Cyclohexene Skeletal Isomerization Research Articles

Highly selective skeletal isomerization of cyclohexene over zeolite-based catalysts for high-purity methylcyclopentene production

Cyclohexene skeletal isomerization towards methylcyclopentene is an economically favorable process due to the higher added value of the product. Traditional oxide-based catalysts face the challenge of achieving both high activity and stability. In this work, cyclohexene skeletal isomerization is achieved under mild conditions over designed zeolite-based catalysts with 96.8 wt.% liquid yield, 95.8 wt.% selectivity towards methylcyclopentene and satisfactory stability for multiple runs. The favorable performance is attributed to the unique acidic, structural and morphological features of the optimized cobalt/NaUZSM-5 catalyst. Further experimental data and DFT studies suggest that a carboncationic mechanism might be followed and that the reaction mainly occurs within the internal pores of the zeolite structures.

Mesoporous Molecular Sieves Modified with Carbonaceous Deposits

There were investigated aluminosilicate MCM-41 samples in the as-prepared form and those modified by the deposition of carbonaceous compounds during conversion of cyclohexene for 12 h. The amount of the deposits decreased with the rising reaction temperature and increased with the Al content of the samples. The cyclohexene conversion followed mainly two mechanisms: cyclohexene skeletal isomerization and hydrogen transfer. The products with 6 carbon atoms in a molecule prevailed in all cases. The process of conversion, proceeding on the Bronsted acid sites, resulted in formation of both coke deposits and volatile products. The creation of coke caused a decrease in the effective concentration of both the Bronsted and the Lewis acid sites. Thermodesorption of pyridine showed that (i) the concentration of these sites before and after the conversion differed only slightly and (ii) the acidic strength of the Bronsted sites was practically independent of their concentration and the sample Si/Al ratio. The chemical composition of the deposits was insignificantly affected by the Al content of the materials and depended mostly on the temperature and duration of the reaction. At relatively low temperatures, both aliphatic and aromatic compounds were formed, being rather weakly bound to the surface of the material. After a longer (55 h) reaction period, some deposits appeared that were strongly bound to the surface. Isotherms of adsorption of water, benzene, and nitrogen were determined, from which a mechanism of this process was derived. It included most probably multilayer adsorption at lower relative pressures, followed by capillary condensation. The sorption capacities of the uncoked samples for benzene and nitrogen were relatively high and independent of the sample Al content. In the case of water, however, an observed reduction in the sorption capacity with the increasing Al content suggested that clusters of the adsorbed molecules formed around the Al centers and caused partial clogging of the material pores. The deposited coke strongly decreased both the surface area and the sorption capacity of the materials.

Structure, Texture, Surface Acidity, and Catalytic Activity of AlPO4–ZrO2(5–50 wt% ZrO2) Catalysts Prepared by a Sol–Gel Procedure

A series of AlPO4–ZrO2(APZr) systems with various zirconia loadings (5–50 wt%) were prepared by sol–gel deposition of zirconia onto AlPO4and characterized by TGA/DTA, XRD, DRIFT, laser-Raman spectroscopy, SEM–EDX, XPS,27Al and31P MAS NMR, and nitrogen adsorption. Surface acid characterization was carried out through pyridine chemisorption and cyclohexene skeletal isomerization reaction. Addition of ZrO2onto AlPO4prevented the formation of monoclinic zirconia and increased the temperature of crystallization into the tetragonal form, so that APZr systems were still amorphous after calcination at 1073 K. This higher thermal stability was responsible for its large surface area as well as pore volume in relation to pure components. SEM showed nonhomogeneous distribution in morphology, texture, and particle size. Also, EDX and XPS indicated that the surface concentration ratios were practically the same as the expected values of stoichiometric composition. Moreover,27Al and31P MAS NMR showed that the local structure of the AlPO4support did not change with zirconia. Good correlations were obtained between catalytic activities in cyclohexene skeletal isomerization and surface acid densities of AlPO4–ZrO2systems (as determined by pyridine chemisorption at 373 and 573 K).

Gas-phase phenyl acetate conversion on AlPO4, γ-Al2O3 and SiO2 catalysts

The conversion of phenyl acetate over AlPO4 (Al/P=1), γ-Al2O3 and SiO2 catalysts generated phenol, by deacetylation, ando-hydroxycetophenone, by Fries rearrangement, as the main reaction products. The activity for Fries rerrangement was in accordance with the acidity data measuredversus cyclohexene skeletal isomerization. Thus, AlPO4 showed the highest activity. Moreover,o-hydroxy-acetophenone formation increased with the reaction temperature. Besides, in AlPO4 catalysts 4-methylcoumarin and 2-methylchromone were also found, although in low amounts.

Acidity and catalytic activity of AlPO 4–B 2O 3 and Al 2O 3–B 2O 3 (5–30 wt% B 2O 3) systems prepared by impregnation

The surface acidic properties of a series of AlPO 4–B 2O 3 (APB) and Al 2O 3–B 2O 3 (AlB) systems, prepared by supporting B 2O 3 from boric acid solutions using an incipient wetness method and containing different boria loading (5–30 wt%), have been studied by adsorption (at 573 K) of pyridine (PY) and 2,6-dimethylpyridine (DMPY) using a pulse-chromatographic method. Acidity measurements show that Lewis and Brønsted sites exist in the APB and AlB systems at all boria loadings. Moreover, the concentration of Brønsted acid sites increases with boria content. This increased surface Brønsted acidity is responsible for their improved performance in the cyclohexene skeletal isomerization (SI) reaction, to 1- and 3-methylcyclopentenes (MCPE), especially at high boria loadings. Comparison of the activity and acidity measurements points to a correlation between the activity and the number of Brønsted acid sites on the APB and AlB catalysts. Moreover, at high boron contents, AlB catalysts are more acidic and hence more active than APB catalysts. Furthermore, the more acidic catalyst (AlB, with 30 wt% boria) also showed catalytic activity for the cyclohexene hydrogen transfer (HT) reaction leading to methylcyclopentane and cyclohexane.

Gas‐phase pinacol conversion on AlPO4, γ‐Al2O3 and SiO2 catalysts

Pinacol (2,3‐dimethyl‐2,3‐butanediol) conversion over AlPO4 (Al/P = 1) and γ‐Al2O3 catalysts proceeded in two parallel reaction pathways with formation of 2,3‐dimethyl‐1,3‐butadiene (by 1,2‐elimination) and 3,3‐dimethyl‐2‐butanone (by rearrangement), the latter being the main reaction product. The activity was in accordance with the surface acidity data as measured versus cyclohexene skeletal isomerization reaction. Thus, AlPO4 showed the highest activity (almost total conversion at 523 K). The 1,2‐elimination/rearrangement ratio depended on the type of catalyst used and diene formation increased with reaction temperature. Moreover, pinacolone was a reaction intermediate for diene production.

Effect of preparation method on the surface acidity and catalytic performance of iron orthophosphates in cyclohexene conversion

A series of FePO4(Fe/P = 1) catalysts have been prepared by precipitation of the corresponding chloride or nitrate iron(III) salt and orthophosphoric acid with aqueous ammonia or propylene oxide at pH 7.0. The resulting solids, after calcination at temperatures in the range 673–1273 K for 3 h, were characterized by surface area, X-ray diffraction (XRD) and diffuse reflectance IR (DRIFT) measurements.All catalysts were amorphous for calcination temperatures below 923 K. Calcination at 923–1273 K develops FePO4 with a quartz-like phase structure (ASTM 29–715). Amorphous samples exhibited relatively high surface areas which fell sharply as crystallization of FePO4 occurred. Moreover, the surface acidity (Bronsted and Lewis), characterized by adsorption of pyridine using a pulse method and DRIFT measurements, strongly depended on the preparation method causing the amorphous FePO4 catalysts, obtained from iron(III) chloride and aqueous ammonia, to be more acidic. This increased surface Bronsted acidity was responsible for their improved performance in cyclohexene skeletal isomerization (CSI). FePO4 catalysts also showed catalytic activity for cyclohexene dehydrogenation to benzene.

Catalytic performance of Fe-modified AlPO4 catalyst in cyclohexene skeletal isomerization

Iron-modified AlPO4 catalysts (0.5–5 wt.% Fe2O3) have been prepared by impregnation until incipient wetness with different iron(III) salts. The resulting solids, after characterization by surface area, XRD and DRIFT measurements, were checked for their surface acidity through their catalytic activity towards cyclohexene skeletal isomerization test reaction. It was found that iron(III) increases the catalytic performance of AlPO4 and besides, the iron(III) salt used strongly influences the activity. Thus, the catalysts prepared using complexed iron(III) salts as starting materials presented higher specific activity (and hence, acidity) than those prepared using iron(III) nitrate. Moreover, a maximum in activity was found for 2 wt.% Fe2O3.

Chromium-aluminium orthophosphates, III. Acidity and catalytic performance in cyclohexene and cumene conversions on CrPO4−AlPO4 (20–50 wt.% AlPO4) catalysts obtained in aqueous ammonia

The modification of CrPO4-A catalyst with AlPO4 leads to CrPO4−AlPO4 (CrAlP-A) catalysts exhibiting not only an increased total acidity but also an increased number of strongest Lewis acid sites as compared to CrPO4 and AlPO4 catalysts. Besides, surface acidity is slightly influenced by AlPO4 loading (5–50 wt.%). This increased surface Lewis acidity is responsible for the improved catalytic activity in cyclohexene skeletal isomerization and cumene dehydrogenation processes. Moreover, the catalytic activity results can be well interpreted through differences in the number and strength of acid sites, measured gas-chromatographically, in terms of pyridine and 2,6-dimethylpyridine chemisorbed at different temperatures (573–673 K).

ChemInform Abstract: Chromium‐Aluminum Orthophosphates. Part 1. Structure, Texture, Surface Acidity and Catalytic Activity in Cyclohexene Skeletal Isomerization and Cumene Conversion of CrPO4‐AlPO4 Catalysts.

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Chromium–aluminium orthophosphates. Part 1.—Structure, texture, surface acidity and catalytic activity in cyclohexene skeletal isomerization and cumene conversion of CrPO4–AlPO4catalysts

A series of CrPO4–AlPO4(5–10wt.% AlPO4, CrAlP) catalysts have been prepared by precipitation in aqueous ammonia or in aqueous ammonia–propylene oxide. The catalysts were calcined at 773–1273 K. All catalysts were amorphous for calcination temperatures below 1073 K. At 1273 K the β-CrPO4 structure was formed. CrAlP catalysts exhibited low surface areas which decreased as the calcination temperature increased. Moreover, surface acidity (acid amount and acid strength), measured by adsorption of pyridine and 2,6-dimethylpyridine at various temperatures (573–673 K) by using a pulse method, was better than for the individual components. This increased surface acidity is responsible for their better performance for cyclohexene skeletal isomerization (CSI) and cumene conversion (CC), reactions that were selected as dynamic methods for surface-acid characterization.

AlP0 4-Al 20 3 catalysts with low-alumina content : II. Acidity and catalytic activity in cyclohexene conversion and cumene cracking of catalysts obtained with aqueous ammonia

The influence of γ-Al 2O 3 loading (5–25 wt.-%) and the calcination temperature of AlPO 4-Al 2O 3 (APALA) catalysts on their surface acidity (number and strength distribution) was studied by means of the chemisorption (gas-chromatographically measured) of pyridine (PY) and 2,6-dimethylpyridine (DMPY) at temperatures in the 473–673 K range. In addition, cyclohexene skeletal isomerization (CSI) and cumene conversion (CC), were selected as dynamic methods for surface acid characterization. The AlPO 4-Al 2O 3 catalysts were found to show greater number and strength of acid sites than each individual component. The addition of Al 2O 3 even in a small amount (5 wt.-%), could significantly improve the activity and catalytic activity of AlPO 4 catalysts. Moreover, the catalytic activity of the APAl-A catalyst for both reactions varied with Al 2O 3 loading and, thus, it increased gradually as the Al 2O 3 content of the catalyst increased up to 15 wt.-%, after which it decreased. Good correlations between catalytic activity and the increase in acidic properties of the AlPO 4 by Al 2O 3 loading (as determined by base chemisorption) have been obtained for both reactions. Qualitative DRIFT spectroscopy of probe molecules [pyridine (PY), 2,6-dimethylpyridine (DMPY) and hexamethyldisilazane (HMDS)] showed that P-OH and Al-OH groups disappeared on adsorption and that both Lewis and Brensted acid sites existed on the surface of APA1-A catalysts. Furthermore, the involvement of strong acid sites in CSI and CC reactions has been investigated by selectively poisoning the acid sites with PY, DMPY and HMDS. The catalytic activity and selectivity of the APAl-A catalyst were strongly affected by this poisoning. Thus, Brønsted acidity was considered to be the common active site on APA1-A catalysts.

Effect of precipitation medium on surface acidity and catalytic performance of chromium orthophosphates in cyclohexene skeletal isomerization and cumene conversion

The influence of the precipitation medium and thermal treatment of CrPO4(Cr: P = 1) catalysts on their surface acidity (number and strength distribution) was studied by means of the gas-phase chemisorption of pyridine (PY) and 2,6-dimethylpyridine (DMPY) at 573 and 673 K. In addition, Cyclohexene skeletal Isomerization (CSI) and cumene conversion (CC), were selected as dynamic methods for surface acid characterization. Catalysts obtained in propylene oxide–aqueous ammonia exhibited a higher number and strength of acid sites and, hence, greater activity in CSI and CC processes, than catalysts obtained in propylene oxide or aqueous ammonia exclusively. Moreover, cumene dehydrogenation (to α-methylstyrene) always predominated over cumene dealkylation (to propylene and benzene) and accounted for 100% selectivity of CrP-A and CrP-P catalysts. Furthermore, a correlation between surface acidity and catalytic activity was found.

New AlPO4-sepiolite systems as acid catalysts, I. Preparation, texture, surface-chemical properties and cyclohexene skeletal isomerization conversion

The effects of the addition of aluminium orthophosphate (Al/P=1) to sepiolite were investigated with respect to texture, porosity, surface chemistry and catalytic performance. The AlPO4-sepiolite catalysts, containing different weight compositions, were prepared by precipitation of AlPO4 (Al/P=1) with propylene oxide or aqueous ammonia on to commercial natural sepiolite. Sepiolite inhibits the AlPO4 crystallization which is found when pure AlPO4 is calcined at 873 K. The surface acid-basic properties have been studied gas-chromatographically through the irreversible adsorption of organic acids and bases at temperatures in the range 473 to 673 K. In addition, the cyclohexene skeletal isomerization (CSI) acid catalysed test reaction to 1- and 3-methylcyclopentenes (1- and 3-MCP), was used to test catalyst performance. Catalytic activity, as an apparent rate constant, fits the Bassett-Habgood kinetic model for first-order processes. Marked changes were found in their catalytic properties as a function of the preparation method and the AlPO4 to sepiolite ratio. AlPO4 addition induced acidity by increasing the strength of acid sites which resulted in a higher conversion which was accompanied by a greater selectivity for 1-MCP. The incorporation of AlPO4 to sepiolite also inhibited the loss of activity with the prolonged use of the catalyst which was found for reference pure natural sepiolite.

Cyclohexene skeletal isomerization activity of sepiolites modified with B3+ or Al3+ ions

The effect of sepiolite modification by B3+ and Al3+ ions on catalytic performance were investigated in cyclohexene skeletal isomerization (CSI) reaction. Catalytic activity to 1- and 3-methylcyclopentenes (1- and 3-MCP) fit the Bassett-Habgood kinetic model for first order processes. These ions induced strong acidity in the sepiolite and the best results (greater activity and selectivity to 1-MCP) were found when the sepiolite was exchanged with an BF3/methanol solution which contained 3 wt.% of the B3+ ion.

Textural properties, surface chemistry and catalytic activity in cyclohexene skeletal isomerization of acid treated natural sepiolites

The structure, texture and distribution of acid and basic sites on the surface of acid-treated natural sepiolites with different thermal treatment has been studied. The pulse Chromatographic technique was used for the determination of surface acidity and basicity and the data obtained have been compared to data on catalytic activity of treated sepiolites towards the cyclohexene skeletal isomerization (CSI) at methylcyclopentene isomers. The results show that catalyst performance correlates with the acidity of the catalysts (an almost linear correlation was obtained) and therefore, samples with the highest concentration of strongly acid sites are shown to be the most active in this reaction. Activities, selectivities and activation parameters (E a, ln A, ΔH ≠ and ΔS ≠) are calculated in terms of Bassett-Habgood's kinetic model for first order processes in which the surface reaction is the controlling step and the partial pressure of the reactant is low. The results also provide direct evidence that acid sites in acid-treated sepiolites are stronger than in natural ones and thus catalyse the CSI at reaction temperatures between 473–673 K more efficiently. The increase in calcination temperature decreases the surface area, surface acid-base character and, hence, catalytic activity for the CSI reaction although this decrease is less pronounced than in the natural sepiolite samples.

Aluminium phosphate—zirconia catalysts: I. Structure, texture, acid—base properties and catalytic activity in cyclohexene isomerization of catalysts obtained with propylene oxide

Nitrogen adsorption, X-ray diffraction measurements and infrared spectroscopy were used to study the effects of surface dehydration (773–1273 K) on the textural properties, crystallinity and structure of AlPO 4 ZrO 2 catalysts of different compositions and prepared by precipitation of aluminium orthophosphate (AlPO4) (Al/P= 1) with propylene oxide on commercial monoclinic zirconia (ZrO 2) (Aldrich Chemie, ceramic grade, 99.9 % ). The surface acid—base properties were studied through the irreversible adsorption of organic bases and acids from cyclohexane solutions at 298 K. In addition, the cyclohexene skeletal isomerization (CSI) acid-catalysed test reaction, giving 1- and 3-methylcyclopentenes, was used as a dynamic method for evaluating the number of strong acid sites, the only ones capable of giving rise to skeletal isomerization. Activation energies and selectivities with respect to 1-methylcyclopentene were calculated in terms of the Bassett—Habgood kinetic model for first-order processes in which the surface reaction is the controlling step and the partial pressure of the reactant is low. An increase in ZrO 2 content and/or calcination temperature decreases the surface area and pore volume. This is a consequence of AlPO 4 crystallization to the tridymite form with T d symmetry, in which the phosphorus atoms are surrounded by tetrahedra of oxygen atoms. The decrease in catalytic activity as calcination increases is consistent with not only the decrease in the amount of acid sites measured vs. weaker organic bases but also with the decrease in Brønsted acidity, as shown by the decrease in OH band intensity.

Textural properties, surface chemistry and cyclohexene conversion of AlPO 4-Al 2O 3 catalysts

The effects of surface dehydration on crystal unity, textural properties (surface area and pore volume) and surface acid-base properties of differently prepared AlPO 4-Al 2O 3 (75:25 wt%) systems have been studied. Physical characterization was carried out by N 2 adsorption, TG and XRD measurements and by infrared spectroscopy. The catalysts were characterized by surface acid-basic site concentrations using a spectrophotometric method and by their catalytic activities in cyclohexene skeletal isomerization (CSI) through the use of a microcatalytic pulse reactor. Al 2O 3 was found to increase both the surface area and pore volume of AlPO 4 through the inhibition of the AlPO 4 crystallization at 1073 K. Apparent rate constants and activation energies in CSI were obtained according to the kinetic model of Bassett-Habgood developed for first order processes. Selectivity studies lead to the conclusion that 1- and 3-methylcyclopentenes are competitive stable primary reaction products coming from cyclohexene across a parallel reaction network. The addition of Al 2O 3 to AlPO 4 catalysts enhances considerably both the catalytic activity to 1-, 3- and 4-methylcyclopentenes (1-, 3- and 4-MCP) and the selectivity to 1-MCP (σ) in these systems for the CSI indicating an increased strong acidity.

Cyclohexene skeletal isomerization on AlPO4 catalysts precipitated with ammonia and promoted with sulfate ions

This study concerns the effect of SO42− (1–10 wt.%) on the surface acid properties of AlPO4 catalysts obtained in aqueous ammonia and based on their catalytic activity in cyclohexene skeletal isomerization (CSI) by using the Bassett-Habgood equation for first order processes. It has been found that AlPO4−SO42− catalysts are much more active in the isomerization process than AlPO4 catalysts, indicating a strongly increased acidity. Maximum catalytic activity was reached at about 2–3 wt.% and varied with further temperature treatment. Furthermore, the sulfate loading brought about a very pronounced increase in crystallinity even at the smallest content.

AlPO4−ZrO2 catalysts, IV. Cyclohexene skeletal isomerization activity of systems obtained in ethylene oxide

The use of AlPO4−ZrO2 (weight ratio AlPO4/ZrO2=3) as catalyst systems obtained with ethylene oxide and subjected to different stages of heat treatments has been studied through their catalytic activity in the skeletal isomerization of cyclohexene to 1- and 3-methylcyclopentenes (1- and 3MCP). The apparent rate constants and selectivity to 1-MCP are used for an evaluation of the presence and amount of strong acid sites, the only ones capable of giving rise to skeletal isomerization. The decrease in catalytic activity as calcination increases is consistent with not only the decrease in the amount of acid sites measured vs. weaker organic bases but also with the decrease in Bronsted acidity, as shown by the decrease in O−H band intensity.