On the role of acidity in bulk and nanosheet [T]MFI (T = Al3+, Ga3+, Fe3+, B3+) zeolites in the methanol-to-hydrocarbons reaction

The influence of framework substituents (Al 3+, Ga 3+, Fe 3+ and B 3+) and morphology (bulk vs. nanometer-sized sheets) of MFI zeolites on the acidity and catalytic performance in the methanol-to-hydrocarbons (MTH) reaction was investigated. The Brønsted acid density and strength decreased in the order Al(OH)Si>Ga(OH)Si>Fe(OH)Si≫B(OH)Si. Pyridine 15N NMR spectra confirmed the differences in the Brønsted and Lewis acid strengths but also provided evidence for site heterogeneity in the Brønsted acid sites. Owing to the lower efficiency with which tervalent ions can be inserted into the zeolite framework, sheet-like zeolites exhibited lower acidity than bulk zeolites. The sheet-like Al-containing MFI zeolite exhibited the greatest longevity as a MTH catalyst, outperforming its bulk [Al]MFI counterpart. Although the lower acidity of bulk [Ga]MFI led to a better catalytic performance than bulk [Al]MFI, the sheet-like [Ga]MFI sample was found to be nearly inactive owing to lower and heterogeneous Brønsted acidity. All Fe- and B-substituted zeolite samples displayed very low catalytic performance owing to their weak acidity. Based on the product distribution, the MTH reaction was found to be dominated by the olefins-based catalytic cycle. The small contribution of the aromatics-based catalytic cycle was larger for bulk zeolite than for sheet-like zeolite, indicating that shorter residence time of aromatics can explain the lower tendency toward coking and enhanced catalyst longevity

The important role of rubidium hydroxide in the synthesis of hierarchical ZSM-5 zeolite using cetyltrimethylammonium as structure-directing agent

Hierarchical ZSM‐5 zeolite with uniform mesopores was synthesized with a simple cetyltrimethylammonium (CTA+) template, which acted as a bifunctional surfactant in a RbOH‐based alkaline synthesis gel. Rb+ plays a key role in obtaining uniform mesopores within ZSM‐5 crystals. The structural, textural properties and the acidity were characterized by XRD, Ar physisorption, TEM, as well as CO IR and 27Al MAS NMR spectroscopy. These data point to partial retention of the initial mesoscale ordered texture of the precursor in the final zeolite. These textural properties result in a strongly improved catalytic performance in the methanol conversion reaction compared to bulk zeolite

The important role of rubidium hydroxide in the synthesis of hierarchical ZSM-5 zeolite using cetyltrimethylammonium as structure-directing agent

Hierarchical ZSM‐5 zeolite with uniform mesopores was synthesized with a simple cetyltrimethylammonium (CTA+) template, which acted as a bifunctional surfactant in a RbOH‐based alkaline synthesis gel. Rb+ plays a key role in obtaining uniform mesopores within ZSM‐5 crystals. The structural, textural properties and the acidity were characterized by XRD, Ar physisorption, TEM, as well as CO IR and 27Al MAS NMR spectroscopy. These data point to partial retention of the initial mesoscale ordered texture of the precursor in the final zeolite. These textural properties result in a strongly improved catalytic performance in the methanol conversion reaction compared to bulk zeolite

Direct synthesis of hierarchical ZSM-5 zeolite using cetyltrimethylammonium as structure directing agent for methanol-to-hydrocarbons conversion

Hierarchical ZSM-5 zeolite can be obtained in a one-step synthesis approach using cetyltrimethyl–ammonium (CTA) as the sole organic template. The reduced crystal domain size and the presence of mesopores result in improved catalytic performance in methanol-to-hydrocarbon (MTH) reaction as compared to bulk ZSM-5. We investigated the role of the base (LiOH, NaOH, KOH, RbOH, CsOH), the H2O/CTA ratio, the Si/Al ratio and counter-ion of CTA (OH−vs. Br−). The crucial role of KOH and RbOH is evident as only these bases allow dissolution of the amorphous silica–alumina precursor to such extent that zeolite crystallization occurs. With other bases, silica dissolution is too limited to start zeolite crystallization, corroborated by the observation that seeding the synthesis gel rapidly led to mesoporous ZSM-5 zeolite for all bases. With KOH, mesoporous zeolite was obtained in the H2O/CTAOH 800–3200 range. The role of Al is also important as without it only ZSM-48 zeolite could be formed, while a too high Si/Al ratio of 20 did not lead to nucleation. A highly crystalline, hierarchical ZSM-5 free from extraframework Al synthesized from a gel with KOH as base, H2O/CTAOH and Si/Al ratios of 800 and 50, respectively displayed the highest catalytic performance in the MTH reaction, outperforming bulk ZSM-5

On the role of acidity in bulk and nanosheet [T]MFI (T = Al3+, Ga3+, Fe3+, B3+) zeolites in the methanol-to-hydrocarbons reaction

\u3cp\u3eThe influence of framework substituents (Al \u3csup\u3e3+\u3c/sup\u3e, Ga \u3csup\u3e3+\u3c/sup\u3e, Fe \u3csup\u3e3+\u3c/sup\u3e and B \u3csup\u3e3+\u3c/sup\u3e) and morphology (bulk vs. nanometer-sized sheets) of MFI zeolites on the acidity and catalytic performance in the methanol-to-hydrocarbons (MTH) reaction was investigated. The Brønsted acid density and strength decreased in the order Al(OH)Si>Ga(OH)Si>Fe(OH)Si≫B(OH)Si. Pyridine \u3csup\u3e15\u3c/sup\u3eN NMR spectra confirmed the differences in the Brønsted and Lewis acid strengths but also provided evidence for site heterogeneity in the Brønsted acid sites. Owing to the lower efficiency with which tervalent ions can be inserted into the zeolite framework, sheet-like zeolites exhibited lower acidity than bulk zeolites. The sheet-like Al-containing MFI zeolite exhibited the greatest longevity as a MTH catalyst, outperforming its bulk [Al]MFI counterpart. Although the lower acidity of bulk [Ga]MFI led to a better catalytic performance than bulk [Al]MFI, the sheet-like [Ga]MFI sample was found to be nearly inactive owing to lower and heterogeneous Brønsted acidity. All Fe- and B-substituted zeolite samples displayed very low catalytic performance owing to their weak acidity. Based on the product distribution, the MTH reaction was found to be dominated by the olefins-based catalytic cycle. The small contribution of the aromatics-based catalytic cycle was larger for bulk zeolite than for sheet-like zeolite, indicating that shorter residence time of aromatics can explain the lower tendency toward coking and enhanced catalyst longevity. \u3c/p\u3

Direct synthesis of hierarchical ZSM-5 zeolite using cetyltrimethylammonium as structure directing agent for methanol-to-hydrocarbons conversion

Hierarchical ZSM-5 zeolite can be obtained in a one-step synthesis approach using cetyltrimethyl–ammonium (CTA) as the sole organic template. The reduced crystal domain size and the presence of mesopores result in improved catalytic performance in methanol-to-hydrocarbon (MTH) reaction as compared to bulk ZSM-5. We investigated the role of the base (LiOH, NaOH, KOH, RbOH, CsOH), the H2O/CTA ratio, the Si/Al ratio and counter-ion of CTA (OH−vs. Br−). The crucial role of KOH and RbOH is evident as only these bases allow dissolution of the amorphous silica–alumina precursor to such extent that zeolite crystallization occurs. With other bases, silica dissolution is too limited to start zeolite crystallization, corroborated by the observation that seeding the synthesis gel rapidly led to mesoporous ZSM-5 zeolite for all bases. With KOH, mesoporous zeolite was obtained in the H2O/CTAOH 800–3200 range. The role of Al is also important as without it only ZSM-48 zeolite could be formed, while a too high Si/Al ratio of 20 did not lead to nucleation. A highly crystalline, hierarchical ZSM-5 free from extraframework Al synthesized from a gel with KOH as base, H2O/CTAOH and Si/Al ratios of 800 and 50, respectively displayed the highest catalytic performance in the MTH reaction, outperforming bulk ZSM-5

A dual-templating synthesis strategy to hierarchical ZSM-5 zeolites as efficient catalysts for the methanol-to-hydrocarbons reaction

\u3cp\u3eA novel dual-templating synthesis strategy is presented to obtain hierarchical ZSM-5 zeolite using a combination of known structure-directing agents for ZSM-5 synthesis and C\u3csub\u3e16\u3c/sub\u3eH\u3csub\u3e33\u3c/sub\u3e-[N\u3csup\u3e+\u3c/sup\u3e-methylpiperidine] (C\u3csub\u3e16\u3c/sub\u3eMP) as mesoporogen. C\u3csub\u3e16\u3c/sub\u3eMP is a cheap surfactant, which can be obtained in a single step by alkylation of N-methylpiperidine. The zeolite materials were extensively characterized for their textural and acidic properties and evaluated on the basis of their ability to convert methanol to hydrocarbons. Bulk and nanosheet (di-quaternary ammonium surfactant) ZSM-5 zeolites served as reference materials. Hierarchical ZSM-5 zeolite can be obtained in this way with diethylamine, n-propylamine, 1,4-diaminobutane, 1,6-diaminohexane. In particular, the combination with diethylamine afforded a material that displayed similar performance in the methanol-to-hydrocarbons reaction as nanosheet ZSM-5. The optimum ZSM-5 zeolite is highly crystalline, contains a large mesopore volume and few silanol groups and external Brønsted acid sites, which contributes to the low rate of deactivation.\u3c/p\u3