PARTICLES

INTRODUCTION

[0001] The present invention relates to a process of making alumina@layered double hydroxide core@shell particles. More particularly, the invention relates to a process of making alumina@layered double hydroxide core@shell particles having an average particle size of ³50 pm.

BACKGROUND OF THE INVENTION

[0002] Layered double hydroxides (LDHs) are a class of compounds which comprise two or more metal cations and have a layered structure. A review of LDHs is provided in Structure and Bonding; Vol. 1 19, 2005 Layered Double Hydroxides ed. X Duan and D.G. Evans. The hydrotalcites, perhaps the most well-known examples of LDHs, have been studied for many years. LDHs can intercalate anions between the layers of the structure.

[0003] Core shell particles are described in the literature by“core@shell” (for example by Teng et ai, Nano Letters, 2003, 3, 261-264), or by“core/shell” (for example J. Am. Chem. Soc., 2001 , 123, pages 7961-7962). We have adopted the“core@shell” nomenclature as it is emerging as the more commonly accepted abbreviation.

[0004] The preparation of core@LDH materials has to date centered predominantly on the use of small particle size core substrates, typically powdered materials having a particle size of <10 pm, thus resulting in core@LDH particles of a similarly small size. For example, WO2017/009664 describes the preparation of zeolite@LDH particles having an average particle size (determined by TEM inspection) ranging from approximately 0.5 to 5 pm. WO2016/1 10698 describes the preparation of silica@LDH microspheres, using silica microsphere starting materials having an average diameter of 0.15 to 8 pm.

[0005] Core@shell particles containing LDH have received industrial interest, particularly in the fields of catalysis and sorption, due to the interesting properties of LDHs and the hybrid nature of this material. However, core@LDH particles of small particle size may be unsuitable for direct use in industrial applications, and are often required to undergo further processing - such as pelletisation - to convert them into a more industrially-manageable particle size. Setting aside the additional time and cost involved in such processes, they can have a negative effect on the properties of the resulting material. In particular, the use of binders and other additives during pelletisation may compromise some of the properties exhibited by the core@shell particles. Moreover, unless carefully controlled, pelletisation is likely to give rise to a wide particle size distribution, which may be unsuitable for the intended industrial application.

[0006] Therefore, there remains a need for processes for the preparation of core@LDH particles of larger particle size, particularly those having an average particle size of ³50 pm.

[0007] The present invention was devised with the foregoing in mind.

SUMMARY OF THE INVENTION

[0008] According to a first aspect of the present invention there is provided a process for the preparation of a plurality of layered double hydroxide-coated alumina particles, wherein the layered double hydroxide comprises Al ³⁺ within its positively-charged layer, and wherein at least a portion of the Al ³⁺ is derived from the alumina particles on which the layered double hydroxide is coated, the process comprising / consisting essentially of / consisting of the steps of:

a) providing an aqueous mixture comprising / consisting essentially of / consisting of:

i. a plurality of alumina particles, the particles having an average particle size of ³50 pm,

ii. a quantity of Al ³⁺ that has leached out of the plurality of alumina particles, iii. at least one metal cation, M ^z+ , wherein z is 1 or 2, and iv. optionally at least one metal cation, M’ ^y+ , wherein M’ is different to M and y is 3 or 4, and

wherein Al ³⁺ , M ^z+ and optionally M’ ^y+ are present in amounts sufficient to form the layered double hydroxide comprising Al ³⁺ within its positively-charged layer; b) adjusting the pH of the aqueous mixture to 9.5 - 11 to form a plurality of layered double hydroxide-coated alumina particles; and

c) optionally isolating the plurality of layered double hydroxide-coated alumina

particles.

[0009] According to a second aspect of the present invention there is provided a plurality of LDH- coated alumina particles obtainable, obtained or directly obtained by the process of the first aspect.

[0010] According to a third aspect of the present invention there is provided a use of a plurality of LDH-coated alumina particles according to the second aspect in a catalytic process.

[0011] According to a fourth aspect of the present invention there is provided a use of a plurality of LDH-coated alumina particles according to the second aspect as a support material. DETAILED DESCRIPTION OF THE INVENTION

Preparation of the plurality of layered double hydroxide-coated alumina particles

[0012] The present invention provides a process for the preparation of a plurality of layered double hydroxide-coated alumina particles, wherein the layered double hydroxide comprises Al ³⁺ within its positively-charged layer, and wherein at least a portion of the Al ³⁺ is derived from the alumina particles on which the layered double hydroxide is coated, the process comprising the steps of:

a) providing an aqueous mixture comprising:

i. a plurality of alumina particles, the particles having an average particle size of ³50 pm,

ii. a quantity of Al ³⁺ that has leached out of the plurality of alumina particles, iii. at least one metal cation, M ^z+ , wherein z is 1 or 2, and iv. optionally at least one metal cation, M’ ^y+ , wherein M’ is different to M and y is 3 or 4, and

wherein Al ³⁺ , M ^z+ and optionally M’ ^y+ are present in amounts sufficient to form the layered double hydroxide comprising Al ³⁺ within its positively-charged layer; b) adjusting the pH of the aqueous mixture to 9.5 - 11 to form a plurality of layered double hydroxide-coated alumina particles; and

c) optionally isolating the plurality of layered double hydroxide-coated alumina

particles.

[0013] As described hereinbefore, the preparation of core@LDH materials has to date centred predominantly on the use of small particle size core substrates, typically powdered materials having a particle size of <10 pm, thus resulting in core@LDH particles of a similarly small size. In particular, WO2017/009664 describes the preparation of zeolite@LDH particles having an average particle size (determined by TEM inspection) ranging from approximately 0.5 to 5 pm. WO2016/1 10698 describes the preparation of silica@LDH microspheres, using silica microsphere starting materials having an average diameter of 0.15 to 8 pm. However, such materials are typically unsuitable for direct use (e.g. as support materials and/or sorbents) in industrial applications due to their small particle size. In order to make such materials industrially- useable (e.g. as supports in fixed bed reactors or fluidised bed processes), the fine core@LDH material powders must first be processed into a more manageable size, typically by pelletisation. However, the low density and particular morphology of LDHs (which are often described as fluffy) can present challenges for effective pelletisation. Moreover, the use of binders and other additives in this process can have a detrimental effect on the properties of the resulting material. In addition, when compared with unpelletised materials, pelletised samples naturally exhibit a reduced quantity of free LDH, which can compromise the suitability of the material for the intended application. Furthermore, unless carefully controlled, pelletisation typically gives rise to a wide particle size distribution, which may be unsuitable for the intended industrial application.

[0014] Through extensive investigations, the inventors have now devised a process of preparing notably larger particle size alumina@LDH core@shell particles in which the alumina core starting material has an average particle size of ³50 pm (e.g. 50 pm to 5 mm). Such alumina@LDH core@shell materials are therefore suitable for direct use (i.e. without pelletisation) in a variety of industrial applications, including as supports in fixed bed reactors or fluidized bed processes.

[0015] The skilled person will be familiar with the structure of LDHs. In particular, (s)he will appreciate that LDHs comprise layers of positively charged metal hydroxides - including, for example, aluminium hydroxide - with layers of negatively charged anions intercalated in the interlayer galleries. The inventors have now devised a process that exploits Al ³⁺ leaching from the larger particle size alumina core particles, and uses it in-situ in the formation of an Al ³⁺ - containing LDH coating. Accordingly, the present process allows for the preparation of larger particle size alumina@(AI ³⁺ -containing)LDH core@shell particles in which at least a portion of the Al ³⁺ of the LDH is derived from the alumina (i.e. AI2O3) particles on which the LDH is coated.

[0016] It will be understood that the LDH coating formed on the surface of the alumina core may be continuous or discontinuous. Suitably, greater than 50% of the surface of the alumina core is coated with the LDH. More suitably, greater than 60% of the surface of the alumina core is coated with the LDH. Even more suitably, greater than 70% of the surface of the alumina core is coated with the LDH. Yet more suitably, greater than 80% of the surface of the alumina core is coated with the LDH. Most suitably, greater than 90% of the surface of the alumina core is coated with the LDH. In an embodiment, substantially all of the surface of the alumina core is coated with the LDH

[0017] Across all aspects of the invention, the terms“plurality of LDH-coated alumina particles” and“alumina@LDH” particles are used interchangeably.

[0018] Acids (such as nitric acid) have previously been used to promote the leaching of Al ³⁺ from alumina. However, such techniques may be unsatisfactory, since the acid can effectively dissolve the basic LDH, once formed. Therefore, step a) (and the subsequent steps) suitably does not use an acid capable of promoting leaching of Al ³⁺ from the alumina particles.

[0019] In step a), the aqueous mixture comprises the plurality of alumina particles, a quantity of Al ³⁺ that has leached from the alumina particles, M ^z+ and optionally M’ ^y+ . It will be understood that M ^z+ and optionally M’ ^y+ are present in the aqueous mixture as a consequence of the dissolution of one or more precursor compounds that dissolve in water to afford M ^z+ and optionally M’ ^y+ . An example of such a compound is magnesium nitrate, which dissolves in water to afford Mg ²⁺ (an example of cation M ^z+ ). Another example of such a compound is aluminium nitrate, which dissolves in water to afford Al ³⁺ (an example of optional cation M’ ^y+ ). It will therefore be appreciated that the present process distinguishes between Al ³⁺ that has leached from the alumina particles, and Al ³⁺ that may optionally be present in the aqueous mixture as the dissolution product M’ ^y+ of a precursor compound, such as aluminium nitrate.

[0020] In an embodiment of the invention, at least 50 wt% of the Al ³⁺ in the layered double hydroxide is derived from the alumina particles provided in step a). Suitably, at least 75 wt% of the Al ³⁺ in the layered double hydroxide is derived from the alumina particles. More suitably, at least 90 wt% of the Al ³⁺ in the layered double hydroxide is derived from the alumina particles. Most suitably, all (or substantially all) of the Al ³⁺ in the layered double hydroxide is derived from the alumina particles. It will be understood that when all (or substantially all) of the Al ³⁺ in the layered double hydroxide is derived from the alumina particles, M’ ^y+ (if it is present) is not Al ³⁺ .

[0021] In an embodiment, step a) comprises the sub-steps of i) providing an aqueous suspension of alumina particles; and ii) adding to that suspension one or more precursor compounds sufficient to generate M ^z+ and optionally M’ ^y+ . It will be understood that the one or more precursor compounds may be added simultaneously or separately.

[0022] In another embodiment, step a) comprises the sub-steps of i) providing an aqueous solution of M ^z+ and optionally M’ ^y+ ; and ii) adding to that solution the plurality of alumina particles.

[0023] The order of addition of the reagents used in the invention is important. In particular, it will be understood that the plurality of alumina spheres is contacted with M ^z+ and optionally M’ ^y+ in water (in step a)) before any base is added.

[0024] The aqueous mixture of step a) suitably has a pH of £7.

[0025] The aqueous mixture of step a) may be provided at a temperature of 20 to 150°C. However, the process of the invention remains operable at notably lower temperatures (including room temperature), thus presenting clear industrial advantages. Suitably, the aqueous mixture of step a) may be provided at a temperature of 20 to 80°C, more suitably from 20 to 50°C and most suitably from 20 to 40°C. Suitably, the aqueous mixture of step a) is stirred.

[0026] The average particle size of the alumina core particles used in step a), and/or that of the resulting alumina@LDH particles, is ³50 pm (e.g. 50 pm to 5 mm). The average particle size can be determined by calculating the mean diameter of a plurality of particles - either by analysis of SEM micrographs, or using callipers for larger particles. Suitably, the average particle size of the alumina core particles and/or that of the resulting alumina@LDH particles is ³100 pm. More suitably, the average particle size of the alumina core particles and/or that of the resulting alumina@LDH particles is ³200 pm. Even more suitably, the average particle size of the alumina core particles and/or that of the resulting alumina@LDH particles is ³500 pm. Yet more suitably, the average particle size of the alumina core particles and/or that of the resulting alumina@LDH particles is 0.70 5.0 mm. Yet more suitably, the average particle size of the alumina core particles and/or that of the resulting alumina@LDH particles is 0.80 3.0 mm. Most suitably, the average particle size of the alumina core particles and/or that of the resulting alumina@LDH particles is 0.80 1.50 mm.

[0027] Alternatively, the average particle size of the alumina core particles and/or that of the resulting alumina@LDH particles is 0.85 5.0 mm. Suitably, the average particle size of the alumina core particles and/or that of the resulting alumina@LDH particles is 0.85 3.0 mm. Most suitably, the average particle size of the alumina core particles and/or that of the resulting alumina@LDH particles is 0.90 1.50 mm.

[0028] In an embodiment, M ^z+ is selected from Li ⁺ , Ca ²⁺ , Mg ²⁺ , Zn ²⁺ , Fe ²⁺ , Mn ²⁺ , Co ²⁺ , Cu ²⁺ and Ni ²⁺ . Suitably, M ^z+ is selected from Ca ²⁺ , Mg ²⁺ , Zn ²⁺ and Ni ²⁺ . Most suitably, M ^z+ is Mg ²⁺ .

[0029] In an embodiment, in step a) the amount of leached Al ³⁺ is determined, and is used to calculate the amount of M ^z+ (and optionally M’ ^y+ ) based on the intended stoichiometry of the resulting LDH. For example, in the preparation of alumina@Mg2AI-NC>3 LDH, if it is determined that 2 moles of Al ³⁺ have leached from the alumina spheres into the aqueous mixture, 4 moles of Mg ²⁺ is added.

[0030] The skilled person will be familiar with the structure of LDHs. In particular, (s)he will appreciate that LDHs comprise layers of positively charged metal hydroxide with layers of negatively charged anions intercalated in the interlayer galleries. Therefore, it will be appreciated that in order for an LDH to form on the surface of the alumina core, at least one anion must be present in the aqueous mixture of step a), or added to the aqueous mixture alongside the base in step b). The anion(s) may be selected from carbonate, bicarbonate, hydroxide, hydrogenphosphate, dihydrogenphosphate, nitrite, nitrate, borate, sulphate, halide and phosphate.

[0031] In an embodiment, the anion may be carbonate, and may be derived from CO2 in the atmosphere.

[0032] In a particularly suitable embodiment, at least one anion is present in the aqueous mixture of step a), or is added to the aqueous mixture alongside the base in step b). Whether the anion is present in the aqueous mixture of step a) will depend on whether it forms, with M ^z+ , M’ ^y+ or Al ³⁺ a water-soluble compound (such as magnesium nitrate or aluminium nitrate). If the anion would form a water-soluble compound with M ^z+ , M’ ^y+ or Al ³⁺ in the aqueous mixture of step a), it is present in the aqueous mixture of step a). For example, if M ^z+ is Mg ²⁺ and the anion is nitrate, the anion would be present in the aqueous mixture of step a). If, however, the anion would form a water-insoluble compound with M ^z+ , M’ ^y+ or Al ³⁺ in the aqueous mixture of step a), it is instead added to the aqueous mixture alongside the base in step b). For example, if M ^z+ is Mg ²⁺ and the anion is carbonate, the anion would be added to the aqueous mixture alongside the base in step b), otherwise it would precipitate in the aqueous mixture of step a) as magnesium carbonate and aluminium carbonate (considering the leaching of Al ³⁺ from the alumina particles).

[0033] In a particular embodiment, the anion is nitrate. Suitably, the anion is present in the aqueous mixture of step a) as magnesium nitrate and aluminium nitrate.

[0034] In a particular embodiment, M ^z+ is either or both of Mg ²⁺ and Zn ²⁺ and the anion is either or both of nitrate and carbonate. When the anion is nitrate, it is present in the aqueous mixture of step a). When the anion is carbonate, it is added to the aqueous mixture during step b).

[0035] In an embodiment, when present, M’ ^y+ is selected from Al ³⁺ , Ga ³⁺ , V ³⁺ , ln ³⁺ , Y ³⁺ and Fe ³⁺ .

[0036] In an embodiment, when M’ ^y+ is Al ³⁺ , the maximum amount of M’ ^y+ used in step a) is 95% by weight relative to the weight of Al ³⁺ within the resulting layered double hydroxide. Suitably, when M’ ^y+ is Al ³⁺ , the maximum amount of M’ ^y+ used in step a) is 70% by weight relative to the weight of Al ³⁺ within the resulting layered double hydroxide. More suitably, when M’ ^y+ is Al ³⁺ , the maximum amount of M’ ^y+ used in step a) is 50% by weight relative to the weight of Al ³⁺ within the resulting layered double hydroxide. Even more suitably, when M’ ^y+ is Al ³⁺ , the maximum amount of M’ ^y+ used in step a) is 30% by weight relative to the weight of Al ³⁺ within the resulting layered double hydroxide. Yet more suitably, when M’ ^y+ is Al ³⁺ , the maximum amount of M’ ^y+ used in step a) is 10% by weight relative to the weight of Al ³⁺ within the resulting layered double hydroxide. Most suitably, M’ ^y+ is not Al ³⁺ and the only Al ³⁺ present in step a) is that which has leached from the alumina particles.

[0037] In a particular embodiment, M ^z+ is Mg ²⁺ , M’ ^y+ is absent or is not Al ³⁺ , and the anion is nitrate or carbonate, and the resulting layered double hydroxide is selected from Mg _x AI-CC>3 and Mg _x AI-NC>3, wherein 1.8£x£5. Suitably, 1.8£x£3.5.

[0038] In a particular embodiment, M ^z+ is Mg ²⁺ , M’ ^y+ is absent, and the anion is nitrate, and the resulting layered double hydroxide is Mg _x AI-NC>3, wherein 1.8£x£5. Suitably, 1.8£x£3.5. In such embodiments, the aqueous mixture of step a) may be prepared by dissolving magnesium nitrate in water, and then adding the plurality of alumina particles. Alternatively, the aqueous mixture of step a) may be prepared suspending the plurality of alumina particles in water and then adding magnesium nitrate.

[0039] In step b), the pH of the aqueous mixture of step a) is adjusted to 9.5 - 11 , suitably with stirring. Adjust the pH of the mixture to within this range results in the coprecipitation of an Al ³⁺ - containing LDH around the outer surface of the alumina particles, thus affording the alumina@LDH core@shell particles. Suitably, step b) comprises adjusting the pH of the aqueous mixture to 10 - 1 1.

[0040] In an embodiment, in step b) the pH of the aqueous mixture of step a) is adjusted to 9.5 - 11 using a basic solution having a molarity ³2.5 M. Suitably, the base used in step b) has a pK _b of £ 5. More suitably, the base used in step b) has a pK _b of £ 2. Even more suitably, the base used in step b) has a pK _b of £ 1. In a particular embodiment, a solution of NaOH and/or Na2CC>3 is used to adjust the pH in step b).

[0041] The base is suitably not a weak base, such as urea.

[0042] In an embodiment, prior to step c), the mixture resulting from step b) is stirred for 10 minutes to 18 hours at a temperature of 18 - 120°C. When compared with conventional techniques for preparing core@LDH particles, the inventors have determined that the present process allows processing times to be significantly reduced. Suitably, the mixture resulting from step b) is stirred for 20 minutes to 15 hours at a temperature of 18 - 120°C. More suitably, the mixture resulting from step b) is stirred for 20 minutes to 8 hours at a temperature of 18 - 120°C.

[0043] In an embodiment, prior to step c), the mixture resulting from step b) is stirred at a temperature of 18 - 80°C for at least 20 minutes (e.g. 20 minutes to 5 hours). Suitably, prior to step c), the mixture resulting from step b) is stirred at a temperature of 20 - 70°C. More suitably, prior to step c), the mixture resulting from step b) is stirred at a temperature of 40 - 70°C.

[0044] In an embodiment, prior to step c), the mixture resulting from step b) is stirred at a temperature of 18 - 70°C. Suitably, prior to step c), the mixture resulting from step b) is stirred at a temperature of 25 - 70°C. More suitably, prior to step c), the mixture resulting from step b) is stirred at a temperature of 40 - 70°C.

[0045] It will be understood that when optional step c) is not employed, the plurality of LDH- coated alumina particles are provided as a dispersion or suspension. Suitably, step c) is employed.

[0046] In step c), the plurality of LDH-coated alumina particles is isolated from the mixture resulting from step b). The particles may be isolated by any suitable means. Suitably, step c) comprises isolating the particles by filtration.

[0047] In an embodiment, the plurality of LDH-coated alumina particles isolated in step c) are then washed with water. Suitably, the isolated particles are washed extensively with water, typically until the pH of the washings is 6.5-7.5. The washed particles may then be dried (e.g. under vacuum). [0048] In an embodiment, the plurality of LDH-coated alumina particles isolated (e.g. by filtration) in step c) are then washed with water, and are then contacted with an organic solvent capable of hydrogen bonding to water. Suitably, the solvent is acetone or ethanol. It has been determined that drying the particles directly from water results in an LDH shell in which the LDH has a lower surface area and/or pore volume. In such embodiments, it is therefore important that the plurality of LDH-coated alumina particles remain wet (e.g. damp or moist) during isolation step c). Without wishing to be bound by theory, the inventors have hypothesised that by employing an intervening solvent washing step using an organic solvent having hydrogen bonding characteristics (e.g. as donor or acceptor), residual water present between the layers of the LDH or on its surface can be efficiently removed. The removal of this residual water greatly reduces the extent to which individual LDH particulates or crystallites aggregate through hydrogen-bonding of residual water present on their surfaces, thereby resulting, upon drying, in a finer LDH powder having high surface area and pore volume. Such properties may be particularly advantageous when the alumina@LDH core@shell is envisaged for use in catalysis or sorption applications, wherein a higher surface area may be key. This solvent treatment process may be referred to herein as ‘AMO treatment’, and the solvent may be referred to herein as an‘AMO solvent’. In a particular embodiment, the plurality of LDH-coated alumina particles isolated in step c) are then washed with water, and are then dispersed in an organic solvent capable of hydrogen bonding to water.

[0049] When compared with currently-available techniques for making similar materials, many of which require high temperatures, the process of the invention remains operable at notably lower temperatures (including room temperature), thus presenting clear industrial advantages.

[0050] In an embodiment, steps a) and b) are carried out at a temperature £105°C (e.g. 5 - 105°C). Suitably, steps a) and b) are carried out at a temperature £75°C. More suitably, steps a) and b) are carried out at a temperature £70°C. Even more suitably, steps a) and b) are carried out at a temperature £50°C.

[0051] The present invention also provides a plurality of LDH-coated alumina particles obtainable, obtained or directly obtained by the process of the first aspect of the invention.

[0052] The following numbered statements 1 to 42 are not claims, but instead serve to define particular aspects and embodiments of the claimed invention:

1. A process for the preparation of a plurality of layered double hydroxide-coated alumina particles, wherein the layered double hydroxide comprises Al ³⁺ within its positively- charged layer, and wherein at least a portion of the Al ³⁺ is derived from the alumina particles on which the layered double hydroxide is coated, the process comprising the steps of:

a) providing an aqueous mixture comprising:

i. a plurality of alumina particles, the particles having an average particle size of ³50 pm,

ii. a quantity of Al ³⁺ that has leached out of the plurality of alumina particles, iii. at least one metal cation, M ^z+ , wherein z is 1 or 2, and

iv. optionally at least one metal cation, M’ ^y+ , wherein M’ is different to M and y is 3 or 4, and

wherein Al ³⁺ , M ^z+ and optionally M’ ^y+ are present in amounts sufficient to form the layered double hydroxide comprising Al ³⁺ within its positively-charged layer; b) adjusting the pH of the aqueous mixture to 9.5 - 11 to form a plurality of layered double hydroxide-coated alumina particles; and

c) optionally isolating the plurality of layered double hydroxide-coated alumina

particles.

The process of statement 1 , wherein at least 50 wt% of the Al ³⁺ in the layered double hydroxide is derived from the alumina particles.

The process of statement 1 , wherein at least 75 wt% of the Al ³⁺ in the layered double hydroxide is derived from the alumina particles.

The process of statement 1 , wherein at least 90 wt% of the Al ³⁺ in the layered double hydroxide is derived from the alumina particles.

The process of statement 1 , wherein substantially all of the Al ³⁺ in the layered double hydroxide is derived from the alumina particles.

The process of any preceding statement, wherein the plurality of alumina particles provided in step a) have an average particle size of ³100 pm.

The process of any preceding statement, wherein the plurality of alumina particles provided in step a) have an average particle size of ³200 pm.

The process of any preceding statement, wherein the plurality of alumina particles provided in step a) have an average particle size of ³500 pm.

The process of any preceding statement, wherein the plurality of alumina particles provided in step a) have an average particle size of 0.80 - 50 mm.

The process of any preceding statement, wherein the plurality of alumina particles provided in step a) have an average particle size of 0.80 - 3.0 mm.

The process of any one of statements 1 to 8, wherein the plurality of alumina particles provided in step a) have an average particle size of 0.85 - 5.0 mm.

The process of any one of statements 1 to 8, wherein the plurality of alumina particles provided in step a) have an average particle size of 0.85 - 3.0 mm. The process of any one of statements 1 to 8, wherein the plurality of alumina particles provided in step a) have an average particle size of 0.90 - 1.50 mm.

The process of any preceding statement, wherein M is selected from Li, Ca, Mg, Zn, Fe, Co, Cu, Mn and Ni.

The process of any preceding statement, wherein M is selected from Ca, Zn, Mg and Ni. The process of any preceding statement, wherein, when present, M’ is selected from Al, V, Ga, In, Y and Fe.

The process of any preceding statement, wherein at least one anion is present in the aqueous mixture of step a), or is added to the aqueous mixture during step b).

The process of statement 17, wherein the at least one anion is nitrate and is present in the aqueous mixture of step a), or the at least one anion is carbonate and is added to the aqueous mixture during step b).

The process of statement 17 or 18, wherein M ^z+ is either or both of Mg ²⁺ and Zn ²⁺ and the anion is either or both of nitrate and carbonate, preferably nitrate.

The process of any preceding statement, wherein the layered double hydroxide comprising Al ³⁺ within its positively-charged layer is one or more layered double hydroxide selected from Mg _x AI-CC>3 and Mg _x AI-NC>3, wherein 1.8£x£5.

The process of statement 20, wherein 1 8£x£3.5.

The process of any preceding statement, wherein in step b) the pH of the aqueous mixture of step a) is adjusted to 9.5 - 11 using a basic solution having a molarity ³2.5 M.

The process of any preceding statement, wherein the base used in step b) has a pK _b of £ 5.

The process of any preceding statement, wherein the base used in step b) has a pK _b of £ 2.

The process of any preceding statement, wherein the base used in step b) has a pK _b of £ 1.

The process of any preceding statement, wherein a solution of NaOH and/or Na2CC>3 is used to adjust the pH in step b).

The process of any preceding statement, wherein step b) comprises adjusting the pH of the aqueous mixture to 10 - 11.

The process of any preceding statement, wherein prior to step c), the mixture resulting from step b) is stirred for 10 minutes to 18 hours at a temperature of 18 - 120°C.

The process of statement 28, wherein prior to step c), the mixture resulting from step b) is stirred at a temperature of 18 - 80°C.

The process of statement 28, wherein prior to step c), the mixture resulting from step b) is stirred at a temperature of 18 - 70°C. 31. The process of statement 28, wherein prior to step c), the mixture resulting from step b) is stirred at a temperature of 25 - 70°C.

32. The process of statement 28, wherein prior to step c), the mixture resulting from step b) is stirred at a temperature of 40 - 70°C.

33. The process of any one of statements 28 to 32, wherein prior to step c), the mixture resulting from step b) is stirred for 20 minutes to 15 hours.

34. The process of any one of statements 28 to 32, wherein prior to step c), the mixture resulting from step d) is stirred for 20 minutes to 8 hours.

35. The process of any one of statements 1 to 28, wherein steps a) and b) are carried out at a temperature £105°C (e.g. 5 - 105°C).

36. The process of statement 35, wherein steps a) and b) are carried out at a temperature £75°C.

37. The process of statement 35, wherein steps a) and b) are carried out at a temperature £70°C.

38. The process of statement 35, wherein steps a) and b) are carried out at a temperature £50°C.

39. The process of any preceding statement, wherein step c) comprises filtering the mixture resulting from step b) to obtain the plurality of layered double hydroxide-coated alumina particles.

40. The process of any preceding statement, wherein the isolated plurality of layered double hydroxide-coated alumina particles resulting from step c) is washed and then dried.

41. The process of any preceding statement, wherein when M’ ^y+ is Al ³⁺ , the maximum

amount of M’ ^y+ used in step a) is 95% by weight relative to the weight of Al ³⁺ within the resulting layered double hydroxide.

42. A plurality of layered double hydroxide-coated alumina particles obtainable by the

process of any preceding statement.

EXAMPLES

[0053] One or more examples of the invention will now be described, for the purpose of illustration only, with reference to the accompanying Figures, in which:

Fig. 1 shows SEM images of a) uncoated 1 mm diameter alumina sphere; b) sphere after 2 hour reaction at 100 °C; c) sphere after 6 hour reaction at 100 °C; d/e) sphere after 12 hours reaction at 100 °C. LDH coating was performed by Method A according to Example 1. Fig. 2 shows SEM images of surface close-up of alumina@LDH spheres prepared by Method A according to Example 1 at 100 °C for a) 6 h; b/c) 12 h.

Fig. 3 shows SEM images of alumina@LDH spheres prepared at a) 60 °C, 1) overview, 2/3) close-up; b) 40 °C, 1) overview, 2/3) close-up; c) room temperature, 1) overview, 2/3) close-up. All reactions carried out by Method A according to Example 1 for 6 hours.

Fig. 4 shows SEM images of alumina@LDH spheres prepared at a) 60 °C, 1) overview, 2/3) close-up; b) 40 °C, 1) overview, 2/3) close-up; c) room temperature, 1) overview, 2/3) close-up. All reactions carried out by Method A according to Example 1 for 3 hours.

Fig. 5 shows SEM images of alumina@LDH spheres prepared by Method A according to Example 1 at 60 °C for a) 2 hours; b) 1 hour; c) 30 minutes.

Fig. 6 shows overview and close-up SEM images of alumina@LDH spheres prepared at room temperature for 3 hours: a) experiment 1 ; b) experiment 3; c) experiment 4; d) experiment 5; e) experiment 6; f) experiment 8; g) experiment 9. All reactions were carried out according to Method B, Example 2.

Fig. 7 shows overview and close-up SEM images of alumina@LDH spheres prepared at room temperature for 1 hour according to experiment 3. All reactions were carried out according to Method B, Example 2.

Fig. 8 shows scratched surface samples of the alumina@LDH spheres, showing portions of LDH (labelled“H”) and portions of alumina (labelled“A”). All reactions were carried out according to Method B, Example 2.

Fig. 9 shows SEM images of alumina@LDH spheres prepared by Method B according to Example 3: a) overview of surface of sphere; b) close up of surface of sphere.

Fig. 10 shows SEM images of alumina@LDH spheres prepared by Method B according to Example 4: a-b) overview of sphere; c-d) close up of sphere.

Materials and methods

[0054] Magnesium nitrate, sodium carbonate and sodium hydroxide were purchased from Sigma Aldrich. Alumina spheres were sent by SCG Chemicals Co., Ltd..

[0055] Scanning electron microscopy (SEM) was performed using a JEOL JSM 6610 microscope, with an accelerating voltage of 20 kV. [0056] Energy dispersive X-ray spectroscopy (EDX), carried out on a JSM-6610LV low vacuum SEM with an accelerating voltage of 20 kV, was used to identify the elements found at the surface of the hybrids.

[0057] Thermogravimetric analyses (TGA) was carried out on a Mettler Toledo TGA/DSC 1 system. Samples (10 mg) were heated in an alumina crucible from 25 to 800 °C (at 5 °C min ^-1 ) under N ₂ flowing at 100 cm ³ min ^-1 .

PART I

Example 1 - Preparation of LDH-coated alumina particles (Method A)

A) General procedure

[0058] Having regard to Scheme 1 below, 100 g of alumina spheres (1 mm diameter) were immersed in a 40 ml_ solution containing Mg(NC> ₃ ) ₂ .6H ₂ 0 (0.246 g). The pH of the mixture was adjusted to ~ 10 using 4M NaOH and the entire reaction was heated at different temperatures (100 and 60 °C) for different times (30 minutes to 12 h). The resulting alumina@LDH spheres were collected by filtration, washed with water and dried.

A) _j O= spheres

Scheme 1 : Synthetic pathway according to Method A

B) Characterisation

[0059] Figures 1 and 2 are SEM images of alumina@LDH spheres prepared at 100°C according to Method A.

[0060] Figure 1 shows SEM images of the spheres before and after the reaction. It is important to underline that, during the process, two reactions need to take place at the same time: the leaching of the Al from the alumina spheres to create Al ³⁺ ions in solution and then the nucleation and growth of the LDH. At 100 °C, the first 2 h favour the leaching of the Al, as can be seen from Figure 1 b, which shows destruction of the outer surface of the sphere without any spongy material (indicative of LDH) present. This is also confirmed from the SEM-EDX, where no Mg was detected. After 6 h of reaction (Figure 1c), the surface of the sphere is covered with the LDH. If the spheres are left in the reaction mixture for 12 h, one can observe the further destruction of the material, on one hand, and the grown of a new one, on the other hand (Figure 1d and 1 e). In this last case we have also observed the presence of a white solid which contain both metals (Mg and Al), detected using SEM-EDX .

[0061] It is unanimously agreed that the hydrothermal synthesis of LDH produces materials with larger platelets. This is also observed in the present case. When zooming in on the surface of the alumina@LDH spheres prepared at 100 °C for 6 h, one observes a spongy-like material, which SEM-EDX confirms is LDH. The same material is observed when zooming in on the alumina@LDH spheres prepared at 100 °C for 12 h, where, in the apparent“holes” on the surface (Figure 1 e), the rosette shape of the LDH was detected (Figure 2b and c).

[0062] Figures 3 to 5 show SEM images of alumina@LDH spheres prepared at room temperature to 60°C according to Method A.

[0063] The results show that after 6 h at all the studied temperatures (room temperature, 40 and 60 °C) LDH can be observed on the surface of the spheres. Figure 3 presents an overview of the materials; and in all cases, one can clearly see the LDH with the rosette shape platelets. The results also show that after only 3 h at all the studied temperatures (room temperature, 40 and 60 °C) LDH can be observed on the surface of the spheres. Figure 4 presents an overview of the materials; and in all cases, one can clearly see the LDH with the rosette shape platelets. In all cases, one can observe that the overall surface of the spheres changes from a shiny one (seen in Figure 1 a) to a matt one (Figures 3a-1 , b-1 , c-1 and Figures 4a- 1 , b-1 and c-1), thus proving that a different material is present.

[0064] The results further show that the duration of the reaction can be reduced even further. Figure 5 shows the presence of LDH in alumina@LDH spheres prepared at 60°C for as little as 30 minutes.

Example 2 - Preparation of LDH-coated alumina particles (Method B)

A) General procedure

[0065] Having regard to Scheme 2 below, 100 g alumina spheres (1 mm diameter) were added to a flask containing 10 ml H2O, to which an amount of Mg(NC> ₃ ) ₂ .6H ₂ 0 is added. The amount of Mg(NC> ₃ ) ₂ .6H ₂ 0 calculated based on an assumed wt% leaching of Al from the alumina spheres to afford a Mg:AI ratio 2:1. The pH is then adjusted to 10.5-10.7 using 4M NaOH. The mixture is then left to stir at room temperatures for different times (1 h to 9 h). Finally the resulting LDH- coated alumina particles are collected by filtration, washed with water and dried.

Scheme 2: Synthetic pathway according to Method B

B) Results and characterisation

[0066] The amount of Mg(N0 ₃ ) ₂ .6H ₂ 0 added during the preparation of the alumina@LDH spheres was calculated as follows:

• consider 0.100 g of AI ₂ O ₃ over which it is desired to grow LDH with the cation ratio Mg:AI 2: 1.

• consider that in a basic medium, 10 wt% of Al is leaching.

• to determine how much Mg(NC> ₃ ) ₂ .6H ₂ 0 should be added, the following calculations are performed:

o Molecular mass of AI ₂ O ₃ is 101.96 g/mol.

o 0.100 g of spheres contain 0.0529 g Al.

o If 10% of this Al is leaching, it means that 0.00529 g of Al is available to form LDH.

This represents 1.96*10 ^-4 moles.

o The moles of magnesium nitrate required to form LDH Mg:AI 2 :1 will be 2 x 1.96*10 ^-4 moles. The mass of magnesium nitrate to be added is then determined (No. moles = Mass / Molecular mass). The pH is adjusted using NaOH 4M.

[0067] The experiments were carried out at room temperature, at pH 10.5-10.7, under stirring for 3 h. Table 1 summarises the experiments.

Table 1 : Preparation of alumina@LDH spheres according to Method B

a theoretical wt%

[0068] All alumina@LDH spheres obtained using the conditions in Table 1 were analysed by SEM and SEM-EDX (Figure 6, omitting the spheres of experiment 2, which were visually identical to the spheres of experiment 1 , as well as the spheres of experiment 7, which were visually identical to the spheres of experiment 6).

[0069] Figures 6a-1 and a-2 show that when very small amounts of magnesium nitrate are used, the surface of the spheres presents, from place to place, spongy-like material. SEM-EDX analysis confirms the presence of both Mg and Al and the TGA analysis shows the presence of 2.92% of a new material.

[0070] Figure 6b-1 shows that the experiment 3 spheres (in which 3.78% wt of Al was assumed) are completely covered with the spongy-like material. A close-up view shows the rosette shape of the LDH. The TGA analysis shows the presence of 8.82% of a new material grown over the spheres.

[0071] Assuming a greater amount of leaching Al did not lead to the formation of more LDH, but instead led to some damage of the spheres, as can be observed in Figure 6c to g. Without wishing to be bound by theory, it is believed that this damage is due to the fact that when a greater amount of leaching Al is assumed; more magnesium nitrate is added, meaning that a greater volume of NaOH is used. It is believed that the increased volume of NaOH leads to rapid leaching of Al from the spheres, thus causing deterioration.

[0072] The results suggest that the optimum conditions for preparing the alumina@LDH spheres were those of experiment 3 (assuming 3.78% wt leaching Al). To confirm this, experiment 3 was repeated, but for a reduced duration of 1 hour. Figure 7 demonstrates that the conditions of experiment 3 are suitable for growing LDH on the surface of the alumina spheres in only 1 hour. [0073] Attempts were made to further study and characterise the spheres, although their large diameter made this challenging. T o address this, the surface of the spheres was scratched in an attempt to analyse the material using TEM. The results (Figure 8) show that the LDH shell is directly connected to the alumina core. Without wishing to be bound by theory, it is believed that this good connectivity between core and shell is likely due to the use of Al leaching from the alumina core (as opposed to using a separate source of Al).

PART - Scale-up studies

Example 3 - Preparation of AMO-treated LDH-coated alumina particles (Method B)

A) General procedure

[0074] Mg(N0 ₃ ) ₂ .6H ₂ 0 (0.942 g of) was added to 4.1602 g of alumina spheres (1 diameter) in 35 ml_ water. The pH of the mixture was adjusted to ~ 10 using 4M NaOH and the entire reaction was stirred for 2 h at room temperature. Finally, the resulting LDH-coated alumina particles were collected by filtration, and then washed with water and ethanol (AMO solvent).

B) Characterisation

[0075] The alumina@LDH spheres were analysed by SEM. Fig. 9A shows the surface of a sphere after growing LDH thereon. Fig. 9B shows a close-up of the sphere, where one can see the presence of small platelets, proving the presence of LDH.

Example 4 - Preparation of LDH-coated alumina particles (Method B)

A) General procedure

[0076] Mg(N0 ₃ ) ₂ .6H ₂ 0 (1.884 g) was added to 4.1619 g of alumina spheres (1 mm diameter) in 30 mL water. The pH of the mixture was adjusted to ~ 10 using a 4M solution of a mixture of NaOH and Na ₂ C0 ₃ . The resulting mixture was then stirred for 2 h at room temperature. Finally, the resulting LDH-coated alumina particles were collected by filtration, and then washed with water.

B) Characterisation

[0077] The alumina@LDH spheres were analysed by SEM. Figs. 10A and B show the surface of a sphere after growing LDH thereon. Figs. 10C and D show close-ups of the sphere, where one can see the presence of material having flower-shaped morphology, thus proving the presence of LDH.

[0078] While specific embodiments of the invention have been described herein for the purpose of reference and illustration, various modifications will be apparent to a person skilled in the art without departing from the scope of the invention as defined by the appended claims.