ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2018, Vol. 92, No. 5, pp. 1018–1024. © Pleiades Publishing, Ltd., 2018. Original Russian Text © E.Yu. Yakovleva, Yu.V. Patrushev, Z.P. Pai, 2018, published in Zhurnal Fizicheskoi Khimii, 2018, Vol. 92, No. 5, pp. 824–830.
PHYSICAL CHEMISTRY OF SEPARATION PROCESSES. CHROMATOGRAPHY
Capillary Columns with a Sorbent Based on Functionalized Poly(1-Trimethylsilyl-1-Propyne) for the Elution Analysis of Natural Gas E. Yu. Yakovlevaa,b,*, Yu. V. Patrusheva,b,**, and Z. P. Paia,*** a
Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia b Novosibirsk State University, Novosibirsk, 630090 Russia *e-mail:
[email protected] **e-mail:
[email protected] ***e-mail:
[email protected] Received July 14, 2017
Abstract—The chromatographic properties of capillary columns prepared using functionalized poly(1trimethylsilyl-1-propyne) (PTMSP) are evaluated and compared with the performance of a commercial column with divinylbenzene polymer sorbent. The loading capacity of a PTMSP column with dimensions of 30 m × 0.53 mm × 0.8 μm is shown to be about 2.5 times higher than that of a divinylbenzene polymer column with a diameter of 0.32 mm and a film thickness of 10 μm. The increased value of the background current for PTMSP columns at 220°C is explained by the presence of non-polar bulky substituents in the polymer chain. Differences in the order of elution are found for the following pairs of compounds: acetylene-ethylene; ethane–water; butene-1–isobutane; and sulfur dioxide–carbonyl sulfide. On a column with the functionalized PTMC, analysis of a mixture composition close to natural gas is found to be complete within 27 min. Keywords: functionalized poly(1-trimethylsilyl-1-propyne), hydrocarbons, inorganic sulfur-containing gases, mercaptans DOI: 10.1134/S0036024418050357
INTRODUCTION Natural gas is one of the most important resources used both as fuel and raw material for the chemical industry. There are no identical samples; in every deposit, the gas composition is unique, since it is formed from different organic residues and under different reaction conditions. The main component of natural gas is methane (98%). In addition, the accompanying components listed below are usually present in natural gas: hydrogen, helium, nitrogen, ethane, propane, butane, hydrogen sulfide, and carbon dioxide. In some cases, natural gas also contains hydrogen sulfide and other sulfur compounds. In addition, some substances with a characteristic odor are added to natural gas (odoration) to prevent leakage. Organosulfur compounds (mercaptans) are usually used for this purpose. Natural gas can also contain negligible amounts of water and methanol [1, 2]. Gas chromatography is the main way of determining the component composition of natural gas. According to both GOST and ASTM standards, natu-
ral gas is currently analyzed using columns packed with silica based sorbents and porous polymers [3–6]. In this work, the separating ability of the capillary columns with a sorbent based on functionalized PTMSP with respect to a mixture with the composition similar to natural gas was assessed. The loading capacity and thermal stability of the porous layer were determined and the selectivity of separation was studied. The chromatographic properties of the column packed with PTMSP were compared with those of a commercial Rt-Q-BOND column. EXPERIMENTAL PTMSP synthesized in our laboratory [7] and nitrous oxide of medical grade (OAO Cherepovetsky Azot) were used. The PTMSP was functionalized via oxidative treatment with N2O as described in [8]. The reaction was conducted in a high-pressure steel reactor (Parr Instruments) with a volume of 100 cm3, and equipped with a pressure gauge and a stirrer; 0.15 g of the polymer and 60 cm3 of toluene as a solvent were placed in the reactor. The reactor was purged
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0.32 mm × 10 μm with divinylbenzene polymer sorbent was used for comparison. This choice is explained by the similarity of chromatographic properties of divinylbenzene polymer as a nonpolar stationary phase and those of PTMSP [8, 10–12].
N/Nmax 1.0 0.8
1
3
2
0.6 0.4 0.2
0
1
2
3
4 logm [ng]
Fig. 1. Column efficiency as a function of the amount of a sample: (1) column I, (2) column II, (3) column III.
with helium, and 0.25 mol of nitrous oxide was added. The reaction was conducted over 12 h at 220°C [8]. Column Preparation A solution of the functionalized PTMSP in toluene was prepared by dissolving 0.05 g of the polymer in 10 mL of toluene. The sorbent was applied to the inner surface of the capillaries by static high-pressure means [9]. The technique for preparing the columns was described in [8]. The following columns were prepared using the functionalized PTMSP: column I 30 m × 0.32 mm × 0.5 μm, column II 30 m × 0.53 mm × 0.8 μm. An Rt-Q-BOND capillary column (Restek, United States, column III) with dimensions of 30 m × Table 1. Composition of calibration mixture of gases (х is the molar fraction of a component) Component
1019
х, %
H 2S
0.0120
CH3SH
0.0130
C2H5SH
0.0123
i-C3H7SH
0.0135
C3H7SH
0.0121
i-C4H9SH
0.0111
C4H9SH
0.0126
He
Diluent
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Determining Loading Capacity To study column overload, column efficiency with different amount of samples was determined as described in [13, 14]. For columns I, II, and III, measurements were made using ethylbenzene as the standard (NPO Ekros, St. Petersburg) and n-hexane as a solvent (99.9% Sigma-Aldrich). Before measurements, absolute calibration of the flame ionization detector was performed using a Kristall 2000M chromatograph (Izhevsk) at temperatures of the injector and detector of 250 and 230°C, respectively; nitrogen was used as a carrier gas (Fig. 1). The concentration of ethylbenzene in n-hexane solutions was varied. The solvent was selected such that there was no overlapping of the peaks of the solvent and standard. Model Mixture Preparation Chromatographic characteristics were studied using a model mixture with a composition close to that of natural gas [3]. The mixture contained individual compounds (C1–C4, carbon dioxide, carbonyl sulfide, sulfur dioxide, and carbon disulfide) and a calibration gas mixture (Table 1; OOO PGS-Service, Zarechnyi, Sverdlovsk oblast). Recording Chromatograms The test mixtures were separated on columns I, II, and III using an Agilent 7890 chromatograph equipped with a thermal conductivity detector (microTCD), and on a Kristall 2000M chromatograph equipped with a flame ionization detector (FID). Chromatography of the mixture components was performed under conditions of temperature programming; the temperatures of the injector and detector were 250 and 230°C, respectively; the carrier gas was nitrogen (FID) or helium (micro-TCD). The temperature of the column oven was maintained with an accuracy of ±0.5 K. ChemStation (Agilent) and NetChrom (Metachrome) software was used for processing the chromatographic data. RESULTS AND DISCUSSION For successful separation of the components of mixtures with compositions close to natural gas, we must determine the amount of substance introduced into columns I, II, and comparison column III corresponding to the onset of separating efficiency loss. A Gaussian shape of the peaks is known to be observed
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on chromatograms when using small amounts of sample. An increase in the amount of substance distorts the peak shape and reduces the efficiency of separation. The loading capacity of the columns must therefore be estimated after absolute calibration of the detector (FID). In the calibration solutions, ethylbenzene concentrations lay in the range of 1–10000 ng/μg. A sample of 1 μL was taken from the prepared calibration solutions with a syringe. The sample was introduced into the injector in the splitless mode. The FID was calibrated using column I. The loading characteristics of the three columns were then studied under calibration conditions. Ethylbenzene samples were added to each column, and the efficiency of the chromatographic peak as a function of the amount of sample was monitored. Maximum efficiency Nmax of a column loaded with the minimum amount of sample, and efficiency N of a column with different amounts of sample, were calculated to estimate the loading capacity. Since the absolute efficiencies obtained for the columns differed, N/Nmax was calculated to compare the results. A column was considered to be overloaded if its efficiency fell by 20% [14]. In Fig. 1, N/Nmax as a function of the amount of sample is shown for columns I and II with functionalized PTMSP films of different thickness, and for column III with divinylbenzene polymer. The data on the maximum loading capacity are given in Table 2. These data clearly show that k grows along with the thickness of the stationary phase films and the inner diameter of the columns. Comparison of the loading properties of the PTMSP columns of different diameters showed k for column II (d = 0.53 mm) to be 6 times higher than for column I (d = 0.32 mm), due to the large volume of the stationary phase in the first case. The volume ratio of the stationary phases in columns II and I was 2.65 (Table 2), this value for columns III and II being 12.5. We would expect the loading capacity of column III to be much higher. However, the loading capacity of column III given in Table 2 is about 2.5 times lower than that of the column II. We may assume that a porous space is much less accessible in divinylbenzene polymer layer than in a PTMSP film. The loading capacity of column II with the functionalized PTMSP was therefore much higher, despite its thinner sorbent layer. Thermal stability is an important characteristic of a chromatographic column with a liquid or solid stationary phase. The thermal stability of a column can be quantitatively estimated from the background signal of a detector at a certain column temperature. When using the FID, the background signal is a current that quantifies the thermal degradation of the stationary phase due to the volatile products entering the detector. The temperature dependence of the FID
Table 2. Loading capacity (k) of the capillary columns Т, °С
d, mm
h, μm
Vrel
k, ng
I
200
0.32
0.5
1.00
9
II
140
0.53
0.8
2.65
54
III
200
0.32
10.0
Column
20.0
22
d is column diameter, h is film thickness, and Vrel is the relative volume of the stationary phase in the column, calculated from the capillary diameter and the thickness of the sorbent layer.
background current allows the mode of column operation to be selected. The temperature dependence of the detector’s background current was studied to estimate the thermal stability of stationary phases (functionalized PTMSP and porous divinylbenzene polymer). Both physical and chemical changes in column properties occur during its operation. Physical changes in the column properties are due to the removal of the stationary phase. Chemical changes can be caused by the interaction between the stationary phase and the column material or impurities in the carrier gas. For the chemical transformation of the stationary phase to be avoided, the carrier gas must not contain oxidizers, and the column walls must not exhibit catalytic activity. In [15, 16] a substantial increase in the background noise of the detector upon raising the column temperature was stated to be one of the main disadvantages of porous layered columns with divinylbenzenebased sorbents. This is because at 270°С, the hightemperature degradation of the porous polymer is accompanied by the formation of gaseous compounds. The products of degradation enter the detector, and the background current rises. The value of the background current depends on the nature of the stationary phase, the presence of polar substituents, the volume of the substituents in the polymer chain, the film thickness and the column diameter, and so on. The temperature dependence of the background current was studied while conditioning the columns. The thermal stability of the divinylbenzene polymer column was shown to be higher than that of the columns packed with PTMSP/N2O (Fig. 2). We can see that with the divinylbenzene polymer column, raising the temperature from 50 to 220°C at a heating rate of 15 K/min brings the background current from 2.0 to 9 pA. Under the same conditions, the increase in the background current is 26 and 32 pA for columns I and II with functionalized PTMSP (Fig. 2), respectively. The higher values of the background current observed in this case were due to the chemical nature of PTMSP.
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40
250
2
30
200
150 20
T, °C
I, pA
1
100 10
3 50
0 0
5
10
15
20
25
t, min Fig. 2. Background current of the detector (FID) for our capillary columns: (1) I, (2) II, (3) III. The dashed line shows the temperature gradient. Temperature: 50°C, 15 K/min–220°C. The carrier gas was helium.
The glass transition temperature is one of the most important characteristics of a polymer, and limits its areas of application. It is known to fall with the fraction of polar groups. An increase in the volume of side substituents (i.e., the looseness of the chain) also results in lower glass transition temperatures [17]. The background current for all of the studied columns was approximately the same up to 200°C. PTMSP is a linear polymer with bulky nonpolar trimethylsilyl substituents [18], so at temperatures above 200°C, the PTMSP structure undergoes transformations. In contrast, divinylbenzene-based sorbent is a crosslinked nonpolar polymer with a three-dimensional structure and rigid framework. It was shown in [9, 19] that the maximum operating temperature for a column with PTMSP is 220°C. In earlier studies, carbon- and sulfur-containing gases were shown to be selectively separated on columns packed with a diatomite carrier modified by PTMSP [20]. On columns packed with Hayesep Q or Porapak Q, the elution of sulfur dioxide was observed as a shoulder at the unseparated peak attributed to propane and propylene [21]. Optimum conditions for the chromatographic separation of hydrocarbon- and sulfur-containing compounds on columns I and III were determined using the data on the loading capacity and thermal stability. The concentrations of mercaptans in the test mixture of gases were low (Table 1). The mixture of hydrocarbons and mercaptans was analyzed using the FID, and the micro-TCD was used for the determination of hydrocarbons, carbon- and sulfur-containing gases, and carbon disulfide. RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
Chromatograms of the test mixtures, obtained on columns with functionalized PTMSP and with divinylbenzene polymer, are shown in Figs. 3 and 4. Retention times of the mixtures’ components are given in Table 3. It should be noted that the order of the components at the column I output differs from the one at column III. In first type of columns, acetylene is eluted before ethylene, sulfur dioxide is eluted before carbonyl sulfide, and butene-1 is eluted before isobutane. The reverse order was observed for column III (Figs. 3a, 3b). On column I, the components eluted according to their molecular sizes, since PTMSP acts as a molecular sieve [7–12]. On column III, the order of the compounds’ elution was determined by their boiling points (Table 3). Each type of column had certain advantages and disadvantages. Peak resolution Rs [22] for pairs of compounds are given below: Pair of compounds
Column I Column III
acetylene/ethylene
1
0.45
ethane/water
0
8.5
butene-1/isobutane
0.45
2
carbonyl sulfide/sulfur dioxide
4
6
The peak resolution for acetylene and ethylene on column III was 0.45, which is insufficient for quantitative determination of the separated compounds. Under the same chromatographic conditions, on column I Rs = 1 for acetylene/ethylene peaks. On the column with PTMSP, a single peak is observed for a
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3 1 2
(a)
5
7
(b)
4 3
1.8
1
6
2.0
2.2
2.4 6
10
2-7
2 11 12
15 14
3
4
5 6 tsp, min
9
10
7
12 13
11
17 13
8 9
2
8 457
14
16
8
9
0
5
10 tsp, min
16
15
15
20
Fig. 3. Chromatograms of carbon- and sulfur-containing components in (a) column I with functionalized PTMSP and (b) commercial column III with divinylbenzene polymer: (1) air, (2) methane, (3) carbon dioxide, (4) acetylene, (5) ethylene, (6) hydrogen sulfide, (7) ethane, (8) sulfur dioxide, (9) carbonyl sulfide, (10) propylene, (11) propane, (12) 1,3-butadiene, (13) butene-1, (14) isobutane, (15) n-butane, (16) carbon disulfide, (17) water.
5 23
(a)
2 3
(b)
4
1
1
6
14 3 2
0.2
0.4
0.6
0.8
1.0
1
16
0.5 1.0 1.5 2.0
13 12
5
4
1-3
15
6 7
8 9 7
0
4
11 10
2
4
13
12
6 tsp, min
8
14
15 16
10
0
5
9 8 11 10
10 tsp, min
15
Fig. 4. Chromatograms of light hydrocarbons С1–С4 and the calibration mixture of mercaptans in (a) column I with functionalized PTMSP and (b) commercial column III with divinylbenzene polymer: (1) methane, (2) acetylene, (3) ethylene, (4) ethane, (5) propylene, (6) propane, (7) methyl mercaptan, (8) 1,3-butadiene, (9) butene-1, (10) isobutane, (11) butane, (12) ethyl mercaptan, (13) isopropyl mercaptan, (14) propyl mercaptan, (15) isobutyl mercaptan, (16) butyl mercaptan.
water/ethane mixture. This could be due the close diameters of these molecules (3.0 and 2.99 Å, respectively). Incomplete separation was also observed for butene-1 and isobutane (Fig. 3a). The low selectivity for water/ethane and butene-1/isobutane mixtures on column I can be explained by the functionalized PTMSP film (0.5 μm) on the walls of the capillary column being too thin. On the column with divinylbenzene-based sorbent (sorbent layer thickness, 10 μm), the water peak with an extended tail is clearly separated from hydrogen sulfide and sulfur dioxide (Fig. 3b). It should be noted that on column I, the
peaks were symmetric for most components, in contrast to those observed using column III (Figs. 3a, 3b). The mixture of light hydrocarbons С1–С4 and the calibration mixture of mercaptans were selectively separated under chromatographic conditions on columns I and III using the FID (Table 3). The rate of separation for this mixture was 1.5 times higher on the column with PTMSP (Fig. 4). CONCLUSIONS The loading capacity of the capillary columns prepared using functionalized PTMSP was deter-
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Table 3. Physicochemical properties and chromatographic parameters of the analyzed components No.
Components
М
tsp(I), min
Тb.p, °С
tsp(III), min
FID
TCD
FID
DTP3
1
Air
28.98
–192
—
1.91
—
2.08
2
Methane
16.0
–161.6*
1.63
1.94
1.85
2.25
3
Carbon dioxide
44.0
–78.6*
—
2.05
—
2.74
4
Acetylene
26
–83.8*
1.70
2.08
3.33
3.70
5
Ethylene
28
–103.7
1.77
2.16
3.19
3.62
6
Hydrogen sulfide
34
–60.7
—
2.23
—
5.27
7
Ethane
30
–88.6
1.90
2.31
4.03
4.29
8
Water
18
100
—
2.31
—
6.52
9
Sulfur dioxide
64
–10.0
—
2.53
—
10.95
10
Carbonyl sulfide
60
–50.2
—
2.79
—
7.17
11
Propylene
42
–47.8
2.84
3.27
14.81
8.89
12
Propane
44
–42.1
3.35
3.70
15.87
9.44
13
1,3-Butadiene
54
–4.4
7.56
6.02
23.53
14.93
14
Butene-1
56
–6.9
8.56
6.40
23.74
15.10
15
Isobutane
58
–11.7
8.97
6.56
22.9
14.39
16
n-Butane
58
–0.5
11.49
7.03
24.23
15.55
17
Carbon disulfide
76
46.0
—
8.15
—
18.96
18
Methyl mercaptan
48
6.0
3.94
21.32
19
Ethyl mercaptan
62
37.0
12.84
27.54
20
Isopropyl mercaptan
76.15
52.5
19.52
31.4
21
Propyl mercaptan
76.15
68.0
21.15
32.92
22
Isobutyl mercaptan
90.18
85.1
25.57
37.14
23
Butyl mercaptan
90.18
98.0
26.37
38.55
tr is retention time, М is molecular weight; asterisks indicate temperatures of sublimation. Conditions of chromatography using the FID (Crystal 2000). The column was kept at 40°C for 11 min, then heated to 200оС at a rate of 7 K/min and kept at 200°С for 10 min. With micro-TCD (Agilent 7890), the column was kept at 35°C for 2 min, then heated to 200°С at a rate of 7 K/min. The carrier gas was helium at a pressure of 0.6 bar.
mined. The limiting load was shown to grow along with the film thickness and internal diameter of the columns. The thermal stability of a porous layer of the capillary columns was studied. At temperatures of up to 200°C, the background current for the columns with PTMSP and divinylbenzene polymer were found to be very close. A capillary column with functionalized PTMSP was shown to be suitable for separating mixtures with compositions close to that of natural gas, the analysis being 1.5 times less time consuming than on a commercial Rt-Q-BOND column. ACKNOWLEDGMENTS This work was conducted within the framework of budget project AAAA-A17-117041710081-1. RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
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Translated by D. Yakusheva
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