Ac

Aa

b

a

ARRAA

KNPSHO

1

histattpStahsh

bcap

nT

1h

Journal of Molecular Catalysis A: Chemical 372 (2013) 167– 174

Contents lists available at SciVerse ScienceDirect

Journal of Molecular Catalysis A: Chemical

j our na l ho me p age: www.elsev ier .com/ locate /molcata

biguanide/Pd-decorated SBA-15 hybrid nanocomposite: Synthesis,haracterization and catalytic application

. Alizadeha,b,∗, M.M. Khodaeia,b,∗, D. Kordestaniaa, M. Beygzadeha

Department of Organic Chemistry, Faculty of Chemistry, Razi University, Kermanshah 67149, IranNanoscience & Nanotechnology Research Center (NNRC), Razi University, Kermanshah 67149, Iran

r t i c l e i n f o

rticle history:eceived 6 December 2012eceived in revised form 25 February 2013ccepted 26 February 2013vailable online 7 March 2013

a b s t r a c t

A stable palladium-decorated SBA-15 nanocomposite was simply fabricated through surface modifica-tion of SBA-15 with biguanide and subsequent metal/ligand coordination with Pd2+ from inexpensivecommercially available starting materials and using standard laboratory techniques. The structure ofthis organic inorganic hybrid material was characterized by SEM, TEM, XRD, elemental analyzer, atomicabsorption spectroscopy, N2 adsorption–desorption (BET), and FT-IR techniques. The catalytic perfor-

eywords:anocompositesalladium catalystsuzuki reactionybrid materials

mance of this novel heterogeneous catalyst was determined for the Suzuki cross-coupling and aerobicoxidation of benzyl alcohols. The composite exhibited an excellent catalytic activity and reuse abilityof various recycles in air for the aforementioned organic transformations. TEM images of the recoveredcatalyst showed retained ordered mesostructure of SBA-15 with no damage in the periodic structure ofthe silicate framework and a good dispersion of in situ generated palladium nanoparticles within the

rganosuper bases SBA-15 structure.

. Introduction

Mesoporous-based palladium catalysts, as organic–inorganicybrid materials have provided some of the most attractive fields

n organic transformations. A wide range of coupling reactionsuch as the Heck [1,2], Suzuki [3,4] and Sonagashira [5,6] reac-ions have been efficiently catalyzed with these hybrid materials. Inddition, other transformations such as hydrogenation and oxida-ion are extensively studied under the catalytic conditions usinghese palladium catalysts [7,8]. Palladium-catalyzed cross cou-ling reactions of aryl halides with aryl boronic acids; known asuzuki–Miyaura reaction, is an increasingly popular method forhe construction of unsymmetrical biaryl structures since biarylss molecular components are important units in pharmaceuticals,erbicides and natural products, as well as in engineering materialsuch as conducting polymers, molecular wires and liquid crystals,as attracted enormous interest [9–11].

Palladium species used as catalyst in the Suzuki reaction haveeen traditionally based on homogeneous palladium/phosphine

omplexes [12–14], which are usually toxic, unstable in airnd moisture, expensive and rarely recoverable. In contrast,hosphine-free heterogeneous Pd catalysts are green alternatives

∗ Corresponding authors at: Department of Chemistry, Nanoscience & Nanotech-ology Research Center (NNRC), Razi University, Kermanshah 67149, Iran.el.: +98 831 4274559; fax: +98 831 4274559.

E-mail address: ahalizadeh2@hotmail.com (A. Alizadeh).

381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.molcata.2013.02.027

© 2013 Elsevier B.V. All rights reserved.

to the conventional homogeneous ones. Several phosphine-freepalladium catalysts immobilized on polymer [15,16], silica [17],zeolite [18] and hybrid materials [19,20] have been examined forthe Suzuki reaction with the advantage that at the end of process,solid-supported palladium moieties can be easily separated fromthe reaction mixture by simple filtration and reused in successivefurther reactions.

The selective oxidation of alcohols to their related aldehydes orketones is also one of the noticeable processes in organic synthe-ses [21,22]. Generally, oxidation is performed using stoichiometricamount of transition metal oxidants or sulfoxides and this stoichio-metric oxidation produces a large amount of undesirable products,which becomes intolerable in today’s environmentally consciousworld. Recently, oxidation of alcohols with molecular oxygen atatmospheric pressure employing complex-tailored heterogeneousnanocatalysts [7] has stimulated great interest since these catalystsare easily recovered and the procedure avoids using toxic and largeamount of oxidizing reagents and no by-product other than wateris produced.

Metformin (Met); a polydentate biguanidine derivative, cancombine with many transition metal ions especially with copper(II), nickel (II), cobalt (II) and palladium (II) to give highly coloredchelate complexes [23–26].

In continuing our previous efforts to develop new

organic–inorganic hybrid materials as heterogeneous cata-lysts [27,28], a new nanocomposite is introduced using a surfacemodified SBA-15 mesoporous system with porous networks. Inthis context, we encountered that SBA-15 functionalized with

dx.doi.org/10.1016/j.molcata.2013.02.027http://www.sciencedirect.com/science/journal/13811169http://www.elsevier.com/locate/molcatamailto:ahalizadeh2@hotmail.comdx.doi.org/10.1016/j.molcata.2013.02.027

1 Catal

mrof[o

2

as1

eo

2(

apsistaTs

((pmaft

2

1Td

2

2

5aa8(trTfT

2

oflt

68 A. Alizadeh et al. / Journal of Molecular

etformin/Pd2+ complex would be served as an efficient andobust catalyst for cross-coupling reactions as well as alcoholxidation processes. Here, we report a convenient procedure forabrication and characterization of a recoverable nanocatalystSBA-15/Met/Pd(II)] for the Suzuki coupling reaction and aerobicxidation of benzyl alcohols.

. Experimental

All chemicals were purchased from Merck except Pluronic123nd metformin hydrochloride which were obtained from Aldrich. IRpectra were determined on a Perkin-Elmer 683 instrument. 1H and3CNMR spectra were recorded on a Bruker (200 MHz) spectrom-ter in CDCl3 as solvent. X-ray powder diffraction patterns werebtained on SE1FERT-3003TT.

.1. Preparation of metformin-functionalized SBA-15SBA-15/Met)

The mesoporous material SBA-15 was prepared according to literature procedure [29] and then was modified with chloro-ropyl tail groups. A 100 ml of round-bottom flask were introduceduccessively 30 ml of anhydrous toluene, 3.0 g of activated sil-ca, and 1.782 g (9 mmol) of 3-chloropropyl trimethoxysilane. Theolution was refluxed for 24 h under an inert atmosphere, fil-ered and washed subsequently with toluene, dichloromethane,nd methanol, and dried under reduced pressure at 80 ◦C for 10 h.hrough this simple procedure, the chloropropyl-functionalizedilica was obtained.

In another 100 ml round-bottom flask, to a solution of 3 g18 mmol) of metformin hydrochloride in 35 ml acetonitrile, 0.72 g18 mmol) NaOH was added. After 1 h, 3 g (18 mmol) KI and 3 g of as-repared 3-chloropropyl functionalized SBA-15 were added to theixture and kept under reflux for 12 h. The solvent was removed

nd then 50 ml distilled water was added to the residue and stirredor 1 h, filtrated and washed with distilled water to afford 3.1 g ofhe metformin-functinalized mesoporous SBA-15.

.2. Immobilization of Pd(II) ions on the surface of SBA-15/Met

A mixture of SBA-15/Met (1 g) and palladium acetate (220 mg, mmol) in acetone (10 ml) was stirred at room temperature for 4 h.he resulting solid was filtered, washed with acetone and THF andried in vacu at 80 ◦C for 3 h to give SBA-15/Met/Pd(II).

.3. Catalytic testing

.3.1. Suzuki–Miyaura coupling reactionIn a typical reaction, to a solution of 1 mmol of the aryl halide in

ml of water/ethanol (1:1) was added 1.1 mmol of phenyl boroniccid, 276 mg of K2CO3 (2 mmol) followed by 15 mg of the solid cat-lyst (1 mol%). The mixture was then stirred for the desired time at0 ◦C. The reaction was monitored by thin layer chromatographyTLC). After completion of reaction, the reaction mixture was cooledo room temperature and the catalyst (SBA-15/Met/Pd(II)) wasecovered by centrifuge and washed with ethyl acetate and ethanol.he combined organic layer was dried over anhydrous sodium sul-ate and evaporated in a rotary evaporator under reduced pressure.he crude product was purified by column chromatography.

.3.2. Aerobic oxidation of benzyl alcohols

A mixture of K2CO3 (1 mmol) and the catalyst (52 mg, ∼3 mol%

f Pd2+) in toluene (5 ml) was prepared in a two necked flask. Theask was evacuated and refilled with pure oxygen. To this solution,he alcohol (1 mmol, in 1 ml toluene) was injected and the resulting

ysis A: Chemical 372 (2013) 167– 174

mixture was stirred at 80 ◦C under an oxygen atmosphere. Aftercompletion of reaction, the reaction mixture was filtered off andthe catalyst rinsed twice with CH2Cl2 (5 ml). The excess of solventwas removed under reduced pressure to give the correspondingcarbonyl compounds.

3. Result and discussion

3.1. Synthesis and characterization of SBA-15/Met/Pd(II)

The schematic pathways for the synthesis of catalyst aredepicted in Scheme 1. First, SBA-15/Met was prepared accordingto the reported method [27a]. Then, palladium acetate was addedto SBA-15/Met in acetone and the mixture was stirred for 4 h atroom temperature to obtain SBA-15/Met/Pd(II).

The SEM micrographs obtained of the catalyst are shown inFig. 1. These micrographs confirmed the formation of a well-ordered structure for SBA-15/Met/Pd(II) consisting of rod-shapedparticles of 1500–2000 nm in length and 450–550 nm in diameter.

In addition, nitrogen adsorption–desorption isotherms of SBA-15, SBA-15/Met and SBA-15/Met/Pd(II) are shown in Fig. 2and the corresponding textural parameters calculated by N2adsorption–desorption isotherms are presented in Table 1. Theisotherms were all Type IV with a H1 hysteresis loop and a steepincrease in adsorption at relative pressures of 0.6–0.8 for SBA-15samples attributed to capillary nitrogen condensation according toIUPAC classification. This is typical for mesoporous materials withordered pore structures [30].

The successful attachment of metformin and subsequent coor-dination of Pd(II) ions within the mesoporous SBA-15 material canbe investigated employing FT-IR spectroscopy. Fig. 3 shows the FT-IR spectra obtained for (a) metformin hydrochloride (Met.HCl), (b)parent SBA-15, (c) SBA-15/Met and (d) SBA-15/Met/Pd(II). Curvea shows the spectrum of Met.HCl and signals appeared at 1568and 1630 cm−1 are attributed to the presence of C N stretchingvibrations [31]. The signals appeared at 3200–3500 cm−1 region canbe assigned to the N H stretching of C N H group on metformin[21]. The unmodified SBA-15 spectrum in curve b shows the typi-cal silica bands associated with the main inorganic backbone: thesignals appeared at 1050 and 1150 cm−1 is assigned to asymmet-ric stretching of Si O Si, at 1660 cm−1 is related to the angularvibration of water bonded to the inorganic backbone and signalappeared at ∼3400 cm−1 can be assigned to O H stretching fre-quency of Si O H groups and/or water in atmosphere and withinporous sample. In curves c and d, the sharp band at 1090 cm−1

is corresponding to Si O Si anti-symmetric stretching vibration,being indicative of the existence of a silica material [32]. The signif-icant difference in FT-IR spectra of Met.HCl and SBA-15/Met (curvesa and c) was the shift of C N stretching frequencies from 1568 and1630 cm−1 to 1580 and 1645 cm−1, respectively due to the removalof HCl when metformin is attached to the silica surface. On the otherhand, the metal–ligand co-ordination [33,34] presumably leads toa shift of these two peaks again to lower frequencies (1568 and1630 cm−1). This shift can be observed comparing curves c and din Fig. 3. All together, the aforementioned observations confirmedthe immobilization of Pd(II) ions on the surface of SBA-15/Met.

Furthermore, elemental analysis showed that the carbon, hydro-gen, and nitrogen content of SBA-15/Met was 12.219, 2.520, and5.194 (wt.%), respectively, which are equivalent to a loading of∼0.7 mmol of metformin per gram of SBA-15. In addition, the

Pd content of the catalyst estimated by atomic absorption spec-troscopy was 0.570 ± 0.001 mmol g−1. This indicated that ∼81% ofthe anchored metformin moieties have efficiently co-ordinatedwith Pd2+ ions providing catalytic active sites.

A. Alizadeh et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 167– 174 169

Scheme 1. Schematic diagram of SBA-15/Met/Pd(II) preparation.

Fig. 1. SEM micrographs of SBA-15/Met/Pd(II).

170 A. Alizadeh et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 167– 174

Fig. 2. N2-adsorption desorption isotherms of SBA-15 (×), SBA-15/Met (�) and SBA-15/Met/Pd(II) (�).

F(

3

mom

3

crbrb(K

Table 2Optimization of the reaction conditions for the Suzuki reaction of bromoacetophe-none with phenylboronic acid.

B(OH)2Br

+SBA15/Met/Pd(II)

80 oC, 60 min

O

O

Entry Solvent Pd2+ (mol%) Base Yield (%)a

1 H2O 1 K2CO3 702 EtOH 1 K2CO3 653 EtOH 1 EtONa 724 H2O/EtOH (1:1) 1 Et3N 915 H2O/EtOH (1:1) 0.5 K2CO3 626 H2O/EtOH (1:1) 1 K2CO3 967 H2O/EtOH (1:1) 2 K2CO3 978 H2O/EtOH (1:1) 1 t-BuONa 869 H2O/EtOH (1:1) 1 NaOAc 82

10 H2O/EtOH (1:1) 1 EtONa 94

TT

ig. 3. FT-IR spectra for samples of (a) metformin hydrochloride, (b) parent SBA-15,c) SBA-15/Met and (d) SBA-15/Met/Pd(II).

.2. Catalytic activities

In order to evaluate catalytic activity of the prepared hybridaterial as a heterogeneous catalyst, Suzuki–Miyaura coupling and

xidation of alcohols using molecular oxygen were selected asodel reactions.

.2.1. Suzuki–Miyaura coupling reactionSBA-15/Met/Pd(II) was applied to catalyze the Suzuki–Miyaura

oupling reaction of various aryl halides and arylboronic acids. Theeactions were conveniently carried out in flask in air and phenyl-oronic acid and 4-bromoacetophenone was chosen as a model

eaction. The influences of various reaction parameters such asase, solvent and catalyst amount on the reaction were testedTable 2). Among the bases evaluated (Et3N, EtONa, AcONa and2CO3), K2CO3 was found to be the most effective and other bases

able 1he structure parameters of SBA-15, SBA-15/Met and SBA-15/Met/Pd(II).

Entry Samples SBET (m2 g−1)

1 SBA-15 1030 2 SBA-15/Met 441 3 SBA-15/Met/Pd(II) 438

11 H2O/EtOH (2:1) 1 K2CO3 87

a Isolated yield.

were substantially less effective. We also investigated the effect ofsolvents on the Suzuki–Miyaura cross-coupling reaction and foundthat H2O/EtOH (1:1) was the best choice. The effect of catalystamount on the reaction was also studied by varying the amountof catalyst while keeping the other parameters constant. It wasobserved that 17.5 mg of mesoporous material (∼0.01 mmol ofPd2+, 1 mol% with respect to arylhalide) was sufficient to completethe reaction. The results are summarized in Table 2.

To investigate the scope and generality of the aforementionedprotocol, we studied the reaction of various kinds of arylboronicacids and a wide range of aryl halides as the substrates underthe optimized reaction conditions. As it is seen in Table 3, entries1–3, aryl bromide, chloride and iodide are sufficiently capable ofundergoing cross-coupling with phenylboronic acid under the opti-mized condition. Both electron-rich (Table 3, entries 4 and 12) andelectron-deficient (Table 3, entries 5–11 and 13) aryl halides wereapplicable for this reaction and the corresponding coupling prod-ucts were obtained in good to excellent yields. It was also foundthat the yield of reaction of aryl bromide bearing substituent atortho-position is lower (Table 3, entry 9) than those of para- ormeta-substituted aryl bromides (Table 2, entries 7 and 8).

It is notably, when 2-iodothiophene was used as coupling part-ner (Table 3, entry 14), desired product was obtained and nopoisoning of palladium catalyst was occurred. Additionally, thecoupling reaction of arylboronic acids with electron-withdrawinggroups such as 3-nitrophenyl and 3,4,5-triflourobenzene boronicacid with bromobenzene and 4-bromobenzaldehyde provided thecorresponding biphenyls in 69–94% yields (Table 3, entries 15–18).

3.2.2. Aerobic oxidation of benzyl alcoholsThe oxidation of benzyl alcohol using various amount of the pre-

pared catalyst was performed under O2 in balloon. In the presenceof the heterogeneous catalyst and optimized conditions, benzalde-

hyde was obtained in excellent yield. It was important to notethat oxidation did not proceed when palladium source (catalyst)was not added. The desired product was obtained only in 28%yield when without using an appropriate base, 3 mol% of catalyst

Total pore volume (cm3 g−1) Mean pore diameter (nm)

1.097 4.250.705 6.390.671 6.48

A. Alizadeh et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 167– 174 171

Table 3Suzuki–Miyaura cross-coupling reactions of aryl halides with arylboronic acids.

Ar1B(OH)2 + Ar2XSBA-15/Met/Pd(II),80 ◦C−→

K2CO3,EtOH/H2OAr1–Ar2

Entry Ar1 Ar2 X Time (min) Yield (%)a

1 Ph Ph I 60 992 Ph Ph Br 55 993 Ph Ph Cl 6 (h) 584 Ph 3-CH3C6H4 Br 35 995 Ph 4-CNC6H4 Br 30 776 Ph 4-CNC6H4 Cl 4 (h) 647 Ph 4-HCOC6H4 Br 30 988 Ph 3-HCOC6H4 Br 30 989 Ph 2-HCOC6H4 Br 4 (h) 61

10 Ph 4-H3CCOC6H4 Br 60 9711 Ph 4-H3CCOC6H4 Cl 4 (h) 5712 Ph 4-H3COC6H4 Br 35 8513 Ph 4-O2NC6H4 Br 35 9214 Ph 2-Iodothiophene I 60 9015 3-O2NC6H4 Ph Br 120 9416 3-O2NC6H4 4-HCOC6H4 Br 120 8917 3,4,5-triflouroC H Ph Br 8 (h) 69

Br 8 (h) 85

wt(

hewgc(icy

3

ricat

Fig. 4. The recycling of the SBA-15/Met/Pd(II) (17.5 mg) carried out under condi-

TA

6 2

18 3,4,5-triflouroC6H2 4-HCOC6H4

a Isolated yield.

as used at 80 ◦C and 24 h (Table 4, entries 1–4). Using K2CO3 ashe appropriate base resulted in 96% yield of the desired productTable 4, entry 8).

As summarized in Table 5, the SBA-15/Met/Pd(II) is an effectiveeterogeneous catalyst for the aerobic oxidation of a wide vari-ty of benzylic alcohols. Primary and secondary benzyl alcoholsere converted to their corresponding aldehydes and ketones in

ood to excellent yield. For instance, under the developed proto-ol, 4-nitrobenzyl alcohol afforded related product in good yieldTable 5, entry 3). In addition, allyl alcohol was efficiently oxidizedn the presence of SBA-15/Met/Pd(II) (Table 5, entry 9) and ferro-enyl carbaldehyde was obtained from ferrocene methanol in goodield (Table 5, entry 7).

.3. Recycling of the catalyst

Reusability of the catalyst was evaluated by carrying outepeated runs of the reaction on the same batch of the catalyst

n the case of the model reaction (Fig. 4). After each run, the solidatalyst was filtered off and washed with different organic solventsnd residue was used at next run alternatively. The results showedhat this nanocomposite could be reused several times without

able 4erobic oxidation of benzyl alcohol in toluene using SBA-15/Met/Pd(II).

Base, O2, toluene, !

OH CHO

SBA-15/Met/Pd(II)

Entry Catalyst amount (mol%) T (◦C)

1 1 25 2 1 80 3 2 80 4 3 80 5 3 80 6 3 80 7 3 80 8 3 80 9 3 80

10 3 80 11 3 80 12 3 80

a Isolated yield.

tions (80 ◦C, EtOH/H2O for 1 h) using a model reaction of 4-bromoacetophenone,phenylboronic acid.

any modification and that no significant loss of activity/selectivity

performance was observed.

TEM images obtained from re-used catalyst showed more evi-dence of the incorporation of palladium species within SBA-15

Base t (h) Run Yield (%)a

– 24 1 NR– 24 1 Trace– 24 1 15– 24 1 28K2CO3 1 1 54K2CO3 5 1 80Et3N 24 1 93K2CO3 24 1 96K2CO3 24 2 92K2CO3 24 3 91K2CO3 24 4 89K2CO3 24 5 89

172 A. Alizadeh et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 167– 174

Table 5Aerobic oxidation of various alcohols using SBA-15/Met/Pd(II).

SBA-15/Met/Pd(II), K2CO 3, O2toluene, 80 oC, 24h

OH

G

CHO

G

Entry Substrate Product Yield (%)a

1OH CHO

96

2OH CHO

95

3

OH

O2N

CHO

O2N80

4OH

Cl

CHO

Cl 95

5OH

Ph

CHO

Ph87

6H3CO

OH CHO

H3CO93

7

OH

Fe

CHO

Fe 78

8

OH O

93

9 OH CHO 95

mcFttopc(fiPtsatpiHstis

a Isolated yield.

aterial. For instance, the TEM micrographs of used SBA-15/Met/Pdatalyst for Suzuki coupling reaction after runs 2–5 are shown inig. 5a–d. The dark spots are presumably the Pd nanoparticles andheir formation is mainly due to the exposure of SBA-15/Met/Pdo reaction medium (H2O/EtOH) which may cause the reductionf Pd2+ ions to Pd(0) species by virtue of EtOH [35]. The Pd0 atomroduced after the first reduction with EtOH is bound to metformin-oated SBA-15 via the coordination of terminus imino moieties

NH) of metformin with Pd0. When the reaction is performedor more runs with the recovered catalyst, Pd0 atoms producedn recycling process are mainly deposited around the preexistingd0 atom and then grow into bigger clusters. As a result, althoughhere is no leaching of Pd2+ into the solvent, however in EtOH–H2Oolvent the catalyst activity is believed to be slightly decreased to

degree due to the agglomeration of the active catalytic sites ofhe catalyst. This might be due to the catalyst aggregates into bigarticle and becoming more difficult to be dispersed and result-

ng in the reduced surface area of the catalyst and activity loss.owever, stirring the reaction mixture for longer time at air atmo-

phere causes the atomic particles of palladium to be back-oxidizedo Pd2+ ions with the help of oxygen [36] and as a result, theres no detectable loss of extent of Pd2+ ions and the spent catalysttill retains its catalytic activity. The effect of reaction time on the

yield of product was also studied and it was found that after 7thrun, due to the above-mentioned reason the longer reaction timeis needed to give the desired product in excellent (>90%) yield. Inaddition, TEM micrographs showed that the ordered mesostructureof SBA-15 was retained, and no damage in the periodic structureof the silicate framework and metal aggregation was observed. Thecatalyst exhibits high stability even after seven recycles.

Furthermore, the XRD patterns (WAXS and SAXS) of thematerials of parent mesoporous silica (SBA-15/Met) and spent SBA-15/Met/Pd(II) are shown in Fig. 6. In curve 6I-a, SBA-15 exhibitsa broad peak centered at 2� = 22.3◦ with co-responding d-spacingof 3.25 Å which confirms the amorphous structure of SBA-15.Curve 6I-b, presents the wide-angle XRD pattern for the recoveredSBA-15/Met/Pd(II) and as seen, after immobilization of palladiumacetate within SBA-15/Met, almost no shift in the center of peakposition (2� = 22.5◦) was observed and only one peak of palladiumcrystalline structure (1 1 1) was found at 40◦. It has to be men-tioned that the presence of silica component in SBA-15/Met/Pd(II)results in a decrease in the intensity of the peaks [37]. In addition,

the low angle XRD patterns of parent SBA-15, SBA-15/Met, SBA-15/Met/Pd(II) and spent SBA-15/Met/Pd(II) (4th run) are shown inFig. 6(II). Observation of three distinct diffraction peaks indexed as(1 0 0), (1 1 0), and (2 0 0) in the low 2� region can be evidenced

A. Alizadeh et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 167– 174 173

Fig. 5. TEM micrographs of used SBA-15/Met/Pd catalyst for Suzuki coupling react

F1

oit

4

doathrae

[

[[[[[[[[

[

ig. 6. XRD wide and small angle patterns of the parent SBA-15, SBA-15/Met, SBA-5/Met/Pd(II) and recovered SBA-15/Met/Pd(II) after 4 times.

rdered hexagonal mesoporous phase for SBA-15 samples and alsot is in a good agreement with the idea that the spent catalyst retainshe SBA-15 structure.

. Conclusion

We have prepared a stable and new SBA-15 based palla-ium nanocomposite with potential catalytic application in variousrganic transformations such as Suzuki–Miyaura cross-couplingnd aerobic oxidation of benzyl alcohols under feasible condi-ions. The SBA-15 decorated with a complex of biguanide/Pd2+ as

eterogeneous palladium nanocatalyst can be conveniently fab-icated through a simple two-step procedure from commerciallyvailable and cheap reagents. This catalytic system showed anxcellent reactivity combined with efficient catalyst recyclability,

[[[

ion: run (a) 2, (b) 3, (c) 4 and (d) 5 (the dark spots are the Pd nanoparticles).

low palladium leaching and reused several times without anydecreases in activity.

Acknowledgements

We are grateful to Razi University, Research Council for thepartial support of this work. We are also thankful to Mr. ChangizKarami, Islamic Azad University, Kermanshah (IAUK), Iran for theuse of FT-IR instrument. D.K. also appreciates the Iran Nanotech-nology Initiative Council (INIC) for their partial support on thisproject.

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1 Catal

[

[

[[

[

[

[

[[

[

[

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A biguanide/Pd-decorated SBA-15 hybrid nanocomposite: Synthesis, characterization and catalytic application1 Introduction2 Experimental2.1 Preparation of metformin-functionalized SBA-15 (SBA-15/Met)2.2 Immobilization of Pd(II) ions on the surface of SBA-15/Met2.3 Catalytic testing2.3.1 Suzuki–Miyaura coupling reaction2.3.2 Aerobic oxidation of benzyl alcohols

3 Result and discussion3.1 Synthesis and characterization of SBA-15/Met/Pd(II)3.2 Catalytic activities3.2.1 Suzuki–Miyaura coupling reaction3.2.2 Aerobic oxidation of benzyl alcohols

3.3 Recycling of the catalyst

4 ConclusionAcknowledgementsReferences

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