Selectivity to Propylene

An excess of 2—10 mol propylene to liydiopeioxide is used to maximize conversion of hydroperoxide and selectivity to propylene oxide. Temperature is... 

EBHP is mixed with a catalyst solution and fed to a horizontal compartmentalized reactor where propylene is introduced into each compartment. The reactor operates at 95—130°C and 2500—4000 kPa (360—580 psi) for 1—2 h, and 5—7 mol propylene/1 mol EBHP are used for a 95—99% conversion of EBHP and a 92—96% selectivity to propylene oxide. The homogeneous catalyst is made from molybdenum, tungsten, or titanium and an organic acid, such as acetate, naphthenate, stearate, etc (170,173). Heterogeneous catalysts consist of titanium oxides on a siUca support (174—176). 

Direct Oxidation of Propylene to Propylene Oxide. Comparison of ethylene (qv) and propylene gas-phase oxidation on supported silver and silver—gold catalysts shows propylene oxide formation to be 17 times slower than ethylene oxide (qv) formation and the CO2 formation in the propylene system to be six times faster, accounting for the lower selectivity to propylene oxide than for ethylene oxide. Increasing gold content in the catalyst results in increasing acrolein selectivity (198). In propylene oxidation a polymer forms on the catalyst surface that is oxidized to CO2 (199—201). Studies of propylene oxide oxidation to CO2 on a silver catalyst showed a rate oscillation, presumably owing to polymerization on the catalyst surface upon subsequent oxidation (202). 

Noncatalytic oxidation of propylene to propylene oxide is also possible. Use of a small amount of aldehyde in the gas-phase oxidation of propylene at 200—350°C and up to 6900 kPa (1000 psi) results in about 44% selectivity to propylene oxide. About 10% conversion of propylene results (214—215). Photochemical oxidation of propylene with oxygen to propylene oxide has been demonstrated in the presence of a-diketone sensitizers and an aprotic solvent (216). 

A qualitatively similar behaviour was obtained during C3H6 epoxidation on Ag 43 Enhancement factor A values of the order of 150 were measured.43 Both the rates of epoxidation and oxidation to C02 increase with I>0 and decrease with I<0. The intrinsic selectivity to propylene oxide was very low, typically 0.03 and could be increased only up to 0.04 by using positive currents. This was again an exploratory study, as no reference electrode was used, thus T and UWr could not be measured 43... 

For both catalysts, as the reaction temperature increases the the selectivity to acrylonitrile (ACN) passes through a maximum at about 480°C. Maximum selectivity is about 30% and 60% for Sb=1.0 and 3.0, respectively. Increasing the temperature, decreases the selectivity to propylene ( 3=) and increases that to carbon oxides (COx). It may be noted that the lower selectivity to ACN in Sb V= 1.0 is... 

The reactor effluent is fractionated to. produce a high purity propylene stream and recycle ethylene and butylene, streams. Selectivity to propylene is greater than 98%. That is, 98% of the converted ethylene and butylene ends up as propylene so the process has few unwanted by-products. 

Figure 1 is the catalytic behavior of VSU545 in propane oxidative dehydrogenation to propylene. Selectivities to propylene in the range of60-80% are obtained up to propane conversions of about 20-25% and reaction temperatures up to around 450- 500 C. For higher reaction temperatures and conversions the selectivity decreases due both to the formation of carbon oxides and of aromatics. As compared to pure silicalite, a significant increase in both the selectivity to propylene and the activity in propane conversion is observed. 

Figure 2. Comparison of the catalytic behavior of VSil samples in propane oxidative dehydrogenation to propylene. Conversion of propane and selectivity to propylene at 470 C. Exp. conditions as in Fig. 1.

Figure 4. Comparison of the behavior of VSil545 in propane oxidative dehydrogenation using N2O or O2 as oxidizing agents. Exp. conditions as in Fig. 1. The dotted lines represent the propane conversion and propylene selectivity observed in the absence of the catalyst (homogeneous gas phase). The activity of the catalyst in the absence of O2 or N2O is similar to that observed in the homogeneous gas phase, but the selectivity to propylene (around 50-60%) is lower.

Smits, Seshan and Ross have studied the selective oxidative dehydrogenation of propane to propylene over Nb20s and Nb2C>5 supported on alumina.32 Vanadia supported on MgO has typically been used in these reactions, although reduced surface vanadyl ions can give rise to decreasing selectivity to propylene. Niobia on the other hand, is much more difficult to reduce than vanadyl but there have been few studies with this oxide in such reactions. 

When unsupported niobia is calcined above 500°C, selectivities to propylene as high as 85 % were observed albeit at conversions less than 2 mol % propane. Supported niobia on alumina gave selectivities as high as 62 % to propylene with conversion of about 1 mol % propane. The good selectivity of niobia in these systems may be related to the inertness of niobia with respect to vanadia and perhaps due to specific Nb-0 interactions in the unsupported oxide catalyst. 

Although the main routes to propylene oxide formation are not based on direct catalytic oxidation of propylene, the direct epox-idation of propylene on silver would be financially preferable if high yield and selectivity to propylene oxide could be achieved. Similarly to ethylene oxidation on silver part of the undesirable byproduct CO2 comes from the secondary oxidation of propylene oxide (2,3). The kinetics of the secondary silver catalyzed oxidation of propylene oxide to CO2 and H2O have been studied by very few investigators (2). 

Yields Process selectivity to propylene is typically greater than 98%. Overall conversion of 2-butenes can reach 90%. 

Yields Process selectivity to propylene is typically greater than 98%. Overall conversion of n-butenes is 85%-92%. Ethylene and butenes feedstreams can come from steam crackers or many refinery sources and in varying concentrations. Alternatively butenes can come from ethylene dimerization, which is also licensed by Lummus. 

It was found that the selective hydrogenation of methylacetylene over ion-exchanged Cu/SiOj produced only oligomeric material (referred to as green oil or foulant) and propylene, but no propane [11]. At temperatures > 160 C the selectivity to propylene Increased drastically and reached 80 % at 280 C. The selectivity increased even more (>99.3 %) when high amounts of propylene were introduced [11]. In order to obtain reliable data for deactivation, operations were carried out under sub optimal reaction conditions at low temperatures (130 "C - 160, where about 20 % olefines and 80 % green oil are produced. 

The behavior of the EDA complexes of graphite with potassium or sodium is also of interest, as the complexes not only chemisorb hydrogen, but also catalyze the hydrogenation of unsaturated hydrocarbons at room temperature. The hydrogenation of methylacetylene proceeded selectively to propylene, and then to propane. 

Table 1 shows both the initial propane conversion (Xg) and the selectivity to propylene (Sg), measured 4 min after the reaction started, and the deactivation parameter (X/Xg obtained in flow reaction experiments). This parameter is defined as the difference between the initial propane conversion (Xg) and the final conversion (X,) measured at 80 min of the reaction time, referred to the initial propane conversion for the different catalysts. 

Values of initial conversion (Xg), initial selectivity to propylene (Sg) and deactivation parameter (X/Xg) for the different mono and bimetallic catalysts. 

Pt/AljOj, but a much higher selectivity to propylene (95%) and a lower deactivation parameter (Table 1). This effect is also reflected in a lower coke deposition (1.95 wt% C for Pt/AljOjand 0.94 wt% for Pt/ZnAljO ), as is observed in TPO profiles (Figure 1). The different behavior of these two catalysts cannot be related to the metal dispersion, since the... 

The high selectivities for propylene which can be as high as 45 % (12) and the low selectivities for ethylene suggest that ethylene could be a primary product in Fischer-Tropsch which could undergo a secondary reaction leading selectivity to propylene. 

Preliminary results show complete selectivity to propylene oxide, suggesting a new route for the synthesis of this important epoxide in good yields under mild conditions. 

For a space-time of 0.074 g-s/pmol, the catalyst prepared at pH 8.5 is the most active with 14.9% propane conversion, two times more active than the catalyst prepared at pH 7.5. Due to this enhancement in propane conversion, the selectivity to propylene decreases from 65% to 60.1%. In addition, it can be observed that at the space-time mentioned above, the propane conversion and propylene selectivity for the catalyst prepared at pH 7.5 and 6.0 are almost the same. 

Increasing the space-time to 0.22 g-s/pmol the catalyst prepared at pH 8.5 remains the most active with a 22.4% propane conversion, followed closely by the catalyst prepared at pH 6.0 with 19.5%. Selectivities to propylene for these two catalysts are almost identical. 

Owing to the fact that the conversion of propane varies inversely with selectivity to propylene, for comparative purposes, we have decided to test the whole series of catalysts at almost the same conversions. So, the catalysts were tested at low (near 7%) and high propane conversion (almost 20%). The activity results corresponding to the different Ni-Co-molybdate samples tested at high conversions are shown in Table 6. At similar propane conversion, ( 20%), small differences in propylene selectivities and yields are observed for all samples. However, the catalyst prepared at pH 8.5 showed the highest productivity in propylene. 

For comparison reasons, the catalysts were tested at the same values of conversion (near 7% and 20%). For a fixed value of conversion, the catalysts showed almost the same level of selectivity to propylene, but working at very different space-time. Therefore, it can be concluded that the preparation pH plays a very important role over the catalytic properties. This study illustrates that an optimum value of precipitation pH of 8.5 provides a minimum space-time to obtain an equally active and selective catalyst (Table 6). 

The Pt black sample active for the epoxidation of 1-hexene and 2-hexenes was also tested in the oxidation of propylene. Figure 5 shows the results of propylene oxidation as a function of the applied voltage across the cell. The oxidation of propylene was initiated at an applied voltage higher than ca. 1.1 V. The formation of propylene oxide and acetone were remarkably enhanced at an applied voltage >1.1 V. The maximum oxidation efficiency for the propylene oxide was 25% and the selectivity to propylene oxide was 53% at 1.7 V. These results indicate that the epoxidation of propylene by the oxidative activation of H O proceeds with fairly good current efficiency and selectivity on the Pt black anode [17]. 

The data in Table 1 summarize catalytic activities for epoxidation of a variety of olefins over an unpromoted 5%Ag/Al203 catalyst. These data illustrate the preferential reactivity at the allylic position relative to addition of oxygen across the C=C bond. While the selectivity to ethylene oxide is typical for an unpromoted catalyst, the selectivities to propylene oxide and butylene oxides are non-existent for propylene, 1-butene, and 2-butene, respectively. In addition to small amounts of the selective allylic oxidation products (acrolein in the case of propylene and butadiene in the case of 1-butene), the only products are those of combustion. However, the results for butadiene reveal it is possible to epoxidize this non-allylic olefin at moderate selectivity and activity. What is not obvious from Table 1 is the short-lived nature of this activity. After 2-3 hours of reaction time, activity and selectivity typically decreased to approximately <1% conversion of C4H6 and approximately 50-75% selectivity to epoxybutene. A typical chromatogram of the activity of an... 

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