An Unusually Acidic and Thermally Stable Cesium Titanate CsxTi2- yMyO4 (x = 0.67 or 0.70; M = vacancy or Zn)

Article abstract of DOI:10.1021/acs.inorgchem.9b00369

Proton-free, alkali-containing layered metal oxides are thermally stable compared to their protonic counterparts, potentially allowing catalysis by Lewis acid sites at elevated temperatures. However, the Lewis acidic nature of these materials has not been well explored, as alkali ions are generally considered to promote basic but to suppress acidic character. Here, we report a rare example of an unusually acidic cesium-containing oxide Cs_xTi_2-yM_yO₄ (x = 0.67 or 0.70; M = Ti vacancy or Zn). These lepidocrocite-type microcrystals desorbed NH₃ at >400 °C with a total acidity of 410 μmol g^-1 at a specific surface area of only 5 m² g^-1, without the need for lengthy proton-ion exchange, pillaring, delamination, or restacking. The soft and easily polarized Cs⁺ ion essentially drives the formation of the Lewis acidic site on the surfaces as suggested by IR of sorbed pyridine. The two-dimensional layered structure was preserved after the oxide was employed in the ethanol conversion at 380 °C, the temperature at which the protonic form could have converted to anatase. The structure was also retained after the NH₃ temperature-programmed desorption measurement up to 700 °C. The production of ethylene from ethanol, well-known to occur over acid sites, unambiguously confirmed the acidic nature of this cesium titanate.

Full text of DOI:10.1021/acs.inorgchem.9b00369

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
An Unusually Acidic and Thermally Stable Cesium Titanate
Cs Ti M O (x = 0.67 or 0.70; M = vacancy or Zn)
x
2−y y 4
,
†,‡
§
§
§
Tosapol Maluangnont,*§
Boonyawat Wuttitham, Panisa Hongklai, Pongsatorn Khunmee,
∥
,‡,§
Sorawat Tippayasukho, Narong Chanlek, and Tawan Sooknoi*
†
College of Nanotechnology, King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520, Thailand
‡
Catalytic Chemistry Research Unit, Faculty of Science, King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520,
Thailand
§
Department of Chemistry, Faculty of Science, King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520, Thailand
∥
Synchrotron Light Research Institute (Public Organization), 111 University Avenue, Muang District, Nakhon Ratchasima 30000,
Thailand
*
S Supporting Information
ABSTRACT: Proton-free, alkali-containing layered metal
oxides are thermally stable compared to their protonic
counterparts, potentially allowing catalysis by Lewis acid
sites at elevated temperatures. However, the Lewis acidic
nature of these materials has not been well explored, as alkali
ions are generally considered to promote basic but to suppress
acidic character. Here, we report a rare example of an
unusually acidic cesium-containing oxide Cs Ti2−yM O (x =
x
y
4
0
.67 or 0.70; M = Ti vacancy □ or Zn). These lepidocrocite-
−
1
2
−1
type microcrystals desorbed NH at >400 °C with a total acidity of ≲410 μmol g at a specific surface area of only 5 m g ,
3
+
without the need for lengthy proton−ion exchange, pillaring, delamination, or restacking. The soft and easily polarized Cs ion
essentially drives the formation of the Lewis acidic site on the surfaces as suggested by IR of sorbed pyridine. The two-
dimensional layered structure was preserved after the oxide was employed in the ethanol conversion at 380 °C, the temperature
at which the protonic form could have converted to anatase. The structure was also retained after the NH temperature-
3
programmed desorption measurement up to 700 °C. The production of ethylene from ethanol, well-known to occur over acid
sites, unambiguously confirmed the acidic nature of this cesium titanate.
INTRODUCTION
application window to relatively mild reaction temperatures,
■
12−16
17
mostly <100 °C.
the lepidocrocite-type H Ti
For example, Sasaki et al. reported that
□ O ·H O (x = 0.7, and □
Acidic materials of the Brønsted (proton-donating) or Lewis
type (electron-accepting) are of paramount importance in
industrial hydrocarbon chemistry, biomass conversion, and
synthesis of fine chemicals.
metal oxides (e.g., titanates and niobates) have received an
increasing amount of attention due to their compositional,
x
2−x/4 x/4
4
2
1
= Ti vacancy; H TO) layered crystals transformed to anatase
0.7
2
−4
at 450 °C with an extensive mass loss of ∼15 wt %. If the
protonic form was pillared with aluminum polyoxocations, the
layered structure was stable up to 500 °C, at which point
Brønsted acid sites disappeared while Lewis acid sites
Recently, protonated layered
5
−7
structural, and microstructural tunability.
The Brønsted
8
remained. A similar decomposition and layered collapse was
acidic properties depend on their two-dimensional (2D)
nature, where protons from the medium exchange with the
interlayer cations and are incorporated in the gallery. On the
other hand, the Lewis acidic properties that originated from
the electron-deficient species, either at the interlayer or at the
sheets, have been explored much less.
16
also observed in the structurally related H Ti O nanotubes.
2
3
7
These shortcomings in protonic layered titanates prompt us
to investigate the possibility of developing a proton-free, Lewis
acidic material. It is well-known that lepidocrocite-type alkali
18−21
titanate
is structurally robust and that the 2D layered
structure is preserved even upon being heated ∼100 °C above
To prepare 2D materials with Brønsted acid sites accessible
to probe molecules or reactants, proton exchange of such
22
the synthetic temperature. Lepidocrocite titanate has a
+
8
,9
10
general composition of A Ti M O (i.e., xA ·
solids is typically followed by pillaring, delamination, and
x
2−y
y
4
x−
5
[
Ti2−yM O ] ), where x = y(4 − n) is the nominal charge
per formula unit; n < 4 is the oxidation state of M, and n = 0
restacking or the synthesis is conducted in the presence of a
y
4
11
structure-directing agent. Otherwise, they typically exhibit
low acidity as a result of relatively large particles often in the
form of microcrystals. In addition, the low thermal stability of
the layered structure containing Brønsted sites limits the
Received: February 8, 2019
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b00369
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
1
7
for M = □. In this structure, the corrugated double-edge-
shared (Ti,M)O6 octahedra join with negatively charged
sheets, extend through the a−c (basal) plane, and stack
reported method with the acid renewed each day. The resulting
material was dried overnight at room temperature. All materials
synthesized in this work were characterized as described in detail in
the Supporting Information.
along the b-direction. The sheets are staggered such that b =
Temperature-Programmed Desorption. For NH TPD, ∼0.2
g of the sample was loaded into the center of a quartz tube reactor and
covered with quartz wool. The sample was activated at T values of
+
3
2
1
d020, with A ions situated in the interlayer space (see Figure
a,b). The Ti/M atoms potentially serve as the Lewis acid
a
sites, especially if they are on the accessible surfaces.
300, 400, and 500 °C for 2 h and cooled to ∼30 °C (both under air
−
1
flowing at a rate of 10 °C min ), followed by the adsorption of 1%
−
1
NH /He (50 mL min ) for 1 h. Then, physisorbed NH was purged
3
3
−
1
with helium (30 mL min ) until the baseline as detected by the
thermal conductivity detector was flat, usually for 1 h. The profile was
−
1
next collected from 50 to 700 °C (10 °C min ) under a flow of
helium. The acidity is expressed as micromoles of NH per gram of
3
the solid, which was quantified by comparison to H-ZSM-5 (PZ-2/
1000H, Zeochem; nominal Si/Al = 500). See the Supporting
Information for details.
Isopropylamine (IPA) TPD, which is specific to only Brønsted acid
Figure 1. Crystal structure of A Ti M O shown (a) as a polyhedral
31,32
x
2−y
y
4
sites,
was performed as follows. An aliquot of 1 μL of IPA was
representation on the a−b plane and (b) as a ball-and-stick model on
repeatedly injected, at RT, onto 0.1 g of Cs TO (activated at 400 °C
23
0.7
the a−c plane. Crystal structures were drawn with VESTA. (c)
−1
under air) under the flow of He (30 mL min ). The total amount of
Representative scanning electron microscopy image of Cs TO.
0
.7
injected IPA was 10 μL. Then, the purging and temperature-
programmed desorption was conducted as described above for NH3
TPD. Propylene, which is the decomposition product of IPA over
Brønsted acid sites, was detected by a flame ionization detector
FID). A similar experiment was performed with H-ZSM-5.
Catalytic Activity Testing. The sample (pressed and sieved to
Our design principle is that the acidic nature of a proton-free
lepidocrocite titanate could be tuned by the inductive effect of
both M at the sheets and the interlayer A cations. If so, a Lewis
acidic material (i.e., relative to the pristine, unmodified
composition) capable of withstanding high temperatures
(
the 600−850 μm fraction) was packed into a vertical glass tube
reactor and covered with glass wool and glass beads. Prior to activity
−
1
testing, the sample was heated from RT to 400 °C (10 °C min ),
(
relative to the protonic form) might be obtained. Reports
held for 2 h, and cooled to 380 °C (all under air flowing at a rate of 30
of such modifications, however, are rather limited. Relevant
−
1
mL min ). Then, 99.99% absolute ethanol (Carlo Erba) was fed into
examples include the ion exchange of H TO with an aqueous
0
.7
−1
24
the reactor by a syringe pump at a rate of 1 mL h . N was used as a
2
solution of the metal ions or the deposition of metal
nanoparticles onto the surfaces of H Ti O nanotubes, yet
−1
carrier gas at a rate of 60 mL min , corresponding to a contact time
(W/F) of 58 g h mol . The catalytic testing was conducted for a total
25
−1
2
3
7
the enhanced acidic nature has not been fully demonstrated.
Quite unexpectedly, we found in this work that the cesium
time on stream of 6 h. The products were periodically analyzed using
an online BUCK Scientific 910 gas chromatograph equipped with the
FID and the FFAP column (30 m × 0.25 mm × 0.25 μm), by
comparison with the retention time and peak area of the standard at a
known concentration.
titanates Cs Ti
□ O (x = 0.67 or 0.70; M = □; Cs TO)
x
2−x/4 x/4
4 x
(x = 0.70; Cs ZnTO), directly
and Cs Ti2−x/2Znx/2
O
x
4 0.7
obtained right after solid state synthesis, were unusually acidic
without the need for proton−ion exchange, pillaring,
delamination, or restacking. These results provide an
interesting example contrasting the general view that soft/
polarizable cesium ions suppress acidic while promoting basic
RESULTS AND DISCUSSION
■
Structural Characteristics. The samples are white,
platelike microcrystals with a particle size of ∼0.5 μm as
shown in Figure 1c for Cs TO as a representative. Figure 2
2
6−29
character.
Acidity and the acidic nature of the micro-
0
.7
crystals of these compositions were evaluated by NH and
3
shows the X-ray diffraction (XRD) patterns of the investigated
isopropylamine temperature-programmed desorption (TPD),
Fourier transform infrared spectroscopy of adsorbed pyridine,
and X-ray photoelectron spectroscopy (XPS). Effects of the Ti
vacancy sites and activation temperature on acidity, including
the stability of the cesium titanate, were also investigated.
Consistent with the observed acid function, the catalytic
activity of the cesium titanate Cs Ti2−x/4□x/4O4 toward
x
conversion of ethanol is highlighted.
EXPERIMENTAL SECTION
■
Synthesis. The samples were prepared following the reported
17,19,21
procedure,
by first calcining the stoichiometric mixture of
A CO (A = K or Cs), anatase-type TiO , and ZnO (when necessary)
2
3
2
at 800 °C for 1 h for decarbonation. Next, the mixture was ground
and subjected to two cycles of calcination (20 h each) at 800 °C
1
7
(
Cs0.7Ti
□0.175O , Cs0.7TO),
9
900 °C (K0.8Zn Ti1.6O4,
1.825
4
0.4
21
1
K ZnTO), or 950 °C (Cs Zn0.35Ti1.65O , Cs ZnTO), all with
intermediate grinding. For Cs0.67Ti1.8325□0.1675O (Cs0.67TO), nano-
0.8
0.7
4
0.7
4
30
sized P25 TiO2 (Degussa, 99.70%) was used as the Ti source
instead of anatase to increase the specific surface area. Proton
exchange of Cs TO to H Ti1.825□0.175O ·H O (H TO) was done
Figure 2. XRD patterns of the cesium titanate samples investigated in
this work, including the respective hkl indices shown atop the peak.
The times signs denote the 011 reflections that overlap or separate
0.7
0.7
4
2
0.7
18,21
three times at room temperature (RT) with 1 M HCl, following the
from the nearby 060 reflection, depending on the composition.
B
DOI: 10.1021/acs.inorgchem.9b00369
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 1. Unit Cell Parameters of the Samples
a (Å)
b (Å)
c (Å)
ref
Cs0.7TO (Cs0.7Ti1.825□0.175O4)
3.815(4)
17.46(2)
17.198(3)
17.54(5)
17.38(2)
17.215
17.12(6)
17.0205(4)
15.67(1)
18.43(3)
18.735(8)
2.961(6)
2.960(1)
2.957(3)
2.939(7)
2.955
2.974(1)
2.9837(1)
2.981(2)
2.953(5)
2.978(2)
this work
17
3
.837(1)
Cs0.7TO-NH (after NH TPD)
3.819(5)
3.803(4)
this work
this work
33, 34
this work
21
36, 38
this work
17
3
3
Cs0.67TO (Cs0.67Ti1.8325□0.1675O4)
3
.823
3.7949(8)
.8143(1)
Cs0.7Zn0.35TO (Cs0.7Zn0.35Ti1.65O4)
3
K0.8ZnTO (K0.8Zn0.4Ti1.6O4)
3.809(3)
3.790(3)
H0.7TO (H0.70Ti1.825□0.175O ·H O)
4
2
3
.783(2)
samples that are all characteristics of the lepidocrocite-type
cesium titanate (Immm). The 011 reflection (× in Figure 2)
typically overlaps with the 060 reflection in the □-containing
similar local environment, either prior to or after the NH TPD
measurement.
3
For comparison, the protonic form H0.7TO (i.e.,
sample Cs TO (x = 0.70 or 0.67), which is in agreement with
H
0.70Ti1.825
□
0.175
O
4
·H
2
O) was prepared via repeated equilibra-
x
17
1
8
tion of Cs TO with HCl. The XRD pattern of H TO and
the report by Grey et al. Upon substitution of Zn for Ti in
0.7
0.7
the corresponding unit cell parameters shown in Figure S1a
Cs ZnTO, these two peaks are clearly separated; this finding
0
.7
17
and Table 1, respectively, agree with the literature. This
is also consistent with the calculated XRD pattern reported by
2
1
material with the expanded interlayer separation (d020 = b/2 ∼
Gao et al. Table 1 lists the corresponding unit cell
parameters, which agree reasonably with the reported
9
.2 Å) contains the interlayer Brønsted acid sites and the water
12−15,17
1
7,21,33,34
molecule.
However, after the NH TPD measurement,
3
41,42
values.
(The differences might be due to small
the characteristic Raman vibrations
of the protonic
variations in stoichiometries from different laboratories.)
3
5
lepidocrocite (H TO) completely disappeared as shown in
0
.7
Taking Cs TO as an example, we used CellCalc to
0
.7
Figure 3b. Instead, several peaks indicative of anatase-type
determine the following unit cell parameters: a = 3.815(4)
Å, b = 17.46(2) Å, and c = 2.961(6) Å, with an interlayer
43
TiO2 are visible, confirming the lepidocrocite-to-anatase
transformation.
spacing d020 (=b/2) of ∼8.6 Å. This distance in Cs TO is >7.8
0.7
Such a limited thermal stability of H TO can also be
19,22,36
0.7
Å in the potassium analogue K ZnTO
but is <9.4 Å in
0
.8
deduced from the massive mass loss [∼15 wt % (Figure 4)]
37
the related structure, e.g., Cs Ti O .
2
5
11
Thermal Stability. Figure 2 also compares the XRD
patterns of Cs TO to those of Cs TO-NH (i.e., after the
0
.7
0.7
3
NH TPD measurement where a temperature of ≤700 °C was
3
reached). Importantly, the similarity in these two patterns
suggests the preservation of the lepidocrocite-type structure.
Using the full width at half-maximum of the 020 peak and the
Scherrer equation, an almost identical crystallite size D was
obtained (42 nm for Cs TO and 38 nm for Cs TO-NH ),
0.7
0.7
3
also indicating minimal structural changes. In addition, the
Raman spectra of the two samples shown in Figure 3a are
3
9,40
generally similar and show bands characteristic
of the
Figure 4. Mass loss curve of several samples recorded under N . The
2
lepidocrocite-type cesium titanate. These bands are typically
dashed lines at 300, 400, and 500 °C indicate the activation
39,40
assigned to the Ti−O stretching vibration of the layers.
temperature T where the NH TPD measurements were carried out.
a
3
−
1
The preservation of the peak at 914−917 cm due to Cs−O
3
9,40
+
stretching
in particular suggests that Cs ions persist in a
upon heating to 700 °C in a separate thermogravimetric
17
analysis. This mass loss has been ascribed to dehydration and
dehydroxylation, accompanied by the subsequent collapse of
the lepidocrocite-type layers and the condensation to anatase.
On the other hand, several compositions of the cesium titanate
show a limited loss (∼2.5−5.0 wt %), which is ascribed to the
liberation of water on the external surfaces. Altogether, these
results highlight the superior thermal stability of the cesium
analogue with respect to the protonic form.
Acid Strength and Acidity. The acid strength and acidity
of the titanate were first investigated by NH TPD after
3
activation of the samples at 400 °C, where the contribution
from surface water can be excluded (Figure 4). The variation
in maximum peak temperature T was within 10 °C, while the
Figure 3. Raman spectra of (a) Cs TO and (b) H TO, either (i)
m
0
.7
0.7
prior to or (ii) after the NH TPD measurement where a temperature
accuracy in acidity was ∼15% (see the Supporting Information
3
of ≤700 °C was reached.
for details).
C
DOI: 10.1021/acs.inorgchem.9b00369
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
As shown in Figure 5, the Cs TO with Ti vacancy □
that anatase-type TiO desorbs NH only at the LT (T ∼ 130
0
.7
2
3
m
desorbs NH in two ranges at 100−300 °C (low-temperature,
°C) without any HT peak. Even for HZSM-5 (Si/Al = 500), its
LT desorption and HT desorption occur at a temperature
slightly lower than that from the cesium titanate samples. For
the protonic H TO with Brønsted acid sites, the layer collapse
3
0
.7
17
and structural transformation (as deduced from the mass loss
curve in Figure 4) certainly contribute to the desorption
49
profile, including that reported previously. Therefore, the
acidic nature of H TO will not be discussed further.
0
.7
Table 2 summarizes the total acidity, which is a combination
of LT and HT acidity. At an activation temperature of 400 °C,
−
1
the acidity of Cs TO (∼280 μmol g ) is ∼5 times larger than
0
.7
that of TiO with a similar S . The small specific surface area
SBET (here ∼5−6 m g ) is rather common for the samples
prepared via a traditional, high-temperature solid state
2
BET
2
−1
synthesis with the micrometer-sized TiO as the reactant. It
2
indicates that the microcrystals are essentially regarded as
nonporous materials and that N adsorption/desorption would
2
mostly occur at the external surfaces. Therefore, the high
acidity observed for Cs TO despite a low S suggests NH3
0
.7
BET
adsorption at the internal surfaces (i.e., intercalation),
requiring in situ XRD for further characterization.
The acidity of the □-containing Cs TO is also found to
0
.7
Figure 5. NH TPD profiles from the samples activated at 400 °C.
The profiles were normalized so that the LT peak is of the same
height. Dashed lines in the HT range are magnified by a factor of 5.
3
increase with a decrease in activation temperature T , reaching
a
−1
∼
410 μmol g at T = 300 °C as summarized in Table 2
a
(
entries 1a−1c). This behavior is in sharp contrast to TiO
2
exhibiting a relatively constant (but low) acidity at T values of
LT) and >400 °C (high-temperature, HT). The respective Tm
values are ∼145 and 470 °C. Similar behavior was observed in
a
4
00 and 700 °C (entries 5a and 5b). [The acidity of TiO2
reported here is within the range of literature values (Table
other compositions tested but with a higher T , including
m
S1).] A different HT desorption pattern for Cs TO activated
Cs0.67TO (also with □) and Cs ZnTO [where the □ sites
0.7
0.7
44−46
at different temperature (Figure 6a) suggests a complex
alteration of surface functionals (e.g., Ti vacancy □) or surface
were filled with Zn (see below)]. While it is well-known
that T is sensitive to several parameters, the desorption of
m
2
2,50,51
rearrangement.
to test these hypotheses. The acidity increased in part with an
increase in S , as demonstrated for Cs0.67TO prepared from
In situ surface analyses will be required
NH from the cesium titanate samples at >400 °C strongly
3
suggests the presence of strong acid sites. The increased acidic
strength of Cs ZnTO (i.e., a higher T especially in the HT
BET
0
.7
m
3
0
range) might be explained by the electron-withdrawing effect
the nanosized P25 TiO (Table 2, entry 2).
2
47,48
21
of Zn, as inferred from Sanderson’s electronegativity
2.223 for Zn and 1.50 for Ti). Considering that the
When the Ti vacancy sites (□) were filled by Zn, the total
acidity of Cs0.7ZnTO is decreased to half of that of Cs0.7TO at
a similar SBET (Table 2). For example at an activation
temperature of 300 °C, the LT acidity and HT acidity are 100
(
2+
intercalated NH3 in Co -exchanged H TO was earlier
released at <300 °C, our results demonstrate the remarkable
acidic nature of the cesium titanate.
0.7
24
−
1
and 108 μmol g , respectively, for Cs ZnTO but 347 and 64
0
.7
−
1
The acidic strength of these cesium titanates appears to be
higher than that observed in relevant samples. It was found
μmol g , respectively, for Cs TO. This behavior was
0.7
observed at all T values studied as also shown in Figure 6b
a
Table 2. Summary of Specific Surface Areas (SBET), Acidities at Low (LT) and High Temperatures (HT), Including the
Maximum Peak Temperature T , and Total Acidities As Determined by NH TPD
m
3
a
SBET (m g−1)
2
LT acidity (μmol g−1) [Tm (°C)]
HT acidity (μmol g−1) [Tm (°C)]
total acidity (μmol g−1
)
entry
sample
1
1
1
2
3
3
3
4
5
5
6
7
a
b
c
Cs0.7TO-300
Cs0.7TO-400
Cs0.7TO-500
Cs0.67TO-400
Cs0.7ZnTO-300
Cs0.7ZnTO-400
Cs0.7ZnTO-500
K0.8ZnTO-400
5
5
5
15
6
6
6
3
347 [143]
264 [145]
252 [133]
397 [152]
100 [139]
74 [148]
66 [145]
not detected
60 [126]
64 [418]
15 [470]
not detected
60 [557]
108 [421]
61 [566]
410
279
252
458
206
135
72
not detected
60
56
311
392
a
b
c
6 [586]
not detected
not detected
not detected
178 [512]
30 [400]
a
b
TiO -400
6
6
2
TiO -700
56 [132]
133 [121]
361 [150]
2
H0.7TO-400
HZSM-5−400
not available
350
a
Cs TO (Cs Ti2−x/4□x/4O , where x = 0.67 and 0.70), Cs ZnTO (Cs Zn0.35Ti1.65O ), K ZnTO (K Zn Ti O ), and H TO
x
x
4
0.7
0.7
4
0.8
0.8
0.4 1.6
4
0.7
(
H Ti1.825□0.175O ·H O). The number after the name indicates the activation temperature T (in degrees Celsius) prior to NH adsorption.
0.7
4
2
a
3
D
DOI: 10.1021/acs.inorgchem.9b00369
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Inorganic Chemistry
Article
Possible Nature of Acid Sites. Figure 7 shows the IR
spectra of pyridine-sorbed Cs TO and Cs ZnTO. The
0
.7
0.7
Figure 6. Temperature profiles of desorption of NH3 from (a)
Cs TO and (b) Cs ZnTO activated at T values of 300, 400, and
0.7
0.7
a
5
00 °C. Each profile was normalized by the mass of the sample.
Figure 7. ATR spectra of pyridine-sorbed Cs TO and Cs ZnTO
0
.7
0.7
with the vibrations characteristic of pyridine adsorbed on Lewis (L),
Brønsted (B), or the combination of these two sites (L+B) indicated.
and in Table 2 (entries 3a−3c). In fact, the decrease in total
acidity upon Zn substitution is due to a notable decrease in LT
acidity, which is accompanied by a moderate increase in HT
acidity. This result suggests that Ti atoms at the proximity of
−
1
strong peak at ∼1440 cm typical of pyridine bound to Lewis
14,58
acid sites
is clearly observed. On the contrary, the signal
the vacancy sites could be specifically responsible for NH LT
characteristics of pyridine bound to Brønsted acid sites is
absent in these two samples. For K ZnTO, we can observe
3
desorption. (The formation of hydroxylated groups due to
0
.8
water adsorption around the □ sites is energetically
only vibrations due to the titanate itself, without any signal due
to sorbed pyridine. This result is in good agreement with the
5
2,53
favorable;
however, the adsorption of NH has not been
3
reported to the best of our knowledge.) Importantly, the
observed increase in acidity upon HT desorption in
flat NH TPD profile mentioned previously. In addition, the
3
TPD experiment with isopropylamine that is specific to
31,32
Cs ZnTO indicates the presence of other types of acid
0
.7
Brønsted acid sites
also showed virtually zero adsorp-
sites, which are not associated with the □ sites (also see the
XPS results below).
tion/desorption (not shown). Accordingly, one can deduce
that Cs TO and Cs ZnTO are the Lewis acids. Because the
0
.7
0.7
The acidic nature of the cesium titanate microcrystals
reported here suggests the novel bifunctional property of these
materials, because only the basic nature of the lattice oxygen
has been reported so far for lepidocrocite-type
+
interlayer Cs ions cannot be the Lewis acid center, it is most
4+
n+
likely that the Ti /M ions on the accessible surfaces of the
sheets (i.e., basal plane or edges) are the acid sites. However,
our attempt to detect the surface Ti species by DRUV was
unsuccessful, likely due to the small SBET of the samples and
consequently the small amount of such species. A careful
surface-sensitive analysis will be further required, which will
provide not only the information about the local structure but
also the possible correlation (if any) with catalytic activities in
the future contribution.
3
6,38
K M Ti2−yO4.
The CO2 TPD measurements were
x
y
performed, and the results are summarized in Figure S2 and
Table S2. Although Cs TO exhibits a typical basicity, the
0.7
(
desorbed NH )/(desorbed CO ) ratio that approximately
3 2
represents the contribution of acidic to basic sites is 1.6. This
ratio as determined in a similar manner for Cs ZnO is 2.9.
0.7
Clearly, these numbers are significantly higher than that of
Accordingly, the Ti 2p XPS spectra of some samples were
recorded and are compared in Figure 8. Note that the Ti 2p
signals are the average of all surface Ti species probed (analysis
area of 300 μm × 300 μm), including Ti at the proximity of □
(and Zn) and Ti that is away from it. In all cases, the binding
TiO (0.9). Again, this result emphasizes the unusually acidic
2
nature of the lepidocrocite-type cesium titanate, contrasting
the basic site-dominating surfaces in other titanates such as the
54
ATiO (A = Sr or Ba) perovskite.
3
+
The essential influence of polarizable Cs ions toward the
4
+
21,22,49,59,60
energy indicates the presence of Ti as anticipated.
acidic properties is further emphasized by comparing the
2
1
Specifically, the □-containing Cs TO shows the peaks at
0.7
□
-free Cs ZnTO (Cs Zn0.35Ti1.65O ; space group Immm;
0.7 0.7 4
2
−1
459.3 eV (Ti 2p3/2) and 465.0 eV (Ti 2p1/2), which are more
SBET = 6 m ·g ) to K ZnTO [K Zn Ti O ; space group
Cm2c1; SBET = 3 m g (Figure S1b)]. It is clear that, while
0.8
−1
0.8
0.4 1.6
4
1
9
2
positive by ∼1 eV than the corresponding signals in TiO
2
(
458.4 and 464.1 eV, respectively). In the Zn-substituted
Cs ZnTO shows considerable NH adsorption/desorption,
0
.7
3
Cs ZnTO, the Ti 2p signals at binding energies (459.3 and
the NH adsorption/desorption from K ZnTO was com-
0.7
3
0.8
4
65.1 eV) similar to those of Cs TO were observed. (The
pletely diminished, resulting in a flat TPD profile (not shown).
This point will be discussed below with regard to the nature of
the acid sites.
0.7
presence of another component at 457.8 and 463.5 eV might
also suggest other sorts of acid sites.) The electropositive
nature of surface Ti atoms in Cs TO and Cs ZnTO in
Altogether, while the SBET of the cesium titanate is rather
low and has not yet been optimized, their acidity is comparable
0.7
0.7
relation to TiO agrees qualitatively with the results from NH
2
3
22
5
5−57
TPD (Figure 5). For K ZnTO, the majority of the Ti 2p
to that of typical acid zeolites
protonic titanate samples
or some high-acidity
0.8
9
,11,12
with a much larger SBET. This is
peak (458.1 eV, Ti 2p3/2; 463.8 eV, Ti 2p1/2) is close to that in
rather exceptional because our samples are in the form of
microcrystalswithout the need for lengthy protonation/ion
exchange, pillaring, delamination, or restackingand have
virtually no porosity. Additional examples comparing the acidic
strength and acidity in our samples versus those reported in the
literature can be found in Table S1.
TiO . Clearly, the surface Ti species in K0.8ZnTO are not as
2
electropositive as those in Cs0.7TO and Cs0.7ZnTO. The
notable difference in the binding energies of Ti 2p in
Cs0.7ZnTO and K0.8ZnTO might be related to the type of
interlayer cation, composition, and also space group. (The
latter can be seen from the XRD pattern in Figure 2 vs Figure
E
DOI: 10.1021/acs.inorgchem.9b00369
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
63,64
Guebert reaction).
The acidic Cs TO was chosen as a
0
.7
representative sample, in comparison to MgO, which is well-
known to predominantly exhibit a basic character. Ethanol
conversion and product yields as a function of time on stream
in a flow reactor at 380 °C are shown in Figure 9.
Figure 9. Conversion of ethanol and the percent yield of products
over Cs TO (left) and MgO (right). Reaction conditions: activation
0
.7
Figure 8. Ti 2p XPS spectra of anatase-type TiO , Cs TO,
in air at 400 °C for 2 h, reaction temperature of 380 °C, contact time
2
0.7
−
1
Cs ZnTO, and K ZnTO.
of 58 g h mol , atmospheric pressure, and flow rate of carrier gas
0.7
0.8
−
1
(
N ) of 60 mL min .
2
S1b, where the peak with the highest intensity is from the
different hkl index.) A better understanding might be obtained
by studying the compositions with mixed interlayer alkali ions
The initial conversion is 7.1% (Cs TO) versus 30.0%
0.7
2
(
g
MgO), which can be ascribed to the difference in S (5 m
BET
−
1
2
−1
vs 63 m g ), suggesting that the observed catalytic
(
i.e., Cs0.7−xK Zn0.35Ti1.65O4 vs Cs Zn0.35Ti1.65O ); this is
x 0.7 4
activity is contributed mainly by the external surfaces. Notably,
being investigated.
The relative distribution of Ti4+ might be indirectly inferred
from the compositional analyses of the Cs TO microcrystals
the major product over Cs TO is ethylene, which could be
0.7
formed over acid sites. On the other hand, butanol, which is
0.7
collectively formed over acid−base pairs, is the major product
(
i.e., Cs Ti1.825□0.175O ) where the nominal Ti/Cs ratio
0.7 4
over MgO. (The yield of butanol is ∼0.1% over Cs TO.)
equals 2.60. The surface Ti/Cs ratio (as determined by XPS)
was 4.1 ± 0.5, larger than that in the bulk (2.94 ± 0.02 for XRF
and 2.9 ± 0.2 for EDX). That is, Ti termination and/or surface
Ti enrichment was observed in Cs TO, potentially rendering
0.7
Clearly, the relative abundance of acidic to basic sites dictates
the product distribution. A full account of catalytic activity
measurements will be reported elsewhere.
0.7
Figure S1c compares the XRD patterns of the Cs TO
the acidic character. For Cs ZnTO (Cs Zn0.35Ti1.65O ), the
0.7
0.7
0.7
4
(
Ti+Zn)/Cs ratios obtained from different techniques were
similar: 2.86 (nominal value), 2.90 (XRF), 2.95 (EDX), and
.30 (XPS). This result suggests a more homogeneous
before and after use (“Cs0.7TO-Spent”) as a catalyst in this
reaction. The layer structure is retained despite a halo and the
slight peak broadening and/or a reduction in crystallite size (D
= 26 nm). This is presumably due to the partial desegregation
of some microcrystal layers by water (i.e., steaming) produced
in situ via ethanol dehydration, yet the possibility of using
Cs0.7TO at a reaction temperature as high as 380 °C greatly
expands the temperature limit from that typically em-
2
distribution of elements on the surface and in the bulk of
Cs ZnO.
0
.7
Similar analyses were conducted on the K ZnTO, but it
0.8
was found that the surface of this material lacks Ti and Zn.
This statement was deduced from the surface (Ti+Zn)/K ratio
that is as low as 1.50, compared to the nominal value of 2.5
for K Zn Ti O .
The low concentration of surface Ti (and Zn) in the non-
acidic K ZnTO contrasts with the high concentration of
surface Ti in Cs TO or the homogeneous distribution of
12−16
22
ployed
in the protonic form (<100 °C).
0
.8
0.4 1.6
4
CONCLUSION
■
In conclusion, we showed that Cs Ti M O (x = 0.67 or
0
.8
x
2−y
y
4
0.70; M = □ or Zn) microcrystals were unusually acidic as
compared to common metal oxides. This is exceptional for
layered metal oxides because (i) protonation/ion exchange,
pillaring, delamination, restacking, or lengthy synthetic
procedures are not required, (ii) cesium ions that are typically
regarded as the promoter for basic character indeed enhance
0
.7
elements in Cs ZnTO. This result hints at the essential role of
0
.7
+
the large and polarizable Cs ions in the interlayer space
toward surface relaxation, in combination with the presence of
the Ti vacancy sites □ and/or the substituted atom. Note that
this is the surface characteristic only, because XRD and Raman
spectroscopy have already confirmed the preservation of the
lepidocrocite structure at long and short range, respectively.
Catalytic Activity. Ethanol was employed as another probe
molecule to further illustrate the dominating acid character in
the lepidocrocite-type cesium titanate; acid sites convert
acidic character, and (iii) the microcrystals have a small SBET
.
The Lewis acidic nature was confirmed by IR of sorbed
4
+
pyridine and very likely is related to the distribution of Ti /
n+
M
species on the surface as suggested by XPS. Meanwhile,
the acidity depends on several factors, including the presence
of Ti vacancy □, the inductive effect of M (i.e., Zn), SBET, and
the activation temperature. The soft and polarizable cesium
61,62
ethanol to ethylene and diethyl ether,
pairs couple two molecules of ethanol into butanol (i.e.,
while acid−base
F
DOI: 10.1021/acs.inorgchem.9b00369
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
ions (but not the potassium analogue) were found to be
essential in this unusually acidic nature. The production of
ethylene from ethanol over Cs TO supports the presence of
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0.7
active acid sites. Importantly, the lepidocrocite-type layered
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(
11) Lee, M.; Seo, Y.; Shin, H. S.; Jo, C.; Ryoo, R. Anatase TiO
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Microporous Mesoporous Mater. 2016, 222, 185−191.
(
(12) Kitano, M.; Nakajima, K.; Kondo, J. N.; Hayashi, S.; Hara, M.
octahedra) will exhibit this unusually acidic property, including
the rationale behind such behavior, is interesting.
Protonated titanate nanotubes as solid acid catalyst. J. Am. Chem. Soc.
2010, 132, 6622−6623.
(13) Kitano, M.; Wada, E.; Nakajima, K.; Hayashi, S.; Miyazaki, S.;
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b00369.
Kobayashi, H.; Hara, M. Protonated titanate nanotubes with Lewis
and Brønsted acidity: relationship between nanotube structure and
catalytic activity. Chem. Mater. 2013, 25, 385−393.
■
*
S
(
14) Wada, E.; Kitano, M.; Nakajima, K.; Hara, M. Effect of
preparation conditions on the structural and acid catalytic properties
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12774.
Characterization, additional XRD patterns, table of
acidities, CO TPD profiles, and basicities (PDF)
(15) Kitano, M.; Kobayashi, H.; Hayashi, S.; Hara, M. Acid
properties of protonated titanate nanotubes. J. Jpn. Pet. Inst. 2017, 60,
2
1
(
13−120.
AUTHOR INFORMATION
Corresponding Authors
E-mail: tosapol.ma@kmitl.ac.th. Telephone: +66 9 0992
076. Fax: +66 2 329 8265.
E-mail: kstawan@gmail.com. Telephone: +66 8 1929 8288.
Fax: +66 2 326 4415.
ORCID
Tosapol Maluangnont: 0000-0002-6213-5539
Notes
The authors declare no competing financial interest.
■
16) de Carvalho, D. C.; Oliveira, A. C.; Ferreira, O. P.; Filho, J. M.;
Tehuacanero-Cuapa, S.; Oliveira, A. C. Titanate nanotubes as acid
catalysts for acetalization of glycerol with acetone: influence of the
synthesis time and the role of structure on the catalytic performance.
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*
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x 4
1
ACKNOWLEDGMENTS
̀
structure
riques
■
́
This work was financially supported by the Thailand Research
Fund (MRG6080004). The assistance of Korawich Trangwa-
chirachai and Kittipong Prakobtham (isopropylamine TPD) is
gratefully acknowledged. T.M. thanks Dr. Adisak Boonchun
x
y
2‑y
4
(
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DOI: 10.1021/acs.inorgchem.9b00369
Inorg. Chem. XXXX, XXX, XXX−XXX