Elsevier

Catalysis Today

Volume 305, 1 May 2018, Pages 192-202
Catalysis Today

CoMoB/Al2O3 catalysts for hydrotreating of diesel fuel. The effect of the way of the boron addition to a support or an impregnating solution

https://doi.org/10.1016/j.cattod.2017.07.004Get rights and content

Highlights

  • For CoMo-B/Al2O3 catalyst, the increase of the B content results in:.

  • Increase of HDS and HDN activity of catalysts.

  • Increase of CoMoS phase contentand the sulfiding degree of active metals.

  • Increase of the concentration of visible sulfide particles in HRTEM images.

  • B in the solution has more pronounced effect on catalyst than B in the support.

Abstract

CoMoB/Al2O3 catalysts with up to 2 wt.% of B have been synthesized by two different methods an introduction of boron into γ-Al2O3 support during the kneading of the paste or by the impregnating of γ-Al2O3 extrudates by the solution of Mo and Co compounds, citric and boric acids. It is shown that the activity in HDS and HDN of diesel fuel increases with increasing boron content in catalysts. The growth of the activity is associated with the increase in CoMoS phase content, Co dispersity, an average slab length of sulfide active component particles and an average number of slabs per 1000 nm2. The positive effect of boron is much more pronounced for catalysts prepared by the introduction of boron to the solution, since boron is located preferentially on the surface of alumina particles.

Introduction

An addition of boron to hydroprocessing catalysts of different oil fractions is known for more than 60 years [1]. However, researches in this area have not stop until nowadays. The positive effect of boron on HDS activity of CoMo/Al2O3 catalysts is described in many papers [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], while an information on boron introduction into sulfide active component (CoMoS phase) is completely absent. The positive influence of boron preferentially consists in adjusting textural and acidic properties of a support— γ-Al2O3. These characteristics influence the composition, structure and morphology of CoMoS phase particles. A boron addition to CoMo/Al2O3 is noted to result in the following effects:

  • 1.

    The total support acidity increases due to the formation of different surface compounds, for example, mixed boron-alumina oxides [3], [9], [10], [11], [13], [14], [17], [19].

  • 2.

    Boron regulates Brønsted acidity of a support and influences electronic properties of sulfide sites [6], [11], [19].

  • 3.

    The speed of α-CoMoO4 formation decreases after calcination of a catalyst. It results in a reduction of the formation of low-active Co9S8[5].

  • 4.

    There is a decrease of chemical interaction between supported metals and alumina. This interaction forms low-active compounds in catalysts [2], [4], [8], [10], [12], [13], [18].

  • 5.

    An introduction of boron to catalysts accelerates isomerization, denitrogenation and hydrogenation reactions. These reactions remove sulfur from oil fractions and impede catalyst deactivation [9], [11], [15], [16], [19].

  • 6.

    Boron influences dispersion of Co and Mo compounds in supported catalysts [4], [7], [8], [10], [11], [17], [19].

  • 7.

    Boron enhances catalyst hydrogenation function [7].

  • 8.

    Boron results in better sulfidation of the supported metals [12], [17].

  • 9.

    Boron adding to a support influences textural properties of a support [13], [14], [16].

  • 10.

    An introduction of boron to a catalyst increases a visualization degree of the active component [19]. Moreover, boron can be used simultaneously with other additives, such as phosphorus [1], [4], [15] or fluorine [7], which intensifies a positive boron influence.

CoMo/Al2O3 catalysts can contain different amounts of boron. However, the highest positive effect on the activity in target hydrotreating reactions will occur, when boron content in a catalyst is in the range of 0,5 to 3,0 wt.%. Such catalysts are widely described in the literature [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [18], [19].

Different methods are used for introduction of boron to hydrotreating catalysts. However, the impregnation of alumina granules by solutions of boric acid or its ammonium salts followed by thermal treatment and supporting of Co and Mo compounds is widely described in the scientific literature [4], [5], [6], [9], [10], [12], [17], [18]. One of the preparation methods was described in [7]. It includes the impregnation of alumina (Sasol PURALOX TH100/150) by H3BO3 solution or borane isoproxyl followed by drying, mixing with compounds of active metals and a binding agent (partially acid-peptized alumina, SASOL, CAPAPAL B), extruding to form a cylindrical shape, drying at 110 °C overnight, and then calcining in the air at 500 °C. Another method is to use the sol gel method that includes synthesis of B–Al2O3 support followed by the supporting of the active metals [13]. Sometimes, H3BO3 solutions in methanol are used for support preparation via impregnating of γ-Al2O3 granules [8], [11] or boehmite powders followed by drying and calcination [16]. All above-mentioned preparation methods of CoMoB/Al2O3 catalysts can be used in scientific researchers. However, these methods are not applicable for industrial conditions due to a large number of technological steps or the use of organic solvents.

The methods described in [14] and [19] are more attractive for industry. The boehmite powder is mixed with a water solution of H3BO3 and a peptization agent (NH3 [14] or HNO3 [19]), then the kneading paste is extruded, dried and calcined. Prepared γ-Al2O3-B support is impregnated by compounds of active metals compound. However, it is shown in [14], [19] that the increase of a boron content in a support results in the increase of a surface area, while a pore volume decreases or remains unchanged. Therefore, there should be inevitable decrease of an average pore diameter. Accordingly, the increase in boron content in a catalyst should reduce the proportion of mesopores, which diameter is optimal for hydrotreating catalysts of diesel and middle distillate petroleum fractions. These are pores with an average diameter of about 10 nm, or more precisely 7–13 nm [20], [21], [22], [23].

Therefore, it seems rational to introduce boron into the composition of CoMoB/Al2O3 hydrotreating catalysts by impregnating a granular support based on γ-Al2O3 with an aqueous solution containing boron, cobalt and molybdenum compounds simultaneously. In a number of studies an undoubted positive mutual influence of boron and citric acid was noted. The positive effect can be achieved by impregnating of calcined CoMoB/Al2O3 catalyst with C6H8O7 aqueous solution [18] or by the use of citric acid as a complexing agent when impregnating γ-Al2O3-B support with solutions containing Co and Mo [4], [12], [19], [23]

We have previously shown in our papers that the addition of boric acid to the impregnating solution containing cobalt, molybdenum, citric and orthophosphoric acid provides high HDS and HDN activities of hydrotreating catalysts of diesel fuel [15]. It was observed in [4] that CoMoB/Al2O3 catalyst prepared by impregnation of Al2O3 with the solution of boron, citric acid, cobalt and molybdenum was more active in HDS than the catalyst prepared by impregnation of Al2O3-B with the solution of citric acid, cobalt and molybdenum compounds. However, only one catalyst, which was prepared by impregnation of Al2O3 with boric and citric acids, cobalt and molybdenum compounds, was studied. In present paper, we compare properties of catalysts differed in a boron content and a way of the boron addition. The first group of catalysts were prepared by supporting of citric acid, cobalt and molybdenum on alumina containing boron (CoMo/AlBx catalysts, hereinafter, x is a boron content in a catalyst, %). The second group of catalysts were prepared by supporting of boric and citric acids, cobalt and molybdenum compounds on alumina (CoMoBx/Al catalysts).

Section snippets

Preparation of supports

Two types of supports were prepared: 1. supports without boron, 2. supports with boron. The boehmite HQ102 B produced by China (292 m2/g; pore volume 1.02 cm3/g; pore diameter 140 Å) was used for a preparation of supports. Three supports were prepared: 1. the support without B; 2. two supports with different content of B.

To prepared the support without B (denoted as Al), an aqueous solution of ammonium was used for paste peptization in all cases. For that case, 80 ml of an aqueous solution of

Textural characteristics

Textural characteristics of supports and catalysts are given in Table 2. Pore size distributions and isotherms of nitrogen adsorption-desorption are shown in Fig. 1, Fig. 2. The comparison of the data for Al, AlB1.5 and AlB2 supports showed that the addition of boron to a kneading paste resulted in the certain increase of a specific surface area of a support proportionally to the reduction of an average pore diameter that agreed with [14], [19]. It is shown in [14] that boron shifts the

Conclusion

  • I

    CoMo/Al2O3 catalysts with 12,5 wt.% of Mo, 3,5% of Co and 0; 1,5 and 2,0 wt.% of B were studied. Boron was introduced to catalyst into the kneading paste of the support or into impregnation solution.

  • II

    Introduction of boron into support results in the increase of a specific surface area and significant decrease of an average pore diameter. Textural characteristics of boron containing CoMoB1.5/Al and CoMoB2/Al catalysts differ a little from the ones of boron-free CoMo/Al catalyst when addition of

Acknowledgement

This work was conducted within the framework of budget project No. 0303-2016-0010 for Boreskov Institute of Catalysis.

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