415
Energy & Fuels 1990, 4 , 415-417 gas phase at room temperature.' However, previous work enables an unequivocal assignment of the feature centered at 1790 cm-' to the monomer and that at 1740 cm-' to the dimer.s Clearly monomer and dimer exist in the pyrolysis gas at somewhat different proportions from those in the standard. As mentioned above, these compounds are present in considerably lower concentrations than the main oxygencontaining gases. However, they are indicative of complex chemical pathways for the release of oxygen from the brown coals. Table I gives yields of these compounds from the rapid pyrolysis of Yallourn coal at 600 "C. At this temperature little secondary decomposition of the tar has occurred. The yields of methane and the total hydrocarbon gas are given for comparison. It is clear from these examples that gas-phase FTIR difference spectroscopy represents a powerful technique for the detection of trace organic species formed during rapid coal pyrolysis. The identity of the oxygenated organic species reported here provides important information on the mechanism of oxygen release from brown coals under rapid heating conditions. The influence of pyrolysis temperature and of exchangeable cations on the formation of these species will be considered in greater detail elsewhere.3 Registry No. Formaldehyde, 50-00-0; methanol, 67-56-1; ketene, 463-51-4; acetic acid, 64-19-7. (7) Sterk, H. Monatsh. Chem. 1968, 99, 457. 1955, 77, 3941. (8) Weltner, W. J. Am. Chem. SOC.
Peter F. Nelson,* Mary J. Wornat CSIRO Division o f Coal Technology P.O. Box 136, N o r t h R y d e N S W , Australia 2113 Received March 19, 1990 Revised Manuscript Received May 11, 1990
Table I. Rate and Selectivity in Thermolysis of =Ph(CH2)aPhat 375 "C surface coverage,b rate x 10': compositionR mmol/g 7 0 s-1 Sd 24e 0.9dJ =DPP 0.59 0.57 18e 0.9da 0.14 1.1e 1.09/-h 0.10 0.72 1.218 =DPP/=NAP 0.12/0.44 1.9 1.08 =DPP/=BP 0.13/0.51 2.3 1.14 0.10/0.51 1.2 1.18' =DPP/=DPM 0.17/0.42 21 0.82' 16 0.93 0.13/0.37 =DPP/=DPM-dz 0.16/0.36 13 0.98 See text for definitions. Coverage in mmol of component per g of derivatized silica. CInitial rates were determined from the slopes of linear regressions of =DPP conversion vs reaction time, which typically employed four to six runs with reaction extents limited to < l o % (estimated error of *lo%). dSelectivity given by average of PhCH=CHz/PhCH3 yields (estimated error of *2%). e Calculated from data in ref 2. 'Includes some data from runs at 345 and 400 "C. #Selectivity was slightly conversion dependent; value given is intercept from linear extrapolation t o zero conversion (estimated error of f270).hCombined with data from a 0.13 mmol/g batch for determination of S. 'Estimated error of *5%. 'Value appears low, based on results for saturated coverages of -DPP and for other =DPP/=DPM batches.
study of thermolysis of silica surfaces containing two covalently anchored model compounds (Si-0-C, linkages). T h e resulti provide evidence t h a t under appropriate conditions, although mass transport is prohibited o n the surface, radical centers can be relayed across the surface by a series of rapid hydrogen-transfer reactions. This novel discovery implies that diffusional limitations for reactive intermediates in coal may also be overcome by similar processes. Two-component surfaces were prepared in order to investigate the influence of the structure of neighboring aromatics on a free-radical chain decay pathway as typified by the thermolysis of surface-attached l,&diphenylpropane (-DPP). The two-component surfaces shown below were
Thermolysis Mechanisms. Evidence for an Alternative Pathway for Radical Migration in Diffusionally Constrained Environments d P P
Sir: New insights into the impact of restricted diffusion and conformational mobility on organic molecular transformations are of great importance in developing an improved understanding of the thermal and chemical reactivity of coal. Our research has been concerned with developing model systems'-3 that probe the potential perturbations in free-radical reaction pathways that may arise in coal because of its cross-linked, network ~tructure.~ This effect may be particularly important in thermal conversions of coals at low temperatures, e.g., 350-400 "C, where bonds begin to break but most of the residual framework is retained.5 In this paper, we present the first (1) Buchanan, 111, A. C.; Dunstan, T. D. J.; Douglas, E. C.; Poutsma, M. L. J. Am. Chem. SOC.1986,108, 7703. (2) Buchanan, 111, A. C.; Biggs, C. A. J. Org. Chem. 1989, 54, 517. (3) Britt, P. F.; Buchanan, 111, A. C.; Biggs, C. A. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1989,34,567. (4) (a) Green, T. K.; Kovac, J.; Brenner, D.; Larsen, J. W., In Coal Structure; Meyers, R. A., Ed.; Academic Press: New York, 1982; Chapter 6. (b) Brenner, D. Fuel 1985, 64, 167. (c) Larsen, J. W.; Green, T. K.; Kovac, J. J. Org. Chem. 1985,50,4729. (d) Lucht, L. M.; Peppas, N. A. Fuel 1987, 66, 803. (5) Berkowitz, N. The Chemistry of Coal, Coal Science and Technology; Elsevier: Amsterdam, 1985; Vol. 7, Chapter 7.
0887-0624/90/2504-0415$02.50/0
/
d
4 P P
P
/
d P M
prepared by coattachment in a single step as described previously for =DPP.*v6 Thermolysis of =DPP in sealed, evacuated (2 X lo4 Torr) tubes at 375 "C and low conversions produces the cracking products shown in eq 1.'
-
-Ph(CH,),Ph PhCH, + -PhCH=CH2
+ -PhCH, + PhCH=CH2 (1)
No new products are detected in the presence of the coattached aromatic molecules.8 The long-chain, freeradical decay pathway (propagation steps shown in eqs (6) The synthesis and purification of pHOC6H,(CH2)sPhhave been described.2 The other starting materials, p-HOC6H4C&, pHOC6H4CH2C6H,,and 2-HOCl&, were commercially available and were purified to >99.9%. The resulting two-component surfaces had final purities of 99.6-99.8%. (7) Volatile products were collected in a cold trap (77 K) as they formed and were analyzed by GC and GC-MS with the use of internal calibration standards; Surface-attached products were liberated as phenols following digestion of the silica in base, silylated to the corresponding trimethylsilyl ethers, and analyzed as above.2 (8) Previous work showed that =PhPh or =PhCH,Ph are stable at 400 "C for 4 h.'
0 1990 American Chemical Society
416 Energy & Fuels, Vol. 4 , No. 4, 1990
2-4) cycles through the two distinct benzylic radicals,
zPhCHCH2CH2Ph (1)
-+
=PhCH=CH2
+ PhCH2’ (2)
zPhCHZCH2CH.Ph (2)
4
=PhCHz’+ PhCH=CH2
-
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CH,Ctl&H&
HZ
a
t n -0 0
(3)
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Communications -
E
0H CH& aHzCH,*
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Hz-0
0
Ha
=PhCH2’ (and PhCH2’) + =DPP -PhCH3 (and PhCH3) + 1 + 2 (4) -PhCHCH2CH2Ph (1) and =PhCH2CH2CH.Ph (21, that undergo subsequent rapid unimolecular decay via 0-scission (eqs 2 and 3).2 Regioselectivity in the thermolysis is determined by the relative concentrations of the two benzylic radicals, [2]/[ 11, and is experimentally monitored by the PhCH=CH2/PhCH3 yield ratio S. In related fluid-phase studies of p-Me3SiOPh(CH2)3Phat 375 “C, a substituent effect, S = 0.91, was observed, indicating a slight inherent stabilization of the benzylic radical para to the siloxy ~ u b s t i t u e n t . ~ The initial rates and selectivities for thermolysis of -DPP at 375 “C as a function of surface coverage and coattached aromatic are given in Table I. For surfaces containing only =DPP, the thermolysis rate decreases dramatically with decreasing surface coverage. Furthermore, S (which is 0.96 for saturated coverages and is comparable to that for fluid-phase Me3SiOPh(CHJ3Ph) increases with decreasing surface coverage, indicating an increasing preference for generating the less stabilized benzylic radical, 2. This selectivity is induced by restricted diffusion, which results in an increasing preference for hydrogen abstraction at the benzylic methylene site farthest from the surface as =DPP molecules and -PhCH2* become increasingly separated. Two-component samples containing coattached p-biphenyl or 2-naphthyl moieties show surprisingly little influence on =DPP thermolysis. The rate of -DPP thers-l) is similar to that for surfaces molysis ((1-2) X containing only =DPP when compared a t similar -DPP coverages. It was anticipated that the rate of the hydrogen transfer propagation step 4 would decrease in the presence of aromatic “spacers”. On the other hand, the rate of termination events involving =PhCH2’ and PhCH2’ may also decrease in a compensating manner. The selectivity values are also similar to those for =DPP alone at similar coverages. This suggests that if and when the aromatics act as barriers to the hydrogen transfer propagation step, no additional selectivity accrues for hydrogen abstraction from the benzylic site farthest from the surface. In dramatic contrast, the presence of coattached diphenylmethane moieties substantially alters both the rate and selectivity of -DPP thermolysis. Batches of =DPP/=DPM at coverages of 0.13/0.37 and 0.17/0.42 and 21 X s-l, remmol/g yield rates of 16 X spectively, comparable to saturated coverages of =DPP. This represents an acceleration by a factor of 15-19 compared with the rate of -DPP thermolysis at a corresponding coverage of 0.14 mmol/g. This behavior is to be distinguished from that of thermolysis of liquid- or vapor-phase DPP at 350-365 “C,where the addition of a hydrogen donor such as tetralin’O or diphenylmethane” had essentially no effect on the DPP thermolysis rate at equivalent concentrations. The presence of =DPM also reduces S to values more typical of saturated coverages of =DPP. (9) Britt, P. F. Unpublished data. (10) Poutsma, M. L.;Dyer, C. W. J. Org. Chem. 1982,47, 4903. (11) Gilbert, K.E.; Gajewski, J. J. J. Org. Chem. 1982, 47, 4899.
Figure 1. Pathway for radical migration under conditions of
restricted diffusion.
These results suggest that rapid hydrogen-transfer steps involving =DPM are occurring that allow radical centers to “migrate” on the surface as illustrated in Figure 1. The result of such a process is to effectively decrease the distance between a -DPP molecule and a radical center on the surface. This enhances the rate at which =DPP reacts by increasing the rate of the hydrogen transfer propagation step, which probably contains a distance dependence in the rate constant when the hydrogen-abstracting radical is also surface attached. The values of S < 1.0 for =DPP/=DPM surfaces are also consistent with a reduced separation between =DPP molecules and radical centers on the surface. Radical migration effectively removes the distance-dependent conformational constraints on the hydrogen-abstraction reactions from -DPP that resulted in the unexpected regioselectivity in product formation at lower coverages. Supporting evidence for the involvement of =DPM in the radical relay mechanism depicted in Figure 1 comes from studies of surfaces containing .=DPP/-DPM-d2.12 Consistent with the results of the protium analogue, we find S C 1.0. Furthermore, the rate is reduced by a factor of ca. 1.6 compared with that of the =DPP/=DPM batch (0.17/0.42 mmol/g) with the most comparable -DPP coverage, suggesting a small kinetic isotope effect at 375 “C for the radical relay process. GC-MS analysis of the products at a =DPP conversion of 15% shows that the gas-phase toluene is 52% PhCHzD and 48% PhCH3, while the surface-attached toluene, following liberation from the surface as cresol, has the composition 47% HOPhCH2D, 14% HOPhCHD2, and 39% HOPhCH3.13 This provides direct evidence for the involvement of H (D) transfer be) both chain-carrying surfacetween -DPM-h2 ( d 2and attached and gas-phase benzyl radi~a1s.I~ This study has demonstrated that facile hydrogenshuttling reactions can occur even under conditions of restricted diffusion. The important new implication for coal reactivity is that this type of process provides a mechanism for radical intermediates to rapidly “migrate” to reaction centers in environments in which classical diffusion is prohibited.
(12) The p-HOC6H4CD2C6H5 starting material was prepared in three steps from p-HOC6H4COC6H5, giving a product with 99.9% chemical purity and >95% isotopic purity. (13) Quantitative analysis of deuterium content in toluene employed GC-MS using known mixtures of PhCH3 and PhCH2D. Analysis of deuterium content in HOPhCH3 was based on GC-MS analysis of the trimethylsilyl ether derivatives using the parent ions at m / e 180,181, and 182 correcting for 13C isotopic contributions. (14) The detection of HOPhCHDz suggests that the surface-attached toluene product is also involved in the hydrogen-transfer/radicalmigration process.
417
Energy & Fuels 1990,4,417-418
Acknowledgment. Research was sponsored by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy, under Contract No. DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc. We appreciate many valuable discussions with Dr. M. L. Poutsma of ORNL.
B
Registry No. DPP, 1081-75-0; BP, 92-52-4; NAP, 91-20-3;
DPM,101-81-5; SiOp, 7631-86-9. A. C. Buchanan, III,* P. F. Britt, C. A. Biggs Chemistry Diuision, Oak Ridge National Laboratory P.O. Box 2008, Oak Ridge, Tennessee 37831-6197 Received April 11, 1990 Revised Manuscript Received May 29, 1990
Microscopic NMR Imaging of Coal Sir: In this Communication, we report the first application of lH NMR imaging techniques for spatially resolving individual microlithotypes (maceral assemblages) within a solid coal.' Distinction of coal microlithotypes that are comprised predominantly of a single maceral has been possible by taking advantage of differences in proton densities and longitudinal relaxation rates, which vary among individual maceral types. Using multipulse proton decoupling (MREV-8) techniques2 in combination with back-projection reconstruction NMR imaging method^,^ we have been able to image and distinguish macroscopic resinite and vitrinite regions within Utah Blind Canyon coal with an in-plane spatial resolution of less than 200 pm. Samples used in the spectroscopic experiments and to construct the image phantom were macroscopic resinite and vitrinite specimens that were handpicked from Utah Blind Canyon high-volatile bituminous coal (APCS No. 6) obtained from the Argonne Premium Coal Sample Program. The phantom consisted of resinite and vitrinite specimens that were each approximately 2 mm X 1.8 mm X 1.4 mm in size. Since it had been observed previously that Utah vitrinite is hygroscopic, specimens were dried a t 100 "C for 48 h and sealed in a Teflon holder prior to the experiments. All spectra and images were acquired on a 2.3-T Bruker CXP-100 NMR spectrometer utilizing a home-built probe and imaging accessory that are described in detail elsewhere.4 A 1-ps 90" pulse width was achieved with approximately 200 W of rf by using a 3.5-mm-i.d. solenoid coil. The imaging pulse sequence employed a back-projection protocol in which MREV-8 line narrowing was implemented concurrent with the application of static field gradients of 20 G/cm. The images were reconstructed from the frequency domain utilizing conventional filtered back-projection image reconstruction techniques5 Standard Bloch decay (90"pulse-acquire) and MREV-8 multipulse 'H NMR spectra of the Utah coal are presented in Figure 1. The Bloch decay spectrum shows little fine structure owing to strong homonuclear dipolar couplings. In the MREV-8 spectrum, there is residual broadening (1) Presented in part at the 31st Experimental Nuclear Magnetic Resonance Spectroscopy Conference, Pacific Grove, CA, April 1-5,1990. (2) Rhim, W. K.; Elleman, D. D.; Vaughan, R. W. J. Chem. Phys. 1973, 59, 3740. (3) Lauterbur, P. C. Nature (London) 1973,242,190. (4) Gopalsami, N.; Forester, G. A.; Dieckman, S. L.; Ellingson, W. A.; Botto, R. E. Development of NMR Imaging Probes for Advanced Ceramics. Reoiews of Progress in Q N D E Thompson, D. O., Chimenti, D. E., Eds.; Plenum Press: New York, 1990, p 9A. ( 5 ) Brooks, R. A.; DiChiro, G. Radiology 1975, 117,561.
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"
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"
"
I
'
250
"
'
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-250
0
-500
Chemical Shift, ppm
Figure 1. (A) Bloch decay spectrum of Utah Blind Canyon coal with 2048 accumulations. (B) Same spectrum obtained with MREV-8 sequence using 512 averages.
Figure 2. Two-dimensional (Xu) images of resinite (left) an1 vitrinite (right) phantom using rotational gradients and MREVback-projection imaging protocol with 90 projections about 180' and 512 averages per projection. The recycle delays used in (A), (B), (C), and (D) were 235,585,1085, and 1585 ms, respectively. 3-
a.5
-
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0887-0624/90 /2504-0417$02.50/0 0 1990 American Chemical Society
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