10082 Chem. Commun., 2013, 49, 10082--10084 This journal is c The Royal Society of Chemistry 2013

Cite this: Chem. Commun.,2013,49, 10082

A robust amino-functionalized titanium(IV) based MOFfor improved separation of acid gases†

Sebastien Vaesen,a Vincent Guillerm,b Qingyuan Yang,cg Andrew D. Wiersum,d

Bartosz Marszalek,e Barbara Gil,e Alexandre Vimont,f Marco Daturi,f Thomas Devic,b

Philip L. Llewellyn,d Christian Serre,*b Guillaume Maurin*c and Guy De Weireld*a

A combination of adsorption, microcalorimetry, infra-red spectroscopy

and modeling has been implemented to reveal the potential of the H2S

resistant amino-functionalized Ti MOF MIL-125 porous solid for the

concomitant elimination of CO2 and H2S from biogas and natural gas.

The transition from total petroleum consumption to alternativeeconomic and green energies is of great interest for the developmentof natural gas (NG) and syngas technologies, and their relatedapplications.1 Further, an important renewable energy source isbiogas whose production stems from the conversion of organicmatter.2 Depending on the origin of NG or the primary matter, andthe type of process, the gas mixtures often contain significantamounts of CO2 and H2S3a that have to be reduced to low concen-trations.3b The elimination of these pollutants can be performed byphysical or chemical absorption, biological removal, cryogenic andmembrane separation or conversion by metal oxides.3c Chemicalscrubbing using amine solutions is the most commonly usedtechnique for acid gas removal4 although it suffers from thedisadvantages of excessive corrosion, oxidative degradation, andtoxicity issues and induces a high regeneration cost.5 Membrane-based processes based on polymers are also envisaged but they arenot selective enough for CO2–CH4 and H2S–CH4 mixtures.6

As an alternative, pressure swing adsorption (PSA) processesare found to be valuable for the selective elimination of CO2 andH2S, but their performance depends highly on the choice of theadsorbents. Activated carbons,7a carbon fiber composite molecularsieves,7b amine-grafted mesoporous silicas,7c ‘‘molecular basket’’sorbents (MBS)7d and more recently Metal Organic Frameworks(MOFs)8,9 have been predominantly envisaged. Some MOFs areattractive for physisorption-based applications due to their highpore volume and surface area combined with their high concen-tration of active sites. Recently, MOFs have been evaluated not onlyfor the selective removal of CO2, but also for the purification ofdiverse gases.9 The incorporation of polar functional groups intoMOFs constitutes a promising approach to enhance their adsorptionproperties.8b,c Several amino terephthalate-based MOFs, such asMIL-53(Al),10 UiO-66(Zr), PAF-1 and ZnMOF11a or other function-alized ZIFs11b have been thus revealed to be highly selective for CO2

over CH4 and N2. Except for the MIL-53-NH2 solid where selectivity isdue to a change in the breathing behavior,10b the importance of theinteraction between CO2 and the –NH2 groups was established.

In contrast, the exploration of the H2S–CH4 separation propertiesof such porous solids is still in its infancy, with no study on theimpact of the functionalization.8b We aim here to probe theperformances of the amino functionalized titanium terephthalateMIL-125(Ti)-NH2 versus its parent MIL-125(Ti) analogue.12a These TiMOFs bear two key features: (i) they are expected to be H2Sresistant,13 and (ii) they can be scaled up to the multi-gram scaleunder ambient pressure conditions.12b

MIL-125(Ti)-NH2 exhibits a quasi-cubic tetragonal structure (seeFig. S0, ESI†), based on octameric Ti8O4(OH)4 oxoclusters and2-amino-terephthalate dianions. This delimits octahedral and tetra-hedral cages with calculated free diameters of 10.7 Å and 4.7 Årespectively,14 slightly smaller than those present in the parentanalogue, accessible through triangular windows of 5–7 Å.

The thermal stability of MIL-125(Ti)-NH2 is close to 573 K (Fig. S1and S2, ESI†). Nitrogen adsorption experiments indicate a type Iisotherm (Fig. S3, ESI†), with BET surface area and a microporevolume of 1245(20) m2 g�1 and 0.54(1) cm3 g�1 respectively, values

a Service de Thermodynamique, Faculte Polytechnique, Universite de Mons,

20 Place du Parc, Mons, 7000, Belgium. E-mail: guy.deweireld@umons.ac.beb Institut Lavoisier, UMR CNRS 8180–Universite de Versailles St Quentin en Yvelines,

45 avenue des Etats-Unis, Versailles, 78035, France. E-mail: serre@chimie.uvsq.frc Institut Charles Gerhardt Montpellier, UMR CNRS 5253, UM2, ENSCM, UM1, Place

E. Bataillon, Montpellier cedex 05, 34095 France. E-mail: gmaurin@univ-montp2.frd MADIREL, UMR CNRS 7246 – Aix-Marseille Univ. Centre de Saint-Jerome,

Marseille cedex 20, 13397, Francee Department of Chemistry, Jagiellonian University in Krakow, Ingardena 3,

Krakow 30-060, Polandf Laboratoire Catalyse et Spectrochimie, ENSICAEN, Universite de Caen Basse

Normandie, CNRS, 6, Boulevard du Marechal Juin, Caen, 14050, Franceg State Key Laboratory of Organic–Inorganic Composites, Beijing University of

Chemical Technology, Beijing 100029, China

† Electronic supplementary information (ESI) available: Synthesis and character-ization. See DOI: 10.1039/c3cc45828h

Received 4th August 2013,Accepted 6th September 2013

DOI: 10.1039/c3cc45828h

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This journal is c The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 10082--10084 10083

lower than the theoretical ones (1730 m2 g�1 and 0.72 cm3 g�1)(see the ESI† and Table S2).15 Such a difference is probably dueto residual organic impurities (methanol and DMF) as confirmedusing IR spectroscopy (Fig. S4, ESI†).

It is noteworthy that MIL-125(Ti)s are stable up to 373 K underwater vapor (Fig. S5, ESI†), with no destruction of the frameworks uponrepeated cycles of adsorption–desorption. The presence of m2-OHhydroxo groups (n(OH) band at 3685 cm�1) was confirmed at roomtemperature (Fig. S4, ESI†)16 with the accessibility of both the OH andNH2 sites demonstrated through specific H/D exchanges (Fig. S6, ESI†).Fig. S7 (ESI†) reports the experimental and simulated adsorptionisotherms for CH4, CO2 and H2S at 303 K. It is noteworthy that theadsorption performances of both MIL-125(Ti)s remain unchangedafter the exposure to H2S. The adsorption isotherms for CH4 measuredon the samples previously submitted to a H2S adsorption–desorptioncycle match very well the ones obtained on the pristine samples(Fig. S8, ESI†). Note also that, despite a slight overestimation of theamounts, Grand Canonical Monte Carlo (GCMC) simulationsreproduce very well the low pressure domain for all isotherms.All together, this unique combination of stability under vapor ofwater and in the presence of H2S is a clear-cut advantage comparedto the majority of Fe and Zn based MOFs reported so far.13

If the impact of functionalization on the adsorption of methane isminor (see Table S1 and Fig. S9, ESI†), consistent with no specific host–guest interactions (Fig. S15, ESI†), this drastically differs for the twoother polar molecules. As shown in Table S1 (ESI†), the initial slopes ofthe adsorption isotherms are increased by 60% and 210% for CO2 andH2S respectively in agreement with the enhancement of the adsorptionenthalpies (see Table S1, ESI†). This increase is for CO2 of 4 kJ mol�1

(�25.8 kJ mol�1 vs. �29.8 kJ mol�1) (Fig. S9, ESI†), a trend fairlyreproduced by the simulations (�26.5 kJ mol�1 vs. �28.5 kJ mol�1).This behavior agrees with previous results on UiO-66(Zr),8c but drasti-cally differs from the increasing order of magnitude recently reportedby other authors on the same systems (>14 kJ mol�1),17a probably dueto the use of a Clausius Clapeyron treatment at low pressure. The samecomment holds true for the high CO2 adsorption enthalpy reported forthe CAU-1 analogue material (�45 kJ mol�1).17b Further, if the resultingCO2 adsorption enthalpy remains lower than those of MOFs bearingunsaturated metal sites,17c zeolite 13X (�45 kJ mol�1)17d or the amine-modified SBA-15 (�65 kJ mol�1),17e the MIL-125(Ti)-NH2 materialshows a clear advantage in maintaining an almost constantD(DH(CO2)– DH(CH4)) B 12 kJ mol�1 in a wide range of pressures up to 20 bar.This is not the case for the other materials mentioned aboveusually characterized by a rapid decrease of the CO2 adsorptionenthalpy at low coverage. The predicted D(DH(H2S) – DH(CH4)) B18 kJ mol�1 for MIL-125(Ti)-NH2 is larger than for CO2 andsuggests a better performance for the removal of H2S (see below).The moderate adsorption enthalpies values for both CO2 and H2Swould nevertheless imply regeneration under mild conditions.

In situ IR spectroscopy indicates that H2S adsorption on MIL-125(Ti) perturbs the hydroxyl groups (Fig. 1A), the free n(OH) bandshifting from 3686 to 3430 cm�1. This is due to the hydrogenbonded species between the m2-OH groups (H-donor) and the sulfuratom of H2S (H-acceptor). This is evidenced by the peak at2570 cm�1 of the n(SH) band (Fig. 1B) similar to silica or zeolites.18a

The magnitude of the shift (DnOH = 256 cm�1) emphasizes that the

hydrogen bond donor ability of the hydroxy groups of MIL-125(Ti)is slightly higher than that of silanol groups of silica (DnOH =200 cm�1). The GCMC simulations performed at 1 bar (Fig. 2a)show the formation of weak hydrogen bonds with a S(H2S)–H-(m2-OH) distance of 2.55 Å (see the radial distribution functions(RDF) in Fig. S18, ESI†). The same scenario lies for MIL-125(Ti)-NH2

with downward shifts of the n(OH) band at 3686 cm�1 and themaximum on the RDF between S(H2S) and H(m2-OH) at ca. the samedistance (2.60 Å) (Fig. S18, ESI†). Interestingly, H2S also perturbs thetwo n(NH2) bands from 3530 to 3498 cm�1 (D = 32 cm�1) and from3395 to 3380 cm�1 (D = 15 cm�1) respectively (Fig. 1C). Such shiftsare similar to those of aniline and hydroxyl groups of aliphaticalcohols.18b This supports the hypothesis that H2S interacts with theamino groups of MIL-125(Ti)-NH2 by hydrogen bonding, the nitro-gen lone pair being the H acceptor and the SH group, the H donor.A supplementary n(SH2) band appears at 2550 cm�1 (Fig. 1B–b)close to the position observed for H2S–amine complexes in solutionwhen H2S is the H-donor molecule.18c GCMC simulations revealednot only interactions with m2-OH groups but also with the aminofunction with H(H2S)–N(NH2) and S(H2S)–H(NH2) distances of 2.4and 2.5 Å respectively (Fig. 2b and RDF Fig. S19, ESI†). Finally theRDF computed for the H(H2S)–S(H2S) (Fig. S20, ESI†) is similar tothe values for H2S in MIL-53(Cr).18d

Fig. 1 (A) Effect of H2S adsorption on the m2-OH groups of MIL-125(Ti): nOH rangeof subtracted spectra of MIL-125(Ti); (B) spectra of MIL-125(Ti) (a) and MIL-125(Ti)-NH2

(b) in the n(H2S) range; (C) nNH2 bands profile before (a) and after H2S adsorption (b)samples activated at 403 K, H2S introduction at 213 K, 10 Torr excess (sample).

Fig. 2 Snapshots extracted from GCMC simulations at 1 bar and 303 K, emphasizingthe interactions between the H2S molecules and (a) the m2-OH groups of the inorganicnode in MIL-125(Ti) and (b) both the m2-OH groups and the NH2 grafted functions inMIL-125(Ti)-NH2. The distances are reported in Å (Ti, light grey; O, red; C, gray;S, yellow and H, white).

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10084 Chem. Commun., 2013, 49, 10082--10084 This journal is c The Royal Society of Chemistry 2013

IR spectroscopy indicates, as for H2S, that the oxygen atom of CO2

molecules interacts at low temperature with the hydroxyl group via alinear 1 : 1 adduct (Fig. S21, ESI†). For MIL-125(Ti)-NH2, weakerinteractions occur between CO2 and the m2-OH groups. The –NH2

groups induce a weak interaction with the CO2 through its carbonatom and the nitrogen lone pair of –NH2. These observations aresupported by molecular simulations (see ESI†). Further, GCMCsimulations carried out at 303 K to predict the separation performancefor CO2–CH4 mixtures indicate (Fig. S22, Tables S7 and S8, ESI†) thatthe amino-function significantly enhances the simulated CO2–CH4

selectivity from 4.5 to 7. These values remain roughly the samewhatever the pressure and the mixture composition and are slightlyhigher than those estimated from the VSM model (values of 3.5 and4.5 for MIL-125(Ti) and its amino-version, respectively, see the ESI†).These results are of similar magnitude to those previously reported forother –NH2 functionalized MOFs (6–9),11a,19 while they remain lowerthan for MIL-68(Al)-NH2 (B45)20 and particularly for MIL-53(Al)-NH2

with an almost infinite selectivity due to a structural change.10

Regarding the H2S–CH4 mixture, MIL-125(Ti)-NH2 is predicted to berelatively highly selective (S B 70 vs. B40 for the non-functionalizedsolid) for a mixture containing a low concentration of H2S (fractionof 10�3) at a total pressure of 10 bar (Fig. 3 and Table S9, ESI†). Thispromising performance is confirmed by the VSM model which leadsto a value of 40. Indeed, this material shows great potential for suchan application with a separation performance similar to that of theconventional zeolites (13X B70) in the same range of composition,21

however with expected milder regeneration conditions.In summary, the combination of both water and H2S stabilities,

the presence of accessible –OH and –NH2 sites together with theabsence of Lewis acid sites, high D(DH(CO2, H2S)–DH(CH4)) adsorp-tion values over all the pressure ranges leading to high H2S–CH4

selectivities (B70) and CO2 and H2S adsorption enthalpies lowerthan those of 13X zeolites, make MIL-125(Ti)-NH2 a promisingcandidate for the capture of acid H2S and CO2 gases. Furthermore,the relatively high CO2–CH4 selectivity (B7) also lets us envisage aone-step process for the concomitant elimination of CO2 and H2Sfrom biogas and natural gas.

The authors thank Dr Emma Gibson and Dr Florence Ragon forassistance, and the UMONS, the CNRS, the UVSQ, the EuropeanCommunity (Macademia FP-7 project, grant agreement no. 228862),the Polish Innovation Economy Operational Program (contract no.

POIG.02.01.00-12-023/08) and the National Natural Science Foundationof China (NSFC no: 21136001 and 21276009) for financial support.

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Fig. 3 Comparison, with logarithmic scale, of the H2S–CH4 selectivities calcu-lated by macroscopic model (VSM) (full lines) and GCMC simulations (dottedlines) at 303 K and 10 bar for MIL-125(Ti) (red) and its NH2 form (blue).

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