Local environment and catalytic property of External Lewis acid sites in hierarchical lamellar titanium Silicalite-1 zeolites

Highlights

• External Lewis acid sites in 2D TS-1 zeolites were quantified.

• 2D TS-1 zeolites have high density of isolated surface silanol and titanol groups.

• Reactivity of external sites decreases with increasing mesoporosity in 2D TS-1.

• Decrease in reactivity of external sites is caused by loss of partial confinement.

• Kinetic decoupling of epoxidation pathways enabled catalytic behavior assessment.

Abstract

Hierarchical two-dimensional (2D) lamellar zeolites represent a young emerging field and a novel expression of the zeolite frameworks. Accompanied with synthesis and catalytic performance investigation, assessment on the local environment and catalytic behavior of acid sites in 2D zeolites are needed. Here, we described the local environment of Lewis acid sites and evaluated catalytic activities of external acid sites (i.e. sites on external surface and in mesopores) in 2D lamellar titanium silicalite-1 (TS-1) zeolites. For comparison purpose, four TS-1 zeolites, including 3D TS-1, multilamellar 2D TS-1 without pillarization and with pillarization by silica and silica/titania, respectively, were prepared. The local environment of Ti-sites, including coordination status of Ti-atom, speciation of hydroxyl (-OH) group that hints degree of confinement, acid site strength and accessibility, was collectively assessed by spectroscopic and organic base titration measurements. Kinetic studies of cyclooctene (C₈H₁₄) epoxidation with hydrogen peroxide (H₂O₂) discerned catalytic activity of external Ti-sites that decreased with increasing mesoporosity due to decrease in site confinement.

Introduction

Since the first disclosure of titanium silicalite-1 (TS-1) zeolite from Enichem in 1983 [1], tremendous efforts have been devoted to study its Lewis acidity from titanium (Ti) centers and catalytic performance in oxidation reactions. Lewis acid catalyzed phenol hydroxylation [[2], [3], [4]], cyclohexanone ammoximation [[5], [6], [7], [8]] and propylene epoxidation [[9], [10], [11]] have been practiced at the industrial scale. The TS-1 catalyst is also studied for benzene hydroxylation [12,13], linear/cyclic olefins epoxidation [[14], [15], [16], [17], [18], [19]], terpenes epoxidation [17,20] and allyl alcohol/chloride epoxidation [[21], [22], [23]] reactions. TS-1 zeolites, however, exhibit poor catalytic performance for the conversion of bulky reactants, when the kinetic diameters of these reactants are larger than the zeolite micropores (10-membered ring (10 MR), 0.51 × 0.55 nm along a-axis and 0.53 × 0.56 nm along b-axis) and, as a result, their transport to the Ti-active centers during catalysis are severely restricted. In order to overcome this constraint, attempts to alter the transport properties through synthetic strategies such as synthesizing Ti-containing zeolites with extra-large (i.e. 12 MR and 14 MR) micropore apertures [[24], [25], [26], [27]] or hierarchical TS-1 zeolite architectures [[18], [19], [20]] have been explored.

Hierarchical zeolites, consisting of dual meso-/micropores, have shown remarkable catalytic performance for reactions involving bulky molecules in the past decades [28]. Among the hierarchical zeolites, two-dimensional (2D) zeolite nanosheet and its assembly structures are unique. The assembly increases the external surface area by introducing interlayer mesopores. Such structures markedly increase the accessibility of reactant to the active sites and at the same time retain the microporosity within zeolitic layers, often required for catalysis. The synthesis of TS-1 nanosheet structures, including unilamellar [18], multilamellar [19,20,29] and pillared [20,29] architectures, have been reported recently. In these TS-1 nanosheet materials, the Ti-centers are positioned tetrahedrally in zeolite framework (similar to that of 3D microporous TS-1 zeolite), which are realized by isomorphic substitution of silicon element during hydrothermal crystallization [30]. The high external surface area of TS-1 zeolite nanosheet architectures enables the post-synthetic method to introduce Ti-active sites. For example, the unilamellar TS-1 nanosheet was grafted with Ti-active centers from the titanium (IV) butoxide precursor on its external surface [17]. Similarly, the delaminated siliceous MWW-type zeolites, ITQ-2 [31,32] and UCB-4 [33], were grafted with Ti-sites from the titanocene and calix [4]arene-Ti^IV precursors. These post-synthetic methods created Ti-sites with increased accessibility to bulky molecules. The pillarization treatment of multilamellar TS-1 nanosheets can also introduce additional Ti-active sites besides those inherently present in the framework. For instance, the intercalation of pillaring precursors by dispersing 2D zeolite in an alkoxide liquid (a mixture of tetrabutoxytitanium (TBOT) and tetraethyl orthosilicate (TEOS)) and hydrolyzing entrapped alkoxide formed the silica/titania (SiO₂/TiO₂) pillars in pillared MWW or TS-1 zeolite. These pillared zeolites showed improvement in converting bulky reactants in epoxidation reactions, when comparing to their non-pillared counterparts [20,29,34,35].

Besides the above studies on the synthesis and catalytic activity of 2D TS-1 zeolites, quantitative assessment on the local environment and catalytic behavior of Lewis acidic Ti-centers is also needed. Comparing to those established for Brønsted acidity [[36], [37], [38], [39]] in 2D zeolites, quantification of Lewis acidity, local Lewis acid site environment and reactivity in catalytic reactions are limited. A recent work by Shamzhy etc. quantified the Lewis acidity, acid strength and accessibility of 2D TS-1 zeolites using the Fourier-transform infrared (FTIR) studies of adsorbed organic bases [40]. Here, we made an effort to characterize the acid site local environment including coordination status of Ti-atom, speciation of hydroxyl (-OH) group that hints for the degree of acid site confinement, site strength and accessibility to bulky molecules of Lewis acidic Ti-sites, in hierarchical 2D TS-1 zeolites that include multilamellar TS-1 (M-TS-1), SiO₂-pillared TS-1 (Si-TS-1) and SiO₂/TiO₂-pillared TS-1 (Si/Ti-TS-1). The combination of spectroscopic measurement, organic base titration and in-situ poisoning of catalytic epoxidation of cyclooctene (C₈H₁₄) with hydrogen peroxide (H₂O₂) was used for this purpose.

In addition, the catalytic activity of external acid sites in these 2D TS-1 zeolites were investigated by running C₈H₁₄–H₂O₂ reactions in the absence of poisoning molecules. The kinetics of C₈H₁₄ conversion can probe exclusively the catalytic activity of Ti-centers on external surface and in mesopores (i.e. external Ti-sites, herein denoted as Ti_ext) because C₈H₁₄ is too bulky (kinetic diameter of 0.77 nm) [29,41] to access to those enclosed within the 10 MR micropores. In contrast, H₂O₂ consumption occurs via the catalytic epoxidation with C₈H₁₄ and undesired, non-productive catalytic decomposition by Ti-sites and non-productive thermal decomposition, the latter occurs even without a catalyst. Kinetic decoupling of these three concomitant H₂O₂ conversion pathways is conducted to enable the assessment on the catalytic behavior of Ti_ext-sites. The present study will guide in-depth understanding of Lewis acidity in 2D zeolite materials.