The effect of Pd(II) chloride complexes anchoring on the formation and properties of Pd/MgAlOx catalysts
Graphical abstract
Introduction
Supported palladium catalysts, particularly the carbon-supported palladium systems, are classical hydrogenation catalysts [1]. Palladium catalysts of the acid type on oxide supports, including anion-modified ones and zeolites, are intended for certain applications [2]. Base-type palladium catalysts are investigated much more rarely. In this series, noteworthy are the catalysts synthesized using layered double hydroxides (LDHs) of different composition, which demonstrated their advantages in reactions of various types: partial oxidation of methane [3], [4], synthesis of methyl isobutyl ketone from acetone [5], [6], cross-coupling reactions [7], [8], hydrogenation of acetylene to ethylene [9], hydrogenation of phenol to cyclohexanone [10], adsorption and reduction of NO2 [11], aromatization of n-hexane [12], [13], [14], and others.
Owing to their structural features, LDHs have a great potential for deliberate variation of properties of both the support and the deposited metal [15]. Layered double hydroxides of hydrotalcite type with the general formula [MII1-xMIIIx(OH)2]x+[An-x/n] mH2O have the structure consisting of the brucite-like layers and the charge-compensating anions that are located in the interlayer space [16], [17]. After controllable heat treatment, LDHs decompose into mixed oxides MII(MIII)O, which are characterized by high specific surface areas, uniform distribution and interaction between elements [17], [18], [19], [20]. Both these materials have attractive properties for their application as the adsorbents and precursors of supports and catalysts [21], [22]. At the same time, it is known [23], [24], [25], [26] that the textural and acid-base properties of LDHs and corresponding mixed oxides as well as the structural and electronic properties of a metal in supported catalysts are affected by various factors. The main factors are the chemical composition of LDHs and the method of their synthesis as well as the nature of the active component precursor, method of its anchoring, and conditions of thermal activation.
Palladium is usually introduced by incipient wetness impregnation of an LDH (freshly prepared or precalcined at temperatures in the region of 773 K) with aqueous solutions of its anionic chloride complexes (commonly [PdCl4]2–) without control of the occurring processes [8], [9], [27], [28], [29], [30]. Aqueous solutions of Pd(NO3)2 and [Pd(NH3)4]Cl2 are employed more rarely. It is noted that the use of anionic chloride complexes results in the formation of more dispersed palladium particles than the use of cationic (amine or acetate) complexes [31]. Palladium precursors in anhydrous organic solvents are also considered, for example, the solutions of PdCl2 in dimethylformamide [8] or palladium acetylacetonate [Pd(acac)2] in toluene [11], [32], [33]. In some cases, coprecipitation from aqueous solutions of Mg, Al and Pd nitrate salts is used [11], [30], [33]. Deposition of any precursor is conventionally followed by calcination at temperatures in the region of 573 K and hydrogen reduction of palladium at 523–573 K.
In some studies [33], [34], [35], [36], [37], [38], the activity of palladium was investigated in dependence on the features of precursor and the method of synthesis. Thus, in [33], Pd/Mg(Al)Ox catalysts were prepared by three methods: impregnation of the calcined LDH with a toluene solution of [Pd(acac)2], coprecipitation of solutions of Mg, Pd and Al nitrates at pH 10, and intercalation of preliminarily synthesized Pd-containing negatively charged colloidal particles (Pd-hydroxycitrate) in the interlayer space of LDHs with interlayer nitrate anions (MgAl-NO3). After a similar pretreatment procedure (calcination at 773 K and reduction at 523 K), the catalysts were tested in the synthesis of 2-methyl-3-phenyl-propanal from benzaldehyde and propanal. Although the sample obtained by impregnation with [Pd(acac)2] had the highest dispersion of palladium, it showed a low hydrogenation rate. The catalyst synthesized from intercalated precursor demonstrated the maximum activity owing to the optimal adsorption strength of reagents [33].
In [38], Pd/MgAl-LDH catalysts were synthesized via the interaction of aqueous solutions of Na2[PdCl4] with MgAl-LDHs having different composition of interlayer anions. The use of MgAl-CO32– led to anchoring of the metal complex on the surface of support, whereas in the case of MgAl-NO3–, the anionic complex was introduced by exchanging with the interlayer nitrate anions. The subsequent liquid-phase reduction using NaBH4 resulted in the formation of palladium particles deposited on LDHs and having different sizes and properties depending on their localization. Thus, the catalyst synthesized by ion exchange contained palladium particles of a smaller size and showed a high selectivity toward the hydrogenation of the CO bond during the conversion of 2-ethylanthraquinone.
Thus, the analysis of numerous studies revealed that only some works on the synthesis of LDH-based palladium catalysts consider the mechanism of the precursor-support interaction and the role of such interaction in the formation of properties of the supported metal. There are virtually no systematic studies analyzing the transformations of LDHs and a metal complex precursor of palladium during the synthesis of such catalysts. Earlier we have performed detailed studies in this direction for the system MgAl-LDH – Pt(IV) chloride complexes [15]. Studies on the synthesis of supported platinum catalysts demonstrated that the dispersed and electronic states of supported metal and its dehydrogenating activity are affected by both the composition of hydroxide layers (the Mg/Al ratio [15], [39], [40] and the presence of Zn, Ga and Sn as the modifying elements [41], [42], [43]) and the nature of interlayer anions in LDHs [44], [45], [46]. In the latter case, the use of interlayer anions with different exchange properties made it possible to anchor platinum(IV) anionic chloride complexes both on the surface of hydroxide layers and in the interlayer space. Different features of the metal complex-support interaction produced differences in the composition of metal complex and structure of the support and later provided the formation of supported platinum particles having different morphology and electronic state. Thus, the constricted conditions of the interlayer space led to the formation of platinum particles with the plane morphology, which had an increased electron density owing to the interaction with the basic magnesium-aluminum support, according to XPS data [45], [46].
The objective of the present study was to elucidate regularities in the formation of supported palladium particles in the system MgAl-LDH – [PdCl4]2–. The study was carried out with the Pd(II) chloride complex H2[PdCl4]. By analogy with H2[PtCl6], it is conventionally used for the synthesis of supported catalysts; however, in distinction to Pt(IV) chloride complexes, it is more labile in the ligand exchange and is considerably hydrolyzed both in aqueous solutions and upon contact with the surface of oxide supports. The synthesis was performed using MgAl-LDHs with the interlayer anions having different anion-exchange properties (CO32– and OH–); in addition, palladium complexes were intercalated in the interlayer space of LDHs also during the rehydration of the mixed oxide in an aqueous solution of H2[PdCl4]. Transformations of the support and palladium species in different steps of the synthesis and palladium properties in finished catalysts were studied by means of XRD, diffuse reflectance electron spectroscopy, temperature-programmed reduction, electron microscopy, XPS and XAFS. The catalytic properties of the obtained palladium sites were investigated in the model reaction of aqueous-phase hydrogenation of furfural. This reaction is important in the biomass processing scheme. In the logic of this work, it is equally important that it proceeds under mild conditions, excluding sintering of the supported palladium and noticeable deactivation of the catalyst.
Section snippets
Catalyst preparation
The synthesis of the MgAl-layered hydroxides having carbonate counter ions was described in detail in [39], [40], [41], [42], [43], [44], [45], [46], [47]. The synthesis procedure included coprecipitation of Mg2+ and Al3+ hydroxides from aqueous solutions (1 mol/L) of nitrate salts upon their interaction with the solutions containing carbonate and hydroxide ions (1 mol/L). The molar ratio of cations Mg2+/Al3+ in the salt solution was 2. The synthesis was carried out at pH 10 and a temperature
The interaction of H2[PdCl4] with LDH
The synthesis of MgAl-CO3 by coprecipitation and the formation of MgAl-OH during rehydration of mixed oxides are well reproducible and provide the specified cationic ratio of metals [16], [17], [18], [19], [20], [21], [22]. Thus, in the performed synthesis, the content of metals in the calcined MgAlOx(CO3) was 17.5 ± 0.6 wt% Al and 31.2 ± 0.5 wt% Mg, which corresponded to the atomic ratio Mg/Al = 2.0 (Table 1S). Structural parameters of these LDHs and corresponding oxides were described in our
Conclusion
The anchoring of Pd(II) chloride complexes on MgAl-LDH (Mg/Al = 2) has been studied. Variation of the support precursor nature made it possible to fix palladium complexes on the surface (using MgAl-CO3) and in the interlayer space (using MgAl-OH and during the rehydration of MgAlOx). Palladium is anchored in the hydrolyzed complexes with the possibility to form polynuclear complexes in the case of its location on the LDH surface. Differences in the anchoring of the active component precursor
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors are grateful to R.M. Mironenko and L.N. Stepanova for helpful discussions and O.V. Maevskaya, C.V. Vysotsky, A.V. Babenko, R.R. Izmaylov and I.V. Muromtsev for testing the compositions and properties of the synthesized samples.
The research was performed using equipment of the Shared-Use Center “National Center for the Study of Catalysts” at the Boreskov Institute of Catalysis.
The work was supported by the Ministry of Science and Higher Education of the Russian Federation in
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