Catalytic performance of sheet-like Fe/ZSM-5 zeolites for the selective oxidation of benzene with nitrous oxide

doi:10.1016/j.jcat.2012.12.002

Journal of Catalysis

Volume 299, March 2013, Pages 81-89

https://doi.org/10.1016/j.jcat.2012.12.002 Get rights and content

Abstract

Hierarchical Fe/ZSM-5 zeolites were synthesized with a diquaternary ammonium surfactant containing a hydrophobic tail and extensively characterized by XRD, Ar porosimetry, TEM, DRUV–Vis, and UV-Raman spectroscopy. Their catalytic activities in catalytic decomposition of N₂O and the oxidation of benzene to phenol with N₂O as the oxidant were also determined. The hierarchical zeolites consist of thin sheets limited in growth in the b-direction (along the straight channels of the MFI network) and exhibit similar high hydrothermal stability as a reference Fe/ZSM-5 zeolite. Spectroscopic and catalytic investigations point to subtle differences in the extent of Fe agglomeration with the sheet-like zeolites having a higher proportion of isolated Fe centers than the reference zeolite. As a consequence, these zeolites have a somewhat lower activity in catalytic N₂O decomposition (catalyzed by oligomeric Fe), but display higher activity in benzene oxidation (catalyzed by monomeric Fe). The sheet-like zeolites deactivate much slower than bulk Fe/ZSM-5, which is attributed to the much lower probability of secondary reactions of phenol in the short straight channels of the sheets. The deactivation rate decreases with decreasing Fe content of the Fe/ZSM-5 nanosheets. It is found that carbonaceous materials are mainly deposited in the mesopores between the nanosheets and much less so in the micropores. This contrasts the strong decrease in the micropore volume of bulk Fe/ZSM-5 due to rapid clogging of the continuous micropore network. The formation of coke deposits is limited in the nanosheet zeolites because of the short molecular trafficking distances. It is argued that at high Si/Fe content, coke deposits mainly form on the external surface of the nanosheets.

Graphical abstract

Nanostructuring of Fe/ZSM-5 results in significantly higher phenol productivity during benzene oxidation with nitrous oxide. Optimal performance is obtained at low iron content.

Highlights

► Nanometer-thin ZSM-5 nanosheets functionalized by iron. ► High hydrothermal stability of the Fe/ZSM-5 nanosheets. ► Improved catalyst stability in the benzene to phenol oxidation reaction. ► Deactivation mainly due to coke deposition in the mesopores. ► Lower Fe content results in less coke formation in micropores.

Introduction

Phenol is an important industrial intermediate for the production of various chemicals such as bisphenol A, phenolic resins, caprolactam, alkylphenols, and adipic acid. The cumene hydroperoxide process is currently the preferred industrial method to produce phenol [1]. One of the alternative routes involves the direct oxidation of benzene with nitrous oxide [2]. A promising catalyst for this process is Fe/ZSM-5 zeolite [3], [4], [5], [6], [7], [8], [9], [10], [11], but the strong catalyst deactivation due to formation of carbonaceous by-products remains an obstacle in its commercialization [5]. Thus, improvements of the stability of this zeolite catalyst remain an important research topic. Steam calcination greatly improves the catalytic activity of Fe/ZSM-5 [12]. The effect of steaming is understood in terms of an increased number of active Fe²⁺ sites [6], [7], [10], [11], [13].

Hierarchical zeolites are also of considerable current interest [14], [15]. Recent examples show that the presence of mesoporosity in Fe/ZSM-5 results in improved catalyst stability during the benzene oxidation reaction with nitrous oxide [16], [17], [18], [19]. It has been observed that the activity and stability of zeolites strongly improve as a result of a decrease in the size of the crystalline zeolite domains. The higher activity is likely caused by the increased mass transport in the smaller crystalline domains, which is most important for the removal of the phenol product from the micropore space. The lower rate of deactivation should be the result of the increased ratio of the external surface over the micropore volume. As a consequence, the residence time of phenol in the micropores will be lower, so that deactivating reactions such as overoxidation and condensation reactions are limited. It has recently been suggested that the formation of phenolate may also block the micropores and precede coke formation [20], [21]. Another effect of the higher external/mesopore surface over micropore volume will be that blockage of micropores will render a smaller fraction of the total micropore volume inaccessible in nanostructured zeolites. As the domain size in mesoporous Fe/ZSM-5 catalysts is still in the order of 20-50 nanometer, the questions arise whether the catalytic performance of Fe/ZSM-5 can be improved by further decreasing the crystalline domain size.

In their studies on the synthesis mechanism of zeolite ZSM-5, Martens and co-workers have reported about the formation of pre-nucleation building blocks (nanoblocks) with dimensions of several nanometers already possessing structural features of the final MFI zeolite [22], [23], [24]. Recently, Koekkoek et al. have shown that nanosized fragments of Fe/ZSM-5 stabilized in a matrix of amorphous silica are active for the selective oxidation of benzene [17]. Attempts to use ZSM-5 nanoblocks containing Fe as catalysts for the oxidation of benzene to phenol were not successful because of their low hydrothermal stability. Ryoo and co-workers have recently shown that is possible to synthesize stable HZSM-5 structures with a dimension of one unit cell in the b-direction of the MFI structure [25], [26]. Their synthesis requires the use of a diquaternary ammonium-type surfactant with a long hydrophobic tail. The two quaternary ammonium centers are separated by a C₆H₁₂ group. Herein, we describe the synthesis of several Fe/ZSM-5 zeolites with the same nanosheet morphology by use of this template. The Si/Fe ratio was varied in the range from 84 to 300. The Si/Al ratio was 38. These materials were extensively characterized for their morphological and textural properties and the nature of the Fe sites. Their catalytic activity in the decomposition of nitrous oxide and the selective oxidation of benzene using nitrous oxide were determined and compared to the catalytic performance of a reference bulk Fe/ZSM-5 zeolite with Si/Al and Si/Fe ratios of 40 and 180, respectively.

Section snippets

Synthesis

The template required for the synthesis of ZSM-5 zeolite nanosheets was prepared according to a known literature procedure [25]. Briefly, N,N,N′N′-tetramethyl-1,6-diaminohexane was reacted with 1-bromo-hexadecane in order to obtain C_16-6(Br). This intermediate was subsequently reacted with 1-bromohexane to obtain C_16-6-6(Br)₂. The hydroxide form of the template was obtained by ion-exchange (MTO-Dowex SBR LCNG OH form, Supelco).

Fe/ZSM-5 zeolite nanosheets were prepared according to a literature

Physicochemical characterization

The chemical composition of the Fe/ZSM-5 nanosheet zeolites and the Fe/ZSM-5 reference zeolite is given in Table 1. The XRD patterns of the calcined and steamed zeolite are shown in Fig. 1. The h 0 l reflections in the patterns of the Fe/ZSM-5-sheet zeolites are sharp, whereas reflections corresponding to crystal planes in the b-direction are either broadened or missing. This is in clear contrast to the pattern of the Fe/ZSM-5 reference zeolite. There is no notable difference between the patterns

Discussion

Sheet-like Fe/ZSM-5 was prepared from a typical synthesis gel for Fe/ZSM-5 in which the conventional structure-directing agent TPAOH was replaced by a surfactant comprising a polar head group with two quaternary ammonium centers and a hydrophobic tail. These zeolites exhibit a similar thin sheet morphology as the zeolites reported by the Ryoo group [25], [26]. The crystallinity of the Fe/ZSM-5-sheet zeolites does not depend on the use of the hydroxyl or bromide forms of the template and is

Conclusions

Structuring Fe/ZSM-5 into thin nanosheets with limited dimensions in the b-direction results in superior catalytic performance, especially in terms of stability, in the selective oxidation of benzene to phenol as compared to conventional Fe/ZSM-5. Due to the small coherent length of the zeolite domains in the direction of the straight channels the probability of secondary reactions of product phenol is substantially decreased. Although spectroscopic investigations do not evidence substantial

Acknowledgements

E.J.M.H. thanks the Technology Foundation STW, the applied science division of the Netherlands Organization for Scientific Research (NWO) and the Programme Strategic Alliances between China and the Netherlands for financial support. R.R. also acknowledges support from the Research Center Program (CA1201) of the Institute for Basic Science in Korea. The authors also thank the Soft Matter Cryo-TEM Research Unit of Eindhoven University of Technology.

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