**1. Introduction**

The hydrogenation of carboxylic acids or their esters poses major challenges, as among the carbonyl compounds, the acid/ester carbonyl is less reactive than that of ketone and aldehyde [1]. The formation of alcohols by the hydrogenation of the carbonyl group is one of the important reactions, for academia as well as industry, particularly for fatty alcohols, which have been extensively manufactured from fatty acids using hydrogenation technology [2]. Hence, there is a need for new type of catalysts that make it possible to hydrogenate carboxylic acids to alcohols under mild reaction conditions. The combination of group VIII metals with metals like rhenium, germanium, and tin has resulted in better catalysts for this reaction. The first report on the hydrogenation using these catalysts was by Louessard et al., who studied the hydrogenation of ethyl acetate to ethanol over Ru-Sn/Al<sup>2</sup> O3 catalyst [3]. Cheath et al. prepared Ru-Sn/Al<sup>2</sup> O3 catalyst by sol-gel method for the hydrogenation of carboxylic acids and esters; this catalyst was found to exhibit high catalytic performance [4]. They have also reported this catalyst for selectively hydrogenating different carboxylic acids; however, for terephthalic and isophthalic acid, the selectivity to the corresponding alcohols was 19 and 9.2%, respectively, with considerable amount of over-hydrogenated products [5].

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© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The bimetallic Ru-Co catalyst has been reported for hydrogenation of succinic acid. The doping of ruthenium in cobalt increases the overall hydrogenation activity and the Ru/Co ratio in the Ru-Co bimetallic catalyst was found to affect the selectivity of gamma butyrolactone and tetrahydrofuran [6]. Catalysts such as Mn-K/CeO<sup>2</sup> -Al<sup>2</sup> O3 are also reported for hydrogenation of benzoic acid to benzaldehyde [7].

Anderson et al. have reported that product distribution for hydrogenation of aromatic acids over Pd/C and Pd supported on carbon nanofiber in aqueous and aprotic organic solvents was almost the same [8]. Chemoselective hydrogenation of benzoic acid to cyclohexane carboxylic acid was found to be efficient over Ni-B[Ni-Zr-B-PEG(800)] amorphous alloy catalyst than noble catalyst [9].

This chapter gives information related to hydrogenation of benzoic acid, which contains two groups, i.e., aromatic ring and carboxylic group, which is not reported in detail.
