Ga改性Ga/ZSM-5分子筛的结构、 性质及其催化丙烷芳构化的固体核磁共振波谱研究

采用不同的方法制备了一系列Ga改性的ZSM-5分子筛催化剂, 通过固体核磁共振波谱(ssNMR)技术对催化剂的结构和酸性进行了表征, 并考察了其催化丙烷芳构化的性能. 结合⁷¹Ga NMR、 吸附三甲基磷(TMP)探针分子的一维³¹P 和二维(2D)¹H-³¹P HETCOR NMR实验, 发现物理混合法制备的Ga₂O₃/ZSM-5样品只含有具有较弱Lewis酸性的Ga₂O₃物种, 浸渍法制备的Ga/ZSM-5-ox和浸渍后还原再氧化法制备的Ga/ZSM-5-redo 2种样品上主要以高分散的氧化镓和阳离子Ga物种为主, 而离子交换法制备的Ga/ZSM-5-IE样品上Ga主要以阳离子Ga物种的形式存在, 高分散的氧化镓和阳离子Ga物种都具有更强的Lewis酸性. Ga/ZSM-5分子筛催化丙烷芳构化反应的结果表明, Ga物种与酸性明显影响催化剂的芳构化性能, 丙烷的转化率和芳烃的选择性顺序为 Ga/ZSM-5-IE>Ga/ZSM-5-redox>Ga/ZSM-5-ox>Ga₂O₃/ZSM-5>H-ZSM-5. 2D ¹H-³¹P HETCOR NMR实验结果表明, Ga/ZSM-5催化剂上产生的Br?nsted酸和Lewis酸(阳离子以及高分散Ga物种)的协同作用提高了分子筛的催化反应活性.     

Ethylene Dehydroaromatization over Ga-ZSM-5 Catalysts: Nature and Role of Gallium Speciation

Bifunctional catalysis in zeolites possessing both Brønsted and Lewis acid sites offers unique opportunities to tailor shape selectivity and enhance catalyst performance. Here, we examine the impact of framework and extra-framework gallium species on enriched aromatics production in zeolite ZSM-5. We compare three distinct methods of preparing Ga-ZSM-5 and reveal direct (single step) synthesis leads to optimal catalysts compared to post-synthesis methods. Using a combination of state-of-the-art characterization, catalyst testing, and density functional theory calculations, we show that Ga Lewis acid sites strongly favor aromatization. Our findings also suggest Ga(framework)–Ga(extra-framework) pairings, which can only be achieved in materials prepared by direct synthesis, are the most energetically favorable sites for reaction pathways leading to aromatics. Calculated acid site exchange energies between extra-framework Ga at framework sites comprised of either Al or Ga reveal a site-specific preference for stabilizing Lewis acids, which is qualitatively consistent with experimental measurements. These findings indicate the possibility of tailoring Lewis acid siting by the placement of Ga heteroatoms at distinct tetrahedral sites in the zeolite framework, which can have a marked impact on catalyst performance relative to conventional H-ZSM-5.     

H(M)ZSM-5杂原子分子筛的表面酸性和裂解催化行为

用IR和ITPD技术研究了H(Al)ZSM-5、H(Ga)ZSM-5和H(Fe)ZSM-5杂原子分子筛的表面酸性。结果表明,这些分子筛表面既存在B酸中心,也存在L酸中心,但是就两种酸的表面浓度比值C_L/C_B而言,H(Ga)ZSM-5和H(Fe)ZSM-5要比H(Al)ZSM-5高得多。在十五烷裂解时,L酸中心起的作用可能更大。     

低硅铝比ZSM-5分子筛的合成及其正庚烷裂化反应性能

以氨水为矿化剂,通过添加NH₄⁺离子水热合成了具有较低骨架硅铝比的ZSM-5分子筛。通过X射线衍射(XRD)、扫描电镜(SEM)、固体核磁共振(MAS-NMR)等表征手段,研究了硅源、铝源、矿化剂、阳离子等对ZSM-5分子筛的结晶度、形貌尺寸和骨架硅铝比等的影响,研究了ZSM-5分子筛的骨架硅铝比对正庚烷催化裂化反应的影响。研究表明,投料硅铝比越低,铝原子越难进入到分子筛骨架中;当氨水为矿化剂、正硅酸四乙酯为硅源时可以合成骨架硅铝比较低的氢型ZSM-5分子筛,添加NH₄⁺离子可以增强骨架铝的嵌入,进一步降低分子筛的骨架硅铝比(24.2)。正庚烷裂化反应结果表明,降低分子筛的骨架硅铝比可以提高正庚烷裂化反应的活性,但会降低低碳烯烃的选择性。     

Framework Effects on Activation and Functionalisation of Methane in Zinc-Exchanged Zeolites

The first selective oxidation of methane to methanol is reported herein for zinc-exchanged MOR (Zn/MOR). Under identical conditions, Zn/FER and Zn/ZSM-5 both form zinc formate and methanol. Selective methane activation to form [Zn-CH₃]⁺ species was confirmed by ¹³C MAS NMR spectroscopy for all three frameworks. The percentage of active zinc sites, measured through quantitative NMR spectroscopy studies, varied with the zeolite framework and was found to be ZSM-5 (5.7 %), MOR (1.2 %) and FER (0.5 %). For Zn/MOR, two signals were observed in the ¹³C MAS NMR spectrum, resulting from two distinct [Zn-CH₃]⁺ species present in the 12 MR and 8 MR side pockets, as supported by additional NMR experiments. The observed products of oxidation of the [Zn-CH₃]⁺ species are shown to depend on the zeolite framework type and the oxidative conditions used. These results lay the foundation for developing structure–function correlations for methane conversion over zinc-exchanged zeolites.     

Physicochemical and catalytic properties of Ga and In pentasils in the reaction of propane aromatization

Ga and In ZSM-5 zeolites are obtained via hydrothermal crystallization from alkali aluminosilicate gels. Their physicochemical and catalytic properties during conversion of propane into aromatic hydrocarbons are studied. These catalysts exhibit different activity and selectivity in propane aromatization process due to their specific physicochemical properties and the localization of promoter atoms in different sites of the zeolite structure. A zeolite containing 1.85 wt % of gallium oxide is the most effective catalyst for propane aromatization.     

金属氧化物和硅沉积改性 ZnO/SiO2/ZSM-5的甲醇择形芳构化反应催化研究

采用真空浸渍法对ZSM-5分子筛进行硅沉积和负载金属修饰,制备了复合改性的ZnO/SiO₂/ZSM-5催化剂,并将它应用于甲醇择形芳构化反应中。采用XRD、BET、NH₃-TPD、SEM、TEM等方法对制备的催化剂进行了物化表征。考察硅沉积次数和ZnO负载量对甲醇芳构化反应(MTA)中对二甲苯(PX)的选择性和收率的影响。结果表明,两次硅沉积改性并负载2.0%金属ZnO的ZSM-5分子筛具有较高的PX收率,在优化的反应温度420 ℃、反应压力0.2 MPa、甲醇空速WHSV为1.25 h^-1下,PX收率在28%以上。     

Catalytic properties,surface composition,and acidity of Ga-ZSM-5 catalysts for aromatization of lower paraffins

A study has been made of the catalytic and acidic properties together with the surface composition of the zeolite HZSM-5 modified by gallium by hydrothermal treatment with a sodium gallate solution and also by impregnation with gallium nitrate solution. It has been established that the first method of introducing gallium produces a more selective catalyst for aromatization of n-butane. It has been shown that under the conditions of the catalytic reaction several types of active centers incorporating Ga are formed: Ga₂O₃ on the zeolite surface, gallium ions inside the zeolite canals, and gallium atoms at the surface of and in the lattice that are bonded to OH groups.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2726–2732, December, 1991.     

Effect of zeolite structure on the selective catalytic reduction of NO with ammonia over Mn-Fe supported on ZSM-5, BEA,MOR and FER

A series of catalysts based on Mn-Fe loaded zeolites was prepared by impregnation and their activity in the selective catalytic reduction of NO with ammonia (NH₃-SCR) was investigated. The highest catalytic conversion was recorded for MnFe-ZSM-5 (MnFe-Z), followed by MnFe-BEA (MnFe-B) and MnFe-MOR (MnFe-M), while MnFe-FER (MnFe-F) showed a very poor activity over the entire temperature range. In order to evidence a correlation between the structure and acidity of the zeolites and NO conversion, the prepared samples were characterized by various techniques (ICP-AES, N₂ physisorption at 77 K, XRD, ²⁷NMR, Raman, FTIR spectroscopy of adsorbed ammonia, H₂-TPR, DRS UV–Vis, EPR and XPS). The superior catalytic activity of MnFe-Z at low temperature is attributed to the abundance of Mn⁴⁺ concentration as revealed by XPS, the highest NH₃-L/NH₄⁺ ratio indicative of the contribution of metals in generating Lewis acidic centers as evidenced by IR-NH3, and the better reducibility of manganese and iron on ZSM-5 which increases the kinetics for red-ox cycles as confirmed in TPR analysis. Fe₃Mn₃O₈ mixed oxide phase is also detected by XRD and XPS and can be associated with the high reducibility of MnFe-Z which generates a high oxidation ability favoring NO to NO₂ oxidation. Raman spectroscopy was also used to confirm the existence of a strong synergy between metals and ZSM-5 support revealed by the shift in the signal position and the decrease in band intensities. The results showed that the zeolite framework and acidity generate catalysts with different textural and structural properties which influence the metal dispersion and speciation and hence influence the catalytic performances.

     

Mechanism of Butane Transformation

Catalytic activity and aromatic selectivity of n‐butane transformation were studied over various MFI type zeolites. From the data obtained, a reaction mechanism is suggested for different catalyst systems. It is visualized that in gallium doped catalysts, Ga³⁺ directly takes part both in cracking and dehydrogenation. The [Ga CH₃]²⁺ and [GaH]²⁺ species formed during cracking and dehydrogenation require protonic sites for regeneration of Ga³⁺ species. An alternative mechanism was suggested for dehydrogenation and cracking by Ga³⁺ without the involvement of protonic sites. However a protonic site would be required for aromatization. In case of gallosilicates a one step mechanism is suggested for cracking and dehydrogenation reaction which does not require the presence of protonic sites in the catalyst system.     

Ga、Zn改性方法对HZSM-5催化剂丙烯芳构化性能的影响

以浸渍法和水热合成法对ZSM-5分子筛进行Ga、Zn改性,制得不同酸性的分子筛催化剂。采用XRD、SEM、NH3-TPD和XPS等表征手段,研究考察了Ga、Zn和不同引入方法对催化剂的孔结构、骨架结构特性和表面酸性的影响,并以丙烯芳构化为模型反应,考察了Ga、Zn改性对ZSM-5催化剂烯烃芳构化催化性能的影响。研究结果表明,Ga、Zn改性对催化剂形貌影响较小,但能明显改变催化剂的表面酸性和烯烃芳构化性能。Zn改性能降低催化剂的酸性,而Ga改性与其引入的方式有关,浸渍法引入催化剂的中强酸位略有下降,而水热合成法引入则显著增加了催化剂的总酸量。Ga、Zn改性均提高了芳构化反应的活性和芳烃选择性,并抑制催化剂表面积炭。     

ZSM-5分子筛酸性质对甲醛合成三聚甲醛催化性能的影响

ZSM-5分子筛是合成三聚甲醛的有效催化剂。本工作通过XRF、XRD、SEM、NH₃-TPD、Py-FTIR和²⁷Al MAS NMR等手段对一系列不同SiO₂/Al₂O₃物质的量比的ZSM-5分子筛催化剂进行了表征,研究了ZSM-5分子筛中Brønsted酸中心和Lewis酸中心对其甲醛合成三聚甲醛催化性能的影响。结果表明,SiO₂/Al₂O₃物质的量比为250的ZSM-5分子筛具有合适的Brønsted酸中心用于催化甲醛缩聚为三聚甲醛的反应,同时其Lewis酸中心量极少,可有效抑制Cannizzaro或Tishchenko等副反应,提高三聚甲醛的选择性,因而具有最佳的合成三聚甲醛催化性能。寿命实验评价结果显示,SiO₂/Al₂O₃物质的量比为250的ZSM-5分子筛具有良好的催化稳定性,单程寿命长达114 h,并且可通过550℃焙烧再生恢复其催化活性。     

不同类型含铁分子筛上N2O催化分解反应

采用固态离子交换法制备了一系列Fe/Al摩尔比为0.33的Fe/分子筛(ZSM-35,ZSM-5,beta和mordenite (丝光沸石))样品,并利用紫外-可见漫反射光谱、原位红外光谱和可见拉曼光谱以及瞬态应答实验考察了分子筛种类对N₂O分解反应性能的影响.研究表明,经高温处理(HT)后分子筛的催化活性顺序为 Fe/ZSM-35 > Fe/beta > Fe/ZSM-5 > Fe/mordenite,与骨架外铝稳定的双核铁物种含量一致.这说明双核铁物种是高温处理后Fe/分子筛样品中N₂O分解的活性位,而它的形成与分子筛种类密切相关.Fe/ZSM-35分子筛十元环孔道中相邻两个α位和八元环孔道中相邻两个β位都能稳定两个Fe(II)离子而形成能高效分解N₂O的双核铁活性中心.Fe/beta和Fe/ZSM-5分子筛中只有落在相邻两个β位上的两个Fe(II)离子才能形成参与N₂O分解的高活性的双铁活性中心.Fe/mordenite-HT分子筛上的铁物种绝大部分以孤立Fe离子的形式存在,因此其催化N₂O分解反应活性很低.     

Study on antimony oxide self-assembled inside HZSM-5

Sb/ZSM-5 was obtained by solid-state reaction with the mixture of Sb₂O₃ and zeolite HZSM-5 under a dry nitrogen flow at 773 K. Characterization of the treated zeolite was undertaken with XRD, ²⁷Al MAS NMR, BET, TGA and FT-IR. The results revealed that part of the antimony oxides migrated into the channels of zeolite, and decreased the Brönsted acid sites in Sb/ZSM-5 remarkably. The other part of antimony oxides together with the amorphous alumino-silicate in the products distributed on the external surface of zeolite ZSM-5 and modified it, while the framework of ZSM-5 in crystal phase was retained. The structure of occluded antimony oxide inside the channels of ZSM-5 was studied by XRD Rietveld method. The result showed that their structure can be described as a chain of non-perfect [Sb₅O₅(H₂O)₂]_n⁵ⁿ⁺, which is parallel to the straight channel of ZSM-5. There is about 0.6 [Sb₅O₅(H₂O)₂]⁵⁺ unit in every cell of the ZSM-5 on an average.     

Study of the role of Zn in aromatization of light alkanes with probe molecules

The role of Zn²⁺ in ZnZSM-5 catalyst in the process of aromatization of alkanes has been studied with the probe molecules - n-hexane, cyclohexane and 1-hexene. IR measurements showed that the parent HZSM-5 zeolite contains predominantly Br?nsted acid sites, while Zn cations in ZnZSM-5 catalyst represent Lewis sites. From the results it follows, that Zn cations in ZnZSM-5 catalyst as Lewis acid sites have strong hydro-dehydrogenation activity and they catalyze the dehydrogenation of n-hexane as well as of cyclic intermediates into the corresponding aromatics. From the results of catalytic tests we conclude that the hydro-dehydrogenation activity of the catalyst plays an essential role in aromatization of light alkanes. The results indicate that for improving the yields of aromatics in aromatization of light alkanes on bifunctional catalysts it is inevitable to increase the participation of the dehydrogenation reactions in the activation of alkane as well as in the formation of the corresponding aromatics from cyclic intermediates. This revised version was published online in August 2006 with corrections to the Cover Date.     

含镓ZSM-5分子筛的制备及其在甲醇芳构化反应中的催化性能

用三种不同方法(水热合成法、浸渍法、离子交换法)制备了含Ga的ZSM-5分子筛,采用XRD、SEM、FT-IR、NH₃-TPD、ICP、XPS和氮吸附等多种技术对其进行了表征,并考察了硅源、Ga含量和Ga的存在状态等对该分子筛结构、酸性及其在甲醇芳构化反应中催化性能的影响。结果表明,硅源种类(硅溶胶、白炭黑、正硅酸乙酯)显著影响分子筛的形貌、晶粒粒径大小及杂原子Ga的存在状态,进而决定其在甲醇制芳烃反应中的芳构化催化活性和稳定性。分子筛中Ga物种主要有骨架Ga、骨架外表面游离态(与分子筛无相互作用)的Ga₂O₃以及与分子筛有相互作用的非骨架Ga三种不同的存在状态。骨架Ga物种以四配位形式存在,提供酸性位点;非骨架Ga作为芳构化活性中心,主要起脱氢芳构化作用。适量的非骨架Ga中心与酸中心协调匹配能有效促进芳烃产物的生成。     

Quantum chemical calculation of the catalytic reaction of ethane dehydrogenation on gallium oxide-hydroxide binuclear clusters in oxidized GaO/ZSM-5 zeolite

The catalytic activity of oxidized GaO/HZSM-5 in the reaction of alkane dehydrogenation can be due to hydrogenated gallium oxide clusters stabilized in the cationic positions of the zeolite. The binuclear gallium oxide clusters [Ga₂O₂]²⁺ in oxidized gallium-substituted high-silica zeolite HZSM-5, which are isomeric to two gallyl ions [GaO]⁺ stabilized on two spatially separated lattice aluminum ions, were considered using the DFT method within the framework of a cluster approach. It was found that, even in the case of a relatively large distance between these aluminum ions, gallium oxide particles in oxidized GaO/HZSM-5 can occur as charged planar [Ga₂O₂]²⁺ four-membered rings. These cluster particles exhibited a high affinity to hydrogen, and they were readily hydrogenated with the retention of their structural integrity. It was demonstrated that this partially hydrogenated cluster could be responsible for the catalytic process of ethane dehydrogenation. In the first step, ethane dissociatively added to the [Ga₂O₂H₂]²⁺ cluster. Then, the ethylene molecule was eliminated from the resulting intermediate to leave the [Ga₂O₂H₄]²⁺ cluster. The cycle was closed by the elimination of a hydrogen molecule with the formation of the initial structure of [Ga₂O₂H₂]²⁺.     

Influence of metal coating methods on the activity of bimetal-containing zeolite catalysts of Co,Pd-ZSM-5 in carbon monoxide oxidation

It has been established that catalytic activity in the CO oxidation of bimetal-containing zeolite Co,Pd-systems based on ZSM-5 and obtained via ion exchange and impregnation at different orders of the introduction of metal cations is higher than that of monometal-containing systems Co-ZSM-5 and Pd—ZSM-5. Through TPD of NH₃, it was determined that coordination-unsaturated bicationic associates are formed in Co,Pd/ZSM-5 zeolites obtained by ion exchange. It was found that the activity of bimetal-containing systems depends on the relation of the active components.     

十元环分子筛在甲醇芳构化反应中催化性能的研究

合成了ZSM-5、ZSM-22、EU-1、MCM-22和ITQ-13具有十元环孔道结构的5种分子筛,研究了分子筛结构、酸性分布等因素对其在甲醇芳构化反应中催化性能的影响。研究表明,不同结构分子筛的形貌、酸性及孔径均存在较大差异,进而影响了其在甲醇制芳烃反应中的催化活性和稳定性。研究的5种分子筛中,ZSM-5表现出最佳的芳构化活性,芳烃收率达34.8%,MCM-22芳烃收率约为21.9%,而其他3种结构的分子筛催化剂基本未表现出甲醇芳构化活性。通过添加具有芳构化性能的Ga物种对ZSM-5和MCM-22进行改性,可显著提升芳烃收率,Ga/ZSM-5上芳烃收率达到40.8%,Ga/MCM-22上芳烃收率可提高到27.1%。另外,采用TG/DTA、GC等方法研究了失活催化剂的积炭情况,发现分子筛结构对积炭量、积炭组成及积炭分布存在显著影响。     

Effect of Fe-Mo promoters on HZSM-5 zeolite catalyst for 1-hexene aromatization

The promotional effect of Fe-Mo species introduced into HZSM-5 (Zeolyst Int., SiO₂/Al₂O₃ ≈ 30) zeolite catalyst by the wetness impregnation method for the 1-hexene aromatization was investigated. The structure and catalytic performance for the aromatization of 1-hexene over xFeyMo-ZSM-5 catalysts in comparison with unmodified HZSM-5 catalysts were studied. The xFeyMo-ZSM-5 catalysts contain fixed loading (5 wt%) and variable Fe/Mo ratio. The catalysts were characterized by BET, ICP-AES, HRSEM-EDS, HRTEM, XRD, FTIR, H₂-TPR, NH₃-TPD, and pyridine DRIFT spectroscopy. The characterization data confirmed the existence of Fe and Mo species in the zeolite matrix. With Fe and Mo species implementation to HZSM-5 zeolite, the amount of the acid sites decreased, but the selectivities to C₉₊ aromatics increased. The catalyst evaluation was performed at 350 °C for 6 h on-stream at atmospheric pressure using a fixed-bed quartz tube reactor. The selectivity to products of different carbon number was affected by the Fe/Mo ratio within the zeolite. It was found the product distribution of grouped fractions of C₁–C₁₇₊ from the liquid product. The results indicate that the optimum ratio of Fe/Mo is 1–1.5. The highest selectivity for gasoline and distillate ranges was obtained for the 2.5wt%Fe2.5wt%Mo- and 3wt%Fe2wt%Mo-ZSM-5 samples, which was higher than that for parent HZSM-5 catalyst.