Role of cerium as a promoter and process optimization studies for dehydration of glycerol to acetol over copper chromite catalyst

Abstract

Cerium-promoted silica supported copper chromite catalyst was synthesized from acid hydrolysis of sodium silicate by sol–gel method. The catalyst was characterized by Brunauer–Emmett–Teller (BET) method, field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), H₂-temperature programmed reduction (H₂-TPR), NH₃-temperature programmed desorption (NH₃-TPD) and pyridine adsorbed Fourier transform infrared spectroscopy (Py-FTIR). Among cerium doped catalysts, 5 wt% of Ce promoted copper chromite supported by 40 wt% of silica (SiCuCr40–Ce5) shows the largest BET surface area. XRD analysis of the reduced form of the catalyst shows both CeO₂/Ce₂O₃ redox system and CuO/Cu₂O/Cu redox system. Py-FTIR shows the maximum number of Lewis acid sites for SiCuCr40–Ce5 than others. The highest acetol selectivity with analytical reagent (AR) grade glycerol conversion is observed for SiCuCr40–Ce5 at 200 °C for 3 h in a batch reactor at atmospheric pressure. Cerium promotion lowers the reaction temperature with enhanced glycerol conversion and increased acetol selectivity. Though the above catalyst shows higher conversion for laboratory reagent (LR) grade glycerol but it reduces acetol selectivity. The addition of glucose into the LR grade glycerol further reduces glycerol conversion and decreases the acetol selectivity to zero. This may be due to the presence of iron as impurity in LR grade glycerol. XRD analysis of spent catalyst shows the absence of redox catalytic system and the pore volume reduces identified by BET analysis. Raman analysis of the spent catalyst shows graphite-like carbon deposition in the spent catalyst.

Introduction

Biodiesel is an alternative diesel fuel obtained from transesterification of oils and fats. The definition of biodiesel is a substitute or additive to diesel fuels that is derived from oils of plants or fats of animals. The development of biodiesel as an alternative fuel has been significant concern in recent years due to its non-toxic, biodegradable properties and environment-friendly renewable nature.¹ Around the world, the awareness of energy and environmental issues related to the burning of fossil fuels has motivated many scientists and/or researchers to look for the possibility of using alternative energy sources rather than fossil fuels and their derivatives. Approximately 1 kg of crude glycerol is generated in producing 10 kg of biodiesel. The estimated global production of biodiesel in 2024 will be 39 billion gallons (OECD-FAO statistics).² Crude glycerol which exceeds its demands further lowers its economic values. Hence urgent attention is needed to find out alternate methods to consume huge byproduct of the biodiesel industries.

Conversion of glycerol to valuable products is a process which is still being thoroughly researched upon and new and better ways to carry out this conversion are being developed.³ To make biodiesel more easily accessible for everyone, it has become necessary to enhance the processes of glycerol conversion.4, 5, 6 Developing this technology is vital for both our environment and our future fuel needs. Novel techniques to obtain value-added products such as acrolein and acetol are being studied and methods to increase the rates of glycerol conversion and selectivity levels to valuable products are being researched.^7,8 Efforts are being made to overcome the drawbacks identified in other processes such as deactivation of the catalyst, difficulty in maintaining high temperatures and pressures.⁹

Copper chromite catalyst has shown high acetol selectivity with 100% glycerol conversion for analytical grade (AR) glycerol.¹⁰ The main limitation of this catalytic system is the loss of activity after reactions due to loss of leaching of active metallic components. In search of that, we have found that rare earth element has the potential to retain the active metals at high temperature reactions. Hence we have chosen cerium as a promoter for copper chromite catalyst in glycerol dehydration. The other objective is to test the catalytic activity of the cerium promoted catalytic system for the lower cost LR grade glycerol in dehydration reactions. Cerium oxide in hydrogen atmosphere forms non-stoichiometric compounds which can be coupled with another redox system for better catalytic performance.11, 12, 13 Literature reported that rare earth additives could restrain the sintering of the catalyst in glycerol dehydration.14, 15, 16 Performances at different loadings of cerium were investigated. The temperature, catalyst weight and glycerol concentration variation of product composition with optimized catalyst were also studied. An important feature of this research is that the rare earth element helps in retaining the metal ion after the reactions and enhances the catalytic activities.