Vikram S. Nayak

Selective poisoning of stronger acid sites on HZSM-5 in the conversion of alcohols and olefins to aromatics

The extent of aromatization and the distribution of aromatics in the conversions of methanol, ethanol, ethylene, propylene, butene-1, 1-hexene and cyclohexene on HZSM-5 zeolite are found to depend strongly on the extent of catalyst poisoning with pyridine, which causes not only a decrease in the stronger sites available for the reactions, but also leads to an increase in the diffusional resistance to the reaction species and the deactivation due to the deposition of hydrocarbon residues in the zeolite channels.

Conversion of alcohols to aromatics on H-SM—5: influence of SiAl ratio and degree of cation exchange on product distribution

Abstract The extent of aromatization and the distribution of aromatic hydrocarbons in the conversion of methanol and ethanol on H-ZSM—5 have been found to be strongly dependent on the Si Al ratio and the degree of H-exchange of the zeolite. The aromatization activity in both the alcohol conversion reactions decreases with the Si Al ratio and increases with the degree of H + -exchange. The reactions on H-silicalite yield mostly aliphatic hydrocarbons. An appreciable extent of hydrocarbon residue is retained in the zeolite channels for both the reactions on H-silicalite and on the zeolites with lower degrees of H + -exchange. The strong acid sites are required for the aromatization and in their absence, the deposition of hydrocarbon residue in the zeolite channels occurs. A good correlation between the acidity (active acid sites) and the aromatization activity of the zeolite has been obtained.

Catalytic activity and product distribution in the cracking of n-hexane over heteropoly oxometalates and ZSM-5 zeolite

Abstract The catalytic properties of microporous and nonporous heteropoly oxometalates and ZSM-5 zeolites prepared in the absence of an organic templating agent have been studied in the cracking of n-hexane at 648, 673 and 698 K. The activities are measured and compared at quasi-steady state and with time-on-stream. The effects of calcination temperature on the cracking activity have also been studied. The dependence of product selectivities on the conversion of n-hexane and on the reaction temperature over ZSM-5 zeolite and (NH4)3PW12O40 are reported. While the products are similar the selectivities are markedly dependent on the catalyst employed.

Effect of degree of H+ exchange on the acidity distribution of HNaZSM5

Abstract The acidity distribution of the HNaZSM5 zeolites was investigated by the adsorption and desorption of pyridine using the two gas chromatographic methods, viz. T.p.d. (temperature programmed desorption) under chromatographic conditions and stepwise thermal desorption (STD), at temperatures close to those employed in the catalytic reactions and also by studying the model reactions, viz. cracking of cumene and isomerization of o-xylene, catalysed by protonic acid sites in a pulse microreactor. The acidity has been found to be strongly dependent on the degree of H+-exchange of the zeolite. When the degree of H+-exchange is increased, an increase is observed not only in the number of strong acid sites, but also in the ratio of strong acid sites to the exchanged H+ and in the catalytic activity for both the cumene cracking and o-xylene isomerization reactions.

Liquid Phase Isomerization of C6C8 Alkenes on Heteropolyoxometalates

Abstract The isomerization of 1-hexene, 1-heptene and 1-octene has been observed in the liquid phase between 303 and 343 K on the solid ammonium salts of 12-tungstophosphoric (H 3 PW 12 O 40 ), 12-molybdophosphoric (H 3 PMo 12 O 40 ) and 12-tungstosilicic (H 4 SiW 12 O 40 ) acid and their parent acids. Double bond and cis — trans isomerizations of the olefins are strongly catalyzed by H 3 PW 12 O 40 , (NH 4 ) 3 PW 12 O 40 and H 4 SiW 12 O 40 and weakly by H 3 PMo 12 O 40 , (NH 4 ) 4 SiW 12 O 40 and (NH 4 ) 3 PMo 12 O 40 at near ambient temperatures. No skeleton isomerization of any of the olefins was observed at temperatures up to 343 K. Variations of the product distributions with reaction time and the dependence of selectivity on the conversion are reported. The isomerization is tentatively attributed to the formation of carbocations and 1,2-hydride shifts.

Catalytic activity and product distribution in the cracking of C6-C8 alkenes on the ammonium salt of 12-tungstophosphoric acid

Abstract The cracking of C6-C8 alkenes has been studied on the ammonium salt of 12-tungstophosphoric acid ( NH4PW ). The products range from C3 hydrocarbons to aromatics, the latter appearing in amounts up to 30% of the products. The primary products from the alkenes provide strong evidence that condensation processes are strongly favoured on these catalysts. The cracking ability of the catalyst decreases rapidly with time on stream but attains a steady state after approximately three hours. The activity of the catalyst is shown to be dependent on the source of the ammonium ion employed in the preparation of the salt with the activity of NH4PW for hezene cracking decreasing in the order NH4Cl > NH4NO3 > (NH4)2CO3 > (NH4)2SO4. The cracking activity of NH4PW for the C6-C8 alkenes decreases in the order: octenes > heptenes > hexenes.

Effect of silicon-to-aluminum ratio and template on the cracking of C6—8 alkenes over ZSM-5 zeolite

Abstract The cracking of 1-hexene, 1-heptene and 1-octene has been studied on ZSM-5 zeolites of various silicon-to-aluminium (Si/Al) ratios prepared with tetrapropyl ammonium bromide (TPAB) as templating agent and one sample (Si/Al=36) prepared by use of ammonia in place of TPAB. The activities and selectivities are found to be similar for the catalysts prepared with the two templating agents. In addition, the cracking activity is shown to decrease with increasing Si/Al while the activity per aluminum atom increases with decrease in the aluminum content of the zeolite up to a Si/Al ratio of approximately 1000. The ease with which the alkenes undergo cracking is found to be in the order octenes> heptenes> hexenes. The cracking of alkenes results mainly in the production of C 3 , C 4 and C 5 alkenes.

Comparison of the surface properties of the metal—oxygen cluster compounds, 12-tungstophosphoric acid and ammonium 12-tungstophosphate from the sorption of benzene, nitrogen and pyridine

Abstract The surface properties of ammonium 12-tungstophosphate and its parent acid, 12-tungstophosphoric acid have been evaluated from measurements of the adsorption of benzene, nitrogen and pyridine. Ammonium 12-tungstophosphate prepared from a number of different ammonium salts is found to have different surface areas, benzene sorption capacities and diffusivities and numbers of Bronsted acid sites, the latter as estimated from the sorption of pyridine. The surface areas, sorption capacities and acidity distributions of both the parent acid and ammonium 12-tungstophosphate show significant differences and are strongly influenced by duration calcination of its temperature and environment.