136
reduced number of processing steps, leads to a substantial reduction in the cost of
capital investment as compared to the existing technology.
1.3.3 Chemistry of Side-Chain Methylation of Toluene with Methanol
In the base-catalyzed alkylation of toluene with methanol, the addition of the methyl
group takes place on the side chain attached to the ring, and hence the process is
called side-chain alkylation of toluene. The main reaction products are ethyl benzene and styrene, as shown in Fig. 13. The side-chain reaction of toluene with methanol has received worldwide attention since it has the potential for a new novel route
to styrene production. Conceptually, this reaction occurs in two parts: the formation
of formaldehyde by endothermic dehydrogenation of methanol and the exothermic
addition of formaldehyde to toluene to produce styrene and ethyl benzene. Both
processes, when properly balanced, make the entire process of side-chain toluene
methylation nearly thermodynamically zero [156].
This reaction was first reported over low Si/Al ratio X zeolites with moderate
basicity wherein high selectivity of styrene and ethylbenzene was obtained in the
toluene methylation [157]. After this work, many catalysts have been reported based
on modified X, Y, and other zeolites. Styrene and ethylbenzene were the main products over Na-, K-, Rb-, and Cs-modified X and Y zeolites, while xylene was observed
over Li-X and Li-Y zeolites during toluene alkylation with methanol [158]. Sidechain alkylation was favored over Cs- and B-modified zeolite 13X deposited with
Cu and Ag in the presence of hydrogen as a carrier gas [152]. Binary zeolites prepared using KX, KY, KM, and KZSM-5 showed higher activity as compared to the
individual zeolites. The binary zeolites, when further modified with KOH and boric
acid, showed improved selectivity to styrene, which implies that the optimum level
of acid–base sites is essential for the reaction [159]. Further studies reinforced that
the selectivity to side-chain alkylation improved with the addition of phosphoric
and H 3 BO 3 to ion exchange solution. High selectivity of >50% for styrene (SM) and
EB calculated on the basis of methanol was observed on Cs-X zeolite modified with
borate. The high selectivity for styrene over Cs-X could be explained on the basis of
the adsorption of toluene among two or more large cations present in the X-zeolite
super-cage with large number of cations in such a way that (1) the toluene molecule
was at higher electrostatic potential than expected and (2) the methyl group only
was easily accessible for alkylation (3) because of the strong interaction of cations
with aromatic molecules. Also, incorporation of the borate in the super-cage slowed
down the decomposition of the real alkylating agent, formaldehyde [160].
Fig. 13 Side-chain alkylation of toluene with methanol
S. M. Pai et al.
reduced number of processing steps, leads to a substantial reduction in the cost of
capital investment as compared to the existing technology.
1.3.3 Chemistry of Side-Chain Methylation of Toluene with Methanol
In the base-catalyzed alkylation of toluene with methanol, the addition of the methyl
group takes place on the side chain attached to the ring, and hence the process is
called side-chain alkylation of toluene. The main reaction products are ethyl benzene and styrene, as shown in Fig. 13. The side-chain reaction of toluene with methanol has received worldwide attention since it has the potential for a new novel route
to styrene production. Conceptually, this reaction occurs in two parts: the formation
of formaldehyde by endothermic dehydrogenation of methanol and the exothermic
addition of formaldehyde to toluene to produce styrene and ethyl benzene. Both
processes, when properly balanced, make the entire process of side-chain toluene
methylation nearly thermodynamically zero [156].
This reaction was first reported over low Si/Al ratio X zeolites with moderate
basicity wherein high selectivity of styrene and ethylbenzene was obtained in the
toluene methylation [157]. After this work, many catalysts have been reported based
on modified X, Y, and other zeolites. Styrene and ethylbenzene were the main products over Na-, K-, Rb-, and Cs-modified X and Y zeolites, while xylene was observed
over Li-X and Li-Y zeolites during toluene alkylation with methanol [158]. Sidechain alkylation was favored over Cs- and B-modified zeolite 13X deposited with
Cu and Ag in the presence of hydrogen as a carrier gas [152]. Binary zeolites prepared using KX, KY, KM, and KZSM-5 showed higher activity as compared to the
individual zeolites. The binary zeolites, when further modified with KOH and boric
acid, showed improved selectivity to styrene, which implies that the optimum level
of acid–base sites is essential for the reaction [159]. Further studies reinforced that
the selectivity to side-chain alkylation improved with the addition of phosphoric
and H 3 BO 3 to ion exchange solution. High selectivity of >50% for styrene (SM) and
EB calculated on the basis of methanol was observed on Cs-X zeolite modified with
borate. The high selectivity for styrene over Cs-X could be explained on the basis of
the adsorption of toluene among two or more large cations present in the X-zeolite
super-cage with large number of cations in such a way that (1) the toluene molecule
was at higher electrostatic potential than expected and (2) the methyl group only
was easily accessible for alkylation (3) because of the strong interaction of cations
with aromatic molecules. Also, incorporation of the borate in the super-cage slowed
down the decomposition of the real alkylating agent, formaldehyde [160].
Fig. 13 Side-chain alkylation of toluene with methanol
S. M. Pai et al.