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Chapter 3 41
exchange. This might stem from the presence of at least two different guest molecules
(acetic acid and pivalic acid), meaning there are more molecules which sustain the whole
framework in some extent through (weak) coordination compared to 1 and 3. Hence, the
stability considerably decreases after solvent exchange while maintaining the structure
(Figure 3.10). The same behavior is observed for Ru-MOF sample 4, most likely due to the
removal of [BPh4] - counter-ions, which previously stabilized the framework (Figure 3.10).
3.1.5.2 Composition determination
To quantitatively determine the composition of Ru-MOFs 1-4_ex elemental analysis (EA)
and Energy-dispersive X-ray spectroscopy (EDX) were subsequently performed. From
our earlier reports, [82, 138] EA results indicated the molar ratio of Ru : Cl being 3 : 1.5. The
XPS studies showed two different Ru-species (Ru III and Ru II ) as well as the presence of
chlorine, suggesting Cl - being the major couter-ion to balance the charge of the
framework. [82] As revealed in the present studies, the molar ratio of Ru : Cl in 1_ex slightly
differs from the sample before solvent exchange (Ru : Cl = 3 : 1.5) [138] and equals to ~ 3 :
1 (Table 3.2). The presence of chlorine was additionally confirmed by the EDX elemental
mapping of Ru and Cl (Figure 3.11). It should be noted, that the analytically determined
Ru-content (weight percentage by AAS) in the exchanged materials (1_ex) decreased from
36.29 % to 30.45 %. [82, 138] Nevertheless, all characterizations proved preservation of
crystallinity, porosity as well as thermal stability (by TGA). Based on these observations,
together with the preserved BET surface area (SBET) observed after solvent exchange (see
the details for next section on porosity), creation of the internal “structural defects” (i.e.
missing of Ru-nodes) in the solvent exchanged sample 1_ex could be expected. As
mentioned earlier, acetic acid is present in the discussed Ru-MOFs. The residual acetic
acid (or acetate) could be included into frameworks mainly in three ways: i) it can be
deprotonated and act as the counter-ion (coordinated to the Ru-center or sitting around
the PW units) to compensate the charge of the [Ru2] 5+ units (Figure 3.12a); ii) inherent
structural defects due to the incomplete substitution by the BTC within used Ru-SBUs
(Figure 3.12b and c); iii) the acetic acid molecules (without deprotonation) might be
accommodated within the pores of the frameworks. Activation procedure of the assynthesized
samples could hardly direct remove the residue of the acetic acid in the
framework. Nonetheless, application of additional solvent exchange method results in
partly elimination of the residual acetic acid, and might consequently create the missing