Ind. Eng. Chem. Res. 1993,32, 1568-1572
1568
Hydrogenation of Aromatics over Hydrodenitrogenation-Selectiveand Hydrodesulfurization-Selective Catalysts Teh C. Ho Corporate Research Laboratories, Exxon Research and Engineering Co., Annandale, New Jersey 08801
A denitrogenation-selectiveFeMo sulfide is evaluated against a commercial desulfurization-selective CoMo catalyst to understand better the effects of catalyst selectivity for heteroatom removal on aromatics hydrogenation. The experiments were done in an upflow fixed-bed reactor with an uncracked gas oil. Combined clay-gel separation and mass spectrometry were used to follow the saturate make and the distributions of aromatic compound classes as functions of the extent of heteroatom removal. The FeMo catalyst, despite its low desulfurization activity, shows a higher volumetric activity for total aromatics reduction than the conventional CoMo catalyst of much higher surface area. The behaviorial differences between the two catalysts in aromatics hydrogenation are discussed. I. Introduction This paper is a continuation of a previous report (Ho et al., 1992a) on the development of a new class of hydroprocessing catalysts. In that report, it was shown that unsupported FeMo and Few sulfides derived from heterometallic metal sulfur complexes gave an unusual combination of high hydrodenitrogenation (HDN) and low hydrodesulfurization (HDS). Also, this bulk catalyst system was found to be thermally stable. These were observed in tests with a highly aromatic cracked distillate. To further exploit this unconventional catalyst system, two questions immediately come to mind. First, to what extent can the high HDN selectivity observed with a cracked distillate be carried over to other feedstocks such as uncracked oils of different boilingpoint range? Second, is the high HDN selectivity a sure sign for high aromatics hydrogenation (or HDA for short, standing for hydrodearomatization)? Model-compound studies with conventional hydroprocessing catalysts have suggested that HDN is linked to HDA because hydrogenation is a kinetically important step in HDN of multiring nitrogen heterocycles (Girgis and Gates, 1991;Ho, 1988). However, in HDS of multiring sulfur heterocycles such as benzonaphthothiophenes, hydrogenation is also a kinetically important step (Girgis and Gates, 1991). These questions motivated the present study. The emphasis of this study is on HDA. It is known that achieving deep HDA with sulfide catalysts generally requires much more severe conditions than those used for HDS and HDN, mainly for the following reasons. First, hydrotreating feedstocks typically contain far more aromatics than sulfur and nitrogen compounds combined. Also, HDS and HDN reactions produce aromatics. Second, aromatic compounds generally have lower adsorptivity than aromatic sulfur and nitrogen compounds. Third, the hydrogenation of monoaromatics is very slow. Finally, current commercial sulfide catalysts are basically developed for heteroatom removal rather than for aromatics hydrogenation. In this study we evaluate the aromatics hydrogenation activity of the bulk FeMo catalyst against that of a supported CoMo catalyst which is HDS selective. The reaction conditions used were relatively mild so that the predominant reactions are HDS and HDN. The results 0888-5885/93/2632-1568~0~.0~/Q0
help us gain a better understanding of the interactions between heteroatom removal and aromatics hydrogenation. 11. Experimental Section 11.1. Catalysts. The precursor to the bulk FeMo sulfide catalyst used here is bis(diethy1enetriamine)iron thiomolybdate, F~(HzNCHZCHZNHCHZCHZNHZ)ZMOS~, which has a well-defined coordinative structure shown in Figure 1. The preparation of this compound has been detailed by Ho e t al. (1986, 1992a). Briefly, the compound is formed as precipitate from the following reaction:
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Fe(H,NCH,CH,NHCH,CH,NH,)~ + MoS," Fe(H,NCH,CH,NHCH,CH,NH,),MoS4~(1) Prior to use, the precursor compound was thermally decomposed to remove their organic constituents. This was done in a separate downflow fixed-bed reactor. It is advantageous to decompose the precursor with a sulfurbearing stream, even though the precursor compound already has sufficient sulfur required for the formation of final working catalysts (Ho et al., 1992a,b). To examine how the aromatic composition of the oil is affected by catalyst selectivity toward HDS and HDN, an HDS-selective CoMo catalyst was also evaluated. Its composition and physical properties are COO,4.5 wt % ; MOOS,16 wt ?4 ; SiOz, 1.0 wt % ;A1203,78.5 wt % ;surface area, 270 m2/g;and pore volume, 0.53 cm3/g. Notice that the support is SiOz-Al203, which is more acidic than the commonly used A1203 support (Kramer and McVicker, 1986). This catalyst was presulfided at 360 "Cand ambient pressure for 1 hour with a 10% HzS-in-Hz gas mixture. 11.2. Feedstock. The feedstock used was a light virgin gas oil. Some of its properties are listed in Table I. The nitrogen content of this feed is much higher than that of the distillate feed used in the previous study (Ho et al., 1992a). The analysis of molecular composition of the feed is described in the section to follow. The results can be found in the Results section (Effects of LHSV). 11.3. Analysis. After purging with nitrogen, the liquid products were analyzed for total sulfur by X-ray fluorescence using a Princeton Gamma-Tech Model 100 with a 55Feradioactive source. Total nitrogen is analyzed by the Antek combustion method, which utilizes chemilumines1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No. 8, 1993 1569
NF
In
m;,
Figure 1. Coordinative structure of bis(diethy1enetriamine)iron thiomolybdate,Fe(H2NCH2CH2NHCH&H2NH2)2MoSd. Table I. Feedstock Properties boiling range, O C density (15"C), kg/m3 sulfur, wt % nitrogen, ppm carbon, wt % hydrogen, w t % GC distillation,wt % 5 10 50 70 90 Table 11. Reaction Conditions temperature, O C pressure, MPa LHSV,(v/v)/h
385-570 0.9378(19.3API at 60 OF) 3 loo0 85 11.98
0 .z
0 .b
1.o
LHSV-I
fF
-T-
437 O C 456 O C 505 O C 527 O C 565 O C 340,370,385 4.4,7.0 1.0-2.25
cent detection of nitric oxide. Total carbon and hydrogen were determined by l3C and 'H NMR (Newport). Clay-gelseparation (ASTM D2007) was used to separate the liquid product into polars, aromatics, and saturates. Following this, the saturate and aromatic fractions were characterized by mass spectrometry (ASTM D2786-71for saturates, ASTM D3239-76 for aromatics) according to the ring numbers (paraffins are classified as zero-ring naphthenes). These procedures provided concentrations of 7 naphthenic ring-number fractions and 21 aromatic ring-number fractions. The latter also included three thiophenoaromatic fractions (benzothiophenes, dibenzothiophenes, and benzonaphthothiophenes). It should be mentioned that the saturate fraction contained a small amount of monoaromatics which could not be analyzed by mass spectrometry. It was assumed that the relative distributions of alkylbenzenes and naphthenobenzenes in the saturate fraction are the same as those in the aromatic fraction. 11.4. Procedure. The activity tests were carried out in a cocurrent, upflow, fixed-bed reactor contained in a fluidized sand bath. The reactor was made of a 3/8-in.i.d. 316 stainless steel pipe and was equipped with a calibrated feed buret, a pump, agas-liquid separator, and a product collector. Hydrogen pressure and flow were controlled by a computer. The reactor was filled with 30 cm3 of catalyst in 20-40 mesh size. It took about 50 h for the catalysts to line out their activities. The bulk FeMo catalyst has apacking density of about 1.1g/cm3,compared to ca. 0.8 g/cm3 for the commercial catalyst. In all runs a large excess of hydrogen was used, corresponding to an Hz-to-oil ratio of 4000 SCF/B (standard cubic feet per barrel). Table I1lists other conditions used in this study. No catalyst deactivation was noted throughout the study, consistent with the results of accelerated aging reported previously (Ho et al., 1992a). For some runs, hydrogen consumption calculations were made. These were based on the increased hydrogen contents in the liquid and gaseous products. The HzS and NH3 contents in the off-gas were calculated by the extents of sulfur and nitrogen removal. The off-gas hydrocarbons were calculated by the difference in carbon contents of feed and product liquids, and the distribution of light ends in terms of carbon number is sssumed to be
L H d Figure 2. (a, top) First-order kinetic plots for HDN on bulk FeMo (open circles) and commercial CoMo/SiOrAlzOs (solid circles) catalysts at 370 "C and 7.0 MPa. NF and Npare ppm of nitrogen in the feed and liquid products, respectively. (b, bottom) Secondorder kinetic plots for HDS on bulk FeMo (open triangles) and commercial CoMo/SiOp4IzOs (solid triangles) Catalysts at 370 "C and 7.0 MPa. SFand Spare wt % of sulfur in the feed and liquid products, respectively. Table 111. HDN and HDS Activity and Selectivity at 370 OC, 7.0 MPa, and 4000 SCF/B
Ksa KNO SN CoMo 10.56 (0.98) 0.85 (0.98) 0.08 FeMo 1.72 (0.92) 1.71 (0.96) 0.99 a KN, cm3 of oil/(cmaof cat-h);Ks, cms of oil/(cms of catah& % ).
uniform. It should be noted that the amount of light ends was found to be very small; the liquid yields for all the runs were around 100% by weight because the reaction conditions are relatively mild. 111. Data Analysis
The HDS volumetric activity was determined by calculating the average of pseudo-second-orderrate constanta (assuming plug flow) for liquid products collected at different on-oil times but at the same condition. The HDN volumetric activity was determined by the rate constant of pseudo-first-order kinetics. For details of the kinetic treatment, the reader is referred to Ho et al. (1992a). For a given feedstock, the selectivity of the catalyst for HDN relative to HDS, denoted by SN,can be conveniently defined as the ratio of the HDN rate constant KN to the HDS rate constant Ks;that is, SN= KN/Ks.
IV. Results IV.1. HDN and HDS. The HDN and HDS kinetic plots for both catalysts at 370 "C and 7.0 MPa are shown in Figure 2. The regression results are summarized in Table 111, in which the numbers in the parentheses are correlation coefficients of the fits. Within the range of conditions employed, the bulk FeMo catalyst is about 12 times more selective for HDN than the commercial CoMo catalyst. This selectivity difference is quite comparable to that observed with a light catalytic cycle oil under a different set of conditions (Ho et al., 1992a).
1570 Ind. Eng. Chem. Res., Vol. 32,No. 8,1993 O C
385
370
O C
340
370
340
4.0
15.6
16.4
Figure 4. Relative selectivitym aa a function of 1/T at 7.0 MPa and 1.0 LHSV.
15.2
16.1
Figure 6. Relative eelectivitym aa a function of pressure a t 370 OC and 1.0 LHSV.
linear Arrhenius behavior for both K N and Ks. This suggests that there is no significant spread among the activation energies of the individual reactions. Another possible explanation is that the temperature dependence of the lumped rate constant is dominated by one of the individual rate constants. The apparent Arrhenius behavior may also be partially attributed to the fact that the range of temperature studied (45 "C) is not particularly wide. To compare SN'Sfor the two catalysts over a range of conditions, we further define
14.5
16.4
Figure 3. (a, top) HDN Arrheniua plots for bulk FeMo (opencirclee) and commercial coMo/sio~&& (solid circles) catalysta at 7.0MPa. (b, bottom) HDS Arrhenius plots for bulk FeMo (open circles) and commercial CoMo/SiOrAl2Os (solid circles) Catalysts at 7.0 MPa.
The temperature dependencies of K N and Ks for both catalysts at 1.0 liquid hourly space velocity (LHSV) and 7.0MPa are depicted in Figure 3a and 3b, respectively. and E m s with The apparent activation energies EHDN the FeMo catalyst are, respectively, 30 and 65 kcal/mol, while those with the commercial CoMo catalyst are 23 and 55 kcal/mol, respectively. Note that K N and Ks are overall, lumped rate constants, representing a complex composite of the rate constants of the individual hydroremoval reactions. If the individual rate constants follow the Arrhenius behavior, there is no a priori reason why the lumped rate constant should also obey the Arrhenius law (Golikeri and Luss, 1972). Yet Figure 3 shows the usual
= SN,FeMdSN,C~M~
(2) Shown in Figures 4 and 5 are the variations of VN with temperature and pressure at 1.0 LHSV,respectively. As can be seen, the FeMo bulk sulfide becomes more selective than the supported CoMo catalyst at high pressures or at high temperatures. The straight lines here are arbitrarily drawn and therefore have no theoretical significance. Having reported the results on heterocyclic aromatics, we next examine how the homocyclic (or carbocyclic) aromatics change accompanying the hydroremoval reactions. By virtue of the difference in heteroatom removal selectivity, the hydrogenations of homocyclic aromatics on the two catalysts proceed in different environments. The environment created by FeMo is relatively rich in NH3 and in organosulfur compounds, while that created by CoMo is relatively rich in H2S and in organonitrogen compounds. IV.2. Changes in Aromatic Composition. To simplify the data analysis, we follow the changes in the following concentration lumps: (1) total homocyclic aromatics, (2) monoaromatics (alkylbenzenes, naphthenebenzenes, dinaphthenebenzenes), (3) diaromatics VN
Ind. Eng. Chem. Res., Vol. 32, No. 8, 1993 1571 =.-I I
Mono-
D1-
3+-
I
0.2
I
I
0.6
I 1 .o
l/LHSV
Figure 6. Concentrations (wt %) of totalhomocyclic aromatics and saturates as functions of contact time (1/LHSV) at 370 OC and 7.0 m a - -,COMO(solidtrianglesand circles); -, FeMo (opentriangles and circles).
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(naphthalenes, acenaphthenes, acenaphthalenes), and (4) higher aromatics (3+-ringaromatics). For the monoaromatic fraction, we also follow the concentrations of two sublumps: alkylbenzenesand naphthenoaromatics. Note that the concentrations of these lumps in the feed are shown in Figures 6-10. IV.2.1. Effects of LHSV. A. Changes in Overall Homocyclic Aromatics. Figure 6 compares the concentrations (wt %) of total homocyclic aromatics and saturates as functions of contact time (l/LHSV) for both catalysts at 370 OC and 7.0 MPa. Again the curves in this figure and those in all figuresto follow are arbitrarily drawn and do not represent the results of regression or modeling. With either catalyst,one immediately sees that a contact time (l/LHSV) of 1h is too short to achieve a net decrease in homocyclic aromatics. Yet, at this condition, CoMo achieved a 97 % HDS and a 57 % HDN while FeMo gave a 83% HDS and a 83 % HDN. Apparently, the increases in homocyclic aromatics and saturates are primarily due to heteroatom removal. In other words, heteroatom removal is the primary reaction occurring on either catalyst. The higher saturate buildup with CoMo results from the catalyst's high HDS activity and the fact that the feed contains far more sulfur than nitrogen. Note that the saturate fraction contains a small amount of monoaromatics which increase as reaction proceeds. The buildup of saturates over the CoMo catalyst is rather fast at first but then becomes increasingly slow as HDS gets deeper and deeper. The accompanying change in total aromatics goes through a maximum and then appears to level off. This suggests that after removing the bulk of the sulfur, the catalyst was rather slow in attacking the aromatic compounds. In contrast, the saturate make on the FeMo catalyst is low initially but increases steadily with time. The concentration of homocyclic aromatics goes through a maximum and then falls. The total aromatics with FeMo at 1/LHSV = 1.0 h was lower than that with CoMo. This suggests that, after removing a substantial amount of nitrogen, the FeMo catalyst becomes effective in hydrogenating aromatics, despite the fact that there is still an appreciable amount of organosulfur compounds.
0.2
0 .b
1 .o
l/LHSV
Figure 7. Concentrations (wt %) of monoaromatics, diaromatics, and 3+-ringaromatics as functions of contact time at 370 OC and 7.0 MPa. - - -, CoMo (solid symbols); -, FeMo (open symbols).
The behavioral differences between the two catalysts can be seen more clearly by looking at the distributions of different aromatic ring-number fractions, as discussed below. B. Intraconversion within Homocyclic Aromatics. The concentrations of three aromatic lumps as functions of 1/LHSV at 370 "C and 7.0 MPa are displayed in Figure 7. As expected, the reduction of multiring aromatics proceeds in a stepwise manner and virtually stops at the monoaromatic step. The result is a significant buildup of monoaromatics. Clearly,in order to achieve an appreciable net reduction of monoaromatics, the reaction time will have to be much longer than 1 h. As will be seen next, over the conditions tested, the hydrogenation reactions are not equilibrium controlled. The buildup of monoaromatics was faster and larger with the HDS-selectiveCoMo catalyst, and there is a rapid initial reduction of 2- and 3+-ringaromatics in the absence of significant HDN. After this initial period, further reduction of these heavy aromatics became quite slow (actually the amount of diaromatics increased slightly). Apparently, the CoMo catalyst had difficulties hydrogenating heavy refractory aromatics, although it was able to hydrogenate the reactive ones. That the catalyst is supported on SiOAl203 may be a factor here: the support offers acidic sites on which these large and heavy aromatics can preferentially adsorb (Ho et al., 1992b) and undergo hydrogenationmost probably via protonation followed by hydride transfer (Pines, 1981). The bulk FeMo catalyst exhibited just the opposite behavior. Initially, there was not much reduction in 2and 3+-ringaromatics. However,the catalyst became more effective in attacking heavy aromatics after removing a significant amount of nitrogen, presumably due to competitive adsorption. This attack occurred at HDS levels much lower than those attained by the CoMo catalyst. This is consistent with the observation that, at a 50% HDN, FeMo consumed about 300 SCF/B of hydrogen vs 600 SCF/B for CoMo. IV.3. Effects of Temperature. The effects of temperature on the reduction of total homocyclic aromatics at 7.0 MPa and 1.0 LHSV are depicted in Figure 8. The data for the FeMo catalyst suggest that the maximum increase in homocyclic aromatics might have occurred at around 340 OC. With the CoMo catalyst, the maximum
1572 Ind. Eng. Chem. Res., Vol. 32, No. 8,1993
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50
\
\
\
\
\ \
-
48
\
\ A' \
\
Hornocycl iC Aromatics 46
W T . yo 44
I
I
I
360
380
'
340
I
TEMP, "C Figure 8. Effect of temperature on the reduction of total homocyclic aromatics at 7.0MPa and 1.0LHSV. - - -,CoMo (solid triangle); -, FeMo (open triangle).
340
380
TEMP,
'C
Figure 9. Effect of temperature on the ratio of alkylbenzenes to naphthenobenzenes at 7.0 MPa and 1.0 LHSV. - - -, CoMo (solid circles); -, FeMo (open circles).
A-------------*
t Homocycl i c Aromatics 48
catalyst (96% HDN with FeMo and 71% HDN with CoMo). It should be mentioned that with either catalyst a reduction in total monoaromatics cannot be achieved at 385 "C. A related question is whether there is interconversion between different types of monoaromatics. As Figure 9 shows, the ratio of alkylbenzenes to naphthenoaromatics at 7.0 MPa and 1.0 LHSV remained virtually constant over 340-385 "C, suggesting that ring opening of naphthenomonoaromatics to alkylbenzenes did not appear to occur. IV.4. Effect of Pressure. The hydrogen pressure dependence of overall hydrogenation is shown in Figure 10. The data were obtained at 340 "C and 1.0 LHSV. As expected, the concentrations of total homocyclic aromatics decreased with increasing pressures. The CoMo catalyst, being preoccupied with HDS, showed a very weak pressure dependence. We may remark that HDS generally has a weaker pressure dependence than HDN (Ho, 1988). V. Concluding Remarks
Using an entirely different feedstock, we have confirmed the high HDN selectivity of the bulk FeMo catalyst reported previously (Ho et al., 1992a). This catalyst most likely provides more active sites for HDN because of the higher HDN activation energy with FeMo than with commercial catalysts, as found here and in Ho et al.'s study (1992a). The present results suggest that catalytic sites for HDN are not the same as those for HDS and that HDA sites are linked to HDN sites. Qualitatively, the differences between the two catalysts can be stated as follows. The bulk FeMo sulfide is a good HDA catalyst because of its high HDN selectivity and activity. However,the downside to this property is that the active sites on FeMo have such a strong affinity for nitrogen compounds that significant HDA does not occur until the bulk of nitrogen compounds is removed. On the other hand, the supported CoMo catalyst has HDS as its primary reaction. Its active sites appear to have less of an ability to discriminate the nitrogen and aromatic compounds. As a result, HDA is less vulnerable to nitrogen inhibition. The acidity of the oxide support also plays a role here: it can facilitate HDA of some large and presumably sterically unhindered aromatic compounds. Literature Cited
WT. ' I .
46
I
4 .o
I
5 .o
I
t
6 .O
7.0
PRESSURE, MPa
Figure 10. Effect of hydrogen pressure on hydrogenation of total homocyclic aromatics at 340 "C and 1.0LHSV. - - -,for CoMo (solid triangles); -, for FeMo (open triangles).
must likely have occurred at a temperature below 340 "C. Neither catalyst was equilibrium limited. At 385 "C FeMo could reduce the total homocyclicaromatics to a level below that in the feed while CoMo could not. At this temperature both catalysts achieved comparable HDS levels (1007% for CoMo and 96% for FeMo). Hence, the difference in HDA activity between the two catalysts should be largely attributed to the highter HDN activity of the FeMo
Girgis, M. J.; Gates, B. C. Reactivities, Reaction Networks, and Kinetics in High-pressure Catalytic Hydroprocessing. Ind. Eng. Chem. Res. 1991,30,2021-2058. Golikeri, S. V.; Luss, D. Analysis of Activation Energy of Grouped Parallel Reactions. AIChE J. 1972,18,277-282. Ho, T. C. Hydrodenitrogenation Catalysis. Catal. Rev.-Sci. Eng. 1988,30 (11,117-160. Ho,T. C.;Young, A. R.; Jacobson, A. J.; Chianelli, R. R. U S . Patent 4591429,assigned to Exxon Research and Engineering Co., 1986. Ho, T. C.; Jacobson, A. J.; Chianelli, R. R.; Lund, C. R. F. Hydrodenitrogenation-Selective Catalyete: 1. Fe Promoted Mo/W Sulfides. J. Catal. 1992a,38, 351-363. Ho, T. C.; Katritzky, A. R.; Cato, S. J. Effect of Nitrogen Compounds on Cracking Catalysts. Znd. Eng. Chem. Res. 199213,31, 15891597. Kramer, G. M.; McVicker, G. B. Hydride Transfer and Olefin Isomerization aa Tools to Characterize Liquid and Solid Acids. Acc. Chem. Res. 1986,19,78-84. Pines, H. The Chemistry of Catalytic Hydrocarbon Conversions; Academic Press: New York, 1981;pp 99-100. Received for review December 9, 1992 Revised manuscript received April 23,1993 Accepted May 14, 1993
