Methanol-to-olefin Reaction Research Articles

Enhanced Catalytic Performance of Copper-Exchanged SAPO-34 Molecular Sieve in Methanol-to-Olefin Reaction

Methanol-to-olefin (MTO) reaction over copper-exchanged SAPO-34 catalysts was investigated in order to extend their catalyst life. The exchange of copper ions into the cages of an SAPO-34 molecular sieve was confirmed by ESR, XPS, and 129Xe NMR techniques. Copper ions located in its cages considerably reduced its deactivation rate in the MTO reaction, while those dispersed on the external surface of the SAPO-34 molecular sieve accelerated the deactivation due to the limited mass transfer through the pore entrances. The 13C NMR and UV-VIS spectroscopy investigations of the materials occluded on the copper-exchanged SAPO-34 catalysts during the MTO reaction clearly showed that the copper ions exchanged in the cages suppressed the further condensation of alkyl aromatics to large, fused polycyclic aromatic hydrocarbons (PAHs). Theoretical calculations for the SAPO-34 and copper-exchanged SAPO-34 molecular sieves supported this observation because copper ions located in the cages stabilized the alkyl aromatics. Therefore, the exchange of copper ions into the SAPO-34 molecular sieve stabilized the reactive intermediates, alkyl aromatics, of the MTO reaction and suppressed their further condensation to PAHs, thereby slowing the deactivation.

Effect of crystallite size of SAPO-34 catalysts on their induction period and deactivation in methanol-to-olefin reactions

Methanol-to-olefin (MTO) reactions were investigated over three kinds of SAPO-34 catalysts with similar acidities but different crystallite sizes to elucidate the effect of crystallite size on their catalytic performance, especially in terms of the induction period and deactivation. The prepared SAPO-34 catalysts showed similar physical and chemical properties, except for their crystallite sizes. The catalyst composed of large crystallites (∼7 μm) was activated after a definite induction period and deactivated rapidly, while that composed of small crystallites (∼0.4 μm) achieved complete conversion without any appreciable induction period and retained its activity for a relatively long time. This variation in catalytic performance according to the crystallite size of the SAPO-34 catalysts was attributed to the number of accessible cages near their external surface and to the diffusion limitation of the materials. Simulated conversion profiles in the MTO reactions over different crystallite sizes supported this explanation.

Silicoaluminophosphate Molecular Sieves STA-7 and STA-14 and Their Structure-Dependent Catalytic Performance in the Conversion of Methanol to Olefins

Details of the synthesis of the small-pore silicoaluminophosphate (SAPO) molecular sieves STA-7 (SAV) and STA-14 (KFI) prepared via a co-templating approach are described. STA-7 includes Si in the framework with Si/(Si+Al+P) ratios varying from 0.04 to 0.17, whereas STA-14 crystallizes from gels with a narrower compositional range (Si/(Si+Al+P) = 0.2−0.3). The main route to Si incorporation is by substitution for P, but in the presence of fluoride, aluminosilicate regions are formed in STA-7. 27Al 3Q MAS NMR studies enable resolution of tetrahedral Al in Al(OP)4 and Al(OP3,OSi) environments in both as-prepared and calcined materials. For STA-7 with low Si content the presence of five- and six-fold Al coordinated with water molecules or fluoride or hydroxide ions has also been confirmed. Upon calcination, all Al adopts tetrahedral coordination. High pressure CO2, CO, and CH4 adsorption at 100 °C indicates that the proton forms of the SAPO D6R zeotypes present effective polarities intermediate between cationic zeolites and pure silica chabazite. The methanol-to-olefin (MTO) performance of the SAPOs H-SAPO-34, H-STA-7, and H-STA-14 was found to depend on both crystallite size and cage connectivity and topology. H-STA-7 with a crystal size of 2−3 μm has MTO stability comparable to that observed for H-SAPO-34, whereas the same materials with larger crystal sizes (≥10 μm) deactivate rapidly. Ex situ GC-MS analyses of the SAPO catalysts after MTO reaction demonstrate that the uniformity in cage shape and size in cage-based, small-pore molecular sieves is a critical factor governing the type of the accumulated aromatic hydrocarbons and, hence, their MTO activity and deactivation behavior.

Methanol to Olefin Conversion on HSAPO-34 Zeolite from Periodic Density Functional Theory Calculations: A Complete Cycle of Side Chain Hydrocarbon Pool Mechanism

For its unique position in the coal chemical industry, the methanol to olefin (MTO) reaction has been a hot topic in zeolite catalysis. Due to the complexities of catalyst structure and reaction networks, many questions such as how the olefin chain is built from methanol remain elusive. On the basis of periodic density functional theory calculations, this work establishes the first complete catalytic cycle for MTO reaction via hexamethylbenzene (HMB) trapped in HSAPO-34 zeolite based on the so-called side chain hydrocarbon pool mechanism. The cycle starts from the methylation of HMB that leads to heptamethylbenzenium ion (heptaMB+) intermediate. This is then followed by the growth of side chain via repeated deprotonation of benzenium ions and methylation of the exocyclic double bond. Ethene and propene can finally be released from the side ethyl and isopropyl groups of benzenium ions by deprotonation and subsequent protonation steps. We demonstrate that (i) HMB/HSAPO-34 only yields propene as the primary ...

IR study on methanol-to-olefin reaction over zeolites with different pore structures and acidities

The effects of the pore structure and acidity of zeolites on their product distribution and deactivation rates in the methanol-to-olefin (MTO) reaction were studied according to the different topologies of CHA, LTA, MFI, BEA, MOR and FAU zeolites. The in situ IR study of the MTO reaction over various zeolites with different pore structures and acidities confirmed the considerable effect exerted by these factors on the species of alkylaromatics formed and on the rates of further condensation of alkylaromatics to polycyclic aromatic hydrocarbons (PAHs). The nature and continuing concentration of alkylaromatics formed in the zeolite pores determined their product composition and deactivation rate in the MTO reaction. LTA and FAU zeolites with large cages and high acid site density were deactivated rapidly due to the high concentration of alkylaromatics with large substituents and their rapid condensation to PAHs. However, the small cages of CHA zeolite and the sinusoidal pores of MFI zeolite suppressed the accumulation of large hydrocarbon molecules in their pores and lengthened their catalyst lives, although the composition of the products formed over them clearly differed because of their different pore structures.

The Weight Change of SAPO-34 Catalyst During MTO Process in TGA Reactor

ABSTRACTThe Thermax 700 thermo gravimetric analysis (TGA) instrument is introduced for the investigation of the weight change of SAPO-34 catalyst during methanol-to-olefin (MTO) process. By the use of a special sample basket with screen bottom, the TGA instrument can be viewed as fixed-bed reactor, while the weight change of SAPO-34 during MTO reaction can be recorded online. It is noticed that the SAPO-34 catalyst experiences a procedure of dehydration as furnace temperature rises; hence the weight increase of SAPO-34 during MTO reaction should be calculated based on the weight after dehydration. It is checked that the increase of weight is mainly coke ignore of adsorbates. In the range of 648∼773K temperature and 7.08∼35.9h-1 (WHSV) space velocity, the maximum coke deposition is roughly 25% independent of reaction condition. The coke formation experiences a stage of fast increase soon after reaction initiates, while slows down apparently after 18% coke is deposited on the catalyst. Higher temperature, space velocity and methanol partial pressure accelerate the coking rate. Temperature plays an important role in the type of coke formation, which can be distinguished directly by the colour of completely deactivated catalyst. The deactivated catalyst in lower temperature experiences a weight loss with partly recovery of catalyst activity as temperature rises in the hydrothermal atmosphere. The phenomenon of weight loss with catalyst regeneration will not occur as reaction temperature decreases for deactivated catalyst, as well as the different DTA curves for coke burning, are also evidences for the different type of coke formation on SAPO-34 catalyst in different reaction temperature.

Space‐ and Time‐Resolved In‐situ Spectroscopy on the Coke Formation in Molecular Sieves: Methanol‐to‐Olefin Conversion over H‐ZSM‐5 and H‐SAPO‐34

Formation of coke in large H-ZSM-5 and H-SAPO-34 crystals during the methanol-to-olefin (MTO) reaction has been studied in a space- and time-resolved manner. This has been made possible by applying a high-temperature in-situ cell in combination with micro-spectroscopic techniques. The buildup of optically active carbonaceous species allows detection with UV/Vis microscopy, while a confocal fluorescence microscope in an upright configuration visualises the formation of coke molecules and their precursors inside the catalyst grains. In H-ZSM-5, coke is initially formed at the triangular crystal edges, in which straight channel openings reach directly the external crystal surface. At reaction temperatures ranging from 530 to 745 K, two absorption bands at around 415 and 550 nm were detected due to coke or its precursors. Confocal fluorescence microscopy reveals fluorescent carbonaceous species that initially form in the near-surface area and gradually diffuse inwards the crystal in which internal intergrowth boundaries hinder a facile penetration for the more bulky aromatic compounds. In the case of H-SAPO-34 crystals, an absorption band at around 400 nm arises during the reaction. This band grows in intensity with time and then decreases if the reaction is carried out between 530 and 575 K, whereas at higher temperatures its intensity remains steady with time on stream. Formation of the fluorescent species during the course of the reaction is limited to the near-surface region of the H-SAPO-34 crystals, thereby creating diffusion limitations for the coke front moving towards the middle of the crystal during the MTO reaction. The two applied micro-spectroscopic techniques introduced allow us to distinguish between graphite-like coke deposited on the external crystal surface and aromatic species formed inside the zeolite channels. The use of the methods can be extended to a wide variety of catalytic reactions and materials in which carbonaceous deposits are formed.

Synthesis, characterization and catalytic studies of CHA zeotype materials containing boron and iron

Abstract Heteroatoms of boron and iron were incorporated into the framework of zeotype materials with CHA topology, and the materials were used as catalysts for the methanol-to-olefin (MTO) process. IR characterization showed that stretching vibration of the hydroxyl groups associated with the framework Fe appeared at 3615 cm −1 . The catalysts exhibited interesting catalytic performance in the MTO reaction, which was attributed to their weak Bronsted acidities as well as the CHA topology.

Product selectivity and catalytic deactivation of MOR zeolites with different acid site densities in methanol-to-olefin (MTO) reactions

The effects of the acid site densities of MOR zeolites on their product selectivity and deactivation were investigated in the methanol-to-olefin (MTO) reactions. The MOR zeolites showed a high conversion and high yield of the lower olefins at the initial time of the MTO reaction, regardless of their acid site density. However, the conversion and the yield of the lower olefins over them rapidly decreased in the order of MOR(103) < MOR(55) < MOR(12) < MOR(5), where the numbers in parenthesis represent their Si/Al ratios. The materials occluded on the MOR zeolites during the MTO reaction were investigated by the IR, 13C NMR and UV–vis spectroscopic methods. A large amount of polycyclic aromatic hydrocarbons (PAHs) with 3–4 fused aromatic rings was occluded on the MOR(5) zeolite with the highest acid site density. However, alkylbenzenes and alkylnaphthalenes were the major components of the materials occluded on the MOR(103) zeolite with the lowest acid site density. The IR spectra recorded in situ during the MTO reaction over the MOR zeolites clearly exhibited the differences in the accumulation rates of PAH on their pores according to the acid site density. The rapid accumulation of PAH on the MOR zeolites with high acid site densities leads to the loss of the active alkylbenzene intermediates as well as to the blockage of the pores, resulting in their rapid deactivation. The sparse distribution of acid sites in the pores of the MOR zeolites suppresses the condensation of alkylbenzenes in their pores and lengthens their catalyst lifetime.

Methanol-to-olefin over gallosilicate analogues of chabazite zeolite

Gallosilicate analogues of chabazite zeolite with CHA topology have been synthesized, characterized and employed as catalysts for the methanol-to-olefin (MTO) process. 71Ga NMR spectra indicate that Ga is incorporated into the framework of these zeolitic materials during the hydrothermal synthesis. IR measurements show that stretching vibration of the hydroxyl groups associated with the framework Ga appears at 3600 cm −1. The gallosilicates with high Ga contents show Lewis acidity, which is possibly caused by degalliation during calcination, whereas only Brönsted acid sites are observed on the sample with relatively low Ga content. The catalysts show promising catalytic performance in the MTO reaction, particularly from the stability point of view, compared with H[Al]CHA zeolites. This is attributed to the weak Brönsted acidity associated with the framework Ga in the gallosilicates.

Effects of cage shape and size of 8-membered ring molecular sieves on their deactivation in methanol-to-olefin (MTO) reactions

Abstract Four kinds of 8-membered ring (8-MR) small-port molecular sieves with CHA (SAPO-34), ERI (UZM-12), LTA (UZM-9), and UFI (UZM-5) topologies were prepared to investigate the effects of the cage shape and the cage size on their catalytic activities and on deactivation behaviors in methanol-to-olefin (MTO) reactions. UZM-5, -9, and -12 zeolites with Si/Al molar ratios of 5–6 showed high initial activities in the MTO reaction owing to their sufficient acidities, and they were highly selective to lower olefins such as SAPO-34 molecular sieve. However, they were rapidly deactivated in the order of CHA

Synthesis and characterization of a microcystalline ZSM-5 molecular sieve

Microcrystalline ZSM-5 zeolite has been prepared and characterized by using XRD, SEM, FT-IR, nitrogen adsorption, MASNMR techniques and the methanol to olefin (MTO) reaction. The selectivity of the MTO reaction is comparatively high.

Methanol conversion on SAPO-34 catalysts prepared by mixed template method

Abstract Effects of templating on the catalytic performance of SAPO-34 catalyst have been investigated in conversion of methanol to olefins (MTO). SAPO-34 catalysts were synthesized using a single agent or a mixture of morpholine or tetraethyl ammonium hydroxide of (TEAOH) as the template during synthesis of gel with nominal composition as 1Al2O3:1P2O5:0.6SiO2:x morpholine:(2 − x)TEAOH:52H2O. Compared with using single template, using a mixed template leads to the particle size reduction and the morphology change to spherical type formed by the aggregation of nano-sized crystals. Although all the catalysts showed similar activity and product distribution in the MTO reaction, the catalyst obtained using the mixture of 75% morpholine and 25% TEAOH gave the longest lifetime.

Phosphorous modified ZSM-5: Deactivation and product distribution for MTO

The product distribution and deactivation in the methanol-to-olefin (MTO) process over a phosphorous modified catalyst containing 10% H-ZSM-5 was studied in small diameter fixed bed reactors. These studies suggest that methane is formed directly from methanol and/or dimethyl ether and that non-aromatic C 5 + hydrocarbons are intermediates in the MTO reaction, forming light olefins ( C 3 = and C 4 = , but not ethylene) by secondary cracking reactions. Based on photographs of the catalyst taken during the course of the reaction, three distinct coking patterns were observed that might be attributable to the different reactions. Deactivation time of the catalyst is highly dependent on the contact time, doubling the feed rate decreased the deactivation time by a factor of 10 and lowered the olefin production. Changing the feed from pure methanol to 10% methanol in nitrogen reduced methanol capacity of the catalyst considerably, but a slight increase in propylene selectivity was also observed.

Supramolecular origins of product selectivity for methanol-to-olefin catalysis on HSAPO-34.

Ethylene selectivity in methanol-to-olefin (MTO) catalysis is related to the number of methyl groups on benzene rings trapped in the nanocages of the preferred catalyst HSAPO-34. By correlating the time evolutions of the catalysts' 13C NMR spectra and the volatile product distribution following abrupt cessation of methanol flow, we discovered that (in the absence of other adsorbates) propene is favored by methylbenzenes with four to six methyl groups but ethylene is predominant from those with two or three methyl groups. We substantially increased ethylene selectivity by operating at lower methanol partial pressures or higher temperatures, either of which reduces the steady-state average methyl substitution. As a step toward a kinetic analysis of the MTO reaction on HSAPO-34, we treated each nanocage with a methylbenzene molecule as a supramolecule capable of unimolecular dissociation into ethylene or propene and a less highly substituted methylbenzene. Addition of a water molecule to a nanocage containing a methylbenzene produces a distinct supramolecule with unique properties. Indeed, co-feeding water with methanol significantly increased the average number of methyl groups per ring at steady state relative to identical conditions without additional water, and also increased ethylene selectivity, apparently through transition state shape selectivity.

Structural and chemical influences on the MTO reaction: a comparison of chabazite and SAPO-34 as MTO catalysts

A comparison of isostructural H-SAPO-34 and H-chabazite in high space-velocity experiments of the methanol to olefins (MTO) reaction has been performed. The initial selectivity was shown to be similar for both catalysts, and also to be independent of acid site density. However, the deactivation rate was critically dependent on the acid site density. Thus, low acid-site density samples exhibited longer lifetimes at low space velocity conditions than their high acid-site density analogues.

The effect of crystal size of SAPO-34 on the selectivity and deactivation of the MTO reaction

The effect of intracrystalline diffusion on the conversion of methanol to olefins (MTO) over SAPO-34 of crystal sizes in the range 0.25–2.5 μm has been investigated at 425°C in a TEOM (tapered element oscillating microbalance) reactor. The 0.25 μm crystals resulted in a relatively large amount of dimethylether (DME) escaping the pores of SAPO-34 before being converted to olefins, hence giving lower olefin yields. The 0.4–0.5 μm crystals gave the largest capacity of olefin formation. The 2.5 μm crystals resulted in diffusion limitation in methanol conversion, with lower olefin yields as a result. The coking rates increased with increasing crystal size at low coke contents, levelling off at a lower level for the largest crystals than for the smaller crystals at high coke contents. The selectivity to different olefins was independent of the crystal size at relatively low coke contents. The selectivity on the fresh catalysts was controlled kinetically, and an effect of transition-state shape selectivity on the selectivity to olefins was induced by coke. The positive effect of coke on the ethene formation was explained in terms of a transition-state shape selectivity mechanism, as the formation of olefins with different sizes went through differently sized intermediates and the reactions with larger intermediates deactivated more quickly.