Regeneration of a Deactivated Hydrotreating Catalyst

V. L. S. Teixeira da Silva,†,§ F. P. Lima,† L. C. Dieguez,† and M. Schmal*,†,‡

NUCAT/PEQ/COPPE, Universidade Federal do Rio de Janeiro, Caixa Postal 68502, CEP 21945-970 Rio deJaneiro, Brazil, and Departamento de Engenharia Quımica, Escola de Quımica, Universidade Federal do Riode Janeiro, CEP 21949-900 Rio de Janeiro, Brazil

A commercial NiMo/Al2O3 catalyst deactivated in a shale oil hydrogenation plant was submittedto several regeneration procedures. The results show that direct coke burnoff with an oxidizingmixture containing low oxygen levels (1.6% v/v O2) was more efficient in recovering the catalyticactivity and the textural properties of the spent catalyst than reductive treatments (5% v/v H2)or sequential treatments of solvent extraction with acetone or cyclohexane/regeneration. Diffusereflectance spectroscopy results suggest that, after the regeneration step, there was some nickelpromoter loss due to nickel spinel (NiAl2O4) formation. The spinel formation explains why thecatalytic activity of the fresh catalyst was not fully recovered after the regeneration of the spentcatalyst, and its formation might be attributed to an unobserved temperature increase in thecatalytic bed during the carbon burnoff step.

Introduction

During Irati shale oil hydrotreating (HDT), com-mercial NiMo/Al2O3 catalyst deactivates, mainly due tocoke and shale oil deposition over the sulfided activephases. Since environmental control laws have becomemore and more severe from year to year, it has becomeincreasingly important to regenerate the deactivatedHDT catalysts instead of discarding them into the openatmosphere.Several works presented in the literature (Arteaga et

al., 1986, 1987; George et al., 1988; Delmon, 1992) haveshown that it is possible to partially recover the catalyticactivity when deactivated HDT catalysts are submittedto either “soft” oxidizing (Arteaga et al., 1986, 1987;Teixeira da Silva, 1992) or reductive (George et al.,1988) treatments in order to remove the coke formedduring deactivation. However, when part of the treatedoil is absorbed in the surface of the deactivated cata-lysts, it is necessary to remove it prior to the regenera-tion procedure. This oil removal can be done byemploying extraction methods which use appropriatesolvents (Babcock et al., 1989; Maraf and Stanislaus,1989; Teixeira da Silva et al., 1994) or acidic solutions(George et al., 1988).The main objective of this work was to study several

procedures for regeneration of a commercial NiMo/Al2O3catalyst deactivated during the hydrotreating of Iratishale oil. A recent study (Teixeira da Silva et al., 1994)has shown that the regeneration of the deactivatedcatalyst by means of direct coke burnoff at 673 K usinga 5% (v/v) O2/N2 mixture as an oxidizing agent orthrough a sequential treatment involving extraction ofthe absorbed shale oil by cyclohexane, followed byoxidation at 673 K, did not lead to a significant recover-ing of the catalytic activity. The fact that the regener-ated catalyst did not recover the activity levels of the

fresh catalyst was explained by the following hypoth-eses: (i) The oxygen concentration of the oxidizingmixture, i.e., 5% (v/v), was high enough to promotea temperature increase in the catalytic bed due to theexothermicity of the coke burnoff reaction, causingsintering of the oxidic precursors of the sulfided phases;(ii) cyclohexane does not efficiently remove the absorbedshale oil, which helps sintering during the regeneration;and (iii) compounds which are present in the shale oilor that are formed during HDT lead to irreversiblepoisoning of the catalyst.In order to answer these questions, the deactivated

catalyst was subjected to several regeneration proce-dures which consisted either of a previous extraction ofthe shale oil using acetone, cyclohexane, or a solutionof oxalic acid before the coke burnoff, or of a direct cokeburnoff using oxidizing or reducing agents. In order tounderstand the transformations occurring during eachof the stages of the regeneration, the fresh sulfidedcatalyst was also subjected to the same extraction/regeneration treatments as those used for the spentcatalyst.

Experimental Section

Catalyst. The fresh catalyst was a commercial NiMo/Al2O3 (Shell S-324), supplied in extrudate form (3 mm× 1 mm), with a nominal composition of 12.0 wt % Moand 2.16 wt % Ni. Before use, the extrudates werecrushed and sieved to 100 mesh Tyler.The spent catalyst originated from the hydrotreating

of shale oil in a fluidized trickle bed reactor operatedat 573 K, 15 MPa, and a reaction time longer than 72 h(Souza et al., 1992). After reaction in the trickle bedreactor, the deactivated catalyst had carbon and sulfurcontents of 15.70 and 6.33 wt %, respectively. Of thetotal carbon content, approximately 6 wt % was due toabsorbed shale oil (Teixeira da Silva et al., 1994).Activation and Catalytic Testing. The regenera-

tion, activation, and catalytic testing experiments werecarried out in a U-shaped Pyrex microreactor with aPyrex frit for holding the catalyst.Activation of HDT catalysts consists of transforming

metallic oxides into corresponding sulfided phases by

* To whom correspondence should be addressed.† NUCAT/PEQ/COPPE.‡ Escola de Quımica.§ Present address: Instituto Militar de Engenharia, SE/5,

Praca General Tiburcio, 80, Praia Vermelha, CEP 22290-270Rio de Janeiro, Brazil.

882 Ind. Eng. Chem. Res. 1998, 37, 882-886

S0888-5885(97)00498-3 CCC: $15.00 © 1998 American Chemical SocietyPublished on Web 01/24/1998

using a gas- or liquid-phase sulfiding agent. Suchtransformations lead to the formation of MoS2, Ni3S2,and a so-called NiMoS phase, which has been suggestedas the possible active phase in HDT reactions (Topsøeet al., 1981).In this study, 0.2 g of oxide was sulfided by flowing a

5% (v/v) H2S/H2 mixture (Oxigenio do Brasil, 99.99%)through the reactor. A local thermocouple monitoredthe temperature of the sample, and a furnace coupledto a controller/programmer (GEMD) heated the reactorfrom 298 to 693 K at a heating rate of 4 K min-1. Once673 K was reached, the temperature was held constantfor 2 h in the flow of the sulfiding gas mixture. Afterthis period, the gas was switched from the 5% (v/v) H2S/H2 mixture to pure H2 (1.2 L h-1; White Martins UP,99.99%), while the temperature was lowered to 553 K.Immediately after the sulfiding step, the reaction was

started by passing a 10:1 molar ratio H2/C4H4S reactantmixture through the reactor. This ratio was obtainedby flowing pure H2 (1.2 L h-1) through two saturatorsconnected in series, maintained at room temperatureby a water bath, and filled with thiophene (Merck, 99%).The reaction conditions were 553 K and 0.1 MPa, withthe H2/C4H4S mixture flow rate adjusted to maintainlow conversions of thiophene (<5%). No diffusionaleffects were observed under the reaction conditionsemployed. Products were analyzed by on-line gas FIDchromatography, and a long-life test was performedwith the fresh sulfided catalyst (72 h), but no deactiva-tion was observed.Although the real industrial process of shale oil

hydrogenation is performed at high pressures, thiophenehydrodesulfurization at atmospheric pressure was cho-sen as reaction test because it gives an idea of theefficiency of the different regeneration procedures.Shale Oil Extraction. Shale oil removal was done

by extraction in a Soxhlet system employing acetone,cyclohexane, and a 1 M oxalic acid solution as extractingagents. In order to understand the influence of thedifferent extracting agents on the properties of the spentcatalyst, the different extractions were also applied tothe fresh sulfided catalyst in a process that, from nowon, will be referred to as “blank extraction”.Regeneration. The regeneration step consisted of

treating the spent catalysts, submitted or not to extrac-tion, with an oxidizing [1.6% (v/v) O2/N2; Oxigenio doBrasil] or reducing [5% (v/v) H2/N2; Oxigenio do Brasil]mixture. The use of an oxidizing mixture allows carbonremoval through the reaction C + O2 f CO2, and theuse of a reducing mixture eliminates it as methane, i.e.,C + 2H2 f CH4.In general, the regeneration procedure involved flow-

ing either the oxidizing or the reducing mixture at aflow rate of 2.4 L h-1 through the reactor containing3.0 g of the catalyst, while the temperature was raised

from room temperature to 673 K at a heating rate of 4K min-1 and held at the final temperature for 24.0 h.Several samples of the fresh sulfided catalyst were

submitted to the regeneration procedure describedpreviously in order to evaluate the influence of carbonon the properties of the regenerated catalysts. Theseexperiments will be referred to, from now on, as “blankregeneration”.Characterization. BET surface areas (Sg) were

measured by means of a commercial BET unit (CG2000) using N2 adsorption at 77 K. Before N2 adsorp-tion, samples were heated at 473 K for 2 h in a flow ofpure N2 (Oxigenio do Brasil, 99.99%).Coke and sulfur contents of the samples submitted

to extraction, blank extraction, regeneration, and blankregeneration were determined by IR absorption in aLECO system (Model CS-224).Diffuse reflectance spectroscopy (DRS) was done using

a Cary 2300 spectrometer with a praying mantisgeometry. The spectral range of 280-980 nm was used,employing a CCI-330 commercial alumina as reference.Before analysis, the samples were crushed and sievedusing the fraction smaller than 325 mesh and then driedin an oven at 523 K for 1 h.Ni and Mo contents of all samples were determined

by atomic absorption in a Perkin Elmer atomic absorp-tion spectrometer (Model 1100B).

Results and Discussion

Table 1 lists and codifies the samples submitted tosolvent extraction. It also presents results of thecatalytic activity, Sg, and Ni, Mo, C, and S contents.The results show that the blank extraction experi-

ments with acetone and cyclohexane did not lead tomodification of the metallic contents, surface area, andactivity values of the fresh catalyst. On the other hand,although the blank extraction with oxalic acid did notchange the Sg of the catalyst, the catalytic activityunderwent a 30% decrease compared to that of the freshcatalyst. This drop in catalytic activity can be explainedby taking into account the fact that the extraction withoxalic acid promoted an extensive removal of both nickel(from 2.4 to 1.1 wt %) and molybdenum (from 11.5 to7.6 wt %). During the oxalic acid extractions of the C0and C1 catalysts, the extracting solution presented agreenish coloration, indicating that some nickel was, infact, removed from the catalyst. The appearance of thegreenish coloration in the extracting solution couldsuggest that, because the oxalic acid has a very lowvolatility, the recycled solvent in the Soxhlet equipmentwould be essentially distilled water, and thus the metalextraction could not take place. However, a blankexperiment with a pH indicator paper inside the Soxhletapparatus showed that the solution was acidic. In thisway, the metals removal during extraction can be

Table 1. Catalytic Activity and Physical and Textural Properties of the Samples Submitted to Solvent Extraction

amount (wt %)

code description C S Ni MoSg

(m2 g-1)

activity(g mol of thiopheneg-1 of catalyst h-1)

C0 fresh catalyst 0.16 2.4 11.5 118 0.108C0-A fresh catalyst submitted to “blank” extraction with acetone 0.22 2.4 11.3 120 0.100C0-C fresh catalyst submitted to “blank” extraction with cyclohexane 0.22 2.4 10.9 119 0.103C0-O fresh catalyst submitted to “blank” extraction with oxalic acid 0.22 1.1 7.6 119 0.0706C1 spent catalyst 15.70 6.33 2.3 11.6 36 0.011C1-A spent catalyst submitted to extraction with acetone 9.85 7.19 2.4 10.6 76 0.019C1-C spent catalyst submitted to extraction with cyclohexane 11.0 6.50 2.4 11.7 93 0.016C1-O spent catalyst submitted to extraction with oxalic acid 16.0 5.70 1.4 12.0 36 0.009

Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998 883

explained; consequently, with lower metal contents, thecatalyst (C0-O) had, after the sulfiding step, a lowernumber of active sites than the fresh catalyst and, thus,a lower activity.The presence of the shale oil on the surface of the

spent catalyst (C1) caused a drastic decrease in bothSg and catalytic activity. Table 1 also shows that, whileacetone (C1-A) and cyclohexane (C1-C) extraction re-moved part of the absorbed oil, the same did not occurwith oxalic acid. However, it is important to point outthat extraction of the spent catalyst with oxalic acidremoved high amounts of nickel but not molybdenum.Since 6 wt % of the total carbon amount in the spent

catalyst is due to the impregnated shale oil (Teixeirada Silva et al., 1994), analysis of Table 1 shows thatextraction with acetone is more efficient than that withcyclohexane. In fact, acetone removed all of the im-pregnated shale oil. However, the nonsoluble coke, i.e.,the carbonaceous sediments associated with compoundsother than shale oil, are still present in concentrationshigh enough to promote blockage of meso- and macro-pores (Teixeira da Silva et al., 1994) as well as inhibitionof the catalytic activity (-rA0 ) 0.019 g‚mol of thiopheneg-1 of catalyst h-1).Because the extraction with acetone led to a total

removal of the absorbed shale oil in the spent catalyst,only theC1-A sample was submitted to the regenerationprocedure, which was intended to eliminate the residualnonsoluble coke. This regeneration was carried out byeither reductive or oxidizing treatments, as shown inTable 2.Tables 1 and 2 reveal that, after treatment at 673 K

with the reductive mixture (C1-A-5H2), the carbonamount was the same as that in the starting material(C1-A). This result indicates that the treatment at 673K with the 5% (v/v) H2/N2 mixture did not promote anynonsoluble carbon removal, as methane and, therefore,the activity of the C1-A-5H2 catalyst did not presentany improvement relative to the spent catalyst C1. Thefact that the carbonaceous deposits associated with thenonsoluble coke were not eliminated might be associatedwith the low treatment temperature, the low hydrogenconcentration in the mixture, or the nature of thenonsoluble coke.Studying the regeneration of a NiMo/Al2O3 spent

catalyst from a gasoil hydroprocessing plant, George etal. (1988) observed that the use of pure H2 as treatmentagent at 773 K did not promote coke removal at all.Since the methanation reaction C + 2H2 f CH4 has anegative free energy for temperatures equal to or higherthan 673 K, the authors concluded that the absence ofcoke removal was associated with its nature and notwith the employed regeneration conditions. Thus, it canbe concluded that the non-removal of coke during thereductive treatment of catalyst C1-A is related to itsnature and not to the regeneration conditions employed.This hypothesis was confirmed because, after reducing

the spent catalyst C1 at 673 K (C1-5H2), 24 wt % of thetotal carbon was removed.On the other hand, Table 2 shows that the regenera-

tion at 673 K with the oxidizing mixture removed allcarbon and sulfur present on catalysts C1 and C1-A.After removal of the carbonaceous deposits, the C1-A-1.6O2 and C1-1.6O2 catalysts exhibited higher activitythan the spent catalyst C1. However, the activity wasnot as high as that of the fresh catalyst. The activityof the catalyst C1-1.6O2 represents a recovery of 77%of the original activity. It is noteworthy that, in anearlier study, the same spent catalyst C1 attained only58% recovery of the activity after regeneration with a5% (v/v) O2/N2 mixture (Teixeira da Silva et al., 1994).This result suggests that the oxygen concentration ofthe oxidizing mixture plays a key role in the regenera-tion procedure.Table 2 also shows that the activity of the catalyst

C1-A-1.6O2 is higher than that of the spent catalyst butis smaller than that of the catalyst C1-1.6O2. Thisdifference might be explained, a priori, by a variationof the number of active sites, by a modification of thenature of the active sites, or by a modification of theiraccessibility. Since the textural properties of thesecatalysts have been fully recovered after the regenera-tion step, the hypothesis of inaccessibility can bediscarded.The product selectivities were identified by gas chro-

matography (butene, 1-butene, and cis- and trans-butene) for catalysts C0,C1-1.6O2, andC1-A-1.6O2 andare presented in Figure 1. The figure shows that,whatever the regeneration procedure used, the productselectivity did not change. Thus, it can be concludedthat, for these catalysts, the nature of the active sitesis essentially the same.In this way, the different activities of catalysts C1,

C1-1.6O2, and C1-A-1.6O2 are associated with the

Table 2. Catalytic Activity and Chemical Properties of the Samples Submitted to Regeneration

amount (wt %)

code description C S Ni Mo

activity(g mol of thiopheneg-1 of catalyst h-1)

C1 spent catalyst 15.70 6.33 2.3 11.6 0.011C1-5H2 spent catalyst regenerated with the 5% H2/N2 mixture 12.0 6.70 2.5 11.7 0.016C1-A-5H2 spent catalyst submitted to extraction with acetone and regenerated

with the 5% H2/N2 mixture9.9 7.20 2.5 10.9 0.014

C1-1.6O2 spent catalyst regenerated with the 1.6% O2/N2 mixture 2.6 11.8 0.083C1-A-1.6O2 spent catalyst submitted to extraction with acetone and regenerated

with the 1.6% O2/N2 mixture2.1 11.9 0.065

Figure 1. Product distribution selectivity.

884 Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998

number of active sites. A hypothesis that can be raisedis that the extraction with acetone in some way causedmodifications that would lead to a small formation ofactive sites after sulfiding. Therefore, this could explainthe lower activity of catalyst C1-A-1.6O2 relative to thatof the catalyst C1-1.6O2. In order to confirm thishypothesis, theC0,C1-1.6O2, andC1-A-1.6O2 catalystswere analyzed by DRS, and the results are presentedin Figure 2.The DRS spectra of the catalysts have different

shapes, confirming that the oxidic phases in the regen-erated catalysts are not the same. The DRS spectrumof the catalyst C0 (Figure 2A) consists of a broadabsorption band with a maximum located in the wave-length region between 710 and 720 nm, which ischaracteristic of NiO (Lo Jacomo et al., 1971). Forcatalyst C1-A-1.6O2, the DRS spectrum shows a distor-tion in the maximum at 710 nm associated with NiOand a small shoulder situated at 630 nm which corre-sponds to the presence of a nickel spinel phase, i.e.,NiAl2O4 (Lo Jacomo et al., 1971; Bouyssieres et al.,1984). Finally, the spectrum of the catalyst C1-1.6O2shows a maximum at 710 nm (NiO), a shoulder at 630nm (NiAl2O4), and another absorption maximum around800 nm which corresponds to a phase of the typeNiMoO4 (Teixeira da Silva et al., 1994).It is well known in the literature (Arteaga et al., 1986,

1987; Delmon, 1992) that NiAl2O4 formation occurs dueto nickel migration to the lattice of the support andrepresents a promoter loss because the nickel aluminateis not sulfided at low temperatures and is not active forHDS reactions. Thus, the presence of NiAl2O4 incatalysts C1-A-1.6O2 and C1-1.6O2 explains the factthat both of them had lower activities than the freshcatalyst: the loss of nickel and the resulting aluminateformation causes a decrease in the NiMoS phase con-centration for the regenerated catalysts. The smallerNiMoS concentration results in a smaller number ofactive sites. The fact that both regenerated catalystshave shown the presence of the characteristic band ofNiAl2O4 in the DRS spectra indicates that increases inthe temperatures of the catalytic bed due to the exo-

thermicity of the coke burnoff occurred during theregeneration. The temperature increase was not ob-served during the regeneration but is supported by thefact that the NiAl2O4 phase can only be formed attemperatures above 773 K. Since the DRS spectrum ofthe fresh catalyst does not exhibit the feature at 630nm which is attributed to NiAl2O4, this phase wasformed either during the shale oil hydrogenation orduring the regeneration procedure. The presence ofNiAl2O4 in both regenerated catalysts explains the loweractivity compared to that of the fresh catalyst. More-over, the DRS spectra of the regenerated catalysts alsoexplain the different activities presented by them. Bothregenerated catalysts present NiO and NiAl2O4 as acommon feature. On the other hand, the DRS spectrumof the catalyst C1-1.6O2 reveals that, after oxidationat 673 K, there is, besides the nickel spinel formation,the appearance of a mixed oxide of nickel and molyb-denum, NiMoO4.Assuming that the regenerated catalysts present an

equal loss of nickel due to the NiAl2O4 formation, thepresence of NiMoO4 in the catalyst C1-1.6O2 wouldexplain its higher catalytic activity if, after sulfiding,the mixed oxide is transformed directly into the NiMoSphase (Teixeira da Silva et al., 1994). Thus, the higheractivity of the catalyst C1-1.6O2 compared to that ofC1-A-1.6O2 might be explained considering that theformer has, after sulfiding at 673 K, a higher numberof active sites than the latter.

Conclusions

The catalytic activity of a spent HDT catalyst con-taining 6 wt % of absorbed shale oil is partiallyrecovered after direct burnoff of the carbonaceousdeposits employing an oxidizing mixture with low levelsof oxygen. Although the extraction of the absorbedshale oil with acetone is more efficient when comparedto those using other solvents, the obtained catalystpresents a lower activity than that submitted directlyto regeneration.Not only the acetone-extracted catalyst but also the

one submitted to a direct coke burnoff have presentedlower activities than the fresh catalyst, basically dueto nickel migration into the support and formation of aspinel phase. The nickel spinel formation has probablyoccurred during the regeneration step, although atemperature increase in the catalytic bed was notobserved.

Acknowledgment

V.T.S. and F.P.L. are grateful to CNPq (ConselhoNacional de Desenvolvimento Cientıfico e Tecnologico,Brasil) for the scholarships received during this work.

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Arteaga, A.; Fierro, J. L. G.; Grange, P.; Delmon, B. CoMo HDSCatalysts: Simulated Deactivation and Regeneration. Role ofthe Various Regeneration Parameters. In Catalyst Deactivation;Delmon, B., Froment, G. F., Eds.; Elsevier: Amsterdam, 1987;pp 59-80.

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Figure 2. DRS spectra of C0 (A), C1-A-1.6O2 (B), and C1-1.6O2(C).

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Received for review July 24, 1997Revised manuscript received November 21, 1997

Accepted November 29, 1997

IE970498M

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