## 1 Introduction  
The need for starting materials in industrial processes makes fine chemistry one of the most relevant sectors in the modern industry, having an expressive economic impact in our society. By definition, fine chemicals are economically relevant products manufactured in a limited content (less than 5000 tons per year) with high added-value associated to its production process and end use [1]. In Brazil, more than 90% of the income from this sector belongs to the production of important starting materials for pharmaceutical, agricultural, and livestock industries [2]. According to data collected by the Brazilian Association of Chemical Industries (ABIQUIM), the cited industries ended 2017 with a net billing of approximately US$25.7 billion altogether, which corresponds to 21.4% of the national chemical industry total income [3]. It is noteworthy that the sustainability of industrial actions must be a major motive to investigate alternative routes for established chemical processes. This concern arises from the main disadvantage of fine chemistry: The generation of about 5 to 50 kg of waste per kg of product due to the use of homogeneous catalysis, stoichiometric reagents, and harsh reaction conditions [4, 5].  
Benzaldehyde (BzD) is an essential fine chemical material for several industrial sectors, being used as a flavoring agent, as a denaturant, as a fragrance, and as a precursor to the manufacture of important products, such as, cinnamaldehyde, mandelic acid, and malachite green [6]. According to an economic report from Transparent Market Research [7], the global revenue of the benzaldehyde market surpassed US$234 million in 2014 and the demand of approximately 80 Mt is expected to increase 3.06% until 2023. Since the last century, benzaldehyde is produced following two main industrial processes [6,8]: (i) The alkali hydrolysis of benzal chloride; and (ii) the aerobic oxidation of toluene. Both reaction pathways present environmental and procedural drawbacks: High basicity of effluents and a large amount of residues in the alkali hydrolysis; and high pressure and temperature at the expense of low conversions in the aerobic oxidation [6,8]. Thus, the development of safer, more efficient, and environmentally friendly technologies to improve the production of benzaldehyde is of great interest to the fine chemical industry. The liquid-phase oxidation of benzyl alcohol (BzOH) is an interesting alternative due to its intrinsic benefits: (i) Generation of low levels of effluents with little or no negative environmental effects [9]; (ii) use of milder conditions than the cited industrial processes, with temperatures and partial pressures (for oxidants, like molecular oxygen) not higher than 150 \({}^{\circ}\)C and 3 atm, respectively [9,10]; and (iii) the protective effect of BzOH towards over-oxidation of benzaldehyde to benzoic acid (HBz), mainly due to the low dissociation energy of the methylene group C-H bond [11]. Hence, the study of this catalytic oxidation process offers good opportunities for the renewal of the knowledge in catalytic oxidative processes, and in the preparation and use of heterogeneous catalysts as well. The application of numerous heterogeneous catalysts for both gas- and liquid-phase oxidation of BzOH has been reported in the literature [12, 13, 14, 15, 16] and microstructural/physicochemical aspects of these materials are decisive in their efficient application. For example, [PERSON] and co-workers [12] reported the gas-phase oxidation of BzOH by air using 1 wt.% of gold and/or copper supported on silica as catalysts. The monometallic materials exhibited lower conversion (75% for Au/SiO\({}_{2}\)) or selectivity (78% for Cu/SiO\({}_{2}\)) when compared to the bimetallic combination.