1,721,012 research outputs found
Methanol to dimethylether on H-MFI catalyst: The influence of the Si/Al ratio on kinetic parameters
No full textA series of H-MFI zeolites at different Si/Al ratio were prepared and characterised in terms of physical and chemical properties and morphology. The zeolites were tested in the methanol dehydration reaction to Dimethyl Ether (DME), aiming at investigate the catalyst stability as a function of the reaction temperature. According to literature the temperature interval was set in the way of keeping a DME selectivity close to one and this required a lower temperature when increasing the catalyst acidity. Conversion data were measured at different catalyst amount/reactant flow rate ratios and the initial rate was extrapolated from the early points. Arrhenius plot of these initial rates as a function of temperature allowed to estimate the (apparent) activation energy as a function of the catalyst acidity (from 50 to 70 kJ/mol). Main findings were the confirmation that the catalyst performance is favoured by the higher acidity, but also samples at the lower is the Si/Al ratio exhibited the higher activation energy, revealing as the more sensitive to temperature variation. © 2013 Elsevier B.V
Hierarchical low si/al ratio ferrierite zeolite by sequential postsynthesis treatment: Catalytic assessment in dehydration reaction of methanol
No full textIn contrast to high silica zeolites, it is difficult to obtain mesoporosity in zeolites with low Si/Al ratio (e.g., <20) via conventional NaOH-based treatment, making the obtainment of hierarchical zeolites with high acidity a challenging target. In this paper, we report the preparation of hierarchical FER-type zeolite at low Si/Al molar ratio (about 10) by postsynthesis etching involving a sequence of three treatments with NaAlO2, HCl, and NaOH solutions and investigate the effect of both NaAlO2 solution concentration and time of treatment on the textural properties. The obtained materials exhibit a mesoporous volume higher than the parent ferrierite with no significant effect on the sample acidity. The catalytic activity of some samples was investigated in vapour-phase methanol dehydration to dimethyl ether, revealing the superiority of hierarchical zeolites in terms of methanol conversion, although the presence of mesopores causes formation of light hydrocarbons at high temperatures
Kinetic analysis of methanol to dimethyl ether reaction over H-MFI catalyst
No full textThis paper reports the kinetic analysis of methanol dehydration to dimethyl ether (DME) on a zeolitic MFI-type catalyst. For this reaction, γ-alumina is the most used catalyst, but the use of a zeolite, such as H-MFI, is gaining greater attention because these materials exhibit a better stability against water presence and can modulate the catalyst acidity acting on different parameters (e.g., Si/Al ratio or postsynthesis treatments). The kinetic analysis of H-MFI is performed by using experimental data of methanol conversion in a differential reactor and in an integral reactor. By including the most important kinetic models proposed in the literature for alumina, kinetic parameters (as a function of temperature) in the case of H-MFI are calculated, and the comparison between different models is also presented and discussed. In addition a new kinetic model is proposed and data fitting is preferable with respect to literature equations: also data of activation energy are in agreement with literature findings
Synthesis and catalytic performances evaluation of FER-based catalysts with different acidity in methanol conversion to DME
No full textIn this work the effect of acidity on catalytic performances of zeolitic catalysts with FER structure in methanol dehydration reaction for DME production is presented. Synthesis condition are tuned in order to obtain a Si/Al ratio in the range of 7-47. NH3-TPD and FT-IR measurements show that aluminum content affect both aluminum incorporation and both strength and distribution of acid sites. Catalytic evaluation suggest that acidity affect methanol conversion while all catalysts exhibit a very high selectivity toward DME suggesting FER structure as a reliable catalyst for DME production
Dimethyl ether synthesis via methanol dehydration: Effect of zeolite structure
No full textIn this paper, the effects of either pore size or topology of zeolites were studied in the methanol dehydration to dimethyl ether reaction by comparing catalytic performances of BEA, MFI and FER structures in the temperature range 180 °C-300 °C. The aim of this study was to investigate how the zeolite catalyst characteristics affect the process performances in terms of methanol conversion and DME selectivity. It was found that the largest-pore 3-D framework zeolite (BEA) was very effective in converting methanol but the channel size and topological connection spaces allowed the fast formation of coke precursors that rapidly decreases the catalyst performances at higher temperatures. Even if lower in intensity, the same phenomenon was observed for MFI structure (medium pores 3-D framework) confirming stable performances in the temperature range used in the real processes. On the contrary, the 2-D small pores FER zeolite showed a very good selectivity at high temperature also exhibiting a promising conversion rate for an industrial application. Continuous catalytic tests at 300 °C, followed by coke deposition analysis, confirmed that BEA rapidly deactivates and exhibited the higher coke formation rate, on the contrary, FER structure exhibited a great stability (conversion and selectivity) as well as a reduced coke formation tendency
Use of Spanish Broom (Spartium junceum L.) Fibers for Removal of Heavy Metal Ions from Aqueous Solutions
No full textIn this work, potential use of Spanish broom fibers for removing from aqueous solutions heavy metal ions such as copper Cu(II) and lead Pb(II) was investigated. The fibers were extracted by DiCoDe (Digestion-Compression-Decompression) process changing experimental conditions. Morphology of fibers was evaluated by scanning electron microscopy and chemical composition, as well as the carboxylic group content, was determined by conventional methods. Various initial concentrations of heavy metal ions were studied to explore potential sorption on cellulose fibers. The results suggest that cellulose fibers, extracted by different conditions of DiCoDe process, are capable of sorbing metal ions
Liquid-like hydrogen in the micropores of commercial activated carbons
No full textThe hydrogen adsorption properties of commercial activated carbon (aC) samples, in particular Nuchar SA-1500, Filtercarb GCC 8x30 and Filtercarb PHA, are evaluated at different temperatures (77 K, 196 K and ambient temperature) and pressure up to 80 bar. A comprehensive characterization is carried out by means of a volumetric Sieverts-type apparatus for hydrogen adsorption measurements and helium picnometry for the skeletal density evaluation, by nitrogen adsorption measurement for the evaluation of the surface area (BET) and pore size distribution (NLDFT), by Scanning Electron Microscopy (SEM) for morphology. All the adsorption data are evaluated by Langmuir/T oth isotherm model with a very high accuracy and the enthalpy of adsorption is calculated through the ClausiuseClapeyron equation. Comparison between the different samples is shown. The probed aC samples show interesting hydrogen storage capacity and reversible behavior up to many cycles with and without any thermal treatment in between. Our data point out the crucial role of the microporosity and ultra-microporosity in the adsorption process at low H2 pressures. The surfaces exhibit an average adsorption enthalpy around 6.5 kJ/mol while for the ultra-microporous sites a 14 kJ/mol value is found. The observed trapping behavior at 77 K is attributed to the ultra-microporous morphology of the porous structure in GCC and PHA samples. A further result is the evaluation of the hydrogen molar density in the micropores with size below 10 A, which is 30 mmol/cm3, a value very close to the liquid hydrogen one. These results could represent an interesting starting point for a real and efficient alternative method to the hydrogen storage using cheap and easy scalable materials
The effect of FER zeolite acid sites in methanol-to-dimethyl-ether catalytic dehydration
No full textIn this paper, the effect of acidity of zeolites with FER framework was studied in the methanol dehydration to dimethyl ether reaction, by comparing catalysts with different Si/Al ratios (namely 8, 30 and 60). The aim of this work was to investigate how the acid sites concentration, strength, distribution and typology (Brønsted and Lewis) affect methanol conversion, DME selectivity and coke formation. It was found that the aluminium content affects slightly acid sites strength whilst a relevant effect on acid sites concentration and distribution (Brønsted /Lewis) was observed as 24% of Lewis sites were found on Al-richest samples, whilst less than 10% of Lewis acid sites were observed on FER at higher Si/Al ratio. All the investigated catalyst samples showed a selectivity toward DME always greater than 0.9 and samples with the lowest Si/Al ratio exhibit the best performances in terms of methanol conversion, approaching the theoretical equilibrium value (around 0.85) at temperatures below 200 °C. Turnover-frequency analysis suggests that this result seems to be related not only to the higher amount of acid sites but also that the presence of Lewis acid sites may play a significant role in converting methanol. On the other hand, the presence of Lewis acid sites, combined with a high acidity, promote the formation of by-products (mainly methane) and coke deposition during the reaction. As final evidence, all the investigated catalysts exhibit very high resistance to deactivation by coke deposition, over 60 h continuous test, and a GC–MS analysis of the coke deposited on the catalyst surface reveals tetra-methyl benzene as main component
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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