Surface species in the direct amination of methanol over Brønsted acidic mordenite catalysts

Quantitative in situ infrared spectroscopy in combination with kinetic analysis is utilized to derive mechanistic aspects for the reaction of methanol with ammonia on Brønsted acidic mordenite. Under non-reactive conditions, a coadsorption complex between methanol and ammonia is found, in which only ammonia is in direct interaction with the Brønsted acid sites of the zeolite. This complex is proposed to be the precursor for the formation of protonated methylamines in the zeolite pores which are formed in sequential order up to tetramethylammonium ions. These methylamines are unable to desorb under reaction conditions in the absence of ammonia. They leave the surface either by ammonia adsorption assisted desorption or by scavenging of methyl groups from protonated methylamines by ammonia. Both steps are concluded to be potentially rate determining.     

Approximation algorithms for TTP(2)

We consider the traveling tournament problem, which is a well-known benchmark problem in tournament timetabling. It consists of designing a schedule for a sports league of n teams such that the total traveling costs of the teams are minimized. The most important variant of the traveling tournament problem imposes restrictions on the number of consecutive home games or away games a team may have. We consider the case where at most two consecutive home games or away games are allowed. We show that the well-known independent lower bound for this case cannot be reached and present two approximation algorithms for the problem. The first algorithm has an approximation ratio of ${3/2+\frac{6}{n-4}}$ in the case that n/2 is odd, and of ${3/2+\frac{5}{n-1}}$ in the case that n/2 is even. Furthermore, we show that this algorithm is applicable to real world problems as it yields close to optimal tournaments for many standard benchmark instances. The second algorithm we propose is only suitable for the case that n/2 is even and n????12, and achieves an approximation ratio of 1?+?16/n in this case, which makes it the first ${1+\mathcal{O}(1/n)}$ -approximation for the problem.     

Pruning in column generation for service vehicle dispatching

Column generation techniques have become a widely used technique to successfully solve large (integer) linear programs. One of the keys to obtaining a practically efficient algorithm is to have a fast method to limit the pricing of new columns to a small set. We study a large scale real-world vehicle dispatching problem with soft time windows which can be modeled as an integer linear program of set partitioning type. We develop a new pruning scheme based on matchings in order to speed up the branch-and-bound enumeration in the column generation process. Computational results on real-world data illustrate the effectiveness of the new pruning scheme.     

A Tailor‐Made Chimeric Thiamine Diphosphate Dependent Enzyme for the Direct Asymmetric Synthesis of (S)‐Benzoins

Thiamine diphosphate dependent enzymes are well known for catalyzing the asymmetric synthesis of chiral α‐hydroxy ketones from simple prochiral substrates. The steric and chemical properties of the enzyme active site define the product spectrum. Enzymes catalyzing the carboligation of aromatic aldehydes to (S)‐benzoins have not so far been identified. We were able to close this gap by constructing a chimeric enzyme, which catalyzes the synthesis of various (S)‐benzoins with excellent enantiomeric excess (>99 %) and very good conversion.     

Extrapolation of Laboratory Reaction Kinetic Data for Relief Vent Design in the Case of a Heterogeneous Runaway Reaction

The relief area sizing of safety devices in the case of a postulated thermal runaway condition requires reaction kinetic data, usually measured in dedicated adiabatic calorimeters. In the course of the design, it is then assumed that the same reaction kinetic data prevail on the laboratory and production scale. This assumption seems to be valid in the case of homogeneous reaction systems. In contrast to this the results of experiments with a heterogeneous system have shown that the stirring intensity and the reactor size have a considerable influence on the reaction progress.This revised version was published online in November 2005 with corrections to the Cover Date.     

Automatic activation analysis

Automatic activation analysis (AAA) is rendered possible by a unique neutron activation analysis facility for short-lived isomeric transitions based on a fast rabbit system with sample changer and sample separation, and an adaptive digital gamma-spectrometer for very high counting rates of up to 10⁶ cps. The system is controlled by a computer program performing irradiation control, neutron flux monitoring, and gamma-spectrometry with real-time correction of counting losses, spectra evaluation, nuclide identification and calculation of concentrations in a fully automatic procedure. As spectrometry is done by means of hundreds of sequentially measured pairs of concurrently recorded loss-corrected and non-corrected spectra, concentrations are derived from an optimally weighted average of all individual occurrences in this sequence of spectra which also enable the separation of isomeric transitions with coinciding energies but different half-lives such as ^116m2In (162.4 keV, T _1/2 = 2.2 s) and ^77mSe (162.2 keV, T _1/2 = 17.4 s). To clear up repeatedly voiced misconceptions concerning the errors of loss-free counting our findings of 1978 and 1981 are reiterated, namely that the counting error of a peak in a corrected spectrum may be derived consistently from the error of the same peak in the respective non-corrected spectrum and from the error of weighting factors in the corresponding region of interest, according to the principle of propagation of errors. Experimental proof is provided for conditions of stationary as well as rapidly varying counting rates and spectral shapes. To the memory of Vincent P. Guinn.     

Progress in Activation Analysis at the Atomic Institute Vienna

Fields in which progress has been achieved at the Atomic Institute Vienna are discussed briefly. The use of short and medium lived nuclides for activation analysis (AA). Construction of out-core parts of a fast transfer system. Combined sample catcher sample-changer for AA allowing cyclic and pseudocyclic AA. A system for discontinuous or continuous irradiation of liquid samples. Counting with an anticompton shield and quantitative correction of random coincidences. Screening analyses with the fast irradiation and measurement system. Improved peak-evaluation of gamma-spectra. Epithermal AA by a neutron converter. Following short-lived nuclide measurement, automatic sample transfer into a well-type detector enabling sensitive counting of nuclides with short half-lives. Use of large samples to increase the detection limits of ng to pg at activation with moderate neutron flux. Flux determination by liquid scintillation counting.     

Gamma-spectrometry shatters the 100 kc/s throughput barrier

Due to the fact that pulse rise time is coincident with the fall time of the previous pulse in the Preloaded Filter pulse processor, it has a definite throughput advantage over all other pulse shaping methods. So, for the first time, it was possible to achieve throughput rates of⁶⁰Co after pileup rejection in excess of 100 kc/s, at substantially better resolutions than those of other high rate shaping amplifiers.     

A Generalized Version of the Abelian Mean Ergodic Theorem with Rates for Semigroup Operators and Fractional Powers of Infinitesimal Generators

This note is concerned with the rate of convergence in a mean ergodic theorem for fractional powers of Abel averages of semigroup operators. Characterizations of optimal and non-optimal approximation in terms of K-functionals are given; they involve fractional powers of the infinitesimal generator of the underlying semigroup.