New Solid Acid Catalyst for Brown Grease Biodiesel Production

There is a big movement in our country to turn many of our processes more sustainable and environmentally friendly.  Alternative fuels have started to play a role in this, with ethanol and biodiesel being added to regular petroleum based gasses to help decrease their environmental impact.  Biodiesel is created when fats and other carbon chains are reacted with small chained alcohols.  Biodiesel is currently made from various vegetable oils and “yellow grease”, used kitchen oils that contain slight amounts of fatty acids, but still easily converted to biodiesel.  But another form of grease, brown grease, can also be used to make biodiesel.  Brown grease commonly refers to sewer and water treatment waste that contains 50-100% FFA (free fatty acids).  Currently brown grease is disposed of by dumping it in landfills or incinerating it.  Neither of the methods are particularly environmentally friendly since the heavy metals trapped in the grease either leak into the land or get released into the air when incinerated.  But if the oil layers of brown grease were to be converted into biodiesel, the stress the waste causes on our environment would lessen and a usable resource would be created.

                Brown grease cannot be prepared in the same way as vegetable oils and “yellow grease” due to the high content of FFA.  When triglyceride rich oils are converted a base catalyst is used as transesterification happens quicker in the presence of a base then in the presence of an acid.  But if FFA is reacted in the presence of a base, any ions or impurities in the brown grease will cause saponification instead of esterification, and soap instead of biodiesel will be created.  So esterification is generally done with an acid as a catalyst.  The most common catalysts for esterification are the readily available hydrochloric acid and sulfuric acid.  Both of the acids are liquids and get washed away when the biodiesel is cleaned with water, so each time new catalyst must be used.  A solid acid catalyst theoretically would be able to catalyze the reaction and be reusable through several reactions since it can be easily separated and retained.

                A new acid catalyst (OMR-[C₄HMTA][SO₄H]) was made from the self-assembly of resol precursors, hexamethylenetetramine (HMTA) cross-linker, and a tri-block copolymer template of Pluronic® F127 (Sigma-Aldrich) (Scheme 1). This created the OMR-[HMTA] backbone which when reacted with 1,4-butanesultone, forms OMR-[C₄HMTA][SO₄H].  This catalyst was made in lab to compare to the commercially available Amberlyst 15 solid acid and the commonly used non-reusable hydrochloric acid.  When the physical proportions of our catalyst was compared to that of the Amberlyst 15, the OMR-[C₄HMTA][SO₄H] had a larger BET surface area and a larger pore volume.  This would suggest that it would be better for reacting with more of the catalyst being available to use per mass.

                Two treatments of the brown grease were tested to compare the catalysts.  The first process starts with 20 mL of brown grease that is reacted with excess methanol and 5%  weight with respect to oil of catalyst at 65°C until there was under 1% FFA by looking at the acid number found using a 0.07 M potassium hydroxide titration using ASTM Method D6751.  After the first reaction, the sample was centrifuged to separate any solid catalyst from the oil/biodiesel sample, and then the sample was washed with distilled water then dried by bubbling air through it.  The sample was then put into a new reactor with excess methanol and 5% (w/w) of KOH at 65°C for one hour to convert the glyceride portions of the grease to biodiesel.  The second way to treat the sample was to try and have esterification and transesterification happen in one step.  10 g of brown grease oil was reacted with a methanol and the acid catalysts for 5 hours at 65°C.  The methanol/oil had about a  40:1 molar ratio (excess), and the catalyst/oil mass ratio was .05:1.

                 Each catalyst was tested under the same conditions above, and for just esterification, the OMR-[C₄HMTA][SO₄H] ended up converting about 99.5% of the FFA to biodiesel after only 1.5 hours.  Both Amberlyst 15 and HCl on the other hand had less than a 98% conversion of FFA after 8 hours.  When the one step esterification and transesterification was done, OMR-[C₄HMTA][SO₄H] ended up converting about 75% of the  triglycerides to biodiesel after 5 hours, which ended up being the equilibrium.  The other catalysts ended up converting less than 65% of the triglycerides after a 5 hour period.  While the triglyceride conversion was very good for an acid catalyzed transesterification, the final biodiesel just missed passing the ASTM D 6751 specifications because of a too high level of total glycerin in the fuel.

                The mesporous solid acid catalyst proved to be extremely reactive for the esterification of FFA in brown grease, and when transesterification was done with KOH after the final biodiesel product passed all ASTM quality checks.