Adsorption methods, characterization silica surface

Adsorption from solutions, research, 653-654 Adsorption isotherm, 253, 254f Adsorption methods, characterization of silica surface, 170, 175-177/... 

Most of the adsorbents used in the adsorption process are also useful to catalysis, because they can act as solid catalysts or their supports. The basic function of catalyst supports, usually porous adsorbents, is to keep the catalytically active phase in a highly dispersed state. It is obvious that the methods of preparation and characterization of adsorbents and catalysts are very similar or identical. The physical structure of catalysts is investigated by means of both adsorption methods and various instrumental techniques derived for estimating their porosity and surface area. Factors such as surface area, distribution of pore volumes, pore sizes, stability, and mechanical properties of materials used are also very important in both processes—adsorption and catalysis. Activated carbons, silica, and alumina species as well as natural amorphous aluminosilicates and zeolites are widely used as either catalyst supports or heterogeneous catalysts. From the above, the following conclusions can be easily drawn (Dabrowski, 2001) ... 

In the author s opinion, the better approach to experimentally study the morphology of the silica surface is with the help of physical adsorption (see Chapter 6). Then, with the obtained, adsorption data, some well-defined parameters can be calculated, such as surface area, pore volume, and pore size distribution. This line of attack (see Chapter 4) should be complemented with a study of the morphology of these materials by scanning electron microscopy (SEM), transmission electron microscopy (TEM), scanning probe microscopy (SPM), or atomic force microscopy (AFM), and the characterization of their molecular and supramolecular structure by Fourier transform infrared (FTIR) spectrometry, nuclear magnetic resonance (NMR) spectrometry, thermal methods, and possibly with other methodologies. 

The progress in the determination of porosity of various types of materials has arisen over the past ten years from advances in application of new spectroscopy techniques. In the present paper the application of small angle X-ray scattering (SAXS), positronium annihilation lifetime spectroscopy (PALS) and low temperature nitrogen adsorption methods to the characterization of mesoporosity is reviewed using different types of silica gels with chemically modified surface. The results from the three methods are compared and discussed. 

This chapter describes the results of the acidity characterization of a selected silica surface with VT-DRIFT spectroscopy. Examples of the capabilities of the method are demonstrated by the qualitative determination of the adsorption and thermal desorption characteristics of pyridine on amorphous, porous silica gel. Procedures for the determination of isothermal desorption rate constants and activation energy of desorption are presented and discussed as a means of assessing acid site strength. 

Substantial progress in the elucidation of the surface structure of crystalline and amorphous silicas has been achieved by means of high-resolution spectroscopic techniques, for example, Si cross-polarization magic-angle spinning NMR spectroscopy and Fourier transform IR spectroscopy. The results lead to a better understanding of the acidity, dehydration properties, and adsorption behavior of the surface. These properties are key features in the design of novel advanced silica materials. The current methods of characterization are briefly reviewed and summarized. 

A third classifying quantity relates to the surface structure of silicas, which is characterized by the coordination of surface silicon atoms the resulting functional groups their density, topology, and distribution the degree of hydroxylation the hydration-dehydration behavior the acidic and basic properties of surface functional groups and their adsorption behavior and chemical reactivity. The pattern of the surface structure in terms of these properties is discussed in the section Current View of the Silica Surface. The following section reviews the methods by which reliable information on these properties is obtained. 

Evidenced has been obtained previously that the reinforcing effect of different grades of fumed silicas on silicone elastomers is influenced by the surface fractality [1] and that the surface roughness increases with the specific surface energy. The aim of the present work is to demonstrate variations by calling on NMR and infrared spectroscopic methods, which are applied to fumed silica samples that have been carefully characterized through adsorption methods including IGC analysis. 

The adsorptive ability of solids is quite another matter. Adsorption is a very general phenomenon, and even common solids will adsorb gases and vapors at least to a certain extent. For example, every student of analytical chemistry has observed with annoyance the increase in weight of a dried porcelain crucible on a humid day during an analytical weighing which results from the adsorption of moisture from the air upon the crucible surface. But only certain solids diibit sufficient specificity and adsorptive capacity to make them useful as industrial adsorbents. Since solids are frequently very specific in their ability to adsorb certain substances in large amounts, the chemical nature of the solid evidently has much to do with its adsorption characteristics. But mere chemical identity is insufficient to characterize its usefulness. In liquid extraction, all samples of pure butyl acetate will extract acetic acid from a water solution with identical ability. The same is not true for the adsorption characteristics of silica gel with respect to water vapor, for example. Much depends on its method of manufacture and on its prior history of adsorption and desorption. 

New data have been obtained for characterizing silica and carbon adsorbents early used in previous adsorption or chroma-tigraphy studies (ref. 5-9). The porous structure characteristics of sorbent samples considered are given in Table 1, where and d are surface area detemined by the BET method,... 

Microporous and, particularly, ultramicropous membranes are more difficult to characterize. Different procedures based on the low-pressure part of the N2 adsorption isotherm have been proposed [36], but they often require knowledge of the shape of the pores and of gas-surface interaction parameters which are not always available. Small angle X-ray scattering (SAXS) is another technique which is well suited to micro-porous powders, but difficult to execute in the case of composite layers, as in microporous membranes. Xenon-129 NMR has recently been proposed [37] for the characterization of amorphous silica used in the preparation of microporous membranes, but the method requires further improvement. Methods based on permeability measurements appear to be limited by the lack of understanding of the mass transport mechanisms in (ultra)microporous systems. 

This well accepted method [28] has been used extensively in the characterization of M41S materials [11-12,14]. From the application of this method to MCM-41, it has been concluded that this material contains no significant amounts of microporosity. This is the main evidence presented so far in order to conclude that MCM-41 is exclusively mesoporous. As it happens with any good method its limitations need to be considered in order to avoid misinformation. In the case of the a method the choice of the reference isotherm is crucial. All the reference silicas should be nonporous in order to allow a reliable analysis of MCM-41. Unfortunately, we observed that most of them have a steep rise in their N2 adsorption isotherms at 77 K at low relative pressures and BET surface areas varying from 40 [29] to 400 mVg [30], For this reason, our sample of MCM-41 was heat-treated so as to sinter the silica particles and thus obtain a nonporous silica (BET surface area 1.5 mVg) and as similar as possible to our MCM-41. The N2 adsorption isotherms for a reference silica [29] and our sintered MCM-41 are shown in Figure 7. 

Textural characterization was performed by N2 adsorption-desorption at 77 K using a Micromeritics ASAP 2010 analyzer. The samples were preheated under vacuum in three steps of Ih at 423 K, Ih at 513 K, and finally 4 h at 623 K. BET specific surface area, Sbet, was calculated using adsorption data in the relative pressure range, P/Po, from 0.05 to 0.2. Total pore volume, Vp , was estimated by Gurvitsch rule on the basis of the amount adsorbed at P/Po of about 0.95. The primary mesopore diameter, Dp, was evaluated using the BJH method from the desorption data of the isotherm. The primary mesopore volume, Vp, and the external surface area, Sext were determined using the t-plot method with the statistical film thickness curve of a macroporous silica gel [5]. 

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