Chemoselective hydrogenation of nitroarenes: Boosting nanoparticle efficiency by confinement within highly porous polymeric framework
Graphical abstract
Highlights
► 99.9% yield to aminocompound in hydrogenation of substituted nitroarenes over Pt/HPS. ► Monodispersed (∼3 nm) Pt0-NPs are confined within the polymer matrix avoiding leaching. ► State-of-the-art Pt catalyst show lower activity, selectivity, and stability relative to Pt/HPS.
Introduction
Selective catalytic hydrogenation is one of the most important topical problems due to the combined increasing demand for waste minimization and complexity of fine chemicals [1]. The key issue in this area is the preferential attack of key functionalities in poly-substituted molecules where the target group is typically the least reactive [2]. Giving that many of these reactions are structure-sensitive [3], that is, distinct response over catalysts bearing different metal nanoparticle (NP) sizes, control of metal dispersion is among the most effective means of adjusting catalytic performance. Nonetheless, for classic heterogeneous catalysts based on supported transition metals, a sound crystal size optimization presents a series of problems; (i) they bear particles covering a range of sizes [4] and (ii) there is a driving force for coalescence of small metal crystallites (agglomeration or sintering) that reduces active surface area causing loss of activity with time-on-stream [5]. A well-known strategy to circumvent these issues is the synthesis of size-controlled metal colloids which can be subsequently anchored onto a support [6]. However, the possibility of NP size alteration during the anchoring step [7] or after the removal of the stabilizing agent [8] are important drawbacks. In addition, typical supported catalysts bear metal atoms at the NP-support interphase that are “inaccessible” for the reactants and, therefore, catalytically inactive, while metal leaching has to be avoided in order to have a real industrial application [9].
Heterogenization of metal complexes via immobilization in porous solid materials is a promising alternative to combine the outstanding properties of both heterogeneous (separation, recycling, and possibility of continuous processing) and homogeneous (increase activity/selectivity and well-defined structure of active sites) catalysts. This approach has been successfully employed with complexes of several metals such as ruthenium [10] or palladium [11] with increased activity/selectivity/stability relative to conventional supported metal catalysts. While applications to date have been mainly directed at selective oxidation [10], [12], [13], [14], recent use in hydrogenations shows promise [11], [15].
Nanostructured polymeric matrix networks, in general, and hyper cross-linked polystyrene (HPS), in particular, are commonly employed as sorbent materials but show potential use as catalytic supports [14]. They exhibit a combination of high specific surface area (SSA > 1000 m2 g−1), well-defined buried interfaces, and thermal stability [14]. The possibility of modifying on-demand the pore size of the polymer allows immobilization and a fine particle size control by confining the active metal within the nanocavities of the polymeric matrix.
Small metal particles (2–4 nm) have been suggested optimal for multiple chemoselective hydrogenation processes [6]. It is proposed in this study the fine tailoring of Pt NP within HPS support as a means of obtaining a highly selective catalyst for the production of functionalized anilines. We have selected the liquid-phase hydrogenation of para-substituted nitroaromatics to the correspondent aminocompounds, as high value chemicals with multiple industrial applications in the manufacture of fine chemicals [16]. Because Pt has shown promising catalytic performance for nitro-group reduction in polyfunctional molecules [17], we have considered the catalytic response of Pt NPs embedded within HPS (Pt/HPS). The potential of Pt/HPS has been examined against three state-of-the-art commercial catalysts, namely Pt supported on activated carbon (C) and Al2O3. We demonstrate that catalytic activity/selectivity response can be altered by modifying the Pt NP size and the hydrogen reaction pressure.
Section snippets
Materials and analytical methods
The reactants p-nitrophenol, p-nitroanisole, p-nitrotoluene, nitrobenzene, p-bromonitrobenzene, p-chloronitrobenzene (p-CNB), p-nitrobenzoic acid, and p-dinitrobenzene (Sigma Aldrich ⩾98%) and solvent (ethanol, Sigma Aldrich 95%) were used as supplied, without further purification. All the gases used in this study (H2, N2, O2, and He) were of ultra-high purity (>99.99%, Carbagas). The composition of the reaction/product mixtures was determined using a Perkin–Elmer Clarus 500 chromatograph
Catalyst characterization
In order to gain a deeper understanding on the source of the distinct catalytic response over the catalysts used, a series of characterization analyses were carried out.
Conclusions
The results presented in this study support the following conclusions:
- (i)
Pt/HPS with mean metal size 3.3 nm promotes the liquid-phase hydrogenation of p-CNB solely to p-CAN at complete conversion of nitroarene (T = 348 K: P = 20 bar) where reaction exclusivity has been maintained over repeated reaction runs with no detectable catalyst deactivation with time-on-stream.
- (ii)
Under the same reaction conditions, Pt/HPS and Pt/C-I with similar (3.1 ± 0.2 nm) Pt crystal size show a similar activity/selectivity response
Acknowledgments
The authors are grateful to E. Yarulin, D. Laub and J.G. Plummer for their contribution to the work. This work was funded by the European Union through the Seventh Framework Program (project POLYCAT; Grant CP-IP 246095-2). IUSTI laboratory (IUSTI-UMR-7343 AMU-CNRS) for the free utilization of iMorph software ©2009 [58] is also acknowledged.
References (58)
- et al.
Appl. Catal. A: Gen.
(2005) - et al.
Catal. Today
(1999) - et al.
Appl. Catal. A: Gen.
(2007) - et al.
Appl. Catal. A: Gen.
(2001) - et al.
Chem. Eng. J.
(2011) - et al.
J. Mol. Catal. A: Chem.
(2007) - et al.
React. Polym.
(1990) - et al.
Catal. Today
(2009) - et al.
J. Colloid Interface Sci.
(1970) - et al.
J. Catal.
(1999)
Appl. Surf. Sci.
J. Colloid Interface Sci.
J. Catal.
Appl. Catal. A: Gen.
Catal. Today
J. Catal.
Appl. Catal. A: Gen.
J. Mol. Catal.
Appl. Catal. A: Gen.
J. Mol. Catal.
J. Mol. Catal.
Appl. Catal. A: Gen.
J. Mol. Catal. A: Chem.
Catal. Today
Fluid Phase Equilib.
Catal. Today
Appl. Catal. A: Gen.
Chem. Eng. Sci.
Catal. Commun.
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