Selective propylene

A process to produce propylene by VPO of propane was patented in the former USSR in 1987 (146). Similar processes have the potential to coproduce hydrogen peroxide. Yields of hydrogen peroxide as high as 1 mol/mol propylene produced have been reported with 60—70% propylene selectivity (147). 

ZSM-5 Methanol to propylene Higher propylene selectivity Higher propylene ethylene ratio [69]... 

ZSM-5 Cracking of n-octane Higher activity Higher propylene selectivity Reduced oligomerization [71]... 

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 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.

Figure 1 Oxygen conversion and propylene selectivity as a function of temperature for niobia with various additives.

Entry Catalyst Productivity Propylene Oxide/ Polyethylene Glycol Selectivity (%) Propylene Selectivity (%)... 

Highly propylene selective catalysts have been developed to meet this challenge. These catalysts, referred to as FCC olefin additives , are generally used in admixture with more traditional FCC catalysts. These additives are based on the MFI (H-ZSM-5) zeolite and the effect of the additive level, which, combined with higher temperatures increases C3 (Figure 5.21) and C4 (Figure 5.22) olefinicity. For instance, at a temperature of 566 °C and 32% Olefin additive, the propylene yield can reach 15% of the feed. 

However, this level of deactivation will be overcome by the catalyst make-up required to replace material lost to physical attrition in the fluidized bed reactors. A slight shift in the ethylene and propylene selectivities was also observed during the initial portion of this run. Characterization of the catalyst during the multi-cycle test shows that there is no change in the microporosity of the SAPO-34 molecular sieve,... 

Olefin selectivities also decrease with increasing bed residence time and chain size on Ru catalysts (4,14). For example, propylene selectivity decreases with increasing bed residence time without a corresponding increase in propane selectivity, leading to a net decrease in the fraction of the converted CO that appears as C3 molecules (Fig. 7b). Readsorbed olefins initiate chains that continue to grow and ultimately desorb as larger olefins or paraffins, ( alitative trends are similar on all supports and on both Ru and Co catalysts. The selectivity details depend on the support physical structure, on the density of exposed surface metal atoms, and on the intrinsic readsorption properties (j8r) of Co and Ru surfaces. 

Fig. 4. Propylene selectivity vs. Space-lime over Ni(i Co

Figs. 3 and 4 show the effect of space-time (W/Fc3Hs) on propane conversion and propylene selectivity over the tested set of catalysts. The selectivity of propylene decreases when propane conversion increases due to the consecutive transformation of the propylene formed to COx products. 

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. 

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. 

As is typical of oxidation catalysts, the selectivity to useful products declines with conversion, as seen for the conversion of propane to propylene (Figure 2) and the conversion of propylene to acrolein (Figure 3), respectively. From these results, measured at low conversions under essentially differential reactor conditions, it is apparent that propylene is the sole primary product of propane oxidation over this catalyst, since extrapolation to zero propane conversion results in 100 percent propylene selectivity, while the conversion to CO (i.e. CO and CO2) waste products at zero propane conversion extrapolates to zero COx selectivity. 

An important example of this kind is a contribution of cracking processes to partial oxidation of propane and higher hydrocarbons. In particular, in the case of catalytic propane ODH, the formation of lower hydrocarbons—first of all ethylene and methane—can substantially reduce propylene selectivity. The analysis of possible homogeneous and heterogeneous pathways of C-C bond breaking can provide valuable guidelines for further improvement of catalyst formulation and/or overall process design. 

However, equilibrium between olefins of 3+ carbon atoms is almost certainly established under conditions of methanol conversion (ref. 11). Thus the olefins observed are products of the quasi-equilibrium, and their relative amounts are determined by the position of equilibrium and rates of diffusion. One expects increase in partial pressure to favour higher olefins and increase in temperature to favour lower olefins. It is clearly not possible for methanol conversion to give propylene selectively or butenes selectively. Equally clearly, it is difficult to obtain evidence on the mechanism of methanol conversion from labelling experiments if carbon and hydrogen are scrambled rapidly in the olefinic products. 

Influence of the addition of silica, as a binder at a concentration of 10 or 50 wt%, to H-gallosilicate (MFI) zeolite on its inter- and intracrystalline acidity, initial activity, product selectivity and distribution of aromatics formed in the propane amortization (at 550°C) and also on its deactivation due to coking in the aromatization process has been thoroughly investigated. Silica binder caused an appreciable decrease in the zeolitic acidity (both external and intracrystalline acid sites) and also in the propane conversion/aromatization activity. Because of it, the deactivation due to coking of the zeolite in the propane aromatization is, however, decreased. The deactivation rate constant for the initial fast deactivation is decreased but that for the later slow deactivation is increased because of the binder. The aromatics selectivity for aromatics and para shape selectivity of the zeolite, particularly at lower conversions, are increased but the propylene selectivity and dehydrogenation/cracking activity ratio are decreased due to the presence of binder in the zeolite catalyst. 

More to the point of this study is that propylene selectivity at atmospheric pressure is almost constant at approximately 38 mole % across the entire isobutane conversion range (Figure 4). Isobutane cracking could be used to produce chemical propylene directly, and the justification to do this instead of producing propylene for alkylate is only a function of price. The price that propylene would have to command to justify its chemical sale, rather than alkylate use, is basically its gasoline value, which is the 3.0 cents/lb figure developed earlier in this paper. 

Figure 4. Pyrolysis of isobutane. Propylene selectivity is almost constant at atmospheric pressure.

Propane conversion is only 54%, and a propylene selectivity and recovery of 76 and 93 mole %, respectively, result in a propylene yield of 38%. Neither methylacetylene nor allene is listed among the reaction products, and coke production amounts to 4% of propane converted. 

By modifying the ZSM-5 catalyst with Mg, ° the stronger acid sites were eliminated, but the number of weaker Lewis acid sites was increased. The Mg-ZSM-5 catalyst was more selective toward propylene. This was alo observed by Juan, who attributed the higher propylene selectivity to the presence of the relatively weak acid sites, active in the /3-scission of carbenium ions. 

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