Propane conversions

Contributions of three types of Ga sites to propane conversion into aromatics were examined by using model catalysts, i.e., gallosilicate of MOR structure with deposited GaaOs particles. The rates of propane conversion and aromatics formation were correlated with the densities of three types of Ga sites determined by NH3-TPD, and it was shown that the propane conversion and the aromatics formation were limited by Ga sites on Ga20j surface. 

The propane aromatization was conducted under the differential condition by using Ga203/Ga-MOR catalysts thus characterized. The contributions of L, HI, and H2 sites to the propane conversion and the aromatics formation were estimated by assuming that the observed reaction rates are the sum of the reaction rate on each site which is equal to the product of the turnover frequency (TFij) and the amount of active sites per weight of catalyst (Aj) ... 

Table 1 Turnover frequencies of propane conversion and aromatics formation over L, HI and H2 sites of Ga203/Ga-M0R catalysts.

It is found that the CNF-HT has not catalytic activity for ODP. After oxidation, all the three samples show hi ly catalytic performances, which are shown in Fig.3. CNF-HL has the longest induction period among the three samples, and it has relatively low activity and propene selectivity at the beginning of the test. During the induction periods, the carbon balance exceeds 105% and then fall into 100 5%, which implies the CNF structure is stable and the surface chemistry of CNF reaches a dynamic equilibrium eventually. These results indicate that the catalytic activity of ODP can be attributed to the existence of surface oxygen complexes which are produced by oxidation. The highest propene yield(lS.96%) is achieve on CNF-HL at a 52.97% propane conversion. 

Fig.2. Propoae selectivity as a fimction of propane conversion over CNF-RA...

According to detailed XRD analyses, the two catalyst preparation procedures under study formed solid solutions. The application of sol-gel method led to improved selectivity to olefins in the reaction of propane ODH, compared to the simple procedure of evaporation and decomposition. However, the propane conversion on the sol-gel catalysts was lower at the same experimental conditions, while the catalysts surface area was higher. Moreover, the sol-gel samples presented higher basicity as shown by C02 TPD. It could be explained by a better incorporation of Nd into the AEO lattice, creating cationic vacancies for attaining electroneutrality and thus rendering the nearby oxide anions coordinatively unsaturated and more basic. 

In the direct ammoxidation of propane over Fe-zeolite catalysts the product mixture consisted of propene, acrylonitrile (AN), acetonitrile (AcN), and carbon oxides. Traces of methane, ethane, ethene and HCN were also detected with selectivity not exceeding 3%. The catalytic performances of the investigated catalysts are summarized in the Table 1. It must be noted that catalytic activity of MTW and silicalite matrix without iron (Fe concentration is lower than 50 ppm) was negligible. The propane conversion was below 1.5 % and no nitriles were detected. It is clearly seen from the Table 1 that the activity and selectivity of catalysts are influenced not only by the content of iron, but also by the zeolite framework structure. Typically, the Fe-MTW zeolites exhibit higher selectivity to propene (even at higher propane conversion than in the case of Fe-silicalite) and substantially lower selectivity to nitriles (both acrylonitrile and acetonitrile). The Fe-silicalite catalyst exhibits acrylonitrile selectivity 31.5 %, whereas the Fe-MTW catalysts with Fe concentration 1400 and 18900 ppm exhibit, at similar propane conversion, the AN selectivity 19.2 and 15.2 %, respectively. On the other hand, Fe-MTW zeolites exhibit higher AN/AcN ratio in comparison with Fe-silicalite catalyst (see Table 1). Fe-MTW-11500 catalyst reveals rather rare behavior. The concentration of Fe ions in the sample is comparable to Fe-sil-12900 catalyst, as well as... 

Figure 1 shows propane conversion and hydrogen production vs. the number of pulses injected. It can be seen that, although propane consumption is large already from the first pulse (figure 1 - left), hydrogen production is initially much smaller in the 2 wt.% Ga catalyst, and is actually zero with the 3 wt.% Ga catalyst (figure 1 - center). 

The oxidative dehydrogenation of propane to give propene catalyzed by TS-1, Ti-beta, Ti-MCM-41, Ti02-silicalite-l, or others was investigated by Schuster et al (259). TS-1 was the best catalyst, with a selectivity of 82% for propene at a propane conversion of 11% (Fig. 42). Sulfation of TS-1 by H2S04 prior to the reaction increased the conversion to 17%, with a selectivity of about 74%. Although conversion of propane was higher on Ti-beta and Ti-MCM-41, selectivity for propene was much lower C02 was the main product. Lewis acid sites were considered to be the major active sites (259). 

Fig. 15.11 (a),(b) Fraction of CNT catalysts combusted after 24 h time on stream in a 02/He gas mixture (a) CNTs, (b) 5 wt% P205/CNTs. (a),(b) Reprinted with permission from [25]. Copyright (2011) American Chemical Society, (c) Catalytic performance of B203-modified CNT catalysts in the ODH of propane. Propene selectivity at 5 % propane conversion ( ) and reaction rate (o) as a function of B203 loading, (d) Reaction scheme of CNT-cataiyzed ODH. (c),(d) Reprinted with permission from [61]. Copyright (2009) Wiley VCH. 

Figure 7.7 Propane conversion percentage versus time (bottom), coke level versus temperature (top), and Raman spectra as coke level increases (inset). Coke level builds to a preset limit, after which a regeneration process is completed and propane production begins again. Reprinted from Bennici et al. (2007) [79]. Copyright Wiley-VCH Verlag CmbH Co. KCaA. Reproduced with permission.

Reaction between methane and propane requires the right conditions. Firstly, it has a positive free energy of AG° = -1- 2kcalmoT at 150 °C for amethane/propane ratio of 1 but this can be overcome by increasing this ratio, which for a value of 1250 allows 98% propane conversion at 250°C. Secondly, it has to be separated from other reactions catalyzed by tantalum hydride, such as propane hydrogenoly-sis, leading to 1 equiv. of methane and 1 equiv. of ethane, or propane metathesis, leading to 0.5 equiv. of ethane ... 

II reaction under similar conditions at temperatures between 80 and 100°C and with a four-fold excess of 2-methylpentanal (to compensate for the low solubility), the selectivity for the Aldol II product (80%) was 20% higher in [BMIMJEF NaOH than in the water/NaOH system, both at 100% propanal conversion. The increased selectivity was attributed to the higher solubility of the reactant 2-methylpentanal in the ionic liquid phase than in the water phase. The higher solubility of 2-methylpentanal effectively suppressed the self-aldol condensation in the ionic liquid. 

Propane reaction. In a series of experiments propane (760 torr) reacted at 773 K over H-ZSM-5 (Si/Al = 15) and H-ZSM-5 modified with Ga or Pt. The conversion of propane was maintained at around 30% by adjusting the flow rate between 1 and 10 l.h , higher flow rates being used for the most active catalysts. The catalytic activities for the different solids were normalized to that of H-ZSM-5. The data are summarized in Table 1. It is apparent that the addition of Ga, Pt, Pt-Cu to the H-ZSM-5 zeolite increased its activity for the propane conversion. 

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.

The results of XRF, BET surface area measurements and catalytic testing are summarized in Table I, while the results of catalytic testing are also shown in Figure 1. The rates displayed in Table I are the rates of reaction of propane per unit surface area as calculated from the interpolated propane conversion. Propane conversion was 3% or less, while oxygen conversion was less than 30%. 

The dependence of selectivity for propene on propane conversion for the better catalysts (Fig. 2) follows a trend similar to that for ethane oxidation. 

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