Direct Conversion of Syngas into Light Olefins with Low CO₂ Emission

Abstract

Direct conversion of syngas into light olefins over bifunctional catalysts has made significant progress; the C 2 = −C 4= selectivity in hydrocarbons reaches >80%. Nevertheless, a relatively harsh reaction condition (>380 °C, 1.0 MPa) led to producing large amounts of CO 2 (>40%) and gave a low olefin/ paraffin (O/P) ratio (<10) as a result of significant promotion of water−gas shift (WGS) reaction and overhydrogenation of olefins. In this context, attempts are made here to develop a highly active lowtemperature composite catalyst. It was found that a zinc−cerium−zirconium solid solution (Zn x Ce 2−y Zr y O 4 ) and a SAPO-34 mixture showed CO conversion, light olefin selectivity in hydrocarbons, and O/P ratios of about 7%, 83%, and 23, respectively, at 300 °C and 1 atm. More interestingly, this catalyst showed CH 4 selectivity and CO 2 emission lower than 5 and 6%, respectively. A combination of experimental, in situ spectroscopy, and theoretical calculation results reveals that doping Ce in Zn x Zr 2.0 O 4 greatly inhibits the WGS reaction by increasing the formation energy barrier of carboxylate intermediate species, but increases surface oxygen vacancy concentration of the composite through formation of a solid solution, and as a consequence, improving the catalytic activity for conversion of syngas at mild conditions by enhancing the interaction of CO with the catalyst, which elongates the C−O bond of the HCO* species.

Boosting the synthesis of value-added aromatics directly from syngas via a Cr2O3 and Ga doped zeolite capsule catalyst

“…S1d †) can be attributed to the desorption of CO 2 molecules adsorbed on the oxygen vacancies of Cr 2 O 3 . The oxygen vacancy-dominated CO activation pathway on the reducible metal oxide has been claried by multiple characterization studies and theoretical analysis, 20,21 during which CO molecules adsorbed and activated by the oxygen vacancies rst combined with the H* species to form formaldehyde (*HCO and *H 2 CO) and formate (*HCOO) intermediates, and the successive hydrogenation step guaranteed the formation of methoxyl (*CH 3 O) species and the nal product, methanol. It should be noted that the formaldehyde or formate intermediates may directly diffuse into the channels of the acidic zeolite for the C-C coupling and cyclization reactions without undergoing the following hydrogenation step mentioned above when two catalytic components, reducible metal oxide and zeolite, are present in the tandem process.…”

Synthesis Gas Conversion to Lower Olefins over ZnCr‐SAPO‐34 Catalysts: Role of ZnO−ZnCr2O4 Interface

“…Thus, formate species should be an important intermediate in the conversion of syngas to methanol. [44] Compared to Zn 1 Cr 2 , the intensity of formate rapidly become weak in Zn 1 Cr 1 , which implies that ZnOÀ ZnCr 2 O 4 interface could be favorable for the transfer of formate species. Nevertheless, these typical bands at 2840-2845 and 2945-2950 cm À 1 corresponding to CÀ H stretching of formate not appear.…”

On the reaction mechanism of MnOx/SAPO-34 bifunctional catalysts for the conversion of syngas to light olefins

“…59 Formate species have been reported as intermediates leading to methanol formation. 20,27,31 However, the as(CH3) band at 2900-2900 cm -1 56-58 typical of methoxy species was not observed in the DRIFT spectra. This feature was explained by a slower formation rate of methoxy species from formates compared to the formation rate of formates and methanol 7 Addition of only 10% wt.…”

“…Most of oxides contained Zn: ZnO, 11 ZnCrOx, 8,[12][13][14][15][16][17][18][19] ZnZrOx, 10,20,21 ZnAlOx, [22][23][24] Zrpromoted ZnAlOx, 25 La-doped ZnAlOx, 26 Zn spinels, 27 ZnCeZrOx. [28][29][30][31] Alternatively, MnOx, 32 MnGaOx, 33,34 or ZrInOx, 35 was used. The zeotype corresponded most of the time to microsized) or to mesoporous M-SAPO, 8,11,32 hierarchical SAPO-34, 21 nanosized SAPO-34, 12,25 SAPO-18, 13,18 SAPO-35, 18 various SAPOs, 24 AlPO-18, 36 SSZ-13, 17,20 .…”

“…On the other hand, methanol and dimethyl ether (DME) formation in significant amounts was evidenced over ZnO, 10 ZnZrOx, 10,20 ZnAlOx, 22,23 , Zn spinels 27 and ZrInOx 35 In this case, non-dissociative activation of CO leads to formates and methoxy species as shown by in situ DRIFT measurements. 20,27,31 Light olefins are then produced over the zeotype similarly to the MTO process and methane arises from consecutive methanol decomposition by C-O cleavage on oxygen vacancies. 35 Therefore, further studies on the reaction mechanism are needed.…”