Helle Mostad

Structural and chemical influences on the MTO reaction: a comparison of chabazite and SAPO-34 as MTO catalysts

A comparison of isostructural H-SAPO-34 and H-chabazite in high space-velocity experiments of the methanol to olefins (MTO) reaction has been performed. The initial selectivity was shown to be similar for both catalysts, and also to be independent of acid site density. However, the deactivation rate was critically dependent on the acid site density. Thus, low acid-site density samples exhibited longer lifetimes at low space velocity conditions than their high acid-site density analogues.

Synthesis, characterization and catalytic testing of SAPO-18, MgAPO-18, and ZnAPO-18 in the MTO reaction

SAPO-18, MgAPO-18 and ZnAPO-18 have been successfully synthesised using TEAOH as organic additive to the synthesis gel. SAPO-18 has catalytic properties comparable to SAPO-34, whereas the MeAPO-18s had much higher selectivities to C3=–C4= and shorter catalytic lifetime than SAPO-18.

Isobutane/2-butene alkylation on faujasite-type zeolites (H EMT and H FAU)

The alkylation of isobutane with 2-butene on hexagonal (protonated form = H EMT, investigated, for the first time on this reaction) and cubic faujasite (protonated form = H FAU) with Si/Al ratios of 3.5 was studied at 80°C and compared with the results obtained for a cerium exchanged Y zeolite (Ce Y). The alkylate composition consisted of 90% and about 80% C8 paraffins for H EMT and H FAU/Ce Y, respectively. Within the C8 fractions the three trimethylpentanes (TMP) 2,2,4-, 2,3,3- and 2,3,4-TMP were the dominating product compounds in all tests, whereas a higher content of dimethylhexanes (DMH) was observed for the H FAU and Ce Y. However, among the four trimethylpentanes 2,3,3,-TMP and 2,3,4-TMP were always the main compounds formed. Oligomerization was favoured only by the H FAU and Ce Y, with a strong increase of C8 olefins after about one third of the observed reaction time. On the contrary, H EMT exhibited much less formation of C8 olefins (and on a constant level) even after 300 min reaction time. The deactivated samples showed only formation of non-aromatic coke. The differences in the product pattern were attributed to the slightly larger size of the second type of supercage in EMT and to the stronger Bronsted acid sites in the hexagonal faujasite.

ISOBUTANE/2-BUTENE ALKYLATION OVER FAUJASITE TYPE ZEOLITES IN A SLURRY REACTOR. : EFFECT OF OPERATING CONDITIONS AND CATALYST REGENERATION

Abstract The isobutane/2-butene alkylation has been catalysed by acidic cubic faujasites (H-FAU and CeY-98) and a hexagonal faujasite (H-EMT) in a semibatch slurry reactor. H-EMT exhibited the best results during the reaction with respect to an improved deactivation profile and moderate loss of alkylate selectivity compared to both CeY-98 and H-FAU. Additionally, H-EMT proved to be regenerable. The different catalytic behaviour between H-EMT and H-FAU has been discussed in terms of differences in their crystal structure. Furthermore, we have focused on the influence of isobutane:2-butene molar ratio and temperature (simultaneously varied) on the catalytic behaviour at constant butene WHSV fed to the slurry reactor at optimum stirring conditions. CeY-98 was used as a model catalyst and the results were treated statistically by principal component and partial least square analyses. The dependence of alkylate selectivity as well as yield related to temperature ( T ) and isobutane:2-butene molar ratio ( M ) were found to be: Selectivity = 0.53 + 0.017 [ M − 12.5] + 0.0003 [ T − 90] + 0.0004 [ T − 90] [ M − 12.5]; Yield = 0.55 + 0.016 [ M − 12.5] + 0.001 [ T − 90] + 0.0004 [ T − 90] [ M − 12.5], within the employed parameters ( T ∈80–100° C , M ∈5–20). Both equations showed optima at low butene concentrations, while the temperature was of minor importance. A high stirring speed was essential to achieve optimal reaction conditions enlarging the alkylate yield and selectivity.

The synthesis and characterization of SAPO-43

The small pore silicoaluminophosphate SAPO-43 has been synthesized as a pure phase, producing crystals of very high quality. Detailed physical and chemical characterization indicates that the structure of this material is not stable after the removal of the template during calcination. Chemical analysis and solid-state n.m.r. studies confirm that silicon substitutes exclusively into the phosphorus site. The high level of silicon incorporation could result in a degree of P/Si ordering within the phosphorus site of the framework. Assuming short-range ordering of the order of the unit cell dimensions, only one model is apparently feasible.

Comparison of the iso-structural H-SAPO-37 and H-faujasite as catalysts for the isobutane/2-butene alkylation

Abstract The alkylation of isobutane with 2-butene has been studied in a slurry reactor on the iso-structural H-SAPO-37 and dealuminated faujasite (FAU) catalysts. It was found that mainly dimerization takes place on H-SAPO-37 by producing C 8 olefins. In the small fraction of C 8 paraffins, the 3,4-dimethylhexane (3,4 DMH) was the most abundant compound. The dealuminated H-FAU catalyst, on the other hand, produced mainly the desired trimethylpentanes (TMPs). The number and strength of the acid sites in H-SAPO-37 and dealuminated H-FAU were investigated by NH 3 -TPD and IR spectroscopy, revealing a considerably higher relative concentration of strong acid sites in the dealuminated H-FAU catalyst compared to H-SAPO-37. Using the ratio of pyridine adsorbed at 400°C and 250°C as a measure of the ratio of strong to weak acid sites in the samples, ratios of 0.40 and 0.65 were obtained for the H-SAPO-37 and dealuminated H-FAU, respectively, ranking the dealuminated H-FAU as a much better catalyst than H-SAPO-37 in the isobutane/2-butene alkylation reaction.

Nafion-H as catalyst for isobutane/2-butene alkylation compared with a cerium exchanged Y zeolite

A solid superacidic Nafion-H polymer was examined as a catalyst for isobutane/2-butene alkylation and compared with a cerium-exchanged Y zeolite. Both catalysts demonstrate initial alkylation activity with rapid decrease in alkylation selectivity. The total product distributions were found to be significantly different after three hours of reaction. Nafion-H showed less selectivity towards iso-octanes, but formed relatively more 2,2,4-trimethylpentane. The differences between the two catalysts suggest dissimilar favoured reaction mechanisms.

Isobutane/2-butene alkylation on dealuminated H EMT and H FAU

The alkylation of isobutane with 2-butene on dealuminated hexagonal H EMT and dealuminated cubic faujasite H FAU with Si/Al ratios of 5–6 was studied at 80°C and compared with results obtained for the as-synthesized and calcined parent material with Si/Al ratios of 3.5. In both cases, the dealuminated samples favour an improved profile with respect to the alkylate yield and selectivity after 3 h reaction time, with dealuminated H EMT as the superior system. The alkylate composition consisted of 76% and about 70% C8 paraffins for dealuminated H EMT and dealuminated H FAU, respectively. Within the C8 fractions the three trimethylpentanes (TMP) 2,3,3-, 2,3,4- and 2,2,4-TMP were the dominating product compounds in all tests, whereas a higher content of dimethylhexanes (DMH) was observed for the H FAUs, both dealuminated and parent material. However, among the four trimethylpentanes (2,2,3-, 2,2,4-, 2,3,3- and 2,3,4-TMP) usually the 2,3,3-TMP and 2,3,4-TMP were the main compounds formed, at least for the dealuminated samples. Oligomerization was suppressed by using the dealuminated samples, however, increasing amounts of C8 olefins after about one half of the monitored reaction time was observed for the H FAUs. By contrast, the H EMTs exhibited much less formation of C8 olefins (and on a constant level) even after 300 min reaction time. The deactivated samples showed mainly formation of paraffinic coke.

Tumbling approach towards template-free synthesis of EMT zeolite

Abstract The zeolite EMT has been synthesized with 18-crown-6/Al 2 O 3 ratios as low as 0.14, by applying continuous tumbling of the synthesis mixture during the entire aging and crystallization period. The crystalline product contained only two crown ether molecules per unit cell, and the mother liquor was essentially free of crown ether. It was found that the lower the amount of crown ether, the slower the rate of crystallization and the smaller the crystallites of the product, whereas the total product yield remained well above 90% based on aluminum. With lower crown ether content the range of synthesis conditions under which EMT was obtained became increasingly narrower, presumably because of the increasing probability of nucleating of other phases such as cubic faujasite when less 18-crown-6 is present.

Isobutane/2-butene alkylation on fresh and regenerated La-EMT-51 compared with H-EMT. The catalysts selectivity changes at high butene conversion in a slurry reactor

Abstract Isobutane/2-butene alkylation was carried out in a semibatch slurry reactor using the hydrogen and lanthanum exchanged form of hexagonal faujasite (EMT, Si Al bulk ratio of 3.5) as catalysts. 51% La exchange in the EMT sample (La-EMT-51) gave a very slight enhanced alkylate activity and yield compared to the parent H-EMT catalyst. The improved activity correlated with a higher relative concentration of strong acid sites as measured by IR. The alkylation experiments were performed with a rather high dilution of butene to be able to investigate the product selectivity for a longer period at high butene conversion. Important changes in the distribution of the trimethylpentanes (TMP) could be observed with time of reaction and were separated into four regions based on catalyst activity. These were the regions of initial activity, high activity, rapid deactivation and complete deactivation. In the high activity period, the fraction of the more spacious and thermodynamically favoured 2,2,4-and 2,2,3-TMP actually increased unlike experiments with fast decay, mainly attributed to kinetic effects such as diffusional limitation in the zeolite pores. The La-EMT-51 was regenerated by burning off coke in air and reused in alkylation under similar conditions as the fresh sample. The catalyst proved to be regenerable with only a slight loss in alkylation activity. Thorough investigations showed that the alkylate selectivity, in particular towards the 2,2,4-TMP among the isooctanes, was improved.