.r.w. Ward (Editor), Catalysis 19871988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

383

PORE STRUCTURE ENGINEERED CATAL YSTS FOR HYDROCRACKING HEAVYFEEDS

M.V.C. SEKHAREnergy Research Laboratories,CAN MET, Energy, Mines and Resources Canada,Ottawa, Canada KIA OGI.

ABSTRACTA technique has been developed for preparing large pore size alumina supported

catalysts starting from commercially available bohemite alumina. These catalysts have beenused to hydrocrack Athabasca oil sand bitumen in a bench-scale high pressure continuousreactor. Viscosity changes in the hydrocracked product and relative activities for asphalteneconversion and hydrodesulphurization (HDS) were found to correlate with the pore volumecontained in the 3-200 nm diameter range. Catalysts having pores between 7 and 20 nmdiameter showed higher activities than those catalysts having pores between 3 and 7 nm.

INTRODUCTIONThe processing of heavy feeds such as those derived from oil sands bitumen and heavy

oil poses a formidable challenge to catalyst development engineers. Apart from their highviscosity and density processing is made more difficult owing to the high concentrations ofheteroatoms, metal contaminants and asphaltenes. Conventional hydrocracking catalystsexhibit severe deactivation when used to process these low grade feedstocks. Several studies(refs.I-4) have shown that deactivation is caused by coke deposition, metal deposition andsubsequent pore mouth plugging. The importance of pore size distributions and in particularthe presence of large pores in alleviating some of the problems associated with catalystdeactivation has been appreciated for a number of years (refs. 5-9). In this paper we addressthe preparation of catalysts having a range of pore sizes and their subsequent use in thehydrocracking of bitumen derived from the Athabasca oil sands in northern Alberta.

A number of studies (refs. 10-11) have been reported on the preparation of catalystshaving a desired or a variable pore structure. The formation of a desired pore structure canbe effected in many stages of the preparation of the catalysts. Many workers have addressedthe problem of pore structure engineering when the components are in the colloidal state (ref.12-14). Others (ref. 15-18) have tried to attack the problem during the extrusion andpelletizing steps. We have adopted a fairly simple technique which utilizes commerciallyavailable pseudo bohemite as the starting material thereby avoiding the operations ofprecipitation and washing.

EXPERIMENTALThe catalysts and the catalyst supports described in this work were derived from alpha

alumina monohydrate supplied by Conoco, N.J. The preparation involved the addition of aweighed amount of alumina to a specific amount of an aqueous solution of nitric acid of acertain molarity. The resulting solution was stirred mechanically for about 24 h and allowed

384

to stand at room temperature for another 24 h. Then the sample was transferred to a dryingoven and dried to a constant weight at 600C. The dried samples were then treated with 1%solution of methyl cellulose, kneaded into a paste and extruded into 3 mm cylinders. Thesamples either as extrudates or as aggregates were then calcined at 5000C for 16 h. The nextstep in the preparation procedure involved the impregnation of active metals, molybdenumand cobalt. The impregnations were carried out sequentially using aqueous solutions ofammonium hepta molybdate and cobalt nitrate. The concentrations of the metal salts werechosen to give a final catalyst composition of 15 m% MoOs and 3 m% CoO. The samplesfollowing each impregnation were dried at 600C and calcined at 5000C for 8 h.

The activities of the catalysts were evaluated using a fixed bed reactor and Athabascabitumen as the feedstock. The experimental details and the properties of the feedstock havebeen described previously (ref. 19). All hydrocracking experiments were performed in theupf'low mode at a 4000C, 13.9 MPa pressure and hydrogen flow at 0.89 mSjL of feed at STP(5000 cfjbbl). Presulphiding of the catalyst was effected with the bitumen feed itself in thepresence of hydrogen between 250 and 4000C. The pore size distributions of the catalystsand the supports were measured by mercury intrusion porosimetry. The contact anglebetween mercury and the solid was assumed to be 1300 for both penetration and retraction ofmercury. The surface areas reported here were calculated from the mercury intrusion testsassuming cylindrical pore geometry. Properties of the hydrocracked products were measuredto evaluate the performance of the catalysts. Sulphur analyses were performed by a LECOSL 32 analyzer and the asphaltene contents were determined by a modified ASTM proceduredeveloped in our laboratories. Viscosities were measured at 380C using ASTM D-445method.

RESULTSThe alumina used in this study is a spray dried high purity bohemite, which converts to

a high surface area gamma alumina on calcination. Addition of acid to this bohemite causesthe aggregates to be peptized into a gel structure. Figure I shows the effect of aluminacontent at three different acid concentrations on the pore structure of the resultant catalysts.For the purposes of this discussion, the IUPAC definition (ref. 20) of pore sizes is beingfollowed. Pores having diameters above 500 A or 50 nm are defined as macropores and thosewith diameters between 50 and 2 nm are termed mesopores. Since the smallest pore accessiblewith mercury intrusion at 515 MPa (60000 psi) is 3 nm, the mesopores reported here representpores having diameters between 50 and 3 nm. Micropores as defined by the IUPAC are notmeasured in this study. At low acid concentrations, the amount of alumina in the mixturedoes not affect the resulting porosity as observed by Tischer (ref. 18). However, as the acidcontent is increased, the effect of alumina content on the porosity becomes more pronounced.At moderate acid molarities mesoporosity increases with alumina content at the expense ofmacroporosity. At the highest acid concentration, mesoporosity almost completely disappearsand macro porosity only remains. In general, lower alumina contents show a greater effectwith changing acid concentrations. This phenomenon is more clearly evident from Figure 2where the cumulative pore volume is plotted as a function of pore diameter for threedifferent acid molarities. Also shown in Figure 2 is the pore size distribution curve for the

case when the sample was treated with water containing no added acid. The pore structurechanges from exclusively unimodal in the mesopore range to bimodal and finally to unimodalin the rnacropore range as the acid concentration is increased from very low to very high.

:J85

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o 20X MACROPORES

40 80o MESOPORES

ALUMINA CONTENT (m%)

Fig. I. Effect of alumina content on porosity. Acid concentrations are expressed in molarities

The presence of acid, if only in small amounts, is essential for the formation ofgelatinous mixture and for peptization. In the absence of acid, the bohemite particles arevery hydrophobic and are not wetted by water. Alumina samples that were treated withwater and acid and dried were found to be unstable when left in contact with air at roomtemperature. They absorbed moisture and became sticky. However, the calcined sampleswere Quite stable and their pore size distributions did not change appreciably even afterseveral weeks of exposure to air.

One of the important features of the preparation procedure is the slow drying of thegel. Fulton (ref. 21) describes the phenomena involved in the drying of porous solids andidentifies four stages. In the first, only the surface liquid is evaporated. In the second,vapour is generated in the pores forcing the evaporating liquid out. If the drying rates areexcessive then the vapour will be generated within the pores faster than moisture can beforced out and the pores will be damaged. In the third, the free liquid in the poresevaporates and in the final stage any adsorbed water is removed. The effect of dryingtemperature on the distribution of macropores and mesopores is shown in Figure 3. In allcases the samples were calcined at 5000C following drying at the specified temperatures.With increasing drying temperatures, the macropore volume increases but simultaneously there

386

is a reduction in the mesopore volume. As the temperature increases, the drying rate can alsobe expected to increase and contribute to the collapse of the pore walls, thereby increasingthe pore sizes and pore volumes. The collapse of the pore structure is more pronounced atthe highest temperature of 1200C. The drying temperature of 600C adopted in this work wasa compromise between ideal drying for pore structure stability and reasonable drying time forpractical experimentation.

15.8 M

ALUMINA CONTENT

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PORE DIAMETER (nrn)

Fig. 2. Effect of acid concentration on porosity.

Other experiments were conducted to establish effects of particle size on the final porestructure. Pseudo bohemite aluminas of two different mean particle sizes were treated withacid and dried and calcined as before and these results are compared in Table I. The smallerparticle alumina shows greater increase in the macropore volume; however, the mesoporevolume is decreased considerably.

387

0.6 f- 5.6 M X

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TEMPERATURE DC

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DRYING

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Fig. 3. Effect of drying temperature on porosity

TABLE IEffect of particle size of precursor bohemite on porosity.

100-500 p.m Particles 30-100 Jlm Particles

a b c a b c

Gel condition dilute gel no gel dilute gel no gelPore volume, mL/g 0.40 0.46 0.30 0.38 0.71 0.58Macropore volume 0.02 0.28 0.22 0.05 0.63 0.55Mesopore volume 0.38 0.18 0.08 0.33 0.08 0.03Surface area, m2/g 202 83 47 233 39 15

• as received. calcined only.b added 3.54 M acid, dried and calcined; alumina content in mixture 4 m%.c calcined, then added 3.54 M acid, dried and calcined; alumina content in mixture 4 m%.

388

Table 2 gives the porosity data for the catalysts at the three stages in their preparation:support only, support with Mo and support with both Mo and Co. In all cases the supportswere impregnated with the metal salts to provide a final composition of 15 m% Mo03 and 3m% CoO. The addition of metals to the calcined support in general does not appreciablyaffect the pore distribution although there is some loss of the mesopore volume reflected as aloss in the surface area.

TABLE 2Effect of metals on pore volumes (ml.yg) and surface areas (m2/g).

Sampled Mesopore volume Macropore volume Surface area

a b c a b c a b c

13 0.20 0.18 0.16 0.04 0.06 0.16 175 135 12415 0.13 0.14 0.19 0.17 0.19 0.15 III 135 12416 0.20 0.16 0.14 0.09 0.18 0.16 158 III 10623 0.16 0.18 0.19 0.23 0.17 0.15 119 110 14931 0.21 0.20 0.17 0.09 0.11 0.10 163 127 11536 0.18 0.16 0.14 0.Q7 0.13 0.07 147 112 11037 0.20 0.19 0.18 0.05 0.06 0.05 165 120 11538 0.17 0.12 0.11 0.13 0.20 0.28 118 79 8839 0.28 0.23 0.24 0.07 0.11 0.06 199 127 139

a support only.b support with Mo.c support with Mo and Co.d numbers in this column refer to selected support specimens

The product properties obtained with selected catalysts (C,D,E) are given in Table 3.These catalysts were selected to represent variations in the pore structure as shown in Table 4.As is evident from Figure 4 there is a reasonable linear correlation between the conversionof both sulphur and asphaltenes and the effective pore space. The effective pore space isdefined as the product of pore volume contained in pores between 3 and 200 nm and the bulkdensity.

389

,p"

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PORE SPACE (3-200 nm Oia. Pores) mL/mL

Fig. 4. Effect of meso pore volume on catalyst activity

TABLE 3Hydrocracked product properties-

TOSb Catalyst Viscosity Percent sulphur(h) (cSt at 38oC) converted

61 C 411 5752 D 99 7852 E 122 81

97 C 369 5795 D 117 7593 E 95 80

a Reaction temperature 4000C, LHSV 0.5 and pressure 13.5 MPa.b Time on stream

Percent asphalteneconverted

577479

576781

390

TABLE 4Catalyst properties

Catalyst

CDE

Surfacearea (m2/g)

106129100

Volume of pores (mL/g) in diameter range (nm)

3500-50 50-3 50-20 20-13 13-7 7-3

0.16 0.14 0.0 0.01 0.04 0.100.14 0.18 0.01 0.01 0.04 0.120.01 0.29 0.01 0.14 0.12 0.02

DISCUSSIONAs mentioned already the addition of acid peptizes the aggregate structure of the

spray-dried bohemite powder into a gel-type structure. The reaction of the acid with thebohemite component of the batch will increase the pH of the system and the final pH willdepend on the extent of reaction between surface OH groups and the acid. These reactionswill in turn depend on time, the temperature and the extent and degree of mixing. Some ofthe potential reactions are listed below.

AIOOH + H+ - AI(OHh+

AI(OHh+ + H2O AI(OH)3 + H+

AI(OHh+ + H+ AI(OH)++ + H2O

AI(OH)++ + H+ A13+ + H2O

AI(OHh+ + N03- AI(OHhN03

A13+ + 3N03- AI(N03h

The variations in the final pore structure that are shown in Figures I and 2 can beattributed to two phenomena that occur when alumina is brought in contact with the acidsolution. One is the formation of hydrogen bonds between the water molecules and thesurfaces of the bohemite and the occlusion of water molecules between the bohemite layers(ref. 22). The other is the interaction of the surface OH groups and Al atoms with the anionsand the cations in nitric acid.

Johnson and Moi (ref. 23) have observed that the pore structure produced in gammaalumina obtained from bohemite comes from the gel-like pore structure present in theoriginal bohemite material. The macropores generally are thought to originate frominter-particle spaces between the aggregates in the mixture, whereas the mesopores originatefrom spaces within the particle, caused, for instance, by removal of water between crystalplanes.

At low acid concentrations, the peptization effects and hydrogen bonding reactionpredominate. As a result the resultant alumina supports possess only mesoporosity and thepore volume is independent of the alumina content of the batch. As the concentration of theacid solution increases, the chemical reaction between the hydrogen and nitrate ions and thesurface species of the bohemite enter into play with the probable formation of aluminum salts

391

such as aluminum nitrate. Some of the aluminum salts, in particular, aluminum formate havebeen known to form what are termed salt bridges and aid in binding and interlocking theprimary particle in the bohemite gel (ref. 17). The presence of these salts presumablyinfluences the electrical double layer around the primary particles and consequently thepacking density as well. This would explain the large increase in macropores when the acidconcentration is the highest and the alumina content is the lowest, conditions that wouldfacilitate increased chemical interactions. The pore structure obtained with the highest acidconcentration and the lowest alumina content very closely resembles the pore structure ofpure aluminum nitrate. Macroporosity is influenced by aggregate formation and henceanything that affects the formation of aggregates and their maintenance during drying andcalcining will affect the macroporosity.

The order of addition of acid water mixture to alumina was also found to be importantto produce reproducible results. When acid was first added to alumina and then followed bywater the final porosity of the sample was different from that when an acid solution of thesame molarity was added to the alumina. One explanation for this would be that the acidsolution is imbibed by the porous bohemite aggregates before the dissolution of water into theacid solution, which would cause the batch to peptize at a higher acid concentration and alsoaffect the extent of reaction between acid and alumina. Also if water was added initially,then because of the hydrophobic nature of bohemite, the bohemite particles tended to float onthe surface of the liquid and the final results obtained were inconsistent. Therefore in allexperiments, the alumina was added to an aqueous solution of the acid of a desiredmolarity.

Work in our laboratory (refs. 1,2,8) has indicated that when hydroprocessing bitumen,the catalytic deactivation is very severe during the initial 20 h. For this reason, comparisonsmust be made at the same time-on-stream. Referring to Tables 3 and 4, all the measuredactivities increase with increase in the mesopore volume. The three catalysts have similartotal pore volumes and total surface areas. Catalyst C is a macroporous catalyst with about50% of the pore volume contained in the macropores, whereas catalyst E is essentially amesoporous catalyst with little or no macropore volume. The observed differences in theactivities can be rationalized in two ways. One is shown in Figure 4 which indicates that thepore volume contained in the mesopores is the main contributing factor for the observedincreases in the conversions. This figure also considers the different amounts of catalyticmaterials that were loaded in the reactor. A slightly different way of looking at the data isshown in Table 4. The mesopore region is further divided into three: between 3-7 nm,between 7-13 nm and between 13-20 nm. None of the three catalysts had any significantpore volume between 20 and 50 nm. The presence of pores in the large rnesopore rangeapparently contributes to the higher activity of Catalyst E and the lower deactivation atlonger hours on stream. Thus it would seem that good catalysts must have large pore volumesin the mesopore range but in addition must have wide pores as well to maintain activity forlonger hours on stream. These results are consistent with the generally accepted mechanismsof catalyst deactivation caused by pore mouth plugging. Wider pores provide not only porespace for accumulation of poisons but also provide access to the pore structure for chemicalreactions to occur.

CONCLUSIONLarge pore catalyst supports are conveniently prepared by mixing commercial bohemite

powders with moderately concentrated nitric acid solutions, followed by drying at low

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temperatures and calcining at 500oC. The engineering of pores of certain sizes in thesesupports can be achieved by varying the concentration of the nitric acid and the solidcontents in the batch. Catalysts prepared from these supports and containing Mo and Co asactive metals have lifetimes of the order of hundred hours when used in hydrocracking heavyfeedstocks. The presence of mesopores in the 13 to 20 nm range was found to enhance theactivity of the catalyst for reaction with large asphaltene species contained in these feeds.

REFERENCES

A.H. Hardin, R.H. Packwood, and M. Ternan, "Effects of Median Pore Diameters inCO/Mo/AlzOs Catalysts on the Conversion of Athabasca Bitumen", Am. Chern. Soc. Div.Petrol. Chem. Preprints, 23 No.4 (1978) 1450.

2 M. Ternan and J.F. Kriz, "Some Effects of Catalyst Composition on Deactivation and CokeFormation when Hydrocracking Athabasca Bitumen", in B. Delmon and G. Froment(Editors), Catalyst Deactivation, Elsevier, Amsterdam, 1980, pp. 283-293.

3 S.T. Sie, "Catalyst Deactivation by Poisoning and Pore Mouth Plugging in PetroleumProcessing", ibid. pp. 545-568.

4 B.B. Silbernagel, and K.L. Riley, "Heavy Feed Hydroprocessing Deactivation: TheChemistry and Impact of Vanadium Deposits", ibid.pp. 313-321

5 K.L. RileY,P.A. Bryant and H.V. Drushel, "The effect of Catalyst Pore Size on ReactionPathway for Heavy Ends Hydrodesulphurization", AIChE Meeting, Houston, Texas, March29-April 2, 1987.

6 M. Shimura, Y. Shiroto and C. Takeuchi, "Effect of Catalyst Pore Structure onHydrotreating of Heavy Oil", Ind. Eng. Chem, Fundam., 25 (1986) 330-337.

7 D.O. Do, "Catalytic Conversion of Large Molecules; Effect of Pore Size and Concentra-tion", AIChE J., 30 (1984) 849-853.

8 M. Ternan:r.:atalytic Hydrogenation and Asphaltene Conversion of Athabasca Bitumen",Can J. Chem. Eng., 61 (1983) 689-696.

9 P.N. Ho, and S.W. Weller, "Effects of Pore Diameter and Catalyst Loading in Hydrolique-faction of Coal with CoO/MoO/AID Catalysts", Fuel Proc. Technol. 4 (1981) 21-29.

10 D.L. Trimm, and A. Stanislaus, "The Control of Pore Size in Alumina Catalyst Supports: AReview", Appl. Cat. 21 (1986) 215-238.

II Preparation of Catalysts III, G. Poncelet, P. Grange and P.A. Jacobs (Editors) Elsevier,Amsterdam 1983.

12 R. Snel, "Control of the Porous Structure of Amorphous Silica-Alumina I. The Effects ofSodium ions and Synersis", Appl. Cat., 11 (1984) 271-280.

13 T. Inui, T. Miyake, K. Fukuda and Y. Takegami, "Control of Pore Structure of gammaAlumina by the Calcination of Bohernite prepared from Gibbsite under specificconditions", Appl. Cat., 6 (1983) 165-173.

14 T. Ono, Y. Ohguchi and O. Togari, "Control of the Pore Structure of Porous Alumina", inG. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III ed, Elsevier,Amsterdam, 1983, pp. 631-641.

15 W. Stoepler, and K.K. Unger, "The Properties of Commercial Alumina base materials andtheir effect on the Manufacture of Active Porous Alumina Supports by means ofExtrusion", ibid., pp.643-651.

16 K. Jiratova.L, Janacek and P. Schneider, "Influence of Aluminum Hydroxide Peptizationon Physical Properties of Alumina Extrudates, ibid., pp. 653-663.

17 U. Hammon, and M. Kotter, "Preparation of Pellets with well defined Pore Structure", Int.Chern. Eng. 26 (1986) 563-573.

18 R.E. Tischer, "Preparation of Bimodal Alumina and Molybdena/Alumina Extrudates", J.Cat., 72 (1981) 255-265.

19 M.V.C. Sekhar, "Effects of Catalyst Pore Structure on the Conversion of Asphaltenes inAthabasca Bitumen", AIChE Meeting, Houston, Texas, March 24-28 1985.

20 K.S.W. Sing, Pure Appl. Chem., 54 (1982) 2201.21 J.W. Fulton, "Making the Catalyst", Chern. Eng., July 7 (1986) 59-63.22 M. Ternan, R.H. Packwood, R.M. Buchanan and B.I. Parsons, "Preparation of High

Porosity Catalysts", Can. J. Chern. Eng.,60 (1982) 33-40.23 M.F.L. Johnson and J. Mooi, "The Origin and Types of Pores in some Alumina Catalysts"

Am. Chem., Soc., Div. Petrol Chern. Preprints, 12 No.3 (1967) 204-220.