Skeletal Isomerization Of N-butenes Research Articles

Seeds Combining Pyrrolidine Control the Framework Al Distribution of FER Zeolite to Enhance Its Performance in the Skeletal Isomerization of n-Butene

FER zeolites have a unique framework structure and acid distribution, and are widely studied as a catalyst for reactions such as n-butene skeletal isomerization and dimethyl ether carbonylation. The Brönsted acid site (BAS) located in the 10-member ring (10-MR) of FER zeolites serves as the active site for the isomerization reaction of skeletal n-butene to produce isobutene. This study prepared five types of FER zeolites using different methods: using pyrrolidine (PY) alone as a template; using Na-form FER as seeds (SN) or H-form FER as seeds without organic structure directing agents (OSDAs); and combining the seeds of SN or SH with PY as OSDAs. The differences in the structure and acid distribution of the five zeolites were investigated, as well as their catalytic performance for the skeletal isomerization of n-butene. Experiments and characterization results showed that under hydrothermal synthesis conditions, the FER-PY+SH zeolites synthesized by using both H-form zeolites seeds and pyridine exhibited the highest aluminum concentrations at T1 and T3 sites, along with the greatest BAS located in the 10-MR. This unique composition contributed to the highest selectivity of isobutene. The FER-PY+SH catalyst was continuously used for 720 h at 350 °C, 0.1 MPa, and an n-butene mass space velocity of 2.0 h−1 for three cycles of 2160 h. During this period, the conversion of n-butene was over 39%, while the selectivity of isobutene exceeded 95%. The FER-PY+SH catalyst exhibited excellent stability and activity.

Engineering pore structure and acidity of ferrierite zeolites by organic structure directing agents for n-butene isomerization

Zeolites are important heterogeneous catalysts in industrial applications, but always suffer from deactivation caused by carbon deposition. In this work, a simple strategy was developed to engineer Ferrierite (FER) zeolites with different pore structure and distribution of Brønsted acid sites by using different organic amine structure directing agents, i.e., pyrrolidine (PY), ethylenediamine (EDA), cyclohexylamine (CyHA). The obtained PY-FER not only exhibited optimal hierarchical porosity, but also showed the highest density of Brønsted acid sites inside micropores and lowest Lewis/Brønsted acid ratio (L/B). Based on the corresponding performance in n-butene skeletal isomerization, it can be concluded that high L/B is prone to coke formation, while a balance between micropores and mesopores is beneficial to the reduction of carbon deposition. Besides, a model y=y0−Aexp(−Kt) was established to describe the behavior of the carbon deposition, which matches well with the properties of the zeolites. These findings are significant for the design of zeolites.

Synthesis of Nano‐Hierarchical Ferrierite with Sole Template and Its Catalytic Application in n‐Butene Skeletal Isomerization

AbstractNano‐hierarchical ferrierite(FER) was successfully synthesized by using pyrrolidine as sole organic structure directing(OSDA) agent under static hydrothermal conditions.SEM images showed that the morphology of the product was a loose three‐stage accumulation structure by nano‐rod crystal grains .The influence of crystallization temperature, gel alkalinity, crystallization time and the amount of OSDA on synthesis of FER was investigated in detail.Moreover,FER catalytic performance in n‐butene skeletal isomerizatio was investigated.The results showed that, compared with the commercial FER, the nano‐hierarchical FER showed better catalytic activity.

Templated coordination as a tool to increase the catalytic activity of metal aluminophosphates: the case of CoAPO-11

Site shifting by templated coordination aimed at enhancing the performance during n-butene skeletal isomerization under severe conditions.

Deactivation of industrial alumina catalyst for the skeletal isomerization of n-butenes

The reasons for deactivation of alumina catalyst during the skeletal isomerization of n-butenes, reducing conversion from 31 to 26%, are investigated. It is shown that contamination with metal Fe and Mg microimpurities lowers n-butene conversion in total by approximately 2.5%. The catalyst’s activity can be restored by washing out metal microimpurities. Water vapor and feedstock pipelines turn out to be the source of metal impurities. The δ-modification of alumina accumulates during catalyst operation, reducing n-butene conversion by ∼3 abs % and creating an irreversible type of deactivation. The drop in the temperature of phase transition in the presence of Fe and Mg microimpurities is one possible reason for the partial phase transition at a temperature lower than 600°C. It is shown that the accumulation of microimpurities and the alumina δ-phase reduces the number of acid sites and thus the efficiency of alumina catalytic conversion. To prolong the catalyst’s operational life, it is recommended that metal impurities be captured from the vapor-feedstock flow by installing a protective layer of a macroporous material with a developed surface inert to the butane fraction upstream of the catalyst.

Effect of crystal size on the skeletal isomerization of n-butene over H-FER zeolite

Three H-FER zeolites with different crystal sizes were successfully obtained via the conventional static hydrothermal method by introducing an aging step to the synthesis procedure. The samples were characterized by XRD, SEM, TEM, NH3-TPD, XRF, and BET analyses. While the number of acid sites decreased with decreasing crystal sizes, the H-FER zeolite with submicron-scale crystal sizes exhibited not only high activity but also good stability during the skeletal isomerization of n-butene. These results are attributed to the large external surface area and abundance of mesopores in the sample. The resulting isobutylene yield reached approximately 45 wt%. The concept of pore mouth catalysis over H-FER zeolites for the reaction was also supported by catalytic performance varied with crystal size.

Influence of hydrothermal treatment conditions on the structure and catalytic activity of alumina during the skeletal isomerization of n-butenes

In order to obtain an efficient alumina catalyst for skeleton isomerization of n-butenes with a high catalytic activity, we study the influence of hydrothermal treatment (HTT) of alumina systems at 150–200°C on the parameters of the crystalline and pore structure, and acid-base properties of industrial γ-Al2O3. It is shown that the HTT of aluminum hydroxide increases the sizes of microcrystallites and reduces the alumina’s specific surface area and the number of acid-base centers. This reduces the activity in the reaction of skeletal isomerization of n-butenes. HTT of the two-phase alumina-aluminum hydroxide system produces smaller crystallites of γ-Al2O3 and raises the acidity of the alumina obtained after calcination at 550°C; as a result, the catalytic activity increases. This method can be used to enhance the activity of industrial samples of alumina in the reaction of skeletal isomerization of n-butenes.

Isomerization of n-butane on Pt (100): Monomolecular mechanism investigation

Density Functional Theory calculations have been performed on a skeletal isomerization of n-butane into isobutane in the absence and presence of hydrogen on a Pt (100) ultra thin film. A pathway implying monomolecular mechanism for the isomerization is assumed. A cyclic type mechanism is induced by the absence of hydrogen while a bond-shift mechanism is favored by its presence. Also, the presence of hydrogen facilitates isomerization by decreasing the energy barrier from 3.41 eV to 2.77 eV. The platinum thin film surface essentially acts as a hydrogen attractor. Afterwards the internal rearrangement of atoms of the activated hydrocarbons results in isomerization.

Characterization of catalysts in their active state by adsorption microcalorimetry: Experimental design and application to sulfated zirconia

A method to characterize the surface sites of catalysts in their active state by adsorption microcalorimetry was developed. A calorimeter cell was used as a flow-type reactor, and the skeletal isomerization of n-butane (1 kPa) at 378 K and atmospheric pressure proceeded at comparable rates and with the same states of induction period, maximum and deactivation phase as in a tubular reactor. The reaction was run for selected times on stream and after the removal of weakly adsorbed species, n-butane or isobutane were adsorbed at 313 K. The surface of activated sulfated zirconia was characterized by at least two different sites for n-butane adsorption, a small group of sites (about 20 μmol g −1) that yielded heats of 50–60 kJ mol −1 and sites that were populated at higher pressures (above about 5 hPa n-butane) and yielded heats of about 40 kJ mol −1. The strongly interacting sites disappear during the induction period and are proposed to be the sites where the isomerization reaction is initiated.

Acid properties and catalytic performance for phosphonic acid modified MCM-49 zeolites in skeletal isomerization of n-butene

Abstract The acid properties of MCM-49 zeolites were modified by phosphonic acid via a simple impregnation method. The physicochemical properties of the samples were well characterized by XRD, NH 3 -TPD, FT-IR of d 3 -acetonitrile adsorption, 27 Al MAS NMR, and their activity in the skeletal isomerization of n -butene was measured as well. It was found that the concentrations of Bronsted and Lewis acid sites have been markedly reduced by the phosphonic acid impregnation. The catalytic results indicated that Bronsted acid sites were active sites for the skeletal isomerization of n -butene, while Lewis acid sites almost had no effect.

N-Butene skeletal isomerization over HFER zeolites: Influence of Si/Al ratio and of carbonaceous deposits

The effect of time-on-stream TOS (from 1 min to 21.5 h) on the rate and selectivity of n-butene transformation at 350 °C was determined over a series of HFER zeolites with different Si/Al ratios (6, 9.5, 10, 19 and 32). The porosity and acidity of the fresh and aged samples were characterized by nitrogen adsorption and by NH 3 adsorption at 50 °C followed by FTIR, respectively. The main differences between the samples were the following: (i) increase with Si/Al in the activity of the protonic sites (turnover frequency) of fresh samples and in their selectivity for isobutene production, (ii) decrease in the rate of isobutene production with increasing TOS for high Si/Al ferrierites as could be expected from coke formation and complex change for low Si/Al ferrierites with in particular an unexpected increase. Furthermore with all the samples, the turnover frequency for isobutene production increases with TOS whereas the one for propene + pentenes decreases or remains constant. All these observations can be explained by considering the zeolite pore system as a series of non-interconnected nanoreactors (the 10-MR channels) in which the molecules diffuse without any possibility of desorption before their exit while undergoing successive bimolecular reactions. In those with few protonic sites, n-butene isomerizes selectively through an autocatalytic process. In those with many sites a thermodynamic equilibrium mixture of propene, butenes and pentenes is formed.

Skeletal Isomerization of n-butenes over Solid Acid Catalysts

Conversion reactions of n-butenes over zeolites and amorphous catalysts have been investigated to deduce the factor that determines the selectivity for the skeletal isomerization producing isobutene. The effects of pore structure and acid site concentration on the selectivity for the skeletal isomerization are discussed on various catalysts. The pore structures of FER and CLI zeolites induce the distant locations of butene molecules, accelerating monomolecular skeletal isomerization. On the other hand, acid site concentration determines the preferred reaction path of n-butenes on amorphous catalysts. Oligomerization followed by cracking that produces various hydrocarbons is suppressed on the catalyst with low acid site concentration, resulting in high selectivity for isobutene. The feasibility of monomolecular skeletal isomerization on zeolites and amorphous catalysts is confirmed by its reversibility with high selectivity.

Selective skeletal isomerization of n-butene over modified MCM-22 zeolite catalysts

Modification of HMCM-22 zeolite with tetraethoxysilane (TEOS) can considerably improve the catalytic performances, especially the selectivity to isobutene, for the reaction of n-butene skeletal isomerization.

Mechanism of n-butene skeletal isomerization over HFER zeolites: a new proposal

The effect of time on stream (from 1 min to 44 h) on the rate and selectivity of n-butene isomerization and on the concentration of protonic sites interacting with ammonia or deuteroacetonitrile was determined over a HFER zeolite with a Si/Al ratio of 10. Aging for 44 h was shown to increase the selectivity from 55 to 95% and the turnover frequency for isomerization from ∼250 to 4000 h −1. All of this can be explained by considering the HFER pore system as equivalent to a series of noninterconnected nanoreactors (the 10-membered ring channels) into which the reactant molecules have to diffuse without any possibility of desorption before their exit while undergoing various successive bimolecular reactions. In nanoreactors with few protonic sites, n-butene isomerizes selectively through an autocatalytic process; in those with many sites a thermodynamic equilibrium mixture of propene, butenes, pentenes, and deactivating carbonaceous species is formed.

On the Effect of the Strength of Acid Sites in Heterogeneous Catalysts on the Activity in the Skeletal Isomerization of n-Butane

The catalytic activity of three groups of acid catalysts different in the nature and strength of acid sites in the skeletal isomerization of n-butane was studied. It was found that the strength of the sites did not correlate with the rate of the reaction.

Gas-phase hydration of ethene over tungstena–zirconia

Hydration of ethene over a variety of tungstena–zirconia (WO 3/ZrO 2) catalysts of different W/Zr ratios using different calcination temperatures is studied at 473 K using a continuous fixed-bed reactor at atmospheric pressure. The maximum activity for ethene hydration is obtained by calcination at 1073 K with a W/Zr ratio of 0.4, while skeletal isomerization of n-butane is found to exhibit a maximum rate at W/Zr=0.1. The most active WO 3/ZrO 2 catalysts is demonstrated to be superior in activity to other typical solid acids such as H 4SiW 12O 40/SiO 2, zeolites (H-ZSM-5 and H-β), various mixed oxides, and polymer resins. Lifetime tests reveal that constant activity of WO 3/ZrO 2 obtained at 72 h is about 70% of initial activity and that the activity is largely recovered by calcination at 773 K in air. The specific activity (per surface area) of WO 3/ZrO 2 is correlated with the hydrophobicity as estimated from the adsorption of water rather than with that estimated from the density of acid sites measured by NH 3-TPD. Therefore, the pronounced activity of WO 3/ZrO 2 is mainly brought about by the high hydrophobicity of the surface along with the inherent acidity.

Oligomerization as an important step and side reaction for skeletal isomerization of linear butenes on H-ZSM-5

The skeletal isomerization of n-butenes on a H-ZSM-5 zeolite with high Si/Al ratio was investigated by using a gradientless recirculation batch reactor. It was concluded that at the very beginning of the reaction, the formation of C 8 surface species should be the first step. These species decompose into propene and pentenes, one part of the just formed and still adsorbed C 3 species immediately dimerizes to hexenes. In the further course of reaction, an oligomerization to C 12 surface species (trimerization) occurs followed by decomposition as shown by the products pentenes, hexenes, and heptenes. The tendency to oligomerization decreased with increasing temperature. For the reaction of lower olefins like propene and ethene, the reaction course was the same as it was observed for n-butene: a gradual increase of the degree of oligomerization with reaction time followed by immediate decomposition.

Dealumination of ferrierite by ammonium hexafluorosilicate treatment: characterization and testing in the skeletal isomerization of n-butene

Ferrierite zeolite has been treated with ammonium hexafluorosilicate (AHFS). The prepared catalysts were characterized by XRD, atomic absorption spectroscopy, ammonia TPD, FT-IR, SEM, BET/pore size distribution and pyridine chemisorption. Their catalytic activity has been evaluated in the skeletal isomerization of n-butene. Ammonium hexafluorosilicate treatment of ferrierite zeolite leads to a little but progressive framework dealumination, although bulk Si/Al ratio determined by chemical analysis only increases slightly, indicating that most of the extracted aluminum atoms remain in the pores. Higher fluorosilicate amounts decrease crystallinity, micropore area, and volume. Ammonium hexafluorosilicate treatment remove mainly ferrierite external acid sites with a marked improvement in the number of medium strength acid sites. The higher dealumination degree the lower isobutene yield, however, the presence of extraframework aluminum species, medium strength acid sites and lower acid site density reduces the conversion of n-butene through consecutive reactions like n-butene dimerization followed by cracking and hydrogen transfer. These reactions are responsible for the formation of by-products and hence dealuminated catalysts show isobutene selectivity values as high as ferrierite zeolite.

Probing the Accessible Sites for n-Butene Skeletal Isomerization over Aged and Selective H-Ferrierite with d3-Acetonitrile

Aged H-ferrierite (H-FER) samples with different contents of deposits were prepared and studied under differential catalytic conditions in a tapered element oscillating microbalance. Subsequently, these samples were examined using infrared spectroscopy to determine the nature of carbonaceous deposits and probing the type and number of accessible sites with d 3-acetonitrile. From these results, for the first time, we have been able to calculate turnover frequencies (TOFs) for n-butene conversion and isobutene formation for both fresh and aged H-FER catalysts. It is observed that the deposition of carbonaceous species significantly lowers the number of accessible Brønsted sites. With short time on stream (TOS), cracking of the alkyl-aromatic deposits contributes to the overall isobutene production but simultaneously harms the selective catalytic action by inducing nonselective side reactions. With prolonged TOS these deposits become nonreactive and as a result the TOF drops, while isobutene is produced with high selectivity. It is demonstrated that at this stage no carbenium ions are detected, while Brønsted OH acid sites are still accessible for d 3-acetonitrile on the extensively aged and highly selective H-FER. This indicates that the latter sites are responsible for the selective catalytic conversion of n-butene into isobutene.

Unusual acceleration of acid-catalyzed reactions by water in the presence of Mo/Zr mixed oxides calcined at high temperatures

Various acid-catalyzed reactions, such as hydrolysis of ethyl acetate, esterification of acetic acid with ethanol, dehydration of 2-butanol, isomerization of 1-butene, and skeletal isomerization of n-butane, in the presence of Mo–Zr mixed oxides and other solid acids have been studied. MoO3/ZrO2 exhibited high activities only for the hydrolysis and esterification reactions, in which these high catalytic activities could not be explained by the acidic property of MoO3/ZrO2 alone, as shown by NH3–temperature programmed desorption (TPD) studies. It was found that the addition of water significantly accelerated the esterification and dehydration in the presence of MoO3/ZrO2 (calcined above 973 K), wheras SiO2–Al2O3 was greatly deactivated by water under similar reaction conditions. Furthermore, in the presence of water vapor, the activity of MoO3/ZrO2 for the isomerization of 1-butene did not decline, whereas SiO2–Al2O3 became inactive completely. IR measurements revealed that the water adsorbed on MoO3/ZrO2 (calcined at 1073 K) gave a sharp peak at 1628 cm−1, and that this peak gradually decreased at 423 K, while ZrO2 only or MoO3/ZrO2 (calcined at 673 K) did exhibit well-defined peaks. Since the adsorption-desorption behavior of water was in good agreement to the catalytic activity for dehydration of 2-butanol, the acceleration by water was presumably a result of the generation of acid sites by the dissociation of water molecules that are coordinated on the Mo species of MoO3/ZrO2.