Elsevier

Applied Catalysis A: General

Volume 452, 15 February 2013, Pages 75-87
Applied Catalysis A: General

One-pot preparation and characterization of bifunctional Ni-containing ZSM-5 catalysts

https://doi.org/10.1016/j.apcata.2012.11.026Get rights and content

Abstract

A convenient method for the incorporation of nickel into an aluminosilicate MFI type zeolite, which avoids the impregnation of nickel, drying and further calcinations steps, to produce a bifunctional catalyst in a cost efficient way, is presented. The one step preparation involves the addition of the nickel source in the zeolite preparation step and anchoring it during the crystallization process. After calcination to remove the organic template, the composite zeolite is ion exchanged to produce the acid form, which already has the nickel incorporated and no further steps are required. The incorporation of high contents of nickel (up to 5 wt% nominal) induced important modifications in the produced materials. As the amount of nickel increases, the textural properties are modified and lower micropore area and volume and higher mesopore and total volumes are observed. The amount of acidic sites decreased and the sites seem to be of the Brønsted kind. The nickel seems to be well dispersed or anchored in the prepared material and then high temperatures are used to reduce the oxidation state of nickel in the material; however, the catalyst with 1 wt% nickel incorporated showed good physical–chemical properties and was as active as other catalysts for the hydroconversion of toluene. The tested catalyst had a good tolerance to sulfur, indicating that part of the nickel incorporated is hindered from sulfur poisoning but is active for the dissociation of hydrogen during hydrogenation in the presence of sulfur.

Highlights

► Convenient method of nickel incorporation to produce a bifunctional Ni-zeolite catalyst. ► Incorporation of nickel in the zeolite synthesis gel produces a well dispersed hydrogenating site. ► Prepared H-Ni-ZSM-5 is sulfur tolerant and active for hydrocracking of toluene.

Introduction

The preparation of nickel-containing catalysts with dispersed nickel or nickel oxide particles on high surface area supports, such as alumina, silica or zeolite-type materials, is well known in the literature. Such prepared catalysts are widely used for many catalytic reactions. For instance, Ni/ZSM-5 catalysts have been used for hydrogenation, isomerization, dehydrogenation, oxidation, aromatization, hydrogen production, carbon nanotube preparation, NOx reduction and hydrocracking reactions [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. The activity, selectivity and stability of these supported nickel catalysts strongly depend on the selected method of nickel incorporation on the surface of the carrier. The peculiarities of the preparation techniques employed result in different dispersion, distribution and states of the metal or oxide particles in these prepared nickel containing catalysts.

Several techniques can be used for the preparation of nickel-containing catalysts including precipitation, impregnation and ion exchange processes, chemical vapor deposition, mechanical mixing of nickel-containing compounds and the support, solid state reactions, introduction of nickel precursor during the synthesis of the support, and so on. In the particular case of zeolitic supports, the modification of the physical–chemical properties of the zeolite by the incorporation of nickel can be achieved most frequently through impregnation or ion exchange procedures. Both methods have their own peculiarities; however, the incorporation of the metal during the zeolite synthesis can produce the desired well-dispersed metal-catalyst in one step [12]. In this way, it is possible to eliminate impregnation or ionic exchange procedures, which will include drying and calcinations steps, to produce a more cost-efficient catalyst preparation method.

The ion exchange of nickel into MFI type materials has been carried out and it is known that the pH of the solution employed, among other parameters, can have an impact on the final material [3], [4], [9], [10], [13], [14], [15]. Several studies of the hydrolysis of nickel salts in water solutions indicate the formation of several polymeric species [16], [17], [18]. The nickel concentrations, as well as the pH of the nickel solutions, have an influence on the ratio of the monomeric to polymeric nickel species. Also, the lower the pH, the higher the amount of monomeric species and these are capable of entering the MFI structural pores without size limitations. On the other hand, the higher the pH, the more abundant the polymeric species that have larger sizes than the pore openings of the zeolite. As they cannot enter the zeolitic pores, this limits the amount of obtainable ion exchange.

Impregnation of nickel into the MFI type material has been carried out [1], [2], [6], [7], [8], [13], [14], [19] and the advantage of this method is the possibility of incorporation of larger amounts of nickel compared with the ion exchange technique method. The metal salt selected, as well as the drying and calcination processes employed, are known to affect the distribution of the nickel in the final catalyst.

The addition of nickel to the gel during the synthesis of the zeolite is another clever method of incorporation. This method is based on the in situ formation of bonds with the other reactants to form part of the zeolite framework, or on the formation of insoluble, tiny nano-particles of salts or hydroxides that are trapped in the crystals of the zeolite formed. Using this method, it has been possible to prepare Ni-containing-zeolitic materials with the MFI structure [12], [20], [21], [22]. The incorporation of nickel can be achieved in the presence of structure-directing organic agents (SDOA) [20], [21], [22] to form nickel-silicates, or without SDOA, to form nickel-aluminosilicates [12].

Preparation of Ni-aluminosilicates with the MFI structure seems to be a suitable approach to produce a Ni-H-ZSM-5 bifunctional hydrocracking catalyst. By standard removal of the SDOA from the Ni-containing zeolite pores by calcination, the nickel is already present in the material and only the normal ion exchange and calcination procedures to obtain the acid form of the material are needed. No secondary steps are required and then the final material can work as a bifunctional catalyst after the reduction step.

In the present work, we aimed at the production of a nickel-containing MFI type bifunctional catalyst which was more cost-efficient to produce than the conventional ones prepared by impregnation. In our previous work [11], we demonstrated that the preparation of a Ni/ZSM-5 catalyst through wet impregnation produced bifunctional catalysts with good hydrogenating and hydrocracking properties. In the method described in Ref. [11], the zeolite ZSM-5 was calcined to remove the organic template, then it was ion exchanged to remove the sodium ions and replace them with ammonium ions, dried and calcined again to obtain the protonic form, and then, the zeolite was impregnated with a solution of the nickel salt, dried and calcined once more. The incorporation of nickel under carefully chosen conditions into the synthesis gel before the crystallization process will eliminate the impregnation, drying and last calcination steps. This will result in a more cost-efficient preparation method and, most importantly will produce a novel Ni-ZSM-5 bifunctional catalyst with the same activity and similar or better selectivity to propane and ethane than our previous catalysts. This work presents the method of preparation, the characterization of these materials, and their activity for the hydrocracking of toluene.

Section snippets

Preparation of nickel-incorporated ZSM-5 catalysts

Sodium silicate, sulfuric acid, sodium hydroxide, aluminum chloride hexahydrate, tetra-propyl ammonium bromide (TPRBr), all from Aldrich, and nickel chloride hexahydrate, from Alfa Aesar, were all used as received without further purification. In a typical preparation, a diluted sulfuric acid solution was prepared with distilled water. The required amount of nickel and aluminum chloride salts was dissolved in the acid solution and, after complete dissolution of the metal salts, the TPABr salt

X-ray diffraction and chemical analysis

Fig. 1 shows the X-ray diffraction patterns of the calcined and ion exchanged Ni-incorporated samples. These X-ray diffraction patterns correspond to a well-crystallized MFI structure and no other peaks corresponding to other phases were observed. This indicates that the nickel in these materials is well dispersed on the support and that the possible NiO or/and nickel-silicate particles are very small and not detectable, even for the sample with the highest nickel content (Ni(5)-INC, Table 1).

Summary

A convenient method for the incorporation of nickel into an aluminosilicate MFI-type zeolite, which avoids the impregnation of nickel, drying and further calcinations steps to produce a bifunctional catalysts in a cost-efficient way, is presented. The one step preparation involves the addition of the nickel source in the zeolite preparation step and anchoring it during the crystallization process. After calcination to remove the organic template, the composite zeolite is ion exchanged to

Acknowledgments

We thank the NSERC Strategic Research Projects program and Nova Chemicals for the support of this work.

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