Silica mercury porosimetry

Values of pore volume of samples of porous silica, determined by ethanol titration (v (EtOH)) and by mercury porosimetry (v (Hg, i) and v (Hg, ii)) ... 

Fig. 2.1.14 Mercury porosimetry data for ordered silica packing structures.

The range of pore sizes important to good catalytic function are from around 1OA in the supported zeolite to perhaps 100,000 A (10 pm) in the zeolite/silica-alumina composite. The technique of gas adsorption is of little use beyond about 250A, so that by far the largest range of important pore sizes (and related interactions between pores which constitute the pore structure of the particle) are assessable only through the mercury porosimetry technique. Many practitioners in catalyst characterisation claim that... 

The monodisperse non aggregated silica sphere slabs undergo only intrusion during mercury porosimetry experiments. Due to the very compact arrangement of the spheres, the pore volume Kng is very small (table 2). Ing increases when the size of the spheres increases from 8 to 206 nm. 

The slabs of non aggregated monodisperse silica particles of the same size range than the xerogels and aerogels (S1-S3) are only intruded by mercury during mercury porosimetry So a necessary condition for the presence of a crushing mechanism is the aggregation of the particles to form a three-dimensional network. 

The results with the slabs of monodisperse non aggregated silica spheres (of the same size range than the xerogels and aerogels) which undergo only intrusion during mercury porosimetry implies that the particles need to be aggregated so that the compaction mechanism takes place... 

Some materials, among the most porous, show a large volume variation due to mechanical compaction when submitted to mercury porosimetry. High dispersive precipitated silica shows, as low density xerogels and carbon black previously experimented, two successive volume variation mechanisms, compaction and intrusion. The position of the transition point between the two mechanisms allows to compute the buckling constant used to determine the pore size distribution in the compaction part of the experiment. The mercury porosimetry data of a high dispersive precipitated silica sample wrapped in a tight membrane are compared with the data obtained with the same sample without memlM ane. Both experiments interpreted by equations appropriate to the mechanisms lead to the same pore size distribution. 

In a previous study [5], we showed that some materials, in particular the low density silica xerogels, exhibit a remarkable behavior when submitted to mercury porosimetry. At low pressure, the volume variation observed is entirely due to a crushing mechanism, generally irreversible with sometimes a weak elastic component. At high pressure, these xerogels are invaded by mercury which intrudes the pore network. The transition fi om the crushing mechanism to intrusion is sudden at a pressure Pi, characteristic of the material. This particular point can be easily located on the curve of cumulative volume versus logarithm of pressure by... 

Mercury porosimetry experiments were performed on a Carlo Erba Porosimeter 2000 allowing measurements in the pressure range 0.01 - 200 MPa. The sample of high dispersive precipitated silica was synthesized and provided by Prayon-Rupel S.A, Belgium. 

Figure 1 shows mercury porosimetry curves on high dispjersive precipitated silica and on a low density xerogel previously examined [5]. The volume variation as a function of logarithm of pressure shows the same behavior. On both curves, one can see a sharp increase of the curve slope for a characteristic transition pressure P,. The value of this transition pressure is 45 MPa for precipitated silica and 27 MPa for the low density xerogel sample. The value of transition pressure Pt is dependent of the compressive strength of the sample. 

In order to identify the volume variation mechanisms on the precipitated silica sample, experiments were performed at various maximum pressure below and near the point of slope change P,.. A monolithic sample of high dispersive precipitated silica was weighted and its specific volume (2.04 cm /g) was determined using mercury pycnometry. It has been submitted to mercury porosimetry until a pressure (40 MPa) just below the characteristic transition... 

Figure 2. Mercury porosimetry curves (Cumulative pore volume versus pressure) obtained on high dispersive precipitated silica samples at maximum experimental pressure 40 MPa (curve a) and 200 MPa (curve b).

A monolithic sample of precipitated silica wrapped under vacuum in a parafilm membrane was measured by mercury porosimetry until a 200 MPa pressure. Figure 3 shows the volume versus pressure curve compared to the curve of the sample without membrane. The mercury porosimetry curves of a sample wrapped in a tight membrane amd the same material without membrane are identical between 1 and 40 MPa. It confirms that the mechanism of volume variation in this pressure domain is truly crushing without intrusion. At pressures above 45 MPa, the two curves are very different as expected because the two mechanisms are different for the sample wrapped in a membrane, the only possible mechanism is the crushing whereas it has been shown that the sample without membrane is invaded by mercury at pressures above 45 MPa. The weak difference between curves observed between 0.1 and 1 MPa can be attributed to a lack of suppleness of the membrane which cannot fit the rough surface of the monolithic sample of precipitated silica fi om the lowest pressures. The volume differences between the two curves which appear progressively below 1 MPa corresponds to the volume comprised between the surface of the sample and the membrane. This volume is... 

Below 45 MPa, the high dispersive precipitated silica sample with or without membrane collapses without mercury intrusion. The buckling mechanism of pores edges can be assumed as in the case of low density xerogels. Consequently, equation (2) can be used to interpret the mercury porosimetry curve in this low pressure domain. The constant A, to be used in equation (2) can be calculated from the P, value using equation (4). With a mercury surface tension 0.485 N/m, a contact angle 0= 130° and P, = 45 MPa, one obtains K = 86.3 nm MPa" . 

Figure 3. Pore size distribution functions (mercury porosimetry data) for A) initial CPG (D = 30.5nm) (solid line) and the same material heated for 20 hrs at 650°C (dashed line) 20 hrs at 720°C (dotted line) 20 hrs at 805°C (dash-dotted line), B) initial silica gel (D = 32.8 nm) (solid line) and the same material heated for 20 hrs at 580°C.

N2 adsorption-desorption isotherms were determined at 77 K after outgassing for 24 h at room temperature. The mercury porosimetry measurements were performed between 0.01 and 200 MPa after outgassing the sample monolith for 2 h at ambient temperature. The size of silica and metallic particles was examined by transmission electron microax)py (TEM). The composition and size of the metallic particles were examined by X-ray diffraction (XRD). 

Hg specific pore volume measured by mercury porosimetry Kc m<75 m diameter between 2 and 7.5 nm determined by Broekhoff-de-Boer theory V volume obtained by addition of tag, Tc m<7 5mii and Sbet specific surface si02 silica particle diameter measured by TEM. 

Figure 46.6 depicts the internal cumulative pore volume as a function of pore diameter for Sorbsil C500. The total pore volumes as measured by the two techniques agree to within 0.05 cm /g the pore volume measured by mercury porosimetry is slightly higher, possibly because this is an extremely wide-pored silica, with some pores too wide to be measured by nitrogen sorption. 

The average pore size of modem analytical HPLC packings is 100 A range 60-120 A. Figure 5 shows the internal surface area versus pore diameter for four commercial 5 im silicas with pore sizes ranging from 60 to 120 A as determined by mercury porosimetry (33). This technique can measure pore diameters down to 30 A, which is the upper limit of the size range for micropores. Note that the data in Figure 5 are biased toward the smallest pore sizes, which... 

C. Ali6, R. Pirard, and J.-P. Pirard, Mercury Porosimetry Applied to Porous Silica Materials Successive Buckling and Intrusion Mechanisms, CoUoids Surf. A, 187-188, pp. 367-74, 2001. 

FIGURE 1.239 Pore size distributions from f =l nm to 10 i,m for silica calculated from the nitrogen adsorption, H NMR spectroscopy data for octamethylcyclotetrasiloxane and the mercury porosimetry. (Adapted from J. Colloid Interface Sci., 336, Ono, Y., Mayama, H., Fur6, I. et al., Characterization and structural investigation of fractal porous-silica over an extremely wide scale range of pore size, 215-225,2009. Copyright 2009, with permission from Elsevier.)... 

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