Elsevier

Applied Catalysis B: Environmental

Volume 210, 5 August 2017, Pages 432-443
Applied Catalysis B: Environmental

Steam catalytic cracking of heavy naphtha (C12) to high octane naphtha over B-MFI zeolite

https://doi.org/10.1016/j.apcatb.2017.04.001Get rights and content

Highlights

  • A controllable Brønsted acid sites borosilicate-1 zeolite was synthesized under microwave irradiation with different silicon to boron (Si/B) ratios.

  • The nature of mineralizer agents has influence on structural coordination system.

  • External surface area, acidity and silanol have a crucial role in steam catalytic cracking of dodecane.

  • Two reaction stages over micro borosilicalite-1 zeolite was observed; primary cracking and secondary reaction.

Abstract

Continues consuming of fuel from fossil oil reservoirs due to the increase in energy demands encouraged scientists to use the pyrolysis of biomass in the production of clean energy. One of the most important product from the pyrolysis of biomass is dodecane. However, dodecane requires a further transformation in order to produce lighter hydrocarbons. Borosilicalite-1 (B-MFI) was synthesized with different mineralizer agents and utilized for steam catalytic cracking of dodecane. Furthermore, the amount of cooperative incorporation of boron to zeolitic framework contributes in adjusting the zeolitic acidity and consequently the amount of weak acid sites were observed to be proportional to the contents of boron. However, from pyridine-FTIR analysis, it was noticed that the acid site nature of borosilicalite-1 samples exhibited Brønsted acid site. Nevertheless, the presence of alkali fluoride as mineralizer agent enhanced the presence of boron in tetrahedral coordination system, which led to lower catalytic activity. While samples synthesized in the presence of alkali hydroxide were catalytically more active in steam catalytic cracking of dodecane due to its exhibit in trigonal coordination system. It was found that borosilicalite-1 was more stable when it was synthesized in the presence of sodium hydroxide as mineralizer agent rather than potassium hydroxide and the conversion was reached to 95% at 350 °C with space velocity of 4 h−1 when the Si/B ratio was 10.

Introduction

Large consumption of fossil oil reservoirs due to the increase in energy demands encouraged scientists to further explore renewable energy sources such as biomass. Many researchers conducted the use of energy crops as clean fuel due to the huge availability of biomass sources such as fast growing trees, switchgrass and agricultural residues. The scientific attention to the biomass has been increasing due to the wishes of global institutions and researchers in utilizing the environmental friendly clean energy. This is an important source, not only for the production of energy, but also for the drastic reduction in the volume of solid residue [1], [2]. Furthermore, the political twists and turns toward the crude oil and price instability, encouraged numerous research organizations to have a closer look to biomass. Moreover, negligible emissions of toxic and hazardous gases such as SO2 and NOx gasses served as the biggest cheerleader in the exploitation of biomass [1].

The technical intermediary for the conversion of biomass and the dismantling of these organic materials is the pyrolysis process. The pyrolysis of biomass can end up with valuable products such as syngas, solid bio-char and liquid bio-oil [1]. However, by further focusing on the bio-oil and the possible products from the pyrolysis of biomass, it was found that dodecane (C12) is one of the most selected product from the pyrolysis process [1]. As the industrial importance of light hydrocarbons in many industrial field, it is important to study the transformation of non-usable chemical compounds to demanded chemical products such as light olefins, light paraffin and aromatics.

The catalytic conversion of dodecane can be carried out over heterogeneous catalysts. Due to the availability of different kinds of heterogeneous catalysts, it is worthy to note that most reactions in petrochemical industries occurs over zeolitic materials due to their adsorption properties for hydrocarbon molecules [3]. The nature of these zeolitic materials made them important to be used in multiple petrochemical processes such as alkylation, catalytic cracking, steam catalytic cracking and methanol-to-olefins [4], [5], [6], [7]. However, the properties of these zeolitic materials should be compatible with the type of the process to notch up the desired goal such as high conversion, high selectivity and high stability. In order to recognize the role of these properties, that are offering the high selectivity, high conversion and long stability in zeolites, it is important that we identify the general factors that help these parameters to be improved.

By considering the nature of silicalite-1, for instance, it is inorganic microporous crystalline catalysts which is mainly consists of T atoms (where T is Si for silicalite-1) in tetrahedra coordination system (TO4) [8], [9]. By looking to the nature of physical properties, silicalite-1 has weak acid site due to the presence of silanol group and the absence of various elements such as aluminum, boron, galium, et cetera. [10]. However, one of the most interested areas in the catalytic modifications is the isomorphous incorporation of tri- and/or tetravalent atoms to the zeolitic framework to modify the physical properties of the zeolites due to its influence in diversifying the catalytic properties of zeolites. Numerous researches studied the isomorphic incorporations of different kinds of heteroatoms such as aluminum [11], [12], [13] and boron [14], [15], [16] to solve classical problems such as low conversion and rapid deactivation.

Various zeolites with several topologies were used in steam catalytic cracking of dodecane [17], [18]. Still, the issue of fast deactivation and low conversion in the catalytic cracking of dodecane need to be solved by modifying the physical properties of the zeolites. One of the most important zeolite is a 3-dimensional ZSM-5 (MFI) zeolite, which has excellent activity in many catalytic reactions [13], [17], [19]. Its wonderful activity has made it one of the most studied zeolite with different tri- and tetravalent atoms. However, the incorporation of boron in MFI topology is expected to add valuable feature due to the influence of boron on the strength of zeolitic acidity. As compared to aluminum, boron has weaker acid sites, which can be interesting for some catalytic cracking. This weaker acid sites are due to the high electronegativity feature of the boron, which make it difficult to chair the pair electrons. Borosilicalite-1 is well known for several kinds of reactions and it was firstly reported by Taramasso et al. [20]. However, further exploration is required for borosilicalite-1 to be used in the catalytic cracking of dodecane.

In this study, we synthesized boron containing MFI type zeolite (borosilicalite-1) under microwave irradiation. Furthermore, borosilicalite-1 was synthesized in the presence of different alkali hydroxide and alkali fluoride sources as mineralizer agent. The effect of isomorphous incorporation of boron in MFI type zeolite was systematically studied by X-ray analysis, N2 physisorption measurements, NH3 temperature program desorption, pyridine adsorption, NMR MAS spectra and it was evaluated in steam catalytic cracking of n-dodecane.

Section snippets

Catalyst preparation

The synthesis of borosilicalite-1 (B-MFI) was achieved by the following chemicals without further purification: (1) colloidal silica (40 wt.% suspension in H2O, LUDOX® HS-40, Sigma-Aldrich), (2) boric acid (ACS reagent, ≥99.5%, Sigma-Aldrich), (3) tetrapropyl-ammonium hydroxide solution (TPAOH, 1.0 M in H2O, Sigma-Aldrich), (4) sodium hydroxide (NaOH, Panreac), (5) sodium fluoride (Sigma-Aldrich), (6) potassium fluoride (Sigma-Aldrich).

Borosilicalite-1 (B-MFI) zeolite was synthesized by further

Effect of mineralizer agent source and boron content on crystallinity, phase purity, and morphology

The synthesis of silicalite-1 and its modification was studied in our previous work [21]. We reported that silicalite-1 can be synthesized in the presence of different mineralizer agents under microwave irradiation. However, in this work, further modifications on the synthesis formulation were performed to synthesize boron containing MFI type zeolite (borosilicalite-1). Three different strategies were used in the synthesis of borosilicalite-1 by adding boron as a heteroatom to the framework of

Conclusions

Microwave irradiation was utilized to synthesize borosilicalite-1 (B-MFI) with different mineralizer agents to improve steam catalytic cracking of dodecane. The isomorphous substitution of boron in zeolitic framework has a curtail role in adjusting the zeolitic acidity. It was observed that the amount of weak acid sites was proportional to the contents of boron and the acid site distributions were shown to be in Brønsted acid site forms. It was found that borosilicalite-1 was more stable when

Acknowledgement

The author would like to acknowledge the financial support provided by Saudi Aramco through contract number 6600011900.

References (32)

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