Deep hydrodesulfurization of DBT and diesel fuel on supported Pt and Ir catalysts

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A PA LL E IYD CP AT SS I GENERAL

A:

Applied Catalysis A: General 137 (1996) 269-286

ELSEVIER

Deep hydrodesulfurization of DBT and diesel fuel on supported Pt and Ir catalysts R. Navarro

a

B. P a w e l e c a J.L.G. Fierro a P.T. V a s u d e v a n J.F. Cambra b P.L. Arias b

a,*

,1

a lnstituto de Catdlisis y Petroleoquimica, CS1C, Campus UAM, Cantoblanco, 28049 Madrid, Spain b Escuela de lngenieros, Alda. Urquijo s~ n, 48013 Bilbao, Spain

Received 5 September 1995; revised 4 December 1995; accepted 4 December 1995

Abstract Deep hydrodesulfurization (HDS) of dibenzothiophene (DBT) and diesel fuel (0.08 wt.-% S) has been carried out on Pt and Ir supported on amorphous silica-alumina (ASA) and on a stabilized HY zeolite under standard industrial conditions. The effect of temperature, two different supports and feedstocks on HDS and hydrogenation (HYD) product selectivities are investigated. Normalized activity data indicate that in the HDS of DBT and of diesel fuel, the platinum catalysts are much more active than the iridium counterparts. In HDS of diesel fuel (at 623 K), both Pt/HY and Pt/ASA are a little more active than a commercial C o - M o / A I 2 0 3 catalyst. The normalized activity in the HDS of DBT (593 K) increases in the following order: Pt/HY > Pt/ASA >> Ir/HY > Ir/ASA, and in the HDS of diesel fuel (623 K), increases according to: Pt/HY >> Pt/ASA >> Ir/HY > Ir/ASA. All spent catalysts were characterized by X-ray photoelectron spectroscopy (XPS). In view of their superior performance, both platinum catalysts were characterized by FTIR spectroscopy of CO probe. Keywords: Hydrodesulfurization;Dibenzothiophene;Diesel fuel; Platinum; Iridium; Silica-alumina; HY

zeolite; XPS characterization;FTIR characterization

1. I n t r o d u c t i o n R e d u c t i o n o f s u l f u r c o n t e n t in fuels is a p r e s s i n g e n v i r o n m e n t a l c o n c e r n , a n d the E n v i r o n m e n t a l P r o t e c t i o n A g e n c y ( E P A ) in the U S has p r o p o s e d r e g u l a t i o n s

* Corresponding author. i Address: Departmentof Chemical Engineering,KingsburyHall, Universityof New Hampshire, Durham, NH 03824, USA. Tel. ( + 1-603) 8622298, fax. ( + 1-603) 8623747, e-mail [email protected]. 0926-860X/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0926-860X(95)00329-0

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limiting the sulfur to less than 0.05 wt.-% in diesel fuels. This low sulfur level is extremely difficult to achieve because some of the sulfur containing compounds are highly refractory due to the steric hindrance of the alkyl substituents to which the sulfur is bound. Published work in the field of deep desulfurization has been progressively increasing in the last few years. Research has been carried out on deep HDS of a variety of petroleum feedstocks [1-6] and of model thiophenic compounds [7-11]. A general account on the subject of deep HDS dealing with new catalysts, effect of solvents and feedstock, kinetics and mechanism, effect of support and catalyst synergy control has recently been reviewed in detail by Vasudevan and Fierro [ 12]. Conventionally, alumina supported cobalt (or nickel) molybdenum catalysts are used in HDS due to their hydrogenation and hydrogenolysis capabilities, and most of the work has centered around these catalysts. In order to reduce sulfur emissions in a variety of fuels, it is important to engineer new catalysts (metals as well as supports). The metals could be the noble and semi-noble transition metal sulfides (TMS), as their activity in the HDS of DBT using bulk catalysts has been well demonstrated by Pecoraro and Chianelli [13]. The authors observed that the activity of unsupported monometallic transition metal sulfides used in HDS of DBT varies according to the position of the elements in the periodic table. After this pioneering work, similar studies of HDS of DBT using TMS were carried out on Y type zeolites by Vrinat et al. [14], on alumina by Dhainaut et al. [15], and on ASA and HY zeolite by Navarro et al. [16,17]. In addition to heterogeneous catalytic systems, homogeneous catalysis using noble metals such as systems derived from tris(triethylphosphine)platinum(0) for the exclusion of sulfur from DBT has recently been proposed [18]. In this work, we selected iridium and platinum as promising candidates for HDS of DBT taking into account the favorable position of the metals in the 'volcano curve' obtained by Pecoraro and Chianelli [13]. Furthermore, both metals possess similar crystallographic properties (both are crystallized as face centered cubics and their atomic radii are 0.135 nm and 0.138 nm, respectively), and the 'structure sensitivity' of any reaction resulting from a geometric orientation of adsorbed reactants is expected to be very similar on both metals [19]. Silica-alumina was selected as a support because it has a greater acidity than conventional alumina, a property which seems to enhance the rate of sulfur removal. Ishihara et al. [20] have investigated the effect of supports on HDS of DBT catalyzed by supported molybdenum carbonyl complexes. They observed that silica-alumina supported catalysts exhibit a much higher catalytic activity than alumina supported catalysts. Similarly, the exceptional properties of zeolites such as high catalytic activity and great resistance to poisoning by sulfur and nitrogen containing organic compounds, provide an incentive to examine zeolites as supports for HDS catalysts. There is considerable chemical and catalytic evidence concerning the formation of electron-deficient metals in acidic

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zeolites, which in part are responsible for the high activity of these catalysts at short times-on-stream [21,22]. As a result, the incorporation of transition metal sulfides into acidic zeolites leads to the formation of bifunctional catalysts by combining both cracking and hydrogenation functions. For instance, the hydrogenolysis rate of neopentane was found to be 40 times larger on 1 nm Pt crystallites in Y zeolites than on Pt supported on conventional supports [23]. Accordingly, the main objective of the present work was to investigate the behavior of Pt and Ir sulfides supported on both silica-alumina and HY zeolite in the HDS of DBT and of diesel fuel. The effects of temperature, of the two different supports and of the two feedstocks on sulfur removal was also investigated and the results are presented.

2. Experimental 2.1. Catalyst preparation Amorphous silica-alumina (ASA) support (Si/A! :-0.62 atomic ratio, surface area 389 m2/g, pore volume 0.72 ml/g), P t / A S A (0.95% Pt loading) and C o M o / A I 2 0 3 catalysts (C444, ca. 3 wt.-% Co, ca. 9.5 wt.-% Mo) were supplied by Shell. The I r / A S A catalyst (1 wt.-% Ir nominal loading) was prepared by impregnation of ASA support with iridium(III) chloride trihydrate (Alfa). Prior to impregnation, the ASA extrudates were crushed to a particle size of 0.25-0.3 mm. The ASA carrier was contacted with 50 ml of solution, and the pH was fixed at a value close to 7. After contacting for 12 h, the excess water was removed in a rotary evaporator. Finally, the catalyst was dried at 383 K for 4 h and calcined in a two step procedure: first at 523 K for 1 h and then at 723 K for 2 h. The I r / H Y and P t / H Y catalysts (nominal loading of 1 wt.-%) were prepared by impregnation of an ultrastable HY zeolite (unit cell 2.454 nm, SiO2/A1203 mole ratio 5.6 and Na20 content 0.14 wt.-%) with aqueous solutions of iridium trichloride trihydrate (Alfa), and hydrogen hexachloroplatinate(IV) hydrate (Sigma, reagent grade), respectively. The method basically consists of an aqueous impregnation of the HY zeolite with a solution of the required amount of noble metal added to 100 ml of water. The water is then removed using a rotary evaporator, and the impregnates are dried at 373 K in air for 4 h, and finally calcined in air at 573 K for 4 h.

2.2. Characterization techniques Nitrogen adsorption isotherms were measured at 77 K using a Micromeritics Digisorb 2600 automatic equipment on samples previously outgassed at 523 K. BET areas were computed from these isotherms using the BET method.

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The IR spectra of adsorbed CO was recorded with a Nicolet 5ZDX Fourier Transform infrared spectrophotometer working with a resolution of 4 c m - 1 over the entire spectral range. The samples were pretreated in an IR cell fitted with greaseless stopcocks and KBr windows which allowed thermal treatments either in flow or under vacuum. The calcined catalyst samples (thickness ca. 10 m g / c m 2) were first outgassed at 723 K under vacuum for 1 h. After cooling to room temperature, they were contacted with ca. 30 Torr CO. Reduced and sulfided samples were also studied. The catalysts were reduced in flowing hydrogen at 573 K for 1 h and then outgassed at the same temperature for 1 h. The sulfiding procedure entailed heating in flowing helium at 673 K for 0.5 h, exposing the samples to a H2S:H 2 mixture (ratio 1:9) for 3 h at the same temperature, followed by purging in flowing helium at 673 K for 0.5 h. Then the samples were outgassed at 723 K for 1 h and exposed to ca. 30 Torr of CO. Photoelectron spectra were recorded with a Fisons ESCALAB Mk II 200R electron spectrometer equipped with a MgK X-ray source (h = 1253.6 eV) and a hemispherical electron analyzer. The X-ray source was operated at 12 kV and 10 mA. The catalysts used in the HDS reaction were deposited into small stainless steel cylinders containing isooctane in order to avoid contact with air and mounted onto a manipulator which allowed transfer from the preparation chamber into the analysis chamber of the spectrometer. The samples were pumped out to 10 -5 Torr (1 Torr =- 133.33 N / m 2) before they were moved into the analysis chamber. The residual pressure in this ion-pumped chamber was maintained below 7 • 1 0 - 9 Torr during data acquisition. Each spectral region of the photoelectrons of interest was scanned a number of times to obtain a good signal-to-noise ratio. Although surface charging was observed on all the samples, accurate binding energies (_+0.2 eV) could be determined by charge referencing with the adventitious C ls peak at 284.9 eV.

2.3. Activity tests HDS of DBT was performed in a high-pressure laboratory scale set-up equipped with a stainless steel fixed bed catalytic reactor (9.5 mm I.D. and 130 mm length). DBT was dissolved in decalin to obtain a 1 wt.-% solution. For the activity tests, 0.3 g of the catalyst with particle size 0.25-0.3 mm were dried under a N 2 flow of 100 m l / m i n at 673 K for 1 h. The activation procedure consisted of heating to the sulfidation temperature of 673 K in N 2 flOW at atmospheric pressure followed by sulfidation in a H2:H2S mixture (ratio 10:1) in a tubular flow reactor at a rate of 50 m l / m i n for 4 h, then purging under a N 2 flow of 100 m l / m i n at 673 K for 1 h and finally storing overnight under a N 2 flow of 3 m l / m i n . Before the experimental run, the N 2 pressure was increased to the desired value, and the catalytic bed was heated to the desired experimental temperature. Hydrogen and DBT were then fed to the reactor. The reaction

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conditions in the HDS of DBT were: temperature 523-593 K; total pressure 30 bar; H 2 flow rate 7 I(STP)/h and WHSV =-35 h -1 Liquid samples were analyzed by GC (Varian chromatograph Model Star 3400 CX) equipped with a 30 m X 0.53 mm column with phase DB-1 (100% methyl-polysiloxane, J & W Scientific), using an initial temperature of 313 K (for 1 min) and then heating at a rate of 18 K / m i n up to 473 K and holding at that temperature for 5 rain. For studies on the effect of temperature of ASA supported catalysts, the procedure was as follows. Activity was measured at four different temperatures: 5 2 3 , 5 5 3 , 5 7 3 and 593 K running from the lowest to the highest and maintaining the reaction for 1 h at each temperature. At each temperature, the purge of the liquid reservoir was carried out for 45 rain, then the liquid effluent was collected for 15 min and analyzed. Once the reaction was carried out at a given temperature, the next temperature was reached while maintaining the catalyst in a flow of inert gas. In order to check if there was any deactivation of the catalysts, the activity was checked at the lowest temperature (523 K) at the end of all the experimental runs. The total conversion was determined by taking the average of three different analyses of the liquid product after 1 h time-on-stream at a given temperature. Activity of plain HY, I r / H Y and P t / H Y catalysts was tested at 593 K as a function of time-on-stream. In order to compare the activity of these catalysts with that of M / A S A , the activity of P t / H Y zeolite was additionally determined at 523, 553 and 573 K. The procedure of purging of the liquid reservoir and sample collection was similar to the one described in the previous paragraph. For ASA supported catalysts, in addition to unreacted DBT, biphenyl (BP) and cyclohexylbenzene (CHB) were the only products detected. However, for HY supported catalysts, several other products arising from the cracking of decalin (solvent) and probably of DBT were identified by GC-mass spectrometry, even though they are difficult to quantify. For this reason, total DBT conversion was calculated as DBT disappearance and HDS and HYD selectivities were defined as ( B P ) / ( C H B + BP) X 100 and ( C H B ) / ( C H B + BP) X 100, respectively. Deep HDS of diesel oil, already catalytically processed to 0.08 wt.-% sulfur content, was performed in a conventional high pressure bench-scale unit equipped with a stainless-steel fixed bed catalytic reactor (11.05 mm I.D. and 400 mm length). The particle size of the catalyst used was 0.42-0.50 mm. The pretreatment of the catalyst was similar to the procedure specified in the previous paragraph. The HDS conversion was determined by analyzing the change in the total sulfur content between the feed and the product on an ASOMA Model 200T sulfur analyzer equipped with an X-ray fluorescence detector. The reaction conditions were: H 2 flow rate = 10 1 (STP)/h, P = 30 bar, W H S V = 21.7 h -~, T = 623 K and 648 K for Ir and Pt supported on ASA, and T = 623 K for Ir and Pt supported on HY.

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3. Results

3.1. Activity tests

Activity data of sulfided M / H Y (M = Pt, Ir) catalysts and plain HY, expressed as total DBT conversion (at 593 K), as a function of time-on-stream are displayed in Fig. 1. It is interesting to note that blank HY displayed quite a high activity, that is, approximately 36% of total conversion of DBT was achieved in the first hour. From the figure, the total DBT conversion increases in the following order: I r / H Y > P t / H Y > HY. However, deactivation is a common problem in zeolites and calculation of deactivation rates (deactivation in this case is the decrease in total conversion per hour between the first and sixth hour of on-stream operation) indicate that deactivation is similar for both I r / H Y and P t / H Y zeolites (Fig. 1). It is quite likely that the catalysts are deactivated to a fairly large extent during the first hour of reaction before samples are collected. It may be noted that for the plain HY zeolite, an additional 10% drop in activity is observed between 3 and 6 h (Fig. 1). Activity data of sulfided It- and P t / A S A , plain ASA and P t / H Y catalysts as a function of the reaction temperature are presented in Fig. 2. In the case of ASA supported catalysts, very little deactivation of the catalyst is observed. In order to effect a proper comparison between the activities of ASA-supported and HY-supported catalysts, the activity data shown in the figure and in the discussion to follow, ensure that the time-on-stream of the catalysts are the same. The results from Fig. 2 indicate that increasing the reaction temperature produces a significant increase in the total DBT conversion on P t / A S A catalyst, a moderate increase for P t / H Y and a much lower increase for ASA and I r / A S A catalysts. Furthermore, the differences in activities between the cata-

80 _-< = 60 o

= 40

o

~

20

Time(h) Fig. 1. Dependence of the overall DBT conversion with time on-stream for ( l I ) HY, ( O ) I r / H Y and ( • ) P t / H Y catalysts. Reaction conditions were: P = 30 bar, T = 593 K and WHSV = 35 h - ~.

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275

100

80 g ~

60

t.o

~

40

"~

20

o

b-•

i

i

i

533

553

573

i

593

Temperature (K) Fig. 2. Influence of reaction temperature on the overall DBT conversion on ( v ) ASA, ( 0 ) I r / A S A , ( O ) P t / A S A and ( • ) P t / H Y catalysts. Reaction conditions were: T = 5 2 3 - 5 9 3 K, P = 30 bar and W H S V = 35 h -I"

lysts are more pronounced at higher temperatures (593 K) than at lower temperatures (523 K). At high reaction temperatures the total DBT conversion increases according to: P t / A S A > P t / H Y > ASA -- Ir/ASA. Note that at 593 K, there is a significant difference in the total DBT conversion between P t / A S A and P t / H Y (ca. 42%). However, at low temperatures (523 K), activities of both Pt catalysts are almost the same. Table 1 shows a comparison of the activity data for all the catalysts. It is clear from Table 1 that the trend in total DBT conversion on ASA and HY supported catalysts (at 593 K) is: P t / A S A > CoMo/A1203 >> I r / H Y > P t / H Y > HY > ASA = Ir/ASA. It is also interesting to note that P t / A S A catalyst is a little more active than the commercial C o M o / A I 2 0 3 catalyst in the HDS of DBT.

Table 1 Activity of M / A S A , M / H Y (M = Pt,Ir) and C o M o / A I 2 0 3 catalysts used in HDS of DBT and of diesel oil Catalyst

Total DBT conv. a (%)

HYD/HDS a

HDS conv. of diesel oil b (%)

Normalized HDS act. of DBT c

Normalized HDS act. diesel oil c

ASA Pt/ASA Ir/ASA HY Pt/HY Ir/HY CoMo/AI203

22.0 83.3 20.5 18.6 48.5 55.3 77.9

4.5 17.8 2.8 4.2 8.7 8.9 9.7

5.0 51.2 18.6 26.6 55.3 52.6 49.2

30.5 5.1 97.0 19.1 -

18.8 4.6 110.6 18.1 -

a Total DBT conversion, HYD and HDS selectivities at 593 K, P = 30 bar. b Total HDS conversion of diesel oil at 623 K, P = 30 bar, data at 4 h time-on-stream. c For M / H Y zeolites, calculation was done according to the formula [Total DBT C o n v . ] / [ M / S i atom ratio]; for M / A S A catalysts this was divided by a weighting factor [ ( S i / A 1 ) H v / ( S i / A 1 ) A s A ].

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100

(a)

,It-

m

o _ - - - - - ~

• o

i

90

(b)

I0 A ._ >

0

i

i

;

i

;

6

7

T i m e (h) Fig. 3. HDS (a) and HYD (b) selectivity for (11) HY, ( 0 ) I r / H Y and (~,) P t / H Y catalysts. Reaction conditions were: P = 30 bar, T = 593 K and WHSV = 35 h t.

Fig. 3 shows a plot of HDS and HYD selectivities of the HY supported catalysts. It is clear from Fig. 3 that both HDS and HYD selectivities in the conversion of DBT are essentially independent of time-on-stream. It is interesting to note that the tendency in HDS selectivity, HY > P t / H Y > I r / H Y , is opposite to the trend observed with total DBT conversion. This suggests that incorporation of metals such as Pt or Ir to HY zeolite leads to an increase in the hydrogenation capability of the catalysts. The influence of reaction temperature on both HDS and HYD selectivities in the conversion of DBT for the M / A S A (M = Pt, Ir) and P t / H Y catalysts are presented in Fig. 4a and 4b, respectively. In agreement with thermodynamic calculations [24,25], a decrease in hydrogenation selectivity is observed with increasing reaction temperature for all catalysts (Fig. 4b). Fig. 5 is a plot of total conversion in deep HDS of diesel fuel for all catalysts at 623 K versus time-on-stream. As can be seen, the collected data represent steady state conditions. In order to compare the activity of these catalysts, the data of a commercial CoMo/A1203 catalyst are also included. It is clear that P t / A S A , P t / H Y and CoMo/A1203 catalysts display the highest catalytic activities after 8 h time-on-stream, and the performance of both platinum catalysts is slightly better than the CoMo/A1203 catalyst. The activity of I r / H Y is a little lower than that of the CoMo catalyst, while the I r / A S A has a much lower activity compared to the CoMo catalyst, but a somewhat similar

R. Navarro et al./ Applied Catalysis A: General 137 (1996) 269-286

277

100

-~ 90 t

/

.~ 80 e-, '-~ 70 30

(b) _~ 20

N 10 )J

533

553

573

593

T e m p e r a t u r e (K) Fig. 4. Influence of temperatures on HDS (a) and HYD (b) selectivities in the HDS of DBT on ( • ) ASA, ( • ) I r / A S A , ( 0 ) P t / A S A and (11) P t / H Y catalysts. Reaction conditions were: T = 523-593 K, P = 30 bar and W H S V = 35 h - J .

activity to that of the blank HY zeolite. Note that in HDS of diesel, the I r / A S A catalyst is more active than the plain ASA carrier. The total conversion in the HDS of diesel fuel at 623 K and after 8 hours is as follows: P t / A S A > P t / H Y > C o M o / A I 2 0 3 > I r / H Y >> HY -- I r / A S A >> ASA (Fig. 5), which differs slightly from data taken after 4 h (Table 1). Generally, the results are in

60 50 e-

--¢ 40 ~" 30 0

e~ 20

<"'----

o o

3

4

5

6

? 7

N

8

T i m e (h) Fig. 5. Dependence of the overall gas oil (0.08 wt.-% S) conversion with time on-stream on ( D ) ASA, ( ~ ) Ir/ASA, ( O ) P t / A S A , (11) HY, ( A ) Ir/HY, ( 0 ) P t / H Y and ( v ) CoMo/AI203 catalysts. Reaction conditions were: P = 30 bar, T = 623 K, WHSV = 21.7 h - j .

278

R. Navarro et al./Applied Catalysis A: General 137 (1996) 269-286

8O e-

52 60 • ,~

--

At

At

e-

8 40 -~ 2 0

o

i

s

r

ii

6

7

Time (h) Fig. 6. Dependence of the overall gas oil (0.08 wt.-% S) conversion with time on-stream on (11) ASA, ( O ) I r / A S A and ( A ) P t / A S A catalysts; and at 648 K: ( D ) ASA, ( O ) Ir/ASA, (O) P t / A S A . Reaction conditions were: P = 30 bar, T = 623 and 648 K, WHSV = 21.7 h t.

agreement with the results obtained with DBT, with the exception that I r / A S A and plain ASA show differences in the HDS of diesel fuel (Fig. 5), but not in the HDS of DBT (Fig. 2). The plain HY support exhibits a higher activity than the plain ASA support, also in agreement with the results obtained with DBT. It should be pointed out that I r / H Y is generally superior to P t / H Y in the HDS of DBT, while the reverse appears to be the case in the HDS of diesel fuel (Table 1). In addition, it is clear from Fig. 5 that over the time period used in the experimental study, the deactivation rate in the HDS of diesel oil on HY supported catalysts is almost the same as the deactivation rate on the ASA supported ones. The effect of temperature on HDS of diesel is shown in Fig. 6. At both reaction temperatures, 623 and 648 K, P t / A S A is the most active catalyst. For this catalyst, an increase in the reaction temperature produces a large increase in activity. However, even at a temperature of 623 K, this catalyst is more active than both the I r / A S A catalyst and the plain ASA carrier tested in reactions at 648 K. At both reaction temperatures, the I r / A S A catalyst displays a better performance in the conversion of diesel fuel than in the conversion of DBT. P t / A S A is the most active catalyst in both reactions and total DBT conversion at 593 K is ca. 80% (Fig. 2), and in the HDS of diesel at 623 K, the conversion is more than 50% (Fig. 5, data at 4 h time-on-stream).

3.2. Infrared spectroscopy of adsorbed CO In order to explain the excellent catalytic behavior of supported Pt catalysts, FFIR of chemisorbed CO was used to identify the surface sites of catalyst subjected to various pretreatments. CO was selected as a chemical probe because it has an asymmetric charge distribution and is easily polarized;

R. Navarro et al./Applied Catalysis A: General 137 (1996) 269-286

279

C I

2300

i

2100 Wavenumber

I

i

1901 (cm ~)

Fig. 7. IR spectra of CO chemisorbed on P t / A S A catalyst: (a) outgassed, (b) reduced in hydrogen, (c) sulfide&

therefore it is sensitive to the strong electrostatic field surrounding Pt atoms and forms chemisorption bonds with metal oxides [26]. The infrared spectra of outgassed, H2-reduced and sulfided P t / A S A and P t / H Y catalysts, in the region corresponding to the C - O stretching vibration modes, are shown in Figs. 7 and 8, spectra a, b, c, respectively. The IR spectra of outgassed and reduced P t / A S A and P t / H Y samples exhibit clear differences between both catalysts as only two bands are common and they are sharp for the former catalyst and very broad for the latter. The poor resolution of the spectra of CO adsorbed on P t / H Y catalyst probably is a result of the local perturbations of CO molecule by the strong electrostatic fields within the channels of the zeolite by the environment Si-O-A1 framework. The spectra of P t / A S A catalyst show one small band at 2226 c m - l and two other overlapping bands around 2090 cm -l and 2101 cm -l. On the other hand, the P t / H Y zeolite exhibits one small band at 2221 c m - l and three other overlapping bands around 2165, 2095 and 2028 cm -~. The small bands at 2226 and 2221 cm -~ present in the P t / A S A and P t / H Y catalysts, respectively, are assigned to CO adsorbed on A I 3+ cations in both carriers [27]. The common band at 2090 cm-~ for P t / A S A (Fig. 6b) and 2095 cm-~ for P t / H Y (Fig. 7b) for outgassed and reduced samples is typical of CO linearly bonded to metallic Pt [28,29]. The presence of metallic Pt in outgassed samples is due to reduction of platinum under vacuum.

280

R. Navarro et al. /Applied Catalysis A: General 137 (1996) 269-286

.<

C

-I

2300

I

I

t

2100 1900 Wavenumber (cmb

Fig. 8. IR spectra of CO chemisorbed on Pt/HY catalyst: (a) outgassed, (b) reduced in hydrogen, (c) sulflded.

The formation of a fraction of completely reduced Pt particles was previously observed in vacuum-decomposed P t / N a X zeolites, although this band was found at lower wavenumbers (2070 cm - l ) [30]. Similarly, a strong and narrow band at 2089 c m - l was observed on reduced Pt/H-ZSM-5 zeolite [29]. It can be noted that in HY and H-ZSM-5 zeolites, the frequency is remarkably higher than in 0.3% P t / A I 2 0 3 and 0.3% P t / S i O 2 catalysts (2060 and 2070 cm -~, respectively) [31]. Tkachenko et al. [29] related this higher frequency in P t / H ZSM-5 catalyst to metal-support interaction and explained it in terms of: (i), electron deficiency of metallic particles in H-ZSM-5 zeolite, a n d / o r (ii), dipole-dipole coupling of adsorbed molecules. The broad band at 2095 c m - z of CO adsorbed on reduced and outgassed P t / H Y catalyst suggests that the vibration of C - O bond is perturbed by the zeolite lattice. The band at 2165 c m - l in P t / H Y outgassed and reduced zeolite (Fig. 8 a, b) can be attributed to CO linearly adsorbed on oxidic Pt species as Pt + or Pt/PtO. Similarly, the band at 2101 cm-~ observed in outgassed P t / A S A catalyst (Fig. 7a) can be assigned to Pt+-CO species [30]. As dicarbonyl species might be formed in the presence of Pt + and P t / P t O species, the band at 2028 cm -~ in P t / H Y zeolite might arise from tetrahedral metal-oxo or hydroxo-dicarbonyls species. A similar band at 2025 c m - 1, observed by Bischoff et al. [30] on incompletely reduced P t / N a X

R. Navarro et al./ Applied Catalysis A: General 137 (1996) 269-286

28t

Table 2 BET areas a and surface atomic ratio (XPS data) b of M / A S A and M / H Y (M = Pt, Ir) catalysts c Catalyst

BET area (m 2)

Si/AI

M/Si

S/M

S/Si

ASA Pt/ASA lr/ASA HY Pt/HY Ir/HY

389 350 347 662 682 630

0.62 0.62 0.68 1.29 1.54 1.45

0.011 0.019 0.003 0.020

0.43 0.00 3.33 1.21

0.00 0.005 0.00 0.02 0.01 0.03

a BET areas of calcined, fresh catalysts. b XPS data of spent catalysts used in HDS of DBT after reaction at 593 K. c M / A S A and M / H Y catalysts after 5 h and 6 h time-on-stream, respectively.

sample, was assigned to Pt(CO) 2 species with supercage size particles ( ' d ' ca. 1.2 nm). Finally, the spectrum of sulfided P t / H Y catalyst shows only two very small bands at 2165 and 2095 cm-~ (Fig. 8c), whereas they are absent in P t / A S A homolog (Fig. 7c). This means that the CO adsorption sites are blocked either by PtS formation or poisoned by adsorption of hydrogen sulfide [32].

4. Discussion

An outstanding procedure to compare catalytic behavior of a series of catalysts is the turnover frequency, which implies a precise knowledge of the number of metal sites involved in the reaction. In the absence of this, it is desirable to compare catalysts with similar surface areas. In this study we have used catalysts with different textural properties (see BET areas in Table 2) and quite different surface acidities. In particular the high acidity, and more specifically the Br~nsted acid groups of HY zeolite is expected to act as an electron acceptor and to decrease the electron density of the metal [29]. In order to compare the activities of the different catalysts, it was assumed that the activity of the surface layer of the catalysts is representative of the total catalyst activity. Furthermore, it was assumed that the catalyst distribution (in other words, a major fraction of the catalyst activity) is confined to a thin outer shell. Consequently, the total conversion was normalized with respect to the M / S i atomic ratio derived from XPS for each catalyst (support) (Table 2). Since the Si/A1 ratios for the two supports are different, the activities for the ASA supported catalysts were adjusted by a weighting factor [(Si/A1)Hv/(Si/A1)AS A] (Table 2), in order to have a proper means of comparison with the HY supported catalysts. Even though this methodology is based on the assumptions discussed earlier, it provides a good basis for comparison (Occam's Razor). According to this methodology, the normalized activity in the HDS of DBT (data from reaction at 593 K, Table 1) increases as follows: P t / H Y >> P t / A S A >>

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I r / H Y >> I r / A S A , and in the HDS of diesel at 623 K (Table 1) increases according to: P t / H Y >> P t / A S A > I r / H Y > Ir/ASA. It must be stated here that in order to compute the normalized activity of the catalysts in diesel fuel, the same M / S i ratios were used. Since the catalysts used in the HDS of diesel and of DBT were from the same batch, it was assumed that these ratios will be more or less the same. On the basis of the normalized activity data in both reactions, it can be seen that the trend in both diesel and in DBT are essentially the same (Table 1). It is clear that the platinum catalysts supported on ASA and on HY are much more effective than the iridium catalysts. Moreover, in HDS of diesel fuel (at 623 K), both P t / H Y and P t / A S A are a little more active than the commercial CoMo/A1203 catalyst (Fig. 5). However, the latter catalyst is more active than P t / H Y in the HDS of DBT (Table 1). As the Pt and Ir metals display very similar crystallographic properties [19], but differ in normalized activity, this precludes participation of any geometric effects on activity, and suggests that the structure sensitivity probably arises from the other factors. It is interesting to note that our normalized activity data totally agree with the observed trend for alumina-supported Pt and Ir catalysts (Pt/A1203 > Ir/A1203) [15]. Consideration of the activity profiles for HDS of DBT (Fig. 1), demonstrates that both Ir and Pt supported on HY zeolite become deactivated during an initial period lasting a few hours, and this behavior can be expected because the plain HY possesses a high acidity, and also undergoes deactivation. It must be emphasized that the rapid initial decrease in conversion is attributed mainly to deactivation of the zeolite support, whereas the gradual loss in conversion after approximately three hours is attributed to the deactivation of the active catalyst which is very difficult to evaluate over the short time periods used in this study. The main origin of catalyst deactivation seems to be the formation of coke residues on the catalyst surface as determined by XPS of the used catalysts, which showed a C l s peak of at least five times the intensity of the fresh counterparts [17]. It is generally accepted that incorporation of transition metal sulfides into acidic zeolites leads to the formation of bifunctional catalysts through combination of both cracking and hydrogenation functions. In this work, this has been confirmed by the observation of different products arising from the cracking of decalin and of DBT, in addition to typical products of HDS of DBT, namely biphenyl and cyclohexylbenzene. However, on ASA supported Ir and Pt catalysts, in addition to unconverted DBT, biphenyl and cyclohexylbenzene were the only products observed. A general observation is that in the conversion of DBT, the HDS reaction is dominant over hydrogenation (Figs. 3 and 4). Literature data indicate that hydrogenation selectivity is in general enhanced over more acidic carriers. Dhainaut et al. [15] observed that platinum on SiO2-A1203 and on montmorillonite possesses greater hydrogenation selectivity in the conversion of DBT than on P t / S i O 2 catalyst. However, from Fig. 4 is clear that the hydrogenation

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selectivity of P t / H Y zeolite is lower than that of P t / A S A catalyst. This suggests that others factors are responsible for the hydrogenation activity. As pointed out by Tanaka et al. [33], three coordinated unsaturation sites (CUS) are needed for hydrogenation; however, the total number of CUS may be different for Ir and Pt on both supports. Comparing the relative intrinsic activities, defined as the ratio of H Y D / H D S selectivities [34], Pt and Ir supported on HY show the same relative activities, but this factor is about two times lower than the more active P t / A S A catalyst, which again contrasts with the very low activity of I r / A S A homolog. This suggests that the total number of CUS sites are probably different for the two catalysts since the active sites for HYD and HDS are known to be different [34]. Other factors which could influence the HDS activity of catalysts include changes in the electronic properties of the metal as a consequence of its dispersion. Although dispersion cannot be precisely determined by XPS, the surface exposure of metals measured by this technique can provide some insight on metal distribution in porous materials. The sequence of surface metal exposure is: I r / H Y >> I r / A S A > P t / A S A > P t / H Y ( M / S i ratio, Table 2). This indicates that surface exposure of iridium on both supports is good, but this fact does not correlate with activity. This is illustrated by the high overall conversion of P t / A S A catalyst, which contrasts with the low conversion of I r / A S A catalyst (Fig. 2) (in terms of normalized activity, identical conclusions are obtained). The behavior of I r / A S A catalyst in the HDS of diesel in terms of overall conversion is quite different (Figs. 5 and 6). However, in terms of normalized activity, the performance is comparable to the HDS of DBT. Generally, conversion data of P t / H Y and I r / H Y in the HDS of DBT (Fig. 1) are in marked contradistinction to ASA supported catalysts (Fig. 2) (especially Ir/ASA), as both I r / H Y and P t / H Y appear to be active. The normalized activity of P t / H Y catalyst however, is much higher than the I r / H Y counterpart similar to the results obtained on the ASA carrier ( P t / A S A >> Ir/ASA). It has been suggested that the zeolite can modify the atomic or electronic structure of the Pt and Ir clusters, or even participate directly in the reaction mechanism. In the case of M / H Y catalysts, this latter assumption is possible as the plain HY zeolite is active in the HDS of DBT (Fig. 1). However, the origin of activity in blank HY is due to the formation of SH groups, as HY zeolite possesses a relatively high S / S i ratio (Table 2). This is the reason why the P t / H Y catalyst possesses an unusually high S / M ratio (Table 2). A similar observation was made by Vrinat et al. [ 14] who observed that the NaY zeolite possesses a higher activity than CoNaY and MoNaY catalysts, and in the latter case, the activation occurs during the first hour of on-stream operation due to the influence of Na + cations. FTIR spectra of adsorbed CO on sulfided P t / A S A and P t / H Y catalysts shed some light on the possibility of metal-support interactions on both supports (Figs. 7 and 8). Comparison of the spectra on the reduced P t / H Y and P t / A S A samples reveals the strong influence of HY zeolite framework on the

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catalytic sites as derived from the band widths. However, as no shifts in the IR bands of CO were observed, strong metal-support interactions can be ruled out. The higher activity of P t / A S A catalyst can probably be rationalized based on the hypothesis of Ledoux et al. [35]. They demonstrated that the most active T M S / C catalysts in HDS of thiophene are those based on metals which in their most stable bulk sulfided forms exhibit different coordinations with sulfur. This observation could agree with the fact that PtS phases have the ability to form higher sulfides (PtS 2) by twisting their crystalline atomic positions, or could be attributed to the reduction of PtS to lower sulfides or even to metal. Considering the concept of Sabatier, these phases could be active in reaction. However, the ability of active sulfides to form different metal sulfide coordinations in equilibrium is a necessary but not sufficient condition for high HDS activity [35]. Ledoux et al. [35] consider that the desulfurization mechanism goes through the formation of extra sulfur-metal bonds and formation of these bonds is easy for amorphous phases, less easy with crystalline bulk sulfides and most difficult with a metal. As the Ir on ASA is only in metallic form (from XPS results, Table 2), this fact could explain why this catalyst is not active. However, by changing the carrier to HY zeolite, both sulfided and metallic iridium species are obtained (Table 2), which yields a slightly better normalized activity in the HDS of DBT and in diesel. It was observed that bulk iridium sulfide is stable under pretreatment conditions ( H 2 / 1 5 % H 2 S at 673 K), but when used in the HDS of DBT at the same temperature, converted to metal plus sulfur, although the catalyst proved very effective in the reaction [13]. As in our case, the I r / A S A catalyst was almost inactive, it can be assumed that the ASA support develops strongly bonded Ir species which are less active in the HDS of DBT than Ir ° + S species detected in unsupported Ir catalyst used in the HDS of DBT

[13]. 5. Conclusions Hydrodesulfurization of DBT and diesel fuel was carried out under deep conditions on Pt and Ir catalysts supported on ASA and HY. The normalized activities of Pt/ASA and P t / H Y catalysts were superior to that of the Ir counterparts, and in all cases the HDS reaction was dominant over hydrogenation. The activity data of I r / H Y and P t / H Y catalysts in the HDS of DBT showed an initial decline during the first 3 h, then a leveling off of activity with no further decline over the remaining time span of the experiments. In both reactions, the two platinum catalysts displayed good stability, and the normalized activity of the P t / H Y catalyst was greater than the Pt/ASA catalyst. In contrast, the Ir/ASA catalyst was not effective in the HDS of DBT at temperatures of 523-593 K but showed a higher conversion in HDS of diesel fuel at a comparatively higher reaction temperature (623 K). The normalized

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activities of the two Ir catalysts were about the same order of magnitude in the HDS of both diesel and DBT.

Acknowledgements Financial support by the Commisi6n Interministerial de Ciencia y Tecnologia of Spain (CICyT) (Project AMB93-1426-CE) and by the Commission of the European Union (Project JOU2-CT93-0409) is kindly acknowledged. P.T. Vasudevan gratefully acknowledges sabbatical grant (SAB95-0084) from the Direcci6n General de Investigaci6n Cient~fica y T6cnica, Ministry of Education and Science, Spain. Thanks are also due to Dr. J.A.R. van Veen, the Netherlands, for providing us the ASA support, Pt/ASA and CoMo/AI203 catalysts.

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