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The diffusion of saturated and unsaturated hydrocarbons is of fundamental importance for many zeolite‐catalyzed processes. Transport of small alkenes in the confined zeolite pores can become hindered, resulting in a significant impact on the ultimate product selectivity and separation. Herein, intracrystalline light olefin/paraffin diffusion through the 8‐ring windows of zeolite SAPO‐34 is characterized by a complementary set of first‐principle molecular dynamics simulations, PFG‐NMR experiments, and pulse‐response temporal analysis of products measurements, yielding information at different length and time scales. Our results clearly show a promotional effect of the presence of Brønsted acid sites on the diffusion rate of ethene and propene, whereas transport of alkanes is found to be insensitive to the presence of acid sites. The enhanced diffusivity of unsaturated hydrocarbons is ascribed to the formation of favorable π–H interactions with acid protons, as confirmed by IR spectroscopy measurements. The acid site distribution is proven to be an important design parameter for optimizing product distributions and separations. [ABSTRACT FROM AUTHOR]

Amorphous silica‐aluminas (ASAs), possessing both Brønsted acid sites (BAS) and Lewis acid sites (LAS), are important bifunctional catalysts in various industrial applications. Solid‐state NMR spectroscopy has been widely used for characterizing the local structure and the surface sites of ASAs with probe molecules. In this work, four‐, five‐ and six‐coordinated Al species have been observed on the flame‐made ASAs by 27Al MQ MAS NMR experiment. 1H/27Al TRAPDOR MAS NMR experiments confirmed that surface Al species contribute to the formation of BAS and protonate ammonia probe molecules. The adsorption of ammonia on Lewis acidic Al sites (δ1H=3.0 ppm) was evidenced by various 1H MAS NMR experiments on samples dehydrated at different temperatures, allowing the distinction from ammonium ions (δ1H=6.7 ppm) formed at BAS. The signal of ammonia adsorbed on LAS increased with increasing Al content in the ASAs. These properties together with the absence of pore diffusional constraints render the flame‐made ASAs excellent catalysts for cyclohexanol dehydration and the conversion of glyceraldehyde in ethanol to ethyl lactate, outperforming the performance of other ASAs or zeolites. [ABSTRACT FROM AUTHOR]

Lactic acid and alkyl lactates are widely applied in the production of food, cosmetics, pharmaceuticals, organic synthesis and biodegradable polymers. They can be prepared via one-pot synthesis from renewable trioses, such as dihydroxyacetone (DHA). Brønsted–Lewis bifunctional solid acids (BAS & LAS) can promote the reaction via a two-step cascade reaction mechanism. BAS catalyses the dehydration of DHA, resulting in the formation of pyruvaldehyde (PA) via the rearrangement of the enol form. Upon alcohol addition, PA can be converted to the desired alkyl lactates at LAS or to pyruvaldehyde hemiacetal (PAHA) at strong BAS. The density and strength control of Brønsted acid sites (BAS) and Lewis acid sites (LAS) and the optimization of their cooperation are essential for the efficient conversion of trioses to the target products. Here, we prepared a series of Sn-containing mesoporous MCM-41 catalysts with various BAS/LAS ratios by room temperature techniques. Sn-doped [Si]MCM-41 having a lower BAS/LAS ratio in this research shows a high initial selectivity to ethyl lactate (EL) and similar EL yield in 6 hours as the reported best Sn catalyst Sn-grafted [Si]MCM-41/carbon network materials in DHA conversion. A relatively large density of LAS in Sn-doped [Si]MCM-41 causes a fast conversion of PA to EL, while the overall yield has been limited by the BAS density for the DHA conversion. New H-form [Sn]MCM-41, having a suitable density of LAS and weak BAS and an optimized BAS/LAS ratio, provides a 100% yield of ethyl lactate in the catalytic conversion of DHA in ethanol within 30 min, showing a superior performance hitherto. [ABSTRACT FROM AUTHOR]

Acetone-2-13C, trimethylphosphine oxide (TMPO), and ammonia were applied as probe molecules for solid-state NMR investigations of Lewis acid sites on γ-Al2O3, TiO2/anatase, and lithium-exchanged zeolite Na-Y. An indication for Lewis acid sites are the 31P MAS NMR signals at 48-51 ppm for TMPO-loaded catalysts. The determination of the Lewis site density through the evaluation of these 31P MAS NMR signals, however, is difficult owing to the demanding TMPO adsorption procedure. Upon ammonia adsorption, the formation of ammonium ions at Brønsted acid sites and the coordination of ammonia at Lewis acid sites cause 1H MAS NMR signals at 6-7 ppm and −0.5-3 ppm, respectively. The integration of these signals results in the densities of Brønsted and Lewis acid sites. Future studies have to clarify whether the different 1H chemical shifts in the range of −0.5-3 ppm for ammonia coordinated at Lewis acid sites on solid catalysts are a hint at the different strengths of these surface sites. [ABSTRACT FROM AUTHOR]

Nanosized-ZSM-5/SBA-15 analog composites (ZSC) were successfully synthesized in a two-step process from zeolite seed solutions containing intact ZSM-5 nanocrystals (nano-ZSM-5) by high-temperature synthesis in mild acidic media (200 °C, pH 3.5). The ZSC solids exhibit domains of nano-ZSM-5 blended homogeneously with highly condensed, well-ordered mesoporous SBA-15 analogs containing zeolite building units. The content of nano-ZSM-5 phase and SBA-15 analog phase can be controlled by a single synthesis parameter, i.e., the precrystallization time of the initially formed zeolite seed solution. Compared to conventional Al-SBA-15, the acidic properties, particularly the surface density of strong Brønsted acid sites, of the ZSC are significantly improved while its micro-/mesostructures are well maintained even upon steaming at 800 °C for 24 h. The catalytic activity of the ZSC solids compared to that of commercial ZSM-5 and Al-SBA-15 samples was evaluated in the gas phase cracking of cumene and 1,3,5-tri-isopropylbenzene (TIPB) as probe reactions. The results show that the ZSC materials are effective for the conversion of both cumene and TIPB due to the increased density of the Brønsted acid sites with high accessibility provided by well-dispersed nano-ZSM-5 in the mesoporous SBA-15 analog matrix. [ABSTRACT FROM AUTHOR]

Phosphate-modified ZSM-5 zeolites were studied by standard characterization techniques and solid-state nuclear magnetic resonance spectroscopy, and in the methanol-to-olefin (MTO) conversion. Considering the physicochemical properties of the ZSM-5 zeolites, the most important effects of the phosphate modification are a deposition of polyphosphates in the pore system and a decrease of the acid site density, but not of the acid site strength. The significant increase in the selectivity to C2 - C4 alkenes and the decrease of C5+ formation in the MTO reaction for a phosphate coverage of about 5 wt % are explained by extra-framework phosphate species near the crossing intersections of the ZSM-5 pore system, which hinder the formation of large intermediates and reaction products. [ABSTRACT FROM AUTHOR]

• Xylene as the solvent in flame-spray pyrolysis promotes AlV-rich silica–alumina. • The new method provides higher Brønsted acidity of amorphous silica–alumina (ASA). • AlV-rich ASAs exhibit superior catalytic performance in phenylglyoxal conversion. Tailoring high-performance aluminosilicates plays a key role in the efficient and clean production of high-value chemicals. Recent work reveals that pentacoordinated Al (AlV) species can significantly enhance the Brønsted acidity of amorphous silica–alumina (ASA), compared with that typically dominated by tetracoordinated Al species. However, the controlled synthesis of AlV-rich ASAs is challenging. Employing xylene as the solvent in a flame-spray pyrolysis process, we synthesized AlV-rich ASAs successfully. The high combustion enthalpy of xylene (36.9 kJ/ml) results in a high flame temperature, promoting the formation and distribution of metastable AlV species in the silica network forming Brønsted acid sites. This provides a promising route for the controlled synthesis of AlV-rich ASAs with higher Brønsted acidity. As an example, AlV-rich ASAs are shown to exhibit superior catalytic performance in phenylglyoxal conversion to ethyl mandelate in ethanol compared with that achieved with other acid catalysts, attaining an ethyl mandelate yield of 99.8%. [ABSTRACT FROM AUTHOR]

Microporous ZSM-22 zeolites (structure type TON) were modified by treatments with different alkaline solutions for introducing mesopores. This surprisingly affected the nature and the density of both SiOH groups and Brønsted acid sites significantly, while the Brønsted acid strength was maintained. The catalyst lifetime in the methanol-to-olefin (MTO) conversion could be enhanced by a factor of up to 2 for an optimum Brønsted acid site density of 0.34 mmol/g, which was also accompanied by the highest propene selectivity of 48%. The product distributions of the parent microporous and the modified mesoporous ZSM-22 catalysts were very similar. This indicates that the shape selectivity of the modified ZSM-22 zeolites in the MTO conversion is mainly determined by the 10-ring pore system and not affected by introduced mesopores. Therefore, it can be excluded that catalysis near the pore mouths plays a significant role in the MTO conversion on ZSM-22 zeolites. [ABSTRACT FROM AUTHOR]