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

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		Dynamic		breathing		behaviour		of	the	titanium-
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.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											†
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		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
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ofChemistry Julia E. Knapp, Borja Ort´ Fabian Kristina Gjorgjevikj,
		Anastasia	Sleptsova,		a Simon	Krause,	a Sebastian		Bette	*a
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		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.
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Introduction Titanium-based MOFs are of particular interest due to the
						earth abundance	of	their	metal source,		high stability,	low
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Metal–organic toxicity, redox activity, and promising photochemical proper-
	frameworks	(MOFs) are	porous	reticular	mate-
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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
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						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
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81377,Germany
						Gastaldo	group, i.e.,	by metal	doping15	or using	hydroxamate-
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†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
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								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-
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								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
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parameters and evaluated for reproducibility towards large syntheticprotocolbyperformingthereactionbetween4and21
different
								times. Indeed,		we found		trends	depending	on the
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singlecrystalsandmicrocrystallinebulkpowders.Furthermore,
solventused(Table1andFig.S1,S2†).
we discovered		a new metastable		form	NTU-9-d	(d	stands for
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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)
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humidity/air.		Our work	establishes	a	reproducible		synthesis
revealedtheformationofcrystalswithvaryingsize(20–50mm)
procedure	and	a detailed		mechanism	of	the	previously
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								and microcrystalline		powder	with	average	crystallite	size
unknownframeworkdynamics.
								smaller 0.5	mm	(Fig. 2c).	S2 syntheses		also resulted	in the
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1d
expectedphaseandshoweduniformlysized(z2.5mm)hexag-
Results		and discussion
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								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
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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
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								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-
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cientgasseparationabilitywithanoptimisedsyntheticproce-
onalcrystals(120mm).Conversely,decreasingthereactiontime
durewerereported.34
											signicant		difference
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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
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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
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.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
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thereactiontime,asshownbyYanetal.37withformicacid,lead
mainlyoriginatesfromaslightchangeofthec-axis(Fig.S9and
toamorphousproductsfortheaceticacidbasedreaction.
TableS2†).Thisbehaviourcanbeexplainedbytheanisotropic
On the	contrary,		we found	that	a microwave-assisted
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structureofNTU-9(Fig.1):intheab-plane,covalentandcoor-
synthesiscouldreducethereactiontimeto15minutesforthe
dinationbondsarestabilisingtheframework,whichischemi-
synthesis	without	modulator		(S2	), yielding	a mixture	of
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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
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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
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tablewithcorrespondingspacegroup,refinedlatticeparameters,and exchange. Dotted lines highlight the peak shifts and red asterisks
unitcellvolume.SEMimagesof(b)S2 and(c)S1 . indicatenewlyformedpeaks.
				1d	5d
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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
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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
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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-
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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
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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.
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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.
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.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- latoryieldsmallercrystalsandshowalowertendencyfordistor- References tion.ThedriedanddistortedMOFisabletotakeupwaterfrom the atmosphere and a variety of polar and non-polar solvent 1 S.L.James,Metal-organicframeworks,Chem.Soc.Rev.,2003, moleculesfromtheliquidphase,leadingtoareversiblestructural 32,276–288. relaxationintotheundistortedformofNTU-9overseveralcycles. 2 H. Furukawa, K. E. Cordova, M. O'Keeffe and O. M. Yaghi, Incorporating different solvent molecules into the structure of The Chemistry and Applications of Metal-Organic NTU-9 leads to signicant changes in the elastic c-axis lattice Frameworks,Science,2013,341,1230444. parameters,whichcorrespondtotheinterlayerdistance.Despite 3 A. J. Howarth, A. W. Peters, N. A. Vermeulen, T. C. Wang, the observed large exibility of the framework, the long-range J. T. Hupp and O. K. Farha, Best Practices for the orderisstillmaintainedaermultiplecyclesofde-andresolva- Synthesis, Activation, and Characterization of Metal– tionandtheassociatedporedistortionandrelaxation. OrganicFrameworks,Chem.Mat.,2017,29,26–39. This work illustrates the oen-overlooked dynamic proper- 4 R.G.Pearson,HardandSoAcidsandBases,J.Am.Chem. ties of reticular materials in general and Ti-based MOFs in Soc.,1963,85,3533–3539. particular under solvent evacuation/resuspension. Our study 5 A. Dhakshinamoorthy, Z. Li and H. Garcia, Catalysis and thus suggests that thorough characterisation of reticular photocatalysis by metal organic frameworks, Chem. Soc. materials for each process is essential to deeply understand Rev.,2018,47,8134–8172. frameworkbehaviouranditsresultingapplications. 6 M. Dan-Hardi, C. Serre, T. Frot, L. Rozes, G. Maurin, C. Sanchez and G. F´erey, A New Photoactive Crystalline Data availability Highly Porous Titanium(IV) Dicarboxylate, J. Am. Chem. Soc.,2009,131,10857–10859. Thedata supportingthisarticle havebeenincludedaspartof 7 A. Cadiau, N. Kolobov, S. Srinivasan, M. G. Goesten, the ESI.† The authors have cited additional references within H. Haspel, A. V. Bavykina, M. R. Tchalala, P. Maity, theESI.†53–59. A. Goryachev, A. S. Poryvaev, M. Eddaoudi, M. V. Fedin, O. F. Mohammed and J. Gascon, A Titanium Metal– Author contributions Organic Framework with Visible-Light-Responsive Photocatalytic Activity, Angew. Chem., Int. Ed., 2020, 59, J.E.K.andS.B.conceivedtheideaoftheproject.J.E.K.syn- 13468–13472. thesisedandcharacterisedthemetal–organicframework.F.H. 8 M. Sachs, L. Harnett-Cauleld, E. Pastor, B. Davies, performedCO sorptionmeasurements.K.Gj.measuredthein D. J. C. Sowood, B. Moss, A. Kazas, J. Nelson, A. Walsh 2 situ Raman data, A. S. the in situ vacuum PXRD data. S. B. andJ.R.Durrant,Ligandeldstatescontrolphotocatalytic performedtheinsiturehydrationmeasurementsandtheRiet- efficiency of transition metal oxides, in preparation, veld renements. J. E. K. wrote the manuscript with contribu- ChemRxiv,2024,DOI:10.26434/chemrxiv-2024-2wn0d. tionsfromallauthors.Allauthorsreadandcommentedonthe 9 B.Bueken,F.Vermoortele,D.E.P.Vanpoucke,H.Reinsch, nalmanuscript. C. C. Tsou, P. Valvekens, T. D. Baerdemaeker, R. Ameloot, C. E. A. Kirschhock, V. Van Speybroeck, J. M. Mayer and Conflicts of interest D. De Vos, A Flexible Photoactive Titanium Metal–Organic Framework Based on a [Ti(IV) (m -O)(O) (COO) ] Cluster, 3 3 2 6 Therearenoconictstodeclare. Angew.Chem.,Int.Ed.,2015,54,13912–13917. 10 T.DevicandC.Serre,Highvalence3pandtransitionmetal Acknowledgements basedMOFs,Chem.Soc.Rev.,2014,43,6097–6115. 11 L.RozesandC.Sanchez,Titaniumoxo-clusters:precursors The authors gratefully acknowledge the help of Viola Duppel for a Lego-like construction of nanostructured hybrid (MPI-FKF)fortheSEMimagesandNadineWeib(MPI-FKF)for materials,Chem.Soc.Rev.,2011,40,1006–1030. the TGA measurements. Financial support was granted by the 12 H.L.Nguyen,Perspectivesontitanium-basedmetal–organic MaxPlanckSocietyandtheEuropeanUnion'sHorizonEurope frameworks,JPhysEnergy,2021,3,021003. 13652 | Chem.Sci.,2025,16,13646–13654 ©2025TheAuthor(s).PublishedbytheRoyalSocietyofChemistry .MP 00:74:5 6202/7/5 no dedaolnwoD .5202 enuJ 52 no dehsilbuP .elcitrA sseccA nepO .ecneciL detropnU 0.3 noitubirttA snommoC evitaerC a rednu desnecil si elcitra sihT View Article Online

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exibility
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	P–(CH )–PO $(H
---	--------------	---	---	---	---	---	---	---	---	---
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