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Macroporous ion exchange resin supported palladium nanoparticle (Pd-NP) catalysts are prepared by an intermatrix synthesis. For the first time, the effect of resin functionality (weak acid, strong acid, strong base) on the NP size, their catalytic activity, and leaching is investigated in the Suzuki cross-coupling of iodobenzene and phenylboronic acid. Whereas the smallest NPs (1.34 nm) are found in the thiol Ambersep GT74 resin, the sulfonic acid Lewatit K2629 and quaternary amine Lewatit MP500 OH resins resulted in NPs of a similar size (2.42 and 2.59 nm, respectively). Despite their smaller size, the NPs on Ambersep GT74 exhibited the lowest conversion (21.6 %), which is attributed to a too strong coordination of the NPs by the thiol groups. The conversions obtained by using Lewatit K2629 (76.8 %) and Lewatit MP500 OH (94.2 %) were considerably higher, the excellent performance of the latter catalyst being ascribed to the promoting effect of the hydroxyl groups on the transmetallation and reductive elimination steps in Suzuki cross-coupling. No Pd leaching was observed when using Ambersep GT74 as the support, compared with Pd leaching amounting to 1.1 % and 4.8 % when using Lewatit MP500 OH and Lewatit K2629, respectively. Such low values indicate that ion exchange resins are ideal supports to stabilize the NPs. Particularly, the combination of high conversion and limited leaching on Lewatit MP500 OH opens up new perspectives for catalyzing Suzuki cross-coupling with a heterogeneous catalyst. [ABSTRACT FROM AUTHOR]

• The leaching behaviour and mechanisms are elucidated via combined data analysis. • A homogeneous-heterogeneous catalytic system prevails for both NP catalysts. • The leaching pathway critically depends on the resin functionality. • Phenylpalladium(II)hydroxide complexes are proposed as the leached Pd2+ species. The Pd leaching behaviour of a strongly acid (Lewatit K2629) versus strongly basic (Lewatit MP500 OH) ion exchange resin supported Pd nanoparticle (Pd-NP) catalyst is elucidated in the Suzuki reaction between iodobenzene and phenylboronic acid with K 2 CO 3 as base in 1:1 (v/v) DMF/H 2 O at 40 °C. Reaction and leaching kinetic analyses combined with hot filtration tests indicated heterogeneous-homogeneous catalysis for both Pd-NP catalysts. More particularly, leaching experiments showed that for Pd-NP Lewatit K2629, Pd-NP as well as molecular Pd2+ leaching (as phenylpalladium(II)hydroxide complexes) occurred due to, respectively, the disturbance of the electrostatic NP stabilization by dissociated K 2 CO 3 and reaction with iodobenzene and hydroxide. For Pd-NP Lewatit MP500 OH, however, molecular Pd2+ leaching induced by iodobenzene, dominates. Hence, the resin functionality determines the contribution of each leaching pathway. Furthermore, at high conversions, Pd re-deposition occurred due to the decreased amount of leaching inducing reagents causing enlarged NPs in the resin which leached less in subsequent runs. [ABSTRACT FROM AUTHOR]

An uncalcined HT supported Pd-NP catalyst was synthesized via co-precipitation, i.e., PdHT_100RED_20, and benchmarked against a Pd impregnated HT catalyst, i.e., Pd@HT_500RED_20. TEM analysis pointed out that the largest NPs (6.5 nm) were found in the co-precipitated catalyst because of its weaker steric NP confinement, compared to the benchmark catalyst (3.2 nm). The PdHT_100RED_20 had a significantly higher catalytic performance (60% conversion, 1.85% leaching) than Pd@HT_500RED_20 (19%, 0.98%) due to lack of calcination resulting in an ordered, and thus, more accessible HT structure in combination with its high support basicity. Both catalysts showed low Pd leaching confirming the suitability of HTs as NP support. Focusing on PdHT_100, the effect of the reduction temperature on the NP size and catalytic performance was investigated. NP size distribution analysis showed that the smallest NPs were found in PdHT_100RED_0 (5.3 nm), resulting in a higher catalytic performance (67%, 2.28%) when compared to PdHT_100RED_20 (6.5 nm). Performing the reduction at higher temperatures, i.e., at 60 °C (5.7 nm), caused an extremely low catalytic performance (13%, 0.20%) due to the less efficient reduction reaction with NaBH 4 at higher reduction temperatures. Reusing PdHT_100RED_20, a decrease in catalytic performance (45%) was observed. However, this decrease was less pronounced (50%) when a re-activation of the catalyst with Na 2 CO 3 between consecutive runs was performed. Moreover, it was shown that the leached Pd species were able to re-deposit onto the support if high conversions were obtained, and thus, an almost quantitatively recovery from the reaction mixture can be achieved with retention of its catalytic activity (>99%). [ABSTRACT FROM AUTHOR]