K.P. Möller

The chemical vapour and liquid deposition of tetraethoxysilane on ZSM-5, mordenite and beta

Abstract The effect of using different silanisation procedures as well as different exposure times and deposition–calcination cycles on the silanisation of three different zeolites, i.e. ZSM-5, mordenite and beta, has been investigated. Tetraethoxysilane (TEOS) was used as a silanisation agent. The three procedures used were chemical vapour deposition (CVD) in a static vacuum system and a vapour phase flow system, respectively, and chemical liquid deposition (CLD). Experiments were carried out with a view to characterising the extent to which the external sites were inertised and the pore openings narrowed. Unidimensional channel structures (e.g. mordenite) or aluminium-rich zeolites (mordenite, beta) were more affected with respect to pore mouth narrowing than the siliceous three-dimensional zeolite ZSM-5. At high temperatures, the presence of water increases the rate of TEOS deposition for zeolites with higher aluminium contents. The adsorbed species resulting from the TEOS deposition reaction or diluents may occupy the pore openings, reducing the extent to which pore mouth blocking occurs. Continuous removal of the adsorbed products resulting from TEOS decomposition results in a faster, less selective deposition. Cyclic TEOS deposition can be used to obtain a more complete coverage by periodic removal of competitively adsorbed species. When agglomerates were crushed after modification, it appeared that acid sites on external surfaces not previously inertised during CVD were exposed, thus partly restoring the external surface acidity.

The characterization and elimination of the external acidity of ZSM-5

The external acidity of ZSM-5 was modified either by coating the crystals with a silicalite shell or by chemical vapour deposition (CVD) of tetraethoxysilane (TEOS). Temperature programmed desorption (TPD) techniques were used to characterize the acidity changes arising from these modifications. Ammonia and pyridine were used as probes for the total acidity, and 4-methylquinoline was used to probe the external acidity of the modified and unmodified ZSM-5 samples. Si/Al ratios of the modified samples were also used to characterize the samples. Both methods of modification were able to make the external acid sites inert. Chemical vapour deposition was found to be a better method, because the external surface acidity was eliminated without significantly changing the catalyst in terms of total acidity. Optimum elimination of the external surface acidity by CVD was obtained for the sample with the higher deposition time (16 h) and temperature (400°C). Optimum elimination of the external surface acidity using the silicalite shell method was obtained using the detemplated NH4-form of the parent crystal as seed material. The channel structures of crystals modified by both processes remained accessible to ammonia and pyridine.

The chemical vapour and liquid deposition of tetraethoxysilane on the external surface of ZSM-5

Abstract The external acidity of ZSM-5 was modified by chemical vapour deposition (CVD) and chemical liquid deposition (CLD) of tetraethoxysilane [Si(OC 2 H 5 ) 4 ] using a static vacuum system, a vapour phase flow system and liquid phase deposition. Temperature programmed desorption (TPD) techniques were used to characterise the acidity changes arising from these modifications. Pyridine was used as a probe for the total acidity and 4-methyl quinoline (MQ) was used to probe the external acidity. The adsorption capacities of the samples were measured using n -hexane, p -xylene, o -xylene and 1,2,4-trimethyl benzene. The extent of Si(OC 2 H 5 ) 4 deposition was strongly dependent on temperature in both vapour phase flow and static vacuum systems. Continuous Si(OC 2 H 5 ) 4 deposition was observed in the presence of H 2 O at relatively high temperatures when decomposition products were removed from the sample. It is proposed that physisorbed species need to be removed by evacuation or calcination to re-expose active sites, thereby enabling complete inertisation of the external surface acidity to occur, and that a more uniform covering can be obtained when a gradual deposition process is used. Such a process may be achieved by the use of diluents or by preventing over-exposure of Si(OC 2 H 5 ) 4 to the sample under conditions where continuous deposition may occur. It was shown that it is possible to reduce the relative external surface acidity by 97% without significant changes in acidity as measured by Py-TPD or adsorption capacity.

The transformation of 1,2,4-trimethylbenzene A probe reaction to monitor external surface modifications of HZSM-5?

Abstract The transformation of 1,2,4-trimethylbenzene is proposed as a new probe reaction to monitor the catalytic effects of inertisation of the external surface of HZSM-5. The external surface has been modified by coating ZSM-5 crystallites with an inert silicalite shell. At 723 K and a WHSV of 0.6 h −1 it has been shown that the isomerisation products 1,2,3-trimethylbenzene and 1,3,5-trimethylbenzene reflect changes in external activity. The disproportionation products, 1,2,4,5-tetramethylbenzene and 1,2,3,5-tetramethylbenzene together are shown to indicate changes in overall activity and shape selective properties of the catalyst sample. The results correlate with those observed for the reaction of 1,3,5-triisopropylbenzene and n-hexane cracking.

Effects of in-situ chemical vapour deposition using tetraethoxysilane on the catalytic and sorption properties of ZSM-5.

The external surface of HZSM-5 crystals was gradually modified by in-situ CVD of tetraethoxysilane. Using the particular procedures described in the paper, it was shown from probe reactions in combination with sorption studies that the external surface was deactivated and the pore-opening size was reduced while the intracrystalline pore space remained unmodified. The positive effect of the CVD treatment procedure on the shape selective properties of HZSM-5 was demonstrated by an enhancement in p-selectivity during toluene disproportionation. The paper discusses the relation between the effects of the CVD treatment procedure on catalytic properties, sorption properties and structural properties and conclusions are made on the use of the conversion of 1,2,4-trimethylbenzene, 1,3,5-triisopropylbenzene, toluene and n-hexane, respectively, to probe the effects of external surface modifications on properties of ZSM-5.

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.

An Investigation of the Ignition Delay Character of Different Fuel Components and an Assessment of Various Autoignition Modelling Approaches

An understanding of the ignition delay behaviour of spark ignition fuels, over a wide range of temperatures and pressures, was an essential prerequisite for an ongoing pursuit to develop a fundamentally-based predictive octane model for gasoline blends. The ignition delay characteristics of certain model fuel compounds such as linear and iso-paraffins, olefins, aromatics and alcohols were investigated by means of chemical kinetic modelling, employing CHEMKIN 3.7 using detailed molecular oxidation mechanisms obtained from the literature. The complexity of these mechanisms necessitated the parallel investigation of reduced kinetic models in some of the applications. Reduced kinetic models were also used to describe the blending behaviour of selected binary combinations of the model fuels. The complex ignition delay response in the temperature/pressure domain that was predicted by the detailed kinetic analyses was reduced to a simple system of three, coupled Arrhenius equations. This simplified expression was used to emulate experimental data that were obtained for the model fuels in a combustion bomb apparatus, the IQT™, as well as data from a single cylinder CFR engine under knocking conditions. A combination of the various approaches has led to new insights regarding the blending behaviour of various classes of fuel molecules in regard to their collective resistance towards autoignition. This is a critical requirement for understanding and modelling the chemical ignition delay as reflected by octane numbers.

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.

The effect of a crystal size distribution on ZLC experiments

This paper reports the results of a theoretical investigation of the effect of a sorbent size distribution on the zero length column (ZLC) method for measuring intracrystalline diffusion. A new model incorporating a continuous log-normal distribution of crystal size is introduced. It is shown that such a distribution introduces tailing into the normally linear long time region of the desorption curve, although this curvature is not obvious when the initial concentration drop is fast. Analysing such a curve with the standard ZLC model causes the diffusional time constant to be underpredicted, whereas the adsorption related parameter L is overpredicted. The error increases with increasing distribution width. It is shown that the curve may be well approximated by summing the desorption curves of the individual, discrete size fractions, weighted by their volume fraction. The systematic error introduced into the measured diffusion coefficient when analysing data for sorbent samples with a size distribution with the standard model may be reduced if the mean particle size is defined as the ratio of the samples volume to its surface area.