J-E Otterstedt

The synthesis of discrete colloidal particles of TPA-silicalite-1

Abstract Discrete colloidal particles of TPA-silicalite-1 with an average particle size of less than 100 nm and with a narrow particle-size distribution have been synthesized in clear homogeneous solutions. The factors influencing particle size have been investigated. High silica contents in conjunction with high alkalinities favor the synthesis of colloidal silicalite suspensions. Polymeric silica sources result in larger particles compared to those synthesized with tetraethoxy silane—presumably due to the different nucleation kinetics. It was found that the base concentration influences nucleation behavior in that high alkalinities result in the nucleation of a relatively large number of particles. The growth rates are constant for a given silica content irrespective of the alkalinity, at least within the alkalinity range studied. This result is ascribed to a rate-limiting surface reaction step that is a consequence of the synthesis mixture composition. The linear growth rates recorded are lower than those typically reported in the literature.

Colloidal zeolite suspensions

Abstract A method is presented whereby it is possible to synthesize colloidal zeolite sols containing discrete zeolite particles with an average particle size of less than 150 nm and with a narrow particle-size distribution. The zeolite sols exhibit typical colloidal characteristics such as Tyndall light scattering and a low rate of sedimentation and coagulation due to electrolyte concentrations in excess of the critical coagulation concentration. The effect of sodium on the crystallization kinetics, product distribution, and particle size has been investigated. Relatively high Na 2 O AL 2 O 3 ratios favor the formation of large crystals of zeolite A. Low Na 2 O AI 2 O 3 ratios result in colloidal crystals of zeolite Y, whereas intermediate ratios yield zeolite Y and/or zeolite A with particle sizes of ca. 100 nm. Midsynthesis addition of sodium allows for a fivefold increase in zeolite yield.

Synthesis of stable suspensions of discrete colloidal zeolite (Na, TPA)ZSM-5 crystals

Synthesis of zeolite ZSM-5 in the form of stable suspensions of colloidal crystals from clear homogeneous solutions has been achieved from synthesis mixtures with low sodium and high TPAOH concentrations. The ultimate size of the discrete crystals is in the range 130-230 nm with a narrow particle size distribution. The crystal growth was monitored by the direct method of dynamic light scattering. An increasing alumina concentration was found to decrease the crystal growth rate, the number of crystals produced, and the ZSM-5 yield. The final crystal size was found to increase with increasing alumina concentration in the synthesis solution. The number of crystals produced decreased with an increased alkalinity in contradiction to what has been found for colloidal TPA-silicalite-1. This together with the presence of the two competing processes, crystal growth and crystal dissolution, which are both catalyzed by hydroxide ions, suggests the presence of an optimum alkalinity (depending on the alumina concentration).

Kinetics of the HDO of methyl-substituted phenols

Abstract The kinetics of the hydrodeoxygenation (HDO) of a series of mono- and dimethyl substituted phenols were determined in a batch autoclave reactor using a sulfided commercial CoMo catalyst. Under the reaction conditions employed (5 MPa, 300°C), the reaction proceeded by two independent parallel paths, one leading to an aromatic product and the other to a naphthenic product. Kinetic analysis showed both paths to be inhibited by reactant. The addition of ammonia suppressed both paths about equally, while H2S depressed the aromatic path appreciably more. Steric hindrance was observed when a methyl group was adjacent to the OH group. It is proposed that different active sites are responsible for the two reaction paths, one for C O bond cleavage and another for hydrogenation.

Analysis of the crystal growth mechanism of TPA-silicalite-1

Abstract The method of chronomal analyses (dimensionless time analyses) according to Nielsen has been applied to the growth of discrete colloidal particles (particle sizes of less than 100 nm) of TPA-silicalite-1 to gain information on the crystal growth mechanism. The increase in particle size in the range 30–95 nm was monitored by dynamic light scattering and yielded the result that the increase in crystal size is a linear function of synthesis time. The linear growth rate of the almost spherical particles at 100°C is 3.79 nm/h — a low value that is ascribed to the synthesis conditions employed. Furthermore, the particle number concentration was shown to be constant, indicating that no secondary nucleation event occurs during the growth process. The crystallization kinetics recorded in the temperature interval 80–100°C correlate with a first-order surface reaction controlled growth mechanism in which the corresponding apparent energy of activation is 42 kJ/mol. A diffusional mechanism as well as a compound growth mechanism in which both surface reaction and diffusion compete for rate control can be ruled out as being operative.

Fluid catalytic cracking of heavy (residual) oil fractions: a review

Abstract The oil crisis of recent times has caused a drastic decrease in the total consumption of oil and changed the demand pattern for the products of petroleumrefining. The demand for heavier fractions or residual oils has steadily decreased, making it imperative to convert these into gasoline, diesel and such lighter fractions. Fluid catalytic cracking (FCC) of these heavier fractions, however, poses several serious problems, caused mainly by their much higher hetero-atom concentration, metal contents and coking tendency, as compared to earlier feedstocks. Several process and catalyst innovations have been made to tackle these problems. A new generation of FCC catalyst technology has emerged with tailor-made catalysts for higher structural stability and attrition strength, more complete CO combustion during regeneration, reducing SOx emissions from FCC stacks, enhancing the gasoline octane number, passivating the harmful effects of metals like Ni and V accumulating on the catalyst, etc., These developments contain valuable lessons for the science and technology of catalysis.

The synthesis of colloidal zeolite hydroxysodalite sols by homogeneous nucleation

Colloidal suspensions of discrete particles of microcrystalline hydroxysodalite have been synthesized from clear homogeneous solutions wherein the average particle size determined by dynamic light scattering is 37 nm with a narrow particle-size distribution. Particle-size analysis performed with various methods yield results that are in agreement. The growth-limiting nutrient has been identified as alumina. As a result, the midsynthesis addition of alumina results in further particle growth to 48 nm with the corresponding increase in zeolite yield. Hence, the fine control of particle size and thereby zeolite yield without the formation of a secondary particle population is shown to be possible. Direct measurements of zeolite yield and particle size allow for a detailed evaluation of the events that occur during crystallization.

Hydrothermal stability of silica as a support for platinum in an oxidation catalyst

Abstract Silica and alumina were subjected to hydrothermal treatment at 750°C for up to 48 h. The sodium content of silica had a strong influence on the stability towards sintering. Silica, containing 0.05 wt.-% sodium, lost less of its original surface area than did alumina during the hydrothermal treatment. Light-off temperatures, and dependence of conversion of carbon monoxide to carbon dioxide over catalysts with platinum on wash-coats of alumina and silica on flow-rate, were determined before and after deactivation. Platinum on sodium-free silica had higher activity than platinum on alumina both before and after deactivation.

Catalytic cracking of heavy oil over catalysts containing different types of zeolite Y in active and inactive matrices

Abstract The effects of addition of alumina to the matrices of cracking catalysts containing different types of zeolite Y, on their cracking performance, were investigated using a micro activity test and two different feed oils. For the heavier feed oil, the alumina addition resulted in a higher conversion at the same catalyst to oil ratio independent of the type of zeolite. This higher conversion was accompanied by a greater selectivity for coke and a lower selectivity for gasoline. For the lighter feed oil the effect of alumina addition on the total conversion was much less pronounced while the effects on the selectivity were similar to those observed using the heavier feed. The performance of the catalysts in a commercial fluid catalytic cracking unit is discussed in view of their coke forming tendencies and the heat balance of the cracker.

Catalytic cracking of heavy oil : use of alumina-montmorillonites both as catalysts and as matrices for rare earth exchanged zeolite Y molecular sieve

Abstract Three types of alumina—montmorillonite complexes were evaluated as cracking catalysts, alone and in admixture with rare earth exchanged zeolite Y (REY), using the micro activity test and three different feed oils. Prior to the test, all catalysts were steam treated at 750°C for 18 h. For cracking of heavy oils, the alumina—montmorillonites showed conversions similar to that of a reference catalyst containing 20% REY in a kaolin-binder matrix. The alumina—montmorillonites showed higher coke and lower gas yield when compared with the reference catalyst while the gasoline yields were essentially the same over the two types of catalysts. The selectivity for light cycle oil was considerably greater for the alumina—montmorillonites. When used as matrices for REY, the alumina—montmorillonites resulted in considerably more active catalysts at the same zeolite content compared with a catalyst having a kaolin-binder matrix, while the selectivity properties differed very little between the two types of catalysts.