Acidic catalysts for the dehydration of glycerol: Activity and deactivation Wladimir Suprun et.al, Journal of Molecular Catalysis A: Chemical 309 (2009) 71–78

Introduction  The conversion of glycerol to acrolein opened a new route for the production of acrylate monomers.  Various solid acid catalysts including sulfates, phosphates, zeolites, supported heteropolyacids have been tested for the dehydration of glycerol.  Catalysts with Ho between −3 and −10 can be chosen from acidic zeolites and from mineral supports (TiO 2, Al 2 O 3 and ZrO 2 ) impregnated with acidic functions such as sulphate, phosphate, tungstate, molybdate or alternatively heteropolyacids.  The biggest disadvantage of these catalysts lies in the formation of a large amount of by-products (25–40%) and catalyst deactivation.  In order to understand the deactivation mechanism, we investigated the catalytic activity of supported phosphates and SAPO samples.  Additionally, the dehydration of 3-HPA and acetol on these catalysts in presence of water, as well as the deactivation of the catalysts was studied.

XRD Pattern Fig. 1. XRD patterns of SAPO-11 and SAPO-34samples and Al 2 O 3 –PO 4 and TiO 2 –PO 4 samples calcined at 530 °C and for SAPO-11 additionally after dehydration of glycerol at 280 °C for 10 h.

BET Results

TPD-NH 3 Fig. 9. NH 3 TPD profiles for SAPO-11 (a) and SAPO-34 (b) samples after calcination, hydrothermal treatment at 300 °C (HTP at 300 °C for 10 h), and after dehydration of glycerol (GD) at 280 °C for 5 and 10 h. Fig. 2. NH3-TPD profiles of SAPO-11 and SAPO-34 samples and of Al 2 O 3 –PO 4 and TiO 2 –PO 4 samples.

 Reactor: Fixed bed catalytic reactor  Reactant: Glycerol  Solvent: Water  Glycerol solution: 5 wt%  Catalyst:200 mg  He flow: 83ml/min  Preheating temperature:320 °C  Reaction temperature: 280 °C  GHSV:43 and 90 h -1 for glycerol,3-HPA and acetol  Products are analyzed using GC and GC/MS. Catalytic tests:

Results

Fig. 3. The conversion of glycerol (■) the selectivity to acrolein (▼) and to acetol (○) over different acidic catalysts is shown as a function of time on stream. Reactions conditions: T: 280 °C, GHSV: 43 h −1.

Conversion of 3-hydroxypropionaldehyde

Conversion of 1-hydroxyacetone

Fig. 10. Carbon dioxide desorption profiles during temperature programmed oxidation of carbonaceous deposited on used catalyst. Heating rate: 10 K min −1 ; air flow: 75 ml/min. TPO

Fig. 11. General reaction scheme of the dehydration of glycerol. Reaction scheme

Conclusions  The total acidity the texture of the supported phosphates and the reaction temperature strongly influenced both the conversion and the distribution of products.  The mesoporous Al 2 O 3 –PO 4 and TiO 2 –PO 4 catalystswith large pores exhibited high activity but limited selectivity towards acrolein.  The comparison of the formation of acrolein and acetol over SAPO-11 and SAPO-34 catalysts showed that the pore size and nature of acidity had at low TOS a significant effect on the selectivity towards acrolein.  The experimental data showed that in the presence of acidic catalysts acetolwas stable until 240° C, whereas 3-HPAwas already dehydrated to acrolein and condensated to cyclic C6 compounds at 120 ° C.  Deactivation was observed for all acidic catalysts, but an oxidative treatment with air at temperatures around 450° C was found to be sufficient to regenerate the deactivated catalysts and to recover acidity and activity.