Controlling the molecular diffusion in MOFs with the acidity of monocarboxylate modulators

Article abstract of DOI:10.1039/d1dt01773j

The catalytic performance of metal-organic frameworks (MOFs) is related to their physicochemical properties, such as particle size, defect chemistry and porosity, which can be potentially controlled by coordination modulation. By combining PXRD, 1HNMR, FT

Full text of DOI:10.1039/d1dt01773j

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Controlling the molecular diffusion in MOFs with
the acidity of monocarboxylate modulators†
Cite this: DOI: 10.1039/d1dt01773j
Isabel Abánades Lázaro, *‡a Catalin Popescu b and Francisco G. Cirujano *‡a
The catalytic performance of metal–organic frameworks (MOFs) is related to their physicochemical
properties, such as particle size, defect chemistry and porosity, which can be potentially controlled by
coordination modulation. By combining PXRD, 1HNMR, FT-IR, and N2 uptake measurements we have
gained insights into the control of different types of defects (missing linker or missing cluster conse-
quence of the spatial distribution of missing linkers, and a combination of both) by the type of modulator
employed. We show that the molar percent of defects, either as missing linkers or as a part of missing
cluster defects, is related to the acidity of a modulator and its subsequent incorporation into the UiO-66
structure. Modulators with strong acidity and small size result in a considerable defect induction that
causes an increase in the external surface area and mesopore volume, which is beneficial for the ring-
opening of epoxides with amines, using UiO-66 defect-modulated MOFs as heterogeneous catalysts.
Received 31st May 2021,
Accepted 14th July 2021
DOI: 10.1039/d1dt01773j
rsc.li/dalton
framework.12–17 In turn, modulators can also act as crystal
growth promoters by controlling the acidity of synthesis media
Introduction
Coordination modulation of Metal–Organic Frameworks without permanent attachment to metal clusters, increasing
(MOFs)1 – highly stable, porous hybrid structures composed of the particle size and enhancing the crystallinity.11 The
metal clusters linked by multidentate organic ligands2–4 – has addition of modulators enables fine-tuning of properties such
emerged as a defect engineering tool to enhance their appli- as size,18 functionality15,19 and defect chemistry,6,14 which are
cations and has received notable attention in the last themselves related to the changes in the polydispersity of
decade.5–7 Defects in Zr MOFs were first reported in 20118 by MOFs,20 mechanical, thermal, and chemical stabilities,21,22
thermogravimetric analysis of modulated Zirconium UiO-66 porosity14 and the density of open metal sites,23,24 beneficial
MOFs revealing a linker deficiency, which was further agreed for several applications such as heterogeneous catalysis.24–28
by EXAFS showing a loss of ligands in the first Zr coordination
Zr MOFs are the benchmark materials regarding defect
shell, compensated by monotopic ligands, the so-called modu- chemistry due to the high stability of the frameworks towards
lators. Modulators are typically monotopic ligands that are the reduction of the cluster’s coordination number.29–31
introduced during MOF synthesis and they compete with mul- Among them, the UiO series of MOFs32 (UiO stands for
titopic linkers for metal nucleation sites.9 Coordination modu- University i Oslo) with UiO-66 (Zr-terephthalate), represented
lation was first reported in 2008 for Zn MOFs,10 was then in Fig. 1, can reduce its coordination number from 12-con-
applied to Zr MOFs in 2011,11 and has boosted the number of nected to 8, resulting in missing cluster consequence of the
reported MOFs ever since.9
spatial distribution of missing linkers within the
Modulators can be attached to the metal clusters of framework.30,33 To the best of our knowledge, the spatial
the resultant structure as capping (size-control) and/or organisation of missing linkers into point defects (non-
defect-compensating ligands, providing functionality to the extended linker deficient metal sites) or missing clusters is
still not fully understood and controlled, with controversial
studies being found in the literature.29 In fact, while missing
aInstituto de Ciencia Molecular (ICMol), Universitat de València, Catedrático José
clusters have been proposed to be the only type of defect,14,29
Beltrán Martínez no 2, 46980 Paterna, Valencia, Spain.
HR-TEM has recently shown the co-existence of different types
of defects, as represented in Fig. 1: ordered missing linkers
from the 12-connected parent fcu 4[Zr6(μ3-O)4(μ3-OH)4L6]
network into a bcu 8-connected 4[Zr6(μ3-O)4(μ3-OH)4L4(Mod)4]
net in which all the BDC ligands are removed from the parent
structure projected along the 〈001〉 axis, the 8-connected
missing cluster reo phase 3[Zr6O4(OH)4L4(Mod)4] in which one
E-mail: isabel.abanades@uv.es, francisco.c.garcia@uv.es
bCELLS–ALBA Synchrotron, E-08290 Cerdanyola del Valles, Barcelona, Spain
†Electronic supplementary information (ESI) available: General experimental
details and remarks, detailed synthesis conditions, exhaustive characterisation
(PXRD, NMR, FT-IR, TGA, SEM, porosity, polydispersity, and water stability) and
relationships between the catalytic activity and the properties of materials. See
DOI: 10.1039/d1dt01773j
‡These authors contributed equally.
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temperature, time, etc. under synthetic and post-synthetic
conditions.23,24,27,41
Although the catalytic performance of the defect-containing
metal-oxo clusters is related to the bulk physicochemical pro-
perties of the materials, studies that enclose both detailed
rationalisation of properties based on the synthetic conditions
and the interrelationship with the catalytic performance of
MOFs are imperative to provide insights into the control of the
interrelationship between synthesis, properties and function,
which will lead to the design of catalytic MOFs with outstand-
ing performance in relevant catalytic/adsorption solid-based
processes. In particular, heterogeneously catalysed synthesis of
industrially relevant compounds, such as fine chemicals and
pharmaceuticals, through the C–C and C–N bond-forming pro-
cesses is a recent example where MOFs can bridge state-of-the-
art homogeneous catalysts with robust reusable solid
catalysts.39,42,43
Herein, we study the effects of the acidity and molecular
size of modulators on UiO-66 properties in relation to its
catalytic performance in the transformation of epoxides
into amino alcohols via MOF-catalysed C–N bond
formation,36,44–47 aiming to provide insights that will lead to
the design of catalytic MOF systems based on their con-
trolled properties. We have chosen UiO-66, a robust system
that allows for precise control of its chemical reactivity
through defect engineering.29 The already demonstrated
acidic properties of this catalytic platform enable the trans-
Fig. 1 Ideal UiO-66 phase (fcu) and different defective phases due to
missing clusters (reo, scu) or linkers (bcu).
cluster and its connected linkers (L) are missing, with linker formation of many organic compounds through hetero-
being compensated by modulators (Mod), and the 8,4-con- geneous catalytic processes taking place at the nodes of the
nected scu net 3[Zr6(μ3-O)4(μ3-OH)4L3(Mod)6] of missing UiO framework.24,27,48,49 When a part of the structure is dis-
cluster defects (Fig. 1).33 By means of formic acid (FA) modu- rupted by the attachment of monocarboxylates, a more open
lation, the authors visualized that under their FA modulated framework is generated, thus favouring the accessibility of
conditions the missing linker domains predominate over the active sites in the form of open metal sites (coordinatively
missing clusters, with FA defect-compensating modulators unsaturated sites), with sufficient stability due to the high
attached to the Zr positions. Recent reports have also shown connectivity of the MOF nodes.14,50
the organization of missing linker defects into other defective
phases such as missing cluster defects in the UiO-66 hcp
network with Zr12(μ3-O)8(μ3-OH)8(μ-OH)6 nodes (paired Zr6O8
Results and discussion
nodes bridged by the OH groups).34–36
Defects play an active role in the chemical reactivity of In order to induce defects into the UiO-66 structure, we intro-
MOFs, and hence in their catalytic activity.23,24 Recent reports duced 10 equivalents (with respect to the linker) of well-
have shown that defective tetravalent metal sites affect the con- known defect promoting modulators. We maintained a fixed
version and selectivity of catalytic processes of industrial rele- excess of linker compared to the metal (1.5 equivalents) to
vance, such as the activation of carbonyl groups in the syn- reduce the particle size and enhance defect promotion by
thesis of biomass-derived platform molecules, biodiesel and speeding the metal–linker complexation and subsequent
other high value-added molecules.37–39 Point defects at the nucleation equilibria.40 We chose benzoic acid (BA), formic
crystalline positions interrupt the periodicity of the framework, acid (FA) and trifluoroacetic acid (TFA) as modulators, paying
increasing the accessibility to coordinatively unsaturated metal special attention to their pKa values (4.2, 3.7 and 0, respect-
sites that interact with the substrates. The presence of defects ively) and molecular sizes (see S.2 in the ESI† for detailed
is often interconnected with the porosity and particle size synthesis conditions). Note that in the MOF structures FA
(nucleation and crystallisation kinetics),40 which have a clear corresponds to formate, TFA to trifluoroacetate and BA to
incidence on the molecular transport in and out of the active benzoate. Recent reports have shown that the pKa values of
sites. In the particular case of zirconium-based MOFs, the acetic acid derivatives as modulators are inversely related to
defective metal-oxo clusters have Lewis/Brønsted acidity which the induction of defects in UiO-66,14 whereas the higher
can be controlled by the use of modulators (e.g. monocar- defect promoting activity of benzoic acid than its pKa ana-
boxylic acids), mineral acids, water, zirconium precursors, logue, acetic acid, with stoichiometric metal/linker ratios is
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attributed to its molecular size, more similar to that of pKa value than BA (see S.3.1 in the ESI†). The reo phase has
benzene dicarboxylate.51
been reported as a broad band at low reflection angles when
Fig. 2a shows that besides the appearance of sharp Bragg measured by conventional PXRD, whereas recent reports have
peaks of the UiO-66 framework in the Powder X-Ray shown that both the 8-connected reo phase and 8,4-connected
Diffraction (PXRD) patterns, the BA and TFA modulated scu missing cluster phase simulated PXRD patterns have new
samples showed the existence of a weak broad band (2 theta = reflection bands (100) and (110) at low angles that are absent
2–3.5), indicated with an arrow. This band, which is con- in both the pristine MOF and the bcu net of ordered missing
sidered indicative of the presence of superlattice peaks,14 is linkers.33 The modulator incorporation into the framework
more significant for the TFA modulated sample, whereas it was analysed by combining NMR and FT-IR profiles. NMR
was not apparent upon FA modulation despite having a lower (Table 1 and S.3.2 in the ESI†) revealed that the incorporation
Fig. 2 (a) Stacked PXRD of modulated UiO-66 in comparison with simulated UiO-66 and simulated reo phase. (b) FT-IR profiles of modulated
UiO-66 in comparison with pristine UiO-66. (c) TGA profiles of modulated UiO-66 with metal residue normalized to 100%. (d) Representation of the
molar percent of missing linkers in Zr6O4(OH)4(L)x(FA)XNMR(Mod)XNMR(OH)Z(H2O)Z as a function of the modulator’s pKa, where missing linkers corres-
pond to 6 − x, x being determined by a combination of NMR analysis of the acid-digested MOF and TGA.
Table 1 Characterisation data extracted by NMR and TGA techniques
NMR
TGA
Technique
Sample
MODa/L
FAb/L
Lc/Zr6
MOD/Zr6
FA/Zr6
Wt% Zr/DH MOF
MLd (%)
BA@UiO-66
FA@UiO-66
TFA@UiO-66
0.19
0.29
0.42*
0.22
0.29
0.18
4.71
4.56
4.26
0.84
1.33
0.92
0.91
0.00
1.80
47.6
52.3
54.0
21.5
24.1
29.0
a MOD refers to the modulator. b FA refers to formic acid. c L refers to linker. d ML refers to missing linker. * Since TFA has no protons, a combi-
nation of fluor and proton NMR with the presence of a calibrant has been used to calculate the molar ratio between modulator and linker.
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of modulators (BA, FA, and TFA) in mole percent (mol%) is the acidity of the modulators (Fig. 2d) due to their incorpor-
inversely related to their pKa values (Fig. S6 and S7 in the ation into the oxo-metallic nodes (see S.3.4 and Fig. S11–18 in
ESI†). However, FA formed by the decomposition of DMF the ESI† for detailed explanation of the mathematical
during MOF synthesis is also incorporated into the BA and models).52 TGA revealed ca. 4.71, 4.56 and 4.26 linkers (6 for
TFA modulated frameworks, although to a less extent than the
pristine
MOF)
with
the
molecular
formula
upon its direct addition, but resulting in a higher number of Zr6O4(OH)4(L)x(FA)y(Mod)z(OH)d(H2O)d for BA, FA and TFA
total monocarboxylate ligands (total modulator) attached to respectively, corresponding to ca. 21, 24 and 29 molar percent
the framework (Fig. S8 in the ESI†). FA co-incorporation com- of missing linkers (Table 1). In the reo phase, one cluster and
pensates for the charge balance upon replacement of ditopic its connected linkers (33.3 molar percent) are removed from
linkers by BA and TFA modulators, possibly making the the pristine 4[Zr6(μ3-O)4(μ3-OH)4L6] MOF upon replacement by
missing cluster defects more favourable. FT-IR profiles, rep- monotopic monocarboxylate modulators (bidentate) or by a
resented in Fig. 2b (see S.3.3 in the ESI†), show the character- combination of bidentate and monodentate monotopic modu-
istic vibration bands of the modulators such as the C–F3 lators (i.e. OH) that coordinate 8 positions in the unit cell with
stretching at ca. 1195 and 1140 cm−1 or the C–F bending at ca. paired neutral species (i.e. water, solvent), resulting in 3
791 cm−1, but the free carboxylate vibration bands are not [Zr6O4(OH)4(L)4(Mod)4] and derivative structures.14,30
present due to the modulator attachment to the Zr cluster.
To rule out the influence of the particle size on the external
Gravimetric analysis of the thermal decomposition profiles surface area53 we measured the particle size of the three modu-
of the MOFs (see S.3.4 in the ESI†), represented in Fig. 2c, lated MOF samples by Scanning Electron Microscopy (see
revealed a high defectivity in agreement with the high incor- section S.3.5 in the ESI†) using ImageJ software to analyze the
poration of monotopic modulators determined by NMR, with particle size distributions. The average particle size had no
the molar percent of missing linkers being directly related to statistical significance between sets, as shown in Fig. 3a. This
Fig. 3 (a) Statistical box chart of the particle size distributions of modulated UiO-66, showing 25% and 75% quartiles, the average size and the stan-
dard distribution. (b) N2 adsorption and desorption isotherms of modulated UiO-66. (c) Representation of the external surface area and mesopore
volume of modulated UiO-66 as a function of the molar percent of missing linkers. (d) Pore size distribution of modulated UiO-66.
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indicates that size will not play a role in assessing the differ- and 6 times smaller in magnitude than the 16 and 19 Å pores
ences in their porosities and catalytic activities.53 Indeed, ana- respectively), BA@UiO-66 conserves the pristine MOF pores,
lysis of the average particle size as a function of incorporation which have a similar magnitude to that of the new pores.
of modulators (Fig. S22 in the ESI†) rules out the influence of FA@UiO-66 presents a different PSD, with no tetrahedral 8 Å
the particle size on the external surface area and mesopore pore (as TFA) but with new pores at ca. 13 and 15 Å, the latter
volume.
being the most significant of all. Although its bigger pore is
The N2 adsorption and desorption isotherms (see S.3.6 in similar to the simulated reo phase (ca. 16 Å),56 its smaller size
the ESI†), represented in Fig. 3b, indicate that the addition of together with the absence of the reo 19 Å pore indicates, in
modulators increases the porosity, with SBET of ca. 1390 m2 g−1 great agreement with the absence of a broad band at low
upon TFA and FA modulation vs. 1200 m2 g−1 for pristine angles in the PXRD pattern, that missing clusters are not the
UiO-66 (Table 2, Fig. S24 in the ESI†). An increase in the poro- most prevalent defects in FA@UiO-66. In addition, the simu-
sity rules out the possibility of linker deficiency arising from a lated accessible surface area (1360/1612 m2 g−1) and pore
metal-rich particle surface. However, the less defective sample volume (0.503/0.557 cm3 g−1) for UiO-66 with 9 linkers out of
(BA) has a smaller SBET value than the pristine MOF. This 12 (4.5 out of 6) compensated by FA or OH− respectively are
could be due to the higher volumes and weights of TFA and similar to the experimental porosimetry values of FA@UiO-66
BA modulators than those of FA51 and due to the fact that TGA (Table 2), with 9 out of 12 linkers calculated by a combination
revealed DMF obstruction in the pores of the BA and TFA of TGA and NMR, with missing linkers compensated by a com-
samples, but not in the pores of the FA sample (see S.3.4 in bination of FA and OH−.57
the ESI† for the assessment of coordinated and obstructed
Although TGA provides very good calculations of the mole-
DMF). However, the external surface area and mesopore cular formula of the materials, it cannot differentiate between
volume (Table 2) are both proportional to the molar percent of the types of defects (missing linkers and/or missing clusters
missing linkers, as represented in Fig. 3c, which was pre- and their topologies).29 Based on the previously discussed
viously found to be related to the modulator’s acidity, while PXRD, TGA and PSD data, we postulate that BA and TFA
there was no relationship with the average particle size induce missing cluster defects upon incorporation, with a
(Fig. S25 in the ESI†). Although it is known that the external partial compensation by FA, while the missing cluster contri-
surface area is related to the particle size for small nano- bution for FA modulation is less significant. The MOF exhibits
particles, given that the average particle size of this sample has ca. 29% of missing linkers upon modulation with highly acidic
no statistical difference, together with defects playing an TFA, close to 33% of missing linkers for the pure reo phase,
important role in the pore size and opening explains that the and its PSD matches well with reo simulations, suggesting
external surface area is directly related to the material defect exclusively missing cluster defects. BA@UiO-66 presents 21%
concentration. High external surface areas are beneficial for of missing linkers, leading to a smaller degree of missing clus-
catalytic applications, where interface phenomena, defective ters. However, the preservation of its micropore structure
metal sites and intra-crystalline diffusion play major roles, in suggests pairing missing linkers that are not part of missing
addition to the micropore area, offering a higher number of cluster defects. In contrast, when only FA is added in the
accessible active sites at the surface of the nanoparticles.54,55
absence of other modulators, it is incorporated into the MOF
The pore size distributions (PSD) represented in Fig. 3c structure compensating for missing linker defects, and its
reveal changes in the pore size distributions from the reported spatial distribution does not favour the formation of missing
tetrahedral (8 Å) and octahedral (11 Å) pores32 that indicate cluster defects under the studied conditions (excess of linker),
nanoregions of missing cluster defects for the TFA and BA at least not in a significant manner in the form of nanore-
modulated samples, which exhibited significant pores at ca. 16 gions, as indicated by PXRD and PSD. The differences in the
and 19 Å, consistent with the reported simulated PSD of the types of defects induced are not related to the pKa values of
reo phase.56 However, while TFA@UiO-66 has completely lost the modulators, but could be a consequence of their molecular
the original pore at 8 Å (and the volume of the 11 Å pore is 3 sizes: steric hindrance due to bigger molecular size favours the
attachment of FA (from the decomposition of DMF) and OH/
H2O pairs in close proximity instead of linkers. Although this
study encloses 3 modulators, further studies on the relation-
ship between the modulator’s molecular size vs. pKa, and the
formation of different defective phases in UiO-type MOFs are
Table 2 Characterisation data extracted from the N2 adsorption/de-
sorption isotherms
underway.
a
a
SBET/SMICRO/SEXT
(m2 g−1
Vmicro/Vmeso
Vt
Overall, characterisation reveals that while the particle size
can be maintained by fixing an excess of linker, the molar
percent of defects, their types and distribution could be tuned
by the modulator choice. The molar percent of defects and the
external surface area are related to the acidity of the modulator
and its subsequent incorporation, but its molecular size seems
to play a more important role in determining its spatial distri-
Sample
)
(cm3 g−1
)
(cm3 g−1
)
BA@UiO-66
FA@UiO/66
TFA@UiO-66
1042/929/113
1393/1226/167
1389/1165/224
0.362
0.494
0.431
0.071
0.121
0.164
0.433
0.615
0.595
a Vmicro was calculated using the t-plot model with the Harkins and
Jura thickness curve based on the BET surface areas. Vtotal was calcu-
lated at P/P0 = 0.9 and Vmeso = Vtotal − Vmicro
.
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bution, resulting in missing linker defects for FA and the co- defectivity – a lower pKa value results in a higher amount of
existence of missing linkers and missing clusters for TFA and deprotonated species that are more effective in capping the
BA, with its consequent impact on the pore size distribution.
Zr6-oxoclusters – and so, the more defective the MOF, the
At this point, we investigated the impact of the defect-modu- higher the catalytic conversion (Fig. 5a). Replacing linkers by
lated MOF properties (Zr sites associated with defectivity modulators results in the opening of the framework, leading
and related porosity features) on the activation of oxygen to higher mesopore volumes and external surface areas. This
and nitrogen-containing substrates (e.g. epoxides, alcohols and results in more accessible Zr active sites, which are available
amines).58 The resulting Zr open metal sites due to the presence for the activation of the reaction substrates. Moreover, bulkier
of different modulators allow the transformation of amines and modulator molecules, like benzoic acid, occupy more space in
epoxides into pharmaceutically relevant functional molecules the MOF cavities, which results in a significant decrease in the
like amino alcohols (i.e. adrenaline). Hence, we studied the porosity in comparison with light monocarboxylates, i.e.
interrelationship between the properties of defective materials formic or trifluoroacetic acid. Thus, the influence of missing
and their catalytic performances in the straightforward syn- linkers on the textural properties of the crystals (improving the
thesis of β-amino alcohols through regioselective ring-opening diffusion of reactants during the ring-opening of cyclohexene
of oxiranes (epoxides) with amines, as represented in Fig. 4a, by epoxide) is the cause of its improved catalytic performance, as
reacting cyclohexene oxide with aniline, which was performed illustrated in Fig. 5b and c. Thus, acidity, defectivity, mesopore
at room temperature in EtOH with and without MOF for volume and external surface area being inter-related pro-
24 hours (see S.4 in the ESI† for detailed conditions).
perties, all are proportional to the modulated UiO-66 catalytic
While the non-catalysed reaction between aniline and cyclo- activity in the order TFA > FA > BA, as illustrated in Fig. 5 and
hexene oxide at room temperature attains a 3% conversion Fig. S28–S31 in the ESI.† The PXRD patterns of the spent
after 24 hours, the defective MOFs are active catalysts for the materials are similar to those of the as-prepared MOFs, indi-
desired C–N bond formation, reaching a ca. 80% conversion cating no damage to the structure (Fig. S33 in the ESI†).
for TFA-modulated UiO-66 after 24 hours and ca. 59% after Additionally, Fig. S34† shows that the high performance
8 hours, as represented in Fig. 4b and c, and tabulated in TFA@UiO-66 maintains its catalytic activity and selectivity for
Table S6 in the ESI.†
4 reaction cycles.
Conversion and selectivity are related to MOF defectivity,
The increase in conversion is notable if we consider pre-
among other inter-related properties. Fig. 4b and c show that vious literature reports showing a 40% conversion in the ring-
the pKa value of the modulator is inversely proportional to con- opening of epoxides after 24 hours at 55 °C for UiO-66 with
version and directly proportional to selectivity towards the 1.75 missing linkers out of 12,58 whereas our conditions are
addition of aniline (instead of the alcohol solvent, which may room temperature and reach up to 80% conversion with ca. 4
act as a nucleophile in the presence of more defective MOFs). out of 12 missing linkers. To put our results in context, we cal-
As previously discussed, the pKa value of the modulator is culated the TONs per Zr site associated with the defects in the
related to its incorporation and the resultant framework’s coordination-modulated MOFs reported here. Assuming ca.
Fig. 4 (a) Conversion of cyclohexene oxide (left axis, red bars) and selectivity to the resulting amino alcohol (right axis, red circles) in the presence
of (Zr)UiO-66 catalysts after 8 h (b) or 24 h (c) prepared with three different monocarboxylate modulators (BA = benzoic acid, FA = formic acid, TFA
= trifluoroacetic acid).
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Fig. 5 Relationship between cyclohexene oxide conversion percent after 24 hours and the structural and textural properties of the UiO type MOFs:
molar percent of missing linkers (a), external surface area (b), and mesopore volume (c).
30 wt% of Zr associated with defective clusters, the TON of
Conflicts of interest
these sites is 8, which could be competitive with TONs of 2–20
obtained with the benchmark Zr-MOFs reported in the litera- There are no conflicts to declare.
ture for the same reaction studied here.44,59
Acknowledgements
Conclusions
FGC and IAL thank the University of Valencia, the Instituto de
We have shown the versatility of the MOF-modulated approach
Ciencia Molecular and the FuniMat Research group for
in tailoring the porosity of the final material. The pKa value of
research facilities.
monocarboxylate molecules (e.g. benzoic, formic and trifluor-
FGC acknowledges the project that gave rise to these results
oacetic acids) used as modulators of crystal growth of zirco-
and received the support of a fellowship from” La Caixa”
nium carboxylate frameworks is key to controlling the degree
Foundation (ID 100010434). The fellowship code is LCF/BQ/
of defects in the crystal. The concentration of defects in the
PI19/11690011. This project has also received funding from
form of missing terephthalate linkers that have been
the European Union’s Horizon 2020 research and innovation
exchanged by the monocarboxylate is inversely proportional to
programme under the Marie Skłodowska-Curie grant agree-
the pKa value of the modulator. Through defect engineering of
ment No 837804. IAL thanks Marie-Curie Actions for the
the framework, the mesoporosity and external surface area can
Individual Fellowship 837804 (DefTiMOFs, MSCA-IF-2018).
be tailored to expose a higher number of Zr acid sites readily
C. P. is thankful for the financial support from the Spanish
available to interact with the reactants involved in catalytic pro-
MINECO Project No. FIS2017-83295-P. Synchrotron powder
cesses. Both cyclohexene oxide and aniline diffusion are
X-ray diffraction experiments were performed at the Materials
favoured for the TFA-modulated MOF, which shows improved
Science and Powder Diffraction beamline of ALBA Synchrotron
textural (higher porosity) and, thus, catalytic properties (higher
(Alba Experiment No. 2020094558).60
conversion in the ring-opening of epoxides).
Although this study encloses 3 modulators, we encourage
MOF researchers to closely look at the relationship between
the modulator’s molecular size and pKa, with the formation of
different defective phases in UiO-type MOFs. This investigation
1 S. Dissegna, K. Epp, W. R. Heinz, G. Kieslich and
also suggests that the formation of either missing clusters or
missing linkers and their distribution do not drastically affect
2 A. Carné-Sánchez, I. Imaz, K. C. Stylianou and D. Maspoch,
the catalytic properties of materials, with overall replacement
of linkers (in either of the defective phases) and its relation-
ship with the material’s porosity driving its catalytic
Notes and references
R. A. Fischer, Adv. Mater., 2018, 10, 1704501–1704524.
Chem. – Eur. J., 2014, 20, 5192–5201.
3 P. Z. Moghadam, A. Li, X.-W. Liu, R. Bueno-Perez, S. Wang,
S. Wiggin, P. A. Wood and D. Fairen-Jimenez, Chem. Sci.,
2020, 11, 8373–8387.
performance.
4 S. Wang, C. M. McGuirk, A. d’Aquino, J. A. Mason and
C. A. Mirkin, Adv. Mater., 2018, 30, 2618–2625.
5 Z. Fang, B. Bueken, D. E. D. Vos and R. A. Fischer, Angew.
Author contributions
The manuscript was written through the contributions of all
authors. All authors have approved the final version of the
manuscript.
Chem., Int. Ed., 2015, 54, 7234–7254.
6 G. Cai and H.-L. Jiang, Angew. Chem., Int. Ed., 2017, 56,
563–567.
This journal is © The Royal Society of Chemistry 2021
Dalton Trans.

View Article Online
Paper
Dalton Transactions
7 W. Xiang, Y. Zhang, Y. Chen, C. Liu and X. Tu, J. Mater. 29 M. Taddei, Coord. Chem. Rev., 2017, 343, 1–24.
Chem. A, 2020, 8, 21526–21546.
30 M. J. Cliffe, W. Wan, X. Zou, P. A. Chater, A. K. Kleppe,
M. G. Tucker, H. Wilhelm, N. P. Funnell, F.-X. Coudert and
A. L. Goodwin, Nat. Commun., 2014, 5, 4176–4184.
31 A. K. Cheetham, T. D. Bennett, F.-X. Coudert and
A. L. Goodwin, Dalton Trans., 2016, 45, 4113–4126.
8 L. Valenzano, B. Civalleri, S. Chavan, S. Bordiga,
M. H. Nilsen, S. Jakobsen, K. P. Lillerud and C. Lamberti,
Chem. Mater., 2011, 23, 1700–1718.
9 R. S. Forgan, Chem. Sci., 2020, 11, 4546–4562.
10 S. Hermes, T. Witte, T. Hikov, D. Zacher, S. Bahnmüller, 32 J. H. Cavka, S. Jakobsen, U. Olsbye, N. Guillou,
G. Langstein, K. Huber and R. A. Fischer, J. Am. Chem. Soc.,
2007, 129, 5324–5325.
C. Lamberti, S. Bordiga and K. P. Lillerud, J. Am. Chem.
Soc., 2008, 130, 13850–13851.
11 A. Schaate, P. Roy, A. Godt, J. Lippke, F. Waltz, M. Wiebcke 33 L. Liu, Z. Chen, J. Wang, D. Zhang, Y. Zhu, S. Ling,
and P. Behrens, Chem. – Eur. J., 2011, 17, 6643–6651.
12 I. A. Lázaro, C. J. R. Wells and R. S. Forgan, Angew. Chem.,
Int. Ed., 2020, 59, 5211–5217.
13 I. A. Lázaro, S. Haddad, J. M. Rodrigo-Muñoz,
R. J. Marshall, B. Sastre, V. del Pozo, D. Fairen-Jimenez and
K.-W. Huang, Y. Belmabkhout, K. Adil, Y. Zhang, B. Slater,
M. Eddaoudi and Y. Han, Nat. Chem., 2019, 11, 622–628.
34 F. C. N. Firth, M. J. Cliffe, D. Vulpe, M. Aragones-Anglada,
P. Z. Moghadam, D. Fairen-Jimenez, B. Slater and
C. P. Grey, J. Mater. Chem. A, 2019, 7, 7459–7469.
R. S. Forgan, ACS Appl. Mater. Interfaces, 2018, 10, 31146– 35 M. J. Cliffe, E. Castillo-Martínez, Y. Wu, J. Lee, A. C. Forse,
31157.
F. C. N. Firth, P. Z. Moghadam, D. Fairen-Jimenez,
M. W. Gaultois, J. A. Hill, O. V. Magdysyuk, B. Slater,
A. L. Goodwin and C. P. Grey, J. Am. Chem. Soc., 2017, 139,
5397–5404.
14 G. C. Shearer, S. Chavan, S. Bordiga, S. Svelle, U. Olsbye
and K. P. Lillerud, Chem. Mater., 2016, 28, 3749–3761.
15 M. Taddei, G. M. Schukraft, M. E. A. Warwick, D. Tiana,
M. J. McPherson, D. R. Jones and C. Petit, J. Mater. Chem. 36 X. Chen, Y. Lyu, Z. Wang, X. Qiao, B. C. Gates and D. Yang,
A, 2019, 7, 23781–23786. ACS Catal., 2020, 10, 2906–2914.
16 I. A. Lázaro, S. Haddad, J. M. Rodrigo-Muñoz, C. Orellana- 37 F. G. Cirujano, Catal. Sci. Technol., 2017, 7, 5482–5494.
Tavra, V. del Pozo, D. Fairen-Jimenez and R. S. Forgan, ACS 38 G. Fu, F. G. Cirujano, A. Krajnc, G. Mali, M. Henrion,
Appl. Mater. Interfaces, 2018, 10, 5255–5268.
17 I. A. Lázaro, N. Almora-Barrios, S. Tatay and C. Martí-
Gastaldo, Chem. Sci., 2020, 12, 2586–2593.
18 C. R. Marshall, S. A. Staudhammer and C. K. Brozek, Chem.
Sci., 2019, 10, 9396–9408.
S. Smolders and D. De Vos, Catal. Sci. Technol., 2020, 10,
4002–4009.
39 F. G. Cirujano and N. Martín, Advanced Functional Solid
Catalysts for Biomass Valorization, Elsevier, Amsterdam,
2020, pp. 187–198.
19 I. A. Lázaro, S. Haddad, S. Sacca, C. Orellana-Tavra, 40 S. Griffin, M. Briuglia, J. ter Horst and R. S. Forgan, Chem.
D. Fairen-Jimenez and R. S. Forgan, Chem, 2017, 2, 561–
578.
20 W. Morris, S. Wang, D. Cho, E. Auyeung, P. Li, O. K. Farha
and C. A. Mirkin, ACS Appl. Mater. Interfaces, 2017, 9,
33413–33418.
– Eur. J., 2020, 26, 6910–6918.
41 C. Jia, F. G. Cirujano, B. Bueken, B. Claes, D. Jonckheere,
K. M. V. Geem and D. D. Vos, ChemSusChem, 2019, 12,
1256–1266.
42 F. G. Cirujano, R. Luque and A. Dhakshinamoorthy,
Molecules, 2021, 26, 1445–1461.
21 S. Dissegna, P. Vervoorts, C. L. Hobday, T. Düren,
D. Daisenberger, A. J. Smith, R. A. Fischer and G. Kieslich, 43 F. G. Cirujano, ChemCatChem, 2019, 11, 5671–5685.
J. Am. Chem. Soc., 2018, 140, 11581–11584.
44 A. Das, N. Anbu, H. Reinsch, A. Dhakshinamoorthy and
22 B. V. de Voorde, I. Stassen, B. Bueken, F. Vermoortele,
S. Biswas, Inorg. Chem., 2019, 58, 16581–16591.
D. D. Vos, R. Ameloot, J.-C. Tan and T. D. Bennett, J. Mater. 45 A. Das, N. Anbu, M. Sk, A. Dhakshinamoorthy and
Chem. A, 2014, 3, 1737–1742. S. Biswas, ChemCatChem, 2020, 12, 1789–1798.
23 F. Vermoortele, B. Bueken, G. L. Bars, B. V. de Voorde, 46 N. E. Thornburg, Y. Liu, P. Li, J. T. Hupp, O. K. Farha and
M. Vandichel, K. Houthoofd, A. Vimont, M. Daturi, J. M. Notestein, Catal. Sci. Technol., 2016, 6, 6480–6484.
M. Waroquier, V. V. Speybroeck, C. Kirschhock and 47 C. Vignatti, J. Luis-Barrera, V. Guillerm, I. Imaz, R. Mas-
D. E. D. Vos, J. Am. Chem. Soc., 2013, 135, 11465–11468.
24 F. G. Cirujano and F. X. L. i Xamena, J. Phys. Chem. Lett.,
2020, 11, 4879–4890.
Ballesté, J. Alemán and D. Maspoch, ChemCatChem, 2018,
10, 3995–3998.
48 J. N. Hall and P. Bollini, React. Chem. Eng., 2018, 4, 207–222.
25 K. Fan, W.-X. Nie, L.-P. Wang, C.-H. Liao, S.-S. Bao and 49 F. Vermoortele, R. Ameloot, L. Alaerts, R. Matthessen,
L.-M. Zheng, Chem. – Eur. J., 2017, 23, 6615–6624.
B. Carlier, E. V. R. Fernandez, J. Gascon, F. Kapteijn and
26 W. Liang, L. Li, J. Hou, N. D. Shepherd, T. D. Bennett,
D. E. D. Vos, J. Mater. Chem., 2012, 22, 10313–10321.
D. D’Alessandro and V. Chen, Chem. Sci., 2018, 9, 3508– 50 F. G. Cirujano, N. Martin and L. H. Wee, Chem. Mater.,
3516. 2020, 32, 10268–10295.
27 N. Martin and F. G. Cirujano, Org. Biomol. Chem., 2020, 18, 51 C. Atzori, G. C. Shearer, L. Maschio, B. Civalleri, F. Bonino,
8058–8073.
C. Lamberti, S. Svelle, K. P. Lillerud and S. Bordiga, J. Phys.
Chem. C, 2017, 121, 9312–9324.
52 I. A. Lázaro, Eur. J. Inorg. Chem., 2020, 2020, 4284–4294.
28 F. G. Cirujano, M. Stalpaert and D. E. D. Vos, Green Chem.,
2018, 20, 2481–2485.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2021

View Article Online
Dalton Transactions
Paper
53 M. Taddei, K. C. Dümbgen, J. A. van Bokhoven and 57 A. W. Thornton, R. Babarao, A. Jain, F. Trousselet and
M. Ranocchiari, Chem. Commun., 2016, 52, 6411–6414. F.-X. Coudert, Dalton Trans., 2015, 45, 4352–4359.
54 N. Martín, M. Dusselier, D. E. D. Vos and F. G. Cirujano, 58 Y. Liu, R. C. Klet, J. T. Hupp and O. Farha, Chem. Commun.,
ACS Catal., 2018, 9, 44–48.
2016, 52, 7806–7809.
55 Q.-L. Zhu and Q. Xu, Chem, 2016, 1, 220–245.
56 W. Liang, C. J. Coghlan, F. Ragon, M. Rubio-Martinez,
59 P. Ji, X. Feng, P. Oliveres, Z. Li, A. Murakami, C. Wang and
W. Lin, J. Am. Chem. Soc., 2019, 141, 14878–14888.
D. M. D’Alessandro and R. Babarao, Dalton Trans., 2016, 60 F. Fauth, I. Peral, C. Popescu and M. Knapp, Powder Diffr.,
45, 4496–4500.
2013, 28, 360–370.
This journal is © The Royal Society of Chemistry 2021
Dalton Trans.