127
activity with increased metal loading. Furthermore, an increase in reduction temperature of the catalyst leads to an increase in the activity of the catalyst without
affecting product selectivities. Nickel content of 3–5 wt%, catalyst reduction temperature of 723–773 K, catalyst calcination temperature of 973–1073 K, and reaction temperature of 473 K are found to be optimal. The best catalyst was 5 wt%Ni
loaded on silica-alumina wherein selectivity of 64% was obtained for CHB at 35%
conversion of benzene.
A four-component catalyst system consisting of zeolite 13X loaded with platinum, nickel, and rare-earth ions has been reported for benzene hydroalkylation
[127]. The study reported the effect of Ni, Pt, and rare-earth metal content on the
performance of the catalyst in terms of its activity and selectivity for benzene
hydroalkylation. It has been reported that only Ni/13X favors the formation of
cyclohexane during benzene hydroalkylation whereas by modification of the Ni/13X
with rare-earth metals favors the formation of CHB by hydroalkylation route. This
improvement in selectivity for rare-earth-modified Ni/13X is due to the reduction in
adsorption strength of benzene by alteration in the electron transfer to the metal,
which favors hydroalkylation over hydrogenation, thus improving the selectivity for
CHB [127]. This study also claims that hydroalkylation is favored at lower temperatures by addition of small quantity of platinum, which facilitates a better reduction
of nickel. With the addition of 0.1 wt% platinum, the temperature required to
achieve similar benzene conversion was reduced from 670 to 450 K. A selectivity of
70% was obtained for CHB on the best catalyst, 5 wt%Ni/10 wt%Re/0.1 wt%Pt
loaded on 13X zeolite at a benzene conversion of 20% [127].
Benzene hydroalkylation activity was also studied on palladium-supported beta
zeolite in a batch process [128]. This study reported the effect of parameters like
metal loading, type of metal, and zeolite silica to alumina ratio (25–200), on the
performance of the catalyst. Zeolite beta with a low Si/Al ratio showed higher selectivity for alkylation due to the presence of higher Brønsted acid sites. The nickelloaded catalyst showed very low activity, whereas ruthenium and rhodium showed
high activity with a benzene conversion of around 100%; however, the selectivity
for CHB was very low (around 2%). On the other hand, the palladium-modified
catalyst gave a selectivity of 67% at a benzene conversion of 28%. 0.2 wt% palladium was reported to be optimal for achieving respectable CHB selectivity. With an
increase in Pd loading beyond 0.2wt%, the activity of the catalyst increased, leading
to higher benzene conversion. However, the CHB selectivity is reported to decrease
due to increased hydrogenation activity leading to complete hydrogenation of the
intermediate cyclohexene. This study also reported the effect of reaction conditions
like temperature, pressure, and reaction time on the activity and selectivity of the
catalyst. With an increase in reaction temperature, benzene conversion increased
linearly; however, the selectivity of CHB decreased due to an increase in the selectivity for cyclohexane and DCHB. A similar trend was seen with an increase in
hydrogen pressure and reaction time. Based on the obtained results, it was concluded that zeolite beta with a silica to alumina ratio of 25 and loaded with 0.2% Pd
having an optimum ratio of metal active sites and acidic sites is a potential catalyst
for benzene hydroalkylation and is the key for selectivity to CHB. This study also
Emerging Trends in Solid Acid Catalyst Alkylation Processes
activity with increased metal loading. Furthermore, an increase in reduction temperature of the catalyst leads to an increase in the activity of the catalyst without
affecting product selectivities. Nickel content of 3–5 wt%, catalyst reduction temperature of 723–773 K, catalyst calcination temperature of 973–1073 K, and reaction temperature of 473 K are found to be optimal. The best catalyst was 5 wt%Ni
loaded on silica-alumina wherein selectivity of 64% was obtained for CHB at 35%
conversion of benzene.
A four-component catalyst system consisting of zeolite 13X loaded with platinum, nickel, and rare-earth ions has been reported for benzene hydroalkylation
[127]. The study reported the effect of Ni, Pt, and rare-earth metal content on the
performance of the catalyst in terms of its activity and selectivity for benzene
hydroalkylation. It has been reported that only Ni/13X favors the formation of
cyclohexane during benzene hydroalkylation whereas by modification of the Ni/13X
with rare-earth metals favors the formation of CHB by hydroalkylation route. This
improvement in selectivity for rare-earth-modified Ni/13X is due to the reduction in
adsorption strength of benzene by alteration in the electron transfer to the metal,
which favors hydroalkylation over hydrogenation, thus improving the selectivity for
CHB [127]. This study also claims that hydroalkylation is favored at lower temperatures by addition of small quantity of platinum, which facilitates a better reduction
of nickel. With the addition of 0.1 wt% platinum, the temperature required to
achieve similar benzene conversion was reduced from 670 to 450 K. A selectivity of
70% was obtained for CHB on the best catalyst, 5 wt%Ni/10 wt%Re/0.1 wt%Pt
loaded on 13X zeolite at a benzene conversion of 20% [127].
Benzene hydroalkylation activity was also studied on palladium-supported beta
zeolite in a batch process [128]. This study reported the effect of parameters like
metal loading, type of metal, and zeolite silica to alumina ratio (25–200), on the
performance of the catalyst. Zeolite beta with a low Si/Al ratio showed higher selectivity for alkylation due to the presence of higher Brønsted acid sites. The nickelloaded catalyst showed very low activity, whereas ruthenium and rhodium showed
high activity with a benzene conversion of around 100%; however, the selectivity
for CHB was very low (around 2%). On the other hand, the palladium-modified
catalyst gave a selectivity of 67% at a benzene conversion of 28%. 0.2 wt% palladium was reported to be optimal for achieving respectable CHB selectivity. With an
increase in Pd loading beyond 0.2wt%, the activity of the catalyst increased, leading
to higher benzene conversion. However, the CHB selectivity is reported to decrease
due to increased hydrogenation activity leading to complete hydrogenation of the
intermediate cyclohexene. This study also reported the effect of reaction conditions
like temperature, pressure, and reaction time on the activity and selectivity of the
catalyst. With an increase in reaction temperature, benzene conversion increased
linearly; however, the selectivity of CHB decreased due to an increase in the selectivity for cyclohexane and DCHB. A similar trend was seen with an increase in
hydrogen pressure and reaction time. Based on the obtained results, it was concluded that zeolite beta with a silica to alumina ratio of 25 and loaded with 0.2% Pd
having an optimum ratio of metal active sites and acidic sites is a potential catalyst
for benzene hydroalkylation and is the key for selectivity to CHB. This study also
Emerging Trends in Solid Acid Catalyst Alkylation Processes