Elsevier

Catalysis Today

Volume 347, 1 May 2020, Pages 48-55
Catalysis Today

Comparison of titanosilicates with different topologies as liquid-phase oxidation catalysts

https://doi.org/10.1016/j.cattod.2018.04.026Get rights and content

Highlights

  • Four titanosilicates were investigated in the liquid phase oxidation reactions.

  • Ti-MCM-68 and Ti-MWW were more active in the epoxidation of alkenes reactions.

  • Ti-MCM-68 was inferior to other three titanosilicates in the ammoximation reactions.

  • Ti-MCM-68 could reach high cyclohexanone conversion and oxime under suitable condition.

Abstract

Titanosilicates, with isolate and tetrahedrally coordinated Ti4+ ions in zeolite framework, have shown their great potentials in the liquid-phase selective oxidation reactions. The usage of H2O2 as the oxidant helped to build up environment-friendly chemical processes by giving H2O as the sole co-product. Four titanosilicates of Ti-MCM-68, Ti-MOR, Ti-MWW and TS-1, with distinct crystalline topologies, were included in present study, whose catalytic performances were compared in the epoxidation of alkenes and the ammoximation of ketones. Ti-MCM-68, as a recently discovered titanosilicate, showed attractive advantages in the epoxidation reactions, while its catalytic performance in the ammoximation reactions was inferior to the other three titanosilicates. However, the cyclohexanone conversion and oxime selectivity could reach ∼99% over Ti-MCM-68 titanosilicate under optimized reaction conditions, indicating that it could also be taken as a promising catalyst for ammoximation reaction.

Introduction

Zeolites, with high framework crystallinity, large surface area, uniform micropores and well-defined cavities, are traditionally composed of tetrahedral Si and Al atoms interconnected by O atoms [1]. Tetrahedral Al atoms create negative charges in the framework, which are usually balanced by organic or inorganic cations. The replacement of these cations with protons endows zeolites with Brönsted acidity, providing them unique properties as solid acid catalysts [2]. The chemical composition of traditional zeolite can be altered by introducing heteroatoms (such as B [3], Ti [4], Ge [5], Fe [6], Sn [7], V [8]) into the framework to replace some or all the Al atoms. The isomorphous substitution of heteroatoms in the framework opens up new applications of zeolite related materials. Among all the heteroatom-containing zeolites, titanosilicates are the most studied one for their remarkable selective oxidation ability in liquid-phase reactions. In 1983, the researchers from ENI company discovered the first titanosilicate TS-1 with MFI structure, analogue to the well-known ZSM-5 aluminosilicate [4]. TS-1 was gradually proved to be highly active in the oxidation reactions using the environmentally benign aqueous H2O2 as oxidant coproducing water as the sole byproduct [9]. The remarkable activity of TS-1 titanosilicate raises hope of a similar achievement in the field of selective oxidation reactions as aluminosilicates in solid acid catalysis. Since then, the researches in the area of titanosilicates focus on establishing mature characterization technique of Ti species in the framework to distinguish TS-1 titanosilicate with good activity, which was characterized by the IR band at 960 cm−1, the sole absorption band at 210 nm in UV–vis spectrum, and the absence of other metal ions (e.g. Al3+ and Na+), etc [10,11].

On the other hand, the bottlenecks of TS-1 titanosilicate in the oxidation reaction involving bulky substrates gradually emerged due to the its bi-direction medium-pore system. In this sense, new titanosilicates with enhanced accessibility were developed, such as Ti-MWW [12], Ti-MOR [13], Ti-Beta [14], Ti-UTD-1 [15], TAPSO-5 [16], Ti-MCM-41 [17] and Ti-SBA-15 [18]. Ti-MWW titanosilicate, constructed form lamellar precursor, possessed a unique pore system composed of two-dimensional sinusoidal 10-ring (10R) channels and independent 12R supercages. These supercages turn to be pockets at the crystal exterior, which serve as reaction spaces for accommodating bulky substrates to enhance the catalytic activities [19]. Moreover, phase delamination [20], interlayer expansion [21] and pillaring [22] can be realized via post-modifications over Ti-MWW lamellar precursor to either expose more Ti4+ active site on the external surface or enlarge the interlayer pore size to release the diffusion constraints and then elevate the catalytic activities. Ti-MOR titanosilicate, with one-dimensional 12R large pore system, favors the diffusion compared to medium pore TS-1 titanosilicate [23]. However, the absence of interconnection between 12R pore channels would cause traffic jam in the reaction. With three-dimensional interconnected 12R pore channels, Ti-Beta zeolite demonstrates its pure strength in catalyzing bulky substrates, such as cyclohexene [24]. The structural defects due to the polymorphs of *BEA structure drag its feet on catalytic performance with lower selectivity. Very recently, a novel titanosilicate Ti-MCM-68, with a straight 12R pore channel intersected by two independent tortuous 10R pore channels and a 12 × 18-R supercage accessible through 10R pore, was prepared via a combination process of dealumination and atom-planting with TiCl4 vapor [25,26]. Ti-MCM-68 exhibited remarkable activity in the phenol oxidation reaction, where TS-1 showed inferior activity and Ti-MWW was almost inactive.

Titanosilicates, with both Ti4+ as Lewis acid site and hydrophobic property of silicate, stand out in catalyzing various hydrocarbons to produce value-added chemicals using hydrogen peroxide as oxidant [27]. The application of green titanosilicates/H2O2 catalytic system is potential to change the backwardness of industry process performed using homogenous catalysts such as corrosive liquid acid and co-producing large amount of useless waste chemicals. As a typical example, the ammoximation of ketones, conventionally carried out using hydroxylamine sulfate, can achieve the clean produce of oxime catalyzed by titanosilicates with H2O2 and NH3 [28]. A large pilot plant for the ammoximation of cyclohexanone using TS-1 catalyst was established in 1994 in Porto Marghera, Italy. As a key material in nylon-66 manufacturing industry, the clean produce process of cyclohexanone oxime reduced the by-product of ammonium sulphate as well as the investment cost in the caprolactam process. Other titanosilicates, such as Ti-MOR [29] and Ti-MWW [30], are also applied in the ammoximation of cyclohexanone, giving superior catalytic performance than TS-1. The most recognized ammoximation mechanism using titanosilicates is the hydroxylamine route, although the imine route is also reported to co-exist in the reaction for specific substrate [31]. The substrates of ammoximation reactions can also be widely extended to dimethyl ketone [32], methyl ethyl ketone [33], acetaldehyde [34], furfural [35], ect. Both crystalline structure and substrate-dependent phenomena were observed for the activity and selectivity of these titanosilicates due to varying local Ti4+ environment in distinct framework with different topology and hydrophilicity/hydrophobicity characters.

However, up to now, no catalytic data have been disclosed for the novel titanosilicate Ti-MCM-68 in the ammoximation of ketones and aldehydes. In this present study, we performed a detail investigation of Ti-MCM-68 titanosilicate in the ammoximation of ketones under different reaction conditions and compared with other titanosilicates of Ti-MOR, Ti-MWW and TS-1. In addition to this, we firstly compared the catalytic activity of the four titanosilicates in the classical model reaction of alkene epoxidation to reveal the activity dependence on pore structure. In the epoxidation of linear alkenes, Ti-MWW titanosilicate gave the highest catalytic activity, while Ti-MCM-68 exhibited superior catalytic performance in the case of cycloalkenes. As for the ammoximation reaction, Ti-MCM-68 was inferior to the other three titanosilicates, although the conversion of cyclohexanone and main product selectivity could reach ∼99% under suitable conditions.

Section snippets

Preparation of titanosilicate catalysts

Ti-MCM-68 zeolite was prepared according to the literature via postsynthesis method [25], including the dealumination with acid leaching and atom-planting process via gas-solid reaction. The aluminsilicate MCM-68 zeolite was firstly synthesized using the N,N,N’,N’-tetraethylbicyclo [2.2.2]oct-7-ene-2,3:5,6-dipyrrolidinium diiodide as the organic structure-directing agent (SDA), denoted as TEBOP2+(I)2, which was prepared strictly according to the literature [25]. In a typical synthesis, 0.333 g

Characterizations of titanosilicates

Four titanosilicates, with distinct pore systems and structural topologies, are included in present study. As shown in Fig. 1, all the titanosilicates exhibited well-resolved and typical diffraction reflections attributed to their corresponding topologies, indicative of high crystallinity. Ti-MCM-68 zeolite, with the MSE topology, possessed the nanoszied crystals with the particle size of 50–100 nm (Fig. 2a). Irregular shaped crystal morphologies (200–500 nm) were observed for Ti-MOR

Conclusions

The catalytic performance of four titanosilicates of Ti-MCM-68, Ti-MOR, Ti-MWW and TS-1 was investigated in the liquid-phase epoxidation and ammoximation reactions. Ti-MCM-68 and Ti-MWW were more active in the epoxidation reactions of both cycloalkenes and linear alkenes. However, Ti-MCM-68 was inferior to other three titanosilicates in the ammoximation of cyclohexanone. By optimizing the reaction condition, Ti-MCM-68 could also give very high cyclohexanone conversion and oxime selectivity

Acknowledgments

The authors gratefully acknowledge financial support from the NSFC of China (21533002 and 21603075) and China Ministry of Science and Technology under contract of 2016YFA0202804.

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