Mesoporous adsorbents

W. D. Machin. Temperature of hysteresis and the pore size distributions of two mesoporous adsorbents. Langmuir 70 1235-1240, 1994. 

K. H. Nam and L. L. Tavlarides, Synthesis of a high-density phosphonic acid functional mesoporous adsorbent, application to Chromium(III) removal, Chem. Mater. 17, 1597-1604 (2005). 

It is now generally accepted that there is no sound reason why isotherms on micro-porous solids should conform any more closely to the classical Langmuir mechanism than isotherms on non-porous or mesoporous adsorbents. Indeed, a considerable amount of the evidence now available gives strong support to the view that the limiting uptake is controlled rather by the accessible micropore volume than by the internal surface area (Gregg and Sing, 1982). 

A closely related problem is the interpretation of physisorption hysteresis, when it appears in the form of an adsorption-desorption loop in association with capillary condensation-evaporation. Until recently, it was generally assumed that some form of hysteresis loop was a distinctive feature of every Type IV isotherm. With some mesoporous adsorbents, the shape of the loop is known to give a useful indication of the type of pore structure (e.g. the uniformity and shape of the pores). 

Confirmation of the validity of the Gurvich rule has been obtained with a number of mesoporous adsorbents for a wide range of adsorptives. The results in Table 7.2 are typical of the level of agreement to be expected between the values of v(sat) derived from a number of isotherms determined on a given mesoporous adsorbent. Although the adsorptives differ widely in their chemical and physical properties, the deviation from the mean value of vp is within c. 5%. 

Hysteresis loops, which appear in the multilayer range of physisoTption isotherms, are generally associated with capillary condensation. It is well known that most mesoporous adsorbents give distinctive and reproducible hysteresis loops (de Boer 1958 Sing era/., 1985). 

The applicability of the Saam-Cole theory has been tested by Findenegg and his co-workers (1993,1994). Their adsorption measurements of certain organic vapours on carefully selected grades of controlled-pore glass provide semi-quantitative confirmation of the theoretical treatment adopted so far. However, it is evident that some refinement is required in the assessment of 0(rp) for materials of small pore size and that the experimental choice of the mesoporous adsorbent is important To make further progress it will be necessary to study adsorbents having narrow size and shape distributions of easily accessible mesopores. 

In fact, most mesoporous adsorbents possess complex networks of pores of different size. It is therefore unlikely that the condensation-evaporation processes can occur independently in each pore. The complexity of capillary condensation in porous materials is illustrated by the recent Monte Carlo computer simulation studies of Page and Monson (1996) and Gelb and Gubbins (1998). The well-defined hysteresis loops observed in the simulation results of both studies were attributed to the presence of thermodynamically metastable states and not to kinetic effects. However, it appears that the extent of die hysteresis was associated with the overall heterogeneity of the adsorbent structure and not simply due to capillary condensation within individual pores. 

The considerable interest now being shown in MCM-41 by both experimentalists and theoreticians is an indication of its potential value as a model mesoporous adsorbent. As we have seen, remarkable progress has already been made in the synthesis and characterization of the non-intersecting assemblage of tubular pores. More work is now required to improve the uniformity of the pore structure and to investigate the stability of different forms of MCM-41 and related materials. Neutron diffraction has... 

Microporous adsorbents Mesoporous adsorbents Macroporous adsorbents ... 

In the present work the meso- and macro-structural characteristics of the mesoporous adsorbent MCM-41 have been estimated with the help of various techniques. The structure is found to comprise four different length scales that of the mesopores, the crystaUites, the grains and of the particles. It was found that the surface area estimated by the use of small angle scattering techniques is higher, while that estimated by mercury porosimetry is much lower, than that obtained from gas adsorption methods. Based on the macropore characterization by mercury porosimetry, and the considerable macropore area determined, it is seen that the actual mesopore area of MCM-41 may be significantly lower than the BET area. TEM studies indicated that MCM-41 does not have an ideal mesopore structure however, it may still be treated as a model mesoporous material for gas adsorption studies because of the large radius of curvature of the channels. 

The discovery of the family of model mesoporous adsorbents MCM-41 has resulted in intensive research in their synthesis and appfications, as well as in the refinement of existing adsorption models. These materials are gaining importance as a result of the reported presence of an ideal pore structure, the ease of tuning the pore diameter between 1.5 and 10 nm, and very high surface areas of the order of 1000 m /g. 

A characteristic feature associated with pore condensation is the occurrence of sorption hysteresis, i.e pore evaporation occurs usually at a lower p/po compared to the condensation process. The details of this hysteresis loop depend on the thermodynamic state of the pore fluid and on the texture of adsorbents, i.e. the presence of a pore network. An empirical classification of common types of sorption hysteresis, which reflects a widely accepted correlation between the shape of the hysteresis loop and the geometry and texture of the mesoporous adsorbent was published by lUPAC [10]. However, detailed effects of these various factors on the hysteresis loop are not fully understood. In the literature mainly two models are discussed, which both contribute to the understanding of sorption hysteresis [8] (i) single pore model. hysteresis is considered as an intrinsic property of the phase transition in a single pore, reflecting the existence of metastable gas-states, (ii) neiM ork model hysteresis is explained as a consequence of the interconnectivity of a real porous network with a wide distribution of pore sizes. 

A three dimensional capillary network model has been developed, aiming to the simulation of sorption by several model mesoporous adsorbents, such as the one mentioned in the previous section. The model offers realistic simulation conditions and is able to provide satisfactory prediction of adsorption-desorption isotherms of CCI4 and C5H1 for different porosities, temperatures and adsorbates. The expected desorption branch hysteresis is estimated as a two component summation of the thermodynamic (single pore) and the network hysteresis. Similarly, the overall sorbed volume is the two component summation of the volume due to multilayer adsorption and to the volume due to capillary condensation. 

Most chromatographic production systems use particulate adsorbents with defined pore structures due to the higher loadability of mesoporous adsorbents compared with non-porous adsorbents. Adsorption of the target molecules on the inner surface of a particle has a tremendous influence on the efficiency of a preparative separation. Several factors with regard to mass transfer that contribute to the total band broadening effect in addition to axial dispersion can be distinguished (Fig. 2.10) ... 

To make further progress it will be necessary to employ well defined micro-porous and small pore mesoporous adsorbents and have available non-porous reference solids of the same surface structure. This will be possible with the current progress in material synthesis procedures (e.g. organised amorphous structures obtained in the presence of templating agents [62,63]. We have to note that zeolites are often not suitable reference materials because the strong electrostatic fields within the crystalline cavities can polarise the gas molecules. Therefore the adsorption equilibrium in these materials is not solely a function of the size of the adsorbate. 

The range of applications for gas and liquid separation and purification by adsorption is large and growing. The strong research and development activity in this area is facilitated by the flexibility of practical adsorptive process designs such as pressure and thermal swing adsorption, and SMB adsorption, as well as by the availability of a large spectrum of new and old micro-and mesoporous adsorbents. 

Table 1 demonstrates the experimental matrix which was followed, and the plot below (Figure 4) shows the ROC curves that were generated from these experiments. The carbonyl peak centered at 1720 cm is used as the metric for phosmet detection. For clarity, 20,000 ppb is omitted since it overlays 2,000 ppb 2 ppb is not shown since this data was not statistically different than the blank. The separation factors, or ROC K-factors, for the curves shown were 6.75, 3.75, and 1.63 for 2,000 ppb, 200 ppb, and 20 ppb respectively. The minimum value of K that meets the Joint Service Agent Water Monitor (JSAWM) requirement of 95% detection at 5% false positives is 3.29. Therefore, these test results clearly demonstrate the detection of a VX surrogate at 200 ppb using these modified mesoporous adsorbents in a simple batch sampling mode. 

Figure 6. Thermal desorption of liquid from a mesoporous adsorbent I - desorption of the bulk liquid out of pores, II - desorption of liquid from the pores. Solid line (1) - quasi-isothermal program, broken line (2) - linear program.

Figure 7. Temperature-time dependence for adsorption of liquid from mesoporous adsorbent. 1 - linear program, 2 - quasi-isothermal program.

Branton, P.J., Hall, P.G., Treguer, M., and Sing, K.S.W. (1995). Adsorption of carbon dioxide, sulfur dioxide and water vapour by MCM-41, a model mesoporous adsorbent. J. Chem. Soc. Faraday Trans., 91, 2041—3. 

Neimark, A.V. (1983). Development of the capillary condensation and adsorption theory in mesoporous adsorbents based on the porous structure lattice models. Dokl. Acad. Sci. USSR, 273, 384-8. 

To study the liquid-solid and solid-solid phase transitions of a confined O2, we performed X-ray diffraction measurements of O2 confined inside the cylindrical pores of six kinds of regular mesoporous adsorbents (MCM-41 and SBA-15) with different pore radii as a function of temperature. Hysteresis effects between the y-to-(3 and p-to-y solid-solid phase transitions are considerably larger than those between freezing and melting and the depression of the P-to-y transition point is slightly smaller than that of the melting point. 

The adsorption of various amino acids from aqueous solutions using MCM-41-type mesoporous molecular sieves is discussed on the basis of their adsorption isotherms. The amounts adsorbed strongly depend on the pH and the nature of the individual amino acid Amino acids with acidic side chains are hardly adsorbed, whereas basic amino acids show very high affinities to the mesoporous adsorbent. The uptake of amino acids with non-polar side chains increases with the chain length. The adsorption complex is proposed to consist of the cationic form of the amino acid attached to the negatively charged silica surface. 

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