Carbon-silica adsorbents

V. M. Gun ko and R. Leboda, Carbon-Silica Adsorbents, in Encyclopedia of Surface and Colloid Science, edited by A.T. Hubbard, pp. 864-878 (Marcel Dekker, 2002). 

Before pyrolysis silicas were dried at 200°C and cooled to room temperature. Different amounts of organic precursors were deposited on dry silica (weight 5 g) to obtain carbon-silica adsorbents (carbosils) with different amounts of carbon deposits. Samples based on acenaphthene (Tables 1 and 2), acetylacetone and glucose (Table 3), were pyrolysed under static conditions in a stainless steel autoclave (0.3 dm3) at 773 K for 6 h. After reaction, all the prepared carbosils were washed in a Soxhlet apparatus with N,N-dimethylformamide and acetone, and then dried at 200°C. 

Table 1 presents the structural characteristics of adsorbents prepared with Si-40 and acenaphthene.15 Larger changes in the Si-40 structure are caused by hydrothermal treatment, despite a relatively low temperature (150°C), than by heating at 500°C. However in the case of carbon-silica adsorbents, both hydrothermal modification and high-temperature pyrolysis changes the pore structure to a large extent. 

R. Leboda, A. Gierak, B. Charmas, and Z. Hubicki, Complex carbon-silica adsorbents preparation, properties and some applications as model adsorbents in Fundamentals of Adsorption, edited by M. D. Le Van (Kluwer Academic Publishers, Boston, 1996), pp. 497-504. 

J. Skubiszewska-Zieba. R. Leboda, O. Seledets, and V. M. Gun ko, Effect of preparation conditions of carbon-silica adsorbents based on mesoporous silica gel Si-100 and carbonized glucose on their pore structure, Colloids Surf. A 231(1-3), 39-49 (2003). 

Abstract. Several series of pyrocarbon/silica adsorbents were prepared using fumed oxides of different specific surface areas, and mesoporous silica gel Si-100, as inorganic matrices. Different synthetic and natural polymers as well as glucose were used as carbon precursors. Solutions of phosphoric acid at various concentrations were utilized to prepare functionalized hybrid carbon-silica adsorbents. Nitrogen, p-nitrophenol and Cd(II) adsorption isotherms as well as AFM, XRD and XRF methods were used to estimate the structural and adsorption characteristics of the adsorbents. 

Keywords carbon-silica adsorbents, fumed silica, pyrolysis, glucose, starch, cellulose, phosphoric acid, polyvinylpyrrolidone, polystyrene, structural characteristics. 

Carbon/silica adsorbents with pure or functionalized carbon deposits, or functionalized silica surfaces, are of interest for many purposes, An improvement of the structural and adsorption characteristics of carbon deposits is desirable.1 Pyrocarbon deposits formed by carbonization of low-molecular organic precursors (dichloromethane, cyclohexene, alcohols, acetylacetone, acenaphthene, etc) at oxide surfaces typically possess a low inner specific... 

Leboda, R., J. Skubiszewska-Zieba and J. Rynkowski, Preparation and porous structure of carbon-silica adsorbents obtained on the basis of Ti, Co, Ni, Cr, Zn, and Zr acetylacetonates and acetylacetone , Colloids and Surfaces A Physicochemical and Engineering Aspects, vol 174, Issue 3,1 December 2000, pp 319-28. 

Figure 1 illustrates differential distributions of the adsorption energy of n-hexane on partially dehydroxylated silica gel (1) and on carbon-silica adsorbents prepared through the pyrolysis of n-heptyl (2) and benzyl (3) alcohols and their different mixtures (4-6) on the solid surface. The energy distribution function X(E) of n-hexane was measured chromatographically from the pressure dependence of the retention volume [24,25]. 

Figure 1. Energy distribution X(E) of n-hexane adsorbed on partially dehydroxylated silica gel (1) and on the carbon-silica adsorbents (2-6) (see text).

Distribution of adsorption centers on the surface of a non-modified silica gel is rather exponential, while in the case of carbon-silica adsorbents it is double or triple gaussian. Correlations were found between the topography and morphology of carbon deposits and the adsorption properties of carbosils (magnitudes of q°(, shape of the function X(E) and TEM photographs)[15-17j. Topography of the carbosils is of an evident patch-wise type, which is indicated by the distinct peaks in the X(E) curves. It also follows from the fact that the global function X(E) for such surfaces may be expressed as [29 ... 

As follows from Eqs. (2 ) and (3) the shape of the X(E) function depends on the local heterogeneity, Ak, of the particular patches of a heterogeneous surface. If the subsurfaces (patches) existing on the surface of the adsorbent radically differ in the Ak values, i.e. Ak i Ak then there are clear maxima in the global X(E) curve, as in the case of carbon-silica adsorbents (Fig. 1). However, if Ak-i Ak, the X(E) function can assume the shape of an exponential distribution, or become similar to the distribution X(E) for a non-modified silica gel (Fig.l). The X(E) function for this adsorbent can also be a superposition of several local quasi-gaussian functions. 

Unique properties of mosaic carboaerosils were pointed out in the further studies on carbon-silica adsorbents by H NMR [56]. Thickness of benzene and water solvation shells near the surface of three adsorbents (Table 2) differing in adsorption site topography and their chemical nature was studied. 

Surface properties of carbon-silica adsorbents modified with water vapour... 

The presented data show possibilities of activation at high temperatures of carbon-silica adsorbents, i.e. microporous structure creation and change of chemical nature of the surface adsorption sites. As an illustration, adsorption from the aqueous solutions of chloro-phenols and methane trihalotanes was discussed [37-39]. Activation and deactivation of carbosils (in physical and chemical adsorption) through their HTT under the water vapour high pressure conditions and at moderate temperatures (250-500°C) were discussed [59]. Some results are presented in Figs. 7 - 10. 

It follows from the works [30,67,68], devoted to the study of the properties of modified carbon-silica adsorbents by means of IR spectroscopy, that the modification of the original silica gel by the pyrolysis products of alcohols does not result in the disappearance of hydroxyl groups on the surface. This makes a chemical modification of the surface of carbo-sils possible [30,59,65,68]. Chemically modified carbosils (with a patch-wise topography) have specific structure of active (adsorption) sites called a topographic-axchitectonic structure [28]. 

Surface properties of unmodified carbon-silica adsorbents (carbosils X, H, B, Y) and the same adsorbents modified with the products of pyrolysis of n-heptanol (H) and benzyl alcohol (B) in an autoclave (A) and dynamic reactor (R). The carbosils were prepared by the pyrolysis of methylene chloride adsorbents X and Y), n-heptyl alcohol (H) and benzyl alcohol (B) on the silica surface... 

It should be added that the method presented here also permits us to obtain adsorbents with an increased specific surface area as compared to the initial carbon-silica adsorbent. This process can be easily accounted for if one takes into consideration the fact that the vapours of the pyrolysed substance transport by the carrier gas, with its adequately high flow rate over the modified carbon-silica adsorbent, may not diffuse into the narrow pores on time but will undergo decomposition either in wide and more easily accessible pores or on active, catalytically acting, corners and walls of the carbon agglomerates formed in the process of pyrolysis of dichloromethane. (Diffusion is a rather slow process, slower than adsorption.) Thus the size of the specific surface area of silica gels modified by dichloromethane may be increased or slightly changed (e.g. Adsorbent X, see Table 7). 

It is the advantage of the rotary reactor that during the pyrolysis of alcohols and other substances only slight amounts of pitch-like substances are formed on the surfaces of the modified adsorbents. Moreover, in the set-up shown in [63,64] one can subject the directly prepared carbon-silica adsorbents to a further thermal modification in the atmosphere of various gases thus obtaining adsorbents with the required properties [35]. 

The paper presents only some studies of preparation and properties of surface carbon-silica adsorbents. The application of these materials as model adsorbents in investigations of various adsorption processes is discussed. They can be also used for testing the theories describing adsorption and catalytic processes on the heterogeneous surfaces. The influence of such surfaces on the properties of adsorbed gases and liquids as well as their application have been studied. 

Carbon/silica adsorbents or carbosils have been prepared by both conventional and microwave heating. These materials were produced by pyrolysis of CH2CI2 on microporous silica gel surfaces for 30 min to 6 h at 550 °C. The resultant materials were hydrothermally treated with steam or liquid water using either a conventional autoclave or a microwave unit. As with the clay materials, hydrothermal treatment using microwave irradiation leads to a significant increase of surface area and total pore volume of the carbosils, as compared to conventional methods. 

Leboda, R. 1980a. Preparation of complex carbon-silica adsorbents with different properties. Chromatographia 9 549-554. 

Leboda, R. 1980b. Modification of surface properties of silica gels in aspect of their application in chromatography. XIX. The effect of the condition of preparation of complex carbon-silica adsorbents on their surface properties. Chem. Anal. 25 979-991. 

Leboda, R. 1980c. The chemical nature of adsorption centres in modified carbon-silica adsorbents prepared by the pyrolysis of alcohols. Chromatographia 11 703-708. 

Leboda, R. 1981. Preparation and modification of complex pyrolytic carbon-silica adsorbents. Chmmatographic 9 524-528. 

Leboda, R. 1987. Thermal behaviour of complex carbon-silica adsorbents (carbosils) J. Thermal Anal. Calorim. 32 1435-1448. 

Rudzinski, W., Gierak, A., Leboda, R., and D browski, A. 1995. Studies of properties of complex carbon-silica adsorbents used in sorption-desorption processes (solid-phase extraction). Fresenius J. Anal. Chem. 352 667-672. 

Seledets, O., Leboda, R., Gizegorczyk, W. et al. 2006. Effect of sulphur on surface properties of complex carbon-silica adsorbents. Micropor. Mesopor. Mater. 93 90-100. 

Seledets, O., Skubiszewska-Zieba, J., Leboda, R., and Gun ko, V.M. 2003. On the surface properties of carbon-silica adsorbents (carboaerosils) prepared by pyrolysis of methylene chloride on the surface of fumed silica (aerosil). Mater. Chetn. Phys. 82 199-205. 

Tracz, E. and Leboda, R. 1985. A microscopic investigation of the surface of carbon-silica adsorbents. 11. Relationships between the type of information obtainable about the surface and the microscopic techniques used for its examination. J. Chromatogr. 364 364—368. 

---