Titanium-doped alumina for catalytic dehydration of methanol to dimethyl ether at relatively low temperatures

Mesoporous Al-Ti oxide composites with molar %Ti of 3, 5, 10, and 20 as well as pure γ-alumina were prepared using a template-free sol-gel method in the absence of a catalyst. The prepared composites were characterized by powder XRD, FTIR spectroscopy and N₂ adsorption for BET surface area and porosity measurements. The composites and the pure alumina possessed relatively high surface areas, 350-410 m²/g, and high porosities after calcination at 500 °C. FTIR spectroscopy was employed to study the products of the catalytic dehydration of methanol to dimethyl ether, DME, over the prepared catalysts at reaction temperatures between 180 and 300 °C. Compared with pure γ-alumina, the Ti-modified alumina with %Ti < 10 showed higher catalytic activity in the methanol dehydration and better selectivity to DME. Composites with %Ti of 3 and 5 showed the highest activity at relatively lower temperatures than the other catalysts where they showed their highest activity at 190 and 200 °C, respectively. The activity of all studied catalysts slightly decreased as the temperature was raised to 300 °C and dropped considerably when the temperature was decreased to 180 °C. However, the activity of Al-Ti-3 dropped only slightly at both temperatures. The selectivity to DME was dependent on the reaction temperature where 100% DME selectivity was obtained at temperatures ?220 °C and as the temperature was raised to 300 °C, some CH₄ and CO₂ formed on the account of DME.     

Statistical analysis of the performance of a bi-functional catalyst under operating conditions of LPDME process

Liquid phase direct synthesis of dimethyl ether (LPDME™) under various operating conditions (temperature, H₂/CO molar ratio of feed) was conducted in a mechanically agitated slurry reactor system. Each run was monitored for 60 h time on stream (TOS) in order to confirm the high activity and long-term stability of a bi-functional catalytic system (CuO–ZnO–Al₂O₃/H-MFI-90). Statistical experimental design was applied for determining the optimum operating conditions under which the catalytic system shows the highest performance. A significant improvement in the performance of the bi-functional catalyst was observed when the temperature and H₂/CO molar ratio of feed were increased from 200 to 240 °C and 1 to 2, respectively at a constant pressure of 35 bar and GHSV equal to 1100 mLn/(g-cat h). CO conversion was increased from 9.1 mol% at T = 200 °C and H₂/CO = 1 to 79.6 mol% at T = 240 °C and H₂/CO = 2 and the yield and selectivity of DME also increased from 7.11% to 47.05% and 41.57% to 59.96%, (molar basis) respectively. No significant deactivation has been observed during 60 h TOS at different operating conditions. Furthermore, from the main effect plots and response table results, it was concluded that the most effective factor on activity and stability of bi-functional catalytic system is temperature.     

杂原子磷铝分子筛BSiAPO-5的合成结构及催化性能的研究

用水热合成法合成了一种硼-硅磷铝分子筛,通过化学分析、测定晶胞参数及用XRD,MAS—NMR方法进行表征,证实了硼、硅原子进入了分子筛骨架,并且具有和AIPO4-5相似的结构;用IR,NH3-TPD表征方法测出了它的酸性,测定结果表明,它具有L酸和B酸中心,以中等强度酸为主。在苯与烯烃烷基化反应中表现出良好的催化活性和长寿命,并且对2-LAB和3-LAB具有高选择性,有可能替代HF成为一种环境友好的新型催化剂。     

Direct synthesis of dimethyl ether from carbon-monoxide-rich synthesis gas: Influence of dehydration catalysts and operating conditions

Various dehydration catalysts were studied in the synthesis of dimethyl ether (DME) directly from carbon-monoxide-rich synthesis gas under a series of different reaction conditions. The investigated catalyst systems consisted of combinations of a methanol catalyst (CuO/ZnO system) with catalysts for methanol dehydration based on γ-Al₂O₃ or zeolites and γ-Al₂O₃ was identified as the most favorable dehydration catalyst. Various reaction parameters such as temperature, H₂/CO ratio and space velocity were studied. The impact of water on Cu/ZnO/Al₂O₃-γ-Al₂O₃ catalysts was investigated and no deactivation could be observed at water contents below 10% during running times of several hours. A running time of several days and a water content of 10% led to a significant increase of CO conversion but the water gas shift reaction became dominating and CO₂ was the main product. After termination of water feeding significant deactivation of the catalyst system was observed but the system returned to high DME selectivity. Catalyst stability and the influence of CO₂ in the gas feed were studied in experiments lasting for about three weeks. The presence of 8% of CO₂ caused an approximately 10% lower CO conversion and an about 5% lower DME selectivity compared to the reaction system without CO₂.     

Dimethyl ether: A review of technologies and production challenges

Dimethyl ether (DME) is a well-known propellant and coolant, an alternative clean fuel for diesel engines which simultaneously is capable of achieving high performance and low emission of CO, NO_x and particulates in its combustion. It can be produced from a variety of feed-stocks such as natural gas, coal or biomass; and also can be processed into valuable co-products such as hydrogen as a sustainable future energy. This review, which also can be counted as an extensive, pioneer review paper on this topic, presents recent developments in synthesis methods of dimethyl ether as an alternative energy while focuses on conventional processes and innovative technologies in reactor design and employed catalysts. In this context, synthesis methods are classified according to their use of raw material type as direct and indirect methods as well as other routes, since different methods need their own operating condition. Also, the available data for the selectivity to DME and its yield as a function of H₂/CO and CO₂ content of the feed is discussed.     

Deactivation and regeneration of hybrid catalysts in the single-step synthesis of dimethyl ether from syngas and CO2

Synthesis of dimethyl ether (DME) has been studied in a single reaction step, from H₂ + CO and H₂ + CO₂, in a fixed bed reactor on CuO-ZnO-Al₂O₃/γ-Al₂O₃ and CuO-ZnO-Al₂O₃/NaHZSM-5 hybrid catalysts. It has been proven that water content in the reaction medium (which is higher when CO₂ is fed) contributes to efficiently decreasing deactivation by coke in both catalysts and, consequently, when water is in the feed deactivation is insignificant for 30 h reaction. Nevertheless, water also decreases the activity of γ-Al₂O₃ acid function, due to its high adsorption capacity on the acid sites. Due to its importance in the viability of the industrial process, a study has been carried on the regeneration of both catalysts by coke combustion under controlled conditions (in order to avoid CuO sintering). For this study, the catalysts have been used under severe deactivation conditions. It has been proven that γ-Al₂O₃ does not have a suitable hydrothermal stability and that CuO-ZnO-Al₂O₃/NaHZSM-5 catalyst has an excellent performance and is suitable for using it in uninterrupted reaction–regeneration cycles.     

Study of the interaction between components in hybrid CuZnAl/HZSM-5 catalysts and its impact in the syngas-to-DME reaction

Hybrid CuZnAl(CZA)/HZSM-5 catalysts were prepared by three mixing methods in order to analyze the possible existence of interactions between components and their impact in the STD process, namely: (a) grinding of powders prior to pelletizing (grinding method), (b) slurrying the two solids in water followed by drying and pelletizing (slurry method), and (c) physical mixture of pre-pelletized components. The materials were characterized by ICP-OES, XRD, N₂ physisorption, H₂-TPR, ²⁷Al MAS NMR, FTIR-pyridine, and EPR spectroscopy. Detrimental interactions producing a drastic reduction in the amount of available zeolitic Brønsted acid sites were observed for the hybrids prepared by slurry and grinding methods. Such interactions involved, on one hand, the partial blockage of micropores by CZA particles and, on the other hand, an inter-cationic exchange of Cu²⁺ (and possibly also Zn²⁺) species. The presence of isolated Cu²⁺ cations occupying exchange positions in the HZSM-5 zeolite was unambiguously evidenced by EPR spectroscopy. The decrease in Brønsted acidity significantly reduced the activity of the zeolite for dehydrating methanol leading to a much lower efficiency of the corresponding hybrids for DME synthesis at typical STD conditions as compared to the catalyst obtained by simple mixture of pre-pelletized components.     

Relationship between surface acidity and activity of solid-acid catalysts in vapour phase dehydration of methanol

A series of solid-acid catalysts comprised of γ-alumina and modified γ-alumina with different of silica were prepared by co-precipitation method. The catalysts were characterized using XRD, TGA, NH₃-TPD and BET techniques. Dehydration of methanol to dimethyl ether (DME) on solid-acid catalysts was studied in a fixed-bed reactor at the same operating conditions (T = 300 °C, P = 16 bar, WHSV = 26.07 h^− 1). According to the experimental results, silica-modified catalysts have shown better performance compared to the pure γ-alumina. It was found that surface areas increase with increasing silica loading. The results of NH₃-TPD analysis showed that the surface acidity of aluminosilicate catalysts increases with increase in SiO₂/Al₂O₃ molar ratio. Also, it was found that the catalysts with highest portion of weak and/or moderate acid sites exhibit the best catalytic performance and stability. The sample with 3 wt.% silica loading has exhibited the best activity for methanol conversion.     

Synthesis, characterization and catalytic properties of benzyl sulphonic acid functionalized Zr-TMS catalysts

Mesoporous zirconium hydroxide, Zr-TMS (zirconium hydroxide with mesostructured framework; TMS, transition metal oxide mesoporous molecular sieves) catalyst has been prepared through the sol–gel method and functionalized with benzyl sulphonic acid (BSA) using post-synthesis route without destroying the mesoporous structure. The benzyl group anchored Zr-TMS (B-Zr-TMS/≡Zr–O–CH₂–Φ) was achieved by etherification reaction of Zr-TMS with benzyl alcohol at 80 °C using cyclohexane as solvent. Further, B-Zr-TMS was subjected to sulphonation reaction with chlorosulphonic acid (ClSO₃H) at 70 °C using chloroform as solvent to yield BSA-Zr-TMS (≡Zr–O–CH₂–Φ–SO₃H). Maximum sulphonic acid (–SO₃H) loading was optimized with respect to time of functionalization and concentration of ClSO₃H. Functionalization was carried out by loading the maximum amount of benzyl group over Zr-TMS and varying the concentration of –SO₃H. The synthesized materials have been characterized by powder XRD, FT-IR, elemental analysis, N₂ adsorption–desorption and TPD of ammonia. The catalytic activity of the synthesized catalyst has been performed in liquid phase benzoylation of diphenyl ether to 4-phenoxybenzophenone (4-PBP) using benzoyl chloride as benzoylating agent at 160 °C under atmospheric pressure. The same reaction was carried out by sulphated zirconia (SO₄²⁻/ZrO₂) and found very poor activity.     

Characterization of an HZSM-5/MnAPO-11 composite and its catalytic properties in the synthesis of high-octane hydrocarbons from syngas

Zeolite HZSM-5 is known to be active for the catalytic conversion of methanol into hydrocarbons, but its strong acidity and narrow channels may lead to high selectivity to aromatics, thus decreasing the quality of synthesized gasoline. In this work, an HZSM-5/MnAPO-11 composite was prepared via hydrothermal synthesis, and the catalytic synthesis of high-octane gasoline from syngas was studied in flow-type fixed-bed reactors. The catalysts were characterized employing X-ray diffraction (XRD), N₂ adsorption-desorption, ammonia temperature-programmed desorption (NH₃-TPD), scanning electron microscopy (SEM), energy-diffusive X-ray spectroscopy (EDS), atomic absorption spectrophotometry (AAS), and Fourier transformed infrared spectroscopy (FT-IR). Compared with HZSM-5 and a mechanical mixture of HZSM-5 and MnAPO-11, the HZSM-5/MnAPO-11 composite showed the highest gasoline yield and iso-paraffin selectivity due to the presence of more mesopores and moderate acid sites. The results provide new perspectives on the synthesis and application of composite molecular sieves in the production of gasoline.     

铜基催化剂的制备及其在催化溴甲烷制二甲醚中的应用

采用浸渍法制备了不同载体和金属组分的系列负载型催化剂,考察对溴甲烷制二甲醚的催化活性,并采用XRD、TPR和BET进行表征。结果表明,铜基催化剂催化性能较好,且以Al2O3-1#为载体制备的催化剂催化性能更优。Cu O负载量较低时,具有较高的分散度;高温焙烧使催化剂的活性中心由分散态Cu O转变为晶相Cu O,导致Cu O利用率下降;添加助剂K可显著提高Cu O利用率。优选的催化剂制备条件为:以Al2O3-1#为载体,活性组分Cu O负载质量分数为5%,助剂K负载质量分数为0.8%,焙烧温度为350℃,此条件下制备的催化剂,CH3Br转化率和二甲醚选择性均为100%,催化剂的Cu O利用率达到88.04%;再生10次后,催化剂性能保持稳定,没有明显下降。     

Synthesis and catalytic properties of eggshell cobalt catalysts for the Fischer-Tropsch synthesis

CO diffusional restrictions decrease C₅₊ synthesis rates and selectivity within large (1–3 mm) catalyst pellets often required in Fischer-Tropsch (FT) synthesis reactors. Eggshell catalysts, where Co is located preferentially near outer pellet surfaces, reduce the severity of these transport restrictions and lead to higher synthesis rates and C₅₊ selectivity. Maximum C₅₊ selectivities occur on catalysts with intermediate shell thickness, within which transport restrictions limit the removal of reactive olefins but not the arrival of reactants at catalytic sites. A new synthetic technique leads to sharp distributions of active sites near outer pellet surfaces by controlling the rate of imbibition of cobalt nitrate melts. Also, slow reduction of the impregnated salt leads to moderate Co dispersions (0.05–0.10) even at high local Co loadings present within shell regions.     

Effect of vanadium dispersion and of support properties on the catalytic activity of V-containing silicas

Two V-SBA-15 and V-MCF materials (containing about 2.5 wt.% vanadium) were prepared by direct synthesis and tested as catalysts in the decomposition of the most stable chlorinated-alkane, dichloromethane (a total oxidation reaction) and in the oxidative dehydrogenation (ODH) of propane (a partial oxidation reaction). Comparison was made with: (i) two V-SBA-15 and V-MCF materials prepared by “traditional” impregnation method and (ii) a non-porous V-SiO₂ catalyst prepared by flame pyrolysis. All catalysts tested had a vanadium content of about 2.5 wt.%. Samples properties were investigated by means of complementary techniques (TEM, IR and DR UV-vis spectroscopies, N₂ sorption at −196 °C) in order to find possible correlations between catalytic properties of the studied materials and their different physico-chemical features. It is shown that direct synthesis allows a better vanadium dispersion to be achieved, a feature that positively affects catalytic performances in both total and partial oxidations. The different porous networks of the SBA-15 and MCF supports also play an important role on catalytic activity: both V-SBA-15 samples gave better results in dichloromethane decomposition, whereas both V-MCF samples were more selective in propane ODH. The latter findings are ascribed to different molecules diffusion and residence time inside the channels of either SBA-15 or MCF networks.     

The influence of a new fabrication procedure on the catalytic activity of ruthenium-selenium catalysts

A new procedure has been introduced to enhance catalytic activity of ruthenium-selenium electro-catalysts for oxygen reduction, in which materials are treated under hydrogen atmosphere at elevated temperatures. The characterisation using scanning electron microscopy, energy dispersive spectroscopy or energy dispersive X-ray spectroscopy exhibited that the treatment at 400 °C made catalysts denser while their porous nature remained, led to a good degree of crystallinity and an optimum Se:Ru ratio. The half cell test confirms feasibility of the new procedure; the catalyst treated at 400 °C gave the highest reduction current (55.9 mA cm⁻² at −0.4 V) and a low methanol oxidation effect coefficient (3.8%). The direct methanol fuel cell with the RuSe 400 °C cathode catalyst (2 mg RuSe cm⁻²) generated a power density of 33.8 mW cm⁻² using 2 M methanol and 2 bar oxygen at 90 °C. The new procedure produced the catalysts with low decay rates. The best sample was compared to the Pt and to the reported ruthenium-selenium catalyst. Possible reasons for the observations are discussed.     

不同制备方法对AlOOH结构和甲醇脱水性能的影响

采用完全液相、水热和沉淀法制备了4种不同的AlOOH催化剂, 选用工业AlOOH(SB粉)作为对照催化剂, 分别用XRD、NH₃-TPD-MS、FT-IR、氮物理吸附和热重分析等方法对催化剂进行了表征并将其用于固定床甲醇脱水制二甲醚反应。活性评价结果显示, 不同方法制备的AlOOH催化剂的甲醇脱水能力存在明显差异, 完全液相法制备的催化剂经焙烧除炭后活性高, 稳定性好, 特别是有良好的低温催化活性, 而沉淀法制备的催化剂活性低且稳定性差。表征结果发现, 与γ-Al₂O₃不同, AlOOH表面拥有强、弱两种酸性中心, 弱酸量相对较高的AlOOH催化剂具有强的甲醇脱水能力, 较多晶格缺陷和晶粒度适中的催化剂有利于甲醇脱水能力的提高。     

Fischer-Tropsch synthesis: characterization and catalytic properties of rhenium promoted cobalt alumina catalysts☆

The unpromoted and promoted Fischer-Tropsch synthesis (FTS) catalysts were characterized using techniques such as X-ray diffraction (XRD), temperature programmed reduction (TPR), X-ray absorption spectroscopy (XAS), Brunauer-Emmett-Teller surface area (BET SA), hydrogen chemisorption and catalytic activity using a continuously stirred tank reactor (CSTR). The addition of small amounts of rhenium to a 15% Co/Al₂O₃ catalyst decreased the reduction temperature of cobalt oxide but the percent dispersion and cluster size, based on the amount of reduced cobalt, did not change significantly. Samples of the catalyst were withdrawn at increasing time-on-stream from the reactor along with the wax and cooled to become embedded in the solid wax for XAS investigation. Extended X-ray absorption fine structure (EXAFS) data indicate significant cluster growth with time-on-stream suggesting a sintering process as a major source of the deactivation. Addition of rhenium increased the synthesis gas conversion, based on catalyst weight, but turnover frequencies calculated using sites from hydrogen adsorption and initial activity were similar. A wide range of synthesis gas conversion has been obtained by varying the space velocities over the catalysts.     

Influence of charge density on rheological properties and dehydration dynamics of weakly charged poly(N-isopropylacrylamide) during phase transition

It is well-known that introduction of charged groups to poly(N-isopropylacrylamide) (PNIPAM) raises its phase transition temperature. However, the influence of charged groups on structural evolution and dehydration dynamics of weakly charged PNIPAM during phase transition still lacks systematic investigation. In the current study, armed with rheometer and two-dimensional Fourier transform infrared spectrometer (2D-FTIR), we investigated on mesoscopic and microscopic scales the phase transition of sodium poly(N-isopropylacrylamide-co-2-acrylamido-2-methylpropanesulfonate), abbreviated as poly(NIPAM-co-NaAMPS), with charge density of 1–10%. At ambient temperature, scaling exponent of poly(NIPAM-co-NaAMPS) varies from that of neutral polymer to polyelectrolytes as charge density increases. Above phase transition temperature, mesoscopic structure of poly(NIPAM-co-NaAMPS) varies from network of physical gel to viscoelastic liquid containing branched aggregates with increase of charge density, indicating increasing hindrance to intra/inter-chain association due to electrostatic repulsion. On a molecular level, poly(NIPAM-co-NaAMPS) exhibits distinctive microdynamic sequence of dehydration during phase transition, in contrast to neutral PNIPAM. In particular, sulfonate groups decouple the cooperative dehydration of alkyl and carbonyl groups, resulting in their distinctive phase transition temperature as well as temperature range. In analogy to hydration of proteins, it is proposed that the microdynamic sequence, implying the hydration stability of each group, is closely related to the density of hydration layer as well as influence of electrostatic field generated by charged groups. For poly(NIPAM-co-AMPS) with charge density of 3%, there still remains 72.3% of hydrogen bonds between carbonyl group and water at 60 °C, meanwhile a highly hydrated network forms with network strands 1–2 times as long as the copolymer chain length.     

三种典型煤化工工艺比较和能量转化率分析

对煤制合成天然气、一步法合成二甲醚、两步法合成二甲醚3种典型煤化工工艺的进行了比较和能量转化率分析,提出一步法合成二甲醚无论在工艺、投资还是在能量转化率上都较其他2种工艺具有优势。     

The comparative kinetic study of non-isothermal and isothermal dehydration of swollen poly(acrylic acid) hydrogel using the Weibull probability function

The isothermal and non-isothermal dehydration of swollen poly(acrylic acid) hydrogel has been investigated. The possibility of calculating the isothermal conversion curves from the non-isothermal conversion curves was examined. It was found that the experimental conversion curves under non-isothermal and isothermal conditions can be described by the Weibull probability distribution function. Changes in the values of distibution parameters with heating rate (non-isothermal dehydration) and operating temperature (isothermal dehydration) were identified. The mathematical procudere for determining the non-isothermal kinetic conversion curves, based on the functional relationship between distribution parameters and the heating rate was developed. In addition, the new mathematical procedure for calculating the isothermal kinetic curves from the non-isothermal ones at a certain temperature was established. It was found that the large deviation exists between the experimental and calculated isothermal conversion curves. In order to explain the difference in the mechanism of hydrogel dehydration under non-isothermal and isothermal conditions, the dependence of the apparent activation energy on the degree of conversion (E_a = E_a(α)) was performed using the isoconversional (model-free) methods. It was found that in the case of non-isothermal dehydration, the apparent activation energy exhibits a concave decrease with the degree of conversion (α), while for the isothermal dehydration, the apparent activation energy exhibits a convex increase with α. Certain changes in a variety of E_a with α are explained on the basis of different relaxation processes, that take place during non-isothermal and isothermal hydrogel dehydration. The characteristic features of density distribution functions of the apparent activation energies are derived in the case of non-isothermal and isothermal dehydration. It was found that the differences in characteristic properties of derived distribution functions are a consequence of different relaxation processes, which taking place under various experimental conditions.     

微波法制备HMS包络合铁配合物催化剂催化苯酚羟基化反应

以表面含氨基的HMS为载体,利用微波法在载体孔内合成8-羟基喹啉铁（FeⅡ-Oxine）、邻苯二酚铁（FeⅡ-Cat）和邻菲洛啉铁（FeⅡ-Phen）负载型催化剂（FeⅡ-L/HMS,L=Oxine,Cat或Phen）,再在载体孔口接枝聚酰胺-酯型二代树状分子(A),制得FeⅡ-L/HMS-A包络合型催化剂。XRD、FT-IR、Fe含量分析和苯酚羟基化反应结果表明,铁配合物能够被有效包络合到HMS孔内,显示出较好的苯酚羟基化活性,具有良好的重复使用性能,是一种介孔分子筛固载金属配合物简便、有效和环境友好的方法。