Quartz catalysts

Similar findings were made by BASF in studies investigating an undisclosed gas-phase reaction in capillaries made of quartz, catalyst material and reactor-wall material [105]. The dimensions were chosen in such a way that they match the of surface-to-volume ratio of a fixed-bed reactor used previously for the same reaction. A quartz capillary shows no conversion, whereas reactor-wall material actually has a greater activity than the catalyst itself Hence BASF came to the, at first sight, surprising conclusion that in their production process it was the reactor wall, and not the catalyst, which catalyzes the reaction. The reactor wall was 70 times more active than the catalyst it needs a temperature increase of about 100 °C to have both at equal conversion. 

For the Pd-silk catalyst,2 PdCl2 was deposited on silk and reduced to Pd° moderate enantioselectivities were obtained for the hydrogenation of a C=C bond (66% enantiomeric excess, ee, which is the difference between enantiomers divided by the sum of enantiomers), but the silk support presented two problems it tended to deteriorate with time on stream and it varied from source to source, so enantioselectivities were not reproducible (Scheme 3.2). On the other hand, deterioration was not a problem with the metal-quartz catalysts. 

Experiments with these catalysts, from an old study by Schwab and Rudolph,3 showed that enantioselectivity occurs at the metal-quartz interface during enantioselective dehydrations of racemic 2-butanol. Klabunovskii and colleagues expanded on this idea and prepared a chiral Ni-quartz catalyst that... 

The propylene-butylene fraction constitutes a large part of the useful hydrocarbons produced by synthesis. It differs from similar fractions derived from petroleum refining in its high olefin (over 80%) and low isobutylene content, but this is no handicap in converting it to high octane gasoline by polymerization or by alkylation, if isobutane is available from another source. Polymerization is effected readily over a phosphoric acid on quartz catalyst with high conversion of propylene as well as butylene. The polymer... 

Fig. 4 and Table 4 summarize some other reaction types and also some very special cases of enantioselection. Quartz catalysts (Ml) have been used for dehydration (45) and isomerization (44) reactions (again with very low ee) and Cu-tartrate (M6) catalyzes the carbene addition 43 with an acceptable optical yield, giving an intermediate in a steroid synthesis [49]. 

The selectivity of the Nafion-H catalyst for monoalkylation has been found to be generally high. With a molar ratio of benzene isopropyl chloride being 5 1, about 94% of the alkylate is monoalkylbenzene. This result is comparable to the highly selective monoalkylation reaction reported by Langlois.236 They alkylated benzene with propylene (5.2 1 molar ratio) over H3P04-quartz catalyst at --200 C and obtained cumene in 95% yield. 

Production of cumene from benzene and propylene using a phosphoric acid on quartz catalyst (Fig. 19-22c). There are four reactor beds with interbed cooling with cold feed. The reactor operates at 260°C. 

At high reaction temperatures (400-500°C) the rate of racemization of the optically active butan-2-ol becomes fast enough that the optical rotation of the product goes through a maximum as a function of reaction time. Results received with a 0.10% Ni-t/-quartz catalyst at 550-560°C are shown below in Table 2.1. 

Figure 2.1. The increase in optical rotation during the decomposition of racemic butan-2-ol over Cu-c/-quartz catalyst at 400°C, after addition of a fresh portion of the catalyst at 8h (Schwab et al.

Table 2.1. Asymmetric decomposition of butan-2-ol over a 0.10% Ni-J-quartz catalyst at 550-560 C length of tube, 20 cm (adapted modified data from Schwab et al.

In 1938 Stankiewicz reproduced the asymmetric 2-butanol decomposition experiments of Schwab (1932) at a higher temperature and atmospheric pressure and extended studies to the asymmetric decomposition of racemic menthol and 3-methylheptan-3-ol. Over a Cu-r/-quartz catalyst the latter substrate produced a maximal optical rotation of -0.26° and the decomposition of 2-butanol gave a reaction mixture with a rotation of +0.25°. 

Table 2.2. Temperature dependence of the optical rotation of the product mixture in the asymmetric dehydrogenation of butan-2-ol over a Cu-tf-quartz catalyst in transport flow (nitrogen + air) (modified adapted data from Stankiewicz...

On a Cu-/-quartz catalyst in flowing N2 dehydration gives a product with rotation up to -0.25° (Table 2.4). 

Increasing the amount of Ni on quartz somewhat diminished the rotation from -0.15 to -0.13°. On Pt-quartz catalysts 2-butanol produced a product with -0.14° at 353°C. Increasing the reaction temperature resulted in larger amounts of butenes and lower rotations. 

In the dehydration of 3-methylheptan-3-ol at 170°C Pt-/-quartz proved to be the most effective catalyst with an observed rotation reaching -0.26°. Thus it was shown that Pt-quartz catalysts give particularly large effects, presumably owing to uniform deposits of Pt-particles on the surfaces of the quartz. Table 2.5. below summarizes the main results received by Stankiewicz... 

Table 2.5. Asymmetric Dehydrogenation-Dehydration of Racemic Alcohols over Metal-Quartz Catalysts (summarized adapted data from Stankiewicz ).

It has been shown that enantioselectivity is increased at low conversions particularly with the catalysts using cathodic scattering of metals (0.4-0.8 atomic layers on the surface of quartz). The temperature dependence of asymmetric decomposition of butan-2-ol revealed two maxima of optical rotation of products at temperatures between 320 - 400° butan-2-ol mainly dehydrogenates and at higher temperatures, above 400°, dehydration takes place. Therefore, in the reaction on Cu-<7-quartz catalysts two maxima of optical rotation were found at 340°C a = 0.21° and at 530°C a = 0.25°... 

Table 2.6. Asymmetric decomposition of racemic butan-2-ol on Cu-<7-quartz catalysts with low metal contents.

It was found that Cu-c/-quartz catalysts are more effective than the others in the asymmetric decomposition of 2-butanol and that the sign of optical rotation of the product after the dehydrogenation reaction always corresponded to the sign of rotation of the quartz. Ag-quartz catalysts prepared by cathodic scattering contained 0.03-0.08% Ag and gave an an of +0.106° on Ag-r/-quartz and -0.07° on Ag-/-quartz at the optimal temperature of 370°C. Two maximal values of optical rotation were observed on Ni-quartz catalysts, too -0.012° on Ni-/-quartz and +0.16° on Ni-r/-quartz. 

Cu-, Ag-, and Pt-quartz catalysts were used for the asymmetric isomerization of the racemic mixture of methyloxirane (Scheme 2.3.). 

Maximal rotations of the partially isomerized products at 180°C on Cu-/-quartz catalyst was -0.055° and on Ag-/-quartz catalyst was -0.052°. Another example of as5mimetric decomposition is the as5mimetric hydrogenation of... 

Rotation was +0.046° on Ni-/-quartz and -0.046° on Ni-c/-quartz. Besides the asymmetric resolution of racemates, metal-quartz catalysts were used to accomplish pure as5mimetric syntheses. The first attempt was reahzed by Schwab (1934) in the hydrogenation of tiglic acid [( )-2-methylbut-2-enoic acid] over Ni-quartz catalysts. 

Terent yev and Klabimovskii hydrogenated the ethyl-(2-phenylciimamate) (Scheme 2.6.) in decalin solution at 135°C on a Ni-r/-piezo-quartz catalyst and received a product with Od = -0.09° 0.007°. On Ni-/-quartz the product had an optical rotation of +0.04°. 

Figure 2.3. Dependence of optical rotation of product with reaction time during the asymmetric cyanoethylation of 2-methylcyclohexanone with acrylonitrile on a NaOH-quartz catalyst (adapted from Terent ev,...

In 1953, Ponomarev and Zelenkova applied the Ni-quartz catalysts for the asymmetric hydrogenation of fiiran derivatives (120 °C, 135-150 bar, liquid phase) in order to elucidate the mechanism of reaction. It was well known that in the hydrogenation of 4-(2-ftiryl)butan-2-ol, 1, 4-(tetrahydrofuran-2-yl)butan-2-ol, 2 and 2-methyl-l,6-dioxaspiro[4.4]nonane, were formed (Scheme 2.8.). 

In this reaction the spirane can be formed but its stereostructure was unknown. The spirane molecule can be chiral only in that case when the two five-membered rings are perpendicular to each other. Optical rotation of the spirane received in this reaction confirmed the structure of the supposed spirane Thus, over Ni-/-quartz catalyst, spirane 3 with assg = -0.066°,... 

Hydrogenations of butan-2-one were carried out at 140-150°C. The optical rotation of the product was observed to be 0.10°, which fell over time to 0.02°. Other generally disappointing irregularities also occurred. Independent of the sign of the rotation of quartz, all products were levorotatory, and the optical activity disappeared in several hours. Quartz crystals without metal layers also catalyzed the dehydration of butan-2-ol at 350° and gave rotations of -0.075° and -0.085°, which after filtration of the products fell to -0.005°. Thus the products obtained with the Pt-/-quartz catalyst at 307°C had an initial rotation of -0.045° that fell to zero in 3 hrs. 

For the mechanism of asymmetric catalysis on metal-quartz catalysts there is no unifying point of view. There are the concepts of "contact activation", "specific adsorption", and "formation of specific chemical bonds" It has been proposed that on the surface of the catalyst intermediate surface diastereomers are formed which are decomposed at different rates. The data on the action of quartz-catalysts have been rather thoroughly discussed in addition positive estimations of data were given by Bonner and Harada... 

The experimental data indicates that metal-quartz catalysts are more selective if they contain a thin, near monoatomic layer of metal on their surfaces. In accordance with this requirement the complicated net of border zones between metal and quartz must be created. Only on these borders can effective asymmetric reactions take place. Guided by this point of view Klabunovskii and Patrikeev considered the role of quartz as an as mimetric adsorbent that resolved the enantiomers and preferentially accumulated one of them on the border zones between metal and quartz where the reaction proceeds. In the case of the reaction of racemic butan-2-ol on Cu-/-quartz catalyst, (S)-(+)-butan-2-ol adsorbed and decomposed preferentially resulting in (-)-optical activity for the product mixture (Scheme 2.9.). 

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