Ammonia Borane and Hydrazine Bis(borane) Confined within Zirconium Bithiazole and Bipyridyl Metal–Organic Frameworks as Chemical Hydrogen Storage Materials

The two ZrIV metal–organic frameworks
(MOFs)
[Zr6O4(OH)4(TzTz)6] (Zr_TzTz) and [Zr6O4(OH)4(PyPy)6] (Zr_PyPy; H2TzTz = [2,2′-bithiazole]-5,5′-dicarboxylic
acid, H2PyPy = 2,2′-bipyridine-5,5′-dicarboxylic
acid) sharing an UiO-67-type crystal structure were used as porous
hosts for the entrapment of the lightweight BN hydrides ammonia borane
(NH3·BH3, AB) and hydrazine bis(borane)
(BH3·NH2–NH2·BH3, HBB). The resulting [hydride@MOF] composites were characterized
in the solid state through a plethora of complementary techniques:
multinuclear (1H, 13C, 15N, 11B) solid-state NMR spectroscopy, synchrotron X-ray powder
diffraction, temperature-programmed decomposition, surface area and
pore size distribution analysis. The NMR evidence shows that, after
nanoconfinement in the MOF pores, the hydrides partially lose H2 through a reaction with the acidic MOF hydroxyl groups, leading
to the formation of direct B–O bonds and dangling boryl-amine
units anchored to the metal nodes. The crystal structures of the adducts
have been solved through an effective combined XRPD/Pair Distribution
Function (PDF) analysis carried out on high-resolution synchrotron
powder X-ray diffraction data, building the initial guess models from
the multinuclear NMR information. To our knowledge, this is the first
example reported to date of a [AB/HBB@MOF] composite crystal structure
and the first example ever reported of a [HBB@MOF] adduct. The amino-boryl
units tend to lose the amine part for prolonged reaction times. However,
these fragments are still capable of releasing additional hydrogen
upon heating the materials at mild temperatures (Tonset for H2 evolution = 57, 55, and 53 °C
for [AB@Zr_TzTz], [HBB@Zr_TzTz], and [AB@Zr_PyPy] respectively, from TPD-MS analysis), demonstrating
the beneficial effect of the MOF scaffold in reducing the hydrogen
evolution temperature compared with the pure hydride (Tdec ≈ 150 °C for AB and 140 °C for HBB).
In particular, Zr_TzTz shows a better performance than Zr_PyPy with the same hydride, confirming the superior catalytic
efficiency of thiazole compared with that of pyridine. From a hydride
perspective, AB can release pure H2 at lower temperatures
than HBB when trapped into the same MOF (Zr_TzTz): Tmax = 103 vs 122 °C, respectively. These
findings may help in the rational design of performant MOF materials
for chemical hydrogen storage purposes.