Ind. Eng. Chem. Res. 2010, 49, 5977–5980
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Gas Chromatography Mass Spectrometric Analysis of Chemically Derivatized Hexahydrotriazine-Based Hydrogen Sulfide Scavengers: 1 Grahame N. Taylor* and Ron Matherly B. J. SerVices Company, Tomball, Texas 77375
A gas chromatography-mass spectrometry method of assaying 1,3,5-tris(2-hydroxyethyl)hexahydro-s-triazine in laboratory and field fluids is presented. This method involves the tris-trifluoroacetylation of anhydrous 1,3,5-tris(2-hydroxyethyl)hexahydro-s-triazine to avoid the undesired thermolysis to oxazolidine. Introduction Hydrogen sulfide occurs in oil-field-produced gases at levels sometimes in excess of 3000 ppm. This highly toxic component must be removed from the gas stream, prior to end use, using hydrogen sulfide scavengers.1 Hexahydrotriazines, which sometimes are incorrectly called simply “triazines”, have been used as hydrogen sulfide scavengers for a considerable time in natural gas production. There is one very clear favorite, 1,3,5-tris(2hydroxyethyl)hexahydro-s-triazine (I), with 1,3,5-trimethylhexahydro-s-triazine (II) being a somewhat distant second.2 There are several reasons why I is a favorite: it is inexpensive to synthesize from readily available raw materials (monoethanolamine and formaldehyde); it is very effective as a hydrogen sulfide scavenger; and the byproduct is readily water-soluble, except at very high concentrations, where separations can occur. Determining when to replace a scavenger fluid is both very necessary and quite challenging. The ultimate effect of spending the fluid is to observe hydrogen sulfide breakthrough. The desire to have a reliable and accurate method to determine the degree to which a fluid is spentsand, hence, when hydrogen sulfide breakthrough is expectedsinitiated this current study. Pretty et al., in their elegant work using electrospray-mass spectrometry, have highlighted the difficulty of direct chromatographic analysis for II and reported that their attempts were unsuccessful, because of thermal instability.3 It was our intention to revisit the possibility of a direct gas chromatography method for use in developing a process that was within the reach of more conventional and readily available equipment. Gas chromatography analysis of II is very straightforward, and an example of this is shown by the partially spent gas tower fluid described by the chromatogram shown in Figure 1. Unreacted II is clearly visible at 9.4 min (peak A), and the two byproducts can be seen at 12.9 min (peak B) and 15.6 min (peak C). Analysis of the byproducts will be discussed in a future publication. A mass spectrum of II is shown in Figure S1 in the Supporting Information and matched a reference spectrum with a confidence level of >90%. Although, at first, it may seem relatively trivial, the gas chromatography mass spectrometric analysis of tris-trifluoroacetylated hexahydrotriazine (III); is less facile than it may seem. Under the conditions that are commonly used, even with an injection block temperature of only 200 °C, which is a practical limit in many cases, the molecule can be observed in the chromatogram but the majority of the material undergoes what is believed to be a very facile thermal rearrangement to * To whom correspondence should be addressed. Tel.: 281-351-3416. E-mail:
[email protected].
oxazolidine (see Figure 2). The hexahydrotriazine is generally synthesized in aqueous solution for the sake of convenience, and the resulting solution is usually ∼65%-70% active. It is certainly true that the thermolysis can be reduced somewhat if the water is removed; however, it is never possible, in our experience, to eliminate this undesired reaction, as shown in the chromatogram and mass spectra of the commercial material. (See Figure S2 in the Supporting Information.) The components were assigned as oxazolidine and II (see Figures S3 and S4, respectively, in the Supporting Information), which both matched a reference spectrum with a confidence level of >90%. With this problem in mind, an investigation was undertaken to
Figure 1. Total ion current chromatogram for partially spent 1,3,5trimethylhexahydro-s-triazine (II) field sample.
Figure 2. Reaction scheme for the thermolysis of 1,3,5-tris(2-hydroxyethyl)hexahydro-s-triazine (I) to oxazolidine.
10.1021/ie100047b 2010 American Chemical Society Published on Web 05/27/2010
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Figure 3. Trifluoroacetylation of 1,3,5-tris(2-hydroxyethyl)hexahydro-s-triazine (I), ethanolamine, and oxazolidine.
find a suitable method to derivatize and analyze this hexahydrotriazine via gas chromatography-mass spectrometry. Experimental Section Reagents. All fine chemical reagentsschlorotrimethylsilane, BSTFA, trifluoroacetic anhydride (TFAA), acetic anhydride (AA), dichloromethane (DCM), and tolueneswere obtained from Aldrich. I was used as a commercial source from Stepan, Hexion, and Conlen or was synthesized from laboratory reagentgrade chemicals. II was also used as both commercial grade and also synthesized from reagent-grade chemicals that were supplied by Aldrich. Gas Chromatography-Mass Spectrometry. Materials were analyzed using an Agilent Technologies Model 7890A GC system with an Agilent Technologies Model 5975C inert XL EI/CI MSD detector. Process conditions: Model HP5-MS gas chromatography column, manufactured by Agilent Technologies, 30 m × 0.25 mm; film thickness, 0.25 µm; injection volume, 0.5 µL; injection block, 200 °C; helium carrier gas flow rate, 1 mL/min; oven temperature profile, 40 °C, hold for 2 min, ramp to 320 at a rate of 5 °C/min. Conditions for Derivatization and GCMS Analysis. Since the decomposition of II is initiated by an internal nucleophilic attack of the hydroxyl group, it was decided that protection and O-derivatization of this functionailty was the correct strategy. Trimethylsilylation has been employed with great success in the gas chromatography of sugars and was the first method employed. Derivatization with chlorotrimethylsilane and BSTFA were both attempted. The criterion for success was a single derivative peak; however, this was not observed in either case. Mixed products were obtained with no clear indication of the trimethylsilated derivative of II. Acetylation was the next option, using either standard acetylation with AA or trifluoroacetylation with TFAA. Since II is known to be thermally labile, TFAA was the derivatization agent of choice, because it is significantly more reactive than AA. In fact, trifluoroacetylation of II with TFAA required only mild heating and occurred readily with a
quantitative yield as described below. Acetylation of II with AA was attempted and was partially successful; however, we did not obtain the same clean gas chromatogram and peak shape quality as that observed with TFAA. For this reason, we turned our attention to TFAA as the derivatization agent of choice. The reaction of II with TFAA required rigorous dehydration of the material prior to reaction. As stated earlier, II is synthesized in aqueous solution and, therefore, the dehydration required is substantial; there is ∼40% water present. Attempts to remove water under vacuum were not successful; since the molecule has three hydroxyl groups, it is very difficult to fully remove the water, because of hydrogen bonding effects. The decision was made to use azeotropic distillation with an aromatic solvent such as toluene for this important step in the derivatization. Typically, 10 g of ∼60% triazine was added to 60 g of toluene and the mixture refluxed under a Dean-Stark condenser for 4-6 h. Initially, the majority of water was easily removed but the distillation was continued to ensure complete dehydration. Anhydrous II was not soluble in hot toluene and remained as oil in the bottom of the flask. Upon cooling, 60 g of DCM was added to this suspension and this yielded complete solution of the dehydrated hexahydrotriazine. An aliquot of this solution (1 mL) was treated in a pressure-sealed Reacti-Vial with a molar excess of TFAA (300 µL). The vial contents were heated under pressure to 80 °C for 30 min, cooled, and analyzed by gas chromatography-mass spectrometry. Great care was taken to ensure that no volatile material was lost under these conditions. Results and Discussion Electron Impact Mass Spectrometry. The reaction sequence is shown in Figure 3, and the chromatogram for trifluoroacetylated hexahydrotriazine is shown in Figure S5 in the Supporting Information. There is one major peak at 11.7 min, which is assigned to III; the associated mass spectrum is shown in Figures S6 and S7 in the Supporting Information. The fragmentation pattern of tris-trifluoroacetylated triazine is believed to be as indicated in Figure 4, and the structure of the base
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Figure 4. Fragmentation of trifluoroacetylated 1,3,5-tris(2-hydroxyethyl)hexahydro-s-triazine (III).
peak at 266 amu is assigned to the 5-trifluoroacetoxyethyl-1,3,5triazabicyclonon-8-ene cation, as shown in Figure 5. Earlier attempts at the tris-trifluoroacetylation of I gave incomplete reaction and yielded some of the monotrifluoroacetyl
Figure 5. Derivation of the 266 amu base peak in mass spectrum of III.
Figure 6. Derivation of oxazolidine during the synthesis of I.
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derivative and bis-trifluoroacetyl derivative at 12.05 and 14.0 min, respectively. The mass spectra of monotrifluoroacetylated hexahydrotriazine and bis-trifluoroacetylated hexahydrotriazine both showed a very strong 266 amu ion, which is undoubtedly the same as that previously described. There are two minor components, at 8.1 and 9.2 min. The 9.2 min component is assigned to N-trifluoroacetyl-O-trifluoroacetyl-ethanolamine (IV, M+ ) 253, first major fragment at 184, loss of 69, CF3) and is shown in Figure S8 in the Supporting Information. The 8.1 min component is assigned to N-trifluoroacetyl-oxazolidine (V, M+ ) 169) and is shown in Figure S9 in the Supporting Information. The presence of monoethanolamine and, hence, its trifluoroacetylated derivative in this chromatogram is not unexpected, since the synthesis of hexahydrotriazine is usually conducted with an excess of this reagent, to ensure no residual formaldehyde. It was somewhat unexpected to see oxazolidine in this dehydrated derivatized sample. Clearly, the oxazolidine must have either been originally present in the sample or possibly produced during the azeotropic dehydration of the sample. To investigate this further, the dehydration was carried out both for longer time periods and also in xylene (in place of toluene),
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to increase the refluxing temperature by 30 °C. In all cases, the same quantity of trifluoroacetylated oxazolidine was observed, and this is thought to be good evidence that it was present in the original commercial material. The reaction sequence shown in Figure 6 seems quite plausible for the source of oxazolidine, alongside the major reaction to produce the triazine. Chemical Ionization Mass Spectrometry. In an attempt to produce a molecular ion for tris-trifluoroacetylated hexahydrotriazine, the material was also analyzed by methane chemical ionization mass spectrometry in both positive- and negative-ion mode. The positive-ion mode spectra of III, IV, and V are shown in Figures S10, S11, and S12, respectively, in the Supporting Information. The spectrum of the hexahydrotriazine derivative shows a very strong 266 peak as before. The spectrum of the monoethanolamine derivative has a strong molecular ion at 254 assigned to [M+H]+ The spectrum of the oxazolidine derivative has a strong molecular ion at 170 assigned to [M+H]+. The negative-ion mode spectrum of III is shown in Figure S13 in the Supporting Information and contains two major ions: 113, which is the trifluoroacetate anion and 492, which the 379 ion from the electron impact spectrum plus 113 or trifluoroacetate. Once again, the largest fragment seen is 379. Since the two minor components give molecular ions very readily in electron impact and positive chemical ionization modes, their negative ion mode spectra are not detailed here. Scavenger Field Fluid Evaluation. Until now, in the opinion of the authors, there has not been a satisfactory method to determine the degree to which an oil field triazine scavenger solution is spent. One application of this method is to observe the presence of unreacted hexahydrotriazine in partially spent natural gas scrubber tower field samples. To perform this analysis, an aliquot of gas tower fluid was subjected to the same azeotropic dehydration as detailed above. The result of the trifluoroacetylation of this sample is shown in Figure S14 in the Supporting Information. The unreacted hexahydrotriazine is clearly visible in this chromatogram at 11.7 min, along with the byproduct of hydrogen sulfide scavenging. The latter will be a subject of a future publication. The quantitative assay of unspent triazine can be accomplished using standard solutions of derivatized, unreacted triazine, since the undesired thermolysis reaction described above can now be avoided. Conclusions A method of determining 1,3,5-tris(2-hydroxyethyl)hexahydro-s-triazine (I), using trifluoroacetylation, has been described.
It avoids the undesirable thermolysis of the parent molecule. Through the rigorous dehydration of I, this process has enabled the assay of partially spent hexahydrotriazine-based scavenger fluids to directly determine the degree to which the fluid has been spent and thereby estimate its expected future lifetime prior to complete exhaustion. This is highly desirable, since it will eliminate unintended gas breakthrough and the discharge of hydrogen sulfide into the commercial gas stream. It will also allow the most-efficient use of the scavenger and optimize the economics of the process for our customers. Supporting Information Available: Mass spectrum of 1,3,5-trimethylhexahydro-s-triazine (II) (Figure S1); total ion chromatogram of commercial 1,3,5-tris(2-hydroxyethyl)hexahydro-s-triazine (I) in acetonitrile (Figure S2); mass spectra of oxazolidine (Figure S3) and I (and reference) (Figure S4); total mass spectra of trifluoroacetylated 1,3,5tris(2-hydroxyethyl)hexahydro-s-triazine (III) (Figures S5 and S6); high-end mass spectrum of III (Figure S7); mass spectra of N-trifluoroacetyl-O-trifluoroacetyl-ethanolamine (IV) (Figure S8) and N-trifluoroacetyl-oxazolidine (V) (Figure S9); positive chemical ionization mass spectra of III (Figure S10), IV (Figure S11), and V (Figure S12); negative chemical ionization mass spectrum of III (Figure S13); and total ion chromatogram of field scavenger fluid derivatized extract (Figure S14). (PDF) This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Nagl, G. J. Removing Hydrogen Sulfide. Hydrocarbon Eng. 2001, 6 (2), 35. (2) Bakke, J. M.; Buhaug, J.; Riha, J. Hydrolysis of 1,3,5-Tris(2hydroxyethyl)hexahydro-s-triazine and Its Recation with Hydrogen Sulfide. Ind. Eng. Chem. Res. 2001, 40, 6051–6054. (3) Pretty, J.; Glaser, R.; Jones, J.; Lunsford, R. A Technique for the Identification and Direct Analysis of Hexahydro-1,3,5-tris(2-hydroxyethyl)s-triazine. Analyst 2004, 129, 1150–1155.
ReceiVed for reView January 8, 2010 ReVised manuscript receiVed March 19, 2010 Accepted April 16, 2010 IE100047B
