Effect of methanol dilution on H-ZSM-5 zeolite deactivation in the process of light olefin formation

Conversion of methanol in the presence of H₂O, O₂, H₂, CO, CO₂ and He was studied at 733 K on H-ZSM-5 zeolite. The effect of diluent on the selectivity to light olefins and resistance to coking of H-ZSM-5 were observed. Among the diluents studied only water exerts a promoting effect on prolonged activity of the catalyst. Some explanations of the deactivation process are given.

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H₂O, O₂, H₂, CO, CO₂ He H-ZSM-5 733K. H-ZSM-5. . .

     

Formaldehyde intermediate participating in the conversion of methanol to aromatics over zinc modified H-ZSM-5

Metal-modified H-ZSM-5 has a high selectivity of aromatics in methanol to aromatics(MTA)reaction,which is often attributed to the metal promoting the aromatization of intermediate olefins.However,the effect of methanol dehydrogenation on aromatics formation over these catalysts is rarely studied.Here,we report that HCHO,which is formed by methanol dehydrogenation over Zn/H-ZSM-5 prepared by Zn impregnation,can participate in the synthesis of aromatics.Methanol conversion can produce more aromatics than olefins(propylene or ethylene)conversion over Zn/H-ZSM-5,indicating the conventional MTA pathway including methanol-to-olefins and olefins-to-aromatics is not complete.Moreover,an MTA mechanism including the conventional pathway and the methanol and HCHO coupling pathway is systematically proposed.     

Conversion of methanol into hydrocarbons over zeolite H-ZSM-5: ethene formation is mechanistically separated from the formation of higher alkenes

The widely debated reaction mechanism for the conversion of methanol to hydrocarbons over acidic zeolite H-ZSM-5 has been investigated using isotopic labeling. The mechanistic findings for H-ZSM-5 are clearly different from those previously described at a detailed level for H-beta and H-SAPO-34 catalysts. On the basis of the current set of data, we can state that, for H-ZSM-5, ethene appears to be formed exclusively from the xylenes and trimethylbenzenes. Moreover, propene and higher alkenes are to a significant extent formed from alkene methylations and interconversions. This implies that ethene formation is mechanistically separated from the formation of higher alkenes, an insight of utmost importance for understanding and possibly controlling the ethene/propene selectivity in methanol-to-alkenes catalysis.     

Production of octyl levulinate biolubricant over modified H-ZSM-5: Optimization by response surface methodology

The present study highlighted the use of modified H-ZSM-5 (Meso-HZ-5) as heterogeneous catalyst for the synthesis of octyl levulinate biolubricant by catalytic esterification of biomass derived renewable levulinic acid (LA) with n-octanol. The process variables such as catalyst loading (X₁), n-octanol to LA molar ratio (X₂) and reaction temperature (X₃) were optimized through response surface methodology (RSM), using Box-Behnken model. Analysis of variance was performed to determine the adequacy and significance of the quadratic model. The yield of octyl levulinate was obtained to be 99% at optimum process parameters. The developed quadratic model was found to be adequate and statistically accurate with correlation value (R²) of 0.9971 to predict the yield of octyl levulinate biolubricant. The study was also extended on the validation of theoretical and experimental data, including catalyst reusability.     

晶粒大小对ZSM-5分子筛甲醇制低碳烯烃催化性能的影响

对三种HZSM-5分子筛进行Ca改性,获得两组酸性接近的催化剂,考察了晶粒大小对甲醇制低碳烯烃(MTO)反应的影响。通过进一步与Na改性的比较,探讨了Ca在催化反应中的作用。采用扫描电镜(SEM)测定晶粒大小及形貌,氨气程序升温脱附(NH3-TPD)及吡啶红外吸附(Py-IR)表征催化剂的酸性。MTO催化性能测定结果表明,HZSM-5的低碳烯烃选择性较低且下降较快,催化活性降低也快;Ca改性降低酸性,提高了低碳烯烃选择性和催化稳定性;晶粒大小主要影响催化稳定性,小晶粒分子筛催化剂稳定性更好。高Ca含量改性效果更好;钠改性也提高了低碳烯烃选择性,但其稳定性较差。对于HZSM-5和Ca/HZSM-5,小晶粒的催化剂具有较好的催化稳定性。提出Ca参与了催化反应,MTO是一个酸碱协同作用的催化过程。     

Effect of Fe-loading and reaction temperature on the production of olefins from ethanol by Fe/H-ZSM-5 zeolite catalysts

Lower loading of Fe (1 wt%) and higher reaction temperature (450 °C) led to higher and stable selectivity of C₃₊ olefins by ethanol conversion using Fe/H-ZSM-5 catalysts. Carbon deposition and framework collapse of zeolite support, which may be the cause of change in selectivity of products, was suppressed in some degree.     

Effect of external surface of HZSM-5 zeolite on product distribution in the conversion of methanol to hydrocarbons

The external surface of HZSM-5 zeolite was passivated by liquid siliceous deposition and by acidic sites poisoning with lepidine, respectively. Then methanol-to-hydrocarbons （MTH） reaction was investigated over the above as-prepared catalysts and the dissoluble coke on these used catalysts was analyzed by GC-MS, to study the role of the external surface of HZSM-5 in the catalytic reaction. Comparison with the experi- mental results based on parent ZSM-5 showed that the product distribution of MTH reaction was obviously influenced by the external surface. Evidences were listed as follows： （1） the final product on parent HZSM-5 showed higher aromatic selectivity, lower olefin selectivity, lower ra- tio of C2/C3＋ aliphatics and higher ratio of C3/C4＋ aliphatics than the reaction mixture produced by the sole catalysis of acidic sites in HZSM-5 channel; （2） a little of pentamethylbenzene and hexamethylbenzene in the product on parent HZSM-5, was produced via multi-methylation of methylbenzene on the external surface. The above conclusion may also be suitable for MTH reaction over other zeolites with 10-ring channel.     

Influence of hydrogen on the aromatization of ethane over Pt/H-ZSM-5 zeolite

Hydrogen influences the activity and selectivity of a Pt/H-ZSM-5 zeolite in ethane aromatization by favoring hydrogenolysis and suppressing dehydrogenation and coking. H₂ formed during the reaction inhibits the aromatization activity.

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Pt/H-ZSM-5 , . H₂, , .

     

Formation of carboxylic acids from small alkanes in zeolite H-ZSM-5

The activation of propane and isobutane in acidic zeolite H-ZSM-5 in the presence of both CO and H2O has been studied by in situ solid-state NMR and GC analysis. Evidence was provided for the conversion of propane to isobutyric acid at 373-473 K by cleavage of the C-C bond; methane and ethane are also produced. Isobutane is transformed into pivalic acid with simultaneous production of hydrogen. The low conversion (1-2%) at this temperature was rationalized by the existence of a small number of sites that are capable of generating carbenium ions which are trapped by CO at this temperature. A formate species was observed when CO and H2O were present on H-ZSM-5. This species disappeared in the presence of the alkane. At 573 K, the generation of large amounts of CO2 indicates a much higher conversion of the alkanes into carboxylic acids which, however, decompose under the reaction conditions.     

HZSM-5分子筛上甲醇制烯烃典型产物的传质行为研究

分子筛催化甲醇制烯烃反应(MTO)是典型的扩散主导反应过程,运用频率响应技术系统研究了几种典型产物分子(乙烯/乙烷、丙烯/丙烷、苯)在HZSM-5分子筛上的扩散行为。结果表明,频率响应法成功辨析了不同产物分子的传质规律,证实C2和C3烃分子在HZSM-5微孔孔道内具有相近的扩散速率,但由于受晶体表面阻碍效应影响不同,乙烷分子可自由进出HZSM-5分子筛孔道,而丙烷分子则受到较显著的微孔孔道扩散限制。另外,苯分子的扩散速率显著小于C2和C3分子,且苯分子受晶体表面阻抗效应的影响较小。本研究结果可用于解释HZSM-5分子筛在MTO反应中产物选择性的特点及表面结焦原因,进而从传质角度为高活性、选择性以及稳定性的高效甲醇转化制烃催化剂的定向开发提供理论指导。     

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.     

酸碱改性HZSM-5分子筛上甲醇制取均四甲苯的研究

对HZSM-5分子筛进行"酸碱两步改性",制备得到了H(0.2)-Na-ZSM-5、H(0.5)-Na-ZSM-5、H(0.2)-K-ZSM-5和H(0.5)-K-ZSM-5四种催化剂。通过XRD、NH₃-TPD、ICP、FT-IR与Py-IR方法对催化剂进行表征,并在连续流动固定床反应装置上对催化剂进行评价。实验结果表明,H(0.2)-K-ZSM-5催化剂由于其孔径变大、弱的B酸量增多,从而提高了均四甲苯的选择性。     

NaOH改性ZSM-5分子筛在苯、甲醇烷基化反应中的应用

用NaOH溶液对高硅ZSM-5分子筛进行处理,考察其应用于苯-甲醇烷基化反应过程的最佳处理条件,并用XRD、SEM、XRF、NH3-TPD、TPO、BET等手段对试样进行表征。结果表明,NaOH在脱除分子筛硅物种的同时,也会脱出晶粒表面附着物,暴露出更多的酸性位,形成一定量的介孔,进而影响苯-甲醇烷基化性能。当NaOH处理量为2.4 mmol/gcat时,在液体积空速为85 h-1下,烷基化反应中苯的转化率达到38%,相比处理前提高了近16%,且积炭量低,稳定性好。     

H-ZSM-5分子筛硅铝比对其催化的甲醇转化丙烯选择性的影响

本研究合成了一系列硅铝比不同（SiO₂/Al₂O₃=50-4000）,但晶粒粒径相近的ZSM-5分子筛,并考察了硅铝比对甲醇转化反应丙烯选择性的影响。采用XRD、N₂吸附-脱附、NH₃-TPD和Py-FTIR方法对合成的HZSM-5分子筛进行物化性质表征。实验结果表明,随着硅铝比的增大,初始甲醇转化率降低,其中,硅铝比为50-1600的样品可以实现甲醇的完全转化。在甲醇完全转化的条件下,随着硅铝比的增大,丙烯选择性单调增加,从机理角度出发,揭示了甲醇转化制丙烯反应中,甲基化/裂化循环相较于甲基化/脱烷基化循环进行程度更大。此外,本研究提出了在甲醇完全转化条件下,保证最大丙烯选择性所需要的临界酸密度值（[AS]_S）,当甲醇进料量为0.162g/min时,该临界值为0.175μmol/m²。     

C_(2~6)烯烃在H-ZSM-5分子筛周期性模型上吸附的理论研究

采用密度泛函理论中的PBE-D方法研究了C2~6直链烯烃以及丁烯异构体在H-ZSM-5分子筛周期性模型上的吸附行为。结果表明,对于C2~6直链烯烃,随着碳数的增加,烯烃的吸附能以-12 kJ/mol的常数线性增大,体系中色散校正能ED以-13 kJ/mol的常数线性增大,而不考虑范德华力的能量E*随碳数增加变化不大,只在C5和C6时由于空间阻力凸显而减小。ED显著大于E*,说明烯烃在分子筛孔道中吸附时范德华力起主要作用,且碳数增加其影响增大。丁烯异构体吸附能大小顺序为:反式-2-丁烯顺式-2-丁烯正丁烯异丁烯。正丁烯的3种异构体ED相近且都比正丁烯的大,3种异构体的吸附能差异由E*不同引起。差分电荷密度分析表明,烯烃双键与酸性中心之间有电子聚集,且聚集程度与π配位作用相一致,而且酸性中心H原子有质子化趋向,O原子及其周围电子增加。     

Effect of zinc introduction on catalytic performance of ZSM-5 in conversion of methanol to light olefins

The effect of Zn on the catalytic performance of ZSM-5 in the methanol-to-olefin conversion was investigated. The samples were characterised by X-ray diffraction, N₂ adsorption, FTIR, temperature-programmed desorption of ammonia and water, and Py-IR. The experimental results revealed Znmodified ZSM-5 to show a lower selectivity to light olefin at the higher reaction temperature of 520°C but a higher selectivity to light olefin at lower temperatures. As a comparison, the catalytic performance of Ca-modified ZSM-5 for the methanol conversion is also given. From the above results, it is concluded that Zn may play another role in the methanol conversion in addition to tuning the surface acidic property after modification.     

Selective production of propylene from methanol over high-silica mesoporous ZSM-5 zeolites treated with NaOH and NaOH/tetrapropylammonium hydroxide

The influence of alkaline treatments with NaOH and NaOH/TPAOH mixtures on the physicochemical properties and catalytic performance of high-silica ZSM-5 zeolites (Si/Al = 175) during the methanol-to-propylene (MTP) reaction have been investigated. It was found that alkaline treatment in an NaOH/TPAOH solution with TPAOH/(NaOH + TPAOH) = 0.4 ensures the formation of narrow and uniform intracrystalline mesoporosity without severely damaging the crystal structure and the intrinsic acidity of the zeolite, leading to the best catalytic performance, including the highest propylene selectivity (47.2) and P/E ratio (4.97) as well as the longest catalyst lifetime (80 h).     

改性ZSM-5分子筛催化裂解湛江等鞭金藻制取低碳烯烃(英)

研究了湛江等鞭金藻(Isochrysis zhanjiangensis)在改性ZSM-5分子筛上催化裂解制取低碳烯烃的过程.与热裂解过程相比,湛江等鞭金藻催化裂解可以得到更高的低碳烯烃选择性和收率.同时还研究了湛江等鞭金藻中不同油脂和藻渣的催化裂解.结果表明,微藻中的油脂能有效转化为烯烃,其中中性脂的烯烃收率最高,可达36.7%.不同溶剂抽提后得到的藻渣也可转化为低碳烯烃,但收率远低于微藻中的油脂.微藻中的油脂,特别是中性脂,是烯烃的主要贡献者,提高微藻中的中性脂含量能够得到更高的低碳烯烃收率.     

Increasing the yield of aromatic hydrocarbons in aromatization of n-butane over Ga/H-ZSM-5 zeolite using a palladium membrane

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Reactivity of C1 surface species formed in methane activation on Zn-modified H-ZSM-5 zeolite

Solid-state (13)C magic angle spinning (MAS) NMR spectroscopy investigations identified zinc methyl species, formate species, and methoxy species as C(1) surface species formed in methane activation on the zeolite Zn/H-ZSM-5 catalyst at T≤573 K. These C(1) surface species, which are possible intermediates in further transformations of methane, were prepared separately by adsorption of (13)C-enriched methane, carbon monoxide, and methanol onto zinc-containing catalysts, respectively. Successful isolation of each surface species allowed convenient investigations into their chemical nature on the working catalyst by solid-state (13)C MAS NMR spectroscopy. The reactivity of zinc methyl species with diverse probe molecules (i.e., water, methanol, hydrochloride, oxygen, or carbon dioxide) is correlated with that of organozinc compounds in organometallic chemistry. Moreover, surface formate and surface methoxy species possess distinct reactivity towards water, hydrochloride, ammonia, or hydrogen as probe molecules. To explain these and other observations, we propose that the C(1) surface species interconvert on zeolite Zn/H-ZSM-5. As implied by the reactivity information, potential applications of methane co-conversion on zinc-containing zeolites might, therefore, be possible by further transformation of these C(1) surface species with rationally designed co-reactants (i.e., probe molecules) under optimized reaction conditions.