The Hydrothermal Synthesis of Zeolites: History and Development from the Earliest Days to the Present Time

An overview is given of the selective catalytic reduction of NOx by ammonia (NH3‐SCR) over metal‐exchanged zeolites. The review gives a comprehensive overview of NH3‐SCR chemistry, including undesired side‐reactions and aspects of the reaction mechanism over zeolites and the active sites involved. The review attempts to correlate catalyst activity and stability with the preparation method, the exchange metal, the exchange degree, and the zeolite topology. A comparison of Fe‐ZSM‐5 catalysts prepared by different methods and research groups shows that the preparation method is not a decisive factor in determining catalytic activity. It seems that decreased turnover frequency (TOF) is an oft‐neglected effect of increasing Fe content, and this oversight may have led to the mistaken conclusion that certain production methods produce highly active catalysts. The available data indicate that both isolated and bridged iron species participate in the NH3‐SCR reaction over Fe‐ZSM‐5, with isolated species being the ...

https://www.tandfonline.com/doi/pdf/10.1080/01614940802480122

The State of the Art in Selective Catalytic Reduction of NOx by Ammonia Using Metal‐Exchanged Zeolite Catalysts

Catalysis for NOx abatement

Increasing demand for sustainable chemicals and fuels has pushed academia and industry to search for alternative feedstocks replacing crude oil in traditional refineries. As a result, an immense academic attention has focused on the valorisation of biomass (components) and derived intermediates to generate valuable platform chemicals and fuels. Zeolite catalysis plays a distinct role in many of these biomass conversion routes. This contribution emphasizes the progress and potential in zeolite catalysed biomass conversions and relates these to concepts established in existing petrochemical processes. The application of zeolites, equipped with a variety of active sites, in Bronsted acid, Lewis acid, or multifunctional catalysed reactions is discussed and generalised to provide a comprehensive overview. In addition, the feedstock shift from crude oil to biomass involves new challenges in developing fields, like mesoporosity and pore interconnectivity of zeolites and stability of zeolites in liquid phase. Finally, the future challenges and perspectives of zeolites in the processing of biomass conversion are discussed.

/pdf/potential-and-challenges-of-zeolite-chemistry-in-the-2jjml8ecsr.pdf

Potential and challenges of zeolite chemistry in the catalytic conversion of biomass

Copper-exchanged zeolites with mordenite structure mimic the nuclearity and reactivity of active sites in particulate methane monooxygenase, which are enzymes able to selectively oxidize methane to methanol. Here we show that the mordenite micropores provide a perfect confined environment for the highly selective stabilization of trinuclear copper-oxo clusters that exhibit a high reactivity towards activation of carbon–hydrogen bonds in methane and its subsequent transformation to methanol. The similarity with the enzymatic systems is also implied from the similarity of the reversible rearrangements of the trinuclear clusters occurring during the selective transformations of methane along the reaction path towards methanol, in both the enzyme system and copper-exchanged mordenite.

https://pure.uva.nl/ws/files/2473815/164930_Single_site_trinuclear_copper_oxygen_clusters.pdf

Single-site trinuclear copper oxygen clusters in mordenite for selective conversion of methane to methanol.

Siting of Al atoms in the framework T sites, in zeolite rings and channel/cavity system, and the distribution of Al atoms between single Al atoms and close Al atoms in various Al-O-(Si-O)n-Al sequences in Si-rich zeolites represent key parameters controlling properties of counter ion species. Framework Al siting and distribution is not random or controlled by simple rules and depends on the conditions of the zeolite synthesis. Al in Al-O-(Si-O)2-Al in one 6-MR and single Al atoms predominate in Si-rich zeolites and their population can be varied to a large extent. The siting and distribution of framework Al atoms dramatically affect catalytic activity/selectivity both of protonic and transition metal ion-containing zeolite catalysts.

Siting and Distribution of Framework Aluminium Atoms in Silicon-Rich Zeolites and Impact on Catalysis

Co2+ ion siting in pentasil-containing zeolites, part 3.

The Al siting in the ZSM-5 zeolite was investigated by 27Al 3Q MAS NMR spectroscopy and QM/MM calculations. It was found that the occupation of the framework T-sites by Al and the concentration of Al in these T-sites are neither random nor controlled by a simple rule. They both depend on the conditions of the zeolite synthesis. At least 12 out of the 24 distinguishable framework T-sites of ZSM-5 are occupied by Al in the set of the investigated zeolite samples. A partial identification of the Al sites is possible. The calculated 27Al NMR shielding values were converted to 27Al isotropic chemical shifts using the experimental isotropic chemical shift of 60.0 ppm referenced to the aqueous solution of Al(NO3)3 and the corresponding calculated NMR shielding of 490.0 ppm of a silicon rich (Si/Al 38) chabazite structure zeolite as a secondary internal standard. The observed 27Al isotropic chemical shifts of 50.0 and 54.7 ppm correspond to Al atoms in the T20 and T6 sites, respectively. The pair of measured isotropic chemical shifts of 52.9 and 53.7 ppm can be assigned to the T4, T8 pair. At the low-shielding end, two assignments are plausible. The smallest deviations between the calculated and observed isotropic chemical shifts are reached for the assignment as follows: T24 (64.8 ppm) is not occupied in the samples and that the observed isotropic chemical shifts 63.6, 62.8, and 60.0 ppm belong to T1, T17, and T7, respectively. It follows then that T-sites T12 (60.8 ppm), T3 (61.7 ppm), and T18 (62.0 ppm) are most likely not occupied by Al in our ZSM-5 samples. If we assume that the calculated isotropic chemical shifts are systematically larger than the observed ones then we can assign the largest observed isotropic chemical shifts of 63.6 and 62.8 ppm to the least shielded T24 and T1 sites, respectively, and 60.0 ppm to T12. Then the sites T3 (61.7 ppm), T18 (62.0 ppm), and T17 (62.5 ppm) would be unoccupied by Al in our ZSM-5 samples. It was further shown that there is no simple linear relationship between the observed 27Al isotropic chemical shifts and the average Al–O–Si angles.

https://www.jh-inst.cas.cz/~sklenak/stepan/publ/36.pdf

Aluminium siting in the ZSM-5 framework by combination of high resolution 27Al NMR and DFT/MM calculations

Activity of Co Ion Sites in ZSM-5, Ferrierite, and Mordenite in Selective Catalytic Reduction of NO with Methane

The occurrence of “Al pairs” in the framework of ZSM-5 and their spatial distribution in zeolites was estimated by using exchanged Co2+ ions as probes for “Al pairs”. The visible spectra of Co2+ ions were employed to monitor the distribution of Co2+ ions, and thus “Al pairs”, among the individual cationic sites, i.e. local framework arrangements at defined positions in the zeolite channels. The effects of concentration of aluminum in the framework and conditions of zeolite synthesis on the presence of “Al pairs” and their spatial distribution in the ZSM-5 framework were investigated. It has been shown that the distribution of Al atoms in the framework is not controlled by statistic rules, but depends on the conditions of zeolite synthesis and aluminum concentration in the framework.

Jiří Dědeček

Papers

Siting and Distribution of Framework Aluminium Atoms in Silicon-Rich Zeolites and Impact on Catalysis

Co2+ ion siting in pentasil-containing zeolites, part 3.

Aluminium siting in the ZSM-5 framework by combination of high resolution 27Al NMR and DFT/MM calculations

Activity of Co Ion Sites in ZSM-5, Ferrierite, and Mordenite in Selective Catalytic Reduction of NO with Methane

Co2+ ions as probes of Al distribution in the framework of zeolites. ZSM-5 study