A simple liquid state 1 H NMR measurement to directly determine the surface hydroxyl density of porous silica

Abstract

Silica is widely used in industrial applications and its performance is partially decided by its surface hydroxyl density αOH. Here we report a quick, simple liquid ¹ H NMR method to...

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12804 | Chem. Commun., 2021, 57, 12804–12807 This journal is © The Royal Society of Chemistry 2021
Cite this: Chem. Commun., 2021,
57, 12804
A simple liquid state
1
H NMR measurement to
directly determine the surface hydroxyl density of
porous silica†
C. Penrose,
a
P. Steiner,
ab
L. F. Gladden,
a
A. J. Sederman,
a
A. P. E. York,
c
M. Bentley
c
and M. D. Mantle *
a
Silica is widely used in industrial applications and its performance is
partially decided by its surface hydroxyl density a
OH
.Herewereporta
quick, simple liquid
1
H NMR method to determine a
OH
using a benchtop
1
H NMR spectrometer. The results show excellent agreement with the
literature with an a
OH
range from 4.16 to 6.56 OH per nm
2
.
Silicas (SiO
2
) are a class of porous materials which exhibit high
surface areas, and whose available surfaces are populated, to
varying degrees, with hydroxyl (silanol) groups. Hydroxyl moi-
eties make silica useful in a number of applications including
chromatography,
1
drug delivery
2–4
and catalysis.
5–8
In drug
delivery, hydroxyls can be functionalised providing controllable
drug release,
9
and in catalysis hydroxyls are sites where metal
ions can coordinate onto the silica surface.
10
Therefore, it
becomes important to characterise, quantify and understand
the behaviour of surface hydroxyls.
A key measurement regarding surface (available) hydroxyls
is their total density on the silica surface. The total surface
hydroxyl density, a
OH
, can impact the performance of silica for
a given application. A higher surface density has a greater
quantity of hydroxyls that can be functionalised,
11
and a low
surface density leads to a more hydrophobic surface more
suited to catalytic systems where water can hinder catalytic
reaction such as olefin hydrogenation.
12
Subsequently, quanti-
fying the total surface hydroxyl density, a
OH
, has been the
subject of much research using analytical techniques including
infrared spectroscopy,
1,13–15
mass spectrometry
16–19
and solid
state NMR.
20–28
Infrared spectroscopy (IR) is able to discrimi-
nate between absorbed water and surface hydroxyls. Gallas
et al.
13
showed that the H
2
O IR stretch peak at 5260 cm
1
representing absorbed water disappeared around 200 1C leav-
ing behind OH stretch peaks between 4200–4800 cm
1
repre-
senting hydroxyls. The presence of internal water complicating
a
OH
measurements was discussed by Davydov et al.
29
who
showed that an infrared peak at 3650 cm
1
persisted following
deuterium (D
2
O vapour) exchange, and its intensity increased
with silica particle size. In most cases, infrared spectroscopy
has been used in conjunction with other techniques to make
their estimates of a
OH
more insightful and less prone to error.
Christy and Egeberg
14
use IR spectroscopy data and partial least
squares analysis to quantify a
OH
with an error of around 10%
on a series of silica gels. However, their method was limited to
silicas with high surface areas (Z400 m
2
g
1
).
Mass spectrometry (MS) paired with temperature pro-
grammed desorption (TPD)
16–19
has been used to quantify the
absorbed water (dehydration) and hydroxyl groups (dehydroxy-
lation) removed by applying heat to silica samples. Zhuravlev
reported that the combined use of MS and TPD can measure
minimal amounts of water from silica ranging 0.04 mLto4mL
with 1–5% relative error.
16
Zhuravlev et al.
17
have also pre-
sented results for the total surface hydroxyl density of 100
different silicas. The results from the different silicas ranged
between a
OH
= 4.2–5.7 OH per nm
2
and had an arithmetic
mean, a
OH,average
= 4.9 OH per nm
2
. These results highlighted
that the surface hydroxyl density is similar regardless of the
geometry, surface area and pore size distribution of the
silica. The total surface hydroxyl density average, a
OH,average
=
4.9 OH per nm
2
, is supported by a theoretical method devel-
oped by Kiselev et al.
30
based on crystallographic data. This
theoretical model also predicted single hydroxyl (silanol)
O
3
–Si–OH was the most probable species on a ‘fully’ hydro-
xylated silica surface.
Solid state
1
H Magic Angle Spinning (MAS) NMR,
29
Si Direct
Polarisation (DP) and
29
Si–{
1
H} Cross Polarisation (CP) have all
been used to quantify hydroxyl densities in silicas.
20–27,31,32
For
example, Sindorf and Maciel,
26
grafted trimethylsilane onto
silica through a reaction that only affects hydroxyls on the
a
Department of Chemical Engineering and Biotechnology, University of Cambridge,
Philippa Fawcett Dr, Cambridge CB3 0AS, UK. E-mail: mdm20@cam.ac.uk;
Tel: +44-1223-766325
b
Gdanska 335/23, Praha 8, 181 00, Czech Republic
c
Johnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading,
RG4 9NH, UK
†Electronic supplementary information (ESI) available. See DOI: 10.1039/d1cc03959h
Received 21st July 2021,
Accepted 9th November 2021
DOI: 10.1039/d1cc03959h
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surface, and by measuring the corresponding weight gained,
they calculated for a ‘fully’ hydroxylated silica an a
OH
E
5.0 OH per nm
2
. One of the major drawbacks with solid state
1
H MAS NMR for direct a
OH
calculation is that a reference silica
with a known total hydroxyl density
24,33
is required. Moreover,
solid state
1
H MAS-NMR does not distinguish between internal
and external hydroxyl groups. To quantify external hydroxyls
Schrader et al.
34
recently reported a liquid state
1
H NMR based
method to calculate silanol densities from a commercial silica.
However, their method involved several steps including the use of
an internal standard, sonication and centrifugation of the sample.
In this paper, we present a simple liquid phase
1
H NMR
technique based upon D
2
O/surface-OH exchange to determine
a
OH
. The following materials were used: D
2
O (deuterium oxide
99.96% in Septum Vial, Eurisotop); H
2
O (deionised water,
15 MO), 5 mm diameter 8 inch length NMR Tubes (Wilmad
Inc, USA); Silica beads (Fuji Silysia Chemical Ltd, Japan
1.70–4.00 mm particle size) were from the CARiACT Q Series
and labelled as: Q6, Q10, Q15, Q30 and Q50 where the number
in the label indicates the average pore size diameter (e.g. Q6 has
a pore diameter of 6 nm).
To remove residual OH groups present on the walls of the 5 mm
NMR tube 0.5 mL of D
2
O was added to the tube and then sealed
with a lid. The NMR tube is then rigorously shaken for duration of
5 minutes. Then the lid was then removed and the D
2
Owas
emptied out. The calibration of the NMR system is as follows:
I. Approximately 1.0 mL of D
2
O (99.96%) is placed into a
5 mm NMR tube. The sample is then weighed using a Precisa
205 A balance capable of weighing to a precision of 0.0001 g.
A ‘background’
1
H signal is then acquired and subsequently
Fourier transformed to give a
1
H NMR spectrum.
II. Approximately 2 mL of deionised H
2
O is then added to the
sample tube in (i). The sample is then capped with a lid,
vigorously shaken for 5 minutes and then a
1
H NMR signal is
acquired from this system.
III. Step (ii) is repeated for the same sample tube until the
cumulative volume of H
2
O in the sample is 10 mL. The mass of
added hydrogen atoms is calculated for each step and the
integral of
1
H signal intensity is then plotted against the known
mass of added hydrogen atoms as shown in the calibration plot
in Fig. 1.
The data points shown in Fig. 1 are fitted to a simple linear
equation to give the mass of hydrogen atoms from a sample
containing an unknown amount of exchangeable hydrogen.
In order to measure hydroxyl densities of the CARiACT
Q-series silicas the following procedure was adopted and
repeated 3 times for each different Q-series silica (n= 3).
(1) Ten Q-series beads are heated in an oven at 120 1C
for 12 h.
(2) Following (1), the silica beads are transferred to a 5 mL
plastic vessel and weighed using a precision scale.
(3) 1 mL of D
2
O (99.96) is syringed into the plastic vessel
containing the silica beads.
(4) The silica beads are left to equilibrate for 3 h. The
equilibration time was confirmed to be sufficient for our system
as described in in ESI†S5.
(5) D
2
O (500–600 mL) is then pipetted from the plastic vessel
into a 5 mm NMR tube.
(6) The NMR tube is then capped and shaken for a duration
of 5 minutes.
(7) The
1
H signal is then acquired, Fourier transformed, and
the resulting spectrum is integrated to obtain the
1
H intensity.
This integral is background corrected by subtracting the inte-
gral obtained by performing only steps (3) to (7), i.e. the same
procedure but without the silica beads.
Using pelletized/granulated materials ensures an easy
separation of liquid and solid materials; in principle the
method is suitable for powdered samples provided a suitable
separation of powder and liquid is possible, e.g., by mild
centrifugation during step (4). Overall, we assume that any
liquid density changes are negligible. The vertical liquid height
must exceed the entire active region of the (RF) coil to ensure all
1
H NMR measurements are from the same control volume. In
these experiments, a sample length of B42 mm was used which
was significantly longer than the length of the coil over which
any signal was received (see ESI†S6).
1
H NMR spectra were
collected using a Magritek Spinsolve 43 MHz NMR benchtop
spectrometer and details of pulse sequence parameters may be
found in ESI†S1.
There are several sources of experimental error that need to
be accounted for when producing the calibration plot in Fig. 1,
the details of which are given in ESI†S2. The calculation of
hydroxyl density is taken directly from Zhuralev
16
using eqn (1):
a
OH
=d
OH
N
A
10
21
(S.A.)
1
g
1
catalyst (1)
where, a
OH
is the hydroxyl density in OH per nm
2
,d
OH
is the
concentration of H atoms, obtained from the calibration plot
expressed in mmol per gram of catalyst, N
A
is Avagadro’s
number and S.A. is the surface area of the catalyst, which was
obtained with BET N
2
(details found in ESI†S3). Table 1
describes the results from the as-received CARiACT Q series
silica beads. The OH density values of silicas with pore sizes
larger than 10 nm (Q15, Q30 and Q50) show excellent agree-
ment with the results reported by Zhuravlev which ranged
Fig. 1 Calibration plot showing the integral of
1
H signal intensity against
the mass concentration of H atoms. A linear regression (blue line) is
displayed and has a gradient of 1.3335e + 08 (intensity units g
1
).
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between 4.1 and 6.1 OH per nm
2
.
18
Theoretically, it is expected
that the OH densities of Q6 and Q10 silicas should also have
been in this range. However, the OH densities calculated for
Q6 and Q10 silicas show statistically significant deviations
from Zhuravlev’s arithmetic mean, with OH densities of
1–2 OH per nm
2
.
It has been previously reported using
1
H magic angle spin-
ning NMR
35
data that, as received CARiACT Q6 and Q10 also
exhibit these lower OH densities. It is unclear what causes the
lower OH densities in as received Q6 and Q10 silicas, but the
evidence from
1
H magic angle spinning NMR
35
and additional
experiments shown in ESI†S5 rule out slower kinetics or
diffusion in smaller pores as a cause of these low OH densities.
We also note that the OH densities reported by Zhuravlev
et al.
17
are based on ‘fully hydroxylated’ samples, and ‘as
received’ does not always equate to ‘fully hydroxylated’. There-
fore, rehydroxylation is required for samples such as Q6 and
Q10 in order to make a fairer comparison with previous
research. Q-series silicas were rehydroxylated by being placed
in boiling water for 100 h in a reflux condenser setup with
heating mantle and cooling water. Fig. 2 highlights that rehy-
droxylation increases the total surface hydroxyl density of Q6
and Q10 silicas. The a
OH
values for Q6 and Q10 following
rehydroxylation (but using the as received surface area) are
2.82 and 3.54 OH per nm
2
respectively and remain below the
expected Zhuravlev constant of around 5.0 OH per nm
2
. How-
ever, by taking a further BET surface area measurement of Q6
and Q10 after rehydroxylation for 100 h in boiling water, both
silicas show a significant reduction in surface area: Q6 reduces
from 407 to 191 m
2
g
1
and Q10 from 312 to 233 m
2
g
1
. The
rightmost columns in Fig. 2 show that when reduction in
surface area is considered, the a
OH
values are more in line with
those seen in literature. This group of ‘fully hydroxylated’
silicas has an a
OH
range of 4.16 to 6.56 OH per nm
2
, and is
consistent with the average Zhuravlev constant. This consis-
tency could not have been achieved without considering the
surface area decrease as a result of rehydroxylation (Fig. 2). The
literature
18,32,36
often does not specify and/or neglects ‘when’
BET measurements for specific surface area are made, though
it is clear that this could be a significant source of error if the
surface area is being affected by any treatment. A surface area
reduction has also been observed by Zhuravlev,
16
who reported
a surface area reduction in an aerosilogel after 60 h in boiling
water from 168 to 108 m
2
g
1
. It is noted that changing
the surface hydroxyl content by heating alone does not change
the BET surface area calculation significantly. Previous
research
19,32,37
limits the discussion of rehydroxylation to a
surface reaction that converts siloxanes (Si–O–Si) into hydroxyls
(Si–OH), and therefore does not consider any structural impli-
cations rehydroxylation may have on the silica. To investigate
the effect of dehydroxylation CARiACT Q15 silica beads were
dehydroxylated by heating under vacuum for a duration of 8 h
under the following temperatures: 325, 425 and 800 1C. The
Q15 samples were then subject to exactly the same the D
2
O
exchange procedure outlined in (1–7) on the previous page.
Table 2 highlights the clear trend that the total surface
hydroxyl density decreases with increasing pre-treatment tem-
perature under vacuum. The surface area variation with the
heat treatments described in Table 2 do not vary significantly
from the as received silica: 207 m
2
g
1
vs. 203, 199, 202 m
2
g
1
for 325, 425, 800 1C respectively. This constant surface area
trend with respect to pre-treatments of 800 1C and below was
also found by Shioji et al.
37
The dehydroxylated total surface
hydroxyl densities obtained through this
1
H liquid phase
technique do not diverge further than 0.4 OH per nm
2
from
the results of Zhuravlev
17,19
(noting slightly different tempera-
tures were used). The dehydroxylation trend and the absolute
values of dehydroxylated total surface hydroxyl densities are
both consistent with the literature, and therefore support the
validity of this technique. In addition, our technique is further
able to measure a significantly dehydroxylated silica sample
heated at 800 1C demonstrating this technique has a suitable
degree of sensitivity.
Table 1 As-received CARiACT Q-series silica beads and their respective
total surface hydroxyl densities as measured by NMR. Further sample
details are given in ESI S4
Sample
Average mass (n=3)
of silica beads (g
1
)
Average mass
concentration d
OH
(mmol g
1
)
Average a
OH
(nm
2
)
Q6 0.1772 g 1.02 1.51 0.13
Q10 0.1454 g 0.67 1.29 0.17
Q15 0.1188 g 1.55 4.52 0.36
Q30 0.1687 g 0.95 5.35 0.81
Q50 0.1705 g 0.60 5.16 0.70
BET N
2
surface areas for the samples were the following: 407 m
2
g
1
(Q6), 312 m
2
g
1
(Q10), 207 m
2
g
1
(Q15), 107 m
2
g
1
(Q30) and
70 m
2
g
1
(Q50). Fig. 2 The total surface hydroxyl density of Q6, Q10 and Q50 silica as
received (0 h) and after rehydroxylation. The rehydroxylation was done by
placing silica in boiling water for 100 h. The specific surface area S.A. was
measured when as received (S.A. as received) and after rehydroxylation
(S.A. rehydroxylated).
Table 2 Total surface hydroxyl density of CARiACT Q15 when heated
under vacuum at pre-treatment temperatures of 325, 425 and 800 1C
Pre-treatment temperature/1C 325 425 800
Average a
OH
/nm
2
3.31 0.44 2.70 0.24 0.90 0.15
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Some of the absolute OH densities, such as rehydroxylated Q6
(with surface area change) and the dehydroxylated measurements
may be considered slightly higher than the literature (by less than
0.3 OH per nm
2
). Small differences between our results and those
presented in the literature are possible since the Q-series silicas
used here were briefly exposed to moisture from the atmosphere
once removed from the oven till full immersion in the D
2
O; there-
fore, through rehydration, some adsorbed surface water could
contribute to the OH density which would increase the a
OH
value
slightly. To reduce this effect, we minimised the exposure time;
alternatively, an inert atmosphere could be used at the expense of
experimental complexity.
A question that this technique does not answer is that of the
ratio of the total hydroxyl density of the silica to that of
exchangeable OH groups on the silica. The amount of intra-
skeletal or intraglobule (internal)
29
OH sites that are inaccess-
ible to D
2
O molecules cannot be determined by this method. It
is unlikely that such sites are involved in catalysis, as they
would be inaccessible to reactant molecules, and hence it is felt
that the question of quantifying the amount of intraskeletal/
intraglobule OH sites is rather superfluous. One final remark
regarding this technique is that it should, in theory, be possible
to measure the concentration of any exchangeable hydrogen
moiety providing (i) the D
2
O is in vast excess and (ii) a suitable
calibration of the NMR spectrometer is performed.
To summarise we have demonstrated a simple benchtop
1
H NMR based liquid deuterium exchange technique that is
able to measure the total surface hydroxyl density a
OH
of silica.
Fully hydroxylated silicas give a
OH
values between 4.16 and
6.56 OH per nm
2
which is in excellent agreement with the
literature. This technique is sensitive enough to measure
samples with low a
OH
values such as CARiACT Q15 dehydroxy-
lated at 800 1C with an a
OH
= 0.90 0.15 OH per nm
2
. It is also
evident that a correction step for specific surface area may be
required for accurate determination of OH densities by this
method, particularly if a sample has undergone any thermal
and/or chemical treatment. Overall, the methods described
demonstrates a similar performance and results to other tech-
niques used in the literature but has the significant advantages
in terms of speed and expense and has the potential to be used
by non-experts.
P. S. and C. P. would like to thank the EPSRC and Johnson
Matthey Plc. for funding this work under grant numbers
GR/R47523/01 and EP/R511870/1 respectively.
Conflicts of interest
There are no conflicts to declare.
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