Ind. Eng. Chem. Res. 2010, 49, 6267–6269
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Gas Chromatographic-Mass Spectrometric Analysis of Chemically Derivatized Hexahydrotriazine-based Hydrogen Sulfide Scavengers: Part II Grahame N. Taylor* and Ron Matherly B.J. SerVices Company, Chemical SerVices Department, 11211 FM 2920, Tomball, Texas 77375
A gas chromatography-mass spectrometry method of assaying various thiadiazines and dithiazines in laboratory and field fluids is described. This method also involves the tris-trifluoroacetylation of anhydrous 5-(hydroxyethyl)dithiazine, which unexpectedly yields a novel derivative. Introduction The gas chromatographic-mass spectrometric analysis of 1,3,5-tris(2-hydroxyethyl)hexahydro-s-triazine (I) (see Figure 1) was described in a previous publication.1 The analysis of the hydrogen sulfide hexahydrotriazine byproduct is of great importance to understanding the chemistry of the scavenging process. Handling the byproduct from oil field application of I is a major concern and has been the subject of considerable study.2 Spent-I-containing fluids, which are produced in volumes up to 5000 gal/week at some locations, must be expelled from a gas tower and discarded in an appropriate manner or eliminated from a gas stream in an atomizer-type spray head application. An elegant NMR-based study has been described by Bakke et al. wherein the byproducts were identified by “in vitro” NMR experiments using I and sodium sulfide as a sulfur source.3 The work described in the current study derives the byproduct from both laboratory gas tower studies using I with hydrogen sulfide at 3000 ppm in a carrier gas. It also details the analysis of actual spent field fluids using I, the less common 1,3,5-trimethylhexahydro-s-triazine (II), and the oil-soluble 1,3,5-tri-t-butylhexahydro-s-triazine (III). The byproducts were fully characterized and identified by mass spectrometry. Experimental Section Reagents. All fine-chemical reagents, namely, monoethanolamine, t-butylamine, formaldehyde, trifluoroacetic anhydride, acetic anhydride, dichloromethane, and toluene, were obtained from Aldrich (Milwaukee, WI). I was used as a commercial source from Stepan Company (Northfield, IL), Hexion Specialty Chemicals (Columbus, OH), and Conlen Surfactant Technology (Conroe, TX) or was synthesized from laboratory-reagent-grade chemicals. II was also both used as a commercial source and also synthesized from reagent-grade chemicals, supplied by Aldrich. Gas Chromatography-Mass Spectrometry. Materials were analyzed using an Agilent Technologies (Palo Alto, CA) 7890A gas chromatography system with Agilent Technologies 5975C inert XL EI/CI MSD detector. The gas chromatography column used was an Agilent 190915-433 column, 30 m × 0.25 mm, 0.25-µm film thickness. 200 °C injection block, 1 mL/min carrier gas flow rate. The oven temperature profile was as follows: start at 40 °C, hold for 2 min, ramp to 320 °C at 5 °C/min. * To whom correspondence should be addressed. Tel.: 281-351-3416. E-mail:
[email protected].
Conditions for Derivatization and Gas Chromatography-Mass Spectrometry Analysis. Trifluoroacetylation, when required, was the chosen method of hydroxyl group derivatization as previously described.1 Results and Discussion 1,3,5-Tris(2-hydroxyethyl)hexahydro-s-triazine (I). Partially spent scavenger fluid from a laboratory gas tower study containing I was continuously extracted with dichloromethane for 8 h. The organic layer was evaporated to dryness and redissolved in approximately 3 mL of dichloromethane. This material was analyzed directly by gas chromatography-mass spectrometry using electron-impact mode. The chromatogram is shown in Figure S1 of the Supporting Information. Components A and B are oxazolidine and I, respectively, from unreacted starting material.1 Component C is assigned to 5-hydroxyethyldithiazine (M+ ) 165) (V) and its mass spectrum is shown in Figure S2 of the Supporting Information. It is very interesting to note that thiadiazine IV was never observed in the scavenging process. This is attributed to the fact that, in the reaction scheme shown in Figure 2, IV is highly reactive toward hydrogen sulfide and, as soon as it is formed, it accepts a second hydrogen sulfide molecule to form V. It is postulated that IV has a greatly enhanced reactivity with hydrogen sulfide because of the neighboring group participation of the adjacent hydroxyethyl group and also a favorable steric positioning of this group. Even though it is theoretically possible for V to react further with hydrogen sulfide to yield 1,3,5-trithiane (VI), such a
Figure 1
10.1021/ie1001247 2010 American Chemical Society Published on Web 06/21/2010
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Figure 2. Reaction scheme for I, II, and III with hydrogen sulfide.
Figure 3. Reaction of VI with trifluoroacetic anhydride.
reaction was never observed, which is attributed to the resistance of V to reaction with hydrogen sulfide. Although V gives excellent chromatographic and mass spectrometric data without derivatization, it was important to investigate the trifluoroacetylation of V because the full analysis of partially spent fluids will contain both I and V. It has been previously established that the successful gas chromatographic analysis of I requires trifluoroacteylation.1 The result of the
reaction of I and V with trifluoroacetic anhydride was very unexpected and yielded a hitherto undescribed molecular species. The reaction sequence is detailed in Figure 3. The gas chromatographic-mass spectral analysis of the derivatized dithiazine alone in shown in Figure S3 of the Supporting Information, and component A is assigned as S,S′trifluoroacteylmethanedithiol (VII, M+ ) 272). The relatively short retention time and mass spectrum of component A shown
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in Figure S4 of the Supporting Information are entirely consistent with this structural assignment. Component B is fully derivatized unreacted I present in the sample. The remaining fragment (VIII) of this reaction was not observed, and it is thought that this species does not survive the thermal impact of gas chromatographic injection block but further reacts to unidentified byproduct. Because VII is produced in a stoicheometric ratio with respect to V, it can be used as a quantitative tool in the desired assay. Only a trace of the originally expected trifluoracetyl dithiazine IX (M+ ) 165) is observed in the chromatogram at a retention time of 23.15 min. 1,3,5-Trimethylhexahydro-s-triazine (II). II was partially spent with hydrogen sulfide in a field-derived fluid. The aqueous spent fluid was continuously extracted with dichloromethane and concentrated as previously described. The product was analyzed by gas chromatography-mass spectrometry, and the results are shown in Figure S5 of the Supporting Information. The mass spectra of components A-C are shown in Figures S6-S8, respectively, of the Supporting Information. These data are entirely consistent with component A being unreacted 1,3,5trimethylhexahydro-s-triazine (II), component B being dimethylthiadiazine (X), and component C being methyldithianze (XI). Unlike the case of I, the thiadiazine is present in partially spent II and is readily observed in the analysis. Clearly, X is less reactive with hydrogen sulfide than V and has a finite lifetime in the system. Because methyltriazine is also easily analyzed by gas chromatography-mass spectrometry, there is no requirement for derivatization. 1,3,5-Tri-t-butylhexahydro-s-triazine (III). The use of the oil-soluble 1,3,5-tri-t-butylhexahydro-s-triazine (III) has been described previously.4 III was dissolved in isooctane and partially spent with hydrogen sulfide. In this case, the organic solvent was simply removed by evaporation, and the residue was dissolved in dichloromethane and analyzed directly. The chromatogram of this material is shown in Figure S9 of the Supporting Information, wherein component A is the thermally cracked monomer of III. It has a molecular ion at M+ ) 85 and a short retention time compared with those of other hexahydrotriazines. It is considered to be the formaldimine of t-butylamine, and its mass spectrum is shown in Figure S10 of the Supporting Information. Although the formation of the cyclic trimer triazine III is well-established by other means, it is
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believed that this molecule reverts back to its monomeric form in the injection block of the gas chromatograph. 5-t-Butyldithiazine (XIII) is assigned to be component B in Figure S11 of the Supporting Information, with its mass spectrum shown in Figure S12 of the Supporting Information (M+ ) 177). 3,5Di-t-butylthiadiazine (XII) is once again not observed. Conclusions Spent fluid samples of both I and II have, on occasion, shown a tendency to crystallize the appropriate dithiazine in quite spectacular fashion. The separation of V is well-documented, and under certain conditions, it can result in the formation of the solid, highly insoluble amorphous form rather than a crystalline material.2 This has led to numerous engineering problems with flushing gas towers, and various remedies have been suggested to circumvent this problem. The characterization of hexahydrotriazine byproduct by gas chromatography offers an easy method of determining the degree to which a fluid is spent, thus enabling a more effective use of the scavenger chemical by customers and service companies. Supporting Information Available: Mass spectra and total ion chromatograms of species discussed in the text. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Taylor, G. N.; Matherly, R. Gas Chromatography Mass Spectrometric Analysis of Chemically Derivatized Hexahydrotriazine-Based Hydrogen Sulfide Scavengers: 1. Ind. Eng. Chem. Res; published online May 27, 2010, http://dx.doi.org/10.1021/ie100047b. (2) Owens, T. R. Formulation for Hydrogen Sulphide Scavenging from Hydrocarbon Streams and Use Thereof. World Patent WO 2008049188 20080502, 2008. (3) Bakke, J. M.; Buhaug, J.; Riha, J. Hydrolysis of 1,3,5-Tris(2hydroxyethyl)hexahydro-s-triazine and Its Reaction with Hydrogen Sulfide. Ind. Eng. Chem. Res. 2001, 40, 6051–6054. (4) Sullivan, D. S., III; Thomas, A. R.; Edwards, M. A.; Taylor, G. N.; Yon-Hin, P.; Garcia, J. M., III. Method of treating sour gas and liquid hydrocarbon. U.S. Patent 5,674,377, Oct 7, 1997.
ReceiVed for reView January 19, 2010 ReVised manuscript receiVed May 12, 2010 Accepted June 9, 2010 IE1001247
