Skeletal Isomerization Of N-butenes Research Articles

The skeletal isomerization of n-butene over ferrierite catalysts

Abstract The catalytic properties of H-ferrierite zeolites, prepared by hydrothermal and dry-state synthesis, were compared in the skeletal isomerization of 1-butene. The activity was found to be strongly influenced by the presence of extra-framework Al (EFAl) species in the zeolite micropores and by the carbonaceous deposit formed during the reaction. The dynamics of mass transport in the zeolite crystallites were studied by the frequency-response (FR) method using acetylene, propane, n - and i -butane as probe molecules. The selectivity of n -butene conversion showed correlation with the dynamics of the n -butane sorption process. Either the diffusion or the sorption was the rate-controlling slowest step of n -butane mass transport, depending on the amount and the distribution of the EFAl species in the sample. In the catalyst sample, free of EFAl, the transport was found to be controlled by the rate of sorption. At an EFAl content of about 0.5–1.0 EFAl/unit cell the diffusion resistance of the micropores governed the n -butane transport rate.

Catalytic Properties of the Thermal Decomposition Products of Zr(SO4)2 · 4H2O

The catalytic activity of the thermal decomposition products of Zr(SO4)2 · 4H2O in the reactions of 1-butene isomerization to 2-butenes, isobutanol dehydration, and n-butane skeletal isomerization was studied. Their behaviors in typical acid reactions and in skeletal isomerization were found to be considerably different. In the first two reactions, which occur with the participation of proton sites, the activity of zirconium sulfates was an extremal function of hydrate calcination temperature. Zirconium sulfate calcined at 400–550°C was the most active catalyst. The reasons for such behavior are discussed. In the skeletal isomerization of n-butane, crystalline zirconium sulfate was practically inactive, and it became active only after degradation. The results suggest that the activation of n-butane molecules did not occur at proton sites.

Catalysis by heteropoly compounds—recent developments

Recent developments for catalysis by heteropoly compounds as both acid and oxidation catalysts have been described. The acid strength of typical heteropoly acid, H 3PW 12O 40, was found to be higher than that of H 2SO 4 in liquid-state and that of H-ZSM-5 in solid-state. The catalytic activity for acid-catalyzed reactions was greatly enhanced by supporting it on mesoporous silica (MCM-41) and the formation of an acidic Cs salt, Cs 2.5H 0.5PW 12O 40. The Cs 2.5H 0.5PW 12O 40 is hydrophobic and active for “water-tolerant catalysis”. Bifunctional catalysts Pt-Cs 2.5H 0.5PW 12O 40 and its SiO 2-supported catalyst were effective for skeletal isomerization of n-alkanes. Isotopic study using 13 C - n-butane revealed that the isomerization of n-butane proceeds mainly via monomolecular mechanism over Pt-Cs 2.5H 0.5PW 12O 40. The Pt-Cs 2.1H 0.9 PW 12O 40 possessed ultramicropores with the width between 0.43 and 0.50 nm and exhibited shape selectivities for oxidation, hydrogenation, and skeletal isomerization of n-butane. As a typical example for oxidation catalyzed by heteropoly compounds, selective oxidation of methane with hydrogen peroxide has been focussed. The H 4PVMo 11O 40 was found to be very efficient for the reaction. The reaction scheme, active species, and the role of heteropoly anion have been proposed.

Direct evidence of intramolecular rearrangement in skeletal isomerization of n-butane over bifunctional catalysts

Mechanisms of skeletal isomerization of n-butane over bifunctional catalysts, Pt–Cs2.5H0.5PW12O40 and Pt-sulfated ZrO2, as well as the corresponding solid acids were studied using 1,4-13C2-n-butane. The isotopic distributions of the reactant and product were analyzed with field ionization mass spectrometry, by which the parent peak patterns were obtained. It was found that 1,4-13C2-n-butane was selectively isomerized to 13C2-isobutane over these catalysts in the presence of H2 at 423–523 K, while the corresponding solid acids gave isobutane with binomial distributions of 13C. These results clearly demonstrate that the skeletal isomerization of n-butane proceeded mainly via a monomolecular path with intramolecular rearrangement on both the bifunctional catalysts, while it occurred through a bimolecular path with intermolecular rearrangement on the solid acids. This difference in reaction mechanism is reflected on that in the selectivity to isobutane.

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.

Deactivation of medium pore zeolite catalysts by butadiene during n-butene isomerization

The deactivation of the 10-membered ring zeolites MFI, FER and TON was studied during the skeletal isomerization of n-butene in the presence of butadiene. The rate of deactivation per butadiene molecule was found to increase with the concentration of butadiene and the contact time. It decreased in the presence of Pt and hydrogen. FER and TON deactivated much faster than MFI. This was attributed to two factors: (i) Deactivation of TON occurred by pore-blocking, whereas direct poisoning of the acid sites by coke deposits took place on MFI. In the case of FER, coke was only deposited in the 10-membered ring channels, whereas the access to the 8-membered ring channels was blocked. (ii) The build-up of carbonaceous deposits on MFI was slower than that on the other zeolites, because MFI had a high activity for cracking oligomeric coke-precursors into smaller fragments, which desorbed. Although this cracking activity is responsible for the moderate selectivity of MFI for butene isomerization, it is beneficial with respect to the stability against deactivation by butadiene.

Comparative study of the catalytic properties of ferrierite zeolite exchanged with alkaline earth metals in the skeletal isomerization of n-butene

Ferrierite zeolites ion exchanged with alkaline earth cations (Mg2+, Sr2+, Ba2+) were prepared and examined as catalysts for the skeletal isomerization of 1-butene. The samples were characterized by XRD, atomic absorption spectroscopy, ammonia TPD, FT-IR, BET/pore size distribution and pyridine chemisorption. The acidity studies indicated that acid strength increases in the order H+>Mg2+>Sr2+>Ba2+, therefore, the number of weak acid sites follows the opposite trend. At the same time, Bronsted acid sites associated with protons disappear and the number of Lewis acid sites increases with metal content. The presence of bulky cations like Mg2+, Sr2+, Ba2+ leads to a slight reduction of surface area and pore size. Ferrierites containing magnesium exhibit the highest isobutene yield. Lewis acid sites are responsible for improvement in catalyst stability as well as suppression of coke formation.

Selective isomerization of n-butenes to isobutene on high Si/Al ratio ferrierite in the absence of coke deposits: implications on the reaction mechanism

High Si/Al ratio ferrierite (Si/Al=59) synthesized using pyridine as structure-directing agent was found to show a high selectivity from very short times on stream (5 min TOS) during the skeletal isomerization of n-butenes. Such a high initial selectivity is accompanied by an unusually high stability with TOS even under reaction conditions that favor the oligomerization of the olefins (low temperatures and high n-butenes partial pressure). In contrast to what has been previously established for ferrierite with lower Si/Al ratios, the isomerization reaction occurs with a high selectivity without appreciable formation of carbonaceous deposits, as determined by elemental and thermogravimetric analysis of the zeolite after reaction. Thus, at 350°C and 0.1 atm partial pressure only about 0.3 wt% carbon was formed after 3.5 h on stream on the high-silica ferrierite (FER59 sample), whereas it amounted to ca. 5 wt% for a commercial low Si/Al ratio ferrierite (Si/Al=8.5, FERTOS sample) under the same reaction conditions. As a result, the acidity and porosity of the FER59 ferrierite remained unaltered after 3.5 h on stream under the above conditions, while both parameters significantly decreased for the FERTOS catalyst. These results, together with the analysis of the initial selectivity with temperature and partial pressure, indicate that isobutene is selectively formed in the ferrierite structure through a monomolecular reaction mechanism without the need of invoking the presence of coke deposits. It appears that reducing the density of acid sites in ferrierite by increasing the framework Si/Al ratio to the required levels is the key parameter to control the stability and selectivity of the zeolite during n-butenes isomerization.

Extent of monomolecular and bimolecular mechanism in n-butene skeletal isomerization to isobutene over molecular sieves

Mechanism of skeletal isomerization of n-butene to isobutene over CoAlPO-11 and H-ferrierite molecular sieves was investigated using 13 C labeled 1-butene. It was confirmed that a high selectivity to isobutene can be reached exclusively via a monomolecular reaction pathway. The bimolecular mechanism is responsible not only for the formation of propylene, pentenes, and other by-products but also for a substantial amount of isobutene. At least 30% of isobutene formed over ferrierite at 620 K was produced via dimerization of butenes followed by isomerization and cracking.

The role of deposits in butene isomerisation

The role of carbonaceous deposits in the skeletal isomerisation of n-butene was studied in a pulse reactor. High activity and selectivity were observed under conditions when the catalyst was almost free of carbonaceous deposits. Carbonaceous deposits deactivate the catalyst and suppress both isobutene and by-product formation. However, since reactions leading to by-products are suppressed more strongly, the relative concentration (yield) of isobutene might increase during the first few pulses. It is concluded that a selective monomolecular mechanism operates on the uncovered OH-groups. Anchoring hydrocarbon molecules on a surface is a more difficult step than subsequent oligomerisation. Deposits (mostly oligomers) that form early, crack non-selectively to isobutene and larger amounts of propene and pentenes.

Isomorphous substitution in ZSM-22 zeolite. The role of zeolite acidity and crystal size during the skeletal isomerization of n-butene

The influence of acid-site density, acid strength and crystal size on the skeletal isomerization of 1-butene using Al-, Ga-, and Fe-ZSM-22 zeolites with different Si/T ratios has been investigated. The highest isobutene selectivity (90% selectivity at 40% conversion) was found with the Fe-ZSM-22 which has milder acid sites. The fact that some iron comes out from the zeolite framework during the treatment of the catalyst, makes that the activity and selectivity of the ironsilicates is similar regardless of the starting Si/Fe ratio. This was not the case for the aluminosilicates, for which the isobutene selectivity increases when decreasing the acid-site density, i.e. when increasing the Si/Al ratio. The zeolite synthesis procedure (static or rotating conditions) produces samples with different crystal sizes. This has important implications on zeolite stability, activity and selectivity. All samples display a stable behavior with time-on-stream, and the kinetic results suggest that isobutene is formed by a monomolecular reaction pathway.

FT-IR Studies on Light Olefin Skeletal Isomerization Catalysis: III. Surface Acidity and Activity of Amorphous and Crystalline Catalysts Belonging to the SiO2–Al2O3System

The surface acidity of both amorphous and crystalline materials belonging to the SiO2–Al2O3system (i.e., amorphous silicas, amorphous silica–aluminas with different Al contents, HZSM5 and H-ferrierite (FER) zeolites, and silicated and pure γ-Al2O3) have been studied using the FT-IR spectroscopy of the surface hydroxy groups and of adsorbed acetonitrile (AN) and pyridine (Py). Lewis bonded and H-bonded species of AN have been observed, while a protonated form of adsorbed AN has been characterized on HZSM5. A molecular sieve effect has been found for Py adsorption on FER and HZSM5. The catalytic activity of these materials in the skeletal isomerization of 1-butene to isobutene has also been investigated. The Brønsted surface acidity scale found correlates well with the catalytic activity in 1-butene conversion, measured after 20–40 min time on stream. The selectivity to isobutene follows nearly an inverse trend, except for FER, which shows both higher activity and higher selectivity than silica–alumina, quite good resistance to coking, and selectivity increasing with time on stream. Comparative analysis of the behavior of these catalysts and of the spectrum of their surface OH groups gives new light on the structure of the acidic OHs on materials belonging to the SiO2–Al2O3system. The usefulness of zeolite and alumina-based catalysts for skeletal isomerization ofn-butene to isobutene is also briefly discussed.

Pore structure and shape selective catalysis of bifunctional microporous heteropoly compounds

The pore structure of Pt-promoted microporous cesium salts of H 3PW 12O 40 and their shape selective catalysis were studied. Pt-Cs 2.1H 0.9PW 12O 40 (0.5 wt% Pt) has only ultramicropores of about 5 Å in width; Pt-Cs 2.5H 0.5PW 12O 40 (0.5 wt% Pt) possesses mesopores as well as micropores. These were revealed by pore size distributions derived from N 2 adsorption-desorption isotherms and adsorption of a variety of molecules having different sizes. While Pt-Cs 2.5H 0.5PW 12O 40 as well as 0.5 wt% Pt/SiO 2 were active for hydrogenation of ethylene, cyclohexene, or cyclooctene, only ethylene was hydrogenated over Pt-Cs 2.1H 0.9PW 12O 40, indicating that Pt-Cs 2.1H 0.9PW 12O 40 exhibits reactant shape selectivity by its constrained pores. In skeletal isomerization of n-butane, the selectivity to isobutane was found to depend significantly on the pore width of Pt-promoted heteropoly compounds and Pt-exchanged zeolites; the selectivity decreased as the pore width became smaller, this is probably due to product shape selectivity.

The role of shape selectivity and intrinsic selectivity of acidic sites of the catalysts in the skeletal isomerization ofn-butenes

The effect upon kinetic behaviour (activity and selectivity) of the physical properties (pore volume distribution) and acidic characteristics (nature of acidic sites and acidic strength distribution) of acidic catalysts in the skeletal isomerization of n-butenes has been studied. The catalysts studied are the following: γ-alumina, alumina modified by introduction of F, Cl and Br, silica–alumina, HZSM-5 zeolite-based catalyst and SAPO-34-based catalyst. It has been proven that the selectivity of acidic catalysts towards the skeletal isomerization of n-butenes is the result of a compromise between pore shape selectivity and intrinsic selectivity of the acidic sites of different strength, this latter property being more significant. Strong acidic sites are needed for the skeletal isomerization, but sites which are too strong produce important amounts of byproducts. A chlorinated alumina is a highly active and selective catalyst for skeletal isomerization. © 1998 SCI.

Skeletal Isomerization ofn-Butenes: II. Composition, Mode of Formation, and Influence of Coke Deposits on the Reaction Mechanism

On a fresh HFER zeolite (Si/Al=13.8) at 623 K, the skeletal isomerization ofn-butenes is accompanied by various reactions: disproportionation into propene and pentenes, coking, hydrogen transfer (in the decreasing order of significance). While there is a rapid deactivation of disproportionation, coking, and hydrogen transfer reactions, owing to the blockage of the pores by carbonaceous compounds (coke), an increase in the rate of isomerization is at first observed followed by a decrease at long time-on-stream. At the maximum of isomerization only 10% of the pore volume remains accessible to nitrogen adsorbent. Moreover, IR spectroscopy shows that 75% of the OH groups are in interaction with coke molecules, the remaining 25% being inaccessible to ammonia and, hence, to the reactant. Coke was found to be composed of slightly condensed aromatics (with two to four aromatic rings) trapped at the intersections of the eight- and 10-membered-ring channels. Coke molecules result from the transformation of secondary benzenic products, slowly desorbed from the zeolite pores, through two different ways, (i) the classical route via alkylation, cyclization, and hydrogen transfer steps and (ii) dehydrogenative coupling. The large significance of this latter reaction in the formation of HFER coke molecules in comparison to the other zeolites, can be related to the proximity of adjacent channel intersections. To explain the initial increase in the rate of butene isomerization a new reaction process is proposed involving as active sites benzylic carbocations formed from coke molecules trapped in pores located near the outer surface of the crystallites (pore mouth catalysis). The deactivation of this reaction which occurs at long time-on-stream is shown to be due to the growth of the active coke molecules.

Alkane Isomerization over Sulfated Zirconia and Other Solid Acids

Some solid acids, including sulfated zirconia and certain industrial isomerization catalysts, catalyze two types of n-butane isomerizations, avoiding primary carbenium ions or carbonium ions: (1) an internal rearrangement of the C atoms in n-butane and (2) skeletal isomerization of n-butane to iso-butane. No superacid sites are required for these reactions. The skeletal isomerization is an intermolecular reaction, involving a C8 intermediate. Easily accessible Bronsted acid sites and small amounts of olefin are crucial. Spectroscopic examination of the acid sites on sulfated zirconia shows that they are not stronger than the acid sites in zeolites such as HY. The butane isomerization rate is suppressed by CO, even when no CO is adsorbed on Lewis sites; formation of oxocarbenium ions is likely. The decisive role of Bronsted acid sites is demonstrated by results on deuterated catalysts.

Deactivation and Regeneration of a Chlorinated Alumina Catalyst Used in the Skeletal Isomerization of n-Butenes

The skeletal isomerization of n-butenes over a chlorinated alumina catalyst has been studied, by operating under reaction−regeneration cycles in an isothermal fixed-bed integral reactor. The limiting values of the reaction and regeneration temperatures for avoiding irreversible deactivation by loss of chlorine have been determined. Reversible deactivation by deposition of coke, which is completely removable by combustion, is unavoidable. The effect of the operating conditions upon the deposition of coke and the deterioration of the porous structure as a consequence of the deposition have been analyzed.

Synthesis, characterization, and catalytic properties of a novel Germano-aluminophosphate molecular sieve with AEL structure: GeAPO-11

A novel GeAPO-11 material has been synthesized and characterized. It is observed that Ge is incorporated in the molecular sieve framework, creating a new Bronsted acid site [Ge(OH)Al]. This catalyst is effective for skeletal isomerization of n-butene.

Kinetic modelling for selective deactivation in the skeletal isomerization of n-butenes

The selective deactivation by coke deposition in the skeletal isomerization of n-butenes over a chlorinated alumina catalyst has been studied. The methodology used is based on that described in previous work (Gayubo et al., 1993), where the selective deactivation of a silica-alumina catalyst used in the double bond isomerization of n-butenes was studied. For this reaction, the triangular kinetic scheme establishes the interconversion between 1-, cis- and trans-butene.

Catalysis by Heteropoly Compounds

Skeletal isomerization ofn-butane has been studied over bifunctional catalysts consisting of noble metals and an acidic cesium salt of12-tungstophosphoric acid, Cs2.5H0.5PW12O40. At 473 K in the presence of 0.05 atm of H2, 1 wt% Pt-Cs2.5H0.5PW12O40(1% in weight of Pt impregnated to Cs2.5H0.5PW12O40) showed activity and selectivity comparable to 1 wt% Pt-sulfated zirconia, while at 573 K and 0.5 atm of H2, 1 wt% Pt- and Pd-Cs2.5H0.5PW12O40became muchmore active and selective for the formation of isobutane than 1 wt% Pt-sulfated zirconia and 1 wt% Pt-exchanged ZSM-5. Negative dependence of the initial reaction rate on H2pressure and little dependence on Pt loading level for Pt-Cs2.5H0.5PW12O40indicate that the reaction proceeds through a bifunctional mechanism in whichn-butane is rapidly dehydrogenated ton-butenes on Pt,n-butenes isomerize to isobutylene on acid sites, and then isobutylene is hydrogenated to isobutane on Pt. A remarkable effect of the presence of hydrogen was the repression of the catalyst deactivation. Thisis probably caused by the hydrogenation of coke and/or its precursor and a decrease in the butene partial pressure. Very interesting was the unique function of protons; protons present in proximity of Pt or Pd particles suppress the hydrogenolysis that takes place on Pt or Pd and greatly increase the selectivity to isobutane.