Desilication of SAPO-34: Reaction Mechanisms from Periodic DFT Calculations

With the aim of understanding the desilication of SAPO-34, we compared three different reaction mechanisms for the hydrolysis of framework silicon by use of density functional theory (DFT) calculations. All three mechanisms are characterized by stepwise hydrolyses of Si–O–Al bonds. In the most favorable mechanism water molecules adsorb strongly to the Lewis acidic Al atoms neighboring the Si atom. Furthermore, evaluation of free energies reveals that an additional water molecule may catalyze the hydrolysis of the first Si–O–Al bond

Mechanistic Comparison of the Dealumination in SSZ-13 and the Desilication in SAPO-34

With the purpose of understanding the behavior of aluminosilicate zeolites and silicoaluminophosphates (SAPOs) in the presence of steam, we carried out a computational density functional theory (DFT) study on the desilication of SAPO-34. The mechanism studied was a stepwise hydrolysis of the four bonds to the Si heteroatom. An analogous process to the desilication of SAPO-34 is the dealumination of SSZ-13. To investigate possible mechanistic differences between the two processes, we compared the results of this study with the results of a previous study on dealumination in SSZ-13. We found that the intermediates along the dealumination path of SSZ-13 have one of the protons bonded to a bridging oxygen atom. In the corresponding intermediates of the desilication path in SAPO-34, the same proton prefers to be part of an aqua ligand coordinated to an Al atom. The principal factor determining the different proton locations is the electronic requirement of the atoms surrounding the proton. The different proton locations in SSZ-13 and SAPO-34 put clear conditions on possible mechanisms, thus causing them to be different for the two materials. We expect the principles determining the proton location also to be valid for other mechanisms of dealumination in SSZ-13 and desilication in SAPO-34

Mechanism of Si Island Formation in SAPO-34

With the aim of understanding the Si island formation in SAPO-34, we have carried out a computational mechanistic study. Briefly, the Si island formation in SAPO-34 is explained by three successive reactions. First, the framework Si atom is removed from the framework through the action of four water molecules. Second, the hydrogarnet defect generated by the desilication is healed by an available H₃PO₄ molecule. Third, the extra framework Si­(OH)₄ species inserts in the framework position of a phosphorus atom while, in a concerted fashion, “kicking out” the phosphorus atom as a H₃PO₄ extra-framework species. When these exchanges of framework and extra-framework species are repeated, the isolated Si atoms may eventually cluster into Si islands

Mechanistic Comparison of the Dealumination in SSZ-13 and the Desilication in SAPO-34

With the purpose of understanding the behavior of aluminosilicate zeolites and silicoaluminophosphates (SAPOs) in the presence of steam, we carried out a computational density functional theory (DFT) study on the desilication of SAPO-34. The mechanism studied was a stepwise hydrolysis of the four bonds to the Si heteroatom. An analogous process to the desilication of SAPO-34 is the dealumination of SSZ-13. To investigate possible mechanistic differences between the two processes, we compared the results of this study with the results of a previous study on dealumination in SSZ-13. We found that the intermediates along the dealumination path of SSZ-13 have one of the protons bonded to a bridging oxygen atom. In the corresponding intermediates of the desilication path in SAPO-34, the same proton prefers to be part of an aqua ligand coordinated to an Al atom. The principal factor determining the different proton locations is the electronic requirement of the atoms surrounding the proton. The different proton locations in SSZ-13 and SAPO-34 put clear conditions on possible mechanisms, thus causing them to be different for the two materials. We expect the principles determining the proton location also to be valid for other mechanisms of dealumination in SSZ-13 and desilication in SAPO-34

Defect Engineering: Tuning the Porosity and Composition of the Metal–Organic Framework UiO-66 via Modulated Synthesis

Presented in this paper is a deep investigation into the defect chemistry of UiO-66 when synthesized in the presence of monocarboxylic acid modulators under the most commonly employed conditions. We unequivocally demonstrate that missing cluster defects are the predominant defect and that their concentration (and thus the porosity and composition of the material) can be tuned to a remarkable extent by altering the concentration and/or acidity of the modulator. Finally, we attempt to rationalize these observations by speculating on the underlying solution chemistry

Kinetics of Zeolite Dealumination: Insights from H‑SSZ-13

When zeolite catalysts are subjected to steam at high temperatures, a permanent loss of activity happens, because of the loss of aluminum from the framework. This dealumination is a complex process involving the hydrolysis of four Al–O bonds. This work addresses the dealumination from a theoretical point of view, modeling the kinetics in zeolite H-SSZ-13 to gain insights that can extend to other zeolites. We employ periodic density functional theory (DFT) to obtain free-energy profiles, and we solve a microkinetic model to derive the rates of dealumination. We argue that such modeling should consider water that has been physisorbed in the zeolite as the reference state and propose a scheme for deriving the free energy of this state. The results strongly suggest that the first of the four hydrolysis steps is insignificant for the kinetics of zeolite dealumination. Furthermore, the results indicate that, in H-SSZ-13, it is sufficient to include only the fourth hydrolysis step when estimating the rate of dealumination at temperatures above 700 K. These are key aspects to investigate in further work on the process, particularly when comparing different zeolite frameworks

Detailed Structure Analysis of Atomic Positions and Defects in Zirconium Metal–Organic Frameworks

We report the structure of the Zr metal–organic frameworks (MOFs) UiO-66 and UiO-67 to very fine detail using synchrotron single-crystal X-ray diffraction and the synthesis method used to obtain single crystals. Zr terephthalate MOF UiO-66 is known to have missing linkers, and the nature of these are shown to be coordinating water and solvent molecules. Single crystals of the isoreticular material UiO-67 does not show such missing linker defects

Detailed Structure Analysis of Atomic Positions and Defects in Zirconium Metal–Organic Frameworks

We report the structure of the Zr metal–organic frameworks (MOFs) UiO-66 and UiO-67 to very fine detail using synchrotron single-crystal X-ray diffraction and the synthesis method used to obtain single crystals. Zr terephthalate MOF UiO-66 is known to have missing linkers, and the nature of these are shown to be coordinating water and solvent molecules. Single crystals of the isoreticular material UiO-67 does not show such missing linker defects

Shape Selectivity in the Conversion of Methanol to Hydrocarbons: The Catalytic Performance of One-Dimensional 10-Ring Zeolites: ZSM-22, ZSM-23, ZSM-48, and EU-1

The methanol-to-hydrocarbon (MTH) reaction, a process in which low-value carbon-rich feedstocks are converted to value-added petrochemical products, is studied over one-dimensional 10-ring zeolites: ZSM-22 (TON), ZSM-23 (MTT), ZSM-48 (*MRE), and EU-1 (EUO). The latter three are little studied as MTH catalysts and were expected to display interesting product-shape-selective properties. The influence of slight differences in channel systems of the materials (size and shape) on product distribution and stability is investigated under various reaction conditions. In addition, the influence of coke deposition on product selectivity is investigated. Temperatures between 350 and 500 °C and WHSV between 2 and 6 g g^–1 h^–1 are investigated using a fixed bed reactor. The products are analyzed using online GC, and hydrocarbons trapped in the channels of the material during the reaction were liberated using the standard HF dissolution procedure and analyzed using GC/MS. Despite the small differences in the channel shape and dimension, the materials displayed very different product spectra. The catalysts converted comparable amounts of methanol before complete deactivation at their optimum MTH condition. Except for EU-1, all the catalysts gave high selectivity for hydrocarbons in the boiling range of gasoline fuel, C₅₊ fraction. Unlike ZSM-22 and ZSM-23, the EU-1 and ZSM-48 catalysts displayed notable amounts of aromatics in their C₅₊ fraction. Such compounds are good octane boosters. However, because of environmental problems, there are limits on aromatics in gasoline. For ZSM-22, ZSM-23, and EU-1 catalysts, the deposition of coke within the channels does not affect the selectivity. Rather, the change in selectivity with reaction time can be regarded as a change in contact time. The involvement of the 12-ring side pocket of EU-1 zeolites for the MTH reaction is indicated both by the unexpected catalytic behavior and by analysis of retained species within the pore structure

Effect of Benzoic Acid as a Modulator in the Structure of UiO-66: An Experimental and Computational Study

The identification and quantification of defects are undoubtedly thorough challenges in the characterization of “defect-engineered” metal–organic frameworks (MOFs). UiO-66, known for its exceptional stability and defect tolerance, has been a popular target for defect-engineering studies. Herein, we show that synthesizing UiO-66 in the presence of an excess of benzoic acid is a reliable method for obtaining UiO-66 samples with a very high concentration of missing-cluster defects, allowing one to modulate specific properties (i.e., surface area and hydrophobicity). This was elucidated by a multitechnique marriage of experimental and computational methods: a combination of PXRD, dissolution/¹H NMR spectroscopy, and N₂ sorption measurements was used to quantify the defect loading, while vibrational spectroscopies (FTIR and Raman) allowed us to unequivocally identify the defect structure by comparison with DFT-simulated spectra and visual analysis of the computed vibrational modes