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Synthesis of mesoporous poly(melamine-formaldehyde) particles by inverse emulsion polymerization
Accepted Manuscript Synthesis of Mesoporous Poly(Melamine-Formaldehyde) Particles by Inverse Emulsion Polymerization Dana Schwarz, Jens Weber PII: DOI: Reference:
S0021-9797(17)30312-0 http://dx.doi.org/10.1016/j.jcis.2017.03.064 YJCIS 22161
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
17 December 2016 12 March 2017 13 March 2017
Please cite this article as: D. Schwarz, J. Weber, Synthesis of Mesoporous Poly(Melamine-Formaldehyde) Particles by Inverse Emulsion Polymerization, Journal of Colloid and Interface Science (2017), doi: http://dx.doi.org/ 10.1016/j.jcis.2017.03.064
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Synthesis of Mesoporous Poly(Melamine-Formaldehyde) Particles by Inverse Emulsion Polymerization
Dana Schwarza,b,*, Jens Weber b,* ––––––––– a) Charles University in Prague, Faculty of Science, Department of Organic Chemistry, Hlavova 2030/8, 12843 Prague 2, Czech Republic b) Hochschule Zittau/Görlitz (University of Applied Science), Fachgruppe Chemie, TheodorKörner-Allee 16, 02763 Zittau, Germany E-mail: [email protected] , [email protected] ––––––––– *Corresponding authors: Dana Schwarz, Jens Weber Keywords: melamine resin, particles, emulsion polymerization, dodecane/Span80, adsorption
Graphical Abstract
Abstract: Mesoporous poly(melamine-formaldehyde) (MF) polymer particles with surface areas of up to 200 m2/g were synthesized by an inverse emulsion polymerization using dodecane and Span80® as continuous phase. The finer details of the shape control (using emulsion techniques) and the porosity control (using silica nanoparticles as hard-template) are discussed. The impact of phase-separation processes on the observable porosity of the 20 to
1
200 µm sized spherical particles is analysed by gas sorption methods and electron microscopy. The high density of amine and triazine functional groups in the porous MF particles make the material a promising adsorber for heavy metal ions and methylene blue. In a preliminary column experiment, the synthesized material exhibited a total capacity of 2.54 mmol/g (≙ 812.4 mg/g) for the adsorption of methylene blue.
Introduction In the last decade, many kinds of porous polymers, including poly(melamine-formaldehyde) (MF) resins, particles and foams, have been designed and synthesized because of their potential
applications
as
separation
media
[
1], gas storage media, [2, 3, 4, 5], thermal insulators or heterogeneous catalyst support [6, 7,8,9,10,11] just to name a few. MF is a low cost resin with a high amount of nitrogen functionality, high chemical resistance (both of interest for separation technologies), and good mechanical properties and is therefore a large scale product in industry, mainly for organic coatings or as construction material. Approaches to obtain nanoporous MF resins have been published within the last years using various strategies, including microemulsion strategies, soft-templating,[12, 13] but also hard-templating using silica nanoparticles as template. That way, mesoporous MF materials have been obtained (using silica particles as templates) that showed specific surface areas of up to 250 m2·g-1 with pores in the range of 2.8 up to 12 nm.[14,15,16,17,18] Frequently, the preparation of monoliths suffers from upscaling issues, as heat transfer issues are problematic and might interfere with the phase separation processes. Hence, the preparation of mesoporous MF resin particles by dispersion polymerization, which is more easy to scale, could be a potential solution. Indeed, synthetic routines towards MF resin particles in the range from some hundred nanometer to a few micrometer have been reported, but the resulting products do usually show only low porosity.[19,20,21,22] Reports on the design and particle formation mechanism of (monodisperse) mesoporous MF spheres exhibiting controllable sizes are, to the best of our knowledge, not existing. Thus, it is of great interest to combine those two key properties (i.e. particle formation and porosity) to achieve a new promising adsorber material for water treatment, for example. Recently, we reported the synthesis of nanoporous poly(melamine-formaldehyde) (MF) xerogels composed of 20 nm sized nanoparticles and intergrown particle networks.[23, 24] The 20 to 80 nm sized particles could be synthesized via a simple, one-step reaction in 2
aqueous solution using inexpensive starting materials. Monodisperse, commercial silica nanoparticles (Ludox® HS-40, ~12 nm in diameter) in aqueous solution were added to the reaction solution as a hard template that could be removed after polymerization to yield a mesoporous polymer. The obtained products had a good porosity, but ill-defined shapes and small particles sizes that hinder application in realistic scenarios. In the present study, the formation of micrometer sized MF particles was investigated by inverse emulsion polymerization in order to overcome the above mentioned issues. The shape of the particles was controlled by surfactants, using a combination of Span80® and ndodecane. Mesopores were obtained by hard-templating with silica nanoparticles (Ludox® HS-40, ~12 nm in diameter), as reported before.[15, 23, 24] The particles were analyzed with regard to their porosity by gas adsorption, small-angle x-ray scattering (SAXS) and electron microscopy (SEM). The obtained particles can be applied as adsorber materials in columns. First results of using the obtained MF particles as adsorber material for heavy metal ions (i.e. copper, nickel, lead, zinc, and iron) as well as the dye methylene blue in batch and column experiments are shown and discussed. The MF particles feature promising adsorption properties.
Results and Discussion MF materials can be produced by a dispersion polycondensation reaction in water using oxalic acid as a catalyst.[23] The starting materials are hydrophilic but the growing oligomers get more hydrophobic as the reaction proceeds. The growing MF agglomerates to particles, phase separate ultimately from the continuous water phase. However, the obtained particles were intergrown, and have relatively small sizes (< 1µm) and ill-defined morphologies. The used procedure starts with the preparation of a hydrophilic, water-based precursor-template mixture and it was anticipated that it should be possible to prepare an inverse emulsion, which could provide better control over particle size and shape. Methylated poly(melamine-co-formaldehyde) (MW = 430 g˙mol-1, 84 wt.% sol. in butanol) was used as MF precursor together with silica nanoparticles (Ludox® HS-40, ~12 nm in diameter) of varying amount dispersed in water as hard-template. Additionally, ethanol was added to decrease the viscosity and phosphoric acid acts as an acid catalyst as well as stabilizing agent towards a transparent and clear precursor-template solution.[15] This mixture is a standard first synthesis step and defines the precursor solution containing all necessary chemicals for the polycondensation of MF. The influence of the individual constituents in the mixture have been discussed in more detail elsewhere.[24] In this study, 3
the precursor-template solution has now been dispersed in n-dodecane containing varying amounts of Span80® as surfactant and polymerization was carried out in the resulting dispersion. Span 80 ® (sorbitan monooleate, S80) is a hydrophobic surfactant (HLB-value ~ 4). According to the Bancrofts rule, [25] its use should result in the formation of a w/o-emulsion, which is needed to obtain spherical particles.[26] The impact of the concentration of S80 on the particle formation process was studied. The synthesis procedures resulted in porous polymer/silica hybrid materials (nomenclature –H), which were analysed with regard to composition, morphology and porosity before the removal of the template by treatment with 1 M NaOH (aq.) yielded the final mesoporous polymeric MF resins (nomenclature –P). 3.1 Composition and Chemical Analysis Discussion General aspects about the composition of MF and typical chemical characteristic features have been intensively discussed in previous publications about MF starting from the same type of MF-Oligomer as the precursor material.[15, 23, 24] Therefore, the main aspects of the materials composition will be stated here only shortly, details can be viewed in the electronic supplementary information (ESI). The main analysis with respect to chemical composition was conducted after purification of the obtained materials. The particles were centrifuged and washed several times with water, ethanol and acetone. Afterwards, the solids were dried at 90 °C for one day. Porous MF particles were obtained after removing the template by washing the solid at least three times with NaOH (aq., 1M) for several days. Adjacent, the washing and drying procedure was repeated as described above for the hybrid. Thermogravimetric analysis (TGA, see ESI Fig. S 2 and Fig. S 3) show that the silica content of the hybrid materials varies only slightly with the increased amount of template in the reaction mixture. The shape of the TGA traces shows two major decomposition steps, in accordance with our previous findings. TGA also showed that the silica could be removed efficiently with a remaining mass of < 7.5 wt.% at 1000 °C. This statement is supported by FTIR spectroscopy of the polymeric materials (see Fig. S 4, ESI). The commonly observed strong bands of silica materials that usually dominate the spectra cannot be observed for the polymeric material due to the removal of silica with NaOH.[24] The elemental analysis data is generally in accordance with the TGA data although the residue values for the hybrid as well as for the polymer materials are somewhat higher than 4
observed with TGA. The mismatch could be due to the presence of oxygen within the materials, which was not analysed by EA. The oxygen could originate either from surfactant residues (namely, the sorbitan moiety) or from the formation of ether-bridged MF materials. The particles obtained using Span80®/n-dodecane were slightly orange leading to the assumption that there could be indeed surfactant residues. Span80®, which had an orange colour, contains hydroxyl groups that could interfere with the MF polycondensation, leading to potential partial incorporation. This could explain the observed difference between residues observed by TGA (only silica) and EA (silica and organic oxygen) as well as the slightly higher C/N ratio (compared to MF materials synthesised from the same precursor-template solution in bulk or water dispersion)
3.2 Impact of the Surfactant Concentration The effect of Span80® (S80) and n-dodecane (D) towards the particle formation of MF was examined first using a constant precursor solution (2.1 g of the MF oligomer, 4.0 g of Ludox® HS40 (40 wt.% silica), 2 mL ethanol and 0.5 mL of concentrated phosphoric acid). This combination of the precursor mixture is based on earlier investigations.[15] The variations of the solvent/surfactant mixture and the consequential resulting total volume of emulsion obtained for the particle formation are summarized in Table 1. Table 1. Composition of the continuous phase, Span80 ® (S80) and n-dodecane (D), used for the formation of MF particles by emulsion polymerization at fixed MF Oligomer (2.1 g solid) and fixed silica (1.6 g solid) content. S80/D
S80/D
S80/D
S80/D
S80/D
S80/D
S80/D
S80/D
(0/40)
(5/35)
(10/30)
(15/25)
(20/20)
(10/10)
(30/30)
(40/40)
VS80
(mL)
0
5
10
15
20
10
30
40
VD
(mL)
40
35
30
25
20
10
30
40
Vtotal
(mL)
40
40
40
40
40
20
60
80
Particle size d (µm)
-
20-200
~20
~20
20-30
~20
20-200
20-200
Particle integrity
-
()
x
x
()
x
Figure 1 presents SEM images of the products obtained by varying the ratio and total volume of S80/D after the purification with ethanol, acetone, and water and drying in the oven at 90 °C. The material obtained using only n-dodecane as continuous phase (see Figure 1 HS80/D(0/40) in comparison with the other images) is of ill-defined morphology, i.e. no spherical particles were obtained. Only the combination of n-dodecane with Span80 ® acting 5
as a surfactant leads to spherical shaped products. We analysed the impact of an increasing amount of S80 while keeping the total volume of the continuous phase constant. It turned out that a 50/50 vol.% mixture of S80/D yielded spherical particles of 20-30 µm diameter, which had best (but not yet sufficient) mechanical stability. Secondly, the ratio S80/D was kept constant at 50/50 vol.%, but the total volume of the continuous phase was varied. Increasing the total volume of S80/D results in a broader particle size distribution but also in better mechanical stability, that increased particle integrity. The increasing particle stability with an increasing total emulsion volume (i.e. lowering the solid content from about ca. 10 wt.% at 40 mL total volume of S80/D to ca. 5 wt.% at 80 mL total volume of S80/D) might be related to differences in the details of the phase separation processes upon polymerization (leading to different mechanical properties of the particles), but also to lowered mechanical stress within the emulsion (lower rate of collision events). Increasing the solid content to about 20 wt.% does indeed give only large, ill-defined pieces and only insular spherical particles. In terms of successful formation of spherical particles having high stability, best results were received using a 1:1 ratio of S80/D and solid contents in the range of 5-7 wt.%.
6
Figure 1: SEM images of the dried hybrid (H) or polymer (P) particles prepared with different compositions of Span80® (S80) and n-dodecane (D) (see Table 1).
Beside the analysis of the macroscopic appearance and the chemical identity, the porosity was analyzed for both type of materials (hybrid, H and polymer, P) by nitrogen adsorption/desorption at 77.4 K (s. Fig. 2). Generally, the hybrid materials as well as the polymer materials had specific surface areas between SBET of 90 and 180 m2 g-1, and 85 and 260 m2 g-1, respectively. Hybrid and polymer materials show however significant differences with regard to their pore size distributions (PSD). The hybrid particles show isotherms of type IV (H2) character and the calculation of the pore size distribution (quenched solid density functional theory (QSDFT) adsorption branch) shows predominantly small mesopores of 4-5 nm radius beside a few micropores. The mesoporous MF polymer materials, obtained after etching of the silica particles shows type IV (H1) hysteresis at high relative pressures, indicative of large mesopores. The above observations are well in line with previous results obtained by bulk polycondensation of the precursor-silica mixture [15]. It was reasoned that the silica particles 7
demix partially from the growing MF polymer due to changes in the free energy upon polymerization. The MF polymer is much more hydrophobic than its oligomers, which can further enhance the demixing tendency. Hence, the pore size observed in the hybrid materials arises mainly from void spaces in between packed silica particles. Indeed a rough estimation of the void in between closely packed 12 nm sized particles gives a size of about 4.4 nm, a result in very good agreement with the pore sizes of the hybrid material as determined by gas adsorption. The isotherm shape (H2-type hysteresis) is well known for ink-bottle-type pores or cavitation effects, both are expected for pores with only narrow entrances. This is again expected for the interstitial voids. Hence, etching of the silica nanoparticles does not lead to well-defined 12 nm-sized pores in the resulting MF polymers, but to larger mesopores and some small macropores. Those arise from the removal of small silica particle agglomerates. Hence, the surface areas of the polymer were always slightly lower than for the corresponding hybrid materials (s. Table 2). This fact can be attributed by the increasing pore size after the removal of the template and the removal of the interparticle voids. No exact values of the pore sizes of the polymer materials could be estimated. This is due to their large size, which is at the upper end of the resolution of nitrogen adsorption. Finally, analysis of the small-angle X-ray scattering data obtained from H and P-type materials supports the above description (see ESI for details). The scattering patterns of the hybrid materials are dominated by the scattering of the spherical silica particles, indicated by the characteristic form factor undulations of spherical objects. However, no significant middle- to long-range ordering of the particles can be observed (absence of a correlation peak), giving support to our thesis that the particles do mainly form small agglomerates. The scattering patterns of the MF polymers are less-defined and show pronounced scattering also at low scattering angles, which are characteristic for the presence of larger structures (large meso- and macropores).
8
Figure 2: selected nitrogen ad-/desorption isotherms of the hybrid (H, with template) material (left-hand-side) and the corresponding nitrogen ad-/desorption isotherms of the polymer (P, template removed) particles (middle); selected pore size distribution (PSD) of the hybrid and polymer materials calculated with N2 at 77K on carbon (slit/cylindrical/spherical pores, QSDFT adsorption branch data model) (right-hand-side).
Table 2: calculated results from the nitrogen ad-/desorption isotherms of the hybrid (H) and polymer (P) materials with QSDFT model on carbon for slit/cylindrical/spherical pores (adsorption branch data model), pore volume (V) calculated for a maximum value at p/p0 = 0.995; shown in Figure 2.) S80/D
20/20 30/30 40/40
S (m2 ·g-1) H
P
0/40
153
182
93
165
Vpore (cm ·g )
0.19
0.20
0.15
0.27
dpore (nm)
4-12
4-12
4-12
4-14
2
131
152
85
262
3
-1
S (m ·g )
-1
In summary, Span80® is acting, as supposed, as a surfactant leading to spherical particles. The added silica particles act as a hard template resulting in porous particles (s. Scheme 1). The size of the particles is strongly dependent on the concentration and ratio of the surfactant. Furthermore, the porosity analysis of the materials shows a very good agreement with the results obtained by bulk polycondensation of the mixture.[15] Hence, we argue, that the polycondensation as such is not significantly influenced by the presence of the continuous phase. The composition of the external phase has however (as expected) a significant impact on the particle characteristics (size, stability, etc.). The presence of a high concentration of Span80® is known to be beneficial for the formation of MF or urea-formaldehyde resins from 9
water-in oil-dispersions.[27] However, we cannot clarify the exact role of the surfactant at this
stage, also given the fact that the silica particles will also act as surface active agents. We suspect that Span80® does more (e.g. changing the continuous phases polarity etc.) than just provide a surface coverage of the droplets, but any discussion of this effects would be too speculative at the moment. Before discussion of the up-scaling of the synthesis concept, the influence of the template content on the characteristics of the materials will be discussed shortly.
Scheme 1: general scheme of mesoporous MF particles formation, precursor solution (MF oligomers and silica particle) is added to a Span80 ® and n-dodecane emulsion at 40 °C leading to the formation of mesoporous MF particles by dispersion polycondensation reaction.
3.3 Impact of the Template Content In our opinion, the material P-S80/D(20/20) had the best potential for further investigations (s. Figure 1), as it showed a compromise between particle size, stability and porosity. The template concentration is expected to have influence on the final porosity, but maybe also on the formation and stability of the dispersion (cf. Pickering effects). Therefore, the silica content was varied between 0 and 60 wt.% (related to the mass of the MF oligomer). The materials discussed previously were prepared using 43 wt.% of silica relative to the MF oligomer. Morphology analysis was again done by SEM and gas adsorption measurements. In general, the template seems to be necessary for a sufficient particle stability, as shown in Figure 3. The
10
SEM pictures display the particles after removing the template, washing and drying in the oven at 90 °C for one day. Without addition of silica nanoparticles to the reaction mixture, the resulting MF particles are completely broken. Furthermore, the particle size is slightly decreasing from 20 to about 10 µm in diameter with increasing template content. The particle integrity seems to be best for silica contents of 53 and 57 wt.% relative to the MF oligomer. At a maximum amount of 60 wt.% template added to the mixture, the particles appear to have a stability problem and broken particles were obtained mainly. The underlying mechanism needs to be clarified in future studies.
P0wt.%
P43wt.%
P49wt.%
P53wt.%
P57wt.%
P60wt.%
Figure 3: SEM images of the polymeric (P) materials in dependence on the amount of silica template added with respect to the MF content. Close-up views and the comparison of the hybrid with the polymeric material are shown in the ESI (s. Fig. S 5).
The N2 ad-/desorption isotherms and SAXS analysis data of the hybrid and polymer materials is very much comparable to the isotherms discussed previously (see ESI). If no silica was present during the synthesis, only a low surface area of SBET ~20 m2g-1 was found for the resulting polymer. Upon substantial increase of the silica content, an increase of specific surface areas of the MF polymers was observed, up to ~ 200 m2g-1 at the maximum.
11
Table 3: calculated results from the nitrogen ad-/desorption isotherms of the hybrid (H) and polymer (P) materials with the QSDFT model on carbon for slit/cylindrical/spherical pores (adsorption branch data model), pore volume (V) calculated for a maximum value at p/p0 = 0.995; shown in Figure S 6; Silica content calculated from the reaction mixture composition MF Oligomer/silica. Silica content 0 wt.% 43 wt.% 49 wt.% 53 wt.% 57 wt.% 60 wt.% 153 137 105 97 114 S (m2g -1) 35 H
P
Vpore
(cm3g-1) dpore (nm) 2
-1
S (m ·g )
0.20
0.19
0.15
0.12
0.12
0.23
(>20)
4-12
4-12
4-12
4-12
7-20
95
116
174
203
21
131
It can be concluded that the absence or presence of silica particles play a significant role in the formation and stabilization process. It is known that there are interactions between silica nanoparticles and the growing MF oligomers.[24] The phase separation properties were shown to be depended on SiO2 concentration. We expect that complex interactions (that are, for instance, also dependent on the pH) play a major role here as well. 3.5 Adsorption Properties The purpose of the present investigation was the establishment of a routine towards mesoporous, micrometer sized spherical polymer particles that combine the advantages of common ion-exchange resins (good applicability in fixed beds or column applications etc.) with the promising properties of highy porous MF materials (high adsorption capacity for e.g. CO2 or micropollutants). Hence, we were interested in a screening of the adsorption properties of the particles for application in water treatment. We targeted two types of micro-pollutants, namely heavy metal-ions and organic compounds. To do so, an up-scaling of the reaction mixture was necessary in order to provide sufficient quantities of material (s. experimental section, ESI). Up-scaling was done using different protocols at constant silica/MF ratio of 43 wt.%. Material S80/D-(20/20) was scaled by a factor of 5 and 10, respectively. Material S80/D-(40/40) was also scaled by a factor of 5. For reasons unknown to us, the particles obtained by scaling factor 5 were broken. Scale-up using a factor 10 did however yield intact particles of 20-40 µm size (s. Fig. S6). We suspect that finer details of the synthesis procedure (such as stirring device and rate as well as mechanical forces during the washing and etching procedures), which are not perfectly comparable for the different batches play a major role here.
12
However, there is no significant difference in the chemical properties (FTIR, see ESI) as well as the porosity of the materials compared to small-scale batches. For example, the specific surface areas for the hybrid (118 – 170 m²g-1) as well as for the polymer materials (~ 160 m²g1
) have the same dimensions as for the initial experiments, please see ESI for more details.
For a general statement on the adsorption properties of MF particles in aqueous solution, we examined the adsorption of heavy metal ions as well as the dye methylene blue (MB) as a preliminary investigation. Five common heavy metal ions in water pollution were investigated in a batch experiment for the adsorption experiments on the particles from P-S80/D(40/40)type. To get a general idea of the maximum uptake, we treated the MF particles with a relatively high initial heavy metal ion concentration between 1 and 2 mmol L-1. The pH of the stock solutions was adjusted to a value of around 4 as the pH of the solution commonly increases upon contact with the MF resins (towards pH=6) due to the alkaline behavior of the melamine resin (s. Table S1). This was important, as the adsorption should not be disturbed by any precipitation of heavy metal hydroxides that are commonly formed at neutral to high pH-values. As shown in many other adsorption studies, amine groups are the main functionality for the adsorption mechanism.[28, 29, 30, 31] In MF, nitrogen is present in the aromatic triazine ring, and as secondary or tertiary amine between the triazine units (s. structure in Scheme 1). At an initial pH of 4, the amine group should be mainly of neutral charge, leading to high potential of coordinative binding with the positively charged heavy metal ions. The adsorption uptakes obtained from the screening experiments are displayed in Figure S1 and calculated with the unit mmol g-1 to neglect the mass of the heavy metal ions in contrast. Additionally, we calculated the adsorption efficiency, shown in Figure 4. All five heavy metal ions were adsorbed on the particles with some selectivity for copper and lead. The preference of copper in comparison to e.g. nickel is in line with results obtained by other researchers, who used also amine based adsorber materials.[32] The adsorption efficiency values are relatively low, which is a consequence to the high initial metal ion concentrations used for the screening. Lately, mesoporous poly(melamine formaldehyde) was investigated as an excellent adsorbent for lead ions in the ppb range for water treatment with a maximum adsorption capacity of q eq ~ 3·10-3 mmol·g-1.[33] A similar adsorption preferential of ions compared to our results was reported (i.e. Pb2+ < Cu2+ ~ Cd2+ >> Zn2+ >> Ni2+). Furthermore, a higher adsorption efficiency was reported, which is however probably related to the much lower initial heavy metal ion concentration (ppb range). The calculated adsorption capacity reported there is for 13
some reasons much lower than the capacity reached in our investigation for lead (q eq ~ 0.09 mmol.g-1). This might be due to the distinct different way of preparing the material. The adsorption capacity of copper on poly(melamine-formaldehyde-thiourea) resin was reported by yet another group with a value of 0.025 mmol.g-1 (at pH 4).[34] This is roughly four times lower than results obtained using P-S80/D(40/40)-5x. On the other hand, melamine resin modified with tetraoxalyl ethylenediamine reached an adsorption capacity value for copper of 0.5 mmol/g.[35] Nevertheless, most common adsorber materials reach far higher adsorption capacities at comparable adsorption parameters.[36, 37, 38, 39] Recently, Schwarz et al. have shown the efficient and favorable adsorption of both, heavy metal ions as well as the commonly corresponding oxyanion by using the amine functional groups of the polymer chitosan.[40] Initial results on the adsorption of oxyanions by porous MF particles indicate also a good binding capacity and significant interactions. In continuative studies, the adsorption of heavy metals in combination with their counter ions on melamine based particles needs to be studied in more detail, as the big picture is still lacking a solid, mechanistic understanding.
Figure 4: heavy metal ion adsorption on P-S80/D(40/40)-5x (SBET = 164 m2 g-1) after removing the template, pH0 ~ 4 in batch experiment (left-hand-side) and adsorption of methylene blue on P-S80/D(20/20)-10x (SBET = 153 m2 g-1) in column experiment (righthand-side). 14
Persistent organic pollutants are another class of micro pollutants that pose new challenges for water treatment by adsorption. Methylene blue (MB) adsorption was analyzed in order to have a first idea on the usability of the MF particles as adsorbents for charged organic molecules. Initially, batch experiments were performed. 100 mg of P-S80/D(20/20)-10x were weight into 100 mL Erlenmeyer flasks. MB concentration as well as the initial pH values were varied. MF particles are commonly positively charged at pH 3 and get negatively charged at higher pH 6, 8 and 10 according to zeta potential measurements on comparable systems.[19, 24] The MB dye is positively charged, therefore the uptake of MB in the solution was expected to increase with increasing pH. Preliminary experiments indicated that the adsorption equilibrium was established rather slowly. We found a time of 24 h however to be sufficient to be near equilibrium. As expected the adsorption capacity was largest at high pHvalues of pH = 10. Exemplarily: at a pH = 3 and an initial c0,MB = 20 mg/L we found an equilibrium concentration of ceq, MB = 7.91 mg/L, while at a pH = 10 and an initial c0,MB = 100 mg/L we found an equilibrium concentration of ceq, MB = 7.4 mg/L. The above experiments are promising. However, the main aim of producing mesoporous MF particles was the benefit of running well packed MF particle columns for adsorption studies. Hence, preliminary column adsorption experiments were investigated which are closer to reality. Figure 4 shows the obtained breakthrough curve. The pattern shows a clear breakthrough point, i.e. no slippage of the MB, which is promising for further detailed studies and would also allow for the removal of trace amounts. Analysis of the breakthrough curve indicates a total capacity of 2.54 mmol/g (≙ 812.4 mg/g). For comparison, some of the highest adsorption capacities for MB are in a comparable range of 980.3 mg/g (specialized, commercial activated carbons).[41]
Conclusion In summary, we have shown a novel synthetic route to obtain meso-/macroporous spherical MF particles of 20-40 µm size. With varying the ratio and total volume of Span80® and ndodecane the particle size distribution can be changed between 20 and 200 µm in diameter. Furthermore, we could prove that the surfactant Span80 ® is necessary for the particle formation and the silica particles are essential to obtain porosity in the hybrid as well as in the polymer material. Hence, we suspect that the mechanism follows an inverse emulsion pathway. With increasing amount of template, the particle stability as well as the specific surface area of the polymer increases. Specific surface areas of up to 200 m2 g-1 could be 15
achieved. Pore sizes are at the upper end of the mesopore regime, which supports fast mass transfer within the particles. The pore sizes do not fit the size of the template particles as a consequence of phase separation issues. Precise pores size control will require careful adjustment of the systems parameters in future studies. The adsorption properties of the particles were screened exemplarily for heavy metal ion pollution as well as for the dye methylene blue by batch and column experiments. The results are promising and high capacities were found. Further and much more detailed analysis of the potential of the herein presented novel MF resins for adsorption applications is however required for a better understanding of the underlying mechanisms.
Acknowledgements This
study
was
supported
by
the
German
Research
Foundation
(Deutsche
Forschungsgemeinschaft, DFG); Grant number: WE4504/3-1. Prof. Dr. Dr. Markus Antonietti is highly acknowledged for his support of the work. Jessica Brandt is acknowledged for laboratory assistance, so are: Björn Kettner for column experiments, Farzad Yazdanbakhsh for preliminary research on particle formation, Simona Schwarz for measuring the concentration of heavy metal ions, Rona Pitschke and Heike Runge for SEM investigations; Ursula Lubahn for TGA analysis; and Sylvia Pirok for EA.
Associated Content Supporting Information Experimental details, characterization methods, and supplementary figures. The supporting Information is available at:
Author Information Corresponding Author [email protected] and [email protected] Notes
16
The authors declare no competing financial interest.
Electronic Supplementary Material is available in the online version of the article.
17
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Leszek Marszall, Bancroft's rule and emulsion inversion, J. Colloid and Interface Science 61 (1977) 202-203. doi:10.1016/0021-9797(77)90431-3 26 Farzad, Y. „Mesoporous Melamine Formaldehyde Resin for Adsorption Applications" Master Thesis 2012, Master of Science program in Polymer Science of the Free University Berlin, Humboldt University Berlin, Technical University of Berlin and University of Potsdam. 27 R. Arshady, M. H. George, Suspension, Dispersion, and Interfacial Polycondensation: A Methodological Survey, Polymer Engineering and Science 1993, 33, 14, 865. 28 Simona Schwarz, Mandy Mende, Dana Schwarz, Chitosan — A Natural Sorbent for Copper Ions, International Journal of Environmental Pollution and Remediation, 4 (2016) 15-19. DOI: 10.11159/ijepr.2016.003 29 S. Bratskaya, Y. Privar, A. Ustinov, Y. Azarova, A. Pestov, Recovery of Au(III), Pt(IV), and Pd(II) Using Pyridylethyl-Containing Polymers: Chitosan Derivatives vs Synthetic Polymers, Ind. Eng. Chem. Res. 55 (2016) 10377–10385. DOI: 10.1021/acs.iecr.6b01376 30 S. Bratskaya, A. Voit, Y. Privar, A. Ziatdinov, A. Ustinov, D. Marinin, A. Pestov, Metal ion binding by pyridylethyl-containing polymers: experimental and theoretical study, Dalton Trans., 45 (2016) 12372-12383. DOI: 10.1039/C6DT01435F 31 A. V. Pestov, A. V. Mehaev, M. I. Kodess, M. A. Ezhikova, Y. A. Azarova, S. Yu. Bratskaya, Imidazolyl derivative of chitosan with high substitution degree: Synthesis, characterization and sorption properties, Carbohydrate Polymers 138 (2016) 252-258. 32 M. Mohsen-Nia, P. Montazeri, H. Modarress, Removal of Cu2+ and Ni2+ from wastewater with a chelating agent and reverse osmosis processes, Desalination 217 (2007) 276–281. doi:10.1016/j.desal.2006.01.043 33 M. X. Tan, Y. N. Sum, J. Y. Ying, Y. Zhang, A mesoporous poly-melamine-formaldehyde polymer as a solid sorbent for toxic metal removal, Energy Environ. Sci. 6 (2013) 3254-3259. DOI: 10.1039/C3EE42216J 34 E. Birinci, M. Gülfen, A. Osman Aydın, Separation and recovery of palladium(II) from base metal ions by melamine–formaldehyde–thiourea (MFT) chelating resin, Hydrometallurgy 95 (2009) 15–21. 35 M.A. Abd El-Ghaffar, Z.H. Abdel-Wahab, K.Z. Elwakeel, Extraction and separation studies of silver(I) and copper(II) from their aqueous solution using chemically modified melamine resins, Hydrometallurgy 96 (2009) 27–34. 36 M. Mihai, I. Bunia, F. Doroftei, C.-D. Varganici, B. C. Simionescu, Highly Efficient Copper(II) Ion Sorbents Obtained by Calcium Carbonate Mineralization on Functionalized Cross-Linked Copolymers, Chem. Eur. J. 21 (2015) 5220 – 5230. DOI: 10.1002/chem.201406011 37 H. Arslanoglu, H. S. Altundogan, F. Tumen, Preparation of cation exchanger from lemon and sorption of divalent heavy metals, Bioresource Technol. 99 (2008) 2699-2705. 38 B. Chen, C. W. Hui, G. McKay, Film-Pore Diffusion Modeling for the Sorption of Metal Ions from Aqueous Effluents onto Peat, Water Res. 2001, 35, 3345-3356. 39 X.S. Jing, X. Liu, P. P. Ling, L.J. Li, C. Long, A.M. Li, Adsorption performances and mechanisms of the newly synthesized N,N′-di (carboxymethyl) dithiocarbamate chelating resin toward divalent heavy metal ions from aqueous media, J. Hazard. Mater. 167 (2009) 589-596. 40 M. Mende, D. Schwarz, C. Steinbach R. Boldt, S. Schwarz, Simultaneous adsorption of heavy metal ions and anions from aqueous solutions on chitosan—Investigated by spectrophotometry and SEM-EDX analysis, Colloids and Surfaces A: Physicochemical and Engineering Aspects 510 (2016) 275-282. 41 M. Rafatullah, O. Sulaimna, R. Hashim, A. Ahmad, Adsorption of methylene blue on low-cost adsorbents: A review, J. Hazard. Mater. 177 (2010) 70-80.
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S0021-9797(17)30312-0 http://dx.doi.org/10.1016/j.jcis.2017.03.064 YJCIS 22161
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
17 December 2016 12 March 2017 13 March 2017
Please cite this article as: D. Schwarz, J. Weber, Synthesis of Mesoporous Poly(Melamine-Formaldehyde) Particles by Inverse Emulsion Polymerization, Journal of Colloid and Interface Science (2017), doi: http://dx.doi.org/ 10.1016/j.jcis.2017.03.064
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Synthesis of Mesoporous Poly(Melamine-Formaldehyde) Particles by Inverse Emulsion Polymerization
Dana Schwarza,b,*, Jens Weber b,* ––––––––– a) Charles University in Prague, Faculty of Science, Department of Organic Chemistry, Hlavova 2030/8, 12843 Prague 2, Czech Republic b) Hochschule Zittau/Görlitz (University of Applied Science), Fachgruppe Chemie, TheodorKörner-Allee 16, 02763 Zittau, Germany E-mail: [email protected] , [email protected] ––––––––– *Corresponding authors: Dana Schwarz, Jens Weber Keywords: melamine resin, particles, emulsion polymerization, dodecane/Span80, adsorption
Graphical Abstract
Abstract: Mesoporous poly(melamine-formaldehyde) (MF) polymer particles with surface areas of up to 200 m2/g were synthesized by an inverse emulsion polymerization using dodecane and Span80® as continuous phase. The finer details of the shape control (using emulsion techniques) and the porosity control (using silica nanoparticles as hard-template) are discussed. The impact of phase-separation processes on the observable porosity of the 20 to
1
200 µm sized spherical particles is analysed by gas sorption methods and electron microscopy. The high density of amine and triazine functional groups in the porous MF particles make the material a promising adsorber for heavy metal ions and methylene blue. In a preliminary column experiment, the synthesized material exhibited a total capacity of 2.54 mmol/g (≙ 812.4 mg/g) for the adsorption of methylene blue.
Introduction In the last decade, many kinds of porous polymers, including poly(melamine-formaldehyde) (MF) resins, particles and foams, have been designed and synthesized because of their potential
applications
as
separation
media
[
1], gas storage media, [2, 3, 4, 5], thermal insulators or heterogeneous catalyst support [6, 7,8,9,10,11] just to name a few. MF is a low cost resin with a high amount of nitrogen functionality, high chemical resistance (both of interest for separation technologies), and good mechanical properties and is therefore a large scale product in industry, mainly for organic coatings or as construction material. Approaches to obtain nanoporous MF resins have been published within the last years using various strategies, including microemulsion strategies, soft-templating,[12, 13] but also hard-templating using silica nanoparticles as template. That way, mesoporous MF materials have been obtained (using silica particles as templates) that showed specific surface areas of up to 250 m2·g-1 with pores in the range of 2.8 up to 12 nm.[14,15,16,17,18] Frequently, the preparation of monoliths suffers from upscaling issues, as heat transfer issues are problematic and might interfere with the phase separation processes. Hence, the preparation of mesoporous MF resin particles by dispersion polymerization, which is more easy to scale, could be a potential solution. Indeed, synthetic routines towards MF resin particles in the range from some hundred nanometer to a few micrometer have been reported, but the resulting products do usually show only low porosity.[19,20,21,22] Reports on the design and particle formation mechanism of (monodisperse) mesoporous MF spheres exhibiting controllable sizes are, to the best of our knowledge, not existing. Thus, it is of great interest to combine those two key properties (i.e. particle formation and porosity) to achieve a new promising adsorber material for water treatment, for example. Recently, we reported the synthesis of nanoporous poly(melamine-formaldehyde) (MF) xerogels composed of 20 nm sized nanoparticles and intergrown particle networks.[23, 24] The 20 to 80 nm sized particles could be synthesized via a simple, one-step reaction in 2
aqueous solution using inexpensive starting materials. Monodisperse, commercial silica nanoparticles (Ludox® HS-40, ~12 nm in diameter) in aqueous solution were added to the reaction solution as a hard template that could be removed after polymerization to yield a mesoporous polymer. The obtained products had a good porosity, but ill-defined shapes and small particles sizes that hinder application in realistic scenarios. In the present study, the formation of micrometer sized MF particles was investigated by inverse emulsion polymerization in order to overcome the above mentioned issues. The shape of the particles was controlled by surfactants, using a combination of Span80® and ndodecane. Mesopores were obtained by hard-templating with silica nanoparticles (Ludox® HS-40, ~12 nm in diameter), as reported before.[15, 23, 24] The particles were analyzed with regard to their porosity by gas adsorption, small-angle x-ray scattering (SAXS) and electron microscopy (SEM). The obtained particles can be applied as adsorber materials in columns. First results of using the obtained MF particles as adsorber material for heavy metal ions (i.e. copper, nickel, lead, zinc, and iron) as well as the dye methylene blue in batch and column experiments are shown and discussed. The MF particles feature promising adsorption properties.
Results and Discussion MF materials can be produced by a dispersion polycondensation reaction in water using oxalic acid as a catalyst.[23] The starting materials are hydrophilic but the growing oligomers get more hydrophobic as the reaction proceeds. The growing MF agglomerates to particles, phase separate ultimately from the continuous water phase. However, the obtained particles were intergrown, and have relatively small sizes (< 1µm) and ill-defined morphologies. The used procedure starts with the preparation of a hydrophilic, water-based precursor-template mixture and it was anticipated that it should be possible to prepare an inverse emulsion, which could provide better control over particle size and shape. Methylated poly(melamine-co-formaldehyde) (MW = 430 g˙mol-1, 84 wt.% sol. in butanol) was used as MF precursor together with silica nanoparticles (Ludox® HS-40, ~12 nm in diameter) of varying amount dispersed in water as hard-template. Additionally, ethanol was added to decrease the viscosity and phosphoric acid acts as an acid catalyst as well as stabilizing agent towards a transparent and clear precursor-template solution.[15] This mixture is a standard first synthesis step and defines the precursor solution containing all necessary chemicals for the polycondensation of MF. The influence of the individual constituents in the mixture have been discussed in more detail elsewhere.[24] In this study, 3
the precursor-template solution has now been dispersed in n-dodecane containing varying amounts of Span80® as surfactant and polymerization was carried out in the resulting dispersion. Span 80 ® (sorbitan monooleate, S80) is a hydrophobic surfactant (HLB-value ~ 4). According to the Bancrofts rule, [25] its use should result in the formation of a w/o-emulsion, which is needed to obtain spherical particles.[26] The impact of the concentration of S80 on the particle formation process was studied. The synthesis procedures resulted in porous polymer/silica hybrid materials (nomenclature –H), which were analysed with regard to composition, morphology and porosity before the removal of the template by treatment with 1 M NaOH (aq.) yielded the final mesoporous polymeric MF resins (nomenclature –P). 3.1 Composition and Chemical Analysis Discussion General aspects about the composition of MF and typical chemical characteristic features have been intensively discussed in previous publications about MF starting from the same type of MF-Oligomer as the precursor material.[15, 23, 24] Therefore, the main aspects of the materials composition will be stated here only shortly, details can be viewed in the electronic supplementary information (ESI). The main analysis with respect to chemical composition was conducted after purification of the obtained materials. The particles were centrifuged and washed several times with water, ethanol and acetone. Afterwards, the solids were dried at 90 °C for one day. Porous MF particles were obtained after removing the template by washing the solid at least three times with NaOH (aq., 1M) for several days. Adjacent, the washing and drying procedure was repeated as described above for the hybrid. Thermogravimetric analysis (TGA, see ESI Fig. S 2 and Fig. S 3) show that the silica content of the hybrid materials varies only slightly with the increased amount of template in the reaction mixture. The shape of the TGA traces shows two major decomposition steps, in accordance with our previous findings. TGA also showed that the silica could be removed efficiently with a remaining mass of < 7.5 wt.% at 1000 °C. This statement is supported by FTIR spectroscopy of the polymeric materials (see Fig. S 4, ESI). The commonly observed strong bands of silica materials that usually dominate the spectra cannot be observed for the polymeric material due to the removal of silica with NaOH.[24] The elemental analysis data is generally in accordance with the TGA data although the residue values for the hybrid as well as for the polymer materials are somewhat higher than 4
observed with TGA. The mismatch could be due to the presence of oxygen within the materials, which was not analysed by EA. The oxygen could originate either from surfactant residues (namely, the sorbitan moiety) or from the formation of ether-bridged MF materials. The particles obtained using Span80®/n-dodecane were slightly orange leading to the assumption that there could be indeed surfactant residues. Span80®, which had an orange colour, contains hydroxyl groups that could interfere with the MF polycondensation, leading to potential partial incorporation. This could explain the observed difference between residues observed by TGA (only silica) and EA (silica and organic oxygen) as well as the slightly higher C/N ratio (compared to MF materials synthesised from the same precursor-template solution in bulk or water dispersion)
3.2 Impact of the Surfactant Concentration The effect of Span80® (S80) and n-dodecane (D) towards the particle formation of MF was examined first using a constant precursor solution (2.1 g of the MF oligomer, 4.0 g of Ludox® HS40 (40 wt.% silica), 2 mL ethanol and 0.5 mL of concentrated phosphoric acid). This combination of the precursor mixture is based on earlier investigations.[15] The variations of the solvent/surfactant mixture and the consequential resulting total volume of emulsion obtained for the particle formation are summarized in Table 1. Table 1. Composition of the continuous phase, Span80 ® (S80) and n-dodecane (D), used for the formation of MF particles by emulsion polymerization at fixed MF Oligomer (2.1 g solid) and fixed silica (1.6 g solid) content. S80/D
S80/D
S80/D
S80/D
S80/D
S80/D
S80/D
S80/D
(0/40)
(5/35)
(10/30)
(15/25)
(20/20)
(10/10)
(30/30)
(40/40)
VS80
(mL)
0
5
10
15
20
10
30
40
VD
(mL)
40
35
30
25
20
10
30
40
Vtotal
(mL)
40
40
40
40
40
20
60
80
Particle size d (µm)
-
20-200
~20
~20
20-30
~20
20-200
20-200
Particle integrity
-
()
x
x
()
x
Figure 1 presents SEM images of the products obtained by varying the ratio and total volume of S80/D after the purification with ethanol, acetone, and water and drying in the oven at 90 °C. The material obtained using only n-dodecane as continuous phase (see Figure 1 HS80/D(0/40) in comparison with the other images) is of ill-defined morphology, i.e. no spherical particles were obtained. Only the combination of n-dodecane with Span80 ® acting 5
as a surfactant leads to spherical shaped products. We analysed the impact of an increasing amount of S80 while keeping the total volume of the continuous phase constant. It turned out that a 50/50 vol.% mixture of S80/D yielded spherical particles of 20-30 µm diameter, which had best (but not yet sufficient) mechanical stability. Secondly, the ratio S80/D was kept constant at 50/50 vol.%, but the total volume of the continuous phase was varied. Increasing the total volume of S80/D results in a broader particle size distribution but also in better mechanical stability, that increased particle integrity. The increasing particle stability with an increasing total emulsion volume (i.e. lowering the solid content from about ca. 10 wt.% at 40 mL total volume of S80/D to ca. 5 wt.% at 80 mL total volume of S80/D) might be related to differences in the details of the phase separation processes upon polymerization (leading to different mechanical properties of the particles), but also to lowered mechanical stress within the emulsion (lower rate of collision events). Increasing the solid content to about 20 wt.% does indeed give only large, ill-defined pieces and only insular spherical particles. In terms of successful formation of spherical particles having high stability, best results were received using a 1:1 ratio of S80/D and solid contents in the range of 5-7 wt.%.
6
Figure 1: SEM images of the dried hybrid (H) or polymer (P) particles prepared with different compositions of Span80® (S80) and n-dodecane (D) (see Table 1).
Beside the analysis of the macroscopic appearance and the chemical identity, the porosity was analyzed for both type of materials (hybrid, H and polymer, P) by nitrogen adsorption/desorption at 77.4 K (s. Fig. 2). Generally, the hybrid materials as well as the polymer materials had specific surface areas between SBET of 90 and 180 m2 g-1, and 85 and 260 m2 g-1, respectively. Hybrid and polymer materials show however significant differences with regard to their pore size distributions (PSD). The hybrid particles show isotherms of type IV (H2) character and the calculation of the pore size distribution (quenched solid density functional theory (QSDFT) adsorption branch) shows predominantly small mesopores of 4-5 nm radius beside a few micropores. The mesoporous MF polymer materials, obtained after etching of the silica particles shows type IV (H1) hysteresis at high relative pressures, indicative of large mesopores. The above observations are well in line with previous results obtained by bulk polycondensation of the precursor-silica mixture [15]. It was reasoned that the silica particles 7
demix partially from the growing MF polymer due to changes in the free energy upon polymerization. The MF polymer is much more hydrophobic than its oligomers, which can further enhance the demixing tendency. Hence, the pore size observed in the hybrid materials arises mainly from void spaces in between packed silica particles. Indeed a rough estimation of the void in between closely packed 12 nm sized particles gives a size of about 4.4 nm, a result in very good agreement with the pore sizes of the hybrid material as determined by gas adsorption. The isotherm shape (H2-type hysteresis) is well known for ink-bottle-type pores or cavitation effects, both are expected for pores with only narrow entrances. This is again expected for the interstitial voids. Hence, etching of the silica nanoparticles does not lead to well-defined 12 nm-sized pores in the resulting MF polymers, but to larger mesopores and some small macropores. Those arise from the removal of small silica particle agglomerates. Hence, the surface areas of the polymer were always slightly lower than for the corresponding hybrid materials (s. Table 2). This fact can be attributed by the increasing pore size after the removal of the template and the removal of the interparticle voids. No exact values of the pore sizes of the polymer materials could be estimated. This is due to their large size, which is at the upper end of the resolution of nitrogen adsorption. Finally, analysis of the small-angle X-ray scattering data obtained from H and P-type materials supports the above description (see ESI for details). The scattering patterns of the hybrid materials are dominated by the scattering of the spherical silica particles, indicated by the characteristic form factor undulations of spherical objects. However, no significant middle- to long-range ordering of the particles can be observed (absence of a correlation peak), giving support to our thesis that the particles do mainly form small agglomerates. The scattering patterns of the MF polymers are less-defined and show pronounced scattering also at low scattering angles, which are characteristic for the presence of larger structures (large meso- and macropores).
8
Figure 2: selected nitrogen ad-/desorption isotherms of the hybrid (H, with template) material (left-hand-side) and the corresponding nitrogen ad-/desorption isotherms of the polymer (P, template removed) particles (middle); selected pore size distribution (PSD) of the hybrid and polymer materials calculated with N2 at 77K on carbon (slit/cylindrical/spherical pores, QSDFT adsorption branch data model) (right-hand-side).
Table 2: calculated results from the nitrogen ad-/desorption isotherms of the hybrid (H) and polymer (P) materials with QSDFT model on carbon for slit/cylindrical/spherical pores (adsorption branch data model), pore volume (V) calculated for a maximum value at p/p0 = 0.995; shown in Figure 2.) S80/D
20/20 30/30 40/40
S (m2 ·g-1) H
P
0/40
153
182
93
165
Vpore (cm ·g )
0.19
0.20
0.15
0.27
dpore (nm)
4-12
4-12
4-12
4-14
2
131
152
85
262
3
-1
S (m ·g )
-1
In summary, Span80® is acting, as supposed, as a surfactant leading to spherical particles. The added silica particles act as a hard template resulting in porous particles (s. Scheme 1). The size of the particles is strongly dependent on the concentration and ratio of the surfactant. Furthermore, the porosity analysis of the materials shows a very good agreement with the results obtained by bulk polycondensation of the mixture.[15] Hence, we argue, that the polycondensation as such is not significantly influenced by the presence of the continuous phase. The composition of the external phase has however (as expected) a significant impact on the particle characteristics (size, stability, etc.). The presence of a high concentration of Span80® is known to be beneficial for the formation of MF or urea-formaldehyde resins from 9
water-in oil-dispersions.[27] However, we cannot clarify the exact role of the surfactant at this
stage, also given the fact that the silica particles will also act as surface active agents. We suspect that Span80® does more (e.g. changing the continuous phases polarity etc.) than just provide a surface coverage of the droplets, but any discussion of this effects would be too speculative at the moment. Before discussion of the up-scaling of the synthesis concept, the influence of the template content on the characteristics of the materials will be discussed shortly.
Scheme 1: general scheme of mesoporous MF particles formation, precursor solution (MF oligomers and silica particle) is added to a Span80 ® and n-dodecane emulsion at 40 °C leading to the formation of mesoporous MF particles by dispersion polycondensation reaction.
3.3 Impact of the Template Content In our opinion, the material P-S80/D(20/20) had the best potential for further investigations (s. Figure 1), as it showed a compromise between particle size, stability and porosity. The template concentration is expected to have influence on the final porosity, but maybe also on the formation and stability of the dispersion (cf. Pickering effects). Therefore, the silica content was varied between 0 and 60 wt.% (related to the mass of the MF oligomer). The materials discussed previously were prepared using 43 wt.% of silica relative to the MF oligomer. Morphology analysis was again done by SEM and gas adsorption measurements. In general, the template seems to be necessary for a sufficient particle stability, as shown in Figure 3. The
10
SEM pictures display the particles after removing the template, washing and drying in the oven at 90 °C for one day. Without addition of silica nanoparticles to the reaction mixture, the resulting MF particles are completely broken. Furthermore, the particle size is slightly decreasing from 20 to about 10 µm in diameter with increasing template content. The particle integrity seems to be best for silica contents of 53 and 57 wt.% relative to the MF oligomer. At a maximum amount of 60 wt.% template added to the mixture, the particles appear to have a stability problem and broken particles were obtained mainly. The underlying mechanism needs to be clarified in future studies.
P0wt.%
P43wt.%
P49wt.%
P53wt.%
P57wt.%
P60wt.%
Figure 3: SEM images of the polymeric (P) materials in dependence on the amount of silica template added with respect to the MF content. Close-up views and the comparison of the hybrid with the polymeric material are shown in the ESI (s. Fig. S 5).
The N2 ad-/desorption isotherms and SAXS analysis data of the hybrid and polymer materials is very much comparable to the isotherms discussed previously (see ESI). If no silica was present during the synthesis, only a low surface area of SBET ~20 m2g-1 was found for the resulting polymer. Upon substantial increase of the silica content, an increase of specific surface areas of the MF polymers was observed, up to ~ 200 m2g-1 at the maximum.
11
Table 3: calculated results from the nitrogen ad-/desorption isotherms of the hybrid (H) and polymer (P) materials with the QSDFT model on carbon for slit/cylindrical/spherical pores (adsorption branch data model), pore volume (V) calculated for a maximum value at p/p0 = 0.995; shown in Figure S 6; Silica content calculated from the reaction mixture composition MF Oligomer/silica. Silica content 0 wt.% 43 wt.% 49 wt.% 53 wt.% 57 wt.% 60 wt.% 153 137 105 97 114 S (m2g -1) 35 H
P
Vpore
(cm3g-1) dpore (nm) 2
-1
S (m ·g )
0.20
0.19
0.15
0.12
0.12
0.23
(>20)
4-12
4-12
4-12
4-12
7-20
95
116
174
203
21
131
It can be concluded that the absence or presence of silica particles play a significant role in the formation and stabilization process. It is known that there are interactions between silica nanoparticles and the growing MF oligomers.[24] The phase separation properties were shown to be depended on SiO2 concentration. We expect that complex interactions (that are, for instance, also dependent on the pH) play a major role here as well. 3.5 Adsorption Properties The purpose of the present investigation was the establishment of a routine towards mesoporous, micrometer sized spherical polymer particles that combine the advantages of common ion-exchange resins (good applicability in fixed beds or column applications etc.) with the promising properties of highy porous MF materials (high adsorption capacity for e.g. CO2 or micropollutants). Hence, we were interested in a screening of the adsorption properties of the particles for application in water treatment. We targeted two types of micro-pollutants, namely heavy metal-ions and organic compounds. To do so, an up-scaling of the reaction mixture was necessary in order to provide sufficient quantities of material (s. experimental section, ESI). Up-scaling was done using different protocols at constant silica/MF ratio of 43 wt.%. Material S80/D-(20/20) was scaled by a factor of 5 and 10, respectively. Material S80/D-(40/40) was also scaled by a factor of 5. For reasons unknown to us, the particles obtained by scaling factor 5 were broken. Scale-up using a factor 10 did however yield intact particles of 20-40 µm size (s. Fig. S6). We suspect that finer details of the synthesis procedure (such as stirring device and rate as well as mechanical forces during the washing and etching procedures), which are not perfectly comparable for the different batches play a major role here.
12
However, there is no significant difference in the chemical properties (FTIR, see ESI) as well as the porosity of the materials compared to small-scale batches. For example, the specific surface areas for the hybrid (118 – 170 m²g-1) as well as for the polymer materials (~ 160 m²g1
) have the same dimensions as for the initial experiments, please see ESI for more details.
For a general statement on the adsorption properties of MF particles in aqueous solution, we examined the adsorption of heavy metal ions as well as the dye methylene blue (MB) as a preliminary investigation. Five common heavy metal ions in water pollution were investigated in a batch experiment for the adsorption experiments on the particles from P-S80/D(40/40)type. To get a general idea of the maximum uptake, we treated the MF particles with a relatively high initial heavy metal ion concentration between 1 and 2 mmol L-1. The pH of the stock solutions was adjusted to a value of around 4 as the pH of the solution commonly increases upon contact with the MF resins (towards pH=6) due to the alkaline behavior of the melamine resin (s. Table S1). This was important, as the adsorption should not be disturbed by any precipitation of heavy metal hydroxides that are commonly formed at neutral to high pH-values. As shown in many other adsorption studies, amine groups are the main functionality for the adsorption mechanism.[28, 29, 30, 31] In MF, nitrogen is present in the aromatic triazine ring, and as secondary or tertiary amine between the triazine units (s. structure in Scheme 1). At an initial pH of 4, the amine group should be mainly of neutral charge, leading to high potential of coordinative binding with the positively charged heavy metal ions. The adsorption uptakes obtained from the screening experiments are displayed in Figure S1 and calculated with the unit mmol g-1 to neglect the mass of the heavy metal ions in contrast. Additionally, we calculated the adsorption efficiency, shown in Figure 4. All five heavy metal ions were adsorbed on the particles with some selectivity for copper and lead. The preference of copper in comparison to e.g. nickel is in line with results obtained by other researchers, who used also amine based adsorber materials.[32] The adsorption efficiency values are relatively low, which is a consequence to the high initial metal ion concentrations used for the screening. Lately, mesoporous poly(melamine formaldehyde) was investigated as an excellent adsorbent for lead ions in the ppb range for water treatment with a maximum adsorption capacity of q eq ~ 3·10-3 mmol·g-1.[33] A similar adsorption preferential of ions compared to our results was reported (i.e. Pb2+ < Cu2+ ~ Cd2+ >> Zn2+ >> Ni2+). Furthermore, a higher adsorption efficiency was reported, which is however probably related to the much lower initial heavy metal ion concentration (ppb range). The calculated adsorption capacity reported there is for 13
some reasons much lower than the capacity reached in our investigation for lead (q eq ~ 0.09 mmol.g-1). This might be due to the distinct different way of preparing the material. The adsorption capacity of copper on poly(melamine-formaldehyde-thiourea) resin was reported by yet another group with a value of 0.025 mmol.g-1 (at pH 4).[34] This is roughly four times lower than results obtained using P-S80/D(40/40)-5x. On the other hand, melamine resin modified with tetraoxalyl ethylenediamine reached an adsorption capacity value for copper of 0.5 mmol/g.[35] Nevertheless, most common adsorber materials reach far higher adsorption capacities at comparable adsorption parameters.[36, 37, 38, 39] Recently, Schwarz et al. have shown the efficient and favorable adsorption of both, heavy metal ions as well as the commonly corresponding oxyanion by using the amine functional groups of the polymer chitosan.[40] Initial results on the adsorption of oxyanions by porous MF particles indicate also a good binding capacity and significant interactions. In continuative studies, the adsorption of heavy metals in combination with their counter ions on melamine based particles needs to be studied in more detail, as the big picture is still lacking a solid, mechanistic understanding.
Figure 4: heavy metal ion adsorption on P-S80/D(40/40)-5x (SBET = 164 m2 g-1) after removing the template, pH0 ~ 4 in batch experiment (left-hand-side) and adsorption of methylene blue on P-S80/D(20/20)-10x (SBET = 153 m2 g-1) in column experiment (righthand-side). 14
Persistent organic pollutants are another class of micro pollutants that pose new challenges for water treatment by adsorption. Methylene blue (MB) adsorption was analyzed in order to have a first idea on the usability of the MF particles as adsorbents for charged organic molecules. Initially, batch experiments were performed. 100 mg of P-S80/D(20/20)-10x were weight into 100 mL Erlenmeyer flasks. MB concentration as well as the initial pH values were varied. MF particles are commonly positively charged at pH 3 and get negatively charged at higher pH 6, 8 and 10 according to zeta potential measurements on comparable systems.[19, 24] The MB dye is positively charged, therefore the uptake of MB in the solution was expected to increase with increasing pH. Preliminary experiments indicated that the adsorption equilibrium was established rather slowly. We found a time of 24 h however to be sufficient to be near equilibrium. As expected the adsorption capacity was largest at high pHvalues of pH = 10. Exemplarily: at a pH = 3 and an initial c0,MB = 20 mg/L we found an equilibrium concentration of ceq, MB = 7.91 mg/L, while at a pH = 10 and an initial c0,MB = 100 mg/L we found an equilibrium concentration of ceq, MB = 7.4 mg/L. The above experiments are promising. However, the main aim of producing mesoporous MF particles was the benefit of running well packed MF particle columns for adsorption studies. Hence, preliminary column adsorption experiments were investigated which are closer to reality. Figure 4 shows the obtained breakthrough curve. The pattern shows a clear breakthrough point, i.e. no slippage of the MB, which is promising for further detailed studies and would also allow for the removal of trace amounts. Analysis of the breakthrough curve indicates a total capacity of 2.54 mmol/g (≙ 812.4 mg/g). For comparison, some of the highest adsorption capacities for MB are in a comparable range of 980.3 mg/g (specialized, commercial activated carbons).[41]
Conclusion In summary, we have shown a novel synthetic route to obtain meso-/macroporous spherical MF particles of 20-40 µm size. With varying the ratio and total volume of Span80® and ndodecane the particle size distribution can be changed between 20 and 200 µm in diameter. Furthermore, we could prove that the surfactant Span80 ® is necessary for the particle formation and the silica particles are essential to obtain porosity in the hybrid as well as in the polymer material. Hence, we suspect that the mechanism follows an inverse emulsion pathway. With increasing amount of template, the particle stability as well as the specific surface area of the polymer increases. Specific surface areas of up to 200 m2 g-1 could be 15
achieved. Pore sizes are at the upper end of the mesopore regime, which supports fast mass transfer within the particles. The pore sizes do not fit the size of the template particles as a consequence of phase separation issues. Precise pores size control will require careful adjustment of the systems parameters in future studies. The adsorption properties of the particles were screened exemplarily for heavy metal ion pollution as well as for the dye methylene blue by batch and column experiments. The results are promising and high capacities were found. Further and much more detailed analysis of the potential of the herein presented novel MF resins for adsorption applications is however required for a better understanding of the underlying mechanisms.
Acknowledgements This
study
was
supported
by
the
German
Research
Foundation
(Deutsche
Forschungsgemeinschaft, DFG); Grant number: WE4504/3-1. Prof. Dr. Dr. Markus Antonietti is highly acknowledged for his support of the work. Jessica Brandt is acknowledged for laboratory assistance, so are: Björn Kettner for column experiments, Farzad Yazdanbakhsh for preliminary research on particle formation, Simona Schwarz for measuring the concentration of heavy metal ions, Rona Pitschke and Heike Runge for SEM investigations; Ursula Lubahn for TGA analysis; and Sylvia Pirok for EA.
Associated Content Supporting Information Experimental details, characterization methods, and supplementary figures. The supporting Information is available at:
Author Information Corresponding Author [email protected] and [email protected] Notes
16
The authors declare no competing financial interest.
Electronic Supplementary Material is available in the online version of the article.
17
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