A study of guaiacol, cellulose, and Hinoki wood pyrolysis with silica, ZrO2&TiO2 and ZSM-5 catalysts

https://doi.org/10.1016/j.jaap.2017.04.004Get rights and content

Highlights

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    Guaiacol, cellulose, and wood were pyrolyzed with silica, ZrO2&TiO2 and ZSM-5 with different SiO2-to-Al2O3 ratios (SAR).

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    High SAR ratio ZSM-5 catalyst increased oxygenated aromatics.

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    However, only low SAR ratio ZSM-5 catalyst converted the oxygenated aromatics to aromatic hydrocarbons.

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    The oxide layer on silica had strong interactions with hydroxyl groups on levoglucosan during pyrolysis.

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    ZrO2&TiO2 increased cyclopentanones with Hinoki wood pyrolysis.

Abstract

Catalytic pyrolysis of biomass is often used to improve bio-oil quality and energy content. In order to better understand the interactions between various catalysts and biomass components, individual components of biomass were isolated and pyrolyzed with different catalysts. Understanding the interactions between pyrolysis vapors and catalysts is important to create catalysts that can improve bio-oil quality and energy content in the future. In this study, cellulose, guaiacol (as a model for lignin), and Hinoki wood were pyrolyzed and catalyzed with silica, ZrO2&TiO2, and ZSM-5 with an SiO2-to-Al2O3 (SAR) ratio of 40 and 1500. GC–MS analysis was used to measure the effects of the different catalysts on the pyrolysis vapors. It was found that silica had a strong interaction with the hydroxyl groups of levoglucosan and significantly decreased sugars in both cellulose and Hinoki wood pyrolysis. ZrO2&TiO2 increased the amount of cyclopentanones and aromatics with Hinoki wood pyrolysis. The ZSM-5 catalysts also increased aromatic hydrocarbons and oxygenated hydrocarbons with Hinoki wood pyrolysis. Based on these results, specific reactions of the catalysts were proposed. Additionally, ZSM-5 with a SAR ratio of 1500 has very few acidic sites, so the steric effects of ZSM-5 could be isolated. These results are discussed and contrasted with ZSM-5 with a SAR ratio of 40. These results show that microporous catalysts and moderate acidity are optimal for improving bio-oil quality.

Introduction

The demand for energy increases every year as countries around the world become more industrialized. Due to environmental concerns and limited supply of fossil fuels, scientists are conducting research on alternative energy sources. Biomass is considered as a potential alternative fuel source due to its carbon neutral process and renewability. Biomass can be converted to bio-oil through pyrolysis with high yields of 60–70% by mass [1]. However, the bio-oil is a complex mixture of oxygenated compounds which decreases the energy density and quality of the bio-oil [2]. For this reason, scientists are using various methods to upgrade the bio-oils. One promising method for improving bio-oils is using catalysts in the pyrolysis vapor stream to promote desired reactions.

Scientists have extensively studied and discussed many different types of catalysts for upgrading pyrolysis vapors [2]. Among these, ZSM-5 and zirconia/titania catalysts are considered to be the most promising because they result in a treated bio-oil with reduced oxygen and high aromatics content [3]. However, the catalytic upgrading mechanism is still not fully understood. Biomass is mainly composed of three structural components: cellulose, hemicellulose and lignin. In order to better understand the interactions between the catalyst and biomass vapors, Stefanidis et al. and Zheng et al. pyrolyzed the individual components of biomass with ZSM-5 and compared them to uncatalyzed products [4], [5]. Other researchers such as Shoucheng Du et al. have studied model compounds (toluene, furane and miscanthus) from biomass and pyrolyzed them over ZSM-5 to get a better understanding of the catalytic mechanism [6]. Haian Xia et al. studied the effects of ZSM-5 and other catalysts on cellulose pyrolysis [7]. ZSM-5 is often studied; however other catalysts are not as well understood. In this research, we also pyrolyzed biomass components with ZSM-5 to verify the findings for benchmarking, and then expanded the study with other catalysts. The results were compared with uncatalyzed products and other researcher’s results. The purpose of this study is to not only to verify the ZSM-5 results, but also understand how other catalysts interact with individual components of biomass.

In this study, cellulose, guaiacol, and Hinoki (Japanese Cypress) wood were chosen as the three types of feed for pyrolysis. Cellulose was chosen because Hinoki wood is composed of about 60% holocellulose (cellulose and hemicellulose). Hemicellulose was not commercially available, so only cellulose was used to model the sugars in the wood. The remaining 40% is composed of lignin and acid-soluble hydrocarbons [8]. Guaiacol was chosen as a model compound for lignin because experimental and computational research has shown that lignin first generates mainly syringol and guaiacol characteristic products [9]. Finally, Hinoki wood was chosen as it was readily available here in Japan for pyrolysis due its use as a building material.

In this study, 4 types of catalysts were used. Two types of ZSM-5 were used. One had a SiO2-to-Al2O3 (SAR) ratio of 40 and the other ZSM-5 catalyst had a SAR ratio of 1500. Lower SAR ratios cause higher acidity. However, changing the SAR ratio does not change the overall structure of the catalyst [10]. By using a ZSM-5 catalyst with high SAR ratio of 1500 we can isolate the steric effects of the catalyst and compare it to the ZSM-5 with increased acid strength and acid site density. ZrO2&TiO2 was chosen as it has been shown to increase hydrocarbon content in bio-oil and improve bio-oil quality [3], [11]. Finally, a blank silica catalyst was also used for comparison with the other catalysts. The catalysts were used for the pyrolysis of each feed and the collected bio-oil was analyzed with GC–MS.

Section snippets

Materials and methods

The biomass feed, consisting of Hinoki (Japanese Cypress) wood chips was supplied by a local store. Wako Pure Chemical Industries supplied the 38 μm powdered cellulose and it was used without modification. Guaiacol (99% purity) was supplied by Acros Oganics and it was used without modification. The Hinoki wood chips were sieved through 2.8 mm and 5.0 mm size trays and the 2.8 mm −5.0 mm wood chips were used. Elemental analysis was performed on the Hinoki wood using a Yanaco CHN corder MT-6 elemental

Yield results of pyrolysis

The catalysts were used to catalyze pyrolysis vapors with various feeds. The bio-oil and char & coke were weighed and the yields were calculated after the pyrolysis process for each experiment. Figs. 2–4 show the mass yields of different feeds. The gas yield is calculated by subtracting the mass of char and oil from the initial mass of wood feed. Each uncatalyzed experiment was conducted 3 times to measure the repeatability of the pyrolysis apparatus.

Guaiacol is a model compound for lignin and

Conclusion

Hinoki wood, cellulose, and guaiacol (as a model compound for lignin) were pyrolyzed with different catalysts to get a better understanding of the catalysis process. The pyrolysis of guaiacol resulted in many different phenolic compounds with very high bio-oil yields of up to 60% with minimal coke formation. Silica, ZrO2-TiO2, ZSM-5 with a SiO2-to-Al2O3 (SAR) ratio of 40, and ZSM-5 with a SAR ratio of 1500 were tested as catalyst; however, the pyrolysis products of guaiacol were remarkably

Acknowledgements

This research was supported in part by the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT) Scholarship to the first author and by the Tokyo Institute of Technology Academy for Co-creative Education of Environment and Energy Science (ACEEES) program. We would like to thank the Kunio Yoshikawa lab in the Tokyo Tech transdisciplinary science and engineering department on the Suzukakedai campus for use of the GC–MS equipment.

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