Comparative Effect of Organosulfur Compounds on Catalysts for the n

672

Ind. Eng. Chem. Res. 1996, 35, 672-682

Comparative Effect of Organosulfur Compounds on Catalysts for the n-Heptane Isomerization Reaction at Medium Pressure: Mo2C-Oxygen-Modified, MoO3-Carbon-Modified, Pt/γ-Al2O3, and Pt/β-Zeolite Catalysts Andrew P. E. York, Cuong Pham-Huu, Pascal Del Gallo, Edd A. Blekkan,† and Marc J. Ledoux* Laboratoire de Chimie des Mate´ riaux Catalytiques, ECPM, Universite´ Louis Pasteur, 1 rue Blaise Pascal, 67008 Strasbourg Cedex, France

Molybdenum oxycarbide formed from oxidized Mo2C or reduced MoO3 is an active and very selective catalyst for the isomerization of n-heptane compared to supported Pt. Deactivation experiments performed on the catalysts with different concentrations of organosulfur compounds show that molybdenum oxycarbide exhibits a very high resistance to deactivation, whereas with platinum-based catalysts deactivation occurs even at low sulfur concentration in the feed. Deactivation can be slowed by increasing the hydrogen partial pressure from 6 to 20 bar. In these conditions, the molybdenum oxycarbide shows no deactivation with sulfur concentrations up to 120 ppm of S. In addition, the deactivated molybdenum catalysts can be easily regenerated by mild oxidative treatment under flowing air at atmospheric pressure and 723 K for 2 h followed by a reactivation period under the hydrogen and hydrocarbon mixture. I. Introduction In the past 20 years, a number of researchers have shown that some transition metal carbides, such as those of molybdenum (Lee et al., 1990) and tungsten (Muller and Gault, 1970; Boudart and Levy, 1973) can show catalytic behavior similar to that of the noble metals. A further development in the use of group VI metal carbides as selective catalysts for the alkane isomerization reaction was the discovery that, while clean carbide surfaces, obtained by reductive treatment after synthesis, gave predominantly hydrogenolysis products, modification by oxidation gave a material which was active and highly selective for the formation of isomers. Iglesia and co-workers (Ribeiro et al., 1991a,b; Iglesia et al., 1991, 1992) reported that oxidative treatment of a WC or β-W2C catalyst prepared by temperature-programmed carburization of WO3 under CH4/H2 mixtures gave a catalyst capable of alkane isomerization without extensive hydrogenolysis. Independent work by Ledoux and co-workers (Ledoux et al., 1993a; Pham-Huu et al., 1993) described a similar effect over a high surface area Mo2C, oxidized under flowing air at 623 K, for the isomerization of n-hexane. This was attributed to the formation of a new catalytically active phase formed during the first few hours of the hydrocarbon reaction; this phase is an oxycarbide consisting of a MoO3 lattice (at the surface of the carbide) where carbon atoms replace some oxygen atoms in the vacancies formed by partial reduction and block the collapse of the oxide to MoO2 (Delporte et al., 1995). It was proposed that over the molybdenum catalyst a metallacyclobutane mechanism was responsible for the alkane isomerization. Later work reported that the molybdenum oxycarbide phase could be synthesized directly from molybdenum metal or molybdenum oxide * To whom correspondence should be addressed. Fax: 33 88 41 68 09. E-mail: [email protected]. † Department of Industrial Chemistry, The Norwegian Institute of Technology (NTH), The University of Trondheim, N7034 Trondheim, Norway. Fax: 47 735 94157.

0888-5885/96/2635-0672$12.00/0

under a mixture of hydrogen and hydrocarbon. More recently, the same group (Ledoux et al., 1993b; Blekkan et al., 1994) has shown that the isomerization of n-heptane can be carried out without cyclization or aromatization over oxygen-modified Mo2C or carbonmodified MoO3 catalysts, even at high conversion; this is a reaction of great interest to industry since, currently, alkanes higher than hexane cannot be isomerized over the conventional platinum catalyst at high conversion, due to the efficiency of this catalyst for the cracking reactions. In addition, a major problem for the reforming catalysts based on Pt, is the presence of permanent poisons, i.e., arsenic, lead, and mercury, and reversible poisons, i.e., nitrogen, halogens, and sulfur (Chauvel et al., 1985). Sulfur poisoning is one of the most severe and commonly encountered poisoning problems, since sulfur-containing compounds are present to some extent in all crude oil. These sulfur compounds can be present in the form of thiols, dithioalkanes, and heterocyclic compounds such as thiophene and its analogs (Kao et al., 1993). Because the presence of sulfur can cause a complete loss of activity, sulfur concentrations must be kept below a few parts per million in the feed. A number of advances have been made over the years in reducing the sensitivity of the supported platinum catalysts, for example the addition of dopants such as rhenium, iridium, and tin, but deactivation is still a major problem. More detailed results were summarized in a review by Biswas et al. (Biswas et al., 1988). The aim of this article is to report the high resistance of the Mo2C-oxygen-modified and MoO3-carbon-modified catalysts, formed after an activation period (Ledoux et al., 1993a; Pham-Huu et al., 1993; Delporte et al., 1995), to different concentrations of thiophene present as impurities during the n-heptane reaction at medium pressure (6-20 bar) when compared to conventional Ptbased catalysts supported either on alumina or on β-zeolite. The influence of the nature of the sulfur source and of the total pressure on the deactivation rate are also investigated. Finally, the effect of an oxidative regeneration under mild conditions was studied over © 1996 American Chemical Society

Ind. Eng. Chem. Res., Vol. 35, No. 3, 1996 673

Figure 1. Schematic diagram of the experimental setup.

completely deactivated catalysts. Structural modifications of the catalysts during the course of the reaction were followed using powder X-ray diffraction (XRD). II. Experimental Section II.1. Materials and Catalysts. High specific surface area molybdenum carbide (Mo2C) was synthesized in a high temperature reaction between MoO3 (Fluka, purity >99.5%) vapor and a high surface area carbon (Fluka Puriss, 1150 m2 g-1, 0.250-0.425 µm) as described previously by Ledoux et al. (Ledoux et al., 1995a, 1992a,b). Before catalytic use, the carbide (60 m2 g-1) was oxidized under flowing air (15 cm3 min-1, 623 K, 14 h) in order to oxidize its surface and thus form the Mo2C-oxygen-modified material. Activation of the material was then carried out by passing a mixture of hydrogen and n-heptane over the catalyst at 623 K for around 2 h (Figure 2), leading to the active molybdenum oxycarbide phase. The unmodified Mo2C was not active and selective for the isomerization reaction (Ledoux et al., 1993) and was not tested. The high specific surface area MoO3-carbon-modified catalyst (145 m2 g-1) was synthesized from the low specific surface area MoO3 (4 m2 g-1) at low temperature (623 K) under a mixture of n-heptane and hydrogen under medium pressure according to Ledoux et al. (Ledoux et al. 1993c, 1995b) and Delporte et al. (Delporte et al., 1995). High-purity γ-alumina (Ketjen CK-300B) was used in the preparation of the Pt-supported sample. This had a BET surface area of 187 m2 g-1, a pore volume of 0.63 cm3 g-1, and impurities in weight percent of Na2O < 0.002, SO4 < 0.001, Fe < 0.004, NiO < 0.001, and MoO3 < 0.001. Before impregnation the support was immersed in a mixture of ethanol:water (50:50 v/v), kept at room temperature for 14 h, and then calcined at 673 K for 2 h. Platinum was added by incipient wetness impregnation of γ-alumina with a water solution of (NH3)4PtCl2‚H2O such that the content of Pt after impregnation was 2% by weight. The impregnated sample was dried at room temperature for 12 h in air and at 393 K for 12 h in an oven. Finally, the catalyst was calcined at 673 K for 2 h. Before the catalytic tests,

the catalyst was reduced in situ with H2 (40 cm3 min-1) at 673 K for 2 h. The Cl content was 0.8%. A typical bifunctional catalyst was also used for comparison. A β-zeolite support (650 m2 g-1) with a Si/ Al ratio of 12.5 was ion-exchanged three times with NH4NO3 (aqueous 1 M), before drying, calcining in air (813 K, 3 h) and impregnation with (NH3)4PtCl2‚H2O to give a Pt load of 1.5% using the incipient wetness technique. After impregnation the catalyst was dried and subsequently calcined in air (573 K, 3 h). Before use the catalyst was reduced in situ at 773 K for 3 h and then cooled to the reaction temperature in flowing H2 before introducing the hydrocarbon feed. The Cl content was 0.5%. The n-heptane (Prolabo) was used as received with a purity of >99.5%. The major organic impurities were methylcyclohexane (0.3%), 3-methylhexane (3MHex) (0.1%), ethylcyclopentane (ECyclPen) (0.06%), 2-methylhexane (2MHex) (0.04%), and 3-ethylpentane (3EPen) (0.04%). These were subtracted from the exit gas analysis before calculating the product distribution. Thiophene, sulfolane and 1-propanethiol were all from Fluka (puriss grade) and hydrogen was obtained from Air Liquide (U grade). II.2. Apparatus. The schematic drawing of the medium pressure micropilot used for the isomerization experiments is shown in Figure 1. Most of the tubing and valves (gray shading in Figure 1) were kept above 423 K in order to avoid any condensation. Reactions were performed in a microreactor consisting of 1/4 in. 316-stainless steel with the catalyst placed between quartz wool plugs in the center of a copper-lined steel tube (4 mm i.d. × 300 mm). The tube furnace temperature was controlled by a Minicor temperature controller and the reactor temperature was monitored using a second thermocouple placed along the side of the reactor. The hydrogen flow rate was controlled using a Brooks 5850 TR mass flow controller monitored by a Brooks 5876 control unit, and the hydrocarbon was delivered via a Gilson 302 HPLC pump. The reactant was passed downward through the catalyst bed. The reactor pressure was regulated using a Grove membrane back pressure regulator and a micrometering valve. Samples were analyzed off-line via a septum.

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Ind. Eng. Chem. Res., Vol. 35, No. 3, 1996

Figure 2. Isomerization of n-heptane over Mo2C-oxygen-modified catalyst. Conditions: 6 bar, 623 K, H2:HC ) 30.

Figure 4. Isomerization of n-heptane over Pt-1.5 wt %/β-zeolite catalyst. Conditions: 6 bar, 563 K, H2:HC ) 30.

Figure 3. Isomerization of n-heptane over MoO3-carbon-modified catalyst. Conditions: 6 bar, 623 K, H2:HC ) 30.

Figure 5. Isomerization of n-heptane over Pt-2 wt %/γ-Al2O3 catalyst. Conditions: 6 bar, 623 K, H2:HC ) 30.

II.3. Product Analysis. Product analysis was carried out on a Hewlett-Packard 5890 Series II gas chromatograph fitted with a flame ionization detector. Separation of the C1 to C7 hydrocarbons was achieved using a HP-PONA capillary column coated with methylsiloxane (50 m × 0.2 mm i.d., film thickness 0.5 mm), and the response factors for the various products were determined and taken into account. Negligible traces of alkenes were detected in the reactor exit gas, and these were therefore omitted from the final data analysis. Kinetic data are reported as reaction rates (mol g-1 s-1) calculated assuming a differential reactor (conversion < 10%) or using pseudo first order kinetics for experiments with higher conversions. The reaction was carried out over a large range of operating conditions: catalyst weight, 0.1-0.3 g; weight hourly space velocity (WHSV), 6-18 h-1; total pressure, 6-20 bar; contact time, 8 × 10-4 to 20 × 10-4 s; total flow rate, 200-700 mL min-1.

time on stream to reach a value of ca. 90% for conversions in the range 37-45% (Tables 1 and 2). Over the two catalysts, no deactivation is observed as a function of time as shown in Figures 2 and 3. The main products from the isomerization of n-heptane are 2-methylhexane (2MHex) and 3-methylhexane (3MHex). Dimethylpentane products contribute to around 13% of the C7 isomers, and only traces of cyclic molecules are detected among the reaction products (ca. 0.4%). Tables 1 and 2 give detailed product distributions over both catalysts. In both cases, the cracking products are initially mainly C3 and C4, but the C6 + C1 fraction increases with time, at the expense of C4 + C3. III.1.2. Pt-1.5 wt %/β-Zeolite. The experiment with the Pt-1.5 wt %/β-zeolite at 6 bar and 563 K presented in Figure 4 shows that the isomerization activity and selectivity are stable over the duration of the experiment. This catalyst is more active than the molybdenumbased catalysts (rate ) 150.0 × 10-7 mol g-1 s-1) and shows a similar selectivity to isomers, i.e., monobranched and dibranched molecules, of about 92% at conversions below 40%; only traces of cyclic molecules are observed. The detailed product distribution is given in Table 3. III.1.3. Pt-2 wt %/γ-Al2O3. Figure 5 shows the development of activity and selectivity over the platinum catalyst at 6 bar and 673 K. The activity and selectivity are stable over the duration of the experiment meaning that, under the reaction conditions, no other deactivation (coking or metal sintering) occurs on the catalyst. The activity is equivalent to the activity observed on Mo2C-oxygen-modified catalyst, i.e., 24.7 × 10-7 mol g-1 s-1. The detailed product distribution is

III. Results III.1. Standard n-Heptane Isomerization Without Sulfur. III.1.1. Mo2C-Oxygen-Modified and MoO3-Carbon-Modified. Figures 2 and 3 show the development of the isomerization activity and the C7 selectivity of the Mo2C-oxygen-modified and the MoO3carbon-modified catalysts as a function of time on stream at 6 bar and 623 K. Over both catalysts the activity is initially low and increases with time on stream to reach a steady state of 28.5 × 10-7 and 53.2 × 10-7 mol g-1 s-1, respectively. The selectivity to the C7 isomers is initially low but rapidly increases with

Ind. Eng. Chem. Res., Vol. 35, No. 3, 1996 675 Table 1. Isomerization of n-Heptane at 6 bar Total Pressure over MoC-Oxygen-Modified Catalyst in the Presence of Different Concentrations of Sulfur in Thiophene Form (Conditions: 623 K, Total Flow 200 cm3 min-1, H2:HC ) 30) sulfur concentration, ppm time on stream, h

0 0.5

8

0

0 26

30 0

30 6

30 24

120 0

120 8

120 26

conversion, % rate, 10-7 mol g-1 s-1 C7 selectivity, % ∑light products, % C7-isomer distribution, % dimethylpentanes 2MHex 3MHex 3EPen ECycPen MCyHex + toluene light products distribution, % C 6 + C1 C5 + C2 C4 + C3 others

15.6 14.2 83 17

37.8 28.8 92 8

36.6 28.5 90 10

39.8 32.0 87 13

39.0 31.6 89 11

39.7 32.0 88 12

50.9 34.3 85 15

40.2 30.3 87 13

21.9 19.1 85 15

14.6 40.4 41.4 3.0 0.0 0.6

11.4 39.8 45.1 3.1 0.0 0.6

10.5 39.4 46.4 3.3 0.0 0.4

10.6 39.4 46.2 3.4 0.0 0.4

11.7 38.8 45.7 3.4 0.0 0.4

11.5 38.2 46.3 3.6 0.0 0.4

12.4 37.9 45.7 3.6 0 0.4

10.6 38.2 47.1 3.6 0.1 0.4

7.7 38.5 49.6 3.7 0.2 0.3

19.4 11.4 61.7 7.5

30.0 16.8 47.2 6.0

37.7 20.0 34.4 7.9

40.7 21.6 30.6 7.1

39.2 19.2 34.0 7.6

43.5 20.4 29.2 6.9

43.1 21.2 27.9 7.8

45.7 21.1 26.5 6.7

46.1 22.6 23.8 7.5

Table 2. Isomerization of n-Heptane at 6 bar Total Pressure over MoO3-Carbon-Modified Catalyst in the Presence of Different Concentrations of Sulfur in Thiophene Form (Conditions: 623 K, Total Flow 200 cm3 min-1, H2:HC ) 30) sulfur concentration, ppm time on stream, h

0


2

0 10

0 25

30 0

30 8

30 34

120 0

120 5

120 30

500 0

500 8

500 27

26.5 37.3 93 7

43.0 53.1 93 7

43.2 53.2 94 6

55.0 57.6 94 6

55.4 58.0 94 6

56.8 58.2 94 6

59.3 60.2 88 12

55.2 59.1 87 13

49.8 56.9 86 14

44.3 53.8 88 12

34.2 27.4 88 12

10.6 10.0 86 14

13.3 41.4 42.4 2.9 0.0 0.0

13.5 39.2 44.1 3.2 0.0 0.0

13.4 39.1 44.2 3.2 0.0 0.0

16.0 38.9 41.9 3.0 0.0 0.2

16.2 38.7 41.8 3.1 0.0 0.2

16.4 38.6 41.7 3.1 0.0 0.2

19.4 36.7 40.0 3.1 0 0.8

19.1 37.3 40.0 3.1 0.1 0.4

19.5 37.0 40.1 3.0 0.0 0.4

17.6 37.4 41.2 3.0 0.0 0.8

16.6 38.2 41.4 2.9 0.0 0.9

17.0 37.6 41.0 3.1 0.0 1.3

20.1 11.1 66.8 2.0

24.9 14.0 58.4 2.7

25.4 16.0 56.3 2.3

11.4 9.3 75.0 4.3

12.3 9.3 74.2 4.2

12.0 9.5 73.2 5.3

8.7 5.1 82.3 3.9

6.6 5.3 83.5 4.6

10.0 8.7 77.6 3.7

7.7 6.9 81.9 3.5

6.0 6.0 85.8 2.2

5.0 6.6 86.4 2.0

Table 3. Isomerization of n-Heptane at 6 bar Total Pressure over Pt-1.5 wt %/β-Zeolite Catalyst in the Presence of Different Concentrations of Sulfur in Thiophene Form (Conditions: 563 K, Total Flow 200 cm3 min-1, H2:HC ) 30) sulfur concentration, ppm time on stream, h conversion, % rate, 10-7 mol g-1 s-1 C7 selectivity, % ∑light products, % C7-isomer distribution, % dimethylpentanes 2MHex 3MHex 3EPen ECycPen MCyHex + toluene light products distribution, % C 6 + C1 C5 + C2 C4 + C3 others

3

0

0 10

0 22

0

30

30 7.5

30 25

120 0

120 7

120 24

500 0

500 7

500 24

45.0 162.4 91 9

41.0 152.0 92 8

40.5 150.0 92 8

38.3 147.0 97 3

33.2 133.1 93 7

32.6 131.1 93 7

43.4 159.6 95 5

23.1 100.3 81 19

21.8 95.3 80 20

44.9 162.7 98 2

12.8 59.0 64 36

11.1 51.8 59 41

15.5 40.5 40.8 2.7 0.0 0.5

14.9 41.0 40.8 2.7 0.0 0.6

14.8 41.2 40.9 2.6 0.0 0.5

12.1 40.9 43.5 3.1 0.0 0.4

13.5 42.8 40.8 2.5 0.0 0.4

13.4 42.6 41.1 2.5 0.0 0.4

16.2 38.7 41.4 3.0 0.0 0.7

15.2 42.1 39.8 2.5 0.0 0.4

15.1 42.0 39.5 2.5 0.0 0.9

13.7 40.0 42.7 3.1 0.0 0.5

13.4 44.2 37.7 2.2 0.0 2.5

13.8 44.5 37.3 2.4 0.0 2

5.8 0.8 91.6 1.8

6.0 0.6 91.4 2.0

6.5 0.5 91 2.0

8.4 0.0 87.7 3.9

8.0 0.0 87.3 4.7

7.6 0.0 88.3 4.2

0.0 0.5 98.2 1.3

0.0 0.3 97.6 2.1

0.0 0.6 97.3 2.1

0.0 1.1 98.2 0.7

0.0 0.6 97.9 1.5

0.0 1.0 98 1.0

given in Table 4. The C7-isomer selectivity observed over this platinum catalyst is rather low (60%) compared to the one observed on molybdenum-based catalysts (85-95%). Along with the monobranched molecules a large fraction of cyclic molecules are observed (25%). The fraction of light products remains stable over the duration of the experiment. III.2. Effect of Thiophene Concentration. Pure n-heptane was passed over the catalysts under the appropriate conditions until the steady state was reached. Then n-heptane containing 30, 120, or 500 ppm of S under the thiophene form was introduced in place of pure n-heptane. The activity of the catalysts was

followed with time on stream, and the deactivation rate expressed as relative rate: ratewith sulfur/ratewithout sulfur, with “rate without sulfur” corresponding to the rate measured at time “zero” in the presence of sulfur. Sometimes (see Tables 1-4) this value is higher (or lower) than the rate at steady state without sulfur because the catalyst was removed for characterization or was regenerated in between. III.2.1. Mo2C-Oxygen-Modified Catalyst. Figure 6 shows the relative rate over the Mo2C-oxygen-modified catalyst as a function of the thiophene concentration with time on stream. The catalyst remains stable up to 30 ppm of S in the feed during more than 25 h of

676


Table 4. Isomerization of n-Heptane at 6 bar Total Pressure over Pt-2 wt %/γ-Al2O3 Catalyst in the Presence of Different Concentrations of Sulfur in Thiophene Form (Conditions: 673 K, Total Flow 200 cm3 min-1, H2:HC ) 30) sulfur concentration, ppm time on stream, h conversion, % rate, 10-7 mol g-1 s-1 C7 selectivity, % ∑light products, % C7-isomer distribution, % dimethylpentanes 2MHex 3MHex 3EPen ECycPen MCyHex + toluene light products distribution, % C 6 + C1 C5 + C2 C4 + C3 others

2

0

0 24

0 48

30 0

30 6

30 26

120 0

120 4,5

120 22

19.7 17.4 58. 42

28.7 23.8 52 48

30.0 24.7 59 41

30.5 24.9 58 42

12.9 11.9 87 13

13.8 12.7 89 11

26.3 22.2 84 16

7.6 7.2 91 9

8.2 7.7 93 7

10.6 22.5 40.0 1.2 0.7 25.0

9.3 23.5 39.8 1.4 0.3 25.7

9.9 22.3 38.7 1.4 0.3 27.4

9.8 22.4 38.9 1.4 0.3 27.2

6.5 31.3 53.6 0.8 0.2 7.6

4.0 34.7 54.1 0.7 0.5 6.0

13.0 33.1 43.6 1.3 5.0 4.0

8.0 30.3 57.0 0.7 3.0 1.0

7.6 30.3 56.9 0.9 3.3 1.0

26.0 21.6 48.0 4.4

21.5 21.8 53.5 2.1

24.3 22.7 50.1 2.9

23.6 22.4 50.5 3.5

21.8 20.5 51.3 6.4

24.0 21.8 49.8 4.4

26.0 21.5 46.9 5.6

44.0 16.0 38.3 1.7

49.8 11.0 36.4 2.8

Figure 6. Effect of sulfur concentrations on isomerization of n-heptane over Mo2C-oxygen-modified catalyst. Conditions: 6 bar, 623 K, H2:HC ) 30.

reaction. However, when the concentration of sulfur is increased to 120 ppm deactivation occurs and the catalyst loses about half its activity after 25 h of reaction. The deactivation observed here is not accompanied by a loss of C7-isomer selectivity (see Table 1) meaning that deactivation is only due to the loss of active sites and not to a modification of the nature of the active phase. Deactivation observed at 120 ppm of S could be due to the formation of some MoS2 on the catalyst surface. Catalytic tests performed over MoS2 (not shown) under similar reaction conditions show that MoS2 exhibits no isomerization activity. Detailed product distributions in the presence of different concentrations of S are given in Table 1. III.2.2. MoO3-Carbon-Modified Catalyst. The effect of sulfur on MoO3-carbon-modified catalyst at 6 bar is presented in Figure 7. As can be seen, the catalyst exhibits a very high thio resistance and almost no deactivation occurs even in the presence of 120 ppm of S in the feed. The isomer selectivity remains unchanged at around 90%. The product distribution as a function of time on stream and as a function of sulfur concentration is given in Table 2. III.2.3. Pt-1.5 wt %/β-Zeolite. The deactivation occurs whatever the sulfur concentration as shown in Figure 8. However, after a rapid deactivation the catalyst isomerization activity reaches a steady state and remains unchanged within the duration of the experiment. The detailed product distribution is given

Figure 7. Effect of sulfur concentrations on isomerization of n-heptane over MoO3-carbon-modified catalyst. Conditions: 6 bar, 623 K, H2:HC ) 30.

Figure 8. Effect of sulfur concentrations on isomerization of n-heptane over Pt-1.5 wt %/β-zeolite catalyst. Conditions: 6 bar, 563 K, H2:HC ) 30.

in Table 3. At low sulfur concentration (e30 ppm of S) the total isomer selectivity remains stable around 9293%. At high sulfur concentration both the isomerization activity and selectivity decrease meaning that the

Ind. Eng. Chem. Res., Vol. 35, No. 3, 1996 677 Table 5. Isomerization of n-Heptane at 20 bar Total Pressure over MoO3-Carbon-Modified and Pt-1.5 wt %/β-Zeolite Catalysts in the Presence of Different Concentrations of Sulfur in Thiophene Form (Conditions for MoO3-Carbon-Modified Catalyst: 623 K; For Pt-1.5 wt %/β-Zeolite Catalyst: 573 K, Total Flow 660 cm3 min-1, H2:HC ) 99)a MoO3 carbon modified 120b ppm

Figure 9. Effect of sulfur concentrations on isomerization of n-heptane over Pt-2 wt %/γ-Al2O3 catalyst. Conditions: 6 bar, 623 K, H2:HC ) 30.

number and the nature of the active sites are changed as a function of deactivation. III.2.4. Pt-2 wt %/γ-Al2O3. The experiment with the Pt-2 wt %/γ-Al2O3 catalyst at 6 bar, in the presence of different concentrations of thiophene (Figure 9), shows an important deactivation phenomenon even at 30 ppm of S. At 30 ppm of S the catalyst remains stable during the first 3 h of reaction and then a rapid deactivation occurs whereas at higher concentrations of sulfur the deactivation starts after 0.25 h on stream. The catalyst rapidly reaches a steady state, 50% for 30 ppm and 30% for 120 ppm of the original rate, and almost no additional deactivation occurs as shown in Figure 9. Table 4 reports the product distribution as a function of the deactivation. It is interesting to note that a large improvment in C7 selectivity is observed in the presence of sulfur, from ∼60% to ∼90%. This phenomenon is well-known and explains why, in many industrial processes, Pt catalysts are sulfided before extensive use. III.3. Effect of Total Pressure. Results obtained at 20 bar total pressure in the presence of g120 ppm of S over the MoO3-carbon-modified catalyst and the two platinum-based catalysts as a function of time on stream are presented in Figure 10. Table 5 reports the product distribution over the MoO3-carbon-modified and the Pt1.5 wt %/β-zeolite catalysts only after 24 h on stream in the presence of different concentrations of sulfur compounds in the feed at 20 bar total pressure. The Mo2C-oxygen-modified catalyst was not tested because it has been shown that this catalyst is not stable at 20 bar (Blekkan et al., 1994). For comparison, the results already shown at 6 bar (Figures 7-9) are repeated. For MoO3-carbon-modified catalyst, no deactivation is observed at 120 ppm of S at high pressure, 20 bar. Increasing the concentration of sulfur from 120 to 500 ppm led to a loss of about 50% of the initial rate of the catalyst (Figure 10a). For Pt-1.5%/β-zeolite (Figure 10b) the effect of the pressure is also very positive, but contrary to what is observed on the Mo-based catalyst, 120 ppm of S whatever the pressure considerably poisons the catalyst. For the Pt-2 wt %/γ-Al2O3 catalyst (Figure 10c), only the deactivation at 120 ppm of S is reported due to the very fast deactivation observed at 500 ppm. Over all the catalysts the decrease in rate due to deactivation is slower when increasing the total pressure. However, over the Pt-2 wt %/γ-Al2O3 catalyst


500 ppm

Pt-1.5 wt %/β-zeolite 120 ppm

500 ppm

43.4 53.3 94 6

23.3 33.7 94 6

31.3 127.9 88 12

25.6 109.1 75 25

13.9 36.5 44.4 3.5 0.0 1.7

12.9 37.2 45.7 3.2 0.0 1.0

15.3 41.0 40.8 2.8 0.0 0.1

14.6 41.8 40.3 2.5 0.0 0.8

34.4 15.9 48.0 1.7

30.0 7.4 56.9 5.7

0.0 0.0 98.2 1.8

0.0 0.2 97.8 2.0

a The results are given after 24 h under time on stream. b Sulfur concentration.

after about 25 h of reaction, almost 60% of the initial activity has been lost meaning that the platinum supported on alumina catalyst is less resistant compared to the molybdenum-based and the Pt-1.5 wt %/βzeolite catalysts under similar conditions and with higher sulfur concentration (500 instead of 120 ppm of S). It should be noted that the two best catalysts (MoO3carbon-modified and Pt-1.5 wt %/β-zeolite) maintain 60% of their initial activity after 30 h under 500 ppm of sulfur at 20 bar. This means that the new catalyst based on molybdenum oxycarbide is at least as good as the platinum catalyst on this point. III.4. Effect of Sulfur Source. Because a large variety of sulfur compounds can be present in industrial feedstocks, variations of the poisoning effect as a function of the sulfur compound type have also been investigated over the molybdenum-based catalysts in order to check the performance of this new material. Figures 11 and 12 show the poisoning effects of these compounds at a constant 120 ppm sulfur level on Mo2Coxygen-modified and MoO3-carbon-modified catalysts, respectively, at 6 bar. It can be seen that the MoO3carbon-modified catalyst exhibits higher resistance than Mo2C-oxygen-modified catalyst for all the tested sulfur molecules (Pt-based catalysts were not tested according to the nature of the sulfur compounds). On Mo2Coxygen-modified catalyst, the worst poison is thiophene. III.5. Regeneration. Regeneration has been carried out for the MoO3-carbon-modified and platinum-based catalysts. Regeneration of the catalyst is very important not only in the development of practical methods to restore catalyst activity, but also to obtain fundamental knowledge about the cause of catalyst deactivation. Since the MoO3-carbon-modified and the Pt-1.5 wt %/β-zeolite catalysts exhibit a very high thio resistance, the deactivation performed at high concentration of thiophene was used in order to obtain a more pronounced effect between the deactivated catalyst and the regenerated one. Therefore, the deactivated catalyst after reaction in the presence of 500 ppm of thiophene was submitted to a different regeneration treatment in

678


a

b

Figure 11. Effect of sulfur source on deactivation rate for n-heptane isomerization over Mo2C-oxygen-modified catalyst. Conditions: 623 K, H2:HC ) 30, 120 ppm of S.

c Figure 12. Effect of sulfur source on deactivation rate for n-heptane isomerization over MoO3-carbon-modified catalyst. Conditions: 623 K, H2:HC ) 30, 120 ppm of S.

Figure 10. (a) Effect of total pressure on deactivation rate for n-heptane isomerization over MoO3-carbon-modified catalyst. Conditions: 623 K, H2:HC ) 30. (b) Effect of total pressure on deactivation rate for n-heptane isomerization over Pt-1.5 wt %/βzeolite catalyst. Conditions: 563 K, H2:HC ) 30. (c) Effect of total pressure on deactivation rate for n-heptane isomerization over Pt-2 wt %/γ-Al2O3 catalyst. Conditions: 623 K, H2:HC ) 30.

order to recover the initial activity. Over the Pt-2 wt %/γ-Al2O3 catalyst, the deactivation was performed in the presence of only 120 ppm of S due to the low thio resistance of the catalyst. Mo2C-oxygen-modified catalyst was not tested according to regeneration.

Figure 13 reports the effect of regeneration of the deactivated catalysts after reaction under air flow at 723 K for 2 h at atmospheric pressure. Over MoO3-carbonmodified catalyst, oxidative treatment followed by an activation period allows the complete recovery of the isomerization activity (Figure 13a). The XRD patterns of MoO3-carbon-modified catalyst are presented in Figure 14. After deactivation under 500 ppm of S (Figure 14B), the diagram shows the diffraction lines of MoO2, MoS2, and Mo3S4; the reflections due to the oxycarbide phase have almost disappeared. After an oxidative regeneration under air flowing at atmospheric pressure and 723 K for 2 h, the XRD pattern (not shown) mostly shows diffraction lines corresponding to MoO3 and no traces of other phases are detected. Activation applied to the regenerated catalyst allows the complete recovery of the isomerization activity and the XRD pattern (Figure 14C) is similar to the one obtained on the fresh catalyst after activation (Figure 14A). Whereas oxidative regeneration allows the almost complete recovery of the isomerization activity of the molybdenum-based catalysts, only 85% of the initial isomerization activity is recovered, after oxidative treat-


a

b

Figure 14. XRD patterns of (A) MoO3-carbon-modified catalyst after activation under pure n-heptane and hydrogen at 20 bar, (B) after reaction in the presence of 500 ppm of S, and (C) after an oxidative regeneration followed by an activation under pure n-heptane and hydrogen.

c

regeneration leading to less dispersed catalysts. The XRD analysis was not performed over the platinumbased catalysts due to the high dispersion of the metal. IV. Discussion

Figure 13. Effect of oxidative regeneration for n-heptane isomerization over (a) MoO3-carbon-modified catalyst and (b) Pt-2 wt %/γAl2O3 at conditions of 623 K, H2:HC ) 30; (c) Pt-1.5 wt %/β-zeolite at conditions of 563 K, H2:HC ) 30.

ment followed by a reduction under hydrogen at 773 K for 3 h, over the platinum catalysts (Figure 13b,c) meaning either that over platinum some sulfur is irreversibly adsorbed according to the results reported in the literature or that sintering occurs during the

The high thioresistance observed on the Mo-based catalysts can be attributed to the presence of molybdenum oxycarbide phase, which mitigates sulfidation of the catalyst. The molybdenum oxycarbide, as explained by Delporte et al. (1995), is formed by reduction of MoO3 via crystallographic shear and lattice contraction where some oxygen atoms are replaced by carbon atoms which stabilize a stable intermediate and stop the process of reduction; similarly the first step of the sulfidation of MoO3 by H2S (Wang et al., 1982) corresponds to the formation of an oxysulfide (Mo4+OxSy) (De Jong et al., 1993). It has been shown that pre-reduction of MoO3 into MoO2 renders the formation of MoS2 very difficult. Similarly Spevack and McIntyre (Spevack et al., 1993) during their investigation of the sulfidation of molybdenum oxide by H2S has reported that MoO2 is more difficult to sulfide even after 2 h of sulfidation. In the present study where almost no Mo(VI) is present (Delporte et al., 1995), the sulfidation of the catalyst is very difficult because of the reduced character of the catalyst, i.e., Mo4+OxCy, Mo5+Ox′Cy′, and MoO2. This hypothesis is verified by performing the activation

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Table 6. Isomerization of n-Heptane at 6 bar Total Pressure over MoO3-Carbon-Modified Catalyst and MoO3-Carbon-Modified Catalyst with and without Sulfur in an Activation Feed at Different Times on Stream (Conditions: 623 K, Total Flow 200 cm3 min-1, H2:HC ) 30) activation under pure n-C7 followed addn of 120 ppm of S 0.5a h conversion, % rate, 10-7 mol g-1 s-1 C7 selectivity, % ∑light products, % C7-isomer distribution, % dimethylpentanes 2MHex 3MHex 3EPen ECycPen MCyHex + toluene light products distribution, % C 6 + C1 C5 + C2 C4 + C3 others a

3.5 h

7h

activated directly under n-C7 and 120 ppm of S 1h

4h

59.3 60.2 88 12

53.2 58.4 88 12

56.3 59.5 86 14

49.6 56.8 86 14

25 h

7.1 11.3 82 18

6.1 24.3 79 21

16.6 25.0 74 26

10 h

14.3 21.8 72 28

24 h

19.4 36.7 40.0 3.1 0.0 0.8

18.8 37.5 40.0 2.9 0.0 0.8

19.5 37.0 39.9 2.9 0.0 0.7

19.6 36.9 39.8 2.9 0.0 0.8

16.2 38.5 40.4 2.9 0.0 2.0

17.7 37.8 40.7 2.9 0.0 0.9

19.4 36.8 39.7 3.1 0.0 1.0

20.1 36.1 39.1 3.1 0.0 1.6

8.7 5.1 82.3 3.9

7.6 4.7 83.9 3.6

6.6 5.3 84.4 3.7

11.1 8.7 78.8 1.4

4.1 8.5 80.8 6.6

6.3 7.3 81.4 5.0

4.5 5.1 84.2 6.2

3.8 4.4 85.5 6.3

Time on stream.

a

b

Figure 16. XRD patterns of MoO3-carbon-modified catalyts (A) after direct activation in the presence of 120 ppm of S at 6 bar and (B) after activation under pure n-heptane and hydrogen followed by reaction in the presence of 120 ppm of S.

Figure 15. Effect of nature of feed during activation period for n-heptane isomerization over MoO3-carbon-modified catalyst: (a) isomerization activity; (b) isomerization selectivity. Conditions: 623 K, H2:HC ) 30.

treatment with feed containing 120 ppm of S instead of pure hydrocarbon. According to the results presented in Figure 15 and Table 6, a lower isomerization activity is obtained when compared to the activity obtained after an activation period under pure n-heptane feed, under otherwise similar conditions. At the beginning of the reaction, sulfur and carbon are in competition to diffuse and replace oxygen, leading to the formation of both molybdenum oxycarbide (Mo4+OxCy) and oxysulfide or sulfide which diminishes the overall activity of the catalyst for isomerization. The XRD pattern of the

catalyst after 24 h of reaction is presented in Figure 16A and reveals the presence of small amounts of MoS2 and Mo3S4 along with diffraction lines corresponding to MoO2 and oxycarbide, while the catalyst activated under the pure hydrocarbon and hydrogen mixture, followed by reaction in the presence of 120 ppm of S (Figure 16b), shows no evidence of the formation of MoS2 or Mo3S4. However, the XRD technique is not a very appropriate analytical tool to investigate such a transformation. New experiments are in progress using X-ray photoelectron spectroscopy (XPS) in order to provide a better description of the nature of the catalyst surface. At high concentrations of sulfur, deactivation occurs on both Mo2C-oxygen-modified and MoO3-carbon-modified catalysts. However, the first one is more affected than the second. On Mo2C-oxygen-modified catalyst the


thin layer of oxycarbide, probably highly dispersed, is much more sensitive to sulfidation than the bulky structure obtained on MoO3-carbon-modified catalyst. This is equivalent to what was observed on metal catalysts where big particles are more sulfur tolerant than small ones (Apesteguia and Barbier, 1982). On Pt-based catalysts the strong decrease in isomerization activity during the first hours is attributed to the suppression of adsorption sites on the Pt surface by formation of surface sulfide. It has been reported by Wang et al. (1982) that sulfidation begins upon exposure to H2S at room temperature on Pt. As shown in Table 4, the decrease in isomerization rate over Pt-2 wt %/γAl2O3 catalyst, whatever the sulfur concentration, is accompanied by an increase of isomerization selectivity as opposed to Mo-based catalysts where isomerization selectivity remains stable as a function of deactivation. Similar results on Pt-2 wt %/γ-Al2O3 have already been reported by several authors in the literature (Maurel et al., 1975; Biloen et al., 1980) and would be due to a size decrease of the active hydrogenolysis ensembles on the catalyst surface, as well as to the blocking of the highly coordinated atoms considered as active for hydrogenolysis. Similarly Dess and Ponec (1989) have observed that modification of the Pt-Ir/γ-Al2O3 catalyst by treatment with thiophene/hydrogen greatly changes the activity and selectivity of the catalyst: almost complete suppression of hydrogenolysis while isomerization increased to a large extent. Actually, this behavior is used by the industrial reformers to improve the selectivity of the industrial catalysts; before running the reforming process, the catalyst is presulfided. As a function of time, the catalysts reaches a steady state whatever the concentration of sulfur as shown in Figures 8 and 9. This result could be due to the equilibrium relationship between the rate of adsorption and the rate of desorption of sulfur species (S + H2 f H2S) at the reaction conditions used here (high H2/ hydrocarbon ratio and medium total pressure). The deactivation observed for all the catalysts is attributed to the formation of H2S by hydrogenolysis of thiophene which can alter the active phase by formation of sulfides. A similar result was observed by Dhainaut et al. (Dhainaut et al., 1982) during dibenzothiophene hydrodesulfurization over supported platinum catalysts where specific activity decreased as the H2S pressure increased. Deactivation is attributed to the decrease of accessible Pt sites by formation of a surface sulfide according to the balanced reaction M + H2S f MS + H2 (where M ) Pt, Mo). Because of the equilibrium relationship, one would expect a greater amount of free surface at high pressures, and thus a lower toxicity effect on reaction rate. Merbott (1954) reported that a high level of sulfur (50-100 ppm) could be used at 20 bar, while high toxicity at pressures of 7-10 bar was achieved with 5 ppm of sulfur in the feed. This is indeed the case: increasing hydrogen partial pressure leads to a decrease in the deactivation rate by sulfur for all the catalysts (Figure 10). Almost no deactivation is observed over molybdenum-based catalysts even in the presence of 120 ppm of S at 20 bar while isomerization activity loss is observed at 120 ppm of S over both the catalysts at 6 bar. MoO3-carbon-modified catalyst shows more thio resistance, and under 500 ppm of S the loss of isomerization activity is only 45% after 25 h of reaction. Oxidative treatment followed by an activation period under the hydrocarbon and hydrogen mixture allows the

complete recovery of the deactivated molybdenum-based catalysts (Figure 13a). It is expected that sulfur poisons present on the catalysts are oxidized during regeneration treatment, leaving the active sites free for the isomerization reaction. The activity and isomer selectivity of MoO3-carbon-modified catalyst were completely recovered after multiple regeneration cycles (three at least). Over the Pt-2 wt %/γ-Al2O3 and Pt-1.5 wt %/βzeolite catalysts, regeneration is not sufficient to allow the complete recovery of the initial activity. In the case of platinum supported on alumina it has been shown that at 773 K under H2 only a fraction of the adsorbed sulfur is rapidly desorbed (Apesteguia et al., 1981), whereas over an alumina support without platinum, the adsorption of H2S is wholly reversible, under otherwise similar conditions. The amount of “irreversible” sulfur is attributed to that in strong interaction with the metal resulting in the incomplete recovery of the isomerization activity over the platinum-based catalysts. V. Conclusion Molybdenum-based catalysts formed either from Mo2C or MoO3 exhibited a very high thio resistance even at a sulfur concentration level of 120 ppm of S. The reduced state of molybdenum (oxycarbide) present in these catalysts is proposed to explain the observed result. As the structure of the oxycarbide comes from the collapse of the layers of the starting MoO3 during an activation period due to carbon incorporation, this results in the formation of a structure into which S diffusion occurs less easily and results in a high thio resistance where the oxycarbide is compared to MoO3. At higher concentrations of sulfur (>120 ppm of, S), sulfidation occurs, resulting in a loss of isomerization activity. Nevertheless, deactivated catalysts are easily regenerated under mild conditions, whereas over the platinum catalyst regeneration, the recovery of only 85% of the initial isomerization activity is possible. In addition, deactivation observed over molybdenum-based catalysts can be lowered or suppressed by increasing the total pressure of the reaction from 6 to 20 bar. This is attributed to the removal of adsorbed sulfur by hydrogen, freeing the active sites. Among the sulfur compounds tested, thiophene is by far the most virulent poison source. However, it is important to note that the experiments carried out in this work involved a high concentration of sulfur in the feed, while in industrial conditions these concentrations are lower, i.e., e10-30 ppm of S. Thus, under real conditions the deactivation of the molybdenum-based catalysts due to sulfur should be negligible. In addition, these new oxycarbide phases allow isomerization of n-alkanes containing more than six carbon atoms with high selectivity (90%) even at high total conversion (70-80%), which is desirable for the enhancement of the octane number of fuel. A further use for isomerization over these materials would be for decreasing the freezing point of gas-oil and lubricant oil fractions (dewaxing) by conversion of the straightchain hydrocarbons into the methyl isomers without cyclic molecules, in line with the new legislation for restriction of aromatic compounds. Acknowledgment This work was supported by the Pećhiney Company. A.P.E.Y. wishes to thank the Royal Society for an ESEP

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Received for review July 5, 1995 Revised manuscript received October 30, 1995 Accepted November 14, 1995X IE950409A

X Abstract published in Advance ACS Abstracts, January 15, 1996.

Comparative Effect of Organosulfur Compounds on Catalysts for the n

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