Synthesis Of Polyoxymethylene Dimethyl Ethers Research Articles

Polymer-based solid acids with adjustable textural and hydrophobic properties for polyoxymethylene dimethyl ethers synthesis from formaldehyde and methanol

Cation exchange resins are widely utilized in polyoxymethylene dimethyl ethers (PODEn) synthesis due to considerable catalytic performance. However, low specific surface area and strong hydrophilicity hinder the generation of long-chain PODEn and industrial applications. Herein, polymer-based solid acids with adjustable textural and hydrophobic properties were prepared by hydrothermal method. Results indicated that the use of tetrahydrofuran and water as solvents in the preparation process resulted in a significant increase in the specific surface area and pore size distribution of the catalysts. Meanwhile, hydrophobic catalysts prepared by copolymerization under optimal solvent exhibited superior performance over commercial resins. Moreover, two reaction pathways for the reaction of hemiformal (HF1, CH3OCH2OH) with formaldehyde and methanol were investigated by DFT calculations. The intermediate formed by combining sulfonic acid group with carbocation is favorable for embedding formaldehyde in HF1 to promote chain growth, while the direct attack of methanol on HF1 is conducive to chain termination.

Cerium sulfate doped ZrO2 as a solid acid catalyst for the synthesis of polyoxymethylene dimethyl ethers: Influence of Brønsted acidity on the catalytic activity and product selectivity

Cerium sulfate doped ZrO2 solid acid catalyst was prepared by a simple wet impregnation method and tested its efficiency for the synthesis of polyoxymethylene dimethyl ethers (PODE). The surface Brønsted acidity contributed significantly for improved selectivity of desired PODE3–5 and higher PODEn>5. The overall acidity of the catalyst was directly related to the dispersion of CeO2 and the sulfate moiety. The polymerization of dimethoxymethane (DMM) and 1, 3, 5-trioxane (TOX) is in direct correlation with the surface Brønsted acid sites present on the cerium sulfate doped zirconia catalyst surface. The recovered catalyst showed consistent activity up to four catalytic cycles. The structural properties of the catalyst were rationalized by using p-XRD, BET-surface area, NH3-TPD, TGA, UV-DRS, IR, pyridine adsorbed IR, XPS and SEM analyses.

Polystyrene-based hybrid solid acids with tunable hydrophobicity for the synthesis of polyoxymethylene dimethyl ethers from formaldehyde and methanol

In the synthesis of polyoxymethylene dimethyl ethers (PODEn) from formaldehyde (FA) and methanol (MeOH) solution, the methanol conversion and PODEn selectivity are unsatisfactory because the presence of water limits the adsorption of reactants on acid sites and leads to the hydrolysis of some products. Herein, polystyrene-based hybrid solid acids with tunable hydrophobicity and high acid density were synthesized by post-synthesis grafting method in this work. The prepared catalysts were investigated by BET, SEM, TEM, FT-IR, and acid-base titration. The results demonstrated that the surface property of the catalyst could be changed from hydrophilicity to hydrophobicity with the increase of organic group chain length, which effectively improved methanol conversion and PODE3-6 selectivity. The correlation between contact angles and turnover frequency (TOF) was investigated, suggesting that the TOF of polystyrene-based hybrid solid acids treated with octadecyl trichlorosilane was 2.0 times higher than that of untreated ones. Moreover, a supposed reaction pathway for the synthesis of PODEn from formaldehyde and methanol was proposed, indicating that the improvement of hydrophobicity promoted the decrease of local water concentration in the catalyst, thus facilitating the stabilization of carbocations.

Thermodynamics of the heterogeneous synthesis of polyoxymethylene dimethyl ethers from paraformaldehyde and dimethoxymethane in presence of methanol and water

Polyoxymethylene dimethyl ethers (PODEn) are promising diesel additives that can reduce the exhaust emission remarkably. Although the thermodynamic model has been proposed for the homogenous synthesis of PODEn, the thermodynamics of heterogeneous synthesis of PODEn from paraformaldehyde and dimethoxymethane is still a challenge because of the complicated characteristics of paraformaldehyde. In this work, a thermodynamic model containing solid paraformaldehyde and liquid dimethoxymethane, methanol and water was established. Experimental validation showed that this model was applicable in a wide range of reaction temperature and feeding composition. This thermodynamic model was then used to study the influence of the reaction temperature, feeding ratio, and addition of water/methanol on the equilibrium conversion. The different starting compositions from heterogeneous and homogenous reactants for the synthesis of PODEn were compared based on thermodynamic analysis, showing that using paraformaldehyde and dimethoxymethane as feedstock gave the highest CH2O conversion and PODE3-5 yield.

Kinetic and thermodynamic studies on [Omim]Cl/ZnCl2 catalyzed synthesis of polyoxymethylene dimethyl ethers

AbstractPolyoxymethylene dimethyl ethers (OMEs) emerging as green additives for soot emission suppression receive significant attention due to their similar physical properties to diesel. Herein, the transformative pathways of dimethoxymethane (DMM) and trioxane reactions for OMEs production catalyzed by [Omim]Cl/ZnCl2 having different ZnCl2 content were demonstrated in combination with the clarification of active species, kinetic and thermodynamic properties, and chemical nature for trioxane decomposition and subsequential formaldehyde insertion steps. The role of [Omim]Cl, which could transform to [Omim]ZnCl3 or [Omim]Zn2Cl5 under different ZnCl2 content, laid in the formation of active zinc species and promotion of their homogeneity with reactants. The DMM chain growth, which followed the same mechanism and kinetic law, showed kinetic and thermodynamic regimes and had identical equilibrium distribution of OMEs. Only Zn2Cl5− species were active for trioxane decomposition, which was the kinetic‐relevant step, while both ZnCl3− and Zn2Cl5− were effective for DMM chain elongation.

Polyoxymethylene dimethyl ethers synthesis from methanol and formaldehyde solution over one-pot synthesized spherical mesoporous sulfated zirconia

The synthesis of polyoxymethylene dimethyl ethers as an ideal diesel fuel additive is the current hot topic of modern petrochemical industry for their expedient properties in mitigating air pollutants emission during combustion. In this work, a series of spherical sulfated zirconia catalysts were prepared by a one-pot hydrothermal method assisted with surfactant cetyltrimethylammonium bromide (CTAB). The prepared sulfated zirconia catalysts were used to catalyze PODEn synthesis from methanol and formaldehyde solution. Various characterization (XRD, BET, SEM, TGA, NH3-TPD, FTIR, and Py-IR) were employed to elaborate the structure–activity relationship of the studied catalytic system. The results demonstrated that S/Zr molar ratio in precursor solution played an effective role on catalyst morphology and acidic properties, where the weak Brønsted acid sites and strong Lewis acid sites were favorable to the conversion of methanol and formation of long-chain PODEn, respectively. The reaction parameters such as catalyst amount, molar ratio of FA/MeOH, reaction time, temperature and pressure were optimized. The speculated reaction pathway for PODEn synthesis was proposed based on the synergy of Brønsted and Lewis acid sites, which suggested that Brønsted and Lewis acid sites might be advantageous to the activation of polyoxymethylene hemiformals [CH3(OCH2)nOH] and methylene glycol (HOCH2OH), respectively.

Synthesis of polyoxymethylene dimethyl ethers from formaldehyde and dimethoxymethane by the coupling of extraction and catalytic reaction

The coupling of catalysis and extraction for the synthesis of polyoxymethylene dimethyl ethers (PODEn) was investigated from low-cost formaldehyde (FA) and dimethoxymethane (DMM). The process used sulfonic acid resins as catalysts and suitable organic solvents as the extractant. The effects of resin acidity, acid content, and pore structure on the catalytic performance were investigated. The correlation between the partition coefficient of FA and catalytic activity for preparation of PODEn was studied. It was found that the yield of PODE3-8 was significantly higher than that without the use of extractant. Dimethoxymethane (EDC) exhibited the highest performance. The effect of various factors on the catalytic reaction, such as the amount of catalyst, the molar ratio of FA to DMM, reaction temperature, reaction time and the types and dosage of different extractants, were explored. Under the optimal conditions of nFA:nDMM = 2:1, 80 °C, 4 wt% D002 catalyst, vEDC:DMM+FA = 1:1 and 8 h, the PODE3-8 yield enhanced from 6.12% to 55.86%.

Analyses and rates of reactions influenced by water in synthesis of polyoxymethylene dimethyl ethers from trioxane and methylal

Polyoxymethylene dimethyl ethers (PODEn) are promising diesel blend components with high oxygen content, high cetane number and similar properties to diesel. In this paper, we conducted a series of experiments for synthesizing PODEn over a HZSM-5 catalyst in a batch stirred autoclave using methylal and trioxane as reactants. Our research focused on the water influence on these syntheses by contrasting the differences from experiments with or without water and built reaction pathway and rate laws. The experimental and kinetic results showed that decrease of water amount could not only increase yields of the PODEn but also reduce the influences on formation rates of the desired products and eliminate some side reactions. Therefore, it is necessary to optimize water amount in the raw materials for balancing costs of the feed and operation as well as yields of the products to maximize the economy in a commercial production of PODEn.

Effects of water and methanol on synthesis of polyoxymethylene dimethyl ethers from dimethoxymethane and paraformaldehyde

Water and methanol are two unavoidable by-products for synthesis of polyoxymethylene dimethyl ethers (PODEn) from dimethoxymethane (DMM) and paraformaldehyde (PF), but their effects have not been clearly studied. As DMM decomposes with water and is formed with methanol, the decomposition ratio of DMM is a key quantity to describe the effects of water and methanol. Using this quantity, a simple and accurate method was proposed for calculating CH2O conversion and a complete model of Schulz-Flory (SF) distribution was established in the presence of water/methanol, both of which were validated experimentally. Then the effects of water and methanol were investigated in detail. The presence of water and methanol had complicated effects on equilibrium of the reaction, and both significantly decreased the yield of PODE3-5. These results showed that both water and methanol should be removed from the recycling stream in an industrial plant to enhance the efficiency of the reactor.

Identification of reaction path for the synthesis of polyoxymethylene dimethyl ethers from methanol and paraformaldehyde catalyzed by Al-MCM-41 zeolite

Polyoxymethylene dimethyl ethers (PODEn) are ideal blending components of diesel oil. However, there are few studies to identify the reaction path of PODEn synthesis, which is essential in understanding the reaction. In this work, the reaction path of PODEn synthesis from methanol and paraformaldehyde on the Al-MCM-41 zeolite was studied by density functional theory calculation and experiment. The structures of intermediates and transition states of PODEn synthesis were designed and optimized. The three possible reaction paths were analyzed by comparing the Gibbs free energy barriers in the calculation and the product distribution in the experiment. The results showed that paraformaldehyde is decomposed into formaldehyde first. Formaldehyde and methanol react to form a hemiacetal intermediate. Then methanol reacts with hemiacetal to form dimethoxymethane. This step has the highest free energy barrier (121.2 kJ·mol−1), which is the rate-determining step. The long-chain PODEn is formed by chain growth from PODEn-1 and formaldehyde.

Controllable synthesis of polyoxymethylene dimethyl ethers by ionic liquids encapsulated in mesoporous SBA-16

Abstract The promising combustion and emission properties of polyoxymethylene dimethyl ethers (PODEn) are of significant interest. However, the synthesis of PODEn products with desired chain lengths is still a problem facing synthetic PODEn. Herein, a series of unique IL@SBA-16-Cx solid catalysts are prepared by encapsulation of ionic liquids (ILs) within the nanocage of SBA-16 through a silylation method. The structure of the encapsulated catalyst was characterized by UV–vis spectra, Fourier transform infrared (FT-IR), N2 adsorption–desorption isotherms, Powder X-ray diffraction (XRD), Transmission electron microscopy (TEM) and Elemental analysis. The encapsulated catalysts show similar catalytic activity to the homogeneous counterparts and display higher selectivity to the targeted PODE3–5 products than their homogeneous counterparts in the synthesis of PODEn from methanol (MeOH) and trioxymethylene (TOM). The encapsulated catalysts exhibit a superior PODE3–5 selectivity and could be the promising catalysts for PODEn synthetic reaction.

Ionic Liquid Encapsulated in MIL-100(Fe): A Green Catalyst for Synthesis of Polyoxymethylene Dimethyl Ethers from Methanol and Trioxane

Polyoxymethylene dimethyl ether (PODEn) is a promising ether fuel. Especially PODE3–5 can be directly mixed into diesel without modification of the diesel engine’s infrastructure for improving thermal efficiency, and significant reduction in particulate matter (PM) and nitrogen oxides (NOx) emissions. In this paper, a series of heterogeneous catalysts, the imidazole sulfonate ionic liquids (ILs) encapsulated in the cages of metal–organic frameworks (MOFs) MIL-100(Fe), were used successfully and efficiently in the synthesis of polyoxymethylene dimethyl ethers (PODEn) from methanol and trioxane (TOX). These catalysts have exhibited excellent catalytic properties because of strong Brønsted acid sites. MIL-100(Fe)@[BSO3HIm][CF3SO3] shows the best catalytic activity with 96.98% conversation of TOX and 47.49% selectivity of the desired product PODE3–5 under a mild reaction condition. Of importance is that the catalyst is recovered easily and reused.

Catalytic Performance and Mechanistic Insights into the Synthesis of Polyoxymethylene Dimethyl Ethers from Dimethoxymethane and Trioxymethylene over ZSM-5 Zeolite

Acidic zeolites are considered a kind of efficient catalysts in the synthesis of polyoxymethylene dimethyl ethers (PODEn) with the fomula of CH3O–(CH2O)n–CH3, a promising green diesel fuel or additive from methanol derivatives; however, the crucial parameters that determine PODEn yield and reaction mechanism over zeolite catalyst remains rather ambiguous. In this work, the relationship between the catalytic performance of HZSM-5 zeolite and Si/Al ratio and reaction temperature was revealed through PODEn synthesis from dimethoxymethane (DMM) and trioxymethylene (TOM). The results indicate that the Si/Al ratio and reaction temperature are interdependent in determining the PODEn yield; High Al HZSM-5 zeolites( Si/Al ≤ 50) show the highest PODE2-8 selectivity and yield only at 70–90 °C, whereas low Al zeolites (Si/Al ≥ 109) possess the better activity to PODE2–8 at 70–150 °C. The reaction mechanism was then investigated through in-situ IR characterization and DFT calculation. The important intermediates ZO-CH2OCH2OCH2OH, ZOCH2OCH3 and ZO(CH2O)nCH3 (ZOH: HZSM-5 zeolite) were detected by In situ IR spectra during the exposure of HZSM-5 to TOM, DMM and co-feed. DFT calculations further verified above reaction intermediates and elucidated the reaction elementary steps. Reaction mechanism was proposed as follows: the Bronsted acidic sites provided by Al–OH of HZSM-5 zeolite promote the dissociation of TOM to monomer CH2O and transform DMM to ZOCH2OCH3 and CH3OH; Next, the CH2O species react with short chain ZO(CH2O)nCH3, forming long chain ZO(CH2O)nCH3; After that, the PODEn products are formed by sealing ZO(CH2)nOCH3 with CH3OH. Ultimately, the difference of catalytic performance resulting from the effect of Si/Al ratio and reaction temperature, based on the proposed mechanisms, was given a logical explanation. The detailed reaction mechanism in the synthesis of polyoxymethylene dimethyl ethers from dimethoxymethane and trioxymethylene over HZSM-5 zeolite were investigated through a series of in-situ IR characterization and DFT calculation. Subsequently, the difference of catalytic performance resulting from the effect of Si/Al ratio and reaction temperature, was given a possible explanation.

Citric acid-modified beta zeolite for polyoxymethylene dimethyl ethers synthesis: The textural and acidic properties regulation

Beta zeolite exhibits satisfactory catalytic activity in the synthesis of polyoxymethylene dimethyl ethers. However, the microporous channels of beta zeolite have adverse effects on mass transfer of macromolecules. In this work, it was found that the textural and acidic properties of catalysts could be improved by citric acid treatment. Extensive characterization confirmed that citric acid induced significant pore-expanding on catalysts, and its dealumination and realumination effect could lead to reduced amount of total acid sites and increased amount of Brønsted acid sites (BAS). Meanwhile, a good positive correlation was found between amount of external weak BAS and initial reaction rate. It was speculated that BAS with relatively weak acid strength might be the primary active sites in this reaction. Compared with the blank sample, the initial reaction rate of optimal catalyst was increased by 100 %, and the conversion of trioxane was increased by 86.5 %, indicating a significantly improved catalytic activity.

Synthesis of polyoxymethylene dimethyl ethers over different microporous and mesoporous zeolites: the effects of acidity and pore size

Polymethoxydimethyl ether (PODEn) is an ideal blending component of diesel oil.

Cover Feature: Nature of Synergy between Brønsted and Lewis Acid Sites in Sn–Beta Zeolites for Polyoxymethylene Dimethyl Ethers Synthesis (ChemSusChem 19/2019)

Cover Feature: Nature of Synergy between Brønsted and Lewis Acid Sites in Sn–Beta Zeolites for Polyoxymethylene Dimethyl Ethers Synthesis (ChemSusChem 19/2019)

Nature of Synergy between Brønsted and Lewis Acid Sites in Sn-Beta Zeolites for Polyoxymethylene Dimethyl Ethers Synthesis.

The role of Lewis and Brønsted acid sites and their potential synergy remains ambiguous for the production of polyoxymethylene dimethyl ethers (OME), which are suitable as a Diesel substitute. Here, this synergistic effect was investigated by using a series of beta polymorph A (BEA) zeolites with various degrees of Brønsted and Lewis acidity. Lewis acidity was introduced in dealuminated zeolites by Sn grafting in dichloromethane. These sites were only active in paraformaldehyde decomposition, OME growth, and acetalization. The Brønsted acid sites arising from bridging hydroxyl groups were active for all reaction steps, and notably for trioxane ring-opening and dissociation to formaldehyde (FA), which did not occur on the Lewis acid sites. Presence of both Lewis and Brønsted acid sites led to a four-fold increase in turnover frequency and a significant decrease of byproduct formation compared with the parent zeolite during OME synthesis from dimethoxymethane and trioxane. The synergistic effect between both types of acid sites is explained by FA insertion on Lewis acid sites leading to OME growth. Interaction between tetrahedral Sn and the carbonyl group of FA resulted in an activated carbonyl bond, which was likely the initial step for insertion of FA into OME.

The Synergistic Effect of Acidic Properties and Channel Systems of Zeolites on the Synthesis of Polyoxymethylene Dimethyl Ethers from Dimethoxymethane and Trioxymethylene

A series of zeolites with different topology structures, including SAPO-34, SUZ-4, ZSM-5, USY, MOR, and beta, were used to synthesize polyoxymethylene dimethyl ethers (PODEn) from dimethoxymethane (DMM) and trioxymethylene (TOM). The influence of acidic properties and channel systems were studied by activity evaluation, characterization, and theoretical calculation. The results confirmed that pore mouth diameter larger than a TOM molecule was an essential prerequisite for the synthesis of PODEn over zeolites, and the synergistic effect between medium-strong Brønsted acid sites (Brønsted MAS) and the maximal space of zeolites available determined the catalytic performance of all studied zeolites. DMM and TOM were firstly decomposed into methoxymethoxy groups (MMZ) and monomer CH2O over Brønsted MAS. Subsequently, the steric constraint of the maximum included sphere, with an appropriate size in zeolite channels, can promote the combination of CH2O and MMZ to form transition species ZO(CH2O)nCH3, which reacted with the methyl-end group to form PODEn over Brønsted MAS. Moreover, the reaction temperature showed different effects on the product selectivity and distribution, which also mainly depends on the size of the maximum space available in zeolite channels.

Cover Feature: Insights into the Nature of the Active Sites of Tin‐Montmorillonite for the Synthesis of Polyoxymethylene Dimethyl Ethers (OME) (ChemCatChem 13/2019)

Cover Feature: Insights into the Nature of the Active Sites of Tin‐Montmorillonite for the Synthesis of Polyoxymethylene Dimethyl Ethers (OME) (ChemCatChem 13/2019)

Insights into the Nature of the Active Sites of Tin‐Montmorillonite for the Synthesis of Polyoxymethylene Dimethyl Ethers (OME)

AbstractIn this study, we report that intercalation of tin into montmorillonite clay (Sn‐MMT) boosts catalytic activity for the synthesis of polyoxymethylene dimethyl ethers, a promising diesel fuel additive. This increase in activity is attributed to the formation of a hierarchical catalyst displaying high surface area with accessible Brønsted and Lewis acid sites. Extensive characterization such as MAS‐NMR, XRD and in‐situ DRIFTS confirmed that tin insertion induced reorganization of the clay layers into a disorganized house‐of‐cards structure. It also revealed that tin resided between the clay layers as defective SnO2 nanocrystals stabilized by the negative charge of MMT. The source of acidity in Sn‐MMT was studied by comparing it with two similar materials, namely Sn(OH)4, and thermally treated Sn‐MMT. Low activity measured for isolated hydroxylated SnO2 nanoparticles illustrates the importance of having clay and tin in close proximity. Meanwhile, deactivation upon thermal treatment occurred via sintering of the tin phase and diffusion of the protons inside the layers. Sn‐MMT acidity is attributed to the combination of Si−OH−Al groups, formed as a by‐product of SnO2 crystallization, and undercoordinated Sn surface sites, stabilized by the negative charge of MMT.