# Some Critical Insights into the Synthesis and Applications of Hydrophobic Solid Catalysts
## 1 Introduction  
Heterogeneous catalysis represents one of the cornerstones of industrial chemistry and due to its versatility can be found in many application fields [1; 2; 3]. A heterogeneously catalyzed reaction is a combination of physical and chemical reaction pathways. These are related to the transport of the reactants towards the solid surface, the reaction and the removal of the products. Therefore, a typical gas-solid catalytic process involves the following steps:  
1. Diffusion of the reaction media through the boundary layer on the catalyst surface. 2. Pore diffusion. 3. Adsorption of the reactants on the inner surface of the pores. 4. Chemical reaction on the catalyst surface. 5. Desorption of the products from the catalyst surface. 6. Diffusion of the products out of the pores. 7. Diffusion of the products away from the catalyst through the boundary layer and into the gas phase [3]. As deducible, it is clear that a catalytic reaction does not involve only a single interaction with the active site but a complex mechanism of adsorption and desorption on the surface of the catalyst. Thus, it is true that chemisorption of the reactants and products on the catalyst surface is of a crucial importance in a catalytic process; however, it could not be considered independently from physisorption. A liquid reaction environment is characterized by the same outlined mechanism, although the description of the adsorption phenomena happening at the liquid/solid surface is more intricated and less studied compared to gas-solid adsorption. Controlling adsorption of species on a solid catalyst surface and its relationship with the active sites is fundamental to achieve a rational catalyst design [4], in particular when reaction takes place in a solvent media. A practical aspect of surface chemistry study, in this specific case, is to understand the wettability properties of a surface. A solid shows a definite surface free energy that plays a key role in the adsorption and desorption of different chemical species. In fact, wettability is the ability of a solid surface to be covered by a liquid therefore describing the affinity between a solid and liquid interface [5; 6; 7]. It is generally defined by the contact angle, which is the apparent result of the balance between interfacial free energies. Measuring the water contact angle (CA) is the easiest way to evaluate wettability. This method relies on the evaluation of the angle formed by a drop of water with a solid surface. If the angle \(\theta\) is smaller than \(90^{\circ}\), the surface is hydrophilic, if \(\theta>90^{\circ}\) the surface is hydrophobic and eventually if \(\theta>150^{\circ}\) the surface is superhydrophobic [7] (Figure 1). Contact angles measurements can be also performed with liquids other than water chosen on the basis of their polarities. The interactions of an adsorbed molecule, usually water, with a solid surface is commonly driven by specific functionalities such as Lewis or Bronsted acids or bases or more in general by polar functional groups (Table 1) [6]. After these statements, it is easily understood that surface wettability evaluation and its proper modulation could play a crucial role in the optimization of a catalytic process. The study of the wettability properties of a solid system can, indeed, give a better outlook on the catalytic mechanism [8] and forecast catalyst stability. In particular, surface hydrophobicity affects multiple variables. Recently, this aspect has gained higher relevance, as witnessed by some recent comprehensive review [9; 10] in particular due to the fact that most reactions involved in the pathway from renewables raw materials to bio-products have to be carried out in water [11]. The use of solid catalysts in aqueous environment is not so trivial: water can lead to a dramatic poisoning of the acidic active sites [12] or a disruption of the structural features and degradation of solid oxides. For example, in the case of silica, aqueous media can cause Si-O-Si and Si-O-M bond hydrolysis [13].