Generation of Basic Centers in High‐Silica Zeolites and their Application in Gas‐Phase Upgrading of Bio‐Oil

Abstract

High-silica zeolites have been reported recently as efficient catalysts for liquid- and gas-phase condensation reactions because of the presence of a complementary source of basicity compared to Al-rich basic zeolites. Herein, we describe the controlled generation of these active sites on silica-rich FAU, BEA, and MFI zeolites. Through the application of a mild base treatment in aqueous Na2CO3, alkali-metal-coordinating defects are generated within the zeolite whereas the porous properties are fully preserved. The resulting catalysts were applied in the gas-phase condensation of propanal at 673 K as a model reaction for the catalytic upgrading of pyrolysis oil, for which an up to 20-fold increased activity compared to the unmodified zeolites was attained. The moderate basicity of these new sites leads to a coke resistance superior to traditional base catalysts such as CsX and MgO, and comparable activity and excellent selectivity is achieved for the condensation pathways. Through strategic acid and base treatments and the use of magic-angle spinning NMR spectroscopy, the nature of the active sites was investigated, which supports the theory of siloxy sites as basic centers. This contribution represents a key step in the understanding and design of high-silica base catalysts for the intermediate deoxygenation of crude bio-oil prior to the hydrotreating step for the production of second-generation biofuels.

Synthesis, characterisation, and catalytic evaluation of hierarchical faujasite zeolites: milestones, challenges, and future directions

“…11a). 39,[58][59][60][61][62] Like in steam treatment, base treatment (Fig. The first is formed by the classical Brønsted acid site generated by the exchangeable charge balancing proton within the micropores, the second is the Lewis acidity generated by EFAl on the external surface of the zeolite crystal.…”

“…In base catalysis, the main way to obtain the basic site is by introducing a large amount of electron donating cations into the framework. 59 The excellent performance of the derived materials (see Section 4.2) was ascribed to the moderate basicity of the sites. Accordingly, CsX was traditionally considered the most active base catalyst, whereas highsilica zeolites were disregarded as potential base catalysts.…”

Advances in porous and nanoscale catalysts for viable biomass conversion

“…51 To circumvent this problem, the development of hierarchical porous zeolites containing a multimodal pore size distribution has attracted a lot of attention, as this strategy combines the advantages of micropores (provide active sites and determine shapeselectivity) as well as mesopores (facilitate rapid mass transfer properties). 31,52 New active sites can also be formed in hierarchical zeolites due to structural modifications and perturbations that occur during their synthesis. For example, the alkaline treatment of zeolites led to the formation of a new type of surface basic sites, along with innate acid-base Different types of shape selectivity in zeolites: (A) reactant shape selectivity, (B) transition state shape selectivity, and (C) product shape selectivity.…”

“…7). 31,52 Owing to their high specific surface area, zeolites are efficient support materials for the stabilization/dispersion of active phase metals/metal oxides (Pd, Pt, Au, Ru, Ag, Ni and V 2 O 5 ). 30,53 Advantages of acid properties of zeolites coupled with the redox properties of metals can lead to the generation of novel, versatile catalysts with improved activities and selectivities.…”

“…Hierarchical porous zeolites, having at least two levels of porosity, are of increasing research interest for catalytic biomass valorization because of their favorable mass transport properties compared to that of conventional microporous zeolites. 30,52 As shown in Fig. 14 (right side, hierarchical zeolite), the introduction of secondary pores into the zeolite structures can provide three key advantages: (1) enhanced active site accessibility due to controllable diffusion of molecules (orange and green spheres). 8 (2) Improved product selectivity (green spheres) driven by the presence of larger external surface.…”