Criterios y sistemas de selección de originales

A range of techniques was used to study the adsorption of 1-pentene on γ- and θ-Al2O3.

olefin systems on alumina. However, a comparison of these adsorption methods is important, as it indicates what has been extracted from this limited study.

As was discussed previously, the use of IR spectroscopy was employed to study the changes occurring on the surface of the alumina upon adsorption of 1-pentene. Flow-through pulsed 1-pentene experiments determined the influence of hydroxyl groups on the interaction of 1-pentene with Al2O3. Additionally, changes in the IR spectra using DRIFTS allowed for

changes in adsorption with pre-treatment temperature to be distinguished. Nevertheless, clear limitations were noted on the sensitivity of the method. It was not possible to distinguish the formation of pentene isomers on alumina pre-treated at high temperature, unlike with 13C NMR T1 relaxometry results (section 4.3.3.3). Short residence times were also characteristic

during IR measurements. The heats of adsorption of 35 – 41 kJ mol-1 were characteristic of weak adsorption and reversible desorption. IR spectroscopy was performed during dynamic measurements vs. batch adsorption of adsorption isotherms and NMR relaxometry. The experimental design would have ideally required a technique with better resolution for low surface density measurements. This would have allowed for strong surface sites to be captured, and for possible reactions to be tracked.

Adsorption isotherms provide a useful method to determine energies of adsorption. Despite the time length of each experimental measurement – equilibrium is required – isotherms contained very interesting information. The type of isotherm curve obtained already revealed information on the adsorption of 1-pentene, and the saturation and condensation pattern (Figure 4.3) showed the extent of interaction with the alumina. Still, the most important aspect was the possibility of extracting heats of adsorption as a function of surface coverage. Those values were useful in order to compare with simulation studies or experimental values from similar hydrocarbons or surfaces. A series of assumptions were made in order to obtain such energies. Literature has discussed in detail the validity of those postulates (Pan et al., 1998). Compared to that study, the pressure and temperature ranges allowed for uptake values to be approximated with the ideal gas method and Clausius-Clapeyron equation (equation 4.7). Additionally, comparison between calorimetry results and isosteric heats of adsorption on the adsorption of simple molecules on zeolites showed similar results were obtained (Shen et al., 2000). Nevertheless, the limiting factor in adsorption isotherms is the vapourisation-condensation range of the adsorbate. Further, only macroscopic information was obtained; hence no physically meaningful evidence was gathered on interaction with the

Lastly, chemical shift resolved 13C T1 NMR relaxometry provided information on relative

strength of interaction of 1-pentene individual carbon atoms with the surface of γ- and θ-Al2O3. Hence, it was possible to determine the adsorption geometry of 1-pentene

interacting with the surface. The technique was sensitive to the alumina type, surface coverage, and dosing temperature, similar to adsorption isotherms. Additionally, it was sensitive to changes in pre-treatment temperature, similarly to IR spectroscopy. Further to this, it was possible to study the adsorption of various hydrocarbons when co-adsorbed on the surface of θ-Al2O3. The technique sensitivity, fingerprint for molecular structure and reduced

number of assumptions made 13C T1 NMR relaxometry very versatile. NMR measurements

have been demonstrated very useful in the past in order to provide relevant catalytic information (Gladden and Mitchell, 2011; Gladden et al., 2012), including the isomerisation and hydrogenation of alkenes (Mantle et al., 2006). Yet, a comparison with other adsorption techniques or previous results from the literature could be broader. Such wider study will be presented in chapter 5.

As was observed, no single adsorption technique could have fully resolved the interaction of 1-pentene with the surface of aluminas. For example, FTIR is widely used in addition to other techniques, such as Temperature Programmed Desorption (TPD), XPS or solid state NMR. Such a combination has been shown to be useful in providing complementary information on the adsorption sites and interaction energies (Tanaka et al., 1999; DeCanio et al., 1993).

4.5 Conclusions

A comprehensive study on the adsorption of 1-pentene on the surface of γ- and θ-Al2O3 was

performed. IR spectroscopy, volumetric adsorption isotherms and 13C NMR T1 relaxometry

were used. As observed, a weak adsorption on the aluminas via the 1-pentene double-bond was present. 1-pentene bonded weakly to OH groups on the surface of porous alumina. Adsorption was confirmed to be more favourable over θ-Al2O3 than over γ-Al2O3 as

expected. Comparison with infrared spectroscopy provided similar information on strength of interaction. Pre-treatment temperature of the alumina exposed Lewis acid sites responsible for isomerisation, only observed with 13C NMR. Studies of adsorption as a function of surface coverage demonstrated the average interaction strength was greater at lower surface coverage, indicating molecules adsorb preferentially on the strongest adsorption sites. Similarly, the temperature of adsorption showed similar effects. Furthermore, the presence of

Energies of adsorption as a function of coverage were obtained via adsorption isotherms and the particular surface-adsorbate interactions were described with IR spectroscopy. Yet, information on atom-specific adsorbate-adsorbent interaction strengths and the molecular conformations of the adsorbates were only inferred with the use of 13C NMR relaxometry. In addition, co-adsorption measurements showed the change in adsorption strength of 1-pentene when in the presence of a second molecule. Important implications were extracted, such as the influence of coke, CO or alkyne in the relative strength of adsorption of olefins. The application of chemical shift resolved 13C NMR T1 relaxometry to the determination of

adsorbate interaction with alumina surfaces is reported. Knowledge of adsorbate geometry and interaction strength is of importance in chemical sensors and heterogeneous catalysis where the interaction of adsorbates with active surface sites influences performance. The results on adsorption of 1-pentene showed that 13C T1 NMR relaxometry is a useful tool to

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Chapter 5:

Adsorption of C₅ and C₆