PROCESSES OF MAKING THEM

INTRODUCTION

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

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 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. 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 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, the process comprising / consisting essentially of / consisting of the steps of:

a) providing a plurality of alumina particles, the particles having an average particle size of ³50 pm;

b) contacting the plurality of alumina particles with a metal cation-containing

solution comprising a mixture of M ^z+ and M’ ^y+ , wherein

M ^z+ is a metal cation of charge z or a mixture of two or more metal cations each independently having charge z,

M’ ^y+ is a metal cation of charge y or a mixture of two or more metal cations each independently having charge y, wherein M is different from M’,

z is 1 or 2,

y is 3 or 4;

c) isolating the plurality of alumina particles resulting from step b) from the metal cation-containing solution,

d) contacting the isolated plurality of alumina particles with a basic solution to

adjust the pH of the particles to 9.5 - 11 ;

e) optionally isolating the plurality of alumina particles resulting from step d) from the basic solution and then repeating steps b) to d) at least one more time;

f) optionally isolating the plurality of alumina particles resulting from step d) or e) and washing the isolated particles with water; and

g) optionally drying the washed plurality of 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 plurality of particles, the particles each comprising / consisting essentially of / consisting of an alumina core having an average particle size of ³50 pm and being coated with a layered double hydroxide comprising metal cations M ^z+ and M’ ^y+ , wherein

M ^z+ is a metal cation of charge z or a mixture of two or more metal cations each independently having charge z,

M’ ^y+ is a metal cation of charge y or a mixture of two or more metal cations each independently having charge y, wherein M is different from M’,

z is 1 or 2, and

y is 3 or 4.

[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 or third aspect in a catalytic process.

[0012] According to a fifth aspect of the present invention there is provided a use of a plurality of LDH-coated alumina particles according to the second or third aspect as a support material.

DETAILED DESCRIPTION OF THE INVENTION

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

[0013] The present invention provides a process for the preparation of a plurality of layered double hydroxide-coated alumina particles, the process comprising the steps of:

a) providing a plurality of alumina particles, the particles having an average particle size of ³50 pm;

b) contacting the plurality of alumina particles with a metal cation-containing

solution comprising a mixture of M ^z+ and M’ ^y+ , wherein

M ^z+ is a metal cation of charge z or a mixture of two or more metal cations each independently having charge z,

M’ ^y+ is a metal cation of charge y or a mixture of two or more metal cations each independently having charge y, wherein M is different from M’,

z is 1 or 2,

y is 3 or 4;

c) isolating the plurality of alumina particles resulting from step b) from the metal cation-containing solution,

d) contacting the isolated plurality of alumina particles with a basic solution to

adjust the pH of the particles to 9.5 - 11 ;

e) optionally isolating the plurality of alumina particles resulting from step d) from the basic solution and then repeating steps b) to d) at least one more time; f) optionally isolating the plurality of alumina particles resulting from step d) or e) and washing the isolated particles with water; and

g) optionally drying the washed plurality of alumina particles.

[0014] As described hereinbefore, 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. 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. In the processes of W02017/009664 and WO2016/1 10698, the various reagents required to form the core@shell material (core substrate, LDH cation precursor solutions, LDH anion precursor solution and base) are reacted in a one-pot manner to yield the small particle size product. 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. The inventors were surprised to discover that the particular core@shell preparation process described in WO2017/009664 and WO2016/110698 was, however, rendered entirely inoperable when the particle size of the core to be coated was increased beyond the small particle sizes described therein.

[0015] 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. The inventors determined that, in contrast to the type of one-pot preparation process described in WO2017/009664 and WO2016/110698, a multi-stage process is necessary in order for an LDH shell to be successfully coated on the surface of the larger alumina core spheres. In particular, it has been found that after first contacting the alumina core spheres with the LDH cation precursor solution (e.g. an aqueous solution of magnesium nitrate and aluminium nitrate), the resulting spheres must be then isolated from that solution prior to being contacted with a base to adjust the pH to cause the formation of the LDH. The inventors were particularly surprised to be able to prepare larger particle size alumina@LDH core@shell particles by such a process, especially considering the emphasis that W02017/009664 and WO2016/1 10698 place on a one-pot technique.

[0016] Across all aspects of the invention, the terms“plurality of LDH-coated alumina particles” and“alumina@LDH” particles are used interchangeably.

[0017] 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.

[0018] 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.

[0019] It will be understood that the metal cation-containing solution comprises a mixture of both M ^z+ and M’ ^y+ , as defined herein, even before the alumina particles come into contact with it. In an embodiment, step b) comprises the sub-steps of: i) providing a metal cation-containing solution comprising a mixture of both M ^z+ and M’ ^y+ , as defined herein; and ii) contacting the plurality of alumina particles with the metal cation-containing solution.

[0020] The metal cation-containing solution of step b) is suitably an aqueous solution.

[0021] In an embodiment, step b) comprises immersing the plurality of alumina particles of step a) in the metal cation-containing solution. Step b) suitably proceeds with stirring. [0022] In an embodiment, the temperature of the metal ion containing solution in step b) is within a range of from 20 to 150°C, suitably from 20 to 80°C, more suitably from 20 to 50°C and most suitably from 20 to 40°C.

[0023] 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⁺ . More suitably, M ^z+ is Mg ²⁺ or Zn ²⁺ . Even more suitably, M ^z+ is Mg ²⁺ . Most suitably, M ^z+ is Mg ²⁺ , and is present in the metal cation- containing solution as magnesium nitrate.

[0024] In an embodiment, M’ ^y+ is selected from Al ³⁺ , Ga ³⁺ , V ³⁺ , ln ³⁺ , Y ³⁺ and Fe ³⁺ . Suitably, M’ ^y+ is Al ³⁺ or ln ³⁺ . More suitably, M’ ^y+ is Al ³⁺ . Even more suitably, M’ ^y+ is Al ³⁺ , and is present in the metal cation-containing solution as aluminium nitrate.

[0025] In a particular embodiment, M ^z+ is Mg ²⁺ or Zn ²⁺ and M’ ^y+ is Al ³⁺ or ln ³⁺ . Suitably, M ^z+ is Mg ²⁺ and M’ ^y+ is Al ³⁺ .

[0026] It will be understood that a small quantity of Al ³⁺ may leach from the alumina particles after the particles come into contact with the metal cation-containing solution during step b). This small quantity of leached Al ³⁺ is not encompassed by the definition of M’ ^y+ appearing herein.

[0027] In a particular embodiment, the metal cation-containing solution is an aqueous solution of magnesium nitrate and aluminium nitrate (in which case M ^z+ is Mg ²⁺ and M’ ^y+ is Al ³⁺ ).

[0028] It will be understood that the relative quantities of M ^z+ and M’ ^y+ must be sufficient to form a layered double hydroxide. For example, when the LDH shell is formed from a MgAI LDH (e.g. MgAI-CCh), the relative amounts of Mg ²⁺ and Al ³⁺ within the metal cation-containing solution may be varied to afford a Mg _x AI LDH in which 1 8£x£5.2.

[0029] In step c), the plurality of alumina particles resulting from step b) is isolated from the metal cation-containing solution. In an embodiment, the plurality of alumina particles resulting from step b) is separated from the metal cation-containing solution, for example by filtering the mixture resulting from step b). It will be understood by the skilled person that residual metal cation- containing solution present on the surface of the alumina spheres following isolation/separation is a key component in the eventual formation of the LDH shell. Therefore, the skilled person will appreciate that isolating, or separating, the plurality of alumina particles resulting from step b) from the metal cation-containing solution does not mean removing all of the metal cation- containing solution from the alumina spheres.

[0030] In step c) the particles isolated from step b) are contacted with a basic solution having a pH of 9.5 - 1 1. Upon contacting the particles with the basic solution, the LDH is formed, by coprecipitation, as a shell coating around the alumina core. Suitably the pH of the basic solution is 10 - 1 1. In an embodiment, the basic solution comprises NaOH. [0031] In an embodiment, step c) comprises immersing the particles isolated from step b) in the basic solution, optionally with stirring.

[0032] 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 one or more of steps b) and c). The anion(s) may be selected from carbonate, bicarbonate, hydroxide, hydrogenphosphate, dihydrogenphosphate, nitrite, nitrate, borate, sulphate, halide and phosphate.

[0033] In an embodiment, the anion may be carbonate, and may be derived from CO2 in the atmosphere.

[0034] In a particularly suitable embodiment, at least one anion is present in either or both of the metal cation-containing solution (in step b)) and the basic solution (in step c)). Whether the anion is present in the metal cation-containing solution will depend on whether it forms, with M ^z+ or M’ ^y+ , a water-soluble compound (such as magnesium nitrate or aluminium nitrate). If the anion would form a water-insoluble compound with M ^z+ or M’ ^y+ , it is instead introduced into the basic solution. For example, if M ^z+ is Mg ²⁺ , M’ ^y+ is Al ³⁺ and the anion is carbonate, the anion would be present in the basic solution, otherwise it would precipitate in the metal cation-containing solution as magnesium carbonate and aluminium carbonate.

[0035] In a particular embodiment, the anion is nitrate. Suitably, the anion is present in the metal cation-containing solution as magnesium nitrate and aluminium nitrate.

[0036] In a particular embodiment, the LDH that forms as a shell on the surface of the alumina core in step d) is a MgAI-NC>3 LDH. More suitably, the LDH that forms as a shell on the surface of the alumina core in step d) is a Mg _x AI-NC>3 LDH, in which 1.8£x£5.2.

[0037] Step e) is an optional step. When step e) is employed, it comprises isolating the plurality of alumina particles resulting from step d) from the basic solution and then repeating steps b) to d) at least one more time. Step e) can be used to increase the quantity of LDH shell on the alumina core. In an embodiment, step e) comprises isolating the plurality of alumina particles resulting from step d) from the basic solution and then repeating steps b) to d) 1 to 5 more times. Suitably, step e) comprises isolating the plurality of alumina particles resulting from step d) from the basic solution and then repeating steps b) to d) 2 or 3 more times. The particles isolated from step d) may be washed with water prior to repeating steps b) to d).

[0038] Optional step f) can follow step d) or e), depending on whether step e) is implemented. Step f) comprises isolating (e.g. by filtration) the plurality of alumina particles resulting from step d) or e) and washing the isolated particles with water. Suitably, the plurality of alumina particles resulting from step d) or e) are washed extensively with water, typically until the pH of the water washings is 6.5-7.5.

[0039] Optional step g) comprises drying the washed plurality of alumina particles. In an embodiment, step g) comprises drying the washed plurality of alumina particles under vacuum.

[0040] It will be understood that when steps f) and g) are not employed, the plurality of LDH- coated alumina particles are provided as a dispersion or suspension. Suitably, both of steps f) and g) are employed.

[0041] In an embodiment, after step f), but prior to step g), the washed particles are contacted with an organic solvent capable of hydrogen bonding to water. Suitably, the solvent is acetone or ethanol. It has been determined that carrying out step g) directly after step f) 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. 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. In a particular embodiment, after step f), but prior to step g), the washed particles are dispersed in the organic solvent capable of hydrogen bonding to water.

[0042] In an embodiment, steps a) to f) are conducted at a temperature of 5-70°C. Suitably, steps a) to f) are conducted at a temperature of 10-45°C.

[0043] 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.

Plurality of layered double hydroxide-coated alumina particles

[0044] The present invention also provides a plurality of particles, the particles each comprising an alumina core having an average particle size of ³50 pm and being coated with a layered double hydroxide comprising metal cations M ^z+ and M’ ^y+ , wherein M ^z+ is a metal cation of charge z or a mixture of two or more metal cations each independently having charge z,

M’ ^y+ is a metal cation of charge y or a mixture of two or more metal cations each independently having charge y, wherein M is different from M’,

z is 1 or 2, and

y is 3 or 4.

[0045] As alluded to hereinbefore, the larger particle size alumina@LDH core@shell particles of the invention offer a plethora of advantages over smaller particle size (e.g. powder) core@LDH particles. In particular, the alumina@LDH core@shell particles of the invention are suitable for direct use in a variety of industrial applications, including as supports in fixed bed reactors or fluidized bed processes, thereby obviating the numerous disadvantages associated with pelletising smaller particle size (e.g. powder) core@LDH particles.

[0046] It will be understood that the LDH coating on 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.

[0047] The average particle size of the alumina core, and/or that of the resulting alumina@LDH particles, is ³50 pm (e.g. 0.5 mm 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.

[0048] 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 .

[0049] 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⁺ . More suitably, M ^z+ is Mg ²⁺ or Zn ²⁺ . Most suitably, M ^z+ is Mg ²⁺ .

[0050] In an embodiment, M’ ^y+ is selected from Al ³⁺ , Ga ³⁺ , V ³⁺ , ln ³⁺ , Y ³⁺ and Fe ³⁺ . Suitably, M’ ^y+ is Al ³⁺ or ln ³⁺ . Most suitably, M’ ^y+ is Al ³⁺ .

[0051] In a particular embodiment, M ^z+ is Mg ²⁺ or Zn ²⁺ and M’ ^y+ is Al ³⁺ or ln ³⁺ . Suitably, M ^z+ is Mg ²⁺ and M’ ^y+ is Al ³⁺ .

[0052] In an embodiment, the layered double hydroxide comprises at least one anion selected from carbonate, bicarbonate, hydroxide, hydrogenphosphate, dihydrogenphosphate, nitrite, nitrate, borate, sulphate, halide and phosphate. Suitably, the at least one anion is selected from carbonate and nitrate.

[0053] In a particular embodiment, the LDH shell/coating is formed from a Mg _x AI LDH in which 1.8£x£5.2. Suitably, the LDH shell/coating is formed from a Mg _x AI-CC>3 or Mg _x AI-NC>3 LDH in which 1.8£x£5.2.

[0054] In an embodiment, each of the particles is of the general formula (I):

Tp @ {[M ^z+ _(1. x ₎ M'x'' ⁺ (0H) ₂ ] ^a+ (X ⁿ -)a _/ n*bH ₂ 0} _q

(l) wherein

M, M’, z and y have any of the definitions appearing hereinbefore,

T is the alumina core,

0 < x < 0.9

b is 0 to 10

p > 0;

q > 0;

p+q = 1 ;

X ^n_ is an anion as defined hereinbefore, with a charge n of > 0

a = z(1-x) + xy-2; and

@ denotes that T is coated with {[M ^z+ _{(i -X)} M' _x ^y+ (0H) ₂ ] ^a+ (X ⁿ ) _a/n* bH ₂ 0}q. [0055] In an embodiment, each of the particles is of the general formula (I), wherein M ^z+ is selected from Ca ²⁺ , Mg ²⁺ and Ni ²⁺ , M’ ^y+ is Al ³⁺ and X ^n_ is selected from carbonate and nitrate.

[0056] In an embodiment, each of the particles is of the general formula (I), wherein M ^z+ is Mg ²⁺ , M’ ^y+ is Al ³⁺ and X ^n_ is selected from carbonate and nitrate.

[0057] As discussed hereinbefore in relation to the first aspect of the invention, prior to being dried, the alumina@LDH particles may be contacted with an organic solvent capable of hydrogen bonding to water. It has been determined that drying LDHs directly from water results in an LDH having a lower surface area and/or pore volume. Without wishing to be bound by theory, the inventors have hypothesised that by employing a 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 in a finer LDH powder having high surface area. 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. Accordingly, in an embodiment, each of the particles may be of the general formula (II):

T _p @ {[M ^z+ _(i -x ₎ M'x ^y+ (OH) ₂ ] ^a+ (X ⁿ )a _/ n*bH ₂ 0 ^, c(solvent)} _q

(II) wherein

M, M’, z and y have any of the definitions appearing hereinbefore,

T is the alumina core,

0 < x < 0.9

b is 0 to 10

0 £ c £ 10 (e.g. 0 < c £ 10)

p > 0;

q > 0;

p+q = 1 ;

X ^n_ is an anion as defined hereinbefore, with a charge n of > 0

a = z(1-x) + xy-2;

solvent is an organic solvent capable of hydrogen bonding to water; and

@ denotes that T is coated with {[M ^z+ _{(i -X)} M' _x ^y+ (0H) ₂ ] ^a+ (X ⁿ )a _/ n*bH ₂ 0}q. [0058] Suitably, the solvent is selected from acetone and ethanol. More suitably, the solvent is ethanol.

[0059] In an embodiment, each of the particles is of the general formula (II), wherein M ^z+ is selected from Ca ²⁺ , Mg ²⁺ and Ni ²⁺ , M’ ^y+ is Al ³⁺ and X ^n_ is selected from carbonate and nitrate.

[0060] In an embodiment, each of the particles is of the general formula (II), wherein M ^z+ is Mg ²⁺ , M’ ^y+ is Al ³⁺ and X ^n_ is selected from carbonate and nitrate.

[0061] It will be understood that preferred, suitable or optional features of the third aspect of the invention may also be preferred, suitable or optional features of the second aspect of the invention.

Applications

[0062] The present invention also provides a use of a plurality of LDH-coated alumina particles according to the second or third aspect in a catalytic process.

[0063] As alluded to hereinbefore, the larger particle size alumina@LDH core@shell particles of the invention offer a plethora of advantages over smaller particle size (e.g. powder) core@LDH particles. In particular, the alumina@LDH core@shell particles of the invention are suitable for direct use in a variety of industrial applications, thereby obviating the numerous disadvantages associated with pelletising smaller particle size (e.g. powder) core@LDH particles.

[0064] In an embodiment, the plurality of LDH-coated alumina particles are used as a support material in a catalytic process. Suitably, the LDH-coated alumina particles are used as a support material in a fixed bed or fluidized bed catalytic process.

[0065] According to a fifth aspect of the present invention there is provided a use of a plurality of LDH-coated alumina particles according to the second or third aspect as a support material (e.g. in a non-catalytic process).

[0066] The following numbered statements 1 to 54 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, the process comprising the steps of:

a) providing a plurality of alumina particles, the particles having an average particle size of ³50 pm; b) contacting the plurality of alumina particles with a metal cation-containing solution comprising a mixture of M ^z+ and M’ ^y+ , wherein

M ^z+ is a metal cation of charge z or a mixture of two or more metal cations each independently having charge z,

M’ ^y+ is a metal cation of charge y or a mixture of two or more metal cations each independently having charge y, wherein M is different from M’,

z is 1 or 2,

y is 3 or 4;

c) isolating the plurality of alumina particles resulting from step b) from the metal cation-containing solution,

d) contacting the isolated plurality of alumina particles with a basic solution to adjust the pH of the particles to 9.5 - 11 ;

e) optionally isolating the plurality of alumina particles resulting from step d) from the basic solution and then repeating steps b) to d) at least one more time; f) optionally isolating the plurality of alumina particles resulting from step d) or e) and washing the isolated particles with water; and

g) optionally drying the washed plurality of alumina particles.

The process of statement 1 , wherein at least one of the metal cation-containing solution of step b) and the basic solution of step d) comprises a source of anions.

The process of statement 2, wherein the anions are selected from one or more of carbonate, bicarbonate, hydroxide, hydrogenphosphate, dihydrogenphosphate, nitrite, nitrate, borate, sulphate, halide or phosphate.

The process of statement 3, wherein the anions are selected from one or more of carbonate and nitrate.

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 - 5.0 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 statement 10, wherein the plurality of alumina particles provided in step a) have an average particle size of 0.85 - 3.0 mm.

The process of statement 11 , 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, Mn, Co, Cu and Ni.

The process of any preceding statement, wherein M is selected from Ca, Mg, Zn and Ni. The process of any preceding statement, wherein M is selected from Mg and Zn.

The process of any preceding statement, wherein M is Mg

The process of any preceding statement, wherein M’ is selected from Al, Ga, V, In, Y and Fe.

The process of any preceding statement, wherein M’ is Al or In.

The process of any preceding statement, wherein M’ is Al.

The process of any preceding statement, wherein M is Mg and is present in the metal cation-containing solution as magnesium nitrate.

The process of any preceding statement, wherein M’ is Al and is present in the metal cation-containing solution as aluminium nitrate.

The process of any preceding statement, wherein step b) comprises immersing the plurality of alumina particles in the metal cation-containing solution.

The process of any preceding statement, wherein the basic solution comprises NaOH. The process of any preceding statement, wherein step d) comprises immersing the isolated plurality of alumina particles in a basic solution.

The process of any preceding statement, wherein step d) comprises contacting the isolated plurality of alumina particles with a base to adjust the pH of the particles to 10 - 11.

The process of any preceding statement, wherein step e) comprises isolating the plurality of alumina particles resulting from step d) from the base and then repeating steps b) to d) 2 or 3 more times.

The process of any preceding statement, wherein step f) comprises isolating the plurality of alumina particles resulting from step d) or e) and washing the isolated particles with water until the pH is 6.5-7.5.

The process of any preceding statement, wherein after step f) but prior to step g), the washed particles are contacted with an organic solvent capable of hydrogen bonding to water.

The process of statement 28, wherein the step of contacting the washed particles with an organic solvent capable of hydrogen bonding to water involves dispersing the washed particles in an organic solvent capable of hydrogen bonding to water. The process of statement 28 or 29, wherein the organic solvent capable of hydrogen bonding to water is selected from acetone and ethanol.

The process of any preceding statement, wherein step g) comprises drying the washed plurality of alumina particles under vacuum.

The process of any preceding statement, wherein steps a) to f) are conducted at a temperature of 5-70°C.

The process of any preceding statement, wherein steps a) to f) are conducted at a temperature of 10-45°C.

A plurality of LDH-coated alumina particles obtainable by the process of any preceding statement.

A plurality of particles, the particles each comprising an alumina core having an average particle size of ³50 pm and being coated with a layered double hydroxide comprising metal cations M ^z+ and M’ ^y+ , wherein

M ^z+ is a metal cation of charge z or a mixture of two or more metal cations each independently having charge z,

M’ ^y+ is a metal cation of charge y or a mixture of two or more metal cations each independently having charge y, wherein M is different from M’,

z is 1 or 2, and

y is 3 or 4.

The plurality of particles of statement 35, wherein the particles each comprise an alumina core having an average particle size of ³100 pm.

The plurality of particles of statement 35, wherein the particles each comprise an alumina core having an average particle size of ³200 pm.

The plurality of particles of statement 35, wherein the particles each comprise an alumina core having an average particle size of ³500 pm.

The plurality of particles of statement 35, wherein the particles each comprise an alumina core having an average particle size of 0.80 - 5.0 mm.

The plurality of particles of statement 35, wherein the particles each comprise an alumina core having an average particle size of 0.80 - 3.0 mm.

The plurality of particles of statement 35, wherein the particles each comprise an alumina core having an average particle size of 0.85 - 5.0 mm.

The plurality of particles of statement 35, wherein the particles each comprise an alumina core having an average particle size of 0.85 - 3.0 mm.

The plurality of particles of statement 35, wherein the particles each comprise an alumina core having an average particle size of 0.90 - 1.5 mm.

The plurality of particles of any one of statements 35 to 43, wherein M is selected from Li, Ca, Mg, Zn, Fe, Co, Cu and Ni. The plurality of particles of any one of statements 35 to 44, wherein M is selected from Ca, Mg, Zn and Ni.

The plurality of particles of any one of statements 35 to 45, wherein M is selected from Mg and Zn.

The plurality of particles of any one of statements 35 to 46, wherein M’ is selected from Al, Ga, In, Y and Fe.

The plurality of particles of any one of statements 35 to 47, wherein M’ is Al or In. The plurality of particles of any one of statements 35 to 48, wherein M’ is Al.

The plurality of particles of any one of statements 35 to 49, wherein the layered double hydroxide comprises at least one anion selected from carbonate, bicarbonate, hydrogenphosphate, dihydrogenphosphate, nitrite, nitrate, borate, sulphate, halide and phosphate.

The plurality of particles of statement 50, wherein the at least one anion is selected from carbonate and nitrate.

The plurality of particles of any one of statements 35 to 51 , wherein the particles each have the general formula (I) shown below:

Tp @ {[M ^z+ _(1.x) M' _x '' ⁺ (0H) ₂ ] ^a+ (X ⁿ -) _a/n* bH ₂ 0} _q (I) wherein

M, M’, z and y are as defined in any one of statements 35 to 49,

T is the alumina core,

0 < x < 0.9

b is 0 to 10

p > 0;

q > 0;

p+q = 1 ;

X ⁿ is an anion as defined in statement 50 or 51 ; with n > 0

a = z(1-x) + xy-2; and

@ denotes that T is coated with {[M ^z V _x) M' _x ^y+ (0H) ₂ ] ^a+ (X ⁿ ) _a/n* bH ₂ 0} _q

The plurality of particles of any one of statements 35 to 51 , wherein the particles each have the general formula (II) shown below:

T _P @ {[M ^z+ _(1.x) MV ⁺ (OH) ₂ ] ^a+ (X ⁿ -) _a/ n*bH ₂ 0*c(solvent)} _q (II) wherein M, M’, z and y are as defined in any one of statements 35 to 49,

T is the alumina core,

0 < x < 0.9

b is 0 to 10

0 £ c £ 10

p > 0;

q > 0;

p+q = 1 ;

X ^n_ is an anion as defined in statement 50 or 51 ; with n > 0

a = z(1-x) + xy-2;

solvent is an organic solvent capable of hydrogen bonding to water; and

@ denotes that T is coated with {[M ^z+ _{(i -X)} M' _x ^y+ (OH) ₂ ] ^a+ (X ⁿ ) _a/n* bH ₂ 0c(solvent)} _q

54. The plurality of particles of statement 53, wherein the solvent is acetone or ethanol.

EXAMPLES

[0067] 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 1 mm diameter alumina spheres: a) overview before coating; b) close-up before coating; c) overview after coating with LDH according to Example 1A; d) close- up after coating with LDH according to Example 1A.

Fig. 2 shows SEM images of 1 mm diameter alumina spheres: a) overview after attempted coating with LDH according to Comparative Example 1A; b) close-up after attempted coating with LDH according to Comparative Example 1A.

Fig. 3 shows the 1 mm diameter alumina spheres after 25 minutes sonication as part of Comparative Example 1A.

Fig. 4 shows SEM images of 2.5 mm diameter alumina spheres: a) after coating with LDH according to Example 2A; b) close-up after coating with LDH according to Example 2A.

Fig. 5 shows SEM images of 2.5 mm diameter alumina spheres: a)-c) after coating with LDH according to Example 3A. Materials and methods

[0068] Magnesium nitrate, aluminium nitrate, sodium carbonate and sodium hydroxide were purchased from Sigma Aldrich. Alumina spheres were provided by SCG Chemicals Co., Ltd., Thailand.

[0069] Scanning electron microscopy (SEM) was performed using a JEOL JSM 6610 microscope, with an accelerating voltage of 20 Kv.

[0070] 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.

Example 1 - Mg ₂ AI LDH-coated alumina particles (1 mm diameter)

A) Preparation

[0071] Having regard to Scheme 1 below, 100 g of alumina spheres (1 mm diameter) were immersed in a 20 ml_ solution containing Mg(NC> ₃ ) ₂ .6H ₂ 0 (0.246 g) and AI(NC> ₃ ) ₃ .9H ₂ 0 (0.180 g). After 1 h, the spheres were removed from the metal precursor solution and immersed in a 20 ml of 2M NaOH and left there for 30 minutes to an hour. The procedure was repeated three times, after which the spheres were washed with deionised water until the pH decreased to 7 and dried under vacuum. The experiment was performed at room temperature.

Scheme 1 - Synthetic pathway

B) Characterisation

[0072] To prove the presence of LDH on the alumina spheres, the particles of Example 1A were analysed using SEM and SEM-EDX. The uncoated alumina spheres having the diameter of 1 mm present a shiny surface with several surface defects, as can be seen from Figs. 1a) and 1 b). After coating using the process of Example 1A, the surface of the spheres totally changes, and is covered by a spongy material, indicative of LDH (see Figs. 1c) and 1d)).

Example 2 - Mq ₃ AI LDH-coated alumina particles (2.5 mm diameter)

A) Preparation

[0073] A 2.5 mm AI ₂ O ₃ sphere was added to a solution containing 0.228 g Mg(N0 ₃ ) ₂ .6H ₂ 0 and 0.11 1 g AI(NC> ₃ ) ₃ 9H ₂ 0 in 3 ml_ H O and was left immersed for 10 minutes. After this, the sphere was removed from the solution and introduced into a 4M NaOH solution for 5 minutes. The sphere was then washed with water and the procedure was repeated 3 times.

B) Characterisation

[0074] The LDH-coated alumina particles formed according to Example 2A were analysed by SEM. Fig. 4a) shows the surface of a sphere after growing the LDH thereon. Fig. 4b) shows a close-up of the sphere depicted in Fig. 4a), where the presence of small platelets are noticeable, proving the presence of LDH.

Example 3 - Zn¾ln LDH-coated alumina particles (2.5 mm diameter)

A) Preparation

[0075] A 2.5 mm AI ₂ O ₃ sphere was added in a solution containing 0.194 g Zn(NC> ₃ ) ₂ .6H ₂ 0 and 0.055 g ln(NC> ₃ ) ₃ 9H ₂ 0 in 3 ml_ H O and was left immersed immersed for 5 minutes. After this, the sphere was removed from the solution and introduced into a 4M NaOH solution for 1 minute. The sphere was then washed with water and the procedure was repeated 3 times.

B) Characterisation

[0076] The LDH-coated alumina particles formed according to Example 3A were analysed by SEM. Figs. 5a), b) and c) show the surface of the sphere after growing the LDH thereon. LDH platelets are clearly visible. Comparative Example 1 - Attempted preparation of LDH-coated alumina particles using conventional technique

A) Preparation

[0077] Adapting the synthetic protocol described in W02017/009664 and WO2016/110698, 100 mg alumina spheres (1 mm diameter) were added to 20 mL of Dl water and sonicated for 25 minutes. Sodium carbonate (Na2CC>3, 0.96 mmol) was then added to the solution and a further 5 minutes of sonication was carried out. Separately, magnesium nitrate hexahydrate (Mg(NC> ₃ ) ₂ .6H ₂ 0, 0.96 mmol) and aluminium nitrate nonahydrate (AI(NC> ₃ ) ₃ .9H ₂ 0, 0.48 mmol) were dissolved in D.l. H2O (20 mL), denoted solution A. Solution A was added to the beaker containing the spheres keeping the pH constant at 10 using 1 M NaOH.

B) Characterisation

[0078] SEM analysis of the product resulting from Comparative Example 1A reveals that the protocol did not afford LDH-coated alumina particles. In particular, Figs. 2a) and b) show that even after attempting to apply a coating of LDH to the alumina sphere, the spheres still possess a smooth and shiny outer surface, which is comparable to that of the uncoated alumina surface depicted in Figs. 1 a) and b).

[0079] Without wishing to be bound by theory, it is believed that the limited dispersability of the larger 1 mm alumina spheres in the reaction medium hampers the formation of an LDH coating according to the W02017/009664 / WO2016/1 10698 synthetic protocol (see Fig. 3). Furthermore, it is to be noted that as soon as magnesium and aluminium nitrate come into contact with sodium hydroxide, the LDH will form instantly. Therefore, although LDH forms in the bulk of the reaction medium, it does not form on the surfaces of the poorly dispersed alumina sphere. As more NaOH is added to keep the pH constant, the alumina spheres start to dissolve.

[0080] 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.