725
significantly with the increase in calcination temperature to 750 °C. These results
emphasize that at high calcination temperatures, a significant drop in catalyst active
sites consequently on 2,6-DMP synthesis rate is observed.
In the later investigation, CuCr 2 O 4 catalyst prepared by the sol-gel method was
further modified with different promoters such as zinc, zirconium, and magnesium
to study their effect on product distribution in the synthesis of 2,6-DMP from crude
glycerol and 1,2-PDA [12]. Among them, it was noticed that the Zn-promoted
Cu-Cr catalyst exhibited a higher rate of 2,6-DMP formation. A detailed investigation on the acid-base sites indicated that the stable activity of Zn-modified CuCr 2 O 4
catalyst was mainly due to stabilization of Cu
0
species and improved Lewis acidity.
In addition, Zn addition (up to 5 wt%) resulted in highly dispersed copper on the
catalyst surface with improved activity and stability. In this study, it was found that
strong basicity with a pair of weak to moderate acid sites (Lewis) is crucial for the
desired selectivity of 2,6-DMP.
Finally, in a recent investigation, a nano-crystalline ZnO-ZnCr 2 O 4 and mixed
oxide hydrotalcite precursor material were prepared at different pH (8, 10, and 12)
and evaluated to convert the crude bio-glycerol to 2,6-DMP [13]. Different aspects
such as the nature and strength of acid-base properties were discussed, and it was
found that the ZnO-ZnCr 2 O 4 catalyst with high ratio of Lewis to Brønsted basic
sites was key to selective 2,6-DMP synthesis. The ZnO-ZnCr 2 O 4 catalyst that prepared a pH of 10 demonstrated highest activity compared to other catalysts that
were mainly due to finely dispersed ZnO-ZnCr 2 O 4 particles on the surface. It was
also found that the presence of strong acidity resulted in lower activity. It can be
concluded that conversion of aqueous (crude) glycerol with corresponding amines
to synthesize alkyl pyrazines entails weak Lewis acidity in conjunction with a pair
of strong basic sites. In this investigation, a comparative study is made in parenthesis to the recent reports on dehydrocyclization of glycerol, and diamine compounds
in reference to 2MP rates are discussed.
2 Experimental
2.1 Preparation of Catalysts
Zn supported on Cr 2 O 3 catalysts was prepared with different loadings of ZnO loadings via wet impregnation method. In this method, 2, 5, 7, 10, and 15 wt% of ZnO
loadings on Cr 2 O 3 support was prepared. In a typical experiment, Cr 2 O 3 was dispersed in double-distilled water and taken in a beaker, and then the desired quantity
of Zn(NO 3 ) 2 ∙6H 2 O was added portion-wise while continuous stirring and heating at
80 °C and oven-dried for 12 h at 100 °C, subsequently calcined at 450 °C in static
air. Catalysts with 2, 5, 7, 10, and 15 wt% loadings of ZnO were denoted as 2ZnO/
Cr 2 O 3 , 5ZnO/Cr 2 O 3 , 7ZnO/Cr 2 O 3 , 10ZnO/Cr 2 O 3 , and 15ZnO/Cr 2 O 3 , respectively.
A Sustainable Process for the Synthesis of Alkylpyrazines by Dehydrocyclization…
significantly with the increase in calcination temperature to 750 °C. These results
emphasize that at high calcination temperatures, a significant drop in catalyst active
sites consequently on 2,6-DMP synthesis rate is observed.
In the later investigation, CuCr 2 O 4 catalyst prepared by the sol-gel method was
further modified with different promoters such as zinc, zirconium, and magnesium
to study their effect on product distribution in the synthesis of 2,6-DMP from crude
glycerol and 1,2-PDA [12]. Among them, it was noticed that the Zn-promoted
Cu-Cr catalyst exhibited a higher rate of 2,6-DMP formation. A detailed investigation on the acid-base sites indicated that the stable activity of Zn-modified CuCr 2 O 4
catalyst was mainly due to stabilization of Cu
0
species and improved Lewis acidity.
In addition, Zn addition (up to 5 wt%) resulted in highly dispersed copper on the
catalyst surface with improved activity and stability. In this study, it was found that
strong basicity with a pair of weak to moderate acid sites (Lewis) is crucial for the
desired selectivity of 2,6-DMP.
Finally, in a recent investigation, a nano-crystalline ZnO-ZnCr 2 O 4 and mixed
oxide hydrotalcite precursor material were prepared at different pH (8, 10, and 12)
and evaluated to convert the crude bio-glycerol to 2,6-DMP [13]. Different aspects
such as the nature and strength of acid-base properties were discussed, and it was
found that the ZnO-ZnCr 2 O 4 catalyst with high ratio of Lewis to Brønsted basic
sites was key to selective 2,6-DMP synthesis. The ZnO-ZnCr 2 O 4 catalyst that prepared a pH of 10 demonstrated highest activity compared to other catalysts that
were mainly due to finely dispersed ZnO-ZnCr 2 O 4 particles on the surface. It was
also found that the presence of strong acidity resulted in lower activity. It can be
concluded that conversion of aqueous (crude) glycerol with corresponding amines
to synthesize alkyl pyrazines entails weak Lewis acidity in conjunction with a pair
of strong basic sites. In this investigation, a comparative study is made in parenthesis to the recent reports on dehydrocyclization of glycerol, and diamine compounds
in reference to 2MP rates are discussed.
2 Experimental
2.1 Preparation of Catalysts
Zn supported on Cr 2 O 3 catalysts was prepared with different loadings of ZnO loadings via wet impregnation method. In this method, 2, 5, 7, 10, and 15 wt% of ZnO
loadings on Cr 2 O 3 support was prepared. In a typical experiment, Cr 2 O 3 was dispersed in double-distilled water and taken in a beaker, and then the desired quantity
of Zn(NO 3 ) 2 ∙6H 2 O was added portion-wise while continuous stirring and heating at
80 °C and oven-dried for 12 h at 100 °C, subsequently calcined at 450 °C in static
air. Catalysts with 2, 5, 7, 10, and 15 wt% loadings of ZnO were denoted as 2ZnO/
Cr 2 O 3 , 5ZnO/Cr 2 O 3 , 7ZnO/Cr 2 O 3 , 10ZnO/Cr 2 O 3 , and 15ZnO/Cr 2 O 3 , respectively.
A Sustainable Process for the Synthesis of Alkylpyrazines by Dehydrocyclization…