N-butane Conversion Research Articles

A new catalyst for n-butane isomerization: persulfate-modified Al 2O 3–ZrO 2

The n-butane isomerization reaction on Al-promoted zirconia catalysts modified by persulfate was studied both at low temperature in the absence of hydrogen and at high temperature in the presence of hydrogen. The addition of Al shows a significant enhancement in the activity and stability of the samples for n-butane isomerization at 250°C. After running on stream for 200 h, the n-butane conversion on the catalyst containing 3 mol% Al 2O 3 remains steady at 72% of its equilibrium conversion and no observable trend of further deactivation has been observed. The different behavior of the with or without Al-promoted catalysts at low temperature and high temperature is probably associated with a change of reaction mechanism from bimolecular to monomolecular. Microcalorimetric measurements of ammonia adsorption on catalysts reveal that the remarkable activity and stability of the Al-promoted catalysts are caused by an appropriate distribution of acid site strengths and an enhancement in the number of acid sites with intermediate acid strengths effective for the n-butane isomerization reaction.

Isomerization of n-butane over acid zeolites : Role of Broensted and Lewis acid sites

Skeletal isomerization of n-butane to isobutane was studied over HM, HZSM-5 and H-beta zeolites. The Lewis/Broensted ratio of acid sites was changed by vacuum treatment of the samples to 650°C. γ-Alumina and CoAlPO containing considerable portion of Lewis acid sites were also employed. It was found that the increased number of Lewis acid sites results in a substantial suppression of the n-butane conversion as well as selectivity to isobutane.

Selective oxidation of light hydrocarbons over promoted vanadyl pyrophosphate

The catalytic behaviour of vanadyl pyrophosphate in the conversion of either n-butane, 1-butene, n-pentane or 1-pentene is reported. The results obtained indicate that the product selectivity and hydrocarbon conversion can be changed by varying the temperature and GHSV. The choice of doping elements has a significant effect on hydrocarbon conversion and product yield, much more than surface area. The maleic anhydride yield increases strongly with an increase in catalyst acidity when n-butane and n-pentane are feed materials. With the corresponding olefins the effect due to acidity is insignificant.

Formation of Nitrogenous Compounds in the Photooxidation of n-Butane under Atmospheric Conditions

The photooxidation of n-butane under atmospheric conditions in the presence of NOx resulted in the formation of the following nitrogenous products: peroxy acetyl nitrate 23, sec-butyl nitrate 16, n-butyl nitrate 1.3, ethyl nitrate 1.3, peroxy n-butyryl nitrate 1.3, and peroxy propionyl nitrate 0.5% of the initially added odd nitrogen. In addition, an electron capturing compound eluting at the retention time of sec-propyl nitrate was also observed accounting for 5% of initial NOx. Butan-2-one was the major product of conversion of n-butane with a yield of 37%. From product ratios it is evident that the formation of sec-butyl nitrate is favored over that of n-butyl nitrate by a factor of 2.1. The rate of reaction of sec-butoxy radicals with oxygen is equal to their decomposition rate.

Synthesis and Characterization of Sulfated Titania Solid Acid Catalysts

Sulfated titania catalysts were prepared by precipitation of titanic acid from titanium tetrachloride using aqueous ammonia, followed by impregnation with sulfuric acid. The sulfate concentrations in the catalysts were in the range 5−15 wt %. These sulfated catalysts were calcined at temperatures in the range 450−650 °C. The characteristics of these sulfated titania catalysts were examined using BET, SEM, powder XRD, ICP−AES elemental analysis, FT-IR, TPD of NH3, TPD of pyridine, and 1H MAS solid state NMR measurements. The activities of these catalysts were evaluated using n-butane isomerization studies to produce isobutane in a fixed-bed microreactor operated at 1 atm, 100−250 °C, and a space velocity (WHSV) of 5−15 h-1. The n-butane conversion and isobutane selectivity were at a maximum with a sulfate concentration of 12.5 wt % and a reaction temperature of 150 °C. Results showed a linear relationship between the acidity of the catalysts and their isomerization activity. The isomerization activity of sulfated titania catalysts were compared with those of other solid acid catalysts such as sulfated zirconia and Pt−HZSM-5 catalysts. This study indicated that isobutane selectivity as well as the acidity increased in the order sulfated zirconia ≫ sulfated titania > Pt−HZSM-5.

Catalytic and non-catalytic oxidative dehydrogenation of n-butane

The thermal pyrolysis and the catalytic and non-catalytic oxydehydrogenation of n-butane were investigated. It was found that, under optimum conditions, the non-catalyzed reaction can be very efficient and selective. The effect of temperature, butane/oxygen ratio and residence time was studied and it was concluded that high selectivity in butenes can be achieved at relatively low temperatures and low butane/oxygen ratios. Conversion of n-butane up to 90%, at 580°C, and 1.8 s residence time with an overall alkenes C 2–C 4 selectivity exceeding 55% were observed. Based on experimental results, it is suggested that oxygen participates in the reaction mechanism of butane decomposition not only in the initiation steps but in the propagation steps as well. Oxidative dehydrogenation was also examined in the presence of mixed oxide catalyst comprising 30 wt% V 2O 5 and 70 wt% MgO. It was observed that the catalyst is very active and quite selective in desirable products at reaction temperatures (540–580°C), similar to those applied in non-catalytic experiments. Very promising results were obtained when n-butane and oxygen were fed over a catalytic bed (VMgO catalyst) combined in series with a void volume (post-catalytic). High conversion of n-butane was achieved with high C 2–C 4 selectivity and relatively low CO x selectivity.

The influence of acidity on zeolite H-BEA catalyzed isobutane/n-butene alkylation

The influence of the concentration of acid sites for isobutane/n-butene alkylation on zeolite BEA with varying degrees of Na+ ion exchange is reported. All catalysts studied showed complete n-butene conversion over a significant time-on-stream. Isooctanes were the dominating products over H-BEA, while the importance of di- and multiple alkylation increased with increasing Na+ concentration in the zeolite. In parallel, the integral number of turnovers per catalytic site markedly decreased. The acid site deactivation is affiliated with blocking by hexadecane species. The higher rate of catalyst deactivation with increasing degree of Na+ exchange is thus attributed to the increase in the probability of having a further butene molecule added to a C8 alkoxy group in place of undergoing hydride transfer from isobutane to desorb as isooctane. This is a direct result of the decreasing concentration of alkoxy groups that can react with n-butene while maintaining the same rate of olefin transport through the pores. Addition of a strong hydride donor/acceptor molecule (adamantane) indicates that the product composition of the isooctane isomers is determined by their lifetime as alkoxy groups. The longer the lifetime before experiencing hydride transfer, the closer the product composition approaches the chemical equilibrium.

Elementary Mechanistic Steps and the Influence of Process Variables in Isobutane Alkylation over H-BEA

Liquid phase conversion of n-butene in excess iso-butane was investigated over zeolite BEA as catalyst in a continuously operated slurry reactor. Single and multiple alkylation and cracking were the main reaction pathways. Only saturated products were observed indicating that hydride transfer reactions were fast, compared to desorption of olefins under the experimental conditions chosen. An optimal reaction temperature of 350 K was identified for single alkylation, higher reaction temperatures favoring cracking of intermediately formed products and lower temperatures, multiple alkylation. Under optimum reaction conditions over 80 wt% butene was converted via single alkylation. The C8product selectivity is determined by the balance between isomerization of the C8alkoxy and the hydride transfer to release iso-octanes from the acid sites. Once desorbed from an acid site, the alkanes do not isomerize. Independent of the space velocities the catalyst and, hence, the individual acid sites deactivate after approximately 30 butene turnovers at 350 K by deposition of polyalkylates. A simplified reaction model for alkylation over solid acid catalysts and the implications for catalyst design are discussed.

Oxidative dehydrogenation of n-butane on MgO-supported vanadium oxide catalysts

Vanadium–magnesium oxide (VMgO) catalysts have been prepared, characterized and tested in the ODH of n-butane. The catalysts were prepared by two variations of the wet-impregnation method using aqueous ammonium metavanadate or organically-based methanolic vanadyl acetylacetonate solutions. The catalyst surface area depends on the vanadium content and the preparation method. Catalyst characterization (i.e. XRD, 51V NMR, FTIR, LRS) results indicate the presence of poor crystalline Mg-orthovanadate (Mg 3V 2O 8), while the presence of other Mg-vanadates is not clear. Oxygen isotopic-exchange experiments on VMgO catalysts indicate an R 2 process: [ 18O 2+2 16O 2− (s) → ( 18O 18O 16O 16O) 4− (s) →2 18 O 2− (s) + 16 O 2(g) ] at temperatures higher than 1023 K, while an R 0-type [ 18O 2(g)+ 16O 2(g)→2 16O 18O (g)] is observed at 823 K, as a consequence of a small activity for homophase isotopic. Both the catalytic activity and the selectivity to oxydehydrogenation products depend on the vanadium content but is independent on the catalyst preparation method. This behavior is observed in the ODH of n-butane with both O 2 and N 2O. However, while conversion of n-butane was higher when using O 2 as oxidant, the selectivity to C 4-olefins was higher with N 2O as oxidant. Pulse experiments show that prereduced surfaces are not effective in producing olefins while selective catalysts are achieved with preoxidized surface. A mechanism for the oxidative dehydrogenation of n-butane is proposed.

Oxidative dehydrogenation of n-butane on V/MgO catalysts. Influence of the type of contactor

A comparative study of the catalytic performance of a selective V-Mg-O catalyst in the oxidative dehydrogenation of n-butane is presented using three different types of reactor: (i) an adiabatic fixed-bed reactor; (ii) a fluidized-bed reactor; and (iii) an in situ redox fluidized-bed reactor. The results obtained indicate that the in situ redox fluidized-bed reactor outperforms the conventional fixed- and fluidized-bed reactors, especially at high n-butane conversions. Thus, a selectivity to C4 olefins of 54% at n-butane conversions of 60% was achieved at 550°C using an in situ redox fluidized-bed reactor while selectivities to C4-olefins lower than 43% were obtained on the other reactor types under the same reaction conditions (isoconversion and reaction temperature).

Sulfated Zirconia and Iron- and Manganese-Promoted Sulfated Zirconia: Do They Protonate Alkanes?

Because of their high activities for alkane conversion, sulfated zirconia and iron- and manganese-promoted sulfated zirconia have been the objects of much recent attention as a possible next generation of solid acid catalysts for alkane conversion. These catalysts have been suggested to be superacidic on the basis of measurements with adsorbed Hammett indicator bases, but published data determined with other adsorbed bases indicate only moderately strong acid sites. The indicator methods are limited by the opaqueness of the materials and by the inability of the methods to probe a possible set of minority sites that might be responsible for the reactivity and catalytic activity for alkane conversions. Another approach to the challenge of estimating the acid strengths of the reactive and catalytic sites is to investigate the reactivities and catalytic activities of the materials for reactions which, for initiation, require donation of protons from a solid acid to a very weakly basic reactant such as an alkane. Such a test reaction is the acid-catalyzed dehydrogenation of alkanes proceeding by the Haag–Dessau mechanism (Olah type chemistry). This review includes a summary of results for conversion of ethane, propane, and n-butane that are consistent with the postulate that iron- and manganese-promoted sulfated zirconia and sulfated zirconia are capable of protonating light alkanes to give carbonium-ion transition states at temperatures as low as 200°C. The data support the postulate that these proton-donation reactions are important at low alkane conversions and in initiating alkane conversions, although conventional carbenium ion reactions predominate at high conversions.

New catalyst of SO 4 2− /Al2O3–ZrO2 for n-butane isomerization

The catalytic behavior of Al-promoted sulfated zirconia for n-butane isomerization at low temperature in the absence of H2 and at high temperature in the presence of H2 was studied. The addition of Al enhances the activity and stability of the catalysts for reaction at 250°C and in the presence of H2 significantly. After on stream for 120 h, the n-butane conversion of the catalyst containing 3 mol% Al2O3 keeps steadily at 88% of its equilibrium conversion and no observable trend of further deactivation has been observed. The difference in behavior of the promoted and unpromoted catalysts at low and high temperature is associated with a change of reaction mechanism from bimolecular to monomolecular. Experimental evidence is presented to show that the promoting effect of Al is different from that of the transition metals. Microcalorimetric measurements of NH3 adsorption on catalysts reveal that the remarkable activity and stability of the Al-promoted catalysts are caused by an enhancement in the number of acid sites effective for the isomerization reaction.

The effect of pretreatment procedures on the activities of Fe- and Mn-promoted sulfated zirconia catalysts

The effect of activation and calcination conditions on the activities of Fe- and Mn-promoted sulfated zirconia (MFSZ) catalysts for the isomerization of n-butane at low temperatures (≤90°C) was investigated. Activation in air resulted in a higher activity of MFSZ than did activation in helium; this is in contrast to the situation for unpromoted sulfated zirconia (SZ) catalysts where the activation atmosphere did not affect the activity. The maximum activity of MFSZ was found to occur when the activation temperature was 450°C, while for unpromoted SZ, the optimum activation temperature was 250°C. Comparison of the n-butane conversions at these two temperatures showed that MFSZ was only about 1.6 times as active as SZ. This activity difference is much less than that reported before in the literature where comparison was made at the same activation temperature. This activity difference is attributed to the different degree of synergism between redox and acid sites on the two catalysts.

Skeletal Isomerization of n-Butane Over Mordeniteand Beta Zeolites

It was found that the addition of platinum (3 wt.%) substantially increases the total n-butane conversion and selectivity to isobutane over H-Beta zeolite. The same holds for H-Mordenite; however, the conversion significantly decreases in time at 350 °C due to the coking of the catalyst. The selectivity to isobutane is the highest at 300 °C over both zeolite types with platinum (over Pt/H-Beta 1.8 times higher than over Pt/H-Mordenite at almost the same conversion levels of 30 and 36%, respectively). The H-forms of both zeolite types are most selective at 250 °C, but the total conversion is low. The results obtained are related to the structural features of both zeolite types.

Kinetic Study of the Oxidation of n-Butane on Vanadium Oxide Supported on Al/Mg Mixed Oxide

The reaction kinetics of the oxidative dehydrogenation (ODH) of n-butane over vanadia supported on a heat-treated Mg/Al hydrotalcite (37.3 wt % of V2O5) was investigated by both linear and nonlinear regression techniques. A reaction network including the formation of butenes (1-, 2-cis-, and 2-trans-butene), butadiene, and carbon oxides by parallel and consecutive reactions, at low and high n-butane conversions, has been proposed. Langmuir−Hinshelwood (LH) models can be used as suitable models which allows reproduction of the global kinetic behavior, although differences between oxydehydrogenation and deep oxidation reactions have been observed. Thus, the formation of oxydehydrogenation products can be described by a LH equation considering a dissociative adsorption of oxygen while the formation of carbon oxides is described by a LH equation with a nondissociative adsorption of oxygen. Two different mechanisms operate on the catalyst: (i) a redox mechanism responsible of the formation of olefins and diol...

Improving the Stability of H–Mordenite forn-Butane Isomerization

The conversion ofn-butane over mordenite-based catalysts in the presence of hydrogen and water was investigated for reaction temperatures between 523 and 623 K. Special attention was given to the influence of Pt upon catalytic activity, selectivity, and stability. With parent mordenite the catalytic activity forn-butane conversion decreased markedly after a short time on stream. Deactivation can be minimized by hydrogen (in the presence of Pt) and water addition. Both measures are thought to reduce the concentration of intermediate olefins in the zeolite pores. The best results with respect to selective conversion ofn-butane to isobutane were obtained for 0.25 wt% Pt on mordenite in the presence of hydrogen. Higher concentrations of Pt in the catalyst are shown to be detrimental forn-butane isomerization, because of increasing selectivity to hydrogenolysis. A detailed mechanistic scheme forn-butane conversion over Pt-containing mordenites is presented.n-Butane conversion is concluded to occur via a bimolecular mechanism involving a complex network of hydrogen transfer, oligomerization/cracking, isomerization, hydrogenation/dehydrogenation, and hydrogenolysis.

Oxydehydrogenation of n-butane over promoted MgV-oxide based catalysts

Magnesium vanadate, Mg 2V 2O x, and promoted systems of the composition Mg 4V 2M 2O x, where M = Mo, W, B, Al, Ga, Sb and (SbP), were synthesized, characterized and evaluated for the oxydehydrogenation of n-butane in the temperature range 400–500°C and atmospheric pressure. The selectivities to butene/butadiene of the majority of the promoted catalysts are, at 500°C and constant n-butane conversion (2%), substantially higher than those of the base system: Sb(87), SbP(82), B(82), Mo(68), Al(62), base(55), W(50). The superior selectivities of the antimony and boron containing compositions are probably primarily due to the formation of ternary MgVSb-oxide Mg 1− x SbV 2/3xO 3.5) and MgVB-oxide (Mg 1− x BV 2/3O 2.5) phases, contributing to the structural site isolation of the paraffin activating vanadium sites; the stabilization of the V 4+ or lower oxidation states; and their possible interaction with an inferred supra-surface Sb-oxide phase. Highest butene/butadiene per pass yields were obtained with a Mg 4V 2Ga 2O x catalyst (5.2% at 7.9% conversion, at 500°C, 1 atm, C 4 0/O 2/N 2 = 5/1/20, and WHSV = 13 h −1). The only composition yielding measurable amounts of furan was the W promoted catalyst.

A comparison of steady-state and unsteady-state reaction kinetics of n-butane oxidation over VPO catalysts using a TAP-2 reactor system

The reaction of n-butane with ‘reactor-equilibrated’ (VO) 2P 20 7 based catalysts has been investigated using a combination of low-pressure transient response experiments and atmospheric-pressure steady-flow reaction experiments. The activation energy for n-butane conversion in both steady-flow and transient response experiments was a function of the oxidation state of the catalyst. Under steady-flow conditions an activation energy of 17 ± 2 kcal/mol was obtained for ‘reactor-equilibrated’ catalysts, and 13 ± 2 kcal/mol for an oxygen-treated catalyst. The activation energy for n-butane conversion in transient-response experiments varied between 12 ± 1 and 23 ± 2 kcal/mol. An analysis of the transient response curves of the reaction products shows that the rates of formation and desorption of products are strong functions of the catalyst oxidation state. Analysis of the kinetic results indicated that the same sites are involved in the steady-state and unsteady-state conversion of n-butane.

Synthesis, characterization and application of H-MCM-22, Ga-MCM-22 and Zn-MCM-22 zeolite catalysts in the aromatization of n-butane

The H-MCM-22, Ga-MCM-22 and Zn-MCM-22 zeolite catalysts were synthesized and characterized by X-ray powder diffraction, scanning electron microscopy, X-ray fluorescence, energy dispersive X-ray analysis, temperature programmed desorption of ammonia and nitrogen adsorption. The study of the acidic properties of the synthesized zeolite catalysts revealed that the catalysts comprised of Bronsted and Lewis acid sites. n-Butane aromatization was investigated over the H-MCM-22, Ga-MCM-22 and Zn-MCM-22 zeolite catalysts. The effect of reaction temperature, weight hourly space velocity and time on stream on the conversion of n-butane and selectivity to aromatic hydrocarbons were studied over the synthesized catalysts. The type and nature of active sites in the aromatization of n-butane over the prepared MCM-22 catalysts have been examined. The reaction mechanism of n-butane conversion and the shape-selective property of the MCM-22 catalysts in light of their unique pore system with supercages are also discussed. The Ga- and Zn-modified MCM-22 zeolite catalysts exhibited high activity in the conversion of n-butane and selectivity to aromatic hydrocarbons. The aromatic hydrocarbons produced over these catalysts were benzene, toluene, ethyl benzene and isomers of xylene.

On the mechanisms of light alkane catalytic oxidation and oxy-dehydrogenation: an FT-IR study of the n-butane conversion over MgCr 2O 4 and a Mg-vanadate catalyst

The interaction of n-butane, 1-butene, 1,3-butadiene and of C 4 oxygenates on the surface of oxidized Mg-chromite and Mg-vanadate catalysts has been studied by FT-IR spectroscopy. The results compare well with the catalytic behavior of these materials one of which is a combustion catalyst (MgCr 2O 4) and the other is a rather selective oxy-dehydrogenation catalyst (Mg-vanadate). A mechanism for these reactions is proposed.