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Chemical yield optimization of the benzene synthesis for radiocarbon dating
InternationalJournal of Applied Radiation anA Isotopes,1975, Vol. 26, pp. 676-682. Pergamon Press. Printed in Northern Ireland
Chemical Yield Optimization of the Benzene Synthesis for Radiocarbon Dating M . A. T A M E R S Nova University,Life Sciences Ce.nter,Ft. Lauderdaie, FL 33314, U.S.A. (Rece/osd 25 October 1974) A benzene synthesisvacuum llnefor radiocarbon dating is dcscrlbed. Operating precautions arc outlinedthatpermit the reductionof secondary reactionsand lead to desiredproduct yields consistentlyin excessof95 ~. The totalprocedure has bccn shortenedfor most archaeological samples by the substitutionof a preliminary carbonization step for the usual combustion. Principal secondary reactionsin the lithium carbide production have been identified. L o w chemical ylclds here are duc to the presence of vacuum line leaks,excessivemoisture, or insufficient heating at the end of the reaction. The production of lithium carbonate and some demental carbon account for the losses. A potassium chromate-activatedsilica-aluminacatalystisnscd for the cyclizationof acetyleneto benzene. It isdeactivatedin secondary reactions by a reductionof the chromium VI to chromium III or IV oxidationstates. The presence of moisture in the acetyleneand/or catalystis responsiblefor low benzene yieldsand increased catalyst deactivationrates. A mechanism is proposed involving reaction of acetylene with water to form acetaldehyde,which is subsequentlyoxidizedto acetate,polymerizationproducts, and potassium carbonate. Thorough dcsiccatlonof the acetylene and catalyst permits closeto quantitativeproduction of benzene. The catalystcan be reactivatedby heating in air to 500°C. INTRODUCTION
metal was adopted and, by 1965, most groups had switched to transition metal ion activated THE OR/OINAL r a d i o c a r b o n dating p r o c e d u r e used solid counting samples consisting of ele- silica-alumlna catalysts for the acetylene cyclimental carbon. (I) This was quickly replaced zation, c8-I°) This permitted total yields up by the more reliable gas methods, cs) Re- to 95+~o and the laboratories wcrc producing cently, however, liquid scintillation counting routine radiocarbon datings. However, faulty syntheses regularly occurred, of synthesized benzene has become the techoften for reasons that were not apparent to nique of choice for n e w installations. T h e the operators. This presented a problem since same chemical and counting cqnipmcnt can hc used for both the radiocarbon and tritium m a n y of the samples processed in radiocarbon dating laboratories are irreplaceable or of age determination methods. ~s) T h e initial work on a practical synthesis limited quantity. It is the purpose of the report of benzene for radiocarbon dating was re- to examine various steps of the current method ported in 1960. c4~ This employed the pyrolysis in order to identify the secondary reactions method of acetylene conversion and a magnesi- responsible for the chemical losses. T h e benzene production is carried out by u m metal-strontium carbonate production of carbide. Further improvements leading to the following reactions: a total yield of 30~o were published in 1961. c5) Sample + O s --~ C O 2 + • • • (a) Concurrently, Soviet scientists developed a or benzene sythcsis for radiocarbon dating using Sample -4- inert gas --* C -4- • • • (b) a Ziegler catalyst, tt~ I n 1962 a diborane acti- o £ vated catalyst was introduced for the acetylene C a r b o n a t e sample A- H C 1 0 4 --~ CO~ + . . . cyclization. ~v) T h e best yields of benzene b y (c) this m e t h o d were 5 0 ~ . Beginning in 1963, 2CO~ -{- lOLi - + Li~C~ + 4 L i f O (d) the carbide synthesis using molten lithium 676
Chemicalyield optimization of the benzene synthesisfor radiocarbon dating
677
dioxide at this position is desired as, for ex-
or
2C
+
2Ig -~
IA,C2
(e) ample, in the case of a combustion operation. The carbon dioxide is quantitatively collected
Li,C 2 + H a O -~ C,H~ + Li,O
(f) by freezing in the pertinent "cold finger" (g)
3C,H, -+ Call,
Figure l shows the design of the vacuum line developed for use in both the routine production of radiocarbon benzene samples and the investigation of yield variations in the individual steps of the synthesis. T h e left hand side pump is used for producing the final vacuum. T h e synthesis line is protected against moisture from this source by a liquid nitrogencooled trap situated between the pump and the system. For preliminary evacuation and catalyst drying, the right hand side pump is used. The benzene synthesis proceeds from left to right on the vacuum line. The initial reaction, conversion of the strontium carbonate to carbon dioxide with dilute perchloric acid, takes place in the flask on the extreme left. All three of the adjacent traps are ordinarily dry ice-cooled to dessicate the carbon dioxide. The third trap in the group can be liquid nitrogen-cooled when retention of the carbon
,o,,
..
':
,t
and then expanded into the double gas storage vessels on the left side. For yield measurements, the gas has to be successively dried by passing over dry ice-cooled traps in the middle of the vacuum system. T h e known volume of the storage vessels and a precise manometer reading permit gas quantity calculations, after the usual atmospheric pressure and temperature corrections, with less than 2 ~o errors. Depending on the final pressure of the carbon dioxide in these vessels, an improperly dried gas could consist of as much as 5 ~o and more water vapor. In addition to using up lithium in the following reaction step, failure to take into account the contribution of this water to the pressure reading results in a miscalculation of acetylene yield. Water is admitted into the cooled lithium chamber to hydrolyse the carbide and, excess lithium, producing acetylene and hydrogen gas. These are separated by the gas trap grouping which follows, the first trap being
''
Fxo. 1. Benzene synthesis gas handling line.
'
678
M. A. Tamers
cooled with dry ice to initially remove water and the second two with liquid nitrogen to completely collect the acetylene. The hydrogen gas is removed by the right side vacuum pump without passing through the phosphoric acid gas washing device. After the hydrolysis, the acetylene is allowed to sublime and is bubbled through the phosphoric acid gas washer. It is collected by liquid nitrogen cooling of one of the gas traps in the first group after circulating to that forward position through the uppermost tubing line. With all the traps in the second group now dry ice-cooled, the acetylene is sublimed over this position and passes into one of the storage vessels on the right hand side without again going through the phosphoric acid bubbler. The acetylene is collected quantitatively using the appropriate "cold finger" and its quantity precisely measured. The acetylene to benzene cyclization is effected in an individual catalyst system previously dried at above 250°C. The vacuum line of Fig. 1 shows three catalyst bottles, each with a thermometer dipping into the pellets for temperature monitoring. There are positions for a total of five catalyst systems. This multiplicity is for convenience in batch changing and drying of the material, generally done only once a week. The benzene is collected under vacuum in the last trap, which is cooled with dry ice. During this operation, the catalyst is heated to approximately 120°C. All of the benzene comes off in less than thirty minutes. EXPERIMENTAL 1. Sample combustion and carbonization
After the pretreatments necessary to remove extraneous carbonaceous matter, low carbon content materials are burned in a Vycor tube to convert all carbon to carbon dioxide. This gas is quantitatively collected in an ammonium hydroxide bubbler system and, subsequently, precipitated with strontium chloride. The resulting strontium carbonate is filtered on a 350 ml, medium porosity Biichner funnel, washed, and dried at I I0°C. The weighed precipitate provides the basis of a convenient gravimetric analysis for total carbon in the
sample and the starting material for the vacuum fine. With ordinary care, there are no carbon losses in this step. Carbonization is used for samples that would produce a mixture having at least 50 ~o carbon at the completion of this procedure. Common organic materials such as wood, cloth, food, flesh, leaves, etc., are treated with this method. Soil samples, ground water carbonates, bones and shells are not suitable. The carbonization takes place in essentially the same system as the combustion, with the difference that in the latter case, nitrogen gas is flowed through the Vycor tube instead of oxygen. The ammonium hydroxide solution is replaced with water that is used to verify the inert gas flow. Heating the sample converts the carbon to the elemental form. This, mixed with the ashes, is used directly with lithium metal for the carbide production in reaction (e). In addition to a simplified synthesis, there is a considerable saving in the amount of lithium required. The production of carbide from carbon dioxide requires five times as much lithium as that from elemental carbon. 2. Carbi& produetion
Carbon dioxide or charcoal are converted with > 9 5 % efficiency to carbide by reaction with lithium metal in the absence of air. Reaction (e) proceeds readily at 800°C (11) and driven toward completion by employing a large excess of lithium and raising the temperature to 900°C, for at least 30 min. Reaction (d) produces best results if the carbon dioxide is not exposed to the lithium until it has been heated to at least 700°C. The stainless steel reaction chamber is shown in Fig. 2. The yields of (d) are reduced by the secondary reactions, 4Li + 2COa --* Li~CO8 + C + Li20 LifO + CO s ~ Li~CO 3
(h) (i)
These losses of carbon can be reduced by heating the reaction mixture to 900°G. The lithium carbonate decomposes to a significant extent starting from 600°C, ~1~ which reverses reaction (i). At the final temperature of 900°G, the regenerated carbon dioxide produced is
Chemical yidd optimization of the bcnxans~algsia for radiocarbondating ~--.---- tOmmO,ID.
~
4"m
1+ Fro. 2. Lithium reaction chamber constructed of 2 mm stainless steel, with water cooling jacket to protect the rubber O-ring seal. eventually converted to lithium carbide by reaction (d) and the carbon of (h) by reaction
(e). It is important to have a tight vacuum system. T h e lithium reaction chamber is thoroughly evacuated initially, but the pump must be disconnected for the entire carbon dioxide reaction operation. A leak in the system will not be readily noticeable since both nitrogen and oxygen combine with lithium metal to produce solid nitride and oxide compounds, a~) Five and a h a F moles of lithium react for each mole of air that enters. Significant quantifies of lithium metal could be consumed in this way, resulting in insufficient amounts of the reactant to produce the desired carbide in the primary reactions. Lithium is also used up in considerable amounts by the high temperature reaction of the metal with water vapor present in the carbon dioxide or generated from the liquid remaining in the water inlet tube from the previous hydrolysis. This produces lithium oxide and hydride. During the hydrolysis reaction in the cooled lithium chamber, the lithium hydride is decomposed to form hydrogen. This gas can
679
produce a false impression of excess lithium metal present in the final mixture. It is inconvenient to maintain the lithium chamber at 900°C for long periods due to the approximately 12 m m Hg vapor pressure of lithium at that temperature. This causes a deposition of lithium metal on the cooler, upper portions of the chamber, with the possibility of yield losses from (h) and (i), here. Raising the temperature above 900°C is not recommended because of excessive volatilization of the metal at higher temperatures. Because the heat of vaporization of lithium metal is high (5000 cal/g approx.) the molten metal surface could be as much as 100°C lower than the temperature reading of the furnace. The indicated temperature should be at least 950-1000°C in order to guarantee the 900°C necessary to insure the completion of reaction (e). Insufficient heating, along with leaks and moisture, are the causes of low carbide yields. The furnace used here was a Thermolyne type 2000. T h e lithium metal was carefuUy selected to be carbon-free since the presence of this contaminant could cause important errors. (aa) Lithium packed in argon gas is necessary for the radiocarbon dating requirements.* T h e hydrolysis of the lithium carbide, cooled to room temperature, proceeds with a quantitative yield, as shown in reaction (f). The excess lithium reacts with the evolution of hydrogen gas. T h e remaining solution should be clear in the case of good yields. A black suspension indicates the predominance of the secondary reaction (h) and a grey suspension, reaction (i). Lithium carbonate has a low solubility in lithium hydroxide solution.
3. Acetylene to benzene cyclization Most benzene laboratories now use transitionmetal activated, silica-alumina catalysts for the cyclization of acetylene to benzene. These materials are common dehydrating agents for various petroleum reforming operations. Those * Lithium Corporation of America, Bessemer City, North Carolina (cat. no. 450 767); Riedel-de I-Iaen AG, Scelze-Hannover, Germany (cat. no. 130 60).
680
M. A. Tamers
generally employed are either Cr vl or V v The presence of carbonates on the deactivated activated.* In our laboratory, the silica- catalyst was verified by acid attack and sub-
alumina catalyst base (in the form of 2 m m pellets) is activated by potassium chromate, 0"2 % by weight. Under the best conditions, 99 ~ conversion of the dry acetylene to benzene is obtained. The catalyst is deactivated to some degree during the reaction, but only a negligible amount of acetylene is used up in the secondary reactions involved. In the case of the potassium chromateactivated catalyst, complete acetylene oxidation reactions arc given below. T h e situation would be similar with the other transition metal ions catalysts. 3CsH~ + 10KsCrO4-~ 6K2CO s + 5Cr~O3 + 3H20 + 4K~O
(k)
CsHs + 5KsCrO4-+ 2K~CO 3 + 5CrO 2 + HsO + 3K~O
(1)
These two catalyst reactions can proceed directly, especially at higher temperatures. However, mechanisms involving intermediates, such as acetaldehyde and acetic acid, would be more likely since it was seen that small amounts of moisture in acetylene caused rapid catalyst deactivation in addition to lower benzene yields. This would begin with a reaction of acetylene with water present in minimal amounts either in the gas or on the catalyst. T h e reactions could stop with the formation of an acetate salt or proceed to potassium carbonate.
C2H s + HsO --*-CHsCHO 3CH3CHO
(m)
+ 2K~CrO~ --~ 3CHsCOOK + Cr~O 3 + KOH.
(o)
3 C H a C O O K + 8KsCrO4 --~ 6K2CO a + 4CrsO a + K O H + 3 K 2 0 + H 2 0
(p)
* The chromium activated catalyst is available from Mobil Oil Corp., N e w York (type Durabead I) and Kali-Chcmic, Harmovcr, Germany (type K C Pcrkator Dl); the vanadium activated material is offered by Task, Inc., Oak Ridge, Tenn. and I-lanshaw Chemical Co., Cleveland, Ohio (type
VO 701 T 118).
sequent collection of evolved carbon dioxide in an ammonium hydroxide solution. Infra-red spectrographic evidence for traces of the carbonyl group of the acetate was obtained. Acetaldehyde being very reactive, (o) would proceed readily at all temperatures. Acetate is more stable and it would be expected that (p) would be most significant at the higher catalyst hearings. Considering that four moles of potassium chromate are reduced for each mole of acetate, the maintenance of low catalyst temperature is important. T h e acetate could also react with acetylene, forming vinyl acetate, which readily undergoes polymerization. Reduction of the chromate by hydrogen gas, formed in small amounts by the decomposition of acetylene, also occurs. This is considerably accelerated by heating, for example, above 200°C. A soot-like carbon produced can be seen in cold traps where acetylene sublimation was accelerated with a heat gun. Likewise, if the catalyst temperature is allowed to rise out of control, a carbon black deposit sometimes results. The colors of the deactivated catalyst provide an indication of the predominating reactions. The production of carbon results in a blackish color, noticeable when the catalyst temperature rises above 230°C during the reaction. The generation of the grey-green or violet CrsO 3 is prevalent when the catalyst is kept below 30°C by constant cooling and slow addition of acetylene. The brown CrOs color is observed with a reaction temperature around 100°C. O f course, the Cr III oxide can be converted to the Cr Iv oxidation state by a subsequent reaction with chromate, further deactivating the catalyst. The effect of temperature on yield with the vanadium activated catalyst was reported by POLACH and STIr'P. ~14) With the moderate temperatures permitted, the chromium activated catalyst used here showed negligible acetylene losses if the gas and catalyst were well dried. This is illustrated in Fig. 3. Thorough drying of the catalyst and the acetylene is very important to insure good yields and a low rate of catalyst deactivation. The effect of moisture on acetylene to benzene
Ghemkal.~ield optimization of the bmzer~ ~mthesisfor radiocarbondating *lam
--~
681
,0
~am
am
t I 8m
amO
MAXI~..fM ~
I
A
I~BAOfNm(°C].
Fio. 3. Acetylene t o benzene yield with maximum temperature recorded by a thermometer embedded in the potassium chromate activated silica-alumina catalyst pellets. production is illustrated in Fig. 4. With exmoisture, the reaction does not start at all. T h e catalyst should be vacuum-dried above 250°C for at least an hour, using the protected left hand side pump of Fig. 1. The acetylene can be dried by repeated passage over dry ice-cooled traps after it passes through the phosphoric acid purifying bubbler. T h e catalyst can be readily reactivated by heating in air at 500°C, which causes the oxidation of the Cr m and Cr Iv to Gr vI. Any carbon produced by acetylene decomposition is burned off at that temperature. It is not necessary to exceed 500°C during this operation. I f higher temperatures are employed, cessive
Ioc
~
m~
the chromium begins to react with the silicaalumina base and the catalyst capacity is permanently reduced. T h e situation is similar in the case of the vandium activated silica-alumina catalyst. The deactivation here involves an oxidation state reduction, from V v to V rv and V m . (15) Reactivation of the catalyst proceeds readily by heating in air. (16) The catalyst used in the present work was capable of converting at least 1 liter S T P acetylene/5 g catalyst without reactivation if the benzene produced was intermittently removed. The deactivation of the catalyst by reaction (o) would mean that less than 0-2 ~ of the
_....._.__.._... C.~.
8G
0 -16a
a ).
gl Z WdO ID lIDO
°o
I m
,l WATIR
V ~
IN
I I s 4 ~J~ETYLIENE ~
~
| s TOTAL1
e
Fro. 4. Acetylene to benzene yield as a function of humidity in the gas reactant.
682
M. A. Tamers
acetylene was taken for these side reaction, if the acetate is not oxidized to any great extent. This explains the fact that at moderate temperatures, nearly quantitative yields are obtained from the dried acetylene cyclization despite the catalyst changes.
SUMMARY The majority of materials submitted to age determination are irreplaceable and the occurrence of a faulty chemical conversion to benzene must be minimized. Studies to optimize conditions for the radiocarbon dating benzene synthesis have been presented here. The principal secondary reactions respomible for reduced yields in the various steps are described as well as methods for their control during the processing of critical samples. For lithium carbide production, it is essential to have a vacuum tight, dry system. The lithium chamber with its connecting lines should be pumped with a good vacuum system for several hours before the metal reactant is introduced. T h e integrity of the vacuum must be verified at this point since, when the lithium is present, pressure changes are small. Additional protection against low yields in the carbide step can be obtained by the final heating of the lithium chamber to an externally measured value above 950°C, and the introduction of the carbon dioxide reactant only after an external temperature of 700°C has been reached. The use of a large excess of Lithium reactant is recommended. Low yields in the acetylene cyclization to benzene can be traced to water vapor contamination of the acetylene and the cataiyst. The gas reactant should be thoroughly dried by repeated passage over a group of dry icecooled traps and, when frozen out by liquid
nitrogen refrigeration, pumped on by a vacuum pump protected with a liquid nitrogen-cooled trap. T h e catalyst should be dried to at least 250°C under good vacuum and with a protecting trap. T h e temperature of the catalyst during benzene production should be mainrained below 200°C by a moderated entrance of acetylene. REICERENCES 1. LmsY W. F. Radiocarbon Dating. University of Chicago Press, Chicago (1952). 2. DmTHRONW. UeAEC Report NYO-6628 (1956). 3. T ~ a s M. A. Mol. C~ysta/s 4, 261 (1968). 4. T ~ g s Y. A. Sdence 1~2) 668 (1960). 5. T ~ R s M . A . , STmx,J.J. and C.~OLLI]gRJ.Geochim. Cosmoddm. Acta 24, 266 (1961). 6. STARIK I. E. eta/. Radiochimiya SSSR 3, 101 (t961). 7. NOAKESJ. E., ISBELL A. F., STreP J. J. and HOOD D. W. Geoehim. Cosmochim. Acta 27, 797 (1963). 8. No,.ms J. •., I ~ S. M. and ST~P J. J. Proc. Int. C-14 and T-3 Dating Conf. Pullman, Washington (1965). 9. T A n g s M. A. Aria Cientif~a Venezolana 16, 156 (1965). 10. PI~.Tm F. and SCHARPI~NK~]gLH. W . Atompraxis 12, 95 (1966). 1I. JuzA R. andWzHLZ V. Naturwiss.52, 537 (1965). 12. A complete description of the properties of lithium and its compounds can be found in: Gmdin~ Handbuch der Anorganischen Chfmie-Lithium, vol. 20. Verlag Chemic, Weinheim (1960). 13. GZYH M. A. J. appl. Radiat. Isotopes 20, 463 (1969). 14. POLACH H. A. and STIPP J. J. appl. Radiat. Isotopes 18, 359 (1967). 15. C O L , ~ D. D., Lw C. L., DXCKEP~O~ D. R.
and FROST g . R. Proe. Int. Conf. Radiocarbon Dating, p. 158, New Zealand (1972). 16. POLACH H. A. Proc. Int. Conf. Radiocarbon Dating, p. 145 New Zealand (1972).
Chemical Yield Optimization of the Benzene Synthesis for Radiocarbon Dating M . A. T A M E R S Nova University,Life Sciences Ce.nter,Ft. Lauderdaie, FL 33314, U.S.A. (Rece/osd 25 October 1974) A benzene synthesisvacuum llnefor radiocarbon dating is dcscrlbed. Operating precautions arc outlinedthatpermit the reductionof secondary reactionsand lead to desiredproduct yields consistentlyin excessof95 ~. The totalprocedure has bccn shortenedfor most archaeological samples by the substitutionof a preliminary carbonization step for the usual combustion. Principal secondary reactionsin the lithium carbide production have been identified. L o w chemical ylclds here are duc to the presence of vacuum line leaks,excessivemoisture, or insufficient heating at the end of the reaction. The production of lithium carbonate and some demental carbon account for the losses. A potassium chromate-activatedsilica-aluminacatalystisnscd for the cyclizationof acetyleneto benzene. It isdeactivatedin secondary reactions by a reductionof the chromium VI to chromium III or IV oxidationstates. The presence of moisture in the acetyleneand/or catalystis responsiblefor low benzene yieldsand increased catalyst deactivationrates. A mechanism is proposed involving reaction of acetylene with water to form acetaldehyde,which is subsequentlyoxidizedto acetate,polymerizationproducts, and potassium carbonate. Thorough dcsiccatlonof the acetylene and catalyst permits closeto quantitativeproduction of benzene. The catalystcan be reactivatedby heating in air to 500°C. INTRODUCTION
metal was adopted and, by 1965, most groups had switched to transition metal ion activated THE OR/OINAL r a d i o c a r b o n dating p r o c e d u r e used solid counting samples consisting of ele- silica-alumlna catalysts for the acetylene cyclimental carbon. (I) This was quickly replaced zation, c8-I°) This permitted total yields up by the more reliable gas methods, cs) Re- to 95+~o and the laboratories wcrc producing cently, however, liquid scintillation counting routine radiocarbon datings. However, faulty syntheses regularly occurred, of synthesized benzene has become the techoften for reasons that were not apparent to nique of choice for n e w installations. T h e the operators. This presented a problem since same chemical and counting cqnipmcnt can hc used for both the radiocarbon and tritium m a n y of the samples processed in radiocarbon dating laboratories are irreplaceable or of age determination methods. ~s) T h e initial work on a practical synthesis limited quantity. It is the purpose of the report of benzene for radiocarbon dating was re- to examine various steps of the current method ported in 1960. c4~ This employed the pyrolysis in order to identify the secondary reactions method of acetylene conversion and a magnesi- responsible for the chemical losses. T h e benzene production is carried out by u m metal-strontium carbonate production of carbide. Further improvements leading to the following reactions: a total yield of 30~o were published in 1961. c5) Sample + O s --~ C O 2 + • • • (a) Concurrently, Soviet scientists developed a or benzene sythcsis for radiocarbon dating using Sample -4- inert gas --* C -4- • • • (b) a Ziegler catalyst, tt~ I n 1962 a diborane acti- o £ vated catalyst was introduced for the acetylene C a r b o n a t e sample A- H C 1 0 4 --~ CO~ + . . . cyclization. ~v) T h e best yields of benzene b y (c) this m e t h o d were 5 0 ~ . Beginning in 1963, 2CO~ -{- lOLi - + Li~C~ + 4 L i f O (d) the carbide synthesis using molten lithium 676
Chemicalyield optimization of the benzene synthesisfor radiocarbon dating
677
dioxide at this position is desired as, for ex-
or
2C
+
2Ig -~
IA,C2
(e) ample, in the case of a combustion operation. The carbon dioxide is quantitatively collected
Li,C 2 + H a O -~ C,H~ + Li,O
(f) by freezing in the pertinent "cold finger" (g)
3C,H, -+ Call,
Figure l shows the design of the vacuum line developed for use in both the routine production of radiocarbon benzene samples and the investigation of yield variations in the individual steps of the synthesis. T h e left hand side pump is used for producing the final vacuum. T h e synthesis line is protected against moisture from this source by a liquid nitrogencooled trap situated between the pump and the system. For preliminary evacuation and catalyst drying, the right hand side pump is used. The benzene synthesis proceeds from left to right on the vacuum line. The initial reaction, conversion of the strontium carbonate to carbon dioxide with dilute perchloric acid, takes place in the flask on the extreme left. All three of the adjacent traps are ordinarily dry ice-cooled to dessicate the carbon dioxide. The third trap in the group can be liquid nitrogen-cooled when retention of the carbon
,o,,
..
':
,t
and then expanded into the double gas storage vessels on the left side. For yield measurements, the gas has to be successively dried by passing over dry ice-cooled traps in the middle of the vacuum system. T h e known volume of the storage vessels and a precise manometer reading permit gas quantity calculations, after the usual atmospheric pressure and temperature corrections, with less than 2 ~o errors. Depending on the final pressure of the carbon dioxide in these vessels, an improperly dried gas could consist of as much as 5 ~o and more water vapor. In addition to using up lithium in the following reaction step, failure to take into account the contribution of this water to the pressure reading results in a miscalculation of acetylene yield. Water is admitted into the cooled lithium chamber to hydrolyse the carbide and, excess lithium, producing acetylene and hydrogen gas. These are separated by the gas trap grouping which follows, the first trap being
''
Fxo. 1. Benzene synthesis gas handling line.
'
678
M. A. Tamers
cooled with dry ice to initially remove water and the second two with liquid nitrogen to completely collect the acetylene. The hydrogen gas is removed by the right side vacuum pump without passing through the phosphoric acid gas washing device. After the hydrolysis, the acetylene is allowed to sublime and is bubbled through the phosphoric acid gas washer. It is collected by liquid nitrogen cooling of one of the gas traps in the first group after circulating to that forward position through the uppermost tubing line. With all the traps in the second group now dry ice-cooled, the acetylene is sublimed over this position and passes into one of the storage vessels on the right hand side without again going through the phosphoric acid bubbler. The acetylene is collected quantitatively using the appropriate "cold finger" and its quantity precisely measured. The acetylene to benzene cyclization is effected in an individual catalyst system previously dried at above 250°C. The vacuum line of Fig. 1 shows three catalyst bottles, each with a thermometer dipping into the pellets for temperature monitoring. There are positions for a total of five catalyst systems. This multiplicity is for convenience in batch changing and drying of the material, generally done only once a week. The benzene is collected under vacuum in the last trap, which is cooled with dry ice. During this operation, the catalyst is heated to approximately 120°C. All of the benzene comes off in less than thirty minutes. EXPERIMENTAL 1. Sample combustion and carbonization
After the pretreatments necessary to remove extraneous carbonaceous matter, low carbon content materials are burned in a Vycor tube to convert all carbon to carbon dioxide. This gas is quantitatively collected in an ammonium hydroxide bubbler system and, subsequently, precipitated with strontium chloride. The resulting strontium carbonate is filtered on a 350 ml, medium porosity Biichner funnel, washed, and dried at I I0°C. The weighed precipitate provides the basis of a convenient gravimetric analysis for total carbon in the
sample and the starting material for the vacuum fine. With ordinary care, there are no carbon losses in this step. Carbonization is used for samples that would produce a mixture having at least 50 ~o carbon at the completion of this procedure. Common organic materials such as wood, cloth, food, flesh, leaves, etc., are treated with this method. Soil samples, ground water carbonates, bones and shells are not suitable. The carbonization takes place in essentially the same system as the combustion, with the difference that in the latter case, nitrogen gas is flowed through the Vycor tube instead of oxygen. The ammonium hydroxide solution is replaced with water that is used to verify the inert gas flow. Heating the sample converts the carbon to the elemental form. This, mixed with the ashes, is used directly with lithium metal for the carbide production in reaction (e). In addition to a simplified synthesis, there is a considerable saving in the amount of lithium required. The production of carbide from carbon dioxide requires five times as much lithium as that from elemental carbon. 2. Carbi& produetion
Carbon dioxide or charcoal are converted with > 9 5 % efficiency to carbide by reaction with lithium metal in the absence of air. Reaction (e) proceeds readily at 800°C (11) and driven toward completion by employing a large excess of lithium and raising the temperature to 900°C, for at least 30 min. Reaction (d) produces best results if the carbon dioxide is not exposed to the lithium until it has been heated to at least 700°C. The stainless steel reaction chamber is shown in Fig. 2. The yields of (d) are reduced by the secondary reactions, 4Li + 2COa --* Li~CO8 + C + Li20 LifO + CO s ~ Li~CO 3
(h) (i)
These losses of carbon can be reduced by heating the reaction mixture to 900°G. The lithium carbonate decomposes to a significant extent starting from 600°C, ~1~ which reverses reaction (i). At the final temperature of 900°G, the regenerated carbon dioxide produced is
Chemical yidd optimization of the bcnxans~algsia for radiocarbondating ~--.---- tOmmO,ID.
~
4"m
1+ Fro. 2. Lithium reaction chamber constructed of 2 mm stainless steel, with water cooling jacket to protect the rubber O-ring seal. eventually converted to lithium carbide by reaction (d) and the carbon of (h) by reaction
(e). It is important to have a tight vacuum system. T h e lithium reaction chamber is thoroughly evacuated initially, but the pump must be disconnected for the entire carbon dioxide reaction operation. A leak in the system will not be readily noticeable since both nitrogen and oxygen combine with lithium metal to produce solid nitride and oxide compounds, a~) Five and a h a F moles of lithium react for each mole of air that enters. Significant quantifies of lithium metal could be consumed in this way, resulting in insufficient amounts of the reactant to produce the desired carbide in the primary reactions. Lithium is also used up in considerable amounts by the high temperature reaction of the metal with water vapor present in the carbon dioxide or generated from the liquid remaining in the water inlet tube from the previous hydrolysis. This produces lithium oxide and hydride. During the hydrolysis reaction in the cooled lithium chamber, the lithium hydride is decomposed to form hydrogen. This gas can
679
produce a false impression of excess lithium metal present in the final mixture. It is inconvenient to maintain the lithium chamber at 900°C for long periods due to the approximately 12 m m Hg vapor pressure of lithium at that temperature. This causes a deposition of lithium metal on the cooler, upper portions of the chamber, with the possibility of yield losses from (h) and (i), here. Raising the temperature above 900°C is not recommended because of excessive volatilization of the metal at higher temperatures. Because the heat of vaporization of lithium metal is high (5000 cal/g approx.) the molten metal surface could be as much as 100°C lower than the temperature reading of the furnace. The indicated temperature should be at least 950-1000°C in order to guarantee the 900°C necessary to insure the completion of reaction (e). Insufficient heating, along with leaks and moisture, are the causes of low carbide yields. The furnace used here was a Thermolyne type 2000. T h e lithium metal was carefuUy selected to be carbon-free since the presence of this contaminant could cause important errors. (aa) Lithium packed in argon gas is necessary for the radiocarbon dating requirements.* T h e hydrolysis of the lithium carbide, cooled to room temperature, proceeds with a quantitative yield, as shown in reaction (f). The excess lithium reacts with the evolution of hydrogen gas. T h e remaining solution should be clear in the case of good yields. A black suspension indicates the predominance of the secondary reaction (h) and a grey suspension, reaction (i). Lithium carbonate has a low solubility in lithium hydroxide solution.
3. Acetylene to benzene cyclization Most benzene laboratories now use transitionmetal activated, silica-alumina catalysts for the cyclization of acetylene to benzene. These materials are common dehydrating agents for various petroleum reforming operations. Those * Lithium Corporation of America, Bessemer City, North Carolina (cat. no. 450 767); Riedel-de I-Iaen AG, Scelze-Hannover, Germany (cat. no. 130 60).
680
M. A. Tamers
generally employed are either Cr vl or V v The presence of carbonates on the deactivated activated.* In our laboratory, the silica- catalyst was verified by acid attack and sub-
alumina catalyst base (in the form of 2 m m pellets) is activated by potassium chromate, 0"2 % by weight. Under the best conditions, 99 ~ conversion of the dry acetylene to benzene is obtained. The catalyst is deactivated to some degree during the reaction, but only a negligible amount of acetylene is used up in the secondary reactions involved. In the case of the potassium chromateactivated catalyst, complete acetylene oxidation reactions arc given below. T h e situation would be similar with the other transition metal ions catalysts. 3CsH~ + 10KsCrO4-~ 6K2CO s + 5Cr~O3 + 3H20 + 4K~O
(k)
CsHs + 5KsCrO4-+ 2K~CO 3 + 5CrO 2 + HsO + 3K~O
(1)
These two catalyst reactions can proceed directly, especially at higher temperatures. However, mechanisms involving intermediates, such as acetaldehyde and acetic acid, would be more likely since it was seen that small amounts of moisture in acetylene caused rapid catalyst deactivation in addition to lower benzene yields. This would begin with a reaction of acetylene with water present in minimal amounts either in the gas or on the catalyst. T h e reactions could stop with the formation of an acetate salt or proceed to potassium carbonate.
C2H s + HsO --*-CHsCHO 3CH3CHO
(m)
+ 2K~CrO~ --~ 3CHsCOOK + Cr~O 3 + KOH.
(o)
3 C H a C O O K + 8KsCrO4 --~ 6K2CO a + 4CrsO a + K O H + 3 K 2 0 + H 2 0
(p)
* The chromium activated catalyst is available from Mobil Oil Corp., N e w York (type Durabead I) and Kali-Chcmic, Harmovcr, Germany (type K C Pcrkator Dl); the vanadium activated material is offered by Task, Inc., Oak Ridge, Tenn. and I-lanshaw Chemical Co., Cleveland, Ohio (type
VO 701 T 118).
sequent collection of evolved carbon dioxide in an ammonium hydroxide solution. Infra-red spectrographic evidence for traces of the carbonyl group of the acetate was obtained. Acetaldehyde being very reactive, (o) would proceed readily at all temperatures. Acetate is more stable and it would be expected that (p) would be most significant at the higher catalyst hearings. Considering that four moles of potassium chromate are reduced for each mole of acetate, the maintenance of low catalyst temperature is important. T h e acetate could also react with acetylene, forming vinyl acetate, which readily undergoes polymerization. Reduction of the chromate by hydrogen gas, formed in small amounts by the decomposition of acetylene, also occurs. This is considerably accelerated by heating, for example, above 200°C. A soot-like carbon produced can be seen in cold traps where acetylene sublimation was accelerated with a heat gun. Likewise, if the catalyst temperature is allowed to rise out of control, a carbon black deposit sometimes results. The colors of the deactivated catalyst provide an indication of the predominating reactions. The production of carbon results in a blackish color, noticeable when the catalyst temperature rises above 230°C during the reaction. The generation of the grey-green or violet CrsO 3 is prevalent when the catalyst is kept below 30°C by constant cooling and slow addition of acetylene. The brown CrOs color is observed with a reaction temperature around 100°C. O f course, the Cr III oxide can be converted to the Cr Iv oxidation state by a subsequent reaction with chromate, further deactivating the catalyst. The effect of temperature on yield with the vanadium activated catalyst was reported by POLACH and STIr'P. ~14) With the moderate temperatures permitted, the chromium activated catalyst used here showed negligible acetylene losses if the gas and catalyst were well dried. This is illustrated in Fig. 3. Thorough drying of the catalyst and the acetylene is very important to insure good yields and a low rate of catalyst deactivation. The effect of moisture on acetylene to benzene
Ghemkal.~ield optimization of the bmzer~ ~mthesisfor radiocarbondating *lam
--~
681
,0
~am
am
t I 8m
amO
MAXI~..fM ~
I
A
I~BAOfNm(°C].
Fio. 3. Acetylene t o benzene yield with maximum temperature recorded by a thermometer embedded in the potassium chromate activated silica-alumina catalyst pellets. production is illustrated in Fig. 4. With exmoisture, the reaction does not start at all. T h e catalyst should be vacuum-dried above 250°C for at least an hour, using the protected left hand side pump of Fig. 1. The acetylene can be dried by repeated passage over dry ice-cooled traps after it passes through the phosphoric acid purifying bubbler. T h e catalyst can be readily reactivated by heating in air at 500°C, which causes the oxidation of the Cr m and Cr Iv to Gr vI. Any carbon produced by acetylene decomposition is burned off at that temperature. It is not necessary to exceed 500°C during this operation. I f higher temperatures are employed, cessive
Ioc
~
m~
the chromium begins to react with the silicaalumina base and the catalyst capacity is permanently reduced. T h e situation is similar in the case of the vandium activated silica-alumina catalyst. The deactivation here involves an oxidation state reduction, from V v to V rv and V m . (15) Reactivation of the catalyst proceeds readily by heating in air. (16) The catalyst used in the present work was capable of converting at least 1 liter S T P acetylene/5 g catalyst without reactivation if the benzene produced was intermittently removed. The deactivation of the catalyst by reaction (o) would mean that less than 0-2 ~ of the
_....._.__.._... C.~.
8G
0 -16a
a ).
gl Z WdO ID lIDO
°o
I m
,l WATIR
V ~
IN
I I s 4 ~J~ETYLIENE ~
~
| s TOTAL1
e
Fro. 4. Acetylene to benzene yield as a function of humidity in the gas reactant.
682
M. A. Tamers
acetylene was taken for these side reaction, if the acetate is not oxidized to any great extent. This explains the fact that at moderate temperatures, nearly quantitative yields are obtained from the dried acetylene cyclization despite the catalyst changes.
SUMMARY The majority of materials submitted to age determination are irreplaceable and the occurrence of a faulty chemical conversion to benzene must be minimized. Studies to optimize conditions for the radiocarbon dating benzene synthesis have been presented here. The principal secondary reactions respomible for reduced yields in the various steps are described as well as methods for their control during the processing of critical samples. For lithium carbide production, it is essential to have a vacuum tight, dry system. The lithium chamber with its connecting lines should be pumped with a good vacuum system for several hours before the metal reactant is introduced. T h e integrity of the vacuum must be verified at this point since, when the lithium is present, pressure changes are small. Additional protection against low yields in the carbide step can be obtained by the final heating of the lithium chamber to an externally measured value above 950°C, and the introduction of the carbon dioxide reactant only after an external temperature of 700°C has been reached. The use of a large excess of Lithium reactant is recommended. Low yields in the acetylene cyclization to benzene can be traced to water vapor contamination of the acetylene and the cataiyst. The gas reactant should be thoroughly dried by repeated passage over a group of dry icecooled traps and, when frozen out by liquid
nitrogen refrigeration, pumped on by a vacuum pump protected with a liquid nitrogen-cooled trap. T h e catalyst should be dried to at least 250°C under good vacuum and with a protecting trap. T h e temperature of the catalyst during benzene production should be mainrained below 200°C by a moderated entrance of acetylene. REICERENCES 1. LmsY W. F. Radiocarbon Dating. University of Chicago Press, Chicago (1952). 2. DmTHRONW. UeAEC Report NYO-6628 (1956). 3. T ~ a s M. A. Mol. C~ysta/s 4, 261 (1968). 4. T ~ g s Y. A. Sdence 1~2) 668 (1960). 5. T ~ R s M . A . , STmx,J.J. and C.~OLLI]gRJ.Geochim. Cosmoddm. Acta 24, 266 (1961). 6. STARIK I. E. eta/. Radiochimiya SSSR 3, 101 (t961). 7. NOAKESJ. E., ISBELL A. F., STreP J. J. and HOOD D. W. Geoehim. Cosmochim. Acta 27, 797 (1963). 8. No,.ms J. •., I ~ S. M. and ST~P J. J. Proc. Int. C-14 and T-3 Dating Conf. Pullman, Washington (1965). 9. T A n g s M. A. Aria Cientif~a Venezolana 16, 156 (1965). 10. PI~.Tm F. and SCHARPI~NK~]gLH. W . Atompraxis 12, 95 (1966). 1I. JuzA R. andWzHLZ V. Naturwiss.52, 537 (1965). 12. A complete description of the properties of lithium and its compounds can be found in: Gmdin~ Handbuch der Anorganischen Chfmie-Lithium, vol. 20. Verlag Chemic, Weinheim (1960). 13. GZYH M. A. J. appl. Radiat. Isotopes 20, 463 (1969). 14. POLACH H. A. and STIPP J. J. appl. Radiat. Isotopes 18, 359 (1967). 15. C O L , ~ D. D., Lw C. L., DXCKEP~O~ D. R.
and FROST g . R. Proe. Int. Conf. Radiocarbon Dating, p. 158, New Zealand (1972). 16. POLACH H. A. Proc. Int. Conf. Radiocarbon Dating, p. 145 New Zealand (1972).