Article pubs.acs.org/JPCC
Hydrogen Bonding Network Disruption in Mesoporous Catalyst Supports Probed by PFG-NMR Diffusometry and NMR Relaxometry Carmine D’Agostino, Jonathan Mitchell, Lynn F. Gladden, and Mick D. Mantle* Department of Chemical Engineering & Biotechnology, University of Cambridge, Pembroke Street, Cambridge, CB2 3RA, U.K. S Supporting Information *
ABSTRACT: The pulsed-field gradient (PFG)-NMR technique has been applied to study molecular diffusion of organic liquids within mesoporous materials used in heterogeneous catalysis, in order to assess the effect of chemical functionalities on the effective self-diffusivity of the probe molecule within the pore space. True tortuosity values of the porous matrix can be calculated from the ratio of the unrestricted free self-diffusivity to the self-diffusivity within the pore space only when the small liquid-phase probe molecules do not have any chemical functionality that interacts within the solid phase (e.g., alkanes). The use of molecules with reactive chemical functionalities gives values heavily dependent on the physical and chemical interactions within the porous medium; hence, these values cannot be defined as tortuosity. Polyols showed an interesting behavior of enhanced rate of self-diffusion within the confined pore space, and this is attributed to the ability of the porous medium to disrupt the extensive intermolecular hydrogen bonding network of polyols.
1. INTRODUCTION PFG-NMR is nowadays a well established tool for probing diffusion properties of molecules in a wide range of applications such as ionic liquids, molecular sieves, and heterogeneous catalysts.1−4 Measuring transport properties in porous media is perhaps one of the fields that fully exploits the potential and the ability of NMR techniques, which are able to probe diffusion in complex systems with a noninvasive and chemically selective approach, monitoring molecular displacements over a wide range of length scales. Mesoporous materials (pores with a diameter between 2 and 50 nm according to the IUPAC5) are of particular interest in this context, as they are used in a large variety of applications, such as heterogeneous catalysis and separation processes. Molecules confined in such porous materials exhibit substantially different properties from those of the bulk liquid. This is due to both liquid−liquid and liquid− surface interactions. These interactions may therefore affect the dynamic properties of fluids in porous media, such as the selfdiffusion coefficient. Diffusion is one of the most important transport properties, and its knowledge may yield important information on the structural properties of the porous medium. PFG-NMR techniques can measure and often provide valuable information on porous structures with an average pore size that ranges well below the micrometer scale,1−3,6 which is beyond the resolution of NMR imaging techniques. Local interactions with the pore wall will also affect the diffusive motion of molecules. For example, Hansen et al.7 studied the effect of surface hydrophobicity on diffusion of n-hexane in mesoporous MCM-41 zeolite probed by NMR. Their NMR analysis revealed a significant change in diffusion versus surface © 2012 American Chemical Society
hydrophobicity. In particular, n-hexane molecules showed a slower diffusivity when the zeolite surface was covered by methyl groups. The observation was rationalized in terms of stronger interactions between the diffusive molecules and the surface of the treated sample. In our recent work, we have reported an interesting phenomenon arising when alcohols and in particular diols diffuse in metal oxide-supported catalysts.8 We studied the selfdiffusion of 1-octanol, 2-octanol, 3-octanol, 1,2-butanediol, and 1,4-butanediol in several heterogeneous gold catalysts. The results revealed that the inhibition effect observed from traditional catalytic activity studies for the oxidation of 2octanol was considered to result from competitive adsorption of the ketone product. Moreover, we found that the selfdiffusivities of butanediols in the porous catalysts were highly enhanced compared to their expected value and were much closer to the free liquid values compared to octanols and noctane. Enhancement of self-diffusion rate in porous materials has previously been observed for water in partially filled porous glass with a low degree of volume filling.9,10 The same findings were reported for n-hexane in partially filled porous media.11 The authors explained this phenomenon in terms of a mechanism involving molecular exchange between the liquid and the vapor phase within the pores. However, in our previous work8 as well as in the current work, the porous materials were completely saturated with liquids and the enhancement in selfReceived: December 21, 2011 Revised: February 29, 2012 Published: February 29, 2012 8975
dx.doi.org/10.1021/jp2123295 | J. Phys. Chem. C 2012, 116, 8975−8982
The Journal of Physical Chemistry C
Article
Figure 1. APGSTE pulse sequence showing gradient pulse duration δ, echo time τ, storage interval T, homospoil gradient (crosshatched pattern), and diffusion time, Δ.
of 300.13 MHz. The PFG-NMR experiments were carried out using a diffusion probe capable of producing magnetic field gradient pulses up to 11.6 T m−1. Diffusion measurements of pure bulk liquids were performed using the pulsed-field gradient stimulated echo sequence12 (PGSTE sequence), while the alternated pulsed-field gradient stimulated echo sequence13 (APGSTE sequence) was used when studying diffusion of liquids within porous pellets, in order to minimize the signal loss due to diffusion occurring under background magnetic field gradients existing in solid samples as a result of different material magnetic susceptibility. The APGSTE sequence is depicted in Figure 1. The sequence is made by combining a series of radiofrequency pulses, RF, with magnetic field gradients, g. The NMR signal attenuation of a PFG-NMR experiment as a function of the gradient strength, E(g), is related to the experimental variables and the diffusion coefficient D by
diffusion rate was only observed for some high molecular weight alcohols and very markedly for diols. Therefore, a different mechanism must take place and must be connected to the molecular nature of the liquids, and particularly the molecular structure of polyols. We explained this diffusion enhancement by presenting a theory on hydrogen bonding network disruption. The dynamic hydrogen bonding network of polyols is disrupted or broken down to some extent by the porous medium, which results in an enhanced self-diffusivity relative to the self-diffusivity of noninteracting molecules, for example, n-octane. In the present work, we aim to give further and stronger evidence on the mechanism of hydrogen bonding disruption of polyols in porous media by combining PFG-NMR measurements with NMR relaxometry. Self-diffusivities and spin−lattice relaxation time constants of organic liquids imbibed in TiO2, γAl2O3, and SiO2 porous supports were measured in order to assess how diffusion and tumbling motion of molecules are affected by the presence of the porous material and what relation exists between the two different types of motion. Several guest molecules were used, including 1,2-propanediol, 1,3-propanediol, and glycerol, molecules with an intermolecular structure mostly dominated by extensive hydrogen bonds and therefore very suitable for the purposes of this study.
⎡ ⎛ E (g ) δ ⎞⎤ = exp⎢ −D γ2g 2 δ2⎜Δ − ⎟⎥ ⎝ ⎣ E0 3 ⎠⎦
(1)
where E0 is the NMR signal in the absence of gradient, γ is the gyromagnetic ratio of the nuclei being studied (i.e., 1H in our case), g is the strength of the gradient pulse of duration δ, and Δ is the observation time (i.e., the time interval between the leading edges of the gradient pulses). Typical values of the acquisition parameters are reported in Table 1. The NMR spin−lattice relaxation time T1 was measured using the inversion recovery technique.14 All the measurements were performed at atmospheric pressure and 20 °C. Values of selfdiffusivity and spin−lattice relaxation time were calculated by considering the aliphatic peak of the organic compound in all cases; hence, any effect due to exchange between hydroxyl groups can be excluded.
2. EXPERIMENTAL SECTION 2.1. Sample Preparation. The metal oxide supports were supplied by Evonik-Degussa (TiO2) and Johnson Matthey (γAl2O3 and SiO2). Cyclohexane, n-hexane, n-octane, n-decane, 1propanol, ethylene glycol, 1,2-propanediol, 1,3-propanediol, glycerol, hydroxyacetone, propionaldehyde, and propionic acid were obtained from Sigma Aldrich; 2-propanol and acetone were supplied by Fisher Scientific. All the chemicals were used as received. Samples were prepared by soaking the support pellets in the liquid for at least 24 h to equilibrate. The pellets were then dried on a presoaked filter paper in order to remove any excess liquid on the external surface and finally transferred to 5 mm NMR tubes. To ensure a saturated atmosphere in the NMR tube, hence minimizing errors due to evaporation of volatile liquids, a small amount of pure liquid was placed onto absorbed filter paper, which was then placed under the cap of the NMR tube. The tube sample was finally placed into the magnet and left for approximately 15 min before starting the measurements, in order to achieve thermal equilibrium. 2.2. NMR Methods. NMR experiments were performed in a Bruker DMX 300 spectrometer operating at a 1H frequency
Table 1. Typical Acquisition Parameters Used for the PFGNMR Experiments
8976
PFG-NMR parameters
pure liquids
liquids in supports
diffusion time, Δ [ms] gradient pulse duration, δ [ms] max. gradient strength [G cm−1] no. of gradient steps no. of scans
50 1 1000 16 16
100 2 1000 16 32−1024
dx.doi.org/10.1021/jp2123295 | J. Phys. Chem. C 2012, 116, 8975−8982
The Journal of Physical Chemistry C
Article
3. RESULTS AND DISCUSSION 3.1. PFG-NMR Studies. Prior to the NMR measurements, all solid samples were characterized by BET and BJH analysis. The pore size for all of the samples was found to be of the order of tens of nanometers (Table 2). To determine the
interact within the porous medium is very important for a quantitative measurement of this parameter. Liquid alkanes seem to be the most suitable molecules to determine a genuine value of tortuosity, as their lack of functional groups minimizes any physical interaction with the porous medium as well as intermolecular interactions. This implies that any change in diffusion with respect to the free bulk liquid is due to the way different pores are connected. Figure 2 shows the log attenuation plots of several alkanes as pure liquids (Figure 2a) and liquids imbibed in TiO2 support pellets (Figure 2b). The experimental data were fitted using eq 1, which gives a straight line when plotted on a logarithmic scale. Similar linear plots were obtained for all the other molecular species, including polyols, in all the supports studied (see the Supporting Information, jp2123295_si_001.pdf). The numerical values of the diffusion coefficient are obtained by taking the negative value of the slope. The lack of any evident curvature for the liquids within the porous support (Figure 2b) indicates that the distance traveled by molecules in the typical observation times probed in a PFG-NMR experiment is much greater than the typical pore dimension; that is, the root-mean-square displacement (rmsd) of molecules is much larger than the pore size.8 As a consequence, molecules will experience many collisions with the pore walls and the measured diffusivity, Deff, will be that of the liquid confined in the porous medium. This behavior is common for mesoporous materials with a macroscopically homogeneous pore structure and is referred to as quasi-homogeneous behavior.16 The numerical values of the PFG-NMR measurements are shown in Table 3. The results clearly show that the values of the PFG interaction parameter obtained using liquid alkanes are very similar in all cases and are independent of molecular weight, chemical structure, and carbon chain length. Conversely, the choice of organic liquids with chemical functionalities as guest molecules may lead to significantly different ξvalues. Clearly, those values cannot be taken as representative of the tortuosity of the porous medium, as they strongly depend upon the chemical species. This leads to the conclusion
Table 2. BET and BJH Characterization of the Porous Supports Used in This Study support
average pore size [nm]
surface area [m2 g−1]
pore volume [cm3 g−1]
TiO2 γ-A12O3 SiO2
22 15 13
40 98 250
0.28 0.48 0.92
influence of physical interactions on self-diffusivity within the porous material, we first need to obtain a good estimation of its tortuosity. We begin our analysis by defining a dimensionless parameter, ξ, given by the ratio of the free bulk liquid selfdiffusivity, D0, to the effective self-diffusivity of the same liquid within the porous medium, Deff: ξ=
D0 Deff
(2)
A similar ratio has also been widely referred to in the literature as being the tortuosity, τ, of a porous medium.15 Tortuosity, as well as surface area, porosity, and pore size, is a structural property of the porous medium. It defines the connectivity between different pores and is therefore a function solely of the pore structure. PFG-NMR experiments allow the calculation of the tortuosity of a porous medium as τ=
D0 Deff
(3)
where Deff represents the effective self-diffusivity of a noninteracting molecule. Indeed, to consider the ratio D0/Deff to be the true tortuosity of a porous medium, certain conditions have to be verified. Strictly speaking, the choice of a guest molecule that does not have chemical functionalities that
Figure 2. Log attenuation plots of alkanes as pure liquids (a) and imbibed in TiO2 support (b). Solid lines are fitting to eq 1. 8977
dx.doi.org/10.1021/jp2123295 | J. Phys. Chem. C 2012, 116, 8975−8982
The Journal of Physical Chemistry C
Article
Table 3. PFG Interaction Parameter Values, ξ, of Several Organic Molecules in Different Supportsa
molecules. There might obviously be other molecules suitable for the purpose, but the use of liquid alkanes seems to be the best choice. We now focus our attention on the PFG interaction parameters, ξ, reported in Table 3. The relative error of each value was estimated to be approximately 2.5%. The tortuosity of each material, i.e., τ ≡ ξAlkanes, was found to be 1.60 for TiO2, 1.70 for γ-Al2O3, and 1.60 for SiO2. The values of tortuosity reported in this work were found to be similar to those reported in the literature for similar porous materials.8,17 It can be clearly seen that molecules such as propanols, acetone, propionaldehyde, hydroxyacetone, and propionic acid show ξvalues that are in many cases completely different from the τvalue of the porous medium. A “student t-test” (see the Supporting Information, jp2123295_si_003.pdf) was performed to assess the significance of the difference in PFG interaction parameter values between the different groups of molecules (i.e., alkanes, monoalcohols, polyols, and carbonyl compounds). The results of the t-test showed that the PFG interaction parameter of groups with chemical functionalities is statistically very different from that of the alkane group (i.e., the probability that the mean of the PFG interaction parameter of the alkane group has the same value of the mean PFG interaction parameter of another group is essentially close to zero). The group of molecules with carbonyl function tends to give the highest values of the PFG interaction parameter. Hydroxyacetone, for example, which is a bifunctional molecule, shows a very large variation in all supports. The value of ξ for this compound in γ-Al2O3 is almost twice the value of τ. It seems that all carbonyl compounds tend to have higher ξ-values with respect to τ. The reason for such a difference is therefore in the chemical functionality of molecules, which reflects the effect of physical interactions of guest molecules within the porous material in addition to the physical structure of the pore space. We now turn our attention to the group of polyols. This class of compounds is also characterized by chemical functionalities (multiple hydroxyl groups), and therefore, we would intuitively
The tortuosity value of each support, τ, is also reported and corresponds to the value of ξ measured for alkanes. Mono-alcohols and carbonyl compounds show higher values than alkanes, whereas polyols exhibit lower values. The relative error is approximately 2.5%.
a
that alkanes are indeed suitable molecules for quantifying pore network connectivity in porous materials. Therefore, we can state that a good estimate of tortuosity using PFG-NMR is given by τ ≡ ξAlkanes
(4)
The meaning of this expression is quite important to understand in order to avoid confusion on the definition and calculation of tortuosity of porous media. It simply states that the tortuosity of a porous medium is given by the PFG interaction parameter determined using alkanes as guest
Figure 3. Normalized values of the PFG interaction parameter, ξN, as a function of type of molecule and type of support. All values are normalized over the tortuosity factor, τ ≡ ξAlkanes, of each porous support. Unity is the reference value for noninteracting molecular species. 8978
dx.doi.org/10.1021/jp2123295 | J. Phys. Chem. C 2012, 116, 8975−8982
The Journal of Physical Chemistry C
Article
Figure 4. T1 inversion recovery plots of pure liquids (a) and liquids within γ-Al2O3 (b). Solid lines are fitting to eq 6.
expect the ξ-values to be higher than τ because of physical interactions with the porous material. However, this is not observed for any of the polyols studied. We notice, on the contrary, that the ξ-values of all polyols are actually lower than the true tortuosity value, and this applies to all the inorganic supports studied here. It is remarkable to notice the behavior of glycerol. The free bulk liquid self-diffusivity of glycerol at 20 °C is 1.26 × 10−12 m2 s−1. If no physical interactions influenced the molecular motion of glycerol, we would expect its effective selfdiffusivity in the porous medium to be that of the free bulk liquid self-diffusivity reduced by the tortuosity factor of each porous material, i.e., glycerol self-diffusivity to be equal to 0.79 × 10−12 m2 s−1 in TiO2, 0.74 × 10−12 m2 s−1 in γ-Al2O3, and 0.79 × 10−12 m2 s−1 in SiO2. What we experimentally find is that the self-diffusivity values of glycerol in all supports are significantly higher than those theoretical values calculated assuming the absence of physical interactions. Glycerol shows a self-diffusion coefficient of 1.18 × 10−12 m2 s−1 in TiO2 and 1.19 × 10−12 m2 s−1 in SiO2. It is remarkable to notice that, within the γ-Al2O3 support, the self-diffusion coefficient of glycerol is even higher than its free bulk liquid value, 1.68 × 10−12 m2 s−1 in γ-Al2O3 against 1.26 × 10−12 m2 s −1 as free bulk liquid. This is a fascinating result, as it is usually the case that the diffusion within a confined pore space is always expected to be slower than the unrestricted free bulk liquid diffusion. The measurements of glycerol in γ-Al2O3 were repeated a second time to check for reproducibility, giving the same results. It appears therefore that all polyols tend to diffuse much faster than theoretically expected (i.e., taking the τ-value as a reference) and for glycerol in γ-Al2O3 even faster than the free bulk liquid. A clear, overall picture is shown in Figure 3. For each support, we define a normalized value of ξ, given by ξN =
ξ τ
influencing the diffusion of polyols in mesoporous media is the disruption of the extensive hydrogen bonding network, which is responsible for the observed enhancement of self-diffusion.8 All polyols, having at least two hydroxyl groups, are able to form networks of molecules held together by hydrogen bonds.18 Hence, when these compounds are diffusing in mesoporous media, the dynamic hydrogen bonding network is disrupted or broken down to some extent by the porous medium, which results in an enhanced effective self-diffusion coefficient relative to the theoretical self-diffusion coefficient obtained using the τvalue. The results shown in the present work give therefore further support to the validity of this theory, as we do observe such diffusion enhancement when ethylene glycol, propanediols, and glycerol are chosen as guest molecules. The case of glycerol in the γ-Al2O3 support is particularly significant, as in this case we observe a very strong enhancement of its selfdiffusion within the pore space, which is even higher than its free bulk liquid value. 3.2. NMR Spin−Lattice Relaxation Studies. NMR studies of spin−lattice relaxation times give further insight into the mechanism of the hydrogen bonding network disruption phenomenon. We have analyzed the ratio between the T1-value of the free bulk liquid to that of the same liquid imbibed in porous material. Figure 4 reports the T1 inversion recovery plot for pure bulk liquids (Figure 4a) and liquids imbibed in γ-Al2O3 (Figure 4b). The experimental data were fitted to the expression given by14 ⎡ ⎛ t ⎞⎤ Mz(t ) = M 0⎢1 − 2 exp⎜ − ⎟⎥ ⎢⎣ ⎝ T1 ⎠⎥⎦
(6)
In terms of NMR relaxation in porous media, it is today commonly accepted that the T1/T2 ratio is an indication of the interaction strength of molecules with the surface of the porous material.19−24 However, there is no clear interpretation of the ratio between T1 of the free bulk liquid to that of the liquid within the pore space. The NMR spin−lattice relaxation time in liquids is caused by time-dependent local magnetic fields, which induce transitions that allow nuclear spins to return to equilibrium.25 The main cause of these fluctuations in local magnetic fields at a nucleus is the rotational motion of molecules, often referred to as molecular tumbling. According
(5)
This factor allows a comparison between supports with different values of tortuosity, τ. By defining this new parameter, unity is taken as a reference value and is associated with alkanes. All the molecular species with ξN > 1 experience reduced diffusivity, while values of ξN < 1 are indicative of enhanced self-diffusivity. All diols and glycerol have a value below unity. In our previous work, we speculated that the main factor 8979
dx.doi.org/10.1021/jp2123295 | J. Phys. Chem. C 2012, 116, 8975−8982
The Journal of Physical Chemistry C
Article
where ηAlkanes is the average of the η-values obtained for cyclohexane, n-hexane, n-octane, and n-decane. Lanin et al.28 have obtained the total energy of adsorption of organic molecules adsorbed onto TiO2 and found that alkanes have the lowest values (30−40 kJ mol−1), followed by alkenes (40−45 kJ mol−1) and oxygenated polar molecules (50 kJ mol−1 or above), which show the highest value of adsorption energy. Table 5
to the BPP (Bloembergen−Purcell−Pound) theory of relaxation,26 the spin−lattice relaxation rate is related to the tumbling motion of molecules according to26 ⎡ ⎤ τc 4τc 1 ⎥ = K⎢ + T1 1 + 4ω02τc 2 ⎥⎦ ⎣⎢ 1 + ω02τc 2
(7)
where K is a constant, ω0 the Larmor frequency, and τc the correlation time of molecular tumbling. Roughly speaking, τc is the time taken by a molecule to end up with a rotation of 1 rad. From eq 7, it is easy to see that, for τc → 0, T1 → ∞. Therefore, small, nonviscous, and fast tumbling molecules will exhibit a slower relaxation rate (hence higher T1) compared to viscous molecules,27 such as diols and glycerol. Analyzing our data, we have noticed an interesting analogy between self-diffusion and spin−lattice relaxation measurements. In particular, the analogy is between the values of the PFG interaction parameter, ξ, and the ratio, η, given by
η=
Table 5. Comparison between the Normalized Values of the T1 Ratio, ηN, and PFG Interaction Parameter, ξNa ηN = η/ηAlkanes
T1,bulk T1,pores
(8)
where T1,bulk is the spin−lattice relaxation time constant of the pure free bulk liquid and T1, pores represents the spin−lattice relaxation constant of the same liquid in the pore space. Table 4 reports the η-values of the different organic molecules in TiO2, γ-Al2O3, and SiO2 support pellets. The numerical values in Table 4 show that η is similar between different alkanes, whereas much higher values are observed for carbonyl compounds. Polyols, on the contrary, show lower values compared to alkanes. This trend is very similar to the trend of the PFG interaction parameter, ξ, found from PFGNMR diffusion measurements. Making the same assumption that we made for self-diffusion, i.e., alkane molecules have minimum interactions within the porous medium and are hence less affected by physical and chemical interactions, we normalize the T1 ratio by taking η ηN = ηAlkanes (9)
TiO2
γ-A12O3
SiO2
1.78 1.75 1.67 1.75 5.26 4.71 1.39 1.20 1.28 1.29 5.69 3.27 23.47 3.35
1.55 1.49 1.49 1.60 2.33 2.31 1.24 1.14 1.12 1.02 4.24 4.34 10.79 3.86
1.37 1.37 1.30 1.48 1.61 1.60 1.17 1.05 1.07 1.07 3.94 3.40 7.77 1.65
γ-A12O3
SiO2
TiO2
γ-A12O3
SiO2
cyclohexane n-hexane n-octane n-decane 1-propanol 2-propanol ethylene glycol 1,2-propanediol 1,3-propanediol glycerol acetone hydroxyacetone propionaldehyde propionic acid
1.02 1.01 0.96 1.01 3.02 2.70 0.80 0.69 0.73 0.74 3.27 1.88 13.49 1.93
1.01 0.97 0.97 1.05 1.52 1.51 0.81 0.75 0.73 0.67 2.77 2.84 7.05 2.52
0.99 0.99 0.94 1.07 1.17 1.16 0.85 0.76 0.78 0.78 2.86 2.47 5.63 1.20
0.99 1.00 1.01 0.99 1.07 1.07 0.92 0.93 0.93 0.66 1.04 1.40 1.33 1.15
1.00 1.00 0.99 1.00 1.15 1.05 0.92 0.79 0.91 0.44 1.18 1.93 1.61 1.56
1.00 0.99 1.01 1.00 1.12 1.06 0.90 0.94 0.95 0.67 1.06 1.42 1.29 1.26
Polyols, in bold font, exhibit values lower than unity for both parameters, indicating an enhancement of both diffusion and molecular tumbling rate. The relative error is approximately 1% for ηN and 3% for ξN.
shows the numerical results obtained from spin−lattice relaxation measurements, comparing those values with the results obtained by the PFG-NMR experiments. Looking at the table, we notice that the trend of the normalized values of the T1 ratio, ηN, is similar to the trend of the normalized values of the PFG interaction parameter, ξN. Obviously, liquid alkanes show values very close to 1, as they are taken as a reference. Monoalcohols and molecules with carbonyl functions show values >1, whereas polyols show values
