Down the Microporous Rabbit Hole of Silicoaluminophosphates: Recent Developments on Synthesis, Characterization, and Catalytic Applications

Enhanced NH3-SCR activity of Cu-SAPO-34 by regulating Si distribution via an interzeolite conversion strategy

An interzeolite conversion (IZC) method was developed for the rapid synthesis of Cu-SAPO-34 from SAPO-37, achieving isolated Si distribution and optimized Cu states. The resulting Cu-SAPO-34 exhibited exceptional NH3-SCR performance, with over 90% NO conversion from 200-600 °C due to proper acidity and Cu status generated from the isolated Si.

Synthesis and Application of SAPO-11 Molecular Sieves Prepared from Reaction Gels with Various Templates in the Hydroisomerization of Hexadecane

Among the most selective catalytic systems for the hydroisomerization of C16+n-paraffins, catalytic systems based on SAPO-11 are quite promising. In order to increase the activity and selectivity of these bifunctional catalysts, it is necessary to reduce the diffusion restrictions for the reacting molecules and their products in the microporous structure of SAPO-11 by reducing the crystal size. To solve this problem, we have studied the influence of different templates (diethylamine, dipropylamine, diisopropylamine, and dibutylamine) on the physicochemical properties of reaction gels and SAPO-11 silicoaluminophosphates during their crystallization. Using XRD, SEM, and NMR techniques, we found that regardless of the template used, the reaction gel after the aging process at 90 °C is an AlPO4·2H2O hydroaluminophosphate. At the same time, the nature of the template affects the morphology and crystal sizes of the intermediate alumophosphate, AlPO4·2H2O, and the molecular sieves, SAPO-11. The acidic properties and the porous structure characteristics of SAPO-11 are also affected by the template. A template was proposed to enable the synthesis of nanoscale SAPO-11 crystals. The influence of the morphology and crystal size of SAPO-11 on the catalytic properties of a bifunctional catalyst based on SAPO-11 in the hydroisomerization of hexadecane was investigated.

Simulating Crystal Structure, Acidity, Proton Distribution, and IR Spectra of Acid Zeolite HSAPO-34: A High Accuracy Study

It is a challenge to characterize the acid properties of microporous materials in either experiments or theory. This study presents the crystal structure, acid site, acid strength, proton siting, and IR spectra of HSAPO-34 from the SCAN + rVV10 method. The results indicate: the crystal structures of various acid sites of HSAPO-34 deviate from the space group of R3¯; the acid strength inferred from the DPE value likely decreases with the proton binding sites at O(2), O(4), O(1),and O(3), contrary to the stability order in view of the internal energy; the calculated ensemble-averaged DPE is about 1525 kJ/mol at 673.15 K; and the proton siting and the proton distribution are distinctly influenced by the temperature: at low temperatures, the proton is predominantly located at O(3), while it prefers O(2) at high temperatures, and the proton at O(4) assumedly has the least distribution at 273.15-773.15 K. In line with the neutron diffraction experiment, a correction factor of 0.979 is needed to correct for the calculated hydroxyl stretching vibration (ν(O-H)) of HSAPO-34. It seems that the SCAN meta-GGA method, compensating for some drawbacks of the GGA method, could provide satisfying results regarding the acid properties of HSAPO-34.

In Situ Multinuclear Magic-Angle Spinning NMR: Monitoring Crystallization of Molecular Sieve AlPO4-11 in Real Time

Molecular sieves are crystalline three-dimensional frameworks with well-defined channels and cavities. They have been widely used in industry for many applications such as gas separation/purification, ion exchange, and catalysis. Obviously, understanding the formation mechanisms is fundamentally important. High-resolution solid-state NMR spectroscopy is a powerful method for the study of molecular sieves. However, due to technical challenges, the vast majority of the high-resolution solid-state NMR studies on molecular sieve crystallization are ex situ. In the present work, using a new commercially available NMR rotor that can withhold high pressure and high temperature, we examined the formation of molecular sieve AlPO₄-11 under dry gel conversion conditions by in situ multinuclear (¹H, ²⁷Al, ³¹P, and ¹³C) magic-angle spinning (MAS) solid-state NMR. In situ high-resolution NMR spectra obtained as a function of heating time provide much insights underlying the crystallization mechanism of AlPO₄-11. Specifically, in situ ²⁷Al and ³¹P MAS NMR along with ¹H → ³¹P cross-polarization (CP) MAS NMR were used to monitor the evolution of the local environments of framework Al and P, in situ ¹H → ¹³C CP MAS NMR to follow the behavior of the organic structure directing agent, and in situ ¹H MAS NMR to unveil the effect of water content on crystallization kinetics. The in situ MAS NMR results lead to a better understanding of the formation of AlPO₄-11.

Crystallization of SAPO-11 Molecular Sieves Prepared from Silicoaluminophosphate Gels Using Boehmites with Different Properties

In this article, we report the results of research the formation of silicoaluminophosphate gels under changing gel aging conditions and the influence of an aluminum source (boehmite), characterized by different properties. The samples of initial gels were characterized by XRF, X-ray diffraction, MAS NMR ²⁷Al and ³¹P, and scanning electron microscopy (SEM). Products of crystallization were characterized by XRF, X-ray diffraction, MAS NMR ²⁷Al and ³¹P, scanning electron microscopy (SEM), N₂-physical adsorption, and IR spectroscopy with pyridine adsorption. It has been established that the chemical and phase composition of aging gels and the products of further crystallization is conditioned by the size of the crystals and the porous structure of boehmite. Methods of management the morphology and secondary porous structure of SAPO-11, including the hierarchical porous structure, are proposed based on the use of boehmits characterized by different properties and changing the aging conditions of the initial gels. SAPO-based catalyst with a hierarchical porous structure showed excellent catalytic performance in dimerization of α-methylstyrene with a high degree of conversion and selectivity for linear isomers.

Investigation into the crystallization of molecular sieve DNL-6

Crystallization of DNL-6, a silicoaluminophosphate (SAPO) based molecular sieve with the RHO topology, was investigated under both the hydrothermal synthesis (HTS) and dry-gel conversion (DGC) conditions. Crystallization of DNL-6 under the HTS conditions is rather fast. But a combination of crystallization under the DGC conditions and reducing reaction temperature slow down the reactions, allowing for intermediates to be captured. Under the DGC conditions, DNL-6 crystallizes through a semi-crystalline layered phase. The nature of this intermediate is aluminophosphate (AlPO) rather than SAPO with most P atoms having a local environment of P(–O–Al)3(OH). The surfactant (cetyltrimethylammonium chloride) used for synthesis appears to be part of the layered intermediate. Si is directly incorporated in the DNL-6 framework via SM II mechanism when the semi-crystalline AlPO phase is transforming to DNL-6 with the assistance of a very small amount of water. Both the structure directing agent and the surfactant play a role in the formation of DNL-6, as they were found within the final synthesized products. SEM data show that hydrothermal synthesis produces a much more crystalline product. The facts that the semi-crystalline layered phase was also observed in the powder X-ray diffraction patterns of the solid samples obtained under the HTS conditions and that the evolution of the local structure around P and Al in the intermediate phases are similar imply that under the reaction conditions employed in the present study, the formation pathways of DNL-6 under the HTS and DGC conditions appear to have some similarities.

Carbene-like reactivity of methoxy groups in a single crystal SAPO-34 MTO catalyst

In situ synchrotron infrared microspectroscopy on single crystals of SAPO-34 reveals that a carbene insertion mechanism is responsible for the first carbon–carbon bond formation from surface methoxy groups.

Activation and conversion of alkanes in the confined space of zeolite-type materials

Microporous zeolite-type materials, with crystalline porous structures formed by well-defined channels and cages of molecular dimensions, have been widely employed as heterogeneous catalysts since the early 1960s, due to their wide variety of framework topologies, compositional flexibility and hydrothermal stability. The possible selection of the microporous structure and of the elements located in framework and extraframework positions enables the design of highly selective catalysts with well-defined active sites of acidic, basic or redox character, opening the path to their application in a wide range of catalytic processes. This versatility and high catalytic efficiency is the key factor enabling their use in the activation and conversion of different alkanes, ranging from methane to long chain n-paraffins. Alkanes are highly stable molecules, but their abundance and low cost have been two main driving forces for the development of processes directed to their upgrading over the last 50 years. However, the availability of advanced characterization tools combined with molecular modelling has enabled a more fundamental approach to the activation and conversion of alkanes, with most of the recent research being focused on the functionalization of methane and light alkanes, where their selective transformation at reasonable conversions remains, even nowadays, an important challenge. In this review, we will cover the use of microporous zeolite-type materials as components of mono- and bifunctional catalysts in the catalytic activation and conversion of C₁⁺ alkanes under non-oxidative or oxidative conditions. In each case, the alkane activation will be approached from a fundamental perspective, with the aim of understanding, at the molecular level, the role of the active sites involved in the activation and transformation of the different molecules and the contribution of shape-selective or confinement effects imposed by the microporous structure.

Investigation on the Promoter-Induced Rapid Non-Aqueous Media Synthesis of SAPO-35 and Methanol-to-Olefin Reaction

Microporous SAPO-35 molecular sieves (Levyne type) were synthesized in non-aqueous media by using different inorganic promoters (HClO₄ ^-, HF, H₃PO₄, and NaNO₃) to enhance the rate of crystallization, and the as-synthesized materials were characterized by using different methods such as powder X-ray diffraction spectroscopy (PXRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), magic angle spinning-nuclear magnetic resonance spectroscopy (MAS-NMR), Brunauer-Emmett-Teller (BET) analysis, and X-ray photoelectron spectroscopy (XPS). From PXRD patterns, it was found that all the materials have a highly crystalline nature without any other impurities. SEM images reveal rhombohedral particles in all synthesis conditions. The framework structure of the synthesized materials was identified by FT-IR spectroscopy, and it reveals that all materials gave a similar framework structure. From BET and XPS, we have confirmed that the pore size and pore diameters along with the elemental compositions have a minor change. The ²⁷Al, ³¹P, and ²⁹Si MAS-NMR spectra of all the promoter-based SAPO-35 materials are close to those of the standard SAPO-35 material. All the above characterization studies reveal the formation of SAPO-35 in a short time with promoters. The catalytic application studies of these synthesized materials for a methanol-to-olefin conversion reaction were performed, and the efficiency of these materials was found to be similar to that of standard materials.

Structure-directing study of 1-methylimidazolium-based dication with tetramethylene as spacer length in the synthesis of microporous silicoaluminophosphates

The molecular sieves, AFO, AEI and CHA-t, were synthesized using the 4BI cation as OSDAs. The crystallization of AEI and AFO phases was through in hydroxide media, while in the presence of fluoride induces the formation of triclinic chabazite (CHA-t).