Effect of Synthesis Conditions on Textural Properties of Silica MCM-41

Document Type: Research Paper

Authors

1 Faculty of Petroleum, Gas and Petrochemical Engineering, Department of Chemical Engineering, Persian Gulf University, Bushehr, Iran

2 Department of Chemical Engineering, Faculty of Petroleum, Gas and Petrochemical Engineering, Bushehr, Iran

3 Faculty of Science, Department of Chemistry, Persian Gulf University, Bushehr, Iran

Abstract

Several samples of mesoporous silica were prepared via sol-gel chemistry in the presence of CTAB as template with variation of stirring intensity and aging time, ratios of CTAB/SiO2 and NH4OH/H2O in synthesis mixture. Effects of aging temperature, hydrothermal treatment and ter-Butanol addition to the synthesis mixture on the textural properties of obtained samples were also studied. Results of XRD, SEM, and N2 adsorption isotherm analyses shows particle size and morphology of mesoporous silica are influenced by synthesis conditions; although no significant change of pore size (

Keywords


1. Introduction

Research on ordered mesoporous silica materials has received increasing attention after the invention of M41S family at Mobil Company in 1992. These materials have attractive properties including good thermal stability, high surface area with controllable particle morphology and mesopore of about 4 nm. MCM-41 as one of the most widely studied ordered mesoporous silica has been the subject of research in various fileds of science and technology from sensors and separation [1] to catalysis [2] and drug delivery [3–5].

Therefore, controlling the formation of this kind of mesoporous to produce crystals of the required particle size and pore system is very important. Attempts to synthesize MCM-41 with controlled pore size started soon after their invention [6,7]; however, most of the investigations have focused on the application of MCM-41 [8–14] and other similar ordered mesoporous materials such as SBA-15 [15–21]. Pore size control is one of the important properties of the ordered mesoporous materials for their application in various fields including catalysis, adsorption, and drug delivery [6,7,22–25].

The influence of water content, silicon source, adding sequence of reactants on pore size, pore shape and product morphology was investigated in [6]. The study showed that the morphology of MCM-41 with uniform partied size and narrow mesopore distribution under the very dilute surfactant solution (~1%) turned into irregular or plate-like aggregates at higher concentrations of the surfactant. The aggregation degree of final products with water content or surfactant concentration was determined. The nature of silica source also significantly affected the morphology and pore size distribution of the products.

Successful enlarging of the pore size of mesoporous silica materials was carried out with larger template species such as the docosyltrimethylammonium chloride [26] in the synthesis of MCM-41 and bulky molecules as micelle swelling agent [27–31] such as mesitylene and tri-isopropyl benzene.

In another study, decane as a swelling agent to enlarge pores of MCM-41 samples was used to study the adsorption and release of bulky biomolecule heparin as a new heparin controlled delivery system [23]. As expected, some mesoporous composites could release heparin in the long term with tuned dosage.

Particle morphology and size is another important property[14,18,32–43] which influences the application of ordered mesoporous silicas, probably most seriously in drug delivery [14]. By using a co-condensation method, functionalized mesoporous silica nanoparticles (MSNs) with a MCM-41 pattern of structures were prepared for the drug delivery application. Highly dispersed MSNs with the surface areas greater than 800 (m2/g) and spherical size lower than 50 (nm) was reported [14].

Aqueous colloidal mesostructured silica nanoparticles (CMSS) were prepared by varying the kind and amount of trialkylbenzenes (TAB) and by varying Si sources [37].

When 1,3,5-triisopropylbenzene (TIPB) was used as TAB and tetrapropoxysilane (TPOS) tetramethoxysilane (TMS) was used as a Si source, both the pore size (from 4 nm to 8 nm and above than 10 nm) and particle diameter (from 20 nm to 380 nm) were enlarged. TIPB can enlarge the pore size and particle diameter more than TMB. A dialysis process was used successfully for the removal of surfactants, TAB and the preparation of CMSS.

In this study, the effect of different parameters such as ter-butanol as cosolvent, concentration of the surfactant and base, aging time and temperature on the final morphology of the mesostructured silica was investigated in the basic synthesis solution. Although many research studies have been conducted on the tuning of morphology of silica MCM-41 material, many of them have dealt with their applications in various fields. To the best of our knowledge, a systematic and comprehensive study of the effects of synthesis conditions on the textural properties of MCM-41 has not been reported previously. Furthermore, the synthesis of MCM-41 wormlike particles of 2 μm by using tertiary alcohol is reported for the first time.

 

2. Experimental

 

2.1.Materials

Hexadecyl tri-methyl ammonium bromide (CTAB ≥ 99.0%, Acros), solution of ammonium hydroxide (NH4OH, 25 wt.%, Merck), tetraethyl orthosilicate (TEOS ≥ 98.0%, Merck),  hydrochloric acid (HCl, 36.5 wt.%, Merck) and ter-butanol (TBA, Merck) were used as received.

 

2.2. Preparation

In a typical synthesis experiment, 0.66 g CTAB was added to the solution of 68.3 ml of NH4OH (25 wt.%) in 90 ml of deionized water and stirred for 30 min at the given temperature. In the case of using ter-butanol, the required amount was introduced after the addition of CTAB. 3.11 g of TEOS was added dropwise to the previous mixture and stirred for a determined time. The molar composition of the suspension was TEOS/NH4OH/CTAB/H2O/TBA = 1.0:32.7:0.120:525:0 (1:a:b:c:d). After washing with deionized water and filtration of the suspension for 3 times, the solid product was dried at 50 °C for 24 h. In order to remove the organic species, the obtained solid calcined in air at 550 °C for 6 h with heating rate of 2.5 °C/min. To investigate the effect of hydrothermal treatment, in one sample the suspension was transferred to a Teflon autoclave kept at 70 °C for 24 h. Table 1 reports starting the composition and other synthesis conditions of the prepared samples.

 

2.3. Characterization

The diffraction patterns were recorded using X-ray diffraction (XRD) with a CuKα (λ=1.5406 Å) 35 kV radiation (D8 Advance, Bruker, Germany) over the range 1°–6° (2θ) at a speed of 0.01°/minute.

The particle morphology of the obtained samples was analyzed by Fe-SEM images taken from Hitachi S-4160 instrument operating at 20 kV.

The mesostructure of the prepared materials was characterized using a Transmission Electron Microscope (TEM, LEO912-AB, LEO) operating at 100 kV and equipped with a high tilt piece achieving a point-to-point resolution of 0.25 nm.

Also, textural properties of the samples were characterized by nitrogen physisorption analysis at 77 K using a Micromeritics Tristar II 3 setup. The calcined samples were degassed at 623 K for 3 h. The specific surface area (SBET) of the materials was calculated using the BET-method in the relative pressure range between 0.06 and 0.25. The pore-size distribution (Dp) and pore volume (Vm) were determined from the desorption branches of the isotherms using the BJH method.

 

Table 1. Synthesis conditions of different samples with yield of synthesis

Hydrothermal

Stirring Intensity (rpm)

Temperature

(oC)

Time

(h)

d

b

a

Sample

-

750

55

2

0

0.121

103.2

M0

(Reference sample)

-

750

55

2

 

0.092

103.2

M-C/S-0.092

-

750

55

2

0

0.05

103.2

M-C/S-0.05

-

750

55

2

0

0.028

103.2

M-C/S-0.028

-

 

750

 

55

 

2

 

0

0.121

60.43

 

M-N/H-0.232

-

750

55

2

0

0.121

30.21

M-N/H-0.116

-

750

55

2

0

0.121

15.1

M-N/H-0.058

-

 

750

 

55

1

 

0

0.121

103.2

 

M-Time-1

-

750

55

4

0

0.121

103.2

M-Time-4

-

750

55

8

0

0.121

103.2

M-Time-8

-

750

20

2

0

0.121

103.2

M-Temp-20

-

750

40

2

0

0.121

103.2

M-Temp-40

-

750

65

2

0

0.121

103.2

M-Temp-65

24 h at 70 C

750

55

2

0

0.121

103.2

M-Temp-55

-H-70

-

500

55

2

0

0.121

103.2

M-500rpm

-

250

55

2

0

0.121

103.2

M-250rpm

-

750

55

2

2

0.121

103.2

M-T/H-0.005

-

750

55

2

4

0.121

103.2

M-T/H-0.011

-

750

55

2

8

0.121

103.2

M-T/H-0.023

-

750

55

2

16

0.121

103.2

M-T/H-0.047

-

500

55

2

0

0.092

60.43

M-Opt1

-

500

55

1

0

0.05

30.21

M-Opt2

 

3. Results and Analysis

 

3.1. Effect of CTAB/SiO2 ratio

Figure 1 shows XRD patterns of Mesoporous silicas obtained by increasing CTAB in the synthesis mixture. According to XRD patterns, the synthesis with CTAB/SiO2 ratio of 0.028 did not lead to highly ordered mesoporous silica (MCM-41) as diffraction peaks of planes (110) and (200) at 2θ more than 4 did not appear. Increasing CTAB/SiO2 ratio to 0.05 resulted in highly ordered mesoporous silicas, MCM-41. However, a reduction of XRD intensity was observed at the higher CTAB/SiO2 ratios. XRD data of samples with the highest CTAB/SiO2 ratio indicate a peak shift to right side in comparison with the sample synthesized with CTAB/SiO2 ratio of 0.05 (designated as M-C/S-0.05). In other words, XRD data represent unit cell length which decreases with the CTAB/Silicon ratio.

TEM micrographs given in Fig 2 and 3 prove the larger cross section of particles of sample M-C/S-0.05 in comparison with M-C/S-0.125 which are in agreement with the SEM results. TEM micrographs confirm the hexagonal crystal structure of the sample M-C/S-0.125. Parallel structure of cylindrical mesoporous silica rod is observed in TEM micrograph of sample M-C/S-0.05 given in Figure 3.

According to Figure 4 & 5, SEM images of M-C/S-0.028 particles represent a worm-like shape with a minimum length of 400 nm whereas M-C/S-0.125 particles are more spherical in shape with a minimum size of ca. 100 nm. This observation can be described by a wider distribution of growth rate of mesoporous silicon layers at higher CTAB concentrations resulting in spherical like particles, whereas layers of similar length  in less concentrations of CTAB would result in worm-like particles [44]. Furthermore, spherical shape of the particles is a better fit to space in a mixture concentrated with micelles. Higher concentrations of CTAB result in the formation of a larger number of micelles and more hexagonal units growing to smaller final size.

N2 isotherm adsorption data given in Figure 6 did not show pore size variation with CTAB/SiO2 ratio indicating constant diameter of micelles at least in the studied range. TEM images of M-C/S-0.05 also showed more inter-connected particles which could be related to small macroporosity observed in the N2 adsorption isotherm.

The reduction of unit cell length with almost constant pore size (Table 2) means that wall thickness of the pores reduces at the higher CTAB concentration.

 

 

Figure 1. XRD pattern of as-synthesized mesoporous silica with different CTAB/SiO2 ratios

 

          

Figure 2. TEM images of Sample M-C/S-0.125

 

         

Figure 3. TEM images of Sample M-C/S-0.05

 

          

Figure 4. SEM images of Sample M-C/S-0.125

 

          

Figure 5. SEM images of Sample M-C/S-0.028

 

 

Table 2. Pore structural parameters of the corresponding samples

Pore volume

(cm3/gr)

Surface area (m2/gr)

Wall thickness

(oA)

Pore size

(oA)

Molar ratio of

CTAB/SiO2

Sample

0.917

1080

15.85

26.0

0.125

M0-C/S-0.125*

0.507

728

17.92

24.9

0.05

M-C/S-0.05

*Reference sample

 

 

 

Figure 6. a) N2 sorption isotherms, b) pore size distribution from Adsorption BJH model of calcined samples synthesized with different CTAB/SiO2 ratios

 

3.2. Effect of NH4OH/H2O ratio

Increasing ammonium hydroxide in the synthesis mixture of the mesoporous silica resulted in the less amount of final product which is in agreement with other studies [45]. This observation may be related to the higher pH of the synthesis mixture. The condensation rate of silica decreases with solution pH after neutral pH [46]. As a result, at lower amounts of NH4OH, more solid particles are formed as a solid product from synthesis through washing and separation steps. More increase of NH4OH resulted in the reduced condensation rate of silica and enhanced silica hydrolysis which reduced the obtained solid product. It should be noted that low concentration of NH4OH has a negative effect on the crystalline hexagonal order of the mesoporous silica as shown in the XRD patterns given in Figure 7. Low hydrolysis with high condensation rate may result in too fast aggregation to establish hexagonal order in all directions.

XRD pattern of samples with different N/H ratios indicates a small peak shift to right side up to the ratio of 0.232 after which the direction of peaks shift reverses. This result implies that NH4OH amount affects the unit cell dimension of the produced mesoporous silica. Reducing condensation rate with NH4OH and subsequent reduction of hexagonal silica tubes wall thickness and the reverse trend at the highest N/H ratio shows a dual effect of NH4OH on the size of the unit cell. A swelling effect on the micelle size may be assumed for NH4OH after a threshold concentration in the solution.

 

 

Figure 7. XRD pattern of as-synthesized mesoporous silica with different NH4OH/SiO2 ratios

 

 

Figure 8. TEM images of Sample M-N/H-0.116

However, NH4OH amount might influence the particle size of MCM-41 obtained at two other concentrations since signal intensity of their XRD pattern enhances. Higher signal intensity usually correlates with larger crystallite size through Scherrer equation possibly to larger particle at favourable conditions.

Parallel cylindrical structure of the mesoporous silica tubes is observed in TEM micrograph of sample M-N/H-0.116 given in Figure 8. At least one dimension of ca. 200 nm is inferred from TEM micrograph, of M-N/H-0.116 which is larger than particle size of M-N/H-0.306 (Figure 2). This result supports past explanations of XRD size-signal intensity relation.

SEM images of M-N/H-0.116  particles (Figure 9) represent a worm-like shape with a minimum length of 200 nm whereas particles of M-N/H-0.306 (Figure 4) have spherical shape with a minimum size of ca. 100 nm. It seems that less concentration of NH4OH results in side by side aggregation of smaller building particles giving rise to cylindrical growth. In the condition of higher concentration of NH4OH, smaller building particles could probably assemble and join together more symmetrically in the presence of sufficient numbers of bridging OH.

 

          

Figure 9. SEM images of Sample M-N/H-0.058

 

Table 3. Pore structural parameters of the corresponding samples

Pore volume

(cm3/gr)

Surface area (m2/gr)

Wall thickness

(oA)

Pore size

(oA)

Molar ratio of

CTAB/SiO2

Sample

0.917

1080

15.85

26.0

0.306

M0-N/H-0.306*

0.676

960

17.5

24.7

0.116

M-N/H-0.116

*Reference sample

 

N­2 Adsorption isotherm of M-N/H-0.116 given in Figure 10 shows typical shape of mesoporous materials with a hysteresis loop formed in the middle of N2 pressure range. BJH pore size distribution does not indicate any significant changes in the pore size (Table 3) between samples M-N/H-0.116 and M-N/H-0.306 (M-C/S-0.05). It means that the amount of NH4OH has no effect on the pore size. Although CTAB amount used in the synthesis of the above- mentioned samples are different, no effect on the pore size of products was observed in the previous section.

 

 

Figure 10. a) N2 sorption isotherms, b) pore size distribution from Adsorption BJH model of calcined samples synthesized with different NH4OH/H2O ratios

 

3.3. Effect of Aging Time

According to XRD spectra given in Figure 11, longer aging time (i.e. time of reaction after the introduction of all species) has a significant effect on unit cell of the obtained mesoporous silica as it varied from 41.3 to 42.7. Hexagonal order of the mesoporous silica is formed in the short time of 1h. These observations all indicate that silica tube wall becomes thicker with time while hexagonal assembly of silica tubes has already been established. Comparing SEM images of the sample obtained after 8 h of aging, given in Figure 12, with SEM images of the reference sample (aging time of 2h) given in Figure 4, it can be inferred that particle size of the mesoporous silica increases with aging time. Also ca has the minimum size of the particles (i.e. 500 nm) in the images of Figure 12. Longer aging time provides more time for the aggregation of grains to form larger product particles.

 

 

Figure 11.  XRD pattern of as-synthesized mesoporous silica obtained at different aging times

 

 

Figure 12. SEM images of Sample M-Time-8h

 

3.4. Effect of Aging Temperature

XRD spectra given in Figure 13 show higher aging temperature (i.e. temperature of mixture after the introduction of all species). It favours mesoporous structure with larger unit cell size and higher hexagonal order. Unit cell size increases with aging temperature by ca. 3 Å.

Another noteworthy observation in the XRD spectra is the intensification of reflections of plane (200) and (110) the sample prepared with hydrothermal treatment at 70 °C.

N2 sorption data reported in Table 4 indicates that pore size increases with aging temperature from 21 to 25 Å at the expense of wall thickness of silica tubes. It may be concluded that the same concentration of CTAB leads to larger micelles at the higher aging temperatures.

SEM images of sample obtained at the aging temperature of 20 °C shows shapeless large particles of ca.1 μm size for the shortest dimension. The lower temperature of mixture enhances grain aggregation to larger particles since the process of aggregation is thermodynamically favourable at lower temperatures. Parallel lines in TEM micrograph in Figure 14 confirm the porous structure of the sample M-Temp-20. Species with hollow structure seen in SEM images of Figure 15 could be responsible for less intensity seen in XRD pattern of sample M-Temp-20.

Hydrothermal treatment at 70 °C for 24 h resulted in mono dispersed particles of nearly hexagonal morphology as seen in SEM graphs given in Figure 16. Particle size increases to at least 500 nm with the applied hydrothermal treatment whereas in the absence of hydrothermal step the particle size reduces to the minimum of 100 nm (M-C/S-0.125). Regarding aging effect on the particle size, the effect of hydrothermal treatment on the particle size may be related to the duration length of this step. The thermal part could influence the pore size of the silica tubes and probably leads to better geometrical order of the hexagonal shape particles.

 

 

Figure 13. XRD pattern of as-synthesized mesoporous silica obtained at different aging temperatures

 

          

Figure 14. TEM images of Sample M-Temp-20

 

          

Figure 15. SEM images of Sample M-Temp-20

 

          

Figure 16. SEM images of Sample M-Temp-55-H-70

 

Table 4. Pore structural parameters of samples synthesized with different aging temperatures

Pore volume

(cm3/gr)

surface area (m2/gr)

Wall thickness

 (Å)

Pore size

(ᵒA)

Temperature

(oC)

Sample

0.917

1080

15.85

26

55

M0-Temp-55*

0.411

986

17.1

21.5

20

M-Temp-20

*Reference sample

 

3.5. Effect of Stirring Intensity

XRD spectra of samples obtained at different stirring rate given in Figure 18 show no significant peak shift and hexagonal order of MCM-41 is confirmed in all samples with the presence of two relevant reflections in 2θ range of 4 to 5°. TEM micrographs of sample M-500rpm show typical image of MCM-41(Figure 19). SEM images of sample M-500 in Figure 20 indicate particle size of 300 to 500 nm which are larger than particles of 100 to 250 nm can be obtained at the stirring speed of 750 rpm. Formation of smaller particles at the higher stirring rate is a known effect in particle growth in liquid phase explained by fluid shear stress exerted on the external surface of the particle. According to BJH, pore size distribution in Figure 21, a considerable contraction of pore size with increasing stirring rate can be observed (Table 5).

 

 

Figure 17. a) N2 sorption isotherms, b) pore size distribution from Adsorption BJH model of calcined samples synthesized with different aging temperatures

 

 

Figure 18. XRD pattern of as-synthesized mesoporous silica obtained at different stirring intensities

 

          

Figure 19. TEM images of Sample M-500rpm

 

          

Figure 20. SEM images of Sample M-500rpm

 

Table 5. Pore structural parameters of samples synthesized with different stirring rates

Pore volume

(cm3/gr)

surface area (m2/gr)

Wall thickness

 (Å)

Pore size

(ᵒA)

Stirring rate

(rpm)

Sample

0.917

1080

15.85

26

750

M0-750rpm*

0.804

972

14.0

28.1

500

M-500rpm

*Reference sample

 

3.6. Effect of ter-Butanol Addition

XRD pattern data, given in Figure 22, confirm hexagonal order of the prepared mesoporous silica MCM-41 at all ratios of ter-butanol as three standard reflections appeared in the expected 2θ.

Crystallographic data reported in Table 6 indicate a decrease of unit cell followed by an increase with ter-butanol/H2O ratio in the synthesis mixture. The reason for initial contraction of unit cell with ter-butanol addition to synthesis mixture is not clear. However, the consumption of a minor amount of CTAB with ter-butanol in the formation of inactive new micellar system for silica condensation and the contraction of normal micellar system with the remaining CTAB may be a probable reason for this observation. Subsequent unit cell enlargement can be attributed to the swelling effect of ter-butanol on the micelles.

 

 

Figure 21. a) N2 sorption isotherms, b) pore size distribution from Adsorption BJH model of calcined samples synthesized with different stirring rates

 

SEM image of sample M-T/H-0.011 in Figure 23 indicates the increase of particle size to the range of 1 μm with a wormlike shape. The particle size increase indicates enhancement of condensation/agglomeration rates which can be attributed to the co-solvent role of the ter-butanol.

 

 

Figure 22. XRD pattern of as-synthesized mesoporous silica obtained at different ter-Butanol ratios

Table 6. Angle of reflection of crystallography planes and unit cell dimension of mesoporous silica prepared with different ter-Butanol ratios

ao

(Å)

Intensity

(100)

2θ (100)

TBA/H2O

 

Sample

41.85

6300

2.43

0

M0-T/H-0.0*

40.34

5012

2.53

5.94e-3

M-T/H-0.005

41.28

5821

2.47

11.88e-3

M-T/H-0.011

40.80

5791

2.50

23.76e-3

M-T/H-0.023

*Reference sample

 

          

Figure 23. SEM graph of sample M-T/H-0.011

 

3.7. Optimum samples

Results of studies on the influence of different synthesis parameters on the pore size, morphology, hexagonal order and particle size of the prepared ordered mesoporous silicas suggest two selected sets of synthesis parameters to intensify XRD signal intensity (and higher crystallinity) of reflection of plane (100). Synthesis conditions of samples designated as M-Opt1 and M-Opt2 have been given in Table 7.

XRD results in Figure 24 show stronger reflection of plane (100) in M-Opt1 and a peak shift to smaller 2θ for M-Opt2. More uniform pore wall of M-Opt1 can be inferred by comparing TEM micrographs in Figure 25 and 26, respectively. The interesting TEM image of plane normal to silica tubes centreline given in Figure 26 shows hexagonal arrangement of the silica tubes bundle.

TEM micrograph of sample M-Opt2 (Figure 26, left) shows large meso porosity as cavities of 10 to 40 nm. Hence, it can be deduced that less amount of NH4OH resulted in enhanced macro porosity formation in the prepared samples.

SEM images of M-Opt2 in Figure 27 shows worm-like particles of 150×250 nm2 which are formed by grains as large as 50 nm. The particles in the reference sample are larger and more spherical.

N2 adsorption isotherm of sample M-Opt1 and M-Opt2 in Figure 28 shows macro porosity formation, especially in the latter case. SEM images (Figure 4) of reference sample indicate smoother external surface which agrees with its smooth N2 adsorption curve near the saturation point (P/P0 ≈ 1).

 

Table 7. Synthesis conditions of samples designated as optimum samples

Hydrothermal

 

Stirring rate

 (rpm)

Temperature

(oC)

Reaction time

(hr)

Molar Ratio

NH4OH/H2O

Molar ratio

CTAB/SiO2

Sample

-

500

55

2

0.232

0.092

M-Opt1

-

500

55

1

0.116

0.05

M-Opt2

 

 

Figure 24. XRD pattern of as-synthesized mesoporous silica obtained at the optimum conditions

 

          

Figure 25. TEM images of Sample M-Opt1

 

          

Figure 26. TEM images of Sample M-Opt2

 

Figure 27- SEM images of Sample M-Opt2

 

Table 8. Pore structural parameters of samples synthesized at the optimum conditions

Pore volume

(cm3/gr)

Surface area

 (m2/gr)

Wall thickness

(oA)

Pore size

(oA)

Samples

0.917

1080

15.85

26

M0*

0.636

874

16.25

25.0

M-Opt1

0.759

784

15.9

26.5

M-Opt2

*Reference sample

 

 

Figure 28. a) N2 sorption isotherms, b) pore size distribution from Adsorption BJH model of calcined samples obtained at the optimum conditions

 

4. Conclusions

Increasing template in the synthesis of MCM-41 showed several effects including particle size decrease, change from worm-shape to spherical, and enhancing hexagonal order.  Also, no significant influence on the pore size was observed.

More mesoporous silica with the spherical particle instead of worm-shape was obtained by increasing ammonium hydroxide in the synthesis. Particle size reduction was observed at higher NH4OH/SiO2 ratios. Low concentration of NH4OH had a negative effect on the crystalline hexagonal order of the mesoporous silica.  No significant influence on the pore size was observed as well.

Longer aging time provides more time for the aggregation of grains to form larger product particles (> 500 nm). Hexagonal order of MCM-41 is formed after 1h of aging time

Thermal effect on the pore size, unit cell and particle morphology was more pronounced than other parameters. The size of pore (≈5 Å) and subsequently unit cell increases due to micelle enlargement with temperature. Larger particles (> 1 μm) are formed at lower temperature due to the enhanced aggregation

Using hydrothermal treatment results in the formation of hexagonal-shape large (> 500 nm) particles with the increased size of the pores and the unit cell.

Stirring rate adversely influenced size of the particles (200-250 nm at 250 rpm) and the pore size (2 Å at 250 rpm) of the mesoporous silica.  The spherical shape remained unvaried.

Size of the formed wormlike particle (>1 μm) of MCM-41 increased with ter-Butanol which indicates enhancement of the condensation/agglomeration rates due to co-solvent role of ter-Butanol.

Results of the characterization analysis of the sample prepared at lower CTAB/SiO2 ratio, less amount of NH4OH, and higher stirring rate indicated formation of MCM-41 particles with smaller size. Sample of larger particles with the reduced crystallinity was obtained at the lowest amount of NH4OH and short aging time.

 

Acknowledgment

The study was financially supported by National Gas Company in Bushehr province and Persian Gulf Science and Technology development Center.

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