Surface Characterization of Ni-Mg-Al and Co-Mg-Al Hydrotalcites Investigated by Inverse Gas Chromatography

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1 Zhiyin Sun et al., J.Chem.Soc.Pak., Vol. 40, No. 04, Surface Characterization of Ni-Mg-Al an Co-Mg-Al Hyrotalcites Investigate by Inverse Gas Chromatography 1 Zhiyin Sun,,4 Guanghua Xia, 1 Wenyuan Tang an 3 Wenchu Wang 1 Taizhou University, Taizhou , P.R.China. Zhejiang University of Technology, Hangzhou , P.R.China. 3 Taizhou Municipal Bureau of Forestry, Taizhou , P.R.China. 4 Taizhou Pollution Control an Engineering Technology Center, Taizhou , P.R.China @qq.com* (Receive on 1 th May 017, accepte in revise form 5 th May 018) Summary: Carbonate pillare hyrotalcite-like compouns (Ni xmg 3-xAl-LDHs an Co xmg 3-xAl-LDHs) with ifferent molar ratios were synthesize through co-precipitation, the samples were characterize by XRD, FTIR an Inverse gas chromatography (IGC) techniques. The surface properties were compare an verifie by computer simulation. The results inicate that with the increasing of Ni + in Ni xmg 3-xAl-LDHs, the surface asorption free energy an the ispersive component of the surface energy ecrease, while the stability increase graually, which was contrary to the Co + in Co xmg 3-xAl LDHs. Besies, the surface free energy of Ni-Mg-Al hyrotalcites was smaller than Co-Mg-Al hyrotalicites when they were in the same molar ratio, an the stability of the former was stronger than the latter. Keywors: Inverse gas chromatography (IGC); Hyrotalcites; Surface characterization. Introuction Layere ouble hyroxies (abbreviate as LDHs), also known as hyrotalcite-like compouns or anion clays, which are well known for their applications in the file of catalysts, sorbents, composite aitives an ion-exchangers etc [1,]. Metal hyroxie with ivalent an trivalent metal ions, with ajustable egeneration, can form the hyrotalcite with catalytic activity by introucing with transition metal ions, such as Cu +, Ni + an Co + etc [3,4]. Recently, the stuy of Co + an Ni + hyrotalcites concentrate in catalysis an asorption etc. Zhao et al.[5] synthesize carbon nanotubes by using the calcine prouct of ternary magnesium nickel aluminum LDHs, the results showe that the catalytic properties is relate to the content of nickel. Liu et al. [6] foun that with the increase of cobalt content in LDHs, the catalytic activity of isopropanol increase, an the catalytic prouct selectivity was also associate with the surface properties of hyrotalcite. Zhao et al.[7] ha researche benzalehye oxiation reaction by grafting Ni-Al-LDHs on carbon nanotubes as catalyst, results showe that Ni-Al-LDHs ha goo catalytic performance, an the main reason for the higher catalytic efficiency was the improvement of the catalyst ispersion. It s obvious that the surface properties of materials play a ecisive role in catalysis an asorption, which have important reference value for practical application. Consiering the research on the surface properties of LDHs with catalytic activity is still very scarce, it s very meaningful to explore the surface properties of Ni-Mg-Al-LDHs an Co-Mg-Al-LDHs. Inverse gas chromatography (IGC) is one of the most sensitive, reliable, an convenient methos to stuy the surface properties [8]. This metho was first use to stuy the interaction between polymer an probe molecules, an the compatibility between polymer an polymer. In recent years, some researchers applie this technology in researching the surface properties of kaolin [9], montmorillonite [10] an other inorganic materials an moifie materials. By using this metho, our workgroup [11,1] ha stuie the ifference between the surface properties of Mg-Al-LDHs an its moifie proucts, incluing the Cu-Mg-Al-LDHs surface properties. In this paper, we applie IGC metho to investigate the surface properties of Ni-Mg-Al-LDHs an Co-Mg-Al-LDHs. Simultaneously, the conclusion was valiate by stuying the stability an Jahn-Teller effect of the two systems with computer simulation [13]. Experimental Sample preparation: In a typical experiment, * To whom all corresponence shoul be aresse.

2 Zhiyin Sun et al., J.Chem.Soc.Pak., Vol. 40, No. 04, Ni xmg 3-xAl-LDHs (x=0-3) were synthesize by a moifie co-precipitation metho [17]. Specifically, an aqueous solution (100 ml) name A was prepare with NaOH (0. mol) an Na CO 3 (0.05 mol), another aqueous solution (100 ml) name B was prepare with Ni(NO 3) 6H O (0.05 mol), Mg(NO 3) 6H O (0.05 mol) an Al(NO 3) 3 9H O (0.05 mol), after that, both A an B were ae ropwise to a three neck roun bottom flask with vigorous stirring, maintain the ph of 9 to 10, then keep stirring 1h. After the reaction, the resulting precipitate was crystallize at 33K for 18 h, an then centrifuge an washe with istille water for several times an was finally rie in vacuo at 338K for 1 h, giving the prouct NiMg Al-LDHs. The preparation of Ni MgAl-LDHs, Ni 3Al-LDHs, Mg 3Al-LDHs, Co 3Al-LDHs, Co MgAl-LDHs an CoMg Al-LDHs were mae by changing the mole ratio of Ni, Al an Mg accoring to the methos escribe above. Pentane, n-hexane an n-heptane, octane are mae from Sinopharm Chemical ReagentCo., Lt. An other chemicals were mae from JuHua group corporation. The instrument an reagent: The chromatographic experiments were performe with a GC7890(Ⅱ) gas chromatograph equippe with thermal conuctivity etector (TCD). Retention times were recore on an N-000 integrator. The probes use were n-pentane, n-hexane, n-heptane, an n-octane, all reagents were obtaine in the higher purity grae possibly an irectly use as receive without further purification. Characterization: Preparation of chromatographic column: A stainless steel column with a length of 15 cm an an internal iameter of 0. cm. To prepare packings for IGC, the clays were presse into pellets using a press at 0 tonne pressure, then crumble an sieve to give aggregates of 50 µm. Clean it with acetone, leave to ry completely an then fill in with the prepare LDHs materials respectively, after that, all the columns were age at 443 K uner a constant nitrogen flow (30mL/min) for 1h. IGC analysis: A GC7890(Ⅱ) gas chromatograph equippe with thermal conuctivity etector, with high purity nitrogen (99.99% pure) as the carrier gas at a flow rate of 50 ml/min, which was measure at the etector outlet with soap bubble flowmeter. Both the etector temperature an gasification chamber temperature were set at 453 K, an the column temperature was conitione by heating at K. X-ray iffraction (XRD) was carrie out using a Shimazu XRD-6000 iffractometer, with Cu-Kα raiation (λ=0.154 nm) at a scan spee of 5 /min. Bruker Vector FT-IR spectra in the range cm -1 was use to analyze the structure of the sample (the mass ratio of the sample to KBr was 1/100). Results an Discussion Inverse gas chromatography Surface asorption free energy: In this paper, C 5-C 8 straight-chain alkanes as probe molecules, the intermolecular interaction force between molecules is neglecte in the infinite ilution region. The relationship between asorption free energy ΔG 0 an retention volume V N is escribe as follows [14,15]: VP 0 N 0 G RT ln 0mS V N (1) T ( tr tm) Fa J () T a 3 ( p i / po) 1 J (3) 3 ( pi / po) 1 where: R is ieal gas constant, where S is the specific surface area of the asorbent, m the weight of sample in the column, π 0 is the surface pressure of the liqui probe an P 0 the equilibrium vapour pressure of the probe uner stanar conitions,t R an t m are the retention time an ea times, T is the column temperature, F a is the flow rate at the en of column at room temperature T a, J is the James-Martin factor for the correction of gas compressibility, p o is outlet pressure, an p i is inlet pressure. The retention volume V N was obtaine by

3 Zhiyin Sun et al., J.Chem.Soc.Pak., Vol. 40, No. 04, testing a series of probe molecules on the Hyrotalcite-Like compouns, an the corresponing surface free energy (ΔG 0 ) of asorption were calculate an summarize in Table-1: ΔG 0 VALUES OF Ni xmg 3-xAl-LDHs AND Co xmg 3-xAl-LDHs (x=0-3) AT 43 K. -ΔG 0 /( KJ mol -1 ) System n-pentane n-hexane n-heptane n-octane Ni3Al-LDHs MgNiAl-LDHs MgNiAl-LDHs Mg3Al-LDHs MgCoAl-LDHs MgCoAl-LDHs Co3Al-LDHs As shown in Table 1, the surface asorption free energy of all LDHs were below zero, inicating that the reaction of n-alkanes asorbing on the soli surface coul be carrie out at room temperature spontaneously. Accoring to the probe arrangements from the above list, n-alkane with more carbon atoms tens to have less surface asorption free energy, this is ue to the increasing of carbon atoms, the alkanes are getting more an more ifficult to enter into the LDHs molecule structure or asorb on it [1]. As we foun in the Ni xmg 3-xAl-LDHs system, the surface free energy ecrease with the increase of Ni +, an this may be ue to the introuction of Ni + that forme a more stable octaheron, which reuce the asorption energy. While, as the content of Co + in the Co/Mg/Al- LDHs systems increasing, a certain egree of istortion may occur in layer boar structure, so the surface asorption free energy increase. By comparing the surface asorption free energy of Ni xmg 3-xAl-LDHs an Co xmg 3-xAl-LDHs, we foun the asorption capacity of Co/Mg/Al-LDHs was larger than Ni/Mg/Al-LDHs if the x were equal, suggesting that LDHs with Co + possess ha more powerful surface activities. Dispersive component of surface energy 1 s G Na CH ( ) CH CH (4) (93 ) (5) CH T where N is Avogaro s number, γch is the surface energy of a hypothetical substance that contain only methylene groups an αch is the cross-sectional area of a methylene group (α CH=0.06 nm ). Thus, at constant temperature, for a series of alkane probes, a plot RTlnV n versus the number of carbon atoms shoul give a straight line, an then obtaine Δ G CH from the linear slope. As shown in Fig.1 an Fig., the RTlnV n of the material show a goo linear relationship with the number of carbon number (the linear slope reflects the increment of the asorption free energy), inicating that Δ G CH is reliable an coul be use to calculate γ s at a high confience level. RTLnVn/(kJ/mol) Mg 3 Al-LDH R = NiMg Al-LDH R = Ni MgAl-LDH R = Carbon number Fig. 1: Plot of RTlnV N values versus carbon number for Ni xmg 3-xAl-LDHs (x=0-) to etermine γ s. RTLnVn/(kJ/mol) Mg 3 Al-LDH R = CoMg Al-LDH R =0.950 Co MgAl-LDH R = Carbon number Fig. : Plot of RTlnV N values versus carbon number for Co xmg 3-xAl-LDHs (x=0-) to etermine γ s.

4 Zhiyin Sun et al., J.Chem.Soc.Pak., Vol. 40, No. 04, Accoring to Dorris an Gray theory, with the temperature of 43 K, the surface energy ispersive component of soli surface coul be calculate by equation (4), an the results are liste in Table. XRD analysis g Table-: γ S values of Ni xmg 3-xAl-LDHs an Co xmg 3-xAl-LDHs (x=0-3). LDHs Mg3Al Mg NiAl ial MgN Ni3Al γs (mj m - ) LDHs Mg3Al MgCoAl MgCoAl Co3Al γs (mj m - ) Intensity f e c b a θ/( ) The γ s can be use to assess the activity of the soli surface [16], the ata in Table suggests that with the increasing of Ni + in Ni xmg 3-xAl-LDHs, γ s showe a ecreasing tren, an the surface activity ecrease, while the stability generally enhance. On the other sie, with the increasing of Co + in Co xmg 3-xAl-LDHs, γ s showe a increasing tren, the surface activity increase, while the stability reuce. This phenomenon was presumably ue to the reason that Mg + was generally replace by Co + through isomorphous substitution to form Co-O octaheron, which increase the istortion egree of the system, leaing the hyrogen boning an electrostatic force between the subject an object ecrease graually, an the absolute value of the bining energy ecrease, so the system stability ecrease. Analyzing the ispersive component of surface energy between Ni xmg 3-xAl-LDHs an Co xmg 3-xAl-LDHs, we foun that the former were smaller, this may be ue to the substitution of Ni +, the electron was transferre from the layer to the guest anion, which le to the enhancement of the supramolecular interaction an the bining energy of the system, result in Ni xmg 3-xAl-LDHs appear more stable. The above conclusions were consistent with the results of relevant literature [13] an theoretical calculation results. Fig. 3: XRD patterns of a-ni 3Al-LDHs, b-mgni Al-LDHs, c-mg NiAl-LDHs, -Mg 3Al-LDHs, e-mg CoAl-LDHs, f-mgco Al-LDHs, g-co 3Al-LDHs. As shown in Fig.3, the crystal sharp of the hyrotalcites were relatively single, an the characteristic iffraction peaks were close to each other, juging that the layer spacing was similar. The ionic raius of Ni + an Co + are nm an nm, which are similar to Mg + (0.070 nm), having little effect on the structure of hyrotalcite, but still presente certain regularity. Compare with Ni + an Co + samples layer spacing, we foun that with the increasing of the incoming element on the plate, layer spacing change slightly. In Co xmg 3-xAl-LDHs system, with more an more Co + supersee Mg + by isomorphous substitution, layer spacing ecrease to a certain extent,one each for nm, nm an nm. While spacing changes in Ni xmg 3-xAl-LDHs showe the similar trens, which was consistent with the tren of lattice constants a an c. The theoretical simulation results showe that with the introucing of Ni +, the valence electron configuration of metalions change, while the plate structure i not cause serious istortion, an each M-O bon length ecrease graually, the average bon length graually reuce from nm (Mg 3Al-LDHs) to nm (Ni xmg 3-xAl-LDHs). Accoring to crystal fiel theory, the interaction between the central ion an the ligan was calle electrostatic effect, for the valence electron of the central metal atom passe out, le to the ecreasing of the electron repulsion between the central ion an the ligan.

5 Zhiyin Sun et al., J.Chem.Soc.Pak., Vol. 40, No. 04, Therefore, the coorination bon woul be stronger, resulting in a more stable octaheron, so the metal ion istance between the plate reuce graually. The same theoretical simulation metho was also suitable for Co xmg 3-xAl-LDHs, the conclusion of the two systems were consistent with the results obtaine from the IGC test. FT-IR analysis Transmittance/a.u g e c b a Wavenumber/cm -1 Fig. 4: FTIR spectra of a-ni 3Al-LDHs, b-mgni Al-LDHs c- Mg NiAl-LDHs, -Mg 3Al-LDHs, e-mg CoAl-LDHs, f-mgco Al-LDHs, g-co 3Al-LDHs. The FTIR spectra of the seven samples at absorption peak of cm -1 are shown in Fig.4. The stretching vibrations between the layer boar hyroxyl an interlayer water molecules, compare with hyroxyl free raical (3600 cm-1), mainly compose them, the peaks move to low wave number, inicating that strong hyrogen boning existe between interlayer water an layer boar hyroxyl or carbonate. The vibration peaks in Fig.4 are liste in Table 3. Table-3: FTIR ata of Ni xmg 3-xAl-LDHs an Co xmg 3-xAl-LDHs(x=0-3). System a b c Ni3Al-LDHs MgNiAl-LDHs MgNiAl-LDHs Mg3Al-LDHs MgCoAl-LDHs MgCoAl-LDHs Co3Al-LDHs a. Stretching vibrations between layer boar hyroxyl an interlayer water, b. H O bening vibration, c. CO 3 - Symmetric vibration,. M-O-M vibration As a certain amount of water inserte into the surface asorbe water an interlayer space of hyrotalcite, a bening vibration peak of crystalline f water rise in the cm -1 Compare the vibration of the interlayer CO 3 - position at cm -1 with the free state CO 3 - (1430 cm -1 ), the former move to low wave number, emonstrating that hyrogen boning existe in interlayer CO 3 - an interlayer water molecules. With the increasing of Ni + an Co +, interlayer CO 3 - remains substantially unchange, meaning that the chemical environment in which it existe still unchange significantly. Computational methos an results Combining the FTIR ata in Table 3 an Mulliken bon population analysis in Table 4, we foun that with the increasing of Ni + in Ni nmg 3-nAl-LDHs, metal oxygen bon vibration peak move to low wave number slightly. It was ue to the increasing of Ni + in Ni nmg 3-nAl-LDHs change the M-O bon in the laminate from covalent to ionic graually, inicating the covalent bons ecrease in the isomorphous substitution process, while the ionic bons increase. Thus, the whole system change from the covalent crystals to the ionic crystal graually, an electrostatic interaction increase. Table-4: Mulliken bon population (e) of Ni nmg 3-nAl-LDHs (n=0-3). System Al-O Mg-O Ni-O H-O I II III IV Combine the ata analysis in table 3 an table 5, with the increasing of Co + in Co xmg 3-xAl-LDHs, metal oxygen bon vibration peak move to high wave number slightly, because the theoretical charge population of Al was e, while Mg was e an Co was e. It was evient that the electrostatic force between metal cations an other anions was in orer of Mg + > Al 3+ > Co +. Thus, the energy of the sample ecrease with the increasing of Co +, an finally move to high wave number. Table-5: Mulliken atomic population (e) of Co nmg 3-nAl-LDHs (n=0-3). System Al Mg Co Layer I II III IV

6 Zhiyin Sun et al., J.Chem.Soc.Pak., Vol. 40, No. 04, Conclusion In summary, Ni xmg 3-xAl-LDHs an Co xmg 3-xAl-LDHs (x=0-3) with catalytic activity were synthesize through a co-precipitation metho, all were characterize by IGC, X-ray iffraction an FT-IR, an get the results as follows: with the increase of Ni + in Ni xmg 3-xAl-LDHs, the surface free energy ecrease, an the stability increase, which were consistent with the computer simulate results. While, with the increase of Co +, the surface energy ispersive component increase, an the stability ecrease. The asorption capacity of Co xmg 3-xAl-LDHs were larger than Ni xmg 3-xAl-LDHs if the x were equal. The γ s values of Co xmg 3-xAl-LDHs were larger than Ni xmg 3-xAl-LDHs, inicating that LDHs with Co + possess ha more powerful surface activities, an LDHs with Ni + showe higher stability than Co xmg 3-xAl-LDHs an Mg 3Al-LDHs. Acknowlegements The authors thank The Public Welfare Analysis of Science an Technology Plan Projects of Zhejiang Province (015C37034) References 1. S. Kannan, Catalytic applications of hyrotalcite-like materials an their erive forms, Catal. Surv. Asia., 10, 117 (016).. A. Tsujimura, M. Uchia an A. Okuwaki, Synthesis an sulfate ion-exchange properties of a hyrotalcite-like compoun intercalate by chlorie ions, J. Hazar. Mater., 143, 58 (007). 3. A. Valleta, M. Bessonb, G. Ovejeroa an J. García, Treatment of a non-azo ye aqueous solution by CWAO in continuous reactor using a Ni catalyst erive from hyrotalcite-like precursor, J. Hazar. Mater., 7, 410 (01). 4. M. Muñoz, S. Moreno an R. Molina, Synthesis of Ce an Pr-promote Ni an Co catalysts from hyrotalcite type precursors by reconstruction metho, Int. J. Hyrogen., 37, 1887 (01). 5. Y. Zhao, Q. Z. Jiao, J. Liang an C. H. Li, Synthesis of Ni/Mg/Al Layere Double Hyroxies an Their Use as Catalyst Precursors in the Preparation of Carbon Nanotubes, Chem. Res. Chinese U., 1, 471 (005). 6. B. H. Liu, H. L. Zhang an J. Y. Shen, Synthesis, Characterization an Isopropanol Catalytic Reaction of Mg/Al, Co /Al an Co/Mg/Al Mixe Oxies from Hyrotalcite an Hyrotalcite-like Precursors, Chinese J. Inorg. Chem., 1, 43 (005). 7. N. Q. Zhao, Master. Thesis, Preparation an catalysis stuies of LDH grafte on carbon nanotubes an electrospinning stuy of PVP/LDH composite fibers, Beijing University of Chemical Technology, (008). 8. G. S. Dritsas, K. Karatasos, C. Panayiotou. Investigation of thermoynamic properties of hyperbranche aliphatic polyesters by inverse gas chromatography, J. Chromatogr. A., 116, 8979 (009). 9. N. E. Thaher an P. Choi, Effect of Preheating Treatment on the Measure Heats of Asorption of Organic Probes on Clays with Different Surface Compositions, In Eng Chem Res., 51, 70 (01). 10. J. M. Chen, N. Yan, Hyrophobization of bleache softwoo kraft fibers via asorption of organo-nanoclay Bioresources., 7, 413 (01). 11. F. Zhang, X. X. Cao an Z. M. Ni, Surface Properties of Mg/Al-Hyrotalcite-like Compoun an Its Moifie Proucts, Chinese J. Inorg. Chem., 5, 71 (009). 1. X. W. Fu, Z. M. Ni an J. Liu, Surface Characterization of Copper-Aluminum-Magnesium Hyrotalcites Investigate by Inverse Gas Chromatography, Acta Chim. Sinica., 70, 968 (01). 13. Z. M. Ni, Y. LI, W. Shi, J. L. Xue an J. Liu, Supramolecular Structure, Electronic Property an Stability of Ni-Mg-Al Layere Double Hyroxies, Acta Phys-Chim. Sin., 8, 051 (01). 14. M. Rückriema, A. Inayatb, D. Enkec, R. Gläserc, W. D. Einickec an R. Rockmann, Inverse gas chromatography for etermining the ispersive surface energy of porous silica, Colloi Surface A., 357, 1 (010). 15. A. Askın an D. T. Yazici, Surface Characterization of Sepiolite by Inverse Gas Chromatography, Chromatographia., 61, 65 (005). 16. A. Voelkel, B. Strzemiecka, K. Aamska, K. Milczewska, Inverse gas chromatography as a source of physiochemical ata, J. Chromatogr. A., 116, 1551 (009). 17. S. K. Yun an T. J. Pinnavaia, Water Content an Particle Texture of Synthetic Hyrotalcite-like Layere Double Hyroxies, Chem. Mat., 7, 348 (1995).