Propylene oxide adsorption

The surface coverage 8 of the i-mer is defined as the fraction of propylene oxide adsorption sites covered by any of the i units of the i-mer. 

A2 is also a known function of T and space velocity since the rate constant K2 is known from the steady state results (eq. 1). The parameters Ai and Af are not known independently however, the ratio Aj/Af equals the adsorption coefficient Kpr of propylene oxide which is a known function of T obtained from the steady state measurements (eq. 1). Since the steady state kinetics indicate that the surface reaction is the rate limiting step it can be concluded that Ai is larger than A2. It was assumed that propylene oxide adsorption is nonactivated and Aj was arbitrarily set equal to be two times larger than A2 at 400°C,for Y =. 002 then Aj was calculated from Af = Ai/Kpro Yp. The numerical simulations indicated that the model predictions are rather insensitive to Aj but are sensitive to the unknown parameters A3 and 0 c Since the Heat of Polymerization of Propylene Oxide is 18 Kcal/mol the parameter 0 was set equal to 0 exp(-18000/RT). 

After epoxidation a distillation is performed to remove the propylene, propylene oxide, and a portion of the TBHP and TBA overhead. The bottoms of the distillation contains TBA, TBHP, some impurities such as formic and acetic acid, and the catalyst residue. Concentration of this catalyst residue for recycle or disposal is accompHshed by evaporation of the majority of the TBA and other organics (141,143,144), addition of various compounds to yield a metal precipitate that is filtered from the organics (145—148), or Hquid extraction with water (149). Low (<500 ppm) levels of soluble catalyst can be removed by adsorption on soHd magnesium siUcate (150). The recovered catalyst can be treated for recycle to the epoxidation reaction (151). 

The initiator usually constitutes less than 1% of the final product, and since starting the process with such a small amount of material in the reaction vessel may be difficult, it is often reacted with propylene oxide to produce a precursor compound, which may be stored until required [6]. The yield of poloxamer is essentially stoichiometric the lengths of the PO and EO blocks are determined by the amount of epoxide fed into the reactor at each stage. Upon completion of the reaction, the mixture is cooled and the alkaline catalyst neutralized. The neutral salt may then be removed or allowed to remain in the product, in which case it is present at a level of 0.5-1.0%. The catalyst may, alternatively, be removed by adsorption on acidic clays or with ion exchangers [7]. Exact maintenance of temperature, pressure, agitation speed, and other parameters are required if the products are to be reproducible, thus poloxamers from different suppliers may exhibit some difference in properties. 

Extensive studies of stereoselective polymerization of epoxides were carried out by Tsuruta et al.21 s. Copolymerization of a racemic mixture of propylene oxide with a diethylzinc-methanol catalyst yielded a crystalline polymer, which was resolved into optically active polymers216 217. Asymmetric selective polymerization of d-propylene oxide from a racemic mixture occurs with asymmetric catalysts such as diethyzinc- (+) bomeol218. This reaction is explained by the asymmetric adsorption of monomers onto the enantiomorphic catalyst site219. Furukawa220 compared the selectivities of asymmetric catalysts composed of diethylzinc amino acid combinations and attributed the selectivity to the bulkiness of the substituents in the amino acid. With propylene sulfide, excellent asymmetric selective polymerization was observed with a catalyst consisting of diethylzinc and a tertiary-butyl substituted a-glycol221,222. ... 

Propylene oxide is a surface active monomer structurally similar to ethylene oxide and therefore of interest as a SHM W-SP, but with more than ten repeating units this polymer is not water soluble. A compositional isomer methyl vinyl ether is water soluble the adsorption behavior of this polymer (PMVE) is illustrated in Figure 4. At 1 ppm the rate of 7T increase is linear over three hours. The diffusion rate could be calculated if the W-SP s molecular weight were monodispersed. The polymer studied had a Gaussian molecular weight distribution, which is true of essentially all W-SPs even after attempts have been made to... 

Diblock copolymers consist of one sequence of anchor segments and a second sequence of backbone segments. The relative lengths of the two sequences can be controlled to provide a wide variety of adsorption and barrier characteristics. Typical commercial dispersants may use alkane -eCH2i- ester -eC5H10---------(CO)—Oi— methyl methacrylate [80-62-6], C(CH3)(COOCH3) —CH2i > propylene oxide... 

Further evidence supporting the bismuth center as a site of propylene activation comes from the analysis of the rates of formation and product distribution of propylene oxidation over bismuth oxide, bismuth molybdate, and molybdenum oxide. Bismuth molybdate is highly active and selective for the conversion of propylene to acrolein. However, the interaction of propylene with its component oxides yields very different results. Haber and Grzybowska (//. ), Swift et al. 114), and Solymosi and Bozso 115) showed that in the absence of oxygen, propylene is converted to 1,5-hexadiene over bismuth oxide with good selectivity and at a high rate, whereas molybdenum oxide is known to be a fairly selective but a nonactive catalyst for acrolein formation. The formation of 1,5-hexadiene over bismuth oxide can be explained if the adsorption of propylene on a bismuth site yields a ir-allylic species. Two of these allylic intermediates can then combine to give 1,5-hexadiene. 

The polymeric chains are assumed to grow on sites available for propylene oxide monomer but not on oxygen adsorption sites. 

Transition metal ion-exchanged zeolites are active catalysts for alkene oxidation but generally result in deep oxidation to carbon dioxide and water (43-45). In common with CO and alkane oxidation, the platinum metal ions are more active than the first-row transition metal ions. Mochida et al. (43) have been able to correlate the catalytic activity of ion-exchanged Y zeolites for propylene oxidation with a so-called Y parameter as shown in Fig. 9. This parameter was considered to express the tendency of the metal ion toward the formation of a dative re-bond with propylene. Further, it was shown that with increasing Y factor there was a decrease in reaction order, which was considered evidence of increased propylene adsorption. In a more recent study of CuX zeolites, Gentry et al. (45) found some evidence... 

FIGURE 10.4 (a) IR spectra after adsorption and subsequent desorption of 1,2-propanediol, propylene oxide (PO), 2-propanol, and 1-propanol on titania (323 K). (b) Most likely structures of the adsorbate species that correspond to the IR spectra in (a) [29]. Reprinted with permission from [29]. Copyright (2005) Wiley VCH. 

Chemical vapor deposition (CVD) using TiC was used to prepare Ti/Si02, Ti/MCM-41, and Ti/MCM-48 catalysts. These catalysts were characterized by inductively coupled plasma-atomic emission spectroscopy (ICP-AES), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy, nitrogen adsorption, and were used to catalyze the epoxidation of propylene to propylene oxide (PO) with in situ prepared ethylbenzene hydroperoxide (EBHP). CVD time and CVD temperature affected the catalyst performance significantly. The optimum temperature range was 800-900 °C, and the optimum deposition time was 2.5-3 h. The maximum PO yields obtained in a batch reactor were 87.2, 94.3, and 88.8% for Ti/Si02, Ti/ MCM-41, and Ti/MCM-48, respectively. Ti/MCM-41 had higher titanium... 

Vapor-phase epoxidation of propylene using H2 and O2 was carried out over gold catalysts supported on mesoporous ordered (MCM-41) and disordered titanosilicates prepared hydrothermally or by modified sol-gel method. Gold nanoparticles were homogeneously dispersed on the titanosilicate supports by deposition-precipitation (DP) method. The catalysts and support materials were characterized by XRD, UV-Vis, surface area measurements (N2 adsorption) and TEM. NaOH was found to be the best precipitant to prepare Au catalysts with optimum propylene oxide yields and H2 efficiency. The extent of catalysts washing during preparation was found to affect the activity of the catalyst. The activity and hydrogen efficiency was found to depend on the type of mesoporous support used. 

Polymers are also essential for the stabilisation of nonaqueous dispersions, since in this case electrostatic stabilisation is not possible (due to the low dielectric constant of the medium). In order to understand the role of nonionic surfactants and polymers in dispersion stability, it is essential to consider the adsorption and conformation of the surfactant and macromolecule at the solid/liquid interface (this point was discussed in detail in Chapters 5 and 6). With nonionic surfactants of the alcohol ethoxylate-type (which may be represented as A-B stmctures), the hydrophobic chain B (the alkyl group) becomes adsorbed onto the hydrophobic particle or droplet surface so as to leave the strongly hydrated poly(ethylene oxide) (PEO) chain A dangling in solution The latter provides not only the steric repulsion but also a hydrodynamic thickness 5 that is determined by the number of ethylene oxide (EO) units present. The polymeric surfactants used for steric stabilisation are mostly of the A-B-A type, with the hydrophobic B chain [e.g., poly (propylene oxide)] forming the anchor as a result of its being strongly adsorbed onto the hydrophobic particle or oil droplet The A chains consist of hydrophilic components (e.g., EO groups), and these provide the effective steric repulsion. 

HUA Huang, Y. and Forciniti, D., Ethylene oxide and propylene oxide random copolymer/sodium chloride aqueous two-phase systems Wetting and adsorption on dodecylagarose and polystyrene, Biotechnol. Bioeng., 77, 786, 2002. 

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