La agenda de Coordinación Interinstitucional para el cumplimiento del Plan

There are various ways to prepare HTs which are presented in Table 1.7. Among all, co-precipitation (co-ppt) is the most commonly used method. Co-ppt can be performed by three different methods; titration, precipitation at low supersaturation and/or precipitation at higher supersaturation160,164,182. The most common and least expensive method is co-precipitation with the base such as, NaOH and/or NaHCO3. This method is somewhat is not advisable because it is believed to be difficult to precipitate pure HTs as they tend to precipitate divalent and trivalent cation at lower pH (Mg²⁺/Al³⁺

at pH 7.7-8.5 while AlOH3 at pH 4-4.5)¹⁶⁰. In addition, it is reported precipitation method using NaOH contributed to leaching due to entrained of sodium ion or sodium salts in LDH¹⁸³.

Table 1.7 Common methods of hydrotalcite synthesis.

Techniques apply Advantages Disadvantages Ref

Co-precipitation

• Inexpensive, simple and no specific

• Progresses slowly, leads to a low degree of supersaturation during precipitation.

• More homogeneous nucleation and growth

25 (sol). Sample

allowed to gelling through precipitation

Ion-exchange method

• Modification from poly (ethylene terephthalate) / aspartic acid on LDH

• Useful for the

Hydrothermal on alkali source

• Combination of co-ppt and followed by hydrothermal ageing treatment

• Then only undergone thermal treatment

• Faster nucleation and uniform growth

Alkali-free and hydrothermal reconstruction

• Alkali-free co ppt species is avoidable

• Versatile, easy and

1.4.3.1 Entrained sodium in LDHs

In particular, the use of alkali as precipitating agents in the co-precipitation method is problematic especially during transesterification, due to the possibility of Na⁺ contamination -in the hydrotalcite lattice as NaOH can act as base catalysts. The NaOH from the lattice can leach out under reaction conditions. This makes hydrotalcite synthesis through alkali co-precipitation route more active and makes it difficult to determine the true activity of the homogeneous catalyst.

A previous study¹⁸⁷ proved addition of alkali in LDH precursor synthesis lowering down the surface area compared to alkali-free LDH, consequently reducing the rate of reaction. Addition of alkali during the synthesis of hydrotalcite results in high levels of residual alkali metal ions, which later requires an extensive of washing and also

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contributes to leach of basicity. Finally, it will affect the catalysis performance, especially in the transesterification reaction.

Hence, promoting alkali-free synthesis of HT is crucial steps for transesterification reaction. Further, existing chemistries employed in the synthesis of HTs have significant environmental drawbacks and we aim to overcome these issues by following the principles of green chemistry¹⁸⁸ (Scheme 1.1).

Among the related principles that we want to achieve are:

1) Waste prevention

-Prioritise the prevention of waste rather than cleaning up and treating waste after it been created.

2) Design less hazardous chemical synthesis

- By designing a safer route of synthesis including the potential hazards towards human health and environment

3) Catalysis

-Use catalyst instead of stoichiometric reaction to enhance selectivity, minimise waste and safer energy.

Scheme 1.1 Twelve principles of Green Chemistry introduced by Paul Anastas in 1998¹⁸⁸.

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1.4.3.2 The x ratio of synthesise hydrotalcite

It has been reported elsewhere, in order to obtain a pure hydrotalcite, the x ratio (Al³⁺/ Al³⁺ + M²⁺) must be in the range of 0.2<x< 0.33¹⁶⁰. If the value is outside the limits, the metal hydroxides could be formed. Below 0.20, normally divalent metal hydroxides will be formed and when x higher than 0.33, amorphous trivalent metal hydroxides (i.e Al(OH)3) tend to form¹⁸⁹. Contrary to that, lowering the x value under 0.2 will increase the metal density (such as Mg²⁺ or Zn²⁺ octahedral sites) in brucite layer hence, forming metal hydroxide such as Mg(OH)2.

Several papers reported LDH can be prepared outside the range of 0.22<x<0.33.

Cantrell et al.¹³¹, for example, have reported MgAl LDHs were prepared between a range of 0.25<x<0.55. They have reported a similar conclusion as above; x value outside the limits of 0.25<x<0.44 were formed Mg(OH)2 and Al(OH)3. ZnNiFe-LDH has been prepared by Touahra et al.¹⁸⁹ in the range of 0.2<x<0.66. Above 0.5 to 0.66, they noticed the formation of NiFe2O4. Some of the reports are tabulated in Table 1.8. In conjunction with that, to obtain a pure hydrotalcite, a certain pH has been suggested as listed below.

This pH is subjected to change depending on the metal cation used in the synthesis process. For example, MgAl HT is favourable to be synthesised at pH 8.5, pH 9.5 in NiAl HT, meanwhile ZnAl HT favours at pH 9-10.

Table 1.8 The range optimum value of x for obtaining pure HT.

pH prepared Results finding References

Lower pH (8 and lower)

• No HT formed, complex pathway, not completed

• Metal hydroxide

impurities will be formed

160,182,190

Suggested pH • A pH range of 8.5–10

can be used to prepare most anionic clays

Higher pH

• At pH 11: Unexpected peaks associated to Al (OH)3 and sodium nitrate

• At pH 13-14: mixed oxide will be formed

1.4.3.3 Hydrotalcite Reconstruction System

The structure of hydrotalcite formed by rehydration of mixed metal oxide is affected by the rehydration conditions, such as the M²⁺/Al³⁺ ratio and pH, parent’s HTs

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calcination temperature and the stirring speed^191–193. Valente et al.¹⁹⁴ have reported that the calcination-rehydration process played enormous roles in changes of morphology, crystallinity and particles size in MgAl hydrotalcite.Traditionally, there are two rehydration techniques applied to hydrotalcites: gas-phase and liquid-phase¹⁹¹. In gas-phase, calcined catalysts were rehydrated by exposing them under N2 or argon passed through a water bubbler. Meanwhile, in the liquid-phase method, samples were treated in decarbonated water at room temperature for at least 5 h. Abello et al.¹⁷⁴ have reported liquid-phase rehydration promised a better physicochemical and catalytic performance compared to gas-phase. Liquid-phase rehydration leads to increased surface area of the LDH up to 270 m²/g and also materials that exhibited higher basicity, the latter enhancing the catalytic activity in aldol condensation. Somehow, these methods have their own drawbacks where there is a tendency to produce an incomplete reconstruction of the lamellar structure¹⁹⁵. Generally, successful reconstruction depends on several conditions such as:

i) Nature of the cation involved; reconstruction becomes more difficult if more than two cations involved. For example, the addition of Ni²⁺ on MgAl compounds deficit the ability of metal oxide to be severe covered under rehydration. Reconstruction becomes challenging if larger cationic atom involved (e.g. Zn²⁺ is harder to reconstruct compare to Mg²⁺).

ii) Temperature, time and ramping rate of the parent calcination.

iii) Rehydration techniques (e.g. gas-phase, liquid-phase, microwave assisted or hydrothermal). NiAl LDH has been reported managed to be reconstructed by microwave assisted but not through non-assisted gas-phase or liquid-gas-phase¹⁹².

iv) Length and temperature of rehydration process.

Recently, hydrothermal treatment has been adapted in hydrotalcite synthesis and results revealed a desirably greater surface area (up to 340 m²/g) with narrow pore size distribution¹⁹⁶ and higher crystallinity^195–197. Apart than that, particle size, morphology and consistency of composition also are feasible and easy to adjust in hydrothermal synthesis method¹⁹⁵. Yet no study has been done on the hydrothermal reconstruction of alkali-free LDH, thus extended knowledge on this approach is essentially required.