Hydrothermal Reconstructing Routes of Alkali-Free ZnAl Layered Double Hydroxide: A Characterization Study

NAZRIZAWATI Ahmad Tajuddin1,2,a, ROZAINA Saleh1, JINESH C. Manayil2,a, MARK Andrew Isaacs2,a, CHRISTOPHER M.A. Parlett2,a, ADAM F. Lee2,a, and KAREN Wilson2,a

1Universiti Teknologi MARA, Perak Branch, Tapah Campus 35000 Tapah, Perak, Malaysia.

2European Bioenergy Research Institute (EBRI), Aston University, Birmingham, B4 7ET, United Kingdom

a*nazriza@perak.uitm.edu.my

Keywords: hydrotalcites, rehydration/reconstruction, hydrothermal, biodiesel, triglycerides

Abstract. A facile, rapid, and noninvasive method for reconstructing ZnAl layered double hydroxide (LDH) is reported. ZnAl LDH series were synthesized at different Zn2+/Al3+ atomic ratio (1.5-4) via an alkali-free method and reconstructed under hydrothermal route (HTM) for the first time. Fresh Zn/Al LDHs were activated at 300°C and reconstructed under hydrothermal process. A better insight and correlation study between the physiochemical properties of reconstructed ZnAl LDH in terms of their crystallinity, surface area and basicity also will be gained here. BET surface area of rehydrated samples increased up to 355 m2/g (Zn:Al ratio 3:1). CO2-TPD probed high number of basic sites density (0.1 mmol/g).

Introduction

Layered material has been scientifically and technologically proven important in many applications such as in environmental benign and energy application including in waste water treatment [1], [2], biomass conversion [3]–[5], photocatalytic activities [6]–[8] and many more. Ability to reconstruct the lamellar structure for example, provides a distinctive effect on catalyst architecture. Tunable basicity strength of divalent and trivalent cation in layered-double hydroxide (LDH) (also known as hydrotalcite (HT)) and reliable inexpensive preparation methods [9] are also among the remarkable gears of layered materials which attain as the great choices for solid bases catalysis reaction. Traditionally, research been focusing on MgAl HT, but nowadays a numerous number of studies on other incorporated metal in LDHs have been explored including zinc [8], [10], lithium [11], chromium [2], [10], nickel [12], [13], cobalt [14], [15] and much more. Unfortunately, most of the LDH preparations reported are still using NaOH co-precipitation method which has a severe contribution towards leaching due to the entrained of the sodium. Furthermore, saponification and emulsification could not be avoided, hence will affecting on the environmental at the same time.

Due to these drawbacks, hereby we synthesized ZnAl LDH via alkali-free method. Fresh HTs were reported inactive in many reactions [16]. Hence, reactivation of the catalyst via calcination and followed by rehydration is anticipated in most LDH synthesis. Through the “Memory effect”, rehydration and of LDH is achievable. Rehydration is classified into two types; gas-phase (GP) and liquid-phase (LP). LP rehydration has been reported conveyed to a higher surface area (400 m2 g-1) and displayed high catalytic activity in aldol condensations compared to GP rehydration [17]. Both rehydration processes are somehow has been identified as complex and time consuming [18]. Hereby, we introduced reconstruction of alkali-free LDH via hydrothermal method (HTM). HTM reconstruction process is a way simpler process, faster as well as energy and cost effective. To the best of our knowledge, physiochemical of reconstruction of alkali-free ZnAl LDH through HTM reconstruction process has not been reported elsewhere. Due to that, total understanding on its specific layer changes from precursor to calcined and rehydrated layer via HTM approaches are essentials in this study.

Experimental methods

Catalyst synthesis ZnAl LDHs have prepared via the alkali-free method as reported before [19]. Metal nitrates, [Zn(NO3)2.6H2O] (100cm3, 1.5M) and [Al(NO3)3.9H2O] (100cm3, 1.5M) solution were added dropwise and simultaneously with ammonium carbonate (100cm3,2M) at room temperature with constant pH 10. The mixture was aged at 65°C overnight under stirring. The Zn/Al atomic ratio was adjusted with the volume of metal nitrates to synthesis ZnAl LDH of atomic ratio 1.5-4. Finally, the solid product was filtered, washed with water until the pH comes ~7. For calcination-rehydration, the catalysts are calcined under a flow of O2 (20 mlmin-1) at 300°C for 5h. The calcined samples were reconstructed under hydrothermal (HTM) phase. In this method, calcined samples were sealed in 50 ml hydrothermal autogenous pressure reactor and heated at 110°C for 24h. The reconstructed LDH was then centrifuged, washed, dried and stored in a vacuum desiccator. The ZnAl LDH prepared via reconstructed methods are denoted as ZnxAly-HTM where x and y stand for Zn/Al atomic ratio.

Catalyst characterization Powder XRD patterns were recorded on a Bruker-AXS D8 ADVANCE diffractometer operated at 40 kV and 40 mA currents using Cu Kα radiation in the range 10-80°. The Scherer equation was used to calculate HT crystallite sizes. XPS was performed on a Kratos Axis HSi X-ray photoelectron spectrometer fitted with a charge neutralizer and magnetic focusing lens employing Al Kα monochromated radiation at 90W. Spectral fitting was performed using CasaXPS version 2.3.16. Binding energies were corrected to the C 1s peak at 284.5 eV. N2 porosimetry was undertaken on a Quantachrome Nova 4000 porosimeter. Samples were degassed from 120°C for 3 hour prior to analysis. The surface area and pore size distribution were calculated by Brunauer-Emmet-Teller (BET) method and Barret-Joyner-Hallender (BJH) method, respectively. Base site densities were determined through a CO2 pulse titration and further Temperature Programme Desorption (CO2-TPD) using Quantachrome ChemBET 3000. 50 mg of catalyst was placed into quartz chemisorption cell plugged with quartz wool and was outgassed for an hour under He gas at 120°C. The sample was then cooled until the temperature dropped to 40°C. Then CO2 was pulse titrated in 50 μL doses until saturation. The sample then was heated to 700°C at 10°C min-1. Thermogravimetric analysis (TGA) was performed on a Mettler Toledo, TGA/MS Stare system under N2 from 40-800°C at a rate of 10 °C min−1. Scanning Electron Microscopy (SEM) images were recorded using HITACHI SU8230, a high-performance cold field emission (CFE) SEM with Oxford Instruments Aztec Energy EDX system with 80mm X-Max SDD detector. Transmission Electron Microscopy (TEM) analysis was carried out using JEOL 2100F FEG STEM operating (200 keV). Samples were set by dispersed in methanol and drop casted onto a copper grid coated with a holey carbon support film (Agar Scientific Ltd).

Results and Discussion

The as-synthesised ZnAl LDH samples were first verified by energy dispersive X-Ray (EDX) spectroscopy to confirms the successful formation of ZnAl LDH series with a varying atomic ratio As presented in Table 1, the results are well in agreement with nominal ratio even though the bulk ratio of Zn:Al is slightly differentiated from the target ratio. These results suggest a lower Al incorporation on Zn bulk as the ratio was increased during synthesized [20]. As expected, the Zn and Al samples composition are parallel with bulk and its compound either on the sheet or on the interlayer. A general formula for HTs; [M(II)1-xM(III)x(OH)2]χ+(An-χ/n)·mH2O where m = 0.81-x, has been applied in order to calculate the theoretical and experimental weight loss [21]. The Zn/Al compositional formula also has been obtained from a combination of this analysis with EDX. Zn, Al, OH and CO3 composition were calculated from atomic percent from EDX, with the H2O composition obtained from percentage weight loss obtained from TGA-MS over 49-340°C attributed to water. As the Zn:Al ratio increases, the amount of H2O in the interlayer is found to increase. Meanwhile, the number of carbonates deposited in interlayer getting decreased as the ratio increase. The x value shows in a range of 0.23< x <0.39, which still in the range of pure HTs (0.2< x < 0.33) [22]. Somehow, at ratio 1.5:1, the x value is higher than permitted value. It has induced towards a non-crystalline or amorphous structure as probed by XRD, shown in Fig. 1.

Table 1 Comparison of actual bulk metal loadings

Nominal ratio

Actual Bulk

Zn:Al ratio

(EDX)

ZnAl LDH Formula*

x = (Al/Al+Zn)

1.50:1

1.59

[Zn0.61Al0.39(OH)2].(CO3)0.190.42H2O

0.39

2.00:1

2.00

[Zn0.67Al0.33(OH)2].(CO3)0.17 0.48H2O

0.33

3.00:1

2.97

[Zn0.75Al0.25(OH)2].(CO3)0.13 0.56H2O

0.25

4.00:1

3.33

[Zn0.77Al0.23(OH)2].(CO3)0.12 0.58H2O

0.23

*obtained from TGA-MS and EDX

The XRD patterns of all ZnAl AS series synthesized at pH 10 are summarized in Fig. 1a. From the XRD, the system of this material is confirmed as hexagonal with R-3m (166) space group. The crystallographic data are in the same agreement as reported in the literature [23]. Sharp peaks diffraction patterns of ZnAl AS were observed at basal plane d(003), d(006), d(012), d(015), d(018), d(1010), d(1011), d(110) and d(113) confirmed the formation of Zn layered double hydroxide.

Calcination removed the interlayer spacing of hydrotalcite which contains hydroxyl group due to a process of dehydroxylation and decarboxylation of carbonate. Thus, no interlayer spacing is recorded by XRD as shown in Fig. 1b. ZnO (zincite) peaks appear at 31.8°, 36.3°, 47.6°, 56.8° in the same agreement with reference [24]. These peaks only appeared at the higher ratio (2.97:1 and 3.33:1) (matched with COD 9011662) with no Al(OH)3 phases were initially detecting. Formation of ZnO normally accompanied by with ZnAl2O4 spinel structure started to form at the lowest ratio. These results are in the same agreement with Carriazo et al. [25]. From these diffraction patterns, we concluded crystallinity of ZnAl LDH are depending on x ratio (Al/(ZnAl)).

Fig.1. XRD patterns of as-synthesised ZnAl LDH prepared via alkali-free method (a). XRD patterns of calcined samples (b), and a series of hydrothermal reconstructed LDH (c). The abbreviation of i, ii, iii and iv represents the ratio from 1.59:1 to 3.33:1 respectively. Effect of ZnAl LDH/ZnO formation and crystallinity over ZnAl HTM ratio (d).

The XRD patterns of ZnAl HTM are given in Fig.1c. The phase purity of all ZnAl HTM series was checked with XRD and patterns showed sharp and intense peaks around at 11.7°, 23.3°, 34.5°, 60.2° and 61.9° attributed to diffraction planes (003), (006), (012), (110) and d(113) respectively resembles characteristic for ZnAl hydrotalcite-like materials [22], [26], [27]. Throughout the series, crystallite size increased marginally with the increase in Zn/Al atomic ratio as shown in Table 2.

The interlayer space drops slightly after rehydration step, might be due to the lesser amount of carbonate in the interlayer and mostly replaced with OH-, which gave BrØnsted basicity to hydrotalcites. To evaluate the purity of the reconstructed calcined ZnAl LDH the ratio of ZnHT (peak 003) over ZnO (peak 012) is calculated. As mentioned before, spinel of ZnAl2O4 appears even though at lower ratio, hence no unique feature of ZnO could be depended on. As these mixed oxides started to form, it is hard to reconstruct the lamellar without formation of these two compounds. For a pure LDH, a ratio of ZnHT/ZnO was calculated and represents the limiting value to be targeted. The calculation has been done by applying this formula: [yZnAl / (yZnAl + yZnO)] x 100%, where y=intensity of y2-y1.

Interestingly, higher purity (82.2 %) of ZnAl LDH was obtained for samples having higher Zn:Al atomic ratio (3.33:1) as compared to lower one. An increase in crystallite size was also observed at higher Zn:Al atomic ratio as depicted in Fig. 1d. All of these results provide important insights into a successful reconstruction of hydrothermally treated calcined ZnAl LDH, regardless to if the removal of ZnO is impossible in the lamellar structure after the calcined-rehydration process due to the rigid formation of ZnO spinel. Most importantly now, a clear understanding of how ZnO influences on the purity of reconstructed LDH is clearly answered.

Unit cell parameters (a and c) calculated from d003, d006, d110 were in well agreement with literature [28] as shown in Table 2. The ‘a’ parameter show bigger values than for MgAl hydrotalcite because of the higher octahedral ionic radius of Zn2+ over Mg2+. All HTM steps exhibit crystallite size of 50 ± 5nm.

Rehydration in HTM leads to a surface area of 230 m2g-1 as depicted in Table 2. Through exfoliation and rehydration process under liquid thermal heating, the crystallinity of the catalyst has been increased, thus increased the surface area simultaneously. Apart than that, no significant effect of ratio on the surface area has been observed here. A larger surface area may facilitate the interaction between the host structure (LDH) and the guest (interlamellar anion).

Table 2 Textural parameter of ZnAl reconstructed LDH through hydrothermal approaches.

Parameter/ catalyst

Crystallite size (XRD) [nm]

aLattice parameter [nm]

Intensity ratio of ZnAl HT/ZnO

[%]

BET surface area [m2/g]

Base site loading [mmol/g]

a

c

Zn:Al1.59:1 HTM

39

0.307

2.24

41.2

168

0.048

Zn:Al2.00:1 HTM

48

0.307

2.28

69.2

109

0.058

Zn:Al2.97:1 HTM

49

0.307

2.25

71.0

355

0.079

Zn:Al3.33:1 HTM

53

0.308

2.28

82.2

230

0.100

Fig. 2a depicted images of Zn3.33:1Al HTM obtained from Scanning Electron Microscopy (SEM) technique. The overlapping platelets suggesting more extensive re-crystallization forming structure [29]. High resolution transmission electron microscopy (HRTEM) was performed to monitor changes in morphology of different rehydration/reconstruction ZnAl LDH at higher magnification. Nanoplatelets sheet of hexagonal structures can clearly be seen in all samples. The lattice fringes obtained in a range of 0.30±0.01 nm corresponds to the lattice parameter, a, in good agreement with values from XRD measurements [30]. The surfaces of ZnAl HTM were subsequence analysed by XPS to understand the surface / near-surface of chemical states of Zn, Al, O and C. Fig. 3(a-c) depicts the Zn 2p, Al 2p and O 1s spectra respectively. The binding energy (BE) of all spectra was calibrated by assigning 284.6 eV to the C1s peak corresponding to adventitious carbon and all spectra were background subtracted. Spectra have been crossed check using NIST standard XPS database [31].

Referring to Fig. 3a, the Zn 2p for Zn/Al HTM exhibited double spectra at 1022 and 1044 eV which corresponding to the binding energy of Zn 2p3/2 and 2p1/2 respectively.

Fig. 2. SEM and HRTEM images of rehydration Zn3.33:1Al LDH through hydrothermal method.

These binding energies were separated about ~24 eV confirming the films are metallic Zn film [32] and this value are correlated to the standard limit value of Zn [33]. The strong intense peak at Zn 2p3/2 also confirms the reconstruction of Zn HT memory effect. The low intense peak of Zn 2p1/2 at 1047.8 eV also confirmed re-cooperation of Zn2+ with Al3+ in LDH phase. Ionic charge and the chemical environments around the Zn atom influence the position of the Zn 2p. It happens due to asymmetric position of Zn atom as ionic charge of Zn-Zn atom is lesser than Zn-O thus increase the BE of Zn 2p3/2 and lowering the BE of Zn 2p1/2 [34]. The intensity increased accordingly with Zn2+ content. The results confirmed Zn2+ content increased well with the ratio.

Al 2p spectra showed two deconvolution peaks at 72.4 and 74.5 eV as depicted in Fig. 3b. These peaks are corresponding to formation of Al in tetrahedral coordination at lower BE and bonded to octahedral Al3+ in higher BE. Again, both formations confirmed the reconstruction of memory effect. Tetrahedral Al3+ is coordinated to Zn2+ and O2- (Al-O-Zn). Contrast to Zn2+ content, we found, intensity of Al3+ decreased as the ratio increased, and they are in good agreement with EDX.

Referring to Fig. 3c, the O 1s spectra exhibit a doublet component at 531 and 533 eV represent the characteristic of oxide in carbonate and hydroxyl environment [19]. Peak deconvolution of all ZnAl HTM series revealed the increment of O2- are well consistent with incorporation of Zn2+. Relationship of Zn/Al surface over bulk Zn/Al can be seen in Fig. 3d as analyzed by XPS and EDX respectively. To calculate surface ratio of Zn/Al, both Zn 2p and Al 2p peaks were calculated simultaneously. Both surface and bulk Zn/Al ratio were increased as the nominal ratio increased. Total surface ratio is found to be lower than bulk of Zn/Al.

Fig 3. Zn 2p, Al 2p and O1s spectra of ZnAl HTM spectra with i-iv are the increasing ratio from 1.59 to 3.33 accordingly (a-c). Zn surface to bulk ratios determined by XPS and EDX analysis for ZnAl HTM LDH (d).

Conclusion

This study makes several noteworthy contributions to application of hydrothermal reconstructed on ZnAl LDH. Alkali-free co-precipitation (ppt) method that has been adapted on this LDH synthesis has shown superior base-sites density and could overcome the common leaching problem. Reconstruction of ZnAl LDH via hydrothermal method show remarkable promising avenue in transesterification reaction matter due to it provides high surface area and basicity. These promising results could be improved via enlargement of pores such as through macroporous-mesoporous study which could help overcomes diffusion limitation of higher TAG and aids the reactivity.

Acknowledgement

We thank the IRAGS Grant (600-RMI/DANA5/3/IRAGS (17/2015)) from Universiti Teknologi MARA for research financial support. NAT would like to thank European Bioenergy Research Institute (EBRI), Aston University for research support and facilities provided throughout her research years. She also like to thank the Malaysian Ministry of Higher Education for Scholarship Funding.

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