Illustrating the overall reaction network of the synthesis-gas-to-hydrocarbons process over iron-zeolite bifunctional catalysis

Evaluating the Role of Descriptor- and Spectator-Type Reaction Intermediates During the Early Phases of Zeolite Catalysis

“…The simultaneous identification of carbonylated (i.e., indicating nondissociative CO activation) and epoxy (i.e., indicating dissociative CO activation) species over zeolites confirmed the coexistence of an oxygenate/Pichler–Schulz mechanism and a carbide/Biloen–Sachtler mechanism in the FTS chemistry. , Methanol/oxygenated species were also identified by NMR (Figure j), produced over the metallic iron phase only (i.e., without any zeolite downstream), via a liquid analysis of the cold-trapped effluent gas . These oxygenated alcohols/ethers were produced over the standalone iron catalyst during the conventional FTS reaction, which could trigger the MTH-like transformations over the zeolite phase downstream, controlling the final product selectivity. ,, Herein, the presence of a metallic catalyst (not the zeolite) is essential to yielding alcohol/ethers from CO 2 /CO, although zeolites could produce carbonylated species from alcohols/ethers. ,, Therefore, detecting all these zeolite-trapped oxygenates/carbonylated species signifies their mechanistic resemblances to the conventional MTH chemistry, as these species are independently capable enough to initiate HCP species formation to govern the autocatalytic stage of the reaction ,, via promoting acid-catalyzed C–C bond formations. ,, Due to this feature, supramolecular hybrid reactive centers (constituted by inorganic zeolites and organic hydrocarbons) are acknowledged as active or working catalysts during the zeolite-catalyzed hydrocarbon conversion, ,, especially in MTH chemistry. We find intangible similarities in the nature of zeolite-trapped organics between both CO/CO 2 hydrogenation and MTH/MTO chemistry.…”

“…Around 70 years after the pioneering work of Emmett and co-workers, we now understand that the oxygenate mechanism in the FTS route has a conceptual resemblance with methanol-related chemistry, especially when involving zeolites in a bifunctional catalytic system. , The independent ability of zeolite to facilitate C–C bond formations from oxygenated/carbonylated species prompted a review of Emmett’s work from a fresh perspective by utilizing solid-state NMR spectroscopy and well-designed control experiments (see Section ). , Although more investigations are required in the future, we demonstrate a few case studies over the last 70 years to venerate the Pichler–Schulz mechanism during the hydrogenation of CO/CO 2 . ,,,,,− ,,, …”

“…The possibility of gas-phase polymerization can be limited upon a combination of zeolites, and the reaction feed from the metallic phase could be tuned to synthesize petrochemicals (e.g., aromatics). Strategically, the following two research strategies were employed for the hydrogenation of CO/CO 2 : (i) Fischer–Tropsch synthesis (FTS) on the metallic catalyst followed by oligomerization/cracking/aromatization reactions on the zeolitic component ,,,− (N.B. : CO 2 -derived FTS is associated with the reverse water-gas shift reaction (RWGS: CO 2 + H 2 ⇌ CO + H 2 O)) and (ii) methanol synthesis over a metallic catalyst (CO + 2H 2 → CH 3 OH; CO 2 + 3H 2 → CH 3 OH + H 2 O) in combination with the zeolite-mediated classical MTH process (Figure ).…”

“…: CO 2 -derived FTS is associated with the reverse water-gas shift reaction (RWGS: CO 2 + H 2 ⇌ CO + H 2 O)) and (ii) methanol synthesis over a metallic catalyst (CO + 2H 2 → CH 3 OH; CO 2 + 3H 2 → CH 3 OH + H 2 O) in combination with the zeolite-mediated classical MTH process (Figure ). ,,− ,− Similar to MTH/MTO chemistry, the zeolite-based hybrid inorganic–organic supramolecular reactive centers also influence the catalysis during the direct hydrogenation of CO/CO 2 . ,,, Therefore, the impact of the organic reaction intermediate is inevitable in zeolite catalysiseither governing the catalysis (descriptor) or promoting the deactivation (spectator). Both deserve equal attention by carefully evaluating their profiles to have the desired selectivity and catalyst lifetime.…”

“… ,,,, During the CO hydrogenation over the same bifunctional catalytic system, four different selectivities were obtained: short olefins (cf. MOR), light paraffins (FER and ZSM-5), long (C 5+ )-chain hydrocarbons (ZSM-22), and aromatics (ZSM-5) (reaction conditions: 20 bar pressure, 375 °C temperature, H 2 /CO = 1, and 5000 mL g –1 h –1 ) (Figure a,b) . During the CO 2 hydrogenation, the identical bifunctional Fe@K/zeolite-based catalytic system also led to four different hydrocarbon selectivities: (i) light olefins (MOR), (ii) paraffins (FER), (iii) long (olefinic) hydrocarbons (ZSM-22), and (iv) aromatics (ZMS-5) , (reaction conditions: 30 bar, 375 °C, H 2 /CO 2 = 3, and 10000 mL g –1 h –1 ) (Figure c,d).…”