Zirconium and Hafnium Hydrogen Monothiophosphates, H2Zr(PO3S)2

Alexander E. Gash, Peter K. Dorhout*, and Steven H. Strauss*. Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523. Inorg...
0 downloads 0 Views 136KB Size
Download PDF
5538

Inorg. Chem. 2000, 39, 5538-5546

Zirconium and Hafnium Hydrogen Monothiophosphates, H2Zr(PO3S)2 and H2Hf(PO3S)2. Syntheses and Selective Ion-Exchange Properties of Sulfur-Containing Analogues of H2M(PO4)2 (M ) Zr, Hf) Alexander E. Gash, Peter K. Dorhout,*,‡ and Steven H. Strauss* Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523 ReceiVed July 1, 1999 The reactions between aqueous solutions of M4+ (M ) Zr, Hf) and PO3S3- each result in the precipitation of a white gel that can be dried to a powder. Elemental analysis results for the white polycrystalline product yield a stoichiometry of H2M(PO3S)2. These new compounds are characterized by thermal analysis (DSC, TG-MS), vibrational spectroscopy (FT-IR, FT-Raman), 31P MAS NMR spectroscopy, energy-dispersive spectroscopy (EDS), and powder X-ray diffraction (XRD). On the basis of the characterizations and the results of trialkylamine intercalation experiments, we conclude that the H2M(PO3S)2 compounds have a layered structure that is likely similar to that of R-H2Zr(PO4)2‚H2O. The interlayer spacing for both H2M(PO3S)2 compounds, determined by XRD, is ∼9.4 Å. Our characterization results suggest that the sulfur atom of each PO3S3- group is preferentially pointed into the interlayer region of the compound and is protonated. Two of the many potentially interesting properties of H2Zr(PO3S)2, its ion-exchange capacity and selectivity, are investigated. H2Zr(PO3S)2 is demonstrated to be an effective and recyclable ion-exchange material for both Zn2+(aq) and Cd2+(aq). Mass balance experiments indicate that the removal of Cd2+(aq) and Zn2+(aq) ions by solid H2Zr(PO3S)2 occurs by an ion-exchange process. Ion exchange results in the formation of the new compounds H0.2Cd0.9Zr(PO3S)2 and H0.50Zn0.75Zr(PO3S)2. The extraction of metal ions is monitored by XRD, vibrational spectroscopy, and elemental analysis. H2Zr(PO3S)2 reversibly intercalates Zn2+(aq) ions through three complete cycles of intercalation and deintercalation without any loss of ion-exchange capacity.

Introduction Thirty-five years ago Clearfield and Stynes first reported the synthesis and characterization of crystalline zirconium phosphate.1 Since then, there have been tremendous research efforts directed toward the syntheses and investigations of the properties of the many phases of crystalline zirconium phosphate.2-5 The fact that after 35 years there is still a great deal of current research in this area is a testament to the synthetic versatility and unique properties possessed by zirconium phosphate-based compounds. In general, all of the zirconium phosphate phases have been shown to have interesting ion-exchange,2,6 separation,7 catalytic,8 ionic conductivity,5 and intercalation properties.9 Intriguing transition metal phosphate chemistry is not limited to the group 4 metals, as Johnson and Jacobson have demon* Corresponding authors. Telephone numbers and e-mail addresses: (970) 491-0624 (P.K.D.); (970) 491-5104 (S.H.S.); [email protected]; [email protected]. ‡ Alfred P. Sloan Research Fellow, 1997-1999; Camille Dreyfus Teacher Scholar, 1997-1999. (1) Clearfield, A.; Stynes, J. A. J. Inorg. Nucl. Chem. 1964, 26, 117. (2) Clearfield, A.; Blessing, R. H.; Stynes, J. A. J. Inorg. Nucl. Chem. 1968, 30, 2249. (3) Clearfield, A.; Duax, W. L.; Medina, A. S.; Smith, G. D.; Thomas, J. R. J. Phys. Chem. 1969, 73, 3424. (4) Clearfield, A.; Medina, A. S. J. Inorg. Nucl. Chem. 1970, 32, 2775. (5) Clearfield, A. Chem. ReV. 1988, 88, 125. (6) Torracca, E. J. Inorg. Nucl. Chem. 1969, 31, 1189. (7) Garcia, M. E.; Naffin, J. L.; Deng, N.; Mallouk, T. E. Chem. Mater. 1995, 7, 1968. (8) Johnstone, R. A. W.; Liu, J.-Y.; Whittaker, D. J. Chem. Soc., Perkin Trans. 2 1998, 1287. (9) Alberti, G.; Casciola, M.; Costantino, U.; Vivani, R. AdV. Mater. 1996, 8, 291.

strated with their intercalation investigations of the layered vanadyl phosphate, VOPO4.10,11 Beginning in 1978, Alberti and co-workers showed that one of the oxygen atoms in the phosphate precursor could be substituted with an organic group (phosphonate) and the resulting zirconium phosphonate compound exhibited the same structural motif seen in the zirconium phosphate phases.12 Zirconium phosphonate chemistry has also been investigated using different XPO32- groups where X is an organic moiety (e.g., a crown ether13 or an amine14). These compounds also have interesting separation, conductivity, sensor, intercalation, and catalytic properties.9 One of the active areas of research in our laboratories involves the remediation of soft heavy metals such as Cd2+, Tl+, Zn2+, Ag+, and Hg2+ from aqueous waste streams.15-17 It is wellknown that several of the H2Zr(PO4)2 phases not only are good ion-exchange materials but also have remarkable thermal and (10) Johnson, J. W.; Jacobson, A. J. Angew. Chem., Int. Ed. Engl. 1983, 22, 412. (11) Jacobson, A. J.; Johnson, J. W.; Brody, J. F.; Scanlon, J. C.; Lewandowski, J. T. Inorg. Chem. 1985, 24, 1782. (12) Alberti, G.; Constantino, U.; Allulli, S.; Tomassini, N. J. Inorg. Nucl. Chem. 1978, 40, 1113. (13) Zhang, B.; Clearfield, A. J. Am. Chem. Soc. 1997, 119, 2751. (14) Casciola, M.; Costantino, U.; Peraio, A.; Rega, T. Solid State Ionics 1995, 77, 229. (15) Gash, A. E.; Spain, A. L.; Dysleski, L. M.; Flashenriem, C. J.; Kalaveshi, A.; Dorhout, P. K.; Strauss, S. H. EnViron. Sci. Technol. 1998, 32, 1007. (16) Strauss, S. H. In Metal-Ion Separation and Preconcentration Progress and Opportunities; Bond, A. H., Dietz, M. L., Rogers, R. D., Eds.; ACS Symposium Series, No. 716; American Chemical Society: Washington, DC, 1999; p 156.

10.1021/ic990775c CCC: $19.00 © 2000 American Chemical Society Published on Web 11/08/2000

Zirconium and Hafnium Hydrogen Monothiophosphates radiolytic stabilities.18 There are numerous reports on the ionexchange properties of different phases of H2Zr(PO4)2 with alkali and alkaline earth metal ions in aqueous solution.1-3,6,19 However, we have no knowledge of any studies involving these materials and their ion-exchange behavior in the presence of aqueous soft heavy metal ions. This is not unexpected because one would assume that H2Zr(PO4)2 and similar compounds, with their hard Lewis basic, oxygen-rich interlayer galleries, would be more likely to undergo ion exchange with hard Lewis acids (i.e., alkali and alkaline-earth metal cations) than with soft Lewis acids such as Cd2+ on the basis of the principles of hard and soft acids and bases (HSAB).20 We proposed that a zirconium phosphate-based material containing interlayer galleries that have a soft Lewis basic character would selectively ion-exchange soft heavy metal ions. The monothiophosphate anion, PO3S3-,21,22 is a likely phosphate-based precursor to employ in the synthesis of such a material. Herein we report the synthesis and characterization of a new class of group 4 metal thiophosphate-based compounds, H2M(PO3S)2 (M ) Zr, Hf). We have demonstrated that these new compounds adopt a layered structure similar to that of the R-H2Zr(PO4)2‚H2O phase.1-3 In R-H2Zr(PO4)2‚H2O, one of the oxygen atoms on each PO43- group points into an interlayer gallery. This oxygen atom is protonated, and the proton can undergo ion exchange. Our data indicate that the new H2M(PO3S)2 compounds adopt a similar layered structure with the PO3S3- groups oriented so that the sulfur atoms, which are protonated, are preferentially oriented toward the interlayer galleries. This structural nuance gives these materials a soft Lewis basic nature that results in a unique ion-exchange selectivity compared with those of H2Zr(PO4)2 phases. Experimental Section Distilled water was purified and deionized (to 18 MΩ) with a Barnstead NANOPure purification system. Hydrochloric acid, nitric acid, and the metal salts were of reagent grade or better. Schlenk, glovebox, and high-vacuum techniques were employed for some experiments.23 Trisodium monothiophoshate, Na3PO3S, was synthesized from aqueous NaOH and PSCl3 according to a literature procedure.24 It should be noted that a slight excess of base was needed to obtain a yield comparable to the reported value of 60%.25 The powder X-ray diffraction (XRD) pattern and FT-IR spectrum of the as-prepared Na3PSO3 salt agreed very well with literature reports for this compound.21,26 Syntheses of the H2M(PO3S)2 compounds were performed by a modification of Clearfield’s method (M ) Zr, Hf).1 In a typical experiment, 2.40 g of ZrCl4 (10.3 mmol; Aldrich) and 5.56 g of Na3PO3S (30.9 mmol) were dissolved in separate 1 M HCl solutions. After the reactants were completely dissolved (typically less than 2 min), the two clear colorless solutions were mixed, resulting in the immediate formation of a white gelatinous precipitate. The precipitation was endothermic, as the reaction vessel became cool to the touch. The white (17) Dorhout, P. K.; Strauss, S. H. In Inorganic Materials Synthesis: New Directions for AdVanced Materials; Winter, C. E., Hoffman, D. M., Eds.; ACS Symposium Series, No. 727; American Chemical Society: Washington, DC, 1999; p 53. (18) Clearfield, A. Ind. Eng. Chem. Res. 1995, 34, 2865. (19) Dyer, A.; Shaheen, T.; Zamin, M. J. Mater. Chem. 1997, 7, 1895. (20) Pearson, R. G. In SurVey of Progress in Chemistry; Scott, A., Ed.; Academic Press: New York, 1969; Chapter 1. (21) Palazzi, M. Bull. Chim. Soc. Fr. 1973, 3246. (22) Thilo, V. E.; Schone, E. Z. Anorg. Allg. Chem. 1949, 259, 227. (23) Shriver, D. F.; Drezdzon, M. A. The Manipulation of Air-SensitiVe Compounds, 2nd ed.; Wiley-Interscience: New York, 1986. (24) Yasuda, S. K.; Lambert, J. L. Inorg. Synth. 1957, 5, 102. (25) Washburn, L. C.; Hayes, R. L. Inorg. Synth. 1977, 17, 193. (26) Brockner, W.; Jendrzok, B.; Menzel, F.; Jensen, V. R.; Ystenes, M. J. Mol. Struct. 1994, 319, 85.

Inorganic Chemistry, Vol. 39, No. 24, 2000 5539 gel mixture was stirred at room temperature for 24 h, after which it was centrifuged and the clear aqueous portion decanted. The gel was treated with 200 mL of water, followed by 30 min of stirring. The washing process was repeated twice. Finally, the gel was isolated and dried under vacuum at room temperature for 24 h. This synthetic procedure yielded a white polycrystalline powder of either H2Zr(PO3S)2 and H2Hf(PO3S)2. Neither compound was soluble in concentrated or dilute HCl, HNO3, or aqua regia. However, both were soluble in dilute aqueous HF. Attempts to improve the crystallinty of the final products using dilute aqueous HF did not result in a more crystalline powder. Syntheses of R-H2M(PO4)2‚H2O (M ) Hf, Zr) for comparison analyses were performed according to literature methods.1-3 A known amount of the appropriate MCl4 salt was dissolved in a solution of 1 M HCl to give a clear colorless solution. An excess of H3PO4 was rapidly added to the clear colorless solution, resulting in the immediate formation of a white gelatinous precipitate. The precipitate was washed and isolated using the same procedure described above for the H2M(PO3S)2 syntheses. Known amounts of the H2M(PO3S)2 compounds were dissolved in 10% aqueous HF for elemental analyses. The digestions were performed in Teflon beakers. The concentrations of Zr, Hf, and P in each solution were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) using a Perkin-Elmer P400 ICP atomic emission spectrometer equipped with a high-salt nebulizer. Calibration curves, which were linear in concentration over the range 5.00-1.00 mM, were constructed using known concentrations of metal salts; matrix matching was used for all experiments. One standard was reanalyzed for every five samples during the course of data collection, and the results were averaged. Qualitative elemental analysis was performed using energy-dispersive spectroscopy (EDS). EDS spectra were obtained on a Philips 505 scanning electron microscope (SEM) equipped with a Kevex detector and analysis software. Powder X-ray diffraction (XRD) measurements were recorded with a Philips diffractometer using Cu KR radiation, and the data were analyzed using Jade (MDI) software. Powder patterns were compared with those on the PDF database. Samples for FT-IR spectroscopy were pressed KBr disks of the compounds. Spectra were recorded at room temperature on a Nicolet 5PC spectrometer. FTRaman spectra were collected on powdered crystalline solids in glass tubes by using a Nicolet Magna-760 spectrometer with a Raman module. Differential scanning calorimetry data were collected on powdered samples sealed in aluminum pans by using a Rheometric Sciences DSC 1000 calorimeter. Thermogravimetric/mass spectroscopic analyses were performed on a TA Instruments TGA 2050 thermogravimetric analyzer interfaced with a Balzars Thermostar mass spectrometer. The thermal unit was ramped at 5 °C/min from room temperature to 200 °C under a constant flow of He carrier gas at a rate of 35 mL/min. Evolved gases were analyzed using scanning (1-300 amu) and multiple-iondetection modes. Solid-state 31P MAS NMR spectra were obtained at room temperature on a Varian Unity-400 spectrometer operating at a frequency of 161.8 MHz. A Varian MAS probe with a 7-mm rotor was used. The typical π/2 pulse width was 7 µs, and the recycle delay was 3 s. The spinning rate was typically 6 kHz. Chemical shifts are reported relative to phosphoric acid (85% H3PO4). The number of scans varied from 128 to 256. Intercalation reactions of H2M(PO3S)2 compounds with trialkylamines were performed by treating known amounts of H2M(PO3S)2 with an excess of either triethylamine or tributylamine (Aldrich). The resulting heterogeneous mixtures were stirred for 24 h for triethylamine and for 48 h for tributylamine. These mixtures were then filtered, and the products of the reactions were washed several times with ethanol before being dried at room temperature in air. All reactions yielded white polycrystalline powders as the products. Heavy-metal extraction experiments were performed for weighed samples of the solid extractants H2M(PO3S)2. A precise volume of 10.00 mM metal nitrate salt in either 0.1 or 0.001 M aqueous HNO3 (the guest solution) was added to each solid extractant sample such that all 6.34 mequiv/g of protons could be exchanged if the reaction proceeded to completion. The mixtures were vigorously stirred at 25 °C for 16 h

5540 Inorganic Chemistry, Vol. 39, No. 24, 2000 and filtered. The filtrates were collected, diluted to a constant volume, and analyzed for metal ions by ICP-AES. Portions of the metal guesthost complexes were digested in dilute aqueous HF until all of the solids had dissolved. The resulting homogeneous solutions were also analyzed by ICP-AES. Mass balance experiments for pH 3 solutions were performed using the same extraction experimental procedure described above. The pHs of the extraction solutions, both before and after extraction, were measured using an Orion 720A pH meter equipped with a glass pH electrode. The heavy-metal concentrations remaining in the extraction solutions were determined by ICP-AES. Back-extraction experiments on heavy-metal-loaded extractants were performed as follows. A sample of the loaded extractant was stirred with an excess of 3 M aqueous HCl for a period of 4-8 h. After the prescribed amount of time, the mixture was filtered, the filtrate collected, and the solid extractant washed with copious amounts of distilled deionized water. The solid extractant was then dried under vacuum at room temperature overnight. The filtrate was analyzed by ICP-AES to determine the amount of heavy metal recovered from the solid by 3 M HCl. In addition, a known portion of the washed extractant was dissolved in a small amount of dilute aqueous HF and the resulting solution analyzed for constituent elements by ICP-AES.

Results and Discussion

Gash et al.

Figure 1. Polyhedral representation of the layer arrangement in H2Zr(PO3S)2. Open circles are SH groups on the PO3S3- ions.

Synthesis and Elemental Composition of H2M(PO3S)2 (M ) Zr, Hf). The synthesis of R-H2M(PO4)2‚H2O involves the relatively straightforward addition of H3PO4 to a dilute HCl solution of a Zr4+ or Hf4+ salt.1 This synthesis results in the immediate precipitation of a white gel whose crystallinity improves upon refluxing the solid in H3PO4. Our synthetic approach to H2M(PO3S)2 (M ) Zr, Hf) was similar to that used for H2Zr(PO4)2 and is summarized in eq 1. Here excess 1 M HCl

(Zr,Hf)Cl4(aq) + 2Na3PO3S(aq) 9 8 24 h H2(Zr,Hf)(PO3S)2(s) + 6NaCl(aq) (1) thiophosphate was used. The products of the reactions were dried under vacuum at room temperature to give white polycrystalline solids. These syntheses were also performed in 2 and 3 M aqueous HCl with no change in either the crystallinity or the physicochemical properties of the products. As stated in the Introduction, we were particularly interested in the orientation of the sulfur atoms in these layered solids resulting from (1). A diagram of what the structure of H2Zr(PO3S)2 may resemble is shown in Figure 1. This representation is based on the following rationale. The monothiophosphate ion contains a phosphorus atom bound to three hard Lewis basic oxygen atoms and one soft Lewis basic sulfur atom. In a mixture with a hard Lewis acid like Zr4+ or Hf4+, we believed, on the basis of HSAB principles, that the oxygen atoms of the monothiophosphate groups would preferentially coordinate to the group 4 metal ions in the layers of the compound, leaving the sulfur atoms pointed into the interlayer spaces (similar to the case of the unique oxygen atom on each of the phosphate groups in H2Zr(PO4)2 phases27). On the basis of electronegativity principles, it is also likely that more than 75% of the negative charge on each PO3S3- anion would be concentrated on the oxygen atoms. ICP-AES measurements of digests of our compounds yielded the following results. For both the zirconium- and the hafniumbased compounds, the phosphorus:metal ratio was 2.0:1.0. Therefore, two PO3S3- anions are present per M4+ cation. This implies that there must be a positive ion or ions that chargebalance with a sum total of 2+. According to (1), there are

only two possible cations present to balance that charge, H+ and Na+. We used EDS to identify the additional chargebalancing ions. The spectrum of H2Zr(PO3S)2 contained peaks corresponding to P, Zr, and S. The spectrum of H2Hf(PO3S)2 contained peaks corresponding to Hf, P, and S. Neither of the spectra showed any peaks for sodium. Therefore, the chargebalancing ion must be H+, which was expected, given the low pHs of the solutions used to synthesize the compounds. Powder X-ray Diffraction of M(HPO3S)2 (M ) Zr, Hf). Powder XRD patterns of H2Zr(PO3S)2 and H2Hf(PO3S)2 are shown in Figure 2. The XRD patterns of the two compounds consist of only a few broad, ill-defined diffraction peaks that are qualitatively like those of several reported H2Zr(PO4)2‚xH2O phases.1,3,4 Attempts to improve the crystallinity of the materials by refluxing in thiophosphate solutions or by recrystallization from aqueous HF did not yield more highly crystalline solids. The XRD patterns for the two H2M(PO3S)2 compounds are indistinguishable (this was also found for H2Zr(PO4)2 and H2Hf(PO4)28). The lowest-angle line of H2Zr(PO3S)2 (presumably the 001 reflection) corresponds to a spacing of ∼9.4 Å, a distance 1.8 Å greater than that observed for R-H2Zr(PO4)2‚H2O. There is a report describing the β phase of H2Zr(PO4)2, the XRD pattern

(27) Troup, J. M.; Clearfield, A. Inorg. Chem. 1977, 16, 3311.

(28) Chernorukov, N. Russ. J. Inorg. Chem. (Engl. Transl.) 1977, 22, 1119.

Figure 2. Powder X-ray diffraction patterns of (a) H2Zr(PO3S)2 and (b) H2Hf(PO3S)2. The asterisks denote the diffraction lines from the internal silicon standard.

Zirconium and Hafnium Hydrogen Monothiophosphates of which exhibits a low-angle peak corresponding to a spacing of ∼9.4 Å.2 However, a comparison of the XRD patterns of β-H2Zr(PO4)2 and H2Zr(PO3S)2 suggests that these phases are not similar in structure. The β-H2Zr(PO4)2 phase is believed to be the dehydrated phase of γ-zirconium hydrogen phosphate,2 which has been shown to have the formula Zr(PO4)(H2PO4)‚ 2H2O. Both the β and γ phases contain two distinctly different types of phosphate groups.9,29 The R phase of H2Zr(PO4)2 (R-H2Zr(PO4)2‚H2O) is a layered compound with a water of hydration as well as pendant OH groups in the interlayer galleries. The difference between the R phase and the β and γ phases lies in the orientation of the phosphate ions within the layers. In the R phase, the pendant oxygen atoms are interdigitated and give rise to an interlayer distance of only ∼7.6 Å. In the β and γ phases, one phosphate unit is completely encased within the layer, as PO43-, and the other is dangling from the top and bottom of the layers as H2PO4-, and this results in larger interlayer distances of 9.4 and 12.2 Å, respectively. On the basis of the observed interlayer distance of H2Zr(PO3S)2 alone, it might seem that the structure of H2Zr(PO3S)2 is similar to that of β-H2Zr(PO4)2, but this is not the case (see below). The XRD patterns of H2Hf(PO3S)2 and H2Hf(PO4)2 are also very similar (the latter compound has the same XRD pattern as R-H2Zr(PO4)2‚H2O and therefore should probably be formulated as R-H2Hf(PO4)2‚H2O).28 If H2Zr(PO3S)2 and H2Hf(PO3S)2 have structures similar to that of R-H2Zr(PO4)2‚H2O, the difference in d spacings could be related to the difference in the sizes of the oxygen and sulfur atoms and/or a difference in the stacking sequences. It is possible that H2Zr(PO3S)2 and H2Hf(PO3S)2 possess a stacking sequence that does not allow the sulfur atoms to become as interdigitated as the pendant oxygen atoms in R-H2Zr(PO4)2‚H2O. Clearfield has reported that the interlayer spacing in R-H2Zr(PO4)2‚H2O is highly dependent on the number of waters of hydration and that d spacings as large as 11.2 Å could be observed.30 However, since we do not find waters of hydration in H2Zr(PO3S)2 and H2Hf(PO3S)2 on the basis of TGA/MS, a hydration argument cannot be used to explain the larger d spacing in these compounds. We believe that the most likely cause of the 9.4-Å d spacings in H2Zr(PO3S)2 and H2Hf(PO3S)2 is the stacking sequence shown in Figure 1. Additionally, the relatively poor crystallinity of H2Zr(PO3S)2 and H2Hf(PO3S)2, which might be a manifestation of imperfect layer stacking, could also be a contributing factor. We collected additional evidence to support the structure and stoichiometry suggested by the elemental analysis and XRD experiments. It is well-known that R and γ phases of zirconium hydrogen phosphate undergo intercalation reactions with a wide variety of molecular, ionic, and polymeric species.7,31,32 One such molecular intercalant family that has been extensively investigated and characterized is organic amines.33,34 It has been shown that the lamellar zirconium hydrogen phosphate phases undergo intercalation with a variety of amines and that the proton attached to the pendant OH group is transferred to the amine nitrogen atom, creating an intercalated ammonium cation. To further demonstrate the similarity in structure and reactivity (29) Alberti, G.; Bernasconi, M. G.; Casciola, M. React. Polym. 1989, 11, 245. (30) Clearfield, A. Mater. Chem. Phys. 1993, 35, 257. (31) Herzog-Cance, M. H.; Jones, D. J.; El Mejjad, R.; Roziere, J. J. Chem. Soc., Faraday Trans. 1992, 88, 2275. (32) Costantino, U.; Casciola, M.; Pani, G.; Jones, D. J.; Roziere, J. Solid State Ionics 1997, 97, 261. (33) MacLachlan, D. J.; Morgan, K. R. J. Phys. Chem. 1990, 94, 7656. (34) MacLachlan, D. J.; Morgan, K. R. J. Phys. Chem. 1992, 96, 3458.

Inorganic Chemistry, Vol. 39, No. 24, 2000 5541

Figure 3. Powder X-ray diffraction patterns of (a) H2Zr(PO3S)2, (b) triethylamine-intercalated H2Zr(PO3S)2, and (c) tributylamine-intercalated H2Zr(PO3S)2.

of our H2M(PO3S)2 compounds to their phosphate analogues, we performed intercalation reactions with both triethylamine and tributylamine. Figure 3 shows the XRD patterns of H2Zr(PO3S)2 and the products of intercalation reactions with triethylamine and tributylamine, respectively. As discussed above, the XRD pattern of H2Zr(PO3S)2 displays a low-angle peak (d ∼ 9.4 Å) that corresponds to the (00l) interlayer distance of this compound. The XRD patterns of the intercalation reaction products containing triethylamine and tributylamine display shifted (00l) peaks that correspond to interlayer spacings of 12.9 and 15.9 Å, respectively. The XRD pattern of the tributylamine intercalate contains a small peak at ∼9.4 Å which is likely due to incomplete intercalation. The new spacings of 12.9 and 15.9 Å correspond to interlayer distance increases of ∼3.5 and ∼6.5 Å for the triethylamine and tributylamine intercalates, respectively. These d-spacing increases are comparable to the those observed for R-H2Zr(PO4)2‚H2O and its amine-intercalated equivalents.35,36 An identical set of intercalation experiments run with H2Hf(PO3S)2 yielded results that were comparable to those run with the zirconium compound. Solid-State 31P NMR Spectrum of H2Zr(PO3S)2. Phosphorus31 MAS NMR spectroscopy has been extensively used to characterize the many phases of zirconium hydrogen phosphate and their intercalation products.33,34,37 The 31P chemical shifts of H2PO4-, HPO42-, and PO43-, which occur at ca. -10, -20, and -30 ppm, respectively, for various zirconium hydrogen phosphate phases, are sufficiently separated that these three species are clearly distinguishable.38 Figure 4 shows the 31P MAS NMR spectrum of H2Zr(PO3S)2, which exhibits an intense resonance at 2.74 ppm and a very weak resonance at -21 ppm. The weak peak is probably due to a small amount of R-H2Zr(PO4)2‚H2O present in the sample, which might have formed during the synthesis of H2Zr(PO3S)2 by the slow hydrolysis of PO3S3- to PO43- under the reaction conditions (vide infra). An alternative explanation that a small percentage of the PO3S3groups are oriented differently from the majority of the PO3S3groups (thereby producing a small percentage of zirconium atoms bonded to five oxygen atoms and one sulfur atom) is less likely because of the >22 ppm difference in δ values for the two peaks. For reference, Na3PO3S showed a resonance at 32 ppm with respect to H3PO4. (35) Gupta, J. P.; Nowell, D. V. J. Chem. Soc., Dalton Trans. 1979, 1178. (36) Alberti, G.; Costantio, U. In Intercalation Chemistry; Whittingham, M. S., Jacobson, A. J., Eds.; Academic Press: New York, 1982; Chapter 5. (37) Clayden, N. J. J. Chem. Soc., Dalton Trans. 1987, 1877. (38) Nakayama, H.; Eguchi, T.; Nakamura, N.; Yamaguchi, S.; Danjyo, M.; Tsuhako, M. J. Mater. Chem. 1997, 7, 1063.

5542 Inorganic Chemistry, Vol. 39, No. 24, 2000

Gash et al.

Figure 4. 31P MAS NMR spectrum of H2Zr(PO3S)2. The arrow indicates the resonance due to a small amount of R-H2Zr(PO4)2‚H2O, and the asterisks denote the spinning sidebands.

Figure 6. FT-IR and Raman spectra of H2Hf(PO3S)2. Table 1. Vibrational Data (cm-1) for H2M(PO3S)2 and H2-xMM′x(PO3S)2 (M ) Zr, Hf; M′ ) Zn, Cd) H2Zr(PO3S)2 IR

Raman

425 618 833 1054

247 437 650 833 1063

2545 3420

Figure 5. FT-IR and Raman spectra of H2Zr(PO3S)2.

The impurity of R-H2Zr(PO4)2‚H2O in samples of H2Zr(PO3S)2 must be less than 5 mol %, since the XRD pattern for H2Zr(PO3S)2 does not exhibit the expected peaks for R-H2Zr(PO4)2‚H2O. The absence of other prominent peaks in the NMR spectrum suggests that there is only one type of 31P environment in H2Zr(PO3S)2, similar to the situation in R-H2Zr(PO4)2‚H2O. In contrast, γ-H2Zr(PO4)2‚2H2O contains both PO43-and H2PO4groups, which give rise to two distinct solid-state 31P NMR resonances.37 These NMR comparisons support the hypothesis, based on the XRD results presented above, that the structure of H2Zr(PO3S)2 is similar to that of R-H2Zr(PO4)2‚H2O. Vibrational Spectroscopy of H2M(PO3S)2. The FT-IR and FT-Raman spectra of H2Zr(PO3S)2 and H2Hf(PO3S)2 are shown in Figures 5 and 6, respectively. Band positions for these and other compounds are listed in Table 1. Vibrational spectroscopy was previously used to characterize various zirconium hydrogen phosphate phases and their intercalates.31,32,39 In R-H2Zr(PO4)2‚ H2O, three of the oxygen atoms on the phosphate groups are bonded to the zirconium atoms; the fourth oxygen atoms on the phosphate groups form OH moieties (ν(OH) ) 3420 cm-1) that form the interlayer galleries. There are three spectral regions of interest. The first region, at 3400-3600 cm-1, indicates that there are some OH groups (39) Slade, R. C. T.; Knowles, J. A.; Jones, D. J.; Roziere, J. Solid State Ionics 1997, 96, 9.

2558

H2Hf(PO3S)2 IR

Raman

614 833 1062

249 440 653 833 1080 1133 2558

2544 3420

H0.50ZrZn0.75(PO3S)2 H0.2ZrCd0.9(PO3S)2 IR 457 625 646 (sh) 1019 1051 1219 3446

Raman 261 355 452 520 669 1040 2555

IR 458 597 629 1012 1033 1210 3467

Raman 247 445 668 1014 1034

in samples of both H2Zr(PO3S)2 and H2Hf(PO3S)2. The frequency of the weak ν(OH) band in each spectrum corresponds to R-H2Zr(PO4)2‚H2O or R-H2Hf(PO4)2‚H2O, which is not surprising since small impurities (