Implications of Cofeeding Acetaldehyde on Ethene Selectivity in Methanol-to-Hydrocarbons Conversion on MFI and Its Mechanistic Interpretation

Abstract

Cofeeding acetaldehyde (1−4 C%) with dimethyl ether (DME) and methanol (DME:methanol ∼9:1, on a carbon basis) on two MFI-type zeolites: a conventional (Conv) MFI zeolite (SiO 2 /Al 2 O 3 ∼80, diffusion length ∼250 nm) and a self-pillared pentasil (SPP) MFI zeolite (SiO 2 / Al 2 O 3 ∼150, diffusion length ∼1.5 nm) at 673 K resulted in a monotonic increase in selectivity toward ethene (from 9.3 to 15 C% on Conv MFI and from 1.4 to 6.4 C% on SPP MFI) and methylbenzenes (from 4.9 to 7.8 C% on Conv MFI and 2.6 to 5.3 C% on SPP MFI). The mechanistic basis for this increase in ethene and methylbenzene (MB) selectivity is acetaldehyde undergoing multiple aldol-condensation reactions to form higher homologues (e.g., sorbaldehyde) that subsequently undergo ring-closure followed by dehydration to form aromatics (e.g., benzene). Cofeeding acetaldehyde, therefore, increases the concentration of aromatics inside the zeolite pores, which in turn enhances the propagation of the aromatics-based methylation/dealkylation cycle and consequentially results in higher ethene production. In an isotopic experiment where 13 C 2 -acetaldehyde (∼4 C%) was coreacted with unlabeled DME and methanol (DME:methanol ∼9:1, on carbon basis) on Conv MFI and SPP MFI at 673 K, ethene present in the effluent was enriched with two 13 C labels and the net 13 C-content in ethene (11−12% on Conv MFI and 45−52% on SPP MFI) was higher than the 13 C-content in MBs (5−6% on Conv MFI and 9−17% on SPP MFI). Ethene, therefore, besides being formed via aromatic-dealkylation, is also being produced from acetaldehyde or its aldol-condensation products via a direct synthesis route.

Formation Pathways toward 2‐ and 4‐Methylbenzaldehyde via Sequential Reactions from Acetaldehyde over Hydroxyapatite Catalyst

“…2,4,6‐Octatrienal has significant selectivities (>95 %) at the lowest conversions, which suggests that 2,4,6‐octatrienal is a primary product of 2‐butenal self‐condensation. The selectivity to 2,4,6‐octatrienal decreases as that for 2‐MB=O and 2‐MB‐OH increases, which together with reported mechanisms for dehydrocyclization reactions, strongly suggests that the 2‐MB products form as secondary products from 2,4,6‐octatrienal. Furthermore, the higher selectivity of 2‐MB=O compared to 2‐MB‐OH within the lower range of conversion (0–50 %) shows that 2‐MB‐OH forms from 2‐MB=O via H‐transfer steps that likely use ethanol as a hydrogen donor (e.g., the MPV reaction) .…”

“…[26] Notably,t he Ca-HAP catalysta nd reactionc onditions used here do not dehydrogenate CÀCb onds of small oxygenates, [7,26] and therefore, saturated aldehydes anda lcohols do not reform the enes or enals needed to create aromatic products. [29,30] Figure 2s hows that the ratio of the concentrationso ft he 2-MB products to that of the 4-MB products (b)i ncreases with acetaldehyde conversion with little dependence on the pressure of acetaldehyde (0.35, 0.56, or 0.87 kPa C 2 H 4 O, 1kPa C 2 H 5 OH, 99 kPa H 2 ,5 48 K) and does not depend on the initial acetaldehyde pressure. The changes in b values suggest that pathways that form 2-and 4-MB products involve different combinationso fr eactive intermediates formed in situ, because the b value would not otherwise appear as as ingle value function of the acetaldehyde conversion.T he increasing of b value shows that the ratio of the formationr ates of 4-MB to 2-MB are greatest at the lowest acetaldehyde conversions,w hich suggestst hat the formation of 4-MB products involves the self-condensation reaction of 2-butenalr ather than the crosscondensation of acetaldehyde and 2,4-hexadienal.…”

“…Besides the 2‐MB and 4‐MB products and their intermediates, other C 4 –C 6 alcohols and aldehydes (e.g., 1‐butanol, 1‐hexanol, butyraldehyde) form as side products via direct (e.g., the Meerwein‐Ponndorf‐Verley (MPV) reaction) or indirect, surface‐mediated H‐transfer from ethanol . Notably, the Ca‐HAP catalyst and reaction conditions used here do not dehydrogenate C−C bonds of small oxygenates, and therefore, saturated aldehydes and alcohols do not reform the enes or enals needed to create aromatic products …”

“…[5-7, 21, 26-28] While the cascades of ethanol coupling( Guerbet reaction) processesp roduce broad distributions of linear and branched alcoholsb yn umerous self-and cross-condensation reactions, [7,26] narrower product distributions that provide selectivity to specific species emerge if unsaturated aldehydes (and enals) mainlyp articipate in condensation reactions. These pathways terminate with the dehydrocyclization of C 6 and C 8 enals (e.g., 2,4-hexadienal, [29] 2,4,6-octatrienal) to form aromatic products [21,30] (e.g.,b enzene, [21,29] trimethylbenzene, [30] tolualdehydes [26][27][28] ), because these products either lack the functional groups needed for furtherc ondensation reactions or possess greatly reduced reactivity as ar esult of combination of steric and electronic effects. [26] Among the aromatics that can form in these reaction networks, ortho-and para-tolualdehydes(specifically,4 -methylbenzaldehyde (4-MB = O) and2 -methylbenzaldehyde( 2-MB = O)) have the potential to replacex ylenes in the productiono ft he large volume monomers terephthalic acid and phthalic anhydride, respectively.T he formation of these C 8 aromatics( 2-/4-MB = O) from acetaldehyde is difficult and provide only moderate yields (30 %), because the multistep addition of acetaldehyde to growing enal chainsc ompetes with H-transfer pathways that produce mixed alcohol products.…”

A mechanistic basis for the effect of aluminum content on ethene selectivity in methanol-to-hydrocarbons conversion on HZSM-5

“…Mechanistically these effects of catalyst composition and morphology and of process conditions can be rationalized as a consequence of the relative extents of propagation of the olefins-and the aromatics-based methylation/cracking cycles [13]. Previously we have ascribed the effects of temperature [13], feedstock composition [13,18,21], reaction conditions [22], and crystallite size [23] on MTH product selectivity to either an enhancement in the number of chain carries of the olefins-or aromatics-based cycles or to transport restrictions which selectively enhance the propagation of the aromatics-based cycle relative to that of the olefins-based cycle. A mechanistic basis relating zeolite morphology and composition to the complex hydrocarbon chemistry in MTH described by the dual-cycle schematic necessitates that: (i) we ascribe terminal products to the olefins-and aromatics-based cycles and relate the yield of these products to the relative propagation of the two catalytic cycles;…”