Influence of precursors on the induction period and transition regime of dimethyl ether conversion to hydrocarbons over ZSM-5 catalysts

ZSM-5 catalysts were subjected to step response cycles of dimethyl ether (DME) at 300 °C in a temporal analysis of products (TAP) reactor. Propylene is the major olefin and displays an S-shaped profile. A 44-min induction period occurs before primary propylene formation and is reduced upon subsequent step response cycles. The S-shaped profile was interpreted according to induction, transition-regime and steady-state stages to investigate hydrocarbon formation from DME. The influence of precursors (carbon monoxide, hydrogen, dimethoxymethane, and 1,5-hexadiene) was studied using a novel consecutive step response methodology in the TAP reactor. Addition of dimethoxymethane, carbon monoxide, hydrogen or 1,5-hexadiene reduces the induction period of primary olefin formation. However, while dimethoxymethane, carbon monoxide and hydrogen accelerate the transition-regime towards hydrocarbon pool formation, 1,5-hexadiene attenuates it. Heavier hydrocarbons obtained from 1,5-hexadiene compete for active sites during secondary olefin formation. A phenomenological evaluation of multiple parameters is presented

Heterogeneous ketonic decarboxylation of dodecanoic acid: studying reaction parameters

Ketonic decarboxylation has gained significant attention in recent years as a pathway to reduce the oxygen content within biomass-derived oils, and to produce sustainable ketones. The reaction is base catalysed, with MgO an economic, accessible and highly basic heterogeneous catalyst. Here we use MgO to catalyse the ketonic decarboxylation of dodecanoic acid to form 12-tricosanone at moderate temperatures (250 °C, 280 °C and 300 °C) with low catalyst loads of 1% (w/w), 3% (w/w) and 5% (w/w) with respect to the dodecanoic acid, with a reaction time of 1 hour under batch conditions. Three different particle sizes for the MgO were tested (50 nm, 100 nm and 44 μm). Ketone yield was found to increase with increasing reaction temperature, reaching approximately 75% yield for all the samples tested. Temperature was found to be the main control on reaction yield, rather than surface area or particle size

Synthetic control of defect structure and hierarchical extra-large/small-pore microporosity in aluminosilicate zeolite SWY

R.G.C. thanks the University of St. Andrews and Johnson Matthey for funding. P.A.W. thanks the Royal Society (Industrial Fellowship INF\R2\192052) for support. R.G.C. acknowledges the support of the EPSRC Light Element Analysis Facility grant EP/T019298/1 and the EPSRC Strategic Equipment Resource grant EP/R023751/1 for the use of the Jeol JSM-IT800 electron microscope at the University of St Andrews. A.M. acknowledges the Spanish Ministry of Science (RYC2018-024561-I) and the Gobierno of Aragon (Nanomidas group, code E13_23R).The SWY-type aluminosilicate zeolite, STA-30, has been synthesized via different routes to understand its defect chemistry and solid acidity. The synthetic parameters varied were the gel aging, the Al source, and the organic structure directing agent. All syntheses give crystalline materials with similar Si/Al ratios (6–7) that are stable in the activated K,H-form and closely similar by powder X-ray diffraction. However, they exhibit major differences in the crystal morphology and in their intracrystalline porosity and silanol concentrations. The diDABCO-C82+ (1,1′-(octane-1,8-diyl)bis(1,4-diazabicyclo[2.2.2]octan)-1-ium)-templated STA-30 samples (but not those templated by bisquinuclidinium octane, diQuin-C82+) possess hierarchical microporosity, consisting of noncrystallographic extra-large micropores (13 Å) that connect with the characteristic swy and gme cages of the SWY structure. This results in pore volumes up to 30% greater than those measured in activated diQuin-C8_STA-30 as well as higher concentrations of silanols and fewer Brønsted acid sites (BASs). The hierarchical porosity is demonstrated by isopentane adsorption and the FTIR of adsorbed pyridine, which shows that up to 77% of the BASs are accessible (remarkable for a zeolite that has a small-pore crystal structure). A structural model of single can/d6r column vacancies is proposed for the extra-large micropores, which is revealed unambiguously by high-resolution scanning transmission electron microscopy. STA-30 can therefore be prepared as a hierarchically porous zeolite via direct synthesis. The additional noncrystallographic porosity and, subsequently, the amount of SiOHs in the zeolites can be enhanced or strongly reduced by the choice of crystallization conditions.Publisher PDFPeer reviewe

Selective Toluene Disproportionation to Produce Para-Xylene over Modified ZSM-5

Industrial demand for p-xylene has been increasing in recent years and the major production is by toluene disproportionation. This research studied the effect of crystal size and the modification by silica deposition on the surface of the zeolite. Shape selective catalysts have been prepared by modifying two different ZSM-5 zeolites with different crystal sizes (0.5 µm, 5 µm) by chemical liquid deposition (CLD) using TEOS (tetraethyl orthosilicate) to deposit inert silica layers. The large ZSM-5 crystals modified with two cycles of silica showed high p-xylene selectivity at typically 84 % with very low conversion. Pressure effect on the reaction was investigated. Selectivity achieved was 40 – 66 % at conversions typically 4-20 % at 10 bar

Performance modelling of zeolite-based potentiometric sensors

Developing simple and cost-effective electrochemical sensors for widespread on-site application is of considerable practical importance in agriculture, environmental monitoring and food science. Among multiple sensing platforms, potentiometry is particularly effective in terms of simplicity, low cost and mass-production. This work is focused on a systematic analysis of the structure – performance relationship using chemometric techniques, which can be applied to sensor arrays with varying response patterns. The potentiometric sensitivity of zeolite-modified electrodes, containing thirteen synthetic and three natural zeolites, in aqueous solutions of Na+, K+, NH4+, Ca2+ and Mg2+ has been correlated with a range of zeolite characteristics using PCA and PLS modelling, thus demonstrating how structural and physical properties impact the performance of zeolite-modified potentiometric sensors. In addition to steric factors, e.g. zeolite pore size, the important characteristics governing the sensor performance are the Si/Al ratio and the presence of specific extraframework cations. For instance, K+ and Na+ show a strong effect on the potentiometric sensitivity towards Ca2+. The level of precision achieved by the PLS models indicates that semi-quantitative predictions are feasible. To improve the computational models, larger sets of data with a wider range of zeolite-modified sensors are necessary. The constituent materials of such sensors should have a set of well-defined properties, which can be controlled and tuned for a particular application. It is anticipated that synthetic rather than natural zeolites would satisfy such requirements

Effect of tapeworm parasitisation on cadmium toxicity in the bioindicator copepod, Cyclops strenuus

The ubiquitous nature of crustaceans has meant that they have been considered as useful bioindicators of water quality. Although crustaceans e.g. Cyclops strenuus serve as intermediate hosts to many metazoan parasites, the effect of infection on toxicity of heavy metal pollution has not been established. The interaction between the procercoid stage of the tapeworm, Bothriocephalus acheilognathi, and its copepod host when exposed to cadmium toxicity was investigated. Exposure to 100 μg Cd/l caused a significant (P < 0.001) decrease in copepod survival of crustaceans exposed to combined parasite infection and metal treatment, compared with all other groups which had been exposed to these stressors individually. This effect was not dependent on infection intensity. Although cadmium could not be detected by EDXMA, optical emission spectroscopy revealed Cd accumulation in both treated uninfected and infected copepods. Overall, the data suggest that parasitic infection is an important consideration in determining the suitability of a bioindicator of water pollution

Impact of aqueous impregnation on the long-range ordering and mesoporous structure of cobalt containing MCM-41 and SBA-15 materials

International audienceThe effect of modification with cobalt via aqueous impregnation on the structure of MCM-41 and SBA-15 materials was studied using X-ray diffraction (XRD) at low scattering angles and low-temperature nitrogen adsorption. It was shown that the introduction of small amounts of cobalt resulted in dramatic changes in the structure of MCM-41 silica. The low angle XRD peaks characteristic of long-range ordering disappeared while the BET surface area and the total pore volume decreased considerably. Only micropores smaller than 20 Å were detected in the cobalt containing MCM-41 materials. A much smaller impact of impregnation with cobalt on the hexagonal structure and porosity was observed for SBA-15 materials. The periodic structure, surface area and total pore volume of SBA-15 mesoporous silica remained almost unchanged on impregnation with cobalt nitrate, catalyst drying and calcination. Low angle XRD patterns typical of hexagonal structure were still well resolved and intense in the SBA-15 materials containing up to 20 wt. % of cobalt. Analysis of XRD intensities suggested that after aqueous impregnation and drying a considerable amount of cobalt ions was located in the SBA-15 mesopores. Nitrogen adsorption-desorption data indicated plugging a part of SBA-15 mesopores in cobalt containing samples. Due to the high stability, SBA-15 silica appears to have a good potential for its use as a support for metal and oxide catalysts

Structural and Acidic Properties of Ion-Exchanged Mazzite

A range of modified MAZ and LTL zeolites have been prepared and ion-exchanged with a series of alkali metal cations. It has been shown that K, Rb and Cs can be readily introduced into the MAZ structure with ~2 cations ion-exchanged per unit cell. In contrast, less than one cation of Li or Na per unit cell has been introduced under similar conditions. Ion-exchanged zeolites have been characterised using Fourier transform infrared spectroscopy, X-ray powder diffraction, scanning electron microscopy, X-ray fluorescence and N2 physisorption in order to gain a better understanding of their structural and acidic properties. The FTIR data indicate considerable heterogeneity of the bridging OH groups in mazzite. The concentration of both Brønsted and Lewis acid sites detected in MAZ using pyridine as a probe molecule is lower than expected from its chemical composition, with the relative accessibility of the bridging OH-groups varying from 16% for H-MAZ to 28% for K-exchanged samples. This is in agreement with the N2 adsorption-desorption data showing a rather low micropore volume for the ion-exchanged materials and with the NH3-TPD results implying considerable transport limitations. This work demonstrates that the channel structure of mazzite is partially blocked resulting in a decreased micropore volume and limited access to the acid sites