Efficient heterogeneous catalysts offer economic benefits in producing biofuels since, unlike homogeneous catalysts, they are easily separated after trans-esterification, and so can be readily recycled, lowering production costs. However this is offset against problems owing to the reaction being dependent on diffusion of reactants to surface/active site.
1.3.2.1 Heterogeneous trans-esterification catalysts
Layered double hydroxides (LDHs) are a promising group of compounds which comprise of positively charged layered materials with charge balancing anions within the interlayer region (See chapter 3). A wide range of anions may be intercalated including organic ions. For example Choudary et al. incorporated tert-butoxide, which was found to catalyse a wide range of trans-esterification reactions,78 including the production of emulsifiers for food products.79 LDH materials with hydrotalcite composition ([Mg(1-x)Alx(OH)2]x+(CO3)x/n
over the range x = 0.25–
0.55) can be formed by co-precipitation of soluble metal salts.63 These solid base materials can have their properties tuned according to the anion intercalated into the layers and the Mg : Al ratio.
The co-precipitation method may be alkali free, leading to no alkaline contaminant in the catalyst.
Using LDHs, Cantrell et al. carried out trans-esterification in a stirred batch reactor for 3 h at 60 °C with 0.05 g MMO, with glyceryl tributyrate, methanol and hexyl ether, and the reaction was periodically sampled with gas chromatography (GC).63 Glyceryl tributyrate was converted into methylbutanoate via di- and monoglycerides. Hydrotalcite, with a Mg : Al ratio of 2.93 : 1, led to the highest conversion, which the authors attributed to increased intralayer electron density (and associated basicity) with increasing Mg content. Both Mg2+ and Al3+ can be completely or partially substituted in the layered structure by other bivalent or trivalent metal ions, respectively.
Calcination of LDH materials leads to the formation of mixed metal oxides (MMOs), which are usually more basic catalysts than the corresponding parent layered samples.80 In a further recent study, calcined LDH samples doped with various metal ions to replace Al3+, were tested for biodiesel production.64 10 % Gallium dopants led to an increase to around 80 % conversion, at 60
°C, of triacetin to the corresponding methyl esters. The use of an Fe based dopant at 5 % and 10 % led to even greater activity with >95 % yield after 40 min at 60 °C, as compared to 1.0 % weight of dopant. The surface area for this catalyst was found to be ~50 % greater than the uncalcined MgAl hydrotalcite, which is typical of hydrotalcite-like or LDH materials. The mixed oxides that derive from calcination of LDHs at temperatures between ca. 400–550 °C, exhibit significantly higher
surface area (ca. 200–300m2 g_1) compared to the parent LDH samples (ca. 50–100 m2 g_1). Upon regeneration, through rehydration, the catalyst was extracted and re-calcined, giving only 50 % of the initial activity of the original catalyst, with further regeneration yielding similar results.
LDHs can become contaminated with sodium base used in their synthesis. This is normally removed by washing at the end of their preparation, however if left entrained can enhance MMO activity. In a study by Cross et al.,81 MMOs were produced from LDHs subjected to varying degrees of washings, leading to intrinsic sodium contaminant. There was a direct correlation found between the amount of sodium contained in the samples and catalyst activity for trans-esterification, with conversion being considerably enhanced with high sodium content. Sodium was also found to leach from LDH samples containing 2.1 % Na by weight, thereby possibly acting as a homogeneous catalyst.
A solid-base catalyst KF/Al2O3 has been utilised for the conversion of palm oil to alkyl esters by Bo et al.65 The catalyst was prepared via impregnation of KF to give a supported catalyst on Al2O3. This was then dried and calcined at 600 °C. The trans-esterification was carried out at atmospheric pressure and with an optimum temperature of 65 °C; above this the volatility of methanol became an issue, leading to a decrease in the methanol : oil ratio from the desired 12 : 1.
A catalyst ratio of KF : Al2O3 0.331 (wt/wt) using 4 % catalyst (wt) over 4 h was found to lead to triglyceride production of over 90 %. Interestingly, calcination of the catalyst at 600 °C led to a new phase of K3AlF6 as characterised by X-ray diffraction (XRD) and Thermogravimetric Analysis (TGA).
A superbase (as denoted by the Hammett scale82, 83) was prepared by calcination of Eu(NO3)3/Al2O3 for 2 h at 300 °C, 2 h at 550 °C and 8 h at 900 °C forming Eu2O3/Al2O3 with an optimal Eu content of >6.75 %.66 This was used to trans-esterify soybean oil in a fixed bed reactor at atmospheric pressure. Again the reaction temperature was optimal at around 70 °C due to the volatility of methanol. Water was removed from the oil and methanol to prevent reaction with the catalyst. No reaction was observed for the first 30 min, as monitored by GC, with a steady increase
in rate from 2 h and a final conversion of 63 % at 8 h. The methanol : oil ratio was ≥4 for the greatest conversion, although continually increasing the methanol ratio can lead to separation problems from the prepared methyl esters, so a value of 5–6 was proposed. After 40 h of use, catalyst activity had decreased, leading only to around 35 % conversion, thought to be due to water and FFAs. After each subsequent regeneration the catalyst had lost surface area and its associated activity had decreased.66
Some potential oils for biodiesel production such as deep frying oils are high in FFA content, making them unsuitable for base catalysed trans-esterification (section 1.3.1.2). In these cases a heterogeneous acid catalyst is preferred. Sulphated zirconia catalyst (S–ZrO2) has been found to catalyse soybean oil to biodiesel with 98.6 % FAME yield.71 Unfortunately the catalyst is deactivated rapidly. Zinc stearate immobilised on silica gel was found to convert waste cooking oil of 15 % FFA to 98 % FAME with no loss of activity after four catalytic cycles, though the reaction temperature was relatively high at 200 °C.84 Carbohydrate-derived heterogeneous acid catalysts have been shown to trans-esterify oils with up to 27.8 wt % FFA content to 92 % FAME after 8 h.85 These catalysts were found to be exceptionally stable in that they were still around 93 % active after 50 successive uses.
The alcohol used in trans-esterification may lead to fuels with differing properties. Usually methanol is the alcohol of choice, but Bokade et al. varied the alcohol used from methanol to n-octanol over 10 wt % of catalyst TPA/K-10.67 The reported percentage conversions were methanol (84 %), ethanol (80 %), n-propanol (76 %) and n-octanol (72 %) showing a decrease in oil conversion, possibly due to the increasing number of carbon atoms leading to a lower rate of reaction. This means less efficiency in the process and so greater costs incurred in the resulting fuel product.
1.3.2.2 Novel energy sources (microwaves) for trans-esterification
Trans-esterification rates may also be increased using microwave heating. Using a potassium
hydroxide catalyst and methanol to trans-esterify rapeseed oil, trans-esterification has been optimised at a temperature of 60 °C and a reaction time of 5 min.68 This is significantly quicker than previously reported reactions, without microwaves, with a biodiesel yield of 93.7 % and a purity of 97.8 % (greater than the required 96.5 % set out in EN 14214 – Automotive fuels – Fatty acid methyl esters (FAME) for diesel engines – Requirements and test methods).86 Using the same microwave methods, sodium hydroxide performed best at 40 °C for 3 min with a FAME yield of 90.9 % and purity of 93.7 %.