Butane Conversion

The highly exothermic nature of the butane-to-maleic anhydride reaction and the principal by-product reactions require substantial heat removal from the reactor. Thus the reaction is carried out in what is effectively a large multitubular heat exchanger which circulates a mixture of 53% potassium nitrate [7757-79-1/, KNO 40% sodium nitrite [7632-00-0], NaN02 and 7% sodium nitrate [7631-99-4], NaNO. Reaction tube diameters are kept at a minimum 25—30 mm in outside diameter to faciUtate heat removal. Reactor tube lengths are between 3 and 6 meters. The exothermic heat of reaction is removed from the salt mixture by the production of steam in an external salt cooler. Reactor temperatures are in the range of 390 to 430°C. Despite the rapid circulation of salt on the shell side of the reactor, catalyst temperatures can be 40 to 60°C higher than the salt temperature. The butane to maleic anhydride reaction typically reaches its maximum efficiency (maximum yield) at about 85% butane conversion. Reported molar yields are typically 50 to 60%. 

Butadiene has the advantage of a relatively low heat of reaction (995 kJ/ mol compared with 1875 kJ/mol in the oxidation of benzene), but the disadvantage of a relatively high price compared with the other -C4 hydrocarbons. Good prospects has the n-butane route. Keeping the n-butane conversion at about 15%, the yield of maleic acid anhydride amounts to 50-60 mol %. 

Figure 2 Conversion - time curves on several Ni - promoted catalysts under n-C4Hio/He mixture. n-Butane conversion to isobutane as a function of time on stream over 0 4 g cat., at 150°C, n-C4Hio flow rate = 5.46 cmVmin, He flow rate = 10.4 cm7min. 3NiSZ(s) (squares) 2NiSZ(s) (triangles) INiSZ(s) (circles).

Figure 5. n-Butane conversion to isobutane as a function of temperature in a temperature programmed reaction experiment conducted over 0.4 g of INiSZ(s) catalyst under an n-butane/ hydrogen mixture (n-C4 molar fraction = 0.34) at a constant heating rate of 2C/min... 

Catalyst Treatment CO/M Temperature (K) Butane conversion (%) Product distribution (mol%) CH4 C2H6 C3H8 ... 

Figure 55.2. Effect of time-on-stream on -butane conversion (O ) and selectivity to MA ( ) of catalysts P/V 1.00 (open symbols) and PA 1.06 (full symbols) at 380°C after treatment of the equilibrated catdysts with air at 380°C for 1 h.

Aromatization activity of gallium containing MEl and TON zeolite catalysts in n-butane conversion effects of gallium and reaction conditions. Appl. Catal. A, 316, 61-67. 

Figure 15.22 Barcharts of (a) butane conversion (%) and (b) product selectivity in n-butane hydrogenolysis on RhPt catalysts supported on Si02, FSM-16 and HMM-1 with and without scCO, treatment.

The oxidation of butane on these orthovanadates were tested at 500°C in a flow reactor using a butane oxygen helium ratio of 4 8 88. The observed products were isomers of butene, butadiene, CO, and CO2. The carbon balance in these experiments were within experimental errors, thus the amount of any undetected product if present should be small. The selectivity for dehydrogenation (butenes and butadiene) was found to depend on the butane conversion and be quite different for different orthovanadates. Fig. 4 shows the selectivity for dehydrogenation at 12.5% conversion of butane [15,18,19]. Its value ranged from a high of over 60% for Mg3(V04)2 to a low of less than 5% for... 

Figure 4. Dependence of selectivity for oxidative dehydrogenation of butane over orthovanadates on the reduction potential of the cations. Reaction conditions 500°C, butane/Oz/He = 4/8/88, butane conversion = 12.5%.

VPO catalyst selectivity is tested by both fixed-bed microreactor measurements and by pulsed microreactor measurements. In the former, the rate constants are measured in a microreactor on about 1 g of catalyst at temperature between 360 and 390 °C in a 1.5% butane/air environment. The pulsed microreactor evaluations are carried out by injecting 0.05 ml pulses of butane using a gas-sampling valve over about 0.5 g of catalyst in a microreactor heated to about 380 °C. /i-butane conversion and selectivity to maleic anhydride (MA)... 

In sill C MAS NMR spectroscopy has also been applied to characterize the scrambling in n-butene conversion on zeolite H-ferrierite (97), n-butane conversion on SZA (98), -butane isomerization on Cs2,5Ho.5PWi204o (99), n-pentane conversion on SZA (100), isopropylation of benzene by propene on HZSM-11 (101,102), and propane activation on HZSM-5 (103-105) and on Al2O3-promoted SZA (106,107). The existence of carbenium ions was proposed to rationalize the experimental scrambling results observed by in situ MAS NMR spectroscopy. 

Butane from natural gas is cheap and abundant in the United States, where it is used as an important feedstock for the synthesis of acetic acid. Since acetic acid is the most stable oxidation product from butane, the transformation is carried out at high butane conversions. In the industrial processes (Celanese, Hills), butane is oxidized by air in an acetic acid solution containing a cobalt catalyst (stearate, naphthenate) at 180-190 °C and 50-70 atm.361,557 The AcOH yield is about 40-45% for ca. 30% butane conversion. By-products include C02 and formic, propionic and succinic acids, which are vaporized. The other by-products are recycled for acetic acid synthesis. Light naphthas can be used instead of butane as acetic adic feedstock, and are oxidized under similar conditions in Europe where natural gas is less abundant (Distillers and BP processes). Acetic acid can also be obtained with much higher selectivity (95-97%) from the oxidation of acetaldehyde by air at 60 °C and atmospheric pressure in an acetic acid solution and in the presence of cobalt acetate.361,558... 

Bluhm H, Havecker M, Kleimenov E, Knop-Gericke A, Liskowski A, Schlogl R, Su DS. In situ surface analysis in selective oxidation catalysis n-butane conversion over VPP. Topics in Catalysis. 2003 23(1) 99—107. 

Discrete bands characteristic of surface hydrocarbon species have been detected during butane conversion catalyzed by zeolites, for example, in the reaction of /(-butane catalyzed by H-mordenite at 573 K (Tzolova-Mtiller et al., unpublished). The spectra in Figure 15 include bands at 293, 330, 400, and about 455 nm. 

Note that although the conversion of 7.11 to 7.12 assumes anti-Markovnikov addition, the Markovnikov product also gives butane. Conversion of 7.9 to 7.11 could also take place by prior coordination of alkene followed by the oxidative addition of dihydrogen. Indeed this parallel pathway for the formation of 7.11 does operate. Like the equilibrium shown between RhClL3, 7.9, and the dimer [RhClL L, there is an equilibrium between 7.9 and the alkene coordinated complex RhCl(alkene)L2. 

The main objective of the present work was to investigate the possibilities of direct (and selective) n-butane dehydroisomerisation into isobutene over Ga-containing zeolites. Another objective was to evaluate the role played by Ga and acid sites in this reaction. For this work such medium pore zeolites, as ferrierite (FER) and theta-1, were chosen because of their superior performance in n-butene isomerisation reaction.3,7 The modifying metal, Ga, was chosen due to the known high dehydrogenation activity of Ga-ZSM-5 catalysts in propane and n-butane conversions. 10 However, Ga-ZSM-5 catalysts were not used in this study because of their high aromatisation activity,8,9 which would not allow to stop the reaction at the stage of formation and isomerisation of butenes. 

Investigation of n-butane conversion over H-forms of the ferrierite and theta-1 zeolites demonstrated that the isobutene selectivities were similar (and low) for these catalysts. The maximum selectivities (7-8 %) were obtained at low n-butane conversions (5-10 %) and decreased with increasing conversion of n-butane due to olefin interconversion and aromatisation reactions. Isobutene was in equilibrium with the other butene isomers due to the high isomerisation activity of the parent zeolites. The maximum selectivity to butenes, which was observed at low conversions, was around 20 %. This value reflects a moderate contribution of the dehydrogenation steps in n-butane transformation over H-forms of the ferrierite and theta-1 zeolites and indicates an important role of the n-butane protolytic cracking steps over these two catalysts. 

Figure 1 Concentrations of the initial reaction products as functions of n-butane conversion over H-FER and Ga-FER catalysts.

Table 1 Maximum Selectivities to Butenes and Isobutene Observed During n-Butane Conversion over Ga-Theta-l and Ga-FER Catalysts...

In order to get better understanding of the role of gallium and acid sites in n-butane transformation over Ga-containing catalysts, we have considered the rate data obtained over H- and Ga-theta-1 catalysts. These catalysts were chosen, since they produced much better results when compared with the ferrierite-based catalysts. Consequently, the activities of the theta-1 catalysts in the initial n-butane dehydrogenation and cracking steps were determined. This was done by the extrapolation of the rate data on formation of the primary reaction products (hydrogen, methane and ethane) to zero n-butane conversions, as shown in Figure 3. 

Table 2 Rates (mmol/(g h)) of Formation of the Initial Products of n-Butane Conversion over H-Theta-1 and Ga-Theta-1 Catalysts...

Conversion of n-butane into isobutene over theta-1 and ferrierite zeolites was studied in a continuous flow microreactor at 530°C and 100% n-butane as a feed. The zeolites were used as catalysts in the H- and Ga-forms. Insertion of Ga into the zeolites resulted in improved isobutene selectivities due (i) to an increase in the dehydrogenation activities and (ii) to a decrease in the cracking activities of the catalysts. The highest selectivities to isobutene (-27%) and butenes (-70%) were obtained with the Ga-theta-1 catalyst at n-butane conversions around 10%. These selectivities decreased with increasing conversion due to olefin aromatisation, which was enhanced considerably by the Ga species present in the catalysts. 

Description N-butane and air are normally fed to a fluidized-bed reactor in the presence of a catalyst to produce maleic anhydride. In this process option, the reactor (1) is operated at a lower butane conversion by either reducing the reaction temperature or by increas-... 

The occurrence of consecutive reactions, leading to combustion, which lower the selectivity to MA when the alkane conversion, is increased. At n-butane conversions, up to 60-70%, the extent of the consecutive reaction to give combustion products is not substantial, but the decrease in selectivity becomes dramatic when the conversion exceeds 70-80%. This observation has been attributed to the development of local catalyst overheating associated with the highly exothermic oxidation reactions and to the poor heat-transfer properties of the catalytic material. This problem is obviously more important in fixed-bed rather than mixed (fluidized) reactors, in which the heat transfer is faster. 

The promotional effects of cobalt (148,150,165,167,169,173,179,182,191-203) and iron (66,148,166,167,171,176,179-181,193-195,201,204-207) have been widely investigated recently. Abdelouahab et al. (193) considered the effects of these promoters on the structure of catalysts prepared with organic solvents. Both cobalt and iron promoters were found to increase the selectivity to MA the butane conversion was found to decrease with cobalt promoters and increase with iron promoters. 

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