Indian Journal of Chemistry Vol. 43A. May 2004, pp. 1083- 1087

Notes

Diamino-functionali zed mesoporous molecular sieve anchoring V(rY) and Mo(YI) complexes as catalyst for alkene epoxidation

Hengquan Yang", Gaoyong Zhang' b, Xinli n Hong" & Yinyan zhu"

"School of Chemistry and M olecular Science, Wuhan Uni versity. Wuhan 430072, P.R. China

*hChina Research Institute of Dail y Chemical Industry, Tai yuan 030001, P.R. Ch ina

Received 19 Febm arv 2003; revised 4 March 2004

The novel hybrid cata lysts have been prepared by anchoring homogeneous catalysts VO(acac)z and Mo02(acach onto the diamino- functionalized mesoporous molecular HMS achieved by sily lation of mesoporous sieve HMS with 3- [(2-aminoethy l)­amino)-propy ldimethoxymethy lsilane. The developed hybrid catalysts demonstrate high acti vity as their homogeneous counterparts for cyclohexene epox idation with I-butyl hydroperox ide as ox idant. The y ield of cyclohexene ox ide is found to be 74.2% for the catalyst NMS-AASP-Mo, and more than 80% of selectiv ity has been obtained for all the deve loped hybrid catalysts. The catalysts exhibit easy recovery, and the metal leaching is found to be less than 6% after two runs. The catalysts have been characteri zed with N2 physical adsorption. x­ray powder diffraction, Ff-JR spectrum, DRUV-vis spectrum and ICP-AES.

IPC Code: lnt. CI 7 BOIJ 23/16; BOlJ 291076

Homogeneous catalysts heterogeni zed onto solid supports form the so-called novel hybrid catalysts. The hybrid catalysts with advantages of both homogeneous and heterogeneous catalysts, e.g. high activity , easy product separation and catalyst recovery, constitute a rapidly expanding research area 1.2. Inorganic solids and organic polymers are usually used as matrices for immobilizing homogeneous catalytic species. The mesoporous molecular sieves disclosed recently by Mobil' s researchers denoted as MCM-41 3 with high specific surface area, pore volume and larger pore size than conventional zeolite have stood for the extension of catalytic application of conventional supports4. In the case of heterogenization of homogeneous catalysts, mesoporous materials have exhibited promlsll1g properties as host materials, and attracted many researchers in catalytic area in recent decadeS.

The industrial manufacture of propylene oxide, which is carried out by liquid phase epoxidation of

propylene with an alkyl hydroperox ide as ox idan t, is catalyzed by a homogeneous Mo(V I)O or a heterogeneous titanium/s ili ca catalyst7

. It is we ll ­known that homogeneous V(lV) and Mo(V I) acety lacetonate co mplexes are effective catalysts for alkenes epoxidation with the alky l hydroperox ides as oxygen sources. For the environmental and engineering reasons, great efforts have been made to immobilize these V(IV) and Mo(VI) onto various functionali zed po lymers8

.'! and inorganic solids such as USY zeolite lO and mesoporous molecular

. I I steves .

In the present note, homogeneou s V(lV) and Mo (VI ) acetylacetonate complexes have been first immobilized onto diamino-functionalized mesoporous molecular s ieve HMS , resulting in the novel hybri d catalysts for alkene epoxidation, cyc lohexene epox idation, as a probe reaction was inves ti gated using these novel catalysts.

Experimental Vanadyl(1V) acetylacetonate [VO(acach l , and

molybdenyl(VI) acety lacetonate [Mo02(acachl were obtained from Aldrich, Anhydrous I-butyl hydroperoxide (TBHP) was obtained by treat ing

. I ' aqueous TBHP (70%) accordll1g to the procedure-and determined with iOdimetryl3 . Cyclohexene and 3-[(2-ami noeth y 1 )ami no lpropy Idi methox ymeth y 1 silane (AASP, China) were distilled before LI se.

The strategy for designing the catalysts. The catalysts were prepared by the procedure

shown in Scheme I

} II 11.1 S

2 M eSi(C II 2) JN U te 1( 2)2N 1-[ 2 .. r 'J l u ~ n c/lt· nu ~

I 0 >,1(C I-1 2)JN II~II ~ 1-- 0 I

Me

11 M S-AASP - Ve] I o r 11 M 5- A ,\5P - Y(],

I-IMS · AASP-M 0

Scheme 1

1084 INDI AN J CI-IEM. SEC A, M A Y 2004

Preparation of HMS and diamillo group grafted IIMS

Mesoporous HMS was prepared according to the

procedure '4 and calcinated at 550°C for 7 h in the air. The diamino-func ti o nali zed mesoporous molecular Sieve HMS-AASP was prepared as g iven 111

lite rature 's.

Immobilization of the V(IV) and Mo(V I) complcxs

Metal acetylacetonate complexes (7.5 mmol ) were added to the suspensio n of HMS-AASP (3 g) in toluene under N2 atmosphere, and re tluxed fo r 48 h with magneti c stirring. The solid product was separated from the mixture and ex trac ted with acetone in sox hl e t apparatu s for 48 h, to remove physica lly

adsorbcd metal complexcs, and thc.n dri ed at 80°C for 8 h in vacuum, resulting in thc cataly ti c materials dcnoted as HMS-AASP- Ve l) and HMS-A ASP-Mo, respecti ve ly. HMS-AASP (3 g) was added to the wate r so luti on of VOSO.j , and the mixture sys tem was st irred for 4 h. The prod uc t was filtered from the sys tem, and washed wi th de ioniscd water, followed by ethano l, and dri ed under vacuu m. T he catal yst HMS-A AS P-V (2) was obtained .

The BET surface area and pore analyses were performcd on an ASAP20 I 0 (m icromcritics) by N2 physica l adsorpti on-desorption at 77.4 K. X-ray Powder diffraction was obtai ned o n a Damx-rA (Rigaka). RT-IR spec trum was recordcd on a TFS-25 PC (Di g ilab Corporation). Diffusc re fl ec tance UV­vis spec trum was o btained o n a Lambda B io 40(PE Corporati on). C, Hand N e lemental anal ys is were co nducted on a vario EL (Elementar). Metal conten t was determi ned wi th i nduc ti vely coupled plasma atom emi ss ion Atomscan 16 (I C P-AES, TJ A Corporati o n) . The quantitative ana lyses of reaction products were performed on 9A gas chromatography (S himadzu). The reac tio n products were confirmed by GC-MS (gas chro matography- mass spectra, 5890A-5970 Bscries, Hew lett-Packard)

Catalytic cpoxidatioll

The catalyst conta ining metal (0.4 mmo l), cyc lohexene ( 150 mmol) as both reac tant and solvent, and internal GC standard sample (b romobenzene, 0.5 ml ) were added to a two-necked round-botto med flask fitt ed with a reflux condenser after being purged

with high pure N2. The sys tem was heated up to 55°C, anhydrous TBHP ( 18 mmo l) was added to it, and remained at thi s temperature before TBHP was co mpletely consumed. Afte r the completion of the reaction , the solid catalyst was filtered and stored in cyC\ohexene for the second use. The homogeneous

catalytic reactio n us ing vanady l or mo lybdeny l acetylacetonate were carried o ut using the simil ar procedure with cyC\ohexene as solvent.

Results and discussion Mesopo rous HMS derived fro m the neutral

template has abundant channel branching and high tex tural mesoporos ity and thicker pore wa ll as compared to mesoporous MCM-4 I I(,. 17, which are

respec ti ve ly believed to fac ilitate di ffusion of reaction mo lecul es to active sites and improve the stab ility.

The N2 phys ical adso rpti on -deso rpti on iso therm plot. The isotherm plo t demonstrates the typica l characteristics o f mesoporous mo lecular s ieves. i.e. type IV3

. N2 mono layer adsorpti on occurs, followed by multi-l ayer adsorption on the surface, and then capi ll ary condensation takes p laCe at re lative pressure between 0.4-0.5 in the channel, indi cating narrow mesopore distribution . It is no ted th at there is a re lati vely bi g hysteres is loop between re lat ive pressures 0.4-1 .0 for the mesoporous HMS prepared in our lab, diffe ring to the desc ri pti on '7. Such a big hysteres is loop indi ca tes that the meso porous HMS has mo re abundant tex tural , framework-confin ed or interparti c le mesoporosity' 7, which is strongly be li eved to faci litate bu lky mo lecu le diffu sion.

The X-ray powder diffrac ti n pattern of HMS shows a s ing le broad peak (re fl ec ti o n ( 100» centered

on around 20(theta)=2°. Additiona l refl ectio ns ( 110) and (200) were no t observed in as-synthesized HMS. which are usually observed in mesoporous MCM-4 1. The difference is att ributed to a broadening of X-ray re fl ec tio n due to re latively small scatte ring domains arising from the di sorder of HMS as compared to MCM-41. The di sorder results fro m the presence of abundant tex tural pore and channe l branching.

The N2 adsorption-desorption curve and BJ H (Barrett-Joyner-Helana) pore di stributi o n of vari o L! s modified HMS were s tudied 18. T he iso therms o f modified HMS were kept in that o f the unmod ifi ed HMS , whereas obvious changes at each step of modifi cation were observed. The BJ H pore size distribution is based on the Ke lvin equation and has been widely used in mesoporous mate rial s. We are inte res ted in the changes in the pore s ize di stributi on because of the modification with the organ ic moieti es and metal complexes. The pore s ize shift graduall y to low value, wh ich indicates that organic moi eti es and metal complexes en te r the channel. Neverthe less, each material s till maintains relati ve ly narrow pore size di stribution . The detailed data were summari zed

NOTES 1085

in Table I. Moreover, it is referred reasonably that the organic mo ieties and metal complexes are broadly and uniformly di spersed in the channel of HMS based on narrow pore size di stribution .

Since the organic moi e ti es or metal co mplexes were successfull y introduced to the channel s, the spec ific surface area and pore volume li sted in Table 1 decreases gradually at each stage of modification. However the final resulting catalysts still exhibit high speci fic surface area (hi gher than no rmal typical organic po lymers ), which is fav ourable to expose the active sites to reacti o n mo lecules and create more effic ient active sites.

Figure I shows FT-IR spectrums of various mod ifi ed HMS . A small peak was obtained between 2900-3000 cm- I for each material , assigned to C-H stretching, which indicates the AASP species were successfully introduced to HMS . In the spectra of both HMS-AASP-V( I) and HMS-AASP-M o, the peak appears around 1530-1560 cm- I as compared to HMS-AASp IS

, which is attribu table to acetylacetonate ligand . Moreover, the bands about 900 cm- I and 675-710 cm- I appear in the spectra of HMS-AASP-M o, ascribed to the Mo=O stretching and Mo-O-Mo bridge stretching, respective ly" ). However no peak is seen appearing of V=O stre tch ing (about 965 cm- I

) ,

which may be explained tha t the reg ion of V=O stretching region is suppressed by Si-O-Si stretch ing band.

The DRUV-vis spectra of various catalytic material s were obtained . HMS-AASP had no adsorption in the range of 250-400 nm, while HMS­AASP-V(l), HMS -AASP-V(2) and HMS-AASP-Mo had strong adsorption in the range of 250-400 nm, which was ascribed to e lec tron shift of metal complexes. The diffe rent UV-vi s adsorption of HMS­AASP and HMS-AASP immobilizi ng metal complexes indicates that the metal complexes were successfully immobilized o n the mesoporous diamino-functionalized HMS .

The mechanism of immobili z ing VO(acac): and Mo0 2(acac)2 onto the surface of diamino-functiona lized HMS is compli cated. Three poss ibi liti es a re cons idered in principal. One of the m is that the two nitrogen atoms of ethy lenedi am ine substitute direc tly one acetylace tonate ligand . The second one is that the te rm ina l nitrogen atom of the ethylenediamine g roup reacts with acety laceto nate li gand , resulting in a schiff base, which may subsequent ly form the bi g cyc le coordinati on with the meta l atom ill situ according to the report20

. The third is that meta l comp lexes are directl y linked to the res idual s ilanol by irreversible adsorption of the complexes via hydrogen bonding or li gand exchange2 1

.

Table I also summarizes the amounts of C. metal in various ca talysts and the ratios of LIM (the mo le ratio o f the orga nic ligand on mesoporo us sieves AASP and metal). The ratios of LIM are all less than I, especia lly for HMS-AASP-M o, the rati o is 0.57. The result is seeming ly in line with the first one and the th ird one. In order to confirm thi s findin g, furth e r

31100 201J() 10011

Fig. I- FT-IR spectra of mod ifi ed HMS [0, HMS-AASP; I, HMS-AASP-V ( I); 2, HMS-AASP-Mol

Table I- Parameters of various HMS

Mesopore mclocule S V D C N M LIM sieves (m2/g) (cm3/g) (nm) (0/0 ) (0/0 ) (%)

HMS 719 0.94 3.2 HMS-AAS P 5 19 0.62 2.8 9.42 3.38 HMS-AAS P-V( I) 3 10 0.37 2.5 13.35 2.65 5.95 0.81 HMS-AAS P-V(2) 436 0.53 2.7 7.36 2.69 5.22 0.93 HMS-AASP-Mo 296 0.3 1 2.5 12.39 2.35 14.2 0.57

S=specific surface area: V=pore volume from single; D=BJH pore distribution; C and N content were determined by e lement analyses: LlM=the mole rat io of the organic ligands on mesoporous sieves AASP and metal.

1086 INDIA N J C HEM. SEC A, MA Y 2004

~ ?if* ~ ?i 0 ?iO* f-IMS L- M,\.J HMS L-M/ "'-M{ '\

II 0- II II cr! o 0 0

MOllloncric cen ter Dimeric or Polymeric cenler

Scheme 2

Tab le 2-T he performance or va rious cata lys ts

Catal ysts

VO(aeae)2 HM S-AASP-V( I) HMS -AAS P-V(2) Mo02(acaeh HM S-AASP-Mo

First run Yi e ld (%") Selec ti vity (%)

40.1 6 1.5 53.3 88.0 58.7 84.6 76.7 89.5 74.2 88.5

Second run Yieid (%) Selecti vity (%)

49.2 87.4 55.2 8 1.2

69.6 88.9

Metal leachi ng afte r two run sh (%)

')~ ~ . .)

4.8

5.7 "The yield is based on the consumpt ion ofTB HP. i.e. 100% yie ld= 18 millo l cyelohexene ox ide from sLO i c hiom~tric amount or 18 11111101 TBHP;bthe rati o of the meta l loss and initialilletal content.

experiment was carried o ut, i.e. the unmodifi ed HMS was e mployed directly to immobilized V(lV) and Mo(VI) acetylacetonate complexes in the same condi ti on as described above. It was found that the unmodified HMS has a poor activity for immobili zing the metal complexes in the same conditi on. So the results are only in agreement with the first mechanism. But C co mpos ition is still seemi ngly lower than the theore tical val ue, whi ch is explained that Mo-O-Mo bridge formation led to the ratio decrease'), confirmed by FT-IR spectrum. So, two metal centers including monomeric and dimeric or polymeri c metal spec;es were produced by the interacti on of the di al1lino-functionalized HMS surface with the metal complexes for the catalyst HMS-AASP-Mo, based on the facts of FT-IR spectrum and elemental analys is. T he proposed catalytically active centers were shown in Scheme 2.

Table 2 shows the yield o f cyclohexelle oxide with various catalysts including heterogeneous catalysts based on HMS and their homogeneous counterparts. Heterogenizatio n of homogeneous species usually leads to the decrease in the catalytic activity. However, it is noteworthy that the developed catalysts have as high catalytic acti vity as comparable to their homogeneous one. In particular fo r the case of V(I V), the activity is hi gher than the activity of homogeneous counterparts and the diamino-functionalized polymer immobil ized V(lV)<J. This may be explained that (i) the catalysts based on mesoporous molecul ar sieves have high surface areas, wh ich favour produci ng isolated active si tes access ible to cyclohexene and TBHP molecules, and ( ii ) heterogenization inhibi ts

the dimerizatio n o r polymeri zation o f metal species. In addition, the developed he terogeneous catalysts have a good selectivity comparable to their homogeneous one. 2-cyclohexene-l-o l or 2-cyclohexene-l-one as major side products were detected for the developed heterogeneous catalysts.

The major advantages of heterogeni zecl homogeneous catalysts over their co unterparts are the ease of recovery and recycle. Whereas metal leaching, probably arises from heterogeni zat ion of homogeneous catalysts. Howe ve r, no obvious metal leach ing was observed after two ru ns in thi s study, which was attributed that the homogeneous species were anchored strongly onto diamino-functi onalized HMS surface by strong lin kage.

Ack!lowledgement Financial support fro m the Research Institute of

Ch ina Daily C hemical Industry IS gratefully acknowl edged.

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