Tobias C. Keller

Hierarchical high-silica zeolites as superior base catalysts

For more than four decades, the design of zeolite base catalysts has relied on the application of aluminium-rich frameworks exchanged with alkali metal cations (preferably Cs+). However, moderate activity associated with access and diffusion limitations, and high manufacturing costs associated with high caesium content (typically over 30%) have hampered their industrial implementation so far. Herein, we have discovered that high-silica USY zeolites outperform their Al-rich counterparts in a variety of base-catalysed reactions of relevance in the fine chemical industry, as well as in the upgrading of biofuels. The benefits of this class of materials are amplified upon the alleviation of diffusion constraints through the introduction of a network of intracrystalline mesopores by post-synthetic modification. For example, the resulting cation-free hierarchical USY provides an up to 30-fold Knoevenagel condensation activity compared to the benchmark Cs–X, and similar observations were made upon application in liquid-phase (nitro)aldol reactions. Moreover, in the gas-phase aldol condensation of propanal, high-silica zeolites provide superior activity, selectivity, and lifetime compared to caesium-containing zeolites and even a strong solid base such as MgO. We decouple the complex interplay between mesoporosity and intrinsic zeolitic properties such as crystallinity, and quantify the increase in catalyst effectiveness upon hierarchical structuring as a function of reactant size. The obtained results are a major step to resolve the drawbacks of zeolites catalysis and thereby revitalise their potential for industrial application.

Hierarchy Brings Function: Mesoporous Clinoptilolite and L Zeolite Catalysts Synthesized by Tandem Acid-Base Treatments

Hierarchical clinoptilolite and L zeolites are prepared using optimized tandem dealumination-desilication treatments. The main challenge in the post-synthetic modification of these zeolites is the high Al content, requiring a tailored dealumination prior to the desilication step. For natural clinoptilolite sequential acid treatments using aqueous HCl solutions were applied, while for L a controlled dealumination using ammonium hexafluorosilicate is required. Subsequent desilication by NaOH treatment yields mesopore surfaces of up to 4-fold (clinoptilolite: 64 m2 g-1, L: 135 m2 g-1) relative to the parent zeolite (clinoptilolite: 15 m2 g-1, L: 45 m2 g-1). A thorough characterization sheds light on the composition, crystallinity, porosity, morphology, coordination, and acidity of the modified clinoptilolite and L zeolites. It is elaborated that, besides the degree of dealumination, the resulting Al distribution is a critical precondition for the following mesopore formation by desilication. Adsorption experiments of Cu2+ and methylene blue from aqueous solutions and the catalytic evaluation in alkylations and Knoevenagel condensation evidence the superiority of the hierarchical zeolites, compared to their purely microporous counterparts. Finally, the post-synthetic routes for clinoptilolite and L are generalized with other recently reported modification strategies, and presented in a comprehensive overview.

Towards Liquid Fuels from Biosyngas: Effect of Zeolite Structure in Hierarchical-Zeolite-Supported Cobalt Catalysts

Wax on, wax off: Bifunctional cobalt-based catalysts on zeolite supports are applied for the valorization of biosyngas through Fischer-Tropsch chemistry. By using these catalysts, waxes can be hydrocracked to shorter-chain hydrocarbons, increasing the selectivity towards the C5 -C11 (gasoline) fraction. The zeolite topology and the amount and strength of acid sites are key parameters to maximize the performance of these bifunctional catalysts, steering Fischer-Tropsch product selectivity towards liquid hydrocarbons.

Hydroxyapatite, an exceptional catalyst for the gas-phase deoxygenation of bio-oil by aldol condensation

Hydroxyapatites with high surface concentrations of calcium exhibit outstanding activity, selectivity, and stability in the gas-phase condensation of propanal in comparison with well-established base catalysts. These abundant and low-cost materials can be attractively used for the deoxygenation of bio-oil, contributing to the sustainable manufacture of renewable second-generation bio-fuels.

Generation of basic centers in high-silica zeolites and their application in gas-phase upgrading of bio-oil.

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.

Immobilizing and de-immobilizing enzymes on mesoporous silica

Beta glucosidase was immobilised as a model enzyme within mesoporous silica (MCF) at a high loading (80 mg g−1). The enzyme was further entrapped within the material by precipitating additional silica within the channels. This entrapment was performed by the polycondensation of tetraethoxysilane under very mild conditions (pure water). Although unreactive while entrapped, in this state the enzyme was highly stable towards heat treatments of 60–70 °C. Upon release from the matrix by a mild silica dissolution step involving a fluoride comprising buffer, the enzyme regained most of its original activity. With this we developed a novel protein entrapment/release scheme, which is designed along the principles of orthogonal protection group chemistry as the protection/deprotection steps do not affect the integrity of the (bio)molecule. The principle can be adopted to many previously developed mesoporous silica/enzyme biocomposites and will allow the application of enzyme dependent diagnostic devices in applications involving demanding environmental storage requirements.