, Description:FIELD OF INVENTION
The present invention relates to development of a novel and cost effective transition metal based heterogeneous catalytic process for the production of dioctyl terephthalate (DOTP) from waste polyethylene terephthalate (PET) plastic under reflux conditions. The present invention particularly relates to synthesis of environmentally benign plasticizer namely dioctyl terephthalate (DOTP) from waste PET using a catalyst comprised of transition metals dispersed on biomass derived activated carbon, required for the synthesis of DOTP. More particularly, the present invention relates to the isolation of a specialty chemical (DOTP) from waste PET plastic using a recyclable and non-toxic catalyst under mild reaction conditions.
BACKGROUND OF THE INVENTION
Plasticizers are indispensable for the production of plastics in vinyl industry such as PVC materials. These chemicals are responsible for the flexibility and increased durability of originally brittle PVC. PVC products may contain up to 50 percent by weight of plasticizers. PVC plastics are crucial part of our civilization. Most of the consumer products are made of PVC based materials which includes synthetic leather, shoe soles, hoses, electric cables, infusion material, plastic toys, medical tubing, car undercoating and many more. Plasticizers, used in the vinyl industry, are mainly diesters of dibasic acids and monohydroxy alcohols, and the most common are phthalates. The annual global production of phthalates is estimated to be 15-20 billion pounds (cf. www.zauba.com). The most widely used phthalate is di(2-ethylhexyl) phthalate (DEHP). But the compound has been identified as reproductive and developmental toxicant (cf. R. Mankidy, S. Wiseman, H. Ma, J. P. Giesy, Toxicology Letters 2013, 217, 50–58). The US EPA classifies DEHP as probable and possible human carcinogens. Use of this phthalate is now restricted from children’s products in the US and European Union (cf. Y. Thangam, P. Ranganayaki, International Journal of Science and Research (IJSR), 2017, 6, 2780-2787).
One of the safe alternatives of DEHP is dioctyl terephthalate (DOTP). Unlike DEHP, DOTP is a non-toxic compound and approved by US EPA (cf. Phthalates and their Alternatives: Health and Environmental Concerns, Lowell centre for sustainable production, 2011, http://ec.europa.eu/environment/aarhus/pdf/35/Annex_11_report_from_Lowell_Center.pdf). The potential for bio-concentration of DOTP in living system is low and the compound is likely to be biodegradable under aerobic and anaerobic conditions. The structural difference between DEHP and DOTP diester is the placement of 2-ethylhexyl substitution at 1,2- and 1,4-position, respectively.
Usually the diester DOTP is commercially prepared via esterification of terephthalic acid with 2-ethyl hexanol in the presence of suitable catalyst (cf. S. N. Lakeev, S. G. Karchevskii, I. I. Iskhakov, Russ. (2017), RU 2612302 C1 20170306; J. G. Kim, J. H. Jang, J. S. Moon, Repub. Korea (2016), KR 1663586 B1 20161010; L. Jinqing, F. Guoyang, J. Chunying, S. Yafei, Z. Cough, Q. Cheng, L. Wenjun, Z. Xiaoling, L. Li, C. Ying, Chinese patent (2015) CN102001948 B; L. Guo, Faming Zhuanli Shenqing (2008), CN 101234355 A; Q. Zhang, Z. Jin, L. Liu, S. Sun, Huaxue Shiji (1997), 19(1), 35-36; P. Jiang, X. Gui, Q. Wang, Fushun Shiyou Xueyuan Xuebao (1998), 18(3), 7-12). Alternatively, it can be prepared by the transesterification of dimethyl terephthalate with 2-ethylhexanol using appropriate titanium based catalyst (cf. Y. Wu, C. Xing, X. Xu, Shiyou Huagong (1996), 25(8), 543-546). Both the process uses petroleum based product terephthalic acid or dimethylterephthalate as raw material. The price of the raw materials varies with the fluctuation in the price of crude petroleum.
An alternative source of the raw material is ‘used polyethylene terephthalate (PET) plastic’. PET is widely used thermoplastic polyester made from terephthalic acid and polyethylene glycol. It is used as a raw material in the production of textiles, photographic films, high strength fibers and soft drink bottles. The global demand for PET is continuously growing by more than 4.5% per year. As a result an enormous amount of PET is getting discarded and eventually posing a serious environmental threat. Thus, recycling the discarded PET has been regarded as one of the most important approach to save resources and protect the environment. For the recycling of PET to DOTP usually the polymeric material is hydrolyzed to terephthalic acid using strong acidic or basic condition and then esterified with iso-octanol (cf. M. Sirek, M. Stepanska, US Patent (2003), 664979; M. Sirek, J. Jirousek, Patent (2001) WO200106858A3; D. Spaseska, M. Civkaroska, Journal of the University of Chemical Technology and Metallurgy, 45, 4, 2010, 379-384; J. Das, A. B. Halgeri, V. Sahu, P. A. Parikh, Indian Journal of Chemical Technology, 2007, 14, 173-177; G. P. Karayannidis, A. P. Chatziavgoustis, D. S. Achilias, Advances in Polymer Technology, 2002, 21, 250–259; G. Grause, W. Kaminsky, G. Fahrbach, Polymer Degradation and Stability, 2004, 85, 571-575). Alternatively, PET is transesterified to dimethyl or dibutyl ester of terephthalic acid and a second step transesterification is carried out to prepare DOTP (cf. M.-J. Cruz-Gomez, C. Rodriguez-Martinez, N. Ramirez-de-Arellano-Aburto, US patent (1999), US 5948934; X. Hongwei, Y. Yong, Y. Jili, X. Yuanyuan, L. Yongwang, Chinese patent (2001), CN 1304924 A). Due to the two step process, these methods are costly and low yielding.
There are a few methods for the direct transesterfication of PET to DOTP. Reference may be made to C. Jinyang, L. Feng, X. Tianjiao, R. Ruyi, Chinese patent (2014), CN102617352 B, wherein, DOTP was prepared from scrapped PET plastic waste by hydrolysis at near critical temperature (300-350 °C). The reaction was carried out in a sealed reaction vessel using PET to alcohol ratio 1: 4.5. Another reference may be made to Y. Shitao, L. Shiwei, X. Qingrong, W. Hongxin, Z. Yinzhang, Chinese patent (2011), CN102241592 A, wherein, DOTP was prepared from polyester waste using a titanate catalyst. The reaction was carried out in a sealed tube at 180-240 °C temperature and under 0.03 MPa pressure. Still another reference may be made to X. Shi, P. Jiang, Y. Lu, J. Hu, H. Wang, Huagong Jinzhan, 2008, 27, 143-146, wherein, DOTP was prepared from waste PET using a tetra-butyl titanate as the catalyst. The reaction required use of an aprotic solvent N-methyl-2-pyrrolidone. A reference by G. Zheng, Faming Zhuanli Shenqing (2015), CN 104722248 A, wherein, DOTP was produced from PET using tetra-butyl titanate as the catalyst in a nitrogen sealed reactor at 210-230 °C. A reference by S. Jia, Z. Xu, Y. Ren, X. Chou, Q. Yu, Jingxi Shiyou Huagong, 2004, 6, 1-3, described development of a new solid super acid catalyst which could convert PET to DOTP at temperature 210-220 °C. Apparently, larger amount of 2-ethyl-1-hexanol (4 times) and considerably higher amount of catalyst (12-13 wt%) was required for the process. A process by C. J. Ambrose, Indian (2002), IN 187128 A1, described synthesis of DOTP from polyester film waste using a two step process where first step involved glycolysis of polyester film using ethylene glycol and Ca(OAc)2 as catalyst at 210-270 °C, preferably 230-260 °C and pressures of 0-3 bar. In the second step the glycolyzed PET was trans-esterified in the presence of a Sn catalyst at 200-250 °C, preferably at 210-230 °C and pressures of 1-3 bar.
Considering the titanium based expensive catalysts employed for the conversion of PET to DOTP by other inventors, it is of utmost important to design a catalyst which can overcome the issue. Thus, development of a comparatively milder, eco-friendly, and cheaper heterogeneous catalytic process for the production of DOTP from PET does have enormous scope for commercial application.
OBJECTIVES OF THE INVENTION
The main highlight of the present invention is development of a novel and cost effective transition metal based heterogeneous catalyst to provide a comparatively milder, eco-friendly, and less costly process for the trans-esterification of PET to DOTP.
SUMMARY OF INVENTION
Accordingly the present invention provides a trans-esterification method to terephthalic acid di-iso-octyl ester by using a transition metal based heterogeneous catalyst. Synthesis method of the present invention has a high conversion rate over the said catalyst which is made of low cost, non-toxic metals and can be reused for several times.
The technical points of the present invention are as follows-
The trans-esterification process consists of addition of a catalyst in a mixture of PET and 2-ethyl-1-hexanol in a reaction vessel. PET to 2-ethylhexanol optimum ratio is kept at 1: 2.5 by weight. Catalyst feed is 1 wt% of waste PET plastic. The mixture is refluxed for 5-8 hours while the entire PET disappeared. After separation of the catalyst; excess alcohol, ethylene glycol and DOTP were obtained by vacuum distillation. The catalyst after separation is used for the further reactions.
The beneficial effects of the present invention are as follows:
i) The key for the conversion of the waste PET into DOTP is a transition metal based heterogeneous catalyst which is prepared from low cost, non-toxic and easily available materials as explained above and catalyst loading for PET to DOTP conversion in only 1wt%.
ii) The reaction conditions claimed for the conversion of PET Plastic waste is moderate as compared to reported methods in the literature.
iii) In addition, alcohol used in the reaction is 2.5 times the weight of the substrate i.e. PET waste.
iv) High PET conversion rate, catalyst reusability (upto 6 times), excess alcohol recovery, product isolation through vacuum distillation and pollution-free green approach are salient points of the process.
DETAILED DESCRIPTION OF THE INVENTION
EXAMPLES
The following examples are given by illustrations and should not construe the scope of the invention.
Example-I
Catalyst was prepared by co-precipitation method using a combination of metal salts deposited on biomass derived activated carbon. For the synthesis of catalyst, activated carbon was dispersed in distilled water to make slurry. The slurry was sonicated for 30 minutes and metal salts were added with vigorous stirring. The pH was adjusted to 10-12 by the addition of KOH solution and the precipitated material was filtered. Excess KOH and chlorides from residue were removed by washing with water. Washing was continued until pH ~ 7.0 and chloride content was very little, as tested using AgNO3 solution. The material was dried at 110 °C for 8 hours.
PET waste (flakes/granules/mixture of flakes and granule) was converted to DOTP by reacting with 2-ethylhexanol over the aforementioned catalyst under reflux condition for 5-8 hours. PET bottles were rinsed with water and dried in an oven at 45 ~ 65 °C for 3 – 4 hours. After drying, the bottles were pulverized into a pulverizer to obtain small size PET flakes/granules (length about 2 ~ 4 mm).
For typical reaction 1.0 g of PET flakes, 10 ml of 2-ethylhexanol and 0.1 g of freshly prepared catalyst was taken in a 100 ml round bottomed flask (RB). The mixture was refluxed using heating mantle for 6 hours by connecting a reflux condenser and a guard tube filled with calcium chloride. All the PET flakes got disappeared in 6 h and the reaction was allowed to cool. Catalyst was separated by centrifugation. Filtrate was taken in RB flask (25 mL) and distilled under reduced pressure. In the distillation a mixture of alcohol and ethylene glycol was obtained in the beginning (Fig. 1 & 2) followed by pure DOTP. Conversion of PET flakes was 100% and pure DOTP obtained was 1.33 g. Purity of the product was confirmed by NMR (Fig. 3 & 4). The physical data of waste PET derived DOTP is shown in Table 1.
Example-II
Catalyst recyclability experiments were performed by using 1.0 g of PET flakes, 10 ml of 2-ethylhexanol and 0.1 g of catalyst as described in example-I, taken in 100 ml round bottomed flask (RB). The mixture was refluxed using heating mantle for 6-8 hours by connecting a reflux condenser and a guard tube. After completion of the reaction the catalyst was filtered and used in a similar and fresh reaction. Upto six recycles the conversion of PET flakes was 100% (Fig. 5).
Example-III
1.0 g of PET flakes, 10 ml of 2-ethylhexanol and varying amount (0.01, 0.05, 0.03, and 0.1 g respectively) of catalyst as described in example-I, was taken in a 100 ml round bottomed flask (RB) and refluxed using heating mantle for 6-8 hours by connecting a reflux condenser and a guard tube. Conversion of PET flakes was 100% in all the cases and DOTP obtained was 1.25 to 1.35 g.
Example-IV
10.0 g of PET flakes, 0.1 g of catalyst as described in example-I and different amount (30, 50, and 100 ml respectively) of 2-ethylhexanol was taken in round bottomed flask (RB). The reaction mixture refluxed using heating mantle for 6-8 hours by connecting a reflux condenser and a guard tube. Conversion of PET flakes was 100% in all the cases and DOTP obtained was 12.7 to 13.2 g.
Example-V
50.0 g of PET flakes, 150 ml of 2-ethylhexanol and 0.5 g of catalyst as described in example-I was taken in 1000 ml round bottomed flask (RB). The mixture was refluxed using heating mantle for 7 hours by connecting a reflux condenser and a guard tube. On completion of the reaction, the mixture was allowed to cool and catalyst was separated by decantation. DOTP was separated by distillation under reduced pressure. The mixture of alcohol and ethylene glycol after distillation was 91 ml. Conversion of PET flakes was 100% and obtained pure DOTP was 64 g.
Example-VI
100.0 g of PET flakes, 300 ml of 2-ethylhexanol and 1 g of catalyst as described in example-I was taken in 2000 ml round bottomed flask (RB). The mixture was refluxed using heating mantle for 7 hours by connecting a reflux condenser and a guard tube. On completion, the reaction mixture was allowed to cool and catalyst was separated by decantation. DOTP was separated by distillation under reduced pressure. The mixture of alcohol and ethylene glycol after distillation was 179 ml. Conversion of PET flakes was 100% and obtained pure DOTP was 133 g.
Example-VII
100 g of PET granules (2 - 4 mm), 300 ml 2-ethylhexanol and 1 g of catalyst as described in example-I, was taken in 2000 ml round bottomed flask (RB). The mixture was refluxed using heating mantle for 12 hours by connecting a reflux condenser and a guard tube. On completion, the mixture was allowed to cool and catalyst was separated by decantation. DOTP was separated by distillation under reduced pressure. The mixture of alcohol and ethylene glycol after distillation was 185 ml. Conversion of PET flakes was 100% and obtained pure DOTP was 121.7 g.
Example-VIII
A mixture (8:2 weight by weight ratio) of 100 g of PET flakes and granules (2 - 4 mm), 300 ml 2-ethylhexanol and 1 g of catalyst as described in example-I, was taken in 2000 ml round bottomed flask (RB). The mixture was refluxed using heating mantle for 10 hours by connecting a reflux condenser and a guard tube. On completion, the mixture was allowed to cool and catalyst was separated by decantation. DOTP was separated by distillation under reduced pressure. The mixture of alcohol and ethylene glycol after distillation was 181 ml. Conversion of PET flakes was 100% and obtained pure DOTP was 124 g.
Example - IX
Reaction temperature optimization experiments were performed by reacting a mixture of 3.0 g of PET flakes, 10 ml of 2-ethylhexanol and 0.3 g of catalyst as described in example-I in a 100 ml round bottomed flask (RB). The reaction was performed by heating the reaction mixture at temperature of 80, 120, 140, 180 and 210 °C respectively for 6-7 hours by connecting a reflux condenser and a guard tube. Conversion of PET flakes was 100% for reaction performed at 210 °C with pure DOTP yield of 3.85 g. No conversion was observed for reaction performed below 210 °C.
Table 1: Physical Data of the product (DOTP)
Sr. No. Particulars Standard method applied Standard Specification Our Results
1 Specific gravity at 27°C ASTM D 1045 0.980-0.986 0.9993
2 Water content (wt%) IS 1448 [P-40] 0.1 <0.1
3 Acidity (mgKOH/g) ASTM D 1045 IP-1/94 BS-2000 P-1 0.05 0.01
4 Pour point (°C) IS-1448 [P-10] -50°C < -39°C
5 Kinematic viscosity at 27°C (cst) IS-1448 [P-25] --- 82.0 cst
6 Saponification value (mgKOH/g) IP-136/89 ASTM-D-94-89 B.S.-2000 --- 287
7 Ester Value (mgKOH/g) IP-136/89 ASTM-D-97-89 284±3 285
8 Flash point (°C) IS-1448 [P-20] >210 >179°C
9 Dynamic viscosity at 27°C (cp) ASTM D7042 --- 81.9 cp

FIGURE:
Figure 1: 1H NMR spectra of reaction mixture and corresponding distilled products
Figure 2: 13C NMR spectra of reaction mixture and corresponding distilled products
Figure 3: 1H NMR of dioctyl terephthalate
Figure 4: 13C NMR of dioctyl terephthalate
Figure 5: Reusability of the catalyst

Advantages:
a) The key for the conversion of the waste PET into DOTP is a transition metal based heterogeneous catalyst which is prepared from low cost, non-toxic and easily available materials and catalyst loading for PET to DOTP conversion is only 1wt%. The catalyst is reusable for a minimum of 6 cycles.
b) The reaction process described here operates in lower temperature (~210 °C) and pressure (atmospheric pressure), and uses less solvent (2.5 times the weight of the substrate) and catalyst (1 wt%) compared to reported methods in the literature.
c) While existing industrial process requires pure terephthalic acid as raw material, the process described here uses PET bottles as raw material. Waste PET bottles as raw material is much cheaper and also the process eventually leads to the recycling of waste PET bottles.

Claims:We claim:
1. A heterogeneous catalytic material is comprised of transition metals supported on carbon.
2. The catalytic material as claimed in claim 1, wherein the catalyst is comprised of transition metals such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn dispersed on activated carbon.
3. The catalytic material as claimed in claim 2, wherein the catalyst is comprised of transition metals such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn, more specifically Fe or a mixture of Fe and one of the metals from first transition metal series dispersed on biomass derived activated carbon.
4. The catalytic material as claimed in claim 2, wherein the said catalyst useful for the preparation of dioctyl terephthalate (DOTP) from waste polyethylene terephthalate (PET) comparatively under mild conditions and its recycling.
5. The claimed catalytic system is capable to convert waste polyethylene terephthalate (PET) into dioctyl terephthalate (DOTP) under atmospheric pressure and reflux conditions (at ~210 °C) for a reaction time of around 6-8 hours.