Continuous Partial Hydrogenation Reactions by Pd@unconventional Bimodal Porous Titania Monolith Catalysts

Abstract

Pd nanoparticles are immobilized by a green procedure onto unconventional dual porosity titania monoliths. The material is used in catalytic continuous-flow hydrogenation reactions showing excellent efficiency, selectivity, and durability. C ontinuous-flow catalytic processes represent a convenient alternative to heterogeneous phase batch systems in terms of efficiency, safety, waste emission, purification, automation, space and energy consumption, 1−3 thus providing a considerable contribution to the sustainability of long-term production of chemical compounds, particularly fine-chemicals. 4,5 To this purpose, different types of microfluidic flow reactors have been developed so far. 6−10 Monolith-based reactors have attracted increasing interest in recent years 11 because of their significant advantages compared to conventional packed-bed systems, including better heat and mass transfer, lower pressure drop, narrow residence time distribution, which ultimately result in higher productivities. 12 Polymeric materials were the first to demonstrate the utility of monoliths in the catalytic fine-chemicals production under flow. 13−16 However, despite their unquestionable interest, polymer-based monoliths may present some drawbacks from an engineering point of view, such as volume and porosity changes with swelling, thermal, mechanical, and chemical stability, shrinking phenomena, back pressure evolution at high flow rate due to limited porosity, which adversely affect their performance as catalyst supports. 17−19 To avoid these problems different types of inorganic monoliths have been synthesized, including conventional ceramic monoliths obtained by extrusion, that are largely employed in conversion of raw materials, pollutant abatement, and automotive exhaust gas treatment. 20,21 Only two types of unconventional inorganic monolith materials were reported for continuous flow operations in fine chemical synthesis, and both based on silica. One was obtained by emulsion templating synthesis and featured a disordered macropores network for transesterification reactions; 22 the other one, showing a well-defined hierarchical porosity network of flow-through macropores (2−10 μm) and diffusive mesopores within the struts (2−20 nm), was obtained by a combination of spinodal decomposition and sol−gel transition, and it was used for diverse organic catalysis. 23 These latter materials can be particularly useful in the synthesis of fine chemicals, being able to address the need of both efficient processing (within small pores) and fast diffusion (by macropores). 24,25 Despite these favorable features, this type of monoliths have never been explored in highly selective transition-metal catalyzed reactions, for example, in hydrogenation reactions. Selective hydrogenation of hydrocarbons with multiple CC and/or CC bonds to achieve partial hydrogenation products is a highly desired and challenging process in the pharmaceutical, agrochemical, and petrochemical industries. 26,27 Particularly, the stereo-and chemo-selective hydrogen...

Advances in monoliths and related porous materials for microfluidics

“…The hydrogenation efficiency was evaluated for a series of reactions, and a conversion rate up to 100% was reported with an increase in H 2 flow. 81 Pd nanoparticles have also been immobilized on a silica monolith by impregnation with an aqueous solution of Pd(NH 3 ) 4 (NO 3 ) 2 . The structure was used for selective hydrogenation of 1,5-cyclooctadiene and 3-hexyn-1-ol, with conversion rates of 95% and 85%, respectively.…”

Synthesis of palladium nanoparticles using continuous flow microreactor

“…It has more oxidation potential than Pt [2,30]. Pd is suitable for hydrogen storage and sensing applications [31].There are several methods available for synthesizing Pd nanoparticles using precursors such as palladium chloride (PdCl 2 ), potassium tetrachloropalladates (K 2 PdCl 4 ),palladium nitrate, palladium acetate with reducing agents such as hydrogen, ethylene glycol, NaBH 4 , xanthan gum, hydrazine, alcohol, sodium ascorbate and sodium citrate and along with capping agents such as cetyl trimethylammonium bromide (CTAB), polyvinylpyrrolidone (PVP) and sodium dodecyl sulfate (SDS) [32][33][34][35][36][37][38][39][40][41][42][43][44][45].…”

Ag/SiO2- and Ag/Co3O4-Based Monolithic Flow Microreactors for Hydrogenation of Dyes: Their Activity and Stability

“…The formation of metal nanoparticles on supports to give heterogeneous catalysts is somewhat of an art form that has been studied over many years [20]. However, there is considerably less work on hierarchically porous materials [4,5,21,22], particularly monolithic materials, which have been evaluated in only a handful of flow experiments [23][24][25].…”