Catalytic pyrolysis of cellulose to bio-oil over Zn/ZSM-5 catalysts
Haian Xia*, Yuejie Ge, Ranran Xu, Xiucong Wang,
Jiangsu Key Lab of Biomass-Based Green Fuels and Chemicals,
Nanjing 210037, China
College of Chemical Engineering, Nanjing Forestry University,
Nanjing 210037, China
* Corresponding
author at: College of Chemical Engineering, Nanjing Forestry
University, Nanjing 210037, China. Tel.: +86-25-85427635; fax: +86-2585418873.
E-mail address: Haianxia@yahoo.com.cn (Haian Xia, assistant professor, PhD).
Abstract
With the increasing concern on energy shortage and environmental
problems, the highly-effective conversion of renewable biomass resources,
such as woody biomass, will play an important role in the future[1]. Cellulose,
a bioploymer which is a bulk component of woody-biomass, is sustainable raw
material in nature. The bio-oil obtained from fast pyrolysis of biomass has
some shortages such as high acid value, low thermal stability, high oxygen
content, etc., and these drawbacks limit its large-scale application. Catalytic
pyrolysis of biomass is an effective thermal conversion method which can
improve the thermal stability of bio-oil by reducing its oxygen content that
mainly affect the thermal value and stability[2-4]. It has been reported that
zeolite catalysts could effectively improve the thermal stability of bio-oil by
decreasing its oxygen content through dehydration, aromatization,
decarboxylation, and decarbonylation during the pyrolysis reaction [5, 6]. More
interestingly, after incorporating some transition metal ions such as Ga, La, and
Fe, zeolite catalysts exhibit some special catalytic properties including olefin
aromatization and oxidation-reduction that zeolite catalysts do not have.
Herein, we use Zn/ZSM-5 catalyst to catalytic pyrolyze cellulose into the
bio-oil in a fix-bed reactor. The influence of Lewis acid sites and Brönsted acid
sites on the pyrolytic behavior of cellulose was investigated by TG and in situ
IR spectroscopy. Physicochemical techniques including XRD, H2-TPR, NH3TPR, UV-Visible diffuse reflectance spectroscopy, and IR spectra of pyridine
adsorption, were employed to characterize the properties of Zn ions. The GCMS, LC-MS, NMR, and FT-IR spectroscopy were used to analyze the
components of bio-oils.
Fig. 1 shows the IR spectra of pyridine adsorption in the range of 14001600 cm-1adsorbed on HZSM-5 and Zn/ZSM-5 with different Zn contents.
Three samples give the band 1450 cm-1due to pyridine adsorbed on Lewis acid
sites and the band at 1490 cm-1 assigned to pyridine related with both Lewis
and Brönsted acid sites [7]. For HZSM-5, the band at 1450 cm-1 could assigned
to the Lewis acid derived from little extra-framework Al species, while
Zn/ZSM-5(1.3wt%Zn) appears a new band at 1460 cm-1, suggesting that the
introduction of Zn species could form new Lewis acid sites. However, for
Zn/ZSM-5(2.7wt%Zn), the band attributed to the Lewis acid sites shifts from
1450 cm-1 to 1455 cm-1 and its band intensity increased compared with HZSM5. In comparison with HZSM-5, the total band intensities associated with the
Lewis acid sites of Zn/ZSM-5 samples increase, indicating that the
incorporation of Zn species form new Lewis acid sites.
Fig. 1 IR spectra of pyridine adsorbed on (a) HZSM-5(Si/Al=25), (b) Zn/ZSM5(1.3 wt%Zn), and (c) Zn/ZSM-5(2.7 wt%Zn) at 200 °C.
Table 1 shows the quantitative analysis of cellulose pyrolytic products over
HZSM-5 and Zn/ZSM-5 catalysts. For catalytic pyrolysis of cellulose over
Zn/ZSM-5 catalysts, the main products are anhydro-sugars such as LGA, LGO
and DGP, as well as furan such as furfural and 2.5-dimethyl furan, carboxyl
acid and aldehyde. More interestingly, it is found that the catalytic pyrolysis of
cellulose over Zn/ZSM-5(1.3 wt.%Zn) could form butanoic acid, 2, 3dimethylbutyl ester, while no-catalytic pryolysis and catalytic pyrolysis over
HZSM-5 did not produce the ester. This could be that highly dispersed Zn
species, i.e. Lewis acid sites evidenced from IR spectra of pyridine adsorption,
catalyzes the reaction. In addition, it is found that the amounts of furfural and
2,5-dimethyl furan increase compared with no-catalytic pyrolysis, especially
for Zn/ZSM-5(2.7 wt.%Zn) producing 8.86% furfural. It has been reported that
ZnCl2 can catalytically convert lignocellulosic biomass into furfural [8]. IR
spectra of pyridine adsorption demonstrates that new Lewis acid sites
originated from Zn/ZSM-5, which could be associated with mono-nuclear Zn
ions or di-nuclear Zn species located in the ion-exchange sites of ZSM-5,
would play the same role as ZnCl2 in the formation of furfural. In addition, the
new Lewis acid sites could promote the formation of esters while extraframework Al species does not have the function.
Table 1 Quantitative Analysis of Pyrolytic Products Formed from Catalytic
Pyrolysis of Cellulose over HZSM-5 and Zn/ZSM-5 Catalyst. The data in the
bracket denotes that the structural similarity of the component.
RT
Component name
3.32
3.90
6.39
Furfural
2,5-dimethyl, furan
5-methyl,2furancarboxaldehyde
7.26
3,4-dihydorxyl-3cyclobutene-1,2-dione
9.29
4-pentenoic acid
9.80 Levoglucosenone(LGO)
12.16
Butanoic acid, 2, 3dimethylbutyl ester
12.9
1,4:3.6-dianhydroalpha-d-glucopyranos
(DGP)
23.8
1,6-Anhydro-.beta.
-D-glucopyranose
(levoglucosan)
HZSM5(Si/Al=25)
Zn/ZSM-5(2.7
wt%Zn)
4.59
1.88
1.36
Zn/ZSM5(1.3
wt%Zn)
1.25
4.45
3.99
/
4.26
0.44
/
45.86
/
2.73
34.31
15.12
1.19
31.83
/
15.68
15.84
20.73
15.34
1.22
21.62
8.86
1.63
0.97
In summary, these results showed that high dispersed Zn ions
coordinated with framework Al species remarkably affects the component
content of bio-oil. The Brönsted acid sites of HZSM-5 effectively lowered the
pyrolysis temperature of cellulose and increased the yields of bio-gas and biochar. The Lewis acid sites originated from Zn ions could promote the
formation of esters and furan compounds.
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