Zeolite HMOR

The transition metal free zeolites (HMOR, HZSM5, NaY) did not show any C-S hydrogenolysis activity for thiophene, but exhibited remarkable coking, and, the acidic zeolites, additionally some cracking activity. 

It was obtained by a pre-treatment of fresh impregnated HMOR in flowing air, up to 773 K. In these conditions, as detected by TEM, EDS and UV-visible spectroscopy (not shown, [12]), a fraction of Co2+ species, exchanged in the pores of HMOR, migrates on the outside of the zeolite grain, to form Co304 on the external surface of the HMOR grain. 

It corresponds to the cobalt initially exchanged into the HMOR porosity. Nevertheless, a fraction of cobalt oxide - Co304 - is produced after calcination, as previously seen in the case of Cat I, on the surface of zeolite grains. 

The transformation of cyclopentanol-cyclohexanone mixture was carried out on aluminas, and compared with a basic (MgO) and acidic (HMOR zeolite, Si/Al = 80) catalysts. Figure 2 shows the activities, expressed as mmol.h 1.m 2, for the two reactions, for the different catalysts. 

Alumina is an amphoteric catalyst, which can difficult to characterize via chemical and physic methods. The transformation of cyclopentanol/cyclohexanone mixture allows us to estimate at the same time the acid-base properties of aluminas. From this transformation, it was shown that aluminas can be classified into two families only basic aluminas, such as theta, which were more basic than MgO, and acido-basic aluminas, eta, gamma and delta, which possess an acidic character less pronounced than dealuminated HMOR zeolite... 

The gas phase isomerisation of o-dichlorobenzene (odCB) was studied over protonic zeolites HZSM5, HMOR, HMAZ, HOFF, HBETA and a pillared clay HPILC. All the catalysts deactivate. The deactivation rate is the highest when dry air is used as carrier gas, and the lowest with nitrogen... 

Among other factors, the strength of the protons in zeolite depends on the framework A1 content, and should go toward a maximum around Si/Al = 10 (ref.20). Indeed, a volcano-shaped dependency between the rate and m = Al/(A1+Si) was reported for odCB isomerisation on HMOR, HBETA and HOFF (ref.7). Moreover, the nature of the zeolite influences the proton strength too. Table 3 reports the reaction rates and intrinsic activities, expressed as turnover number (TON), on various zeolites at a Si/Al content close to the optimum. The TON was calculated by dividing the rate by the proton concentration in the zeolite. 

Anisole and veratrole acetylation with AA were carried out over three large pore zeolites HFAU, HBEA and HMOR and one average pore size zeolite HMFI.[3] With both substrates, the initial rates as well as the maximum yield were found lower over the monodimensional MOR zeolite and with MFI, which was explained by diffusion limitations. Anisole acetylation was shown to be quicker and the maximum yield higher over HBEA than over HFAU, whereas the reverse was found with veratrole acetylation.[3,4] Such behaviour could be explained from the relative sizes of the acetylation intermediates and of the micropores (transition shape selectivity).[3] Whereas HFAU and HBEA zeolites with similar Si/Al ratios have practically the same activity for 2-MN acetylation, HMFI is practically inactive. Furthermore, HBEA is more active than HFAU for 1-AMN isomerization. 

A relation exists between the T-O-T bond angles and the acid strength of zeolites20. Thus, the protonic sites of HMOR (bond angle range of 143-180°) and HMFI (133-177°) zeolites are stronger than those of HFAU (133-147°). This explains why HMOR is active for butane and n-hexane isomerizations at 200-250°C which require very strong acid sites whereas it is not the case for HFAU. 

The accessibility of the protonic sites also plays a significant role in the catalytic activity of zeolites. Obviously this accessibility depends both on the location of the OH in the zeolite and on the size of the reactant molecules. Thus, the portion of protonic sites of HFAU zeolites located in the supercages is accessible to many organic molecules whereas the others, located in the hexagonal prisms, are inaccessible to all the organic molecules. HMOR also has protonic sites accessible (in the large channels) to many organic molecules and less accessible sites (in the... 

Fig. 1.5 Schematic representation of shape selective effects a) Reactant selectivity Cracking of an n-iso C6 mixture, b) Product selectivity Disproportionation of toluene into para-xylene over a modified HFMI zeolite, c) Spatioselectivity Disproportionation of meta-xylene over HMOR. The diphenylmethane intermediate A in formation of 1,3,5 trimethylbenzene is too bulky to be accommodated in the pores, which is not the case for B...

Fig. 9.8 Turnover frequency values TOF (h1) of the protonic sites for ethylbenzene disproportionation over various PtA/HMOR catalysts with different proportions of PtA and HMOR and different framework Si/Al ratios of the zeolite versus the concentration of protonic sites nH+ per gram of zeolite...

Anisole acetylation, which was one of the main reactions investigated, was first shown to be catalysed by zeolite ten years ago by Bayer (13), which was confirmed by Harvey et al. (14), then by Rhodia (15). Large pore zeolites and especially those with a tridimensional pore structure such as HBEA and HFAU were found to be the most active at 80°C, in a batch reactor with an anisole/acetic anhydride molar ratio of 5 and after 6 hours reaction, the yield in methoxyacetophenone (MAP) was close to 70% with HBEA and HFAU zeolites, to 30% with HMOR and 12% with HMFI. With all the zeolites and also with clays and heteropolyacids, the selectivity to the para-isomer was greater than 98%, which indicates that this high selectivity is not due to shape selective effects but rather to the reaction mechanism (electrophilic substitution). The lower conversion observed with HMOR can be related to the monodimensional pore system of this zeolite which is very sensitive to blockage by heavy secondary products. Furthermore, limitations in the desorption of methoxyacetophenone from the narrow pores of HMFI are probably responsible for the low activity of this intermediate pore size zeolite. 

The etherification of vanillic alcohol with ethanol was compared over three large pore zeolites HBEA, HFAU and HMOR and over an average pore size zeolites (HMFI) with different Si/Al ratios. Whatever the large pore zeolite and its Si/Al ratio, a total conversion of the benzylic alcohol and a 100% yield in ether are obtained. With the HMFI samples, conversion and yields were equal to 55% only for an Si/Al ratio of 15 and to approximately 70% for Si/Al = 40. It could be expected that the reaction is limited by diffusion of the bulky reactant and product molecules in the narrow pores of this zeolite. 

For similar coke contents the composition of coke depended very much on the zeolite, which confirmed that coking is a shape-selective process (9-10). At low coke contents, the major components were alkylbenzenes in the case of HZSM-5 and HERI, while they were polyaromatic compounds with HUSY and HMOR. At high coke contents the coke components were polyaromatic with all zeolites. However, the size and the shape of coke molecules were different ... 

Figure 5. n-Heptane cracking at 450 C over various protonic zeolites. Relative decrease in activity (1-AR) as a function of the ratio of the number of coke molecules to the number of strong acid sites (nK/nA2). Experimental values USHY (A) HZSM-5 ( ) Straight lines with slope = 1, 2, 3 HMOR ( ) HERI ( ) slope = 10,20. Reproduced with permission from ref.9. 

Quantitative data on the relation of Pt loading, concentration of acid sites, and activity for -heptane conversion have been published by Guisnet et al. for HY, HMor, and HZSM-5 40). Their findings are illustrated in Fig. 22. Using p,/ a as a parameter for the relative concentrations of metal and acid sites, respectively, it appears that the minimum pt/ A values required for maintaining catalytic stability are different for different zeolites. 

Syngas conversion has also been studied with physically mixed Pd/SiOi and acidic zeolites, including HY, HZSM-5, and HMor. The mixture of Pd/SiOi with HY was found to produce mainly aliphatic higher hydrocar-... 

In this paper we describe the surface chemistry of adsorption of thiophene on acidic (HZSM5, H-Mordenite (HMOR)) and non-acidic (NaY) zeolites, as well as on their partially nickel or cobalt exchanged forms. The thiophene HDS activity of Ni or Co exchanged zeolites was studied and connection was established between the adsorption and the catalytic properties. 

HZSM5 (Si/AI = 27.5), HMOR (Si/AI = 10) and NaY (Si/AI = 2.65) zeolites were partially ion-exchanged with NiClg or CoClg solutions. The composition of the ion-exchanged zeolites (Table 1) was determined by Scanning Electron Microscopy -Energy Dispersive X-Ray Absorption Spectroscopy (SEM-EDS). 

Figure 3. Difference of infrared spectra obtained with and without adsorbed thiophene for MOR zeolites at 1 mbar thiophene equilibrium pressure and the difference spectrum for the spent HMOR sample.

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