Toluene molecular dimensions

For this study, p-xylene and triisopropylcyclohexane (TIPcyC6) were the two molecular probes chosen, using toluene as a solvent. Their molecular dimensions were obtained from the shadow of the three-dimensional molecule projected onto a plane according to the method of Rohrbaurgh et al. [5] (Table 2). A molecular probe is considered not to penetrate into a cylindrical pore if two of its dimensions are greater than the pore diameter [6], As the free diameter of the window of the supercage of the Y zeolite is equal to 0.74 nm, it is considered that only TIPCyC6 cannot penetrate into the zeolite microporosity. 

One of the zeolite catalysts, H-ZSM-5, has been mainly studied because the pore size of ZSM-5 (0.55 nm) exists between the molecular dimensions of p-xylene (0.51 nm) and o- and m-xylene (0.58 nm). p-Xylene is selectively formed in the pore of ZSM-5. However, the para-selectivity is not so high because of the presence of acid sites on the external surface and the size of the pore-openings. Therefore, many techniques [4-9] have been proposed to enhance the para-selectivity such as controlling of acidity, surface modification and narrowing pore openings. In each technique, however, toluene conversion was decreased at the expense of the enhancement ofpara-selectivity. 

A. Miyamoto et al. of Kyoto University used the computer to show [75] that, in an RbLi exchange NaX zeolite, the distance from the strongly basic center Rb to the weakly acidic center at Li optimally matches the molecular dimensions of the toluene. Fixing the toluene and abstracting the benzylic proton are optimally tailored to one another in the X zeolite. In the Y zeolite. ZSM-5 and mordenite, on the other hand, this good match is absent. The computer prediction is in agreement with the experimental findings. 

It may be noted that thermo-osmosis of water and methanol occurs through cellophane membrane but not for ethyl alcohol or toluene. Thermo-osmosis occurs if the radii of the pores are smaller than the Van der Waals radii, but not if the pore size is greater than the molecular radii but smaller than the mean free path. Since the mean free path of the molecules in the liquid is of molecular dimensions, the range of pore size for thermo-osmosis to occur is limited. The size of ethyl alcohol molecules appears to be comparable to the pore size, whereas for toluene the pore size is perhaps greater than the molecular radii and the mean free path. Probably for methyl alcohol and water the pore diameter of the membrane is between the mean free path and the Van der Waals radii. However, there is no independent evidence for the magnitude of mean free path of liquid water. 

Bansal, studied the adsorption desorption isotherms of benzene, toluene and o-xylene on sugar charcoal associated with varying amounts of the carbon-oxygen surface groups and observed that the area of the hysteresis loop decreases as the molecular dimensions of the adsorbate increase from benzene to o-xylene (Table 2.7). The point of inception of the hysteresis loop was also found to shift to lower relative vapor pressures as we move up the series of hydrocarbons, which is due to an increase in the molecular size of the adsorbate. The point of inception of hysteresis loop was calculated using Cohn postulates and compared with the values read from the experimental curves (Table 2.8). It is seen that the two values agree closely for all the adsorbate-adsorbent systems. Higute, from thermodynamic considerations, also proposed that the critical radius for the inception of capillary condensation is equal to four times the molecular radius of the adsorbate. 

The value of a for poly(propylene oxide) was obtained from the data of Moacanin 189) for polyurethanes prepared by condensation of toluene-2,4-diisocyanate with atactic polypropylene glycols of molecular weights about 1000 and 2000. The small quantity of diisocyanate present in these compositions can make only a negligible effect on the chain dimensions (less than one per cent on the freely rotating chain, as is... 

The first systematic evaluation of chain dimensions of stereoregular polymers in solution were reported by Danusso and Moraglio2). They studied both isotactic and atactic polystyrene by viscometry and osmometry, in benzene and in toluene, (both thermodynamically good solvents). They concluded that no difference could be observed in the intrinsic viscosity-molecular weight relationship, [tj] = k Ma, for the two stereoregular forms [see Fig. I]. However, they did find a noticeable difference between the second varial coefficients for the two stereoregular forms of equivalent molecular weight. These results were consistent with subsequent measurements on... 

Figure 4.20. Electron micrographs of 40/60 poly(styrene-h-isoprene) systematically diluted with polystyrene and polyisoprene homopolymers. The two homopolymers have the same molecular weights as their respective blocks, and are added in a 40/60 ratio in order to maintain the same overall composition. The major effect is a progressive increase in phase domain dimensions, which eventually approach those of the cast blend structure (Kawai and Inoue, 1970). Compositions are shown in Figure 4.21 films are cast from 5 % solutions in toluene and are stained with osmium tetroxide.

Traditional zeolite-based molecular sieves most commonly exhibit incommensurate adsorption. This can be attributed to two main criteria (1) the dimensions of their channels or cages are, on average, much larger than that of the adsorbate molecules and (2) symmetry mismatches are often present. For example, zeolite-Y exhibits incommensurate adsorption for benzene and toluene molecules adsorbed within its super cage (Figure 4). The incommensurate adsorption type depends also on external parameters such as temperature. Single crystal and PXRD studies on the -hexane-loaded silicate-I MFI-type zeolite shows that at room temperature the adsorbed molecules are dynamically disordered and distributed over the entire channel, whereas at sufficiently low temperature, the adsorbate becomes well ordered within the channel, which is commensurate with the framework type. ... 

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