Mechanisms of glycerol dehydration

Dehydration of neutral and protonated glycerol was investigated using quantum mechanical calculations (CBS-QB3). Calculations on neutral glycerol show that there is a high barrier for simple 1,2-dehydration, E(a)=70.9 kcal mol(-1), which is lowered to 65.2 kcal mol(-1) for pericyclic 1,3-dehydration. In contrast, the barriers for dehydration of protonated glycerol are much lower. Dehydration mechanisms involving hydride transfer, pinacol rearrangement, or substitution reactions have barriers between 20 and 25 kcal mol(-1). Loss of water from glycerol via substitution results in either oxirane or oxetane intermediates, which can inter-convert over a low barrier. Subsequent decomposition of these intermediates proceeds via either a second dehydration step or loss of formaldehyde. The computed mechanisms for decomposition of protonated glycerol are supported by the gas-phase fragmentation of protonated glycerol observed using a triple--quadrupole mass spectrometer.     

Deactivation of acid catalysts in vapor-phase dehydration of glycerol into acrolein

An effect of acid properties and structure of catalysts on their deactivation and selectivity of the acrolein formation was studied in dehydration of glycerol. It was found that γ-Al₂O₃ is effective catalyst for vapor-phase dehydration of glycerol into acrolein.     

磷酸锆催化甘油气相脱水制备丙烯醛

以沉淀法、水热合成法和浸渍法制备了磷酸锆催化剂,通过X射线衍射、热重分析、氮气物理吸附、红外光谱和Hammett指示剂法对催化剂进行了表征,并将该催化剂用于甘油气相脱水反应.研究表明,由沉淀法得到的磷酸锆经过400℃焙烧后能达到最佳催化活性,在温和条件下,甘油可完全转化,丙烯醛选择性为81%,反应24h内催化剂失活不明显.不同方法制备的磷酸锆其结构和表面酸性显著不同,催化剂表面酸性对催化剂活性、丙烯醛选择性和催化剂的寿命均有较大影响.     

Kinetics and mechanism of the dehydration of γ-FeOOH

The dehydration of γ-FeOOH to γ-Fe₂O₃ in vacuo has been investigated by thermoanalysis. Results have been checked by electron microscopy and diffraction and by x-ray diffraction. Authors find that formal kinetics are not conclusive. Electron micrographs, however, show directly that random nucleation occurs, producing perfectly oriented but disordered γ-Fe₂O₃ crystals of about 70 Å size. The results can be accomodated with an atomistic model of lattice collapse.     

催化甘油脱水反应的酸活化蒙脱石负载WOx催化剂的研究

甘油是一种可由生物资源生产、可持续的、可降解的平台化学品,是生物柴油、肥皂化工等工业生产过程中的主要副产物.催化甘油脱水反应生产丙烯醛,有望能替代丙烯等石油裂解产物合成丙烯醛的传统工业路线.丙烯醛是一种重要的化工中间体,被用于合成蛋氨酸、丙烯酸、3-甲基吡啶和1,3-丙二醇,并被广泛地应用于农药、医药、高分子材料等领域.随着全球可持续能源发展,生物柴油生产迅速发展,将产生大量的副产物甘油.利用甘油为原料,通过合适的催化剂的催化脱水反应生成丙烯醛,是近十多年来国内外工业催化的研究热点之一.用于催化甘油脱水合成丙烯醛的酸催化剂有杂多酸、金属氧化物、沸石与酸性粘土矿物等.钨磷杂多酸(H3PW12O40)负载的催化剂虽然具有较强的酸性,有利于催化甘油脱水,但容易导致结焦,而且热稳定差,容易失活.钨磷杂多酸负载于SiO2,TiO2,Al2O3,SiO2-Al2O3,K-10蒙脱石上表现出不同的催化活性,表明催化剂和载体的表面酸性和孔结构影响催化性能.近来研究发现,负载于ZrO2,Al2O3的钨氧化物(WOx)催化剂热稳定性好、酸性高,在甘油脱水反应生成丙烯醛中表现出良好的催化性能.但有关钨氧化物(WOx)结构、催化活性受载体组成、酸性影响的本质和规律一直不清楚.本文采用20 wt%的硫酸、盐酸、磷酸和乙酸对蒙脱石进行酸改性,并在磷酸改性的蒙脱石上负载W含量为4–16 wt%的WOx作为催化剂,用于甘油气相脱水反应.X-射线衍射(XRD)、热重-差热法(TG-DTG)、氨程序升温脱附(NH3-TPD)、红外光谱(FT-IR)和紫外漫反射可见光谱(DR UV-vis)等表征,探讨了酸改性和负载WOx的蒙脱石对催化剂催化性能的影响.蒙脱石经过20wt%的硫酸、盐酸、磷酸和乙酸的活化,酸性增加.四种酸改性的蒙脱石对甘油气相脱水反应均有催化活性,这是因为在蒙脱石酸活化过程中,H+经过阳离子交换反应进入蒙脱石层间,同时蒙脱石八面体中的部分Al3+被浸出,使层板上出现不饱和Al3+,为催化剂提供了L酸位,蒙脱石硅氧四面体上的Si?OH以及[AlO4]上吸附的H3O+提供了B酸位.XRD分析表明,负载WOx的蒙脱石表面存在WO2.72,WO2.9和WO3三种不同类型的WOx,当钨负载量从12 wt%增至16 wt%,孤立的单斜晶系WO3晶粒增多.NH3-TPD和DR UV-vis结果表明,WOx负载在蒙脱石表面以[WO5/WO6](B酸位)、[WO4]和单斜晶系WO3相(L酸位)形式存在.蒙脱石上负载WOx能够调节催化剂的酸强度、酸量和酸位.随着钨负载量从4 wt%增至12 wt%,丙烯醛收率从40.9%增加到67.3%;进一步增加钨负载量到16 wt%,丙烯醛收率降为50.7%.结果发现,随着钨负载量的增加,催化活性组分含量增加,[WO5/WO6](B酸位)增加,使催化活性增加;当W负载量达到16 wt%时,WOx分散性降低,且在催化剂表面形成孤立的单斜晶系WO3相(L酸位),不利于提高丙烯醛选择性.当反应温度为320 oC,甘油水溶液浓度为15 wt%时,磷酸活化蒙脱石负载12 wt%W的催化剂上甘油转化率为89.6%,丙烯醛收率达到73.3%.     

Platinum/phosphotungstic acid/(Zr)MCM-41 catalysts in glycerol dehydration

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Effect of structure on the mechanism of dehydration of clay minerals

The kinetics of the isothermal dehydration of two clay minerals, attapulgite and powdered vermiculite were studied both in vacuo and in the presence of constant water vapour pressure. The mechanism of dehydration was found to be interfacial where diffusion plays a dominant role and it is called “activated diffusion”. A comparison is made with some layered structure clay minerals.     

铯钨青铜的合成及其对甘油脱水制丙烯醛的催化性能

以仲钨酸铵、硝酸铯为原料,乙二胺四乙酸(EDTA)为模板剂和还原剂,采用水热法合成了铯钨青铜,考察了EDTA用量、水热前驱液浓度、铯钨比等条件对水热产物组成及形貌的影响,并以所得产物为催化剂在固定床反应器中进行甘油脱水制丙烯醛的反应研究,发现铯含量对催化性能有较大影响,在Cs/W摩尔比为0. 2∶1时催化性能最好,甘油转化率达到81. 3%,丙烯醛的选择性达到92. 9%。     

A study of the preparation conditions of aluminum oxide on its catalytic activity and stability in vapor-phase dehydration of glycerol to acrolein

Effect of the preparation conditions of aluminum oxide on its catalytic activity and stability in the course of vapor-phase dehydration of glycerol to give acrolein was studied. The conditions were determined in which γ-Al₂O₃ is prepared so that the maximum selectivity of acrolein formation (～60%) and glycerol conversion (～80%) is in 10 h. The results of the study suggest that systems based on γ-Al₂O₃ are the most promising catalysts for the process under consideration.     

23Na NMR study of the mechanism for the dehydration of zeolite NaA

Conclusions Water molecules in partially dehydrated zeolite directly interact with Na⁺ cations. The final three or four water molecules per unit cell removed upon the dehydration of zeolite are apparently bound to Na⁺ cations of the eight-membered windows, while the previous 10 molecules are bound to Na⁺ cations of six-membered windows.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1160–1162, May, 1988.     

Further evidence for the substitution mechanism of 2-propanol dehydration on alumina

The mechanism of adsorption of 2-propanol on alumina, as the first step of its dehydration to propene and water, has been studied by determination of active hydrogen concentration on the surface of the catalyst and by oxygen isotope exchange in water at 150°C. The results confirmed the adsorption of the alcohol by substitution of the surface hydroxy groups and with displacement of water which contains the oxygen atom and at least one hydrogen atom from the surface.

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Preparation of alumina with different precipitants for the gas phase dehydration of glycerol and their characterization by thermal analysis

Journal of Thermal Analysis and Calorimetry - Mesoporous aluminas were prepared using different precipitants at different pH of precipitation and evaluated in the gas phase dehydration of glycerol....     

Structures and reaction mechanisms of glycerol dehydration over H-ZSM-5 zeolite: a density functional theory study

The initial stage of glycerol conversion over H-ZSM-5 zeolite has been investigated using density functional theory (DFT) calculations on an embedded cluster model consisting of 128 tetrahedrally coordinated atoms. It is found that glycerol dehydration to acrolein and acetol proceeds favourably via a stepwise mechanism. The formation of an alkoxide species upon the first dehydration requires the highest activation energy (42.5 kcal mol(-1)) and can be considered as the rate determining step of the reaction. The intrinsic activation energies for the first dehydration are virtually the same for both acrolein and acetol formation, respectively, suggesting the competitive removal of the primary and secondary OH groups. A high selectivity to acrolein at moderate temperatures can be attributed to the selective activation of the stronger adsorption mode of glycerol through the secondary OH group and the kinetically favoured subsequent consecutive steps. In addition, the less reactive nature of acrolein relative to acetol precludes it from being converted to other products upon conversion to glycerol. In accordance with typical endothermic reactions, the forward rate constant for glycerol dehydration significantly increases with increasing reaction temperature.     

Unravelling the mechanism of glycerol hydrogenolysis over rhodium catalyst through combined experimental-theoretical investigations

We report herein a detailed and accurate study of the mechanism of rhodium-catalysed conversion of glycerol into 1,2-propanediol and lactic acid. The first step of the reaction is particularly debated, as it can be either dehydration or dehydrogenation. It is expected that these elementary reactions can be influenced by pH variations and by the nature of the gas phase. These parameters were consequently investigated experimentally. On the other hand, there was a lack of knowledge about the behaviour of glycerol at the surface of the metallic catalyst. A theoretical approach on a model Rh(111) surface was thus implemented in the framework of density functional theory (DFT) to describe the above-mentioned elementary reactions and to calculate the corresponding transition states. The combination of experiment and theory shows that the dehydrogenation into glyceraldehyde is the first step for the glycerol transformation on the Rh/C catalyst in basic media under He or H(2) atmosphere.