Printer Friendly

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

Byline: Zhiyin Sun, Guanghua Xia, Wenyuan Tang and Wenchu Wang

Summary: Carbonate pillared hydrotalcite-like compounds (NixMg3-xAl-LDHs and CoxMg3-xAl-LDHs) with different molar ratios were synthesized through co-precipitation, the samples were characterized by XRD, FTIR and Inverse gas chromatography (IGC) techniques. The surface properties were compared and verified by computer simulation. The results indicated that with the increasing of Ni2+ in NixMg3-xAl-LDHs, the surface adsorption free energy and the dispersive component of the surface energy decreased, while the stability increased gradually, which was contrary to the Co2+ in CoxMg3-xAl-LDHs. Besides, the surface free energy of Ni-Mg-Al hydrotalcites was smaller than Co-Mg-Al hydrotalicites when they were in the same molar ratio, and the stability of the former was stronger than the latter.

Keywords: Inverse gas chromatography (IGC); Hydrotalcites; Surface characterization.

Introduction

Layered double hydroxides (abbreviated as LDHs), also known as hydrotalcite-like compounds or anion clays, which are well known for their applications in the filed of catalysts, sorbents, composite additives and ion-exchangers etc [1,2]. Metal hydroxide with divalent and trivalent metal ions, with adjustable degeneration, can form the hydrotalcite with catalytic activity by introducing with transition metal ions, such as Cu2+ , Ni2+ and Co2+ etc [3,4].

Recently, the study of Co2+ and Ni2+ hydrotalcites concentrated in catalysis and adsorption etc. Zhao et al.[5] synthesized carbon nanotubes by using the calcined product of ternary magnesium nickel aluminum LDHs, the results showed that the catalytic properties is related to the content of nickel. Liu et al. [6] found that with the increase of cobalt content in LDHs, the catalytic activity of isopropanol increased, and the catalytic product selectivity was also associated with the surface properties of hydrotalcite. Zhao et al.[7] had researched benzaldehyde oxidation reaction by grafting Ni-Al-LDHs on carbon nanotubes as catalyst, results showed that Ni-Al-LDHs had good catalytic performance, and the main reason for the higher catalytic efficiency was the improvement of the catalyst dispersion.

It's obvious that the surface properties of materials play a decisive role in catalysis and adsorption, which have important reference value for practical application. Considering 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 and Co-Mg-Al-LDHs.

Inverse gas chromatography (IGC) is one of the most sensitive, reliable, and convenient methods to study the surface properties [8]. This method was first used to study the interaction between polymer and probe molecules, and the compatibility between polymer and polymer. In recent years, some researchers applied this technology in researching the surface properties of kaolin [9], montmorillonite [10] and other inorganic materials and modified materials.

By using this method, our workgroup [11,12] had studied the difference between the surface properties of Mg-Al-LDHs and its modified products, including the Cu-Mg-Al-LDHs' surface properties. In this paper, we applied IGC method to investigate the surface properties of Ni-Mg-Al-LDHs and Co-Mg-Al-LDHs. Simultaneously, the conclusion was validated by studying the stability and Jahn-Teller effect of the two systems with computer simulation [13].

Experimental

Sample preparation: In a typical experiment, NixMg3-xAl-LDHs (x=0-3) were synthesized by a modified co-precipitation method [17]. Specifically, an aqueous solution (100 mL) named A was prepared with NaOH (0.2 mol) and Na2CO3 (0.025 mol), another aqueous solution (100 mL) named B was prepared with Ni(NO3)2*6H2O (0.025 mol), Mg(NO3)2*6H2O (0.05 mol) and Al(NO3)3*9H2O (0.025 mol), after that, both A and B were added dropwise to a three neck round bottom flask with vigorous stirring, maintain the pH of 9 to 10, then keep stirring 1h. After the reaction, the resulting precipitate was crystallized at 323K for 18 h, and then centrifuged and washed with distilled water for several times and was finally dried in vacuo at 338K for 12 h, giving the product NiMg2Al-LDHs.

The preparation of Ni2MgAl-LDHs, Ni3Al-LDHs, Mg3Al-LDHs, Co3Al-LDHs, Co2MgAl-LDHs and CoMg2Al-LDHs were made by changing the mole ratio of Ni, Al and Mg according to the methods described above.

Pentane, n-hexane and n-heptane, octane are made from Sinopharm Chemical ReagentCo., Ltd. And other chemicals were made from JuHua group corporation.

The instrument and reagent: The chromatographic experiments were performed with a GC7890(II) gas chromatograph equipped with thermal conductivity detector (TCD). Retention times were recorded on an N-2000 integrator.

The probes used were n-pentane, n-hexane, n-heptane, and n-octane, all reagents were obtained in the higher purity grade possibly and directly used as received without further purification.

Characterization: Preparation of chromatographic column: A stainless steel column with a length of 15 cm and an internal diameter of 0.2 cm. To prepare packings for IGC, the clays were pressed into pellets using a press at 20 tonne pressure, then crumbled and sieved to give aggregates of 250 um. Clean it with acetone, leave to dry completely and then fill in with the prepared LDHs materials respectively, after that, all the columns were aged at 443 K under a constant nitrogen flow (30mL/min) for 1h.

IGC analysis: A GC7890(II) gas chromatograph equipped with thermal conductivity detector, with high purity nitrogen (99.99% pure) as the carrier gas at a flow rate of 50 mL/min, which was measured at the detector outlet with soap bubble flowmeter. Both the detector temperature and gasification chamber temperature were set at 453 K, and the column temperature was conditioned by heating at 403-433 K.

X-ray diffraction (XRD) was carried out using a Shimadzu XRD-6000 diffractometer, with Cu-K[alpha] radiation (I>>=0.1542 nm) at a scan speed of 5Adeg/min.

Bruker Vector 22 FT-IR spectra in the range 4000 - 400 cm-1 was used to analyze the structure of the sample (the mass ratio of the sample to KBr was 1/100).

Results and Discussion

Inverse gas chromatography

Surface adsorption free energy: In this paper, C5-C8 straight-chain alkanes as probe molecules, the intermolecular interaction force between molecules is neglected in the infinite dilution region. The relationship between adsorption free energy IG0 and retention volume VN is described as follows [14,15]:

(Equations)

Where: R is ideal gas constant, where S is the specific surface area of the adsorbent, m the weight of sample in the column, I0 is the surface pressure of the liquid probe and P0 the equilibrium vapour pressure of the probe under standard conditions ,tR and tm are the retention time and dead times, T is the column temperature, Fa is the flow rate at the end of column at room temperature Ta, J is the James-Martin factor for the correction of gas compressibility, po is outlet pressure, and pi is inlet pressure.

The retention volume VN was obtained by testing a series of probe molecules on the Hydrotalcite-Like compounds, and the corresponding surface free energy (IG0) of adsorption were calculated and summarized in

Table-1: IG0 VALUES OF NixMg3-xAl-LDHs AND CoxMg3-xAl-LDHs (x=0-3) AT 423 K

###I-G0/(KJ*mol-1)

###System###n-Pentane###n-Hexane###n-Heptane###n-Octane

Ni3Al-LDHs###10.76###7.49###-###-

MgNi2Al-LDHs###14.82###11.22###8.78###7.27

Mg2NiAl-LDHs###16.12###12.96###11.11###8.53

Mg3Al-LDHs###18.34###15.58###13.54###12.47

Mg2CoAl-LDHs###10.97###7.80###5.35###5.35

MgCo2Al-LDHs###16.56###13.88###12.12###10.64

Co3Al-LDHs###16.61###14.29###-###-

As shown in Table 1, the surface adsorption free energy of all LDHs were below zero, indicating that the reaction of n-alkanes adsorbing on the solid surface could be carried out at room temperature spontaneously.

According to the probe arrangements from the above list, n-alkane with more carbon atoms tends to have less surface adsorption free energy, this is due to the increasing of carbon atoms, the alkanes are getting more and more difficult to enter into the LDHs molecule structure or adsorb on it [12].

As we found in the NixMg3-xAl-LDHs system, the surface free energy decreased with the increase of Ni2+, and this may be due to the introduction of Ni2+ that formed a more stable octahedron, which reduced the adsorption energy. While, as the content of Co2+ in the Co/Mg/Al- LDHs systems increasing, a certain degree of distortion may occur in layer board structure, so the surface adsorption free energy increased.

By comparing the surface adsorption free energy of NixMg3-xAl-LDHs and CoxMg3-xAl-LDHs, we found the adsorption capacity of Co/Mg/Al-LDHs was larger than Ni/Mg/Al-LDHs if the x were equal, suggesting that LDHs with Co2+ possess had more powerful surface activities.

Dispersive component of surface energy

(Equations)

Where N is Avogadro's number, I3CH2 is the surface energy of a hypothetical substance that contain only methylene groups and [alpha]CH2 is the cross-sectional area of a methylene group ([alpha]CH2=0.06 nm2). Thus, at constant temperature, for a series of alkane probes, a plot RTlnVn versus the number of carbon atoms should give a straight line, and then obtained I GCH2 from the linear slope.

As shown in Fig.1 and Fig.2, the RTlnVn of the material show a good linear relationship with the number of carbon number (the linear slope reflects the increment of the adsorption free energy), indicating that I G CH2 is reliable and could be used to calculate I3sd at a high confidence level.

According to Dorris and Gray theory, with the temperature of 423 K, the surface energy dispersive component of solid surface could be calculated by equation (4), and the results are listed in Table 2.

Table-2: I3s dvalues of NixMg3-xAl-LDHs and CoxMg3-xAl-LDHs (x=0-3).

###LDHs###Mg3Al###Mg2###MgN###Ni3Al

###NiAl###i2Al

Sd(mJ*m-2)###48.02###36.49###26.31###13.58

###LDHs###Mg3Al###Mg2CoAl###MgCo2Al###Co3Al

Sd (mJ*m-2)###48.02###53.77###52.65###66.24

The I3sd can be used to assess the activity of the solid surface [16], the data in Table 2 suggests that with the increasing of Ni2+ in NixMg3-xAl-LDHs, I3s showed a decreasing trend, and the surface activity decreased, while the stability generally enhanced. On the other side, with the increasing of Co2+ in CoxMg3-xAl-LDHs, I3sd showed a increasing trend, the surface activity increased, while the stability reduced.

This phenomenon was presumably due to the reason that Mg2+ was generally replaced by Co2+ through isomorphous substitution to form Co-O2 octahedron, which increased the distortion degree of the system, leading the hydrogen bonding and electrostatic force between the subject and object decrease gradually, and the absolute value of the binding energy decreased, so the system stability decreased.

Analyzing the dispersive component of surface energy between NixMg3-xAl-LDHs and CoxMg3-xAl-LDHs, we found that the former were smaller, this may be due to the substitution of Ni2+, the electron was transferred from the layer to the guest anion, which led to the enhancement of the supramolecular interaction and the binding energy of the system, result in NixMg3-xAl-LDHs appear more stable. The above conclusions were consistent with the results of relevant literature [13] and theoretical calculation results.

As shown in Fig.3, the crystal sharp of the hydrotalcites were relatively single, and the characteristic diffraction peaks were close to each other, judging that the layer spacing was similar. The ionic radius of Ni2+ and Co2+ are 0.0690 nm and 0.0745 nm, which are similar to Mg2+ (0.0720 nm), having little effect on the structure of hydrotalcite, but still presented certain regularity. Compared with Ni2+ and Co2+ samples' layer spacing, we found that with the increasing of the incoming element on the plate, layer spacing changed slightly. In CoxMg3-xAl-LDHs system, with more and more Co2+ supersede Mg2+ by isomorphous substitution, layer spacing decreased to a certain extent,one each for 0.7887 nm, 0.7813 nm and 0.7708 nm. While spacing changes in NixMg3-xAl-LDHs showed the similar trends, which was consistent with the trend of lattice constants a and c.

The theoretical simulation results showed that with the introducing of Ni2+, the valence electron configuration of metalions changed, while the plate structure did not cause serious distortion, and each M-O bond length decreased gradually, the average bond length gradually reduced from 0.2045 nm (Mg3Al-LDHs) to 0.1986 nm (NixMg3-xAl-LDHs). According to crystal field theory, the interaction between the central ion and the ligand was called electrostatic effect, for the valence electron of the central metal atom passed out, led to the decreasing of the electron repulsion between the central ion and the ligand.

Therefore, the coordination bond would be stronger, resulting in a more stable octahedron, so the metal ion distance between the plate reduced gradually.

The same theoretical simulation method was also suitable for CoxMg3-xAl-LDHs, the conclusion of the two systems were consistent with the results obtained from the IGC test.

The FTIR spectra of the seven samples at absorption peak of 3443-3496 cm -1 are shown in Fig.4. The stretching vibrations between the layer board hydroxyl and interlayer water molecules, compared with hydroxyl free radical (3600 cm-1), mainly composed them, the peaks moved to low wave number, indicating that strong hydrogen bonding existed between interlayer water and layer board hydroxyl or carbonate. The vibration peaks in Fig.4 are listed in Table 3.

Table-3: FTIR data of NixMg3-xAl-LDHs and CoxMg3-xAl-LDHs(x=0-3).

###System###a###b###c###d

###Ni3Al-LDHs###3478###1640###1380###415

###MgNi2Al-LDHs###3485###1633###1367###422

###Mg2NiAl-LDHs###3478###1631###1367###436

###Mg3Al-LDHs###3451###1629###1382###441

###Mg2CoAl-LDHs###3489###1636###1373###422

###MgCo2Al-LDHs###3496###1642###1373###428

###Co3Al-LDHs###3481###1650###1373###436

As a certain amount of water inserted into the surface adsorbed water and interlayer space of hydrotalcite, a bending vibration peak of crystalline water rise in the 1629-1650 cm-1 Compared the vibration of the interlayer CO 2-position at 1367-1382 cm-1 with the free state CO 2- (1430 cm-1), the former moved to low wave number, demonstrating that hydrogen bonding existed in interlayer CO3 and interlayer water molecules. With the increasing of Ni2+ and Co2+, interlayer CO3 remains substantially unchanged, meaning that the chemical environment in which it existed still unchanged significantly.

Computational methods and results

Combining the FTIR data in Table 3 and Mulliken bond population analysis in Table 4, we found that with the increasing of Ni2+ in NinMg3-nAl-LDHs, metal oxygen bond vibration peak moved to low wave number slightly. It was due to the increasing of Ni2+ in NinMg3-nAl-LDHs changed the M-O bond in the laminate from covalent to ionic gradually, indicating the covalent bonds decreased in the isomorphous substitution process, while the ionic bonds increased. Thus, the whole system changed from the covalent crystals to the ionic crystal gradually, and electrostatic interaction increase.

Table-4: Mulliken bond population (e) of NinMg3-nAl-LDHs (n=0-3).

System###Al-O###Mg-O###Ni-O###H-O

I###0.398###-0.723###----###0.575

II###0.370###-0.784###0.275###0.570

III###0.351###-0.857###0.242###0.578

IV###0.318###----###0.203###0.575

Combined the data analysis in table 3 and table 5, with the increasing of Co2+ in CoxMg3-xAl-LDHs, metal oxygen bond vibration peak moved to high wave number slightly, because the theoretical charge population of Al was 1.370-1.660 e, while Mg was 1.680-2.010 e and Co was 0.586-0.860 e. It was evident that the electrostatic force between metal cations and other anions was in order of Mg2+ > Al3+ > Co2+. Thus, the energy of the sample decreased with the increasing of Co2+, and finally moved to high wave number.

Table-5: Mulliken atomic population (e) of ConMg3-nAl-LDHs (n=0-3).

System###Al###Mg###Co###Layer

###I###1.420###1.668###----###0.64

II###1.510###1.810###0.586###0.67

III###1.580###1.970###0.705###0.69

IV###1.700###----###0.860###0.72

Conclusion

In summary, NixMg3-xAl-LDHs and CoxMg3-xAl-LDHs (x=0-3) with catalytic activity were synthesized through a co-precipitation method, all were characterized by IGC, X-ray diffraction and FT-IR, and get the results as follows: with the increase of Ni2+ in NixMg3-xAl-LDHs, the surface free energy decreased, and the stability increased, which were consistent with the computer simulated results. While, with the increase of Co2+, the surface energy dispersive component increased, and the stability decreased. The adsorption capacity of CoxMg3-xAl-LDHs were larger than NixMg3-xAl-LDHs if the x were equal.

The I3s values of CoxMg3-xAl-LDHs were larger than NixMg3-xAl-LDHs, indicating that LDHs with Co2+ possess had more powerful surface activities, and LDHs with Ni2+ showed higher stability than CoxMg3-xAl-LDHs and Mg3Al-LDHs.

Acknowledgements

The authors thank The Public Welfare Analysis of Science and Technology Plan Projects of Zhejiang Province (2015C37034)

References

1. S. Kannan, Catalytic applications of hydrotalcite-like materials and their derived forms, Catal. Surv. Asia., 10, 117 (2016).

2. A. Tsujimura, M. Uchida and A. Okuwaki, Synthesis and sulfate ion-exchange properties of a hydrotalcite-like compound intercalated by chloride ions, J. Hazard. Mater., 143, 582 (2007).

3. A. Valleta, M. Bessonb, G. Ovejeroa and J. Garcia, Treatment of a non-azo dye aqueous solution by CWAO in continuous reactor using a Ni catalyst derived from hydrotalcite-like precursor, J. Hazard. Mater., 227, 410 (2012).

4. M. Munoz, S. Moreno and R. Molina, Synthesis of Ce and Pr-promoted Ni and Co catalysts from hydrotalcite type precursors by reconstruction method, Int. J. Hydrogen., 37, 18827 (2012).

5. Y. Zhao, Q. Z. Jiao, J. Liang and C. H. Li, Synthesis of Ni/Mg/Al Layered Double Hydroxides and Their Use as Catalyst Precursors in the Preparation of Carbon Nanotubes, Chem. Res. Chinese U., 21, 471 (2005).

6. B. H. Liu, H. L. Zhang and J. Y. Shen, Synthesis, Characterization and Isopropanol Catalytic Reaction of Mg/Al, Co /Al and Co/Mg/Al Mixed Oxides from Hydrotalcite and Hydrotalcite-like Precursors, Chinese J. Inorg. Chem., 21, 43 (2005).

7. N. Q. Zhao, Master. Thesis, Preparation and catalysis studies of LDH grafted on carbon nanotubes and electrospinning study of PVP/LDH composite fibers, Beijing University of Chemical Technology, (2008).

8. G. S. Dritsas, K. Karatasos, C. Panayiotou. Investigation of thermodynamic properties of hyperbranched aliphatic polyesters by inverse gas chromatography, J. Chromatogr. A., 1216, 8979 (2009).

9. N. E. Thaher and P. Choi, Effect of Preheating Treatment on the Measured Heats of Adsorption of Organic Probes on Clays with Different Surface Compositions, Ind Eng Chem Res., 51, 7022 (2012).

10. J. M. Chen, N. Yan, Hydrophobization of bleached softwood kraft fibers via adsorption of organo-nanoclay Bioresources., 7, 4132 (2012).

11. F. Zhang, X. X. Cao and Z. M. Ni, Surface Properties of Mg/Al-Hydrotalcite-like Compound and Its Modified Products, Chinese J. Inorg. Chem., 25, 271 (2009).

12. X. W. Fu, Z. M. Ni and J. Liu, Surface Characterization of Copper-Aluminum-Magnesium Hydrotalcites Investigated by Inverse Gas Chromatography, Acta Chim. Sinica., 70, 968 (2012).

13. Z. M. Ni, Y. LI, W. Shi, J. L. Xue and J. Liu, Supramolecular Structure, Electronic Property and Stability of Ni-Mg-Al Layered Double Hydroxides, Acta Phys-Chim. Sin., 28, 2051 (2012).

14. M. Ruckriema, A. Inayatb, D. Enkec, R. Glaserc, W. D. Einickec and R. Rockmann, Inverse gas chromatography for determining the dispersive surface energy of porous silica, Colloid Surface A., 357, 21 (2010).

15. A. Askin and D. T. Yazici, Surface Characterization of Sepiolite by Inverse Gas Chromatography, Chromatographia., 61, 625 (2005).

16. A. Voelkel, B. Strzemiecka, K. Adamska, K. Milczewska, Inverse gas chromatography as a source of physiochemical data, J. Chromatogr. A., 1216, 1551 (2009).

17. S. K. Yun and T. J. Pinnavaia, Water Content and Particle Texture of Synthetic Hydrotalcite-like Layered Double Hydroxides, Chem. Mat., 7, 348 (1995).
COPYRIGHT 2018 Asianet-Pakistan
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2018 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Publication:Journal of the Chemical Society of Pakistan
Article Type:Report
Date:Aug 31, 2018
Words:3456
Previous Article:Experimental Study on Microwave-Assisted Preparation of Hydrophobic Modified Guar Gum for Fracturing Fluid.
Next Article:A Facile Single Pot Synthesis of Highly Functionalized Tricyclic Heterocycle Compounds via Sequential Knoevenagel-Michael Addition and their...
Topics:

Terms of use | Privacy policy | Copyright © 2022 Farlex, Inc. | Feedback | For webmasters |