University of Groningen
Multifunctional catalytic systems for the conversion of glycerol to lactates Tang, Zhenchen
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Tang, Z. (2019). Multifunctional catalytic systems for the conversion of glycerol to lactates. University of Groningen.
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Chapter 6
Chapter 6
Conclusive remarks
Biomass of vegetable origin is the dominant renewable alternative source of carbon compared to depleting fossil resources, and as such has received considerable attention in the last decades. Among biobased products, biodiesel is one of the most important sustainable fuels. Biodiesel is produced from a transesterification process, in which glycerol is the main by-product. In this context, the conversion of glycerol into useful chemical products over heterogeneous catalysts is an important research topic that received increasing attention in recent years. This PhD project focussed on the design and development of enhanced catalysts for the conversion of glycerol into lactic acid and lactates. The several possible reaction pathways for the conversion of glycerol into valuable chemicals were systematically discussed in Chapter 1 from the point of view of catalyst design and catalytic process optimization. Among the discussed routes, the conversion of glycerol into lactic acid or alkyl lactate was identified as one of the most promising pathways. This multi-step reaction requires the combination of two types of catalytic sites: metallic sites for the oxidative dehydrogenation and acid (or base) sites for the rearrangement step. In the research work that led to this PhD thesis, several different catalytic systems were designed and optimised for the conversion of glycerol to methyl lactate or lactic acid with high activity and selectivity.
In the first part of this thesis, Au-based noble metal catalysts combined with Sn-MCM-41-XS solid acid were designed, synthesised and tested for the multistep synthesis of methyl lactate from glycerol without adding a base. In Chapter 2, the best catalytic system consisting of a physical mixture of Au/CuO and Sn-MCM-41-XS achieved higher yield compared to Au/Sn-MCM-41-XS and to systems based on Au supported on other metal oxides. The catalyst reached 66% selectivity towards methyl lactate at 96% glycerol conversion after 10.5 h of reaction. FT-IR characterization of the adsorption of CO and ethanol indicated that metallic Au NPs are the active sites for oxidation and suggested that the relatively weak interaction between reaction intermediates and Au/CuO was beneficial for their desorption and further rearrangement over Sn-MCM-41-XS. The catalytic system based on Au/CuO was further optimized by fine-tuning the
size of the Au NPs supported on CuO by preparing the material with different Au loadings and different thermal treatments. The optimum balance between activity and selectivity was found for an average particle size of 3 nm. Furthermore, the suitability of Sn-MCM-41-XS as solid acid catalyst was confirmed by its much better selectivity compared to USY in the second rearrangement step, i.e. from trioses to methyl lactate. Importantly, the Au/CuO - Sn-MCM-41-XS system exhibits excellent reusability either through a simple washing step or by mild thermal treatment at 200 oC. These results are of practical importance in the context of the conversion
of a bio-based platform molecule as glycerol into a valuable product as methyl lactate.
The observation that the physical mixture of supported Au catalysts and Sn-MCM-41-XS solid acid worked better than Au nanoparticles supported directly on Sn-MCM-41-XS, prompted a further study involving the modification of the Au based metallic sites as well as the supports, which was reported in Chapter 3. A series of multifunctional heterogeneous catalytic systems consisting of (i) bimetallic Au-Pd nanoparticles supported on (functionalised) CNTs and (ii) Sn-MCM-41-XS were used for the first time for the conversion of glycerol into methyl lactate in a base-free one-pot reaction. Among these materials, the catalytic system employing CNTs functionalised by a HNO3-H2SO4 treatment showed the best performance and gave 85% yield
of methyl lactate at 96% conversion of glycerol, which is the best catalytic performance reported so far in the literature for base-free systems. The use of bimetallic Au-Pd nanoparticles was shown to lead to higher activity compared to the mono-metallic systems (either Au or Pd alone), proving the synergy between Au and Pd. The observed catalytic trends could be rationalised on the basis of a characterisation study, which highlighted the interaction between Au and Pd, the small particle size and the high degree of functionalisation with acidic groups of the CNTs functionalised by a HNO3-H2SO4 treatment. The latter feature is considered crucial for
the final catalytic activity, as it led to: (i) increased interaction between the bimetallic nanoparticles and the CNTs surface, with consequent small particle size (average size = 3.5 nm); (ii) presence of surface Brønsted acid sites that promote the dehydration of the trioses intermediates to pyruvic aldehyde; (iii) enhanced dispersion of the AuPd/CNTs-NS material in the polar reaction medium. A kinetic study revealed that the reaction is first order with respect to glycerol, thus identifying the initial oxidation of our starting material as the rate-determining step. Moreover, the role of the carboxylic acid and ester side-products as possible inhibitors for the active sites was investigated, showing that the former negatively affect the catalytic activity. The reusability of the catalytic system consisting of AuPd/CNTs-NS and Sn-MCM-41-XS was excellent, as proven by the retention of the original activity in 5 consecutive batch recycle
Chapter 6
experiments. Our strategy of combining a material bearing the catalyst for the oxidation of glycerol (AuPd/CNTs-NS) with one providing the active sites for the consecutive rearrangement of the triose intermediates into the desired lactate product (Sn-MCM-41-XS) proved successful in achieving unsurpassed catalytic performance.
In the second part of this thesis, the aim was to utilise the hydrogen obtained from glycerol in its conversion to lactic acid. For this purpose, a reaction system was designed for the transfer hydrogenation between glycerol and a H2 acceptor to produce lactic acid and hydrogenated
target product, respectively. In Chapter 4, a catalytic system based on highly dispersed Pt supported on ZrO2 was prepared and tested for the transfer hydrogenation from glycerol to
cyclohexene, in which lactic acid and cyclohexane were obtained as target products in a one-pot batch reaction. The atomically dispersed Pt species were prepared by wet-impregnation and calcination with ZrO2, while the active sites for this transfer hydrogenation reaction were
Pt nanoparticles that were obtained by reducing the atomically dispersed Pt species (Pt2+ and
Pt4+). The study of the conditions for the calcination and reduction of the Pt/ZrO2 catalysts
demonstrated that a carefully tuned calcination and reduction temperature (550 and 250 oC,
respectively) is crucial to obtain extra-fine metallic Pt nanoparticles. The 2Pt/ZrO2-550-R250
catalyst exhibited significantly high activity (96% glycerol conversion) and selectivity to lactic acid (99%) at 160 oC, 4.5 h under N2 atmosphere. This reaction also gave a 36% selectivity in
the transfer hydrogenation from glycerol to yield cyclohexane. Moreover, adding NaOH turned out to be critical to initiate this reaction, which is probably related to the first dehydrogenation step of glycerol as well as to the reaction with lactic acid to push the equilibrium concentrations to the right. The reusability was also tested by batch recycle experiments, showing gradual loss of activity of catalyst, which is due to the aggregation Pt nanoparticles as proven by TEM image of the reused catalyst.
To further optimise this catalytic transfer hydrogenation system, bimetallic Ni-Co catalysts supported on CeO2 were prepared and tested, leading to comparable catalytic performance,
higher stability and much lower cost (Chapter 5). Introducing Co into the catalyst formulation was crucial to obtain finely dispersed bimetallic Ni-Co nanoparticles on CeO2, which led to
increased activity compared to the monometallic Ni or Co counterparts. The bimetallic 10NiCo/CeO2 catalyst exhibited very high activity (91% glycerol conversion) and selectivity to
lactic acid (94%) at 160 oC, 4.5 h under N2 atmosphere. Moreover, various H2 acceptors
(levulinic acid benzene, nitrobenzene decene and cyclohexene) were hydrogenated with the in‐
reusability was also tested by batch recycle experiments, showing slight loss of catalytic activity, which is probably due to the leaching of active components.
In conclusion, this PhD thesis presented two different catalytic systems with enhanced catalysts to obtain either alkyl lactate or lactic acid from glycerol. Besides the very high yields of methyl lactate (85%) or lactic acid (96%) achieved here, this work underlined the relevance of a rational design of catalysts and of an approach that combines improved catalytic results in the upgrading of bio-based glycerol with an understanding of the relation between the physicochemical properties of the catalyst and its performance.
