Full Paper DOI: 10.1002/prep.201300016
Selective Extraction of N-Heterocyclic Precursors of 1,3,5,7Tetranitro-1,3,5,7-tetraazacyclooctane (HMX) Using Molecularly Imprinted Polymers Lei Wang,[a] Zhi-Bin Xu,[a] Peng Wang,[a] Min Xue,[a] Xue-Min Dong,[a] Zi-Hui Meng,*[a] Zhong-Liang Lou,[a] Zhi-Hui Lin,[a] and Cheng-Yi Lu[a]
Abstract: N-heterocyclic compounds are key nitration precursors for some high energy density explosives such as 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX). Nitration of 1,3,5,7-tetraacetyl-1,3,5,7-tetraazacyclooctane (TAT) yields HMX in high yields and purity. However, the analogue 1,3,5-triacetyl-1,3,5-triazacyclohexane (TRAT) is easily
co-produced via the condensation of acetonitrile and 1,3,5trioxan. To selectively extract TAT from a mixture of TAT and TRAT, the molecular imprinting technology (MIT) was developed in this study. The capacity of the dry polymer is 16 mg g 1 and the recovery surpasses 75 %.
Keywords: 1,3,5,7-Tetraacetyl-1,3,5,7-tetraazacyclooctane · 1,3,5-Triacetyl-1,3,5-triazacyclohexan · Molecularly imprinted polymers · Solid phase extraction · Frontal chromatography
1 Introduction The application of 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX), one of the most powerful explosives, is limited by its high production cost. Currently, HMX is produced using a modified Bachmann process (Scheme 1a), in which the nitration of hexamine (HA) with ammonium nitrate and nitric acid was carried out in a mixture of acetic acid/acetic anhydride [1–5]. This process is also restrained by the slow production rate and the poor yield. The synthesis of HMX by nitration of 1,3,5,7-tetraacetyl-1,3,5,7-tetraazacyclooctane (TAT) (Scheme 1b) now attracts increasing attention as an economical and environmentally friendly alternative [6–10]. The nitration of TAT in this process requires less nitration agent and occurs under mild conditions, the yield and purity (> 99 %) of HMX are also promising. However, synthesis of TAT by acetolysis of HA (Scheme 2a) still requires a large amount of acetic anhydride [11–13]. Recently, an
Scheme 1. Synthesis of HMX by (a) Bachmann process; (b) TAT nitration.
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Scheme 2. Synthesis of TAT from (a) hexamine; (b) acetonitrile and trioxane.
economical, mild, and quick synthetic method for TAT via the condensation of acetonitrile and 1,3,5-trioxan (Scheme 2b) has been developed [14, 15]. Unfortunately, the product is a mixture of TAT and 1,3,5-triacetyl-1,3,5-triazacyclohexane (TRAT), which is the nitration precursor of 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX). Therefore, urgent demand for a selective extraction method for TAT exists. [a] L. Wang, Z.-B. Xu, P. Wang, M. Xue, X.-M. Dong, Z.-H. Meng, Z.-L. Lou, Z.-H. Lin, C.-Y. Lu School of Chemical Engineering & Environment Beijing Institute of Technology Beijing, 100081, P. R. China, *e-mail:
[email protected] [email protected]
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Table 1. Composition of MIPs prepared. Polymers
Template
Monomer
Cross linker
Template:monomer:crosslinker (in molar ratio)
Solvent
MIP1 MIP2 MIP3 NIP
TAT TRAT MBA –
MAA MAA MAA MAA
TRIM TRIM TRIM TRIM
1:8 :8 1:8 :8 1:8 :8 1:8 :8
acetonitrile acetonitrile acetonitrile acetonitrile
Differentiating N-heterocyclic compounds with similar sizes is difficult. Chen et al. reported a complicated method to separate a mixture of HMX and RDX [16], in which a two-step filtration and extremely long equilibration time were involved. A mixture of HMX and RDX was stirred in mixed alcohols at 90 8C for 40 h, the obtained solid-liquid suspension was filtered to obtain solid HMX with a purity of 98 %, and a yield of 81 %. The filtration was then evaporated under reduced pressure to obtain a mixture of RDX and HMX, and the mixture was stirred at 90 8C in a mixture of butyrolactone and alcohol for 240 h. The obtained suspension was filtered to obtain RDX with a purity of 98 % and a yield of 78 %. The recrystallization process is tedious and uneconomical. Moreover, potential impurities may be introduced into the final crystal products. Molecularly imprinted polymers (MIPs) can be used as promising adsorbents with predetermined selectivity [17, 18]. MIPs have been used in various separation processes [19–22]. Although numerous efforts have been reported for the explosive-imprinted polymers, these MIPs are mainly used for sensing or as SPE materials for chromatographic analysis [23–26]. Only recently, we used MIP beads prepared by suspension polymerization to adsorb TNT from waste waters [27]. Lordel et al. also reported molecularly imprinted silica to selectively extract nitroaromatic explosives from aqueous samples [28]. It is still a great challenge to use MIPs to selectively extract N-heterocyclic targets from interference with high similarity in size and functionality. TAT and TRAT differ slightly in the size of ring structure and the number of nitroamine groups. Herein, we describe our effort to selectively extract TAT from a mixture of TAT/ TRAT by using MIP adsorbents in an SPE cartridge.
stationary phase while exposed to an aqueous mobile phase. Our studies on the mechanism of HMX synthesis from condensation of small molecules [5, 33, 34] suggest that N,N’-methylenebisacetamide (MBA) is a key intermediate for this reaction. An MBA-imprinted MIP was therefore synthesized in order to capture unknown intermediates with chain structure from the reaction solution. However, results showed that the MBA-imprinted polymer can selectively extract TRAT from a mixture of TAT and TRAT. Thus, it might be possible to differentiate N-heterocyclic compounds with different sizes on MIPs. Table 1 shows the recipes used to prepare the MIPs by precipitation polymerization, which is a promising method for MIP microspheres used as SPE sorbents [22, 35, 36]. These recipes are classic for small molecules containing a rigid ring structure, so they were directly adopted without further optimization. The SEM micrographs of the polymer microspheres, as shown in Figure 1, confirmed their spherical morphology with a narrow size distribution of 0.5–1 mm. Usually, MIPs are evaluated by a static batch-wise absorption experiment. The theoretical binding capacity and equilibration constant, which represents the affinity of the MIPs are calculated from the binding isotherms. However, this tedious method usually produces very large error for the theoretical data, which is not suitable to be adopted in the scenario of adsorption on a large column. Therefore, a simple frontal chromatography setup was used to evaluate the performances of the MIPs in a flash column [27, 29]. The data obtained by this method, especially the apparent capacity at the breakthrough point of frontal chromatography, are useful for the scale-up experiments. In order to selectively extract N-heterocyclic intermediates from an aqueous sample, batches of 0.5 mL (10 mg mL 1) TAT/TRAT aque-
2 Results and Discussion Separation and purification of HMX and RDX are well reported till now. Both HMX and RDX were extracted from environmental samples using SPE fibber by Gaurav et al. [30, 31]. Borch et al. separated HMX, RDX, and their metabolites using a C8 column [32]. A chromatographic method separating TAT and TRAT was developed in our group with a silica HPLC column [14]. Although high purity of TAT up to 97 % or more was obtained in this method, it cannot be applied to large scale separation due to the high cost of high pressure preparative LC and the poor stability of silica 782
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Figure 1. SEM of (a) MIPs; (b) NIPs.
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Selective Extraction of N-heterocyclic Precursors of HMX
Figure 3. Separation scheme of TAT and TRAT using a MIP absorbents flash column. Table 2. Relative amount [%] of TAT and TRAT in the loading and washing fractions. Figure 2. HPLC chromatogram of TAT and TRAT.
Polymers
ous solutions (4/6, w/w) were loaded on a SPE cartridge containing 3 g MIP microspheres at a flow rate of 0.1 mL min 1. TAT and TRAT in each fraction were analyzed by HPLC (Figure 2). For the NIP, the ratios of TAT/TRAT in every loading fraction stayed constant, which suggests that the affinity of the NIP is the same to TAT and TRAT. However, for the MIP1, which was imprinted with TAT, no TAT but TRAT was detected in the first 12 mL loading fraction, indicating a total adsorption of TAT. The breakthrough point for the cartridge was determined to be 12 mL. After the breakthrough point, TAT can be detected in the fraction. The ratio of TAT/TRAT in the loading fraction maximized at 4/6 (w/w) after a loading of 20 mL mixture sample, and the saturation was reached. The apparent capacity is arbitrarily defined as the amount of TAT absorbed on the column at the breakthrough point (12 mL fraction). For MIP1, the apparent capacity to TAT is 16 mg g 1 dry polymer. In practice, after the breakthrough of TAT, the flash column should be washed to obtain pure TRAT. However, the solubility of TAT in acetic anhydride was found to be much lower than that of TRAT. Therefore, the column was washed with 6 mL acetic anhydride first. No TRAT was detected in the washing fraction and only trace amount of TAT was detected. This suggests that no TRAT was adsorbed on MIP1. Subsequently, the column was washed with 12 mL methanol at 0.1 mL min 1. About 3 mg mL 1 TAT (purity > 99.5 %) was detected in the washing fraction, corresponding to a recovery of 75 % for TAT. Still, no TRAT was detected. Therefore, the flash column packed with MIP1 selectively extracted TAT from the mixture, leading to the separation of TAT and TRAT. As shown in Figure 3, MIP1 has predetermined recognition sites for TAT. Although the apparent capacity is not the true saturation capacity, it is more useful for preparative separations on a large scale. The MIP1 column requires regeneration at the breakPropellants Explos. Pyrotech. 2013, 38, 781 – 785
MIP1 MIP2 MIP3 NIP
Loading
Washing
TAT
TRAT
TAT
TRAT
0 44 51 40
100 56 49 60
100 39 29 40
0 61 71 60
through point in order to obtain pure TRAT in the fraction. It was found that MIP1 could be regenerated for at least 5 rounds without obvious deterioration of capacity. According to Table 2, MIP2 did not show confirmed imprinting effect to its template TRAT, no matter the loading fraction (44/56), or the washing fraction collected (39/61), the change of TAT/TRAT is negligible, MIP2 has almost the same affinity to TRAT and TAT. The imprinting protocol of MIP1 does not work for TRAT. MIP3, an MBA-imprinted polymer, can selectively adsorb TRAT to some extent. For the first 0.5 mL loading fraction, the TAT/TRAT ratio changed from 40/60 to 50/50. Further washing with methanol resulted in a ratio of 29/71. This phenomenon suggests that MIP3 has a higher affinity to TRAT, which has a hexatomic ring structure. It is interesting that an MIP imprinted with a chain structure can selectively adsorb molecules of a hexatomic ring structure. In fact, the carbonyl group and amino group of MBA can form an intramolecular hydrogen bond, which further induces a stable hexatomic ring structure (Figure 4). During the imprinting of MBA, it is actually the hexatomic ring structure of the intramolecular hydro-
Figure 4. Intramolecular hydrogen bonding formed within MBA.
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gen bond that was imprinted, hence induce the selectivity to TRAT.
3 Experimental 3.1 Reagents and Chemicals
Trimethylolpropane trimethacrylate (TRIM), methacrylic acid (MAA), and azobisisobutyronitrile (AIBN) were purchased from TCI (Tokyo, Japan). Acetonitrile, 1,3,5-trioxan, H2SO4 (98 %), MgSO4, and the other materials (all analytical grade) were purchased from Beijing Chemical Plant and used directly without purification. TAT, TRAT and N,N’-methylenebisacetamide (MBA) were synthesized according to literature methods [14, 15]. 3.2 Preparation of MIPs
The MIP microspheres were prepared by a dispersion polymerization procedure as described in the literature [22]. The compositions are shown in Table 1. In brief, templates (0.5 mmol) were mixed with MAA (4 mmol) in 40 mL acetonitrile, followed by the addition of TRIM (4 mmol) and) AIBN (0.04 mmol). The pre-polymerization mixture was thoroughly purged with nitrogen for 5 min and polymerized at 4 8C for 12 h. The polymerization was initiated by UV irradiation at 365 nm. The resulting microspheres were collected by centrifugation, washed overnight with methanol/acetic acid (30 mL, 4/1, v/v), followed by five rounds of 15 mL 1 h washing with methanol, and dried under vacuum before use. The corresponding non-imprinted polymers (NIPs) were prepared in the same manner in the absence of template. The morphology of the polymers was characterized by a HITACHI S4800 field emission scanning electron microscope (Tokyo, Japan) with an accelerating voltage of 15 kV. 3.3 Chromatographic Analysis
HPLC analyses were performed using a Shimadzu LC-20 system equipped with an auto sampler and a diode array detector. The LC Solution software was utilized for instrument control, data acquisition, and analysis. Aliquots (5 mL) of sample solutions were injected into a SHISEIDO PC HILIC (4.6 250 mm, 5 mm) column. The mobile phase was acetonitrile/water (99/1, v/v) and the flow rate was 1.0 mL min 1. The detection wavelength was set at 215 nm. 3.4 Selective Extraction of TAT
Separation of TAT and TRAT was carried out using a flash chromatography method as described in reference [27, 29]. The MIP-flash columns were prepared by packing MIPs (3 g) in a 12 mL polypropylene SPE column containing micropore filters. Prior to sample loading, the MIP-flash column was rinsed with loading solvent. Batches of 0.5 mL 784
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aqueous sample containing 10 mg mL 1 TAT/TRAT (40/60, w/w) were loaded on the column continuously. When TAT was detected in the loading fraction, it marked the breakthrough point of the column. Finally the column was washed with acetic anhydride and methanol respectively. The loading and washing fractions was analyzed by HPLC to determine the amount of TAT and TRAT.
4 Conclusions Although a separation method of TAT and TRAT based on the recrystallization in ethyl acetate and precipitation by deionized water is succeed [37], the process is tedious and solvent wasted, together with no guarantee of the product purity. As a more efficient alternative to recrystallization, we prepared MIP to extract TAT from the binary mixture of TAT and TRAT. Using a flash chromatography column packed with this selective adsorbent for TAT, TAT can be selectively extracted from an aqueous sample containing TAT and TRAT on a preparative scale with purity higher than 99.5 %, meanwhile less solvent is needed. Generally, flash columns packed with MIPs for N-heterocyclic compounds could be considered for the preparation of pure nitration precursors for high energy explosives.
Acknowledgments Scientific advice from Dr. Michael Whitcombe from Cranfield University, and linguistic advice from Prof. K. J. Shea from UCI are appreciated
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