Methanol-to-olefin Reaction Research Articles

Hierarchical low-silica SAPO-34 with enhanced MTO performance: Mesopore template- and fluoride-free synthesis

The inherent mass transport limitations and undesired acidity of silicoaluminophosphate SAPO-34 catalysts are two key factors that result in a fast deactivation rate in the methanol-to-olefin (MTO) reaction. Herein, we demonstrate that SAPO-34 with hierarchical pore architecture and different silicon contents can be rationally synthesized by hydrothermal treatment of homogeneous amorphous silicoaluminophosphate precursors in the absence of mesopore templates and fluoride. It reveals that the hierarchical architecture is formed by in situ alkaline TEAOH etching of framework atoms. The silicon content (acidity) of the products can be facilely tuned in wide range. 29Si MAS NMR spectra reveal silicon mainly exists in the form of Si(4Al) in low-silica SAPO-34 catalysts free from silica islands. The hierarchical SAPO-34 catalyst with a moderate silicon content (nSi/n(Si + Al + P) = 0.048) and small particle size (265 ± 94 nm) exhibits a prolonged catalytic lifetime as well as a higher selectivity in MTO reaction, attributing to the synergetic effect of a fast diffusion speed and an appropriate acidic density.

Mechanisms and Kinetics of the Dehydrogenation of C6–C8 Cycloalkanes, Cycloalkenes, and Cyclodienes to Aromatics in H-MFI Zeolite Framework

This work employs periodic density functional theory to elucidate dehydrogenation mechanisms of C6–C8 cycloalkanes, cycloalkenes, and cyclodienes into aromatics during methanol-to-olefin (MTO) chemistries in H-MFI zeolites. Aromatic compounds act as co-catalysts that predominantly form ethylene over propylene products and lead to site-blocking polyaromatic compounds; thus, understanding the formation of aromatic compounds during MTO is critical to understanding its selectivity and catalyst stability. Ring dehydrogenation reactions occur via sequential hydride transfer followed by deprotonation. The rate-controlling hydride transfer reactions were investigated with surface-bound protons (Z–H) and alkyls (Z–CnH2n+1) as hydride-accepting species. Hydride transfers via proton-mediated routes occur with intrinsic free energy barriers (623 K) of 215 kJ mol–1 for C6H12, 164 kJ mol–1 for C6H10, and 141 kJ mol–1 for C6H8, whereas methyl-mediated counterparts occur with intrinsic free barriers of 101, 95, and 81 kJ mol–1, respectively. Transition states for hydride transfer reactions prefer channel intersections within MFI networks, and their activation barriers suggest that methyl-mediated routes are favored over proton-mediated counterparts during MTO. Methyl substituents on hydrocarbon rings generally increase activation barriers for hydride transfers that occur proximal to the −CH3, partly because of steric hindrances between the hydride acceptor and ring-bound −CH3. Rapid double-bond isomerization within cyclohexenes and cyclohexadienes allows hydride transfer reactions to occur away from sterically hindering methyl substituents in methylated and dimethylated C6 ring hydrocarbons. The impact of carbocation substitution in hydride-accepting species was explored by contrasting barriers of methyl (Z–CH3), ethyl (Z–C2H5), 2-propyl (Z–C3H7), and tert-butyl (Z–C4H9) surface-bound alkyls. Intrinsic activation barriers decrease with increasing substitution of the alkyl hydride acceptor, consistent with those alkyls forming more stable carbocations. However, when accounting for steric hindrances associated with the co-adsorption of hydrocarbon rings near surface-bound alkyls, apparent free barriers are larger for more-substituted alkyls. Taking these apparent barriers into account, we predict that methyl-mediated hydride transfer reactions are responsible for the aromatization of hydrocarbon rings during MTO, leading to the CH4 formed during MTO reactions as the aromatic pool is enriched.

Selective production of light olefins over zeolite catalysts: Impacts of topology, acidity, and particle size

Compared to conventional steam cracking, catalytic cracking of naphtha and methanol-to-olefins (MTO) over zeolite catalysts have been proved as the most successful processes for producing light olefins. To achieve the maximum light olefins yield and the longest lifetime, zeolite catalysts with excellent catalytic performance should be designed, by choosing suitable structure and regulating their physicochemical properties. In this review, the impacts of topology, acidity, and particle size of zeolites on the catalytic performance of naphtha cracking and MTO reactions, especially the light olefins selectivity and lifetime are systematically discussed. We hope that this review can be helpful not only to deeply understand the structure–property-performance relationship of various zeolite catalysts, but also to design more excellent zeolite catalysts for these two processes.

Insight into induction period of methanol conversion reaction: Reactivity of ethene-precursors over H-ZSM-5 zeolite is independent of Brϕnsted acid site density

A series of H-ZSM-5 (TZ-m, m represents different Si/Al ratios) has been prepared and their Methanol to Olefin (MTO) performance are investigated. By adjusting the catalyst weight of H-ZSM-5 loaded in the pulse micro-reactor, the total Brϕnsted acid sites (BAS) amount of TZ-m catalysts is unified. The resultant similar methanol conversion (∼24 %) over different TZ-m zeolites confirms that the MTO reaction is dominantly catalyzed by BAS. Nevertheless, the BAS density considerably influences the reaction pathways; more condensed BAS enhances the relative propagation of aromatic-based cycle to alkene-based cycle, and vice versa. However, isotopic co-feeding and switching experiment reveals that the reactivity of aromatic hydrocarbon pool (HCP) species, which has been considered as main ethene precursors, is almost independent of acid site density. Although large amounts of aromatics are generated via hydrogen transfer reactions on the sample with high acid site density, most of the confined aromatics function as coke precursors instead of active HCPs. The accumulation of such confined aromatics in zeolite channels further limits olefins mobility and their access to BAS, leading to decreased reactivity of alkene HCPs on high acid site-density samples.

Methanol‐to‐Light Olefins Reaction using Zn‐Ag Modified SSZ‐13 Catalyst

AbstractThe 2 wt.% of Zn and Ag (Zn1‐x‐Agx; x=0.2, 0.4, 0.6 %) modified SSZ‐13 (SZ‐13) zeolite was prepared by wet‐impregnation method, and its catalytic activity was demonstrated for methanol‐to‐olefin (MTO). The Zn0.6‐Ag0.4/SZ‐13 catalyst exhibited high catalytic activity due to change in properties of catalyst. The Zn particles were highly dispersed inside the SZ‐13; while Ag particles were dispersed on the surface of SZ‐13, confirmed by HRTEM and surface area. The small % of Ag over SZ‐13 promotes ethylene production via aromatization of reaction, and highly dispersed Zn on SZ‐13 help to produce stable ethylene and propylene. The Zn0.6‐Ag0.4/SZ‐13 catalyst showed (∼10 %) higher light olefin (87 %) selectivity than parent SZ‐13 as well as mono metal incorporated SZ‐13. The higher catalytic activity was attributed due to effect of both metals (Zn and Ag), where Ag promotes partial aromatization on the surface, and Zn increases the Lewis acidity in the zeolite pore. This catalyst can help to improve the selectivity of light olefin using MTO reaction.

Enhanced methanol-to-olefins (MTO) performance over SAPO-34 molecular sieves synthesized using novel sources of silicon and aluminium

Abstract As the core technology of methanol-to-olefins (MTO), the development of a high-efficiency and low-cost molecular sieve catalyst has always been the most important challenge. Herein, SAPO-34 molecular sieves were synthesized by using three different compositions of amorphous aluminium silicate as both Si and Al source under the same conditions. The structure and composition of synthesized SAPO-34s were investigated using X-ray diffraction and X-ray fluorescence, while the morphology and physical properties were characterized using scanning electron microscopy, N2 adsorption–desorption and ammonia temperature-programmed desorption measurements. Compared to the silica sol as the Si source, the SAPO-34s prepared with amorphous aluminium silicate show the higher solid yields and crystallinity, larger surface area and lower numbers of acid sites. Besides the effect of Si, the effect of the Al source on the textural structure and catalytic performance of SAPO-34s in MTO reactions was investigated in detail. The results confirmed that the SAPO-34s prepared using amorphous aluminium silicate have low Si content and hierarchical structure, which results in a longer catalytic lifetime and higher selectivity for light olefins. The sample S2 exhibited the longest lifetime of 214 min and the highest selectivity for light olefins of 85.37%. This also provides a potential for the approach to obtain a high-efficiency catalyst with enhanced catalytic performance and low cost.

Numerical simulation of commercial MTO fluidized bed reactor with a coarse-grained discrete particle method — EMMS–DPM

Particle scale information of commercial fluidized beds is important for its design and optimization, e.g., the catalyst performance evolution along time. However, it is difficult to be obtained by in-situ measurements or continuum-based numerical simulations due to the limited accuracy in space and time. Thus, the newly developed reaction model for methanol-to-olefins (MTO) is integrated into the coarse-grained discrete particle method (EMMS-DPM) to study the evolution of coke content distribution on the catalyst. The main gaseous products of the commercial MTO reactor are well predicted. Then, the operation optimization for a commercial MTO reactor is conducted by changing the annual processing capacity and the size distribution of catalysts. Results show that the reactor operation is slightly more difficult when the annual processing capacity is increased by 20%, and increasing the mass fraction of finer catalyst may result in more homogeneous MTO reaction and reduce the energy consumption.

Structural Transformation-Involved Synthesis of Nanosized ERI-Type Zeolite and Its Catalytic Property in the MTO Reaction.

Nanosized ERI-type aluminophosphate was prepared by the calcination of a precursor material (denoted as ECNU-38P) synthesized using 1,1,6,6-tetramethyl-1,6-diazacyclododecane-1,6-diium hydroxide (TDDH) as a structure-directing agent. The structure of ECNU-38P is related to ERI topology but exhibits a highly disordered manner and contains both four- and six-coordinated Al atoms. In situ XRD patterns revealed a rarely reported temperature-induced three-dimensional (3D)-to-3D structural transformation from ECNU-38P to the ordered ERI-type ECNU-38 zeolite at 573-623 K. Nanosized ERI-type silicoaluminophosphate Si-ECNU-38 was also obtained by introducing Si atoms into the synthetic system of ECNU-38P. The catalytic performance of ERI-type silicoaluminophosphates in the methanol-to-olefin (MTO) reaction was revealed to be highly related to the crystal sizes.

New CHA-Type aluminoborosilicates as efficient catalysts for MTO and NH3-SCR of NOx reactions

The small-pore CHA-type zeolites have received intensive attentions due to their outstanding performance in both of the methanol-to-olefin (MTO) process and ammonia selective catalytic reduction of nitrogen oxides (NH3-SCR of NOx) reactions. However, the costly synthesis of CHA-type aluminosilicate is a great issue for the practical application. In this study, a novel and cheap organic structure-directing agent of N,N,N-trimethylisobutylammonium hydroxide was developed and used to synthesize CHA-type zeolites (name as ECNU-25) with boric acid as the supporting agent. The Si/Al ratio in the synthetic gel could be tailored in a wide range of 50 - ∞. [Al,B]-ECNU-25 zeolite thus synthesized served as an effective catalyst for the MTO reaction. After Cu2+-exchange, the Cu-[Al,B]-ECNU-25 was also highly active in the NH3-SCR of NOx.

Understanding the effects of acid strength of active center and local confinement environment on the conversion of methanol to olefins in H-RUB-50

Brønsted acid (BA) strength and confinement effect of the cavity-type zeolites play an important role in the formation of products during methanol to olefins (MTO) conversion. In the present study, olefin-based cycle using 2,3-dimethyl-2-butene (iso-C6) as the hydrocarbon-pool (HCP) species was systematically investigated in H-RUB-50 zeolite with 8-membered ring (MR) pore opening and small LEV cavity by using periodic density functional theory calculations combined with microkinetic modeling, so as to evaluate the effects of different local confined space, active acid sites and acid strength caused by different Al sites on the catalytic performance of MTO reaction. The free energy profiles at four different BA sites (Al1O2, Al1O4, Al2O2 and Al2O5) generated by Al sitting at T1 and T2 sites in 8-MR window show that the olefin-based cycle is conducive to the formation of ethene at the four BA sites in H-RUB-50 zeolite, and Al1O2 BA site with the strongest acid strength is the most active site for MTO reaction. The change trend of TOF of ethene and propene shows that the acid strength is closely related to the formation of ethene, but for the formation of propene, not only the acid strength, but also the local confined space of the active center plays a more important role. In H-RUB-50 zeolite, the selectivity to ethene based on the olefin-based cycle is higher than to propene, which is consistent with the previous theoretical and experimental results. It is shown that the overall catalytic activity and ethene selectivity of H-RUB-50 at T1 site is much better than that at T2 site. This study provides a useful guidance for the experimental synthesis and adjustment of Al sites to improve the catalytic performance of MTO.

SAPO-34 crystals with nanosheet morphology synthesized by pyrophosphoric acid as new phosphorus source

The nanosheet SAPO-34 zeolite crystals with a diameter of 300 nm and a thickness of 50 nm using a new phosphorus source (pyrophosphoric acid, H4P2O7) were synthesized. The roles of the pyrophosphate (P2O74−) species in the crystallization of SAPO-34 were elucidated. The results suggested that one P atom from the P2O74− species participated in the formation of the CHA framework structure while the other P atom served as a crystallization control agent, facilitating preferential crystal growth in [100] crystal plane. The SAPO-34 nanosheets have silica-rich shell and phosphorus-rich core leading to superior catalytic performance in methanol-to-olefin reaction (MTO). The catalytic lifetime of the SAPO-34 nanosheets was significantly improved due to the larger amount of medium-strong surface acidity and short diffusion pathway in comparison to the conventional SAPO-34 catalyst synthesized in the presence of the classical phosphorus source (H3PO4).

Frustrated Lewis Pair in Zeolite Cages for Alkane Activations

AbstractThe frustrated Lewis pair (FLP) concept in homogeneous catalysis was extended to heterogeneous catalysis via the supramolecular system of FLP between deprotonated zeolite framework oxygens and confined carbocations in methanol‐to‐olefin (MTO) reactions. In this FLP, the polymethylbenzenium (PMB+) functioned as the Lewis acid to accept an electron pair, and the deprotonated framework oxygen site acted as the Lewis base to donate an electron pair. This FLP theoretically demonstrated the ability to undergo H2 heterolysis and alkanes dehydrogenation, and this was further confirmed by gas chromatography–mass spectrometer (GC‐MS) catalytic experiments inside FLP‐containing chabazite zeolites. All these findings not only bring new recognition to the carbocation chemistry in zeolite cages but also put forward a new reaction pathway as one part of MTO reactions.

Frustrated Lewis Pair in Zeolite Cages for Alkane Activations.

The frustrated Lewis pair (FLP) concept in homogeneous catalysis was extended to heterogeneous catalysis via the supramolecular system of FLP between deprotonated zeolite framework oxygens and confined carbocations in methanol-to-olefin (MTO) reactions. In this FLP, the polymethylbenzenium (PMB+ ) functioned as the Lewis acid to accept an electron pair, and the deprotonated framework oxygen site acted as the Lewis base to donate an electron pair. This FLP theoretically demonstrated the ability to undergo H2 heterolysis and alkanes dehydrogenation, and this was further confirmed by gas chromatography-mass spectrometer (GC-MS) catalytic experiments inside FLP-containing chabazite zeolites. All these findings not only bring new recognition to the carbocation chemistry in zeolite cages but also put forward a new reaction pathway as one part of MTO reactions.

Tuning product selectivity in the MTO reaction over Zn-modified SSZ-13 catalysts

In this work, Zn-modified SSZ-13 catalysts with different spatial distributions of zinc species were prepared by ion exchange (IE), wet impregnation (WI) and in situ synthesis (IS) methods employed for the methanol-to-olefin (MTO) reaction.

Clariant unveils groundbreaking catalysts developed jointly with Linde Engineering for novel ethylene production technology

Eight-membered ring small-pore zeolite of RHO-type topology has been synthesized, characterized and tested for methanol-to-olefin (MTO) reaction. The zeolite was hydrothermally crystallized from the gel with Si/Al ratio of 5.0. It showed a high BET specific surface area (812 m2 g–1), micropore volume (0.429 cm3 g–1), and acid amount (2.53 mmol g–1). Scanning electron microscopy observations showed small crystallites of about 1 μm. The zeolite was active for MTO reaction with 100% methanol conversions at 623–723 K, whereas selectivity to lower olefins changed with time.

Cover Picture: Insight into Carbocation‐Induced Noncovalent Interactions in the Methanol‐to‐Olefins Reaction over ZSM‐5 Zeolite by Solid‐State NMR Spectroscopy (Angew. Chem. Int. Ed. 51/2021)

Cyclic carbocations are important intermediates in methanol-to-olefins (MTO) reaction over zeolite. In their Research Article on page 26847, Jun Xu and co-workers report that the carbocations formed in the MTO reaction can capture small hydrocarbon molecules via non-covalent interactions. Multiple non-covalent interactions have been identified by solid-state NMR spectroscopy and confirmed to be the key factors in modulating reaction properties.

Insight into Carbocation-Induced Noncovalent Interactions in the Methanol-to-Olefins Reaction over ZSM-5 Zeolite by Solid-State NMR Spectroscopy.

Carbocations such as cyclic carbenium ions are important intermediates in the zeolite-catalyzed methanol-to-olefins (MTO) reaction. The MTO reaction propagates through a complex hydrocarbon pool process. Understanding the carbocation-involved hydrocarbon pool reaction on a molecular level still remains challenging. Here we show that electron-deficient cyclopentenyl cations stabilized in ZSM-5 zeolite are able to capture the alkanes, methanol, and olefins produced during MTO reaction via noncovalent interactions. Intermolecular spatial proximities/interactions are identified by using two-dimensional 13 C-13 C correlation solid-state NMR spectroscopy. Combined NMR experiments and theoretical analysis suggests that in addition to the dispersion and CH/π interactions, the multiple functional groups in the cyclopentenyl cations produce strong attractive force via cation-induced dipole, cation-dipole and cation-π interactions. These carbocation-induced noncovalent interactions modulate the product selectivity of hydrocarbon pool reaction.

Insight into Carbocation‐Induced Noncovalent Interactions in the Methanol‐to‐Olefins Reaction over ZSM‐5 Zeolite by Solid‐State NMR Spectroscopy

AbstractCarbocations such as cyclic carbenium ions are important intermediates in the zeolite‐catalyzed methanol‐to‐olefins (MTO) reaction. The MTO reaction propagates through a complex hydrocarbon pool process. Understanding the carbocation‐involved hydrocarbon pool reaction on a molecular level still remains challenging. Here we show that electron‐deficient cyclopentenyl cations stabilized in ZSM‐5 zeolite are able to capture the alkanes, methanol, and olefins produced during MTO reaction via noncovalent interactions. Intermolecular spatial proximities/interactions are identified by using two‐dimensional 13C–13C correlation solid‐state NMR spectroscopy. Combined NMR experiments and theoretical analysis suggests that in addition to the dispersion and CH/π interactions, the multiple functional groups in the cyclopentenyl cations produce strong attractive force via cation‐induced dipole, cation–dipole and cation–π interactions. These carbocation‐induced noncovalent interactions modulate the product selectivity of hydrocarbon pool reaction.

Facile and Rapid Synthesis of Durable SSZ-13 Catalyst Using Choline Chloride Template for Methanol-to-Olefins Reaction

In the current study, a facile and rapid synthesis approach for a SSZ-13 catalyst using choline chloride (CC) as a template was proposed, and the catalytic performance for the methanol-to-olefins (MTO) reaction was examined. With a proper amount of CC addition (i.e., m(CC)/m(SiO2) = 0.14), uniform and homogeneously distributed cubic SSZ-13 crystals were obtained within 4 h with lower aggregation. The synthesized catalyst demonstrated excellent porous features with a total specific surface area and mesopore volume of 641.71 m2·g−1 and 0.04 cm3·g−1, respectively. The optimized strong and weak acid sites on SSZ-13 were obtained by regulating the m(CC)/m(SiO2) ratio. The less strong acid sites and a larger amount of weak acid sites in the synthesized catalyst were conducive to the catalytic performance of the MTO reaction under a lower reaction temperature (450 °C). The appropriate acidity and well-developed pore structure of synthesized SSZ-13 could also slow down the carbon deposition rate and, thus, significantly improve the catalytic lifetime of the catalyst. The methanol conversion rate and initial selectivity of light olefin using the synthesized catalyst could maintain over 95% and 50%, respectively, and a lifetime of 172 min was achieved. Although the low olefin selectivity of the synthesized SSZ-13 catalyst was slightly lower than that of the purchased one, its desirable features were thought to have good potential for industrial application.

Conversion of Methanol to Olefins over Modified OSDA-Free CHA Zeolite Catalyst

Methanol to olefins (MTO) is an essential process that produces olefins from both renewable and fossil fuels resources. Chabazite (CHA) is an efficient catalyst for MTO process due to its distinct properties that promote light olefins (i.e., ethylene and propylene) production. Herein, we report a series of chabazite-based catalysts (OSDA-free with enhanced textural properties) that facilitate the MTO reaction. The synthesized CHA was modified by steam injection (CHA-S), acid treatment (CHA-A), steam and acid treatment (CHA-SA), and metal incorporation (CHA-SA (Mg), CHA-SA (Ca), and CHA-SA (Ba)). The catalysts were characterized by several characterization techniques. The structure was investigated using X-ray diffraction (XRD) and nuclear magnetic resonance, while the morphology and physical properties were studied by field emission scanning electron microscopy (FE-SEM), N2 adsorption/desorption, and ammonia temperature-programmed desorption (NH3-TPD). The catalysts were evaluated at 450 °C until deactivation. CHA modified by only acid treatment showed higher selectivity to olefins from ca. 92 to 95 wt.% but suffered quick deactivation. CHA modified with both steam and then acid treatment was more stable in the MTO process.