Andrei-Nicolae Parvulescu

Generalized high-temperature synthesis of zeolite catalysts with unpredictably high space-time yields (STYs)

As a class of important catalysts and adsorbents, zeolites are normally prepared through hydrothermal synthesis, whereby a relatively long crystallization time and use of a large amount water solvent strongly hinder the enhancement of zeolite space-time yields (STYs), which is a critical factor for the industrial manufacturing. To overcome this limitation, herein we report a novel strategy for highly efficient zeolite synthesis by means of fast crystallization at high temperatures (200–240 °C) in the absence of water solvent. This concept significantly enhances the crystallization rates and allows drastic reduction of the time required for crystallization of the zeolite frameworks such as the crystallization of MFI from 12–24 h at 180 °C to 0.5 h at 240 °C and RUB-36 from 14 days at 140 °C to 1.5 days at 200 °C. Together with much better utilization of the reactor volume, the space-time yields (STYs) for zeolites prepared from high-temperature synthesis in the absence of water solvent can be remarkably increased. The STYs of MFI and RUB-36 are as high as 11 000 and 178 kg m−3 per day, which are almost two orders of magnitude higher than those of conventional hydrothermal synthesis. This novel synthesis method should be applicable for synthesizing a wide variety of zeolite structures and bears the potential for highly efficient zeolite synthesis on an industrial scale.

An efficient, rapid, and non-centrifugation synthesis of nanosized zeolites by accelerating the nucleation rate

Access to nanosized zeolites is a topical subject due to the advantageous mass transfer in zeolite catalyzed reactions. Herein we report a low-cost and generalized methodology to decrease zeolite crystal sizes by accelerating the nucleation rate when reducing the solvent amount in the synthesis. As anticipated, nanosized zeolites like MFI (100–200 nm, Si/Al ratios from 100 to ∞ in the starting gels) and BEA (50–200 nm, Si/Al ratio of 13) have been successfully synthesized with a H2O/SiO2 ratio of 2.0–3.0. The lower consumption of water as the solvent in the synthesis leads to non-centrifugation for nanosized zeolites, giving higher yields of the zeolite product. Experimental data confirm lower apparent activation energy for nucleation at higher gel concentrations in the synthesis of ZSM-5 zeolites. Catalytic examination of the conversion of methanol to olefins (MTO) demonstrates that the thus-obtained nanosized ZSM-5 zeolite (NS-ZSM-5) exhibits a much longer catalyst lifetime than conventional ZSM-5 (C-ZSM-5) and even nanosized ZSM-5 crystals (NC-ZSM-5) obtained from a hydrothermal route.

Efficient synthesis of aluminosilicate RTH zeolite with good catalytic performances in NH3-SCR and MTO reactions

2,6-methyl-N-methylpyridinium, as a novel organic template, is employed for the synthesis of RTH aluminosilicate zeolite with a SiO2/Al2O3 ratio of 17.6. The amount of 2,6-methyl-N-methylpyridinium template, the Na2O/SiO2 ratio, the SiO2/Al2O3 ratio, and the H2O/SiO2 ratio in the starting gel significantly influence the crystallization of RTH zeolite. Several analytical methods such as XRD, SEM, N2 sorption, TG-DTA, DRIFT and NMR were employed for the characterisation of the obtained RTH zeolites and to understand the crystallization process with the new template. Very interestingly, the crystallization of RTH zeolite with the new template takes a very short time (12 h at 130 °C and 50 min at 240 °C) compared with conventional RTH zeolite synthesis reported in the literature (72 h at 130 °C). Theoretical calculations show that this novel organic template has lower interaction energies for zeolite cage space filling than those of the organic templates previously reported in the literature, which lead to stronger structural directing. Kinetic results show that the activation energy of this novel organic template is much lower than that of the traditional one. Catalytic tests show that copper exchanged RTH zeolite (Cu-RTH) exhibits good catalytic properties in the NH3-SCR reaction and the H-RTH zeolite catalyst has excellent selectivities for ethylene and propylene in MTO reactions.

Excellent Performances of Dealuminated H-Beta Zeolites from Organotemplate-Free Synthesis in Conversion of Biomass-derived 2,5-Dimethylfuran to Renewable p-Xylene

Direct synthesis of renewable p-xylene (PX) by cycloaddition of biomass-derived 2,5-dimethylfuran (2,5-DMF) and ethylene was achieved over Al-rich H-beta zeolites synthesized by an organotemplate-free approach and their dealuminated counterparts with different Si/Al ratios. Among them, H-beta zeolite with an Si/Al ratio of 22, obtained from an Al-rich parent by dealumination, was found to be an excellent catalyst for the synthesis of PX. A PX yield of 97 % and 2,5-DMF conversion of 99 % were obtained under optimized conditions. These results are even better than those of a commercial H-beta zeolite prepared using a organotemplate synthesis with a similar Si/Al ratio of 19. The excellent performance of the H-beta zeolite with Si/Al ratio of 22 is closely related to its acidity and porous structure. A moderate Brønsted/Lewis acid ratio can improve the conversion of 2,5-DMF to as high as 99 %. Furthermore, dealuminated H-beta zeolite has a secondary pore system that facilitates product diffusion, which increases the selectivity to PX. In addition, this catalyst shows better regeneration. After five successive regeneration cycles, the yield of PX was still as high as 85 % without obvious dealumination. This work provides a deeper understanding of the more general Diels-Alder cycloaddition of furan-based feedstocks and olefins and significantly improves the potential for the synthesis of chemicals from lignocellulosic biomass.