26
ZSM-5 and mordenite [89]. The zeolite pore size affects the formation of aromatics
and coke, and larger pores are related to higher yields of coke [156]. Another relevant factor is the acidity of the zeolite, where lower acidity levels produce more
coke and fewer aromatics [157].
HZSM-5 and ZSM-5 zeolite catalysts are the most tested in catalytic pyrolysis.
ZSM-5 has less selectivity, acidity and thermochemical stability than HZSM-5.
Therefore, HZSM-5 is a more attractive catalyst for aromatics production, and it
promotes catalytic reactions in the biomass such as aromatization, decarbonylation,
cracking, isomerization, decarboxylation, cyclization, oligomerization, deoxygenation and alkylation [89, 158]. HZSM-5 was tested in a pyrolysis fixed bed reactor
for converting microalgae, and the produced bio-oil presented a lower oxygen content (19.6 wt%) and a higher calorific value (32.6 MJ/kg) compared to no catalytic
treatment at the same conditions [159]. Zhang et al. [160] and Stephanidis et al.
[161] used the same zeolite to pyrolyse corncobs and beech wood respectively, and
the bio-oil in both studies showed a higher calorific value (30–35 MJ/kg) and aromatics content and a decrement of oxygenated compounds.
Some works have experimented ZSM-5 catalyst in a pyrolysis fixed bed reactor
using glucose, rice husks, maple wood and furans as feedstock [156, 162]. The general results were a substantial reduction in the yield and oxygen content of the biooil. The chemical structure of ZSM-5 was also altered by adding some metals such
as Zn, Ni, Mo, Ga, Co, Pd and Pt. This modification influenced the alkylation and
cyclization reactions, increasing the zeolite acidity and the aromatics yields, especially using Ga metal [163, 164]. Zeolite catalysts favour the production of coke
which is deposited on the surface of catalyst filling the respective pores.
Consequently, the catalyst is deactivated, reducing its durability, and the formed
coke also absorbs the bio-oil organics, resulting in lower quality and quantity, thus
increasing the cost of the process [89].
Mesoporous catalysts, including Al-SBA-15, MCM-41 and Al-MCM-41, are
also an option for catalytic pyrolysis of materials. The last works developed in a
circulating fluid bed reactor demonstrated the effect of mesoporous catalysts in the
pyrolysis oil composition, increasing the phenolics content and reducing the concentration of acids (carboxylic acid) and carbonyls [165]. Moreover, an increment
of the silica/alumina ratio in this kind of catalyst led to an increase of the organics
and aromatics in the bio-oil but a lower liquid production [166]. The use of mesoporous catalysts shows a clear indication of upgrading treatment; however, the
deoxygenation process is incomplete [165].
Some works investigated the application of mineral catalysts such as MgO, dolomite, SiO 2 , CaO, limestone and K 2 O through pyrolysis of cellulose, pine sawdust,
lignin and poplar wood using a fixed bed reactor at 500 °C [167, 168]. CaO and
MgO (basic oxides) inhibited the production of tar and phenolics in the bio-oil,
while coke and gaseous products presented higher levels [168]. Alternatively, the
use of acid oxides indicated more tar formation. In another research, Na 2 CO 3 catalyst produced a very unstable bio-oil, but the application of Pt enhanced the quality
and the stability of the same pyrolysis oil [157].
H. Jahangiri et al.
ZSM-5 and mordenite [89]. The zeolite pore size affects the formation of aromatics
and coke, and larger pores are related to higher yields of coke [156]. Another relevant factor is the acidity of the zeolite, where lower acidity levels produce more
coke and fewer aromatics [157].
HZSM-5 and ZSM-5 zeolite catalysts are the most tested in catalytic pyrolysis.
ZSM-5 has less selectivity, acidity and thermochemical stability than HZSM-5.
Therefore, HZSM-5 is a more attractive catalyst for aromatics production, and it
promotes catalytic reactions in the biomass such as aromatization, decarbonylation,
cracking, isomerization, decarboxylation, cyclization, oligomerization, deoxygenation and alkylation [89, 158]. HZSM-5 was tested in a pyrolysis fixed bed reactor
for converting microalgae, and the produced bio-oil presented a lower oxygen content (19.6 wt%) and a higher calorific value (32.6 MJ/kg) compared to no catalytic
treatment at the same conditions [159]. Zhang et al. [160] and Stephanidis et al.
[161] used the same zeolite to pyrolyse corncobs and beech wood respectively, and
the bio-oil in both studies showed a higher calorific value (30–35 MJ/kg) and aromatics content and a decrement of oxygenated compounds.
Some works have experimented ZSM-5 catalyst in a pyrolysis fixed bed reactor
using glucose, rice husks, maple wood and furans as feedstock [156, 162]. The general results were a substantial reduction in the yield and oxygen content of the biooil. The chemical structure of ZSM-5 was also altered by adding some metals such
as Zn, Ni, Mo, Ga, Co, Pd and Pt. This modification influenced the alkylation and
cyclization reactions, increasing the zeolite acidity and the aromatics yields, especially using Ga metal [163, 164]. Zeolite catalysts favour the production of coke
which is deposited on the surface of catalyst filling the respective pores.
Consequently, the catalyst is deactivated, reducing its durability, and the formed
coke also absorbs the bio-oil organics, resulting in lower quality and quantity, thus
increasing the cost of the process [89].
Mesoporous catalysts, including Al-SBA-15, MCM-41 and Al-MCM-41, are
also an option for catalytic pyrolysis of materials. The last works developed in a
circulating fluid bed reactor demonstrated the effect of mesoporous catalysts in the
pyrolysis oil composition, increasing the phenolics content and reducing the concentration of acids (carboxylic acid) and carbonyls [165]. Moreover, an increment
of the silica/alumina ratio in this kind of catalyst led to an increase of the organics
and aromatics in the bio-oil but a lower liquid production [166]. The use of mesoporous catalysts shows a clear indication of upgrading treatment; however, the
deoxygenation process is incomplete [165].
Some works investigated the application of mineral catalysts such as MgO, dolomite, SiO 2 , CaO, limestone and K 2 O through pyrolysis of cellulose, pine sawdust,
lignin and poplar wood using a fixed bed reactor at 500 °C [167, 168]. CaO and
MgO (basic oxides) inhibited the production of tar and phenolics in the bio-oil,
while coke and gaseous products presented higher levels [168]. Alternatively, the
use of acid oxides indicated more tar formation. In another research, Na 2 CO 3 catalyst produced a very unstable bio-oil, but the application of Pt enhanced the quality
and the stability of the same pyrolysis oil [157].
H. Jahangiri et al.