Anion-controlled cation-exchange process: Intercalating α-Titanium phosphate through direct ion exchange with alkylammonium salts

Article abstract of DOI:10.1021/acs.inorgchem.7b03030

Several alkylammonium salts were used in the study of α-Titanium phosphate (α-TiP) intercalation chemistry. The characterization results demonstrated that the expected intercalation by direct ion exchange could be successfully achieved without any additio

Full text of DOI:10.1021/acs.inorgchem.7b03030

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Anion-Controlled Cation-Exchange Process: Intercalating α‑Titanium
Phosphate through Direct Ion Exchange with Alkylammonium Salts
Chunyang Wang,† Qingyan Cheng,*,† and Yanji Wang
Key Laboratory of Green Chemical Technology and High Efficient Energy Saving of Hebei Province, School of Chemical
Engineering, Hebei University of Technology, Tianjin 300132, People’s Republic of China
ABSTRACT: Several alkylammonium salts were used in the study of α-
titanium phosphate (α-TiP) intercalation chemistry. The characterization
results demonstrated that the expected intercalation by direct ion exchange
could be successfully achieved without any addition of an extra amine
substance. Our findings are different from the current opinion that by the
ion-exchange method, without the assistance of bases, large cations are
difficult to exchange into the narrow interlayer space of α-tetravalent metal
phosphate directly because of the small interlayer distance. Studies found
that alkylammonium cations, for example, n-butylammonium cation, could
be directly exchanged into the interlayer space merely by choosing salts
with appropriate anions such as phosphate, phosphite, sulfite, citrate, and malate ions. In the case of phosphates, besides n-
butylammonium, the exchange of n-hexylammonium, cyclohexylammonium, and pyridinium with interlayer protons was
investigated and successfully accomplished as well. The uptake values for these four cations were 0.420, 0.595, 0.571, and 0.335
g/g, respectively. A mechanism study revealed that although the relevant exchange reaction seemed only to involve the proton of
α-TiP and the alkylammonium cation of the salt, the strength of the conjugate acid of the anion from the salt−the counterion−
was proven to be the key factor in this process.
on the molecular structure and relevant functional groups it
bears, α-MPs intercalated with different structured amines will
present diverse choices in the interlayer distance for the
following introduction of large sized objects, making them
more flexible in practical applications than Na+-exchanged
precursors. Usually they can be acquired through directly
exposing α-MPs to an amine vapor or solution,11,20−23 resulting
in the amine molecules combined with interlayer protons,
which is considered to be a reaction of a Brønsted acid and
base. Another way to induce intercalation is to treat α-MP with
an alkylammonium salt solution in order to have the cation
moiety of the salt exchanged into the interlayer space.11,24
However, it was not found to be very successful in that
alkylammonium salts themselves, unless with amine additives,24
could not trigger the ion-exchange process due to the large size
of their cations.25 This is normally attributed to the
introduction of OH−, which is produced from additionally
added amine. According to the published article,26 small
hydroxide ions can diffuse into the interlayer space and then
create sorption sites through the reaction O3PO−H + OH− →
O3PO− + H2O, consequently providing enough energy to move
metal phosphate layers apart so that the large cations can be
allowed to reach the interlayer space. The added basic
substance is supposed to be the actual activator of this process,
and alkylammonium salts themselves only play the role of
supplying alkylammonium cations. Therefore, whether it is
possible to activate ion exchange by alkylammonium salts
1. INTRODUCTION
Nanostructured layered compounds have attracted a great
amount of attention in recent years due to their unique physical
and chemical properties and find wide applications in fields of
adsorption, catalysis, electrochemistry,1−7 and so forth. One of
the interesting features that the layered compounds have is that
exotic species are capable of being introduced into their
interlayer spaces among laminates, bringing them new
specialties and leading to the extension of their applications.8,9
For example, the properties of α-tetravalent metal phosphates,
a well-known category of layered compounds with a chemical
formula of α-M(HPO4)2·H2O (α-MP, M = Ti, Zr, Sn, Ge, Pb,
etc.),10 can be modified in this way according to various needs.
Its characteristic sandwiched structure, composed of negatively
charged metal phosphate laminates [M(PO4)2]n2n− and
positively charged interlayer protons, makes it possible to
introduce exotic species into their interlayer spaces either by
combination with the interlayer protons or through a cation-
exchange process.11−13 However, due to the hindrance of the
narrow interlayer distance, which is about 0.75 to 0.78 nm,11 it
is generally acknowledged that large species cannot be
exchanged into α-MP directly. Measures to increase the
interlayer distance must be taken before their introduction.11,14
For example, α-ZrP half exchanged with small metal cation
Na+ (ZrNaH(PO4)2·5H2O15) or intercalated with an amine can
provide a larger interlayer distance, thus they are often used as
the precursors for the further uptake of species such as a metal
ion with a large ionic radius,16 a metal oxide cation,17 an
alkanol/glycol,18 a quaternary ammonium cation,19 etc. In the
case of amine intercalation, since its size is directly dependent
Received: November 30, 2017
© XXXX American Chemical Society
A
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themselves and achieve consequent intercalation of α-MP is still
an interesting question.
3. RESULTS AND DISCUSSION
3.1. Structural Analysis of Alkylammonium Cation-
Exchanged α-TiP. In this section, without any base additives,
we employed alkylammonium phosphates and their counter-
part chlorides that bear the same cations to reveal their different
behavior on the ion-exchange intercalation process.
XRD patterns of α-TiP treated by four kinds of
alkylammonium phosphates are shown in Figure 1. Compared
In this work, we employed α-TiP, which is a representative of
α-MPs and has the highest ion exchange capacity in this
category, to present a novel method to acquire the amine
intercalated product. By cation exchange with alkylammonium
salts without any additives, the large alkylammonium cation can
be driven into the interlayer space directly. To the best of our
knowledge, there is no published research on this process. By
focusing on the acid radical moieties of alkylammonium salts,
which were rarely paid attention, our work elucidated the
crucial role played by anions and stressed the importance of the
strength of their conjugate acids in controlling the intercalation
procedure, and further demonstrated their complicated
influences on the relevant large cation-exchange process. This
investigation not only realized the goal of intercalating α-MP
through the exchange of alkylammonium salts in the absence of
additional additives but also achieved the goal of the direct
introduction of large cations of salts into the unexpanded
interlayer space without additional assistance, which was
acknowledged to be difficult. This may bring about a one-
step method for the introduction of large groups in the future
instead of the current two steps, in which an amine first
preintercalates and then performs an ion-exchange process.
2. EXPERIMENTAL SECTION
Figure 1. XRD patterns of (A) original α-TiP and α-TiP treated with
different alkylammonium phosphates: (B) n-butylammonium, (C) n-
hexylammonium, (D) cyclohexylammonium, and (E) pyridinium. The
amount of each alkylammonium phosphate used was 0.05 mol.
2.1. Reagents. n-Hexylamine (AR, 99.0%) was purchased from
Macklin. Tetrabutyl titanate (CP, 98.0%), phosphoric acid (AR,
85.0%), hydrochloric acid (AR, 36.0%), phosphorous acid (AR,
99.0%), sulfurous acid (AR, 6.0%), citric acid (AR, 99.5%), malic acid
(dl-) (AR, 99.5%), and nitric acid (AR, 65.0%) were purchased from
Tianjin Fengchuan Chemical Reagent Company. Pyridine (AR,
99.5%), n-butylamine (AR, 99.5%), and cyclohexylamine (AR,
99.5%) were purchased from Tianjin Damao Chemical Reagent
Factory. None of the chemicals were further purified before use, and
deionized water was used throughout this work.
2.2. Ion-Exchange Operation of α-TiP. α-TiP was synthesized
through the hydrothermal method described elsewhere.27 Alkylammo-
nium phosphate aqueous solutions were prepared by adding amine
dropwise to a phosphoric acid solution at 308 K under magnetic
stirring to acquire a 50 mL solution of pH 6.0−6.5, where the typical
concentration of the alkylammonium cation was 1 mol/L (i.e., amount
of the alkylammonium cation was 0.05 mol). Then the solution was
moved into a 100 mL round-bottomed flask to which 0.5 g α-TiP had
been added. The flask was placed in 323 K water bath for 200 min
under magnetic stirring in order to perform the ion-exchange process
by batch methodology. This procedure was also applied in the
preparation and ion-exchange reaction of other alkylammonium salts
involved in this work. The obtained products were collected and
thoroughly washed until clean by conducting centrifugation operations
repeatedly at a speed of 4000 r/min, dried at 343 K overnight, and
finally pulverized into a powder for characterization. In comparison
with the intercalation results of alkylammonium cations, a heterocyclic
and aromatic pyridine is employed, and its salts are prepared through
the method described above as well. For the sake of simplicity, its
corresponding cation pyridinium cation is also classified as an
alkylammonium cation in the following section.
to the original α-TiP (d002 ≈ 0.76 nm, characteristic peak: 2θ =
11.60°), the phosphate-exchanged ones exhibited a larger
interlayer distance, providing clear evidence of successful
intercalation with the target exchanger in the absence of extra
amine species. The characteristic peak of each product shown
in Figure 1 gave interlayer distances of 1.89 nm (n-
butylammonium phosphate, 2θ = 4.68°), 2.34 nm (n-
hexylammonium phosphate, 2θ = 3.78°), 1.79 nm (cyclohexyl-
ammonium phosphate, 2θ = 4.92°), and 1.14 nm (pyridinium
phosphate, 2θ = 7.76°), respectively. In comparison with the
data acquired from published articles,21,28,29 cyclohexyl-
ammonium phosphate-treated α-TiP demonstrated a larger
interlayer distance while the other three phosphates gave
almost the same interlayer distance to the products obtained
when reacting α-TiP with an amine. Although the reaction time
chosen in this research was very short, the intercalated
compounds with an expected large interlayer distance had
been successfully synthesized from what the XRD patterns
revealed, indicating that intercalated α-TiP could be obtained
quickly by a direct ion exchange operation.
However, XRD patterns for the alkylammonium chloride-
involved ones, which are depicted in Figure 2, demonstrated
totally different results. It was clear that without adding
additional amine, alkylammonium chlorides basically showed
no intercalation activity for α-TiP, since the alkylammonium
cations of chlorides were incapable of diffusing into the
interlayer space. As both the selected phosphate and its chloride
counterpart bear the same cation moiety, it was obvious that
the anions of alkylammonium salts played an important role in
the ion-exchange intercalation process. That was to say that
although the cations of alkylammonium salts were the same, it
2.3. Characterization. To measure the interlayer distance, powder
X-ray diffraction (XRD) patterns were recorded on a Rigaku D/MAX-
2500 with Cu Kα radiation (λ = 1.541 Å) from 2θ = 2° to 80° (rate =
5°/min). A Bruker Vertex-70 Fourier transform infrared (FT-IR, IR)
spectrometer was used to detect the functional groups of the products.
The element content of surface layers of the intercalated α-TiP was
performed through X-ray photoelectron spectroscopy (XPS) using a
Thermo Fisher Escalab 250Xi.
B
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the 2θ = 11.60° peak had become much smaller. In D−F, 0.01,
0.03, and 0.05 mol n-butylammonium phosphate were added,
respectively. XRD patterns revealed that the degree of
completion of intercalation had become higher and higher,
and finally the peak of 2θ = 11.60° became almost invisible
when the amount of the n-butylammonium cation was much
larger than the amount of H+, proving that the intercalation
reaction had generally finished.
From Figure 3, one could also find that, along with the
growth of the n-butylammonium cation content, the character-
istic peak of intercalated α-TiP kept moving left until 2θ =
4.68° (corresponding interlayer distance is 1.89 nm, F in Figure
3), representing a basically completed intercalation process.
This revealed that the variation of interlayer distance was a
consecutive process; it would grow along with the growing
amount of the n-butylammonium cation carried by the
exchanger until the process of ion exchange reached a
maximum degree of completion, and it demonstrated a
comparatively different phenomenon from the alkylammonium
ion-exchange intercalation of α-ZrP with the assistance of an
amine (which was a multistep and segmented process).24,30
The IR spectra that were obtained further confirmed the
above conclusion. Figure 4 demonstrates the different IR
Figure 2. XRD patterns of (A) original α-TiP and α-TiP treated with
different alkylammonium chlorides: (B) n-butylammonium, (C) n-
hexylammonium, (D) cyclohexylammonium, and (E) pyridinium. The
amount of each alkylammonium chloride used was 0.05 mol.
was the anion of the salts which took control of the relevant
process and thus decided whether the large cations could be
intercalated into the interlayer space of the exchanger.
Since the XRD results could reflect the progression of an
intercalation process, n-butylammonium phosphate was taken
as an example to show how the amount of the alkylammonium
cation affected the intercalation results. In Figure 3, from A to
Figure 4. Infrared (IR) spectra of (A) original α-TiP and α-TiP
treated with alkylammonium phosphates. From (B−E): n-butylammo-
nium phosphate with 0.001, 0.002, 0.005, and 0.05 mol. From (F−H):
0.05 mol alkylammonium phosphates with cations n-hexylammonium,
cyclohexylammonium, and pyridinium.
spectra between the original α-TiP and alkylammonium
phosphate-treated ones. The P−O−H band of the intact
exchanger was very clear at 1250 cm−1,31 but, along with the n-
butylammonium phosphate content varying from 0.001 to
0.005 mol (B−D of Figure 4), more and more protons were
replaced by n-butylammonium cations, resulting in a gradually
weakened absorption band. Finally, when 0.05 mol of
alkylammonium cations was introduced into the solution, the
P−O−H band became nearly invisible (E−G of Figure 4). But
H (pyridinium phosphate-treated) showed a strong peak at
1250 cm−1, indicating that a large number of protons remained
in the interlayer space and were not replaced by pyridinium
cations. As referred to by the XRD pattern of pyridinium
phosphate given above (E of Figure 1), it could be concluded
that the intercalation procedure was basically finished (peak of
2θ = 11.60° was only around 4% of the area ratio to the peak of
Figure 3. XRD patterns of α-TiP treated with n-butylammonium
phosphate. The amounts of n-butylammonium phosphate used were
(A) 0.001, (B) 0.002, (C) 0.005, (D) 0.01, (E) 0.03, and (F) 0.05 mol.
F, along with the content variation of n-butylammonium
phosphate in the solution, an obvious change with the
corresponding XRD patterns could be easily observed. When
the n-butylammonium cation content was low enough
(compared to the amount of 0.5 g of α-TiP ≈ 0.002 mol,
which contained approximate 0.004 mol of protons), as shown
in A and B of Figure 3, the characteristic peak of the original α-
TiP (2θ = 11.60°, corresponding to the incipient interlayer
distance) was still obvious, indicating an incomplete
intercalation process. While the n-butylammonium cation
content went up to 0.005 mol as C in Figure 3, the area of
C
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2θ = 7.76°, of which the corresponding interlayer distance is
1.14 nm). This meant that although the quantity of
alkylammonium salts applied was equal and the completion
of the intercalation process was close, the cation amount inside
the exchanged α-TiP was still different. Data obtained from
XPS analysis verified this result. The exchanger treated with
0.05 mol of alkylammonium phosphate (with aliphatic
ammonium cations) presented their N/Ti element ratio as
1.46 (n-butylammonium, with an uptake value of 0.420 g/g),
1.50 (n-hexylammonium, 0.595 g/g), and 1.47 (cyclohexyl-
ammonium, 0.571 g/g), respectively, which theoretically would
be 2 from the chemical formula of α-TiP (Ti(HPO4)2·H2O) if
all of the protons were exchanged by alkylammonium cations.
But in the case of pyridinium phosphate, only 1.08 (0.335 g/g)
N/Ti element ratio was given by XPS analysis, showing
evidence that pyridinium cations replaced a fewer number of
protons than the aliphatic ammonium cations did, just as what
was observed from IR spectra. This data also revealed that
incomplete bilayer cation insertions were formed in the
alkylammonium phosphate-exchanged products while mono-
layer pyridinium cations were formed in the interlayer space of
the pyridinium phosphate-treated one. The reason that the ion-
exchange rate of the pyridinium cation was slower than that of
aliphatic ammonium cations might be attributed to the circular
π-bond of the pyridinium cation: its high electron density
would actually be a hindrance to the cation smoothly diffusing
into the negatively charged titanium phosphate laminates.
There were two obvious bands with the original exchanger in
its IR spectrum at 3557 and 3480 cm−1 due to asymmetric and
symmetric stretching vibration of the inside crystal water
molecules.32,33 The intensity of these bands decreased along
with the progression of intercalation (B−D of Figure 4), and
they became very weak while the intercalation reaction
approached its end, implying an almost complete loss of all
of their interlayer water molecules. This phenomenon was
agreed with the insertion of some bases into α-ZrP.33
Cyclohexylammonium phosphate-treated α-TiP, however,
showed some differences. Although the degree of completion
of the ion-exchange intercalation process was almost the same
in n-butylammonium and n-hexylammonium phosphate ones,
as the XRD patterns and XPS data demonstrated, IR spectra
revealed that there was still a certain amount of crystal water
remaining inside. This explained why cyclohexylammonium
phosphate-treated α-TiP showed a larger interlayer distance
than did the pure amine vapor or solution-treated ones as
mentioned in the discussion of XRD patterns above: the “co-
intercalated” crystal water would affect the interlayer distance of
α-MP. That is, when the amount of crystal water located in the
interlayer space increased, the interlayer distance would also
increase accordingly34 so that the absence of crystal water that
α-TiP treated with pure cyclohexylamine shown in ref 28 would
lead to a relatively small interlayer distance. This result
indicated that it was possible that, by ion exchange with α-
TiP, some alkylammonium cations of salts would exhibit
intercalation behavior different from reacting the exchanger
with the corresponding amine vapor or solution.
and their corresponding functional groups are listed in Table 1.
Combined with the XRD results that the interlayer distance had
Table 1. Bands of IR Spectra of Alkylammonium Phosphate-
a
Treated α-TiP from 1600 to 1300 cm−1
N−H
C−H
aromatic ring-
stretching
vibration
bending
vibration
bending
vibration
n-butylammonium
n-hexylammonium
cyclohexylammonium
pyridinium
1541
1541
1541
1472, 1395
1472, 1397
1454, 1391
1404
1549, 1487
a
E to H of Figure 4.
been expanded, there was no doubt that the cations combined
with α-TiP were in the spaces between two titanium phosphate
layers and gave rise to the formation of intercalated products.
The IR spectra of α-TiP reacted with alkylammonium
chlorides are shown in Figure 5. They were almost the same as
Figure 5. Infrared (IR) spectra of (A) original α-TiP and α-TiP
treated with different alkylammonium chlorides: (B) n-butylammo-
nium, (C) n-hexylammonium, (D) cyclohexylammonium, and (E)
pyridinium. The amount of each ammonium chloride used was 0.05
mol.
the intact exchanger. First, the bands at 3557 and 3480 cm−1,
which represented the vibration of crystal water, were all very
strong, indicating the neglectable loss of crystal water. Second,
no clear bands around 1500 cm−1 proved that alkylammonium
cations failed to be intercalated into the interlayer space. Third,
obvious bands in 1250 cm−1 for the P−O−H group indicated
that there were still many protons in the interlayer space. In
other words, few interlayer protons were exchanged by
alkylammonium cations. These three aspects indicated that
alkylammonium chlorides could not conduct an intercalation
procedure with α-TiP, which agreed with the XRD results.
3.2. Mechanism Study of Ion-Exchange Intercalation.
According to the experimental results, even with the same
cation moiety and pH environment, alkylammonium chlorides
showed no ion-exchange intercalation capability in the absence
of a basic substance, whereas phosphates could achieve this
process by sending their cations into the space between two
titanium phosphate laminates. This fact strongly suggests that
in the situation of alkylammonium phosphates, there might be a
The bands around 1500 cm−1 were ascribed to alkylammo-
nium cations.22,35,36 As no obvious absorption bands around
1500 cm−1 could be observed with the original α-TiP, bands
appearing in this region with samples B to H were clear
evidence that alkylammonium cations existed inside the
products, although bands of B were not as clear as the others
due to its low cation content inside. The bands' wavenumbers
D
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phosphate anion-related process involved in the corresponding
intercalation.
was established and resulted in a net effect that a part of the
interlayer protons remained outside the solid phases and finally
formed a weak acid environment. However, when α-TiP was
dispersed in an alkylammonium phosphate solution, there
would be some differences. The anion moiety of the
Kotov et al.37 investigated the influence of some anions on
the sodium ion exchange kinetics of α-ZrP and found that the
diffusion coefficient and rate of the ion-exchange process
−
−
increased in the order of I− < Br− < Cl− and ClO4− < NO3
<
alkylammonium phosphates would take the form of H2PO4 ,
SO42−. They concluded that the surface density of the negative
charges of anions was the key point of influence of the different
anions. As the conclusion they drew was from the small sodium
ion and could not be spontaneously used in describing the large
cation situation such as with alkylammonium cations, which
were generally believed to stand no chance of being exchanged
into the interlayer space without the assistance of a basic
substance, their theory seemed difficult to explain as to why
alkylammonium phosphates behaved so differently from that of
the alkylammonium chlorides in the α-TiP ion-exchange
intercalation process.
H2PO4 , and PO43− in aqueous solution, thus they fabricated a
−
buffer system capable of absorbing surplus protons in the
environment due to the relatively high pKa of their
corresponding conjugate acids. They should act as the acetate
anions did in the divalent metal ion-exchange process but had a
much better capability of buffering than the acetate due to the
multistep ionization of phosphoric acid. While the exchanger
was added to the phosphate solution, the protons which
diffused into the liquid phase due to the concentration
difference would be absorbed by the phosphate buffer system
and were prevented from going back to the interlayer space.
This behavior led to the established proton equilibrium moving
toward the direction of releasing more protons into the liquid
phase from the exchanger. Along with the release of the
interlayer protons, the electrostatic repulsion between two
negatively charged titanium phosphate layers would keep
increasing due to the continuous loss of positive charges in the
interlayer space and moving the titanium phosphate layers apart
gradually. When the process proceeded to the extent to where
the interlayer distance had been enlarged widely enough to
allow the alkylammonium cations of phosphate to be
introduced, the expected ion exchange started and kept going
until the reaction was stopped. This process further enlarged
the interlayer distance attributed to the large size of the
alkylammonium cation. This proposed mechanism coincided
with the area variation of the characteristic peak in the XRD
pattern and finally was reflected in the gradually increased
interlayer distance as shown in Figure 3. In alkylammonium
chlorides, however, due to the low pKa of HCl, Cl− ions could
not form a buffer system, so they were unable to prevent the
released protons from going back to the interlayer space.
Consequently, the interlayer distance would not be enlarged
widely enough to let the alkylammonium cations in, and the
intercalation process did not happen either. Therefore, as in the
case of the large alkylammonium cations, the phosphates
exhibited much better ion-exchange capabilities than chlorides
did. A schematic diagram of an alkylammonium phosphate
intercalating α-TiP is shown as Figure 6.
Like what had been described first, it is generally acknowl-
edged that in the absence of additional bases, alkylammonium
salts themselves (alkylammonium chloride for instance, as we
had demonstrated) cannot trigger the ion-exchange intercala-
tion reaction,11,24 but by adding amine additives, the exchange
between alkylammonium cations and protons can be achieved.
This is believed to be attributed to the introduction of OH−,26
though there were very few hydroxide ions in the pH 6.0−6.5
solution. Meanwhile, the alkylammonium phosphate and
chloride solutions have the same pH value with the same
quantity of hydroxide ions, so the initiator of this process
should not be OH−. Clearfield et al.38 found that by using
divalent metal acetate as an alternative of the insoluble
hydroxides, the exchange rate of α-ZrP could be faster than
that of other salts, which had the same cation moiety, resulting
in a high loading amount of target cation. They concluded that
the acetate anions played the role of a base, analogously to what
the hydroxide ion did, preventing the pH of the solution from
quickly descending so that the ion-exchange reaction could be
initiated and sustained. However, the acetate ions are so large
that they should not be able to diffuse into the interlayer space
of the exchanger as the hydroxide ions did. It was also very
difficult to imagine that large phosphate ions (and even much
larger anions, for example some organic anions can achieve this
process as well, which will be shown later) used in this work
could diffuse into the narrow interlayer space of the original α-
MP. Therefore, there must be an explanation for the different
situation between the large anions and hydroxide ions, although
they are all thought to accelerate the ion-exchange process as a
Brønsted base: the large anions should accelerate the exchange
process from the exterior of the exchanger. Meanwhile, the
cations involved in the research of Clearfield et al. were only
metal ions, which were much smaller than the alkylammonium
cations shown in this research. Therefore, their conclusion
could not simply be applied to the situation of large cations.
Considering that the phenomena described in this work
cannot be well explained by the current theories, we proposed a
possible mechanism to explain why the alkylammonium
phosphates can cause the intercalation process whereas
chlorides cannot. When α-TiP was dispersed in pure water,
some of its interlayer protons would diffuse into the nearby
aqueous area due to the concentration difference between the
two phases, and the process would keep going until the electric
charge balance force started to drive some of the protons back
to the α-TiP phase to compensate for the loss of the positive
charges. The equilibrium between the two opposite processes
Consequences deduced from this hypothesis are also in
accordance with the results of the influence of different anions
in the Na+ ion-exchange kinetics of α-ZrP obtained by Kotov et
al.,37 though the explanation here is different. The pKa data
(298 K) of the corresponding conjugate acids for I−, Br−, and
Cl− are −10.0, −9.0, and −8.0, while for ClO4 , NO3 , and
−
−
SO42− are −10.0, −1.3, and 2.0, respectively. Therefore, this will
2−
lead to an order of I− < Br− < Cl− and ClO4− < NO3− < SO4
.
On the basis of the above analysis, some different anions in n-
butylammonium salts were employed in the study to testify to
our hypothesis. The anions we tested were acid radicals of
phosphite, sulfite, citrate, malate, and nitrate. The first four
anions were all from weak polyprotic acids, such as phosphoric
acid, that are capable of forming buffer systems like the
phosphoric acid radical system did. The characterization results
are shown in Figure 7 and 8. XRD patterns of the acquired
products demonstrated that n-butylammonium phosphite,
sulfite, citrate, and malate could also have an interlayer distance
expanded like phosphate did, while n-butylammonium nitrate
E
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Article
Figure 8. IR spectra of (A) original α-TiP and α-TiP treated with n-
butylammonium salts composed of different acid radicals from (B)
phosphite, (C) sulfite, (D) citrate, (E) malate, and (F) nitrate.
Figure 6. Schematic diagram of the α-TiP intercalation process by ion
exchange with alkylammonium phosphates. (A) α-TiP dispersed in
pure water, (B) α-TiP dispersed in alkylammonium phosphate
solution, (C) enlarged interlayer distance stemmed from increased
electrostatic repulsion between laminates due to proton loss, and (D)
further expanded interlayer space caused by a large intercalated
alkylammonium cation.
exchanger, as inferred above. Moreover, it was found in the
XRD patterns of Figure 7 that even if the cations of the salts
were all n-butylammonium cations, the enlargement results of
ion-exchange intercalation would still be very different,
implying that the interlayer distance could be modified to
some extent by choosing an appropriate anion. In addition, XPS
data of the product exchanged with n-butylammonium sulfite
gave the S/P element ratio as only 2.2%, illustrating that the
possible anion exchange on laminate was negligible, so it was
reasonable to consider that the ion-exchange intercalation
surely involved only cations and was not assisted by a potential
laminate-exchange process.
The pKas (under 298 K) of the conjugate acids of the anions
involved in our experiments are provided in Table 2, and the
Table 2. pKa Data (298 K) of the Acids Employed and Area
Ratios for the Un-Intercalated and Intercalated
Characteristic Peak Which Were Calculated from XRD
a
Patterns in Figure 7 and B of Figure 1
pKa 1
pKa 2
pKa 3
peak ratio
phosphorous acid
sulfurous acid
phosphoric acid
citric acid
2.0
1.9
6.6
7.0
7.2
4.8
5.2
4.0%
4.9%
2.2
12.3
5.4
2.6%
Figure 7. XRD patterns of (A) original α-TiP and α-TiP treated with
n-butylammonium salts composed of different acid radicals from (B)
phosphite, (C) sulfite, (D) citrate, (E) malate, and (F) nitrate.
3.1
16.9%
29.2%
malic acid
3.4
nitric acid
−1.3
−8.0
hydrochloric acid
All pKa data were from elsewhere.39.
a
could not carry out an intercalation procedure as expected since
its conjugate acid, nitric acid, had fully dissociated in a dilute
water solution. IR spectra for the four polyprotic acid radicals
involved showed clear bands at 1541, 1472, and 1395 cm−1,
which were exactly the same as that of the phosphoric acid
radical involved, whereas nitrate salt-treated α-TiP showed no
obvious bands around 1500 cm−1 like the chloride salt-treated
one did. These results underline the importance of the buffer
system established by weak polyprotic acid radicals and
confirmed that the strength of the anion’s conjugate acid
certainly played a deciding role in the ion-exchange
intercalation. Even in an anion moiety as large as the citrate
ion, the intercalation could still be achieved, proving that the
anion part could influence the ion exchange from outside of the
peak area ratios between the characteristic peaks of
unintercalated (2θ = 11.60°) and intercalated in XRD patterns
of α-TiP, which was treated with 0.05 mol of the corresponding
n-butylammonium salt, were also listed. It could be found from
the peak area ratios of the ones which were successfully
intercalated that n-butylammonium salts with organic anions
showed much worse performance than did the inorganic
anions, indicating the existence of steric hindrance caused by
the very large organic anions while the n-butylammonium
cation was about to diffuse into the interlayer space. From
Table 2, it could also be concluded that for weak polyprotic
acids whose pKas were close, the control factors of the ion-
F
DOI: 10.1021/acs.inorgchem.7b03030
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Author Contributions
exchange intercalation processes were complicated and pKas of
the anions’ conjugate acids might not be the only ones.
Although the strength of phosphoric acid was weaker than that
of phosphorous acid and the intercalation completion of the n-
butylammonium phosphate-treated exchanger was higher than
that of the phosphite-treated one, which could be observed
from the peak ratio in Table 2, the XRD patterns showed that
the phosphite-exchanged α-TiP demonstrated a 1.94 nm
interlayer distance (B of Figure 7), which was larger than
that of the phosphate-exchanged one.
Interestingly, further experiments revealed that it was not
every kind of acid radical of weak polyprotic acids which could
form a buffer system that had the capability of achieving an
intercalation procedure with its n-butylammonium salt; when
oxalic acid was employed, n-butylammonium oxalate would
prefer to smash the α-TiP crystals into extremely thin particles
rather than intercalating them, and the particles could not be
precipitated through a centrifugation operation even at a speed
of 12 000 rpm. This was probably due to an exfoliation process.
The mechanism of the abnormal phenomenon presented by
oxalate-employed reaction is still under investigation.
†These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support from the National Natural
Science Foundation of China (21576069, 21236001, and
21106029) and the Natural Science Foundation of Hebei
Province (B2016202335 and B2015202369).
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: chengqingyan@hebut.edu.cn. Telephone: +86-022-
60203540.
ORCID
Chunyang Wang: 0000-0001-8203-8675
G
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