Propene selectivity

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. 

TPSR results are presented in Fig. 4. Propene is produced when the sample temperature is above 350 TC on both samples, which means converting of propane over CNF catalysts could occur without oxygm. The desorption products amounts are 0.35 and 0.26 mmol/g for CNF-RA and CNF-HA respectively while the percentages of propene in llie desorption substances over these two sample are 51.4% and 87.7%. These results imply that the propene selectivity may increase, at least partly, due to restriction of oxidation of propane to COx by heat treatment at the cost of catalytic activity. 

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. 

Oxide catalyst Temp. (°C) Propane conv. (%) Propene select. (%) Ref. 

A very interesting finding can perhaps modify the vision we have presently of the reaction. In the case of the homogeneous reaction, we found that a partial pressure of water in the feed promotes propane conversion. Fig. 10 shows the dramatic difference [98]. This makes the performance of the homogeneous reaction at a given temperature very close to those of the catalysed reaction at this temperature. An interesting observation is that the production of byproduct ethylene is very little affected by conversion and almost not at all by the presence of water [98]. Fig. 11 gives propene selectivity as a function of propane conversion [98]. This seems to exceed the performances indicated by Burch and Crabb. It is not yet known whether similar effects could take place in catalysed reactions. 

The 1 1 Sb-V and 1 1 5 Nb-V-Si systems only were tested at the different space velocity of 100 ml min" g". Almost all the prepared systems exhibited propane conversions of about 30% and propene selectivities higher than 20%. The most selective catalysts with respect to propene formation were the 1 1 Nb-V prepared via the hydrolytic method and 1 1 Sb-V systems (Scshs ca 40%). The 1 1 Sb-V and 1 1 5 Nb-V-Si prepared via the non-hydrolytic method gave the best results in terms of propane conversion and yield in propene. In these two cases, while the higher conversion is in contradiction with... 

Figure 1. Propene yield as a function of Figure 2. Propene selectivity at 20% propane...

The conversions of propane to propene have been studied on the CeNix mixed oxides as a function of temperature. Appropriate blank runs showed that, under our experimental conditions, the contribution of the gas phase reaction is negligible. On Figures 1 and 2, the evolutions of the conversion and selectivity as a function of temperature obtained on CeNio.2 and CeNio.s at the stationnary state are presented as examples. For the sake of comparison the catalytic activity of Ce02 was also evaluated. Propene and CO2 were the only products detected. No CO was observed whatever the temperature. On Ce02, at 373 K a propane conversion of 3% is observed with a selectivity to propene of 1.6%. As a function of temperature the conversion and selectivity increase and at 673 K, a propane conversion of 10% is obtained with a propene selectivity of 6%. 

As many as 10 patents have been assigned to ARCO Chemical, all describing improvements of the Union Carbide Patent for the epoxidation of propene. Selectivities as high as 50% have been claimed, in the range of 2-5% conversion, based on a 10% propene and 5% oxygen feed. The following modifications have been claimed ... 

Various borosilicates have been reported in the methanol conversion process. In a study reported in 1984, Holderich gave details for the preparation of propene selectively from methanol using a borosilicate molecular sieve of the MFI structure type (10 ). Autocatalysis was observed when small amounts of olefin were added to the feed. Modification of the borosilicates using HF, HC1, or extrusion with amorphous silica-alumina led to changes in the observed product distribution to yield more C2-C4 olefins. Use of borosilicates of the MOR and ERI structure types for methanol conversion was reported by lone, et al. (28). The selectivity to olefins was improved for borosilicates with these structures relative to the silicate of the same structure and aluminum impurity level. 

The model of the ODH of propane on NiMo04 catalyst, based on a simplified mechanism taking into account the formation of propene and its further oxidation to carbon oxides may be used both with transient and steady state results. In the first case, the evolution of partial pressures of propene and carbon oxides (lumped into COJ is reproduced. When applied to steady state results, the model gives a good representation of the reaction rate, but overestimates the propene selectivity. It should be stressed, that the mechanism proposed takes into account the participation of the lattice oxygen only, both in the formation of propene and in its destruction. Under steady state conditions, where the oxygen gas is present, its participation seems restricted to the propane destruction (but is not significant in the propane activation). 

Propane conversions and propene selectivities over rare-earth orthovanadates (La, Pr, Yb, Er, Sm, Ce, Tb, Nd) at 673 K suggests that the activity (conversion) was higher with Er and Sm orthovanadates ( 11.5%) with 29% propene selectivity [54]. Fang et al. [55] studied a series of rare-earth orthovanadate catalysts for the low-temperature ODH of propane. At 773 K and for a propane/oxygen molar ratio of 2 1 Y-doped VO4 was able to form propene with a selectivity of 49% at 23% conversion [55,56]. Michaels et al. [15] carried out the ODH of propane over Mg-V-Sb oxide catalysts and observed that the propene selectivity decreases from 75 to 5% as the propane conversion increases from 2 to 68%. 

Various Nb containing catalysts were also studied for the ODH of propane. Smits and coworkers [61] observed that niobium oxide shows a very high selectivity in this reaction although the conversion was very low. The activity of the Nb containing catalysts was improved without diminishing its selectivity by adding elements, such as V, Cr, or Mo, all of which are reducible transition metals. For a series of V-based catalysts and at isoconversion levels ( 30% conversion), the propene selectivity for 1 1 Nb-V and Sb-V catalysts was 20 and 40%, respectively. The 1 1 Sb-V and 1 1 5 Nb-V-Si prepared via the nonhydrolytic method offer the best results in terms of propane conversion and propene yield. With the 1 1 5 Nb-V-Si catalysts the propene productivity of 0.23 kg of propene per kilogram of catalyst per hour was achieved at 823 K [62]. 

Though V-P-0 catalysts are nonselective for ODH of propane, vanadium aluminophosphate (VAPO) catalysts displayed higher propene selectivity, especially for samples with low vanadium content. Moreover, ALPO-5 has an initial selectivity of about 100%, which is associated to the specific stmcture that promotes the accessibility of propane to the dehydrogenation sites, thus contributing to the high selectivity of VAPO catalysts [63]. The vanadium species in the framework of the ALPO-5 was considered as more selective than the polymeric V species. As for their total activity, it appears to be associated with the presence of the vanadium species. Other phosphate containing catalysts have also been tested, such as the manganese pyrophosphate, which shows a propene yield of 16% at 823 K [64]. 

The ODH of propane over titanium and vanadium containing zeolites and nonzeolitic catalysts revealed that Ti-silicalite was the most active. The addition of water caused an increase in selectivity, probably due to a competitive adsorption on the active sites. The reaction is proposed to occur on the outer surface of the Ti-silicalite crystallites on Lewis acid sites, and a sulfation of the catalyst, which increases the acidity of these sites, results in a further increase of the catalytic activity. The maximum conversion obtained was 17% with a propene selectivity of up to 74% [65]. Comparison of propane oxidation and ammoxidation over Co-zeolites shows an increase in conversion and propene selectivity during ammoxidation. For a conversion of 14%, 40% propene selectivity was obtained with ammonia, whereas, at 10% conversion the propene selectivity was only 12% with oxygen. The increase in activity and selectivity can be due to the formation of basic sites via ammonia adsorption [38]. 

Ce-Ni-0 catalysts have been found to be active and selective at relatively low temperatures of about 573 K. For the CeNil precipitated catalyst ( Ni/ Ce (bulk) = 1), the yield of propene amounted to 11% with the selectivity of about 60%. The potassium addition to the Ce-Ni-O impregnated sample led to an increase in the propene selectivity but a decrease in conversion [73] in accordance with the use of alkali dopants discussed previously. 

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