bed reactor system with a lab-scale continuous feeder. A schematic diagram of this self-designed experimental apparatus was depicted in Fig.1. The effects of catalysts proportions on product distribution and chemical components were investigated. These JS cake/OrgCaO/NZSM-5 ratios (1:0:3, 1:0.5:3, 1:1:3, 1:3:3, and 1:5:3) were to study the effects of Org-CaO proportions, and these JS cake/Org-CaO/NZSM-5 ratios (1:1:0, 1:1:0.5, 1:1:1, 1:1:3, and 1:1:5) were to study effects of NZSM-5 proportions. The quartz reactor was followed by a twin-chamber condenser which condensed bio-oil by liquid nitrogen. Prior to pyrolysis, the CaO and ZSM-5 catalysts were
respectively filled in the upper and lower quartz cradles. Then, the system was flushed with pure N2 (99.99%) at a flow rate of 200 mL/min for 30 min to ensure an inert atmosphere and two reaction zones were heated at desired temperature. The catalytic temperature of CaO layer is 500 ℃ and the catalytic temperature of ZSM-5 layer is 550 ℃. In all pyrolysis runs, the biomass particles were fallen into reactor from the continuous feeding device as 0.1 g/min. 2.3. Products analysis and catalyst characterization Compositions in bio-oil were qualitatively and semi-quantitatively detected by GCMS (Angilent 7890A/5975C, capillary chromatographic HP-5MS). Helium (99.9999%) is used as the carrier gas with 1.0 mL/min. The injector temperature of GC was 250 oC, and the split ratio of the carrier gas was 1:10. The oven temperature program as follow: the temperature was firstly held at 50 oC for 3 min, and the temperature increased from 50 oC to 320 oC at 5 oC/min. Nitrogen adsorption/desorption isotherms at -196 oC were recorded using a Quantachrome Autosorb iQ instrument. Before the measurements, the samples were heated to 200 oC in avacuum for at least 12 h. The Brunauer Emmett Teller (BET) equation was used to calculate the specific surface area. The size distribution was derived from the adsorption branch by using the Barrett Joyner Halenda (BJH) method.
3. Results and discussion 3.1 Effect of Org-CaO/NZSM-5 on aromatics production
To investigate whether the Org-CaO/NZSM-5 system having heightened selectivity toward aromatics compared with conventional CaO/ZSM-5 system, Figure 2(a) exhibited the effect of Org-CaO/NZSM-5 on the distribution of predominant products in bio-oil obtained from the catalytic pyrolysis of JS cake. CH-CaO/MZSM-5 system was applied as the conventional CaO/ZSM-5 system in general works. The mass ratio (biomass/CaO/NZSM-5) in this section were all 1:1:3. For raw bio-oil without catalyst, the total content of aromatic hydrocarbons was lower than 20% and the content of BTX and naphthalene was little. Interestingly, the proportion of aromatic hydrocarbons in bio-oil obtained from pyrolysis of JC cake with Org-CaO/NZSM-5 is up to 93% which is much more than that by CH-CaO/MZSM-5. The relative content of BTX in bio-oil by CH-CaO/MZSM-5 were just around 26%, whereas the relative content of BTX in bio-oil by Org-CaO/NZSM-5 is around 54% which is almost twice as that by CH-CaO/MZSM-5. Besides, Org-CaO/NZSM-5 strongly promoted the formation of naphthalenes mainly as naphthalene and methylnaphthalene, likely because Org-CaO/NZSM-5 catalyzed more conversion of oxygenated compounds especially for acids into aromatics especially for BTX and naphthalenes. According to our previous work, Org-CaO catalysts have stronger basic strength and more basic sites than CHCaO catalyst. The stronger basicity of Org-CaO has strong potential in improving OrgCaO deoxygenation ability and cracking capability compared with CH-CaO (Yi et al., 2019). This probably improved the synergistic catalytic performance of OrgCaO/NZSM-5 and enhanced the selectivity for aromatics and BTX. Comparing with
CH-CaO/MZSM-5, the better catalytic performance of Org-CaO/NZSM-5 system was attributed to both effects of Org-CaO and NZSM-5. However, this section is difficult to separate their respective contributions to this upgraded system. Thus, the specific experiments were designed to ascertain the effects of Org-CaO and NZSM-5 in 3.2 and 3.3 section. Different Org-CaO and NZSM-5 proportions in Org-CaO/NZSM-5 system were respectively applied to explore the effects of two catalysts proportions and find better catalytic conditions. Figure 2(b) shows the products distribution of aromatics from CFP of JC cake with varying Org-CaO proportions or NZSM-5 proportions. As the increase in NZSM-5 catalyst, the proportion of aromatics was promoted obviously from 20.9% with only Org-CaO to 46.7% with biomass/Org-CaO/NZSM-5 at 1:1:1. The increase of NZSM-5 promoted more formation of BTX and naphthalenes when the proportion of NZSM-5 in Org-CaO/NZSM-5 system is relatively low. But there was no significant increase in the relative content of BTX when further increasing the proportion of NZSM-5 to 1:1:3, indicating that the satisfying production should be obtained as ZSM-5 proportion varying from 1:1:1 to 1:1:3. However, the BTX yield decreased with increasing bulky monocyclic aromatic hydrocarbons (MAHs) when the NZSM-5 proportion increased to 1:1:5, probably because an excess of NZSM-5 could prolong the residence time of vapors in NZSM-5 pores and boost the polymerization of organics. According to Fig. 2(b), low proportions of Org-CaO (1:0.5:3) promoted the conversion into naphthalene over NZSM-5. With increasing CaO proportions, more BTX were
formed over NZSM-5. When the ratio of CaO to NZSM-5 was 1:1 (including 3:3), the maximum yield of BTX could be obtained, which might be as a consequence of the optimal catalytic performance of dual catalytic beds system. The synergistic catalysis of Org-CaO/NZSM-5 showed high selectivity for aromatics when the mass ratio of OrgCaO to NZSM-5 is 1:1. These phenomena reflected that the synergistic catalysis of OrgCaO/NZSM-5 could achieve a better selectivity for BTX during the CFP of biomass probably when Org-CaO/NZSM-5 system integrated and balanced the basicity and acidity. 3.2 Role of Org-CaO in aromatics production over Org-CaO/NZSM-5 In this section, the pyrolysis of four kinds of biomass waste with only NZSM-5, as well as two dual catalytic systems (CH-CaO/NZSM-5 and Org-CaO/NZSM-5) were conducted. Figure 3 showed the effects of different catalytic systems on relative content of aromatic hydrocarbons in bio-oil. The pyrolysis with only NZSM-5 and CHCaO/NZSM-5 were applied as experimental control groups to the investigate the role of Org-CaO in effects of Org-CaO/NZSM-5 on aromatics formation during in-situ catalytic pyrolysis. According to the above results in 3.2 section, Org-CaO showed relatively stronger effects during synergistic catalysis of Org-CaO/NZSM-5 when the mass ratio (biomass/CaO/NZSM-5) was 1:3:3. Thereby, the mass ratio in this section were all 1:3:3. As shown from Fig.3, aromatic hydrocarbons were classified into four groups: BTX (benzene, toluene and xylene), naphthalene, methylnaphthalene and other aromatics. The relative content of BTX in bio-oil catalyzed by only NZSM-5 were just
around 27% and these aromatics were mainly as bulky benzene derivatives. The proportion of aromatic hydrocarbons in bio-oil obtained from pyrolysis of JC cake with Org-CaO/NZSM-5 was much more than that by CH-CaO/MZSM-5 and only NZSM-5. The relative content of BTX in bio-oil by CH-CaO/NZSM-5 were just 30%, whereas the relative content of BTX in bio-oil by Org-CaO/NZSM-5 is around 54% which is almost twice as that by CH-CaO/NZSM-5. In contrast, the bio-oil by Org-CaO/NZSM-5 contained less naphthalene and methyl-naphthalene than that by CH-CaO/NZSM-5, indicating that the pre-catalysis of Org-CaO suppressed the formation of polycyclic aromatic hydrocarbons over NZSM-5 compared with CH-CaO. Therefore, OrgCaO/NZSM-5 system has higher selectivity for aromatics and promoted higher conversion of bulky aromatic hydrocarbons and oxygenated compounds into BTX than CH-CaO/MZSM-5 system. Notably, both two CaO/ZSM-5 catalytic systems have not obviously positive effects on the formation of aromatic hydrocarbons compared with only NZSM-5 catalysis. Especially for corncob and bagasse, the relative content of aromatics and BTX in upgraded bio-oil catalyzed by CaO/NZSM-5 were even slightly less than that by only NZSM-5, which indicated that the addition of CaO slightly suppressed the formation of aromatics over NZSM-5 for corncob and bagasse. Apparently, primary pyrolytic vapors firstly underwent the CaO catalyst bed to form some intermediates and then these intermediates further reacted with ZSM-5 were converted to BTX and other aromatics. Thus, it is very important for the effects of CaO pre-catalysis on the formation mechanism of aromatics over NZSM-5 during in-situ
upgrading of bio-oil. In order to further investigate the effects of CaO pre-catalysis on the formation of intermediate products during dual catalytic pyrolysis, the experiments of CFP of biomass with only Org-CaO were conducted. Figure 4 (a) exhibited the relative content of oxygenated compounds and hydrocarbons in bio-oil without and with Org-CaO catalyst. Raw bio-oil mainly contained oxygenated compounds that accounted for 6883%. It is worth to notice that Org-CaO showed excellent deoxygenation activity during JS cake pyrolysis. The addition of Org-CaO effectively facilitated the conversion of oxygenated compounds to hydrocarbons and the amounts of oxygenated compounds decreased from 83% in raw bio-oil to 18% in upgraded bio-oil by Org-CaO. But for pyrolysis of corncob and peanut shell with Org-CaO, the proportion of oxygenated compounds slightly decreased. The proportion of oxygenated compounds in bio-oil from the pyrolysis of bagasse with Org-CaO even increased a little. These phenomena imply that Org-CaO have little effects on the removal of oxygenated compounds from pyrolysis of peanut shell, corncob and bagasse. These differences of deoxygenation process were consistent with the mentioned above differences about BTX yields, which probably was resulted from the different characteristics of bio-oil from four biomass. To make a deeper insight, the proportions of main oxygenated compounds in raw bio-oil and the difference values of the main oxygenated compounds in bio-oil after CaO catalyzing were analyzed as shown in Fig.4(b). It is need to explain that the difference values of the predominant oxygenated compounds were obtained from the
proportion in CaO bio-oil minus the proportion in raw bio-oil. Firstly, there is an obvious difference for characteristic products in raw bio-oil from pyrolysis of four biomass. The characteristic products in raw bio-oil obtained from pyrolysis of JS de-oil cake were mostly fatty acids, like oleic acid, stearic acid and palmitic acid. Whereas, the raw bio-oil of peanut shell, corncob and bagasse contained much furans and phenols which respectively accounted for 4%-20% and 43%-56%. These phenols mainly were composed of methoxy phenols and methoxyphenols, dimethoxyphenol. It was reported that methoxy phenols were the most characteristic products of lignin pyrolysis and furans were the most characteristic products of cellulose/hemicellulose pyrolysis (Mohan et al., 2006; Patwardhan et al., 2011; Vinu and Broadbelt, 2012). Likewise, Org-CaO also showed selective deoxygenation during catalytic pyrolysis of various biomass waste. According to Fig.4(b), Org-CaO enhanced the removal of fatty acids in JC cake bio-oil, whereas it has little effects on the removal of furans and phenols. It was reported that pyrolytic vapors with higher H/Ceff can produce more aromatics over ZSM-5 catalysts (Vispute et al., 2010; Zhang et al., 2011). The effective hydrogen to carbon ratio (H/Ceff) were defined as following Eq.1: H/Ceff =
𝑛(H) ― 2𝑛(o) 𝑛(𝑐)
(1)
Org-CaO showed good deoxygenation via dehydration, decarboxylation, alkylation, and ketonization to remove fatty acids and form more hydrocarbons and low-oxygen compounds like ketones and alcohols (Yi et al., 2019). But Org-CaO hardly promoted the conversion of phenols and furans to hydrocarbons. Therefore, the H/Ceff in
vapors after CaO catalyzing obtained from JS cake pyrolysis were higher than that from pyrolysis of peanut shell, corncob and bagasse, which partly caused that OrgCaO/NZSM-5 exhibited higher selectivity for BTX during CFP of JS cake. Meanwhile, it also proved that the selectivity for aromatics of Org-CaO/NZSM-5 catalytic system is closely related with the biomass characteristics of these feedstocks. However, it is unknown for the in-depth mechanism of effects of biomass characteristics on the formation of aromatics during catalytic pyrolysis of biomass with Org-CaO/NZSM-5. Our next work will further systematically expound the formation pathways of BTX among CFP of biomass with different characteristics over Org-CaO/NZSM-5. 3.3 Role of NZSM-5 in aromatics production over Org-CaO/NZSM-5 The role of NZSM-5 in effects of Org-CaO/NZSM-5 system on the generation of aromatics was studied. Figure 5 illustrates the relative content of aromatics, BTX and naphthalenes (i.e., naphthalene derivatives) during catalytic pyrolysis of JS cake over the nano and MZSM-5 catalysts combined with two types of CaO. According to the above results, NZSM-5 can show relatively strong and obvious effects during synergistic catalysis of Org-CaO/NZSM-5 when the mass ratio (biomass/CaO/NZSM-5) was 1:1:3. Thereby, the mass ratio (biowaste: CaO: ZSM-5) in these experiments was uniform as 1:1:3 in order to preferably investigate the role of NZSM-5 in effects of OrgCaO/NZSM-5 system. As shown from Fig.5, the nanosized ZSM-5 combined with two CaO both showed better performance on the formation of aromatics compared to the MZSM-5. When CaO type is uniform, it is observed that NZSM-5 principally exhibited
higher selectivity to naphthalenes than MZSM-5. It was reported that diffusion resistance in pores increases with increasing the particle size of ZSM-5(Zheng et al., 2014). Notably, the proportion of aromatics and BTX with Org-CaO/NZSM-5 catalytic system is 20% and 10% more than that with the CH-CaO/MZSM-5 system. The effects of particle size on the physical characteristics of ZSM-5 were investigated by N2 adsorption method to further find the reasons of different effects for nanosized and micro-sized ZSM-5. There is a significant difference in the shape of isotherms of both two samples according to N2 adsorption/desorption isotherms for isotherms of NZSM-5 and MZSM-5 catalysts as shown in Figure 6(a). MZSM-5 exhibited type I isotherm which is characteristic of microporous material. Whereas, according to its hysteresis loop of N2 appears appearing at relatively high relative pressure, it can be known that NZSM-5 possesses both microporous and mesoporous because the accumulation of nanosized particles caused a large secondary accumulation mesopore volume (Jia et al., 2017; Su et al., 2016). BJH pore size distribution curves of two ZSM-5 catalysts given in Fig. 6(b) also indicate the presence of higher mesopore volume in NZSM-5. NZSM-5 contained much more mesopores of 20-500 Å diameter range and macropores of >500 Å diameter range than MZSM-5. During in-situ catalysis of pyrolytic vapors, bulky compounds cannot diffuse into the pores of MZSM-5 and even were condensed into coke deposited on the catalyst surface or blocked the pores, which rapidly caused deactivation of ZSM-5 and reduced the catalytic efficiency. In contrast, NZSM-5 with improved porosity firstly facilitated the diffusion of larger
molecules into mesopores/macropores, and subsequently enhanced more conversion of bulky compounds into BTX and naphthalene in pores. In addition, it is beneficial to slow down the polymerization of aromatics and olefins inside the micropores, and then relief catalyst deactivation resulting from the coke clogging the pore. The total pore volume of NZSM-5 and MZSM-5 illustrated in Table 3 were 0.29 cm3/g and 0.18 cm3/g, respectively. Besides, NZSM-5 possesses higher external surface area than MZSM-5, and then NZSM-5 can provide more acid sites. Higher external specific surface area and shorter pore passage of smaller crystallized NZSM-5 can enhance accessibility of active sites and shorten residence time of pyrolytic vapors, which relief the polymerization of organics and loss of acid sites covered by coke deposition. Thus, NZSM-5 catalyst with greater catalytic activity significantly promoted the formation of aromatics over the acid sites located on the external surface.
4. Conclusions During catalytic pyrolysis of biomass, Org-CaO/NZSM-5 promoted that the proportions of aromatics and BTX in bio-oil increased to 93% and 54% which were much more than that by CH-CaO/MZSM-5. The stronger pre-deoxygenation of OrgCaO made volatiles containing more intermediates with high H/Ceff, like hydrocarbons, low-oxygen alcohols and ketones. Besides, NZSM-5 with improved microporous/mesoporous also allowed the entrance of bulky compounds into pores. Thus, more BTX and naphthalene were generated under Org-CaO/NZSM-5 system. The
pyrolysis catalyzed by Org-CaO/NZSM-5 of Jatropha de-oiled cake provided much higher aromatics, especially for BTX, than that of peanut shell, corncob and bagasse.
Supplementary data Supplementary data associated with this article can be found in the online version.
Acknowledgments The authors are grateful to National Natural Science Foundation of China (No. 51661145010) for the financial supports. They also express thanks to the Analytical and Testing Center of Huazhong University of Science and Technology for the testing.
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The BTX production by Org-CaO/NZSM-5 is almost twice as that by conventional CaO/ZSM-5.
The proportion of aromatics and BTX with Org-CaO/NZSM-5 were up to 93% and 54%.
Org-CaO contributed to BTX formation during dual catalytic pyrolysis.
NZSM-5 promoted the conversion of bulky molecules into naphthalenes.
Figure captions Fig. 1. Schematic diagram of in-situ upgrading of pyrolytic vapors during catalytic pyrolysis of biomass over various CaO/ZSM-5 systems Fig. 2. (a)Products distribution of main chemical compounds in bio-oil from CFP of JC cake; (b) Products distribution of aromatics from CFP of JC cake with varying Org-CaO proportions or NZSM-5 proportions. Fig. 3. Relative content of various aromatic hydrocarbons during catalytic pyrolysis of JS cake, peanut shell, corncob and bagasse Fig. 4. Relative content of compounds in bio-oil from pyrolysis of JS cake, peanut shell, corncob and bagasse (a) Yields of hydrocarbons (HCs) and oxygenated compounds (OCs);(b) Yields and difference value* of main oxygenated compounds respectively in raw bio-oil and in bio-oil catalyzed by Org-CaO. difference value*=the proportion of X in bio-oil catalyzed by Org-CaO - the proportion of X in raw bio-oil (X means acids, phenols or furans) Fig. 5. Relative content of aromatics, BTX and naphthalenes in bio-oil catalyzed by various dual catalytic system Fig. 6. (a) N2 adsorption/desorption isotherms for isotherms of Nano- and MicroZSM-5 catalysts; (b) BJH pore size distribution of Nano- and Micro- ZSM-5 catalysts.
Fig. 1. Schematic diagram of in-situ upgrading of pyrolytic vapors during catalytic pyrolysis of biomass over various CaO/ZSM-5 systems
( a)
( b)
Fig. 2. (a)Products distribution of main chemical compounds in bio-oil from CFP of JC cake; (b) Products distribution of aromatics from CFP of JC cake with varying Org-CaO proportions or NZSM-5 proportions.
Fig. 3. Relative content of various aromatic hydrocarbons during catalytic pyrolysis of JS cake, peanut shell, corncob and bagasse
( a)
( b)
Fig. 4. Relative content of compounds in bio-oil from pyrolysis of JS cake, peanut shell, corncob and bagasse (a) Yields of hydrocarbons (HCs) and oxygenated compounds (OCs);(b) Yields and difference value* of main oxygenated compounds respectively in raw bio-oil and in bio-oil catalyzed by Org-CaO. difference value*=the proportion of X in bio-oil catalyzed by Org-CaO - the proportion of X in raw bio-oil (X means acids, phenols or furans)
Fig. 5. Relative content of aromatics, BTX and naphthalenes in bio-oil catalyzed by various dual catalytic system
(
180
Nano ZSM-5 Micro ZSM-5
160
Volume adsorbed (cm3/g)
a)
140 120 100 80 60 40 20 0
-0.1 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
Relative pressure (P/Po) (
0.15 Micro ZSM-5 Nano ZSM-5
dV(logd) (cm3/g)
b) 0.10
0.05
0.00
1
10
100
Pore diameter (nm) Fig. 6. (a) N2 adsorption/desorption isotherms for isotherms of Nano- and MicroZSM-5 catalysts; (b) BJH pore size distribution of Nano- and Micro- ZSM-5 catalysts.
Table list Table 1. Ultimate analysis and proximate analysis of biomass wastes (wt.%) Table 2. Various CaO/ZSM-5 catalytic systems Table 3. Characteristics of NZSM-5 and MZSM-5
Table 1. Ultimate analysis and proximate analysis of biomass wastes (wt.%) C
Ultimate analysis H O N
Proximate analysis S
a
JS cake
4 5.3
Peanut shell
.2 4
9.7 Corncob
5
4
Bagasse
4
5
4
0
4
6
.7
sh
18.2
7
84.1
14.5
1 .4
0
86.9
11.8
.2 0
A
.3 0
0
4
73.5
.1
.4
5.8
0
Fixed carbon
.2
.6
7.0
.7
4 .5
3.7
.8
6.4
4 3.8
.8
6.6
a:
6
Volatile matter
1 .3
0
87.4
9.7
.4
2 .9
by difference Table 2. Various CaO/ZSM-5 catalytic systems Definition
Conventional CaO/ZSM-5 Control group Control group Improved CaO/ZSM-5
Label
CaO types
CHCaO/MZSM-5 CHCaO/NZSM-5 OrgCaO/MZSM-5 OrgCaO/NZSM-5
CaO from Ca(OH)2 CaO from Ca(OH)2 CaO from organic calcium precursor CaO from organic calcium precursor
ZSM-5 types Micro ZSM-5 Nano ZSM-5 Micro ZSM-5 Nano ZSM-5
Table 3. Characteristics of NZSM-5 and MZSM-5 Catalysts
BET (m2/g)
External
Total pore
Average
surface area
volume (mL/g)
pore volume
(m2/g)
(nm)
MZSM-5
262
56
0.19
2.08
NZSM-5
385
82
0.27
4.08