The pyrolysis of bisphenol A (BPA), an essential process ingredient used in industry và many everyday life products, helps produce low-industrial-demand chemicals such as isopropenyl- & isopropyl-phenols (IPP và iPrP). In this study, tandem micro-reactor gas chromatography/mass spectrometry combined with an H2 generator (H2-TR-GC/MS) was employed for the first time lớn investigate the selective recovery of phenol via simultaneous hydrogenation/dealkylation of IPP and iPrP. After investigating the iPrP dealkylation performances of several zeolites, we obtained full iPrP conversion with over 99% phenol selectivity using the Y-zeolite at 350 °C. In contrast, when applied khổng lồ IPP, the zeolite acid centres caused IPP polymerisation and subsequent IPP-polymer cracking, resulting in many byproducts and reduced phenol selectivity. This challenge was overcome by the addition of 0.3 wt% Ni on the Y-zeolite (0.3Ni/Y), which enabled the hydrogenation of IPP into iPrP & subsequent dealkylation into phenol (full IPP conversion with 92% phenol selectivity). Moreover, the catalyst deactivation & product distribution over repetitive catalytic use were successfully monitored using the H2-TR-GC/MS system. We believe that the findings presented herein could allow the recovery of phenol-rich products from polymeric waste with BPA macro skeleton.
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Introduction
Polycarbonate (PC) is currently the largest consumer of bisphenol A (BPA). Therefore, pyrolysis of PC waste has been widely studied1,2,3 because it allows the recovery of oil và gas from polymeric waste4,5. This, by itself, represents a significant advantage khổng lồ treating polymeric waste combined with other resins & organic additives, which cannot be treated by mechanical recycling & solvolysis6,7,8,9,10,11,12,13. However, there are several drawbacks associated with this approach. First, this method generates a mixture of various products like phenol, 4-isopropenylphenol (IPP), 4-isopropylphenol (iPrP) & other alkyl phenols14,15,16, some of which are important chemical feedstock that cannot be properly utilised due lớn their difficult purification. Secondly, the oil obtained through this approach contains oxygen as part of the phenolic compound, which reduces the calorific value when used as a fuel. Even though IPP & iPrP are the major products in this case, they have very low industrial demand. Moreover, these issues are also observed with other BPA-skeletal polymers such as epoxy resins, polysulfones, bismaleimides, triazines, polyarylates, và BPA-type flame retardants. Nevertheless, the global demand for BPA should increase annually17 due to lớn the rapid worldwide growth of automotive, construction, & electrical and electronic application markets. Therefore, it is crucial that new methods are developed for the recovery of useful chemicals from polymeric wastes with BPA skeleton via the pyrolytic approach.
Various researches have investigated catalytic pyrolysis using earth-alkali oxides, hydroxides & zeolites14,15,16,18,19. Among them, Grause et al.18 achieved the largest BPA yield of 91% at 300 °C in the presence of steam and MgO. However, further decomposition of BPA lớn phenol, IPP, and iPrP is unavoidable during pyrolysis because of the high temperatures required for this process14,15,16,18,19,20. Thus, the purity of BPA recovered via pyrolysis of PC waste is too low for it khổng lồ be used as a process ingredient in the production of resins & flame retardants. Moreover, considering the widespread demand for phenol as a petrochemical feedstock for resins, agrichemicals, and medicinal chemicals, it is more industrially useful to lớn let the low-purity BPA further decompose to lớn phenol than to employ it as obtained.
One challenge that must be overcome to achieve selective phenol recovery from BPA via the pyrolytic approach is the conversion of iPrP và IPP into phenol. In this work, we focused on the use of zeolites as inexpensive heterogeneous shape-selective catalysts capable of participating in gas-solid reactions. This is an important step forward for the field of process technology. The applicability of zeolites to lớn different reactions including alkylation21,22,23,24,25, dealkylation24,25,26,27,28,29,30, và transalkylation28,29,30,31,32,33,34,35 of (alkyl)benzenes, such as benzene, toluene, ethylbenzene, xylene, and trimethylbenzenes, has been widely studied. These reactions have been mainly investigated using medium-pore zeolites such as ZSM-5 & MCM-22, as well as large-pore zeolites such as Y-zeolite, β-zeolite, mordenite, and faujasites. Apart from the pore size, these types of zeolites also differ in terms of the type and strength of their acid sites. Moreover, Pradhan and Rao36 reported that large-pore zeolites such as mordenite, Y-zeolite, và β-zeolite allow for the transalkylation of the isopropyl group of diisopropyl benzene, thus yielding benzene.
To the best of our knowledge, the dealkylation of alkyl phenol has only been reported by Verboekend et al.37, who investigated the dealkylation of n-propyl phenol, as a representative of depolymerised lignins and coal, using HZSM-5. Therefore, the knowledge on the dealkylation of alkyl phenols is not well established, và research on the dealkylation of iPrP và IPP remains scant. Furthermore, it is assumed that the direct dealkylation of IPP would be an unfavourable và complicated process because the reactive isopropenyl unit in IPP triggers the polymerisation of the IPP molecules at pyrolytic temperatures, thus enhancing product diversification20.
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Therefore, we devised a process to lớn selectively recover phenol. This included the hydrogenation of IPP into iPrP and the subsequent dealkylation of iPrP into phenol on Ni-loaded zeolites under H2 purge flow (Fig. 1). Ni is known lớn be a rather inexpensive hydrogenation catalyst of alkyl C=C bonds38,39,40,41. A number of protocols for the hydrogenation/hydrodeoxygenation procedures of (alkyl)benzenes & phenolic compounds, using Ni-supported catalysts in pressurised batch systems42,43,44,45,46, as well as continuous high-pressure H2 flow systems and flow systems under atmospheric pressure47,48,49,50,51,52, have been reported. In all these cases, cyclohexane & cyclohexanone are the final products from the deep hydrogenation of aromatic rings or removal of oxygen-containing compounds in sản phẩm oil by hydrodeoxygenation. Therefore, the selective hydrogenation of the isopropenyl unit and subsequent dealkylation, while preventing the hydrogenation of both the phenol group and the benzene ring on the single catalyst, would be a novel & challenging approach.