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The room temperature chemistries of isocyanates with zeolite NaX
Microporous and Mesoporous Materials 139 (2011) 110–119
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The room temperature chemistries of isocyanates with zeolite NaX Jared B. DeCoste, David C. Doetschman ⇑, Miranda J. Lahr, Charles W. Kanyi, Jürgen T. Schulte Department of Chemistry, Binghamton University, Binghamton, NY 13902-6000, USA
a r t i c l e
i n f o
Article history: Received 16 June 2010 Received in revised form 20 September 2010 Accepted 18 October 2010 Available online 26 October 2010 Keywords: Zeolite Isocyanate Di-isocyanate Nucleophilic Addition
a b s t r a c t Isocyanates and di-isocyanates are used in the production of various chemicals and materials, such as pesticides and polyurethane. However, since the disastrous accidental release of isocyanate in Bhopal in 1984 this class of toxic compounds has been under scrutiny, with growing interest in means to respond to isocyanate release. The susceptibility of isocyanates to nucleophilic substitution [D.P.N. Satchell, R.S. Satchell, Chem. Soc. Rev. 4 (1975) 231] and the known nucleophilicity of NaX [Yang, et al., Microporous Mesoporous Mater. 92 (2006) 56; Doetschman, et al., Microporous Mesoporous Mater. 92 (2006) 292; Kanyi, et al., Microporous Mesoporous Mater. 108 (2008) 103] led to this study of a spectrum of isocyanates representing cyclic and aromatic isocyanates, di-isocyanates, 1°, 2°, and 3° isocyanates, and alkyl chain isocyanates of various lengths. Additional experiments were done in order to understand the nature of a tightly bound chemical adduct product that forms in the isocyanate reactions with the zeolite. The isocyanate functional group was shown to be susceptible to nucleophilic attack by Faujasite NaX. A carbamate framework adduct, with the anionic nitrogen stabilized by the sodium cation is proposed. The product is very tightly bound to the zeolite. In two, but not all, isocyanates in which there are methyl protons b to the isocyanate moiety, an elimination reaction took place to form the a, b olefin, competing with the nucleophilic addition reaction. The types of chemistry occurring and the tenacity with which the products are retained, suggests that zeolites may have practical applications in hazardous materials cleanup or controlling isocyanate vapor release. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction Isocyanates and di-isocyanates have been very useful in the production of various chemicals and materials. Bayer discovered that di-isocyanates are ideal for the production of polymers such as polyurethane [1]. But since the deaths of an estimated 20,000 people in 1984 in Bhopal from an accidental release of methyl isocyanate used as an intermediate in pesticide production, isocyanates, as well as other industrial chemicals, have been under much legislative and risk management scrutiny [2]. The Environmental Protection Agency, along with other authorities, lists methyl isocyanate, toluene 2,4- and 2,6-di-isocyanates (TDI), and other isocyanates, as hazardous wastes and potential pollutants [3,4]. It has been shown that conventional decontamination methods, such as aqueous scrubbing, are not effective methods for TDI removal. The reaction between water and TDI to produce the corresponding amine and carbon dioxide is slow without the presence of a catalyst [5]. Sorption onto solid materials has been shown to be an effective method for TDI removal. Both activated carbon and high-density polyurethane foam have been shown to be effective scrubbing
⇑ Corresponding author. Tel.: +1 607 777 2298; fax: +1 607 777 4478. E-mail address: [email protected] (D.C. Doetschman). 1387-1811/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2010.10.027
materials [6]. The drawback of this method is that it is effective only over short periods of time before breakthrough occurs. Faujasite NaX zeolite has been shown to be effective in the adsorption and subsequent destruction of materials that are susceptible to nucleophilic attack. Dimethyl methylphosphonate, a type G and X nerve agent simulant, and alkyl halides have been shown to undergo adsorption by NaX with subsequent nucleophilic chemistry with the zeolite [7–9]. A delocalized negative charge between the supercage O atoms and Al atoms in NaX, which is stabilized by Na cations, gives zeolite X its nucleophilic character. Additionally the supercage cation also serves to assist the departure of an anionic leaving group. Zeolite X (NaX) has been well characterized structurally [10– 13] and has a supercage diameter of 11 Å, a 7 Å diameter window into the supercage, and a Si/Al ratio of 1.23 [7–9]. Thus the structure of the zeolite poses a size limitation to the adsorption of molecules, depending upon their rigidity. The structure of NaX is not compromised under the vacuum and high temperatures employed to dry it thoroughly [12]. The susceptibility of isocyanates to nucleophilic substitution [14] and the known nucleophilicity of NaX [7–9] led to a study of the following spectrum of isocyanates and di-isocyanates: n-butyl isocyanate (BtIC), cyclohexyl isocyanate (CHIC), 1,6 diisocyanato hexane (16DIH), n-dodecyl isocyanate (DDIC), ethyl isocyanate (EIC), isopropyl isocyanate (IPIC), n-octyl isocyanate (OIC), n-propyl
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isocyanate (PIC), toluene 2,4 di-isocyanate (T24DI), tert-butyl isocyanate (TBIC). CHIC and T24DI represent a cyclic and an aromatic isocyanate, respectively, of comparable size and also, together with 16DIH, permit a comparison of mono- and di-isocyanates. BtIC, DDIC, EIC, OIC, and PIC represent primary isocyanates, IPIC represents a secondary isocyanate, and TBIC represents a tertiary isocyanate, permitting a study of the dependence of isocyanate zeolite chemistry on the isocyanate connectivity. Also, BtIC, DDIC, EIC, OIC, and PIC make possible a study of the dependence of the isocyanate zeolite chemistry on the length of the aliphatic chains connected to the isocyanate moiety. Additional experiments were done in order to understand the nature of a tightly bound chemical adduct product that forms in the isocyanate reactions with the zeolite. Solid-state 23Na NMR experiments on the BtIC-exposed NaX were compared with the NMR of the untreated zeolite. Also NaOH-catalyzed hydrolysis and extraction experiments were performed on the BtIC exposed zeolite. The exposed NaX was treated with NaOH solution in an attempt to remove the tightly bound proposed carbamate framework species from the zeolite, having unsuccessfully attempted to hydrolyze the product with neutral water or to perform extraction with a variety of non-aqueous solvents.
2. Experimental The preparation of the NaX zeolite and the introduction of water and the various isocyanate compounds into the zeolite were done with methods similar to those published previously [7–9]. The details of these methods are described in full in the Supplementary material. NaX (Sigma–Aldrich) was prepared by heating it under vacuum for an extended period of time. In certain experiments the dried NaX was loaded with stoichiometric amounts of water, ranging from 0 to 10 water molecules per supercage, which were also transferred through the vapor phase into the sample. The addition of water [15] was done either before isocyanate transfer or subsequent to it, depending upon the purpose of the experiment. The isocyanate compounds were introduced into the dry NaX at 3 molecules of isocyanate per supercage of zeolite by methods similar to the introduction of water. After introduction of the isocyanate and water (where employed) the samples were equilibrated at room temperature and then the products in the zeolite or products released from the zeolite were characterized. Attempts to extract products with neutral solvents from the zeolite yielded no detectable products, the details of which are described in the Supplementary material. Residual, unreacted isocyanate and gaseous products, which emanated from the exposure of the zeolite to some of the isocyanates, were released and captured in an IR cell using the Canulla technique. A Schlenk-line with a rotary oil vacuum pump connected to it was used to argon-flush and evacuate the cell and to backfill with argon after introducing the gases from the reaction vessel. The stopcock to the IR cell was closed and the cell was taken to the IR spectrometer for measurement on an Equinox 550 FTIR instrument. (Scans (128) were made at 2 cm1 resolution with background subtraction.). In select instances gas chromatographic–mass spectrometric (GC–MS) head space analyses, using a Varian 4000 Ion Trap GC–MS, were done on the gases that were present after exposure of the NaX to isocyanate in the head space analysis vial. Details may be found in the Supplementary material. Products and residual isocyanate trapped in the isocyanate exposed NaX were characterized in the zeolite by solid-state 13C cross-polarized (CP) magic angle spinning (MAS), nuclear magnetic resonance (NMR), 23Na MAS NMR, and IR spectroscopy using the KBr pellet technique. The solid-state NMR experiments were per-
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formed with a Bruker AC 300 spectrometer, modified for solidstate NMR experiments on the particular nuclei. The 13C CP MAS NMR experiments were similar to those described in previous publications [7–9]. The IR experiments on the exposed zeolites also employed the Equinox 550 FTIR instrument. Details about these experimental methods may be found in the Supplementary material. In order to perform NaOH-catalyzed hydrolysis of the product of the reaction of BtIC with NaX, 0.25 g of NaX (3BtIC/SC), (3.8 104 moles BtIC) was stirred in a round bottom flask with 5.0 mL of 0.25 M NaOH (10 NaOH per supercage) for approximately 1 h. The slurry was filtered and the filtered solid was then placed in a round bottom flask with 5.0 mL of CDCl3 and stirred for 1 h. The slurry was gravity-filtered and the extract was kept for solution NMR analysis. A similar attempt to hydrolyze the BtIC-exposed NaX with neutral water was made and the extract was examined. The products of the hydrolysis were characterized as follows. The filtrates were analyzed on a Bruker AM 360 NMR spectrometer, which has an 8.45 T magnetic field. The sample was placed in a 5 mm NMR tube and spun at 20 Hz. The 360.14 MHz 1H NMR spectrum was recorded, employing a 3.0 ls pulse, a relaxation delay of 35 ls, followed by an acquisition time of 2.34 s and a recycle time of 500 ms. A total of 1000 transients was recorded and processed with a Fourier transformation. The spectra were referenced internally to the solvent. The 90.57 MHz 13C NMR spectrum of the extract was recorded, employing a 3 ls pulse, a relaxation delay of 35 ls, followed by an acquisition time of 1.09 s and a recycle time of 500 ms. Inverse gated decoupling was applied on the proton channel at a frequency of 360.14 MHz. A total of 20,000 transients were obtained. The spectrum was referenced internally to the solvent. The H, H COSY spectrum of the extract was recorded on the 360.14 MHz 1H channel, employing a 13 ls pulse, followed in succession by a delay of 3 ls, a pulse of 13 ls, a delay of 10 ls, an acquisition time of 256 ms, and a delay of 500 ms on the 1H channel. This sequence was repeated for 8 transients at each of the 1024 2-dimensional points. There were 1024 acquired points over a spectral width of 2000 Hz. The C, H COSY spectrum of the extract was recorded on the 90.56 MHz 13C channel, employing a 10.4 ls pulse, followed in succession by a variable delay, another pulse of 10.6 ls, a delay of 1.75 ms, a relaxation delay of 10 ls, an acquisition time of 51.2 ms, and a delay of 1 s on the 360.14 MHz 1H channel. A pulse of 12 ls was applied on the 13C channel immediately after the second 1H channel pulse. The sequence was repeated for 256 scans at each of the 256 2-dimensional points. There were 1024 acquired points over a spectral width of 10,000 Hz. 3. Results In view of the number of isocyanates examined and the number of characterization experiments performed, a set of typical results will be presented here that are sufficient to convey the nature of the isocyanate chemistry. These examples are represented by BtIC, OIC, T24DI, and TBIC. Other results of a similar nature will be given in the Supplementary material. Moreover, many of the experiments on authentic samples of various compounds, performed to support the identification of reaction products and residual, unreacted isocyanates, are relegated to the Supplementary material. 3.1. n-Octyl isocyanate exposed NaX The results for OIC are typical of single, straight-chain or closedchain substituted isocyanates with a single isocyanate functional group. The 13C CP MAS NMR spectrum of NaX exposed to OIC at
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3 molecules per supercage is shown in Fig. 1, together with a 13C NMR spectrum of a solution of authentic OIC in CDCl3. Similar pairs of 13C spectra for the other isocyanates studied, after adsorption into NaX and in CDCl3 solution are shown in the Supplementary material. The IR spectrum of OIC exposed NaX (3/sc) in comparison with that of NaX alone shows the appearance of a few new bands. The FTIR spectrum of NaX exposed to OIC is presented in the Supplementary material. There is a medium sized band at 1577 cm1 and there are also weak bands at 1478, 1310, 1258, and 1246 cm1. Substantially similar FTIR results are found for the other monoisocyanates and the results for OIC are thus typical of the chemistry of a number of differently branching substituted isocyanates with one isocyanate functional group (see the Supplementary material).
The GC–MS of the headspace over NaX exposed to TBIC (2.3/sc) shows the presence of 2 species. These have been found to vary with the amount of NaX added to the headspace vial (see Supplementary material). One of these species has a retention time of 1.6 min and the other one a retention time of 2.2 min. The mass spectrum at the retention time of 1.6 min is shown in Fig. 3. This was determined to be the major species present, other than unreacted TBIC. The mass spectrum at a retention time of 2.2 min is identical to that of neat TBIC and is given in the Supplementary material. An FTIR spectrum of NaX exposed to 3 molecules of TBI is presented in the Supplementary material.
3.2. Tert-butyl isocyanate exposed NaX
The results for the bifunctional T24DI differ from those of isocyanates with a single functional group. Fig. 4 shows (a) the solid state CP MAS 13C NMR spectrum of T24DI (3/sc) exposed NaX and (b) the 13C NMR spectrum of a solution of authentic T24DI in CDCl3, illustrating the behavior of a di-isocyanate in the zeolite. The solid-state NMR spectrum in this case exhibits pronounced sidebands of the 120–140 ppm peaks.
Fig. 2 shows (a) the solid state CP MAS 13C NMR spectrum of TBIC-exposed (3/sc) NaX and (b) the 13C NMR spectrum of a solution of TBIC in CDCl3. The peak in the neighborhood of 110 ppm and the two nearly overlapping peaks between 150 and 160 ppm are evidence for an isocyanate chemistry that will be shown to yield an olefin.
3.3. Toluene 2,4 di-isocyanate exposed NaX
3.4. Sequential chemistry in di-isocyanates The FTIR spectrum of the bifunctional isocyanate T24DI in NaX subsequently exposed to various amounts of water was recorded,
Fig. 1. (Top) Solid-state 13C CP NMR spectrum of NaX exposed to 3 molecules per supercage of OIC. (Bottom) Liquid 13C NMR spectrum of authentic OIC in CDCl3 solvent. The horizontal axis scale designates chemical shift d in units of parts per million (ppm).
Fig. 3. Mass spectrum of the headspace over NaX exposed to 2.3 TBIC/sc at a retention time of 1.6 min.
Fig. 2. (Top) Solid-state 13C CP NMR spectrum of NaX exposed to 3 molecules per supercage of TBIC. (Bottom) Liquid 13C NMR spectrum of authentic TBIC in CDCl3 solvent. The horizontal axis scale designates chemical shift d in units of parts per million (ppm).
Fig. 4. (Top) Solid-state 13C CP NMR spectrum of NaX exposed to 3 molecules per supercage of T24DI. (Bottom) Liquid 13C NMR spectrum of T24DI solution in CDCl3 solvent. The horizontal axis scale designates chemical shift d in units of parts per million (ppm).
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in order to characterize the nature of the hydrolysis chemistry and its implications for the sequence of the two isocyanate group chemistries. Shown in Fig. 5 are the FTIR spectra of (3/sc) exposed NaX, to which the indicated amounts of water have been added after the NaX exposure to T24DI. For reference, the FTIR spectrum of NaX alone is presented in the Supplementary material.
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solution hydrolysate of the BtIC-exposed NaX, shown in the Supplementary material, is rich. The regions 0.8 to 1.8 ppm and
3.5. Changes in Na cation environment The solid-state NMR spectrum of the Na cation was examined after exposure of the zeolite to BtIC, in order to investigate what interaction, if any, the Na cation has with the zeolite bound product after exposure. Unlike TBIC, BtIC yields no volatile products and was chosen for investigation because of this simplifying behavior. Fig. 6 (Top) shows the 23Na MAS NMR spectrum of NaX exposed to BtIC (3/sc). For reference the 23Na MAS NMR spectrum of dry, unexposed NaX is presented in Fig. 6 (Bottom). The spectra are given from 20 to –100 ppm to focus on the major features. 3.6. Base hydrolysis of the framework bound reaction product The 1H NMR spectrum (not shown) from the water filtrate of the hydrolysate of BtIC-exposed NaX contains no non-solvent peaks. However, the 1H NMR spectrum of CDCl3 extract of the NaOH
Fig. 7. Segments of the 1H NMR spectrum from 0.8 to 1.8 ppm of the CDCl3 extract from BtIC-exposed NaX that had been treated with NaOH solution and centrifuged. The sample was acidified to emphasize the molecular form of the n-butyl carbamic acid anhydride.
Fig. 5. FTIR spectrum of T24DI (3/sc) with H2O (x/sc), where x = (a) 0, (b)1, (c) 1.5, (d) 2, (e) 2, and (f) 3.5, from 2400 to 2200 cm1. The weak spectra (c–f), obtained with 1.5–3.5 H2O per supercage, decrease successively in intensity with increasing H2O and are not individually labeled.
Fig. 8. Segments of the 1H NMR spectrum from 2.5 to 4.0 ppm of the CDCl3 extract from BtIC-exposed NaX that had been treated with NaOH solution and centrifuged. The sample was acidified to emphasize the molecular form of the n-butyl carbamic acid anhydride.
Fig. 6. (Top) Solid-state 23Na MAS NMR spectrum of NaX exposed to BtIC (3/sc). (Bottom) Solid-state 23Na MAS NMR spectrum of NaX alone. The horizontal axis scale designates chemical shift d in units of parts per million (ppm).
Fig. 9. 13C NMR spectrum of the CDCl3 extract from BtIC-exposed NaX that had been treated with NaOH and centrifuged. The sample was acidified to emphasize the molecular form of the n-butyl carbamic acid anhydride.
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2.5–4.0 ppm of a sample, acidified to favor the acid form, are expanded in Figs. 7 and 8, respectively. The spectra will be shown to include n-butyl carbamic acid anhydride. Likewise, the rich 13C NMR spectrum of the CDCl3 extract is shown in Fig. 9 (The regions 10–50 ppm and 140–180 ppm of it are expanded in the Supplementary material). The H, H and C, H COSY 2-dimensional spectra provide a basis for determining how many closely related residual BtIC and BtIC product species are present in the extract and which peaks belong to which species. The H, H COSY 2-dimensional spectrum and the C, H COSY 2-dimensional spectra of the CDCl3 filtrate are also given in the Supplementary material. 4. Analysis 4.1. n-Octyl isocyanate exposed NaX In the 13C CP MAS NMR spectrum of OIC (3/sc) exposed NaX shown in Fig. 1, the four chemical shifts of d 14, 23, 29, and 43 correspond, by comparison with the spectrum of authentic OIC, to the eight different carbon atoms in the alkyl chain. These are presented in Table 1. Uncertainties in the 13C solid-state NMR chemical shifts are 1 ppm or less. For the 13C CP NMR spectrum of authentic OIC, shown in Fig. 1, the eight chemical shifts between d 12 and 43 ppm represent the eight carbon octane chain. The closer the carbon is to the isocyanate moiety, the more downfield is the chemical shift. The chemical shift of authentic OIC at d = 121.9 ppm is assigned to the carbon within the isocyanate moiety. Uncertainties in the solution 13C NMR shifts are 0.1 ppm or less. The line broadening of the solid-state NMR spectrum causes these chemical shifts to overlap and to be indistinguishable from one another. The solid-state 13C NMR chemical shift at d 159 ppm is assigned to a carbon in the resulting functional group of a proposed framework carbamate, shown in Structure 1, that is derived from the C atom of the original isocyanate functional group. A peak in this region, often of greater intensity than in this example, is observed in all of the isocyanate exposed NaX samples (see Supplementary materials). The new infrared bands appearing in the OIC exposed NaX have not been assigned.
Corresponding assignments have been made for the other isocyanates (see Supplementary material). 4.2. Tert-butyl isocyanate exposed NaX The 13C CP MAS NM R spectrum of TBIC-exposed NaX (3/sc) in Fig. 2 exhibits chemical shifts of d 23, 29, 52, 109, 149, and 159 ppm. In view of the 13C NMR spectrum of authentic TBIC in solution, also shown in Fig. 2 and assigned in the Supplementary material, there appear to be too many chemical shifts to arise from just one product. The chemical shifts at d 109 and 149 ppm probably indicate the presence of an olefinic product that is assigned tentatively to 2-methyl propene. The chemical shifts at d 29, and 23 belong to methyl groups in 2-methyl propene and a proposed framework tert-butyl carbamate, Structure 2, respectively. The chemical shift at d 52 ppm probably belongs to the carbon to which the functional group is attached, in view of the results from other isocyanates. The functional group is proposed to be a framework carbamate of carbamic acid or simply the framework hydrogen carbamate, shown in Structure 3, according to a mechanism proposed in the Section 5. The weak complex peak with the chemical shift at d 159 ppm evidently also belongs to the carbons in the framework carbamate functional groups of the products, being around a chemical shift that is different than that of the TBIC isocyanate group. The chemical shift assignments made here are summarized in Table 2. As most of the framework carbamates occur at essentially the same chemical shift and the lines are relatively broad, this peak is consistent with the presence of two different framework carbamate species. It will become clearer that the production of the 2-methyl
Structure 2. Proposed structure of framework tert-butyl carbamate. Table 1 Chemical shift assignments for the solid-state 13 C CP MAS NMR of the product of NaX exposure to octyl isocyanate (OIC). Octyl group carbons are numbered consecutively from the C atom attached to the isocyanate group. Chemical shift (ppm)
Carbon assignment
159
Framework carbamate Chain – 1 Chain – 2–6 Chain 7 Chain 8
43 29 23 14
Structure 3. Proposed structure of framework hydrogen carbamate.
Table 2 Chemical shift assignments for the solid-state 13C CP MAS NMR of the products of NaX exposed to tert-butyl isocyanate.
Structure 1. Proposed structure of framework octyl carbamate.
Chemical shift (ppm)
Carbon assignment
159 149 109 52 29 23
Framework carbamate groups C atoms Interior olefin C atom Terminal olefin C atom Tertiary carbon Atom Methyl group C atoms of framework carbamates Methyl group C atoms of 2-methyl propene
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propene probably accompanies the formation of another framework isocyanate. The IR spectrum of TBIC (3/sc) exposed NaX shows the appearance of new bands in comparison with the IR spectrum of NaX alone, indicating that chemistry has taken place (see spectrum in the Supplementary material). There is a medium sized band at 1369 cm1 and weak bands at 2235, 2169, 1733, and 1692 cm1. These bands have not been assigned. The GC–MS spectra observed in the headspace of the TBICexposed NaX were presented in Fig. 3 and in the Supplementary material. At the 1.6 min retention time, the base peak is at 41 m/z, which represents an M 15 peak for 2-methyl propene product. The parent peak is at 56 m/z, for which an M 1 peak is observed at 55 m/z. These assignments are summarized in Table 3. The mass spectrum for the species observed at the retention time of 2.2 min, shown in the Supplementary material, has a base peak at 84 m/z, which represents an M 15 peak. The parent peak is at 99 m/z. The other major peak is at 56, representing an M 43 peak. These are summarized in Table 4. The retention time and spectrum are in agreement with authentic, neat TBIC. 4.3. Toluene 2,4 di-isocyanate exposed NaX
Table 5 Chemical shift assignments for the solid-state 13C CP MAS NMR of the product of NaX exposed to toluene 2,4 di-isocyanate. Chemical shift (ppm)
Carbon assignment
159 131 121
Framework carbamate group C atom Ring – 1,3,5,6 C atoms Ring – 2,4 and isocyanate group C atoms Methyl group C atom
17
Al
O
Si
C
O
N
C N
O
Na
Structure 4. Proposed structure of framework toluene-2-isocyanato-4-carbamate.
13
In the C CP NMR of neat toluene 2,4 di-isocyanate, displayed in Fig. 4, the chemical shift at d 17.8 ppm belongs to the methyl group. The chemical shift at d 124.7 ppm is assigned to the carbons within the isocyanate moieties. Even though these two carbons are not chemically equivalent they are very close, and evidently share a common chemical shift. The other chemical shifts at d 120.9, 122.2, 130.5, 131.3, 131.9 and 133.1 correspond to the carbons in the 3, 5, 1, 6, 4, and 2 positions, respectively. The intensity of the chemical shifts at the 1, 2, and 4 positions are significantly smaller than the other chemical shifts, because there are no adjacent protons with which to couple with the CP NMR sequence. In the 13C CP MAS NMR spectrum of T24DI-exposed NaX (3/sc) there are chemical shifts of d 17, 121, 131, and 159 ppm, as can be seen in Table 5 with reference to the proposed Structure 4. The chemical shift at 17 ppm corresponds to a methyl group. The chemical shifts at d 121 and 131 are from aromatic carbons in a benzene ring. The peaks at d 69, 77, 176, and 187 are spinning
Table 3 Mass spectrometry data for NaX exposed to 2.3 molecules of TBIC per supercage at a retention time of 1.6 min, assigned to 2-methyl propene. M represents the parent 2-methyl propene mass of 56. m/z
Relative abundance (%)
Species
41 55 56 57
100.0 63.0 70.5 21.7
M 15 M1 M M+1
Table 4 Mass spectrometry data for NaX exposed to 2.3 molecules of TBIC per supercage at a retention time of 2.2 min, assigned to residual TBIC by comparison with the spectrum of authentic TBIC. M represents the parent TBIC mass of 99. m/z
Relative abundance (%)
Species
56 84 99 100
42.46 100.00 .67 .04
M 43 M 15 M M+1
sidebands of the benzene carbon peaks. It is impossible to determine from this spectrum whether the 13C of the functional group of residual isocyanate is present or not, because the broad aromatic peaks mask the region where it would show up. The chemical shift at d 159 ppm represents the carbon in the functional group of the framework carbamate product. Similar results were found for the di-isocyanate, 16DIH (see Supplementary material). 4.4. Sequential chemistry in di-isocyanates The IR spectrum of T24DI-exposed NaX (3/sc), not shown, exhibits many new bands in comparison with those of NaX alone. (There are strong bands at 2341, and 2281 cm1, and weak bands at 1510, 1423, 1404, 1362, 1304 and 1242 cm1.) The trace shown in Fig. 5 exhibits the presence of new bands at 2281 and 2341 cm1. According to the literature, the NCO asymmetric stretch appears at approximately 2268 cm1 [16]. If the observed peaks indicate unreacted isocyanate functional groups, then there are still isocyanate functional groups present after the exposure of NaX to T24DI. This observation contrasts with the IR spectra in the mono-functional isocyanate reactions, in which all of the isocyanate functional groups react with the zeolite. As can be seen in Fig. 5, as more H2O is added to the NaX, the size of both of these peaks decrease. The absorbance of these peaks was measured and found to decrease progressively with the amount of water added (see Supplementary material). This result appears to indicate that one of the T24DI isocyanate groups is employed in forming the observed framework carbamate species. The second isocyanate group does not react with the zeolite, but appears to be susceptible to hydrolysis by water, as indicated by its progressive disappearance when the T24DI exposed zeolite is further exposed to water (see Supplementary material). 4.5. Na cation environment The 23Na MAS NMR spectrum of NaX in Fig. 6 (Top) shows two main chemical shifts at d 5 and 46 ppm corresponding to cation sites I and III0 , respectively [17]. The 23Na MAS NMR spectrum of NaX exposed to BtIC (3/sc) in Fig. 6 (Bottom) shows one main
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chemical shift at d 14 ppm. This does not correspond to any of the known cation sites reported for empty NaX. Cation sites in Faujasite zeolites are described in detail by Olsen in Ref. [12].
Table 8 n-Butyl carbamic acid anhydride 1H and 13C NMR chemical shifts and H,H coupling constants. C atom and corresponding protons are numbered consecutively from the N atoms to the methyl groups. 1
NH
H1
H2
H3
H4
d (ppm) Multiplicity JHH (Hz) NH H1 H2 H3 H4
7.25 Broad
3.65 Doublet of triplets
1.54 Multiplet
1.37 Multiplet
0.94 Triplet
– 5.55 – – –
5.55 – 7.05 – –
– 7.05 – – –
– – – – 7.05
– – – 7.05 –
– –
C1 40.3
C2 31.7
C3 20.1
C4 13.8
H
4.6. Base hydrolysis of the framework bound reaction product 1
The H NMR of the H2O filtrate, presented in the Supplementary material, shows no evidence of products being extracted from NaX (3 BtIC/SC) after being treated with neutral water. This appears to be evidence that either the tightly bound chemical product or products of the exposure of the NaX to BtIC do not extract in water or that the hydrolysis product of those exposure products do not extract in water. The 1H and 13C NMR spectra of the CDCl3 extract in Figs. 7–9 show evidence of several products. With the help of the H, H and C, H COSY 2-dimensional spectra of the CDCl3 extract, given in the Supplementary material, an assignment was made from the correlation of the peaks with one another. There are three major products in the CDCl3 extract. The assignments of the spectra to n-butyl amine, N,N0 -dibutyl urea, and n-butyl carbamic acid anhydride are given in Tables 6–8, respectively. The structures of these compounds can be seen in Fig. 11 in Section 5. The appearance of additional NMR peaks near those of the C, H, and N positions of the n-butyl carbamic acid anhydride numbered 1 (at dC = 42.9, dH = 3.25) and 2 (at dC = 30.9, dH = 1.45), near the C peak at the carbamate functionality (dC = 156.2) and at the N atom (dH = 7.05) was taken as an indication of the presence of an anionic form in solution that froms by deprotonation at a N atom of the carbamic acid anhydride. The 13C NMR peaks in the 10– 45 ppm range all correspond to carbons in butyl groups. The 1H NMR of the CDCl3 extract shows the presence of butyl amine with peak assignments in agreement with an authentic sample. The presence of the product, N,N0 -dibutyl urea, was also confirmed by comparison with the 1H NMR spectrum of an authentic, synthesized sample. The product, identified as n-butyl carbamic acid
Table 6 n-Butyl amine 1H NMR chemical shifts and coupling constants. C atom and corresponding protons are numbered consecutively from the N atom to the methyl group. 1
NH
H1
H2
H3
H4
d (ppm) Multiplicity JHH (Hz) NH H1 H2 H3 H4
0.92 –
2.68 Triplet
1.51 Multiplet
1.38 Multiplet
0.90 –
– – – – –
– – 6.84 – –
– 6.84 – – –
– – – – –
– – – – –
H
Table 7 N,N0 -dibutyl urea 1H and 13C NMR chemical shifts and H,H coupling constants. C atom and corresponding protons are numbered consecutively from the N atoms to the methyl groups. 1
NH
H1
H2
H3
H4
d (ppm) Multiplicity JHH (Hz) NH H1 H2 H3 H4
4.16 Broad
3.15 Doublet of triplets
1.49 Multiplet
1.37 Multiplet
0.91 Triplet
– 5.99 – – –
5.99 – 7.05 – –
– 7.05 – – –
– – – – 7.27
– – – 7.27 –
13
– –
C1 40.4
C2 32.2
C3 19.9
C4 13.8
H
C d (ppm)
13
C d (ppm)
anhydride, has the chemical shifts that are appropriate to the assigned protons and carbon nuclei. The 13C NMR chemical shift in the carbamic acid anhydride product, dC = 158.1 ppm, is consistent with other carbamate compounds [19]. The slight shifting of the peaks in the ionic form provides some additional evidence for this assignment. 5. Discussion 5.1. Nucleophilic chemistry The solid-state 13C CP MAS NMR chemical shifts observed in isocyanate exposed zeolites at d 159 ± 5 ppm have been assigned to the respective framework carbamates. Weak peaks with chemical shifts in this region were seen in exposures of NaX to all types of isocyanate adsorbates studied. A chemical shift in this region has been observed in the literature for carbamate species [19]. In the reaction of NaX with TBIC, 2-methyl propene is formed, an indication of an elimination type of nucleophilic reaction taking place in addition to the formation of framework carbamate adduct. These results support the hypothesis that all of the isocyanates undergo a reaction with the zeolite to form a framework carbamate. The reaction for the formation of the framework carbamate adduct with the zeolite is shown in Fig. 10. The bonding would have to occur in the NaX supercage regions of the zeolite, because the sizes of the isocyanate molecules would not permit them to enter the sodalite cages. In previous work from this laboratory the supercage oxygen atoms have been implicated as the attacking nucleophile of the zeolite [7–9,20–22]. The supercage oxygen atoms adjacent to the aluminum atoms of the framework are available to perform the nucleophilic attack, because of the delocalized negative charge from the aluminum atoms and the lone pair of electrons that can attack a susceptible moiety. The electrons on the oxygen atom attack the isocyanate carbon, allowing electrons to move toward the electronegative nitrogen atom. This electronrich center is then free to attract the sodium cations in the zeolite, leading to the proposed, stable framework carbamate adduct. This is supported by the 23Na NMR evidence, showing the creation of a new Na environment in NaX after exposure to BtIC (3/sc), at the expense of the supercage site I and III0 Na ions. The relative size of the
Al Si
O
Na
Al
N
+
R
C
O
N R
Na
C
O
Si
O
Fig. 10. Proposed mechanism for the reaction of isocyanate with NaX.
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Al
Al Si
O
O
C
Si
Na
H2N
Bt
N
Si
C
Si
O Na
O
Na
N
Bt _
Al
_
NaOH
O
O
Bt
Bt
N H
Al
Bt N H
- CO2
O Na
O O
O OH
C
Na N
Bt
H2O -NaOH
C
OH
N H
Bt
OH
C
N H
Bt
O
O Bt
N H
O
N H
Bt
Fig. 11. Proposed mechanisms for the formation of n-butyl amine, N,N0 -dibutyl urea, and n-butyl carbamic acid anhydride.
isocyanate, within the range of sizes examined, appears to have a negligible effect on the adsorption into the zeolite, since no residual isocyanate peaks have been detected in the 13C CP MAS NMR of NaX exposed to any of the isocyanates, except the di-isocyanate. Further support for the formation of the proposed framework carbamate is provided by the base hydrolysis experiments performed on BtIC. The framework butyl carbamate is tightly bound and could not be extracted by solvents, including neutral water solvent. However, it underwent a nucleophilic substitution reaction with the stronger hydroxide ion nucleophile in an aqueous medium to form n-butyl amine, n-butyl carbamic acid anhydride, and N,N0 -dibutyl urea, which were observed in the CDCl3 extract. The proposed reaction mechanism for the formation of these species is presented in Fig. 11. Central to the proposed mechanism is base (OH) hydrolysis of the framework carbamate species to form the fleeting butyl carbamic acid intermediate[18,23,24,14]. The proposed dissociation of the n-butyl carbamic acid intermediates into n-butyl amine and CO2 is known to occur readily in aqueous media [25]. The formation of the n-butyl carbamic acid anhydride, also observed, can be rationalized as a bi-product of the framework carbamate and n-butyl carbamic acid in either of two ways. The dehydration of two n-butyl carbamic acid molecules may occur (shown). Alternatively, a nucleophilic substitution of the butyl carbamate framework species by n-butyl carbamic acid may occur (not shown). The reaction to form carbamic acid anhydrides from carbamic acids has not been reported in the literature, which suggests that the reaction is catalyzed by the zeolite framework. However, Naegelim et al. have proposed that the n-butyl carbamic acid
anhydride does exist as an intermediate in the formation of urea [26]. Similar results were found in the hydrolysis of EIC exposed NaX, as described in the Supplementary material. The zeolite has clearly been shown to sequester the isocyanate into a chemically new, framework bound species. The type of hydrolysis chemistry observed here shows that this sequestration product is susceptible to hydroxide nucleophilic substitution. The proposed framework carbamate ester species should exhibit this susceptibility, as it indeed does. 5.2. Tert-butyl isocyanate reaction with NaX The reaction of tert-butyl isocyanate appears to have competing nucleophilic addition and elimination reactions occurring. Based on the results from the 13C CP MAS NMR and GC–MS, it was ascertained that 2-methyl propene is formed along with two different framework carbamates. In Fig. 12 the proposed addition reaction is shown with the formation of a tert-butyl framework carbamate by the top route. The framework carbamate formation follows the same addition mechanism as other isocyanates (see Fig. 10). In parallel with addition, shown in the bottom route, the overall elimination reaction is proposed to form the observed olefin and the framework hydrogen carbamate that is consistent with NMR and FTIR observations. It is possible that the overall elimination reaction involves several steps to give the observed products, in line with the roles of the framework oxygen atom and the sodium ion proposed in previous work on the elimination reactions of alkyl halide exposed zeolites [9]. The first step would be the elimination reaction that
Al N Si
Al O Na
N
C
O
Na
+
O C O
H +
Si
Na N
O C
O Si
Fig. 12. Proposed mechanism for the reaction of TBIC with NaX.
Al
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removes a methyl group proton, leading to the observed olefin, with the assistance of a charged supercage oxygen, also leading to an acid zeolite site at the framework oxygen that removed the proton. The sodium cation assists the departure of the NCO group, leading to sodium isocyanate. Evidently the relative basicities of the NCO and the framework oxygen favor subsequent conjugate acid-base formation of the hydrogen isocyanate and the Na-zeolite site. Finally, the hydrogen isocyanate forms the adduct framework hydrogen carbamate. Thus the TBIC substitution and elimination reactions together would account for the three final products observed, namely the framework hydrogen carbamate, consistent with solid-state 13C NMR and FTIR spectra, the observed framework tert-butyl carbamate, and 2-methyl propene. No intermediate HNCO was extracted and observed in the 13C NMR or detected as an emitted gas by GC–MS. 5.3. Toluene 2,4 di-isocyanate reaction with NaX T24DI showed results that differed slightly from the chemistry of other isocyanates, evidently because it has two isocyanate functional groups. In the reaction of NaX with T24D it appears that only one of the isocyanate functional groups reacts with the framework. The other is left unreacted, as indicated by the IR results. The second isocyanate moiety is proposed to undergo hydrolysis with the addition of water. Only one of the original isocyanate groups was able to react with the NaX supercage, presumably because of the rigid structure of the cage and T24DI itself. It seems improbable for the molecule to contort itself so as to find a second active supercage oxygen atom that can add to the second isocyanate group. The second functional group is thus left available for chemistry, such as for hydrolysis to the corresponding amine with loss of CO2 [25]. The CP MAS 13C NMR results of T24DI exposed NaX, namely the peak at d 159 ppm, show that a framework carbamate is formed. The region near d 120 ppm, indicative of residual isocyanate, is masked by the chemical shifts of the aromatic carbons, and therefore it cannot be determined solely from the 13C NMR whether all of the isocyanate moieties have reacted. As an alternative approach, an FTIR study of the reaction was done to determine if all of the isocyanate moieties had reacted. The spectrum shows a peak at 2269 cm1 characteristic of an unchanged isocyanate moiety, which decreases progressively with the addition of water. These results taken together are consistent with one of the isocyanate moieties in T24DI being attacked by the nucleophile, while the other is left unchanged in the absence of water. It is not clear how the formation of the framework carbamate is protected from water hydrolysis and why the other isocyanate functional group is susceptible to water hydrolysis. The two functional groups of 16DIH were shown to behave in a similar manner (see Supplementary material).
an E2 type elimination reaction [9]. This tendency is evidently also being observed in the reactivity of NaX with the isocyanates. 6. Conclusions The isocyanate functional group is susceptible to nucleophilic attack by Faujasite NaX. A framework carbamate product, with the anionic nitrogen stabilized by the sodium cation, is proposed as a product common to the chemistry of all isocyanates studies. The product is very tightly bound to the zeolite. Two, but not all, observed cases suggest that where there are methyl protons b to the isocyanate moiety present, an elimination reaction may also form the a, b olefin, competing with the nucleophilic addition reaction. Based on the types of chemistry occurring and the tenacity with which the products are retained, zeolites have the promise of practical applications in hazardous materials cleanup and possibly in controlling isocyanate vapor release in the workplace and in the vicinity of chemical plants. Acknowledgements We acknowledge the support of a US Army/DTRA Project, Numbers W911NF-07-1-0042/AA05CBT019, a NATO Cooperative Linkage Grant No. 982991, and a Binghamton University, Innovative Technologies Center instrument user grant. We thank Professor Apostolos K. Rizos for a number of helpful suggestions in preparation of this paper. Appendix A A detailed description of the experimental procedures, which are substantially the same as in previously published studies, is relegated to the Supplementary material. The NMR data of compounds that give substantially the same results as the examples presented here in the paper, are collected in the Supplementary material. Some of the experiments on authentic samples of various compounds, performed to support the identification of reaction products and residual, unreacted isocyanates, are placed in the Supplementary material. In experiments designed to address some of the practical issues of the exposure of NaX to isocyanates, the headspace of zeolite exposed to isocyanates under various conditions of NaX exposure to isocyanate was examined, including experiments where the isocyanate adsorption and reaction was observed in zeolite previously exposed to various amounts of water. GC–MS headspace analyses are presented in the Supplementary materials over successively greater amounts of NaX zeolite exposed to a fixed amount of isocyanate. These experiments are also analyzed in the Supplementary material.
5.4. Dependence on connectivity of the isocyanate
Appendix A. Supplementary data
Primary (OIC), secondary (IPIC), and tertiary (TBIC) saturated isocyanates were investigated. The OIC and IPIC showed very similar results leading exclusively to the formation of the framework carbamate adduct, through the proposed nucleophilic addition mechanism in Fig. 10. The reaction between TBIC and NaX formed the elimination product, 2-methyl propene, as well as the nucleophilic addition product, framework tert-butyl carbamate. This shows that all isocyanates, regardless of connectivity, are susceptible to some type of nucleophilic attack by NaX, with both nucleophilic addition and elimination observed, not only for tertiary TBIC but also for primary EIC. In work done by Kanyi et al. it has been shown that NaX has a tendency to act as both a nucleophile, doing an SN2 type (substitution) reaction, or as a base, doing
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.micromeso.2010.10.027. References [1] O. Bayer, Agnew. Chem. 59 (1947) 257. [2] P. Shrivastava, Bhopal Anatomy of a Crisis, Paul Chapman, London, 1992. [3] EPA’s Hazardous Wastes A Consolidated List Inter, Face Associates, Middletown, CT, 1986. [4] H.J. Beim, J. Sperro, L. Sperro, Rapid Guide to Hazardous Air Pollutants, Van Nostrand Reinhold, New York, 1986. [5] K. Verschuenren, Handbook of Environmental Data on Organic Chemicals, 4th ed., Wiley-Interscience, New York, 2001. [6] C. De Gopan, J.D. Nauratil, J.A. Raper, J.M. Starling, J. Eniviron. Sci. Health A27 (1915) 17.
J.B. DeCoste et al. / Microporous and Mesoporous Materials 139 (2011) 110–119 [7] S.W. Yang, D.C. Doetschman, J.T. Schulte, J.B. Sambur, C.W. Kanyi, J.D. Fox, C.O. Kowenji, B.R. Jones, N.D. Sherma, Microporous Mesoporous Mater. 92 (2006) 56. [8] C.W. Kanyi, D.C. Doetschman, J.T. Schulte, K. Yan, R.E. Wilson, B.R. Jones, C.O. Kowenji, S.W. Wang, Microporous Mesoporous Mater. 92 (2006) 292. [9] C.W. Kanyi, D.C. Doetschman, S.-W. Yang, J. Schulte, B.R. Jones, Microporous Mesoporous Mater. 108 (2008) 103. [10] A.R. Armstrong, P.A. Anderson, L.J. Woodall, P.P. Edwards, J. Am. Soc. 117 (1995) 9087. [11] J.J. Charnell, Cryst. Gr. 8 (1971) 2758. [12] D. Olsen, Zeolites 15 (1995) 439. [13] Ch. Baerlocher, W.M. Meier, D.H. Olson, Atlas of Zeolite Framework Types, Elsevier, Netherlands, 2001. [14] D.P.N. Satchell, R.S. Satchell, Chem. Soc. Rev. 4 (1975) 231. [15] H.J. Lee, Y.M. Kim, O.S. Kweon, I.J. Kim, J. Eur. Ceram. Soc. 27 (2007) 561. [16] J.R. Durig, R.J. Berry, C.J. Wurrey, J. Am. Chem. Soc. 110 (1988) 718.
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[17] M. Feuerstein, M. Hunger, G. Engelhardt, J.P. Amoureux, Solid State Nucl. Magn. Reson. 7 (1996) 95. [18] E.A. Castro, R.B. Moodie, P.J. SanSom, J. Chem. Soc. Perkin Trans. 2 (1985) 737. [19] Sadtler Research Laboratories Carbon-13 NMR Collections, Researchers, United States, 1985. [20] C.W. Kanyi, D.C. Doetschman, J.T. Schulte, Microporous Mesoporous Mater. 117 (2009) 48–54. [21] C.W. Kanyi, D.C. Doetschman, S.-W. Yang, J.T. Schulte, Microporous Mesoporous Mater. 119 (2009) 23. [22] C.W. Kanyi, D.C. Doetschman, J.T. Schulte, Microporous Mesoporous Mater. 124 (2009) 232. [23] R.G. Arnold, J.A. Nelson, J.J. Verbanc, Chem. Rev. 57 (1957) 47. [24] N.V. Sidgwick, L.T. Millar, H.D. Springall, The Organic Chemistry of Nitrogen, Oxford Press, London, 1966. [25] M.L. Bender, R.B. Homer, J. Org. Chem. Soc. 30 (1965) 3975. [26] C. Naegelim, A. Tyabji, L. Conrad, F. Litwan, Helv. Chim. Acta 63 (1938) 6867.
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
The room temperature chemistries of isocyanates with zeolite NaX Jared B. DeCoste, David C. Doetschman ⇑, Miranda J. Lahr, Charles W. Kanyi, Jürgen T. Schulte Department of Chemistry, Binghamton University, Binghamton, NY 13902-6000, USA
a r t i c l e
i n f o
Article history: Received 16 June 2010 Received in revised form 20 September 2010 Accepted 18 October 2010 Available online 26 October 2010 Keywords: Zeolite Isocyanate Di-isocyanate Nucleophilic Addition
a b s t r a c t Isocyanates and di-isocyanates are used in the production of various chemicals and materials, such as pesticides and polyurethane. However, since the disastrous accidental release of isocyanate in Bhopal in 1984 this class of toxic compounds has been under scrutiny, with growing interest in means to respond to isocyanate release. The susceptibility of isocyanates to nucleophilic substitution [D.P.N. Satchell, R.S. Satchell, Chem. Soc. Rev. 4 (1975) 231] and the known nucleophilicity of NaX [Yang, et al., Microporous Mesoporous Mater. 92 (2006) 56; Doetschman, et al., Microporous Mesoporous Mater. 92 (2006) 292; Kanyi, et al., Microporous Mesoporous Mater. 108 (2008) 103] led to this study of a spectrum of isocyanates representing cyclic and aromatic isocyanates, di-isocyanates, 1°, 2°, and 3° isocyanates, and alkyl chain isocyanates of various lengths. Additional experiments were done in order to understand the nature of a tightly bound chemical adduct product that forms in the isocyanate reactions with the zeolite. The isocyanate functional group was shown to be susceptible to nucleophilic attack by Faujasite NaX. A carbamate framework adduct, with the anionic nitrogen stabilized by the sodium cation is proposed. The product is very tightly bound to the zeolite. In two, but not all, isocyanates in which there are methyl protons b to the isocyanate moiety, an elimination reaction took place to form the a, b olefin, competing with the nucleophilic addition reaction. The types of chemistry occurring and the tenacity with which the products are retained, suggests that zeolites may have practical applications in hazardous materials cleanup or controlling isocyanate vapor release. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction Isocyanates and di-isocyanates have been very useful in the production of various chemicals and materials. Bayer discovered that di-isocyanates are ideal for the production of polymers such as polyurethane [1]. But since the deaths of an estimated 20,000 people in 1984 in Bhopal from an accidental release of methyl isocyanate used as an intermediate in pesticide production, isocyanates, as well as other industrial chemicals, have been under much legislative and risk management scrutiny [2]. The Environmental Protection Agency, along with other authorities, lists methyl isocyanate, toluene 2,4- and 2,6-di-isocyanates (TDI), and other isocyanates, as hazardous wastes and potential pollutants [3,4]. It has been shown that conventional decontamination methods, such as aqueous scrubbing, are not effective methods for TDI removal. The reaction between water and TDI to produce the corresponding amine and carbon dioxide is slow without the presence of a catalyst [5]. Sorption onto solid materials has been shown to be an effective method for TDI removal. Both activated carbon and high-density polyurethane foam have been shown to be effective scrubbing
⇑ Corresponding author. Tel.: +1 607 777 2298; fax: +1 607 777 4478. E-mail address: [email protected] (D.C. Doetschman). 1387-1811/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2010.10.027
materials [6]. The drawback of this method is that it is effective only over short periods of time before breakthrough occurs. Faujasite NaX zeolite has been shown to be effective in the adsorption and subsequent destruction of materials that are susceptible to nucleophilic attack. Dimethyl methylphosphonate, a type G and X nerve agent simulant, and alkyl halides have been shown to undergo adsorption by NaX with subsequent nucleophilic chemistry with the zeolite [7–9]. A delocalized negative charge between the supercage O atoms and Al atoms in NaX, which is stabilized by Na cations, gives zeolite X its nucleophilic character. Additionally the supercage cation also serves to assist the departure of an anionic leaving group. Zeolite X (NaX) has been well characterized structurally [10– 13] and has a supercage diameter of 11 Å, a 7 Å diameter window into the supercage, and a Si/Al ratio of 1.23 [7–9]. Thus the structure of the zeolite poses a size limitation to the adsorption of molecules, depending upon their rigidity. The structure of NaX is not compromised under the vacuum and high temperatures employed to dry it thoroughly [12]. The susceptibility of isocyanates to nucleophilic substitution [14] and the known nucleophilicity of NaX [7–9] led to a study of the following spectrum of isocyanates and di-isocyanates: n-butyl isocyanate (BtIC), cyclohexyl isocyanate (CHIC), 1,6 diisocyanato hexane (16DIH), n-dodecyl isocyanate (DDIC), ethyl isocyanate (EIC), isopropyl isocyanate (IPIC), n-octyl isocyanate (OIC), n-propyl
J.B. DeCoste et al. / Microporous and Mesoporous Materials 139 (2011) 110–119
isocyanate (PIC), toluene 2,4 di-isocyanate (T24DI), tert-butyl isocyanate (TBIC). CHIC and T24DI represent a cyclic and an aromatic isocyanate, respectively, of comparable size and also, together with 16DIH, permit a comparison of mono- and di-isocyanates. BtIC, DDIC, EIC, OIC, and PIC represent primary isocyanates, IPIC represents a secondary isocyanate, and TBIC represents a tertiary isocyanate, permitting a study of the dependence of isocyanate zeolite chemistry on the isocyanate connectivity. Also, BtIC, DDIC, EIC, OIC, and PIC make possible a study of the dependence of the isocyanate zeolite chemistry on the length of the aliphatic chains connected to the isocyanate moiety. Additional experiments were done in order to understand the nature of a tightly bound chemical adduct product that forms in the isocyanate reactions with the zeolite. Solid-state 23Na NMR experiments on the BtIC-exposed NaX were compared with the NMR of the untreated zeolite. Also NaOH-catalyzed hydrolysis and extraction experiments were performed on the BtIC exposed zeolite. The exposed NaX was treated with NaOH solution in an attempt to remove the tightly bound proposed carbamate framework species from the zeolite, having unsuccessfully attempted to hydrolyze the product with neutral water or to perform extraction with a variety of non-aqueous solvents.
2. Experimental The preparation of the NaX zeolite and the introduction of water and the various isocyanate compounds into the zeolite were done with methods similar to those published previously [7–9]. The details of these methods are described in full in the Supplementary material. NaX (Sigma–Aldrich) was prepared by heating it under vacuum for an extended period of time. In certain experiments the dried NaX was loaded with stoichiometric amounts of water, ranging from 0 to 10 water molecules per supercage, which were also transferred through the vapor phase into the sample. The addition of water [15] was done either before isocyanate transfer or subsequent to it, depending upon the purpose of the experiment. The isocyanate compounds were introduced into the dry NaX at 3 molecules of isocyanate per supercage of zeolite by methods similar to the introduction of water. After introduction of the isocyanate and water (where employed) the samples were equilibrated at room temperature and then the products in the zeolite or products released from the zeolite were characterized. Attempts to extract products with neutral solvents from the zeolite yielded no detectable products, the details of which are described in the Supplementary material. Residual, unreacted isocyanate and gaseous products, which emanated from the exposure of the zeolite to some of the isocyanates, were released and captured in an IR cell using the Canulla technique. A Schlenk-line with a rotary oil vacuum pump connected to it was used to argon-flush and evacuate the cell and to backfill with argon after introducing the gases from the reaction vessel. The stopcock to the IR cell was closed and the cell was taken to the IR spectrometer for measurement on an Equinox 550 FTIR instrument. (Scans (128) were made at 2 cm1 resolution with background subtraction.). In select instances gas chromatographic–mass spectrometric (GC–MS) head space analyses, using a Varian 4000 Ion Trap GC–MS, were done on the gases that were present after exposure of the NaX to isocyanate in the head space analysis vial. Details may be found in the Supplementary material. Products and residual isocyanate trapped in the isocyanate exposed NaX were characterized in the zeolite by solid-state 13C cross-polarized (CP) magic angle spinning (MAS), nuclear magnetic resonance (NMR), 23Na MAS NMR, and IR spectroscopy using the KBr pellet technique. The solid-state NMR experiments were per-
111
formed with a Bruker AC 300 spectrometer, modified for solidstate NMR experiments on the particular nuclei. The 13C CP MAS NMR experiments were similar to those described in previous publications [7–9]. The IR experiments on the exposed zeolites also employed the Equinox 550 FTIR instrument. Details about these experimental methods may be found in the Supplementary material. In order to perform NaOH-catalyzed hydrolysis of the product of the reaction of BtIC with NaX, 0.25 g of NaX (3BtIC/SC), (3.8 104 moles BtIC) was stirred in a round bottom flask with 5.0 mL of 0.25 M NaOH (10 NaOH per supercage) for approximately 1 h. The slurry was filtered and the filtered solid was then placed in a round bottom flask with 5.0 mL of CDCl3 and stirred for 1 h. The slurry was gravity-filtered and the extract was kept for solution NMR analysis. A similar attempt to hydrolyze the BtIC-exposed NaX with neutral water was made and the extract was examined. The products of the hydrolysis were characterized as follows. The filtrates were analyzed on a Bruker AM 360 NMR spectrometer, which has an 8.45 T magnetic field. The sample was placed in a 5 mm NMR tube and spun at 20 Hz. The 360.14 MHz 1H NMR spectrum was recorded, employing a 3.0 ls pulse, a relaxation delay of 35 ls, followed by an acquisition time of 2.34 s and a recycle time of 500 ms. A total of 1000 transients was recorded and processed with a Fourier transformation. The spectra were referenced internally to the solvent. The 90.57 MHz 13C NMR spectrum of the extract was recorded, employing a 3 ls pulse, a relaxation delay of 35 ls, followed by an acquisition time of 1.09 s and a recycle time of 500 ms. Inverse gated decoupling was applied on the proton channel at a frequency of 360.14 MHz. A total of 20,000 transients were obtained. The spectrum was referenced internally to the solvent. The H, H COSY spectrum of the extract was recorded on the 360.14 MHz 1H channel, employing a 13 ls pulse, followed in succession by a delay of 3 ls, a pulse of 13 ls, a delay of 10 ls, an acquisition time of 256 ms, and a delay of 500 ms on the 1H channel. This sequence was repeated for 8 transients at each of the 1024 2-dimensional points. There were 1024 acquired points over a spectral width of 2000 Hz. The C, H COSY spectrum of the extract was recorded on the 90.56 MHz 13C channel, employing a 10.4 ls pulse, followed in succession by a variable delay, another pulse of 10.6 ls, a delay of 1.75 ms, a relaxation delay of 10 ls, an acquisition time of 51.2 ms, and a delay of 1 s on the 360.14 MHz 1H channel. A pulse of 12 ls was applied on the 13C channel immediately after the second 1H channel pulse. The sequence was repeated for 256 scans at each of the 256 2-dimensional points. There were 1024 acquired points over a spectral width of 10,000 Hz. 3. Results In view of the number of isocyanates examined and the number of characterization experiments performed, a set of typical results will be presented here that are sufficient to convey the nature of the isocyanate chemistry. These examples are represented by BtIC, OIC, T24DI, and TBIC. Other results of a similar nature will be given in the Supplementary material. Moreover, many of the experiments on authentic samples of various compounds, performed to support the identification of reaction products and residual, unreacted isocyanates, are relegated to the Supplementary material. 3.1. n-Octyl isocyanate exposed NaX The results for OIC are typical of single, straight-chain or closedchain substituted isocyanates with a single isocyanate functional group. The 13C CP MAS NMR spectrum of NaX exposed to OIC at
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3 molecules per supercage is shown in Fig. 1, together with a 13C NMR spectrum of a solution of authentic OIC in CDCl3. Similar pairs of 13C spectra for the other isocyanates studied, after adsorption into NaX and in CDCl3 solution are shown in the Supplementary material. The IR spectrum of OIC exposed NaX (3/sc) in comparison with that of NaX alone shows the appearance of a few new bands. The FTIR spectrum of NaX exposed to OIC is presented in the Supplementary material. There is a medium sized band at 1577 cm1 and there are also weak bands at 1478, 1310, 1258, and 1246 cm1. Substantially similar FTIR results are found for the other monoisocyanates and the results for OIC are thus typical of the chemistry of a number of differently branching substituted isocyanates with one isocyanate functional group (see the Supplementary material).
The GC–MS of the headspace over NaX exposed to TBIC (2.3/sc) shows the presence of 2 species. These have been found to vary with the amount of NaX added to the headspace vial (see Supplementary material). One of these species has a retention time of 1.6 min and the other one a retention time of 2.2 min. The mass spectrum at the retention time of 1.6 min is shown in Fig. 3. This was determined to be the major species present, other than unreacted TBIC. The mass spectrum at a retention time of 2.2 min is identical to that of neat TBIC and is given in the Supplementary material. An FTIR spectrum of NaX exposed to 3 molecules of TBI is presented in the Supplementary material.
3.2. Tert-butyl isocyanate exposed NaX
The results for the bifunctional T24DI differ from those of isocyanates with a single functional group. Fig. 4 shows (a) the solid state CP MAS 13C NMR spectrum of T24DI (3/sc) exposed NaX and (b) the 13C NMR spectrum of a solution of authentic T24DI in CDCl3, illustrating the behavior of a di-isocyanate in the zeolite. The solid-state NMR spectrum in this case exhibits pronounced sidebands of the 120–140 ppm peaks.
Fig. 2 shows (a) the solid state CP MAS 13C NMR spectrum of TBIC-exposed (3/sc) NaX and (b) the 13C NMR spectrum of a solution of TBIC in CDCl3. The peak in the neighborhood of 110 ppm and the two nearly overlapping peaks between 150 and 160 ppm are evidence for an isocyanate chemistry that will be shown to yield an olefin.
3.3. Toluene 2,4 di-isocyanate exposed NaX
3.4. Sequential chemistry in di-isocyanates The FTIR spectrum of the bifunctional isocyanate T24DI in NaX subsequently exposed to various amounts of water was recorded,
Fig. 1. (Top) Solid-state 13C CP NMR spectrum of NaX exposed to 3 molecules per supercage of OIC. (Bottom) Liquid 13C NMR spectrum of authentic OIC in CDCl3 solvent. The horizontal axis scale designates chemical shift d in units of parts per million (ppm).
Fig. 3. Mass spectrum of the headspace over NaX exposed to 2.3 TBIC/sc at a retention time of 1.6 min.
Fig. 2. (Top) Solid-state 13C CP NMR spectrum of NaX exposed to 3 molecules per supercage of TBIC. (Bottom) Liquid 13C NMR spectrum of authentic TBIC in CDCl3 solvent. The horizontal axis scale designates chemical shift d in units of parts per million (ppm).
Fig. 4. (Top) Solid-state 13C CP NMR spectrum of NaX exposed to 3 molecules per supercage of T24DI. (Bottom) Liquid 13C NMR spectrum of T24DI solution in CDCl3 solvent. The horizontal axis scale designates chemical shift d in units of parts per million (ppm).
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in order to characterize the nature of the hydrolysis chemistry and its implications for the sequence of the two isocyanate group chemistries. Shown in Fig. 5 are the FTIR spectra of (3/sc) exposed NaX, to which the indicated amounts of water have been added after the NaX exposure to T24DI. For reference, the FTIR spectrum of NaX alone is presented in the Supplementary material.
113
solution hydrolysate of the BtIC-exposed NaX, shown in the Supplementary material, is rich. The regions 0.8 to 1.8 ppm and
3.5. Changes in Na cation environment The solid-state NMR spectrum of the Na cation was examined after exposure of the zeolite to BtIC, in order to investigate what interaction, if any, the Na cation has with the zeolite bound product after exposure. Unlike TBIC, BtIC yields no volatile products and was chosen for investigation because of this simplifying behavior. Fig. 6 (Top) shows the 23Na MAS NMR spectrum of NaX exposed to BtIC (3/sc). For reference the 23Na MAS NMR spectrum of dry, unexposed NaX is presented in Fig. 6 (Bottom). The spectra are given from 20 to –100 ppm to focus on the major features. 3.6. Base hydrolysis of the framework bound reaction product The 1H NMR spectrum (not shown) from the water filtrate of the hydrolysate of BtIC-exposed NaX contains no non-solvent peaks. However, the 1H NMR spectrum of CDCl3 extract of the NaOH
Fig. 7. Segments of the 1H NMR spectrum from 0.8 to 1.8 ppm of the CDCl3 extract from BtIC-exposed NaX that had been treated with NaOH solution and centrifuged. The sample was acidified to emphasize the molecular form of the n-butyl carbamic acid anhydride.
Fig. 5. FTIR spectrum of T24DI (3/sc) with H2O (x/sc), where x = (a) 0, (b)1, (c) 1.5, (d) 2, (e) 2, and (f) 3.5, from 2400 to 2200 cm1. The weak spectra (c–f), obtained with 1.5–3.5 H2O per supercage, decrease successively in intensity with increasing H2O and are not individually labeled.
Fig. 8. Segments of the 1H NMR spectrum from 2.5 to 4.0 ppm of the CDCl3 extract from BtIC-exposed NaX that had been treated with NaOH solution and centrifuged. The sample was acidified to emphasize the molecular form of the n-butyl carbamic acid anhydride.
Fig. 6. (Top) Solid-state 23Na MAS NMR spectrum of NaX exposed to BtIC (3/sc). (Bottom) Solid-state 23Na MAS NMR spectrum of NaX alone. The horizontal axis scale designates chemical shift d in units of parts per million (ppm).
Fig. 9. 13C NMR spectrum of the CDCl3 extract from BtIC-exposed NaX that had been treated with NaOH and centrifuged. The sample was acidified to emphasize the molecular form of the n-butyl carbamic acid anhydride.
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2.5–4.0 ppm of a sample, acidified to favor the acid form, are expanded in Figs. 7 and 8, respectively. The spectra will be shown to include n-butyl carbamic acid anhydride. Likewise, the rich 13C NMR spectrum of the CDCl3 extract is shown in Fig. 9 (The regions 10–50 ppm and 140–180 ppm of it are expanded in the Supplementary material). The H, H and C, H COSY 2-dimensional spectra provide a basis for determining how many closely related residual BtIC and BtIC product species are present in the extract and which peaks belong to which species. The H, H COSY 2-dimensional spectrum and the C, H COSY 2-dimensional spectra of the CDCl3 filtrate are also given in the Supplementary material. 4. Analysis 4.1. n-Octyl isocyanate exposed NaX In the 13C CP MAS NMR spectrum of OIC (3/sc) exposed NaX shown in Fig. 1, the four chemical shifts of d 14, 23, 29, and 43 correspond, by comparison with the spectrum of authentic OIC, to the eight different carbon atoms in the alkyl chain. These are presented in Table 1. Uncertainties in the 13C solid-state NMR chemical shifts are 1 ppm or less. For the 13C CP NMR spectrum of authentic OIC, shown in Fig. 1, the eight chemical shifts between d 12 and 43 ppm represent the eight carbon octane chain. The closer the carbon is to the isocyanate moiety, the more downfield is the chemical shift. The chemical shift of authentic OIC at d = 121.9 ppm is assigned to the carbon within the isocyanate moiety. Uncertainties in the solution 13C NMR shifts are 0.1 ppm or less. The line broadening of the solid-state NMR spectrum causes these chemical shifts to overlap and to be indistinguishable from one another. The solid-state 13C NMR chemical shift at d 159 ppm is assigned to a carbon in the resulting functional group of a proposed framework carbamate, shown in Structure 1, that is derived from the C atom of the original isocyanate functional group. A peak in this region, often of greater intensity than in this example, is observed in all of the isocyanate exposed NaX samples (see Supplementary materials). The new infrared bands appearing in the OIC exposed NaX have not been assigned.
Corresponding assignments have been made for the other isocyanates (see Supplementary material). 4.2. Tert-butyl isocyanate exposed NaX The 13C CP MAS NM R spectrum of TBIC-exposed NaX (3/sc) in Fig. 2 exhibits chemical shifts of d 23, 29, 52, 109, 149, and 159 ppm. In view of the 13C NMR spectrum of authentic TBIC in solution, also shown in Fig. 2 and assigned in the Supplementary material, there appear to be too many chemical shifts to arise from just one product. The chemical shifts at d 109 and 149 ppm probably indicate the presence of an olefinic product that is assigned tentatively to 2-methyl propene. The chemical shifts at d 29, and 23 belong to methyl groups in 2-methyl propene and a proposed framework tert-butyl carbamate, Structure 2, respectively. The chemical shift at d 52 ppm probably belongs to the carbon to which the functional group is attached, in view of the results from other isocyanates. The functional group is proposed to be a framework carbamate of carbamic acid or simply the framework hydrogen carbamate, shown in Structure 3, according to a mechanism proposed in the Section 5. The weak complex peak with the chemical shift at d 159 ppm evidently also belongs to the carbons in the framework carbamate functional groups of the products, being around a chemical shift that is different than that of the TBIC isocyanate group. The chemical shift assignments made here are summarized in Table 2. As most of the framework carbamates occur at essentially the same chemical shift and the lines are relatively broad, this peak is consistent with the presence of two different framework carbamate species. It will become clearer that the production of the 2-methyl
Structure 2. Proposed structure of framework tert-butyl carbamate. Table 1 Chemical shift assignments for the solid-state 13 C CP MAS NMR of the product of NaX exposure to octyl isocyanate (OIC). Octyl group carbons are numbered consecutively from the C atom attached to the isocyanate group. Chemical shift (ppm)
Carbon assignment
159
Framework carbamate Chain – 1 Chain – 2–6 Chain 7 Chain 8
43 29 23 14
Structure 3. Proposed structure of framework hydrogen carbamate.
Table 2 Chemical shift assignments for the solid-state 13C CP MAS NMR of the products of NaX exposed to tert-butyl isocyanate.
Structure 1. Proposed structure of framework octyl carbamate.
Chemical shift (ppm)
Carbon assignment
159 149 109 52 29 23
Framework carbamate groups C atoms Interior olefin C atom Terminal olefin C atom Tertiary carbon Atom Methyl group C atoms of framework carbamates Methyl group C atoms of 2-methyl propene
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propene probably accompanies the formation of another framework isocyanate. The IR spectrum of TBIC (3/sc) exposed NaX shows the appearance of new bands in comparison with the IR spectrum of NaX alone, indicating that chemistry has taken place (see spectrum in the Supplementary material). There is a medium sized band at 1369 cm1 and weak bands at 2235, 2169, 1733, and 1692 cm1. These bands have not been assigned. The GC–MS spectra observed in the headspace of the TBICexposed NaX were presented in Fig. 3 and in the Supplementary material. At the 1.6 min retention time, the base peak is at 41 m/z, which represents an M 15 peak for 2-methyl propene product. The parent peak is at 56 m/z, for which an M 1 peak is observed at 55 m/z. These assignments are summarized in Table 3. The mass spectrum for the species observed at the retention time of 2.2 min, shown in the Supplementary material, has a base peak at 84 m/z, which represents an M 15 peak. The parent peak is at 99 m/z. The other major peak is at 56, representing an M 43 peak. These are summarized in Table 4. The retention time and spectrum are in agreement with authentic, neat TBIC. 4.3. Toluene 2,4 di-isocyanate exposed NaX
Table 5 Chemical shift assignments for the solid-state 13C CP MAS NMR of the product of NaX exposed to toluene 2,4 di-isocyanate. Chemical shift (ppm)
Carbon assignment
159 131 121
Framework carbamate group C atom Ring – 1,3,5,6 C atoms Ring – 2,4 and isocyanate group C atoms Methyl group C atom
17
Al
O
Si
C
O
N
C N
O
Na
Structure 4. Proposed structure of framework toluene-2-isocyanato-4-carbamate.
13
In the C CP NMR of neat toluene 2,4 di-isocyanate, displayed in Fig. 4, the chemical shift at d 17.8 ppm belongs to the methyl group. The chemical shift at d 124.7 ppm is assigned to the carbons within the isocyanate moieties. Even though these two carbons are not chemically equivalent they are very close, and evidently share a common chemical shift. The other chemical shifts at d 120.9, 122.2, 130.5, 131.3, 131.9 and 133.1 correspond to the carbons in the 3, 5, 1, 6, 4, and 2 positions, respectively. The intensity of the chemical shifts at the 1, 2, and 4 positions are significantly smaller than the other chemical shifts, because there are no adjacent protons with which to couple with the CP NMR sequence. In the 13C CP MAS NMR spectrum of T24DI-exposed NaX (3/sc) there are chemical shifts of d 17, 121, 131, and 159 ppm, as can be seen in Table 5 with reference to the proposed Structure 4. The chemical shift at 17 ppm corresponds to a methyl group. The chemical shifts at d 121 and 131 are from aromatic carbons in a benzene ring. The peaks at d 69, 77, 176, and 187 are spinning
Table 3 Mass spectrometry data for NaX exposed to 2.3 molecules of TBIC per supercage at a retention time of 1.6 min, assigned to 2-methyl propene. M represents the parent 2-methyl propene mass of 56. m/z
Relative abundance (%)
Species
41 55 56 57
100.0 63.0 70.5 21.7
M 15 M1 M M+1
Table 4 Mass spectrometry data for NaX exposed to 2.3 molecules of TBIC per supercage at a retention time of 2.2 min, assigned to residual TBIC by comparison with the spectrum of authentic TBIC. M represents the parent TBIC mass of 99. m/z
Relative abundance (%)
Species
56 84 99 100
42.46 100.00 .67 .04
M 43 M 15 M M+1
sidebands of the benzene carbon peaks. It is impossible to determine from this spectrum whether the 13C of the functional group of residual isocyanate is present or not, because the broad aromatic peaks mask the region where it would show up. The chemical shift at d 159 ppm represents the carbon in the functional group of the framework carbamate product. Similar results were found for the di-isocyanate, 16DIH (see Supplementary material). 4.4. Sequential chemistry in di-isocyanates The IR spectrum of T24DI-exposed NaX (3/sc), not shown, exhibits many new bands in comparison with those of NaX alone. (There are strong bands at 2341, and 2281 cm1, and weak bands at 1510, 1423, 1404, 1362, 1304 and 1242 cm1.) The trace shown in Fig. 5 exhibits the presence of new bands at 2281 and 2341 cm1. According to the literature, the NCO asymmetric stretch appears at approximately 2268 cm1 [16]. If the observed peaks indicate unreacted isocyanate functional groups, then there are still isocyanate functional groups present after the exposure of NaX to T24DI. This observation contrasts with the IR spectra in the mono-functional isocyanate reactions, in which all of the isocyanate functional groups react with the zeolite. As can be seen in Fig. 5, as more H2O is added to the NaX, the size of both of these peaks decrease. The absorbance of these peaks was measured and found to decrease progressively with the amount of water added (see Supplementary material). This result appears to indicate that one of the T24DI isocyanate groups is employed in forming the observed framework carbamate species. The second isocyanate group does not react with the zeolite, but appears to be susceptible to hydrolysis by water, as indicated by its progressive disappearance when the T24DI exposed zeolite is further exposed to water (see Supplementary material). 4.5. Na cation environment The 23Na MAS NMR spectrum of NaX in Fig. 6 (Top) shows two main chemical shifts at d 5 and 46 ppm corresponding to cation sites I and III0 , respectively [17]. The 23Na MAS NMR spectrum of NaX exposed to BtIC (3/sc) in Fig. 6 (Bottom) shows one main
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chemical shift at d 14 ppm. This does not correspond to any of the known cation sites reported for empty NaX. Cation sites in Faujasite zeolites are described in detail by Olsen in Ref. [12].
Table 8 n-Butyl carbamic acid anhydride 1H and 13C NMR chemical shifts and H,H coupling constants. C atom and corresponding protons are numbered consecutively from the N atoms to the methyl groups. 1
NH
H1
H2
H3
H4
d (ppm) Multiplicity JHH (Hz) NH H1 H2 H3 H4
7.25 Broad
3.65 Doublet of triplets
1.54 Multiplet
1.37 Multiplet
0.94 Triplet
– 5.55 – – –
5.55 – 7.05 – –
– 7.05 – – –
– – – – 7.05
– – – 7.05 –
– –
C1 40.3
C2 31.7
C3 20.1
C4 13.8
H
4.6. Base hydrolysis of the framework bound reaction product 1
The H NMR of the H2O filtrate, presented in the Supplementary material, shows no evidence of products being extracted from NaX (3 BtIC/SC) after being treated with neutral water. This appears to be evidence that either the tightly bound chemical product or products of the exposure of the NaX to BtIC do not extract in water or that the hydrolysis product of those exposure products do not extract in water. The 1H and 13C NMR spectra of the CDCl3 extract in Figs. 7–9 show evidence of several products. With the help of the H, H and C, H COSY 2-dimensional spectra of the CDCl3 extract, given in the Supplementary material, an assignment was made from the correlation of the peaks with one another. There are three major products in the CDCl3 extract. The assignments of the spectra to n-butyl amine, N,N0 -dibutyl urea, and n-butyl carbamic acid anhydride are given in Tables 6–8, respectively. The structures of these compounds can be seen in Fig. 11 in Section 5. The appearance of additional NMR peaks near those of the C, H, and N positions of the n-butyl carbamic acid anhydride numbered 1 (at dC = 42.9, dH = 3.25) and 2 (at dC = 30.9, dH = 1.45), near the C peak at the carbamate functionality (dC = 156.2) and at the N atom (dH = 7.05) was taken as an indication of the presence of an anionic form in solution that froms by deprotonation at a N atom of the carbamic acid anhydride. The 13C NMR peaks in the 10– 45 ppm range all correspond to carbons in butyl groups. The 1H NMR of the CDCl3 extract shows the presence of butyl amine with peak assignments in agreement with an authentic sample. The presence of the product, N,N0 -dibutyl urea, was also confirmed by comparison with the 1H NMR spectrum of an authentic, synthesized sample. The product, identified as n-butyl carbamic acid
Table 6 n-Butyl amine 1H NMR chemical shifts and coupling constants. C atom and corresponding protons are numbered consecutively from the N atom to the methyl group. 1
NH
H1
H2
H3
H4
d (ppm) Multiplicity JHH (Hz) NH H1 H2 H3 H4
0.92 –
2.68 Triplet
1.51 Multiplet
1.38 Multiplet
0.90 –
– – – – –
– – 6.84 – –
– 6.84 – – –
– – – – –
– – – – –
H
Table 7 N,N0 -dibutyl urea 1H and 13C NMR chemical shifts and H,H coupling constants. C atom and corresponding protons are numbered consecutively from the N atoms to the methyl groups. 1
NH
H1
H2
H3
H4
d (ppm) Multiplicity JHH (Hz) NH H1 H2 H3 H4
4.16 Broad
3.15 Doublet of triplets
1.49 Multiplet
1.37 Multiplet
0.91 Triplet
– 5.99 – – –
5.99 – 7.05 – –
– 7.05 – – –
– – – – 7.27
– – – 7.27 –
13
– –
C1 40.4
C2 32.2
C3 19.9
C4 13.8
H
C d (ppm)
13
C d (ppm)
anhydride, has the chemical shifts that are appropriate to the assigned protons and carbon nuclei. The 13C NMR chemical shift in the carbamic acid anhydride product, dC = 158.1 ppm, is consistent with other carbamate compounds [19]. The slight shifting of the peaks in the ionic form provides some additional evidence for this assignment. 5. Discussion 5.1. Nucleophilic chemistry The solid-state 13C CP MAS NMR chemical shifts observed in isocyanate exposed zeolites at d 159 ± 5 ppm have been assigned to the respective framework carbamates. Weak peaks with chemical shifts in this region were seen in exposures of NaX to all types of isocyanate adsorbates studied. A chemical shift in this region has been observed in the literature for carbamate species [19]. In the reaction of NaX with TBIC, 2-methyl propene is formed, an indication of an elimination type of nucleophilic reaction taking place in addition to the formation of framework carbamate adduct. These results support the hypothesis that all of the isocyanates undergo a reaction with the zeolite to form a framework carbamate. The reaction for the formation of the framework carbamate adduct with the zeolite is shown in Fig. 10. The bonding would have to occur in the NaX supercage regions of the zeolite, because the sizes of the isocyanate molecules would not permit them to enter the sodalite cages. In previous work from this laboratory the supercage oxygen atoms have been implicated as the attacking nucleophile of the zeolite [7–9,20–22]. The supercage oxygen atoms adjacent to the aluminum atoms of the framework are available to perform the nucleophilic attack, because of the delocalized negative charge from the aluminum atoms and the lone pair of electrons that can attack a susceptible moiety. The electrons on the oxygen atom attack the isocyanate carbon, allowing electrons to move toward the electronegative nitrogen atom. This electronrich center is then free to attract the sodium cations in the zeolite, leading to the proposed, stable framework carbamate adduct. This is supported by the 23Na NMR evidence, showing the creation of a new Na environment in NaX after exposure to BtIC (3/sc), at the expense of the supercage site I and III0 Na ions. The relative size of the
Al Si
O
Na
Al
N
+
R
C
O
N R
Na
C
O
Si
O
Fig. 10. Proposed mechanism for the reaction of isocyanate with NaX.
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Al
Al Si
O
O
C
Si
Na
H2N
Bt
N
Si
C
Si
O Na
O
Na
N
Bt _
Al
_
NaOH
O
O
Bt
Bt
N H
Al
Bt N H
- CO2
O Na
O O
O OH
C
Na N
Bt
H2O -NaOH
C
OH
N H
Bt
OH
C
N H
Bt
O
O Bt
N H
O
N H
Bt
Fig. 11. Proposed mechanisms for the formation of n-butyl amine, N,N0 -dibutyl urea, and n-butyl carbamic acid anhydride.
isocyanate, within the range of sizes examined, appears to have a negligible effect on the adsorption into the zeolite, since no residual isocyanate peaks have been detected in the 13C CP MAS NMR of NaX exposed to any of the isocyanates, except the di-isocyanate. Further support for the formation of the proposed framework carbamate is provided by the base hydrolysis experiments performed on BtIC. The framework butyl carbamate is tightly bound and could not be extracted by solvents, including neutral water solvent. However, it underwent a nucleophilic substitution reaction with the stronger hydroxide ion nucleophile in an aqueous medium to form n-butyl amine, n-butyl carbamic acid anhydride, and N,N0 -dibutyl urea, which were observed in the CDCl3 extract. The proposed reaction mechanism for the formation of these species is presented in Fig. 11. Central to the proposed mechanism is base (OH) hydrolysis of the framework carbamate species to form the fleeting butyl carbamic acid intermediate[18,23,24,14]. The proposed dissociation of the n-butyl carbamic acid intermediates into n-butyl amine and CO2 is known to occur readily in aqueous media [25]. The formation of the n-butyl carbamic acid anhydride, also observed, can be rationalized as a bi-product of the framework carbamate and n-butyl carbamic acid in either of two ways. The dehydration of two n-butyl carbamic acid molecules may occur (shown). Alternatively, a nucleophilic substitution of the butyl carbamate framework species by n-butyl carbamic acid may occur (not shown). The reaction to form carbamic acid anhydrides from carbamic acids has not been reported in the literature, which suggests that the reaction is catalyzed by the zeolite framework. However, Naegelim et al. have proposed that the n-butyl carbamic acid
anhydride does exist as an intermediate in the formation of urea [26]. Similar results were found in the hydrolysis of EIC exposed NaX, as described in the Supplementary material. The zeolite has clearly been shown to sequester the isocyanate into a chemically new, framework bound species. The type of hydrolysis chemistry observed here shows that this sequestration product is susceptible to hydroxide nucleophilic substitution. The proposed framework carbamate ester species should exhibit this susceptibility, as it indeed does. 5.2. Tert-butyl isocyanate reaction with NaX The reaction of tert-butyl isocyanate appears to have competing nucleophilic addition and elimination reactions occurring. Based on the results from the 13C CP MAS NMR and GC–MS, it was ascertained that 2-methyl propene is formed along with two different framework carbamates. In Fig. 12 the proposed addition reaction is shown with the formation of a tert-butyl framework carbamate by the top route. The framework carbamate formation follows the same addition mechanism as other isocyanates (see Fig. 10). In parallel with addition, shown in the bottom route, the overall elimination reaction is proposed to form the observed olefin and the framework hydrogen carbamate that is consistent with NMR and FTIR observations. It is possible that the overall elimination reaction involves several steps to give the observed products, in line with the roles of the framework oxygen atom and the sodium ion proposed in previous work on the elimination reactions of alkyl halide exposed zeolites [9]. The first step would be the elimination reaction that
Al N Si
Al O Na
N
C
O
Na
+
O C O
H +
Si
Na N
O C
O Si
Fig. 12. Proposed mechanism for the reaction of TBIC with NaX.
Al
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removes a methyl group proton, leading to the observed olefin, with the assistance of a charged supercage oxygen, also leading to an acid zeolite site at the framework oxygen that removed the proton. The sodium cation assists the departure of the NCO group, leading to sodium isocyanate. Evidently the relative basicities of the NCO and the framework oxygen favor subsequent conjugate acid-base formation of the hydrogen isocyanate and the Na-zeolite site. Finally, the hydrogen isocyanate forms the adduct framework hydrogen carbamate. Thus the TBIC substitution and elimination reactions together would account for the three final products observed, namely the framework hydrogen carbamate, consistent with solid-state 13C NMR and FTIR spectra, the observed framework tert-butyl carbamate, and 2-methyl propene. No intermediate HNCO was extracted and observed in the 13C NMR or detected as an emitted gas by GC–MS. 5.3. Toluene 2,4 di-isocyanate reaction with NaX T24DI showed results that differed slightly from the chemistry of other isocyanates, evidently because it has two isocyanate functional groups. In the reaction of NaX with T24D it appears that only one of the isocyanate functional groups reacts with the framework. The other is left unreacted, as indicated by the IR results. The second isocyanate moiety is proposed to undergo hydrolysis with the addition of water. Only one of the original isocyanate groups was able to react with the NaX supercage, presumably because of the rigid structure of the cage and T24DI itself. It seems improbable for the molecule to contort itself so as to find a second active supercage oxygen atom that can add to the second isocyanate group. The second functional group is thus left available for chemistry, such as for hydrolysis to the corresponding amine with loss of CO2 [25]. The CP MAS 13C NMR results of T24DI exposed NaX, namely the peak at d 159 ppm, show that a framework carbamate is formed. The region near d 120 ppm, indicative of residual isocyanate, is masked by the chemical shifts of the aromatic carbons, and therefore it cannot be determined solely from the 13C NMR whether all of the isocyanate moieties have reacted. As an alternative approach, an FTIR study of the reaction was done to determine if all of the isocyanate moieties had reacted. The spectrum shows a peak at 2269 cm1 characteristic of an unchanged isocyanate moiety, which decreases progressively with the addition of water. These results taken together are consistent with one of the isocyanate moieties in T24DI being attacked by the nucleophile, while the other is left unchanged in the absence of water. It is not clear how the formation of the framework carbamate is protected from water hydrolysis and why the other isocyanate functional group is susceptible to water hydrolysis. The two functional groups of 16DIH were shown to behave in a similar manner (see Supplementary material).
an E2 type elimination reaction [9]. This tendency is evidently also being observed in the reactivity of NaX with the isocyanates. 6. Conclusions The isocyanate functional group is susceptible to nucleophilic attack by Faujasite NaX. A framework carbamate product, with the anionic nitrogen stabilized by the sodium cation, is proposed as a product common to the chemistry of all isocyanates studies. The product is very tightly bound to the zeolite. Two, but not all, observed cases suggest that where there are methyl protons b to the isocyanate moiety present, an elimination reaction may also form the a, b olefin, competing with the nucleophilic addition reaction. Based on the types of chemistry occurring and the tenacity with which the products are retained, zeolites have the promise of practical applications in hazardous materials cleanup and possibly in controlling isocyanate vapor release in the workplace and in the vicinity of chemical plants. Acknowledgements We acknowledge the support of a US Army/DTRA Project, Numbers W911NF-07-1-0042/AA05CBT019, a NATO Cooperative Linkage Grant No. 982991, and a Binghamton University, Innovative Technologies Center instrument user grant. We thank Professor Apostolos K. Rizos for a number of helpful suggestions in preparation of this paper. Appendix A A detailed description of the experimental procedures, which are substantially the same as in previously published studies, is relegated to the Supplementary material. The NMR data of compounds that give substantially the same results as the examples presented here in the paper, are collected in the Supplementary material. Some of the experiments on authentic samples of various compounds, performed to support the identification of reaction products and residual, unreacted isocyanates, are placed in the Supplementary material. In experiments designed to address some of the practical issues of the exposure of NaX to isocyanates, the headspace of zeolite exposed to isocyanates under various conditions of NaX exposure to isocyanate was examined, including experiments where the isocyanate adsorption and reaction was observed in zeolite previously exposed to various amounts of water. GC–MS headspace analyses are presented in the Supplementary materials over successively greater amounts of NaX zeolite exposed to a fixed amount of isocyanate. These experiments are also analyzed in the Supplementary material.
5.4. Dependence on connectivity of the isocyanate
Appendix A. Supplementary data
Primary (OIC), secondary (IPIC), and tertiary (TBIC) saturated isocyanates were investigated. The OIC and IPIC showed very similar results leading exclusively to the formation of the framework carbamate adduct, through the proposed nucleophilic addition mechanism in Fig. 10. The reaction between TBIC and NaX formed the elimination product, 2-methyl propene, as well as the nucleophilic addition product, framework tert-butyl carbamate. This shows that all isocyanates, regardless of connectivity, are susceptible to some type of nucleophilic attack by NaX, with both nucleophilic addition and elimination observed, not only for tertiary TBIC but also for primary EIC. In work done by Kanyi et al. it has been shown that NaX has a tendency to act as both a nucleophile, doing an SN2 type (substitution) reaction, or as a base, doing
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.micromeso.2010.10.027. References [1] O. Bayer, Agnew. Chem. 59 (1947) 257. [2] P. Shrivastava, Bhopal Anatomy of a Crisis, Paul Chapman, London, 1992. [3] EPA’s Hazardous Wastes A Consolidated List Inter, Face Associates, Middletown, CT, 1986. [4] H.J. Beim, J. Sperro, L. Sperro, Rapid Guide to Hazardous Air Pollutants, Van Nostrand Reinhold, New York, 1986. [5] K. Verschuenren, Handbook of Environmental Data on Organic Chemicals, 4th ed., Wiley-Interscience, New York, 2001. [6] C. De Gopan, J.D. Nauratil, J.A. Raper, J.M. Starling, J. Eniviron. Sci. Health A27 (1915) 17.
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