Microporous Polymer Networks Made by Cyclotrimerization of

Letter pubs.acs.org/macroletters

Microporous Polymer Networks Made by Cyclotrimerization of Commercial, Aromatic Diisocyanates Eduard Preis, Nicole Schindler, Sven Adrian, and Ullrich Scherf* Bergische Universität Wuppertal, Macromolecular Chemistry Group (buwmakro) and Institute for Polymer Technology, Gauss-Str. 20, D-42199 Wuppertal, Germany S Supporting Information *

ABSTRACT: The cyclotrimerization of commercial, aromatic diisocyanates allows for the formation of monolithic, microporous polymer networks with SBET surface areas up to 1300−1500 m2/g. The process has been up-scaled for production of 100 g batches. The monolithic materials show a promising potential for the removal of lipophilic components from aqueous mixtures.

M

icroporous polymer networks (MPNs) today attract strongly increasing attention due to prospective applications in gas storage/separation, catalysis and sensor devices or as superabsorbers for hydrophobic solvent or oil traces in aqueous media. 1−13 However, especially for applications as absorbers or in gas storage, a simple fabrication scheme is needed that allows for their mass production in an inexpensive way. Noble metal catalyzed aryl−aryl coupling routes and schemes using expensive monomers do, of course, not fulfill these requirements. Here, we describe a metal-free MPN fabrication route starting from commercially available, aromatic diisocyanates leading to high-porosity micoporous polymer networks in a simple cyclotrimerization scheme (Figure 1). Cyclotrimerizations of other monomers have been reported in the literature until now: (i) Dicyanoarenes, as terephthalodinitrile, can be cyclotrimerized into polymer networks with 1,3,5-triazine cores under ionothermal conditions at elevated temperatures, for example, in molten salts.14−16 (ii) Diacetyl-substituted arenes can be condensed into MPNs with 1,3,5-trisubstituted benzene cores.17,18 (iii) Bisindanone derivatives as well as macrocycles with two −CH2−CO− functions can be coupled into laddertype MPNs with truxene or other multicyclic cores.19,20 (iv) Various diethynyl-substituted arenes can be cyclotrimerized into microporous, benzene-cored materials, in the form of soluble hyperbranched polymers or insoluble polymer networks.21,22 In the now presented study, the cyclotrimerization of commercial, aromatic diisocyanates as low-priced monomers used for polyurethane mass production leads to microporous, triarylisocyanurate-cored, microporous polymer networks with SBET surface areas of up to 1320 m2/g. The cyclotrimerization of aromatic isocyanates under triarylisocyanurate formation have been extensively described by using several catalysts as © XXXX American Chemical Society

Figure 1. Chemical structures of the commercially avaible diisocyanates used as well as of the resulting aromatic polymer networks.

metal salts of carboxylic acids,23 tertiary amines,24,25 sodium para-toluenesulfinate (p-TolSO2Na)/1,3-dimethyl-2-imidazolidinone,26 tetrabutylammonium fluoride (TBAF),27 tetrakis(dimethylamino)ethylene (TDAE),28 or N-heterocyclic carbenes.29 Cyclotrimerization of isocyanate groups is known to improve such properties of polyurethanes as thermal and chemical resistance,30 flame retardance,31 and film forming characteristics.32 Nonporous polymer networks with isocyanurate knots have been produced in the cyclotrimerization of Received: October 9, 2015 Accepted: October 28, 2015

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DOI: 10.1021/acsmacrolett.5b00726 ACS Macro Lett. 2015, 4, 1268−1272

Letter

ACS Macro Letters Table 1. Specific SBET surface Areas and Yields of MPNs Derived from Monomers M1−M3 (Reaction Time: 24 h)

*

p-TolSO2Na is very poorly soluble in this solvent; addition as suspension (after sonification).

catalyst/monomer ratio should influence the cross-linking density. Table 2 summarizes our results for reduction of the catalyst/ monomer ratio. The results show, at least for the catalyst

commercial, araliphatic diisocyanates as methylenediphenyldiisocyanat (MDI) and aliphatic diisocyanate as hexamethylenediisocyanate (HMI).33 Our novel synthetic procedure toward microporous, triarylisocyanurate-based polymer networks also allows for the generation of MPN monoliths by carefully adjusting the ratio between monomers and solvents. These monolithic, microporous materials have be used for the removal of hydrophobic/ lipophilic components from aqueous media simply by contacting the compact porous sponges with hydrocarboncontaminated water. Hereby, the presence of the porous sponge as compact material represents a strong advantage over powdery, microporous materials since a very simple separation from liquid media is possible after absorption of the hydrocarbon component. Finally, we have been able to demonstrate a practically relevant up-scaling potential for our MPN materials by running batches up to the 100 g scale while maintaining the high porosity of the products. Results and Discussion: Based on the available literature three catalyst systems were chosen for our investigations into the cyclotrimerization of aromatic, bifunctional isocyanates as tectons for the formation of microporous polymer networks: (i) p-TolSO2Na, (ii) tetrabutylammonium fluoride (TBAF), and (iii) TDAE (solvents: tetrahydrofuran (THF), 1,3dimethyl-2-imidazolidinone (DMI) and 1,2-dichlorobenzene (ODB) for catalysts (i) and (ii), as well as DMI, acetonitrile (MeCN) and tetramethylurethane (TMU) for catalyst (iii)). Table 1 summarizes yields and specific SBET surface areas (as derived from nitrogen sorption isotherms) under variation of catalysts, solvents, and reaction temperatures. The following monomer concentrations were used in this first round of experiments: 0.24 (M1), 0.11 (M2), and 0.12 g/mL (M3), corresponding to concentrations near the solubility limits of the monomers. Based on these orienting experiments (Table 1), the batches P1, P2, and P3 (for catalyst TBAF), P4 (for catalyst pTolSO2Na), and P8 (for catalyst TDAE) were selected for further optimization, all based on monomer M1. Please note that although P5 for monomer M2 showed a high specific surface area, we did not further follow this conditions, since the yield of only 51% wasn’t encouraging. Within the second (optimization) round of experiments, we only varied the amount of catalyst, since this parameter should have a strong impact on the course of the reaction, especially in the synthesis of polymer networks, where the connecting points are created in the cyclotrimerization reaction and, hence, the

Table 2. SBET Surface Area Data and Yields of the MPNs Produced in the Optimization Experiments for Different Catalyst Loadings (Monomer: M1)

systems p-TolSO2Na DMI at 150°C (batch P11−1%) and TBAF THF at RT (batch P14−1%), a significant increase of SBET to 1320 and 600 m2/g, respectively. The following results are all based on networks made from monomer M1. The nitrogen adsorption isotherm of P11−1% shows some unique features (Figure 2): First, it shows a rapid gas uptake at low pressure (P/P0 > 0.05), which indicates a predominantly microporous structure (type I isotherm). However, the desorption branch displays a strong hysteresis. There are common models to categorize the hysteresis loops.34

Figure 2. Nitrogen gas sorption isotherms for P11−1% (monomer: M1; at a temperature of 77 K). 1269


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ACS Macro Letters According to this classification of hysteresis loops, the best suited description here is a “type H2” hysteresis. In such systems, the hysteresis between adsorption and desorption isotherms is usually understood as a sign of an inhomogeneous distribution of interconnected pores,35 with pores of smaller diameter acting as bottlenecks, as so-called “ink-bottle” pores.36 Table 3 summarizes the gas sorption properties of sample P11−1%. A high specific surface area of 1320 m2/g with a total Table 3. Gas Sorption Data of MPN P11-1% (monomer: M1) BET surface area (m2/g)

pore volume (cc)

H2 uptake (%)

CO2 uptake (%)

CH4 uptake (%)

selectivity (CO2/CH4) (273 K)

selectivity (N2/H2) (77 K)

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1.35

1.3

14.4

1.1

4.69

5.99

pore volume of 1.25 cc was calculated. Reasonable gas uptakes of 1.3 w% for hydrogen (at 77 K), 1.1 w% for methane (at 273 K), and 14.4 w% for carbon dioxide (at 273 K) were measured. Considering the ease of MPN production, these values are remarkable, also, in relation to literature values. From the adsorption isotherms, the selectivity values for carbon dioxide over methane and nitrogen over hydrogen, respectively, were calculated. The observation that monolithic products are formed under suited reaction conditions prompted us to investigate the influence of the monomer concentration on appearance and porosity of the resulting products. For this series of experiments monomer M1 and TBAF as catalyst (in THF at RT) were selected (see conditions P3, Table 1). Figure 3a presents a plot of the calculated specific BET surface area versus monomer concentration. Decreasing the monomer concentration causes a decrease of the specific SBET surface area most probably due a reduced cross-linking density. After reaching a certain threshold (dashed line in Figure 3a) monoliths are no longer formed, the networks are isolated as fine powders. The corresponding nitrogen sorption isotherms of three MPN samples made at different monomer concentrations are depicted in Figure 3b (for more details, see Tables S16 and S17, SI). For the highest concentration (0.23 g/mL, sample M1P3−1), the isotherm can be clearly assigned as type I (according to the IUPAC classification), with a rapid gas uptake at low relative pressure (P/P0 < 0.05), which indicates a predominant microporous structure, followed by a second adsorption step at higher pressure (P/P0 > 0.9). The “type H4” hysteresis for all samples is attributed to the slit pore effect. Some hysteresis in the low pressure range for all samples indicates a somewhat restricted access to the pores.37 Increasing the solvent/monomer ratio leads to a reduction of the cross-linking density and specific SBET surface area of the resulting monoliths. With increasing amount of solvents the resulting monoliths become brittle and morphologically instable. After passing the concentration threshold for monolith formation the nitrogen adsorption isotherms of the formed powders shows clear type III appearance (see sample M1P3−5) with low specific SBET surface areas of about 130 m2/g and with dominant gas uptake at higher relative pressure, due to a filling of the voids between the polymer particles. A further increase of the solvent/ monomer ratio had no further impact on the properties of the formed MPN powders (sample M1P3−6). The obtained microporous materials were also analyzed by FTIR-spectroscopy to confirm the (nearly) complete con-

Figure 3. (a) Monomer concentration vs specific surface area (monomer: M1, conditions: P3); the dashed line represents the threshold concentration for the formation of stable monoliths. (b) Nitrogen adsorption (filled symbols) and desorption (open symbols) isotherms at 77 K for three samples (M1P3-1, M1P3-3, M1P3-5, see (a)).

sumption of the isocyanate functions and to extract information on the formation of the isocyanurate rings. Figure S1 (SI) shows typical FTIR spectra of MPNs obtained from monomer M1 with the catalyst systems TBAF and p-TolSO2Na (P11−1% and P14−1%). For both MPNs, the characteristic IR-band for isocyanate functions at around 2250 cm−1 is not observed, a novel band at about 1695−1710 cm−1 for the formed isocyanurate rings (Figure S1, SI) appears, except for batch P10, where the band is observed at 1630/1640 cm−1. In the case of P10, isocyanurate formation seems questionable. Jones et al. reported a carbonyl absorption band at 1694−1711 cm−1 for cyclic isocyanurate functions.28 The removal of oil contaminations from seawater became a global challenge, especially related to oil spill accidents. So there is a need for the development of efficient and economical 1270


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(4) Akhavan, B.; Jarvis, K.; Majewski, P. ACS Appl. Mater. Interfaces 2013, 5, 8563−8571. (5) Lei, W. W.; Portehault, D.; Liu, D.; Qin, S.; Chen, Y. Nat. Commun. 2013, 4, 1777. (6) Zhu, Y.; Zhang, L.; Schappacher, F. M.; Pottgen, R.; Shi, J.; Kaskel, S. J. Phys. Chem. C 2008, 112, 8623−8628. (7) Wu, D.; Xu, F.; Sun, B.; Fu, R.; He, H.; Matyjaszewski, K. Chem. Rev. 2012, 112, 3959−4015. (8) Zhao, W.; Hou, Z.; Yao, Z.; Zhuang, X.; Zhang, F.; Feng, X. Polym. Chem. 2015, 6, 7171−7178. (9) Zhuang, X.; Gehrig, D.; Forler, N.; Liang, H.; Wagner, M.; Hansen, M. R.; Laquai, F.; Zhang, F.; Feng, X. Adv. Mater. 2015, 27, 3789−3796. (10) Zhuang, X.; Zhang, F.; Wu, D.; Feng, X. Adv. Mater. 2014, 26, 3081−3086. (11) Han, S.; Feng, Y.; Zhang, F.; Yang, C.; Yao, Z.; Zhao, W.; Qiu, F.; Yang, L.; Yao, Y.; Zhuang, X.; Feng, X. Adv. Funct. Mater. 2015, 25, 3899−3906. (12) Zhuang, X.; Zhang, F.; Wu, D.; Forler, N.; Liang, H.; Wagner, M.; Gehrig, D.; Hansen, M. R.; Laquai, F.; Feng, X. Angew. Chem., Int. Ed. 2013, 52, 9668−9672. (13) Schwab, M. G.; Fassbender, B.; Spiess, H. W.; Thomas, A.; Feng, X. J. Am. Chem. Soc. 2009, 131, 7216−7217. (14) Thomas, A.; Fischer, A.; Goettmann, F.; Antonietti, M.; Müller, J.-O.; Schlögl, R.; Carlsson, J. M. J. Mater. Chem. 2008, 18, 4893− 4908. (15) Kuhn, P.; Antonietti, M.; Thomas, A. Angew. Chem., Int. Ed. 2008, 47, 3450−3453. (16) Bojdys, M. J.; Müller, J. O.; Antonietti, M.; Thomas, A. Chem. Eur. J. 2008, 14, 8177−8182. (17) Rose, M.; Klein, N.; Senkovska, I.; Schrage, C.; Wollmann, P.; Böhlmann, W.; Böhringer, B.; Fichtnerd, S.; Kaskel, S. J. Mater. Chem. 2011, 21, 711−716. (18) Wisser, F. M.; Eckhardt, K.; Wisser, D.; Böhlmann, W.; Grothe, J.; Brunner, E.; Kaskel, S. Macromolecules 2014, 47, 4210−4216. (19) Sprick, R. S.; Thomas, A.; Scherf, U. Polym. Chem. 2010, 1, 283−285. (20) Samanta, S. K.; Preis, E.; Lehmann, C. W.; Goddard, R.; Bag, S.; Maiti, P. K.; Brunklaus, G.; Scherf, U. Chem. Commun. 2015, 51, 9046−9049. (21) Liu, J.; Zheng, R.; Tang, Y.; Häussler, M.; Lam, J. W. Y.; Qin, A.; Ye, M.; Hong, Y.; Gao, P.; Tang, B. Z. Macromolecules 2007, 40, 7473−7486. (22) Hu, R.; Lam, J. W. Y.; Deng, M.; Li, H.; Li, J.; Tang, B. Z. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4752−4764. (23) Frentzel, W. Ber. Dtsch. Chem. Ges. 1888, 21, 411−413. (24) Kogon, I. C. J. Org. Chem. 1959, 24, 83−86. (25) Kogon, I. C. J. Am. Chem. Soc. 1956, 78, 4911−1914. (26) Moritsugu, M.; Sudo, A.; Endo, T. J. Polym. Sci., Part A: Polym. Chem. 2011, 49, 5186−5191. (27) Nambu, Y.; Endo, T. J. Org. Chem. 1993, 58, 1932−1934. (28) Giuglio-Tonolo, A. G.; Spitz, C.; Terme, T.; Vanelle, P. Tetrahedron Lett. 2014, 55, 2700−2702. (29) Duong, H. A.; Cross, M. J.; Louie, J. Org. Lett. 2004, 6, 4679− 4681. (30) Samborska-Skowron, R.; Balas, A. Polym. Adv. Technol. 2002, 13, 653−662. (31) Pearce, E., Ed. Flame-Retardant Polymeric Materials; Springer Science & Business Media Verlag: Berlin, Heidelberg, 2012; ISBN: 1468421484. (32) Ahmadabadi, H. Y.; Rastegar, S.; Ranjbar, Z.; Allahdini, A. Prog. Org. Coat. 2014, 77, 1688−1694. (33) Moritzu, M.; Sudo, A.; Endo, T. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2631−2637. (34) Naumov, S. Dissertation, Universität Leipzig, Leipzig, Germany, 2009. (35) Mason, G. J. Colloid Interface Sci. 1982, 88, 36−46. (36) McBain, J. W. J. Am. Chem. Soc. 1935, 57, 699−700. (37) Jeromenok, J.; Weber, J. Langmuir 2013, 29, 12982−12989.

schemes for the elimination of oil contaminations in aqueous systems.38−40 To test the suitability of the microporous monoliths for absorption of lipophilic compounds, they have been contacted with dodecane-water mixtures (Figure 4). A monolith of

Figure 4. Oil-soaking test for the absorption of dodecane (dyed with Sudan I) from an aqueous mixture.

sample M1P3−3 (see Figure 3, contact angle with water ca. 93°) was used for a preliminary test of the oil-recovery potential by dipping a monolith into a dodecane (dyed with Sudan I)/water mixture. Due the low MPN density of ca. 0.18 g/cm3 the monolith remains on the surface of the fluid mixture.2 Treatment of the MPN monolith with the dodecane/water mixture is depicted in Figure 4. A complete oil uptake was accomplished in a short time and a 4-fold mass increase of the MPN monolith was observed in this initial experiment, from 125 mg dry mass to 485 mg after dodecane uptake. The oilfilled MPN still shows good mechanical stability, what may be important for “real” applications. Finally, we have tested the up-scaling potential for reaction conditions P11−1%. Also an up-scaling to the 100 g monomer (M1)-scale leads to a compact, highly microporous material that shows, interestingly, a slightly increased (!) SBET surface area (1510 m2/g, see Figure S3, SI). We report a very simple protocol for the formation of monolithic MPN materials by cyclotrimerization of commercial, aromatic diisocyanates. The monoliths show high SBET surface areas up to 1300−1500 m2/g. The monoliths possess a high ability for the removal of lipophilic/hydrophobic components from aqueous mixtures, what may be useful for applications in the treatment of oil-contaminated water.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.5b00726. Video of the oil-uptake experiment (AVI). Experimental details, FTIR-spectra, and gas sorption isotherms (PDF).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Du, R.; Zheng, Z.; Mao, N.; Zhang, N.; Hu, W.; Zhang, J. Adv. Sci. 2015, 2, 1400006. (2) Wang, B.; Liang, W.; Guo, Z.; Liu, W. Chem. Soc. Rev. 2015, 44, 336−361. (3) Arbatan, T.; Fang, X. Y.; Shen, W. Chem. Eng. J. 2011, 166, 787− 791. 1271


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ACS Macro Letters (38) Zhang, L.; Zhong, Y.; Cha, D.; Wang, P. Sci. Rep. 2013, 3, 2326. (39) Li, A.; Sun, H.-X.; Tan, D.-Z.; Fan, W.-J.; Wen, S.-H.; Qing, X.J.; Li, G.-X.; Li, S.-Y.; Deng, W. Q. Energy Environ. Sci. 2011, 4, 2062− 2065. (40) Yang, R.-X.; Wang, T.-T.; Deng, W.-Q. Sci. Rep. 2015, 5, 10155.

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Microporous Polymer Networks Made by Cyclotrimerization of

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