An anionic In(III)-based metal-organic framework with Lew is basic sites for the selective adsorption and separation of organic cationic dyes

Qi Yang,Bin Wang,Ya Chen,Yabo Xie*,Jianrong Li

Beijing Key Laboratory for Green Catalysis and Separation and Department of Chemistry and Chemical Engineering,College of Environmental and Energy Engineering,Beijing University of Technology,Beijing 100124,China

Keywords:Anionic metal-organic framework Lew is basic sites Selective adsorption Separation Organic dye

ABSTRACT In this paper,a new anionic metal-organic framework,[In(PBPTTBA)][(CH3)2NH2](BUT-29)has been synthesized through the reaction of tetratopic acid ligand with double Lew is pyridine sites,,40,400,4000-(4,40-(1,4-phenylene)bis(pyridine-6,4,2-triyl))tetrabenzoic acid(H4PBPTTBA)and In(NO3)2? 5H2O and fully characterized by single-crystal X-ray diffraction(SXRD),pow der X-ray diffraction(PXRD),thermogravimetric analysis(TGA),infrared spectroscopy(IR),and elemental analysis(EA).BUT-29 can be used as an efficient adsorbent for the selective removal of organic cationic dyes in N,N0 -dimethylformamide(DMF)solution.The adsorption capacities of BUT-29 tow ard methylene blue and crystal violet at 298 K can reach 1119 mg/g and 832 mg/g,respectively.Moreover,the adsorbed dyes can be released in the DMF solution of LiNO3 gradually.

Dyes are w idely used in many industrial manufacturing processes,such as textiles,plastics,paper,printing,and so on.However,the industrial wastew ater containing dyes molecules are usually discharged into w ater directly without puri fi cation,leading to serious w ater pollution.In addition,most dyes molecules are immune tow ard light and oxidation so that they are difficult to degrade in nature[1–3].Therefore,the removal of dyes from w ater is essential.Till now,the removal of organic dyes from w ater is mainly based on traditional methods such as photocatalysis degradation[4–6]and membrane separation[7].Compared with the above-mentioned methods,the adsorption technology based on physical absorption is more promising because of its lowenergy-cost and user-friendly control.As a new class of p orous materials,metal-organic frameworks(MOFs),formed by organic ligands and inorganic nodes through coordination bond have been intensively investigated for many applications such as gas storage/separation[8–10],solid separation[11],ion exchange[12],catalysis[13–18]and sensing[19,20],due to their advantages of ultrahigh porosity,high surface area,and tunable functionalities[21–23].specifically,as many published works showed that MOFs have show n excellent performances in the removal of dyes[24–28].

Ionic MOFs(I-MOFs)is a unique subclass of MOFs and consists of charged frameworks and extra-framework counter ions[29,30].Strong electrostatic interactions between the charged framework and guest molecules are bene fi cial to increase the adsorption capacity and improve the ef fi ciency of separation processes.Since organic dyes are divided into electrically positive,negative,and neutral charged ones,I-MOFs have been considered as promising candidates for selective adsorption and separation of organic dyes[30,31].

In this work,a tetratopic acid ligand with double Lew ispyridine sites,4,40,400,4000-(4,40-(1,4-phenylene)bis(pyridine-6,4,2-triyl))tetrabenzoic acid(H4PBPTTBA)has been designed and synthesized.The assembly of H4PBPTTBAwith In(NO3)2?5H2Oin DMFyielded a new anionic MOF,[In(PBPTTBA)][(CH3)2NH2](BUT-29,w here BUT=Beijing University of Technology).BUT-29 can rapidly and selectively adsorb organic cationic dyes including[23_TD DIFF]acri fl avine hydrochloride(AH),acridine red(AR),safranine O(SO),methylene blue(MB),crystal violet(CV)and rhodamine 6G(R6G)through an ion-exchange process.Organic anionic and neutral dyes such as[24_TD DIFF]congo red(CR),orange G,naphthol yellow S(NYS),orange II,acid fuchsin(AF),methyl orange(MO),methyl yellow(MY),tartrazine,and isatin cannot be adsorbed by BUT-29.Furthermore,adsorption isotherms measured at 298 K demonstrate that the maximum adsorption amounts of MBand CV in BUT-29 are 1119 and 832 mg/g,respectively,and BUT-29 could be fully reused after washing with DMF solution of LiNO3several times.These results indicate that BUT-29 is promising adsorbents for efficient capture of MB and CV from waste w ater.

Solvothermal reaction of H4PBPTTBA and In(NO3)2?5H2O in the presence of HBF4as competing reagents in DMFat 120?Cfor 48 h yielded rhombic shaped crystals of BUT-29.Its phase purity has been characterized by PXRD.As show n in Fig.S3(Supporting information ),the experimental PXRD pattern match well with the one simulated from the single-crystal data,indicating that BUT-29 isin the pure phase.The TGAplot of the prepared sample of BUT-29 is showed in Fig.S1 in Supporting information,con fi rming that BUT-29 is stable up to ca.370?C.In the FT-IR spectra of BUT-29,slight blue shifts of carbonyl group characteristic bands compared with corresponding ligands were observed,illustrating the metal coordination of carboxylate groups in these ligands(Fig.S2 in Supporting information).

Single-crystal X-ray diffraction reveals BUT-29 crystallizes in an orthorhombic chiral space group of C2/c.In the structure of BUT-29,the InIIIion adopts a tetrahedral linkage geometry through coordinating to eight Oatoms from four carboxylate groups of four different PBPTTBA4?ligands(Fig.1a).The In-Odistances are in the range of 2.2617(1)? to 2.2905(1)?,being comparable to those of reported indium complexes[32,33].As show n in Fig.1c,the carboxylate groups in PBPTTBA4?ligand exhibit chelating coordination mode,with each one coordinates to one InIIIion.Thus,one InIIIconnect to four PBPTTBA4?ligands and one PBPTTBA4?ligand connect to four InIIIatoms to form a 3D framework with one dimensional(1D)rhombic channel,the edge length of which is 16.8?(atom to atom distance).Furthermore,tw o such frameworks are mutually interpenetrated(Fig.1d).After removal of free solvent molecules,the total solvent-accessible volume of BUT-29 is estimated to be 71.3%by PLATON.From the topological view point,each In(COO)4node and PBPTTBA4?ligand can be viewed as 4-connected nodes,the 3D structure of BUT-29 can thus be simpli fied as a 4,4-c net with the point symbol of(42?84),corresponding to a Pts-type topology.It should be noted that in the framework of BUT-29,every InIIIatom connect with four carboxylic groups,thus,in its pore,there should exist countercations to balance the charge of the framework.However,due to the disordered nature,these counter-cations could not be identi fied through single-crystal X-ray diffraction analysis.We speculate that these counter-cations should be(CH3)2NH2+,which is a common by-product in the solvothermal reaction w hen DMF was used as solvent.EA analysis was then carried out,which showed extra N content inside the pore,which implied the presence of(CH3)2NH2+.Furthermore,in the TGA curve of the assynthesized BUT-29,there exist a 7.1%weight loss between 200?C and 340?C,which is well corresponding to the presence of one(CH3)2NH2+ion per formula(Fig.S).

Based on the large rhombic channels and cationic framework of BUT-29,we sought to explore its application in the removal of organic dyes.Fifteen dyes including[13_TD DIFF]AH,AR,SO,MB,CV,R6G,CR,orange G,NYS,orange II,AF,MO,MY,Tartrazine,and Isatin were checked.Freshly prepared BUT-29 was well activated and then immersed in DMF solutions of these dyes at room temperature.The adsorption abilities of BUT-29 tow ard these dyes were determined by UV–vis spectroscopy.As show n inFigs.2a–d and Figs.S4(Supporting information),nearly all the cationic dyes can be removed within 15 min,while the anionic and neutral dyes could not be absorbed.To future demonstrate the selective adsorption of BUT-29 tow ard cationic dyes,the MOFsamples were soaked in the binary mixtures of MB/MO,MB/RB with the concentration ratio is 50/50,respectively.The results were as expected:only the MBwas absorbed by the BUT-29 in the solution mixture and the solutions exhibited the color of MO or RB at last and the color of the BUT-29 changed from yellow to blue(inset photograph of Figs.2e and f).Moreover,compared to other absorbents using a few days,the absorption rate of cationic dyesby BUT-29 was really fast,and nearly 100%of dye molecules can be absorbed within 10 min.The high adsorption rate is attributed to the large size of the cage w indow s(12.4?7.1?)and negative charged framework.

In order to verify w hether the absorption process tow ard cationic dyes is caused by ionic interactions between the cationic dye and framework,dye-releasing experiments were performed both in pure DMF and a saturated DMF solution of LiNO3,respectively.The release process was monitored through UV–vis spectroscopy.The results demonstrate that the cationic dye molecules in MB@BUT-29 and CV@BUT-29 can be gradually released in the presence of LiNO3,w hereas they are hardly released without LiNO3(Fig.3).These results suggest that the absorption can be assigned to the ionic interaction of the dyes with the anionic framework.In addition,the absorbed dyes can be released within 20 min(Figs.3c and d).

Fig.2.UV–vis spectra changes of DMFsolutions of(a)MB,(b)MO,(c)CV,and(d)AF;selective adsorption of BUT-29 tow ard MBin the binary mixtures of(e)MB/MOand(f)MB/RB with the concentration ratio is 50/50;inset show s the digital photograph of the color change of BUT-29 before and after organic dye adsorption.

In the above experiments,it was showed that MBand CV can be completely adsorbed within 6 min and 4 min in BUT-29,respectively(Figs.2a and c).Thus,we further explored the adsorption isotherms of MB and CV in BUT-29 at 298 K.As show n in Fig.4a,the maximum adsorption amounts of MBand CV in BUT-29 are 1119 and 832 mg/g,respectively.These values are comparable and even higher than those in other porous materials reported so far(Table S2 in Supporting information).In addition,we further measured the of FT-IRspectra of MBloaded BUT-29 and found the appearance of an extra characteristic peak in the w avenumber of 1323 cm?1,which should be the stretching vibration peak of C-N bond of MB molecules,indicating that MB molecules are indeed adsorbed into the pores of BUT-29(Fig.S5 in Supporting information).The superior performances of BUT-29 in MB and CV adsorptions could be ascribed to their large specific surface areas,suitable pore size,as well as the ionic interactions between the cationic dye and anionic framework structure of BUT-29.Moreover,Langmuir[34]and Freundlich[35]models were used to fi t and examine above adsorption isotherms respectively.The related parameters are given in Figs.S6 and S7 in Supporting information.Obviously,the data are well fi tted by the Langmuir model,indicating a homogeneous and monolayer adsorption occurring in BUT-29 with a fi nite number of identical sites.

In order to red uce the cost,reusability of an adsorbent is important for its p ractical app lications.To explore the reusability of BUT-29,the MOF sample after CV adsorption was regenerated by dispersing it in saturated DMF solution of LiNO3,and then,the regenerated MOFagain used to absorb CV.As show n in Fig.4b,after 4 repeated cycles,BUT-29 almost regained its initial adsorption capacities tow ard CV,demonstrating its high stability and good reusability.In addition,as con fi rmed from PXRD pattern,BUT-29 retained crystallinity after cyclic tests(Fig.S3).

In summary,a new In(III)-based MOF,BUT-29,has been designed,synthesized,and used in the selective adsorption of organic cationic dyes in DMFsolution.BUT-29 has a 3Dframework with one dimensional(1D)rhombic channel,which is occupied by(CH3)2NH22+counter ions.BUT-29 can be used as an excellent candidate for the selective adsorption of organic cationic dyes and its maximum adsorption amounts tow ard MB and CV are 1119 mg/g and 832 mg/g,respectively,comparable or higher than those in other porous materials reported so far.The adsorption isotherms of BUT-29 toward MB and CV can be fi tted by the Langmuir model,indicating a homogeneous and monolayer adsorption process.In addition,BUT-29 can be fully reused after washing with DMF solution of LiNO3several times.

Fig.3.UV–vis spectra monitored the process of dye release of[1_TD DIFF]MB@BUT-29(a)and CV@BUT-29(b)in a saturated DMF solution of LiNO3;comparison of dye release ofMB@BUT-29(c)and CV@BUT-29(d)in pure DMFand in a saturated DMF solution of LiNO3.

Fig.4.(a)Adsorption isotherms of BUT-29 tow ard MBand CV(adsorption conditions:at 298 K,50 m Lof solution,15 mg of MOFs,contact time of 4 h);(b)Cyclic application of BUT-29 in adsorbing CV(adsorption conditions:298 K,50 m L solution,15 mg MOFs,and 4 h adsorption time).

Ack now led gm ent

This work was fi nancially supported by the9IF]National Natural Science Foundation of China(NSFC,No.U1407119).

Appendix A.Supplem entary data

Supplementary data associated with this article can be found,in the online version,at https://doi.org/10.1016/j.cclet.2018.03.023.