Porous pellets, methods of production and uses thereof

Abstract

In various embodiments, the present invention provides a method for forming a porous pellet, a porous pellet obtained or obtainable according to such methods, and uses of such porous pellets The method comprising steps of forming a slurry from slurry components including a particulate inorganic material, a first solvent, and polymeric binder; immersing droplets of the slurry in a second solvent, the second solvent being selected to be miscible with the first solvent and immiscible with the polymeric binder, to form a precursor pellet; and performing a heat treatment on the precursor pellet to thereby form the porous pellet.

Description

[0001] Porous pellets, methods of production and uses thereof

[0002] This application claims priority from GB 2317430.3 filed 14 November 2023, the contents and elements of which are herein incorporated by reference for all purposes.

[0003] Field of the Invention

[0004] The present invention relates to porous pellets, methods of production and uses thereof and particularly, although not exclusively, to porous pellets having a degree of hierarchical microstructure which are suitable for use as catalyst substrates.

[0005] Background

[0006] Porous pellets have a wide variety of possible applications - for example, they may find use as catalyst supports, as adsorbents, in filtration applications, and may other application. The use of porous pellets as catalyst supports is particular prevalent. Typically, such pellets are formed by processes including tableting, extrusion or granulation to form a pellet from one or more precursor materials. The resultant pellets typically have an amorphous microstructure, including a random pore network that is isotropic in nature. This network typically consists of macro-pores formed by the random packing of micro- or meso- porous particles, which is sometimes referred to as a bi-dispersed pore structure. However, this porous structure presents a challenge when it comes to optimizing the diffusion and specific surface area of interest. The structural parameters, such as porosity, pore size, pore size distribution, and tortuosity, are interdependent and affect each other. As a result, the effective diffusivity (De) where DAB= bulk diffusivity, E = pellet porosity, oc= construction factor, and T = tortuosity) is not easily tunable in such known pellets, and it is difficult to optimize <p, the Thiele modulus, which is a parameter relating the reaction rate to the diffusion rate, with a lower <p value indicating a more uniform concentration profile within the catalyst for stoichiometric reactions.

[0007] Industrial pellets, which are commonly used to reduce pressure drops in fixed-bed reactors, are typically 1-3 mm in size and can suffer from diffusion limitation issues. Alternative reactor configurations, such as fluidized and slurry bed reactors, can mitigate these issues by using fine catalyst particles with negligible diffusion problems. However, these configurations come with their own set of challenges, such as complex control, high energy costs, clogging, and separation / recovery of catalyst. Since the first report of the Thiele modulus in 1939, controlling the dimensions of catalyst particles / pellets has remained the primary approach for adjusting <p and diffusion limitations. Although various modeling studies have revealed the importance of catalyst pore structure, the random isotropic pore network, sometimes modified with pore templates for a higher void fraction, still dominates catalyst pore structures. This creates a dilemma between catalyst pellet size and diffusion limitation.

[0008] The present invention has been devised in light of the above considerations. Summary of the Invention

[0009] It is known in the art to use micro-structured inorganic substrates as catalyst supports - typically such supports are in the form of ceramic membranes, which are formed by various processes, including precipitation from the vapor phase, precipitation through controlled evaporation, thermally induced phase separation, and immersion precipitation.

[0010] The present inventors have realised that phase inversion immersion precipitation techniques applied to create micro-structured inorganic substrates such as ceramic membranes can be modified in order to create porous pellets having an anisotropic or asymmetric pore network, and in particular a pore network having degree of hierarchical microstructure.

[0011] Accordingly, in a first aspect, the present disclosure provides a method for forming a porous pellet, the method comprising steps of: forming a slurry from slurry components including a particulate inorganic material, a first solvent, and polymeric binder; immersing droplets of the slurry in a second solvent, the second solvent being selected to be miscible with the first solvent, and immiscible with the polymeric binder, to form a precursor pellet; and performing a heat treatment on the precursor pellet to thereby form the porous pellet.

[0012] In a second aspect, the present disclosure provides a porous pellet obtained or obtainable according to the first aspect.

[0013] In this above method, when the droplets of the slurry are immersed in the selected second solvent which is both miscible with the first solvent of the slurry and immiscible with the polymeric binder of the slurry, a phase inversion process takes place, thereby allowing for formation of pellets having an anisotropic or asymmetric pore network. The pore network may comprise a hierarchical microstructure: in particular, the pore network may comprise radial micro-channels in a central region, surrounded by a denser surface layer (also referred to herein as a ‘skin layer’). Provision of pellets having such a microstructure may provide a significant improvement over conventional pellets comprising a conventional isotropic pore network. This is because circuitous diffusion routes in conventional pellets are replaced with e.g. straight radial macro-channels, resulting in a diffusion process that is more akin to normal bulk diffusion. This leads to fewer restrictions from the conventional random pore network, and as a result, the effective diffusivity inside catalyst pellets may be significantly increased in comparison to conventional pellets.

[0014] Moreover, an anisotropic pore network may have a large portion of void fraction provided by radial macrochannels, which can be controlled during the fabrication process. This can decouple the interdependence between porosity, pore size, pore size distribution, and reduce tortuosity significantly. By designing the anisotropic pore structure, the effective diffusivity and, therefore, the Thiele modulus of the pellets can be suitably controlled.

[0015] The term “hierarchical microstructure” is used herein to define that the pellet exhibits an ordered structure with n levels of microscopic length scales (n being 2 or greater). The precise structure of the resultant pellet will be discussed in further detail below. However, it will be appreciated that in view of the method of formation of the pellets, the structure of the pellets produced according to this method differ significantly from pellets produced by known, prior art methods such as tabletting, extrusion, or granulation.

[0016] The particulate inorganic material may comprise ceramic particles, salt particles, metal particles, or mixtures thereof. It is contemplated that the phase inversion technique may be applicable to a wide range of different particulate materials. Preferably the particulate material is not soluble in the first solvent or the second solvent, at least at room temperature (25°C).

[0017] Where the particulate inorganic material comprises one or more ceramic materials (typically understood to be inorganic crystalline or amorphous material compounds of a metal and a non-metal), the ceramic may be selected from alumina, zirconia (including yttrium-stabilized zirconia), titania, silicon dioxide, silicon carbide, clay and mixtures thereof.

[0018] Where the particulate inorganic material comprises one or more salts, the salts may be selected from carbonates, hydroxides, oxides, phosphates, borates, sulfites, chromates, arsenates, sulfates, halides, silicates, and mixtures thereof.

[0019] Where the particulate inorganic material comprises one or more metals, the metals may be selected from iron-based alloys including stainless steel, or FeCr alloys, aluminium-based alloys, and mixtures thereof.

[0020] The first solvent may be selected from N-methyl-2-pyrrolidone (NMP), dimethyl sulphoxide (DMSO), tetra hydrofuran (THF) or Dimethyl acetamide, and mixtures thereof. Such solvents have been found to be particularly convenient for use in phase-inversion techniques. However, it is also contemplated that various other solvents may be used. The first solvent may be a water-miscible solvent.

[0021] The second solvent may be selected from water, ethanol, acetone, isopropanol and mixtures thereof. Water is especially preferred since it is cheap, readily available and non-toxic

[0022] The polymeric binder may comprise an invertible polymer. Such polymers are known in the art from known phase-inversion techniques. Suitably, the polymeric binder may comprise a polymer selected from polyethersulfone (PESf), Poly(methyl methacrylate) (PMMA), cellulose derivatives such as cellulose acetate, polyacrylonitrile, polyimides, polyvinylidene fluoride, polyurethane, polyamide, and mixtures thereof. The polymeric binder is preferably soluble in the first solvent.

[0023] The slurry may comprise the particulate inorganic material in an amount of from 5 wt% to 90 wt%, based on total weight of the slurry. The particulate organic material may be present in the slurry in an amount of 7 wt% or more, 10 wt% or more, 15 wt% or more, 20 wt% or more, 25 wt% or more, 30 wt% or more, 35 wt% or more, 40 wt% or more, 45 wt% or more, 50 wt% or more, 55 wt% or more, 60 wt% or more, 65 wt% or more, 70 wt% or more, 75 wt% or more, or 80 wt% or more. As the person skilled in the art understands, the wt% amount of particulate inorganic material in the slurry may be largely dependent on the density of the particulate inorganic material, with denser materials typically constituting a larger wt% of the slurry. For example, where the particulate inorganic material comprises a perovskite ceramic material, the wt% of the particulate material may be in a range of from 75-80 wt%. Where the particulate inorganic material comprises mesoporous SiO2, the wt% of the particulate material may be in a range of from 7-13 wt% The slurry may comprise the polymeric binder in an amount of from 0.1 wt% to 20 wt%, based on total weight of the slurry. The polymeric binder may be present in the slurry in an amount of 0.5 wt% or more, 1 wt% or more, 2 wt% or more, 5 wt% or more, 10 wt% or more, or 15 wt% or more. In some embodiments, the amount of polymeric binder may be selected to be in a weight ratio against the particulate inorganic material (i.e. polymeric binder : particulate inorganic material) of from 1 :8 to 1 :12, although it is contemplated that wider ranges may also be feasible.

[0024] The slurry may comprise the first solvent in an amount of from 20 wt% to 80 wt%, based on total weight of the slurry. The first solvent may be present in the slurry in an amount of 25 wt% or more, 30 wt% or more, 35 wt% or more, 40 wt% or more, 45 wt% or more, 50 wt% or more, 55 wt% or more, 60 wt% or more, 65 wt% or more, 70 wt% or more, or 75 wt% or more. The first solvent may constitute the balance of the slurry, in addition to the particulate inorganic material and polymeric binder.

[0025] The term “slurry” is used herein to refer to semi-liquid mixtures comprising insoluble matter suspended in one or more liquids. The slurry may be formed in any suitable manner, e.g. by mixing or milling of the slurry components.

[0026] The method may include a step of degassing the slurry prior to the step of immersing droplets of the slurry in the second solvent. This can help ensure removal of trapped air from the slurry, therefore reducing the risk of incorporation of air bubbles (i.e. unwanted large pores) in the resulting porous pellets. Such a degassing step may be performed in any suitable manner. One suitable method includes placing the slurry under vacuum (e.g. by placing the slurry in a chamber connected to a vacuum pump). The slurry may be stirred during the degassing step. This can effectively disperse bubbles and expedite the degassing process. The duration of degassing may vary. The degassing step may be stopped when no new bubbles are observable at the surface of the slurry.

[0027] The step of immersing droplets of the slurry in a second solvent may be performed by immersing the droplets into a bath comprising the second solvent. The term “bath” is used herein to define a volume that is large relative to the volume of the droplets. The volume of the bath is not particularly limited but may be e.g. 1 L or more, e.g. 2.5L or more, 5 L or more, 10 L or more, up to 25L or more. Where the method is performed on an industrial scale, the volume of the bath may be significantly larger than this.

[0028] The bath may be a composite solvent bath, i.e. a bath comprising two or more solvents. Preferably the solvents in the solvent composite bath are selected to be immiscible with one another, so that the composite solvent bath comprises two or more solvent layers. In one particularly preferred arrangement, the bath is a composite solvent bath comprising a first layer comprising or consisting of a liquid which is immiscible with both the first and second solvents, and a second layer comprising the second solvent. In this arrangement, the liquid which is immiscible with both the first and second solvents may be selected to have a density that is lower than the density of the second solvent, such that the first layer sits above the second solvent in the composite solvent bath. In such an arrangement, the droplets will contact this first layer before being immersed in the second solvent. This can assist in forming the droplets into an approximately or substantially spherical shape, before they are immersed in the second solvent, thereby allowing formation of pellets of approximately or substantially spherical shape. The first layer may be referred to herein as a ‘spherification layer’. In some embodiments, the liquid which is immiscible with both the first and second solvents may be selected from e.g. heptane, hexane, isooctane, diethyl ether, cyclohexane, pentane, xylene and mixtures thereof.

[0029] The slurry may have a viscosity in a range of from 0.1 to 500 Pa.s, e.g. 1 to 100 Pa.s, or 10 to 60 Pa.s as measured using cone and plate geometry at a shear rate of 30 s-1, and a temperature or 20°C. In some embodiments, the viscosity may be 10 Pa.s or more, 20 Pa.s or more or 30 Pa.s or more. The viscosity may be 60 Pa.s or less, 50 Pa.s or less or 40 Pa.s or less. It has been found that selecting the viscosity of the slurry to be in this range can more readily allow for formation of pellets having an approximately or substantially spherical shape.

[0030] The droplets may be formed by extrusion of the slurry via a nozzle having at least a first orifice. Extrusion of the slurry has been found to be a particularly convenient method for production of droplets in the present process. The extrusion rate may be in a range of from 0.1 to 10 ml / min. For example, the extrusion rate may be 0.5 ml / min or more, 1 ml / min or more, 2 ml / min or more, 3 ml / min or more, 4 ml / min or more, or 5 ml / min or more. The extrusion rate may be 10 ml / min or less, 9 ml / min or less, 8 ml / min or less, 7 ml / min or less, or 6 ml / min or less. It has been found that extruding the slurry at extrusion rates in the above range can suitably provide for formation of pellets having approximately or substantially spherical shape. The extrusion rate may affect the size of the resultant pellets: generally, increasing the extrusion rate tends to lead to the formation of smaller droplets, and thus smaller pellets. This is because a higher rate of slurry being pushed through the nozzle results in more frequent breakups of the continuous stream of slurry, creating smaller individual droplets. On the other hand, a lower extrusion rate may lead to larger droplets (and therefore larger pellets) being formed due to less frequent breakups of the slurry stream.

[0031] The nozzle may be spaced a distance of from 0 cm to 5 cm above the surface of the second solvent, or above the surface of the bath as appropriate, e.g. it may be spaced from the surface of the second solvent or surface of the bath by 0.5 cm or more, 1 cm or more, 2 cm or more, 3 cm or more or 4 cm or more. It may be spaced by 5cm or less, 4 cm or less or 3 cm or less. Suitably, the nozzle may be spaced at a distance of about 2cm from the surface of the second solvent or bath. It has been found that these ranges may provide for formation of pellets having approximately or substantially spherical shape. If the gap between the nozzle and the second solvent / bath is significantly higher or lower than this range, the resultant pellets may have a less suitable shape. Preferably the nozzle is only spaced at a distance of less than 1 cm from the bath (e.g. 0.5 cm or less, or 0cm from the bath) where the bath is a composite bath as described above. In such arrangements, the depth of the first layer of the bath can allow for formation of pellets having approximately or substantially spherical shape even despite the close proximity of the nozzle to the bath.

[0032] In some arrangements, the nozzle may have plural orifices e.g. may have at least first and second orifices. In some arrangements, the first and second orifices may be concentrically arranged (i.e. the second orifice may be disposed within the first orifice). In other arrangements, the first and second orifices may be separately arranged. Where the nozzle comprises plural separate orifices, multiple droplets can be extruded simultaneously.

[0033] Where the nozzle comprises concentrically arranged first and second orifices, this can allow for extrusion of multi-layer droplets - i.e. pellets having a plurality of different compositional layers. For example, a pellet produced according to this method may have two or more, three or more, or four or more concentrically-arranged layers, wherein the composition of one of said layers is different to a composition of at least one of the other layers. In some arrangements, the composition of each layer of the multi-layer pellet differs from the composition of each other layer.

[0034] Each orifice may have a diameter in a range of from of 1 / 16 - % inches (1 .59 mm - 6.35 mm). The diameter of each orifice may be 2 mm or more, 3 mm or more, 4 mm or more, 5 mm or more, or 6 mm or more. It has been found that extruding the slurry from orifices having a size in the above range can suitably provide for formation of pellets having approximately or substantially spherical shape.

[0035] Where the first and second orifices are concentrically arranged, it will be appreciated that one of the orifices will be of different sizes. In such an arrangement, the first orifice may have a diameter in a range as noted above, and the second orifice may have a diameter which is 0.5 mm - 3 mm, e.g. about 1 mm larger than the diameter of the first orifice.

[0036] In some embodiments, the nozzle has concentrically arranged first and second orifices, and the method includes concurrently extruding droplets of a first slurry and a second slurry from the first and second orifices respectively. That is, the first slurry is extruded from the first orifice at the same time as the second slurry is extruded from the second orifice. The extrusion rate of the first and second slurries may be substantially equal, and in the ranges noted above. Alternatively, the extrusion rate of the first and second slurries may differ. The selected rate of extrusion from each orifice may be selected based on factors such as slurry viscosity and preferred thickness of each layer in the pellets. For example, if a thicker outer layer is preferred, a higher extrusion rate may be used for extrusion from the radially outwardly arranged nozzle as compared with the radially inner nozzle, and vice versa, if a thin outer layer is preferred, a lower extrusion rate may be used for extrusion from the radially outwardly arranged nozzle as compared with the radially inner nozzle.

[0037] The second slurry may comprise a particulate inorganic material, a solvent, and a polymeric binder, in an analogous manner to the first slurry. Optional features as set out above in relation to the composition and properties of the first slurry may apply equally to the second slurry.

[0038] The first slurry and second slurry preferably have a composition that differ from one another. The compositions of the first and second slurry may differ from one another in respect of the relative amounts of particulate inorganic material, solvent, and polymeric binder in the slurries. The compositions of the first and second slurry may additionally or alternatively differ from one another in respect of the composition of the particulate inorganic material, solvent, and polymeric binder in the slurries.

[0039] The precursor pellet may be soaked in the second solvent for a predetermined time period prior to the step of performing a heat treatment on the precursor pellet. This soaking step may be performed for a time of 1 hour or more, e.g. 2 hours or more, 3 hours or more, 4 hours or more, 5 hours or more, 6 hours or more, 12 hours or more, 18 hours or more, or 24 hours or more. Performing such a soaking step can help to ensure removal of any residual first solvent remaining in the precursor pellet (i.e. allow for full exchange of the first solvent with the second solvent).

[0040] The method may include a step of drying the precursor pellets before the step of performing a heat treatment on the precursor pellet to thereby form the porous pellet. The drying step may be a natural drying step, e.g. air-drying. The drying step may be performed at a predetermined temperature, which may be room temperature (about 25 °C) or may be at an elevated temperature above room temperature. The drying step may be performed for any suitable time period, e.g. 1 hour or more, 2 hours or more, 3 hours or more, 4 hours or more, 5 hours or more, 6 hours or more, 12 hours or more, 18 hours or more, or 24 hours or more. In some embodiments, the precursor pellets are air dried at room temperature for a period of at least 24 hours.

[0041] The step of performing a heat treatment on the precursor pellet to thereby form the porous pellet includes a first heating step of heating the precursor pellet to a first predetermined temperature for a first predetermined time period, and a second heating step of heating the precursor pellet to a second predetermined temperature for a second determined time period.

[0042] The first heating step may remove some of all of the polymeric binder present in the precursor pellets. The first heating step may cause the polymeric binder to decompose into gaseous components (be burnt off). The first predetermined temperature may be a temperature in a range of from 100 °C to 1000 °C, e.g. 200 °C or more, 300 °C or more, 400 °C or more, 500 °C or more or 600 °C or more. In preferred embodiments, the first predetermined temperature is a temperature in a range of from 500 °C to 700 °C, or 550 °C to 650 °C, e.g. about 600 °C. The first heating step may comprise applying a ramped heating profile to raise the temperature of the precursor pellet from approximately room temperature to the first predetermined temperature. The first heating step may be performed for a time period of from 1 to 24 hours, e.g. 2 to 12 hours, e.g. about 2 hours.

[0043] The second heating step may sinter the particulate inorganic material present in the precursor pellets to thereby form the porous pellet. The second predetermined temperature may be a temperature in a range of from 800 °C to 2400 °C, e.g. 1000 °C or more, 1200 °C or more, 1400 °C or more, 1600 °C or more,1800 °C or more, 2000 °C or more or 2200 °C or more. The second predetermined temperature may be selected based on the composition of the particulate inorganic material present in the precursor pellets. For example, the second predetermined temperature may be selected to be a temperature that is within 600 °C of the melting point of the primary constituent component of the particulate inorganic material present in the precursor pellets, and preferably within 500 °C, within 300 °C or within 200 °C of said temperature. For example, where the primary constituent component of the particulate inorganic material present in the precursor pellets is alumina, which has a melting temperature of 2072 °C, the second predetermined temperature may be selected to be 1500 °C or greater. The first heating step may be performed for a time period of from 1 to 24 hours, e.g. 2 to 12 hours, e.g. 4 to 6 hours.

[0044] After performing the heat treatment, the porous pellet may be cooled. This cooling may be natural cooling (e.g. in air, to room temperature) or may be forced cooled (e.g. by application of a cooling medium). As noted above, methods according to the above disclosure allow for formation of porous pellets having an anisotropic or asymmetric pore network. The pore network may comprise a hierarchical microstructure. In particular, pellets according to the present invention may comprise radial microchannels in a central region, surrounded by a denser surface layer (also referred to herein as a ‘skin layer’).

[0045] For avoidance of doubt, the term “porous” is used herein to define that the pellets have a structure comprising a plurality of pores. Preferably, the pores are not filled with any solid material, i.e. they constitute voids within the pellets. The pores may provide connected porosity within the pellet and may thereby act as flow-paths e.g. for liquid or gases impregnating the pellets. Preferably the pellet comprises a plurality of radially-extending micro-channels. The radially extending channels may have a pore size in a range of from 5000 to 300000 nm, (i.e. 0.005 to 0.3 mm), as determined by mercury porosimetry.

[0046] The porous pellet may have a porosity in a range of from 30 to 70 vol%, as determined by mercury porosimetry. The porosity may be e.g. 40 vol% or more, 50 vol% or more, or 60 vol% or more.

[0047] The porous pellet may have a multimodal pore size distribution.

[0048] The pellet is preferably approximately or substantially spherical. In such cases, the pellet may be referred to as a ‘bead’ - references to ‘beads’ herein will therefore be understood as defining pellets having an approximately or substantially spherical shape. The diameter of the pellet is not particularly limited, although in some embodiments, the pellet may have a diameter in a range of from 1 / 16 - % inches (1 .59 mm - 6.35 mm). The diameter of the pellet may be 1 mm or more, 2 mm or more, 3 mm or more, 4 mm or more, 5 mm or more, or 6 mm or more. In some embodiments, the diameter of the pellet may be up to 10 mm.

[0049] Where the pellet is formed by co-extrusion of two or more slurries having different compositions from concentric orifices, the pellet may comprise two or more concentrically-arranged layers having different compositions.

[0050] One or more further processing steps may be performed after initial formation of the porous pellets. For example, the method may include a step of milling or grinding the porous pellet to remove at least part of a surface layer (also referred to as a ‘skin layer’) of the porous pellet. A skin layer of the pellet may be defined as a layer having an average pore size of 200 nm or less, 150 nm or less, 100 nm or less, or 50 nm or less, e.g. in a range of from 20 to 200 nm. The skin layer may extend for a depth of up to around 100 pm from an outer surface of the pellet, e.g. may extend for 10 pm or more, 20 pm or more, 30 pm or more, 40 pm or more, 50 pm or more, 60 pm or more, 70 pm or more, 80 pm or more, or 90 pm or more from an outer surface of the pellet, as measured in a radial direction.

[0051] Removal of the skin layer of the porous pellet may be advantageous as it can allow exposure of radially- extending micro-channels present in a central region of the pellet, thereby providing a pellet with increased effective diffusivity De, and significantly reduced tortuosity as compared with pellets which have not had the skin layer removed. Pellets that have had the skin layer removed may demonstrate a tortuosity of 5 or less, or 4 or less e.g. about 3, and measured using mercury porosimetry data, e.g. using a method as discussed in S.C. Carniglia, “Construction of the tortuosity factor from porosimetry”, Journal of Catalysis, Volume 102, Issue 2, 1986, Pages 401 -418. Pellets that have not had the skin layer remove may demonstrate a tortuosity of 100 or more.

[0052] The method may include a step of impregnating the porous pellet with a catalyst. Pellets according to the present invention may act as support for a wide range of catalysts e.g. any heterogenous catalyst.

[0053] Accordingly, in a further aspect, the present disclosure provides the use of a porous pellet according to the second aspect as a catalyst support.

[0054] In addition to their use as a catalyst support, pellets according to the present invention may find further use in various other applications e.g. as adsorbent media.

[0055] The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided.

[0056] Summary of the Figures

[0057] Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which:

[0058] Figure 1 shows SEM images of (A) an alumina pellet according to the present invention, in cross-section; (B) the alumina pellet of Fig. 1 A, having had a surface skin-layer removed; and (C) the outer surface of the alumina pellet shown in Fig. 1 B.

[0059] Figure 2 shows various micrographs of (A) a plurality of alumina pellets according to the present invention; (B) an alumina pellet according to the present invention, in cross-section; (C) a single alumina pellet according to the present invention, immediately after formation, and (D) a single alumina pellet according to the present invention, having had a surface skin-layer removed.

[0060] Figures 3 (a) and (b) are schematic diagrams illustrating the internal structure of various pellets according to the present invention.

[0061] Figure 4 is a graph showing pore size distribution of sintered alumina beads according to the present invention, as formed (52AMs) and having had a surface skin-layer removed (52AMs-ps).

[0062] Figure 5 shows (a) a graph of degradation profiles of Sulfamethoxazole (SMX) in different reaction systems with initial catalyst dosages of 0.25 g / 100 mL; (B) and 0.5 g / 100 mL; (C) a graph of catalytic degradation of SMX over 50AEs (0.25 g / 100 mL) in a packed bed column reactor with continuous flow rates; and (D) cobalt leaching test (0.5 g / 100 mL catalyst). Reaction Condition: [PMS]o = 2 g / L, [SMX]o = 20 ppm, and T = 25 °C.

[0063] Figures 6 (A)-(H) show various micrographs of SiO2 pellets according to the present invention.

[0064] Figure 7 is a graph showing pore size distribution of SiO2 pellets according to the present invention.

[0065] Figures 8 (A)-(D) show various micrographs of SiO2 pellets according to the present invention, having been impregnated with 2 wt% Co catalyst. Detailed Description of the Invention

[0066] Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.

[0067] EXAMPLE 1: production & characterisation of porous alumina pellets

[0068] Sample a -AI2O3 powder NMP PMMA PESf Arlacel P135

[0069] 52AM 52 42.44 5.20 0 0.36

[0070] Table 1 presents an example of the suspension / slurry composition for preparing the alumina pellets. Values are given in wt%.

[0071] Preparation of the slurry: taking the preparation of 100 gram of the slurry as an example of Table 1 . First, measure 0.36 grams of Arlacel P135 and add it to 42.44 grams of NMP (solvent), then dissolve it. Gradually add 52 grams of alumina powder into this solution, and continuously ball mill the mixture for 48 hours to ensure thorough mixing. Next, add 5.2 grams of PMMA (polymeric binder) to the mixture and continue mixing for an additional 48 hours. To remove any trapped air inside, place the formed slurry in a vacuum chamber before extrusion. PESf is another type of polymeric binder playing the same role as PMMA. It can be used to replace PMMA wherever needed.

[0072] Beads extrusion, sintering and treatment: The slurry is extruded through a thin tube with an inner diameter of 1 / 8 or 1 / 16 inches, using a high-pressure syringe pump, at an extrusion rate of 0.1-5 ml / min. The air gap, which is the distance between the tube end and the water surface, should be set between 0.5-5 cm. The slurry is extruded into a water bath at room temperature, forming spherical beads (pellets) that settle down inside the water bath. After 24 hours, the beads are collected, dried at room temperature, and then sintered at 1500°C for 4 hours to obtain the 52AM beads (Fig. 1 (A)). 52AMs has a thin skin layer on its outer surface, which can be removed using various methods, such as gentle grinding with milling balls. The skin layer has a thickness of less than 100 pm.

[0073] Samples with the skin layer removed are referred to herein as ‘52AMs-ps’, ‘ps’ here standing for ‘porous surface’, as after removing the skin layer, larger holes on the surface become observable, as shown in Fig. 1 (B) and (C). By exposing these holes, it is easier for materials to transport inside the beads, and as thus reduce diffusional resistance for applications of catalysis and adsorption.

[0074] Fig 2 (A)-(D) show various micrographs of the same pellets before and after removal of the skin layer. Fig. 2 (A), (B), and (C) are image of the 52Ams samples prior to skin layer removal. Fig. 2(D) is an image of the 52AMs-ps sample after skin layer removal. In this figure, pores are visible at the surface of the pellet.

[0075] The 52Ams pellets produced according to this example have a general structure as shown in schematic Fig. 3(a). In other words, the pellets comprise a less-dense central region 3, and a more-dense outer skin layer 5. In this example, the outer skin layer 5 has a substantially identical composition to the central region 3, but varies only in its level of porosity (and accordingly, in its density). The central region 3 comprises a plurality of radially-extending microchannels 7.

[0076] Whilst the example shown here has a substantially homogeneous composition throughout the pellet, it is also possible to produce pellets having a composition that varies through the pellet body. Fig. 3(b) is a schematic figure showing an example of a pellet which has a composition that varies in a radial direction. A pellet having this structure can be produced by co-extruding slurries of different compositions from concentrically-arranged nozzles. Here, the central region 3’ of the pellet includes a first layer 9, and a second layer 1 1 which vary in composition. The first and second layer are concentrically arranged. The pellet also comprises an outer skin layer 5’ which has a substantially identical composition to the second layer 11 but varies only in its level of porosity (and accordingly, in its density). Similarly to the arrangement shown in Fig. 3(a), the central region 3’ comprises a plurality of radially-extending microchannels 7. Whilst this pellet shown in this schematic example includes portions of two different compositions, it will be understood that the principle is more generally applicable to formation of pellets having a plurality of different compositional layers, e.g. a pellet may have three or more, or four or more concentrically-arranged layers

[0077] Characterisation of 52Ams and 52Ams-ps pellets

[0078] Figure 4 displays the pore size distribution of the alumina pellets, measured using a mercury porosimeter.

[0079] As noted above, the sample labelled as 52AMs represents alumina pellets with a surface skin layer, which requires higher pressure for the mercury to penetrate through the skin layer and reach the radial macro-channels. The average pore size of 447.6 nm corresponds to the size of the pores within the skin layer.

[0080] On the other hand, in the case of 52AMs-ps, where the skin layer has been removed, the macro-channels are directly exposed. Consequently, the mercury can directly fill the macro-channels. The peaks observed between 5000 and 300000 nm indicate the size of the holes which are shown in Fig. 1 (C) and Fig. 2 (D).

[0081] The degree of measured porosity for each sample is given in Table 2, below.

[0082] Sample Porosity VT Tortuosity

[0083] (%) (mL / g)

[0084] 52AMs 53.1 1 0.2852 111.23*

[0085] 52AMs-ps 52.57 0.2852 3.15

[0086] Table 2: Mercury intrusion results for alumina pellets, VT: Total intrusion volume

[0087] Table 2 provides additional insights into the pore structure information depicted in Fig. 4. The porosity of alumina pellets, both with and without the skin layer, is quite similar. This similarity arises because the skin layer is relatively thin, measuring less than 100 micrometers, and has minimal impact on the overall porosity and total intrusion volume. However, the samples without the skin layer exhibit significantly lower calculated tortuosity, indicating a higher effective diffusivity. This enhanced effective diffusivity is not attainable with existing ceramic pellets lacking radial macro-channels. It is noted that the calculated tortuosity value for the 52Ams sample (*) is perhaps somewhat higher than would typically be expected, even for the skin being present: it is theorised that this value may result from discrepancies in the software used to calculate the tortuosity value of this sample.

[0088] In order to assess the impact of the pellet microstructure on catalysis by a catalyst supported on the pellets, the alumina pellets were loaded with a 2% CO3O4 catalyst. Comparative pellets were also analysed - samples indicated as 50AEs are porous alpha-alumina pellets with an isotropic pore structure (not according to the present invention). Fig. 5 displays relevant information regarding the utilization of alumina pellets loaded with a 2% Co3O4 catalyst for sulfamethoxazole (SMX) degradation through peroxymonosulfate (PMS) activated advanced oxidation processes. Fig.5 shows (a) a graph of degradation profiles of Sulfamethoxazole (SMX) in different reaction systems with initial catalyst dosages of 0.25 g / 100 mL; (B) and 0.5 g / 100 mL; (C) a graph of catalytic degradation of SMX over 50AEs (0.25 g / 100 mL) in a packed bed column reactor with continuous flow rates; and (D) cobalt leaching test (0.5 g / 100 mL catalyst). Reaction Condition: [PMS]o = 2 g / L, [SMX]o = 20 ppm, and T = 25 °C. Overall, the samples demonstrate favourable catalytic performance with less cobalt leaching. Furthermore, the samples with macro-channels exhibit higher performance compared to those without macro-channels. Additionally, the samples lacking a skin layer outperform those with a skin layer.

[0089] EXAMPLE 2: production & characterisation of porous SiO? pellets

[0090] Sample SiO2 powder NMP PMMA

[0091] SiO2bead 11 79 10

[0092] Table 3 presents an example of the suspension / slurry composition for preparing the SiO2pellets. Values are given in wt%.

[0093] Preparation of the slurry: Initially, 11 wt.% of SiO2powder was added into 79 wt.% of the solvent NMP within a glass bottle. The resulting mixture underwent milling for approximately 48 hours using milling balls as the grinding media. Following this, 10 wt.% of PMMA (polymeric binder) was introduced, and the suspension was further milled for another 48 hours. The well-mixed suspension was then transferred into a beaker and subjected to a vacuum within an air-tight vessel for 3 hours, eliminating any trapped air within the suspension. Subsequently, the suspension was gradually transferred into a 100 mL stainless steel syringe.

[0094] The SiO2powder used in formation of these pellets had an amorphous structure, with BET surface area of around 263 m2 / g.l

[0095] Beads extrusion, sintering and treatment: By configuring the syringe pump to an extrusion mode with a flow rate of 0.2 mL / min, the suspension was extruded through a 1 / 4-inch pipe and dropped into a water bath. It was found that maintaining an air gap of approximately 4.0 cm allowed for generation of spherical beads (pellets). The beads were allowed to remain in the water bath for 24 hours, before being removed and dried at room temperature. Finally, these beads were sintered in the surface at different temperatures (1 OOO °C / 1050 °C / 1100 °C / 1200 °C).

[0096] Characterisation of SiO2 pellets

[0097] Fig. 6 (A)-(H) show various micrographs of the resultant SiO2 pellets. Fig. 6 (A) and (B) are micrographs of an SiO2 pellet sintered at 1000 °C. Fig. 6 (C) and (D) are micrographs of an SiO2 pellet sintered at 1050 °C. Fig. 6 (E) and (F) are micrographs of an SiO2 pellet sintered at 1100 °C. Fig. 6 (G) and (H) are micrographs of an SiO2 pellet sintered at 1200 °C.

[0098] It can be seen from these micrographs that, similarly to the alumina pellet produced in Example 1 , pellets produced according to this example have a general structure as shown in schematic Fig. 3(a). In other words, the pellets comprise a less-dense central region, and a more-dense outer skin layer. The central region of each pellet comprises a plurality of radially-extending microchannels. From this is can be concluded that methods according to the present invention are applicable to a wide range of material systems.

[0099] It can be seen from these micrographs that with increasing sintering temperatures, the SiO2 beads shrink more significantly, however, the presence of radial macro-channels is maintained until the temperature reaches 1200°C - it can be seen that the density and thickness of the skin layer of the pellets generally increases with sintering temperature.

[0100] The pellets were then characterised using mercury porosimetry . Figure 7 displays the pore size distribution of the SiO2 pellets, measured using a mercury porosimeter.

[0101] Sample Total pore Av. Pore Porosity area Diameter (%)

[0102] (m2 / g) (nm)

[0103] SiG2-1000 150.80 41.60 75.88

[0104] SiO2-1050 66.29 38.10 57.70

[0105] SiO2-1100 11.12 29.80 15.32

[0106] SiO2-1200 0.63 105.00 3.70

[0107] Table 3: Mercury intrusion results for SiO2 pellets

[0108] Table 3 provides additional insights into the pore structure information depicted in Fig. 7. However, it is theorised that because this data was performed on pellets that had not had the dense skin layer removed, the resultant values may not be representative of the true bulk porosity and pore size values of the pellets, in particular for the pellets formed at higher temperatures close to the glass transition temperatures of the pellet material (the SiO2-1100 and SiO2-1200 samples), because at such temperatures, small pores in the outer skin layer can disappear, thereby preventing intrusion of the mercury past the dense skin layer.

[0109] In order to assess the feasibility of using the pellets as a catalyst support, the resultant SiO2 pellets were loaded with 2 wt% of Co as a catalyst using an impregnation technique. Fig. 8 shows various micrographs of the resultant pellets. Catalyst was effectively loaded onto the SiC2-1000 pellets (shown in Fig. 8 (A) and (B)), and onto the SiO2-1050 pellets (shown in Fig. 8 (C) and (D)). It can be seen that the general structure of the pellets was maintained even with catalyst loaded.

[0110] The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

[0111] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

[0112] For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

[0113] Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

[0114] Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

[0115] It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / - 10%.

Claims

Claims:1 . A method for forming a porous pellet, the method comprising steps of: forming a slurry from slurry components including a particulate inorganic material, a first solvent, and polymeric binder; immersing droplets of the slurry in a second solvent, the second solvent being selected to be miscible with the first solvent and immiscible with the polymeric binder, to form a precursor pellet; and performing a heat treatment on the precursor pellet to thereby form the porous pellet.

2. The method according to claim 1 wherein the particulate inorganic material comprises ceramic particles, salt particles, metal particles, or mixtures thereof.

3. The method according to claim 1 or claim 2 wherein the first solvent is selected from N-methyl-2- pyrrolidone (NMP), dimethyl sulphoxide (DMSO), tetrahydrofuran (THF) or Dimethyl acetamide, and mixtures thereof.

4. The method according to any one of the preceding claims wherein the second solvent is selected from water, ethanol, acetone, isopropanol, and mixtures thereof.

5. The method according to any one of the preceding claims wherein the polymeric binder comprises an invertible polymer.

6. The method according to claim 5 wherein the polymeric binder comprises a polymer selected from polyethersulfone (PESf), poly(methyl methacrylate) (PMMA), a cellulose derivative, polyacrylonitrile, polyimides, polyvinylidene fluoride, polyurethane, polyamide, and mixtures thereof.

7. The method according to any one of the preceding claims wherein the step of immersing droplets of the slurry in a second solvent is performed by immersing the droplets into a bath comprising the second solvent.

8. The method according to claim 7 wherein the bath is a composite solvent bath comprising a first layer comprising a liquid which is immiscible with both the first and second solvents, and a second layer comprising the second solvent.

9. The method according to any one of the preceding claims wherein the droplets are formed by extrusion via a nozzle having at least a first orifice.

10. The method according to claim 9 wherein the first orifice has a diameter of 1 / 16 - % inches (1 .59 mm - 6.35 mm).

11. The method according to claim 9 or claim 10 wherein the nozzle has concentrically arranged first and second orifices, and the method includes concurrently extruding droplets of a first slurry and a second slurry from the first and second orifices respectively.

12. The method according to claim 11 wherein the second slurry has a composition that differs from the composition of the first slurry.

13. The method according to claim 11 or claim 12 wherein the second orifice has a diameter which is 0.5 mm - 3 mm larger than the diameter of the first orifice.

14. The method according to any one of the preceding claims as dependent from claim 9, wherein the step of extruding droplets of the slurry into second solvent bath is performed at an extrusion rate of 0.1-10 ml / min.

15. The method according to any one of the preceding claims wherein the method includes a step of degassing the slurry prior to the step of immersing droplets of the slurry in a second solvent.

16. The method according to any one of the preceding claims wherein the precursor pellet is soaked in the second solvent for a predetermined time period prior to the step of performing a heat treatment on the precursor pellet.

17. The method according to any one of the preceding claims wherein the step of performing a heat treatment on the precursor pellet to thereby form the porous pellet includes a first heating step of heating the precursor pellet to a first predetermined temperature for a first predetermined time period, and a second heating step of heating the precursor pellet to a second predetermined temperature for a second determined time period.

18. The method according to any one of the preceding claims wherein the method includes a step of milling or grinding the porous pellet to remove at least part of a surface layer of the porous pellet.

19. The method according to any one of the preceding claims wherein the method includes a step of impregnating the porous pellet with a catalyst.

20. A porous pellet obtained or obtainable according to the method of any one of claims 1 to 19, wherein the porous pellet comprises an anisotropic or asymmetric pore network.

21. The porous pellet according to claim 20 wherein the pellet is approximately or substantially spherical.

22. The porous pellet according to claim 20 or 21 wherein the pellet has a porosity in a range of from 30 to 70 % as measured using mercury intrusion porosimetry.

23. The porous pellet according to any one of claims 20 to 22 wherein the pellet has a diameter in a range of from 1 mm to 10 mm.

24. Use of a porous pellet according to any one of claims 20 to 23 as a catalyst support.

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