Surface Methoxy Group Chemistry

METHANOL TO DME MECHANISM

One of the first species to form within the zeolite catalyst is dimethyl ether (DME). There are two mechanisms proposed for this. The first being the indirect route where methanol (MeOH) reacts with the surface to form surface methoxy, and this surface methoxy reacts with MeOH to form DME.

Step 1)

indirect1

Step 2)

indirect2

The second mechanism is the direct route, where two MeOH molecules interact with the surface to yield DME.

direct

But how do we know which (if any) of these reactions take place? The authors reacted MeOH on the acidic zeolite and was found that DME and surface methoxy was formed. The formation of surface methoxy indicates that the indirect route is a part of the formation of DME, but it does not prove that it is important in kinetic formation of DME. The formation of surface methoxy might also be a parallel reaction that we haven't taken into account. Without further testing both reaction pathways are possible.

So how can we further investigate the reaction pathways? Well, if we could prove that the surface methoxy on the zeolite catalyst could lead to the formation of DME, it would be a good indication that the indirect route is a contributor to the DME formation.

So how can we do this experimentally? Well, if one prepared a zeolite catalyst covered by only surface methoxy and allowed it to react with MeOH, one would expect to see a decrease of the surface methoxy levels as the reaction progressed.

img12
To prepare a catalyst with surface methoxy (i.e. methylating the catalyst), the authors used a procedure previously developed by Wang et al. (J. Phys. Chem. B 105 (2001) 12553). This was done by flowing carbon-13 enriched MeOH through acidic zeolite catalysts at room temperature. Followed by a purge with dry nitrogen at high temperatures. As we can see below, the system that was initially only methanol is now mostly surface methoxy.

surfacecheck

To test the reactivity of the surface methoxy, the catalyst was methylated with C-13 rich MeOH. This means that the surface is covered with surface methoxy groups consisting of the C-13 isotope. As normal MeOH was introduced with the C-13 methylated catalyst, it was observed that the amount of C-13 surface methoxy decreased and the amount of DME increased. This shows that the surface methoxy groups are indeed interacting with the gas phase methanol. It is important to use to different types of isotopes to do this because the MeOH can form more surface methoxy, keeping the level of surface methoxy constant. By measuring the level of C-13 surface methoxy, one can see whether the original methoxy is decreasing or not (i.e. we are labeling the surface methoxy). The decrease in the C-13 surface methoxy indicates that the following reaction is occurring:

meth to dme

But what does this really tell us? This experiment shows that the surface methoxy do indeed react to DME, and the initial experiment showed that methanol do form surface methoxy. So the indirect method does contribute to the formation of DME, but it does not mutually exclude the formation of DME from the direct mechanism. The direct mechanism could still be a parallel reaction, and even the preferred reaction path. There is some controversy on the point of which of the reaction paths is most preferred. Through DFT computational methods, Gale and his co-workers suggest that both of the reaction paths are energetically reasonable, however, Blaszkowsky and van Santen found that the direct route is the preferred route energetically.

To this day no one has definitively been able to distinguish which of the two reaction pathways is the leading. We can only conclusively say from this data that the indirect method is a possibility.

FURTHER STUDIES ON SURFACE METHOXY

Under batch conditions it's hard to detect the formation of surface methoxy when reacting with MeOH. Why is this hard to do? One theory is that the water that is formed when the MeOH forms surface methoxy, will react back with the surface methoxy to form MeOH in a reversible reaction.

H2O rxnTo investigate this theory a specimen of methylated zeolite catalyst was allowed to react with water. It was observed that methanol is indeed formed and and the surface methoxy levels do decrease (see data below). This is a good sign that the theory is correct, and that the methylation reaction is reversible.

methylation

OTHER SPECIES

If we believe the hydrocarbon pool mechanism and that methoxy groups play a role in the initiation of the MTO process, then we need to describe how the large assortment of chemical species in the hydrocarbon pool are formed from the simple methoxy group.

The reactivity of methoxy groups with toluene was investigated. A methylated catalyst was prepared and combined with toluene. As the temperature was increased from 298 K, the formation of xylene, ethylbenzene and other poly methylated benzene was observed.

other

This shows that the methoxy groups are strong methylating agents at temperatures below that of the steady state operation of the MTO process (523 K). The reactivity of methoxy groups with aliphatic compounds was also investigated and similar results were found. Thus methoxy groups are strong methylating agents of water, toluene and aliphatic species. These reactions occur appreciably below 523 K and their products are similar to those of the hydrocarbon pool.

INITIAL HYDROCARBON FORMATION

We have still left out how the initial hydrocarbons are formed from the methoxy groups. We know DME can be created and that the methoxy groups can methylate higher carbon number species, but how are these made?

To find out a sample of pure methylated zeolite sealed in a glass ampoule in the MAS NMR setup was heated at temperatures in excess of 523 K and the surface methoxy groups were found to decompose to propane, isobutane and aromatics. These products when further methylated are characteristic of the hydrocarbon pool.

meth decomposition

The authors conclude that the methoxy groups alone are thus capable of creating the hydrocarbon pool. In the next section as well as the conclusion, we will discuss some other theories.

WHAT ABOUT IMPURITIES?

Around the time of this paper, Haw and colleagues also published on this topic (Haw et al. J. Am. Chem. Soc. 124 (2002) 3844). They claim that the initiation of the MTO process is caused by organic impurities and NOT from any direct route from methanol. The authors of this paper refute this argument by providing more data that a methylated sample deemed pure from UV/Vis spectra starts to form aromatics upon heating in a sealed vessel. It is important to note, however, that Haw's claims are that it only requires very small (a few ppm) of organic impurities to initiate the process. These are below measurable limits for both NMR and UV/Vis and thus it appears impossible to completely prove either theory at this time.

An important detail would be the determination of the MeOH to DME mechanism. Further work on this topic is mentioned lightly here. DFT calculations were completed by Blaszkowski et al. and Gale et al. who determined that the direct pathway is preferred and that neither pathway is preferred energetically, respectively.

Below is an illustration of the two competing transition states (1 - indirect, 2 - direct):

tst

Blaszkowski and colleagues actually looked at three different mechanism, the direct, the indirect, and a combination of the two. The lowest energy pathway is shown below:

tstpath

 

Blaszkowski et al. J. Phys. Chem. B 101 (1997) 2292.
Gale et al. J. Am. Chem. Soc. 121 (1999) 3292.