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Anal. Chem. 1982,54, 1730-1733
Differential-Pulse Voltammetry at the Hanging-Mercury-Drop Electrode for Identification of Aromatic Structures in Coal Extracts Keith D. Bartle,” Christopher Gibson, Derek Mllls, Michael J. Mulllgan, and Norman Taylor Department of Physical Chemistry, The University, Leeds LS2 9JT, United Kingdom Terence 0. Martin and Colin E. Snape National Coal Board, Coal Research Establlshment, Stoke Orchard, Cheltenham, United Klngdom
Differential-pulse voltammetry at the hanging-mercury-drop electrode was used to characterize coal derivatives. Polycyclic aromatic structures present both in anthracene oils and in the more complex molecules of the asphaltenes from high-yield coal extracts were identified by comparison of haw-wave potentials with those of a range of pure compounds. The aromatic structures Identified in the asphaltenes are similar to those in the “average” structures deduced by nuclear magnetic resonance methods. For coal-tar distlllate olls, quantitative analyses were possible and are in satisfactory agreement with results from capillary gas chromatography.
The chemical nature of the aromatic groups present in coal extracts and derivatives, important in any consideration of further treatment by cracking or catalytic reduction, is usually deduced from structural analysis schemes based on 13Cand lH NMR spectrometry (1, 2). However, NMR signals only give information on averaged behavior of each atom in the different clusters of aromatic rings which are linked together by alkyl and heterocyclic groups. A range of unknown structural types with widely varying degrees of condensation can all contribute to a given average. Oxidation procedures can, however, delineate the structures in the original coal (3). Polarographic and voltammetric methods represent a promising solution to the determination of the distribution of structures about the NMR-derived average. The polarographic reduction potentials of a large number of polynuclear aromatic hydrocarbons and their heterocyclic analogues have been determined (4-6) and provide a basis for the detection of various groups in coal structure. As an example, quinones have been identified in coal extracts by conventional DC polarography (7). Microprocessor-based polarographic instruments, coupled with “dmpense-type”dropping mercury electrodes (DME) have many advantages over those used in this early work, including the reduction of contribution of charging current of the electrical double layer a t current sampling and the use of a wider range of applied voltage wave forms in a variety of pulse modes. These, coupled with the use of three-element polarographic cells to reduce potential-drop effects in the electrolyte help to ensure the reproducibility and precision of the measured reduction potentials. In our work using a microprocessor-controlledinstrument, we found a high degree of mechanical instability of the DME in the nonaqueous solvent and electrolyte systems used, and this prevented full use of the potential of the technique. We have therefore turned to pulse voltammetry at a hanging-mercury-drop electrode (HMDE). Voltammetric curves for asphaltenes derived from highyield extracts of coals confirm that the average aromatic structures deduced by NMR are indeed those present. Oxidation of the original coals with peroxyacetic acid yielded 0003-2700/82/0354-1730$01,25/0
similar aromatic fragments to those identified in the extracts. EXPERIMENTAL SECTION Voltammetric analyses were carried out with an EG and G Model 303 mercury-drop electrode system in (small) hanging-drop mode linked to a microprocessor-controlled polarograph with 16-bit DAC and a three-channel Servogor 460 recorder; the reference electrode was Ag/AgN03 (0.01 M) in 0.1 M tetraethylammonium perchlorate (TEAP) in dimethylformamide (DMF). Analyses were carried out with a variety of polarographic techniques; initially the standard wave form of differential pulse polarography was employed, i.e., pulses on a linearly increasing voltage ramp; but in later experiments the wave form shown in Figure 1 was used. This wave form, previously applied at the DME by Osteryoung (8)and at the stationary platinum disk electrode by Rifkin and Evans (91,allows easier computation of theoretical pulse voltammograms. The equation used was a general one derived by the latter authors (10) by solution of the boundary-value problem for any step-functional continuous series of potential changes for a reversible polarographic process. Since the basis of this treatment is fast-electrontransfer of a given number of electrons, adherence to this model was tested by a comparison of computed and experimental peak widths at half-maximum height. Additionally, the Rifkin and Evans equation (10) allowed a more exact relationship between voltage (E,) for differential pulse current (i,) and the polarographic half-wave potential (El,&to be obtained; t h s is particularly important if the ramp commences in the vicinity of electrochemical activity; otherwise El12was taken as E, - AE/2, where AE is the applied voltage pulse. Three-element differential-pulse voltammograms were recorded for -10 cm3of solutions and between M in PAC or between 0.1 and 20 g dm-3 in coal extract fractions or polymers, in DMF containing 0.1 M TEAP. The solutions were deoxygenated by passing nitrogen. The cell and referenced potential were recorded simultaneously with the differential current. The parameters determining the wave form (Figure 1)were in the following ranges: pulse height, -40 to -120 mV; voltage step, -1.5 to -9.5 mV; pulse width, 7 to 42 ms; delay time, 80 to 130 ms. Ancillary analyses by gas chromatography of two anthracene oils (distillation cuts of coal tar), and for one of the oils after catalytic hydrogenation, were carried out on an SE-52 coated 12 m X 0.25 mm glass capillary column, deactivated by HCl leaching and by high-temperature silylation. Chromatographic peaks were identified from retention indexes by the procedure of Lee et al. (11).
Oxidation of coals previously extracted with benzene/methmol was carried out with peroxyacetic acid by the method of Hayatau et al. (12). Oxidized producb were methylated with diazomethane and their mass spectra recorded. RESULTS AND DISCUSSION Values of El12determined for a range of polycyclic aromatic compounds (PAC) with TEAP electrolyte and Ag/Ag+ reference electrode (Table I) differed by 0.43 f 0.02 V from available reported (4-6) values in DMF with 0.1 M tetraethylammonium iodide electrolyte and aqueous saturated calomel electrode reference. 0 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 11, SEPTEMBER 1982
1 1,
1
CURRENT SAMPLE TIMES
T
NEGATIVE VOLTS
PULSE HEIGHT
I
m
TI
I _--- - _.- - - .* _-- - -- - - - - -- -- -- - - - -. -
$ I
-
,TIME
Figure 1. Voltage wave form used In voltammetric measurements.
1
ANTHRACENE l-2.18v OIL 1 1
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Table I. Voltammetric E , Values of Model Compounds a -EpIV naphthalene 1-methylnaphthalene biphenyl fluorene carbazole dibenzofuran anthracene 2-methylanthracene phenanthrene fluoranthene pyrene chrysene phenanthrene resin polyacenaphthylene
2.95 2.93 3.02 3.10
3.08 2.95 2.38, 2.40, 2.88 2.18, 2.50, 2.68, 2.90 2.97
2.88 2.90 2.74 3.00 3.06
Electrolyte 0.1 mol dm-3 TEAP in DMF. Reference electrode Ag/AgNO, (0.01 M) in 0.1 M TEAP/DMF. n phenanthrene nuclei linked by Cathode HMD. methylenegoups. n = 2, 3, and 4. Molecular mass = 2300; M,/M, = 1.03.
k
\
I t
4 x d AMPS
ANTHRACENE OIL 2
r.
I
0 0L
-
J :25
56
50
CONCENTRATION
ANTHRACENE OIL 2 AFTER HYOROOENATION
Figure 2. Graph of differential pulse current against concentratlon for anthracene (100% = M), fluoranthene (100% = M), and anthracene oil 1 (100% = 4.04 g dm-3).
Graphs of pulse currenil; against concentration (e.g., Figure 2) were linear in the above concentration range. Ell2.values are little affected by methyl substitution (Table I) and hnkages of ring systems through alkyl groups should, therefore, not influence Ell2values significantly. We have confirmed this (Table I) by determining Ellzfor (a) a virtually monodisperse polyacenaphthylene sample, the naphthalene rings of which are reduced at a potential within 40 mV of naphthalene itself and (b) an oligomeric resin composed of phenanthrene residues joined by methylene groups, for which Ellz is only 20 mV greater than that of phenanthrene. Single-ring compounds are not reduced below 3,!5 V in our experiments. The presence of different PAC structures in coal-derived materials is easily demonstrated by HMDE differential pulse voltammetry. For example, Figure 3 illustrates how anthracenes, fluoranthenes, pyrenes, and chrysenes, as well as a range of other PAC, contribute t,o the pulse-voltammetric curve for two anthracene oils. Foir the hydrogenated anthracene oil, however, which was shown by capillary-column GC to contain
~
-3
/
POTENTIAL
VOLTS
-1
-2 y8
/
Ag &NO3
REFERENCE
Figure 3. Differential-pulsevoltammograms of anthracene oils in 0.1 M tetraethylammonium perchlorate in dimethylformamide: peak 1, chrysenes; 2, pyrenes, 3, anthracenes;4, fluoranthenes. Wave form parameters: pulse height, -40 mV; voltage step, -2.5 mV; pulse width, 30 ms; delay time, 100 ms.
lower concentrations of condensed-ring compounds, the voltammetric peaks are correspondingly less intense (Figure 3). With the assumption that the peak current response at a given molar concentration is the same for alkylated derivatives as for the parent PAC, quantitative analyses of these samples for certain compound types can be made (Table 11) by comparison of i, vs. concentration graphs. Agreement with results from GC is satisfactory (Table 11). Voltammetric curves for asphaltenes, with number average relative molecular masses between 370 and 940, derived from high-yield extracts of coals (13) (Figure 4 and Table 111) c o n f i i that the structures deduced from NMR measurements (2,13)are indeed present in the extracts and that the average
ANALYTICAL CHEMISTRY, VOL. 54, NO. 11, SEPTEMBER 1982
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Table 11. Voltammetric and Capillary Gas Chromatographic Analyses of Anthracene Oils % w/w of oil
oil 1 voltammetry anthracenes fluoranthenes pyrenes chrysenes
oil 2 GC
3 3 4
4 4 5 1
1
oil 2 after hydrogenation
voltammetry a 5 8 7 2
a Includes alkyl derivatives. Includes methyl derivatives. PAC each less than 2%. ND = not detected, Le.,
