30
Amphoteric metal oxide catalysts such as ZrO 2 [198, 199], MnO [200, 201],
CeO 2 [202, 203] and UO 2 [204, 205] are the most active catalysts for the ketonization reaction. The basic and acidic characteristics of catalysts play a significant part
in the ketonization reaction. Different characterizations showed that CeO 2 , TiO 2 ,
ZrO 2 and MnO have both acidic and basic properties which confirmed the amphoteric characteristics of these metal oxides [206–209]. In the metal oxide catalysts
generally, metal cations are Lewis acid sites, but oxygen anions are Lewis base sites.
Pham et al. reported that oxygen anions must perform as Brønsted basic sites
instead of Lewis basic sites for surface carboxylate formation in the ketonization
reaction. It is stated that the adjacent Lewis acid (stabilizes and activates another
carboxylic acid) and presence of Brønsted basic (carboxylates formation) on most
oxide catalysts are the keys for the ketonization reaction [210].
There are substantially fewer studies about ketonization of carboxylic acid over
zeolite catalysts in comparison with metal oxide catalysts. The following catalysts
were tested in the ketonization reaction which are HZSM-35, HZSM-34, HZSM-11,
HZSM-5, zeolite beta, erionite and mordenite. It was found that HZSM-5, HZSM-11
and zeolite beta are the most active zeolite catalysts in comparison with other zeolites for acetic acid conversion into acetone [211, 212].
8 Current State of the Art for Furfural
and 2-Methylfuran Production
Furfural is an essential oxygenated compound having C=O and C=C bonds with the
high stability of the C=C bond due to its location inside a one five-member ring and
six electrons [213]. Furfural can be produced from renewable biomass and agricultural wastes rich in xylose (such as sugarcane bagasse, oat hulls and corncobs), and
it can be applied for the formation of essential non-petroleum-derived chemicals
(such as 2-methylfuran (2-MF) and furfuryl alcohol), competing with crude
oil [214].
This compound can also be used in many industrial processes such as pharmaceutical, plastics, oil refining and agrochemical industries where it is transformed
into perfume intermediates, chemical solvents, medicine compounds and pesticides
[215]. Furfural is obtained through the following steps: production of pentose such
as xylose, submitting the hemicellulose material to hydrolysis and the final conversion into furfural by acid-catalysed dehydration of pentose (Fig. 12) [216].
Presently, furfural production is an energy-intensive process due to long side
reactions which causes a decrease of furfural yield because of extensive residence
times and the necessity for substantial waste disposal. The need for high-pressure
steam to heat the reaction is also a drawback in this process [216]. Concentrated
sulphuric acid, which is particularly corrosive and extremely toxic, is used as a catalyst. This brings serious issues compared to homogeneous catalytic reactions such
as problematic separation and reprocessing of the product contamination and
H. Jahangiri et al.
Amphoteric metal oxide catalysts such as ZrO 2 [198, 199], MnO [200, 201],
CeO 2 [202, 203] and UO 2 [204, 205] are the most active catalysts for the ketonization reaction. The basic and acidic characteristics of catalysts play a significant part
in the ketonization reaction. Different characterizations showed that CeO 2 , TiO 2 ,
ZrO 2 and MnO have both acidic and basic properties which confirmed the amphoteric characteristics of these metal oxides [206–209]. In the metal oxide catalysts
generally, metal cations are Lewis acid sites, but oxygen anions are Lewis base sites.
Pham et al. reported that oxygen anions must perform as Brønsted basic sites
instead of Lewis basic sites for surface carboxylate formation in the ketonization
reaction. It is stated that the adjacent Lewis acid (stabilizes and activates another
carboxylic acid) and presence of Brønsted basic (carboxylates formation) on most
oxide catalysts are the keys for the ketonization reaction [210].
There are substantially fewer studies about ketonization of carboxylic acid over
zeolite catalysts in comparison with metal oxide catalysts. The following catalysts
were tested in the ketonization reaction which are HZSM-35, HZSM-34, HZSM-11,
HZSM-5, zeolite beta, erionite and mordenite. It was found that HZSM-5, HZSM-11
and zeolite beta are the most active zeolite catalysts in comparison with other zeolites for acetic acid conversion into acetone [211, 212].
8 Current State of the Art for Furfural
and 2-Methylfuran Production
Furfural is an essential oxygenated compound having C=O and C=C bonds with the
high stability of the C=C bond due to its location inside a one five-member ring and
six electrons [213]. Furfural can be produced from renewable biomass and agricultural wastes rich in xylose (such as sugarcane bagasse, oat hulls and corncobs), and
it can be applied for the formation of essential non-petroleum-derived chemicals
(such as 2-methylfuran (2-MF) and furfuryl alcohol), competing with crude
oil [214].
This compound can also be used in many industrial processes such as pharmaceutical, plastics, oil refining and agrochemical industries where it is transformed
into perfume intermediates, chemical solvents, medicine compounds and pesticides
[215]. Furfural is obtained through the following steps: production of pentose such
as xylose, submitting the hemicellulose material to hydrolysis and the final conversion into furfural by acid-catalysed dehydration of pentose (Fig. 12) [216].
Presently, furfural production is an energy-intensive process due to long side
reactions which causes a decrease of furfural yield because of extensive residence
times and the necessity for substantial waste disposal. The need for high-pressure
steam to heat the reaction is also a drawback in this process [216]. Concentrated
sulphuric acid, which is particularly corrosive and extremely toxic, is used as a catalyst. This brings serious issues compared to homogeneous catalytic reactions such
as problematic separation and reprocessing of the product contamination and
H. Jahangiri et al.