Probe molecules cyclohexane

Commercial catalysts consist essentially of Ni snpported on a-alumina. Mg-promoted catalysts showed a greater difficulty for Ni precursor s reduction besides different probe molecules (H and CO) adsorbed states. In the conversion of cyclohexane, Mg inhibited the formation of hydrogenolysis products. Nonetheless, the presence of Ca did not influence the metallic phase. The impregnated Ni/MgO-catalyst performed better than the other types (Santos et al., 2004). 

The reactivity of surface methoxy species was further investigated with various probe molecules that were thought to possibly be involved in the MTO process, including water, toluene (representing aromatics), and cyclohexane (representing saturated hydrocarbons) (263). It was found that surface methoxy species react at room temperature with water to form methanol, which indicates the occurrence of a chemical equilibrium between these species at low reaction temperatures (Scheme 15) (263). 

The solvatochromic probe molecule chosen for this work was 2-nitroanisole (Aldrich Chemical Co.). The s value reported in the literature for 2-nitroanisole is -2.428 + 0.195 ( 1 ). A known s value for the solute allows one to calculate the change in the supercritical fluid solvents it value as temperature or pressure changes. The reference absorption maxima for 2-nitroanisole is 32.56 x 10J cm"1 (vQ) in cyclohexane (1). 

A similar technique has been used to determine the acidic character of niobium oxide and niobyl phosphate catalysts in different solvents (decane, cyclohexane, toluene, methanol and isopropanol) using aniline and 2-phenyl-ethylamine as probe molecules [27, 28]. The heat evolved from the adsorption reaction derives from two different contributions the exothermic enthalpy of adsorption and the endothermic enthalpy of displacement of the solvent, while the enthalpy effects describing dilution and mixing phenomena can be neglected owing to the differential design and pre-heating of the probe solution. 

The catalysts were characterized by using various techniques. X-ray diffraction (XRD) patterns were recorded on a Siemens D 500 diffractometer using CuKa radiation. The specific surface areas of the solids were determined by using the BET method on a Micromeritics ASAP 2000 analyser. Acid and basic sites were quantified from the retention isotherms for two different titrants (cyclohexylamine and phenol, of p/Ta 10.6 and 9.9, and L ,ax 226 and 271.6 nm, respectively) dissolved in cyclohexane. By using the Langmuir equation, the amount of titrant adsorbed in monolayer form, Xm, was obtained as a measure of the concentration of acid and basic sites [11]. Also, acid properties were assessed by temperature-programmed desorption of two probe molecules, that is, pyridine (pKa= 5.25) and cyclohexylamine. The composition of the catalysts was determined by energy dispersive X-ray analysis (EDAX) on a Jeol JSM-5400 instrument equipped with a Link ISI analyser and a Pentafet detector (Oxford). 

The probe molecule used for the studies on the solvatochromic behavior of supercritical fluids was 2-nitroanisole, which has an S value of -2.428 0.195 (15) and a reference absorbance maximum of 32,560 cm-1 in cyclohexane ( )o) Absorption spectra of 2-nitroanisole in the supercritical fluids were obtained with a Varian Model 2200 spectrophotometer operated in the dual beam mode with an air reference. The gases used as supercritical solvents were SFC grade C02 high purity grade N2O and NH3, and research grade for the other gases. 

It is possible to evaluate quantitatively the location of acid sites in zeolites by using different probe molecules, e.g., on one hand, ammonia, which is able to reach all acid sites, and, on the other hand, pyridine, cyclohexane, benzene, etc., probes that can only attach to sites in the main channels. 

The emission spectra of C-153 in microanulsions are markedly different from the emission spectra in cyclohexane. The red shift in onission maximum indicates C-153 molecules have experienced more polarity compared to cyclohexane. It indicates that probe molecules are gradually encapsulated in [Bntim][BF4] pool of the microemulsions. TX-lOO can form nticro ulsions in cyclohexane [140,141]. Due to this reason we have observed a large red shift from cyclohexane to w = 0. Now as we add [Bmim][BF4] in this system the polarity again increases and emission peak become further red shifted. But the observed emission peak at w = 1.5 (516nm) is much less than emission peak in neat [Bmim][BF4] (537nm) [32]. It indicates that the polarity of the microemulsions is much lower compared to neat [Bmim][BF4]. 

The observed rotational relaxation time of C-153 in cycloheaxne is 135 ps. In microemulsions the rotational relaxation time is bimodal in nature. The biexponential nature of rotational relaxation in TX-lOO/water reverse micelles has been reported [142,143]. Both rotational relaxation times in microemulsions are slower compared to cyclohexane. It strongly suggests that the probe molecules are residing at the core of the microemulsions. With an increase in w value the number of [BmimJlBFJ molecules increases in the core of the microemulsions, thus microviscosity also increases. The average rotational relaxation time also increases due to the increase in the viscosity of the core due to the addition of highly viscous [BmimJlBFJ. From the above discussion it is clear C-153 is located in the core of the [BmimKBFJ/TX-lOO microemulsions. 

Figure 2.1 Quenching rate coefficient versus polymer concentration in the good solvent benzene (—) and the poor solvent cyclohexane (- -) for various molar masses of the probe molecules (see text). Figure is reprinted with permission from [85]. Copyright 1981,...

The investigation of high-critical-temperature supercritical fluids is a more challenging task. One of the significant difficulties associated with these studies is probe-molecule thermal stability many molecular probes commonly used with ambient supercritical fluids decompose at the temperatures required by these high-critical-temperature fluids. Fortunately, pyrene can be employed for such tasks. Several reports have been made of the use of pyrene as a molecular probe to investigate solute-solvent interactions in high-critical-temperamre supercritical fluids (e.g., pentane, hexane, heptane, octane, cyclohexane, meth-cyclohexane, benzene, toluene, and water) (44,48,49). In supercritical hexane... 

Eor another example at the liquid/liquid interface. Steel and Walker used two different solvatochromic probe molecules, para-nitrophenol (PNP) and 2,6-dimethyl-para-nitrophenol (dmPNP), to study the polarity of the water-cyclohexane interface. These probes give spectral shifts as a function of bulk solvent polarity that are very similar because both solutes are mainly sensitive to the nonspecific solvent dipolar interactions. However, when these two dye molecules are adsorbed at the water/cyclohexane interface, they experience quite different polarities. The more polar solute (PNP) has a maximum SHG peak that is close to that of bulk water, and thus it reports a high-polarity environment. In contrast, the less polar solute (dmPNP) reports a much lower interface polarity, having a maximum SHG peak close to that of bulk cyclohexane. Clearly, the more polar solute is adsorbed on the water side of the interface, keeping most of its hydration shell, and thus reports a higher polarity than does the nonpolar solute. Other examples of the surface polarity dependence on probe molecules are discussed in Ref. 363. 

The example furthermore shows that diffusion from the bulk fluid phase toward the volume near the IRE, which is probed by the evanescent field, has to be accounted for because it may be the limiting step when fast processes are investigated. The importance of diffusion is more pronounced when a catalyst layer is present on the IRE, because of the diffusion in the porous film is much slower than that in the stagnant liquid film. Indeed, the ATR method, because of the measurement geometry, is ideally suited to characterization of diffusion within films (50,66-68). Figure 16 shows the time dependence of absorption signals associated with cyclohexene (top) and i-butyl hydroperoxide (TBHP, bottom). Solutions (with concentrations of 3mmol/L) of the two molecules in cyclohexane and neat cyclohexane were alternately admitted once to... 

Succinimidyl 6-((((4-iodoacetyl)amino)methyl)cyclohexane-l-carbonyl)amino)-hexanoate (SIACX) is an analog of SIAC that contains an additional aminohexanoate spacer group next to its NHS ester end (Molecular Probes). The result is the creation of an approximately 16-atom spacer arm between conjugated molecules. All other properties of SIACX are similar to those of SIAC. 

We have developed a novel ultrasensitive detection method, thermal lens microscopy (TLM), for nonfluorescent species [13]. TLM is photothermal spectroscopy under an optical microscope. Our thermal lens microscope (TLM) has a dual-beam configuration excitation and probe beams [13]. The wavelength of the excitation beam is selected to coincide with an absorption band of the target molecule and that of the probe beam is chosen to be where the sample solution (both solvent and solute) has no absorption. For example, in determination of methyl red dye in water, cyclohexane, and n-octanol, a 514-nm emission line of an argon-ion laser and a 633-nm emission line of a helium-neon laser were used as excitation and probe beams, respectively [21], Figure 4 shows the configuration and principle of TLM [13]. The excitation beam was modulated at 1 kHz by an optical chopper. After the beam diameters were expanded, the excitation and probe beams were made coaxial by a dichroic mirror just before they were introduced into an objective lens whose magnification and numerical aper-... 

The well characterized and stable surface phases observed on the Sn-Pt(l 11) have provided researchers in the chemisorption and catalysis field with a substrate of great interest for studying the properties of bimetallic interfaces. Simple probe gases such as CO have been studied after adsorption on this system [45] as well as a variety of organic molecules such as acetylene [46], cyclohexane and benzene [47, 48], butane and isobutane [49], methanol, ethanol and water [50]. Several surface reactions of the above gases were also studied. 

The standard adsorption enthalpy can also be used to gain information on changes in the surface characteristics of the carbon materials. For this purpose, molecular probes with different sizes and shapes can be used n-hexane, benzene, cyclohexane and 2,2 DMB. All these molecules have six carbon atoms and are linear (n-hexane), cyclic (benzene and cyclohexane) and branched (2,2 DMB), and their mean molecular sizes range from 0.405nm (n-hexane) to 0.62nm (2,2 DMB). 

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