Analytical pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS) of sawdust with Al/SBA-15 catalysts

Abstract

Pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS) was employed to achieve fast pyrolysis of sawdust and on-line analysis of the pyrolysis vapors. A mesoporous SBA-15 catalyst and four Al/SBA-15 catalysts with different Si/Al ratios were prepared, and tests were performed to determine their effects on cracking the pyrolysis vapors. After catalysis, levoglucosan was significantly reduced or even completely eliminated. The yields of heavy furans and heavy phenols decreased significantly, while light furans and light phenols increased. Moreover, the catalytic cracking reduced the yields of light aldehydes and ketones, while increased the formation of acetic acid. Catalytic cracking also resulted in the formation of hydrocarbons, but their yields were not high. In regard to the four Al/SBA-15 catalysts, their effects on cracking the pyrolysis vapors were enhanced with the reducing of Si/Al ratios.

Introduction

Fast pyrolysis of biomass to produce bio-oils has gained extensive attentions in recent years, because it can offer an alternative way to solve the liquid fuel shortage problem. However, bio-oils are low-grade liquid fuels. They are highly oxygenated, acid and corrosive to common metals, chemically and thermally unstable, as well as non-miscible with petroleum fuels [1]. These poor fuel properties inhibit bio-oils to be directly used in existing thermal devices [2].

There are several ways to upgrade the fuel properties of bio-oils. One of these is catalytic cracking that is performed under atmospheric pressure by using cracking catalysts to crack the highly oxygenated bio-oils into products with lower molecular weight and oxygen content, and oxygen will be removed as H₂O, CO and CO₂ [3]. Catalytic cracking can be performed either on bio-oils or on pyrolysis vapors. It seems more reasonable to perform on pyrolysis vapors, because direct conversion of pyrolysis vapors eliminates costly condensation and re-evaporation procedures of bio-oils, and also avoids the instability problems that will be caused during heating of bio-oils.

Catalytic cracking of pyrolysis vapors can be conducted in two ways. The first way is to introduce catalysts into pyrolysis reactor and thus pyrolysis and catalysis take place in the same reactor. The second way is that pyrolysis and catalysis proceed in different reactors, so that the two processes can be performed under different conditions. Lots of studies have been conducted by employing various catalysts, mainly zeolites such as HZSM-5 [4], [5], [6], [7], [8], [9], [10]. Significant modification of the pyrolysis products can be achieved by these catalysts. However, many problems were encountered, such as the fast deactivation of the catalysts resulted from the severe coke formation, and the low yields of organic liquids. These problems can be partly attributed to the presence of non-volatile oligomers in bio-oils or pyrolysis vapors. To overcome these problems, mesoporous catalysts, whose pore sizes are much larger than that of traditional zeolites, have attracted great interest for their potential to convert large molecules, so as to reduce catalyst deactivation rates and give high organic yields.

Since the discovery of M41S by Mobil's scientists in 1992, a series of ordered mesoporous silicate materials (M41S, SBA, MSU, HMS, etc.) have been successfully synthesized. Some of the mesoporous materials have already been studied for catalytic cracking of biomass fast pyrolysis vapors. Adam et al. [11] investigated the cracking effects of several Al-MCM-41 catalysts by using the pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS) instrument. The results revealed that the utilization of these catalysts eliminated all the levoglucosan, reduced large molecular mass phenols, and increased the yields of acetic acid, furans, small phenols and hydrocarbons. A further study was conducted to test these catalysts in a lab-scale fixed bed reactor. All the catalysts were found to increase the desirable products in catalytic bio-oils [12]. In addition, some Me-Al-MCM-41 (Me = Fe, Cu or Zn) catalysts were tested, and they were all shown to improve the phenol yields [13], [14]. Triantafyllidis et al. [15] compared the performance of two mesoporous aluminosilicate materials (MSU-S_BEA) to Al-MCM-41. The use of the MSU-S catalysts resulted in high yields of cokes and chars, and significantly low yields of organic liquids. In regard to the organic liquids, the catalysts favored the formation of PAHs and heavy fractions, while produced almost no acids, alcohols and carbonyls, and very few phenols. Pattiya et al. [16] tested the catalytic effects of four catalysts by Py–GC/MS: the zeolite ZSM-5, two aluminosilicate mesoporous materials Al-MCM-41 and Al-MSU-F, and a proprietary commercial catalyst alumina-stabilised ceria MI-575. The results showed that all the catalysts produced aromatic hydrocarbons and reduced oxygenated lignin derivatives. ZSM-5 was the most active to all the changes in pyrolytic products.

SBA-15 is by far the largest pore size mesoporous material with highly ordered hexagonally arranged mesochannels, thick walls, adjustable pore size from 3 nm to 30 nm, and high hydrothermal and thermal stability. These properties make SBA-15 a promising catalyst for treating biomass pyrolysis vapors which contain quite a lot of water steams and large molecules. However, SBA-15 is a purely silica material lack of acidity. To create acid sites, it can be achieved by the incorporation of Al in the framework of the mesoporous silica by doping (one-pot synthesis) or post-grafing. Adam et al. [12], [17] prepared the Al/SBA-15 catalyst via one-pot synthesis, and tested its cracking effects with the Py–GC/MS instrument as well as in a fixed bed reactor. In this study, four Al/SBA-15 catalysts with different Si/Al ratios were prepared by post-synthesis alumination. Py–GC/MS experiments were performed to investigate how the Al/SBA-15 catalysts will affect the biomass fast pyrolysis vapors, and the results were compared with those found in previous studies to reveal whether the way Al/SBA-15 prepared would affect its catalytic properties.