Ceric ammonium nitrate is an orange-red crystalline solid at room temperature. It is highly soluble in water and alcohols; its aqueous solution is strongly oxidizing and acidic. The solid is hygroscopic and readily deliquesces. Cerium exists in the +4 oxidation state and exhibits outstanding oxidizing properties. It is a commonly used single-electron oxidizing agent in organic synthesis, employed in the oxidation of alcohols, deprotection, and the nitration of aromatic rings. It is also used as an etching reagent, a standard oxidizing agent in titrimetric analysis, and a raw material for fluorescent materials.

Ceric ammonium nitrate as a catalyst in organic synthesis

Cerium belongs to the family of the lanthanides, which constitute (together with scandium and yttrium) the so-called rare earth elements. Cerium is the most abundant of these elements and has been estimated to constitute about 0.0046% of the Earth’s crust by weight. In fact, cerium cannot be considered “rare” at all, because its abundance is similar to or higher than that of better-known elements such as copper, bromine, cobalt, zinc, and tin. Cerium has a property, unique among the lanthanides, that explains its ability to participate in one-electron transfer reactions, its ability to exist in two stable, adjacent oxidation states +3 and +4 with configurations [Xe]4f1 and [Xe]4f0, respectively, due to enhanced stabilization of the latter due to the presence of the vacant f shell. The high reduction potential of Ce(IV) (1.61 V vs normal hydrogen electrode) makes Ce(IV) a very efficient oxidizing reagent as compared to other cations, and for this reason its salts, and especially the commercially available ceric ammonium nitrate (CAN), have found widespread use as one-electron oxidants.  More specifically, CAN has been found to be chemically superior in many respects to the widely employed manganese triacetate for the generation of radicals. Ceric ammonium nitrate has the additional advantages of having a low toxicity besides being inexpensive, reasonably soluble in many organic media, air-stable, and easily handled, allowing for a considerable degree of experimental simplicity.[1]

The possibility of employing Ce(IV)-generated radicals for the synthetically useful generation of carbon?carbon bonds was first discovered by Heiba and Dessau and has been subsequently developed to a considerable extent, together with the use of ceric ammonium nitrate for the generation of carbon?heteroatom bonds. CAN is an excellent, multipurpose catalyst that can be used to promote a wide range of synthetically relevant reactions that go well beyond its traditional role as an oxidant. These reactions are characterized by their experimental simplicity and mild reaction conditions. From a mechanistic point of view, they may proceed via processes initiated by a one-electron oxidation, or alternatively ceric ammonium nitrate may act as a Br?nsted acid catalysis, due to the generation of protons by hydrolysis of the nitrate anion, or as a Lewis acid. In this connection, the low cost and air stability of CAN may make it a useful alternative to the expensive, hygroscopic lanthanide triflates. While the results described here are a more than adequate proof of the validity of ceric ammonium nitrate as a catalyst, most of the work that we summarize here has been published in the past few years, meaning that there is still much new ground to explore. We hope that this Review will serve to stimulate research in this fascinating and very useful area of catalysis.

Removal of PFAS from water by ceric ammonium nitrate

Per- and polyfluoroalkyl substances (PFAS) are toxic and xenobiotic compounds that have been widely used in fire extinguishers, carpet guards, paper, and non-stick cookware. However, these groups of organofluorines are highly resistant to degradation and are not easily separable by the usual water purification techniques. Scientists report a new approach for precipitation of the commercially available PFOS (as potassium salt) from water (<20 μM) using ceric ammonium nitrate (CAN). This study was initiated from the perspective of generating in situ high valent/mixed valent oxide species/nanoparticles from FeCl2 and CoCl2 that can oxidatively degrade PFAS. In preliminary studies, ceric ammonium nitrate was used as an oxidant and resulted in significant reduction in the concentration of PFAS of the supernatant aqueous solution whenever CAN was used. Thereafter a range of concentrations of CAN (from 0.094 mM to 0.75 mM) were tested in consecutive experiments keeping PFOS concentration unchanged at 15 μM. After the treatment with 0.38 mM concentration ceric ammonium nitrate, the supernatant shows ～80% removal of PFOS

In conclusion, ceric ammonium nitrate effectively precipitates out both PFOS and PFOA even at low concentration (<20 μM) and clearly reacts more efficiently with the former. A clear trend of CAN > Ce(SO4)2 > CeCl3 > CoCl2 ≥ FeCl2 > NH4NO3 was observed for precipitating out PFOS and PFOA from the aqueous medium. These PFASs are reported among the most difficult ones to degrade. The CeIV center plays an important part in this, as reflected by the comparative studies using CAN, Ce(SO4)2, CeCl3 and AN. The oxidation state of the cerium center, as well as the size of the PFASs play crucial roles in the successful removal/precipitation from the aqueous medium. Thus, a similar simple strategy can potentially be used in future for the design of highly effective filter beds (e.g., using a combination of GAC/PAC and ceric ammonium nitrate) for purifying PFAS contaminated water at larger scale.

References

[1]Sridharan, V., & Menéndez, J. C. (2010). Cerium(IV) Ammonium Nitrate as a Catalyst in Organic Synthesis. Chemical Reviews, 110(6), 3805–3849. https://doi.org/10.1021/cr100004p

[2]Sun J, Jennepalli S, Lee M, O'Carroll DM, ?kermark B, Manefield MJ, Das B, Kumar N. Removal of per- and polyfluoroalkyl substances (PFAS) from water by ceric(iv) ammonium nitrate. RSC Adv. 2021 May 13;11(29):17642-17645. doi: 10.1039/d1ra02635f. PMID: 35480216; PMCID: PMC9032701.