Surface species methoxy

REAPDOR method to measure the C-Al distances by numerical simulations of the time-dependence of the REAPDOR evolution effect. An intemuclear C-Al distance of 3.1 A was determined for a signal at 57.3 ppm in ZSM-5, which is in excellent agreement with quantum chemical calculations of surface methoxy species [232]. Larger C-Al distances (>4 A) were determined for nearby signals at 60.0 and 61.7 ppm. 

Until now, the detailed mechanism involved in the MTG/MTO process has been a matter of debate. Two key aspects considered in mechanistic investigations are the following the first is the mechanism of the dehydration of methanol to DME. It has been a matter of discussion whether surface methoxy species formed from methanol at acidic bridging OH groups act as reactive intermediates in this conversion. The second is the initial C—C bond formation from the Ci reactants. More than 20 possible mechanistic proposals have been reported for the first C-C bond formation in the MTO process. Some of these are based on roles of surface-bound alkoxy species, oxonium ylides, carbenes, carbocations, or free radicals as intermediates (210). 

A.l. Formation of Surface Methoxy Species during the Conversion of Methanol to DME on Acidic Zeolites... 

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... 

A.4. Formation of Higher Hydrocarbons by Surface Methoxy Species... 

The reactivity of surface methoxy species was further investigated with various probe molecules that were thought to possibly be involved in the MTO process, including water, toluene (representing aromatics), and cyclohexane (representing saturated hydrocarbons) (263). It was found that surface methoxy species react at room temperature with water to form methanol, which indicates the occurrence of a chemical equilibrium between these species at low reaction temperatures (Scheme 15) (263). 

In a similar way, the reaction of surface methoxy species with cyclohexane on zeolite CHs-Y was investigated (263). At temperatures T>493K. surface methoxy species most probably act as precursors of carbene intermediates, which can undergo the sp insertion into C-H single bonds. In the absence of water and other organic species, conversion of methoxy groups alone to alkanes and aromatics was observed at temperatures r>523K (Scheme 17) (263). This observation indicates a... 

The initial activation of methanol by a Br0nsted acid proton is merely the first of many stages in the MTG process (213). Many mechanisms have been proposed on the basis of experimental studies of these the surface methoxonium ion pathway of Hutchings and Hunter (240) has received a partial experimental justification. The first stage of this mechanism involves the dehydration of adsorbed methanol to give water and a surface methoxy species. This reaction has been investigated theoretically by a number of authors (221, 222, 241). 

The transition state that leads to the formation of the surface methoxy species is similar to that of Sinclair and Catlow (241) insofar as it exhibits little strain around the planar CH3 component. However, the activation energy for methoxide formation (referenced to the initial sorption complex of two methanol molecules) is lower in the calculations of Blaszkowski et al. (245). The value is 160 kJ/mol, compared with the value of Sinclair and Catlow (241) of 180-190 kJ/mol. The reason for this difference lies in the mode of interaction of the methanol molecules. The twofold interaction modeled by Sinclair and Catlow results in a costly rotation to allow formation of the transition state. Such a rotation is not required in the case of a threefold interaction. 

The overall selectivity to C2 compounds is thus determined by the relative rates of the combination of CH3 radicals [Eq. (3.35)] and their reaction with oxygen molecules [Eq. (3.36)] or with surface O- [Eq. (3.37)] to form methyl peroxy radical and surface methoxy species, respectively ... 

Bronkema and Bell (2007) analyzed the Raman bands of surface methoxy species and of supported vanadia to elucidate the mechanism of methanol oxidation to formaldehyde. In their detailed investigation, insight from Raman spectroscopy was combined with information from EXAFS and XANES spectroscopies. The authors discussed the reaction pathways in the presence and absence of 02, and identified the roles of various lattice oxygen sites. Formaldehyde was found to decompose to H2 and CO in the absence of 02 (Bronkema and Bell, 2007). Similar observations were reported by Korhonen et al. (2007) for methanol conversion on supported chromia catalysts. 

The mechanism for the oxidation of methanol to formaldehyde on iron molybdate catalysts (illustrated in Figure 4) envisages the H-abstraction and electron transfer of surface methoxy species desorption of the products and reoxidation of metal sites complete the cycle. 

Figure 28 clearly shows the importance of diffusion within a chemisorbed layer to surface reaction processes (Leibsle and Bowker, in prep.). In this series of STM images the surface methoxy species on Cu(110) is decomposing (evidenced by the loss of total area of methoxy islands), but diffusion is taking place between islands since big islands get bigger at the expense of smaller ones, which eventually disappear. This kind of diffusion phenomenon can be classified as surface mediated Ostwald ripening. 

Formation of methane, carbon monoxide, and hydrogen can be explained by a sequence (Equation 2-3a) involving collapse of TTMA cation to trimethylamine and a surface methoxy species (I), analogous to that observed on Aerosil and alumina (3, 20). 

W. M. Meier (Eidgenossische Technische Hochschule, Zurich) In connection with the postulated formation of a surface methoxy species on heating TTMA-Y, it might be of interest to note that we have observed pronounced methyl-oxygen interaction in the structures of TTMA-soda-lite and TTMA-gismondine (C. Baerlocher and W. M. Meier, Helv. Chim. Acta 1969, 52, 1853, and in press). 

Tp = 190-220 C), as well as the surface formate intermediates (Tp = 265-285 C), possess essentially the same Idnetics on all the supported vanadia catalyst. The desorption of H2O exhibited a very broad peak with a maximum at about 300 C for the supported vanadia catalysts. The slight variations among the different TPRS runs are relat to the use of several parallel reactor systems to expedite the experiments, and are not due to different decomposition kinetics. Identical peak temperatures were obtained when two different catalysts were studied in the same reactor system. Thus, the TPRS experiments demonstrate that the dramatically different TOFs measured during the steady state methanol oxidation to formaldehyde over the supported vanadia catalyst are not related to kinetic differences in the rate determining surface reaction step, the decomposition of the surface methoxy species... 

It is also important to establish if methanol is directly coordinated to one or two surface vanadia sites, mono-doitate vs. bidentate, or if two surface vanadia sites are required because of lateral interactions among the surface methoxy species at monolay surface vanadia coverage. Comparative IR studies of adsorbed methoxy on vanadia catalysts with known molecular structural reference compounds reveal that the adsorbed methoxy species is only coordinated to one surface vanadia species [20]. This coordination is consistent with the almost constant methanol oxidation TOF as a function of surface vanadia coverage and the insensitivity of the methanol oxidation TOF to the presence of secondary surface metal... 

Samson JAR, Ederer DL (eds) Academic Press, San Diego, p 225-261 Aiken GR, McKnight DM, Wershaw RL, MacCarthy P (1985) Humic Substances in Soil Sediment and Water Geochemistry, isolation, and characterization. Wiley, New York Amemiya K, Kitajima Y, Yonamoto Y, Terada S, Tsukabayashi H, Yokoyama T, Ohta T (1999) Oxygen -edge X-ray-absorption fine-structure study of surface methoxy species on Cu(lll) and Ni(lll). Phys Rev 59 2307-2312... 

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