Studies on chemistry of unsaturated cyclic systems consisting of germanium atoms

Thesis (Ph. D. in Science)--University of Tsukuba, (A), no. 2854, 2002.3.25Includes bibliographical reference

White Paper from Workshop on Large-scale Parallel Numerical Computing Technology (LSPANC 2020): HPC and Computer Arithmetic toward Minimal-Precision Computing

In numerical computations, precision of floating-point computations is a key factor to determine the performance (speed and energy-efficiency) as well as the reliability (accuracy and reproducibility). However, precision generally plays a contrary role for both. Therefore, the ultimate concept for maximizing both at the same time is the minimal-precision computing through precision-tuning, which adjusts the optimal precision for each operation and data. Several studies have been already conducted for it so far (e.g. Precimoniuos and Verrou), but the scope of those studies is limited to the precision-tuning alone. Hence, we aim to propose a broader concept of the minimal-precision computing system with precision-tuning, involving both hardware and software stack. In 2019, we have started the Minimal-Precision Computing project to propose a more broad concept of the minimal-precision computing system with precision-tuning, involving both hardware and software stack. Specifically, our system combines (1) a precision-tuning method based on Discrete Stochastic Arithmetic (DSA), (2) arbitrary-precision arithmetic libraries, (3) fast and accurate numerical libraries, and (4) Field-Programmable Gate Array (FPGA) with High-Level Synthesis (HLS). In this white paper, we aim to provide an overview of various technologies related to minimal- and mixed-precision, to outline the future direction of the project, as well as to discuss current challenges together with our project members and guest speakers at the LSPANC 2020 workshop; https://www.r-ccs.riken.jp/labs/lpnctrt/lspanc2020jan/

Alkoxysilane production from silica and dimethylcarbonate catalyzed by alkali bases: A quantum chemical investigation of the reaction mechanism

Density functional theory (DFT) calculations were carried out to investigate mechanistic details of the reaction between silica and dimethylcarbonate (DMC) catalyzed by an alkali base. Various experimental studies have reported that several silicon dioxide sources can react with DMC in the presence of alkali base catalysts to produce tetramethoxysilane (TMOS), but details of the reaction mechanism are still elusive. Our DFT calculations suggest that the reaction can be characterized by four mechanistic steps. The first two steps include the activation of Si-O bonds by the alkali base catalyst, and the cleavage of the Si-O bond forming -Si+ and -O-. In the third step, the O- moiety reacts with the methyl group of DMC to form the -O-CH3 moiety; this is the rate-determining step of the overall reaction. Finally, transfer of a methoxy group from DMC to the silicon occurs to produce a compound in which two Si-O bonds in silica are replaced by two Si-OCH3: i.e., dimethoxylsilyloxide. DFT calculations reveal that the rate-determining step of the reaction depends strongly on the nature of the cationic part of the alkali base catalysts. The order of barrier height of the rate-determining step was computed to be LiOH > KOH > CsOH, and this trend is in agreement with previous experimental studies. The Li cation was found to interact with DMC to form a very stable intermediate compound that causes the barrier height of the reaction between the O- moiety of the activated SiO2 and DMC to be higher than that of the reaction catalyzed by other cations

Direct use of low-concentration CO2 in the synthesis of dialkyl carbonates, carbamate acid esters, and urea derivatives

The CO2 in thermal power plant and factory exhaust gases and the air must be addressed because all have low-concentration CO2. However, applying low-concentration CO2 to developed reactions that use high-purity CO2, energy- and cost-intensive pretreatments, such as CO2 purification, concentration, and compression, are required. Thus, a technology that directly converts low-concentration CO2 into useful chemicals without these pretreatment processes is attractive for energy saving and cost reduction. Such technology has been achieved in the fields of methane and methanol synthesis via the hydrogen reduction of CO2. Recently, the concept of directly using low-concentration CO2 has been applied to the synthesis of high value-added chemicals without hydrogen reduction. The synthesis of useful chemicals from CO2 without hydrogen reduction is a technology that will be put into practical use without waiting for the widespread use of green hydrogen. Thus, this review introduces the technology for the direct synthesis of useful chemicals, such as dialkyl carbonates (DRCs), carbamic acid esters (CAEs), and urea derivatives from low-concentration CO2 contained in exhaust gas or air, without hydrogen reduction. DRCs, CAEs, and urea derivatives are synthesized via the non-hydrogen reduction route of CO2. They are expected to contribute to long-term, large-volume CO2 fixation because they are raw materials of functional polymers, which have long-material-life and large market potential. Details on how to synthesize these compounds directly from low-concentration CO2 are presented, focusing on the efforts to devise CO2 capture processes, reactants, and catalysts