H.R. Reinhoudt

Catalysts for second-stage deep hydrodesulfurisation of gas oils

Abstract The interest for new deep hydrodesulfurisation (HDS) processes is expected to rise since more stringent legislation for the maximum sulfur concentration in automotive diesel fuel has been proposed. A realistic option is the application of a separate deep HDS reactor following the existing HDS process in which alternative catalysts may be applied. It was shown that ASA-supported Pt and PtPd catalysts are very active in model feed deep HDS reactions. Moreover, in the deep HDS of a pre-hydrotreated straight-run gas oil (P-SRGO) under relevant industrial conditions, PtPd/ASA showed a very promising performance. The applicability of ASA-supported noble metal catalysts in practice will be largely determined by the H 2 S concentration in the second-stage reactor and the price of noble metals. In addition, NiW/γ-Al 2 O 3 is also considered to be a promising catalyst for second-stage deep HDS. From the differences in the relative performance between model and real feed experiments, it is found that the suitability of a catalyst for deep HDS of gas oils cannot be evaluated by single-component model studies alone. The H 2 S concentration and the presence of other competing reactants largely determine the outcome of model experiments and should therefore be chosen carefully.

On the difference between gas- and liquid-phase hydrotreating test reactions

Abstract In industrial practice, hydrotreating of oil fractions is carried out in either a gas-phase process or a trickle flow process. We previously noticed that a remarkable difference exists between the relative activity of mixed sulfide catalysts in gas-phase and liquid-phase hydrodesulfurization (HDS) reactions. In the literature, however, no satisfying explanation with respect to the possible fundamental differences between these reactions can be found. In this paper, we report an elaborate investigation on the effect of reaction conditions, type of reactant and type of the catalyst on the occurrence of differences between the relative activity, i.e. ranking, of mixed sulfide catalysts in gas- and liquid-phase reactions. Striking differences were observed between the ranking of nitrilo-triacetic acid (NTA) and conventionally prepared NiMo catalysts in thiophene gas-phase HDS and liquid-phase dibenzothiophene (DBT) HDS. Importantly, these differences did not depend on the nature of the reacting sulfur-containing compound. This allows the generalisation that NTA-based Ni(Mo) catalysts are relatively more active in gas-phase HDS reactions, whereas conventionally prepared NiMo catalysts are relatively more active in liquid-phase HDS reactions. An analogous behaviour was observed for low- and high-temperature sulfided NiW/γ-Al 2 O 3 catalysts, of which the latter is much more active in gas-phase HDS reactions and the former is more active in liquid-phase HDS reactions. It is concluded that this so-called ‘gas–liquid-phase controversy’ is a generic phenomenon in hydrotreating reactions over metal sulfide catalysts. It was verified that mass transfer limitations do not play a role in this matter. The active sites of stacked slabs of the type II catalysts are more affected than those of type I catalysts, in which the active phase is in a more close interaction with the support. It is proposed that the phenomenon is related to a non-selective competitive adsorption of the a-polar solvent molecules on sites protruding from the catalyst surface. Apparently, the proximity of the ionic surface of the alumina support hinders the adsorption of the a-polar hydrocarbon molecules on the non-stacked systems, whereas the sulfur- and nitrogen-containing molecules are not so much affected in their adsorption behaviour on these active sites.

Testing and characterisation of Pt/ASA for deep HDS reactions

Abstract In the search for active catalysts for the conversion of refractory sulfur compounds in diesel fuel, the activity, the role of the support, and the nature of the active sites on Pt/ASA catalysts in deep HDS reactions were studied and compared to Pt/γ-Al 2 O 3 and Pt/XVUSY (stabilised Y zeolite). Pt/ASA appears to be much more active than Pt/γ-Al 2 O 3 but is initially less active than Pt/XVUSY. The latter however, showed strong deactivation after short reaction times. It is concluded that an appropriate tuning of the support acidity is crucial for this reaction. In contrast to the activity, the H 2 S sensitivity of the tested Pt based catalysts is hardly influenced by acidity of the support. Adsorption of H 2 S on these sulfur vacancies leads to strong competitive adsorption with the reacting sulfur compound. It is proposed that the stabilisation of small platinum clusters in the presence of H 2 S is an important effect of acidic supports. In addition, the strength and nature of the acidic sites on the support may affect the Pt–S bond strength of the active sites on small platinum particles. It is concluded that no sulfur-free platinum metal sites are present under the applied reaction conditions. It is therefore proposed that the conversion of 4-E,6-MDBT over Pt/ASA proceeds over sulfur vacancies on small platinum particles. The creation of sulfur vacancies on these small platinum particles may be related to their electron-deficient character on acidic supports.

Application of ASA supported noble metal catalysts in the deep hydrodesulphurisation of diesel fuel

Abstract The potential of Amorphous Silica Alumina (ASA) supported Pt and Pd catalysts for deep hydrodesulphurisation (HDS) of diesel fuels was investigated. It appeared that the ASA supported catalysts exhibit an excellent activity for the conversion of 4-Ethyl, 6-Methyl Dibenzothiophene (4-E, 6-M DBT) under model conditions as compared to conventional HDS catalysts and γ-Al 2 O 3 supported noble metal catalysts. Pt/ASA was also tested under practical conditions using a diesel fuel feed. ThePt/ASA catalyst showed a comparable activity to the NiW/γ-Al 2 O 3 catalyst which was higher than that of the conventional CoMo/γ-Al 2 O 3 catalyst. The main difference of the catalyst was the better hydroconversion of the 4,6 di-alkylated DBTs. The better performance of Pt/ASA in the testing under model conditions as compared to the diesel fuel HDS can be attributed to poisoning of part of the active phase by basic nitrogen compounds like quinoline. It is concluded that ASA supported noble metal catalysts have a promising potentialfor deep HDS processing.

The sulfidation mechanism of NiW/γ-Al2O3 as a function of the calcination temperature studied with 57Fe-MAS and temperature programmed sulfidation

Abstract Currently the production of low sulfur diesel fuel ( 57 Fe-MAS shows that re-dispersion of Ni-sulfide species occurs after sulfidation at 573 K for a dried catalyst to form the so-called NiWS phase. The NiWS phase is also formed on calcined catalysts but only at sulfidation temperatures of 673 K. It is demonstrated that the formation of WS2 slabs is a prerequisite for the re-dispersion of Ni-sulfide species.

Testing and characterisation of Pt/ASA and PtPd/ASA for deep HDS reactions

Abstract The development of a dedicated, second stage deep HDS reactor to meet the low sulfur levels in diesel fuel is a challenging option, since it allows to process the feed under low H 2 S partial pressures. Because ASA supported noble metal catalysts have a high activity in the HDS of 4,6 di-alkylated DBTs in real gas oil, they constitute a new option. The nature of the active sites on ASA supported noble metal catalysts has been studied by electron microscopy and infrared spectrometry on adsorbed CO (FTIR(CO)). It is concluded that the support acidity plays a role in the genesis of active sites for HDS. Alloying Pt and Pd on ASA results in very active catalysts for conversion of 4-E,6-MDBT and DBT. Pt/ASA has a high selectivity for hydrogenolysis whereas PtPd/ASA shows a high selectivity for hydrogenation reactions. FTIR(CO) and the catalytic activity in DBT HDS indicate the presence of two different sites on PtPd/ASA: HDS sites, similar to those present in Pt/ASA and hydrogenation sites. The former consist of electron deficient Pt which facilitates the formation of sulfur vacancies, whereas the latter are are sulfur free, metal-like sites. The strong competitive adsorption between H 2 S, 4-E,6-MDBT and DBT suggests that the sulfur containing compounds are all adsorbed through the sulfur atom.

The design of base metal catalysts for hydrotreating reactions : temperature programmed sulphidation of NiW/Al2O3 catalysts and their activity in the hydrodesulfphurisation of thiophene and dibenzothiophene

NiW based hydrotreating catalysts have a good performance in hydrodesulphurisation reactions. It appeared that the their performance in the hydrodesulphurisation of dibenzothiophene and thiophene strongly depends on their sulphiding degree, which can be controlled by both the calcining and sulphiding temperature. By combining temperature programmed sulphiding with quasi in-situ XPS and activity measurements, it was concluded that the active phase for the HDS of DBT consist of either micro-crystalline sulphided Ni on a WO3 substrate, or of Ni2+ dissolved in the in NiWO4. In contrast, the active phase for the HDS of thiophene seems to consist of Ni-promoted WS2 structures.

A versatile infrared cell for in situ catalyst pretreatment and measurements at temperatures between 120 and 773 K: Application to alumina supported NiO and WO3

Abstract A new infrared (IR) transmission cell has been developed which allows in situ catalyst pretreatment under well-controlled conditions, followed by the characterization with IR spectroscopy on adsorbed CO at 120 K. The temperature of the sample can be controlled accurately from 120 up to 773 K while the sample is present in the IR beam. In an integrated reactor, the sample can be heated from 300 up to 1273 K under reactive conditions, for instance in H 2 S. To ease the comparison of different samples pretreated under identical conditions, for instance for background corrections, the sample holder may contain two samples. The transmission cell is high vacuum compatible and has been built in a transportable frame allowing a flexible use of the IR spectrometer. The performance of the cell is demonstrated for the characterization of an oxidic Ni/Al 2 O 3 sample and a reduced WO 3 /Al 2 O 3 sample.

Design of a model activity test for second stage deep HDS catalysts

Abstract The availability of a fast model feed test for the prediction of catalyst performance in a real feed would be helpful for the development of new catalysts for the deep HDS of diesel fuel. In this work, the relation between the model feed composition and its predictive value for the trend in activity of a CoMo/, NiMo/, and NiW/γ-Al 2 O 3 and PtPd/ASA catalyst for the deep HDS of a pre-hydrotreated gas oil was studied. It appeared that the choice of the sulfur compounds, the H 2 S concentration and the presence of nitrogen containing compounds strongly affects the catalyst activity ranking. It was found that the ranking and relative activity of the four catalysts for deep HDS of a pre-hydrotreated gas oil could be well predicted by applying a model feed containing 4-ethyl, 6-methyl dibenzothiophene, DMDS and carbazole.