Continuous flow, catalytic partial hydrogenation reactions by Pd@sulfonated silica monoliths

1. Scope Continuous flow fine chemical synthesis by mean of supported heterogeneous catalysts is receiving increasing interest in large scale applications, due to the considerable safety, environmental and economic benefits compared to conventional batch operations. However, the need to overcome the common drawbacks associated with the mesoporous catalysts usually employed to this purpose requires a constant effort to develop innovative materials with improved properties . Monoliths featuring an isotropic, hierarchically porous structure of interconnected flow-through macropores and diffusive mesopores have shown a unique hydrodynamic behavior in the liquid-phase. Indeed, few inorganic monoliths with grafted functionalities have been described.A facile synthesis of functionalized, dual porosity silica monoliths is herein described by post-grafting of sulfonic groups and in-situ growth of Pd nanoparticles. The system was explored in the continuous flow, catalytic partial hydrogenation reaction of halonitroarene and alkynes, showing very good conversion, selectivity and resistance under mild conditions. 2. Results and discussion Hierarchical macro- / mesoporous silica monoliths were first synthesized using the combination of sol-gel process and spinodal decomposition accordingly to our previous works. 1 The resulting monoliths (MonoSil) were then sulfonated by grafting of 2-(4-phenylsulfonic)ethyl silane (MonoSil-ArSO3) with a one-pot, batch treatment of monolith, hence Pd nanoparticles were grown in-situ onto MonoSil-ArSO3 by a simple two-steps, one-pot flow procedure. The procedure afforded PdNP of small size evenly distributed within the monolith and resulting in a 4.6% wt bulk metal loading. Compared to previuous synthetic methods,2 the actual strategy provides significant advantages in terms of higher metal content and smoother, environmentally friendlier, procedure. The catalytic performance of Pd@MonoSil-ArSO3 were investigated in the selective hydrogenation of 3-chloro-nitrobenzene (fig. 1) and 3-hexyn-1-ol under continuous flow conditions, as test reactions showing remarkable versatility and very good activity and chemo/stereoselectivity. 3-chloro-nitrobenzene and 3-hexyn-1-ol were hydrogenated in > 75.5% and > 90% selectivity at nearly complete conversion, respectively, under very mild conditions. 3. Conclusions A facile method was described for the synthesis of PdNP- doped hierarchical porosity silica monoliths, based onto an ion-exchange / reduction sequence of post-functionalized sulfonated materials. The materials were scrutinized as catalysts for the continuous flow, partial hydrogenation reaction of various substrates. The catalyst showed pretty nice constant efficiency over prolonged time on stream, with no regeneration treatments required. Compared to the known catalysts for the flow processes, the reported catalysts provide a good compromise solution between product purity and productivity, with no need of additives to achieve satisfactory selectivity. References 1. A. Galarneau, A. Sachse, B. Said, C. H. Pelisson, P. Boscaro, N. Brun, L. Courtheoux, N. Olivi-Tran, B. Coasne, F. Fajula, C. R. Chimie 2016, 19, 231-247. 2. A. Sachse, N. Linares, P. Barbaro,F. Fajula , A. Galarneau,. Dalton Trans. 2013, 42, 1378-1384.