Investigations on the development of MCM-41 as a potential mesoporous silica based reference material for the analysis of multi-textural properties

Abstract

Investigations on the time dependent analysis of the structural and textural properties of the mesoporous silica material MCM-41 were performed by analyzing the samples at regular intervals for one year by powder X-ray diffraction (PXRD) and nitrogen sorption techniques for samples stored (i) under a dry nitrogen atmosphere in a glove box and (ii) under dry ambient atmospheric conditions. The stability and durability of this high surface area material gave promising results under both storage conditions in terms of its structural and textural properties. Under the typical storage conditions employed, the values of various textural properties, including the surface area (SA, m² g⁻¹), pore volume (PV, cm³ g⁻¹), pore diameter (PD, Å) and wall thickness (WT, Å) determined by nitrogen adsorption were found to be 1030 ± 49, 1.015 ± 0.066, 27.55 ± 0.60 and 17.04 ± 0.61 respectively for the glove box samples. Almost identical values (SA = 1022 ± 45 m² g⁻¹), (PV = 0.933 ± 0.065 cm³ g⁻¹), (PD = 27.54 ± 0.14 Å), (WT = 16.58 ± 0.81 Å) were obtained for the samples under ambient atmospheric conditions. The experimental analysis indicated that this material has potential as a standard reference material for the analysis of the multi-textural characteristics of high surface area mesoporous materials.

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1. Introduction

Porous materials are of scientific and technological importance because of the presence of voids of controllable dimensions at the atomic, molecular, and nanometer scales, enabling them to discriminate and interact with molecules and clusters, and interact with atoms, ions and molecules not only at their surfaces but through the bulk of the materials. The distribution of sizes, shapes and volumes of the void spaces in porous materials directly relates to their ability to perform the desired functions of a particular application. The need to create uniform pore sizes, shapes and volumes has steadily increased over recent years because these factors can lead to superior properties which find applications in various fields such as catalysis,^1–5 adsorption,⁶ hybrid optics,⁷ biomedical devices,⁸ sensors,⁹ drug delivery¹⁰ separation processes¹¹ and many more. All of these applications are largely dependent on the textural properties of the materials. This interest in various porous matrices has triggered the need for a new generation of environmentally stable and durable standard reference materials for textural analysis. The characterization of novel and nanoporous films, spheres, fibers and monoliths requires a new standard reference material for textural analysis. Thus, studies on the textural properties are very important for such materials. The surface area (SA), pore volume (PV), pore diameter (PD) and wall thickness (WT) constitute the major textural properties that are of importance to the above applications.

Every instrument and apparatus has to be standardized and calibrated at appropriate intervals to ensure accurate and precise performance and data collection. Many of these instrument techniques need a reference material or standard for calibration. Surface area and pore size analyzers need standard reference materials for calibration. The Institute for Reference Materials and Measurements (IRMM), the Bundesanstalt für Materialforschung und-prüfung (BAM) and the National Institute of Standards and Technology (NIST) are major suppliers of surface area standards,^12–14 and reference standards with surface areas ranging from 0.0686 to 258.0 m² g⁻¹ are available. The BAM is the major producer and supplier of reference standards for pore size analysis. CRM BAM-PM-103 and CRM BAM-PM-104 are the standards for pore size analysis. The high surface area standards are activated nanoporous carbon BAM-P108 with a BET surface area of 550 ± 5 m² g⁻¹ and BAM-P109 with a BET surface area of 1396 ± 24 m² g⁻¹.

Although different materials like alumina and silica are used as reference materials for surface area analysis, only alumina based materials have been developed as reference materials for pore size analysis. However, majority of the reference standards for surface area and pore size analysis are related to alumina. Studies have been conducted to develop more silica based reference standard materials. Gel-derived silica was investigated for its textural properties and was developed as a standard reference material.^15–17 This gel-derived silica had a surface area of 165.5 m² g⁻¹ and a pore radius of 11.9 nm. So it is well understood that most of these standard reference materials are of lower surface area when compared with high surface area carbon, mesoporous silica and MOFs. Many of the recently developed porous materials are mesoporous materials of very large surface area. The prominent features of periodic mesoporous materials are: very high surface areas, well defined pore shapes, narrow distributions of pore sizes, very high degrees of pore order, the ability to tailor and fine tune the pore dimensions, large pore volumes, large amounts of internal hydroxyl groups, the ease of modification of surface properties, enhanced catalytic selectivity and excellent thermal and mechanical stabilities. The advantages associated with these properties include: (i) permitting free ingress of reactants and egress of product species that have cross-sections smaller than the diameter of the pores, (ii) offering a greater scope for the grafting of organometallic moieties onto the inner surface of the pores for the heterogenization of homogeneous catalysts and (iii) opening up new strategies for the production of novel materials like porous carbons and other composite materials. There is a pressing need to develop a standard from these high surface area materials. For a material to have the potential to be developed as a reference material, it is necessary to qualify the durability and stability. Durability refers to the performance over a period of time and stability refers to the performance on repeated usage.

Mobil Composition of Matter, MCM-41,¹⁸ has a large surface area and nanometer-sized pore sizes (from 20 to 100 Å) and finds application in adsorption,¹⁹ separation,²⁰ catalysis^21,22 and sensing.²³ It also offers a special environment for the chemical separation of large molecules.²⁴ It has a simple structure and an easy synthesis procedure. It is the most suitable model mesopore adsorbent available for studying some of the fundamental features of adsorption, such as the effects of pore size, hysteresis, etc., owing to its relatively uniform cylindrical/hexagonal pore channels. It is characterized by parallel and ideally shaped pore structures without the complications of a network. The cylindrical pore structure and high degree of pore symmetry associated with MCM-41 merit it as a representative mesoporous material for the testing of existing adsorption and diffusion models.²⁵ Our intense research activities on mesoporous material based catalysis^{1–5,26–28} and these specific properties of MCM-41 led us to carry out investigations on its use as a standard. Therefore to the best of our knowledge, we herein report the first investigation into the development of the ordered mesoporous material MCM-41 (ref. 18), associated with a high surface area, as a reference material with the potential to analyze properties such as surface area, pore volume, pore diameter and wall thickness as a single standard.

In the present investigations the well-known material MCM-41 was hydrothermally synthesized and studied for its durability and stability at regular intervals for a period of one year. A nitrogen adsorption–desorption technique is used for the characterization of the textural properties of solid materials. Microporous and mesoporous materials can be studied using this technique to measure the surface area, pore volume, pore diameter and wall thickness etc. Additionally, powder X-ray diffraction (PXRD) of MCM-41 was performed over a period of one year to study the effect of storage on the structural properties of the material.

2. Experimental

2.1. Materials

Sodium silicate solution and sulphuric acid were procured from S D Fine Chemicals, Mumbai, India. Cetyltrimethylammonium bromide (CTAB) was purchased from Sigma-Aldrich, USA. Deionized Milli Q water was used throughout the synthesis.

2.2. Synthesis of MCM-41

MCM-41 was synthesized using sodium silicate and CTAB by a procedure described by Beck et al.¹⁸ The ratio of 1 : 0.5 : 74 of SiO₂ : CTAB : H₂O was used for the starting gel formation. In a typical synthesis procedure, 18.2 g of CTAB was dissolved in 100 mL of deionized water and stirred using a mechanical stirrer. A 20.3 g sample of sodium silicate was dissolved in 20 mL of deionized water and added to the CTAB solution at a rate of 5 mL min⁻¹ using a peristaltic pump while stirring. The stirring was continued for 15 min and then the pH of the formed gel was found to be ∼11.3. The pH was then adjusted to 10 by the addition of a solution of 1 : 1 (v/v) H₂SO₄ : H₂O. The final gel was transferred to a 500 mL glass bottle and kept in oven at 100 °C for 6 days. It was then filtered, washed and dried at 80 °C for 4 h and the formed product was designated SN1. This as-synthesized (SN1) product was calcined at 600 °C for 6 h under dynamic air and named SN2. This calcined material (SN2) was typically transferred to different screw-top dry plastic vials with an amount of ∼500 mg in each vial. The vials were stored under two sets of conditions. For one set of samples, the vials were stored under dry conditions in a glove box (MBRAUN, Germany) and designated SN2-GB. For the second set of samples, the closed plastic vials were stored under dry ambient atmospheric conditions and designated SN2-AC.

PXRD analysis was recorded using a Rigaku-MiniFlex instrument, Japan, with Ni filtered Cu Kα radiation (1.54056 Å) at 30 kV and 15 mA in the 2θ range of 1.0° to 8.0° at a scan speed of 0.25° min⁻¹. The peak positions and peak intensity were obtained from the instrument’s software. The surface area, pore volume, pore diameter and wall thickness were determined by N₂ adsorption at 77.3 K using an automated surface area and pore size analyzer (Quadrasorb SI, Model no: 3SI-9, Quantachrome Instruments, USA) with a pressure tolerance (ads/des) of 0.050/0.050, equilibrium time (ads/des) of 200/200 s, and equilibrium timeout (ads/des) of 400/400 s. The samples were degassed at 300 °C for 3 h prior to analysis. A multi-point BET plot was plotted between P/P₀ of 0.08 to 0.234. The pore size distribution analysis was performed using a BJH method using the desorption branch of the isotherm, and the total pore volume was calculated from the adsorption data at ∼0.99 P/P₀.

3. Results and discussion

3.1. Preliminary evaluation of MCM-41 formation by PXRD

PXRD patterns of the fresh as-synthesized and calcined samples are given in Fig. 1. The MCM-41 formation was confirmed by the existence of diffractions at 2θ values, in degrees, of 2.018 (100), 3.58 (110), 4.11 (200) and 5.376 (210) for the as-synthesized sample. For the calcined MCM-41 sample, these diffraction peaks were shifted to slightly higher 2θ values with a significant increase in intensity owing to the removal of CTAB.

Fig. 1 PXRD patterns of the as-synthesized and calcined MCM-41 samples.

The data obtained from the PXRD analyses are given in Table 1. The values of 2θ for the (100) plane were calculated from the experimental values of 2θ for the 110, 200 and 210 planes by eqn (1)–(3).

2θ(d₁₀₀) = 2θ(d₁₁₀)/√3 (1)

2θ(d₁₀₀) = 2θ(d₂₀₀)/2 (2)

2θ(d₁₀₀) = 2θ(d₂₁₀)/√7 (3)

Table 1 PXRD data of the as-synthesized and calcined MCM-41 samples^a

Sample	Parameter	d100 (Å)	d110 (Å)	d200 (Å)	d210 (Å)	Standard deviation	Unit cell a0, Å
a XRD conditions: 2θ = 1–10°, scan speed = 0.25, sampling width = 0.02, kV = 30, mA = 15.
SN1 (as synthesized)	2θ experimental	2.018	3.580	4.110	5.376		49.38
	2θ(d100) calculated	2.018	2.067	2.055	2.032	0.0221
	d Spacing experimental	43.70	24.69	21.48	16.43
	d Spacing (d100) calculated	43.70	42.76	42.96	43.47	0.4354
	Intensity, cps	1407	178	123	93
SN2 (freshly calcined)	2θ experimental	2.274	3.957	4.540	6.010		44.62
	2θ(d100) calculated	2.274	2.284	2.270	2.271	0.0065
	d Spacing experimental	38.81	22.31	19.43	14.70
	d Spacing (d100) calculated	38.81	38.64	38.86	38.89	0.1113
	Intensity, cps	4460	538	328	120

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Similarly the d spacing of 100 planes were calculated by eqn (4)–(6).

d₁₀₀ = d₁₁₀ × √3 (4)

d₁₀₀ = d₂₀₀ × 2 (5)

d₁₀₀ = d₂₁₀ × √7 (6)

There is an improvement in the regularity of the material after calcination as indicated by the decrease in values of standard deviation for 2θ and d₁₀₀ spacing.

a₀ = (2 × d₁₀₀)/√3 (7)

3.2. Durability and stability studies

Studies on the effect of aging on the structural and textural properties (stability) of the MCM-41 silica have been undertaken. The stability of the calcined MCM-41 sample was determined by analyzing the structural and textural properties of the samples at different time intervals by PXRD and N₂ adsorption analysis at liquid N₂ temperature (∼77 K).

3.3. Effect of aging on textural properties

N₂ sorption analysis of the samples stored under the two different conditions was carried out at regular time intervals. All of the analysis was done under the degassing conditions of 300 °C for 10 h. For the BET surface area analysis, adsorption values at 5 points in the range 0.07–0.2 (±0.01) P/P₀ were considered. The BJH pore diameter adsorption branch of the isotherm was used for the calculation of surface area (SA), pore volume (PV), pore diameter (PD) and wall thickness = (a₀ − PD) [a₀ = unit cell parameter obtained from PXRD, according to eqn (7)]. Analysis was performed and detailed results are given in the supplementary information (Tables 1S and 2S† for samples stored in closed vials in a glove box and under atmospheric conditions in closed vials respectively). A representative N₂ adsorption isotherm, BET plot and pore size distribution curve are given in Fig. 2–4 respectively.

Fig. 2 N₂ adsorption isotherm of calcined sample (SN2).

Fig. 3 Multi-point BET plot of calcined sample (SN2).

Fig. 4 Pore size distribution curve of calcined sample (SN2).

The first analysis was carried out in quadruplicate for the freshly calcined material and the rest of the analyses were done at regular intervals of two months in triplicate and their mean and standard deviation are given in Tables 1S and 2S (ESI†). The observed surface area, pore diameter, pore volume and wall thickness at different months are given in Fig. 5–8. The mean values of the surface area for the samples in the glove box (Table 1S†) were in the range 975–1099 m² g⁻¹ with a standard deviation range of 2.2–70.6. Under atmospheric conditions in closed vials (Table 2S†), the mean values of the surface area were in the range 977–1082 m² g⁻¹ with a standard deviation range of 2.2–71.6. The standard deviations for the surface area analyses carried out at 4 months were the highest for both the glove box sample and atmospheric conditions sample. The variation in the results may be attributed to experimental error. This variation is prominent only in the case of 4 month samples, which is indicative of a problem associated with the analytical handling of the sample, either from human or instrumental error. The surface area was calculated from a 5 point BET plot. Therefore any variations in these points can lead to higher percentage error in the surface area. Even a small change in the degassing or even the analytical conditions can affect the result. From the PXRD results (Fig. 9 and 10) it is clear that there is no change in its structural feature.

Fig. 5 Variation of BET surface area with time.

Fig. 6 Variation of pore diameter with time.

Fig. 7 Variation of pore volume with time.

Fig. 8 Variation of wall thickness with time.

Fig. 9 PXRD patterns of samples stored under glove box conditions.

Fig. 10 PXRD patterns of samples stored under atmospheric conditions.

Analytical results of textural properties of the material stored under glove box and atmospheric conditions for different time intervals revealed that there is good reproducibility in the values of the surface area, pore volume, pore diameter and wall thickness under the employed time of analysis over a period of one year. To assign a single value for all these textural properties for both the samples stored in glove box and atmospheric conditions, overall means of all the values and standard deviations were taken corresponding to all the four properties and are given in Table 2. Almost identical values were found for all the four parameters for the MCM-41 stored under glove box and atmospheric conditions.

Table 2 Values of the textural properties of MCM-41 stored under two conditions for one year

Storing system	Value	BET SA, m^2 g^−1	Total PV, cm^3 g^−1	BJH adsorption PD, Å	Wall thickness, Å
Glove box conditions	Mean value	1030	1.015	27.55	17.04
	Standard deviation	49.0	0.066	0.60	0.61
	% Standard deviation	4.8	6.5	2.2	3.6
Atmospheric conditions	Mean value	1022	0.933	27.54	16.58
	Standard deviation	45.0	0.065	0.14	0.81
	% Standard deviation	4.4	6.9	0.5	4.9

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The wall thickness of the material is amorphous in nature. Therefore it can vary with changes in the synthetic procedure. The rate of addition of reagents and the hydrothermal temperature can contribute to the wall thickness. The wall thickness for each batch can vary if the synthesis is not performed according to a uniform procedure. However, by calculating the wall thickness of the standard materials, one can prove whether the structure has collapsed during storage. The collapse of the structure results in a variation of the wall thickness and thus a stable wall thickness means a stable porous material.

It is of interest to compare MCM-41 with existing high surface area standards BAM-P108 and BAM-P109. They are nanoporous carbon based systems with a high surface area. The lower surface area standard BAM-P108 has a surface area of 550 ± 5 m² g⁻¹. The high surface area standard BAM-P109 has a surface area of 1396 ± 24 m² g⁻¹. It has to be noted that these standards are microporous materials and hence the BET surface area analysis is conducted between the relative pressure ranges of 0.001 and 0.1. There are many surface area instrumental systems which do not go to such low relative pressures. These instruments are intended to be used for the analysis of mesoporous materials. Therefore standards based on microporous materials are not suitable for the standardization of such surface area analyzers. In the present study, a BET plot was plotted between P/P₀ values of 0.08 to 0.25 and the surface area was found. Therefore MCM-41 like materials are better suited to study surface areas in this range.

3.4. Effect of aging on structural properties

The PXRD patterns of the samples stored in the glove box and under atmospheric conditions are shown in Fig. 9 and 10 and the corresponding data are given in Tables 3S and 4S† respectively. The PXRD patterns of the samples remained the same throughout the year. The contribution of planes (110), (200) and (210) towards 2θ (100) and d₁₀₀ was determined by calculating the weighted average values of 2θ (100) by eqn (8)–(12) and the d spacing (d₁₀₀).

I(d₁₀₀) × 2θ(d₁₀₀) = A (8)

I(d₁₀₀) × 2θ(d₁₀₀ calculated by eqn 1) = B (9)

I(d₂₀₀) × 2θ(d₁₀₀ calculated by eqn 2) = C (10)

I(d₂₁₀) × 2θ(d₁₀₀ calculated by eqn 3) = D (11)

2θ(d₁₀₀)WA = (A + B + C + D)/(I(d₁₀₀) + I(d₂₁₀) + I(d₂₀₀) + I(d₂₁₀)) (12)

where, I = intensity in cps and WA = weighted average.

Similarly the WA of the d spacing was calculated by replacing the 2θ values with d spacing in eqn (8)–(12).

In both cases, for the samples stored in the glove box and under ambient conditions, there was not much change in the 2θ values, and hence d spacing, with time. Considering the results of the PXRD analyses of samples aged for 2 months and 4 months (Fig. 10), a decrease in the peak intensity is observed for the samples stored under atmospheric conditions. However, there is not much change in the intensity upon further aging. Furthermore, there were no regular trends of decrease or increase in the values of the peak intensities with time. However, they vary randomly and that may be attributed to the experimental errors of analysis. Qualitatively it can be concluded from the results of PXRD analyses of the samples stored in a glove box and under atmospheric conditions that the structure of MCM-41 remained the same during the period of analysis.

3.5. Reproducibility of synthesis method

The synthesis of MCM-41 was repeated following the same method three times, and the surface area and pore size distributions of these three individual samples were calculated. The sodium silicate used in the present study had a SiO₂/Na₂O ratio of 3.77. The surface areas obtained for these samples were in the range 980 to 1050 m² g⁻¹ with pore diameters of ∼27.3–27.7 Å. Variations in the SiO₂/Na₂O ratio, addition time of the surfactant solution, final pH of the gel and hydrothermal treatment time etc. affect the packing of the mesoporous silica particles and the formation of amorphous wall.²⁸ These factors can effectively contribute to the surface areas, pore volumes and pore diameters of the samples. Hence these factors are quite controllable and it is possible to minimise the error elegantly during synthesis by strictly employing identical conditions.

4. Conclusions

At regular time intervals, analyses of the textural and structural characteristics by nitrogen sorption and PXRD techniques were carried out on the calcined MCM-41 samples in the glove box and under ambient atmospheric conditions. The results demonstrated that the high surface area material is durable and stable for the long analysis period of one year. The durability and stability of the calcined MCM-41 is almost identical for the samples stored under the two conditions, which suggested that the storing conditions do not have much effect, provided the atmosphere over the sample is dry. No significant change in the values of SA, PV, PD and WT occurred during the analysis of one year. The values of the textural properties, SA (m² g⁻¹), PV (cm³ g⁻¹), PD (Å) and WT (Å) assigned after analyzing the samples for one year are 1030 ± 49, 1.015 ± 0.066, 27.55 ± 0.60 and 17.04 ± 0.61 respectively for the glove box samples, and 1022 ± 45, 0.933 ± 0.065, 27.54 ± 0.14, and 16.58 ± 0.81 respectively for the samples stored under atmospheric conditions. This high surface area mesoporous material, MCM-41, has shown potential to act effectively as a single reference material to analyse the multi textural properties SA, PV, and PD.

Acknowledgements

CSMCRI communication no. IMC-03, CSIR-CSMCRI - 157/2014. The authors thank the Council of Scientific and Industrial Research (CSIR), New Delhi, India for the financial support through the Network Project on the Advancement in Metrology (NWP 0045) and the Analytical Division and Central Instrumentation Facility of CSIR-Central Salt and Marine Chemicals Research Institute, Bhavnagar, for providing instrumental analysis. NS and MV thank CSIR, New Delhi for the award of Senior Research Fellowships.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra11509k

‡ Present address: Department of Chemistry, N. S. S. College, Pandalam, Pathanamthitta, Kerala – 689501, India.

§ Present address: Department of Chemistry, St. Gregorious College, Kottarakkara, Kollam, Kerala – 691531, India.

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