528
condensation and ketonization of acetaldehyde to acetone which is converted to
propylene by dehydration of isopropyl alcohol formed by reduction of acetone.
Though the metal oxide catalysts display better stability vis-à-vis zeolites, achieving higher C 3 –C 4 olefins selectivity remains a challenge.
4.5 Conversion of Ethanol to Isobutene
Two processes, based on mixed metal oxide catalysts, are available for the conversion of ethanol to isobutene [166]. The first one is a two-step process involving
formation of acetone from ethanol in the first step followed by its conversion to
isobutene in the second step according to the equations:
2
4
3
2
2
3
3
2
2
CH CH OH H O CH COCH CO
H
+
→
+
+
(1)
3
2
3
3
4 8
2
2
CH COCH
i C H CO H O
→ −
+
+
(2)
The first step involves base-catalysed dehydrogenation of ethanol to yield acetaldehyde, which on aldol condensation and de-carbonylation results in acetone formation [185, 186]. An alternative route [187] consists of oxidation of acetaldehyde to
acetic acid, which can undergo ketonization to acetone. Amongst the different catalysts explored [166], Cu supported on La 2 Zr 2 O 7 catalyst [188] displayed 72% carbon selectivity for acetone (96% of theoretical yield) at 400 °C, with water/ethanol
mole ratio of 9:1 The second step for the conversion of acetone to isobutene is a
complex process, involving three molecules of acetone and many reactions, like
condensation, dehydration and decomposition, probably requiring Bronsted or
Lewis acid-base pair sites [189, 190], where acetone condensation reaction is initiated. Condensation is envisaged between two acetone molecules, one in gas phase
and the other adsorbed on Lewis acid sites, while it occurs through two molecules
adsorbed on Bronsted acid sites [191]. Such a complex pathway involving two types
of acid sites with varying strengths results in further conversion of isobutene and
formation of mesityl oxide intermediates, and higher reaction temperatures lead to
formation of aromatics [166]. Zeolite β [192] with moderate acidity displays 87%
isobutene selectivity at 400 °C and better stability in comparison with ZSM-5 [193].
Zhu et al. observed that use of nanosize zeolites helps to control the rate of deactivation, possibly by promoting facile diffusion and reducing residence time of reactants and intermediates within the pores [194].
In contrast, the one-step process developed by Zhu et al. [195] uses Zn x Zr y O z
mixed oxide catalyst with balanced acidity-basicity characteristics. It was observed
that addition of ZnO moderated strong acid sites present in ZrO 2 , favouring dehydrogenation of ethanol to acetaldehyde against dehydration to ethylene [196, 197].
Aldol condensation of acetaldehyde followed by ketonization leads to the formation
of acetone. Presence of weak Bronsted acid sites brings out further conversion of
acetone, via diacetone alcohol and mesityl oxide, to the final product isobutene. The
R. Vinayagamoorthi et al.
condensation and ketonization of acetaldehyde to acetone which is converted to
propylene by dehydration of isopropyl alcohol formed by reduction of acetone.
Though the metal oxide catalysts display better stability vis-à-vis zeolites, achieving higher C 3 –C 4 olefins selectivity remains a challenge.
4.5 Conversion of Ethanol to Isobutene
Two processes, based on mixed metal oxide catalysts, are available for the conversion of ethanol to isobutene [166]. The first one is a two-step process involving
formation of acetone from ethanol in the first step followed by its conversion to
isobutene in the second step according to the equations:
2
4
3
2
2
3
3
2
2
CH CH OH H O CH COCH CO
H
+
→
+
+
(1)
3
2
3
3
4 8
2
2
CH COCH
i C H CO H O
→ −
+
+
(2)
The first step involves base-catalysed dehydrogenation of ethanol to yield acetaldehyde, which on aldol condensation and de-carbonylation results in acetone formation [185, 186]. An alternative route [187] consists of oxidation of acetaldehyde to
acetic acid, which can undergo ketonization to acetone. Amongst the different catalysts explored [166], Cu supported on La 2 Zr 2 O 7 catalyst [188] displayed 72% carbon selectivity for acetone (96% of theoretical yield) at 400 °C, with water/ethanol
mole ratio of 9:1 The second step for the conversion of acetone to isobutene is a
complex process, involving three molecules of acetone and many reactions, like
condensation, dehydration and decomposition, probably requiring Bronsted or
Lewis acid-base pair sites [189, 190], where acetone condensation reaction is initiated. Condensation is envisaged between two acetone molecules, one in gas phase
and the other adsorbed on Lewis acid sites, while it occurs through two molecules
adsorbed on Bronsted acid sites [191]. Such a complex pathway involving two types
of acid sites with varying strengths results in further conversion of isobutene and
formation of mesityl oxide intermediates, and higher reaction temperatures lead to
formation of aromatics [166]. Zeolite β [192] with moderate acidity displays 87%
isobutene selectivity at 400 °C and better stability in comparison with ZSM-5 [193].
Zhu et al. observed that use of nanosize zeolites helps to control the rate of deactivation, possibly by promoting facile diffusion and reducing residence time of reactants and intermediates within the pores [194].
In contrast, the one-step process developed by Zhu et al. [195] uses Zn x Zr y O z
mixed oxide catalyst with balanced acidity-basicity characteristics. It was observed
that addition of ZnO moderated strong acid sites present in ZrO 2 , favouring dehydrogenation of ethanol to acetaldehyde against dehydration to ethylene [196, 197].
Aldol condensation of acetaldehyde followed by ketonization leads to the formation
of acetone. Presence of weak Bronsted acid sites brings out further conversion of
acetone, via diacetone alcohol and mesityl oxide, to the final product isobutene. The
R. Vinayagamoorthi et al.