Fast pyrolysis of biomass is one of the most promising technologies to utilize renewable biomass resources, and has attracted extensive interest in recent years. It offers a convenient way to convert biomass mainly into a liquid product, known as bio-oil, which covers a wide applications [
1,
2].
Bio-oil has been regarded a promising candidate to replace petroleum fuels. However, it is a low-grade liquid fuel, because it is highly oxygenated, acidic and corrosive to common metals, chemically and thermally unstable, as well as non-miscible with petroleum fuels [
3,
4]. As a result, it is difficult to directly use crude bio-oil in various thermal devices, especially internal combustion engines [
5]. These poor fuel properties can be attributed to the presence in bio-oil of large amounts of water, acids, aldehydes and large molecular oligomers, therefore, it is necessary to upgrade bio-oils to eliminate these undesirable compounds or convert them to more desirable ones. Several methods have been applied to upgrade bio-oils, and one of them is the catalytic cracking which can be performed either on liquid bio-oils or on pyrolytic vapors immediately after the pyrolysis process. The key problem for catalytic cracking is the selection of suitable catalysts. In early studies, traditional zeolites (such as HZSM-5, HY,
etc.) have been widely studied. They were effective to convert the highly oxygenated compounds to hydrocarbons, but many problems were encountered, such as fast deactivation of the catalysts by coke deposition, low organic liquid yield and the formation of polycyclic aromatic hydrocarbons (PAHs) [
6,
7]. Recently, mesoporous catalysts (such as MCM-41, SBA-15, MSU,
etc.) have been applied for their potential to upgrade the large molecular oligomers [
8,
9,
10,
11,
12,
13]. However, due to their poor hydrothermal stability and high production cost, these catalysts cannot be utilized industrially at present.
Chemically, bio-oil contains many valuable compounds, and thus, has the potential for the recovery of useful chemicals. However, most of the compounds in bio-oil are present in low amounts, making the recovery not only technically difficult but also economically unattractive at present [
14]. Hence, the commercialization of bio-oil for value-added chemicals requires production of specific bio-oils with high contents of target products. Till now, many special catalysts have been reported to be effective to maximize levels of various chemicals, such as the production of levoglucosenone by using H
3PO
4 [
15], 1-hydroxy-3,6-dioxabicyclo[3.2.1]octan-2-one by nano aluminium titanate [
16], acetol by NaOH or Na
2CO
3 [
17], light furans by sulfated metal oxides [
18], and furfural by MgCl
2 [
19].
In recent years, nano metal oxides have attracted extensive attention in various catalytic processes due to their unique properties, but they are not widely used in catalytic treatment of biomass fast pyrolysis vapors. Li
et al. prepared nano NiO and tested its activity during biomass pyrolysis using a thermogravimetric analyzer [
20]. In our previous study, nano TiO
2 and its modified catalysts were used for experiments and confirmed to have some good catalytic activity [
21]. In this study, six nano metal oxides were used as catalysts to test whether they had the capability to upgrade the fuel properties of bio-oil or maximize the formation of some valuable chemicals. The experiments were performed using an analytical Py-GC/MS instrument which allows direct analysis of the pyrolytic products. The catalytic and non-catalytic products were compared to reveal the catalytic capabilities of these catalysts.