Vapor-solid reactions, catalyst

A clear example of the possible use of acid and/or superacid solids as catalysts is the alkylation of isobutane with butenes. Isobutane alkylation with low-molecular-weight olefins is one of the most important refining process for the production of high-octane number (RON and MON), low red vapor pressure (RVP) gasoline. Currently, the reaction is carried out using H2SO4 or HF (Table 13.1), although several catalytic systems have been studied in the last few years. 

The catalyst is mixed with air where it picks up oxygen atoms, then is blown together with butane into a reactor where the chemical reaction takes place. The effluent from the, reactor is a mixture of catalyst, MA, water vapor, and a little feedstock. The catalyst is removed by a contraption called a cyclone, which uses centrifugal force to spin the solid., heavier catalyst particles out of the mixture. The MA and feedstock (butane, etc.) are then separated for recovery (the MA) or recycle (the feedstock). 

A more recent development in ethylbenzene technology is the Mobil-Badger process,161,314-316 which employs a solid acid catalyst in the heterogeneous vapor-phase reaction (400-45O C, 15-30 atm). A modified H-ZSM-5 catalyst that is regenerable greatly eliminates the common problems associated with... 

In the process (Figure 8-12), the feedstock is vaporized upon contacting hot regenerated catalyst at the base of the riser and lifts the catalyst into the reactor vessel separation chamber where rapid disengagement of the hydrocarbon vapors from the catalyst is accomplished by both a special solids separator and cyclones. The bulk of the cracking reactions takes place at the moment of contact and continues as the catalyst and hydrocarbons travel up the riser. The reaction products, along with a minute amount of entrained catalyst, then flow to the fractionation column. The stripped spent catalyst, deactivated with coke, flows into the Number 1 regenerator. 

In the DCC unit, the hydrocarbon feed is dispersed with steam and cracked using a hot solid catalyst in a riser, and enters a fluidized bed reactor. A known injection system is employed to achieve the desired temperature and catalyst-to-oil contacting. This maximizes the selective catalytic reactions. The vaporized oil and catalyst flow up the riser to the reactor where the reaction conditions can be varied to complete the cracking process. The cyclones that are located in the top of the reactor effect the separation of the catalyst and the hydrocarbon vapor products. The steam and reaction products are discharged from the reactor vapor line and enter the main fractionator where further processing ensure the separation of the stream into valuable products. 

Styrene. Styrene is the largest benzene derivative with annual consumption about 11.5 billion lb in the United States. It is produced mainly by catalytic dehydrogenation of high-purity ethylbenzene (EB) in the vapor phase. The manufacture process for EB is based on ethylene alkylation with excess benzene. This can be done in a homogeneous system with aluminum chloride catalyst or a heterogeneous solid acid catalyst in either gas or liquid-phase reaction. In the past decade, the liquid-phase alkylation with zeolite catalyst has won acceptance. Those processes have advantages of easier product separation, reducing waste stream, and less corrosion. In addition, it produces less xylene due to lower... 

Reaction of ammonia with various combinations of aldehydes, over solid acid catalysts in the vapor phase, is a convenient route for producing pyridines [77]. For example, amination of a formaldehyde/acetaldehyde mixture affords pyridine and 3-picoline (Fig. 2.25). Mobil scientists found that MFI zeolites such as H-ZSM-5 were particularly effective for these reactions. 

HPAs, however, is their solubility in polar solvents or reactants, such as water or ethanol, which severely limits their application as recyclable solid acid catalysts in the liquid phase. Nonetheless, they exhibit high thermal stability and have been applied in a variety of vapor phase processes for the production of petrochemicals, e.g. olefin hydration and reaction of acetic acid with ethylene [100, 101]. In order to overcome the problem of solubility in polar media, HPAs have been immobilized by occlusion in a silica matrix using the sol-gel technique [101]. For example, silica-occluded H3PW1204o was used as an insoluble solid acid catalyst in several liquid phase reactions such as ester hydrolysis, esterification, hydration and Friedel-Crafts alkylations [101]. HPAs have also been widely applied as catalysts in organic synthesis [102]. 

In a homogeneously catalyzed reaction the determination of the kinetic factors for the process is usually straightforward. In a solution, reactants and the soluble catalysts are uniformly distributed throughout the reaction medium and the reaction rate can be expressed as a function of the concentrations of these substances. A heterogeneously catalyzed process is more complex because the catalyst is not uniformly distributed throughout the reaction medium. Consider a two phase system, either vapor/solid or liquid/solid, with the solid phase the catalyst. In such a system several steps are needed to complete the catalytic cycle ... 

As in the two phase liquid reactions there are fewer mass transport steps in vapor phase reactions than in three phase processes. These steps are shown in Fig. 5.13. The gaseous reactants must pass through the gas/solid interface to reach the catalyst particle. They then migrate through the particle to become adsorbed on the active sites. After reaction the product desorbs, migrates back through the particle to the solid/vapor interface which it passes through to enter the vapor phase in the reactor. 

Fig. 5.13. Mass transport steps in vapor phase reactions catalyzed by a solid catalyst.

Historically, oxide catalysts have been used primarily for vapor phase reactions in the petroleum and petrochemical industries. Recent work, however, has shown that these catalysts can also be effective in promoting a number of synthetically useful reactions. While simple oxides show activity for some oxidations they are more commonly used as solid acids or bases. Complex oxides can act as acids or bases as well as oxidation catalysts. Complex oxides can range in composition from the simple, amorphous, binary oxides to the more complex ternary and quaternary systems. The use of zeolites and clays can impart shape selectivity to a number of reactions, a feature that makes these systems particularly appealing for use in synthesis. 

These materials are thermally stable so they find use as solid acid catalysts for a variety of vapor phase reactions. They can be used as the bulk oxyacid but are more efficient when they are supported on materials such as alumina, silica or carbon. The supported heteropoly acids are usually prepared by either incipient wemess or equilibrium impregnation procedures. 3 Carbon supported heteropoly... 

The sulfonic acid resins such as Dowex-50 and Amberlyst-15 have been used to promote the alkylation of the more active aromatic rings but attempts to increase their acidity generally resulted in the degradation of the solid. 2 The more strongly acidic perfluorinated resin sulfonic acid, Nafion-H,2>3 has, however, been used to promote the alkylation of benzene and other aromatic compounds. Nafion-H catalyzed the vapor phase reaction between toluene and methanol. When nm at 185°C a 12% yield of the isomeric xylenes was obtained with the ortho isomer the major product. 0 Methylation of phenol at 205°C over this catalyst gave, at 63% conversion. 37% anisole and 10% of a mixture of the ortho and para cresols in a 2 1 ratio. Reaction of anisole with methanol under these conditions resulted in a 14% selectivity to the methyl anisoles at 40% conversion, with the ortho and para isomers formed in nearly equal amounts. ... 

Heterogeneous reduction processes still involve the reaction of gases, but in these cases the reaction occurs in the presence of a suitable solid phase catalyst. Sulfur dioxide may be reduced to sulfur with hydrogen sulfide, if this is available, and the sulfur vapor condensed out of the gas stream by cooling, as in the second half of the Claus process (Eq. 3.17). 

The use of a support allowing the HPA to be dispersed over a large surface may result in an increase of its catalytic activity. The performance of supported HPA catalysts depends on the carrier, the HPA loading, conditions of pretreatment, among other variables. Acidic or neutral solids such as active carbon, Si02 and ZrOz are suitable as supports [1]. But HPA often leaks out of catalyst supports even in vapor-phase reactions. It is important, for practical purposes, to develop supported catalysts which can be applied to several reactions with no leakage of HPA. 

The presence of catalytically active Lewis acid sites in sulfated zirconia catalysts is much debated [1-5]. The conventional preparation of sulfated zirconia catalysts involves reaction of freshly precipitated zirconium hydroxide with diluted sulfiiric acid or impregnation of zirconium hydroxide with sulfuric acid or ammonium sulfate [6,7]. The final solid acid catalyst results by calcination at a temperature of 723 to 873 K. Provided thermodynamic equilibrium has been reached, all water and free sulfuric acid should have evaporated upon calcination at 673 to 873 K and only chemically bonded sulfete groups remain [8]. Above 890 K, bulk anhydrous Zr(S04)2 decomposes [1]. When uptake of water by the calcined catalyst is prevented or after loading of the catalyst in the reactor physisorbed water is removed by thermal treatment, only Lewis acid sites are present. Since it is difficult either to prevent the uptake of water vapor or to remove adsorbed water completely, it is difficult to attribute the acid activity of sulfeted zirconia catalysts unambiguously to Lewis acid sites. 

It was well known from the literature that aniUnes can be mono- or di-alkylated with alcohols in presence of acidic catalysts. However, preliminary experiments on several solid acidic catalysts in the vapor-phase gave complex mixtures of the desired product (NAA), as well as N-methyl-, N-dimethyl-, N-propyl- and N-isopropyl-M L4 as by-products. These can be explained by cleavage of the ether group of MOIP followed by reaction of the fragments with MEA. All atten Jts to improve the selectivity of this acid catalyzed alkylation Med. Therefore, the reductive alkylation route was chosen for further investigations. 

The general principles of modeling a vapor-phase reaction on a solid catalyst in a fixed-bed reactor are illustrated by applying them to the specific case of nitrobenzene reduction to aniline. Several models of varying complexity are discussed. 

If you have never run a catalytic vapor-phase reaction in a hot tube, pick one that could use a solid acid or base as a catalyst and try it. 

Solid-eatalyzed reactions can occur in either the liquid or gas phase. Gas-phase reactions are not very common in the production of fine chemieals, beeause eom-plex molecules with limited volatility and thermal stability are usually involved, which makes operation at the high temperatures required for their vaporization impossible. Gas-liquid reactions with a solid catalyst probably encompass the largest number of applications in fine-chemical and pharmaceutical processes [1]. Two other classes of solid-eatalyzed reaction taking place in the liquid phase are liquid-solid reactions and liquid-liquid-solid reactions, but these are much less eommon. We shall, therefore, foeus on gas-liquid-solid reaetions, in which the solid is a heterogeneous catalyst. 

Although many solid-acid catalysts have been reported for the vapor-phase Beckmann rearrangement [2], their performance has been less than satisfactory from an industrial standpoint and the heterogeneously catalyzed Beckmann rearrangement has not yet been commercialized. In this chapter heterogeneous catalysis of the Beckmann rearrangement, its mechanism, and acid properties and reaction conditions suitable for the reaction will be reviewed. 

Class 1 contains most of the commercial operations, but vapor-phase reactions are becoming increasingly important. The passage of chloro benzene vapor and steam at an elevated temperature over a solid catalyst to produce phenol and hydrochloric acid is an example of the latter type, as is the direct hydration of ethylene to produce ethyl alcohol. 

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