Methanol-to-olefin Reaction Research Articles

Control of Sequential MTO Reactions through an MFI-Type Zeolite Membrane Contactor.

A membrane for controlling methanol-to-olefin (MTO) reactions was developed, which featured an MFI-type zeolite membrane (Si/Al = 25) that was synthesized on a porous α-alumina substrate using a secondary growth method. Here, the H2/SF6 permeance ratios were between 150 and 450. The methanol conversion rate was 70% with 38% ethylene selectivity and 28% propylene selectivity as determined using a cross-flow membrane contactor. In order to improve the olefin selectivity of the membrane, the MFI zeolite layer (Si/Al = ∞) was coated on an MFI-type zeolite membrane (Si/Al = 25). Using this two-layered membrane system, the olefin selectivity value increased to 85%; this was 19% higher than the value obtained during the single-layer membrane system.

Long-time simulation of catalytic MTO reaction in a fluidized bed reactor with a coarse-grained discrete particle method — EMMS-DPM

Long-time behaviors widely exist in gas–solid reactors, e.g., the coke deposition and catalyst deactivation in the methanol to olefins (MTO) process. Due to the limitation of the computational cost, previous simulations of MTO reactors were mainly conducted with two-fluid model at short-time scales and assumed that the catalyst are perfectly mixed. Efficient simulation methods are, therefore, highly desirable, even for evaluating the assumptions through very long-time investigation. In this study, the MTO reaction kinetics is incorporated into a coarse-grained discrete particle method, namely EMMS-DPM, to simulate a pilot-scale reactor for 8 h of physical time. The simulation is validated by comparing the obtained gaseous products and coke contents with the experimental data. The perfect mixing assumption is validated by observing the residence time distribution of the catalyst for the whole reactor. The detailed coke content could provide more accurate data for accelerating steady state simulations. Furthermore, the relation between the coke content and age of the catalyst is directly obtained for the first time, which is valuable to effective use of the catalyst before discharging. The coke content is positively correlated to the age, but is also influenced by local conditions. For example, near the distributor, the catalyst particles are not well mixed and the maximum reaction rate fluctuates significantly even at large time scales. Thus, long-time particle-scale simulation would provide more dynamic details to characterize the reaction in the critical regions of the reactor, thus being very helpful to optimize operating conditions and the reactor designs.

Highly Selective MTO Reaction over a Nanosized ZSM-5 Zeolite Modified by Fe via the Low-Temperature Dielectric Barrier Discharge Plasma Method

Nanosized ZSM-5 zeolites were synthesized by an in situ seed-induced hydrothermal method, and samples modified with an Fe promoter were prepared by the traditional wet impregnation-thermal decomposition and dielectric barrier discharge plasma (DBD) methods, respectively. The physico-chemical properties of the catalysts were studied by XRD, SEM, TEM, BET, XPS, H2-TPR, NH3-TPD and Py-IR techniques. The catalytic performance was eValuated by the methanol-to-olefin (MTO) reaction. The results showed that the acidity of the catalysts, the dispersity of the Fe promoter and the interaction degree with the ZSM-5 zeolite are closely related to product selectivity in the MTO reaction. Compared with the Fe-NZ5 sample prepared by the traditional impregnation-calcination method, the FeD-NZ5 samples prepared by the DBD method exhibited the higher selectivity of C2—C4 light olefins and the lower coke deposition during long-term eValuation (100 h), which can be attributed to the weaker acid strength, more uniform Fe promoter dispersion and strong interaction with the ZSM-5 zeolite. The developed Fe-modified catalysts have high potential for application in the MTO reaction.

Insights into NH4-SAPO-34 preparation procedure: Effect of the number of ammonium exchange times on catalytic performance of Zn-modified SAPO-34 zeolite for methanol to olefin reaction

Abstract Zn-modified SAPO-34 zeolites via NH4-SAPO-34 intermediates were prepared, and the effect of the number of ammonium exchange times on their catalytic performance in methanol to olefin (MTO) reaction was comprehensively investigated. The physicochemical properties of parent and Zn-modified SAPO-34 zeolites were characterized by XRD, SEM, N2 adsorption-desorption, NH3-TPD, XRF, XPS and TGA. The reaction results show that the catalytic lifetime and product selectivity of Zn-modified SAPO-34 zeolites is closely related to the number of ammonium exchange times, and the samples with more than two times of ammonium exchange exhibit excellent catalytic performance in MTO reaction with ca. 50 C-% of ethene selectivity, ca. 75 C-% of light olefins selectivity and ca. 2.0 of ethene to propene ratio. Besides, the effect of the number of ammonium exchange times on the structural integrity and the location of the introduced Zn-related species of Zn-modified SAPO-34 zeolites is described.

Insight into the Coke Precursor in the Process of the Methanol-to-Olefins Reaction

In this work, formaldehyde formed in the methanol-to-olefins (MTOs) reaction was found to be a coke precursor on SAPO-34 and the amount of formaldehyde during the entire MTO reaction was measured a...

Synthesis of Ga-containing CON-type material and its catalytic performance in methanol-to-olefins reaction

Abstract Ga-containing CON-type materials were synthesized and its catalytic performance in methanol-to-olefins (MTO) reaction were evaluated. The state of Ga atoms in the CON framework was investigated by 71Ga MAS NMR and UV–vis measurements. The effect of incorporation of Ga on the acidic properties of CON-type materials was investigated by NH3-TPD and FT-IR measurements with pyridine and CO as molecular probes. When Ga-containing CON-type zeolite was employed in MTO reaction, the yield to propylene were 60 C-atom %, as high as Al-CON, but with the lower ethylene selectivity, leading to the higher propylene-to-ethylene (P/E) ratio.

Organosilane surfactant-assisted synthesis of mesoporous SSZ-39 zeolite with enhanced catalytic performance in the methanol-to-olefins reaction

SSZ-39 zeolite with AEI framework structure is a good catalyst candidate for the methanol-to-olefins (MTO) reaction. However, the diffusion limitation and coke formation often results in fast deactivation of the SSZ-39 zeolite catalyst. One solution for this challenge is to introduce mesoporosity in the SSZ-39 zeolite. Herein, we report the synthesis of mesoporous SSZ-39 zeolite using an organosilane surfactant, N,N-dimethyl-N-(3-(trimethoxysilyl)propyl)octan-1-aminium chloride, as a mesopore template and N,N-dimethyl-cis-2,6-dimethylpiperidinium as a micropore template. The obtained zeolites were characterized by X-ray diffraction, N2 sorption, scanning electron microscopy, temperature programmed desorption of ammonia, and magic angle spinning nuclear magnetic resonance of 27Al. The results show that the mesoporous SSZ-39 zeolite has high crystallinity, meso/microporosity, high surface area, cuboid morphology, and abundant acidic sites. More importantly, this mesoporous SSZ-39 zeolite exhibits enhanced catalyst lifetime in the MTO reaction due to the presence of mesoporosity for fast mass transfer, compared with a conventional SSZ-39 zeolite without mesoporosity.

Synthesis of hierarchical SAPO-34 catalysts modified with manganese nanoparticles for conversion of methanol to light olefins: a deactivation and regeneration study

The hierarchical type 34 silicoaluminophosphate (SAPO-34) catalysts containing manganese were synthesized in the presence of carbon nanotube as mesopore template and tested in the transformation of methanol to light olefin. Also, the regenerability of spent catalysts after methanol to olefins (MTO) reaction was studied. In order to investigate the effect of manganese on the framework and extra framework of SAPO-34 structure, manganese species were added to SAPO-34 by isomorphous substitution and impregnation methods. The physicochemical properties of modified hierarchical samples such as crystallinity, acidity, porosity and reducibility were strongly dependent on the metal addition method. The modified hierarchical samples showed superior performance in the terms of activity and longevity compared to unmodified hierarchical SAPO-34. Among the samples, hierarchical MnAPSO-34 catalyst prepared by isomorphous substitution exhibited the best performance in the MTO reaction. The characterization of the used hierarchical catalysts by TGA analysis revealed that the deposited coke on modified samples was lower compared to unmodified hierarchical SAPO-34. The regenerated modified hierarchical SAPO-34 samples showed more enhanced ethylene selectivity in the MTO reaction and longer lifetime due to the existence of more methylated naphthalenes and methylated benzenes in the residual coke after regeneration.

Cobalt-isomorphous substituted SAPO-34 via milling and recrystallization for enhanced catalytic lifetime toward methanol-to-olefin reaction

Silicoaluminophsophate (SAPO)-34 is considered as a promising catalyst for methanol-to-olefin (MTO) reaction, because its cylinder-like cages with small 8-ring pore openings restrict the diffusion of large hydrocarbons and exceptionally give high selectivity for light olefins. However, its main drawback is rapid catalytic deactivation by heavy hydrocarbons confined inside the cages. Here, we report a facile synthetic strategy for submicro-sized cobalt (Co)-isomorphous substituted SAPO-34 with controlled Co content through milling and Co-incorporated recrystallization. Co-isomorphous substitution on the CHA framework of SAPO-34 was thoroughly proven by XRD patterns, HADDF scanning TEM images, EDS mapping images, H2-TPR profiles, and diffuse reflectance UV-Vis spectra. Regarding the modified particle size, pore structure and acidity of the as-synthesized Co-isomorphous substituted SAPO-34, evaluation of MTO reaction revealed the correlation between the catalytic lifetime and the content of Co-isomorphous substitution. As an efficient platform to overcome the intrinsically low catalytic lifetime toward MTO reaction, this work provides a versatile strategy to generate various transition metal-isomorphous substituted SAPO-34 materials on the submicro scale.

Evolution of Hydrocarbon Pool over SAPO‐34 Catalyst during Methanol to Light Olefins

AbstractThe coke deposition behaviors and the evolution of hydrocarbon pool species of SAPO‐34 during the methanol to olefins (MTO) process were investigated. Retained hydrocarbon analysis was employed to explore the amount and the category of coke in the cages of thermally quenched SAPO‐34 samples which had been used as the catalyst in the MTO reaction. The coke lead to the distinct microstrain of unit cell. The UV‐vis spectra and GC‐MS studies showed the evolution of hydrocarbon pool species from monocyclic to polyaromatic. Furthermore, the linearly increasing of coke content and linearly reducing of surface area of SAPO‐34 indicated that the deactivation of catalyst was mainly caused by the blockage of SAPO‐34 pore.

Tuning adsorption properties of GaxIn2−xO3 catalysts for enhancement of methanol synthesis activity from CO2 hydrogenation at high reaction temperature

Abstract Light olefins can be produced from CO2 hydrogenation in a single reactor using a combination of a methanol synthesis catalyst and a methanol-to-olefin (MTO) catalyst. However, commercial methanol synthesis catalysts are active at low temperatures (200–260 °C), while MTO reaction is feasible at higher temperatures (>300 °C). Herein, we report the CO2 hydrogenation to methanol at high temperatures (320–400 °C) over GaxIn2−xO3 catalysts. By tuning the Ga/In ratios, phase, crystallinity, pore structure, morphology, electronic properties as well as adsorptive properties of GaxIn2−xO3 catalysts can be modified. At the lowest temperature (320 °C), the pure In2O3 shows the highest methanol yield. However, the maximum methanol yield declines significantly with increasing reaction temperatures. Incorporation of Ga into the In2O3 crystal lattices at x = 0.4 (Ga0.4In1.6O3) maximizes the methanol yield at higher reaction temperatures of 340–360 °C. This enhancement can be attributed to an increased binding energy of adsorptive molecules with the catalyst surface to promote the hydrogenation of CO2 to methanol. Further increasing Ga content (x > 0.4) leads to greatly strengthen the binding for adsorptive molecules, resulting in a lower methanol yield and the formation of methane. The surface chemisorbed oxygen is found to be a key factor determining the CO yield.

Synthesis and characterization of nano-sized hierarchical porous AuSAPO-34 catalyst for MTO reaction: Special insight on the influence of TX-100 as a cheap and green surfactant

Abstract Hierarchical SAPO-34 and AuSAPO-34 were synthesized as a catalyst in methanol to olefins (MTO) reaction by utilizing gold (Au) as a novel promoter and Triton X-100 (TX-100) as a cheap, non-toxic and innovative mesoporogen. In this work, independent and simultaneous effects of introducing different amounts of TX-100 and Au to the SAPO-34 framework were studied by XRD, ICP-AES, UV–vis/DRS, FESEM, HRTEM, BET, NH3-TPD, FTIR and TGA-DTA analyses. The characterization results revealed that when the concentration of TX-100 is lower than the critical micelle concentration (CMC), the effects of surfactant are only limited to the covering and steric effects and its result can only be seen on the size of SAPO-34 crystals. Nevertheless, when the concentration of the surfactant exceeds CMC, in addition to the mentioned effects, micellization of the TX-100 molecules effectively occurs and results in the formation of small SAPO-34 crystals with hierarchical porosity. The presence of a small amount of gold demonstrates a significant effect on acidic properties and coke deposition rate of the samples. Consequently, the simultaneous presence of TX-100 and Au is accompanied with a superior catalytic performance of about 97% selectivity towards light olefins and 331 min lifetime.

H-ZSM-5 Materials Embedded in an Amorphous Silica Matrix: Highly Selective Catalysts for Propylene in Methanol-to-Olefin Process

H-ZSM-5 materials embedded in an amorphous silica were successfully synthesized with three different Si/Al ratios (i.e., 40, 45, and 50). The presence of the MFI structure in the synthesized samples was confirmed by X-ray diffraction (XRD), Fourier transform infra-red (FT-IR), and solid state-nuclear magnetic resonance (SSNMR) techniques. The morphology and textural properties of the samples were investigated by scanning electron microscopy (SEM), TEM, and N2-physisorption measurements. Furthermore, acidic properties of the synthesized catalysts have been studied by NH3-TPD and FT-IR spectroscopy of CO adsorption studies. Variation of the Si/Al ratio affected the crystal morphology, porosity, and particle size, as well as the strength and distribution of acid sites. The synthesized zeolite materials possessed low acid-site density and exhibited high catalytic activity in the methanol-to-olefin (MTO) reaction. To study the intermediate species responsible for catalyst deactivation, the MTO reaction was carried out at high temperature (500 °C) to accelerate catalyst deactivation. Interestingly, the synthesized catalysts offered high selectivity towards the formation of propylene (C3=), in comparison to a commercial microporous crystalline H-ZSM-5 with Si/Al = 40, under the same reaction conditions. The synthesized H-ZSM-5 materials offered a selectivity ratio of C3=/C2= 12, while it is around 2 for the commercial H-ZSM-5 sample. The formation of hydrocarbon species during MTO reaction over zeolite samples has been systematically studied with operando UV-vis spectroscopy and online gas chromatography. It is proposed that the strength and type of acid sites of catalyst play a role in propylene selectivity as well as the fast growing of active intermediate species. The effective conversion of methanol into propylene in the case of synthesized H-ZSM-5 materials was observed due to possession of weak acid sites. This effect is more pronounced in H-ZSM-5 sample with a Si/Al ratio of 45.

Cage-Defining Ring: A Molecular Sieve Structural Indicator for Light Olefin Product Distribution from the Methanol-to-Olefins Reaction

The methanol-to-olefins (MTO) process produces high-value-added light olefins from nonpetroleum sources. Acidic zeotypes containing cages bounded by 8-ring (small-pore) windows can effectively catalyze the MTO reaction, since their cages can accommodate the necessary aromatic intermediates that produce the light olefin products that escape. While progress on the mechanisms of the MTO reaction continues, zeotype structure–reaction property relationships have yet to be elucidated. Here, we report MTO reaction results from various small-pore, cage-containing silicoaluminophosphate/metalloaluminophosphates (SAPO/MAPOs) and zeolites under the same reaction conditions. The MTO behaviors of microporous materials having the following topologies are investigated: LEV, ERI, CHA, AFX, SFW, AEI, DDR, RTH, ITE, SAV, LTA, RHO, KFI, and UFI. The previous observation that light olefin product distributions from a series of small-pore, cage-containing zeolites can be classified into four structural categories is further supported by the results shown here from zeolite structures not investigated in the previous study and SAPO and MAPO materials with isostructural frameworks to all the zeolites. Additionally, these data reveal that light olefin product distributions are very similar over a given topology independent of framework composition. To develop a structure–property relationship between the framework topology and the MTO light olefin product distribution, the concept of the cage-defining ring size is introduced. The cage-defining ring size is defined as the minimum number of tetrahedral atoms of the ring encircling the center of the framework cages in the molecular sieve topology. It is shown that the cage-defining ring size correlates with MTO light olefin product distribution.

Tuning the product selectivity of SAPO-18 catalysts in MTO reaction via cavity modification

The AEI cavity of SAPO-18 catalyst was modified with zinc cations with the conventional ion exchange procedure. The cavity modification effectively tunes the product selectivity, and shifts the products from mainly propylene to comparable production of ethylene and propylene in methanol to olefin (MTO) reaction. The incorporation of zinc ions and the generation of bicyclic aromatic species in the AEI cavity of SAPO-18 catalysts introduce additional diffusion hindrance that exert greater influence on the relatively bulky products (e.g. propylene and higher olefins), which increase the selectivity to small-sized products (e.g. ethylene). It appears that the incorporated zinc cations facilitate the generation of lower methylbenzenes which promote the generation of ethylene. The cavity modification via incorporating zinc ions effectively tunes the product selectivity over SAPO molecular sieves with relatively larger cavity, which provides a novel strategy to develop the potential alternative to SAPO-34 catalysts for industrial MTO reaction.

Achieving a Superlong Lifetime in the Zeolite-Catalyzed MTO Reaction under High Pressure: Synergistic Effect of Hydrogen and Water

Zeolites are usually considered to be acid catalysts, which are prone to deactivation due to the coke deposition in the hydrocarbon conversions such as methanol-to-olefins (MTO) reaction. Herein, a high-pressure MTO process with cofeeding H2 and H2O is reported, which can effectively prolong the catalytic lifetime of SAPO-34. The corresponding methanol handling capacity is about 200 times larger than that under the normal-pressure condition. Investigation reveals that the ultralong lifetime originates from the hydrogenation ability of the acid sites on SAPO-34 for aromatic species, which can hydrogenate the heavy aromatic deposits (especially the phenanthrene composed of three benene rings) to active aromatic intermediates (methylbenzenes and methylnaphthalenes) and thus slow down the evolution of coke species. A positive synergistic effect between H2 and H2O on prolonging the catalyst lifetime is observed at higher H2O partial pressure, likely resulting from the reduced barriers of hydrogenation reaction...

GeAPSO-34 molecular sieves: Synthesis, characterization and methanol-to-olefins performance

GeAPSO-34 molecular sieves were successfully synthesized with various Ge stoichiometries and used as catalysts in the conversion of methanol to olefins (MTO). The XRD and SEM characterizations proved that the presence of Ge species in the initial gel could accelerate the nucleation and crystallization of the zeolites. The XRD and XRF results indicated that the Ge element has been incorporated into the CHA frame structure of GeAPSO-34. NH3-TPD results showed relatively weak acidity and less acid sites for Ge-incorporated samples. Compared with SAPO-34, GeAPSO-34 showed improved light olefins selectivity in MTO reaction.

Influence of Diffusion and Acid Properties on Methane and Propane Selectivity in Methanol-to-Olefins Reaction

ZSM-5 zeolites with different particle sizes and mesoporous structures were evaluated in a methanol-to-olefins (MTO) reaction. The results show that the efficient diffusion leads to a low methane selectivity in the initial stage of reaction and a slight increase in methane selectivity throughout the reaction. ZSM-5 zeolites with different acid amounts were also evaluated, and the results indicate that ZSM-5 with a few acid sites exhibits a low initial methane selectivity and a slight increase with prolonged reaction time. In addition, a low initial propane selectivity was observed. [Fe]-[Al]-ZSM-5 zeolites with different Fe/Al ratios were also evaluated in MTO reaction, and a low propane selectivity was observed over ZSM-5 with a weak acid strength (high Fe/Al ratio). Those results are very useful for the understanding of methane and propane formation mechanism. Furthermore, it contributes to a further study of methanol conversion mechanism and deactivation mechanism.

Performance Enhanced SAPO-34 Catalyst for Methanol to Olefins: Template Synthesis Using a CO2-Based Polyurea

Introducing mesopores into the channels and cages of conventional micropores CHA (Chabazite) topological structure SAPO-34 molecular sieves can effectively improve mass transport, retard coke deposition rate and enhance the catalytic performance for methanol to olefins (MTO) reaction, especially lifetime and olefins selectivity. In order to overcome the intrinsic diffusion limitation, a novel CO2-based polyurea copolymer with affluent amine group, ether segment and carbonyl group has been firstly applied to the synthesis of SAPO-34 zeolite under hydrothermal conditions. The as-synthesized micro-mesoporosity SAPO-34 molecular sieve catalysts show heterogeneous size distribution mesopores and exhibit slightly decrease of BET surface area due to the formation of defects and voids. Meanwhile, the catalysts exhibit superior catalytic performance in the MTO reaction with more than twice prolonged catalytic lifespan and improvement of selectivity for light olefins compared with conventional microporous SAPO-34. The methodology provides a new way to synthesize and control the structure of SAPO-34 catalysts.

Origin and evolution of the initial hydrocarbon pool intermediates in the transition period for the conversion of methanol to olefins over H-ZSM-5 zeolite

In the conversion of methanol to olefins (MTO), it is now well accepted that ethene and propene are formed via a direct mechanism in the initial stage; after a transition period, olefins are then mainly produced via a hydrocarbon pool (HCP) mechanism in the steady stage. However, there was still no clear consensus over what actually occurred in the transition period and how the initial ethene and propene products construct the first HCP intermediates. In this work, the formation of initial HCP intermediates in the transition period and their evolution tendency were investigated for MTO over H-ZSM-5 zeolite by density functional theory and microkinetic modeling. The results illustrate that ethene and propene formed in the initial stage can quickly construct methylcyclopentadiene and methylbenzene; acting as the initial HCP intermediates, these cyclic species can build the light olefins through both the pairing route and the side-chain route and then promote the MTO reactions into the steady stage governed by the HCP mechanism. Meanwhile, the formation of methylcyclopropene and methylnaphthalene needs to conquer a high free energy barrier, which should be less significant as the HCP intermediates in the transition period. The insight attained in this work helps to clarify the origin and evolution of initial cyclic HCP intermediates in the transition period and is thus beneficial for a better understanding of the MTO reaction mechanism.