钛硅分子筛的改性对环己烷氧化反应的影响

使用不同改性液H_2SO_4-H_2O_2、(NH_4)_2CO_3-H_2O_2、(NH_4)HF_2-H_2O_2和(NH_4)_2CO_3+(NH_4)HF_2-H_2O_2混合溶液对中空钛硅分子筛进行改性。采用XRD、UV-Vis和拉曼光谱进行表征分析,考察改性前后钛硅分子筛在环己烷氧化反应的催化性能。结果表明,改性过程没有破坏钛硅分子筛的MFI拓扑结构,但提高了钛硅分子筛相对结晶度,并脱除了部分锐钛矿相TiO_2;与未改性钛硅分子筛相比,环己醇和环己酮选择性及H_2O_2有效利用率明显提高,以改性液(NH_4)_2CO_3+(NH_4)HF_2-H_2O_2改性钛硅分子筛效果最佳,醇酮选择性提高12.78个百分点,H_2O_2有效利用率提高17.33个百分点;(NH_4)_2CO_3+(NH_4)HF_2-H_2O_2混合溶液改性钛硅分子筛显著降低H_2O_2用量,在己内酰胺生产过程中有很好的应用前景。     

钛丝光沸石(Ti-MOR)的研究进展

综述了钛丝光沸石即MOR拓扑结构钛硅分子筛(Ti-MOR)的制备方法及催化应用的研究进展。合成方法的选择直接影响骨架钛的化学状态和液相氧化催化活性。因Ti-MOR分子筛具备孤立四配位状态的骨架钛物种、12元环大孔结构和廉价的制备成本，Ti-MOR分子筛在催化酮醛类氨肟化反应和大分子芳香烃羟化反应中显示优于传统MFI拓扑结构的钛硅杂原子分子筛(TS-1)的催化活性与工业应用前景。此外，通过调控钛催化活性中心的化学微环境、降低孔道内传质阻力可进一步提升Ti-MOR分子筛在液相催化氧化反应中的活性与稳定性。     

V-ZSM-5上氨选择性催化还原NO降解机理的DFT研究

运用密度泛函理论对NO在V-ZSM-5的周期性晶包模型上的氨选择性催化还原降解路径进行了系统的研究。计算结果表明,NO优先于NH_3吸附于V上,NO的氨选择性催化还原降解反应经过生成不稳定的中间产物NH_2NO,随着中间产物NH_2NO分解成H_2O和N2,NO被分解。不稳定的中间产物NH_2NO在[VO]1+活性位点上也可以分解成H_2O和N_2。     

NO与NH_3在不同构型分子筛催化剂中吸附和扩散的分子模拟

基于巨正则蒙特卡洛和分子动力学,对NH_3-SCR反应体系中吸附质分子(NO与NH_3)在不同拓扑结构沸石分子筛(LTL、FER、LEV、BEA、MOR、FAU、CHA和MFI)上的吸附和扩散特性进行系统研究。结果表明,对于全硅分子筛而言,其分子筛的拓扑结构影响NO与NH_3在分子筛上的吸附,综合吸附量及吸附作用能发现,MFI和LEV分子筛对NO具有较优的吸附特性;MFI和BEA分子筛对NH_3具有较优的吸附特性。研究了Si与Al物质的量比对BEA分子筛吸附性能影响,结果表明,随着Si与Al物质的量比降低,分子筛自由体积逐渐增加,进而有助于分子筛催化剂对NO和NH_3的吸附。采用分子动力学模拟计算NO与NH_3在不同构型全硅分子筛上的扩散系数,发现具有三维直通道且孔径较大的分子筛催化剂有利于NO和NH_3在其孔道内部的扩散,MFI虽然具备三维孔道结构,但由于存在Z型交叉通道,一定程度阻碍了反应物分子的扩散。     

多组分钼铋催化剂上丙烯氨氧化反应的原位漫反射红外光谱研究

在原位条件下利用漫反射红外技术对丙烯选择氨氧化反应进行了研究.结果表明:(1)丙烯氨氧化反应的速率控制步骤是丙烯脱除α-H生成烯丙基中间物种,其吸收峰在1487 cm-1;(2)催化剂表面上氨中间物种存在四种结合方式:(i)氨解离形成NH2中间物种(1553 cm-1);(ii)Bronsted酸性位上形成NH4 中间物种(1659 cm-1和1443 cm-1);(iii)Lewis酸性位上形成配位氨中间物种(1257 cm-1),它是由丙烯醛生成丙烯腈所需的氨物种;(iv)通过估算法初步确认催化剂表面上存在Mo=NH中间物种(1029 cm-1),它是由丙烯直接生成丙烯腈所需的氨物种;(3)丙烯腈生成路线有两条:一是丙烯在钼亚氨物种作用下直接生成丙烯腈,反应机理模型为Grasselli模型;二是丙烯选择氧化生成的中间产物丙烯醛在配位氨物种作用下通过加成反应生成丙烯腈.以上结果不仅完善了丙烯氨氧化反应机理,而且对丙烯醛在反应过程中作用的认识更进了一步.     

联氨制造法的改进

联氨是经由氨与氯氨NH_2Cl反应而制备的。在详细研究NH_3与NH_2Cl的反应机理后,得出以下的改进方法。即是氯氨须在液体氨中,温度保持-78℃时进行。开始时NH_2Cl的浓度不可高;NH_2Cl的浓度高时,可与生成的NH_2·NH_2反应而使其分解为NH_4Cl及N_2。     

含氟TS-1分子筛的制备、表征及催化性能

以NH_4F为氟源、正硅酸四乙酯为硅源、钛酸四丁酯为钛源、四丙基氢氧化铵为模板剂,通过后处理过程成功制备了含氟TS-1(F-TS-1)分子筛,通过XRD、UV-Vis、ICP、SEM、XPS、FTIR等手段对其形貌和结构进行表征,并以正己烯环氧化反应为探针反应考察其催化性能。结果表明,后处理过程可以有效地将氟植入TS-1分子筛骨架,Si/F比为41;F-TS-1分子筛具有典型的MFI拓扑结构,粒径约为300nm,其中的钛均以四配位骨架钛(Ⅳ)形式存在,疏水性显著提升;F-TS-1分子筛的催化性能优于常规TS-1分子筛,正己烯和H_2O_2的转化率分别为28.1%和35.2%。     

氢气选择性催化还原氮氧化物催化剂的研究进展

综述了H_2-SCR的反应机理以及近年来国内外氢气选择性催化还原氮氧化物催化剂的研究进展。重点介绍了Langmuir-Hinshelwood和双功能两种反应机理。机理不同,其中间产物也不相同。对不同负载金属(如Pt、Pd)对H_2-SCR反应的影响进行了介绍。之后对不同类型载体(传统氧化物、复合氧化物、钙钛矿型氧化物、分子筛、其它类型载体)对H_2-SCR反应催化性能的影响进行了阐述。最后对氢气选择性催化还原氮氧化物的发展方向进行了展望。     

丙酮氨氧化制备丙酮肟过程中副反应研究

通过GC-MS、LC-MS等方法表征TS-1催化丙酮氨氧化制备丙酮肟过程中的副产物结构,分析副反应类型。结果表明:在此反应体系中,副反应主要产生途径有:(1)碱催化酮缩合,导致副产物骨架的生成;(2)亲核氨解,生成各种有机胺,导致体系碱性增强,起到自催化的作用;(3)消除脱水生成烯烃,然后烯烃被H2O2环氧化;(4)酮类副产物肟化;(5)酮肟氧化偶联反应生成2,3-二硝基-2,3-二甲基丁烷(DMNB);(6)其它氧化过程生成不饱和度高的副产物等。     

TS-1催化环己酮氨肟化反应的研究进展

钛硅分子筛TS-1是具有MFI结构的沸石分子筛,其特点为以Ti4+取代全硅分子筛骨架中的少量Si4+,使其具有独特的催化氧化性能,尤其对于有H_2O_2或有机过氧化合物参与的一系列有机物的选择氧化反应,能够使得反应具有很高的选择性。本文综述了TS-1催化环己酮氨肟化制环己酮肟反应的基础研究和工业应用现状及存在的问题,并对其今后的研究及应用作了展望。     

Ammonia activation over catalysts for the selective catalytic reduction of NOx and the selective catalytic oxidation of NH3. An FT-IR study

The adsorption and transformation of ammonia over V₂O₅, V₂O₅/TiO₂, V₂O₅-WO₃/TiO₂ and CuO/TiO₂ systems has been investigated by FT-IR spectroscopy. In all cases ammonia is first coordinated over Lewis acid sites and later undergoes hydrogen abstraction giving rise either to NH₂ amide species or to its dimeric form N₂H₄, hydrazine. Other species, tentatively identified as imide NH, nitroxyl HNO, nitrogen anions N₂⁻ and azide anions N₃⁻ are further observed over CuO/TiO₂. The comparison of the infrared spectra of the species arising from both NH₃ and N₂H₄ adsorbed over CuO/TiO₂ strongly suggest that N₂H₄ is an intermediate in NH₃ oxidation over this active selective catalytic reduction (SCR) and selective catalytic oxidation (SCO) catalysts. This implies that ammonia is activated in the form of NH₂ species for both SCR and SCO, and it can later dimerize. Ammonia protonation to ammonium ion is detected over V₂O₅-based systems, but not over CuO/TiO₂, in spite of the high SCR and SCO activity of this catalyst. Consequently Brönsted acidity is not necessary for the SCR activity.     

An investigation of alumina-supported catalysts for the selective catalytic oxidation of ammonia in biomass gasification

Alumina-supported catalysts containing different transition metals (Ni, Cu, Cr, Mn, Fe and Co) were prepared and tested for their activity in the selective oxidation of ammonia reaction at high temperatures (between 700 and 900 °C) using a synthetic gasification gas mixture. The catalysts were also characterised for their acidic properties by infrared studies of pyridine and ammonia adsorption and reaction/desorption. The Ni/Al₂O₃ and Cr/Al₂O₃ catalyst displayed the highest selective catalytic oxidation (SCO) activity in that temperature range with excellent N₂ selectivities. FT-IR studies of adsorbed pyridine and NH₃ indicate that Lewis acid sites dominate and that NH₃ adsorption on these sites is likely to be the first step in the SCO reaction. FT-IR studies on less active catalysts, particularly on Cu/Al₂O₃ allowed the detection of oxidation intermediates, amide (NH₂), and possibly hydrazine and imido and nitroxyl species. The amide and hydrazine intermediate gives credence to a proposed SCO mechanism involving a hydrazine intermediate, while the proposed imide, =N–H, and/or nitroxyl, HNO species could be intermediates in incomplete oxidation of NH₃ to N₂O.     

A study of the mechanism of selective conversion of ammonia to nitrogen on Ni/γ-Al2O3 under strongly oxidising conditions

The activity and excellent selectivity (>90%) of γ-Al₂O₃-supported Ni for the selective catalytic oxidation (SCO) of NH₃ to N₂ with excess O₂ has been shown by microreactor studies. Further studies of the mechanism involved in this reaction have been carried out using TPD, TPO, TPReaction as well as DRIFTS. N₂H₄ and NO have been used to model the intermediates of the SCO mechanism (direct formation of N₂ via the recombination of two NH_x species) and of the in situ SCR mechanism (two-step formation of N₂ via the reduction of an in situ produced NO species by a NH_x species), respectively. Two IR absorption bands appear during the TPO of NH₃ in the temperature range of N₂ formation and have been assigned to stable bidentate nitrate surface species. This represents strong evidence that under the present conditions, formation of N₂ occurs via the in situ SCR mechanism. This also explains the sudden “NO jump” observed on various systems once the temperature is high enough to activate 50% of the NH₃ molecules fed to the catalyst. The fact that NO and NH₃ are able to react to give N₂ at low temperature (from 100°C) confirms that activation of NH₃ is the limiting step. In contrast, no evidence has been found to support the possibility of the SCO mechanism.     

Selective catalytic reduction of NOx with propylene in the presence of oxygen over Co–Pt promoted H-MFI and HY

Kinetic and in situ spectroscopic studies of Co–Pt/MFI and Co–Pt/HY catalysts for the selective reduction of NO_x with propylene in the presence of oxygen were carried out. The results of catalytic tests of Co–Pt/MFI showed that the addition of Pt to Co based catalyst improved the activity, but a small increase in selectivity to N₂O (15–20%) was observed. In the case of Co–Pt/HY catalyst, the addition of Pt improved the activity more significantly and however, a larger increase in selectivity to N₂O (6–72%) was obtained. It was also found from the results of FT-IR studies of Co–Pt/MFI that the reduction of NO to N₂ was as follows: firstly the oxidation of NO to NO₂ occurred over metallic Pt and NO₂ forms Co–NO₂, Co–ONO, and/or Co–ONO₂; secondly, the partial oxidation of C₃H₆ was happened over Brønsted acid sites and the reaction of NO₂ formed on Co sites with partial oxidized C₃H₆ produced organo-nitro species. These species were dehydrated and isomerized to form isocyanate. Finally, [NCO] type intermediates react with NO from gas phase to selectively yield N₂.     

Nitrous oxide reduction with ammonia over Co---MgO catalyst and the influence of excess oxygen

N₂O decomposition on Co---MgO was studied under high (6.67 kPa) and low (75 Pa) N₂O pressure and the effect of reductant (C₂H₆, NH₃) was studied. The activity decays because of the strong adsorption of oxygen produced, while the reductant removed the adsorbed oxygen giving a steady catalysis. The reaction between NH₃ and excess amount of O₂ produced N₂O as a main product. Although N₂O gives the same intermediate (O) as O₂ does, the former decomposition seems to proceed faster than the latter on Co---MgO. The reaction mechanism studied here was compared with the SCR (NO---NH₃ in O₂) reaction on V₂O₂-TiO₂. Since NH₃, N₂O and O₂ gives only N₂ and water, Co---MgO can be a possible catalyst used in the boiler exhaust to reduce N₂O concentration by adding ammonia.     

Effect of different oxygen species originating from the dissociation of O2, N2O and NO on the selectivity of the Pt-catalysed NH3 oxidation

An effect of oxygen species formed from O₂, N₂O and NO on the selectivity of the catalytic oxidation of ammonia was studied over a polycrystalline Pt catalyst using the temporal analysis of products (TAP) reactor. The transient experiments were performed in the temperature range between 773 and 1073 K in a sequential pulse mode with a time interval of 0.2 s between the pulses of the oxidant (O₂, N₂O and NO) and NH₃. In contrast to adsorbed oxygen species formed from NO, those from O₂ and N₂O reacted with ammonia yielding NO. It is suggested that the difference between these oxidising agents may be related to the different active sites for dissociation of O₂, N₂O and NO, where oxygen species of various Pt-O strength are formed. Weaker bound oxygen species, which are active for NO formation, originate from O₂ and N₂O rather than from NO. These species may be of bi-atomic nature.     

NO reduction with NH3 over chromia–vanadia catalysts supported on TiO2: an in situ Raman spectroscopic study

In situ Raman spectroscopy was used for studying the ternary 2% CrO₃–6% V₂O₅/TiO₂ catalyst, for which a synergistic effect between vanadia and chromia leads to enhanced catalytic performance for the selective catalytic reduction (SCR) of NO with NH₃. The structural properties of this catalyst were studied under NH₃/NO/O₂/N₂/SO₂/H₂O atmospheres at temperatures up to 400 °C and major structural interactions between the surface chromia and vanadia species are observed. The effects of oxygen, ammonia, water vapor and sulfur dioxide presence on the in situ Raman spectra are presented and discussed.     

Selective oxidation of ammonia to nitrogen on transition metal containing mixed metal oxides

The selective catalytic oxidation of ammonia to nitrogen (NH₃-SCO) has been studied over hydrotalcite derived mixed metal oxides containing Cu, Co, Fe or Ni. XRD, BET, NH₃-TPD and TPR techniques were used for catalysts characterization. Results of NH₃-SCO were compared with those of selective catalytic reduction of NO with NH₃ (NO-SCR). Reaction mechanism was studied by temperature-programmed surface reaction (TPSR) and activity tests with a various contact time. Catalytic performance of the studied samples depends on both kind and loading of transition metals in the mixed metal oxide system. The Cu-containing samples have been found to be the most active catalysts of the NH₃-SCO process. Transition metal loading strongly influences distribution of ammonia oxidation products. The highest selectivity to N₂ was measured for the catalysts with the lowest transition metal content.     

The selective catalytic oxidation of NH3 over Fe-ZSM-5

The abatement of NH₃ from waste streams has become an important environmental issue. Selective catalytic oxidation (SCO) of NH₃ to N₂ has emerged as a potential technology for taking care of NH₃ slips and NH₃ in waste streams. In this work, we describe the catalytic activity of Fe-zeolite catalysts prepared by incipient wetness technique, ion exchange and hydrothermal synthesis in the SCO of NH₃ to N₂ using a fixed bed flow reactor. Selective catalytic oxidation was carried out at 573–723 K and 10⁵ Pa with gas hourly space velocities (GHSV) between 24 000 and 240 000 h⁻¹. Results obtained showed that Fe-ZSM-5 catalysts prepared by incipient wetness technique were active for NH₃ conversion (77–100%) and selectivity to N₂ (65–100%). Fe-ASA and Fe-Beta showed good catalytic activity and selectivity, but their activity and selectivity were less than that of Fe-ZSM-5. The effects of water vapour, Fe loading, and activation method on the performance of these catalysts was also investigated.     

Characteristics of Fe-exchanged natural zeolites for the decomposition of N2O and its selective catalytic reduction with NH3

Natural zeolites obtained from various regions of Turkey and their iron-exchanged forms were characterised by XRD, BET, H₂-TPR and NH₃-TPD methods. Transient experiments with N₂O showed that the iron introduced into natural zeolites have appreciable oxygen deposition capacity due to isolated iron species involved. Atomic surface oxygen species in these zeolites are formed at 250 °C, which is released through increasing the temperature until 900 °C, similar to Fe-containing ZSM-5 zeolite. The steady-state experiments indicate that the iron-containing zeolite of the Yavu-Sivas region, in particular, has high activity in selective catalytic reduction of N₂O with NH₃ as a consequence of isolated cationic and/or dimeric iron content.