Calcium Vapor Adsorption on the Metal–Organic Framework NU-1000: Structure and Energetics

Abstract

The nature and energy of the reactions between calcium vapor and the internal surfaces of the metal–organic framework (MOF) NU-1000 have been studied by adsorption microcalorimetry, low energy He+ ion scattering spectroscopy (LEIS), X-ray photoelectron spectroscopy (XPS), and Kohn–Sham density functional theory (DFT). NU-1000 is one of the most stable MOFs with transition-metal-oxide nodes, and thus it is of interest as a potential catalyst or catalytic support when modified with other metals. The reaction heats of Ca with NU-1000 are high below 2 monolayers (ML) Ca coverage (570–366 kJ/mol), attributed (based on DFT) to Ca reacting first with free benzoic acid functionalities or water impurities, then with H2O and OH groups on the Zr6 nodes to produce Ca­(OH)2 clusters. With higher Ca doses, the heat of Ca reaction decreases asymptotically to the sublimation enthalpy of bulk Ca (178 kJ/mol), attributed to the formation of Ca­(solid) nanoparticles on the external surface, which only occurs after all of the H2O and OH groups are titrated deeply enough (∼20 nm) such that slow Ca diffusion prevents further reaction.

Heats of Adsorption of N2, CO, Ar, and CH4 versus Coverage on the Zr-Based MOF NU-1000: Measurements and DFT Calculations

“…We had published a previous paper about the adsorption energy of calcium vapor onto this same type of NU-1000 sample synthesized by that same group, in which they were coauthors. 12 The Experimental Section of that joint paper with them says regarding defects: " 1 H NMR of the digested material confirmed that activation was complete, with the only carboncontaining component being the tetra-acid of the MOF organic linker, pyrene-tetraphenylcarboxylate. Nitrogen adsorption measurements of the activated MOF yielded the Brunauer− Emmett−Teller (BET) surface area, the pore volume, and the isotherm shape expected for a clean and undamaged sample.…”

Pyrene-based metal organic frameworks: from synthesis to applications

“…Furthermore, the bonding energetics of metal vapor adsorption onto NU-1000 for the metalation of the nodes has been investigated using different experimental techniques and density functional theory (DFT). 139 The other common method for cluster metalation in a pyrene-based MOF is solvothermal deposition in MOF (SIM), which is complementary to gas-phase metalation of the node (AIM). In this method, the modifying metal species are dissolved in a non-reactive solvent (e.g.…”

Computational Design of Functionalized Metal–Organic Framework Nodes for Catalysis

“… 95 , 96 The M06-L functional has been validated for many applications relevant to catalysis 97 − 119 (including some cases very similar to the cases discussed in the next section but also including studies relevant to the broader applicability for transition metal catalysis and barrier heights) and MOFs. 120 , 121 …”

“…Among the density functionals that have proven useful in the KS-DFT context, we single out the Minnesota functional M06-L, , which has shown a good performance in reproducing experimental trends and being able to accurately predict spin-state energetics in MOFs in good agreement with high-level methods, such as the CASPT2 method. , The M06-L functional has been validated for many applications relevant to catalysis − (including some cases very similar to the cases discussed in the next section but also including studies relevant to the broader applicability for transition metal catalysis and barrier heights) and MOFs. , …”

“…The OH and OH/H 2 O motifs of metal-oxide nodes can be functionalized with a wide variety of metal precursors, in analogy to the functionalization of hydroxylated bulk surfaces. − To date there are reports on gas-phase and solution functionalization of Zr- and Hf-based MOF nodes with Mg­(II), Al­(III), ,, Si­(IV), Ca­(II), V­(V), Fe­(II), Fe­(III), Co­(II), ,,,− Ni­(II), ,, Cu­(II), ,, Zn­(II), ,, Zr­(IV), Nb­(V), Mo­(VI), , Rh­(I), In­(III), Ir­(I), , and Au­(I) . In many cases, modeling has been used to understand the structures that have been synthesized.…”