A tracer study of the metathesis of simple olefins over molybdena-alumina catalysts

Metathesis of perdeuterio- and perhydroethene, propene, 1-butene, and their mixtures with cis-2-butene was studied over a molybdena-alumina catalyst using recirculation and pulse systems. No metathesis was observed over a fresh unreduced catalyst, but after pretreatment with cis-2-butene the reactivity of olefins in metathesis reactions increased in the sequence of ethene ? 1-butene < propene < cis-2-butene. The activity was higher for the metathesis of propene or 1-butene after the catalyst was pretreated with the corresponding olefin instead of cis-2-butene, but no metathesis was induced by ethene. Metathesis of cis-2-butene was suppressed by the addition of equal amounts of olefins in the sequence: ethene ? 1-butene < propene. Studies of mixtures revealed that the rate of cis- to trans-2-butene isomerization by metathesis ～- the rate of degenerate metathesis of 1-butene or the cross metathesis of 1-butene with 2-butene.     

Co-reaction of ethene and methanol over modified H-ZSM-5

Co-reaction of ethene and methanol was carried out over HZSM-5, P-La modified ZSM-5 (PLaHZ) and hydrothermal-treated PLaHZ catalysts. Hydrothermal treatment at high temperature sharply reduced the acidity of the catalyst, on which the direct conversion of ethene or methanol/dimethyl ether was almost completely suppressed. Co-feeding of ethene and methanol over the said catalyst resulted in considerable conversion of both reactants. Meanwhile, high propene selectivity (ca. 80%) was obtained at lower conversions. The methylation of ethene by methanol was responsible for the enhancement of conversions and propene selectivity in the co-reaction system. The further methylation of propene and the cracking of higher olefins were also operative under current reaction conditions.     

Converting olefins to propene: Ethene to propene and olefin cracking

Demand for propene as a petrochemical building block keeps growing, while its availability has been decreased by the adoption of shale gas resources, among others. Efforts to optimize its production by conventional means (including modified fluid catalytic cracking) and new on-purpose production technologies (including ethene to propene (ETP) and olefin cracking) are being pursued. This work reviews the progress made on olefin conversion processes, including the ETP reaction, which is still under development, and the cracking of butenes and higher olefins (C₅–C₈). The factors analyzed include the catalytic performance of different zeolite materials and their modifications to increase catalyst stability, yield, and selectivity to propene, as well as the effect of operating conditions, reaction thermodynamics, and mechanisms involved. The work is complemented by a survey of commercial technologies and developments on olefin conversion processes.     

Optimizing activity of tungsten oxides for 1-butene metathesis by depositing silica on γ-alumina support

A series of catalysts made of tungsten oxide loaded on SiO₂, γ-Al₂O₃, SiO₂–Al₂O₃ and silica deposited γ-alumina are tested for 1-butene metathesis. Among these catalysts, the catalyst 6W/20SiO₂/Al₂O₃ gives the highest activity for 1-butene metathesis reaction with 1-butene conversion up to 71 mol% and the yield of propene up to 21 mol%. The excellent catalytic activity is related to the moderate dispersion of tungsten oxide and the suitable acidity of the support. The dispersion of WO_x species and the acidity of supports were studied by characterization of XRD, Raman spectra, UV–vis, H₂-TPR and NH₃-TPD in detail. The surface properties of silica modified γ-alumina leads to the moderate aggregation of supported tungsten oxide, which appears to be more effective for 1-butene metathesis at the low temperature of 453 K. Optimized activity was realized by tuning the dispersion of tungsten species on silica deposited alumina.     

Selective cracking of natural gasoline over HZSM-5 zeolite

This work presents a study on the catalytic cracking of natural gasoline (extracted from natural gas) over HZSM-5 zeolite. A factorial planning was carried out to evaluate the effect of temperature and W/F ratio on the cracking of natural gasoline, analyzing their effects on conversion and product distribution using an analysis based on surface response methodology. The process was optimized focusing on the maximization of the mass fractions and the production of specific products such as ethene, propene and butanes. The results have shown that the maximum selectivity and hourly mass production of ethene is obtained at high temperature (450 °C) and low catalyst weight to flow rate ratio (W/F) (7.2 to 8.2 g_cat h/mol). Maximum selectivity of propene is obtained at 350 °C and 7.0 g_cat h/mol, while the best condition for maximum mass production is found at 421 °C and 5.7 g_cat h/mol. The highest mass production of butanes is favored by high temperature (450 °C) and mid range W/F ratios (12.1 g_cat h/mol), while the highest selectivity is found at low temperature (350 °C).     

Highly effective P-modified HZSM-5 catalyst for the cracking of C4 alkanes to produce light olefins

A series of HZSM-5 zeolites modified by different amounts of phosphorus (P/HZSM-5) were prepared. The physicochemical features of P/HZSM-5 catalysts were characterized by means of XRD, BET, NH₃-TPD, FT-IR spectra of adsorbed pyridine, etc., and their performances for the catalytic cracking of the mixed C₄ alkanes to produce light olefins were investigated. The results indicated that phosphorus (P) modification not only modulated the amount of acidic sites and the percentage of weak acidic sites in total acidic sites, but also regulated the acid type, i.e., the ratio of L/B (Lewis acid/Brönsted acid). The introduction of P also altered the basic characteristics of HZSM-5 which was evidenced by CO₂-TPD analysis. Consequently, P modification with suitable amount was favorable for enhancing the selectivity to light olefins, especially to propene. At the temperature of 650 °C, the maximum yields of propene and ethene were achieved 25.6 and 33.9%, which were higher than those over parent HZSM-5 by 7 and 4.5%, respectively. Aromatics yield was found to be decreased with the increasing P loading due to the reduction of strong acid and the formation of new basic site which inhibited the hydrogen transfer reaction. All this indicates that P-modified HZSM-5 zeolites are effective catalysts for the cracking of mixed C₄ alkanes to produce more light olefins.     

Catalytic cracking of tricyclo [5.2.1.0] decane over HZSM-5 molecular sieves

The catalytic cracking of a high density hydrocarbon fuel, tricyclo [5.2.1.0^2.6] decane (JP-10) over HZSM-5 molecular sieves with different Si/Al mole ratios of 25, 38, and 50 was investigated at the temperature range from 773 to 873 K. Compared with the thermal cracking and the catalytic cracking over ZSM-5, conversions of JP-10 from the catalytic cracking over HZSM-5 molecular sieves at the same temperature were evidently heightened. The predominant hydrocarbon products from the catalytic cracking, checked at room temperature and atmospheric pressure, were methane, ethane, ethene, propane and propylene in the gaseous phase and benzene, indene, naphthalene and their homologues in the liquid phase. The contents of ethane, propane and propene decrease with increasing Si/Al mole ratio of a catalyst while those of methane and ethene increase simultaneously with the increase of Si/Al mole ratio of HZSM-5. The contents of the main components in the liquid products produced on the catalyst surface at a given temperature also decreased with the increase of Si/Al mole ratio. To keep high yields of alkenes, the HZSM-5 catalyst with high Si/Al mole ratio could be chosen.     

Butene Catalytic Cracking to Propene and Ethene over Potassium Modified ZSM-5 Catalysts

Catalytic cracking of butene over potassium modified ZSM-5 catalysts was carried out in a fixed-bed microreactor. By increasing the K loading on the ZSM-5, butene conversion and ethene selectivity decreased almost linearly, while propene selectivity increased first, then passed through a maximum (about 50% selectivity) with the addition of ca. 0.7–1.0% K, and then decreased slowly with further increasing of the K loading. The reaction conditions were 620 °C, WHSV 3.5 h⁻¹, 0.1 MPa 1-butene partial pressure and 1 h of time on stream. Both by potassium modification of the ZSM-5 zeolite and by N₂ addition in the butene feed could enhance the selectivity towards propene effectively, but the catalyst stability did not show any improvement. On the other hand, addition of water to the butene feed could not only increase the butene conversion, but also improve the stability of the 0.7%K/ZSM-5 catalyst due to the effective removal of the coke formed, as demonstrated by the TPO spectra. XRD results indicated that the ZSM-5 structure of the 0.07% K/ZSM-5 catalyst was not destroyed even under this serious condition of adding water at 620 °C.     

Metathesis of Ethene and Decene to Propene over a WO3/SiO2 Catalyst

Metathesis between decene and ethene to propene over a WO₃/SiO₂ catalyst was studied. The dependency of the conversion of decene, selectivity to propene, and working lifetime of the catalyst on ethane‐to‐decene molar ratio and temperature was evaluated. Low temperature was found to be favorable to the production of C₆–C₉ olefins, while high temperature enhanced C₁₀₊ olefins. The working lifetime of the catalyst decreased with the weight hourly space velocity. The optimum reaction conditions for the metathesis process of decene and ethene to propene were determined. An obvious induction period was found to exist in the metathesis reaction.     

Ga2O3/HZSM-48 for dehydrogenation of propane: Effect of acidity and pore geometry of support

The catalytic performance of propane dehydrogenation over HZSM-48 supported Ga₂O₃ catalysts in the presence of CO₂ was investigated, and compared with that of HZSM-5 supported ones. The activity decreases with the increase of Si/Al ratio of catalyst support while the selectivity to propene shows a contrary trend. Ga₂O₃/HZSM-48 with Si/Al ratio of 130 has the best propene yield of 22%. HZSM-48 supported catalysts exhibit higher selectivities to propene than the HZSM-5 supported ones at similar propane conversion, due to their weak acid strength. However, their stabilities are not so good as those of the latters, owing to their more weak acid sites and unidimensional pore structures.     

Effects of catalyst acidity and HZSM-5 channel volume on the catalytic cracking of poly(ethylene)

The effects of catalyst acidity and the restricted reaction volume afforded by the HZSM-5 zeolite structure on the volatile cracking products derived from poly(ethylene) are investigated. The effectiveness of silica-alumina, HZSM-5, and sulfated zirconia acid catalysts for poly(ethylene) cracking are compared. When high catalyst to polymer ratios are employed and volatile products are rapidly removed during cracking, the most abundant volatile products generated by poly(ethylene) cracking are propene and isoalkenes. The relative amount of propene produced and the temperatures corresponding to the maximum rate of volatile hydrocarbon production are found to be related to catalyst acidity. The restricted volume inside HZSM-5 channels facilitates oligomerization and the production of small alkyl aromatics. © 1995 John Wiley & Sons, Inc.     

Effect of Surface Tungstate W<Superscript>5+</Superscript> Species on the Metathesis Activity of W-Doped Spherical Silica Catalysts

The high surface area W-doped spherical silica (SSP) catalysts were prepared with different sequences of W and Si addition (W–Si_(Alt), Si₁–W_2, and W₁–Si₂) by the sol–gel method with CTAB as a structure directing agent and compared with the impregnated one (W/SSP). All the catalysts exhibited high specific surface area (～?1100 m² g^?1) with a closely perfect spherical shape. The presence of surface/sub-surface tungstate W⁵⁺ species, crystalline bulk WO₃, and tetrahedral tungsten oxide species on the prepared catalysts was investigated by means of X-ray photoelectron spectroscopy depth profile analysis, X-ray diffraction, and Raman spectroscopy. Without in situ reduction by the reactants/products, tungstate W⁵⁺ species was found on the top surface of the as-prepared W–Si_(Alt) whereas for the Si₁–W₂, W/SSP, and W₁–Si₂, the W⁵⁺ appeared only on the sub-surface of the catalysts after 5 and 15 s Ar⁺ etching. The abundance of surface W⁵⁺ species is suggested to facilitate the establishment of the active tungsten carbenes and was correlated well to the catalytic activity in propene metathesis. The surface W⁵⁺-activity relationship of the WO₃-based metathesis catalysts is useful especially when the catalyst activity did not depend solely on the amount of active tetrahedral coordinated tungsten oxides.     

Effects of catalyst acidity and HZSM-5 channel volume on polypropylene cracking

Effects of catalyst acidity and the restricted reaction volume afforded by HZSM-5 on the catalytic cracking of polypropylene are described. Polypropylene cracking by silica—alumina and HZSM-5 catalysts yields olefins as primary volatile products. In addition, HZSM-5 channels restrict carbenium ion rearrangements and facilitate formation of significant amounts of propene and alkyl aromatic volatile products. The higher acidity of sulfated zirconia compared to the other catalysts results in an increase in the frequency of hydride abstractions, resulting in the formation of significant yields of saturated hydrocarbons and organic residue for this catalyst. Primary polypropylene cracking products can be derived from carbenium ion reaction mechanisms. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67: 341–348, 1998     

Hydrogenation of CO2 to dimethyl ether on La-, Ce-modified Cu-Fe/HZSM-5 catalysts

Cu–Fe–La/HZSM-5 and Cu–Fe–Ce/HZSM-5 bifunctional catalysts were prepared and applied for the direct synthesis of dimethyl ether (DME) from CO₂ and H₂. The catalysts were characterized by X-ray diffraction (XRD), N₂ adsorption–desorption, H₂-temperature programmed reduction (H₂-TPR), and X-ray photoelectron spectroscopy (XPS). The results showed that La and Ce significantly decreased the outer-shell electron density of Cu and improved the reduction ability of the Cu–Fe catalyst in comparison to the Cu–Fe–Zr catalyst, which may increase the selectivity for DME. The Cu–Fe–Ce catalyst had a greater specific surface area than the Cu–Fe–La catalyst. This promoted CuO dispersion and decreased CuO crystallite size, which increased both the DME selectivity and the CO₂ conversion. The catalysts were stable for 15 h.     

Catalytic cracking of 1-butene to propylene by Ag modified HZSM-5

Silver modified HZSM-5 (AgHZ) zeolite catalysts were prepared by ion exchange method and their catalytic properties in the 1-butene cracking reaction were measured. The catalysts were characterized by infrared spec-troscopy with pyridine adsorption (Py-IR), N2 adsorption and X-ray diffraction (XRD). The effects of Ag loading and steaming treatment on catalytic performances were studied. It is found that the activity of HZSM-5 (HZ) cat-alyst significantly decreases with the steaming time, whereas AgHZ catalysts show stable activity in the steaming time of 24–48 h and their activities increase with the Ag loading. When the steaming time is 24–48 h, the yield of propylene over HZ catalyst significantly decreases, whereas it is stable over AgHZ catalysts. The AgHZ catalysts with Ag loadings of 0.28%–0.43%(by mass) show similar propylene yields (~30%), which are higher than that over the AgHZ catalyst with a Ag loading of 0.55%(by mass). These results indicate that the steam-treated AgHZ catalysts with optimum Ag loadings have higher yield of propylene and are more stable than the steam-treated HZ catalyst. The regeneration stability measurement in butene cracking also shows that the AgHZ catalyst steam-treated under a suitable condition has better stability than the HZ catalyst.     

1-丁烯催化裂解制丙烯和乙烯反应性能的研究

以MCM-49分子筛为催化剂,纯1-丁烯为原料,考察了反应压力和空速对烯烃催化裂解制丙烯,乙烯反应性能的影响.选择适宜的反应压力和空速条件能够有效地抑制副反应,从而提高丙烯,乙烯的总产率。     

1-丁烯催化裂解制丙烯和乙烯反应性能的研究

以MCM-49分子筛为催化剂,纯1-丁烯为原料,考察了反应温度对烯烃催化裂解制丙烯、乙烯反应性能的影响。选择适宜的反应温度条件能够有效地抑制副反应,从而提高丙烯、乙烯的总产率。     

Methane and ethane activation without adding oxygen: promotional effect of W in Mo-W/HZSM-5

The conversions of methane and ethane over Mo/HZSM-5 and W/HZSM-5 catalysts are compared. A reaction model for hydrocarbon formation over Mo/HZSM-5 catalysts is proposed, which involves heterolytic splitting of methane and a molybdenum-carbene intermediate. Ethene is shown to be the initial product of methane conversion, and it undergoes further reaction to form aromatics in a solid acid environment. The promotional effect of addition of tungsten in the Mo-W/HZSM-5 catalyst in methane conversion reaction suggests the formation of Mo-W mixed oxide. The product selectivity patterns of Mo/HZSM-5 and W/HZSM-5 catalysts in ethane conversion reaction are consistent with a dual-path model involving dehydrogenation and cracking (or hydrogenolysis) of ethane. The rates of both these reactions over Mo/HZSM-5 are higher than over W/HZSM-5.     

Dual effects of supported W catalysts for dehydroaromatization of methane in the absence of oxygen

The screening of a series of W-based catalysts on different supports i.e. HZSM-5, Hβ, USY and Al₂O₃ for the dehydroaromatization of methane (DHAM) revealed that HZSM-5 emerged as the best support. Next, the performance of W/HZSM-5 and W-H₂SO₄/HZSM-5 catalysts for the DHAM reaction was compared to study the effect of acidic treatment in the impregnation method. The results showed that the optimum activity of W-H₂SO₄/HZSM-5 catalyst exceeded that of W/HZSM-5 catalyst. Finally, the influence of Si/Al ratio in the W-H₂SO₄/HZSM-5 catalyst was studied and the catalyst with Si/Al ratio = 30 was found to be the most promising for the DHAM reaction. The remarkable activity of the catalyst is attributed to the presence of dual effects: suitable content of octahedral polymeric and tetrahedral monomeric tungstate species accompanied by proper amount and strength of acid sites in the catalyst.     

Catalytic cracking of n-dodecane over HZSM-5 zeolite under supercritical conditions: Experiments and kinetics

The catalytic cracking of n-dodecane over HZSM-5 zeolite catalyst was investigated at 400–450 °C under supercritical and subcritical pressures (0.1–4.0 MPa). The results show that both the activity of the catalyst and its stabilization towards deactivation decrease with increasing pressure, and the catalyst maintains substantially higher activity when feed rate exceeds 4.00 ml/min under supercritical conditions. A first-order Langmuir kinetic model with a novel decay function is developed for the supercritical catalytic cracking of hydrocarbon incorporating supercritical extraction effect on catalyst stability, which is satisfactory to describe the kinetic behaviors of catalytic cracking of supercritical n-dodecane. According to the estimated reaction rate and adsorption constant of n-dodecane on HZSM-5 at different temperature, the activation energy of 125.4 kJ/mol and adsorption heat 109.5 kJ/mol were calculated. An index of CRSE is proposed to define contribution ratio of supercritical extraction to the activity of the HZSM-5 catalyst in the developed kinetics model, and it is found that the CRSE increases with increasing hydrocarbon feed rates and decreasing catalytic activities, and reaches maximum value when the coke formation rate equals to the coke removal rate by supercritical hydrocarbon.