Continuous flow transformations of glycerol to valuable products: an overview
© Len and Luque; licensee Chemistry Central Ltd. 2014
Received: 9 November 2013
Accepted: 3 January 2014
Published: 10 January 2014
Glycerol conversion to valuable products has been a research avenue that attracted a significant interest in recent years due to its large available volumes (as by-product of biodiesel production) and the different possibilities for chemical and biological conversion into high added value chemicals profiting from the unique presence of three hydroxyl groups in its structure. The utilization of continuous flow processes in combination with transformation of platform chemicals (e.g. glycerol) can offer several advantages to batch processes in view of their potential implementation in industry. This minireview has been aimed to highlight most recent key continuous flow systems for glycerol valorization to valuable products using different types of catalysts and processes.
KeywordsGlycerol Continuous flow Green chemistry
Society faces daunting challenges for the 21st century. Resource, food and water scarcity combined with an ever increasing population, decreasing of fossil fuel resources and increasing energy demands for the next years to come are some of the prospects for future generations. There is an ever increasing pressure on scientists, governments and politicians to enforce and promote more sustainable practises for a switch to a bio-based future society. This is however not an easy task and requires of significant multidisciplinary joint efforts from various disciplines directed to the development of greener technologies and more environmentally friendly methodologies.
The concept of biorefineries and the utilisation of renewable-derived resources (e.g. biomass) has been increasingly popular in recent years as a promising alternative to meet some of these future challenges . The development of analogous refinery-type processes based on adding value to biomass and waste via transformation into valuable chemicals, materials, fuels and energy is the way forward to advanced sustainable practises for future generations. Biomass holds a remarkable potential in terms of diversity, composition, variability and abundance to be the core of biorefinery concepts in potentially future industrial ventures .
Profiting from its unique nature and complex structure, scientists have devised ways to deconstruct different biomass feedstocks into simpler entities (the so-called platform chemicals) from which several transformations to valuable products can in fact make possible the conversion of biomass-derived feedstocks in a more rational and understandable way. In this regard, simpler and more understood chemistries can be in principle designed to provide similar end products (e.g. solvents, fuels, plastics, pharmaceuticals, agrochemicals, etc…) to those currently obtained from crude oil . Significant challenges are still however to be addressed in terms of developing chemistries under aqueous processing conditions, design of stable and active catalysts and essentially different processing of biomass as compared to petrol-derived feedstocks for chemicals and fuels production (e.g. de-functionalisation through mainly deoxygenation instead of functionalisation/upgrading/molecular weight-structure adjusting steps) .
Interestingly, only few reports are currently available on continuous flow glycerol valorisation processes which however can offer several advantages in terms of future industrial implementation. Indeed, continuous flow chemical processes for biomass valorization to fuels and chemicals hold significant potential for future development in our aim to drive our chemistries to more efficient and scalable approaches, while being environmentally sound and sustainable at the same time. Continuous flow processes can offer faster and safer reactions and reaction optimization, allowing in some cases to conduct chemistries that are not possible under batch conditions . Simpler setups can be then subsequently scaled up to larger scale processes in which process intensification can also allow an efficient control of parameters (e.g. temperature, flow of gases and/or reagents etc.).
In view of the possibilities of continuous flow processes applied to biomass conversion, this contribution has been aimed on providing a short overview on selected possible valorization reactions and added-value chemical obtained from glycerol under liquid phase-continuous flow processing in recent years.
Only few literature examples are available concerning acrolein synthesis from glycerol using continuous flow processes. The double dehydration of glycerol could be achieved under sub- and supercritical water as reaction media [12–15]. The critical temperature and pressure of water were 374°C and 220 bar, respectively. Studies related to the continuous evaporation of acrolein upon formation were not mentioned in this work.
Hydrogenolysis of glycerol
Aqueous phase reforming (APR) of glycerol is an attractive methodology able to grant access to relevant chemicals (e.g. propanediols) as well as a sustainable source of hydrogen .
Comparatively, the industrial hydrogenolytic conversion of glycerol to propan-1,3-diol (1,3-PDO) can be achieved using metabolically engineered microorganisms (e.g. Clostridium acetobutylicum) . In terms of chemical production of 1,3-PDO, an aqueous-phase glycerol degradation protocol was reported to employ a Pt/WO3/ZrO2 (3.0 wt.% Pt and 10 wt.% W) catalyst in a fixed-bed continuous flow reactor at 130°C under 40 bar (0.5 mL.h-1 aqueous glycerol, 10 mL.min-1 hydrogen gas, 24 hours residence time) . Under such conditions, a 70% conversion of glycerol with 32% yield (46% selectivity) to 1,3-PDO was obtained. n-propanol was another major compound observed under the investigated conditions (instead of i-propanol) which pointed to a high secondary deoxygenation selectivity as compared to that of the primary hydroxyl group in glycerol.
DMSO afforded the best conversion of glycerol probably due to a better desorption of the product from the hydrophilic surface of the catalyst. Different catalysts including calcined (mainly Lewis basic sites) or rehydrated (mainly Bronsted basic sites) materials were tested. Optimum activities were obtained using rehydrated catalysts, suggesting a more important contribution of Bronsted basic sites in the observed enhancement of catalytic performance. The high performance of the rehydrated hydrotalcite supported on α-Al2O3 led to a higher GDC yield. Increasing Mg vs Al (Mg/Al molar ratio of 4 instead 2) quantities provided improved glycerol conversion but decreased catalyst stability, a fact that was attributed to the presence of an extra MgO(H) phase in the catalyst .
Analogously, the catalytic synthesis of glycerol monoacetate was reported using a continuous bed column reactor packed with cation exchange resin Amberlyst 16. Starting from a solvent-free equimolar mixture of glycerol and acetic acid (7.67 mol L-1) with a column reactor containing Amberlyst 16 (5 g), the selective esterification (50°C, 30 min. residence time) selectively provided the corresponding monoacetate with a good selectivity .
Importantly, the scope of the reaction was also extended to the valorization of crude glycerol (GlyBio) from a biodiesel company . The biocatalyst selectively converted crude glycerol into monoacetin with moderate conversion at low flow rate (0.5 mL.min-1, 4.8 min of residence time) in the presence of ethyl acetate at 60°C. Promisingly, the use of vinyl acetate promoted triacetin production (84%) at low flow rate (0.5 mL.min-1, 4.8 min of residence time), with only small quantities of diacetins observed under the investigated conditions. Diacetin production increased to 70% and 60% (1.5 mL.min-1 and 3.0 mL.min-1, respectively) at higher residence times (14.4 and 28.8 min for 1.5 mL.min-1 and 3.0 mL.min-1, respectively).
In contrast with the use of pure glycerol, the choice of the acyl agent allowed a flexible and controllable protocol to maximise mono-, di-, and triacetin proportions starting from crude glycerol. Ethyl acetate as less reactive acylating agent can lead to the production of monoacetin-enriched acetin mixtures.
Future prospects and conclusions
Best experimental conditions for continuous flow transformations of glycerol
Additives and solvent
CH2 = CHOAc
Platform molecules derived from biomass such as glycerol have been mostly investigated to date as feedstocks to be converted into a wide variety of fuels, materials and chemicals using a range of green technologies (e.g. microwaves, mechanochemistry, etc).
The highlighted examples demonstrate the potential of a range of transformations of biomass-derived platform molecules under continuous flow conditions. Several high added value chemicals and biofuel precursors can be obtained using different continuous flow chemical methodologies which possess already established markets and developed applications to replace fossil-derived commodities. Many of these and related routes to convert platform molecules into valuable end products under continuous flow conditions offer a significant industrial potential, with some already being developed or under development, taking advantage of the important benefits of continuous flow processing. We envisage a series of topics including the design of novel flow processes, water-tolerant and stable catalysts for aqueous chemistries as well as low environmental impact technologies based on multi-step reactors and mild conditions to be part of future key investigations in the implementation of continuous flow chemical processing of biomass feedstocks.
Regardless of the industrial potential benefits of the implementation of continuous flow processes in biomass valorization practices, the environmental advantages of these methodologies in the processing of platform chemicals has to be taken into account. In this regard, continuous flow processing is the future of biomass valorization practices and we hope this manuscript can serve as momentum to both academia and industry to join efforts in order to carry on designing flow processes for biomass processing envisaging their implementation at industrial scale.
The author gratefully acknowledges Spanish MICINN for financial support via the concession of a RyC contract (ref: RYC-2009-04199) and funding under project CTQ2011-28954-C02-02 (MEC). Consejeria de Ciencia e Innovacion, Junta de Andalucia is also gratefully acknowledged for funding project P10-FQM-6711. The author is also indebted to Prof. Guohua Chen, the Department of Chemical and Biomolecular Engineering (CBME) and HKUST for the provision of a Visiting Professorship at the CBME in 2013.
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