Fibrillas de colágeno y minerales

In this section, the results from the different adsorption techniques will be reviewed, according to the hydrocarbons and conditions studied on each case. In chapter 4 the interaction of the alkene with γ- and θ-Al2O3 was already discussed in terms of the adsorption

sites of the alumina. Here 1H 2D T1-T2 NMR relaxometry correlations, TEOM and TPD were

performed for adsorbates on both γ- and θ-Al2O3 to complete such observations.

Notable differences in the adsorption strength of the hydrocarbons were noted. These results showed the difference in the adsorption strength of 1-pentyne, with a triple bond, vs. the rest of the hydrocarbons, all olefins. Both IR spectroscopy and volumetric adsorption isotherms showed a marked difference in this respect. Taking 1-pentene as an exemplar olefin, the decay in the IR signal corresponding to the olefin presented a value of τ = 2.3 min (Table

4.2), as compared to τ = 3.1 min for 1-pentyne on θ-Al2O3 pre-treated at 673 K. Adsorption

isotherm results also pointed towards enhanced 1-pentyne adsorption. Heats of adsorption at zero coverage, using the Tóth fitting, indicated that for 1-pentyne (-ΔHads)zero = 93.8 kJ mol-1

as compared to 45.6 kJ mol-1in the case of 1-pentene. These results were in agreement with previous literature adsorption studies on aluminas (Hoffmann et al., 1966). Energies of

adsorption of an alkyne compared to an alkene have been reported in other catalysts. For example, values of 62 kJ mol-1 _{for the adsorption of acetylene vs. 37 kJ mol}-1 _{for ethylene}

were previously reported on hydrogenated silicon surfaces (Takeuchi et al., 2004). Other metals, such as Pt or Pd, have also shown preferential alkyne adsorption 1-hexyne / 1-octene, 1-hexyne / 1-heptene mixtures, which related to preferential hydrogenation (Dobrovolná et

al., 1998) Significantly, the selective hydrogenation of alkynes in alkene-alkyne mixtures

relies on the preferential adsorption of the alkyne (Teschner et al., 2006). Previous 13C T1

NMR relaxometry results also revealed important differences between 1-pentyne and the rest of the alkenes (Huang, 2008). Taking 1-pentene again as an example of the olefins, and considering the average T1,ads/T1,bulk value of the carbons constituent of the unsaturation,

results were in agreement with previous findings. Thus, the average value for C1 and C2 was

T1,ads/T1,bulk = 6.3 × 10-3 for 1-pentyne, while T1,ads/T1,bulk = 2.64 × 10-2 for 1-pentene on

θ-Al2O3. TPD results did not show such behaviour.

In addition to the differences between double-bond and triple-bond interactions, it was also possible to distinguish adsorption trends within olefins. As shown in Tables 5.4 and 5.5, results at zero coverage revealed notable differences within the C5 molecules and with

cyclohexene. Both 1,4-pentadiene and 1-pentene had similar interaction energies, with 1,4-pentadiene showing higher heats of adsorption. These results would be in agreement with an additional interaction provided by the added terminal unsaturation in 1,4-pentadiene. A comparison was also established between the 2-pentene isomers and 1-pentene. A difference of about 10 kJ mol-1 _{in the heats of adsorption of three isomers was observed. The interaction}

of an internal unsaturation could be expected to present some steric hindrance. The double bond is free of substitution in one carbon in 1-pentene, in contrast to the presence of substituents in both carbons for 2-pentenes. Similar unsaturated systems have shown a reduction in adsorption caused by substitution. However, the higher adsorption energy of

trans-2-pentene is not well understood. Isomerisation of the trans isomer might be occurring

in the alumina, with the subsequent additional energy being released. Previous studies have shown the formation of one isomer upon adsorption on aluminas. Adsorption of cyclohexene revealed the lowest heat of adsorption of the series, with a value of 10.5 kJ mol-1_.

In contrast to the adsorption isotherm results, desorption energies obtained from TPD were significantly higher than adsorption energies. Energies of desorption were in the range 85 – 130 kJ mol-1, considering peaks I to III, and similar among the different hydrocarbons. These

1-pentene on γ-Al2O3 (Clayborne et al., 2004), with a value of 134 kJ mol-1. Similar energies

were observed in the energies of activation of alcohol dehydration to alkenes on γ-Al2O3.

Values of 93.3 kJ mol-1 for 1-pentene, 97.9 to 122.2 kJ mol-1 for trans-2-pentene, 95.4 to 109.6 kJ mol-1 for cis-2-pentene, or 107.1 kJ mol-1 for cyclohexene (Knözinger et al., 1972). In addition, these results were similar to those observed in the formation of isobutylene, with a value of 114 kJ mol-1 (Swecker and Datye, 1990). The contrast between such values is discussed in the context of the results from other adsorption methods in section 5.3.2. It was observed that high coverage adsorption was dominated by weak Brønsted adsorption sites, generally of low energy vs. Lewis acid sites for TPD, where reactivity is present. As observed, weak adsorption of 1-pentene via Brønsted acid sites was considered in section 4.4.1. Here, TPD revealed different adsorption sites. This was shown more clearly in the results of adsorption on high temperature pre-treated aluminas, seen also in IR, TEOM and

1_{H T}

1/T2, presented vide infra. Previously, different heat of adsorption values have been

reported on alumina, i.e., in the σ-coordination of CO onto cus Al3+ sites. Calorimetric values ranged between a minimum of 8.5 kJ mol-1 and a maximum of 60 kJ mol-1, whereas most calculated isosteric heat values were scattered in the narrow range 20-35 kJ mol-1 _{(Morterra et} al., 1994). As shown in chapter 3, no Brønsted acid sites were observed by means of pyridine

adsorption, and two types of Lewis acid sites were considered. The presence of various Lewis sites would link with the appearance of peaks I to III during TPD.

Although not measured directly, the presence of adsorbate-adsorbate interactions was noted. Such interactions were present in this system, especially at higher coverages, as observed during adsorption isotherm results, but also during 1-pentene co-adsorption experiments, discussed in section 4.4.2. This interaction was also apparent in the results of the Tóth fitting. The values of the parameter t in the Tóth equation were particularly high during the adsorption of 2-pentenes and 1,4-pentadiene, as seen in Table 5.3. As can be seen in Figure

5.3, those species presented the trends on dominant adsorbate-adsorbate interactions starting

at the lowest coverages. These results are in agreement with the description of t, previously proposed (Tóth, 1995). 1H 2D T1-T2 relaxometry correlations in 1-pentene and cyclohexene

showed in detail the interaction of the double-bond and the aliphatic chain (Tables 5.19 and

5.20). The presence of lateral interactions via the aliphatic chain was noted.

The pre-treatment of the alumina resulted in an activation process whereby the adsorption sites were modified to stronger acid sites, as discussed in section 4.4.1. While not presented

in the figures, desorption of hydroxyl groups from the alumina was significant. Thus, the rates of desorption for m/z = 18 were of the same order of magnitude compared with m/z = 15, 39 and 44 for pre-adsorbed 1-pentyne on θ-Al2O3 pre-treated at 673 K. A difference of

over an order of magnitude was seen for m/z = 18 as compared to the fragment species. Analysis of the TPD showed the influence of the hydroxyl groups with the adsorption of the hydrocarbons. A simultaneous desorption of m/z = 18 and m/z = 15, 39 and 44 for peaks II and III was observed. These peaks were also present for m/z = 42 and 43 (1-pentene and

n-pentane, respectively). The desorption of hydroxyl species could reduce the number of

adsorption sites, and induce the desorption of hydrocarbons (Wischert et al., 2012). Infrared results for the adsorption of hydrocarbons on γ- vs. θ-Al2O3 showed some relation with the

removal of OH. Similar findings were obtained with TEOM. These results relate with the findings during TPD desorption of 1-pentyne from alumina pre-treated at 673 K. Similarly,

1_{H 2D T}

1-T2 relaxometry correlations with T1/T2 values of 1-pentyne, 1-pentene and

cyclohexene pre-treated at 393 K contrasted with those obtained on pre-treated alumina at 673 K.

The analysis of desorption peaks from TPD was performed considering purely adsorption of the hydrocarbons. However, it is important to note that some reactivity was present in the aluminas. As mentioned in chapter 3, aluminas are widely used as catalyst supports, but have been also used as catalysts in their own right, e.g., in hydrogenation or isomerisation reactions (Knözinger and Ratnasamy, 1978). 13C T1 NMR results shown during the

adsorption of 1-pentene on pre-treated alumina at 673 K (section 4.3.3.3) already indicated reactivity on the alumina surface upon adsorption of these hydrocarbons. However, as discussed in chapter 4 and section 5.2.1, no other species were observed in the IR spectra. An overall weak adsorption of the hydrocarbons was assumed then. Despite IR spectra, results from TPD clearly showed some reactivity on both γ- and θ-Al2O3. As presented in section

5.2.4.1, isomerisation of 2-pentenes to 1-pentene, and hydrogenation of 1-pentyne were noted. Table 5.11 presented the energies of desorption obtained from TPD analysis. As noted, those values were in the range 85 – 130 kJ mol-1, considering peaks I to III, from highest to lowest coverage, respectively. These values are typical for hydrogenation-dehydrogenation processes, and have been previously cited for highly active adsorption sites on alumina surfaces (Digne et al., 2002). It is important to note that the surface coverage was much lower during TPD. For example, an initial coverage of 1-pentyne of < 1 × 10-5 mol g-1, equivalent to < 0.3 % ML or < 1 % of adsorption sites on alumina

resulted in the high energies of desorption obtained. Notwithstanding, these results were also noted during TEOM experiments in the adsorption of 1-pentyne on both aluminas. The increased uptakes with increasing adsorption temperature suggested some reactivity. Again, uptake values were in the line of the initial coverages during TPD experiments. The higher interaction strength of 1-pentyne on the aluminas allowed for TEOM experiments to show the trend observed in Figure 5.9.

As observed, the interaction of hydrocarbons on alumina is a complex process. Different strength of adsorption was noted as a function of type of unsaturation. The relative position of the unsaturation also provided different heats of adsorption. Finally, the presence of adsorbate-adsorbate interactions was noted. The heterogeneity of the alumina also played an important role. Thus, while the interaction of C5 and C6 hydrocarbons was predominantly a

weak adsorption process, reactivity was noted. Isomerisation of 2-pentenes, as well as 1-pentyne hydrogenation was observed. Such processes were considered to be occurring on a small fraction of high energetic adsorption sites.