(560ib) To Leach or Not to Leach? Combined Data Analysis to Elucidate the Leaching Behavior of Strongly Acid Resin Supported Pd and Pdco NP Catalyst in Suzuki Coupling

To leach or not
to leach? Combined data analysis to elucidate the leaching behavior of strongly
acid resin supported Pd and PdCo NP catalysts in Suzuki coupling

Introduction

Suzuki reactions
are frequently investigated due to their relevance in pharmaceutical and fine
chemical industries and are often carried out with Pd complexes. However, present
research focuses on replacing these homogeneous catalysts by suitable
heterogeneous counterparts due to the well-known challenges in product
separation and catalyst recyclability in the area of homogeneous catalysis [1]. In heterogeneous catalysis, however, metal
leaching undeniable needs to be assessed [2]. Ion exchange resin supported Pd nanoparticle
(NP) catalysts have attracted a lot of attention in this respect because of
their excellent performance in terms of NP stabilization (i.e., electrostatic
as well as steric NP stabilizers), resulting in highly active NP catalysts with
low leaching [3,4]. Moreover, the introduction of non-noble metals (e.g., Co
[5]) induces synergistic effects between Pd and the former enhancing the
catalytic performance in terms of activity, selectivity as well as stability and
significantly reduces the catalyst cost. However, to date, literature still lacks
of in-depth mechanistic leaching studies with combined data analysis for these NP catalysts, causing great
ambiguity about the actual leaching behavior in Suzuki reactions.

Methodology

The leaching behavior and corresponding
mechanism(s) of strong acid resin (Lewatit K2629, i.e., a macroporous,
cross-linked polystyrene based resin with sulfonic acid groups), supported Pd
and 1:1 Pd:Co (mol:mol) alloyed NP catalysts were investigated in the model Suzuki
reaction between iodobenzene and phenylboronic acid with K₂CO₃
as base in 1:1 (v/v) DMF/H₂O at 40°C. The Pd content in the solution
and iodobenzene conversion were monitored as function of time. Each point on
the reaction/leaching curve represents one batch reactor experiment with a
specific reaction time.

A hot filtration test was performed to unambiguously identify the type
of catalysis that is occurring, i.e., heterogeneous versus (pseudo-)homogeneous.

Leaching tests (i.e., single and two component leaching tests),
Transmission Electron Microscopy (TEM) analyses (i.e., NP size and distribution
analysis) and recycle reaction and leaching kinetic experiments were performed and
combined to elucidate their leaching mechanism in detail.

Type of catalysis

For Pd-NP/Lewatit
K2629, the Pd leaching exhibited a maximum (35.1%) after 60 minutes (i.e., 0.23
min · mmol Pd) at 30% conversion and significantly decreased to 5.6% after 3h (i.e.,
0.68 min · mmol Pd) at 75% conversion (Figure 1) [6]. The measured reaction and
leaching profiles were not directly correlated, i.e., when the Pd concentration
in the liquid phase rapidly decreased, the reaction rate did not change
significantly. This implies a heterogeneous contribution to the overall
catalytic activity.

Figure 1. Iodobenzene conversion
(●) and Pd leaching () as function of time for
Pd-NP/Lewatit K2629 [6]. Lines are added to guide the eye.

Moreover, a hot filtration test pointed
out that the leached Pd species were also catalytically active (i.e., from 30% after
1h reaction to 84% conversion after another 2h upon filtration). As a result, a
homogeneous-heterogeneous catalytic system is proposed.

For PdCo-NP/Lewatit K2629, the leached Pd
species in solution sharply decreased from 20.0% after 2h at 40% conversion to
1.4% after 3h at 100% conversion. Remarkably, the reaction rate significantly
increased as function of time which is in strong contrast to traditional
reaction profiles for which the reaction rate decreases as function of time due
to decreased reactant concentrations. This indicates a significant homogeneous
contribution of leached Pd to the overall activity. Besides Pd, Co also leached (i.e., 5.5% and 9.5% after 2h and 3h
respectively). In order to more thoroughly explain its catalytic nature,
further research is focused on completing the reaction and leaching kinetic
profiles as well as performing a hot filtration test.

Leaching inducing character of the
involved chemicals

For Pd-NP Lewatit K2629, single and two
component leaching tests showed that the leaching behavior occurs via two
pathways. On the one hand, Pd-NP leaching is induced by dissociated K₂CO₃
which causes a disturbance of the electrostatic NP stabilization (i.e., 45.8% and
44.4% after 60 minutes and 24h respectively). On the other hand, iodobenzene in
the presence of the base induced molecular Pd²⁺ leaching as
phenylpalladium(II)hydroxide complexes (i.e., 13.6% and 100% after 60 minutes
and 24h respectively). In that respect, the re-deposition of leached Pd species
at high conversions is due to the decreased amount of these leaching inducing
reagents. This caused enlarged NPs in the resin (from an initial NP size of 2.4
nm to 4.1 nm after the 2nd run) as observed by TEM. In addition, the decreased
catalytic activity (i.e., 75% versus 47% conversion after 3h) as well as the
decreased and shifted leaching peak to higher site times (i.e., 35.1% Pd
leaching after 60 minutes versus 25.2% after 90 minutes) in a second run confirms
the leaching/re-deposition behavior of Pd-NP Lewatit K2629. For PdCo-NP/Lewatit
K2629, research concerning its leaching behaviour and corresponding mechanism
is ongoing.

Conclusions

For Pd-NP/Lewatit K2629, the reaction and
leaching kinetic analysis in combination with the hot filtration test
unambiguously point towards heterogeneous-homogeneous catalysis. Single and two
component leaching tests showed that both Pd-NP and molecular Pd²⁺
leaching (as phenylpalladium(II)hydroxide complexes) occurs. For PdCo-NP/Lewatit
K2629, the increased reaction rates as function of time indicate a more significant
homogeneous contribution of the leached Pd species to the overall activity than
for Pd-NP/Lewatit K2629. For both NP catalysts, the leached Pd species re-deposit
at high conversion.

Overall, it is clear that well-combined
data analysis is crucial to unambiguously elucidate the leaching behavior and
corresponding mechanism of supported metal nanoparticle catalysts in Suzuki
reaction.

References

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[6] Van
Vaerenbergh, B., Lauwaert, J., Thybaut, J.W., Vermeir, P., and De Clercq, J., Pd
nanoparticle and molecular Pd²⁺ leaching pathways for a strongly
acid versus strongly basic resin supported Pd nanoparticle catalyst in Suzuki
coupling. Chemical Engineering Journal, xx (2019) xx-xx. Manuscript
submitted for publication.