Vanadium- and chromium-containing mesoporous MCM-41 molecular sieves with hierarchical structure

Microporous and Mesoporous Materials 43 (2001) 227±236

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Vanadium- and chromium-containing mesoporous MCM-41 molecular sieves with hierarchical structure Z.Y. Yuan a,b,*, J.Z. Wang a, Z.L. Zhang b, T.H. Chen a, H.X. Li a b

a Department of Chemistry, Nankai University, Tianjin 300071, People's Republic of China Beijing Laboratory of Electron Microscopy, Institute of Physics and Center for Condensed Matter Physics, Chinese Academy of Sciences, P.O. Box 2724, Beijing 100080, People's Republic of China

Received 2 May 2000; received in revised form 20 December 2000; accepted 20 December 2000

Abstract Vanadium- and/or chromium-containing MCM-41 molecular sieves were synthesized and characterized by spec A hiertroscopic techniques. These materials have bimodal pore size distributions with diameters of 26 and 39 A. archical structure with paintbrush-like morphology was observed by HRTEM. The reason for the formation of such structures might be that there were two types of micelles growing simultaneously in the same direction and in the same particles. V and Cr could be incorporated alone or simultaneously and competitively in the framework through interaction with the silanol groups on the internal walls of the hexagonal tubular silicate MCM-41. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: MCM-41; Mesoporous molecular sieve; Bimodal pore size distribution; Vanadium; Chromium; Hierarchical structure

1. Introduction Recently, much attention has been paid to mesoporous MCM-41 molecular sieves [1,2], and many kinds of metals were incorporated into the MCM-41 framework [3±6]. The occurrence of pores with a bimodal pore size distribution in these

* Corresponding author. Address: Beijing Laboratory of Electron Microscopy, Institute of Physics and Center for Condensed Matter Physics, Chinese Academy of Sciences, P.O. Box 2724, Beijing 100080, People's Republic of China. Tel.: +86-10-8264-9453; fax: +86-10-6256-1422. E-mail address: [email protected] (Z.Y. Yuan).

mesoporous materials may be important and useful for an engineering of pore systems [7,8]. Moreover, mesoporous materials with tubular, spherical and ®brous morphology are interesting for possible uses in embedding conducting materials, optical communications, chromatographic separation, and biomaterials [6]. Here, we report on the synthesis of the siliceous, V- and Cr-substituted mesoporous molecular sieve MCM-41 with a paintbrush-like morphology in a mild alkali condition using cetylpyridinium bromide (CPBr) as a template. These materials have bimodal pore  size distributions with diameters of 26 and 39 A. The simultaneous incorporation of V and Cr into this kind of mesoporous MCM-41 molecular sieve has also been studied.

1387-1811/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 1 ) 0 0 1 8 8 - 3

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2. Experimental 2.1. Synthesis Vanadyl acetylacetonate (VO(acac)2 ) and chromium acetylacetonate (Cr(acac)3 ) were used as the metal precursors in the synthesis of mesoporous metallosilicate MCM-41 samples. A typical procedure is as follows: 5.76 g of CPBr was dissolved in 70 ml of water, 18.8 ml of ammonia solution (25%) were then added under stirring, resulting in a clear solution. 6.72 g of tetraethoxysilane (TEOS) were then added slowly using a pipette under stirring, followed by adding a proper amount of metal precursors. The pH value of the ®nal mixture was about 9.2. The molar composition of the reaction mixture is 100 TEOS:50 CPBr:920 NH3 :a VO(acac)2 :b Cr(acac)3 :15 000 H2 O. The samples with di€erent amounts of metal precursors are denoted as ÔYVaCrbÕ. After stirring for more than 30 min, the mixture was loaded into an autoclave and statically heated at 80°C for three days. Siliceous MCM-41 was prepared by the same method without the addition of a metal source, and denoted as ÔYSiCPBÕ. The resulting solid product was recovered by ®ltration, washed with distilled water and dried in air at room temperature. In order to remove the organic species in the mesopores, the as-synthesized material was calcined in air at 150±250°C for 1.5 h, then at 540°C for 5 h. 2.2. Characterization Powder X-ray di€ractograms (XRD) of the solids were recorded on a Rigaku D/max 2500 di€ractometer using CuKa radiation between 1.4 and 10° (2h) with a scanning rate of 4°/min and a step size of 0.01°. N2 adsorption±desorption isotherms were obtained at liquid nitrogen temperature on a Micromeritics ASAP 2010 apparatus. The sample was outgassed at 300°C prior to adsorption. The speci®c surface area was determined by the Brunauer±Emmett±Teller (BET) method, and the pore-size distribution was obtained from the N2 desorption branch using the Barrett±Joyner±Halenda (BJH) method. Thermogravimetric analysis (TGA) was performed on a Netzsch TG

209 analyzer in which the sample was heated in a N2 atmosphere at a rate of 10°C/min. SEM micrographs of the calcined solids were obtained in a Hitachi X-650 scanning electron microscope using an accelerating voltage of 20 keV. High-resolution transmission electron microscopy (HRTEM) was carried out on a Jeol JEM-2010 and a Philips CM-200 electron microscope operating at 200 kV. The specimens for the HRTEM studies were prepared by dispersing the particles in alcohol by ultrasonic treatment, dropping onto a holey carbon ®lm supported on a copper grid, and air drying. Di€use re¯ectance (DR) spectra were recorded between 200 and 800 nm on a Shimadzu UV-240 spectrometer in the UV±VIS region using BaSO4 as a reference. ESR spectra of the solids were recorded on a Jeol JES-FEIXG spectrometer at ambient temperature using Mn2‡ (MnO) as an external standard. 3. Results and discussion 3.1. X-ray di€ractograms analysis The powder XRD patterns of both the as-synthesized and the calcined samples for every material exhibit four sharp di€raction peaks, re¯ecting a typically well-de®ned MCM-41 structure [1,2]. The XRD patterns are almost ¯at in the 2h range of 20±30°, indicating that no signi®cant amount of conventional amorphous silica was present in the products. Upon introduction of V and/or Cr into the MCM-41 structure, the di€raction peaks shifted to higher 2h values compared to siliceous MCM-41, corresponding to a decrease of the unit cell parameter (a). Moreover, after V and Cr substitution, there is a signi®cant loss of crystallinity compared with the siliceous sample, indicating that the order of the hexagonally arranged pores decreased due to the incorporation of the transition-metal atoms. The results of the XRD analyses of some representative mesoporous silicates are listed in Table 1. The contraction of the hexagonal unit cell parameter after calcination is very small, suggesting a high thermal stability. It is noted that two groups of overlapping di€raction peaks can be

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229

Table 1 XRD analysis results of purely siliceous MCM-41 and metal substituted samples   Samples d-spacing (A) Unit cell parameter (A) hk l

As-synthesized

Calcined

As-synthesized

Calcined

Contraction

YSiCPB (pure siliceous)

100 110 200 210

42.03 24.06 20.82 15.63

41.25 23.73 20.48 15.41

48.54

47.63

0.91

YV1 …V=Si ˆ 0:01†

100 110 200

41.84 24.12 20.82

39.23 22.87 19.75

48.31

45.30

3.01

YCr2 …Cr=Si ˆ 0:02†

100 110 200

40.87 23.48 20.39

39.94 22.75 19.84

47.19

46.12

1.07

YV2Cr1 …V=Si ˆ 0:02; Cr=Si ˆ 0:01†

100 110 200

41.44 23.86 20.72

39.41 22.81 19.66

47.85

45.50

2.35

observed in the sample ÔYCr2Õ after calcination (Fig. 1a). This implies that there are two kinds of mesopores in this material. 3.2. N2 adsorption data Representative N2 adsorption±desorption isotherms for the calcined metallosilicate samples and their corresponding pore size distribution curves calculated using the BJH method based on the desorption isotherm branch are presented in Fig. 1b. A typical irreversible type IV adsorption isotherm with a type H2 hysteresis loop, as identi®ed by the IUPAC [9], is observed. This result suggests that the metallosilicate samples have slit-shaped pores [9]. The adsorption isotherm exhibits a large increase in the P =P0 range from 0.2 to 0.3 which is characteristic of capillary condensation within uniform mesopores [9]. The P =P0 position of the in¯ection points is clearly related to the diameter in the mesopore range, and the step indicates the mesopore size distribution. From a plot of the pore-size distribution in Fig. 1b, we can see a wellde®ned pore size distribution centered around 26  and a shoulder peak at 39 A.  The pore size A distribution curves of the siliceous and all metallosilicate samples are bimodal in the range from 25  and from 35 to 40 A.  The pore size disto 30 A tribution curves of the metallosilicate samples

show a slight increase in their half-height width of  if compared with that of the peak at around 26 A the siliceous sample. This indicates that the mesoporosity was somewhat degraded after incorporation of the transition metal which could be veri®ed by HRTEM data. The pore size distributions of these mesoporous materials, their corresponding BET surface areas and the cumulative BJH desorption pore volumes are listed in Table 2. It can be seen that, upon introducing the transition metal(s) into the bimodal mesoporous MCM-41, the pore size increases. 3.3. Thermogravimetric analysis The TGA results together with the DTGA curve are shown in Fig. 2. The TGA patterns have at least three distinct stages of weight loss. As to the sample ÔYV1Õ, a weight loss due to the desorption of water amounting to 2.14% was observed between room temperature and 100°C. The stage of 100±380°C corresponding to a weight loss of 28.80% can be ascribed to the decomposition of the cetylpyridinium surfactant species [10]. The weight loss of 18.55% from 380°C to 650°C can be assigned to coke calcination and the loss of silanol groups (dehydroxylation). For ÔYV2Cr1Õ, three stages of weight loss amounting to 1.90%, 29.55% and 17.06% were observed. As to the ÔYCr2Õ

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Fig. 1. (a) XRD patterns of the as-synthesized (bottom) and calcined (top) MCM-41 sample ÔYCr2Õ; (b) N2 adsorption±desorption isotherm at liquid N2 temperature for the sample ÔYCr2Õ and its corresponding pore-size distribution curve (inset).

sample, the weight loss is 1.93% till 130°C between 130°C and 400°C, it is 29.53% due to the decomposition of the surfactant template from 400°C to 580°C, it is 10.51%, mainly due to the calcination of coke. The total weight losses of these samples are also listed in Table 2. Additionally, there is a

shoulder in the DTGA curves in the range between 200°C and 300°C in our samples. Since the processes of removal and decomposition of the surfactant molecules are complicated, this shoulder cannot be regarded as evidence for the existence of secondary pores.

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231

Fig. 2. TGA measurement with the corresponding DTGA curve from the bimodal mesoporous MCM-41 samples: (a) ÔYV1Õ; (b) ÔYCr2Õ; (c) ÔYV2Cr1Õ.

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Table 2 Structural parameters for the representative bimodal mesoporous materials and their total weight losses from thermal analysis  Sample Surface area (m2 /g) Pore volume (cm3 /g) Pore diameter (A) Total weight loss (wt.%) YSiCPB YV1 YCr2 YV2Cr1

749.1 794.5 728.9 752.5

0.742 0.741 0.700 0.972

25.0, 26.6, 25.7, 25.6,

35.1 39.2 39.0 39.0

39.41 49.49 41.97 48.51

3.4. Electron microscopy analysis Although the N2 adsorption±desorption studies  respectively, no showed two peaks at 26 and 39 A direct evidence for a bimodal nature of the samples was found in the XRD and TGA experiments. This is probably due to a low proportion and/or disordering of the large pores. HRTEM was then employed to examine the samples. This technique has been demonstrated as one of the most powerful means to study the detailed structures of mesoporous materials recently [11]. During the present HRTEM experiments, it was noticed that the samples were relatively stable under the electron beam irradiation in comparison with most other MCM-41 materials. It is believed that this is due to their relatively thick framework as in SBA-15 [12]. Fig. 3 shows a typical HRTEM image of the calcined metallosilicate specimen viewed down the (0 0 1) zone axis, indicating the long-range regular hexagonal arrangement of the pores and the pore±pore distance corresponding to the XRD results. Many local distortions and defects can also be found in the TEM image (see arrows). The hexagonal array of the mesopores is locally disturbed by some larger pores, which are  in most likely the larger mesopores around 39 A diameter as revealed by the N2 adsorption±desorption studies. These large pores are parallel to the small ones and form some cylindrical domains in the crystals. The sizes of the domains can be as  in diameter or as small as a single large as 200 A mesopore. The overall proportion of the large mesopores is much less than that of the small ones which is in agreement with the N2 adsorption± desorption measurements. The lattice strain built up from the di€erence of the pore sizes is probably

Fig. 3. HRTEM image of the calcined metallosilicate MCM-41 samples viewed down the [0 0 1] direction. The domains of large mesopores and local distortions nearby can be easily seen along the directions indicated by arrows.

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partially overcome by varying the local thickness of the silica wall. Further evidence for the bimodal pore size distribution come from HRTEM images perpendicular to the c-axis (the pore direction) of the particle, as shown in Fig. 4. There are at least two particles in Fig. 4a and b. In Fig. 4a, one particle shows regular hexagonal pore ordering, including some distorted domains (i.e., the larger pores). Many bundles of nanotubes grown from the inner of another particle can be clearly seen on a direction perpendicular to the c-axis. As shown in Fig. 4b, on the (0 0 1) surfaces of these particles many bundles of nanotubes appear, making the morphology of the (0 0 1) surface paintbrush-like. It is very interesting to see on the left side of Fig. 4a that single nanotubes of metallosilicate can exist.

233

Such pore symmetry and morphology suggest the existence of a bimodal mesopore distribution in the present metallosilicate MCM-41. Siliceous MCM-41 has a similar hierarchical structure with paintbrush-like morphology. The reason for the formation of a hierarchical structure with the paintbrush-like surface might be that there were two types of micelles growing simultaneously in the same direction and in the same particles. Since the synthesis was performed in mild alkaline conditions with NH4 OH, where strong hydrogenbonding forces are often formed, NH4 OH might act as an auxiliary which combines with part of the surfactant to form ¯exible and extended micelles. When one group of micelles stops to grow and the other group of micelles continues to grow, or the growth rates for these two micelles are di€erent, the lengths of the mesopores can be di€erent in the specimen, resulting in the formation of a hierarchical structure with a paintbrush-like morphology on the (0 0 1) surface. Very recently Lin et al. [13] reported on a post-synthesis hydrothermal treatment with ammonia to simultaneously re-structure the pore size, the nano-channel regularity and morphology of the mesoporous materials prepared from acid route. The nano-sized tubular form of the mesoporous silica they obtained is similar in its external appearance to our bundles of silica nanotubes. Although the formation mechanism of these fascinating structures is still not completely understood, we believe that ammonia might play an important role in the synthesis. The SEM photographs of the metallosilicates MCM-41 reveal that the particles of all samples show both a ``grainy cobblestone''-like shape up to 3.8 lm size and a ``worm''-like shape of about 0:9  2:5 lm2 . However, no paintbrush-like shape could be observed in the SEM photographs, due to the limited resolution of scanning electron microscopy. 3.5. ESR spectroscopy

Fig. 4. HRTEM images of the bimodal mesoporous MCM-41 when viewed on a direction perpendicular to the c-axis: (a) two particles viewing either on a direction parallel or perpendicular to the c-axis; (b) paintbrush-like (0 0 1) surfaces of two particles.

All as-synthesized V-MCM-41 samples give similar ESR signals (gk ˆ 1:941, Ak ˆ 174 G and g? ˆ 1:976, A? ˆ 61 G), which are characteristic of well-dispersed vanadyl ion (VO2‡ ) species in an

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axially symmetric crystal ®eld and consist of eight nearly equally spaced lines coupled to its own nuclear spin (51 V, In ˆ 7=2). The relative intensity of the ESR signals increases with the vanadium loading in the samples. The ESR signal of vanadyl species completely disappeared upon calcination, indicating that all the VO2‡ ions have been oxidized to V5‡ (d0 ) state. The as-synthesized Cr-MCM-41 sample displayed one broad isotropic ESR signal at g ˆ 1:981 (Fig. 5a), which has been assigned to the

chromium species of the ion-exchange cations [Cr(H2 O)6 ]3‡ and chromium oxide-like compounds occluded in the channels outside the framework [14,15]. According to Giannetto et al., the existence of other overlapping ESR signals cannot be excluded [16]. Upon calcination, this broad band became very weak, and we ascribe this to the remaining clusters of Cr(III), due to the oxidation of most of the Cr(III) species to Cr(VI), which is diamagnetic, and a second narrow hyper®ne signal appeared at g ˆ 1:98 (Fig. 5b), attributed to the Cr(V) species either tetrahedrally or octahedrally coordinated, probably linked to oxygen atoms of the framework [16]. The change in the oxidation state of Cr(III) was also con®rmed by the change in color of the samples to orange after being thermally treated. As to the MCM-41 samples substituted simultaneously with V and Cr when the content of V is low …V=Si ˆ 0:01†, i.e., sample ÔYV1Cr2Õ, an ESR spectrum results (Fig. 5c) with a weak vanadyl signal with hyper®ne splitting besides the isotropic ESR signal of Cr(III). The ESR signal of Cr(III) is weaker than in the Cr-MCM-41 sample ÔYCr2Õ with the same content of Cr, similar to the signal of vanadyl ions in the respective samples. The vanadyl signal in the sample ÔYV2Cr1Õ is stronger than that in the sample ÔYV1Cr2Õ, while the Cr(III) signal is weaker. Whereas the sample ÔYV2Cr2Õ shows one isotropic ESR signal just like the CrMCM-41 samples, the signal attributed to vanadyl ions is very weak. If the chromium content increases to Cr=Si ˆ 0:036 in the as-synthesized sample, the vanadyl signal is too weak to be observed. After calcination, the sample ÔYV1Cr2Õ shows only one hyper®ne signal of Cr(V) (Fig. 5d). This result shows that there is a competition relationship when V and Cr are incorporated simultaneously into MCM-41 samples, since V(IV) is sensitive to electron spin resonance due to its very high natural abundance.

Fig. 5. ESR spectra of Cr-containing samples (a) as-synthesized ÔYCr2Õ, (b) calcined ÔYCr2Õ, (c) as-synthesized ÔYV1Cr2Õ, and (d) calcined ÔYV1Cr2Õ. The signals of Mn reference are also shown in the spectra of (a) and (b) as pointed out with the arrows, and the sixth line of the Mn reference in the spectrum of (b) is not shown.

3.6. Di€use re¯ectance UV±VIS spectroscopy The as-synthesized MCM-41 molecular sieve samples synthesized with cetylpyridinium bromide in this study are white-yellow. The DR spectrum

Z.Y. Yuan et al. / Microporous and Mesoporous Materials 43 (2001) 227±236

of as-synthesized siliceous MCM-41 presents bands at 220, 260 and 420 nm, which interfere with the electron-absorption signals of metallosilicates. Upon calcination to remove the organic surfactant, the color of the siliceous samples is white, and the bands at 260 and 420 nm disappear. Therefore, the calcined metallosilicate samples cannot be a€ected by the cetylpyridinium bromide surfactant in the electron absorption spectroscopy, and the coordination state and property of these heteroatoms could still be re¯ected clearly. Fig. 6 shows the DR spectra of the V-, Cr-, and VCrMCM-41 samples before and after calcination. Assynthesized VMCM-41 shows one weak band at 340 nm, which can be attributed to the low-energy charge transfer band associated with V±O electron transfer for tetrahedrally coordinated and colorless V5‡ ions [17], suggesting the framework incorporation of vanadium. One very weak band at about 605 nm was assigned to the d±d transition of

Fig. 6. DR UV±VIS spectra of V and Cr substituted samples (a) as-synthesized ÔYV2Õ, (b) calcined ÔYV2Õ, (c) as-synthesized ÔYCr1Õ, (d) calcined ÔYCr1Õ, (e) as-synthesized ÔYV1Cr2Õ (f) calcined ÔYV1Cr2Õ.

235

VO2‡ ions [18]. This indicates that most of the VO2‡ ions were oxidized to V5‡ ions during synthesis. After calcination the band around 600 nm disappears due to the oxidation of V4‡ to V5‡ , and the sample shows one absorption band at 270 nm and one very broad band around 340±420 nm (broadening towards lower energy), indicating the charge transfer transition of the V@O double bond of square-pyramidal V5‡ ions coordinated to the framework, and the presence of V5‡ species with distorted octahedral coordination [17,18]. The DR UV±VIS spectrum for the calcined CrMCM-41, given in Fig. 6d, shows bands at 270 and 370 nm and a shoulder around 440 nm, besides a weak band around 620 nm. The bands centered at 440 and 620 nm can be assigned to the 4 A2g ! 4 T1g and 4 A2g ! 4 T2g transitions in the octahedral Cr(III) ions surrounded by four oxygen atoms of the framework and two oxygen atoms of H2 O in the channel, respectively [15,19], implying the presence of unoxidized Cr(III) framework sites even after calcination. The bands at 270 and 370 nm are usually assigned to O ! Cr(VI) charge transfer absorption bands [15], indicating that most of chromium is oxidized to Cr(VI) after calcination. The DR UV±VIS spectra of VCr-MCM-41 samples have both the characters of VMCM-41 and CrMCM-41 molecular sieves. One weak absorption band at 340 nm present in the as-synthesized sample, corresponding to a charge transfer band of V(V) in tetrahedral coordination, and the band around 640 nm should be assigned to the d±d transition of octahedral Cr(III), probably bonded to oxygen atoms of the framework. It is noted that the band based on the 4 A2g ! 4 T2g transition in the absorption spectra of VCr-MCM41 was shifted towards longer wavelengths, compared with that of Cr-MCM-41. This suggests that the bond strength between Cr(III) and the oxygen atoms of the VCr-MCM-41 framework is weaker than that between Cr(III) and the oxygen atoms of the Cr-MCM-41 framework, perhaps due to the simultaneous incorporation of vanadium in the VCr-MCM-41 silicate. Upon calcination the band at 340 nm disappeared and one new absorption band appeared at about 370 nm, indicating that Cr(III) ions were oxidized to Cr(VI) by heating.

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4. Conclusion Bimodal mesoporous siliceous and metallosilicate MCM-41 with a hierarchical structure of a paintbrush-like morphology have been synthesized in a mild alkaline condition, and the coexistence of two distinguished mesopores has been directly observed by using HRTEM technique. The introduction of the hetero-atoms a€ects the mesoporosity of the materials and structural stability. Vanadium and chromium can be incorporated into the tubular walls of the bimodal mesoporous materials through interaction with surface silanol groups. V and Cr atoms can also be simultaneously incorporated into the bimodal mesoporous MCM-41, but there exists one competitive relationship for V and Cr atoms in the synthesis of VCr-MCM-41.

Acknowledgements This work was supported by the National Natural Science Foundation of China.

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