Manganese occurs in a variety of oxidation states in its oxides, most notably +2, +3, and +4 in nature, and up to +7 in synthetic compounds. Because these different oxidation states are available to manganese, it is possible for manganese containing oxides to engage in oxidation chemistry, converting various compounds to more oxidized and often more useful forms. In order for manganese oxide systems to engage in oxidation chemistry, the average oxidation state of the manganese must be greater than +2, since the +2 oxidation state is the lowest stable oxidation state of Mn in its oxides. Hence, it is the compounds containing some manganese in the +3 and higher oxidation states that can engage in oxidation chemistry and catalysis. Manganese (IV) compounds are well known and are used in a variety of oxidation reactions. For example, manganese dioxide (MnO.sub.2) has been used in the manufacture of chlorine gas from hydrogen chloride and the oxidation of aniline to hydroquinone. See "Chemistry of the Elements", N. N. Woodward and A. Earnshaw, Pergammon Press, Oxford, pp 1219-20 (1984). A molecular manganese-oxo cluster is involved in the oxidation of water to oxygen in the photosynthesis process used by plants. See Yachandra et.al., Science, 260, 675-679 (1993). Because manganese has stable oxidation states of +4, +3 and +2, manganese oxides can be used in batteries.
Manganese oxides can have layered structures or three-dimensional microporous structures. S. Bach et al., Electrochimica Acta, 36, 1595-1603 (1991), P. LeGoff et al., Mat. Res. Bull., 31, 63-75 (1996), P. Strobel et al., Mat. Res. Bull., 28, 93-100 (1993), Y. Shen et al., Science 260, 511-515 (1993). Finally, the ion-exchange properties of manganese oxide compositions have been reported by Q. Feng et al. in Chem. Mater., 7, 148-153 and 1722-1727 (1995).
A handful of Mn(III)-containing phosphate compounds are known, some occurring in nature, while others have been synthesized. Among the mineral phosphates containing Mn(III) are
__________________________________________________________________________ Bermanite Mn.sup.2+ (H.sub.2 O).sub.4 [Mn.sup.3+ .sub.2 (OH).sub.2 (PO.sub.4).sub.2 ] Kampf and Moore, American Mineralogist, 61, 1241 (1976) Robertsite Ca.sub.3 Mn.sup.3+ .sub.4 (OH).sub.6 (H.sub.2 O).sub.2 [PO.sub.4 ].sub.4 Moore and Ito, American Mineralogist, 59, 48 (1974) Pararobertsite Ca.sub.2 Mn.sup.3+ .sub.3 (PO.sub.4).sub.3 O.sub.2 *3H.sub.2 O Roberts et al., Can. Mineral., 27, 451 (1989) Mitridatite Ca.sub.6 (H.sub.2 O).sub.6 [(Fe.sup.3+ .sub.8.2 Mn.sup.3+ .sub.0.8)O.sub.6 (PO.sub.4).sub.9 ]* Rogers and Brown, Am. Mineral., 64, 3H.sub.2 O 169 (1979) Purpurite/Heterosite (Fe,Mn)PO.sub.4 Blanchard and Abernathy, Florida Scientist, 43, 257, (1980) -- MnPO.sub.4 *1.5H.sub.2 O Schwab, Soil Sci. Soc. Am. J., 53, 1654, (1989) __________________________________________________________________________
More information on Mn(III)-containing minerals, especially structure can be found in the review by Hawthorne in Z. Kristallogr., 192, 1 (1990). These materials fall in the classification of octahedral-tetrahedral framework structures, where Mn(m) is always found in octahedral coordination.
There are a number of examples of Mn(III)-containing phosphates that have been prepared by hydrothermal synthesis, e.g., KMn.sub.2 O(PO.sub.4)(HPO.sub.4), Lightfoot et. al., J. Solid State Chem., 73, 325-329, (1988), and NH.sub.4 Mn.sub.2 O(PO.sub.4)(HPO.sub.4).H.sub.2 O, Lightfoot et. al, J. Solid State Chem., 78, 17-22, (1989). These materials were obtained via hydrothermal transformation of Mn.sub.3 O.sub.4 in the presence of KH.sub.2 PO.sub.4 or NH.sub.4 H.sub.2 PO.sub.4 at 400.degree. C. and 220.degree. C. respectively, relatively harsh conditions. H.sub.3 PO.sub.4, and water at 200.degree. C., Lightfoot et. al., Inorg. Chem., 26, 3544-3547, (1987). Solid-state ion-exchange of MnPO.sub.4 *H.sub.2 O with LiNO.sub.3 (4 weeks at 200.degree. C.) led to LiMn(PO.sub.4)(OH), Aranda et. al., Angew. Chem. Int. Ed. Engl., 31, 1090-1092, (1992). A number of Mn(III) pyrophosphates, e.g., NH.sub.4 MnP.sub.2 O.sub.7, are known and have found use as pigments, Lee et. al., J. Chem. Soc. (A), 559-561, (1968). Mn(III) has also been substituted for up to one quarter of the VO.sup.3+ groups in the VOPO.sub.4 * 2H.sub.2 O structure, forming [(Mn(H.sub.2 O)).sub.x (VO).sub.1-x PO.sub.4 ]*2 H.sub.2 O, Richtrova et. al., J. Solid State Chem., 116, 400-405, (1995). Finally, an example of a Mn(III)-phosphate complex is the water-soluble dipyridyl complex [Mn(III)(bpy)(HPO.sub.4)(H.sub.2 PO.sub.4)].sub.x, Sarneski et.al., Inorg. Chem., 32, 3265-3269, (1993).
In contrast to these references, applicant has synthesized crystalline manganese phosphate compounds which contain Mn(III) and which have an extended network. By extended network is meant that the defining Mn--P--O structural unit of the material repeats itself into at least two adjacent unit cells without termination of bonding, i.e., the material is not molecular. See "Structural Inorganic Chemistry, Fifth Edition," A. F. Wells, Clarendon Press, Oxford, pp. 11-15, (1984). The network can be one-dimensional (a linear chain), two-dimensional (layered) or three-dimensional. The three dimensional network may or may not be a microporous network. By Mn(III)-containing phosphate, it is meant that the average oxidation state of Mn is greater than 2.0 but less than or equal to 3.0, indicating the presence of some Mn(III). These compositions are prepared by trapping the desired manganese oxidation state via titrimetric methods, reduction of a novel manganese(IV) phosphate solution, or hydrothermal transformations of birnessite-like (e.g., Na.sub.4 Mn.sub.14 O.sub.27 *xH.sub.2 O) materials, all in the presence of excess phosphate at specific reaction conditions. Besides MnPO.sub.4 *H.sub.2 O, which is ubiquitous in Mn(III) phosphate chemistry and not part of this invention, these methods have not yielded any of the synthesized compounds or mineral structures noted above. This may be due to the mild conditions and the higher reactivity of the reagents employed. In addition, applicant discloses the first manganese(III) and mixed valence Mn(III)/Mn(III) phosphates containing organoammonium cations. Further, applicant has also synthesized metallo manganese phosphates where a portion of the manganese is replaced by a metal such as iron (III), aluminum, gallium, etc.