q123/random_k4/random_3.md

Showcasing research from Professor Bettina V. Lotsch’s
laboratory, Nanochemistry Department, Max Planck Institute for As featured in:
Solid State Research, Stuttgart, Germany.
Dynamic breathing behaviour of the titanium-based
metal–organic framework NTU-9 upon adsorption of
water and organic solvents
What happens to the structure of a framework material when
the pore content is changed or reduced? While investigating and
optimising the reproducibility of synthesising the two-dimensional
titanium-based MOF NTU 9, we discovered a dynamic behaviour
of NTU-9 triggered by guest adsorption and desorption. After
applying additional stimuli (i.e., vacuum), the material exhibits
reversible pore distortion by compression in the lateral dimension,
forming a new metastable form, NTU-9-d (d stands for distorted),
with a reduction in unit cell volume, pore size, and crystal symmetry. See Sebastian Bette,
Bettina V. Lotsch et al.,
Image reproduced by permission of Julia Knapp from Chem. Sci., Chem. Sci., 2025, 16, 13646.
2025, 16, 13646.
rsc.li/chemical-science
Registered charity number: 207890

Chemical
Science
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|     |     | Dynamic |     | breathing |     | behaviour |     | of  | the | titanium- |     |     |
| --- | --- | ------- | --- | --------- | --- | --------- | --- | --- | --- | --------- | --- | --- |
.ecneciL detropnU 0.3 noitubirttA snommoC evitaerC a rednu desnecil si elcitra sihT  based metal – organic framework NTU-9 upon
| Citethis:Chem.Sci.,2025,16,13646 |     |            |     |     |          |     |         |     |          |     | †   |     |
| -------------------------------- | --- | ---------- | --- | --- | -------- | --- | ------- | --- | -------- | --- | --- | --- |
|                                  |     | adsorption |     |     | of water | and | organic |     | solvents |     |     |     |
 .MP 00:74:5 6202/7/5 no dedaolnwoD .5202 enuJ 52 no dehsilbuP .elcitrA sseccA nepO
Allpublicationchargesforthisarticle
havebeenpaidforbytheRoyalSociety
|     |     |     |     | ab  |     | ın-Rubio, | a   | Heck,ac |     |     |     | ab  |
| --- | --- | --- | --- | --- | --- | --------- | --- | ------- | --- | --- | --- | --- |
ofChemistry Julia E. Knapp, Borja Ort´ Fabian Kristina Gjorgjevikj,
|     |     | Anastasia   | Sleptsova, |        | a Simon | Krause, | a Sebastian |     | Bette | *a  |     |     |
| --- | --- | ----------- | ---------- | ------ | ------- | ------- | ----------- | --- | ----- | --- | --- | --- |
|     |     | and Bettina | V.         | Lotsch | *abc    |         |             |     |       |     |     |     |
Understandingthestructuralresponseofframeworkmaterialstoexternalstimulihasbeenofgreatinterest.
However,littleisknownaboutthestimuliresponsivenessoftitanium-basedmetal–organicframeworks.We
investigatethereproducibilityofthesynthesisofthetwo-dimensionalmetal–organicframeworkNTU-9
(NTU = Nanyang Technological University) composed of Ti4+ cations and 2,5-dihydroxyterephtalate as
theorganiclinkerandtheflexibleresponseoftheframework
structuretoexternalstimuli.Usingacetic
acid simultaneously as mediator and solvent leads to the reproducible formation of large NTU-9
crystallites after long reaction times. The MOF synthesis in isopropanol:acetonitrile (i-PrOH:MeCN)
significantly
mixtures without a modulator is faster but yields smaller NTU-9 crystallites. Pure heat
treatment under ambient conditions removes a significant amount of the incorporated host solvent
moleculeswithoutalterationoftheframework'sstructure.Afterapplyingadditionalexternalstimuli(i.e.,
vacuum),thematerialexhibitsaporedistortionby compressioninthelateraldimension,dependingon
the synthetic procedure. The new, distorted, metastable form NTU-9-d shows a reduction in unit cell
volume, pore size, and crystal symmetry. Under humidity/air exposure or solvent resuspension, the
frameworkrevertsintoitsoriginalstate.Thesynthesisconditionssignificantlyaffecttheflexibilityofthe
Received7thApril2025 MOF structure, wheresamples synthesized withoutmodulator showed a lower tendency for distortion.
Accepted24thJune2025
Our results emphasise the importance of an in-depth understanding of the structure–property
DOI:10.1039/d5sc02585k relationshipsinflexibleMOFsthroughadetailedcharacterisationofthematerial'sstimuliresponsiveness
| rsc.li/chemical-science |     | process. |     |     |     |     |     |     |     |     |     |     |
| ----------------------- | --- | -------- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
Introduction Titanium-based MOFs are of particular interest due to the
|     |     |     |     |     |     | earth abundance | of  | their | metal source, |     | high stability, | low |
| --- | --- | --- | --- | --- | --- | --------------- | --- | ----- | ------------- | --- | --------------- | --- |
Metal–organic toxicity, redox activity, and promising photochemical proper-
|     | frameworks | (MOFs) are | porous | reticular | mate- |     |     |     |     |     |     |     |
| --- | ---------- | ---------- | ------ | --------- | ----- | --- | --- | --- | --- | --- | --- | --- |
rials consisting of inorganic nodes (e.g., metal ions, clusters) ties.6Thelatterpavethewayforattractiveapplications,suchas
linked by organic bridging ligands through coordination in photocatalysis, because tetravalent titanium cations exhibit
bonds.1–3
Among them, MOFs constructed from tri- and tetra- anemptyd-shellconstitutingtheconductionbandminimum,
valent metal-based clusters combined with carboxylate-based whichcangeneratelong-livedchargecarrier,resultinginhigh
|              |                  |           |        |      | so | solar-to-chemical |     | conversion | efficiencies.7,8 |     | However, | the |
| ------------ | ---------------- | --------- | ------ | ---- | --- | ----------------- | --- | ---------- | ---------------- | --- | -------- | --- |
| ligands show | higher stability | according | to the | hard | and |                   |     |            |                  |     |          |     |
Ti4+-clusters
acidsandbases(HSAB)principle,4whichmakesthemsuitable synthesis of carboxylate-based is challenging due
forapplicationssuchasgasstorageandseparation,aswellas tothehighreactivity(pronetofastolationandoxolation)and
catalysis.2,5 thehydrolyticinstabilityoftitaniumprecursors,impedingthe
|                                                                             |     |     |     |     |     | crystallisation | process.9–11                         |           | Ti4+ ions    | are highly | susceptible    | to      |
| --------------------------------------------------------------------------- | --- | --- | --- | --- | --- | --------------- | ------------------------------------ | --------- | ------------ | ---------- | -------------- | ------- |
|                                                                             |     |     |     |     |     | hydrolysis,     | which                                | results   | in the rapid | formation  | of             | various |
|                                                                             |     |     |     |     |     | molecular       | titanium-oxo-clustersand,eventually, |           |              |            | intheprecip-   |         |
|                                                                             |     |     |     |     |     | itation of      | TiO . As                             | a result, | signicantly | fewer      | titanium-based |         |
| aMaxPlanckInstituteforSolidStateResearch,Heisenbergstraße1,Stuttgart,70569, |     |     |     |     |     |                 | 2                                    |           |              |            |                |         |
Germany.E-mail:s.bette@f.mpg.de;b.lotsch@f.mpg.de MOFs have been reported in the literature compared to other
Pfaffenwaldring
bDepartment of Chemistry, University of Stuttgart, 55, Stuttgart, MOFs based on transition metals such as Zn2+, Cu2+, and
70569,Germany Zr4+.9,12–14Usefulsynthesisstrategiesforenablingtheformation
cDepartment of Chemistry, University of Munich, Butenandtstraße 5-13, Munich, Mart´ı-
|     |     |     |     |     |     | of Ti-MOF | single crystals |     | have been | introduced | by the |     |
| --- | --- | --- | --- | --- | --- | --------- | --------------- | --- | --------- | ---------- | ------ | --- |
81377,Germany
|     |     |     |     |     |     | Gastaldo | group, i.e., | by metal | doping15 | or using | hydroxamate- |     |
| --- | --- | --- | --- | --- | --- | -------- | ------------ | -------- | -------- | -------- | ------------ | --- |
†Electronicsupplementaryinformation(ESI)available:Methods,additionaldata
andanalysis.SeeDOI:https://doi.org/10.1039/d5sc02585k
13646 | Chem.Sci.,2025,16,13646–13654 ©2025TheAuthor(s).PublishedbytheRoyalSocietyofChemistry

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basedlinkers.16AnotherstrategywasproposedbyLietal.using
achelatingcoordinationmodulationmethod.17
Oneofthemostinterestingfeaturesofreticularmaterialsis
| the combination |     | of robust | and | exible | entities | in  | a collective |     |     |     |     |     |     |     |
| --------------- | --- | --------- | --- | ------- | -------- | --- | ------------ | --- | --- | --- | --- | --- | --- | --- |
structure,whichisalsoreferredtoassoporouscrystals18,19or
exibleframeworkmaterials20–23whichreacttoexternalstimuli
| (e.g.,    | temperature, | pressure,        | light) | by                  | reversibly | changing | the         |     |     |     |     |     |     |     |
| --------- | ------------ | ---------------- | ------ | ------------------- | ---------- | -------- | ----------- | --- | --- | --- | --- | --- | --- | --- |
| framework | structure    | without          |        | bond-breaking.18,19 |            | These    | mate-       |     |     |     |     |     |     |     |
| rials     | can respond  | to incorporation |        | and                 | release    | of       | guest mole- |     |     |     |     |     |     |     |
.ecneciL detropnU 0.3 noitubirttA snommoC evitaerC a rednu desnecil si elcitra sihT
culeswithstructuraldeformationsoftheframework,resulting
|     |     |     |     |     |     |     |     | Fig.1 ImageofthecrystalstructureofNTU-9(ref.32)(a)alongthe |     |     |     |     |     |     |
| --- | --- | --- | --- | --- | --- | --- | --- | ---------------------------------------------------------- | --- | --- | --- | --- | --- | --- |
c-axis)
inexpansionorshrinkageoftheporespace,alsoreferredtoas pore (view along and (b) visualising the layer structure (view
 .MP 00:74:5 6202/7/5 no dedaolnwoD .5202 enuJ 52 no dehsilbuP .elcitrA sseccA nepO b-axis), i.e.,
breathing behaviour. Since then, framework materials have along with the pores and pore content, positionally
|            | classied    |            |             |               |           |      | so-       | disorderedsolventmolecules. |     |     |     |     |     |     |
| ---------- | ------------ | ---------- | ----------- | ------------- | --------- | ---- | ---------- | --------------------------- | --- | --- | --- | --- | --- | --- |
| been       |              | into three | categories  |               | according | to   | their      |                             |     |     |     |     |     |     |
| ness:18,19 | exibility   | through    | synergistic |               | effects   | of   | the metal  |                             |     |     |     |     |     |     |
| nodes'     | coordination | symmetry,  |             | the rotatable |           | axis | of organic |                             |     |     |     |     |     |     |
linkers,andstronghost–guestinteractions.22,24–27 synthetic routes differ in terms of solvent (acetic acid vs. i-
PrOH:MeCNmixture)andreactiontime(5dvs.1d).Gaoetal.
F´ereyandco-workersrstreportedonthebreathing/swelling
rst
behaviourinMOFsin2002.28Todate,thedynamicbehaviourof reported the solvothermal synthesis S1 (Solvent
|     |     |     |     |     |     |     |     |     |     |     |     |     | 5d  | reaction- |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --------- |
)inaceticacid(1),actingasbothmodulatorandsolvent.32
| titanium-basedMOFs |     | israrely |     | describedinthe |     | literature,and |     | time |     |     |     |     |     |     |
| ------------------ | --- | -------- | --- | -------------- | --- | -------------- | --- | ---- | --- | --- | --- | --- | --- | --- |
onlyafewreportsarepublished.9,29–31InCOK-69,theframework The acidic medium during synthesis promotes a slow seeding
exibility byshiingthedeprotonationequilibriumofthelinkerH dhta
|                                             | results | from the | dynamic | conformational |        |      | change  | of         |           |       |      |         |          | 4         |
| ------------------------------------------- | ------- | -------- | ------- | -------------- | ------ | ---- | ------- | ---------- | --------- | ----- | ---- | ------- | -------- | --------- |
|                                             |         |          |         |                |        |      |         | toward the | precursor | side, | thus | slowing | down the | reaction, |
| the (cis/trans)1,4-cyclohexanedicarboxylate |         |          |         |                | linker | upon | solvent |            |           |       |      |         |          |           |
removal.9Anotherexampleistheheterometallic(TiMn )MUV- resultinginlargercrystals.33,38Incontrast,Yuetal.useda1:1
2
mixtureofi-PrOHandMeCN(2)inanalternativesynthesisS2
35, where the single-crystal transformation is controlled by 1d
linkerconformationinopen,intermediate,andclosedstates.30 intheabsenceofmodulatoranddecreasedthereactiontimeto
onedayinsteadofve.35
| In    | the following, | we      | report    | the synthesis, |          | activation | proce-    |        |         |                 |     |                  |     |             |
| ----- | -------------- | ------- | --------- | -------------- | -------- | ---------- | --------- | ------ | ------- | --------------- | --- | ---------------- | --- | ----------- |
|       |                |         |           |                |          |            |           | As the | limited | reproducibility |     | of MOF syntheses |     | is a recur- |
| dure, | and the        | dynamic | behaviour | of the         | Ti-based |            | MOF NTU-9 |        |         |                 |     |                  |     |             |
eld,39,40
(NTU = Nanyang Technological University) triggered by guest ring problem that is gaining attention in the we
statisticallystudiedtheinuenceofthereactiontimeforeach
| adsorption |     | and desorption. |     | We optimised |     | the | synthesis |     |     |     |     |     |     |     |
| ---------- | --- | --------------- | --- | ------------ | --- | --- | --------- | --- | --- | --- | --- | --- | --- | --- |
parameters and evaluated for reproducibility towards large syntheticprotocolbyperformingthereactionbetween4and21
different
|     |     |     |     |     |     |     |     | times. Indeed, |     | we found |     | trends | depending | on the |
| --- | --- | --- | --- | --- | --- | --- | --- | -------------- | --- | -------- | --- | ------ | --------- | ------ |
singlecrystalsandmicrocrystallinebulkpowders.Furthermore,
solventused(Table1andFig.S1,S2†).
| we discovered |     | a new metastable |     | form | NTU-9-d | (d  | stands for |     |     |     |     |     |     |     |
| ------------- | --- | ---------------- | --- | ---- | ------- | --- | ---------- | --- | --- | --- | --- | --- | --- | --- |
distorted),whichexhibitsastrongporeandunitcelldistortion Half of the S1 5d syntheses resulted in the formation of an
amorphousproduct(Fig.S3†),ashasbeenpreviouslyobserved
uponactivation.Weinvestigatedthestructuraltransitionsbyin
|             |       |                   |     |          |         |       |         | byothergroups.41ThesuccessfulS1 |      |               |         | attemptsresultedinthe |           |             |
| ----------- | ----- | ----------------- | --- | -------- | ------- | ----- | ------- | ------------------------------- | ---- | ------------- | ------- | --------------------- | --------- | ----------- |
| situ powder |       | X-ray diffraction |     | (PXRD)   | and THz | Raman | spec-   |                                 |      |               |         | 5d                    |           |             |
|             |       |                   |     |          |         | rst  |         | formation                       | of a | heterogeneous | mixture | of                    | dark red, | crystalline |
| troscopy.   | These | investigations    |     | revealed | the     |       | dynamic |                                 |      |               |         |                       |           |             |
hexagonalNTU-9andmicrocrystallinepowder(Fig.2candS4†).
breathingbehaviourofatitanium-basedmetal–organicframe-
conrmed
work with a rigid linker upon solvent loss and exposure to We the phase purity of the bulk powder by PXRD
|               |     |          |             |     |              |     |           | measurements |     | (Fig. 2a). Scanning |     | electron | microscopy | (SEM) |
| ------------- | --- | -------- | ----------- | --- | ------------ | --- | --------- | ------------ | --- | ------------------- | --- | -------- | ---------- | ----- |
| humidity/air. |     | Our work | establishes | a   | reproducible |     | synthesis |              |     |                     |     |          |            |       |
revealedtheformationofcrystalswithvaryingsize(20–50mm)
| procedure | and | a detailed |     | mechanism | of  | the | previously |                      |     |        |      |         |             |      |
| --------- | --- | ---------- | --- | --------- | --- | --- | ---------- | -------------------- | --- | ------ | ---- | ------- | ----------- | ---- |
|           |     |            |     |           |     |     |            | and microcrystalline |     | powder | with | average | crystallite | size |
unknownframeworkdynamics.
|     |     |     |     |     |     |     |     | smaller 0.5 | mm  | (Fig. 2c). | S2 syntheses |     | also resulted | in the |
| --- | --- | --- | --- | --- | --- | --- | --- | ----------- | --- | ---------- | ------------ | --- | ------------- | ------ |
1d
expectedphaseandshoweduniformlysized(z2.5mm)hexag-
| Results |     | and discussion |     |     |     |     |     |          |          |           |       |       |            |         |
| ------- | --- | -------------- | --- | --- | --- | --- | --- | -------- | -------- | --------- | ----- | ----- | ---------- | ------- |
|         |     |                |     |     |     |     |     | onal red | crystals | (Fig. 2a, | b and | S4†). | The rened | lattice |
Synthesis,reproducibilityandmaterialscharacterisation parameters (Fig. 2a, inset) differ slightly from the reported
In 2014, Gao et al. synthesised NTU-9 (C H O Ti ) as a pho- valuesforthecrystallographicc-axisand,hence,cellvolume.32
|     |     |     |     |     | 24  | 6 18 | 2   |     |     |     |     |     |     |     |
| --- | --- | --- | --- | --- | --- | ---- | --- | --- | --- | --- | --- | --- | --- | --- |
Additionally,FT-IRdata32,35,42(Fig.S5†)aswellasTHzRaman
| toactive | titanium(IV)-based |     | MOF | with | p-type | semiconductor |     |     |     |     |     |     |     |     |
| -------- | ------------------ | --- | --- | ---- | ------ | ------------- | --- | --- | --- | --- | --- | --- | --- | --- |
spectroscopy43(Fig.S6†)ofthesynthesisedsolidsalignwiththe
behaviourandutiliseditforthephotochemicaldegradationof
reportedliteratureandindicatesuccessfulformationofNTU-9
| organic | dyes.32 | This 2D | MOF (Fig. | 1) results |     | from | the reaction |          |           |           |     |               |             |     |
| ------- | ------- | ------- | --------- | ---------- | --- | ---- | ------------ | -------- | --------- | --------- | --- | ------------- | ----------- | --- |
|         |         |         |           |            |     |      |              | for both | synthesis | pathways. |     | More detailed | information |     |
betweentitanium(IV)-isopropoxideand2,5-dihydroxyterephtalic
regardingthesyntheticconditionscanbefoundintheESI.†
acid(H dhta)inaceticacid,formingdarkredhexagonalcrystals
4
WewereabletosolvethereproducibilityproblemforS1by
composedofasingleTiion-basedhexagonalframeworkinan
underlying hcb net.32,33 Later, investigations on NTU-9's effi- doubling the reaction time (S1 10d ). SEM images of S1 10d
|     |     |     |     |     |     |     |     | revealed | a uniform | distribution | of  | larger agglomerated |     | hexag- |
| --- | --- | --- | --- | --- | --- | --- | --- | -------- | --------- | ------------ | --- | ------------------- | --- | ------ |
cientgasseparationabilitywithanoptimisedsyntheticproce-
onalcrystals(120mm).Conversely,decreasingthereactiontime
durewerereported.34
|     |     |     |     |     |     |     |     |     |     |     | signicant |     | difference |     |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---------- | --- | ---------- | --- |
We rst synthesised NTU-9 by reproducing different to three days did not yield any (S1 3d ) in
literature.32,35–37 terms of reproducibility, and decreasing it further did not
| synthesis | conditions | reported |     | in the |     |     | These |     |     |     |     |     |     |     |
| --------- | ---------- | -------- | --- | ------ | --- | --- | ----- | --- | --- | --- | --- | --- | --- | --- |
©2025TheAuthor(s).PublishedbytheRoyalSocietyofChemistry Chem.Sci.,2025,16,13646–13654 | 13647

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Table1 SyntheticconditionsforNTU-9varyinginsolvent(aceticacid reactionsolventmightexhibitastrongerbindinginteractionwith
vs.i-PrOH:MeCN)andreactiontime(between15minand10days).
|     |     |     |     |     |     |     | the framework. | This leads | to an extension |     | or shrinking | of the |
| --- | --- | --- | --- | --- | --- | --- | -------------- | ---------- | --------------- | --- | ------------ | ------ |
Reactionsyieldedanamorphousphase(red),phasemixture(yellow),
interlayerdistance(Fig.1b),whichindicatesatendencyforNTU-9
| orpurephase(green)ofNTU-9. |     |     |     | (successfulsynthesis/total |     |     | reaction |     |     |     |     |     |
| -------------------------- | --- | --- | --- | -------------------------- | --- | --- | -------- | --- | --- | --- | --- | --- |
tobeexibletowardsdifferentsolvents.
attempts)
Stabilitytowardssolvents–guestmoleculesforstructural
stabilisation
|     |     |     |     |     |     |     | We studied | the framework–solvent |     | interaction |     | by solvent |
| --- | --- | --- | --- | --- | --- | --- | ---------- | --------------------- | --- | ----------- | --- | ---------- |
.ecneciL detropnU 0.3 noitubirttA snommoC evitaerC a rednu desnecil si elcitra sihT
|     |     |     |     |     |     |     | exchange       | of the incorporated | solvent   | molecules, |          | letting the |
| --- | --- | --- | --- | --- | --- | --- | -------------- | ------------------- | --------- | ---------- | -------- | ----------- |
|     |     |     |     |     |     |     | NTU-9 crystals | soak in             | polar and | non-polar  | solvents | (hexane,    |
 .MP 00:74:5 6202/7/5 no dedaolnwoD .5202 enuJ 52 no dehsilbuP .elcitrA sseccA nepO
H O,EtOH,acetone,i-PrOH,THF,DMF,MeCN)forthreedays
2
aAcetic acid as solvent. b1:1 ratio ofi-PrOH:MeCN as solvent. atroomtemperature(Fig.3).TheoverallPXRDpatternsofthe
cMicrowave-assistedreaction.
resultingsolvent-exchangedNTU-9samplescanbeindexedby
thereportedtrigonalcell,whichshowsthattheMOFisstablein
|     |     |     |     |     |     |     | differentsolvents. |     | however,aresignicantlyshif- |     |     |     |
| --- | --- | --- | --- | --- | --- | --- | ------------------ | --- | ---------------------------- | --- | --- | --- |
Somepeaks,
|         |                 |     |          |     |         |           | ted, e.g.,        | 002 or 102. This       | could        | indicate | that different-sized |             |
| ------- | --------------- | --- | -------- | --- | ------- | --------- | ----------------- | ---------------------- | ------------ | -------- | -------------------- | ----------- |
|         |                 |     |          |     |         |           | solvent molecules | are                    | accommodated | within   | the                  | pores. With |
| produce | any crystalline |     | product. | The | attempt | to change | from              |                        |              |          |                      |             |
|         |                 |     |          |     |         |           | different         | solvents incorporated, | the          | unit     | cell volume          | slightly    |
solvothermaltomicrowave-assistedsynthesistofurtherreduce
|     |     |     |     |     |     |     | varies between | 2174(2) | Å3 and 2208(2) | Å3. | The volume | change |
| --- | --- | --- | --- | --- | --- | --- | -------------- | ------- | -------------- | --- | ---------- | ------ |
thereactiontime,asshownbyYanetal.37withformicacid,lead
mainlyoriginatesfromaslightchangeofthec-axis(Fig.S9and
toamorphousproductsfortheaceticacidbasedreaction.
TableS2†).Thisbehaviourcanbeexplainedbytheanisotropic
| On the | contrary, |     | we found | that | a microwave-assisted |     |     |     |     |     |     |     |
| ------ | --------- | --- | -------- | ---- | -------------------- | --- | --- | --- | --- | --- | --- | --- |
structureofNTU-9(Fig.1):intheab-plane,covalentandcoor-
synthesiscouldreducethereactiontimeto15minutesforthe
dinationbondsarestabilisingtheframework,whichischemi-
| synthesis | without | modulator |     | (S2 | ), yielding | a mixture | of  |     |     |     |     |     |
| --------- | ------- | --------- | --- | --- | ----------- | --------- | --- | --- | --- | --- | --- | --- |
mw cally stiff. Perpendicular to the ab-plane, the 2D sheets are
different-sized
| polycrystalline | powder     |     | and      |      | hexagonal |      | crystals.  |                                          |     |     |     |     |
| --------------- | ---------- | --- | -------- | ---- | --------- | ---- | ---------- | ---------------------------------------- | --- | --- | --- | --- |
|                 |            |     |          |      |           |      | stabilised | betweeneachotherbyhydrogenbondsandvander |     |     |     |     |
| Instead,        | increasing | the | reaction | time | resulted  | in a | phase      |                                          |     |     |     |     |
Waalsforces,whicharechemicallylessstrongandmoreelastic.
reections
| mixture | (S2 3d , S2 | 5d , S2 | 10d ). Additional |     | broad |     | in the |     |     |     |     |     |
| ------- | ----------- | ------- | ----------------- | --- | ----- | --- | ------ | --- | --- | --- | --- | --- |
Thelayeredstructureinthec-directioncanthusbemoreeasily
diffractionpatternscouldnotbeassignedtooneofthereported expandedandaccommodatesdifferent-sizedsolventmolecules,
polymorphsofNTU-917,44(Fig.S7andS2†).
alsodependingonthespecicsolvent–frameworkinteractions.
| S1 ,S1 | ,S1 | andS2 | ,S2 | resultedinphase-pureNTU-9 |     |     | Aer |     |     |     |     |     |
| ------ | --- | ----- | --- | ------------------------- | --- | --- | ---- | --- | --- | --- | --- | --- |
3d 5d 10d mw 1d resuspension of NTU-9 in DMF, new, unindexed peaks
| with slight | peak | shis | in the | PXRD | patterns. | The peaks | of the |     |     |     |     |     |
| ----------- | ---- | ----- | ------ | ---- | --------- | --------- | ------ | --- | --- | --- | --- | --- |
shied
| resulting | products | of synthesis |     | route | S2 are slightly |     | to  |     |     |     |     |     |
| --------- | -------- | ------------ | --- | ----- | --------------- | --- | --- | --- | --- | --- | --- | --- |
lower2qvalues,indicatingalargerunitcellvolume,comparedto
S1,whichisconrmedbyRietveldrenement45foreachsample
(Fig.2,S8andTableS1†).Allproductscrystalliseinthetrigonal
(cid:1)
spacegroupP31c;butthec-axisincreasesfortheS2.Thisvariance
|            |      | different |      | llings |                          |     |     |     |     |     |     |     |
| ---------- | ---- | --------- | ---- | ------- | ------------------------ | --- | --- | --- | --- | --- | --- | --- |
| may result | from |           | pore |         | and solventinteractions. |     |     |     |     |     |     |     |
Despitemultiplewashingstepsofeachsamplewithethanol,itis
possiblethatacompletesolventexchangedidnotoccur,andthe
Fig.2 (a)PXRDpatternofNTU-9calculatedfromthesinglecrystal
(black)32includingtherefinedandreportedlatticeparametersofthe
as-synthesised S1 (red) and S2 (blue) isolated crystallites. Inset: Fig.3 PXRDpatternsofS1withdifferentsolventsusedforsolvent-
|     |     | 5d  |     | 1d  |     |     |     |     |     |     |     |     |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
tablewithcorrespondingspacegroup,refinedlatticeparameters,and exchange. Dotted lines highlight the peak shifts and red asterisks
unitcellvolume.SEMimagesof(b)S2 and(c)S1 . indicatenewlyformedpeaks.
|     |     |     |     | 1d  | 5d  |     |     |     |     |     |     |     |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
13648 | Chem.Sci.,2025,16,13646–13654 ©2025TheAuthor(s).PublishedbytheRoyalSocietyofChemistry

View Article Online
| EdgeArticle |             |           |     |            |     |           |        |     |     |     |     |     | ChemicalScience |     |     |
| ----------- | ----------- | --------- | --- | ---------- | --- | --------- | ------ | --- | --- | --- | --- | --- | --------------- | --- | --- |
| emerge      | in the PXRD | patterns, |     | indicating | the | very slow | forma- |     |     |     |     |     |                 |     |     |
tionofanadditional,unknownphase(Fig.3,S9andS10†).
Thermalstabilityatambientpressureandundervacuum
| Next, we    | explore | the thermal |         | and solvolytic |                   | stability | of NTU-9 |     |     |     |     |     |     |     |     |
| ----------- | ------- | ----------- | ------- | -------------- | ----------------- | --------- | -------- | --- | --- | --- | --- | --- | --- | --- | --- |
| (S1 5d ) in | air and | under       | dynamic | vacuum.        | Thermogravimetric |           |          |     |     |     |     |     |     |     |     |
analysis(TGA)insyntheticairshowsthatthematerialisstable
uptoaround290°C.Twomainmasslossstepscanbedistin-
.ecneciL detropnU 0.3 noitubirttA snommoC evitaerC a rednu desnecil si elcitra sihT
guished.StepI(20wt%massloss)inthetemperaturerangeof
30–220°Ccanbeassignedtothereleaseofsolventmolecules.
 .MP 00:74:5 6202/7/5 no dedaolnwoD .5202 enuJ 52 no dehsilbuP .elcitrA sseccA nepO different
The combustion of the linkers, which is associated with the Fig. 5 PXRD pattern of S1 (left) and S2 (right) at stages of
deconstruction of the framework structure, leads to step II drying: as-synthesised (black), 120 °C heated (red) and exposed to
(48 wt% mass loss) in the range of 220–460 °C. At 460 °C, dynamicvacuumat60°C(blue).
asmall,additionalmasslossoccurs(3wt%massloss),andthe
TG-curveeventuallyreachesaplateauat550°C(Fig.4a).Exsitu
different
|               |         |        |                 |             |          |       |         | vacuum    | for 24 | h.36 Surprisingly, |       | PXRD  | revealed | a    |        |
| ------------- | ------- | ------ | --------------- | ----------- | -------- | ----- | ------- | --------- | ------ | ------------------ | ----- | ----- | -------- | ---- | ------ |
| PXRD analyses |         | show   | that combustion |             | of NTU-9 | leads | to the  |           |        |                    |       |       |          |      |        |
|               |         |        |                 |             |          |       |         | behaviour | when   | activating         | NTU-9 | using | heat     | only | (under |
| formation     | of pure | rutile | (TG             | temperature | 1000     | °C).  | TGA was |           |        |                    |       |       |          |      |        |
measuredtwice(5Kmin −1vs.10Kmin −1)forthesamebatchto ambient pressure) (Fig. 5, red lines) compared to applying
additionaldynamicvacuum(Fig.5,bluelines).Forthelatter,we
| distinguish | the | solvent | loss, | which | is assigned | to  | be around |     |     |     |     |     |     |     |     |
| ----------- | --- | ------- | ----- | ----- | ----------- | --- | --------- | --- | --- | --- | --- | --- | --- | --- | --- |
20 wt% (19 and 21 wt% for the different heating rates). We observedapronouncedpeaksplittingthatimpliesastructural
cannotbesurethatallsolventisremovedintherststepand distortion. The distorted structure is characterised by a peak
|     |     |     |     |     |     |     |     | splitting | of the | 100 reection | and | a shi | of peaks, | i.e., | 110 or |
| --- | --- | --- | --- | --- | --- | --- | --- | --------- | ------ | ------------- | --- | ------ | --------- | ----- | ------ |
remainingcoordinatingsolventmoleculescanbeexpecteddue
todiffuse,residualelectrondensityinthepores,asestablished 002,inthePXRDpattern.ForS2 1d ,theconversiontoNTU-9-dis
byRietveldrenementofthedriedNTU-9(Fig.S11–S13†). incomplete,resultinginaphasemixtureofNTU-9andNTU-9-d.
Inthemechanisticstudytounderstandthedistortion,wefocus
| For | structural | insights, |     | we  | performed | temperature- |     |     |     |     |     |     |     |     |     |
| --- | ---------- | --------- | --- | --- | --------- | ------------ | --- | --- | --- | --- | --- | --- | --- | --- | --- |
dependent in situ PXRD using the same sample (S1) within onsynthesisS1,whichresultsinNTU-9-duponevacuationand
notinaphasemixture(Fig.5,S14andS15†).
thetemperaturerangeofsolventrelease,i.e.,upto220°C.The
| heating | results | in changes | in  | peak | intensity, | attributed | to the |     |     |     |     |     |     |     |     |
| ------- | ------- | ---------- | --- | ---- | ---------- | ---------- | ------ | --- | --- | --- | --- | --- | --- | --- | --- |
Characterisationofthenewform(NTU-9-d)
| release    | of adsorbed | solvent | molecules         |     | and         | peak broadening, |             |     |     |     |     |     |     |     |     |
| ---------- | ----------- | ------- | ----------------- | --- | ----------- | ---------------- | ----------- | --- | --- | --- | --- | --- | --- | --- | --- |
| indicating | a decrease  |         | in crystallinity. |     | The absence |                  | of a strong |     |     |     |     |     |     |     |     |
WederivedastructuralmodelofthedistortedNTU-9-dfromthe
| peak shi | illustrates | the | low | thermal | expansion | of  | NTU-9 and |     |     |     |     |     |     |     |     |
| --------- | ----------- | --- | --- | ------- | --------- | --- | --------- | --- | --- | --- | --- | --- | --- | --- | --- |
PXRDdata.Startingfromthetrigonalstructureofnon-distorted
showsitsratherstrongthermalstability,astheoverallpattern NTU-9,wegraduallyreducedthespacegroupsymmetryandper-
| and, therefore, |     | the framework |     | of NTU-9 | remains | unchanged. |     |     |     |     |     |     |     |     |     |
| --------------- | --- | ------------- | --- | -------- | ------- | ---------- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
formedglobaloptimisationofthelatticeparametersbyperform-
The 100 and 110 peaks do not change positions but broaden, ingmultipleRietveldrenements45(furtherdetailsaregiveninthe
whereas the other peaks shi to slightly higher 2q values, ESI†).Eventually,wewereabletodescribethediffractiondataof
| indicating | smaller | lattice | parameters |     | as the | loss | of guest |     |     |     |     |     |     |     |     |
| ---------- | ------- | ------- | ---------- | --- | ------ | ---- | -------- | --- | --- | --- | --- | --- | --- | --- | --- |
thedistortedNTU-9byusingatriclinicunitcellwithspacegroup
moleculeslikelyleadstoasmallerlatticecontraction(Fig.4b). P1 (cid:1) (Table2).Aerdistortion,NTU-9iscontractedalongtheaaxis
A typical activation procedure for MOFs employs heating andthebanglesignicantlydecreases.Thisleadstoadecreaseof
Different
under dynamic vacuum.46 activation procedures for 9%inunitcellvolumefrom2192(2)Å3to1995(4)Å3(activationvia
NTU-9 are reported, e.g., 120 °C under vacuum overnight,32 vacuumat60°C)andtoanellipsoidalporedistortion.(TableS3†)
| dried at | 60 °C,35 | without | dynamic |     | vacuum, | or 80 | °C under |     |     |     |     |     |     |     |     |
| -------- | -------- | ------- | ------- | --- | ------- | ----- | -------- | --- | --- | --- | --- | --- | --- | --- | --- |
Thisbehaviourisunusualin2Dframeworksasitisnottheelastic
axisc(layerdistance)thatchanges,butthedistortionoccursinthe
ab-plane,whichresultsintheporedeformation.Theporechannel
runningalongthec-directionisslightlytiltedandperpendicularto
theab-plane(Fig.6).AertheRietveldrenementusingonlythe
signicant
|     |     |     |     |     |     |     |     | node and   | the linkers, | a              |     | amount      | of residual | electron |        |
| --- | --- | --- | --- | --- | --- | --- | --- | ---------- | ------------ | -------------- | --- | ----------- | ----------- | -------- | ------ |
|     |     |     |     |     |     |     |     | density is | still        | visible within | the | pores (Fig. | S13†).      | The      | broad, |
signicant
|     |     |     |     |     |     |     |     | anisotropic | peak | shape of | NTU-9-d | indicates | a   |     | struc- |
| --- | --- | --- | --- | --- | --- | --- | --- | ----------- | ---- | -------- | ------- | --------- | --- | --- | ------ |
turaldisorderofthedistortedframework.Thisledtoinstabilities
inthenalRietveldrenements,whenweattemptedtorenethe
|     |     |     |     |     |     |     |     | cation and | linker | position. | Consequently, |     | we can | neither | report |
| --- | --- | --- | --- | --- | --- | --- | --- | ---------- | ------ | --------- | ------------- | --- | ------ | ------- | ------ |
(a)TGAdataofS1,heatingrate5Kmin−1undersyntheticair
| Fig.4 |     |     |     |     |     |     |     | reliablebondanglesandlengthsbetweenthetitaniumnodesand |     |     |     |     |     |     |     |
| ----- | --- | --- | --- | --- | --- | --- | --- | ------------------------------------------------------ | --- | --- | --- | --- | --- | --- | --- |
atmosphere(flowrate70mLmin−1)(black)andthederivationofthe
|     |     |     |     |     |     |     |     | the linker | molecules | nor | quantify | the residual | amount | of  | guest |
| --- | --- | --- | --- | --- | --- | --- | --- | ---------- | --------- | --- | -------- | ------------ | ------ | --- | ----- |
TGAdata(lightgreen).(b)Temperaturedependent(30–220°C)insitu
moleculeswithinthepores.
| PXRD measurements |     | including  |       | selected     | reflection | indices  | using an |                 |     |            |     |                 |     |            |     |
| ----------------- | --- | ---------- | ----- | ------------ | ---------- | -------- | -------- | --------------- | --- | ---------- | --- | --------------- | --- | ---------- | --- |
|                   |     |            |       |              |            |          |          | We investigated |     | the nature | of  | this structural |     | distortion | by  |
| open capillary    | of  | S1. Dotted | lines | are included | to         | indicate | the peak |                 |     |            |     |                 |     |            |     |
differentactivationmethods:evaporationatroomtemperature/
shifts.
©2025TheAuthor(s).PublishedbytheRoyalSocietyofChemistry Chem.Sci.,2025,16,13646–13654 | 13649

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| --------------- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ----------- | --- |
CellparametersofNTU-9andtheresultingparametersofthemodelfromNTU-9-dbydifferentactivationmethods
Table2
Activationprocedure As-synthesised 120°C,7h 120°C,N 2 ,7h scCO 2 Vacuum,rt Vacuum,60°C
Polytype NTU-9 NTU-9 NTU-9-d(83.8wt%)NTU-9(16.2wt%) NTU-9-d NTU-9-d NTU-9-d
|            |     | (cid:1)  |     | (cid:1)  |     | (cid:1)  |     |     |     | (cid:1)  | (cid:1)  |     | (cid:1)  |     |
| ---------- | --- | -------- | --- | -------- | --- | -------- | --- | --- | --- | -------- | -------- | --- | -------- | --- |
| Spacegroup |     | P31c     |     | P31c     |     | P1       |     |     |     | P1       | P1       |     | P1       |     |
| a/Å        |     | 14.63(1) |     | 14.62(1) |     | 13.24(1) |     |     |     | 14.15(1) | 13.42(1) |     | 13.24(2) |     |
| b/Å        |     | 14.63(1) |     | 14.62(1) |     | 15.23(1) |     |     |     | 15.00(1) | 15.36(1) |     | 15.14(2) |     |
| c/Å        |     | 11.82(1) |     | 11.53(1) |     | 11.35(1) |     |     |     | 11.50(1) | 11.37(1) |     | 11.31(2) |     |
| a/°        |     | 90       |     | 90       |     | 94.5(1)  |     |     |     | 92.3(1)  | 94.2(1)  |     | 94.3(1)  |     |
| b/°        |     | 90       |     | 90       |     | 79.9(1)  |     |     |     | 85.6(1)  | 80.6(1)  |     | 79.9(1)  |     |
.ecneciL detropnU 0.3 noitubirttA snommoC evitaerC a rednu desnecil si elcitra sihT  g/° 120 120 116.0(1) 118.5(1) 117.1(1) 116.6(1)
V/Å3
|     |     | 2192(1) |     | 2133(1) |     | 2026(1) |     |     |     | 2138(2) | 2060(2) |     | 1995(4) |     |
| --- | --- | ------- | --- | ------- | --- | ------- | --- | --- | --- | ------- | ------- | --- | ------- | --- |
 .MP 00:74:5 6202/7/5 no dedaolnwoD .5202 enuJ 52 no dehsilbuP .elcitrA sseccA nepO
ambientpressure(overnight),heatingat120°Cunderambient To the best of our knowledge, no external stimuli-induced
exible
pressure (7 h), heating at 120 °C under continuous nitrogen behaviour for polymorphs of NTU-9 such as MIL-167-
stream (7 h), supercritical CO activation (scCO ), and under 169 or FIR-117-119 has been reported as yet.17,44 However,
|     |     |     | 2   |     | 2   |     |     |     |     |     |     |     |     |     |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
exibility
dynamic vacuum (with or without additional heating). We some studies were reported for FIR-131-138, with
different
observed that NTU-9-d formation was triggered by dynamic sulfonated linker, when soaking crystals in
| vacuum, scCO | ,   | or heating | in  | a continuous | nitrogen | ow, | solvents.49,50 |     |     |     |     |     |     |     |
| ------------ | --- | ---------- | --- | ------------ | -------- | ---- | -------------- | --- | --- | --- | --- | --- | --- | --- |
2
inuence
whereas drying the sample up to 220 °C at 1 bar (ambient To investigate the vacuum on the distortion of
pressure) did not distort the framework (Fig. 4b). The rened NTU-9 in more detail, we performed in situ PXRD under
cellparametersofthedifferentlyactivatedsamplesS1showthat dynamicvacuum.SampleS1wasrstheatedto60°Cfollowed
byastepwisereductioninpressure,eventuallyreachinganal
| the cell distortion |     | varies | depending | on the | activation | process. |     |     |     |     |     |     |     |     |
| ------------------- | --- | ------ | --------- | ------ | ---------- | -------- | --- | --- | --- | --- | --- | --- | --- | --- |
For the sample dried with only heat under nitrogen stream dynamicvacuumof3×10 −3mbarwhileconstantlymeasuring
|     |     |     |     |     |     |     | diffraction |     |     |     | rst, |     |     |     |
| --- | --- | --- | --- | --- | --- | --- | ----------- | --- | --- | --- | ----- | --- | --- | --- |
a mixture of NTU-9 and NTU-9-d is present. For the scCO 2 - patterns (Fig. 7a). At NTU-9 remained undis-
activatedsample,weobservedlesspronouncedchangesinthe torted when heating at 60 °C and under ambient pressure. A
cellparameterscomparedtothevacuum-activatedanalogue;by slight decrease of the c-axis (Fig. 7b, green squares) indicates
dynamicvacuumexposure,weobtainedamoredistortedpore the incipient release of solvent molecules. Further pressure
withanoverallsmallercellvolume,whichcanbeampliedby decreaseto200mbarinitiatestheformationofNTU-9-d,indi-
catedbythedegenerationofthea-andb-axesandthea-andb-
additionalheat.Wecanstatethatdryingundervacuumdistorts
theporemostefficiently(Table2,Fig.S16andS17†). angles(Fig.7bandc).Thegradualdistortionofthetrigonalunit
Although vacuum-induced structural transformations in celltowardstriclinicmetricsandthereductionoftheunitcell
rst
MOFs have been investigated,47 this is the time that the volumecontinueswithfurtherreducingthepressure.Interest-
exibility behaviour ofNTU-9 hasbeen reported. We hypothe- ingly,thetransitionfromNTU-9toitsdistortedformdoesnot
sisethatthedifferencesbetweensynthesisprotocols(S1vs.S2) affectthecontractionbehaviouroftheelasticc-axis.
in the activation process (NTU-9-d vs. phase mixture) can be TheformationofNTU-9-dcanalsobefollowedbyinsituTHz
either explained by the difference in crystallite size which is Raman spectroscopy in dynamic vacuum at 60 °C. Changes
known to impact dynamic framework transitions,48 or by the from the initial spectrum compared to the vacuum evacuated
differenceinthekindorpositionoftheguestmoleculeswithin sample (S1) at different pressure values between 0 and 1100
the pores. We noticed a difference by nuclear magnetic reso- mbar and 60 °C can be observed (Fig. S6b and c†). The vibra-
nance(NMR)ofthedigestedsampleswithandwithoutmodu- tions of the organic H dhta linker dominate the Raman spec-
4
lator. In synthesis S1 , the digested sample shows residual trum.HerewemainlyfocusontheTi–Ostretchingbandaround
5d
| aceticacid(Fig.S18†). |     |     |     |     |     |     | −1.43,51    |           |          |                  |           |             |          |          |
| --------------------- | --- | --- | --- | --- | --- | --- | ----------- | --------- | -------- | ---------------- | --------- | ----------- | -------- | -------- |
|                       |     |     |     |     |     |     | 683 cm      | The       | band     | broadening       | in        | the process | of       | vacuum   |
|                       |     |     |     |     |     |     | exposure    | indicates | changes  | in               | the Ti–O  | bond        | upon     | pore     |
|                       |     |     |     |     |     |     | distortion. | We can    | assume   | that,            | as proven | for         | Zr-MOFs, | node     |
|                       |     |     |     |     |     |     | distortion  | caused    | by guest | removal/addition |           | can         | also     | occur in |
Ti-MOFs.52Broadeningcanalsobeobservedinthesymmetrical
−1.43
|     |     |     |     |     |     |     | stretching | vibration | mode | around | 700 | cm  | This | spectro- |
| --- | --- | --- | --- | --- | --- | --- | ---------- | --------- | ---- | ------ | --- | --- | ---- | -------- |
scopicresulthintsattheformationofthedistortedformNTU-9-
|     |     |     |     |     |     |     | d and, | in particular, |     | indicates | changes | in  | the octahedral |     |
| --- | --- | --- | --- | --- | --- | --- | ------ | -------------- | --- | --------- | ------- | --- | -------------- | --- |
titanium-linkercoordination(Fig.S6bandc†).
CO gasadsorptionexperimentswereconductedtoanalyse
2
|     |     |     |     |     |     |     | porosity     | and adsorption        |            | behaviour | of      | NTU-9-d     | (S1).         | The CO 2 |
| --- | --- | --- | --- | --- | --- | --- | ------------ | --------------------- | ---------- | --------- | ------- | ----------- | ------------- | -------- |
|     |     |     |     |     |     |     | adsorption   | isotherm              | was        | collected | at      | 273 K       | and resembles |          |
|     |     |     |     |     |     |     | a type I     | isotherm,             | indicating | the       | present | micro-pores |               | (<2 nm)  |
|     |     |     |     |     |     |     | (Fig.S19a†). | NTU-9-dadsorbed72cm3g |            |           |         | −1ofCO      |               |          |
Fig.6 StructuralmodelofNTU-9-d'sdistortedcrystalstructureupon 2 witha total
vacuumexposure(a)intheab-planeand(b)alongthec-axis.Green pore volume of 0.15 cm3 g −1 and a calculated Brunauer–
ellipsoidal cylinders represents the random distribution of solvent Emmett–Teller = m2 −1
|     |     |     |     |     |     |     |     |     | (BET) | surface | area of | S BET | 199 | g   |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | ----- | ------- | ------- | ----- | --- | --- |
moleculeswithinthechannels.
13650 | Chem.Sci.,2025,16,13646–13654 ©2025TheAuthor(s).PublishedbytheRoyalSocietyofChemistry

View Article Online
| EdgeArticle |     |     |     |     |     |     |     |     |     |     |     | ChemicalScience |     |     |
| ----------- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --------------- | --- | --- |
.ecneciL detropnU 0.3 noitubirttA snommoC evitaerC a rednu desnecil si elcitra sihT
 .MP 00:74:5 6202/7/5 no dedaolnwoD .5202 enuJ 52 no dehsilbuP .elcitrA sseccA nepO
Fig. 7 (a) In situ diffraction data of sample S1. The pressure was (a)InsituPXRDmeasurementofopencapillaryofvacuumdried
Fig.8
gradually reduced from ambient pressureto dynamic vacuumat 10 ±
|         |        |           |        |         |           |          |        | S1 over | time (RH | of 25 | 5%). Evolution | of (b) | lattice | parameter |
| ------- | ------ | --------- | ------ | ------- | --------- | -------- | ------ | ------- | -------- | ----- | -------------- | ------ | ------- | --------- |
| mbar at | 60 °C. | Evolution | of (b) | lattice | parameter | changes, | volume |         |          |       |                |        |         |           |
changes,volumechanges,and(c)theunitcellanglechangesduring
changes, and (c) the unit cell angle changes during in situ vacuum therehydrationofNTU-9-dbyambientatmosphere.
PXRD.
theambientatmosphere,indicatingarelativelyslowwateruptake
(Fig.S19b†).Thecalculatedporesizedistributionsiscentredat
fromtheatmosphere(Fig.S6d†).
S19c†).
0.6 nm (Fig. In comparison to a reported similar WeconrmedbyexsituPXRDthatresuspendingNTU-9-din
| procedure | for | S2 from | Cai | et al. (2 | d reaction |     | time, without |     |     |     |     |     |     |     |
| --------- | --- | ------- | --- | --------- | ---------- | --- | ------------- | --- | --- | --- | --- | --- | --- | --- |
differentsolvents(ethanol,water,acetonitrile)overnightresults
ethanol solvent exchange), the CO 2 uptake of NTU-9 (no peak S21andS22†). Differentsolvent
inthe recoveryofNTU-9(Fig.
| splitting | observed) | is  | about | 12% higher | with | a   | largely higher |     |     |     |     |     |     |     |
| --------- | --------- | --- | ----- | ---------- | ---- | --- | -------------- | --- | --- | --- | --- | --- | --- | --- |
moleculesinsidethelledchannelsleadtoslightdifferencesin
| BET surface | area | (917 | m2 g −1 | from | N sorption |     | data).34 These |                                                            |     |     |     |     |     |     |
| ----------- | ---- | ---- | ------- | ---- | ---------- | --- | -------------- | ---------------------------------------------------------- | --- | --- | --- | --- | --- | --- |
|             |      |      |         |      | 2          |     |                | thelatticeparameters,inparticularinthelengthoftheelasticc- |     |     |     |     |     |     |
differences
|     | could | originate | from | the | tendency | of  | S1 to distort, |          |               |             |     |              |      |        |
| --- | ----- | --------- | ---- | --- | -------- | --- | -------------- | -------- | ------------- | ----------- | --- | ------------ | ---- | ------ |
|     |       |           |      |     |          |     |                | axis, of | the resulting | undistorted |     | NTU-9 (Table | S5), | demon- |
whichcausesloweraccessibilityofthematerial'spores,result-
stratingtheadaptivenatureoftheframeworktowardsdifferent
inginloweruptakeandBETsurfacearea.PXRDdataaergas
guestmolecules.Interestingly,weobservethatthereversibility,
| adsorption | experiments |     | indicate | the | stability | of  | the distorted |              |     |        |                 |        |     |            |
| ---------- | ----------- | --- | -------- | --- | --------- | --- | ------------- | ------------ | --- | ------ | --------------- | ------ | --- | ---------- |
|            |             |     |          |     |           |     |               | from NTU-9-d | to  | NTU-9, | is signicantly | faster | in  | the liquid |
pore(Fig.S19d,S20andTableS4†).
|     |     |     |     |     |     |     |     | phase compared |         | to the                | gas phase. | We demonstrated |      | full     |
| --- | --- | --- | --- | --- | --- | --- | --- | -------------- | ------- | --------------------- | ---------- | --------------- | ---- | -------- |
|     |     |     |     |     |     |     |     | reversibility  | of this | desolvation-solvation |            | process         | over | 5 cycles |
byPXRD(Fig.S23†).
ReversibilityofthedeformationofNTU-9
Aerapplyingvacuum,NTU-9-dcanbetransformedbackinto
its original state by exposure to moisture. We monitored the Conclusions
| structural | changes | upon | water | uptake | of  | NTU-9-d | from the |     |     |     |     |     |     |     |
| ---------- | ------- | ---- | ----- | ------ | --- | ------- | -------- | --- | --- | --- | --- | --- | --- | --- |
ambient atmosphere (22 °C, 25, 5% R.H.) in an open glass In this work, we explore the impact and reproducibility of the
capillary with an opening diameter of 0.7 mm by PXRD over synthesis conditions on the formation of the layered Ti-MOF
aperiodof42hours(Fig.8). NTU-9. We observe that the selection of solvent, additive, and
inuenced
The c-lattice parameter is hardly by the water reactiontimeiscrucialforareliable,reproduciblesynthesisof
uptake (Fig. 8b, light green), whereas the a- and b-axis exhibit the phase pure material. Using pure acetic acid as modulator
signicant
changes. Initially, both the a- and the b-axis expand and solvent at 120 °C leads to the reproducible formation of
upon exposure to ambient conditions (Fig. 8d, cyan and olive), large NTU-9 crystals. When long reaction times of at least 10
whichleadstoarapidincreaseintheunitcellvolume(Fig.8b, daysareapplied,thepresenceofaceticacidintheframework
|     | Aer |     |     |     |     |     |     |     |     | aer |     |     |     |     |
| --- | ---- | --- | --- | --- | --- | --- | --- | --- | --- | ---- | --- | --- | --- | --- |
black). approximately three hours, the b-axis starts con- can be detected, even vigorous post-synthetic washing
tractingwhileakeepsexpanding,leadingtoanoverallexpansion steps. Solvothermal synthesis in an i-PrOH:MeCN mixture
oftheunitcellvolume.Bothunitcellaxesbecomemoresimilarin without a modulator at 120 °C is faster (1 day) and can be
length, and the g-angle between a and b expands towards 120° further sped up using microwave-assisted synthesis (15 min),
(Fig.8c,lightgreen).Whilethea-angleslightlydecreasestowards butyieldssmallerNTU-9crystals.
90°,theb-anglerapidlyexpandsto90°(Fig.8c,cyanandolive). Pure heat treatment of the solvated MOFs up to 220 °C and
Consequently, water uptake from the atmosphere distorts the ambient pressure removes large amounts of the incorporated
ellipsoidalporebacktowardstheroundshapeofNTU-9,andthe guestsolventmoleculeswithoutchangingtheframework'sstruc-
tilted channel becomes more perpendicular to the ab-plane. ture. However, activating the material with heating under
Despitethefactthatthecellrelaxestowardsthetrigonalmetricof a nitrogen ow, scCO , or vacuum results in a previously
2
NTU-9,itdoesnotreachthisstateaer42hoursofexposureto
|     |     |     |     |     |     |     |     | unknown, | distorted | structure, | abbreviated | as  | NTU-9-d. | The |
| --- | --- | --- | --- | --- | --- | --- | --- | -------- | --------- | ---------- | ----------- | --- | -------- | --- |
©2025TheAuthor(s).PublishedbytheRoyalSocietyofChemistry Chem.Sci.,2025,16,13646–13654 | 13651

ChemicalScience EdgeArticle
structuraldistortiontransformsthecircularporechannelsintoan research and innovation programme under the Marie
ellipsoidal shape and reduces the unit cell volume by 9%. The Skłodowska-Curie Actions (Grant Agreement No. 101208213,
distortion mechanism is primarily governed by compression MUVCATMOF,HORIZON-MSCA-2024-PF-01-01).TheFondsder
withinthehardcoordinationplane(ab-plane,metal–ligandcoor- Chemischen Industrie (Kekul´e stipend to J. E. K.) is gratefully
dinationconnectivity),withonlyminorchangesintheelasticaxis acknowledged.B.O.-R.gratefullyacknowledgesapostdoctoral
c(layerstackingviahydrogenbonding).Thisresultisparticularly scholarship from the Max Planck Society and the European
interesting, as it contradicts the prevailing assumption that the Union's Horizon Europe research and innovation programme
soconnectivitybetweenlayerspredominantlydrivesexibilityin for the receipt of a Marie Skłodowska-Curie Actions Post-
2Dlayeredframeworks.Ramanspectroscopyindicatesadistortion doctoral Fellowship. Deutsche Forschungsgemeinscha (DFG)
oftheoctahedralcoordinationgeometryofthetitaniumcations. (project-ID358283783,SFB1333/2),S.K.acknowledgessupport
Interestingly,thesynthesisconditionsstronglyaffecttheexibility fromtheCarl-Zeiss-StiungNEXUSprogram.
oftheMOFstructure,wheresamplessynthesisedwithoutmodu-
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13652 | Chem.Sci.,2025,16,13646–13654 ©2025TheAuthor(s).PublishedbytheRoyalSocietyofChemistry
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