Energy & Fuels 1991,5, 934-936
934 I
1
1.2r
at a much higher temperature. Char A also contains 13 wt 90 ' oxygen. This char was prepared by pyrolyzing Highvale coal at 600 O C . P. L. Silveston Department of Chemical Engineering University of Waterloo Waterloo, Ontario, Canada Received May 24, 1991 Revised Manuscript Received August 22, 1991
z
5) V 3
0.2 6.0
200
300 400 500 TEMPERATURE ( ' C )
600
Figure 1.
a 1.0 w I-
0.8
Preparation of Deuterium-Labeled Petroleum Residuum and Asphaltene
0 w07Lr 1 x 7
> 0.6 0
z 2 z 0
L
2
0.4
0.2 0.0 200
300 400 500 TEMPERATURE ( " C )
600
Figure 2.
Miura et al. used the results presented in Figures 5-7 from their paper to conclude that the two-step process occurs in gasification rather than pyrolysis. These figures show that demineralizing the pyrolysis char eliminates two-step gasification. Another explanation of these figures is possible. In my proposal, flash pyrolysis leads to the formation of a highly active carbon which anneals to a lower activity material on extended exposure to pyrolysis temperatures above 700-800 "C. Figure 5 may be explained by rapid gasification of this "reactive" carbon at the beginning of the TPR measurement if it is assumed that this gasification is catalyzed by the minerals in the char. Thus, when the minerals are removed, gasification of the "reactive" material is slower and the two-step phenomenon vanishes. In Figures 6 and 7, demineralization seemed to remove the two-step behavior but had only a small influence on the conversion-temperature profile in the TPR measurement. In my view, the samples used showed little two-step behavior so that I believe these experiments are inconclusive. I agree with Miura et al. that two-step gasification results from carbon heterogeneity in a char sample. However, I believe that this is due to the "reactive" carbon arising in flash pyrolysis rather than "unreactive" carbon created during gasification. For the subbituminous Forestburg coal, char contact times less than 0.5 s at pyrolysis temperatures greater than 700 O C or pyrolysis temperatures of 600 "C or less lead to the formation of a "reactive" char whose gasification is catalyzed by the char mineral matter. As the contact time increases or if the char is reheated at a higher temperature, the char "anneals" and becomes less reactive. This is a slow, activated process and two-step gasification appears only when "annealing" is incomplete. Perhaps the "reactive" carbon expresses the volatile matter or oxygen content, or perhaps the internal structure as seen in pore volume or surface area. "Annealing" may represent volatilization and/or structural rearrangement of a char sample. There is some support for my hypothesis in data presented by Miura et al. Char A which shows strong two-step gasification has 20.3 w t % volatile matter vs 11.5 and 12.2 for chars B and C which show the second step
Sir: In coal conversion technologies, for instance, in coprocessing, it is sometimes desirable to trace the reaction pathway of a particular reaction component, such as an asphaltene. For example, one may want to establish whether fragments of a particular component of a resid have been incorporated into another component of the system such as a coal molecule. By isotopically labeling the component, and by subsequently analyzing the producta of the reaction by appropriate spectroscopic methods, knowledge can be obtained about the complex reaction systems. Isotopic labels provide one method to accomplish this goal, but no selective methods have been elaborated for the deuteration of coal molecules or resids. We have therefore investigated methods of preparing deuteriumlabeled resids and report the results of base and acidcatalyzed hydrogen4euterium exchange reactions of whole Lloydminster resid and the asphaltene' derived from it. Base-Catalyzed Labeling. Base-catalyzed deuterium exchange was carried out by refluxing resid and asphaltene with Lochmann's base (a combination of potassium tertbutoxide and n-butyllithium)2in hexane and subsequently quenching the reaction mixtures with deuterium oxide. The degree of deuteration was measured by comparing the areas of 2HNMR signals of the resid with the signal of an internal standard. Typical results are shown in Table I. Table I. Base-CatalyzedDeuterium Labeling Experiments" degree of deuteration, mmol of D/z benzyl aliphatic I -
expt. no. 1 2
3
0.17 M t-BuO-
material; concn, g/L resid; 8.9 resid; 17.8
0.12 M n-butyllithium 0.4 M t-BuO-
resid; 18
1.35
asphaltene; 24 asphaltene; 19
0.072
base 0.44 M t-BuO-
D
D
0.045 0.72
0.16
0.86
0.36 M n-butyllithium 4
5
0.55 M LDAb 0.20 M t-BuO0.15 M n-butyllithium
1.0
0.64
The materials and bases were mixed and refluxed in hexane for 8-12 h before being quenched with DzO. bLithium diisopropyl-
amide. This experiment was carried out in tetrahydrofuran at 25 "C. (1) Lloydminster
resid and asphaltene were supplied by John Catsis
of Universal Oil Products, Inc. The microelemental analyses of these
materials are reported in the following reference: Ceylan, K.; Stock,L. M. Energy Fueis. 1991,5,482. (2)Bates, R. B.;Siahaan, T. J. J . Org. Chem. 1986, 51, 1432.
0887-0624/91/2505-0934$02.50/00 1991 American Chemical Society
Energy & Fuels, Vol. 5, No. 6, 1991 935
Communications
Table 11. Acid-Catalyzed Labeling Experimentsa expt. no. 1 2 3
acid 3.5 M CH3COOD 2.6 M CFSCOOD 0.75 M CF3SO3D
resid concn, g/L 53 52 78
4
1.12 M CF3SOaD
75
time, h 24 24 0.5. 2 24 0.5 2 36
degree of deuteration, mmol of D/g total D alkyl D aryl D 0.00 0.00 0.00 0.44 0.13 0.31 1.36 0.51 0.85 1.68 0.86 0.82 1.57 1.18 0.39 4.34 1.74 2.6 3.93 1.84 2.09 3.39 2.49 0.9
“The acid-catalyzed exchange reactions were carried out in methylene chloride at 25 “C.
The 2H NMR spectra indicate that the resid and asphaltene were labeled predominantly in the benzylic position (6 2-4 ppm); deuteration at aliphatic positions (6 1-2 ppm) also occurred. When potassium tert-butoxide was used as a catalyst, the extent of deuteration was insignificant (less than thrice the natural deuterium abundance). Lithium diisopropylamide was only moderately more successful with 0.072 g atom of deuterium incorporated into the benzylic position for every gram of the resid. On the other hand, Lochmann’s base provided significant deuteration, and experiments 2 and 3 reveal that an increase in the concentration of Lochmann’sbase increases deuteration at the aliphatic positions. The labeled products could be readily isolated for further study. Acid-Catalyzed Deuterium Labeling. Acid-catalyzed deuterium incorporation into Lloydminster resid and its asphaltene was investigated with several Bronsted acids in methylene chloride. Again, the extent of deuterium incorporation was determined by 2H NMR spectroscopy. Typical results are shown in Table 11. These experiments indicate that Lloydminster resid and asphaltene were labeled predominantly at the aromatic (6 6.6-8.4 ppm) and aliphatic (6 0-2 ppm) positions. The results indicate that, although acetic acid-d is too weak to react with the resid, trifluoroacetic acid-d does introduce deuterium at the aryl and aliphatic positions. Triflic acid-d, which is known to be one of the strongest Bronsted effects a much greater deuteration of the resid. Experiments 3 and 4 show that, initially, deuteration occurs predominantly at the aromatic positions. However, after 30 minutes, the extent of deuteration in the aromatic positions appears to steadily decrease while aliphatic deuteration increases. Whereas Lloydminster resid exchanged with triflic acid-d at room temperature, the asphaltene showed insignificant deuteration under the same conditions. Deuterium could be incorporated into the asphaltene when the reaction mixture was refluxed, but this process was usually accompanied by the formation of undesirable methylene chloride insoluble materials. Aromatic deuteration is obviously the result of electrophilic deuteriodeprotonation and it is not surprising that aromatic deuteration predominates at the early stage of the reaction. However, the subsequent increase in aliphatic deuteration with a loss of deuteration from the aromatic positions is surprising. The reactions of triflic acid-d with several hydrocarbons were therefore examined to shed light on this puzzling feature. The reaction of 1-phenyltridecane with triflic acid-d in methylene chloride at 25 OC led to aromatic (3) Howells,R. D.; Mc Cown,J. D. Chem. Reu. 1977, 77, 69, (4) Stang, P. J.; White, M. R. Aldrichim. Acta 1983, 16, 15.
deuteration only. There was no evidence of labeling at the aliphatic and benzylic positions. The reaction of 2,6,10,14-tetramethylpentadecanewith triflic acid-d in methylene chloride at 25 “C led to extensive aliphatic deuteration with almost complete disappearance of the reactant accompanied by the formation of a multitude of smaller hydrocarbon products, confirming that the acid is powerful enough to initiate aliphatic substitution reactions and that these reactions occur independently of electrophilic aromatic substitutions. The reactions of these two hydrocarbons with triflic acid-d imply that petroleum resid possesses branched aliphatic compounds. This conclusion is in accord with the report of Farcasiu and associates,58who concluded that the aliphatic substituents in Arabian Light Vacuum Resid contained straight chain or branched chain alkyl groups with 2-16 carbon atoms. There are two possible explanations for the decrease in aromatic deuteration and the general loss of deuterium in the resid. Farcasiu and associates reported that the triflic acid catalyzed transalkylation of alkylnaphthalenes gave rise to small amounts of alkyltetralins.’ These hydrogenation reactions may lead to the decrease in aromatic deuteration and the apparent loss of deuterium from aromatic positions as shown in Table 11. Another possible explanation for the apparent loss of deuterium is the formation of high molecular weight products as indicated by the formation of insoluble materials. Furthermore, when samples of the untreated asphaltene, the methylene chloride soluble portion of the acid-treated asphaltene and base-treated asphaltene and were subjected to molecular weight determination by osmometry, the results indicated that, whereas triflic acid reduced the molecular weight of the asphaltene substantially, treatment with the base did not reduce the molecular weight of the asphaltene. Conclusions. Lochmann’s base and triflic acid-d accomplish hydrogen-deuterium exchange of Lloydminster resid and asphaltene. Moreover, the reagents catalyze the exchange of complementary protons. However, spectroscopic and molecular weight measurement suggest that triflic acid changes the chemical nature of the asphaltene specially after prolonged reaction times. Therefore, the deuteration of asphaltenes and resids is preferably carried out with Lochmann’s base at concentrations and reaction times specified in Table I. Acknowledgment. Financial support for this project was provided by the US. Department of Energy under Grant No. DE-AG22-88PC88811. We are gratefully in(5) Farcasiu, M.; Forbus, T. R.; LaPierre, R. B. Prepr.-Am. Chem. Soc., Diu. Pet. Chem. 1983, 28, 279.
(6) Farcasiu, M.; Rubin, B. R. Energy Fuels. 1987, I , 381. (7) Farcasiu, M.; Forbus, T. R.; Rubin, B. R. Energy Fuels 1987,1,28.
936 Energy & Fuels, Vol. 5, No. 6, 1991
debted to Dr. John Gatsis of Universal Oil Products for carrying out the molecular weight measurements by osmometry. Registry No. Triflic acid, 1493-13-6;trifluoroacetic acid, 76-05-1; K-tert-butoxide, 865-47-4;butyllithium, 109-72-8;1phenyltridecene, 123-02-4;2,6,10,14-tetramethylpentadecane, 1921-70-6.
Communications Leon M. Stock,* Carlos Cheng
Department of Chemistry The University of Chicago Chicago, Illinois 60637 Received April 15, 1991 Revised Manuscript Received June 28, 1991
