19Bio-additives obtained from the acetylation of biodiesel-derived glycerol has been 20 extensively synthesized because of their nature as value-added products and their 21 contribution to environmental sustainability. Glycerol acetylation with acetic acid produces 22 commercially important fuel additives. Considering that the recovery of individual 23 monoacetin, diacetin (DA), and triacetin (TA) is complicated, many endeavors have 24 enhanced the selectivity and total conversion of glycerol using acetic acid during catalytic 25 acetylation. In this work, we extensively review the catalytic activity of different 26 heterogeneous acid catalysts and their important roles in glycerol acetylation and product 27 selectivity. In addition, the most influential operating conditions to attain high yield of 28 combined DA and TA are achieved by closely examining the process. This review also 29 highlights the prospective market, research gaps, and future direction of catalytic glycerol 30 acetylation.31 3anhydride is not manufacturing-friendliness especially for large-scale production (as above 47 49 °C explosive vapour/air mixtures may be formed).
48On the contrary, the sudden decline in crude oil prices has significantly reduced the 49 prices of biodiesel during the second half of 2014. Figure 1(a) shows the biodiesel prices 50 declined strongly from 112 USD/hL (2013) to less than 80 USD/hL (2014); the ten-year 51 forecast for biodiesel prices are expected to recover in nominal terms close to those in 2014 52 level (prices vary from 85-90 USD/hL). Figure 1(b) indicates that the global biodiesel 53 production is expected to reach almost 39 billion liters by 2024; moreover, the projected 54 production volume of biodiesel is stable and is mostly policy driven 3 . Nevertheless, 55 conversion of biodiesel-derived glycerol into value-added products is necessary to support 56 long-term growth of the oleochemical market. The price reported for 80% pure crude glycerol 57 is $0.24/kg and that for United States Pharmacopeia (USP)-grade glycerol is $0.9/kg in mid-58 2014 4 . 59 Various studies on transforming glycerol into different value-added derivatives, such 60 as propylene glycerol, polyglycerols, succinic acid, gaseous hydrogen, glycerol carbonate, 61 acrolein, fuel additives, ethanol, glycerol esters, and lubricant additive, were conducted 5-12 . 62 This review aims to study the role of heterogeneous acid catalysts in glycerol acetylation 63 using acetic acid given that the low selectivity of the desired products (diacetin (DA) and 64 triacetin (TA)) remains the greatest challenge in catalytic acetylation. In addition, recovery of 65 key derivatives is a very complicated work because the mono-, di-, and tri-substituted 66 derivatives that constitute a mixture exhibit indistinguishable boiling points 1 . This review 67 then focuses on the important features of solid acid catalysts and on the influence of 68 operating parameters in enhancing the product selectivity of glycerol acetylation. To our best 69 knowledge, this work i...
