Solvent-free and Mesoporogen-free Synthesis of Mesoporous Aluminosilicate ZSM‑5 Zeolites with Superior Catalytic Properties in the Methanol-to-Olefins Reaction

Large crystalline particles of ZSM-5 zeolites (S-ZSM-5) have been successfully synthesized by adjusting crystallization time and Si/Al ratios in the starting solid mixtures in the absence of water solvent. Catalytic tests in the methanol-to-olefins (MTO) reaction show that these S-ZSM-5 zeolites obtained from the solvent-free route exhibit superior catalytic properties including excellent propylene selectivity and extraordinarily long life in the MTO reaction. Particularly, when the crystallization time is 30 h and the Si/Al ratio in the starting solid mixture is adjusted at 150 under the solvent-free conditions, the S-ZSM-5-30h-150 catalyst with the large particle sizes (10–20 μm) gives propylene selectivity as high as 50.0% and catalyst life as long as 9 h, which are much better than those (propylene selectivity at 38.9% and catalyst life of 3 h) of the conventional ZSM-5 zeolite with smaller crystals (ca. 5 μm) synthesized from the hydrothermal route. High-resolution TEM images of these S-ZSM-5 zeolites demonstrate that there is mesoporosity in the samples. More interestingly, these mesopore size distributions could be adjusted by the crystallization time during the solvent-free synthesis. Obviously, the presence of mesoporosity is very favorable for the mass transfer, and an appropriate Si/Al ratio in the zeolite framework could offer suitable acidic density for the catalytic conversion, which should be responsible for the superior catalytic properties in the MTO over the S-ZSM-5-30h-150 catalyst. The features of mesoporosity in the S-ZSM-5 crystals and sustainability for the solvent-free synthesis could be potentially important for wide applications of these S-ZSM-5 zeolites in the future

Solvent-Free Synthesis of Zeolites from Solid Raw Materials

As important industrial materials, microporous zeolites are necessarily synthesized in the presence of solvents such as in hydrothermal, solvothermal, and ionothermal routes. We demonstrate here a simple and generalized solvent-free route for synthesizing various types of zeolites by mixing, grinding, and heating solid raw materials. Compared with conventional hydrothermal route, the avoidance of solvents in the synthesis not only significantly reduces the waste production, but also greatly increases the yield of zeolite products. In addition, the use of starting solid raw materials remarkably enhances the synthesis efficiency and reduces the use of raw materials, energy, and costs

Sustainable Synthesis of Zeolites without Addition of Both Organotemplates and Solvents

The development of sustainable and environmentally friendly techniques for synthesizing zeolites has attracted much attention, as the use of organic templates and solvents in the hydrothermal synthesis of zeolites is a major obstacle for realizing green and sustainable synthesis ways. Recently, the introduction of the organotemplate-free synthesis method allowed avoiding the use of organic templates, but water as solvent was still required; solvent-free routes on the other hand beared the potential to significantly reduce the amount of polluted wastewater, but organic templates were still present. In this work, we have demonstrated a combined strategy of both organotemplate- and solvent-free conditions to synthesize aluminosilicate zeolites Beta and ZSM-5 (S-Beta and S-ZSM-5), two of the most important zeolites relevant for industry. The samples are thoroughly characterized by XRD patterns, SEM images, N₂ sorption isotherms, UV-Raman spectra, and ²⁹Si and ²⁷Al MAS NMR spectra. The results demonstrate that S-Beta and S-ZSM-5 zeolites exhibit almost the same textural parameters (e.g., BET surface area and pore volume) and catalytic performance in cumene cracking and <i>m</i>-xylene isomerization as those of conventional Beta and ZSM-5 zeolites synthesized under hydrothermal conditions (C-Beta and C-ZSM-5). The organotemplate- and solvent-free syntheses of S-Beta and S-ZSM-5 take place at a low-pressure regime and are free of harmful gases as well as give high product yields together with highly efficient consumption of the starting raw materials. These advantages plus the very simple procedures opened the pathway to a highly sustainable zeolite synthesis protocol compared to conventional methods currently employed for C-Beta and C-ZSM-5. Very interestingly, this simple synthesis is a good model for understanding zeolite crystallization. The detail characterizations indicate that the S-Beta crystals are formed from the assembly of zeolite building units, mainly 4MRs, while the 5MRs in the framework are just formed in the crystallization of S-ZSM-5, rather than existence in the starting solid mixture. During the crystallization processes, small traces of water play an important role for the hydrolysis and condensation of silica and/or aluminosilicate species

Solvent-Free Synthesis of Zeolites from Anhydrous Starting Raw Solids

Development of sustainable routes for synthesis of zeolites is very important because of wide applications of zeolites at large scale in the fields of catalysis, adsorption, and separation. Here we report a novel and generalized route for synthesis of zeolites in the presence of NH₄F from grinding the anhydrous starting solid materials and heating at 140–240 °C. Accordingly, zeolites of MFI, BEA*, EUO, and TON structures have been successfully synthesized. The presence of F^– drives the crystallization of these zeolites from amorphous phase. Compared with conventional hydrothermal synthesis, the synthesis in this work not only simplifies the synthesis process but also significantly enhances the zeolite yields. These features should be potentially of great importance for industrial production of zeolites at large scale in the future

Highly Mesoporous Single-Crystalline Zeolite Beta Synthesized Using a Nonsurfactant Cationic Polymer as a Dual-Function Template

Mesoporous zeolites are useful solid catalysts for conversion of bulky molecules because they offer fast mass transfer along with size and shape selectivity. We report here the successful synthesis of mesoporous aluminosilicate zeolite Beta from a commercial cationic polymer that acts as a dual-function template to generate zeolitic micropores and mesopores simultaneously. This is the first demonstration of a single nonsurfactant polymer acting as such a template. Using high-resolution electron microscopy and tomography, we discovered that the resulting material (Beta-MS) has abundant and highly interconnected mesopores. More importantly, we demonstrated using a three-dimensional electron diffraction technique that each Beta-MS particle is a single crystal, whereas most previously reported mesoporous zeolites are comprised of nanosized zeolitic grains with random orientations. The use of nonsurfactant templates is essential to gaining single-crystalline mesoporous zeolites. The single-crystalline nature endows Beta-MS with better hydrothermal stability compared with surfactant-derived mesoporous zeolite Beta. Beta-MS also exhibited remarkably higher catalytic activity than did conventional zeolite Beta in acid-catalyzed reactions involving large molecules