Valorization of cigarette butts for top value-added chemicals: Levulinic Acid

Levulinic acid (LA), one of the top value-added intermediates of chemical industry, can be obtained by thermal hydrolysis (at 200 °C) from cigarette butts (as cellulose feedstock) catalysed by phosphoric acidic. The protocol avoids the use of more aggressive H2SO4 and HCl, that are generally employed on several cellulose sources (e.g. sludge paper), thus minimizing corrosion phenomena of plants. Neither chemical pre-treatment of butts nor specic purication procedure of LA are required. Notably, by simply modifying acid catalyst (e.g. using CH3COOH), another top value-added ne chemical such as 5-hydroxymethylfuraldehyde (HMF) is obtained, thus widening the scope of the method.Being cigarette lters a waste available in quantities of megatonnes per year, they represent an unlimited at no cost source of cellulose, thus enabling the up-scale to an industrial level of LA production.


Introduction
The conscious use of the planet's natural resources has become mandatory to ensure the survival of life on earth. To this end, the imperative contribution that the scienti c community can provide is to develop new sustainable process and materials, with a signi cant impact on the social level and with a reduced environmental repercussion. In this context, the tuning of new chemical approaches to exploit the waste has an extreme importance in the realization of the virtuous path that Circular Economy is encouraging, to create environmental and social bene ts, in a "Rethinking Progress" approach for sustainable development and sustainability.
Exploiting biomass is the true weapon to face this challenge, the true road for producing energy, ne chemicals and bio-based manufacts in a sustainable manner, thus de nitively eliminating the dependence on fossil sources, without loss of soil 1 .
Important examples of biomass are wood and energy crops, such as for example soy, useful for producing biofuels but also bio-based chemicals and polymers 2 . Recent years have witnessed a rapid growth in the production of fuels 3 and materials entirely deriving from biomasses. 4 However, this has led to many troubles such as improper exploitation of soils (non-food applications), increase of raw materials price (especially in Third World countries), biodiversity reduction, soil erosion and increased risk of insects and bacteria that destroy crops 1 .
Biomass wastes can be the right solution to these problems 2 , constituting a widely available and no cost reservoir of carbohydrates, lipids and proteins, with possible on-site processing, coming from scraps of forests, yards, farms, or municipal waste foods, the proportion of which has been estimated at hundreds of megatonnes (Mt) per year around the world 5 .
Carbohydrates, the main component of these vast reservoir, can be converted via biological or chemical routes into Levulinic Acid (LA) 6,7 , which is considered one of the twelve most promising industrial biointermediates and amongst the most innovative building blocks of chemical industry, due to its conversion in several high-value bio-based chemicals and materials (Fig. 1) 6 The main end users of LA are agricultural, pharmaceutical, and cosmetic sectors, although this natural molecule also contributes to the creation of new ecological fuels, fertilizers, and pesticides. It is also used in the biodegradable plastics eld and as intermediate element for creating high-performance plastic materials, medicines, and many other new concept "green" products, thus allowing to broaden its scope of application. According to the most recent studies, it is estimated that the world market demand for LA will grow 150-200 times over the next 7-8 years 7 .
The well-known approach to convert lignocellulosic materials (wood, paper, food crops wastes) into levulinic acid is the thermal treatment with strong Brønsted acids (e.g. H 2 SO 4 ) as homogeneous catalysts 8,9 . To date, a two-step continuous process is used to obviate the deterioration of the plants.
Hemicellulose and cellulose fractions of biomass are preliminarily hydrolyzed in a rst reactor (at 210 -230 °C, for few seconds in the presence of 1% -5% of mineral acid) producing hydroxymethylfurfural (HMF), that is removed in ow feeding continuously a second reactor where it is further hydrolyzed to produce LA ( Figure 2) [9][10][11][12] Despite the high yields, this strategy is di cult to apply at an industrial level, due to the harsh conditions and plants corrosion 9 . To date, only few companies can produce LA at commercial scale directly from biomass 9,12 . In recent years, much attention has been paid to producing LA by means of milder and more eco-sustainable conditions, 13 for example employing heterogeneous acid catalysts and green solvents such as water or ionic liquids (ILs) 7,14 .
As regards the cellulosic starting material, beside agricultural scraps 9 , municipal paper wastes are gaining attention 14 . Among these latters, cigarette lters represent a neglected and no cost reservoir of cellulose acetate 15 , that is virtually boundless if considering that about 5.5 trillion cigarettes are produced each year 16 . Notably, dirty cigarette butts (CBs) are considered a dangerous waste, because of the content of organic and heavy metals contaminants, therefore their use as starting raw materials is rather complex and essentially limited to production of asphalts, mesoporous carbon, and cellulose pulp [16][17][18] .
Recently, bioethanol has been produced by fermentation of cellulose obtained by deacetylation of CBs 19 , but no attempts have been reported until now on their use to produce LA or other ne chemicals.
Following our ongoing interest in developing green protocols obeying to circular economy principles 20,21 we report herein, unprecedented in the literature, a protocol that exploits cigarettes lters as source of Levulinic acid avoiding the strong acidic conditions and extendable at an industrial level.

Results And Discussion
In the proposed procedure the preliminary digestion with strong acids (H 2 SO 4 ) was circumvented using a one-pot procedure involving the less aggressive H 3 PO 4 22 , that preserves bres structure compared to analogous strong Brønsted acids 11 . According to most of reported procedures, catalytic hydrolysis experiments were conducted in a batch reactor processing 250 mg of lters in 15 mL of aqueous H 3 PO 4 at different times and temperatures 12 ( Table 1). Formation of Levulinic Acid was surveyed by GC/MS and NMR techniques. Both unsmoked and smoked cigarette butts were tested as source of cellulose biomass, whereas acetic and formic acids were formed as by-products together with HMF as an intermediate ( Figure 3) 13,23 .
Blank reaction carried out in the absence of H 3 PO 4 led to the complete recovery of unreacted lters, thus con rming that Brønsted acids are true catalysts for the process ( Attempts were done to increase the yields of levulinic acid, re-submitting residual humins by-products to the hydrolysis conditions at higher temperatures and prolonged reaction times. The total absence of products indicated that such conditions are not su ciently hard to give the cleavage of the furan-based polymeric skeleton of humins (eq. 1). Table 1 showed that the best hydrolysis conditions are 240 °C for 2 hours or 200 °C for 6 hours (Table 1, entries 6 and 14). Nevertheless, in both these cases, greater quantities of solid residue were observed probably due to the higher temperature and the longer times. Therefore, milder conditions of 200 °C for 2 hours were selected for the successive experiments aimed at studying both real waste samples such as the smoked lters and the in uence of their pre-treatment (e.g. washing).

Results in
At this end, smoked cigarette lters were washed with 100 ml of water at 80°C for three times. The collected water fractions were extracted with ethyl acetate and the organic phase was analysed by GC/MS revealing triacetin (triacetylglycerin) as the main product, which is a humectant additive, and trace amounts of phenolic compounds.
As reported in Table 2 (entries 1-2), almost identical results in terms of yields were obtained with washed and unwashed cigarette butts. In addition we have also avoided the thermal pre-treatment, which although it increases the yield of levulinic acid, requires a greater expenditure of energy. (entries 3) Moreover, NMR analyses ( Figure 4) of crude reaction product of unwashed cigarette butts, revealed that levulinic acid was obtained with the same high degree of purity of that obtained with unsmoked lters (besides a little solvent residue removable in vacuo).

Data in
These results suggest that that dirt or contaminants of the smoked cigarettes do not interfere with the reaction outcome 17 and that the method is highly selective and doesn't require neither special pretreatment of the starting waste material nor speci c puri cation procedure of the reaction product.
Further aspects that represent crucial advantages for a plausible industrial application of this method concern: i) the possibility of recycling humins wastes through thermal valorisation (burning) or syn-gas production 24 , although more recently they have been used for producing macroporous foam-like materials 25 ; ii) the prompt recycle (by distillation) of Ethyl Acetate used for extracting Levulinic acid; iii) the possibility of recovering water phase by eliminating phosphoric acid through precipitation 26 and COD by Fenton treatments 27 .
A complete process diagram of this protocol is listed in Figure 5. In line with Green Chemistry and Circular Economy principles, an E-factor of 19.08 (about 9 with H 3 PO 4 , but with heat pre-treatment 11 ), very close to that of the pharmaceutical industries and chemical industry 28 , was calculated taking into account that most of material involved can be recycled and valorized such as in the case of humins that represent a new platform for production of mesoporous carbons.

Conclusion
Unprecedented in the literature, cigarette butts can be used as cellulose feedstock for producing Levulinic acid, one of the top value-added intermediates of chemical industry, by means of thermal hydrolysis (at 200 °C) catalysed by phosphoric acidic. The proposed protocol avoids the use of more aggressive H 2 SO 4 and HCl, that are generally use for promoting this transformation from other cellulose sources (e.g. sludge paper), thus minimizing corrosion phenomena of plants. Further bene ts that enable this method to be suitable for industrial applications are the following: reaction doesn't require neither pre-treatment of the starting material nor speci c puri cation procedure of levulinic acid product; the possibility of recycling all the process components, ranging from humins by-products (by thermal valorisation), to the extracting solvent Ethyl acetate (by distillation), until to water phase residue (by Fenton COD abatement); the opportunity of obtaining, by simply modifying acid catalyst (e.g. using CH 3 COOH), another top value-added ne chemical such as 5-hydroxymethylfuraldehyde (HMF), thus widening the scope of the protocol.
Further advantages such as the huge amount of cigarette lters (megatonnes per years) that provide a no cost unlimited source of cellulose, suggest that this protocol marks a signi cant step forward compared to the current literature on this important issue.

Materials And Methods
Materials: Ethyl acetate (>99%) was purchased by Honeywell, Phosphoric acid (85%) and Levulinic acid were purchased from Sigma-Aldrich. All the reagents and solvents were used as received, without any further treatment. GC-MS analyses were run on a Shimadzu GLC 17-A instrument (Shimadzu, MI, Italy) using a SLB-5MS column (30 m x 0.25 mm id, lm thikness 0.25 µm). Mass spectra were performed in EI mode (70 eV) and yields of LA were determined via GC-MS by means of a calibration curve. NMR spectra were recorded on a Bruker 500 MHz spectrometer: 1 H NMR (500 MHz) spectra were referenced to residual isotopic impurity of CDCl 3 (7.25 ppm) and 13 C-NMr (125 MHz) spectra were referenced to 77.00 ppm.
FTIR analyses were carried out on a Perkin-Elmer UATR Two spectrophotometer equipped with a single re ection diamond ATR crystal (refractive index of 2.4). Spectra were acquired with 32 scans in the range 4000 -600 cm −1 by applying both the baseline and the ATR corrections.

Levulinic acid synthesis
Weighed amounts of cigarette butts (250 mg ca. of "Rizla + ultra slim 5,7 mm") were nely chopped in small pieces and suspended into 15 mL of aqueous H 3 PO 4 . Three different concentrations of H 3 PO 4 were explored: 7,5% w/w, 15 % w/w, and 20 % w/w. Each suspension was charged into a 100 mL stainless steel autoclave and heated at temperatures in the range 160 -260 °C for different times (1 -6 hours). After cooling, mixture was ltered and/or centrifugated to separate solid "Humins", that were dried and weighed to give from 20% to 80% of yield (depending on the reaction conditions), while supernatant was extracted with ethyl acetate (2 × 20 mL). Combined organic phases were dried and the solvent removed in vacuo to give levulinic acid as crude oil.
Optimized procedure was then applied to washed and non-washed smoked cigarette butts recovered in Chemistry Department of Bari University, that were previously disinfected under UV rays and mechanically separated by the surrounding paper. A test was also carried out using unsmoked lter and 15 mL of aqueous CH 3 COOH 4M in place of H 3 PO 4 as catalyst. Dirty cigarette butts were washed in 3 cycles with 100 mL of water at 80 °C.

Synthesis of Levulinic Acid on grams scale
To validate the protocol, reaction was repeated on grams scale. At this end, 5 g of unwashed smoked lter were treated, in autoclave, with 300 mL of aqueous H 3 PO 4 15% (w:w) for 2 hours. Mixture was ltered and aqueous solution transferred into a separating funnel and extracted with Ethyl Acetate. The combined organic phases were distillated in vacuum to give 0.85 gr of Levulinic acid, while humins fraction was 1.4 g (corresponding to 28 % w/w respect to the starting waste material).

Calculations and data analysis
Two different yields in Levulinic acid were calculated based on weight of lters and on theoretical amounts of LA. The rst one, was calculated with the ratio Levulinic acid (g) obtained after the reaction/cigarette butts(g) x100 The theoretical maximum yield 11,12 of Levulinic acid is calculated on 250 mg of cigarette butts that contain 245 mg (98% ca.) of cellulose acetate (C.A.). 16 Considering a 2:1 stoichiometric ratio of transformation (a dimeric C.A. unit leads to 2 molecule of Levulinic acid) and that C.A. dimeric unit molecular weight (MW C.A. ) is 492.428 mg/mmol., the C.A. millimoles can be calculated as follows: