membranes for the separation of gases because of their ... tures and ensure the selective separation of permanent gases and hydrocarbons on their ...
ISSN 10619348, Journal of Analytical Chemistry, 2013, Vol. 68, No. 12, pp. 1044–1051. © Pleiades Publishing, Ltd., 2013. Original Russian Text © E.Yu. Yakovleva, I.K. Shundrina, T.A. Vaganova, 2013, published in Zhurnal Analiticheskoi Khimii, 2013, Vol. 68, No. 12, pp. 1171–1178.
ARTICLES
Specific Features of the Separation of Inorganic and Organic Compounds on Alumina Modified with Fluorinated Polyimide E. Yu. Yakovleva, I. K. Shundrina, and T. A. Vaganova Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, pr. Akad. Lavrent’eva 5, Novosibirsk, 630090 Russia Vorozhtsov Institute of Organic Chemistry, Siberian Branch, Russian Academy of Sciences, pr. Akad. Lavrent’eva 5, Novosibirsk, 630090 Russia Received July 5, 2012; in final form, February 19, 2013
Abstract—Highly fluorinated thermally stable polyimide is synthesized and used to modify alumina. The effects of polyimide concentration (10 and 15 wt %) and thermal treatment conditions on the adsorption and chromatographic properties of the prepared adsorbents are studied by the lowtemperature adsorption of nitrogen and gas chromatography on packed columns. The adsorbents prepared possess mesoporous struc tures and ensure the selective separation of permanent gases and hydrocarbons on their simultaneous pres ence. Keywords: gas chromatography, packed column, alumina, fluorinated polyimide (PI) modifier, inorganic and organic gases, hydrocarbons DOI: 10.1134/S1061934813100110
Gas chromatography is one of the best methods for the analysis of inorganic and organic gases and com plex mixtures of hydrocarbons and volatile organic compounds in the industry, catalytic processes, oil and gas prospecting, and ecology [1]. In the analysis of purely hydrocarbon mixtures, packed columns are obtained mainly using adsorbents with highly devel oped surfaces: silica and alumina, carbopacks, car bosieves, polymeric adsorbents (porapacks, chro mosorbs, hiseps, polysorbs, composite polymeric material poly(1trimethylsilyl1propyne)/poly(1 phenyl1propyne), etc. [2–9]. Capillary chromatog raphy on columns with stationary phases of different nature or on capillary columns with porous layers of alumina modified by potassium chloride [1, 12, 13] is also used. If the hydrocarbon mixture also includes permanent gases (hydrogen, oxygen, nitrogen, and carbon mono and dioxide), twocolumn systems (zeolites and porous polymeric adsorbents, silica, alu mina, etc.) or capillary columns with porous layers (zeolite 5A or Carboplot P7) can be used. However, the analysis is long; for example, on a column with Carboplot P7 oxygen is eluted in the second, methane in the 7th, and ethane in the 78th minute [1, 13].
and satisfactory gas permeability. One of the directions of regulating the selectivity and permeability of poly imide membranes is the introduction of fluorine atoms into their chains. Polyimides with high concentrations of fluorine atoms possess lower chain packing densities and, as consequence, higher transmittance factors, which increases the free volume in the polymer and allows one to control its hydrophily [14–16]. However, highly fluorinated polyimides are poorly available and no works were found in the literature on their use as chromatographic phases. Researchers of the Institute of Organic Chemistry of the Siberian Branch of the Russian Academy of Science in recent years have syn thesized highly flyorinated polyimides with various architecture of polymer chains [17–23].
Recently analysts have widely used polyimide membranes for the separation of gases because of their high thermal and chemical stability, high selectivity,
Materials. 2,7Diaminohexafluoronaphthalene (99 %) was synthesized according to the procedure [18]. 2,2Bis(3',4'dicarboxyphenyl)hexafluoropro
In this work we determined the conditions of the formation of a selective layer based on highly fluori nated polyimide applied onto alumina and studied the adsorption and chromatographic properties of the adsorbent prepared. EXPERIMENTAL
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pane dianhydride (99%) before use was dryed in vacuo at 140°C for 6 h.
Synthesis and properties of polyimide (PI) based on 2,7diaminohexafluoronaphthalene and 2,2bis(3',4' dicarboxyphenyl)hexafluoropropane dianhydride (see scheme). The cyclic solidphase chain growth method [21] was used.
NMethylpyrrolidone (MP) was purified by distil lation over P 2 O 5 and stored over 3E molecular sieves, residual moisture < 0.02%. F
O
CF3
O
+ O
CF3
O
F
H2N
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NH2
F
F F
O
O
F F
F
* F
O
CF 3
O
N
CF 3
N
F O F
*
O
PI
F
n
Scheme. Synthesis of the polyimide.
Ten milliliters of MP and equimolar amounts of 2,7diaminohexafluoronaphthalene (0.0004 moles) and 2,2bis(3',4'dicarboxyphenyl)hexafluoropro pane dianhydride (0.0004 moles) were sequentially placed in a flask with a magnetic stirrer and a system of noble gas (argon) admittance. The reaction mass was stirred for 24 h at 70°C to the disappearance of signals from the initial monomers in 19F NMR spectra; then the obtained solution of the polyamide acid was poured in a Petri dish and dried from the solvent at 70°C. The solidphase chain growth was performed in a vacuum heating cabinet by gradually rising tempera ture (90, 120, 150, 180, 250°C) and storing the reac tion mixture for 1 h at each temperature. The obtained polymer was dissolved in MP and two or three times dried according to the presented scheme. Instruments and equipment. 19F NMR spectra of 5% solutions of the polyimide in dimethylacetamide were recorded on a Bruker AV300 spectrometer (282.36 MHz) using C6F6 (δ = 0 ppm) as an internal standard. Fouriertransform infrared spectra were recorded on a Bruker Vector22 spectrometer using KBr disks. Thermogravimetry (TG) and differential scanning calorimetry (DSC) analyses were performed on a NETZSCH STA 409 synchronous TG/DSC analyzer in inert (He) and oxidative (He : О2 = 80 : 20, v/v) atmosphere at a heating rate of 10°C/min.
teristic for fivemembered imide cycles: 1797, 1739, 1347, and 718 cm–1. The molecular weight of PI was determined by the ratio of the integral intensities of signals from the inner diimide fragments and the terminal amine–imide fragments (according to the data of 19F NMR): n ~ 21.2, Mn ~ 15000 (the molecular weight of the struc tural unit C29H6N2F12O4 is 674). The stability of the polyimide obtained to heating to high temperatures in inert and oxidative atmo sphere was determined by the data of thermogravime try (Fig. 1). The temperatures of 5% weight loss were 506°C in the inert and 480°C in the oxidative atmo sphere. The glass transition point of the polymer was % 100 80
40 20
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The structure of PI was confirmed by 19F NMR spectra: δ 100.5 (C(CF3)2), 43.7 (F1,8), 35.6 (F1, terminal fragment), 25.9 (F4,5), and 16.4 (F3,6). The IR spectrum contains absorption bands charac JOURNAL OF ANALYTICAL CHEMISTRY
1
60
200
400
600
800
1000 T, °C
Fig. 1. Diagrams of thermogravimetric analysis for the polyimide in (1) inert and (2) oxidative aerosphere. No. 12
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YAKOVLEVA et al.
not detected before the start of polymer decomposi tion, which is typical for rigidchain polyimides. The sample of γAl2O3 selected for studies was of chemically pure grade (Sasol Limited, Republic of South Africa) and contained 0.015% Si, 0.015% Fe, 0.015% Ti, and 0.01% Na, (S sp. = 200–220 m2/g). Modified samples of alumina were prepared according to the standard procedure [13] and then impregnated in a PI solution in dimethylacetamide (10 or 15% of the adsorbent mass) within a day. The suspension was transferred to a porcelain cup, the solvent was removed at 90°C under stirring, and the residue was dried to a constant mass. The layers based on polyimide applied onto alumina were finally formed in a furnace. The adorbents γAl2O3 + 10 wt % PI and γAl2O3 + 15 wt % PI were put into a quartz reactor and conditioned at 250°C in an argon flow. Small portions of adsorbents were taken for the study of their adsorption properties. The adsorption studies of the initial aluminas, those modified by the fluorinated polyimide, and adsorbents thermally treated at 250°C were performed using isotherms of N2 adsorption–desorption at 77 K, which were measured on an ASAP2400 setup (Micromeritics, United States) after training samples under vacuum at 150°C. These isotherms were used to calculate the total accessible surface area by the method of Brunauer–Emmett–Teller (BET), specific surface, and the distribution of macropores by charac teristic sizes (by the desorption branch of the iso therm). The chromatographic properties of the adsorbents were studied in the separation of inorganic and organic gases and of saturated and unsaturated hydrocarbons. For this purpose, column I packed with γAl2O3 + 10 wt % PI and column II packed with γAl2O3 + 15 wt % PI were prepared from the remained major samples of adsorbents. The reference columns were packed with the known adsorbents: column III (γAl2O3 + 8 wt % NaHCO3), column IV (activated charcoal AG5), column V (polar polymer Porapak S), and a column with Silochrome C80. The prepared columns were 3 m × 2 mm. Studies of chromatographic properties were per formed on a Kristall2000 chromatograph (Izhevsk, Russia) with a flame ionization detector (FID) and a thermal conductivity detector (TCD). The thermostat temperature was maintained with an accuracy of ±0.5°C. In the work with FID, the flow rate of the car rier gas (nitrogen) was 30 mL/min; of hydrogen, 30 mL/min, and of air, of 300 mL/min. In the work with TCD, the flow rate of the carrier gas (helium) was 30 mL/min. The separation of compounds of different nature was attained in the isothermal mode or under the conditions of temperature programming. The chromatographic data were processed using the Netchrom software. Calibration mixtures on the basis of argon with cer tain concentrations of adsorbates were prepared in
metal cylinders of the volume 10 L using the volume manometric method. Cylinder 1 contained a mixture of С1–С2 hydrocarbon gases and cylinder 2, a mixture of C3 and C4 gases (10 vol % of each gas). Liquid and unsaturated hydrocarbons С5–С12 were mixed in sep arate penicillin vials without retaining certain concen trations of components. Model mixtures of gaseous hydrocarbons from gas cylinders and of liquid and unsaturated hydrocarbons from penicillin vials were sampled with a syringe and injected into the chromatograph. RESULTS AND DISCUSSION The surface of the prepared adsorbent samples was studied by the method of lowtemperature nitrogen adsorption followed by thermal desorption. The total specific surface area (SBET) calculated by the BET method, the sizes meso and micropores (Vmeso and Vmicro, respectively), and effective pore diameters in the desorption curve, DM, calculated by a comparative method, are listed in Table 1. The analysis of desorption curves has shown that the adsorbents γAl2O3 + 10 (15) wt % PI after treat ment at 250°C possessed more homogeneous surfaces; the effective pore diameter varied in the range 64.7– 70.0 Å; no micropores were found. An increase in the concentration of the applied fluorinated polyimide slightly reduced surface area and the size of mesopores (Table 1, Fig. 2). If we take into account the percent age of the applied modifier per one gram of the initial adsorbent, we can say that the surface of the initial and modified alumina virtually did not change and that a decrease in the size of mesopores with an increase in the amount of the modifier is indicative of the incom plete coating of the exterior adsorbent surface. The chromatographic properties of the obtained adsorbents substantially differ from the properties of both unmodified γAl2O3 and alumina modified with inorganic salts [13]. The chromatographic properties of the studied adsorbates are summarized in Table 2, columns I and II. Previously the separation of permanent gases was attained only on zeolites (CaA, NaX) and carbon molecular sieves (carboxenes, capbopacks) due to the molecularsieve effect typical for these adsorbents [24]. On alumina, silicon oxide, and porous polymers, permanent gases, except for carbon dioxide, are eluted as one peak. Carbon molecular sieves selectively sepa rate permanent gases and hydrocarbons С1–С4; heavy carboncontaining molecules are more strongly retained on their surface [1, 2, 13, 24]. On the sorbent γAl2O3 + 10 wt % PI, we could for the first time sep arate a mixture consisting of permanent gases (hydro gen, oxygen, nitrogen, carbon oxide), carbon dioxide, and hydrocarbons С1–С10 with reducing the time of analysis; this previously was not possible on the known stationary phases (Fig. 3).
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Table 1. Adsorption properties of adsorbent surfaces γAl2O3
Property SBET, m2/g
γAl2O3 + 10 wt % PI
γAl2O3 + 15 wt % PI
200.1
191.8
227
Vmicro + Vmeso, cm3/g (17⎯3000 Å)
0.54
DM, Å
0.45
77.52
SBET, m2/g*
227
Vmeso, cm3/g
0.41
66.5
64.7
220.1
220.47
0.52
0.50
0.47
* Taking into account the mass fraction of the modifier per one gram of initial γAl2O3.
It is important that, on the proposed adsorbent, one can separate hexane isomers; the peaks exhibit rather symmetrical shapes, the asymmetry factor is close to unity (Table 2, Fig. 3). The order of hydrocar bon elution in the core corresponds to their behavior on the initial unmodified alumina or on alumina mod
ified with inorganic salts [1, 13], though here again we have found a difference. Thus, if on alumina modified with NaHCO3 the components are eluted in the order ethane, ethylene, propane, propene, nbutane, acety lene, on alumina with 10 (15) wt % of polyimide, acet ylene is eluted after propene (Fig. 3). The column
Vpore, cm3/g 0.012
0.010
0.008
0.006
0.004
0.002
0 0
100
200
300
Pore diameter, Å Fig. 2. Curves of nitrogen desorption from the surface γAl2O3 + 10 wt % PI. JOURNAL OF ANALYTICAL CHEMISTRY
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Table 2. Physicochemical properties and retention times (min) of the studied compounds on columns packed with γAl2O3 with various modifiers Adsorbate
Mol. weight
Tb.p., °C
8 wt % NaHCO3 (Fas)
γAl2O3 + 10 wt % γAl2O3 + 15 wt % PI (Fas) PI (Fas)
H2
2
–252.7
0.91
0.813
0.633
O2
32
–182.9
0.91
0.933
0.703
N2
24
–210
0.91
0.933
0.703
CO
28
–192
0.91
1.023
0.753
CO2
44
–
5.093
3.713
CH4
16
–161.6
0.91
1.122
0.542
C2H2
26
–83.8
7.841
C2H4
28
–103.7
1.551
5.142
3.742
C2H6
30
–88.6
1.241
3.412
2.162
C3H6
42
–47.8
3.591
12.662
10.682
C3H8
44
–42.1
2.071
8.832
6.982
nC4H10
58
–0.5
4.461
15.922
13.972
C5H12
72
36.1
10.921
21.782
19.782
Cyclohexane
84
81
15.811
26.32 (0.9)
24.512
C6H14
86
68.7
17.261
27.332 (0.9)
25.312
1Hexene
84
63.4
19.111 (1.4)
30.22 (0.85)
29.692
C7H16
100
98.4
20.671 (1.5)
32.352 (1.0)
30.322
C8H18
114
125.7
25.651 (1.7)
36.772 (1.1)
34.742
C9H20
128
150.8
34.561 (1.9)
44.592 (1.2)
39.332
C10H22
142
174.1
38.761 (2.0)
57.902 (1.2)
48.572
1
–78.6 (0.52 MPa)
14.12
12.072
Storage at 100°C for 500 s then heating to 230°C at a rate of 11°C/min; 2 storage at 100°C for 500 s then heating to 240°C at a rate of 11°C/min; 3 30°C. Fas is peak asymmetry factor.
Table 3. Efficiency and selectivity of the studied columns γAl2O3 + 10 wt % PI
8 wt % NaHCO3
AG5
HETP, mm (for C3H6)
0.12
0.31
0.18
α (C2H2/C2H4)
3.07
10.67
*
α (C3H6/C3H8)
1.47
2.3
*
α (C10H22/C9H20)
1.3
1.12
*
Property
* Missing experimental data for some adsorbates are due to the impossibility of their elution from the column with AG5 in the studied tem perature range. JOURNAL OF ANALYTICAL CHEMISTRY
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SPECIFIC FEATURES OF THE SEPARATION
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(а) 2
6
5
9
8
10
11
3
4
7 1
0.65
1.03
1.42
2.60
5.21
7.82
(b)
1 8
10.43
13.03
tR, min
13
10 9
12
5 4
2 3
6 7 14
11
15
0.00
18.14
36.28
tR, min
Fig. 3. Chromatograms of mixtures of permanent gases and hydrocarbons on a column packed with γAl2O3 + 10 wt % PI: (a) 1, hydrogen; 2, oxygen + nitrogen; 3, carbon monoxide; 4, methane; 5, ethane; 6, ethylene; 7, CO2; 8, propane; 9, propene; 10, acetylene; 11, nbutane; (b) 1, methane; 2, ethane; 3, ethylene; 4, propane; 5, propene; 6, acetylene; 7, nbutane; 8, pentane; 9, cyclohexane; 10, hexane; 11, 1hexene; 12, heptane; 13, octane; 14, nonane; 15, decane on γAl2O3 + 10 wt % PI. Conditions of analysis: column kept at 32°C for 1 min, then heating to 240°C at a rate of 10°C/min. JOURNAL OF ANALYTICAL CHEMISTRY
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YAKOVLEVA et al. Fig. 4. Chromatogram of mixtures of permanent gases on columns packed with (a, b) active carbon AG5, (c) polar polymer Porapak S, and (d) Silochrome C80: (a) 1, hydrogen; 2, oxygen; 3, nitrogen; 4, carbon monox ide; 5, C1–C4 hydrocarbons; (b) 1, methane; 2, acetylene; 3, ethylene; 4, ethane; 5, propene; 6, propane; 7, nbutane; (c) 1, hydrogen + oxygen + nitrogen + CO + methane; 2, CO2; 3, ethylene; 4, acetylene; 5, ethane; 6, propene; 7, propane; 8, propadiene; 9, isobutane; 10, nbutane. Conditions of analysis: (a, b) storage at 32°C for 1 min then heating to 240°C at a rate of 10°C/min; (c) storage at 30°C for 100 s then heating to 190°C at a rate of 13°C/min; (d) isothermal mode 30°C.
(а)
3
4
2 5
1
0.0
01.10 02.20 03.30 04.40
05.50 tR, min
(b) 5
6
1 2 3 4 7
0
10
20
30
40
50
60 tR, min
(c)
REFERENCES
8
1 7 6
9 34 5 10 2
0.0
3.3
6.6
10.0
13.3
16.6 tR, min
(d) 1 4 5 7 2 3 6
0.0 00.97
01.94
02.91
packed with γAl2O3 + 10 wt % PI (Table 3) is charac terized by high performance and selectivity. The height equivalent to a theoretical plate (HETP) and the selectivity coefficients α were calculated by the equations [25] taking the time of hydrogen elution as to. The efficiency of a column with AG5 for the sep aration of a similar mixture was a little lower, 0.18 (Table 3); nbutane on this column was eluted only in the 64th minute, whereas on the column with γAl2O3 + 10 (15) wt % PI, it was eluted in the 17th minute. Mixtures of similar composition are separated on porous polymers of different polarity and on silicon oxides. However, permanent gases, except for СО2 and hydrogen on porous polymers, cannot be separated on these substrates (Figs. 4c and 4d).
03.88
04.85 tR, min
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