Methanol conversion on zeolites

Autocatalysis, Retardation, Reanimation and Deactivation during Methanol Conversion on Zeolite HZSM5... 

K during the methanol conversion on zeolite HY ( si/KAi = 2.7) under flow conditions (74). In these experiments, a flow of C-enriched methanol with a modified residence time of WIF — lOOgh/mol was continuously injected into the spinning MAS NMR rotor reactor. Simultaneously, the yields of DME, Tdme, were determined by on-line gas chromatography (Fig. 32, middle). 

To shed more light on this issue, the steady state of methanol conversion on zeolites HZSM-5, H-SAPO-34, and H-SAPO-18 was characterized by CF MAS NMR spectroscopy under CF reaction conditions (49,261). 

Schulz, H., Zhao Siwei and Baumgartner, W. (1987), Coke forming reactions during methanol conversion on zeolite catalyst, in B. Delraon and G.F. Froment (eds.). Studies in Surface Science and Catalysis, Catalyst Deactivation, Elsevier, Amsterdam, pp. 479-492. 

The approach of this work is to measure product compositions and mass balances in much detail in a time resolved manner and to relate this to the controlling kinetic principles and elemental reactions of product formation and catalyst deactivation. Additionally the organic matter, which is entrapped in the zeolite or deposited on it, is determined. The investigation covers a wide temperature range (250 - 500 °C). Four kinetic regimes are discriminated autocatalysis, retardation, reanimation and deactivation. A comprehensive picture of methanol conversion on HZSM5 as a time on stream and temperature function is developed. This also explains consistently individual findings reported in literature [1 4]. 

The simultaneous investigation of the methanol conversion on weakly dealuminated zeolite HZSM-5 by C CF MAS NMR and UV/Vis spectroscopy has shown that the first cyclic compounds and carbenium ions are formed even at 413 K. This result is in agreement with UV/Vis investigations of the methanol conversion on dealuminated zeolite HZSM-5 performed by Karge et al (303). It is probably that extra-framework aluminum species acting as Lewis acid sites are responsible for the formation of hydrocarbons and carbenium ions at low reaction temperatures. NMR spectroscopy allows the identification of alkyl signals in more detail, and UV/Vis spectroscopy gives hints to the formation of low amounts of cyclic compounds and carbenium ions. 

Figure 2. Comparison of methanol conversion on various ZSM-34 zeolites prepared by different methods.

Tsitsishvili et al. have carried out experiments of methanol conversion on H-offretite and TMA-offretite. TMA-offretite zeolites were calcined at 200 and 450 °C. H-offretite zeolites were prepared by ammonium ion-exchange and then calcined at 300 and 450 C. TMA-offretite calcined at 200 C was inactive, probably because the channels are blocked by the large Me N ions so that the acid sites become inaccessible for methanol molecules. A hydrocarbon fraction containing principally propylene, propane, n-butane, and n-butene was obtained in the cases of TMA-offretite and H-offretite calcined at 450 "C. At reaction temperatures lower than 210 C only dimethyl ether was detected. H-offretite zeolites are active in the isomerization of xylenes, indicating that the removal of TMA-cations enlarged the pore opening. 

Shape Selectivity. One of the most important features of zeolite catalysts is their ability to act as a molecular sieve because the channels have molecular dimensions. Three types of shape selectivity can be distinguished reactant, product, and restricted transition state selectivity, depending on whether reactants can enter, products can leave, or intermediates can be formed in the zeolite catalyst, respectively. Medium-pore zeolites have been shown to have excellent restricted transition state selectivity. The high resistance toward coke formation on medium-pore zeolites has also been attributed to this type of shape selectivity. Transition state selectivity and product selectivity have been observed directly in the methanol conversion on ZSM-5 by means of magic-angle-spinning NMR. ... 

Balkrishnan et al. suggested that, even though the tortuosity in the zeolite channels is increased, the observed higher selectivity toward Cj-C olefins in the methanol conversion on P-modified ZSM-5 is due more to a change in the acidity than to any steric effect. According to Cai et al., ° phosphorus affects both Bronsted and Lewis sites of various acid strength. As a result, an increased ethylene selectivity was attained. 

Schulz, H., W. Bohringer, W. Baumgartner and Z. Siwei, 1986, Comparative investigation of time on stream selectivity changes during methanol conversion on different zeolites, in New Developments in Zeolite Science and Technology, eds Y. Murakami, A. Iijima and J.W. Ward, Vol. 28 of Studies in Surface Science and Catalysis (Elsevier, Amsterdam) pp. 915-922. 

Dejaifve, P., Auroux, A., Gravelle, P.C., Vedrine, J.C., Gabelica, Z. and Derouane, E.G. (1981) Methanol conversion on acidic ZSM5, offretite and mordenite zeolites A comparative study of the formation and stability of coke deposits J. Catal. 70, 123-136. 

Qi L, Wei Y, Xu L, Liu Z Reaction behaviors and kinetics during induction period of methanol conversion on HZSM-5 zeolite, ACS Catal 5 3973—3982, 2015. 

With the use of these sensitivity-enhancement approaches the stable alkoxide intermediates were indeed detected by C CP/MAS NMR spectroscopy. Isopropoxide was the first alkoxy intermediate reliably identified in propylene labeled with C in the CH= group) conversion on zeolite HY [15]. Isopropoxide exhibits the signal at 87 ppm from the labeled C atom, which is characteristic of the (CH3)2CH fragment bonded to an oxygen atom of the zeolite framework (Fig. 20). Later, other alkoxide intermediates were detected and characterized. It was demonstrated that methoxides [121,122] and ethoxides [122,123] formed from methyl and ethyl iodides and also from methanol and ethanol on H-ZSM-5 and CsX zeolites. Isobutoxy [124] and tert-butoxy [90] intermediates resulted from the dehydration of isobutanol and tert-butanol on HZSM-5. Alkoxides are highly reactive species. For example, surface methoxides are effective methylating agents in their reactions with methanol, water, ammonia, alkyl halides, HCl, CO, acetonitrile, and aromatic compounds [125]. 

Dejaifve P, Auroux A, Gravelle PC, Vedrine J, GabelicaZ, Derouane EG. Methanol conversion on acidic ZSM-5, offretite, and mordenite zeolites a comparative study of the formation and stability of coke deposits. J Catal 1981 70 123-36. 

Kikhtyanin OV, Mastikhin VM, lone KG. Methanol conversion on aluminophosphates with zeolite structure. Appl Catal 1988 42 1-13. 

Acid catalysis in hydrocarbon conversion. In terms of the transformation of substrates, our mechanistic understanding has reached a high level, mainly because the systems can be largely (but not totally) explained in terms of classical organic chemistry. Many mechanistic details remain to be elucidated, however, such as how the first C-C bond is formed in the methanol-to-hydrocarbon conversion on zeolites and other solid acids. The steric and topologic constraints that are specific to zeolites have been identified and used to predict catalytic properties. Much more needs to be understood about how structure and composition at the surface sites control the chemistry. 

The first mode of the high resolution C-NMR of adsorbed molecules was recently reviewed Q-3) and the NMR parameters were thoroughly discussed. In this work we emphasize the study of the state of adsorbed molecules, their mobility on the surface, the identification of the surface active sites in presence of adsorbed molecules and finally the study of catalytic transformations. As an illustration we report the study of 1- and 2-butene molecules adsorbed on zeolites and on mixed tin-antimony oxides (4>3). Another application of this technique consists in the in-situ identification of products when a complex reaction such as the conversion of methanol, of ethanol (6 7) or of ethylene (8) is run on a highly acidic and shape-selective zeolite. When the conversion of methanol-ethylene mixtures (9) is considered, isotopic labeling proves to be a powerful technique to discriminate between the possible reaction pathways of ethylene. 

The small molecules entrapped in the H-ZSM-5 zeolite channels because of coke deposition which occured at the outer surface of the particles, have been examined by CP/MAS- C-NMR spectroscopy (10). Figure 3 shows that isoparaffins are more abundant than linear chains, which agrees with other classical data on methanol conversion (47,48). 

As an illustration, the isomerization of 1-butene adsorbed on NaGeX or mixed tin-antimony oxides has been carried out. In the methanol to hydrocarbon conversion on the shape selective H-ZSM-5 zeolite, the surface methylation could be observed, the role of... 

The effect of the Si/Al ratio of H-ZSM5 zeolite-based catalysts on surface acidity and on selectivity in the transformation of methanol into hydrocarbons has been studied using adsorption microcalorimetry of ammonia and tert-butylamine. The observed increase in light olefins selectivity and decrease in methanol conversion with increasing Si/Al ratio was explained by a decrease in total acidity [237]. 

Fig. 27. CF MAS NMR spectra recorded at 723 K during the conversion of pure C-enriched methanol on zeolite CsOH/Cs,NaX fa), after purging with dry nitrogen (b), and during the conversion of mixtures of toluene and C-enriched methanol with molar ratios of 3 1 (c) and 1 1 (d). Reproduced with permission from (23f). Copyright 2000 Elsevier Science.

The left-hand side of Figs 28a-c shows C CF MAS NMR spectra which were recorded during the conversion of pure C-enriched methanol on zeolite CsOH/ Cs,NaY under CF conditions at reaction temperatures of 473—523 K (242). As was observed previously (234-236), the conversion of methanol (49 ppm) on the basic zeolite catalysts caused the formation of surface formate species, leading to a C MAS NMR signal at 166 ppm. Upon cessation of the methanol flow at the reaction... 

Among the early investigations of methanol adsorption and conversion on acidic zeolites, most of the H and C MAS NMR experiments were performed under batch reaction conditions with glass inserts in which the catalyst samples were fused. Zeolites HZSM-5 76a,204,206,264-272), HY 71,72), H-EMT 273), HZSM-12 274), HZSM-23 275), H-erionite 275), H-mordenite 271,272), and H-offretite 275,276), silicoaluminophosphates H-SAPO-5 271,274), H-SAPO-11 274), and H-SAPO-34 76,277,278), as well as montemorillonite 279) and saponite 279) were investigated as catalysts. 

To unambiguously elucidate the reactivity of surface methoxy species, the preparation of pure methoxy species on the catalyst surface is an important prerequisite. This preparation can be achieved by a SF protocol, which starts with a flow of C-enriched methanol into acidic zeolites at room temperature, followed by a purging of the catalyst with dry nitrogen at room temperature and subsequently at higher temperatures (74,262. The latter step progressively removes the surplus of methanol and DME, together with water produced by the conversion of methanol. 

The role of surface methoxy species during the conversion of methanol to DME was investigated by SF MAS NMR spectroscopy (f4). After the preparation of pure surface methoxy species by conversion of C-cnrichcd methanol on zeolite HY ( si/ Ai — 2.7) (Fig. 34a), a flow of methanol with a natural abundance of C-isotopes ( CII3OI1) was injected at 433 K for 10 min into the spinning MAS NMR rotor reactor. In the " C CP/MAS NMR spectrum shown in Fig. 34b, weak signals are evident at 60.5 and... 

Fig. 34. SF CP/MAS NMR spectra recorded at 433 K after stopping the conversion of C-enriched methanol (tt, / = 40gh/mol) on zeolite HY (nsi/tiAi = at 423 K and purging the catalyst with dry carrier gas (a). Spectra (b) and (c) were obtained 10 min and 1.0 h, respectively, after starting the flow of C-enriched methanol (Wj F = 40 g h/mol) at 43 3 K. Asterisks denote spinning sidebands. Reproduced with permission from (74). Copyright 2001 American Chemical Society.

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