P. A.
Vecino
a,
Z.
Huang
a,
J.
Mitchell
*b,
J.
McGregor†
a,
H.
Daly
c,
C.
Hardacre
c,
J. M.
Thomson
c and
L. F.
Gladden
a
aDepartment of Chemical Engineering and Biotechnology, University of Cambridge, Pembroke Street, Cambridge CB2 3RA, UK
bSchlumberger Gould Research, High Cross, Madingley Road, Cambridge CB3 0EL, UK. E-mail: JMitchell16@slb.com
cSchool of Chemistry and Chemical Engineering, Queen's University, Belfast, BT9 5AG, UK
First published on 21st July 2015
Relative strengths of surface interaction for individual carbon atoms in acyclic and cyclic hydrocarbons adsorbed on alumina surfaces are determined using chemically resolved 13C nuclear magnetic resonance (NMR) T1 relaxation times. The ratio of relaxation times for the adsorbed atoms T1,ads to the bulk liquid relaxation time T1,bulk provides an indication of the mobility of the atom. Hence a low T1,ads/T1,bulk ratio indicates a stronger surface interaction. The carbon atoms associated with unsaturated bonds in the molecules are seen to exhibit a larger reduction in T1 on adsorption relative to the aliphatic carbons, consistent with adsorption occurring through the carbon–carbon multiple bonds. The relaxation data are interpreted in terms of proximity of individual carbon atoms to the alumina surface and adsorption conformations are inferred. Furthermore, variations of interaction strength and molecular configuration have been explored as a function of adsorbate coverage, temperature, surface pre-treatment, and in the presence of co-adsorbates. This relaxation time analysis is appropriate for studying the behaviour of hydrocarbons adsorbed on a wide range of catalyst support and supported-metal catalyst surfaces, and offers the potential to explore such systems under realistic operating conditions when multiple chemical components are present at the surface.
Nuclear magnetic resonance (NMR) techniques offer the potential for probing adsorbate–adsorbent interactions. Previous studies have focused on the change in the chemical shift δ of the NMR spectral line associated with an atom that occurs on adsorption. When a chemical species is adsorbed, the distribution of electrons within the molecule is distorted, altering the degree of chemical shielding experienced by the nucleus. A larger chemical shift change Δδ indicates stronger adsorption.5 This interpretation can be complicated by external factors such as the change of polarisation within an adsorbed molecule6 and the variation with surface coverage.7 Nevertheless, this method has been used to study the chemisorption of hydrogen on Pt and Pd surfaces using 1H NMR,7 and of fluorine-containing adsorbates on a variety of surfaces, including aluminas, using 19F NMR.813C NMR has been used to investigate oligomers grafted onto silica surfaces,9 ionic surfactants adsorbed on silica,5 and in applications to catalysis; for example, hydrocarbon adsorption on zeolites.6 The use of 13C NMR has practical advantages over 1H NMR. Significant line broadening of the spectral resonances occurs for adsorbed species as a result of differences in magnetic susceptibility at the liquid–solid interface. This line broadening can prevent the identification of individual 1H resonances due to the limited chemical shift range associated with the 1H nucleus. In contrast, the much larger chemical shift range of 13C means that it is possible to distinguish the individual resonances even for the broadened lines of adsorbed molecules,10 hence the surface interaction of each individual carbon atom in an adsorbed hydrocarbon can be examined.
An alternative NMR probe of surface interaction is the longitudinal relaxation time T1. The nuclear spin relaxation behaviour of a chemical species adsorbed onto a surface is modified by the strength of that interaction.11–13 The molecular motion (translation and rotation) of the adsorbate on the adsorbent surface is described by the autocorrelation function G(τ) averaged over the time interval τ. The Fourier-transform of this function is proportional to the longitudinal relaxation time T1. The autocorrelation function of the molecule will change on adsorption, and hence a change in T1 is observed. In general, energetically stable adsorption conformations (corresponding to reduced mobility of the surface-bound species) are associated with stronger surface interactions. The ratio of T1 relaxation times for surface adsorbed to free diffusing (bulk) molecules T1,ads/T1,bulk can therefore be used as an indicator of the relative strength of surface interaction assuming the motional narrowing regime applies always, such that ω0τ ≪ 1, where ω0 is the Larmor frequency. A low T1,ads/T1,bulk ratio is associated with low mobility and hence stronger adsorption.
Previously, we have used the ratio of longitudinal to transverse relaxation times T1/T2 to probe the relative strengths of interaction between chemical species in liquid saturated porous catalysts using two-dimensional 1H relaxometry techniques.14 In a fully saturated porous medium, this ratio of relaxation times is preferable as it removes the influence of pore geometry that would be significant in, say, the T1,pore/T1,bulk ratio. Here, we do not need concern ourselves with the geometric effects because we are probing only a monolayer on the pore surface and as such can directly compare the relaxation time of the adsorbed liquid molecules to those in the bulk. Also, the use of T2 relaxation time measurements is suboptimal in these high-field NMR experiments because the magnetic susceptibility differences between the solid, and the adsorbed liquid and vapour phases may prevent accurate interpretation of the results;15 it is established that T1 relaxation times are independent of such artefacts.16
The inference of adsorption properties from NMR relaxation parameters has been investigated previously by Monduzzi et al. with regard to the interaction of cyclic molecules, including pyrrole, with the surface of zeolites.13 It was observed that the C3 carbon atoms of pyrrole (i.e. those furthest from the nitrogen heteroatom) had a shorter T1,ads relaxation time than the C2 carbon atoms. This was explained as a loss of mobility for the C3 carbons due to their involvement in the solid–liquid interaction. Molecular modelling calculations confirmed that the species binding to the surface through the C3 carbons represented the most energetically stable configuration. Elsewhere, Popova and co-workers have employed 13C NMR in order to investigate the variation in T1 along the carbon-chain of a surfactant inside porous SiO2.5 In this case the pore space was filled with liquid surfactant and hence the adsorbate–adsorbent interaction was not probed directly; it was modified by molecular exchange with bulk liquid in the pore. As neither of these studies considered the ratio of the measured relaxation time against that of the bulk liquid, only general trends could be established. It is important to consider a ratio, as different carbon atoms within a molecule can have different T1 values in the bulk phase and it is the variation of T1 on adsorption that reveals the relative strength of surface interaction.
To date, 13C T1 measurements of adsorbed species have not, to the best of our knowledge, been exploited as a tool for comparing the relative surface interaction strengths of each carbon in an adsorbed hydrocarbon molecule and hence used as an indicator for adsorbate configuration. We have inferred information previously on the relative strength of adsorbate–adsorbent interactions from two-dimensional 1H relaxation analysis (without chemical shift information) in catalytic systems.14 Here, we demonstrate the use of chemical shift resolved 13C NMR T1 measurements to probe the adsorption of individual carbon atoms within their parent molecules. These measurements are straightforward to conduct and analyse, and yield direct information on the molecular configuration and relative adsorption strength of adsorbate molecules; the technique is described in Section 2. In Section 3.1 we present a study of the adsorption of a series of acyclic C5 and cyclic C6 hydrocarbons on θ-Al2O3 and γ-Al2O3. Acyclic C5 molecules provide a proxy for molecules of interest in many industrially-relevant catalysed reactions, such as hydrogenation or isomerisation.17 Cyclic C6 species are important as intermediates in the processing of petrochemical feedstocks.18 These NMR data are supported by conventional Diffuse Reflectance Infrared Fourier-Transform Spectroscopy (DRIFTS) results. The influences of surface coverage, surface pre-treatment, and temperature on the geometry of adsorbed hydrocarbons are explored in Section 3.2. To highlight the versatility of the NMR technique, we explore the conformation of 1-pentene when co-adsorbed with carbon monoxide and other hydrocarbons (see Section 3.3). It is not possible to characterise the surface geometry of co-adsorbed species using conventional techniques such as DRIFTS. The presence of a co-adsorbate allows us to consider how the interaction of the alkene with the active surface sites might be modified during a catalytic reaction where products and solvents will be present alongside the reactant. Two cases are considered; the solid phase used is θ-Al2O3 in both cases. First, we consider the co-adsorption of species representative of those relevant to industrially catalytic processes of interest. In particular, the co-adsorption of CO or 2-butyne is considered. The co-adsorption of CO with 1-pentene has been selected because of its relevance to the industrial semi-hydrogenation of acetylene where CO is added as a co-adsorbate to improve selectivity.19 The co-adsorption of 1-pentene and 2-butyne allows us to explore the relative strength of triple-bond and double-bond interactions. In both cases, two experiments are performed. In the first experiment, 1-pentene is pre-adsorbed prior to admitting the CO or 2-butyne to the system. In the second experiment, the order of adsorption is reversed, with 1-pentene adsorption occurring after adsorption of CO or 2-butyne. The second case considered was the adsorption of 1-pentene following adsorption of cyclohexane or benzene; these cyclic species were chosen as models for soft and hard coke, respectively.
For the NMR relaxation time measurements of the adsorbates in the adsorbed state, T1,ads, the hydrocarbons were adsorbed onto the aluminas via a glass vacuum line (Soham Scientific Ltd). A glass basket containing 3.5 g of Al2O3 trilobes was attached to the vacuum line and evacuated to a pressure <10−2 mbar. The liquid hydrocarbon was degassed via several freeze–evacuate–thaw cycles and admitted to the vacuum line in the gas-phase at a known pressure. The sample basket was cooled using liquid nitrogen to condense the hydrocarbon onto the alumina surface. Adsorption was confirmed by a corresponding reduction in pressure in the vacuum line. The basket was then flame-sealed. This process was repeated for each of the C5 and C6 hydrocarbons on both θ-Al2O3 and γ-Al2O3. Surface coverages were calculated assuming a uniform distribution of the hydrocarbon, estimating the surface area occupied by an individual molecule from the excluded molecular area of n-pentane (4.2 × 10−19 m2) for the C5 adsorbates and 2-butyne, and cyclohexane (4.7 × 10−19 m2) for the C6 adsorbates and benzene.20 For example, 3.5 g of γ-Al2O3 has a total surface area of 665 m2 and can therefore accommodate 1.58 × 1021 molecules of n-pentane, equivalent to 2.6 × 10−3 moles. Therefore, in order to obtain a theoretical coverage approximating but not exceeding 1 monolayer (ML), approximately 2 × 10−3 moles of the C5 species were introduced into the alumina trilobes. NMR measurements were also performed on individual liquid species in order to obtain the T1,bulk values. For these studies, the hydrocarbons were dosed into empty glass baskets as received. Co-adsorption experiments were performed with 1-pentene on θ-Al2O3. For cyclohexane and benzene, adsorption of 0.5 ML of 1-pentene was performed after 0.9 ML of the co-adsorbate was dosed on the alumina. In the case of 1 pentene and CO, and 1-pentene and 2-butyne, 0.5 ML of 1-pentene was adsorbed, followed by 0.5 ML of the co-adsorbate, and vice-versa. A T1,bulk relaxation time for CO in the gas phase could not be determined due to the low 13C density at atmospheric pressure. Consequently, the T1,ads/T1,bulk ratio of physisorbed CO was not obtained.
The NMR experiments were performed on a Bruker DMX spectrometer with a vertical bore 7 T superconducting magnet. The radio frequency (r.f.) excitation and detection was achieved using a birdcage resonator tuned to 75 MHz for 13C. T1 relaxation properties of both bulk liquid and adsorbed hydrocarbons were measured using the standard inversion-recovery pulse sequence, with signal acquired in the presence of 1H broad-band decoupling. T1 values were determined by fitting a single-component exponential recovery function to the data. Due to the limited homogeneity of the applied r.f. magnetic field, it was necessary to acquire the 13C spectral information in two separate experiments spanning chemical shift ranges of 0–100 ppm and 100–200 ppm. The experiments were performed at 20 °C, unless reported otherwise; bulk and surface relaxation times were compared at the same temperature in all cases. The spectra were referenced to the 13C resonance of tetramethylsilane (TMS). The errors associated with fitting the bulk T1,bulk relaxation times were all within ±2%, whilst the errors associated with fitting the adsorbed T1,ads relaxation times were all within ±5%. Therefore, the total errors on the T1,ads/T1,bulk ratios are considered to be less than ±7%. The measurements with 1-pentene were repeated several times on fresh samples and found to be consistent. The repeat data fell within the errors quoted here. The theoretical foundation which enables us to correlate T1,ads/T1,bulk with adsorption strength has been discussed in detail elsewhere.12,21
DRIFTS measurements were acquired for the adsorption of 1-pentene over θ-Al2O3 and γ-Al2O3 powders. The alumina trilobes were crushed in order to prepare the sample as a powder. Adsorption of the alkene over KBr was also performed to provide a reference spectrum for the 1-pentene on a non-adsorbing surface. All spectra were recorded at 4 cm−1 resolution, using a Bruker Equinox 55 spectrometer equipped with an MCT detector. The alumina samples were treated in situ to partially dehydroxylate the surface, prior to adsorption of the alkene. This treatment consisted of heating to 500 °C with a 10% O2 in Ar atmosphere flowing at 100 ml min−1, holding for 1 hour at 500 °C, and then cooling to 35 °C under Ar flow. The 1-pentene was then adsorbed onto the alumina at 35 °C. Once the alumina surface was saturated (detected by no further change in the spectra) the feed was switched to Ar at 20 ml min−1 and desorption of the alkene was monitored. During desorption, spectra were recorded every 15 s.
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Fig. 2 The ratio of surface to bulk relaxation times T1,ads/T1,bulk for the individual carbon atoms in (a) 1-pentene, (b) 1-pentyne, (c) cis-2-pentene, (d) trans-2-pentene, and (e) 1,4-pentadiene adsorbed on θ-Al2O3 (squares) and γ-Al2O3 (circles). The carbon numbers correspond to the schematic diagrams in Fig. 1. Solid symbols represent olefinic or acetylinic carbons associated with the unsaturated carbon–carbon bonds, and open symbols represent aliphatic carbons. |
Comparing an alkene (1-pentene, Fig. 2(a)) and an alkyne (1-pentyne, Fig. 2(b)), we see a more pronounced difference between the T1,ads/T1,bulk ratios of the acetylenic and aliphatic carbons in the alkyne than between the olefinic and aliphatic carbons in the alkene. This variation corresponds to the greater adsorption strength of alkynes through the carbon–carbon triple bond compared to alkenes through the carbon–carbon double bond, as has been established previously.24
The observed T1,ads/T1,bulk ratios are lower, consistently, for species adsorbed on θ-Al2O3, Fig. 2 (squares), than on γ-Al2O3, Fig. 2 (circles). This result implies stronger adsorption on θ-Al2O3, as expected from the higher surface Lewis25 and Brønsted26 acidities of θ-Al2O3 compared to γ-Al2O3. However, it is important to note that, for a given molecule, the same general trends in T1,ads/T1,bulk ratio are seen on both surfaces when compared on an atom-by-atom basis. Therefore, the type of interaction and molecular configuration adopted upon adsorption is similar on the two forms of alumina.
The use of 13C NMR to study individual carbon atoms allows additional information to be obtained. Notably, the geometry occupied by the adsorbate on the adsorbent surface can be considered. Since T1 is associated with the strength of interaction of an atom with the adsorbent surface, it can also be used as a qualitative indicator of the distance between the atom and the surface. Again considering cis-2-pentene as a representative example, the olefinic C2 and C3 carbons have the greatest surface interaction and so are assumed to be located closer to the surface than the aliphatic carbon atoms. The different T1,ads/T1,bulk ratios of C1 and C4 suggests the molecule does not adsorb in a planar (symmetric) fashion around the double-bond. This interpretation is consistent with the molecular configuration proposed for alkene adsorption on catalytically-relevant surfaces from other spectroscopic studies.4,23,27 Buchbinder and co-workers employed broadband vibrational sum-frequency generation spectroscopy (SFG) to study the adsorption of molecules including 1-pentene and cis-2-pentene on an α-Al2O3 (0001) surface.4 The results reported in this paper are consistent with the conclusions from SFG spectroscopy. However, the latter does not provide information on individual carbon atoms but rather explores the behaviour of functional groups, e.g., methyl or methylene functionalities. Analysis of the NMR data can yield additional information, is more straightforward to interpret and is not dependent upon selection rules that render vibrations in particular planes undetectable. Furthermore, the NMR experiments are conducted over adsorbates in an industrially relevant physical form, while a single crystal surface has to be employed as the adsorption surface in the SFG studies.
Molecular conformations are inferred assuming adsorption on a flat surface. However, the measurements are actually giving us an average measurement of the interaction strength of each carbon atom with the surface – whether that surface is or is not flat. Thus, given that the surface of the real alumina surface is unlikely to be either energetically or physically ‘flat’, and that the observed T1,ads times are weighted heavily by the strongest surface adsorption sites, the measurements provide us with the ability to identify differences in average adsorbate configuration between different adsorbates and different substrates in real materials. The sensitivity of the measurement to the strongest interaction sites between adsorbate and solid surface is an advantage in understanding the role that differences in adsorption configuration play in subsequent catalytic behaviour.
The data for 1,4-pentadiene are shown in Fig. 2(e). The symmetric nature of the molecule means that only three distinct carbon resonances are observed in the bulk liquid. On adsorption, a single T1,ads relaxation time was observed for the aliphatic carbons, consistent with the molecule adsorbing symmetrically. If the olefinic carbons adsorbed to different extents, a two-component relaxation time for one or both of the T1,ads values for the C1 and C2 carbons would be observed.
In addition to NMR relaxation time analysis, the adsorption of 1-pentene adsorbed on KBr, θ-Al2O3 and γ-Al2O3 was investigated by DRIFTS studies. Comparison of the spectra of 1-pentene over the KBr and the alumina surfaces in Fig. 3 shows no significant change in position or relative intensity of the 1-pentene bands. The loss of OH groups from the alumina surfaces (indicated by negative bands in Fig. 3) suggests that the alkene interacts with the OH groups via H-bonds.28,29 The ν(CC) band is present at 1641 cm−1 over all the surfaces indicating that there is no perturbation of the C
C bond following adsorption. This observation is consistent with the relaxation time data in Fig. 2 which suggests that the olefinic carbons interact more strongly than the aliphatic carbons with the surface, with C1 and C2 having a very similar interaction strength. DRIFTS was used to follow desorption of 1-pentene from the saturated surface. Bands due to 1-pentene were no longer observed after approximately 10 minutes for desorption from γ-Al2O3, whereas on θ-Al2O3, the ν(C
C) band was still detectable after 20 minutes. These data support the T1,ads/T1,bulk analysis that suggest that 1-pentene adsorbs more strongly on θ-Al2O3 than γ-Al2O3.
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Fig. 4 The ratio of surface to bulk relaxation times T1,ads/T1,bulk for the individual carbon atoms in (a) cyclohexene, (b) 1,4-cyclohexadiene, and (c) 1,3-cyclohexadiene adsorbed on θ-Al2O3 (squares) and γ-Al2O3 (circles). The carbon numbers correspond to the schematic diagrams in Fig. 1. Solid symbols represent olefinic carbons associated with the unsaturated carbon–carbon double bonds, and open symbols represent aliphatic carbons. |
A stronger interaction is observed for the adsorbed olefinic carbons than for the aliphatic carbons in the cyclic hydrocarbons. There is no indication in the NMR spectra of the C6 hydrocarbons that the double-bonds break, suggesting that adsorption occurs again via a weak interaction with surface OH groups. A more detailed examination of the relative strength of interaction of each carbon in a given molecule allows adsorption conformations to be proposed. For many cyclic molecules, a number of different adsorption geometries are theoretically possible and experimental evidence is required to determine which conformation exists on a given surface. Considering 1,4-cyclohexadiene adsorption on θ-Al2O3, the data are consistent with a boat conformation where the C1 carbons are in close proximity to the surface whilst the C2 carbons are slightly further from the surface. In a similar manner, the data for adsorbed cyclohexene suggest a di-σ boat conformation. The implied geometries of all three cyclic C6 species are consistent with conclusions reported previously as a result of experimental surface science and theoretical investigations on Pt(111)3,30
In a comparison of Fig. 2 and 4, similar T1,ads/T1,bulk ratios are obtained in the acyclic and cyclic compounds. For example, the olefinic carbons in cyclohexene adsorbed on θ-Al2O3 exhibit T1,ads/T1,bulk = 0.044, comparable to an average T1,ads/T1,bulk = 0.046 for the olefinic carbons in cis-2-pentene. Similarly, the aliphatic carbons in cyclohexene on θ-Al2O3 have an average T1,ads/T1,bulk = 0.112, comparable to an average T1,ads/T1,bulk = 0.102 for the aliphatic carbons in cis-2-pentene. These data suggest that the bonding mechanisms, and relative strengths of the bonds, are similar for both types of organic molecule.
Thus, while significant information on the nature of adsorption can be revealed through measurements of Δδ, we suggest that the use of atom-specific T1,ads/T1,bulk ratios provides a more detailed and less ambiguous description of the surface interaction. For instance, no change in δ is observed (suggesting negligible surface interaction) for the olefinic C1 carbon in 1-pentene (Fig. 5), whereas the T1,ads/T1,bulk ratio indicates a stronger interaction of the C1 carbon with the alumina surface compared to the aliphatic carbons (Fig. 2(a)), as expected. In general, changes in chemical shift upon adsorption do correlate with adsorption strength but the change in δ, unlike the relaxation time analysis which is sensitive only to molecular mobility, will be determined by the change in electronic configuration introduced by the adsorption interaction of the nucleus of interest which results in a modification to the chemical shielding effects around that nucleus. If that change in electron distribution is influenced by factors other than the adsorption interaction, a direct correlation between Δδ and adsorption strength will not hold. Further, the much broader linewidths associated with the nuclei of molecules in the adsorbed state introduces additional uncertainties in the precise value of Δδ characterising a given system. Overall, we conclude that T1,ads/T1,bulk offers a more direct indication of the surface interaction strength than Δδ in these systems.
The results in Fig. 7 indicate that 1-pentene adsorption via the double-bond is negligibly affected by temperature in the range 0–20 °C. The interaction of the alkyl chain with the surface is very similar at 0 and 10 °C, but a significant decrease in interaction with the surface is observed in increasing the temperature from 10 to 20 °C. These results imply a decrease in the alkyl chain mobility with decreasing temperature, consistent with an increase in lateral molecule–molecule interactions, as observed during adsorption of propane–propylene mixtures.35
The T1,ads/T1,bulk ratios of the carbon atoms in trans-2-pentene show a stronger interaction for the olefinic carbons than the aliphatic carbons, as expected. In particular, C3 shows the strongest interaction. The interaction through the double-bond is notably stronger than in the case of trans-2-pentene adsorbed as a single component, see Fig. 2(d).
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Fig. 9 The ratio of surface to bulk relaxation times T1,ads/T1,bulk for the individual carbon atoms in 1-pentene co-adsorbed with CO on θ-Al2O3. Data are shown for case (i) adsorption of 0.5 ML 1-pentene followed by 0.5 ML CO (circles) and (ii) adsorption of 0.5 ML CO followed by 0.5 ML 1-pentene (triangles). For case (iii), adsorption of 0.5 ML 1-pentene only (squares), data are repeated from Fig. 6. Solid symbols represent olefinic carbons and open symbols represent aliphatic carbons. |
Further comparison with the data for 0.5 ML and 1 ML 1-pentene (Fig. 6) suggest that pre-adsorption of pentene followed by adsorption of CO causes the adsorption interaction of 1-pentene to be very similar to the adsorption of single component 1-pentene at 1 ML on θ-Al2O3, albeit with a slightly lower interaction of the C5 carbon with the surface than in the co-adsorbed case. However, we may deduce that the conformation of 1-pentene during adsorption remains the same, with lateral interactions reducing the interaction of the C1 and C2 with the surface. In contrast, when CO is pre-adsorbed, whilst the behaviour with respect to the C1–C4 carbons is the same as when 1-pentene is pre-adsorbed, the C5 carbon appears to be held more closely to the surface. The sample was remade twice and the experiment repeated to confirm the validity of this measurement and it was found that the value of T1,ads/T1,bulk for the C5 carbon was reproducible to within experimental error. These data suggest that when CO is pre-adsorbed, the alkyl chain of 1-pentene is more restricted, perhaps by stronger lateral interactions with the adsorbed CO.
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Fig. 10 The ratio of surface to bulk relaxation times T1,ads/T1,bulk for the individual carbon atoms in 1-pentene co-adsorbed with 2-butyne on θ-Al2O3. Data are shown for case (i) adsorption of 0.5 ML 1-pentene (triangles) followed by 0.5 ML 2-butyne (×) and (ii) adsorption of 0.5 ML 2-butyne (+) followed by 0.5 ML 1-pentene (circles). For case (iii), adsorption of 0.5 ML 1-pentene only (squares), data are repeated from Fig. 6. Solid symbols represent olefinic carbons and open symbols represent aliphatic carbons in 1-pentene. |
The aliphatic carbons of 1-pentene are less mobile in case (i) than in case (ii) and (iii) due to lateral molecule–molecule interactions with excess 2-butyne. Similarly, in case (ii), C1 and C4 of 2-butyne present higher T1,ads/T1,bulk ratios (weaker interaction) than in case (i), most likely due to higher 2-butyne effective adsorption coverage and possibly due to interactions with adsorbed 1-pentene.
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Fig. 11 The ratio of surface to bulk relaxation times T1,ads/T1,bulk for the individual carbon atoms in 1-pentene co-adsorbed with benzene and cyclohexane on θ-Al2O3. Data are shown for adsorption of 0.5 ML 1-pentene following adsorption of 0.9 ML cyclohexane (circles), and adsorption of 0.5 ML 1-pentene following adsorption of 0.9 ML benzene (triangles). For comparison, data for the adsorption of 0.5 ML 1-pentene only (squares) are repeated from Fig. 6. Solid symbols represent olefinic carbons and open symbols represent aliphatic carbons. The single T1,ads/T1,bulk ratios for benzene (solid line) and cyclohexane (dash-dot line) are included. |
For the specific molecules studied, it was observed that all the unsaturated molecules bonded weakly to the porous alumina surface. An acetylenic functionality was observed to have a stronger interaction than olefinic functionalities. Adsorption was confirmed to be more favourable over θ-Al2O3 than over γ-Al2O3, as expected from the higher surface acidity of the θ-Al2O3. Studies of adsorption as a function of surface coverage demonstrated the average interaction strength was greater at lower coverage, indicating molecules adsorb preferentially on the strongest adsorption sites. In addition to information on atom-specific, adsorbate–adsorbent interaction strengths, the molecular conformations of the adsorbates were inferred. Finally, the co-adsorption studies demonstrate the application of the 13C NMR T1,ads/T1,bulk relaxation time measurement to gain insight into how the presence of one species modifies the adsorption behaviour of a second species. As long as distinct spectral resonances can be identified, more complex mixtures can be studied. In ongoing work, this approach is being used to both understand reaction pathways in heterogeneous catalysis and aid the selection of materials for specific catalytic processes.
Footnote |
† Present address: Department of Chemical and Biological Engineering, The University of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheffield S1 3JD, UK. |
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