PII S0016-7037(99)00348-8

Low temperature iron- and nickel-catalyzed reactions leading to coalbed gas formation

JUAN CARLOS MEDINA,1 STEVEN J. BUTALA ,1 CALVIN H. BARTHOLOMEW,2 and MILTON L. LEE1,*

1Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 84602-5700, USA2Department of Chemical Engineering, Brigham Young University, Provo, Utah 84602-5700, USA

(Received April26, 1999;accepted in revised form September9, 1999)

Abstract—Hydrocarbon hydrogenolysis and CO2 hydrogenation in the presence of Fe/SiO2 and Ni/SiO2

catalysts were evaluated as potential mechanisms contributing to natural gas formation in coalbeds. Thehydrocarbons used as reactants in hydrogenolysis included butane, octane, 1-octene, and 1-dodecene. Thereactions carried out in a laboratory batch reactor produced gas that contained methane concentrations greaterthan 90%, which resembles the composition of natural gas. Reaction temperatures were selected to resemblenatural coalbed conditions. Evidence is presented to show that iron and nickel minerals, which can be presentin coals at levels of 2000 and 10 ppm, respectively, can become active under geologic conditions. The oxides(Fe2O3 and NiO) used as precursors of the active catalysts (Fe and Ni metals) were reduced at 200°C undera hydrogen atmosphere. Mo¨ssbauer spectroscopy showed that ca. 6% of the iron oxide was converted to themetal; in the case of nickel, oxygen titration showed that the extent of reduction to the metal was ca. 29%. Theresultant fractions of the active metals in coals are adequate to catalyze generation of appreciable amounts ofmethane over geologic time.Copyright © 2000 Elsevier Science Ltd

1. INTRODUCTION

Coal seams are important potential sources of natural gas. Itis commonly assumed that coals function as self-source reser-voirs (Gayer and Harris, 1996) for hydrocarbon gases and oilsformed by temperature-dependent thermolysis (cracking) of thebulk coal organic matter. However, recent experimental obser-vations raise questions about the applicability of the thermol-ysis model. For example, in artificial maturation experiments,clays exert a catalytic effect on formation of gaseous hydro-carbons from sedimentary organic matter and the whole, min-eral-containing coal generates more hydrocarbon gas than themineral-free coal kerogen (Sundararman, 1995; Espitalie et al.,1980; Elie et al., 1995). These results suggest that mineralcatalysis may play a role in hydrocarbon gas formation. Inaddition, product compositions from acid catalysis and hydro-carbon thermolysis under either dry or wet conditions do notresemble those of natural gas (Andresen et al., 1993; Price andSchoell, 1995). Thermolysis experiments typically generate agas containing 50–65% methane, while methane accounts for85–95% of natural gas (Hamak and Sigler, 1991).

Generation of methane and light hydrocarbons via catalyzeddecomposition of petroleum by reduced transition metal-bear-ing compounds such as NiO and acetates of Ni, Fe, V, Co, Cr,and Mn was observed by Mango and co-workers (Mango,1992a; Mango, 1992b; Mango et al., 1994; Mango, 1996;Mango and Hightower, 1997). Their studies were conductedusing both continuous flow and batch reactors at one atmo-sphere of hydrogen, and the reaction has been proposed as aroute to natural gas formation in petroleum fields. We recentlyobserved (Medina et al., 2000) that methane is generated athigh ratesvia CO2 hydrogenation in the presence of a reduced10% Fe/SiO2 catalyst at 180–200°C, after reduction of theprecursor oxide (Fe2O3). Reactions were carried out under

batch reaction conditions at 180, 190, and 200°C and theygenerated methane at rates comparable to those observed dur-ing nickel-catalyzed hydrogenolysis of hydrocarbons reportedby Mango (1996). In this paper, we report data for the hydrog-enolysis of butane, octane, 1-octene, and 1-dodecene on thesame Fe/SiO2 catalyst. Also, data are reported for a Ni/SiO2

catalyst containing only 120 ppm Ni for both CO2 hydrogena-tion and hydrogenolysis of 1-octene, under batch reactor con-ditions. To our knowledge, this is the first time that catalyticactivity for generation of natural gas on a transition metal atlow concentration typical of coalbeds has been demonstrated.

2. EXPERIMENTAL

2.1. Catalyst Preparation

The catalysts were prepared by addition of an aqueous solution ofeither Fe(NO3)3 z 9H2O (Aldrich, Milwaukee, WI, USA) or Ni(NO3)2 z6H2O (Fisher Scientific, Fair Lawn, NJ, USA) onto a silica support(Cab-O-Sil, grade M-5 fumed SiO2, Cabot, Tuscola, IL, USA). In thepreparation of the iron catalyst, a solution containing 14.3 g of the ironsalt in 200 mL of purified water was added in aliquots of 25 mL to 18 gof support (previously dried for 2 h at100°C). In the preparation of thenickel catalyst, 100 mL of an aqueous solution containing 500 mg ofthe nickel salt, were diluted to obtain a concentration of 120 ppm in9.0 g of the silica support. After each addition, the material was driedat 90°C. The catalysts were then calcinated at 150°C for 75 min.Reduction of the catalysts was carried out for 96 h in a fixed bed reactorplaced in a GC oven at 200°C under a hydrogen space velocity of 13.5min21.

2.2. Mossbauer Spectroscopy

Mossbauer spectra of the iron precursor and iron catalyst werecollected before and after hydrogenolysis using an Austin-600 spec-trometer with a laser absolute velocity calibrator, which enabled peakposition determination to within60.01 mm/s. A rhodium foil contain-ing 87.3 mCi57Co served as the radioactive source. Prior to collectionof spectra, samples were pressed into wafers. Typically, 290 mg sam-ples of catalyst were used. Data acquisition required approximately20 h. Mossbauer data were collected at room temperature, converted bycomputer to gamma ray counts as a function of radioactive sourcevelocity (adjusted relative to metallic iron), and fitted to obtain isomer

* Author to whom correspondence should be addressed (Milton_Lee@byu.edu).

Pergamon

Geochimica et Cosmochimica Acta, Vol. 64, No. 4, pp. 643–649, 2000Copyright © 2000 Elsevier Science LtdPrinted in the USA. All rights reserved

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643

shift, quadrupole splitting, and hyperfine splitting using a nonlinearleast-squares routine.

2.3. Temperature Programmed Reduction (TPR) and Extent ofReduction (ER)

TPR and ER experiments were carried out on the Ni catalyst usinga Thermogravimetric Analyzer 7 (Perkin Elmer). Gases were passedthrough molecular sieves and Drierite traps to remove impurities.Hydrogen was also passed through a Pt/Pd oxygen-removal trap. Allgases were ultrahigh purity grade (99.999%) from Matheson. Thereduction was carried out by flowing 10% H2 dissolved in Ar whileheating at a ramp of 1°C/min to the reduction temperature of either200°C or 400°C over 4–7 mg of powdered catalyst. The catalyst washeld for 70 h at 200°C and for 7 h at400°C. After reduction, the H2flow was discontinued for approximately 1 h to obtain an accurateweight reading. Oxygen gas was then slowly introduced into the systemduring a period of 2 h until a 10% oxygen concentration in argon wasobtained. The catalyst was kept in this mixture for 3 h, after which theoxygen flow was terminated to enable measurement of the actualweight of the oxidized catalyst.

2.4. Catalytic Reactions

Hydrogenolysis of the various hydrocarbons was performed underone atmosphere hydrogen gas pressure at temperatures of 180, 190, and200°C. Each run began with in situ activation of 0.80 g (1.4 mmoles Feor 1.6 3 1023 mmoles Ni) of catalyst for 4 h at the same conditionsused during reduction in order to reverse any oxidation that may haveoccurrred during catalyst handling. After catalyst activation, 55mL ofhydrocarbon were injected through a septum into the reactor. Thisprocedure was used for hydrogenolysis of octane, 1-octene, and 1-do-decene. For CO2 hydrogenation, CO2 and H2 gases were introducedinto the reactor in a stoichiometric ratio of 1 : 4 at a total pressure of 1atm. In addition, three control reactions were performed in which (1) nocatalyst was added, (2) the support, without metals, was used as apotential cracking catalyst, and (3) a nonreduced supported oxide wasused as a potential catalyst. The activities of the Ni and Fe catalysts

were tested during CO2 hydrogenation in the presence of water in orderto simulate wet conditions that may be more realistic of geologicconditions. For these experiments, 50mL of distillated water wereintroduced into the batch reactor, in addition to CO2 and H2, aspreviously described. A second set of wet reactions was carried out inwhich excess water (100mL) was added at the beginning of the reactions.

Special instrumentation enabled computerized recording of pressureand temperature, and on-line gas chromatographic (GC) analysis ofboth reactants and products as described in greater detail elsewhere(Medina et al., 1999). Briefly, a 500 mL round-bottom glass flask wasused as batch reactor. The gas chromatograph (HP 5890, HewlettPackard, Little Falls, PA, USA) was equipped with a Carboxen 1006PLOT column (Supelco, Bellefonte, PA, USA), a flame ionizationdetector, and a microthermal conductivity detector. Sample was intro-duced using a valve with a 10mL external loop. The reactor temper-ature and pressure were monitored using a Pentium 120 PC withWindows 95. This computer was also connected to two GC integrators(HP 3392A, Hewlett Packard, Little Falls, PA, USA). The data collec-tion/system control software was written in Microsoft Visual BasicVersion 5.0.

3. RESULTS

Catalyst characterization.The following reactions describethe preparation of the iron catalyst (unbalanced equations):

Fe(NO3)3 z 9 H2OO¡�

Fe2O3 1 NOx~ g! 1 H2O (1)

Fe2O3 1 H2O¡�

Fe1 (other iron oxides, i.e., Fe3O4) 1 H2O.

(2)

Mossbauer spectroscopy was used to characterize the differentiron species during decomposition of the salt to the oxide,

Fig. 1. Mossbauer spectrum of the iron oxide used as precursor to the catalyst.

644 J. C. Medina et al.

reduction of the oxide to the metal, and characterization of thecatalyst after the methane-forming reactions. Figure 1 shows aMossbauer spectrum of the iron oxide after thermal decompo-sition of iron nitrate. The dots represent experimental data andthe solid line is the least-squares fit. The spectrum contains adoublet with Mossbauer parameters characteristic of superpara-magnetic Fe2O3. Figure 2shows a Mo¨ssbauer spectrum of thecatalyst after use in the hydrogenolysis of butane and exposureto air. The spectrum shows signals characteristic of a materialcomposed of superparamagnetic and magnetically orderedFe2O3 species. Also, a fraction of Fe12 ions with parameterstypical of octahedral symmetry is present. These ions could beproduced after chemisorption of butane, a phenomenon previ-ously observed in supported iron catalysts exposed to gases(Clausen and Topsoe, 1989). The sextet corresponding to me-tallic iron is not present. The spectrum of the active catalystwas reported elsewhere (Medina et al., 2000). Basically, it is

composed of four species, which are listed in Table 1 alongwith the parameters used in the identification of the differentiron species. Peak positions, isomer shifts (IS), hyperfinesplitting (HFS), and quadruple splitting (QS) are reportedwith respect to metallic iron. Under these mild reducingconditions (200°C, 1 atm H2), only ca. 6% of the iron (usingthe same recoil free fraction for all species) is converted tothe metal state. Proton induced x-ray emission (PIXE) anal-ysis indicated that final total iron and nickel concentration inthe reduced Fe/SiO2 and Ni/SiO2 were 10% and 120 ppm,respectively. Using oxygen chemisorption, we found that theNi catalyst used in this study (reduction temperature of200°C) has an extent of reduction to the metal state of onlyca. 29%. The fact that the Ni precursor is reduced moreeasily than the Fe precursor (29% vs 6%), is in good agree-ment with thermodynamic calculations by Huang andAnderson (1975).

Fig. 2. Mossbauer spectrum of the iron catalyst after reaction and exposure to air.

Table 1. Mossbauer magnetic parameters for different iron species found in this study.

Material Iron phase HFS (kOe) IS (mm/s) QS (mm/s) % area

Catalyst precursor Fe2O3 (sp)a 0 0.35 0.75 100Active catalyst Fe3O4 (A) 478 0.24 0 37

Fe3O4 (B) 444 0.58 0.02 52Fe° 321 0 20.08 6Fe2O3 (sp)a 0 0.26 1.01 5

After reaction Fe2O3 (sp)a 0 0.39 0.80 10Fe2O3 468 0.23 20.16 85Fe12 0 0.49 2.20 5

a sp 5 superparamagnetic material.

645Low temperature iron- and nickel-catalyzed reactions

CO2 hydrogenation.Methane was detected as the only prod-uct from CO2 hydrogenation when Ni catalyzed the reaction,while small fractions of ethane and propane were found inaddition to methane in the products from Fe-catalyzed reac-tions. Figure 3 shows that the amounts of methane generated byboth catalysts increased continuously and near linearly withtime. This indicates that both catalysts are active and stable forthis reaction since no signs of deactivation were observed evenafter 67 h of reaction (in the case of Fe).

Hydrocarbon hydrogenolysis.In Ni-catalyzed hydrocarbonhydrogenolysis, methane, ethane, and propane were detectedimmediately after the beginning of the reactions. However,ethane and propane were converted to methane after severalhours of reaction, so by the end of the reaction, methane wasthe only component of the gas. In the Fe-catalyzed reactions,methane and propane appeared after several hours of reaction,but in contrast to the Ni-catalyzed reactions, their amountsincreased steadily.

Figure 4 shows the amount of methane (cumulative) gener-ated during hydrogenolysis of 1-octene at 180°C using iron andnickel catalysts. It reveals a major difference between hydro-carbon hydrogenolysis and CO2 hydrogenation, i.e., the amountof methane generated through hydrogenolysis reached a max-imum. In the case of Ni, this maximum occurred at approxi-mately 12 h, while for Fe it occurred approximately after 6 h.To investigate whether or not the catalyst was temporarily or

irreversibly poisoned, the gaseous reactants and products wereremoved from the batch reactor by the application of vacuum.Then, the system was purged with H2 for 4 h following whichreactions were reinitiated by adding the same amount of hy-drocarbon as in the original reaction. Both the Ni and Fecatalysts recovered their activity, but then exhibited the samebehavior again. This indicates that the catalyst is not irrevers-ibly poisoned, but merely temporarily deactivated. Possiblecauses for deactivation include oxidation of the catalyst, ad-sorption of products on the catalyst surface, and formation ofcarbides. Evidence from the catalysis literature shows thatmetals rather than their oxides are the active species duringmethane formation. Even though we intended to remove all airfrom the reactor, oxygen was still detected by the thermalconductivity detector. Table 1 and Fig. 2 show that the materialafter reaction did not contain metallic iron. This means that the6% active iron metal was most likely oxidized during thereaction and consequently lost its activity.

No gases were observed even after 24 h in either of thecontrol experiments, i.e., hydrogen thermolysis of 1-octenecarried out in the absence of a catalyst at a temperature of200°C, and reaction of hydrogen and 1-octene in the presenceof the silica support or the supported metal oxide. These resultsindicate that the presence of a reduced transition metal is key ingenerating natural gas. The rate of methane formation in thehydrogenolysis of butane at 180°C was 2.52 mmol CH4 mol21

Fe day21. Ethane was also found as a product. A butaneconversion of ca. 20% was achieved after 18 h of reaction. Thisrate of metal-catalyzed gas formation contrasts markedly withthe thermal stability shown by gaseous hydrocarbons in theabsence of a catalyst under the same temperature conditions.For example, Laidler et al., (1962) have calculated that at200°C, the half-life for propane is;8 3 108 years.

4. DISCUSSION

The gaseous products generated in all of the transition-metal-catalyzed reactions reported here have compositions matchingclosely that of natural gas, with a percentage of methane greaterthan 90%; while methane accounts for 85–95% in U.S. naturalgas (Hamak and Sigler, 1991). Table 2 compares rates ofmethane generation for the reactions studied. Apparently, thereactions on Ni are the most promising for generation ofmethane. However, as has been discussed and is shown in Fig.4, both Ni and Fe catalysts slowly become deactivated duringhydrogenolysis. A more realistic picture is given in the thirdcolumn of Table 2, for which the catalyst activities are normal-ized per g of coal to represent metal concentrations in coalbeds.Concentrations of 10 and 2000 ppm for Ni and Fe were as-sumed. Under these conditions carbon dioxide hydrogenationon Fe is the most promising reaction for generation of naturalgas, with the advantage of no catalyst deactivation. The data inTable 2 also show low activation energies (;20 kcal/mol) forall of the catalytic reactions compared to the 56–66 kcal/molactivation energies required for thermal cracking reactions ofliquid hydrocarbons (Ungerer, 1990).

These results show that hydrocarbon hydrogenolysis andespecially hydrogenation of carbon dioxide on iron and nickelminerals are viable mechanisms for coalbed methane forma-tion. The experimental temperatures used both in the catalyst

Fig. 3. Cumulative plot of methane generated with time at 180°Cduring CO2 hydrogenation. (For conditions, see Experimental section.)

Fig. 4. Cumulative plot of methane generated with time at 180°Cduring 1-octene hydrogenolysis. (For conditions, see Experimentalsection.)

646 J. C. Medina et al.

reduction and in the catalytic reactions were carefully selectedto resemble natural coalbed conditions. For example, 180–200°C is representative of coalbed temperatures (Quigley andMackenzie, 1988; Price, 1993; Freudenberg et al., 1996). Inaddition to an adequately high temperature, the catalytic reac-tions require the presence of hydrocarbons or carbon dioxide,hydrogen gas, and the transition-metal catalyst. A large numberof hydrocarbons, including olefins, paraffins, and aromatics arefound in coal (Law and Rice, 1993). The presence and origin ofcarbon dioxide in coalbeds are discussed in our previous paper(Medina et al., 2000). Hydrogen gas is required for both re-duction of the metal precursor and natural gas formation.Mango (1994) reported that hydrogen is present in natural gasdeposits at levels of approximately 700 ppm. This amountcorresponds to a partial pressure of approximately one atmo-sphere, at a total pressure of 1300 atmospheres in a geologicformation. Some coal gases are reported to have hydrogenconcentrations of 0.02% by volume (Kim, 1973). A survey ofanalytical data for produced natural gas compiled by the USBureau of Mines from 1917 to 1992 shows that 253 of 1067 gassamples in the states of Colorado, New Mexico, and Wyomingcontained a concentration of hydrogen gas higher than 0.05%(mol%). Hydrogen contents ranged mostly from 0.1 to 0.3%,with a few having more than 1%. The average hydrogen con-tent of these 253 samples was 0.22%.

Formation of hydrogen during mild pyrolysis of kerogen and

coal is well documented (Harwood, 1977; Rohrback et al.,1984). It should be noted that lack of hydrogen in gases evolvedfrom coal samples does not necessarily indicate lack of forma-tion since it could have been readily consumed in metal-reduction and metal-catalyzed reactions. Moreover, liquid hy-drocarbon hydrogenolysis can occur in the absence of hydrogengas, because hydrogen is also available through hydrogen trans-fer from hydrocarbons present in the coal matrix. This processhas already been observed in laboratory studies using petro-leum (Mango and Hightower, 1997). Another potential route tohydrogen formation during coal maturation is free radical de-hydrogenation of aromatic clusters (Butala et al., 1997). In anycase, it is expected that generation of hydrogen gas for reduc-tion of metals and/or reaction with hydrocarbons and/or CO2

would be the rate-determining step in methane generation.Both Ni and Fe compounds are commonly found in coal. The

Ni concentrations in the eight argonne premium coals rangebetween 6 and 21 ppm, while Fe concentrations range between2.67 and 0.31% (Palmer, 1990). From PIXE analysis of 11coals, we found the average Ni and Fe concentrations to be 9.8ppm and 0.33%, respectively. Table 3 lists the samples ana-lyzed and their Ni and Fe contents. The data in this tabledemonstrate that adequate concentrations of minerals, espe-cially iron, are available for reduction. The most commoniron-bearing mineral in coals is pyrite, FeS2. However, iron isalso found in the form of oxides and carbonates, such as Fe2O3

Table 2. Catalyst activities in reactions leading to methane generation in coalbeds.

Substrate/Catalyst

Catalyst activity

Activation energyb

(kcal/mol)Measured

[mmol CH4/(mol metal) day]Normalized per g coala

[mmol CH4/(g coal) day]

CO2/120 ppm Ni 3810 0.65 22.2CO2/10% Fe 80 2.71 17.01-Octene/120 ppm Ni 1800 0.33 18.51-Octene/10% Fe 20 0.67 19.0

a Assuming concentrations of 10 and 2000 ppm Ni and Fe, respectively.b From Arrhenius plots of reactions carried out at 180, 190, and 200 °C.

Table 3. Iron and nickel concentrations in various coal samples.

Coal sample Formation Coal rank Fe concentration (ppm) Ni concentration (ppm)

University #9-2a Fruitlande High volatile A bituminous 40606 300 136 2University #8-1a Fruitlande High volatile A bituminous 6506 30 106 2University #12-1a Fruitlande High volatile A bituminous 19506 75 96 3Champlin 336b Almonde Bituminous 6006 65 66 1Champlin 242 D-1b Almonde Bituminous 10756 75 86 1Davis #504c Fruitlande High volatile A bituminous 58006 250 96 1Vanderslice #100c Fruitlande High volatile A bituminous 27006 200 76 2Valencia Canyon #29-1a Fruitlande High volatile A bituminous 6006 20 126 1Valencia Canyon #29-2a Fruitlande High volatile A bituminous 132006 900 166 2Black Thunderd Fort Unionf Subbituminous 17526 18 36 1Belle Ayrd Fort Unionf Subbituminous 23016 62 86 1

a San Juan Basin, La Plata County, Colorado, USA.b Green River Basin, Carbon County, Wyoming, USA.c San Juan Basin, San Juan County, New Mexico, USA.d Powder River Basin, Campbell County, Wyoming, USA.e Cretaceous.f Tertiary.

647Low temperature iron- and nickel-catalyzed reactions

and siderite, FeCO3 (Huffman and Huggins, 1978). Iron (II)-bearing mica-like clay minerals include illite; other iron min-erals in coal are rozenite, FeSO4 z 4H2O, and melanteriteFeSO4 z 7H2O (Taneja and Jones, 1984). So-called organicallybound iron is found in coal in the form of porphyrins, protein-type structures (Herod et al., 1996), ferrous acetate,Fe(C2H3O2)2 (Lefelhocz et al., 1967), and ferrous iron associ-ated with carboxylic groups (Schafer, 1977).

Among the different iron minerals, we propose that oxidesand carbonates in the coal are more likely to undergo partialreduction to dispersed Fe during coal maturation in the pres-ence of hydrogen. It is also possible that the reduced Fe and Nimetals can be reoxidized after mining and exposure to air.Indeed, we observed this phenomenon in our experiments.When the support was initially loaded with the iron salt andthen dried, the color of the catalyst was dark yellow (Fe2O3).Upon reduction, the catalyst turned black (Fe metal), but afteruse in the catalytic reaction and exposure to air for one month,the material developed a lighter color (Fe2O3). The spectrum inFig. 2 shows that metal iron is no longer present in thismaterial. We believe that the reducing, anaerobic conditions ofdiagenesis present in coalbeds (Surdam et al., 1993; Ram et al.,1998; Shock, 1988) keep Fe and Ni metals active for longperiods of time. No information on nickel speciation is avail-able, because of its low concentration in coal. However, nickelis likely to be present in the form of metal-porphyrin complexesand bunsenite (NiO).

The discovery that under low temperatures present in coal-beds, namely 200°C, fractions of iron and nickel oxides arereduced and convert into catalytic materials is unique to thisstudy. Mango and co-workers (Mango et al., 1994; Mango,1996; Mango and Hightower, 1997) have reported catalysis byseveral transition-metal compounds (acetyl acetonates, etiopo-phyrins, and oxides). The most promising of these catalysts forliquid hydrocarbon hydrogenolysis were the metal oxides, NiOand Fe3O4. These oxides were reduced at temperatures near400°C for 24 h in flowing hydrogen. These conditions effectalmost complete reduction (90%) of the iron precursor to themetal state, as analysis using Mo¨ssbauer spectroscopy hasdemonstrated (Rankin and Bartholomew, 1986). Using oxygenchemisorption, we found that NiO/SiO2 could be reduced com-pletely to the metal state at 400°C; on the other hand, whenreduced at 200 °C the extent of reduction to the metal state wasca. 29%. However, the objective of this study was not tomaximize the extent of reduction and catalytic activity, butrather to test the catalyst under conditions representative ofnatural coalbed environments.

Certain industrial processes use Fe catalysts for hydrogena-tion of CO and CO2; however, the reduction is carried out attemperatures up to 480°C (Weatherbee and Bartholomew,1982) at hydrogen space velocities 2–4 times higher than usedin this study. These conditions lead to almost complete reduc-tion of the oxide precursor. However, the fraction of iron metalpresent in the catalyst used in this study is able to generatemethane at a rate that is significant over geologic time.

Finally, it is acknowledged that the rates obtained under theconditions of this study could be higher than those in nature.This is due to the fact that the reaction conditions in thelaboratory are relatively clean. In geochemical environments,mineral matter content and composition are not uniform, but

vary with respect to mineral matter grain size, pore watercontent, seam thickness, and accessibility of the reactants to thecatalytically active sites (Goldstein, 1983; Nelson et al., 1998).

Water can affect the reaction mechanisms, including itspotential involvement as a source of hydrogen (Seewald et al.,1998; Lewan, 1997). In this study, three different concentra-tions of water were considered. First, if water is a by-product ofCO2 hydrogenation, two moles of water would be produced permole of methane. At this level (see Fig. 3), no signs of deac-tivation were observed for either of the catalysts. Second, when50 mL of water (amount equivalent to the number of moles ofCO2) were added at the beginning of the reaction, the result wasdependent on the catalyst used. In the case of Fe, the amount ofmethane generated increased; in the case of Ni, the activitydropped to ca. 40% of the activity obtained under dry condi-tions. Finally, when the amount of water added at the beginningof the reaction was twice the number of moles of reactant CO2,both catalysts experienced a considerable reduction in theiractivity towards methane generation. After removing the prod-ucts from the batch reactor and flowing H2 through the systemfor 4 h, catalyst activity was recovered. However, the catalystsexhibited different behavior. Figure 5 shows that the Fe catalystrecovered its activity and generated methane at the same rate asin the original reaction. On the other hand, the Ni catalystrecovered only 35% of its original activity. Both catalystsgenerated gases with compositions similar to those produced inthe original reactions. The different behaviors of the catalystsafter addition of water can be explained by taking into accountthat the number of moles of Ni are ca. 1000 times less than thenumber of moles of Fe. As can be seen in Table 3, the Ni andFe concentrations used in this study are typical of those foundin geologic environments.

5. CONCLUSIONS

Based on the product distributions and amounts of methanegenerated in these experiments, hydrogenolysis of hydrocar-bons and, especially carbon dioxide hydrogenation on Fe andNi minerals, merit consideration in models used to explainnatural gas formation during coal maturation. Results demon-strate that catalyst activation and metal-catalyzed gas formationcould occur under natural conditions. This, in turn, suggeststhat consideration of the chemical characteristics of the coal,

Fig. 5. Cumulative plot of methane generated with time at 180°Cduring Fe-catalyzed CO2 hydrogenation in the presence and absence ofwater. (For conditions, see Experimental section.)

648 J. C. Medina et al.

including mineral matter content, transition-metal content, andiron speciation may be important in performing gas resourceassessments.

Acknowledgments—Useful discussions on Mo¨ssbauer spectroscopywith Matthew Stoker are gratefully acknowledged. Help from DallanAndrus on the experimental setup is appreciated. George Hubber fromthe Department of Chemical Engineering carried out TPR and ERexperiments, and Dr. Nolan Mangelson and Brett Clark conducted thePIXE analysis. Technical assistance of Dr. P. Yin of the Institute forEnergy Research, University of Wyoming, is gratefully acknowledged.The Gas Research Institute (GRI) provided financial support for thisresearch under Contract No. 5096-260-3600.

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649Low temperature iron- and nickel-catalyzed reactions