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Recent improvements in benzene chemistry for radiocarbon dating
1094
Geochemical
notes
In general, the dispersions for rubidium and strontium results are greater on both levels t,han those for potassium and argon. F tests show that there are no significant differences in the interlaboratory and intralaboratory precisions except for the rubidium measurements. The wi& variation in results for common strontium may be because strontium is a t,race element in P-ZO’i or because of contamination in mineral analyses as described by WASSERBVL’R~ et al. (1964,. Rubidium-strontium ages calculated using the mean rubidium and 87Sr,,d values from each laboratory have a total range of IO.7 m.y. and the mean Rb-Sr age is significantly different tluau the mean K-Ar age at the 5% level of signiilcance if the “geologically determined” half-lifct of 50 x IO9 yr is used for *‘Rb. It is interesting to note that for this mineral, the K-Ar end Rb-Sr ages (81.0 and 83.2 m.y., respectively) are in good agreement if t,he 47 ‘, IO9 yr half-lifts determined by liquid scintillation counting is used for 87Rb. REFERENCES CROW E. L., DAVIS F. A. and MAXFIELD M. W. (1960) &atistica _Ifon~clZ. Dover Publications. LANPHERE M. A. and DAI.RYMPLE G. B. (1965) P-207: An interlaboratory standard musoovite for ergon and potassium analyses. J. Geophys. Res. 70, 3497-3503. In The IWmk of Geolog!j MCINTYRE D. B. (1963) Precision and resolution in geochronometry. (editor C. C. Albritton), pp. 112-134. Addison-Wesley Publishing Co., Inc., Reading, Mtws. WASSERBURG G. J., WEN T. and ARONSOX J. (1964) Strontium contamination in mineralanalyses. Geochim. Cosmochim. Acta 28, 407-410.
Recent improvementsin benzene chemistry for radimkn dating Joas
SO~KES,
Special Training
(Receiwd
STEPHEN
KIM
and LAWRENCE
AKERS
Division, Oak Ridge Institute of Nuclear Oak Ridge, Tennessee 37830
20 September
1966; accepted
in revised form
Studies.
11 Jam&my
1967)
AI&r&.-Benzene radiocarbon dating is discussed in view of recent improved chemistry. ;i high yield benzene camlyst is described which eliminates concern for carbon isotope fraotionat,ion, benzene purity and extensive sample prepar&on time. Radiocarbon dating, utilizing benzene sample preparation and liquid scintillation counting, had its conception and early development with the works of LEGER et al. (1958), TAMERS (1960), NOAKES et al. (1961, 1963), and STIPP et al. (1962). In the last eight years continuous advancement in the benzene chemistry has been made with the present state of the art, as practiced by most benzene C-14 dating laboratories, shown in Table 1. The most recent improvements in the benzene sample preparation has been the utilization of the acetylene chemistry described by BARKER (1953) and the application of a vanadium catalyst for the near quantitative conversion of acetylene gas to benzene. Since the aaetylenr: chemistry has elready been well described by BARICER (1953), comments on the improved benzene chemistry will be restricted to the development and use of the new benzene catalyst. The benzene synthesis adopted by the earliest benzene dating laboratories utilized a siica alumina diborane activated catalyst for the synthesis of acetylene gas to benzene. Although the method had the favorable attributes of producing only pure benzene, the chemical yields were Bt best only SO’%. The mein objection to the catalyst, other than the long catalysis time (approximately one day) and low benzene yields, was the hazard involved with handling the tosic and highly explosive diborane gas (BsH,) which was needed to act,ivate t,ho catalyst. The
Geochemical Table
1. Benzene
notes
synthesis chemical reaction time
dioxide
c _t O,a_ CO,=
98-100
Time
(min)
30-60
CO, + H,O 120-150
L&C,
2C0,
+ 1OLi a’
L&C,
+ 2H,O
3C,H,
and
chemistry
2C + 2Li 7
Benzene
( “/,)
yields
chemistry co,
+ 2H+ ----f
Acetylene
equations,
Yields
Equations Carbon
1095
--f
L&C, C,H,
+ 4Li,O + 2LiOH
95-98
30-60
80-98
30-43
73-96
“lo-315
chemistry
vmsdium catillyst+ C,H6
Cumulative
development of a high yield benzene catalyst which required no activat,ion with diboranc The areas of investigation in this resulted from a study carried out by NOAKES et ul. (1965). catalyst study were (1) benzene catalyst reaction mechanism, (2) benzene catalysts which rcquirod no diboranr activation, (3) chemical purity of the synthesized benzene, and (4) carbon isotope fractionat,ion studies. -4 brief discussion of the pertinent point’s (1-4 list& above) \vill bc doscribed. 1. Benzene cutulyst reaction mechanism. The catalytic reaction mechanism of silica alumina catalyst in the unreact,ed and diborane reactivated condition was studied to determine the: role These st#udies revealed that boron and alumina of the boron hydride in catalyst, activation. ---3 ions were not preferred sites of acetylene att)achment but rather the silicon +4 ions. The role of the boron hydride in activating the catalyst was attributed to boron’s affinity for electrons associated with silicon ions which effectively increased their Lewis Acid propert,ies. The postulated catalyst mechanism is that acetylene molecules in the gaseous state coming ill contact with strong Lewis Acids silicon ions become attached and polarized. Additional acetylonc molecules couple with the attached acetylene through induced polarization. JVhcn tfhree molccldes of acetylene are thns att,ached, angular rotat#ion is possible ant1 tlrll benzcnc ring structure is formed. 2. Xon-tlibornne activated catalyst. To verify the Lewis Acid theory, mctai osidc alml:ina cat,alpst,s of stronger Lewis Acid properties were fabricated. Mt%al ions of valences from J- 5 t,o -17 from the 5b, 6b, and 7b groups of the Periodic Chart were rmploycd. Thcscs catalysts gal-e benzene yields ranging from 60 to great,er than 90 ‘A, w&h the vanadirun osiclr allunina rntalyst giving superior results. Th? reactivity of thcase catalysts was found not only to be related to Txwin Acid strength of th(* act,ivo metal ions but also to their concentrat,ion and surface area of t,h(l alumina base Optimum conditions were d~(~mincd at mcital ion lnatcrinl t,o which t’hey were at,tached. concentration not, exceeding 20% of total catalyst weight when combined with allunina base luatcrial of surface area greater than 200 m2/g. 3. Ben-rne pz~rity. Gas chromatography and i.r. analysis were made on the catalytice.ll> synthesized benzene samples to check for benzene purity. The gas chromatogra?>‘n utilizrd XVRS a Uarbcr Coleman IDS Model 20, equipped with a 100 ft column filled wit,h Apiozon I,, a solid absorbent which is well suited for analyzing non polar aromatic material up t)o B.iO”C. Infrared analyses were made with a Beckman In-10 model, using a CaP, cell with a 200 ,J lcn&h ligllt loath. All samples analyzed by these two methods showed only benzene to be prcsent. Mass spcctrometric analyses of acetylene and benzene 4. 13C/12Cma.~s spectrom~etric analysis.
1090
Geochemiualnotes
synthesized from the acetylene were carried out to determine if any carbon isot,opefractionation was occurring during the benzene synthesis. The ratio of the atomic mass units 28 and 27 for acetylene and 79 and 78 for benzene was measured. No variation in 13C/1zCmass ratios was observed between the acetyiene and benzene samples. A recent study by TAMERSet al. (1966) using an empiricalapproach also found no evidence of carbon isotope fraction occurringthroughout the total benzene sample preparation. Vanadium catalyst can be prepared by incorporating vanadium oxide onto the surface of activated alumina material in the form of an aqueous solution of sodium vanadate (Na~VO,lOHsO~. The aqueous phase is removed by drying in a vacuum oven which allows storage of catalyst in this form with unlimited shelf life. Prior to immediate use of the catalyst, it is heated to 300°C under vacuum conditions (1 mm Hg) for a period of 2 hr. One hundred g of catalyst have been found to be an ample amount for synthesis of 5 cm3 of benzene at yields greater than SO%. Reuse of the catalyst is possible but not desirable because of possible cross contamination of samples. Vanadium catalyst can be prepared in the laboratory as described or co~ereially purchased at a cost of $S/lb. The resulting cost of catalyst material ix approximately $2 for synthesis of a 5 ems benzone sample. The benzene synthesis is accomplished by allowing acetylene gas, which has been purified by passing it through powdered phosphorus pentoxide (PsO,) and ascarite, to come in cont.act with the prepared vanadium catalyst. The interaction with the catalyst results in a slight exothermic reaction with a noticeable darkeningof the catalyst. Critical control of the acetylono flow rate onto the vanadium catalyst is not needed as with the silica alumina diborane catalyst, so catalysis time is reduced to l/l0 previous reaction time. On completion of the acetylene to benzene conversion, the benzene is moved from the catalyst with applied heat (1OO’C) and vacuum (1 mm Hg) for a duration of approximately one hour. The recent advances cited in the chemical preparation of benzene samples for radiocarbon dating have resultedin the following improvements in the method. The introductionOf BARlDrR’S acetylene chemistry has enabled (1) more rapid and reproducible acetylene production, (2) routine quantitative yields in the 95-98 ok range, and (3) elimination of possible sample contamination through the use of a closed chemical tram. The advantages gained througb use of a vanadium catalyst for the catalysis reaction are (1) increased chemical yields to greater than SO%, (2) catalysis reaction time reduced to 1110 previous reaction and catalyst cost reduction to l/5, (3) elimination of the need for diborane gas catalyst activation and handling hazards to personnel, (4) synthesis of pure benzene samples without additional purification steps, and (0) preparation of benzene with no carbon isotope fractionation. REFERXGNCES BARTER H. (1953) Radiocarbon Dating; large scale preparation of acetylene from organic materials. Nature 172, 631-632. LEGXR C., DELIBRIASG., PICHATL. and BARET C. (1958) On a new method for inkodueing natural Cl4 in a liquid scintillation. Tn Conferc%ceson LigplidSc&@&~&Xr Count&g, _&TOrth,westentUraiwereity 1957, pp. 261-267. Pergamon Press. NOAKESJ. E., ISBX~ A. F. and HOOD D. W. (1961) Conversion of carbon dioxide to benzene for liquid scintillation counting. Paper presented at 42nd Ammal Meeting of the American Geophysical Union. NOAKESJ. E., ISBELLA. F., STDP J. 5. and Hoon D. W. (1963) Benzene synthesis by low temperature catalysis for radiocarbon dating. Cfertchimka Cmchimb Acta $37, 797-808. NOA~ESJ. E., KIM S. M., THOMASG. A. and Arrnn8L. K. (1965) A ohemiaelstudy of an ambient temperature catalytic benzene synthesis used in radiocarbon dating. USAEC Report ORINS50, I-11. STIP~J. J., DAMPSE. M., NOAKEE~ J. E. and HOOVERT. E. (1962) University of Texas radiocarbon dates I. Radiocarbon4,43-60. TAXERSM. A. (1980) Carbon-14 d&kg with the liquid scintillation counter: Total synthesis of the benzene solvent. Sc&.cc i@3, 668-609. Tm M. A. and PEARSOXF. 5. (1965) Isotope effect in the liquid scintillation method for radiocarbon dating. Nature , 120%1207.
Geochemical
notes
In general, the dispersions for rubidium and strontium results are greater on both levels t,han those for potassium and argon. F tests show that there are no significant differences in the interlaboratory and intralaboratory precisions except for the rubidium measurements. The wi& variation in results for common strontium may be because strontium is a t,race element in P-ZO’i or because of contamination in mineral analyses as described by WASSERBVL’R~ et al. (1964,. Rubidium-strontium ages calculated using the mean rubidium and 87Sr,,d values from each laboratory have a total range of IO.7 m.y. and the mean Rb-Sr age is significantly different tluau the mean K-Ar age at the 5% level of signiilcance if the “geologically determined” half-lifct of 50 x IO9 yr is used for *‘Rb. It is interesting to note that for this mineral, the K-Ar end Rb-Sr ages (81.0 and 83.2 m.y., respectively) are in good agreement if t,he 47 ‘, IO9 yr half-lifts determined by liquid scintillation counting is used for 87Rb. REFERENCES CROW E. L., DAVIS F. A. and MAXFIELD M. W. (1960) &atistica _Ifon~clZ. Dover Publications. LANPHERE M. A. and DAI.RYMPLE G. B. (1965) P-207: An interlaboratory standard musoovite for ergon and potassium analyses. J. Geophys. Res. 70, 3497-3503. In The IWmk of Geolog!j MCINTYRE D. B. (1963) Precision and resolution in geochronometry. (editor C. C. Albritton), pp. 112-134. Addison-Wesley Publishing Co., Inc., Reading, Mtws. WASSERBURG G. J., WEN T. and ARONSOX J. (1964) Strontium contamination in mineralanalyses. Geochim. Cosmochim. Acta 28, 407-410.
Recent improvementsin benzene chemistry for radimkn dating Joas
SO~KES,
Special Training
(Receiwd
STEPHEN
KIM
and LAWRENCE
AKERS
Division, Oak Ridge Institute of Nuclear Oak Ridge, Tennessee 37830
20 September
1966; accepted
in revised form
Studies.
11 Jam&my
1967)
AI&r&.-Benzene radiocarbon dating is discussed in view of recent improved chemistry. ;i high yield benzene camlyst is described which eliminates concern for carbon isotope fraotionat,ion, benzene purity and extensive sample prepar&on time. Radiocarbon dating, utilizing benzene sample preparation and liquid scintillation counting, had its conception and early development with the works of LEGER et al. (1958), TAMERS (1960), NOAKES et al. (1961, 1963), and STIPP et al. (1962). In the last eight years continuous advancement in the benzene chemistry has been made with the present state of the art, as practiced by most benzene C-14 dating laboratories, shown in Table 1. The most recent improvements in the benzene sample preparation has been the utilization of the acetylene chemistry described by BARKER (1953) and the application of a vanadium catalyst for the near quantitative conversion of acetylene gas to benzene. Since the aaetylenr: chemistry has elready been well described by BARICER (1953), comments on the improved benzene chemistry will be restricted to the development and use of the new benzene catalyst. The benzene synthesis adopted by the earliest benzene dating laboratories utilized a siica alumina diborane activated catalyst for the synthesis of acetylene gas to benzene. Although the method had the favorable attributes of producing only pure benzene, the chemical yields were Bt best only SO’%. The mein objection to the catalyst, other than the long catalysis time (approximately one day) and low benzene yields, was the hazard involved with handling the tosic and highly explosive diborane gas (BsH,) which was needed to act,ivate t,ho catalyst. The
Geochemical Table
1. Benzene
notes
synthesis chemical reaction time
dioxide
c _t O,a_ CO,=
98-100
Time
(min)
30-60
CO, + H,O 120-150
L&C,
2C0,
+ 1OLi a’
L&C,
+ 2H,O
3C,H,
and
chemistry
2C + 2Li 7
Benzene
( “/,)
yields
chemistry co,
+ 2H+ ----f
Acetylene
equations,
Yields
Equations Carbon
1095
--f
L&C, C,H,
+ 4Li,O + 2LiOH
95-98
30-60
80-98
30-43
73-96
“lo-315
chemistry
vmsdium catillyst+ C,H6
Cumulative
development of a high yield benzene catalyst which required no activat,ion with diboranc The areas of investigation in this resulted from a study carried out by NOAKES et ul. (1965). catalyst study were (1) benzene catalyst reaction mechanism, (2) benzene catalysts which rcquirod no diboranr activation, (3) chemical purity of the synthesized benzene, and (4) carbon isotope fractionat,ion studies. -4 brief discussion of the pertinent point’s (1-4 list& above) \vill bc doscribed. 1. Benzene cutulyst reaction mechanism. The catalytic reaction mechanism of silica alumina catalyst in the unreact,ed and diborane reactivated condition was studied to determine the: role These st#udies revealed that boron and alumina of the boron hydride in catalyst, activation. ---3 ions were not preferred sites of acetylene att)achment but rather the silicon +4 ions. The role of the boron hydride in activating the catalyst was attributed to boron’s affinity for electrons associated with silicon ions which effectively increased their Lewis Acid propert,ies. The postulated catalyst mechanism is that acetylene molecules in the gaseous state coming ill contact with strong Lewis Acids silicon ions become attached and polarized. Additional acetylonc molecules couple with the attached acetylene through induced polarization. JVhcn tfhree molccldes of acetylene are thns att,ached, angular rotat#ion is possible ant1 tlrll benzcnc ring structure is formed. 2. Xon-tlibornne activated catalyst. To verify the Lewis Acid theory, mctai osidc alml:ina cat,alpst,s of stronger Lewis Acid properties were fabricated. Mt%al ions of valences from J- 5 t,o -17 from the 5b, 6b, and 7b groups of the Periodic Chart were rmploycd. Thcscs catalysts gal-e benzene yields ranging from 60 to great,er than 90 ‘A, w&h the vanadirun osiclr allunina rntalyst giving superior results. Th? reactivity of thcase catalysts was found not only to be related to Txwin Acid strength of th(* act,ivo metal ions but also to their concentrat,ion and surface area of t,h(l alumina base Optimum conditions were d~(~mincd at mcital ion lnatcrinl t,o which t’hey were at,tached. concentration not, exceeding 20% of total catalyst weight when combined with allunina base luatcrial of surface area greater than 200 m2/g. 3. Ben-rne pz~rity. Gas chromatography and i.r. analysis were made on the catalytice.ll> synthesized benzene samples to check for benzene purity. The gas chromatogra?>‘n utilizrd XVRS a Uarbcr Coleman IDS Model 20, equipped with a 100 ft column filled wit,h Apiozon I,, a solid absorbent which is well suited for analyzing non polar aromatic material up t)o B.iO”C. Infrared analyses were made with a Beckman In-10 model, using a CaP, cell with a 200 ,J lcn&h ligllt loath. All samples analyzed by these two methods showed only benzene to be prcsent. Mass spcctrometric analyses of acetylene and benzene 4. 13C/12Cma.~s spectrom~etric analysis.
1090
Geochemiualnotes
synthesized from the acetylene were carried out to determine if any carbon isot,opefractionation was occurring during the benzene synthesis. The ratio of the atomic mass units 28 and 27 for acetylene and 79 and 78 for benzene was measured. No variation in 13C/1zCmass ratios was observed between the acetyiene and benzene samples. A recent study by TAMERSet al. (1966) using an empiricalapproach also found no evidence of carbon isotope fraction occurringthroughout the total benzene sample preparation. Vanadium catalyst can be prepared by incorporating vanadium oxide onto the surface of activated alumina material in the form of an aqueous solution of sodium vanadate (Na~VO,lOHsO~. The aqueous phase is removed by drying in a vacuum oven which allows storage of catalyst in this form with unlimited shelf life. Prior to immediate use of the catalyst, it is heated to 300°C under vacuum conditions (1 mm Hg) for a period of 2 hr. One hundred g of catalyst have been found to be an ample amount for synthesis of 5 cm3 of benzene at yields greater than SO%. Reuse of the catalyst is possible but not desirable because of possible cross contamination of samples. Vanadium catalyst can be prepared in the laboratory as described or co~ereially purchased at a cost of $S/lb. The resulting cost of catalyst material ix approximately $2 for synthesis of a 5 ems benzone sample. The benzene synthesis is accomplished by allowing acetylene gas, which has been purified by passing it through powdered phosphorus pentoxide (PsO,) and ascarite, to come in cont.act with the prepared vanadium catalyst. The interaction with the catalyst results in a slight exothermic reaction with a noticeable darkeningof the catalyst. Critical control of the acetylono flow rate onto the vanadium catalyst is not needed as with the silica alumina diborane catalyst, so catalysis time is reduced to l/l0 previous reaction time. On completion of the acetylene to benzene conversion, the benzene is moved from the catalyst with applied heat (1OO’C) and vacuum (1 mm Hg) for a duration of approximately one hour. The recent advances cited in the chemical preparation of benzene samples for radiocarbon dating have resultedin the following improvements in the method. The introductionOf BARlDrR’S acetylene chemistry has enabled (1) more rapid and reproducible acetylene production, (2) routine quantitative yields in the 95-98 ok range, and (3) elimination of possible sample contamination through the use of a closed chemical tram. The advantages gained througb use of a vanadium catalyst for the catalysis reaction are (1) increased chemical yields to greater than SO%, (2) catalysis reaction time reduced to 1110 previous reaction and catalyst cost reduction to l/5, (3) elimination of the need for diborane gas catalyst activation and handling hazards to personnel, (4) synthesis of pure benzene samples without additional purification steps, and (0) preparation of benzene with no carbon isotope fractionation. REFERXGNCES BARTER H. (1953) Radiocarbon Dating; large scale preparation of acetylene from organic materials. Nature 172, 631-632. LEGXR C., DELIBRIASG., PICHATL. and BARET C. (1958) On a new method for inkodueing natural Cl4 in a liquid scintillation. Tn Conferc%ceson LigplidSc&@&~&Xr Count&g, _&TOrth,westentUraiwereity 1957, pp. 261-267. Pergamon Press. NOAKESJ. E., ISBX~ A. F. and HOOD D. W. (1961) Conversion of carbon dioxide to benzene for liquid scintillation counting. Paper presented at 42nd Ammal Meeting of the American Geophysical Union. NOAKESJ. E., ISBELLA. F., STDP J. 5. and Hoon D. W. (1963) Benzene synthesis by low temperature catalysis for radiocarbon dating. Cfertchimka Cmchimb Acta $37, 797-808. NOA~ESJ. E., KIM S. M., THOMASG. A. and Arrnn8L. K. (1965) A ohemiaelstudy of an ambient temperature catalytic benzene synthesis used in radiocarbon dating. USAEC Report ORINS50, I-11. STIP~J. J., DAMPSE. M., NOAKEE~ J. E. and HOOVERT. E. (1962) University of Texas radiocarbon dates I. Radiocarbon4,43-60. TAXERSM. A. (1980) Carbon-14 d&kg with the liquid scintillation counter: Total synthesis of the benzene solvent. Sc&.cc i@3, 668-609. Tm M. A. and PEARSOXF. 5. (1965) Isotope effect in the liquid scintillation method for radiocarbon dating. Nature , 120%1207.