Methane-to-chemicals: a pathway to decarbonization.

The utilization of methane for chemical production, often considered as the future of petrochemistry, historically could not compete economically with conventional processes due to higher investment costs. Achieving sustainability and decarbonization of the downstream industry by integration with a methane-to-chemicals process may provide an opportunity to unlock the future for these technologies. Gas-to-chemicals is an efficient tool to boost the decarbonization potential of renewable energy. While the current implementation of carbon capture utilization and storage (CCUS) technologies is of great importance for industrial decarbonization, a shift to greener CO2-free processes and CO2 utilization from external sources for manufacturing valuable goods is highly preferred. This review outlines potential options for how a methane-to-chemicals process could support decarbonization of the downstream industry.

Visualizing the Flexibility of RHO Nanozeolite: Experiment and Modeling.

Structural flexibility is an intrinsic feature of zeolites, and the characterization of such dynamic behavior is key to maximizing their performance and realizing their potential in both existing and emerging applications. Here, the flexibility of a high-aluminum nano-sized RHO zeolite is directly visualized with in situ TEM for the first time. Variable temperature experiments directly observe the physical expansion of the discrete nanocrystals in response to changes in both guest-molecule chemistry (Ar vs CO2) and temperature. The observations are complemented by operando FTIR spectroscopy verifying the nature of the adsorbed CO2 within the pore network, the desorption kinetics of carbonate species, and changes to the structural bands at high temperatures. Quantum chemical modeling of the RHO zeolite structure substantiates the effect of cation (Na+ and Cs+) mobility in the absence and presence of CO2 on the flexibility behavior of the structure. The results demonstrate the combined influences of temperature and CO2 on the structural flexibility consistent with the experimental microscopy observations.

Interplay between alkali-metal cations and silanol sites in nanosized CHA zeolite and implications for CO2 adsorption.

Silanols are key players in the application performance of zeolites, yet, their localization and hydrogen bonding strength need more studies. The effects of post-synthetic ion exchange on nanosized chabazite (CHA), focusing on the formation of silanols, were studied. The significant alteration of the silanols of the chabazite nanozeolite upon ion exchange and their effect on the CO2 adsorption capacity was revealed by solid-state nuclear magnetic resonance (NMR), Fourier-transform infrared (FTIR) spectroscopy, and periodic density functional theory (DFT) calculations. Both theoretical and experimental results revealed changing the ratio of extra-framework cations in CHA zeolites changes the population of silanols; decreasing the Cs+/K+ ratio creates more silanols. Upon adsorption of CO2, the distribution and strength of the silanols also changed with increased hydrogen bonding, thus revealing an interaction of silanols with CO2 molecules. To the best of our knowledge, this is the first evidence of the interplay between alkali-metal cations and silanols in nanosized CHA.

Exploration, explanation and exploitation of hydroxyls in zeolites.

The precise location and role of all types of hydroxyls in zeolites are still enigmatic, and their control permits tailoring of novel properties increasing the efficiency of catalysts and adsorbents in industrial processes for cleaner energy.

The role of mixed alkali metal cations on the formation of nanosized CHA zeolite from colloidal precursor suspension.

A clear understanding of the crystal formation pathways of zeolites remains one of the most challenging issues to date. Here we investigate the synthesis of nanosized chabazite (CHA) zeolites using organic template-free colloidal suspensions by varying the time of aging at room temperature and the time of hydrothermal treatment at 90 °C. The role of mixed alkali metal cations (Na+, K+, Cs+) on the formation of CHA in the colloidal suspensions was studied. Increasing the aging time of the precursor colloidal suspension from 4 to 17 days resulted in faster crystallization of CHA nanocrystals (3 h instead of 7 h at 90 °C) to afford significantly smaller particles (60 nm vs 600 nm). During the crystallization a considerable change in the content of inorganic cations in the recovered solid material was observed to coincide with the formation of the CHA nanocrystals. The Na+ cations were found to direct the formation of condensed and pre-shaped aluminosilicate particles in the colloidal precursor suspensions, while K+ cations facilitated the formation of secondary building units (SBUs) of the CHA type framework structure such as d6r and cha cages, and the Cs+ cations promoted the long-range crystalline order facilitating the crystallization of stable zeolite nanocrystals.

Synthesis of Discrete CHA Zeolite Nanocrystals without Organic Templates for Selective CO2 Capture.

Small-pore zeolites such as chabazite (CHA) are excellent candidates for the selective separation of CO2 ; however, the current synthesis involves several steps and the use of organic structure-directing agent (OSDA), increasing their cost and energy requirements. We report the synthesis of small-pore zeolite crystals (aluminosilicate) with CHA-type framework structure by direct synthesis in a colloidal suspension containing a mixture of inorganic cations only (Na+ , K+ , and Cs+ ). The location of CO2 molecules in the host structure was revealed by 3D electron diffraction (3D ED). The high sorption capacity for CO2 (3.8 mmol g-1 at 121 kPa), structural stability and regenerability of the discreate CHA zeolite nanocrystals is maintained for 10 consecutive cycles without any visible degradation. The CHA zeolite (Si:Al=2) reaches an almost perfect CO2 storage capacity (8 CO2 per unit cell) and high selectivity (no CH4 was adsorbed).

Emphasis on the Properties of Metal-Containing Zeolites Operating Outside the Comfort Zone of Current Heterogeneous Catalytic Reactions.

The development of catalysts that can operate under exceptionally harsh and unconventional conditions is of critical importance for the transition of the energy and chemicals industries to low-emission and renewable chemical feedstocks. In this review we will highlight materials and more specifically metal-containing zeolite catalysts that have been tested under harsh reaction conditions such as high temperature light alkane conversion and biomass valorization. Particular attention will be given to studies that explore the stability and recyclability of metal-containing zeolite catalysts operating in continuous modes. Metal-containing zeolites are considered as an important class of catalysts operating outside the comfort zone of current heterogeneous catalytic reactions in both gas and liquid phase reactions. The relationship between the properties of the metal-containing zeolite and catalytic performance will be explored.

Novel Strategy for the Synthesis of Ultra-Stable Single-Site Mo-ZSM-5 Zeolite Nanocrystals.

The current energy transition presents many technological challenges, such as the development of highly stable catalysts. Herein, we report a novel "top-down" synthesis approach for preparation of a single-site Mo-containing nanosized ZSM-5 zeolite which has atomically dispersed framework-molybdenum homogenously distributed through the zeolite crystals. The introduction of Mo heals most of the native point defects in the zeolite structure resulting in an extremely stable material. The important features of this single-site Mo-containing ZSM-5 zeolite are provided by an in-depth spectroscopic and microscopic analysis. The material demonstrates superior thermal (up to 1000 °C), hydrothermal (steaming), and catalytic (converting methane to hydrogen and higher hydrocarbons) stability, maintaining the atomically disperse Mo, structural integrity of the zeolite, and preventing the formation of silanols.

Electrochemical investigation of [Co4(µ3-O)4(µ-OAc)4(py)4] and peroxides by cyclic voltammetry.

Two oxidative redox processes of the neutral cobalt(iii) cubane, [Co4(µ3-O)4(µ-OAc)4(py)4], were investigated by cyclic voltammetry at a glassy carbon electrode in acetonitrile. In addition to the first quasi-reversible one-electron oxidation at E1/2 = 0.283 V vs. Fc0/+, a second quasi-reversible one-electron oxidation was observed at E1/2 = 1.44 V vs. Fc0/+. Oxidation at this potential does not facilitate water oxidation. In the presence of tert-butylhydroperoxide the peak current of this second oxidation increases, suggesting oxidation of the peroxide by the doubly oxidised cubane.

Molecular Cobalt Clusters as Precursors of Distinct Active Species in Electrochemical, Photochemical, and Photoelectrochemical Water Oxidation Reactions in Phosphate Electrolytes.

Three cobalt model molecular compounds, Co-cubane ([Co4 (µ3 -O)4 (µ-OAc)4 py4 ]), Co-trimer ([Co3 (µ3 -O)(µ-OAc)6 py3 ]PF6 ), and Co-dimer ([Co2 (µ-OH)2 (µ-OAc)(OAc)2 py4 ]PF6 ), are investigated as water oxidation reaction (WOR) catalysts, using electrochemical, photochemical, and photoelectrochemical methodologies in phosphate electrolyte. The actual species contributing to the catalytic activity observed in the WOR are derived from the transformation of these cobalt compounds. The catalytic activity observed is highly dependent on the initial compound structure and on the particular WOR methodology used. Co-cubane shows no activity in the electrochemical WOR and negligible activity in the photochemical WOR, but is active in the photoelectrochemical WOR, in which it behaves as a precursor to catalytically active species. Co-dimer also shows no activity in the electrochemical WOR, but behaves as a precursor to catalytically active species in both the photochemical and photoelectrochemical WOR experiments. Co-trimer behaves as a precursor to catalytically active species in all three of the WOR methodologies.