Novel Routes to Ethylene Glycol Synthesis via Acid-Catalyzed Carbonylation of Formaldehyde and Dimethoxymethane
- Author(s): Celik, Fuat Emin;
- Advisor(s): Bell, Alexis T;
- et al.
Carbon-carbon bond forming carbonylation reactions were investigated as candidates to replace ethene epoxidation as the major source of ethylene glycol production. This work was motivated by the potentially lower cost of carbon derived from synthesis gas as compared to ethylene. Synthesis gas can be produced from relatively abundant and cheap natural gas, coal, and biomass resources whereas ethylene is derived from increasingly scarce and expensive crude oil. From synthesis gas, a range of C1 compounds containing no C-C bonds, such as methanol, formaldehyde and its closely related acetals such as dimethoxymethane (DMM), can be readily obtained.
Formaldehyde carbonylation was once used commercially to produce precursors to ethylene glycol. Previous investigations of this reaction were carried out in the liquid phase, and required high carbon monoxide pressures (tens to hundreds of atmospheres) to overcome the low solubility of carbon monoxide. At lower carbon monoxide pressures, the reaction of formaldehyde with itself, the Cannizzaro disproportionation reaction, becomes the dominant process. The focus of this work was to carry out the carbonylation of formaldehyde and DMM with high selectivity and activity towards ethylene glycol precursors without requiring harsh conditions.
Formaldehyde carbonylation was investigated in the liquid-phase using methyl formate (MF) as the source of CO using silicotungstic acid and other heteropoly acids as the catalyst. Methyl glycolate (MG) and methyl methoxyacetate (MMAc), both precursors to ethylene glycol, were formed along with DMM and dimethyl ether (DME), the primary byproducts. Using MF as the CO source avoided the need to pressurize the headspace with high pressures of CO gas. The effects of formaldehyde source, reaction temperature, reaction time, and catalyst were investigated. Methoxymethanol, paraformaldehyde, 1,3,5-trioxane, and DMM were examined as sources of formaldehyde. The highest yields of methyl glycolate and methyl methoxyacetate were obtained using 1,3,5-trioxane as the source of formaldehyde. Release of carbon monoxide from MF was found to be slow and limited the rate of carbonylation. Of the heteropoly acids investigated, silicotungstic acid produced the highest yields of MG and MMAc, whereas methanesulfonic acid did not produce these products at similar acid loading. The difference in the effectiveness of heteropoly acids and methanesulfonic acid is ascribed to the role of the anion of the heteropoly acid, a soft base, in stabilizing the reactive intermediates involved in the carbonylation of formaldehyde.
While using MF as the CO source provided milder conditions, the selectivity to ethylene glycol precursors was still low. To achieve high selectivity under mild conditions, a novel vapor-phase process was developed. By carrying out the reaction in the vapor phase, the need for high pressure to dissolve CO in a liquid was avoided, and by using the dimethyl acetal of formaldehyde, DMM, the need for water or alcohol was avoided. Using an acid zeolite, Faujasite (FAU), as the catalyst it was possible to produce MMAc with a selectivity of up to 79% and a yield of up to 20% based on DMM at 3 atm of CO pressure. The disproportionation of DMM to produce DME and MF was the only competing process observed. The rate of disproportionation was minimized by operating at high CO to dimethoxymethane feed ratios.
By selecting zeolites of different frameworks and Si/Al ratios, the effects of pore size and connectivity and the proximity of acid sites on the carbonylation of dimethoxymethane to produce methyl methoxyacetate were revealed. FAU, ZSM-5 (MFI), Mordenite (MOR), and Beta (BEA) showed very similar activity for DMM carbonylation. However, FAU had the highest selectivity compared to the other zeolites because of its very low activity towards disproportionation. The higher rate of DMM disproportionation observed for MFI, MOR, and BEA is ascribed to the small pores of these zeolites, which facilitate the initial and critical step in the formation of dimethyl ether and methyl formate. Ferrierite showed very low activity for both carbonylation and disproportionation. Increasing the Si/Al ratio for both FAU and MFI led to an increase in the turnover frequency for DMM carbonylation. The low rate of MMAc formation found at low Si/Al ratios was proposed to be due to repulsive interactions occurring between adsorbed species located within the same supercage of FAU or channel intersection of MFI.
Mechanisms were proposed for both DMM carbonylation and disproportionation reactions over acid zeolites and were evaluated using in situ infrared spectroscopy. Surface intermediates for both carbonylation and disproportionation reactions were observed spectroscopically, and their responses to changes in reaction conditions were consistent with steady-state kinetic experiments and the predictions of density functional theory (DFT) calculations. For DMM carbonylation, the solvation of the carbocationic transition state of the CO insertion step was observed when gaseous nucleophiles promoted the formation of the CO insertion product, a methoxyacetyl surface species. The surface concentration of the methoxyacetyl species at steady state, as measured by infrared spectroscopy, was 10 times smaller on zeolite FAU than on MFI, despite the higher rate of DMM carbonylation on FAU. This was supported by DFT calculations, which predicted a very small barrier for the reaction of the methoxyacetyl species over FAU, but a substantial barrier over MFI, leading respectively to smaller and larger concentrations of this species. The rate expression derived from the proposed mechanisms was used in a plug-flow reactor model to predict the rates of carbonylation and disproportionation over FAU as functions of reaction temperature and DMM and CO partial pressures. The results showed good agreement with steady-state rate measurements.