Silicon redistribution, acid site loss and the formation of a core–shell texture upon steaming SAPO-34 and their impact on catalytic performance in the Methanol-to-Olefins (MTO) reaction

Minova, Ivalina B.; Barrow, Nathan S.; Sauerwein, Andrea C.; Naden, Aaron B.; Cordes, David B.; Slawin, Alexandra M. Z.; Schuyten, Stephen J.; Wright, Paul A.

doi:10.1016/j.jcat.2021.01.012

Cited by 16 publications

(12 citation statements)

References 52 publications

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“…Direct spectroscopic proof of this migration is difficult to obtain. In contrast to SAPO materials (Si-islanding , ) or gallosilicates, T-atom reshuffling cannot readily be traced by NMR techniques in zeolites and ferrosilicates. , The further development of novel NMR solutions (potentially challenging with paramagnetic Fe) or a combination of indirect validation methods (e.g., cobalt titration) would be needed.…”

Section: Discussionmentioning

confidence: 99%

Selective Formation of α-Fe(II) Sites on Fe-Zeolites through One-Pot Synthesis

et al. 2021

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α-Fe(II) active sites in iron zeolites catalyze N2O decomposition and form highly reactive α-O that selectively oxidizes unreactive hydrocarbons, such as methane. How these α-Fe(II) sites are formed remains unclear. Here different methods of iron introduction into zeolites are compared to derive the limiting factors of Fe speciation to α-Fe(II). Postsynthetic iron introduction procedures on small pore zeolites suffer from limited iron diffusion and dispersion leading to iron oxides. In contrast, by introducing Fe(III) in the hydrothermal synthesis mixture of the zeolite (one-pot synthesis) and the right treatment, crystalline CHA can be prepared with >1.6 wt % Fe, of which >70% is α-Fe(II). The effect of iron on the crystallization is investigated, and the intermediate Fe species are tracked using UV–vis-NIR, FT-IR, and Mössbauer spectroscopy. These data are supplemented with online mass spectrometry in each step, with reactivity tests in α-O formation and with methanol yields in stoichiometric methane activation at room temperature and pressure. We recover up to 134 μmol methanol per gram in a single cycle through H2O/CH3CN extraction and 183 μmol/g through steam desorption, a record yield for iron zeolites. A general scheme is proposed for iron speciation in zeolites through the steps of drying, calcination, and activation. The formation of two cohorts of α-Fe(II) is discovered, one before and one after high temperature activation. We propose the latter cohort depends on the reshuffling of aluminum in the zeolite lattice to accommodate thermodynamically favored α-Fe(II).

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Section: Discussionmentioning

confidence: 99%

Selective Formation of α-Fe(II) Sites on Fe-Zeolites through One-Pot Synthesis

et al. 2021

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“…After adding fluoride, a small number of diffraction peak appear near −90.7 ppm, indicating that a small amount of Si (4Al) was produced during the initial gel drying process. 31 In contrast to the 27 Al MAS NMR spectrum (Figure 9B), the dry gels mainly consisted of tetrahedral (−41 ppm) and amorphous aluminum (6 ppm) and a small amount of octahedral aluminum (−11 ppm). After adding fluoride, there was a weak acromion at −40 ppm, which was a fluorinated pentacoordinate aluminum structure, indicating that the addition of fluoride has the effect of inducing nucleation.…”

Section: ■ Results and Discussionmentioning

confidence: 83%

“…The 29 Si MAS NMR spectrum (Figure A) shows that the silica species of dry gels were mainly amorphous silicon. After adding fluoride, a small number of diffraction peak appear near −90.7 ppm, indicating that a small amount of Si (4Al) was produced during the initial gel drying process . In contrast to the 27 Al MAS NMR spectrum (Figure B), the dry gels mainly consisted of tetrahedral (−41 ppm) and amorphous aluminum (6 ppm) and a small amount of octahedral aluminum (−11 ppm).…”

Section: Resultsmentioning

confidence: 93%

“…20 When the ratio of SiO 2 /Al 2 O 3 was reduced to 0.3, the reaction lifetime was prolonged, indicating that the acid density of the molecular sieve was significantly reduced. 31 The shorter lifetime of V-0.15T/0.3S was because it had low crystallinity and was accompanied by AEI and a small amount of Si (3Al).…”

Section: ■ Results and Discussionmentioning

confidence: 99%

“…The methanol conversion curve in Figure a shows that V-0.15T/0.5S has the shortest lifetime (84 min) in the reaction due to its silicon island, which was prone to side reactions such as hydrogen transfer cyclization and oligomerization . When the ratio of SiO 2 /Al 2 O 3 was reduced to 0.3, the reaction lifetime was prolonged, indicating that the acid density of the molecular sieve was significantly reduced . The shorter lifetime of V-0.15T/0.3S was because it had low crystallinity and was accompanied by AEI and a small amount of Si (3Al).…”

Section: Resultsmentioning

confidence: 99%

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A Green and Cost-Effective Synthesis of Hierarchical SAPO-34 through Dry Gel Conversion and Its Performance in a Methanol-to-Olefin Reaction

et al. 2021

Ind. Eng. Chem. Res.

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In this work, a low-cost and high-performance methanol-to-olefin (MTO) catalyst was synthesized by dry gel conversion (DGC). Using tetraethylammonium hydroxide (TEAOH) as the structure-directing agent (SDA) to prepare the dry gel, nano-SAPO-34 in a range of 50−500 nm was synthesized under the effect of the vapor of triethylamine (TEA) and H 2 O. The TEAOH consumption using this method was only 7.5% of that in the hydrothermal synthesis (HTS), and the yield of nano-SAPO-34 products was up to 94%. When scaling up to the pilot test, obvious heterocrystals appeared with the decrease in SiO 2 content in the dry gel. Our results showed that the heterocrystals were completely eliminated after the introduction of fluoride into the dry gel. A low acid density nanosheet-SAPO-34 molecular sieve product with good micropore, mesopore, and macropore connectivity was obtained. The obtained multikilogram-scale molecular sieve sample can be directly shaped to obtain MTO microsphere catalysts by spray drying without separation and washing. No wastewater was produced during the whole process, demonstrating an entirely green production way. The MTO performance of the microsphere catalysts was tested in a fluidized bed reactor, and results showed that the catalysts exhibited the characteristics of a slow carbon deposition rate and a high activity utilization ratio. Their lifetime was extended by 14%, and their selectivity (C 2 = + C 3 = ) was increased by 1.55%, so they are very suitable for industrial application. This work has provided the theoretical and practical basis for the industrial production and application of DGC in the future.

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