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Effect of ball milling on the structural and textural features of MCM-41 mesoporous material
Microporous and Mesoporous Materials 218 (2015) 153e159
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Effect of ball milling on the structural and textural features of MCM-41 mesoporous material Bahaa M. Abu-Zied a, b, c, *, Wilhelm Schwieger d, Abdullah M. Asiri a, b a
Center of Excellence for Advanced Materials Research (CEAMR), King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia Chemistry Department, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia c Chemistry Department, Faculty of Science, Assiut University, 71516 Assiut, Egypt d €t Erlangen-Nürnberg, Egerlandstraße 3, 91058 Erlangen, Germany Lehrstuhl Für Chemische Reaktionstechnik, Friedrich-Alexander-Universita b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 March 2015 Received in revised form 13 July 2015 Accepted 15 July 2015 Available online 22 July 2015
This paper reports on the ball-milling process and its influence on the structural, textural and morphological features of mesoporous MCM-41 material. MCM-41 and the as-milled products were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), N2-adsorption, a field emission scanning electron microscope (FE-SEM) and a thermogravimetric analysis (TGA). The XRD showed that the mechanical treatment of MCM-41 in a ball mill leads to a gradual crystallinity loss with time until 2 h, and complete crystallinity loss was observed after 4 h of ball milling. N2 sorption measurements revealed a sharp surface area decrease as well as mesoporosity loss accompanying the milling process. The FE-SEM investigation demonstrated severe damage of the spherical morphology of MCM-41 upon milling for 8 h. Moreover, it was found that these structural, textural and morphological changes strongly affected the water content of the milled MCM-41 material. © 2015 Elsevier Inc. All rights reserved.
Keywords: MCM-41 Ball milling Mesoporous material Texture analysis
1. Introduction Mesoporous silica, MCMe41, belongs to a group of mesoporous materials known as M41S, and it is characterized by its onedimensional hexagonally ordered pore structure. MCM-41 has high specific surface area (~1000 m2/g) and pore volume (~1 cm3/ g), very narrow pore size distribution, tunable pore size between 1 nm and 10 nm or more, adjustable hydrophobicity and very good thermal stability [1e3]. However, mesoporous MCM-41 possesses a neutral framework, a weak surface acidity and a poor hydrothermal stability, which limit its application in industry and as a catalyst. In this context, many efforts have been made to improve its acidity and stability, including (i) the incorporation of active metal species into MCM-41 by direct synthesis or impregnation methods and (ii) the addition of other materials with acidic features into the MCM41 matrix. During the last few years, many transition metals and main group elements have been introduced into MCM-41, showing
* Corresponding author. Chemistry Department, Faculty of Science, Assiut University, 71516 Assiut, Egypt. Tel.: þ20 882412429; fax: þ20 88 2342708. E-mail addresses: [email protected], [email protected], babuzied@ yahoo.com (B.M. Abu-Zied). http://dx.doi.org/10.1016/j.micromeso.2015.07.012 1387-1811/© 2015 Elsevier Inc. All rights reserved.
interesting catalytic applications. In this way, Li et al. [4] prepared a series of Fe-MCM-41 with high iron content, up to 13.21 wt. %, using a direct hydrothermal method under alkaline conditions. These samples exhibited excellent catalytic performance for oxidative desulfurization (ODS) of dibenzothiophene (DBT) by hydrogen peroxide at 50 C, and the sulfur removal did not decline after being recycled four times. Mn-MCM-41 powders with different Mn/Si molar ratios were prepared using the incipient wetness method [5]. The catalytic activity of the prepared systems towards complete nhexane oxidation revealed that the efficiency of this type of catalytic system increases with temperature and manganese quantity [5]. Recently, increased interest has been sparked in the synthesis and application of mesoporous MCM-41 composites with microporous molecular sieves. A composite molecular sieve with biporous structure, i.e., mesoemicro porous materials, combined with the benefits of the individual pore size regime could potentially improve the acidity and hydrothermal stability of molecular sieve catalysts [6,7]. In this regard, Jiang et al. [7] synthesized a Y/ MCM-41 composite molecular sieve using kaolin as the main silica source via a hydrothermal method. They demonstrated that, after calcination at 800 C for 3 h or hydrothermal treatment at 100 C for 10 days, the mesoporous framework in the composite
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still maintained. Moreover, based on NH3eTPD measurements, it was shown that more strong acid sites on the H-Y/MCM-41 sample occur than on the H-MCM-41 sample. Ball milling is a common physical technique used to produce subemicrometer particles. During the milling process, the grinding media is accelerated to higher velocities that provide a high transfer of kinetic energy from the balls to the sample, resulting in the production of fine powders. Ball sizes, rotation speed, and milling time have great influence on the experimental results. Using this technique, Hampsey et al. [8] obtained micrometer-to subemicrometeresized mesoporous silica particles starting from a silica xerogel prepared by a surfactant selfassembly solegel process. Concurrently, Hedayati et al. [9] prepared a good dispersion and distribution of nano-silica particles with ball milling. Ball milling is a practical method for reducing the size of zeolites crystals. Smaller zeolite crystals have higher catalytic effectiveness, lower coke formation, faster diffusion, higher external surface area and easier template extraction [10]. The changes occurring in the structural and textural features accompanying the ball milling of Y, X, A, and ZSM-5 zeolites have been well investigated [10e13]. In this context, Zielinski et al. [13] investigated the effect of milling time on the properties of NaA, CaA, KL, Silicalite-1, H-ZSM-5 and HY zeolites using high-energy ball milling. Their results indicated the direct correlation of the mechanical resistance of the zeolite and its Si/Al ratio. Moreover, the smaller-pore zeolites exhibited an initial increase of their catalytic activity, for conversion of hydrocarbons, with the obtained increase in their surface areas. The ball milling technique is a well-established method for inducing crystalline structure modifications (to the extent of amorphization) of other materials, such as Na-montmorillonite (NaeMMT) and organomontmorillonite (Cloisite 30B) [14], graphiteeFe mixtures [15], CeO2 powders [16], multi-walled carbon nanotubes (MWNTs) [17], cellulose [18] and cassava and maize starches [19,20]. Moreover, this technique is used in the preparation of some drug formulations such as ibuprofen (IBU) with mesoporous SBA-15 [21]. There are a limited number of publications dealing with the effect of mechanical action on the crystallinity of MCM-41 material. For example, Hagen et al. [22] investigated the influence of mechanical action (through applied pressure and ball milling) on the stability of Ti-MCM-41 material. They concluded that the mechanical stability of the studied Ti-MCM-41 was rather low; subjecting it to higher pressures or ball milling caused a destruction of the hexagonal structure accompanied by channel blocking. SpringueleHuet et al. [23] studied the effect of pressure on the structural and textural properties of Alecontaining MCM41 and SBA-15 materials. They showed that the mesopore structure is partially destroyed when the powder is compressed at high pressure (up to 520 MPa). Moreover, for both MCM-41 and SBA-15 materials the Alecontaining samples are more fragile than the purely siliceous materials. Tsoncheva et al. [24] prepared small vanadium oligomeric species via the ball milling reduction technique using V2O5 and mesoporous silica of types MCM-41, SBA-15 and MCM-48. They demonstrated that the reduction usually leads to increase in the catalytic activity for partial oxidation of ethyl-acetate and facilitates the process of partial oxidation [24]. Concurrently, Qi et al. [25] employed the wet ball milling in order to disperse the aggregates of the particles during the synthesis of MCM-41 single crystals. Accordingly, the objective of this study is to precisely understand the variation of crystalline structure and to elucidate the morphology as well as the pore and surface characteristics of mesoporous MCM-41 materials during ball milling through X-ray
diffraction, scanning electron microscope investigation, thermogravimetry and nitrogen adsorption. 2. Experimental procedures 2.1. Preparation of MCM-41 The parent MCM-41 was synthesized according to the procedure reported by Grün et al. [26]. Briefly, 5 g of n-hexadecyltrimethylammonium bromide (C16TMABr, 0.014 mol) was dissolved in 100 g of deionized water, 30.2 g of aqueous ammonia (28 wt. %, 0.25 mol) and 120.0 g of absolute ethanol (EtOH, 2.6 mol) were added to the surfactant solution. The solution was stirred for 15 min (500 rpm), and 9.4 g of TEOS (0.044 mol) was added at one time, resulting in a gel formation. After stirring for 2 h, the gel was aged for another 2 h; it was then filtered and washed with 200 ml of deionized water and 200 ml of methanol. After drying overnight at 90 C, the sample was heated to 550 C (at a rate of 2 C min1) in airflow. It was kept at that temperature for 5 h to remove the template and then cooled to room temperature. 2.2. Milling of the calcined MCM-41 Milling of the calcined MCM-41 material was carried out in a laboratory with a stirred media mill (Fritsch, Idar-Oberstein Germany). The media mill consisted of a grinding chamber (0.5 l) and five balls (3 cm diameter each) were used for milling. Approximately 5 g of MCM-41 were charged into the grinding chamber, which were, then milled for 0.25 h, 0.5 h, 1 h, 2 h, 4 h and 8 h. 2.3. Analysis of the milled products Powder X-ray diffractograms were obtained in the 2q range 1.5 e8 with a Philips X-Pert-Pro diffractometer, using Ni-filtered CuKa radiation. Adsorptionedesorption isotherms of nitrogen (at 196 C) were conducted with the aid of a Quantachrome (Nova 3200 series) gas adsorption apparatus. The different milled samples were degassed at 250 C for 4 h before the nitrogen-adsorption measurements. FT-IR spectra were recorded using a Thermo-Nicolet iS50 FT-IR spectrometer equipped with attenuated Total Reflectance (ATR) sampling accessory. Thermal analysis measurements were completed using a TA instrument apparatus (model TGA-Q500) using a heating rate of 10 C min1 in a nitrogen atmosphere (40 ml min1). The average mass of the samples was 5 mg. The morphology of the samples was analyzed by a field-emission scanning electron microscope (FE-SEM) (JEOL model JSMe7600F). Transmission electron microscopy (TEM) investigations were performed on a TECNAI G2 Spirit EWIN microscope operated at 120 kV. 3. Results and discussion 3.1. Structural characterization On the basis of TGA-DTA analysis, the MCM-41 sample was calcined at 550 C for 3 h. The XRD patterns of the calcinated MCM41 and its milling products (milled for 0.25 he8 h), are presented in Fig. 1. MCM-41 mesoporous material exhibits three Bragg diffraction peaks attributable to the highly ordered structure with the hexagonal pore arrays. The first peak is related to the (1 0 0) plane, which exhibits the highest intensity. In addition to this peak, the less intense secondary reflections ascribed to (1 1 0) and (2 0 0) are well-distinguished and appear at the 2q range of 3.5 e6.7. These features indicate that the synthesized MCM-41 material possesses an ordered hexagonal mesostructure [4e7,26e32]. The low angle XRD diffraction patterns of the un-milled MCM-41 material, pattern
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Fig. 1. XRD patterns of the calcined MCM-41 (a) and the different MCM-41 samples being milled for 15 min (b), 30 min (c), 1 h (d), 2 h (e), 4 h (f), and 8 h (g).
(a) in Fig. 1, reveals the existence of these three peaks at the 2q values of 2.51 (1 0 0), 4.33 (1 1 0), and 4.97 (2 0 0). No marked change were observed in the XRD patterns of the MCM-41 material when it was milled for 15 min or 30 min (Fig. 1, diffractograms b and c). Subjecting the MCM-41 material to milling for 1 and 2 h is accompanied by a sharp decrease in the intensity of the (1 0 0) peak, together with the disappearance of the other two peaks (patterns (d) and (e) in Fig. 1). Further increase in the milling time to 4 and 8 h led to the complete loss of all reflections suggesting a full amorphization process. In the open literature, there are many interesting papers discussing the structural modification of MCM-41 as a result of different physical or chemical treatments. For instance, Chung et al. [28] reported a broadening and a shift, slightly to a higher angle, of the (1 0 0) diffraction peak in Pd-MCM-41 material. In agreement, it was shown that increasing the Cr concentration in Cr-MCM-41 is accompanied by a continuous slight shift of its 2q values towards the higher region, along with an intensity decrease of the XRD peaks [27]. In a study by Lehmann et al. [29], they also observed the decreasing (1 0 0) intensity with an increase of the nickel content. A similar trend of order decrease with the increase in the metal loading was reported for MCM-41 loaded with Ca, Co, Fe and Ti [30e32]. Regarding the mechanical action, Hagen et al. [22] reported on a gradual decrease of the hexagonal arrangements features of Ti-MCM-41 material with increasing pressure. A complete destruction of the hexagonal structure of Ti-MCM-41 was obtained at 30 MPa of pressure or after 60 min of treatment in the ball mill [22]. In light of these interesting literature results, the observed changes in the XRD patterns shown in Fig. 1(d) and (e) suggest that the mesoporous crystallinity was reduced for the samples milled for 1 and 2 h. Therefore, although the samples milled for 1 h and 2 h have the mesoporous MCM-41 framework, the ordering of these
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samples is poor. The obtained XRD patterns for the samples milled for 4 h and 8 h, as seen in Fig. 1(f) and (g), reveal the disappearance of all reflections characterizing the MCM-41 material. This, in turn, suggests that the MCM-41 structure was severely damaged by a ball milling duration 4 h. FT-IR spectroscopy has been used extensively for the characterization of modified molecular sieves. Several bands are observed for the different MCM-41 samples between 400 cme1 and 1400 cme1. The FT-IR spectrum of the calcined MCM-41 sample (Fig. 2(a)) shows two bands at 1234 cme1 and 1085 cme1 that were assigned to nas(SieOeSi) [29,33e37]. Additional three bands at 968 cme1, 808 cme1 and 463 cme1, were attributed to nas(SieOH), ns(SieOeSi) and d(SieOeSi) modes of vibration, respectively [29,33e37]. The absorptions located at 1085, 968, 808 and 463 cme1 are also characteristic absorption peaks of amorphous SiO2 [36,38]. The obtained FT-IR spectra of the milled samples (Fig. 2(b) e (g)) show the existence of all of the aforementioned bands of the calcined MCM-41 material. However, two points could be raised from close inspection of spectra (a) e (g) in Fig. 2. First, a gradual shift of the band from 968 cme1 to 939 cme1 appears as a result of an increased the milling time from 0.0 h to 8.0 h. In this context, it was reported that the band at 968 cme1 is sensitive to the introduction of metal ions into the silica framework of MCM-41 [37]. Du et al. [36] reported that this band is a characteristic adsorption peak of MCM-41 material. Moreover, they demonstrated that the partial replacement of Si atoms by Al atoms leads to a noticeable shift of this peak to a lower wavenumbers [36]. Secondly, the two absorptions at 1234 cme1 and 1085 cme1 gradually merged into one shallow broad band, in the region of 1250 cme1 e 1040 cme1, as a result of increasing the milling time to 8 h. Under such conditions, low angle XRD data showed the disappearance of the reflections characterizing the MCM-41 material. This is not
Fig. 2. FT-IR spectra of the calcined MCM-41 (a) and the different MCM-41 samples being milled for 15 min (b), 30 min (c), 1 h (d), 2 h (e), 4 h (f), and 8 h (g).
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surprising as SieOH or SieOeSi vibration modes of amorphous silica and MCM-41 have similar wavenumbers. 3.2. Sample textures and morphologies In addition to X-ray diffraction examination, nitrogen physisorption can provide the information of the textural modifications accompanying the milling process. The surface area, pore size and pore diameter were calculated and used to compare the quality of different milled MCM-41 samples in comparison with those of the un-milled sample. The obtained adsorptionedesorption isotherms of nitrogen for the investigated samples are shown in Fig. 3(a). The isotherm of the calcined MCM-41 sample shows the typical type IV character according to the IUPAC nomenclature, which is
Fig. 3. N2 adsorptionedesorption isotherms (a) and BJH pore size distribution curves (b) of the different MCM-41 samples.
characteristic of mesoporous MCM-41 materials [39]. At the adsorption branch, a fairly smooth increase of adsorbed volume at low pressures is observed, where the nitrogen molecules were adsorbed on the inner surface of the mesopores via monolayer and multilayer formation. At a relative pressure P/Po ¼ 0.20e0.40, a steep increase in nitrogen uptake is observed, which is caused by capillary condensation of nitrogen molecules inside of the mesopores. This sudden increase in adsorbed gas volume indicates a uniform pore size distribution of the un-milled MCM-41. A plateau with a slight inclination can be observed at a high relative pressure, indicating multilayer adsorption on the external surface of the materials [39]. The un-milled MCM-41 exhibits nearly the expected H1 hysteresis loop for mesoporous materials, which indicates that the material contains a narrow pore size distribution. As shown in Fig. 3(a), an obvious difference in the isotherm curve of the N2 sorption is found on the samples milled for different time durations, suggesting that the milling process has marked influence on the pore structure of the MCM-41 material. An analysis of the pore size distribution curves of the milled samples proved this finding (Fig. 3(b)). The obtained isotherms for the samples milled for 15 min and 30 min (Fig. 3 (a)) resembles that of the unmilled sample, i.e. they correspond to the type IV curves that indicate that the order of hexagonal arrays of mesopores in MCM41 is not much affected after 30 min of milling. However, the amount of adsorbed N2 decreased, so the increase in nitrogen uptake becomes even less steep; the position of the capillary condensation step shifted, to some extent, towards the lower partial pressures. This, in turn, suggests that these two samples possessed less-uniform pore distributions and smaller pore volumes and surface areas, which agrees with the data given in Table 1. After 1 h of milling, the obtained isotherm still showed the characteristics of type IV, but the volume adsorbed was markedly decreased. Increasing the milling time to 2 h, the obtained isotherm no longer showed type IV behavior and the volume adsorbed further decreased. The desorption branches of the isotherms nearly coincide with the adsorption branches. The isotherms of the MCM41 samples subjected to milling durations between 4 h and 8 h are type I isotherms, suggesting that the mesoporous structure in the MCM-41 sample was entirely destroyed, which is consistent with the results of the XRD analysis. The pore size distributions (PSD) of the calcined and milled MCM-41 samples were obtained from the N2 desorption branch using a BJH method (Fig. 3(b)). The milling process had a great effect on the pore size distribution. A mono-disperse peak can be observed at 2.86 nm. Milling the calcined MCM-41 for 15 min did not greatly alter the pore diameters. After 30 min of milling, the PSD curve still exhibits the mesoporous characteristics, but the pore diameter was shifted to 3.15 nm. Further increase in the milling time to 1 h is accompanied by an intensity decrease of the mesoporous peak, together with a shift in the pore diameter to 3.02 nm. It is evident from Fig. 3 (b) that the height of the peak located at the mesoporous range decreases noticeably when the milling time is increased to 1 h. This feature indicates a decrease in the number of pores already present with nearly the same shape and the same dimensions in the mesoporous range. Milling the calcined MCM-41 for a 2 h e 8 h period resulted in the complete disappearance of its mesoporosity, as indicated by the absence of any peak at the mesoporous range (Fig. 3(b)). This picture suggests a gradual decrease of the hexagonal arrangement features with an increased milling time to 1 h and the complete destruction of the hexagonal structure at a milling time 2 h. The specific surface areas, pore sizes and pore volumes data, calculated by BET, t-plot and BJH methods, of the calcined and milled MCM-41 samples are summarized in Table 1. The un-milled MCM-41 exhibited a surface area, a pore volume and a pore diameter of 920 m2 g1, 0.71 cm3 g1
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Table 1 Texture data for the calcined MCM-41 mesoporous material as well as its milled samples. Milling time [h]
SBET [m2/g]
Total pore volume [102 cc/g]
Micropore volume [102 cc/g]
Mesopore volume [102 cc/g]
Average pore diameter [nm]
0.00 0.25 0.50 1.00 2.00 4.00 8.00
920 791 636 294 251 46 27
71.2 58.7 48.7 20.5 17.1 4.3 1.9
0.0 0.0 0.0 1.9 14.6 4.1 1.8
71.2 58.7 48.7 18.6 2.5 0.2 0.1
3.09 2.97 3.06 2.78 2.73 2.59 2.44
and 3.09 nm, respectively. From the data listed in Table 1, it is clear that increasing the milling time from 15 min to 8 h resulted in a dramatic surface area and pore volume decrease. Moreover, the obtained values of micro- and mesopore volumes given in Table 1 reveal the absence of microporosity for the samples milled for 15 and 30 min. For the samples milled for longer period a development of microporosity and a loss of mesoporosity is evident. These findings are a direct response to the sorption isotherms and the PSD curves, which consistently suggest severe damage of the MCM-41 structure by the ball milling action. The results show an empirical exponential relation between both the surface area (SBET) and the pore volume (Vp) with the milling time of the type;
collapse of the pores with increasing the ball-milling time. Fig. 6 shows some representative TEM images of the MCM-41 and its 8 h milled product. For the un-milled MCM-41 (Fig. 6 (a, b)), one can observe quite well-ordered spherical surfaces having diameters in the range 170e600 nm. This, in turn, indicates that this sample has a system of symmetrical pores, which is in agreement with the information obtained by XRD, N2-adsorption and SEM investigations. The images obtained for the 8 h milled MCM-41 sample (Fig. 6(c, d)) clearly indicate that the milling process had essentially damaged the regular mesoporous array structure of MCM-41. The obtained material is characterized consists of agglomerated smaller particles with irregular shape.
1=SBET ¼ 1=ðk1 tn1 Þ; 1 Vp ¼ 1=ðk2 tn2 Þ; where k and n are constants. This results in a linear relation of log(1/SBET) or log(1/Vp) vs. log(1/t) as shown in Fig. 4. A linear relationship can be observed in Fig 4. The computed values of k1 and k2 were 270.3 and 0.203, respectively, and the values of n1 and n2 were 1.03 and 1.01, respectively. The FE-SEM images obtained for the un-milled MCM-41 along with those of the sample milled for 8 h are shown in Fig. 5. It can be seen that most particles of the un-milled MCM-41 (Fig. 5 (a)) have a sphere-like morphology with particle size ranges from 300 nm to 700 nm. Moreover, these particles exhibit some sort of aggregation, where a large distributed space with no characteristic size or shape occurs. Dramatic morphological change is obvious when the MCM41 sample is subjected to 8 h of milling (Fig. 5 (b)). Clearly, the spherical morphology of the MCM-41 is lost after 8 h of milling: the MCM-41 spheres were fragmented into smaller particles with irregular shape. The use of the TEM in combination with SEM enabled the accurate characterization of the deformation or
Fig. 4. Plot of log(1/SBET) and log(1/VP) against log(1/t) at different milling times.
Fig. 5. FE-SEM micrographs of calcined MCM-41 (a) and MCM-41 milled for 8 h (b).
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Fig. 6. TEM images of calcined MCM-41 (a, b) and MCM-41 milled for 8 h (c, d).
3.3. Samples thermal behaviors Fig. S1 shows the TGA, DTG, and DTA curves obtained for the assynthesized MCM-41 in air. The total weight loss obtained as a result of heating the parent material from ambient temperature to 900 C is approximately 38.5%. Fig. S1 shows that decomposition occurred through four weight loss steps. Based on the previous thermal analysis studies [33], the first step below 100 C is assignable for the loss of the adsorbed and included water molecules and/or organic structures. This step is followed by two other steps; the first is a composite step with two maxima at 244 C and 277 C, whereas the second step is maximized at 334 C. These two thermal events, which represent the major portion of the weight loss process, are mainly ascribable to the loss of the template (decomposition and/or combustion). In this respect, the DTA thermogram indicates that these steps are exothermic in nature. Finally, the last weight loss step, maximized at 540 C, showing a small weight loss of approximately 4%. This step could be related to the loss of the charcoal formed during the template release and/or the condensation of the adjacent silanol (SieOH) groups to form a siloxane bond [27]. At temperatures higher than 700 C, an approximate constant sample weight can be observed, suggesting that MCM-41 mesoporous materials have high thermal stability. According to the thermal analysis results, the parent MCM-41 was calcined at 550 C for 5 h with a ramp of 2 C/min. A thermogravimetric analysis was carried out to estimate the water content of the milled MCM-41 samples in comparison to the un-milled specimens. Fig. 7 shows the TGA curves of the un-milled MCM-41 sample and those milled for 0.5 h, 2 h, 4 h, and 8 h. All samples show two weight loss steps. The first one starts from
ambient temperature to approximately 150 C, and the second step extends from 150 through approximately 700 C. However, the results clearly indicate that the TGA curves of the milled samples exhibited a gradual weight-loss decrease as a result of the milling time increase with respect to that of the original MCM-41 sample.
Fig. 7. TGA curves of the un-milled MCM-41 (a) and its products milled for 0.5 h (b), 2 h (c), 4 h (d) and 8 (e) h.
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Final weight losses of 26.9%, 20.2%, 18.0%, 9.9% and 9.2% at 800 C were obtained for the samples milled at 0.0, 0.5, 2, 4, and 8 h, respectively. In this context, it was reported that the mesoporous MCM-41 material has a great number of silanol groups (SieOH) with pronounced hydrophilic characteristics. These OH groups are distributed on the inner surface of the MCM-41 channels [40]. In combination with the previous characterization results, it is plausible to suggest that the gradual structure damage and mesoporosity loss in response to the milling process are responsible for the observed water content decrease with the milling time increase. Therefore, the damage of the MCM-41 channels during the milling process is responsible for the inner OH group losses and, hence, the water content decreases. 4. Conclusions In this paper, the effects of ball milling on mesoporous MCM-41 structure, texture and morphology properties have been studied in detail. The results showed that increasing the milling time accelerates the amorphization process of mesoporous MCM-41 material. This material maintains its initial crystallinity until 30 min of milling time; thereafter, a severe crystallinity loss can be obtained by milling the MCM-41 for a period 1 h e 2 h. Moreover, subjecting MCM-41 to 4 h of milling is sufficient for a complete amorphization. N2 adsorptionedesorption isotherms suggested a gradual mesoporosity loss, although the overall ordered mesoporous structure was retained for the samples milled for 1 h. The “stepped” gas adsorption isotherm no longer exists for the samples milled for more than 1 h. The surface area and pore volume showed continuous decreases with the milling time increase. The FE-SEM analysis showed a drastic destruction of the MCM-41 spherical morphology in the case of the sample milled for the longest time (8 h). Mechanical milling had major effects on the water content decrease of MCM-41; the observed structural and textural changes in response to the milling process may be responsible for that decrease. Due to the fragile MCM-41-framework, caution is recommended during the preparation, modification and catalytic testing of MCM-41based materials when mechanical action is applied. Acknowledgment B.M. Abu-Zied thanks the financial support from the Deutscher Akademischer Austausch Dienst (DAAD) (grant code number: A/11/ 07705) during his research visit to Germany. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.micromeso.2015.07.012. References [1] H. Saputra, R. Othman, A.G.E. Sutjipto, R. Muhida, J. Membr. Sci. 367 (2011) 152e157.
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Effect of ball milling on the structural and textural features of MCM-41 mesoporous material Bahaa M. Abu-Zied a, b, c, *, Wilhelm Schwieger d, Abdullah M. Asiri a, b a
Center of Excellence for Advanced Materials Research (CEAMR), King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia Chemistry Department, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia c Chemistry Department, Faculty of Science, Assiut University, 71516 Assiut, Egypt d €t Erlangen-Nürnberg, Egerlandstraße 3, 91058 Erlangen, Germany Lehrstuhl Für Chemische Reaktionstechnik, Friedrich-Alexander-Universita b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 March 2015 Received in revised form 13 July 2015 Accepted 15 July 2015 Available online 22 July 2015
This paper reports on the ball-milling process and its influence on the structural, textural and morphological features of mesoporous MCM-41 material. MCM-41 and the as-milled products were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), N2-adsorption, a field emission scanning electron microscope (FE-SEM) and a thermogravimetric analysis (TGA). The XRD showed that the mechanical treatment of MCM-41 in a ball mill leads to a gradual crystallinity loss with time until 2 h, and complete crystallinity loss was observed after 4 h of ball milling. N2 sorption measurements revealed a sharp surface area decrease as well as mesoporosity loss accompanying the milling process. The FE-SEM investigation demonstrated severe damage of the spherical morphology of MCM-41 upon milling for 8 h. Moreover, it was found that these structural, textural and morphological changes strongly affected the water content of the milled MCM-41 material. © 2015 Elsevier Inc. All rights reserved.
Keywords: MCM-41 Ball milling Mesoporous material Texture analysis
1. Introduction Mesoporous silica, MCMe41, belongs to a group of mesoporous materials known as M41S, and it is characterized by its onedimensional hexagonally ordered pore structure. MCM-41 has high specific surface area (~1000 m2/g) and pore volume (~1 cm3/ g), very narrow pore size distribution, tunable pore size between 1 nm and 10 nm or more, adjustable hydrophobicity and very good thermal stability [1e3]. However, mesoporous MCM-41 possesses a neutral framework, a weak surface acidity and a poor hydrothermal stability, which limit its application in industry and as a catalyst. In this context, many efforts have been made to improve its acidity and stability, including (i) the incorporation of active metal species into MCM-41 by direct synthesis or impregnation methods and (ii) the addition of other materials with acidic features into the MCM41 matrix. During the last few years, many transition metals and main group elements have been introduced into MCM-41, showing
* Corresponding author. Chemistry Department, Faculty of Science, Assiut University, 71516 Assiut, Egypt. Tel.: þ20 882412429; fax: þ20 88 2342708. E-mail addresses: [email protected], [email protected], babuzied@ yahoo.com (B.M. Abu-Zied). http://dx.doi.org/10.1016/j.micromeso.2015.07.012 1387-1811/© 2015 Elsevier Inc. All rights reserved.
interesting catalytic applications. In this way, Li et al. [4] prepared a series of Fe-MCM-41 with high iron content, up to 13.21 wt. %, using a direct hydrothermal method under alkaline conditions. These samples exhibited excellent catalytic performance for oxidative desulfurization (ODS) of dibenzothiophene (DBT) by hydrogen peroxide at 50 C, and the sulfur removal did not decline after being recycled four times. Mn-MCM-41 powders with different Mn/Si molar ratios were prepared using the incipient wetness method [5]. The catalytic activity of the prepared systems towards complete nhexane oxidation revealed that the efficiency of this type of catalytic system increases with temperature and manganese quantity [5]. Recently, increased interest has been sparked in the synthesis and application of mesoporous MCM-41 composites with microporous molecular sieves. A composite molecular sieve with biporous structure, i.e., mesoemicro porous materials, combined with the benefits of the individual pore size regime could potentially improve the acidity and hydrothermal stability of molecular sieve catalysts [6,7]. In this regard, Jiang et al. [7] synthesized a Y/ MCM-41 composite molecular sieve using kaolin as the main silica source via a hydrothermal method. They demonstrated that, after calcination at 800 C for 3 h or hydrothermal treatment at 100 C for 10 days, the mesoporous framework in the composite
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still maintained. Moreover, based on NH3eTPD measurements, it was shown that more strong acid sites on the H-Y/MCM-41 sample occur than on the H-MCM-41 sample. Ball milling is a common physical technique used to produce subemicrometer particles. During the milling process, the grinding media is accelerated to higher velocities that provide a high transfer of kinetic energy from the balls to the sample, resulting in the production of fine powders. Ball sizes, rotation speed, and milling time have great influence on the experimental results. Using this technique, Hampsey et al. [8] obtained micrometer-to subemicrometeresized mesoporous silica particles starting from a silica xerogel prepared by a surfactant selfassembly solegel process. Concurrently, Hedayati et al. [9] prepared a good dispersion and distribution of nano-silica particles with ball milling. Ball milling is a practical method for reducing the size of zeolites crystals. Smaller zeolite crystals have higher catalytic effectiveness, lower coke formation, faster diffusion, higher external surface area and easier template extraction [10]. The changes occurring in the structural and textural features accompanying the ball milling of Y, X, A, and ZSM-5 zeolites have been well investigated [10e13]. In this context, Zielinski et al. [13] investigated the effect of milling time on the properties of NaA, CaA, KL, Silicalite-1, H-ZSM-5 and HY zeolites using high-energy ball milling. Their results indicated the direct correlation of the mechanical resistance of the zeolite and its Si/Al ratio. Moreover, the smaller-pore zeolites exhibited an initial increase of their catalytic activity, for conversion of hydrocarbons, with the obtained increase in their surface areas. The ball milling technique is a well-established method for inducing crystalline structure modifications (to the extent of amorphization) of other materials, such as Na-montmorillonite (NaeMMT) and organomontmorillonite (Cloisite 30B) [14], graphiteeFe mixtures [15], CeO2 powders [16], multi-walled carbon nanotubes (MWNTs) [17], cellulose [18] and cassava and maize starches [19,20]. Moreover, this technique is used in the preparation of some drug formulations such as ibuprofen (IBU) with mesoporous SBA-15 [21]. There are a limited number of publications dealing with the effect of mechanical action on the crystallinity of MCM-41 material. For example, Hagen et al. [22] investigated the influence of mechanical action (through applied pressure and ball milling) on the stability of Ti-MCM-41 material. They concluded that the mechanical stability of the studied Ti-MCM-41 was rather low; subjecting it to higher pressures or ball milling caused a destruction of the hexagonal structure accompanied by channel blocking. SpringueleHuet et al. [23] studied the effect of pressure on the structural and textural properties of Alecontaining MCM41 and SBA-15 materials. They showed that the mesopore structure is partially destroyed when the powder is compressed at high pressure (up to 520 MPa). Moreover, for both MCM-41 and SBA-15 materials the Alecontaining samples are more fragile than the purely siliceous materials. Tsoncheva et al. [24] prepared small vanadium oligomeric species via the ball milling reduction technique using V2O5 and mesoporous silica of types MCM-41, SBA-15 and MCM-48. They demonstrated that the reduction usually leads to increase in the catalytic activity for partial oxidation of ethyl-acetate and facilitates the process of partial oxidation [24]. Concurrently, Qi et al. [25] employed the wet ball milling in order to disperse the aggregates of the particles during the synthesis of MCM-41 single crystals. Accordingly, the objective of this study is to precisely understand the variation of crystalline structure and to elucidate the morphology as well as the pore and surface characteristics of mesoporous MCM-41 materials during ball milling through X-ray
diffraction, scanning electron microscope investigation, thermogravimetry and nitrogen adsorption. 2. Experimental procedures 2.1. Preparation of MCM-41 The parent MCM-41 was synthesized according to the procedure reported by Grün et al. [26]. Briefly, 5 g of n-hexadecyltrimethylammonium bromide (C16TMABr, 0.014 mol) was dissolved in 100 g of deionized water, 30.2 g of aqueous ammonia (28 wt. %, 0.25 mol) and 120.0 g of absolute ethanol (EtOH, 2.6 mol) were added to the surfactant solution. The solution was stirred for 15 min (500 rpm), and 9.4 g of TEOS (0.044 mol) was added at one time, resulting in a gel formation. After stirring for 2 h, the gel was aged for another 2 h; it was then filtered and washed with 200 ml of deionized water and 200 ml of methanol. After drying overnight at 90 C, the sample was heated to 550 C (at a rate of 2 C min1) in airflow. It was kept at that temperature for 5 h to remove the template and then cooled to room temperature. 2.2. Milling of the calcined MCM-41 Milling of the calcined MCM-41 material was carried out in a laboratory with a stirred media mill (Fritsch, Idar-Oberstein Germany). The media mill consisted of a grinding chamber (0.5 l) and five balls (3 cm diameter each) were used for milling. Approximately 5 g of MCM-41 were charged into the grinding chamber, which were, then milled for 0.25 h, 0.5 h, 1 h, 2 h, 4 h and 8 h. 2.3. Analysis of the milled products Powder X-ray diffractograms were obtained in the 2q range 1.5 e8 with a Philips X-Pert-Pro diffractometer, using Ni-filtered CuKa radiation. Adsorptionedesorption isotherms of nitrogen (at 196 C) were conducted with the aid of a Quantachrome (Nova 3200 series) gas adsorption apparatus. The different milled samples were degassed at 250 C for 4 h before the nitrogen-adsorption measurements. FT-IR spectra were recorded using a Thermo-Nicolet iS50 FT-IR spectrometer equipped with attenuated Total Reflectance (ATR) sampling accessory. Thermal analysis measurements were completed using a TA instrument apparatus (model TGA-Q500) using a heating rate of 10 C min1 in a nitrogen atmosphere (40 ml min1). The average mass of the samples was 5 mg. The morphology of the samples was analyzed by a field-emission scanning electron microscope (FE-SEM) (JEOL model JSMe7600F). Transmission electron microscopy (TEM) investigations were performed on a TECNAI G2 Spirit EWIN microscope operated at 120 kV. 3. Results and discussion 3.1. Structural characterization On the basis of TGA-DTA analysis, the MCM-41 sample was calcined at 550 C for 3 h. The XRD patterns of the calcinated MCM41 and its milling products (milled for 0.25 he8 h), are presented in Fig. 1. MCM-41 mesoporous material exhibits three Bragg diffraction peaks attributable to the highly ordered structure with the hexagonal pore arrays. The first peak is related to the (1 0 0) plane, which exhibits the highest intensity. In addition to this peak, the less intense secondary reflections ascribed to (1 1 0) and (2 0 0) are well-distinguished and appear at the 2q range of 3.5 e6.7. These features indicate that the synthesized MCM-41 material possesses an ordered hexagonal mesostructure [4e7,26e32]. The low angle XRD diffraction patterns of the un-milled MCM-41 material, pattern
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Fig. 1. XRD patterns of the calcined MCM-41 (a) and the different MCM-41 samples being milled for 15 min (b), 30 min (c), 1 h (d), 2 h (e), 4 h (f), and 8 h (g).
(a) in Fig. 1, reveals the existence of these three peaks at the 2q values of 2.51 (1 0 0), 4.33 (1 1 0), and 4.97 (2 0 0). No marked change were observed in the XRD patterns of the MCM-41 material when it was milled for 15 min or 30 min (Fig. 1, diffractograms b and c). Subjecting the MCM-41 material to milling for 1 and 2 h is accompanied by a sharp decrease in the intensity of the (1 0 0) peak, together with the disappearance of the other two peaks (patterns (d) and (e) in Fig. 1). Further increase in the milling time to 4 and 8 h led to the complete loss of all reflections suggesting a full amorphization process. In the open literature, there are many interesting papers discussing the structural modification of MCM-41 as a result of different physical or chemical treatments. For instance, Chung et al. [28] reported a broadening and a shift, slightly to a higher angle, of the (1 0 0) diffraction peak in Pd-MCM-41 material. In agreement, it was shown that increasing the Cr concentration in Cr-MCM-41 is accompanied by a continuous slight shift of its 2q values towards the higher region, along with an intensity decrease of the XRD peaks [27]. In a study by Lehmann et al. [29], they also observed the decreasing (1 0 0) intensity with an increase of the nickel content. A similar trend of order decrease with the increase in the metal loading was reported for MCM-41 loaded with Ca, Co, Fe and Ti [30e32]. Regarding the mechanical action, Hagen et al. [22] reported on a gradual decrease of the hexagonal arrangements features of Ti-MCM-41 material with increasing pressure. A complete destruction of the hexagonal structure of Ti-MCM-41 was obtained at 30 MPa of pressure or after 60 min of treatment in the ball mill [22]. In light of these interesting literature results, the observed changes in the XRD patterns shown in Fig. 1(d) and (e) suggest that the mesoporous crystallinity was reduced for the samples milled for 1 and 2 h. Therefore, although the samples milled for 1 h and 2 h have the mesoporous MCM-41 framework, the ordering of these
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samples is poor. The obtained XRD patterns for the samples milled for 4 h and 8 h, as seen in Fig. 1(f) and (g), reveal the disappearance of all reflections characterizing the MCM-41 material. This, in turn, suggests that the MCM-41 structure was severely damaged by a ball milling duration 4 h. FT-IR spectroscopy has been used extensively for the characterization of modified molecular sieves. Several bands are observed for the different MCM-41 samples between 400 cme1 and 1400 cme1. The FT-IR spectrum of the calcined MCM-41 sample (Fig. 2(a)) shows two bands at 1234 cme1 and 1085 cme1 that were assigned to nas(SieOeSi) [29,33e37]. Additional three bands at 968 cme1, 808 cme1 and 463 cme1, were attributed to nas(SieOH), ns(SieOeSi) and d(SieOeSi) modes of vibration, respectively [29,33e37]. The absorptions located at 1085, 968, 808 and 463 cme1 are also characteristic absorption peaks of amorphous SiO2 [36,38]. The obtained FT-IR spectra of the milled samples (Fig. 2(b) e (g)) show the existence of all of the aforementioned bands of the calcined MCM-41 material. However, two points could be raised from close inspection of spectra (a) e (g) in Fig. 2. First, a gradual shift of the band from 968 cme1 to 939 cme1 appears as a result of an increased the milling time from 0.0 h to 8.0 h. In this context, it was reported that the band at 968 cme1 is sensitive to the introduction of metal ions into the silica framework of MCM-41 [37]. Du et al. [36] reported that this band is a characteristic adsorption peak of MCM-41 material. Moreover, they demonstrated that the partial replacement of Si atoms by Al atoms leads to a noticeable shift of this peak to a lower wavenumbers [36]. Secondly, the two absorptions at 1234 cme1 and 1085 cme1 gradually merged into one shallow broad band, in the region of 1250 cme1 e 1040 cme1, as a result of increasing the milling time to 8 h. Under such conditions, low angle XRD data showed the disappearance of the reflections characterizing the MCM-41 material. This is not
Fig. 2. FT-IR spectra of the calcined MCM-41 (a) and the different MCM-41 samples being milled for 15 min (b), 30 min (c), 1 h (d), 2 h (e), 4 h (f), and 8 h (g).
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surprising as SieOH or SieOeSi vibration modes of amorphous silica and MCM-41 have similar wavenumbers. 3.2. Sample textures and morphologies In addition to X-ray diffraction examination, nitrogen physisorption can provide the information of the textural modifications accompanying the milling process. The surface area, pore size and pore diameter were calculated and used to compare the quality of different milled MCM-41 samples in comparison with those of the un-milled sample. The obtained adsorptionedesorption isotherms of nitrogen for the investigated samples are shown in Fig. 3(a). The isotherm of the calcined MCM-41 sample shows the typical type IV character according to the IUPAC nomenclature, which is
Fig. 3. N2 adsorptionedesorption isotherms (a) and BJH pore size distribution curves (b) of the different MCM-41 samples.
characteristic of mesoporous MCM-41 materials [39]. At the adsorption branch, a fairly smooth increase of adsorbed volume at low pressures is observed, where the nitrogen molecules were adsorbed on the inner surface of the mesopores via monolayer and multilayer formation. At a relative pressure P/Po ¼ 0.20e0.40, a steep increase in nitrogen uptake is observed, which is caused by capillary condensation of nitrogen molecules inside of the mesopores. This sudden increase in adsorbed gas volume indicates a uniform pore size distribution of the un-milled MCM-41. A plateau with a slight inclination can be observed at a high relative pressure, indicating multilayer adsorption on the external surface of the materials [39]. The un-milled MCM-41 exhibits nearly the expected H1 hysteresis loop for mesoporous materials, which indicates that the material contains a narrow pore size distribution. As shown in Fig. 3(a), an obvious difference in the isotherm curve of the N2 sorption is found on the samples milled for different time durations, suggesting that the milling process has marked influence on the pore structure of the MCM-41 material. An analysis of the pore size distribution curves of the milled samples proved this finding (Fig. 3(b)). The obtained isotherms for the samples milled for 15 min and 30 min (Fig. 3 (a)) resembles that of the unmilled sample, i.e. they correspond to the type IV curves that indicate that the order of hexagonal arrays of mesopores in MCM41 is not much affected after 30 min of milling. However, the amount of adsorbed N2 decreased, so the increase in nitrogen uptake becomes even less steep; the position of the capillary condensation step shifted, to some extent, towards the lower partial pressures. This, in turn, suggests that these two samples possessed less-uniform pore distributions and smaller pore volumes and surface areas, which agrees with the data given in Table 1. After 1 h of milling, the obtained isotherm still showed the characteristics of type IV, but the volume adsorbed was markedly decreased. Increasing the milling time to 2 h, the obtained isotherm no longer showed type IV behavior and the volume adsorbed further decreased. The desorption branches of the isotherms nearly coincide with the adsorption branches. The isotherms of the MCM41 samples subjected to milling durations between 4 h and 8 h are type I isotherms, suggesting that the mesoporous structure in the MCM-41 sample was entirely destroyed, which is consistent with the results of the XRD analysis. The pore size distributions (PSD) of the calcined and milled MCM-41 samples were obtained from the N2 desorption branch using a BJH method (Fig. 3(b)). The milling process had a great effect on the pore size distribution. A mono-disperse peak can be observed at 2.86 nm. Milling the calcined MCM-41 for 15 min did not greatly alter the pore diameters. After 30 min of milling, the PSD curve still exhibits the mesoporous characteristics, but the pore diameter was shifted to 3.15 nm. Further increase in the milling time to 1 h is accompanied by an intensity decrease of the mesoporous peak, together with a shift in the pore diameter to 3.02 nm. It is evident from Fig. 3 (b) that the height of the peak located at the mesoporous range decreases noticeably when the milling time is increased to 1 h. This feature indicates a decrease in the number of pores already present with nearly the same shape and the same dimensions in the mesoporous range. Milling the calcined MCM-41 for a 2 h e 8 h period resulted in the complete disappearance of its mesoporosity, as indicated by the absence of any peak at the mesoporous range (Fig. 3(b)). This picture suggests a gradual decrease of the hexagonal arrangement features with an increased milling time to 1 h and the complete destruction of the hexagonal structure at a milling time 2 h. The specific surface areas, pore sizes and pore volumes data, calculated by BET, t-plot and BJH methods, of the calcined and milled MCM-41 samples are summarized in Table 1. The un-milled MCM-41 exhibited a surface area, a pore volume and a pore diameter of 920 m2 g1, 0.71 cm3 g1
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Table 1 Texture data for the calcined MCM-41 mesoporous material as well as its milled samples. Milling time [h]
SBET [m2/g]
Total pore volume [102 cc/g]
Micropore volume [102 cc/g]
Mesopore volume [102 cc/g]
Average pore diameter [nm]
0.00 0.25 0.50 1.00 2.00 4.00 8.00
920 791 636 294 251 46 27
71.2 58.7 48.7 20.5 17.1 4.3 1.9
0.0 0.0 0.0 1.9 14.6 4.1 1.8
71.2 58.7 48.7 18.6 2.5 0.2 0.1
3.09 2.97 3.06 2.78 2.73 2.59 2.44
and 3.09 nm, respectively. From the data listed in Table 1, it is clear that increasing the milling time from 15 min to 8 h resulted in a dramatic surface area and pore volume decrease. Moreover, the obtained values of micro- and mesopore volumes given in Table 1 reveal the absence of microporosity for the samples milled for 15 and 30 min. For the samples milled for longer period a development of microporosity and a loss of mesoporosity is evident. These findings are a direct response to the sorption isotherms and the PSD curves, which consistently suggest severe damage of the MCM-41 structure by the ball milling action. The results show an empirical exponential relation between both the surface area (SBET) and the pore volume (Vp) with the milling time of the type;
collapse of the pores with increasing the ball-milling time. Fig. 6 shows some representative TEM images of the MCM-41 and its 8 h milled product. For the un-milled MCM-41 (Fig. 6 (a, b)), one can observe quite well-ordered spherical surfaces having diameters in the range 170e600 nm. This, in turn, indicates that this sample has a system of symmetrical pores, which is in agreement with the information obtained by XRD, N2-adsorption and SEM investigations. The images obtained for the 8 h milled MCM-41 sample (Fig. 6(c, d)) clearly indicate that the milling process had essentially damaged the regular mesoporous array structure of MCM-41. The obtained material is characterized consists of agglomerated smaller particles with irregular shape.
1=SBET ¼ 1=ðk1 tn1 Þ; 1 Vp ¼ 1=ðk2 tn2 Þ; where k and n are constants. This results in a linear relation of log(1/SBET) or log(1/Vp) vs. log(1/t) as shown in Fig. 4. A linear relationship can be observed in Fig 4. The computed values of k1 and k2 were 270.3 and 0.203, respectively, and the values of n1 and n2 were 1.03 and 1.01, respectively. The FE-SEM images obtained for the un-milled MCM-41 along with those of the sample milled for 8 h are shown in Fig. 5. It can be seen that most particles of the un-milled MCM-41 (Fig. 5 (a)) have a sphere-like morphology with particle size ranges from 300 nm to 700 nm. Moreover, these particles exhibit some sort of aggregation, where a large distributed space with no characteristic size or shape occurs. Dramatic morphological change is obvious when the MCM41 sample is subjected to 8 h of milling (Fig. 5 (b)). Clearly, the spherical morphology of the MCM-41 is lost after 8 h of milling: the MCM-41 spheres were fragmented into smaller particles with irregular shape. The use of the TEM in combination with SEM enabled the accurate characterization of the deformation or
Fig. 4. Plot of log(1/SBET) and log(1/VP) against log(1/t) at different milling times.
Fig. 5. FE-SEM micrographs of calcined MCM-41 (a) and MCM-41 milled for 8 h (b).
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Fig. 6. TEM images of calcined MCM-41 (a, b) and MCM-41 milled for 8 h (c, d).
3.3. Samples thermal behaviors Fig. S1 shows the TGA, DTG, and DTA curves obtained for the assynthesized MCM-41 in air. The total weight loss obtained as a result of heating the parent material from ambient temperature to 900 C is approximately 38.5%. Fig. S1 shows that decomposition occurred through four weight loss steps. Based on the previous thermal analysis studies [33], the first step below 100 C is assignable for the loss of the adsorbed and included water molecules and/or organic structures. This step is followed by two other steps; the first is a composite step with two maxima at 244 C and 277 C, whereas the second step is maximized at 334 C. These two thermal events, which represent the major portion of the weight loss process, are mainly ascribable to the loss of the template (decomposition and/or combustion). In this respect, the DTA thermogram indicates that these steps are exothermic in nature. Finally, the last weight loss step, maximized at 540 C, showing a small weight loss of approximately 4%. This step could be related to the loss of the charcoal formed during the template release and/or the condensation of the adjacent silanol (SieOH) groups to form a siloxane bond [27]. At temperatures higher than 700 C, an approximate constant sample weight can be observed, suggesting that MCM-41 mesoporous materials have high thermal stability. According to the thermal analysis results, the parent MCM-41 was calcined at 550 C for 5 h with a ramp of 2 C/min. A thermogravimetric analysis was carried out to estimate the water content of the milled MCM-41 samples in comparison to the un-milled specimens. Fig. 7 shows the TGA curves of the un-milled MCM-41 sample and those milled for 0.5 h, 2 h, 4 h, and 8 h. All samples show two weight loss steps. The first one starts from
ambient temperature to approximately 150 C, and the second step extends from 150 through approximately 700 C. However, the results clearly indicate that the TGA curves of the milled samples exhibited a gradual weight-loss decrease as a result of the milling time increase with respect to that of the original MCM-41 sample.
Fig. 7. TGA curves of the un-milled MCM-41 (a) and its products milled for 0.5 h (b), 2 h (c), 4 h (d) and 8 (e) h.
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Final weight losses of 26.9%, 20.2%, 18.0%, 9.9% and 9.2% at 800 C were obtained for the samples milled at 0.0, 0.5, 2, 4, and 8 h, respectively. In this context, it was reported that the mesoporous MCM-41 material has a great number of silanol groups (SieOH) with pronounced hydrophilic characteristics. These OH groups are distributed on the inner surface of the MCM-41 channels [40]. In combination with the previous characterization results, it is plausible to suggest that the gradual structure damage and mesoporosity loss in response to the milling process are responsible for the observed water content decrease with the milling time increase. Therefore, the damage of the MCM-41 channels during the milling process is responsible for the inner OH group losses and, hence, the water content decreases. 4. Conclusions In this paper, the effects of ball milling on mesoporous MCM-41 structure, texture and morphology properties have been studied in detail. The results showed that increasing the milling time accelerates the amorphization process of mesoporous MCM-41 material. This material maintains its initial crystallinity until 30 min of milling time; thereafter, a severe crystallinity loss can be obtained by milling the MCM-41 for a period 1 h e 2 h. Moreover, subjecting MCM-41 to 4 h of milling is sufficient for a complete amorphization. N2 adsorptionedesorption isotherms suggested a gradual mesoporosity loss, although the overall ordered mesoporous structure was retained for the samples milled for 1 h. The “stepped” gas adsorption isotherm no longer exists for the samples milled for more than 1 h. The surface area and pore volume showed continuous decreases with the milling time increase. The FE-SEM analysis showed a drastic destruction of the MCM-41 spherical morphology in the case of the sample milled for the longest time (8 h). Mechanical milling had major effects on the water content decrease of MCM-41; the observed structural and textural changes in response to the milling process may be responsible for that decrease. Due to the fragile MCM-41-framework, caution is recommended during the preparation, modification and catalytic testing of MCM-41based materials when mechanical action is applied. Acknowledgment B.M. Abu-Zied thanks the financial support from the Deutscher Akademischer Austausch Dienst (DAAD) (grant code number: A/11/ 07705) during his research visit to Germany. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.micromeso.2015.07.012. References [1] H. Saputra, R. Othman, A.G.E. Sutjipto, R. Muhida, J. Membr. Sci. 367 (2011) 152e157.
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