J.
Ryan
,
M. T.
Elsmore
,
E. R.
Binner
,
D. S. A.
De Focatiis
,
D. J.
Irvine
and
J. P.
Robinson
*
Coates Building, Faculty of Engineering, University of Nottingham, NG7 2RD, UK. E-mail: John.robinson@nottingham.ac.uk
First published on 8th December 2020
This work demonstrates a novel approach to add value to pyrolysis liquids by exploiting the diverse range of alcohol functional groups present within the mixture to yield a non-energy product, without requiring extensive separation. It is shown that 79.2% of the alcohol functional groups can be converted by esterification and subsequently polymerised (85.7%) to produce a range of polymer products with peak molecular weight (Mp) ranging from 22.9–36.9 kDa. Thermal and rheological properties of the most promising pyrolysis material have been compared with conventional poly(butyl methacrylate) (pBMA) of similar molecular weight, showing viability as a potential replacement owing to similarities in its thermorheological behaviour. A low molecular weight wax component of the novel polymer has been identified as a possible plasticizing agent, causing some decreases in viscosity. Production of the monomer is achieved in one reaction step and without separation or the use of toxic reagents. The overall mass balance and relevance to a biorefinery process is highlighted and strategies to tune the process to vary glass transition temperature (Tg) and Mp are discussed.
A broad range of polymeric materials can be obtained from pyrolysis liquid owing to variability in structure of the monomers either found in or derived from the pyrolysis liquid. Works by Qu et al., Mialon et al., Stanzione et al. and Nowakowska et al. have demonstrated the viability of polymer production from tightly-controlled feedstocks such as lignin, vanillin, syringol, guaiacol and eugenol.11–15 It has further been reported that acyl chlorides and anhydrides offer potential for the synthesis of methacrylate monomers from specifically controlled model feedstocks or pyrolysis liquid fractions. These methacrylate polymers show viability in a wide range of applications, from resins and adhesives to bitumen binders.15–21 It has been shown that polymers developed from biomass can be tuned to a wide range of thermoset and thermoplastic applications including bio-based polyurethanes, melt-spinnable fibres and pressure sensitive adhesives.4,13,15,20,22 However, numerous additional steps are required to extract or fractionate useful parts of the pyrolysis oil involving the use of toxic and corrosive solvents and reagents.15,22 Ideally, for its eventual use in any integrated bio-refining process, pyrolysis liquid should be processable to the desired value added product from this impure state.23
Transesterification is an industrially applied technology that can transfer hydroxyls onto to methacrylate monomers.24 Our approach uses a transesterification of an industrially produced pyrolysis liquid to generate monomers. Subsequent free radical polymerisation (FRP) on the functionalised pyrolysis oil should enable phase separation of polymer product and unreacted components of the functionalised pyrolysis liquid, see Fig. 1.
Fig. 1 Pyrolysis liquid to methacrylate polymer via transesterification24 and radical polymerisation.25 |
Unlike previous studies the entire crude pyrolysis liquid is used as feedstock for synthesis of a sustainably-sourced replacement for methacrylates, without the need for extensive, complex separation stages or significant upgrading. The aim of this work is to investigate the effectiveness of the proposed transesterification–FRP method. The objectives are (1) determine the yield and physical properties of polymer produced from crude pyrolysis oils (2) present an initial mass balance and (3) benchmark the thermal and rheological properties against pBMA.
Fig. 2 Decrease in hydroxyl content in the distilled pyrolysis liquid during the transesterification reaction at 145 °C along with transesterification reaction shown in the insert. |
The concentration of alcohol was measured in the reaction vessel with the butanol (BuOH) by-product being removed by distillation throughout the reaction. Control reactions of distilled pyrolysis liquid and BMA, with TNBT confirm that this decrease in OH is due to transesterification reaction. Consequently, as conversion is less than 100% and an excess of BMA was added there is unreacted BMA blended with functionalised pyrolysis liquid. This means that any polymer produced was likely a copolymer of BMA and pyrolysis liquid methacrylate monomers.
Fig. 3 Block flow diagram to outline the streams in the proposed transesterification of pyrolysis liquid. |
Pyrolysis liquid, as received | Pyrolysis liquid, volatiles | Pyrolysis liquid, distilled | BMA & TNBT | Transesterification by-product | Upgraded pyrolysis liquid | Wax | Pyrolysis liquid monomer | ||
---|---|---|---|---|---|---|---|---|---|
Stream | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | |
Mass (kg) | 1 | 0.4 | 0.6 | 0.6 | 0.12 | 0.078 | 0.03 | 0.972 | |
Composition from GC (%) | Acid | 6 | 4 | — | — | 2.3 | 18.8 | 17.1 | |
Alcohol | 29 | 5 | 63.5 | — | 70.4 | — | 33.4 | ||
Aldehyde | 4 | 2 | — | — | 4.2 | — | 6.3 | ||
Aromatic | 3 | 1 | 17.3 | — | 6 | — | 3.2 | ||
Ester | 1 | 1 | — | — | — | — | 8.1 | ||
Furans | 26 | 15 | 11.2 | — | 4.8 | — | 3.2 | ||
Ketone | — | — | — | — | 5.9 | — | 18.1 | ||
Non identified | 2 | 1 | 8 | — | 6.5 | — | — | ||
Sugars | — | — | — | — | — | 81.2 | 10.7 | ||
Excluded | <1 | 1.17 | <1 | — | <1 | <1 | <1 | — | |
Water content (%) | 29 | 72 | <1 | — | <1 | <1 | <1 | <1 |
The block flow masses in Table 1 show that the main components of the pyrolysis liquid volatiles, removed in the drying step, are water and furans. Alcohols, aromatics and some non-identified species remain in pyrolysis liquid after distillation. The transesterification by-product is composed of 70% butanol and other species that could be useful in combustion. The upgraded pyrolysis liquid stream, Fig. S6 and S7,† shows an enrichment of acid and sugar derivatives compared to the pyrolysis liquid as supplied, confirmed with analytical standards as acetic acid and levoglucosan. The wax comprises unreacted alcohols, acid, aldehyde, ketone and sugar derivatives miscible with the pyrolysis monomer. Titanium catalyst fate was in fraction 6 confirmed by gravimetric ash measurement of fractions 6 & 8 in 550 °C oven.
Reaction yield is a one dimensional way of measuring the quality of a reaction, there are several inter-dependant steps, so there are mass yields of polymer with respect to the pyrolysis liquid monomer (8), BMA (4) and the pyrolysis liquid, as received (1), these are 85.7%, 138.8% and 83.3% respectively. These mass yields, particularly with respect to BMA, show that pyrolysis liquid is incorporated into the mass of the polymer.
Estimations of how much mass in stream 8 is derived from BMA, itself a derivative of natural gas via methacrylic acid from the ACH process, can also be made from the masses in Table 1. However, bio-based routes to methacrylic acid have now been identified and are in the process of being commercialised, thus sustainability footprint of this process in the near future will exhibit significant improvement.29 With respect to defining an crude estimation of the final copolymer BMA:pyrolysis polymer ratio. The known input of BMA was 0.6 mass equivalents and the quantity of pyrolysis liquid monomer was estimated by taking accounting for the 0.0845 (0.12 × 0.704) equivalents of BuOH removed by distillation. This gave a mass ratio for BMA:pyrolysis liquid monomer of 0.515:0.972 or 53.0 wt%. It should be noted that this value includes both excess BMA and the mass of the methacrylate functional group in the pyrolysis liquid monomer. The addition of a BMA recycling step prior to polymerisation could further reduce the amount of BMA, and hence petrochemical derivatives, in the product. The choice of initiator is interesting grounds for further work, as there is a limited number of thermally triggered radical initiators available to us. Thus, there would be fertile ground in trying to produce more sustainable options. Although such a study would need to reflect on the fact that, as the initiator only accounts for an extremely small percentage of the reaction mass, its influence of the overall sustainability if the process is very limited. Furthermore, as it also determines the reaction parameters that can be used, for instance 4,4-azobis(4-cyanovaleric acid) allows for lower initiation temperatures than AIBN. However, at lower temperatures the viscosity is usually much higher, which may result in the need to use a solvent which would introduce much greater sustainability issues. Fermentation routes to C3–C12 methacrylates have been identified elsewhere that could reduce the mass of natural gas derived methacrylate.30
However, the commercial prospect of pyrolysis polymers produced using this approach will also depend in part on the utilisation of the coproduct streams shown in Table 1. There are many options that can be considered here, e.g. utilisation of the volatiles stream as a source of platform furan chemicals and the upgraded pyrolysis liquid as a fermentation feedstock. Full consideration of co-product utilisation is beyond the scope of this work, however there is an exciting opportunity for future work to use different technologies such as microwave pyrolysis to change the starting chemistry of the pyrolysis liquid5 and subsequently to understand how the pyrolysis polymer properties and the overall mass balance are affected. Future work will also consider catalysts that can tolerate water within the pyrolysis liquids, thereby removing the need to undertake the first distillation step and purification or control agent strategies to increase the molecular weight could improve rheological properties.
Fig. 4 Size exclusion chromatograph of upgraded pyrolysis liquid prior to purification including annotation for pyrolysis polymer, wax and monomer peaks from left to right. |
The SEC chromatogram shows that an additional molecular weight species is present in the polymer sample. A kinetic study of the evolution of the polymer peaks given in Fig. S8† demonstrated that this material was present from the outset of the polymerisation and did not increase during reaction. The only peaks noted to change during the reaction are the depletion of the monomer peak and the increase of a monomodal polymer peak. Thus, it is proposed that this additional peak may be an unreactive wax. The molecular weight of this wax, 0.5–2 kDa is similar to that of pyrolytic lignin, the origin of this wax is unclear. SEC analysis of the; (a) pyrolysis liquid as received, (b) pyrolysis liquid after drying, (c) transesterified pyrolysis liquid and (d) transesterification residue are presented in Fig. S9.† The apparent change in the wax concentration/molecular weight after the drying step was attributed to both (a) volatile low molecular weight compounds being removed contributing the wax being a greater percentage of the total mass and (b) a small amount of wax crosslinking as has been reported in the higher temperature experiments within this study. There were instances where there was incomplete baseline separation between polymer and wax peaks, this artificially increased Mn and to a lesser extent Mw values and for this reason Mp was used in this comparison because of its lack of dependence on the distribution of the sample molecular weight.
Additionally, the mono-modal nature of the SEC polymer peak was unexpected. There is such a wide variety of alcohols present in the pyrolysis liquid that it was anticipated that their transesterification would have produced a wide range of different monomers with disparate levels of reactivity. A range of different polymers formed at different rates and resulting in different molecular weight products would give rise to polymers with different hydrodynamic volumes and hence a multi-modal SEC. Analysis of the light scattering derived molecular weight of the pyrolysis polymer is shown in Fig. 5.
Fig. 5 Molecular weight analysis of pyrolysis polymers using light scattering, dn/dc value of 0.087 mL g−1. |
The refractive index increment (dn/dc) value of BMA is 0.087 mL g−1 and was used for molecular weight analysis of polymer as BMA is likely to be the most abundant single motif in the system. There are multiple different shoulder peaks apparent in Fig. 5, indicating the presence of other high molecular weight species, distinct from a BMA homopolymer. These components are likely to have a range of different dn/dc values and full characterisation of the components and their respective dn/dc values would be needed to validate these molecular weights. However, the existence and relative position of these peaks is consistent with results elsewhere supporting the idea of multiple copolymers.15
The effect of pyrolysis liquid concentration on the polymerisation was investigated further by varying the molar amount of BMA added to the pyrolysis liquid prior to transesterification. The stoichiometry and nomenclature used is provided in Table S3.† By increasing the amount of residual BMA present in the transesterification step the effect of the pyrolysis liquid monomers on the polymerisation was reduced.
It was not possible to define the exact BMA:pyrolysis liquid monomer ratio in the transesterification product via NMR analysis due to the overlap of coincidental peaks in the spectra. Instead, the mass of butanol condensate distilled out of the reaction vessel in the transesterification step was used as a measure of the amount of pyrolysis monomer formed. This value could then be compared to the amount of BMA in the feed to give an indicative measure of the BMA:pyrolysis liquid monomer ratio. The data used for this calculation is included in Table S3† and show that using the distillate mass to estimate the BMA:pyrolysis liquid monomer ratio overestimates the concentration of BMA in the monomer mixture, this is not the case for 31P-NMR, the difference between the two calculations becomes more pronounced as the amount of distillate increases.
The initiator concentration was varied to investigate the effect of a change in concentration on the polymer peak molecular weight and conversion, Table 2.
Entry # | Ratio of BMA:pyrolysis liquid | [AIBN] (wt%) | Conversion (%) | M p (kDa) | M n (kDa) | M w (kDa) | Đ |
---|---|---|---|---|---|---|---|
Control | 0.6:1 | 0 | * | * | * | * | * |
1 | 0.6:1 | 0.01 | * | * | * | * | * |
2 | 0.6:1 | 0.11 | * | * | * | * | * |
3 | 0.6:1 | 0.98 | 79.04 | 22.9 | 16.1 | 28.4 | 1.77 |
Control | 1.2:1 | 0 | * | * | * | * | * |
4 | 1.2:1 | 0.02 | 8.26 | 29.8 | 6.99 | 35.7 | 5.11 |
5 | 1.2:1 | 0.17 | 10.71 | 27.4 | 9.86 | 33.4 | 3.39 |
6 | 1.2:1 | 0.80 | 82.49 | 24.6 | 15.8 | 29.5 | 1.87 |
7 | 1.2:1 | 1.00 | 85.12 | 22.9 | 20.9 | 34.3 | 1.64 |
Control | 2.4:1 | 0 | * | * | * | * | * |
8 | 2.4:1 | 0.02 | 7.41 | 36.9 | 19.0 | 40.1 | 2.11 |
9 | 2.4:1 | 0.10 | 27.01 | 32.2 | 21.7 | 41.3 | 1.90 |
10 | 2.4:1 | 0.62 | 87.03 | 13.4 | 6.57 | 15.5 | 1.65 |
Table 2 shows increasing initiator concentration in the polymerisations resulted in a decrease in Mp, increased conversion and lower Đ. When comparing the initiator concentrations, the effects of different BMA:pyrolysis liquid ratios are most apparent in their effect on Mp. In all cases when initiator concentration was increased the expected decrease in Mp was observed, The differences are more pronounced at higher BMA:pyrolysis liquid ratios. For example, comparing 5 with 7, a 5.9 fold increase in initiator concentration decreased Mp by 17% where in the case of 9 and 10, a 6.2 factor increase in initiator concentration reduced Mp by approximately 40%. For the same examples, the effect on dispersity is even more pronounced, with entries 5 & 7 showing a 52% decrease and 9 & 10 showing only a 13% decrease. Comparing samples with comparable initiator concentrations, entries 4 & 8, similar conversions but entry 8 had higher molecular weight and lower dispersity. This suggests that the wide variety of monomers in the functionalised pyrolysis liquid decrease the conversion and molecular weight and increase polydispersity.
In order to study this in more detail, isothermal frequency sweeps were carried out at temperature intervals of 30, 35, 40, 45, 55, 65, 80, 100 °C for the pyrolysis polymer and 40, 50, 60, 80, 95, 125 °C for p(BMA), to provide sufficient overlap of individual data curves to form mastercurves. Temperature intervals were predicted using the Williams–Landel–Ferry equation and universal C1 and C2 constants aiming for an overlap of 0.5 decades of frequency.31,32 The individual isothermal curves were manually shifted in frequency to produce the Tg-normalised frequency mastercurves of G′ and G′′ shown in Fig. 7.
Fig. 7 Frequency mastercurves for (a) the pyrolysis polymer and (b) p(BMA) at Tref = Tg + 50 °C. G′, G′′ and tanδ are shown as squares, triangles and circles respectively for both materials. |
Although the behaviour is similar, there are two important differences. Firstly, the pyrolysis polymer exhibits lower moduli by approximately an order of magnitude across the temperature range. Secondly, in the low frequency flow region, the p(BMA) exhibits the expected power-law behaviours with gradients very close to 2 and 1 in G′ and G′′ respectively,33 but the pyrolysis polymer does not, particularly in G′. These differences point to the role of the wax component in reducing the elasticity and the viscosity and preventing a clear entanglement plateau. In addition, structural changes are apparent at high temperatures in the pyrolysis polymer in processes related to those that eventually lead to char formation.
TGA data obtained for the pyrolysis polymer and p(BMA) of a similar molecular weight is included in Fig. S10.† A photograph of the pyrolysis polymer is included in Fig. S11.† Temperatures at 1%, 10% and 50% mass loss are reported in Table 3 along with molecular weight information and Tg values established from DSC experiments. Despite the similar molecular weight, the presence of the low molecular weight wax species is the likely cause of the reduction in Tg, the earlier onset of degradation and the residual char in the same manner as found for related bimodal blended systems.14,34
Property | Pyrolysis polymer | p(BMA) |
---|---|---|
M p (kDa) | 35.4 | 37.6 |
M n (kDa) | 18.6 | 25.1 |
M w (kDa) | 37.9 | 47 |
Đ | 2.04 | 1.87 |
T g (°C) | 13.9 | 26.7 |
T 1% (°C) | 145.6 | 274.9 |
T 10% (°C) | 245.8 | 302.2 |
T 50% (°C) | 363.9 | 329.2 |
Mass residue, 500 °C (%) | 11.2 | 0 |
tanδmin | 1.3 | 1.01 |
f (Hz) | 1.16 | 1.51 |
τ (s) | 0.87 | 0.66 |
G′ (kPa) | 39.85 | 217.58 |
G′′ (kPa) | 51.64 | 220.45 |
η* (kPa s) | 41.33 | 492.97 |
Table 3 gives a comparison of rheological parameters measured at the frequency where the loss tangent, tanδ, is a minimum for the pyrolysis polymer and p(BMA). Across the relevant processing temperature range, the pyrolysis polymer exhibits moduli that are smaller than those of the p(BMA) by almost an order of magnitude. Similarly, the complex viscosity η* was found to be lower for the pyrolysis polymer by just over an order of magnitude. The pyrolysis polymer's reduced viscosity could be due to the diluting and plasticizing effects of the wax. The frequency at which the loss tangent is closest to unity is comparable for the two polymers, suggesting similar terminal relaxation times, τ. To probe further the rheological behaviour of the pyrolysis polymer at elevated temperature, a 15 mm disc was held at 110 °C and subjected to small angle oscillations of γ = 0.1% and f = 1 Hz for 1 hour. The complex viscosity rose a little, at a constant rate, from 181 Pa s to 193 Pa s, suggesting an overall small increase in molecular weight.35 SEC experiments were carried out before and after the 1 hour test. Although similar, there was an increase in Mp of the waxy peak by a factor of ∼4.7, although remaining oligomeric in nature, and a very small reduction in Mp of the polymer peak by 8.4%. This suggests that there has been some degree of polymerisation of the waxy component, and possibly a small degree of degradation or scission of the polymer.
Based on the results shown, the rheological performance of the pyrolysis polymer is similar to that of the p(BMA) but distinct, likely due to the presence of the oligomeric wax. Further investigation into identifying and controlling the mass fraction and thermal stability of the wax in the pyrolysis polymer may offer a facility for tuning processing performance to desired applications. It has been noted that the rheological performance of this novel material is comparable to that of numerous bitumen binders from crude oil and synthetic sources across similar temperature ranges.36–38 Works published in the road surfacing sector have expressed the need for renewable acrylates and methacrylates for use as synthetic bitumen binders which offers a potential application for this polymer and will be the subject of further work.39,40 As this is a process that stems from the use of naturally occurring reagents, there will likely be variability in the monomer/polymer make up if the source of the bio-reagent is changed. Thus, in this study, pyrolysis liquid from the same batch source was used. Should an industrial outlet be found from a potentially variable feedstock then at that point, a full parametric study would need to be conducted to draw conclusions on the compositions of the products and their effect on the properties with specific relevance to a chosen application.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0re00419g |
This journal is © The Royal Society of Chemistry 2021 |