Transformation Of N-butane Research Articles

1H MAS NMR In Situ Reaction Monitoring Reveals the Particular Effects of Zn2+ and ZnO Species on the Kinetics of Isobutane Transformation on Zn-Modified Zeolites

Establishing the role of metal species in the reaction of alkane aromatization on metal-modified zeolites is a challenging task aiming at understanding the mechanism of the alkane transformation and improving the zeolite catalyst performance. Herein, the kinetics of isobutane transformation on BEA zeolites, modified with either Zn2+ cations (Zn2+/H-BEA) or ZnO clusters (ZnO/H-BEA), has been monitored with 1H MAS NMR spectroscopy in situ at 543–573 K. It is established that the rates for the alkane dehydrogenation, hydrogenolysis, and aromatization steps are higher for Zn2+/H-BEA than for ZnO/H-BEA zeolite, implying higher efficiency of Zn2+/H-BEA compared to ZnO/H-BEA zeolite for isobutane transformation. An occurrence of the reaction of isobutane with methane to yield propane on both zeolites has been evidenced by kinetic analysis. The kinetic parameters of the reaction have been compared with earlier derived parameters for n-butane transformation on the same zeolites. It is inferred that Zn2+/H-BEA exhibits higher activity than ZnO/H-BEA for both n-butane and isobutane aromatization. At the same time, Zn2+/H-BEA is more active for n-butane aromatization than for isobutane transformation to aromatics.

N-Butane transformation on Zn/H-BEA. The effect of different Zn species (Zn2+ and ZnO) on the reaction performance

Using solid-state 1H and 13C MAS NMR spectroscopy, the performance of H-BEA zeolite, modified with either Zn2+ cations (Zn2+/H-BEA) or ZnO clusters (ZnO/H-BEA), has been investigated with respect to n-butane transformation by aromatization and hydrogenolysis pathways. 13C-labeled n-butane has been used to follow the main stages of n-butane transformation on both Zn2+/H-BEA and ZnO/H-BEA with 13C MAS NMR at 298–623 K. Similar surface species, including n-butylzinc, n-butene, allyl-like oligomers, are formed as the intermediates on both zeolites. The kinetics of n-butane transformation has been monitored with 1H MAS NMR in situ at 543–573 K. Kinetics modeling reveals that Zn2+/H-BEA is more active for n-butane transformation than ZnO/H-BEA. A remarkable difference in the rates and the pathways of hydrogenolysis for Zn2+/H-BEA and ZnO/H-BEA has also been established. Propane and methane are hydrogenolysis products on ZnO/H-BEA whereas ethane is produced by the reaction on Zn2+/H-BEA. 13C NMR data and the kinetics analysis provide an insight on the occurrence of joint methane and n-butane conversion on Zn-modified zeolites under non-oxidative conditions.

Tagra extends Azelis distribution territory to include India, Australia & New Zealand

MCM-41 mesoporous materials and silicoaluminophosphates (SAPO-5) were prepared by gel hydrothermal crystallization. Both series of materials were modified by transition metals (chromium or iron close to 1 wt%). The synthesized solids were characterized by chemical analysis, XRD, nitrogen adsorption–desorption isotherms, SEM, EPR, XPS, IR of ammonia or pyridine adsorption studies, and 29Si, 27Al and 31P NMR. The catalysts were evaluated in the n-butane isomerization in order to assess their properties with regard to alkane transformations. The properties of the solids were modified by adding metals that altered the morphology, structural features and acidic character of the solids and result in an efficient performance in the transformation of n-butane in comparison with the pure counterparts. The introduction of Cr or Fe enhanced the catalytic ability of silicoaluminophosphates to produce isobutane. Fe and Cr modified silicoaluminophosphates were shown to be more efficient in the transformation of n-butane in comparison with the MCM-41 analogues. However, the activity of these iron-containing molecular sieves was considered too low in comparison with performances of iron-modified modernite. This was ascribed to an initial redox C4 activation step occurring in synergy with Brønsted sites with sufficient acid strength which is indeed higher than that provided by MCM-41 and SAPO molecular sieves.

Kinetic Modeling of n-Butane Cracking on HZSM-5 Zeolite Catalyst

A kinetic model of lumps has been established for n-butane cracking over HZSM-5 zeolite catalyst (SiO2/Al2O3 = 30) in the 400−550 °C range, based on the results obtained in a fixed bed reactor (space time, up to 2.4 (g of catalyst) h (mol CH2)−1; He/n-butane molar ratio in the feed, up to 6/1; time on stream, 5 h). The model allows quantifying the distribution of the lumps of products (C2−C4 olefins, C2−C4 paraffins, methane, and C5−C10 components) in a wide range of temperatures, partial pressures of hydrocarbons in the reaction medium, and space times. The kinetic model steps for the transformation of n-butane into olefins and of olefins into paraffins and C5−C10 are second order with respect to the reactant, whereas the remaining steps are first order with respect to each reactant. When the target is the production of C2−C4 olefins, the yield is limited to 12%, at 550 °C.

Catalyst discrimination for olefin production by coupled methanol/n-butane cracking

The joint transformation of n-butane and methanol has been studied on catalysts prepared based on HZSM-5 zeolites (with different SiO 2/Al 2O 3 ratios and doped with Ni) and SAPO-18. A methanol/n-butane molar ratio of 3 has been used, which corresponds to an energy-neutral integrated process. The kinetic behaviour of the catalysts has been compared based on C 2–C 4 olefin yield, deactivation by coke deposition for 5 h time on stream and hydrothermal stability by operating in reaction–regeneration cycles. The results show a good performance of the catalyst prepared based on HZSM-5 with SiO 2/Al 2O 3 = 30, which has high levels of acidity and acid strength (≥120 kJ mol NH 3 −1). This catalyst allows obtaining 24.4% of C 2–C 4 olefin yield (11.5% of propene), with a selectivity of 43% at 575 °C and a space time of 1.1 g cat h mo l C H 2 − 1 . The catalyst fully recovers its initial kinetic behaviour after 10 reaction–regeneration cycles.

Synergies in the production of olefins by combined cracking of n-butane and methanol on a HZSM-5 zeolite catalyst

The production of olefins by the joint transformation of n-butane and methanol has been studied in a fixed bed reactor on a HZSM-5 zeolite catalyst in the 400–575 °C temperature range. A methanol/n-butane molar ratio of 3 has been used in order to obtain an energy-neutral integrated process, which enhances the activity in the cracking reaction. Moreover, the joint transformation of the two reactants generates a reaction medium with favourable composition for improving the yield and selectivity of C 2–C 4 olefins compared to the two individual reactions. The olefins formed in n-butane cracking play a significant role for both activating the methanol transformation mechanism and attenuating coke formation. Likewise, the water formed in methanol dehydration also plays a significant role for attenuating the hydrogen transfer reactions in n-butane cracking. These synergistic effects contribute to increasing olefin yields for low values of space time, which has been quantified by means of a coefficient.

Structural, acidic and catalytic features of transition metal-containing molecular sieves in the transformation of C 4 hydrocarbon

MCM-41 mesoporous materials and silicoaluminophosphates (SAPO-5) were prepared by gel hydrothermal crystallization. Both series of materials were modified by transition metals (chromium or iron close to 1 wt%). The synthesized solids were characterized by chemical analysis, XRD, nitrogen adsorption–desorption isotherms, SEM, EPR, XPS, IR of ammonia or pyridine adsorption studies, and 29Si, 27Al and 31P NMR. The catalysts were evaluated in the n-butane isomerization in order to assess their properties with regard to alkane transformations. The properties of the solids were modified by adding metals that altered the morphology, structural features and acidic character of the solids and result in an efficient performance in the transformation of n-butane in comparison with the pure counterparts. The introduction of Cr or Fe enhanced the catalytic ability of silicoaluminophosphates to produce isobutane. Fe and Cr modified silicoaluminophosphates were shown to be more efficient in the transformation of n-butane in comparison with the MCM-41 analogues. However, the activity of these iron-containing molecular sieves was considered too low in comparison with performances of iron-modified modernite. This was ascribed to an initial redox C 4 activation step occurring in synergy with Brønsted sites with sufficient acid strength which is indeed higher than that provided by MCM-41 and SAPO molecular sieves.

Comparative study of transformation of linear alkanes over modified mordenites and sulphated zirconia catalysts: Influence of the zeolite acidity on the performance of n-butane isomerization

Zeolite mordenite was dealuminated by steam treatment followed by acid leaching and further impregnated with 1 wt.% of iron or sulphated. The various solids were characterized by XRD, 29Si and 27Al NMR, IR of adsorbed pyridine as well as ammonia adsorption calorimetry and evaluated in n-butane isomerization. BET surface area of non-dealuminated mordenite was 462 m 2 g −1 and this value showed marked decreases if compared with the dealuminated (310 m 2 g −1), iron impregnated (302 m 2 g −1) and sulphated mordenite (247 m 2 g −1). It suggested the pore plugging or blocking by EFAL species, further confirmed by NMR experiments. The dealuminated mordenite showed Brønsted and Lewis acid sites with medium strength. The sulphated modified mordenite showed highest acidity strength, however its isomerization activity remains modest. The activity in the transformation of n-butane to isobutane was markedly improved by dealumination and subsequently iron impregnation (from 4 × 10 −9 to 2 × 10 −7 mol s −1 g −1); a significant initial activity was observed at a temperature as low as 200 °C. At higher temperatures, the conversion level did not change. The positive effect of iron impregnation could be related to the increase in acid strength and/or to the participation of the redox properties of iron to the initial activation step of the reaction. Nevertheless, sulphated zirconia, used as a reference catalyst, presented the highest performance (7 × 10 −7 mol s −1 g −1) in comparison with modified mordenites.

Acid and superacid solids for the transformation of n-butane

Acid and superacid solids were evaluated in n-butane isomerization to produce isobutane. The combined dealumination and sulfatation of the starting H-mordenite zeolite improves the activity and selectivity of the catalyst.

Active Sites in Working Bifunctional GaH-TON Aromatization Catalysts: Kinetic Evaluation

The conversion of light alkanes to high value aromatics proceeds with a high selectivity over bifunctional, gallium (Ga) containing zeolite catalysts. It is generally agreed that Ga sites are involved in dehydrogenation reaction steps and that the zeolite acid sites catalyze cracking, oligomerization, and cyclization reactions. However, understanding of the precise roles of the acid and Ga sites in the reaction mechanisms is significantly hampered since the number of these sites in working catalysts is not known. This paper describes a kinetic approach to evaluation of the acid and Ga active sites in working Ga containing TON zeolite catalysts that relies on the analysis of the rates of formation of the primary products of a n-butane aromatization reaction. Our results show that the rate of ethane formation at low n-butane conversions can be used as a quantitative estimate of acidity in working bifunctional zeolite catalysts and demonstrate, for the first time, a significant decrease in the number of Brønsted acid sites in the Ga containing catalysts under reaction conditions: around 47 and 79% for the catalysts with Ga loading of 1.5 and 2.5 wt %, respectively. We conclude that the reduction in acidity is associated with the formation of catalytically active Ga(+) ions and obtain estimates for the number and steady-state turnover activity of the acid and Ga active sites in n-butane transformation. We anticipate that our work will facilitate understanding of the precise roles of the acid and Ga sites in the mechanisms of alkane aromatization and, as a far-reaching implication, will prompt wider use of detailed kinetic studies for the evaluation of active sites in working catalysts.

One-step conversion of n-butane to isobutene over H-beta supported Pt and Pt,M (M = Cu, In, Sn) catalysts: An investigation on the role of the second metal

The one-step transformation of n-butane to isobutene was studied over H-beta zeolite supported Pt and Pt,M (M = Cu, In, Sn) catalysts. Catalytic performance of monometallic Pt/H-beta samples resulted to be affected by the support acidity, a lower number of acid sites leading to higher iso- and n-butenes selectivities and lower by-products formation. Addition of Cu, In or Sn to Pt enhanced both isobutene and n-butenes selectivities, which were in the order: Pt,In > Pt,Sn ≫ Pt,Cu > Pt. All Pt,M samples exhibited also a higher stability than the corresponding monometallic Pt samples, the sequence of deactivation rates being: Pt > In,Pt > Cu,Pt ≈ Sn,Pt. On the basis of characterization results it was stated that the addition of Cu, In or Sn to Pt affects the n-butane dehydroisomerization modifying both the surface structure of Pt clusters and the support acidity. In particular the observed order of isobutene selectivity was related to the degree of Pt–M interaction leading to a dilution of Pt clusters, which inhibits hydrogenolysis reactions and enhances dehydrogenation processes. The decrease in the number of acid sites caused by addition of the second metal was instead accounted for the improved resistance to deactivation of Pt,M catalysts.

Synthesis and characterization of Pd-MCM-22 and Pt-SAPO-11 catalysts for transformation of n-butane to aromatic hydrocarbons

Pd-Na-MCM-22, Pd-H-MCM-22 and Pt-SAPO-11 catalysts were synthesized and characterized using techniques of X-ray powder diffraction, scanning electron microscopy, FTIR, nitrogen adsorption and X-ray fluorescency. The FTIR spectra of Pd and Pt modified MCM-22 and SAPO-11 exhibited lower amounts of Brønsted (B) and Lewis (L) acid sites than the unmodified MCM-22 and SAPO-11. The catalysts were tested in the transformation of n-butane to aromatic hydrocarbons. Aluminosilicate type of zeolite catalysts Pd-Na-MCM-22 and Pd-H-MCM-22 exhibited higher selectivity to aromatics than aluminophosphate Pt-SAPO-11. The pretreatment of the catalysts with oxygen and hydrogen was found to influence the n-butane conversion and selectivity to aromatic hydrocarbons. The hydrogen pretreated Pt-SAPO-11 exhibited higher n-butane conversion than the oxygen pretreated. The catalysts exhibited deactivation with time on stream due to coke formation and pore blocking. The specific surface areas of the Pt-SAPO-11, Pd-Na-MCM-22 and Pd-H-MCM-22 deactivated catalysts were found to be lower than the fresh because of coke deposits inside the pores.

The transformation of n-butane over Ga/SAPO-11 : The role of extra-framework gallium species

A series of Ga-supported SAPO-11 catalysts with Ga-loading between 0.25 and 2.2 wt.% were studied during the direct transformation of n-butane to iso-butene at 773 K and atmospheric pressure. With the exception of the richest Ga sample H 2-TPR profiles showed only one signal, centred at around 973 K and designated as β-peak, associated to [Ga-O] + species. A model was advanced to illustrate a probable reduction mechanism to Ga + species, which have been thought to be the active catalytic centres. The model found support on the invariability of the turn-over frequency (TOF) with respect to Ga-charge. The deviation from this behaviour observed for the catalyst with the highest Ga content was explained in terms of the presence of free Ga 2O 3 detected also by H 2-TPR. that would lead to the formation of reduced Ga 2O clusters in the neighborhood of the active Ga + sites, strongly inhibiting their catalytic activity. The catalyst with 0.5 wt.% of Ga showed an excellent activity and selectivity for the direct transformation of n-butane to iso-butene. In fact, dehydrogenation yield at the thermodynamic limit of about 15% with a skeletal isomerization efficiency of around 70% was achieved.

Mechanism of Butane Transformation

Catalytic activity and aromatic selectivity of n-butane transformation were studied over various MFI type zeolites. From the data obtained, a reaction mechanism is suggested for different catalyst systems. It is visualized that in gallium doped catalysts, Ga3+ directly takes part both in cracking and dehydrogenation. The [Ga CH3]2+ and [GaH]2+ species formed during cracking and dehydrogenation require protonic sites for regeneration of Ga3+ species. An alternative mechanism was suggested for dehydrogenation and cracking by Ga3+ without the involvement of protonic sites. However a protonic site would be required for aromatization. In case of gallosilicates a one step mechanism is suggested for cracking and dehydrogenation reaction which does not require the presence of protonic sites in the catalyst system.

Transformation of propane, n-butane and n-hexane over H3PW12O40 and cesium salts. Comparison to sulfated zirconia and mordenite catalysts

Over H3PW12O40 and its acidic cesium salts at 250°C, alkane transformations occur through the mechanisms previously proposed for sulfated zirconia and mordenite catalysts: propane is mainly transformed into butanes through a trimerization–isomerization–cracking process, n-butane into isobutane, propane and pentanes through a dimerization–isomerization–cracking process, n-hexane into methylpentanes and 2,3-dimethylbutane through a monomolecular mechanism. With all the samples, n-butane transformation is initially much faster than propane transformation, the difference in rate increasing significantly with the Cs content: from 25 times with H3PW12O40 to 350 times with Cs2.4H0.6PW12O40. On the other hand, n-hexane transformation is 2.3 to 7 times faster than n-butane transformation. A decrease in acid strength and in acid site density with Cs introduction is proposed to explain the increase in the rate ratios. For all the reactions, sulfated zirconia pretreated at 600°C is 2–3 times more active than the heteropolycompounds. HMOR10 which is the most active catalyst for n-hexane transformation is the least active for n-butane and especially propane transformation. This very low activity of mordenite for these bimolecular processes can be related to particularities of its pore system: bimolecular reactions are strongly unfavoured in the narrow non-interconnected channels of this zeolite.

Aromatization of butane over gallosilicates

Gallosilicates of various Si/Ga ratios were synthesized, characterized and were found to be MFI-type zeolites. A catalytic study of n-butane over gallosilicate showed that gallium concentration has a pronounced effect on conversion and aromatics selectivity. Butene selectivity at very low conversion showed that dehydrogenation and cracking are the primary activation reactions of butane. Direct dehydrogenation reaction, which does not involve protonic sites, was proposed to occur in the transformation of n-butane over gallosilicate.

Transformation of n-butane over HZSM-5 and other MFI type zeolites

Transformation of n-butane was studied over HZSM-5, silicalite, ferrisilicate and modified HZSM-5 zeolites. It was found that silicalite gives the lowest conversion and no aromatic selectivity. Modification with gallium has increased conversion as well as aromatic selectivity. HZSM-5 gave the highest conversion and Ga-HZSM-5 (ion exchange) the highest aromatic selectivity. In case of ferrisilicate, conversion was better than silicalite and aromatic selectivity was better even than that of HZSM-5. Both conversion and aromatic selectivity increased profoundly when ferrisilicate was impregnated with gallium. Gallium is thought to enhance both conversion and aromatization.

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.

Isomerization ofn-Butane over H-Mordenites under Nitrogen and Hydrogen: Influence of the Acid Site Density

n-Butane transformation was investigated in a flow reactor at 523 K over a series of H-mordenite samples with Si/Al ratios of 6.6, 10, 20, 60, and 80. Under nitrogen, butane isomerization is accompanied by the formation in significant amounts of propane and pentanes. The reaction order with respect ton-butane is close to 2 for isobutane, for propane, and for pentane formation. These observations are in favor of a bimolecular mode ofn-butane transformation involving successively formation of C8carbenium ions, isomerization, and cracking of these C8carbenium ions. A large positive effect of the density of acid sites is found, which can be related to the demanding character of this bimolecular process: two acid sites are required for the formation of the twosec-butyl carbenium ions involved as intermediates. Hydrogen strongly inhibits the transformation ofn-butane, which can be explained by a decrease in the concentration ofsec-butyl carbenium ions. An increase in the selectivity to isobutane and a decrease in the reaction order are observed, which seems to indicate participation of the very difficult monomolecular process (through the very unstable isobutyl carbenium ion) in butane isomerization.

Comparative study of n-butane transformation over [formula omitted] zirconia and [formula omitted

The hydroconversion of n-butane catalyzed by sulfated zirconium oxide (SZ), H-beta (HBEA) and H-mordenite (HMOR) zeolites of different Si Al ratios with the promotion of platinum has been studied at temperature ranges of 240–340°C and at 1–27.2 atm. The results indicate that, with increased pressure and with an increased Si Al ratio of zeolites the cracking as well as the hydrogenolysis enhance the formation of C 1C 3 alkane products. The formation of isobutane by metal-acid bifunctional mechanism is favorable at low pressure and the bimolecular mechanism is enhanced with a higher density of acid sites over Pt Hzeolite catalysts. The transformation of n-butane via an acid-type unimolecular mechanism is an important process on Pt SZ , since Pt SZ possesses less metallic function than Pt ZrO 2 and Pt HBEA as detected by the activity of benzene hydrogenation at 20.4 atm. On Pt SZ , the conversion increases significantly with hydrogen pressure up to a total pressure of 6.8 atm and becomes constant for a total pressure of > 6.8 atm to 27.2 atm.