Bifunctional metal-loaded micro-mesoporous zeolites for waste plastics conversion to high quality liquid product

A series of micro-mesoporous zeolitic catalysts synthesized using recrystallization of two different zeolites (HZSM-5 and beta) were impregnated with Pt and other notable metals. The resulting bifunctional catalysts were characterized and tested for the hydrocracking of a model municipal waste plastic mixture. Actual waste plastic mixture of the same composition was also employed for the comparison. The hydrocracking experiments were carried out in a Parr stirred batch reactor at 325, 350, and 375 °C. The other reaction conditions were fixed at the initial cold H2 pressure of 20 bar, reaction time of 60 min, and plastic to catalyst ratio of 20:1 by wt. The inclusion of the Pt metal has shown to increase the hydrocracking activity of the micro-mesoporous supports. The Pt micro-mesoporous ZSM-5 catalysts have shown the better activity while Pt micro-mesoporous beta catalysts have shown better selectivity towards liquid. The presence of Pt metal has also produced a better quality liquid product with fewer unsaturated hydrocarbons and results in lower coke formation as determined by the TGA analysis of the spent catalysts.


Introduction
In the modern world, humans are heavily dependent on plastics. No doubt, plastics have some unique characteristics and find numerous applications, but the waste generated by plastics is becoming a serious threat to the environment. The recycling of all types of plastic waste, at a high rate, is extremely required not only to reduce the waste, but to utilize the potential resource of the wastes [1,2]. In one of the methods of recycling, known as chemical recycling, plastic materials are cracked in the absence of air and often in the presence of a catalyst to transform the waste plastics to more useful gas and liquid products. Hydrocracking of plastics is one such method that occurs in the presence of hydrogen gas. It is one of the most favorable methods of cracking that is exothermic and requires a lower reaction temperature [3,4], produces a high quality liquid product with lower olefinic mass [5][6][7], and inhibits coke formation during the cracking reaction.
A typical hydrocracking catalyst is a bifunctional metalloaded solid acid catalyst [7]. The metal part takes care of the dehydrogenation-hydrogenation reactions while the acidic part, the support, carries out the cracking and isomerization reactions. The metal part not only dehydrogenates the initially produced polymeric segments to olefins that are protonated at the acid sites of the support, but it also hydrogenates the deprotonated product and the coke precursors to maintain the activity of the acid support. Commonly used metals for the hydrocracking catalysts are Pt, Pd, Ni, Co, Mo, W, etc., while the support is most often a zeolite [5,[8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25]. Zeolites are microporous crystalline materials that, owing to their strong acidic character, show a very high cracking ability. However, due to their microporous nature, they provide diffusional resistance [26][27][28][29] to the feed as well as to the product molecules. This may result in the decreased rate of cracking for large molecular feed and significant amount of gases in the product [30]. Mesoporous catalysts, on the other hand, have weak acidity [31,32] and quite a low cracking ability. However, due to the presence of bigger pores in their structure, larger reactant and product molecules diffuse more easily and results in increasing the activity and liquid selectivity of the catalysts. A catalyst that has both the characters of a zeolite (activity and stability) and a mesoporous material (large pore size), a mesozeolite notably a layered zeolite [33,34] or a micro-mesoporous zeolitic material [35], is thus expected to be more active and more selective than both of its respective microporous and mesoporous counterparts.
Surveying the literature, it is found that the bifunctional metal-loaded microporous and mesoporous catalysts [8,10,18,20,[36][37][38] have been largely applied for the hydrocracking of plastic materials. However, no such study is found with metal-loaded composite micro-mesoporous catalysts. A few studies are found where such catalysts have been applied for the hydrocracking of alkanes and are therefore needed to be discussed below. Wang et al. [39] prepared Pt incorporated micro-mesoporous core-shell ZSM-5 catalysts for the hydrocracking of n-hexadecane. Compared to the conventional PtZSM-5 catalyst, they observed increase in both the conversion and the C5-C11 selectivity over the micro-mesoporous form of the Pt-ZSM-5. Yin et al. [40] also synthesized a micromesoporous core-shell catalyst, but with Y zeolite. They loaded Ni and W metals over the support and used the catalyst for the hydrocracking of n-decane. They found that the hydrocracking reaction is easier to carry out over the metal-loaded micro-mesoporous catalyst. Hydrocracking of n-decane is also studied by Huyen et al. [41]. They used PtZSM-5-SBA-15 and Pt/Al-SBA-15 catalysts and reported that PtZSM-5-SBA-15 is a better hydrocracking catalyst. Azkaar et al. [42] also studied hydrocracking of n-hexadecane, but with Ru/micro-mesoporous Y zeolite. They have found that modifying micro-mesoporous zeolite Y with Ru behaves better than without Ru. Imyen et al. [43] studied the introduction of mesoporosity in ZSM-5 and used Pt/meso ZSM-5 and conventional PtZSM-5 to compare the hydrocracking reaction of n-hexadecane. They found a substantial increase in the catalytic activity for the Pt mesoZSM-5 than PtZSM-5 and reasoned that it might be due to the small size and well distribution of the Pt particles over the surface of the former catalyst.
In our previous studies, aluminum-modified mesoporous catalysts [23,44,45] and micro-mesoporous catalysts of USY, beta, and HZSM-5 zeolites [23][24][25] were tested for the hydrocracking of plastic mixtures. Among the catalysts tested, micro-mesoporous zeolite beta and zeolite HZSM-5 catalysts showed highly favorable results with respect to both the activity and the selectivity towards liquid product. These results have led us to work with the metal-impregnated form of these catalysts, which are expected to perform even better in a hydrocracking reaction. Pt metal as it is a highly active metal for the hydrocracking reaction is loaded on five micro-mesoporous supports of zeolite ZSM-5 and zeolite beta. Moreover, being cheaper than Pt metal, combinations of other metals that are common in industrial hydrotreating and hydrocracking catalysts are also tried to compare their hydrocracking performance with Pt. To the best of our knowledge, the metal-loaded micro-mesoporous catalysts have never been tested for the hydrocracking reaction of a plastic material.

Catalyst preparation
Five micro-mesoporous cracking catalysts were used as supports and were impregnated with metals to prepare bifunctional metal-loaded micro-mesoporous catalysts. The five catalyst supports used for the impregnation are shown in Table 1. The detailed synthesis procedures for the supports are provided in Munir et al. [24] and Munir and Usman [25].

Monometallic (Pt-loaded) catalysts
The micro-mesoporous catalysts were impregnated with 0.5wt% platinum. A calculated amount of tetraammineplatinum(II) chloride hydrate was dissolved in 40 ml of water. 5 g of catalyst support was then mixed with the solution and allowed to stir vigorously for 15 min at 40 °C. NH 4 OH solution was then added dropwise so that the pH of the solution reached 9. Afterwards the solution was stirred for 1 h at 40 °C. The solution was then allowed to equilibrate for additional 24 h at room temperature. The product was rinsed with doubly distilled water, dried in an oven at 100 °C, and clacined at 500 °C for 4 h.

Bimetallic and trimetallic impregnated catalysts
One of the micro-mesoporous HZSM-5 catalysts, ZC-FP, was impregnated with additional metals. The catalytic support was co-impregnated with 0.5wt% of platinum and 0.3wt% of palladium to produce 0.5%Pt-0.3%PdZC-FP. Tetraammine platinum(II) chloride hydrate and tetraammine palladium(II) chloride monohydrate were used for the impregnation of Pt and Pd. Additionally, 0.3%Pd-0.7%RuZC-FP was prepared using tetraamminepalladium(II) chloride monohydrate and ruthenium(III) chloride hydrate; 10%Co-0.7%Ru/ZC-FP using cobalt(II)nitrate hexahydrate and ruthenium(III) chloride hydrate; and 7%Ni-12%Mo-15%WZC-FP using nickel(II) nitrate hexahydrate, ammonium molybdate tetrahydrate, and ammonium meta tungstate hydrate. In all of these bi-and tri-metallic impregnated catalysts, the same procedure of impregnation was adopted as that used for the monometallic platinum impregnated catalysts. All the synthesized catalysts are shown listed in Table 1.
The above mono-, bi-, and trimetallic supported catalysts were reduced ex-situ in the presence of hydrogen flow before using them for the hydrocracking reaction. A catalyst to be reduced was placed in a glass tube which was located inside a tubular furnace. The temperature of the furnace was increased up to 500 °C and the reduction of the catalyst was carried out under hydrogen flowrate of 100 ml/min for 2 h. The temperature of the furnace was then decreased to the room temperature under hydrogen atmosphere (to avoid oxygen chemisorption at high temperature) and the reduced catalyst was then used in the autoclave reactor for the hydrocracking reaction.

Catalyst characterization
The metal impregnated catalysts were subjected to the important characterization techniques, namely, N 2 -BET analysis, wide-angle XRD, TEM, and TGA for the spent catalysts. N 2 -BET analysis was performed by Micromeritics TriStar II-3020. Prior to the analysis, the sample was degassed using Micromeritics Smart Prep (Programmable Degas System) at 200 °C for 2 h under vacuum. PANalytical X'Pert diffractometer was available for the wide angle XRD analysis. For quality of diffraction, a fine powdered sample was used for an XRD analysis. For detailed morphology of the catalysts, TEM images were obtained by Joel (JEM-2100F) field emission transmission electron microscope. Before the imaging, a pulverized sample was dispersed in ethyl alcohol and tiny volume of the suspension was seated on a copper grid and evaporated subsequently. Shimadzu TGA-50 was used for thermogravimetric analysis of spent catalysts to evaluate the coke deposited on their surface during the hydrocracking reaction. The dried spent catalysts were used and thermal effects were studied up to 600 °C. The micro-mesoporous supports were characterized in even more detail as given in our previous studies [24,25].
Catalytic experiments were performed at three temperatures (325 °C, 350 °C, and 375 °C) and at a fixed initial cold pressure of 20 bar H 2 . A high pressure 500 ml batch stirred reactor manufactured by Parr Instrument Co. was used for this purpose. The extended details of the experimental system can be found in Munir [46]. In each experiment, initially the reactor was loaded with 10 g plastic and 0.5 g catalyst and a 20 bar hydrogen pressure was applied at cold conditions. After that the contents were heated at the rate of 4.6 °C/min and the temperature was raised to the reaction temperature where a 60 min residence time was provided. At the end of reaction, the heater was immediately turned off and removed from the reactor vessel. The vessel was allowed to cool down to the lab temperature and the obtained products were analyzed. Figure 1 shows the analysis scheme adopted for the analysis of the reaction products. The contents of the reactor were undergone solvent Table 2 Structural properties of the supports and the metalloaded hydrocracking catalysts Data for supports (catalysts without metal loading) is taken from our previous studies [24,25] Percentage metal loading is as synthesized a surface area by using BET method, b obtained by t-plot, c S BET -S micro , d v micro + v meso , e BJH adsorption method

Catalyst
Zeolite base S BET a (m 2 /g) extraction by n-heptane (≥ 99.0% purity, Sigma-Aldrich) and tetrahydrofuran (THF, ≥ 99.0% purity, Sigma-Aldrich), to obtain n-heptane soluble fraction and the total "liquid yield". The part of the total liquid, soluble in n-heptane was called as "oil yield". The THF insoluble content that may contain unconverted plastic, any coke formed, and the spent catalyst was subjected to drying in an oven at 110 °C. The dried solid obtained was weighed and the conversion of the hydrocracking reaction was based on this dried solid residue. The amount of the gas formed was measured by knowing the mass of the reactor at the end of an experiment after removing the gases and the mass of the reactor at the start of the experiment, before introducing the H 2 gas. The n-heptane soluble fraction, i.e., oil fraction was analyzed in GC-FID (Shimadzu GC-2014) equipped with a 30 m long and 0.25 mm inner diameter capillary column (Agilent DB-1MS). The yields of C5-C12 (gasoline), C13-C18 (diesel), and C19 + fraction were obtained by gas chromatography.
Additionally, FTIR analysis of the n-heptane soluble fraction was also carried out primarily to detect the presence of alkenes in the reaction products.

Characterization of catalysts
The structural properties of Pt impregnated catalysts from nitrogen physisorption study are shown in Table 2. The highest surface area is found in PtBC27 followed by PtBC48. It can be seen that after the platinum impregnation, compared to their respective supports, the surface area of all the five catalysts is decreased. However, the decrease in surface area is more prominent in the two composite catalysts of zeolite beta, i.e., PtBC27 and PtBC48. There is a considerable loss in the surface area compared to the HZSM-5 composite catalysts, i.e., PtZC-F, PtZC-FP, and PtZC-P. Along with the surface area, PtBC27 and PtBC48 both exhibit high BJH volume and between the two, the highest BJH volume is present in PtBC48 indicating both these beta catalysts have the highest mesoporous content. PtZC-F, PtZC-FP, and PtZC-P exhibit higher micropore volume with smaller BJH volume.
In these three catalysts, the micropore volume remains intact when compared with their corresponding supports without Pt impregnation. However, the BJH volume of these catalysts is greatly reduced. On the other hand, in PtBC27 and PtBC48, there is a great reduction of microporous volume after platinum impregnation. The reason for these observations corresponds to the effect of basic pH that was maintained during the platinum impregnation procedure. Mesoporous volume may be greatly reduced due to the basic pH of the impregnated solution. Recalcination of the catalysts when impregnated which may cause sintering and the presence of Pt crystallites within the micropores may contribute to the decrease in the micropore volume of the catalysts. Figure 2 shows the Transmission Electron Microscopic (TEM) images of the five Pt-loaded catalysts. Additional images of these catalysts are shown in Fig. S-1, in the electronic supplement. In each micrograph, the dark and distinct particles laid over the surface of the support are the Pt metal particles. Clearly, in each case, the particle size of the Pt metal is of nano-sized and roughly ranges between 10 to 100 nm. Also, the particle shape varies and there are cubic and polyhedral shaped particles. Additionally, the particles are observed to be well dispersed and rather uniformly distributed. High resolution views of the Pt particles are shown in Fig. S1 (d-e). Unlike the arrangement of the lattice fringes in PtBC27 (Fig. S1e), the arrangement of the lattice fringes in Pt/BC48 (Fig. S1d) shows that there are at least two different directions of the lattice fringes. A change in fringes direction may indicate different facets of the particles. The Pt particles have various facets as can be seen from Fig. 2d. By looking at the supports surface only, it is apparent that ZC-F, ZC-FP, and ZC-P supports (synthesized from ZSM-5 zeolite) have shown the presence of ordered mesoporous phase. A more detailed TEM analysis of these supports is provided in our previous study [25]. On the other hand, for the supports synthesized using zeolite beta, i.e., BC27 and BC48, ordered mesoporous phase is not observed by the TEM analysis. Figure 3 depicts the wide angle X-ray diffraction patterns of the catalysts. It is found that all the five Pt catalysts (Fig. 3B) show a crystalline structure suggesting that the crystalline structure of the supports (Fig. 3A) is retained after the impregnation with platinum metal. The appearance of three new small peaks though small at 2θ of around 40, 67, and 82° may show the presence of Pt crystallites on the surface of the catalysts. The three new peaks are more prominent in HZSM5 based mesoporous catalysts than those of beta based catalysts. For BC27, the impregnated species are found to be amorphous or their crystal size is too small (less then 4 nm) to be detected by XRD analysis [47].

Mono-metallic Pt-loaded catalysts
The hydrocracking performance of the platinum impregnated catalysts for the model plastic mixture is shown in Fig. 4. At the lowest reaction temperature of 325 °C, it is observed that the maximum conversion of 69.83% is produced by PtZC-FP while PtBC27 provides the lowest conversion. The conversion is decreased in the order of PtZC-FP˃PtZC-F˃PtZC-P˃ PtBC48˃PtBC27. Although, PtBC27 and PtBC48 both yielded the least conversion, but they produced quantities of liquid products (~ 22%) equivalent to the other catalysts with extremely reduced amount of gaseous products. This shows their enhanced ability to convert model plastic mixture to liquid products. On the other hand, PtZC-FP, PtZC-F, and PtZC-P though showing increased conversion, but they are more selective towards gaseous products.
At 350 °C, the maximum conversion of 88.90% is exhibited by PtZC-P which is followed by PtZC-F and PtZC-FP. Again, PtBC48 and PtBC27 showed the least conversion, but increased selectivity towards liquids with decreased selectivity towards gaseous products. The highest liquid yield (53.68%) and the lowest gas yield (13.75%) are produced by PtBC27 that is followed by PtBC48. However, PtZC-FP, PtZC-F, and PtZC-P again delivered high conversions, but at the cost of increased gas yields. This PtZC-F, c) PtZC-P, d) PtBC48, and e) PtBC27. Data for A (catalysts without Pt loading) is taken from our previous studies [24,25] is especially true for Pt/ZC-P which yielded the maximum gaseous products.
At 375 °C, every catalyst showed a very high conversion. Apart from PtBC27 and PtBC48, all the other catalysts offered 100% conversion. PtBC27 produced the highest liquid quantity equals to 70.48wt% closely followed by PtBC48. PtBC48 yielded the least gaseous products. The catalysts PtHZSM-5, PtZC-F, PtZC-FP, and PtZC-P were highly selective towards gas yield and produced the least quantity of liquid products.
As discussed above, at each reaction temperature studied, PtZC-P, PtZC-FP, and PtZC-P produced much higher gas yield as compared to PtBC27 and PtBC48. Structural properties of these catalysts in Table 1 show that PtZC-P, PtZC-FP, and PtZC-P have much less BJH volume as oppose to PtBC27 and PtBC48. These catalysts have smaller mesoporous content and mostly comprised of micropores. Due to the presence of majority of the micropores in these catalysts they have produced increased gaseous products. On the other hand, PtBC27 and PtBC48 are highly selective towards liquid yield due to the presence of higher mesoporous content in them. This idea is further strengthened by the fact that PtHZSM-5, a microporous catalyst, produced the highest gas yield, much higher than any of its composite catalysts at 375 °C reaction temperature. An increased activity of PtZC-P, PtZC-FP, and PtZC-P catalysts may also the reason for the more gas yield and less liquid yield over these catalysts.
Overall with increase in temperature, the conversion over each of the catalysts is increased. For PtZC-F, PtZC-FP, and PtZC-P catalysts, the gas yield increases with an increase in temperature whereas the liquid yield is only slightly varied. For PtBC27 and PtBC48, the liquid yield increased with an increase in temperature, but the gas yield for the case of PtBC48 was first increased and then virtually remained constant.
When comparing the Pt-loaded catalysts with catalysts with no Pt impregnation (respective supports), in Fig. 4 (375 °C), it is observed that all the platinum impregnated catalysts showed higher activity. Significantly increased selectivity towards gases and reduced selectivity of liquids are obtained over PtHZSM-5 and its platinum impregnated composites compared to their corresponding catalysts without Pt impregnation. At 350 °C (Fig. S2), it can be seen again that the addition of Pt has greatly increased the activity of the catalysts and therefore the hydrocracking reaction Fig. 4 Results of the hydrocracking of model plastic mixture over micro-mesoporous catalysts with and without Pt impregnation. 500 ml autoclave reactor, 20 bar initial cold hydrogen pressure, 60 min residence time, and 20:1 feed to catalyst ratio (by weight). Data for supports (catalysts without Pt loading) is taken from our previous studies [24,25] can be carried out at a much lower reaction temperature if a Pt-impregnated micro-mesoporous support will be used.
Gas chromatography analysis of n-heptane soluble fraction, over the platinum impregnated catalysts, at three reaction temperatures is displayed in Fig. 5. Each catalyst shows adequate selectivity towards gasoline content. At 325 and 350 °C reaction temperatures, the highest gasoline fraction is produced by PtZC-P whereas at 375 °C the highest gasoline fraction is obtained with PtBC27. The PtZC-P catalyst exhibits the highest microporous volume than the other catalysts. These micropores facilitated the lower molecular weight molecules of gasoline to leave the active site which might be the cause for an increased gasoline content at lower temperatures. It is observed that compared to the other Pt-impregnated catalysts though PtBC27 and PtBC48 are found highly selective towards liquid yield, but their selectivity towards gasoline is generally not better than the other catalysts.
Comparing the gasoline and diesel yields at 375 °C for the catalysts with and without Pt impregnation (Fig. 5), it is observed that all the catalysts without Pt impregnation produced higher gasoline fraction compared to their corresponding platinum impregnated catalysts.
The FTIR spectra of n-heptane soluble liquids obtained by hydrocracking of model plastic mixture at 375 °C over the catalysts with and without Pt impregnation are shown in Fig. 6. It is found that for all the catalysts there exists a choppy peak at 2900 cm -1 . This corresponds to the presence of alkanes in the liquid samples. Among the catalysts without impregnation, Fig. 6A, the highest amounts of alkanes are observed with BC27. However, for platinum impregnated catalysts, Fig. 6B, the most alkanes are found with PtBC48. All the catalysts without impregnation show a small peak at 1600 cm -1 attributed to the presence of C=C bond stretching. The highest amount of unsaturated compounds is produced by BC27 and the lowest unsaturated compounds are produced by ZC-FP. Among the platinum impregnated catalysts, the highest unsaturated compounds exist in PtZC-P. All the other catalysts have negligible quantities of unsaturated compounds. The characteristic sharp peak between 1450 and 1600 cm -1 in all the catalysts indicates the presence of aromatic double bond stretch. For the catalysts without impregnation, this peak is the highest with Fig. 5 Results of GC analysis of n-heptane soluble liquids obtained by the hydrocracking of model plastic mixture over micromesoporous catalysts with and without Pt impregnation. 500 ml autoclave reactor, 20 bar initial cold hydrogen pressure, 60 min residence time, and 20:1 feed to catalyst ratio (by weight). Data for supports (catalysts without Pt loading) is taken from our previous studies [24,25] BC27 therefore showing the maximum aromatic content produced by BC27. However, this peak is relatively weak with ZC-FP and BC48. Among the platinum catalysts the highest aromatic content is present with PtBC48 and the least amount of aromatics is present with PtBC27.
When compared, it is observed that the oils obtained over platinum catalysts contain more alkanes and less unsaturated components than that of the oils from their corresponding catalysts with no Pt impregnation. Generally, all the catalysts after the platinum metal impregnation produced higher amounts of alkanes, less aromatic compounds, and negligible unsaturated compounds. This was expected due to hydrogenating ability of Pt metal.

Bi-and trimetallic supported catalysts
The hydrocracking ability of ZC-FP was further tested by impregnating different combination of metals. The results of the hydrocracking experiments are shown in Fig. 7.
It is observed that at the lowest reaction temperature PtZC-FP has been found as the most active catalyst. The highest conversion and gas yield of 69.82 and 42.67%, respectively, are produced by PtZC-FP. The second most active catalyst is still a Pt containing catalyst, however, the inclusion of Pd with Pt has considerably improved the liquid selectivity with an added benefit of reduced gas selectivity. CoRuZC-FP catalyst is the least active catalyst whereas the performance of PdRuZC-FP catalyst and NiMoWZC-FP catalyst is virtually comparable. It is observed that compared to the reactions at lower temperatures, the gas selectivity increased more rapidly than the liquid selectivity at higher temperatures. This may be due to the addition of hydrogenation-dehydrogenation function which has increased the activity of the catalysts by increased dehydrogenation, at high temperature, of the initially formed polymeric fragments. It is important to note that dehydrogenation reaction rate increases with temperature and thereby generates more alkenes which are cracked at higher rates than their corresponding alkanes.
At 350 °C PdRuZC-FP catalyst has shown the highest activity though not much different than PtPdZC-FP catalyst. Both of these catalysts along with NiMoWZC-FP have delivered nearly the same liquid yields of about 32.0%. At the highest temperature of 375 °C, it is observed that PtZC-FP shows 100% conversion closely followed by all the other catalysts. CoRuZC-FP catalyst has shown the highest selectivity towards liquid (43.13%) and also provided the least amount of gaseous products. NiMoWZC-FP catalyst has shown the least conversion, least liquid and oil selectivities, and very high gas yields. At this temperature, PtZC-FP, PdRuZC-FP, and NiMoWZC-FP catalysts are probably not the suitable catalysts for the hydrocracking of plastics.
At all the temperatures the conversion and gas yield over each of the catalyst is considerably enhanced with increase in temperature. For PtZC-FP catalyst, the gas yield is always on the higher side and for CoRuZC-FP there is a substantial increase in liquid yield with increase in temperature. At 325 °C PtPdZC-FP is found the most suitable catalyst whereas at the highest temperature CoRuZC-FP has shown the potential to be the most suitable catalyst for the hydrocracking reactions.
The GC analysis of the n-heptane soluble liquids obtained at 375 °C is shown in Fig. 7D. It is found that the highest gasoline selectivity is obtained by PtZC-FP. Although, CoRuZC-FP catalyst yielded the highest liquid amount at 375 °C, but the gasoline percentage in the liquid product is found less than that obtained with PdRuZC-FP and PtZC-FP catalysts.

Actual waste plastic mixture
Activity and selectivity of PtBC27 is also evaluated by using it with actual waste plastic mixture and the results are compared to the performance of the catalyst with model plastic Fig. 6 FTIR spectra of n-heptane soluble liquids obtained by the hydrocracking of model plastic mixture over: A) Catalysts without Pt impregnation, B) Catalysts with Pt impregnation. 500 ml autoclave reactor, 20 bar initial cold hydrogen pressure, 60 min residence time, 375 °C reaction temperature, and 20:1 feed to catalyst ratio (by weight). Data for A (catalysts without Pt loading) is taken from our previous studies [24,25] mixture under the same experimental conditions, Fig. 8. PtBC27 was selected owing to its striking ability to produce liquid product. It is found that actual waste plastic mixture resulted relatively lower conversion, lower gas yield, and lower liquid yield compared to the model plastic mixture.
The presence of heteroatoms and trace metals due to stabilizers, colorant, etc., might have interfered with cracking reactions with actual waste plastic mixture that ultimately resulted in reduced conversion and reduced gas yield. As there is not a significant reduction in conversion and liquid Fig. 7 Results of the hydrocracking experiments with model plastic mixture over various metal impregnated catalysts. 500 ml autoclave reactor, 20 bar initial cold hydrogen pressure, 60 min residence time, and 20:1 feed to catalyst ratio (by weight) yield when PtBC27 is used with actual waste plastic mixture, it is recognized that PtBC27 could be an appropriate catalyst for the hydrocracking of waste plastics. The gas chromatography analysis of the oils obtained over model plastic mixture and actual waste plastic mixture using PtBC27 is shown at the bottom of Fig. 8. It is observed that PtBC27 has produced high concentration of gasoline range components with both types of feedstocks. With actual waste plastic mixture it has given slightly decreased gasoline yield. 68.16% of gasoline is obtained with model plastic mixture compared to 65.76wt% with actual waste plastic mixture. These findings further suggest PtBC27 to be an appropriate catalyst for the hydrocracking of waste plastics.

Thermogravimetric analysis (TGA) of the spent catalysts
The spent or used catalysts obtained in the hydrocracking of model plastic mixture at 375 °C are examined by TGA. TGA is a temperature programmed technique in which the weight loss of a material is studied by change in carefully controlled temperature. Figure 9 shows the TGA results of the spent catalysts with and without Pt impregnation. For clarity of the findings, the TGA results are also summarized in Table 3. For each catalyst three clear thermal regions are observed. The weight change of the catalyst from 30 to 300 °C corresponds to the desorption of the reactants/ products, adsorbed in the pores of the catalysts [48]. The next most drastic weight loss centered at around 300 to 450 °C and another less significant weight lose from 450 to 600 °C attributed to the decomposition of coke deposited on the catalyst pores. The coke decomposition at two temperature ranges indicates the presence of the two different types of coke deposited in the catalysts pores such as soft coke and hard coke produced perhaps due to the deposition of paraffinic and aromatic coke deposition on the catalysts [49].
It is found that all the catalysts without impregnation produced significantly higher coke compared to their corresponding platinum impregnated catalysts. This indicates the higher hydrogenation ability of the platinum impregnated catalysts that inhibits the coke deposition Fig. 8 Comparison of the hydrocracking experiments with model plastic mixture and with actual waste plastic mixture over PtBC27. 500 ml autoclave reactor, 20 bar initial cold hydrogen pressure, 60 min residence time, 375 °C reaction temperature, and 20:1 feed to catalyst ratio (by weight) Fig. 9 Results of TGA with the spent catalysts resulted from the hydrocracking of model plastic mixture conducted at 375 °C reaction temperature, 500 ml autoclave reactor, 60 min of residence time, initial cold hydrogen pressure of 20 bar, and 20:1 feed to catalyst ratio (by weight): A) Without Pt impregnation, B) With Pt impregnation. Data for A (catalysts without Pt loading) is taken from our previous studies [24,25] by hydrogenating the unsaturated coke precursor species adsorbed on the surface of the catalysts.

Conclusion
All the catalysts showed a reduction in the surface area after platinum impregnation. It is observed that after platinum loading, the catalysts showed higher conversion and produced higher gas yield than that of their corresponding catalysts without impregnation under the same reaction conditions. However, the increase in gas yield was more drastic in the case of PtHZSM-5 and its composites catalysts (PtZC-F, PtZC-FP, PtZC-P), whereas for PtBC27 and PtBC48 there was a minor increase in gas yield with significant increase in the liquid yield. GC analysis showed that platinum impregnated catalysts produced lower gasoline with higher light diesel and heavy diesel components. Among the five platinum impregnated catalysts PtBC27 is found the most promising catalyst with respect to the liquid yield as it offered the highest liquid yield at 350 and 375 °C and the least gas yield at all the reaction temperatures studied. GC analysis of oil obtained over this catalyst also showed the higher selectivity of this catalyst towards gasoline. Liquid FTIR of the oil showed that the quality of the oil was good over PtBC27 with lower aromatics and negligible unsaturated compounds than that of the other catalysts. PtBC27 was also tested with actual waste plastic mixture. With waste plastic mixture virtually similar performance was shown by the catalyst as that observed with model plastic mixture. Comparing impregnation of ZC-FP with Pt and with other metals showed that generally Pt-PdZC-FP is found a better catalyst than PtZC-FP with regards to higher liquid selectivity with virtually the same conversion.