Abstract
Catalyst leaching is a major impediment to the development of commercially-viable processes conducted in a liquid-phase. To date, there is no reliable technique that can accurately identify the extent and dynamics of the leaching process in a quantitative manner. In this work, a tandem flow-reactor system has been developed, which allowed us to distinguish between surface-catalyzed reactions from those occurring in solution by comparing%conversion at the exit of each reactor (S1, S2) corresponding to predominance of heterogeneous/homogeneous reactions (spatial) and two different residence times (temporal). A multiscale model is subsequently established to quantify the two types of reaction rate and simulate the catalyst leaching from a cross-coupling catalyst, PdEncat™ 30; including: (1) a multi-particle sizes model for catalyst scale; and (2) a dispersion model for reactor scale. The results show that catalyst leaching occurs via more than one process, and that the homogeneous Pd-catalyst (leached from the immobilized catalyst and dissolved in the flow) dominates the reaction and possesses a much higher activity than the heterogeneous (immobilized) Pd-catalyst. Additionally, the change of leached Pd stream inside reactors can be predicted along with the axial direction and the reaction time through the reactor-scale dispersion model.
Highlights
The modern chemical industry is powered by catalysis; the global demand for industrial catalysts is projected to exceed US$20 billion by 2020, with heterogeneous catalysts accounting for approximately 80% of the total market share [1]
Either end of the block was fitted with appropriate Swagelok fittings: at the bottom of the packed bed reactor (PBR), connections to a 1/8” metallic tubing; at the top, connections to an outlet with an inserted Ktype thermometer to measure the temperature of the fluid (RS-53II digital thermometer) this outlet is connected via 1/8” steel tubing to a three-way solenoid valve, one of the outlets of the valve is connected to a fraction collector (S1) and the other one is connected to a coil of 1/8” PTFE tubing (PFR,1 m, internal Ø = 1/16”) that is submerged in an oil bath
The following conclusions can be drawn: 1) Product formation can be attributed almost entirely to catalysis occurring in the homogeneous solution due to leached species; the surface-catalyzed reaction is negligible as changes in the khet and Ehet do not affect the outcome of the fitting of all data sets
Summary
The modern chemical industry is powered by catalysis; the global demand for industrial catalysts is projected to exceed US$20 billion by 2020, with heterogeneous catalysts accounting for approximately 80% of the total market share [1]. There has been much impetus to eschew the use of traditional batch reactors in favor of continuous flow (CF) technology, in combination with catalytic processes, for the manufacturing of pharmaceutical products [2] This is greatly championed by the ACS Green Chemistry Institute and the FDA, for reasons of promoting greater sustainability, process efficiency, as well as better quality control by eliminating the batch-to-batch variability [3]. For the synthesis of molecules of medium to high complexity, it is often necessary to employ solvents to dissolve the reactants in the liquid phase In this regard, it is more desirable to employ heterogeneous catalysts for commercial synthesis: As the catalyst remains in a different phase from the reaction mixture, it simplifies the workup procedure and facilitates catalyst discovery (reducing units of operation); it is highly amenable to CF operation. This is especially problematic for the manufacture of consumer products (including pharmaceuticals), as the amount of impurities in the final product is strictly regulated [4]
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