## 4 Solketal Synthesis
### Recent Advances on the Continuous Process
The same researcher investigated the performance of zeolites with crude glycerol and achieved good conversion rates after just 1 h of reaction, but there was a fast deactivation of the catalysts and there was almost no conversion after 6 h [111]. Still on a fixed-bed reactor, but with a different type of packing, Dominguez-Barroso et al. built a carbon-based monolithic structure from spiral-wound laminated cellulose, constituting a structured packed reactor. The structure was impregnated with methanesulfonic acid solution with SO3H to provide the catalyst acidic sites. The authors reported a complete glycerol conversion at WHSV = 2.9 h\({}^{-1}\) and no catalytic activity loss, which they attributed to the possible hydrophobic character of the monolithic structure [109]. Solketal synthesis was also tested in capillary microreactors with zeolite ZSM5 film, as proposed by Huang et al. and Zhang et al. ; however, the results are not encouraging when considering scaling-up the process [112, 113]. \begin{table}
\begin{tabular}{c c c c c} \hline
**Author** & **Catalyst** & **Catalyst Loading** & **Reaction Conditions** & **Results** & **Ref** \\ \hline Kowalska-Kus, & ZSM5 (H) & & G:A = 1:3 & X\({}_{\text{gly}}\) = 85\% & \\ Kowalska-Kus, & & T = 323 K & Sel \(>\) 95\% & \\
2020 & Beta(P) & 0.083 g\({}_{\text{cat}}\)/g\({}_{\text{gly}}\) & P = 1 bar & X\({}_{\text{gly}}\) = 69\% & \\  & & & Solv. = Methanol & X\({}_{\text{gly}}\) = 85\% & \\  & USY(P) & & Space time = 5 min & Sel \(>\) 95\% & \\ \hline Capillary Microreactor & & & & \\ \hline Huang, 2020 & ZSM5 Film & 0.050 g\({}_{\text{cat}}\)/g\({}_{\text{gly}}\) & G:A = 1:2 & \\  & & T = 323 K & & \\ Huang, 2020 & ZSM5 Film & 0.050 g\({}_{\text{cat}}\)/g\({}_{\text{gly}}\) & P = N.A. & Y\({}_{\text{solk}}\) = 30\% & [112] \\  & & Solvent Free & & \\  & & Space time = 2.86 min & & \\ \hline  & & & G:A = 1:8 & \\  & & & T = 323 K & \\ Zhang, 2020 & ZSM5 Film & 4.7 \(\mu\)m film & P = N.A. & X\({}_{\text{gly}}\) = 62\% & [113] \\  & & thickness & Solvent Free & \\  & & Space time = 2.86 min & & \\ \hline Continuously Stirred Tank Reactor & & & & \\ \hline  & & & G:A = 1:5 & \\  & & & T = 308 K & \\ Dmitriev, 2018 & Sulfuric Acid & 0.00066 g\({}_{\text{cat}}\)/g\({}_{\text{gly}}\) & P = N.A. & X\({}_{\text{gly}}\) = 68.4\% & [102] \\  & & Solvent Free & & \\  & & Space time = 1.73 h & & \\ \hline \end{tabular} \({}^{1}\)**Weight hourly space velocity. \({}^{2}\)** Not available. \({}^{3}\) Only result provided.  
\end{table} 
Table 3: _Cont._Most of the studies on the continuous production of solketal are still done on a very small scale, which inhibits its industrial application, required to solve the problem of the gut of glycerol. Furthermore, many continuous technologies still focus on catalyst performance rather than on intensifying the overall operation by handling the limitations inherent to this reaction. Process Intensification (PI) is one of the most promising approaches to attain environmental sustainability at the industrial level. According to Stankiewicz and Moulijn, by definition, PI is an engineering strategy for the development of new equipment and techniques expected to spawn in \"substantially smaller, cleaner, safer and more energy-efficient technologies\" [114, 115]. It is divided into two main areas: process-intensifying equipment (new reactors, intensive heat, and mass transfer devices) and process-intensifying methods (integration of reaction and separation, heat exchange, or phase transition--multifunctional reactors--hybrid separations, and techniques using alternative energy sources--solar energy, ultrasound--and new process-control methods--intentional unsteady-state operation) [114]. For the solketal synthesis, there are a few registries in the literature about the use of such strategies to minimize the mass transfer and thermodynamic limitations, or even to couple biodiesel and ketal production, as listed in Table 4.