steric effects in hydrogenation of nitroarenes over the heterogeneous [email protected] and [email protected] catalysts

Journal Pre-proof Electronic/steric effects in hydrogenation of nitroarenes over the heterogeneous Pd@BEA and Pd@MWW catalysts ´ a, ´ Martin Kubu, ˚ Michal Mazur, Yuyan Zhang, Katar´ına Fulajtarov ˇ Milan Hronec, Jiˇr´ı Cejka

PII:

S0920-5861(19)30638-8

DOI:

https://doi.org/10.1016/j.cattod.2019.11.020

Reference:

CATTOD 12565

To appear in:

Catalysis Today

Received Date:

25 June 2019

Revised Date:

28 October 2019

Accepted Date:

19 November 2019

ˇ ´ a´ K, Kubu˚ M, Mazur M, Hronec M, Cejka Please cite this article as: Zhang Y, Fulajtarov J, Electronic/steric effects in hydrogenation of nitroarenes over the heterogeneous Pd@BEA and Pd@MWW catalysts, Catalysis Today (2019), doi: https://doi.org/10.1016/j.cattod.2019.11.020

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Electronic/steric effects in hydrogenation of nitroarenes over the heterogeneous Pd@BEA and Pd@MWW catalysts

Yuyan Zhang1, Katarína Fulajtárová2, Martin Kubů1, Michal Mazur1, Milan Hronec2, Jiří Čejka1 1Department

of Physical and Macromolecular Chemistry, Faculty of Science, Charles

University, Hlavova 2030/8, 128 43, Prague, Czech Republic 2Department

of Organic Technology, Slovak University of Technology, Radlinského 9,

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812 37 Bratislava, Slovakia

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Palladium doped zeolites synthesised for catalysis Pd nanoparticles of controllable size incorporated into the zeolite pore system Pd@MWW and Pd@BEA zeolites shape-selectively catalyzes hydrogenation Active and highly selective catalyst for hydrogenation of nitroarenes

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Highlights

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Graphical Abstract

Abstract

Hydrogenation of nitroarenes is a catalytic reaction of high interest owing to the importance of the resulting aromatic amines in the chemical industry. Up to date, various metal@zeolite catalysts have been reported for this transformation. Herein, Pd@Beta and Pd@MWW catalysts were synthesized by ion exchange with

Pd(NH3)4(NO3)2 and characterized by XRD, nitrogen sorption, ICP-OES, electron microscopy and FTIR(CO). Structural and textural analysis proved no significant changes of the zeolites after the Pd precursor was introduced. STEM images proved the uniform distribution of metal species in zeolite crystals. Average size of Pd NPs is 2.6 nm and 1.7 nm for Beta and MWW zeolites, respectively. The resulting Pd@Beta catalyst exhibited higher catalytic activity compared to Pd@MWW in the hydrogenation of nitroarenes. The electronic/steric effects of substrates and products were also investigated for both catalysts. Nitroarenes with electron-donating groups exhibit higher initial reaction rates than nitroarenes containing electron-withdrawing groups. The nitroarenes bearing functionalized groups in the ortho-position are

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harder to hydrogenate than corresponding meta- and para- isomers because of the steric hindrance effect.

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Keywords: Pd nanoparticles; MWW zeolite; Beta zeolite; hydrogenation of

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nitroarenes

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Introduction

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Aromatic amines, especially those with functional groups, are important compounds and key precursors used as building blocks in the manufacture of dyes, pigments, agrochemicals, pharmaceuticals, pesticides, herbicides, and fine chemicals [1-3].

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Therefore, much attention has been paid to the development of efficient and practical methods for the synthesis of aromatic amines [4, 5]. In terms of sustainability, the reduction of parent nitro compounds is preferred for the synthesis

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of amines and related compounds owing to its environmental friendliness, atomic efficiency, and compatibility with industrial processes [6, 7]. Reducing agents, such as sodium borohydride [8, 9], hydrazine derivatives [10, 11], formic acid [12, 13], etc. serve as hydrogen transfer agents that can accurately target and reduce nitro groups to yield functionalized amines. However, these agents are hazardous and generate undesirable chemical wastes. To transform nitroarenes into aromatic amines, H2 represents an economically and environmentally responsible alternative, with water as the only byproduct, making hydrogenation with H2 the most atom-efficient

approach [14-16]. Heterogeneous noble metal catalysts, such as Pd or Pt nanoparticles (NPs) are well known to catalyze hydrogenation, as they efficiently activate molecular H2 to transform nitroarenes to aromatic amines [17-19]. However, the low thermal stability and severe aggregation of metal NPs during the hydrogenation process limit their applications because it lowers the number of exposed active sites, thus decreasing the yield of aromatic amines [20-22]. To prevent aggregation of metal NPs, metal NPs are usually anchored onto solid supports which create stable and finely dispersed metal NPs with a high catalytic activity [23]. Various supports, including metal oxides

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[24, 25], polymers [26] and carbon materials [27, 28] have been investigated. Zeolite structures differ in the size, shape, connectivity of their channels and the

presence or absence of cavities allowing a reversible uptake of molecules up to 1 nm

in size [29-31]. Due to their unique characteristics, zeolites have proven to be a class of ideal hosts to encapsulate metal NPs in their pores and channels to protect them

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against sintering [19, 32-38]. Corma et al. [39] confined single Pt atoms and Pt

clusters into MCM-22 through the transformation of 2D zeolite to 3D with

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exceptionally high thermal stability, even after the treatment in air up to 540 °C. A novel synthetic approach was developed by Román-Leshkov’s group [40]. PtZnx

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nanoclusters were encapsulated inside zeolite micropores by introducing Pt2+ cations into a zinc silicate framework via ion exchange, followed by controlled demetalation and alloying with the framework Zn. These materials possessed shape selectivity features demonstrated by the selective production of p-chloroaniline and

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1,3-dimethyl-5-nitrobenzene [40]. Chen et al. reported Pd NPs encapsulated inside mesoporous silicalite-1 nanocrystals (Pd@mnc-S1) by a one-pot method. The

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hydrogenation of nitroarenes on Pd@mnc-S1 catalyst, independent of their molecule size, yields corresponding aromatic amines [41]. However, only few studies compared the catalytic activity and discuss steric and/or electronic effects on metal NPs

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encapsulated within zeolite catalysts with respect to the hydrogenation of nitroarenes [42-47]. Herein, we presented the introduction of Pd NPs to Beta and MWW zeolites by ion-exchange and further investigation of their catalytic activity and electronic/steric effects in the hydrogenation of nitroarenes. The initial reaction rates were compared between Pd@Beta amd Pd@MWW. Nitroarenes with electron-donating and electron-withdrawing substituents at the meta-position were chosen for the

investigation of the initial reaction rates and conversions. To describe steric effects, nitroarenes bearing differently located electronic groups were investigated.

Experimental details 2.1 Preparation of Pd@Beta and Pd@MWW catalysts Full details of the sample preparation by ion exchange are given elsewhere [48-50], but in short it was as follows. Beta zeolite (Si/Al ratio of 12.5) was purchased from Zeolyst (CP814E). All chemicals were purchased from Sigma-Aldrich. Prior to the

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exchange, Beta zeolite was calcined in air at 540 °C for 300 min to obtain the zeolite in the H+ form. Pd@Beta sample was prepared by stirring Beta zeolite in an aqueous

solution of Pd(NH3)4(NO3)2 at room temperature for 24 h. The product was

centrifuged, washed with deionized water and dried at 65 °C overnight, followed by reduction in H2 at 250 °C for 2 h. The Pd@MWW catalyst with Pd loading 0.36 wt%

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was prepared in the same way.

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2.2 Characterization

XRD measurements were performed on a Bruker AXS D8 Advance diffractometer

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equipped with a Vantec-1 detector in the Bragg–Brentano geometry using CuKα (λ = 0.15 nm) radiation with a step size of 0.25 (2θ/min). Micromeritics ASAP 2020 static volumetric apparatus was used to determine the

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adsorption isotherms of nitrogen at −196 °C. In order to attain accuracy adsorption data, the ASAP 2020 was equipped with pressure transducers covering the 133 Pa, 1.33 kPa and 133 kPa ranges. Before the measurement, zeolites were outgassed

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under turbo molecular pump vacuum. Starting at the ambient temperature zeolite was outgassed at 110 °C until the residual pressure of 0.5 Pa was obtained. After

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further heating at 110 °C for 1 h, the temperature was increased until the temperature 250 °C was achieved. This temperature was maintained for 6 h. The surface area, SBET, was calculated using adsorption data in the range of relative pressures p/p0 = 0.05 - 0.20. The external surface area (Sext) of Beta, Pd@Beta and MWW, Pd@MWW materials was evaluated by the t-plot method. The adsorbed amount at relative pressure p/p0 = 0.99 reflects the total adsorption capacities (Vtot). NLDFT algorithm using standard Micromeritics software for cylindrical pores (nitrogen on oxides at -196 °C) was applied to estimate the micropore volumes (Vmic).

The Pd loading of Pd@Beta and Pd@MWW catalysts was determined by ICP-OES (ThermoScientific iCAP 7000) analysis. The mixture of 1.8 mL of HF, 5.4 mL of HCl, 1.8 mL of HNO3 and 50 mg of Pd@Beta catalyst or Pd@MWW catalyst was added into a closed vessel and placed in the microwave. After cooling down, 13.5 mL of H3BO3 was added and retreat in the microwave in order to complexation of the HF surplus. Then, the solutions were diluted by distilled water before analysis. The morphology of samples was imaged by scanning electron microscopy (SEM), using a JEOL JSM-5500LV microscope. For the measurement, crystals were coated with a thin platinum (~10 nm) layer by sputtering in the vacuum chamber of a BAL-TEC SCD-050. STEM imaging was performed using JEOL NeoARM 200F operated

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at 200 kV. The alignment was prepared using the standard gold nanoparticles film

method. The size distributions of Pd NPs in Pd@Beta and Pd@MWW were analyzed by ImageJ software.

A Nicolet iS50 spectrometer with a transmission MCT/B detector was used to

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measure the FTIR spectra of non-reduced samples. Zeolites were pressed into self-supporting wafers with a density of ~10 mg/cm2 and activated in situ at T =

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450 °C and p = 5∙10–5 Torr for 4 h. All spectra were recorded with a resolution of 4 cm–1 by collecting 128 scans for a single spectrum at room temperature. The CO

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adsorption took place at room temperature and p = 15 Torr, followed by desorption at T = 25, 50, 100, 150, 200 °C. 2.3 Catalytic test

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The hydrogenation measurements were carried out in a stirred batch isobaric reactor equipped with H2 consumption monitoring [51]. The reaction was carried out in a 50 mL stainless steel reactor connected with a flexible metal capillary to a H2 supply

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system recording the H2 consumption in defined reaction times at constant operating pressure. For a typical reaction, n-octane (5 mL), Pd@Beta or Pd@MWW catalyst

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(50 mg) and nitroarenes (1.5 mmol) were loaded to the reactor. After sealing, the reactor was flushed several times with low H2 pressure and then pressurized with H2 usually to 0.6 MPa (ambient temperature). After reaching the desired temperature (130 °C), the reactor was shaken using an orbital rotator. After an appropriate reaction time the reactor was quickly cooled down and filtered. The reaction mixture was analyzed by gas chromatography (Hewlett Packard 5990 with FID detector, column 0.3 x 60 cm packed with 10 % SE 30 on Chromaton N AW). The kinetic diameters of the nitroarenes were estimated by Chem 3D software.

The conversion was calculated as:

(1)

The selectivity was calculated as:

(2)

The mass of the active phase (Pd) was calculated as:

(4)

N0 – initial amount of reactant: nitroarenes;

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The initial reaction rate was calculated as:

(3)

Nr(t) – the molar number of reactant: nitroarenes at particular time; Np(t) – the molar number of product: aromatic amines;

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NPd – the molar number of Pd species;

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t – reaction time.

3.1 Characterization

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Results and discussion

The X-ray diffraction (XRD) patterns of Beta, Pd@Beta (Fig. 1a) and MWW,

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Pd@MWW (Fig. 1b) samples confirm that all samples are highly crystalline, corresponding to Beta and MWW type zeolite structure [52-54]. After the Pd precursor was introduced via ion exchange and reduced by H2, the crystal structures

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of Beta and MWW were preserved. Fig. 2 shows nitrogen adsorption isotherms of the parent and Pd-loaded zeolites, Table 1 provides the corresponding textural

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parameters. There are no distinct differences in textural properties between the Pd@zeolite catalysts and the parent materials. Slightly lower BET areas and micropore volumes in comparison to parent zeolites without Pd NPs might be caused by a partial pore blocking in the zeolites structure due to the introduction of Pd NPs. These results evidences that zeolite structures are stable during the ion exchange and calcination processes.

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Fig. 1 - XRD patterns of Beta, Pd@Beta (a) and MWW, Pd@MWW (b)

Fig. 2 - Nitrogen adsorption (●) and desorption (○) isotherms of Beta, Pd@Beta (a) and MWW, Pd@MWW (b)

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Table 1 - Textural parameters and Pd content of Beta, Pd@Beta and MWW, Sext

Vmic

Vtot

Pd content

m2/g

m2/g

cm3/g

cm3/g

(wt%)

Beta

577

187

0.17

0.78

0

Pd@Beta

529

177

0.15

0.80

0.38

MWW

489

79

0.19

0.45

0

Pd@MWW

439

78

0.15

0.39

0.36

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SBET

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Pd@MWW samples

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Fig. 3 - SEM images of Beta (a), Pd@Beta (b) and MWW (c), Pd@MWW (d)

SEM micrographs of the parent Beta (Fig. 3a) and Pd-containing Beta (Fig. 3b) catalyst illustrate uniform morphology with the average spherical particles size

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around 3.0 - 5.0 μm. SEM images of MWW and Pd@MWW samples consist of thin, randomly oriented hexagonal platelets of the size about 1.6 μm (Fig. 3c-d). It is noticeable that the morphology of the parent zeolites and Pd-containing catalysts

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exhibits no significant changes after Pd NPs introduced. The EDS measurements were performed twice by choosing different selected areas. The EDS spectrum of Pd@Beta

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and Pd@MWW catalysts revealed the presence of Pd elements. The Pd content in Pd@Beta and Pd@MWW tested by ICP-OES were 0.38 wt% and 0.36 wt%, respectively. Pd NPs were formed by ion exchange with the parent zeolite, followed by the reduction process.

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Fig. 4 - STEM images and size distribution of Pd@Beta (a,b,c) and Pd@MWW (d,e,f),

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the white spots visible in the pictures are the Pd NPs.

Scanning transmission electron microscopy (STEM) images (Fig. 4) showed the

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presence of Pd NPs in Beta and MWW zeolites. White spots observed in the images are Pd nanoparticles. The size distributions of Pd NPs in zeolites are presented (Fig. 4 c, f) showing the average sizes of 2.6 nm and 1.7 nm for Beta and MWW, respectively.

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The Pd@Beta and Pd@MWW prepared by the ion-exchange of parent Beta and MWW with Pd(NH3)4(NO3)2, followed by calcination in air, and reduction in H2. Most probably, due to different pore sizes of zeolite, the average size of Pd NPs formed in

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Beta (channels diameter of 0.59 nm) is larger than in MWW zeolite (channels diameter of 0.49 nm). The reason for having Pd species bigger than zeolite cavities

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may be location of Pd nanoparticles along the voids.

Fig. 5 - FTIR spectra of CO (15 Torr equilibrium pressure) adsorbed at 25°C on

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Pd@Beta (a) and Pd@MWW (b) catalysts (red lines) and evolution of the spectra in dynamic vacuum at different temperatures.

Fig. 5 shows the FTIR spectra taken at room temperature after exposing of Pd@Beta and Pd@MWW catalysts to CO atmosphere following by the desorption at T = 25, 50,

100, 150, 200 °C. CO adsorption on Pd@Beta and Pd@MWW catalysts produces a

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series of bands in the 2300 – 2000 cm-1 region. Physically adsorbed CO exhibits a characteristic band at ~2140 cm-1, while the bands of CO adsorbed on Lewis acid

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sites of different strength are observed in 2200 – 2240 cm-1 region for all Pd@Beta and Pd@MWW catalysts (Fig. 5) and for Pd-free aluminosilicate zeolites (the results

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are not shown). The pair of the bands at 2180/2152 cm-1 for Pd@Beta (Fig. 5a) and 2190/2152 cm-1 for Pd@MWW (Fig. 5b) is attributed to Pd2+-CO complexes[55]. The pair of the bands at ~2138 /2117 cm-1 was assigned to Pd+-CO complex for both

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catalysts. These bands are stable compared to the carbonyls of Pd2+ on Pd@Beta and Pd@MWW catalysts, vanishes upon evacuation at 150 °C, which accounts for a significant π component interaction. In contrast to Pd@MWW, the FTIR(CO) spectra

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of Pd@Beta catalyst show additional bands at < 2100 cm-1, attributed to Pd0-CO complexes. The result reveals that Pd n+ species were easier to reduce to Pd0 in CO

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atmosphere when supported on Beta vs. MWW zeolite. 3.2 Catalytic test 3.2.1 The catalytic activity of Pd@Beta and Pd@MWW The catalytic activity of the Pd@Beta and Pd@MWW were investigated in the hydrogenation of nitroarenes. They catalyzed the reactions to give the corresponding aromatic amines without by-products detected by GC. Fig. 6 summarized the initial reaction rates for the hydrogenations over Pd@Beta and Pd@MWW catalysts. Tested

catalysts were active for the most of investigated nitroarenes. The substituent groups of nitroarenes have strong electronic and steric influences, thus various catalytic activities were obtained. Those two types of effects are discussed separately in the

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Fig. 6 - The comparison of the initial reaction rates for the hydrogenation of

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nitroarenes on Pd@Beta and Pd@MWW catalysts

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Graph showing the comparison of the initial reaction rates is presented in Fig. 6. Pd@Beta catalyst showed higher catalytic activity than Pd@MWW. Both samples

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have a similar Pd content (0.38 wt% for Beta and 0.36 wt% for MWW) and comparable average size of Pd nanoparticles (2.6 nm for Beta and 1.7 nm for MWW). The worse catalytic performance of Pd@MWW is probably attributed to diffusion limitations of the MWW zeolite having smaller micropores (0.55 × 0.40 and 0.51 × 0.41 nm) [54]. Therefore, lower initial reaction rates were obtained for Pd@MWW sample. Better performance of Pd@Beta catalyst could be explained by the large micropores of Beta (0.73 × 0.71 and 0.56 × 0.56 nm) facilitating the transport of nitroarenes and aromatic amines. The higher stability of linear Pd2+-CO complexes

and easier reduced from Pdn+ to Pd0 on Pd@Beta compared with Pd@MWW would also contribute to this effect.

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3.2.2 Electronic effect

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Fig. 7 - Hydrogenation reaction kinetic profiles of 3-nitrotoluene (■), 3-nitroanisole (●) and methyl 3-nitrobenzoate (▲) on Pd@Beta (a) and Pd@MWW (b)

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Hydrogenation of nitroarenes with H2 requires an efficient catalyst to simultaneously promote the activation of H2 and nitro group of substrates. In Pd@Beta and Pd@MWW catalysts, palladium activates H2 molecules and facilitates subsequent

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hydrogenation of nitro group because it is abundant of delocalized electrons. The performance of the hydrogenation of nitroarenes depends on the interaction between Pd NPs and nitro group with the same H2 pressure. Therefore, the electron

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density of nitro group can be a crucial factor. To explore the electronic effect, several nitroarenes with different functional groups were subjected to the optimized reaction condition for the synthesis of aromatic amines (Fig. 7). Electron-donating

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and electron-withdrawing substituents in meta-position on the aromatic ring of nitroarenes were chosen to investigate details of hydrogenation. In general, various

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functionalized nitroarenes with the size smaller than the pore size of Beta and MWW zeolites could be efficiently reduced considering the shape-selectivity of zeolite. However, the initial reaction rates highly depend on the nature of the substituents. 3-nitrotoluene, with a kinetic diameter of 0.59 x 0.55 nm, is small enough to diffuse through the pore aperture of the Beta framework. The reaction of 3-nitrotoluene reached >99 % conversion after 5 min on Pd@Beta catalyst with an initial rate 26.9 mmol/s/gMe (Table 2). 3-nitrotoluene can be almost completely converted to

m-toluidine. As the molecular size of 3-nitroanisole (0.60 x 0.57 nm) is also smaller than the pore size of Beta, 3-nitroanisole can be reduced on Pd@Beta catalyst. The initial reaction rate of hydrogenation of 3-nitroanisole is 5.7 mmol/s/gMe. Those numbers are lower than for 3-nitrotoluene, suggesting dominating electronic effect of substituents on aromatic ring. Considering the electron density of nitro group, the electron-donating effect of methyl group is stronger than that of methoxy group on the aromatic ring, resulting in lower electron density of nitro group on 3-nitroanisole than 3-nitrotoluene. It indicates that electronic effect of substituents on the aromatic ring is an important factor for initial reaction rates. Furthermore, the molecular size of 3-nitroanisole is slightly larger than 3-nitrotoluene and might influence the

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diffusion rate to some extent. Despite the lower initial hydrogenation rate of

3-nitroanisole the yield of corresponding aromatic amines is nearly >99 % without

any signs of by-products. The initial hydrogenation reaction rate of methyl 3-nitrobenzoate (0.8 mmol/s/gMe) is noticeably lower than other meta-position

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substituted nitroarenes on Pd@Beta catalyst. The electron-withdrawing groups on aromatic ring significantly reduce the electron density of nitro group comparing to the electro-donating substituted group, resulting only in 45 % conversion after 100

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min - this indicates that electronic factor is significant.

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Similar results were obtained on Pd@MWW catalyst even through the lower initial reaction rates compared with Pd@Beta (Fig. 7b and Table 3). Different initial reaction rates were found for nitroarenes with various types of substituted groups on

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Pd@Beta and Pd@MWW catalysts. Variations in initial reaction rates might be attributed to the electron density of nitro group caused by different groups substituted on aromatic ring. Nitroarenes containing electron-donating group with a

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high electron density on nitro group are more active than nitroarenes with electron-withdrawing group [42, 45, 56-58]. Nevertheless, due to smaller pore size of MWW the contribution of the steric effect should be more prominent than in case of

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Beta.

3.2.3 Steric effect To explore the steric hindrance and molecular sieving effects of Pd@Beta and Pd@MWW catalysts, nitroarenes bearing same groups in different positions were investigated under the optimized reaction condition. In these cases, steric effect dominates. The hydrogenation kinetic profiles of substrates with the same electron-donating group (-CH3) or electron-withdraw group (-COOCH3) on

ortho/meta/para position are shown in Fig. 8 (Pd@Beta) and Fig. 9 (Pd@MWW). 3-nitrotoluene and 4-nitrotoluene react effectively yielding desired products and exhibiting similar initial reaction rates (~27 mmol/s/gMe) on Pd@Beta catalyst. However, the steric hindrance of accessibility of -NO2 group has a negative impact on the reaction which is visible in case of 2-nitrotoluene. In the latter reaction, product was obtained in lower initial reaction rate (9.6 mmol/s/gMe). This result shows that

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the steric effect of the substrate plays an important role in the catalytic activity.

Fig. 8 - Hydrogenation reaction kinetic profiles of nitrotoluene (a) and methyl

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nitrobenzoate (b) on ortho/meta/para position on Pd@Beta catalyst Similarly, substrates (methyl nitrobenzoate) with electron-withdrawing group attached on the aromatic ring were also tested. The electronic effect on the aromatic

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ring is not significant due to the same electron-withdrawing group. The steric hindrance of methyl nitrobenzoates in ortho-, meta- and para-position have evident influence on the catalytic performance. For methyl 4-nitrobenzoate, the steric effect

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of the ester group in the para-position of the aromatic ring could be neglected. It shows high initial reaction rate 20.3 mmol/s/gMe and >99 % conversion on Pd@Beta

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catalyst. In contrast, the electronic repulsion of the ester group and nitro group in the meta position causes lower initial reaction rate 0.8 mmol/s/gMe and 45 % conversion. For methyl 2-nitrobenzoate, steric effects possibly arise from stronger repulsion forces of two adjacent groups resulting in no hydrogenation reaction, however it is even more probable that this molecule is too big to be transport through the pores.

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Fig. 9 - Hydrogenation reaction kinetic profiles of nitrotoluene (a) and methyl nitrobenzoate (b) on ortho/meta/para position on Pd@MWW catalyst

Similar results were obtained using Pd@MWW as catalyst (Fig. 9). In the

hydrogenation of nitrotoluene, i.e. lower initial reaction rate of 2-nitrotoluene compared with 3-nitrotoluene and 4-nitrotoluene. The initial reaction rate of methyl

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4-nitrobenzoate was larger than methyl 3-nitrobenzoate, and almost no conversion

was achieved in the case of methyl 2-nitrobenzoate (Table 3). These results

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demonstrate that steric effects are crucial for the hydrogenation rate of nitroarenes (both hindrance of the groups and molecular sieving effect of the zeolite). The

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substituent groups attached to the ortho-position of the aromatic ring are more difficult to reduce than corresponding meta- and para- isomers, as steric hindrance causes a tilting of the nitro group out of the arene plane, making charge

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delocalization in the reduced species [44, 56, 59-63].

Table 2 - Reaction results for the hydrogenation of substituted nitroarenes on the

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Pd@Beta catalyst; Reaction conditions: temperature 130 °C; hydrogen pressure

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0.6 MPa; 0.5 mL n-octane; 1.5 mmol nitroarenes, 50 mg Pd@Beta catalyst Substrate

Product

rate (mmol/s/gMe)

Time (min)a

26.9

1

5.7

4

Max conversion

Selectivity

(%)

(%)

>99

>99

>99

>99

45

>99

27.2

0.5

>99

>99

9.6

2

>99

>99

20.3

1

>99

>99

0

--

0

Time required to reach 15 % conversion

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26

--

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a

0.8

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Table 3 - Reaction results for the hydrogenation of substituted nitroarenes on the Pd@MWW catalyst; Reaction conditions: temperature 130 °C; hydrogen pressure

Product

rate

(mmol/s/gMe)

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Substrate

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0.6 MPa; 0.5 mL n-octane; 1.5 mmol nitroarenes, 50 mg Pd@MWW catalyst Time (time)a

Max conversion

Selectivity

(%)

(%)

20.0

1

>99

>99

5.0

5

>99

>99

0

45

9

>99

14.4

1.5

>99

>99

3.6

6

>99

>99

a

18.1

1

>99

>99

0

--

0

--

Time required to reach 15 % conversion

3.2.4 Reusability

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Reusability is an important feature of the heterogeneous catalysis which should be examined in catalytic reactions. The resusability of the Pd@Beta catalyst under the studied condition is shown in Fig. 10. For this purpose, hydrogenation of

3-nitrotoluene was carried out with recovered Pd@Beta catalyst. After the first cycle, the spent catalyst sample was washed with n-octane to remove substances retained

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on Pd@Beta catalyst, and directly reused in the next run up to five cycles. These

results suggest that the catalyst remains high conversion up to three cycles, whereas

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the selectivity to m-toluidine was maintained >99 %. A decrease in the conversion was observed in the fourth and fifth run.

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After the fifth cycle, the spent Pd@Beta catalyst was “cleaned” by cyclopentyl methyl ether overnight and reuse for the sixth and seventh run. The conversion was clearly recovered up to >99 %, and the selectivity remained almost constant at the same

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time. These results suggest that the deactivation of Pd@Beta catalyst is ascribed mainly to the contamination by unidentified deposits or coke formation on Pd NPs during the hydrogenation reaction process, which cannot be easily removed. The

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hydrogenation resusability performance of Pd@Beta further demonstrates the high

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activity of these well dispersed Pd@Beta catalyst.

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Fig. 10 - The recycle hydrogenation of 3-nitrotoluene on Pd@Beta catalyst

Conclusion

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The hydrogenation of nitroarenes on metal NPs encapsulated within zeolites can be controlled through the intrinsic characteristics of individual molecules. In order to investigate the catalytic activity and handle electronic/steric effects, Pd@Beta and

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Pd@MWW catalysts were prepared by ion exchange with Pd(NH3)4(NO3)2. The structure and textural properties of Pd@Beta and Pd@MWW catalysts were preserved well after the successful encapsulation of Pd NPs (proven by XRD, SEM and

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nitrogen sorption measurements). STEM images showed Pd NPs supported on the Beta and MWW zeolites with uniform distribution and the average size of Pd NPs is

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2.6 nm and 1.7 nm, respectively. The catalytic activity and electronic/steric effects on the hydrogenation of nitroarenes were investigated by adjusting the functionalized groups and substituent positions of nitroarenes on Pd@Beta and Pd@MWW catalysts.

Nitroarenes

with

electron-donating

and

electron-withdrawing

functionalized groups were subjected to reveal electronic effects. Nitroarenes containing an electron-withdrawing substituent are less active than nitroarenes containing electron-donating groups. In the case of nitroarenes with the same functionalized groups, the ortho substituent are more difficult to reduce than

corresponding meta- and para- isomers because of the steric hindrance effect.

Acknowledgement This study is based upon work supported by the US Army RDECOM-Atlantic under Award No. W911NF-17-S-0003. Authors would like to thank the OP VVV “Excellent Research Teams”, project no. CZ.02.1.01/0.0/0.0/15_003/0000417 – CUCAM. JČ acknowledges the Czech Science Foundation for the ExPro project (19-27551X). YZ would like to thank the GAUK project (No. 1372819). Authors would like to thank Dr.

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Mariya Shamzhy for FTIR measurements.

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