Importance of oxygen-containing functionalities and pore structures of biochar in catalyzing pyrolysis of homologous poplar

Abstract

Biochar and bio-oil are produced simultaneously in one pyrolysis process, and they inevitably contact and may interact, influencing the composition of bio-oil and modifying the structure of biochar. In this sense, biochar is an inherent catalyst for pyrolysis. In this study, in order to investigate the influence of functionalities and pore structures of biochar on its capability for catalyzing the conversion of homologous volatiles in bio-oil, three char catalysts (600C, 800C, and 800AC) produced via pyrolysis of poplar wood at 600 or 800 °C or activated at 800 °C, were used for catalyzing pyrolysis of homologous poplar wood at 600 °C, respectively. The results indicated that the 600C catalyst was more active than 800C and 800AC for catalyzing cracking of volatiles to form more gas (yield increase by 40.2%) and aromatization of volatiles to form more light or heavy phenolics, due to its abundant oxygen-containing functionalities acting as active sites. The developed pores of the 800AC showed no such catalytic effect but could trap some volatiles and allow their further conversion via sufficient aromatization. Nevertheless, the interaction with the volatiles consumed oxygen on 600C (decrease by 50%), enhancing the aromatic degree and increasing thermal stability. The dominance of deposition of carbonaceous material of a very aromatic nature over 800C and 800AC resulted in net weight gain and blocked micropores but formed additional macropores. The in situ diffuse reflectance infrared Fourier transform spectroscopy characterization of the catalytic pyrolysis indicated superior activity of 600C for removal of –OH, while conversion of the intermediates bearing C=O was enhanced over all the char catalysts.

1. Introduction

Biochar is homologous with bio-oil, which is produced by the same pyrolysis process [1]. The interaction of the organics in bio-oil with biochar is inevitable during pyrolysis as the majority of volatiles are generated in the inner structures of a biomass particle [2,3]. The travel of volatile organic components from the inner section of a particle in pyrolysis would contact biochar and might interact with biochar as biochar is not an inert material [4,5]. Similar to the organics in bio-oil, biochar has its own oxygen-containing functionalities and skeleton structures, which can be regarded as a “macro” reactive molecule [6e9]. The interaction between bio-oil and biochar shapes the composition of bio-oil and the structure of biochar [2,4,10,11].

There have been many studies about the catalytic effects of biochar in the pyrolysis of biomass [12e22]. For instance, Ren et al. [12] investigated catalytic pyrolysis of Douglas Fir pellets over biochar catalyst and found that the biochar catalyst reduced the production of organic acids in bio-oil while enhancing the formation of hydrocarbons and phenols. The study by Yang et al. [13] showed that corn straw biochar activated with H3PO4 could also promote the formation of phenolic compounds and aromatics (up to 67%) at the expense of oxygenic compounds and acetic acid in pyrolysis of Douglas Fir sawdust pellets. Bai et al. [14] performed catalytic pyrolysis of sewage sludge with self-derived char and found that the char catalyst could promote the formation of aliphatics while suppressing the generation of nitrogen-containing organics. Liu et al. [15] conducted catalytic pyrolysis of anise in a fixed-bed reactor, and the results showed that the Fe/biochar catalyst had high catalytic activity. Xu et al. [16] prepared three Ni/biochar catalysts with different K content for toluene steam reforming, and the results showed that K presence accelerated the consumption of biochar support at higher temperatures. These results demonstrated the catalytic effects of biochar on the evolution of the organics in bio-oil in pyrolysis, but further studies are required for understanding the structural evolution of biochar catalysts during catalytic pyrolysis.

In fact, except for the influence on bio-oil, the interaction between volatiles in bio-oil and biochar also impacted the structural features of biochar [10,23]. In various aspects of the properties of biochar, functionality and pore structure are important features that play essential roles in the determination of ...