1648
I n d . E n g . Chem. Res. 1988,27, 1648-1653
Sarian, S. J. Appl. Phys. 1968, 39, 3305. Sarian, S. J . Phys. Chem. Sol. 1972, 33, 1637. Scherrer, P. Gott. Nachr. 1918, 2, 98. Shigehara, Y. Nippon Kagaku Kaishi 1977,4, 474. Sinfelt, J. H.; Yates, D. J. C. Nature (London) Phys. Sci. 1971,229, 27. Tammann, G. Z. Anorg. Allg. Chem. 1926, 39, 869. Tammann, G.; Mansuri, Q. A. 2.Anorg. Allg. Chem. 1923,126,119. Toth, L. E. Transition Metal Carbides and Nitrides; Academic: New York, 1971.
Vidick, B.; Lemakre, J.; Leclercq, L. J . Catal. 1986, 99, 439. Vilk, Yu. N.; Nikolskii, S. S.; Avarbe, R. G. Teplofiz. Vys. Temp. 1967, 5, 607. Volpe, L.; Oyama, S. T.; Boudart, M. Prep. Catal. 3, Int. Symp., 3rd 1983, 147. Volpe, L.; Boudart, M. J. Solid State Chem. 1985a, 59, 332. Volpe, L.; Boudart, M. J. Solid State Chem. 198513, 59, 348. Received f o r review March 30, 1987 Accepted March 22, 1988
Catalytic Behavior of Selected Transition-Metal Carbides, Nitrides, and Borides in the Hydrodenitrogenation of Quinoline James C. Schlatter and S. Ted Oyama*,+ Catalytica, 430 Ferguson Drive, Building 3, Mountain View, California 94043
Joseph E. Metcalfe, 111, and Joseph M. Lambert, Jr.$ Standard Oil Company of Ohio,9101 East Pleasant Valley Road, Independence, Ohio 44131 High surface area carbides and nitrides have been synthesized and tested for hydrodenitrogenation activity. Some of the novel materials, particularly MozC and MozN, provided hydrodenitrogenation activity of the same magnitude as commercial sulfided Ni-Mo/Alz03. In the absence of sulfur in the feed, the refractory catalysts could achieve denitrogenation with less hydrogen consumption than the commercial sample. Sulfur addition was detrimental to the selectivity, but not the activity, of MozC and MozN. Heteroatom removal is an important processing step in upgrading hydrocarbon feedstocks to commercially useful products. Organic nitrogen and sulfur are most commonly removed via reaction with hydrogen at 300-400 "C and 50-150 atm of pressure. Under these severe conditions, hydrogen is consumed not only in breaking carbon-nitrogen and carbon-sulfur bonds but also in saturating aromatic components in the feed. In fact, the latter reactions generally consume at least as much hydrogen as the desired denitrogenation and desulfurization reactions (Katzer and Sivasubramanian, 1979). There are economic incentives to decrease hydrogen consumption in hydrotreating operations-not only is hydrogen an expensive raw material in many situations, but also the required capital investment is directly related to the amount of hydrogen consumed (Kirk-Othmer Encyclopedia of Chemical Technology, 1980). Sulfur can be extracted from aromatic organosulfur compounds using only enough hydrogen to react with the sulfur atom and to heal the broken carbon-sulfur bonds (Katzer and Sivasubramanian, 1979; Rollmann, 1977). On the other hand, removing a nitrogen heteroatom using existing catalysts requires complete saturation of the associated aromatic rings in the molecule (Katzer and Sivasubramanian, 1979; Rollmann, 1977; Cocchetto and Satterfield, 1981). Thus, development of a catalyst for denitrogenation without saturation of the surrounding aromatic rings could represent a significant advance in treating high-nitrogen feedstocks. Processing of shale-derived materials in particular would benefit from such a catalyst, since nitrogen levels of about 2 wt % are typical of shale oil. Hydrogen consumptions in excess of 1500 scf/bbl (standard cubic feet per barrel) are common in hydrotreating shale oil, while the amount theoretically t Present address: Department of Chemical Engineering, Clarkson University, Potsdam, NY 13676. f Present address: Chemical Data Systems, 7000 Limestone Road, Oxford, PA 19363.
0888-5885/88/2627-1648$01.50/0
required for selective heteroatom removal is only about 600 scf/bbl (Robinson, 1978). The reaction pathways accessible using sulfided Co-Mo or Ni-Mo hydrotreating catalysts (standard in industrial practice) favor ring saturation prior to denitrogenation. Because transition-metal sulfides are able to catalyze sulfur removal with high hydrogen selectivity, we reasoned that nitrides might behave analogously for selective nitrogen removal. Also, certain carbides have shown interesting catalytic properties for hydrogenation reactions (Levy, 1977). For these reasons, we investigated several novel catalytic materials, specifically transition-metal carbides, nitrides, and borides which might allow direct hydrogenation and removal of the nitrogen atom without the need for prior ring saturation. The primary point of interest is the selectivity for denitrogenation in preference to ring saturation, since such selectivity will decrease the amount of hydrogen required for heteroatom removal. Thus, for our model substrate, quinoline, the desired product is n-propylbenzene rather than the saturated analogue, npropylcyclohexane (see Figure 1). The theoretical hydrogen consumption for the selective pathway is 43% lower than for the pathway involving ring saturation (4 mol versus 7 mol of hydrogen).
Experimental Section The gases employed in this study consisted of H2 (Linde High Purity Grade, 99.995%),CH4 (Linde Research Grade, 99.99%), NH3 (Linde UHP Grade, 99.995%), HzS (Matheson Certified Purity Grade, 99.5%), Nz (Linde High Purity Grade, 99%), Oz (Linde Specialty Grade 99.99%), and CO (Linde Research Grade, 99.97%). The H2, H2S, N2, and CO were used without further purification. The CH, was purified from sulfur impurities by passage through a bed of Ni/Si02 catalyst previously reduced at 673 K. The NH3 was dried by passage through a bed of BaO (Fischer Technical Anhydrous Grade) in the form of porous lumps. The CO was purified by passage through 0 1988 American Chemical Society
Ind. Eng. Chem. Res., Vol. 27, No. 9, 1988 1649 O r t h o p r o p y l a n i I ,ne (OPA)
QQ Qulnollne
P r o p y l benzene
IPBJ
CQ 707
\2;t
NH3
~ r o p ~ i c yomexane ci
IP)
Decahydroquinollne
OQ
(""'
(QHQ)
5 , s ,7 .&Tetrdhydroqul no1 ( n e 15-THQJ
Figure 1. Reaction network for quinoline hydrodenitrogenation.
a bed of activated carbon (VWR Y-12). The chemicals used in this study consisted of MOO, (Aldrich Gold Label, 99.999%), MOO, (Mallinckrodt ACS Spec., 99.5%), W 0 3 (Aldrich Gold Label, 99.999%), Nb206 (Aldrich Gold Label, 99.99%), 7-A1203 (Union Carbide, Linde Division, alumina extrudate 60-501), MOB (Alfa Products, 99%), WB (Alfa Products, 99%), Mo2C (Alfa Products, 99.8%), Mo2N (Alfa Products, unanalyzed, lot 090880), MoCh (Alfa, 98%), L i a (Alfa, 99%), ethyl acetate (Baker Analyzed Reagent, 99%), quinoline (Aldrich, 99%), tetradecane (MCB reagents, 99%), CS2 (Baker Analyzed Reagent, 99%),1,2,3,4-tetrahydroquinoline(Aldrich, 97%), 5,6,7,8-tetrahydroquinoline(Aldrich, unanalyzed), opropylaniline (Aldrich, 97%), o-toluidine (Aldrich, 99%), propylbenzene (Aldrich, 98% ), n-propylcyclohexane(Pfaltz and Bauer, 99%), and cyclohexane (Aldrich, 99%). The 7-A1203had a reported composition of 99.85 wt % A1203, 0.1 wt % Si02,0.02 wt % Fe203,0.005 wt ?& SO4,and 0.003 wt % Na20. The Ni-Mo/A1203 (Shell 324) was composed of 2.7 wt % Ni and 13.2 wt % Mo as the oxides. The preparation of NbN, Mo2C,Mo2N, and WC is described elsewhere (Oyama et al., 1988). Briefly, the preparation consists of treating a precursor oxide with a reactive gas stream in a temperature-programmed manner while monitoring the composition of the exit gases. The reactive gas is a mixture of CHI and H2 for the carbides and NH, for the nitrides. A passivation procedure, consisting of exposing the catalysts to controlled amounts of O2in He, deposits a small layer of oxygen on their surfaces so as to protect them from bulk oxidation. The passivation layer is removed by reduction at 673 K prior to chemisorption measurements and catalytic testing. The commercial samples of MOB,MqC, Mo2N,and WB were also reduced at 673 K before measurement of their chemisorption and catalytic properties. The commercial samples of Mo2Cand Mo2Nwill henceforth be referred to as Mo2C-Comand MqN-Com to identify their origin. The prepared samples of Mo2C and Mo2N will be referred to as Mo2C-1 and Mo2N-1 in consistency with the nomenclature in Oyama et al., (1988) where their syntheses are reported in detail. A MoS2 samples was prepared by dissolving 10.0 g of MoC14in 300 cm3 of ethyl acetate and slowly adding 3.86 g of Lips with vigorous stirring. The resulting brown precipitate was collected and treated in 39% H2S/H2at 673 K for 1h. The material was then reacted with 12% acetic acid, and the resulting solid was again collected and treated in 39% H2S/Hz at 673 K for 1 h to give MoSz in the form of a gray powder. Prior to chemisorption and catalytic testing, MoS2, as well as Ni-Mo/A120s, was treated in a flowing 10% H2S/H2stream at 633 K for 2 h.
Figure 2. Schematic diagram of batch autoclave reactor for HDN studies.
Supported Mo2C/A1203and Mo2N/A1203samples were prepared by treating a Mo03/A1203precursor at 850 K in 20% CH4/Hz and NH, streams, respectively. Space velocities based on bed volume were on the order of 5000 h-l. The precursor was prepared by dissolving 50 g of MOO, (Mallinckrodt) in 150 cm3of NH40H with heating, adding 150 cm3 of HzO, and forming a slurry with 150 g of A1203. This corresponded to a nominal Moo3 loading of 20 wt % . The slurry was stirred for a few minutes and then filtered. The solid was dried and then impregnated with the original filtrate to form a paste. The paste was dried at 493 K for 3 h. The bulk crystallographic phase of the materials was determined by powder X-ray diffraction (XRD) by using a Philips Norelco 4227510 diffractometer. The surface of the catalysts was characterized by irreversible chemisorption of CO or O2 and by surface area measurements by the N2 BET method. The measurements were carried out with samples of about 1g in a volumetric adsorption apparatus equipped with a Texas Instruments differential pressure gauge. Pretreatment consisted of flowing H2 at 673 K or, in the case of the sulfides, 10% H2S/H2at 633 K, evacuating the sample at the pretreatment temperature, and, finally, cooling to room temperature in vacuum. The irreversible uptake of CO or O2was obtained by taking the difference between two adsorption isotherms, the first measured on the freshly pretreated sample and the second measured on the same sample after evacuating the sample cell at room temperature. Figure 2 presents a schematic of the testing unit. The reactor was an Autoclave Engineers (Model BC 0030 SS 05 AH) 300-cm3stainless steel autoclave operated as a semibatch stirred tank reactor. The reaction was carried out in the liquid phase with tetradecane as a solvent. Hydrogen pressure was regulated at the desired value, and the progress of the reaction was characterized via periodic withdrawal of gas and liquid samples for gas chromatographic analysis. With the exception of gas cylinders and gas chromatographs, the reactor system was totally contained within a vented enclosure made of 1/4-in.steel and lined with 1/2-in.plywood. All controls were accessible from outside the enclosure, and interlocks and relief devices were included to provide for safe operation. The typical experiment was started by loading the autoclave with the catalyst and reducing it at 673 K in a hydrogen flow. The reactor was then cooled and charged with tetradecane without exposure to the atmosphere. In the case of the sulfided catalysts, the fiist step was omitted and the catalyst was placed in the tetradecane under a blanket of inert gas. To initiate the reaction, the autoclave
1650 Ind. Eng. Chem. Res., Vol. 27, No. 9, 1988 Table I. Surface Properties of Catalysts surface area, CO uptake, material m2 g-' &molg-' This Study Mo&-1 37 38 Mo2N-1 88 288 MoS, 17 48" wc 6.7 4.5 NbN 3.6 1.2 Commercial Samples Mo2C-Com 0.5 1.o 1.0 1.4 Mo2N-Com 0.6 MOB 0.05 0.7 WB 0.2 492" Ni-Mo/A1203 122
,
r---0
01
0 2
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,-.-
03
O L
05
06
'L
0 7
TimeIHours
Figure 3. Sample chromatogram of reaction mixture. A 50-m OV-101column, initially at 373 K, was used. Thirty-one minutes after sample injection, the column temperature was raised to 478 K at a rate of 16 K min-' for a total analysis time of 45 min.
was brought to the desired temperature at a hydrogen pressure somewhat below that to be used during the reaction. Then quinoline was injected into the Hz supply line (via valve 14 in Figure 2) as the Hz pressure was quickly increased to the final value. In this manner, a precise zero time for the reaction was established. This scheme also allowed subsequent quinoline additions to be made without opening the autoclave. Liquid-phase samples were withdrawn via valve 17. A large (-2-cm3) loop and a small (-0.75-cm3) sample loop connected in series were fiied simultaneously. The 2-cm3 sample flushed the lines and was discarded. The small sample was collected in a septum-capped vial outside the reactor enclosure, and a portion could be injected, via syringe, into a gas chromatograph. This approach was developed in preference to an on-line direct injection into the chromatograph to allow the injected amount to be varied easily, to make reactor sampling independent of analysis time, and to allow repeated injections of a particular sample. Gas-phase samples, primarily to provide an NH3 analysis, were injected directly into the chromatograph via sampling valve 15. A large (2-cm3) flushing loop and a small (0.020-cm3)injection loop were arranged in the same way as those on the liquid sample valve. Each gas-phase analysis required approximately 5 min. When several runs were made with a single catalyst charge, the liquid phase could be removed from the autoclave via valves 21 and 25. The 0.5-pm filter on the dip tube prevented the catalyst from leaving the reaction vessel. A change in catalyst required that the autoclave cover be removed. Analysis of the products of the reaction was carried out with two Varian Instruments Model 3700 gas chromatographs. Gas-phase ammonia and light hydrocarbon products were analyzed by using a 3-m Porapak Q column and a thermal conductivity detector. Liquid-phase products were analyzed by using a 50-m OV-101 column whose effluent was split into a flame ionization detector and a nitrogen-specific detector (Varian thermionic specific detector Model 03-949626-90). The signals from both detectors were recorded simultaneously by a laboratory data acquisition system (Hewlett-Packard 3357 LAS), allowing the identification of nitrogen-containing compounds in the complex mixture of hydrocarbon products. Specific compounds were identified and quantified by the injection of
" Uptake of 02.
Assumes two sites per
site density, ioi4 cm-2 0.64 1.9 3.4* 0.40 0.20
1.2 0.84 0.050
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standards. A sample chromatogram is given in Figure 3. By use of the gas-phase analysis for NH3, the overall nitrogen balance was normally closed to within 5-lo%, with an imbalance of more than 15% being rare. The reactor system was designed for operation a t pressures up to 20 600 kPa (3000 psig) and temperatures up to 723 K. However, most measurements were carried out at 6900 kPa (1000 psig) and 673 K. Typically hydrodenitrogenation (HDN) testing was carried out at a nitrogen level of 0.1 wt % in the liquid phase (-0.9 wt % quinoline in tetradecane) with about 130 cm3 of liquid volume. These conditions will be referred to as standard conditions. Usually 1 g of catalyst was used, but with the low surface area commercial samples, 10 g was used. Later in the experimental program, HDN in the presence of sulfur was carried out by including CSz with the quinoline injection. CSz is rapidly converted to HzS and CHI at reaction conditions (Shih et al., 1977). These gases were detected in the gas-phase analysis. The amount of CS2 added to the liquid phase was 0.46 wt % which yielded an initial gas-phase HzS level of 3.9 vol %. Except for the CS2 addition, the experimental procedures and analyses in the presence of sulfur were performed just as in the sulfur-free experiments. To characterize HDN performance, liquid samples collected at approximately 1-h intervals were analyzed to determine the activity and selectivity of the reaction. Activity is expressed as the percentage removal of organic nitrogen from the liquid phase as a function of time. It is determined by calculating the total number of moles of nitrogen-containing species in each liquid sample. As mentioned in the introduction, a convenient measure of selectivity is the ratio of propylbenzene (PB) to propylcyclohexane (PCH). Since other HDN products such as cyclohexane (CH) represented only minor fractions relative to PB and PCH, they were not included in the calculations.
-
Results Materials and Blanks. Table I lists the catalysts that were tested for HDN along with their surface properties. X-ray analysis verified the purity of all compounds except NbN, which retained some lower niobium oxide. Figure 4 shows the blank denitrogenation activity of the reactor at standard conditions without catalyst. The blank corresponded to 10-15% conversion in 6 h and was constant throughout the early phase of the experimentation. The use of sulfided catalysts did not measurably increase the blank activity in the early work. However, in later tests when sulfur was included in the feed, the blank activity rose to 25-30% conversion in the same time period. This change occurred over a span of about 10 experiments. It was found that treatment of the autoclave with concen-
Ind. Eng. Chem. Res., Vol. 27, No. 9, 1988 1651 100
y
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100
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alter 10 S experiments
i
25 alter 2 S experiments
e=====-
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Figure 4. Denitrogenation activity of the empty reactor as work proceeded with sulfur-containing reaction mixtures.
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Figure 6. Catalytic behavior of Mo2C-1and sulfided Ni-Mo/A1203 in quinoline HDN in the absence of gas-phase sulfur: (a) activity, (b) selectivity.
0
0
4
2
6
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O 50 0
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Figure 5. Catalytic behavior of Mo2C-1and Mo2N-1in quinoline HDN in the absence of sulfur: (a) activity, (b) selectivity.
15' V
trated nitric acid at 340 K for several hours restored the blank activity to 10%. Testing under Sulfur-Free Conditions. It was found that only the MozC-1, Mo2N-1, Mo2C/A1203, and Mo2N/A1203samples prepared in this study were active in hydrodenitrogenation. The Mo2C-Com,Mo2N-Com, WC, NbN, MOB, and WB conversions at standard sulfur-free conditions were not appreciably different from the reactor blank. This implies a catalyst contribution of less than 5% HDN in 6 h. Figure 5 compares the HDN activity and selectivity of the molybdenum compounds, Mo2C-1and Mo2N-1,synthesized in this study. One gram of each catalyst was tested at standard conditions. Both catalysts were similar in activity, but MozC-1 showed considerably higher selectivity than Mo2N-1. In a similar fashion, Figure 6 compares the HDN performance of Mo2C-1 to that of Ni-Mo/A120,. Again 1.0-g quantities of the catalysts were tested at standard sulfur-free conditions. Testing with Sulfur Present. Figure 7 compares the HDN activity and selectivity of 1.0-g quantities of Ni-
e -
E
1.0
Ni -Mo/AI2Oj
/
u1 0
1
2 Time/Hours
Figure 7. Catalytic behavior of Mo,C-1 and Ni-Mo/A1203 in quinoline HDN in the presence of sulfur: (a) activity, (b) selectivity.
Mo/Al,O, and Mo2C-1. Reaction conditions were standard except for the addition of sulfur. The Ni-Mo/A1203 behavior was identical with that in the sulfur-free conditions. For Mo2C-1,a second test was made on the used catalyst without removing it from the autoclave. The selectivity, our primary figure of merit, was quite consistent between the two experiments. It is not clear whether the observed increase in %HDN after sulfur exposure is a real effect or a consequence of analytical uncertainties in the two experiments. Figure 8 compares the HDN performance of Mo2C-1 reported in Figure 7 to that of MoS2 Within experimental
1652 Ind. Eng. Chem. Res., Vol. 27, No. 9, 1988 NI-MoIA120 3
-
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Figure 8. Catalytic behavior of Mo2C-1and MoS2 in quinoline HDN in the presence of sulfur: (a) activity, (b) selectivity. tal
100
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-
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=,X
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Figure 10. Time dependence of the product distribution when propylbenzene is added to the initial reaction mixture: (a) NiMo/A1209, (b) Mo&-l. Table 11. HDN Performance of Various Catalysts"
$. /'
20
L TimeIHours
catalyst Mo&-1 Mo&-1 Mo2N-1 Mo2N-1 Mo2C/A120, Mo2N/A120, Ni-Mo/Al,O,
sulfur? no yes no yes yes yes yes
initial rateb 1 0.5 0.5 1 2 2 2
initial %N selec- removal tivityC after 6 h 1.8 95 75 0.5 0.8 96 0.4 0.4 0.4 0.4
100
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0.5 0.1 0.2
0.1 0.1
"In all cases, T = 673 K, P = 6900 kPa, catalyst charge = 1 g. Units: IO4 moles organic nitrogen removed per second per gram of catalyst. Propylbenzene/propylcyclohexane ratio.
A summary of the HDN performance of the various catalysts is given in Table 11. >*/A1203 -6,
Mo2N/
. a - - A - - - - - - A -
0 1
I
0
2
I L
6
TlmelHours
Figure 9. Catalytic behavior of Mo&/A1203 and M%N/A1203 in quinoline HDN in the presence of sulfur: (a) activity, (b) selectivity.
scatter, the activity and selectivity of the two samples are quite similar. Figure 9 displays the HDN behavior of the supported refractory molybdenum compounds, Mo2C/A1203and Mo2N/Al2O3. The results are for 1.0-g quantities of the materials used at standard conditions in the presence of sulfur. Figure 10 presents the time evolution of the products of HDN over Ni-Mo/A1203 and Mo2C-1 catalysts. The experiments were carried out by adding propylbenzene to the standard reaction mixture. As usual, each experiment was done with a 1.0-g catalyst sample.
Discussion The experimental work reported here was part of an exploratory effort, so the results do not warrant a detailed kinetic or mechanistic analysis. Nevertheless, the studies of hydrodenitrogenation over novel refractory materials have revealed catalytic properties of potential practical interest. The high-area molybdenum compounds (Mo2C-1and Mo2N-1) were of similar HDN activity on a mass basis, implying that the Mo2C activity is higher than that of Mo2N on a unit area or site basis (cf. Table I). The selectivity is significantly higher over Mo2C (Figure 5b), so Mo2C was chosen for further evaluation. Figure 6 shows that, in the absence of sulfur, Mo2C-1 has about half the initial specific activity of a sulfided Ni-Mo/A1203 HDN catalyst. It is rather remarkable that an unsupported bulk material can so closely approach the specific activity of a supported catalyst. On the basis of the chemisorption values in Table I, the individual Mo2C
Ind. Eng. Chem. Res., Vol. 27, No. 9, 1988 1653 sites are actually more active than those on the commercial catalyst. A key difference between the supported Ni-Mo catalyst and the high-area Mo2C catalyst is evident in Figure 6b. While Mo2Cfavors the unsaturated PB product, the commercial catalyst favors complete hydrogenation to PCH. The 5 1 0 % PB remaining after 6 h over the Ni-Mo/Alz03 catalyst represents the equilibrium level of that compound in a PB PCH mixture (Cocchetto and Satterfield, 1981). I t is noteworthy that after 6 h the percent HDN is equivalent on both the MozC and the Ni-Mo/A1203 catalysts, yet the PB/PCH ratio is much greater for MozC than for Ni-Mo/A120s. Thus, the Mo2C offers the possibility of accomplishing denitragenation with considerably lower H, consumption than typical commercial HDN catalysts. When 4 mol % sulfur is present in the gas phase, the HDN activity of the Mo2C-1 catalyst is decreased slightly, if at all (Figute 7). However, the selectivity advantage over Ni-Mo/A1203 is diminished considerably. A second exposure to the sulfur-containing environment did not cause any further change in the Mo2C performance. It is probable that a less-selective surface layer of MoS2 forms on the Mo2C under the reaction conditions, since the sulfide is thermodynamically preferred over the carbide (Levy, 1977). This hypothesis is supported by the experiment using a bulk MoS2catalyst (Figure 8). The MoSz catalyst is quite similar in both activity and selectivity to the Mo2C catalyst when tested in the presence of sulfur. Bulk sulfidation apparently does not occur to an appreciable extent on the time scale of these experiments, since X-ray analysis of the used sample showed only the MozC structure. I t may be that even surface sulfidation would not occur if sulfur were present at ppm levels rather than the 4 % used here. We were not equipped to investigate sulfur effects at such low concentrations, however. Like the bulk refractory materials, the Al,O,-supported MozC and MozN samples are effective HDN catalysts (Figure 9). When sulfur is present, though, there does not appear to be a selectivity advantage over a Ni-Mo/A1203 catalyst. The supported samples generate noticeably more solvent cracking products than the unsupported materials, presumably because of reactions occurring on the acidic A1203. This enhanced cracking activity is probably responsible for the higher initial HDN rates as well. It is reasonable to ask whether the low selectivity of the commercial catalyst is a consequence of high activity for hydrogenating propylbenzene (PB) to propylcyclohexane (PCH), thus eliminating the desired product. Experiments directed at this question monitored the fate of PB added to the standard quinoline reaction mixture (Figure 10). The evolution of the product distribution is complex, and a detailed analysis is not appropriate. It is evident, however, that Ni-Mo/Al,O, is a more active hydrogenation catalyst per unit mass than the Mo2C sample. With the commercial catalyst, quinoline equilibrates more rapidly with 1-THQ (Shih et al., 1977), 1-THQ reacts faster, and propylbenzene is more rapidly converted to propylcyclohexane. With Mo2C, appreciable amounts of o-propylaniline, an intermediate in the selective HDN pathway, are observed, whereas only traces appear with the commercial catalyst. Although some of the propylbenzene is converted to propylcyclohexane over the commercial sample, the hydrogenation reaction is not rapid enough to
+
decrease significantly the PB concentration in the reaction medium. This observation, plus the absence of opropylaniline as a product over the commercial catalyst, implies that the selective pathway is not operative to any appreciable extent. With sulfur present in the reaction mixture, the Mo2C catalyst likewise does not produce significant quantities of the desired propylbenzene, but the appearance of o-propylaniline indicates that carbon-nitrogen bonds are being broken without prior saturation of the adjacent aromatic ring. Conclusions Tests of quinoline hydrodenitrogenation in a batch autoclave reactor showed that a high surface area molybdenum carbide can catalyze nitrogen removal with less hydrogen consumption than a commercial Ni-Mo/Al,O, sample. The refractory catalyst is about half as active as the commercial catalyst on an equal mass basis. The HDN activity is retained in the presence of sulfur, although the selectivity advantage is diminished considerably. I t is likely that a layer of nonselective MoSz forms on the Mo2C surface when sulfur is present. In situations where the nitrogen level is high and the sulfur level is low, these novel carbide and nitride catalysts may offer the opportunity to selectively remove the nitrogen and thus lower the total cost of hydrogen for the process. A two-stage hydrotreating scheme could possibly provide the right environment. Relatively mild conditions in the first stage would remove sulfur with good hydrogen selectivity, while more aggressive conditions and a MozC catalyst would remove the nitrogen with minimal hydrogen consumption in the essentially sulfur-free second stage. Acknowledgment We thank T. F. Kellett and the Shell Chemical Company for supplying the sample of Shell 324 Ni-Mo/Al,O, catalyst. We also thank Michael A. Faber for the preparation of the MoSz sample. We acknowledge the substantial contribution of Alex A. Draganescu in the setup of the reactor and the testing of the catalysts. Finally, we are grateful to the Standard Oil Company of Ohio and Catalytica for permission to publish this work. Registry No. Mo2C, 12069-89-5; Mo2N, 12033-31-7; MoS2, 1317-33-5;WC, 12070-12-1;NbN, 24621-21-4; quinoline, 91-22-5; propylbenzene, 103-65-1; propylcyclohexane, 1678-92-8.
Literature Cited Cocchetto, J. F.; Satterfield, C. N. Ind. Eng. Chem. Process Des. Dev. 1981, 20, 49. Katzer, J. R.: Sivasubramanian, R. Catal. Rev.-Sci. Eng. - 1979, 20(2), 155. Kirk-Othmer EncvcloDedia o f Chemical Technoloev: -" Wilev: New York, 1980; Voi. 12; pp 944-982. Levy, R. B. In Advanced Materials in Catalysis; Burton, J. J., Garten, R. L., Eds.; Academic: New York, 1977; pp 101-127. Oyama, S. T.; Schlatter, J. C.; Metcalfe, J. E.; Lambert, J. M. Ind. Eng. Chem. Res. 1988, preceding paper in this issue. Robinson, E. T. In 12th Oil Shale Symposium Proceedings; Gary, J. H., Ed.; Colorado School of Mines Press: Golden, CO, 1978; pp 120-134. Rollmann, L. D. J. Catal. 1977,46, 243. Shih, S. S.; Katzer, J. R.; Kwart, H.; Stiles, A. B. Prep.-Am. Chem. SOC.,Diu. Pet. Chem. 1977, 22, 919. I
Received f o r review March 30, 1987 Accepted March 22, 1988
