Comprehensive investigation of the effect of adding phosphorus and/or boron to NiMo/γ-Al₂O₃ catalyst in diesel fuel hydrotreating

Highlights

• Phosphorus and boron have positive effects on the HDS and HDN catalyst preparation.

• The method of P and/or B addition affects catalytic performance in diesel HDS/HDN.

• Addition of P/B to catalyst supports enables higher conversion of diesel HDS/HDN.

Abstract

In order to comprehensively investigate the effect of boron (B) and phosphorous (P) promoters on the catalytic performance of a hydrodesulfurization catalyst (NiMo/γ-Al₂O₃, separate or simultaneous addition of the two promoters and the method of their addition to the catalyst solution or support have been studied by physical, chemical and reactor evaluation of nine synthetic catalysts using a diesel fuel feed containing 11,500 ppm of sulfur and 250 ppm of nitrogen. For better evaluation of the synthetic catalysts, their surface area, morphology, crystalline structure, regeneration temperature and acidity have been studied by ICP, XRD, Raman, FTIR, TPR, BET, FESEM and TPD analyses. Reactor evaluations have been performed at a temperature of 340 °C, pressure of 65 bar, LHSV of 0.8 h⁻¹ and hydrogen to hydrocarbon ratio of 250 and the amounts of sulfur and nitrogen resulting from each catalyst have been determined. The catalysts showed very different performances in the absence or presence of P and B promoters in the desulfurization and hydrodenitrogenation processes. The presence of P and B promoters seems to be necessary in the hydrodesulfurization process. However, the presence of P and B promoters in the HDN process depends on the type and method of the promoter addition.

Introduction

Today, most of the major investments in the oil industry are extensively accomplished by widespread and deep research to produce more and cleaner distillation products in accordance with environmental standards and regulations (Liu et al., 2005; de Mello et al., 2018). One of the most significant problems in refining and petrochemical processes is the environmental and process aspects related to presence of harmful substances. The most important troubles that should be overcome is undesirable sulfur compounds including thiols, sulfides, thiophenes, alkyl thiophenes, tetrahydrothiophene, thiophenols, benzothiophenes, dibenzothiophenes, alkyl benzothiophenes, and alkyl dibenzothiophenes (Saleh et al., 2019).

Using catalyst in a chemical process called hydrodesulfurization (HDS), which is used as a process for desulfurization and hydrogenation is the conventional method for eliminating organic sulfur compounds of petroleum products (Javadli and Klerk, 2012; Argyle and Bartholomew, 2015). HDS catalysts in the petroleum industry function as catalytic converters by postponing deactivation of expensive catalysts such as reforming and isomerization catalysts. This operation is executed by eliminating sulfur and nitrogen containing poisons and heavy metals such as lead, arsenic, vanadium and nickel (Hajjar et al., 2018).

The catalysts used in these processes are catalysts not poisoned by nitrogen and sulfur. The saturation and hydrocarbon cracking properties of these catalysts are minimized and thus their hydrogen consumption is low (Hajjar et al., 2018; Hajjar et al., 2017). Generally, HDS process catalysts consist of a single layer coating and a homogeneous distribution of a group VIB metal oxide such as molybdenum oxide and a group VIII metal oxide such as cobalt or nickel oxide on a gamma alumina support (Shafi and Hutchings, 2000; Ma et al., 1994; Vasudevan and Fierro, 1996). CoMo and NiMo catalysts are among the most active and common catalysts in the process. Ordinary desulfurization catalysts are not suitable for the hardly removal of thiophene compounds from fuels and it is necessary to increase their activity to four or five times in order to reduce sulfur content to global standards (Babich and Moulijn, 2003; Furimsky, 2017). The complete desulfurization of diesel fuels is very hard to achieve using the current industrial catalysts due to the presence of such resistant molecules as 4,6-dimethyldibenzothiophene (4,6-DMDBT). Dibenzothiophene (DBT) and 4,6- DMDBT are generally believed to convert by two parallel pathways: direct desulfurization pathway (DDS) via CS bond cleavage, which forms biphenyls and hydrogenation pathway (HYD) to yield cyclohexylbenzenes. The comparison of the reactivity of the commercial catalysts of DBT and 4,6-DMDBT shows that the alkyl groups in positions 4 and 6 hardly affect the reactivity in the HYD pathway. However, these groups considerably influence the reactivity in the DDS pathway (Richard et al., 2007). Therefore, different strategies have been studied for improving the activity and selectivity of these catalysts. For example, the application of new catalyst supports, active phases, noble metals, intermediate metal phosphates and secondary reinforces and changes in the synthesis method have been extensively investigated (Poulet et al., 1993; Muralidhar et al., 1984; Leonova et al., 2014; Rana et al., 2008). One of the methods to improve the activity of a catalyst is to create more active sites. By adding different elements to the support or active phase, more active sites can be formed. In addition, the application of chelating agents, phosphorus and boron affects the performance of the catalyst by improving the catalyst acidity, reducing the interaction of active metals and support and producing highly active structures of type II, CoMoS and NiMoS layers. A lot of research has been carried out on the effect of phosphorus and boron on the activity of desulfurization catalysts. Based on numerous publications and patents, phosphorous is categorized as second promoter in molybdenum containing catalysts used in hydrotreating for production of clean fuels.

The addition of boron increases the catalyst activity in the following ways: (1)Improvement of alumina acidity, which results in increased cracking reactions, isomerization and hydrodesulfurization, by overcoming the severe interruption of the binding of alkyl dibenzothiophenes of heavy oils; (2)Increasing the dispersion of the active metal phase on alumina by reducing the interaction of the support with active metal phase, which facilitates the formation of metallic sulfide blade (edge) structures; (3)Preventing the blocking of methyl groups for the adsorption of alkyl dibenzothiophenes (Li et al., 1998; Saih and Segawa, 2009; Vatutina Yu et al., 2016; Maity et al., 2011; Rashidi et al., 2013).

Phosphorus is considered the second promoter of hydrotreating catalysts. The effects of phosphorus on increasing the catalytic activity and reduction of deactivation are as follows: (1) Improvement of the distribution of catalytic sites for the formation of metal sulfide particles; (2) Changes in surface hydroxyl groups; (3) Increasing the formation of the octahedral structure of nickel and cobalt ions, which increases the active phases; (4) Reducing polymerization of Mo-S bonds; (5) Increasing the degree of sulfidation of Co or Ni particles by reducing the interaction with the support; (6) Improvement of the power and distribution of Bronsted acid sites of the support; (7) Prevention of the blocking of methyl groups for the adsorption of alkyl dibenzothiophenes; (8) Improvement of the hydrogenation of aromatic rings; (9) Improvement of HDN reaction when hydrogenation of aromatic rings determines the reaction rate; (10)Improvement of the stability of the impregnation solution, which results in better metal distribution on the support; (11) Prevention of the formation of inactive NiAl₂O₄ particles; (12) Improvement of the thermal stability of alumina; (13) Prevention of coke formation during hydrotreating reaction; (14) Increasing the activity of hydrocracking (Poulet et al., 1993; Rashidi et al., 2013; Morales and Ramirez de Agudelo, 1986; Jian and Prins, 1996; Kushiyama et al., 1990; Lewis et al., 1992; Maity et al., 2008; Maity et al., 2005; Kwak et al., 1999; Cruz et al., 2002; Rayo et al., 2012).

Catalytic hydrotreating is one of the most effective methods for preparation of clean fuels. Molybdenum and/or tungsten based catalysts promoted by nickel have been widely used in commercial hydrotreating units. Much attention has been paid to the understanding of the structure of catalytic active sites, reaction mechanism and the effect of promoters on these catalysts. The role of nickel compounds has been seriously studied and the effect of nickel and cobalt has been studied in more details. In addition, the effects of supports such as γ-Al₂O₃, SiO₂, carbon, TiO₂, ZrO₂, Al₂O₃-SO₂, and Al₂O₃-TiO₂ have been investigated (Prins and de Beer VHJ, 1989; Saleh, 2017; Spojakina et al., 1989).

Phosphorous positively affects isomerization and hydrogenation reactions, which require acidic character. In addition, phosphorous improves the activity of hydrogenation of aromatic rings; especially in NiMoP/Al catalyst. Phosphorous is less effective on hydrodesulfurization of thiophene and may be effective on the HDS acidity of catalysts for heavier sulfur containing molecules, such as the ones in VGO, in which the hydrogenation of aromatic ring may occur prior to the breaking of CS bond. In addition, phosphorous improves HDN reactions when the hydrogenation of aromatic rings is not rate determining. Regardless of the type of hydrotreating reaction, too much phosphorous is always harmful (Iwamoto and Grimblot, 1999). Spojakina et al. have proposed that the presence of phosphorous increases the distribution of molybdenum as well as preventing penetration of nickel into the alumina support. Therefore, phosphorous can contribute to the optimization of Ni/Mo ratio in the active phase and increasing the available nickel in the Ni-Mo-S active phase effective for hydrogenation (Iwamoto and Grimblot, 1999).

Iwamoto and Grimblot have shown that phosphorous addition increases the thiophene HDS activity in sol-gel NiP/Al catalyst due to the increased sulfidation capability of nickel particles (Andreev et al., 1994). Andreev et al. have reported high thiophene HDS activity for the catalyst prepared by mixing NiPS₃ and Al₂O₃ while being less selective for hydrogenation compared with Ni/Mo and CoMo/Al catalysts (Kim and Woo, 1992).

The effects of phosphorous have also been proposed from different perspectives. Firstly, phosphorous may cause decreased polarization of Mo-S bond and thus increase its covalent characteristics. Since molybdenum based catalysts with numerous Mo-S binds have high HDS activity. Phosphorous can improve HDS activity (DeCanio et al., 1991). Secondly, the presence of phosphorous increases the formation of octagonal molybdenum, cobalt and nickel, which can be the active precursors of the catalytic active site (Morales et al., 1988; Gishti et al., 1984). Ultimately, phosphorous severely causes catalytic hydrogen in MoP catalysts (Vafaeian et al., 2013). This can be useful for all hydrotreating reactions.

In the present work, the method of boron and phosphorous addition to HDS catalyst will be investigated. The effect of these two promoters added simultaneously or separately, on the performance of HDS catalysts has been investigated. In addition, the effect of the addition of these promoters to the impregnation solution and/or prior to the impregnation of the active phase has also been studied. TPR, mapping, FESEM, BRT, ICP, XRD, FTIR, Raman, and TPD analyses have been used for the chemical and physical evaluation of the synthetic catalyst.