Effects of Zeolite Structural Confinement on Adsorption Thermodynamics and Reaction Kinetics for Monomolecular Cracking and Dehydrogenation of n-Butane.

The effects of zeolite structure on the kinetics of n-butane monomolecular cracking and dehydrogenation are investigated for eight zeolites differing in the topology of channels and cages. Monte Carlo simulations are used to calculate enthalpy and entropy changes for adsorption (ΔHads-H+ and ΔSads-H+) of gas-phase alkanes onto Brønsted protons. These parameters are used to extract intrinsic activation enthalpies (ΔHint‡), entropies (ΔSint‡), and rate coefficients (kint) from measured data. As ΔSads-H+ decreases (i.e., as confinement increases), ΔHint‡ and ΔSint‡ for terminal cracking and dehydrogenation decrease for a given channel topology. These results, together with positive values observed for ΔSint‡, indicate that the transition states for these reactions resemble products. For central cracking (an earlier transition state), ΔHint‡ is relatively constant, while ΔSint‡ increases as ΔSads-H+ decreases because less entropy is lost upon protonation of the alkane. Concurrently, selectivities to terminal cracking and dehydrogenation decrease relative to central cracking because ΔSint‡ decreases for the former reactions. Depending on channel topology, changes in the measured rate coefficients (kapp) with confinement are driven by changes in kint or by changes in the adsorption equilibrium constant (Kads-H+). Values of ΔSint‡ and ΔHint‡ are positively correlated, consistent with weaker interactions between the zeolite and transition state and with the greater freedom of movement of product fragments within more spacious pores. These results differ from earlier reports that ΔHint‡ and ΔSint‡ are structure-insensitive and that kapp is dominated by Kads-H+. They also suggest that ΔSads-H+ is a meaningful descriptor of confinement for zeolites having similar channel topologies.

Environmental and Economic Assessment of Electrothermal Swing Adsorption of Air Emissions from Sheet-Foam Production Compared to Conventional Abatement Techniques.

A life-cycle assessment (LCA) and cost analysis are presented comparing the environmental and economic impacts of using regenerative thermal oxidizer (RTO), granular activated carbon (GAC), and activated carbon fiber cloth (ACFC) systems to treat gaseous emissions from sheet-foam production. The ACFC system has the lowest operational energy consumption (i.e., 19.2, 8.7, and 3.4 TJ/year at a full-scale facility for RTO, GAC, and ACFC systems, respectively). The GAC system has the smallest environmental impacts across most impact categories for the use of electricity from select states in the United States that produce sheet foam. Monte Carlo simulations indicate the GAC and ACFC systems perform similarly (within one standard deviation) for seven of nine environmental impact categories considered and have lower impacts than the RTO for every category for the use of natural gas to produce electricity. The GAC and ACFC systems recover adequate isobutane to pay for themselves through chemical-consumption offsets, whereas the net present value of the RTO is $4.1 M (20 years, $0.001/m(3) treated). The adsorption systems are more environmentally and economically competitive than the RTO due to recovered isobutane for the production process and are recommended for resource recovery from (and treatment of) sheet-foam-production exhaust gas. Research targets for these adsorption systems should focus on increasing adsorptive capacity and saturation of GAC systems and decreasing electricity and N2 consumption of ACFC systems.

New trends in the kitchen: propellants assessment of edible food aerosol sprays used on food.

New products available for food creations include a wide variety of "supposed" food grade aerosol sprays. However, the gas propellants used cannot be considered as safe. The different legislations available did not rule any maximum residue limits, even though these compounds have some limits when used for other food purposes. This study shows a preliminary monitoring of propane, butane and dimethyl ether residues, in cakes and chocolate after spraying, when these gases are used as propellants in food aerosol sprays. Release kinetics of propane, butane and dimethyl ether were measured over one day with sprayed food, left at room temperature or in the fridge after spraying. The alkanes and dimethyl ether analyses were performed by headspace-gas chromatography-mass spectrometry/thermal conductivity detection, using monodeuterated propane and butane generated in situ as internal standards. According to the obtained results and regardingthe extrapolations of the maximum residue limits existing for these substances, different delays should be respected according to the storage conditions and the gas propellant to consume safely the sprayed food.

Quality of random number generators significantly affects results of Monte Carlo simulations for organic and biological systems.

We have simulated pure liquid butane, methanol, and hydrated alanine polypeptide with the Monte Carlo technique using three kinds of random number generators (RNG's)-the standard Linear Congruential Generator (LCG), a modification of the LCG with additional randomization used in the BOSS software, and the "Mersenne Twister" generator by Matsumoto and Nishimura. While using the latter two RNG's leads to reasonably similar physical features, the LCG produces significant different results. For the pure fluids, a noticeable expansion occurs. Using the original LCG on butane yields, a molecular volume of 171.4 Å(3) per molecule compared to about 163.6-163.9 Å(3) for the other two generators, a deviation of about 5%. For methanol, the LCG produces an average volume of 86.3 Å(3) per molecule, which is about 24% higher than the 68.8-70.2 Å(3) obtained with the RNG's in BOSS and the generator by Matsumoto and Nishimura. In case of the hydrated tridecaalanine peptide, the volume and energy tend to be noticeably greater with the LCG than with the BOSS (modified LCG) RNG's. For the simulated hydrated extended conformation of tridecaalanine, the difference in volume reached about 87%. The uniformity and periodicity of the generators do not seem to play the crucial role in these phenomena. We conclude that, it is important to test a RNG's by modeling a system such as the pure liquid methanol with a well-established force field before routinely employing it in Monte Carlo simulations.

Control of electrothermal heating during regeneration of activated carbon fiber cloth.

Electrothermal swing adsorption (ESA) of organic gases generated by industrial processes can reduce atmospheric emissions and allow for reuse of recovered product. Desorption energy efficiency can be improved through control of adsorbent heating, allowing for cost-effective separation and concentration of these gases for reuse. ESA experiments with an air stream containing 2000 ppm(v) isobutane and activated carbon fiber cloth (ACFC) were performed to evaluate regeneration energy consumption. Control logic based on temperature feedback achieved select temperature and power profiles during regeneration cycles while maintaining the ACFC's mean regeneration temperature (200 °C). Energy requirements for regeneration were independent of differences in temperature/power oscillations (1186-1237 kJ/mol of isobutane). ACFC was also heated to a ramped set-point, and the average absolute error between the actual and set-point temperatures was small (0.73%), demonstrating stable control as set-point temperatures vary, which is necessary for practical applications (e.g., higher temperatures for higher boiling point gases). Additional logic that increased the maximum power application at lower ACFC temperatures resulted in a 36% decrease in energy consumption. Implementing such control logic improves energy efficiency for separating and concentrating organic gases for post-desorption liquefaction of the organic gas for reuse.

TIGER2: an improved algorithm for temperature intervals with global exchange of replicas.

An empirical sampling method for molecular simulation based on "temperature intervals with global exchange of replicas" (TIGER2) has been developed to reduce the high demand for computational resources and the low computational efficiency of the conventional replica-exchange molecular dynamics (REMD) method. This new method overcomes the limitation of its previous version, called TIGER, which requires the assumption of constant heat capacity during quenching of replicas from elevated temperatures to the baseline temperature. The robustness of the TIGER2 method is examined by comparing it against a Metropolis Monte Carlo simulation for sampling the conformational distribution of a single butane molecule in vacuum, a REMD simulation for sampling the behavior of alanine dipeptide in explicit solvent, and REMD simulations for sampling the folding behavior of two peptides, (AAQAA)(3) and chignolin, in implicit solvent. The agreement between the results from these conventional sampling methods and the TIGER2 simulations indicates that the TIGER2 algorithm is able to closely approximate a Boltzmann-weighted ensemble of states for these systems but without the limiting assumptions that were required for the original TIGER algorithm. TIGER2 is an efficient replica-exchange sampling method that enables the number of replicas that are used for a replica-exchange simulation to be substantially reduced compared to the conventional REMD method.

Multiple Markov transition matrix method: obtaining the stationary probability distribution from multiple simulations.

We herein propose the multiple Markov transition matrix method (MMMM), an algorithm by which to estimate the stationary probability distribution from independent multiple molecular dynamics simulations with different Hamiltonians. Applications to the potential of mean force calculation in combination with the umbrella sampling method are presented. First, the performance of the MMMM is examined in the case of butane. Compared with the weighted histogram analysis method (WHAM), the MMMM has an advantage with respect to the reasonable evaluation of the stationary probability distribution even from nonequilibrium trajectories. This method is then applied to Met-enkephalin nonequilibrium simulation.

Reconstructing atomistic detail for coarse-grained models with resolution exchange.

Coarse-grained (CG) modeling has emerged as a promising tool to bridge the gap between the temporal and spatial scales of all-atom (AA) simulations and those of many important biological processes. Resolution exchange, a variant of the replica exchange method, combines the efficiency of CG simulation and the accuracy of AA simulation by swapping configurations between AA and CG simulations. The crucial step in a resolution exchange move is to rigorously reconstruct the high-resolution system from models at coarser resolutions. In this paper, configurational-bias Monte Carlo is adopted as a general method to rebuild the missing degrees of freedom rigorously for CG models and for the first time combined with resolution exchange. The new approach is demonstrated on an alkane and a peptide system. It is found that the efficiency of resolution exchange depends significantly on the quality of the CG model.

Probing the sub-microsecond photodissociation dynamics in gas-phase retinal chromophores.

The photoinduced fragmentation of a retinal model chromophore (all-trans-n-butyl protonated Schiff-base retinal) was studied in vacuo using a new experimental technique. The apparatus is able to record the photodissociation yield of gas-phase biomolecular ions in the first microseconds after absorption. Together with the existing ion storage ring ELISA, which operates on the millisecond to second time scale, the complete decay dynamics of such molecules can now be followed. In the case of retinal, the time-dependent fragmentation yield observed after irradiation with a 410 nm laser pulse exhibits contributions from one- and two-photon absorption, which decay non-exponentially with lifetimes on the order of 1 ms and 1 micros, respectively. The decay can be simulated using a statistical model, yielding good agreement with the experimental findings on both the millisecond and the microsecond time scales. No indication for nonstatistical processes is found for this molecule, the upper limit for a possible direct rate being a factor of 10(4) below the observed statistical dissociation rate.

CPI-1189. Centaur.

The efficacy of the proapoptotic cytokine tumor necrosis factor (TNF) alpha-inhibiting compound CPI-1189 has been demonstrated in various cell culture and animal models of chronic neurodegenerative and inflammatory diseases. CPI-1189 intracellularly inhibits the p38 mitogen-activated protein kinase phosphoactivation, thereby protecting against TNF alpha-induced neurodegeneration. Clinical proof-of-concept phase IIa trials in patients with Parkinson's disease and AIDS dementia complex were successful. These studies demonstrated clinical relevance for treatment with CPI-1189 (50 to 100 mg/day), which attenuated the deterioration in cognitive and/or motor function without any relevant side effects. Since the importance of neuroprotection is emerging, in particular in neurodegenerative diseases with concomitant observed immunological pro-apoptotic alterations in the central nervous system, long-term application of CPI-1189 could represent a promising future therapeutic alternative, in addition to neuroprotective compounds such as selegiline.