MTO Reaction Research Articles

The effect of hierarchical pore structure SAPO-34 catalyst on the diffusion and reaction behavior in MTO reaction

Establishing the hierarchical pore structure is considered as a feasible approach for enhancing diffusion and improve catalytic efficiency of SAPO-34 zeolite in the methanol-to-olefin process. Nevertheless, under reaction conditions, the correlation among the diffusion behavior in the crystal, coke deposition behavior and catalytic performance of SAPO-34 catalysts with varying pore structures remains to be clarified. Herein, we revealed the structure–activity relationship between the hierarchical pore structure, the catalytic performance and the anti-coke deposition performance of the catalyst in the MTO reaction via the synergy of reaction–diffusion experiments and molecular simulation, then the mechanism of the hierarchical pore SAPO-34 catalyst to eliminate the diffusion limit and improve the accessibility of acid sites was clarified. The results demonstrated that the establishment of a hierarchical pore structure strengthened the diffusion of reactant methanol in the pores, leading to the diffusion resistance being reduced by 5 times and the diffusion coefficient being improved by two orders of magnitude. The key reaction rate-determining step shifted from micropores diffusion to the adsorption of acid sites in hierarchical pores, and the catalytic efficiency of the catalyst was increased by 3 times. Furthermore, with the establishing hierarchical pore technique, the expansion of the accommodation space for coke deposition led to a reduction in the blocking of pores due to coke deposition, particularly around the orifices. Moreover, the accessibility of the reactant molecules to the acid sites within the pores was enhanced, resulting in a balanced ratio between pore blockage and acid site coverage, thereby improving the anti-coke deposition performance of the SAPO-34 catalyst. This work can provide important foundational data and theoretical guidance for the study of reaction–diffusion synergistic interaction in zeolite catalysis reactions.

Controlling product selectivity and catalyst lifetime by altering acid strength, cavity size of SAPO, and diffusion rate of methanol in the MTO reaction: DFT and MD calculations.

The initiation mechanisms of the MTO process over silicoaluminophosphate (SAPO) catalysts with zeolite-like structures using first-principles calculations have been investigated. The supramolecular system of silicoaluminophosphates consisting of inorganic cages with Brønsted acid sites and trapped organic compounds was used as a catalyst in the MTO reaction. To study the structure-property relationship in more detail, the effect of acidity and cage size of different types of SAPOs (SAPO-18, SAPO-34, and SAPO-17 with CHA, AEI, and ERI structures, respectively) in the aromatic cycle of hydrocarbon pool mechanism was investigated. The differences in reaction barriers can be explained by the cage size, pore topology, and environment of framework protons of materials. Product selectivity was controlled by using cavity-type zeolite, the steric constraint of the cavity for the formation of critical intermediates, and acidic strength. The results show that ethylene selectivity increases as the cavity size decreases, and the elliptical pore size of the structures decreases, thereby decreasing the acidity of the zeolite structure, leading to an increase in propylene selectivity. SAPO-18 exhibits the longest reaction lifetime and has the highest amount of carbonaceous material after reaction completion. SAPO-17 with small pore and cavity size is selective to ethylene, although it shows a rapid catalyst deactivation rate.

An investigation on the effect of cross-linking state of silicon species on the distribution of framework Al atoms in ZSM-5 zeolite and its catalytic performance for MTO reaction

Controllable regulation of Al atoms distribution in ZSM-5 zeolite framework can effectively modulate product selectivity for MTO reaction. Here, the Al atoms distribution of ZSM-5 zeolite can be adjusted by controlling the cross-linking state of silicon species. Detailed explorations show that, the surface of ZSM-5 zeolite is rich in silicon when low cross-linking state silicon species is silicon source, while the surface of ZSM-5 zeolite is rich in aluminum when high cross-linking state silicon species is silicon source. When the cross-linking state of silicon species presents a “gripper-like” states, Al atoms of ZSM-5 zeolites are homogeneously distributed, and thus, Al atoms tends to be distributed in straight or sinusoidal channels, resulting in high olefin selectivity and low double cycle index in the catalytic MTO reaction process. This work would be beneficial to the design of ZSM-5 zeolite catalyst with desirable acid site distribution for specific reactions.

Optimal mixing method of ZnZrOx and MOR-type zeolite to prepare a bifunctional catalyst for CO2 hydrogenation to lower olefins

In recent years, the issue of global warming caused by CO2 emissions has become a critical problem and poses a worldwide challenge. To address this issue, we have developed a bifunctional catalyst that can convert CO2 to lower olefins in a single stage. Our bifunctional catalysts are a combination of two catalysts, a CO2-to-methanol hydrogenation catalyst (Zn-doped ZrO2, named ZnZrOx) and a methanol-to-olefin catalyst (MOR-type zeolite, named MOR104). In this research, we examined the impact of various mixing modes of the two catalysts on product distribution. We tested four different mixing modes using a down-flow fixed bed reactor: (a) ZnZrOx in the upper layer and MOR104 in the lower layer, (b) catalysts were mixed randomly after being pelletized separately, (c) MOR104 in the upper layer and ZnZrOx in the lower layer, and (d) granulated catalyst produced by physically mixing both catalyst powders. Our findings indicated that the best performance was achieved with catalyst (d), where the two catalysts were mixed in close proximity. This proximity resulted in efficient supply of methanol produced on ZnZrOx to MOR104. In other words, the MTO reaction in MOR104 efficiently consumed methanol molecules produced via equilibrium-limited CO2-to-methanol hydrogenation. When ZnZrOx and MOR104 were thoroughly mixed, the conversion of CO2 to methanol shifted towards the product side, resulting in a greater overall utilization of CO2. Furthermore, the bifunctional catalyst we developed was stable for six hours. Since there have been few studies of bifunctional catalysts containing zeolites other than ZSM-5 and SAPO-34, this study opens up new opportunities for bifunctional catalysts specialized for one-pass hydrocarbon synthesis through CO2 hydrogenation.

Controlling the SAPO-34 Particle Size to Enhance Reaction-Diffusion Behavior for Improving Catalytic Efficiency in the MTO Reaction

The SAPO-34 zeolite as an important catalyst is widely used in the methanol-to-olefin reaction, but understanding the internal diffusion behavior of its reactant methanol is still a challenge. In this work, a series of SAPO-34 catalysts with different particle sizes were prepared, among which the thin-layer SAPO-34 zeolite could be synthesized when the ratio of TEAOH/Mor is 8:1, which exhibits a high selectivity of 95% for light olefins and a long lifetime of 410 min. Through 29Si NMR and N2 physical adsorption–desorption, it indicates that more Si(4Al) coordination structures and a larger external volume jointly determine the stability of SAPO-34. Combining the TG and BET results of deactivated catalysts, it was found that the main reason for deactivation is the micropore blocked by coke deposition. The kinetic results demonstrated that the particle size of the zeolite is inversely proportional to its apparent reaction rate. The calculation of the Thiele modulus shows that the intramolecular diffusion rate of the small particle zeolite is faster. The catalytic efficiency can be enhanced by reducing the particle size due to the partly removed internal diffusion and the improved utilization of acid sites.

Combining hierarchization and Mg2+ ions insertion in ZSM-5: acidity-modulation effect on MTO reaction

Acid catalysis is recognised as the most reliable industrial ì way to convert hydrocarbons into various products (methanol to olefins and aromatics, glycerol to olefins, methane dehydroaromatization, isomerization, etc.). In this concern, ZSM-5 is extensively used, also because of peculiar type of active sites of such zeolite. In this paper, a fine tuning of the acidity content and strength of ZSM-5 was investigated through a combination of post-synthesis treatments: desilication via alkaline solution and wet impregnation with Mg2+ ions followed by calcination. Results show that the insertion of Mg2+ has a large influence on the distribution and strength of Brønsted and Lewis acid sites of the hierarchical ZSM-5, as well as an improved crystallinity with respect to the desilicated-one. The hierarchical ZSM-5 zeolite incorporating Mg-species was tested in Methanol-to-olefins reaction and the reduction of acid strength of catalyst increased the selectivity toward light olefins (mainly propylene) as well as it suppressed oligomerisation reactions delaying the coke formation.

Dimethyl Ether to Olefins on Hybrid Intergrowth Structure Zeolites

A series of catalysts based on hybrid intergrowth structure zeolites MFI-MEL, MFI-MTW, and MFI-MCM-41 are studied in the reaction of olefins synthesis from dimethyl ether at atmospheric pressure and a temperature of 340 °C. The total acidity of hybrid zeolite-based catalysts is shown to correlate with their activity. However, the use of zeolite with the structure MFI-MCM-41, which is characterized by a high content of medium acid sites, additionally catalyzes the methanol dehydration reaction, resulting in a decrease in the observed DME conversion. The obtained product distributions are brought into correlation with the texture of catalysts. It is shown that the use of hybrid zeolites does not change the mechanism of reaction, but the structural features of zeolites influence the priority of the competing MTO reactions: high ethylene yield is observed for catalysts with high micropore volume. The topology of the hybrid zeolite has been shown to influence the hydrogen transfer reaction rate, but not to change the isomerizing activity of the catalyst.

Tailoring structure and acidity of ZSM-5 zeolite by algae carbon modification: Promoting effect in the MTO reaction

A biochar obtained from algae (Trachydiscus minutus, TRA) with rich surface functional groups and metal ions was used as a hard template to efficiently synthesize modified ZSM-5 zeolites with high external surface area (ESA) and mild acidity. Compared with pristine ZSM-5, which exhibited an ESA of 121 m2/g, TRA/ZSM-5 modified with algal carbon yielded an ESA of 171 m2/g. In addition, the catalytic lifetime of TRA/ZSM-5 zeolite in the methanol-to-hydrocarbons (MTH) reaction was also greatly improved compared with H-ZSM-5: TRA/ZSM-5 maintained a methanol conversion >95% during 16 h (at 400 °C, WHSV = 8 h−1), whilst H-ZSM-5 could only last for 10 h. After further optimization, the lifespan of TRA/ZSM-5 could be further enhanced to 24 h. Finally, it could be shown that the presence of hydrophilic functional groups and heteroelements were the key parameters to enhance the ESA of the zeolite and reduce its Brønsted acidity.

Efficient synthesis of high-silica SSZ-13 zeolite and its catalytic performance in MTO reaction

Highly efficient synthesis of zeolites is an important goal given the many catalytic applications of zeolites in industrial chemical processes. The conventional zeolite syntheses, i.e., hydrothermal synthesis in a batch setup are usually challenged by certain limiting factors deriving from the long synthesis period or the low yield. Herein, we report that a combined strategy of organic promoter and zeolite seeding can remarkably improve the synthetic efficiency of high silica SSZ-13 zeolite. The low-cost promoter diethylamine (DEA) and zeolite seeds could both remarkably accelerate the crystallization rate of SSZ-13 zeolite, and DEA has a much higher beneficial effect on the solid yield as compared to the zeolite seeds. Consequently, the crystallization time could be unprecedentedly shorted by a factor of 30, instead of multiple days normally required. Besides, this novel protocol also exhibits high atom efficiency (93% of yield). The resultant high-silica nano-sized SSZ-13 zeolite shows good (hydro)thermal stability along with excellent catalytic performance for the MTO reaction.

Synthesis of hierarchical ZSM‐5 microspheres with superior performance for catalytic methanol‐to‐olefin conversion

AbstractMethanol‐to‐olefin (MTO) conversion on zeolites has encountered severe coke deposition and rapid deactivation. Creating different levels of porosity is essential to mitigate such issues. Herein, we demonstrate a facile and green strategy to synthesize uniform and hierarchically macro/mesoporous ZSM‐5 microspheres by combining spray‐freeze drying and steaming‐assisted crystallization (SAC). The structure, crystallinity, and porosity of the zeolite microspheres are controlled by adjusting the water/gel mass ratio and time in the SAC process. The structure evolution during the SAC process is revealed. In the catalytic MTO reaction, the representative hierarchically porous ZSM‐5 catalyst exhibits superior catalytic performance. At a very high weight hourly space velocity of 18 h−1, it shows a dramatically prolonged lifetime (47 h at >99% conversion) and much‐improved selectivity to ethylene and propylene compared with the conventional microporous ZSM‐5 and nano‐sized ZSM‐5. The enhanced performance is originated from the hierarchical structure and suitable acidity of the ZSM‐5 microspheres.

Effect of silica source on zeolite MFI crystallization in fluoride media and its physicochemical and catalytic properties

A series of MFI zeolites were synthesized in fluoride media from fumed silica hydrogel, fumed silica and TEOS to elucidate the influence of silica source on the physicochemical and catalytic properties of MFI zeolites. The materials obtained were characterized by nitrogen adsorption, XRD, SEM, TPD and 27Al NMR MAS methods and evaluated in MTO and hexane/3-methyl pentane cracking catalytic reactions. It has been demonstrated that crystallization proceeds via solution-mediated mechanism independently on silica source. However, the nature of silica source substantially affects the crystallization rate, the crystal size, the dynamics of Al incorporation into the MFI framework and Al distribution in the zeolite crystals. The application of fumed silica hydrogel and fumed silica with high polymerization degree yields crystals with an Al-enriched core and a siliceous shell. These materials demonstrate faster deactivation in the MTO reaction and higher selectivity to ethylene as compared to the MFI obtained from TEOS with a uniform Al distribution in the crystals. The effect observed is attributed to a longer diffusion path length of the reagent to active sites located mainly in the crystal core and the reaction products outside from the crystal core, resulting in a higher contribution of secondary reactions.

CFD simulation of an industrial MTO fluidized bed by coupling a population balance model of coke content

The coke distribution over shape-selective catalysts in fluidized bed reactors is critical to the selectivity of target products. A population balance model (PBM) is developed in this study to describe the evolution of coke distribution due to both reactions and circulation of catalysts. The PBM is coupled with multiscale computational fluid dynamics (CFD) to simulate a methanol-to-olefins fluidized bed reactor. A wide, uneven distribution of coke content can thus be predicted. The mass fractions of the desired products, ethylene and propylene, show better agreement with the experimental data than those without PBM. And the selectivity ratio of ethylene to propylene is also better predicted. Furthermore, the relaxation time of fresh catalyst is considered to express the effects of the induction period in MTO reactions. The reasonable prediction suggests that coupling PBM with multiscale CFD is helpful to understand MTO reactors and may find more application with respect to its optimization and scale-up.

Robust CON-type zeolite nanocatalyst in methanol-to-olefins reaction: downsizing, recrystallisation and defect-healing treatments toward prolonged lifetime

A CON-type zeolite underwent bead-milling, recrystallisation and defect-healing treatments to yield highly crystalline zeolite nanoparticles, showing prolonged lifetime in MTO reactions.

Rapid synthesis of hierarchical nanosized SSZ-13 zeolite with excellent MTO catalytic performance

An efficient approach for the rapid synthesis of nanosized SSZ-13 zeolite with hierarchical structure was rationally developed by the combination of additive and seed induced crystallization. Compared with the conventional hydrothermal synthesis, the addition of additive and seed during the synthesis of SSZ-13 could significantly shorten the crystallization time from 144 h to 6 h. The obtained nanosized SSZ-13 with average crystal size of ca. 400 nm possessed abundant mesopores within the crystals. The crystal size could also be regulated in the range of 400–6000 nm by adjusting the crystallization time and the type and amount of the additives. UV–Vis results showed that the strong chelation ability of citric acid to Al ions as well as the formation of new metastable phase accelerated the crystallization rate in the synthesis of SSZ-13. The fast-synthesized hierarchical nanosized SSZ-13 exhibited prolonged catalytic lifetime and improved selectivity to light olefins in comparison with the conventional SSZ-13 catalyst in the MTO reaction. This work not only provides a simple and facile approach for the rapid synthesis of hierarchical nanosized aluminosilicate zeolite, but also might help to develop the fast synthesis methods for other aluminosilicate zeolites. • Rapid synthesis of hierarchical nanosized SSZ-13 with the addition of additive and seed. • SSZ-13 crystal size could be regulated in the range of 400–6000 nm. • Hierarchical nanosized SSZ-13 exhibited superior MTO catalytic performance.

Metal–organic framework-assisted synthesis of Zr-modified SAPO-34 zeolites with hierarchical porous structure for the catalytic transformation of methanol to olefins

Zr-Modified SAPO-34 zeolites with hierarchical porous structure were synthesized in the presence of UiO-66. The Zr0.50-SAPO-34 zeolites exhibited long lifetimes (>1500 min) and excellent selectivity for light olefins (98.66%) in the MTO reaction.

Controlling Sized Sapo-34 Zeolite to Enhance Reaction-Diffusion of Methanol for Improving Catalytic Efficiency in Mto Reaction

Controlling Sized Sapo-34 Zeolite to Enhance Reaction-Diffusion of Methanol for Improving Catalytic Efficiency in Mto Reaction

Efficient Synthesis of High-Silica Ssz-13 Zeolite and its Catalytic Performance in Mto Reaction

Efficient Synthesis of High-Silica Ssz-13 Zeolite and its Catalytic Performance in Mto Reaction

Efficient Synthesis of High-Silica Ssz-13 Zeolite and its Catalytic Performance in Mto Reaction

Efficient Synthesis of High-Silica Ssz-13 Zeolite and its Catalytic Performance in Mto Reaction

Catalytic and deactivated behavior of SAPO-34/ZSM-5 composite molecular sieve synthesized by in-situ two-step method

This work aims at revealing the reaction and carbon behavior of SAPO-34/ZSM-5 composite molecular sieve (SZ-TS) in MTO reaction, which is synthesized via in situ two-step crystallization method. Several tests are used to explore the structure–activity relationship of the SZ-TS. The results show that the strength of weak and medium-strong acid is suitable, the particle size of two-phase molecular sieve is decreased, the hierarchical pore structure with abundant mesoporous is formed. By using the SZ-TS in MTO reaction, the methanol conversion and light olefins selectivity are 98% and 92.7%, respectively. These results can be attributed to the interface phase effect produced by etching and dissociation on the skeleton of the nuclear phase SAPO-34. The result of in situ IR and analysis of carbon deactivation indicate that polymethylbenzene is generated as the active intermediate of MTO reaction.SZ-TS with appropriate acid sites and rich mesoporous distribution could alleviate the formation rate of methyl-substituted benzene and polycyclic aromatic hydrocarbons, smaller particle size can avoid the occurrence of secondary reactions of olefins, therefore displaying the excellent resistance to the carbon deposition inactivation, with a catalytic lifetime up to 1380 min.

Facile Synthesis of an Mg‐incorporated ZSM‐5 Zeolite from Dual Silicon Sources and Its Application for Conversion of Methanol to Olefins

AbstractIn this work, an Mg‐incorporated ZSM‐5 zeolite with superior catalytic properties is synthesized directly from dual silicon sources(laponite and tetraethylorthosilicate) as raw material via hydrothermal method. The ZSM‐5 synthesized from laponite as one of the Si sources exhibits unique aggregated spherical morphology composed of sheet‐like crystals, which is due to the depolymerized laponite can act as crystalline seeds and facilitate the ZSM‐5 zeolite nucleation. Furthermore, the Mg species in laponite can subsequently enter into the ZSM‐5 zeolite during the process of crystallization. In comparison with ZSM‐5(0) synthesized by using tetraethylorthosilicate(TEOS) as single Si source, ZSM‐5 zeolite synthesized from dual silicon sources exhibited better catalytic performace for MTO reaction. Our work provides a straightforward and green route for the synthesis of an Mg‐incorporated ZSM‐5 zeolite from dual silicon sources with excellent MTO performance.