GREEN CATALYTIC STRATEGIES FOR THE PRODUCTION OF BIOBASED BUILDING-BLOCKS

Green chemistry is an important tool in the roadmap towards a sustainable economy system. Biomass-waste valorization, in particular ligno-cellulosic wastes, and the use of sustainable processes are pillars of the green chemistry approach. Notably, biobased lactones can be valorized into new valuable molecules such as diols and amines that are ubiquitous chemicals in today’s chemical industry. The use of simple and cheap heterogeneous copper catalysts supported on various materials, prepared with the Chemisorption-Hydrolysis method, will avoid toxic and expensive noble metal catalysts. These materials are easy to prepare, resistant and provide reliable results. Depending on the nature of the support they can have different acidic/basic or wettability properties making them very versatile in organic synthesis. Deep characterization was performed on the catalytic system to enlighten the relationship between physicochemical properties of the catalysts with their performances. Particularly, a focus was given on the study of copper phase morphology, wettability and acidity of the surfaces. The first part of the Ph.D project was devoted to study the hydrogenation of γ- valerolactone (GVL) to 1,4-pentanediol (1,4-PDO) (scheme A1), using silica supported copper catalysts under the green solvent cyclopentylmethylether (CPME). Scheme A1. Hydrogenation of GVL to give 1,4-PDO. Notably, two different methods have been implemented to obtain low hydrophilic catalysts namely the right choice of the support and the post-metal deposition hydrophobization through silane grafting. In the first case, two different SiO2 materials, namely one less hydrophilic pyrogenic and one hydrophilic gel, were chosen to prepare two copper catalysts CuO/SiO2 A and CuO/SiO2 B respectively. The wettability properties of the two supports were confirmed through static contact angle measurements and with hydroxyl density determination using TGA. The catalysts were tested in the hydrogenation of GVL to give 1,4-PDO (T=160 °C, P(H2) = 50 bar, t = 22h) after catalysts pre-reduction under CPME. The best performances were obtained using Cu/SiO2 A achieving 78% in the diol while 47% was achieved by Cu/SiO2 B. The catalysts performances were ascribed to the SiO2 support wettability: a low hydrophilicity allows the diol product a better desorption, preventing methyltetrahydrofuran formation by dehydration side reaction. Moreover, as confirmed by XPS analysis, a low -OH surface density allows to achieve a higher Cu dispersion, thus a higher hydrogenation activity compared to the catalyst CuO/SiO2 B prepared with the more hydrophilic silica. Furthermore, the activity was found to be dependent on the nature and on the effective number of the acid sites on the surface measured in CPME through FT-IR after pyridine adsorption and through liquid-solid titration using phenylethylamine (PEA) as a probe molecule respectively. In fact, the presence of a lower quantity and density (1 μmolPEA/m2 vs 5 μmolPEA/m2 for CuO/SiO2 A and CuO/SiO2 B respectively) of strong acid sites observed with CuO/SiO2 A in CPME could therefore be the main reason for its much higher selectivity (98% vs 82% for CuO/SiO2 A and CuO/SiO2 B respectively). This has a more pronounced masking effect on the case of CuO/SiO2 A Lewis acid sites. Successively, an uncomplicated procedure of surface functionalization of the more hydrophilic CuO/SiO2 B catalyst using trietoxyoctylsilane (TEOCS), was set up to explore the possibility to obtain low hydrophilic systems with a post metal deposition method. Four samples with different silane loading were obtained (namely 1, 5, 10 and 15% wt/wt) and the catalysts properties were investigated by several characterization techniques. The effective silane grafting was confirmed by IR and XPS while the stability at the reaction temperature (T=160 °C) was assessed using TGA. The progressive reduction of hydrophilicity was confirmed by SCA measurements and water adsorption isotherms. The results obtained showed that the functionalization significantly lowers the hydrophilicity of the surface with a decrease of SCA of 20% from the bare CuO/SiO2 to the CuO/SiO2 B - 15% TEOCS. The materials were tested in the hydrogenation reaction under the same conditions and an increase in the diol yield from 47% in the case of the unfunctionalized one to 80% in the case of the 10% wt/wt TEOCS was observed. However, a further organosilane loading, as in the case of 15%, gives rise to a decrease in activity due to the scarce accessibility of GVL to the copper sites. This behavior was corroborated by 29Si CP MAS solid state NMR measurements that is able to distinguish the silicon of the bulk silica and the organosilicon. The spectra showed a dense surface silane layer on the 15% loaded sample with a T3 configuration, namely in a reticulated-type structure, if compared to the 10% one. From the results it was possible to deduce that the capability to easily tune the wettability properties of the metal supported catalysts paves the way to a wide range of applications in which both the metallic phase activity and the polarity of the surface play a pivotal role in affecting the catalytic reaction. 7 The second part of the Ph.D thesis was focused on the synthesis of N-heterocycles once again starting from GVL. N-alkyl-5-methyl-2-pyrrolidinones and pyrrolidines are an important class of chemicals that can be used as benign alternative solvent or as the starting material for the synthesis of agrochemicals and pharmaceuticals. GVL has been coupled with butylamine to obtain N-butyl-5-methyl-2-pyrrolidone (NBPO) and N-butyl-5-methyl-2- pyrrolidine (NBPE) (Scheme A2) utilizing similar copper catalysts supported on different materials. Scheme A2. Reaction of amination of GVL to give NBPO and NBPE. Initially, copper was deposited on three different supports widely used in catalysts preparation: one amphoteric, namely hydroxyapatite (HAP), one acidic, namely SiO2-Al2O3 and one moderately acidic, namely SiO2 and the acidity of the readily made catalysts was deeply characterized through FT-IR after pyridine adsorption and solid-liquid acid-base titration using PEA as basic probe molecule. Copper phase morphology was investigated through TEM, UV-DRS and TPR analysis. Subsequently the catalysts were tested in the lactamization of GVL with butylamine. (T=200 °C, P(H2) = 10 bar and GVL:BuNH2 = 1:1) The reaction conditions and the set up were improved to lower the impact avoiding catalyst pre- treatment and high hydrogen pressure. The best results were achieved by CuO/HAP with the highest conversion and selectivity towards the green solvent product NMPO (77% and 61% respectively) while the most selective toward NBPE was CuO/SiO2. This behavior was ascribed to the high density while low strength Lewis acidity of the CuO/HAP if compared to the other samples. Once again, this moderate Lewis acidity allows to coordinate the C=O group and activate the GVL to the amine nucleophilic attack. On the other hand, the higher reducibility of CuO/SiO2 gives account for its higher activity in the deoxygenation of NBPO to NBPE, due to the in situ easier formation of reduced copper phase. Considering these results, other supports, namely Al2O3 A, Al2O3 B, ZrO2 and TiO2, were chosen to prepare the corresponding copper catalysts that were also tested in the lactamization reaction. The best performances were observed with the CuO/Al2O3 B achieving 75% conversion of GVL with a selectivity towards NMPO of 75% while the worst results were obtained using CuO/Al2O3 A (30% conversion and 70% selectivity). The reasons were found in the higher copper dispersion and in the lower particle size of Cu on the CuO/Al2O3 B catalyst 8 investigated by TEM and XRD. The system also showed its efficacy for the lactamization of other biobased lactones. Also in this case, the characterization of the morphology of the copper phase and of the acidity of the systems was of great importance to understand the catalytic behavior of the samples. Moreover, the activity and selectivity can be easily tuned by varying the hydrogen pressure in the reaction environment and the acidic properties of the support. In particular, modulating the acidity properties of the catalysts is an important tool to drive the reaction selectivity towards each product.