Energy & Fuels 1987,1, 323-331 Our fluidized-bed procedure appears to be an excellent means of determining kinetics for char oxidation. The large mass of inert quartz sand, relative to the mass of shale, provides a suitable heat sink that enables highly exothermic reactions to be investigated under nearly isothermal conditions. In addition, the specially designed gas distributor gave a very well-stirred reaction medium, with uniform temperature and no stagnation regions. Attrition and elutriation of the shale are the principal potential disadvantages. These were held to an acceptable level by using the minimum gas flow rate that gave a flow-independent reaction rate. Less than 10% of the shale was
323
elutriated. Most of this occurred during the pyrolysis step of the process, which did not interfere with subsequent measurement of oxidation kinetics. It would be desirable to measure the rate of oxidation of identical samples of retorted shale by different methods in order to observe the role of the apparatus and procedures.
Acknowledgment. Helpful discussions with A. K. Burnham and H. Y. Sohn are gratefully acknowledged. This work was performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under Contract No. W-7405-ENG-48.
Catalytic Hydrodeoxygenation of Dibenzofuran Vito LaVopa and Charles N. Satterfield" Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received July 24, 1986. Revised Manuscript Received February 1 1 , 1987
The hydrodeoxygenation of dibenzofuran (DBF) on a sulfided NiMo/A1203catalyst was studied at 350-390 "C and 7.0 MPa. The major products isolated were single-ring hydrocarbons, cyclohexane predominating; the remainder were double-ring hydrocarbons, cyclohexylbenzene predominating. No oxygen-containing species other than water were isolated in any significant amount. The initial reactions in the hydrodeoxygenation of DBF are rate-limiting. The non-sulfided ("oxide") catalyst is much less active, and double-ring products predominate over single-ring products. From studies of possible intermediates it appears that on a sulfided catalyst two pathways operate in parallel for the hydrodeoxygenation of dibenzofuran (Figure 14): (1)hydrogenation of DBF to hexahydro DBF, which reacts via 2-cyclohexylphenol to form single-ring hydrocarbons; (2) direct hydrogenolysis via 2-phenylphenol, without prior ring hydrogenation, to form biphenyl and cyclohexylbenzene (a minor route). On this catalyst the overall reaction is first order with respect to hydrogen and to DBF and exhibits an apparent activation energy of 67 kJ/mol.
Introduction Liquids derived from coal and oil shale differ from petroleum in their substantially higher oxygen contents and types of oxygen compounds present. Oxygen removal from coal liquids is required to ensure fuel stability during storage1 and may be required before further processing. Important interactions may occur between hydrodeoxygenation (HDO) and other hydroprocessing reactions, which as yet are poorly understood. The oxygen-containingcompounds found in coal liquids are phenols, aryl ethers, and benzofurans, as shown by Petrakis et a1.,2 who analyzed a heavy distillate from an SRC-I1 liquid, and Furimsky? who analyzed a coal hydrogenation liquid. Even in a California petroleum distillate phenols and dibenzofuran accounted for 11% and 17% respectively of the total oxygen ~ o n t e n t . ~ A considerable literature on HDO has developed since the early 1970s. Shah and Cronauer5 reviewed 0, N, and S removal reactions in donor solvent coal liquefaction, and (1) White,C. M.; Jones, L.; Li, N. C.Fuel 1983,62, 1397. (2) Petrakis, L.; Young, D. C.; Ruberto, R. G.; Gates, B. C. Znd. Eng. Chem. Process Des. Deu. 1983,22, 298. (3) Furimaky, E. Fuel Process. Technol. 1982, 6, 1. (4) Snyder, L. R. Prepr.-Am. Chem. SOC.,Diu. Pet. Chem. 1970, 15(2), C44. ( 5 ) Shah, Y. T.; Cronauer, D. C. Catal. Rev.-Sci. Eng. 1979,20,209.
0887-0624/87/2501-0323$01.50/0
Furimsky6 reviewed the chemistry of catalytic HDO. Several model compound studies have investigated the HDO of specific furans and phenols over conventional hydroprocessing catalysts. The only previous detailed study of the products formed from dibenzofuran (DBF) and its reaction network is that of Krishnamurthy et al.7 done in a batch autoclave a t 343-376 "C and hydrogen pressures of 6.9 to 13.8 MPa, the DBF being dissolved in n-dodecane. A presulfided NiMo/A120, catalyst was used, and CS2 was added to maintain a H2Spartial pressure during reaction. They also reported studies of 2-phenylphenol and 2-cyclohexylphenol. Lee and Ollis8reported on the catalytic HDO of benzofuran and 2-ethylphenol at 69 atm and temperatures up to 340 "C, but the reaction network of these compounds is significantly different from that of dibenzofuran. A study of furan by Pratt and Christover~on~ and of tetrahydrofuran by Furimsky'O were performed under atmospheric pressure and therefore may not be representative of expected behavior under industrial processing conditions. (6) Furimsky, E. Catal. Rev.-Sci. Eng. 1983, 25, 421. (7) Krishnamurthy, S.; Panvelker, S.; Shah,Y. T. AIChE J. 1981,27,
W-_A-.
(8) Lee, C.-L.; Ollis, D. F. J. Catal. 1984, 325, 87.
Pratt, K. C.;Christoverson, V. Fuel Process. Technol. 1983,8,43. (10)Furimsky, E. Ind. Eng. Chem. Prod. Res. Dew 1983, 22, 31.
(9)
0 1987 American Chemical Society
LaVopa and Satterfield
324 Energy & Fuels, Vol. 1, No. 4, 1987 Badilla-Ohlbaum et a1.l1J2and Rollmann13found that dibenzofuran was the most unreactive of a group of compounds tested in mixtures, including heterocyclic S and N compounds. Rollmann also substituted several other
phenol and furan derivatives one at a time for DBF in the mixture and showed that DBF was the least reactive of the group. Li e t al.14 reported that DBF was 1 order of magnitude less reactive than the phenols present in an SRC-I1
liquid. We chose dibenzofuran for this study since industrially, removal of the least reactive oxygenated compounds is the bottleneck for obtaining a high degree of conversion. We have also explored some of the effects of HzO and H2Son t h e catalytic reaction since HzO is formed as a HDO product a n d HzS is usually present from hydrodesulfurization reactions.
Experimental Section The reactor consisted of a vertical 0.52-cm4.d. tube immersed in a fluidized sand bath to maintain isothermal conditions. Liquid was fed by a high-pressure liquid chromatography pump through a preheater coil and to the top of the catalyst bed. Hydrogen was similarly fed through a preheater coil. The hydrogen and liquid feeds mixed at the reactor inlet, passed downward through the catalyst bed, and flashed through a back-pressure regulator. This same equipment has been used for previous studies of hydrodenitrogenation and hydrodeoxygenationand is described in more detail e1~ewhyre.l~ Reactants. Dibenzofuran (99+% from Aldrich) was dissolved in a c16 paraffin mixture to form a solution with a concentration of 0.245 mol/L (-5.25 wt %). H2Sor H20 were produced in situ by addition of 1-dodecanethiol or 1-decanol, respectively, to the liquid feed. These react rapidly under reaction conditions to produce the desired products. All experiments described here were performed at 350-390 "C and 7.0 MPa total pressure, except for one series of studies to determine the reaction order with respect to hydrogen. Hydrogen was fed at a rate of 1605 mL at STP/mL liquid, equivalent to 9000 SCF/barrel (SCF = standard cubic feet), producing an initial dibenzofuran partial pressure of 24 kPa. This gas-to-liquidratio is high enough to ensure complete vaporization of the liquid.16 Note that space time is defined here per mole of DBF, not total moles of liquid fed. A space time of 300 (h g of catalyst)/mol of DBF corresponds to a liquid hourly space velocity (LHSV) of 10 h-' for the DBF concentration used here. Catalyst. Several different samples of the same NiMo/A1203 hydrotreating catalyst, Cyanamid HDS-3A (15.4 wt % Moos and 3.2 wt % NiO), were used. These are identified as samples number 21,22,23,26, and 28. The catalyst pellets were crushed and sieved to a size range of 150-212 pm. After being dried in air at 110 "C overnight, 1.6 g of catalyst was diluted 4:l with inert Sic and packed into the reactor tube. After the reactor was purged under helium flow for 12 h at 100 "C, the catalyst was presulfided with a 10 mol % H2S in hydrogen gas mixture. The gas flow rate was 40 mL/min at a total pressure of 0.24 MPa. Sulfiding temperature was maintained at 175 "C for 6 h, and the temperature was then raised to 315 "C at a rate of 1 "C/min. The gas flow was switched to helium after sulfiding had proceeded for 1 h at 315 "C. The activity of this and hydrotreating catalysts in general decreases during reaction for the first 100-200 h on stream. To achieve a steady-state activity within a reasonable length of time, we treated this presulfided catalyst with a feed containing 5 wt % quinoline at 375 "C and 3 MPa without a sulfur compound added until a steady state was observed with respect to HDN of (11)Badilla-Ohlbaum, R.; Chadwick, D. Fuel 1979, 58,834. (12) Badilla-Ohlbaum, R.; Pratt, K. C.; Trimm, D. L. Fuel 1979, 58,
309.
(13) Rollmann, L. D.J. Catal. 1977, 46, 243. (14) Li, C.-L.; Xu, 2.-X.; Gates, B. C.; Petrakis, L. Ind. Eng. Chem. Process Des. Deu. 1985, 24, 92.
(15) Satterfield, C. N.;Yang,S. H. Ind. Eng. Chem. Process Des. Deu.
1984, 23, 11.
(16) Lin, H.; Sebastian, H. M.; Chao, K. J. Chem. Eng. Data. 1980,25,
252.
Table I. Specie6 Identified by GC/MS" methylcyclopentane (cyclopentylmethy1)benzene (CPMB) benzene dicyclohexyl (DCH) cyclohexane cyclohexylbenzene (CHB) cyclohexene biphenyl methylcyclohexane dibenzofuran (DBF) (cyclopentylmethy1)cyclo- tetrahydrodibenzofuran (TH-DBF) hexane (CPMCH) a
In order of increasing retention time on SE-30 column.
quinoline. This generally occurs within 150 h on stream. In a few cases the catalyst was deactivated with dibenzofuran in the presence of H2S. The resulting steady-state activity was slightly higher, but there was no effect on product distribution. We also performed a limited number of studies on the same catalyst, under hydrogen pressure, but without presulfiding and without addition of a sulfur compound. The relative distribution of reaction producta was substantidy different, as will be shown. Analysis. Both liquid- and gas-phase samples of the reactor effluent were collected downstream from the low-pressure phase separator, and concentrations were related to the starting liquid feed. Carbon mass balances on total products compared to dibenzofuran disappearance ranged from 99 to 107%. Reaction products were routinely analyzed on a Perkin-Elmer Sigma 1B gas chromatograph equipped with dual FID detectors. Separation of reaction species was accomplished by using bonded SE-30 methyl silicone or Carbowax capillary columns. The SE-30 is a nonpolar phase and Carbowax is polar. Compounds were identified by comparing their retention times to those of pure standards. One representative liquid sample was also subjected to GC/MS analysis with a 30-m SE-30 column for positive component identification. Mass spectra of individual species were compared with published spectra if these were available,17and the compounds that were identified are listed in Table I. We believe that cyclopentane was also present although it was not detected by GC/MS. We attribute this to evaporation from this particular sample during storage. Cyclopentane is the most volatile of the species formed, and we generally observed an initial GC peak corresponding to the retention time expected for cyclopentane. This peak did not appear in the sample subjected to GC/MS. A product identified as (cyclopentylmethy1)benzene (CPMB) was detected at 13% of the concentration of its isomer cyclohexylbenzene (CHB). No published spectrum was found, and identification was based upon the fragmentation pattern. A parent ion was observed at 160 amu, and a base peak was found at 92 amu, which is characteristic of alkylphenyl compounds such as n-hexylbenzene. For routine sample analysis a Carbowax GC column was usually used. On this column CPMB was masked by the solvent peak, and hence data on CPMB are not available from our results. Inclusion of CPMB would have increased the mass balances by 1-2%. A compound identified as tetrahydrodibenzofuran was present in very low concentration and is not shown on any of the figures of results. Its assignment is based on the similarity of its spectrum to that of tetrahydrodibenzothiophene,which has an analogous structure.
Results: Studies of Dibenzofuran (DBF) Product Distribution, Sulfided Catalyst. T h e products isolated were essentially all hydrocarbons. No oxygenated species other than water were isolated in any significant amount. Single-ring compounds comprised the major portion of the hydrocarbons (about 75%), as shown in t h e upper portion of Figure 1. As shown in Figure 2, cyclohexane predominated, but also present in significant but smaller amounts were methylcyclopentane, cyclopentane, benzene, methylcyclohexane, a n d cyclohexene. Note that cyclohexane is referred to t h e right-hand scale, with t h e others referred to t h e left-hand scale. (The (17) Heller, $. R.; Milne, G. W. A. EPAINIH Mass Spectral Data Ease: NSRDS-NBS 63, Suppl.; US. Government Printing Office: Washington, DC, 1980.
Catalytic Hydrodeoxygenation of Dibenzofuran
Energy & Fuels, Vol. 1, No. 4, 1987 325
100 SI NGLE-RING
HYDROCARBONS
CYCLOHEXYL BENZENE -o-----
p---o-,
-0
SPACE TIME,(HR)(G-CAT)/MOL DBF
OF
Figure 1. Product distribution of DBF at 375 "C,7.0 kPa of H2S, and 7.0 MPa. Individual double-ring compounds are shown on lower plot.
w U " c L
o r
I
4
0
0
200
400
CYCLOHEXENE
-
600
.
0 800
SPACE T I M E , ( H R ) ( G - C A T ) / M O L OF DBF
Figure 2. Product distribution of DBF at 360 "C, 7.0 P a of H2S, and 7.0 MPa for individual single-ring compounds.
chemical acronyms used here are shown in Table I.) Selectivity is defined here and in subsequent figures as percent of DBF reacted that was converted to the specified product; i.e., allowance is made for the fact that two molecules of single-ring products are formed from each molecule of DBF reacted in this way. Except for cyclohexene the ratios of the minor single-ring hydrocarbon products to one another did not vary with space time, suggesting that they are formed by parallel reactions. No linear paraffins were detected. Cyclohexene would be rapidly hydrogenated to cyclohexane. It can plausibly be formed by elimination of water from phenol, via intermediate hydrogenation to form cyclohexanol, or directly from 2-cyclohexylphenol (see later). Figure 1 (lower portion) shows the distribution of double-ring hydrocarbon products as a function of space time. The principal species was cyclohexylbenzene. Biphenyl also appeared, indicating that one pathway involves direct removal of oxygen without prior ring hydrogenation.
Dicyclohexyl (DCH) and (cyclopentylmethy1)cyclohexane (CPMCH) increased with contact time, whereas cyclohexylbenzene and biphenyl decreased, indicating secondary hydrogenation. Comparison with Previous Literature. Our product spectrum is similar to that of Krishnamurthy et aL7in that they reported the same four double-ring compounds. Cyclohexane was found in major amounts, but they did not report any of the other single-ring compounds shown in Figure 2. (The compound we identify as (cyclopentylmethy1)cyclohexane is termed (cyclohexylmethy1)cyclopentane by them.) They also reported that a phenylhexanol was present in their isolated products, but we believe this probably was a misidentification. We compared a chromatogram of the DBF reaction products obtained with a Carbowax (polar) packed column as reported in the thesis by Krishnamurthyls to our chromatograms on a Carbowax capillary column. The elution orders of the various species were identical on the two columns. However, the peak eluting between dicyclohexyl and cyclohexylbenzene was assigned to 6-phenyl-1-hexanol by Krishnamurthy et al.,7 while we identified it as (cyclopentylmethy1)benzene (CPMB), on the basis of our GC/ MS analysis described above. Furthermore, when we spiked a representative reaction sample with known 6phenyl-1-hexanol,the alcohol was found to elute after both cyclohexylbenzene and DBF on a Carbowax column. We also compared the MS data of Krishnamurthy18with our own data for the same GC peak. The MS results of Krishnamurthy indicate contamination by air (high intensities a t mass numbers 28 and 32), but the higher mass numbers showed good agreement with ours. We conclude that the high intensities a t mass numbers 91 and 92 together with a parent peak a t mass number 160 found in both studies allow identification of this species as CPMB rather than a phenylhexanol. A phenylhexanol might plausibly be formed, but if so, it would be expected to be very reactive and unlikely to survive. On an oxide catalyst (see later) we also found a small amount of 2-phenylphenol, the latter identified by GC retention time compared to that of a known standard. Effect of Hydrogen Sulfide. While the reactor was unattended, it was maintained at 7.0 MPa of hydrogen and 360 OC, but no DBF was fed to it. Instead a solution of hexanethiol (rather than dodecanethiol) in xylene was continuously introduced. This generated 13.9 kF'a of H2S in situ. With this treatment the activity of the deactivated catalyst with respect to HDO of DBF remained stable over at least 150 h on stream when H,S was also present during reaction. Standard activity was measured at 360 OC, 7.0 MPa, and a space time of 160 (h g of catalyst)/mol of DBF. During reaction of DBF, when the thiol was removed from the feed stream, the conversion of DBF increased dramatically and then declined fairly rapidly toward its original value as shown in Figure 3. Reintroduction of H2S to the system, via the thiol, caused a drop in conversion, with the catalyst requiring over 10 h to return to normal activity. Removal of H2S a second time again increased activity. The same behavior was shown on another sample of the catalyst (sample 26) in which the effect of partial pressure of H2Swas determined more quantitatively. As shown in Figure 4, increasing the H2S partial pressure from 0 to about 5 kPa dropped the percent HDO from about 60% to about 44%, but further increases in the partial pressure to 75 kPa showed no further effect. Decreasing the H2S (18)Krishnamurthy,S. Ph.D. Thesis, University of Pittsburgh, 1980.
LaVopa and Satterfield
326 Energy & Fuels, Vol. 1, No. 4, 1987 I
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A 13.9 k Pa of H2S NO HS ,
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Total Dou bla Ring Produc t r
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5
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14
and
V
a W a tZ
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Figure 5. Increased H2Spartial pressure decreasing double-ring products. The remainder are single-ring products.
400
Figure 3. Removal of H2S increasing HDO activity. L
m
n
10
1
k Partial Pressure
Table 11. Selectivity toward Single-Ring Products on CoMo and NiMo Catalystsa % selectivitv species CoMo NiMo cyclopentane 7.1 7.0 methylcyclopentane 6.6 9.0 cyclohexane 47 50 methylcyclohexane 2.5 2.5 b b cyclohexene 3.3 3.6 benzene
Tot. single-ring products
71
72
Note that the selectivities are identical for both catalysts. Varies.
Partial Pressure
L
301 W
a
L 0
H2S
I
I
I
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PARTIAL
PRESSURE, k P a
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Figure 4. Lowering of H2S partial pressure increasing HDO activity. Transient effects are observed; 24 P a of DBF was used. partial pressure to 15 kPa from 7 5 kPa resulted in a substantial increase in activity, which increased further as H2S was removed completely. These last values are transients, however. Judging from Figure 3 these high levels of activity would not be maintained. This behavior is the opposite of that observed in the hydrodenitrogenation of quinoline-there the addition of H2S to a feed stream in contact with a presulfided NiMo/A1203catalyst increased activity instead of decreasing it.19 Removal of H2S increased the content of unsaturated products (benzene, biphenyl, cyclohexylbenzene) formed per mole of DBF reacted from about 20 mol % to 30 mol %. Figure 5 (catalyst sample 26) shows that a decrease in H2S partial pressure also increased the percent of double-ring hydrocarbon products present. This selectivity was not affected significantly by the length of time for which the catalyst had been previously exposed to the specified partial pressure of H2S. Effect of Water. The effect of water in the presence of H2Swas investigated by the addition of decanol to the dibenzofuran feed in a quantity that would double the partial pressure of water in the reactor (from 24 to 48 kPa), if dibenzofuran were completely reacted. The effects are minor. Conversion remained essentially the same. The ratio of (cyclohexylmethy1)cyclopentane to dicyclohexyl (CHMCP/DCH) increased from about 1.5 to 1.7, indi-
cating a slight increase in isomerization activity. CoMo Catalyst. The HDO of dibenzofuran was also studied briefly on a CoMo/A1203 catalyst (American Cyanamid HDS-1442B; 15.9 % Moo3 and 3.2 wt % COO) to see if there were any significant differences in the reaction network compared to that on a NiMo catalyst. After being sulfided by the standard procedure, the catalyst was exposed to dibenzofuran (24 kPa) and H2S (7 kPa). After a steady state was reached, the conversion of DBF was determined as a function of space time. The reaction was first order with respect to DBF as it is on a NiMo catalyst (see later), but the CoMo catalyst was about one-third as active as the NiMo catalyst. Table I1 shows that for this limited study the selectivities for single-ring products were identical on the two catalysts. However, in studies of other oxygenated compounds such as phenol derivatives, significant differences have been reported in the product distribution over the two types of catalysts.2o Oxide Catalyst. If the NiMo/A1203 catalyst is not sulfided, its behavior is substantially different. Sample 21 was operated under hydrogen pressure but was not presulfided, and no sulfur compound was added to the feed. This catalyst will be termed “oxide catalyst”, although the actual forms of the metallic elements present are not known. During an initial 140 h on stream at 360 “C, deactivation was completed by passing a feed containing 0.245 mol/L of dibenzofuran through the reactor a t a space time of 544 (h g of catalyst)/mol of DBF. To investigate the possible effect of water as such, 0.245 mol/L of decanol was added to the dibenzofuran feed to produce an additional 24 kPa of H 2 0 in situ beyond that formed by HDO of DBF. This increase in water partial pressure dropped the conversion from 30 to 20% at 160 (h g of catalyst)/mol. In order to reduce the proportionate change in water partial pressure with conversion, in further experiments with the oxide catalyst, we utilized feeds
(19) Satterfield,C.N.;Gultekin, S. Ind. Eng. Chem. Process Des. Dew 1981, 20, 62.
(20) Haider, G . Ph.D. Thesis, University of Utah, 1981.
Catalytic Hydrodeoxygenation of Dibenzofuran
Energy & Fuels, Vol. 1, No. 4, 1987 327 7011 I A L
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Figure 6. Double-ring product distribution over oxide catalyst. LL 0
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i
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/
I/.
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SPACE T I M E , ( H R ) ( G - C A T ) / M O L OF DBF
Figure 7. Single-ringproduct distribution over oxide catalyst.
containing 0.245 mol/L of decanol. The oxide form of the NiMo catalyst was less active than the sulfide form. The percent of HDO over the oxide catalyst a t 360 "C,7 MPa, and 160 (h g of catalyst)/mol was only 19% (with 24 kPa of water added) compared to a percent HDO of 55% (with 7.0 kPa of H2S added) on the sulfided catalyst. The product distribution as a function of space time for the oxide catalyst is shown in Figure 6 (for double-ring products) and Figure 7 (for single-ring products) for a study a t 360 O F and 7.0 MPa and with 24.0 kPa of HzO formed from decanol. The distribution differed greatly from that obtained over a sdfided catalyst. The selectivity to single-ring hydrocarbons was only 25% on the oxide catalyst, as compared to a 75% selectivity on the sulfide catalyst. The major double-ring products were cyclohexylbenzene and biphenyl. 2-Phenylphenol was detected a t low concentrations and appears to be an intermediate. Saturated products such as dicyclohexyl and CPMCH were not found to any significant extent. Among the single-ring products, cyclohexane predominated as with the sulfided catalyst; benzene, methylcyclopentane, and cyclopentane were found as before, but in somewhat different ratios. Two minor products found previously, methylcyclohexane and cyclohexene, were not found here. Figures 6 and 7 also show that in the presence of 24.0 kPa of water, the rate of HDO is zero order in DBF over the oxide catalyst, indicating that adsorption of DBF may be quite strong. The curves for biphenyl, cyclohexylbenzene, cyclohexane, and benzene show a linear increase in concentration with space time. After the above studies, the catalyst was sulfided by using a liquid feed containing 30 w t % dimethyl disulfide (DMDS) in xylene. It has been reported that DMDS decomposes at a relatively low temperature, making it useful as a spiking compound.21 Sulfiding conditions were 1 h
SPACE TIME , ( H R ) ( G - C A T ) I MOL
Figure 8. Hydrodeoxygenation of 2-phenylphenol using sulfided sample 21.
at 360 "C and a total pressure of 7.0 MPa, which would produce an HzS partial pressure of 0.7 MPa. After this severe sulfiding, the product distribution for dibenzofuran HDO closely resembled that found for a catalyst sulfided in the normal manner. The only difference in selectivity observed was an increase in the CPMCH/DCH ratio from 1.5 on a normal catalyst to 2.2. However, the activity of the catalyst was slightly lower than that for catalyst sulfided without prior exposure to hot hydrogen alone. Results: Studies of Possible Reaction Intermediates Five possible intermediates in the HDO network for dibenzofuran were studied separately on either the severely sulfided form of sample 21, the cataryst originally studied as an oxide, or on sample 28 of the sulfided standard catalyst. Both catalyst samples were studied at 7.0 MPa and 360 "C, but the H,S partial pressure was 13.9 kPa for sample 21 and 7 kPa for sample 28. The product distribution as a function of space time was essentially the same for both catalysts when DBF, 2-phenylphenol, or 2-cyclohexylphenol was fed, the three compounds studied on both catalysts. Of particular interest is the extent to which single-ring compounds are formed vs. double-ring compounds, since this provides important evidence for determining the main reaction pathway. Also of interest is the ratio of CPMCH to DCH, which is a measure of isomerization activity of the catalyst. Four of the possible intermediates reacted so fast that they were completely converted at the shortest space times studied. The least reactive was 2-phenylphenol, but it reacted much faster than DBF. Results below were those obtained after the catalysts had reached steady-state activity. 2-Phenylphenol. As shown in Figure 8,2-phenylphenol reacts to form biphenyl and cyclohexylbenzene (CHB). Although both biphenyl and cyclohexylbenzene undergo secondary hydrogenation, the hydrogenation of biphenyl by itself proceeds much slower than the HDO of 2phenylphenol. Thus we found that the first-order hydrogenation rate constant for biphenyl, measured separately, is in the range of 0.0045 mol/ (h g of catalyst) while that for 2-phenylphenol is about 0.016 mol/(h g of catalyst) * If CHB were formed only through biphenyl as an intermediate in a series reaction, applying the above rate constants, one can calculate that the biphenyl concentration would reach a maximum value of 61 % of the initial (21)Hallie, H.Oil Gas J. 1982, 80, 69.
La Vopa and Sat terfield
328 Energy &Fuels, Vol. 1, No. 4, 1987 loo
7 2
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-CYC LO H EX Y LPHE NOL
'"
60
5 n
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2
1
3
4
SPACE TIME , ( H R ) ( G - C A T ) / MOL
Figure 10. Hydrodeoxygenation of 2-cyclohexylphenolat mod200 400 600 800 SPACE TIME,(HR)(G-CAT) / M O L
0
Figure 9. Hydrodeoxygenation of 2-cyclohexylphenol using sulfided sample 28.
concentration of 2-phenylphenol rather than the 10% observed. Hence we conclude that most of the cyclohexylbenzene was formed directly from 2-phenylphenol. A parallel, less important reaction was the formation of CHB via biphenyl. The ratio of CPMCH to its isomer DCH was approximately 0.5. Single-ringhydrocarbons comprised only 16% of the final products. Krishnamurthy et al.' reported a study of 2-phenylphenol at 325 "C and 10.34 MPa. Cyclohexylbenzene and biphenyl were found, as here. They also reported that a phenylhexanol was isolated, but we suggest that this was erroneously identified. 2-Cyclohexylphenol. A total of 91% of the products were single-ring compounds, cyclohexane predominating (Figure 9). The double-ring compounds were CPMCH, DCH, and CHB. Cyclohexylbenzenewas apparently slowly hydrogenated to DCH as a secondary reaction. Krishnamurthy et al. studied 2-cyclohexylphenol at 287 "C in their autoclave reactor and reported finding cyclohexylbenzene and dicyclohexyl, as here, but also phenol. Cyclohexane was a major isolated product in both studies. Since we did not find phenol under our usual reaction conditions, we performed a brief study a t 360 "C and 7 MPa using only 10 mg of sulfided NiMo catalyst, less than 1% of the usual catalyst loading. By variation of space time, conversions were varied from 8% to 85%. As shown in Figure 10, the primary products were phenol and cyclohexene, formed in nearly equal amounts. Cyclohexane appeared at higher conversions, evidently formed by hydrogenation of cyclohexene. Small amounts of 3-cyclohexylphenol, cyclohexylbenzene,and dicyclohexyl were also identified (not shown). Krishnamurthy et al., also concluded that even in the presence of catalyst this reaction may have been noncatalytic a t least in part. To test this possibility under our conditions, the reactor was assembled in the normal configuration with the catalyst replaced by silicon carbide of the same particle size. The reaction was carried out in the presence of hydrogen a t a gas-to-liquid feed ratio of 1605 mL a t STP/mL. Liquid feed containing 0.245 mol/L of cyclohexylphenol was metered to the reactor a t 0.34 mL/min. A t 390 "C and 7.0 MPa total pressure conversion of 2-cyclohexylphenol was only 2.5%. By comparison, the catalytic reaction produced 100% conversion at 350 "C
erate conversions. 83
pi I
-
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800
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1200
SPACE T I M E , ( H R ) ( G - C A T ) / M O L
Figure 11. Hydrodeoxygenation of trans-2-phenylcyclohexanol using sulfided sample 21.
over the entire range of space times tested. The products of the thermal reaction were phenol and cyclohexene, as we observed at moderate conversions for the catalytic reaction. By either mechanism the primary products thus appear to be principally phenol and cyclohexene. Hydrogenation of phenol would yield cyclohexanol, which would be expected to be very reactive. Removal of oxygen by elimination of water would form cyclohexene, which, produced by either route, is hydrogenated fairly rapidly to cyclohexane. This is consistent with the studies of dibenzofuran shown in Figure 2. In an earlier study of HDO of methylphenol we also found considerable quantities of ethyl cyclohexene formed as an intermediate.22 trans -2-Phenylcyclohexanol.Figure 11shows that this compound was converted rapidly and selectively to cyclohexylbenzene (CHB). The CHB became hydrogenated to DCH and isomerized to CPMCH at approximately equal rates. 2-Cyclohexylcyclohexanol. The major products formed were DCH and CPMCH, as shown in Figure 12. At low space times selectivity to DCH was high, producing a CPMCH to DCH ratio of 0.5. The DCH produced then slowly isomerized to form CPMCH. CHB was also detected, possibly formed directly during the dehydrogenation of the cyclohexanol structure, but the amounts were small. (22) Satterfield, C. N.; Yang,
S.H.J. Catal. 1983, 81,336.
Energy & Fuels, Vol. 1 , No. 4, 1987 329
Catalytic Hydrodeoxygenation of Dibenzofuran
Table 111. Product Selectivity from DBF and Three Possible Reaction Intermediates % selectivity
DBF TH-DBF tot. single-ring products individual species cyclopentane methylcyclopentane cyclohexane methycyclohexane
88
72 7.0
2-cyclohexylphenol 91
2-phenylphenol 16
0.9
0.8
81.9 0.1
11.4 0.4 1.9
13 12
9.0 50
5.2
53 5.9 3.8
2.5 3.6
m+m - a - w-
benzene
SINGLE - RING
2.4
1.2
HYDROCARBONS SINGLE -RING
H
0
200 4 0 0 600 800 1000 1200 SPACE T I M E . ( H R ) ( C A T ) / M O L
1400
DIBENZOFURAN LDBF)
TETRAHYDRODBF
HEXAHYDRODBF
2 -CYCLOHEXYL PHENOL
Figure 12. Hydrodeoxygenation of 2-cyclohexylcyclohexanol using sulfided sample 21. 20,
I
I
0-0
I
I
v
Q-p 2
w
>
CYCLOHEXYLBENZENE
BIPHENYL
0
u c
6)
2%-
\
-PHENYLPHENOL
ICYCDPENTYLMETHYLI
z
DICYCLOHEXYL
BENZENE
~
ICYCDPENTYLMETHYLI -CYCLOHEXANEICPMCHI
Figure 14. Proposed hydrodeoxygenation reaction network for dibenzofuran.
10
~EXAHYDRO-DBF
0
I
I
I
200 400 600 000 SPACE T I M E , ( H R ) ( G - C A T ) / M O L
0
Figure 13. Hydrodeoxygenation of 1,2,3,4-tetrahydrodibenzofuran using sample 28.
1,2,3,4-Tetrahydrodibenzofuran(TH-DBF). This compound was custom synthesized for this study by the method published by Ebe1.23 Figure 13 shows that 88% of the TH-DBF reacted was converted to single-ring compounds. The cyclohexane concentration is given by the right-hand scale; those of the other species are given by the left-hand scale. Complete conversion was obtained even a t the lowest space time studied. At very short space times a new product was found to be present in small concentrations. We have identified it as hexahydrodibenzofuran by its retention order on a polar (Carbowax) GC column. For identification we synthesized a small quantity of perhydrodibenzofuran by hydrogenation over Rh catalyst, following published procedures. On the polar column the retention order of the DBF derivatives is perhydrodibenzofuran, tetrahydrodibenzofuran (TH-DBF), and DBF. Since the new product was eluted between the peak for perhydrodibenzofuran and TH-DBF, we identify it as hexahydrodibenzofuran. It is plausible that hexahydrodibenzofuran can be formed from the TH-DBF and that it reacts rapidly. (23) Ebel, F. Helu. Chim. Acta 1929, 12, 3.
Discussion Reaction Network. A high selectivity to single-ring products was observed from the HDO of DBF, but of the various intermediates studied, a high proportion of single-ring products occurred only from the reaction of TH-DBF or 2-cyclohexylphenol. Table I11 compares the selectivity to various single-ring products from these three substances to that from 2-phenylphenol. The predominance of single-ring products from DBF indicates that the major reaction pathway over the sulfide catalyst was through saturation of one ring of DBF, forming TH-DBF, hexahydrodibenzofuran, and then 2-cyclohexylphenol. This last substance reacted to form largely single-ring products. A second, less important parallel pathway, was through direct hydrogenolysis of DBF to form 2-phenylphenol. (2-Phenylphenol was also detected in the reaction of DBF over the oxide catalyst.) The 2-phenylphenol then reacts to form biphenyl and cyclohexylbenzene. One can estimate the relative importance of the two pathways by the weighted contribution from TH-DBF and 2-phenylphenol to the 72% single-ring selectivity found for DBF. By this measure, the route via TH-DBF and 2-cyclohexylphenol accounted for about 77% of the DBF reaction and that via 2-phenylphenol, about 23%. The selectivity to single-ring compounds was not affected by temperature within the range 350-390 "C, indicating that the relative importance of the two parallel pathways was not altered by temperature within this range. At low space times the ratio of CPMCH to DCH in the products was higher for DBF than for other species studied. At high space times this ratio was about 1.2 for DBF and TH-DBF, and it ranged from 0.5 to 0.8 for all other species. The discussion above suggests a reaction network as shown on Figure 14. The rate-limiting reaction for the overall hydrogenation is that of DBF by the two parallel initial reactions, the sum of which is measured by the first-order rate constant (see below). Our proposed network agrees with that of Krishnamurthy et al. in that we
LaVopa and Satterfield
330 Energy & Fuels, Vol. 1, No. 4, 1987
both postulate that in the first step DBF can be converted to either 2-phenylphenol or (via TH-DBF) to 2-cyclohexylphenol. We differ in that we have no evidence to suggest that 2-phenylcyclohexanol,cyclohexylcyclohexanol, or phenylhexanol are formed as intermediates. All the oxygen-containing intermediates are more reactive than DBF and hence were not found in any significant amounts in the products when a substantial conversion of DBF occurred. Our observations are thus consistent with the results of Li et al.,14 who studied the catalytic HDO of a number of phenolic compounds in the acidic fraction isolated from a coal liquid. They concluded that the phenols reacted 1order of magnitude faster than dibenzofuran in the neutral oil fraction of the same coal liquid. Sulfide vs. Oxide Catalyst. The sulfide form of the NiMo catalyst was more active than the oxide form. The overall reaction was first order in both DBF and hydrogen (see below), and the products were largely single-ring compounds. On the oxide catalyst, zero-order kinetics with respect to DBF were observed; double-ring compounds predominated in the products, and these were less saturated. Evidently hydrogenation functionality is much weaker on the non-sulfided catalyst than on the sulfided catalyst. The two catalysts also responded somewhat differently to water vapor. The present results emphasize the fact that a sulfided catalyst can behave quite differently' from its oxide precursor. Effects of H2S. Results obtained over the sulfided NiMo catalyst demonstrate that H2Sinhibits the HDO of dibenzofuran, but the cause for the changes in catalyst activity and in product distribution with H2S partial pressure remains unclear. As shown above, with increased H2S partial pressure, selectivity to double-ring hydrocarbons decreased (Figure 5) and that to single-ring hydrocarbons increased. Thus the product distribution in the complete absence of H2S shows some resemblance to that found on the oxide catalyst (Figures 6 and 7). However, these changes in product distribution occurred immediately upon the removal of H2S, which would not be expected if deactivation were due to oxysulfide formation. A more likely explanation is that labile sulfur is lost from the catalyst surface in the presence of high-temperature hydrogen and absence of H2S. The fact that increased H2Sconcentration decreased the selectivity to double-ring products suggests that H2S inhibits hydrogenolysis more than hydrogenation. Similar results have been reported for the hydrodesulfurization of diben~othiophene,~~ the sulfur analogue of DBF. An increase in H2S concentration lowered the conversion of dibenzothiophene and lowered the biphenyl/cyclohexylbenzene product ratio. The change in selectivity was interpreted as preferential inhibition of the hydrogenolysis reaction, which produces biphenyl. Hydrogen Consumption. Hydrogen consumption is an important cost in hydroprocessing to remove heteroatoms. The consumption is determined not only by the quantity of heteroatoms to be removed but also by the reaction selectivity. Among the principal product species found here, the moles of H2 consumed per mole of DBF reacted varies from 2 for formation of biphenyl to 9 for cyclohexane or methyl cyclopentane. The hydrogen consumed dwing the HDO of DBF at 360 "C and 7 MPa was calculated for both the oxide and sulfided forms of a NiMo catalyst. Over the oxide catalyst, in the presence of 24 kPa of water, the H2 consumption (24) Broderick, D. H.; Gates, B. C. MChE J. 1981,27, 663.
CL
0.1I
I
I
I
I
0 200 400 600 SPACE T I M E ,(HR)(G-CAT)/MOL OF DBF
Figure 15. SuEde catalyst hydrodeoxygenation of dibenzofuran (DBF). The reaction is first order in DBF. Corresponding rate constants are shown in units of mol of DBF/(h g of catalyst).
was 5.1 mol of H2/mol of DBF reacted. Over the sulfided catalyst in the presence of 7 kPa of H2S,it was 7.5 mol of H2/mol of DBF, 50% greater. Reaction Kinetics. H2SPresent. The percent conversion of DBF was measured as a function of space time over the sulfided catalyst in the presence of 7.0 kPa of H2S, produced in situ, at each of four temperatures, with an initial DBF partial pressure of 24 kPa. Semilogarithmic plots of the percent of initial DBF remaining as a function of space times are shown in Figure 15. The linear relationships obtained establish that the overall reaction is firat order with respect to DBF partial pressure over a wide range of conversions. The discrepancies in the data at zero conversion may be due to calibration error in the analysis: this would tend to shift the entire line vertically. Least-squares regression was used to fit first-order rate constants to the data. The rate equation was of the form d(l - X ) -- -k(l - X ) dt where x = conversion of DBF to hydrocarbons, t = space time [(h g of catalyst)/mol of DBF], and k = first-order rate constant [mol/(h g of catalyst)]. From the effect of temperature on the rate constant an Arrhenius plot gives an activation energy of 67 kJ/mol. The overall HDO rate was shown to be directly proportional to hydrogen partial pressure by the following procedure. Hydrogen pressure was varied by changing the ratio of hydrogen to helium flow rates at 14.0 MPa total pressure, keeping H2Spartial pressure fixed a t 13.9 kPa. The percent HDO a t 360 "C and a space time of 160 (h g of catalyst)/mol of DBF was used to calculate first-order rate constants for DBF a t each of several hydrogen pressures. These constants were directly proportional to the hydrogen partial pressure. Further details are given by LaV~pa.~~ H2S Absent. The above study at 360 "C was repeated, but with no sulfur compound added to the feed. Results are compared to those observed in the presence of H2S in Figure 16. A t high space times the degree of conversion (25) LaVopa, V. Sc.D. Thesis, MIT, 1987.
Catalytic Hydrodeoxygenation of Dibentofuran
A
7.0 k P a of H2S
0 N O H2S
Prasant
0
0
200
400
600
SPACE TIME, (HR)(G-CAT) /MOL OF DBF
Figure 16. Space time conversion. The conversion at high space times is greater in the absence of H2S.
is substantially greater when H2S is omitted. Other data (not shown) indicate that water enhances the HDO of dibenzofuran in the absence of H2Sso the results in Figure 15 can be attributed to a self-enhancement stemming from water formed in increasing amounts a t high conversions. When H2S is present, water has no significant effect on activity or selectivity, as discussed previously. It appears that adsorption of HzS onto the catalyst competes with that of H 2 0 and that the latter is much more weakly adsorbed. A somewhat similar effect was observed in studies in our laboratory on the effects of H2S and H 2 0 on the hydrodenitrogenation of quinoline, on the same sulfided catalyst.26 There water vapor and H2S, studied separately, each caused some enhancement of activity, the effects of H2Sbeing considerably greater than that of HzO. In a mixture of the two, however, each a t 14 kPa partial pressure, the percent conversion was essentially the same as that with 14 kPa of HzS by itself. In later experiments (sample 23), the HDO reaction exhibited first-order behavior over the range of conversion in Figure 16 in the absence of H2S when the equivalent of 24.0 kPa of water was added to the feed as decanol. Evidently there are some concentration effects, which were not explored here further. (26)Satterfield, C. N.; Smith, C. M. Ind. Eng. Chem. Process Des. Deu. 1986,25, 942.
Energy & Fuels, Vol. 1 , No. 4, 1987 331 Summary a n d Conclusions 1. In the hydrodeoxygenation (HDO) of dibenzofuran (DBF) a t 360 OC and 7.0 MPa on a sulfided catalyst, the major products found were single-ring hydrocarbons. Cyclohexane predominated, but also present were methylcyclopentane, cyclopentane, benzene, methylcyclohexane, and cyclohexene. Lesser quantities of double-ring products were isolated. Among them cyclohexylbenzene predominated. All oxygenated species formed as intermediates are much more reactive than DBF. 2. The major reaction pathway under our conditions appears to be hydrogenation of one ring of DBF to form hexahydrodibenzofuran, which is converted to 2-cyclohexylphenol. This in turn reacts to form phenol and cyclohexene. We propose that the phenol in turn is hydrogenated to cyclohexanol, which, by elimination of water, is converted to cyclohexene. Cyclohexene, formed by either path, is hydrogenated fairly rapidly to cyclohexane. 3. A second parallel pathway, of lesser importance, is direct hydrogenolysis without prior ring hydrogenation to form 2-phenylphenol, which is converted to biphenyl and cyclohexylbenzene. 4. Removal of HzSfrom the reaction caused an increase in catalyst activity for HDO, which persisted for hours. Returning H2S to the system caused a repression of catalyst activity. Removal of H2S decreased formation of single-ring hydrocarbons and increased the selectivity to unsaturated products in the double-ring group. In the absence of H a , added water (24 kPa) accelerated the HDO reaction. In the presence of H2S,added H20 had no effect. 5. The catalyst in the oxide form, not presulfided, behaves quite differently than the sulfided catalyst. The activity was lower, and doublie-ringproducts predominated over single-ring compounds, saturated double-ring compounds being minor in amount. Among single-ring compounds, however, cyclohexane predominated as with a sulfided catalyst. In contrast to the sulfided catalyst, added water (24 kPa) inhibited the HDO reaction. 6. For a sulfided NiMo/A1203catalyst in the presence of H2S a t 350-390 "C and at Hz partial pressures of 2-14 MPa, the reaction is first order in hydrogen and in DBF. The activation energy is 67 kJ/mol. On the oxide form of the catalyst, not presulfided, however, the reaction is zero order with respect to DBF. Acknowledgment. This study was supported in part by the National Science Foundation in the form of a Fellowship to V.L. I t was also supported in part by the Office of Fossil Energy, US. Department of Energy, under Grant Nos. DE-FG22-83PC60798 and DE-FG2285PC80516. Registry No. DBF,132-64-9; Ni, 7440-02-0; Mo,7439-98-7.
