Elsevier

Catalysis Today

Volume 213, 15 September 2013, Pages 198-205
Catalysis Today

Unsupported NiMoAl hydrotreating catalysts prepared from NiAl-terephthalate hydrotalcites exchanged with heptamolybdate

https://doi.org/10.1016/j.cattod.2013.02.029Get rights and content

Highlights

  • NiMoAl LDHs were prepared from NiAl-terephthalate LDHs by exchange with Mo7O246−.

  • Mixed oxides with up to 122 m2 g−1 BET area were obtained from NiMoAl hydrotalcites.

  • NiMoAl catalysts obtained from LDHs are selective for hydrogenation route in HDS.

Abstract

Unsupported NiMoAl hydrotreating catalysts were prepared starting from NiAl-terephthalate layered double hydroxides (LDHs) with x values (Al/(Al + Ni) ratios) in the 0.3–0.8 range by ion exchange with ammonium heptamolybdate, followed by calcination at 723 K. Mixed oxides containing ca. 33–43 wt% molybdenum and Ni/Mo atomic ratios in the 0.5–1.4 range were obtained. There was loss in the long-range ordering of the LDHs in the c direction during the ion-exchange, but the local structure of the brucite layers was maintained. The calcined mixed-oxides had surface areas ranging from 26 to 122 m2 g−1. The catalysts were sulphided in situ and subsequently tested in the simultaneous dibenzothiophene (DBT) HDS and tetralin (THN) hydrogenation reactions in a high pressure batch reactor at 613 K and 70 bar. The catalysts had higher specific activity for both the HDS and the HDA reactions and much higher selectivity for the ring hydrogenation (HYD) route in the HDS of DBT than a conventional NiMo/Al2O3 catalyst. Their activities in both reactions were similar to those of an Al-free unsupported NiMo catalyst, but their preference for the HYD route was higher than that of the latter.

Introduction

There is nowadays a global and growing concern about environmental protection. In terms of energy for transportation, this is reflected increasingly stringent environmental in terms of sulphur content, giving rise to the need of producing the so-called ultra-low sulphur diesel (ULSD), containing less than about 10 ppm sulphur [1]. In several countries, cetane number and aromatic hydrocarbons content are also specified [2].

In petroleum refineries, the main process used for improving the quality of diesel with respect to these properties is hydrotreating (HDT). In this process, unwanted components present in petroleum fractions are removed by reaction with hydrogen at high pressures [3]. For many decades, alumina-supported mixed sulphides comprised of a group 6 element (Mo and/or W) together with an element of groups 9 (Co) and/or 10 (Ni) of the periodic table, have been used in HDT.

The principal organosulphur components in diesel belong to the family of condensed-ring aromatic compounds, such as dibenzothiophenes (DBTs). However it has been observed that the most refractory compounds towards HDS are condensed-ring organosulphur compounds with alkyl groups in the vicinity of the sulphur atom, such as in 4,6-dialkyldibenzothiophenes [3]. For the HDS of these compounds the so-called hydrogenation (HYD) route is required [4], where the previous hydrogenation of one of the condensed aromatic rings has to occur before the sulphur atom is removed. With the conventional alumina-supported catalysts, severe temperature and pressure operating conditions are necessary for this route to be significant, impairing process economics.

Recently, unsupported mixed sulphides of the same family as the supported ones have come into commercial use [5]. They are reported to have higher selectivity for the HYD route in the HDS of DBTs and therefore higher activity for ULSD production than the alumina-supported catalysts. Furthermore, due to their stronger hydrogenating function, they are also reported to be useful for hydrodenitrogenation (HDN) and hydrodearomatisation (HDA), which leads to cetane improvement under appropriate operating conditions [5]. This opens promising lines of research for new formulations and preparation methods of this type of catalysts and also for understanding the reasons for their improved hydrogenation performance.

The present work is focused on the use of layered double hydroxides (LDHs), also known as hydrotalcite-type materials, as precursors for unsupported NiMo mixed-sulphide catalysts. The structure of LDHs is based on the stacking of brucite-like (Mg(OH)2) layers with an excess positive charge that is compensated by interlayer anions [6]. Their general formula is [(M2+)1−x(M3+)x(OH)2]x+(An)x/n·mH2O, where M2+is a divalent cation (Ni, Mg, Cu, Zn, etc.), M3+ is a trivalent cation (Al, Cr, Fe, V, etc.) and An a charge compensation anion (commonly NO3, Cl, SO42−, CO32−). This subject has been reviewed several times [6], [7], [8].

LDHs are widely used as catalyst and support precursors due in part to the possibility of preparing homogeneous mixed oxides from them [6], [9], [10]. Furthermore, different interlayer metal oxy-anions, such as vanadates [7], [11], molybdates [12], [13] and tungstates [14], [15], may be used to obtain useful catalysts for many applications, such as hydrotreating of oil fractions [15], [16] or selective oxidations [17]. Recently Wang et al. have reported on the activity for 4,6-dimethyldibenzothiophene HDS of NiMoWAl catalysts obtained by direct sulphidation of LDH-containing materials obtained by ion exchange of a NiAl-nitrate LDH with isopolymetallate salts of Mo and W, in the presence of a surfactant and ethylene glycol [16].

In the present work, NiMoAl catalysts were prepared by sulphidation of mixed-oxides obtained by calcination of NiAl-heptamolybdate HDLs. The latter were, in their turn, obtained by ion-exchange with ammonium heptamolybdate (AHM) of NiAl-terephthalate HDLs. Terephthalate was chosen as the initial compensation anion partly because, due it is length, it facilitates the diffusion of bulky anions such as heptamolybdate into the interlayer gallery and is easily replaced by this anion [13], [18], [19], but an even more important factor is that it allows the obtention of LDH phases with high aluminium content. It is frequently said in the literature that the maximum possible x value in the formula of LDHs is 0.33 due to the so-called cation avoidance rule, which states that the second coordination sphere of a 3+ cation cannot contain another 3+ cation, due to electrostatic repulsion [6], [8]. We have recently found, however, that it is possible to prepare LDHs with x values around 0.5 with terephthalate as the compensation anion [20].

The sulphided catalysts were tested in the simultaneous HDS of dibenzothiophene and hydrogenation of tetrahydronaphthalene (THN) and their behaviour in these reactions was compared to those of a conventional, alumina-supported NiMo catalyst and an aluminium-free NiMo unsupported catalyst.

Section snippets

Preparation of the materials

The preparation of the NiAl-terephthalate LDH starting materials was described in detail elsewhere [20]. Briefly, they were prepared by simultaneous dropwise addition, to a water-containing beaker, of an aqueous solution of the metal nitrates and another containing terephthalic acid dissolved in sodium hydroxide, so as to maintain the pH at 6.5 ± 0.3. The precipitation was carried-out at 323 K and the precipitate obtained was aged in the mother-solution for 16 h at room-temperature. The resulting

Elemental analysis

Table 1 shows the elemental analysis results for the molybdenum exchanged materials after calcination at 773 K, compared to the values obtained with the terephthalate LDHs calcined at the same temperature (except for the carbon analysis performed with the uncalcined materials). It is first clear that large amounts of molybdenum were incorporated into the materials by ion-exchange.

Carbon analysis shows a large decrease in the content of this element upon exposure of the terephthalate LDHs to AHM

Conclusions

NiMoAl LDHs were prepared by ion-exchange of NiAl-terephtalate LDHs with heptamolybdate anion. The original terephthalate LDHs had Al/(Ni + Al) proportions in the brucite layers considerably above the commonly accepted 0.33 limit. Exchange with heptamolybdate led to the incorporation of large amounts of molybdenum and it was possible, by controlling the amount of aluminium in the LDHs, to control the Ni/Mo ratio in the molybdenum-exchanged materials. Molybdenum incorporation caused a pronounced

Acknowledgements

The authors wish to thank PETROBRAS for financial support to this project, the Laboratório Nacional de Luz Síncrotron (Campinas, Brazil) for approval of research proposals D04B-XAFS1 10916 and SXS-12702 and financial support during XAS measurements, to NUCAT/COPPE/UFRJ for XRF analyses, Grupo Catalizadores y Adsorbentes from Instituto de Química/Universidad de Antioquia for the AAS analyses, the Laboratório Multi-Usuário de Raios X, Inorganic Chemistry Department and NMR laboratory of the

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