Solvent effect and reactivity trend in the aerobic oxidation of 1,3-propanediols over gold supported on titania: Nmr diffusion and relaxation studies

Article abstract of DOI:10.1002/chem.201300502

In recent work, it was reported that changes in solvent composition, precisely the addition of water, significantly inhibits the catalytic activity of Au/TiO₂ catalyst in the aerobic oxidation of 1,4-butanediol in methanol due to changes in diffusion and adsorption properties of the reactant. In order to understand whether the inhibition mechanism of water on diol oxidation in methanol is generally valid, the solvent effect on the aerobic catalytic oxidation of 1,3-propanediol and its two methyl-substituted homologues, 2-methyl-1,3-propanediol and 2,2-dimethyl-1,3-propanediol, over a Au/TiO₂ catalyst has been studied here using conventional catalytic reaction monitoring in combination with pulsed-field gradient nuclear magnetic resonance (PFG-NMR) diffusion and NMR relaxation time measurements. Diol conversion is significantly lower when water is present in the initial diol/methanol mixture. A reactivity trend within the group of diols was also observed. Combined NMR diffusion and relaxation time measurements suggest that molecular diffusion and, in particular, the relative strength of diol adsorption, are important factors in determining the conversion. These results highlight NMR diffusion and relaxation techniques as novel, non-invasive characterisation tools for catalytic materials, which complement conventional reaction data. In solvent company: The solvent effect on the aerobic catalytic oxidation of 1,3-propanediol and its two methyl-substituted homologues, 2-methyl-1,3-propanediol and 2,2-dimethyl-1,3-propanediol, over a Au/TiO ₂ catalyst has been studied. The results show that diol conversion is significantly lower when water is present in the initial diol/methanol mixture. A reactivity trend within the group of diols was also observed. Copyright

Full text of DOI:10.1002/chem.201300502

FULL PAPER
DOI: 10.1002/chem.201300502
Solvent Effect and Reactivity Trend in the Aerobic Oxidation of
1,3-Propanediols over Gold Supported on Titania: NMR Diffusion
and Relaxation Studies
Carmine DꢀAgostino,[a] Tatyana Kotionova,[b] Jonathan Mitchell,[a] Peter J. Miedziak,[b]
David W. Knight,[b] Stuart H. Taylor,[b] Graham J. Hutchings,[b] Lynn F. Gladden,[a] and
Mick D. Mantle*[a]
Abstract: In recent work, it was report-
ed that changes in solvent composition,
precisely the addition of water, signifi-
cantly inhibits the catalytic activity of
Au/TiO2 catalyst in the aerobic oxida-
tion of 1,4-butanediol in methanol due
to changes in diffusion and adsorption
properties of the reactant. In order to
understand whether the inhibition
mechanism of water on diol oxidation
in methanol is generally valid, the sol-
vent effect on the aerobic catalytic oxi-
dation of 1,3-propanediol and its two
methyl-substituted homologues, 2-
methyl-1,3-propanediol and 2,2-dimeth-
yl-1,3-propanediol, over a Au/TiO2 cat-
alyst has been studied here using con-
ventional catalytic reaction monitoring
in combination with pulsed-field gradi-
ent nuclear magnetic resonance (PFG-
NMR) diffusion and NMR relaxation
time measurements. Diol conversion is
significantly lower when water is pres-
ent in the initial diol/methanol mixture.
A reactivity trend within the group of
diols was also observed. Combined
NMR diffusion and relaxation time
measurements suggest that molecular
diffusion and, in particular, the relative
strength of diol adsorption, are impor-
tant factors in determining the conver-
sion. These results highlight NMR dif-
fusion and relaxation techniques as
novel, non-invasive characterisation
tools for catalytic materials, which
complement conventional reaction
data.
Keywords: diffusion · gold cataly-
sis · heterogeneous catalysts · NMR
spectroscopy · solvent effects
Introduction
ing the equilibrium towards the product. Although the ad-
sorption of the reactant onto the catalyst surface is not the
only process involved in oxidation reactions, it is a funda-
mental step required for the reaction to occur.
The oxidation of alcohol groups over heterogeneous cata-
lysts in the liquid-phase is an important class of reactions,
particularly from a “green” chemistry perspective. Among
the variety of reagents, polyhydroxy alcohols, such as diols
and glycerol, represent a class of compounds which are cur-
rently attracting enormous attention, due to their sustaina-
bility. Several reaction mechanisms have been proposed to
describe the oxidation of alcohol groups.[1] According to the
classical and widely accepted dehydrogenation mechanism,[2]
the oxidation of alcohol groups over catalytic surfaces
begins with adsorption of the reactant in solution onto the
Another important aspect to consider in these heteroge-
neously catalysed reactions is that of molecular diffusion.
Porous catalysts are used in order to provide a very large
catalytic surface in a small volume; the pores are usually of
nanometre dimensions and diffusion of reactants into the
catalyst, and products back out, may dissipate a large part
of the available chemical potential. This is particularly the
case for viscous substrates, such as glycerol and its derivates,
the high viscosity of which makes the use of solvents neces-
sary. For example, Bianchi et al.[4] studied the oxidation of
ethylene glycol over gold supported on carbon and reported
that reactivity was limited by internal diffusion of the re-
agent to gold particles inside the catalyst. Further, Mertens[5]
noted that the temperature dependence of the reaction rate
in the oxidation of 1,2-diols catalysed by colloidal gold cata-
lysts was too small for the reaction to be purely under chem-
ical control. They proposed that diffusion of reagents to the
Au colloids limits the turnover frequency. This hypothesis
was supported by the observation that an excess of encapsu-
lating polymer around the Au particles led to a 50% de-
crease in turnover frequency. Unlike many alcohols, diols
have high viscosity and so diffusion is expected to influence
overall catalyst activity.
ꢀ
catalyst surface, which occurs at equilibrium. The O H
bond of the alcohol breaks upon adsorption on the surface
site, yielding hydrogen and an alkoxide;[3] adsorbed oxygen
is necessary to oxidise the co-product hydrogen, thus shift-
[a] Dr. C. DꢀAgostino, Dr. J. Mitchell, Prof. L. F. Gladden,
Dr. M. D. Mantle
Department of Chemical Engineering and Biotechnology
University of Cambridge, Pembroke Street
Cambridge CB2 3RA (UK)
E-mail: mdm20@cam.ac.uk
[b] Dr. T. Kotionova, Dr. P. J. Miedziak, Prof. D. W. Knight,
Dr. S. H. Taylor, Prof. G. J. Hutchings
Cardiff Catalysis Institute, School of Chemistry, Cardiff University
Main Building, Park Place, Cardiff CF10 3AT (UK)
Chem. Eur. J. 2013, 19, 11725 – 11732
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11725

The majority of liquid-phase catalytic reactions of diols
are carried out in solvents, the primary role of which is to
enhance mass and heat transfer during reactions. Water
tends to be the solvent of choice,[4,6–8] but lower alcohols,
such as methanol, are also frequently used.[9–12] Many au-
thors have reported that the same reaction carried out in
different solvents gives completely different catalytic per-
formance in terms of activity and selectivity.[13–18] It is, of
course, quite likely that diffusion and adsorption properties
within porous catalysts will be greatly affected by the nature
of the solvent. In our previous work,[19] a novel type of study
on the catalytic activity of gold nanpoparticles supported on
titania for the oxidation of 1,4-butanediol in methanol was
reported. The addition of water to diol/methanol mixtures
was seen to have a detrimental effect on the catalytic activi-
ty. Diffusion and adsorption studies using pulsed-field gradi-
ent (PFG)-NMR and NMR relaxometry, respectively, re-
vealed that the addition of water decreased the diffusion
rate of the 1,4-butanediol reactant within the porous cata-
lyst; moreover, adsorption of 1,4-butanediol over the cata-
lyst surface was strongly inhibited by the presence of water,
as inferred from the T1/T2 ratio of the NMR relaxation
measurements. The results suggested that water inhibits cat-
alytic activity by limiting the access of 1,4-butanediol mole-
cules to the catalytic sites.
PFG-NMR and NMR relaxometry have recently been
recognised as powerful tools to probe diffusion and adsorp-
tion in porous materials.[20–22] These techniques have very re-
cently been extended to porous heterogeneous catalysts and
proved to be useful in elucidating several aspects of cataly-
sis.[19,23,24] For example, we recently combined PFG-NMR
diffusion and spin-lattice relaxation time studies to probe
structural properties and dynamics of organic molecules
confined in TiO2, g-Al2O3 and SiO2 supports.[25] The results
showed that the dynamics of polyols, in particular glycerol,
is enhanced in the confined pore space of a porous medium
as compared to the unrestricted free bulk liquid, and it was
proposed that this was due to the ability of the pore matrix
to disrupt the extensive hydrogen bonding network formed
by polyol molecules. Our previous work[19] on 1,4-butanediol
oxidation in methanol over a Au/TiO2 catalyst leads us to
ask if the water inhibition mechanism is generally valid for
other diol/methanol oxidations over gold nanoparticles sup-
ported on titania. Further, we wish to explore the relative
importance of the adsorption strength of the reactant com-
pared to its molecular diffusivity in determining catalytic
conversion.
Experimental Section
Materials and chemicals: 1,3-Propanediol (98%), 2-methyl-1,3-propane-
diol (99%), 2,2-dimethyl-1,3-propanediol (98%), methanol (99.8%) and
sodium hydroxide (97%, powder) were purchased from Sigma–Aldrich;
deionised water was obtained from
system.
a laboratory water purification
For the catalyst preparation, an aqueous solution of HAuCl4·3H2O of the
desired concentration was prepared. Polyvinylalcohol (PVA; 1 wt.% sol-
ution, Aldrich, MW =10000, 80% hydrolysed) was added (PVA/Au (by
wt.)=1.2); a 0.1m freshly prepared solution of NaBH4 (>96%, Aldrich,
NaBH4/Au (mol/mol)=5) was then added to form a dark-brown sol.
After 30 min of sol generation, the colloid was immobilised by adding ti-
tanium dioxide (Degussa, P25, SA=49 m2gꢀ1, 80% anatase), acidified at
pH 1 by sulfuric acid, under vigorous stirring conditions. The amount of
support material required was calculated so as to give a total final metal
loading of 1 wt.%. After 2 h the slurry was filtered, the catalyst was
washed thoroughly with distilled water (neutral mother liquors) and
dried at 1208C, overnight. The prepared monometallic gold catalyst sup-
ported on titania is denoted as 1% Au/TiO2. Nitrogen adsorption mea-
surements on the porous Au/TiO2 catalyst gave a unimodal pore size dis-
tribution centred at approximately 15 nm.
Catalytic experiments: A pressurised Radleyꢀs reactor with a nominal
volume of 50 mL and a maximum working pressure of 4 bar was charged
with 1,3-propanediol or 2-methyl-1,3-propanediol or 2,2-dimethyl-1,3-
propanediol (1.5 mmol), sodium hydroxide (1.5 mmol, 0.06 g), methanol
(7.9 g) or 50% water in methanol by volume (8.95 g), and catalyst 1%
Au/TiO2 (substrate/metal=300:1, 90 mg). The reaction vessel was sealed,
purged three times with oxygen and pressurised to 3 bar; the oxygen
inlet remained open throughout the reaction so that oxygen was replen-
ished as it was consumed. The reaction mixture was stirred (1000 rpm)
and heated to 808C. Completed reactions were cooled to 208C periodi-
cally and samples were taken. The reactor was then recharged with
oxygen and reheated to 808C. The isolated reaction mixtures were con-
centrated under vacuum and analysed by gas chromatography, using a
Varian 3800 chromatograph equipped with a CP 8400 autosampler and
CP-wax 52 column. Products were identified by comparison with authen-
tic samples and quantification was carried out using an external standard.
NMR spectroscopy experiments: All the NMR spectroscopy experiments
were performed on a Bruker Biospin DMX 300 operating at a 1H fre-
quency of 300.13 MHz. The 1H PFG-NMR spectroscopy experiments
were carried out using a Bruker Biospin Diff-30 diffusion probe capable
of producing magnetic field gradient pulses up to 11.76 Tmꢀ1. Diffusion
measurements of pure bulk liquids were performed using the pulsed gra-
dient stimulated echo (PGSTE)[26] pulse sequence, whereas the alternat-
ing pulsed gradient stimulated echo (APGSTE)[27] sequence was used
when studying diffusion of liquids within catalyst particles to minimise
the effects of background magnetic field gradients. The measurements
were carried out holding the gradient pulse duration, d, constant and
varying the magnetic field gradient strength, g, for sixteen points. The ob-
servation time, D, was set to 50 ms for pure bulk liquid samples and
100 ms for liquids in catalyst particles. The gradient pulse duration, d,
was set to 1 ms. Values of the diffusion coefficient were obtained by fit-
ting the PFG-NMR experimental data to the expression:
EðgÞ
In the present work the aerobic oxidation of 1,3-propane-
diol and its methyl substituted homologues, namely 2-
methyl-1,3-propanediol and 2,2-dimethyl-1,3-propanediol,
carried out in methanol over Au/TiO2 catalyst has been
studied. The effect of water addition on catalytic activity
was assessed; PFG-NMR studies were used to probe diffu-
sion of reactants in the Au/TiO2 porous catalyst in the ab-
sence and in the presence of water. The adsorption proper-
ties of the reactants under these different conditions were
probed using NMR relaxation time measurements.
ð1Þ
¼ exp½ꢀbDꢁ
E0
where E(g) and E0 are the NMR echo signal intensity in the presence
and absence of magnetic field gradient, respectively; b is the b-factor,
b ¼ ꢀg2g2d2ðD ꢀ d=3Þ, where
g is the gyromagnetic ratio of the
1H nucleus; D is the self-diffusion coefficient of the species of interest.
NMR relaxation times T1 and T2 were measured using standard inversion
recovery[28] and CPMG[29] techniques, respectively. Experimental data
were fitted using single exponential functions. The T1 relaxation time
constant was obtained by fitting the experimental data to the equation[28]
:
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Aerobic Oxidation of 1,3-Propanediols
FULL PAPER
ꢂ
ꢀ
ꢁꢃ
t
T1
Table 1. Catalytic oxidation of 1,3-propanediols in methanol over 1% Au/TiO2. Con-
version is at 4 h reaction time. The letters in square brackets identify the species for
which selectivity is reported.
SðtÞ ¼ S0 1 ꢀ 2 exp
ꢀ
ð2Þ
The T2 relaxation time constant was obtained by fitting the ex-
perimental data to the equation[28]
Reactant
Conversion
[%]
Ester
selectivity [%]
Acid
selectivity [%]
:
ꢀ
ꢁ
1,3-propanediol
2-methyl-1,3-propanediol
2,2-dimethyl-1,3-propanediol
18
12
10
84[a]
75[c]
60[e]
15[b]
14[d]
40[f]
t
T2
SðtÞ ¼ S0 exp
ꢀ
ð3Þ
In Equations (2) and (3), S represents the NMR signal intensi-
ty.
[a] Methyl 3-hydroxypropionate. [b] 3-Hydroxypropanoic acid. [c] Methyl hydroxyiso-
butyrate. [d] Hydroxyisobutyric acid. [e] Methyl 3-hydroxypivalate. [f] 3-Hydroxypival-
ic acid.
NMR spectroscopy samples were prepared using 5 mm NMR
tubes. For bulk liquid measurements, the tube was filled with
liquid to a height of approximately 10 mm. Solid samples were
Table 2. Conversion data at 24 h reaction time for the oxidation of 1,3-
propanediols in methanol and methanol/water mixtures.
prepared by soaking catalyst particles in the liquid for at least 24 h. The
catalyst was dried on a pre-soaked filter paper in order to remove any
excess liquid on the external surface and finally transferred into 5 mm
NMR tubes. To ensure a saturated atmosphere inside the NMR tube,
hence minimising errors due to evaporation of volatile liquids, a small
amount of liquid was placed onto absorbed filter paper, which was then
placed under the cap of the NMR tube. All the NMR measurements
were carried out at atmospheric pressure and 208C. Tetramethylsilane
Reactant
Conversion [%]
1,3-propanediol in MeOH
39
34
32
25
20
5
2-methyl-1,3-propanediol in MeOH
2,2-dimethyl-1,3-propanediol in MeOH
1,3-propanediol in MeOH/H2O
2-methyl-1,3-propanediol in MeOH/H2O
2,2-dimethyl-1,3-propanediol in MeOH/H2O
1
(TMS) was used as a H reference standard.
The criterion chosen for the selection of the NMR mixture composition
was to keep it as close as possible to that used in catalytic experiments
but at the same time ensure a good signal-to-noise ratio for the H NMR
1
signal of the reactant in a given mixture inside the 1% Au/TiO2 catalyst.
For the diol/methanol mixture, a 10% mole fraction of diol in methanol
was used. Lower diol concentrations did not yield a good signal-to-noise
ratio for the signal of the reactant within the catalyst. For the methanol/
water mixture, a volumetric amount of water equal to that of methanol
was added to the initial mixture; again the diol concentration was main-
tained at 10% mole fraction. The mixtures were all base-free. It is noted
that the NMR characterisation is carried out ex-situ, and therefore not
under reaction conditions. As explained in the previous work,[19] the
NMR spectroscopy experiments are conducted under equilibrium condi-
tions (i.e., no reaction occurring). Considering the type of reaction (aero-
bic oxidation), the preparation method, and the high resistance of gold to
over-oxidation,[30,31] it is reasonable to assume that the state of the surface
in the NMR measurements will be very similar to that under reaction
conditions. Furthermore, given that for each diol oxidation the nature of
the solvent is the only variable in our experiments, any change in catalyt-
ic activity is expected to be attributed to the nature of the solvent. This
was confirmed in our previous work on 1,4-butanediol,[19] in which we
also showed that the addition of base does not influence the solvent be-
haviour.
propanediol has a negative effect on the substrate reactivity:
the general trend of reactivity of the different propanediols
shows that the rate of oxidation decreases with the addition
of methyl groups, following the order 1,3-propanediol>2-
methyl-1,3-propanediol>2,2-dimethyl-1,3-propanediol. One
of the reasons may be the different steric hindrance around
ꢀ
the OH group in the different alcohols as previously sug-
gested.[32]
The detrimental effect of water observed in the present
work is very similar to that observed in the 1,4-butanediol
oxidation on a similar catalyst.[19] The results presented
show that the influence of water on the aerobic oxidation of
a homologous series of diols is consistent with the conclu-
sions drawn from that earlier work. Consideration of a
series of diols also allows us to explore the relative impor-
tance of reactant adsorption and diffusion in determining
conversion.
NMR diffusion and adsorption studies: The rate of molecu-
lar diffusion is an important factor to take into account in
heterogeneous catalysts, especially for viscous molecules,
such as diols. For the case of the present work, catalytic ex-
periments on smaller catalyst particles showed a faster reac-
tion rate, indicating the presence of intra-particle mass
transfer. Therefore, a quantitative analysis of diffusion
within the catalyst was carried out using the PFG-NMR
spectroscopy technique.
A typical NMR spectrum of diol/solvent mixtures imbibed
in the catalyst is reported in Figure 1, which shows the
1H NMR spectrum of 10% mole fraction 2,2-dimethyl-1,3-
propanediol in methanol imbibed in 1% Au/TiO2 catalyst.
Results and Discussion
Oxidative esterification of propanediols in methanol: The
oxidative esterification of 1,3-propanediol and the methyl-
substituted homologues was carried out using a 1% Au/TiO2
catalyst. The data for a reaction time of 4 h are shown in
Table 1. In all cases the main product observed was the
ester and the amount of the acid product (formed as the
sodium salt) increased with the degree of methyl substitu-
tion. The product acid may arise from the hydrolysis of the
ester. The activity of the 1% Au/TiO2 catalyst was assessed
in methanol and 50% by volume water/methanol mixtures;
the conversion data for the reaction after 24 h is shown in
Table 2. It is clear that the addition of water leads to a de-
crease in the conversion in all cases. At the same time, the
data show that introducing the methyl substituents to 1,3-
ꢀ
ꢀ
The peak of the two CH3 groups of 2,2-dimethyl-1,3-pro-
panediol, at approximately 0.9 ppm, is well resolved on the
1
chemical shift axis of the H NMR spectrum; it is the signal
intensity of this peak that is monitored in the PFG-NMR ex-
Chem. Eur. J. 2013, 19, 11725 – 11732
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11727

M. D. Mantle et al.
Figure 1. 1H NMR spectrum of 10% 2,2-dimethyl-1,3-propanediol in
methanol imbibed in 1% Au/TiO2 catalyst. The aliphatic peak of 2,2-di-
methyl-1,3-propanediol is well resolved on the chemical shift axis at
0.9 ppm. All chemical shifts are quoted relative to the 1H resonance in
tetramethylsilane (TMS).
periment. The NMR spectra of 1,3-propanediol and 2-
methyl-1,3-propanediol mixtures showed similar features to
those shown in Figure 1. Diffusion coefficients of diols in
methanol and methanol/water mixtures were obtained for
bulk liquid mixtures and liquid mixtures within the porous
1% Au/TiO2 catalyst. PFG-NMR log attenuation plots are
shown in Figure 2; the value of the diffusion coefficient of
each diol is obtained by taking the negative of the slope of
the plot. The plots are approximately linear indicating that
the molecular displacements probed during the PFG-NMR
spectroscopy experiment (on the order of microns) are sig-
nificantly larger than the dimension of a single pore (nano-
metres), and that the porous material is homogenous over a
macroscopic length scale.[33,34]
Inspection of the data presented in Figure 2 shows that
the slope characterising diol diffusion in the presence of
methanol (open circles) is steeper than that obtained for
methanol/water mixtures (closed circles), which means that
the diffusion rate of the diol decreases upon addition of
water. This behaviour has also been reported in our previ-
ous work[19] on 1,4-butanediol oxidation and is attributed to
changes in hydrogen bonding and to the non-ideal mixing of
methanol/water mixtures, widely reported in the litera-
ture,[35–38] where the hydrogen bonding between water and
methanol molecules is responsible for the slower dynamics
within the mixture.[39] Values of self-diffusion coefficients in
bulk liquid mixtures and in mixtures confined within the
1% Au/TiO2 catalyst are reported in Table 3, along with the
PFG interaction parameter, x, that is, the ratio between the
self-diffusivity of bulk liquid to self-diffusivity of the same
liquid within the pore space. It can be noted that 1,3-pro-
panediol diffuses faster than 2-methyl-1,3-propanediol,
which is in turn faster than 2,2-dimethyl-1,3-propanediol.
This is due to the presence of the additional methyl substitu-
ents, which increase molecular weight and size of the mole-
cule, hence decreasing the molecular mobility. The tortuosi-
ty of the catalyst, t, was estimated by measuring the PFG in-
teraction parameter of cyclohexane. The concept of using al-
kanes as probe molecules to estimate the “intrinsic” value
of tortuosity (i.e., that determined by the physical pore
Figure 2. PFG-NMR log attenuation plots for different 1,3-propanediols
in methanol (open symbols) and methanol/water (closed symbols) mix-
tures within 1% Au/TiO2 catalyst: a) 1,3-propanediol, b) 2-methyl-1,3-
propanediol, and c) 2,2-dimethyl-1,3-propanediol. Fits to the data using
Equation (1) are shown by the solid lines.
structure alone) of these types of materials with PFG-NMR
measurements has already been presented in a previous
work.[25] It is also noted that in general the PFG interaction
parameter of each diol is higher in methanol than in metha-
nol/water mixtures, suggesting that the diffusing diol experi-
ences a more tortuous path (associated with either or both
enhanced molecule–molecule and molecule–surface interac-
tions) in methanol than in methanol/water mixtures. Methyl-
substituted 1,3-propanediols exhibit higher values of x,
which in pure methanol are higher than the catalyst tortuos-
ity, t. In all other cases x values are generally slightly lower
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Aerobic Oxidation of 1,3-Propanediols
FULL PAPER
Table 3. PFG-NMR self-diffusivity of diols in methanol and methanol/water mixtures
as bulk liquid (Dbulk) and liquid in 1% Au/TiO2 (Dinpores). The PFG interaction param-
eter is also reported. Values are compared with those of cyclohexane. The PFG inter-
action parameter of cyclohexane represents the tortuosity of the porous catalytic
matrix.
ing to note that the introduction of methyl substitu-
ents to 1,3-propanediol also decreases the T1/T2
ratio compared to the case with no substituents.
This observation may be explained by the steric
ꢀ
hindrance around the OH group of the diol
Mixture
Dbulk
Dinpores
x
AHCTUNGTRENNUNG
[m2 sꢀ1] 10ꢀ10 [m2 sꢀ1] 10ꢀ10
caused by the additional methyl groups, which in-
hibit the interaction of the molecule with the cata-
lyst surface.
1,3-propanediol in MeOH
7.74ꢂ0.15 3.98ꢂ0.12
6.85ꢂ0.14 3.03ꢂ0.09
6.31ꢂ0.13 2.57ꢂ0.08
4.66ꢂ0.09 2.44ꢂ0.07
3.90ꢂ0.08 1.97ꢂ0.06
1.95ꢂ0.06
2.26ꢂ0.06
2.46ꢂ0.07
1.90ꢂ0.06
1.97ꢂ0.06
2.05ꢂ0.06
t
2-methyl-1,3-propanediol in MeOH
2,2-dimethyl-1,3-propanediol in MeOH
1,3-propanediol in MeOH/H2O
2-methyl-1,3-propanediol in MeOH/H2O
2,2-dimethyl-1,3-propanediol in MeOH/H2O 3.50ꢂ0.07 1.71ꢂ0.05
Relaxation measurements using cyclohexane as a
“reference” probe molecule were also performed.
This liquid shows a considerably lower T1/T2 ratio
as compared to that of diols; this is expected as cy-
cyclohexane
13.50ꢂ0.10 6.42ꢂ0.20
2.10ꢂ0.06 clohexane is a non-polar molecule with no function-
al groups that can adsorb onto the solid surface of
the catalyst. Hence, we expect a minimal interac-
tion strength with the surface.[41]
than t, indicating a faster self-diffusion coefficient in the
pore space than would be expected based purely on the
physical structure of the pore space. This behaviour seems
to be common to polyols imbibed in porous materials and is
ascribed to a disruption of the intermolecular hydrogen
bonding network within the diol when it is confined within
mesoporous materials.[23,25] The conversion is clearly seen to
decrease with decrease in diol diffusion coefficient as might
be expected in a diffusion-limited reaction.
The adsorption strength of all reactants in the absence
and presence of water was also investigated by using NMR
relaxation techniques, measuring the longitudinal T1 and
transverse T2 relaxation times of the diols within the Au/
TiO2 catalyst. Relaxation measurements were also per-
formed on bulk liquid mixtures. In all cases, for the diol in
the bulk liquid mixture T1 ~T2, as expected from the NMR
relaxation theory of liquids.[40] Plots of T1 and T2 of each
diol in different liquid mixtures imbibed in the 1% Au/TiO2
catalyst are reported in Figure 3 and Figure 4, respectively.
The numerical values of the relaxation measurements are in-
cluded in Table 4. It is noted that the addition of water has
a strong influence on the T1 relaxation time of the diol for
both bulk liquids and for the liquid mixtures inside the cata-
lysts. This result, along with the observed decrease in T1/T2
ratio of the diol when water is present within the pore space
of the catalyst, is a strong indicator that the presence of
water weakens the interaction of the diol with the catalyst
surface and that water is selectively accessing the surface.
This result is very similar to that observed for the 1,4-buta-
nediol oxidation system previously studied.[19] It is interest-
Comparison between catalytic reaction data and NMR diffu-
sion and adsorption results; reactivity trend and solvent
effect: The catalytic and NMR spectroscopy data are sum-
marised in Figure 5, which reports conversion, diffusion and
T1/T2 ratio of 1,3-propanediol and its methyl substituents in
methanol and methanol/water mixtures imbibed within the
1% Au/TiO2 catalyst. It is clear that the detrimental effect
of water in terms of conversion is consistent with a decrease
in measured diol diffusivity in the pore space and with the
decrease in adsorption strength of the diol on the catalyst
surface as measured by the T1/T2 ratio. The same agreement
in trend is also seen as a function of methyl substituents for
a given solvent composition. The reactivity of 1,3-propane-
diols is affected by the presence of methyl substituents and
shows the following reactivity trend: 1,3-propanediol>2-
methyl-1,3-propanediol>2,2-dimethyl-1,3-propanediol.
The NMR T1/T2 ratio data suggest that the steric hin-
drance caused by the additional methyl substituents inhibits
the adsorption of the diol over the catalyst surface. The diol
diffusion and T1/T2 data presented also suggest that the
strength of diol interaction with the surface is perhaps more
important than the molecular diffusivity in determining con-
version. Considering Figure 5, we see that for the case of
methanol as a pure solvent, both conversion and T1/T2 ratio
decrease far less rapidly than does the diol diffusivity. For
the case of the methanol/water solvent, although the diol
diffusivity remains relatively constant, the decrease in con-
Table 4. Values of T1 and T2 NMR relaxation time constants and their ratio T1/T2 for diols in methanol and methanol/water mixtures in bulk liquid and
within 1% Au/TiO2. Cyclohexane is reported as a reference compound for comparison.
Bulk liquid
Liquid in Au/TiO2
Mixture
T1 [ms]
T2 [ms]
T1/T2
T1 [ms]
T2 [ms]
T1/T2
1,3-propanediol in MeOH
1630ꢂ33
1340ꢂ27
1080ꢂ22
1330ꢂ27
1090ꢂ23
830ꢂ17
1430ꢂ29
1140ꢂ23
1080ꢂ22
1270ꢂ25
1000ꢂ20
830ꢂ17
1.14ꢂ0.03
1.18ꢂ0.04
1.00ꢂ0.03
1.05ꢂ0.03
1.09ꢂ0.03
1.00ꢂ0.03
1.00ꢂ0.03
470ꢂ10
527ꢂ11
525ꢂ10
268ꢂ5
10ꢂ0.2
14ꢂ0.3
16ꢂ0.3
8ꢂ0.2
47ꢂ1.4
38ꢂ1.3
34ꢂ1
2-methyl-1,3-propanediol in MeOH
2,2-dimethyl-1,3-propanediol in MeOH
1,3-propanediol in methanol/H2O
2-methyl-1,3-propanediol in MeOH/H2O
2,2-dimethyl-1,3-propanediol in MeOH/H2O
cyclohexane
33ꢂ1
288ꢂ6
10ꢂ0.2
15ꢂ0.3
128ꢂ3
29ꢂ0.9
18ꢂ0.5
10ꢂ0.3
275ꢂ6
2900ꢂ58
2900ꢂ58
1327ꢂ27
Chem. Eur. J. 2013, 19, 11725 – 11732
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www.chemeurj.org
11729

M. D. Mantle et al.
Figure 3. T1 inversion recovery plots for 1,3-propanediols in methanol
(open symbols) and methanol/water (closed symbols) mixtures within
1% Au/TiO2 catalyst: a) 1,3-propanediol, b) 2-methyl-1,3-propanediol,
c) 2,2-dimethyl-1,3-propanediol. Fits to the data using Equation (2) are
shown by the solid lines.
Figure 4. T2 CPMG plots for 1,3-propanediols in methanol (open sym-
bols) and methanol/water (closed symbols) mixtures within 1% Au/TiO2
catalyst: a) 1,3-propanediol, b) 2-methyl-1,3-propanediol, c) 2,2-dimethyl-
1,3-propanediol. Fits to the data using Equation (3) are shown by the
solid lines.
version again seems to follow the trend in T1/T2 ratio more
closely.
and co-workers[16] studied the aerobic epoxidation of stil-
bene over Au/TiO2 catalyst and also found that the choice
of the solvent is important. These workers attempted to ex-
plain the solvent effect by correlating catalytic activity in
each solvent with the solubility of oxygen in the solvent.
However, the influence of solvent on catalytic activity did
not follow any trend in terms of oxygen solubility, suggest-
ing that the solvent effect on the catalysed epoxidation of
It has to be pointed out that solvents may also affect the
catalytic activity by changing the solubility properties of dis-
solved oxygen. However, this would influence the oxidation
reaction only in the case of oxygen supply being the rate-
limiting process, which is not the case here as these reac-
tions occur in an excess of oxygen. In earlier work, Lignier
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Chem. Eur. J. 2013, 19, 11725 – 11732

Aerobic Oxidation of 1,3-Propanediols
FULL PAPER
of support) and reaction medium (solvent). NMR diffusom-
etry and relaxometry, therefore, have significant potential as
routine tools in catalyst characterisation. These measure-
ments will provide new insights into the physical chemistry
involved in heterogeneous catalysis and, combined with con-
ventional catalyst characterisation techniques, will give a
more comprehensive picture of catalytic reactions over sup-
ported catalysts.
Acknowledgements
The authors would like to acknowledge the Technology Strategy Board
for funding this work, Grant No.: TP/7/ZEE/6/I/N0262B. The Technology
Strategy Board is a government body, which promotes and supports the
development and exploitation of technology and innovation for the bene-
fit of UK business, in order to increase economic prosperity and improve
quality of life. For more information see: www.innovateuk.org. We also
wish to thank the EPSRC for funding the CASTech consortium (EP/
G011397/1) and Platform Grant (EP/F047991/1), both of which supported
this work.
Figure 5. Comparison of catalytic conversion (at 24 h reaction time) with
NMR diffusion and relaxation data for different diols in methanol and
methanol/water mixtures. The reactivity trend observed for different 1,3-
propanediols is determined by the steric hindrance of methyl substitu-
ents, which inhibit the adsorption of the diol over the catalyst surface.
The decrease in catalytic activity due to water addition is accompanied
by a decrease of both self-diffusivity and T1/T2 ratio of all homologous
1,3-propanediols within the 1% Au/TiO2 catalyst and indicates that the
overall catalytic activity is influenced by reactant diffusivity rate as ex-
pected, but is particularly strongly correlated to the adsorption strength
(T1/T2) of the reactant onto the catalyst surface.
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Received: February 7, 2013
Published online: July 19, 2013
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Chem. Eur. J. 2013, 19, 11725 – 11732