C sulfide catalysts prepared by slurry impregnation with molybdic acid

~

AP PA LE IY D C AT L SS I A: GENERAL

ELSEVIER

Applied CatalysisA: General 138 (1996) 13-26

Effect of loading on hydrodesulfurization activity of Mo/A1203 and Mo/C sulfide catalysts prepared by slurry impregnation with molybdic acid E. I-Iillerovfi, M. Zdra~il * Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, Rozvojovd 135, 165 02 Prague 6 - Suchdol, Czech Republic

Received 28 November 1994; revised22 September 1995; accepted 25 September 1995

Abstract The MoO3/alumina and Mo/active carbon catalysts of various loadings were prepared by the new slurry impregnation method (SIM). The slurry of MoO 3 in water was added to the support. The pH of the slurry was 2.6 and this was favourable for the adsorption of molybdate anions on alumina and active carbon with the point of zero charge 7.0 and 7.5, respectively. The low solubility of MoO 3 was sufficient to transport molybdenum species from solid MoO 3 to the adsorbed phase. The equilibrium was achieved after 5 weeks at 30-50°C. The calcination was not necessary and was left out. This was especially advantageous for active carbon support which is susceptible to oxidative damage. The maximum loading achieved was 18 and 31 wt.-% MoO 3 for Al203 (195 m 2 g-~) and active carbon (1100 m 2 g-~), respectively. The activity was tested in the hydrodesulfurization of thiophene at 1.6 MPa and 250-400°C. The activity of the SIM samples was similar to the activity of the catalysts prepared by the conventional impregnation method (CIM) with ammonium heptamolybdate. The dependence of the activity on loading for the SIM samples was the same as reported in the literature for CIM catalysts. On alumina the activity per unit of molybdena was constant in the whole range of the loading 0-18 wt.-% MoO 3. On active carbon it gradually decreased in the Whole range of the loading from 0 to 31 wt.-% MoO 3. It is concluded that the nature of the SIM catalysts is the same as of the CIM catalysts but the SIM possesses several advantages. Keywords: Molybdenumsulfide catalyst; Aluminasupported molybdena;Carbon supported molybdena;Slurry impregnation;Hydrodesulfurization

* Corresponding author. Fax. (+ 42-2) 342073, e-mail [email protected]. 0926-860X/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0926-860X(95)00241-3

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E. Hillerov6, M. Zdra~il /Applied Catalysis A: General 138 (1996) 13-26

l. Introduction

Molybdenum based supported catalysts in oxidic, reduced, or sulfidic forms possess activity for many reactions. The common precursor of these catalysts is supported M o O 3.

The hydrotreating over sulfide catalysts belongs to the most important applications of molybdenum catalysts. The usual support of the industrial catalysts is alumina. However, carbon supported sulfide catalysts are also intensively studied recently because they exhibit interesting catalytic properties

[1,2]. The most widely used preparation of the molybdenum catalysts is aqueous impregnation of the support with ammonium heptamolybdate [3-5]. However, a number of alternative methods have been described: adsorbing MoO2(OH) 2 from the gas phase [6], gas phase adsorption of Mo(CO) 6 [7], thermal spreading of M o O 3 [4,8], adsorption of metal complexes [9], thermal spreading of MoS 2 [10], impregnation with molybdenum sulfur cluster compounds [11], homogeneous precipitation of MoS 2 on alumina [12], etc. We have recently described a new procedure named slurry impregnation method with the acronym SIM [13,14]. Alumina or active carbon support is mixed with a slurry of required amount of M o O 3 in water. After some time (depending on the temperature and the size of the support particles) all powder of the solid M o O 3 disappears. The low solubility of M o O 3 is sufficient to transport molybdenum species from solid M o O 3 via the solution to the supported phase. All deposited species are adsorbed. The precipitation of molybdena species in pores during drying is negligible because the concentration of the solution is very low at the end of the impregnation. Another advantage of the SIM method over the conventional impregnation with ammonium heptamolybdate concerns calcination. Calcination is applied in conventional impregnation to decompose ammonium heptamolybdate and to remove ammonia from the catalyst. In the slurry impregnation method the calcination is not necessary because no other chemicals beside M o O 3 and water are used. Evolution of nitrogenous gases formed from ammonium heptamolybdate is thus avoided. Carbon supported molybdenum catalysts cannot be calcined in air because the carrier is susceptible to oxidative damage. That is the reason why the carbon supported ammonium heptamolybdate is often decomposed during in situ sulfidation in the catalytic reactor (e.g. [15-20]). The disadvantage is that ammonium sulfide is released which can cause plugging problems in downstream parts of the apparatus. With the SIM samples, calcination is left out and only water is produced during in situ sulfidation. The previous papers described the SIM preparation of MoO3/A1203 [13] and M o O 3 / C [14] catalysts with usual loadings of about 15 wt.-% M o O 3. The deposition of M o O 3 was achieved by refluxing the mixture for several hours and

E. Hillerov6, M. Zdra~il / Applied Catalysis A: General 138 (1996) 13-26

15

the pH of the slurry was not measured. The purpose of the present paper was to test milder conditions (five weeks at 30-50°C), to determine the maximum achievable loading of alumina and active carbon, and to investigate pH changes of the slurry during impregnation. The activity was tested in the hydrodesulfurization of thiophene. The dependence of the activity on loading of the catalysts prepared by SIM using MoO 3 was compared with the dependence reported in literature for the catalysts prepared by the conventional impregnation using (NH4)6MOTO24.

2. Experimental

2.1. Supports Alumina (A1) T-A1203 Norton 6173 (Norton Chemical Process Products, UK) had BET area of 195 m 2 g - t and pore volume 0.59 cm 3 g - l (mercury porosimetry). The extrudates were crushed to the particle size of 0.16-0.32 mm. Active carbon (C) GA-1 (Slovensk6 lu~obn6 z~vody, Slovak Republic) had BET surface area of 1100 m 2 g- 1, volume of micropores 0.35 m 3 g - 1 (nitrogen adsorption) and pore volume 0.73 cm 3 g - l (mercury porosimetry). The extrudates were crushed to the particle size of 0.16-0.32 mm. They were intensively washed with hot distilled water (50 1 of water per 100 g of carbon) and dried before use.

2.2. Catalysts The catalysts are summarized in Tables 1 and 2. The acronyms SIM and CIM designate the slurry impregnation method using M o O 3 and the conventional Table 1 Composition and hydrodesulfurization activity of alumina supported catalysts Catalyst

Content of MoO 3 (wt.-%) a

Relative activity b k ( j ) / k ( r )

Nominal

Actual

2 4 8 15 24 35

2.0 3.9 8.0 13.5 14.8 18.1

0.05 0.35 0.96 1.50 1.54 1.92

Slurry impregnation with MoO 3 SIM-2MoO 3/AI SIM-4MoO 3/AI SIM-8MoO 3/AI SIM- 15MoO 3/AI SIM-24MoO 3/AI SIM-35MoO 3/AI

Conventional impregnation with (NH4 )6 Mo7024 CIM- 10.3MOO3 / A !

10.3

10.3

1.00

Commercial catalyst BASF M-8-30

-

15.0

1.10

a I00.[ M o O 3 / ( M O O 3 +

carrier)].

b k is apparent first-order rate constant, reference catalyst r is CIM-10.3MoO 3/AI.

16

E. H illerovr, M. Zdra~il / Applied Catalysis A: General 138 (1996) 13-26

Table 2 Composition and hydrodesulfurization activity of carbon supported catalysts Catalyst

Content of MoO 3 (wt.-%) a

Relative activity bk(j)/k(r)

Nominal

Actual

2 4 8 15 24 35

1.9 3.9 7.9 14.7 23.5 31.2

O.10 0.95 2.23 3.08 4.30 4,88

12.4

2.80

Slurry impregnation with MoOz SIM-2MoO 3 / C SIM-4MoO 3 / C SIM-8MoO 3 / C SIM- 15MoO3 / C SIM-24MoO 3 / C SIM-35MoO 3 / C

Conventional impregnation with (NH4 )6 M°7 024 CIM- 12.4M003/C

12.4

a 100' [MoO3/(MOO 3 + carrier)]. b k is apparent first-order rate constant, reference catalyst is CIM-10.3MoO 3/AI from Table 1.

impregnation method using ammonium heptamolybdate, respectively. The catalysts are named accordingly: SIM-15MoO3/AI is the catalyst with the nominal content 15 wt.-% MoO 3 and 85 wt.-% A1203, and prepared by the slurry impregnation method. The nominal content of MoO 3 corresponds to the amount of M o O 3 or M o O 3 in the form o f (NHn)6Mo7024 used in the preparation of the catalyst. In the preparation of CIM catalysts and low- and medium-loaded SIM catalysts, all (NHa)6MOTO24 or M o O 3 used in the preparation was in fact deposited. In the preparation of SIM catalysts with the highest loading, part of the M o O 3 w a s not deposited and remained in the form of slurry; the achieved loading was lower than the nominal loading. The procedure used to determine the deposited amount of MoO 3 is described henceforward and the actually achieved loadings are shown in Tables 1 and 2.

2.2.1. CIM-IO.3MoO 3 / A l The solution of (NH4)6MOTO24 " 4H20 (1.41 g) in water (10 ml) was mixed with alumina support (10 g) and it was left standing overnight at room temperature. It was dried in a rotary vacuum evaporator and calcined in a flow of air. The temperature was linearly increased to 400°C during 1 h and then kept at 400°C for 1 h. 2.2.2. CIM-12.4MoO 3 / C The solution of ( N H 4 ) 6 M o 7 0 2 4 " 4H20 (1.73 g) in water (10 ml) was mixed with the active carbon (10 g). After standing at 95°C for 1 h it was dried in a rotary vacuum evaporator. The catalyst was not calcined. 2.2.3. SIM catalysts M o O 3 (Fluka, p.a.) was ground in an agate mortar. The mixture of the appropriate amount of MoO 3, 20 ml of water and 1 g of support were prepared

E. Hillerov6, M. Zdra[il/Applied Catalysis A: General 138 (1996) 13-26

17

in a set of flasks. For instance, the flask of the SIM-24MoO3/C sample contained 0.316 g MoO 3, 1 g of carbon and 20 ml of water. The closed flasks were left standing for 5 weeks at 30-50°C (on the grate above the central heating radiators) with occasional shaking about five times per day. The pH was measured with a combined glass electrode at room temperature. The fine powder of MoO 3 disappeared after five weeks; the originally turbid liquid surrounding support particles became almost clear. However, with the samples of the highest nominal content of MoO 3, saturation of the support was reached, a part of MoO 3 was not deposited and remained as fine powder in the turbid liquid. The support was separated from the clear or turbid liquid by decantation. It was dried in a rotary vacuum evaporator and was not calcined. The amount of MoO 3 in the remaining clear or turbid liquid in each flask was determined. Concentrated NH 4OH (1 ml) was added and it was slowly heated to 100°C and left standing for 30 min to dissolve M o O 3. The solution was filtered and molybdenum was determined by atomic absorption. The commercial catalyst M o O 3 / A 1 2 0 3, BASF M 8-30 containing 15% M o O 3 was also included for the comparison of activity. It was ground to particles of 0.16-0.32 mm.

2.3. Catalytic activity The model reaction was hydrodesulfurization of thiophene (TH). Integral conversions were obtained in a fixed bed flow reactor (I.D. 2.5 mm). The pressure was 1.6 MPa, the initial partial pressure of TH 40 kPa, the feed rate F(TH) 0.6 mmol h - l , and the hydrogen flow 1.5 mol h - 1. The catalyst charge W was 0.01 g and it was diluted with 0.03 g of inert low surface area a-alumina (8 m 2 g-1). The catalyst was in situ presulfided with H 2 S / H 2 mixture (1:10) at atmospheric pressure. The temperature was linearly increased in 1 h to 400°C and kept at this value for 2 h. The feed was introduced at the pressure of 1.6 MPa and the conversion of TH, x(TH), was determined at several temperatures from 400 to 240°C, changing the temperature in steps of 20°C. The conversion was defined as x ( T H ) = ( n ° ( T H ) - n(TH))/n°(TH), where n o and n are the initial and final number of moles, respectively. The conversions were in the range 0-0.85, depending on the catalyst and temperature. The test lasted about 9 h and no deactivation of the catalysts was observed. The reproducibility of the activity in terms of relative rate constants (see below) was better than + 10%.

3. Results and discussion

3.1. Position of SIM among other impregnations As for the conventional impregnation from the solutions of ammonium heptamolybdate, the molybdenum species adsorbed on the surface during im-

18

E. Hillerov6, M. Zdra~il/ Applied Catalysis A: General 138 (1996) 13-26

Table 3 Preparation of supported molybdena catalysts by equilibrium adsorption from a solution of ammonium heptamolybdate (AHM) at low pH Support

"y-Al203 3~-A1~O3 y-A1203 Active carbon Carbon black Carbon black 7-A1203

Surface area (m 2 g- t)

AHM solution Concentration (g MoO 3/100 ml H20)

Catalyst loading (wt.-% MoO 3)

Ref.

pH

192 256 213 800 200 200 170

0.7 4.9 0.8 1.5 0.15 0.15 0.79

2.1 2.0 2.0 2.3 2.6 4.8 4.5

13.2 13.7 17.8 24.3 2.1 4.3 9.4

[22] [26] [6] [27] [ 17] [ 19] [29]

pregnation are generally considered better dispersed and more important for activity than the molybdenum species precipitated in the pores during drying. Various impregnation methods (incipient wetness, with excess of the solution, etc.) lead to different ratio adsorbed/precipitated species. The highest portion of adsorbed species is achieved by equilibrium adsorption with filtration of the excess solution [4,21 ]. At a solution pH below the point of zero charge (PZC) of the support, the surface is positively charged and attracts molybdate anions from the impregnation solution of ammonium heptamolybdate (adsorption on alumina [6,22-26], adsorption on active carbon [17,27]). The PZC of aluminas and active carbons usually falls into the range 4-9. The natural pH of the ammonium molybdate solution is about 5.5 [28]. The solution has to be acidified (usually with nitric acid) to adjust the pH to the range 2-3 favourable for the adsorption. However, the concentrated ammonium heptamolybdate solutions precipitate when acidified [28], and precipitation can also be initiated by the addition of alumina [24]. The diluted solutions only have to be used for the equilibrium adsorption impregnation from solutions of ammonium heptamolybdate at low pH. This is illustrated in Table 3 showing low concentrations chosen by previous authors. High loadings were reported in Table 3 even when the solution concentrations were low. This demonstrates the high affinity of molybdenum anions to the surface at low pH. Molybdenum(VI) oxide is dissolved to molybdic acid with pK l 3.6 and pK 2 4.1 at 25°C [30]. The solubility of M o O 3 is 0.21 g per 100 ml of water at 25°C [31]. The pH of the slurry at 25°C calculated from the saturated concentration and pK values is 2.6. This value has also been found experimentally (see below). The important advantage of the SIM over CIM is that the low natural pH of the molybdic acid saturated solution falls into the pH range 2-3 favourable for the adsorption of molybdate anions. The SIM mixture does not contain ammonium cations and no additive is necessary to adjust the low pH.

E. Hillerov6, M. Zdra~il / Applied Catalysis A: General 138 (1996) 13-26

19

Molybdenum(VI) oxide has not been considered to be a suitable impregnation compound in the literature because of its low solubility. Its solubility is 0.13 and 1.08 g MoO 3 per 100 ml of water at 20 and 60°C, respectively [31], while the values for ammonium heptamolybdate (expressed as MoO 3) are 40 and 125 g, respectively [28]. However, the low solubility of MoO 3 is only an illusory disadvantage. The concentrations of ammonium heptamolybdate solutions used in papers in Table 3 are similar to the solubility of MoO 3. It is shown in the present work that high loadings of MoO 3, comparable or even higher than those in Table 3, are achieved in SIM in the absence of ammonia (introduced in CIM as component of ammonium heptamolybdate) and added acid (introduced in CIM to adjust pH).

3.2. Molybdenum uptake Table 1 shows that the highest actual loading of alumina achieved with SIM was 18.1% MoO 3. This corresponds to one molecule MoO 3 occupying 0.21 nm 2 of the surface. The area of one molecule of MoO 3 at the surface of solid MoO 3 calculated from its density by the method of Emmet and Teller is 0.154 nm 2 [32]; this value defines the 'geometrical monolayer'. Previous authors concluded from various types of experiments that alumina is saturated with molybdena, or covered with 'saturation monolayer' (or with 'uppermost dispersion capacity monolayer'), at somewhat higher ratio of 0.17-0.25 nm2/MoO3 [6,33-37]. The above SIM value 0.21 nm2/MoO3 falls into this range and this suggests that the 'saturation monolayer' is achieved with SIM. It is seen in Table 2 that the highest actual loading of active carbon achieved with SIM was 31.2% M o O 3. Using the ratio 0.21 nm2/MoO3, observed in the present work with alumina, the surface of the active carbon available to adsorption of M o O 3 w a s calculated to be 380 m 2 g - ~. It is almost twice higher than the area of typical industrial aluminas with an area of 200-250 m 2 g-1. Other parts of the active carbon surface do not adsorb MoO 3 for either geometrical (microporosity) of chemical reasons (chemical heterogeneity of the surface). However, the saturated SIM loading observed in the present work was higher than the highest loading reported in the literature for the conventional adsorption from ammonium heptamolybdate solution which was 27.3 and 24.3% MoO 3 for active carbons of 1100 and 800 m 2 g-~, respectively [27].

3.3. pH of the slurry The pH of the slurries for the nominal contents 2, 4, 8, 15, 24, and 35% MoO 3 were measured in blank experiments in the absence of the support, and the results are shown in Fig. 1. The values obtained after 48 h were 2.5-2.6 in good agreement with the value 2.6 calculated from pK 1, pK 2, and the solubility of molybdic acid. These values also correspond to the PZC 2.9 reported for

20

E. Hillerov~, M. Zdra~il / Applied Catalysis A: General 138 (1996) 13-26 1

I

'"1

I

I

I

.,~

pH

I

0

I

0.2

i

0,4

0,6 w (Mo0s) ' g

Fig. I. Dependence of pH on composition of the slurry and time; the experimental points correspond to nominal composition of catalysts 2, 4, 8, 15, 24 and 35 wt.-% MOO3, respectively. (Q) slurry w(MoO 3) + 20 ml H20, (®) slurry w(MoO3)+20 ml H20+ I g A]203, (O) slurry w(MoO3)+ 20 ml H20+ I g C; (A) after 1 h, (B) after 48 h, (C) 5 weeks after the preparation of the slurry.

MoO 3 in literature [37]. The slightly higher pH observed for the lowest nominal loading of 2% M o O 3 w a s caused by too small an amount of M o O 3 which was lower than required for the saturated solution. The data for blank experiments in Fig. 1 also illustrate that the dissolution of M o O 3 is slow at the temperature used, 30-50°C. 1 h after the preparation of the MoO3-water slurries, the pH was higher than the equilibrium value observed after 48 h. The rate of the dissolution is proportional to the surface area of solid M o O 3 present in the flask. This is the reason why the non-equilibrium values of pH measured after 1 h increased with decreasing amount of M o O 3 in the flask. The pH of the MoO3-water-support slurries was measured 48 h and 5 weeks after their preparation and they are shown in Fig. 1. The equilibrium values after 5 weeks are determined by a combination of three factors. (i) The concentration of molybdic acid solution: the pH decreases from 7.0 to 2.6 as the concentration increases from zero to saturation. (ii) The ratio of solid M o O 3 to A1203 or C in the slurry: the pH of the slurry of M o O 3 alone is 2.6 and that of the present alumina and carbon 7.0 and 7.5, respectively; the pH of the mixtures is intermediate. (iii) The surface coverage of alumina or carbon with MOO3: the PZC of the supported MoO3/A1203 samples prepared by thermal spreading decreased from PZC of alumina at zero loading to PZC of M o O 3 at 'saturation monolayer' loading [37]; the same trend was observed for MoO3/A1203 prepared by wet impregnation from ammonium heptamolybdate solution [38]. The data o n MoO3//C are not available in the literature but a similar behaviour should be expected.

E. Hillerov6, M. Zdra~il /Applied Catalysis A: General 138 (1996) 13-26

21

In the mixtures with the lowest nominal loading of 2% MoOa/A120 3 and 2% MoO3//C, all MoO 3 was dissolved and adsorbed, and the molybdic acid concentration in the solution was negligible. The surface loading was much lower than the 'saturation monolayer'. The pH was high and close to that of the slurry water-support alone, which was 7.0 and 7.5 for the present alumina and active carbon, respectively. In the mixtures with the highest nominal loading of 35% MoO3//A120 3 and 35% MoO3//C, the support surface was covered by the 'saturation monolayer' and the solution was saturated with molybdic acid. The pH was low and close to that of the water-MoO 3 slurry alone, which was 2.6. In the mixtures with the medium nominal loadings 4, 8, 15, and 24% MoOa/A1203 and / C , the situation was intermediate between the two above extremes. As the nominal loading increased, the support was more covered with molybdena and not all molybdic acid was adsorbed; the equilibrium pH decreased. Fig. 1 also shows the pH values after 48 h when the slurries were far from equilibrium. These pH were similar to equilibrium values for MoO3//Csamples but higher than the equilibrium values for MoO3/A1203 catalysts. The interpretation is difficult because of the complexity of the system. One possible reason might be the different kinetic situation. The rate determining step for A1203 seems to be the adsorption of anions; the molybdic acid solution is more concentrated (and acidic) after 48 h than in the equilibrium after five weeks. The rate determining step for carbon seems to be the dissolution of MOO3;the molybdic acid solution is far from saturation after 48 h and does not lower the pH of the slurry.

3.4. Hydrodesulfurizationactivity The catalysts were tested under fixed pressure of hydrogen, initial concentration of thiophene and space time W/F(TH). The obtained dependencies of the conversion of thiophene on temperature are illustrated by examples shown in Fig. 2. The relative activities of the catalysts were evaluated by the procedure used already in our previous papers dealing with other catalysts [20,39]. It was checked previously that under fixed conditions of our test the dependence x(TH) =f[W/F(TH)] follows in the range of x(TH) from 0 to 0.90 a pseudo-first order rate equation with the apparent rate constant k. All catalysts were tested at the same fixed space time W/F(TH) and at several temperatures. Eq. (1) can be written for each temperature ( j ) In[1 - x(TH,r)] ln[1 - x ( T H , j ) ] -- kk(r)

(1)

where j and r denote the j-th and the reference catalyst, respectively. The

22

E. Hillerov&, M. Zdra[il / Applied Catalysis A: General 138 (1996) 13-26 1.0

I

I

I

280

32O

36O

xCTH)

0.5

0

260

z~)0

t, °C Fig. 2. Dependence of thiophene conversion on temperature, ( ~ ) SIM-2MoO3/C, ( ~ ) SIM-4MoO3/AI, ((])) CIM-10.3MoO 3/AI, (®) SIM-15MoO3/AI, (©) S1M-15MoO3/C, (circle with double triangle) SIM-35 MoO 3 / C .

sample CIM-10.3MoO3/AI was chosen as the reference catalyst. Straight lines were obtained by plotting the conversions x(TH) at various temperatures according to Eq. (1). These linear correlations indicated that the apparent activation energies of the samples tested were the same. The ratios k(j)/k(r) were obtained from the slopes of the lines. They are independent of temperature and characterize the relative activity of catalysts in the whole range of temperatures measured. The values of them are summarized in Tables 1 and 2 and they are plotted against the catalyst loading in Fig. 3. The carbon supported samples were more active than alumina supported catalysts which is in agreement with the well known behaviour of Mo, C o - M o and N i - M o sulfides prepared by the CIM using ammonium heptamolybdate [1,2,39,40]. The dependence of activity o n M o O 3 loading of SIM catalysts was compared with literature data on hydrodesulfurization of thiophene over catalysts prepared by other methods. For this purpose each set of data for the catalysts prepared and tested by the same method was normalized in the following way. The data were plotted in coordinates of activity against loading. The smooth curve was drawn by hand through the experimental points and the activity at the loading of 10% MoO 3 was read out from the curve. The experimental activities at various loadings were then normalized to this activity: relative activity at a given loading was defined as experimentally observed activity divided by activity at the loading of 10% M o O 3. The comparison of alumina supported catalysts is shown in Fig. 4. Thomas et al. [41] prepared catalysts by the incipient wetness method or by drying the suspension of the support with volumetric excess of ammonium heptamolybdate

E. Hillerov~, M. Zdra~il /Applied Catalysis A: General 138 (1996) 13-26

23

kci) /

k(r)3I / 1

2 ~ ~ ~ 1 0

®

I "

10

2O

3O wt.% MoO3

Fig. 3. Dependence of relative hydrodesulfurization activity on actual support loading, (O) SIM catalystson alumina, (O) CIM-10.3MoO 3/Al, (circle with double triangle) BASF M-8-30; ( 0 ) SIM catalysts on carbon, ( ~ ) CIM-12.4MoO 3/C.

solution; surface area of the alumina was 213 m 2 g-~. An almost linear increase of activity was observed up to a loading of about 20%. Another set of catalysts was prepared by Thomas et al. [41] by equilibrium

I

I

10

20

I

2 kCreO 1

0

30 40 wt.°/oMoO3

Fig. 4. Relative rate constant of hydrodesulfurization of thiophene as function of loading of MoO 3/AI203 catalysts; each set of data is normalized to activity at 10% MOO3, (O) Ref. [41], wet and dry impregnation with ammonium heptamolybdate, ( ~ ) Ref. [41], equilibrium adsorption of ammonium heptamolybdate, ( 0 ) Ref. [42], ammonium heptamolybdate deposited in three portions with intermediate drying, (®) present work, slurry impregnation with MoO 3.

24

E. Hillerov6, M. Zdra[il /Applied Catalysis A: General 138 (1996) 13-26 I

1

I

2 k (ret)

I

10

I

20

I

30

40

wt.% Mo0~ Fig. 5. Relative rate constant of hydrodesulfurization of thiophene as function of loading of MoO 3 / C catalysts; each set of data is normalized to activity at 10% MOO3, ( O ) Ref. [16], impregnation with ammonium heptamolybdate, ( ® ) present work, slurry impregnation with MoO 3.

adsorption from a solution of ammonium heptamolybdate where the loading was varied by changing the pH of the solution from 6 to 1. A slight deviation from linearity is observed for this set of catalysts in Fig. 4. A similarly small deviation from linearity was also observed in data of the present work. However, a different behaviour was observed in plotting the data by Pratt et al. [42]. A strong deviation from linearity is seen in Fig. 4 already at relatively low loading of about 10-15%, with the support surface area of 240 m 2 g - l . This indicates poor dispersion of the active phase which was probably caused by the unusual preparation method: the ammonium heptamolybdate was deposited in three portions by repeated pore filling impregnation with intermediate drying. The comparison of carbon supported catalysts is shown in Fig. 5. Duchet et al. [16] prepared the catalysts by pore filling impregnation with ammonium heptamolybdate of the active carbon with a BET area of 1000 m 2 g - t. It is seen in Fig. 5 that the dependence obtained in the present work with SIM catalysts was exactly the same as observed by Duchet et al. with conventional catalysts. Observations in Figs. 4 and 5 suggest that the character and dispersion of the active phase obtained in SIM is the same as obtained by impregnation from ammonium heptamolybdate solution. With alumina of a surface area of about 200-250 m 2 g - l, effective utilization of MoO 3 can be achieved up to a loading of about 20%: the activity per unit of MoO 3 is almost constant up to this loading. With active carbon of surface area of about 1000 m 2 g-1, the activity per unit of MoO 3 is slightly decreasing already from low loadings below 10% but the degree of the molybdenum utilization is still reasonable up to high loading of about 30%.

E. Hillerovdt, M. Zdra~il / Applied Catalysis A: General 138 (1996) 13-26

25

4. Conclusions Molybdena catalysts supported on alumina or active carbon were prepared by the new version of the equilibrium adsorption impregnation denominated slurry impregnation method: slurry of molybdenum(VI) oxide in water was used instead of commonly used solution of ammonium heptamolybdate. The highest loading achieved with alumina was 18 wt.-% MoO 3 which corresponded to monolayer formation. Active carbon was saturated at the loading of 31 wt.-% M o O 3. The hydrodesulfurization activity of the catalysts increased with loading in the same way as reported in the literature for catalysts prepared with the conventional impregnation with ammonium heptamolybdate. The activity per unit of molybdena was almost constant up to the highest achieved loading of alumina. In the case of active carbon the activity per unit of molybdena slowly decreased in the whole range of the loading from zero to maximum. With the same support and molybdena loading the activity of the slurry impregnation and the conventional impregnation catalysts was the same. However, the slurry impregnation is advantageous: it is clean because no foreign ions are added, calcination can be left out which is especially important for active carbon support, all deposited species are adsorbed, precipitation of molybdena species does not occur during drying, and molybdenum waste solutions are not produced.

Acknowledgements The authors thank to Dr. O. Solcov~i for technical assistance in surface area measurements and to companies Norton Chemical Process Products (UK) and Slovensk6 luEobn6 z~ivody (Slovak Republic) for samples of supports. The financial support by Grant Agency of the Academy of Sciences of the Czech Republic is gratefully acknowledged (grant No. 472101).

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