Shape selectivity in toluene disproportionation into para-xylene generated by chemical vapor deposition of tetramethoxysilane on MFI zeolite catalyst

https://doi.org/10.1016/j.micromeso.2017.01.022Get rights and content

Highlights

  • High para-xylene selectivity 99.7% in toluene disproportionation was achieved at conversion = 10% on the silica/MFI.

  • The parent zeolite with small number of Brønsted acid sites on the external surface brought the high selectivity.

  • The selectivity was predominantly dependent on the diffusion rate of bulky molecule.

Abstract

Dependence of shape selectivity for para-xylene production by toluene disproportionation on conditions of chemical vapor deposition (CVD) of tetramethoxysilane on MFI (ZSM-5) zeolite were investigated in detail. The CVD after pelletization was necessary to obtain 0.7–1 mm particles with high selectivity. The influences of preparation conditions on the selectivity were investigated in detail to find the optimum conditions. The parent zeolite with small number of Brønsted acid sites on the external surface brought the high selectivity after the CVD. The catalyst prepared in the optimized conditions showed the selectivity 99.7% at ca. 10% of the toluene conversion.

Graphical abstract

(Left) Schematic drawing of shape selective formation of para-xylene and benzene by toluene disproportionation on silica-modified MFI with suitable pore-opening size. (Right) Selectivity-conversion plots on various catalysts. Only the present study gave higher selectivity than the conventional process.

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Introduction

para-Xylene (1,4-dimethylbenzene) is one of the most valuable hydrocarbon compounds, because it is the raw material of polyethylene terephthalate (PET) [1]. para-Xylene is contained in crude oil and naphtha produced by the fluid catalytic cracking, but the purification from an isomer mixture consumes huge energy and resources [2], and therefore the selective production of para-xylene has been a subject of study. Zeolites, aluminosilicates with microporous structure, have shape selectivity in methylation of benzene or toluene (methylbenzene) and disproportionation of toluene, where para-xylene is preferentially formed compared to meta- and ortho-isomers (1,3- and 1,2-dimethylbenzene) with bulky molecular shapes [2], [3]. The toluene disproportionation is the most efficient reaction for the production of xylenes [3], because toluene is relatively useless among benzene and its derivatives, and no other expensive reagent such as methanol is necessary. MFI type zeolite (ZSM-5) with 10-ring pores is known to have the para-xylene selectivity in the toluene disproportionation [2].

However, even the MFI zeolite shows the selectivity only at a very low conversion of toluene. In industrially operated conditions, namely at a certainly high conversion, the shape selectivity of unmodified MFI zeolite is negligible, although the shape selectivity has been introduced to be a good example of usage of microporous material. In a fact, many attempts have been made to improve the selectivity of MFI zeolite, and some of them have been industrially applied.

Modification of a zeolite with rare earth, boron, phosphorous and silicon compounds have been known to generate shape selectivity [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. Formation of silicalite crystal (siliceous MFI) on the external surface of MFI zeolite [26], [27], [28] should have similar effects. Coke, as a byproduct of the reaction or designed to have suitable nature, has also been known to improve the shape selectivity [29], [30], [31], [32]. There has been a controversy about the origin of selectivity, because the discussed structure is always smaller than nanometric scale; the inactivation of active sites on the external surface [14], [17], [23], [30], [32], the control of pore-opening size [6], [8], [11], [13], [15], [19], [25] and both of them with each role [5], [10], [16], [18], [20], [21], [31] have been proposed, whereas increased tortuosity has also been a candidate [22].

Among them, the modification with silica has been investigated well as a practically important method to construct a stable structure. The first attempt of modification of the micropore size and pore-opening size of a zeolite with silane has been presented by Barrer et al. [4] Then, tetramethoxysilane, a safer silicon source, has been utilized to control the pore-opening size by Niwa et al. [8] These methods utilize a chemical vapor deposition (CVD) technique. Beck et al. have industrialized an impregnation method in an organic solvent [12], in other words, a method of chemical liquid deposition (CLD) [15], [17], [20], [24], [25] of silica using polyalkylsiloxane compounds. The industrial process according to this patent has been operated with 93% of the para-xylene selectivity (the rate of formation of para-xylene/the total rate of formation of xylene isomers and ethylbenzene) at 30% of the toluene conversion, and it has greatly improved the economic efficiency and saved the consumption of energy and resources for the production of PET. The CVD of tetraethoxysilane has also been developed in an industrial scale and applied to the production of para-diethylbenzene (1,4-diethylbenzene), a similar reaction [11], [13], [33]. We believe these studies approaching one of the most precise technologies which mankind has created and practically applied, because key of the works is the structure control in an atomic scale, the product is really useful, and the usefulness is related with the molecular shape of product.

However, even at the selectivity 93%, a post treatment (currently pressure swing adsorption using a zeolite) for the separation of para-isomer from a mixture of xylene isomers (or C8 fraction) is necessary, because the para-xylene purity higher than 99.7% is required for the production of PET. We can count potential ways to reduce the cost and energy consumption of the separation process, improvement of the pressure swing adsorption [34], [35] and development of molecular sieving membranes [36], [37], [38], [39], [40]. However, more efficient way is believed to be development of a catalyst with higher selectivity which can produce para-xylene with selectivity >99.7% at a sufficiently high conversion such as 10%. Efforts have been continued, but to the best of our knowledge, selectivity higher than that achieved by Beck et al. [12] has not been reported.

We have developed the CVD method of tetramethoxysilane on zeolites [41], [42], including its application to the toluene disproportionation [10], [18], [43] and other materials [44], [45], [46], [47], [48]. Here detailed influences of the conditions of CVD on the shape selectivity of MFI zeolite in the toluene disproportionation were investigated. The selectivity was compared with the conventional techniques.

Section snippets

Experimental

An H-ZSM-5 (zeolite with the MFI structure) sample purchased from Mizusawa Chemical Industry with Si/Al2 molar ratio 30 was utilized as the parent zeolite in most cases, while other zeolite species [H-ZSM-5 with Si/Al2 = 24 ion exchanged from Tosoh Na-ZSM-5, H-ZSM-5 with Si/Al2 = 48 from Mizusawa Chemical Industry and H-mordenite (zeolite with MOR structure) with Si/Al2 = 20 from Reference Catalyst Division, Catalysis Society of Japan as a reference catalyst JRC-Z-HM20] were employed in some

Change in catalytic activity and selectivity by CVD

The amount of deposited silica was determined by weighing the sample in some experiments. Fig. S1 shows that the deposition of silica proceeded quickly in the first several hours at 473 K on the powder sample up to ca. 1 wt% of SiO2, corresponding to ca. 8 Si atoms nm−2 on the external surface. Then, slow deposition continued in the following several days. The preparation of catalysts employed in the present study was mainly carried out in the region where the secondary slow deposition

Conclusions

Application of CVD of tetramethoxysilane on MFI to the toluene disproportionation was examined. The pelletization after the CVD significantly decreased the shape selectivity, presumably due to the destruction of crystallite or surface silica layer, and therefore the CVD after the pelletization was necessary. The investigation on influences of preparation variables clarified that the CVD at 473 K for suitable reaction time (in this scale, ca. 20 h) in 20 μmol s−1 of tetramethoxysilane

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