H-ZSM-5 catalyst

As described in the previous section, the silica-alumina catalyst covered with the silicalite membrane showed exceUent p-xylene selectivity in disproportionation of toluene [37] at the expense of activity, because the thickness of the sihcahte-1 membrane was large (40 pm), limiting the diffusion of the products. In addition, the catalytic activity of silica-alumina was not so high. To solve these problems, Miyamoto et al. [41 -43] have developed a novel composite zeohte catalyst consisting of a zeolite crystal with an inactive thin layer. In Miyamoto s study [41], a sihcahte-1 layer was grown on proton-exchanged ZSM-5 crystals (silicalite/H-ZSM-5) [42]. The silicalite/H-ZSM-5 catalysts showed excellent para-selectivity of >99.9%, compared to the 63.1% for the uncoated sample, and independent of the toluene conversion. 

The excellent high para-selectivity can be explained by the selective escape of p-xylene from the H-ZSM-5 catalyst and inhibition of isomerization on the external surface of catalysts by silicalite-1 coating. In addition to the high para-selectivity, toluene conversion was still high even after the silicalite-1 coating because the silicalite-1 layers on H-ZSM-5 crystals were very thin. 

Figure 4. Comparison of Propane Aromatization Performances of a Palladium Membrane Reactor (PMR) and a Conventional Reactor (CR) using a Ga-H-ZSM-5 Catalyst...

In our previous work [11], it has been shown that the reduction of NO with CH4 on Ga/ and ln/H-ZSM-5 catalysts selectively proceeds in the following two stages ... 

Na-ZSM-5(a molar SiOz/AlaOa ratio=23.8) provided by Tosoh Corp. was used. ln(4wt%)/H-ZSM-5 and lr(1wt%)/H-ZSM-5 catalysts were prepared by the ion exchange method using NH4-ZSM-5 derived from the Na-ZSM-5 with aqueous solutions of ln(NOs)3 at 368 K for 8 h and lrCI(NH3)sCl2 at room temperature for 24 h, respectively. Addition of precious metals, 1wt% platinum and iridium to ln/H-ZSM-5 was carried out by impregnating the ln/NH4-ZSM-5 in aqueous solutions of Pt(NH3)4Cl2 and lrCI(NH3)5Cl2, respectively. The catalysts were calcined at 813 K for 3 h. 

Table 1. Summary of kinetic data for NOX-CH4-O2 reaction on ln/H-ZSM-5 and lr/ln/H-ZSM-5 catalysts.

As shown in Table 1, the reaction order with respect to NO2 on ln/H-ZSM-5 was smaller than that of NO. This is in accordance with the proposed reaction sequence that NO is firstly oxidized to NO2 and the NO2 reacts with CH4. Coincidence in the order of reaction for NO2 between ln/H-ZSM-5 and lr/ln/H-ZSM-5 catalysts means that NO2 react on a common active site which should be In species. 

In our previous work [11], it has been shown that the reduction of NO with CH4 on Ga and ln/H-ZSM-5 catalysts proceeds through the reactions (1) and (2), and that CH4 was hardly activated by NO in the absence of oxygen on these catalysts. Therefore, NO2 plays an important role and the formation of NO2 is a necessary step for the reduction of NO with CH4. In the works of Li and Armor [17] and Cowan et al. [18], the rate-determining step in NO reduction with CH4 on Co-ferrierite and Co-ZSM-5 catalysts is involved in the dissociative adsorption of CH4, and the adsorbed NO2 facilitates the step to break the carbon-hydrogen bond in CH4. It is suggested that NO reduction by use of CH4 needs the formation of the adsorbed NO2, which can activate CH4. 

Fig. 3.1.10 Molecular lifetimes xintra and. aii in H-ZSM-5 crystallites obtained using the NMR tracer desorption technique and calculated via Eq. (3.3.15), respectively. Tracing by probe molecules (methane, measurement at 296 K) after an H-ZSM-5 catalyst has been kept for different coking times in a stream of n-hexane (filled symbols) and mesitylene (open symbols) at elevated temperature. The inserts present the evidence provided by a comparison of xintra and r]1,]]], with respect to the distribu-...

A pore-size-regulated 5% ZnO-H-ZSM-5 catalyst was studied in the aromatization of C4-Cg hydrocarbon stream at 500-540°C.421 Modification of the pore size was performed by chemical vapour deposition of (EtO)4Si to form Si02. A significant enhancement of p-xylene selectivity as high as 99% was achieved with this catalyst with the production of benzene and toluene remaining unaffected. 

In connection with the aromatization of methane induced by Mo-H-ZSM-5 catalysts (see Section 3.5.2), Mo2C on H-ZSM-5 prepared by carburation of M0O3 was studied in the aromatization of ethane,427 ethylene,428 and propane 429 The high dehydrogenation activity of Mo2C and the ability of H-ZSM-5 to induce aromatization of alkenes makes this an active and selective catalyst for aromatics production. 

Experimental work using a pulse-quench catalytic reactor650 to probe transition between induction reactions and hydrocarbon synthesis on a working H-ZSM-5 catalyst has resulted in the suggestion that stable cyclopentenyl cations are formed during the induction period from small amounts of olefins formed in an induction reaction 647 One study reports a surprising observation, namely, enhanced aromatic formation over the physical mixture of Ga203 and H-ZSM-5 (1 1) (18.2% of benzene and methylbenzenes).651... 

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... 

Figure 3. Illustration of the decrease in % C2H2 conversion with time on stream in reactions of C2H2 + H2O mixtures over pure H-ZSM-5 catalysts at 350 °C and 400 °C.

The zeolite catalyst can be tailored depending on the desired product distribution. For instance, pyridine/picoline ratios are higher with H-ZSM-5 catalysts than with other zeolites (10) ... 

The structure of the hydrocarbons produced can be modified by the use of catalyst. Catalytic cracking consumes less energy than the noncatalytic process and results in formation of more branch-chain hydrocarbons. On the other hand the addition of the catalyst can be troublesome, and the catalyst accumulates in the residue or coke. There are two ways to contact the melted polymer and catalysts the polymer and catalyst can be mixed first, then melted, or the molten plastics can be fed continuously over a fluidized catalyst bed. The usually employed catalysts are US-Y, and H-ZSM-5. Catalyst activity and product structure have been reported [7-11]. It was found that the H-ZSM-5 and ECC catalysts provided the best possibility to yield hydrocarbons in the boiling range of gasoline. 

Acetaldoxime was converted to acetonitrile in about 80% yield over an H-ZSM-5 catalyst at 300°C (Eqn. 22.23).5 Heating aldoximes with montmorillonite KSF in refluxing toluene gave the nitriles in 65-85% yields. Rupe Reaction... 

The vapor phase N-alkylation of aniline with ethanol took place over an H-ZSM-5 catalyst at temperatures between 250° and 500°C. The maximum selectivity for N-monoethylation occurred at the lower temperatures with increasing amounts of diethyl aniline produced at higher reaction temperatures. v/ith montmorillonite catalyst, reaction at 400°C gave a 64% selectivity for N-ethyl aniline formation at 77% conversion. A vanadium exchanged montmorillonite was more active but less selective giving N-ethyl aniline in 48% selectivity and N, N-diethyl aniline in 37% selectivity at 97% conversion.94... 

The transformation of light alkanes (C2-C4) over H-ZSM-5 and Ga or Zn modified H-ZSM-5 catalysts to aromatic hydrocarbons has been studied intensively in recent years, since it would expand the raw material base for the manufacture of aromatics [1, 2]. The aromatics produced can be used as feed-stock for plastics, as chemical source for many chemical processes, as additives for increasing the octane number in gasoline, etc. 

The transformation of n-butane over Ga-H-ZSM-5, Zn-H-ZSM-5 and H-ZSM-5 catalysts was carried out in a continuous-flow quartz reactor with 0.5 g of catalyst at atmospheric pressure. Inert silica grains and anti bumping granules were placed at both ends of the catalyst bed. The reactor was heated in an electric oven and the temperature of the catalyst bed was... 

Figure 3. Temperature-programmed desorption of ammonia for Zn-H-ZSM-5 catalyst.

The transformation of n-butane over the Ga and Zn modified ZSM-5 catalysts produced similar aromatic hydrocarbons and gaseous products as over H-ZSM-5. Ethyl benzene was the only aromatic which was not formed with the proton form catalyst. Ga-H-ZSM-5 and Zn-H-ZSM-5 exhibited higher catalytic activity and selectivity to aromatics than the H-ZSM-5 catalyst The amount of cracking products formed for Ga- and Zn- modified catalysts were smaller than for ZSM-5 in its proton form. Toluene constituted almost 50 % of the aromatics formed while benzene, xylenes and ethylbenzene formed the rest The conversion of n-butane and selectivity to aromatic hydrocarbons increased with increasing temperature. The effect of temperature on n-butane conversion and aromatic selectivity over the catalysts is given in Figures 4 and 5. The product selectivity obtained from the transformation of n-butane over the H-ZSM-5, Ga-H-ZSM-5 and Zn-H-ZSM-5 catalysts at 803 K is given in Table 1. 

The reaction of n-butane at different WHSV (1.5 h to 5.5 h ) over H-ZSM-5, Ga-H-ZSM-5 and Zn-H-ZSM-5 catalysts resulted in the same type of products as those formed at different temperatures, indicating that the reaction products formed at longer contact time are not adsorbed on the surface of zeolite or trapped in the zeolite channel system. The n-butane conversion and selectivity to aromatics over H-ZSM-5, Ga-H-ZSM-5 and Zn-H-ZSM-5 catalysts decreased with increase in the space velocity. The effect of space velocity on n-butane conversion and selectivity to aromatic hydrocarbons is given in Figures 6 and 7. 

The catalyst stability test was performed for all the catalysts at 803 K for 4.5 hours at WHSV equal to 2.5 h. The n-butane conversion and aromatic selectivity over the catalysts were observed to be stable even after 4.5 hours. The coke formation over Ga-H-ZSM-5 and Zn-H-ZSM-5 catalysts was found to be smaller than over H-ZSM-5 catalysts, showing that Bronsted sites when combined with metal species are more resistant to coke formation. 

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