Influence of the Template Removal Method on the Mechanical Stability of SBA‐15

Abstract Removing the template from the pores after the polycondensation of the silica precursor is a necessary step in the synthesis of mesoporous silica materials. In our previous work, we developed a method for the efficient and spatially controlled functionalization of SBA‐15. First, the silanol groups on the particle surface and in the pore entrances were passivated. After extraction of the template, a pretreatment step in N2 converted the silanol groups to the single and geminal state. Afterwards, an azide functionality was introduced exclusively into the mesopores. This ensured that the catalyst could afterwards be immobilized unambiguously in the mesopores. The mechanical stability of a material functionalized in such a spatially controlled manner is studied and compared to other template removal methods. Even though several studies investigated the influence of the calcination temperature, the presence or the absence of oxygen during the template removal, the specific conditions used during the herein reported selective functionalization procedure have not been covered yet.


Explanation of the Nomenclature
For a better understanding of the present work, the nomenclature of the different samples is introduced in Table S1. Table S1. Explanation of the sample nomenclature.

Mechanical Stability of SBA-15-calc
To better understand if there is an influence of the template removal method on the mechanical stability of SBA-15 against pressure, the mechanical stability of SBA-15-calc was investigated first. Therefore, SBA-15-calc was pressed with pressures between 8 and 156 MPa. Afterwards, the different samples were investigated with SAXS and N2 physisorption measurements.
The SAXS curves of the calcined and the pressed samples are pictured in Figure S1. All diffractograms shown have been normalized to the same intensity of the attenuated primary beam. Considering that the sample preparation and the measurement set-up were the same for every sample, this relative value depends on the X-ray absorption of the sample and the amount of scattered X-rays only. [1] The intensity of the attenuated beam I follows the equation with I0 the intensity of the unattenuated primary beam, x the sample thickness and µ the linear attenuation coefficient. The latter relates to the total cross section tot via with Na the number of attenuating centers, i.e., atoms, per volume. This total cross section may be further divided into contributions from the photoelectric absorption (σpe), pair production (σpp), Compton (σinel) and Thomoson (σel) cross sections. Except for the Thomoson scattering, which arises due to the form or structure factor of the investigated sample, all of these contributions to the total cross section solely depend on the type and density of the atoms within the sample as well as the energy of the X-ray beam used. Taking into account that all samples consist of the same atoms, i.e., silicon and oxygen in a ratio of 1:2, differences in the scattering intensities after normalization to the attenuated primary beam may thus directly be attributed to a variation in the ratio of total silica to nanostructured silica.
As seen from Figure S1, the scattering intensity of SBA-15-calc-8MPa increases compared to the unpressed sample. This intensity increase indicates that the amount of unstructured silica is larger after applying pressure, which shows that a part of the SBA-15 is destroyed by this rather small pressure, already. At the same time, the full width at half maximum of the scattering peaks, which is plotted in Figure S2 for the example of the (100) reflection, stays constant, revealing that the properties of the nanostructured part of the sample stay unchanged.
When increasing the pressure, the scattering intensities of the three characteristic peaks as well as their width stay almost constant up to a value of 39 MPa. When increasing the pressure to 78 MPa, the scattering intensity, especially of the (200) peak, starts to decrease, and the width of the peaks increases, a trend which continues for even higher pressures. This broadening of the scattering maxima was previously attributed to a reduction of the average diameter of the crystallites within the pressed sample. [2] However, if this was the case, the relative intensity of the individual peaks should not decrease, [3,4] but rather increase as the amount of unstructured silica rises further. Thus, it seems more likely to us that the observed broadening is due to an increasing disorder of the second kind, which means that the positions of the hexagonally arranged pores become more and more "blurred". [5] Such blurring may arise, if parts of the pores collapse, leading to plugged or partially filled pores. Chytil et al. [6] came to the same conclusion in their work. In addition to the SAXS measurements, N2 physisorption measurements were performed. The resulting isotherms are shown in Figure S3(a). Unpressed SBA-15-calc shows the typical type IV isotherm. [7] With increasing pressure, the type of the isotherms changes, and the hysteresis gets smaller. This is an indication for a loss of mesoporosity within SBA-15-calc. [8] The results evaluated from the N2 physisorption measurements are listed in Table S2. The decrease in surface area and pore volume, as described in the literature and expected from the change in isotherms of the N2 physisorption measurements, are not so pronounced. The comparison of the pore size distribution of the different samples shows a change with increasing pressure ( Figure S3(b)). It can be seen that the amount of pores with the main pore diameter decreases with increasing pressure. [6,7] This leads to the conclusion that SBA-15-calc is mechanically stable against pressures up to 39 MPa. At higher pressures, there is an increasing loss of the hexagonal structure of SBA-15.  The SBA-15-as of one batch was used as starting material for all experiments in this work.

Removal of Pluronic® P-123 -Calcination
To open the pores, SBA-15-as was calcined for 6 h at 550°C in an air flow of 150 l h -1 . The heating rate was 1 K min -1 .

Refilling of the Pores
To refill the pores with template, Pluronic® P-123 was dissolved in ethanol overnight.

Removal of Pluronic® P-123 -Soxhlet Extraction
The triblock copolymer Pluronic® P-123 was removed from the pores of SBA-15-as or SBA-15-calc-re by Soxhlet extraction for 112 h using ethanol as extracting agent. The extracted materials (SBA-15-as-E and SBA-15-calc-re-E) were dried in an oven at 80°C.

Pretreatment in N2
SBA-15-as-E was treated in an oven at 400°C or 550°C for 6 h in N2. The treatment was performed with a heating rate of 2°C min -1 and a N2 flow of 58 l h -1 . The product obtained was named SBA-15-as-E-p400 or SBA-15-as-E-p550 depending on the temperature during the pretreatment. 8

Investigation of the Mechanical Stability
For the investigation of the mechanical stability, the press FluXana (Vaneox® Pressing Technology) was used. For each experiment, 150 mg of the respective sample was pressed with different pressures. The pressed tablets were then carefully crushed again in a mortar in order to be able to characterize the samples.

Small angle X-ray scattering (SAXS)
The powdery samples were filled into mark capillaries with a diameter of 1 mm (Hilgenberg, glass no. 14) and flame-sealed. For measurements, a SAXSess mc 2 diffractometer (Anton Paar) in the line collimation geometry was used for which the sample to detector distance was calibrated with cholesteryl palmitate. X-ray radiation with a wavelength of λ(Cu-Kα) = 0.1542 nm was generated by an ID 3003 X-ray generator (Seifert) operated at 40 kV and 40 mA. The sample housing was evacuated prior to measurements, which were carried out at 25°C with the help of a TCS 120 hot stage (Anton Paar) and averaged over 60 individual measurements. The scattered X-ray intensity was detected with a one-dimensional CMOS Mythen 2K detector (Dectris). The semi-transparent beam-stop allowed for an additional measurement of the transmitted primary X-ray beam. Using the software SAXSquant TM , the measured scattering profiles were normalized to the same intensity of the attenuated primary beam, background-corrected and deconvoluted. Subsequently, the scattering curves were Lorentz-and polarization-corrected. The scattering associated to the form factor of the mesopores as well as further incoherent scattering was removed by fitting and subtracting a double exponential decay, leaving only the part of the scattering, which originates from the hexagonal structure. The obtained Bragg-like diffraction maxima were fitted with Lorentzian functions to extract the exact peak positions and full widths at half maximum.

N2 physisorption
The surface area as well as the pore size of the SBA-15 samples were analyzed by N2 physisorption measurements. The adsorption and desorption isotherms were recorded using Autosorb 3B from Quantachrome Instruments. Before the measurements, the samples were outgassed under vacuum at 200°C for 16 h. After the pretreatment, the N2 physisorption measurements were performed in a liquid N2 bath at −196°C. From the adsorption isotherms, the surfaces were calculated using the BET method, whereas the pore sizes and pore size distributions were determined with the DFT method, taking into account the hexagonal structure.

Elemental analysis
The amount of carbon and hydrogen was measured with an Elemental Analizer 1106 from the company Carlo Erba Strumentazione.
The results of the elemental analysis (Table S3) Table S3.

Influence of the Temperature during the Pretreatment in N2
As described in the main article, SBA-15-as-E was pretreated at 400°C and 550°C in N2 and subsequently analyzed by elemental analysis, SAXS measurements, and N2 physisorption measurements.
Observation of the calculated fractions of removed Pluronic® P-123 (Table S3) shows that after thermal treatment at 400°C and 550°C, the remaining template was removed from the pores and all pores were accessible.
The SAXS curves of SBA-15-as-E-p400 in Figure S9 and of SBA-15-as-E-p550 in Figure S7 are can be adumbrated. Therefore, it can be assumed that SBA-15-as-E-p550 was not damaged to the same extent as SBA-15-as-E-p400 and, thus, higher temperatures during pretreatment in N2 have a positive effect on the mechanical stability of SBA-15 against pressure.
Furthermore, a decrease of the lattice parameter with increasing temperature can be observed compared to SBA-15-as, due to the pretreatment in N2 (Table S4). This shrinkage is accompanied by a shrinkage of the pores (Table S4).
These results are confirmed by the N2 physisorption measurements. While the shapes of the N2 physisorption isotherms and their relatively narrow pore size distribution of the two unpressed samples are very similar, the samples pressed at 156 MPa differ significantly from the unpressed materials ( Figure S11). Looking at the different surface areas, pore volumes and pore sizes determined from the N2 physisorption measurements for SBA-15-as-E-p400 and SBA-15-as-E-p550 in Table S4, smaller differences are seen. This can be explained by the stronger shrinkage of the pores at higher temperatures. It is known from the literature that the unit cell shrinks at higher temperatures in air due to further condensation of the silica framework. [9] It seems that the oxygen in the air as well as from desorbed water molecules acts as catalyst for reordering the silica bonds in the lattice. [10] However, it is also known, that the temperature range in which the strongest shrinkage is observable is between 300°C and 500°C. [9] Accordingly, it can be assumed that a similar effect is also obtained when heating in N2, so that further condensation reactions make the pore walls more ordered and thus more stable to pressure. In this work, the pretreatment is done in N2 at 400°C and at 550°C, and the shrinkage is more pronounced for SBA-15-as-E-p550. This leads to the assumption that the higher temperature is necessary to overcome the binding energies to reorder the silica lattice.

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This leads to a more ordered silica lattice and results in more stable pore walls for SBA-15-as-E-p550 compared to SBA-15-as-E-p400.
A comparison of the unpressed materials with those pressed at 156 MPa shows large differences between the N2 physisorption isotherms and pore size distributions. The analysis shows the expected decrease in surface areas and pore volumes due to pressing at 156 MPa.
In addition, the comparison of SBA-15-as-E-p400-156MPa and SBA-15-as-E-p550-156MPa shows further differences. Although the two N2 physisorption isotherms look similar, large differences between the pore size distributions are evident. While the amount of the original main pores has almost completely disappeared for SBA-15-as-E-p400-156MPa, it has also decreased for SBA-15-as-E-p550-156MPa, but is still perceived as the main fraction.
Accordingly, the pore structure of SBA-15-as-E-p550-156MPa was less damaged. This confirms the assumptions of the SAXS studies and shows that a higher temperature during the pretreatment has a positive effect on the mechanical stability of SBA-15 against pressure.