W. Böhringer

The use of a jet loop reactor to study the effect of crystal size and the co-feeding of olefins and water on the conversion of methanol over HZSM-5

Abstract The conversion of methanol into olefins has been carried out in a gradientless quartz jet-loop reactor, thus ensuring the absence of mass and heat transfer effects and the absence of wall reactions. The catalysts used were three samples of H-ZSM-5, each with a Si:Al ratio of about 100 and with a different crystal size. A thermodynamic analysis of the methanol (MeOH)–dimethylether (DME)–[CH2]n system showed that in the jet loop reactor the MeOH–DME reaction was far removed from equilibrium, and this increased with increasing temperature. No fundamental difference was observed in the selectivities of the hydrocarbon fraction at the same conversion of oxygenates between the jet loop reactor and published data using fluidized bed and fixed bed reactors. Co-fed water reduced the conversion, probably by reducing the number of available sites due to preferential adsorption. In the jet loop reactor the alkylation of olefins with oxygenates appeared to occur to a lesser extent than that which is usually observed in a fixed bed reactor. Moreover, significantly different behaviour was observed in the case of each crystal size, with the least amount of DME forming when the largest crystals were used. It was possible to explain these differences in terms of the diffusional resistance experienced by DME inside the crystals. Pseudo rate constants were derived using a mechanistic model typical of the MTO reaction. Methanol and DME were both involved to a similar extent in the alkylation of the [CH2]n species. The model predicted that the first C–C bond formation was the slow step in the reaction sequence. Alkylation was faster than the reversible MeOH to DME reaction.

Rediscovery of the paring reaction: The conversion of 1,2,4-trimethylbenezene over HZSM5 at elevated temperature

The paring reaction was first described 4 decades ago as an acid catalysed sequence of reactions to apparently “pare” methyl groups from polymethylated cyclic hydrocarbons thereby joining the methyl groups to longer side chains and subsequently splitting off these chains to form low aliphatics. Since then the paring reaction of polymethylated aromatics was seldom reported in literature, presumedly because it hardly plays a role as a side reaction with the frequently studied isomerisation and transalkylation of low aromatics (such as xylenes) below 400°C. In contrast, the paring reaction was found to play a decisive role during the conversion of 1,2,4-trimethylbenzene over HZSM5 at elevated temperature (450°C). 1,2,4-Trimethylbenzene disproportionates to form xylenes and tetramethylbenzenes. Over amorphous silica-alumina the reaction almost terminates at the disproportionation step. Over HZSM5 the tetramethylbenzenes almost completely undergo the classical paring dealkylation to produce benzene. The benzene rapidly transalkylates with other feed molecules and eventually goes into an interconverting C 6 to C 9 pool. Product distributions from conversions over CVD treated HZSM5 and the effect of temperature indicate, that both the transalkylation of 1,2,4-TMB and the paring dealkylation of the tetramethylbenzenes take place inside the pore system. The diffusion resistances for these bulky intermediates inside the zeolite crystals decisively promote the paring reaction.

Selective Fischer-Tropsch wax hydrocracking : opportunity for improvement of overall gas-to-liquids processing

Abstract Fischer-Tropsch (F-T) based Gas-to-Liquids (GTL) processing is recognized as an industrially proven and economically competitive route to high quality diesel. Furthermore, it is generally accepted that for this purpose, GTL processing is most effective when comprising an F-T synthesis driven to wax production, followed by hydrocracking to produce middle-distillate products. Applying a CoMo/SiO 2 -Al 2 O 3 catalyst, optimised for hydrocracking crude oil refinery feedstocks in a sulphur-containing environment, to the processing of a linear paraffin F-T wax model compound, n-tetradecane, shows that a significant opportunity exists for utilisation of base metal catalyst, having the advantage of producing less branched hydrocracking products, i.e. high cetane number diesel via a hydrogenolytic cracking mechanism. A drawback of such a catalyst, if applied in non-sulphided form and in a non-sulphur containing environment, is the comparably high yield of light gases, in particular methane. It is shown, and proved by a simple kinetic model, that methane is formed via ‘methanolysis’, i.e. successive hydrogenolytic demethanisation reaction of the feed compounds, presumably of islands of metallic cobalt on the catalyst.

Effect of the deposition temperature on the chemical vapour deposition of tetraethoxysilane on ZSM-5

The effect of deposition temperature during the chemical vapour deposition of tetraethoxysilane (TEOS) on zeolite H-ZSM-5 was studied over the temperature range from 50 to 400 °C. Cracking of 1,3,5-triisopropylbenzene and disproportionation of toluene (T-DP) showed the extent of modification by the silica deposition. Ethanol dehydration at elevated temperatures as subsequent step to TEOS deposition leads to non-uniform deposition and reduced catalyst activity. From both shape-selectivity improvements and catalytic activity it is seen, that low deposition temperatures and frequent modification cycles are the preferred conditions for obtaining catalysts with improved shape selectivity at preserved conversion levels.

Alkylation of phenol with methanol over zeolite H-MCM-22 for the formation of p-cresol

The alkylation of phenol with methanol over H-MCM-22 was investigated and the performance of this catalyst compared with that of H-ZSM-5 and amorphous silica-alumina. The reaction was studied in the liquid phase (batch reactor) at temperatures between 200–250°C and pressures between 20–42 bar and in the gas phase (flow reactor) at temperatures between 250–400°C and feed partial pressures of 0.2 bar. The C (ring) : O (side chain)-alkylation ratio was found to be more dependent on reaction conditions (i.e. gas/liquid phase, pressure and temperature) than catalyst type. The highest C : O alkylation ratios were obtained using gas phase reactions and were similar for all three catalysts. The highest p/o -cresol ratios were obtained using H-MCM-22 in the liquid phase at 200°C. Reasons are proposed to explain the different ring vs side chain alkylation and cresol isomer distributions for the different catalysts and the different reaction conditions.

The effect of different Si/Al ratios and reaction conditions on the phenol methylation activity of MCM-22

Abstract MCM-22 samples of different Si/Al ratios were prepared using various silica sources and agitation conditions and were characterised using BET, SEM, XRD and elemental analysis. Two samples with Si/Al ratios of ∼10 and ∼100 were used in a study of the alkylation of phenol with methanol in a flow reactor. Phenol conversions did not vary much with Si/Al ratio in the gas phase but were greater at high Si/Al ratios in the liquid phase. For the Si/Al = 10 sample, selectivities to anisole and cresols were hardly influenced by the reaction phase but varied considerably at Si/Al = 100. p-Cresol selectivity increased significantly in liquid phase reactions but was not greatly influenced by changes in the Si/Al ratio. A series –parallel mechanism is proposed to explain these results. The primary products are anisole, formed via a rapid alkylation of the side chain, and o-cresol, formed via a slower alkylation of the aromatic ring. Secondary reactions, which may occur, involve a slow monomolecular transalkylation to convert anisole to o-cresol or a reaction between anisole and phenol to form cresol and phenol.

Pressure induced enhancement of shape selective phenol methylation

Abstract In the alkylation of phenol with methanol at low reaction temperatures (200–250°C) over zeolite H-MCM-22, the p:o-cresol ratio was found to increase significantly with increasing pressure. At high reaction pressure (23–41.5 bar, autogeneous, in liquid phase) high p:o-cresol ratios of 1.2–1.3 were found whereas at low feed partial pressure (0.2 bar in gas phase) low p:o-cresol ratios around 0.5 were found. The influence of crystal size on the p:o-cresol ratio indicates product shape selectivity, i.e. transport control of isomer formation. Also, phenol strongly adsorbs at low temperature and fills the zeolite pores eventually resulting in zero reaction order for the phenol. It is concluded that this occupancy is reinforced at high pressure and effectively increases resistances for products diffusing out of the zeolite crystals that way increasing shape selectivity i.e. preference for the “slimmest” isomer.

11-O-02 - Unravelled from the back: kinetics of alkoxysilane CVD on zeolites and evidence for pore mouth plugging determined from model conversion over stepwise silanised samples

Publisher Summary This chapter discusses the chemical vapor deposition (CVD) of tetraethoxysilane on HZSM5. It also presents evidence for pore mouth plugging determined from model conversion over stepwise silanised samples. Several test reactions are performed over the series of modified samples. Under mild conditions, CVD follows first-order kinetics with respect to uncovered external sites on the zeolite crystals. The external surface is homogeneous with respect to both CVD and catalytic activity. Reactions that are controlled by strong internal mass-transfer restrictions, do respond in a way, which indicates that CVD causes pore mouth plugging rather than pore mouth narrowing.

28-P-15-The influence of pore geometry on the alkylation of phenol with methanol over zeolites

Publisher Summary This chapter discusses the alkylation of phenol with methanol, in the liquid phase, using zeolites H-ZSM-5, H-beta, H-MCM-22, H-mordenite, H-USY, and amorphous silica alumina. At the low temperature of 200°C, anisole is the major product over all the catalyst investigated, second is cresols. H-Beta, H-USY, H-ZSM-5, H-mordenite, and amorphous silica alumina show similar cresol distributions. H-MCM-22, which has the smallest pore openings and the narrowest channel system among all zeolites studied, shows the highest preference for p-cresol.

The conversion of methanol to hydrocarbons over ZSM5: Fixed bed vs. recycle reactor

The conversion of MeOH was investigated in a jetloop recycle reactor and fixed bed reactor over ZSM5. The recycle reactor showed no deactivation, had a lower reaction rate and favoured the formation of longer chain hydrocarbons and aromatics. The fixed bed reactor had much higher reaction rates and favoured the formation of light olefins. Co-feeding water slowed the reaction down without changing selectivity while propene accelerated the reaction and increased alkylation activity. It has been suggested that the C-C formation step is responsible for deactivation while DME as well as MeOH are responsible for alkylation.