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Structural Elucidation of the Solid Byproduct from the Use of 1,3,5-Tris(hydroxyalkyl)hexahydro-s-triazine Based Hydrogen Sulfide Scavengers Grahame N. Taylor* and Ron Matherly B.J. SerVices Company, Tomball, Texas 77375, United States
Two forms of the solid byproduct from the use of hydroxyalkyl hexahydrotriazine as a hydrogen sulfide scavenger were investigated. The crystalline monomeric dithiazine and the intractable solid, known formerly as amorphous dithiazine. It was implied that the latter was simply another solid form of the same chemical species. The exact chemical structure and derivation of amorphous dithiazine were investigated in this study and the analytical data suggest that the material is polymeric in nature. Introduction It is very well established that the byproduct from the use of 5-(2-hydroxyethyl)hexahydrotriazine (I) is 5-(2-hydroxyethyl)dithiazine (II) (see Figure 1).1 In a previous publication we have described the chemical derivatization of starting material and byproduct in order to provide a complete analysis for the use of this very common hydrogen sulfide scavenger.2 There is however one aspect of the chemical process that has received little if any study. The practical outcome is well-known, and there have been attempts to mitigate the results of this phenomenon.3 When initially produced, II exists in the form of a lower, dense liquid layer in the gas tower which can under certain conditions actually crystallize to form a low melting, highly crystalline solid. However another form of this material is also very common and has been referred to as amorphous dithiazine.3 It is a fine, powdery, highly insoluble solid that represents a “so-called” different physical form of the same chemical species. We have examined this material in some detail and have also been able to synthesize it under laboratory conditions. Experimental Section A full field strength solution (30% by mass of active hexahydrotriazine), typical for the oilfield application of I, was spent with pure hydrogen sulfide in a glass gas tower. The mass of hydrogen sulfide absorbed by the fluid was measured by the mass increase of the gas tower. A considerable exotherm of between 20 and 30 °C was observed during the reaction to form the expected dithiazine. At the end of the experiment, a full theoretical mass of hydrogen sulfide had been absorbed, namely, 4 mol of hydrogen sulfide per mol of I. Initially II was seen to separate in the bottom of the gas tower as a lower colorless liquid, but within one hour a very heavy white solid deposited in the fluid of the gas tower and the lower layer solidified into a fine white powder. Attempts to arrest the solidification of the lower layer by separating and dissolution in methanol were not successful. It was deduced that this fine white solid was the laboratory analogue amorphous dithiazine observed in the field use of I. Several field locations where I is in use were known to deposit heavy quantities of amorphous dithiazine as a somewhat troublesome byproduct. It made unloading the gas tower at times difficult requiring physical line clearing and hot fluid treatments. On occasions samples of spent fluid were obtained that contained both liquid II and amorphous dithiazine solids. It was found
very convenient to separate these as follows. At elevated temperatures II existed in a liquid state and contained amorphous dithiazine floating through the bulk of the fluid. This heterogeneous fluid was filtered hot under reduced pressure. The pure liquid dithiazine, when collected in the filtration flask, was now free from amorphous dithiazine, and it would very often solidify and form large, high quality crystals of II. The oil field derived sample of amorphous dithiazine was washed with methanol and dried under vacuum to produce a fine off-white to gray freeflowing powder. The laboratory synthesized amorphous dithiazine (a-DTZ-l) and the purified field amorphous dithiazine (aDTZ-f) were examined by X-ray diffraction analysis, elemental analysis, IR spectroscopy and NMR spectroscopy. X-ray Diffraction. The X-ray diffraction patterns of a-DTZ-l and a-DTZ-f are shown in Figure 2, and that of c-DTZ is shown in Figure 3. It is clear that the two amorphous samples are very similar and yet completely different from the c-DTZ sample. Elemental Analysis. Samples of a-DTZ-f and a-DTZ-l were filtered, resuspended in water, filtered again and washed with methanol. They were then dried to constant mass in a vacuum oven. The empirical formulas for a-DTZ-f and a-DTZ’-l from the elemental analysis are shown in Table 1 together with the value for c-DTZ. If a-DTZ-l and a-DTZ-f were indeed different physical forms of the same chemical species (II) they would be expected to have the same elemental analysis. Clearly they do not. Both samples of amorphous dithiazine have very significantly more sulfur than would be expected for II. Further it was noted that the laboratory generated sample, a-DTZ-l, was a more extreme departure from the theoretical c-DTZ than the field sample a-DTZ-f. This discovery has led the authors to believe that previous reports where this material is referred to
Figure 1. 1,3,5-Tris(hydroxyethyl)hexahydro-s-triazine and 5-(2-hydroxyethyl)dithiazine.
10.1021/ie101985v 2011 American Chemical Society Published on Web 12/15/2010
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Figure 2. X-ray diffraction for a-DTZ-l and a-DTZ-f.
Figure 3. X-ray diffraction for c-DTZ-l.
as amorphous dithiazine are not correct and that it is another chemical species altogether.3 One possibility considered was that a-DTZ-f and a-DTZ-l are, in fact, clathrate crystal structures of the monomeric dithiazine and hydrogen sulfide. It is known that clathrates have very different physical properties from the parent compound.6 This possibility was, however, discounted since the predicted elemental analyses for sequentially more hydrogen sulfide molecules per molecule of monomeric dithiazine were calculated as shown in Table 2.
Table 1. Empirical Formulas Calculated from Elemental Analysis for c-DTZ, a-DTZ-l and a-DTZ-f element
theoretical c-DTZ (II)
a-DTZ-l
difference from c-DTZ
a-DTZ-f
difference from c-DTZ
carbon hydrogen nitrogen sulfur oxygena
5.0 11.0 1.0 2.0 1.0
9.8 21.5 1.0 7.8 1.0
4.8 10.5 0.0 5.8 0.0
7.2 15.1 1.0 4.6 1.2
2.2 4.1 0.0 2.6 0.2
a
Oxygen was actively determined, not calculated by difference.
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Table 2. Theoretical Clathrate Calculated Elemental Analysis no. of H2S
C
H
N
O
S
MW
%C
%H
%N
%O
%S
0 1 2 3 4 5 a-DTZ-f
5 5 5 5 5 5
11 13 15 17 19 21
1 1 1 1 1 1
1 1 1 1 1 1
2 3 4 5 6 7
165 199 233 267 301 335
36.4 30.2 25.8 22.5 19.9 17.9 29.1
6.7 6.5 6.4 6.4 6.3 6.3 5.3
8.5 7.0 6.0 5.2 4.7 4.2 3.5
9.7 8.0 6.9 6.0 5.3 4.8 6.7
38.8 48.2 54.9 59.9 63.8 66.9 61.6
NMR Analysis. The extreme insolubility of the a-DTZ-f and a-DTZ-l made the NMR analysis quite challenging. It was found possible to dissolve sufficient material in deuterated dimethyl sulfoxide by heating the material to 80 °C for approximately one hour followed by filtration of the undissolved material. By this means the proton NMR spectra shown in Figures 4 and 5 were obtained. These were compared with c-DTZ in the same solvent for reference purposes. The proton NMR of 5-(2-hydroxethyl)dithianze has been reported previously, and the proton assignments are well established.1 The proton NMR of both a-DTZ-l and a-DTZ-f clearly show residual resonances that match well the c-DTZ spectra but also additional multiple resonances in the region 3.8 to 4.4 ppm (methylene adjacent to sulfur) and 3.2 to 3.7 ppm (methylene adjacent to oxygen) when compared with c-DTZ.1 This is interpreted as good evidence for many additional CH2-S units of various magnetically nonequivalent environments in the proposed structure over and above the simple monomeric dithiazine. It was deduced that II was involved in further reaction to produce a-DTZ thus increasing the carbon, hydrogen and sulfur content. Since the increase in carbon and hydrogen was in the ratio of 1:2, it was thought
Figure 4. Proton NMR of a-DTZ-f, a-DTZ-l and c-DTZ in d6-DMSO.
that this indicated methylene groups had been added, a deduction supported by the NMR analysis discussed above. Since a very substantial increase in sulfur was seen, it was thought some type of polymeric structure with multiple methylene and sulfur linkages was inferred. Structure Activity Study. In order to understand which chemical reaction was taking place to produce the deduced polymeric structure of a-DTZ-l and a-DTZ-f, other hydroxyalkyl hexahydrotriazines, tris-1,3,5-(3-hydroxypropyl)hexahydrotriazine (III), tris-1,3,5-(2-hydroxypropyl)hexahydrotriazine (IV) and tris-1,3,5-(1-ethyl-2-hydroxyethyl)hexahydrotriazine (V) (see Figure 6) were synthesized from the appropriate hydroxyalkylamine by well established procedures.5 In every case when spent with pure hydrogen sulfide, as described above, an analogous heavy white solid resulted after approximately one hour following complete consumption of the hexahydrotriazine. If the same complete spend process is undertaken for 1,3,5-trimethylhexhydrotriazine (VI) after complete consumption a heavy crystalline deposit of 5-methyldithiazine is observed with no analogous formation of a high sulfur containing amorphous material.
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Figure 5. Expanded proton NMR of a-DTZ-f and a-DTZ-l in d6-DMSO.
Figure 7. 1,3,5-Tris(methoxypropyl)hexahydro-s-triazine and 5-(methoxypropyl)dithiazine.
Figure 6. 1,3,5-Tris(hydroxyalkyl)hexahydro-s-triazines.
As further proof for necessity of a terminal hydroxyl functionality the hexahydrotriazine from 3-methoxypropylamine (VII) (see Figure 7) was synthesized and completely spent as described above. In this case a two liquid phase system was produced with no white solid production. The lower layer was quite stable and did not solidify upon standing. It was analyzed by GCMS and found to contain exclusively 5-(3-methoxypropyl)dithiazine (VII) with a molecular ion at M+ 193 as shown in Figure S1 in the Supporting Information. Clearly the terminal hydroxyl functionality is involved in this reaction to yield the high sulfur amorphous by product.
A GCMS study was undertaken to follow the formation of the a-DTZ-l with time. During this experiment, as solid deposition was observed, the concentration of II in the supernatant significantly decreased, indicating that II was actually consumed in the chemical process that was taking place to yield a-DTZ-l. IR Spectroscopy. Although the samples of amorphous dithiazine, so-called, were not significantly soluble in any solvent investigated, it was possible to obtain a partial infrared absorption spectrum using a diamond ATR (attenuated total reflectance) cell. The spectra of a-DTZ-l, a-DTZ-f and c-DTZ are presented in Figures S2, S3 and S4 respectively in the Supporting Information. a-DTZ-l is found, not surprisingly, to be a cleaner spectrum than a-DTZ-f, however the most obvious conclusion from
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Figure 8. Proposed polymeric structures for a-DTZ-l and a-DTZ-f.
Figure 9. Proposed mechanism for polymer XII propagation.
these spectra is that a-DTZ-l and a-DTZ-f are similar but vastly different from c-DTZ. This confirms the conclusion that these two materials are not the same chemical species. The strong absorbance at the very far end of the observable region (700 cm-1) is assigned to the carbon sulfur stretch in all cases.4 Conclusion The evidence presented in this paper is consistent with the following structures for both a-DTZ-l and a DTZ-f. The structures shown, IX and X (see Figure 8), represent repeating units and are somewhat idealized versions of the true polymeric structure. The molecular structures shown depict the average content of the side chains which have a certain degree of variability within the bridging portion. This type of structure can best be explained by the transient generation of the highly reactive species, thioformaldehyde, which rapidly reacts with the hydroxyl terminus of the dithiazine and builds a linking chain. Sulfur insertion must occur at some point to generate the polysulfide linkages. At some point the terminus reacts with another dithiazine molecule via nucleophilic substitution and ring-opening via protonation of the nitrogen (XI and XIII; see Figure 9). Once
the second dithiazine molecule has been incorporated into the growing chain, further thioformaldehyde molecules are added and the process is repeated (XII and XIV) (see Figure 10). This type of polymerization process must be invoked to explain the nitrogen and oxygen content of the elemental analysis. Polymeric structures of this type were postulated for laboratory generated amorphous dithiazine but no analytical data were cited.7 Further evidence for the involvement of thioformaldehyde is presented as follows. The analysis of the byproduct fluid from hydrogen sulfide reacting with I shows II as the major byproduct, but there are other multiple sulfur containing species as significant minor byproducts, most notably 1,2,4trithiolane (M+ ) 124) and to a lesser extent 1,2,3,5tetrathiepane (M+ ) 156), identified by GCMS analysis of spent scavenger fluids and shown in Figures S5 and S6 in the Supporting Information. These species are thought to be derived from thioformaldehyde cyclization, presumably coupled with sulfur insertion from hydrogen sulfide to create the polysulfide linkages. While the exact mechanism whereby thioformaldehyde is generated is open for speculation, one possibility is that, as the fluid is spent with hydrogen sulfide and the pH drops, an
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to thioformaldehyde by hydrogen sulfide. The further reaction of thioformaldehyde with the hydroxyl terminus of II is the only explanation of the entire body of analytical data presented. Supporting Information Available: Mass spectra of VIII, 1,2,4-trithiolane, and 1,2,3,5-tetrathiepane and IR spectra of a-DTZ-l, a-DTZ-f, and c-DTZ. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) 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. (2) Taylor, G. N.; Matherly, R. Gas Chromatographic-Mass Spectrometric Analysis of Chemically Derivatized Hexahydrotriazine-based Hydrogen Sulfide Scavengers: Part II. Ind. Eng. Chem. Res. 2010, 49, 6267– 6269. (3) Owens, T. R. Formulation For Hydrogen Sulfide Scavenging From Hydrocarbon Streams And Use Thereof. World Patent WO 2008049188 20080502, 2008. (4) Schrieber, K. C. Anal. Chem. 1949, 21 (10), 1168. (5) Dillon, E. T. Composition and method for sweetening hydrocarbons. US Patent 4,978,512, Dec 18, 1990. (6) Ukegawa, H.; Matsuo, T.; Suga, H.; Leadbetter, A. J.; Ward, R. C.; Clark, J. W. Static and dynamic properties of hydrogen sulphide in hydroquinone clathrates. Can. J. Chem. 1988, 66, 743. (7) Titley, C. W.; Wieninger, P. H. Method and Composition for Removing Sulfides from Hydrocarbon Streams. U.S. Patent 7,115,215. Figure 10. Proposed mechanism for polymer XIV propagation.
increasing degree of I hydrolysis back to formaldehyde and monoethanolamine occurs.1 As soon as formaldehyde is formed, it would compete with I and instantly be converted
ReceiVed for reView September 30, 2010 ReVised manuscript receiVed November 24, 2010 Accepted December 1, 2010 IE101985V
