Selectivities in Post-Synthetic Modification of Borosilicate Zeolites

Advances and challenges in designing active site environments in zeolites for Brønsted acid catalysis

Zeolites contain proton active sites in diverse void environments that stabilize the reactive intermediates and transition states formed in converting hydrocarbons and oxygenates to chemicals and energy carriers. The catalytic diversity that exists among active sites in voids of varying sizes and shapes, even within a given zeolite topology, has motivated research efforts to position and quantify active sites within distinct voids (synthesis-structure) and to link active site environment to catalytic behavior (structure-reactivity). This Feature Article describes advances and challenges in controlling the position of framework Al centers and associated protons within distinct voids during zeolite synthesis or post-synthetic modification, in identifying and quantifying distinct active site environments using characterization techniques, and in determining the influence of active site environments on catalysis. During zeolite synthesis, organic structure directing agents (SDAs) influence Al substitution at distinct lattice positions via intermolecular interactions (e.g., electrostatics, hydrogen bonding) that depend on the size, structure, and charge distribution of organic SDAs and their mobility when confined within zeolitic voids. Complementary post-synthetic strategies to alter intrapore active site distributions include the selective removal of protons by differently-sized titrants or unreactive organic residues and the selective exchange of framework heteroatoms of different reactivities, but remain limited to certain zeolite frameworks. The ability to identify and quantify active sites within distinct intrapore environments depends on the resolution with which a given characterization technique can distinguish Al T-site positions or proton environments in a given zeolite framework. For proton sites in external unconfined environments, various (post-)synthetic strategies exist to control their amounts, with quantitative methods to distinguish them from internal sites that largely depend on using stoichiometric or catalytic probes that only interact with external sites. Protons in different environments influence reactivity by preferentially stabilizing larger transition states over smaller precursor states and influence selectivity by preferentially stabilizing or destabilizing competing transition states of varying sizes that share a common precursor state. We highlight opportunities to address challenges encountered in the design of active site environments in zeolites by closely integrating precise (post-)synthetic methods, validated characterization techniques, well-defined kinetic probes, and properly calibrated theoretical models. Further advances in understanding the molecular details that underlie synthesis-structure-reactivity relationships for active site environments in zeolite catalysis can accelerate the predictive design of tailored zeolites for desired catalytic transformations.

ZeoSyn: A Comprehensive Zeolite Synthesis Dataset Enabling Machine-Learning Rationalization of Hydrothermal Parameters

Zeolites, nanoporous aluminosilicates with well-defined porous structures, are versatile materials with applications in catalysis, gas separation, and ion exchange. Hydrothermal synthesis is widely used for zeolite production, offering control over composition, crystallinity, and pore size. However, the intricate interplay of synthesis parameters necessitates a comprehensive understanding of synthesis-structure relationships to optimize the synthesis process. Hitherto, public zeolite synthesis databases only contain a subset of parameters and are small in scale, comprising up to a few thousand synthesis routes. We present ZeoSyn, a dataset of 23,961 zeolite hydrothermal synthesis routes, encompassing 233 zeolite topologies and 921 organic structure-directing agents (OSDAs). Each synthesis route comprises comprehensive synthesis parameters: 1) gel composition, 2) reaction conditions, 3) OSDAs, and 4) zeolite products. Using ZeoSyn, we develop a machine learning classifier to predict the resultant zeolite given a synthesis route with >70% accuracy. We employ SHapley Additive exPlanations (SHAP) to uncover key synthesis parameters for >200 zeolite frameworks. We introduce an aggregation approach to extend SHAP to all building units. We demonstrate applications of this approach to phase-selective and intergrowth synthesis. This comprehensive analysis illuminates the synthesis parameters pivotal in driving zeolite crystallization, offering the potential to guide the synthesis of desired zeolites. The dataset is available at https://github.com/eltonpan/zeosyn_dataset.

Rationalizing kinetic behaviors of isolated boron sites catalyzed oxidative dehydrogenation of propane

Boron-based catalysts exhibit high alkene selectivity in oxidative dehydrogenation of propane (ODHP) but the mechanistic understanding remains incomplete. In this work, we show that the hydroxylation of framework boron species via steaming not only enhances the ODHP rate on both B-MFI and B-BEA, but also impacts the kinetics of the reaction. The altered activity, propane reaction order and the activation energy could be attributed to the hydrolysis of framework [B(OSi≡)3] unit to [B(OSi≡)3-x(OH···O(H)Si≡)x] (x = 1, 2, "···" represents hydrogen bonding). DFT calculations confirm that hydroxylated framework boron sites could stabilize radical species, e.g., hydroperoxyl radical, further facilitating the gas-phase radical mechanism. Variations in the contributions from gas-phase radical mechanisms in ODHP lead to the linear correlation between activation enthalpy and entropy on borosilicate zeolites. Insights gained in this work offer a general mechanistic framework to rationalize the kinetic behavior of the ODHP on boron-based catalysts.

Synthesis and catalytic behavior of FCC catalysts obtained from kaolin by the in-situ method

In-situ zeolites NaY and Na[B]Y were synthesized on microspherical matrices of kaolin to obtain FCC catalysts. An alkaline pretreatment of the matrix was investigated in order to evaluate its effect on matrix properties and crystallization of the in-situ synthetized zeolites. Catalysts were characterized by SEM, TEM, Ar adsorption, XRD and NH3-TPD. It was observed an increase in the surface area and mesoporosity of the alkaline treated catalysts either synthetized with the presence of boron or with no boron in the hydrothermal reaction mixture. Ammonia TPD analyses have shown an increase in the amount and strength of the acidity of the catalysis with the zeolites crystallized on the pretreated matrices and exchanged with lanthanum ions. Thus, a combination between higher concentration of stronger acid sites and higher proportion of mesoporous generated in the matrices treated with alkaline solution had resulted in more active catalyst as shown by the triisopropylbenzene cracking experiments conducted here.

Graphical abstract

Modified reverse ADOR assembles Al-rich UTL zeolite from IPC-1P layers

Extra-large pore zeolites clear the way for the synthesis of fine chemicals and bulky compounds unable to enter the channels of medium- and large-pore zeolites.

Tuning supported Ni catalysts by varying zeolite Beta heteroatom composition: effects on ethylene adsorption and dimerization catalysis

The influence of zeolite heteroatom composition on the electron density and catalytic activity of a supported Ni cation is examined.

Dynamic Reorganization and Confinement of TiIV Active Sites Controls Olefin Epoxidation Catalysis on Two-Dimensional Zeotypes

The effect of dynamic reorganization and confinement of isolated Ti^IV catalytic centers supported on silicates is investigated for olefin epoxidation. Active sites consist of grafted single-site calix[4]arene-Ti^IV centers or their calcined counterparts. Their location is synthetically controlled to be either unconfined at terminal T-atom positions (denoted as type-(i)) or within confining 12-MR pockets (denoted as type-(ii); diameter ∼7 Å, volume ∼185 ų) composed of hemispherical cavities on the external surface of zeotypes with *-SVY topology. Electronic structure calculations (density functional theory) indicate that active sites consist of cooperative assemblies of Ti^IV centers and silanols. When active sites are located at unconfined type-(i) environments, the rate constants for cyclohexene epoxidation (323 K, 0.05 mM Ti^IV, 160 mM cyclohexene, 24 mM tert-butyl hydroperoxide) are 9 ± 2 M^-2 s^-1; whereas within confining type-(ii) 12-MR pockets, there is a ∼5-fold enhancement to 48 ± 8 M^-2 s^-1. When a mixture of both environments is initially present in the catalyst resting state, the rate constants reflect confining environments exclusively (40 ± 11 M^-2 s^-1), indicating that dynamic reorganization processes lead to the preferential location of active sites within 12-MR pockets. While activation enthalpies are Δ H^‡_app = 43 ± 1 kJ mol^-1 irrespective of active site location, confining environments exhibit diminished entropic barriers (Δ S^‡_app = -68 J mol^-1 K^-1 for unconfined type-(i) vs -56 J mol^-1 K^-1 for confining type-(ii)), indicating that confinement leads to more facile association of reactants at active sites to form transition state structures (volume ∼ 225 ų). These results open new opportunities for controlling reactivity on surfaces through partial confinement on shallow external-surface pockets, which are accessible to molecules that are too bulky to benefit from traditional confinement within micropores.

Boosting the turnover number of core–shell Al-ZSM-5@B-ZSM-5 zeolite for methanol to propylene reaction by modulating its gradient acid site distribution and low consumption diffusion

Hierarchical Al-ZSM-5@B-ZSM-5 core–shell zeolite was prepared to boost turnover number of MTP reaction.