Hierarchically Porous Microparticles via Spraydrying

Transcription

1 Hierarchically Porous Microparticles via Spraydrying Kathryn Waldron 1, Winston Wu 1, Zhangxiong Wu 1, Wenjie Liu 1, Cordelia Selomulya 1 *, Dongyuan Zhao 1,2, and Xiao Dong Chen 1,3 1 Department of Chemical Engineering, Monash University, Clayton Campus, Victoria 3800, Australia Cordelia.Selomulya@Monash.edu 2 Department of Chemistry and Laboratory of Advanced Materials, Fudan University, Shanghai, , P. R. China dyzhao@fudan.edu.cn 3 College of Chemistry, Chemical Engineering and Material Science, Soochow University,Suzhou City, Jiangsu PR China Abstract A micro-fluidic jet spray-dryer, developed at Monash University, was used to produce monodisperse microparticles with mesoporous characteristics. We demonstrated that hierarchical structures could be generated through the addition of co-templates in the precursors. Microparticles produced with pluronic block copolymer (F127) with the addition of Eudragit RS 30D, a polymer-colloidal dispersion of ~ 120 nm nanoparticles as a co-template, produced microparticles with very open, porous structure with macropores of ~ nm, along with ~9 nm mesopores. These particles exhibited highly ordered hexagonal mesostructures in addition to the macropores created by the removal of the polymer, with surface areas up to 460 m 2 /g and pore volumes up to 0.68 cm 3 /g. Through the variation in weight ratio of the cotemplates and the silica different hierarchical structures can be obtained. Keywords- mesoporous silica; spray-drying; hierarchical pores I. INTRODUCTION Mesoporous materials, in particular mesoporous silicas, have been attracting a lot of attention since they were first developed in 1992 by Mobil [1, 2] due to their characteristic high surface areas [3], controllable mesostructures and mesopore sizes [3, 4], coupled with high chemical and thermal stability [4] and availability for surface functionalization. Recently there has also been a focus on hierarchical pore architecture, which can be defined as an arrangement of welldefined pores of differing sizes in a 3D arrangement, with the smaller pores establishing a connection between the larger pores due to their location in the walls between the large pores [5]. For example a macro-mesoporous structure would have macropores embedded in a matrix containing mesopores [5]. The benefits of this type of structure are numerous from the enhancement of the pore wall functionality and interactions with various species, within inorganic materials producing mass transport pathways to enhancing the diffusion though the porous matrix of foreign molecules [6]. This structure also features the potential for larger pore sizes coupled with high pore volumes and large surface areas [5]. The applications are numerous and diverse such as catalysis, separation, adsorption, electrode materials, biomedical and drug delivery as multifunctional carriers [5, 6]. Sel et al. (2006) theorised that the final mesopore morphology could be tuned through the choice of appropriate surfactant and block co-polymers type and concentration, however sighted the rarity of well-defined hierarchical mesopore architecture through both the nanocasting and hydrothermal methods as indications of fundamental problems in their synthesis [5]. This could be attributed to the thermodynamically unfavourable creation of hierarchical micellar systems, as the two template molecules have a tendency to form either separate macroscopic phases or mixed micelles [5]. Hence there have been difficulties in controlling the final structure with most materials produced lacking, on at least one length scale, a well-defined pore structure (pore shape and size) [5]. Another method of synthesis of mesoporous materials is evaporation-induced self-assembly (EISA) which involves the competition between condensation of silica and self-assembly of micelles [7]. The mesostructure is formed via the preferential evaporation of the more volatile components in the solution, which enrich the vapour-liquid interface in water, while surfactant and silica trigger the formation and coassembly of micelles into a liquid-crystalline mesophase [8, 9]. In 2012 Wei et al. synthesised ordered dual-mesoporous silica materials via solvent-evaporation-induced step-by-step aggregating process involving dual templates[5]. While Areva et al. (2004) stated that aerosol assisted EISA could be used to produce particles exhibiting multimodal porosity if another means for pore generation is used in parallel also be generated with aerosol assisted EISA if another means for pore generation is used in parallel[10]. The core-shell structure of the particles they produced was ascribed to an earlier solidification of the droplet surface, and is normally observed for aerosol-generated mesoporous particles [10]. Hard templates such as latex beads or emulsion droplets can be utilized as a direct approach for the synthesis of hierarchically porous particles, providing spherical or interconnect networks of reasonably large pores [10].

2 Spray drying is a robust particle production method with applications in industrial processes [11] such as in food, dairy, chemical, pharmaceutical, and ceramics applications [12]. It enables fast and continuous production of particles over a reasonable size range [3, 13, 14]. The main drawback associated with conventional spray drying is the generation of polydisperse droplets undergoing diverse trajectories and drying profiles, culminating in poor reproducibility of particle properties and functionalities between batches (or even in the same batch) [13]. The recently developed microfluidic jet spray drying [15], however, can produce monodisperse microparticles in the size range of µm in a single step. The method relies on a specially designed nozzle which a solution is forced to jet through, to be broken up by vibrating piezoceramics into monodisperse droplets. These monodisperse droplets pass through the drying chamber in a single stream where they come into contact with hot air to produce particles that are uniform in size, shape, and properties [13, 16-20]. In this paper we demonstrate a method for producing hierarchically ordered porous silica microparticles via microfluidic jet spray drying. The continuous phase for these materials is the mesopores templates by the surfactant F127, with the presence of macropores template by the colloidal polymer suspension Eudragit RS within this matrix, these macropores are of the range of less than 100 nm which has been identified as desirable [5]. The effect of increasing the initial concentration of polymer in the solution was investigated with the materials being characterised via nitrogen physisorption, mercury porosimetry, SEM and TEM. A. Materials II. EXPERIMENTALS Tetraethyl orthosilicate (TEOS 99.0 %) purchased from Aldrich Chemicals, used as supplied, 0.2 M Hydrochloric acid solution (prepared by dilution of 1 M analytical reagent grade HCl purchased from Univar with distilled H 2 O, amphiphillic pluronic block co-polymer F127 (purchased from Sigma- Aldrich used as supplied), Eudragit RS 30D a copolymer of ethyl acrylate, methyl macrylate, and a low content of methacrylic acid ester with quaternary ammonium groups, 30 % aqueous solution (kindly donated by Evonik Industries used as supplied) with a uniform colloidal size of ± 1.2 nm (measured from a 1 % (w/v) Eudragit RS dispersion by the dynamic light scattering method on the Malvern Zetasizer, Nano-ZS and confirmed by SEM not shown here) and distilled water prepared in a Bio Nex Power 1000 Water Purification System. B. Precursor Preparation 13.3 g TEOS (equivalent to 3.84 g silica) is hydrolysed with 6.4 g 0.2 M HCl in a glass bottle under magnetic stirring for 2 hours; while 6.4 g F127 is dissolved in distilled water in a water bath at 35 o C, also under magnetic stirring for two hours. The solutions are then combined while being stirred and the Eudragit RS 30D, if required, is added. The solution is then diluted to 10 wt% using distilled water and stirred magnetically for a further 30 mins. The amount of Eudragit RS added ranged from 0 to 70 wt% of the silica added for example for the 70 wt% Eudragit RS: 0.7 x 3.84 = 2.69 g Eudragit RS or 8.96 g of the 30 wt% solution. C. Micro-fluidic Jet Spray Drying Monodisperse droplets were formed from the precursor solution by a micro-fluidic aerosol nozzle system[13] with an orifice diameter of 75 µm, resulting in droplets of approximately 150 µm in diameter. The solution was fed into a standard steel reservoir with dehumidified instrument air to force the liquid to jet through the nozzle. The jet was broken up by disturbance from vibrating piezoceramics. Droplet formation was monitored using a digital SLR (Nikon D90) with a speed light (Nikon SB-400) and micro-lens (AF Micro- Nikkor 60 mm f/2.8d). Droplet spacing was optimized via adjustment of frequency and period of stimulation of the piezoelectric nozzle under observation using high-speed photography. Due to the efficient contact between drying air and monodisperse droplets in a single stream, every single droplet was converted to a particle on one-to-one basis. The setup temperature was 210 oc producing inlet and outlet temperatures of ~160 and ~ 115 oc, respectively. The droplet diameter for all samples was 158 µm, the air flow rate was 1000 L min-1, and the liquid flow rate was 1.05 g min-1. The particle collection efficiency was > 70 % by weight, and collected particles were then oven dried at 100 oc overnight to remove excess water before being placed in a tube furnace to remove the F127 and Eudragit RS 30D. Calcination was conducted in air environment at 550 C for 5 hours with a 2 C min-1 ramp. D. Characterisation Samples of (intentionally) crushed and intact particles were placed on carbon tapes attached to spindles and coated with ~ 1 nm of Pt for morphological examination of microparticles taken using a FEI va NanoSEM 450 Field Emission Scanning Electron Microscope. To observe pore structure and distribution, TEM samples were prepared by crushing a small quantity of silica particles in butanol in an agate pestle and mortar to form a weak suspension of fragments. A droplet was placed on a carboncoated copper grid and then air-dried. The samples were examined using a FEI Tecnai G2 T20 Twin LaB6 TEM, operated at 200 kv. Nitrogen adsorption and desorption isotherms were obtained using an ASAP 2020 instrument. The samples were dried in an oven overnight prior to a pre-treatment of 30 min at 100 C to remove all the water followed by 240 min at 180 C under vacuum. Surface areas and pore volumes were calculated using the Brunauer-Emmett-Teller (BET) method, while the pore size distribution was determined by the adsorption branch of the isotherm with the Barret-Joyner- Halenda (BJH) method. Hg intrusion and extrusion analysis were conducted over a wide range of pressures starting with vacuum up to 60,000 psi with a Micromeritics Autopore III. Samples were oven dried overnight at 180 o C prior to analysis.

3 III. RESULTS AND DISCUSSIONS A1 A2 A3 50 nm B1 B2 Pores B3 Skin 50 nm 50 µm Figure 1: SEM (inset TEM) images for A: no Eudragit, B 30 wt% Eudragit RS, 1: non-calcined, 2 and 3: calcined TABLE 1: SUMMARY OF SAMPLES AND RESULTS 2 BET Surface Area (m /g) Pore Volume (cm3/g) Pore size (nm) (BJH ads) Morphology change after calcination? Ordered hexagonal meso? Ordered large pore? 0 wt% Ea wt% Ea wt% Ea wt% Ea wt% Ea a. E stands for Eudragit RS material, thus the skin may be attributed to the effects of the Eudragit RS being combined with the F127 and silica. Previous works conducted by this group combined TEOS and Eudragit RS in water based sols and spray dried them on Otherwise the morphology of the materials in this study were the MFJSD[19]. It was found that the Eudragit RS diffused to similar to uniform, bowl-like microparticles that keep their the surface, while the silica diffused to the centre during the shape after calcination. The bowl-like morphology is the drying process, forming core-shell morphology due to the result of the vapour-liquid interface of the droplet acting as a difference in size of the molecules. This core-shell structure nucleating surface for the liquid-crystal growth which exerts a was not evident in this study with the addition of the surfactant pressure and causes the shell to buckle[9], and is typical of F127. Instead, for all Eudragit RS containing samples, there skin forming materials[21]. This morphology is typical of all was a thin skin evident after spray drying, often joining the the mesoporous silica samples spray dried on this particular particles together (Figure 1 B1), however the structure of this drier, regardless of the template used (unpublished works). skin did not differ from the rest of the particle indicating there The addition of Eudragit RS to the precursor solution was a uniform distribution of the materials in the particle. This produced a significant increase in surface area, pore volume skin was not evident in the non-eudragit RS containing and pore size of the material; this is evident in both the

4 Figure 2: Comparison of N2 isotherms and pore size distributions of A: Eudragit RS, B: 30 wt% Eudragit RS ( adsorption, desorption) A B Figure 3: Comparison of pore size distributions of A: Eudragit RS, B: 30 wt% Eudragit RS ( differential pore volume, cumulative pore volume) nitrogen physisorption (Table 1 and Figure 2, 3) and electron microscopy (Figure 1). The effects of Eudragit RS may be rate of evaporation of the water from the inside of the particle, thus allowing the surfactant and silica micelles to continue to self-assemble after the particles are formed and collected, this is known as a wet-pocket phenomena, with the final structure being produced after the oven drying and calcination. This formation of a wet pocket, or core-shell, has also been noted as normal for aerosol-generated mesoporous particles and is attributed to an earlier solidification of the droplet surface[10]. This phenomenon allows the formation of large microparticles while possessing the highly ordered mesostructure of the typically synthesized materials, which are smaller. attributed to the skin mentioned in the previous paragraph. This skin forms very early in the drying process and slows the The increase in pore size from ~ 7 nm with a sharp distribution to ~ 9 nm with a broad size distribution (Figure 3 A and B respectively) with the addition of the Eudragit RS can be attributed to the partial melting of the colloidal polymer dispersion due to the presence of solvents in the precursor solution and the temperature it was exposed to while drying. The hydrophobic components of the melted polymer would be attracted to the hydrophobic components of the surfactant and thus swell the size of the F127-silica micelles, and hence increase the size of the pores. This partial melting of the polymer can be seen in both the SEM cross-section and in the mercury porosimetry by the broad range of pore sizes from 20

5 Figure 4: Mercury porosity data -120 nm left by the Eudragit RS, compared to the uniform initial colloidal dispersion of 120 nm. From examination of the mercury porosimetry data it is evident that the only the 70 wt% Eudragit RS material produces a significant increase in the volume of large meso- and macropores when compared with the other 3 materials, while there are large meso- and macropores present in all Eudragit RS materials (Figure 4). This significant increase in pore volume for the large pores in the 70 wt% Eudragit RS sample may be attributed to the increased concentration of polymer present, further increases may result in even higher pore volumes. The presence of the macropores can also be seen in the nitrogen adsorption isotherm (Figure 2B) as the adsorption at pressure ratios of ~ 0.8 to 1 instead of plateauing, which is typical of mesoporous materials, continues to rise indicating the partial filling of macropores. The pore volume of the materials also increased with increasing amount of Eudragit RS above 50 wt% Eudragit, this can be attributed to the increased presence of macropores in the materials as a result of the higher concentrations of the polymer template present in each droplet. While the surface areas and pore size remained relatively constant within the errors of nitrogen physisorption. The material without the addition of Eudragit RS displayed very different results. The surface area and pore volume were lower due to the disordered mesostructure. The pore size distribution was narrow due to the pore size being dictated only by the surfactant F127. The overall morphology was similar to the after calcination Eudragit RS containing materials due to the curvature imposed by the surfactant-silica micelles present in the drying process, as previously mentioned, creating the bowl-like morphology. However the Eudragit RS free material did not form a skin of polymer during the drying process (Figure 1 A1). The lack of the skin indicated that the wet pocket phenomena would not have occurred in the drying of the material, hence the surfactantsilica micelles would not have had time to self-organise into the highly ordered hexagonal mesostructure more typically observed. This would have resulted in the disordered mesostructure evident in Figure 1 A3 which also results in lower surface areas and pore volumes. Interestingly the 10 wt% Eudragit RS sample exhibits an additional peak centred around 115 nm, while this sample was also the only Eudragit RS containing sample that did not display the highly ordered hexagonal mesostructure. The difference could be attributed to a slower formation of the skin while drying which would cause the samples to dry more fully prior to the drying in the oven after collection. If the particle was more fully dried prior to this step there would be less of the wet pocket self-assembly phenomenon of micelles thus the polymer could not move as freely. Therefore the structure would not be as affected by its melting during the subsequent drying step, leading to the presence of the larger pore sizes closer to the initial size of the Eudragit RS evident in the mercury porosimetry. The polymer did melt sufficiently to act as a swelling agent in the pores, and the surface area and pore volume were still higher than the material without Eudragit RS to indicate that there was some wet pocket phenomena exhibited in this material.

6 CONCLUSIONS Hierarchically porous materials with uniform mesopores ~ 9 nm and non-uniform large pores ranging from nm have been produced via spray drying. These materials were template with the pluronic F127 block co-polymer surfactant and the colloidal polymer dispersion Eudragit RS. These materials possess larger pore sizes due to the swelling properties of the Eudragit RS as it melts in the precursor solution and while drying. The mesostructure is formed via the wet pocket phenomenon in which a skin forms during spray drying which slows the rate of drying from the inside of the particle, and allows more time for the self-organisation of the surfactant silica micelles. As a result, a high degree of ordering in the mesopores and hence increasing surface area were observed for the samples. The increased concentration of the Eudragit RS triggered the wet pocket phenomena and increased the pore volume of the materials due to the presence of large pores left behind by the Eudragit RS. These materials would have applications in bio-immobilisation or catalysis with the large pores enabling higher mass transfer rates. ACKNOWLEDGMENTS The authors acknowledge use of facilities within the Monash Centre for Electron Microscopy. This research was supported under the Australian Research Council s Centres of Excellence funding scheme (COE for Design in Light Metals). K.W. thanks the Australian Postgraduate Award Scheme for the provision of a scholarship. REFERENCES [1] C. T. Kresage, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, and J. S. Beck, Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism. Nature, : p [2] J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz, C. T. Kresage, K. D. Schmitt, C. T. W. Chu, D. H. Olson, E. W. Sheppard, S. B. McCullen, J. B. Higgins, and J. L. Schlenker, A new family of mesoporous molecular sieves prepared with liquid crystal templates. Journal of the American Chemical Society, (27): p [3] M. Colilla, M. 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