Open Access

Carbon isotopic evidence for the catalytic origin of light hydrocarbons

  • Frank D. Mango1
Geochemical Transactions20001:38

DOI: 10.1186/1467-4866-1-38

Received: 18 July 2000

Accepted: 22 August 2000

Published: 6 September 2000

Abstract

The molecular proportionality between C6 and C7 isomers reported recently (F. D. Mango, Geochim. Cosmochim. Acta, 2000, 64, 1265; ref. 1) is probably the strongest evidence for catalysis yet published. It implicates two cyclopropane-like precursors, [S6] and [S7] (where S denotes any substrate), of similar structures, each yielding three isomers along similar kinetic pathways:

[S6] → n-hexane + 2-methylpentane + 3-methylpentane

[S7] → n-hexane + 2-methylhexane + 3-methylhexane

This view is supported here by the carbon isotope ratios of these isomers in 36 oils from Western Canada (M. J. Whiticar and L. R. Snowdon, Org. Geochem., 1999, 30, 1127; ref. 2). They exhibit strong correlations in δ13C, consistent with their being formed in triads through isotopically indistinguishable precursors. These results add significantly to the growing body of evidence supporting catalysis.

Introduction

There can be little doubt that light hydrocarbons (C1–C9) can be produced thermally from decomposing hydrocarbons in sedimentary rocks.[3] Although other pathways have always seemed possible (e.g., catalysis[4, 5]), they were rarely given serious consideration until it became clear that (a) ordinary hydrocarbons should remain stable under the time–temperature conditions typically seen in sedimentary rocks, [69] and (b) thermal cracking in the laboratory does not produce a gas resembling natural gas. [8, 1016] Catalysis gained additional recognition in 1987 when an invariance in isoheptanes was disclosed.[17] That work introduced steady-state kinetics as a critical, if not necessary, element to light hydrocarbons (LH) genesis, thereby undermining thermal cracking as the sole explanation.

Catalysis by acidic clay minerals [1820] and reduced transition metals[21] were offered as alternative sources of LHs. However, only the latter has reproduced the composition of natural gas in the laboratory. [2225] LHs exhibit a striking molecular proportionality consistent with a catalytic origin through cyclopropane-like intermediates,[1] a mechanism independently supported elsewhere.[26, 27] An isotopic analysis of these same hydrocarbons is reported here. The data used are from Whiticar and Snowdon[2] who reported the molecular and isotopic compositions for 26 LHs in 42 oils and condensates from Western Canada.

Results and discussion

Assume that hexane and heptane isomers originate as suggested by Mango[1] and illustrated in Fig. 1.
Figure 1

A kinetic scheme for catalytic isomerization through a cyclopropane-reduced metal oxide intermediate: [S6] (n = 1), and [S7] (n = 2); Fig. 1 of Mango.[1] [S] is a cyclopropyl-transition metal complex formed from some substrate S. The actual structure of [S] is unspecified and should not be inferred from the figure (see Mango[1] for discussion). Cyclopropanes may or may not exist as distinct entities. However, their inclusion as distinct entities coordinated to a catalytic site best illustrates the hypothetical process where three isomers are kinetically linked to a common intermediate.

If the kinetic pathways [S6] → [n-C6 + 2-MP + 3-MP] and [S7] → [n-C7 + 2-MH + 3-MH] are energetically similar, as would be the case in Fig. 1 for example, then the following proportionality obtains:

[(n-C6)(MHs)]/[(MPs)(n-C7)] = α     (1)

(where MHs = 2-methylhexane + 3-methylhexane; MPs = 2-methylpentane + 3-methylpentane).

The LH in crude oils obey eqn. (1) to a remarkable degree. Fig. 2 shows the correlation between [(n-C6)(MHs)] and [(MPs)(n-C7)] in concentrations of wt.% total oil (r2 = 0.99; ref. 1). α is tightly constrained to a mean of 0.75 with a standard deviation (s) of 0.20 (mean centered), significantly below those of the ratios composing α: s = 0.42 for (n-C6/n-C7), 0.46 for (n-C6/MPs), 0.51 for (n-C6/MHs), and 0.41 for (MPs/MHs).
Figure 2

A plot of [(n-C6)*(MHs)]1/2 vs. [(MPs)*(n-C7)]1/2 for 900 crude oils; Fig. [2] in Mango (ref. 1). MHs = (2-MH + 3-MH); MPs = (2-MP + 3-MP). Concentrations are in wt.% total oil. The data are plotted as square root to constrain the scale to average concentrations. The line is the linear regression line: intercept = -0.068; slope = 1.01; r2 = 0.991. Mean for the ratio [(n-C6)*(MHs)]/[(MPs)*(n-C7)] = 0.75 ± 0.31s. Mean-centered s = 0.20.

Moreover, the variability of α is unique to its particular combination of product functions. Its s of 0.20 increases by a factor of four in [(MPs)(MHs)]/[(n-C6)(n-C7)] and by a factor of almost five in [(n-C7)(MHs)]/[(n-C6)(MPs)] (ref. 1). Thus, at all concentrations, [(n-C6)(MHs)] and [(MPs)(n-C7)] express a strong and significant correlation in crude oils, perhaps the strongest yet disclosed among LHs, while a remains nearly constant.

This relationship establishes a genetic link between [n-C6 + 2-MP + 3-MP] and [n-C7 + 2-MH + 3-MH] pointing to structurally similar precursors. It would be reinforced if it could also be shown that the six LH reflect isotopically similar precursors. Although δ13C for [S6] and [S7] cannot be measured directly, they can be calculated from the weighted sums:

δ13C([S6]) = a δ13C(2-MP) + b δ13C(3-MP)+ c δ13C(n-C6)     (2)

δ13C([S7]) = d δ13C(2-MP) + e δ13C(3-MP) + f δ13C(n-C6)     (3)

(where a, b and c (d, e, and f) are the molecular fractions of the respective isomers; a + b + c = 1, and d + e + f = 1). Thus, δ13C's for [S6] and [S7] can be calculated from the δ13C's and molecular fractions of the six isomers.

Whiticar and Snowdon[2] published this data for the LH in 42 oils from Western Canada. Table 1 was constructed from their data. It contains 36 of their oils, including all with sufficient data to calculate δ13C ([S6]) and δ13C ([S7]) except for two, possibly altered oils. Fig. 3 shows [S6] to be isotopically indistinguishable from [S7]. The mean for δ13C ([S6])/δ13C ([S7]) = 1.00 ± 0.024s, which is within the experimental error reported for this data (± 0.5s).
Table 1

Light hydrocarbon data taken from Whiticar and Snowdon.[2] δ13C values are averages of multiple analyses, δ13C [C6] and δ13C [C7] (the last two columns) were calculated from eqn. (2) and eqn. (3), respectively with coefficients a, b and c calculated from the respective C6 concentrations normalized to 1 and coefficients d, e, and f calculated from the respective C7 concentrations normalized to 1. The amount of 2-MP (in %) was taken from column five of Whiticar–Snowdon's Table 3 labeled 3DMC4 incorrectly. Six oils in Whiticar–Snowdon's set of 42 oils were excluded from this set: Brazeau PA was excluded because of possible thermochemical sulfate reduction (TSR), Fusilier was not included because of low n-alkanes and thus possible biodegradation. Four other oils were excluded because they did not contain the full suite of data required to calculate δ13C [C6] and δ13C [C7]: Brazeau River F, Chester, Foothills 8, and Manyberries.

 

2MP (%)

3MP (%)

nC6 (%)

δ13C 2MP

δ13C 3MP

δ13C n C6

2MH (%)

3MH (%)

nC7 (%)

δ13C 2MH

δ13C 3MH

δ13C nC7

δ13C [C6]

δ13C [C7]

Blueberry

5.48

3.89

4.13

-28.3

-27.45

-28.1

8.09

7.08

3.42

-26.65

-27.9

-28.9

-27.9939

-27.5400

Bonanza

6.27

4.56

4.94

-29.03

-29.4

-30

3.3

5.67

2.86

-28.9

-29.47

-30.8

-29.4408

-29.6325

Brazeau C

4.49

3.29

3.26

-27.9

-25.7

-29.7

2.53

4.16

2.3

-26.6

-26.8

-30.5

-27.7759

-27.6903

Brazeau D

1.67

1.28

3.49

-26.8

-25

-29

3.25

4.3

10.09

-26.2

-26.8

-29.9

-27.6345

-28.4626

Brazeau M

4.72

3.28

2.44

-24.6

-24.25

-26.5

5.52

4.58

1.45

-24.2

-25.3

-27.2

-24.9341

-25.0128

Brazeau Y

2.39

1.75

3.79

-21.8

-21.9

-23.7

6.92

5.56

11.16

-21.6

-23.5

-24.8

-22.7301

-23.5575

Cecil

5.58

4.18

2.86

-28.6

-27.6

-29.4

3.95

6.35

2.26

-26.5

-27.9

-29.5

-28.4501

-27.7476

Cherhill

6.34

4.98

3.91

-27.5

-26.3

-28.8

3.93

7.22

3.46

-25.6

-26.55

-29.1

-27.4414

-26.8984

C. del Bonita

8.78

5.95

6.41

-27.2

-26.7

-28.4

9.91

5.49

1.72

-26.3

-27.2

-29

-27.4231

-26.8599

Dunvegan

2.9

1.88

6.34

-25.1

-24.8

-26

4.02

4.01

12.91

-24

-26.1

-26.7

-25.5624

-26.0668

Foothills 12

3.7

2.37

7.57

-25.1

-23.8

-25.4

6.21

5.06

12.62

-24.2

-25.9

-26.5

-25.0406

-25.7751

Glenevis

8.89

6.06

5.86

-27.5

-26.85

-28.1

5.44

6.36

2.53

-25.55

-27.2

-28.5

-27.4797

-26.8031

Home 34

3.64

2.33

6.95

-25.3

-25.3

-25.9

6.1

4.84

11.38

-24.9

-26.3

-27

-25.6228

-26.2743

Hutch

6.12

5.59

1.78

-26.7

-25.9

-27.1

4.09

7.16

1.21

-23.9

-25.6

-27.4

-26.4213

-25.2168

Loon

3.4

2.82

1.75

-27.4

-25.6

-29.3

2.51

4.33

1.49

-25.3

-26.25

-29.5

-27.1803

-26.5451

Lousana

4.62

4.22

1.95

-28.7

-26.3

-30.1

1.6

3.64

1.03

-26.2

-27.1

-30.7

-28.0144

-27.4617

Medicine River

5.14

3.58

4.2

-25.55

-24.95

-25.7

5.04

5.71

3.41

-24.2

-26.15

-26.55

-25.4325

-25.5523

Miracle 1

6.05

3.73

9.54

-26

-25.2

-26.4

6.44

5.03

9.01

-24.7

-26.1

-27

-26.0431

-26.0557

Moose Mtm

4.92

3.93

6.42

-24.55

-23.85

-25.85

9.64

6.34

6.74

-23.85

-23.9

-25.9

-24.9164

-24.4721

Moose Mtm 2

3.77

2.89

8.24

-24.7

-23

-25.3

7.17

4.71

13.38

-23.2

-23.6

-25.8

-24.7021

-24.6518

Otter

4.22

2.98

6.7

-27.8

-26.1

-29.7

1.97

4.66

9.65

-26

-26.7

-29.9

-28.3514

-28.5121

Plato

5.26

4.11

2.88

-25.9

-25.5

-26.9

4.19

5.49

1.11

-23.9

-25.8

-27.4

-26.0009

-25.2268

Provost

5.21

3.68

3.61

-25.9

-26

-26

4.92

5.5

2.59

-24.4

-26.2

-26.9

-25.9583

-25.6586

Rainbow 1

5.49

4.09

8.5

-25.55

-24

-27.75

5.15

7.1

14.13

-23.9

-24.2

-28.9

-26.2337

-26.6589

Rainbow 2

3.85

3.3

6.36

-26.9

-25.2

-29.3

5.05

7.48

14.11

-24.3

-24.7

-28.4

-27.6146

-26.5839

Redwater

3.55

2.8

4.78

-27.5

-25.8

-29

3.11

4.68

8.05

-25.6

-26.4

-29.9

-27.7165

-28.0217

Rycroft

4.15

2.83

7.1

-27.05

-26.01

-29.15

3.78

4.92

11.89

-27.25

-28.9

-30.95

-27.8999

-29.7809

S. Eureka

4.91

3.23

7.48

-26.5

-26

-27.15

5.48

4.81

9.22

-24.85

-26.5

-27.95

-26.7079

-26.7218

Sage Creek

0.64

0.56

0.97

-25.15

-25.25

-24.9

5.56

4.77

8.8

-23.2

-25.55

-25.55

-25.0641

-24.8670

Sylvan Lake

6

4.21

4.03

-25.9

-25.8

-25.8

2.13

5.29

2.19

-24.25

-25.3

-25.55

-25.8421

-25.1242

Turner Valley

0.78

0.66

1.26

-23.3

-22.8

-24.05

4.88

5.78

7.53

-22.7

-24.1

-25.4

-23.5278

-24.2626

Virginia Hills

4.71

3.51

7.86

-24.9

-24.3

-26.9

8.27

6.44

7.19

-24.55

-25.35

-28.1

-25.7466

-25.9508

Wainwright

6.19

4.19

3.19

-25.7

-25.6

-26

5.76

6.78

1.69

-23.8

-25.8

-26.5

-25.7396

-25.0736

Wallesden Gr.

5.05

3.83

4.37

-26.5

-25.8

-27.7

4.62

5.7

2.91

-24.7

-26.3

-27.9

-26.6934

-26.0932

Wembley

5.44

3.38

6.71

-29.7

-28.5

-30.4

4.93

6.04

6.14

-28.1

-28.9

-30.8

-29.7413

-29.3513

Zama

4.71

3.06

7.66

-25.9

-25.3

-28

5.19

6.74

15.43

-23.4

-24.8

-28.8

-26.8235

-26.7903

Figure 3

A plot of carbon isotope ratios (‰) for presumed intermediates [S6] and [S7], where δ13C ([S6]) = a δ13C (2-MP) + b δ13C (3-MP) + c δ13C (n-C6) and δ13C ([S7]) = d δ13C (2-MH) + e δ13C (3-MH) + f δ13C (n-C7). Data shown in Table 2. The reported analytical error for δ13C's is ± 0.5‰s (ref. 2). The mean for δ13C ([S6])/δ13C ([S7]) = 1.00 ± 0.02s. For the linear regression, r2 = 0.83, slope = 0.90 and intercept = -2.48.

Fig. 2 (ref. 1) provides molecular evidence for structurally similar precursors, like the hypothetical intermediates [S6] and [S7] in Fig. 1. Fig. 3 is consistent with this, implicating isotopically indistinguishable precursors. Whiticar and Snowdon[2] came to a similar conclusion: "these isotopic distributions among isomers are strong evidence suggesting that the formation of these gasoline-range hydrocarbons is intricately linked to the isotopic signature of the precursor molecules from which they are derived".

Fig. 3 does not exclude the conventional view that LH are thermal descendents of higher isoprenoids and n-alkanes (ref. 28). But it is difficult to explain the two correlations (Fig. 2 and 3) by this mechanism. They suggest a catalytic agent guiding the course of reaction through structurally similar intermediates. Irrespective of how these six LH might originate (catalytically or thermally), however, their molecular and isotopic correlations establish a genetic link (↔) between n-alkanes and isoalkanes that traverses carbon number and is fundamental to the origin of LH:

[n-C6 ↔ 2-MP ↔ 3-MP] ↔ [n-C7 ↔ 2-MH ↔ 3-MH]

Declarations

Acknowledgements

I am grateful to the US Department of Energy for Grant DE-FG05-92ER14295. I thank Alan Young for his review of the manuscript and helpful comments.

Authors’ Affiliations

(1)
Department of Chemical Engineering, Rice University

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Copyright

© The Royal Society of Chemistry and the Division of Geochemistry of the American Chemical Society 2000