Temperature-programmed desorption (TPD)

TPD-MS experiments were performed in a CATlab microreactor module (Hiden Analytical) and desorbed species were detected with a QIC-20 quadrupolar mass spectrometer (QMS). Details of the equipment were described in section 2.2.1.2. Approximately 100 mg of γ- or θ-Al2O3 trilobes were pre-treated at 393 or 673 K, after which the hydrocarbon was adsorbed

until saturation. This procedure was either preformed in situ with the aid of saturator, or via the glass manifold, as detailed in section 5.1.5. In the latter case, the alumina with the pre-adsorbed hydrocarbon was transferred into the reactor gas tube. Desorption of excess

hydrocarbon was achieved at 313 - 323 K under flowing He, at a flow rate of 40 cm3 min-1, for a duration of 1 h. The samples were then heated from 323 K to 1073 K with a ramp rate of 10 K min-1 _{at a flow rate of 40 cm}3 _min-1 _{of He. Desorption of pre-adsorbed hydrocarbon was}

monitored observing m/z = 42 for 1-pentene; m/z = 67 for 1-pentyne, 1,4-pentadiene and cyclohexene; and m/z = 55 for cis- and trans-2-pentene. Also, desorption and isomerisation of 2-pentene products to 1-pentene isomer were recorded simultaneously via m/z = 55 and 42. Possible hydrogenation reactions during 1-pentyne desorption were detected following the hydrogenated species up to the alkane, with m/z = 43 for n-pentane. Finally, the presence of secondary reactions and cracking of the hydrocarbon was monitored via a full range scan, set for m/z = 2 to 71, during 1-pentene and 1-pentyne desorption.

Calibration of the QMS signal was performed by integration of the respective m/z following the injection of a known amount of each hydrocarbon, usually 5.0 ± 0.1 µL. Thus, desorption rates were expressed in molecules desorbing by specific surface area of each alumina per unit of time (molec nm-2 s-1). The Polanyi-Wigner equation (equation 1.3) was used to determine the energies of desorption. Analysis of the energetics of desorption followed the methods described in section 2.2.1.2.

5.2 Results

The adsorption of C5 and C6 unsaturated hydrocarbons was investigated. In addition to IR

spectroscopy and adsorption isotherms via the volumetric method, other experimental techniques were implemented. TEOM and NMR relaxometry techniques were employed. In the case of TEOM and 1H 2D T1-T2 relaxometry correlations, the study was limited to

1-pentene, 1-pentyne and cyclohexene. This allowed for geometric effects as well as double-bond vs. triple-bond interactions to be studied.

5.2.1 Infrared spectroscopy

The adsorption of 1-pentyne was studied via DRIFTS. The type of interaction with the surface, via an adsorbed bond or functional group, was followed. Semi-quantitative information on strength of adsorption was also studied. As shown in section 3.3.1, and as will be evident here, the adsorbed species had only a short residence time on the surface, as shown by IR. Hence, only major differences, such as those between double-bond and triple-bond adsorption were studied.

Figure 5.1 shows the infrared spectrum of the adsorption of 1-pentyne at 298 K on γ- and

θ-Al2O3 pre-treated at 393 and 673 K. Difference spectra with respect to each pre-treated

alumina are shown. Table 5.1 shows the bands present in the spectra and their assignment. As can be seen, negative bands appeared at 3735 cm-1 and 3700 cm-1, and a positive band was observed at 3631 cm-1 in the OH stretching region. Similar negative bands have been observed in the adsorption of butene isomers on γ-Al2O3 (Trombetta et al., 1997). In the

adsorption of 1-pentyne, the bands were related to the types of hydroxyl groups interacting with the hydrocarbons. Thus, the adsorption of 1-pentyne was similar to that of 1-pentene shown in section 4.3.1, whereby double-bridged and triple-bridged OH coordinated to AlVI participated (Morterra and Magnacca, 1996; Knözinger and Ratnasamy, 1978, Tsyganenko and Filimonov, 1972). Comparatively, these bands were more pronounced in the adsorption of 1-pentyne, indicating a stronger interaction with the alumina. All bands decayed after 6 min, indicative of the fast, weak and reversible adsorption-desorption of the alkyne from the surface hydroxyls.

Figure 5.1. DRIFTS spectra for the adsorption of 1-pentyne on γ- (dashed) and θ-Al2O3 (continuous). Initial desorption after signal saturation (a, b and c) and after 5 min He flow (d, e and f). The black lines represent samples pre-treated at 673 K, while the grey lines show the spectra on samples pre-treated at 393 K.

Table 5.1. Summary of the observed IR bands for the adsorption of 1-pentyne on γ- and θ-Al2O3. The type of vibration, the position in wavenumbers (cm-1), and the assignment to a corresponding bond are shown.

bond vibration wavenumber / cm-1 assignment

O-H stretch 3730 double-bridged OH (II) 3716 triple-bridged OH (II) 3700 3650 - 3630 bulk OH C-H stretch 3338 v(≡CH) 3330 3322 2967 v(CH3) and v(CH2) 2945 2879 C-C stretch 2133 a v(C≡C) 2113 C-H bend 1456 δ(CH3)as 1440 _δ(CH2)scissors 1382 δ(CH3)s 1336 w(CH2)b

a) Shoulder of main band; b) wagging

Interaction of the hydrocarbons with the aluminas was also shown by the presence of characteristic bands of the hydrocarbon. C-H stretching bands, corresponding to v(CH3) and v(CH2), appeared at 2967 cm-1, 2945 cm-1 and 2879 cm-1. Characteristic bands of 1-pentyne

unsaturation were noted. Bands at 3330 cm-1, split into a triplet at 3338 cm-1, 3330 cm-1 and 3322 cm-1, appeared with the adsorption of 1-pentyne irrespective of the type of alumina or pre-treatment temperature. These bands corresponded to v(≡CH) from excess vapour and weakly interacting liquid (Crowder and Fick, 1986; Horn et al., 2011), indicative of the weak type of interaction with the surface. Additionally, a weaker band was observed at 2113 cm-1, with a shoulder at 2133 cm-1, corresponding to v(C≡C) for liquid adsorption and vapour, respectively. Finally, in the fingerprint region (<1500 cm-1), bands appeared at 1456 cm-1, with a shoulder at 1440 cm-1, 1382 cm-1 and 1336 cm-1, during the adsorption of 1-pentyne. These bands corresponded to δ(CH3)as, δ(CH2) scissors, δ(CH3)s, and w(CH2) (Crowder and

Fick, 1986; SDBS, 1997). Comparing with Figure 4.1, bands were more intense for 1-pentyne, indicating more molecules of 1-pentyne were interacting with the alumina, as compared to 1-pentene. In Figure 5.1 a considerable reduction in these bands with purging was observed after 5 min (spectra c, d and e). Noticeably, a redshift from 2112 cm-1 _to

triple bond, in line with interactions of the functional group with the surface OH. No other bands related to products of reactions, such as cracking or hydrogenation, were present, contrary to results from adsorption of butene and butane on aluminas (Garbowski and Primet, 1985). It was inferred that the interaction was too weak to dissociate the molecule (Busca et

al., 1987; Trombetta et al., 1997). However, a yellow colouration was observed on the

alumina post-adsorption. This is in line with previous studies that have manifested the presence of strongly-adsorbed hydrocarbon. For example, a yellow coloration of the alumina was seen after prolonged adsorption of butane isomers (Peri, 1961).

Fitting the integrated area of the v(C≡C) band in the IR spectrum with a single exponential decay (equation 4.1) resulted in the values summarised in Table 5.2. As can be seen, pre-treatment temperature modified the time constant. Thus, values of τ = 3.1 ± 0.5 min and τ = 2.4 ± 0.4 min were obtained for adsorption of 1-pentyne on θ-Al2O3 pre-treated at 673 K

and 393 K, respectively. Similar values of the time constants were observed for the adsorption of 1-pentyne on γ- and θ-Al2O3, indicative of a weak effect on adsorption.

Comparison of these time constants with results on the adsorption of 1-pentene in section 4.3.1 (Table 4.2) was indicative of stronger interaction of the triple bond. Values of τ = 3.1 ± 0.5 min and τ = 2.3 ± 0.1 min were obtained for adsorbed 1-pentyne and 1-pentene, respectively, on θ-Al2O3 pre-treated at 673 K. Additionally, I0 values were higher for

1-pentyne adsorption, in agreement with higher surface density of 1-pentyne molecules interacting with the alumina. These values are semi-quantitative, as the experiments were performed using diffuse-reflectance mode. Overall, IR results were indicative of a weak interaction of the hydrocarbon via the unsaturation with surface hydroxyl groups. These adsorption sites can be related with weak Brønsted acid sites on the aluminas. Minor differences in adsorption were observed between both aluminas. Pre-treatment temperatures showed a marked influence on adsorption, which might suggest the participation of Lewis acid sites discussed in section 3.3.2. Differences in the strength of interaction of alkyne vs. alkene were noted, following the lower time constant for 1-pentene adsorption.

Table 5.2. Parameters resulting from the fitting of an exponential decay to the integrated IR bands for

the desorption of 1-pentyne on γ- and θ-Al2O3. Parameters were described in Table 4.2

surface pre-treatment T / K I0 / - τ / min

θ-Al2O3 673 0.65 ± 0.07 3.1 ± 0.5

γ-Al2O3 673 0.8 ± 0.2 3.0 ± 0.4