History and Classification of Non-Siliceous Hybrid Materials - Mesoporous Organic-Inorganic Non-Siliceous Hybrid Materials - page 13

synthesis of homogeneous hybrids. The homogeneous and efficient incorporation
of organic functional groups into the framework of the materials can be realized,
allowing for uniform physicochemical properties from the external surface to the
internal skeleton.
Metal phosphonate chemistry is originated from the corresponding inorganic
phosphate counterparts. In the beginning, gels were refluxed in strong H 3 PO 4 and
crystallized into what later came to be known as ʱ -zirconium phosphate ( ʱ -ZrP)
[ 45 ]. This compound has a clay-like structure in which the ZrO 6 octahedra are
sandwiched between layers of phosphate tetrahedral. Dines et al. first conceived
of producing porous materials by cross-linking the ʱ -zirconium phosphate-type
layers using diphosphonic acids, H 2 O 3 P-R-PO 3 H 2 , where R may be an alkyl or
aryl group [ 46 ]. The strategy was to choose the cross-linking groups that are large
and then space them such that different size of pores would result. However, the
area subtended by a phosphate group on the ʱ -ZrP layer was 24 Å 2 . Given the
fact that an alkyl or aryl group spaced every 5.3 Å apart on the layer occupies
most of the area between pillars, there should be no microporosity. To overcome
this restriction, the Dines group used phosphorous acid as a spacer group, together
with biphenyl as the R group. The idea was to space the biphenyl pillars two or
three positions apart, thus creating microporosity. Clearfield and co-workers con-
tributed a lot to the intimately relevant work concerning diphosphonate derivatives
[ 47 ], which contains 2D sheets of ZrO 6 octahedra sandwiched between phosphate
(or phosphonate) layers, which were akin to many other inorganic clays. Porous
zirconium diphosphonates are synthesized by combining both a rigid diphospho-
nate (such as biphenylene bis(phosphonate)) and phosphite (HPO 3

−

) or phosphate,
where the average pore size could be adjusted by varying the ratio of acids used in
the synthesis [ 48 ]. Figure 2.5 illustrates a possible structure for nanoporous zirco-
nium diphenylenebis(phosphonate)/phosphate.
One important advantage of this approach is that materials with pores in the
range of micropores and mesopores may be obtained. That is to say, an advantage

Fig. 2.5 An idealized
structure of a porous
zirconium phosphonate. ZrO 6
octahedra are shown in gray,
PO 3 C tetrahedra in light gray,
and carbon atoms as light
gray spheres