Neopentane, adsorption

Nitrogen and neopentane adsorption on activated Kevlar 29 chars... 

The thermal decomposition of magnesium hydroxide under carefully controlled conditions, followed by subsequent rehydration, has been studied by means of nitrogen and neopentane adsorption isotherm measurements. Up to about 85% decomposition a uniform layered particle structure, consisting of plate-like microcrystallites intercalated by slit-shaped pores of width ca. 0.93 nm, gradually spreads from the outside towards the centre of each crystal. At this stage each particle consists of MgO of normal... 

Nitrogen adsorption isotherms at 77 K were determined using a Carlo-Erba Sorptomatic. Neopentane adsorption isotherms at ice melting temperature (ca. 273 K) were determined using a Cl Robal microbalcince with pressure measurement by means of a calibrated CEC strain gauge pressure transducer. 

The pore size of Cs2.2 and Cs2.1 cannot be determined by the N2 adsorption, so that their pore sizes were estimated from the adsorption of molecules having different molecular size. Table 3 compares the adsorption capacities of Csx for various molecules measured by a microbalance connected directly to an ultrahigh vacuum system [18]. As for the adsorption of benzene (kinetic diameter = 5.9 A [25]) and neopentane (kinetic diameter = 6.2 A [25]), the ratios of the adsorption capacity between Cs2.2 and Cs2.5 were similar to the ratio for N2 adsorption. Of interest are the results of 1,3,5-trimethylbenzene (kinetic diameter = 7.5 A [25]) and triisopropylbenzene (kinetic diameter = 8.5 A [25]). Both adsorbed significantly on Cs2.5, but httle on Cs2.2, indicating that the pore size of Cs2.2 is in the range of 6.2 -7.5 A and that of Cs2.5 is larger than 8.5 A in diameter. In the case of Cs2.1, both benzene and neopentane adsorbed only a little. Hence the pore size of Cs2.1 is less than 5.9 A. These results demonstrate that the pore structure can be controlled by the substitution for H+ by Cs+. 

Figure 1. Relationship between the measured adsorption volumes, Fp (measd) and calculated void volume Vp of several zeolites. The dashed line corresponds to Vp (measd) = Vp (calcd). The symbols represent the zeolites as described in Tables I-VI A, X, L, Z (mordenite Zeolon), omega (to), and offretite-type 0. Vertical shaded areas containing plotted values of Vp (measd) correspond to calculated values of Vp for the main pore systems. The narrow area, 0, corresponds to the main c-axis void of zeolite 0. The value of Vp for Zt = Vp for zeolite 0. Symbols with the subscript t (At Xt) etc.) represent values of Vp for the total void volume shown by narrow shaded areas. The neopentane (NP) volumes lie consistently below the dashed line thus showing a paeking effect. In all of these zeolites of varying structure, the H20 and N2 volumes correspond with complete filling of the total voids even though this is not possible in the case of N2 in zeolites A, X, and L.

The results presented in this paper provide a simple explanation for the compensation effect. Neopentane (C-(CH3)4) is chosen as a specific example since neopentane can only adsorb via one of the methyl groups, and therefore its adsorption can be approximated by CH3 adsorption, which has been considered in this paper. Other, more straight alkanes like n-hexane can adsorb in a variety of geometries, which complicates the picture. 

Neopentane adsorbs dissociatively on the Pt surface. A C-H bond is broken and a Pt-C and Pt-H bond are formed. The cleavage of the neopentane C-H bond is not influenced by the support. Moreover, as argued above, the adsorption enthalpy of H2 is > 45 kJ/mol regardless of the support, and on a basic support an additional H2 molecule is formed prior to the adsorption of neopentane. Thus, according to equation (6) the difference in Eapp between the basic and acidic support is given by the sum of the difference in the intrinsic Pt-C bond energy (16 kJ/mol lower for the basic support) and the H2 desorption enthalpy limit at the reaction temperature ... 

These new results can explain the compensation effect and the negative order in the partial pressure of hydrogen found for the hydrogenolysis of neopentane catalyzed by supported Pt particles. The kinetics of the catalytic reaction is driven by the mode of the adsorption of hydrogen on the Pt surface. 

Adsorption. The adsorption properties for selected AlP0,-based molecular sieves are summarized in Table VIII. The adsorption data are arranged for each structure-type in order of increasing adsorbate size. The large pore structure-types (5, 36, 46) with pores defined by 12-rings of oxygen readily adsorb neopentane (kinetic dia. 0.62 nm>. The 5 and 46 structure-types have been... 

Of the medium pore structures (11,31,41), only the 11 structure has been published (22,25) and it has parallel, elliptical, non-intersecting 10-ring channels. Adsorption data indicate that cyclohexane is readily adsorbed but neopentane is excluded. The other two structures show less differentiation between cyclohexane and neopentane and may, in fact, have elliptical 12-ring pores. 

The early work of Kiselev (1957) revealed that the adsorption isotherms of n-pentane and n-hexane on non-porous quartz were intermediate in character between Types II and m. Values of C(BET) <10 were obtained and the differential enthalpies of adsorption decreased steeply at low surface coverage. More recently, the isotherms of isobutane (at 261 K) and neopentane (at 273 K) on TK800 have been found to be of a similar shape (Carrott et al., 1988 Carrott and Sing, 1989). Unlike those of benzene, these alkane isotherms do not undergo a pronounced change of shape as a result of surface dehydroxylation. This is consistent with the non-specific nature of their molecular interactions (see Chapter 1). 

The molecular sieving behaviour of Silicalite-I, as illustrated in Table 11.5 by the low saturation uptakes of neopentane and o-xylene, is primarily dependent on size exclusion. It is of interest that n-nonane has been found to give an isotherm of essentially Type I character at 296 K (Grillet et al., 1993). The initial part of this isotherm was completely reversible, but a small sub-step at p/p0 0.2 was followed by a long plateau and associated narrow, Type H4, hysteresis loop. The plateau was located at N° = 4 molec uc"1. This level of pre-adsorption was sufficient to block the whole of the intracrystalline pore structure. The accessibility to nitrogen was gradually restored by the progressive removal of the nonane. These results confirm the complexity of the nonane pre-adsorption and entrapment in relation to the pore network and indicate that there is no simple relation between the thermal desorption of n-nonane and the adsorbent pore structure. 

Figure 12.15. Adsorption-desorption isotherms on VPI-5 of methanol (squares), isobutane (diamonds). neopentane (circles) and propane (triangles) determined at 298, 261, 273 and 196 K, respectively (Kenny et al., 1992).

Among the chemical reactions of interest catalyzed by zeolites, those involving alkanes are specially important from the technological point of view. Thus, some alkane molecules were selected and a systematic study was conducted, on the various steps of the process (diffusion, adsorption and chemical reaction), in order to develop adequate methodologies to investigate such catalytic reactions. Linear alkanes, from methane to n-butane, as well as isobutane and neopentane, chosen as prototypes for branched alkanes, were considered in the diffusion and adsorption studies. Since the chemical step requires the use of the more time demanding quantum-mechanical techniques, only methane, ethane, propane and isobutane were considered. 

Figure 4. Two orientations of a neopentane molecule with respect to site Sjj of type X zeolite which are assumed in the calculation of potential energy of adsorption...

Table III shoves the comparison of the calculated potential energy of neopentane on NaX zeolite with the experimental value of the heat of adsorption (O. M. Dzhigit, A. V. Kiselev, L. G. Ryabukhina, Zh. Fix. Khim. 1970, 44, 1790). Two models, 1 and II (Figure 3), of the distribution of the force centers in zeolite NaX and two orientations, (1) and (2), of the neopentane molecule at site Sn (Figure 4) were used. The calculations show that for approximate evaluation of potential energy, it is only necessary to take into consideration about 50 nearest charged oxygen atoms and 4 cations. In model II, the influence of other cations (which are situated in sites Sn of the supercage) were taken into consideration. The other cations, which are distributed more randomly, do not influence seriously the electrostatic fleld in the space occupied by the neopentane molecule near one of the Sn sites (calculations by L. G. Ryabukhina and A. A. Lopatkin).

Early adsorption studies indicated that the effective micropore openings in natural and synthetic mordenites were about 4A (2) and not ca. 7A as required by structural data. Later it was shown that this probably was caused by partial blocking of the main channels by calcium and/or sodium cations (3, 22). Sorption in the side channels is limited normally to molecules smaller than n-butane (4.9A) (3), while the main channels in hydrogen-mordenite are nearly filled by the larger cyclohexane (22), benzene, and neopentane molecules (3) of critical diam-... 

In the original paper one of the major advantages put forward in favour of the Os method over the contenq)ary t method was that it allows a similar type of analysis of adsorption isotherms of other adsorptives, besides nitrogen, to be made. This is of particular importance in the case of activated carbons where it is customary to make use of a range of probe molecules of different size, shape, polarizability and polarity in order to carry out a more complete characterization. A number of authors have since demonstrated the general feasability of doing this and reference data for the adsorption of neopentane and butane, for... 

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