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Catalysis Letters 43 (1997) 149-154

Catalyticoxidationofstyrene bymanganese(II) bipyridine complexcations immobilized inmesoporousAl-MCM-41Seong-SuKim,WenzhongZhang andThomasJ. Pinnavaia *

DepartmentofChemistry andCenter for FundamentalMaterialsResearch, Michigan State University.East Lansing. MI48824, USAReceived12August 1996; accepted 23 October 1996

Manganese 2,2'-bipyridine (bpy) complex cations, [Mn(bpYh]2+, have been immobilized in mesoporous AI-MCM-41 (SilAI = 9) and used as a catalyst forthe oxidationof styreneby iodosylbenzene, H202 and tert-buty1 hydroperoxide (TBHP). The oxidation products includedepoxide, diol and aldehyde. AI-MCM-41-immobilized (Mn(bpYh]2+ exhibited a higher catalytic activityfor styrene oxidation than the corresponding homogeneous catalyst and showed no significant loss of catalytic activity whenrecycled.Keywords: AI-MCM-41, Mn(II), immobilization, oxidationof styrene

149

1. IntroductionManganese(II) bipyridinecomplexes havebeenrecog

nized as potential catalysts for the oxidation of alkanesand alkenes [1-4]. However, their catalytic activity insolution is limited by their tendency to decomposeH202[5]. Moreover, these homogeneous catalytic processesrequire liquid-liquidphase-transfer reaction conditions,which limits the reaction rate.Immobilization of these Mn(II) complexes in thechannels of zeolites has been of special interest since itoffers the possibility of achievingmolecular isolation ofthe complexes which are very promising catalytic sites[6]. However, complex immobilization in the zeolitesrequires multi-step "ship-in-bottle" synthesis, whereinthe complex is assembled from the ligand-metal ions atthe exchange site of the zeolite [6-8]. The major drawback for the use of conventional zeolites is the smallporesizes (4-13 A), which makes it difficult for substrates todiffuse and access the active sitesand for products to diffuse out of thepores [7].The recently discovered mesoporous aluminosilicateMCM-41 (AI-MCM-41) affords new opportunities forthe catalytic conversion of substrates with largermolecular size [9,10). These materials also are promising forthe immobilizationoflargemetal complexesand clustersof potential catalytic significance [II]. In the presentstudywe report the immobilization ofMn(II) bipyridinecomplex cations in Al-MCM-41 channels by direct ionexchange. The catalytic activity of the AI-MCM-41immobilized complex for oxidation of styrenewith iodosylbenzene (PhIO), H202 and tert-butyl hydroperoxide(TBHP) as oxidants has been investigated. Oxidation of To whomcorrespondence shouldbe addressed.

J.C. BaltzerAG, Science Publishers

styrene over the AI-MCM-41-immobilized catalystshows higher turnover number than thatof the homogeneous complex itself, which reveals thatmolecularly isolated sites are very efficientcatalysts.

2.ExperimentalAl-MCM-41 with a SilAl ratio of9 was prepared following a proceduresimilar to previouslydescribedmethods [9,10].NaOHwas added under constant stirring to asolution ofcetyltrimethylammoniumbromide (Aldrich,

[CTMA]Br). Then Ah03 sol (10%, Nacol Chemicals)and tetraethylorthosilicate (Aldrich) were added. Thecomposition of the resultant gel was ISi02:O.04Ah03:0.24CTAB:0.3Na20:150H20. The mixture was stirredat room temperature for I h, and thenheated at 383 Kinan autoclave for 4 days to complete the crystallization.Theproductwasfiltered, thoroughlywashedwith deionizedwater, driedin air and calcined at 813 Kfor 4h i nN2and 6 h in air in order to remove the template. Elementalanalysis for the SilAl ratio was performed with inductively coupledplasma emission spectroscopy. The SilAlratio of thefinal productwas 9.Two AI-MCM-41-[MnL2j2+ (where L = 2,2'-bipyridine) samples were prepared by ion exchange of the calcined Al-MCM-41 (200 mg) with known amounts ofMnL2(N03h (0.073 or 0.2 g) [I] dissolved in 20 ml of aI :9 (vol: vol) DMF and acetonitrile mixture at roomtemperature for 24 h. The samples were filtered, washedwith acetonitrile until no MnL2(N03h was detected inthe filtrate, and then dried in vacuum at 80C. Chemicalanalyses show that the two products haveMnlAl ratioof only 0.05 (5%) and 0.07 (or 7%). Even though a higherconcentration manganese complex solution was used,

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150 S.-S.Kimetal. / Al-MCM-41-immobilizedfMn(bpy)212+

Fig. 1. X-ray powderdiffraction patterns of the (A) as-synthesizedAtMCM-41, (8) calcined At-MCM-41 and (C) A t - M C M - 4 1 - [ M ~ j 2 +(5%).

B

A"\: A\.-..4 8 12 16 20

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with pore wall thickness less than 10 A. Therefore, theAI-MCM-41 prepared in this work is sufficiently stableto be used as a catalyst support for liquid phase oxidation reactions.Ion exchange of the zeolite with MnL2(N03h in aDMF and acetonitrile mixture did not affect the structure of the mesoporous Al-MCM-41. Both AI-MCM

41-[MnL2]2+ (5%) and Al-MCM-41-[MnL2j2+ (7%)exhibit the hexagonal phase with at least two resolved(100) and (110) peaks in the X-ray diffraction pattern(fig. Ie). In addition, the AI-MCM-41-immobilized[MnL2]2+ complexes exhibit a pink color instead of thepale yellow color characteristic of the originalMnL2(N03h. This color change is consistent with theabsence ofN0i" in the coordination sphere of the manganese (seebelow).The reflectance spectra of the pink AI-MCM-41[MnL2]2+ samples showed a broad band at around530 nm (fig. 2). This band is not observed in the reflectance spectra for AI-MCM-41, MnL2(N03h, or puresiliceous MCM-41 impregnated with a known quantityofMnL2(N03)z. Therefore, this band can be assigned tothe metal-to-ligand charge transfer transition of thenitrate-free [MnL2]2+ cation bonded to the zeolite framework [6,16]. Also, the analytical data of the catalystare in agreement with the calculatedMn : N : Cratios ofabout 1 : 4 : 20 for the immobilized complex cations.These results indicate that [MnL2j2+ is anchored to themesopore walls of AI-MCM-41 probably via coordination to the anionic framework. The binding of [MnL2j2+to zeolite X or Y through oxygen bridginghas also beenobserved [6].The nitrogen adsorption-desorption isotherm andthe HK pore size distribution for AI-MCM-41 and AlMCM-41-[MnL2]2+ samples are presented in fig. 3. A

the Mn/Al ratio could not be increased further.Therefore, 7% represents the saturation loading of thezeolite.Oxidation of styrene was carried out using 100 mg ofAI-MCM-41-[MnL2j2+ (contains 0.83 x 10-2 and1.16 x 10-2mmol ofMn for 5% and 7%, respectively) ascatalyst, 0.087mmolof styreneas substrate,Smlof acetonitrile in 50 ml round flask, into which 0.47 mmol ofPhIO or 3.45 mmol of H202 or TBHPwas added as anoxidant. The products during the reaction were periodically sampled and analyzed by a gas chromatograph(HP 5890) using a SPB-20capillary column (fused silica,30 m, 0.5 mm i.d.) and a flame ionization detector. Theproducts were identifiedbyGC-MS.X-ray diffraction patterns of powdered samples wereobtained with a Rigaku X-ray diffractometer equippedwith a rotating anode and Cu Ka radiation. Diffusereflectance UV-vis spectra were recorded with aShimadzu UV-265 equipped with a Harrick diffusereflectance integration sphere. Nitrogen adsorption/desorption isotherms at liquid nitrogen temperature weremeasured on a Coulter Omnisorp 360CX sorptometerusing super-pure nitrogen (99.999%). Samples (about 60mg) were pre-outgassed overnight at 358 K undervacuum (10-5 Torr). Surface areas were obtained by theBETmethod. The sizeof framework-confined mesoporewas determined by Horvath-Kawazoe (HK) analysis[12] of the nitrogen adsorptionisotherms.

3.Results and discussionX-ray powder diffraction patterns of the as-synthesized, calcined and [MnL2j2+ ion-exchanged Al-MCM

41 (Si/Al = 9) samples are presented in fig. 1. The assynthesized sample exhibits a very strong peak at d spacingof41.7A (100) and threeweakerpeaksat23.8 (110),20.1 (200) and 15.5 A (210). These four peaks fit a hexagonal unit cell with ao = 48.2 A (where ao = 2dlOo/v'3).The calcined sample exhibits three peaks at 37.4 (100),21.4 (110) and 18.2 A (200) with an unit cell size of 43.2A. The lattice contraction of about 4.3 Aupon calcination is smaller than the 7-10 A contraction reported inthe literature [13,14]. The mesopore wall thickness is14.2A, ascalculated from the differenceof the HorvathKawazoe (HK) pore (29.0 A) and the unit cell size (43.2A). Thus, the wall thickness is larger than the 8-10 Avalues reportedfor silicaMCM-41 [10,15]. We note thatthe AI-MCM-41 prepared by reaction at 100C for 2days becomes amorphous upon stirring in H20 for 24 hat room temperature. For this latter unstable form ofAlMCM-41, the pore wall thickness is less than 10 A. TheAI-MCM-41 in this work was prepared at 100C for 4days. No decomposition of the structure was noted evenafter exposure to liquid water for 3 days. The BET surface area of our calcined sample is 920 m2g-l, which issmaller than the value (>1000 m2 g-l) for the samples

IIII'il I

..i...

c

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S.-s. Kim etal. / AI-MCM-41-immobilizedfMn(bpYhJ2+ 151

Fig. 2. Reflectance spectra of the (A) Al-MCM-41, (B) Al-MCM-41[MnL2]2+ (5%), (C) Al-MCM-41-[MnL2]2+ (7%), (0 ) MnL2(N03h

and (E) M n ~ ( N 0 3 h impregnatedpure siliceousMCM-41.

Wavelength (nm)

...uClJ

..o.,,l:l-