Conspectus
The 2-D
layers of the inorganic ion exchanger α-zirconium
phosphate (Zr(HPO4)2·H2O, α-ZrP)
make this compound particularly stable to low pH, high temperature,
and ionizing radiation. Initially studied for its ion exchange properties,
once the conditions for its synthesis in crystalline form was accomplished
by James Stynes and Abraham Clearfield in 1964, numerous other types
of studies and applications followed. Extensive studies in the 1960s
and 1970s on the thermodynamics of ion exchange led to insights into
the intercalation mechanism of this material. The Clearfield group
solved the crystal structure in 1968 and refined it in 1977. Powder
methods were pioneered by the Clearfield group to solve the structure
of this type of materials. In 1968 Giulio Alberti reported means to
prepare zirconium phosphonates expanding the chemistry of these layered
compounds. New phases of ZrP were also discovered (e.g., γ,
θ, λ, τ) and the applications ranged from heterogeneous
catalysis to intercalation chemistry and solid-state proton conductors.
Methods to exfoliate the layers of ZrP were developed in the 1990s
as interest grew in new applications of these types of materials.
For example, protein and enzyme intercalation was accomplished starting
in the 1990s by the McLendon, Mallouk, and Kumar groups. In the early
2000s, the Colón group pioneered the use of the θ phase
of ZrP for the direct intercalation of large inorganic metal complexes
that could not be directly intercalated into the α phase. Initial
studies in the Colón group ranged from applications of these
directly intercalated ZrP derivatives in photophysics and photochemistry,
amperometric biosensors, vapochromism, and vapoluminescence. Over
the past decade, new applications of these materials have been developed
in anticancer drug delivery and electrocatalysis of the oxygen evolution
reaction (OER). ZrP has now proven to be a promising drug nanocarrier
and its unique chemical microenvironment provided by the α-type
layers and the interlayer space enhances catalytic activity for numerous
types of reactions. Further elucidation of the catalytic active species
under operando conditions as well as the chemical structure of drug-intercalated
derivatives should provide new insights that will advance the design
and development of new compounds with desired properties. The initial
pioneering efforts of Clearfield and Alberti are being continued by
numerous research groups providing new exciting areas of development
on the chemistry of layered M(IV) phosphate and phosphonate compounds.
In this Account we present the efforts of the Colón group during
the past decade on studies of the applications of ZrP for anticancer
drug delivery and electrocatalysis of the OER.
