Robust Zirconium PhosphateâPhosphonate Nanosheets Containing

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Robust Zirconium Phosphate-Phosphonate Nanosheets containing Palladium Nanoparticles as Efficient Catalyst for Alkynes and Nitroarenes Hydrogenation Reactions. Ferdinando Costantino, Morena Nocchetti, Maria Bastianini, Alessandro Lavacchi, Maria Caporali, and Francesca Liguori ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00193 • Publication Date (Web): 10 Apr 2018 Downloaded from http://pubs.acs.org on April 10, 2018

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ACS Applied Nano Materials

Robust Zirconium Phosphate-Phosphonate Nanosheets containing Palladium Nanoparticles as Efficient Catalyst for Alkynes and Nitroarenes Hydrogenation Reactions. Ferdinando Costantino,⸸ Morena Nocchetti,§ Maria Bastianini,# Alessandro Lavacchi, ǂ Maria Caporali*,ǂ and Francesca Liguori*,ǂ, ⸸

Dipartimento di Chimica Biologia e Biotecnologia, Via Elce di Sotto 8, 06123 University of

Perugia §

Dipartimento di Scienze Farmaceutiche, Via del Liceo 1, 06123 University of Perugia

#

PROLABIN & TEFARM srl, Via Dell’Acciaio, 9 06134 Ponte Felcino, Perugia

ǂ

Istituto di Chimica dei Composti Organo Metallici - Consiglio Nazionale delle Ricerche (CNR

ICCOM), Via Madonna del Piano 10, 50019 Sesto Fiorentino; Firenze, Italy. Keywords: Zirconium Phosphonates, Layered 2D materials, Heterogeneous Catalysis, Palladium Nanoparticles, Partial Hydrogenation, Alkynes, Nitroarenes,

ACS Paragon Plus Environment

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ABSTRACT:

Thin nanosheets based on layered zirconium mixed phosphate-phosphonate of formula Zr2(PO4)[(PO3CH2)2NCH2COOH]2⋅H2O were obtained by exfoliation in aqueous solution of propylamine of the pristine crystalline precursor. Highly dispersed palladium nanoparticles, of 2 to 5 nm average size were generated using palladium acetate as precursor and immobilized on the nanosheet surface. The resulting layered nanohybrids differ in the palladium loading (from 5 to 22 % in weight of Pd) and in the amount of propylamine intercalated between the nanosheets. The new 2D material was investigated as catalyst in the partial hydrogenation of C≡C bonds and in the selective reduction of nitroarenes, showing very good selectivity at full conversion under room temperature, with pretty constant activity upon recycle. Metal leaching in solution was negligible as well as no additives nor regeneration steps were needed for repeated use.

INTRODUCTION In recent years, 2D materials have gained a relevant importance in many fields of material sciences, energy storage and catalytic application.1,2 Nanosheets are considered as bi-dimensional objects with lateral thickness of the order of few nanometers and an extended surface of micrometric or sub-micrometric size.3 They are usually obtained by exfoliation of a crystalline precursor with the aid of a proper compound, able to intercalate into the interlayer region and large enough to separate the sheets. A high number of inorganic nanosheets with fascinating properties exist in the literature, such as titanate, oxides, hydroxides, dichalcogenides and black phosphorus.4,

5, 6, 7, 8

On the contrary, the number of


2

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hybrid nanosheets, containing exposed organic functional groups covalently linked to inorganic layers is limited. In recent years hybrid crystalline materials containing zirconium received a huge attention due to their high chemical and thermal stability. As an example, zirconium MOF, 3D materials built from the connection of zirconium oxoclusters and carboxylic ligands are actually considered the benchmark compounds for many application in solid state chemistry, including heterogeneous catalysis.9,10 MOF based on zirconium phosphonates have received lower attention for the tendency of phosphonate ligands to yield layered compounds. The formation of 2D nanosheets makes these materials very promising for the obtainment of robust and stable nanosheets obtained for exfoliation of the pristine precursors. Layered zirconium phosphates and phosphonates are 2D insoluble materials already used for a large number of applications in heterogeneous catalysis.11,12 They are chemically stable in a wide range of temperature and acidity conditions and therefore they are amenable for use in heterogeneous catalysis, as support for active metallic nanoparticles.13,14 Recently, some of us reported on the synthesis and application in Suzuki-Miyaura and Heck flow reactions of Pd NPs dispersed onto the

surface

of

a

zirconium

phosphate-carboxyphosphonate

of

formula

Zr2(PO4)[(PO3CH2)2NCH2COOH]2⋅H2O (hereafter named Pd@ZPGly) exfoliated in 2-3 nm thick nanosheets by using propylamine as exfoliating agent.15,16 The compound possesses a polar surface due to the presence of exposed carboxylic and phosphonic groups able to interact with propylammonium ions and with Pd(II) cations to be reduced in Pd nanoparticles. The composite catalysts showed a high activity and recyclability with a very low Pd leaching. Herein we report on the modification of the synthesis of the Pd@ZPGly catalyst where the Pd loading was varied by changing the initial amount of propylamine respect to the ion exchange capacity of the ZPGly in acidic form. In this work, propylamine was used at 20, 40, 60 and 80% of the ion exchange


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capacity (IEC), respectively, affording materials with different Pd loading. In order to study the effect of the amount of propylamine and of the palladium loading the catalysts were used in the selective partial hydrogenation reactions of alkynes and in the reduction of nitroarenes under batch condition. The catalytic semi-hydrogenation reaction of multiple C≡C bonds to obtain Zalkenes is a main strategy for the large scale production of fine-chemicals including pharmaceuticals, food additives and fragrances17,18 and for the complete elimination of alkynes and alkadienes from alkene feedstock.19,20 The choice of palladium nanoparticles was due to their high versatility as chemo and regioselective catalyst for substituted alkynes. On industrial scale, this hydrogenation process is usually carried out using Lindlar-type catalyst

21,22

or by the

addition of contaminants such as organic bases (e.g. quinoline) or metal ions (Cu, Pb)23 with serious drawbacks in terms of overall sustainability. EXPERIMENTAL SECTION Materials and methods All reactions and manipulations were performed under nitrogen by using standard Schlenk techniques, unless otherwise stated. Methanol was distilled from magnesium prior of use. All the reagents were commercial products and were used as received without further purification. The reaction products were unequivocally identified by GC retention times and GC-MS. GC analyses were performed on a Shimadzu GC-2010 gas chromatograph equipped with a flame ionization detector and a 30 m (0.25 mm ID, 0.25 mm FT) Varian VF-WAXms capillary column. GC-MS analyses were performed on a Shimadzu QP2010SE spectrometer equipped with an identical capillary column. Hydrogenation reactions with alkynes under a controlled pressure of hydrogen (1 bar) were performed using a H2 generator Parker–Balston H2PEM-260 and a simple twonecked glass-flask. Hydrogenation reactions with nitroarenes under a controlled pressure of


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hydrogen (5 bar) were performed using a 100 mL stainless steel PARR 4565 reactor equipped with a bottom drain valve and a pressure controller. Zirconium, phosphorus and palladium contents of samples were obtained by inductively coupled plasma−optical emission spectrophotometry (ICP−OES) using a Varian Liberty Series II instrument working in axial geometry after the mineralization of samples with hydrofluoric acid. XRPD patterns were collected with Cu Kα radiation on a PANalytical X’PERT PRO diffractometer, PW3050 goniometer equipped with an X’Celerator detector. The long fine focus (LFF) ceramic tube operated at 40 kV and 40 mA. To minimize preferential orientations of the microcrystals, the samples were carefully side-loaded onto an aluminum sample holder with an oriented quartz monocrystal underneath. Transmission electron microscope (TEM) (Philips, model 208) operating at 80 kV of beam acceleration was used to investigate the morphology of the samples. A drop of the aqueous dispersions Pd@ZPGly was deposited on a 200 mesh copper-coated with a Formvar/carbon support grid and then evaporated in air at room temperature. SEM and STEM images were acquired on TESCAN GAIA 3 2016 FIB/SEM electron microscope equipped with a STEM detector capable of collecting bright field, dark field, and high angle dark field signals. All the images were acquired at an accelerating voltage of 30 kV with a spot size of 1.7 nm. A Micromeritics 2010 apparatus was used to obtain the adsorption and desorption isotherms with nitrogen at 77 K. Before the adsorption analysis, the samples were outgassed at 100 °C under vacuum overnight.

Synthesis of microcrystalline catalyst Pd@ZPGly The synthesis of Zr2(PO4)[(O3PCH2)2NHCH2COOH]2⋅H2O (ZPGly) was carried out following the previously reported procedure.24 Samples of ZPGly exfoliated with different amount of


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propylamine were prepared. The exfoliated samples will be indicated as ZPGlyx-y (with x = 20, 40, 60, 80, corresponding to the amount of propylamine added and y = A, B and C corresponding to the % Pd loading, reported in table S1). In particular, 100 mg of ZPGly (MW = 834 g/mol, IEC=3.60 mequiv/g) were suspended in 12 ml of deionized water and then treated with 0.7, 1.5, 2.2, 2.9 ml of aqueous solution of propylamine 0.1 M corresponding to 20, 40, 60 and 80% of IEC, respectively. The dispersions were kept under stirring for 1 day and then the pH of the dispersions was measured. To each ZPGlyx dispersion, 5.4 ml of Pd(CH3COO)2 0.05 M in acetone (0.27 mmol of Pd(II), meq Pd/meq H=1.5) were added drop by drop under vigorous agitation. The dispersions were left under magnetic agitation for 2-3 days. The solids were recovered by ultracentrifugation (15000 rpm for 10 minutes) and washed once with acetone and twice with deionized water. The solids were dried over P2O5 and worded as Pd@ZPGlyx-A. Samples with lower content of Pd(II) were prepared starting from dispersion of ZPGly20 and ZPGly60 by adding 2.7 ml and 1.8 ml of Pd(CH3COO)2 0.05 M in acetone (0.135 mmol of Pd(II), meq Pd/meq H=0.75 and 0.09 mmol of Pd(II), meq Pd/meq H=0.5 respectively). The solids were dried over P2O5 and worded as Pd@ZPGlyx-B and Pd@ZPGlyx-C (Pd amount is reported in table S1). Hydrogenation reactions of alkynes by Pd@ZPGly under batch conditions In a general experiment, the supported catalysts were added under nitrogen into a two-necked 50 mL flask containing a degassed solution of the substrate in methanol (0.2 M). The amount of catalyst and substrate used were calculated for each experiment considering the Pd-loading of the selected catalyst, the type of substrate and the substrate/catalyst ratios pursued (Sub/Cat=170250-400-800). In a typical experiment, 6.6 mg of catalyst (19.8%-Pd@ZPGly20-A) were added to a 0.2 M degassed solution of 3-hexyn-1-ol in methanol (28 ml, Sub/Cat ratio = 400). Pure


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hydrogen gas, produced by a hydrogen-generator Parker- Balston, was bubbled while keeping the reaction mixture stirring at 1 bar and at room temperature, using an orbital stirrer at 200 rpm. The reaction was monitored by sampling periodically 1 µL of solution and analyzing by GC and GC-MS until to nearly complete conversion. The reaction products were unequivocally identified by the GC retention times and mass spectra of those of authentic specimens. After the desired time, the reaction mixture was centrifuged (12000 rpm for 30 min) and the methanol solution was completely removed under a stream of hydrogen using a gas-tight syringe. A sample of this solution was analyzed by ICP-OES, the amount of leached palladium was negligible for every type of catalysts, never above 0.7 ppm. In any case the contribution of homogeneous phase catalysts could be ruled out, since in no case catalytic activity was shown by the solution recovered after catalysts (Maitlis catalyst leaching test).25 After use in catalysis, the solid catalyst was washed with hydrogen-degassed methanol (3 x10 ml), dried in a stream of nitrogen overnight and stored for later characterization or used for other experiments without decrease of catalytic activity. Hydrogenation reactions of nitroarenes by Pd@ZPGly20-B under batch conditions For each experiment, the catalyst selected Pd@ZPGly20-B (2 mg) was suspended in degassed methanol (3 mL) by one minute of sonication, then the amount of substrate, calculated considering the desidered substrate/catalyst ratios, was added and the vial transferred in a stainless steel reactor that was closed, purged with hydrogen gas for three times and pressurized with 5 bar of hydrogen. The mixture was stirred at room temperature (1200 rpm) for the required time and afterwards the gas was vented. To the reaction mixture, degassed methanol (3 mL) was added and after centrifugation (14000 rpm for 30 min) the clear surnatant was removed and


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analysed by GC. The reaction products were unequivocally identified by the GC retention times and mass spectra of those of authentic specimens. In the recycling tests, first the catalyst was precipitated from the reaction mixture by ultracentrifugation, then it was re-dispersed in methanol (3 mL) by sonication (2 minutes). Fresh substrate was added and the mixture was transferred by a gas-tight syringe into a glass vial inside the stainless steel reactor. Hydrogen gas was pressurized up to 5 bar and a new cycle was launched. RESULTS AND DISCUSSION Catalyst preparation and characterization The polyhedral structure of ZPGly is shown in Fig. S1. The compound has layers 8 Å thick made of two planes of ZrO6 octahedra connected by an internal PO4 phosphate group. The bisphosphonic glyphosine moieties are placed in the external part of the layer with a doubly connected PO3C group binding two different Zr atoms and the other PO3C group monoconnected to a single Zr atom. In this way there are two free P-O groups pointing towards the interlayer region, one of which is protonated. The amino-carboxylic groups are not involved in any bond and they are also exposed on the layer surface, thus designing a polar acidic region able to interact with cations through ion-exchange reaction with the acidic protons. Figure 1 reports the schematic synthetic procedure for the exfoliation of the crystalline ZPGly precursor in propylamine and the successive deposition of 2 nm Pd nanoparticles, obtained from reduction of Pd2+ ions exchanged onto the acidic surface. Table S1 reports the weight percentage of Zr, P and Pd for each propylamine–exfoliated compound, wherein pH values of ZPGly dispersions before and after the Pd(CH3COO)2 addition were reported. The pH of the ZPGly dispersions at different propylamine content increases with the content of propylamine. In particular, for low


8

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propylamine content (20-40 % of IEC), the pH is close to those of the ZPGly suspension (7-8) indicating that the amine is mainly present as propylammonium involved in ionic interactions with the exposed PO- and COO- groups of the layers. When the content of propylamine increases (60-80 % of IEC) the pH is more than 10 and it is in line with the presence of free propylamine in the dispersions. The pH measured after the contact with the Pd(CH3COO)2 0.05 M solution was always around 5 very likely due to the formation of acetate buffer arising from the protonation of a fraction of acetate with the proton released by the ion exchange Pd2+/H+.


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Figure 1. Schematic representation of the exfoliation of ZPGly precursor and successive deposition of Pd NPs onto the nanosheets surface (colors code: green = Zr, red = O, purple = P, grey = C, blue = N).


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Figure 2 shows the XRPD patterns of the sample reported in Table S1. Samples of Pd@ZPGly20-A, B and C containing the lowest amount of propylamine, show basal planes (002) around 15 Å (samples B and C) and a peak at 18 Å in the case of A indicative of cointercalation of propylamine and Pd(II) ions. The same d-spacing was already reported in a previous paper.15 In the sample Pd@ZPGly20-B and C the basal plane of the crystalline ZPGly compound (15.1 Å) is present as predominant phase and this suggests that the use of small amount of propylamine leads to the intercalation only of a small fraction of the pristine compound. The presence of two basal planes at 14.9 and 18.4 Å suggests that a higher amount of palladium induced a better co-intercalation. The fraction of intercalated phase has strongly increased in compound Pd@ZPGly40-A, as the intensity of the reflection at 22.2 Å is much stronger than that of non intercalated ZPGly compound. In compounds Pd@ZPGly60-A and Pd@ZPGly80-A only the peak at 22.2 Å is visible whereas the peak around 15 Å is absent.


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Figure 2. XRPD of Pd@ZPGlyx-y. Interlayer distances: 15.1 Å (1), 18.1 Å (2-4), 14.9 Å (3), 20.2 Å (5-6), 22.2 Å (7). The morphology of the flakes was analysed by STEM and SEM on a fresh sample of Pd@ZPGly20-B (see Fig. S3). TEM pictures in Fig. S2a-d refer to the samples Pd@ZPGlyx-A. In all cases, the nanosheets have an extended surface of micrometric size and Pd nanoparticles are uniformly distributed over it. The nanoparticles have an average size from 2 to 5 nm even if some larger clusters (up to 20 nm) are also detected. The reduction of palladium ions to metallic palladium during the synthetic procedure has been already observed and reported.15 The size distribution of the smallest population of Pd NPs has been determined on the Pd@ZPGly20-B (Fig. S2e) as representative sample and the histogram is reported in Fig. S2f. The particles have a narrow size distribution centred at 2.0 nm. The interaction between Pd and the nanosheet surface looks to be quite strong since Pd NPs are stable do not show any tendency to aggregate. The high dispersion degree is due to the ionexchange mechanism of the –PO3H2 and COOH groups exposed on the surface that are easily deprotonated by the propylamine used for the exfoliation and thus become able to bind Pd ions over the whole nanosheet surface. Pd NPs are then stabilized by the surface aminocarboxylic groups. Specific surface area of microcrystalline ZPGly sample and of Pd@ZPGly_60 nanosheets was measured by BET analysis using N2 at 77 K. The isotherms are shown in Figure S8. The microcrsytals of ZPGly have a BET surface area of 34 m2/g with 2x10-3 cm3/g of micropores volume. The external surface area is equal to 30 m2/g. On the contrary, nanosheets exfoliated with 60% of propylamine and containing Pd NPs displayed a drastic reduction of the surface area to about 7 m2/g, suggesting that after separation from liquid phase by centrifugation, the


12

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nanosheets tend to strongly aggregate forming a thin film and the porosity due to the particle contacts disappears.

Selective hydrogenation reactions of alkynes In order to achieve a preliminary estimate of the catalyst performance and an optimization of the reaction conditions in term of Substrate/Catalyst ratio, Pd loading and amount of propylamine (20-40-60-80), all the Pd@ZPGly catalysts were tested in the hydrogenation reaction of 3-hexyn-1-ol 1 (see Scheme 1). The experiments were monitored for conversion, selectivity, productivity and metal leaching. Other substrates such as terminal (2, 5), internal alkynes (1,6), alkyn-esters (3) and diols (4), were investigated only using the best catalyst selected.

Scheme 1. Sketch of substrates investigated and products detected.


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The choice of 3-hexyn-1-ol 1 as benchmark substrate was due to the relevance of the semihydrogenation product (Z)-3-hexen-1-ol 1a, an important market flavour ingredient 26, 27 which is industrially manufactured with a 96% selectivity at 99% conversion by a batch process with Lindlar catalyst.28,29 PdNP@titanate nanotubes30 and Dowex resin-supported Pd NPs31 were explored in the past years aiming at a more sustainable Pd-based catalyst. Continuous flow systems for this substrate were also recently developed32,33 to be competitive on a large scale.34


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Aimed at obtaining catalysts with a high surface area, layered-zirconium-phosphonates were selected as appropriate support for PdNPs and for partial hydrogenation reaction of alkynes. The first catalyst tested was Pd@ZPGly-20-A containing a high Pd loading (19.80% w/w) and low amount of propylamine (20% respect to the ion-exchange capacity EIC) in mild reaction condition (P = 1 bar, r.t., 0.2 M in MeOH). With Substrate/Catalyst ratio = 400 a good compromise in term of conversion (99.9%), ene-selectivity (92.5%) Z-selectivity (91.2%) and TOF (480 h-1) was achieved (Table 1, entry 3), comparable to the values obtained with conventional catalysts.

Table 1. Hydrogenation of 3-hexyn-1-ol with Pd@ZPGly-20-A, at different substrate/catalyst ratio.a

Entry

Sub/Cat

Conversion (%)

TOF (h-1)b ene-Sel. (%)c Z-Sel. (%)d

1

167

99.9

333

74.8

1a 70.6

2

250

99.9

600

89.2

1a 88.9

3

400

99.9

480

92.5

1a 91.2

4

800

99.9

480

84.4

1a 82.5

a

Reaction conditions: substrates in methanol solution 0.2 M, room temperature, 1 bar H2, orbital stirrer 200 RPM. Dry catalyst Pd-ZPGly-20-A 19.8 % (w/w). H2 pressure produced by Hydrogen generator. Reactor volume 50 mL. Data from GC analysis. b TOF calculated on total conversion c eneselectivity, for example, 1a+1b=(1a+1b)/(1a+1b+1c+1d+1e+1f).d Zselectivity, for example 1a=1a/(1a+1b).

Maintaining the Substrate/Catalyst ratio equal to 400 and standard reaction conditions, four samples of Pd@ZPGly catalyst with comparable Pd loading (19-21%) but exfoliated with different amount of propylamine (respectively 20, 40, 60 and 80% of base respect to the EIC)


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were tested in the hydrogenation of 3-hexyn-1-ol achieving good results in term of TOF especially with Pd@ZPGly-20A, and Pd@ZPGly-60A. (Table 2, entry 1-3)

Table 2. Hydrogenation of 3-hexyn-1-ol with Pd@ZPGly catalysts containing different % of propylamine.a

Entry Catalyst

Pd% (w/w)

Conversion Pd TOF (h-1)b ene-Sel. (%)c Z-Sel. (%)d (%) (ppm)e

1

Pd-ZPGly-20-A

19.8

99.9

480

92.5

1a 91.2

0.1113

2

Pd-ZPGly-40-A

19.4

99.0

297

96.3

1a 95.9

0.0557

3

Pd-ZPGly-60-A

21.7

99.2

397

97.2

1a 96.5

0.1011

4

Pd-ZPGly-80-A

19.4

99.9

150

89.4

1a 89.8

0.0166

a

Reaction conditions: substrates in methanol solution 0.2 M, room temperature, 1 bar H2, orbital stirrer 200 RPM, Substrate/catalyst ratio = 400, H2 pressure produced by Hydrogen generator. Reactor volume 50 mL. Data from GC analysis. b TOF calculated on total conversion c eneselectivity, for example, 1a+1b = (1a+1b)/(1a+1b+1c+1d+1e+1f).d Z-selectivity, for example 1a=1a/(1a+1b). e Pd leached in ppm, measured with ICP-OES.

Table 3. Hydrogenation of 3-hexyn-1-ol with Pd@ZPGly catalysts. Effect of Pd loading on

Selectivity.a

Entry

Catalyst

Pd% Conversion Pd TOF (h-1)b ene-Sel. (%)c Z-Sel. (%)d (w/w) (%) (ppm)e

1

Pd-ZPGly-20-A

19.8

99.9

480

92.5

1a 91.2

0.1113

2

Pd-ZPGly-20-B

8.6

99.6

478

90.6

1a 89.9

0.0168

3

Pd-ZPGly-20-C

5.3

99.9

343

92.6

1a 91.9

0.2556

4

Pd-ZPGly-60-A

21.7

99.2

397

97.2

1a 96.5

0.1011

5

Pd-ZPGly-60-B

13.9

99.7

109

95.7

1a 95.9

0.2719

a

Reaction conditions: substrates in methanol solution 0.2 M, room temperature, 1 bar H2, orbital stirrer 200 RPM, Substrate/catalyst ratio = 400, H2 pressure produced by hydrogen generator. Reactor volume 50 mL. Data from GC analysis. b TOF calculated on total conversion c ene-


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selectivity, for example, 1a+1b = (1a+1b)/(1a+1b+1c+1d+1e+1f).d Z-selectivity, for example 1a=1a/(1a+1b). e Pd leached in ppm, measured with ICP-OES.

Considering that the catalyst Pd@ZPGly-20-B shows a comparable activity to Pd@ZPGly-20A (Table 3, entry 2), but contains less than half of palladium, it was selected as the best catalyst for the hydrogenation reaction of various alkynes and nitroarenes as showed in Table 4 and 5 respectively.

Table 4. Hydrogenation of alkynes with Pd@ZPGly-20-B catalyst a

Entry Substrate

Conversion TOF (h-1)b ene-Sel. (%)c (%)

Z/E-Sel. (%)d

Pd (ppm)e

1a 89.9

0.0168

1

3-hexyn-1-ol (1)

99.6

478

(1a+1b) 90.6

2

2-methyl-3-butyn-2-ol (2)

99.9

685

2a

40.5

-

0.5279

3

”

75.3

602

2a

94.9

-

-

4

methyl-3-phenylpropiolate (3)

99.9

400

(3a+3b) 62.3

3a 94.0

0.6707

5

”

53.5

428

(3a+3b) 84.3

3a 95.4

-

6

2-butyn-1,4-diol (4)

98.3

79

(4a+4b) 97.3

4a 99.9

0.4871

7

ethynylbenzene (5)

95.6

170

5a

-

0.1045

8

3-phenyl-2-propyn-1-ol (6)

95.4

95

(6a+6b) 93.7

6b 96.8

0.10050

94.8

a

Reaction conditions: substrates in methanol solution 0.2 M, room temperature, 1 bar H2, orbital stirrer 200 RPM, Substrate/catalyst ratio = 400, H2 pressure produced by hydrogen generator. Reactor volume 50 mL. Data from GC analysis. b TOF calculated on total conversion c ene-selectivity, for example, 1a+1b = (1a+1b)/(1a+1b+1c+1d+1e+1f).d Z-selectivity, for example 1a = 1a/(1a+1b) e Pd leached in ppm, measured with ICP-OE.

2-Methyl-3-butyn-2-ol 2, an important intermediate for the synthesis of vitamins (A, E), perfumes and pharmaceuticals35 can be manufactured with high yields but with fast catalyst deactivation using the Lindlar and other catalysts under batch conditions.36,37,38 The use of


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Pd@ZPGly-20-B provided 2a with high selectivity (94.9%), moderate conversion (75.3%) and a high TOF value (602 h-1) (Table 4, entry 3). A decrease of selectivity was observed at nearly complete conversion (Table 4, entry 2). High performance was obtained in the reduction of methyl-3-phenylpropiolate 3 in term of Z/E selectivity (94.0%) at nearly complete conversion (Table 4, entry 4). Comparable results were obtained using conventional palladium catalysts, either Lindlar’s Type,39 pumice supported40 or in presence of amine as additive41 (Table 4, entry 4,5). 2-Butyn-1,4-diol 4 was partially hydrogenated obtaining cis-2-butene-1,4-diol 4a, an important pharmaceutical and agrochemical intermediate42 manufactured in ca 5000 t/y under batch conditions and high pressure and/or temperature, with Pd/Al2O3 catalysts

43

and on the

laboratory scale with variable selectivity (70-99% at 80-90% conversion) with different catalyst.44, 45, 46, 47 In our case, under mild conditions (1 bar, r.t.) the catalyst Pd@ZPGly-20-B provided the mixture 4a+4b in excellent selectivity (97.3%) 100% of which the Z isomer 4a, at 99.9% conversion (Table 4, entry 6). Comparable results in term of conversion and selectivity were obtained for the hydrogenation of 3-phenyl-2-propyn-1-ol 6 (Table 4 , entry 8), thus cinnamyl alcohol (6a+6b) was obtained in 93.7% selectivity, 96.8% of which the Z isomer 6a at 95.4% conversion. Considering the highest selectivity for the hydrogenation of 6 recently observed with dendron-stabilized Pd NPs and quinoline additives (97% ene-selectivity, 98% Zselectivity),48 the use of Pd-ZPGly-20-B as heterogeneous catalysts couples the inherent advantage of efficiency with the absence of toxic contaminants. Finally the efficiency of the same catalyst towards unsubstituted alkynes was evaluated using ethynylbenzene 5,49, 50, 51 herein hydrogenated with high selectivity to styrene (94.8%) at 95.6% conversion (Table 4, entry 7). Selective hydrogenation reactions of nitroarenes


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Functionalized anilines are high valuable intermediates for the manufacture of many agrochemicals, pharmaceuticals, polymers and dyes,52 therefore the selective reduction of nitroarenes to the corresponding anilines has been the subject of substantial research efforts over the last decades. The main challenge is to find a catalytic system capable of reducing the nitro group with high chemoselectivity in substrates containing unsaturated C-C or C-N bonds, carbonyl groups or halogens. Having observed a very good performance of Pd@ZPGly-20-B in the semihydrogenation of alkynes, we decided to extend its application to the selective hydrogenation of nitroarenes, see Scheme 2. Very mild reaction conditions were chosen, i.e. room temperature and a low hydrogen pressure (5 bar). For each experiment, the catalyst was suspended in methanol (by one minute of ultrasonication), the substrate was added and the mixture stirred at room temperature in a stainless steel reactor pressurized with 5 bar of hydrogen. In ethanol the performance of the catalyst was the same as in methanol, whereas in tetrahydrofuran the conversion was close to zero.

Scheme 2. Sketch of the investigated nitroarenes and of the corresponding identified reaction products.


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Table 5. Hydrogenation of nitroarenes with Pd@ZPGly-20-B catalysta

Entry Substrate (cycle)

Sub/catb

Time Conversion (%) [min]

TOF (h-1)c

Selectivity (%)

1

nitrobenzene

100

120

99.9

50

7a 100

2

1-chloro-3nitrobenzene

300

45

99.7

400

8a 84.0

3

1-chloro-3nitrobenzene

300

20

75.3

683

8a 84.9

4

1-fluoro-3nitrobenzene (1st)

200

60

99.9

200

9a 100

5

1-fluoro-3nitrobenzene (2nd)

200

60

99.9

200

9a 100


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6

1-fluoro-3nitrobenzene (3rd)

200

60

98.5

197

9a 100

7

1-bromo-4nitrobenzene

200

60

72.4

145

10a 0.6

8

4-nitrobenzaldehyde

200

105

99.3

137

11a 84.0

a

Reaction conditions: substrates in methanol, room temperature, 5 bar H2, magnetic stirring 1200 RPM. Data from GC analysis. b Substrate/Catalyst ratio (mol/mol). c TOF calculated on total conversion.

Intriguingly, the catalytic efficiency changed remarkably depending on the substituent in the benzene ring: nitrobenzene resulted the least reactive, while the presence of a halogen, either fluoro or chloro, in meta position, speeded up the kinetic of the process, reaching a TOF of 683 h-1 for the chloro derivative, which is much higher if compared to other Pd-heterogeneous catalysts based on supported Pd NPs and used in similar experimental conditions (see entry 3 in Table 5).53 The chemoselectivity was complete in the case of the fluoro derivative, while with chloro or aldehyde substituent, the chemoselectivity droped to 84%, being formed respectively the dehalogenated product, aniline or the over-hydrogenated product, aminobenzyl alcohol. The observed selectivity towards 3-chloroaniline is higher in comparison to known heterogeneous catalysts based on supported Pd NPs.54 In the presence of bromine as substituent, the dehalogenated products aniline and nitrobenzene were obtained almost exclusively, being much easier the dehalogenation of the C-Br bond in comparison to C-F and C-Cl bonds in the presence of palladium-based heterogeneous catalyst.54 Catalyst reuse was tested in the case of 1-fluoro-3-nitrobenzene and the conversion and selectivity were unaltered after three consecutive runs (Table 5, entry 4,5,6), denoting that the catalyst structure is robust . No catalytic activity was shown by the recovered reaction solution, thus ruling out the loss of active species.


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Further evidence of the robustness of our system came from the characterization performed on the recycled nanocatalyst. Indeed, no substantial changes were observed in the PXRD of Pd@ZPGly-20-B (see Figure S7), SEM (Fig. S4) and TEM analysis (see Fig. S5-S6) showed the

morphology of the flakes is unaltered and the size distribution of Pd NPs results centered at 6.5 nm, only a small degree of agglomeration in comparison with fresh catalyst.

CONCLUSIONS Exfoliation of bulk zirconium phosphate-carboxyphosphonate was carried out varying the amount of propylamine (20, 40, 60 and 80% of the ion exchange capacity (IEC)) thus affording materials with different Pd loading. An array of new 2D nanohybrid catalysts, having Pd NPs on the surface of the nanosheets was isolated in order to study the effect of the amount of propylamine and of Pd loading in catalysis. All the synthesized catalysts were tested in the semihydrogenation of a wide range of alkynes showing in every case very high selectivity versus the desired Z-alkene at complete conversion even if the increasing amount of propylamine caused a detrimental effect on the reaction rate. Therefore, only the catalyst bearing the lower amount of base (20%) and a medium loading of Pd (8.69%), Pd@ZPGly20-B, was tested in the reduction of nitroarenes. High chemoselectivity was observed for nitroarenes bearing the halo (F, Cl) group and in particular the presence of chloro substituent was accompanied by a drastic increase of turnover number in comparison to the other substrates. A constant activity upon recycle was afforded, with negligible metal leaching in solution nor regeneration steps for the catalysts. These results can be considered relevant to the development of an efficient, cost


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effective, reproducible and environmentally friendly heterogeneous catalyst for the synthesis of important fine-chemicals. AUTHOR INFORMATION Corresponding Author Francesca Liguori and Maria Caporali, CNR ICCOM, Via Madonna del Piano 10, 50019 Sesto Fiorentino; Firenze, Italy. Fax. +39 055 5225203, Tel: +39 055 5225291-5225249. *E-mail: [email protected]; *E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS Thanks are due to Dr Matteo Vanni, to Dr. Fabio Marmottini (BET analysis) and to Mr. Carlo Bartoli for technical assistance.

Supporting Information Available

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TOC: Table of Content


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251x216mm (96 x 96 DPI)



169x136mm (300 x 300 DPI)


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33x50mm (299 x 299 DPI)


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Robust Zirconium PhosphateâPhosphonate Nanosheets Containing

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