Synthesis, Characterization, and Anion Exchange Reactions of Zn2Cr Layered Double Hydroxides Intercalated with Acetate and Chloride

Layered double hydroxides (LDH) with the composition Zn2Cr, intercalated with acetate and chloride ions were synthesized by co-precipitation with increasing pH and characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and inductively coupled plasma-optical emission spectrometry (ICP-OES). The samples synthesized at optimized pH showed XRD patterns with basal diffraction peaks typical of layered structures, with basal distances of 7.88 and 12.69 Å for intercalated chloride (Zn2Cr/Cl) and acetate (Zn2Cr/Ac), respectively. After optimization experiments, exchange reactions were performed with different anions, using an excess of five times the anions to be intercalated. Zn2Cr/Cl was exchanged with CH3COO-, F-, Br-, I-, SO42- and NO3-, and Zn2Cr/Ac was exchanged with F-, Cl-, Br-, I-, SO42- and NO3-. Not all reactions were effective, indicating that among the evaluated anions, CH3COO- was preferred for exchange, mainly attributed to the pre-expansion with bigger anions, facilitating the exchange with smaller anions.


Introduction
Recently, the properties of two-dimensional materials have been gaining importance due to advances in nanoscience and nanotechnology in producing new combinations of materials with novel properties, expanding the range of applications.
Layered double hydroxides (LDH) are compounds belonging to this class of materials, especially those with brucite-like structures (Mg(OH) 2 ), in which Mg 2+ cations are coordinated with six hydroxyls and the octahedra share edges to form two-dimensional layers, which are packed along the basal direction and held together with hydrogen bonds. 1 In the LDH structure, a partial replacement of divalent metals by trivalent metals occurs, generating an excess of positive charge in the layers, which are compensated by the intercalation of hydrated anions.The traditional family of LDHs is represented by the formula [M 2+ 1-x M 3+ x (OH) 2 ](A n− ) x/n• yH 2 O, where M 2+ and M 3+ represent the metals present in the layer and (A n− ) x/n• yH 2 O denotes the intercalated hydrated anions. 2,3ese compounds are attractive due to the wide range of applications, and although LDHs exist as minerals, it is possible to obtain them at a relatively low cost, using different synthetic approaches.Thus, it is important to study new phases aiming at new applications of these compounds. 4DHs are known for their high anion exchange capacity (AEC) and the intercalation of different anions can provide different properties to the materials.Some anions have been widely studied, such as carbonate, nitrate, chloride and phosphate, and several anions have been cited as exchangeable, both organic and inorganic. 3,57][8][9] However, there are still few reports in the literature of LDHs intercalated with some anions, as is the case of acetate, either the compound obtained by anion exchange or by direct synthesis.
][20][21][22] An element less present in LDH structures is chromium (Cr 3+ ), since most of the compounds described are synthesized with different metals and using Al 3+ .To expand knowledge of this class of layered materials, we synthesized compounds containing Zn/Cr in the Zn 2+ :Cr 3+ molar ratio of 2:1 and intercalated them with chloride and acetate, and then evaluated the compounds as exchangers of different anions.

Experimental
LDHs with Zn 2+ :Cr 3+ with molar ratios of 2:1 were synthesized by co-precipitation with increasing pH.First, experiments were performed to determine the best pH for the formation of crystalline compounds.Using the Zn 2 Cr/Cl phase as an example, ZnCl 2 97% (Vetec, Rio de Janeiro, Brazil), CrCl 3 98% (Êxodo Científica, Sumaré, Brazil), and NaCl 98.5% (Reatec, Colombo, Brazil) were dissolved in 100 mL of Milli-Q (Millipore-simplicity UV, Bedford, USA) decarbonated water.The solution was kept at room temperature and slowly titrated with a solution of NaOH 98% (Sigma-Aldrich, Saint Louis, USA) 1 mol L -1 in an automatic glass titration reactor (UP Control, Porto Alegre, Brazil) operating at room temperature, coupled to a peristaltic pump and with pH control, under N 2 flow.All the chemicals were of analytical grade and used without further purifications.
The dispersions were removed from the reactor at desired pH and hydrothermally treated at 90 °C for 120 h in oven (Tecnal TE-395, Piracicaba, Brazil) and later the solids were separated by centrifugation (centrifuge Sigma 3-16P, rotor 11133, An der Unteren Söse, Germany) at 4000 rpm, repeatedly washed and redispersed in a portion of decarbonated and distilled water and finally dried at room temperature.The same procedure was performed for the Zn 2 Cr/Ac phase, using Zn(C 2 H 3 O 2 ) 2 99.5% (Química Moderna, Barueri, Brazil), CH 3 COONa 99% (Alphatec, São José dos Pinhais, Brazil), and Cr 3 (OH) 2 (CH 3 CO 2 ) 7 99.5% (Carlo Erba, São Paulo, Brazil).The chemicals and the corresponding amounts used in the synthesis of the samples are presented in Table 1.
After determination of the optimal pH for the synthesis of more crystalline phases, the samples intercalated with chloride (Zn 2 Cr/Cl) and acetate (Zn 2 Cr/Ac) were again synthesized, and for the anion exchange reactions, an aqueous dispersion of the solid was magnetically stirred with excess of salts (five times the concentration of the intercalated anion) for 7 days at room temperature and under N 2 flow.The exchange time was optimized and to avoid the loss of water, the Becker containing the dispersion was protected with a polymeric film with small holes to release the N 2 flow.
The materials were characterized by X-ray diffraction (XRD) using a Shimadzu XRD-6000 diffractometer (Kyoto, Japan) with Cu Kα 1.5418 Å radiation.The slurry obtained after the last centrifugation and washing procedure was dropped on the glass sample holder and allowed to dry at room temperature to form a film.The analyses were performed using voltage of 40 kV and current of 30 mA, with a dwell time of 2° min -1 and step of 0.02° in 2θ.
Fourier-transform infrared (FTIR) spectra were obtained in transmission mode using a Bruker Vertex 70 spectrophotometer (Billerica, USA) with KBr pellets containing around 1% (m/m) of the sample.The spectra were collected from 400-4000 cm -1 , with 32 scans and using a resolution of 2 cm -1 .
Scanning electron microscopy (SEM) image were obtained with a Tescan Vega3LMU microscope with AZ Tech software.The samples were deposited on copper tapes, and after collecting the energy-dispersive X-ray (EDS) spectra, the samples were sputtered with a thin gold layer to obtain the SEM images.
The elements were quantified with a Thermo Fisher Scientific inductively coupled plasma atomic emission (ICP-OES) spectrometer (model iCAP 6500) (Waltham,

Results and Discussion
The XRD patterns of Zn 2 Cr/Cl (Figure 1A) and Zn 2 Cr/Ac (Figure 1B) showed series of basal peaks characteristic of layered structures.
5][26] The low crystallinity (or the small number of packed layers along the basal direction) did not vary greatly with the change of synthesis pH and small intensity diffraction peaks were observed in the region of 60° (in 2θ) but the polytype could not be identified.Due to this low crystallinity and stacking disorder, the cell parameters along the layers could not be determined.
Also, Cr 3+ oligomeric species were present in the pH range of 6 to 8, affecting the formation of crystalline LDH and resulting in materials with layer-stacking faults. 279][30] Based on the high intensity and the sharp basal diffractions peaks, the crystallinity varied a lot with changing pH, being most crystalline at pH close to 9 (Figure 1Bc).
At higher pH (Figure 1Bg), a small diffraction peak (indicated with an asterisk) was observed with a basal distance of around 21 Å, which can be attributed to the intercalation of Cr 3+ complexes like [Cr(OH) 4 ] − , the predominant species at pH = 11. 31he FTIR spectra (Figure 2) showed the same pattern for the different pH values of the same sample, Zn 2 Cr/Cl (Figure 2A) or Zn 2 Cr/Ac (Figure 2B).
][39][40] According to the higher intensity and sharper diffraction peaks in the XRD patterns, indicating a bigger number of packed layers and consequently higher crystallinity, the best pH values for the syntheses were close to 6 for the Zn 2 Cr/Cl and close to 9 for Zn 2 Cr/Ac, using the chemical concentrations and the synthesis procedure of the present work.The samples were again synthesized at the optimized pH values, and from these, anion exchange reactions were performed using different anions.The XRD patterns and FTIR spectra of the phases synthesized at the optimized pH values are shown in Figure 3.
With the optimized synthesis conditions, Zn 2 Cr samples showed XRD patterns characteristic of layered compounds, with the presence of series of typical basal peaks.The Zn 2 Cr/Cl phase (Figure 3Aa) presented a basal distance of 7.88 Å, which is consistent with the intercalation of chloride ions. 24,41The Zn 2 Cr/Ac phase synthesized at pH 8.95 (Figure 3Ab) had a basal distance of 12.69 Å, indicating the presence of acetate ions in a bilayer arrangement. 30,42he FTIR spectra (Figure 3B) presented a broad band in the region of 3400 cm -1 , attributed to the OH bond of the hydroxyls of the LDH structure and of the intercalated/adsorbed water molecules, in addition to a band at 1620 cm -1 , corresponding to the bending of the water molecules.The Zn 2 Cr/Ac (Figure 3Bb) sample had bands at 568 and 618 cm -1 , attributed to the M−OH bonds, while the Zn 2 Cr/Cl (Figure 3Ba) sample only had a band at 568 cm -1 and a broad band close to 600 cm -1 , possibly due to structural disorder, which was also detected by the noisier XRD pattern (Figure 3Aa).The other bands in the region below 750 cm -1 are related to the vibrations of the M−O bonds. 32,43][46] Despite the N 2 flow during synthesis and use of decarbonated water in the samples obtained at different pH values (Figure 2A), after synthesis at pH 6, the Zn 2 Cr/Cl (Figure 3Ba) sample also had a small band in the region of 1365 cm -1 , attributed to carbonate contamination.
Figure 4 shows the X-ray diffraction patterns of Zn 2 Cr/Cl before and after the anion exchange reactions.Zn 2 Cr/Cl (Figure 4Aa) had a basal distance of 7.88 Å, and after the exchange with acetate (Figure 4Ab) the value remained practically constant (7.89 Å), indicating that the exchange reaction was not effective.However, after the anion  exchange, the FTIR spectrum (Figures 4Ba and 4Bb) had bands at 1555 and 1410 cm -1 , 39,40 indicating the presence of acetate, but probably only exchanged at the surface of the particles, not intercalated.
After the exchange reaction of chloride with iodide (Figure 4Ac), the obtained basal distance of 7.79 Å indicated the absence of reaction, since intercalated iodides have basal distance close to 8 Å, due the ionic radius of hydrated I − (2.10 Å) (predicted basal distance = 4.8 + 2.10 × 2 = 9.02 Å) was greater than that of Cl − (1.56Å) (predicted basal distance = 4.8 + 1.56 × 2 = 7.92 Å). 47 The small change in the basal distance in relation to the precursor was associated with the exchange of chloride anions with some carbonate anions, and the broadening of the diffraction peaks hampered determination of the values.The contamination was confirmed by the FTIR spectra (Figure 4Bc), with the presence of a band at 1360 cm -1 , attributed to carbonate. 48Carbonate contamination was observed in most of the cases, the exception being the bromide exchange sample.
In the case of the sample after the bromide exchange reaction (Figure 4Ad), the basal distance changed from 7.88 to 7.93 Å.This increase was attributed to the fact that the ionic radius of Br − (1.80 Å) is greater than that of the Cl − that was previously intercalated in the LDH structure. 49,50he predicted basal distance of Br − intercalated LDH would be 4.8 + 1.80 × 2 = 7.96 Å. 47 The same behavior has already been observed in other studies, where intercalation of bromide in LDH led to a greater basal distance than with Cl − (around 8.1 Å). [51][52][53] The FTIR spectra showed no changes compared to the precursor Zn 2 Cr/Cl (Figures 4Ba  and 4Bd), as expected.
For the Zn 2 Al/Cl after the exchange with F − , there was a reduction in the basal distance in relation to the precursor phase (Figures 4Aa and 4Ae), since the ionic radius of the fluoride (1.11 Å) is smaller than that of chloride.
][56] After the reaction with Li 2 SO 4 (Figures 4A and 4Bf), the crystallinity decreased and the basal distance increased to 11.01 Å, indicating the presence of sulfate ions in the interlayer spaces, in the form of a double layer.It is difficult to predict the basal distance in this case since sulfate can be intercalated with variable numbers of water molecules, with the basal distance ranging from 7.1 to 11 Å. 47,57,58 The FTIR spectra also confirmed the anion replacement due to the presence of typical ν 1 , ν 3 and ν 4 bands of the S−O bond at 1115, 980 and 617 cm -1 , respectively, characteristic of distorted octahedral symmetry. 51,57,59s reported for other Zn 2 Cr-SO 4 layered double hydroxides, 27 SO 4 2− was probably intercalated with the C 3 v symmetry where one of the S−O bonds was oriented parallel to the c-crystallographic axis.Probably these ions were disordered between the layers and the layers were tilted, reducing the crystallinity of the synthesized materials.
After the reaction with NaNO 3 (Figure 4Ag), an increase of the basal distance to 8.91 Å was also observed, which is consistent with the NO 3 − intercalation (predicted basal distance = 4.8 + 1.89 × 2 = 8.58 Å).This exchange was also confirmed by the FTIR spectra (Figure 4Bg), in which a band at 1382 cm -1 was present, attributed to the N−O bond. 33or the Zn 2 Cr/Ac compound (basal distance of 12.69 Å), all samples showed decreased crystallinity after the anion exchange reactions (Figure 5A).There was a shift in the position of the basal diffraction peaks to 7.82 Å (Figures 5Aa  and 5Ab), consistent with chloride intercalation. 60After the abrupt reduction of the basal distance, the layers stacking faults and reduction of the sizes of the crystalline domains are expected, which broadens the basal diffraction peaks.Unlike the phase synthesized with Zn 2 Cr/Cl, in the Zn 2 Cr/Ac phase (Figure 5Ac) the sample had a basal distance of 8.13 Å, indicating exchanged reactions with iodide.This value is close to the values reported for LDHs intercalated with iodide, which were also obtained by anion exchange reactions. 30,61,62or samples after exchange of acetate with bromide (Zn 2 Cr/Ac-Br) and fluoride (Zn 2 Cr/Ac-F), (Figures 5Ad  and 5Ae), the obtained basal values of 7.95 Å and 7.59 Å also confirmed the exchange reactions. 50,52,63,64In the case of the Zn 2 Cr/Ac-Cl, Zn 2 Cr/Ac-I, Zn 2 Cr/Ac-Br and Zn 2 Cr/ Ac-F samples (Figures 5Bb, 5Bc, 5Bd, 5Be), the typical bands of acetate in the FTIR spectra were not observed, suggesting complete anionic exchange reactions.
The XRD patterns and the FTIR spectra of the sample after the exchange reaction with Li 2 SO 4 (Figures 5Af and  5Bf) showed changes in relation to the precursor Zn 2 Cr/Ac.In addition to the reduction in crystallinity, the basal distance changed to 9.70 Å.This result differed from the other values obtained for the sulfate intercalation in the different phases synthesized in this work and from the values of 11 and 8.9 Å reported in the literature for the basal distance of sulfate intercalated in Zn 2 Cr LDH. 57,65][68] The same behavior was observed in the sample after exchanging chloride with nitrate.In the XRD pattern (Figure 5Ag), the diffraction peaks shifted to 8.78 Å.In addition, there was splitting of the first basal diffraction peak, with a new peak at 7.75 Å, indicating carbonate intercalation.The FTIR spectra (Figure 5Bg) showed a band located at 1380 cm -1 , attributed to nitrate, and this band had a shoulder on the right side, which is associated with overlapping of the carbonate band, corroborating the results of X-ray diffraction. 69,70e SEM images (Figure 6) indicated agglomerated platelet-like particles, typical of LDHs prepared by evaporating the slurry in the sample holder.The sample Zn 2 Cr/Ac (Figure 6a) contained particles in the range of 1 µm and smaller, but the particles along the platelets were larger than those of Zn 2 Cr/Cl (Figure 6h), which presented nanometric dimensions.
The particle sizes along the basal axis (or platelet-like particles thicknesses) were calculated through the Scherrer equation (Table 2) and the values ranged from 77 to 92 Å for the Zn 2 Cr/Cl system in the whole pH range and from 42 to 72 Å for the Zn 2 Cr/Ac system, again for the whole pH range.It is important to mention that raw data were used, without including the instrumental corrections, so the data are only estimated values.The number of packed layers could the calculated by dividing the thicknesses of the platelet-like particles by the basal distance, which is an indication of the distance between two adjacent layers (also the thickness of one layer plus the interlayer space) (Figure S1, Supplementary Information (SI) section).The number of packed layers was almost the same in the Zn 2 Cr/Cl system in the whole pH range investigated, from 10 to 12 layers, while these values were in the range of 4 to 6 for the Zn 2 Cr/Ac system.
As observed through the SEM images, during the exchange reactions performed with the Zn 2 Cr/Ac sample, the particles had increased sizes along the platelets due to Ostwald ripening, since the reactions were processed in solution for 7 days at room temperature.The same effect was also observed in the sample Zn 2 Cr/Cl (Figure 6h), except for the sample after exchange with KBr (Figure 6k), in which the particles were smaller and more compacted.Except for the sample exchanged with Li 2 SO 4 , the number of packed layers remained almost constant for the Zn 2 Cr/Cl phases after the exchange reactions, while the number slightly increased in the Zn 2 Cr/Ac system (Table 2).
The results obtained in the analysis of ICP-OES (Table 3) showed that in general the samples presented the expected compositions related to the M 2+ :M 3+ ideal molar ratios (Zn 2+ = 0.667 and Cr 3+ = 0.333).
The presence of sulfate ions was identified in the samples Zn 2 Cr/Cl-SO 4 and Zn 2 Cr/Ac-SO 4 , confirming the exchange reactions.][75][76] Since most intercalated anions could not be evaluated through ICP-OES analysis, qualitative EDS spectra were obtained for the samples (SI section), confirming the exchange reactions by the presence of typical signals of the respective intercalated anions.
No elements were observed in the region of 1 to 5 eV in Zn 2 Cr/Ac (Figure S2, SI section), and after the exchange reactions, signals were present at 2.6 eV, attributed to chloride (Figure S3, SI section), at 4 eV, attributed to iodine (Figure S4, SI section), at 1.5 eV, attributed to bromide (Figure S5, SI section), at 0.7 eV, attributed to fluoride (Figure S6, SI section), at 2.3 eV, attributed to sulfur from sulfate (Figure S7, SI section), and at 0.4 eV when exchanged with NaNO 3 (Figure S8, SI section).
For the synthesized Zn 2 Cr/Cl sample, a signal attributed to chloride was observed at 2.6 eV (Figure S9, SI section).This signal was still present in most samples after the exchange reactions and had greater intensity in samples in which anion substitutions were not observed by XRD analysis, as was the case of Zn 2 Cr/Cl-Ac (Figure S10, SI section) and Zn 2 Cr/Cl-NaI (Figure S11, SI section).Although in the EDS spectrum of the latter, the signals referring to iodide appeared in the region of 3.5-4.5 eV, indicating there was a partial exchange, but not enough to change the basal parameter to values consistent with the intercalation of iodide ions, therefore being considered negligible.For the other samples after the exchange reactions, signals were observed at 1.5 eV, attributed to bromide (Figure S12, SI section), at 0.7 eV, attributed to fluoride (Figure S13, SI section), at 2.3 eV, attributed to sulfur after exchange with Li 2 SO 4 (Figure S14, SI section), and at 0.4 eV, attributed to nitrogen after exchange with NaNO 3 (Figure S15, SI section).
For the sample Zn 2 Cr/Cl, the anion exchange reactions occurred when the sample was added to solutions containing bromide, fluoride, sulfate, and nitrate anions.Furthermore, there was partial substitution by acetate ions and absence of reactions when iodine was used.For the sample synthesized with acetate, Zn 2 Cr/Ac, anion exchange reactions occurred in all cases when the sample was added to solutions containing fluoride, chloride, iodide, bromide, sulfate, and nitrate anions.
The series of anionic stabilization in the interlayer space in the order CO 3 2− > SO 4 2− > OH − > F − > Cl − > Br − > NO 3 − > I − , 24 indicated LDH intercalated with Cl − were likely replaced with Br − , SO 4 2− and NO 3 − .However, the occurrence of unfavorable exchange reactions can be explained by the presence of an excess of the anions of interest (five times in relation to the intercalated anions).In addition, sulfate and chloride are very close in the series, facilitating anion exchange, which has been observed in other studies. 77,78Regarding acetate, which in all cases was exchanged for other anions, our results indicated it was the least stable among the evaluated anions, so its position in the series was CO 3

Conclusions
Layered double hydroxides of Zn 2 Cr intercalated with chloride (Zn 2 Cr-Cl) and acetate (Zn 2 Cr-Ac) were successfully synthesized by co-precipitation with increasing pH, followed by hydrothermal ripening at 90 °C.To obtain phases with the best crystallinity of the precipitated materials, optimization of the synthesis was carried out at different pH values, and after determining the best value of pH, new syntheses were performed for the two compositions.
XRD patterns indicated that Zn 2 Cr/Cl and Zn 2 Cr/Ac had respective basal distances of 7.88 and 12.69 Å, consistent with the intercalation of chloride and acetate ions.The FTIR spectra showed the characteristic bands of the structure of the compounds, in addition to the bands attributed to the acetate ions, and a small contamination with carbonate in Zn 2 Cr/Cl.The results obtained by XRD, FTIR and EDS showed the effectiveness of the exchange reactions.In general, there was a reduction in the crystallinity of the compounds, but the particles were bigger along the platelet particles.The basal distances increased or decreased as a function of the size of the ionic radius of the intercalated anions.
The results of ICP-OES indicated a molar ratio close to 2:1, according to the amounts used in the syntheses.Also, sulfate and lithium contents obtained by ICP-OES analyses after anion exchange were lower than expected for a shigaite-like composition [Na 0.111 (H 2 O) 0.667 ][Mn 0.667 Al 0.333 (OH) 2 (SO 4 ) 0.222 ]. 77  For both the Zn 2 Cr/Cl and Zn 2 Cr/Ac samples, exchanges were achieved with bromide, fluoride, sulfate, and nitrate.In addition to these, in Zn 2 Cr/Cl there was partial exchange with acetate, and for Zn 2 Cr/Ac there were also exchanges with chloride and iodide, indicating that the acetate anion is the most labile among the evaluated anions.
The synthesized compounds are under investigation as semiconductors for photocatalytic degradation of substrates of environmental interest, results that will be the subject of future publications.

Figure 3 .
Figure 3. XRD patterns (A) and FTIR (KBr pellets) spectra (B) of Zn 2 Cr/Cl synthesized at pH = 6.47(a) and Zn 2 Cr/Ac synthesized at pH = 8.95 (b).Values indicated in the XRD patterns are attributed to basal distances and indicated in Å.

Figure 5 .
Figure 5. XRD patterns (A) and FTIR spectra (B) of Zn 2 Cr/Ac before (a) and after exchange with NaCl (b), NaI (c), KBr (d), NaF (e), Li 2 SO 4 (f) and NaNO 3 (g).Values indicated in the XRD patterns are attributed to basal distances and indicated in Å.

Table 1 .
Chemicals, amounts and conditions used during the syntheses
a Along the basal axis;

Table 3 .
Compositions of the samples obtained by ICP-OES However, the sulfate contents were above the values for a traditional LDH structure [Zn 0.667 Al 0.333 (OH) 2 ](SO 4 ) 0.167• yH 2 O, indicating that the last composition is possible when part of the hydroxide anions are replaced with sulfate, or the excess of sulfate is retained through -OH 2 +… SO 4 2− bonds.The phases Zn 2 Cr/Cl and Zn 2 Cr/Ac could not be converted to shigaite-like compounds with the composition [Na 0.111 (H 2 O) 0.667 ][Mn 0.667 Al 0.333 (OH) 2 (SO 4 ) 0.222 ], as reported for Mn 2 Al layered double hydroxide, 77 but the question is still open if similar compounds can be directly obtained by co-precipitation of Cr, Al and alkali metal salts in the presence of sulfate, carbonate, vanadate, molybdate or other divalent anions.