Ind. Eng. C h e m . Res. 1995,34, 3970-3973
3970
GENERAL RESEARCH
Model Compounds for Light Cycle Oil Conversion L. Deane Rollmann,” Paul A. Howley, Dominick N. Mazzone, and Hye Kyung C. Timken Mobil Research and Development Corporation, P.O. Box 480, Paulsboro, New Jersey 08066
The reaction of dimethyl- and trimethylbenzenes with dibenzothiophene (DBT), phenanthrene, and a number of other polynuclear aromatics characteristic of light cycle oils (LCOs) has been explored over zeolites ZSM-5, Y, and Beta. In catalyzing the reaction between trimethylbenzene and DBT, for example, zeolites Y and Beta yielded methyl-substituted DBTs in greater than 80% selectivity at DBT conversion levels exceeding 90%. With smaller ring structures, e.g., in the reaction between rn-xylene and benzothiophene, ZSM-5 was similarly effective (but somewhat more prone to aging). It was also shape-selective in its product distribution. Transmethylation provides a simple and effective synthesis route to the polymethyl polynuclear aromatics and thiophenes (PNAs and PNTs, respectively) needed in the search for improved catalytic routes to very low sulfur distillate and other clean fuels products.
Introduction
MDBTr
New regulations are strongly impacting the sulfur and aromatics content of diesel fuels and thus the operating configuration of many refineries (Lee, 1991). One result has been significant growth in worldwide hydrogenative desulfurization (des) capacity. From a research perspective, the new deep des requirements focus attention on the large reactivity differences which exist among the various sulfur components of distillate fuels. The greater reactivity of benzo- vs dibenzothiophenes,for example, has long been known (Frye and Mosby, 1967; Girgis and Gates, 1991). In the face of very low sulfur specifications, additional reactivity relationships become important, namely, distinction in the ease of des among the various methyl and polymethyl isomers of benzo- and dibenzothiophene (BT and DBT, respectively). The relatively low reactivity of certain sterically hindered methyl and dimethyl DBTs, most notably the 4- and 4,6-derivatives, has been quantitatively defined in model compound experiments at the University of Delaware and confirmed elsewhere (Kilanowskiet al., 1978;Houalla et al., 1980; Girgis and Gates, 1991; Shih et al., 1992; Kabe et al., 1992, 1993). (See Figure 1 for the isomer numbering convention.) Methyl and polymethyl PNAS/€”Ts typically arrive at a refinery hydrogenation unit from one of three sources, a distillation tower, a coker, or a fluid catalytic cracking (FCC) unit. They are well-recognized components of individual crude oils and, in many cases, are useful geochemical characterization tools. For example, isomer distribution among methyl- and dimethylnaphthalenes, phenanthrenes and DBTs is reported to change in a regular fashion when kerogen-containing petroleum source rock is subjected t o increasing temperature for increasing lengths of time, i.e., during “maturation” (Radke, 1987). The more stable 2- and 3-methylphenanthrenes (2- and 3-MPhen, respectively) grow in abundance relative to the 1-,4-, and g-MPhen’s, for example. Similarly, 2- and 3- methyldibenzothiophenes (2- and 3-MDBT)increase relative to the 1and 4-isomers. 4-MDBT reportedly increases relative t o 1-MDBT as crude oil matures. 0888-588519512634-3970$09.00/0
Figure 1. Product distribution example for methylated DBT. Peak assignments based on Vassilaros et al., 1982, and Chawla and Di Sanzo. 1992.
The LCO product from a cracking unit is rich in methyl and polymethyl PNAsPNTs. It is not uncommon to find 70-80% aromatics and 2-4% S in an LCO, largely in the form of methyl-substituted naphthalenes (Naph‘s), Phen’s, BTs, DBTs, and other related molecules (McPherson and Bourgeais, 1989; Danaher and Palmer, 1988;Anabtawi and Mi, 1991). Survival in the FCC unit provides strong evidence for the chemical stability of these polynuclear ring structures. Individual methyl and polymethyl PNA/€”T isomers can be synthesized. To use DBT as an example, however, the preparations are often complex and sometimes even uncertain with respect to ultimate product selectivity (Gerdil and Lucken, 1965; Campaigne et al., 1969;Katritzky and Perumal, 1990). The present paper describes a simple route to mixtures of methyl and polymethyl PNAsPNTs, mixtures useful in exploratory fuels-related sulfur and aromatics removal experiments (des and deA, respectively). It also demonstrates the relative ease of methyl group transfer among the various aromatic ring structures characteristic of refinery unit product streams.
Experimental Section Three catalysts were used in the transmethylation experiments, all generated internally, all containing 0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 3971 35% alumina binder, and all in the “hydrogen” form: ZSM-5 (Argauer and Landolt, 1972), Beta (Wadlinger et al., 1967), and “ultrastable” Y (USY)(Breck, 1974). Framework sioz/&o3 ratios for the three were approximately 70, 50 and 200, and pore diameters were 5.3 x 5.6 A, 6.4 x 7.6 A, and 7.4 A, respectively (Meier and Olson, 1992). Transmethylation experiments were conducted in a downflow tubular reactor, using 24/60 mesh catalyst. Typical reaction conditions were 3.5 MPa (500 psig), 440 “C inlet temperature, 2-6 WHSV, and 2/1 Hhydrocarbon (HfiC) mole ratio. The standard feed was 10 mol % PNAPNT in rn-xylene (mX), 1,2,44rirnethylbenzene (124TMB) or mesitylene (135TMB). A thermal blank was run with mX and DBT at 485 “C, 2 WHSV, using 14/60 mesh vycor silica. Conversion was -1%. Products were analyzed by gas chromatography on a DB-1 column (60 m x 0.25 mm i.d., 0.25 pm film thickness, from J & W Scientific, Folsom, CA) with a flame ionization detector (FID) and a temperature program as follows: 30 min at 25 “C, 3 “C/min t o 250 “C, and 250 “C for 30-60 min. This temperature program did not resolve m-and p-xylene, but o-xylene was invariably 23-24% of the product xylenes, indicating the presence of strong aromatic isomerization activity. Thermodynamic equilibrium for o-xylene at standard run conditions is 24.6% (Stull et al., 1969). Peaks were identified on the basis of literature reports (Vassilaros et al., 1982; Andersson, 1986; Chawla and Di Sanzo, 1992). In addition, as noted in the Acknowledgement section, certain key DBT derivatives were synthesized for GC identification purposes at Chestnut Hill College, Philadelphia, PA. FID GC response factors were determined for both BT and DBT and used for both the unsubstituted parents and all iosmers of their respective methylated products. Subsequent spot-check analyses with a Sievers Model 350B Sulfur Chemiluminescence Detector confirmed the FID results. In every case, transmethylation material balance and conversion data were taken after several hours on stream with a particular feed, but aging behavior was not a target in this study. Catalysts were “on stream” continuously for 1-2 weeks, and model compounds were normally fed only during the day. Hydrogen flow was continued overnight and over weekends. Note that “conversion” of BT and DBT does not indicate desulfurization. It indicates formation of methyl and dimethyl derivatives. Feeds were changed by simply flushing the unit with hydrogen overnight at reaction temperature. All PNAsPNTs were purchased from Aldrich Chemical and were used without further purification. Our sample of LCO was obtained from Mobil’s Paulsboro refinery. Hydrogenation experiments were conducted with a commercial NiMo/AlzOs catalyst a t 6.3 MPa (900 psig), 7 HfiC, 0.5-3 WHSV, and 288-400 “C. The catalyst was sized t o 24/60 mesh and contained 3% Ni and 16% Mo. Sulfur distribution in the feed and liquid products was determined with the Sievers Model 350B.
Results and Discussion ZSM-6. An initial scoping experiment with ZSM-5 demonstrated effective methyl group transfer from xylene t o BT, smallest of the multiring thiophenes. The experiment can be summarized as follows: (Details are in Table 1.) BT, with an estimated critical dimension of I6 A, was able to enter the ZSM-5 pore system and was readily
Table 1. ZSM-5 Run Summary, 440 “C thiophene BT WHSV 4.2 conversion, w t % BT, DBT 70 xylene 28 thiophene distribution in product, mol % BT, DBT 39 MBT, MDBT 39 DMBT, DMDBT 22 TMBT, TMDBT €1 H2S, wt % 0.00 a
DBT 4.3
DBT 2.1
-5 35
-10 46
98 2 0 0
96 3 1 0 0.05=
0.00
Corresponds to 1% DBT desulfurization.
Table 2. Zeolite Beta Run Summary, 440 “C, 5 WHSV feed mX+DBT mX+DBTD m X + P h e n conversion, wt % PNMNT 83 59 36 xylenes 38 22 12 PNAPNT distribution in product, mol % parent 19 44 78 monomethyl 40 39 18 dimethyl 30 15 4 trimethyl+ 11 3 0 HzS, wt % 0.32* 0.13b 0.00 a After 33 h of liquid contact. 0.1%H2S corresponds to 3% DBT des.
converted to MBTs and DMBTs, as shown below. Even at very significant conversion, methyl group transfer to BT was a relatively “clean”reaction. Selectivity to MBT and DMBT exceeded 80%. DBT, on the other hand, was largely unconverted, presumably due t o its large size. The BT transmethylation process in ZSM-5 was “shape selective,” namely, it yielded preferentially the smallest of the various possible MBT and DMBT isomers. At 70% BT conversion, over 95% of the MBTs were the 2-, 3-, 5-, and 6-position isomers. The ratio of (2-MBT 3-MBT)/(5-MBT 6-MBT)was 2.0. (Whereas 7-MBT was nearly absent with ZSM-5, later experiments with the large-pore USY yielded products with 7-MBTI2-MBT ratios of 0.6-0.7.) Moreover, only 3 of the 15 possible DMBT isomers were significant in concentration, and virtually no trimethylBT (TMBT) was obtained with ZSM-5. Beta. With the feasibility of methyl transfer demonstrated, focus was shifted t o a high Si02/A1203zeolite with a larger pore, Beta. Note that liquid was fed to the catalyst only during workday hours. Hydrogen alone was the feed overnight and over weekends, with reactor temperature maintained at 440 “C. An activity decline was often observed during the 8-h on-stream time, and catalyst reactivation by overnight hydrogen contact was apparent. As shown in Table 2, DBT transmethylation occurred readily in the large pore system of Beta. Despite some apparently irreversible carbon deposition and initial catalyst aging, over 80% DBT transmethylation was achieved with a selectivity approaching 90%. As would be expected, Phen was less reactive than DBT, but it too was converted to mono- and dimethyl derivatives in high selectivity. An example product distribution from DBT is shown in Figure 1. USY. USY is an alternative example of a large-pore zeolite, and its catalytic behavior in the mX/DBT and -hen reactions was explored for comparison with that of Beta, as summarized in Table 3. Both zeolites were found to be effective transmethylation catalysts, but differences do exist. For example, the USY sample
+
+
3972 Ind. Eng. Chem. Res., Vol. 34, No. 11,1995 Table 3. Transmethylation with Xylene, Zeolite USY, 440 “C. 5-6 WHSV feed mx+ mx+ mX+ mx+ mx+ BiP DBT Phen Flu0 Naph conversion, wt 9% 40 PNAPNT 80 40 46 44 23 xylene 28 9 12 13 PNAPNT distribution in product, mol % 58 26 68 54 61 parent 22 43 28 35 31 monomethyl 10 31 4 11 8 multimethyl 0.00 HzS, wt % 0.15” 0.00 0.00 0.00 a
Methyl-substituted dlbenzolhlophenes DBT
I
. MDBT
Rn
-
DMDBT
n
TMDBT
Light cycle oil
Corresponds to 4% desulfurization of DBT.
Table 4. Results with Trimethylbenzenes, Zeolite USY, 440 “C. 5-6 WHSV Phen Phen substrate PNAPNT DBT DBT methyl donor 135TMB 124TMB 135TMB 124TMB conversion, wt B 58 93 PNMPNT 96 66 PNMPNT distribution in product, mol B 32 40 parent 3 6 45 43 26 monomethyl 18 19 16 38 dimethyl 37 2 4 29 trimethyl+ 42 0.00 0.00 HzS, wt R 0.09Q 0.10“ a
I
Corresponds to 4-MDBT > 4,6-DMDBT TMDBTs
’
-
Very satisfactory agreement is noted between the above sequence, taken with an actual refinery feed and a NiMo catalyst, and the CoMo results published earlier for individual MDBT and DMDBT compounds (Houalla et al., 1980). The University of Delaware group reported 2,8-DMDBT > 3,7the following ordering: DBT DMDBT > 4-MDBT > 4,6-DMDBT.
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Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 3973
Conclusions Transmethylation over large-pore zeolites was shown to be an effective route t o a broad range of polymethyl polynuclear aromatics and thiophenes. Protonation of the single-ring aromatic (the methylating agent) appeared t o dominate the process. DBT (and BT) showed particularly high reactivity in this acid-catalyzed reaction, a reactivity attributed to the S atom in the molecule. The molecular integrity of DBT was very largely maintained, under all conditions tested. The resultant product mixtures afford additional insight into the severe process requirements which accompany new fuel sulfur and aromatics specifications.
Acknowledgment We are grateful to Dr. Helen Burke and the chemistry students at Chestnut Hill College, Philadelphia, PA, for several GC identification samples of methyl- and dimethyldibenzothiophenes. In addition, special thanks go to W. R. Morgan, Jr., and W. Weimar for their laboratory assistance and to B. Chawla, N. A. Collins, F. G. Di Sanzo, G. A. Jablonski, D. 0. Marler, and R. A. Wolny for their GC help and advice.
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Received for review November 1, 1994 Accepted April 3, 1995@ IE940632S
Abstract published in Advance A C S Abstracts, October 15, 1995. @
