Spectroscopic investigation into the design of solid–acid catalysts for the low temperature dehydration of ethanol

Abstract

The increased demand for bulk hydrocarbons necessitates research into increasingly sustainable, energy-efficient catalytic processes. Owing to intricately designed structure-property correlations, SAPO-34 has become established as a promising material for the low temperature ethanol dehydration to produce ethylene. However, further optimization of this process requires a precise knowledge of the reaction mechanism at a molecular level. In order to achieve this a range of spectroscopic characterization techniques are required to probe both the interaction with the active site, and also the wider role of the framework. To this end we employ a combination of in situ infra-red and neutron scattering techniques to elucidate the influence of the surface ethoxy species in the activation of both diethyl ether and ethanol, towards the improved formation of ethylene at low temperatures. The combined conclusions of these studies is that the formation of ethylene is the rate determining step, which is of fundamental importance towards the development of this process and the introduction of bio-ethanol as a viable feedstock for ethylene production.

Combining catalysis and computational fluid dynamics towards improved process design for ethanol dehydration

“…[35][36][37] Acid Catalysed dehydration of ethanol SAPO-34 has previously been shown to be a stable, ( Figure S7 Please do not adjust margins Please do not adjust margins (C2H5OH) to ethylene (C2H4) at low (< 250 o C) temperatures; it is therefore is an ideal candidate for use as our model catalyst. In our previous work, [22,23] and in the wider literature, [24][25][26][27][28][38][39][40] the dehydration of C2H5OH to C2H4 proceeds through three main reaction pathways (scheme 1); a -direct formation of C2H4 from C2H5OH, b -dehydration of C2H5OH to form C4H10O and cdegradation of C4H10O to give C2H4 and C2H5OH. Our preliminary studies have suggested that C4H10O is not a competing byproduct, rather an intermediate for the formation of C2H4 (following reaction b then reaction c).…”

“…[22,23] Powder Xray diffraction (XRD, Figure S1) confirmed that the sample was highly crystalline and that CHA (Chabazite) was the only crystalline phase observed, with unit cell parameters (Table S1) of a and b falling in the range of 13.7 -13.8 Å, with c = 15.0 Å (α = β = 90 o and γ = 120 o ). [22,23,31,32] Similarly the surface area and pore volume were in good agreement with previous literature at 580 m 2 g -1 , with micropore volume of 0.27 cm 3 g -1 , and total pore volume of 0.33 cm 3 g -1 . [31,32] ICP analysis of the sample showed a silicon loading of 3.4 wt% (Table S1), corresponding to a maximum possible acidity of 1.2 mmol/g.…”

“…In our previous experimental work we described our targeted approach to designing a solid-acid heterogeneous catalyst for this process. By isomorphous incorporation of small quantities of dopant species within a microporous aluminophosphate framework, highly active and selective Brønsted acid sites were generated, affording near-quantitative yields of ethylene at temperatures as low as 300 o C. [22,23] We found that the industrially deployed Methanol To Olefins (MTO) catalyst, silicon substituted aluminophosphate, SAPO-34, was particularly effective. This was attributed to a combination of isolated strong Brønsted acid sites, favoring the selective formation of ethylene, within a chabazite (CHA) framework (possessing a pore diameter of just 3.8 Å) that bestows product selectivity on the reaction.…”

“…This was attributed to a combination of isolated strong Brønsted acid sites, favoring the selective formation of ethylene, within a chabazite (CHA) framework (possessing a pore diameter of just 3.8 Å) that bestows product selectivity on the reaction. [23] A large body of work has been dedicated to understanding the mechanism for this reaction, using a range of solid acid catalysts. [24][25][26][27][28] The precise mechanism is still under debate, particularly on the role of diethyl ether (C4H10O).…”

Unraveling the Homologation Reaction Sequence of the Zeolite‐Catalyzed Ethanol‐to‐Hydrocarbons Process

“…Only the first step of the ETH reaction (that is, dehydration) is relatively better understood in the literature. [8,[22][23][24][25] Unfortunately,t he rest of the reaction sequences (that is,homologation, cyclization, aromatization, and cracking) have yet to be unraveled. In particular,t he exact mechanistic routes to the origin of > C 2 even-numbered (that is,homologated products such as C 4 -butylene) and > C 2 odd-numbered (that is,n on-homologated products such as C 3 -propylene) carbon-containing hydrocarbon pool (HCP) species from C 2 -ethanol are still under scrutiny within the ETH process.T he importance of the mechanistic understanding of this industrial reaction should not be underestimated;assuch, information is crucial to maximizing yields and developing new and/or improved heterogeneous catalysts.…”