Synthesis and characterization of V, Mo and Nb incorporated micro–mesoporous MCM-41 materials

Materials Chemistry and Physics 125 (2011) 148–155

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Synthesis and characterization of V, Mo and Nb incorporated micro–mesoporous MCM-41 materials Arzu Solmaz a,1 , Suna Balci a,∗ , Timur Dogu b,2 a b

Gazi University, Faculty of Engineering, Chemical Engineering Department, 06570 Ankara, Turkey Middle East Technical University, Faculty of Engineering, Chemical Engineering Department, 06531 Ankara, Turkey

a r t i c l e

i n f o

Article history: Received 15 November 2009 Received in revised form 22 August 2010 Accepted 26 August 2010 Keywords: Nanostructure Crystal structure Microporous materials Nitrogen adsorption

a b s t r a c t Highly microporous metal-MCM-41 ordered mesoporous structure catalysts having different metal/Si (V, Mo, Nb) atomic ratios and combinations of metal sources were hydrothermally synthesized. The structural properties estimated using different techniques were found to be in agreement with each other. Metals were successfully incorporated into MCM-41 without deteriorating the ordered hexagonal structure. The metal ions in the synthesis solutions probably settled on the hydrophilic end of the template hence the metal incorporation resulted improvements in the micropore structure. Low loading of metals caused an increase in the surface area and pore volume values of the catalysts. The highest total (1310 m2 g−1 ) and micropore surface area values (1083 m2 g−1 ) were obtained by Nb incorporation. The micro- and mesopore dimensions of MCM-41 increased from 0.5 to 1.1 nm and from 2.5 to 2.8 nm, respectively, with metal incorporation. Low V/Si ratios and presence of Nb in the starting solution enhanced narrow mesopore size distribution. The pore dimension and wall thickness values estimated from nitrogen adsorption and X-ray diffraction methods were consistent with the corresponding values obtained using transmission electron microscopy. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Silica based mesoporous MCM-41 has unique properties, such as high specific surface area and pore volume, tunable pore size and narrow pore size distribution. It has received the most attention because of its simple structure, as well as the ease of synthesis and tailoring of its structural and surface properties. Because of having these properties it is generally used as adsorbent and catalysts support. Metal incorporation into the matrix also enhances the catalytic activity. MCM-41 and metal containing MCM-41 type materials are synthesized by different methods. The direct hydrothermal synthesis has important advantages over the post synthesis methods such as economy of one pot use, uniform metal distribution and non-blockage of especially micropores [1–5]. Vanadium, molybdenum and niobium based oxide catalysts are known to have high activity in several reactions especially for the selective oxidation of aromatics, olefins and alcohols [6–22]. Most of the literature studies related with

∗ Corresponding author. Tel.: +90 312 582 35 06; fax: +90 312 230 84 34. E-mail addresses: [email protected] (A. Solmaz), [email protected] (S. Balci), [email protected] (T. Dogu). 1 Tel.: +90 312 498 21 50/11 26; fax: +90 312 498 24 40. 2 Tel.: +90 312 210 26 31; fax: +90 312 210 26 00. 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.08.086

MCM-41 incorporated with these metals generally involved the catalytic application rather than the investigation of the effect of metal incorporation to the micro- and mesopore structure [13–17,23–27]. Determination of the solid catalysts’ structural properties is an important task. X-ray diffraction (XRD), nitrogen adsorption, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses are the most valuable methods for the interpretation of the structural properties of MCM-like materials. The studies that involve the different characterization methods have been reported in the literature. However, in those studies the main focus was on the crystalline and mesopore structure analysis, little attention was given to the microstructure analysis [9,10,13–17,23–27]. The surface area and pore volume values along with the pore size distributions obtained using different methods enable real pore structure analyses which are highly important in selecting the proper catalytic application for the synthesized samples. In the present study, vanadium, molybdenum and niobium sources with different combinations were used in the preparation of the metal-MCM-41 samples and they were characterized by different techniques. The data obtained by several characterization techniques were compared and related to each other to form a thorough interpretation of the structural properties. Additionally it was aimed the detailed micropore analyses of the synthesized samples for proving the explicit structural information.

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Table 1 Physicochemical properties of MCM-41 and metal-MCM-41 samples. Sample

Chemical properties

Physical properties

Metal/Si atomic ratio in solution

MCM-41 V3-MCM-41 V6-MCM-41 V9-MCM-41 VMo4-MCM-41 VMo5-MCM-41 VMo6-MCM-41 VMoNb3-MCM-41 Nb3-MCM-41

Metal/Si atomic ratio in solid

V/Si

Mo/Si

Nb/Si

V/Si

Mo/Si

Nb/Si

– 0.03 0.06 0.09 0.04 0.05 0.06 0.03 –

– – – – 0.04 0.05 0.06 0.03 –

– – – – – – – 0.03 0.03

– 0.06 0.07 0.068 0.03 0.055 0.06 0.03 –

– – – – 0.028 0.011 0.021 0.003 –

– – – – – – – 0.056 0.07

2. Experimental 2.1. Synthesis of MCM-41 and metal-MCM-41 type catalysts MCM-41 type catalysts were prepared as reported by Gucbilmez et al. using sodium silicate (27 wt% SiO2 in 14 wt% NaOH, Aldrich as silica source), cetyltrimethylammonium bromide (CTMABr, Merck as surfactant) and deionized water (as solvent) [10]. A synthesis solution of 100 g was prepared taking a molar ratio of CTMABr/H2 O/SiO2 in the starting solution as 0.51/55/1. The metal-MCM-41 samples were hydrothermally synthesized with mono-, dual- and trio-combinations of metals using vanadium sulfate trihydrate (VOSO4 ·3H2 O, Aldrich), molybdenum oxide (MoO3 , Merck), and ammonium niobium oxalate (Nb2 C10 H8 N2 O33 , Aldrich). Metal/Si atomic ratios in the starting solution were taken as x, y and z for V, Mo and Nb sources, respectively, per mole SiO2 in the parent MCM-41 solution (x, y, z values, 0.03 ≤ x ≤ 0.09, 0.03 ≤ y ≤ 0.06, z = 0.03). In the previous work the metal sources in solid form were added directly to synthesis solution [10]. However in the present study to obtain good solubility and homogeneous distribution of the metals in the synthesis solution, V, Mo, Nb sources were dissolved in deionized water separately and then mixed with starting solution in the given order. The final pH value was around 11.5 and adjusted to 11.0 with 4N H2 SO4 solution to allow the formation of crystalline structure orderly. The final mixture was transferred to a teflon-lined stainless steel autoclave and kept at 120 ◦ C for 96 h; hereafter the gel was filtered and washed until the pH of the filtrate was equilibrated around 7.0. The solid product was dried at room temperature for 24 h and then calcined in a tubular furnace under 150 cm3 min−1 dry air flow by heating the sample from ambient temperature to 550 ◦ C with a heating rate of 1 ◦ C min−1 and keeping the sample at 550 ◦ C for 6 h. The samples were named according to their metal sources and atomic percent of the metal in the synthesis mixture (Table 1). 2.2. Characterization of MCM-41 and metal-MCM-41 samples The powder X-ray diffraction (XRD) patterns of MCM-41 and metal-MCM-41 samples were recorded on a Philips PW 3040 diffractometer equipped with CuK␣ radiation ( = 0.15406 nm) in the 2 range of 1–90◦ with a scan speed of 0.025 s−1 and a step size of 0.02. Nitrogen adsorption/desorption isotherms of the samples were obtained by means of a Quanthrocrome Autosorb 1C gas adsorption system at liquid nitrogen temperature. Prior to the measurement, samples were dried at 110 ◦ C for 12 h and outgassed at a temperature of 300 ◦ C under high vacuum for 3 h. Measurements were started at a relative pressure of around 10−7 . The total pore volume (Vt ) and the mesopore volume including micropores (V␮+m ) were estimated from the desorption data at P/P0 values of ∼0.99 and ∼0.96, respectively. Multi point BET surface area (SBET ) values were calculated within 0.05 < P/P0 < 0.30. Barrett–Joyner–Halenda (BJH) method was applied to the desorption data for P/P0 values above 0.35 to determine the mesopore surface area (SBJH ) and the mesopore volume (Vm,BJH ). de Boer adsorbate thickness t-plots were used for the estimation of the total (St from the slope of the first linear segment) and external (Sext from the slope of the second linear segment) surface areas and the micropore volumes (V␮,t from the intercept of the second linear segment). The micropore surface area (S␮,DR ) and micropore volume (V␮,DR ) values were also calculated by use of Dubinin–Radushkevich (DR) method. Saito–Foley (SF) and Dubinin–Astakhov (DA) methods were applied for the micropore size distribution and Barrett–Joyner–Halenda (BJH) method was used for mesopore size distribution [5,27,28]. By the use of solid densities of the oven dried samples measured with Micromeritics Multivolume He-Air Pycnometer 1305 and nitrogen adsorption data, the porosity values of samples were calculated. Particle size distributions of MCM-41 were analyzed with Malvern Mastersizer (2000 Hydro MU) laser particle analyzer. Scanning electron microscopy (SEM) and MAP images of some samples were taken by a JEOL-JSM-6360 with accelerating voltage of 20 kV. The near surface compositions of metals and Si in the synthesized solids were determined using an energy dispersive X-ray spectroscopy (EDS) equipped in the electron microscope JEOL-JSM-6400. Transmission electron microscopy (TEM)

d1 0 0 (nm)

a (nm)

ı (nm)

Solid density (g cm−3 )

Porosity

3.42 3.68 3.74 3.87 3.87 3.87 3.91 3.85 3.71

3.95 4.25 4.32 4.47 4.47 4.47 4.28 4.44 4.28

1.58 1.61 1.78 1.91 1.79 1.79 1.85 1.80 1.62

2.23 1.93 1.46 1.50 1.80 1.97 2.08 1.84 1.88

0.75 0.72 0.47 0.45 0.65 0.64 0.67 0.73 0.64

images were obtained using a JEOL-3010 instrument with an electron beam accelerating voltage of 300 kV. The TEM specimen was prepared by dispersing the powder in 2-propanol by ultrasonic treatment and dropping onto a holey carbon film supported on a copper grid, and then dried in air.

3. Results and discussion 3.1. XRD patterns XRD patterns of all the samples are presented in Fig. 1a and their estimated d1 0 0 -spacing, lattice parameter (a) and pore wall thickness (ı) values are tabulated in Table 1. The characteristic of long range ordered hexagonality as reflection peak (1 0 0) and the other two or three reflection peaks (1 1 0), (2 0 0), (2 1 0) were observed in the samples as described by Beck et al. [2]. The incorporation of Nb or dual metal into the matrix could cause improvements in the crystalline structure hence the characteristic peaks of MCM-41 became more evident. By the metal loading the first characteristic reflection peak (1 0 0) of the MCM-41 at a Bragg angle of 2.58◦ was moved to slightly lower values and the cell parameters (d1 0 0 , a, ı) increased with the corresponding decrease in the solid densities. The increase in the metal/Si ratio also resulted in further increase in the cell parameters’ values. These increases in d1 0 0 values resulted an increase especially in the micropore dimensions as it was supported by nitrogen sorption results (Tables 1 and 2). At the large Bragg angle range of 10–90◦ (Fig. 1b), no obvious diffraction peaks contributing to the crystalline metals/metal oxides were observed clearly. This might be due to the incorporation of the metal species into the amorphous walls of the ordered mesoporous silica with the small particle sizes or the nonhomogeneous distributions of different forms of metals at the studied low loadings to be detected by XRD. 3.2. Nitrogen adsorption/desorption isotherm behaviours The nitrogen sorption isotherm of pure MCM-41 reflected high nitrogen uptake in the low relative pressure range due to the micropore filling and a sharp increase in the adsorbed quantity around the saturation pressure showing the evidence of macropores (Fig. 2). The narrow hysteresis in the desorption branch supported the formation of the well-ordered mesoporous structure. As consistent with the literature the sorption isotherms of all metal containing samples corresponded to Type IV of IUPAC classification showing no significant increase in the adsorbed quantity at high relative pressures [5,27–29]. The observed significant increases in the adsorbed volume at low relative pressures (P/P0 < 0.01) indicated the presence of considerable amounts of micropores, and the sharp inflections (around P/P0 values of 0.25 and 0.45) in the adsorption branch and H1 type narrow hysteresis loop showed the presence of cylindrical mesopores in all samples. For low metal loaded samples,

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(a)

(b) V3-MCM-41

MCM-41 V6-MCM-41

V3-MCM-41

V9-MCM-41

V6-MCM-41 V9-MCM-41

Intensity (a.u.)

VMo4-MCM-41

VMo5-MCM-41 VMo4-MCM-41 VMo6-MCM-41 VMo5-MCM-41 VMo6-MCM-41 VMoNb3-MCM-41

Nb3-MCM-41 VMoNb3-MCM-41 Nb3-MCM-41 0

2

4

6

8

10 10

30

50

70

90

2 theta

2 theta

Fig. 1. XRD patterns of MCM-41 and metal incorparated-MCM-41 samples at (a) 2 ≤ 10◦ (b) 2 > 10◦ .

the observed sharpening of the inflection reflected the uniformity of the mesopore sizes and the high nitrogen uptake indicated the increase in mesopore volume values (see Sections 3.3 and 3.5). For the most of the samples the desorption branches conjoined with the adsorption isotherms around P/P0 values of 0.4. The opening in the hysteresis loop became significant and the conjunction occurred above 0.4 for the niobium containing samples (Fig. 2C) In the following sections, the structural analyses were carried out using several approaches and the relations among them are summarized. The estimated structural variables are reported in Table 2.

3.3. Pore volumes The total (Vt ) and the mesopore volume including micropores (V␮+m ) values estimated as 1.30 and 1.21 cm3 g−1 for MCM-41 did not change significantly with the low V content. The use of dual metal incorporation and the increase in metal concentration resulted a decrease in both values while the presence of Nb caused an increase in these values. The same behaviour was also observed in the BJH-mesopore volume (Vm,BJH ) values (Table 2). t-Plots, estimating the adsorbate thickness from de Boer; equation are given in Fig. 3 and the structural properties estimated from

Table 2 Pore structure analyses’ results of MCM-41 and metal-MCM-41 samples. Sample

MCM-41 V3-MCM-41 V6-MCM-41 V9-MCM-41 VMo4-MCM-41 VMo5-MCM-41 VMo6-MCM-41 VMoNb3-MCM-41 Nb3-MCM-41

Pore volume (cm3 g−1 )

Surface area (m2 g−1 )

Pore diameter (nm)

V␮,t

V␮,DR

Vm,BJH

V␮+m

Vt

Micropore volume (%)

SBET

St

SBJH

Sext

S␮ ,DR

dBJH

dSF

dDA

0.54 0.54 0.20 0.18 0.70 0.57 0.52 0.55 0.64

0.30 0.32 0.14 0.11 0.36 0.33 0.27 0.35 0.33

0.81 0.81 0.33 0.37 0.63 0.44 0.59 0.92 0.75

1.21 1.24 0.53 0.50 1.04 0.91 0.87 1.26 1.29

1.30 1.30 0.59 0.53 1.10 0.96 0.97 1.40 1.37

23 25 24 21 33 34 28 25 24

1050 1110 458 390 1084 967 872 1109 1211

1072 1096 499 397 1098 988 888 1193 1310

377 559 145 216 556 340 464 542 532

280 417 165 155 630 250 185 285 686

853 901 396 305 1023 947 756 996 1083

2.5 2.8 2.7 2.8 2.8 2.8 2.8 2.8 2.8

0.5 0.7 0.5 0.6 0.9 0.6 0.5 1.1 1.0

1.7 1.7 1.7 1.7 1.6 1.6 1.7 1.7 1.6

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151

1000

1000 1000 800

800

1000

(b)

800 600

600

400

400

400

200

200

200

0.0001

0.1

0 1E-07

1

700

0.0001

0.1

(c)

800

600

0 1E-07

3

-1

Volume of adsorbed N2 at STP (cm .g )

900

1000

(a)

0 1E-07

1

900 800 0.0001

0.1

1

700

600

600

500

500

400

400

300

300

200

200 MCM-41 V3-MCM-41 V6-MCM-41 V9-MCM-41

100

MCM-41

MCM-41

VMo4-MCM-41

100

Nb3-MCM-41

VMo5-MCM-41

VMoNb3-MCM-41

VMo6-MCM-41

0

0

0

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

P/P0 Fig. 2. N2 adsorption/desorption isotherms (filled symbols for adsorption, empty symbols for desorption) of MCM-41 and metal incorparated-MCM-41 samples.

the plots are summarized in Table 2. The micropore volume (V␮,t ) of MCM-41 estimated as 0.54 cm3 g−1 constituted around 40% of the total pore volume. Low metal loadings resulted no changes in the pore volumes for V-MCM-41 while it caused a slight decrease in the total pore volume and increase in the micropore volume of dual

metal incorporation. Further increases in metal content caused significant decreases in both pore volumes. Micropore volume values estimated from the DR method were found to be around half of those obtained from t-plots. The summation of the micropore volume from the DR (V␮,DR ) and the mesopore volume from the BJH 1.5

1.5

(b)

(c)

1.2

1.2

0.9

0.9

0.6

0.6

0.3

0.3

3

-1

Liquid volume of adsorbed N (cm .g )

(a)

V3-MCM-41 V6-MCM-41 V9-MCM-41 MCM-41

0

0

0.5

1

1.5

VMo4-MCM-41

2

0

0.5

VMo5-MCM-41

Nb3-MCM-41

VMo6-MCM-41

VMoNb3-MCM-41

1

1.5

2

0

Thickness (nm) Fig. 3. t-Plots of MCM-41 and metal incorporated MCM-41 samples.

0.5

1

1.5

2

0

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(Vm,BJH ) methods gave nearly the same pore volumes as estimated from the desorption data around P/P0 = 0.96. 3.4. Surface areas In the literature, mostly BET surface area values were reported. Although in some studies surface area values estimated from different methods were given, the relation among them and with the other structural properties was rarely reported for metal-MCM-41 samples. The BET surface area estimations are generally applicable for Type II isotherms (non-porous or mesoporous samples), however, isotherms of metal-MCM-41 samples resembled Type IV (micro- and mesoporous solids) isotherms. For accurate pore size analyses, the total surface area (St ) and external (mesopore) surface area (Sext ) from t-plots, the mesopore surface area (SBJH ) from BJH approach and the micropore surface area (S␮,DR ) from DR method were also estimated. The BET surface area of the MCM-41 sample was similar to the values found in the literature studies [10,13,18,25,30]. As in consistency with the pore volume behaviour, the low metal loaded samples had nearly the same BET surface area values as that of MCM-41. The increase of metal/Si ratio in the solution caused drastic decrease in BET surface area values for V-MCM-41 samples while it caused small declining in that of VMo-MCM-41 samples as in consistency with the literature [9,10,13,15]. Since Nb incorporation resulted more ordered crystalline (Fig. 1a) and uniformity in the pore structure (Fig. 2c), the Nb incorporated samples gave the highest BET surface area values similar to the results cited in the literature [8,30]. The BJH desorption mesopore surface area of the MCM-41 catalyst obtained as 377 m2 g−1 drastically increased (approximately 1.5 times) with the low metal loadings. In parallel to the decreases in the mesopore volume values, further increase of the metal content caused a decrease in the BJH surface area values. Interactions between the structure directing agent (surfactant) and silica entail, the replacement of water by inorganic species and the geometry of the surfactant form the shape and size of the pore structure. The addition of inorganic species leads the assembly of surfactants into ordered arrays and the charge density of the inorganic species affects the number of surfactants that are associated to each inorganic entity [4,31]. The micropores on the silica wall resulted in the hydrophilic head group of surfactant in the synthesized MCM-41. It could be said that the inorganic metal ions in the synthesis solutions probably settled on the hydrophilic end of the template and the charge density and concentration of them affected the orientation of the head groups and the average distance between them. The suggested mechanism caused improvements in the micropore properties of metal-MCM-41 samples. So the measured micropore surface area (853 m2 g−1 ) of MCM-41 from the DR method increased in parallel with the increase of micropore volumes. For most of the samples, the micropore surface area contributed above 80% of the total surface area values. Nb and dual metal incorporation resulted rather improvements in the micropore structures. The total surface area values from t-plots and BJH-mesopore surface areas were generally found to be somewhat above those of BET surface areas and external-mesopore surface areas from t-plots, respectively. The various types of surface area values estimated from different approaches represented the same behaviour as observed in BET surface area values by metal incorporation. 3.5. Pore size distribution (PSD) The BJH-mesopore size distribution (BJH-PSD) estimated from both adsorption and desorption data and the SF-micropore size distribution (SF-PSD) of samples are plotted in Fig. 4 and the DA-

micropore size distributions (DA-PSD) are given in Fig. 5. The estimated average pore dimensions are summarized in Table 2. Fig. 4 exhibits broad peaks for the micropores with sizes up to 1.5 nm and sharp peaks for the mesopores with maximums in the range of 2.0–3.0 nm and the adsorption–desorption BJH-PSD curves coincided in this range for all the samples. The peaks contributing to the mesopores became narrower by the addition of Nb or second metal to the matrix (Fig. 4d–h). Both the neat BJHPSD and the coincidence of the BJH-PSD curves obtained from the adsorption and desorption data in this range confirmed the formation of homogeneity in the crystalline and pore size distribution in these samples as consistent with the XRD results (Figs. 1 and 4d–h). A small peak in the desorption curve centered at 4.0 nm for the MCM-41 sample showed slight increases in the peak height by dual metal loading. Since Nb containing samples displayed different hysteresis behaviours, the second peak in the mesopore range became more evident. The continuing increase in the adsorption isotherm at high relative pressures (Fig. 2) for the MCM-41 also resulted in a small broad peak above 10 nm showing the presence of some quantity of large dimension pores (Fig. 4i). The PSD peaks’ heights for the MCM-41 sample showed similar behaviour both in the micropore and mesopore region with the maxima at 0.5 and 2.5 nm, respectively, and the maximums increased up to 1.1 and 2.8 nm, respectively, by metal incorporation. Nb- and dual metal incorporation caused sharpening and approximately twofold increases in the meso-peak heights while the increase of V incorporation resulted decreases in the mesopore peak height. The resembling average mesopore dimensions from the adsorption and desorption data also indicated the presence of the similar sized (regular shaped) pores with uniform dimensions except for the Nb containing samples (Fig. 4g–h, Table 2). Metal incorporation caused improvement both in the quantity and homogenous pore size distribution quality in mesopore range while it resulted a decrease in the height of the SF-PSD curves in parallel to the broadening of them. The broadening of SF-PSD curves and decreases in peak heights in both ranges become more evident by the increase of metal/Si ratio. The micro-PSD by the DA method demonstrated sharper distributions than those of the SF method. Decrease in DA-PSD peak heights was in consistency with SF-PSD. The DA dimensions were estimated to be around 1.7 nm for all samples (Fig. 5, Table 2). 3.6. Particle size, TEM and SEM images and chemical analysis The typical hexagonal pore openings of MCM-like material were observed in the TEM images of MCM-41 sample (Fig. 6a and b). Fig. 6b also shows non-uniform distribution of the pores as expected from the XRD pattern. The narrow channel arrays were observed in the metal-MCM-41 samples (Fig. 6c–h). The similarities in the pore dimension and wall thickness within the particle were observed. The pore dimensions were measured around 2 nm from the both hexagonal pore opening and arrays with an approximate wall thickness. These cell parameters were found to be in an agreement with the ones obtained from XRD/nitrogen sorption data. The parallel darkly stacking comprising the layered hexagonal voids (arrays) ranged up to approximately 40–100 nm lengths in agreement with the previous studies (Fig. 6) [32]. It was seen that particle sizes of the MCM-41 ranged within 2 and 100 ␮m and the average particle size of it was found to be 20 ␮m from particle size analysis. The MAP images of the V6, VMo5- and VMoNb3-MCM-41 samples confirmed the uniform metal distributions within the framework (Fig. 7A–C). The elemental composition of the samples obtained from EDS analyses showed that, an increase in the V/Si atomic ratio in the synthesis solution resulted in an increase in the V content of the solid product while

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Fig. 4. Micropore and mesopore size distribution of MCM-41 and metal incorparated-MCM-41 samples using SF and BJH methods (straight lines obtained from adsorption data, dashed line obtained from desorption data).

no significant variations were observed with an increase of Mo/Si atomic ratio in the solution (Fig. 7I–III, Table 1). The metal sources were dissolved and introduced to MCM-41 synthesis solution. The incorporation of more metals into the matrix compared with the literature [9] might be caused due to the followed synthesis procedure. For instance, Kilos and coworkers [9] reported that although in the synthesis solution both vanadium (Si/V) and molybdenum

(Si/Mo) values were adjusted as 32 (V/Si or Mo/Si = 0.03125), in the solid products they obtained these values as 259 (V/Si = 0.00386) and 2000 (Mo/Si = 0.0005), respectively. In Nb containing samples, relative amounts of niobium in the solid product were higher than the relative amounts in the synthesis solutions. Presence of niobium particles in the solid matrix more than the stoichiometric ratio could be resulted from the solubility behaviour of this metal

1.2

1.2 MCM-41 V3-MCM-41 V6-MCM-41 V9-MCM-41

Dv(d) [cm3.(nm.g)-1]

(a)

MCM-41 VMo4-MCMC-41 VMo5-MCMC-41 VMo6-MCM-41

(b)

(c)

MCMC-41 Nb3-MCM-41 VMoNb3-MCMC-41

0.8

0.8

0.4

0.4

0

0 1

2

3

4 1

2

3

4 1

2

Pore Diameter (nm) Fig. 5. DA-micropore size distribution of MCM-41 and metal incorparated-MCM-41 samples.

3

4

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Fig. 6. TEM images of (a) and (b) MCM-41, (c) V6-MCM-41, (d)–(f) VMo4-MCM-41, (g) and (h) VMoNb3-MCM-41.

Fig. 7. SEM and MAP images of (A) V6-MCM-41, (B) VMo5-MCM-41 and (C) VM6oNb3-MCM-41 and EDS spectrums of (I) V6-MCM-41, (II) VMo5-MCM-41 and (III) VMoNb3MCM-41.

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source or the settling of the particles (non-homogeneity) mostly within the measurement length of the EDS technique (Fig. 7C, III). 4. Conclusions The micro- and mesoporous pure MCM-41 and metal incorporated MCM-41 samples were hydrotermally synthesized. Characterization studies showed that metal incorporation improved the crystal and pore structures of MCM-41. The settlement of metal ions to the hydrophilic end of template affected the orientation of the head groups and caused improvements especially in the micropore structure. The observed similarities in the pore structure properties estimated from different methods confirmed the applicability of any approaches to the pore structure analyses of MCM-41 and metal-MCM-41 samples. Uniform hexagonality of mesopores exhibited sharp increases at the beginning of the nitrogen adsorption hysteresis while broad micropore size distributions were seen for all samples. The surface area values calculated from different approaches were found to be in harmony with each other and did not change too much with the low metal loadings and Nb incorporation. For all samples, the micropore surface area and the micropore volume values covered considerable percentages of the total. The cell parameters estimated from the XRD method, nitrogen adsorption isotherms and TEM analyses were close to each other. The EDS, SEM/mapping and TEM images exhibited achievement of the metal incorporation into the matrix. It was observed that the presence of Nb metals in MCM-41 improved its structural properties but an increase in the concentration of V in MCM-41 with and without Mo caused some decrease in surface parameters. The synthesized uniform mesopores structured metal-MCM-41 samples are also practicable catalyst for high molecules applications. Acknowledgements This work is partially supported by the Scientific Research Project Department of Gazi University (project codes BAP-06/200612 and BAP-18/2007-01). The authors also thank to the X-Ray Diffraction and Scanning Electron Microscopy Laboratory of Metallurgical and Materials Engineering Department, METU for the XRD and SEM/EDS experiments and to the TEM Laboratory of Physics Department of Kirikkale University for TEM experiments.

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