Continuously operated falling film microreactor for selective hydrogenation of carbon–carbon triple bonds

[Display omitted] •Process intensification with microstructured reactor equipment.•Continuous-flow gas–liquid–solid reaction.•Novel catalyst development with atomic layer deposition and PVP-Pd NPs.•Water as selectivity-enhancing and environmental friendly solvent. Despite significant advances in the fabrication and applications of microreactors for production of chemicals, their use for catalytic reactions remains a challenge, especially in fine chemical synthesis where the selectivity towards the desired product is an issue. A falling film microstructured reactor (FFMR) was tested in the selective hydrogenation of 2-butyne-1,4-diol (1) to its olefinic derivative (2). The FFMR plates were coated with Al2O3 or ZnO followed by the deposition of Pd nanoparticles (NPs). The oxides were deposited on the microstructured reaction plates using either conventional washcoating or atomic layer deposition (ALD) in the liquid or gas phase, respectively. The Pd-NPs were either formed via impregnation of an organometallic precursor with subsequent reduction, or with pre-fabricated Pd-NPs stabilized in poly(vinyl pyrrolidon) (PVP) with subsequent pyrolysis of the organic matrix and activation in H2 atmosphere. The palladium loading was varied in the range of 1.1–13.6wt%. Different solvents including water, 2-propanol and mixtures with organic bases were tested aiming at their environmental impact and highest activity/selectivity. In this work the best performing catalyst was 1.1wt% of Pd on ZnO which was prepared by washcoating and pre-fabricated Pd NPs. Under optimized conditions with water as solvent 98% of selectivity at 96% conversion was obtained, which was close to the results of the benchmark reaction in batch mode (with 98% selectivity at 99% conversion). Finally, the FFMR demonstrated a 15-fold higher performance in comparison with a batch-operated reactor showing important process intensification for the hydrogenation of (1) to (2) in continuous-flow mode.