- Email: [email protected]
A novel approach to prepare MCM-41 supported CuO catalyst with high metal loading and dispersion
Microporous and Mesoporous Materials 88 (2006) 38–47 www.elsevier.com/locate/micromeso
A novel approach to prepare MCM-41 supported CuO catalyst with high metal loading and dispersion Xiang-Ying Hao
a,b,c,*
, Yin-Qing Zhang a,b, Jun-Wei Wang Cui Zhang a,b, Shuangxi Liu a,b,*
a,b
, Wei Zhou
a,b
,
a
b
Institute of New Catalytic Materials Science, College of Chemistry, Nankai University, Tianjin 300071, People’s Republic of China Joint Research Institute of Nankai University and Tianjin University, Tianjin 300071, People’s Republic of China c College of Chemistry, Inner Mongolia Normal University, Huhhot 010022, People’s Republic of China Received 15 March 2005; received in revised form 25 July 2005; accepted 25 August 2005 Available online 3 October 2005
Abstract In the present research, CuO was abundantly and evenly dispersed onto the mesoporous molecular sieves MCM-41 via the following novel method: MCM-41 support was above all modified with organic silicanes; the organofunctionalized MCM-41 then coordinated with cupric ions; the ultrasonic washing and calcination were afterwards orderly performed on the as-synthesized sample. The resultant catalyst was characterized by nitrogen adsorption–desorption analysis, temperature-programmed reduction (TPR), transmission electron microscopy (TEM), powder X-ray diffraction (XRD), Fourier Transform infrared spectroscopy (FT-IR), and ICP-AES techniques. The influences of cupric ion concentration, different organic functional groups and the effects of the ultrasonic washing on the dispersion and loading amount of CuO were also investigated and discussed. A comparison was made between the present method and the simple ion-exchange of AlMCM-41 with copper nitrate. Shown by experiment, MCM-41 maintains its ordered mesoporous structure after the dispersion of CuO. Meanwhile, some CuO are dispersed on the external surface of MCM-41 in very small particles, while the others are dispersed inside the pores of MCM-41. The effective preparative strategy introduced in this paper is a very promising method for preparing MCM-41 supported metal oxide catalyst with high metal loading and dispersion. 2005 Elsevier Inc. All rights reserved. Keywords: MCM-41; Organofunctionalized; CuO; Loading; Dispersion
1. Introduction The discovery of periodic, ordered mesoporous molecular sieves M41s [1,2] received considerable attention in the heterogeneous catalysis area by applying these materials as support due to their high specific surface area (700– 1500 m2 g1) as well as their uniform pore size (varying from 1.5 to 10 nm). The synthesis of M41s can be a supra-
*
Corresponding authors. Address: Institute of New Catalytic Materials Science, College of Chemistry, Nankai University, Tianjin 300071, Peoples Republic of China. Tel./fax: +86 022 2350 9005 (X.-Y. Hao). E-mail addresses: [email protected] (X.-Y. Hao), sxliu@ nankai.edu.cn (S. Liu). 1387-1811/$ - see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.08.019
molecular self-assemble process in the presence of ionic surfactants as templates during the mesophase formation; the pore size can be tailored on the basis of the surfactant chain length and the synthetic conditions applied. In comparison with crystalline microporous zeolites, the M41s materials break through the pore size limit and provide the opportunity to process bulky molecules larger than 1.2 nm in diameter. A recent review on the preparation and essential properties of ordered mesoporous catalysts as well as the catalytic processes over such materials summarizes the literature up to 2004 [3]. Many previous researches have been documented in the introduction of transition metal elements into the mesoporous silica materials to prepare redox catalysts. Copper is particularly interesting due to its special redox properties and
X.-Y. Hao et al. / Microporous and Mesoporous Materials 88 (2006) 38–47
polarizability; copper-based catalysts have been therefore a central focus of recent research [4–22]. Grunert and coworkers [6–8] loaded Cu/ZnO on the PMS (periodic mesoporous silicates) by the conventional wet impregnation and metal-organic chemical vapour deposition (MOCVD) techniques, and nano-disperse CuO particles have been obtained. MCM-41 and MCM-48 materials containing copper and zinc were synthesized by Hartmann et al. [9,10]; the factors including copper concentration, the pore diameter of the supporting mesoporous material, and the presence of zinc are found to influence CuO dispersion and particle size. They also investigated the coordination geometry of the copper(II) complex in mesoporous siliceous MCM-41 material by ESR [11]. Kevan and coworkers studied Cu(II) ion coordination and the adsorbate interaction in ion-exchanged AlMCM-41 mesoporous materials by ESR and ESEM spectrometry; the result reveals some information about the nature of the Cu(II) ion-exchanged site in Cu–AlMCM-41 [12,23]. The presence of Cu, CuO, or Cu2O is related to the preparation method applied. Two preparative methods are generally available to introduce catalytically active species into the MCM-41 support: (1) metallic or non-metallic elements are added to the gel formed during the synthesis of MCM-41 prior to the hydrothermal treatment [5,24– 26] and, (ii) the post-synthesis modification of as-synthesized molecular sieves [8,15,27,28]. The direct synthesis may be disadvantageous since the introduced heteroatoms may lower the long-range order of the support by distorting the liquid crystalline array. The post-synthesis offers the opportunity to start with highly ordered MCM-41 with high specific surface area. The post-synthetic treatment involves two possible methods known as the grafting of precursor species under gas phase conditions and the wet impregnation technique. The gas phase introduction hardly affects the porous host, however, it sometimes results in relatively low loading content. On the other hand, the wet impregnation technique allows higher loading but requires very careful preparation [17]. This is because the host structure may be destroyed since the calcined MCM41 is not resistant to hydrothermal or solvothermal conditions. Thus the liquid phase treatment may bring about a local framework collapse. Meanwhile, the dispersion of metal species is difficult to control in the wet impregnation and tiny condensed oxide particles are commonly formed, even at relatively low metal concentration [29]. In the present work, a novel method to assemble MCM-41 supported CuO catalyst with high loading and dispersion is brought forward. Mesoporous MCM-41 was modified with organic functional groups. Then, copper(II) ions were grafted and fixed compactly onto the organofunctionalized MCM-41 via the coordination bonds. Ultrasonic washing was performed afterwards to remove the copper ions deposited or physisorbed on the surface of the MCM-41 support and to make the dispersion more uniform. Calcination of the catalyst was adopted to receive CuO/MCM-41 composite. Detailed characterization of
39
the catalyst was performed to acquire information about location and dispersion of the metal species on the MCM-41 support, which may influence the catalytic performance significantly. 2. Experimental 2.1. Materials Tetraethylorthosilicate (TEOS, A.R. 98%) was used as silica source, ammonia (25 wt.%, A.R.) as alkaline source, and the surfactant cetyltrimethylammonium bromide (CTABr, A.R.) as template for preparing MCM-41. N-[3(Trimethoxysilyl)-propyl]-ethylenediamine (97%, ACROS ORGANICS) and 3-aminopropyltrimethoxysilane (99%, ACROS ORGANICS) were the silicanes employed to organofunctionalize MCM-41. Cu(NO3)2 Æ 3H2O (A.R.) was the copper precursor. All the chemicals used in the present investigation were in the as-received forms without any further purification. 2.2. Synthesis of MCM-41 Parent silica-based MCM-41 was synthesized according to the literature method [30]. A typical synthetic procedure was carried out as follows: 60.0 mL of ammonia (25 wt.%) and 4.80 g of cetyltrimethylammonium bromide (CTABr, A.R.) were dissolved in 225 mL distilled water under vigorous stir until homogeneous. 25.2 mL of tetraethylorthosilicate (TEOS, A.R. 98%) was then added dropwise into the solution. After continuous 2-h-stir, a gel with a molar composition of SiO2:CTABr:NH4OH:H2O = 1:0.12:8:114 was obtained. The gel was then transferred into a Teflon-lined autoclave that was heated at 383 K for 72 h. The solid product was recovered by filtration, washed with deionized water until the filtrate was neutral, and dried in air overnight. The as-synthesized material was calcined at 823 K for 6 h in air with a temperature-programmed rate of 2 K min1. 2.3. Organofunctionalization of MCM-41 Two grams of calcined MCM-41 was suspended by 60 mL toluene in a round flask and then 3.0 mL of organic silicane was added dropwise. The refluxing lasted for 8 h and the resulting solid was recovered by centrifugation, washed carefully with ethanol and distilled water, and dried at room temperature for 12 h. In the rest of the paper, N-[3-(trimethoxysilyl)-propyl]ethylenediamine organofunctionalized MCM-41 will be noted as MCM-41-ed and 3aminopropyltrimethoxysilane organofunctionalized MCM-41 as MCM-41-am. 2.4. Immobilization of Cu2+ on MCM-41-ed and MCM-41-am Forty milliliters of Cu(NO3)2 solution (0.02 mol L1) was mixed with 1.0 g of MCM-41-ed or MCM-41-am.
40
X.-Y. Hao et al. / Microporous and Mesoporous Materials 88 (2006) 38–47
The mixture was then continuously stirred for 12 h at room temperature. The solid product was washed repeatedly with distilled water by taking advantage of an ultrasonic cleaner and finally dried at 323 K in a vacuum oven. Copper(II) immobilized MCM-41-ed and MCM-41-am are given, respectively, the denotations of MCM-ed-Cu(II) and MCM-am-Cu(II).
3.5. Transmission electron microscopy (TEM) Transmission electron microscopy (TEM) measurements were performed with a PHILIP EM400ST electron microscope operated at 100 kV. Sample material was mounted on a holey carbon film supported on a Cu grid by drying a droplet of a suspension of ground sample in ethanol on the grid.
2.5. Preparation of CuO/MCM-41 3.6. Temperature-programmed reduction (TPR) The MCM-ed-Cu(II) and MCM-am-Cu(II) (which appeared, respectively, blue and pale blue) were treated in a vacuum oven at 313 K for 8 h and were afterwards temperature-programmed calcined in a muffle furnace at 823 K for 6 h in air with a rate of 2 K min1. The calcined samples are abbreviated to CuO/MCM-41-ed and CuO/ MCM-41-am. 3. Characterization 3.1. Powder X-ray diffraction (XRD) The XRD data were collected on a Rigaku D/max-2500 diffractometer with Cu Ka radiation (40 kV and 100 mA). The phase development of CuO in the samples was examined in the scan range of 10 6 2h 6 70 and for MCM41 phase 1 6 2h 6 10.
The reducibility of the calcined copper catalyst was measured by the TPR method with a homebuilt apparatus. 100 mg of the catalyst (20–40 mesh) was mounted in a quartz tube and calcined in argon flow at 773 K for 1 h (temperature-programmed rate of 10 K min1) with the aim to remove the substances physisorbed. Then the sample was cooled to ambient temperature in argon before the reduction by a mixing gas of hydrogen and argon (5% H2 in volume percentage). In the H2-TPR analysis, the heating rate was 10 K min1 and the gas flow was 30 mL min1. The cold trap consisted of liquid nitrogen and acetone. The hydrogen consumed during the reduction was detected by a thermal conductivity detector. 4. Results and discussion 4.1. Powder X-ray diffraction
3.2. N2 adsorption–desorption analysis N2 adsorption–desorption analysis was operated at 77 K on a Micromeritics ASAP2010 micropore analysis system. About 100 mg of the sample was dehydrated in vacuum at 373 K for 1 h and then degassed at 473 K for 4 h. The analytical data were processed by the BET equation for the surface area and by Barrett–Joyner–Halenda (BJH) method for pore size distribution.
XRD patterns at low angles, from 1 to 10, of CuO/ MCM-41-am, CuO/MCM-41-ed, Cu–AlMCM-41, and parent MCM-41 are shown in Fig. 1. Three low-angle reflections corresponding to d100, d110 and d200, which are characteristic of the ordered mesoporous structure of hexagonal MCM-41 [1] are shown in Fig. 1. As is shown in Fig. 1b and c, the decrease of the corresponding d100 peak
3.3. Induction-coupled plasma-atomic emission spectrometry (ICP-AES) ICP-AES measurement was carried out by an IRIS Advantage ICAP full spectrum direct-reading emission spectrometer manufactured by TJA Solutions (US). To accomplish this measurement, the sample was quantified with great accuracy and adequately dissolved with requisite hydrogen fluoride. Then the solution was vapourized with the aim to remove silicon fluoride. The remaining residue was dissolved by 0.2 mol L1 hydrogen chloride and the solution formed was adjusted to weak acidity by ammonia. The resulting solution was diluted and was ready for ICPAES measurement. 3.4. Fourier transform infrared spectrometry (FT-IR) FT-IR spectra were taken on a BRUKER VECTOR 22 spectrometer using self-supporting wafers.
Fig. 1. XRD patterns of samples at low angles (a) parent MCM-41, (b) CuO/MCM-41-am, (c) CuO/MCM-41-ed, and (d) Cu–AlMCM-41 (Si/ Al = 11).
X.-Y. Hao et al. / Microporous and Mesoporous Materials 88 (2006) 38–47
intensity and the lack of the d210 peak, compared with parent MCM-41, reflect a less ordered hexagonal mesoporous structure [29]. However, organofunctionalized MCM-41 (not coordinated with copper ions), after calcination, maintained its typical hexagonal structure, with no obvious decrease in peak intensity observed. This indicates that the decrease in the peak intensity is mainly related to the introduction of metal species instead of the thermal instability of the support. Fig. 2 displays the higher-angle XRD patterns of CuO/ MCM-41-ed before and after ultrasonic washing treatment together with the mechanical mixture of parent MCM-41 with CuO. The peaks at 2h = 35.5, 38.7, and 48.7 are ascribed to the CuO phase. Intense and well resolved CuO peaks are observed in the CuO/MCM-41 XRD pattern before ultrasonic washing (Fig. 2b) which indicates the existence of relatively large CuO particles. Besides, the broad onsets of these peaks reveal that small CuO particles are also present. After ultrasonic washing, very weak and broad reflections are resolved in the CuO/MCM-41-ed catalyst (Fig. 2c), characteristic of very small CuO particles formed exclusively on the external surface of the MCM-41. The size of CuO particles treated with and without ultrasonic washing, calculated from the CuO reflections of XRD according to the Scherrel equation, was about 6 and 20 nm, respectively. The same trends were found in TEM: large CuO particles could be observed before ultrasonic washing, whereas after ultrasonic washing very small particles were found. The ultrasonic washing can remove the cupric ions deposited and/or adsorbed (not coordinated with organic functional groups) on the surface of the organofunctionalized MCM-41 as well as to make the ions dispersed more uniform. Therefore after calcination, the copper(II) complex coordinated with functional groups are transformed to the copper oxide on the nano-scale. The ratio of the corresponding diffraction peak area, which is related to the CuO loading before and after ultrasonic washing, amounts to 2.0, while the ICP-AES gives a basically consistent rate value of 2.1 (Table 2), indicating the quantity ratio of CuO before and after ultrasonic washing. In addition, comparing Fig. 1d with b and c, the ion-exchanged Cu–AlMCM-41 reveals a less ordered hexagonal
Fig. 2. XRD patterns of samples at high angles (a) mechanical mixture of MCM-41 and CuO (b) CuO/MCM-41-ed before ultrasonic washing, and (c) CuO/MCM-41-ed after ultrasonic washing.
41
mesoporous structure since the corresponding diffraction peaks are poorly resolved and less intensive. This may be attributed to the effect of introduction of heteroatoms into the silica-based mesoporous materials [29]. 4.2. BET surface area, pore volume and pore size Nitrogen adsorption–desorption is a common method to characterize mesoporous materials, which can provide information upon the specific surface area and the average pore diameter. Although the use of the BET theory for calculating small mesoporous pore diameters (<5 nm) is not perfect, it does offer proper information about the differences of various mesoporous materials; on the other hand, the BJH theory is commonly accepted for determining average pore diameters. These calculations are based on the Kelvin equation and underestimate the average diameter of small mesopores in the size range of 1–2 nm [31]. Comprehensive studies have shown that BJH compared to other theories for pore size calculations like Saito–Foley (SF) [32,33] or density functional theory (DFT) gives more appropriate results to determine the differences between pristine and modified MCM-41 silica materials. Figs. 3 and 4 show the nitrogen adsorption–desorption isotherms and BJH pore size distribution of CuO/MCM-41-ed before and after ultrasonic washing and parent MCM-41. These isotherms are of type IV according to the IUPAC classification of adsorption isotherms [34], typical of mesoporous solids. A well defined step occurs in p/p0 range of 0.2–0.3 that represents the spontaneous filling of the mesopores due to capillary condensation. An inflection at p/p0 0.2– 0.3 in the CuO/MCM-41-ed isotherm compared with that of parent MCM-41 at p/p0 0.25–0.35 is obviously representative of mesopores filling. It is shown that the pore diameter of the mesopores in CuO/MCM-41-ed (Fig. 4) is smaller than that of parent MCM-41 (Fig. 4a) since the inflection position on the p/p0 axis is closely related to
Fig. 3. Nitrogen adsorption/desorption isotherms at 77 K of calcined MCM-41 and CuO/MCM-ed.
42
X.-Y. Hao et al. / Microporous and Mesoporous Materials 88 (2006) 38–47
Fig. 4. Pore diameter distribution of calcined MCM-41, CuO/MCM-41ed (bf) before ultrasonic washing, and CuO/MCM-41-ed (af) after ultrasonic washing.
the pore diameter, which is also evidenced by the BJH distribution (Fig. 4). The amount of physisorbed nitrogen decreases when the loading of CuO on mesoporous material is high; on the contrary, the volume of nitrogen increases if the ultrasonic washing is used, indicating cupric ions that physisorbed on the organofunctionalized MCM-41 have been removed. The decrease in pore size in comparison with parent MCM-41 is attributed to the dispersion of CuO inside the mesoporous MCM-41 channels. Since CuO/MCM-41 preserves the mesoporosity, in combination with the data on pore diameter distribution, it is reasonable to conclude that the coating in the inner surface of MCM-41 was successful. Nevertheless, complete filling of the pores could not be observed in any case of the host/guest compounds. The BET surface area, pore volume and pore size of the sample conformably decrease after dispersing CuO into the mesoporous MCM-41. Table 1 shows these physical parameters for the samples before and after ultrasonic washing and parent MCM-41. Two causes may account for the specific surface area reduction: the intrapore formation of the copper oxide nano-particles within the mesopores reduces the mean pore diameter and meanwhile and, the introduction of metallic elements onto MCM-41 decreases the specific surface area can be also ascribed to an increase in density of the composites after loading copper elements.
Table 1 Structural parameters derived from nitrogen adsorption–desorption data for different samples SBET (m2/g) Vp (cm3/g) dBJHa (nm) wdb (nm) MCM-41 CuO/MCM41-ed (bf) CuO/MCM41-ed (af) a b
705 129 605
BJH average pore diameter. Primary mesopore diameter.
0.475 0.122 0.470
2.76 2.99 2.63
2.62 1.93 2.28
The structural parameters of CuO/MCM-41-ed and parent silica MCM-41 derived form isothermal nitrogen adsorption data are given in Table 1. The pore size decrease of CuO/MCM-41-ed (after ultrasonic washing) compared with parent MCM-41 reflects that some CuO particles are dispersed inside the channels of MCM-41. However, the BET surface area and pore size of CuO/MCM-41-ed evidently decrease before ultrasonic washing compared with parent MCM-41, which is mainly because of the poreblocking in the support MCM-41. The specific surface area and pore volume have an evident increase after the ultrasonic washing, indicating that copper ions absorbed on the channel surface can be removed from the surface of the support. A similar behaviour can be deduced from TEM. According to the data of pore diameters, the thickness of CuO layer can be calculated to be about 0.17 nm. Compared the calculated thickness with the data of CuO crystallite (the radii of Cu2+ and O2 are 0.070 nm and 0.126 nm, respectively), it is therefore concluded that a part of the CuO particles are dispersed on the internal surface of the MCM-41, and the molecular assembly of CuO within the pores indicates that the method employed here was successful. The pore diameter of MCM-41 calculated from BJH method is 2.76 nm. Obviously, the channels of MCM-41 would not limit the dispersion of CuO considering rO2 = 0.126 nm and rCu2+ = 0.070 nm. Assuming that CuO particles are highly dispersed on the internal surface of MCM-41 in the form of close-packed monolayer [27], the inner surface area occupied by each CuO unit can be calculated by the following equation: 2 d 2 A¼p ¼ p ð0:196 nmÞ ¼ 1:21 1019 m2 ð1Þ 2 where d stands for the dynamic diameter of CuO, i.e., the diameter of a single CuO ‘‘molecule’’ and A is the internal surface area occupied by each CuO unit. Since the specific surface area of MCM-41 is 705 m2 g1, every gram of MCM-41 is capable of accommodating 5.83 · 1021 CuO species: N¼
S 705 ¼ ¼ 5:83 1021 A 1:21 1019
ð2Þ
where N symbolizes the molecules number of CuO dispersed inside the pores of MCM-41 and S is the specific surface area of MCM-41. Based on Eqs. (1) and (2), the theoretical capacity of CuO inside MCM-41 channels is calculated as N 5:83 1021 W ¼ M ¼ 79:5 ¼ 0:750 g 6:02 1023 6:02 1023 ð3Þ where W represents the theoretical amount of CuO dispersed on each gram of MCM-41 support and M is the molecular weight of CuO.
X.-Y. Hao et al. / Microporous and Mesoporous Materials 88 (2006) 38–47
43
Notably, the theoretical value is larger than the actual amount given by ICP-AES analysis: 0.08 g CuO for each gram of CuO/MCM-41-ed and 0.13 g for CuO/MCM-41am. Therefore, CuO does not actually disperse on the surface of MCM-41 as a close-packed monolayer. There is just a sub-monolayer of CuO on the internal surface of MCM41 due to the size-confine of support inside the pore system, i.e., only part of the internal surface of MCM-41 is occupied by the dispersed CuO. Voegtlin et al. [35] have determined the external surface area of MCM-41 by nitrogen adsorption analysis of template-containing samples and found a maximum value of 10 m2 g1. Employing a value of 10 m2 g1 for the external surface area, by means of a simple calculation we learn that if the copper oxide particles were located exclusively at the external surface 3–7 monolayers, depending on orientation, external deposited CuO would occupy about 5 wt.% of the total loading amount. Therefore the very small copper oxide particles present on the catalyst can by far not be located at the external surface of the support, because there is simply not enough space for them. Consequently, combining the N2 adsorption, TEM and XRD results, it can be concluded that some CuO particles are located inside the mesopores of MCM-41, while the other very small CuO particles are dispersed on the external surface of the MCM-41.
sonic washing. The above result is basically consistent with that acquired from XRD and nitrogen adsorption–desorption analysis. Similar results can be obtained from CuO/ MCM-41-ed.
4.3. Transmission electron microscopy (TEM)
4.4. Fourier transform infrared spectrometry (FT-IR)
TEM micrographs of CuO/MCM-41-am treated with and without ultrasonic washing are given in Figs. 5 and 6. Fig. 5 shows small CuO particles are decorated on the external surface of the MCM-41 support and are slightly agglomerated. In Fig. 6, it can be clearly observed that the regular and ordered straight-channel characteristics of MCM-41 [1] are preserved and unaffected by the presence of CuO particles. Furthermore, in Fig. 6, the CuO particles are more uniformly dispersed on the external surface of the MCM-41 with sizes from 2 nm to 5 nm as a result of ultra-
Fig. 7 shows the FT-IR spectra in the scan range from 1200 to 4000 cm1 for template-free MCM-41, organofunctionalized MCM-41, organofunctionalized MCM-41 coordinated with Cu(II), and calcined catalyst CuO/ MCM-41. The IR spectrum of template free MCM-41 shows specific bands at 1082, 796, and 455 cm1 (not shown). Bands at similar wave numbers in the spectra of crystalline and amorphous SiO2 have been assigned to characteristic vibrations of Si–O–Si bridges crosslinking
Fig. 5. TEM image of CuO/MCM-41-am before ultrasonic washing.
Fig. 6. TEM image of CuO/MCM-41-am after ultrasonic washing.
Fig. 7. FT-IR spectra of (a) template free MCM-41, (b) MCM-41-ed, (c) MCM-ed-Cu(II), and (d) calcined CuO/MCM-41-ed.
44
X.-Y. Hao et al. / Microporous and Mesoporous Materials 88 (2006) 38–47
the silicate network [21]. These absorption peaks characterizing the MCM-41 framework are maintained after the dispersion of CuO. The broad band around 3450 cm1 is attributed to the adsorbed H2O molecules, while deformational vibrations of adsorbed molecules have the absorption bands at 1633 cm1 [24]. The peak at 3746 cm1 is ascribed to the fundamental stretching vibrations of terminal Si–OH group [24], which is invisible after the condensation of Si–OH groups with organic silicane and subsequent coordination with cupric ions. However, the peak is regenerated after calcination is performed on the metal-coordinated organicfunctionalized samples. The successful modification of –(CH2)3NH(CH2)2NH2 organic functional group is reflected in the IR peaks at high wave number part: the absorption bands at 2926 and 2850 cm1 corresponding to the asymmetric and symmetric vibrations of –CH2 as well as absorption bands at 1463, 1411 and 1349 cm1, respectively. The coordination of cupric ions with amino groups alters the constant k in the simple harmonic vibration model. Therefore, the vibration frequency and intensity will change as well. A transformation of metal nitrate into metal oxide is reflected in the decrease of the IR peak at 1380 cm1 corresponding to NO 3 [28]. The analysis drawn from the IR spectra is illustrated in Fig. 10. The IR spectra of aminopropyl modified MCM41, which are not shown, allow similar conclusion.
Fig. 8. A comparison with the reduction plot of bulky CuO, obtained from Cu(NO3)2 by calcination, is also included. Bulky CuO is reduced in one step, the onset temperature being 593 K. Both CuO/MCM-41-ed and CuO/MCM-41-am catalysts display two reduction stages as in Fig. 8. The first low-temperature reduction stage (533 K in Fig. 8b and 556 K in Fig. 8c) is ascribed to the presence of some larger particles, which are most likely located at and relatively weakly interact with the external surface of the MCM-41. The second reduction stage (718 K in Fig. 8b and 629 K in Fig. 8c) is associated with the presence of very small copper oxide particles that have a more intense interaction with support, stabilised by the MCM-41 framework and demonstrated by the high temperature of reduction. These particles are most likely located at the internal surface of the MCM-41. These results are in agreement with the literature [36,37]. Therefore, the TPR process is most likely a one-step reduction. Grunert and coworkers also investigated the reduction of copper oxide species dispersed in mesoporous MCM-48, without the detection of intermediate Cu(I) [38]. On the other hand, it clearly appears from the TPR profiles that the peak area of CuO/MCM-41-am is larger due to more consumption of hydrogen, indicating more abundant CuO in CuO/MCM-41-am than its diamino functionalized counterpart. This conclusion is in agreement with ICP-AES data (Table 2). Summarizing from
4.5. Temperature-programmed reduction (TPR) TPR was used to attain information about the reducibility and the state of the deposited copper oxide of the calcined catalysts. It is often difficult to compare results obtained in different laboratories or reported literature data since there is no general agreement on the optimum experimental parameters for conducting TPR experiments such as the heating rate, the composition of the reducing gas mixture, and their flow-rate. These factors can, apart from the particle size, greatly affect the rate of reduction [9]. Since TPR is a bulky process, not all particles are exposed to the reducing gas at the same time and thus a dependence on Tm (temperature at maximum) on the particle size is expected. For bulky oxides, Tm increases with increasing particle size. However, metal oxides supported on inert carriers may exhibit different reduction behaviour compared with the unsupported counterparts. TPR profiles of supported copper catalysts are in general more complex than that of bulky CuO because there exists a wide range of variables including the particle size of CuO and its interaction with the matrix. The effects of particle size and the interaction of the particles with the matrix however are generally inconsistent. A decrease in the size of the metal oxide particles makes the reduction faster due to a higher surface area/volume ratio while smaller particles interact more intensely with the support and slow down the reduction. The cooperative consequence lies on the competition of the two factors. TPR profiles of the CuO/MCM-41-am, CuO/MCM-41-ed, and Cu–AlMCM-41 are shown in
Fig. 8. H2-TPR profiles of (a) Cu–AlMCM-41 (Si/Al = 11), (b) CuO/ MCM-41-ed, (c) CuO/MCM-41-am, and (d) bulk CuO.
Table 2 The results of ICP-AES analysis Catalysts
CuO/MCM-41-ed CuO/MCM-41-am Cu–AlMCM-41 (Si/Al = 11)
Cu content (%) Before ultrasonic washing
After ultrasonic washing
13.8 21.8 –
6.43 10.5 2.05
X.-Y. Hao et al. / Microporous and Mesoporous Materials 88 (2006) 38–47
TPR analysis, one may conclude that some CuO particles are dispersed inside the MCM-41 pore system while others exist on the external surface of the MCM-41. The reduction plot of the Cu–AlMCM-41 catalyst is given in Fig. 8a. Two peaks with equivalent area are observed in this profile. The first peak at 587 K is ascribed to the reduction of the exchanged Cu2+ to Cu+. The hightemperature reduction peak at 795 K represents Cu+ to Cu0. 4.6. ICP-AES In this study all the other experiment conditions were held constant except for the concentration of Cu(NO3)2. The loading amount of CuO along with varying concentration of Cu(NO3)2 is analyzed by ICP-AES and is plotted in Fig. 9 (investigation was made on 3-aminopropyltrimeth-
Fig. 9. Content of CuO on MCM-41 supports vs different concentration of Cu(NO3)2.
45
oxysilane organofunctionalized MCM-41). The figure clearly shows the loading amount of CuO increases along with the increase of the concentration of Cu(NO3)2 however, it tends to be an invariable maximum when Cu(NO3)2 concentration arrives around 0.45 mol L1 due to maximum occupation of amino groups. Table 2 lists the ICP-AES analysis results of CuO/ MCM-41 loaded by using different organic functional groups and Cu–AlMCM-41 by ion-exchange of AlMCM41 with copper nitrate. Comparing two loading methods (organofunctionalized and ion-exchanged methods), obviously, the CuO content loaded by using organofunctionalized method is higher than that loaded by ion-exchanged method. In general, the intensity of the d100 reflection in the XRD patterns of incorporating heteroatoms into the Si framework of MCM-41 tends to decrease with the amount of incorporated metal [29]. It is also confirmed by our XRD results. This shows the difficulty to obtain well defined, highly ordered mesoporous materials with high metal contents by ion-exchange. Meanwhile, AlMCM-41 modified with transition ions is unsuitable to the catalytic processes that require basic support due to its acidity. Concerning the organofunctionalized method, the effect of the different organic groups is shown in Table 2. The CuO content loaded on the 3-aminopropyltrimethoxysilane organofunctionalized MCM-41 is higher than that modified by N-[3-(trimeth-oxysilyl)propyl]ethylenediamine. The loading amount of CuO dispersed on MCM-41 is related both to the amount of organic functional groups grafted onto the MCM-41 support and the coordination state of cupric ions with organic groups, as is illustrated in Fig. 10. In the non-aqueous solvent, methoxyls of the grafted organic silicanes condense with the hydroxyls to the surface of the pore wall of MCM-41 during the organofunctionalization. Subsequently the cupric ions coordinate with the amino groups and H2O molecules are supplementary ligands, through which the active sites are introduced
Fig. 10. Possible coordinations of cupric ions with organofunctionalized MCM-41 by 3-aminopropyltrimethoxysilane and N-[3-(trimethoxysilyl)propyl]ethylenediamine (L = H2O).
46
X.-Y. Hao et al. / Microporous and Mesoporous Materials 88 (2006) 38–47
and fixed. The ultrasonic washing can remove the cupric ions deposited and/or adsorbed (not coordinated with organic functional group) on the surface of the organofunctionalized MCM-41 as well as to make the ions dispersed more uniform. Then the copper oxide is formed on the surface of the MCM-41 support during calcination of the copper(II) complex functionalized MCM-41, and dispersed uniformly at nano-size on the surface of the MCM-41. The different CuO loading in the two organofunctionalized MCM-41 supports may be attributed to different coordination states: the cupric ions are likely to be chelated by the diamine species and thus the fixed cupric ions are less. The possible coordination complexes are illustrated in Fig. 10 marked as (a), (b), (c) and (d). When propylamine species are grafted, coordination types (a) and (b) are possible. The formation of type (b) involves the exact orientation of two adjacent flexible organic chains, which is of low possibility. When the diamine species are grafted, the coordination type (d) is dominant due to the stability of this chelate; the coordination type (c) is also very likely to occur as a stable five-membered ring with a chelate structure. Under the present experimental conditions, the organic functional groups grafted are in excess. It is reasonable to believe the two types of grafted organic functional groups are approximately the same in molar amount. In theory, the cupric ions immobilized on MCM-41-am are twice as many compared to that immobilized on MCM-41-ed assuming the exaggerated situation that only type (d) is present when Cu2+ is fixed on MCM-41-ed. The ICPAES analysis data (Table 2) reveals however, a ratio less than 2 (13.08/8.041 = 1.6) which probably resulted from the high concentration of coordination type (c). 5. Conclusion Organofunctionalizing the parent MCM-41 to coordinate with cupric ions and using ultrasonic washing play the key roles in this novel approach. The ultrasonic washing is capable of removing cupric ions deposited or adhered (not coordinated with organic functional group) on the surface of the organofunctionalized MCM-41 as well as making the dispersion more uniform. The loading content varies when different organic functional groups are employed due to different coordination states. The prepared MCM-41 supported CuO catalyst possesses high metal loading and uniform dispersion. The characterization information acquired by XRD, nitrogen adsorption–desorption, TEM, TPR, FT-IR and ICP-AES techniques demonstrate that some CuO are dispersed inside the pores of MCM-41, while the others are dispersed on the external surface in nano-size. The typical structural properties of mesoporous material are preserved after the dispersion of CuO. From the metal loading and the type of support as well as its structure point of view, the present novel strategy is advantageous over simple ion-exchange of AlMCM-41 with Cu(II).
Acknowledgements We appreciate the financial support to this research by the National Science Foundation of China (Grant Nos. 29973016 and 20233030) and the Ministry of Education of PR China. References [1] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [2] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.-W. Chu, D.H. Olsen, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. [3] A. Taguchi, F. Schuth, Micropor. Mesopor. Mater. 77 (2005) 1. [4] L.Z. Wang, S. Velu, S. Tomura, F. Ohashi, K. Suzuki, M. Okazaki, T. Osaki, M. Maeda, J. Mater. Sci. 37 (2002) 801. [5] Z.P. Li, L. Gao, J. Phys. Chem. Solids 64 (2003) 223. [6] R. Becker, H. Parala, F. Hipler, O.P. Tkachenko, K.V. Klementiev, W. Gru¨nert, H. Wilmer, O. Hinrichsen, M. Muhler, A. Birkner, C. Wo¨ll, S. Scha¨fer, R.A. Fischer, Angew. Chem. Int. Ed. 43 (2004) 2839. [7] O.P. Tkachenko, K.V. Klementiev, E. Loffler, I. Ritzkopf, F. Schuth, M. Bandyopadhyay, S. Grabowski, H. Gies, V. Hagen, M. Muhler, L. Lu, R.A. Fischer, W. Grunert, Phys. Chem. Chem. Phys. 5 (2003) 4325. [8] H. Fies, S. Grabowski, M. Bandyopadhyay, W. Grunert, O.P. Tkachenko, K.V. Klementiev, A. Birkner, Micropor. Mesopor. Mater. 60 (2003) 31. [9] M. Hartmann, S. Racouchot, C. Bischof, Micropor. Mesopor. Mater. 27 (1999) 309. [10] M. Hartmann, S. Racouchot, C. Bischof, Chem. Commun. (1997) 2367. [11] A. Poppl, M. Hartmann, W. Bohlmann, R. Bottcher, J. Phys. Chem. B 102 (1998) 7752. [12] M. Hartmann, A. Poppl, L. Kevan, J. Phys. Chem. 99 (1995) 17251. [13] M. Hartmann, Studies in Surface Science and Catalysis, vol. 128, Elsevier, Amsterdam, 2000, p. 215. [14] S. Wellach, M. Hartmann, S. Ernst, J. Weitkamp, in: Proceedings of the 12th International Zeolite Conference, vol. II, Baltimore, 1998, p. 1409. [15] K. Hadjiivanov, T. Tsoncheva, M. Dimitrov, C. Minchev, H. Knozinger, Appl. Catal. A: Gen. 241 (2003) 331. [16] S. Zheng, L. Gao, J.K. Guo, J. Solid State Chem. 152 (2000) 447. [17] R. Kohn, M. Froba, Catal. Today 68 (2001) 227. [18] J. Okamura, S. Nishiyama, S. Tsuruya, M. Masai, J. Mol. Catal. A 135 (1998) 133. [19] J. Xu, J. Yu, S.J. Lee, B.Y. Kim, L. Kevan, J. Phys. Chem. B 104 (2000) 1307. [20] M. Ziolek, I. Sobczak, P. Nowak, Studies in Surface Science and Catalysis, vol. 125, Elsevier, Amsterdam, 1999, p. 633. [21] Shan Zheng, Lian Gao, Jingkun Gao, Mater. Chem. Phys. 71 (2001) 174. [22] S.K. Bhargava, D.B. Akolekar, J. Colloid Interface Sci. 281 (2005) 171. [23] A. Pdppl, M. Newhouse, L. Kevan, J. Phys. Chem. 99 (1995) 10019. [24] M. Selvarej, A. Pandurangan, K.S. Seshadri, Appl. Catal. A: Gen. 242 (2003) 347. [25] S. Velu, L. Wang, M. Okazaki, K. Suzuki, S. Tomura, Micropor. Mesopor. Mater. 54 (2002) 113. [26] I. Sobczak, M. Ziolek, M. Renn, P. Decyk, I. Nowak, M. Katuri, J.C. Lavalley, Micropor. Mesopor. Mater. 74 (2004) 23. [27] Z.P. Li, L. Gao, S. Zheng, Appl. Catal. A: Gen. 236 (2002) 163. [28] W.J. Lu, G.Z. Lu, Y. Luo, J. Mol. Catal. A: Chem. 188 (2002) 225.
X.-Y. Hao et al. / Microporous and Mesoporous Materials 88 (2006) 38–47 [29] N. Lang, P. Delichere, A. Tuel, Micropor. Mesopor. Mater. 56 (2002) 203. [30] T. Asefa, M.J. Maclachlan, N. Coombs, G.A. Ozin, Nature 402 (1999) 867. [31] U. Ciesla, F. Schu¨th, Micropor. Mesopor. Mater. 27 (1999) 131. [32] A. Saito, H.C. Foley, Micropor. Mater. 3 (1994) 337. [33] A. Saito, H.C. Foley, Micropor. Mater. 3 (1995) 531. [34] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T. Siemieniewska, Pure Appl. Chem. 57 (1985) 603.
47
[35] A.C. Voegtlin, A. Matijasic, J. Patarin, C. Sauerland, Y. Grillet, L. Huve, Micropor. Mater. 10 (1997) 137. [36] D.J. Lensveld, J.G. Mesu, A.J. Dillen, K.P. Jong, Micropor. Mesopor. Mater. 44–45 (2001) 401. [37] X.Y. Hao, W. Zhou, J.W. Wang, Y.Q. Zhang, S.X.. Liu, Chem. Lett. 7 (2005). [38] O.P. Tkachenko, K.V. Klementiev, N. Koc, X. Yu, M. Bandyopadhyay, S. Grabowski, H. Gies, W. Grunert, Studies in Surface Science and Catalysis, vol. 154, Elsevier, Amsterdam, 2004, p. 1670.
A novel approach to prepare MCM-41 supported CuO catalyst with high metal loading and dispersion Xiang-Ying Hao
a,b,c,*
, Yin-Qing Zhang a,b, Jun-Wei Wang Cui Zhang a,b, Shuangxi Liu a,b,*
a,b
, Wei Zhou
a,b
,
a
b
Institute of New Catalytic Materials Science, College of Chemistry, Nankai University, Tianjin 300071, People’s Republic of China Joint Research Institute of Nankai University and Tianjin University, Tianjin 300071, People’s Republic of China c College of Chemistry, Inner Mongolia Normal University, Huhhot 010022, People’s Republic of China Received 15 March 2005; received in revised form 25 July 2005; accepted 25 August 2005 Available online 3 October 2005
Abstract In the present research, CuO was abundantly and evenly dispersed onto the mesoporous molecular sieves MCM-41 via the following novel method: MCM-41 support was above all modified with organic silicanes; the organofunctionalized MCM-41 then coordinated with cupric ions; the ultrasonic washing and calcination were afterwards orderly performed on the as-synthesized sample. The resultant catalyst was characterized by nitrogen adsorption–desorption analysis, temperature-programmed reduction (TPR), transmission electron microscopy (TEM), powder X-ray diffraction (XRD), Fourier Transform infrared spectroscopy (FT-IR), and ICP-AES techniques. The influences of cupric ion concentration, different organic functional groups and the effects of the ultrasonic washing on the dispersion and loading amount of CuO were also investigated and discussed. A comparison was made between the present method and the simple ion-exchange of AlMCM-41 with copper nitrate. Shown by experiment, MCM-41 maintains its ordered mesoporous structure after the dispersion of CuO. Meanwhile, some CuO are dispersed on the external surface of MCM-41 in very small particles, while the others are dispersed inside the pores of MCM-41. The effective preparative strategy introduced in this paper is a very promising method for preparing MCM-41 supported metal oxide catalyst with high metal loading and dispersion. 2005 Elsevier Inc. All rights reserved. Keywords: MCM-41; Organofunctionalized; CuO; Loading; Dispersion
1. Introduction The discovery of periodic, ordered mesoporous molecular sieves M41s [1,2] received considerable attention in the heterogeneous catalysis area by applying these materials as support due to their high specific surface area (700– 1500 m2 g1) as well as their uniform pore size (varying from 1.5 to 10 nm). The synthesis of M41s can be a supra-
*
Corresponding authors. Address: Institute of New Catalytic Materials Science, College of Chemistry, Nankai University, Tianjin 300071, Peoples Republic of China. Tel./fax: +86 022 2350 9005 (X.-Y. Hao). E-mail addresses: [email protected] (X.-Y. Hao), sxliu@ nankai.edu.cn (S. Liu). 1387-1811/$ - see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.08.019
molecular self-assemble process in the presence of ionic surfactants as templates during the mesophase formation; the pore size can be tailored on the basis of the surfactant chain length and the synthetic conditions applied. In comparison with crystalline microporous zeolites, the M41s materials break through the pore size limit and provide the opportunity to process bulky molecules larger than 1.2 nm in diameter. A recent review on the preparation and essential properties of ordered mesoporous catalysts as well as the catalytic processes over such materials summarizes the literature up to 2004 [3]. Many previous researches have been documented in the introduction of transition metal elements into the mesoporous silica materials to prepare redox catalysts. Copper is particularly interesting due to its special redox properties and
X.-Y. Hao et al. / Microporous and Mesoporous Materials 88 (2006) 38–47
polarizability; copper-based catalysts have been therefore a central focus of recent research [4–22]. Grunert and coworkers [6–8] loaded Cu/ZnO on the PMS (periodic mesoporous silicates) by the conventional wet impregnation and metal-organic chemical vapour deposition (MOCVD) techniques, and nano-disperse CuO particles have been obtained. MCM-41 and MCM-48 materials containing copper and zinc were synthesized by Hartmann et al. [9,10]; the factors including copper concentration, the pore diameter of the supporting mesoporous material, and the presence of zinc are found to influence CuO dispersion and particle size. They also investigated the coordination geometry of the copper(II) complex in mesoporous siliceous MCM-41 material by ESR [11]. Kevan and coworkers studied Cu(II) ion coordination and the adsorbate interaction in ion-exchanged AlMCM-41 mesoporous materials by ESR and ESEM spectrometry; the result reveals some information about the nature of the Cu(II) ion-exchanged site in Cu–AlMCM-41 [12,23]. The presence of Cu, CuO, or Cu2O is related to the preparation method applied. Two preparative methods are generally available to introduce catalytically active species into the MCM-41 support: (1) metallic or non-metallic elements are added to the gel formed during the synthesis of MCM-41 prior to the hydrothermal treatment [5,24– 26] and, (ii) the post-synthesis modification of as-synthesized molecular sieves [8,15,27,28]. The direct synthesis may be disadvantageous since the introduced heteroatoms may lower the long-range order of the support by distorting the liquid crystalline array. The post-synthesis offers the opportunity to start with highly ordered MCM-41 with high specific surface area. The post-synthetic treatment involves two possible methods known as the grafting of precursor species under gas phase conditions and the wet impregnation technique. The gas phase introduction hardly affects the porous host, however, it sometimes results in relatively low loading content. On the other hand, the wet impregnation technique allows higher loading but requires very careful preparation [17]. This is because the host structure may be destroyed since the calcined MCM41 is not resistant to hydrothermal or solvothermal conditions. Thus the liquid phase treatment may bring about a local framework collapse. Meanwhile, the dispersion of metal species is difficult to control in the wet impregnation and tiny condensed oxide particles are commonly formed, even at relatively low metal concentration [29]. In the present work, a novel method to assemble MCM-41 supported CuO catalyst with high loading and dispersion is brought forward. Mesoporous MCM-41 was modified with organic functional groups. Then, copper(II) ions were grafted and fixed compactly onto the organofunctionalized MCM-41 via the coordination bonds. Ultrasonic washing was performed afterwards to remove the copper ions deposited or physisorbed on the surface of the MCM-41 support and to make the dispersion more uniform. Calcination of the catalyst was adopted to receive CuO/MCM-41 composite. Detailed characterization of
39
the catalyst was performed to acquire information about location and dispersion of the metal species on the MCM-41 support, which may influence the catalytic performance significantly. 2. Experimental 2.1. Materials Tetraethylorthosilicate (TEOS, A.R. 98%) was used as silica source, ammonia (25 wt.%, A.R.) as alkaline source, and the surfactant cetyltrimethylammonium bromide (CTABr, A.R.) as template for preparing MCM-41. N-[3(Trimethoxysilyl)-propyl]-ethylenediamine (97%, ACROS ORGANICS) and 3-aminopropyltrimethoxysilane (99%, ACROS ORGANICS) were the silicanes employed to organofunctionalize MCM-41. Cu(NO3)2 Æ 3H2O (A.R.) was the copper precursor. All the chemicals used in the present investigation were in the as-received forms without any further purification. 2.2. Synthesis of MCM-41 Parent silica-based MCM-41 was synthesized according to the literature method [30]. A typical synthetic procedure was carried out as follows: 60.0 mL of ammonia (25 wt.%) and 4.80 g of cetyltrimethylammonium bromide (CTABr, A.R.) were dissolved in 225 mL distilled water under vigorous stir until homogeneous. 25.2 mL of tetraethylorthosilicate (TEOS, A.R. 98%) was then added dropwise into the solution. After continuous 2-h-stir, a gel with a molar composition of SiO2:CTABr:NH4OH:H2O = 1:0.12:8:114 was obtained. The gel was then transferred into a Teflon-lined autoclave that was heated at 383 K for 72 h. The solid product was recovered by filtration, washed with deionized water until the filtrate was neutral, and dried in air overnight. The as-synthesized material was calcined at 823 K for 6 h in air with a temperature-programmed rate of 2 K min1. 2.3. Organofunctionalization of MCM-41 Two grams of calcined MCM-41 was suspended by 60 mL toluene in a round flask and then 3.0 mL of organic silicane was added dropwise. The refluxing lasted for 8 h and the resulting solid was recovered by centrifugation, washed carefully with ethanol and distilled water, and dried at room temperature for 12 h. In the rest of the paper, N-[3-(trimethoxysilyl)-propyl]ethylenediamine organofunctionalized MCM-41 will be noted as MCM-41-ed and 3aminopropyltrimethoxysilane organofunctionalized MCM-41 as MCM-41-am. 2.4. Immobilization of Cu2+ on MCM-41-ed and MCM-41-am Forty milliliters of Cu(NO3)2 solution (0.02 mol L1) was mixed with 1.0 g of MCM-41-ed or MCM-41-am.
40
X.-Y. Hao et al. / Microporous and Mesoporous Materials 88 (2006) 38–47
The mixture was then continuously stirred for 12 h at room temperature. The solid product was washed repeatedly with distilled water by taking advantage of an ultrasonic cleaner and finally dried at 323 K in a vacuum oven. Copper(II) immobilized MCM-41-ed and MCM-41-am are given, respectively, the denotations of MCM-ed-Cu(II) and MCM-am-Cu(II).
3.5. Transmission electron microscopy (TEM) Transmission electron microscopy (TEM) measurements were performed with a PHILIP EM400ST electron microscope operated at 100 kV. Sample material was mounted on a holey carbon film supported on a Cu grid by drying a droplet of a suspension of ground sample in ethanol on the grid.
2.5. Preparation of CuO/MCM-41 3.6. Temperature-programmed reduction (TPR) The MCM-ed-Cu(II) and MCM-am-Cu(II) (which appeared, respectively, blue and pale blue) were treated in a vacuum oven at 313 K for 8 h and were afterwards temperature-programmed calcined in a muffle furnace at 823 K for 6 h in air with a rate of 2 K min1. The calcined samples are abbreviated to CuO/MCM-41-ed and CuO/ MCM-41-am. 3. Characterization 3.1. Powder X-ray diffraction (XRD) The XRD data were collected on a Rigaku D/max-2500 diffractometer with Cu Ka radiation (40 kV and 100 mA). The phase development of CuO in the samples was examined in the scan range of 10 6 2h 6 70 and for MCM41 phase 1 6 2h 6 10.
The reducibility of the calcined copper catalyst was measured by the TPR method with a homebuilt apparatus. 100 mg of the catalyst (20–40 mesh) was mounted in a quartz tube and calcined in argon flow at 773 K for 1 h (temperature-programmed rate of 10 K min1) with the aim to remove the substances physisorbed. Then the sample was cooled to ambient temperature in argon before the reduction by a mixing gas of hydrogen and argon (5% H2 in volume percentage). In the H2-TPR analysis, the heating rate was 10 K min1 and the gas flow was 30 mL min1. The cold trap consisted of liquid nitrogen and acetone. The hydrogen consumed during the reduction was detected by a thermal conductivity detector. 4. Results and discussion 4.1. Powder X-ray diffraction
3.2. N2 adsorption–desorption analysis N2 adsorption–desorption analysis was operated at 77 K on a Micromeritics ASAP2010 micropore analysis system. About 100 mg of the sample was dehydrated in vacuum at 373 K for 1 h and then degassed at 473 K for 4 h. The analytical data were processed by the BET equation for the surface area and by Barrett–Joyner–Halenda (BJH) method for pore size distribution.
XRD patterns at low angles, from 1 to 10, of CuO/ MCM-41-am, CuO/MCM-41-ed, Cu–AlMCM-41, and parent MCM-41 are shown in Fig. 1. Three low-angle reflections corresponding to d100, d110 and d200, which are characteristic of the ordered mesoporous structure of hexagonal MCM-41 [1] are shown in Fig. 1. As is shown in Fig. 1b and c, the decrease of the corresponding d100 peak
3.3. Induction-coupled plasma-atomic emission spectrometry (ICP-AES) ICP-AES measurement was carried out by an IRIS Advantage ICAP full spectrum direct-reading emission spectrometer manufactured by TJA Solutions (US). To accomplish this measurement, the sample was quantified with great accuracy and adequately dissolved with requisite hydrogen fluoride. Then the solution was vapourized with the aim to remove silicon fluoride. The remaining residue was dissolved by 0.2 mol L1 hydrogen chloride and the solution formed was adjusted to weak acidity by ammonia. The resulting solution was diluted and was ready for ICPAES measurement. 3.4. Fourier transform infrared spectrometry (FT-IR) FT-IR spectra were taken on a BRUKER VECTOR 22 spectrometer using self-supporting wafers.
Fig. 1. XRD patterns of samples at low angles (a) parent MCM-41, (b) CuO/MCM-41-am, (c) CuO/MCM-41-ed, and (d) Cu–AlMCM-41 (Si/ Al = 11).
X.-Y. Hao et al. / Microporous and Mesoporous Materials 88 (2006) 38–47
intensity and the lack of the d210 peak, compared with parent MCM-41, reflect a less ordered hexagonal mesoporous structure [29]. However, organofunctionalized MCM-41 (not coordinated with copper ions), after calcination, maintained its typical hexagonal structure, with no obvious decrease in peak intensity observed. This indicates that the decrease in the peak intensity is mainly related to the introduction of metal species instead of the thermal instability of the support. Fig. 2 displays the higher-angle XRD patterns of CuO/ MCM-41-ed before and after ultrasonic washing treatment together with the mechanical mixture of parent MCM-41 with CuO. The peaks at 2h = 35.5, 38.7, and 48.7 are ascribed to the CuO phase. Intense and well resolved CuO peaks are observed in the CuO/MCM-41 XRD pattern before ultrasonic washing (Fig. 2b) which indicates the existence of relatively large CuO particles. Besides, the broad onsets of these peaks reveal that small CuO particles are also present. After ultrasonic washing, very weak and broad reflections are resolved in the CuO/MCM-41-ed catalyst (Fig. 2c), characteristic of very small CuO particles formed exclusively on the external surface of the MCM-41. The size of CuO particles treated with and without ultrasonic washing, calculated from the CuO reflections of XRD according to the Scherrel equation, was about 6 and 20 nm, respectively. The same trends were found in TEM: large CuO particles could be observed before ultrasonic washing, whereas after ultrasonic washing very small particles were found. The ultrasonic washing can remove the cupric ions deposited and/or adsorbed (not coordinated with organic functional groups) on the surface of the organofunctionalized MCM-41 as well as to make the ions dispersed more uniform. Therefore after calcination, the copper(II) complex coordinated with functional groups are transformed to the copper oxide on the nano-scale. The ratio of the corresponding diffraction peak area, which is related to the CuO loading before and after ultrasonic washing, amounts to 2.0, while the ICP-AES gives a basically consistent rate value of 2.1 (Table 2), indicating the quantity ratio of CuO before and after ultrasonic washing. In addition, comparing Fig. 1d with b and c, the ion-exchanged Cu–AlMCM-41 reveals a less ordered hexagonal
Fig. 2. XRD patterns of samples at high angles (a) mechanical mixture of MCM-41 and CuO (b) CuO/MCM-41-ed before ultrasonic washing, and (c) CuO/MCM-41-ed after ultrasonic washing.
41
mesoporous structure since the corresponding diffraction peaks are poorly resolved and less intensive. This may be attributed to the effect of introduction of heteroatoms into the silica-based mesoporous materials [29]. 4.2. BET surface area, pore volume and pore size Nitrogen adsorption–desorption is a common method to characterize mesoporous materials, which can provide information upon the specific surface area and the average pore diameter. Although the use of the BET theory for calculating small mesoporous pore diameters (<5 nm) is not perfect, it does offer proper information about the differences of various mesoporous materials; on the other hand, the BJH theory is commonly accepted for determining average pore diameters. These calculations are based on the Kelvin equation and underestimate the average diameter of small mesopores in the size range of 1–2 nm [31]. Comprehensive studies have shown that BJH compared to other theories for pore size calculations like Saito–Foley (SF) [32,33] or density functional theory (DFT) gives more appropriate results to determine the differences between pristine and modified MCM-41 silica materials. Figs. 3 and 4 show the nitrogen adsorption–desorption isotherms and BJH pore size distribution of CuO/MCM-41-ed before and after ultrasonic washing and parent MCM-41. These isotherms are of type IV according to the IUPAC classification of adsorption isotherms [34], typical of mesoporous solids. A well defined step occurs in p/p0 range of 0.2–0.3 that represents the spontaneous filling of the mesopores due to capillary condensation. An inflection at p/p0 0.2– 0.3 in the CuO/MCM-41-ed isotherm compared with that of parent MCM-41 at p/p0 0.25–0.35 is obviously representative of mesopores filling. It is shown that the pore diameter of the mesopores in CuO/MCM-41-ed (Fig. 4) is smaller than that of parent MCM-41 (Fig. 4a) since the inflection position on the p/p0 axis is closely related to
Fig. 3. Nitrogen adsorption/desorption isotherms at 77 K of calcined MCM-41 and CuO/MCM-ed.
42
X.-Y. Hao et al. / Microporous and Mesoporous Materials 88 (2006) 38–47
Fig. 4. Pore diameter distribution of calcined MCM-41, CuO/MCM-41ed (bf) before ultrasonic washing, and CuO/MCM-41-ed (af) after ultrasonic washing.
the pore diameter, which is also evidenced by the BJH distribution (Fig. 4). The amount of physisorbed nitrogen decreases when the loading of CuO on mesoporous material is high; on the contrary, the volume of nitrogen increases if the ultrasonic washing is used, indicating cupric ions that physisorbed on the organofunctionalized MCM-41 have been removed. The decrease in pore size in comparison with parent MCM-41 is attributed to the dispersion of CuO inside the mesoporous MCM-41 channels. Since CuO/MCM-41 preserves the mesoporosity, in combination with the data on pore diameter distribution, it is reasonable to conclude that the coating in the inner surface of MCM-41 was successful. Nevertheless, complete filling of the pores could not be observed in any case of the host/guest compounds. The BET surface area, pore volume and pore size of the sample conformably decrease after dispersing CuO into the mesoporous MCM-41. Table 1 shows these physical parameters for the samples before and after ultrasonic washing and parent MCM-41. Two causes may account for the specific surface area reduction: the intrapore formation of the copper oxide nano-particles within the mesopores reduces the mean pore diameter and meanwhile and, the introduction of metallic elements onto MCM-41 decreases the specific surface area can be also ascribed to an increase in density of the composites after loading copper elements.
Table 1 Structural parameters derived from nitrogen adsorption–desorption data for different samples SBET (m2/g) Vp (cm3/g) dBJHa (nm) wdb (nm) MCM-41 CuO/MCM41-ed (bf) CuO/MCM41-ed (af) a b
705 129 605
BJH average pore diameter. Primary mesopore diameter.
0.475 0.122 0.470
2.76 2.99 2.63
2.62 1.93 2.28
The structural parameters of CuO/MCM-41-ed and parent silica MCM-41 derived form isothermal nitrogen adsorption data are given in Table 1. The pore size decrease of CuO/MCM-41-ed (after ultrasonic washing) compared with parent MCM-41 reflects that some CuO particles are dispersed inside the channels of MCM-41. However, the BET surface area and pore size of CuO/MCM-41-ed evidently decrease before ultrasonic washing compared with parent MCM-41, which is mainly because of the poreblocking in the support MCM-41. The specific surface area and pore volume have an evident increase after the ultrasonic washing, indicating that copper ions absorbed on the channel surface can be removed from the surface of the support. A similar behaviour can be deduced from TEM. According to the data of pore diameters, the thickness of CuO layer can be calculated to be about 0.17 nm. Compared the calculated thickness with the data of CuO crystallite (the radii of Cu2+ and O2 are 0.070 nm and 0.126 nm, respectively), it is therefore concluded that a part of the CuO particles are dispersed on the internal surface of the MCM-41, and the molecular assembly of CuO within the pores indicates that the method employed here was successful. The pore diameter of MCM-41 calculated from BJH method is 2.76 nm. Obviously, the channels of MCM-41 would not limit the dispersion of CuO considering rO2 = 0.126 nm and rCu2+ = 0.070 nm. Assuming that CuO particles are highly dispersed on the internal surface of MCM-41 in the form of close-packed monolayer [27], the inner surface area occupied by each CuO unit can be calculated by the following equation: 2 d 2 A¼p ¼ p ð0:196 nmÞ ¼ 1:21 1019 m2 ð1Þ 2 where d stands for the dynamic diameter of CuO, i.e., the diameter of a single CuO ‘‘molecule’’ and A is the internal surface area occupied by each CuO unit. Since the specific surface area of MCM-41 is 705 m2 g1, every gram of MCM-41 is capable of accommodating 5.83 · 1021 CuO species: N¼
S 705 ¼ ¼ 5:83 1021 A 1:21 1019
ð2Þ
where N symbolizes the molecules number of CuO dispersed inside the pores of MCM-41 and S is the specific surface area of MCM-41. Based on Eqs. (1) and (2), the theoretical capacity of CuO inside MCM-41 channels is calculated as N 5:83 1021 W ¼ M ¼ 79:5 ¼ 0:750 g 6:02 1023 6:02 1023 ð3Þ where W represents the theoretical amount of CuO dispersed on each gram of MCM-41 support and M is the molecular weight of CuO.
X.-Y. Hao et al. / Microporous and Mesoporous Materials 88 (2006) 38–47
43
Notably, the theoretical value is larger than the actual amount given by ICP-AES analysis: 0.08 g CuO for each gram of CuO/MCM-41-ed and 0.13 g for CuO/MCM-41am. Therefore, CuO does not actually disperse on the surface of MCM-41 as a close-packed monolayer. There is just a sub-monolayer of CuO on the internal surface of MCM41 due to the size-confine of support inside the pore system, i.e., only part of the internal surface of MCM-41 is occupied by the dispersed CuO. Voegtlin et al. [35] have determined the external surface area of MCM-41 by nitrogen adsorption analysis of template-containing samples and found a maximum value of 10 m2 g1. Employing a value of 10 m2 g1 for the external surface area, by means of a simple calculation we learn that if the copper oxide particles were located exclusively at the external surface 3–7 monolayers, depending on orientation, external deposited CuO would occupy about 5 wt.% of the total loading amount. Therefore the very small copper oxide particles present on the catalyst can by far not be located at the external surface of the support, because there is simply not enough space for them. Consequently, combining the N2 adsorption, TEM and XRD results, it can be concluded that some CuO particles are located inside the mesopores of MCM-41, while the other very small CuO particles are dispersed on the external surface of the MCM-41.
sonic washing. The above result is basically consistent with that acquired from XRD and nitrogen adsorption–desorption analysis. Similar results can be obtained from CuO/ MCM-41-ed.
4.3. Transmission electron microscopy (TEM)
4.4. Fourier transform infrared spectrometry (FT-IR)
TEM micrographs of CuO/MCM-41-am treated with and without ultrasonic washing are given in Figs. 5 and 6. Fig. 5 shows small CuO particles are decorated on the external surface of the MCM-41 support and are slightly agglomerated. In Fig. 6, it can be clearly observed that the regular and ordered straight-channel characteristics of MCM-41 [1] are preserved and unaffected by the presence of CuO particles. Furthermore, in Fig. 6, the CuO particles are more uniformly dispersed on the external surface of the MCM-41 with sizes from 2 nm to 5 nm as a result of ultra-
Fig. 7 shows the FT-IR spectra in the scan range from 1200 to 4000 cm1 for template-free MCM-41, organofunctionalized MCM-41, organofunctionalized MCM-41 coordinated with Cu(II), and calcined catalyst CuO/ MCM-41. The IR spectrum of template free MCM-41 shows specific bands at 1082, 796, and 455 cm1 (not shown). Bands at similar wave numbers in the spectra of crystalline and amorphous SiO2 have been assigned to characteristic vibrations of Si–O–Si bridges crosslinking
Fig. 5. TEM image of CuO/MCM-41-am before ultrasonic washing.
Fig. 6. TEM image of CuO/MCM-41-am after ultrasonic washing.
Fig. 7. FT-IR spectra of (a) template free MCM-41, (b) MCM-41-ed, (c) MCM-ed-Cu(II), and (d) calcined CuO/MCM-41-ed.
44
X.-Y. Hao et al. / Microporous and Mesoporous Materials 88 (2006) 38–47
the silicate network [21]. These absorption peaks characterizing the MCM-41 framework are maintained after the dispersion of CuO. The broad band around 3450 cm1 is attributed to the adsorbed H2O molecules, while deformational vibrations of adsorbed molecules have the absorption bands at 1633 cm1 [24]. The peak at 3746 cm1 is ascribed to the fundamental stretching vibrations of terminal Si–OH group [24], which is invisible after the condensation of Si–OH groups with organic silicane and subsequent coordination with cupric ions. However, the peak is regenerated after calcination is performed on the metal-coordinated organicfunctionalized samples. The successful modification of –(CH2)3NH(CH2)2NH2 organic functional group is reflected in the IR peaks at high wave number part: the absorption bands at 2926 and 2850 cm1 corresponding to the asymmetric and symmetric vibrations of –CH2 as well as absorption bands at 1463, 1411 and 1349 cm1, respectively. The coordination of cupric ions with amino groups alters the constant k in the simple harmonic vibration model. Therefore, the vibration frequency and intensity will change as well. A transformation of metal nitrate into metal oxide is reflected in the decrease of the IR peak at 1380 cm1 corresponding to NO 3 [28]. The analysis drawn from the IR spectra is illustrated in Fig. 10. The IR spectra of aminopropyl modified MCM41, which are not shown, allow similar conclusion.
Fig. 8. A comparison with the reduction plot of bulky CuO, obtained from Cu(NO3)2 by calcination, is also included. Bulky CuO is reduced in one step, the onset temperature being 593 K. Both CuO/MCM-41-ed and CuO/MCM-41-am catalysts display two reduction stages as in Fig. 8. The first low-temperature reduction stage (533 K in Fig. 8b and 556 K in Fig. 8c) is ascribed to the presence of some larger particles, which are most likely located at and relatively weakly interact with the external surface of the MCM-41. The second reduction stage (718 K in Fig. 8b and 629 K in Fig. 8c) is associated with the presence of very small copper oxide particles that have a more intense interaction with support, stabilised by the MCM-41 framework and demonstrated by the high temperature of reduction. These particles are most likely located at the internal surface of the MCM-41. These results are in agreement with the literature [36,37]. Therefore, the TPR process is most likely a one-step reduction. Grunert and coworkers also investigated the reduction of copper oxide species dispersed in mesoporous MCM-48, without the detection of intermediate Cu(I) [38]. On the other hand, it clearly appears from the TPR profiles that the peak area of CuO/MCM-41-am is larger due to more consumption of hydrogen, indicating more abundant CuO in CuO/MCM-41-am than its diamino functionalized counterpart. This conclusion is in agreement with ICP-AES data (Table 2). Summarizing from
4.5. Temperature-programmed reduction (TPR) TPR was used to attain information about the reducibility and the state of the deposited copper oxide of the calcined catalysts. It is often difficult to compare results obtained in different laboratories or reported literature data since there is no general agreement on the optimum experimental parameters for conducting TPR experiments such as the heating rate, the composition of the reducing gas mixture, and their flow-rate. These factors can, apart from the particle size, greatly affect the rate of reduction [9]. Since TPR is a bulky process, not all particles are exposed to the reducing gas at the same time and thus a dependence on Tm (temperature at maximum) on the particle size is expected. For bulky oxides, Tm increases with increasing particle size. However, metal oxides supported on inert carriers may exhibit different reduction behaviour compared with the unsupported counterparts. TPR profiles of supported copper catalysts are in general more complex than that of bulky CuO because there exists a wide range of variables including the particle size of CuO and its interaction with the matrix. The effects of particle size and the interaction of the particles with the matrix however are generally inconsistent. A decrease in the size of the metal oxide particles makes the reduction faster due to a higher surface area/volume ratio while smaller particles interact more intensely with the support and slow down the reduction. The cooperative consequence lies on the competition of the two factors. TPR profiles of the CuO/MCM-41-am, CuO/MCM-41-ed, and Cu–AlMCM-41 are shown in
Fig. 8. H2-TPR profiles of (a) Cu–AlMCM-41 (Si/Al = 11), (b) CuO/ MCM-41-ed, (c) CuO/MCM-41-am, and (d) bulk CuO.
Table 2 The results of ICP-AES analysis Catalysts
CuO/MCM-41-ed CuO/MCM-41-am Cu–AlMCM-41 (Si/Al = 11)
Cu content (%) Before ultrasonic washing
After ultrasonic washing
13.8 21.8 –
6.43 10.5 2.05
X.-Y. Hao et al. / Microporous and Mesoporous Materials 88 (2006) 38–47
TPR analysis, one may conclude that some CuO particles are dispersed inside the MCM-41 pore system while others exist on the external surface of the MCM-41. The reduction plot of the Cu–AlMCM-41 catalyst is given in Fig. 8a. Two peaks with equivalent area are observed in this profile. The first peak at 587 K is ascribed to the reduction of the exchanged Cu2+ to Cu+. The hightemperature reduction peak at 795 K represents Cu+ to Cu0. 4.6. ICP-AES In this study all the other experiment conditions were held constant except for the concentration of Cu(NO3)2. The loading amount of CuO along with varying concentration of Cu(NO3)2 is analyzed by ICP-AES and is plotted in Fig. 9 (investigation was made on 3-aminopropyltrimeth-
Fig. 9. Content of CuO on MCM-41 supports vs different concentration of Cu(NO3)2.
45
oxysilane organofunctionalized MCM-41). The figure clearly shows the loading amount of CuO increases along with the increase of the concentration of Cu(NO3)2 however, it tends to be an invariable maximum when Cu(NO3)2 concentration arrives around 0.45 mol L1 due to maximum occupation of amino groups. Table 2 lists the ICP-AES analysis results of CuO/ MCM-41 loaded by using different organic functional groups and Cu–AlMCM-41 by ion-exchange of AlMCM41 with copper nitrate. Comparing two loading methods (organofunctionalized and ion-exchanged methods), obviously, the CuO content loaded by using organofunctionalized method is higher than that loaded by ion-exchanged method. In general, the intensity of the d100 reflection in the XRD patterns of incorporating heteroatoms into the Si framework of MCM-41 tends to decrease with the amount of incorporated metal [29]. It is also confirmed by our XRD results. This shows the difficulty to obtain well defined, highly ordered mesoporous materials with high metal contents by ion-exchange. Meanwhile, AlMCM-41 modified with transition ions is unsuitable to the catalytic processes that require basic support due to its acidity. Concerning the organofunctionalized method, the effect of the different organic groups is shown in Table 2. The CuO content loaded on the 3-aminopropyltrimethoxysilane organofunctionalized MCM-41 is higher than that modified by N-[3-(trimeth-oxysilyl)propyl]ethylenediamine. The loading amount of CuO dispersed on MCM-41 is related both to the amount of organic functional groups grafted onto the MCM-41 support and the coordination state of cupric ions with organic groups, as is illustrated in Fig. 10. In the non-aqueous solvent, methoxyls of the grafted organic silicanes condense with the hydroxyls to the surface of the pore wall of MCM-41 during the organofunctionalization. Subsequently the cupric ions coordinate with the amino groups and H2O molecules are supplementary ligands, through which the active sites are introduced
Fig. 10. Possible coordinations of cupric ions with organofunctionalized MCM-41 by 3-aminopropyltrimethoxysilane and N-[3-(trimethoxysilyl)propyl]ethylenediamine (L = H2O).
46
X.-Y. Hao et al. / Microporous and Mesoporous Materials 88 (2006) 38–47
and fixed. The ultrasonic washing can remove the cupric ions deposited and/or adsorbed (not coordinated with organic functional group) on the surface of the organofunctionalized MCM-41 as well as to make the ions dispersed more uniform. Then the copper oxide is formed on the surface of the MCM-41 support during calcination of the copper(II) complex functionalized MCM-41, and dispersed uniformly at nano-size on the surface of the MCM-41. The different CuO loading in the two organofunctionalized MCM-41 supports may be attributed to different coordination states: the cupric ions are likely to be chelated by the diamine species and thus the fixed cupric ions are less. The possible coordination complexes are illustrated in Fig. 10 marked as (a), (b), (c) and (d). When propylamine species are grafted, coordination types (a) and (b) are possible. The formation of type (b) involves the exact orientation of two adjacent flexible organic chains, which is of low possibility. When the diamine species are grafted, the coordination type (d) is dominant due to the stability of this chelate; the coordination type (c) is also very likely to occur as a stable five-membered ring with a chelate structure. Under the present experimental conditions, the organic functional groups grafted are in excess. It is reasonable to believe the two types of grafted organic functional groups are approximately the same in molar amount. In theory, the cupric ions immobilized on MCM-41-am are twice as many compared to that immobilized on MCM-41-ed assuming the exaggerated situation that only type (d) is present when Cu2+ is fixed on MCM-41-ed. The ICPAES analysis data (Table 2) reveals however, a ratio less than 2 (13.08/8.041 = 1.6) which probably resulted from the high concentration of coordination type (c). 5. Conclusion Organofunctionalizing the parent MCM-41 to coordinate with cupric ions and using ultrasonic washing play the key roles in this novel approach. The ultrasonic washing is capable of removing cupric ions deposited or adhered (not coordinated with organic functional group) on the surface of the organofunctionalized MCM-41 as well as making the dispersion more uniform. The loading content varies when different organic functional groups are employed due to different coordination states. The prepared MCM-41 supported CuO catalyst possesses high metal loading and uniform dispersion. The characterization information acquired by XRD, nitrogen adsorption–desorption, TEM, TPR, FT-IR and ICP-AES techniques demonstrate that some CuO are dispersed inside the pores of MCM-41, while the others are dispersed on the external surface in nano-size. The typical structural properties of mesoporous material are preserved after the dispersion of CuO. From the metal loading and the type of support as well as its structure point of view, the present novel strategy is advantageous over simple ion-exchange of AlMCM-41 with Cu(II).
Acknowledgements We appreciate the financial support to this research by the National Science Foundation of China (Grant Nos. 29973016 and 20233030) and the Ministry of Education of PR China. References [1] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [2] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.-W. Chu, D.H. Olsen, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. [3] A. Taguchi, F. Schuth, Micropor. Mesopor. Mater. 77 (2005) 1. [4] L.Z. Wang, S. Velu, S. Tomura, F. Ohashi, K. Suzuki, M. Okazaki, T. Osaki, M. Maeda, J. Mater. Sci. 37 (2002) 801. [5] Z.P. Li, L. Gao, J. Phys. Chem. Solids 64 (2003) 223. [6] R. Becker, H. Parala, F. Hipler, O.P. Tkachenko, K.V. Klementiev, W. Gru¨nert, H. Wilmer, O. Hinrichsen, M. Muhler, A. Birkner, C. Wo¨ll, S. Scha¨fer, R.A. Fischer, Angew. Chem. Int. Ed. 43 (2004) 2839. [7] O.P. Tkachenko, K.V. Klementiev, E. Loffler, I. Ritzkopf, F. Schuth, M. Bandyopadhyay, S. Grabowski, H. Gies, V. Hagen, M. Muhler, L. Lu, R.A. Fischer, W. Grunert, Phys. Chem. Chem. Phys. 5 (2003) 4325. [8] H. Fies, S. Grabowski, M. Bandyopadhyay, W. Grunert, O.P. Tkachenko, K.V. Klementiev, A. Birkner, Micropor. Mesopor. Mater. 60 (2003) 31. [9] M. Hartmann, S. Racouchot, C. Bischof, Micropor. Mesopor. Mater. 27 (1999) 309. [10] M. Hartmann, S. Racouchot, C. Bischof, Chem. Commun. (1997) 2367. [11] A. Poppl, M. Hartmann, W. Bohlmann, R. Bottcher, J. Phys. Chem. B 102 (1998) 7752. [12] M. Hartmann, A. Poppl, L. Kevan, J. Phys. Chem. 99 (1995) 17251. [13] M. Hartmann, Studies in Surface Science and Catalysis, vol. 128, Elsevier, Amsterdam, 2000, p. 215. [14] S. Wellach, M. Hartmann, S. Ernst, J. Weitkamp, in: Proceedings of the 12th International Zeolite Conference, vol. II, Baltimore, 1998, p. 1409. [15] K. Hadjiivanov, T. Tsoncheva, M. Dimitrov, C. Minchev, H. Knozinger, Appl. Catal. A: Gen. 241 (2003) 331. [16] S. Zheng, L. Gao, J.K. Guo, J. Solid State Chem. 152 (2000) 447. [17] R. Kohn, M. Froba, Catal. Today 68 (2001) 227. [18] J. Okamura, S. Nishiyama, S. Tsuruya, M. Masai, J. Mol. Catal. A 135 (1998) 133. [19] J. Xu, J. Yu, S.J. Lee, B.Y. Kim, L. Kevan, J. Phys. Chem. B 104 (2000) 1307. [20] M. Ziolek, I. Sobczak, P. Nowak, Studies in Surface Science and Catalysis, vol. 125, Elsevier, Amsterdam, 1999, p. 633. [21] Shan Zheng, Lian Gao, Jingkun Gao, Mater. Chem. Phys. 71 (2001) 174. [22] S.K. Bhargava, D.B. Akolekar, J. Colloid Interface Sci. 281 (2005) 171. [23] A. Pdppl, M. Newhouse, L. Kevan, J. Phys. Chem. 99 (1995) 10019. [24] M. Selvarej, A. Pandurangan, K.S. Seshadri, Appl. Catal. A: Gen. 242 (2003) 347. [25] S. Velu, L. Wang, M. Okazaki, K. Suzuki, S. Tomura, Micropor. Mesopor. Mater. 54 (2002) 113. [26] I. Sobczak, M. Ziolek, M. Renn, P. Decyk, I. Nowak, M. Katuri, J.C. Lavalley, Micropor. Mesopor. Mater. 74 (2004) 23. [27] Z.P. Li, L. Gao, S. Zheng, Appl. Catal. A: Gen. 236 (2002) 163. [28] W.J. Lu, G.Z. Lu, Y. Luo, J. Mol. Catal. A: Chem. 188 (2002) 225.
X.-Y. Hao et al. / Microporous and Mesoporous Materials 88 (2006) 38–47 [29] N. Lang, P. Delichere, A. Tuel, Micropor. Mesopor. Mater. 56 (2002) 203. [30] T. Asefa, M.J. Maclachlan, N. Coombs, G.A. Ozin, Nature 402 (1999) 867. [31] U. Ciesla, F. Schu¨th, Micropor. Mesopor. Mater. 27 (1999) 131. [32] A. Saito, H.C. Foley, Micropor. Mater. 3 (1994) 337. [33] A. Saito, H.C. Foley, Micropor. Mater. 3 (1995) 531. [34] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T. Siemieniewska, Pure Appl. Chem. 57 (1985) 603.
47
[35] A.C. Voegtlin, A. Matijasic, J. Patarin, C. Sauerland, Y. Grillet, L. Huve, Micropor. Mater. 10 (1997) 137. [36] D.J. Lensveld, J.G. Mesu, A.J. Dillen, K.P. Jong, Micropor. Mesopor. Mater. 44–45 (2001) 401. [37] X.Y. Hao, W. Zhou, J.W. Wang, Y.Q. Zhang, S.X.. Liu, Chem. Lett. 7 (2005). [38] O.P. Tkachenko, K.V. Klementiev, N. Koc, X. Yu, M. Bandyopadhyay, S. Grabowski, H. Gies, W. Grunert, Studies in Surface Science and Catalysis, vol. 154, Elsevier, Amsterdam, 2004, p. 1670.