Effect of catalyst precursors on coal reactivity in catalytic hydropyrolysis

Effect of catalyst precursors on coal reactivity in catalytic hydropyrolysis C. E. Snape, C. J. Lafferty,

H. P. Stephens*,

R. G. Dosch” and E. Klavetter”

Department of Pure andApplied Chemistry, University of Strathclyde, * Sandia National Laboratories, Albuquerque, NM 87185, USA (Received 6 September 1990; revised 31 October 1990)

Glasgow

G 7 IXL, UK

The use of dispersed sulphided molybdenum and hydrous titanium oxide (HTO) catalysts enable tar yields in excess of 60% daf coal to be obtained for bituminous coals in fixed-bed hydropyrolysis using relatively

mild conditions. However, it was found that a key difference between hydropyrolysis and batchwise hydrogenation is that the active form of the catalyst must be formed at a lower temperature in hydropyrolysis in order to be effective because of the much higher heating rates used. Thus, ammonium dioxydithiomolybdate which decomposes to form a sulphided MO compound below 250°C and Pd-exchanged HTO, where the Pd is reduced below 100°C have been found to be particularly effective (-0.2%. MO required to achieve maximum conversion). Molybdenum naphthenates and iron sulphides are much less effective in hydropyrolysis than in direct liquefaction because the active phases (MO& and pyrrhotite) are not appreciably formed below about 400°C. Preliminary results indicate that low concentrations of MO (about 0.02%) have considerable activity when ion-exchanged onto HTO-coated coals. (Keywords:catalyst;

Recent

studies

reactivity;

hydropyrolysis)

have shown

that tar yields in excess of in fixed-bed hydropyrolysis for bituminous coals using dispersed sulphided molybdenum catalystsrm3. For lower rank coals, the improvements in oil yields on the addition of catalysts are much more variable, which probably reflects the differing degrees of success in limiting retrogressive char-forming reactions”. Relatively low temperatures (506520°C) have been used to maximize the selectivity to tar (%tar/%hydrocarbon gases > 5) and a hydrogen pressure of 150 bar has been 60% daf coal can be obtained

sufficient to achieve the maximum conversion. Although dispersed MO catalysts have been widely used for batchwise hydrogenation4s5 and direct liquefaction6-a, the early use of these catalysts in hydropyrolysis failed to achieve a high selectivity to liquid productsg-I5 because of the high temperatures and pressures used. In this work, the effects of iron, molybdenum and palladium concentrations and precursors on the conversion rate were investigated for a small suite of coals. The formation of the active catalytic species at sufficiently low temperatures was addressed. The importance of catalyst dispersion was investigated by ion exchanging active metal species onto hydrous titanium oxide (HTO) coated coals1,2,‘6.

EXPERIMENTAL Coals and catalysts

The proximate, elemental and maceral analyses of the coals used are listed in Table I. The MO precursors used for impregnating the coals were ammonium heptamolybdate, ammonium dioxydithiomolybdate [(NH,),MoO,S,], molybdenum chloride, a Presented at ‘Coal Structure and Reactivity’, Cambridge,

UK

0016-2361/91/03039343 0 1991 Butterworth-Heinemann

Ltd

5-7 September

1990,

molybdenum naphthenate (octanate), a dithiocarbamate and a C8-dithiophosphate17. Aqueous methanol solutions were used for impregnating the coals with MO for all instances except the naphthenate and dithiophosphate, which are soluble in hexane, in the range 0.02-1.0% MO (daf coal). For Fe, colloidal iron(I1) sulphide (prepared by adding sodium sulphide to iron(I1) sulphate18) and iron(I1) sulphate were used as precursors with a nominal Fe loading of 1.0% daf coal. The HTOs were prepared from titanium tetraisopropoxyl and sodium hydroxide in methanol as described previously’v16. Their coatings on the coals were used at about 2% daf. The dispersions of MO obtained on one of the bituminous coals (Linby) were investigated by scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDAX). Hydropyrolysis

The fixed-bed hydropyrolysis reactor and experimental procedure have been described previously’. The following conditions were used in this work: temperature, 520°C; pressure, 150 bar; heating rate, 5°C s- ‘; hold time, 10 min; volumetric flow-rate, 10 dm3 min-‘; mass of coal, 5 g mixed with 10 g of sand. The char yields were determined from the mass loss of the reactor tube and tar yields from the mass gain of the dry-ice cooled trap (the amount of water condensed in the tray was determined using the Dean-Stark method). The gases evolved were recovered, sampled and analysed for Cl-C4 hydrocarbon gases.

RESULTS AND DISCUSSION Molybdenum

catalysts

The yields of tar, char and hydrocarbon gases obtained from one of the UK bituminous coals (Linby) with the various precursors for a nominal loading of 1% daf MO

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Catalyst precursors Table 1

Proximate,

and coal reactivity: C. E. Snape et al.

ultimate

and maceral

analysis

for the coals investigated

in this work Coal

Analysis

Pt of Ayr”

Linby”

Gedling”

Herrin

N. Dakota lignite*

Moisture (X ar) Ash (X db) Volatile matter (% daf)

2.4 10.0 36.3

9.8 6.7 37.9

10.0 2.2 39.2

8.0 15.5 47.4

32.2 9.7 49.8

C (% H (% 0 (% N (% Total Pyritic

87.2 5.8 4.4 1.6 1.68 1.03

83.0 5.5 8.7 1.9 1.87 0.82

81.6 5.2 9.4 1.7 0.98 0.07

80.7 5.2 10.1 1.4 4.83 2.81

74.1 4.9 19.1 1.2 0.80 0.14

14 66 19

10 74 16

8 72 20

5 85 10

dmmf) dmmf) dmmf) dmmf) sulphur (% db) sulphur (% db)

Exinite (X vol’) Vitrinite (% ~01’) Inertinite (X vol’)

_ _ _

(ar) As received; (db) dry basis ‘Sample and analyses provided by British Coal b Argonne Premium Coal Sample ‘Mineral matter/shale free basis

a

b %Y leld

‘O0i

0.2

0.4

0.6

MO Concentration Figure 1 Effect of MO concentration (+) tar yield

Table 2 Effect of molybdenum yields for Linby coal

on conversions

and iron precursors

0.8

1

1.2

Concentration of metal (%)

and tar yields for a, ammonium

on hydropyrolysis

Char

Tar

C,-C4 gases

38 55 64 65 54

I 9 10 6 _

58 59 56 42 40

10 8 9 4 8

None MO naphthenate (NH,),MoW, MWJM,WN (NH,),M0,0,,.4H,O

A.2

48 21 21 20 27

C,-MO DTP MoCl, FeS, FeS0,.7H,O

1.0 1 1 0.3 1

25 23 28 49 47

C8 - MoDTP

394

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is (R0)2P,s,Mo,s,Mo,s,

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0.2

0.4

0.6

0.8

MO Concentration

Yield (% daf coal)

Catalyst

0

(W daf)

dioxydithiomolybdate

1

1.2

(% daf)

and b, dithiocarbamate.

(m) Conversion;

are given in Table 2. Figure I shows the effect of MO on conversions (lOt- % char) for the ammonium dioxydithiomolybdate and dithiocarbamate. As previously reportedrm3, total conversions of about 80% daf coal and tar yields in excess of 60% are achieved with ammonium dioxydithiomolybdate. Figure I shows that for both the dioxydithiomolybdate and dithiocarbamate, the maximum conversion is achieved at MO concentrations as low as 0.2%. In contrast, a significant reduction in conversion is obtained with the heptamolybdate as the loading is reduced from 1 to 0.2% (Table 2). Table 2 also shows that the naphthenate and chloride are considerably less effective than the sulphur-containing precursors for a MO loading of 1%. The conversion obtained with the C,-dithiophosphate is only slightly lower than that for the two other sulphur-containing compounds. The low conversion obtained with the naphthenate is surprising in view of the well documented activity of these compounds in both batchwise hydrogenation4 and

Catalyst precursors Table 3 Comparison of Pd and MO HTOs and sulphided as catalysts in hydropyrolysis

molybdenum

Yield (% daf coal)

Coal

type

Concentration of active metal (% daf coal)

Point of Ayr

None Pd-HTO MoS, None Pd-HTO MoS, None Pd-HTO MoS, None MO-HTO MoS,

0.0 0.3 1.0 0.0 0.3 1.0 0.0 0.3 1.0 0.0 0.02 1.0

Catalyst

Gedling

Herrin

Linby

North

Dakota

0.0 None MO-HTO 0.03 MoS, 1.0

Char

Tar

C,-C, gases

54 34 39 55 31 32 45 16 16 48 37 20

38 53 48 34 56 57 46 73 75 38 50 62

7 8 11 6 8 10 6 10 9 7 9 10

23 18 19

52 57 57

-

direct liquefaction (including co-processing’, often with low MO loadings ( < 0.1%)). The SEM-EDAX analysis indicated qualitatively that the dispersions achieved with the naphthenate and dioxydithiomolybdate were comparable for Linby coal. It is therefore proposed that the lower conversions obtained with the naphthenate and other compounds not containing sulphur are due to the inability of these compounds to form the active sulphided MO phase at sufficiently low temperatures in hydropyrolysis ( < 400°C) to promote hydrogenation and heteroatom removal reactions with the relatively high heating rate used compared to direct liquefaction. Thermal gravimetric analysis confirmed that both the dioxydithiomolybdate and the dithiocarbamate decompose to species corresponding closely to molybdenum disulphide at 400”Crg. For the dioxydithiomolybdate, a species corre-sponding to molybdenum trisulphide l9 is formed at 250°C which is probably also catalytically active. Therefore in hydropyrolysis the choice of MO precursor is much more critical than in batchwise hydrogenation and direct liquefaction. In the latter processes, heating rates are slower to temperatures close to 400°C. Once formed, sulphided MO and indeed all hydroprocessing catalysts are considerably more effective at 400°C than at the higher temperatures needed to achieve high tar yields in hydropyrolysis where dehydrogenation reactions are thermodynamically favoured. Iron

catalysts Table 2 shows that the Fe based catalysts only increased

yields by < 5% daf coal compared to about 25% for sulphided MO. The difference in activity for Fe and MO based catalysts is much more pronounced than in direct liquefaction6. As for the MO compounds which do not contain sulphur, the ineffectiveness of the Fe based catalysts in hydropyrolysis is attributable to the fact that in the active phase, sufficient amounts of pyrrhotite are not formed below 400°C under the relatively rapid heating conditions used. Hydrous

titanium

oxides

Table 3 compares the yields of tar, char and hydrocarbon gas obtained with Pd-exchanged HTOs and sulphided

and coal reactivity:

C. E. Snape et al.

MO (prepared from ammonium dioxydithiomolybdate, 1.0% loading of MO) for the coals investigated. In addition, the first results obtained for HTOs exchanged with low concentrations of MO are included. It was reported previously’,2 for Linby coal that Pd HTO with a Pd loading of about 0.8% daf coal is as effective as MoS,. With a loading of 0.3% daf coal for Pd, similar results have been obtained for the other three bituminous coals and the lignite (Table 3). The activity of the Pd HTO is attributed to the fact that Pd is formed at low temperatures (about 1OoOC)and the high dispersion of Pd appears to prevent rapid deactivation via the formation of sulphides. In the two tests conducted with MO HTOs using MO concentrations of about 0.015 and 0.2%, the catalysts display reasonable levels of activity (Table 3). Blank HTOs not containing metals or metal sulphides have been found to have essentially no catalytic activity in hydropyrolysis 2o. Also, sulphided MO catalysts prepared from sulphur-containing precursors have low activities at MO concentrations less than about 0.02% (Figure 1). The influence of preparation procedures on the activity of MO and Pd-exchanged HTOs in both hydropyrolysis and batchwise hydrogenation is the subject of continuing investigations. ACKNOWLEDGEMENT The authors thank the EC (Contract No. EN3V-0048UK (H)) and the US Department of Energy (Contract No. DE-AC04-76DPOO789) for financial support, and Dr Y. Inukai of the Japanese Government Industrial Research Institute for providing a sample of Cs-MoDTP catalyst. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

18 19 20

Snape, C. E., Bolton, C., Dosch, R. G. and Stephens, H. P. Am. Chem. Sot. Dia. Fuel Chem., Prep. 1988, 33, 251 Snape, C. E., Bolton, C., Dosch, R. G. and Stephens, H. P. Energy & Fuels 1989, 3, 421 Snape, C. E. and Lafferty, C. J. Am. Chem. Sot. Div. Fuel Chem. Prepr. 1990, 35(l), 1 Hawk, C. 0. and Hiteshue, R. W. US Bur. Mines Bull., No. 622, 1965 Derbyshire, F. J., Davis, A., Lin, R., Stansberry, P. G. and Terrer, M. T. Fuel Process Technol. 1986, 12, 127 Garg, D. and Givens, E. N. Am. Chem. Sot. Diu. Fuel Chem. Prepr. 1983, 28(5), 200 Ruether, J. A., Mima, J. A., Kornosky, R. M. and Ha, B. C. Energy and Fuels, 1987, l(2), 198 Curtis, C. W. and Pellegrino, J. L. Energy and Fuels, 1989, 3(2), 160 Hiteshue, R. W., Friedman, S. and Madden, R. US But-. Mines Rep. 6027, 1962 Hiteshue, R. W., Friedman, S. and Madden, R. US Bur. Mines Rep. 6125, 1962 Hiteshue, R. W., Friedman, S. and Madden, R. US Bur. Mines Rep. 6376, 1964 Hiteshue, R. W., Friedman, S. and Madden, R. US Bur. Mines Rep. 6470, 1975 Schroeder, W. C. US Pat. 3,030,297, 1962 Schroeder, W. C. US Pat. 3,152,063, 1964 Schroeder, W. C. US Pat. 3,926,775, 1965 Bolton, C., Snape, C. E. and Stephens, H. P. Fuel 1989,68,161 Inukai, Y., Arita, S. and Nakamizo, M. Proceedings of the International Conference on Coal Science, Tokyo, Japan, 1989, Vol. 2, p. 839 Nakao, Y., Yokoyama, S., Maekana, Y. and Kaeriyama, K. Fuel 1984, 63, 721 Prasad, T. P., Diemann, E. and Muller, A. J. Inorg. Nucl. Chem. 1973, 35, 1895 Klavetter, E., Sylvester, A., Wilcoxon, J., Snape, C. E. and Lafferty, C. J., unpublished

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