Higher Dehydration Activity Research Articles

On the role of the nature and density of acid sites on mesostructured aluminosilicates dehydration catalysts for dimethyl ether production from CO2

In this work, we designed four different mesostructured acidic materials to be used as methanol dehydration catalysts for the one-pot CO2-to-DME process, in the form of physical mixtures with a Cu/ZnO/Al2O3-based commercial redox catalyst (CZA). The studied systems consist in a mesostructured γ-Al2O3 and three mesostructured aluminosilicates (namely Al-MCM-41, Al-SBA-15, and Al-SBA-16) with the same Si/Al ratio (= 15) but significantly different textural properties. The main goal of this work is to understand how the textural features can influence the acidic properties (typology, amount, strength, surface density) and, consequently, how catalytic performances can be correlated with acidic features. On this note, we found that the systems presenting both Brønsted and Lewis sites (namely the three aluminosilicates) show much better catalytic performances than γ-Al2O3, that only features Lewis sites, thus implying that Brønsted sites are more active towards methanol dehydration than Lewis sites. The three aluminosilicates, despite presenting comparable amounts of Brønsted sites, show significantly different performances in terms of selectivity to DME; particularly, Al-SBA-16, the system with the lowest surface area, proved to be the most efficient catalyst. This finding led us to infer that, besides Brønsted acidity, a high surface density of acid sites is a key factor to obtain a high dehydration activity; being methanol dehydration a bi-molecular reaction, the close proximity of two acid sites would indeed favor the kinetics of the process.

Modeling of a membrane integrated catalytic microreactor for efficient DME production from syngas with CO2

Conversion of syngas (CO + CO2 + H2) to dimethyl ether (DME) with in–situ steam separation is modeled in a membrane integrated, isothermal catalytic microchannel reactor. Reaction channel, involving washcoated form of physically mixed Cu–ZnO/Al2O3 and HZSM–5 catalysts, is separated from the permeate channel by a supported sodalite (SOD) membrane layer. Conservation of momentum and mass within the porous washcoat and the channels, and cross–membrane material transport are modeled in two dimensions at steady–state to elucidate the effects of temperature, pressure, syngas composition and permeate flow properties. Dosing syngas to both channels at 523 K, 50 bar, CO2/COx (COx: CO + CO2) = 0.5 and H2/COx = 2.0 gives CO and CO2 conversions, and DME yield of 33.2, 12.4 and 15.3 %, respectively, which are 29.9, 7.2 and 12.7 % without membrane. These findings are coherent with relaxed thermodynamic limitations on methanol synthesis and dehydration upon selective H2O removal. Higher permeate syngas flow rates promote steam efflux and H2 influx across the membrane and further elevate CO2 conversion and DME yield up to 15.8 and 17.4 %, respectively. These metrics can be improved up to 23.4 and 22.9 %, respectively upon dosing pure H2 as the permeate fluid. However, a positive pressure gradient from reaction to permeate channel does not improve reactor performance. Coherent with its higher dehydration activity, HZSM–5 responds to membrane assistance stronger than γ–Al2O3.

Nanostructured composite TiO2/carbon catalysts of high activity for dehydration of n-butanol

Abstract A novel method of wood impregnation with titanium ions is presented. Titanium(IV) ions were complexed to peroxo/hydroxo complexes which were obtained by treating a TiCl4 water solution with H2O2. The solution of chelated titanium ions was used for the impregnation of living stems of Salix viminalis wood. Saturated stems were carbonized at 600–800 °C, yielding a microporous carbon matrix, in which nanoparticles of TiO2 were uniformly distributed. A series of composite TiO2–carbon catalysts was manufactured and tested in the process of n-butanol conversion to butane-1. The composite catalysts exhibited very high selectivity (ca. 80%) and yield (ca. 30%) despite a low content of titanium (ca. 0.5% atomic). The research proved that the proposed functionalization led to high dispersion of the catalytic phase (TiO2), which played a crucial role in the catalyst performance. High dispersion of TiO2 was achieved due to a natural transport of complexed titanium ions in living plant stems.

Efficient and selective conversion of fructose to 5-hydroxymethylfurfural over metal exchanged heteropoly tungstate supported on tin oxide catalysts

A series of Al exchanged heteropoly tungstate (AlTPA) supported on SnO2 catalysts were prepared and characterized by FT-Infrared, X-ray diffraction and temperature programmed desorption of ammonia. The catalysts were evaluated for dehydration of fructose to 5-hydroxymethylfurfural (5-HMF). The catalysts showed high activity for dehydration of fructose and the catalyst with 20 % AlTPA on SnO2 exhibited highest activity. The high activity of AlTPA catalysts related to the presence of Lewis acidic sites apart from Bronsted acidic sites generated due to presence of Al. The active catalyst was further treated at different temperature to understand the surface-structural variations and thereby their influence on fructose dehydration activity. High temperature calcinations of catalysts leads to destabilization of Keggin unit and resulted a decrease in acidity. The dehydration of fructose activity and selectivity towards 5-HMF product also depends on the reaction temperature, time and catalyst weight.

Performance of modified H-ZSM-5 zeolite for dehydration of methanol to dimethyl ether

The conversion of methanol to dimethyl ether was carried out over various commercial zeolites and modified H-ZSM-5 catalysts to evaluate their catalytic performance. A series of commercially available zeolite samples were used for vapor-phase dehydration of methanol to DME. Catalyst screening tests were performed in a fixed-bed reactor under the same operating conditions ( T = 300 °С, P = 16 barg, WHSV = 3.8 h − 1). It was found that all the H-form zeolite catalysts in this study were active and selective for DME synthesis. According to the experimental results MDHC-1 catalyst exhibited the highest activity in dehydration of methanol. After finding the most active catalyst, the H-MFI90 zeolite was modified with Na content varying from 0 to 120 mol%, via wet-impregnation method to further improve its selectivity. All of catalysts were characterized by BET, XRD, NH 3-TPD, ICP, TGA, SEM, FT-IR and TPH techniques. It was found that these materials affected activity of MDHC-1 zeolite by changing its acidity. Ultimately, among all the catalysts studied, Na 100-modified H-MFI90 zeolite exhibited optimum activity, selectivity and stability at methanol dehydration reaction.

Synthesis of Dimethyl Ether over Modified H-Mordenite Zeolites and Bifunctional Catalysts Composed of Cu/ZnO/ZrO2 and Modified H-Mordenite Zeolite in Slurry Phase

Synthesis of dimethyl ether (DME) via methanol dehydration were investigated over various catalysts, and via direct CO hydrogenation over hybrid catalysts composed of Al-modified H-Mordenite zeolite and Cu/ZnO/ZrO2. H-Mordenite zeolite exhibited the highest activity in dehydration of methanol. However, its selectivity toward dimethyl ether was rather low. For this reason, the H-Mordenite was modified. Modification of zeolites was performed by wet impregnation method and considered catalysts were characterized by AAS, XRD and NH3-TPD analyses. Results of catalytic tests indicated that H-Mordenite modified with 8 wt% aluminum oxide was the best catalyst for synthesis of dimethyl ether from methanol, in which methanol conversion and DME selectivity were 99.8 and 96.8%, respectively, without noticeable change in catalyst stability. For direct synthesis of dimethyl ether from synthesis gas, hybrid catalysts were prepared by coprecipitation sedimentation method. It was found that optimum ratio of methanol synthesis catalyst to methanol dehydration catalyst is 2:1. In this case, CO conversion and DME selectivity were 64 and 78.8%, respectively, with good catalyst stability. Ultimately, it was concluded that the hybrid catalyst composed of Cu/ZnO/ZrO2 and Al-modified H-Mordenite zeolite is an appropriate catalyst for direct synthesis of dimethyl ether from the synthesis gas.

Determining an optimum catalyst for liquid-phase dehydration of methanol to dimethyl ether

Abstract The liquid-phase dehydration of methanol to dimethyl ether was investigated over various materials including synthetic zeolites, namely, ZSM-5, Y, Mordenite, Ferrierite and Beta as well as silica and alumina. The key characters investigated were the Si/Al ratio and cation exchange. The results showed that the Mordenite zeolite exchanged with H+ exhibited the highest activity in dehydration of methanol. After finding the most active catalyst, the Mordenite zeolite was modified with Cu, Zn, Ni, Al, Zr, Mg and Na via wet-impregnation method to further improve its selectivity, and characterized by AAS, XRD, NH3-TPD, NH3-FT-IR and BET surface area techniques. It was found that these materials affected activity of HM zeolite by changing its acidity. Ultimately, amongst all catalysts studied, Al-modified Mordenite zeolite exhibited optimum activity, selectivity and stability for methanol dehydration to dimethyl ether.

Evaluation of Zeolites in Production of Tetrahydrofuran from 1,4-Butanediol: Performance Tests and Kinetic Investigations

The liquid-phase dehydration of 1,4-butanediol to tetrahydrofuran was investigated over various materials including synthetic zeolites, namely, ZSM-5, Y, Mordenite, Ferrierite, and Beta as well as silica and alumina. The key characters investigated were the Si/Al ratio and cation exchange. The results showed that the ZSM-5 zeolite exchanged with NH4+ exhibited the highest activity in dehydration of 1,4-butanediol. In all tests, nearly 100% selectivity to tetrahydrofuran was observed. After finding the most active catalyst, the kinetics of dehydration of 1,4-butanediol to tetrahydrofuran was studied over this material. The influence of operating parameters such as temperature, pressure, and substrate concentration on reaction was investigated through the Taguchi analysis of experimental data. A strong dependence of reaction rate on temperature was observed. Pressure, however, was found to have a negligible effect on reaction rate. The contribution of external and intraparticle mass transfer limitations on ...

The effect of the gradual thermal decomposition of surface oxygen species on the chemical and catalytic properties of oxidized activated carbon

The physicochemical properties, surface chemical structure and some catalytic properties of a series of carbons prepared by nitric acid oxidation of an activated carbon and subsequent heat treatment under vacuum and mild temperature conditions (423–573 K) were studied. The porous structure characteristics of the partially evacuated samples were estimated from low-temperature nitrogen adsorption data. The thermal analysis and the quantitative determination of surface functional groups by selective neutralization of bases and pH-metric titration were carried out. The dehydration of 2-methylpropan-2-ol was used as a test reaction. While gradual annealing in vacuum alters the surface only slightly, it does differentiate strongly the number and the acidic strength of the surface groups. Progressive heating under mild conditions removes mainly those surface groups that are located in macropores or on the outer surface of the carbon. According to TPD results, the decomposed surface groups are single carboxylic groups, as expected. The decomposition of single, strong carboxylic groups is accompanied by rearrangements of other carboxylic groups with the simultaneous formation of additional cyclic structures like anhydrides, lactones or lactols. Catalytic tests support our previous findings that oxidized carbons have a high dehydration activity. This activity is controlled not only by the number and the strength of acidic groups, but also by their accessibility. There exists an optimum concentration of surface acidic groups above which the activity decreases due to steric restrictions.

Recovery of ethene-selective FeO x/Al 2O 3 ethanol dehydration catalyst from industrial chemical wastes

Boehmite, γ-AlO(OH), recovered from aluminium dross tailings (ADT) and steel-pickling chemical waste (SPW) were used to prepare 1–10 wt.% FeO x /Al 2O 3 catalysts. Surface and bulk properties of the catalysts were characterized by nitrogen sorptometry, thermogravimetry (TG) of adsorbed pyridine, X-ray photoelectron spectroscopy and X-ray powder diffractometry. The results have shown the catalysts to consist of highly dispersed α-Fe 2O 3 particles on high-surface area γ-Al 2O 3, but cannot exclude the coexistence of monolayer species of Fe(II)–O and/or Fe(III)–O, as well as Fe(III) incursions into the support bulk structure. Activity tests of the catalysts towards ethanol dehydration (at 140–250°C) in the gas phase, have revealed that increasing amounts of supported FeO x species could turn γ-Al 2O 3 from being diethyl ether selective to ethene selective, without changing its originally high dehydration activity towards the alcohol. Consequent modifications of the surface acid site density have been correlated to observed changes in the alcohol dehydration rate. Hence, the switching of the catalyst selectivity to ethene formation has been suggested to follow suppression (or elimination) of bi-molecular reaction mechanism necessary for ether formation due to Fe(III) incursion into the support, and/or ether decomposition on Fe 2O 3 particles.

Catalytic properties of H2-reduced MoO3 with noble metal for the conversions of heptane and propan-2-ol

Effects of H2 reduction on the catalytic properties of MoO3 with Pt, Pd, Rh, Ir, or Ru for the conversions of heptane and propan-2-ol were studied. The catalytic activity of MoO3 with noble metal for the isomerization of heptane was strongly dependent on the period of H2 reduction at 623 K. This behavior was almost the same as that of MoO3 without noble metal. Among the catalysts tested, Pt/MoO3 was the most active for this reaction. The catalytic activities of MoO3 for dehydration and dehydrogenation of propan-2-ol also increased in proportion with the period of H2 reduction. In the case of MoO3 with noble metal, a higher dehydration activity was obtained by a longer period of H2 reduction, while the dehydrogenation activity was almost independent of the reduction period. Pt/MoO3 exhibited a high dehydration activity compared with the other catalysts, indicating the most acidic property of Pt/MoO3. We conclude from these results that the high isomerization activity of Pt/MoO3 can be attributed to its high acidity as well as to the hydrogenative and dehydrogenative properties of Pt metal.

PLS vs. zeolites as sorbents and catalysts. 5. Evidence for Brønsted/Lewis acid crossover and high acidity in conversions of C 1–3 alcohols in some alumina-pillared smectite clays

Conversions of methanol, ethanol, and iso-propanol have been investigated in the activated alumina-pillared montmorillonite (BP-PILC), beidellite/montmorillonite (FAZA), and saponite (ATOS)-based PILCs obtained from standard [Al 13O 4(H 2O) 12(OH) 24] 7+ Keggin-ion. Product outcomes for MeOH differ from those on smectite clays themselves and on alumina (or transition metal ion oxides): all give rise to dimethylether, rather than methylformate or formic acid. Systematic study of conversion, yields and selectivity for cation-exchanged FAZA show that the large changes observed must be ascribed to both steric effects and selective blocking of proton-containing active sites. The latter are most important in Ni 2+-exchanged FAZA-containing catalysts and are attributed to generation of highly acidic Lewis sites, e.g. hydrocarbons alone (in a distribution similar to the MTG process) are obtained on Ni 2+-FAZA. Apart from ethene and acetaldehyde, EtOH conversion also gives diethylether, not obtained on smectites themselves, and produced via a bimolecular reaction. The total dehydration/dehydrogenation ratio varies in the order BP-PILC>FAZA>>AZA=ATOS, BP-PILC being the most acidic and most selective. Dehydration and dehydrogenation activity with temperature go through a crossover point. This is ascribed to “iso-acidity”, i.e. the iso-acidic point is where Lewis and Brønsted acids are equally strong. Trends in iso-acidity with metal-ion exchange in FAZA materials suggest that the Lewis acid sites are at the alumina pillar. Detailed study of reaction kinetics and contact times leads to the conclusion that saponite surfaces have higher dehydration activity than those in montmorillonite.

Effect of spinel (ZnCr 2O 4) formation on the texture, electrical conduction and catalytic behaviour of the ZnOCr 2O 3 system

Structural and phase changes accompanying the calcination in air, up to 1000 °C, of the ZnOCr 2O 3 system were monitored using differential thermal analysis, differential scanning calorimetry, infrared spectrophotometry, X-ray diffractometry and chromium ion estimation. The texture was assessed by analyzing nitrogen sorption isotherms measured at −196 °C. The results indicate that ZnOCr 2O 3 solid solution becomes a major product at 500 °C, and the ZnCr 2O 4 spinel phase starts to form at 400 °C. The electrical conductance properties of the samples were investigated before and after admission of 2-propanol in the temperature range 100–400 °C. The electrical conduction is attributed to the existence of a surface mobile-electron Zener phase that maximizes the conductivity. The obtained interstitial Zn 2+ cations and free electrons (charge carriers) are considered to be localized at the ions or vacant sites, and the conduction occurs via a hopping-type process, which implies a thermally activated electronic mobility. The catalytic activity data of the vapor-phase decomposition of 2-propanol were obtained in the temperature range 200–400 °C, using a flow system technique. It was found that propylene is the main reaction product, with a minor yield of acetone. In the solid solution of ZnO in Cr 2O 3, the Cr ion sites will be electronically more isolated than either disordered or ordered spinel phases. The neutralization of these sites will not occur by bulk electron transfer, but they tend to trap electrons from the oxygen end of the 2-propanol molecule, leading to a high dehydration activity. Correlations were attempted between the structure of the catalysts and their catalytic activity.

Catalytic conversion of propan-2-ol on carbon catalysts

The decomposition of propan-2-ol was studied using carbon catalysts with a different chemical nature of the surface. They were prepared from poly(furfuryl alcohol). The catalytic tests were performed in a flow reactor in the temperature range 348–463 K. Dehydration as well as dehydrogenation of the substrate occur under the conditions applied. The products are di-2-propyl ether plus propene, and acetone, respectively. The study supports our previous findings of a high dehydration activity of oxidized carbons. It was found that the dehydration occurs on the outer surface of the carbon catalyst, whereas the dehydrogenation takes place also within the catalyst pores. There exists an optimum concentration of acidic surface oxides, above which there is no further increase of the dehydration activity. In the contrary, a decrease was observed. Effects of poisoning with substances of different acceptor-donor properties support our earlier findings that the dehydration activity results from the presence of carboxyl groups of varying strength, whereas the dehydrogenation activity results from the simultaneous presence of the acidic and basic Lewis sites.

The catalytic decomposition of ethers on evaporated tungsten films

The reactions of diethyl and di- n -propyl ethers have been studied in the presence of hydrogen on evaporated films of tungsten. In the temperature range from 200 to 260°C ethane and ethylene were formed from diethyl ether and small amounts of butenes were also produced. Each film had an initial high activity, especially for the formation of ethane, but the activity declined to a steady value during a transitional period. Subsequently, the decomposition of the ether occurred with zero order kinetics. A similar transitional period was observed during the decomposition of di- n -propyl ether but the change in the character of the reactions was more marked. Propane and propylene were formed initially, but very little further propane was produced after the initial period. If the surface of the tungsten was oxidized before exposure to ether, a high activity for the dehydration of the propyl ether was observed. Evidence from a number of experiments showed that irreversible changes were occurring to the catalyst during the transitional periods in which the metal surface was being converted to a different type of surface under the combined action of the ether and water vapour which was either added or formed by reaction. Most of the results could be interpreted on the assumption that two types of surface were formed—an oxidized surface of high activity for dehydration and an inactive surface covered by strongly adsorbed hydrocarbon residues. Subsidiary experiments were carried out with n -propanol on oxidized tungsten and evidence was found that the dehydration of the alcohol which was strongly adsorbed probably controlled the rate of reaction of the ether.