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investigated the effect of zeolite acidity, nature of metal, and metal wt% to derive
optimum catalyst composition for hydroalkylation of benzene.
Hydroalkylation activity of mordenite and beta zeolites modified with 0.2 wt%
of ruthenium has been performed in a batch reactor with cyclohexane as a solvent
[137]. The study demonstrated that beta zeolite with 0.2 wt% Ru possessed higher
activity than mordenite loaded with 0.2 wt% Ru due to easy access of acid sites for
hydroalkylation, leading to the selectivities of 60–40% for CHB and DCHB cumulatively at a benzene conversion of 30 and 80%, respectively. Comparing the results
at the same benzene conversion showed higher CHB selectivity using beta zeolite
with 0.2 wt% Ru as compared to mordenite loaded with 0.2 wt% Ru.
More recently, hydroalkylation of benzene using more complex Pd/HBeta core–
shell catalyst with extra acid sites produced by coating Si-Al framework outside the
conventional Pd/HBeta has been reported [138]. Thus, prepared zeolites were modified with Palladium metal 0.1–4.0 wt%) and evaluated for hydroalkylation of benzene using a batch reactor. By employing the above approach, the Lewis and
Bronsted acid sites of Pd/HBeta increased by 19 and 58%, respectively, as compared to Pd/HBeta. Furthermore, Pd@HBeta showed 44% CHB selectivity at 85%
conversion of benzene as compared to 25% CHB selectivity using Pd/HBeta at
similar benzene conversion. The enhanced activity and CHB selectivity in the case
of Pd@HBeta as compared to Pd/HBeta were attributed to the presence of higher
Bronsted acidity in the case of Pd@HBeta. Additionally, increased Pd metal loading
on Pd@HBeta and Pd/HBeta increased benzene conversion; however, CHB selectivity decreased, thus confirming the fact that higher metal sites favor hydrogenation
instead of hydroalkylation. Hydroalkylation performance of different metalmodified HBeta catalyst were also evaluated under similar operating conditions.
Beta zeolite modified with 0.2 wt% Rh and Ru yielded benzene conversion of 85%
followed by Pd with 56% benzene conversion. On the other hand, Ni-modified beta
showed very low benzene conversion (1.3%) without any CHB yield. Among all the
metal tested, CHB selectivity was the highest for Pd (25.1%), followed by Rh
(18.3%) and Ru (15.8%) at the same benzene conversions of 85%. Similarly, the
effect of support acidity on hydroalkylation performance was studied using different catalyst supports (HY, HMCM-41, γ-Al 2 O 3 , and SiO 2 ). Among all the catalysts
tested, Pd/ SiO 2 showed the least activity for benzene conversion as there are no acid
sites present on the catalyst support followed by Pd/MCM-41, Pd/HY, and Pd/γ- -
Al 2 O 3 . However, on catalysts Pd/ SiO 2 , Pd/MCM-41 and Pd/γ-Al 2 O 3 selectivity for
CHB was insignificant, thus confirming the fact that Brönsted acid sites are vital for
the hydroalkylation reaction. In addition to this, the effect of the ratio of Brönsted
acid sites and Lewis acid (n B /n L ) and Brönsted acid/metal sites (n B /n m ) were also
studied. As per obtained results, higher ratio of n B /n L is found to favor hydroalkylation. Furthermore, the effect of metal particle size has been investigated. The
obtained results showed that catalysts with smaller metal particles showed marginally higher benzene conversion with higher selectivity for cyclohexane, whereas
catalysts with higher metal particle size favored hydroalkylation.
Summary of suitable features for the literature-reported bi-functional hydroalkylation catalysts is listed in Table 7. Several bifunctional catalyst recipes
S. M. Pai et al.
investigated the effect of zeolite acidity, nature of metal, and metal wt% to derive
optimum catalyst composition for hydroalkylation of benzene.
Hydroalkylation activity of mordenite and beta zeolites modified with 0.2 wt%
of ruthenium has been performed in a batch reactor with cyclohexane as a solvent
[137]. The study demonstrated that beta zeolite with 0.2 wt% Ru possessed higher
activity than mordenite loaded with 0.2 wt% Ru due to easy access of acid sites for
hydroalkylation, leading to the selectivities of 60–40% for CHB and DCHB cumulatively at a benzene conversion of 30 and 80%, respectively. Comparing the results
at the same benzene conversion showed higher CHB selectivity using beta zeolite
with 0.2 wt% Ru as compared to mordenite loaded with 0.2 wt% Ru.
More recently, hydroalkylation of benzene using more complex Pd/HBeta core–
shell catalyst with extra acid sites produced by coating Si-Al framework outside the
conventional Pd/HBeta has been reported [138]. Thus, prepared zeolites were modified with Palladium metal 0.1–4.0 wt%) and evaluated for hydroalkylation of benzene using a batch reactor. By employing the above approach, the Lewis and
Bronsted acid sites of Pd/HBeta increased by 19 and 58%, respectively, as compared to Pd/HBeta. Furthermore, Pd@HBeta showed 44% CHB selectivity at 85%
conversion of benzene as compared to 25% CHB selectivity using Pd/HBeta at
similar benzene conversion. The enhanced activity and CHB selectivity in the case
of Pd@HBeta as compared to Pd/HBeta were attributed to the presence of higher
Bronsted acidity in the case of Pd@HBeta. Additionally, increased Pd metal loading
on Pd@HBeta and Pd/HBeta increased benzene conversion; however, CHB selectivity decreased, thus confirming the fact that higher metal sites favor hydrogenation
instead of hydroalkylation. Hydroalkylation performance of different metalmodified HBeta catalyst were also evaluated under similar operating conditions.
Beta zeolite modified with 0.2 wt% Rh and Ru yielded benzene conversion of 85%
followed by Pd with 56% benzene conversion. On the other hand, Ni-modified beta
showed very low benzene conversion (1.3%) without any CHB yield. Among all the
metal tested, CHB selectivity was the highest for Pd (25.1%), followed by Rh
(18.3%) and Ru (15.8%) at the same benzene conversions of 85%. Similarly, the
effect of support acidity on hydroalkylation performance was studied using different catalyst supports (HY, HMCM-41, γ-Al 2 O 3 , and SiO 2 ). Among all the catalysts
tested, Pd/ SiO 2 showed the least activity for benzene conversion as there are no acid
sites present on the catalyst support followed by Pd/MCM-41, Pd/HY, and Pd/γ- -
Al 2 O 3 . However, on catalysts Pd/ SiO 2 , Pd/MCM-41 and Pd/γ-Al 2 O 3 selectivity for
CHB was insignificant, thus confirming the fact that Brönsted acid sites are vital for
the hydroalkylation reaction. In addition to this, the effect of the ratio of Brönsted
acid sites and Lewis acid (n B /n L ) and Brönsted acid/metal sites (n B /n m ) were also
studied. As per obtained results, higher ratio of n B /n L is found to favor hydroalkylation. Furthermore, the effect of metal particle size has been investigated. The
obtained results showed that catalysts with smaller metal particles showed marginally higher benzene conversion with higher selectivity for cyclohexane, whereas
catalysts with higher metal particle size favored hydroalkylation.
Summary of suitable features for the literature-reported bi-functional hydroalkylation catalysts is listed in Table 7. Several bifunctional catalyst recipes
S. M. Pai et al.