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

Abstract

The two Zr^IV 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 (T_onset 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 (T_dec ≈ 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): T_max = 103 vs 122 °C, respectively. These findings may help in the rational design of performant MOF materials for chemical hydrogen storage purposes.