Anion-Controlled Cation-Exchange Process: Intercalating α-Titanium

Mar 21, 2018 - Several alkylammonium salts were used in the study of α-titanium phosphate (α-TiP) ... of α-tetravalent metal phosphate directly bec...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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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 αtitanium 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 nbutylammonium, 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.

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(PO 4) 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 cationexchange 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 © XXXX American Chemical Society

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 Received: November 30, 2017

A

DOI: 10.1021/acs.inorgchem.7b03030 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry themselves and achieve consequent intercalation of α-MP is still an interesting question. 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 onestep 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.

3. RESULTS AND DISCUSSION 3.1. Structural Analysis of Alkylammonium CationExchanged α-TiP. In this section, without any base additives, we employed alkylammonium phosphates and their counterpart 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

2. EXPERIMENTAL SECTION Figure 1. XRD patterns of (A) original α-TiP and α-TiP treated with different alkylammonium phosphates: (B) n-butylammonium, (C) nhexylammonium, (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 Alkylammonium 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. 2.3. Characterization. To measure the interlayer distance, powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/MAX2500 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.

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 (nbutylammonium phosphate, 2θ = 4.68°), 2.34 nm (nhexylammonium phosphate, 2θ = 3.78°), 1.79 nm (cyclohexylammonium 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 cyclohexylammonium 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 chlorideinvolved 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 B

DOI: 10.1021/acs.inorgchem.7b03030 Inorg. Chem. XXXX, XXX, XXX−XXX

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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 characteristic 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) nhexylammonium, (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-butylammonium 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 nbutylammonium 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 αTiP (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

DOI: 10.1021/acs.inorgchem.7b03030 Inorg. Chem. XXXX, XXX, XXX−XXX

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and their corresponding functional groups are listed in Table 1. Combined with the XRD results that the interlayer distance had

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 (cyclohexylammonium, 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 monolayer pyridinium cations were formed in the interlayer space of the pyridinium phosphate-treated one. The reason that the ionexchange 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 “cointercalated” 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 αTiP, some alkylammonium cations of salts would exhibit intercalation behavior different from reacting the exchanger with the corresponding amine vapor or solution. The bands around 1500 cm−1 were ascribed to alkylammonium 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

Table 1. Bands of IR Spectra of Alkylammonium PhosphateTreated α-TiP from 1600 to 1300 cm−1a

n-butylammonium n-hexylammonium cyclohexylammonium pyridinium a

N−H bending vibration

C−H bending vibration

aromatic ringstretching vibration

1541 1541 1541

1472, 1395 1472, 1397 1454, 1391 1404

1549, 1487

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-butylammonium, (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 D

DOI: 10.1021/acs.inorgchem.7b03030 Inorg. Chem. XXXX, XXX, XXX−XXX

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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 alkylammonium phosphates would take the form of H2PO4−, 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 pK a 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. 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 lead to an order of I− < Br− < Cl− and ClO4− < NO3− < SO42−. On the basis of the above analysis, some different anions in nbutylammonium 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

phosphate anion-related process involved in the corresponding intercalation. 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− < 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. Like what had been described first, it is generally acknowledged that in the absence of additional bases, alkylammonium salts themselves (alkylammonium chloride for instance, as we had demonstrated) cannot trigger the ion-exchange intercalation 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 αMP. 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 E

DOI: 10.1021/acs.inorgchem.7b03030 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 8. IR spectra of (A) original α-TiP and α-TiP treated with nbutylammonium 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 Patterns in Figure 7 and B of Figure 1a phosphorous acid sulfurous acid phosphoric acid citric acid malic acid nitric acid hydrochloric acid

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. 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

pKa 1

pKa 2

2.0 1.9 2.2 3.1 3.4 −1.3 −8.0

6.6 7.0 7.2 4.8 5.2

pKa 3

12.3 5.4

peak ratio 4.0% 4.9% 2.6% 16.9% 29.2%

All pKa data were from elsewhere.39.

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 ionF

DOI: 10.1021/acs.inorgchem.7b03030 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry 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 nbutylammonium 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. According to this discussion, it can be summarized that the influences of an anion moiety may be complex, suggesting that more efforts shall be devoted to the clarification of the specific roles that anions play in the cation-exchange process of α-TiP.

†

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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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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4. CONCLUSIONS Experimental results showed that without any addition of amine species, some alkylammonium salts still had the capability of sending their cations into α-TiP, which led to an intercalation effect. This fact also demonstrated that large cations could be introduced into the interlayer space of α-TiP directly, which was thought to be difficult according to current opinions. Mechanism studies revealed that although the exchange reaction seemed to involve only positive ions, the strength of the anion’s corresponding conjugate acid was the key factor in this process. Through consecutive catching the interlayer protons diffused away from α-TiP with the buffer system established by the anions of a weak polyprotic acid, electrostatic repulsion between the two negatively charged laminates will significantly increase, and moving the laminates apart. Consequently, the introduction of large cations can be implemented. Further investigation revealed that the influences of an anion moiety may be complicated, indicating the great importance of studying the roles different anions play in the ion-exchange process of α-MP. It is also suggested that while conducting intercalation chemistry with similarly structured materials via an ionexchange process, more attention shall be paid to the influence of counterions, which is usually ignored. By the careful selection of suitable counterions, it can be reasonably expected that more products intercalated with large cations will be prepared by using the time-saving one-step procedure presented in this research.

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Chunyang Wang: 0000-0001-8203-8675 G

DOI: 10.1021/acs.inorgchem.7b03030 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.7b03030 Inorg. Chem. XXXX, XXX, XXX−XXX