Formation Of Isobutene Research Articles

The importance of acid site locations for n-butene skeletal isomerization on ferrierite

The role of the acid site location in H-ferrierite (H-FER) on the skeletal isomerization of linear butenes was studied. Assignment of OH groups observed in FTIR analysis is addressed taking into account previous NMR and computational studies regarding the FER structure. Possible locations of alkali ions in the zeolite framework are discussed. Close structure activity relationship has been observed between Brønsted acid sites located in the 10 member ring (MR) channels and selective isobutene formation. Molecular dynamics (MD) calculations of possible products observed during n-butene transformation at reaction temperatures support the experimental findings.

Structure–Activity Correlations for TON, FER, and MOR in the Hydroisomerization of n-Butane

n-Butane hydroconversion was studied over (Pt-loaded) molecular sieves with TON, FER, and MOR morphology. The conversion occurs via a complex interplay of mono- and bimolecular bifunctional acid mechanism and monofunctional platinum-catalyzed hydrogenolysis. Hydroisomerization occurs bimolecularly at low temperatures. This is strongly indicated by the reaction order in n-butane of 2 for isobutane formation and the presence of 2,2,4-trimethylpentane among the products. Intracrystalline diffusion limitations of the reaction rates seem to be important for TON. Due to diffusion-controlled reaction rates for TON, the presence of Pt in TON was detrimental for the isomerization selectivity. As the ratio of utilized acid sites to accessible Pt becomes low (approximately 1:75), diffusion of the feed molecules to the acid sites is too slow to prevent Pt hydrogenolysis of n-butane. Reactions on H-FER occur predominantly on the outer surface and the pore mouth of the molecular sieve, presumably owing to rapid pore filling following a transient period of single-file diffusion. Due to high intrinsic activity toward (hydro)cracking this does not lead to high selectivity toward isobutane. Addition of Pt (bifunctionality) was in this case beneficial. Reaction at the external surface is not diffusion limited, allowing bifunctional nC4 isomerization to occur. Although PtFER was found to approach selectivity levels as found for PtMOR, the latter has a significant advantage as the larger concentration of accessible acid sites leads to much higher activity.

N-GERMYL DERIVATIVES OF 4-NITROPHENYLAMINE: GERMYLAMINES, STABLE GERMA-IMINE

N-germyl secondary amines of 4-nitrophenylamine were obtained by transamination from N-trialkylgermyldimethylamine, by transmetallation from the N-lithium derivative of 4-nitrophenylamine, or by intermolecular dehydrohalogenation between the corresponding halogermane and p-nitrophenylamine. Halogermylamines formed by the same methods are always obtained as a mixture with the corresponding diamine bis(4-nitrophenylamino)dimesitylgermane, and cannot be used as germa-imine precursors. However, elimination reactions from N-dimethylaminodimesitylgermyl-4-nitrophenylamine and N-dimesitylchlorogermyl-4-nitrophenylamine, formed from N-triethylgermyl N-dimesitylchlorogeryl-4-nitrophenylamine, yielded the thermally stable N-(4-nitrophenyl)dimesitylgerma-imine as a deep red amorphous powder. This germa-imine is moisture-sensitive, yielding (4-nitrophenylamino) dimesitylgermanol which adds to the germa-imine forming bis(4-nitrophenylamino)dimesitylgermyl) oxide which was isolated as an orange powder. N-4-nitrophenyl)dimesitylgerma-imine adds readily to chloroform yielding N-dimesitylchlorogermyl-4-nitrophenylamine. The 3+2 addition of the germa-imine to N-t-butyl-phenylnitrone gave an adduct whose thermal decomposition began at 100°C yielding benzylidene-4-nitrophenylamine and dimesitylgermoxane. The germa-imine addition to 4,5-di-t-butyl ortho-quinone led to the corresponding dimesitylgermadioxolane through decomposition at room temperature of a transient adduct. The formation of isobutene in this reaction is constitent with a Single Electron Transfer mechanism in the first step.

Active sites on ZrO 2 for the formation of isobutene from CO and H 2

The selective formation of isobutene from CO and H 2 over ZrO 2 has been investigated. ZrO 2 catalysts having different fraction of monoclinic phase were prepared by changing pH value in the mother solution at the precipitation of zirconium hydroxide. The rate of isobutene formation increased with an increase in the volumetric fraction of monoclinic phase in ZrO 2, while those of C 1, C 2, C 3, and C 5+ were independent of the fraction. The amounts of adsorbed methoxy and formate species during the reaction and also of the surface sites with strong basicity increased with an increase in the fraction of monoclinic phase. Chemical trapping experiment showed that the amount of surface methoxy species is comparable to that of site with the strong basicity. These findings were explained by both coordinate unsaturation and stronger basicity based on the configuration of ZrO 7 group in the monoclinic structure.

Synthesis of isobutene from synthesis gas over nanosize zirconia catalysts

Performances of nanosize zirconium oxides prepared by precipitation, supercritical fluid drying (SCFD) and freeze-drying (FD) methods were evaluated for the selective synthesis of isobutene from synthesis gas (syn-gas). The crystal structure, particle size and specific surface area were measured by XRD, TEM and BET. The acidic and basic properties of the catalysts were studied by NH 3- and CO 2-TPD, respectively. The results showed that the crystal phases, acidic and basic properties of nanosize zirconia depend remarkably on the drying conditions. The catalysts over which the formation of isobutene is favored show a higher ratio of basic to acidic sites on their surfaces. The results of adding acidic and basic components into zirconia suggested that both the acidic and basic sites on the catalysts are required for the isosynthesis. The conversion of CO was increased by adding acidic component into zirconia, and the selectivity for i-C 4H 8 was increased greatly by adding basic component into zirconia. It is suggested that the acidic sites are responsible for the activation of reactant molecules and the basic sites of the catalysts are significant for the formation of i-C 4H 8 from CO hydrogenation.

Oxidative dehydrogenation of isobutane on Cr–Ce–O oxide: II. Physical characterizations and determination of the chromium active species

Cr–Ce–O catalysts with different Cr contents and prepared by different methods have been characterized by X-ray diffraction (XRD), XPS, ISS and UV–Vis spectroscopies, and the surface potential technique. On the basis of these studies, a model of catalysts has been proposed and the correlations between the structure of the catalysts and their performance in the oxidative dehydrogenation (ODH) of isobutane have been discussed. At low Cr content corresponding to the coverage of the ceria surface with chromium θ<35%, as determined by the ISS method, Cr 6+O x species are the only Cr species, irrespective of the preparation method. In the θ range 35–61%, well dispersed Cr 3+ species appear as Cr 2O 3. The maximum coverage of 75% is obtained for the samples prepared by impregnation at 20 Cr atoms per nm 2. From this Cr content, agglomeration of Cr 2O 3 is observed. The specific rate of isobutene formation increases with the increase in Cr content up to the coverage of about 61% and then remains constant. It is concluded that the activity is due to the well dispersed Cr 6+O x species on the ceria surface, whereas agglomerates of Cr 2O 3 do not contribute to the reaction.

99/03600 The mineral oil industry at the territory of the GDR: Bittrich, H.-J. and Matschke, F. W. Chem. Tech., 1999, 51, (1), 32–37. (In German)

The hydrogenation of carbon dioxide was studied using composite catalysts comprised of Fe-Zn-M (M= Cr, Al, Ga, Zr) catalysts and the HY zeolite, where the methanol synthesis and the methanol-to-gasoline (MTG) reaction are combined. The results show that light olefins are important intermediates for iso-butane formation. In all of the cases, the selectivity of iso-butane, which can be used as a reactant in subsequent methyl-tert-butyl ether (MTBE) synthesis, was the highest in hydrocarbons.

99/03597 Transport method of sludge-containing tar from coke oven: Takahashi, Y. et al. Kokai Tokkyo Koho JP 10 330,763 [98 330,763] (Cl. C10C1/00), 15 Dec 1998, Appl. 97/162,001, 3 Jun 1997, 4 pp. (In Japanese)

The hydrogenation of carbon dioxide was studied using composite catalysts comprised of Fe-Zn-M (M= Cr, Al, Ga, Zr) catalysts and the HY zeolite, where the methanol synthesis and the methanol-to-gasoline (MTG) reaction are combined. The results show that light olefins are important intermediates for iso-butane formation. In all of the cases, the selectivity of iso-butane, which can be used as a reactant in subsequent methyl-tert-butyl ether (MTBE) synthesis, was the highest in hydrocarbons.

Linear Relationship of the Rate of Isobutene Formation from CO and H2 on ZrO2 to the Monoclinic Phase Fraction

Abstract The rate of formation of isobutene from CO and H2 over ZrO2 is proportional to the volume fraction of the monoclinic phase. The active site was explained by both vacant site and stronger bascity due to the shorter Zr–O bond in the monoclinic structure.

Kinetic Model for Skeletal Isomerization of n-Butene over ZSM-22

A kinetic model for n-butene transformation over ZSM-22 was developed. The effects of temperature, partial pressure, and deactivation on the formation of isobutene and byproducts were included in the model. The model was able to predict n-butene transformation over ZSM-22 reasonably well, although several assumptions were made in order to reduce the number of adjustable parameters. The major assumptions were that isobutene is only produced through monomolecular reactions and byproducts are only produced in reactions between two n-butene molecules or n-butene and isobutene.

A study of manganese-silicoaluminophosphate molecular sieves

Surface and chemical characterization were performed on a manganese-substituted silicoaluminophosphate molecular sieve (MnAPSO-11), and on a manganese-supported silicoaluminophosphate molecular sieve (Mn/SAPO-11). For comparison purposes, the characterization process was also carried out over the parent SAPO-11 molecular sieve. Different characterization techniques were used: XPS, redox cycles, 31 P MAS NMR, and acidity measurements. The transformations of n-butane were carried out over the corresponding platinum promoted solids (Pt/MnAPSO-11, Pt/Mn/SAPO-11 and Pt/SAPO-11). Platinum dispersion was measured by H 2 chemisorption. XPS results indicated that manganese was better dispersed on the MnAPSO-11 solid than on the supported Mn/SAPO-11 catalyst. Redox cycles showed a strong difference between the H 2 (or O 2) consumed by each solid. The Mn/SAPO-11 consumed nearly three times as much H 2 (or O 2) per Mn atom as the MnAPSO-11 solid. 31 P MAS NMR results showed an increase in the intensity of the side bands, probably due to an anisotropic paramagnetic shift caused by a stronger dipolar interaction between the 31 P and the paramagnetic Mn(II) ions on the MnAPSO-11 sample, when compared with the Mn/SAPO-11 solid. These results suggest a better dispersion of the manganese species on the MnAPSO-11 solid, which would facilitate the above mentioned 31 P –Mn(II) interaction, in agreement with the XPS results. Acidity was measured by pyridine chemisorption at different temperatures. A larger number of (moderate+strong) Brönsted acid sites was found for the MnAPSO-11 solid compared with the SAPO-11 and Mn/SAPO-11 samples. The addition of platinum decreased the acidity. The Pt dispersions were 83%, 68% and 54% for the Pt/SAPO-11, Pt/Mn/SAPO-11 and Pt/MnAPSO-11 solids, respectively. The catalytic results indicate higher yields for the production of isobutane and isobutene over the Pt/MnAPSO-11. A severe decrease in the yield of formation of hydrocarbons with less than four carbon atoms (undesirable side reaction) was also observed for the Pt/MnAPSO-11 system compared with the Pt/SAPO-11 and Pt/Mn/SAPO-11 systems. An ensemble effect is suggested as responsible for the differences observed in the yield of formation of hydrocarbons with less than four carbon atoms. The higher yield and selectivity observed for the formation of isobutane (iso-C4) and isobutene (iso-C4) hydrocarbons over the Pt/MnAPSO-11 solid, was accounted for in terms of the largest number of (moderate+strong) Brönsted acid sites found on this solid. The catalytic and characterization results suggest the incorporation of manganese into the molecular sieve structure for the substituted MnAPSO-11 solid.

Oligomerization of n-Butenes on the Surface of a Ferrierite Catalyst: Real-Time Monitoring in a Packed-Bed Reactor during Isomerization to Isobutene

A packed-bed-configured oscillating balance reactor (OBR) was used to study the surface oligomerization reactions that accompany the isomerization of linear butenes to isobutene on a ferrierite (FER) catalyst. The modeling of the surface deposition process of high molecular weight species indicates that H-FER undergoes site blockage, followed by a pore blockage phase that sets in after ca. 70% of the total uptake is achieved. Prior to reaching saturation of the carbonaceous residue uptake, a significant fraction of selective sites for isobutene formation are lost. This suggests that such sites are unlikely to exist exclusively at the pore mouths of H-FER, as proposed earlier. In situ DRIFTS was also used to discriminate between oligomerization and true coke formation. Continuous mass spectrometric monitoring of the OBR product stream reveals that the production of octenes and/or higher oligomers goes through a maximum at short times on stream (i.e., before the zeolite pores reach saturation uptakes). This is consistent with the idea that bimolecular processes dominate the catalysis of fresh H-FER. In a temperature range representative of those normally used for reaction, the hydrocarbon residues formed on the surface of the catalyst upon a few hours of exposure to a 1-butene flow are more properly described as hydrogen-deficient high molecular weight species that do not reach the hard coke category.

Bis(acetylacetonato)dioxomolybdenum(VI) catalysed rearrangement of methylbutynol in the presence of ultrasound

The molybdenum (VI) catalysed rearrangement of methylbutynol was carried out under the influence of ultrasound (20 kHz). Surprisingly, 2-methylpropene (isobutene) was found as the main product. The formation of isobutene can be explained by rearrangement to prenal, oxidation to 3,3-dimethylacrylic acid and then decarboxylation. 3,3-Dimethylacrylic acid was decarboxylated in the presence of ultrasound and a catalyst; without a catalyst or without ultrasound (at 50 degrees C) it remained unchanged.

Benzene Formation from the Flow Reactor Oxidation of Methyl t-Butyl Ether

The stoichiometric oxidation of methyl t-butyl ether at temperatures of 973 and 1273 K using a fused silica tubular flow reactor is reported. H abstraction by OH radicals is expected to become effective in competing with the previously proposed four-center methanol elimination initiation pathway above 973 K. This is based on the nine equivalent C-H abstraction sites on the t-butyl group coupled with OH being nonselective towards the abstraction sites at these temperatures. In addition to the formation of isobutene and methanol as primary organic byproducts, gas chromatographic-mass spectrometric analysis indicated the facile formation of benzene and other aromatic compounds at 1273 K. The decomposition of isobutene through propargyl radical intermediates is the most likely source of the formation of these compounds, subject to model verfication.

Comparative Study of the Catalytic Properties of ZSM-22 and ZSM-35/ Ferrierite Zeolites in the Skeletal Isomerization of 1-Butene

The catalytic activities of the two catalysts ZSM-22 and ZSM-35 were compared in the skeletal isomerization of 1-butene. ZSM-22 demonstrated higher activity in 1-butene transformation, compared to that of ZSM-35. ZSM-35 was not selective to isobutene until nonselective acid sites had been poisoned by coke deposits, while ZSM-22 was selective already from the beginning. Information about the extent of coke formation was obtained by FTIR experiments and surface area measurements. ZSM-22 was more resistant towards coke formation, compared to that of ZSM-35. In order to obtain information about the reaction mechanism, 2,4,4-trimethyl-2-pentene and 1-octene were cracked over the catalysts. The selective mechanism for isobutene formation in the skeletal isomerization of 1-butene was most likely monomolecular. The bimolecular mechanism is not selective to isobutene, although it can contribute to the overall isobutene production.

Comparative study of the transformation of n-butane, n-hexane and n-heptane over H-MOR zeolites with various Si/Al ratios

The transformations of n-butane, n-hexane and n-heptane were carried out in a flow reactor at 523 K, p alkane=0.1 bar, p nitrogen=0.9 bar over a series of H-mordenite samples with framework Si/Al ratios from 6.6 to 80. Because of the rapid deactivation of the samples (mostly during n-hexane and n-heptane transformations), a series of product analyses was performed at a very short time-on-stream in order to obtain, with good accuracy, the activity and selectivity of the fresh samples. With all the samples, n-heptane is slightly more reactive than n-hexane and much more reactive than n-butane (15–100 times). The effect of the acid-site density on the mordenite activity is different for n-butane, n-hexane and n-heptane transformations, which suggests that these reactions occur through different mechanisms: bimolecular with n-butane; monomolecular with n-hexane and n-heptane. The bimolecular mechanism of n-butane transformation is confirmed by simultaneous formation of isobutane, propane and pentanes as primary products. With all the H-mordenite samples, isomers and C 3–C 5 alkanes appear as primary products of n-hexane transformation. From n-heptane, C 3–C 5 alkenes are observed as primary products as well as isomers and C 3–C 6 alkanes. The isomer/light products ratio is approximately equal to 2 from n-hexane and 0.2 from n-heptane, as is expected from the relative difficulty in the modes of cracking: difficult C mode (involving two secondary carbenium–ion intermediates) from n-hexane and relatively easy B mode (one tertiary and one secondary carbenium–ion intermediates) from n-heptane. However, most of the light products do not result from direct cracking of C 6 and C 7 compounds. Whatever the reactant, the product distribution is practically identical for all the dealuminated samples. Very different distributions of the C 3–C 6 products are observed with the non-dealuminated sample: faster formation of C 3 at the expense of C 4–C 6, in particular C 4. This large change in selectivity should be due to the presence of mesopores in the dealuminated samples rather than the larger density of acid sites in the non-dealuminated one.

Oxidative dehydrogenation of iso-butane to iso-butene I. Metal phosphate catalysts

Metal pyrophosphates catalyse the oxidative dehydrogenation of iso-butane to iso-butene at 450–550°C using a feed gas of 75 mol% iso-butane and 5% O 2. Ni 2P 2O 7 is the most selective catalyst with the iso-butene selectivity reaching to a maximum value of 82.2% at 550°C. Ag 4P 2O 7 and Zn 2P 2O 7 are also effective, but the iso-butene selectivities were slightly lower than that of Ni 2P 2O 7. Pyrophosphates of Mg, Cr, Co, Mn, and Sn catalyse the oxidative dehydrogenation, but the iso-butene selectivity was 43.8–65.7% at the temperature where the maximum iso-butene yield is observed. The optimum oxygen concentration for iso-butene formation was 5–15 mol%, but the increase in O 2 concentration did not increase the iso-butene selectivity. No adsorbed oxygen species was found by means of TPD. The lattice oxygen of the pyrophosphates began to react with H 2 at 200–400°C. Reactivity of the lattice oxygen of pyrophosphates can be estimated from the value of ΔH f 0 for the corresponding oxide. More than 2 desorption peaks were observed in the TPD spectra of NH 3 adsorbed on the pyrophosphates, and a linear correlation was found between the concentration of acid amount of the catalysts and the specific rate of iso-butene formation. This strongly suggests that the acidic sites play a key role in the iso-butene formation.

Kinetic study of methyl tert-butyl ether (MTBE) oxidation in flames

Three premixed MTBE/H2/O2/Ar flat flames burning at low pressure (30 torr) have been investigated by molecular beam mass spectrometry for equivalence ratios ranging from 0.18 to 1.84. Profiles of stable, atomic, and radical species concentrations have been measured as well as the temperature ones. The structure of these flames has been established in order to deduce the rate coefficients of H-atom abstraction reactions (R1-R3) from MTBE by highly reactive species like H and O atoms and OH radicals. C5H12O+H→Products (R1) C5H12O+O→Products (R2) C5H12O+OH→Products (R3) The rate coefficient of reaction R1 has been deduced in the rich flame k1=3.92×1013 exp(−2830/T) cm3 mole−1 s−1 between 700 and 1000 K. Rate coefficients of reactions R2 and R3 have been determined in the lean flame k2=1.78×1014 exp(−4000/T) cm3 mole−1 s−1 between 1000 and 1300 K and in the the stoichiometric flame k3=4.1×1013 exp(−1450/T) cm3 mole−1 s−1 between 650 and 900 K, respectively. Using the deduced rate coefficients k1, k2, and k3, and the experimental mole fraction profiles of the involved species, the relative contribution of reactions (R1-R3) to the MTBE consumption has been determined in the investigated flames. The analysis of the flame structures has allowed also to evaluate the pathways leading to the formation of isobutene and acetone that are intermediate species arising from the first steps of the MTBE decay. It has been demonstrated that all MTBE conversion proceeds through isobutene formation whereas reactions leading to the formation of acetone have a weak contribution.

Selective isomerization of n-butenes into isobutene over aged H-ferrierite catalyst : nature of the active species

The nature of the active species responsible for butene isomerization over aged HFER samples is reexamined in the light of the change in the product yields at very short time-on-stream and of the reversible and irreversible increases in weight of the zeolite during the reaction. At very short time-on-stream, the selectivity of butene isomerization is that expected from a dimerization-cracking process, in particular simultaneous formation of isobutene, propene and pentenes. A rapid decrease of all the yields is observed with time-on-stream; however, for isobutene but not for the other products, the initial decrease is followed (after 10 minutes-on-stream) by an increase. The decrease in the yield can be related to the formation of carbonaceous compounds (``coke'') which block the access to the pores, while the increase in isobutene yield can be explained by the development of a new isomerization mode which is very selective to isobutene. This new mode could be catalyzed by carbonaceous compounds and/or by reaction products which are shown to be retained inside the pores during the reaction. It is proposed that at short time-on-stream the increase in isobutene yield is due to an autocatalytic reaction, n-butene isomerization occurring on t-butyl carbenium ions formed by adsorption of isobutene molecules (which are slowly desorbed from the pores) on the protonic sites of the zeolite. At long time-on-stream, the active species would be benzylic carbocations formed from carbonaceous compounds trapped in the pores near the outer surface of the crystallites.

Simultaneous dehydrogenation and isomerization of η‐butane to isobutene over ZSM‐22 and zinc‐modified ZSM‐5 zeolites

Direct formation of isobutene from η‐butane was investigated over a zinc‐impregnated potassium‐ion‐exchanged ZSM‐5 dehydrogenation catalyst and an acidic shape‐selective ZSM‐22 skeletal isomerization catalyst. The experiments were performed in a fixed‐bed microreactor system operating at near‐atmospheric pressure. High selectivity to η‐butene was obtained over the zinc‐impregnated potassium‐ion‐exchanged ZSM‐5 dehydrogenation catalysts. The yield of isobutene increased after adding the acidic ZSM‐22 skeletal isomerization catalyst, although the selectivity to butene isomers slightly decreased because the skeletal isomerization of η‐butene was competing with other acid‐catalyzed reactions such as cracking and aromatization.