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NMR of petroleum cokes II: Studies by high resolution solid state NMR of 1H and 13C
Carbon, Vol. 32, No. 1, pp. 41-49, 1994 Copyright0 1994Elsevier ScienceLtd Printed in Great Britain. All rights reserved 0008-6223194 $6.00+ .OO
Pergamon 0008-6223(93)EOO53-N
NMR OF PETROLEUM COKES II: STUDIES BY HIGH RESOLUTION SOLID STATE NMR OF ‘H AND 13C Department
M. PRUSKI” and B. C. GERSTEIN of Chemistry and Ames Laboratory,** Iowa State University, Ames, Iowa 50011, U.S.A.
and Fachbereich
D. MICHEL Physik, Universitgit Leipzig, D-04103 Leipzig, Germany
(Received 19 June 1992; accepted Abstract-Chemical
in revised form 8 July 1993)
of hydrogen and carbon in a series of cokes obtained from heavy coker pilot unit are probed with high-resolution solid state NMR. The fractions of aromatic hydrogen and carbon, as determined from ‘H combined rotation and multiple-pulse spectroscopy (CRAMPS), and “C magic angle spinning (MAS) experiment with and without cross polarization (CP), varied only slightly between 0.49 and 0.65 and between 0.88 and 0.92, respectively, for the samples studied. A comparison with the results of direct excitation (‘)C MAS) NMR showed that CPiMAS NMR spectra taken with a contact time of 1 ms well represented relative carbon intensities. The high-resolution spectra, in combination with previously reported wideline ‘H NMR data and the results of elemental analysis, are used to derive several structural parameters, crude
functionalities
oils in a Mobile
continuous
flow laboratory
including aromatic and aliphatic hydrogen to carbon ratios and the average formula per 100 carbon atoms. Finally, the “average” structures for studied cokes are proposed and discussed. Most cokes are concluded to consist of molecules having approximately 10 aromatic rings bearing only few substitutions. Key Words-Petroleum structure
coke,
high-resolution
solid state
1. INTRODUCTION
correspondence
CRAMPS,
cross-polarization,
coke
To analyze the CRAMPS spectra, it is advantageous to use a superposition of the proton resonance lines by means of a nonlinear procedure for a spectral simulation, although the shortcomings of such computational methods, (c~.g., the dependence of the results on the selection of starting parameters) compared to linear prediction techniques (LPSVD[7]), are known. We will show that all spectra of studied cokes may be fit to a superposition of two groups of resonance frequencies centered approximately at 2 ppm (mostly methylene and methyl protons) and 7 ppm (aromatic protons). By using cross polarization (CP) in combination with magic angle spinning (CPIMAS) and strong dipolar decoupling[8], high-resolution “C spectra of coals and cokes can be obtained. Usually aromaticities of fossil fuel samples have been reported: however, more detailed information on carbon functionalities is also available. Several “resolution enhancement” experiments utilizing CP have been successfully applied in such systems. Most of these materials are heterogenous in structure and, as such, undergo multicomponent relaxation. Variable-contact time experiments and dipolar dephasing[9], based on differences in cross-polarization rates and on differences in the ‘H-13C interactions, respectively, are the most prominent examples of the techniques used in combination with CP/MAS. We based the present study of 13C NMR on the use
The development of high-resolution techniques in solid state NMR of ‘H and 13C has enabled nondestructive structural analysis of solid fossil materials[ I-31. The aim of this study is to use ‘H combined rotation and multiple-pulse spectroscopy (CRAMPS) and 13C magic angle spinning (MAS) with high-power proton decoupling to determine the average chemical composition and structure of several petroleum-derived cokes of diverse origin. These data will be combined with the results of quantitative analysis of hydrogen[4] and with the results of elemental analysis to provide insight into the structure of studied cokes and to develop better understanding of coke chemistry. The line separation in the CRAMPS NMR spectra of ‘H in fossil fuels is rather limited due to a broad distribution of chemical shifts in such systems [3]. Consequently, the application of more sophisticated multipulse sequences like BR-24[5] does not lead to a better resolution in these materials, and in the present work only the MREV-8 pulse sequence [6] is used. *To whom
NMR,
should
be addressed. at Ames Laboratory by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, under Contract No. W-7405-Eng-82, and by a grant from Mobil Research and Development.
**The authors’ work is supported
41
M. PRUSK~et
42
of CP/MAS, including variable contact time and dipolar dephasing experiments. In particular, we considered the quantitative aspect of CPJMAS measurements. For that reason, we observed i3C NMR signals of selected samples using direct excitation (Bloch decay experiment) under high-resolution conditions of proton decoupling. The i3C lineshapes thus obtained proved to be exact reproductions of CPiMAS spectra. Finally, i3C spin counting was performed on these samples to assure that the i3C intensities observed were representative for i3C nuclei in cokes.
tuning was found with the use of frequency sweep generator. During the cross-polarization experiments, the delay times of at least 5 times the largest value of T, observed for protons in the sample in question were used, while the cross-polarization times and the dipolar diphasing times varied from 10 ps to 15 ms and from 0 to 400 ks, respectively. A 180” refocussing pulse was used during the delay period in the dipolar dephasing experiment. All ‘H and i3C chemical shifts are referred to liquid TMS (zero shift) using the 6 scale with negative values being upfield.
2. EXPERIMENTAL The samples used in this study were Mobile cokes obtained in a coker pilot unit as described in 141. The ‘II NMR CRAMPS measurements were performed on an MSL-300 Bruker spectrometer at room temperature. The powdered samples (-200 mesh) were spun in an alumina rotor at 3.6 kHz, and the MREV-8 multipulse sequence with a cycle time tc = 127 = 36 ps was used to eliminate homonuclear dipolar broadening. The 90” pulse length was 1.5 ps. Eight signals were accumulated for each measurement, using a repetition time of 20 seconds. 13CNMR spectra were obtained on a home-built spectrometer operating at 25.15 MHz and 100.06 MHz for i3C and ‘H, respectively. A double-tuned, single-coil probe equipped with a modified Shoemaker-Apple type magic angle spinner[lO] was used for both cross-polarization and direct excitation experiments. No background signal was detected under the experimental conditions used for the cokes. One hundred and eighty to 250 mg of samples were sealed in 5-mm glass tubes and spun at 4.2-5 kHz using an air drive. In all i3C experiments, a spin temperature inversion method was used to reduce baseline artifacts and the effect of deadtime of the spectrometer[l 11. A free induction decay (FID) with a size of 2K data points was acquired using quadrature detection and Fourier transformed after extending the size of the FID to 8K by zero filling. All CPlMAS NMR spectra were obtained utilizing rf fields of 60 kHz: the proton field was then kept at the same level during the high-power proton decoupling period. As Stejskal and Schaefer pointed out[ 121, spinning may have a striking effect on the polarization process. To avoid experimental errors, the Hartmann-Hahn matching condition was carefully controlled by monitoring the i3C CPiMAS intensities of hexamethylbenzene versus i3C rf field. The characteristic oscillation pattern was observed for both CH3 and ring carbons similar to those reported in the earlier studies[ 131. The rf field corresponding to the center of symmetry of the pattern represented the exact Hartman-Hahn matching condition. No measurable impact of the presence of studied samples in the NMR coil on the probe
al.
3. RESULTS AND DISCUSSION
3. I CHAMPS Two examples of proton NMR CRAMPS spectra of cokes are shown in Figure la and b. All samples exhibited a strong absorption centered betwen 7.0 and 7.5 ppm with a full width at half height (fwhh) between 3.4 and 4.2 ppm (- l-l.25 kHz), associated with aromatic CH protons. Phenolic protons also resonate in this region. However, phenolic compounds undergo condensation reactions at temperatures lower than 400”C[14,151 and are not likely to exist as structural fragments of cokes. On the aliphatic side, the spectra consisted of two shoulders located at -1.5 (1-0.5) ppm and -4 (‘0.5) ppm, representing methyl groups a to aromatic rings and ring-joining methylenes, respectively. This is evidenced by the observed chemical shifts and the results of 13CNMR (vide infra), which showed the presence of CH, and CH, carbons. This conclusion is further supported by the aliphatic hydrogen-to-carbon ratios derived below. Olefinic =CHprotons, which typically resonate at -5-6
20
10
0
PPM FROM TMS Fig. 1. High-resolution solid state ‘H NMR (CRAMPS) spectra of cokes 7 (spectrum a) and 14 (spectrum b).
NMR of
than two lines did not lead to unambiguous results. Therefore, we included all aliphatic intensity into one peak and restricted its position between 2 and 2.5 ppm during the simulation.
ppm, are not expected to be found in abundance in cokes. The lineshape in Figure la is typical of most cokes investigated in this work in that the relative fractions of aromatic and aliphatic protons are almost comparable. In only one sample[l4, Figure lb], does the aromatic band strongly dominate the spectrum. The simulation of the experimental data was performed by means of DATAPLOT software on a MicroVAX 3600 computer. For the analysis of the spectra, a superposition of two Gaussian lines was assumed, F(o) = A,, exp[ -(w - o~J~/~A&,] + A,, exp] -(o - 0,,)*/2Ao$];
This approach resulted in fits that should best represent relative aromatic and aliphatic proton intensities in the CRAMPS spectra of cokes. Still, the above-mentioned difficulties caused the reliability of hydrogen aromaticity values derived in this work to be estimated at only -lo%, which is below that of 2-3% achieved by the CRAMPS method in more favourable cases[l6]. The results are given in Table 1. column 3, and will be used later in the discussion.
(1) 3.2 CPIMAS NMR Studies The purpose of the CPIMAS experiment was to obtain t3C NMR spectra that accurately represent all carbons in the cokes studied. It has been pointed out earlier (see, for example, ref. l] and the references therein) that the time evolution of carbon magnetization during the CP/MAS NMR experiment is a result of a complicated interplay between the spin-lattice relaxation processes in the rotating frame and cross polarization processes (described by relaxation times cp and Tcu), which can be further complicated by sample spinning. To optimize the conditions of our CP/MAS experiment, the aromatic “C intensities were monitored as a function of contact time for three cokes (samples 7, 2, and 14) characterized by the shortest, medium, and longest c,‘s. For sample 7 the r3C magnetization reaches its peak at a contact time of 0.25 ms, whereas for cokes 2 and 14 the maximum signal is observed at contact times of CCI0.75 ms and ca 1.O ms, respectively (see Figure 2). The experimental data were fitted to the three-parameter equation of
where A,, , A,, , war, and o,, are the amplitudes and positions of the aromatic and aliphatic peaks, respectively, hoi, and A& are the second moments of those peaks. Subsequently, the proton aromaticities, f,“, were calculated using the equation f,“, = (A,&,,)/(A,,Aw,,
+ A,,Aw,,).
(2)
Several steps were taken to assure the best possible reliability of the results obtained: l
l
l
43
petroleum cokes II
All spectra were acquired sequentially under the same experimental conditions and were subject to identical phase corrections. To minimize the effect of baseline distortions around the carrier frequency (- - 10 ppm), only the portions of spectra downfield from -5 ppm were analyzed. An attempt to simulate the CRAMPS results by partitioning the whole spectrum between more
Table I. Aromaticities, average peak locations, obtained
from
“C CP/MAS
and average linewidths NMR experiments for cokes “C NMR
.f,“,
.fz
Av. peak location (ppm)
0.53 0.51 0.53 0.56 0.51 0.56 0.49 0.55 0.55 0.55 0.53 0.54 0.55 0.65 0.52
.90 .91 .88 .91 .90 .90 .90 .92 .91 .90 .89 .91 .91 .92 .89
128 127 127 128 127 127 129 129 127 126 128 126 128 125 128
‘H NMR No.
Name resid resid refinery resid resid resid resid resid refinery refinery refinery refinery resid aromatic resid
I
2 3 4 5 6 7 8 9 10 11 12 13 14 15 Error
mix
mix mix mix mix
esimates:
ref. stream
f:
(-+O.l);Ec
(-20.02).
Av. line width
19 19 19 18 20 16 22 21 19 19 19 18 20 15 21
(ppm)
44
M. PRUSKI et al.
M42,) = &A-‘[I
- exp( - htCPITcH)l expf - r&7$,
(3)
where Iw, is the equilibrium magnetization of the ensemble of carbon spins, fcP is the contact time, and A = 1 - Tcu/~P[171. Figure 2 shows that the least-square fits thus obtained do not match the experimental results for cokes 2 and 14. The deviation can be understood, however, since neither the cross polarization nor the rotating frame relaxation processes are exponential. The time constant for cross polarization transfer is known to be strongly dependent on the proton-carbon distance and the mobility[ 171. For rigid, aromatic C-H functionai groups, Tcu is typically 0. I-O.2 ms and increases by a factor of three or more for nonprotonated aromatic carbons; aliphatic carbons are generally polarized faster than aromatic carbons. Our previous ‘H NMR studies[4] showed that lr”, relaxation processes in cokes are complex. A distribution of eP values was found in all samples, with a significant fraction (4559%) being less than 300 pus (see Table 3 in [41). The kinetics of cross polarization in cokes was not further investigated. However, it should be noticed that if the monoexponential decay (4) was fitted to the tail of the carbon magnetization curve (where it is governed primarily by T& processes), the resulting Tt values of 2.1 ms, 1.8 ms, and 9.0 ms obtained for cokes 2, 7, and 14, respectively, do not appreciably differ from the long qP
values derived with ‘H NMR (see Table 3 in [4]). The above discussion concerned only aromatic carbon intensities; however, no significant differences were noticed in the time evolution of aliphatic carbon magnetization for contact times greater than 0.5 ms. Consequently, a contact time of 1.O ms was chosen as a good compromise between sensitivity and quantitative accuracy requirements for further studies. The CPlMAS NMR spectra of cokes exhibit two distinct peaks located in the aliphatic and aromatic regions (Figure 3). The aliphatic part consists of a sharp peak with a linewidth of 12 ppm located at -21 ppm, assigned to -CH, groups on the basis of dipolar dephasing experiments. A second, superimposed broader feature of similar integrated intensity centered at -35 ppm can be distinguished and is primarily assigned to methylene carbons. In all cokes studied, the aromatic portion of spectra consists of a single, strong peak at 127.5 (L42.5) ppm with a linewidth of -19 ppm (see Table 1). There were observed differences in Iinewidth from this value for cokes 6 (16 ppm), 14 f 15 ppm), 7 (22 ppm), and 8 (21 ppm). Also, a 2-3 ppm upfield shift of the first moment was observed for samples 6 and 14, whereas a shift of about 2 ppm downfield was measured for samples 7 and 8. The detection of these lineshape changes may reflect differences in the dist~bution of chemical functionalities in the studied samples. In several spectra, a shoulder on the downfield side of the aromatic peak can be clearly seen. Attempts to perform a computer decomposition of aromatic intensities to extract more information on chemical composition did not, however, give unambiguous results (vide infra). Finally, a very weak
EXF’ER. FIT
0
1 0
I
2
I
I
4
1
COKE 2
0
...........
COKE 7
+
-
--__ *-__ ---___ ------.____ ----______ I I 1 I
6
8
10
12
CP CONTACT TIME (MS) Fig.2.
Variation in CPiMAS NMR intensities of aromatic carbons with cross-poiarization contact time for Mobil cokes and the least-square fits of experimental data to biexponential function (3).
NMR of petroleum
cokes
4s
II
cessfully used by applying a two-component Gaussian-Lorentzian decay to fit the experimental results[18,22,23] as in MU& = Mot exp[ - 0.5(tdlT&)Zl + MO,_expl- tdlTZ’,l. (6)
300
0
100
200 PPM
FROM
-100
ThtS
Fig. 3. “C CPIMAS NMR spectrum of coke 6 (tcp = I.0 ms, spinning speed U, = 4.4 kHz).
feature, not exceeding 2% of the total carbon intensity, could be distinguished for several samples (1,2, 6, 9) with the chemical shifts indicative of aromatic C-O carbons (- 180-200 ppm). Table 1 gives the fraction of aromatic carbons, as determined from CPiMAS spectra using the equation
where I”’ represents the integrated aromatic carbon intensity (chemical shifts >90 ppm), including intensities from spinning sidebands; r”’ is the remaining (aliphatic) carbon intensity. 3.3 Dipolar dephasing To differentiate carbons with directly bonded protons from those without protons in ‘)C CPiMAS NMR spectra of a solid sample, a dipolar dephasing experiment can be used[9,18,19]. In this experiment a short delay t, (dipolar dephasing period) is introduced at the end of cross polarization and prior to data acquisition in which both ‘H and ‘jC rf fields are turned off. During the dephasing period “C magnetization decays because of static ‘jC-‘H dipolar interactions, which can be modified by molecular motion, spin diffusion, and sample spinning[20,21]. The typical decay time constants in rigid systems are 15-32 p.s for protonated carbons and from 50 to several hundred ps for carbons that are mobile (CH,) and/or two or more bonds away from a nearest proton. The initial part of the decay can be described by a Gaussian function with a time constant proportional to the heteronuclear carbon-proton second moment MZLs. For longer dephasing times the evolution of magnetization can be better characterized by a Lorentzian decay[20,21]. It is noted that spinning of the sample modulates the interactions responsible for dephasing, which may lead to oscillations in the “C magnetization versus td, especially in case of high spinning frequencies and inefficient spin diffusion[20,2 I]. However, in several applications the dipolar dephasing technique has been suc-
where M,,, MO,_are the initial intensities, and T,,; and TZ’_are the respective time constants of the two decays. For rigid protonated carbons the (larger) heteronuclear second moments may be derived from the rapidly decaying Gaussian component. For carbons located two or more bonds away from a nearest proton, it has been shown[20] that the (smaller) values of Mzls can be evaluated from T?L using the formula M I,s = (CUT&‘. The parameter (Ydepends on the MAS frequency and has been estimated for nonprotonated carbons to be u = 1.93 (kO.06 x lo-’ s for MAS frequency of 4 kHz[20]. In this work “C aromatic intensities have been monitored as a function of dephasing time for cokes 2 and 7 by integrating the Fourier transformed ‘“C NMR signals. The data were subsequently analyzed using eqn (6). For cokes 2 and 7, the Gaussian component of dipolar dephasing signal decayed with a time constant TZGof 27 (+3) ps and represented 47% (~5%) and 37% (~5%) of aromatic carbons, respectively (Figure 4). The remaining magnetization decayed with a time constant TZLof 500 (2200) ps. The reported decay constants have values similar to those found previously for coals[22]. A TzGof 27 ps corresponds to the heteronuclear second moment Mzls of 1.4 x 10’ s -*, typical for carbons strongly coupled to protons (C-H). The second moment Mzls of - 10’ s ’ evaluated from T?,_(for MAS frequency of 4 kHz) is typical of carbons located two or more bonds away from a nearest proton. Surprisingly, almost no change in lineshape of the aromatic peak was observed as the dephasing time increased from 0 to 400 ws. The result may be contrasted with that found in coals, where a dipolar de-
0
EXPER GAUSSIAN
DIPOLAR
DEPHASMG
TIME,
(US)
Fig. 4. Fit of the dipolar dephasing data for coke 7 (aromatic intensities only).
PRUSKI et al.
M.
46
phasing time of 60 ps led to a decrease of intensity in the upfield portion of the aromatic region associated with aromatic C-H perimeter carbons. Only a very slight (about 3 ppm) downfield shift was monitored; this indicated that most of the nonprotonated carbons resonate at around 130 ppm. This chemical shift most likely corresponds to bridged carbons between the aromatic rings. It also indicates that for the cokes studied, an attempt to fit the aromatic portion of the r3C spectrum with a superposition of peaks will lead to results that are devoid of physical meaning and do not represent different chemical functionalities of carbon. The results of the dipolar dephasing experiment will be analyzed later in comparison with those obtained from high-resolution solid state ‘H NMR and elemental analysis. 3.4 NMR Studies of 13C by direct excitation To address the potential quantitation problem in CP/MAS spectra, the direct excitation (Bloch decay), r3C NMR lineshape, and spin-counting measurements were performed with selected samples 2, 5, 7 under high resolution conditions of MAS and proton decoupling. It is known that under proper experimental conditions, the initial intensity of the Bloch decay signal is proportional to the number of nuclei of interest in the sample. However, in the presence of paramagnetic centers, which are abundant in cokes under study[4], some of the resonances may be broadened and/or shifted beyond the limits of observation. To assure that the repetition time will be 2 5 Ty, the progressive saturation experiment [24] was carried out for coke 5 to estimate the value of carbon longitudinal relaxation time. Five delays, ranging from 1 to 20 seconds, were used, which yielded an estimated T,c of 3.8 s (kO.5) for this sample. No direct measurement of T,c was performed on samples 2 and 7; however, judging from the similarities in v and Tt relaxation between samples 2, 5, and 7, we assumed that the
Table Coke
2 3 4 5 6 7 8 9 10 11 12 13 14 15
%H”
%Cb
f,“,
3.4 3.3 3.4 3.7 3.4 3.9 3.6 3.2 3.8 3.8 3.6 3.7 3.5 4.0 3.4
87.7 87.3 87.6 90.7 86.1 92.8 83.3 87.0 88.6 87.3 88.4 87.8 87.7 90.9 86.6
0.53 0.51 0.53 0.56 0.51 0.56 0.49 0.55 0.55 0.55 0.53 0.54 0.55 0.65 0.52
2. Structural
0.90 0.91 0.88 0.91 0.90 0.90 0.90 0.92 0.91 0.90 0.89 0.91 0.91 0.92 0.89
delay of 15 seconds satisfied the requirement for quantitative accuracy for these three samples. The Bloch decay experiment showed that no measurable change in the lineshape was found in the three cokes studied compared to the spectra taken under conditions of CPIMAS. The spin-counting experiment showed that 80% (* 10%) of all 13C nuclei in the samples were detected using direct excitation. In this experiment the integrated intensities of cokes’ spectra, including first-order spinning sideband intensities to the aromatic peak, were compared with the intensity of a standard sample of the same geometry (99.3% r3C enriched acetic acid). A correction for the intensity losses due to the spectrometer deadtime was performed by extrapolation of the FID to the initial value in time domain. The loss of 20% (5 10%) in intensity can be partly attributed to a direct influence of paramagnetic centers in cokes. The above results indicate that the r3C NMR spectra observed using cross polarization with a contact time of 1 ms accurately represented carbons in studied cokes and were justifiably used for quantitative analysis. Due to unfavorably fast eQ relaxation, the signal enhancement per one scan is CP experiments is below 2.0. However, the measurement time is still significantly reduced because of the ability to use must shorter delays between data acquisitions. 3.5 Summary of NMR results on petroleum cokes In this section, we will set forth the important conclusions that the techniques of solid state NMR used here and in our previous study[4] offer regarding coke chemistry. The proton and carbon aromaticities of petroleum cokes obtained in this work are summarized in Table 2 together with other structural parameters measured using NMR of ‘H and elemental analy-
parameters
0.43 0.33 -
Error estimates: f,“, (--kO.l);~$ (-+0.02);j$-H c--C 0.08). aEstimated from ‘H NMR and elemental analysis [4]. bAfter [4].
of petroleum
cokes
H/C
H/C,
H/C,
0.47 0.45 0.47 0.49 0.47 0.50 0.52 0.44 0.51 0.53 0.49 0.51 0.48 0.53 0.47
2.2 2.5 2.0 2.4 2.3 2.2 2.5 2.5 2.6 2.4 2.1 2.6 2.4 2.3 2.1
0.27 0.26 0.28 0.30 0.27 0.31 0.28 0.26 0.31 0.32 0.29 0.30 0.29 0.37 0.28
Average
formula/
1OOC
C$ HZ; C$, H$ OS,
C;:, HR C:l, H$! OS?
NMR of
petroleum
sis[4]. Included in Table 2 are hydrogen mass concentration in coals from the previous study[4], %H (because of some differences in hydrogen concentrations obtained from various elemental analyses and NMR[4J, the average of the obtained results is listed in Table 2); carbon mass concentration, %C, as determined by combustion analysis [4]; hydrogen and carbon aromaticities, f,“, and fFr, as obtained from high-resolution solid state NMR of ‘H and ‘“C; and the fraction of aromatic carbons with directly bonded protons,f!$“, obtained from t3C NMR with dipolar dephasing. Also, the total hydrogen to carbon (H/C) mole ratio was evaluated. The mole fractions (H/C),, and (H/C),, were then calculated usmg the formulae (H/C),, = (H/C) (fz!f,“, and (H/C),, = (H/C) (f,H,/f:$, I3ased on these results, the average stochiometric formuia per 100 carbon atoms was calculated for each coke and listed in the last column of Table 2. The general observation is that despite diversity of origin of the crude oils used in coking process, the products studied do not vary appreciably with respect to most of the experiments performed. With some exceptions, the respective parameters listed in Table 2 are the same within limits of experimental errors. The results of hydrogen spin counting yielded concentrations of hydrogen in cokes between 3.04 and 3.79 mass-%[4]. These values are generally slightly below those obtained from elemental anaiysis. Despite the possibility that a few percent of hydrogen may not be visible because of the interaction with paramagnetic centers, we concluded that NMR is a fast and reliable method for such determinations. Both 7’rand Tfff exhibit a distribution of values that could be interpreted as evidence for existence of macroscopic domains in cokes, collecting protons with different mobilities and/or concentrations[4]. However, the detailed analysis of the FID and the solid echo indicated monoexponential decay of proton magnetization with values of T,in the range of 19 to 20 gsec, in contrast to coals, where more than one component of the transverse relaxation is typically foundl251. The lack of components with longer T? indicates a rigid character of the coke structure, where only hindered motions of functional groups (e.g., CH,) take place. Also, the presence of a significant amount of isolated hydrogen atoms can be excluded; the cokes appear to have a rather homogeneous distribution of hydrogen. Assuming the homonuclear dipolar interaction to be the dominating broadening mechanism in ‘I-I NMR, the mean distance between protons in cokes of - 1.91 A can be calculated from the measured values of T,. The CRAMPS results indicate that a considerable portion of aliphatic hydrogen exists in the cokes studied. The spectra did not vary appreciably for different samples. Most of the cokes have hydrogen aromaticities between 0.49 and 0.56, with only one
cokes 11
47
sample being outside of this range (coke 14, with f,“, = 0.65). Still, the variation of hydrogen aromaticities is wider than the variation of similar values for carbon. Carbon aromaticities are higher than those of hydrogen for all cokes. The ‘%ZCPiMAS spectra indicate that despite some differences in lineshape and in relaxation behavior, as presented in 141. a remarkable similarity exists in the values of j?j,. which are all found to be in the range of 0.90 t 0.02. The aromatic peaks are narrower than those typically found for coals and represent aromatic C-H carbons and bridged carbons between aromatic rings. In the aliphatic portion of “C spectra, methyl carbons are distinguishable and comprise roughly 50% of the aliphatic peak intensity. No measurable concentrations of quaternary carbons were detected in this peak via dipolar dephasing technique. In most samples the (H/C),, values range between 2.2 and 2.5 with the lowest value of 2.0for coke 3 and the highest of 2.6 for coke 9. On the aromatic side, the (H/C),, values of 0.26-0.32 were found, which suggests a high degree of polycondensation of aromatic rings in cokes. The _f&--” values as obtained from dipolar diphasing experiments with variable delays have the same physical interpretation as (H/C),,. In sample 7 the two values are close. However, the discrepancy of 0.17 in sample 12 is larger than error estimates. This may be attributed to errors in reconstructed intensities of nonpr~~tonated carbons in a dipolar dephasing experiment[?O]. Another possible explanation of this result may be that some intermolecular dipolar dephasing occurs because of intermolecular interactions in a densely packed hydrocarbon system. In this case, the dipolar dephasing experiment yields overestimated results (also, assuming the j’;-” value for sample 2 as correct yields unrealistically low values of (H/C),,). It is noted that a good agreement between structural parameters of several coals derived from dipolar dephasing and CRAMPS data was found in a recent study by Hanna er fri.[26]. 4. ESTIMATION
OF AVERAGE CHEMlCAL FUNCTIONALITIES IN COKES
In studies of coals, the identification of individual constituents is very difficult, if not impossible. to accomplish[ 141. Chemical analyses and spectroscopic techniques can give indications of structural types present in these materials. Often, the “average structure” of coals is proposed as a result of such investigations. However, heterogeneity limits usefulness of “average” models for coals. Although caution should also be applied while searching for average structures in petroleum cokes, the results ofthe present studies by ‘H and 13CNMR indicate that these materials are more homogeneous than coals. An attempt to determine the structure of cokes is also facilitated by the low concentration
48
M.
PRUSKI et al.
Fig. 5. Structures of cokes invoking data from Table 2: (a) samples l-13 and 15; (b) sample 14.
of heteroatoms. The data in Table 2 show that in spite of different origins of the heavy crude oils used in the coking process, the products obtained exhibit similar structural parameters. With the exception of sample 14, the differences between average formulae derived in this work are too small to enable identification of distinctly different structural types for these cokes. Therefore, two basic structures are proposed, as illustrated in Figure 5: (a) a structure based on structural parameters and the average formula calculated using -35 carbons for cokes 1-13 and 15 (Figure 5a), and (b) a similar structure for sample 14 (Figue 5b). The term “basic” is used to underline the notion that Figure 5 indicates the structural types present in studied materials. Also, because of possible experimental errors in estimation of some of the structural parameters by NMR (see footnotes to Tables 1 and 2), relative errors of up to 20% in our estimations of (H/C),, and (H/C),, ratios cannot be excluded. The cokes studied in the present work may be viewed as high-molecular-weight conglomerates of structures formed by covalent bonds or methylene
bridges. As seen in Figure 5a, the bulk of cokes is well represented by polycyclic aromatic, hydrogendeficient structures with typical cluster size of the order of 10 hexagonal rings. These clusters carry relatively few substituents and result from aromatization with progressive loss of hydrogen and components of lower molecular weight. From 13CNMR it appears that the fraction of total carbon contained in methyl groups is of the order of 5%. It is reasonable to assume that most methyl groups in cokes exist in position (Yto aromatic rings, because the presence of long aliphatic chains is unlikely in the materials studied, which are products of pyrolysis. A similar finding was also reported for coal tar pitches[ 141. The remaining aliphatic hydrogen and carbon must exist in the form of methylene groups (see the (H/C),, ratios in Table 2), most likely in indane and flourene structures. Although no direct information on chemical functionalities of heteroatoms was derived from NMR, we assumed that oxygen and sulphur form furan- and thiophene-type structures, respectively. These types of structures represent predominant oxygen- and sulphur-con-
NMR of petroleum
species, found using high-resolution mass spectrometry in coals pyrolized at 45O”C[27]. Another study[28] also suggested that the coalification process causes the organic sulphur to change from thiols (-SH) through aliphatic sulfides (R-S-R) to thiophenic in condensation reactions. Even if small fractions of oxygen and sulphur existed in other functional groups (e.g., hydroxylic, carboxylic, or carbonylic oxygen, or thiolic or sulfidic sulphur), they would have marginal effect on the structures proposed in Figure 5, because of small overall concentrations of these heteroatoms. For coke 14 the (H/C),, ratio is higher, and smaller ring systems are expected (Figure 5b). The average structure has about five aromatic rings carrying less than one methyl group, methylene group, and heteroatom. Care must be exercised in comparing the present results with previous studies. From numerous investigations on petroleum-derived materials (carbonaceous mesophases. pitches cokes, graphites), it is well known that their structure and properties strongly depend on feedstock and the thermal processes used during their production[29,30]. Nevertheless, it is noted that the structures derived in this work are similar to those suggested earlier for several petroleum-derived carbons. For example, by using conventional transmission electron microscopy (TEM). the sizes of basic structural units of pitches of less than 12 fused benzene rings were obtained[3 1,321. Similar results were obtained by Smith er ~1. (reference [29], p. 63), who utilized liquid state NMR spectroscopy to study petroleum pitches with elemental composition very similar to those studied in this work. The analysis of their NMR spectra indicated proton aromaticity of 0.5 I, carbon aromaticity of 0.79, average alkyl group composition of C,,,H,,, and 3. I substituent groups in an average molecule. Average aromatic structures were based on seven-ring systems, which is less than proposed in this work, perhaps due to less advanced graphitization.
cokes
taining
A[,knoM,/rdgemc,nt-The authors are indebted to Dr. Duane Whitehurst of Mobil Research and Development for supplying the cokes studied in this work. We are also grateful to Dr. Whitehurst and to Dr. W. S. Trahanovsky of the Iowa State University Chemistry Department for invaluable advice regarding interpretation of the results.
7. 8. 9. IO. II. 12. 13 I4 1s 16 17 18. 19. 20. 21. 22. 23.
24. 25.
26. 27.
28. 29.
REFERENCES R. M. Davidson, Nuclecrr- Magnetic Resonance Studies of Coal. Report No. ICTISITR32, IEA Coal Research, London, UK (1986). 2. B. C. Gerstein and C. R. Dybowski, Transient Twh-
30.
I
31. 32.
II
49
niques in NMR oj’So/ids: An Inirodrcction to the Thcjory crnd Praclice. Academic Press, New York (1985). G. E. Maciel, C. E. Bronnimann, A. Jurkiewicr. R. A. Wind, and V. H. Pan, Flrc~l70, 925 ( 1991). D. Michel. M. Pruski, and B. C. Gerstein. Part 1. Carbon 32, D. Burum and W. K. Rhim, J. Mogn. Re.con. 34, 247 (1979); J. Che,n. Phys. 71, 944 (1979). W. K. Rhim. D. D. Elleman. and R. W. Vaughan. J. Chetn. Phys. 58, 1771 (1973); P. Mansfield, J. P/I?.\. c 4, 1444 (1971). H. Barkhuisen. R. de Beer, and D. van Ormondt, J. Mugn. Reson. 67, 371 (1986). J. Schaefer and E. 0. Stejskal, J. Am. Chem. SW. 98, 1031 (1976). S. J. Opella and M. H. Frey. J. Am. Chem. SW. 101, 5854 (1979). M. Pruski, D. K. Sanders, T. S. King. and B. C. Gerstein, J. Magn. Reson. 96, 574 (1992). E. 0. Stejskal and J. Schaefer. J. Mcr~n. Reson. 28, 560 (1977). E. 0. Stejskal and J. Schaefer. J. Morn. Rc.von. 28, I05 (1977). F. Engelke, T. Kind, D. Michel, M. Pruski, and B. C. Gerstein. J. Mugn. Reson. 95, 286 (1991). J. G. Speight. Chernicul Industries, Vol. 12, Dekker. New York, Ch 9. IO (1983). P. R. Solomon. In Adu. in Chew. Ser. No. 192. p. IO.5 (1981). C. E. Bronnimann, B. L. Hawkins. M. Zhang, and G. E. Maciel. Anal. Chern. 60, 1743 (1988). M. Mehring, PriwipleJ of High Resolrrrion NMR in Solids, Springer-Verlag. New York (1983). P. D. Murphy. T. J. Cassady. and B. C. Gerstein. Fltel61, 1233 (1982). K. D. Schmitt and E. W. Sheppard. F/rr/ 63, 1241 (1984). F. Witt, Ph.D. thesis, University of Leipzig (1991). R. H. Newman, /. Mug. Reson. 86, 176 (1990). M. S. Solum, R. J. Pugmire, and D. M. Grant, Enc,rg> & Fllel.7 3, I87 (1989). M. Pruski, X. Wu, M. W. Smale, B. C. Gerstein. and T. S. King, In Catalyst Deactivation (Edited by C. H. Bartholomew and J. B. Butt), p. 699. Elsevier Science Publishers B. V., Amsterdam (1991). D. Michel. Gmndlagen und Methoden dcr Kernrnrrgnetischen Resonunz, Akademie-Verlag. Berlin (1979). B. C. Gerstein, M. Pruski, and D. Michel. Proton NMR Spectroscopy of Coals, Cokes, and Coal-Derived Liquids. In Advances in CouI Spec~rrosc~op> (Edited by H. L. C. Meuzelaar), Ch. 9 Plenum Press. New York (1992). J. V. Hanna, A. M. Vassallo, and M. A. Wilson. E,lergy & Fuels, 6, 28 (1992). R. E. Winans and P. H. Neill, In Geochernis/ry of Sulphur in Fossil FueIs (Edited by W. L. Orr and C. M. White), ACS Symp. Ser. No. 429, p. 249 (1990). A. Attar and F. Dupuis. Preprint.\, Am. Chem. Sot.. Div. Fuel Chem., 24(l). 166 (1979). Petroleum Derived Carbons (Edited by R. F. Gould). ACS Symp. Ser. No. 21 (1976). Petroleum-Derived Carbons (Edited by J. D. Bacha. J. W. Newman. and J. L. White). ACS Symp. Ser. No. 303 (1986). A. Oberlin. Curbon, 22, 521 (1984). A. Oberlin, J. L. Boulmier. and B. Durand, Geochirn. Cosmoch. Acfa. 38, 647 (1974).
Pergamon 0008-6223(93)EOO53-N
NMR OF PETROLEUM COKES II: STUDIES BY HIGH RESOLUTION SOLID STATE NMR OF ‘H AND 13C Department
M. PRUSKI” and B. C. GERSTEIN of Chemistry and Ames Laboratory,** Iowa State University, Ames, Iowa 50011, U.S.A.
and Fachbereich
D. MICHEL Physik, Universitgit Leipzig, D-04103 Leipzig, Germany
(Received 19 June 1992; accepted Abstract-Chemical
in revised form 8 July 1993)
of hydrogen and carbon in a series of cokes obtained from heavy coker pilot unit are probed with high-resolution solid state NMR. The fractions of aromatic hydrogen and carbon, as determined from ‘H combined rotation and multiple-pulse spectroscopy (CRAMPS), and “C magic angle spinning (MAS) experiment with and without cross polarization (CP), varied only slightly between 0.49 and 0.65 and between 0.88 and 0.92, respectively, for the samples studied. A comparison with the results of direct excitation (‘)C MAS) NMR showed that CPiMAS NMR spectra taken with a contact time of 1 ms well represented relative carbon intensities. The high-resolution spectra, in combination with previously reported wideline ‘H NMR data and the results of elemental analysis, are used to derive several structural parameters, crude
functionalities
oils in a Mobile
continuous
flow laboratory
including aromatic and aliphatic hydrogen to carbon ratios and the average formula per 100 carbon atoms. Finally, the “average” structures for studied cokes are proposed and discussed. Most cokes are concluded to consist of molecules having approximately 10 aromatic rings bearing only few substitutions. Key Words-Petroleum structure
coke,
high-resolution
solid state
1. INTRODUCTION
correspondence
CRAMPS,
cross-polarization,
coke
To analyze the CRAMPS spectra, it is advantageous to use a superposition of the proton resonance lines by means of a nonlinear procedure for a spectral simulation, although the shortcomings of such computational methods, (c~.g., the dependence of the results on the selection of starting parameters) compared to linear prediction techniques (LPSVD[7]), are known. We will show that all spectra of studied cokes may be fit to a superposition of two groups of resonance frequencies centered approximately at 2 ppm (mostly methylene and methyl protons) and 7 ppm (aromatic protons). By using cross polarization (CP) in combination with magic angle spinning (CPIMAS) and strong dipolar decoupling[8], high-resolution “C spectra of coals and cokes can be obtained. Usually aromaticities of fossil fuel samples have been reported: however, more detailed information on carbon functionalities is also available. Several “resolution enhancement” experiments utilizing CP have been successfully applied in such systems. Most of these materials are heterogenous in structure and, as such, undergo multicomponent relaxation. Variable-contact time experiments and dipolar dephasing[9], based on differences in cross-polarization rates and on differences in the ‘H-13C interactions, respectively, are the most prominent examples of the techniques used in combination with CP/MAS. We based the present study of 13C NMR on the use
The development of high-resolution techniques in solid state NMR of ‘H and 13C has enabled nondestructive structural analysis of solid fossil materials[ I-31. The aim of this study is to use ‘H combined rotation and multiple-pulse spectroscopy (CRAMPS) and 13C magic angle spinning (MAS) with high-power proton decoupling to determine the average chemical composition and structure of several petroleum-derived cokes of diverse origin. These data will be combined with the results of quantitative analysis of hydrogen[4] and with the results of elemental analysis to provide insight into the structure of studied cokes and to develop better understanding of coke chemistry. The line separation in the CRAMPS NMR spectra of ‘H in fossil fuels is rather limited due to a broad distribution of chemical shifts in such systems [3]. Consequently, the application of more sophisticated multipulse sequences like BR-24[5] does not lead to a better resolution in these materials, and in the present work only the MREV-8 pulse sequence [6] is used. *To whom
NMR,
should
be addressed. at Ames Laboratory by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, under Contract No. W-7405-Eng-82, and by a grant from Mobil Research and Development.
**The authors’ work is supported
41
M. PRUSK~et
42
of CP/MAS, including variable contact time and dipolar dephasing experiments. In particular, we considered the quantitative aspect of CPJMAS measurements. For that reason, we observed i3C NMR signals of selected samples using direct excitation (Bloch decay experiment) under high-resolution conditions of proton decoupling. The i3C lineshapes thus obtained proved to be exact reproductions of CPiMAS spectra. Finally, i3C spin counting was performed on these samples to assure that the i3C intensities observed were representative for i3C nuclei in cokes.
tuning was found with the use of frequency sweep generator. During the cross-polarization experiments, the delay times of at least 5 times the largest value of T, observed for protons in the sample in question were used, while the cross-polarization times and the dipolar diphasing times varied from 10 ps to 15 ms and from 0 to 400 ks, respectively. A 180” refocussing pulse was used during the delay period in the dipolar dephasing experiment. All ‘H and i3C chemical shifts are referred to liquid TMS (zero shift) using the 6 scale with negative values being upfield.
2. EXPERIMENTAL The samples used in this study were Mobile cokes obtained in a coker pilot unit as described in 141. The ‘II NMR CRAMPS measurements were performed on an MSL-300 Bruker spectrometer at room temperature. The powdered samples (-200 mesh) were spun in an alumina rotor at 3.6 kHz, and the MREV-8 multipulse sequence with a cycle time tc = 127 = 36 ps was used to eliminate homonuclear dipolar broadening. The 90” pulse length was 1.5 ps. Eight signals were accumulated for each measurement, using a repetition time of 20 seconds. 13CNMR spectra were obtained on a home-built spectrometer operating at 25.15 MHz and 100.06 MHz for i3C and ‘H, respectively. A double-tuned, single-coil probe equipped with a modified Shoemaker-Apple type magic angle spinner[lO] was used for both cross-polarization and direct excitation experiments. No background signal was detected under the experimental conditions used for the cokes. One hundred and eighty to 250 mg of samples were sealed in 5-mm glass tubes and spun at 4.2-5 kHz using an air drive. In all i3C experiments, a spin temperature inversion method was used to reduce baseline artifacts and the effect of deadtime of the spectrometer[l 11. A free induction decay (FID) with a size of 2K data points was acquired using quadrature detection and Fourier transformed after extending the size of the FID to 8K by zero filling. All CPlMAS NMR spectra were obtained utilizing rf fields of 60 kHz: the proton field was then kept at the same level during the high-power proton decoupling period. As Stejskal and Schaefer pointed out[ 121, spinning may have a striking effect on the polarization process. To avoid experimental errors, the Hartmann-Hahn matching condition was carefully controlled by monitoring the i3C CPiMAS intensities of hexamethylbenzene versus i3C rf field. The characteristic oscillation pattern was observed for both CH3 and ring carbons similar to those reported in the earlier studies[ 131. The rf field corresponding to the center of symmetry of the pattern represented the exact Hartman-Hahn matching condition. No measurable impact of the presence of studied samples in the NMR coil on the probe
al.
3. RESULTS AND DISCUSSION
3. I CHAMPS Two examples of proton NMR CRAMPS spectra of cokes are shown in Figure la and b. All samples exhibited a strong absorption centered betwen 7.0 and 7.5 ppm with a full width at half height (fwhh) between 3.4 and 4.2 ppm (- l-l.25 kHz), associated with aromatic CH protons. Phenolic protons also resonate in this region. However, phenolic compounds undergo condensation reactions at temperatures lower than 400”C[14,151 and are not likely to exist as structural fragments of cokes. On the aliphatic side, the spectra consisted of two shoulders located at -1.5 (1-0.5) ppm and -4 (‘0.5) ppm, representing methyl groups a to aromatic rings and ring-joining methylenes, respectively. This is evidenced by the observed chemical shifts and the results of 13CNMR (vide infra), which showed the presence of CH, and CH, carbons. This conclusion is further supported by the aliphatic hydrogen-to-carbon ratios derived below. Olefinic =CHprotons, which typically resonate at -5-6
20
10
0
PPM FROM TMS Fig. 1. High-resolution solid state ‘H NMR (CRAMPS) spectra of cokes 7 (spectrum a) and 14 (spectrum b).
NMR of
than two lines did not lead to unambiguous results. Therefore, we included all aliphatic intensity into one peak and restricted its position between 2 and 2.5 ppm during the simulation.
ppm, are not expected to be found in abundance in cokes. The lineshape in Figure la is typical of most cokes investigated in this work in that the relative fractions of aromatic and aliphatic protons are almost comparable. In only one sample[l4, Figure lb], does the aromatic band strongly dominate the spectrum. The simulation of the experimental data was performed by means of DATAPLOT software on a MicroVAX 3600 computer. For the analysis of the spectra, a superposition of two Gaussian lines was assumed, F(o) = A,, exp[ -(w - o~J~/~A&,] + A,, exp] -(o - 0,,)*/2Ao$];
This approach resulted in fits that should best represent relative aromatic and aliphatic proton intensities in the CRAMPS spectra of cokes. Still, the above-mentioned difficulties caused the reliability of hydrogen aromaticity values derived in this work to be estimated at only -lo%, which is below that of 2-3% achieved by the CRAMPS method in more favourable cases[l6]. The results are given in Table 1. column 3, and will be used later in the discussion.
(1) 3.2 CPIMAS NMR Studies The purpose of the CPIMAS experiment was to obtain t3C NMR spectra that accurately represent all carbons in the cokes studied. It has been pointed out earlier (see, for example, ref. l] and the references therein) that the time evolution of carbon magnetization during the CP/MAS NMR experiment is a result of a complicated interplay between the spin-lattice relaxation processes in the rotating frame and cross polarization processes (described by relaxation times cp and Tcu), which can be further complicated by sample spinning. To optimize the conditions of our CP/MAS experiment, the aromatic “C intensities were monitored as a function of contact time for three cokes (samples 7, 2, and 14) characterized by the shortest, medium, and longest c,‘s. For sample 7 the r3C magnetization reaches its peak at a contact time of 0.25 ms, whereas for cokes 2 and 14 the maximum signal is observed at contact times of CCI0.75 ms and ca 1.O ms, respectively (see Figure 2). The experimental data were fitted to the three-parameter equation of
where A,, , A,, , war, and o,, are the amplitudes and positions of the aromatic and aliphatic peaks, respectively, hoi, and A& are the second moments of those peaks. Subsequently, the proton aromaticities, f,“, were calculated using the equation f,“, = (A,&,,)/(A,,Aw,,
+ A,,Aw,,).
(2)
Several steps were taken to assure the best possible reliability of the results obtained: l
l
l
43
petroleum cokes II
All spectra were acquired sequentially under the same experimental conditions and were subject to identical phase corrections. To minimize the effect of baseline distortions around the carrier frequency (- - 10 ppm), only the portions of spectra downfield from -5 ppm were analyzed. An attempt to simulate the CRAMPS results by partitioning the whole spectrum between more
Table I. Aromaticities, average peak locations, obtained
from
“C CP/MAS
and average linewidths NMR experiments for cokes “C NMR
.f,“,
.fz
Av. peak location (ppm)
0.53 0.51 0.53 0.56 0.51 0.56 0.49 0.55 0.55 0.55 0.53 0.54 0.55 0.65 0.52
.90 .91 .88 .91 .90 .90 .90 .92 .91 .90 .89 .91 .91 .92 .89
128 127 127 128 127 127 129 129 127 126 128 126 128 125 128
‘H NMR No.
Name resid resid refinery resid resid resid resid resid refinery refinery refinery refinery resid aromatic resid
I
2 3 4 5 6 7 8 9 10 11 12 13 14 15 Error
mix
mix mix mix mix
esimates:
ref. stream
f:
(-+O.l);Ec
(-20.02).
Av. line width
19 19 19 18 20 16 22 21 19 19 19 18 20 15 21
(ppm)
44
M. PRUSKI et al.
M42,) = &A-‘[I
- exp( - htCPITcH)l expf - r&7$,
(3)
where Iw, is the equilibrium magnetization of the ensemble of carbon spins, fcP is the contact time, and A = 1 - Tcu/~P[171. Figure 2 shows that the least-square fits thus obtained do not match the experimental results for cokes 2 and 14. The deviation can be understood, however, since neither the cross polarization nor the rotating frame relaxation processes are exponential. The time constant for cross polarization transfer is known to be strongly dependent on the proton-carbon distance and the mobility[ 171. For rigid, aromatic C-H functionai groups, Tcu is typically 0. I-O.2 ms and increases by a factor of three or more for nonprotonated aromatic carbons; aliphatic carbons are generally polarized faster than aromatic carbons. Our previous ‘H NMR studies[4] showed that lr”, relaxation processes in cokes are complex. A distribution of eP values was found in all samples, with a significant fraction (4559%) being less than 300 pus (see Table 3 in [41). The kinetics of cross polarization in cokes was not further investigated. However, it should be noticed that if the monoexponential decay (4) was fitted to the tail of the carbon magnetization curve (where it is governed primarily by T& processes), the resulting Tt values of 2.1 ms, 1.8 ms, and 9.0 ms obtained for cokes 2, 7, and 14, respectively, do not appreciably differ from the long qP
values derived with ‘H NMR (see Table 3 in [4]). The above discussion concerned only aromatic carbon intensities; however, no significant differences were noticed in the time evolution of aliphatic carbon magnetization for contact times greater than 0.5 ms. Consequently, a contact time of 1.O ms was chosen as a good compromise between sensitivity and quantitative accuracy requirements for further studies. The CPlMAS NMR spectra of cokes exhibit two distinct peaks located in the aliphatic and aromatic regions (Figure 3). The aliphatic part consists of a sharp peak with a linewidth of 12 ppm located at -21 ppm, assigned to -CH, groups on the basis of dipolar dephasing experiments. A second, superimposed broader feature of similar integrated intensity centered at -35 ppm can be distinguished and is primarily assigned to methylene carbons. In all cokes studied, the aromatic portion of spectra consists of a single, strong peak at 127.5 (L42.5) ppm with a linewidth of -19 ppm (see Table 1). There were observed differences in Iinewidth from this value for cokes 6 (16 ppm), 14 f 15 ppm), 7 (22 ppm), and 8 (21 ppm). Also, a 2-3 ppm upfield shift of the first moment was observed for samples 6 and 14, whereas a shift of about 2 ppm downfield was measured for samples 7 and 8. The detection of these lineshape changes may reflect differences in the dist~bution of chemical functionalities in the studied samples. In several spectra, a shoulder on the downfield side of the aromatic peak can be clearly seen. Attempts to perform a computer decomposition of aromatic intensities to extract more information on chemical composition did not, however, give unambiguous results (vide infra). Finally, a very weak
EXF’ER. FIT
0
1 0
I
2
I
I
4
1
COKE 2
0
...........
COKE 7
+
-
--__ *-__ ---___ ------.____ ----______ I I 1 I
6
8
10
12
CP CONTACT TIME (MS) Fig.2.
Variation in CPiMAS NMR intensities of aromatic carbons with cross-poiarization contact time for Mobil cokes and the least-square fits of experimental data to biexponential function (3).
NMR of petroleum
cokes
4s
II
cessfully used by applying a two-component Gaussian-Lorentzian decay to fit the experimental results[18,22,23] as in MU& = Mot exp[ - 0.5(tdlT&)Zl + MO,_expl- tdlTZ’,l. (6)
300
0
100
200 PPM
FROM
-100
ThtS
Fig. 3. “C CPIMAS NMR spectrum of coke 6 (tcp = I.0 ms, spinning speed U, = 4.4 kHz).
feature, not exceeding 2% of the total carbon intensity, could be distinguished for several samples (1,2, 6, 9) with the chemical shifts indicative of aromatic C-O carbons (- 180-200 ppm). Table 1 gives the fraction of aromatic carbons, as determined from CPiMAS spectra using the equation
where I”’ represents the integrated aromatic carbon intensity (chemical shifts >90 ppm), including intensities from spinning sidebands; r”’ is the remaining (aliphatic) carbon intensity. 3.3 Dipolar dephasing To differentiate carbons with directly bonded protons from those without protons in ‘)C CPiMAS NMR spectra of a solid sample, a dipolar dephasing experiment can be used[9,18,19]. In this experiment a short delay t, (dipolar dephasing period) is introduced at the end of cross polarization and prior to data acquisition in which both ‘H and ‘jC rf fields are turned off. During the dephasing period “C magnetization decays because of static ‘jC-‘H dipolar interactions, which can be modified by molecular motion, spin diffusion, and sample spinning[20,21]. The typical decay time constants in rigid systems are 15-32 p.s for protonated carbons and from 50 to several hundred ps for carbons that are mobile (CH,) and/or two or more bonds away from a nearest proton. The initial part of the decay can be described by a Gaussian function with a time constant proportional to the heteronuclear carbon-proton second moment MZLs. For longer dephasing times the evolution of magnetization can be better characterized by a Lorentzian decay[20,21]. It is noted that spinning of the sample modulates the interactions responsible for dephasing, which may lead to oscillations in the “C magnetization versus td, especially in case of high spinning frequencies and inefficient spin diffusion[20,2 I]. However, in several applications the dipolar dephasing technique has been suc-
where M,,, MO,_are the initial intensities, and T,,; and TZ’_are the respective time constants of the two decays. For rigid protonated carbons the (larger) heteronuclear second moments may be derived from the rapidly decaying Gaussian component. For carbons located two or more bonds away from a nearest proton, it has been shown[20] that the (smaller) values of Mzls can be evaluated from T?L using the formula M I,s = (CUT&‘. The parameter (Ydepends on the MAS frequency and has been estimated for nonprotonated carbons to be u = 1.93 (kO.06 x lo-’ s for MAS frequency of 4 kHz[20]. In this work “C aromatic intensities have been monitored as a function of dephasing time for cokes 2 and 7 by integrating the Fourier transformed ‘“C NMR signals. The data were subsequently analyzed using eqn (6). For cokes 2 and 7, the Gaussian component of dipolar dephasing signal decayed with a time constant TZGof 27 (+3) ps and represented 47% (~5%) and 37% (~5%) of aromatic carbons, respectively (Figure 4). The remaining magnetization decayed with a time constant TZLof 500 (2200) ps. The reported decay constants have values similar to those found previously for coals[22]. A TzGof 27 ps corresponds to the heteronuclear second moment Mzls of 1.4 x 10’ s -*, typical for carbons strongly coupled to protons (C-H). The second moment Mzls of - 10’ s ’ evaluated from T?,_(for MAS frequency of 4 kHz) is typical of carbons located two or more bonds away from a nearest proton. Surprisingly, almost no change in lineshape of the aromatic peak was observed as the dephasing time increased from 0 to 400 ws. The result may be contrasted with that found in coals, where a dipolar de-
0
EXPER GAUSSIAN
DIPOLAR
DEPHASMG
TIME,
(US)
Fig. 4. Fit of the dipolar dephasing data for coke 7 (aromatic intensities only).
PRUSKI et al.
M.
46
phasing time of 60 ps led to a decrease of intensity in the upfield portion of the aromatic region associated with aromatic C-H perimeter carbons. Only a very slight (about 3 ppm) downfield shift was monitored; this indicated that most of the nonprotonated carbons resonate at around 130 ppm. This chemical shift most likely corresponds to bridged carbons between the aromatic rings. It also indicates that for the cokes studied, an attempt to fit the aromatic portion of the r3C spectrum with a superposition of peaks will lead to results that are devoid of physical meaning and do not represent different chemical functionalities of carbon. The results of the dipolar dephasing experiment will be analyzed later in comparison with those obtained from high-resolution solid state ‘H NMR and elemental analysis. 3.4 NMR Studies of 13C by direct excitation To address the potential quantitation problem in CP/MAS spectra, the direct excitation (Bloch decay), r3C NMR lineshape, and spin-counting measurements were performed with selected samples 2, 5, 7 under high resolution conditions of MAS and proton decoupling. It is known that under proper experimental conditions, the initial intensity of the Bloch decay signal is proportional to the number of nuclei of interest in the sample. However, in the presence of paramagnetic centers, which are abundant in cokes under study[4], some of the resonances may be broadened and/or shifted beyond the limits of observation. To assure that the repetition time will be 2 5 Ty, the progressive saturation experiment [24] was carried out for coke 5 to estimate the value of carbon longitudinal relaxation time. Five delays, ranging from 1 to 20 seconds, were used, which yielded an estimated T,c of 3.8 s (kO.5) for this sample. No direct measurement of T,c was performed on samples 2 and 7; however, judging from the similarities in v and Tt relaxation between samples 2, 5, and 7, we assumed that the
Table Coke
2 3 4 5 6 7 8 9 10 11 12 13 14 15
%H”
%Cb
f,“,
3.4 3.3 3.4 3.7 3.4 3.9 3.6 3.2 3.8 3.8 3.6 3.7 3.5 4.0 3.4
87.7 87.3 87.6 90.7 86.1 92.8 83.3 87.0 88.6 87.3 88.4 87.8 87.7 90.9 86.6
0.53 0.51 0.53 0.56 0.51 0.56 0.49 0.55 0.55 0.55 0.53 0.54 0.55 0.65 0.52
2. Structural
0.90 0.91 0.88 0.91 0.90 0.90 0.90 0.92 0.91 0.90 0.89 0.91 0.91 0.92 0.89
delay of 15 seconds satisfied the requirement for quantitative accuracy for these three samples. The Bloch decay experiment showed that no measurable change in the lineshape was found in the three cokes studied compared to the spectra taken under conditions of CPIMAS. The spin-counting experiment showed that 80% (* 10%) of all 13C nuclei in the samples were detected using direct excitation. In this experiment the integrated intensities of cokes’ spectra, including first-order spinning sideband intensities to the aromatic peak, were compared with the intensity of a standard sample of the same geometry (99.3% r3C enriched acetic acid). A correction for the intensity losses due to the spectrometer deadtime was performed by extrapolation of the FID to the initial value in time domain. The loss of 20% (5 10%) in intensity can be partly attributed to a direct influence of paramagnetic centers in cokes. The above results indicate that the r3C NMR spectra observed using cross polarization with a contact time of 1 ms accurately represented carbons in studied cokes and were justifiably used for quantitative analysis. Due to unfavorably fast eQ relaxation, the signal enhancement per one scan is CP experiments is below 2.0. However, the measurement time is still significantly reduced because of the ability to use must shorter delays between data acquisitions. 3.5 Summary of NMR results on petroleum cokes In this section, we will set forth the important conclusions that the techniques of solid state NMR used here and in our previous study[4] offer regarding coke chemistry. The proton and carbon aromaticities of petroleum cokes obtained in this work are summarized in Table 2 together with other structural parameters measured using NMR of ‘H and elemental analy-
parameters
0.43 0.33 -
Error estimates: f,“, (--kO.l);~$ (-+0.02);j$-H c--C 0.08). aEstimated from ‘H NMR and elemental analysis [4]. bAfter [4].
of petroleum
cokes
H/C
H/C,
H/C,
0.47 0.45 0.47 0.49 0.47 0.50 0.52 0.44 0.51 0.53 0.49 0.51 0.48 0.53 0.47
2.2 2.5 2.0 2.4 2.3 2.2 2.5 2.5 2.6 2.4 2.1 2.6 2.4 2.3 2.1
0.27 0.26 0.28 0.30 0.27 0.31 0.28 0.26 0.31 0.32 0.29 0.30 0.29 0.37 0.28
Average
formula/
1OOC
C$ HZ; C$, H$ OS,
C;:, HR C:l, H$! OS?
NMR of
petroleum
sis[4]. Included in Table 2 are hydrogen mass concentration in coals from the previous study[4], %H (because of some differences in hydrogen concentrations obtained from various elemental analyses and NMR[4J, the average of the obtained results is listed in Table 2); carbon mass concentration, %C, as determined by combustion analysis [4]; hydrogen and carbon aromaticities, f,“, and fFr, as obtained from high-resolution solid state NMR of ‘H and ‘“C; and the fraction of aromatic carbons with directly bonded protons,f!$“, obtained from t3C NMR with dipolar dephasing. Also, the total hydrogen to carbon (H/C) mole ratio was evaluated. The mole fractions (H/C),, and (H/C),, were then calculated usmg the formulae (H/C),, = (H/C) (fz!f,“, and (H/C),, = (H/C) (f,H,/f:$, I3ased on these results, the average stochiometric formuia per 100 carbon atoms was calculated for each coke and listed in the last column of Table 2. The general observation is that despite diversity of origin of the crude oils used in coking process, the products studied do not vary appreciably with respect to most of the experiments performed. With some exceptions, the respective parameters listed in Table 2 are the same within limits of experimental errors. The results of hydrogen spin counting yielded concentrations of hydrogen in cokes between 3.04 and 3.79 mass-%[4]. These values are generally slightly below those obtained from elemental anaiysis. Despite the possibility that a few percent of hydrogen may not be visible because of the interaction with paramagnetic centers, we concluded that NMR is a fast and reliable method for such determinations. Both 7’rand Tfff exhibit a distribution of values that could be interpreted as evidence for existence of macroscopic domains in cokes, collecting protons with different mobilities and/or concentrations[4]. However, the detailed analysis of the FID and the solid echo indicated monoexponential decay of proton magnetization with values of T,in the range of 19 to 20 gsec, in contrast to coals, where more than one component of the transverse relaxation is typically foundl251. The lack of components with longer T? indicates a rigid character of the coke structure, where only hindered motions of functional groups (e.g., CH,) take place. Also, the presence of a significant amount of isolated hydrogen atoms can be excluded; the cokes appear to have a rather homogeneous distribution of hydrogen. Assuming the homonuclear dipolar interaction to be the dominating broadening mechanism in ‘I-I NMR, the mean distance between protons in cokes of - 1.91 A can be calculated from the measured values of T,. The CRAMPS results indicate that a considerable portion of aliphatic hydrogen exists in the cokes studied. The spectra did not vary appreciably for different samples. Most of the cokes have hydrogen aromaticities between 0.49 and 0.56, with only one
cokes 11
47
sample being outside of this range (coke 14, with f,“, = 0.65). Still, the variation of hydrogen aromaticities is wider than the variation of similar values for carbon. Carbon aromaticities are higher than those of hydrogen for all cokes. The ‘%ZCPiMAS spectra indicate that despite some differences in lineshape and in relaxation behavior, as presented in 141. a remarkable similarity exists in the values of j?j,. which are all found to be in the range of 0.90 t 0.02. The aromatic peaks are narrower than those typically found for coals and represent aromatic C-H carbons and bridged carbons between aromatic rings. In the aliphatic portion of “C spectra, methyl carbons are distinguishable and comprise roughly 50% of the aliphatic peak intensity. No measurable concentrations of quaternary carbons were detected in this peak via dipolar dephasing technique. In most samples the (H/C),, values range between 2.2 and 2.5 with the lowest value of 2.0for coke 3 and the highest of 2.6 for coke 9. On the aromatic side, the (H/C),, values of 0.26-0.32 were found, which suggests a high degree of polycondensation of aromatic rings in cokes. The _f&--” values as obtained from dipolar diphasing experiments with variable delays have the same physical interpretation as (H/C),,. In sample 7 the two values are close. However, the discrepancy of 0.17 in sample 12 is larger than error estimates. This may be attributed to errors in reconstructed intensities of nonpr~~tonated carbons in a dipolar dephasing experiment[?O]. Another possible explanation of this result may be that some intermolecular dipolar dephasing occurs because of intermolecular interactions in a densely packed hydrocarbon system. In this case, the dipolar dephasing experiment yields overestimated results (also, assuming the j’;-” value for sample 2 as correct yields unrealistically low values of (H/C),,). It is noted that a good agreement between structural parameters of several coals derived from dipolar dephasing and CRAMPS data was found in a recent study by Hanna er fri.[26]. 4. ESTIMATION
OF AVERAGE CHEMlCAL FUNCTIONALITIES IN COKES
In studies of coals, the identification of individual constituents is very difficult, if not impossible. to accomplish[ 141. Chemical analyses and spectroscopic techniques can give indications of structural types present in these materials. Often, the “average structure” of coals is proposed as a result of such investigations. However, heterogeneity limits usefulness of “average” models for coals. Although caution should also be applied while searching for average structures in petroleum cokes, the results ofthe present studies by ‘H and 13CNMR indicate that these materials are more homogeneous than coals. An attempt to determine the structure of cokes is also facilitated by the low concentration
48
M.
PRUSKI et al.
Fig. 5. Structures of cokes invoking data from Table 2: (a) samples l-13 and 15; (b) sample 14.
of heteroatoms. The data in Table 2 show that in spite of different origins of the heavy crude oils used in the coking process, the products obtained exhibit similar structural parameters. With the exception of sample 14, the differences between average formulae derived in this work are too small to enable identification of distinctly different structural types for these cokes. Therefore, two basic structures are proposed, as illustrated in Figure 5: (a) a structure based on structural parameters and the average formula calculated using -35 carbons for cokes 1-13 and 15 (Figure 5a), and (b) a similar structure for sample 14 (Figue 5b). The term “basic” is used to underline the notion that Figure 5 indicates the structural types present in studied materials. Also, because of possible experimental errors in estimation of some of the structural parameters by NMR (see footnotes to Tables 1 and 2), relative errors of up to 20% in our estimations of (H/C),, and (H/C),, ratios cannot be excluded. The cokes studied in the present work may be viewed as high-molecular-weight conglomerates of structures formed by covalent bonds or methylene
bridges. As seen in Figure 5a, the bulk of cokes is well represented by polycyclic aromatic, hydrogendeficient structures with typical cluster size of the order of 10 hexagonal rings. These clusters carry relatively few substituents and result from aromatization with progressive loss of hydrogen and components of lower molecular weight. From 13CNMR it appears that the fraction of total carbon contained in methyl groups is of the order of 5%. It is reasonable to assume that most methyl groups in cokes exist in position (Yto aromatic rings, because the presence of long aliphatic chains is unlikely in the materials studied, which are products of pyrolysis. A similar finding was also reported for coal tar pitches[ 141. The remaining aliphatic hydrogen and carbon must exist in the form of methylene groups (see the (H/C),, ratios in Table 2), most likely in indane and flourene structures. Although no direct information on chemical functionalities of heteroatoms was derived from NMR, we assumed that oxygen and sulphur form furan- and thiophene-type structures, respectively. These types of structures represent predominant oxygen- and sulphur-con-
NMR of petroleum
species, found using high-resolution mass spectrometry in coals pyrolized at 45O”C[27]. Another study[28] also suggested that the coalification process causes the organic sulphur to change from thiols (-SH) through aliphatic sulfides (R-S-R) to thiophenic in condensation reactions. Even if small fractions of oxygen and sulphur existed in other functional groups (e.g., hydroxylic, carboxylic, or carbonylic oxygen, or thiolic or sulfidic sulphur), they would have marginal effect on the structures proposed in Figure 5, because of small overall concentrations of these heteroatoms. For coke 14 the (H/C),, ratio is higher, and smaller ring systems are expected (Figure 5b). The average structure has about five aromatic rings carrying less than one methyl group, methylene group, and heteroatom. Care must be exercised in comparing the present results with previous studies. From numerous investigations on petroleum-derived materials (carbonaceous mesophases. pitches cokes, graphites), it is well known that their structure and properties strongly depend on feedstock and the thermal processes used during their production[29,30]. Nevertheless, it is noted that the structures derived in this work are similar to those suggested earlier for several petroleum-derived carbons. For example, by using conventional transmission electron microscopy (TEM). the sizes of basic structural units of pitches of less than 12 fused benzene rings were obtained[3 1,321. Similar results were obtained by Smith er ~1. (reference [29], p. 63), who utilized liquid state NMR spectroscopy to study petroleum pitches with elemental composition very similar to those studied in this work. The analysis of their NMR spectra indicated proton aromaticity of 0.5 I, carbon aromaticity of 0.79, average alkyl group composition of C,,,H,,, and 3. I substituent groups in an average molecule. Average aromatic structures were based on seven-ring systems, which is less than proposed in this work, perhaps due to less advanced graphitization.
cokes
taining
A[,knoM,/rdgemc,nt-The authors are indebted to Dr. Duane Whitehurst of Mobil Research and Development for supplying the cokes studied in this work. We are also grateful to Dr. Whitehurst and to Dr. W. S. Trahanovsky of the Iowa State University Chemistry Department for invaluable advice regarding interpretation of the results.
7. 8. 9. IO. II. 12. 13 I4 1s 16 17 18. 19. 20. 21. 22. 23.
24. 25.
26. 27.
28. 29.
REFERENCES R. M. Davidson, Nuclecrr- Magnetic Resonance Studies of Coal. Report No. ICTISITR32, IEA Coal Research, London, UK (1986). 2. B. C. Gerstein and C. R. Dybowski, Transient Twh-
30.
I
31. 32.
II
49
niques in NMR oj’So/ids: An Inirodrcction to the Thcjory crnd Praclice. Academic Press, New York (1985). G. E. Maciel, C. E. Bronnimann, A. Jurkiewicr. R. A. Wind, and V. H. Pan, Flrc~l70, 925 ( 1991). D. Michel. M. Pruski, and B. C. Gerstein. Part 1. Carbon 32, D. Burum and W. K. Rhim, J. Mogn. Re.con. 34, 247 (1979); J. Che,n. Phys. 71, 944 (1979). W. K. Rhim. D. D. Elleman. and R. W. Vaughan. J. Chetn. Phys. 58, 1771 (1973); P. Mansfield, J. P/I?.\. c 4, 1444 (1971). H. Barkhuisen. R. de Beer, and D. van Ormondt, J. Mugn. Reson. 67, 371 (1986). J. Schaefer and E. 0. Stejskal, J. Am. Chem. SW. 98, 1031 (1976). S. J. Opella and M. H. Frey. J. Am. Chem. SW. 101, 5854 (1979). M. Pruski, D. K. Sanders, T. S. King. and B. C. Gerstein, J. Magn. Reson. 96, 574 (1992). E. 0. Stejskal and J. Schaefer. J. Mcr~n. Reson. 28, 560 (1977). E. 0. Stejskal and J. Schaefer. J. Morn. Rc.von. 28, I05 (1977). F. Engelke, T. Kind, D. Michel, M. Pruski, and B. C. Gerstein. J. Mugn. Reson. 95, 286 (1991). J. G. Speight. Chernicul Industries, Vol. 12, Dekker. New York, Ch 9. IO (1983). P. R. Solomon. In Adu. in Chew. Ser. No. 192. p. IO.5 (1981). C. E. Bronnimann, B. L. Hawkins. M. Zhang, and G. E. Maciel. Anal. Chern. 60, 1743 (1988). M. Mehring, PriwipleJ of High Resolrrrion NMR in Solids, Springer-Verlag. New York (1983). P. D. Murphy. T. J. Cassady. and B. C. Gerstein. Fltel61, 1233 (1982). K. D. Schmitt and E. W. Sheppard. F/rr/ 63, 1241 (1984). F. Witt, Ph.D. thesis, University of Leipzig (1991). R. H. Newman, /. Mug. Reson. 86, 176 (1990). M. S. Solum, R. J. Pugmire, and D. M. Grant, Enc,rg> & Fllel.7 3, I87 (1989). M. Pruski, X. Wu, M. W. Smale, B. C. Gerstein. and T. S. King, In Catalyst Deactivation (Edited by C. H. Bartholomew and J. B. Butt), p. 699. Elsevier Science Publishers B. V., Amsterdam (1991). D. Michel. Gmndlagen und Methoden dcr Kernrnrrgnetischen Resonunz, Akademie-Verlag. Berlin (1979). B. C. Gerstein, M. Pruski, and D. Michel. Proton NMR Spectroscopy of Coals, Cokes, and Coal-Derived Liquids. In Advances in CouI Spec~rrosc~op> (Edited by H. L. C. Meuzelaar), Ch. 9 Plenum Press. New York (1992). J. V. Hanna, A. M. Vassallo, and M. A. Wilson. E,lergy & Fuels, 6, 28 (1992). R. E. Winans and P. H. Neill, In Geochernis/ry of Sulphur in Fossil FueIs (Edited by W. L. Orr and C. M. White), ACS Symp. Ser. No. 429, p. 249 (1990). A. Attar and F. Dupuis. Preprint.\, Am. Chem. Sot.. Div. Fuel Chem., 24(l). 166 (1979). Petroleum Derived Carbons (Edited by R. F. Gould). ACS Symp. Ser. No. 21 (1976). Petroleum-Derived Carbons (Edited by J. D. Bacha. J. W. Newman. and J. L. White). ACS Symp. Ser. No. 303 (1986). A. Oberlin. Curbon, 22, 521 (1984). A. Oberlin, J. L. Boulmier. and B. Durand, Geochirn. Cosmoch. Acfa. 38, 647 (1974).