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Nitrogen-containing aromatic compounds: quantitative analysis using gas chromatography with nitrogen phosphorus detector

Nitrogen-containing aromatic compounds: quantitative analysis using gas chromatography with... The nitrogen-containing aromatic compounds found in the petrochemical industry are varied and extend beyond classes such as the anilines, pyrroles and pyridines. Quantification of these nitrogen-containing compounds that may occur in complex mixtures has practical application for quality assurance, process development and the evaluation of conversion processes. Selective detection of nitrogen-containing species in complex mixtures is possible by making use of gas chromatography coupled with a nitrogen phosphorous detector (GC-NPD), which is also called a thermionic detector. Despite the linearity of the NPD response to individual nitrogen-containing compounds, the response factor is different for different compounds and even isomers of the same species. Quantitative analysis using an NPD requires species-specific calibration. The reason for the sensitivity of the NPD to structure is related to the ease of forming the cyano-radical that is ionized to the cyanide anion, which is detected. The operation of the NPD was related to the processes of pyrolysis and subsequent ionization. It was possible to offer plausible explanations for differences in response factors for isomers based on pyrolysis chemistry. Due to this relationship, the NPD response can in the same way be used to provide information of practical relevance beyond its analytical value and a few possible applications were outlined. Keywords Thermionic detector · Nitrogen phosphorus detector (NPD) · Cyano-radical · Pyridine pyrolysis Introduction nitrogen-containing heterocycles are contaminants in feed material to petrochemical processes. Even at low concen- Nitrogen-containing aromatic compounds in the petrochem- tration level, of the order 100 μg/g, nitrogen compounds ical and pharmaceutical industries are varied and include can cause noticeable inhibition of hydroprocessing and acid compound classes that extend well beyond the anilines, pyri- catalysts as is found in processes such as gas oil hydrotreat- dines and pyrroles. In terms of abundance, coal tar histori- ing, hydrocracking, and fluid catalytic cracking [3 ]. Various cally used to be the primary source of nitrogen-containing strategies for removal of nitrogen-containing compounds compounds and most aromatic chemicals, but this situa- have been suggested, but nitrogen-removal is challenging tion changed in the period after the 1950s, with petroleum when it is applied to bulk petroleum cuts [4]. becoming the main source [1]. Despite a sizable market for When only the total nitrogen or basic nitrogen content of these compounds, the market resembles that of specialty petroleum is of interest, then CHNS analysis or acid titration petrochemicals [2] and it has considerable product diversity. may be sufficient for analytical quantification. If both the The constructive petrochemical use of nitrogen-con- nature and the amount of the nitrogen-containing species taining aromatic compounds is unfortunately not the only matter, then analysis must involve separation and quantifi - aspect of these compounds to consider. More often, the cation. Gas chromatography is particularly useful for such analyses, but the complexity of petroleum derived materi- als may make clean chromatographic separation difficult. * Arno de Klerk For such cases gas chromatography in combination with a deklerk@ualberta.ca thermionic detector can be useful. Department of Chemical and Materials Engineering, The thermionic detector, also called a nitrogen phosphorous University of Alberta, 9211-116th Street, Edmonton, detector (NPD), is selective towards nitrogen and phospho- AB T6G 1H9, Canada rous compounds. Nitrogen selective detection of compounds Anton Paar, Canada Vol.:(0123456789) 1 3 130 Applied Petrochemical Research (2021) 11:129–136 in petroleum is facilitated by the low natural abundance of NPD response (Table 1). HPLC grade (> 99%) toluene and phosphorus compounds. It was found to be a useful analytical acetone were purchased from Fisher Scientific as solvents technique for profiling different crude oils [5 –7], and to quan- for dilution. tify nitrogen-containing compounds in different petroleum cuts [5, 8–10]. Procedure The application of an NPD to quantify nitrogen-containing compounds in petroleum and petrochemicals initially appeared Samples were gravimetrically prepared by dissolving a to be straightforward. However, a more careful reading of the measured quantity of the analyte in a measured quantity of literature revealed some inconsistencies. solvent. Toluene was employed as solvent, unless the nitro- The nature of the NPD response is reportedly mass depend- gen-containing compound was poorly soluble, or insoluble ent [11] and the NPD response factor for nitrogen in different in toluene, in which case acetone was employed. compounds was reportedly nearly the same [8]. Yet, Albert Two aspects were experimentally investigated: [5] stated that the NPD response was only approximately pro- portional to the nitrogen content and that the response was 1. The linearity of the NPD response was determined by strictly speaking dependent on the compound structure. This analysis of a set of samples with a range of different was related to the chemistry taking place in the detector, which requires the formation of a cyano-radial (·C≡N) that is reduced Table 1 Nitrogen-containing compounds employed in this study to the cyanide anion (CN ) and it is the cyanide anion that is a b Compound Formula CASRN Purity detected [12]. Structural features of the molecule that would affect the formation of a cyano-radial, would therefore affect 2-Methylpyridine (2-picoline) C H N 109-06-8 0.98 6 7 its detection and detector response. 3-Methylpyridine (3-picoline) C H N 108-99-6 0.99 6 7 Carlsson et al. [13] provided a more general description of 4-Methylpyridine (4-picoline) C H N 108-89-4 0.98 6 7 the process. At or close to the hot surface of the NPD, ther- 2,3-Dimethylpyridine (2,3-lutidine) C H N 583-61-9 0.99 7 9 mal decomposition takes place to produce free radical frag- 2,4-Dimethylpyridine (2,4-lutidine) C H N 108-47-4 0.99 7 9 ments. Only those free radicals that readily form anions by 2,5-dimethylpyridine (2,5-lutidine) C H N 589-93-5 0.95 7 9 abstracting electrons from the detector would be detected. 2,6-Dimethylpyridine (2,6-lutidine) C H N 108-48-5 0.98 7 9 Thus, quantification of nitrogen-containing species was not 3,4-Dimethylpyridine (3,4-lutidine) C H N 583-58-4 0.98 7 9 necessarily proportional to the nitrogen-content, or the mass 2-Ethylpyridine C H N 100-71-0 0.97 7 9 of the nitrogen-containing species, but to depended on how 4-Ethylpyridine C H N 536-75-4 0.98 7 9 much cyano-radicals were formed in relation to the amount of 2,4,6-Trimethylpyridine C H N 108-75-8 0.99 8 11 nitrogen in the species. 2,3,5-Trimethylpyridine (2,3,5-col- C H N 695-98-7 0.99 8 11 lidine) The literature on quantification of nitrogen-containing com- 3-Cyanopyridine (3-pyridinecarboni- C H N 100-54-9 0.98 pounds with an NPD is therefore to some extent contradictory. 6 4 2 trile) The justification of this study in the context of petroleum Quinoline C H N 91-22-5 0.98 9 7 and petrochemicals was twofold. First, to determine how Isoquinoline C H N 119-65-3 0.98 9 7 the structure of nitrogen-containing aromatic compounds 4-Methylquinoline (lepidine) C H N 491-35-0 0.99 10 9 affected their quantification using gas chromatography with 6-Methylquinoline C H N 91-62-3 0.98 10 9 nitrogen phosphorus detector. Quantification of these com- 8-Methylquinoline C H N 611-32-5 0.97 10 9 pounds has practical application for quality assurance, pro- aniline C H N 62-53-3 0.995 6 7 cess development and evaluation of conversion processes. 2-Methylaniline (o-toluidine) C H N 95-53-4 0.99 7 9 Second, to explore whether the response of the nitrogen 3-Methylaniline (m-toluidine) C H N 108-44-1 0.99 7 9 phosphorus detector to different nitrogen-containing com- 2,4-Dimethylaniline (2,4-xylidine) C H N 95-68-1 0.99 8 11 pounds could be used to learn something about the free radi- 2,6-Dimethylaniline C H N 87-62-7 0.99 8 11 cal decomposition of such species in petroleum and petro- N,N-Dimethylaniline C H N 121-69-7 0.99 8 11 chemical processes that employed thermal conversion. indole C H N 120-72-9 0.99 8 7 3-Methylindole C H N 83-34-1 0.98 9 9 Acridine C H N 260-94-6 0.97 13 9 Experimental Phenanthridine C H N 229-87-8 0.98 13 9 Pyrazine C H N 290-37-9 0.99 4 4 2 Materials CASRN Chemical Abstracts Services Registry Number A variety of nitrogen-containing compounds were com- This is the mass fraction purity of the material guaranteed by the mercially obtained from Sigma-Aldrich to evaluate the supplier; material was not further purified 1 3 Applied Petrochemical Research (2021) 11:129–136 131 concentrations. The analytes selected were pyrazine that 4000 N contains two nitrogen atoms and quinoline that contains pyrazine r = 0.999 one nitrogen atom. Acetone solutions were prepared that contained both pyrazine and quinoline at equal nitro- gen concentration (μg N/g solution). Acetone was used as solvent for this specific analysis because pyrazine is soluble in acetone, but poorly soluble in toluene. quinoline 2. The proportionality of the NPD response to nitrogen in r = 0.992 different compounds was measured at a fixed concentra- tion of 1600 μg/g. For this study the compounds were analyzed individually and each analysis was repeated 0 200 400 600 800 three times. Toluene was employed as solvent for most Concentration (µg N/g) of these analytes. Fig. 1 NPD response with respect to the nitrogen concentration in the Analytical form of pyrazine (black circle) and quinoline (black square) The study was performed with an Agilent 7890A gas chro- concentration of specific nitrogen-containing compounds, matograph with nitrogen phosphorus detector. The gas chromatograph was equipped with a low polarity Agilent this can be accomplished using calibration for those specific compounds. Once a linear calibration is established, the con- HP-5 ms (19091S-433) fused silica column, 30 m in length, 250 μm inner diameter, and with 0.25 μm film thickness. centration can be quantified over a few orders of magnitude as indicated by Poole [11]. A split/splitless injector was employed with a split ratio of 1:100. Helium was the carrier gas with flow rate of 1 mL/ Proportionality of the NPD min. Since the separation required was only between the analyte and solvent, a fairly short temperature program was The second question that was addressed was to determine employed. The initial oven temperature was 80 °C, which was increased to 170 °C with a ramp rate of 6 °C/min and whether the NPD response was proportional to the amount of the nitrogen-containing compounds as suggested by Li then increased to 300 °C with a ramp rate of 15 °C/min. The column was held at 300 °C for 5 min. et al. [8], or approximated the total nitrogen content as sug- gested by Albert [5]. For this purpose the data in Fig.  1 was employed to calculate the proportionality of the NPD response with respect to nitrogen concentration and the con- Results and discussion centration of the nitrogen-containing compound (Table 2). The relationship between the amount of the two nitrogen- Linearity of the NPD containing compounds and the NPD response was not the same for the two compounds, 2.05 vs. 0.38 pC/(μg/g), for The first question that was addressed was to determine whether analysis using an NPD is linear over several orders pyrazine and quinoline respectively. The difference in NPD response was so large that it was clear that there was no of magnitude as reported by Poole [11]. For this evaluation pyrazine and quinoline were selected. The measured peak compound-independent universal relationship between the NPD response the amount of different nitrogen-containing area in relation to the concentration of each compound is shown in Fig. 1. compounds. Comparatively, the die ff rence between the NPD response and the total nitrogen content for the two test com- It can be seen from Fig. 1 that the response for both com- pounds was linear with respect to concentration. The NPD pounds was less, 5.85 vs. 3.54 pC/(μg N/g), for pyrazine and quinoline respectively. Still, it was clear that there was no response for pyrazine passed through the origin, but this was not the case for quinoline. The detection limit for qui- compound-independent universal relationship between the NPD response the total amount of nitrogen. noline based on the intercept with the baseline was around 85 μg N/g. There was also a more pronounced deviation These observations provided indirect support for the claim that the relationship between the NPD response and from the otherwise near linear response at the lowest con- centration tested. nitrogen-containing analytes depends on the pyrolysis chem- istry and therefore the nature of the species being analyzed. The NPD response area in the chromatograms in pico- coulomb (pC), i.e. picoampere (pA) × seconds (s), was lin- This was investigated further by expanding the number of nitrogen-containing species that were analyzed. ear with respect to concentration. For specific petrochemi- cal applications where it is necessary to track the change in 1 3 NPD peak area (pC) 132 Applied Petrochemical Research (2021) 11:129–136 Table 2 NPD response slopes of pyrazine and quinoline Table 3 NPD response slopes of different nitrogen-containing com- pounds Description Pyrazine Quinoline CompoundNPD response Uncertainty in Linear regression r -value 0.999 0.992 analysis (%) pC/(µg N/g) pC/(µg/g) NPD proportionality  nitrogen mass, pC/(µg N/g) 5.85 3.54 Pyridines  nitrogen molar, pC/(µmol N/g) 0.42 0.25 2-Methylpyridine 4.85 0.73 4.1  nitrogen-compound mass, pC/(µg/g) 2.05 0.38 3-Methylpyridine 6.99 1.05 3.4  nitrogen-compound molar, pC/(µmol/g) 164 50 4-Methylpyridine 6.65 1.00 0.6 2,3-Dimethylpyridine 6.11 0.80 0.4 The proportionality is expressed as the ratio of the integrated area in 2,4-Dimethylpyridine 5.30 0.69 1.2 the chromatogram of the NPD in picocoulomb (pC) and four differ - ent way to express the concentration of the nitrogen-containing com- 2,5-Dimethylpyridine 6.27 0.82 3.7 pound in the sample solution that was analyzed 2,6-dimethylpyridine 4.16 0.54 4.1 3,4-Dimethylpyridine 4.41 0.58 2.3 2-Ethylpyridine 4.74 0.62 3.6 NPD response slope for different 4-Ethylpyridine 5.74 0.75 0.4 nitrogen‑containing compounds 2,4,6-Trimethylpyridine 5.54 0.64 0.3 2,3,5-Trimethylpyridine 4.93 0.57 0.7 Nitrogen-containing compounds (Table 1) were selected 3-Cyanopyridine 7.90 1.06 1.9 to evaluate isomeric diversity of species and to see the Quinolines influence of differences in compound class. The list of  Quinoline 3.95 0.43 4.0 compounds is neither exhaustive, nor is it meant to be  Isoquinoline 3.77 0.41 3.1 representative of the major species found in petroleum or  4-Methylquinoline 3.55 0.35 3.9 the petrochemical industry. For example, it will be noticed  6-Methylquinoline 2.01 0.20 4.2 that the pyrroles and carbazoles are under-represented.  8-Methylquinoline 5.59 0.55 1.6 The NPD response slopes were determined for individu- Anilines ally for solutions of each compound in toluene. Results Aniline 2.28 0.34 4.2 were ordered according to compound class and are pre- 2-Methylaniline 2.47 0.37 0.6 sented in Table 3. The variability in results is expressed 3-Methylaniline 2.19 0.29 4.5 in terms of a relative sample standard deviation of the 2,4-Dimethylaniline 2.07 0.24 4.0 integrated values in picocoulomb for analyses performed 2,6-Dimethylaniline 2.62 0.30 1.4 in triplicate. The relative sample standard deviation never N,N-Dimethylaniline 5.29 0.61 2.2 exceeded 5%. Indoles The pyridine derivatives in Table 3 can be classified into  Indole 2.72 0.32 4.7 three groups: monomethyl pyridines (C H N), dimethyl 6 7  3-Methylindole 1.37 0.15 2.7 and monoethyl pyridines (C H N), and trimethyl pyridines 7 9 Trinuclear (C H N). The compounds in each group not only have the 8 11  Acridine 1.25 0.10 3.2 same molar mass, but also have the same number of moles  Phenanthridine 1.12 0.09 3.9 in the toluene solution. Despite this, the experimental results indicated that the NPD response slopes were different for a The proportionality is expressed as the ratio of the integrated area in structurally similar pyridine derivatives. For example, three the chromatogram of the NPD in picocoulomb (pC) and the analyte concentration in either microgram nitrogen per gram of solution (µg tested methyl pyridines with methyl substitution on ortho-, N/g), or microgram nitrogen-containing compound per gram of solu- meta-, and para-positions have response slopes of 4.85, 6.99, tion (µg/g) and 6.65 pC/(μg N/g), respectively. The NPD response was Relative sample standard deviation of NPD response area in pico- sensitive to the position of substitution, which presumably coulomb (pC) for analyses performed in triplicate affected the ease with which cyano-radicals were formed. The chain length of the alkylated substituent on the slope. The NPD response did not display any apparent trend pyridines also affected the NPD response, but to a lesser for mono-, di-, and trimethylpyridines (Table 3). extent than the substituent position. For example, the NPD As a compound class, the anilines that had amino-groups response slopes of 2-methylpyridine and 2-ethylpyridine (–NH ) had similar NPD response slopes (Table 3), with were 4.85 and 4.74 pC/(μg N/g), respectively, and those for values in the range of 2.07–2.62 pC/(μg N/g), respectively. 4-methylpyridine and 4-ethylpyridine were 6.65 and 5.74 The only N-substituted aniline derivative that was analyzed, pC/(μg N/g), respectively. The number of alkylated substitu- was N,N-dimethylaniline, and it had a significantly higher ents was not a key factor that determined the NPD response 1 3 Applied Petrochemical Research (2021) 11:129–136 133 NPD response, 5.29 pC/(μg N/g). Having two methyl groups These are pyrolysis conditions and produces radical species. attached to the nitrogen atom might have assisted the forma- Selectivity is achieved due to ionization on the alkali metal tion of cyano-radicals, which could explain the higher NPD bead in the NPD that acts as an electron donor to free radi- response compared to the other anilines. cal species with a high electronegativity, such as those that Quinoline and isoquinoline had near similar NPD contain nitrogen and phosphorus as ·CN, ·PO, and ·PO radi- response slopes (Table  3), 3.95 and 3.77 pC/(μg N/g), cals [13]. During the competitive ionization process, more respectively. Considering the 5% uncertainty, the different in electropositive radicals, such as most hydrocarbon radicals, these values were not meaningful. Alkylated derivatives of are not ionized. quinoline displayed the position sensitive variation in NPD The pyrolysis chemistry taking place in the NPD will response as seen with the alkylated pyridines, with values produce free radical products. However, to detect nitrogen- varying from 2.01 to 5.59 pC/(μg N/g) depending on the containing species the pyrolysis chemistry must lead to the position of the methyl group on the quinoline. formation of the cyano-radical. If the nitrogen is in a more The indoles and trinuclear nitrogen-containing aromat- hydrogen rich environment, it is more likely that pyrolysis ics all had NPD response slopes of around 1–3 pC/(μg N/g) will lead to ammonia (NH ). It should therefore be antici- (Table 3). pated that nitrogen-containing compound classes with more In conclusion, the NPD response slopes of different hydrogen-rich nitrogen, such as anilines, would on average nitrogen-containing compounds in Table 3, were not only have a lower NPD response than hydrogen-poor nitrogen, affected by compound class, but also by isomeric differences such as pyridines. This is indeed also what was observed of species. In the case of isomeric species, the observations (Table 3). The higher NPD response of N,N-dimethylaniline were consistent with an interpretation that related the NPD with –N(CH ) compared to anilines with –NH can simi- 3 2 2 response to the pyrolysis chemistry of the compounds. The larly be explained. potential relationship between the pyrolysis chemistry of the nitrogen-containing compounds and the NPD response was Impact of molecular structure of isomers worthwhile exploring in more detail. on cyano‑radical formation Selective ion formation in the NPD What remains to be explained, is the difference in NPD response of structural isomers of the same compound. In To relate the pyrolysis chemistry of nitrogen-containing an attempt to understand the impact of molecular structure compounds to NPD response, it is useful to first look at of isomers on NPD response, we looked more closely at the ionization detectors in more detail. methylpyridines. There are different kinds of ionization detectors that are Of the three isomers, 3-methylpyridine is the least reac- employed in conjunction with gas chromatography [11]. tive [18], but under pyrolysis conditions the decomposi- The most commonly used ionization detector, is the flame tion reactivity is reversed, with 3-methylpyridine being ionization detector, which achieves a flame temperature the most reactive for decomposition at around 800 °C [19]. of the order 1500–2000 °C. The flame ionization detector From Table 3, the relative NPD responses were in the order: response is roughly related to the amount of carbon in the 3-methylpyridine (1) > 4-methylpyridine (0.95) > 2-methyl- analyte. For flame ionization detectors the response factor pyridine (0.69). The highest NPD response was the same as for heteroatom-containing compounds can be estimated the highest pyrolysis reactivity, which was consistent with using the effective carbon number concept [14– 16]. Another NPD operation as explained in Sect. 3.4. modelling approach to relate the analyte structure to detector The pyrolysis studies of pyridine, 2-methylpyridine and response was proposed by Katritzky et al. [17], but as with 3-methylpyridine by the group of Mackie [20–24] and the the effective carbon number concept, the number of carbons work by Memon et al. [25] provided some insights into the in the analyte remains a key parameter. The effective car - high temperature chemistry relevant to the NPD. The follow- bon number, as opposed to the actual carbon number, takes ing observations relevant to the present study follow from into account the impact of different functional groups on the their work: efficiency of carbon-based ion formation due to chemical ionization taking place in the hydrogen flame. a. In all instances hydrogen cyanide was a major prod- The charge carriers in the NPD, which are also respon- uct. This is consistent with the cyanide anion being the sible for its selectivity, are thermal decomposition products charge carrier for selective detection of nitrogen-con- that are produced at much lower temperature than in a flame taining compounds by the NPD. ionization detector. In the NPD the flow of reaction gases b. Propagation reactions leading to decomposition of 2- in less and insufficient to maintain combustion as a flame and 3-methylpyridine formed hydrogen (H ), methane and the temperature achieved is in the range 400–800 °C. (CH ), pyridine (C H N), acetylene (C H ), hydrogen 4 5 5 2 2 1 3 134 Applied Petrochemical Research (2021) 11:129–136 cyanide (HCN), and cyanoacetylene (HCCCN) as major convenient resonance structure as shown in Fig. 3, which products [21, 24]. Hydrogen, acetylene, hydrogen cya- would explain why decomposition of 3-methylpyridine nide and cyanoacetylene were major products from pyri- is easier than the other isomers. dine pyrolysis [20], and at short context time pyridine e. There is also a difference in the NPD response between pyrolysis produced acetylene, butadiene (C H ) and 2- and 4-methylpyridine that should be explained. The 4 6 methylnitrile (CH CH) as major initial products [25]. pyrolysis of 2-methylpyridine was faster than that of pyr- Any differences in the decomposition pathways between idine and loss of the methyl appeared to be an important these compounds that could give rise to a difference in parallel pathway [23]. It would imply that both 2- and NPD response must therefore be related to the initial 4-methylpyridine should resemble pyridine in further rate and amount of cyanide formation and not due to the decomposition. If there was a difference in decomposi- overall decomposition reaction network. tion after the loss of the methyl group, to explain the c. The likely first step in pyrolysis of the methylpyridine difference in NPD response, it could therefore not be is loss hydrogen from the methyl group attached to the due to pyridyl radical decompositon per se, because the pyridine (Fig. 2). This does not imply that no other ini- NPD response of 4-methylpyridine > 2-methylpyridine. tiation steps are possible, such as the dissociation of There had to be a different explanation and possibly one C –CH and C –H [22], but these are likely related to the formation of an ortho-pyridyl radical by pyridine 3 pyridine minor initiation pathways at the conditions in the NPD. 2-methylpyridine and a para-pyridyl radical by 4-meth- Based hydrocarbons, the bond dissociation energies for ylpyridine. One side-reaction that may have bearing on benzylic C–H, aromatic C–CH , and aromatic C–H are the difference between the isomers, is the formation of of the order 375, 433, and 472 kJ/mol respectively [26]. heavier products during the pyrolysis of 2-methylpyri- If initiation by breaking a bond to the heterocycle was dine. Ikeda and Mackie [23] analyzed the heavier prod- the dominant pathway, then the loss of a methyl radical ucts and observed that “many of the multiring pyrolysis would cause methyl pyridines and pyridine all to pro- products have a nitrogen atom adjacent to a benzene ceed by decomposition of a pyridyl radical. This would ring but not in the β-position.” One of the major primary make it difficult to reconcile the observed difference in products from pyridine pyrolysis is butadiene [25], and NPD response of the different methylpyridine isomers the addition reaction of butadiene to the ortho-pyridyl listed in Table 3. radical could be assisted by the adjacent nitrogen (Fig. 4) d. If initiation by C–H dissociation on the methyl group in a way that is not possible for the para-pyridyl radical. is indeed the main initiation pathway, then it may be Forming a different stable product as side-reaction from possible to explain the higher NPD response of 3-meth- 2-methylpyridine decomposition would not necessarily ylpyridine. As mentioned before, 3-methylpyridine had affect the overall decomposition, but it would reduce the the highest pyrolysis reactivity of the methylpyridine initial decomposition rate to release a cyano-radical to isomers [19], and this might be due to the resonance reduce the NPD response of 2-methylpyridine relative structure that is possible after ‘benzylic’ C–H dissocia- to that of 4-methylpyridine. tion (Fig. 3). A similar argument as was presented for the formation of the ortho-pyridyl radical to explain ini- tial decomposition that would result in a terminal cyano- The present study was not designed to explore the pyrolysis group, which was consistent with the decomposition chemistry of the nitrogen-containing compounds. Neverthe- products formed by pyridine decomposition [20]. The less, combining the results in Table 3 with literature observa- 2-, or 4-methylpyridyl radical does not have the same tions showed that a relationship between the pathway of initia- tion leading to initial decomposition and the response of the NPD was able to provide a plausible, but unproven explanation for the difference in NPD response. NCH - H - H • • - CH CH CH CH 3 2 2 • • NCH NCH 2 3 - H major N N N pathway N Fig. 2 Initiation of 2-methylpyridine decomposition at NPD condi- Fig. 3 Possible decomposition sequence responsible for the higher tions decomposition rate and higher NPD response of 3-methylpyridine 1 3 Applied Petrochemical Research (2021) 11:129–136 135 CH • 3 • • NCH N N N N H H Fig. 4 Side-reaction of the ortho-pyridyl radical formed by 2-methylpyridine decomposition that is assisted by the adjacent nitrogen to reduce initial decomposition rate and decrease the NPD response of 2-methylpyridine Relevance to petrochemical and other processes the detector response and concentration of nitrogen-contain- ing compounds. It was found that: In the previous section it was shown how NPD response could a. Despite the linearity of the NPD response to individual be related to pyrolysis chemistry of isomeric species. The same type of approach can be used to use the NPD response to assist nitrogen-containing compounds, the response factor of the NPD is different for different compounds and even with elucidating decomposition pathways of interest to petro- chemical transformations. structural isomers of the same species. Quantitative analysis using an NPD requires species-specific calibra - In the same way it could also be an indicator of likely emis- sion products during high temperature processing or destruc- tion. b. The NPD response can be related to the pyrolysis chem- tion of nitrogen-containing materials. The same pyrolytic decomposition that is involved in forming the cyano-radical istry leading to cyano-radical formation. In the same way the NPD response can be used to provide informa- also leads to the formation of hydrogen cyanide during com- bustion processes. For example, the nature of the nitrogen- tion of practical relevance beyond its analytical value. containing species in fuels with the same total nitrogen content affected the HCN yield during combustion [27], which is likely reflected in the NPD response of those materials too. Open Access This article is licensed under a Creative Commons Attri- Analyses using an NPD also have analytical uses beyond bution 4.0 International License, which permits use, sharing, adapta- those that are self-evident from its selective detection. It may tion, distribution and reproduction in any medium or format, as long be possible to obtain some indication of the compound class as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes composition of a complex material, such as a petroleum prod- were made. The images or other third party material in this article are uct, by relating the NPD response to the total nitrogen content included in the article’s Creative Commons licence, unless indicated by CHNS elemental analysis. For example, pyridines on aver- otherwise in a credit line to the material. If material is not included in age have a higher NPD response than anilines, which would the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will assist with differentiation between nitrogen bases. Similarly, it need to obtain permission directly from the copyright holder. To view a may be possible to differentiate between isomers that are diffi- copy of this licence, visit http://creativ ecommons .or g/licenses/b y/4.0/. cult to differentiate using mass spectrometry, because isomers of the same species may have different NPD response factors. These are just some of the potential uses where the NPD response can be related to practical applications through the References description of the structure-sensitive reaction chemistry taking place in the NPD. 1. Waddams AL (1973) Chemicals from petroleum, 3rd edn. John Murray, London, pp 209–212 2. Goe GL (1982) Pyridine and pyridine derivatives. Kirk-Othmer encyclopedia of chemical technology, 3rd edn, vol 19. Wiley, New Conclusions York, pp 454–483 3. Kaiser MJ, De Klerk A, Gary JH, Handwerk GE (2020) Petro- The quantification of nitrogen-containing compounds with leum refining. Technology, economics, and markets, 6th edn. CRC a nitrogen-phosphorus detector (NPD) was investigated fol- Press, Boca Raton lowing on contradictory reports on the relationship between 1 3 136 Applied Petrochemical Research (2021) 11:129–136 4. Prado GHC, Rao Y, De Klerk A (2017) Nitrogen removal from 17. Katritzky AR, Ignatchenko ES, Barcock RA, Lobanov VS, Karel- oil: a review. Energy Fuels 31:14–36 son M (1994) Prediction of gas chromatographic retention times 5. Albert DK (1978) Determination of nitrogen compound distri- and response factors using a general qualitative structure-property bution in petroleum by gas chromatography with a thermionic relationships treatment. Anal Chem 66:1799–1807 detector. Anal Chem 50:1822–1829 18. Tenenbaum LE (1961) Alkylpyridines and arylpyridines. In: 6. Bakel AJ, Philip RP (1990) The distribution and quantitation of Klingsberg E (ed) Pyridine and its derivatives. Part II. Intersci- organonitrogen compounds in crude oils and rock pyrolysates. Org ence, New York, pp 155–298 Geochem 16:353–367 19. Hurd CD, Simon JI (1962) Pyrolytic formation of arenes. III. 7. Frame GM, Carmody DC, Flanigan GA (1978) An atlas of gas Pyrolysis of pyridine, picolines and methylpyrazine. J Am Chem chromatograms of oils using dual flame-ionization and nitro- Soc 84:4519–4524 gen phosphorus detectors. Report of the United States Coast 20. Mackie JC, Colket MB III, Nelson PF (1990) Shock tube pyrolysis Guard Research and Development Center, CGR/DC-3/78, of pyridine. J Phys Chem 94:4099–4106 USCG-D-A054966 21. Terentis A, Doughty A, Mackie JC (1992) Kinetics of pyrolysis 8. Li N, Ma X, Zha Q, Song C (2010) Analysis and comparison of a coal model compound, 2-picoline, the nitrogen heteroaro- of nitrogen compounds in different liquid hydrocarbon streams matic analogue of toluene. 1. Product distributions. J Phys Chem derived from petroleum and coal. Energy Fuels 24:5539–5547 96:10334–10339 9. Machado ME (2019) Comprehensive two-dimensional gas chro- 22. Doughty A, Mackie JC (1992) Kinetics of pyrolysis of a coal matography for the analysis of nitrogen-containing compounds in model compound, 2-picoline, the nitrogen heteroaromatic ana- fossil fuels: a review. Talanta 198:263–276 logue of toluene. 2. The 2-picolyl radical and kinetic modeling. J 10. Von Mühlen C, De Oliveira EC, Morrison PD, Zini CA, Caramão Phys Chem 96:10339–10348 EB, Marriott PJ (2007) Qualitative and quantitative study of nitro- 23. Ikeda E, Mackie JC (1995) Thermal decomposition of two coal gen containing compounds in heavy gas oil using comprehensive model compounds—pyridine and 2-picoline. Kinetics and product two-dimensional gas chromatography with nitrogen phosphorus distributions. J Anal Appl Pyrolysis 34:47–63 detection. J Sep Sci 30:3223–3232 24. Jones J, Bacskay GB, Mackie JC (1996) The pyrolysis of 3-pico- 11. Poole CF (2015) Ionization-based detectors for gas chromatogra- line: ab initio quantum chemical and experimental (shock tube) phy. J Chromatogr A 1421:137–153 kinetic studies. Isr J Chem 36:239–248 12. Kolb B, Bischoff J (1974) A new design of a thermionic nitrogen 25. Memon HUR, Bartle KD, Taylor JM, Williams A (2000) The and phosphorus detector for GC. J Chromatogr Sci 12:625–629 shock tube pyrolysis of pyridine. Int J Energy Res 24:1141–1159 13. Carlsson H, Robertsson G, Colmsjö A (2001) Response mecha- 26. Blanksby SJ, Ellison GB (2003) Bond dissociation energies of nisms of thermionic detectors with enhanced nitrogen selectivity. organic molecules. Acc Chem Res 36:255–263 Anal Chem 73:5698–5703 27. Moreea-Taha R (2000) NOx modelling and prediction. IEA Coal 14. Sternberg JC, Gallaway WS, Jones DTL (1962) The mechanism Research, London, p 12 of response of flame ionization detectors. In: Brenner N, Callen JE, Weiss MD (eds) Gas chromatography. Academic Press, New Publisher’s Note Springer Nature remains neutral with regard to York, pp 231–267 jurisdictional claims in published maps and institutional affiliations. 15. Scanlon JT, Willis DE (1985) Calculation of flame ionization detector relative response factors using the effective carbon num- ber concept. J Chromatogr Sci 23:333–340 16. Jorgensen AD, Picel KC, Stamoudis VC (1990) Prediction of gas chromatography flame ionization detector response factors from molecular structures. Anal Chem 62:683–689 1 3 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Applied Petrochemical Research Springer Journals

Nitrogen-containing aromatic compounds: quantitative analysis using gas chromatography with nitrogen phosphorus detector

Rao, Yuan; de Klerk, Arno
Applied Petrochemical Research , Volume 11 (2) – Mar 6, 2021
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Springer Journals
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2190-5525
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10.1007/s13203-021-00265-z
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Abstract

The nitrogen-containing aromatic compounds found in the petrochemical industry are varied and extend beyond classes such as the anilines, pyrroles and pyridines. Quantification of these nitrogen-containing compounds that may occur in complex mixtures has practical application for quality assurance, process development and the evaluation of conversion processes. Selective detection of nitrogen-containing species in complex mixtures is possible by making use of gas chromatography coupled with a nitrogen phosphorous detector (GC-NPD), which is also called a thermionic detector. Despite the linearity of the NPD response to individual nitrogen-containing compounds, the response factor is different for different compounds and even isomers of the same species. Quantitative analysis using an NPD requires species-specific calibration. The reason for the sensitivity of the NPD to structure is related to the ease of forming the cyano-radical that is ionized to the cyanide anion, which is detected. The operation of the NPD was related to the processes of pyrolysis and subsequent ionization. It was possible to offer plausible explanations for differences in response factors for isomers based on pyrolysis chemistry. Due to this relationship, the NPD response can in the same way be used to provide information of practical relevance beyond its analytical value and a few possible applications were outlined. Keywords Thermionic detector · Nitrogen phosphorus detector (NPD) · Cyano-radical · Pyridine pyrolysis Introduction nitrogen-containing heterocycles are contaminants in feed material to petrochemical processes. Even at low concen- Nitrogen-containing aromatic compounds in the petrochem- tration level, of the order 100 μg/g, nitrogen compounds ical and pharmaceutical industries are varied and include can cause noticeable inhibition of hydroprocessing and acid compound classes that extend well beyond the anilines, pyri- catalysts as is found in processes such as gas oil hydrotreat- dines and pyrroles. In terms of abundance, coal tar histori- ing, hydrocracking, and fluid catalytic cracking [3 ]. Various cally used to be the primary source of nitrogen-containing strategies for removal of nitrogen-containing compounds compounds and most aromatic chemicals, but this situa- have been suggested, but nitrogen-removal is challenging tion changed in the period after the 1950s, with petroleum when it is applied to bulk petroleum cuts [4]. becoming the main source [1]. Despite a sizable market for When only the total nitrogen or basic nitrogen content of these compounds, the market resembles that of specialty petroleum is of interest, then CHNS analysis or acid titration petrochemicals [2] and it has considerable product diversity. may be sufficient for analytical quantification. If both the The constructive petrochemical use of nitrogen-con- nature and the amount of the nitrogen-containing species taining aromatic compounds is unfortunately not the only matter, then analysis must involve separation and quantifi - aspect of these compounds to consider. More often, the cation. Gas chromatography is particularly useful for such analyses, but the complexity of petroleum derived materi- als may make clean chromatographic separation difficult. * Arno de Klerk For such cases gas chromatography in combination with a deklerk@ualberta.ca thermionic detector can be useful. Department of Chemical and Materials Engineering, The thermionic detector, also called a nitrogen phosphorous University of Alberta, 9211-116th Street, Edmonton, detector (NPD), is selective towards nitrogen and phospho- AB T6G 1H9, Canada rous compounds. Nitrogen selective detection of compounds Anton Paar, Canada Vol.:(0123456789) 1 3 130 Applied Petrochemical Research (2021) 11:129–136 in petroleum is facilitated by the low natural abundance of NPD response (Table 1). HPLC grade (> 99%) toluene and phosphorus compounds. It was found to be a useful analytical acetone were purchased from Fisher Scientific as solvents technique for profiling different crude oils [5 –7], and to quan- for dilution. tify nitrogen-containing compounds in different petroleum cuts [5, 8–10]. Procedure The application of an NPD to quantify nitrogen-containing compounds in petroleum and petrochemicals initially appeared Samples were gravimetrically prepared by dissolving a to be straightforward. However, a more careful reading of the measured quantity of the analyte in a measured quantity of literature revealed some inconsistencies. solvent. Toluene was employed as solvent, unless the nitro- The nature of the NPD response is reportedly mass depend- gen-containing compound was poorly soluble, or insoluble ent [11] and the NPD response factor for nitrogen in different in toluene, in which case acetone was employed. compounds was reportedly nearly the same [8]. Yet, Albert Two aspects were experimentally investigated: [5] stated that the NPD response was only approximately pro- portional to the nitrogen content and that the response was 1. The linearity of the NPD response was determined by strictly speaking dependent on the compound structure. This analysis of a set of samples with a range of different was related to the chemistry taking place in the detector, which requires the formation of a cyano-radial (·C≡N) that is reduced Table 1 Nitrogen-containing compounds employed in this study to the cyanide anion (CN ) and it is the cyanide anion that is a b Compound Formula CASRN Purity detected [12]. Structural features of the molecule that would affect the formation of a cyano-radial, would therefore affect 2-Methylpyridine (2-picoline) C H N 109-06-8 0.98 6 7 its detection and detector response. 3-Methylpyridine (3-picoline) C H N 108-99-6 0.99 6 7 Carlsson et al. [13] provided a more general description of 4-Methylpyridine (4-picoline) C H N 108-89-4 0.98 6 7 the process. At or close to the hot surface of the NPD, ther- 2,3-Dimethylpyridine (2,3-lutidine) C H N 583-61-9 0.99 7 9 mal decomposition takes place to produce free radical frag- 2,4-Dimethylpyridine (2,4-lutidine) C H N 108-47-4 0.99 7 9 ments. Only those free radicals that readily form anions by 2,5-dimethylpyridine (2,5-lutidine) C H N 589-93-5 0.95 7 9 abstracting electrons from the detector would be detected. 2,6-Dimethylpyridine (2,6-lutidine) C H N 108-48-5 0.98 7 9 Thus, quantification of nitrogen-containing species was not 3,4-Dimethylpyridine (3,4-lutidine) C H N 583-58-4 0.98 7 9 necessarily proportional to the nitrogen-content, or the mass 2-Ethylpyridine C H N 100-71-0 0.97 7 9 of the nitrogen-containing species, but to depended on how 4-Ethylpyridine C H N 536-75-4 0.98 7 9 much cyano-radicals were formed in relation to the amount of 2,4,6-Trimethylpyridine C H N 108-75-8 0.99 8 11 nitrogen in the species. 2,3,5-Trimethylpyridine (2,3,5-col- C H N 695-98-7 0.99 8 11 lidine) The literature on quantification of nitrogen-containing com- 3-Cyanopyridine (3-pyridinecarboni- C H N 100-54-9 0.98 pounds with an NPD is therefore to some extent contradictory. 6 4 2 trile) The justification of this study in the context of petroleum Quinoline C H N 91-22-5 0.98 9 7 and petrochemicals was twofold. First, to determine how Isoquinoline C H N 119-65-3 0.98 9 7 the structure of nitrogen-containing aromatic compounds 4-Methylquinoline (lepidine) C H N 491-35-0 0.99 10 9 affected their quantification using gas chromatography with 6-Methylquinoline C H N 91-62-3 0.98 10 9 nitrogen phosphorus detector. Quantification of these com- 8-Methylquinoline C H N 611-32-5 0.97 10 9 pounds has practical application for quality assurance, pro- aniline C H N 62-53-3 0.995 6 7 cess development and evaluation of conversion processes. 2-Methylaniline (o-toluidine) C H N 95-53-4 0.99 7 9 Second, to explore whether the response of the nitrogen 3-Methylaniline (m-toluidine) C H N 108-44-1 0.99 7 9 phosphorus detector to different nitrogen-containing com- 2,4-Dimethylaniline (2,4-xylidine) C H N 95-68-1 0.99 8 11 pounds could be used to learn something about the free radi- 2,6-Dimethylaniline C H N 87-62-7 0.99 8 11 cal decomposition of such species in petroleum and petro- N,N-Dimethylaniline C H N 121-69-7 0.99 8 11 chemical processes that employed thermal conversion. indole C H N 120-72-9 0.99 8 7 3-Methylindole C H N 83-34-1 0.98 9 9 Acridine C H N 260-94-6 0.97 13 9 Experimental Phenanthridine C H N 229-87-8 0.98 13 9 Pyrazine C H N 290-37-9 0.99 4 4 2 Materials CASRN Chemical Abstracts Services Registry Number A variety of nitrogen-containing compounds were com- This is the mass fraction purity of the material guaranteed by the mercially obtained from Sigma-Aldrich to evaluate the supplier; material was not further purified 1 3 Applied Petrochemical Research (2021) 11:129–136 131 concentrations. The analytes selected were pyrazine that 4000 N contains two nitrogen atoms and quinoline that contains pyrazine r = 0.999 one nitrogen atom. Acetone solutions were prepared that contained both pyrazine and quinoline at equal nitro- gen concentration (μg N/g solution). Acetone was used as solvent for this specific analysis because pyrazine is soluble in acetone, but poorly soluble in toluene. quinoline 2. The proportionality of the NPD response to nitrogen in r = 0.992 different compounds was measured at a fixed concentra- tion of 1600 μg/g. For this study the compounds were analyzed individually and each analysis was repeated 0 200 400 600 800 three times. Toluene was employed as solvent for most Concentration (µg N/g) of these analytes. Fig. 1 NPD response with respect to the nitrogen concentration in the Analytical form of pyrazine (black circle) and quinoline (black square) The study was performed with an Agilent 7890A gas chro- concentration of specific nitrogen-containing compounds, matograph with nitrogen phosphorus detector. The gas chromatograph was equipped with a low polarity Agilent this can be accomplished using calibration for those specific compounds. Once a linear calibration is established, the con- HP-5 ms (19091S-433) fused silica column, 30 m in length, 250 μm inner diameter, and with 0.25 μm film thickness. centration can be quantified over a few orders of magnitude as indicated by Poole [11]. A split/splitless injector was employed with a split ratio of 1:100. Helium was the carrier gas with flow rate of 1 mL/ Proportionality of the NPD min. Since the separation required was only between the analyte and solvent, a fairly short temperature program was The second question that was addressed was to determine employed. The initial oven temperature was 80 °C, which was increased to 170 °C with a ramp rate of 6 °C/min and whether the NPD response was proportional to the amount of the nitrogen-containing compounds as suggested by Li then increased to 300 °C with a ramp rate of 15 °C/min. The column was held at 300 °C for 5 min. et al. [8], or approximated the total nitrogen content as sug- gested by Albert [5]. For this purpose the data in Fig.  1 was employed to calculate the proportionality of the NPD response with respect to nitrogen concentration and the con- Results and discussion centration of the nitrogen-containing compound (Table 2). The relationship between the amount of the two nitrogen- Linearity of the NPD containing compounds and the NPD response was not the same for the two compounds, 2.05 vs. 0.38 pC/(μg/g), for The first question that was addressed was to determine whether analysis using an NPD is linear over several orders pyrazine and quinoline respectively. The difference in NPD response was so large that it was clear that there was no of magnitude as reported by Poole [11]. For this evaluation pyrazine and quinoline were selected. The measured peak compound-independent universal relationship between the NPD response the amount of different nitrogen-containing area in relation to the concentration of each compound is shown in Fig. 1. compounds. Comparatively, the die ff rence between the NPD response and the total nitrogen content for the two test com- It can be seen from Fig. 1 that the response for both com- pounds was linear with respect to concentration. The NPD pounds was less, 5.85 vs. 3.54 pC/(μg N/g), for pyrazine and quinoline respectively. Still, it was clear that there was no response for pyrazine passed through the origin, but this was not the case for quinoline. The detection limit for qui- compound-independent universal relationship between the NPD response the total amount of nitrogen. noline based on the intercept with the baseline was around 85 μg N/g. There was also a more pronounced deviation These observations provided indirect support for the claim that the relationship between the NPD response and from the otherwise near linear response at the lowest con- centration tested. nitrogen-containing analytes depends on the pyrolysis chem- istry and therefore the nature of the species being analyzed. The NPD response area in the chromatograms in pico- coulomb (pC), i.e. picoampere (pA) × seconds (s), was lin- This was investigated further by expanding the number of nitrogen-containing species that were analyzed. ear with respect to concentration. For specific petrochemi- cal applications where it is necessary to track the change in 1 3 NPD peak area (pC) 132 Applied Petrochemical Research (2021) 11:129–136 Table 2 NPD response slopes of pyrazine and quinoline Table 3 NPD response slopes of different nitrogen-containing com- pounds Description Pyrazine Quinoline CompoundNPD response Uncertainty in Linear regression r -value 0.999 0.992 analysis (%) pC/(µg N/g) pC/(µg/g) NPD proportionality  nitrogen mass, pC/(µg N/g) 5.85 3.54 Pyridines  nitrogen molar, pC/(µmol N/g) 0.42 0.25 2-Methylpyridine 4.85 0.73 4.1  nitrogen-compound mass, pC/(µg/g) 2.05 0.38 3-Methylpyridine 6.99 1.05 3.4  nitrogen-compound molar, pC/(µmol/g) 164 50 4-Methylpyridine 6.65 1.00 0.6 2,3-Dimethylpyridine 6.11 0.80 0.4 The proportionality is expressed as the ratio of the integrated area in 2,4-Dimethylpyridine 5.30 0.69 1.2 the chromatogram of the NPD in picocoulomb (pC) and four differ - ent way to express the concentration of the nitrogen-containing com- 2,5-Dimethylpyridine 6.27 0.82 3.7 pound in the sample solution that was analyzed 2,6-dimethylpyridine 4.16 0.54 4.1 3,4-Dimethylpyridine 4.41 0.58 2.3 2-Ethylpyridine 4.74 0.62 3.6 NPD response slope for different 4-Ethylpyridine 5.74 0.75 0.4 nitrogen‑containing compounds 2,4,6-Trimethylpyridine 5.54 0.64 0.3 2,3,5-Trimethylpyridine 4.93 0.57 0.7 Nitrogen-containing compounds (Table 1) were selected 3-Cyanopyridine 7.90 1.06 1.9 to evaluate isomeric diversity of species and to see the Quinolines influence of differences in compound class. The list of  Quinoline 3.95 0.43 4.0 compounds is neither exhaustive, nor is it meant to be  Isoquinoline 3.77 0.41 3.1 representative of the major species found in petroleum or  4-Methylquinoline 3.55 0.35 3.9 the petrochemical industry. For example, it will be noticed  6-Methylquinoline 2.01 0.20 4.2 that the pyrroles and carbazoles are under-represented.  8-Methylquinoline 5.59 0.55 1.6 The NPD response slopes were determined for individu- Anilines ally for solutions of each compound in toluene. Results Aniline 2.28 0.34 4.2 were ordered according to compound class and are pre- 2-Methylaniline 2.47 0.37 0.6 sented in Table 3. The variability in results is expressed 3-Methylaniline 2.19 0.29 4.5 in terms of a relative sample standard deviation of the 2,4-Dimethylaniline 2.07 0.24 4.0 integrated values in picocoulomb for analyses performed 2,6-Dimethylaniline 2.62 0.30 1.4 in triplicate. The relative sample standard deviation never N,N-Dimethylaniline 5.29 0.61 2.2 exceeded 5%. Indoles The pyridine derivatives in Table 3 can be classified into  Indole 2.72 0.32 4.7 three groups: monomethyl pyridines (C H N), dimethyl 6 7  3-Methylindole 1.37 0.15 2.7 and monoethyl pyridines (C H N), and trimethyl pyridines 7 9 Trinuclear (C H N). The compounds in each group not only have the 8 11  Acridine 1.25 0.10 3.2 same molar mass, but also have the same number of moles  Phenanthridine 1.12 0.09 3.9 in the toluene solution. Despite this, the experimental results indicated that the NPD response slopes were different for a The proportionality is expressed as the ratio of the integrated area in structurally similar pyridine derivatives. For example, three the chromatogram of the NPD in picocoulomb (pC) and the analyte concentration in either microgram nitrogen per gram of solution (µg tested methyl pyridines with methyl substitution on ortho-, N/g), or microgram nitrogen-containing compound per gram of solu- meta-, and para-positions have response slopes of 4.85, 6.99, tion (µg/g) and 6.65 pC/(μg N/g), respectively. The NPD response was Relative sample standard deviation of NPD response area in pico- sensitive to the position of substitution, which presumably coulomb (pC) for analyses performed in triplicate affected the ease with which cyano-radicals were formed. The chain length of the alkylated substituent on the slope. The NPD response did not display any apparent trend pyridines also affected the NPD response, but to a lesser for mono-, di-, and trimethylpyridines (Table 3). extent than the substituent position. For example, the NPD As a compound class, the anilines that had amino-groups response slopes of 2-methylpyridine and 2-ethylpyridine (–NH ) had similar NPD response slopes (Table 3), with were 4.85 and 4.74 pC/(μg N/g), respectively, and those for values in the range of 2.07–2.62 pC/(μg N/g), respectively. 4-methylpyridine and 4-ethylpyridine were 6.65 and 5.74 The only N-substituted aniline derivative that was analyzed, pC/(μg N/g), respectively. The number of alkylated substitu- was N,N-dimethylaniline, and it had a significantly higher ents was not a key factor that determined the NPD response 1 3 Applied Petrochemical Research (2021) 11:129–136 133 NPD response, 5.29 pC/(μg N/g). Having two methyl groups These are pyrolysis conditions and produces radical species. attached to the nitrogen atom might have assisted the forma- Selectivity is achieved due to ionization on the alkali metal tion of cyano-radicals, which could explain the higher NPD bead in the NPD that acts as an electron donor to free radi- response compared to the other anilines. cal species with a high electronegativity, such as those that Quinoline and isoquinoline had near similar NPD contain nitrogen and phosphorus as ·CN, ·PO, and ·PO radi- response slopes (Table  3), 3.95 and 3.77 pC/(μg N/g), cals [13]. During the competitive ionization process, more respectively. Considering the 5% uncertainty, the different in electropositive radicals, such as most hydrocarbon radicals, these values were not meaningful. Alkylated derivatives of are not ionized. quinoline displayed the position sensitive variation in NPD The pyrolysis chemistry taking place in the NPD will response as seen with the alkylated pyridines, with values produce free radical products. However, to detect nitrogen- varying from 2.01 to 5.59 pC/(μg N/g) depending on the containing species the pyrolysis chemistry must lead to the position of the methyl group on the quinoline. formation of the cyano-radical. If the nitrogen is in a more The indoles and trinuclear nitrogen-containing aromat- hydrogen rich environment, it is more likely that pyrolysis ics all had NPD response slopes of around 1–3 pC/(μg N/g) will lead to ammonia (NH ). It should therefore be antici- (Table 3). pated that nitrogen-containing compound classes with more In conclusion, the NPD response slopes of different hydrogen-rich nitrogen, such as anilines, would on average nitrogen-containing compounds in Table 3, were not only have a lower NPD response than hydrogen-poor nitrogen, affected by compound class, but also by isomeric differences such as pyridines. This is indeed also what was observed of species. In the case of isomeric species, the observations (Table 3). The higher NPD response of N,N-dimethylaniline were consistent with an interpretation that related the NPD with –N(CH ) compared to anilines with –NH can simi- 3 2 2 response to the pyrolysis chemistry of the compounds. The larly be explained. potential relationship between the pyrolysis chemistry of the nitrogen-containing compounds and the NPD response was Impact of molecular structure of isomers worthwhile exploring in more detail. on cyano‑radical formation Selective ion formation in the NPD What remains to be explained, is the difference in NPD response of structural isomers of the same compound. In To relate the pyrolysis chemistry of nitrogen-containing an attempt to understand the impact of molecular structure compounds to NPD response, it is useful to first look at of isomers on NPD response, we looked more closely at the ionization detectors in more detail. methylpyridines. There are different kinds of ionization detectors that are Of the three isomers, 3-methylpyridine is the least reac- employed in conjunction with gas chromatography [11]. tive [18], but under pyrolysis conditions the decomposi- The most commonly used ionization detector, is the flame tion reactivity is reversed, with 3-methylpyridine being ionization detector, which achieves a flame temperature the most reactive for decomposition at around 800 °C [19]. of the order 1500–2000 °C. The flame ionization detector From Table 3, the relative NPD responses were in the order: response is roughly related to the amount of carbon in the 3-methylpyridine (1) > 4-methylpyridine (0.95) > 2-methyl- analyte. For flame ionization detectors the response factor pyridine (0.69). The highest NPD response was the same as for heteroatom-containing compounds can be estimated the highest pyrolysis reactivity, which was consistent with using the effective carbon number concept [14– 16]. Another NPD operation as explained in Sect. 3.4. modelling approach to relate the analyte structure to detector The pyrolysis studies of pyridine, 2-methylpyridine and response was proposed by Katritzky et al. [17], but as with 3-methylpyridine by the group of Mackie [20–24] and the the effective carbon number concept, the number of carbons work by Memon et al. [25] provided some insights into the in the analyte remains a key parameter. The effective car - high temperature chemistry relevant to the NPD. The follow- bon number, as opposed to the actual carbon number, takes ing observations relevant to the present study follow from into account the impact of different functional groups on the their work: efficiency of carbon-based ion formation due to chemical ionization taking place in the hydrogen flame. a. In all instances hydrogen cyanide was a major prod- The charge carriers in the NPD, which are also respon- uct. This is consistent with the cyanide anion being the sible for its selectivity, are thermal decomposition products charge carrier for selective detection of nitrogen-con- that are produced at much lower temperature than in a flame taining compounds by the NPD. ionization detector. In the NPD the flow of reaction gases b. Propagation reactions leading to decomposition of 2- in less and insufficient to maintain combustion as a flame and 3-methylpyridine formed hydrogen (H ), methane and the temperature achieved is in the range 400–800 °C. (CH ), pyridine (C H N), acetylene (C H ), hydrogen 4 5 5 2 2 1 3 134 Applied Petrochemical Research (2021) 11:129–136 cyanide (HCN), and cyanoacetylene (HCCCN) as major convenient resonance structure as shown in Fig. 3, which products [21, 24]. Hydrogen, acetylene, hydrogen cya- would explain why decomposition of 3-methylpyridine nide and cyanoacetylene were major products from pyri- is easier than the other isomers. dine pyrolysis [20], and at short context time pyridine e. There is also a difference in the NPD response between pyrolysis produced acetylene, butadiene (C H ) and 2- and 4-methylpyridine that should be explained. The 4 6 methylnitrile (CH CH) as major initial products [25]. pyrolysis of 2-methylpyridine was faster than that of pyr- Any differences in the decomposition pathways between idine and loss of the methyl appeared to be an important these compounds that could give rise to a difference in parallel pathway [23]. It would imply that both 2- and NPD response must therefore be related to the initial 4-methylpyridine should resemble pyridine in further rate and amount of cyanide formation and not due to the decomposition. If there was a difference in decomposi- overall decomposition reaction network. tion after the loss of the methyl group, to explain the c. The likely first step in pyrolysis of the methylpyridine difference in NPD response, it could therefore not be is loss hydrogen from the methyl group attached to the due to pyridyl radical decompositon per se, because the pyridine (Fig. 2). This does not imply that no other ini- NPD response of 4-methylpyridine > 2-methylpyridine. tiation steps are possible, such as the dissociation of There had to be a different explanation and possibly one C –CH and C –H [22], but these are likely related to the formation of an ortho-pyridyl radical by pyridine 3 pyridine minor initiation pathways at the conditions in the NPD. 2-methylpyridine and a para-pyridyl radical by 4-meth- Based hydrocarbons, the bond dissociation energies for ylpyridine. One side-reaction that may have bearing on benzylic C–H, aromatic C–CH , and aromatic C–H are the difference between the isomers, is the formation of of the order 375, 433, and 472 kJ/mol respectively [26]. heavier products during the pyrolysis of 2-methylpyri- If initiation by breaking a bond to the heterocycle was dine. Ikeda and Mackie [23] analyzed the heavier prod- the dominant pathway, then the loss of a methyl radical ucts and observed that “many of the multiring pyrolysis would cause methyl pyridines and pyridine all to pro- products have a nitrogen atom adjacent to a benzene ceed by decomposition of a pyridyl radical. This would ring but not in the β-position.” One of the major primary make it difficult to reconcile the observed difference in products from pyridine pyrolysis is butadiene [25], and NPD response of the different methylpyridine isomers the addition reaction of butadiene to the ortho-pyridyl listed in Table 3. radical could be assisted by the adjacent nitrogen (Fig. 4) d. If initiation by C–H dissociation on the methyl group in a way that is not possible for the para-pyridyl radical. is indeed the main initiation pathway, then it may be Forming a different stable product as side-reaction from possible to explain the higher NPD response of 3-meth- 2-methylpyridine decomposition would not necessarily ylpyridine. As mentioned before, 3-methylpyridine had affect the overall decomposition, but it would reduce the the highest pyrolysis reactivity of the methylpyridine initial decomposition rate to release a cyano-radical to isomers [19], and this might be due to the resonance reduce the NPD response of 2-methylpyridine relative structure that is possible after ‘benzylic’ C–H dissocia- to that of 4-methylpyridine. tion (Fig. 3). A similar argument as was presented for the formation of the ortho-pyridyl radical to explain ini- tial decomposition that would result in a terminal cyano- The present study was not designed to explore the pyrolysis group, which was consistent with the decomposition chemistry of the nitrogen-containing compounds. Neverthe- products formed by pyridine decomposition [20]. The less, combining the results in Table 3 with literature observa- 2-, or 4-methylpyridyl radical does not have the same tions showed that a relationship between the pathway of initia- tion leading to initial decomposition and the response of the NPD was able to provide a plausible, but unproven explanation for the difference in NPD response. NCH - H - H • • - CH CH CH CH 3 2 2 • • NCH NCH 2 3 - H major N N N pathway N Fig. 2 Initiation of 2-methylpyridine decomposition at NPD condi- Fig. 3 Possible decomposition sequence responsible for the higher tions decomposition rate and higher NPD response of 3-methylpyridine 1 3 Applied Petrochemical Research (2021) 11:129–136 135 CH • 3 • • NCH N N N N H H Fig. 4 Side-reaction of the ortho-pyridyl radical formed by 2-methylpyridine decomposition that is assisted by the adjacent nitrogen to reduce initial decomposition rate and decrease the NPD response of 2-methylpyridine Relevance to petrochemical and other processes the detector response and concentration of nitrogen-contain- ing compounds. It was found that: In the previous section it was shown how NPD response could a. Despite the linearity of the NPD response to individual be related to pyrolysis chemistry of isomeric species. The same type of approach can be used to use the NPD response to assist nitrogen-containing compounds, the response factor of the NPD is different for different compounds and even with elucidating decomposition pathways of interest to petro- chemical transformations. structural isomers of the same species. Quantitative analysis using an NPD requires species-specific calibra - In the same way it could also be an indicator of likely emis- sion products during high temperature processing or destruc- tion. b. The NPD response can be related to the pyrolysis chem- tion of nitrogen-containing materials. The same pyrolytic decomposition that is involved in forming the cyano-radical istry leading to cyano-radical formation. In the same way the NPD response can be used to provide informa- also leads to the formation of hydrogen cyanide during com- bustion processes. For example, the nature of the nitrogen- tion of practical relevance beyond its analytical value. containing species in fuels with the same total nitrogen content affected the HCN yield during combustion [27], which is likely reflected in the NPD response of those materials too. Open Access This article is licensed under a Creative Commons Attri- Analyses using an NPD also have analytical uses beyond bution 4.0 International License, which permits use, sharing, adapta- those that are self-evident from its selective detection. It may tion, distribution and reproduction in any medium or format, as long be possible to obtain some indication of the compound class as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes composition of a complex material, such as a petroleum prod- were made. The images or other third party material in this article are uct, by relating the NPD response to the total nitrogen content included in the article’s Creative Commons licence, unless indicated by CHNS elemental analysis. For example, pyridines on aver- otherwise in a credit line to the material. If material is not included in age have a higher NPD response than anilines, which would the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will assist with differentiation between nitrogen bases. Similarly, it need to obtain permission directly from the copyright holder. To view a may be possible to differentiate between isomers that are diffi- copy of this licence, visit http://creativ ecommons .or g/licenses/b y/4.0/. cult to differentiate using mass spectrometry, because isomers of the same species may have different NPD response factors. 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Journal

Applied Petrochemical Research – Springer Journals

Published: Mar 6, 2021

Keywords: Thermionic detector; Nitrogen phosphorus detector (NPD); Cyano-radical; Pyridine pyrolysis

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