## 4 HPS-Based Catalysts for the Synthesis and Transformation of Platform Chemicals
### Hydrogenation of Levulinic Acid
Interestingly, the LA conversion, achieved in 2 h at a temperature of 70 \({}^{\circ}\)C and a pressure of 3.0 MPa, increased from 87.9% to 92.1% after the first hydrogenation and gradually to 97.2% in the fourth cycle suggesting in situ catalyst activation. Despite a variety of LA hydrogenation catalysts [211], supported ruthenium NPs remain one of the most common catalytic systems. We have previously shown that HPS can be successfully used as a support for the creation of Ru [212; 213] and Ru\(-\)Co [185] catalysts for the hydrogenation of LA to GVL in an aqueous environment. It was shown that the HPS support allows to synthesize Ru-containing NPs, predominantly consisting of RuO\({}_{2}\) and exhibiting high activity and selectivity in the LA hydrogenation. At the same time, HPS functionalized with tertiary amino groups (HPS-NR\({}_{2}\)) gives tiny Ru-containing NPs (with a diameter of 1-2 nm), which ensures 100% yield of GVL at a temperature of 100 \({}^{\circ}\)C and a partial hydrogen pressure of 2.0 MPa for 100 min of reaction at a LA-catalyst ratio of 100 g/g. The activity of the catalyst based on non-functionalized HPS (5%-Ru/HPS), containing NPs with a diameter of about 4 nm, is inferior to the activity of the 5%-Ru/HPS-NR\({}_{2}\) sample: the conversion of LC in the case of 5%-Ru/HPS is 83% in 100 min reactions. For the 3%-Ru/HPS-NR\({}_{2}\) catalyst, GVL yields of 77% and 99% can be achieved under similar conditions at LA-catalyst ratios of 100 g/g and 50 g/g, respectively. It is interesting to note that in non-functionalized HPS, Ru-containing NPs tended to form cluster-like aggregates located closer to the outer surface of the polymer granules, to a greater extent than in the SPH-NR\({}_{2}\), which is due to different hydrophobicity of the polymers [212]. In addition, during the study of the kinetic of LA hydrogenation in the presence of the 5%-Ru/HPS-NR\({}_{2}\) catalyst, it was found that the apparent activation energy is about 28 kJ/mol, and it was assumed that the LA hydrogenation in an autoclave-type reactor can be partially limited by the mass transfer of hydrogen from the gas phase to the aqueous solution [213]. In a study of bimetallic samples, it was shown that the most promising modifier metal for ruthenium is cobalt. It was found [183] that bimetallic Ru\(-\)Co catalysts based on HPS-NR\({}_{2}\) containing 3% (mass) Ru provide 99% yield of GVL at 120 \({}^{\circ}\)C and a partial hydrogen pressure of 2 MPa for 60 min of reaction. Compared to the monometallic analogue (3%-Ru/HPS-NR\({}_{2}\)), the most promising bimetallic catalytic system (3%-Ru-0.1%-Co/HPS-NR\({}_{2}\)) increases the initial reaction rate by approximately 1.5 fold, probably due to the redistribution of RuO\({}_{2}\) NPs inside the polymer after its impregnation with a cobalt precursor. It is noteworthy that no chemical reaction products of Co and Ru were detected on the catalyst surface, which could be a consequence of the low cobalt amount. A study of the stability of the 3%-Ru-0.1%-Co/HPS-NR\({}_{2}\) catalyst showed only slight decrease in activity: the LA conversion was 82% after three cycles. The results are very promising for an industrial application.