Can Metal Promotion of SAPO-34 Genuinely Improve Its Catalytic Performance in Methanol Conversion to Light Olefins Reaction?

To genuinely assess the effect of secondary metal promotion on improving the SAPO-34 catalytic performance in MTO reaction, a broad spectrum of metals from different groups of the periodic table (alkali and alkaline earth metals, transition metals, rare earth metals, and basic metals) were investigated. Metals were added through a direct incorporation route with Me/Al2O3 molar ratio of 0.05. Some metals seamlessly incorporated into the SAPO-34 framework and replaced the Si and Al atoms, while others partially merged or even failed to be combined with SAPO and emerged as amorphous phases. Although, in some cases, the surface area of the metal-promoted samples increased due to enhanced nucleation rate and smaller particle formation, the majority of the promoted samples suffered from a severe loss in crystallinity that resulted in inferior catalytic performance. It was also illustrated that hydrogen co-feeding with methanol (H2/MeOH molar ratio of 1.5) at ambient pressure could extend the catalyst lifetime by 27 % due to hydrogenation and cracking of the coke species and improve the light olefins selectivity.

Synthesis and Crystallization Mechanism for SAPO-34 Zeolite Derived from Magadiite

In this work, we explored the hydrothermal synthesize and crystallization process of SAPO-34 zeolites from two-dimensional layered silicate magadiite by using tetraethylammonium hydroxide (TEAOH) as a templating agent. Comprehensive characterization was conducted by XRD, SEM, FTIR, Raman, and BET. Time-resolved PXRD analysis revealed that SAPO-34 zeolite exhibited a steep growth curve when the crystallization time was 30 h, and the crystallinity reached 98.65 % at 48 h. Specifically, the disruption of the magadiite layer exposed charged silanol groups on the surface, fostering an affinity for AlO4 and PO4 species, thereby initiating the nucleation process. Under the guidance of TEAOH, these nucleation sites transformed into SAPO-34 nuclei, gradually advancing towards crystallization. FTIR and Raman analyses affirmed the presence of 6Rs, followed by D6R and 4Rs SBUs, along with the characteristic CHA structure. Combined with 29Si NMR established that disaggregated silicate minerals served as zeolite synthesis "seeds", enhancing nucleation sites and overall crystallization efficiency.

Synthesis of SAPO-34 Nanoplates with High Si/Al Ratio and Improved Acid Site Density

Two-dimensional SAPO-34 molecular sieves were synthesized by microwave hydrothermal process. The concentrations of structure directing agent (SDA), phosphoric acid, and silicon in the gel solution were varied and their effect on phase, shape, and composition of synthesized particles was studied. The synthesized particles were characterized by various techniques using SEM, XRD, BET, EDX, and NH₃-TPD. Various morphologies of particles including isotropic, hyper rectangle, and nanoplates were obtained. It was found that the Si/Al ratio of the SAPO-34 particles was in a direct relationship with the density of acid sites. Moreover, the gel composition and preparation affected the chemistry of the synthesized particles. The slow addition of phosphoric acid improved the homogeneity of synthesis gel and resulted in SAPO-34 nanoplates with high density of acid sites, 3.482 mmol/g. The SAPO-34 nanoplates are expected to serve as a high performance catalyst due to the low mass transfer resistance and the high density of active sites.

The hydrothermal synthesis of hierarchical SAPO-34 with improved MTO performance

A two-step hydrothermal approach for the synthesis of hierarchical SAPO-34 in the absence of any mesoporogen.

CFD Modeling of Methanol to Light Olefins in a Sodalite Membrane Reactor using SAPO-34 Catalyst with In Situ Steam Removal

AIMS AND OBJECTIVE

In this work, the performance of a sodalite membrane reactor (MR) in the conversion of methanol to olefins (MTO process) was evaluated for ethylene and propylene production with in situ steam removal using 3-dimensional CFD (computational fluid dynamic) technique.

METHODS

Numerical simulation was performed using the commercial CFD package COMSOL Multiphysics 5.3. The finite element method was used to solve the governing equations in the 3- dimensional CFD model for the present work. In the sodalite MR model, a commercial SAPO-34 catalyst in the reaction zone was considered. The influence of key operation parameters, including pressure and temperature on methanol conversion, water recovery, and yields of ethylene, propylene, and water was studied to evaluate the performance of sodalite MR.

RESULTS

The local information of component concentration for methanol, ethylene, propylene, and water was obtained by the proposed CFD model. Literature data were applied to validate model results, and a good agreement was attained between the experimental data and predicted results using CFD model. Permeation flux through the sodalite membrane was increased by an increase of reaction temperature, which led to the enhancement of water stream recovered in the permeate side.

CONCLUSION

The CFD modeling results showed that the sodalite MR in the MTO process had higher performance in methanol conversion compared to the fixed-bed reactor (methanol conversion of 97% and 89% at 733 K for sodalite MR and fixed-bed reactor, respectively).

Recent Progress in Methanol-to-Olefins (MTO) Catalysts

Methanol conversion to olefins, as an important reaction in C1 chemistry, provides an alternative platform for producing basic chemicals from nonpetroleum resources such as natural gas and coal. Methanol-to-olefin (MTO) catalysis is one of the critical constraints for the process development, determining the reactor design, and the profitability of the process. After the construction and commissioning of the world's first MTO plant by Dalian Institute of Chemical Physics, based on high-efficiency catalyst and fluidization technology in 2010, more attention has been attracted for a deep understanding of the reaction mechanism and catalysis principle, which has led to the continuous development of catalysts and processes. Herein, the recent progress in MTO catalyst development is summarized, focusing on the advances in the optimization of SAPO-34 catalysts, together with the development efforts on catalysts with preferential ethylene or propylene selectivity.

Synthesis of sub-micrometric SAPO-34 by a morpholine assisted two-step hydrothermal route and its excellent MTO catalytic performance

SAPO-34 with a sub-micrometer crystal size was synthesized by a double hydrothermal treatment employing cost-effective morpholine as a structure directing agent, which presented an enhanced catalytic lifetime (nearly 3 times the conventional one) in the reaction of methanol to olefins with a higher light olefin selectivity (total selectivity of 97.1%). Detailed studies of the sample after different time intervals in the second crystallization with and without additional morpholine were carried out, which offered insight into crystal degradation and re-crystallization phenomena. The samples with different morpholine concentrations during the second hydrothermal treatment were also prepared, in which the sample with 80% MOR aqueous solution exhibited the smallest crystal size and the longest MTO lifetime. Furthermore, the investigation on the additional amount of mother liquor (from the first crystallization) required for the second crystallization showed that the presence of half the amount of the mother liquor (nutrients) could give us the required results effectively.

Assembly of Sub-Crystals on the Macroscale and Construction of Composite Building Units on the Microscale for SAPO-34

The nucleation and growth of SAPO-34 crystals with triethylamine (TEA) as a single template was monitored with ex situ time-resolved characterization methods. The investigation focused on the evolution of the intermediate phases at different crystallization stages of SAPO-34. The morphology transformation of the intermediate phases at different crystallization times revealed the unique self-assembly process of the sub-crystals. The cubic SAPO-34 crystals can be constructed from eight pyramidal subunits. Additionally, the construction order of cha cages and double-six-membered ring (d6r) units in the initial crystallization stage was determined. The appearance of cha cages prior to d6r units can be attributed to the structure-directing effect of protonated TEA, which is charge balanced with the negative charge of the framework from Si incorporation. Further analysis showed that Si species were incorporated into the framework by direct participation in the initial crystallization stage and substitution for framework P atoms during the later stage.

CTAB-assisted size controlled synthesis of SAPO-34 and its contribution toward MTO performance

SAPO-34 shows higher light olefin selectivity in the reaction of methanol to olefin (MTO), but its small pore system implies diffusion limitations to bigger molecular products and results in coking too.

Complex relationship between SAPO framework topology, content and distribution of Si and catalytic behaviour in the MTO reaction

Three small-pore silicoaluminophosphates containing relatively large cavities in their structure (LEV, LTA and SAV) have been hydrothermally synthesized with various silicon concentrations.

Conversion and coking of olefins on SAPO-34

Olefins' reactions lead to the formation of soluble and insoluble coke at the near-surface region of a SAPO-34 crystal.

Recent advances of the nano-hierarchical SAPO-34 in the methanol-to-olefin (MTO) reaction and other applications

The synthesis and catalytic applications of nano-hierarchical SAPO-34 for MTO conversion have received great significance.

Nanoseed-assisted synthesis of nano-sized SAPO-34 zeolites using morpholine as the sole template with superior MTO performance

Nano-sized SAPO-34 catalysts have been prepared using morpholine as the sole template via a nanoseed-assisted method showing superior MTO performance.

Insights into the catalytic cycle and activity of methanol-to-olefin conversion over low-silica AlPO-34 zeolites with controllable Brønsted acid density

The catalytic cycle and activity of methanol-to-olefin conversion over low-silica AlPO-34 zeolites can be effectively altered by changing the Brønsted acid density.

Pioneering In Situ Recrystallization during Bead Milling: A Top-down Approach to Prepare Zeolite A Nanocrystals

Top-down approach has been viewed as an efficient and straightforward method to prepare nanosized zeolites. Yet, the mechanical breaking of zeolite causes amorphization, which usually requires a post-milling recrystallization to obtain fully crystalline nanoparticles. Herein we present a facile methodology to prepare zeolite nanocrystals, where milling and recrystallization can be performed in situ. A milling apparatus specially designed to work under conditions of high alkalinity and temperature enables the in situ recrystallization during milling. Taking zeolite A as an example, we demonstrate its size reduction from ~3 μm to 66 nm in 30 min, which is quite faster than previous methods reported. Three functions, viz., miniaturization, amorphization and recrystallization were found to take effect concurrently during this one-pot process. The dynamic balance between these three functions was achieved by adjusting the milling period and temperature, which lead to the tuning of zeolite A particle size. Particle size and crystallinity of the zeolite A nanocrystals were confirmed by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and water adsorption-desorption. This work presents a pioneering advancement in this field of nanosized zeolites, and will facilitate the mass production as well as boost the wide applications of nanosized zeolites.

Organophosphorous surfactant-assistant synthesis of SAPO-34 molecular sieve with special morphology and improved MTO performance

SAPO-34s with special morphologies have been synthesized by using a self-designed organophosphorous surfactant and various microporous templates. The prepared SAPO-34s were evaluated by the MTO reaction, which showed improved catalytic performance.

Evolution of confined species and their effects on catalyst deactivation and olefin selectivity in SAPO-34 catalyzed MTO process

A “three sections, three periods” mechanism is proposed to discuss the formation and transformation of confined species and its effects on catalyst deactivation and product selectivity.

Insights into the growth of small-sized SAPO-34 crystals synthesized by a vapor-phase transport method

Under VPT conditions, a dry gel derived from a conventional hydrogel (or an ultrasonically treated hydrogel) can be converted into SAPO-34 crystals around 300–500 nm (or below 100 nm).

Seed-assisted, solvent-free synthesis of a CHA-type aluminophosphate molecular sieve

Triclinic AlPO₄-34 with CHA topology has been successfully synthesized under seed-assisted, solvent-free conditions.