Silica-alumina Catalyst Research Articles

Synthesis of pyridine and picolines over modified silica-alumina and ZSM-5 catalysts

In the reaction of acetaldehyde, formaldehyde and ammononia over HZSM-5 (Si/Al-280), PbZSM-5 and WZSM-5 catalysts at 420°C, 0.5 h−1 weight hourly space velocity, the total yields of pyridine and 3-picoline obtained were 58.2, 42.8 and 78.3 wt.% based on aldehydes, respectively. In the reaction of acetaldehyde and ammonia over typical Pb−SiO2−Al2O3 (20% PbO), W−SiO2−Al2O3 (10% W), Pb−Cr−SiO2−Al2O3 (F) and Pb−Cu−SiO2−Al2O3 (E) catalysts at 420°C, 0.5 h−1 W.H.S.V., the yields of 2- and 4-picolines obtained were 51.1, 66.1, 80.6 and 53.7 wt.%, respectively.

The characterization of modified silica-alumina catalysts active in the synthesis of picolines

The SiO2−Al2O3 catalyst is modified by promoters like Pb, Cr, Cu. The SiO2−Al2O3 catalyst is also modified using two elements by a simultaneous or stepwise impregnation method. The catalysts are characterized by XRD, IR and ESR techniques, and are useful in the synthesis of picolines.

31P NMR Investigation of Surface Acidity Using Adsorbed Tricyclohexylphosphine as a Probe

A new probe molecule (tricyclohexylphosphine) has been used to study acidity on silica, alumina, and silica-alumina catalyst surfaces. It was found that tricyclohexylphosphine (PCy 3 ) rotates on the SiO 2 surface at room temperature, and the motion is nearly isotropic in the first adsorbed layer. Larger amounts of PCy 3 on silica are solidlike. Lewis sites were detected on alumina, but not on a silica-alumina catalyst. This is probably due to steric effects. On the silica-alumina catalyst, the concentrations of Bronsted acid sites were determined. These appeared to increase on raising the adsorption temperature. At 200 °C, the maximum concentration of PCy 3 H + found is 0.26 μmol/m 2 .

Preparation of silica-alumina catalyst by the sol-gel method and its activity for the Diels-Alder reaction of isoprene and acrylaldehyde

In the presence of 2-PrOH and oxalic acid, a porous silica-alumina has been prepared by the sol-gel method from Si(OEt) 4 and Al(NO 3) 3 · 9H 2O. The surface area, the average pore size, the pore volume and most importantly, the number of the acidic sites in the range of p K a ≤ −3.0 are greatly increased by the use of oxalic acid as the organic additive. The silica-alumina prepared by the new method is found to catalyze the Diels-Alder reaction of isoprene and acrylaldehyde efficiently, improving the yield and the regioselectivity.

Kinetic modelling of the deactivation of a commercial silica—alumina catalyst during isopropylbenzene cracking

In the deactivation of commercial catalysts by coke formation, complete deactivation of the catalyst is seldom observed and frequently a certain lower activity level is maintained. In the present work, several kinetic models that predict partial catalyst deactivation have been analysed. The presence of active sites with different strengths on the catalyst surface and reversible coke formation have been considered as possible causes of the residual activity of the catalyst. In order to check these kinetic models, they have been applied to the isopropylbenzene cracking reaction over silica-alumina catalyst. From the results obtained, the model which gave the best fit to the experimental data was that of reversible coke deposition.

Hydrocarbons from synthesis gas; effect of Mg addition over Pd impregnated silica-alumina catalysts

The effect of Mg addition to Pd impregnated silica-alumina catalysts and the behavior of these catalysts in synthesis gas to hydrocarbons transformation have been studied. Measurements of H2 chemisorption and temperature programmed NH3 desorption allow us to consider that Mg addition to Pd containing catalysts increases the Pd dispersity, the support acidity not being modified. The presence of Mg enhances hydrocarbon production. Meanwhile, the hydrocarbon distribution is not modified. A large amount of light alkanes in the reaction products is obtained.

Influence of Chromium Endowment and Surface Area of Silica-Alumina Catalysts and of Reaction Conditions on Benzene Synthesis

Radiocarbon laboratories using liquid scintillation counting depend on the availability of a catalyst for benzene synthesis. One of the two commonly used is a commercially available chromium-activated silica-alumina catalyst, PKN/D1. As this catalyst will no longer be produced, we have tested similar catalysts as possible replacements. We measured benzene purity by gas chromatography and mass spectrometry, and found that chromium endowment was crucial for a proper catalyst function. The surface area of the catalyst also significantly affected benzene yield and purity. We also studied the effects of different reaction conditions of acetylene absorption and benzene desorption on benzene purity. A catalyst with half of the Cr endowment and doubled specific surface area created yield and purity comparable to PKN/D1.

Computer Simulation of the Performance of Fluid Catalytic Cracking Risers and Downers

Newly developed zeolite catalysts for the fluid catalytic cracking (FCC) process are typically characterized by high activity and rapid deactivation. Modifications of conventional FCC reactors are necessary to avoid severe backmixing which decreases selectivities and yields and to take full advantage of new catalysts. Recent studies suggest that more uniform flow, better reaction control, shorter residence times with narrower residence time distributions (RTD), and higher catalyst/oil ratios can be achieved in downflow systems (downers) compared to upflow systems (risers). In this work, a novel computer simulation of a FCC downer reactor is introduced. Comparison of downer and riser models for the FCC process shows that downers are slightly more beneficial than risers when commercial silica-alumina catalysts are used. In the case of the FCC process using zeolite catalysts, the benefits of downers become more significant.

Contributions to the calculation of coke deactivation kinetics. A comparison of methods

The deactivation of catalysts by coke deposition can be described in various ways. Several methods are compared with experimental data. The experimental system studied is the deactivation of a silica—alumina catalyst used in the dehydration of 2-ethylhexanol in differential and integral reactors (at constant temperature and using a linear ramp of temperature—time). The limitations of the conventional simplified methods of data analysis are proven. The kinetics calculated by using the simplified methods may differ from the real kinetics and be useless for reactor simulation.

Role of catalysis in achieving environmentally sustainable growth in the 21st century

“The physicist does not discover, he creates his universe.” So proclaimed physicist Henry Margenau in an articled entitled ‘Metaphysical Elements in Physics,’ published in 1941 in Reviews of Modern Physics. In much the same spirit, we catalytic scientists have the opportunity to create the future, rather than let it happen. To do so, it might be helpful for us to envision what could be, in both the near and longer term. History teaches that the future need not be just a natural extension of the present. For example, the automobile was not a simple extension of the horse and buggy, any more than polyethylene was a simple extension of phenolic plastics such as Bakelite. Similarly, the use of zeolites in catalytic cracking did not follow directly from the use of amorphous silica-alumina catalysts, nor did the Oxirane Process for propylene oxide follow from epichlorohydrin technology. These inventions, as many of those that have brought about significant changes, represent discontinuities in the way things were done at the time. But they were not completely serendipitous. In some instances, they might be viewed as ‘planned’ successes which came about by working on the right problem, at the right time, in the right way. I believe we can continue these successes, if we so choose.

Hydrocarbon production from synthesis gas over ZnO−Cr2O3+SA physical mixtures and Pd/SA impregnated catalysts

The performances of ZnO−Cr2O3+silica-alumina physically mixed and Pd impregnated on silica-alumina catalysts in the transformation of synthesis gas to hydrocarbons are compared in the present work. ZnO−Cr2O3 or Pd and silicaalumina are used as methanol synthesis and the hydrocarbon formation catalysts, respectively. The highest CO conversion corresponds to the highest relative amount of methanol synthesis active sites. The highest proximity between both types of active sites in the Pd imprenated on silica-alumina produces higher hydrocarbon selectivity and higher C1 fraction than when using the physically mixed ZnO−Cr2O3+silica-alumina catalysts.

Intermolecular transalkylation of methylanilines

Solid-acids such as Cr 2O 3/Al 2O 3, SiO 2/Al 2O 3 and large port zeolites such as faujasites catalyze intermolecular transalkylation of methylanilines by an electrophilic mechanism. This contrasts to the intramolecular, 1,2-shift isomerization mechanism observed with the zeolite HZSM-5. Aniline and 2,4,6-trimethylaniline give p-toluidine and 2,6-dimethylaniline in the presence of silica—alumina catalysts. They produce o-toluidine and 2,4-dimethylaniline in the presence of metal oxides on alumina. 2,4- and 2,6-dimethylaniline equilibrate in the presence of excess 2,4,6-trimethylaniline, o- or p-toluidine. Aniline as the methyl acceptor interconverts o- and p-toluidine with little formation of the meta isomer. The three dimethylanilines with one meta-methyl (2,3- 2,5- and 3,4-) equilibrate if m-toluidine is the methyl acceptor. We have determined kinetic and thermodynamic parameters for some of these reactions.

Effect of coke deposition on the effective diffusivity of catalyst pellets

A chromatographic pulse injection technique has been used with a modified Wicke-Kallenbach diffusion cell to measure the diffusivity of He-N 2 mixtures in silica-alumina, a commercial zeolitic cracking catalyst and chromia-alumina pelleted catalysts containing increasing amounts of coke. While the results confirmed that diffusivity decreases with increasing coke content, the variation in the measured diffusivity with temperature suggested that diffusion occurs in the transition region for all pellets. The pore size distribution analysis indicated that the coke has a considerable blocking effect on the mesopores for silica-alumina and chromia-alumina catalysts while for zeolites the large mesopores and some macropores are affected.

Kinetic study of methylamine reforming over a silica-alumina catalyst

The performance of a commercial silica-alumina catalyst was investigated in the methylamine reforming reaction system. A mass action based kinetic model was derived from experiments conducted in a laboratory backmixed reactor. Application of this model identified a kinetic route to dimethylamine formation that indicates that operation under kinetic control clearly is necessary to achieve maximum dimethylamine selectivity over this catalyst

Shape selective methylamines synthesis: Reaction and diffusion in a CMs-SiO-AlO composite catalyst

Product shape selective effects, to date observed only with zeolites and a few other crystalline molecular sieves, should be generalizable and thus useful in the design of other novel catalyst materials. To be a viable approach in general for a given reaction, the only requirement for product shape selectivity is that the molecules involved be sufficiently different in size that they transport through micropore structures at different rates. Here, it is reported for the first time that non-crystalline composite catalysts, consisting of silica-alumina and amorphous carbogenic molecular sieves, are able to produce mono- and dimethylamines from methanol and ammonia with higher selectivities than the silica-alumina catalyst, currently used in the commercial practice. With the standard catalyst at ∼ 85% methanol conversion a ratio of (MMA + DMA)/TMA = 1 results, while the new catalyst, designated CMS-SiO 2 -Al 2 O 3 , at the same conditions and methanol conversion, produces ratios of at least (MMA + DMA)/TMA = 2.7. Although the rate of conversion per gram on the new catalyst is only one-third that of the standard catalyst, this is more than adequate activity for a commercial fixed-bed process. A model that accounts for reaction on silica-alumina and diffusion in the molecular sieve carbon is described. By adjusting the diffusivities of the three amines good agreement between prediction and experiment is obtained.

Shape selective methylamines synthesis: Reaction and diffusion in a CMs-SiO 2-Al 2O 3 composite catalyst

Product shape selective effects, to date observed only with zeolites and a few other crystalline molecular sieves, should be generalizable and thus useful in the design of other novel catalyst materials. To be a viable approach in general for a given reaction, the only requirement for product shape selectivity is that the molecules involved be sufficiently different in size that they transport through micropore structures at different rates. Here, it is reported for the first time that non-crystalline composite catalysts, consisting of silica-alumina and amorphous carbogenic molecular sieves, are able to produce mono- and dimethylamines from methanol and ammonia with higher selectivities than the silica-alumina catalyst, currently used in the commercial practice. With the standard catalyst at ∼ 85% methanol conversion a ratio of (MMA + DMA)/TMA = 1 results, while the new catalyst, designated CMS-SiO 2-Al 2O 3, at the same conditions and methanol conversion, produces ratios of at least (MMA + DMA)/TMA = 2.7. Although the rate of conversion per gram on the new catalyst is only one-third that of the standard catalyst, this is more than adequate activity for a commercial fixed-bed process. A model that accounts for reaction on silica-alumina and diffusion in the molecular sieve carbon is described. By adjusting the diffusivities of the three amines good agreement between prediction and experiment is obtained.

Measurement of Surface Acidity by 31P NMR of Adsorbed Trimethylphosphine: Application to Vapor Deposited SiO 2 on Al 2O 3 Monolayer Catalysts

Experimental procedures for quantitative estimation of surface acid by 31P NMR of adsorbed phosphines are discussed. These procedures are illustrated by application to a commercial silica-alumina cracking catalyst, and are applied to a series of silica on alumina monolayer catalysts prepared by chemical vapor deposition. The monolayer catalysts show a maximum in Brønsted acidity for a deposition of 8 SiO 2 groups per nm 2 of A1 2O 3 surface. The evolution of Brønsted acid concentration as a function of amount of deposited silica can be accounted for by a simple kinetic model.

Acid properties of silica-alumina catalysts and catalytic degradation of polyethylene

A relationship between the acid strengths and amounts of silica-alumina catalysts and the compositions of products formed by the catalytic degradation of polyethylene at 673 K was studied. The acid strengths and amounts were varied with SiO{sub 2}/Al{sub 2}O{sub 3} weight ratio in the catalysts. Although the resulting products and amounts were varied with SiO{sub 2}/Al{sub 2}O{sub 3} weight ratio in the catalysts. Although the resulting products consisted of gases, oils, and wax, the fraction of gases increased, and, inversely, the fraction of oils decreased, as the acid amounts over the catalysts increased. The fraction of aromatics in the oils was enhanced, however, as the acid amounts over the catalysis increased, which was discussed in terms of the acid types: Broensted and Lewis acids generated on silica-aluminas. Since some inorganic compounds such as MgO, ZnO, TiO{sub 2}, and carbon are incorporated into plastics, the catalytic activities and selectivities of these additives for polyethylene degradation were also discussed.

Hydrocarbons from synthesis gas over Pd supported on silica-alumina catalyst

We have studied the operational conditions which allow the synthesis gas to hydrocarbon transformation in only one step using Pd supported over an amorphous silica-alumina catalyst. The increase of temperature, pressure and time factor favors carbon monoxide conversion. Methanol synthesis reaction can be considered as the limiting step of the overall process. The product distribution is governed by the reaction medium (type and distribution of active sites and the presence of hydrogen).

Investigation of the acidity of Ni/Al2O3 and Ni/SiO2−Al2O3 catalysts

Acidity of Ni-modified alumina and silica-alumina catalysts was determined using n-butylamine titration and pyridine adsorption methods. Strong influence of Ni2+ ions on the Bronsted acidity of silicaalumina was observed. Improved Bronsted acidity of such system was confirmed by the results of the test reaction.