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Centinel Technology catalysts for distillate desulphurisation:

Science and Applications

T.J. Remans (*), W.H.J. Stork (*), J.A.R. van Veen (*), A. Gabrielov (°), B.J. van der Linde (#),

J. Swain (+), D. di Camillo (%) and R.S. Parthasarati (!).

1. Introduction Refineries convert a wide range of crude oils into products, such as transportation fuels and feedstocks for petrochemical industries. Conversion processes, among which hydroprocessing, play a key role in modern refineries, removing heteroatoms and changing chemical structures. In the drive towards clean fuels, tightening product specifications pose increasing challenges to the refiners. For improvement of the hydroprocesses, in addition to the development of better catalysts, based on mechanistic research, attention is being given to technology improvements as well. In September 2000, CC&T introduced its new catalyst technology, Centinel, for refinery hydroprocess applications. Both CoMo and NiMo type catalysts were launched simultaneously in distillate HDS, First-Stage Hydrocracking (FSHC) (DN-3120, DN-3100) and Cat feed Hydrotreating (CFH) (DC-2118, DN-3110). At present, more than 100 batches of Centinel catalysts have been sold, in a combined amount of over 20 MM lbs, for applications in distillate HDS, First Stage Hydrocracking and Cat cracker Feed Hydrotreating. Centinel technology catalysts generate much higher value in a world of ever-tightening specifications, as the performance advantages of Centinel over conventional catalysts, already important at deep desulphurisation levels, will increase strongly upon Ultra Deep desulphurisation. In the present paper the focus is on the novel catalyst technologies and on a better understanding of the chemistry involved. In order to derive the maximum benefits out of high performance catalysts, also the reactor designs and process conditions should be optimized. It is essential to use an optimized combination of catalysis and reactor/process technology to arrive at optimum results in demanding applications such as Ultra Deep HDS. In the present paper, however, we will strongly focus on the catalysis aspects and only briefly deal with optimized reactor design.

(*) Shell International Chemicals BV (#) Shell Global Solutions International BV

Shell Research and Technology Center, Amsterdam,

Badhuisweg 3, 1031 CM Amsterdam, The Netherlands

(°) Shell Chemicals LP Westhollow Technology Center

3333 Highway 6 South, Houston, TX 77082, USA

(+) Criterion Catalysts & Technologies

1650 Parkway, The Solent Business Park, Whiteley, Fareham, Hampshire, PO15 7AH,

England (

%) Criterion Catalysts & Technologies

16825 Northchase Drive, Suite 1000 Houston, Texas 77060-6029, USA

(!) Criterion Catalysts & Technologies

298 Tiong Bahru Road #07-03/04/05 Tiong Bahru Plaza

Singapore 168730

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2. Future specifications Ever tightening product specifications continue to pose increasing challenges to the refiners. In the European Union, the sulphur specification for automotive gasoil has steadily decreased from 3000 ppmw S in 1989 to 2000 ppmw in 1993, 500 ppmw in 1996 and 350 ppmw in 2000. Other diesel specifications, e.g. for cetane, density, T95 and Di+ aromatics have also tightened during these years. In the near future, the specification for sulphur will continue to decrease to 50 ppmw S by 2005 (Auto Oil II program) and sub 10 ppmw S by 2008-2011 (Clean Air For Europe or “CAFE” initiative). The US mandates 15 ppmw S for diesel by the year 2006. Gasoline sulphur specifications are not discussed here but in general follow the same trend. The paper focuses on diesel application only. The complete overview of past and future specifications for diesel as specified by the European Union is given in Table 1. Table 1: Diesel specifications as specified by the European Union 1989 1993 1996 2000

(AO-I) 2005

(AO-II) 2008-2011 (CAFE*)

S, ppmw 3000 2000 500 350 50 "0" (< 10) d 15/4, kg/m3 860 860 860 845 845 845 Cetane No 49 49 49 51 51 51 T95, °C 370 370 370 360 360 360 Di+ Arom., wt% - - - 11 11 11 *: Press release Dec 2001 states a compromise has been made on CAFE specs between

the 2008 and 2011 targets and the new data for total implementation would be 2009, phased-in from 2005 onwards.

A strong drive for reducing sulphur in diesel even before the European Unions legislation becomes effective comes from tax incentives set by specific states. A recent overview of current and proposed tax incentives for low sulphur diesel is presented in reference 1. 3. Increased workload For an average EU refinery feedstock containing 1.2 wt% S, the tightened S-specification from 3000 to 10 ppmw S implies an increase in HDS conversion level from 75 % to 99.92 %. A graphical presentation of the S-specification and concomitant HDS conversion level, see Figure 1, suggests that the largest steps were taken in the past and future specifications seem to require only a marginal increase in conversion. A more accurate way of illustrating the effect of the tightening S-specifications on the HDS reaction is by calculating the workload on the catalyst, as given in Figure 2. The workload quantifies the required additional catalyst volume to reach the tightened specifications for a given existing HDS unit, the feed it processes and the catalyst loaded. Now, it becomes clear that there is a continuous and steady increase in the workload of the HDS catalyst over the years. On average, every new specification has increased the workload by a factor of about 2. Or in other words, the catalyst activity had to increase each time by a factor of about 2 to achieve the new specification. In total, a 20 times increase in workload is required to go from the 1989 specification to the 2011 specification. It is clear that only catalyst improvements will not get us there.

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Figure 1: EU S-specification and associated HDS conversion level for a 1.2 wt% S feed.

Figure 2: HDS conversion level for a 1.2 wt% S feed to reach the imposed EU

specification in the given year and associated workload on the HDS catalyst.

These large increases in workload ask for an optimum match between the use of high performance catalysts as well as customer specific unit modifications. To bridge the gap from one workload level to the next, one can apply higher activity catalysts, upgrade the technology by e.g. installing Shell Global Solutions’ High Dispersion (HD) trays or expanding in reactor volume. The first step –the drop-in solution of a more active catalyst- was sufficient when going from 3000 to 2000 ppmw S. The step from 2000 to 500 ppmw S in 1996 required additional measures to reach the new workload level. Improving the gas/liquid dispersion and catalyst wetting becomes more critical as the specifications tighten. The installation of an HD tray can on average yield a 30 % higher

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catalyst activity as well as a reduced deactivation rate. The combined effect of these two effects results in a reduction of the workload by 50 %. More details on the technology improvements can be found in paragraph 7. The remaining gap between effective workload and required workload will have to be –and historically has been- bridged by expanding reactor volume. Figure 3 shows to what extent each of these factors contribute to reaching the increased workload levels. Taking into account the activity level of the catalysts in the specific time frames, Figure 3 shows that the gap requiring reactor extension has been reduced upon going from 1996 to 2000. Improvements in catalyst activity outpaced the increase in required workload such that the step from 500 to 350 ppmw S could easily be met by a drop-in solution of a newer generation catalyst. From Figure 3, it becomes also clear that this gap becomes much larger again when targeting the 2005 specifications of 50 ppmw S with the current generation of highest performance catalysts. Based on the integrated approach required here, CC&T’s Clean Fuels Team is specialized to help out by creating customer-tailored solutions for achieving both the 2005 and 2011 specifications, considering both process and catalyst options.

Figure 3: Bridging the gap between required workload and existing by

applying more active catalysts and installing a HD tray.

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4. Catalyst improvements Recent advances in hydroprocessing catalyst development by CC&T have taken a high flight in a field that is already decades old. The large log scale in Figure 3 masks this somewhat, but when expressed on a natural scale, it becomes clear that we are on the steep edge of a new S-curve in catalyst development for refinery applications, see Figure 4.

Figure 4: Recent advances in hydroprocessing catalyst development. This has been achieved by the fast track development of a new way of producing hydroprocessing catalysts, named CC&T’s Centinel Technology. The combination of this new technology and the knowledge already built up on conventional type catalysts yielded a remarkable series of catalysts with optimized pore structure, metals dispersion and active phase selectivity. An example would be improved hydrogenation activity. Without being in a position to offer catalyst preparation details, the Centinel technology leads to catalysts that are very different to the conventional hydroprocessing catalysts. This difference is schematically shown in figure 5, where a Centinel catalyst is compared with a conventionally prepared one with the same composition. The following detailed characterization studies should underpin these conclusions.

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Figure 5: schematic representation of Centinel catalysts In the hydrotreating Co(or Ni)/Mo catalysts the active metals are largely present in sulphided form, specifically as layers of MoS2 (MoS2 has a layered structure) promoted with Co or Ni; these latter are considered to be the active sites (ref. 2). Electron microscopic studies on CoMo catalysts show (figure 6) that, as indicated, the size of the MoS2 layer in the DC 2118 Centinel catalyst is smaller than in the DC 130 conventional one, while the number of layers tends to be larger.

Catalyst Average slab size (nm) Average stacking

DC-130 Reference Base Base

DC-2118 Base – 21 % Base – 4 %

Figure 6: electron microscopic results of DC 2118 Centinel catalyst This is due to the deliberately selected catalyst recipe, where the interaction between the alumina support and the active metals during preparation is closely controlled. As a consequence, the metals are more completely sulphided than in conventional catalysts (see figure 5). Evidence for this is shown in figure 7, where X-ray photoelectron spectroscopic measurements (XPS) were carried out on conventional and Centinel catalysts after various sulphidation treatments, showing that clearly in the Centinel catalyst the sulphidation proceeds faster, further than in the conventional catalyst. In addition, Temperature Programmed Sulphidation (TPS) studies were carried out on conventional and Centinel catalysts: results are given in figure 8, and show that in Centinel catalysts, molybdenum is quickly converted to a MoS3 intermediate species, while in conventional catalysts it is directly, but less completely, converted to MoS2.

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Centinel versus conventional on fundamental level

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Figure 7: Temperature Programmed Sulfidation of Centinel NiMo.

Percentage of sulfided phase indicated for both Centinel and conventional catalysts (conventional value in brackets).

Figure 8: Temperature Programmed Sulfidation for Centinel versus conventional in inset (same scale) demonstrating the typical Type II behaviour of high S uptake by MoVI to MoS3 prior to reduction to MoS2.

As is As is

100 % (84 %)

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Inset: S-uptake and release for conventional catalysts

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Applied T ramping

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Inset: S-uptake and release for conventional catalysts

S-uptake and release for Centinel

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Figure 9: NO-chemisorption on Centinel and conventional catalyst showing the same types of sites and also the higher amount of Co at the MoS 2 edges relative to the available sites.

As shown in figure 5, this means that the Centinel catalyst has a structure that brings about a large fraction of so-called type II sites, but does not appear to involve any new or unconventional type of sites. This is also the conclusion that is drawn from in-situ IR studies, using NO as a probe gas. Figure 9 shows the spectra typical of NO adsorbed to the Co on MoS2 and to vacant MoS2 sites, and do not indicate any novel type of site. The magnitude of the peak at 1850 cm-1 underlines the high number of active sites relative to the available sites at the MoS2 edge. In conclusion, taking all fundamental characterization results together, one arrives at figure 5, showing very well dispersed metals, well sulphided, with a high proportion of type II sites.

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5. Chemistry of desulphurization The chemistry of hydrodesulphurization has been studied in recent years extensively. With the detailed analysis of sulphur compounds that is now possible by element selective detection in a GC, or even better, using two-dimensional GC, (ref. 1), we can now identify the most refractory compounds (figure 10a,b), the alkyl substituted dibenzthiophenes.

Figure10a: S-distribution of a European refinery gasoil. Figure10b: S-distribution of liquid product after HDS down to 75 ppmw S.

Examples of identification: *: 4,6-dimethyldibenzothiophene **: 4-methyl, 6-ethyldibenzothiophene There are strong indications that the mechanism for the refractory compounds is different to that for the other compounds: “”easy sulphur”” reacts via direct desulphurization, the substituted dibenzothiophenes react via prior hydrogenation of an aromatic ring (figure 11).

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Figure 11: Overview of possible pathways for the HDS of highly refractive S-compounds. Originally this was attributed to steric hindrance of the sulphur atom; at present it rather seems caused by the electronic factors (ref. 3). The overall conclusion remains, however, that deep desulphurization requires prior hydrogenation, and is hence effectively done on CoMo catalysts with a high proportion of type II sites. Indeed, at equal conversion different product distributions are obtained over different catalysts, indicating different relative contributions of both reaction pathways (table 3). As can be seen, Catalyst B is more effective in removing the most refractive S-compounds than Catalyst A such that the former catalyst is preferred for deeper conversion levels as the last S-compounds are all highly refractive. The overall HDS activity is the same for the applied catalyst formulations, but the selectivity is significantly different. Alternatively, NiMo catalysts, preferably also with a high type II site content, can be used.

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Catalyst Ratio of highly refractive DBT over less refractive DBT CoMo A 10.8 CoMo B 7.4

Table 3: Example of changed ratio of S-compounds at comparable

conversion level applying two CoMo based HDS catalysts It has also become clear that in HDS the catalyst is easily poisoned. A detailed study in our laboratory (ref. 4) showed that basic nitrogen compounds are really the main catalyst poisons. In Figure 12 the catalyst activity for HDS of dibenzothiophene in a gasoil matrix is shown as a function of the basic nitrogen content, and clearly the HDS activity suffers substantially at basic nitrogen levels of 5-20 ppm.

Figure12: DBT conversion activity as a function of basic nitrogen content. As nowadays the nitrogen compounds can also be separated (as for sulphur), as shown in figure 13; one may expect that detailed identification of the nitrogen compounds at various stages in the process (conversion of more reactive species, intermediate hydrogenated compounds) will add further to our understanding. References for detailed identification of these N-compounds can be found in Reference 7. One interesting point in this respect is the observation that at comparable HDN conversion level, Centinel catalysts remove more of the most difficult N-compounds than conventional catalysts do. This is shown in Figure 13 as the surface area under the curve between the two vertical bars that demarcate the region of the highly refractive N-compounds. For both Centinel catalysts, this area is significantly smaller than for the conventional catalyst at the same N-slip. Therefore, Centinel catalysts are not only for HDS but also for HDN more non-selective in the removal of the normal, refractive and highly refractive S- and N-compounds yielding a higher conversion of the more difficult S- and N-species at comparable conversion levels. This makes them especially attractive for attacking those final highly refractive S- and N-compounds.

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Figure 13: Separation of N compounds in hydrotreated gasoil by N-specific

chromatography for Centinel NiMo (DN-3110), Centinel CoMo (DC-2118) and conventional CoMo (DC-130) at about 20 ppmw N level in liquid product. Thin lines indicate drift in base line. Vertical thick lines demarcate region of highly refractive N-compounds.

Separately, a study showed that extraction of nitrogen compounds from a gasoil results in a product that was clearly easier to desulphurize. (ref. 5). In contrast with the effect of nitrogen compounds, the presence of polyaromatics is not very harmful, as can be seen from figure 14 (ref. 4).

Figure14: Effect of poly cyclic aromatics on HDS activity Basis the above results the differences between conventional CoMo and NiMo catalysts in the gasoil HDS are easily explained: the CoMo catalysts are the more active for HDS, but the NiMo catalysts, with their higher HDN activity, can be preferred at times: in a stacked bed NiMo over CoMo, or with high N/cracked feeds at higher pressures.

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Interestingly enough, data in ref. 4 suggests that at very deep HDS, possibly where the hydrogenation reaction path prevails, NiMo catalysts are more active than CoMo catalysts, even on (synthetic) feedstocks without any N compounds. The results reported in ref. 4 were obtained on conventional catalysts; with the high proportion of type II sites in the Centinel catalysts, one would expect the NiMo and CoMo catalysts to come a little closer in performance (as reported in ref 1). Next to the above poisoning effects, it has also been shown that at the production of Ultra Low Sulphur Diesel (ULSD), effects of apparent thermodynamic constraint can easily play a role. Under certain conditions, with increasing temperature the effective activation energy decreases. A similar effect has been observed in denitrogenation over NiMo/alumina catalysts, and has been attributed to the shifting between e.g. the substituted acridines and their hydrogenated counterparts, well-known intermediates in the denitrogenation reaction (ref. 6). In ultra deep HDS similar effects are observed: under specific conditions (low pressure, cracked feedstocks, etc) the increase in reactor temperature does no longer result in the rate increases expected basis the traditional activation energy. A real example of the onset of thermodynamic limitation at low PP(H2) in targeting sub 50 ppmw S (Ultra Deep-HDS) is presented in Figure 15. Here, 3 conditions on a typical European refinery feed after more than 2000 h on stream time are highlighted out of a long lab experiment on DC-2118. Condition 1, the base condition, yields a S-slip of 25 ppmw at low pressure and medium temperature. In condition 2, all conditions remained the same except for the temperature that was significantly increased. With a S-slip still as high as 21 ppmw in the new condition, it is clear that the temperature response at low pressure in UD-HDS is lower than anticipated. The normalized k-value indicates a 35 % lower response. Increasing the H2 partial pressure, as done upon going from condition 2 to 3, shows that more than half of the lower response has now been removed. After correcting the k-value for the temperature only, the k-value now is higher than in condition 1 but upon including the pressure correction as well, it still hasn’t reached the activity level of condition 1. A higher PP(H2) shifts the thermodynamic limitation to lower concentrations (actually ratio’s of concentrations) such that the pure kinetics become the major player again. The thermodynamic limitation on the aromatics is reflected in the change in di+ aromatics, also in Figure 10. The first condition was repeated at the end to ensure that deactivation could be ruled out as a cause.

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Figure 15: Thermodynamic effects in deep HDS The effect has severe consequences, in that a higher reaction temperature becomes a less effective tool to reach ultra deep sulphur levels, making such levels unattainable in a number of existing low-pressure units except by means such as severely reducing the endpoint of the feedstock. 6. Performance of Centinel catalysts In chapter 4 it was established that Centinel catalysts predominantly have type II active sites, which are known to have higher selectivity to hydrogenation and hydrodenitrogenation than type I sites (ref. 2). In chapter 5 the chemistry of deep desulphurization was reviewed, highlighting the importance of an indirect desulphurization route in deep HDS, and the poisonous effect of nitrogen compounds under those conditions. In addition, it emerged that apparent thermodynamic effects can limit the reaction rate at high reaction temperatures. Figures 16, 17 and 18 show the performance of the Centinel CoMo 2118 catalyst in HDS and HDN, and at various HDS levels. The catalyst has a higher selectivity to HDN than a conventional CoMo catalyst (DC 130, also supplied by Criterion), and shows it largest activity benefit at very deep HDS levels, where we have seen the effects of nitrogen poisoning and of the hydrogenation step required in the indirect route. With the limited response to temperature in mind, these catalyst activity gains are of course of paramount importance in commercial units to achieve the required sulphur target at an acceptable catalyst life.

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Figure 16: RVA for HDS and HDN of Centinel CoMo (DC-2118 and Centinel NiMo (DN-3110) versus conventional CoMo (DC-130) SRGO Feed, medium PP(H2), high LHSV, S-slip around 200 ppmw S.

Figure 17: RVA for HDS of Centinel CoMo (DC-2118 and Centinel NiMo (DN-3110) versus conventional (DC-130). N-slip around 0 for both Centinel catalysts. SRGO Feed, medium PP(H2), low LHSV, S-slip around 10 ppmw S.

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Figure 18: HDS activity improvement of Centinel over conventional catalysts as a

function of S-slip. The good performance of DC 2118 shown in laboratory testing is also confirmed by commercial experience. Figure 19a shows the performance of DC2118 (in combination with a conventional CoMo catalyst, DC 185) in ultra deep HDS, with a clear performance gain over the previous conventional catalyst, giving products with sulphur contents between 10 and 50 ppm. Figure 19b gives commercial results for a unit with DC 2118 and DN 3110 producing consistently sub-10 ppm S.

Figure 19a: Commercial performance of DC-2118 (with some DC-185) at 35 ppmw S.

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Figure 19b:Commercial performance of DC-2118 (with some DN-3110) at 8 ppmw S. As was pointed out above, NiMo catalysts become more and more attractive when going to lower Sulphur levels, to increasing amounts of cracked feedstocks, and to higher hydrogen pressures. Figure 20 schematically shows at what conditions the Centinel NiMo catalyst DN-3110 is better than its CoMo analogue.

Figure 20: Application area for full bed CoMo or NiMo as a function of S-level and PP(H2).

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Upon targeting sub 15 ppmw S (US market) and sub 10 ppmw S (EU market), it will become crucial to fully understand the kinetics of the various reaction pathways in distillate HDS. In general, the discussion usually revolves around the direct and indirect HDS reaction pathway, the indirect pathway being the one in which full hydrogenation of the neighboring aromatic ring precedes the S-removal step. Figure 11 gives an overview of all theoretically possible pathways to remove sulphur from a highly refractive S-compound in hydrodesulfurization. The normal HDS pathway for the non-refractive S-compounds is via direct HDS, this in clear contradiction to HDN where the indirect pathway is always followed due to the stronger C-N bond in the aromatic ring structure. For removal of the last refractive S-compounds, the indirect reaction pathways, with aromatics hydrogenation as a first step, are crucial. 7. Technology improvements CC&T and Shell Global Solutions have successfully demonstrated on numerous occasions that the net reactor performance can be increased up to50% at a fraction of the investment cost of a new vessel and with implementation within a matter of months (typically during next planned shutdown). Consider the example in Figure 21 which demonstrates that improved space utilization can either result in (1) construction of a 20% smaller reactor if considering a grassroots design or ( 2) a 28% increase in catalyst volume within an existing vessel.

Figure 21: Improved reactor space utilization can decrease size of a reactor design or increase available catalyst volume in an existing vessel.

Maximum catalyst utilization can only be achieved if the gas and liquid reactants are

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uniformly distributed both volumetrically and thermally before they are introduced into the very top of each and every one of the catalyst beds. As mentioned previously, even some designs as recent as the late 1990’s don’t use distribution trays that have high dispersion. Shell High Dispersion (HD) and Ultra Flat Quench (UFQ) trays have been developed to incorporate the use of the unique HD nozzles that attain near 100% catalyst utilization for trickle phase reactors. The HD tray is of the latest proprietary Shell design, creating an almost perfectly uniform liquid distribution as depicted in Figure 22. The UFQ in turn additionally provides nearly perfect mixing of process liquid and gas, and quench medium between the beds, while using minimum reactor volume. Compared with many distribution trays being offered today, the Shell internals can boost the catalyst utilization from 80 to nearly 100%, generating an equivalent improvement in activity of 25% RVA HDS or 7°C.

Figure 22: Visualization of distributor tray flow patterns 8. Conclusions The paper focused on the need for optimally using existing distillate HDS units for achieving the ever-tightening sulphur specifications. The huge increase in workload both in the near past and future quantifies this need. Recent catalyst development by CC&T has led to the commercialization of the Centinel technology catalysts that maximize the HDS conversion level with balanced H2 consumption. Currently, 36 distillate HDS units have been started-up with Centinel Technology catalysts. In distillate, as well as in other catalytic processes as CFH and FSHC, clear advantages over conventional catalyst systems have been demonstrated. Centinel catalysts have been thoroughly characterized; given the fact that the Centinel catalysts are rich in type II sites, their performance could be well understood. In most cases, a drop-in catalyst option is not enough to reach the increased workload levels, especially when targeting 50 or 10 ppmw S. Thermodynamics will start to play an important role which can result in higher than expected start-of-run WABTs as well as in a less efficient use of the available operating window.

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NNoo GGrraaddiinngg MMaatteerriiaall AAllll CCaattaallyysstt BBeedd

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Figure 23: Novel catalysts currently being developed. In addition to the catalyst, the need for perfect gas/liquid dispersion as well as for complete wetting of the catalyst bed becomes increasingly more important to reach the required workload level. The use of HD trays was shown to bridge a major part of the gap between available workload and required workload for the new specifications. It should be clear that the combination of an optimum catalyst and prime technology know how will be crucial for implementing the future sulphur specifications. Of course catalyst research does not stop, and, using fundamental insights as presented here, several new and improved catalysts are currently under development (figure 23). These will help the refiner to manufacture the 10 and 50 ppmw S products in existing units with their existing constraints.

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1980 1985 1990 1995 2000 2005 2010

Relative Catalyst Volume Activity (RVA) of commercially producedcatalysts for Hydrodesulfurisation (HDS) of diesel range gasoilRVA (%)

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9. References [1] T.J. Remans, J.A.R. van Veen, A. Gabrielov, B.J. van der Linde,

J. Swain, D. di Camillo and R.S. Parthasarati, Centinel Technology catalysts for refinery hydroprocess applications: Recent advances in the HDS catalyst portfolio of Criterion Catalysts & Technologies and the integrated approach with optimum process conditions and reactor internals. European Catalyst and Technology Conference (ECTC), February 26-27, 2002, Amsterdam, The Netherlands (2002).

[2] H.R. Reinhoudt, C.H.M. Boons, A.D. van Langeveld, J.A.R. van Veen, S.T. Sie and J.A. Moulijn, On the difference between gas- and liquid phase hydrotreating test reactions, Appl. Catal. A: General, 207, 25-36 (2001). H. Topsoe, B.S. Clausen, and F.E. Massoth, Hydrotreating catalysis, Science and Technology, Springer Verlag (1996).

[3] F. Bataille, J. Lemberton, P. Michaud, G. Perot, M. Vrinat, M. Lemaire, E. Schulz, M. Breysse, and S. Kasztelan, Alkyldibenzthiophenes HDS promotor effect, J.Catal, 191, 409-422 (2000)

[4] F. van Looij, P. van der Laan, W.H.J. Stork, D.J. Dicamillo, and J. Swain, Key parameters in deep hydrodesulfurization of diesel fuel, Applied catalysis. A: General, Vol. 170, No. 1 (1998) 1.

[5] M. Macaud, E. Schulz, M. Vrinat and M. Lemaire , A new material for selective removal of nitrogen compounds from gasoils towards more efficient HDS processes, Chem. Comm 2340-2341 (2002)

[6] J.K. Minderhoud and J.A.R. van Veen, First-stage hydrocracking: Process and catalytic aspects, Fuel Processing Technology, 35 (1993) 87.

[7] P. Wiwel, K. Knudsen, P. Zeuthen and D. Whitehurst, Assessing Compositional Changes of Nitrogen Compounds during Hydrotreating of Typical Diesel Range Gas Oils Using a Novel Preconcentration Technique Coupled with Gas Chromatography and Atomic Emission Detection, Ind. Eng. Chem. Res., 39, 533-540 (2000). P. Zeuthen, K. Knudsen and D. Whitehurst, Organic nitrogen compounds in gas oil blends, their hydrotreated products and the importance to hydrotreatment, Catalysis Today, 65, 307-314 (2001). G.C. Laredo, S. Leyva, R. Alvarez, M. T. Mares, J. Castillo and J.L. Cano, Nitrogen compounds characterization in atmospheric gas oil and light cycle oil from a bland of Mexican crudes, Fuel, 81, 1341-1350 (2002).