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Applied Catalysis A: General 399 (2011) 117–125

Contents lists available at ScienceDirect

Applied Catalysis A: General

journa l homepage: www.e lsev ier .com/ locate /apcata

elective hydrogenation of chloronitrobenzenes with an MCM-41 supportedlatinum allyl complex derived catalyst

rindam Indraa, Pattuparambil R. Rajamohananb, Chinnakonda S. Gopinathc,∗, Sumit Bhadurid,∗,outam Kumar Lahiri a,∗

Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, IndiaCentral NMR Facility, National Chemical Laboratory, Pune 411008, IndiaCatalysis Division, National Chemical Laboratory, Pune 411008, IndiaDepartment of Chemistry, Northwestern University, Evanston, IL 60208, USA

r t i c l e i n f o

rticle history:eceived 25 January 2011eceived in revised form 25 March 2011ccepted 27 March 2011

a b s t r a c t

A platinum precatalyst (1) has been prepared by reacting [(�3-C3H5)4Pt4Cl4] with surface function-alized MCM-41 with pendant –(CH2)3NH(CH2)2NH2 groups. For the hydrogenation of o-, m- andp-chloronitrobenzenes to the corresponding chloroanilines, 1 is found to be a highly active catalyst withgood selectivities for the m- and p-isomers. Its performance is superior to that of its palladium analogue

vailable online 2 April 2011

eywords:eterogeneous catalystelective hydrogenationehydrohalogenation

and far superior to that of commercial (5%) Pt/C or (5%) Pt/Al2O3. Comparison of solid state and solu-tion NMR data and other evidences indicate that on treatment with the functionalized MCM-41 support;[(�3-C3H5)4Pt4Cl4] loses the allyl ligand. XPS data show that in the fresh catalyst Pt is present in the 2+oxidation state. Based on these and analytical data, co-ordination by surface diamine and hydroxo groupsto Pt2+ in 1 is suggested. In the used catalyst both Pt2+ and Pt0 are present but the amount of metallic

otal.

hloronitrobenzenesCM-41

platinum is ∼16% of the t

. Introduction

Discovery of a high-performance catalyst for the hydrogena-ion of substituted nitrobenzenes has recently been the subjectf much research. Nanoparticles of many transition metals suchs platinum, palladium, gold, and nickel have been found to haveigh selectivity and/or activity [1–25]. Efficient hydrogenation of-, m- and p-chloronitrobenzene (CNB), with low hydrodehalo-enation, is a reaction of considerable industrial importance as theroducts chloroanilines (CAN) are intermediates for the commer-ial manufacture of many pharmaceuticals [26–35]. The objectiveherefore is to maximize the yields of o-, m- and p-chloroanilinesnd minimize the hydrodehalogenation byproducts nitrobenzeneNB) and aniline (AN) that are formed due to hydrogenolysis of thearbon–chlorine bond.

The earlier reports show that both in terms of turnovers and

electivities, Pt-catalysts generally give better results than allther metals. Although some of the reported catalysts have excel-ent selectivities, they have been tested only on either o-, m- or-CNB and not on all the three isomers [36–44]. The stereoelec-

∗ Corresponding authors.E-mail addresses: s-bhaduri@northwestern.edu (S. Bhaduri),

ahiri@chem.iitb.ac.in (G.K. Lahiri).

926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.apcata.2011.03.044

© 2011 Elsevier B.V. All rights reserved.

tronic demands for selective hydrogenation of the three isomers ofchloronitrobenzene are different. It is important therefore to estab-lish that under optimum conditions the same catalyst performswell for all the three isomers. From a practical point of view it isalso desirable to have a catalyst that is distinctly and demonstrablysuperior to commercial Pt-catalysts under identical conditions.

There are several recent review articles that deal with theadvantages and challenges of supported and tethered metal com-plexes as catalysts [45–47]. Our interest in supported and tetheredmetal complexes as precatalysts arises from the fact that theyhave the potential to generate small metal particles of unusualselectivity under the catalytic conditions. We have used tetheredand supported metal complexes as precursors of nanoparticlesand showed that organometallic derived MCM-41 supported Ru-and Pt-nanocatalysts exhibit high chemo- and enantioselectivityin hydrogenation reactions [48–50]. Of special relevance in thepresent context, is our earlier observation that a palladium allylcomplex tethered onto diamine functionalized MCM-41 is a selec-tive precatalyst for the hydrogenations of o- and m-CNB [51].

This precatalyst was prepared by the treatment of [(�3-

C3H5)2Pd2Cl2] with diamine functionalized MCM-41. In the viewof the encouraging results with Pd, we wanted to find out if theplatinum analogue (precatalyst 1) of the Pd-precatalyst (2) couldbe prepared by a similar synthetic approach and if so whether itcould provide better performance. A Pt-precatalyst can indeed be

1 sis A:

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18 A. Indra et al. / Applied Cataly

btained by the treatment of [(�3-C3H5)4Pt4Cl4] with diamine func-ionalized MCM-41. From NMR (solid state and solution) and XPSe find that in the fresh precatalyst (1) the surface anchored molec-lar species has a coordination environment distinctly differentrom that of the Pd catalyst (2). The Pt-catalyst (1) gives excellentonversions with high selectivities in the hydrogenation of all thehree o-, m- and p-CNB. Its performance is distinctly superior tohat of the Pd catalyst (2) and far superior to two commercial plat-num catalysts (5% Pt/C and 5% Pt/Al2O3). In the used catalyst somet-nanoparticles are seen but the extent of reduction of Pt2+ to Pt0

n catalyst 1 is far less than what was observed for the Pd catalyst2).

. Experimental methods

.1. Chemicals

Ludox HS-30 (30 wt% silica solution in water),etyltrimethylammonium chloride, N-[3-(trimethoxysilyl)propyl]thylenediamine ((CH3O)3SiCH2CH2CH2NHCH2CH2NH2), d6-imethylsulfoxide were obtained from Aldrich. Nitrobenzene,niline, ortho-, meta- and para-chloronitrobenzene (o-, m- and-CNB), ortho-, meta- and para-chloroaniline (o-, m- and p-CAN),thylenediamine, sodium hydroxide, sodium chloride, ammoniumydroxide solution (30%), acetic acid (glacial), allyl chloride,odium, iodine, methanol, tetrahydrofuran, toluene and CDCl3ere obtained from Merck India Limited, Mumbai, India. Com-ercial catalyst (5%) Pt on charcoal and (5%) Pt on alumina were

urchased from Lancaster and Aldrich Chem. Co., respectively.2PtCl4 was purchased from S. D. Fine Chemicals, Mumbai, India.nhydrous stannous chloride was obtained from Spectrochem Pvt.td., Mumbai, India. Hydrogen gas cylinder was supplied by BOCimited, India.

MCM-41 [52], [(�3-C3H5)4Pt4Cl4] [53] and [(�3-C3H5)2Pd2Cl2]54] were synthesized according to the literature reported proce-ures. The solvents were dried by following the standard literaturerocedure [55].

.2. Instrumentation

A JEOL-JEM-2100F FEG-TEM and Philips CM200 (operatingoltages: 20–200 kV) transmission electron microscope, 400 MHzarian FT-NMR spectrometer and Bruker AV III 400 MHz FT-NMRpectrometer (solution), Bruker DRX 500 MHz NMR (solid state)nd a Micromass Q-ToF mass spectrometer were used for TEM,MR and mass spectrometric measurements, respectively. Magna50 from Nicolet Instruments, USA has been used for recording IRpectra. Elemental analysis and platinum and palladium content ofresh precatalyst 1 and 2 was measured by using a Perkin-Elmer40C elemental analyzer, and Arcos from M/s. Spectro, GermanyCP-AES instrument, respectively.

.3. Synthesis of 1

MCM-41 (1 g) was heated under vacuum at 473 K forh and taken in 20 mL of dry toluene. The reagent N-[3-

trimethoxysilyl)propyl]ethylenediamine (4 mL) was added to thebove mixture and heated to reflux at 383 K for 120 h under dinitro-en atmosphere. The product was then separated by filtration andashed several times with dry toluene followed by dry methanol.

0.25 g [(�3-C3H5)4Pt4Cl4] was dissolved in 50 mL of dry tetrahy-rofuran at 338 K to get a yellow solution. To this solutionried N-[3-(trimethoxysilyl)propyl]ethylenediamine functional-

zed MCM-41 (1 g) (as stated above) was added and the mixtureas stirred at 338 K for 6 h under dinitrogen atmosphere. The mix-

General 399 (2011) 117–125

ture was filtered out in hot condition to get the solid materialand washed thoroughly with hot tetrahydrofuran. The faint yel-low material (precatalyst 1) thus obtained was stored in a vacuumdesiccator.

2.4. Synthesis of 2

The palladium based catalyst 2 is synthesized from [(�3-C3H5)2Pd2Cl2] with the procedure previously reported by us(Scheme 1) [51].

2.5. Catalytic experiment

The catalytic runs in general were carried out at 300 K withspecified amounts of catalysts and substrates in fixed volumesof methanol in glass vials with magnetic stirring. The glass vialwas placed in an autoclave and specified hydrogen pressure wasapplied. At the end of the catalytic run the reaction mixturewas subjected to GC and extent of conversion was calculatedon the basis of the ratio of areas of starting material and theproduct by using toluene as an external standard. Conversionsand chemo selectivity of the hydrogenation reactions with differ-ent substrates were monitored by gas chromatographic techniquewith FID detector (Shimadzu GC-2014 gas chromatograph) usinga capillary column (Sigma–Aldrich, Supelco, Astec, Chiraldex B-DM; length 50 m, inner diameter 0.25 mm, thickness 0.12 �m). Allhydrogenated products were initially identified by using authenticcommercial samples of the expected products.

3. Results and discussion

3.1. Synthesis and characterization of 1

The reaction of [(�3-C3H5)4Pt4Cl4] with N-[3-(trimethoxysilyl)propyl]ethylenediamine functionalized MCM-41(Scheme 1) leads to the formation of surface species where thePt-co-ordination environment is best formulated as shown by 1.

The structure of the [(�3-C3H5)2Pd2Cl2] derived catalyst, 2reported in our earlier work is in marked contrast and given forcomparison in Scheme 1. The justification for the suggested Pt-co-ordination environment as shown by 1 comes from bulk and surfaceanalytical, solution and solid state NMR, and XPS data.

The preparative procedures for 1 and 2 are somewhat differentindicating that the Pt and Pd precursors have different reactiv-ity. Unlike 2 which is obtained at room temperature, 1 cannot beprepared at room temperatures, and a measurable amount of Ptloading is achieved only at 338 K. The amounts of silyl and aminofunctionalities as well as platinum loading for 1 are given in Table 1.For comparison the data on the reported Pd-catalyst 2 are alsoshown. The bulk silyl and the diamine contents of 1 and 2 are verysimilar but the metal contents are strikingly different.

The slight difference between the silyl and the diamine contentsof the two precatalysts arises due to the heterogeneous nature ofthe functionalization reaction and batch to batch variation. How-ever, about two orders of magnitude difference in the bulk metalcontent (mM/g) could only be explained on the basis of a plausibledifference in the co-ordination environments of the two catalysts.The bulk analytical data may be contrasted with that of the surfaceas measured by XPS. The surface nitrogen to metal atom percent-age ratio as measured in fresh and used 1 and 2 by XPS is close to2:1. The Cl to Pt atom percentage ratio in fresh and used 1 also by

XPS is about one. The surface analytical data therefore clearly indi-cate that while in both 1 and 2 the diamine functionalities act aschelating ligands, in 1 a coordinated chloride ion is present.

In both the precatalysts only some of the surface diamine groupsare used for ligation but the participation of surface Si–O– as lig-

A. Indra et al. / Applied Catalysis A: General 399 (2011) 117–125 119

O OO OOO

Si Si

NH

NH2Pt

O Cl

O O O O O O

Si Si

HN NH2

OH

OOO OOOSi Si

HN NH2Pd

OH

Cl

0.5 [Pt(allyl)Cl]4-C3H6 , -C3H5Cl

[Pd(allyl)Cl]2

Precatalyst 1 Precatalyst 2THF, 338 KN2 ,

Toluene, 300 KN2 , 4 hMCM-41 MCM-41 MCM-41

6 h

Scheme 1. Synthetic routes for the preparation of precatalysts 1 and 2. For clarity only T3 and Q3 surface sites are shown.

Table 1Bulk and surface concentrations of diamine and metal in 1 and 2.

Catalyst Metal present Bulk concentration (fresh catalyst) Surface concentrationc

Silyl group present(mM/g)a

Diamine moiety(mM/g)a

Metal wt% (mM/g)b N (atom %) Metal (atom %) Cl (atom %)

Fresh Used Fresh Used Fresh Used

1 Pt 1.3 1.3 0.109 (0.0056) 2.46 2.1 1.1 0.9 1.2 1.12 Pd 1.5 1.5 4.0 (0.4) 2.58 1.3 1.3 0.85 – –

aoddwptiwfNaoo

aots41tfo

fscidCv(wosas1tc

a Estimated via elemental (C, H, N) analysis.b Estimated by ICP–AES.c Estimated by XPS analysis.

nds are also required for the suggested co-ordination environmentf 1. This causes a notable difference in the percentage of the coor-inated diamines, which is ∼25% in 2 but only ∼0.4% in 1. Theifference in the percentage of coordinated diamines is consistentith the different co-ordination environments of Pt in 1 and 2. Thearticipation of Si–O– as a ligand in 1 requires close proximity ofhe diamine functionalities to the silanol groups. The functional-zation of the surface by the reaction of the surface silanol groups

ith (CH3O)3Si(CH2)3NHCH2CH2NH2, reduces the number of theree silanol groups on the support. This is evident from the IR andMR spectra of MCM-41, the diamine functionalized support [51]nd 1 (Supplementary Information). As a result of this, the fractionf silanol groups that are in proximity of the diamine, and capablef occupying the fourth co-ordination site, is substantially reduced.

Although the IR spectra of the diamine-functionalized MCM-41nd 1 do not provide evidence about the coordination environmentf Pt in 1, they do show a progressive and substantial reduction inhe concentration of the –OH groups. The area ratios of the twotrong IR bands at ∼3500 (�OH) and ∼1100 (�Si–O) cm−1 in MCM-1, the diamine functionalized support and 1 are approximately.7, 0.4 and 0.2, respectively. Thus the conversion of MCM-41 tohe diamine functionalized support and then to 1, brings about aour and a eight fold reduction, respectively, in the concentrationf the hydroxyl groups.

Further support for the above explanation and the proposedormulation of 1 comes from NMR and XPS data. The solid-tate NMR spectra (29Si and 13C) of 1 are shown in Fig. 1. Foromparison, the 13C NMR spectrum of solid (�3-C3H5)4Pt4Cl4s also shown. Detailed solid state 29Si and 13C chemical shiftata of MCM-41, diamine functionalized MCM-41, 1 and (�3-3H5)4Pt4Cl4 are given in Table 2. The 29Si NMR spectrum of 1 isery similar to that of 2 and MCM-41 functionalized with N-[3-trimethoxysilyl)propyl]ethylenediamine, reported in our earlierork [51]. Two peaks of approximately 2:3 intensity ratio are

bserved at ı = −59.6 and −68.9 ppm corresponding to T2, T3 siliconites (Tm = RSi(OSi)m(OMe)3−m by convention) (Fig. 1a). Three peaks

t −94.4, −102.8 and −112.8 ppm corresponding to Q2, Q3 and Q4

ilicon sites (Qn = Si(OSi)n(OH)4-n by convention) in approximately:9:8 intensity ratios are also observed (Fig. 1a). These belong tohe unfunctionalized surface silicon (Q2, Q3) and frame-work sili-on atoms, respectively. In Scheme 1 only one type of surface site

Fig. 1. (a) Solid state CPMAS 29Si NMR spectrum of 1. Solid state CPMAS 13C NMRspectra of (b) (�3-C3H5)4Pt4Cl4 and (c) 1.

120 A. Indra et al. / Applied Catalysis A: General 399 (2011) 117–125

Table 2Solid state 13C and 29Si chemical shift data of MCM-41, functionalized MCM-41, (�3-C3H5)4Pt4Cl4 and 1.

System Chemical shifts (ppm)

29Si 13C

MCM-41 −110.7(Q4), −100.9(Q3), −91.8(Q2) –MCM-41–(CH3O)3SiCH2CH2CH2NHCH2CH2NH2 −111.4(Q4), −102.1(Q3), −49.9(T1), −59.6(T2),

−68.2(T3)50.9(–Si–OCH3, –CH2––NH–CH2), 39.4(–CH2–NH2),21.7(Si–CH2–CH2–CH2), 9.4(–Si–CH2–CH2)

(�3-C3H5)4Pt4Cl4 – 94.3(–CH2–CH CH2), 60.1(–CH2–CH CH2),20.5(–CH2–CH CH2)

1 −112.8(Q4), −102.8(Q3), −94.4(Q2), −68.9(T3),−59.6(T2)

51.8(–Si–OCH3, –CH2–NH–CH2),46.5(NH2–CH2–CH2–), 37.7(NH2–CH2–CH2–),20.7(Si–CH2–CH2–CH2–), 10.1(–Si–CH2–CH2–)

Fi

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θ = 84 oUsed

θ = 20 o

Inte

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4f7/2

Fresh

Pt 2+Pt0

Used

3.2. Catalytic hydrogenation reaction

ig. 2. 13C NMR spectrum of the mixture of [(�3-C3H5)4Pt4Cl4] and ethylenediaminen (CD3)2SO.

T3) corresponding to each diamine functionalized silicon atoms shown, and the T2 sites are omitted for clarity. This is reason-ble since both T3 and T2 sites provide a neutral bidentate diamineigand to the platinum coordination sphere.

The solution 13C NMR of the tetramer [(�3-C3H5)4Pt4Cl4] hasever been reported probably because of its very low solubility. Itsingle crystal X-ray structure however is known and found to beonsistent with its solid state 13C (CPMAS) NMR [56]. The X-raytructure shows that in [(�3-C3H5)4Pt4Cl4], the allyl ligands havehree structurally inequivalent carbon atoms with three and twoifferent Pt–C and C–C distances, respectively. Accordingly three3C signals are observed at � ∼ 94, 60 and 20 ppm as shown inig. 1b. However, in the analogous Pd-complex [(�3-C3H5)2Pd2Cl2],he two terminal carbon atoms of the bridging �3-C3H5 ligand are

agnetically equivalent. Consequently instead of three, two 13CMR signals are observed at 112 and 66 ppm.

The structural differences between the Pt and Pd precursors ofand 2 are reflected in their reactions with the functionalized sup-ort. As reported in our earlier work, 2 retains the �3-C3H5 ligandhich can be clearly seen by 13C NMR. In contrast the 13C NMR

CPMAS) of 1 (Fig. 1c) fails to show the presence of any signal thatould be attributed to an unsaturated carbon atom of “C3H5

′′. This

ndicates that on treatment with the support, the allyl group of therecursor complex [(�3-C3H5)4Pt4Cl4] is lost.

Further evidence for the loss of “C3H5′′

as propene and/or allylhloride comes from the in situ solution NMR of a mixture of [(�3-3H5)4Pt4Cl4] and ethylenediamine (Fig. 2). The chemical shifts ofhe down field signals are practically identical to that of the alkenearbons of propene and/or allyl chloride. The proton required forhe formation of propene probably comes from the trace quantitiesf water.

A comparison between the spectra of the functionalized supportnd 1 reveals one additional signal for the latter at ı ∼ 46.5 ppm.ll the other signals of the functionalized support are also seen

n 1 with marginal changes (≤|1.7| ppm) in the chemical shifts.

Binding Energy / eV

Fig. 3. Normal and grazing angle XPS spectra of fresh and used 1.

The new signal at ı 46.5 ppm is assigned to the ˇ-carbon of the‘–CH2CH2NH2’ group. The formulation of surface species as shownfor 1 requires the metal to be present in 2+ oxidation state. XPSdata (Fig. 3) show that this indeed is the case. In freshly prepared 1signals for Pt 4f7/2 and core levels at a characteristic binding energy(BE) of Pt2+ (Pt 4f7/2 at 73 ± 0.1 eV) are clearly seen. Direct evidencefor Pt–O co-ordination in 1 also comes from a comparison of N 1sBE in 1 and 2. There is a clear shift in the N 1s BE in 1 towards higherenergy (Fig. 4). This shift is best attributed to the increased electrondonation from N to O through Pt in 1. In NH3 and primary or sec-ondary amines BE of N 1s core level is around 399.5 eV. In 2 the N1s BE of 399.4 eV is very close to this value. The shift to 400.2 eV in1 is consistent with its formulation as shown in Scheme 1, wherehigher electronegativity of oxygen is expected to reduce the elec-tron density on the amino groups. For both 1 and 2, between freshand used samples there is no observable change in the N 1s bindingenergy.

The turnover numbers (TON) and selectivities in the hydrogena-tions of all the three CNBs with 1, 2, and commercial (5%) Pt/C and(5%) Pt/Al2O3 as catalysts were first measured. As both the com-

A. Indra et al. / Applied Catalysis A: General 399 (2011) 117–125 121

Table 3Selective hydrogenation of chloronitrobenzenes with 1, 2 and commercial catalysts Pt/C and Pt/Al2O3.a

Entry Catalyst Pressure (bar) o-CNB m-CNB p-CNB

Conversion (selectivity) TON Conversion (selectivity) TON Conversion (selectivity) TON

1

1

10 32 (72) 1428 61 (80) 2723 71 (75) 31692 20 55 (73) 2455 100 (78) 4464 100 (76) 44643 30 80 (69) 3571 100 (90) 4464 100 (84) 44644 40 100 (75) 4464 100 (92) 4464 100 (96) 44645 50 100 (69) 4464 100 (81) 4464 100 (82) 44646 40 24 (82)b 1071 49 (98)c 2187 62 (98)c 2767

7 2 40 60 (60) 2678 50 (62) 2232 36 (57) 16078 Pt/C 40 41 (16) 1782 61 (43) 2652 74 (66) 32169 Pt/Al2O3 40 19 (8) 826 19 (4) 826 32 (50) 1391

a All the catalytic runs were carried out for 45 min under 40 bar H2 pressure in duplicate in a teflon coated autoclave at 300 K with a stirring rate of 900 rpm unlessstated otherwise. 1 mM substrate, 2.24 × 10−4 mM Pt (40 mg catalyst) or Pd (0.60 mg catalyst), 5 mL methanol were used for catalysts 1 and 2; 10 mM substrate, 9 mgcatalyst (2.3 × 10−3 mM), 20 mL methanol were used for the commercial catalysts. The conversion (amount of CNB reacted) and selectivity (concentration of CAN/sum ofc

mns(

t44staaw

oncentrations of CAN, AN, NB) values are in percentages.b 15 min.c 10 min.

ercial Pt-catalysts give measurable and reproducible turnoverumbers (TON) at a hydrogen pressure of 40 bar, the initial mea-urements were carried out for all the three catalysts under 40 barTable 3, entries 4, 7–9).

The performance of 1 is clearly far superior to that of the otherhree catalysts. Complete conversions corresponding to a TON of464 and a TOF of 99 min−1 (TOF = TON/time) could be achieved in5 min for all the three CNBs. As these TON and TOF values corre-pond to complete conversions, they represent the lower limits of

he catalytic activity. Under identical conditions for o-CNB 2, Pt/Cnd Pt/Al2O3 give ≤60% conversion and the TOF are only 59.5, 40nd 18 min−1, respectively. The maximum conversion for m-CNBith any of the other three catalysts are ≤61% (TON ≤ 2652) and

404401398395

N 1S XPS

2-Used

2-Fresh

1-Used

1-Fresh

Inte

nsity

/ A

rb. U

nits

Binding Energy / eV

Fig. 4. N 1s core level spectra (XPS) of fresh and used 1 and 2.

for p-CNB it is ≤74% (TON ≤ 3216) (Table 3). The activity of the fourcatalysts for o-CNB is in the order 1 > 2 > Pt/C > Pt/Al2O3, while form-CNB and p-CNB it is 1 > Pt/C > 2 > Pt/Al2O3.

In terms of selectivity, 1 is also distinctly superior to the otherthree catalysts (2 (5%) Pt/C and (5%) Pt/Al2O3). The maximum selec-tivities for o-, m- and p-CNB with any of the other three catalysts,i.e., 2 (5%) Pt/C and (5%) Pt/Al2O3, are 60, 62 and 66%, respectively(Table 3). In contrast with 1 the selectivities under these conditionsare 75, 92 and 96%, respectively. The overall conversion of CNB andselectivity towards CAN depend on several competing reactions.The hydrogenation of –NO2 to –NH2 group is known to involveintermediates with functionalities such as –NO and –NHOH, andeven in the hydrogenation of nitrobenzene to aniline many sideproducts in small quantities are usually formed [18,22].

In the present context the rate of formation of CAN, and thatof dehydrochlorinations of CNB and CAN to give nitrobenzeneand aniline, respectively, are the important ones. It may be notedthat nitrobenzene formed by dehydrochlorination of CNB can alsoundergo hydrogenation to give aniline. Representative time versusconcentration plots of o- and m-CNB to o- and m-CAN and the sideproducts are shown in Fig. 5. It is apparent that the relative ratesof product formations and their overall effect on selectivity to CANare different for the two isomers.

Since hydrogen pressure and reaction time are expected to influ-ence the relative rates of the several simultaneous and consecutivereactions, the effect of these two parameters on conversions andselectivities have been studied in some details (Table 3, entries1–5). The effect of pressure on conversion is more pronounced foro-CNB than for m- or p-CNB (Fig. 6 and Table 3, entries 1–5). At40 bar H2 pressure full conversion is achieved for o-CNB, but for m-or p-CNB only 20 bar H2 pressure is required. It is therefore reason-able to propose that the lower reactivity of the o-isomer towardshydrogenation is primarily a result of increased steric hindrancenear the reaction center.

In contrast a change in pressure has little effect (∼6%) on theselectivity of o-CAN formation, but more pronounced effects (∼14and 21%) on the formations of the other two isomers (Fig. 7and Table 3, entries 1–5). Conversion of the precatalyst 1 to theproposed catalytically active intermediate 3, and the main prod-uct forming reactions are shown in Scheme 2. The oxidationstate of Pt in 3, is proposed to be 2+ for which there is direct

XPS evidence. Under the hydrogenation conditions hydrogenol-ysis of the “Pt–O–Si O3

′′moiety leads to the formation of a

reactive platinum hydride and surface “HO–Si O3′′. As shown in

Scheme 2, this reactive intermediate is proposed to catalyze thehydrogenation of CNB to CAN, dehydrochlorinations of CNB and

122 A. Indra et al. / Applied Catalysis A: General 399 (2011) 117–125

504030201000

20

40

60

80

100

m-CNB

m-CAN

AN

NB% C

once

ntra

tion

Time (min)

504030201000

20

40

60

80

100

o -CNB%

Con

cent

ratio

n

Time (min)

o-CANANNB

Fig. 5. Change in concentration of the substrate and products in the reaction mixtureatfA

Ct

orbtotrwt

Fb

50403020100

20

40

60

80

100o-CNB

m-CNB

p-CNB

Sele

ctiv

ity

H pressure (bar)

reducing the reaction time to 10–15 min (Table 3, entry 6). Underthese conditions TON for all CNBs remain >1000 and the selec-tivities for ortho-, meta- and para- isomers are 82, 98 and 98%,respectively.

s a function of time for the hydrogenation of m-CNB by 1. Inset: Change in concen-ration of the substrate and products in the reaction mixture as a function of timeor the hydrogenation of o-CNB by 1. (CNB, chloronitrobenzene; CAN, chloroaniline;N, aniline; NB, nitrobenzene).

AN, and hydrogenation of nitrobenzene (reactions a–d, respec-ively).

It is, therefore, reasonable to propose that coordination to Pt byne of the oxygen atom of the –NO2 group is a pre-requisite for itseduction to –NH2. In o-CNB, the close proximity of –NO2 to –Clrings the C–Cl bond within bonding interaction with the metal,hereby making its hydrogenolysis a relatively facile process. With

-CNB rather than the m- and p-isomers consistent lower selectivityowards CAN is therefore expected and observed. The dehydrochlo-inations of m- and p-CNB would require interaction of the metalith the –Cl rather than –NO2 group. At a given time the concentra-

ions of such species and their conversions to dehydrochlorinated

5040302010

30

40

50

60

70

80

90

100

110

o-CNB

m-CNB

p-CNB

% C

onve

rsio

n

H2 pressure (bar)

ig. 6. Effect of H2 pressure on conversion in the hydrogenation of o-, m- and p-CNBy 1.

2

Fig. 7. Effect of H2 pressure on selectivity in the hydrogenation of o-, m- and p-CNBby 1.

products must depend on a multitude of hydrogen pressure depen-dent competing reactions. This possibly explains why selectivitygoes through a maximum with increasing pressure, i.e., maximumselectivity is observed at an optimum pressure.

From the time versus concentration plot (Fig. 5) it is apparentthat selectivity towards CAN is sensitive to reaction time. Depend-ing on the reaction time the selectivity (%) of o-CAN, AN and NBranges from 68 to 82, 20 to 8 and 15 to 7, respectively. The high-est (82) and lowest (68) selectivity towards o-CAN are observedat time intervals 15 and 5 min, respectively. Similarly, in m-CNBhydrogenation, the selectivity (%) of m-CAN, AN and NB ranges from98 to 74, 16 to 1 and 9.5 to 0.5, respectively. Here the highest (98)and lowest (74) selectivity towards o-CAN are observed at timeintervals 10 and 20 min, respectively. Thus without seriously com-promising the TON, the selectivity could be further improved by

O O O O O O

Si Si

HN NH2Pt

OHCl

MCM-41

H

3NO2

NO2 NH2 NH2

3H2

2H2O

H2

HCl (a)(b)

(c)(d)Cl

Cl

3H2 2H2O H2HCl

1 H2

Scheme 2. Activation of precatalyst 1 to the catalytically active intermediate 3under hydrogenation conditions and the main product forming reactions.

A. Indra et al. / Applied Catalysis A:

3210

20

40

60

80

100

% Conversion

% Selectivity

% C

onve

rsio

n/Se

lect

ivity

Number of cycle

Fb

3

btgsifc

investigated any further.

ig. 8. Effect of recycle on conversion and selectivity in the hydrogenation of p-CNBy 1.

.3. XPS, TEM and recycling of 1

In our earlier work with 2, the recyclability of the catalyst had toe tested in nitrobenzene hydrogenation reaction. This is becausehe recycled catalyst showed negligible selectivity in CNB hydro-enation. The recyclability of 1 in terms of loss in activity and

electivity has been studied by using it for p-CNB hydrogenationn three successive batches and unlike recycled 2, recycled 1 isound to retain substantial selectivity (Fig. 8). Over three batchesorresponding to a total TON > 104, no measurable Pt-loss has been

Fig. 9. TEM images of (a) fresh 1 at 20 nm resolution, (b) used 1 after TON 4464

General 399 (2011) 117–125 123

detected. However, there is a progressive drop in selectivity, about∼15% per batch. The drop in conversion between the first twobatches is small (8%), but on the third recycle there is a drastic drop(>50%). To gain an insight into the possible mechanism for the lossin activity and selectivity, the used catalysts at two different stagesbeen subjected to XPS (Figs. 3 and 4) and TEM studies (Fig. 9).

Characteristic MCM-41 fringes are clearly seen in the TEMimages of fresh 1, where no Pt particles are visible. This is expectedand provided the anchored species retain their molecular struc-tures, and/or aggregation of the molecular species do not producenanoparticles bigger than the fringe width of MCM-41. However,it must be emphasized that HRTEM in any case cannot provideunambiguous molecular level characterization of the active sitesfor two reasons. First, HRTEM is known to modify small metal clus-ters on supports during the imaging process [57]. Secondly, the sizeof a tethered mononuclear complex is expected to be beyond thereliable resolution limit of HRTEM.

After the first cycle platinum particles ≥3 nm can be seen whileafter the third cycle (TON > 10300) the size of the Pt particlesincreases further to 5–6 nm. The characteristic MCM-41 fringes alsodisappear after third cycle which suggests that the structure ofMCM-41 is destroyed. As instability of MCM-41 in protic mediumis well known, formation of HCl in the dehydrochlorination reac-tion is probably the main reason for the collapse of the MCM-41structure. After the third cycle the fate of the tethered functional-ities and the co-ordination environment around Pt have not been

As mentioned earlier, XPS of freshly prepared 1 shows the pres-ence of only Pt2+. After its use in the first batch there is very littlediscernible change in the spectrum. At normal emission angle forused 1, Pt 4f7/2 core level shows features almost exclusively due to

at 10 nm resolution and (c) used 1 after TON 10000 at 10 nm resolution.

1 sis A:

Pi8

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24 A. Indra et al. / Applied Cataly

t2+. Very little Pt0 can be seen even in grazing angle spectra, theres very little difference between the grazing angle spectra of 20 and4◦. Quantitatively the amount of Pt0 is found to be ∼16%.

Although as mentioned Pt particles ≥3 nm could be seen by TEMfter third cycle, no clearly defined regular shape, core–shell struc-ure or otherwise, is observed. However, the changes in the surfacetomic percentage ratio of metal to nitrogen for fresh and used 1nd 2 (Table 1) are notably different and consistent with the graz-ng angle XPS data. The metal to nitrogen ratio changes from 0.45 to.43 and 0.5 to 0.65 for 1 and 2 respectively. In other words, while

n 1 there is a small (4%) decrease in surface platinum, in 2 surfaceoncentration of Pd increases by ∼30%.

The facts that the metal to nitrogen ratio in used 1 change littlerom that of the fresh catalyst and is close to 0.5 suggesting that the

olecular structures of the surface species are largely retained. Inontrast the relatively large changes in the surface concentrationsf nitrogen and Pd of used 2 suggest that the molecular structuresf the surface species are lost. As reported in our earlier publica-ion, TEM and XPS analyses show that in used 2 some of the diamineunctionalities are buried underneath Pd-particles (∼20 nm) [51].his results in an increase in the Pd to nitrogen ratio. The over-ll decrease in the surface Pd and nitrogen percentages due to thetructural changes at the molecular level, is made up by an increasen the oxygen percentage, i.e., increase in the surface “Si–OH” func-ionalities. The drop in selectivity on recycles is correlated to thencreasing amounts of Pt0. Oxidative addition of aryl chloride to Pt0

s a very well known reaction. A similar reaction with CNB followedy hydrogenolysis of the aryl–Pt bond would result in an increase

n the amount of the dehydrochlorinated products. Reduction of 3o zero valent platinum followed by aggregation to nanoparticles,ecreases the number of active sites and results in a drop in con-ersion. On the first recycle the extent of aggregation is less ando is the drop in the conversion. However, on the second recycleecause of significant aggregation the drop in conversion is sub-tantial.

The TEM and XPS data of used 1 are notably different fromhat of used 2. For the latter facile reduction of Pd2+ to Pd0 underhe hydrogenation conditions was observed and in used 2, theurfaces of the metal particles (TON ∼ 1000) were found to belmost exclusively Pd0. Underneath Pd0, a little Pd2+ was alsoeen and a core-shell structure for the resultant nanoparticles wasroposed. The reduced Pd-nanoparticles were found to be highlyobile and over a TON ∼ 2500, underwent facile aggregation to give

d-particles as large as 20 nm. With recycled 2 this was consid-red to be the main reason for the partial loss in activity in NBydrogenation and total loss in selectivity in CNB hydrogenation51].

The above mentioned TEM and XPS data for 1 clearly showhat the anchored molecular species of platinum are far moreesistant towards reduction and agglomeration, than that of pal-adium in 2. It is reasonable to propose that the retention of

olecular catalytic sites over considerably more TON in 1 thann 2 is the primary reason behind the superior performance of.

. Conclusions

In conclusion we described a simple method for tethering at-diamine complex onto MCM-41 starting with a Pt-allyl pre-ursor, and characterization of the tethered species. The resultantaterial 1 is a high performance precatalyst, notably superior to

wo commercial Pt-catalysts (5%) Pt/C and (5%) Pt/Al2O3 and ad-analogue, for the selective hydrogenation of o- m- and p-CNB.PS and TEM analyses indicate that reduction and formation of Pt-anoparticles by hydrogen reduction of the tethered species arelow processes.

[[[[[

General 399 (2011) 117–125

Acknowledgements

Financial assistance from Reliance Industries Limited, Mumbaiand Council of Scientific and Industrial Research, New Delhi, Indiais gratefully acknowledged.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.apcata.2011.03.044.

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