Activity of noble metal-promoted hydroprocessing catalysts for pyridine HDN and naphthalene hydrogenation

FUEL PROCESSING TECHNOLOGY ELSEVIER

Fuel Processing Technology 49 ( 1996) 137- 155

Activity of noble metal-promoted hydroprocessing catalysts for pyridine HDN and naphthalene hydrogenation H.S. Joo *, James A. Guin Auburn Uniuersity, Chemical Engineering Department, Auburn, AL 36849, USA

Received 10 October 1995; accepted 17 April 1996

Abstract An important step in the upgrading of coal and coal-waste coprocessing liquids is the removal of heteroatoms, particularly nitrogen, which tend to poison the downstream cracking operations required to lower boiling points to the transportation fuels range. This requirement has motivated research to improve the activity of current hydroprocessing catalysts. In this regard we have examined the effects of Pt, Ru, and Ir promotion on three commercial Al,O,-supported catalysts (NiMo, CoMo, and NiW) using the model compounds, pyridine and naphthalene, to investigate hydrodenitrogenation and ring hydrogenation reactions, respectively. For pyridine hydrodenitrogenation (HDN), the activity of the sulfided catalysts was better than that of the oxide forms. For sulfided forms of NiMo, all noble metals tested improved the HDN activity. The activity of CoMo was improved by Pt and Ir while NiW was not improved by any noble metals promotion. For ring hydrogenation (HYD) of naphthalene, the P&promoted catalysts were more active than the original catalysts in the oxide forms, while the activities of both catalysts were similar after sulfidation. However for ring HYD in the presence of N, the Pt-promoted catalysts exhibited a distinct advantage. Keywords: Naphthalene hydrogenation; Noble metal-promoted bydroprocessing catalysts; Pyridine hydrodeni-

trogenation

1. Introduction Solid fossil fuels such as coal, which are relatively abundant in comparison with petroleum, will become major fuel sources in the future [Il. A variety of processes for

??

Corresponding author.

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producing coal-derived transportation fuels have been developed in past years. There is also a great deal of current interest in the coprocessing of coal with waste plastics and/or rubber to produce transportation fuels [2-51. The primary liquid products from these processes usually contain relatively high quantities of heteroatoms, especially nitrogen, oxygen, and sulfur. To produce clean transportation fuels, the first stage liquids must be upgraded by eliminating the heteroatoms through hydrodenitrogenation (HDN), hydrodesulfurization (HDS), and hydrodeoxygenation (HDO) reactions. Nitrogen compounds are especially detrimental because they poison acid catalysts used to effect molecular weight reduction via hydrocracking [6]. Among the heteroatom removal reactions, C-N bond scission in heterocyclic aromatic compounds is more difficult than C-S and C-O scission [7]. Ring hydrogenation (HYD) is also necessary as a preliminary step in N removal, as well as in hydrocracking polyaromatics to reduce molecular weight ranges and lower boiling points. Bimetallic catalysts such as NiMo, CoMo, and NiW supported on Al,O, are currently used in commercial hydrotreating processes. However, when coal liquids are upgraded, it is difficult to obtain adequate heteroatom removal with these conventional catalysts [81. For this reason, more active hydrotreating catalysts need to be developed. One pathway toward the development of more active catalysts is the consideration of polymetallic catalysts. For example, Pt-promoted NiMo/Al,O, catalysts were used to upgrade a coal-derived middle distillate and found to be better than a commercial NiMo/Al,O, catalyst in HDN activity and rate of deactivation [9]. Similarly, studies of the effects of Ru promotion on CoMo/Al,O, catalyst showed that the Ru-promoted catalyst exhibited improved quinoline HDN activity [ 10,111, but interestingly had no effects on conversion of residual oil and HDS activity [12]. Ir was found to have good hydrotreating (HDN, HDS, and HDO) activities in model compound (quinoline, thiophene, and diphenylether) reactions with Al,O, [13] and carbon 114-171 supported transition metal catalysts and bulk transition metal sulfides [18]. Our work extends these various previous investigations by comparing in a single unified study the effects of noble metal promotion on all three types of commercial hydrotreating catalysts. Specifically, the promotion of three commercial bimetallic catalysts through the addition of three noble metals, namely, Pt, Ru, and Ir was investigated. Most of the attention was focused on Pt based on the results of exploratory experiments. In addition, a detailed study of effects of catalyst sulfidation was made by comparing the performance of the oxide and sulfide forms of the catalyst. For reasons detailed in the following, naphthalene and pyridine were chosen as model compounds for our study of catalyst HYD and HDN activity. 1.1. Pyridine HDN

Many N-containing compounds found in petroleum and synthetic liquids are heterocyclic aromatic forms which are resistant to HDN. The literature on the HDN mechanisms of these compounds has been summarized [7,19,20]. For our work, pyridine was chosen as an HDN model compound due to the fact that pyridine derived compounds are the simplest N-heterocycles found in petroleum and coal liquids and consequently, the

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Pyridine

Pipetidine

Pentylamine

139

Pentane + etc.

Q+/vv Pentyipiperidine

O+Q Pentyipiperidine Fig.

1.Reaction pathways for pyridine HDN.

pyridine HDN reaction network involves relatively few intermediates as compared to polycyclic N compounds. Several previous investigations of pyridine HDN have been performed. The kinetics of pyridine HDN have been studied with both sulfided [20-261 and reduced catalysts [27]. As a result of these studies, pyridine HDN has been found to occur through three reactions as depicted in Fig. 1. In the first reaction, pyridine is initially saturated to form piperidine by first-order kinetics followed by the formation of pentylamine through piperidine hydrogenolysis [22]. Mcllvried [23] reported that the piperidine hydrogenolysis step is slower than pyridine HYD with NiMo/Al,O,. Subsequent nitrogen removal from pentylamine results in pentane and ammonia, as well as some minor isomerized and lower boiling products. This reaction rate is faster than pyridine HYD and piperidine hydrogenolysis [23]. In the second and third reactions, as shown in Fig. 1, pentylpiperidine and ammonia are formed by disproportionation of piperidine and pentylamine molecules (alkyl-transfer reaction) and/or two piperidine molecules. In one study, it was noted that pentylpiperidine did not react further due to its relative stability [28]. In our experiments, di-n-pentylamine was also found to a minor extent. More than ten products have been identified from the pyridine reaction with CoMo/Y zeolite hydrocracking catalyst [281. With regard to the effects of S in pyridine HDN, it has been reported that thiophene enhanced the overall conversion of pyridine and piperidine [24] and that H,S promoted piperidine hydrogenolysis but not pyridine HYD [25]. We note here that the primary purpose of our work was to compare the noble metal-promoted and unpromoted versions of the hydroprocessing catalysts, rather than a determination of the detailed mechanism of pyridine HDN.

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N@hthalMle

Tetralin

Decalins

Fig. 2. Reaction pathways for naphthalenehydrogenation.

As noted previously, naphthalene was selected as a model compound for hydrogenation activity measurements in our work. Naphthalene HYD is sequential to form tetralin and cis and trans decalin as shown in Fig. 2.

2. Experimental 2.1. Materials Four commercial Al,O,-supported catalysts (Criterion 424 NiMo, Cyanamid 1442B CoMo, Crosfield 550s NiW, and Harshaw Al-3391, a pure alumina), as shown in Table 1, were used in this study. Chemicals used as received include pyridine (Aldrich, 99%) and naphthalene (Fisher, purified) as model compounds, hexadecane (Fisher, certified) as a solvent and H,PtCl, (Aldrich, 8%) RuCl, (Strem, anhydrous), Ru,(CO),, (Strem, 99%) and H,IrCl, (Aldrich, 4%) as catalyst precursors. 2.2. Catalyst preparation Catalysts were crushed and sieved to between 100 and 200 mesh for reactions. The noble metals were impregnated on the catalysts by an incipient wetness technique. Catalysts were dried in air overnight and calcined at 450 “C for 3 h. An aqueous solution of 37% HCl and cyclohexane were used to dissolve RuCl, and Ru,(C0)i2, respectively. These two different Ru precursors are denoted as Ru(A) and Ru(B), respectively. 2.3. Reaction procedure All reactions were performed in 20 cm3 316 ss tubing bomb microreactors (TBMR) which were agitated in a fluidized sand bath as shown in Fig. 3 and replicated at least twice. Catalysts were presulfided at 300 “C for 2 h using dimethyl disultide (DMDS) in a separate TBMR charged with 1000 psig cold hydrogen pressure. The quantity of sulfur added for each catalyst was based upon its metals content using 50% excess of the stoichiometric amount needed for bulk sulfidation (MoS,, Ni,S,, Co,!&, and WS,). All catalysts in this paper are sulfide forms unless specifically noted as oxide forms. In these

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Table 1 Properties of reference catalysts Catalyst

Compositions

Surface area cm* g- ‘1

Pore vohmle (ml g-‘1

Criterion 424

MOO,. 19.5%; NiO, 4%; P,O,, 8%; AI,O,, balance MOO,, 13.2%; COO, 3.5%; SO,, 0.5%; Al,O,, balance WO,, 18-228; NiO, 4-78; Al,O,, balance Al,O,, 99.9%

155

0.47

300

0.82

230

0.5

230

0.69

Cyanamid HDS 1442B Crosiield Sphexicat 550s Harshaw Al-3991

studies, a reactant solution (6 g) containing 2 wt.% pyridine and (or) 2 wt.% naphthalene in hexadecane was used with 0.1 g catalyst loading at 1000 psig cold hydrogen pressure. In reactions with sulfided catalysts, an additional 1.5 wt.% DMDS was added as a sulfur source to the TBMR to maintain catalyst sulfidation.

Tbcrmocarple 1

1 CompressedAir

Fig. 3. Schematic diagram of tubing bomb microreactor apparatus.

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142

2.4. Analysis

Analysis was performed on a temperature-programmed Varian 3200 GC with 30 m X 0.32 mm X 1.0 p_rnRestek Stabilwax-DB capillary column for products of pyridine reactions and 30 m X 0.32 mm X 0.25 pm J&W DB-5 capillary column for products of naphthalene reactions. GC-MS and standard additions were performed to identify the products. Percentage HDN and HYD were calculated based on the product distribution from the GC analysis. Typical average standard deviations for %HDN and %HYD measurements from replicate experiments were 3.7% and 2.0% on a scale of O-100%, respectively.

3. Results and discussion 3.1. Naphthalene hydrogenation

To simplify the presentation of hydrogenation activity, an extent of HYD, A, was defined according to Eq. (1): A,=

2M,+5M, (1)

5(M,+MT+Mo)

where M,, M,, M, are moles of naphthalene, tetralin, and decalin present after reaction, respectively [29]. The sum in the denominator of Eq. (1) was essentially equal (within about 2%) to the initial moles of naphthalene charged confirming the conserva-

Oxide

? ?Unpromoted

Sulfide

B

Pt Pmnoted

Fig. 4. Extent of naphthalene hydrogenation at 310 “C for 30 min.

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Processing Technology 49 (19%) 137-1.55

143

tion of moles of two-ring aromatic species as shown in Fig. 2. According to this definition, A, corresponds to the moles of H, actually reacting divided by the maximum H, usage possible (all naphthalene converted to decalins). Thus, A, ranges from 0 to 1 as the products vary from totally aromatic (no HYD) to completely saturated (100% HYD to decalins). Fig. 4 shows the effect of Pt promotion and sullidation on ring HYD of naphthalene in terms of A, defined by Eq. (1). Oxide and sulfide catalyst forms are shown on the left and right sides of the figure, respectively. As a point of reference, the A, of naphthalene in the absence of any catalyst at 350 “C for 30 min was measured to be 0.00, i.e. no reaction. As shown on the left side of Fig. 4, in the original oxide forms, which could be partially Hz-reduced under reaction conditions, the two Ni-based commercial catalysts evidence much greater HYD activity than the Co-promoted catalyst, in keeping with the known good HYD activity of Ni metal. Likewise, with the oxide catalysts, the extent of hydrogenation (A,) for Pt-promoted catalysts was 100% and much superior to those of unpromoted ones. It is especially notable that Au for CoMo was increased from 0 to 1 by Pt. This high HYD activity is in keeping with the

Time (min)

04 0.20

Unpromoted

pt Promoted

Time (min)

Fig. 5. Extent of naphthalene hydrogenation in a mixture of pyridine and naphthalene: (a) 350 “C, (b) 380 “C.

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known excellent HYD activity of Hz-reduced noble metals such as F’t and suggests that the impregnated Pt has adequate dispersion on the catalysts. As expected, this HYD activity is, however, significantly poisoned by sulfidation as indicated by the results for the sulfide forms on the right side of Fig. 4; however, it is worth noting a recent paper which shows that the noble metals themselves do retain some hydrogenation activity even in the presence of sulfur [30]. Following sulfidation, both Pt-promoted and original catalysts showed essentially the same activity with NiMo > CoMo > NiW. For all catalysts, the hydrogenation activities were decreased by sulfidation except for CoMo, showing that generally the hydrogenation sites of the catalysts are poisoned by sulfur. Because of the effect of sulfur on HYD activity it would appear that little benefit would accrue from Pt promotion regarding HYD of polyaromatics, however a somewhat expanded perspective can be gained by referring to Fig. 5 which shows naphthalene HYD in the presence of added N in the form of pyridine at two temperatures. In comparison with the sulfide catalysts of Fig. 4, the predominant effect in Fig. 5 is the N inhibition of the HYD reaction, due to competitive adsorption effects, as evidenced by the much lower overall A, values. However a second point apparent from Fig. 5, is that in every case, the Pt-promoted catalyst has a higher A, value as compared to the corresponding unpromoted catalyst. Taken together, Figs. 4 and 5 suggest that, in the presence of N poisons the Pt-promoted catalysts can provide enhanced HYD of polyaromatics, as compared to the original catalysts. Such a behavior could provide a significant benefit to downstream hydrocracking operations in that polyaromatic saturation is required prior to cracking of the ring. Of course, coal liquids contain many additional N compounds and their behavior may be somewhat different from the simple model compound system used here. 3.2. Pyridine HDN With the catalysts used in this study, analysis of the liquid products revealed only five compounds, namely, pyridine, piperidine, pentylamine, pentylpiperidine, and di-npentylamine as the major N-containing compounds (more than 0.01%). The major terminal product found was n-per&me, although minor amounts of additional lower boiling products were also observed at severe conditions. From the liquid-phase product analysis, two measures of HDN activity were derived. The first is based upon the wt.% N in the product liquid as calculated from amounts of the five N-containing products noted above. Thus, if Nr and NP represent the wt.% nitrogen in the feed and product mixture, respectively: %--

%HDN = -

Np

N, In spite of the apparent simplicity of the above procedure as represented by Eq. (2), there are some difficulties in application of this method. In batch reactions, such as performed in this study, some N-removal from the liquid phase will occur due to irreversible preferential adsorption of N compounds on the catalyst surface, rather than catalytic HDN reaction per se. To examine the extent of N adsorption, a mixture of pyridine, piperidine, and pentylamine in hexadecane (6 g solution and 2 wt.% each) and

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137-155

Table 2 Product distribution with Al,O, and Pt-, Ru-, and k-loaded AI,O, at 350 “C for 20 min Catalyst

None A’203

wt.% Pyridine

Piperidine

Pentylamine

Pentylpiperidine

n-Pentane

1.89 1.69

0.00 0.00 0.33 0.02 0.21

0.00 0.00 0.04 0.02 0.03

0.00 0.00 0.03 0.01 0.02

0.00 0.00 0.00 0.00 0.00

WA1203

1.32

Ru/Al,O, Ir/Al,O,

1.62 1.42

0.1 g of sultided NiMo catalyst were charged to a TBMR and agitated at room temperature for 20 min. The removal of pyridine, piperidine, and pentylamine from the liquid phase by room-temperature adsorption on the catalyst in this experiment were 9, 14, and 22%, respectively. Since the main purpose of this study was to compare the noble metal-promoted catalysts with the original unpromoted catalysts, it was of interest to determine the effect of the promotion procedure on the adsorption of N compounds. In this regard, a pure alumina (Table 1) was impregnated with three noble metals and used in pyridine HDN experiments. Table 2 shows the liquid product distribution from these experiments using 0.1 g of catalyst as well as a blank run in which the HDN of pyridine without catalyst was examined at 350 “C for 20 min. For the blank run, no products were observed although a 6% pyridine loss was found. In the case of pure A&O,, no products are formed, however, there is a significant adsorption of pyridine on the Al,O, as the product concentration of 1.69% is significantly less than the feed concentration of 2%. In the case of the impregnated aluminas, some reaction products are formed, however in all four cases the total N-containing species sum to 1.69 f 0.04%. These four experiments thus suggest that the total adsorption of N species is approximately the same, regardless of the noble metal promotion, even though the product distribution may be different. Nonetheless, because of the possible contribution of adsorption, it is probably not possible to attach significance to small differences in product distribution in the pyridine experiments and for this reason attention will only be focused on the major differences, i.e. the values of pyridine HDN determined in this work are considered as relative indicators assuming that adsorption is about the same in comparable (promoted vs. unpromoted) experiments. One additional point to note from Table 2 is confirmation that the sulfided noble metals themselves do have catalytic activity, most notably in the HYD of pyridine to piperidine; however, no hydrogenolysis activity to further products, e.g. pentane, is observed. The second indicator used as a measure of catalyst activity in the pyridine experiments was pentane production. Experiments using the NiMo catalyst showed that pentane did not adsorb preferentially on the catalysts surface and that it did not react to form other products. Based on its appearance as a major terminal HDN product (Fig. l), the pentane concentration in the liquid product thus provides good indicator of overall HDN catalytic activity. Generally, reaction product mixtures with a higher %HDN calculated from Eq. (1) also had higher wt.% pentane concentrations, showing the two measures of catalyst activity to be consistent.

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? ?NiMo

gjj

COMO

? ?NM

Fig. 6. Effect of Pt loadings on pyridine HDN at 350 “C for 30 min.

Fig. 6 shows the effect of Pt loading on HDN activity in the range of O-2 wt% Pt. For the three original unpromoted catalysts shown as 0% Pt, the ranking of pyridine HDN activity is NiMo > CoMo > NiW. The PtNiMo and PtCoMo catalysts are more active for HDN than the original catalysts, while NiW showed no significant benefit due to Pt promotion. The lack of improvement with the NiW catalyst, contrasted with the improvement for the other two catalysts, suggests that Pt addition is not simply an additive effect, but that there is some interaction between the added Pt and the original catalysts. Had the effect been simply additive, one would expect about the same degree of improvement with each catalyst. An additional point from Fig. 6 is that any improvements in HDN activity from Pt promotion are likely to be realizable with small amounts, ca. < l%, of Pt. Based on the results in Fig. 6, further experiments as described below were conducted using 0.8 wt.% Pt loading. Ru(A) and Ir promotion effects were also examined in parallel experiments in the same ranges of loading. The Ru(A)- and Ir-promoted NiMo catalysts exhibited highest activities around 1.0 wt.%. Therefore, in subsequent experiments, 1.0 wt.% Ru- and Ir-loaded catalysts were examined. Fig. 7 compares the pyridine HDN activities of the oxide and the sulfided original and promoted catalysts. The HDN activity ranking of the unpromoted sulfided catalysts is NiMo > CoMo > NiW, in agreement with the data shown in Fig. 6 at 30 min. Except for the NiW catalyst, all sulfided noble metal-promoted catalysts showed higher HDN activity than the original unpromoted catalysts. In contrast to the HYD activity shown in Fig. 4, all catalysts showed higher HDN activity in their sulfide forms except for Ru(A and B)NiMo. This is in general agreement with known enhancement effects of sulfidation on HDN [6]. Sulfided catalysts are known to have acidic functionalities which

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? ?oxide Fig. 7. Pyridine HDN with oxide and sulfided

147

? ?sulfide

original and promotedcatalysts at 350 “C for 20min(*, NiMo

catalysts).

enhance hydrogenolysis, as opposed to hydrogenation reactions. Both Ru-promoted NiMo catalysts, in the oxide form, also exhibited an unusual activity for hydrocracking, as well as being very active for pyridine HDN (100% HDN activity); however, this activity was reduced by sulfur. Ir promotion was very effective in the sulfide form but not in the oxide form. In general, the HDN activities were improved by the noble metal promotion. Figs. 8 and 9 show the effect of Pt promotion on pyridine HDN liquid product distribution with oxide and sulfide catalysts, respectively. For the oxide catalysts (Fig. 8), HYD of pyridine to piperidine was much greater with the PtCoMo and PtNiW. For example, comparing the oxide NiW and PtNiW shows that the concentration of pyridine with PtNiW (0.04) was much lower than that of NiW (1.3 1) as a result of greater HYD (pyridine-to-piperidine) with the Pt-promoted catalyst. A similar result is obtained with CoMo. These effects are in agreement with the increased ring HYD activity of the promoted oxide catalysts as observed earlier in Fig. 4. For the NiMo oxide, the result of increased pyridine to piperidine HYD is not as apparent because of the increased subsequent conversion of the piperidine to other products, i.e. the NiMo catalyst is generally more active for HDN than the CoMo or NiW catalysts. For sullided catalysts (Fig. 91, a similar trend of higher pyridine HYD and HDN for the Pt-promoted catalysts compared to the unpromoted forms was observed, except for the NiW catalyst. This trend is most apparent from the increased pentane concentration for the Pt-promoted NiMo and CoMo catalysts. There were no major differences in product distributions between NiW and PtNiW in the sulfide catalysts showing that Pt promotion was not effective in increasing the activity of the NiW catalyst.

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Fig. 8. Effect of Pt promotion on pyridine HDN product distributions with oxide catalysts at 350 “C for 20 min.

The effect of sulfidation with Pt-promoted catalysts is shown in Fig. 10 where S denotes the sulfided catalysts. The sulfided catalysts were generally less active for pyridine HYD but more active for pyridine HDN than their oxide counterparts, as

Fig. 9. Effect of Pt promotion on pyridine HDN product distributors with sulfided catalysts at 350 “C for 20 min.

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149

Fig. 10. Effect of sultidation on pyridine HDN product distribution with Pt-promoted catalysts at 350 “C for 20 min.

evidenced by higher pyridine concentration and the appearance of substantial quantities of the terminal product pentane in the product distributions. The reaction products with high n-pentane concentrations also contained several unidentified low boiling products. Considering the product distributions of Fig. 10, together with the first reaction pathway in Fig. 1, it is apparent that the effect of sulfidation is to enhance hydrogenolysis of piperidine as opposed to the hydrogenation of pyridine. Since this hydrogenolysis step is the slow (rate determining) step in the overall pyridine HDN process [23], the sulfided catalysts have significantly greater HDN activity and produce much larger amounts of terminal product pentane. All three types of catalysts evidence this same trend, although the NiW catalyst is less pronounced. This trend is more apparent in Fig. 11, where selectivities for the five major products expressed as their mole percentages are compared from experiments with oxide and sulfided catalysts, at the same level of pyridine conversion, obtained by systematically varying the reaction time. These data show that the concentrations of piperidine and pentylpiperidine were dominant with oxide catalysts while the sulfided catalysts had much higher n-pentane and much lower pentylpiperidine than the oxide forms. These results indicate slower C-N scission through the ring opening of piperidine and more selective disproportionation reactions (second and third reactions in Fig. 1) with the oxide forms as compared to the sulfide forms. This observation agrees with a previous study 1251in which pentylpiperidine was observed to only a minor extent with the sulfided catalyst. Overall, as noted in the preceding paragraph, the sulfided catalysts are more effective for pyridine HDN, as indicated by the higher terminal product (pentane) concentration.

150

H.S. Joo. J.A. &in/

MM0

Fuel Processing Technology 49 (1996) 137-lS5

SNiMo

PtNiMo SPlNiMo

PICOMOSPtcoMo

Fig. 11. Effect of sulfidation on pyridine HDN selectivity at the same level of pyridine conversion at 350 “C.

Pyridine HDN activity as a function of time for unpromoted and Pt-promoted sulfided catalysts is shown in Fig. 12. PtNiMo and PtCoMo show higher HDN activities than the unpromoted versions yielding essentially 100% HDN in 30 min. The HDN activity of the NiW catalyst was not enhanced significantly by Pt promotion. The order of pyridine

0 0 A 0

NiMo COMO NiW PtNiMo ?? PtCoMo A PtNiW

R, ./ 0 0

, 10

5 20

Time f(min) _ . Fig. 12. Pyridine HDN activity vs. time at 350 “C.

I

30

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151

Time (min) Fig. 13. n-Pentane

concentration

vs. time in pyridine HDN experiments

at 350 “C.

HDN activity was PtNiMo > PtCoMo > NiMo > CoMo > (PtNiW = NiW). Fig. shows the associated pentane concentrations vs. time from the same experiments. noted earlier, pentane concentration is another index of pyridine HDN and yields same catalyst relative activity ranking as that of %HDN in Fig. 12. Assuming that

. PtCoMo

Time (min) Fig. 14. First-order

plot of pyridine concentration

conversion

at 3.50 “C.

13 As the the

1.52

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Table 3 Rate constants for pyridine hydrogenation Catalyst

Rate constant (min-’ )

PtNiMo PtCoMo

0.16 0.12

NiMo CoMo

0.12 0.091 0.067 0.058

PtNiW NiW

hydrogen pressure is approximately constant so that the pyridine to piperidine HYD is an apparent first-order reaction, Eq. (3) is obtained:

=

(3)

k't

where Cpyr is the pyridine concentration. Based on Eq. (3), the first-order plot in Fig. 14 and the apparent rate constants in Table 3 were obtained. The catalyst rate constant ranking for pyridine HYD is the same as that of HDN. This trend is in agreement with that in Fig. 5 in which the Pt-promoted catalyst showed improved HYD activity in the presence of N. For both the promoted and unpromoted catalyst versions, NiMo was more active for HDN than CoMo. This observation is in agreement with the results of other investigations showing NiMo to be more effective for HDN [7,31]. While the most attention was focused on Pt, other noble metals showed significant potential for promotion of HDN. The HDN activity with the unpromoted and Pt-, Ru-, and Ir-promoted catalysts is shown in Fig. 15. The most obvious result in Fig. 15 is an increase in HDN activity of the NiMo and CoMo catalysts due to both Pt and Ir

Y

NiMo

COMO

NiW

Fig. 15. %HDN of pyridine with unpromoted and Ru-, Ir-, and Pt-promoted catalysts at 350 “C for 20 min.

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153

0.601

L

0.00

COMO

NIW

Fig. 16. Pentane concentration in pyridine HDN reactions with unpromoted and Ru-, Ir-, and Pt-promoted catalysts at 350 “C for 20 min.

promotion. Also the NiW catalyst was not improved significantly by any metal. As noted earlier, these results suggest that the noble metal promotion is not simply an additive effect, but that it depends on the original catalyst composition in synergistic fashion. Fig. 16 shows the pentane concentrations corresponding to the experiments in Fig. 15. Again, the relative catalyst activity rankings are the same as those based on %HDN, with the Ir- and Pt-promoted NiMo and CoMo catalysts being most active.

4. Conclusions 4.1. Naphthalene

HYD

For the oxide catalysts, the Pt-promoted catalysts showed much higher activities than the unpromoted ones, whereas the activities of both promoted and unpromoted sulfide catalysts were similar. In the oxide forms, the Ni-promoted catalysts were more active than Co-promoted versions for HYD. The sulfide catalysts were less active than the oxide ones, except for CoMo. In the presence of N, the Pt-promoted catalysts exhibited higher ring HYD activities than the unpromoted versions. 4.2. Pyridine HDN

All noble metals used improved the HDN activity in the NiMo catalysts with Pt promotion being best. With CoMo, Pt and Ir improved the HDN activity to the same

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extent. With NiW, none of the noble metals increased the activity. Generally, the NiMo catalyst was more efficient for HDN than the other catalysts tested either with or without promotion. Among all the catalysts, PtNiMo was most active for pyridine HDN. For both original and Pt-promoted catalysts, the sulfide forms were more effective for HDN than the oxide ones. The results of kinetics studies suggest that the noble metals promotion accelerates HYD of the N ring, which is the first step in N removal. Finally, we note that experiments currently ongoing indicate an improved HDN activity for the Pt-promoted catalysts with both coal and coal-plastics coprocessing liquids [32]. The completed results of these studies will be reported in a subsequent paper.

Acknowledgements This work was supported by the US Department of Energy as part of the research program of the Consortium for Fossil Fuel Liquefaction Science.

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