On the Location and Accessibility of Active Acid Sites in MFI Zeolites Modified by Alkaline Treatment
Abstract
:1. Introduction
- Si/Al ratio: Since Al atoms protect neighboring Si atoms, high-aluminum zeolites are rarely attacked by alkali solutions. On the other hand, low-aluminum zeolites suffer from extensive removal of silicon, with significant destruction of the framework. Thus, the optimum range for the application of this method is Si/Al = 15–50 [21,23,24,25,26].
- Temperature: Temperature’s effect is dual. On the one hand, an increase in temperature leads to an increase in solubility for both silicates and aluminates, decreasing the selectivity of the treatment. On the other hand, the process is very slow at temperatures below 25 °C, with the optimum temperature being between 30 and 70 °C [21,27].
- Alkali: The treatment requires strong bases. It was found that kinetics follow the order LiOH < NaOH < KOH, in accordance with the effective ionic diameter of the cations. However, NaOH is more conducive to mesoporosity development than KOH, since silicate anions are more stable in the presence of Na+, preventing silica redeposition [21]. Weak bases (for example, NaHCO3 or tetraalkylammonium hydroxides) lead to very slow silicon removal, and have proven useful for generating mesopores in a controlled way over low-aluminum frameworks [25,29,30].
- Alkali concentration: The alkali concentration reflects the aggressiveness of the treatment. Higher concentrations lead to higher silicon removal and porosity generation, as well as lower yields and, ultimately, framework collapse [27]. The optimum concentration depends strongly on the Si/Al ratio of the parent material and its crystalline structure [23,24,25,31].
2. Materials and Methods
3. Results
3.1. Evaluation of Treatment Conditions
3.2. Evaluation of the Accessibility of the Acid Sites
3.3. Dynamic Adsorption Experiments
- The first contribution is 425–450 K: butenes, which enable the isomerization of 1-butene.
- The second contribution is 450–525 K: light olefins, which result from polymerization reactions.
- The third contribution is 500–575 K: species released from coke, coinciding with those typically observed in TPO experiments.
3.4. Catalytic Evaluation
3.4.1. Isobutane/1-Butene Alkylation
3.4.2. Glycerol Esterification with Acetic Acid
4. Discussion
Supplementary Materials
Author Contributions
Funding
Data Availability Statement
Conflicts of Interest
References
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Material | Surface Area (m2 g−1) | Pore Volume (cm3 g−1) | Si/Al (mol/mol) | |||
---|---|---|---|---|---|---|
SBET | External | Total | Microp. | Mesop. | ||
Z40 | 430 | 44 | 0.231 | 0.166 | 0.065 | 40.55 |
Z40(0.05/25) | 394 | 63 | 0.226 | 0.137 | 0.089 | 41.02 |
Z40(0.1/25) | 379 | 50 | 0.218 | 0.135 | 0.083 | 39.62 |
Z40(0.2/25) | 384 | 47 | 0.221 | 0.138 | 0.083 | 42.27 |
Z40(0.5/25) | 389 | 50 | 0.222 | 0.137 | 0.085 | 41.00 |
Z40(0.1/45) | 425 | 91 | 0.263 | 0.140 | 0.123 | 41.13 |
Z40(0.2/45) | 407 | 91 | 0.271 | 0.143 | 0.129 | 33.55 |
Z40(0.5/45) | 432 | 141 | 0.332 | 0.122 | 0.209 | 25.93 |
Z40(0.7/45) | 375 | 84 | 0.368 | 0.118 | 0.250 | 25.40 |
Z40(0.1/65) | 424 | 88 | 0.274 | 0.142 | 0.132 | 35.25 |
Z40(0.2/65) | 492 | 177 | 0.496 | 0.110 | 0.385 | 33.39 |
Z40(0.3/65) | 500 | 240 | 0.745 | 0.117 | 0.627 | 25.21 |
Z40(0.5/65) | 441 | 239 | 0.806 | 0.090 | 0.717 | 8.51 |
Z40(0.7/65) | 254 | 172 | 0.609 | 0.036 | 0.573 | 4.88 |
Z40(0.5/45)-1h | 408 | 103 | 0.567 | 0.122 | 0.445 | 17.71 |
Z15 | 370 | 37 | 0.198 | 0.139 | 0.059 | 15.04 |
Material | Cristallinity (%) | Acid Sites a (mmol g−1) | Brønsted Acid sites b (%) | External Acid Sites c (mmol g−1) |
---|---|---|---|---|
Z40 | 100 | 0.39 | 80.6 | 0.10 |
Z40(0.5/45) | 56.0 | 0.77 | 69.6 | 0.27 |
Z40(0.7/45) | 69.0 | 0.96 | 75.4 | n. d. |
Z40(0.1/65) | 67.5 | 0.69 | 76.9 | n. d. |
Z40(0.2/65) | 88.9 | 0.58 | n. d. | n. d. |
Z40(0.3/65) | 79.9 | 0.69 | n. d. | n. d. |
Z40(0.5/65) | 40.8 | 0.70 | n. d. | n. d. |
Z40(0.7/65) | 22.8 | 0.44 | n. d. | n. d. |
Z40(0.5/45)-1h | 35.3 | 1.35 | 67.3 | 0.31 |
Z15 | 100 | 1.44 | 81.0 | 0.23 |
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Tonutti, L.G.; Vergara, L.; Querini, C.A.; Dalla Costa, B.O. On the Location and Accessibility of Active Acid Sites in MFI Zeolites Modified by Alkaline Treatment. Processes 2024, 12, 2567. https://doi.org/10.3390/pr12112567
Tonutti LG, Vergara L, Querini CA, Dalla Costa BO. On the Location and Accessibility of Active Acid Sites in MFI Zeolites Modified by Alkaline Treatment. Processes. 2024; 12(11):2567. https://doi.org/10.3390/pr12112567
Chicago/Turabian StyleTonutti, Lucas G., Lourdes Vergara, Carlos A. Querini, and Bruno O. Dalla Costa. 2024. "On the Location and Accessibility of Active Acid Sites in MFI Zeolites Modified by Alkaline Treatment" Processes 12, no. 11: 2567. https://doi.org/10.3390/pr12112567
APA StyleTonutti, L. G., Vergara, L., Querini, C. A., & Dalla Costa, B. O. (2024). On the Location and Accessibility of Active Acid Sites in MFI Zeolites Modified by Alkaline Treatment. Processes, 12(11), 2567. https://doi.org/10.3390/pr12112567