Montmorillonites have both, Bronsted and Lewis type acidity. Bronsted acidity results from the dissociation of adsorbed H2O:

http://www.clays.org/journal/archive/volume%2022/22-5-435.pdf

Lewis acid sites are related to isomorphus substitution of Si4+ by Al3+, and can be also found on the crystal edges:

http://rsta.royalsocietypublishing.org/content/311/1517/333

http://www.clays.org/journal/archive/volume%2048/48-5-528.pdf

Solid acids are active in biodiesel production:

http://www.sciencedirect.com/science/article/pii/S0926669016303752

Therefore, one would say, that montmorillonites could be also used as catalyst in the reaction.

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 Modification  of clay by acid-activation    is one of the efficient ways to increases   surface acidity and catalytic activity in biodiesel production  .Acid –activation decreasing the  aggregation of clay particles, elimination of impurities , and altering  the  chemical composition  of clay. Improving of surface acidity and catalytic activity  relates with  increasing of   surface area, porosity and number of acidic sites compared to the pristine clays.

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