Application of quasi- equilibrated thermodesorption of hexane and cyclohexane for characterization of porosity of zeolites and ordered mesoporous silicas. Zeolites. The QE- TPDA profiles of hexane and cyclohexane obtained for the studied zeolites are compared in Figs. 1 and 2. For the narrow and medium pore zeolites large differences in thermodesorption of hexane and cyclohexane were found (Fig. 1). While QE- TPDA profiles of hexane showed considerable intensity for both ZSM- 5 and 5. A, in the profiles of cyclohexane no desorption was observed for 5.
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Application of quasi-equilibrated thermodesorption of hexane and cyclohexane for characterization of porosity of zeolites and ordered mesoporous silicas. Organic chemistry is the chemistry of carbon compounds. All organic compounds contain carbon; however, there are some compounds of carbon that are not. Butane (/ ˈ b juː t eɪ n /) is an organic compound with the formula C 4 H 10 that is an alkane with four carbon atoms. Butane is a gas at room temperature and. A family of faujasite (FAU) zeolites with different Si:Al ratio, and/or hierarchical porosity introduced via post-synthetic alkaline desilication treatment, have been.
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A and only a high temperature peak for ZSM- 5. The two step profiles of hexane observed for 5. A and ZSM- 5 zeolites are in agreement with the earlier findings (Sivasankar and Vasudevan 2. Smit and Maesen 1.
Fig. 1. QE- TPDA profiles of hexane and cyclohexane on zeolites ZSM- 5 and 5.A, measured at 1. on this page. C/min. Partial pressures of hexane and cyclohexane in the carrier gas equal to 6 and 7 mbar, respectively.Fig. 2. QE- TPDA profiles of hexane and cyclohexane on zeolites 1.X, Y and Na. MOR, measured at 1.C/min. For 1. 3X and Y partial pressures of hexane and cyclohexane equal to 6 and 7 mbar, respectively.
For Na. MOR they were equal to 8 and 6 mbar, respectively. For the wide pore zeolites the profiles observed for both hydrocarbons are quite similar, only in the case of Na. MOR intensity of the thermodesorption peaks was smaller for cyclohexane. For this zeolite also some effects due to catalytic reaction hydrocarbons were observed at high temperatures—increase of the detector signal indicating cracking, accompanied by coking leading to decrease of the adsorption capacity. These effects were not observed in the additional QE- TPDA measurements were performed in the temperature range 2. C (not shown here).
The thermodesorption data obtained for the studied zeolites were interpreted quantitatively by fitting with functions based on the Langmuir adsorption model, according to the procedure published earlier (Makowski and Ogorzałek 2. The QE- TPDA profiles were integrated and normalized to one. Adsorption isobars were calculated by averaging of the integral desorption and adsorption profiles. In the case of Na. MOR the QE- TPDA profiles recorded up to 4. C, not affected by catalytic reactions, were used. Temperature derivatives of the experimental isobars exhibiting a single minimum were fitted with the temperature derivative of the Langmuir adsorption function: $$ \frac{d\theta }{d.
T} = \frac{{{\text{p}}\Updelta H_{ads} \exp \left( { - \frac{{\Updelta G_{ads} }}{RT}} \right)}}{{\left[ {1 + {\text{p}}\exp \left( {\frac{{ - \Updelta G_{ads} }}{RT}} \right)} \right]^{2} RT^{2} }} $$(1)$$ \Updelta G_{ads} = \Updelta H_{ads} - T\Updelta S_{ads} , $$(2)where T is the temperature, p the partial pressure of the adsorptive, ΔHads and ΔSads are the adsorption enthalpy and entropy. In the case of the two step desorption profiles observed for hexane on 5. A and ZSM- 5 zeolites a dual site Langmuir (DSL) function i. Langmuir functions (Eq. 1) was used. Least square fitting was performed using the Microsoft Excel Solver procedure. The results of the fitting are shown in Fig. 3, and the fitting parameters (i. Table 1, together with values of the micropore volume, determined by integration of the QE- TPDA profiles.
Very good agreement between the experimental data and the fitted functions in almost the whole temperature range was obtained for hexane and cyclohexane on ZSM- 5, and also for hexane on 5.A (not shown here).For all the other adsorbate/adsorbent systems good fit could be obtained only in the high temperature range.Fig. 3. Fitting of the Langmuir and the DSL functions (solid lines) to the temperature derivatives of the normalized isobars determined by integration of the QE- TPDA profiles of hexane and cyclohexane (dashed lines)Table 1. Creating Dynamic Forms With Adobe Livecycle Designer Download . Values of the parameters determined for zeolites from QE- TPDA profiles of hexane and cyclohexane.Values of the adsorption enthalpy and entropy of hexane calculated as the fitting parameters are in agreement with our earlier findings and the literature data (Makowski and Majda 2.Differences of the corresponding parameters found for a given zeolite reflect differences in interactions of hexane and cyclohexane molecules during adsorption.
In the case of zeolites 1. X and Y values of the adsorption enthalpy and entropy of cyclohexane are quite close to those found for hexane, thus indicating that interactions of both types of molecules with the walls of the supercages in the FAU framework and extraframework Na+ cations do not differ considerably. Slightly larger value of the adsorption entropy loss (−ΔSads) obtained for cyclohexane on Na. MOR may indicate that these molecules adsorbed in the 1. MR channels of this zeolite have less rotational or translational freedom than the adsorbed hexane molecules.
On the other hand, lower micropore volume calculated from the adsorption capacity of cyclohexane for this zeolite suggests that a part of the void space in the MOR framework (most probably 8. MR side pockets perpendicular to the main 1.
MR channels) are not accessible for cyclohexane molecules, in contrary to those of hexane. The parameters found for the two step desorption profiles of hexane observed for 5. A and ZSM- 5 zeolites are consistent with the “commensurate freezing” concept. Large values of the entropy loss (−∆Sads) found for the low temperature peaks corroborate a considerable ordering of the adsorbed molecules and suppressing of their mobility at high adsorption degrees. The lack of the low temperature desorption step in the QE- TPDA profile of cyclohexane on ZSM- 5 indicates that such an ordering of these molecules does not occur.
However, it should be noticed that values of the adsorption enthalpy end entropy loss (−∆Hads and −∆Sads) for the high temperature desorption peak observed for ZSM- 5 are smaller in the case of cyclohexane. . This may mean that the adsorbed cyclohexane molecules are located in the intersections of 1.MR channels of MFI framework where they have more rotational freedom than hexane molecules adsorbed within the channels.Mesoporous silicas.
The QE- TPDA profiles of both hydrocarbons measured for all the studied mesoporous silicas are plotted in Fig. 4. The partial pressures of hexane and cyclohexane during the experiments were about 2. However, as their saturation values at 2. C are much higher (2.
The profiles consist of one low temperature desorption maximum and adsorption minimum typical for the multilayer adsorption on the silica surface (Makowski et al. The differences in intensity for the thermodesorption profiles of hexane and cyclohexane result from the lower value of the cyclohexane partial pressure applied. But these differences were not always the same—it is worth noting that both QE- TPDA profiles obtained for SBA- 1. Additionally, these profiles are broader than those obtained for any other mesoporous material. These facts might reflect differences in the morphology or roughness of the SBA- 1. Fig. 4. QE- TPDA profiles of hexane and cyclohexane on the MCM- 4.
MCM- 4. 1/TMB, SBA- 1. HMS mesoporous materials, measured at 1. C/min. Partial pressures (in mbar) of hexane: 2. MCM- 4. 1, SBA- 1. HMS), 2. 2 (MCM- 4.
TMB); of cyclohexane: 1. MCM- 4. 1, SBA- 1. HMS), 1. 6 (MCM- 4. TMB)The observed minima related to the adsorption of hydrocarbons have lower intensity of the signal in comparison to the maxima.
This effect is caused by non- effective cooling down the sample in the low temperature range, shown in the inset in the Fig. 4. In the range of 2.
C the temperature decrease is slower than 1. C/min, which results in a slower uptake of the adsorptive from the carrier gas, and consequently in a decreased intensity of the adsorption minimum. However, this limitation does not affect the quasi equilibrium control of the thermodesorption measurements.
The QE- TPDA profiles of hexane and cyclohexane found for the studied mesoporous materials were converted into the adsorption isobars, according a similar procedure to that used for transformation of the profiles obtained for zeolites. The only difference was that the integrated profiles were not normalized. The resulting isobars (Fig. 5) were fitted with the BET function: $$ V_{ads} = \frac{{V_{m} \left( {\frac{p}{{p_{e} }}} \right)C}}{{\left( {1 - \frac{p}{{p_{e} }}}.