Solid-state NMR as a probe of porous catalysts and catalytic processes

The power of solid-state NMR for the interrogation of porous catalytic materials is illustrated using three examples. First, for the investigation of catalytic processes occurring within the confines of a microporous catalyst NMR is shown to reveal both the details of shape-selectivity and the nature of internal surface species. Second, NMR is shown to be a powerful short-range tool to reveal precise structural information on highly disordered microporous titanosilicates. Despite long-range disorder the short-range order is maintained and can be easily studied. Finally, the same utility of probing short-range chemical phenomena is shown to be crucial for the investigation of novel-ordered-amorphous-mesoporous materials known generically as M41S. This class of material is currently one of the most important with potential catalytic application.     

Catalysis at the toluene/water interface: shape-selective hydrolysis of esters having some rings and lactones promoted by octadecyl immobilized H-ZSM-5 catalyst

Shape-selective properties of octadecyltrichlorosilane-treated H-ZSM-5, abbreviated as H-ZSM-5-C₁₈, have been observed in the hydrolysis of esters having some rings and lactones in toluene-water solvent system. The shape-selectivity for the reaction has been evaluated by the ratio of the relative rate constants in comparison with the rate constant of methyl acetate. The selectivity became higher with increase in bulkiness of the substrate. Substrates having the minimum diameter larger than 6.5 Å, significantly larger in size than the pore openings of ZSM-5, could not react in this system.     

Pore Structure and Shape Selectivity of Platinum-Promoted Cesium Salts of 12-Tungstophosphoric Acid

Pore-widths and pore-size distributions of 0.5 wt% Pt-Cs_xH_3-xPW₁₂O₄₀ have been studied by means of adsorption of various molecules. For the distributions of micropore and mesopore, isotherms of Ar and N₂ adsorption were analyzed, respectively. Pt-Cs_2.1H_0.9PW₁₂O₄₀ possessed only ultramicropores. On the other hand, the pores of Pt-Cs_xH_3-xPW₁₂O₄₀ (x = 2.3, 2.5, 2.8 and 3.0) showed bimodal distributions in the range from micropore to mesopore, and the widths of both pores tended to increase as the Cs content increased. From the amounts and rates of adsorption for n-butane and isobutane, the pore width of Pt-Cs_2.1H_0.9PW₁₂O₄₀ was determined to be close to the molecular size of n-butane, that is, 0.43 nm. The fraction of external surface area in the total surface area of Pt-Cs_2.1H_0.9PW₁₂O₄₀ was estimated to be only 0.06 from the adsorption of 1,3,5-trimethylbenzene and t-plot of N₂ adsorption. Pt- Cs_2.1H_0.9PW₁₂O₄₀ exhibited a shape selectivity due to the uniform ultramicropores and small external surface area; it catalyzed the oxidation of n-butane but not that of isobutane. SEM and TEM measurements revealed the primary crystallites and their aggregated states.     

重油特征分子尺寸的精确计算方法

 采用量子化学方法计算得到重油特征分子的几何结构，提出了用量子化学方法计算重油特征分子的最小横截面直径和描述分子尺寸的方法。预测了代表性的重油特征分子的最小横截面直径。结果表明，理论计算与实验测定的分子尺寸一致。大庆渣油窄第一馏分、第二馏分和残渣馏分的特征分子的最小横截面直径为别为1.8703、3.0080和3.4929nm，因此这3种渣油分子均不能直接进入孔径较小的沸石分子筛，而只能在其表面发生一级催化反应。研究结果可为筛选和设计合成分子筛提供有用的理论依据。     

分子筛对甲醇制轻烯烃反应烃类产物选择性的影响

采用XRD、SEM、XRF、BET、NH₃-TPD等方法表征USY、Beta、ZSM-5、ZSM-22、SAPO-41和SAPO-34分子筛的物相、组成、结构和酸性,并采用脉冲微反技术考察这些分子筛催化甲醇转化反应活性及烃类产物选择性随反应温度的变化。结果表明,三维十二元环的USY和Beta分子筛的甲醇转化催化活性最高,其次为二维十元环的ZSM-5分子筛,一维十元环的ZSM-22分子筛最低。分子筛的孔结构与孔径尺寸具有择形效应,与反应温度共同影响高选择性烃类产物的碳链长度。大孔分子筛在反应温度450℃以下时的C₄ 烃选择性最高;在反应温度400℃以上时,中孔ZSM-5、ZSM-22和SAPO-41分子筛的C₃烃选择性最佳,小孔SAPO-34分子筛的C₃烃和C₂烃选择性最高。随着反应温度升高,高选择性烃产物的碳数降低。ZSM-5和SAPO-34分子筛是特别适合于甲醇制丙烯和乙烯的择形催化剂,这2种分子筛在催化选择性方面还具有优异的升温特性,在400~550℃范围,随着反应温度升高,产物中乙烯和丙烯的总选择性提高,副产物中丙烷、C₄、C₅和C₆₊烃的选择性降低,甲烷选择性略有增加。     

The advantages and future potential of single-site heterogeneous catalysts

Using open-structure (nanoporous) solids, advantage may be taken of single-site catalytically active centres to effect an enormous range of conversions of organic compounds where regio-selectivity and shape-selectivity loom large. When the single-site active centres are accommodated within a well-defined nanospace (typically pores of ca 30 ? diameter), uniquely powerful enantio-selective conversions may also be achieved; thus organometallic complexes, composed of cheap (nitrogen-based) chiral ligands anchored to the concave walls of the nanoporous solid, are especially powerful enantio-selective hydrogenation catalysts for pharmaceutically significant precursor species. The design, characterization and performance of single-site nanoporous catalysts are described and their application in (1) aerobic, shape-selective oxidation of cyclohexane to adipic acid; (2) region-specific oxidation of linear alkanes; (3) bromine-free synthesis of terephthalic acid from p-xylene using air as oxidant; (4) the in situ generation of hazardous reagents for Baeyer–Villiger oxidation of ketones to lactones and for the epoxidation of olefins; and (5) the single-step production of ε-caprolactam from cyclohexanone are outlined. Atomically engineered, multinuclear nanoparticle (bimetallic) catalysts are also described and their application in (1) one-step, solvent-free hydrogenation of polyenes; (2) asymmetric synthesis of pharmaceutical intermediates; (3) region-selective and stereo-selective allylic aminations; (4) single-step hydrogenation of dimethyl tererpthalate to 1,4-cyclohexane-dimethanol; and (5) renewable nylons from glucose and its biocatalytic derivatives is summarized. The key features of these solid catalysts (which facilitate separation of products from reactants) are that (a) they dispense with the use of corrosive reagents (and often of solvents); (b) permit the replacement of aggressive oxidants (e.g. HNO₃, CrO₃, etc) or even H₂O₂ and alkyl hydroperoxides, with molecular oxygen as the oxidant, thereby facilitating use of low-temperature and otherwise environmentally benign processes; (c) they are generally stable and recyclable and require no chemical initiators; and (d) they offer significant processing advantages in large scale and industrial operations when compared to homogeneous catalysts, especially in the generation of vital intermediates (surfactants, detergents, pharmaceuticals, polymers, perfumes and cosmetics). Typically, just one of the products (nylon-6), made by the “green”, environmentally benign bifunctional catalysts described herein, is in great demand industrially for the manufacture of (i) apparel and other textiles; (ii) floor coverings; (iii) industrial yarns; (iv) engineering plastics and (v) films for food and industrial packaging.     

H-SAPO-34分子筛磷改性机理及作用

选取n(Si)/n(Si+Al+P)为0120和0086两种H SAPO 34分子筛进行磷改性,制备出一系列(n(P)/n(Si)为033~167)磷改性H SAPO 34分子筛样品。采用XRD、XRF、SEM、BET、NH3 TPD和29Si NMR、27Al NMR、31P NMR手段分析磷改性H SAPO 34分子筛的物相、孔结构和酸性以及骨架元素Si、P、Al的化学配位,并通过脉冲微反实验考察磷改性对H SAPO 34分子筛催化甲醇制烯烃（MTO）反应活性和烃产物分布及选择性的影响。〖JP2〗结果表明,对H SAPO 34进行磷改性时,在高温水蒸气作用下,磷氧化物能够均匀分布在H SAPO 34〖JP〗分子筛晶体的外表面,并随机与表面的骨架Al、P发生化学配位,引起骨架结构中的P-O-Al键和Si-OH-Al键断裂;随着n(P)/n(Si)增加,表面骨架铝的配位数提高,促进了AlPO4鳞石英相生成,以及Si-O-Al键断裂和Si(OSi)4无定形相析出,伴随着孔结构破坏、相对结晶度下降以及酸量降低。采用n(P)/n(Si)为033~133范围内的磷改性H SAPO 34分子筛催化MTO反应,随着n(P)/n(Si)的增加,产物中C4、C5烃的含量有所下降,C2烃的含量明显增加,择形性提高,乙烯选择性提高。     

二甲苯在分子筛孔道中扩散特性的分子模拟研究

应用分子模拟技术研究了甲苯歧化反应中的产物二甲苯各同分异构体在MFI、MEL和CAN分子筛孔道中的扩散特性,得到二甲苯各同分异构体与不同分子筛之间的相互作用能,以及吸附位和扩散特性等相关信息。计算结果表明,吸附质分子扩散的难易程度受到孔径和孔道结构的共同影响。MFI分子筛对于p-xylene有很好的选择性,不论是直孔还是弯孔,均有此特性,但弯孔中的扩散能垒较直孔中高,因此吸附质分子更倾向于在MFI分子筛直孔中进行扩散;MEL分子筛的孔径略小于MFI分子筛,各吸附质的扩散能垒较在MFI分子筛孔道中高,二甲苯各同分异构体中p-xylene的扩散能垒远小于m-xylene和o-xylene,因此,MEL分子筛也表现出对于p-xylene的选择性;在CAN分子筛的孔道中,二甲苯各种同分异构体的扩散能垒按p-xylene＜m-xylene＜o-xylene的顺序依次升高,但由于数值整体偏低,所以CAN分子筛不能够表现出对于p-xylene的择形性。     

Shape-selective isopropylation of naphthalene over dealuminated mordenites. Increasingβ-substitution selectivity by adding water

Isopropylation of naphthalene was examined over two dealuminated H-mordenites (HM) having SiO₂/Al₂O₃ molar ratio of 38 (HM38) and 74 (HM74). Experiments were done in batch reactors at 200°C. With propylene as the alkylating agent, selectivity for-substitution of naphthalene increased when water was added, and reached a maximum with a water-to-catalyst mass ratio of 0.80 (ca. 45 mmol water/g-cat). Of particular importance was the increase in-selectivity to 2,6- and 2,7-diisopropylnaphthalene (2,6-DIPN and 2,7-DIPN) with added water. Without added water,-selectivities were 56 and 77% for HM38 and HM74, respectively. Selectivities to the highly desired 2,6-DIPN isomer were 37 and 54%, respectively. With added water,-selectivities increased to 90 and 95%, and 2,6-DIPN selectivities increased to 60 and 70% for HM38 and HM74, respectively. A large amount of added water always decreased the activity; however, mixed effects were observed for intermediate amounts of added water. Sorption of water on the catalyst surface seems to explain changes in isomer selectivity and catalytic activity. Coke deposits on the catalysts also decreased when water was added. Furthermore, it was shown that higher-selectivity could be obtained with isopropanol (i-PrOH) as the alkylating agent, rather than propylene, under the same conditions. Comparison experiments showed that this was due to water formed fromi-PrOH.     

烃分子动力学直径与分子筛择形催化性能的相关性

作为分子筛研究领域的一个热点,择形催化理论在工业上已经被研究了近60年。尽管在理论上很多研究还不够深入,但随着对催化反应类型、新型催化材料合成、分子筛结构测定、催化性能表征、催化反应作用机理等方面研究的深入,对择形催化反应的认识仍取得了令人欣慰的进步。本文在现有的研究基础上,对不同尺寸的烃分子在分子筛孔道内的吸附和扩散行为进行了总结,并对烃分子反应动力学直径与分子筛的择形相关性做了一定的探究。分析表明,烃分子的反应动力学直径与分子筛择形催化相关性存在一个适用规律区间,当其略小于或接近分子筛孔径时,分子筛的择形催化性能表现最好。此外,分子筛本身的拓扑结构等因素也会对分子筛催化性能造成影响。