Structure and reactivity of single site Ti catalysts for propylene epoxidation

Abstract

Propylene epoxidation using molecular oxygen and hydrogen mixture on Au-based catalysts has attracted attention because of high propylene oxide selectivity and the use of an inexpensive and environmental friendly oxidant. Single-site titanium on metal oxide supports plays an important role in achieving high reactivity and selectivity in propylene epoxidation. Here we used TiO₂ atomic layer deposition (ALD) to synthesize single-site titanium imbedded in the SiO₂ framework for propylene epoxidation. High temperature calcination was used as post-treatment to control the titania structure and TiO coordination number. Using UV–vis spectroscopy and X-ray absorption spectroscopy, we successfully established that under similar propylene conversion the selectivity to propylene oxide (PO) is strongly correlated to the TiO coordination number and bond length. Using a cluster model, density functional theory (DFT) calculations indicate that the partial charges of single TiSiO₂ sites scale linearly as a function of the coordination number. Also, the predicted TiO bond lengths follow the same trend as found in the experiments, providing additional support for the observed experimental activity relationships.

Introduction

Propylene epoxidation using molecular oxygen is a promising, alternative route to synthesize propylene oxide (PO), a chemical of great industrial importance [1], [2], [3], [4], [5]. Since Haruta first discovered that supported gold nanoparticles on TiO₂ are active and selective for the direct epoxidation of propylene in the presence of molecular oxygen and hydrogen, it had been gradually realized that the reactivity, PO selectivity and hydrogen efficiency could benefit from synergic effects of small nano-sized gold particles and the neighboring TiO₂ [6] . Studies suggest that highly dispersed isolated Ti sites serve as the preferential sites to adsorb propylene, while the gold nanoparticles are responsible for the formation of the OOH* species from oxygen and hydrogen [7], [8], [9]. The OOH* species then migrate toward nearby Ti sites to produce PO from propylene. The isolation of Ti sites was found to increase the PO selectivity with less byproduct formation. The adjacent Ti sites were found to favor bidentate adsorption of propylene, leading to the formation of undesired byproducts. Understanding the TiO₂ structure is of great importance for designing highly efficient AuTiO₂ based catalysts for propylene epoxidation.

To establish the Ti structure and performance relationship, Haruta and coworkers deposited TiO₂ on non-porous silica using incipient wetness impregnation (IWI) and subsequently calcined the support up to 1000 °C [10]. The PO yield and selectivity improved by increasing the calcination temperature. The improved PO yield and selectivity were ascribed to the formation of the site-isolated tetrahedral TiO₂ in the SiO₂ framework with the presence of TiOSi. However, as TiO₂ prepared by IWI tends to initially form clusters and nanoparticles on the SiO₂ support, it could be difficult to drive all of the TiO₂ to form single-site Ti in SiO₂ framework using high temperature calcination. Another potential concern is the possible loss of surface area due to the collapse of pores during high temperature calcination. Other efforts have been used to synthesize single-site catalysts for propylene epoxidation. To generate single-site Ti, Nijhuis and co-workers used a Ti-grafted silica method prepare Ti-doped SiO₂ for propylene epoxidation with H₂ and O₂, and achieved promising activity with 2.7% propylene conversion and PO selectivity at 89.4% [11]. A sol-gel method was used to prepare site-isolated Fe in the SiO₂ framework for propylene epoxidation with O₂, and showed good activity without the use of Au [12], however, the catalysts showed 33.6% PO selectivity at 5.5% conversion.

The establishment of the Ti structure and performance relationship can benefit from the use of uniformly dispersed single-site Ti on the SiO₂ support. In this study, we investigated the isolated TiO₂ single-site initially deposited on porous SiO₂ by atomic layer deposition (ALD), a method that has been used to prepare single-site catalysts [13], [14], [15]. Advanced catalyst synthesis is advancing from obtaining understandings of the catalytic active sites [16] to assemble catalytic active architecture [17]. ALD represents a promising technique to construct catalytic active architecture in a “bottom-up” manner to achieve high activity, selectivity, and stability [18], [19]. It is a thin film deposition technique that uses organometallic compounds as precursors. It enables conformal coatings because the deposition conditions are optimized to ensure the surface self-limiting reaction between the organometallic precursors and the reaction sites on the substrates. Under surface self-limiting reaction conditions, the gaseous organometallic precursor molecules, e.g., titanium isopropoxide or TTIP, preferentially react with the surface reaction sites, e.g., surface hydroxyl groups, forming stable intermediates on the substrate [20], [21]. These stable, chemisorbed intermediates will not decompose to form TiO₂ until a co-reactant is dosed, e.g., H₂O. The steric hindrance effect from the bulky TTIP ligands ensures that the intermediates distance from each other and prevent the possible formation of TiOTi bonds, and the chemisorption nature of the intermediates allows the facile formation of TiOSi bonds. In a previous study, we have prepared site-isolated Ti for Au-based catalysts using TiO₂ ALD for propylene epoxidation with H₂ and O₂ at 100 °C [22]. The catalysts showed 90% selectivity at 100 °C, however the PO formation rate was relatively low, about 1–2 g_PO h⁻¹ kg⁻¹cat. A substantial gain in PO formation rate and propylene conversion could be obtained by carrying out the reaction at higher reaction temperature. To maintain high PO selectivity at high temperature, it is necessary to optimize the catalyst structure that favors the PO formation pathway. In this work, we used ALD to generate single-site Ti, followed by a post-ALD calcination to alter the coordination of Ti, and connect the TiO structure and its performance in propylene epoxidation with H₂ and O₂. Using ultraviolet–visible (UV–vis) diffuse reflectance spectroscopy and X-ray absorption spectroscopy (XAS), we investigated the structure of single-site Ti in the SiO₂ framework as a function of calcination temperature. The PO selectivity shows a linear correlation to the TiO coordination number and bond distance, and DFT calculations predict a linear scaling of the partial charge of the Ti site as a function of the coordination number and bond distance.