Titania-zirconia mixed oxide aerogels as supports for hydrotreating catalysts

Applied Catalysis A." General, 94 ( 1993 ) 45-59

45

Elsevier Science Publishers B.V., A m s t e r d a m A P C A T A2425

Titania-zirconia mixed oxide aerogels as supports for hydrotreating catalysts J.G. Weissman Texaco Inc., P.O. Box 509, Beacon, N Y 12508 (USA)

and E.I. Ko and S. Kaytal Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213 (USA) (Received 20 May 1992, revised manuscript received 29 October 1992)

Abstract Supercritical fluid (SFC) extraction was used to make aerogels of T i Q , Zr02 and two TiO2/ZrO2 mixed oxides, having surface areas from two to five times greater than their conventionally prepared equivalents; additionally the mixed oxides have higher surface acidities than the two single component oxides. Heat treatments, either during catalyst preparation or reactor testing, always resulted in small to significant decreases in surface areas in the aerogel-containing samples. These samples were used as supports for Mo Ni catalysts for the hydroprocessing of gas oil in a pilot-plant scale reactor. The high Z r Q containing materials were found to be unstable under reaction conditions and were nearly inactive; in contrast, the high TiOe containing catalysts, while somewhat unstable, are more active on a surface area basis than AIeQ or conventional TiO z equivalent supported Mo-Ni catalysts. This improvement is attributed to properties inherent in the SCF prepared supports; these results also indicate that support acidity contributes to hydrotreating activity.

Keywords: acidity; aerogel; hydrodenitrogenation; hydrodesulfurization; hydrotreating; mixed oxides; super-critical fluid extraction; surface acidity; titania; zirconia.

INTRODUCTION

Hydroprocessing catalysts prepared with alternative supports can sometimes have higher sulfur and nitrogen removal activities when compared to conventionally prepared alumina supported catalysts. Some of the more promising of these supports are titanium and zirconium oxides and their mixtures [1-7]. Ti02 and ZrO2 supports give rise to different active metal-support inCorrespondence to. Dr. J.G. Weissman, Texaco Inc., P.O. Box 509, Beacon, NY 12508, USA. Tel. ( + 1-914)8387654, fax. ( + 1-914)8387120.

0926-860X/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.

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J.G. Weissman et al./Appl. Catal. A 94 (1993) 45-59

teractions when compared to AI203. When cobalt or nickel promoted MoS2 is supported on Ti02 or Zr02, the activity per molybdenum atom is higher [3 ], perhaps due to a weaker molybdenum-support interaction or to smaller MoS2 particles than on an A1203 support; in either case, the production of more coordinately unsaturated sites on MoS2 crystals having a greater number of edge sites is thought to occur [ 1-3 ]. As a consequence, catalysts prepared using Ti02 or Zr02 supports are at least two to three times more active than the equivalent A1203 supported catalyst, measured on either a surface area, weight, or atomic basis [1,3,5-7]. However, because of the typical low surface area of these supports, usually less than 50 m2/g, the volume activity is much lower than that of A1203 supports, which usually have surface areas of at least 200 m2/g. An additional drawback to hydrotreating catalysts prepared using TiO2 or Zr02 supports is the reported severe deactivation due to rapid coke build-up [1,4]. This coke deposition may be in part due to increased surface acidity of Ti02 and ZrO2 supports as compared to A1203 supports. Better molybdenum dispersion may also result in increased coke build-up, as coordinately unsaturated sites are thought to be acidic in nature [8]. However, increased acidity is also beneficial, as surface acidity has been noted to be directly proportional to hydrodenitrogenation (HDN), and to a lesser degree hydrodesulfurization (HDS), activities [9,10]. The exact role of the acidic nature of the support and MoS2 on HDS and HDN catalyst properties is not clear. The problem of low surface area in TiO2 and Zr02 supports can be addressed through alternative preparation methods. These materials are conventionally prepared via precipitation or gelation from alkoxide or chloride organic solutions, followed by calcining to produce the oxide. A general review of sol-gel preparation techniques [ 11 ] and the effects of preparation variables on reaction kinetics and product properties [12-14] describes these procedures and products. We report the use of supercritical fluid (SCF) extraction to prepare high-surface-area Ti02, Zr02, and two TiOffZr02 mixed oxides for use as supports for Ni-Mo hydrotreating catalysts. Catalysts were also prepared using conventional Ti02 and A1203 supports for comparison. The SCF method extracts solvent from a gel without collapsing the structure of the gel, as would occur in conventional drying. The result is an aerogel having high surface area. The use of SCF to prepare aerogels for catalytic applications has been extensively reviewed [ 15 ]. The SCF supports were characterized by their surface areas and surface acidities; while the catalysts were characterized by their surface areas and X-ray diffraction patterns in both their fresh and used states and by hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) activities in the sulfided state using light straight-run gas oil (LSRGO).

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J.G. Weissman et al./Appl. Catal. A 94 (1993) 45 59

EXPERIMENTAL

Supercritical fluid extracted single and mixed oxides were formed by a twostep process, involving gelation of the precursor chemicals, and then drying and calcining of the gel. Zirconium propoxide and/or titanium butoxide in npropanol were mixed with water and a small concentration of nitric acid as a homogeneous catalyst to form a gel or precipitate. The gels were dried by conventional heating, or by supercritical fluid extraction. SCF involves the removal of alcohol and water from the gel through replacement with supercritical carbon dioxide at high pressures, followed by release of pressure to vent the carbon dioxide, leaving behind a fine, highly porous powder. The preparation and extraction procedure is similar to that described elsewhere [16]. After extraction, the samples were dried at l l 0 ° C and then calcined at 500°C, in flowing oxygen, for two hours. Specifically, the precursor metal alkoxides, either or both titanium n-butoxide, Ti(OC4H~)4 (Alfa Products) and zirconium n-propoxide, Zr(OCaHT)4 (Alfa Products) were mixed with dried n-propanol, in a nitrogen atmosphere. Separately, a solution of n-propanol and deionized water was also prepared. The pH of the gelling solution was adjusted by adding small amounts of aqueous 6 M HN03 to the alkoxide mixture. All quantities used are listed in Table 1. The two solutions were rapidly and thoroughly mixed to form a gel at room temperature. We find that pH (which is inversely proportional to acid concentration ) has a significant effect on gel quality, ranging from no gel formation at very low pH values to precipitate formation at high pH values; at intermediate pH values gels of varying transparency can be formed depending on composition. For each composition listed in Table 1, the pH was systematically varied over a range wide enough to observe the transparent to precipitate transition. All samples were prepared at the highest pH values that still produced transparent gels. TABLE 1 Quantities used in preparation of single and mixed oxide titania and zirconia aerogels Product, composition (wt.-%/wt.-%)

Ti/n-Pr

Ti02 100 TiO2/Zr0~ 85/15 Ti0z/ZrO~ 26/74 Zr02 100

1.00 1.00 0.39

Zr/n Pr

H~O/(Ti+Zr)

HNOa/n-Pr

0.16 1.00 1.00

4.00 2.10 1.97 4.00

0.010 0.030 0.058 0.040

Ti as Ti(OC4H9)4, Zr as Zr(0C:~H7)4, mpr is n-propanol, HN0a is aqueous 6 M HNOa. Ratios expressed as mmol per ml tbr T i / n Pr and Zr/n-Pr, mmol per mmol for H20/(Ti + Zr), and ml per ml fi)r HNOJn-Pr. See text for preparation procedures.

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J.G. Weissman et al./Appl. Catal. A 94 (1993) 45-59

Solvents were extracted from the gels with supercritical CO2, using a standard autoclave, operated at about 70°C and 207 bars (3000 psi) [16]. Supercritical CO2 was used at a rate of 1400 standard cm 3 per min, for two to three hours, at the end of which no additional solvent was extracted from the sample. The dried product was passed through a 100 mesh sieve and then calcined, prior to characterization and support formation, in oxygen at 500°C for two hours. Surface acidity measurements were performed on two single oxide SCF compositions, TiO2 and Zr02, and on three mixed oxide compositions, 26/74, 49/ 51, and 85/15 wt.-%/wt.-% TiOffZr02. Surface acidity was measured by titration with n-butylamine following a procedure similar to the Benesi method [17]. H a m m e t t indicators and corresponding acid strengths are: benzalacetophenone, - 5.6, dicinnamalacetone, - 3.0, and methyl red, + 4.8. Commercially available A1203 and Ti02 supports were used for comparative conventional catalyst supports; in addition, four supports were prepared using the SCF oxides: Zr02, TiO2, TiOffZrO2 85/15 and TiOffZrO2 26/74, ratios expressed as weight percent; supports and catalysts are characterized in Tables 2, 3, 5 and 6. All of the Ti02 and ZrO2 SCF oxides were mixed with water into a paste, extruded, and then calcined at 400 ° C to form hardened extruded supports. Because of difficulty in extruding Ti02, the SCF extracted powder was mixed with 20 wt.-% A1203 binder prior to extrusion and calcining. A similar conventional TiOffA12Q 80/20 sample was also prepared. The supports are identified in Table 2. Drying steps were done in a vacuum oven at l l 0 ° C for TABLE2 Surface areas of supercritical fluid extracted oxides, supports and corresponding fresh and used Ni-Mo catalysts Sample support

SCF aerogel

Support

Catalyst Fresh

A. conventional A1203 B. conventional Ti02 C. SCF Ti02 D. conventional TiO2/A1203 80/20 E. SCF TiOJA1203 80/20 F. SCF TiO2/ZrO2 85/15 G. SCF TiO2/ZrO2 26/74 H. SCF ZrO2

580

260 50 156

390 315 335

133 120 205 130

Used

259 44

179 42

76 118

53 38 34 (2) (16)

Surface areas reported for SCF aerogels after calcining at 500 ° C, for supports after extruding and calcining at 400 ° C, fresh catalysts after final metal deposition and 400 ° C calcining step, and used catalysts as removed from the reactor, after washing with acetone or tetrahydrofuran to remove residual oil. Surface areas reported in square meters per gram, as measured by the BET method. Numbers in parenthesis may be inaccurate due too small mass of sample analyzed.

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J.G. Weissman et al./AppI. Catal. A 94 (1993) 45 59

TABLE3 Phases found by X-ray diffraction of calcined supports and corresponding fresh and used Ni-Mo catalysts Sample support

A. conventional A120:, B. conventional Ti02 C. SCF TiO2 D. conventional TiO2/A1203 80/20 a E. SCF TiQ/A1203 80/206 F. SCF TiOe/Zr02 85/15 G. SCF TiO2/ZrO2 26/74 H. SCF Zr02

Calcined

GA AN AN AN pc-AN AN, pc-ZR ZR

Catalyst Fresh

Used

GA AN, RU AN, RU pc-AN, RU pc-AN pc-AN, ZR -

GA AN, RU AN, RU AN, RU AN AN, ZR ZR

Phases indicated by: GA, ),-alumina; AN, anatase; pc, poorly crystallized; and ZR, a mixture of zirconia phases, mostly the tetragonal form. a Note that A120:~in samples D and E was added as a binder to conventional Ti02 (sample B ) or SCF Ti02 (sample C ); it was not a co-gelled material as samples F and G.

one hour, while calcining steps were completed by heating the samples to 400 ° C in forty minutes in flowing nitrogen, holding at 400 °C for two hours in nitrogen, followed by four hours in cylinder air, then cooling in flowing air. All gas flow-rates were 0.5 1 h -1. All supports were treated identically prior to metals deposition; however, the conventional alumina support, sample A, was only dried prior to metals deposition as it was already shaped into a support. Molybdenum and nickel were deposited onto the supports prepared above to form hydrotreating catalysts. On a weight basis, the same amounts of nickel and molybdenum were deposited onto each support, using the same procedure. (NH4)6MovO24" 6H20 in aqueous solution was deposited by incipient wetness, in a single step, followed by drying in vacuum at l l 0 ° C for one hour and calcining at 400 ° C in air at a flow-rate of 0.51 h - 1. Ni (NO3) 2"6H20 was deposited in the same manner. Loadings corresponded to 2 wt.-% Ni and 6 wt.-% Mo. SCF oxides, extruded supports, and fresh and used catalysts were characterized by surface area measurements, X-ray diffraction and by elemental analysis. The catalysts listed in Table 2 were tested for HDS and H D N activities. From 3 to 10 cm 3 of catalyst was placed in an up-flow bench scale hydrotreating reactor with a feed stream consisting of light straight run gas oil at 2.0 liquid hourly space velocity (cm 3 catalyst per cm 3 liquid per hour) and 100% H2 at a ratio of 270 standard cm 3 of gas per cm :~or liquid, equivalent to 1500 standard cubic feet per barrel. The gas oil feed contains 1.37 wt.-% S, 83 ppm N, 30 wt.-% aromatic content, and has a density of 0.87 g/cm 3 (31.1 ° API gravity). Catalysts were sulfided in situ prior to testing by passing a mixture of 10% H2S in H2, at a rate of 6 cm ~ of gas per cm 3 catalyst per minute, for three hours at

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J.G. Weissman et al./Appl. Catal. A 94 (1993) 45-59

300°C. Catalysts were tested at 300°C and 27.6 bars (400 psi) for 24 h, which provided sufficient time for steady-state operations to be established. After 24 h three two-hour samples were collected for analysis for sulfur and nitrogen content. In all cases results for each sampling interval were consistent, reported results represent an average of the three measurements for each catalyst. RESULTS AND DISCUSSION

Support properties Surface areas of titania prepared by different methods are listed in Table 4. Supercritical fluid extraction produces a material with a significantly higher surface area; this advantage is retained after calcining. As discussed in the original work by Kistler [ 18], supercritical drying eliminates the liquid-vapor interface which would exist during conventional drying, thus preventing capillary forces from collapsing the porous material. Comparison of the two SCF samples of Table 4 show that once the solvent has been removed by supercritical drying, further heat treatment in an oven has little additional effect on the calcined surface areas of the TiO2 supports. In contrast, drying of the gelled sample without supercritical extraction, followed by calcining, leads to a significant decrease in surface area, as shown by the second entry in Table 4. Surface areas of most of the SCF oxides, extruded supports, and catalysts in fresh and used states, are listed in Table 2. The SCF oxides, samples C, E, F, and G, lost between one-third and two-thirds of their surface area when formed into extruded supports; however, these are still greater than that of the comparative conventional support composition, sample B. This advantage is retained after preparation of the catalysts, indicated by comparing the surface areas of the fresh catalysts D and E in Table 2. The SCF oxides were X-ray amorphous while the conventional A120~ and TiO2 containing materials were highly crystalline, being either y-A1203 or anTABLE4 Surface areas of titania prepared by different methods Preparation method

Dried

Calcined

Precipitation Oven dry SCF SCF, oven dry

270 152 531 580

85 0.2 160 156

Surface areas reported in square meters per gram, as measured by the BET method. Samples calcined at 500°C, oven dried samples heated at l l 0 ° C for three hours, SCF: supercritical fluid extracted.

J.G. Weissman et al./Appl. Catal. A 94 (1993) 45 59

51

atase. After forming into extrudates, all of the single component supports were well crystallized, to either tetragonal ZrO2 or anatase TiO2, while the mixed oxide supports were only poorly crystallized, having difficult to interpret Xray diffraction (XRD) patterns, Table 3. TiOffZrO2 26/74 crystallized to mostly anatase while TiO2/ZrO2 85/15, Fig. 1, crystallized to what appears to be a mixture of poorly crystalline ZrO2 phases. Two predominate mechanisms may account for these changes in the SCF single oxides: sintering of adjacent small crystallites into larger crystallites, and growth of crystallites due to aggregation of poorly crystallized material and phase transformations, through a surface diffusion mechanism [ 19 ]. Crystallization was not as extensive in the mixed oxides, samples E, F and G, as oxide mixtures are known to suppress collapse and transformations due to surface diffusion [20], leading to the relative less crystalline state of the SCF mixed oxides after extrusion, measured by XRD, as compared to the single oxide and conventional oxide supports. Mixed oxide sample D, having been formed from well crystallized conventional A1203 and TiO2 precursors, did not noticeably change crystallinity during the various heating steps. For the SCF mixed oxides, TiO2/ZrO2 26/74 and 85/15, the intimate mixing of the two immiscible oxides retards solid-state diffusion, limiting adverse changes to the material due to heating. The precursor gels are formed through condensation reactions involving partially hydrolyzed alkoxides, leading to

[

$

25

45

65

DEGREES 2 0

Fig. 1. X-ray diffraction patterns of sample series F, TiO2 ZrO2 85/15, The pattern illustrates a trend towards increased crystallinity as the material is taken from the extruded catalyst support state to the used catalyst state.

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J.G. Weissman et al./Appl. Catal. A 94 (1993) 45-59

cross-linking and polymeric growth of the gel [ 11,21 ]. This behavior has been observed experimentally for the hydrolysis of zirconium and silicon alkoxides [ 14 ], a system similar to our TiO2/ZrO2 aerogels. By analogy we contest that the same types of Ti-O-Zr linkages occur in our mixed alcogels, although we have only indirect evidence for this, the stability of the mixed oxide aerogels towards sintering. The interaction between the two oxides also results in a significantly greater surface acidity than that of the single component oxides. Fig. 2 shows that the single oxides, TiO2 and Zr02, have little surface acidity, as measured by Hammett indicators, while the mixed oxides have various degrees of increasing surface acidity, with the maximum occurring at about 50 wt.-% Ti02 (a Ti02/ Zr02 49/51 sample was prepared for the acidity measurements). These findings are consistent with the model of Tanabe [22 ], in which new acid sites are associated with Ti-O-Zr linkages. It is also possible that as transitional metal oxide particle size decreases, the number of surface oxygen anion vacancies increases, and so new and stronger acid sites are created, with particles of smallest diameter having the strongest acidity [23]. Our mixed oxides can be thought of as a mixture of relatively small particles contributing acidity from anion vacancies, in addition to surface acidity arising from interactions postulated by Tanabe. 0.8

0.7

0.6

0.5

g iz 0.4 .-~ o a

0.3

0.2

0.1

20

40

60

80

100

WEIGHT PERCENT TiO 2, BALANCE ZrO 2

Fig. 2. Surface acidity of single and mixed TiO2-ZrQ oxides as a function of acid strength, as measured by Hammett indicator titration procedure described in the text. Acid strengths increase from pKA'S of (A) +4.8, to (0) --3.0 to (11) --5.6; surface acid sites with the highest acid strength having occurred at lower concentrations.

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J.G. Weissman ct al./Appl. Catal. A 94 (I993) 45 59

Catalytic properties Little additional support surface area was lost as a result of catalyst preparation, indicating that the extruded supports had been stabilized being calcined at 400 ° C. The used catalysts, however, showed considerable physical changes as a result of exposure to reaction conditions. Significant decreases in surface area and increases in crystallinity, Table 3, occurred for the catalysts prepared

z

,< I-

z

Ilc is

u. o

5

25

45 DEGREES

65

20

Fig. 3. X-ray diffraction patterns of used catalysts. Sample letters correspond to those listed in Table 3, phases present are listed in Table 3. Ti02 and ZrO2 single oxide supported used catalysts, I), E, and H, have a greater degree of crystallinity than the mixed oxide supported used catalysts, F and G, as indicated by the increased sharpness of the diffraction peaks and intensity above background.

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J.G. Weissman et al./Appl. Catal. A 94 (1993) 45-59

with SCF supports. Catalysts prepared with the conventional supports, A, B, and D in Table 2, had relatively little physical differences between the fresh and used states, retaining most of their surface area and having little increase in crystallinity. Small to no changes to X-ray diffraction patterns were observed between calcined and corresponding fresh and used catalysts for conventional single oxide supported samples A, B, and D; indicating the relative stability of these supports and resistance to further transformations. In the case of the two TiO2 containing support, B and D, the well crystallized nature of the material contributes to this stability, as does the inherent stability of y-A1203 in sample A. In contrast, the four samples containing SCF supports, E, F, G, and H, consist of poorly crystallized material, as determined by X-ray diffraction, which is susceptible to crystallization when exposed to additional heat treatments, as during catalysts preparation and testing. Fig. 1 illustrates such behavior for sample F. Fig. 3 shows that there is a general trend in the SCF supported catalysts towards increased crystallinity with higher zirconia content, as cryso tallinity of the used catalysts increases from sample F ( 15 wt.-% ZrO2 ) through G (74 wt.-% ZrO2) to H (100% ZrO2). Additionally, the three mixed oxide samples, E, F, and G, are less well crystallized after exposure to reaction conditions than the single oxide and conventional oxide samples. These results indicate the interactive nature of the SCF oxides contained in the extruded supports, either with a conventional oxide, as in sample E, or other SCF oxides, F and G, leads to the observed relative stability of the final products. Although the reaction temperature was 200 °C lower than the greatest calcining temperatures used in the preparation of the catalysts, the SCF materials were destabilized by reaction conditions. The higher ZrO2 content samples proved to be especially unstable, forming well crystallized materials with very low surface areas. The reasons for this collapse are not clear; however, the sulfiding and reducing environments of the reaction are likely causes, accelerating sintering and agglomeration of the support material. Additionally the materials are exposed to reasonably high temperatures, 300 ° C, for a prolonged time interval, which can accelerate collapse; even though they have been exposed to higher temperatures during catalyst formation. In some cases moisture is known to cause sintering of oxides; however, there is a minimum amount of moisture in our reactor system. Any water adsorbed onto the catalysts is removed during the initial heating stage prior to reaching sulfiding temperatures, and no additional sources of water are present. The conventional A1203 and Ti02 materials were not susceptible to this collapse, as both were crystalline stabilized materials prior to catalyst preparation.

Hydrotreating activities The seven catalysts were evaluated for HDS and HDN activities at 300 °C and 400 psi using light straight-run gas oil. Sulfur and nitrogen removal con-

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J.G. W e i s s m a n et al./Appl. Catal. A 94 (1993) 4 5 - 5 9

versions are listed in Table 5. None of the TiO2 or ZrO2 based catalysts had HDS conversions, on a volume basis, comparable to the conventional A1203 supported catalyst, A, although the high TiO2 compositions exhibited significantly more H D N conversion on a volume basis. Sulfur reaction rates (Rs), Table 5, reveal more about the behavior of the SCF catalysts. While none of the TiO2 or ZrO2 supported catalysts had Rs's better than the A1203 supported catalyst, the high TiO2 supported, and especially the SCF supported catalysts E and F, had much higher Rs's on a surface area basis (Rs/SA) than the conventional A1203 catalyst. These results confirm the previously discussed reports, finding that hydrotreating catalysts prepared using a TiO2 support are much more active for HDS on a surface area basis than conventional A120:~ supported catalysts [ 1,3,5,7 ]. Catalyst supports prepared using SCF have a slightly higher per-surface-area sulfur removal rate, Rs/SA, E compared with D. The presence of 15 wt.-% ZrO2 does not significantly alter the HDS activity of the SCF TiO2 supported catalyst, compare E and F. Higher HDS activity, on a surface area basis, as found for our SCF supports, is explainable from previous work. Molybdenum oxides or sulfides supported on Ti02, Zr02, or TiOJZr02 mixed oxides have a greater dispersion, or smaller particle size, than equivalent amounts on A1203. Smaller crystal size leads to a TABLE 5 HDS a n d H D N conversions and sulfur reaction rates of single and mixed oxide supported Ni Mo catalysts Sample support

Volume (cm a )

Density ( g / c m a)

HDS a (%)

HDN a (%)

Sb

Rs ~

Rs/SN'

A. B. C. D.

10 10

0.57 0.86

68 41

17 18

0.16 0.38

3.1 1.3

0.017 0.030

0.96 1.20 1.15 1.45 1.65

54 57 45 19 20

23 28 49 8 5

0.34 0.39 1.1 0.40 0.23

1.5 1.2 1.0 0.34 0.32

0.028 0.032 0.030 (0.17)d (0.02)d

conventional AI20~ conventional TiO2 SCF TiO2 conventional TiQ/A12Oa 80/20 E. SCF TiO2/AI~O3 80/20 F. SCF T i Q / Z r Q 85/15 G. SCF T i O J Z r O 2 26/74 H. SCF Z r Q

5.0 4.7 3.0 3.5 3.0

Volumes and densities are measured as loaded into reactor. Gas and liquid flow rates adjusted proportionally for each run. a HDS a n d H D N are sulfur and nitrogen removal conversions, calculated on an equal volume basis. b S is selectivity of reaction, defined as the ratio of nitrogen and sulfur pseudo-first order reaction rates, given by S = In ( 1 0 0 / 1 0 0 - H D N ) / l n ( 1 0 0 / 1 0 0 - H D S ) . c Rs is sulfur reaction rate based on catalyst weight, expressed in (mol/g s), while R s / S A is based on catalyst surface area, expressed in ( m o l / m e s). d Numbers in parenthesis are likely to be inaccurate due to poorly defined low surface areas.

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J.G. Weissman et al./Appl. Catal. A 94 (I993) 45-59

greater number of coordinately unsaturated sites, or anion vacancies, which have been found to correlate with HDS activity [2,3,10,24]. MoS2 crystal morphology is known to vary between different supports, but the nature of the coordinately unsaturated site is the same [2,3,7,25,26]. The underlying difference between catalysts of different HDS activities is the strength of molybdenum-support interactions on TiO2 and ZrO2 is less than that on A1203. This leads to greater molybdenum reducibility and sulfiding ability on TiO2 and ZrO2, and consequently a relatively higher dispersion of sulfided molybdenum. Additionally, there is less chance of molybdenum support compound formation, such as A12(MoO4)3, due to the weaker interaction [6,26,27]. The higher Rs/SA found for the SCF supported catalysts is a consequence of the active metals on these supports having a higher activity per active site. In contrast, HDN activity seems to depend more on the support composition. In general, HDN activity of mixed oxide supported catalysts, D-H, is greater than that of the single oxide supported catalysts, A and B. The exception is that the high ZrO2 content supported catalysts, G and H, showed little HDN activity. SCF TiOffZrO2 85/15 supported catalyst had the highest HDN activity of all the catalysts tested, much greater than the conventional A1203 supported catalyst at 300 ° C. These results point to the role of support surface acidity in HDN activity, as our mixed oxides have a greater surface acidity than the corresponding single oxides, as long as other factors do not become important, as in the very low surface area used catalysts G and H. Additionally the TiO2 and TiOffZrO2 supported Ni-Mo catalysts have HDN selectivities greater than the A1203 supported catalysts. Selectivity is defined here as the ratio of nitrogen to sulfur first order removal rates. The TiOffZrO2 85/15 supported catalyst showed the greatest selectivity for HDN over HDS. Because of the low nitrogen level of the feed, adsorption of nitrogen onto the catalysts may be a competitive nitrogen removal process; however, elemental analysis, Table 6, found that total amount of nitrogen adsorbed onto the catalysts cannot account for the increased HDN activity; a nitrogen mass balance TABLE 6 Elemental analysis of used Ni-Mo catalysts Sample support

S

C

H

N

A. conventional A1203 E. SCF TiO2/A12Q 80/20 F. SCF TiO2/ZrO2 85/15 G. SCF TiO2/ZrO2 26/74 H. SCF Zr02

4.9 4.9 3.4 1.7 1.7

3.2 3.0 4.2 1.1 1.2

1.7 1.1 0.98 0.56 0.36

0.08 0.06 0.18 0.75 0.33

Analysis by pyrolysis, values given as weight percent.

J.G. Weissman et al./Appl. Catal. A 94 (I993) 45-59

57

reveals that most nitrogen entering the system was removed as a reaction byproduct. Acidic supports are thought to give an increase in activity, especially for HDN. While the TiO,~ support is only weakly acidic, the TiO2/ZrO~ mixed oxide supports have high acid strengths. Results in Fig. 2 shows that the TiO2/ ZrO~ 85/15 sample, which has the highest HDN activity, possesses significantly more acid sites than the TiO,~ at PKA values of -- 3.0 and + 4.8. The 26/ 74 sample, having similar acidic properties, was not as active, as discussed below. MoS~ has acidity in the sulfided state; addition of cobalt or nickel promoters results in the removal of the strongest acid sites. These sites are identified with coordinately unsaturated sites noted previously [8,9,28]. However, additional acid sites can be contributed by the support [5]. Additional activity, accounting for the enhanced HDN reaction rate, as compared to the HDS rate, arises from the ability of the TiOJZrO,~ mixed oxide supports to contribute to the reaction, in the form of promoting hydrogenation and possibly C-N bond cleavage, due to the strong acidic nature of the support [5]. Greater molybdenum dispersion and support acidity, together, account for the high, per surface area, activity of our sample F. The surface acidity measurements were made on the calcined supports, prior to metals deposition and sulfiding. Apparently, acidity is preserved during the subsequent preparation steps, and remains great enough to effect catalytic activity. Recent measurements of acidity on calcined and sulfided Ni-Mo/A120:~ catalysts found similar acid site densities on the two states [28], indicating that surface acidity occurring on the oxide form of the catalyst is preserved to a great degree in the sulfided state. This model is consistent with the amount of coking observed. Specifically, the carbon content of the most active HDN catalyst was the highest, Table 6, again indicating that strong acidity is present on the supports during the hydrotreating reaction. Little or no deactivation was observed on the high TiO~ supported catalysts over the time of the experiments, despite the high carbon loading. While the SCF high TiO~ catalysts, E and F, proved to be active, the TiO~/ ZrO~ 26/74 and ZrO,~ catalysts, G and H, were not, both having low HDN and HDS conversions. The two corresponding used catalyst samples also contained less sulfur than the other used catalysts, Table 6. The low activity of these materials can be ascribed to collapse of the support, as found by surface area measurements and XRD patterns, resulting in active metals becoming inaccessible. Although the TiO~/ZrO~ 26/74 support has high intrinsic acidity, the benefit of this enhanced acidity cannot be realized due to structural collapse under hydrotreating conditions. The low sulfur content of used catalysts G and H, indicates not all of the active metals became sulfided, supporting the idea of active metals being inaccessible. Review of applications of SCF prepared supports, for partial oxidation or hydrogenation of chemicals, found no evi-

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dence of collapse or instability in a hydrogen environment, the opposite was noted [15]. The severe sulfur containing reducing environment of the hydrotreating reaction is a likely contributor towards destabilization of our Zr02 supports, leading to inaccessibility of the active metals and the observed low activity. CONCLUSION Supercritical fluid extraction can be used to prepare Ti02, ZrO2, and TiOff ZrO2 mixed oxide catalyst supports having surface areas greater than the equivalent conventionally prepared materials. This advantage is retained in the corresponding calcined extruded supports. For the SCF oxide supported samples, there is a small to significant loss in surface area after each preparation step, which becomes more severe after reactor testing. After super-critical extraction and calcining at 500 ° C, the mixed oxide aerogels have higher surface acidities than the pure components, as measured by H a m m e t t indicators, and are poorly crystalline, both indicative of a strong interaction between the two component oxides. This interaction helps to stabilize the mixed oxides and provides an underlying reason for the high surface areas and acidities observed. These materials were subsequently used to form catalyst supports; the performance of the resulting catalysts can be explained by assuming these acidity increases and stabilizing interactions are retained by the aerogel components in the final product. TiO2 single oxide and TiOe/Zr02 and TiOe/Al203 mixed oxide supported Ni-Mo catalysts have greater HDS and HDN activities, on a surface area basis, than the equivalent AleO~ supported catalyst. The same catalysts also have a greater selectivity towards HDN as compared to AlzQ supported catalysts. Mixed oxide supported catalysts have a greater HDN activity on a volume basis than the equivalent A1203 supported catalyst. The SCF supported catalysts were not stable under the reaction conditions, suffering significant loss in surface area and showing increase in crystallinity after exposure to hydrotreating conditions. High ZrO2 content supports were exceptionally prone to collapse, forming low-surface-area materials having almost no catalytic activity. The high Ti02 content SCF materials exhibited enhanced HDN activities and selectivities, and suffered only partial collapse.

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