I n d . E n g . Chem. Res. 1995,34, 1107-1113
1107
Reactions of n-Heptane and Methylcyclopentane over an Oxygen-ModifiedMolybdenum Carbide Catalyst. Study of Coke Formation, Catalyst Deactivation, and Regeneration Cuong Pham-Huu,Andrew P. E. York, Mohamed Benaissa, Pascal Del Gallo, and Marc J. Ledoux* Laboratoire de Chimie des Matiriaux Catalytiques, EHZCS, Uniuersith Louis Pasteur, 1 rue Blaise Pascal, 67000 Strasbourg, France
After a n oxidative treatment, molybdenum carbide is an active and very selective catalyst for the isomerization of n-heptane. A n important parameter in the choice of a catalyst for application in industry is its resistance to deactivation by coke formation, which can be caused by the presence of cyclic molecules. In this work, a study of the effect of methylcyclopentane (MCP) on the deactivation of the oxygen-modified molybdenum carbide catalyst has been performed. It has been found that the catalyst deactivates in the presence of pure MCP and that a lower activity for heptane isomerization is obtained after the reaction under MCP. TEM showed that coke formation is the cause of the deactivation. It has also been shown that increasing the total pressure when the MCP is reacted leads to a large decrease in the catalyst deactivation and that heptane isomerization can be carried out over this deactivated sample with only a small loss in activity. Further, the catalyst can be totally regenerated in air.
Introduction In the petroleum refining industry, catalytic reforming transforms low-octane number naphtha to higheroctane motor fuel. Two reactions are desirable: isomerization of linear to branched alkanes and aromatization, although strict regulations are currently being employed t o regulate the addition of aromatics to fuel. Metalbased catalysts, especially platinum, are used widely to catalyze such reactions. However, one of the major problems with the traditional platinum-based (monoor bimetallic) catalysts is the deactivation caused by sulfur and nitrogen compounds and by coke formation via the condensation of highly dehydrogenated cyclic or polyolefinic compounds formed in the reaction or present in the industrial feed. Recently, it has been shown that transition metal carbides treated by an oxidation process give an active and selective catalyst for the isomerization of alkanes. On WC and P-W2C, Iglesia and co-workers (Iglesia et al., 1991;Ribeiro et al., 1991a,b)reported that oxidation gave a surface capable of alkane isomerization via a bifunctional mechanism involving a heterogeneous surface containing both tungsten carbide and tungsten oxide. Ledoux and co-workers (Ledoux et al., 1993a; Pham-Huu et al., 1993) reported a similar effect with high specific surface area Mo2C oxidized at 623 K for the isomerization of CS hydrocarbons, but in this case the isomerization activity was attributed to the formation of a new, catalytically active phase formed during the first hours of the hydrocarbon reaction, possibly an oxycarbide consisting of a layer of Moo3 where carbon from the gas phase replaces some oxygen vacancies created in the course of the activation period. More recently extensive use of physical techniques such as high-resolution transmission electron microscopy (HRTEM), XRD, XPS, etc..., has shown that this active phase is an oxycarbide where carbon atoms fill oxygen vacancies formed during the reduction process and stabilize an intermediate structure between Moo3 and MoO2, called MoO,C, (Delporte et al., in press). The
* To whom correspondence should be addressed.
MozC-oxygen-modified material was also shown to be an active and very selective catalyst for n-heptane isomerization (Ledoux et al., 1993b; Blekkan et al., 1994), a reaction which cannot be carried out at high conversion over platinum catalysts due to the extensive cracking of the C7+ species. This has been identified as a challenge in catalysis for cleaner fuels, as nonaromatic octane enhancement of petrol becomes more important (Maxwell and Naber, 1992). It has already been shown that the molybdenum carbide-oxygen-modified catalyst is also deactivated by the presence of cyclic compounds (especiallythe Cg ring) at atmospheric pressure, and this is attributed to the fast formation of carbonaceous residues (Pham-Huu et al., 1993). The aim of this article is t o show the behavior of the MozC-oxygen-modified catalyst for n-heptane isomerization before and afier reaction with methylcyclopentane and also the effect of the total pressure on the deactivation rate of the catalyst. In addition to the catalytic experiments, transmission electron microscopy (TEM) coupled with energy dispersive X-ray microanalysis has been used in order to characterize the nature of the fresh and coked catalysts. Oxidative regeneration is also performed on the deactivated catalyst since easy regeneration is an important parameter for industrial use.
Experimental Section Materials and Catalysts, n-Heptane was obtained from Prolabo and used as received with a purity of 99.3%, and methylcyclopentane was from Fluka with a purity of 299%. HZwas obtained from Air Liquide and of grade U quality. Mo&-Oxygen-Modified. The high-surface area molybdenum carbide (Mo2C)was synthesized by a hightemperature reaction between Moo3 vapor and active charcoal as described before (Ledoux et al., 1989). After synthesis the material had a surface area of 125 m2g-l (BET, liq Nz), and the apparent bulk density was 770 kg m-3, close t o the value reported by Oyama (759 kg m-3> for Mo2C synthesized by the temperature-programmed reaction method (Oyama, 1981). From the
0888-588519512634-1107$09.0OlO 0 1995 American Chemical Society
1108 Ind. Eng. Chem. Res., Vol. 34, No. 4, 1995 line broadening of the XRD carbide peaks, the mean crystallite size was calculated, and the actual carbide surface area was estimated to be at least 40 m2 g-l, the remainder being surface area attributed to unreacted charcoal. However, this estimate of the actual carbide surface area is conservative, since spherical particles are assumed, whereas SEM analysis indicates that the unreacted charcoal forms the core of the carbide particles, which have a very rugged appearance. Before use, the carbide was treated with air a t 623 K for 14 h. The oxidized catalysts developed their catalytic activity and selectivity over a period of several hours under reaction conditions. Detailed characterization of these materials after various activation treatments have already been reported (Ledoux et al., 1993a). Catalyst Testing Apparatus. Isomerization of nheptane was performed at elevated pressures (2001700 kPa) in a reactor unit described in detail elsewhere (Ledoux and Djellouli, 1989). Briefly, the reactor consists of a copper-lined stainless steel tube with an i d . of 4 mm. The liquid feed was dosed using an HPLC pump (Gilson model 302), and injected into the H2 stream (regulated by a Brooks 5850 TR mass flow controller linked to a Brooks 5876 control unit), and subsequently vaporized a t the top of the reactor. The catalyst was placed between quartz wool plugs in the center of the reactor allowing preheating of the gases. The reactor was heated in a vertical electric furnace, controlled by a Minicor series 41 temperature controller, and the reactor wall temperature was monitored by a separate thermocouple. The reactor pressure was regulated by a Grove membrane regulator, and samples were analyzed off-line. The reaction made at normal pressure were performed in an all-glass greaseless micropilote already described many times (Pham-Huu et al., 1993). Experimental Procedure. The catalyst (after oxidation) was flushed with H2 (10 mL min-l) at room temperature and the temperature ramped to 623 K (10K min-l). When the reaction temperature was attained, the hydrogen was replaced by a mixture of n-heptane and hydrogen and the catalyst activity followed with time (see tables for more details). Deactivation tests were performed, at steady state, by replacing the n-heptane with methylcyclopentane (MCP). The catalyst behavior was followed by gas chromatography, and when the catalyst reached a new steady state, the MCP was replaced and n-heptane reintroduced. The extent of the catalyst deactivation was then monitored. Analysis and Calculations. The products were analyzed by GC-FID, using an HP-PONA capillary column coated with methyl siloxane (50 m x 0.2 mm id., film thickness 0.5 mm), allowing the efficient separation of hydrocarbons from C1 t o C,. Reference data and pure component injections were used to identify the major peaks; some minor C7 peaks, including traces of alkenes in the reactor exit gas, were not identified and omitted from the data analysis. Response factors for all the products and reactants were determined and have been taken into account in the calculations. The conversion and the product distribution were calculated from the GC analysis of the exit gas. Kinetic data are reported as reaction rates (mol g-l s-l), calculated assuming a differential reactor (conversion < 10%)or using pseudo-first-order kinetics for experiments with higher conversions. Selectivities are re-
I { -
40000
Y
!2 &
15000 10000
v)
5000
loo
/
i
0 1
C7
-Sel.
1
I
I
f
I
0
ported on the basis of the number of carbon atoms in the reactant hydrocarbon. Catalyst Characterization. Transmission electron microscopy was performed on a Topcon EM200B microscope operating at 200 kV. To prevent artifacts due to contamination, no solvents were used at any stage. Samples were prepared by grinding the catalysts between glass plates and bringing the powder into contact with a holey carbon-coated copper grid.
Results and Discussion n-Heptane Isomerization Activity. The behaviour of the Ma&-oxygen-modified catalyst for the n-heptane isomerization reaction has already been published (Blekkan et al., 1994), and a short summary of the results obtained is presented below. The development of the activity and selectivity for the isomerization of n-heptane on the MozC-oxygen-modified catalyst at 6 bar is presented in Figure 1. At the beginning of the reaction no activity is observed, meaning that the upper layers of the Moo3 are not active for catalysis of the isomerization reaction. The activity begins to increase after a few hours under the hydrocarbon and hydrogen mixture to reach the steady state after about 7 h. Table 1 gives a detailed product distribution taken during the early part of the activation period and after the steady state was established. The main C7 isomers formed are 2-methylhexane (2MHex) and 3-methylhexane (3MHex), in a ratio, at steady state, of 0.850.90. Other isomers are 3-ethylpentane and dimethylpentanes, contributing around 3.0% and 10.5-11.5% to the C7 distribution, respectively. Almost no cyclic products are observed, thus excluding the possible contribution of a cyclic mechanism over this material (Gault, 1981). Table 1 also shows that the amount of the Cq + C3 fraction in the cracked products continuously decreases as a function of the reaction duration, while the amount of the CS C1 and C g C2 increases. An earlier publication concerning the isomerization of n-hexane over the MonC-oxygen-modifiedcatalyst (Pham-Huu et al., 1993) provided evidence that the isomerization is performed via a non-bifunctional mechanism; the metallocyclobutane mechanism was proposed. Influence of Methylcyclopentane on the n-Heptane Isomerization Activity. Table 2 gives the product distribution for methylcyclopentane (MCP) hydrogenolysis at 6 bar as a function of time on stream over the Mo2C-oxygen-modifiedcatalyst after an activa-
+
+
Ind. Eng. Chem. Res., Vol. 34, No. 4, 1995 1109 Table 1. n-Heptane Isomerization on Mo&-Oxygen-ModifiedCatalyst?
Table 3. Effect of Total Pressure on Methylcyclopentane Hydrogenolysis on MozC-Oxygen-ModifiedCatalyst?
time on stream (h) 0.5
2.0
6.5
10.0
total pressure (bar) 26.0
1
6 ~~
conversion (%) 15.6 32.0 38.0 37.8 rate (lo-’ mol 8-l s-l) 14.2 26.0 29.5 29.2 C7 selectivity % 83 90 89 92 Distribution of the Isomer Products (%) DM-2,2P 1.4 1.3 1.5 1.5 DM-2,4P 4.8 4.2 3.9 3.5 0.8 DM-3,3P 0.9 0.7 0.7 DM-2,3P 7.4 6.1 5.6 5.4 3EP 3.0 3.0 3.0 3.2 2MHex 40.3 41.1 40.3 39.7 3MHex 41.4 43.2 44.6 45.4 Ccyclic 0.7 0.3 0.3 0.4 Distribution of the Cracking Products (%) CS + c1 10.0 19.8 24.3 30.1 c5 + cz 9.2 14.3 18.0 18.2 c4 + c3 74.2 59.5 47.9 44.9 others 6.6 6.5 9.7 6.9
37.0 28.5 90 1.4 3.2 0.8 5.2 3.3 39.4 46.4 0.3 37.7 20.1 34.4 7.9
Reaction conditions: total pressure = 6 bar, total flow rate = 200 mL min-’, reaction temperature = 623K, Hz/n-C7H16 = 30. Abbreviations: DM-x yP, x y-dimethylpentane; 3EP, 3-ethylpentane; ZMHex, 2-methylhexane, BMHex, 3-methylhexane. a
Table 2. MethylcyclopentaneHydrogenolysis on Mo2C-Oxygen-ModifiedCatalyst” time on stream (h) 19.5
24.0
conversion (%) 6.3 4.9 4.6 2.8 rate (10-7molg-1 s-l) 6.8 5.3 4.9 3.0 CS selectivity (%) 96 97 97 96 Distribution of the Isomer Products (%) DM-2,2B and -2,3B 1 2 1 0.5 2MPen 4 4 4 4 3MPen 3 3 3 3 11 13 14 23 n-Hex Ben + Cyclo 81 78 78 70 Distribution of the Cracking Products (%) 38 36 c5 + c1 38 40 32 31 c4 + cz 30 30 13 13 12 15 2c3 0 0 0 0 3c2 18 18 17 19 6C1
2.0
3.0
4.5
3.0 3.1 97 0.5 4 3 23 70
31 31 16 0 21
a Reaction conditions: total pressure = 6 bar; total flow rate = 200 mL min-’; reaction temperature = 623 K; Hz/C&12 = 30. Abbreviations: DM-xyB, x y-dimethylbutane; 2MPen, 2-methylpentane; 3MPen, 3-methylpentane; Ben, benzene; Cyclo, cyclohexane; n-Hex, n-hexane.
tion period under a mixture of n-heptane and hydrogen. The specific rate of the total reaction drastically decreases at the beginning of the reaction and reaches a steady-state after about 20 h of reaction at 623 K. Inside the Cg products, n-hexane and cyclohexane are formed preferentially, 11%and 81%)respectively, at the start of the reaction, to reach a steady state after 20 h with 23% and 70%) respectively. It is important to emphasize that this catalyst has some activity for the decyclization reaction. The large decrease in specific rate of the reaction of MCP can be attributed to the fast formation of carbonaceous residues resulting from the polymerization of highly dehydrogenated Cg rings (cyclopentadienyl). The TEM micrograph in Figure 2A shows a typical particle of Mo2C before reaction with MCP, and the microdiffraction pattern of this particle (Figure 2C) confirms the presence of Mo2C in the hcp structure. No carbonaceous deposits are observed on this sample. Figure 2B presents a particle after reaction with MCP and shows
time on stream (h) 15 24 conversion (%) 1 3 rate (10-7 mol g-1s-1) 0.5 3.1 CS selectivity (%) 97 97 Distribution of the Isomer Products (%) DM-2,2B and -2,3B 0 0.5 2MPen 1 4 3MPen 1 3 n-Hex 23 23 Ben Cyclo 75 70 Distribution of the Cracking Products (%) 53 31 c s + c1 18 31 c4 + cz 12 16 2c3 2 0 3cz 15 21 6C1
+
18
~
19 4.5 4.8 93 1 6 5 18 69
46 25 13 0 17
a Reaction conditions: total pressure = 6 bar; total flow rate = 200 mL min-’; reaction temperature = 623 Hz/C,jH12 = 30. Abbreviations: DM-xyB, x y-dimethylbutane, 2MPen, 2-methylpentane; SMPen, 3-methylpentane; Ben, benzene; Cyclo, cyclohexane; n-Hex, n-hexane.
well-ordered filamentous carbon surrounding the Mo2C particle. Measurements of d-spacings from the selected area diffraction pattern (Figure 2D)indicate $he presence of graphite, molybdenum oxycarbide, and Mo2C. The corresponding energy dispersive microanalysis spectrum (Figure 2E)shows the presence of Mo, 0, and C. Again, the formation of n-hexane, cyclohexane, and a large amount of C€& can be explained by the existence of a metallocyclobutane mechanism over the Mo2Coxygen-modified catalyst. Figure 3 shows that methylcyclopentane can adsorb via a metallocyclobutane species. Following route 1, the metallocyclobutane intermediate can split into a methylidene radical and an adsorbed cyclopenteneby rupture of an external C-C bond. The methylidene radical is then hydrogenated to CHI, while cyclopentene polymerizes to yield carbonaceous residues which can rapidly block the access of the reactant to the active sites resulting in deactivation. Route 2 shows the rupture of an internal C-C bond and the formation of a “c-C6 adsorbed intermediate. This species then forms either n-hexane by hydrogenation or cyclohexane by ring closure followed by dehydrogenation t o give benzene. From the above mechanism, one expects to find that the specific rate of MCP hydrogenolysis is strongly dependent on the total pressure and that at higher total pressures the hydrogenation of coke intermediates will be faster. Therefore, an experiment was carried out by increasing the total pressure of the methylcyclopentane reaction to 18 bar. Table 3 reports the specific rate and the product distribution for MCP hydrogenolysis for three pressures, 1, 6, and 18 bar. As expected the specific rate increases from 3.1 x t o 4.8. x lo-’ mol g-’ s-l on increasing the total pressure from 6 to 18 bar. In Figure 4 are shown the activity and selectivity for n-heptane isomerization a t 6 bar, after deactivation of the catalyst by the MCP reaction at 1, 6, and 18 bar. Presented in Table 4 is the detailed product distribution for n-heptane isomerization as a function of time on stream. A drop of about 50% (Figure 4A) is observed for the activity of n-heptane isomerization after the MCP reaction a t 6 bar, indicating that a large amount
1110 Ind. Eng. Chem. Res., Vol. 34, No. 4,1995
II E
Mo
I
1
1
1
2
cu
3
4
5
6
7
89-
Energy (keV)
Figure 2. (A) TEM micrograph of MmC after synthesis. (B)TEM micrograph of MmC a h r reaction under methylcyclopentane. (C) Diffraction pattern of sample shown in panel A. (D)Diffraction pattern of sample shown in panel B. (E) Energy dispersive microanalysis spectrum of sample shown in panel B. of the active sites has been poisoned after the MCP hydrocarbons with a C g ring (cyclopentane, methylcy-
reaction. An even more drastic deactivation is observed after the MCP reaction a t atmospheric pressure. Severa1 researchers (Pham-Huu et al., 1993; Beltramini et al., 1983; Barbier et al., 1984a,b) have emphasized that
clopentane) are the most effective coke precursors, and therefore the coking capacity of the catalysts with compounds having a cyclopentanic ring depends on the dehydrogenating capacity of the catalyst to form a
Ind. Eng. Chem. Res., Vol. 34,No. 4, 1995 1111 Table 4. n-Heptane Isomerization on MozC-Oxygen-ModifiedCatalyst after the Methylcyclopentane Reaction at 1 , 6 and 18 BalD MCP reaction pressure (bar) 1
6 18 19.5 2.0 18.0 1.5 3.0 4.0 22.0 13.0 14.5 17.0 27.0 4.6 29.1 30.8 12.0 13.2 15.4 23.2 1.9 25.4 26.2 89 91 91 90 81 92 89 Distribution of the Isomer Products (%) DM-2,2P 0 0 0.9 1.0 1.3 1.6 1.7 1.8 DM-2,4P 2.2 1.6 2.4 2.4 2.9 3.8 3.8 3.7 0 0.5 0.5 0.7 0.8 0.8 0.9 DM-3,3P 0.6 DM-2,3P 2.8 0.7 4.0 4.2 4.5 5.8 6.0 6.1 3EP 2.5 3.5 3.2 3.3 3.2 3.1 3.2 3.4 2MHex 38.8 44.1 40.1 40.0 40.1 39.7 39.1 38.5 3MHex 41.3 46.0 48.3 48.0 46.6 44.7 44.9 45.1 Ecyclic 7.2 4.0 0.6 0.5 0.6 0.4 0.4 0.4 Distribution of the Cracking Products (%) c6 + c1 76.5 46.7 46.5 44.0 32.1 32.2 35.3 38.9 c5 + cz 3.4 18.8 17.7 17.8 16.9 15.0 16.5 17.8 c4 + c3 14.6 28.5 28.6 30.2 37.4 46.3 41.6 36.2 others 5.5 6.0 7.2 8.0 13.6 6.5 6.6 7.2 Reaction conditions: total pressure = 6 bar; total flow rate = 200 mL min-l; reaction temperature = 623 K Hdn-C7H16 = 30. Abbreviations: DM-xyP, xy-dimethylpentane; 3EP, 3-ethylpentane; 2MHex, 2-methylhexane; 3MHex, 3-methylhexane. time on stream (h) conversion (%) rate (10-7 mol g-1 s-1) c6 selectivity (%)
2.0 3.5 1.5 51
n U
9-
H-M-H
CHI + cyclopentane
or Polycyclopentadienyl
-
62
H-M-H
0 Figure 3. Mechanistic scheme for the reaction of methylcyclopentane via a bond-shift mechanism involving a metallocyclobutane intermediate.
cyclopentadienicring. This ring has a conjugate double bond which is very reactive and can condense with other similar rings by a diene-dienophile Diels-Alder type condensation giving an indenic structure (Barbier et al., 1984b). Nevertheless, it is significant to note that the deactivation observed here is less pronounced compared to that obtained at atmospheric pressure for n-hexane isomerization, reported in a previous publication (PhamHuu et al., 19931, where only 37% of the initial activity was recovered and for n-heptane isomerization where only 30% of the initial activity was recovered after 20 h of reaction. This result is probably due to the fact that at higher pressure the coke formation from poly(cyc1opentadienyl) is less favored. In fact, hydrogen pressure keeps the active sites of the catalyst free of hydrogendeficient hydrocarbon residues which can undergo polymerization to form carbonaceous residues.
In order t o verify the above assumption, a new experiment was performed in which the MCP reaction was performed on a fresh catalyst (after an activation period under an n-heptane and hydrogen mixture at 6 bar until steady state) under the same reaction conditions as above but a t a total pressure of 18 bar. A hydrogedn-heptane mixture was then reintroduced and the catalyst activity followed at 6 bar. The results are reported in Figure 4A and presented in Table 4, so that a comparison can be made with those obtained after the MCP reaction a t 1 and 6 bar. It can be seen that, as expected from the discussion above, the deactivation of the catalyst is much less pronounced for the n-heptane isomerization reaction &r the MCP reaction at 18 bar. It is also important to note that some of the lost activity is recovered after reintroduction of the heptanehydrogen mixture and reaches a steady-state after about 15 h. A 20% drop in activity is observed after the MCP reaction a t 18 bar, and after 20 h only 10% of the activity has been lost, compared to the decrease of around 50% after the MCP reaction at 6 bar and 70% after MCP reaction at atmospheric pressure. Further, Figure 4B and Table 4 show that the isomer selectivity for n-heptane remained constant after MCP poisoning whereas at atmospheric pressure, for nheptane isomerization, a large decrease was observed (85 t o 66%). Similar results were observed for n-hexane isomerization at atmospheric pressure (Pham-Huu et al., 1993). This decrease was attributed to the recarbidation of the oxycarbide phase by methylidene radicals to form surface carbide. The stable isomer selectivity observed in this study for higher pressure is attributed to the fast hydrogenation of the methylidene radicals which may subsequently desorb t o the gas phase, avoiding the reduction of the oxycarbide surface. Regeneration of the Deactivated MozC-OgygenModified Catalyst. 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. Therefore, the deactivated catalyst obtained from the MCP reaction at 6 bar was treated oxidatively in situ in flowing air (rate = 10 mL min-l) at 623 K for 2 h at atmospheric pressure. Figure 5 shows that the isomerization specific rate
1112 Ind. Eng. Chem. Res., Vol. 34,No. 4, 1995
A
'. 0.8
3
.A
3 'G u
0.6-
4 before reaction with MCP
(d
0
after MCP and 2 hrs under H,/n-C, after MCP and 20 hrs under H2/n-C7
06
1
18
MCP reaction pressure (bars)
4 H2/n-C7 reaction before MCP H,/n-C, reaction after MCP
T
T
1
6
18
MCP reaction pressure (bars) Figure 4. (A) Activity for n-heptane isomerization after deactivation of the catalyst under methylcyclopentane (MCP). Effect of total pressure during the MCP reaction (total pressure = 1, 6, and 18 bar, Hz/n-C7 = 30). (B)Selectivity for n-heptane isomerization after deactivation of the catalyst under methylcyclopentane (MCP). Effect of total pressure during the MCP reaction (total pressure = 1,6, and 18 bar, Hdn-C7 = 30).
returned to the initial value (before deactivation with MCP) and that a similar activation rate was observed, indicating that the poisoning is due to carbonaceous residues, as already evidenced by TEM, which are burned off in the oxidative regeneration process. TEM observations indicated that all the carbonaceous residues had been removed by the regenerative treatment (not shown). Also, the isomerization selectivity was constant after the catalyst regeneration a t 90-92%. It is important to point out that the initial oxycarbide active phase was obtained after oxidative treatment of the starting Mo2C for at least 14 h, while the activity of the deactivated catalyst is completely recovered after an oxidative treatment under the same conditions for only 2 h. This observation shows that the thickness of the deactivated phase is only superficial and not deeper in the bulk of the catalyst.
t
35000
A
w
'm 30000
w
'bD
"0 25000
6
7 20000 2 ' e! 15000 E *z
10000
3 cn
5000
i1f-
o !
0
I
10
I
I
I
I
!
I
1
1
B
I I I
I I
I I I I I
I I I I I
I
1
20
30
1 1
40
I
1
I
50
60
70
Time on stream (h) Figure 5. Activation, deactivation, and regeneration of the M02C-
Conclusion Molybdenum carbide treated oxidatively followed by an activation period under a hydrogedn-heptane reac-
oxygen-modified catalyst: .(A)Activation-n-heptane isomerization until the steady state (623 K, 6 bar). (B)Deactivation-n-heptane isomerization after catalyst deactivation with methylcyclopentane (6 bar). (C) Regeneration-n-heptane isomerization after oxidative regeneration with air (6 bar).
Ind. Eng. Chem. Res., Vol. 34, No. 4,1995 1113 tant mixture is transformed to an oxycarbide species, which is active and very selective for the n-heptane isomerization reaction, even at high conversion (Blekkan et al., 1994). No catalyst deactivation is observed after more than 100 h under the reaction conditions. This is due t o the metallocyclobutane mechanism in operation over this material, resulting in the formation of almost no cyclic intermediates which can polymerize to form carbonaceous residues. The introduction of methylcyclopentane as pure reactant onto the catalyst results in catalyst deactivation and a loss in activity for the n-heptane isomerization. This is due t o the formation of coke from the dehydrogenated Cg ring precursor, which encapsulates the particles of the active phase, as observed by TEM. When methylcyclopentanehydrogenolysis was carried out at higher pressure (18 bar), the deactivation was less pronounced. This is probably due to the faster hydrogenation of olefinic species adsorbed on the catalyst surface, which block the active sites. The molybdenum oxycarbide deactivated by MCP could easily be regenerated by a short oxidative treatment under mild conditions, showing that the deactivation of the catalyst was only superficial. It is important to note that the experiments carried out in this work involved the use of pure methylcyclopentane which is present only as a minor constituent of an industrial feed. Thus, the resistance of the oxygenmodified molybdenum carbide catalyst to coke formation from cyclic intermediates at elevated pressures, as shown in this study, indicates that deactivation would be slow in an industrial environment.
Acknowledgment This work was supported by the PBchiney Co., and A.P.E.Y. would like to thank the Royal Society for a European Science Exchange Program fellowship.
Literature Cited Barbier, J.; Elassal, L.; Gnep, N. S.; Guisnet, M.; Molina, W.; Zhang, Y. R.; Bournonville, J. P.; Franck, J. P. Formation de Coke sur les Catalyseurs MBtalliques Support6s 1. Effets d'ajouts au platine sur la s6lectivitB de transformation du cyclopentane et sur la vitesse de formation de coke. (Coke formation on supported metallic catalysts. 1.Effects of additives to platinum on the selectivity of cyclopentane reactions and on the rate of coke formation.) Bull. 5".Chim. Fr. 1984a,1,245249. Barbier, J.; Elassal, L.; Gnep, N. S.; Guisnet, M.; Molina, W.; Zhang, Y. R.; Bournonville, J. P.; Franck, J. P. Formation de Coke sur les Catalyseurs Mbtalliques Support6s 2. Sites responsables de la formation de coke au cours de la transformation du cyclopentane sur PUAl203. (Coke formation on supported metallic catalysts. 2. Sites responsible for coke formation during reactions of cyclopentane on platinudalumina.) Bull. SOC. Chim. Fr. 1984b,l,250-254.
Beltramini, J. N.; Martinelli, E. E.; Churin, E. J.; Figoli, N. S.; Parera, J. M. WAl2O3-Cl in Pure Hydrocarbon Reforming. Appl. Catal. 1983,7,43-55. Blekkan, E. A.; Pham-Huu, C.; Ledoux, M. J.; Guille, J. Isomerization of n-Heptane on an Oxygen Modified Molybdenum Carbide Catalyst. Znd. Eng. Chem. Res. 1994,33,1657-1664. Delporte, P.; Meunier, F.; Pham-Huu, C.; VennBgues, P.; Ledoux, M. J.; Guille, J. Physical characterization of Molybdenum Oxycarbide Catalyst; TEM, XRD and XPS. In Catalysis Today; Ledoux, M. J., Ed.; Elsevier, in press. Gault, F. G. Mechanisms of Skeletal Isomerization of Hydrocarbons on Metals. Adv. Catal. 1981,30,1-95. Iglesia, E.; Baumgartner, J. E.; Ribeiro, F. H.; Boudart, M. Bifunctional Reactions of Alkanes on Tungsten Carbides Modified by Chemisorbed Oxygen. J . Catal. 1991,131,523-544. Ledoux, M. J.; Djellouli, B. Hydrodenitrogenation Activity and Selectivity of Well-Dispersed Transition Metal Sulphides of the Second Row on Activated Carbon. J . Catal. 1989,115, 580590. Ledoux, M. J.; Guille, J.; Pham-Huu, C.; Marin, S. Obtention de carbures de mBtaux lourds a surface specifique Blevee. (Synthesis of high specific surface area carbides of heavy metals.) Eur. Pat. Appl. No. 0 396 475 A l , 1989. Ledoux, M. J.; Pham-Huu, C.; Dunlop, H. M.; Guille, J. n-Hexane Isomerization on High Specific Surface MozC Activated by an Oxiditive Treatment. In Proc. 10th International Congress on Catalysis; Guczi, L., Solymosi, F., TBtBnyi, P., Ed.; AkadBmiai Kiado: Budapest, 1993a, Vol. B, p 955-967. Ledoux, M. J.; Guille, J.; Pham-Huu, C.; Blekkan, E. A.; Peschiera, E. ProcBd6 d'isom6risation d'hydrocarbures lineaires contenant plus de six atomes de carbone a l'aide de catalyseurs 51 base d'oxycarbure de molybdhne. (Isomerization process for linear saturated hydrocarbons containing more than six carbon atoms.) Eur. Pat. Appl. No. 93-14199, 199313. Maxwell, I. E.; Naber, J. E. New and Improved Processes for Clean Fuels. Catul. Lett. 1992,12, 105-116. Oyama, S. T. Ammonia Synthesis and Decomposition on Molybdenum and its Interstitial Alloys. Ph.D. Dissertation, University of Stanford, 1981. Pham-Huu, C.; Ledoux, M. J.; Guille, J. Reactions of 2- and 3-Methylpentane, Methylcyclopentane, Cyclopentane and Cyclohexane on Activated MozC. J. Catal. 1993,143,249-261. Ribeiro, F. H.; Dalla Betta, R. A.; Boudart, M.; Baumgartner, J. E.; Iglesia, E. Reactions of Neopentane, Methylcyclohexane and 3,3-Dimethylpentane on Tungsten Carbides: the Effect of Surface Oxygen on Reaction Pathways. J . Catal. 1991a,130, 86-105. Ribeiro, F. H.; Boudart, M.; Dalla Betta, R. A.; Iglesia, E. Catalytic Reactions of n-Alkanes on Tungsten Carbide (P-WzC and WC): the Effect of Surface Oxygen Pathways. J . Catal. 1991b,130, 498- 5 13.
Received for review August 15, 1994 Revised manuscript received November 30,1994 Accepted December 13, 1994" IE940495Z
@
Abstract published in Advance A C S Abstracts, February
15, 1995.
