Novel Mo-Containing Organometallic Compounds as Dispersed

activities are compared to those of the typically used ammonium tetrathiomolybdate (TTM). Two types of liquefaction experiments (single stage at 350 a...
1 downloads 0 Views 173KB Size
Download PDF
Energy & Fuels 1997, 11, 411-415

411

Novel Mo-Containing Organometallic Compounds as Dispersed Catalysts for Liquefaction and Hydropyrolysis of Coals Bele´n Lastra, Roberto Garcı´a,* and Sabino R. Moinelo Instituto Nacional del Carbo´ n (CSIC), Apartado 73, 33080 Oviedo, Spain Received May 21, 1996X

In this work, several new Mo-containing organometallic compounds (OM1, OM2, OM3, and OM4) are used as catalyst precursors in the liquefaction and hydropyrolysis of four coals of different rank (Mequinenza lignite and Wyodak, Maria Luisa, and Upper Freeport coals). Their activities are compared to those of the typically used ammonium tetrathiomolybdate (TTM). Two types of liquefaction experiments (single stage at 350 and 450 °C, two stage at 225-425 °C) were carried out, with conversions and liquid compositions being studied. For the hydropyrolysis experiments a fixed-bed reactor was used, heating at 300 °C up to a final temperature of 600 °C. Good conversions both in liquefaction and in hydropyrolysis, similar to or better than those obtained with TTM, were achieved with the new precursors. High-rank coals are more affected by the catalytic activity promoted by the precursors.

Introduction It is generally accepted that coal is primarily constituted by fused ring structures (polycyclic or heterocyclic) of relatively low molecular weight joined together by various types of cross-links (alkyl chains, ether or thioether linkages, hydrogen bridges) to form an extensive three-dimensional macromolecular network. Additionally, lower molecular weight species, referred to collectively as the mobile phase, are weakly bonded or physically trapped in the hollows or cavities of the macromolecular matrix. The conversion of coal to useful liquid fuels or chemical feedstocks implies the release of the mobile phase, the breaking of cross-links, and the removal of heteroatoms.1,2 Coal liquefaction and pyrolysis are complex processes leading to numerous and different types of compounds. During coal liquefaction and pyrolysis the thermally induced rupture of cross-links gives rise to free radical fragments of different molecular size, which can be stabilized by hydrogen addition, resulting in the formation of liquid compounds. When hydrogen is not available or it is in low concentrations, the free radical species tend to react with each other, resulting in compounds of high molecular weight. The latter retrogressive reactions are responsible for the formation of solid products (char).2 Coal liquefaction occurs in two loosely defined stages, consisting of coal dissolution followed by upgrading of the solubilized products. These processes are significantly improved by the presence of an agent capable of suplying hydrogen to stabilize the thermally produced fragments. Hence, the use of an appropriate solvent promotes the formation of larger quantities of lowboiling aromatic species. Alternatively, the transfer of hydrogen to the reactive sites in the coal structure can be improved by a suitable catalyst, even in the absence X Abstract published in Advance ACS Abstracts, February 15, 1997. (1) Snape, C. E. Fuel 1991, 70, 285. (2) Derbyshire, F.; Davis, A.; Lin, R. Energy Fuels 1989, 3, 431.

S0887-0624(96)00077-1 CCC: $14.00

of a hydrogen-donor substance. Current research in coal liquefaction places great emphasis on the development of new catalysts, essential to achieve a more efficient consumption of hydrogen and reduce the costs of the process.3 One of the most promising approaches to more efficient coal liquefaction and hydropyrolysis is the development of novel multicomponent dispersed catalysts. The presence of a second metallic atom can improve the catalytic activity by a synergistic effect.4 Additionally, there is the possibility of significantly reducing the costs of coal liquefaction in the presence of an expensive catalyst by its combination with a cheaper cocatalyst. The use of highly dispersed catalysts is known to improve overall conversion and also the quality of the liquid products in a number of coal liquefaction regimes including hydrogenation, both with and without (dry) hydrogen-donor solvents,5-7 and hydropyrolysis.6-10 In this study, several newly developed synthetic Mo organometallic compounds are tested as catalyst precursors in the liquefaction and hydropyrolysis of several coals. The effect on the conversion and the nature of the liquid products is tested and compared with the results obtained with a more typical precursor, ammonium tetrathiomolybdate (TTM), extensively used in coal liquefaction and hydropyrolysis investigation.6,11-17 (3) Derbyshire, F. J. Catalysis in coal liquefaction: New directions for research; IEA Coal Research: London, 1988. (4) Chianelli, R. R. Catal. Rev., Sci., Eng. 1984, 26, 361. (5) Derbyshire, F. J. Energy Fuels 1989, 3, 273. (6) Snape, C. E.; Bolton, C.; Dosch, R. G.; Stephens, H. P. Energy Fuels 1989, 3, 421. (7) Mastral, A. M.; Rubio, B.; Izquierdo, M. T.; Mayoral, C.; Pardos, C. Fuel 1994, 73, 925. (8) Parfitt, D. S.; Song, C.; Schobert, H. H. Proceedings of the 9th Annual Pittsburgh Coal Conference; University of Pittsburgh, Pittsburgh, PA, 1992; p 503. (9) Mastral, A. M.; Izquierdo, M. T.; Burchill, P.; Harbottle, S. J.; Lowe, A. J.; McCaffrey, D. J. A.; Way, D. S. Fuel 1994, 73, 449. (10) Mastral, A. M.; Rubio, B.; Izquierdo, M.; Mayoral, C.; PerezSurio, M. J. Fuel 1994, 73, 897. (11) Artok, L.; Davis, A.; Mitchell, G. D.; Schobert, H. H. Energy Fuels 1993, 7, 67.

© 1997 American Chemical Society

412 Energy & Fuels, Vol. 11, No. 2, 1997

Lastra et al. Table 1. Analytical Data of the Coals % dry basis

These new organometallic precursors were previously studied in the reaction of model compounds under liquefaction conditions.18 They showed similar or improved performances compared with more typical precursors, together with a lower activation temperature. However, although reactions with model compounds can provide an indication of the precursor activity, results are not directly extrapolable to coal liquefaction. The reaction environment is completely different, so the catalyst activity has to be ultimately checked with a real coal, making any predictions based on reactions with individual compounds difficult. Experimental Section The organometallic molybdenum compounds, containing chelating sulfur ligands, used as catalyst precursors were as follows: [Mo(SnPhCl2)(CO)3(NH2C6H4CH3){S2P(OEt)2}], referred to as OM1; [Mo(η3-C3H5)(CO)2{S2P(OEt)2}(NH2C6H4CH3)], OM2; [Et4N][{Mo(η3-C3H5)(CO)2(S2P(OEt)2)}2(µ-CN)], OM3; and [MoCo(η3-C3H5)(CO)4(µ-S2CPCy3)], OM4. Their spatial arrangements are shown in Figure 1. They were prepared according to the methods described elsewhere,19-21 and their activities in liquefaction and hydropyrolysis were compared with the more typical precursor TTM, (NH4)2MoS4, prepared according to the procedure of Artok et al.17,22 Four coals of different rank were used in this study: two Spanish, namely Mequinenza lignite and Maria Luisa (bitu(12) Derbyshire, F. J.; Davis, A.; Epstein, E.; Stansberry, P. Fuel 1986, 65, 1233. (13) Garcı´a, A. B.; Schobert, H. H. Fuel Process. Technol. 1990, 24, 179. (14) Garcı´a, A. B.; Schobert, H. H. Fuel Process. Technol. 1990, 26, 99. (15) Hirschon, A. S.; Wilson, R. B., Jr. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1991, 36 (1), 103. (16) Hirschon, A. S.; Wilson, R. B., Jr. Fuel 1992, 71, 1025. (17) Artok, L.; Davis, A.; Mitchell, G. D.; Schobert, H. H. Fuel 1992, 71, 981. (18) Lastra, B.; Garcı´a, R.; Moinelo, S. R. Unpublished results. (19) Barrado, G.; Miguel, D.; Pe´rez Martı´nez, J. A.; Riera, V.; Garcı´aGranda, S. Organomet. Chem. 1993, 463, 127. (20) Barrado Paz, G. Ph.D. Thesis, University of Oviedo, Spain, 1994. (21) Miguel, D.; Pe´rez Martı´nez, J. A.; Riera, V.; Garcı´a-Granda, S. Organometallics 1994, 13, 1336. (22) Artok, L.; Schobert, H. H.; Mitchell, G. D.; Davis, A. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1991, 36 (1), 36.

S

Oa

Re

ash

VM

C

H

N

Mequinenza Wyodak Maria Luisa Upper Freeport

0.27 0.32 0.78 1.16

18.0 8.8 10.6 13.2

50.6 44.7 37.3 27.5

73.6 75.0 84.1 85.5

5.7 5.4 5.6 4.7

1.0 11.5 8.2 0.93 1.1 0.5 18.0 0.86 1.7 1.7 6.9 0.80 1.6 0.7 7.5 0.66

a

Figure 1. Spatial arrangements of organometallic precursors.

% daf basis

coal

H/C

By difference.

minous), and two American, namely Upper Freeport (bituminous) and Wyodak (subbituminous) from the Argonne Premium coal bank. The analytical data of these coals are listed in Table 1. The coals were impregnated with the catalyst precursors, following a procedure based on the incipient wetness method.23,24 Either a solution in dichloromethane (for the new precursors) or a suspension in methanol (for TTM) were used in the minimum volume required to cover the whole amount of carbon in each case. The suspension was immersed for 15 min in a refrigerated ultrasonic bath to achieve a good dispersion, and then the solvent was evaporated slowly, using a rotary evaporator at 40 °C and under an argon flow. The final catalyst nominal loading was 1% Mo (daf coal). Coal liquefaction experiments were conducted in a 25 mL stainless steel microreactor, using 2-3 g of impregnated coal and 5 MPa of hydrogen measured at ambient temperature. In the single-stage experiments, the loaded reactor was immersed for 60 min in a fluidized bed sand bath preheated to the desired temperature (350 or 450 °C). This long reaction time may be suitable to promote retrogressive reactions; however, preliminary experiments showed that conversion continuously increased from 30 min, reaching an almost constant value after 60 min. In the two-stage liquefaction experiments, an initial temperature of 225 °C was maintained during 30 min; then the sand bath was heated at 3 °C/min up to 425 °C, and the temperature was kept constant again for 30 min. After the runs, gases were collected and analyzed by gas chromatography (GC). The content of the reactor was extracted with THF to obtain the whole liquid product (THF solubles). After the solvent was removed, the THF solubles were treated with boiling toluene to obtain the preasphaltenes (toluene insolubles). Finally, the toluene solubles were treated with n-hexane to separate oils (solubles) and asphaltenes (insolubles). For the hydropyrolysis experiments a fixed-bed reactor described previously6 was used. Experiments were conducted using 2-3 g of impregnated coal with 15.2 MPa of hydrogen and a gas velocity of 10 cm3/min. The loaded reactor was heated at 300 °C/min to 600 °C and held at this temperature for 10 min. Tars were collected in a cold trap (-78 °C, dry ice) and gases quantified and analyzed by GC. Both in liquefaction and in hydropyrolysis experiments, conversions were calculated on a daf basis using the solid residue (THF insolubles) yields. The residual catalyst, although representing a very low proportion of the residue, was discounted considering the mass losses obtained in previous TGA analyses of the precursors.18 The HPLC analysis of the THF solubles was carried out using a Waters system incorporating two columns (300 mm × 7.5 mm of i.d.) packed with polystyrene/divinylbenzene copolymer of different nominal pore sizes (500 and 100 Å, respectively) and connected in series. A UV detector operating at 254 nm was used. The mobile phase was THF at a flow rate of 1 mL min-1. (23) Serio, M. A.; Kroo, E.; Charpenay, S.; Solomon, P. R. Prepr. Pap.sAm.Chem. Soc., Div. Fuel Chem. 1993, 38, 1021. (24) Song, C.; Parfitt, D. S.; Schobert, H. H. Energy Fuels 1994, 8, 313.

Mo Compounds as Catalysts

Energy & Fuels, Vol. 11, No. 2, 1997 413

Table 2. Conversions in Liquefaction Runs at 350 °C catalyst coal

No

TTM

OM1

OM2

OM3

OM4

Mequinenza Wyodak Maria Luisa Upper Freeport

44.7 17.5 32/8 15.4

67.8 59.0 46.8 18.0

92.8 54.0 61.7 26.4

84.4 60.3 57.9 17.4

66.2 47.4 54.1 17.5

90.8 52.8

Results and Discussion Liquefaction. All of the OM precursors tested in this study contain at least one Mo atom and several S atoms directly bonded to the former. The presence of sulfur in the precursor molecule avoids any pretreatment to obtain Mo sulfide, a compound extensively reported as active in coal liquefaction.17,25 Table 2 shows the results obtained in the liquefaction runs developed at 350 °C. The presence of any precursor increases the conversion despite the coal rank. In most cases, the new organometallic compounds give rise to better or similar results compared with the more typical TTM. Results are especially good for Mequinenza lignite and Maria Luisa coal, with the new precursors yielding conversions in the ranges of 84-93 and 5362%, respectively, while TTM only reaches 68 and 47%. In the case of Wyodak coal conversions are similar with all of the precursors, while OM1 gives rise to the best result with Upper Freeport coal. The advantage of solubility in organic solvents, not possible in the case of TTM, allows a better dispersion of the precursor in the porous structure of coal, avoiding the necessity of using a solvent during the liquefaction process.5,15,16,24,26 Furthermore, the temperature at which the active phase forms is much lower than that corresponding to TTM,18 well below the liquefaction temperature used in this study. Both facts contribute to a more efficient conversion of coals into liquids. Among the four new organometallic precursors tested, OM1 promotes the highest conversions, except in Wyodak runs. This could be due to the active phase forming at a lower temperature (between 250 and 300 °C, according to unpublished results obtained by the authors18) for this precursor and facilitating the thermolysis of the coal structure and the hydrogenation of the radicals formed. However, the possibility of the presence of a more active phase, promoted by the spatial arrangement of this particular precursor, and/or the synergistic effect due to the presence of an Sn atom in the molecule of this precursor24,26,27 cannot be disregarded. The quality of the liquid products obtained by liquefaction was studied by fractionation in preasphaltenes, asphaltenes, and oils. The yield of these coal liquid fractions and that of gases are shown in Figures 2-5. As expected, the general conversion decreases with coal rank, reaching the highest values for Mequinenza lignite and the lowest for Upper Freeport coal. However, although the yield of oils + gases generally follows the same trend, there is the exception of Wyodak coal, (25) Mitchell, G. D.; Davis, A.; Derbyshire, F. J. Proceedings of the 1989 International Conference on Coal Science, Tokyo, Japan; New Energy and Industrial Technology Development Organization: Tokyo, 1989; p 751. (26) Curtis, C.; Pellegrino, J. L. Energy Fuels 1989, 3, 160. (27) Laneman, S. A.; Stanley, G. G. In Homogeneous Transtition Metal Catalyzed Reactions; Moser, W. R., Slocum, D. W., Eds.; Advances of Chemistry Series 230; American Chemical Society: Washington, DC, 1992; Chapter 24, p 349.

Figure 2. Product yields in liquefaction of Mequinenza lignite.

Figure 3. Product yields in liquefaction of Wyodak coal at 350 °C.

Figure 4. Product yields in liquefaction of Maria Luisa coal.

for which the proportion of oils + gases is similar to or higher than that for Mequinenza lignite, a lower rank coal. This can be attributed to the presence of a high proportion of functional groups containing oxygen, which content in Wyodak coal is the highest. Most of this oxygen appears in hydroxylic and carboxylic functional groups in this coal28 suitable to form labile H bridge cross-link bonds, breakage of which can promote a more extensive rupture of the macromolecular net(28) Winans, R. E.; McBeth, R. L.; Melnikov, P. E.; Botto, R. E. Proceedings of the 1993 International Conference on Coal Science, Banff (Alberta), Canada; 7th International Conference on Coal Science: Devon, AB, Canada, 1993; Vol. II, p 515.

414 Energy & Fuels, Vol. 11, No. 2, 1997

Lastra et al.

Figure 5. Product yields in liquefaction of Upper Freeport coal at 350 °C.

Figure 6. GPC profiles of the THF solubles (preasphaltenes + asphaltenes + oils) obtained in some liquefaction experiments of Mequinenza lignite at 350 and 450 °C.

work of the coal, giving rise to smaller radicals, quenched by the hydrogen excess. At 350 °C, the proportions of oils, asphaltenes, and preasphaltenes are not significantly affected by the presence of a catalyst precursor in the case of Mequinenza lignite and Maria Luisa bituminous coal (Figures 2 and 4). However, the new organometallic precursors promote a great increase in the yield of oils for Wyodak coal (Figure 3). In the case of the higher rank Upper Freeport coal the yield of asphaltenes increases in the presence of any precursor (Figure 5). Globally, the Mo-S precursors tested in this study favor the hydrogenolysis of coal increasing the liquefaction conversion. To test their capacity for promoting the hydrogenation of coal liquids, liquefaction runs in two stages8,24 (225 and 425 °C) and at 450 °C were also carried out for Mequinenza lignite and Maria Luisa coal. Conversions decrease in general as the liquefaction temperature increases due to the retrogressive reactions, but they are always higher than those obtained in the absence of precursor (Figures 2 and 4). Differences are more significant in the high-sulfur Mequinenza, in which sulfur-containing cross-links (thioether) bonding the aromatic units, present in higher proportion, can be easily broken, giving rise to radicals and contributing to coal depolymerization. These radicals take part in two kinds of reactions. On one hand, when the amount of hydrogen present is insufficient or the coal reactive centers are difficult to reach,2,11 they react with each other, as occurs in the absence of catalyst, to yield insoluble compounds with a subsequent decrease of conversions. On the other hand, they are quenched by hydrogen, giving rise to low molecular weight compounds, with a concomitant increase in the yield of oils and gases (Figures 2 and 4). In this sense, the THF solubles (preasphaltenes + asphaltenes + oils) were analyzed by GPC, and the profiles corresponding to Mequinenza lignite runs at 350 and 450 °C, normalized to 100%, are shown in Figure 6. The liquids obtained at 450 °C are clearly constituted by a higher proportion of low molecular weight compounds, eluting after 16 min, corroborating the higher proportion of oils found in these products. Differences are also observed between catalytic and noncatalytic experiments at 350 °C; the presence of an organometallic precursor, especially any of the OM series, gives rise to greater areas in the region of low molecular weight compounds, indicating a better quality of the liquids obtained.

Gas yields are especially high in two-stage reactions, although a temperature of only 425 °C is reached. The soluble products obtained at the first reaction temperature are involved in molecular weight reduction reactions, which improve the yield of oils but also increase the gas production. According to the results, retrogressive reactions predominate over hydrogenation, reducing the conversion at 450 °C. This fact suggests that the asphaltenes and preasphasfaltenes initially formed and then still next to the reactive sites of the catalyst are more likely to receive its effect yielding oils and gases. Thus, the amount of catalyst available for the radicals newly formed from coal structure is limited. Hydropyrolysis. The characteristics of the hydropyrolysis unit (swept fixed-bed reactor) used in this study, with the products being immediately removed from the reactor, reduce the risk of secondary reactions. For this reason, the catalytic ability of the precursors to hydrogenate coal free radicals can be more precisely tested. The conversions obtained, significantly higher than the liquefaction ones, support this point. The conversions and tar yields obtained in the hydropyrolysis reactions at 600 °C are shown in Figure 7. The presence of a catalyst precursor favors the conversion of coal to liquid and gaseous products for the four coals studied. As expected and according to the results obtained in liquefaction experiments,1 tar yield decreases with increasing rank. Tar yield improvements are, however, proportionally higher in the highest rank coal tested (Upper Freeport), as shown in Table 3. The new organometallic precursors tested produce similar conversions in the range of those promoted by TTM. In general, OM4 provides the higher values, indicating the existence of some synergistic effect in the catalytic active phase, due to the Co atom present in the precursor molecule. The increase in conversion obtained when hydropyrolysis is carried out in the presence of any of the new precursors is principally due to the formation of tars, especially for high-rank coals, in spite of the conditions used, favorable to the gas production.1 Table 4 lists the tar/gas yield ratio values. For all coals OM1 gives rise to the better conversions relative to gases. The rest of the precursors produce ratios very similar to those obtained for Mequinenza and Wyodak in the absence of precursor. In contrast, Maria Luisa and Upper Freeport, according to their higher rank, give rise to

Mo Compounds as Catalysts

Energy & Fuels, Vol. 11, No. 2, 1997 415

Figure 8. Methane proportion in the gaseous product of hydropyrolysis reactions. Table 4. Tar/Gas Yield Ratio in Hydropyrolysis Experiments catalyst coal

No

TTM

OM1

OM2

OM3

OM4

Mequinenza Wyodak Maria Luisa Upper Freeport

3.6 2.3 1.2 1.9

3.3 2.2 2.1 3.1

8.6 4.9 2.7 4.6

3.5 2.0 2.4 3.3

3.5 4.7 2.7 3.2

2.2 1.1 3.7 1.5

effect promoted by the precursors is also significant. Taking methane as representative, a lower proportion is observed when a precursor is used (Figure 8), indicating that the organometallic compounds tested not only favor the production of liquid products but also reduce the hydrogen consumption. Conclusions

Figure 7. Conversions and tar yields in hydropyrolysis reactions. Table 3. Tar Yield Increases Promoted by Catalysts in the Hydropyrolysis Runs tar yield coal

without catalyst

with OM1

tar yield increase (%)

Mequinenza Wyodak Maria Luisa Upper Freeport

64 57 40 36

86 78 63 60

34 37 58 67

higher values of this ratio in the catalytic experiments. Therefore, for similar conversions, better yields of liquid products are obtained when OM1 is used as precursor, with the benefit of using the other organometallic compounds being only evident for the higher rank coals tested. If gas composition is considered, the catalytic

The better dispersion achieved with the organometallic precursors tested in this study, together with the lower activation temperature needed for the formation of the active phase, gives rise to good conversions both in the liquefaction and in the hydropyrolysis experiments, similar to or better than those obtained with the typical precursor TTM. The better results obtained in general with OM1 (higher conversions in liquefaction at any temperature and better tar/gas ratios in hydropyrolysis) can be attributed to synergistic effects promoted by the presence of an Sn atom in its molecule. The catalytic effect promoted by these precursors is more significant for high-rank coals, for which more asphaltenes and less preasphaltenes were obtained in the liquefaction experiments, while in hydropyrolysis higher proportions of tars with respect to gases are observed. Acknowledgment. We thank G. Barrado and D. Miguel of the University of Oviedo (Department of Organometallic Chemistry) for supplying the organometallic precursors, FICYT (Project PA-CAR91-3) for financial support, and HUNOSA for a contract awarded to B.L. R.G. gratefully acknowledges the Spanish Ministry of Education and Science (MEC) for funding his research at Instituto Nacional del Carbo´n. EF9600772