Mechanisms of n-decane hydrocracking on a sulfided NiW on silica-alumina catalyst (2023)


High-pressure hydrocracking is a catalytic refining process allowing an upgrade of rather heavy petroleum fractions such as vacuum distillates into gasoline or middle distillates, kerosene, and gas oil[1], [2], [3], [4]. It is also used to upgrade some products obtained from other processes, such as deasphalted oils and cycle oils from fluid catalytic cracking[1], [5]. A major interest of hydrocracking is its great versatility since it is possible to equilibrate supply (gasoline or middle distillates) and demand. The growing demand for middle distillates, which cannot be obtained using fluid catalytic cracking, makes hydrocracking a strategic process in a modern refinery[2], [6], [7].

Moreover, in addition to the cracking of the molecules, the hydrocracking process ensures the elimination of sulfur- and nitrogen-containing compounds as well as a deep saturation of aromatics. This allows the production of high-quality fuels which will match the future and more stringent specifications[1], [8].

Hydrocracking catalysts are bifunctional, associating a hydro-dehydrogenating function with an acidic one. Since the feeds to be treated contain significant amounts of heteroatoms (sulfur and nitrogen), the catalysts must resist poisoning by hydrogen sulfide and ammonia. That is why the hydro-dehydrogenating function of industrial hydrocracking catalysts is generally provided by mixed sulfides of group VI and VIII metals, such as molybdenum or tungstene promoted by nickel or cobalt.

If gasoline is the required product, the acidic component of the catalyst will be preferably a zeolite, the strong acidity of which will favor successive cracking reactions of the feed molecules with formation of the desired light products[9]. On the contrary, the catalysts which would allow an optimal gas oil yield which would possess a moderate acidity, halfway between that of zeolites (low selectivity) and that of aluminas (too low activity). Amorphous supports like silica-aluminas could have such an acid strength[2], [10], [11], [12].

Our objective was to study hydrocracking catalysts allowing a selective transformation of vacuum distillates into gas oil. For the reasons explained above, a sulfided NiW/silica-alumina catalyst was chosen, and was compared to a strongly acidic NiW/USHY zeolite catalyst. The model reaction used was the hydrocracking of n-decane, under conditions similar to the industrial ones: fixed-bed dynamic reactor, high hydrogen pressure, presence in the feed of sulfur- and nitrogen-containing compounds. In this work the nature and the repartition of the reaction products were examined andthe reaction scheme established. The different selectivities were measured, in particular the isomerization/cracking selectivity which can be considered as representative of the selectivity of the catalyst for the production of middle distillates. To obtain a better understanding of the behavior of the NiW/silica-alumina catalyst, we also studied the transformation of n-decane on a nonacidic NiW/alumina catalyst, on an unpromoted W/silica-alumina catalyst, and finally on NiW/silica-alumina in the absence of nitrogen-containing compound in the feed.

Section snippets


n-Decane hydrocracking was carried out in a flow reactor at 380°C under a 6 MPa total pressure, with a hydrogen/n-decane molar ratio equal to 20. Dimethyl disulfide and aniline were added to n-decane in order to generate H2S (pH2S= 6.1 kPa) and NH3 (pNH3= 5kPa), respectively. The catalysts were first sulfided in the reactor by a n-heptane/dimethyl disulfide/aniline mixture, under the same pressures as those used for the n-decane hydrocracking reaction, and at a 0.27-min contact time (1/WHSV).

Acidity of the supports

Silica-alumina possessed only one type of terminal OH groups, already observed by Knözinger and colleagues[14]. On the contrary, several OH groups were observed on the USHY zeolite[13]: terminal OH groups, similar to those observed on silica-alumina, bridging OH in large cavities or in small cavities, and OH interacting with extraframework aluminum species. Five OH bands were observed on alumina[15]: OH coordinated to octahedral or to tetrahedral Al, and OH bridging tetrahedral and


Pyridine adsorption experiments clearly confirm that silica-alumina is much less acidic than the USHY zeolite, which is widely known. Compared to zeolite, silica-alumina has only few Brønsted acid sites. Moreover the strength of this sites is rather moderate, while a wide distribution in the acid strength is observed on zeolite. However, one can reasonably suppose that the strongest Brønsted acid sites of the zeolite do not participate in n-decane hydrocracking because they are irreversibly


The results obtained in the present work indicate that the behavior of the NiW/silica-alumina catalyst for n-decane hydrocracking in the presence of nitrogen-containing compounds is very different from that of classical hydrocracking catalysts such as NiW/USHY zeolite. The main reaction observed is the transformation of n-decane into monobranched isomers, the bifunctional transformation of which into multibranched isomers and into cracking products being hardly observed. On NiW/zeolite the

  • Effect of SiO<inf>2</inf>/Al<inf>2</inf>O<inf>3</inf> ratio in Ni/Zeolite-Y and Ni-W/Zeolite-Y catalysts on hydrocracking of heptane

    2022, Molecular Catalysis

    This study investigated the effect of the SiO2/Al2O3 ratio in the range of 5-80 in zeolite Y (ZY) as a support for the bi-functional reaction of heptane hydrocracking. Bi-metallicity impact, through the addition of W on Ni in the supported metal catalysts was also examined. The catalytic activity was assessed at 350°C and 400°C in order to deduce an optimized composition of the catalyst in terms of metal composition and Si/Al ratio. The results were correlated to the catalysts’ surface and bulk properties, the latter after employing a number of material characterization techniques. It was shown that Ni-W bimetallic catalysts demonstrated better catalytic activity (conversion, 78% to 91%) than Ni-based monometallic counterpart catalysts (conversion, 74.2% to 82.7%), with NiO-WO3-ZY30 (SiO2/Al2O3 ratio equal to 30) exhibiting the highest conversion. This was attributed to the bimetallic's enhanced metal dispersion and smaller particle size, evaluated using temperature-programmed desorption (TPD) of H2 and high-resolution transmission electron microscopy (HR-TEM) imaging. The stronger acidity, as quantified by total acidity calculations, of zeolite Y having higher Si/Al ratio, and their balanced ratio of micro- and meso-porosity played a vital role in their catalytic performance. This study provides useful design guidelines on how to adjust both the Si/Al ratio in the zeolite Y support and the catalyst's bimetallicity for enhanced hydrocracking performance.

  • Heteropolyacids supported on clay minerals as bifunctional catalysts for the hydroconversion of decane

    2021, Applied Catalysis B: Environmental

    The role of heteropolyacid (HPA) was studied as a precursor for the generation of NiMo and CoMo catalysts supported on a vermiculite and a bentonite previously delaminated and bolstered with the incorporation of AlZr and AlCe species. The solids were characterized using X-ray diffraction (XRD), N2 adsorption, Scanning Electron Microscopy (SEM) and High-Resolution Transmission (HR-TEM), Raman spectroscopy, Temperature-Programmed Reduction (H2-TPR), IR spectroscopy with probe molecule (NH3-DRIFTS) and acidity analysis “in-situ”, electronic X-ray spectroscopy (XPS,) and the catalytic performance was evaluated in the hydroconversion of decane. The controlled modulation of the properties of natural clay minerals combined with the anchoring of the active phase from the HPA type precursor, generates supported catalysts with different ranges of catalytic performance. The stability of the HPA type structure was evidenced even after the calcination and pre-reduction process. The catalysts in the present work register better physicochemical properties (textural, acidic, reducible species and metallic dispersion) and a superior catalytic performance in the hydroconversion of decane, compared with catalysts obtained from a conventional salt reported in the literature.

  • Homogeneous catalyst for in-situ hydrotreating of heavy oils

    2019, Applied Catalysis A: General

    Citation Excerpt :

    It is known that the disappearance rates of thiophene type compounds with increasing numbers of aromatic rings decrease while nonthiophenic aromatic sulfur compounds react more quickly than dibenzothiophenes, and due to the inherent difficulty of the alkyl substituted dibenzothiophenes for reacting, it is clear that they persist in the hydrotreated products. As for hydrodesulfuration, it can proceed via two pathways; the first one is hydrogenolysis in which both carbon-sulfur bonds are replaced by carbon-hydrogen bonds, which lead to ring opening; in the second one, hydrogenation can occur initially and then, the intermediate product undergoes a hydrogenolysis step [45–47]. The predominant HDN pathway proceeds via the breaking of the C (sp3) bond.

    The upgrading of the physical and chemical properties of heavy oil by hydrotreating using a liquid Ni-Mo catalyst was studied through bench-scale tests aiming at developing "inside-reservoir" technology. The liquid Ni-Mo catalyst was synthesized by a very simple method consisting in preparing and mixing solutions at room temperature under acidic conditions. Mixtures of liquid Ni-Mo catalyst and heavy oil were evaluated in a 500-mL batch reactor at 350, 390 and 400 °C for 30, 60 and 90 min. The results showed that the liquid Ni-Mo catalyst improved the crude oil properties by increasing the API gravity from 12.5 to 22 and decreasing the kinematic viscosity from 13,490 to 72 cST at 15.6 °C; the sulfur content from 5.5 to 3.1% and the nitrogen content from 750 to 392 ppm were also reduced. Furthermore, the volume of gasoline and diesel by Simulated Distillation were increased to 8 and 14 vol. %, due to the change in chemical composition. Aromatic and saturated hydrocarbon compounds increased at the expense of asphaltenes and resins.

  • n-Decane hydro-conversion over bi- and tri-metallic Al-HMS catalyst in a mini-reactor

    2018, Chinese Journal of Chemical Engineering

    Bi-metallic (Pt–Sn and Sn–Ni) and tri-metallic (Pt–Sn–Ni) catalysts, supported on Al-containing hexagonal mesoporous silica (Al-HMS) (Si/Al = 20) materials, were synthesized. N2 adsorption–desorption, X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) test, and temperature programed desorption (NH3-TPD) were used to characterize physicochemical characteristics and textural properties of the Al-HMS catalysts. Catalytic performances on hydro-cracking of n-decane at different reaction conditions were studied in a micro-reactor. Comparison between Pt–Sn, Sn–Ni and Pt–Sn–Ni catalyst under different hydro-cracking conditions was made. The experimental results indicate that the proper balance between the acid and metal functions is the key in synthesizing a catalyst with a better performance in hydro-cracking. Tri-metallic catalyst exhibits the best catalytic performance in n-decane hydro-cracking than two bi-metallic catalysts.

  • Incorporation of Ni and Mo on delaminated clay by auto-combustion and impregnation for obtaining decane hydroconversion catalysts

    2017, Catalysis Today

    Citation Excerpt :

    As noted above, the autocombustion method increases the redox properties of the catalyst and probably maintains the appropriate distance between the metallic and acidic sites so as to accomplish the balance required in the bifunctional reaction. It is important to emphasize that the excellent yield of cracking products of Mo15Ni5-EQ suggests a good performance for hydrocracking of heavy molecules [38,39]. Supported catalysts of Ni–Mo were synthesized on mineral clay delaminated (Bentonite).

    A series of solids was synthesized by the incorporation of Ni and Mo active species on delaminated clay with novel acidic properties, by impregnation and auto-combustion. The resulting solids were characterized by X-ray fluorescence, X-ray diffraction (XRD), textural analysis by N2 physisorption, temperature-programmed reduction (TPR), transmission electron microscopy (TEM) and, in situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS). Decane hydroconversion was used to evaluate the potential of the materials as hydrocracking catalysts for vacuum bottoms. XRD was used to confirm modification of the material and good active phase dispersion. TPR revealed an important decrease in the reduction temperatures with auto-combustion method. Low-temperature reducible species play a key role in the catalytic performance for decane hydroconversion.

  • Multiscale Aspects in Hydrocracking: From Reaction Mechanism Over Catalysts to Kinetics and Industrial Application

    2016, Advances in Catalysis

    Citation Excerpt :

    The nitrogenous poisons generally comprise five- and six-membered heteroatom rings and anilines, of which the latter two have the most strongly inhibiting character. Pyridine and aniline are therefore often used as model components to investigate the corresponding poisoning effect (7,76,398). Galperin (234) on the other hand used tributylamine which rapidly decomposed to ammonia that covers the acid sites of the catalyst.

    Hydroisomerization and hydrocracking are widely recognized as versatile reactions. They allow not only converting feeds of various origin and quality into high-value blendstocks but also identifying the opportunities brought about by different solid acids with characteristic framework structures to tailor activity and product selectivity. The bifunctional reaction mechanism is an essential feature in this respect. It comprises acid-catalyzed rearrangement and cracking in addition to metal-catalyzed (de)hydrogenation and is effective at relatively mild operating conditions. Innumerable combinations of metal and acid functions, ranging, respectively, from sulfided transition metals to noble ones and from crystalline, microporous to wider pore, amorphous materials are available for ensuring the required catalytic performance to convert the feed into the desired product slate. An adequate understanding of the detailed reaction mechanism represents a crucial element in this endeavor. Over the years an interesting evolution from simple, lumped model toward advanced ones accounting for all potentially occurring elementary steps could be discerned. Hydrocracking has been among the first reactions involved in hydrocarbon fuel production and regained popularity in the last years because of the processing of ever more heavy crudes. Its horizon, however, extends beyond the fossil era with applications in bio-fuel production and plastic waste valorization. It ensures a bright future for a historical and reliable conversion process.

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