Molten carbonate pyrolysis of digestate with metal-modified HZSM-5 for bio-based monophenols: Kinetics and mechanism study

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Highlights

  • Molten carbonates pyrolysis digestate integrated with catalytic cracking for monophenols.

  • Fe-modified ZSM-5 promoted the generation of monophenols up to 79.22 %.

  • The obtained activation energy of lignin was reduced obviously by molten carbonate.

  • Alkali metal icon attached to β-O-4 bonds and methoxy group of lignin and broke these groups.

Abstract

Molten carbonate pyrolysis is proposed to convert digestate (DE) into biobased monophenols coupled with a metal-modified HZSM-5 catalyst. The results indicated that Fe/HZSM-5 was the most suitable catalyst for this process. The optimum reaction conditions for the process were WHSV = 15 h−1 at a pyrolysis temperature of 430 °C and a catalytic cracking temperature of 475 °C, and 79.22 % of monophenol components were achieved in the bio-oil. A kinetic study on the digestate with carbonates indicated that the alkali metal ion in the molten carbonates attached to the β-O-4 bonds and branch methoxy group of lignin, which promoted the removal of these functional groups, resulting in a reduction in the activation energy during monophenol generation. The Fe/HZSM-5 catalyst reinforced the generation of monophenol compounds in both concentration and yield. Compared with molten carbonate pyrolysis, the Fe on the catalyst not only benefited the breaking of β-O-4 bonds and methoxy groups of lignin but also lowered the temperature during molten carbonate pyrolysis to minimize the secondary cracking of phenolic compounds.

Introduction

The conversion of biomass to energy sources such as biogas and biofuel currently has drawn great attention because of the limitation on the storage of fossil fuels as our primary energy source [1,2]. The demands for renewable carbon in fuels and chemicals, such as monophenols, impel the development of technology towards more efficient and economic pathways. Phenol and its derivatives are vital industrial chemical compounds found in myriad industrial products. Industrial products using phenol as feed, such as phenol formaldehyde and phenolic resins, are relatively expensive at phenol prices [3]. Among all the main components in biomass, lignin has been recognized as an important natural phenolic resource [4]. Related research has been conducted on conversion from lignin to syngas, char and phenol rich bio-oil [[5], [6], [7]]. In addition, remarkable recovery of energy has also been found during the process [8]. Digestate (DE) is certified to be suitable for phenol production because of its considerable lignin content [1,9]. However, the superfluous nitrogen and heavy metals retained in DE have negative effects on the environment. Thermal pyrolysis provides a promising pathway to deal with DE on a large scale instead of as land fertilizer or soil conditioners, not only protecting the environment without heavy metals retained but also enhancing the quality of bio-oil. However, low thermal conductivity prohibits heat transfer during digestate pyrolysis, which further leads to an uneven distribution of temperature and results in a high yield of char rather than bio-oil [10,11].

With the intention to overcome the heat transfer barrier, huge efforts have been devoted to lignin pyrolysis. Microwave pyrolysis and catalytic pyrolysis have been reported in the research literature regarding phenol generation [4,12]. Our prior study found that a molten carbonate pyrolysis system significantly contributed to the depolymerization of digestate [13]. Molten carbonates are transparent ionic liquids, owing great potential as reaction media for pyrolysis, with remarkable stability, heat capacity, and thermal conductivity and less corrosion compared with other molten salts. The alkali metal ions, especially sodium and potassium ions, promote the primary degradation of digestate and the secondary monomolecular dissociation reaction of methoxyphenol. In addition, the molten carbonate medium reduced the reaction temperature dramatically in the atmosphere. Molten carbonate pyrolysis opens a new horizon for the thermal conversion of lignocellulosic biomass. To evaluate the effects of biomass on molten salt pyrolysis, various experiments have been performed using thermogravimetric analysis (TGA). Hathaway et al. [14] studied the kinetics of pyrolysis in ternary carbonate systems and showed an increase of 74 % compared with conventional pyrolysis. On the other hand, Robert Bedoić et al. [15] reported that the digestate from anaerobic digestion could consume less energy for subsequent pyrolysis in comparison with untreated biomass by kinetic research. Relative kinetic studies have been performed on the combined anaerobic digestion-pyrolysis process for biomass/waste material with its digestate [[16], [17], [18]] but without molten salt. To the best of our knowledge, there are still quite few reports on molten carbonate pyrolysis, especially the kinetic study of the medium on digestate.

Furthermore, the results of our previous experiment exhibited poor deoxidation ability in the oxygenated group, as nonnegligible amounts of guaiacol, acid, ketone and alcohol have been observed in bio-oil. From this perspective, catalytic pyrolysis has been integrated with molten carbonate pyrolysis to enhance the deoxidation ability during the process. A considerable number of studies have concentrated on zeolite catalysts, especially ZSM-5, due to their promising performance in bio-oil upgrading [[19], [20], [21], [22]]. ZSM-5, particularly the acid sites in its porous structure, facilitates the cracking of bonds and removes the oxygen in the forms of carbon monoxide and carbon dioxide, resulting in the generation of hydrocarbons. However, a large amount of coke has also been found on the catalysts. Related work has found that coke was synthesized by polymerization of phenolic and guaiacol compounds [23]. According to previous work, small active molecules, such as aldehydes and ketones, attached to the active site in the inner pore and worked as extra sites extending out of the moderate pore of the ZSM-5 catalyst to attach guaiacol molecules and cause coke formation [24]. To solve this problem, many different kinds of metal ions, such as Ni, Fe, Zn, Cu and Co, have been uploaded to the ZSM-5 support [25,26]. Nevertheless, there is still an urgent demand to find suitable catalysts integrated with molten carbonate pyrolysis that can remove oxygen from pyrolysis products with high selectivity while reducing coke formation from phenol and guaiacols.

In this case, digestate was utilized as feedstock in molten carbonate pyrolysis and integrated with ex situ catalytic reforming. Four typical metal catalysts of Ni/ZSM-5, Fe/ZSM-5, Zn/ZSM-5 and Cu/ZSM-5 were used for catalysis of the volatiles from pyrolysis compared with the HZSM-5 catalyst, and the pyrolysis characteristics and product distribution were investigated. Meanwhile, a kinetic study on digestate in molten carbonates was also conducted to determine the mechanism of the pyrolysis reaction.

Section snippets

Materials

Digestate (DE) was the solid residue of anaerobic digestion in the laboratory using Sargassum horueri as the fermentation feedstock. The DE was oven-dried at 105 °C for 24 h, crushed by a grinder and sieved into 0.60−0.85 mm size fractions as previously mentioned [13].

Li2CO3 (99 %, AR, CAS: 554-13-2), Na2CO3 (99.5 %, ACS, CAS: 497-19-8), and K2CO3 (99 %, AR, CAS: 584-08-7) were purchased from Aladdin Bio-Chem Technology Co, Ltd. (Shanghai, China.). The alkali carbonates were mixed with Li2CO3,

Effect of metal-modified HZSM-5 on product yield and monophenol selectivity

Table 2 shows the yield of the bio-oil product after being refined with different catalysts. A significant effect on the reduction of bio-oil yield was observed using metal-modified HZSM-5 compared with the unmodified HZSM-5, accompanied by the moisture content. It is mainly caused by the deoxidation reaction that converts the oxy-compounds to small molecule gases (e.g., CO, CO2, CH4) and coking with polymerization from phenolics [34]. In contrast, the yield of syngas dramatically increased to

Conclusions

In this study, an integrated molten carbonate pyrolysis of digestate with ex situ catalytic cracking for bio-oil-based monophenols was successfully developed. The optimum catalyst and reaction conditions were investigated. Some key conclusions from this work are summarized as follows:

Fe/HZSM-5, with a proper porous structure and mild acidity, demonstrated a distinct catalytic performance for enhancing the yield of monophenol in bio-oil. The main factor was attributed to the remarkable

CRediT authorship contribution statement

Chaoyue Shen: Writing - review & editing. Xiying Jia: Validation. Yifei Chen: Data curation. Licong Lu: Investigation. Fangqi Wang: Writing - original draft. Yi Wei: Conceptualization, Methodology, Supervision. Fengwen Yu: Formal analysis, Supervision.

Acknowledgments

The study was funded by the Nature Science Foundation of China [grant No. 21808207], and the Zhejiang Provincial Natural Science Foundation of China [LQ18B060006]. The research was supported by Zhejiang Province Key Laboratory of Biofuel.

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