5.2.1 Materials and surface passivation
Mesoporous γ-alumina (γ-Al2O3), θ-alumina (θ-alumina) and silica (SiO2) were
obtained from Johnson Matthey, while mesoporous anatase-titania (A-TiO2) was
obtained from Evonik-Degussa. Each material was functionalised with surface octyl groups through a simple liquid-phase treatment with triethoxyl(octyl)silane
(TEOS, Sigma Aldrich, ≥ 96 %).[27] This treatment generates a polymeric surface layer which passivates hydroxyl groups at the pore surface; the passivation mechanism is illustrated in Figure 5.1. To prevent excessive polymerisation of the liquid-phase TEOS before agregation at the surface the majority of physisorbed water was removed by first drying the oxide pellets for 2 hours at 105 ◦C. [28] The pellets were then soaked in excess TEOS for 12 hours under ambient conditions; remaining physisorbed water leads to hydrolysis of the ethoxy groups (Figure 5.1). The resulting hydroxysilane molecules then aggregate with the pore surface through hydrogen bonding interactions with surface hydroxyl groups, and condensation reactions between adjacent molecules form a polymeric surface layer which passivated the oxides. [28] Following this treatment the pellets were washed several times in cyclohexane to remove the evolved ethanol and unreacted TEOS, before being dried at 105◦C for a further 12 hours. The high TEOS concentration utilised here (excess, no solvent) suggests that a highly cross-linked polymeric surface layer is formed at the pore surface. [28] This approach is advantageous in the present study as it limits the ability of any remaining surface hydroxyl groups to interact with liquids imbibed within the passivated pore system.
5.2.2 Materials characterisation
DRIFTS
Diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) measurements were performed to confirm successful passivation of the oxide supports. Measurements were performed on a ThermoFischer Nicolet iS50 FT-IR spectrometer equipped with a Praying Mantis diffuse reflectance cell and high temperature reaction chamber. Samples were ground by hand using a pestle and mortar, and were analysed without dilution. Approximately 50 mg of each catalyst support was loaded into the reaction cell supported by a small amount of quartz wool. To remove the influence of physisorbed water all samples were heated to 150 ◦C at a rate of 10 ◦C min−1 under a low flow of helium (10 ml min−1); this temperature was maintained for 30 minutes, after which the samples were cooled to 25◦C at the same rate. Infrared absorption spectra were acquired with 64 repeat scans with a
Figure 5.1: Illustration of the three-step mechanisms for the formation of polymeric surface layers at oxide surfaces by liquid-phase treatment with silanes, such as triethoxyl(octyl)silane
resolution of 4 cm−1, relative to a KBr background measurement acquired under identical conditions.
Nitrogen porosimetry
N2 isotherm measurements were carried out by technical support staff at the
Department of Chemical Engineering and Biotechnology, University of Cambridge. Measurements were performed using a Micromeritics TriStar 3000 automated gas adsorption analyser. Specific surface areas SBET were obtained using the Brunauer-
Emmett-Teller (BET) method. Pore volumes VBJ H were calculated using the
Barrett-Joyner-Halenda (BJH) method. All N2 adsorption measurements were
carried our at −196◦C.
5.2.3 NMR measurements
Sample preparation
Unfunctionalised oxides were dried for at least 12 hours at 105 ◦C before use. To ensure saturation, each material (both functionalised and unfunctionalised) was soaked in excess methanol (Sigma Aldrich, ≥ 99.8 %) for at least 24 hours; this allowed the pore structure to fill through the process of capillary imbibition. It has been shown elsewhere that soaking porous catalyst support materials in this way is sufficient to saturate the pore network. [29] Separately, each unfunctionalised material was also saturated with cyclohexane (Sigma Aldrich, ≥ 99.5 %) to provide a weakly-interacting reference. The saturated oxide materials were then removed from each liquid and rolled across a pre-soaked filter paper. This process removed any excess liquid on the outer surface of the pellets without extracting the imbibed liquid from the pore structure; it is necessary that this extrapellet excess be removed as it typically exhibits different nuclear spin relaxation characteristics to liquid within the porous network. Finally, the samples were transferred to sealed 5 mm NMR tubes for analysis; each sample consisted of between 5 and 10 saturated catalyst pellets so as to provide a well-averaged measurement of the surface-adsorbate interactions present between the imbibed liquids and the pore surfaces throughout each oxide support.
0.0 10
δ/ ppm
OH
CH3
Figure 5.2: 1H NMR spectrum obtained from methanol-saturated γ-Al 2O3.
Nuclear spin-lattice relaxation
Nuclear spin relaxation measurements were performed using a Bruker DMX 300 NMR spectrometer equipped with a 7.1 T superconducting magnet, corresponding to a 1H frequency of 300.13 MHz. Figure 5.2 illustrates a typical1H NMR spectrum obtained from the methanol-saturated catalyst materials under examination; no significant changes in peak shape or chemical shift were observed upon functionalisation. Separate resonance signals from the methyl and hydroxyl 1H
groups are clearly distinguishable; this allowed their relaxation properties to be evaluated individually without the use of complex correlation measurements. Values of the 1H spin-lattice relaxation time constant T1 were acquired using the inversion
recovery method detailed in Chapter 3. [30] The appropriate pulse sequence is given in Figure 3.10a; here 16 τ1 recovery delays were employed ranging logarithmically
from 1 ms to ∼ 5T1. By selecting appropriate integration limits designed to minimise
the effects of peak overlap the observed relaxation was found to exhibit near-single exponential behaviour; as such, T1 values were obtained by fitting the acquired
nuclear spin relaxation data to the simple expression
S(τ1) S0 = 1 − 2 exp −τ1 T1 . (5.1)
Here S(τ1) is the signal acquired from the time-dependent longitudinal magnetisation
and S0 is the signal acquired at thermal equilibrium. All NMR measurements were