Zeolite catalysts advantages

A solution of trifluoroacetic acid in toluene was found to be advantageous for cydization of pyruvate hydrazoncs having nitro substituents[4]. p-Toluene-sulfonic acid or Amberlyst-15 in toluene has also been found to give excellent results in preparation of indole-2-carboxylale esters from pyruvate hydra-zoiies[5,6J. Acidic zeolite catalysts have been used with xylene as a solvent to convert phenylhydraziiies and ketones to indoles both in one-flask procedures and in a flow-through reactor[7]. 

Compared to amorphous silica-alumina catalysts, the zeolite catalysts are more active and more selective. The higher activity and selectivity translate to more profitable liquid product yields and additional cracking capacity. To take full advantage of the zeolite catalyst, refiners have revamped older units to crack more of the heavier, lower-value feedstocks. 

The activity advantage of zeolite catalysts over amorphous silica-alumina has well been documented, Weisz and his associates [1] reported that faujasite Y zeolite showed 10 to 10 times greater activity for the cracking of n-hexane than silica-alumina. Wang and Lunsford et al. [2] also noted that acidic Y zeolites were active for the disproportionation of toluene while silica-alumina was inactive. The activity difference between zeolite and silica-alumina has been attributed to their acidic properties. It is, however, difficult to explain the superactivity of zeolite relative to silica-alumina on the basis of acidity, since the number of acid sites of Y-type zeolite is only about 10 times larger than that of silica-alumina. To account for it, Wang et al. [2] proposed that the microporous structure of zeolite enhanced the concentration of reactant molecules at the acid sites. The purpose of the present work is to show that such a microporous effect is valid for pillared clay catalysts. 

C, 10-50 atm). Xylene benzene ratios of 1-10 may be obtained. Metal catalysts were later replaced by zeolites.210,211,326-328 The most recent development is the Mobil selective toluene disproportionation process,329 which takes advantage of the high para shape selectivity of a zeolite catalyst.210 The catalyst activated by a novel procedure ensures a p-xylene content of up to 95%. After the successful com-mercialization at an Enichem refinery in Italy, the process is now licensed. The catalysts and technologies applied in toluene disproportionation may be also used for transalkylation324,325,331 [Eq. (5.74)] ... 

In recent years alkylations have been accomplished with acidic zeolite catalysts, most nobably ZSM-5. A ZSM-5 ethylbenzene process was commercialized jointly by Mobil Co. and Badger America in 1976 (24). The vapor-phase reaction occurs at temperatures above 370°C over a fixed bed of catalyst at 1.4—2.8 MPa (200—400 psi) with high ethylene space velocities. A typical molar ethylene to benzene ratio is about 1—1.2. The conversion to ethylbenzene is quantitative. The principal advantages of zeolite-based routes are easy recovery of products, elimination of corrosive or environmentally unacceptable by-products, high product yields and selectivities, and high process heat recovery (25,26). 

Zeolite-Based Alkylation. Zeolites have the advantage of being noncot-rosive and environmentally benign. The Mobil-Badger vapor-phase ethylbenzene process was ihe lirsl zeolite-based process to achieve commercial success. It is based on a synthetic zeolite catalyst. ZSM-5. and has the desirable characteristics of high activity, low oligomerization, and low coke formation. See also Molecular Sieves. 

Recently Mikae et al. (66) showed that NaY zeolite impregnated with NaCl solution can be used as a para selective catalyst for chlorination of chlorobenzene, which is indeed another example of zeolite modified advantageously by means of monolayer dispersion of salts. 

In principle, the zeolite catalyst system would offer advantages over the existing homogeneous catalyst, particularly with respect to corrosion due to the absence of HC1 and chlorine-containing by-products. However, acetaldehyde and acetic-acid production via ethylene has recently become less economically attractive compared to methanol carboxylation chemistry. 

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... 

In the manufacture of ethylbenzene and cumene, the cost of benzene feedstock is a major factor in the overall economics. Thus, it is critical to have efficient technology for the alkylation of benzene. Zeolitic catalysts have the advantages of achieving higher purity and higher yield of product relative to aluminum chloride and SPA catalysts. Table 4.10 compares purities and yields and also shows the breakdown of impurities for both ethylbenzene and cumene. In both cases, extremely high purities can be achieved, 99.96 and 99.97%, respectively. The product yields are also extremely high, 99.6% and 99.7%. 

Process Catalyst Advantages in using zeolite-based catalysts... 

We can readily obtain an optimum zeolite catalyst for achieving high efficiency and high selectivity by exchanging cations in the zeolite. The advantage of zeolite over alumina is the easy adjustability of the chemical properties. 

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