137
The Cs-X zeolite shows higher alkylation activity as compared to boronimpregnated Cs
+
exchanged X, and this may be due to the increase in acidity of the
boron-impregnated catalyst [161–164]. On the other hand, boron-impregnated catalyst exhibited higher selectivity to toluene alkylation and activity compared to Cs-X
zeolites impregnated with CsO since it increased the dehydrogenation activity for
methanol [165]. This was because the incorporation of boron results in a lowering
of the overall basic strength of the CsO cluster by producing B 2 Cs 2 O 4 species inside
the zeolite channels.
The primary aim of the addition of boron to Cs-X was to poison the higher
strength basic sites in order to reduce the decomposition of formaldehyde to carbon
monoxide during the side-chain toluene methylation without inhibiting the active
sites required for alkylation [166]. In another study, it was reported that alkaliexchanged X and Y zeolites with high basicity were active for toluene alkylation but
also caused decomposition of methanol to CO to a higher extent. However, the
Cs-exchanged L and beta zeolites exhibited low formic acid decomposition to CO
but required higher temperatures to attain similar aromatic yields during the sidechain alkylation compared to X and Y zeolites [167].
Theoretical studies involving quantum chemical calculation have suggested that
the requirement of basic sites for side-chain alkylation is indispensable. However,
specific combinations of acidic and basic sites promote side-chain alkylation to
achieve better selectivity [168]. Rb-X promoted with a small amount of Li
+
ions was
more active as compared to Rb-X for p-xylene side-chain alkylation [169, 170] due
to the presence of slightly stronger acid sites in Li-Rb-X. The decomposition of
formic acid intermediate was suppressed due to the incorporation of Li
+
ions.
It is reported that in toluene alkylation process, the ability to dehydrogenate
methanol and the selectivity to side-chain methylation of toluene are directly proportional to the basicity imposed by the alkali metal cation-exchanged zeolites. The
topology of the zeolite and the Si/Al ratio are responsible for the proximity of acid–
base sites and the zeolite basicity. The introduction of metal oxide clusters into the
zeolite pores imparts high basicity to the zeolite and can promote side-chain toluene
alkylation with methanol [165, 171, 172]. However, large amounts of CO are formed
from methanol decomposition over such strongly basic material.
It was reported that Fe-Mo dual catalyst oxide deposited over Cs
+
-exchanged
zeolites leads to enhancement in the styrene/ethylbenzene ratio by a factor of 4.5.
The Fe-Mo promoted zeolites were found to be active at 598 K [163] as against the
requirement of temperatures higher than 648 K for the unpromoted single Cs
+
-
exchanged catalysts. This may be due to the concerted interaction of toluene with
Cs
+
-exchanged zeolite and Fe-Mo oxide.
The use of layered double hydroxides (LDH) for side-chain toluene alkylation is
also reported [173]. As shown in Table 10, Mg-Al-LDH was selective for toluene
side-chain alkylation with methanol by mainly producing styrene and EB, while the
other LDHs synthesized using Co, Ni, Cu, and Zn gave a mixture of side chain as
well as ring-alkylated products, viz., xylene and mesitylene. The selectivity of styrene was found to be higher over Mg-Al-LDHs with low Al content. Higher ethyl
benzene formation was observed at 673 K.
Emerging Trends in Solid Acid Catalyst Alkylation Processes
The Cs-X zeolite shows higher alkylation activity as compared to boronimpregnated Cs
+
exchanged X, and this may be due to the increase in acidity of the
boron-impregnated catalyst [161–164]. On the other hand, boron-impregnated catalyst exhibited higher selectivity to toluene alkylation and activity compared to Cs-X
zeolites impregnated with CsO since it increased the dehydrogenation activity for
methanol [165]. This was because the incorporation of boron results in a lowering
of the overall basic strength of the CsO cluster by producing B 2 Cs 2 O 4 species inside
the zeolite channels.
The primary aim of the addition of boron to Cs-X was to poison the higher
strength basic sites in order to reduce the decomposition of formaldehyde to carbon
monoxide during the side-chain toluene methylation without inhibiting the active
sites required for alkylation [166]. In another study, it was reported that alkaliexchanged X and Y zeolites with high basicity were active for toluene alkylation but
also caused decomposition of methanol to CO to a higher extent. However, the
Cs-exchanged L and beta zeolites exhibited low formic acid decomposition to CO
but required higher temperatures to attain similar aromatic yields during the sidechain alkylation compared to X and Y zeolites [167].
Theoretical studies involving quantum chemical calculation have suggested that
the requirement of basic sites for side-chain alkylation is indispensable. However,
specific combinations of acidic and basic sites promote side-chain alkylation to
achieve better selectivity [168]. Rb-X promoted with a small amount of Li
+
ions was
more active as compared to Rb-X for p-xylene side-chain alkylation [169, 170] due
to the presence of slightly stronger acid sites in Li-Rb-X. The decomposition of
formic acid intermediate was suppressed due to the incorporation of Li
+
ions.
It is reported that in toluene alkylation process, the ability to dehydrogenate
methanol and the selectivity to side-chain methylation of toluene are directly proportional to the basicity imposed by the alkali metal cation-exchanged zeolites. The
topology of the zeolite and the Si/Al ratio are responsible for the proximity of acid–
base sites and the zeolite basicity. The introduction of metal oxide clusters into the
zeolite pores imparts high basicity to the zeolite and can promote side-chain toluene
alkylation with methanol [165, 171, 172]. However, large amounts of CO are formed
from methanol decomposition over such strongly basic material.
It was reported that Fe-Mo dual catalyst oxide deposited over Cs
+
-exchanged
zeolites leads to enhancement in the styrene/ethylbenzene ratio by a factor of 4.5.
The Fe-Mo promoted zeolites were found to be active at 598 K [163] as against the
requirement of temperatures higher than 648 K for the unpromoted single Cs
+
-
exchanged catalysts. This may be due to the concerted interaction of toluene with
Cs
+
-exchanged zeolite and Fe-Mo oxide.
The use of layered double hydroxides (LDH) for side-chain toluene alkylation is
also reported [173]. As shown in Table 10, Mg-Al-LDH was selective for toluene
side-chain alkylation with methanol by mainly producing styrene and EB, while the
other LDHs synthesized using Co, Ni, Cu, and Zn gave a mixture of side chain as
well as ring-alkylated products, viz., xylene and mesitylene. The selectivity of styrene was found to be higher over Mg-Al-LDHs with low Al content. Higher ethyl
benzene formation was observed at 673 K.
Emerging Trends in Solid Acid Catalyst Alkylation Processes