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1.
 采用HNO3溶液对HZSM-5分子筛进行预处理,并以处理后的分子筛为载体制备了Mo/HZSM-5 催化剂. 结果表明,改性的Mo/HZSM-5催化剂在甲烷无氧脱氢芳构化反应中表现出很好的稳定性,显著地抑制了积炭物种在催化剂表面的形成. SEM,XRD和1H MAS NMR等表征结果表明,酸处理在一定程度上降低了HZSM-5分子筛的结晶度,使部分Al物种脱离骨架结构,迁移到骨架外形成新的表面Al羟基,从而使HZSM-5分子筛上B酸中心的数目明显减少. 未经改性的Mo/HZSM-5催化剂表面,平均每个晶胞中有1.12个B酸位,而改性的Mo/HZSM-5催化剂表面,平均每个晶胞中仅有0.88个B酸位. 这表明过多的酸性位存留在催化剂上会引起积炭的生成,降低催化剂的稳定性.  相似文献   

2.
 在固定床反应器中进行了HZSM-5分子筛催化剂催化苯氧化生成苯酚时的积炭和失活实验,催化剂经高温水蒸气处理,反应条件为: 9.7%N2O-90.3%N2混合气流速115 ml/min,苯流速6 g/h,反应温度400 ℃. 采用TG,吸附吡啶的IR谱,13C NMR和低温N2吸附法对催化剂进行了表征,并用连续流动吸附法测定催化剂上正己烷和环己烷的吸附量. 结果表明,苯氧化生成苯酚的反应过程中,催化剂表面积炭导致分子筛微孔孔口堵塞是造成催化剂失活的主要原因. 与其他有机物在ZSM-5上的积炭相似,B酸中心是积炭的活性中心. 积炭物种主要为带有烷基的芳烃和多环芳烃,同时含有少量带有羟基的多环芳烃,这可能是由在L酸中心上生成的苯酚进一步氧化、脱氢和聚合形成的.  相似文献   

3.
不同方法制备的Mo/HZSM-5催化剂上甲烷的芳构化反应   总被引:8,自引:0,他引:8  
 采用机械混合、机械混合后焙烧和机械混合后微波处理等方法制备Mo/HZSM-5催化剂,并对催化剂上甲烷芳构化反应性能进行了考察.结果表明,与浸渍法相比,用机械混合法、固相反应法和微波法制备的催化剂,在保持甲烷转化率不变的前提下,能明显提高芳烃选择性并减少积炭的生成;不同方法制备的Mo/HZSM-5催化剂上Mo物种的落位不同,机械混合法、固相反应法和微波法使Mo物种较多地落位于分子筛外表面.结合反应结果可以得出,落位于分子筛外表面的Mo物种对甲烷芳构化反应更为有利,而且明显减少积炭的生成.  相似文献   

4.
采用两种不同的脱铝方法对HZSM-5分子筛进行了预处理,并利用MAS NMR和吸附吡啶的FT-IR对分子筛的结构和酸性质进行了表征,考察了分子筛的脱铝程度对Mo基催化剂上甲烷芳构化反应性能的影响.结果表明,HZSM-5分子筛的酸性过强或B酸量不足,均会导致催化剂严重积炭,但积炭成因不同.母体HZSM-5分子筛上的强B酸中心的存在可促使催化剂上反应中间物种深度脱氢,造成催化剂在反应过程中严重积炭.经水热处理的HZSM-5分子筛,骨架铝脱出严重,造成B酸活性中心不足以及部分微孔阻塞,不利于C2中间物种芳构化,导致芳烃选择性显著降低.经高温N2处理的HZSM-5分子筛,骨架铝脱出相对缓和,在消除母体分子筛上强B酸中心的同时,保留了较多的弱B酸中心,既可满足C2中间物种芳构化反应的需要,又可有效抑制催化剂积炭,导致甲烷芳构化反应性能显著改善.  相似文献   

5.
采用连续流动的激光诱导超极化129Xe为探针分子,利用高分辨固体核磁共振技术研究了不同电子结构的金属阳离子(Na+,Ag+Cu2+,Cu+,Cs+)交换的FAU型分子筛与超极化129Xe之间的相互作用.结果表明,超极化129Xe能够很灵敏地检测不同电子结构的金属离子对限阈空间中电场梯度的影响.Na+对超笼内电场梯度的影响很小,NaX与NaY分子筛中Xe的化学位移表现出相似的行为.对于具有nd10电子结构的Ag+或Cu2+经自还原生成的Cu+,其与Xe的5d0轨道形成dπ-dπ键,导致Xe的化学位移明显向商场方向位移.Cuπ由于具有一定的顺磁性,使得129Xe NMR谱展宽,甚至无法观测.Cs+的电子结构与Xe原子相同,它能吸附更多的Xe原子导致Xe的化学化移明显向低场方向偏移.  相似文献   

6.
采用三聚氰胺作为N源,N掺杂改性HZSM-5沸石分子筛后负载Mo活性金属组分,制备了一种用于甲烷无氧芳构化反应(MDA)的催化剂。采用XPS、N_2吸附-脱附、XRD、H2-TPR、TEM和NH3-TPD对催化剂性质和Mo金属组分状态进行了分析表征,并考察了催化剂的甲烷无氧芳构化反应催化性能。结果表明,HZSM-5经过N掺杂改性后,会在分子筛表面生成一层含氮基团,有序调控了分子筛的酸性位点;同时会诱导Mo金属组分在催化剂表面更好的锚定落位。此方法制备的Mo/HZSM-5-CN催化剂能有效提高MDA反应的甲烷转化率和芳烃选择性,减缓了积炭的生成,展现出更优良的催化性能。  相似文献   

7.
ZSM-5分子筛催化剂酸性对积炭的影响   总被引:1,自引:0,他引:1  
研究了几种分子筛在乙基化反应中的积炭过程,并与分子筛的酸性、孔结构等参数关联,阐明了ZSM-5分子筛催化剂的酸中心对积炭起着主要作用。酸强度越高,越容易积炭,酸量越大积炭越多。积炭的形成部位与酸中心在晶内外的分布关系密切。分子筛外表面酸中心比晶内的更易生成积炭,体现了积炭的择形性。  相似文献   

8.
采用水热合成法,在合成过程中通过添加矿化剂、尿素和改变硅源,制备了不同骨架铝落位的ZSM-5分子筛。通过SEM、XRD、BET、XRF、MAS NMR、NH_3-TPD和Py-FTIR等表征手段对分子筛的形貌、织构、骨架铝落位和酸性进行了系统研究,同时考察了不同ZSM-5分子筛催化剂甲醇制芳烃的催化性能。研究结果表明,制备的ZSM-5分子筛均具有结晶度高和形貌均一等特点,但在骨架铝落位和酸性方面存在显著差异。椭球状ZSM-5分子筛的骨架铝主要分布于直通孔道或正弦孔道中,并表现出较多的酸性位。块状分子筛中骨架铝主要落位在孔道交叉处,且具有较低的强酸量。在甲醇制芳烃反应中,骨架铝主要位于直通或正弦孔道并表现出较多酸性位的椭球状ZSM-5分子筛催化剂具有较高的活性稳定性和芳烃选择性。  相似文献   

9.
 在Fe-Zn-Zr/分子筛复合催化剂上考察了不同类型的分子筛对CO2加氢反应性能的影响. 结果表明,不同分子筛对复合催化剂性能的影响不同,Fe-Zn-Zr/HY是合成异构烷烃有效的复合催化剂. 分子筛的酸性及酸强度对复合催化剂性能有较大的影响,中等强度和较高强度的酸性位有利于异构烃的生成.  相似文献   

10.
Fe-ZSM-5 分子筛催化剂上N2O一步氧化苯制苯酚的积炭动力学   总被引:2,自引:0,他引:2  
 采用等温固定床反应器和热重分析相结合的方法研究了 Fe-ZSM-5 分子筛上 N2O 一步氧化苯制苯酚的积炭行为, 考察了反应时间、温度和原料配比对积炭量的影响. 依据实验现象和积炭物组分分布的气相色谱-质谱分析结果, 提出了 Fe-ZSM-5 分子筛上 N2O 一步氧化苯制苯酚的积炭机理. 结果表明, Fe-ZSM-5 分子筛上的积炭物种主要来源于氧化产物苯酚的深度氧化, 属连串失活. 基于所提出的积炭机理和实验结果, 建立了 Fe-ZSM-5 分子筛催化剂的积炭动力学模型, 统计检验和残差分布检验表明, 所建模型与实验结果相容, 可靠性强.  相似文献   

11.
利用129XeNMR技术,催化剂裂化活性及裂化反应性能的评价方法研究了高岭土型FCC催化剂上的稀土离子分布及裂化反应特性,129XeNMR测试结果表明,当催化剂中稀土含量不高时,大部分稀土离子分布在分子筛上,当稀土含量超出其中分子筛的理论交换量时,分子筛的超笼内无多余的稀土离子,多出的稀土一部分在白土基质上,一部分可能分布在分子筛的某些缺陷位上;裂化性能的测试结果显示出,随着稀土含量的增加,催化剂的活性、汽油和焦炭产率增加,汽油的研究法辛烷值降低.  相似文献   

12.
采用固体硅源、铝源和四乙基氢氧化铵(TEAOH)在极浓体系中成功合成了硅铝摩尔比(SiO2/Al2O3)分别为30和150的β沸石。同时用硅铝摩尔比为30的β沸石为原料,在70℃下采用草酸脱铝制备了硅铝摩尔比接近150的样品。直接合成或经脱铝改性的β沸石采用XRD、SEM、吡啶吸附红外光谱和N2吸附对其晶相、酸性、比表面积和孔体积进行了表征。将三种β沸石分别作为烃类裂化催化剂的活性组分添加剂,对其催化剂的性能进行了评价。结果表明,三种β沸石的引入都可以提高裂化催化剂的活性和改善产品分布,低硅β沸石催化剂能改善催化剂的异构化反应性能和提高乙烯、丙烯的选择性,高硅β沸石或脱铝β沸石催化剂有利于裂化汽油中烯烃的芳构化和C4烯烃产率的提高,并可减少结焦。  相似文献   

13.
2,6-二烷基萘的氧化产物2,6-萘二甲酸与乙二醇反应得到的聚2,6-萘二甲酸乙二醇酯(PEN),是一种具有广阔市场前景的新一代聚酯产品,其开发的关键在于降低2,6-二烷基萘的生产成本[1~3]。利用裂解重油的2-甲基萘副产烷基化获得2,6-二烷基萘,将大大降低PEN的生产成本,同时也帮助石化  相似文献   

14.
Hierarchical porous architecture with interconnected trimodal micro-meso-macroporous systems constructed from uniform zeolite Zr-doped silicalite-1 nanocrystals has been prepared. The synthesis has been made by using glycerin as a reaction medium via a quasi-solid-state crystallization of hierarchically meso-macroporous zirconosilicate precursor under the effect of the structure directing agent TPAOH. The presence of glycerin is crucial in the synthesis systems to maintain the porous hierarchy. The pores inter-connectivity, Zr location in the framework, the acidity and the catalytic activity have been studied by laser-hyperpolarized (129)Xe NMR spectroscopy, UV-visible spectroscopy, temperature-programmed desorption of ammonia and the catalytic isopropylbenzene cracking probe reaction, respectively. The products possess well-defined macrochannels interconnected with mesopores located in the macropore walls, which in turn have been constructed from microporous MFI-type zeolite units. (129)Xe NMR study indicated that the hierarchically micro-, meso-, macro-pore systems are homogeneously distributed throughout the final materials and well interconnected, which is important for molecular diffusion. The TPD-NH(3) investigation revealed that the hierarchically micro-meso-macroporous materials constructed from zeolite Zr-Silicalite-1 nanocrystals present strong acidity.  相似文献   

15.
The dependence of the 129Xe NMR chemical shift value of XeF2 on temperature and concentration was determined in a variety of prototypic media: in acidic (anhydrous HF, aHF), nonprotic but polar (dichloromethane), and basic (CD3CN-EtCN, 1:3 v/v) solvents. The 129Xe NMR spectra of a representative series of organoxenon(II) salts [RXe][Y] (R = C6F5, heptafluoro-1,4-cyclohexadien-1-yl (cyclo-1,4-C6F7), pentafluoro-1,4-cyclohexadien-3-on-1-yl (cyclo-3-O-1,4-C6F5), CF2=C(CF3), (CF3)2CFC[triple bond]C, C4H9C[triple bond]C; Y = BF4, AsF6) in aHF showed, compared with XeF2-aHF, a quantitatively less distinct but qualitatively related dependence of delta(129Xe) vs temperature. The dependence of their delta(129Xe) values on concentration in aHF is negligible. An explanation for the different behavior of [RXe][Y] and XeF2 is offered.  相似文献   

16.
Results of the first solid-state 131Xe NMR study of xenon-containing compounds are presented. The two NMR-active isotopes of xenon, 129Xe (I=1/2) and 131Xe (I=3/2), are exploited to characterize the xenon magnetic shielding and quadrupolar interactions for two sodium perxenate salts, Na4XeO6.xH2O (x=0, 2), at an applied magnetic field strength of 11.75 T. Solid-state 129/131Xe NMR line shapes indicate that the local xenon environment in anhydrous Na4XeO6 adopts octahedral symmetry, but upon hydration, the XeO6(4-) anion becomes noticeably distorted from octahedral symmetry. For stationary, anhydrous samples of Na4XeO6, the heteronuclear 129/131Xe-23Na dipolar interaction is the principal contributor to the breadth of the 129/131Xe NMR lines. For stationary and slow magic-angle-spinning samples of Na4XeO(6).2H2O, the anisotropic xenon shielding interaction dominates the 129Xe NMR line shape, whereas the 131Xe NMR line shape is completely dominated by the nuclear quadrupolar interaction. The xenon shielding tensor is approximately axially symmetric, with a skew of -0.7+/-0.3, an isotropic xenon chemical shift of -725.6+/-1.0 ppm, and a span of 95+/-5 ppm. The 131Xe quadrupolar coupling constant, 10.8+/-0.5 MHz, is large for a nucleus at a site of approximate Oh symmetry, and the quadrupolar asymmetry parameter indicates a lack of axial symmetry. This study demonstrates the extreme sensitivity of the 131Xe nuclear quadrupolar interaction to changes in the local xenon environment.  相似文献   

17.
Zeolite nanoclusters coated onto the mesopore walls of SBA-15   总被引:6,自引:0,他引:6  
Ultrahigh-field 27Al MAS and MQMAS NMR and 129Xe NMR were used to characterize the zeolite-coated SBA-15 materials. This study shows evidence that zeolite nanoclusters are coated on the mesopore surface of SBA-15. These techniques are useful for the detection of zeolite nanoclusters and different aluminum environments in zeolite/aluminosilicate composites, which are difficult or even impossible to detect by conventional techniques.  相似文献   

18.
Localization of PdCl2 clusters supported on multi-wall carbon nanotubes (MWCNT) has been investigated using 129Xe NMR of adsorbed xenon. As-made MWCNTs with channels initially inaccessible for adsorption and ball-milled MWCNTs with the totally accessible internal surface were used as supports. The observed 129Xe NMR spectra were determined by the dynamics of xenon exchange between the aggregate pores and nanotube channels. No considerable changes of the 129Xe NMR spectrum with the concentration of supported PdCl2 were observed for the as-made MWCNT, while an additional resonance appeared for the ball-milled nanotubes. The 129Xe NMR experiments evidenced the supported species to be localized on the internal surface of the ball-milled MWCNT.  相似文献   

19.
The 129Xe NMR line shapes of xenon adsorbed in the nanochannels of the (+/-)-[Co(en)3]Cl3 ionic crystal have been calculated by grand canonical Monte Carlo (GCMC) simulations. The results of our GCMC simulations illustrate their utility in predicting 129Xe NMR chemical shifts in systems containing a transition metal. In particular, the nanochannels of (+/-)-[Co(en)3]Cl3 provide a simple, yet interesting, model system that serves as a building block toward understanding xenon chemical shifts in more complex porous materials containing transition metals. Using only the Xe-C and Xe-H potentials and shielding response functions derived from the Xe@CH4 van der Waals complex to model the interior of the channel, the GCMC simulations correctly predict the 129Xe NMR line shapes observed experimentally (Ueda, T.; Eguchi, T.; Nakamura, N.; Wasylishen, R. E. J. Phys. Chem. B 2003, 107, 180-185). At low xenon loading, the simulated 129Xe NMR line shape is axially symmetric with chemical-shift tensor components delta(parallel) = 379 ppm and delta(perpendicular) = 274 ppm. Although the simulated isotropic chemical shift, delta(iso) = 309 ppm, is overestimated, the anisotropy of the chemical-shift tensor is correctly predicted. The simulations provide an explanation for the observed trend in the 129Xe NMR line shapes as a function of the overhead xenon pressure: delta(perpendicular) increased from 274 to 292 ppm, while delta(parallel) changed by only 3 ppm over the entire xenon loading range. The overestimation of the isotropic chemical shifts is explained based upon the results of quantum mechanical 129Xe shielding calculations of xenon interacting with an isolated (+/-)-[Co(en)3]Cl3 molecule. The xenon chemical shift is shown to be reduced by about 12% going from the Xe@[Co(en)3]Cl3 van der Waals complex to the Xe@C2H6 fragment.  相似文献   

20.
Among rare gases, xenon features an unusually broad nuclear magnetic resonance (NMR) chemical shift range in its compounds and as a non-bonded Xe atom introduced into different environments. In this work we show that (129)Xe NMR chemical shifts in the recently prepared, matrix-isolated xenon compounds appear in new, so far unexplored (129)Xe chemical shift ranges. State-of-the-art theoretical predictions of NMR chemical shifts in compounds of general formula HXeY (Y = H, F, Cl, Br, I, -CN, -NC, -CCH, -CCCCH, -CCCN, -CCXeH, -OXeH, -OH, -SH) as well as in the recently prepared ClXeCN and ClXeNC species are reported. The bonding situation of Xe in the studied compounds is rather different from the previously characterized cases as Xe appears in the electronic state corresponding to a situation with a low formal oxidation state, between I and II in these compounds. Accordingly, the predicted (129)Xe chemical shifts occur in new NMR ranges for this nucleus: ca. 500-1000 ppm (wrt Xe gas) for HXeY species and ca. 1100-1600 ppm for ClXeCN and ClXeNC. These new ranges fall between those corresponding to the weakly-bonded Xe(0) atom in guest-host systems (δ < 300 ppm) and in the hitherto characterized Xe molecules (δ > 2000 ppm). The importance of relativistic effects is discussed. Relativistic effects only slightly modulate the (129)Xe chemical shift that is obtained already at the nonrelativistic CCSD(T) level. In contrast, spin-orbit-induced shielding effects on the (1)H chemical shifts of the H1 atom directly bonded to the Xe center largely overwhelm the nonrelativistic deshielding effects. This leads to an overall negative (1)H chemical shift in the range between -5 and -25 ppm (wrt CH(4)). Thus, the relativistic effects induced by the heavy Xe atom appear considerably more important for the chemical shift of the neighbouring, light hydrogen atom than that of the Xe nucleus itself. The predicted NMR parameters facilitate an unambiguous experimental identification of these novel compounds.  相似文献   

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