*3.1.5. Hydrogenation of benzoic acid with 1%Ru-6%Re/C catalyst*

*3.1.3. Hydrogenation of benzoic acid with 5% Re/C catalyst*

106 New Advances in Hydrogenation Processes - Fundamentals and Applications

*3.1.4. Hydrogenation of benzoic acid with 5%Pd-2%Re/C bimetallic catalyst*

cyclohexyl methanol (CHM), toluene (TOL), and methyl cyclohexane (MCH).

**Scheme 2.** Reaction scheme for hydrogenation of benzoic acid using 5%Ru/C catalyst.

**Scheme 3.** Reaction scheme for hydrogenation of benzoic acid with 5% Re/C catalyst.

cyclohexyl methanol was 50% each.

Hydrogenation of benzoic acid over 5% Re/C is very slow as compared to 5% Ru/C and 5% Pd/C catalysts. The reaction pathway for hydrogenation of benzoic acid over 5% Re/C is also different from that for the other two catalysts as shown in **Scheme 3**. In the literature, ReO<sup>3</sup> is reported to be active for selective hydrogenation of terephthalic acid to p-hydroxymethyl benzoic acid [10]. Conversion of benzoic acid was 6.1%, and selectivity to benzyl alcohol and

The activity of bimetallic 5%Pd-2%Re/C catalyst was higher than that for monometallic 5% Pd/C and 5% Re/C catalysts in terms of benzoic acid conversion (98.5%). But the selectivity to benzyl alcohol, cyclohexane carboxylic acid, or cyclohexyl methanol was low as compared to the other catalysts such as 5% Re/C and 5% Pd/C. This is because hydrogenation of benzoic acid with 5%Pd-2%Re/C catalyst gives benzyl alcohol which undergoes hydrogenolysis. The reaction is thus both consecutive and parallel, which gives a series of products such as CCA, As compared to 5%Pd-2%Re/C catalyst, 1%Ru-6%Re/C showed lower activity in terms of BZA conversion (8.1%). This may be attributed to the low percentage of Ru in the catalyst, which alone is a good hydrogenation catalyst for benzoic acid. With 1%Ru-6%Re/C as catalyst, it was observed that –COOH group was hydrogenated in preference to the aromatic ring. The advantage of 1%Ru-6%Re/C catalyst over 5%Pd-2%Re/C and 5%Ru/C is that the benzyl alcohol (33%) is obtained at the end of reaction.

#### *3.1.6. Hydrogenation of benzoic acid with 5%Ru-29%Sn/Al2 O3 catalyst*

Hydrogenation of benzoic acid with Ru catalyst on different supports such as carbon (C), alumina (Al<sup>2</sup> O3 ), titania (TiO<sup>2</sup> ), and magnesia (MgO) was studied. The activity and selectivity of the catalyst varied with support as seen in **Table 2**.

5%Ru/MgO was the least active catalyst and hydrogenated only the aromatic ring, while Ru on carbon was very active for hydrogenation of aromatic ring but relatively less active for hydrogenation of carboxylic group. With Ru on carbon and magnesia, hydrogenation of BZA to BZOH was not observed. Ru on supports like alumina, silica, and titania was active for hydrogenation but not a selective as seen in **Table 2**.

When Sn is doped in 5% Ru/Al<sup>2</sup> O3 catalyst, the selectivity of the 5%Ru/Al<sup>2</sup> O3 catalyst is changed completely. This catalyst is observed to hydrogenate only the carboxylic group of benzoic acid to give benzyl alcohol as shown in **Scheme 4**. The selectivity to benzyl alcohol is 100% with 24% conversion of benzoic acid.
