C NMR and Mass Spectrometry - American Chemical Society

Chapter 8

Characterization of Fluid Catalytic Cracking Catalyst Coke by C NMR and Mass Spectrometry 13

1

1

1

2

B. J. McGhee , J. M . Andresen , C. E. Snape , R. Hughes , C. L. Koon , and G. Hutchings 2

3

Downloaded by TUFTS UNIV on June 5, 2018 | https://pubs.acs.org Publication Date: June 6, 1996 | doi: 10.1021/bk-1996-0634.ch008

1

Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow G1 1XL, United Kingdom Department of Chemical Engineering, University of Salford, Salford M5 4WT, United Kingdom Department of Chemistry, Leverhulme Centre for Innovative Catalysis, Liverpool University, Liverpool L69 3BX, United Kingdom 2

3

Coke has been concentrated from deactivated FCC catalysts via demineralisation to facilitate characterisation by solid state C NMR and mass spectrometry. The catalysts were obtained from refinery operations with a residue feed and a hydrotreated vacuum gas oil and from microreactivity runs with n-hexadecane and various additives. As for solid fuels, the use of a low-field spectrometer in conjunction with the single pulse excitation (or Bloch decay) technique has enabled quantitative carbon skeletal parameters to be obtained from C NMR for the cokes. Internal standard measurements demonstrated that most of the carbon is detectable by SPE and, therefore, NMR-invisible graphitic layers are not thought to be major structural features of the cokes. Differences in feedstock composition were reflected in the structure of the cokes with the aromatic nuclei being most highly condensed in the residue-derived coke and corresponding to 15-20 peri-condensed aromatic rings. 13

13

In view of the importance of fluid catalytic cracking (FCC) to petroleum refining, the deactivation of cracking catalysts via coke deposition has been the subject of considerable investigation over the past 50 years (1-3). As well as being formed via the actual cracking reactions associated with the strongly acidic catalytic sites, coke can arise in F C C units from (i) normal thermal reactions, (ii) dehydrogenation reactions promoted by metals - Ni and V in heavy feeds and (iii) entrained catalyst products, symptomatic of incomplete stripping in the regenerator. The contributions of these different mechanisms is clearly going to be dependent on the type of feedstock, together with the design and operation of particular FCC units. However, the lack of detailed basic knowledge on the chemical nature of coke is preventing progress towards a proper overall mechanistic understanding and a clear identification of the different coke-forming routes in FCC operations.

0097-6156/96/0634-0117$15.00/0 © 1996 American Chemical Society

O'Connor et al.; Deactivation and Testing of Hydrocarbon-Processing Catalysts ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

118

DEACTIVATION AND TESTING OF HYDROCARBON-PROCESSING CATALYSTS

Fundamental deactivation studies involving NMR on zeolites thus far have generally involved excessively high concentrations of coke in relation to normal FCC operation where catalysts are regenerated typically after only oa 1% w/w of carbon has been deposited. The high concentrations have been necessary to achieve sufficient sensitivity for characterising the coke on the deactivated catalysts by solid state C NMR. For example, Groten et al (4) have investigated coke formation on zeolite USHY with 1-hexene as the feed with coke levels of oa 5% w/w. Lange and coworkers (5) used C-enriched ethene to follow the formation of polyaromatic structures on H-mordenite. Deactivated y -alumina-supported hydroprocessing catalysts have also been characterised by solid state C NMR (6) but, again due to the relatively low sensitivity, the spectra have been obtained by cross polarisation (CP) which fails to observe all the carbon in coals and oil shales and usually discriminates heavily against aromatic carbon (7). Further problems are posed by high magnetic field strengths where, to eliminate spinning sidebands, special pulse sequences or extremely rapid magic-angle spinning (MAS, >10 kHz) are needed. There is a general consensus that the use of low field strengths with Bloch decay or single pulse excitation (SPE) offers the best compromise for quantitative C NMR analysis of coals and solid fuels (7-10), albeit with a considerable sacrifice in sensitivity since long recycle times (with 90° pulses, 5 times the C thermal relaxation times -Tis) are required to ensure that the C magnetisation fully regains equilibrium. In principle, the only carbon not observed is that in the vicinity of paramagnetic centres which obviously includes graphite. The only way this methodology can be applied successfully to deactivated catalysts is by demineralising the aluminosilicate matrices to concentrate the coke. This approach has been used to characterise two coke concentrates isolated from deactivated FCC catalysts containing only oa 1% w/w carbon. The samples chosen for investigation were from refinery runs with an atmospheric residue and a hydrogenated vacuum gas oil (HVGO) in order to ascertain whether the use of these two vastly different feedstocks had a significant effect on coke composition. The degree of condensation of the aromatic structure has been assessed from the proportions of non-protonated aromatic carbon derived by the SPE C NMR technique. Information on the coke concentrates from the refinery catalysts and the MAT runs with n-hexadecane has also been obtained by mass spectrometry. The latter extends our earlier study on the roles of quinoline and phenanthrene as poisons and coke inducers in n-hexadecane cracking where only the whole catalysts, as opposed to coke concentrates, were characterised by mass spectrometry (77, 12). A laboratory-scale fluidised-bed reactor has been used to enable sufficently large quantities of deactivated catalyst containing oa 1% coke (oa 80 g) to be obtained from n-hexadecane for the demineralisation-quantitative C NMR methodology. 1 3

13


1 3

1 3

1 3

1 3

1 3

1 3

Experimental Catalysts and their demineralisation. The FCC catalysts were typical commercial formulations and the deactivated samples were obtained from units processing (i) a heavy feedstock containing oa 1.5% sulphur, a Conradson carbon content of 5.0% w/w and Ni and V contents of 5 ppm each and (ii) a HVGO containing only 0.1% sulphur and below 2 ppm of Ni and V. The catalyst samples deactivated in the MAT reactor were from a series runs with n-hexadecane as the base feed with either phenanthrene or quinoline as a co-feed present at concentrations of 1 and 10% v/v (77, 72). The all-silica laboratory-scale fluidised-bed reactor used had a 4 cm diameter bed and, during each run, oa 30 g n-hexadecane was fed into the reactor containing 80 g of catalyst. The catalysts "as received" all had carbon contents close to 1%. They were


8. McGHEE ET AL.

FCC Catalyst Coke by

C NMR & Mass Spectrometry 119

first refluxed in chloroform for 3 hours to remove any entrained molecular species and then vacuum-dried prior to demineralisation. This initial treatment reduced the carbon content of the catalyst deactivated with the atmospheric residure from 1.08 to only 1.00% (estimated error of ± 0.05%), indicating that molecular species only account for an extremely small proportion of the coke. This observation is consistent with the fact that both catalysts were removed from the FCC units after stripping (collected at the base of the stripper). The cokes were concentrated by applying the standard demineralisation procedure for solid fuels (13,14) to the chloroform-extracted catalysts. This involved successive extraction with 2M hydrochloric acid (stirring overnight at 60°C) and 40% hydrofluoric acid (HF), the HCl-extracted sample being stirred at room temperature for 4 hours with 20 cm of HF being used per gram of sample. The coke concentrates werefinallywashed with dilute hydrochloric acid to remove any remaining inorganic paramagnetics prior to collection in plastic filtration equipment. The vacuum-dried coke concentrates recovered from the refinery catalysts deactivated by the residue and HVGO feedstocks had carbon contents of 55 and 39%, respectively. The n-hexadecane-derived coke concentrate had a carbon content of 33%.


3

1 3

1 3

C NMR. The CP and SPE C NMR measurements on the coke concentrates were carried out as previously for coals (8-10) at 25 MHz on a Bruker MSL100 spectrometer with MAS at 4.5-5.0 kHz to give spectra in which the sideband intensities are only ca 3% of the central aromatic bands. Known weights of tetrakis (trimethylsilyl)silane (TKS) were added to the samples as the internal reference and to facilitate estimation of the fraction of the total carbon observed by SPE. Approximately ca 150 mg of sample was packed into the zirconia rotors. The H decoupling and spin-lockfieldwas ca 60 kHz and, for SPE, the 90° C pulse width was 3.4 ms. A recycle delay of 50 s was employed between successive 90° pulses in SPE since the C Ti for the non-protonated aromatic carbons were 10 s as measured by Torchia's CP-based method (15) for both coke concentrates (Table 1). Normal CP spectra of the residue-derived coke were obtained using a range of contact times between 0.05 and 8 ms for the residue-derived coke to facilitate determination of the time constants for CP (TCH) and H rotating-frame relaxation (Ti ). The CP spectrum of the HVGO and n-hexadecane-derived cokes were obtained using a contact time of 1 ms. Dipolar dephasing (DD) was combined with both SPE and CP to estimate the proportions of protonated and non-protonated aromatic carbon, and at least 8 separate dephasing periods in the range 5-500 ms were used (500 scans each). In order to check that the tuning had remained virtually constant throughout the duration of the DD experiments, the delays were arranged in a random order and between 500 and 1000 scans were accumulated for each delay. The spectra were processed using exponential line broadening factors of either 30 or 50 Hz. No background signal was evident in the SPE spectra from the Kel-F rotor caps The measurement of aromatic and aliphatic peak areas manually was found to be generally more precise than using the integrals generated by the spectrometer software. !

1 3

1 3

l

r

Mass Spectrometry. Mass spectrometry was conducted on the coke concentrates using a VG instrument in which the probe was heated from ambient to 500°C at a rate of 20°C min and spectra over the mass range 50-600 were recorded every 5 s. Spectra were recorded in both electron impact (EI) and chemical ionisation (CI, with ammonia) modes. Field ionisation (FI) spectra of some of the deactivated catalysts from the n-hexadecane MAT runs were obtained at the Stanford Research Institute as described elsewhere (16). 1


120 1 3


C NMR analysis of the cokes from the refinery feeds 1 3

Quantitative aspects. Figures 1 and 2 compare the CP and SPE C spectra of the highly aromatic refinery coke concentrates and Figure 3 presents the decays of the aromatic peak intensities in the CP and SPE-DD experiments on the residuederived coke. The carbon skeletal parameters obtained from the SPE and CP C NMR experiments for both samples are summarised in Table 1. The first coke concentrate obtained from the catalyst deactivated with the residue feedstock gave a broad aromatic band, possibly due to the incomplete removal of the rare-earth during the HC1 wash. Indeed, it was found that, after the final HC1 wash, much narrower spectral bands were obtained and the quality of the spectra shown are comparable to those obtained for low-volatile coals and anthracites (with similar aromaticities as the cokes„ see following). The sensitivity is obviously superior in the CP spectra (7-10,000 scans for CP compared to 1000-2000 for SPE, Figures 1 and 2) but, as found for many coals < \ CP significantly underestimates the carbon aromaticity using relatively short contact times (Table 1, CP values for 1 ms contact, estimated error ± 0.01). This arises from the non-protonated aromatic carbons, in particular, cross polarising much more slowly than the aliphatic carbons (both CH2 and CH3 in the residue coke have much shorter TCHS than the aromatic carbon, Table 1). At longer contact times, the discrimination against aromatic carbon is not quite as acute, but the aromaticity values obtained are still lower than that by SPE (Figure 4). This is indicative that some aromatic carbon, presumed to be in the vicinity of free radicals, is polarised at all ( H TifS are too short - < 02 0.2 ms, the values in Table 1 are only for hydrogens adjacent to the carbons being polarised). The fact that 70 and 90% of the carbon in the residue and HVGOderived coke concentrates, respectively, has been observed by SPE demonstrates that the procedure is reasonably quantitiatve and that graphitic layers are probably not present in significant amounts. If these were major structural features, the resultant paramagnetism would have detuned the probe resulting in little of the carbon being detectable.


1 3

911

!

Aromatic structure. As for the total aromaticity, CP also grossly underestimates the fraction of non-protonated aromatic carbon determined by dipolar dephasing (Table 1, estimated error ± 0.02). This can be seen in the decay of the aromatic peak intensity (Figure 3); the faster relaxing Gaussian component for the protonated aromatic carbon has virtually decayed completely after oa 60 ms with the slower relaxing exponential component from the non-protonated carbon having a time constant of over 500 ms. After the intial decay of the protonated aromatic carbon, the intensity of the remaining non-protonated carbon is modulated by the rotation of the rotor at 5 kHz, this effect being particularly evident in Figure 3 for the CP experiment (note that this modulation is not encountered in variable contact time CP measurements due to the much longer timescale). From the fractions of non-protonated aromatic carbon derived by SPE (Table 1), it is estimated that bridgehead aromatic carbons (CBR/CAR) account for oa 67 and 56% of the total aromatic carbon in the residue and HVGO-derived cokes, respectively. The only assumptions needed are that (i) each aliphatic carbon is bound to one aromatic carbon, which is not unreasonable in light of the distribution of aliphatic carbon (see following) and (ii) the concentrations of heteroatoms in the cokes are relatively small (total concentration corresponding to less than oa 2 mole % carbon). If peri-condensed aromatic strucures are drawn to fit the CBR/CAR values (Table 1), 15-20ringsare required for the residue feedstock coke compared to only 812ringsfor the HVGO sample. This represents a significant difference in aromatic structure which is considered to arise primarily from the major differences in feedstock


McGHEE ET AL.

FCC Catalyst Coke by

1 3

C NMR & Mass Spectrometry

Aromatic


carbon

i . . . .

200 Figure 1

150

100 PPM

.1 . . . .

50

i

. . . .

t

0 1 3

CP (1 ms contact time) and SPE C NMR spectra of the coke concentrate from FCC catalyst deactivated using the residue feedstock.




200 Figure 2

150

100 PPM

50

0 1 3

CP (1 ms contact time) and SPE C NMR spectra of the coke concentrate from FCC catalyst deactivated using the HVGO feedstock.



8. McGHEE ET AL.

13

FCC Catalyst Coke by C NMR & Mass Spectrometry 123

B CP \

• SPE

—

—

I

100

I

—

300

200

—

I

—

400

—

I

500

Tau/us

Figure 3

Decay of the natural logarithm of the aromatic peak intensity in the CP and SPE C NMR dipolar dephasing experiments on the coke concentrate from FCC catalyst deactivated using the residue feedstock (initial intensity assigned value of 10). 1 3




124

Figure 4

1 3

Variation of aromaticity as function of contact time in CP C NMR for the coke concentrate from FCC catalyst deactivated using the residue feedstock.


13

8. McGHEE ET AL.


composition and, along with other factors, particularly the Ni and V concentrations, could well affect combustion behaviour in the regenerator. Table 1. NMR and other structural parameters for the coke concentrates Parameter C T\l s for non-protonated aromatic carbon.

Residue feedstock 10.0

1 3

CP-derived parameters obtained from variable contact time experiment, TCH/ ms, aromatic C CH CH Ti,7 ms, aromatic CH CH CH


b

2

3

2

3

330 40 70 8 6 5

HVGO feedstock 11.5

N.D. N.D. N.D. N.D. N.D. N.D.

a

Carbon aromaticity, f , (± 0.0 l) CP (1ms) SPE

0.91 0.97

0.85 0.96

Fraction of aromatic carbon that is nonprotonated from dipolar dephasing experiments (± 0.02) , CP (1 ms) SPE

0.55 0.67

N.D. 0.56

Fraction of aromatic carbon that is bridgehead, C R/CAR (SPE, ± 0.03)

0.63

0.51

Fraction of aliphatic carbon that is CH3 (10-24 ppm range, SPE, ± 0.1 )

0.75

0.50

a

a

a

B

a

% of carbon observed by SPE

70

90

N.D. = not determined. SPE = single pulse excitation. CP = cross-polarisation = estimated errors. = average value for protonated and non-protonated carbons. a

b

Aliphatic structure. Although the cokes are clearly highly aromatic in character, some information on the distribution of aliphatic groups can also be obtained from the C NMR spectra. Intuitively, one would expect virtually all the aliphatic carbon to be adjacent to aromatic rings in either arylmethyl or diarylmethylene groups. The fraction of aliphatic carbon present as methyl has been estimated from the intensity of the 10-24 ppm chemical shift range (Table 1, a fairly clear separation between CH2 and CH3 chemical shifts for the aliphatic strucures in high-rank coals occurs at 23-24 ppm). Figure 1 indicates that CP overestimates the fraction of CH3 for the residue-derived coke; CH3 cross polarises at a slower rate than CH2 which is reflected in characteristic time constants, TCH (Table 1). The SPE spectrum (Figure 1) 1 3


126


suggests that arylmethyl groups account for oa 75% of the aliphatic carbon in the residue-derived coke (Table 1). Although the signal to noise levels of the aliphatic bands are not good, this fraction appears to be much higher than for the HVGOderived coke where CH3 accounts for approximately only half of the aliphatic carbon (Figure 2 and Table 1). The larger proportion of C H (and CH if present) in the HVGO-derived coke is again consistent with the differences in composition between the two feedstocks. By definition, the more aliphatic HVGO contains higher concentrations of both long chain alkyl and naphthenic moieties than the atmospheric residue. 2


Mass Spectrometry Cokes from the refinery feeds. Figure 5 shows two of the probe EI mass spectra obtained from the refinery coke concentrate prepared from the residue feedstock. Very little material evolved below 100°C which is probably an indication that the chloroform extraction was reasonably successful at removing the entrained products. Those which evolved were characteristic of alkylated species (major peaks at 55 and 57 obtained in spectrum at 55°C) and clearly, in view of the extremely high aromaticity of the coke (Table 1), these represent only a negligible quantity of the total organic matter. Most of the volatiles from the coke evolved above 400°C and gave a series of peaks characteristic of 4-8 ring polynuclear aromatics (Figure 5, bottom spectrum). Again, these volatile species represent only a small fraction of the organic matter and are clearly much less condensed than the bulk of the coke structure (15-20 rings, see earlier). Cokes from n-hexadecane M A T runs. Mass spectrometry indicates that the coke concentrate from n-hexadecane possesses considerably more aliphatic character than the refinery cokes (Figure 6). Even at high probe temperatures, the EI mass spectra are still dominated by fragments characteristic of alkylated species (m/z 43 and 44) and 1 and 2 ring aromatics (e.g. m/z 77,91, 105 and 117) with only minor peaks being observed for >3 ring polynuclears. It should be noted that the coke concentrates gave far better sensitivity than previously obtained for the whole coked catalysts (77, 12), allowing the higher molecular mass aromatic fragments to be observed. FIMS has the advantage of giving parent ions for the volatiles evolved from the coke and the principal classes of species identified are alkanes, mono and dialkenes and alkylbenzenes up to C o- Two catalyst samples coked using nhexadecane were analysed, the first sample being cooled immediately after reaction under a nitrogen purge. The second was deactivated with a feed containing 10% / phenanthrene in n-hexadecane and was left for 30 minutes at reaction temperature under a nitrogen purge before cooling to simulate the stripping action of an FCC unit. Figure 7 compares the relative ion intensities of dialkene alkyl benzene species from both samples and it can be seen that the stripping action simulated with the phenanthrene-derived coke gives rise to increasing proportions of both the dialkene and alkyl benzene species. Although, just as for EI, FIMS data are clearly not representative of the total coke, these results clearly suggest that there is a considerable increase in aromatic content during the stripping period used here in the MAT. 2

w

w


n}



8. McGHEE ET AL.



100

10(h

41

55

^43

.57 73

Figure 6

1

|^

140

,fT

194 207208.221 /

i

-

r(t-fffp'itf •(•rrt*rp-rff|"rrfr|fTT^f|ffrr(tTt t-jff tTfTfTf |*f

239 253 / 277 312 180 200 220 240 260 280 300 320 340

JV"1

160

1 4 9

490 °C

rr-TT ft t 11 j n

65 °C

r p-rrr|' i M r p t i i |

EI mass spectra of coke concentrates obtained from nhexadecane as the feed in a MAT run, top scan taken at 65°C and the bottom scan at 490°C.

120

LlC

105 117

104


f i jt

McGHEE ET AL.

U

FCC Catalyst Coke by C NMR & Mass Spectrometry


a) Dialkene Intensities from FIMS

Carbon Number

b) Alkyl Benzene Intensities from FIMS

6

7

8

9

10

11

12

Carbon Number

Figure 7

FI mass spectra relative intensities of a) dialkenes and b) alkyl benzenes from coke concentrates obtained from neat nhexadecane and 10% / phenanthrene in n-hexadecane as the feeds in MAT runs. w

w


130



Aromatic carbon


i J r

8. McGHEE ET AL. 1 3


C NMR analysis of coke from n-hexadecane feed 1 3

Figure 8 shows the CP and SPE C NMR spectra of the n-hexadecanederived coke concentrate obtained from thefluidised-bedreactor. The coke is again largely aromatic but to a much lesser degree than for the refinery cokes with the aromaticity (total sp carbon) values derived from the CP and SPE spectra both being close to 0.80. The sp carbon envelope contains a small contribution from carboxyl and carbonyl groups (Figure 8 , - 5 mole % carbon, 175-205 ppm). These probably arose from accidental exposure of the coked catalyst to hot air during recovery from the fluidised-bed reactor. The presence of a reasonably high proportion of aliphatic carbon is consistent with the EI mass spectrometry results where alkyl fragments were the most intense for the corresponding MAT cokes. The distribution of intensity in the aliphatic envelope which is centred at ca 30 ppm indicates that CH2 dominates over methyl which is again in agreement with the mass spectrometry results where the major alkyl fragments observed arise from chains of at least 4/5 carbons. 2


2

Conclusions The coke formed in FCC refinery operations is highly aromatic in nature and comprises large polynuclear groups. This structure is dependent, however, on the nature of the feedstock. n-Hexadecane in MAT and fluidised-bed tests givesriseto a coke with a considerable greater degree of aliphatic character with only small contributions from polynuclears being observed in probe mass spectrometry. The demineralisation-quantitative C NMR methodology successively demonstrated here for refinery catalysts is now being extended to other cokes prepared from nhexadecane in the laboratory-scale fluidised-bed reactor to further elucidate the influence of different compound classes on coke structure. 1 3

Acknowledgements The authors thank the Engineering & Physical Sciences Research Council (EPSRC) for financial support of this work through Grant No. GR/H/24990 (including a studentship for BJ. McGhee) and the Mass Spectrometry Service Centre at the University of Wales. Dr. N.J. Gudde of BP Oil International Ltd. (Oil Technology Centre, Sunbury-on-Thames, Middlesex TW16 7LN, UK) supplied the deactivated refinery catalysts. The authors are indebted to Dr. R. Malhotra of Stanford Research Institute for obtaining the FIMS results. Literature Cited 1. 2. 3. 4. 5. 6.

Butt, J.B., Catalyst Deactivation, Adv. Chem. Ser. 1972, 109, 259. Hughes, R. Deactivation ofCatalysts, Academic Press, London 1984. Wolf, E.H.; Alfani, A, Cat. Rev. Sci. Eng., 1982, 24, 329 and references therein. Groten, W.A.; Wojciechowski, B.W.; Hunter, B.K, J. Catal. 1990, 125 311. Lange, J.P.; Gutsze, A.; Allgeier, J.; Karge H.G., Appl. Catal. 1988, 45, 345. Egiebor, N.O.; Gray, M.R.; Cyr, N. Appl. Catal. 1989, 55, 81.


132

7. 8. 9. 10. 11. 12.


13. 14. 15. 16.


Snape, C.E.; Axelson, D.E.; Botto, R.E.; Delpuech, J.J.; Tekely, P., Gerstein, B.C.; Pruski, M.; Maciel, G.E.; Wilson, M.A., Fuel, 1989, 68 547 and references therein. Franz J.A.; Garcia R.; Linehan J.C.; Love G.D.; Snape C.E. Energy & Fuels, 1992, 6 , 598. Love G.D.; Law R.V.; Snape C.E., Energy & Fuels, 1993,7 , 639. Maroto-Valer M.M.; Love G.D.; Snape C.E., Fuel, 1994, 73, 1926. Hughes R.; Hutchings G.; Koon C.L.; McGhee B.J.; Snape C.E., Preprints, Am. Chem. Soc., Div. of Petrol. Chem., 39, 1994(3) , 379. Hughes, R.; Hutchings; G., Koon, C.L.; McGhee, B.; Snape, C.E., in Catalyst Deactivation , Elsevier, 1994 , Delmon, B.; Froment, G.F., Eds., Studies in Surface Science and Catalysis, 1994, 88, 377. Saxby, J.B Chem. Geol., 1970, 6 , 173 and in Yen T.F.; Chilingarian G.V. (Eds.), Oil Shale, Elsevier, 1976, 103. Durand B.; Nicaise G., Kerogen-insoluble organic matter from sedimentary rocks; Durand B. Ed., Editons Technip, Paris, 1980, 35. Torchia D.A., J. Magn. Reson., 1978, 30 , 613. Malhotra, R., McMillen, D.F. and Huestis, D.L., Preprints, Am. Chem. Soc. Div. Fuel Chem., 1991, 36(3), 1252.


C NMR and Mass Spectrometry - American Chemical Society

Recommend Documents