Gambling on better fuelTo perform at their best, top athletes need to have a controlled diet comprising a careful mixture of foods. The same is true of your car or motorbike. If you put the wrong fuel in the tank you won't get the right sort of performance - your engine will become sluggish and inefficient, like an athlete on an imbalanced diet. A measure of how potent your engine's fuel can be is the octane rating. Contrary to what the name might suggest, this has nothing to do with the amount of octane in the fuel but instead it is a measure of what fraction of the fuel is made up of branched molecules. Petrol consists of a mixture of hundreds of different hydrocarbons along with additives to reduce the polluting effects of the fuel. The hydrocarbons fall into three main categories : linear, branched and cyclic - the branched hydrocarbons providing most energy when they are ignited. So how do petrol manufacturers increase the octane rating? They use special compounds called Zeolites. 
Zeolites are crystalline solids consisting of silicon, aluminium, and oxygen which are bonded in such a way that the resulting solid has many microscopic pores running through it -like a sponge. The figure shows a widely used zeolite called silicalite-1 consisting of straight channels (top view) and zig-zag channels (bottom view) which intersect with the straight channels. The red spheres represent the oxygen atoms and the purple spheres represent the silicon atoms. The channels are around one hundred thousand times narrower than the width of a human hair but are just large enough to allow some molecules to use them as a roadway to move around inside the zeolite. Thus zeolites can be used to separate certain mixtures, but they are more than just molecular sieves. To balance the total electrical charge of the zeolite, wherever there is an aluminium atom in the framework there must be an acidic proton close by. These protons are the origin of the catalytic effect. At these special sites, chemical reactions such as cracking and isomerisation can take place. It is the combination of the sieving and catalytic properties which make zeolites indispensable in industry. To improve fuels, scientists must know how to increase the fraction of branched isomers. But to understand how this is done they must delve into the competitions taking place within the channels of the zeolite. Probing the complexities of what takes place inside the channels of a zeolite is not easy. For many years, computers have been used in this area, evolving from graphics (used to visualise the zeolites themselves) in the 1990s to present day simulations with real predictive capabilities harnessing the abundant and increasingly affordable cpu cycles that sit in desktop machines. One of the most powerful and computationally expedient algorithms, the so called Monte Carlo method, relies on little more than the roll of a dice.
Monte Carlo simulations can provide microscopic information such as the position of molecules within a zeolite. This allows a map to be built up of areas in the complex maze of channels in which individual molecules are most likely to be found. Indeed, refining this data, it is possible to create a 3D map of the parts of a zeolite which are inaccessible to a particular molecule. This can provide a quick test to see if new zeolites are suitable for industrial scale catalysis. The figure (above right) shows how this data can be presented to highlight the localisation of certain molecules within a zeolite. The pink areas represent favourable locations and the grey areas are inaccessible. The zeolite is shown as the red and yellow network and the black line represents a distance of 10A.
Clearing the computational hurdle between length scales is an important step on the road towards the simulation of a complete industrial catalytic process. Using the flexibility and power of the Monte Carlo technique and harnessing the increasingly affordable computational resources, the use of computer simulations to make real predictive contributions to industrial scale processes will only become more widespread. By
Joseph P. Fox, University of Edinburgh. |
A
Monte Carlo simulation aims to find out where in the zeolite a molecule
will be found - without having to follow it on
its potentially tortuously slow path getting there. A 'dice roll' is
used at each step of the simulation when a random number is required
to decide the fate of the molecule. The dice roll determines if the
molecule is moved, rotated, regrown, destroyed or another added. The
chosen outcome is accepted if it lowers the energy of the whole system
(energy raising moves have a small chance of being accepted, depending
on the amount by which they increase the energy). Molecules which only
just fit into the zeolite pores, in particular cyclic molecules,
are the most difficult to simulate, yet they are of great importance
to many industrial applications. By using smarter algorithms - by 'growing'
the molecule in situ and removing the subsequent bias to the
conformation - it becomes possible to simulate large molecules and,
with further refinement to these algorithms, the behaviour of cyclic
molecules can also be investigated. 
Although conceptually
simple, the Monte Carlo simulation technique is extremely powerful and
can be used both to explain experimental findings and to provide
information that would otherwise be impossible to find in a conventional
laboratory. Indeed, in 1995 Berend Smit, one of the key developers of
advanced zeolite simulation methods, [Nature, 374, 42-44] was able to
offer an explanation, using computer simulations, for a feature of the
adsorption of hexane and heptane in silicalite-1 that for years had
eluded experimentalists. 
Whilst
finding the adsorption location for a given molecule is useful, real
processes are much more complex, involving mixtures of many molecules.
Instead of spending weeks or even months in a laboratory conducting
elaborate experiments, Monte Carlo simulations can calculate the location
of molecules within a new zeolite and determine which component of a
complex mixture is most likely to be admitted into the zeolite and undergo
catalysis. The figure (left) shows the results of just such a simulation;
it depicts the adsorption isotherms for a mixture of linear (blue squares),
branched (green crosses), and cyclic (red circles) hydrocarbons in the
zeolite ITQ-22. This graph is a measure of how many molecules of each
type will manage to adsorb, from an equimolar mixture, onto the internal
surface of the zeolite at a given temperature and pressure. It is clear
and somewhat counter-intuitive that the bulkier cyclic molecules are
accommodated much more easily than either the linear or branched. This
information is vital when determining if a zeolite has potential use
in industry since the separation of mixtures is a key factor in many
processes. The synthesis of zeolites is a huge and still growing industry
-with massive financial rewards if the zeolites proves to have useful
commercial properties. Computer simulations can be used to analyse these
potential money making structures to determine if they are suitable
for industrial use. 
The
efficiency of the Monte Carlo technique means that it is possible to
increase the length scale of the simulations. Thus, the simulation of
mesoporous materials (which have pores that are up to an order of magnitude
larger than zeolites) is possible - allowing the investigation of the
fluid properties of the molecules moving around within the pores.
The final figure shows a snapshot of a simulation of the
adsorption of hexane (depicted in green) in a mesopore (MCM-41), once
again, the black line is a distance of 10A.