## 4 HPS-Based Catalysts for the Synthesis and Transformation of Platform Chemicals
### Hydrogenation of Levulinic Acid
Under optimal conditions (100 \({}^{\circ}\)C, 3 MPa), a GVL yield of more than 99% was achieved in 10 h. [PERSON] et al. [195] synthesized nickel catalysts (10 wt.% Ni) based on CNTs, which showed high activity in the hydrogenation of LA in a continuous mode in a trickle bed reactor at 180 \({}^{\circ}\)C and 30 bar. The influence of the nature of nickel precursor (acetate, nitrate, and hydroxide) and the arrangement of nickel particles in CNTs was studied and shown that, depending on the catalyst synthesis conditions, nickel crystallites with diameters from 4 nm to 16 nm were formed. Recently, [PERSON] et al. [196] reported a bimetallic Ni/Ru (Ni:Ru = 10:1) catalyst based on ordered mesoporous carbon (Ni/Ru@WOMC). The catalyst was obtained by the self-assembly method using organo-solve lignin as a carbon precursor and Ni\({}^{2+}\) as a crosslinking agent followed by the addition of Ru. A close to 100% yield of GVL in 2-propanol was attained in 4 h of reaction at 80 \({}^{\circ}\)C.  
In recent years, there has been a trend to create carbon supports doped by heteroatoms, in particular by nitrogen, for the catalytic transformation of LA [197; 198; 199; 200; 201]. For example, [PERSON] et al. [198] carried out the hydrogenation of LA using formic acid as a source of hydrogen. The catalyst, Ru-decorated and N-doped carbon nanoplates, provided 99.8% LA conversion and 100% GVL selectivity. [PERSON]. [199] developed N-doped hierarchical carbons with incorporated Co particles (Co@NC). The Co@NC-700 catalyst (carbonated at 700 \({}^{\circ}\)C) provided 100% LA conversion at 190 \({}^{\circ}\)C and 1.9 MPa H\({}_{2}\) for 2 h. Also, the catalyst showed high stability (up to five reuses), which was likely due to the synergy of the metal active sites Co, Co-Ox, and Co-Nx. [PERSON] et al. [200] developed catalysts containing ultrafine Ru NPs stabilized in hierarchically porous N-doped carbon nanospheres (HPNCs) obtained by nano-emulsion self-assembly. The Ru/HPNC catalyst demonstrated excellent catalytic performance in the hydrogenation of LA to GVL under solvent-free conditions: GVL yield > 99% in 2 h. [PERSON] et al. [201] synthesized Ru NPs stabilized in three-dimensional hierarchical carbon nanoflowers containing pyridinic nitrogen (Ru/PNC). The role of pyridine N compounds was attributed to the formation of electron-rich Ru, which provided weaker adsorption of LA but stronger adsorption of H\({}_{2}\) on Ru. This resulted in high activity (TOF 5042.5 h\({}^{-1}\)) and selectivity to GVL > 99%. In addition, a direct correlation was found between TOFs and surface pyridine-N/Ru\({}^{0}\) ratios. There are also reports of polymer carriers (dendrimers, polystyrenes, metal\(-\)organic frameworks (MOFs), porous organic polymers (POPs), etc.) being used for LA hydrogenation. Polymers are used both for the immobilization of catalytically active complexes [202] and metal-containing (for example, Co, Cu, Pt, Pd, and Ru) nanoparticles [203; 204; 205; 206; 207; 208; 209]. It is important that natural polymers can be used as active phase carriers to create catalysts for the hydrogenation of LA. Thus, in the work of [PERSON] et al. [208] the self-assembly method was applied to synthesize colloidal nanospheres based on lignin containing Co\({}^{2+}\) ions. In this case, heavy metal ions acted as crosslinking agents. After calcination at 500 \({}^{\circ}\)C, particles of CoO and metallic Co were formed, providing a 99.8% yield of GVL with 100% conversion of LA in 60 min at 200 \({}^{\circ}\)C and 2 MPa H\({}_{2}\). Among polymer carriers, it is worth highlighting polymers containing functional groups that play the role of acid sites (-SO\({}_{3}\)H). However, reports on such polymers in LA transformation are scarce [203; 209; 210]. For example, [PERSON] et al. [203] developed a bifunctional catalyst based on Ru NPs (diameter about 3 nm) immobilized in crosslinked sulfonated polyether sulfone for the selective hydrogenation of LA to GVL. The combination of acid and metal centers lead to high activity.