Use of dispersed catalysts for fossil fuel upgrading

® 1997 Elsevier Science B. V. All rights reserved. Hydrotreatment and hydrocracking of oil fractions G.F. Froment, B. Delmon and P. Grange, editors

Use of Dispersed Catalysts for Fossil Fuel Upgrading A. S. Hirschon and R. B. Wilson SRI International, 333 Ravenswood Ave, Menlo Park, CA 94025, USA 1.

ABSTRACT

Because of dwindling supplies of premium oil reserves, we are striving to find economical methods to utilize altemative fossil fuels, such as coal and heavy oils, which are far more abundant. Conventional hydrotreating catalysts are not adequate for these types of feedstocks because of the heavy asphaltenes and metal contaminates. One approach to convert these feedstocks is to use dispersed catalysts. The advantage of dispersed catalysts is that they should not be subject to deactivation as readily as conventional supported catalysts, while at the sametime,should provide more catalytic activity with a lower metal loading. We have tested the effect of several preparations of dispersed catalysts to convert feedstocks such as coal, recycle solvents, and heavy oils. These dispersed catalysts include those containing Fe, Mo, and Ni metals or metal sulfides. We found that the dispersed catalysts are more effective than those of conventional catalysts, with the nK)st effective ones being those that are easily activated under the reaction conditions. 2.

INTRODUCTION

Worldwide crude oil production is already close to capacity and is expected to decline due to diminishing reserves. Estimates of the decline range from 2015 to 2020.1*2 Yet estimates of demand for premium oil is expected to increase by over 30% over this time pericxi and the demand could exceed the supply before the year 2005. Due to these factors and frequent political uncertainties, the cost of oil is expected to be significantiy higher in the near future and the need to economically utilize altemative fossil fuels to usable feedstocks is essential. Vast amounts of altemative feedstocks exist world wide, but because of the difficulties in upgrading these feedstocks, they are not fully utilized. Coal, resids, and heavy oil reserves could account for decades of future energy usage. The problem with these materials is that they contain heavy amounts of asphaltenes and metals which rapidly deactivate conventional supported catalysts. The asphaltenes contain coke precursors which tend to accumulate in pores of conventional hydrotreating or liquefaction catalysts. Furthermore, metals, especially those associated with porphrins such as Ni and V, deposit as the sulfides in the pores of the catalyst support.^"^ Our approach to upgrade and utilize these altemative feedstocks is to use dispersed catalysts. The advantage of dispersed catalysts is that since they are not supported they are not subjected to deactivation by pore plugging, and since they are highly dispersed, there is an intimate contact between the oil and/or cod and solvent with the hydrogen atmosphere.

499

500 In the ideal case, they can be activated and transfer hydrogen directiy to the asphaltene molecules even in the absence of a good H-shuttiing solvent Thus a more efficient hydrogen utilization can be used, potentially lowering pressure and perhaps temperature requirements. Furthermore, with proper design, cat^ysts with high surface areas with less metal loadings will be required. One of the problems in developing dispersed catalysts is to be able to produce the catalyst in the most active state. The reason fortfiisrequirement is so that retrogressive reactions or coking will not occur while waiting for the catalyst to activate. In our work we try to design the dispersed catalyst so that it is already in the most active stoichiometry and oxidation state, and is therefore more easily activated. The following work describes our efforts to determine the most effective catalysts for the various altemative feedstocks. 3,

EXPERIMENTAL

The hydrotreating catalyst Shell 424 NiMo alumina supported catalysts was received as a gift from Criterion Catalyst Co. and was ground and sieved to 60-200 mesh. The catalyst was presulfided under flowing 10% H2S/H2 at 400°C prior to use. Ammonium tetrathiomolybdate (NH4)2(MoS4) was obtained from Alfa Chemicals and is referred to as M0S4. The organometallic molybdenum catalyst was (C5H5)2Mo2(^SH)2(^-S)2, referreid to as Mo(OM), and was prepared by modifying the method of Cowens et al.^ Coal conversions and oil upgrading experiments were conducted in 300 mL Autoclave Engineers stirred reactor. The coals tested were either Illinois #6, North Dakota Lignite, or Black Thunder (obtained from the Wilsonville coal liquefaction facility). The Black Thunder recycle solvent as well as the MolyVanL oil (used as a dispersed catalyst) were also obtained from Wilsonville. Maya ATB was used to test the effectiveness of the various catalysts for heavy oil upgrading. The feedstock or solvent containing the desired amount of catalyst was charged with 1200 psia H2 containing 3% H2S. The coal conversions in synthetic solvents were conducted using approximately 5 g of coal in 30 g of solvent. Fortiieconversions in recycle solvents, 20 g of coal (Black Thunder) and 50 g of solvent were used. In the case of the heavy oil, 50 g of the oil was used. The Mo loading for the coal and oil experiments was approximately 500 ppm. In the case of the NiMo supported catalyst conversion of the Maya, approximately 1.0 gram of catalyst was used. After the autoclave was charged it was heated to 400°C or 425°C for up to one hour. Microdistillations were conducted on Perkin-Elmer TGA apparatus. Mass spectra were obtained on a Field Ionization Mass Spectrometer (FIMS) built by SRI. The distribution of preasphaltenes, asphaltenes, and oils, were determined by solubilities in THF, toluene, and hexane, respectively. Coal and oil conversions were based on THF solubilities. 4.

RESULTS AND DISCUSSION

The purpose of our work was to examine dispersed catalysts for several types of feedstocks, each of which has the characteristic in that they are hard to upgrade using conventional catalysts. In this paper we compare conventional dispersed catalysts such as M0S4, MolyVanL, and a thiolato molybdenum catalyst precursor. These feedstocks have different properties in that they have varying degrees of H/C ratio, and varying amounts of aliphatic and aromatic components in the hydrocarbon material. Thus these feedstocks offer a wide spectrum of properties to which we can test our catalysts. The H/C ratio, ranging from 0.79 for the coals, 0.96 for the recycle solvent to 1.49 for the ATB oil is less than what would be desired for a premium petroleum feedstock. Furthermore, these materials

501 contain high degrees of asphaltenes, which easily forms coke. During the upgrading process these heavy asphaltenes tend to phase separate, making the coking process even more of a problem. Furthermore, these feedstocks have high degree of meml contents which tend to rapidly deactivate conventional supported catalysts. In order to examine the various dispersed catalysts we conducted a series of experiments designed to simplify the assessment and comparison of the catalysts. This step was necessary due to the complexity of fossil fuel feedstocks. For our fct approach to compare the dispersed catalysts, we examined coals in synthetic solvents such as tetralin, and n-hexadecane (CI6). This comparison allows us to determine the relative effects of a good H-donation solvent with a totally inert solvent in conjunction with the dispersed catalysts. The more inert solvent allows us to magnify the comparison of the dispersed catalysts. Furthermore, a catalyst which works well in conjunction with such a solvent would most likely be useful in cases where we wish to upgrade heavy oils. We tested two ranks of coals. A subbituminus coal with a relative high H/C ratio, and moderately easy to convert, and in an addition, a lignite coal which is difficult to convert, and contains many precursors to retrograde reactions such as free radical chemistry during the loss of carboxylates. Of the various feedstocks, coal, recycle solvent, and heavy oils, Ae coal is the most challenging due to the low H/C content and the large amounts of metal and ash which can readily deactivate conventional supported catalysts. The retrogressive reactions can occur even under moderate temperatures so dispersed catalysts tiiat can be rapidly activated during the heat-up period of the feedstock is essential. Table 1 compares the effects of the two molybdenum dispersed catalysts and a supported CoMo catalyst for the conversion of Illinois #6 coal in both tetralin and in the more inert solvent, C16. Noted in this figure is that in the tetralin solvent, even though more metal catalyst was used for the supported catalyst conversion, the conversion of the coal to liquid products was far worse witii the alumina support. In contrast, both the dispersed catalysts gave much better yields to soluble products, with the Mo(OM) catalyst giving the better results. Also note that for the thiolato complex the conversion is almost as good in the inert C16 solvent as is in the tetralin, suggesting that this dispersed catalyst should be useful in the conversions using recycle solvents, resids, or heavy oils. The fact that the supported catalyst behaved so poorly suggests that the ^umina support also promoted retrogressive reactions, perhaps centered on the acidic functionalities of the support. Table 2 shows the results where we substituted authentic heavy fossil solvents for the synthetic solvent. In the case of the coal conversion experiment, the ratio of solvent to coal was approximately 2.5 to 1 by weight. In each case the amount of Mo catalyst added was approximately 500 ppm. The coal conversion (Black Thunder Coal) was calculated by determining the THF soluble fraction, and allowing form the contributions of the heavy recycle solvent We found the hexane soluble fractions (oils) to be approximately the same in all cases (approximately 20-25%). The products (from the Maya ATB conversion) was taken up in hexane and filtered through a medium porosity filter. Again, the hexane fractions or oils were approximately the same in all cases (12%). The hexane insoluble portion was then taken up and filtered in THF. We compare the Maya conversions in terms of THF solubility. As seen in both cases, the coal conversion and Maya conversion, the thiolato catalysts gave superior results. In the case of the coal conversion, the value increased from 45% using the MolyVanL catalyst to 99% for the Mo(OM) catalyst. Likewise in the case of the heavy oil upgrading, the final product using the Mo(OM) was essentially totally THF soluble, indicating very littie coking, if any, had occurred. In the

502 case of the MolyVanL and NiMo catalyzed runs material, the THF fraction could only be filtered difficulty, most likely do the high molecular weight species formed after reaction. TGA distillation curves of the oils were quite similar, however the Mo(OM) treated product showed a higher proportion of lower boiling point materials. FIMS spectral data of the two fractions likewise showed an improvement for the Mo(OM) product, with a higher fraction of low molecular weight products in the range of 300 mass units. The number average molecular weights of the various products were fairly similar with values of 679,645, and 725 for the Mo(OM), MolyVanL and NiMo catalyzed conversions, respectively.

Table 1 Effect of catalyst for conversion of coal to toluene-soluble products in synthetic solvents^^ Catalyst

Coal

Solvent

None

Illinois # 6

Tetralin

M0S4

Illinois #6

Tetralin

M0/AI2O2

Illinois #6

Tetralin

Mo(OM)

Illinois #6

Tetralin

None

Illinois #6

Hexadecane

M0S4

Illinois #6

Hexadecane

Mo(OM)

Illinois #6

Hexadecane

None

Lignite

Hexadecane

Mo(OM)

Lignite

Hexadecane

T(°Q

400 400 400 400 400 400 400 425 425

%"TS—

48 51 41 61 25 41 61 24 49

^Reaction conducted in a 300-mL autoclave with 5 g coal, 3 mmol catalyst, 30 g solvent and 500 psi H2 for 20 min at temperature. 'fields calculated on daf basis for Illinois #6 coal and on cartx)n basis for the lignite.

Table 2 Effect of catalyst for conversion of coal or oil in petroleum products^ Catalyst MolyVanL Mo(OM)

Feedstock Black Thunder Coal Black Thunder Coal

% Conversion 45 99

^Reaction conducted in a 300-mL reactor with 500 ppm metal In catalyst, 50 g solvent for 1 hr at425X.

503 5.

CONCLUSIONS

Dispersed catalysts appear to be an effective way to convert these difficult to process feedstocks, coal, resids, and heavy oils. These catalysts are less susceptible to catalyst deactivation, they can increase distillate yields while at the sametimedecreasing coke formation. The dispersed catalysts precursors that are akeady in the correct stoichiometry and oxidation state of the active catalyst are more easily activated during the reaction, and give superior conversions. In future work we hope to explore these catalysts to determine the most active metals and combination of metals that will further increase their performance during the hydrocarbon conversion process. The authors gratefully acknowledge the partial support of this work by the Department of Energy under Contract No. DE-AC22-91PC91039.

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Report from DOE Direct Liquefaction Contractors' Review Meeting, Pittsburgh, Pennsylvania, September 1995.

2.

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I. A. Wiehe, Am. Chem. Soc. Div. Petroleum Prepr., 38(2) (1993) 428-433.

6.

B. A. Cowens, R. C. Haltiwanger, and M. R. DuBois, Qrganometallics 6 (1987) 995-1004.