Highly efficient heterogeneous catalysts for phenol oxidation: Binuclear pyrrolyl-azine metal complexes encapsulated in NaY zeolite

Microporous and Mesoporous Materials 227 (2016) 272e280

Contents lists available at ScienceDirect

Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso

Highly efficient heterogeneous catalysts for phenol oxidation: Binuclear pyrrolyl-azine metal complexes encapsulated in NaY zeolite Iwona Ku
zniarska-Biernacka a, *, M. Manuela M. Raposo a, Rosa Batista a, Pier Parpot a, ~es b, Anto
nio M. Fonseca a, Isabel C. Neves a, ** Krzysztof Biernacki b, Alexandre L. Magalha a

Centro de Química, Departamento de Química, Universidade do Minho, Campus de Gualtar, 4710-057, Braga, Portugal UCIBIO/REQUIMTE Departamento de Química e Bioquímica, Faculdade de Ci^ encias, Universidade do Porto, Rua do Campo Alegre, 4169-007, Porto, Portugal

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 October 2015 Received in revised form 17 January 2016 Accepted 2 March 2016 Available online 3 March 2016

Phenol was successfully oxidized by the heterogeneous catalysts based on iron(II) or copper(II) pyrrolylazine complexes encapsulated into NaY zeolite, under mild conditions. The heterogeneous catalysts were prepared using two different methods and characterized by different spectroscopic techniques and chemical analysis. The pyrrolyl-azine complexes are formed with a metal/ligand molar ratio of 2:1. Their structures were further confirmed by DFT calculations. The catalytic experimental conditions (e.g., amount of catalyst, presence or absence of the oxidant, PhOH:tBuOOH molar ratio) were optimized. The presence of the encapsulated metal pyrrolyl-azine complexes in NaY zeolite enhances higher conversion of phenol to catechol. The catalytic activities of the encapsulated metal complexes in NaY zeolite were compared with the homogeneous counterpart, and the catalysts were easily recovered and reused. © 2016 Elsevier Inc. All rights reserved.

Keywords: NaY zeolite Pyrrolyl-azine derivatives Encapsulation Phenol oxidation DFT

1. Introduction The oxidation process of phenol into catechol has been used in several chemical industries including polymerization inhibitors, petrochemical, paint, textile, oil-refineries, food, photographic chemicals, antioxidants and flavouring agents [1,2]. Chemical oxidation using Fenton process is one of the more usual oxidation techniques. This process has many disadvantages, such as the requirement of further treatments for the iron ions, the acidification of reaction medium, and the neutralization of treated solutions before disposal [3]. Recently, heterogeneous Fenton-like systems using iron supported catalysts, e.g., zero valent iron (Fe0) [4e6], goethite (-FeOOH) [7,8], magnetic nanoparticles (Fe3O4) [9,10] and Fe0/Fe3O4 composites [11,12] have been developed. Many of these systems are slow and need additional energy sources such as UV or visible light irradiation and ultrasound that increase the cost of

* Corresponding author. Current address: REQUIMTE/LAQV, Departamento de ^ncias, Universidade do Porto, Rua do Campo Química e Bioquímica, Faculdade de Cie Alegre s/n, 4169-007 Porto, Portugal. ** Corresponding author. E-mail addresses: [email protected], [email protected] (I. Ku
zniarskaBiernacka), [email protected] (I.C. Neves). http://dx.doi.org/10.1016/j.micromeso.2016.03.003 1387-1811/© 2016 Elsevier Inc. All rights reserved.

equipment and operation [13e15]. The transition metal complexes of Cu(II) and Fe(II) with ligands containing nitrogen and/or oxygen donor atoms, are also active for phenol oxidation reaction [2,16e18]. Heterogeneous catalysis plays a central role in the development of sustainable processes that are important to produce chemicals, considering that they reduce the energy and treat the effluents without significant impact for the environment. Keeping that in mind, the preparation of stable catalysts on a solid support is a usual and suitable procedure in order to maintain the active catalytic sites of the homogeneous catalysts, while at the same time providing a separation and recycling [19]. One of the strategies is the encapsulation of the transition metal complexes in zeolites [20e23]. Zeolites have well organized structures based on building blocks arranged in a periodic way to form channels and cavities. They are very stable in different media and exhibit a large specific surface area [24]. The aim of this study is to develop new heterogeneous catalysts based on the transition metal complexes with heterocyclic pyrrolyl-azine ligands encapsulated in zeolites for oxidation of phenol. The current interest in Schiff-base and azine derivatives in coordination chemistry arises from a wide range of applications in diverse areas such as optics, electronics and catalysis [25e29].

I. Ku
zniarska-Biernacka et al. / Microporous and Mesoporous Materials 227 (2016) 272e280

Additionally, heterocyclic Schiff-base and azine ligands bearing N, O and S heteroatoms in their structure should exhibit improved coordination ability and different catalytic activity. In this context, pyrrole-containing Schiff base ligands have been used as versatile building blocks for supramolecular chemistry [30e34] especially due to their ability to form dinuclear dimers or mononuclear monomers [30]. The presence of two metal centres in complexes might lead to better catalytic activities. On the other hand, less attention has been drawn to their study as ligands for metal complexes preparation with further application in catalysis, contrary to the corresponding salicyl analogues (Schiff base of the salen type). Bearing this fact in mind we decided to synthesize a pyrrolyl-azine derivative in order to be used as ligand on the preparation of new heterogeneous catalysts based on the transition metal complexes in zeolites. In this work we report the preparation of copper(II) and iron(II) pyrrolyl-azine complexes encapsulated in NaY zeolite. The novel heterogeneous catalysts were tested in phenol oxidation in acetonitrile as a model reaction for further study in water solution.

273

and the precipitates were formed as dark-brown crystals. After cooling, the solid products were filtered off and stored for 24 h at room temperature. The obtained solid was washed with cold methanol (2  5 mL) then ethanol (2  3 mL) and dried in vacuum. The complexes obtained are denoted as [M2LCl2]. [Cu2LCl2(MeOH)2]. Brown solid (yield 0.2 g, 94 %). IR (KBr) n 1639 (C]N), 1530, 1397, 1100, 1021, 875, 802 cm1 Anal. Calc. For C12H16Cu2N4O2Cl2: C, 32.30; H, 3.61; N, 12.55; Cu, 28.48. Found: C, 32.75; H, 3.88; N, 12.72; Cu, 28.33. HRMS: m/z (NBA) for C12H16Cu2N4O2Cl2; calcd: 443.9237; found: 443.9231. [Fe2LCl2(H2O)6]. Brown solid (yield 0.2 g, 96 %). n 1645 (C]N), 1532, 1402, 1095, 877, 803 cm1. Anal. Calc. For C10H20Fe2N4O6Cl2: C, 25.29; H, 4.25; N, 11.80. Found: C, 25.31; H, 3.78; N, 11.95. HRMS: m/z (NBA) for C10H20Fe2N4O6Cl2; calcd: 473.9453; found: 473.9456. Several methods are used for the encapsulation of active sites into the zeolite framework as heterogeneous catalysts [36e38]. In this work, the encapsulated metal(II) pyrroleazine complexes in NaY zeolite were prepared by two different methodologies reported by us elsewhere [39,40].

2. Experimental 2.3. Method A 2.1. Materials and methods 1H-Pyrrole-2-carbaldehyde was from Acros Organics. Hydrazine hydrate, cuprizone, KBr (FTIR grade), phenol (PhOH), chlorobenzene (PhCl), tert-butyl hydroperoxide solution e 5.0e6.0 M in decane (tBuOOH), acetonitrile (HPLC grade), dichloromethane and methanol (UVeVis spectroscopy grade), copper(II) chloride hydrate (CuCl2.2H2O), iron(II) chloride hydrate (FeCl2.20H2O) were supplied from Aldrich and were used as receive. NaY zeolite (Si/Al ¼ 2.83) in powder form was obtained from Zeolyst International. 2.2. Preparation of neat compounds 2.2.1. Synthesis of ligand: 1,2-bis((1H-pyrrol-2-yl)methylene) hydrazine. The pyrrolyl-azine derivative denoted as H2L was obtained by refluxing hydrazine hydrate with pyrrole-2-carbaldehyde in ethanol for 5 h (Scheme 1). Purification of the crude solid, which precipitated on cooling, by recrystallization from ethanol gave the pure heterocyclic ligand in a better yield (94%) compared to the previous reported procedure [30] in which an acid catalysis methodology was used (yield ¼ 60%). The ligand was characterized through the usual spectroscopic techniques. Dark yellow solid (1.0 g, 94%). Mp: 163.4e164.9  C, IR (KBr) n 3212 (NH), 1633 (C]N), 1540, 1407, 1294, 1132, 1028, 953, 881, 810 cm1. 1H NMR (DMSOd6) d ¼ 6.18 (m, 2H, 2  H-4), 6.59 (m, 2H, 2  H-5), 6.97 (m, 2H, 2  H-3), 8.37 (s, 2H, 2 ¼ CH), 11.54 (s, 2H, 2  NH). 13C NMR (DMSO-d6) d ¼ 109.66 (2  C4), 114.79 (2  C5), 123.21 (2  C3), 127.32 (2  C2), 150.54 (2 ¼ CH). 2.2.2. Synthesis of copper(II) and iron(II) complexes. The copper(II) and iron(II) complexes are prepared according to the reported procedure [35]. A solution of the ligand H2L (0.10 g, 0.54 mmol) in 15 mL of methanol was added to 5 mL of methanol solution of copper(II) or iron(II) chloride with metal/ligand molar ratio of 2:1. The resulted solution was stirred and refluxed for 5 h

A suspension of 1.5 g NaY zeolite (previously dehydrated at 120  C overnight) in 50 mL methanol was added to a solution of the H2L ligand (22.3 mg, 0.12 mmol) and Cu(II) or Fe(II) chloride (0.24 mmol). The mixture was further stirred for 24 h at 80  C. The solid fraction was filtered and washed with methanol. The resulting catalysts were washed by Soxhlet extraction using dichloromethane. Finally, the new catalysts (M2L@YA) were dried in the oven at 120  C overnight under reduced pressure to obtain Cu2L@YA as brown powder and Fe2L@YA as drack brown powder. The resulting heterogeneous catalysts were kept at 100  C until its use in the catalytic tests. 2.4. Method B In this method, NaY zeolite was first ion-exchanged with an aqueous solution of copper(II) or iron(II) chloride (0.80 mmol, liquid/solid ¼ 20 mL g1) for 24 h at room temperature, and dried at 120  C overnight under reduced pressure. MY solid (1.5 g) was suspended in the solution of 40 mg (0.21 mmol) H2L ligand in 50 mL methanol. The mixture was refluxed for 24 h at 80  C under constant stirring. The ligand consumption/adsorption was monitored by UVeVIS in the range at 200e900 nm at different time. Previous to the absorbance measurements, a calibration curve for free ligand was obtained that showed a good linear relationship between the absorbance at 357 nm (characteristic wavelength of the H2L ligand) and the concentration up to 0.25 mM 0.2 mL of the aliquots was withdrawn from the mixture and was diluted in methanol in a volumetric flask of 5 mL. The results suggest that the ligand adsorption/consumption was achieved faster (20 h) for iron based catalyst. After 24 h the ligand loading was 0.13 and 0.14 mmol/g for iron and copper complexes encapsulated in zeolite, respectively. The catalysts were filtered, washed with ethanol and Soxhlet extracted with dichloromethane. The new heterogeneous catalysts (M2L@YB) were dried in the oven at 120  C overnight under reduced pressure to obtain Cu2L@YA as brown powder and Fe2L@YA as dark brown powder. The resulting heterogeneous catalysts were kept at 100  C until their use in the catalytic tests. 2.5. Physical measurements

Scheme 1. Synthesis of pyrrolyl-azine derivative.

Room temperature Fourier Transform Infrared (FTIR) spectra of ligand, complex and solid samples in KBr pellets were measured using a Bomem MB104 spectrometer in the range 4000-500 cm1

274

I. Ku
zniarska-Biernacka et al. / Microporous and Mesoporous Materials 227 (2016) 272e280

by averaging 32 scans at a maximum resolution of 4 cm1. NMR spectra were obtained on a Bruker Avance III 400 at an operating frequency of 400 MHz for 1H NMR and 100.6 MHz for 13C NMR using the solvent peak as internal reference at 25  C. All chemical shifts are given in ppm using dH Me4Si ¼ 0 ppm as reference. Mass spectrometry analyses were performed at the “C.A.C.T.I. e Unidad de Espectrometria de Masas” at the University of Vigo, Spain. Scanning electron micrographs (SEM) were collected on a LEICA Cambridge S360 Scanning Microscope equipped with EDS system. To avoid the surface charging, prior to the analysis the samples were coated with gold in vacuum, by using a Fisons Instruments SC502 sputter coater. Powder X-ray diffraction patterns (XRD) were recorded using a Philips Analytical X-ray model PW1710 BASED diffractometer system. The solids samples were exposed to the Cu Ka radiation at room temperature in a 2q range between 5 and 65 . The relative crystallinity of the heterogeneous catalyst was estimated by comparing the intensities of the peaks with NaY used as a standard sample (100% crystalline) according to ASTM D-3906-80 method. The unit cell parameters (a0) were determined using the quartz as an internal standard by ASTM D-3942-80 method. Chemical analysis of C, H, and N was carried out on a Leco CHNS-932 analyzer. Copper(II) in neat complex was determined by spectrophotometric method using cuprizone as a complexing agent. The metal loading in zeolite based catalysts and iron content in neat complex were performed by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) using a ICP-AES Horiba Jobin-Yvon model Ultima Spectrometer with SMEWW 3120 method, after acid digestion
lises” of the Instituto Superior
rio de Ana of the samples in “Laborato
cnico (Portugal). Mp was determined on a Gallenkamp appaTe ratus. The phenol oxidation products were analysed by SRI 8610C Gas Chromatograph equipped with CP-Sil 8CB capillary column. Nitrogen was used as the carrier gas. The identification of reaction products were confirmed by GC-MS (Varian 4000 Performance). Helium was used as the carrier gas.

2.6. Computational details A series of DFT calculations have been carried out using a variety of exchangeecorrelation functional including hybrid schemes such as B3LYP and BHHLYP [41,42], and also the M06-2X meta-GGA functional developed by Zhao and Truhlar [43e45]. In all cases the calculations were carried out within the unrestricted (spin-polarized) formalism. The large, triple-zeta (z) Pople basis set 6e311þþG(d,p) [46] was employed, which ensures a superior electronic description by adding combinations of diffuse and polarization functions of the p-type, d-type, and specially f-type for all hydrogen, non-hydrogen atoms, and metal ions, respectively. Full geometry optimizations of the molecular system were performed in the gas phase without any symmetry restriction, and starting always with the metal ion out of plane defined by the coordinating atoms, in order to avoid metastable structures. The vibration frequencies for all forms of studied complex were calculated employing a harmonic model considering the molecules as being isolated in the gas phase. The vibrational frequencies of these optimized structures were predicted using the same basis set of atomic functions. No imaginary frequencies were obtained, which confirms that the molecular structures correspond to local minima on the potential energy surface. All geometry optimizations, energy and frequency calculations were performed using the Gaussian-09 package of programs [47]. Graphical representations of the optimized structures and the molecular orbitals were produced with the MOLEKEL 4.3 [48] and Gauss-View 5.0 molecular visualization programs [49].

2.7. Catalytic oxidation of phenol The oxidation of phenol was studied under argon atmosphere with constant stirring. The quantities of substrate, oxygen source and catalysts were changed due to optimize the best reaction conditions. The reaction was carried out in 6.0 mL of acetonitrile at 80  C (±0.5  C) under constant stirring, and the composition of reaction medium was phenol (0.09 g, 1.0 mmol, substrate), chlorobenzene (0.11 g, 1.0 mmol, GC internal standard) and 0.10 g of heterogeneous catalyst (previously dried at 120  C for 2 h). The oxygen source, tBuOOH (0.54 mL of 5.5 M in decane), was progressively added to the reaction medium at a rate of 0.02 mL,min1. Periodically, samples were taken from the solution with a hypodermic syringe, filtered through 0.2 mm syringe filters, and directly analysed by GC-FID. At the end of each run, the catalysts were sequentially extracted/centrifuged three times with 15 mL of ethanol and one time with 15 mL of dichloromethane to remove occluded reactants and products, and then dried in an oven at 120  C before the new catalytic cycle or the FTIR characterization. To ensure that the oxidation was only catalysed heterogeneously, the leaching test was performed. The eventual catalytic activity of the support itself in the oxidation of phenol, reactions using the same experimental conditions (vide supra) were also carried out in the presence of the support (0.1 g of the parent NaY zeolite). The identities of the products were confirmed by comparison with authentic samples or by GC-MS. The products were quantitatively determined by the internal standard method. The reusability was studied under the same experimental conditions. 3. Results and discussion 3.1. Characterization of compounds and heterogeneous catalysts In this work, the pyrrolyl-azine ligand was used for the preparation of the heterogeneous catalysts. This ligand presents the same arrangement of potential donor sites than the diazine ligands derived from hydrazine [35]. Due to position of donor atoms these compounds may form many possible mononucleating and dinucleating coordination modes. The synthesis of copper(II) complex with diazine ligand namely picolinamide azide with metal/ligand molar ratio of 2:1 [35] yields complex with the metal/ligand/ chlorine stoichiometry 2:1:4. Where, two copper(II) ions are bound to one picolinamide azide ligand each via a pyridine and a diazine nitrogen with nominal four coordination being completed by two chlorines. Taking account this fact, we assumed that in copper(II) complex, two copper(II) ions are bonded to pyrrolyl-azine ligand and the four coordination of each metal centre are completed by chlorine and MeOH (metal/ligand/chloride/MeOH stoichiometry 2:1:2:2). In the case of iron(II) complex, the six-coordinated chromophores are achieved with the metal/ligand/chlorine/H2O stoichiometry 2:1:2:6. Elemental analysis of free complexes, [M2LCl2], confirmed the purity of compounds obtained and proved that the pyrrolyl-azine complexes are formed with the metal/ligand molar ratio 2:1. These results are different from obtained by Zhang et al. [31] for copper(II) complex with the same ligand. In that studies the metal centre is coordinated to both pyrrole- and imino-nitrogen donor atoms from two separate ligands. This might be due to the different metal/ligand molar ratio used during synthesis of this complex. Elemental analysis data show that C/N ratio for both copper heterogeneous catalysts is similar to that obtained for free complex (2.3 and 2.8 for Cu2L@YA and Cu2L@YB respectively). This suggests that no ligand destruction took place during ligand diffusion process. Also, Cu/N ratio in free complex is 2.2 and it is similar for Cu2L@YA, indicating that all copper was coordinated. The copper

I. Ku
zniarska-Biernacka et al. / Microporous and Mesoporous Materials 227 (2016) 272e280

loading was 79 mmol g1 for the method A. However, the method B leads to higher copper loading, 142 mmol g1. As a consequence Cu/ N ratio (3.7) for Cu2L@YB catalyst is higher than in the free complex. The higher Cu/N ratio in Cu2L@YB catalyst suggests the presence of a fraction of not coordinated copper with the pyrrolyl-azine ligand (0.3 % wt). According to the literature data, part of copper ions could be located in framework sites that are inaccessible for the ligand [50]. In the case of iron(II) based catalysts, the iron loading reach 430 mmol g1 and 394 mmol g1 for methods A and B, respectively. Like the encapsulated copper(II) complexes, C/N ratio for both iron catalysts is similar to that obtained for free complex (2.1 and 2.0 for Fe2L@YA and Fe2L@YB, respectively) justify that the ligand structure was kept inside the zeolite. Both methods lead to higher amount of exchanged iron than that corresponding to the free complex with Fe/N ratios 25.4 and 9.4 for Fe2L@YA and Fe2L@YB, respectively. These results show the presence of different iron species which are not coordinated with the pyrrolyl-azine ligand as observed for the heterogeneous catalyst prepared with copper using method B. The powder X-ray patterns of NaY and the heterogeneous catalysts are shown in Fig. 1. Detailed inspection of XRD analysis of heterogeneous catalysts with comparison to NaY zeolite leads important observations. The heterogeneous catalysts obtained by methods A and B display the expected patterns of hydrated NaY zeolite, and no diffraction lines assigned to any new phases were detected [51]. Moreover, the zeolite framework has not undergone any significant structural change during the metal exchange and complex encapsulation by both methods. However, there are differences in the relative peak intensities of 331, 311 and 220 upon introducing the Cu(II) or Fe(II) complexes. For NaY, the order of peak intensity was found: I331 > I220 > I311, while for all heterogeneous catalysts, the order of peak intensity became I331 > I311 > I220. This reversal in peak intensities can be attributed to a redistribution of randomly intra zeolite charge balancing cations [52] and is also correlated with the presence of the complexes in the supercages of NaY [52]. The quantitative results obtained from XRD analysis based in ASTM D-3942-80 and ASTM D-3906-80 methods are quoited in Table 1. The data show that the method B leads a minor impact on the crystallinity of the zeolite host with over 85% while the catalysts

275

Table 1 Structural characterization of heterogeneous catalysts obtained XRD analysis.

Si/Ala Crystallinity(%)b a b

NaY

Cu2L@YA

Cu2L@YB

Fe2L@YA

Fe2L@YB

2.80 100

2.57 72.0

2.60 87.0

2.56 74.7

2.66 92.1

Framework Si/Al ratio obtained by ASTM D-3942-80 method. Degree of crystallinity obtained by ASTM D-3906-80 method.

obtained by method A show above 70% of crystallinity. The similarity of Si/Al ratio of the framework obtained for all heterogeneous catalysts shows that the dealumination of zeolite does not occur during both methods used. These results indicate that under encapsulation conditions, the zeolite are not severely affected by the introduction of the pyrrolyl-azine complexes into their structure and are supported with our previous work with Mn(III) complex with a tetradentade Schiff-base ligand encapsulated in NaY [39,40]. In order to obtain more structural information of the free complexes and the heterogeneous catalysts, FTIR analysis was carried out. The FTIR spectrum of the free ligand shows absorption bands corresponding to pyrrole (NeH) at 3212 cm1 [53] and at 1633 cm1 usually attributed to C]N [54]. After coordination with the transition metals, the band assigned to the pyrrole NeH vibration is not observed in the FTIR spectra, suggesting the formation of M-Npyrrole bond in both copper(II) and iron(II) complexes. Moreover, position of C]N vibrations in spectra of the complexes show shift to the higher energy bands at 1639 cm1 and 1645 cm1 for copper and iron complexes respectively, which suggest that the C]N linkage is also involved in formation of coordination bond MNimine. As expected, the FTIR spectra of NaY and of all heterogeneous catalysts are very similar and the strong bands assigned to framework vibration bands of zeolite dominated all samples [20,23,38]. The broad band in the 3700-3300 cm1 region is attributed to surface hydroxyl groups, while bands corresponding to the lattice vibrations are observed between 1200 and 500 cm1. No shift of the bands characteristic for vibration of zeolite framework was observed in spectra of the heterogeneous catalysts. This implies that the zeolite remains unchanged upon encapsulation pyrrolylazine complexes in agreement with XRD analysis. The lack of additional bands from the encapsulated complexes in the region 1200-1600 cm1 where the zeolite does not absorbed, was due to low complexes loading. The morphology of the encapsulated Cu(II) and Fe(II) complexes in NaY was studied by SEM analysis. As examples, Fig. 2 displays the SEM micrographs of NaY and the heterogeneous catalysts Cu2L@YB and Fe2L@YA prepared by different methods. In the SEM micrographs of NaY (Fig. 2A) and of the heterogeneous catalysts (Fig. 2B and C) we can perceive that the morphology of the encapsulated pyrrolyl-azine complexes was similar of zeolite with regular small particles. Namely, the preparation of the catalysts by both methods doesn't cause any modification on the morphology of the zeolite. The average particle diameter of the parent NaY was 700 nm and for the heterogeneous catalysts the same particle diameter was preserved ca. 500e750 nm. 3.2. Theoretical studies

Fig. 1. XRD patterns of (1) NaY, (2) Cu2L@YA, (3) Cu2L@YB, (4) Fe2L@YA and (5) Fe2L@YB

The experimental studies suggest that the metal pyrrolyl-azine complexes retain their geometry into NaY zeolite cavity after encapsulation. In order to confirm the structure of the copper(II) and iron(II) pyrrolyl-azine complexes obtained in this work, several structures were optimized by DFT calculations, namely [Cu2LCl2(MeOH)2], [Fe2LCl2(H2O)6], complexes type [Cu2L2] [31] for which

276

I. Ku
zniarska-Biernacka et al. / Microporous and Mesoporous Materials 227 (2016) 272e280

Fig. 2. SEM micrographs with different resolutions of (A) NaY x5000, (B) Cu2L@YB and (C) Fe2L@YA x10000.

Fig. 3. Optimized structures of the copper(II) complexes a) [Cu2LCl2(MeOH)2], b) [Cu2L2] c) [Fe2LCl2(H2O)6] and d) [Fe2L2(H2O)4] determined by DFT calculations at the M062X/6311þþG(d,p) level of theory. The boxes indicate the molecular dimensions, size and volume, are estimated from the electronic isodensity surfaces of 0.005 e/a3o.

the X-ray structure is known, and [Fe2L2(H2O)4] as well. The linear dimensions for all complexes i.e., size and volume, were estimated from the electronic isodensity surfaces of 0.005 e/a3o, and are illustrated in Fig. 3. Although the optimizations were performed

using the three different methods B3LYP, BHHLYP and M062X, only the results of the latter are presented as they are all comparable. The most relevant geometrical parameters of optimized structures are depicted in Table 2.

I. Ku
zniarska-Biernacka et al. / Microporous and Mesoporous Materials 227 (2016) 272e280 Table 2 Optimized values of the most relevant geometrical parameters of the copper(II) and iron(II) complexes; obtained at the M062X/6-311þþG(d,p) level. Distances/Åa,b

Cu:L 2:1

M(1)-N(1) (M(1)-N(8)) M(1)-N(2) (M(1)-N(7)) M(2)-N(3) (M(2)-N(6)) M(2)-N(4) (M(2)-N(5)) M(1)-M(2) Angles/ a N(1)-M(1)-N(2) N(3)-M(2)-N(4) Dihedral/ a N(1)-N(2)-N(3)-N(4) (N(8)-N(7)-N(6)-N(5)) N(1)-N(2)-O(1)-Cl(1) (N(1)-N(2)-N(7)-N(8)) eCH]NeN]CHe a b c

1.938 2.069 2.069 1.938 5.180

Fe:L 2:2 1.963 (1.963) 2.03 (2.030) 2.03 (2.030) 1.963 (1.963) 4.000

2:1 c

1.942 (1.932)c 2.002c (1.993)c 2.006c (2.000)c 1.940c (1.942)c 3.950c c

1.998 2.029 2.024 1.993 4.330

2:2 2.052 (2.052) 2.064 (2.064) 2.064 (2.064) 2.046 (2.046) 3.992

83.05 83.05

82.47 82.47

82.49 82.42c

81.47 81.60

79.95 80.45

175.72

50.71 (50.71) 29.40 (29.40) 43.48

e

37.54

168.50 (168.52) 8.74 (11.49) 32.06 (32.06)

40.22 (40.20) 179.99

c

24.40 (26.3)c 43.20c (43.20)c

6.28 133.34

Atom numbering as referred in Fig. 3. M ¼ Cu(II) or Fe(II). From ref. 23.

The metal centre is tetra-coordinated in both copper(II) complexes. The chromophore of the complex [Cu2LCl2(MeOH)2] consists of two nitrogen donor atoms derived from pyrrole- and iminogroups from pyrrolyl-azine ligand, and completed by chlorine and MeOH. Whereas for [Cu2L2] the chromophore consists of four nitrogen donor atoms derived from two separate pyrrolyl-azine ligands. Both complexes adopt a distorted square-planar geometry. The linear dimensions of both copper(II) complexes are comparable, but there are noticeable differences in the geometry of pyrrolylazine ligand inside the complex structure. These differences are the result of higher flexibility of the complex when one molecule of the pyrrolyl-azine ligand is involved in the complex formation with two copper(II) ions. In the case of two copper(II) ions bound to two pyrrolyl-azine ligands, the obtained complex structure becomes more rigid. The distances between the nitrogen-donor atoms and copper(II) ion are similar in both complexes. These distances are in good agreement with those obtained from the X-ray data of the [Cu2L2] complex [31]. The calculated angles between nitrogendonor atoms and copper(II) ion are ca. 83 which is very close to those from X-ray data. However, in the case of the complex [Cu2LCl2(MeOH)2] the nitrogen-donor atoms lie almost in the same plane of ligand, whereas for the [Cu2L2] complex both ligands are distorted. In addition, the copper(II) centres in the 2:1 complex are located in opposite directions relative to the ligand, whereas in the 2:2 complex they are located in the same side. This difference results in longer distance between copper(II) ions for the former complex, ca. 1 Å, and significant reduction of the torsion angle eCH]NeN]CHe for the latter one, ca. 140 . In the case of iron(II) complexes, each iron(II) centre is sixcoordinated. The chromophore of the complex [Fe2LCl2(H2O)6] consists of two nitrogen donor atoms derived from pyrrole- and imino-groups of the pyrrolyl-azine ligand and completed by one chloride ion and three water molecules. Whereas for [Fe2L2(H2O)4] complex, the chromophore consists of four nitrogen donor atoms derived from two pyrrolyl-azine ligands and completed by two water molecules. Both iron(II) complexes adopt a pseudooctahedral coordination geometry, but their linear dimensions and conformations adopted by the ligands are slightly different. These differences are the result of the higher flexibility of the

277

[Fe2LCl2(H2O)6], as in the case of the copper(II) complexes. The distances between the nitrogen-donor atoms and iron(II) ion are similar in both complexes, and they are slightly longer than for copper(II) counterparts. The calculated angles, between nitrogendonor atoms and iron(II) ion are ca. 80 , which are smaller 2 e3 than in copper(II) complexes. The nitrogen-donor atoms are not in the same plane and the whole ligand is distorted for the [Fe2LCl2(H2O)6] complex, contrary to the corresponding copper complex where the ligand is planar. The iron(II) centres in [Fe2LCl2(H2O)6] complex are located in opposite directions, similar to the copper(II) complex. The distances between metal centres for both iron(II) complexes do not differ as much as for the copper(II) complexes. In order to evaluate the encapsulation process of the copper(II) and iron(II) complexes into zeolite cavities, the linear dimensions for all complexes were estimated and are illustrated in Fig. 3. All the complexes are too large to effectively pass through the free aperture of the zeolite supercages (ca. 7.4 Å), but they are small enough to be confined within the supercages (internal diameter ca. 13 Å) [55,56]. The pyrrolyl-azine ligand can easily diffuse into the zeolite and form the [(M2LCl2) (solv)2] complex, where solv is MeOH or H2O, with previously exchanged metal ions inside the zeolite. Based on the structural data it is possible to get some information about the complex stability. Morokuma et al. [57e59], defined the energy stabilization as the difference between the energy of the global system and the total energy of separate components: DE ¼ Ecomplex  Ecomponents. The stability of the optimized forms can also be confirmed by the hardness parameter (h). The hardness can be calculated by DFT, in general, by using simple orbital theory, which allows the hardness to be computed as the energy difference between the lowestunoccupied molecular orbital (LUMO) and the highest-occupied molecular orbital (HOMO), h ¼ (ELUMO  EHOMO)/2 [60]. The values of gap energy as well as values of hardness for studied complexes show that they all have comparable stability (Table 3). The differences in the structure and flexibility of the complexes with both metal ions obtained in this work, suggest that during catalytic reaction the oxidant molecules and the substrate have easier access to the active sites. This fact can explain high phenol conversion in the catalytic study when compared to other complexes studies [61e63]. 3.3. Oxidation of phenol All catalysts obtained were tested for the oxidation of phenol in acetonitrile (ACN), in the presence of tBuOOH as oxygen source, under mild conditions. The activity of the encapsulated metal pyrrolyl-azine complexes was confirmed using a blank reaction with the parent NaY where the conversion of phenol is negligible. Also, no significant reaction was observed either in the absence of oxygen source nor in the absence of the catalyst. Consequently, the presence of the metal pyrrolyl-azine complexes in NaY zeolite framework as heterogeneous catalysts improves the phenol conversion into catechol (CAT). In the present study no para-

Table 3 Energy stabilization (DE) and hardness (h) of the studied copper(II) complexes (M062X/6-311þþG(d,p) level). Complex

DE [eV]

h [eV]

[Cu2LCl2(MeOH)2] [Cu2L2] [Fe2LCl2(H2O)6] [Fe2L2(H2O)4]

60.07 62.51 59.86 66.40

2.60 2.43 2.72 2.50

I. Ku
zniarska-Biernacka et al. / Microporous and Mesoporous Materials 227 (2016) 272e280

278

benzoquinone and hydroquinone (HQ) formation was observed, this is in agreement with the results presented by other authors [64] where the absence or traces amount of HQ were mentioned. The experimental results of catalytic tests obtained are summarized in Tables 4 and 5 and Fig. 4 The encapsulated transition metal complexes are able to oxidize phenol to CAT and HQ [65]. The oxidation of phenol depends on various factors such as: catalyst amount, phenol concentration, oxidant concentration and temperature. In order to achieve suitable reaction conditions for the maximum hydroxylation of phenol several parameters were studied. Three different PhOH/tBuOOH molar ratios (1:1, 1:2 and 1:3) were tested, entries 5, 6 (Table 4) and 8 (Table 5). The 1:2 or 1:3 M ratios lead to high phenol conversion (62% and 77% respectively), while 1:1 M ratio yields only 40% conversion. Based on these results, the 1:3 PhOH/tBuOOH molar ratio was chosen as suitable for the catalytic studies. The effect of the catalyst amounts was also studied using Fe2L@YB as representative catalyst. Fig. 4 presents kinetic curves for phenol conversion in the presence of different amounts of the catalyst while all other parameters were maintained constant (0.54 mL of tBuOOH, 4 mL of ACN, 1 mmol of PhCl and 1 mmol of PhOH at 80  C). After 48 h of reaction, the conversion of phenol increase from 31% (entry 4, Table 4) to 77% (entry 8, Table 5), increasing the amount of the catalyst from 10 to 100 mg, respectively. It was found that ca. 65% of the total conversion was achieved in the initial 6 h for 10 and 50 mg of catalyst, while for 100 mg of catalysts the conversion reached almost 100% (Fig. 4). The reaction using 50 mmol of PhOH and 50 mg of Fe2L@YB leads to 37% conversion (entry 1), while only 15% conversion was obtained (entry 3) for 10 mg of the catalyst. Comparing the efficiencies of these experimental reactions, it is clear that the best conditions for phenol oxidation are: 0.54 mL of tBuOOH, 4 mL of ACN, 1 mmol of PhOH and 100 mg of heterogeneous catalysts (Table 5). All heterogeneous catalysts show the activity for phenol oxidation and the maximum conversion of the initial substrate reach 100% when Fe2L@YA was used (entry 10). The catalyst Fe2L@YB leads to slightly lower PhOH conversion (entry 8). The opposite effect was observed for copper(II) catalysts considering that the catalyst obtained by method B, Cu2L@YB, leads to higher phenol conversion with comparison to Cu2L@YA (entries 12 and 14). The latter catalyst shows more stable behaviour and can be reused without lost in catalytic activity. However, for all other catalysts a

1 2 3 4 5 6 a b c d e f g h i j)

Run

1 1 1 1 1 1

Catalyst

Fe2L@YBe Fe2L@YBf Fe2L@YBg Fe2L@YBh Fe2L@YBi Fe2L@YBj

t (h)a

48 48 48 48 24 24

%Cb

37 54 15 31 40 62

CAT

Entry

7 8 9 10 11 12 13 14 15 16 17 18

Run

1 1 2 1 2 1 2 1 2 1 1 1

Catalyst

NaY Fe2L@YB Fe2L@YA Cu2L@YB Cu2L@YA Fe2Lf,g Cu2LBf,h Cu2LAf,i

t (h)b

48 6 24 24 24 6 24 24 <24 <24 <24 <24

%Cc

4 77 23 100 47 86 67 34 38 59 100 89

CAT

Others

%Sd

%Ye

%Sd

%Ye

e 78 100 100 0 61 100 50 100 100 100 100

e 56 23 100 0 53 67 17 29 59 100 89

e 22 0 0 100 39 0 50 0 0 0 0

e 21 0 0 47 33 0 17 0 0 0 0

a Reaction conditions: PhOH 1 mmol, 1:3 PhOH: tBuOOH molar ratio, heterogeneous catalyst 100 mg. b Reaction time at which PhOH conversion start to become constant. c PhOH conversion determined by GC against internal standard. d Product selectivity, determined by GC against internal standard. e Product yield, determined by GC against internal standard. f Reaction runs under homogeneous conditions. g Equivalent 100 mg of Fe2L@Y h Equivalent 100 mg of Cu2L@YB i Equivalent 100 mg of Cu2L@YA.

▪

Fig. 4. Effect of catalyst amount on phenol oxidation ( ) 100 mg (A) 50 mg and (:) 10 mg of Fe2L@YB catalyst in the presence of 1 mmol of phenol (entries 3, 5 and 8, Tables 4 and 5).

Table 4 Effect of concentration of phenol and catalyst on oxidation of phenol. Entry

Table 5 Oxidation of phenol with tBuOOH by the metal pyrrolyl-azine complexes catalysts under optimized conditionsa.

Others

%Sc

%Yd

%Sc

%Yd

100 100 100 100 100 79

37 54 15 31 40 43

0 0 0 0 0 31

0 0 0 0 0 19

Reaction time at which PhOH conversion start to become constant. PhOH conversion determined by GC against internal standard. Product selectivity, determined by GC against internal standard. Product yield, determined by GC against internal standard. 50 mg heterogeneous catalyst: 50 mmol of PhOH. 50 mg of heterogeneous catalyst: 1 mmol of PhOH. 10 mg of heterogeneous catalyst: 50 mmol of PhOH. 10 mg of heterogeneous catalyst: 1 mmol of PhOH. 1:1 PhOH mmol: tBuOOH mmol ratio. 1:2 PhOH mmol: tBuOOH mmol ratio.

significant decrease in phenol conversion was observed for the second cycle (entries 9, 11 and 13). This can be due to leaching/ deactivation of the catalyst. The iron based catalysts show higher phenol conversion than the homogeneous counterparts (entries 8, 10 and 16) for the first reaction cycle. The higher substrate conversion could be also due to the acidity of the zeolite support which might acts as co-catalyst, as Fenton type catalysts lead to higher phenol conversion at low pH. The opposite effect is observed for the copper(II) based catalysts, when compared with homogeneous counterparts. Here, when reaction is running in homogeneous phase the conversion of substrate is higher. In the filtrate, after removal of the heterogeneous catalysts, no catalytic activity was observed, which indicates that the reaction was fully catalysed heterogeneously and no active phase is leached into

I. Ku
zniarska-Biernacka et al. / Microporous and Mesoporous Materials 227 (2016) 272e280

solution. Low catalytic activity after catalysts removal was detected for Cu2L@YB, where phenol conversion was C~10%. The leaching of complex into the reaction media can cause the reduction in reaction time and the decrease of phenol conversion for the second cycle. Nevertheless the decrease in phenol conversion upon reuse was observed for all catalysts except Cu2L@YA. This suggests that this effect is determined not only by complex leaching, but also by some metal complex deactivation during the catalytic cycles. The catalytic sites can be deactivated by decomposition of the complex inside the zeolite by the oxidant [66] or by phenol adsorption. The adsorbed or/ and bounded (chelate formation with metal ion) phenol [2,67] could block the accessibility of the active sites and leads to decrease of substrate conversion in the second cycle. The study of the stability of the encapsulated metal pyrrolylazine complexes in NaY zeolite was performed by FTIR analysis after the second reaction cycle. Fig. 5 presents the FTIR spectra of the heterogeneous catalyst obtained before and after the second cycle (entries 9, 11, 13 and 15) for Cu2L@YA, as a representative example. The results show additional bands at 1727, 1704 and 1407 cm1. These bands might be due to some oxidant, substrate or/and products occluded during the catalytic experiments which cannot be removed by washing procedure. Conversely, the bands typical of the zeolite matrix do not show significant changes after the catalytic reaction. These observations suggest that no structural changes take place in the zeolite during consecutive catalytic cycles under the catalytic experimental conditions. The direct comparison of the catalytic results presented in this work with literature data is not easy especially due to different active species and the oxygen source used. The results and the conditions from these studies are summarised in Table 6. The encapsulated copper(II) phthalocyanine complexes in zeolites were tested as catalysts in phenol hydroxylation in acetonitrile using H2O2 as an oxygen source [61] leading to less than 20% of phenol conversion. Similar studies were carried out by Abbo and Titinchi [62] with copper exchanged NaY zeolite under similar experimental conditions to those in this work. The catalyst leads to 15% of phenol conversion. The iron complex of amidate ligand namely 1,2-bis(2-hydroxybenzamido)ethane encapsulated in the supercages of zeolite-Y was also used as a catalyst in phenol

279

Table 6 Comparison of the results of catalytic test of zeolite based catalysts.a Catalyst

Amountb

PhOHc

Time (h)

%Cd

Ref.

Fe(Hybe)CleY

25 25 5 40 25 25 750 750 750 750 750 750 750 750 750 750 750 750 750 750 750 25 25

50 25 50 50l 50 100 1:1e 2:1e 3:1e 4:1e 5:1e 10:1e 3:1e,f 3:1e,g 3:1e,h 1:1e 2:1e 3:1e 4:1e 5:1e 10:1e 50 50

<1 6 <1 <1 6 6 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 6 6

43 53 32 43 48 32 20.9 21.4 19.7 17.6 15.1 7.9 7.3 1.1 2.3 10.8 7.7 7.0 6.4 6.2 5.9 14.9 27.1

63 63 63 63 63 63 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 62 62

CuCl14Pc-NaY

Cu(NO2)4Pc-NaY

CueNaY Fe(III)eNaY

a Oxygen source H2O2, solvent ACN, temperature 80  C, Hybe stands for 1,2-bis(2hydroxybenzamido)ethane and Pc stands for phthalocyanine. b Catalyst amount in [mg]. c PhOH amount in [mmol]. d Conversion of PhOH [%]. e PhOH/H2O2 molar ratio. f In methanol. g In acetone. h In water.

oxidation [63]. The authors tested different conditions such as: amount of catalyst, solvent, H2O2 and substrate concentration. The maximum phenol conversion achieved was 50%. This comparison show that the presence of two metal centres in the complexes with pyrrolyl-azine ligand encapsulated in NaY zeolite enhances the conversion of phenol into catechol. The heterogeneous catalysts presented in this work are highly active and very promising.

Figure 5. Infrared spectra of the heterogeneous catalyst (A) in the region 4000-500 cm1 and (B) in the region 1800-600 cm1:(1) Cu2L@YA; entries: (2) - 9, (3) - 11, (4) - 13, (5) e 15.

280

I. Ku
zniarska-Biernacka et al. / Microporous and Mesoporous Materials 227 (2016) 272e280

4. Conclusions The heterogeneous catalyst based on copper(II) and iron(II) complexes with the 1,2-bis((1H-pyrrol-2-yl)methylene)hydrazine ligand were synthetized and encapsulated in NaY zeolite using two different methods (A and B). The methods for the encapsulation are efficient, simple and do not demand prior functionalization processes. The characterisation techniques confirm that the heterocyclic pyrrolyl-azine ligand act as bidentate chelate, and the metal: ligand molar ratio is 2:1 for both free complexes and heterogeneous catalysts. The spectroscopy and DFT studies confirm that the zeolite structure improved the catalytic behaviour of the encapsulated metal pyrrolyl-azine complexes for phenol oxidation. All heterogeneous catalysts show high conversion for the phenol oxidation under optimized reaction conditions with elevated selectivity towards catechol. Acknowledgement IKB thanks to Foundation for the Science and Technology (FCT, Portugal), for the contract under ‘Programa Ci^ encia 2007 and for the contract under the project “n-STeP - Nanostructured systems for Tail” (NORTE-07-0124-FEDER-000039, supported by Programa Operacional Regional do Norte (ON.2). FCT and FEDER (European Fund for Regional Development)-COMPETE-QREN-EU for financial support to the Research Centres, CQ/UM [PEst-C/QUI/UI0686/2013 (F-COMP-01-0124-FEDER-037302) and project NORTE-07-0124FEDER-000039. This work is funded also by FEDER funds through the Operational Programme for Competitiveness Factors COMPETE and by National Funds through FCT e Foundation for Science and Technology under the grant PEST-C/EQB/LA0006/2013/ 37285. FCT is also acknowledged for financial support to the NMR Portuguese network (PTNMR, Bruker Avance III 400-Univ. Minho). Also, the authors are grateful to Dr A. S. Azevedo for collecting the powder diffraction data. References [1] O.B. Ayodele, J.K. Lim, B.H. Hameed, Chem. Eng. J. 197 (2012) 181e192 (and cited therein). [2] A. Babuponnusamia, K. Muthukuma, Chem. Eng. J. 183 (2012) 1e9.
s, M.A. Oturan, Chem. Rev. 109 (2009) 6570e6631. [3] E. Brillas, I. Sire [4] T. Zhou, Y.Z. Li, J. Ji, F.S. Wong, X.H. Lu, Sep. Purif. Technol. 62 (2008) 551e558. [5] M. Kallel, C. Belaid, T. Mechichi, M. Ksibi, B. Elleuch, Chem. Eng. J. 150 (2009) 391e395. [6] Y. Segura, F. Martínez, J.A. Melero, R. Molina, R. Chand, D.H. Bremner, Appl. Catal. B 113e114 (2012) 100e106. [7] G.B. Ortiz de la Plata, O.M. Alfano, A.E. Cassano, Appl. Catal. B 95 (2010) 1e13. [8] G.B. Ortiz de la Plata, O.M. Alfano, A.E. Cassano, Appl. Catal. B 95 (2010) 14e25. [9] J.B. Zhang, J. Zhuang, L.Z. Gao, Y. Zhang, N. Gu, J. Feng, D.L. Yang, J.D. Zhu, X.Y. Yan, Chemosphere 73 (2008) 1524e1528. [10] S.X. Zhang, X.L. Zhao, H.Y. Niu, Y.L. Shi, Y.Q. Cai, G.B. Jiang, J. Hazard. Mater 167 (2009) 560e566. [11] R.C.C. Costa, F.C.C. Moura, J.D. Ardisson, J.D. Fabris, R.M. Lago, Appl. Catal. B 83 (2008) 131e139. [12] F.C.C. Moura, M.H. Araujo, R.C.C. Costa, J.D. Fabris, J.D. Ardisson, W.A.A. Macedo, R.M. Lago, Chemosphere 60 (2005) 1118e1123. [13] H.S. Son, J.K. Im, K.D. Zoh, Water Res. 43 (2009) 1457e1463. [14] T. Zhou, Y.Z. Li, F.S. Wong, X.H. Lu, Ultrason. Sonochem 15 (2008) 782e790. [15] T. Zhou, X.H. Lu, T.T. Lim, Y.Z. Li, F.S. Wong, Chem. Eng. J. 156 (2010) 347e352. [16] M.R. Maurya, S.J.J. Titinchi, S. Chand, I.M. Mishra, J. Mol. Catal. A Chem. 180 (2002) 201e209. [17] K.C. Gupta, A.K. Sutar, J. Mol. Catal. A Chem. 272 (2007) 64e74. [18] I.U. Castro, D.C. Sherrington, A. Fortuny, A. Fabregat, F. Stüber, J. Font, C. Bengoa, Catal. Today 157 (2010) 66e70. [19] C.E. Song, S. Lee, Chem. Rev. 102 (2008) 3495e3524. [20] P. Parpot, C. Teixeira, A.M. Almeida, C. Ribeiro, I.C. Neves, A.M. Fonseca, Microporous Mesoporous Mater 117 (2009) 297e303. [21] A.M. Fonseca, S. Gonçalves, P. Parpot, I.C. Neves, Phys. Chem. Chem. Phys. 11 (2009) 6308e6314. [22] H. Figueiredo, B. Silva, C. Quintelas, M.M.M. Raposo, P. Parpot, A.M. Fonseca, ~ ares, I.C. Neves, T. Tavares, Appl. Catal. B 94 A.E. Lewandowska, M.A. Ban (2010) 1e7.

~es, A.M. Fonseca, I.C. Neves, [23] I. Ku
zniarska-Biernacka, K. Biernacki, A.L. Magalha J. Catal. 278 (2011) 102e110. [24] A. Corma, Chem. Rev. 97 (1997) 2373e2419. [25] K.C. Gupta, A.K. Sutar, Coord. Chem. Rev. 252 (2008) 1420e1450. [26] B. Pedras, L. Fernandes, E. Oliveira, L. Rodríguez, M.M.M. Raposo, J.L. Capelo, C. Lodeiro, Inorg. Chem. Comm. 12 (2009) 79e85. [27] M.D.H. Bhuiyan, A. Teshome, G.J. Gainsford, M. Ashraf, K. Clays, I. Asselberghs, A.J. Kay, Opt. Mater 32 (2010) 669e672. [28] S. Dufresne, W.G. Skenea, J. Phys. Org. Chem. 25 (2012) 211e221. [29] M. Grigoras, L. Vacareanu, T. Ivan, A.M. Catargiu, Dyes Pigm 98 (2013) 71e81. [30] L. Yang, X. Shan, Q. Chen, Z. Wang, J.S. Ma, Eur. J. Inorg. Chem. (2004) 1474e1477. [31] G. Zhang, L. Yang, J. Ma, G. Yang, J. Mol. Struct. 1006 (2011) 542e546. [32] N.A.H. Male, M. Thornton-Pett, M. Bochmann, J. Chem. Soc. Dalton Trans. (1997) 2487e2494. [33] L.-Y. Yang, Q.-Q. Chen, G.-Q. Yang, J.-S. Ma, Tetrahedron 59 (2003) 10037e10041. [34] Y. Wang, H. Fu, F. Shen, X. Sheng, A. Peng, Z. Gu, H. Ma, J. Ma, J. Yao, Inorg. Chem. 46 (2007) 3548e3556. [35] Z. Xu, L.K. Thompson, D.O. Miler, Inorg. Chem. 36 (1997) 3985e3995. [36] F. Davar, M. Salavati-Niasari, M. Shakouri-Arani, Microporous Mesoporous Mater 116 (2008) 77e85. [37] B. Fan, H. Li, W. Fan, C. Jin, R.Y. Li, Appl. Catal. A 340 (2008) 67e75. [38] C. Jin, W. Fan, Y. Jia, B. Fan, J. Ma, R. Li, J. Mol. Catal. A Chem. 249 (2006) 23e30. [39] I. Ku
zniarska-Biernacka, O. Rodrigues, M.A. Carvalho, P. Parpot, K. Biernacki, A.L. Magalh~ aes, A.M. Fonseca, I.C. Neves, Eur. J. Inorg. Chem. (2013) 2768e2776. [40] I. Ku
zniarska-Biernacka, O. Rodrigues, M.A. Carvalho, I.C. Neves, A.M. Fonseca, Appl. Organometal. Chem. 26 (2012) 44e49. [41] A.D. Becke, J. Chem. Phys. 98 (1993) 5648e5652. [42] P.J. Stephens, F.J. Devlin, C.F. Chabalowski, M.J. Frisch, J. Phys. Chem. 98 (1994) 11623e11627. [43] Y. Zhao, D.G. Truhlar, J. Chem. Phys. 125 (2006) 194101e194118. [44] Y. Zhao, D.G. Truhlar, J. Phys. Chem. A 110 (2006) 13126e13130. [45] Y. Zhao, D.G. Truhlar, Theor. Chem. Acc. 120 (2008) 215e241. [46] M. Frisch, M. Head-Gordon, J. Pople, Chem. Phys. Lett. 166 (1990) 281e289. [47] Gaussian 09, Revision A.1, M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, Jr., J.A. Montgomery, J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J.J. Dannenberg, S. Dap€ Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, prich, A.D. Daniels, O. Gaussian, Inc., Wallingford CT, 2009. [48] P. Flükiger, H.P. Lüthi, S. Portmann, J. Weber, MOLEKEL 4.3: Molecular Visualization Software, Swiss Center for Scientific Computing, Manno, Switzerland, 2000. [49] R. Dennington, T. Keith, J. Millam, GaussView, Version 5, Semichem Inc., Shawnee Mission KS, 2009. [50] H. Figueiredo, B. Silva, M.M.M. Raposo, A.M. Fonseca, I.C. Neves, C. Quintelas, T. Tavares, Microporous Mesoporous Mater 109 (2008) 163e171. [51] T.M. Salama, A.H. Ahmed, Z.M. El-Bahy, Microporous Mesoporous Mater 89 (2006) 251e259. [52] M. Salavati-Niasari, Polyhedron 28 (2009) 2321e2328. [53] A. Bacchi, M. Carcelli, L. Gabba, S. Ianelli, P. Pelagatti, G. Pelizzi, D. Rogolino, Inorg. Chim. Acta 342 (2003) 229e235. [54] R. Kannappan, S. Tanase, I. Mutikainen, U. Turpeinen, J. Reedijk, Polyhedron 25 (2006) 1646e1654. [55] R.I. Kureshy, N.H. Khan, S.H.R. Abdi, I. Ahmad, S. Singh, R.V. Jasra, J. Catal. 221 (2004) 234e240. [56] Q.-H. Fan, Y.-M. Li, A.S.C. Chan, Chem. Rev. 102 (2002) 3385e3466. [57] K. Morokuma, J. Chem. Phys. 55 (1971) 1236e1244. [58] K. Kitaura, K. Morokuma, Int. Quantum Chem. 10 (1976) 325e340. [59] H. Umeyama, K. Morokuma, J. Am. Chem. Soc. 99 (1977) 1316e1332. [60] R.G. Pearson, Proc. Natl. Acad. Sci. U. S. A. 83 (1986) 8440e8441. [61] R. Raja, P. Ratnasamy, Appl. Catal. A 143 (1996) 145e158. [62] H.S. Abbo, S.J.J. Titinchi, Appl. Catal. A 356 (2009) 167e171. [63] M.R. Maurya, S.J.J. Titinchi, S. Chand, J. Mol. Catal. A Chem. 214 (2004) 257e264. [64] A. Santos, P. Yustos, A. Quintanilla, S. Rodríguez, F. García-Ochoa, Appl. Catal. B 39 (2002) 97e113. [65] H.S. Abbo, S.J.J. Titinchi, Top. Catal. 53 (2010) 254e264. [66] J.M. Fraile, J.I. Garcia, J. Massam, J.A. Mayoral, J. Mol. Catal. A Chem. 136 (1998) 47e57. [67] I. Neves, F. Jayat, P. Magnoux, G. Perot, F.R. Ribeiro, M. Gubelmann, M. Guisnet, J. Mol. Catal. 93 (1994) 169e179.