122
Energy & Fuels 1991,5, 122-125
chemical properties of zeolites, it seems reasonable to suggest that the acidic sites on the external surfaces of the mordenite crystals also made a contribution. On this basis the phenomena shown in Figure 3 are attributed to a combination of a physical effect (diffusion)and a chemical effect (acidic reaction sites). The catalyst surface acidity was measured by adsorbing benzofuran on the catalysts and then performing temperature-programmed desorption (TPD). Some TPD results are shown in Figure 9. The peak at 120 O C increased (and became a shoulder in the catalyst containing 20% H-mordenite) when more mordenite was present in the catalyst. The increase in the high-temperature peaks (120 and 130 "C) is taken as an increase in the number of acidic sites. Thus the improved HDM conversions caused by the mordenite component can be attributed partly to improved diffusion toward the catalyst surface and partly to the increased number of acid sites on the external surfaces of the mordenite crystals.
Figure 9. Benzofuran temperature-programmed ,.sorption. Change in weight per unit time (% of sample per minute) versus temperature ("C).The curves are labeled by H-mordenite content, 0%, lo%, and 20%,respectively. Peaks at 90,120, and 130 "C are indicated.
0.39 to 0.49 with only a modest increase in PTOF/B. In contrast, when it increases from 0.49 to 0.59, there is a substantially greater increase in PTOF/B. If the effective diffusivity was the only factor affecting the curves in Figure 8, they would be linear or perhaps have a concave downward shape. The preceding discussion indicates that the addition of mordenite had an influence on diffusion. However, the shape of the curve in Figure 8 suggests that diffusion is not the complete explanation. Based on the known
Conclusions The addition of hydrogen-mordenite to a Co-Mo/A1203 catalyst affected several geometric properties of the catalyst. The catalyst bulk density, extrudate particle density, and surface area measured by mercury porosimetry all decreased. However, the catalyst pore volume per unit surface area increased, suggesting an enhancement in the rate of diffusion of the large resid molecules toward the reaction sites on the catalyst surface area. The addition of hydrogen-mordenite caused an enhancement in HDM when measured either in terms of overall conversion or in terms of PTOF. This enhancement was attributed to two factors. A greater rate of diffusion to the reaction sites was caused by the increased catalyst pore volume. A greater rate of reaction on the catalyst surface was attributed to the increase in number of reaction sites, caused by the acidity of additional mordenite crystals' external surfaces. Registry No. Co, 7440-4&4;Mo, 7439-98-7; Ni, 7440-02-0; V, 7440-62-2.
Fractionation of Coal Tar Pitch with Iodine Masahide Sasaki,* Tetsuro Yokono, Masaaki Satou, and Yuzo Sanada Metals Research Institute, Faculty of Engineering, Hokkaido University, Sapporo 060, Japan Received August 6, 1990. Revised Manuscript Received September 24, 1990
Coal tar pitch was fractionated by charge-transfer fractionation (CTF) with iodine as an electron acceptor. Chemical analyses of each fraction were made by means of flame thermionic detector gas chromatography (FTD-GC), FID-GC, and GC/MS. Results of CTF would indicate that basic nitrogen compounds (pyridine type) were concentrated in the component and interact strongly with iodine. On the other hand, neutral nitrogen compounds (pyrrole type) were not concentrated by CTF. The fractionation behavior of the aromatic hydrocarbons has been discussed. Introduction Coal tar pitch consists of a variety of compounds with wide molecular weight distributions. The structure of coal tar pitch is principally aromatic in nature, consisting mostly of aromatic carbon, The ring positions are largely occupied by carbon atoms, though there are many heterocyclic rings which include nitrogen or oxygen atoms. In
order to upgrade the quality of pitch for use as a fine carbon, and to obtain the feedstock of fine chemicals, various physical and chemical separation methods have been applied to selective compounds. However, there is still a demand for an effective pitch separation technique. Recently, Zander et al. proposed the "charge-transfer fractionation (CTF)" of coal tar pitch with the use of picric
0887-0624/91/2505-0l22$02.50/00 1991 American Chemical Society
Energy & Fuels, Vol. 5, No. 1, 1991 123
Fractionation of Coal Tar Pitch with Iodine Table I. Elemental Analyses of Coal Tar Pitch and Fractionated Pitch O(diff), SP, C H N S % OC 1.42 91.1 PK-E, original 92.82 4.24 1.14 0.38 1.38 N.D. PK-E, CHClS0solubles 92.73 4.20 1.20 0.48 92.47 4.21 1.52 0.43 PK-E SAo#* 1.38 N.D. 92.24 3.43 2.48 0.44 1.42 N.D. Fr-Cc
PK-E CHCl,-S
I
100-265 O C 3 *C/mln
CHC&-free basis. PK-E SA 5 wt % of indole and carbazole are added. e CHC13- and iodine-free basis.
iodine (low%) I
sdution
prdipit ate
FPA
CHCIjim)
0
20
60
40
60
""k
" n
Figure 2. FID gas chromatogram of CHC13 soluble.
pnch moafi.d(CnCl,part)
with rddtlon ot Mole md
carbarole
F i g u r e 1. Separation scheme of coal tar pitch with iodine.
acid or elementary iodine as an electron acceptor.'-!' This fractionation technique yields pitch fractions with different degrees of thermal rea~tivity.~More recently, chargetransfer fractionation of pitch with regard to the chemical composition of the fractions obtained has been further investigated."' The authors revealed that resin was concentrated in components of iodine adducts! The lack of detailed analysis for chemical constitution of the fractions does not allow discussion of the mechanisms of fractionation. In this study, complexes formed with iodine were isolated into three fractions and their characterizations were carried out using flame thermionic detector gas chromatography (FTD-GC), FID-GC, and GC/MS. The difference of the chemical compositions of the fractions will be discussed in further detail. Experimental Section The elemental composition of the coal tar pitch used in this study is shown in Table I. The softening point of this coal tar pitch is 91.9 "C. Iodine, without purification, supplied by Wako Chem. Co. Ltd.,was used as the electron-acceptor reagent. Coal tar pitch was dissolved in chloroform. The soluble part of pitch in chloroform was about 60 wt % of the whole pitch. Coal tar pitch consists mainly of aromatic hydrocarbons and small amounts of nitrogen-containing compounds. Most of the nitrogen-containing compounds in pitch are basic nitrogen compounds (pyridine type). Neutral nitrogen compounds (pyrrole type) are lower in concentration than basic nitrogen compounds. In order (1)Zander, M. Fuel 1987,66, 1459. (2) Gemmenke, W.; Collin, G.; Glaser, H.; Marrett, R.; Zander, M. Proc. 3rd Int Conf. on Carbon 1980, 288. ( 3 ) Zander, M.; Palm, J. Erdoel Kohle Erdgas Petrochem. 1987,40, 408. (4) Stadehofer,J, W.; Gemmenke, W.; Zander, M. Light Metals 1983, 1211. (5) Yokono, T.; Imamura, T.; Sanada, Y. J. Jpn. Pet. Inst. 1987,30, 59. (6) Yokono, T.; Sasaki, M.; Sanada, Y. Ext. Abstr. 18th Bienn. Conf. Carbon, Worcester, MA 1987, 19, 7-24, 7. (7) Sato, M.; Matsui, Y.; Fujimoto, K. Ext. Abstr. 17th Bienn. Conf. Carbon, Lexington KY 1985, 19, 6-21, 6, 326.
to evaluate the neutral nitrogen compound effect on chargetransfer fractionation, 5 wt % of two types of neutral nitrogen compounds was added to the chloroform-soluble part of the coal tar pitch. In this study, we used indole and carbazole as neutral nitrogen compounds. The separation scheme with iodine is shown in Figure 1. Iodine was added to pitch sample (10 wt % relative to the sample). The chloroform solution of the pitch was first placed in an ultrasonic bath for 10 min. Afterwards, it was stirred and kept at room temperature for 1h. A precipitate was obtained. The precipitate of iodine complex was separated from the solution by filtration. The remaining solution was defined as Fr-A. The fractionated precipitate was dried in a vacuum oven at 60 OC for 1h. After which 10 mL of chloroform was added to remove contaminants from the mother liquid. We define the chloroform-soluble fraction as Fr-B and the precipitate as Fr-C. The resultant precipitate and the filtrate were treated with aqueous ammonia solution in order to remove iodine. The fractionated samples were analyzed with a Hitachi Model 163 gas chromatograph with an SE-52 fused silica capillary column (0.25 mm i.d. X 25 m). The temperature program used was an increase of 3 "C/min from 150 to 265 "C. Identification of components in each fraction was carried out with a Hitachi Model M-52 GC/MS system equipped with the same capillary column as the Hitachi gas chromatograph. Electron impact spectra were obtained a t an ionization energy level of 20 eV and then stored in and analyzed by a Hitachi 002B and M-003 data processing system. Most of the compounds were identified by comparison to standard mass spectra data, employing the retention index system proposed by Lee et al.,"'O and with standard compounds. Furthermore, we carried out an experiment to clarify the mechanism of separation by CTF using the model compounds of coal tar pitch. Anthracene, carbazole, and acridine without purification were used as model compounds of coal tar pitch. The model compound was dissolved in chloroform and iodine was added in the region of molar ratio 0.05-1.50. The solution was placed in an ultrasonic bath for 10 min and then stirred and kept at room temperature for 1 h. After stirring, the chloroform was evaporated and the iodine complex was thus obtained. Evaluation of the complexes were obtained by means of infrared spectroscopy. Diffuse reflectance FT--1R spectra were obtained with a Nicolet 20-SB infrared spectrometer, and 'H NMR spectra were obtained with a Varian XL-200 FT-NMR spectrometer.
Results and Discussion Elemental analyses of the original pitch and its fractions (8) Lee, M. L.; Vassilaros, D. L.; White, C. M.; Novotny, M. Anal. Chem. 1979,51,6, 768. (9) Vassilaros, D. L.; Kong, R. C.; Later, D. W.; Lee, M. L. J . Chromatogr. 1982, I , 252. (10) Satou, M.; Tanabe, K.; Uchino, H.; Yokoyama, S.; Sanada, Y. J. Chem. SOC.Jpn. 1984, 1984, 1964.
Sasaki et al.
124 Energy & Fuels, Vol. 5, No. 1, 1991
Fr-B
I
b) Fr-A
0
20
40
60
80
retatIan timr (mh)
Figure 4. FID gas chromatogram of fractionated pitch; Fr-B.
0
20
40
00
80
rotontlon tlmo (mln)
Figure 3. FID gas chromatograms of fractionated pitch (a) Fr-C, (b) Fr-A.
are displayed in Table 1. The Fr-C interacts strongly with iodine resulting in precipitation. It is clear that nitrogen content was concentrated in Fr-C from 1.20 to 2.48. This result suggests that nitrogen compounds in the pitch are concentrated in Fr-C. Figure 2 shows FID gas chromatogram in the chloroform soluble part in pitch. Fluoranthene, pyrene and phenanthrene are major components, while five or six rings of aromatic hydrocarbons and nitrogen compounds are minor components in the fraction. Figure 3 shows FID gas chromatograms of the fractionated pitches. The peaks marked with an arrow in Figure 3 correspond to nitrogen-containing compounds which are confirmed by means of FTD gas chromatography. The majority of nitrogen-containing compounds in Fr-C are basic nitrogen compounds, although basic nitrogen compounds are rarely to found in the pitch sample. It is easy to detect many basic nitrogen compounds, for example, phenanthridine, azapyrene, and azachrysene. This means that basic nitrogen compounds are concentrated in Fr-C. On the other hand, for Fr-A, the pattern of the chromatogram is similar to that of the parent pitch sample. The peak intensities of neutral nitrogen compounds were almost unchanged. Neutral nitrogen compounds such as indole and carbazole were not concentrated as precipitate by the addition of iodine. In other words, interaction is weak between neutral nitrogen compounds and iodine. Figure 4 shows the FID gas chromatogram of Fr-B. If the components of Fr-B were due to contaminants from those in the mother liquid, the pattern of chromatogram
might be expected to be the same as that of Fr-A. However, the distribution pattern differs clearly from both those of Fr-A and Fr-C. This result indicates that the components in Fr-B are not only due to the contaminants from the mother liquid. Let us look more closely at the compounds in Fr-B. The component in Fr-B consists mainly of polyaromatic hydrocarbons. Comparing the chromatogram of Fr-A with that of Fr-B, the components of Fr-B were concentrated aromatic hydrocarbons of five or six rings, for example, benzo[a]pyrene and perylene. Interaction of the components in Fr-B with iodine is presumably between in that of Fr-C and Fr-A. From the results mentioned above, the compounds in chloroform solubles in coal tar pitch can be separated into three fractions by difference of the interaction with iodine. 1. The compounds which interact strongly with iodine are basic nitrogen compounds such as azapyrene and azachrysene. 2. The compounds which interact moderately with iodine are five- or six-ring aromatic hydrocarbons such as benzo[a]pyrene and perylene. 3. The compounds which interact weakly with iodine are aromatic hydrocarbons with less than four rings, and neutral nitrogen compounds, for example, pyrene, phenanthrene, indole, and carbazole. As a general rule, the electron affinity of the acceptor molecule is constant. The formation of the charge-transfer complex of the aromatic hydrocarbon depends on the ionization potential of the electron donor molecules. In other words, the smaller the ionization potential of donor molecule, the easier the charge-transfer complex is formed. Ionization potentials for the selected compounds were calculated by the Huckel method. Table I1 shows the results of Huckel calculation. First, in the case of polyaromatic hydrocarbon, the larger the aromatic ring is, the smaller its ionization potential. The relation between the charge-transfer complex and ionization potential of electron donor molecules has been supported by experimental data of Fr-B. The degree of interaction of the aromatic hydrocarbons studied with iodine could be explained by the ionization potential of the donor molecules. However, in the case of nitrogen compounds, the degree of interaction can not be explained by its ionization potential. The strong interaction of basic nitrogen compounds with iodine is considered to be due to unpaired electrons attached to the nitrogen atom, while neutral nitrogen compounds have no such unpaired electrons.
Fractionation of Coal Tar Pitch with Iodine
Energy & Fuels, Vol. 5, No. 1, 1991 125
Table 11. Results of Biickel Calculation compound
structure
ionizn potential, eV
benzene
9.550
naphthalene
8.599
phenanthrene
8.567
lo
r------
O.o/ 8.0
mi--0:
4.0
A: phenanthrldlns
8.168
0
0
8.355
1.o
2.0
3 .O
mol ratio
Figure 5. Relation between molar ratio and the quantity of shift in FT-IR spectra: (m) acridine, (a)phenazine, (A) phenanthridme.
0
200 MHz
7.984
benzo[a]pyrene \
perylene
phanazlne
\
0
@)
7.925
0
7.784
dibenzo[def,mno]chrysene \
to46 0.00
\
indole carbazole acridine
9.129 8.814 8.340
Table 111. Variation of FT-IRSpectrum for Acridine/Iodine Complex acridine/iodine (molar ratio) u,O cm-' shift, cm-' acridine alone 1268.1 1:0.05 1267.7 -0.4 1:O.lO 1267.9 -0.2 1:0.25 1261.1 -7.0 1:0.50 1261.5 -6.6 1:l.W 1260.9 -7.2 1:1.50 1260.8 -7.3 a
Aromatic C-N stretching vibration.
From the results of the IR measurement of iodine complexes, it was proven that aromatic C-N stretching vibration of basic nitrogen compound shifts to shorter wave number due to the formation of the complex. Table I11 displays the result of IR measurements for the acridineiodine complex. In the case of molar ratios 1:0.25 and above, the shift of aromatic C-N stretching vibration is observed clearly. From the result, it is considered that unpaired electrons of basic nitrogen compound play an important role in formation of the iodine complex. Even other basic nitrogen compounds observed a similar tendency of their IR spectra. The relationship between the degree of shift and the molar ratio can be depicted as Figure 5. The degree of shift is converted to energy by equation of E = hu. A similar result is observed in measurements of 'H NMR for acridine-iodine complex. From Figure 6, it is clear that the proton nearest the nitrogen atom in the acridine molecule is shifted to lower a magnetic field. The other protons on the acridine molecule do not
0.20
0.40
0.60
0.80
1.00
mol rallo
Figure 6. Variation of chemical shift in acridine/iodine system.
shift. In the case of carbazole and anthracene, the shift of its characteristic absorbance was not observed accompanying the formation of the iodine complex. In general, it was found that benzene-bromine complex is a sandwich structure in which the two components lie parallel to each other." The sandwich structure of complex influences in each atom in components is very small. It is proposed that the complex of neutral nitrogen compound-iodine and that of aromatic hydrocarbon-iodine are a sandwich structure. While, the most attractive model for basic nitrogen compound-iodine complex is a edgeto-edge model12 in which an iodine molecule attacks directly binding sight of nitrogen atom. This structure for the complexes is consistent with the results of Uchida12 who found a similar structure for the phenazine-iodine complex in crystalline using X-ray methods. From these results, it is concluded that the complex structure of basic nitrogen compounds differs from that of neutral nitrogen compounds and aromatic hydrocarbon. The difference of complex structure is affected by mechanism of separation by the CTF method. This technique has potential for the isolation and concentration of basic nitrogen compounds in tar pitch. The next investigation is going to clarify the mechanism of separation by CTF using the model compounds of coal tar pitch. Registry No. I*, 7553-56-2. (11) Foster, R. Organic Charge-Transfer Complexes; Academic Presa: New York, 1969; p 216. (12)Uchida, T.Bull. Chem. SOC.Jpn. 1967, 40, 2244.
