Hydroconversion of resids with dispersed molybdenum catalysts derivedfrom phosphomolybdates

B. Demirel* , E.N. Givens

Center for Applied Energy Research, University of Kentucky, 2540 Research Park Drive, Lexington, KY 40511-8410, USA

Received 18 June 1999; received in revised form 7 December 1999

Abstract

Phosphomolybdic acid (PMA) and cobalt, nickel and potassium phosphomolybdates have been found very active catalyst precursors forthe conversion of coal- and petroleum-derived resids when they are impregnated onto coal and Al2O3. They are mostly stable up to 400–4508C in the presence of He and H2, but significant change in stability occurs in the presence of H2S, transforming these materials into anactive form of catalyst. Their solubility in water provides highly dispersed catalysts in the reaction media. PMA and these bimetallicmaterials were tested at the concentration of 15, 150 and 1500 mg Mo per kg feed for reaction times of 30 and 90 min, and compared toa commercial NiMo/Al2O3 catalyst (AKZO-60). In the 30 min reactions, increasing Mo concentration did not provide a significant improve-ment in resid conversions compared to the non-catalyzed case. However, in the 90 min reactions, improvements were observed in conversionof coal and Mayan resids to distillate boiling below 5258C. The results indicate that thermal reactions play an important role in the 30 minreactions, and catalytic reactions resulting in increased resid conversions become more important in the 90-min reactions. Higher conver-sions with nickel phosphomolybdate supported on Al2O3 were observed with Mayan resid compared with coal resid. Nickel phosphomo-lybdate has been found to have promising catalytic activity for hydroconversion processes.q 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Resid; Hydroconversion; Phosphomolybdic acid; Molybdenum catalysts

1. Introduction

Resid is defined as the fraction of petroleum, bitumen orcoal liquids that does not distill under vacuum at atmo-spheric equivalent boiling points over 5258C. Resids aretypically coked, hydroprocessed or used in manufacturingasphalts [1–5]. In the near future, the worldwide crudesupply quality is expected to move towards heavier feed-stocks because of the potential increase in the oil price.Therefore, it becomes necessary to upgrade these fractionsinto distillable fractions. The conversion of resids into morevaluable products requires a very efficient hydrogenationwith a very active catalyst, which is commonly generatedin-situ from inorganic or organometallic molybdenumcompounds as precursors [6]. The metal precursors typicallydecompose at high temperatures reacting with a sulfursource being thermally transformed into highly dispersedmetal sulfides [7]. Although the exact form of the catalystis not known, it appears to be some a form of molybdenumsulfide, although apparently not MoS2. Lopez et al. reported

that the atomic ratio of sulfur to molybdenum was less thantwo in the active catalysts isolated from heavy petroleumfractions with ammonium molybdate and thiomolybdateprecursors [8]. Hydrotreatment of Belayim atmosphericresidue was examined with dispersed catalysts derivedfrom thiomolybdates and molybdenyl acetylacetonate, andmost interesting results were obtained for S/Mo# 4 with[Mo3S9]

22 as the best precursor [9]. Our studies withPMA indicates that molybdenum oxysulfides are formedunder coal liquefaction conditions [10].

PMA and most bimetallic complexes of heteropoly acids,such as cobalt and nickel phosphomolybdates, are water-soluble forms of Mo that can be prepared by a very simpleprocedure starting from MoO3 [11]. Polyoxomolybdateshave been known to create active, well-dispersed catalystsfor converting both coal and resids. Although there has beenconsiderable interest in the use of catalysts prepared fromPMA for conversion of heavy hydrocarbons and carbon-aceous materials, Co, Ni and K phosphomolybdates havenever been tested before for these applications. In one ofthe earliest patents, Gleim and Gatsis [12] claimed a processfor hydrorefining a petroleum oil at 4008C and 10 MPa. Inthis case, PMA was added to the reaction mixture as anisoamyl alcohol solution, which presumably forms a colloidal

Fuel 79 (2000) 1975–1980

0016-2361/00/$ - see front matterq 2000 Elsevier Science Ltd. All rights reserved.PII: S0016-2361(00)00056-9

www.elsevier.com/locate/fuel

* Corresponding author.E-mail address:demirel@earthlink.net (B. Demirel).

dispersion when the isoamyl alcohol is removed by distilla-tion at 1308C [13]. In the hydroconversion of coal, a higheroil yield and lower coke make were reported for PMA rela-tive to molybdenum naphthenate, both of which wereclaimed to be soluble in the heavy distillate solvent [14].Addition of phenolic solution of PMA to coal slurries wasreported to give higher liquid yields [15]. Several casesclaim preparation of catalyst concentrates by mixing eitheraqueous or isopropyl alcohol solutions of PMA with oil andheating at 3858C for 30 min in 5% H2S in H2 [16–19].Addition of phosphoric acid or hydrogen halides to PMAhas been reported to improve catalyst activity [20–21].Combinations of MoO3 and PMA have also been reportedto give better resid conversion than the individualcompounds [22–24]. Our studies showed that PMA is read-ily transformed in the reaction media to produce an activeform of catalyst for liquefaction of Wyodak coal [5,10]. It iswell known that the addition of a second metal, such as Niand Co, can increase hydrogenation, hydrodesulfurizationor hydrodenitrogenation activities. Chianelli et al. usedbimetallic complexes to study the promotional effects ofNi and Co for MoS2 [25]. Garg and Givens reinvestigatedthe catalytic activity of several impregnated transitionmetals in coal liquefaction and showed that addition ofNi, Co and Mo salts was effective [26]. Eccless and deVaux reported that Ni–Mo or Co–Mo gives a very highyield of light oil fractions [27]. Our earlier studies on directliquefaction of Wyodak coal with nickel, cobalt and potas-sium phosphomolybdates showed that these precursors areas good as PMA and nickel phosphomolybdate gives rela-tively higher resid conversion [28].

Processing of coals with petroleum resid has been thesubject of a number of studies in recent years with theintention of integrating coal into conventional petroleumrefining operations [2,3,29–33]. Studies have suggestedthat combining coal liquefaction and heavy residue upgrad-ing is economically feasible. It has been claimed that thepresence of coal will reduce the deposition of coke andmetals on the catalyst. In addition, heavy resids will alsoact as hydrogen transfer agents for coal conversion [2–4,34–35]. We have processed metal impregnated-Wyodakcoal with coal-derived solvents [5,28], and further expandedour work on PMA derived catalysts to investigate variousmethods of dispersing the molybdenum in the reaction

mixture. The use of coal as a carrier is intriguing since itis inexpensive and largely converts to liquid product duringthe reaction thereby eliminating the necessity of recoveringand disposing of a solid byproduct. Unfortunately, coal doesnot possess the unique activating effect that alumina impartsto molybdenum catalysts.

This work here reports the activities of PMA, cobaltphosphomolybdate (CoPM), nickel phosphomolybdate(NiPM) and potassium phosphomolybdate (KPM) for theconversion of both coal- and petroleum-derived residswhen they are impregnated on coal and Al2O3. These phos-phomolybdates have never been used before as precursors inprocessing petroleum fractions although PMA has foundsome applications. We have evaluated these bimetallicprecursors for the conversion of resids to distillates boilingbelow 5258C, and compared their activities to a commercialcatalyst.

2. Experimental

Heavy distillate as a solvent (,5658C, Wilsonville VesselNumber V-1074, Period B) and solids-free resid (.5658C,Wilsonville Vessel Number V-130, Period A) used in thisstudy were produced in Run 258 at the Advanced CoalLiquefaction R&D Facilities in Wilsonville, Alabamawhen the plant was operating in a close-coupled configura-tion and feeding Wyodak coal from the Black Thunder mine[36]. Mayan resid was supplied by Ashland Petroleum.Resid from coal contains 8.3% distillate and 0.1% ashwhereas Mayan resid has 6.7% distillate and 0.1%ash(Elemental analysis of coal resid: 91.1% C, 6.0% H, 1.2%N, 1.4% S, 0.3% O; elemental analysis of Mayan resid:82.9% C, 10.3% H, 0.7% N, 4.5% S, 1.6% O). Wyodakcoal from the Black Thunder Mine in Wright, Wyomingwas supplied by Hydrocarbon Technologies, Inc. Proximateand ultimate analysis of the coal are given in Table 1. Tetra-hydrofuran was obtained from Burdick & Jackson; hydro-gen sulfide (99.5% purity) and hydrogen (UHP 6000#) wereobtained from Air Products and Chemicals, Inc. Aluminawas supplied by United Catalyst (surface area, 180 m2/g).Phosphomolybdic acid (PMA) was obtained from AldrichChemicals Inc. Cobalt phosphomolybdate, Co3(PMo12O40)2,nickel phosphomolybdate, Ni3(PMo12O40)2, and potassiumphosphomolybdate, K3PMo12O40, were synthesized in ourlaboratory [37]. NiMo/Al2O3 (AKZO-60) was obtainedfrom AKZO Chemicals (2.6% Ni and 12.25% Mo; surfacearea, 286 m2/g), and ground to the size of2200 mesh priorto use.

PMA, NiPM, CoPM and KPM were impregnated ontocoal and Al2O3 by adding aqueous solutions that containedthe appropriate concentrations of the individual metal saltsto provide a final loading of 15, 150 or 1500 mg of Mo perkg feed. During addition, powdered coal or Al2O3 support(2200 mesh) was continually stirred to assure even disper-sion. The potassium salt was soluble only after adding a few

B. Demirel, E.N. Givens / Fuel 79 (2000) 1975–19801976

Table 1Analysis of Wyodak black thunder coal

Proximateanalysis

Wt% Ultimateanalysis

Wt%(dry)

Sulfuranalysis

Wt%

Moisture 8.89 Carbon 70.62 Total 1.94Ash 5.76 Hydrogen 5.03 Pyritic 0.80Volatile matter 39.88 Nitrogen 1.13 Sulfate 0.80Fixed carbon 45.47 Sulfur 0.52 Organic 0.34

Oxygen (diff) 16.38Ash 6.32Ash, SO3-free 5.47

drops of KOH to the water. Metal impregnated coals weredried in a vacuum oven at 968C and 33 kPa overnight toremove essentially all of the moisture. Metal supported onAl2O3 was calcined at 2508C for 3 h prior to use. The calci-nation temperature was selected low since phosphomolyb-dates have tendency to decompose at higher temperatures.

Activity tests were carried out in a 50-cm3 micro auto-clave at 4408C and 1350 psig (cold) for 30 and 90 min. Thereactor was equipped with a thermocouple, and connected toa pressure transducer for monitoring temperature and pres-sure during the reaction. Experiments were repeated at leastonce to confirm reproducibility. In a typical experiment,2.5 g of heavy distillate, 4.0 g of deashed resid or 3.9 g ofMayan resid and 0.35 g of metal impregnated coal or Al2O3

were added to the reactor, pressurized with H2S/H2 (3 wt%H2S in H2), submerged in a fluidized sand bath and agitatedcontinuously at the rate of 400 cycles per minute at thespecified temperature. At the end of the reaction, the reactorwas removed from the sand bath and quenched to ambienttemperature. The solid and liquid products were removedfrom the reactor using tetrahydrofuran (THF) and themixture was extracted in a Soxhlet extractor for 18 h. TheTHF-insoluble fraction was dried in a vacuum oven at 808Cand 17 kPa and weighed. The soluble fraction was distilledunder vacuum (modified ASTM D-1160-87) to an atmo-spheric equivalent end point of 5248C. Cut points wereadjusted based upon a GC simulated distillation procedurethat had been developed at the CAER. The product distribu-tions are calculated by subtracting the weights of residue,distillate and insoluble organic matter (IOM) in the productfrom the weight of each in the feed. Resid conversion was

calculated as follow:

Resid Conv� 1 25248C1Residout

5248C1 ResidFeed�madf�

!× 100

where madf is moisture, ash and distillate free. The amountof resid in the original coal in feed was also included to thecalculation although it was very low.

3. Results and discussion

PMA is a unique Mo precursor since it is relatively stableeven in the presence of hydrogen to temperatures in excessof 3008C. The stability of PMA has been associated with theKeggin structure in which the 12 Mo atoms surround thecentral phosphorus atom. The catalytic activity of PMA in anumber of oxidation reactions has been associated with theKeggin structure. Although H2S causes limited sulfidation atsomewhat lower temperatures, the limited introduction ofsulfur in the structure suggests that some ionic features maystill be intact at relatively high temperatures, thoughcertainly not at liquefaction temperatures. Yong et al. recog-nized that, in some cases, associated cations impart addi-tional thermal stability on the Keggin structure [38]. For thisreason, the thermal and catalytic activity of three metallicsalts of PMA were investigated, namely the Co, Ni and K,and compared with the activity of PMA.

The thermal stability of the three salts in He, H2 and H2S/H2 were determined by thermogravimetric analysis [39].Dehydration of these materials proceeds quite differentlyfor these salts. The K salt appears to dehydrate most rapidly,

B. Demirel, E.N. Givens / Fuel 79 (2000) 1975–1980 1977

Table 2Conversion of coal resids and product distribution using phosphomolybdates supported on coal

Catalystprecursor

Reaction time,min

Mo concentration,mg Mo/kg feed

% Residconversion

% Product distribution

Resid Distillate IOM Gasa

None 30 – 11.4 50.0 40.9 2.9 6.290 – 15.9 47.4 34.3 4.7 13.6

PMA 30 15 11.9 50.2 43.7 2.1 4.0150 14.3 48.2 47.2 2.5 2.1

90 15 27.0 41.2 43.3 1.6 13.9150 31.9 38.4 45.8 1.5 14.3

CoPM 30 15 12.0 49.6 40.2 5.1 5.1150 12.8 49.2 45.9 2.1 2.8

90 15 24.5 43.3 39.5 3.5 13.7150 31.4 38.7 39.5 4.1 17.7

NiPM 30 15 13.6 48.5 40.2 2.2 9.1150 14.1 48.5 45.0 1.3 5.2

90 15 27.4 40.9 44.4 2.8 11.9150 32.2 38.2 48.2 2.0 11.6

KPM 30 15 12.1 49.6 44.7 1.0 4.7150 13.9 48.3 44.6 1.9 5.2

90 15 23.9 42.9 41.3 2.4 13.4150 29.6 39.7 46.3 2.0 12.0

a Calculated from difference to 100%.

even more rapidly than PMA. Dehydration of the Ni and Cosalts occurs over a much wider temperature range of 330–4008C, which is lower than for PMA. This has been ascribedto the collapse of the Keggin structure. Such a spike is notobserved in the K structure until 5008C suggesting that theKeggin ion may be stable at this temperature. In H2, theinitial loss of water from KPM occurs over a more narrowtemperature range than for PMA. The material formed at1508C is quite stable. By contrast, the Ni salt and, to a lesserextent, the Co salt appear to continually lose weight over thewhole region suggesting that dehydration is quicklyfollowed by further changes in the ionic structure. Afterdehydration, the K salt is unusually stable at temperaturesbeyond 4508C suggesting that the Mo may have remained inthe 1 6 oxidation state. When H2S is added along with H2,the PMA and Ni salt show a major loss in weight at approxi-mately 3008C, probably indicating breakdown of the Kegginion. The K salt showed initially a weight gain suggestingincorporation of sulfur into the structure. However, at3508C, a very significant loss in weight occurs in an exother-mic reaction. This suggests formation of a significant amountof water, destruction of the Keggin structure and reductionof Mo, probably to the1 4 state. Clearly, the H2S leads to asignificant change in the stability of the K salt.

Activities of PMA and the Co, Ni and K salts of PMAwere tested at the 15 and 150 mg Mo per kg feed for the 30and 90 min reactions. Table 2 compares the activities of thephosphomolybdates when impregnated on Wyodak coal.Resid conversion in the absence of a catalyst was 11.4%after 30 min, and reached only 15.9% for the 90 min

reaction time. The addition of phosphomolybdates at thelevel of 15 or 150 mg Mo per kg feed for the 30 min reac-tions gave about the same resid conversions as the non-catalyzed case, however, significant improvements in residconversions (24–27%) were observed for the 90 min reac-tions. Increasing the metal concentration to 150 mg exhib-ited a sizable increase in conversion. Both PMA and NiPMgave about 32% conversion of resid to distillate followed bythe CoPM. The KPM is nearly as active as PMA, CoPM andNiPM. It is likely that conversions up to 30 min are mainlydue to thermal reactions with catalytic reactions becomingmore dominant at higher reaction times (up to 90-min).

Significant improvement in resid conversion wasachieved when those phosphomolybdates were supportedon Al2O3 (Table 3). For the 30 min reactions, resid conver-sions at the 15 mg Mo concentration reached a value whichis about the same as the 90 min thermal reaction. At the150 mg Mo concentration, the conversion with PMA andCoPM was about the same, KPM was less effective, andNiPM was more active. For the 90-min reactions at the15 mg Mo level, PMA gave a significant increase in conver-sion (31.3%) followed by the KPM (25%), and CoPM andNiPM exhibited the highest conversions (38%) indicatingthat Co and Ni have promotional effects on hydrogenation.Increasing Mo concentration to 150 mg in the period of90-min reactions did not significantly improve residconversion.

Supporting precursors on coal or Al2O3 gave about thesame conversions for the 30-min reactions, however, signif-icantly higher conversions were observed with those on

B. Demirel, E.N. Givens / Fuel 79 (2000) 1975–19801978

Table 3Conversion of coal resids and product distribution using phosphomolybdates supported on Al2O3

Catalystprecursor

Reaction time,min

Mo concentration,mg Mo/kg feed

% Residconversion

% Product distribution

Resid Distillate IOM Gasa

None 30 – 11.4 50.0 40.9 2.9 6.290 – 15.9 47.4 34.3 4.7 13.6

PMA 30 15 15.2 47.8 44.5 1.6 6.1150 18.7 45.8 45.7 2.0 6.5

90 15 31.3 38.7 44.4 2.2 14.7150 33.4 37.6 49.1 2.1 11.2

CoPM 30 15 15.4 47.7 45.5 2.0 4.8150 18.1 46.1 46.0 1.4 6.5

90 15 36.8 35.6 50.3 2.8 11.3150 39.2 34.3 49.8 1.3 14.6

NiPM 30 15 19.4 45.4 46.8 2.4 5.4150 22.3 43.8 46.3 1.8 8.1

90 15 37.6 34.2 49.6 1.5 14.7150 39.4 34.8 48.9 1.1 15.2

KPM 30 15 15.4 47.6 41.9 1.4 9.1150 15.7 47.5 44.7 1.5 6.3

90 15 25.0 42.2 45.4 1.9 10.5150 32.3 38.2 46.6 1.6 13.6

a Calculated from difference to 100%.

Al2O3 for the 90 min reactions. There is no doubt that Al2O3

has much higher active surface area than coal that increaseshydrogenation activity. The influence of phosphorus onhydrotreatment processes has been discussed for a numberof catalysts, and an increase of the phosphorus content inNiMo catalyst was reported to have a significantly positiveeffect on hydrogenation and hydrodenitrogenation activity[40–42]. If that is the case, phosphorus in the Keggin struc-ture provides higher activities for conversion of resids todistillate fractions. It is also reported that phosphoric aciddevelops extensive porosity in carbons, which in turn willincrease surface area [43]. Although the effect of PMA andits salts on the surface area of coal is not known, there is nodoubt that they are well dispersed on coal as well as onAl2O3. The lower activity of KPM on Al2O3 relative to theother phosphomolybdates differs from its activity whenimpregnated on coal. This may be a result of an ionexchange of K with the functional groups in coal.

Products from the conversion of coal resids were sepa-rated into resid, distillate and IOM fractions, and gas frac-tions are determined from difference to 100% (Tables 1 and2). Distributions of these fractions are about the same at thesame reaction conditions using impregnated coals. Nosignificant changes were observed in IOM fractions at thevarious reaction conditions. For 30-min reactions, 40–45%

distillate was observed whereas resid fractions were about48–50% at concentrations of 15 and 150 mg Mo/kg feed aswell as in the non-catalyzed case. For 90-min reactions,resid fractions decreased from 38 to 42% and the distillateincreased slightly. In all these experiments, increased reac-tion time results in increased gas production. It appearsthermal effects are generally responsible for the conversionof resid to gas products whereas catalytic effects promoteproduction of distillates.

Activity test results show that the nickel phosphomolyb-date supported on Al2O3 is active for converting coal resid todistillate fractions. NiMo/Al2O3 from Akzo Chemicals wastested to compare conversions of coal and Mayan resids atthe level of 1500 mg Mo per kg feed for 90 min (Table 4).Conversion of coal-derived resid reached to 39.6% withNiPM on Al2O3 whereas AKZOB60 (NiMo/Al2O3) gave32.1% conversion. This is 5% lower than that observedwith NiPM on Al2O3 at the 15 mg Mo load per kg feed.With Mayan resid, higher conversions were also obtainedwith NiPM on alumina and AKZO-60 which are 62.4% and54.9%, respectively. These results show that Mo concentra-tion on Al2O3 does not have a very significant effect onconversion (Fig. 1). Addition of NiPM at the level of 15,150 or 1500 mg Mo gave about the same conversions vary-ing from 37.6% to 39.6%. It is likely that initial activities ofNiPM are as good at 15 mg Mo/kg feed as AKZO-60 is at1500 mg Mo/kg feed.

4. Conclusion

Thermal stability tests of PMA and bimetallic phospho-molybdates showed that these materials are mostly stable upto 400–4508C in the presence of He or H2. The H2S leads toa significant change in the stability, leading to transforma-tion of these materials into active catalyst. These phospho-molybdates are water soluble and easily impregnated oncoal or Al2O3 resulting in the formation of highly dispersedcatalysts in the reaction media. Catalytic activity tests ofPMA and Co, Ni and K phosphomolybdates showed thatthey have significant activity for conversion of resid. Coalsimpregnated with each of these precursors at the level of15 mg Mo/kg feed gave resid conversions that were aboutthe same as for untreated coal for 30-min reactions, and

B. Demirel, E.N. Givens / Fuel 79 (2000) 1975–1980 1979

Table 4Conversion of coal and petroleum resids using NiPM and AKZO-60 at the 1500 mg Mo/kg feed and 90-min reaction time

Resid Catalyst precursor % Resid conversion % Product distribution

Resid Distillate IOM Gasa

Coal resid NiPM on alumina 39.6 34.1 45.7 1.2 19.0Coal resid AKZO-60 32.1 38.2 48.2 1.5 12.1

Mayan resid NiPM on alumina 62.4 22.0 56.6 1.6 19.8Mayan resid AKZO-60 54.9 26.4 50.1 0.8 22.7

a Calculated from difference to 100%.

Fig. 1. Comparison of conversion of coal and petroleum resids using NiPMon Al2O3 and AKZO60 at the reaction time of 90 min.

considerable improvements were observed at loadings of150 mg Mo. Increasing reaction time significantly increasedresid conversions with PMA, CoPMA and NiPMA. WhenAl 2O3 supported nickel phosphomolybdate was used, signif-icant increases in resid conversions were obtained at load-ings of 15 to 1500 mg Mo/kg feed for both 30 and 90 min ofreaction times compared to the non-catalyzed cases. NiPMand CoPM gave the highest activity consistent with higherhydrogenation reactivity. Coal does not possess the activat-ing effect that Al2O3 imparts to Mo catalyst, the latter maybe the result of an increase in surface area. The data indi-cates that catalytic effects are insignificant for 30-min reac-tions that are driven primarily by thermal reactions althoughcatalyst has a very strong impact in the 90-min reactions.Comparing the conversions of coal and petroleum resids atloadings of 1500 mg Mo/kg feed for the 90 min reactiontime, NiPM on Al2O3 gave higher conversions of Mayanresid than AKZO-60.

Acknowledgements

The authors gratefully acknowledge financial supportprovided by the U.S. Department of Energy Federal EnergyTechnology Center under contract number DE-AC22-91PC91040.

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B. Demirel, E.N. Givens / Fuel 79 (2000) 1975–19801980