Differential–algebraic equations in multibody system modeling

Numerical methods for the efficient integration of both stiff and nonstiff equations of motion of multibody systems having the form of differential-algebraic equations (DAE) of index 3 are discussed. Linear multi-step ABM and BDF methods are considered for the non-iterational integration of nonstiff DAE. The Park method is proposed for integration of stiff equations. This revised version was published online in June 2006 with corrections to the Cover Date.     

Comparative FT infrared study of structural changes occuring on dealumination of zeolite Na-Y with ethylenediaminetetraacetic or hydrochloric acid

IR lattice vibration spectra were used to monitor the changes in the structure of zeolite Na-Y upon its dealumination with ethylenediaminetetraacetic acid (EDTA) or with HCl. The terminal Si-O(H,Na) bonds thus formed (which are detectable by characteristic absorbance at v 900–950 cm^–1) are energetically less uniform when EDTA has been used as the dealuminating agent. This inhomogeneity is connected with the local deformations of zeolite structure which result in lowering the symmetry of silicon-oxygen tetrahedra. As a consequence, two novel bands appear at v 1090 cm^–1 and 1200 cm^–1 in addition to the fundamental absorption bands, v_as (TO₄) (T = Si, Al), at 1030 cm^–1 and 1145 cm^–1. A mechanism of dealumination is proposed, which takes into account the topochemical peculiarities involved in the chelation of the framework aluminum ions with EDTA.Translated fromIzyestiya Akademii Nauk. Seriya Khimicheskay, No. 2, pp. 284–287, February, 1993.     

HZSM－5分子筛的水热脱铝研究 Ⅰ．氨－FTIR与吡啶－FTIR考察

运用ＦＴＩＲ手段，以氨和吡啶为探针研究了水热处理对分子筛结构及酸性的影响。研究结果表明，水热处理温度对分子筛中铝的状态有很大影响，随着水热处理温度的升高，骨架铝不断从分子筛的骨架中脱除出来，Ｂ酸位亦随之减少；脱出来的铝在孔道中沉积形成非骨架铝物种，其结构十分复杂，部分地形成了Ｌ酸位，从而使Ｌ酸位随着水热处理温度的升高而有所增加。Ｂ酸位与骨架铝有很好的对应关系。     

Mo/HZSM-5催化剂上甲烷无氧芳构化反应——HZSM-5分子筛脱铝的影响

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.     

多步法制备纳米钛酸钡

采用醋酸钡、碳酸钾、钛酸丁酯等为初始反应物，通过多步反应的方法，制得了纳米钛酸钡。所得产品采用XRD、TEM和SEM等进行了表征，并对生长机理进行了分析。研究结果表明，得到的碳酸钡为直径约100 nm的纳米棒，再经室温下钛酸丁酯水解产生二氧化钛对碳酸钡纳米棒进行包覆，可得到一维核-壳结构的碳酸钡/二氧化钛，最后于700 ℃下经固相反应而得到钛酸钡的纳米颗粒。这种制备纳米钛酸钡的方法可以有效地控制产品形貌及粒径。     

Multi-Step Continuous-Flow Organic Synthesis: Opportunities and Challenges

Continuous-flow multi-step synthesis takes the advantages of microchannel flow chemistry and may transform the conventional multi-step organic synthesis by using integrated synthetic systems. To realize the goal, however, innovative chemical methods and techniques are urgently required to meet the significant remaining challenges. In the past few years, by using green reactions, telescoped chemical design, and/or novel in-line separation techniques, major and rapid advancement has been made in this direction. This minireview summarizes the most recent reports (2017–2020) on continuous-flow synthesis of functional molecules. Notably, several complex active pharmaceutical ingredients (APIs) have been prepared by the continuous-flow approach. Key technologies to the successes and remaining challenges are discussed. These results exemplified the feasibility of using modern continuous-flow chemistry for complex synthetic targets, and bode well for the future development of integrated, automated artificial synthetic systems.     

求解PageRank问题的多步幂法修正的内外迭代法

引用两种加速计算PageRank的算法,分别为内外迭代法和两步分裂迭代算法.从这两种方法中,得到多步幂法修正的内外迭代方法.首先,详细介绍了算法实施过程.然后,对此算法的收敛性进行证明,并且将此算法的谱半径与两步分裂迭代算法的谱半径进行比较.最后,数值试验说明该算法的计算速度比两步分裂迭代法要快.     

Improved linear multi-step methods for stochastic ordinary differential equations

We consider linear multi-step methods for stochastic ordinary differential equations and study their convergence properties for problems with small noise or additive noise. We present schemes where the drift part is approximated by well-known methods for deterministic ordinary differential equations. In previous work, we considered Maruyama-type schemes, where only the increments of the driving Wiener process are used to discretize the diffusion part. Here, we suggest the improvement of the discretization of the diffusion part by also taking into account mixed classical-stochastic integrals. We show that the relation of the applied step sizes to the smallness of the noise is essential in deciding whether the new methods are worthwhile. Simulation results illustrate the theoretical findings.     

A Unified Continuous Flow Assembly-Line Synthesis of Highly Substituted Pyrazoles and Pyrazolines

A rapid and modular continuous flow synthesis of highly functionalized fluorinated pyrazoles and pyrazolines has been developed. Flowing fluorinated amines through sequential reactor coils mediates diazoalkane formation and [3+2] cycloaddition to generate more than 30 azoles in a telescoped fashion. Pyrazole cores are then sequentially modified through additional reactor modules performing N-alkylation and arylation, deprotection, and amidation to install broad molecular diversity in short order. Continuous flow synthesis enables the safe handling of diazoalkanes at elevated temperatures, and the use of aryl alkyne dipolarphiles under catalyst-free conditions. This assembly-line synthesis provides a flexible approach for the synthesis of agrochemicals and pharmaceuticals, as demonstrated by a four-step, telescoped synthesis of measles therapeutic, AS-136A, in a total residence time of 31.7 min (1.76 g h⁻¹).