The surface chemistry of nanocrystalline MgO catalysts for FAME production: An in situ XPS study of H2O, CH3OH and CH3OAc adsorption

Abstract

An in situ XPS study of water, methanol and methyl acetate adsorption over as-synthesised and calcined MgO nanocatalysts is reported with a view to gaining insight into the surface adsorption of key components relevant to fatty acid methyl esters (biodiesel) production during the transesterification of triglycerides with methanol. High temperature calcined NanoMgO-700 adsorbed all three species more readily than the parent material due to the higher density of electron-rich (111) and (110) facets exposed over the larger crystallites. Water and methanol chemisorb over the NanoMgO-700 through the conversion of surface O 2-sites to OH -and coincident creation of Mg-OH or Mg-OCH3 moieties respectively. A model is proposed in which the dissociative chemisorption of methanol occurs preferentially over defect and edge sites of NanoMgO-700, with higher methanol coverages resulting in physisorption over weakly basic (100) facets. Methyl acetate undergoes more complex surface chemistry over NanoMgO-700, with C-H dissociation and ester cleavage forming surface hydroxyl and acetate species even at extremely low coverages, indicative of preferential adsorption at defects.Comparison of C 1s spectra with spent catalysts from tributyrin transesterification suggest that ester hydrolysis plays a key factor in the deactivation of MgO catalysts for biodiesel production.

Sol–gel La 2 O 3 –ZrO 2 mixed oxide catalysts for biodiesel production

“…For La 2 O 3 -ZrO 2 catalysts, it can be seen that the larger methyl ester production was obtained using 3% La 2 O 3 -ZrO 2 (56% at 4 h) and 2% La 2 O 3 -ZrO 2 (27% at 4 h), whereas lower conversion levels were observed for the 1% La 2 O 3 -ZrO 2 catalyst (1.4%) and 5% La 2 O 3 -ZrO 2 catalyst (3%). Although the obtained results are lower than those of the supported alkaline metal oxide catalysts, such as K 2 O/Al 2 O 3 [13,34] , CaO/zeolite [12] or Li 2 O/ZrO 2 [38] , [27] . Meanwhile, the characterization of the La 2 O 3 -ZrO 2 mixed oxides allow one to propose that La 3 + could be active for CH 3 Oadsorption, and O 2-, with a low coordination number, could allow H + adsorption.…”

“…There is a small shift towards lower BE of around 0.7 eV when introducing lanthanum in the ZrO 2 mixed oxide suggesting a decrease in the oxidation state of lanthanum. The BEs of O 1 s in Table 2 are at 530 and 532 eV [27,59,60] , indicating two types of surface oxygens: the larger component at 530 eV is associated with the O 2species and the chemisorbed oxygen at 532 eV is associated with Ospecies at lower amounts. According to Sato et al .…”

“…According to the literature, the basic sites present in the heterogeneous catalysts of alkaline metals are responsible for the catalytic activity in transesterification reactions [20,27,35,36,39,42] . However, our mixed oxides appear to be active, even without an alkaline metal presence.…”

“…According to reports, bulk metal oxides, such as MgO [27] , CaO [18,19,[28][29][30] and zeolites [17,31,32] , can serve as active catalysts in transesterification reactions. Some reports show that active supported metal oxides, such as KNO 3 /Al 2 O 3 [13,33] , KI/Al 2 O 3 [34] , CaO/MCM-41 and CaO/SBA-15 [12] , KOH/ZrO 2 [35] , strontium oxides [36] , modified strontium nanocatalysts supported on ZSM-5 [31] , HZSM-5-supported lanthanum, [17] and cesium-impregnated sodium zirconate [37] , work as basic catalysts in transesterification reactions.…”

“…Some reports show that active supported metal oxides, such as KNO 3 /Al 2 O 3 [13,33] , KI/Al 2 O 3 [34] , CaO/MCM-41 and CaO/SBA-15 [12] , KOH/ZrO 2 [35] , strontium oxides [36] , modified strontium nanocatalysts supported on ZSM-5 [31] , HZSM-5-supported lanthanum, [17] and cesium-impregnated sodium zirconate [37] , work as basic catalysts in transesterification reactions. The trend is clear, for the transesterification reaction, it is necessary to use basic metal oxide catalysts, such as Li 2 O [38] , K 2 O and potassium titanate [13,26,33,39] , NaZrO 3 [14,37] , CaO [19,[28][29][30] , SrO [31,36] , MgO [27] and La 2 O 3 [17][18][19][20][21] . To date, studies have been focused on materials that simultaneously carry out esterification and transesterification reactions depending on the starting triglycerides [40][41][42] .…”