Hydrothermal synthesis and characterization of a layered zirconium silicate

A layered zirconosilicate composed of SiO4 tetrahedra and ZrO6 octahedra was hydrothermally synthesized in the presence of tetramethylammonium hydroxide. The layered material with a nominal composition Na2ZrSi4O11·xH2O was further characterized by XRD, SEM, thermogravimetric analysis, UV-Raman spectroscopy and single- and triple-quantum 23Na and 29Si MAS NMR spectroscopy. There are no Zr–O–Zr chains in the framework structure implying that the ZrO6 octahedra are isolated by SiO4 tetrahedra. 23Na MAS NMR indicates at least two different Na sites in the framework of SZS and the Na located in the interlayer can be exchanged by TMAOH, leading to an increase of the interlayer spacing. The layered zirconosilicate SZS has a high activity in the isomerization of glucose to fructose in water and can be reused.status: publishe

Dual template synthesis of a highly mesoporous SSZ-13 zeolite with improved stability in the methanol-to-olefins reaction

The dual template synthesis of zeolite SSZ-13 by use of trimethyl-adamantanammonium hydroxide and a diquaternary-ammonium mesoporogen induces considerable mesoporosity without impeding zeolite microporosity. The strongly improved accessibility of Brønsted sites in mesoporous SSZ-13 increases its stability during application as an acid catalyst in the methanol-to-olefins reaction.status: publishe

Structure and Reactivity of Zn-Modified ZSM-5 Zeolites: The Importance of Clustered Cationic Zn Complexes

A novel route for the introduction of well-defined zinc species into ZSM-5 zeolite via chemical vapor deposition of dimethylzinc (CVD(DMZ)) is explored. The structural properties and catalytic reactivity of the synthesized material (Zn/ZSM-5-CVD(DMZ)) are investigated against a set of Zn/ZSM-5 catalysts prepared by incipient wetness impregnation (IWI), ion exchange (IE), and high-temperature reaction with zinc vapor (CVD(m)). The materials are characterized by a range of physicochemical methods including temperature programmed reduction (TPR), in situ FTIR, magic-angle spinning (MAS) NMR, and X-ray absorption spectroscopy (XAS). The catalysts are tested for their activity in the dehydrogenation of propane. Catalysts prepared by IE and IWI exhibit a high degree of heterogeneity of extraframework zinc species. These include, besides isolated Zn2+ cations, multinuclear oxygenated zinc clusters and bulk zinc oxide aggregates. The CVD(m) method results in quantitative replacement of all Brønsted acid protons by isolated Zn2+. In CVD(DMZ) the Brønsted acid sites (BAS) react stoichiometrically with dimethylzinc Zn(CH3)2 (DMZ) yielding grafted [Zn-CH3]+ species, which can further be transformed to isolated Zn2+ ions by reduction in hydrogen. The presence of zinc in ZSM-5 enhances the rate of alkane dehydrogenation. The initial activity of Zn/ZSM-5 prepared by IWI and IE correlates with the Zn content. The samples with a more heterogeneous distribution of extraframework Zn species are more active than the samples with isolated Zn2+. The activity of reduced Zn/ZSM-5-CVD(DMZ) containing predominantly isolated Zn2+ ions can be substantially increased by oxidation prior to the reaction. However, the resulting oxygenated complexes easily decompose during the reaction. Propane dehydrogenation and catalyst stability of Zn/ZSM-5-CVD(DMZ) can be improved by addition of steam to the hydrocarbon feed. This rate enhancement is ascribed to an increase of the steady-state concentration of the reactive oxygenated sites.status: publishe

Selective production of biobased phenol from lignocellulose-derived alkylmethoxyphenols

\u3cp\u3eLignocellulosic biomass is the only renewable source of carbon for the chemical industry. Alkylmethoxyphenols can be obtained in good yield from woody biomass by reductive fractionation, but these compounds are of limited value for large-scale applications. We present a method to convert lignocellulose-derived alkylmethoxyphenols to phenol that can be easily integrated in the petrochemical industry. The underlying chemistry combines demethoxylation catalyzed by a titania-supported gold nanoparticle catalyst and transalkylation of alkyl groups to a low-value benzene-rich stream promoted by HZSM-5 zeolite. In this way, phenol can be obtained in good yield, and benzene can be upgraded to more valuable propylbenzene, cumene, and toluene. We demonstrate that intimate contact between the two catalyst functions is crucial to transferring the methyl groups from the methoxy functionality to benzene instead of phenol. In a mixed-bed configuration, we achieved a yield of 60% phenol and 15% cresol from 4-propylguaiacol in a continuous one-step reaction at 350 °C at a weight hourly space velocity of ∼40 h \u3csup\u3e-1\u3c/sup\u3e. \u3c/p\u3

Selective production of biobased phenol from lignocellulose-derived alkylmethoxyphenols

\u3cp\u3eLignocellulosic biomass is the only renewable source of carbon for the chemical industry. Alkylmethoxyphenols can be obtained in good yield from woody biomass by reductive fractionation, but these compounds are of limited value for large-scale applications. We present a method to convert lignocellulose-derived alkylmethoxyphenols to phenol that can be easily integrated in the petrochemical industry. The underlying chemistry combines demethoxylation catalyzed by a titania-supported gold nanoparticle catalyst and transalkylation of alkyl groups to a low-value benzene-rich stream promoted by HZSM-5 zeolite. In this way, phenol can be obtained in good yield, and benzene can be upgraded to more valuable propylbenzene, cumene, and toluene. We demonstrate that intimate contact between the two catalyst functions is crucial to transferring the methyl groups from the methoxy functionality to benzene instead of phenol. In a mixed-bed configuration, we achieved a yield of 60% phenol and 15% cresol from 4-propylguaiacol in a continuous one-step reaction at 350 °C at a weight hourly space velocity of ∼40 h \u3csup\u3e-1\u3c/sup\u3e. \u3c/p\u3

Bronsted acidity of Al/SBA-15

To increase the content of well-dispersed aluminum in Al/SBA-15 and its Brønsted acidity, a water-free synthesis method involving a Si–O–Al precursor was employed to prepare a series of Al/SBA-15 of widely varying composition. The materials were characterized with ICP, BET, TEM, 27Al MAS NMR and IR spectroscopy and compared to conventionally prepared Al/SBA-15. For low Al content (Si/Al > 10), the conventional approach leads to highly dispersed, predominantly isolated Al with well-ordered mesopores. At higher Al content, the order decreased or was even completely lost. In general, this led to the agglomeration of Al species on the surface. Although in most cases a substantial part of Al atoms exhibit tetrahedral coordination indicating their high dispersion and interaction with the silica, the overall Brønsted acidity of these Al/SBA-15-type materials is very low. The overall acidity of all Al/SBA-15 is of the same order as amorphous silica–alumina. By IR spectroscopy of H/D exchange with perdeuterobenzene, it was found that the intrinsic acidity of the acid sites in Al/SBA-15 is similar to that in zeolites and amorphous silica–alumina, thus further confirming that acid site concentration dominates differences in acidity of aluminosilicas.status: publishe

Facile synthesis of the DD3R zeolite: performance in the adsorptive separation of buta-1,3-diene and but-2-ene isomers

Small pore size and hydrophobic nature of DD3R make this material a unique zeolite with high potential in industrial separation applications. However, the reproducible rapid synthesis of this zeolite is still a problem. In this work, a thorough assessment of different synthetic methods revealed that synthesis reproducibility relies on two main pillars: the use of properly cleaned autoclave liners and the synthesis composition. High quality DD3R crystals are obtained when KOH is used as a cleaning agent, eliminating memory effects, and when KF is used in the synthesis as a mineralizing agent. The effect of fluoride addition is investigated by use of several characterization techniques (13C, 19F and 29Si MAS-NMR and (2D) 29Si–1H correlation spectra), while monitoring the temporal crystallization of DDR. 29Si–1H NMR reveals that template molecules accommodated within the cages are sticking to these 8-ring windows through their amine group. High quality DD3R crystals are applied in the adsorptive separation of buta-1,3-diene and but-2-ene isomers, one of the most energy intensive separations in chemical industry. Mixture separation experiments revealed that the 8-ring apertures of the DD3R cages are only accessible to trans-but-2-ene and buta-1,3-diene, while excluding but-1-ene and cis-but-2-ene molecules, resulting in shape-selective separation in the presence of C4 mixtures.status: publishe

Influence of Extraframework Aluminum on the Bronsted Acidity and Catalytic Reactivity of Faujasite Zeolite

A series of faujasite zeolites was modified by extraframework Al (AlEF) with the goal to investigate the influence of such species on the intrinsic Brønsted acidity and catalytic activity towards paraffin cracking. The chemical state of AlEF and zeolite acidity were investigated by 27Al MAS NMR and COads IR spectroscopy, H/D exchange reaction, and propane cracking. Strongly acidic defect-free Y zeolites were obtained by substitution of framework Al by Si with (NH4)2SiF6. In accordance with the next-nearest-neighbor model, the intrinsic acidity of the protons increased with decreasing framework Al density. This increased acidity was evidenced by an increased shift of the OH stretching vibration upon CO adsorption in COads IR spectroscopy and by an increased H/D exchange rate in H/D exchange reactions with perdeuterobenzene. All of the acid sites in these zeolites were of equal strength beyond a certain Si/Al ratio. The increased acidity resulted in an enhanced propane cracking activity. Modification of a model dealuminated Y zeolite by AlEF only resulted in a small fraction of cationic AlEF species, because it was difficult to control the ion exchange process. In comparison, commercial ultrastabilized Y zeolites contained less AlEF and these species were predominantly present in cationic form. The rate of propane cracking strongly correlated to the concentration of Brønsted acid sites perturbed by cationic AlEF species. The results of MQMAS 27Al NMR spectroscopy confirmed the presence of sites perturbed by AlEF and unaffected framework Al sites. Zeolites with higher intrinsic cracking activities contained a higher proportion of perturbed sites. Although CO_ads IR and H/D exchange methods proved to be suitable methods to probe the acidity of Y zeolites free from AlEF, they were less suitable to predict the reactivity if the Brønsted acid sites were affected by cationic AlEF species.status: publishe

"Extracting" the Key Fragment in ETS-10 Crystallization and Its Application in AM-6 Assembly

The mechanism of crystallization of microporous titanosilicate ETS-10 was investigated by Raman spectroscopy combined with 29Si magic-angle spinning (MAS) NMR spectroscopy, DFT calculations, and SEM imaging. The formation of three-membered ring species is shown to be the key step in the hydrothermal synthesis of ETS-10. They are formed by means of a complex process that involves the interaction of silicate species in the reaction mixture, which promotes the dissolution of TiO2 particles. These insights into the mechanism of ETS-10 growth led to the successful development of a new synthesis route to the vanadosilicate AM-6 that involves the use of intermediates that contain three-membered ring species as an initiator.status: publishe