Process for the preparation of supported heteropolyacid catalysts

Abstract

A process for the preparation of a supported heteropolyacid catalyst comprises contacting the support with a solution of the heropolyacid under conditions such that the formation of a sticky support / solution mixture is avoided. The technique is very efficient and makes it possible to produce a catalyst that can be used in a catalytic depolymerization process.

Description

PROCESS FOR THE PREPARATION OF SUPPORTED HETEROPOLYACIDCATALYSTSFIELD OF THE DISCLOSURE

[0001] This disclosure relates to a process for the preparation of a catalyst comprising heteropolyacid compounds supported inorganic support and to a method for depolymerizing plastic feedstock in the presence of supported heteropolyacids catalysts.BACKGROUND OF THE DISCLOSURE

[0002] Plastics are inexpensive and durable materials, which can be used to manufacture a variety of products that find use in a wide range of applications, so that the production of plastics has increased dramatically over the last decades. Due to the durability of the polymers involved in plastic production, an increasing amount of plastics are filling up landfill sites and occupying natural habitats worldwide, resulting in environmental problems. Even degradable and biodegradable plastics may persist for decades depending on local environmental factors, like levels of ultraviolet light exposure, temperature, presence of suitable microorganisms and other factors.

[0003] Currently plastic recycling primarily includes mechanical recycling and chemical recycling. Globally speaking, mechanical recycling is the most used method for new uses of plastics, and through this method, plastics are mechanically transformed without changing their chemical structure, so they can be used to produce new materials. Typical mechanical recycling steps include collecting plastic wastes; sorting plastic wastes into different types of plastics and colors; packaging plastics by pressing or milling plastics; washing and drying the plastics; reprocessing the plastics into pellets by agglutinating, extruding and cooling the plastics; and finally recycled raw materials are obtained. This is the most widely used technology for the polyolefins like polyethylene (PE) and polypropylene (PP).

[0004] Chemical recycling, on the other hand, reprocesses plastics and modify their structure so that they can be used as raw material for different industries or as a basic input or feedstock for manufacturing new plastic products. Chemical recycling typically includes the steps of collecting plastics, followed by heating the plastics to a temperature at which the polymers break down into small fragments. This process, also called depolymerization, is a basic process whereby plastic waste material is converted to liquid fuel by thermal degradation(cracking) in the absence of oxygen. Plastic waste is typically first melted within a stainless steel chamber under an inert purging gas, such as nitrogen. This chamber then heats the molten material to a gaseous state that is drawn and then condensed in one or more condensers to yield a hydrocarbon distillate comprising straight and branched chain aliphatic, cyclic aliphatic and aromatic hydrocarbons. The resulting mixture can then be used as a fuel or used as a feedstock for further thermocatalytic process in order to obtain refined chemicals such as monomers that can be reintroduced into the plastic manufacturing cycle.

[0005] The step of converting the molten plastic mass into a gaseous stream can in principle take place only by the action of the heat (thermal depolymerization). However, it has been proved that the presence of a catalyst in this stage allows the depolymerization to take place at a lower temperature and more efficiently.

[0006] To this end, various catalysts have been proposed often based on depolymerization tests carried out on virgin polymers or accurately presorted recycled plastics composed of a substantially single polymer. Under these “non real” conditions the catalysts may show a somewhat higher depolymerization activity with respect to thermal depolymerization only. However, these tests does not provide any information as to how the catalyst may perform under real conditions and in particular no information as to whether it will be affected, and to what extent, by the poisoning effect coming from the components of the real plastic waste.

[0007] Zeolites based catalyst for example, show a good depolymerization activity with virgin or singled out recycled plastics but when used with more complex plastic waste feedstock suffer from a pronounced decay of catalyst activities.

[0008] Catalysts based on heteropolyacids have been found useful in the depolymerization of real plastic waste as they show a less pronounced deactivation trend.

[0009] In order to work as depolymerization catalysts said heteropolyacid structures are conveniently deposited on porous supports such as silica or alumina. Due to the fact that the extent of depolymerization activity is also depending on the amount of heteropolyacid deposited on the support, it results that the process for the catalyst preparation should be able to efficiently incorporate the heteropolyacid in the support thereby making possible to prepare catalyst with a higher load of heteropolyacid.

[0010] The typical supportation process is carried out by slurrying the support in a water solution of heteropolyacid. The relatively high volume of water needed for creating theslurry makes necessary using a high amount of heteropolyacid if a high load catalyst is needed. However, since after the step of contact between the solution and the support the liquid portion is removed, it results that also a substantial amount of heteropolycaid is removed with the liquid. Accordingly, the process has a very low efficiency in terms of heteropolyacid fixation. On the other hand, the step of removal the liquid portion from the slurry by drying it would long and inefficient as well due to the high boiling point of water and its strong interactions with both support and heteropolyacid.

[0011] It has now been found that a process for the preparation of supported heteropolyacid catalyst can be carried out in non-slurry conditions thereby improving the efficiency of supportation.SUMMARY OF THE DISCLOSURE

[0012] It is therefore an aspect of the present disclosure a process for the preparation of a supported heteropolyacid catalyst comprising the steps of: a) contacting a solution of heteropolyacid (HP A) in a polar solvent with a solid inorganic support having a pore volume measured with liquid titration method of at least 0.3 cm3 / g, preferably at least 0.5 cm3 / g and most preferably at least 0.7 cm3 / g and a particle size (D50) ranging from 5 to 200 pm under conditions and amount such that the ratio (PFR) between the volume of HP A solution and the total pore volume relative to the amount of support used is equal to, or lower than 1.20; b) drying the product obtained from previous step under the following conditions:- the product is kept in substantially continuous motion;- the drying is carried out under vacuum;- the drying temperature ranges from 40 to 150°C with the proviso that if the temperature chosen is higher than 90°C the drying time at that temperature is lower than 16 hours.

[0013] Moreover, it is another object of the present application a process for depolymerizing plastics, comprising the steps of: i) providing a melt plastic waste feedstock, preferably comprising at least recycled polypropylene and polyethylene; and ii) subjecting the melt product obtained in (i) to a temperature ranging from 280°C to 600°C to obtain a depolymerization product;said process being characterized by the fact that either or both of the melt product and depolymerization product are contacted with a catalyst comprising a supported heteropolyacid obtained by the method described above.DETAILED DESCRIPTION OF THE DISCLOSURE

[0014] Heteropolyacids (HP A) can be considered to belong to a class of complex oxygen-containing acids formally deriving from condensation of two or more different inorganic acids and consequent elimination of water. In turn, one of the acid can regarded as formed by combination of several molecules of an acid anhydride of a transition metal (for example molybdenum trioxide or tungsten trioxide) while the other acid would derive from a non-metal (for example phosphorus or silicon) anhydride in which the transition metal portion contains transition metals selected from the group consisting of W, Mo and V and the non- metal portion contains non-metal elements selected from Si, P and As.

[0015] The HPA can have the formula HnfXMnCho], where X is a heteroatom selected from Si, P and As, M is a transition metal selected from W, Mo and V and n is a number balancing the remaining negative valences of oxygen atoms.

[0016] Preferably the transition metal compound is W or Mo and especially W. It constitutes a preferred embodiment the presence of additional transition metal compounds (ATMC) in amount such that the molar ratio between W or Mo and ATMC ranges from 0.5 to 100, more preferably from 5 to 100, and especially from 20 to 100. It constitutes an especially preferred embodiment the absence of any additional transition metal compound (ATMC). Preferably, the heteropolyacid is selected from those in which the non-metal element is Si or P and especially Si. In particular, supported tungstosilicic acid (TSA) is especially preferred. The silica supported TSA is especially preferred.

[0017] The amount of heteropolyacid on the support, expressed by the amount of transition metal, ranges from 0.5 to 20wt% with respect to the total amount of supported catalyst, preferably from 1 to 15%wt and more preferably from 1.5 to 10%wt. If the weight heteropolyacid complex is considered, its amount based on the total weight of supported catalyst could range from 1 to 26%wt, preferably from 1.5 to 20% and more preferably from 2 to 15%wt.

[0018] Particularly preferred are the catalysts having a content of transition metal from 1 to 15%wt preferably from 2 to 12% and especially from 2 to 7%wt based on the total weight of the catalyst.

[0019] The porous inorganic support can be spherical or granular. Examples of solids of this type, are aluminum oxide, silicon dioxide (silica gel), titanium dioxide or their mixed oxides or cogels, or aluminum phosphate. Preferably, in the process according to the present disclosure, the solid inorganic support is selected from inorganic oxides and more preferably from AI2O3, SiCh and TiCh. Preferably, the support is based on AI2O3, SiCh, or mixed silica / alumina composition. Support based on SiCh is especially preferred.

[0020] Preferably, the support particles have a pore volume which is preferably in the range between 0.5 and 3.0 cm3 / g, more preferably in the range from 0.7 and 3.0 cm3 / g, and especially in the range from 0.8 cm3 / g to 2.0 cm3 / g.

[0021] Preferably the support particles have a pore diameter which is preferably in the range below 200A, more preferably in the range below 150 A, particularly preferably in the range from 50 A to 130A.

[0022] Preferably the support particles have a particle size (D50 in volume) measured with laser diffraction, ranging from 10 to 200 pm, more preferably from 20 to 110pm.

[0023] The surface area of the inorganic support can range from 100 m2 / g to 1000 m2 / g, preferably in the range from 150 m2 / g to 700 m2 / g and particularly preferably, especially when the support is silica, in the range from 200 m2 / g to 600 m2 / g. The specific surface area of the support particles is the surface area of the particles determined by means of nitrogen adsorption in accordance with the BET technique.

[0024] The apparent density of the inorganic supports for catalysts is preferably in the range from 250 g / 1 to 1200 g / 1, with the apparent density being able to vary as a function of the water content of the support. Particularly when the support is silica, the apparent density of water-containing support particles is preferably in the range from 500 g / 1 to 1000 g / 1, more preferably in the range from 600 g / 1 to 950 g / 1 and particularly preferably in the range from 650 g / 1 to 900 g / 1. In the case of supports which contain very little if any water, the apparent density is preferably from 250 g / 1 to 600 g / 1.

[0025] Silica supports of the above defined features are commercially available.

[0026] Silica supports can be anyway prepared by several methods known in the art. According to one of them, which is also preferred, the support is prepared starting from a silica hydrogel by acidic or basic precipitation from water glass as described in EP 1778748 Al the specific disclosure of which is herein incorporated by reference.

[0027] Preferably, the silica support is a non-fumed silica.

[0028] The support preferably comprises a high proportion of SiCh. Preference is given to the silicon content of the support being in the range >10% by weight, preferably in the range >15% by weight, more preferably in the range >20% by weight, particularly preferably in the range >25% by weight, more particularly preferably in the range >30% by weight, especially in the range > 40% by weight, very particularly preferably in the range > 50% by weight, based on the total weight of the support.

[0029] The support material can also be partially or fully modified before use in the process of the invention. The support material can, for example, be treated under oxidizing or nonoxidizing conditions at temperatures of from 200 to 1000°C, if appropriate in the presence of fluorinating agents such as ammonium hexafluorosilicate. In this way, it is possible, inter alia, to vary the water content and / or OH group content. Also, it is possible to dope the support with metal compounds different from those on which the support is based on.

[0030] The polar solvent used in step (a) can be either protic or aprotic. With the term polar solvents are denoted solvents that are liquid at room temperature and have a permanent dipole moment.

[0031] Protic solvents are, for example, alcohols, Ci-Cs-carboxylic acids and inorganic aqueous acids such as dilute hydrochloric acid or sulfuric acid, water, aqueous ammonia or mixtures thereof with water being especially preferred.

[0032] Preferred alcohols are those of formula RJ-OH where the radicals R1are each, independently of one another Ci-C2o-alkyl, C2-C2o-alkenyl, Ce-C2o-aryl groups. Preferred alcohols RkQH are methanol, ethanol, 1 -propanol, 2-propanol, 1 -butanol, 2-butanol, 1- pentanol, 2-pentanol, 1 -hexanol, 2-ethylhexanol, 2,2-dimethylethanol or 2,2- dimethylpropanol, in particular methanol, ethanol, 1 -propanol, 1 -butanol, 1 -pentanol, 1- hexanol or 2-ethylhexanol with methanol being the most preferred.

[0033] Aprotic solvents are, for example, ketones, ethers, esters and nitriles, without being restricted thereto.

[0034] As previously disclosed, the solution of HP A in the polar solvent is brought in contact with the support ensuring that the volume of solution is not higher than 1.20 times the total pore volume of the amount of support used. The ratio between the volume of HP A solution and the total pore volume relative to the amount of support used is herein called Pore Filling Ratio (PFR). For the sake of clarity it is specified that the total pore volume is obtained bymultiplying the amount (in grams) of the support by its specific porosity (cm3 / g). Preferably the PFR is lower than 1.10 and more preferably kept in the range 0.8-1.0 and especially in the range 0.9-1.0 By the proper selection of the PFR in fact, it is possible to run a smooth and efficient deposition of the solution into the support while avoiding excessive wetness or slurry formation.

[0035] Although not preferred from the point of view of the productivity, it is in principle possible to contact the support with a first aliquot of solution and successively with one or more additional aliquots. However, as mentioned above, it is preferred to contact the whole volume of solution in one single stage of contact.

[0036] Preferably during stage (a) the support is kept in continuous motion. Most preferably the motion is obtained by a rotating device which rotates at a velocity that is correlated with the feeding of the solution.

[0037] The solution can be added in droplets, for example by using a dropping funnel or by spraying through a nozzle on the support in motion.

[0038] The temperature at which stage (a) is carried out is not critical as long as it is a temperature at which the solvent remains in liquid form. Typically such a temperature ranges from 10 to 120°C preferably from 20 to 100°C.

[0039] The addition time may require from 1 minute to 50 hours preferably from 30 minutes to 20 hours.

[0040] After addition is completed it is preferred to keep the contact product of step (a) in motion (mixing step) in order to increase homogeneity and penetration of the solution into the pores. Preferably the mixing time is at least equal to the addition time and more preferably the mixing time is longer than the addition time. In a more preferred embodiment the mixing time is more than 1.5 times longer than the addition time and in the most preferred embodiment it is at least 2.0 times longer than addition time.

[0041] Step (b) is carried out under conditions allowing the product to be kept in a substantially continuous motion. This means that also intermittent motion is contemplated, a motion being considered intermittent when the length of stopping time is shorter than the motion time of the preceding and / or successive motion step.

[0042] Moreover, the drying step (b) is carried out under vacuum at a pressure ranging from 5 to 600 mmHg, preferably from 10 to 400mmHg especially from 15 to 200 mmHg.

[0043] The drying temperature ranges from 40 to 150°C preferably in the range 60- 90°C and more preferably in the range 65-85°C.

[0044] Higher temperatures can be chosen, however, in order to reduce the risks of catalyst deactivation, when the drying temperature is higher than 90°C, the drying time, at that temperature, must be lower than 16 hours, preferably lower than 13 hours and more preferably lower than 10 hours.

[0045] Suitable devices for performing the drying step are available in the art. In particular those capable at the same time of rotating, providing heat and reduced pressure, such as rotary evaporator are used as small scale apparatus. Commercial tumble driers can be used in a bigger scale production.

[0046] As explained the so obtained catalyst can be used in a plastic waste depolymerization process.

[0047] Preferably, the amount of catalyst used ranges from 0.1 to 20 wt.%, more preferably 0.1-10 wt.% and especially from 0.1 to 5 wt.% with respect to the total weight of plastic waste feedstock and catalyst.

[0048] Preferably, the plastic waste feedstock comprises a mixture of polyethylene and polypropylene in a weight ratio 85:15 to 15:85 more preferably 80:20 to 20:80. The polyethylene can be one or more of high density polyethylene (HDPE), low-density polyethylene (LDPE), linear low density polyethylene (LLDPE). Polypropylene (PP) can be either propylene homopolymer or a propylene copolymer with lower amount of ethylene and / or butene. In addition, the feedstock may comprise other polyolefins like polybutene. In a particular embodiment, the feedstock may comprise also polymeric mixtures that incorporates other materials like polystyrene (PS), ethyl-vinyl acetate copolymer (EVA), ethyl-vinyl alcohol copolymer (EVOH), polyvinyl chloride (PVC), or mixtures thereof. In a preferred embodiment, the feedstock is constituted by more than 80% wt of a mixture between polyethylene and polypropylene in which polypropylene accounts for more than 50%wt of the polypropylene / polyethylene mixture.

[0049] When carrying out the depolymerization process, care should be taken for not introducing oxygen containing atmosphere into the depolymerization system. The barrier to the potentially oxygen-containing atmosphere can be obtained with a series of expedients such as nitrogen blanketing and vacuum system connected to a barrel of the extruder.

[0050] More specifically, the plastic feedstock mixture, can be charged into the feeding system of the depolymerization reactor by means of a hopper, or two or more hoppers in parallel, and the oxygen present in the atmosphere of the plastic waste material is substantially eliminated inside the hopper(s).

[0051] Plastic feedstock can be fed directly into the depolymerization reactor for small scale tests. For larger scale it is preferred to fed to the depolymerization reactor by means of an extruder which is turn fed with the plastic feedstock.

[0052] Preferably, plastic scrap is brought to a temperature at which substantially all the mass is melted and then injected into the depolymerization reactor. The extruder receives the plastic scrap cut in small pieces into the feed hopper, conveys the stream in the melting section and heat the polymer by combined action of mixing energy and heat supplied by barrel heaters. Usually, the melting temperature ranges from 250°C to 350°C.

[0053] Additives can optionally be incorporated in the melt aimed at reducing corrosivity of plastic scrap or improving depolymerization efficiency.

[0054] During the extrusion, one or more degassing steps can be foreseen to remove residual humidity present in the product.

[0055] Before being fed to the reactor, the melt stream can be filtered by in order to remove solid impurities present in the plastic waste.

[0056] Any extrusion systems can be applied, as single screw extruders, twin screw extruders, twin screw extruders with gear pump, or combination of the above.

[0057] The mixing of the plastic waste feedstock and catalyst can take place either directly into the depolymerization reactor or beforehand outside the reactor. When the mixing takes place in the depolymerization reactor, the catalyst can be fed according to several options. The simplest one, preferably used in small scale systems, is to directly pour the solid catalyst in the reactor under a nitrogen atmosphere. According to another option, the powdery catalyst may be fed to the reactor in a form of a liquid hydrocarbon slurry or a semisolid paste using dedicated devices.

[0058] As an alternative, the mixing can take place outside the depolymerization reactor. Also in this case several options are possible. According to one of them, catalyst is mixed with plastic scrap in a homogenizer apparatus and the mixture is then pelletized. The so obtained pellets, which can also contain other additives, may then be charged to the extruderhopper which is used to feed the polymerization reactor. It is also possible to charge into the hopper plastic scrap and catalysts separately. In this case, the mixing can take place into the extruder at the time of plastic scrap melting which is subsequently fed to the depolymerization reactor.

[0059] The depolymerization reactor is preferably a vessel equipped with a device to keep the reactor content in motion and operated at temperature ranging from 300°C to 550°C, more preferably from 350°C to 500°C and especially from 350°C to 450°C with inlet for plastic feedstock and catalyst and outlet for the gaseous depolymerization product.

[0060] In fact, as a result of the depolymerization process, a gaseous stream is generated that is sent to a condensation unit which totally or partially liquifies said stream.

[0061] The condensation section receives effluent gases from the depolymerization reactor and partially condense them in an oily depolymerized product substantially made up of hydrocarbons. A fraction of incondensable gases can be collected and stored separately. The condensation section can be composed by one or more stages, operated in pressure or not, at different temperatures in order to recover the maximum amount of products according to the volatility of the resulting formed compounds. The temperature range can vary of course depending on the operative pressure.

[0062] Preferably the condensation section has at least two condensation stages preferably operating at descending temperatures. As an example, in small scale equipment the first condensation stage is operated at a temperature range of 100-120°C and the second at a temperature range of from 2°C to -20°C:

[0063] It is also possible to subject the depolymerization product coming from the condensation stage to a second depolymerization stage carried out in the presence of the supported heteropolyacid already described. The second depolymerizaztion stage can be carried out under similar conditions described for the previous depolymerization stage. When this set-up is issued, it is also preferred to recycle back the catalyst and part of the liquid or semiliquid mass to the first depolymerization reactor from which the solid residue is discharged. In analogy with the first depolymerization step, the gaseous effluent can be condensed in a subsequent condensation stage.

[0064] At the end of the process preferably at least 80% wt., and preferably at 90% wt., of the plastic feedstock has been converted in liquid or gaseous depolymerization product.

[0065] As mentioned above, the main use of the depolymerization product according to the present disclosure can be as a cracker feedstock. In this connection, it would be preferred to generate from the depolymerization process a high yield in liquid depolymerization product. In a preferred embodiment the amount of liquid depolymerization product is higher than 60%wt more preferably from 65 to 85% wt. of the plastic waste feedstock.

[0066] Moreover, it would also be preferable for the liquid depolymerization product to have a composition as much as possible suited for a cracker feedstock. This involves having a very low amount, or even absence, of fractions with C28 or higher. Preferably, in the liquid depolymerization product the amount of the higher than C28 fraction is equal to, or lower than, 4%, preferably lower than 3% and more preferably lower than 2% with respect to the total amount of liquid depolymerization product.

[0067] Also, the quality of cracker feedstock is higher when the depolymerization oil obtained from real plastic waste has low values of C6-C8 aromatics and Internal Olefin Index (I.O.I.). This latter is defined as the molar ratio between internal double bonds with respect to double bond in chain end position (alfa-olefins) determined as described in the characterization section. Preferably, in the liquid depolymerization product the I.O. I. is lower than l%wt and more preferably lower than 0.5%wt.

[0068] As used herein, “C6-C8 aromatics” refers to a hydrocarbon with sigma bonds and delocalized pi electrons between carbon atoms forming a circle, wherein total of 6 to 8 carbon atoms are present.

[0069] According to a preferred embodiment, a poison-suppressing agent can be used in association with the catalyst. Preferably, it is selected from the group consisting of Ca(OH)2, Mg(0H)2, Ba(OH)2, Sr(OH)2, CaO, phyllosilicates, aluminosilicates, and Zr(HPO4)2. Among them, the use of Zr(HPO4)2, Ca(OH)2 and phyllosilicate is preferred. Among phyllosilicates, use of bentonite is preferred.

[0070] The data reported in the present disclosure show that the process according to the present disclosure allows conversion of virgin resins, and also complex plastic waste, in a liquid depolymerization product which is obtained in high yields and composition that makes it suitable for use as a cracker feedstock.CHARACTERIZATION

[0071] The properties are determined according to the following methods.Analytical Methods

[0072] Characterization of liquid products: The liquid products from the two traps were characterized by Gas Chromatography (GC) and proton NMR (1H NMR).

[0073] The GC analysis of the liquid product for each run was performed using an Agilent 7890 GC (Agilent Technologies, Santa Clara, CA) equipped with a standard non-polar column and a flame ionization detector. For the GC data, the weight percent for x < nC7 (having boiling point <98°C named LF1), nC7 < x < nCl l (having boiling point 98°C <BP< 203°C named LF2), nC12 < x < nC28 (having boiling point 203°C <BP< 434°C named LF3) , x > C28 (having boiling point >434°C named LF4) were used to characterize the liquid product.

[0074] NMR data were used to characterize the percent of aromatic protons, paraffinic protons and olefinic protons in the liquid product. The examples were analyzed with an addition of CDC13 (0.6 g of depolymerize polymer / metal oxide mixture with 0.4 g of CDC13). The data were collected on a Bruker AV500 MHz NMR spectrometer (Bruker Corporation, Billerica, MA) at 25°C with a 5mm Prodigy probe. One dimension 1H NMR data were processed using TOPSPIN® software (Bruker) with an exponential line broadening window function. Quantitative measurements were performed with a 15 second relaxation delay, a 30° flip angle pulse, and 32 scans to facilitate accurate integrals. The spectral integrations for aromatic olefinic, and paraffinic protons were obtained and used to quantify relative ratios of these protons.

[0075] Commercial samples of Silicon Dioxide SiCh (White sand) and Aluminum (III) oxide AI2O3 as well as Titanium Dioxide TiCh are commercially available from Sigma Aldrich, while CBV400 HY Zeolite is commercially available from Zeolyst International.

[0076] Heteropoly acids used in the different examples are commercially available from common suppliers like Merk, Alfa Aesar, ABCR.

[0001] Average Particle Size

[0077] Determined by a method based on the principle of the optical diffraction of monochromatic laser light with the "Malvern Instr. 2600" apparatus. The average size is given as D50

[0078] Determination of Al, Ti

[0079] The determination of Al, Ti content in the solid catalyst component has been carried out via inductively coupled plasma emission spectroscopy on “TC P Spectrometer ARLAccuris”. The sample was prepared by analytically weighting, in a “Fluxy” platinum crucible”, 0.1- ).3 grams of catalyst and 2 grams of lithium metaborate / tetraborate 1 / 1 mixture. After addition of some drops of KI solution, the crucible is inserted in a special apparatus "Claisse Fluxy” for the complete burning. The residue is collected with a 5% v / v HNO3 solution and then analyzed via ICP at the following wavelengths: aluminum, 394.40 nm; titanium, 368.52 nm.

[0080] Determination of Si, W

[0081] The determination of Si, W content in the solid catalyst component has been carried out via inductively coupled plasma emission spectroscopy on “TC P Spectrometer leap 7000”. The sample was prepared by analytically weighting, in a plastic 100 mL volumetric flask 0.01^-0.10 grams of catalyst. 20 mL of hydrofluoric acid (48%) were diluted at ten percent in demineralized water and added into the flask. Subsequently, a cold solution of 1.5 g of boric acid (purity > 99.5%) in 50 mL of demineralized water was also added. Finally, the content of the flask is make up to the mark with demineralized water and mix. The resulting solution was then directly analyzed via ICP at the following wavelengths: tungsten, 224.875 nm; silicon, 212.412 nm.

[0082] Determination of pore volume

[0083] Throughout the whole application the pore volume refers to a value measured as follows:

[0084] Methanol is added in portions to the pulverulant substance to be tested with constant mixing until all pores are saturated with liquid, which is evident from the powder losing its flowability and starting to form lumps. The volume of liquid required per gram of sample corresponds to the pore volume of the sample.

[0085] Specifically, the pore volume was determined as follows: 5 g of the substance to be tested were weighed cut into a dry powder bottle (150 cm3) with screw cap. Distilled water was added in decremental portions (starting from initially 2 cm3) from a burette, the bottle was sealed with the screw cap, and the contents were mixed by vigorous shaking. The bottle was then placed on a cork mat and subsequently rotated. When about 1 / 3 of the sample remains stuck to the base of the bottle during this operation, the pores are saturated. The methanol consumption was read off and converted to 1 g of the sample. The standard deviation of the pore measurement values is ± 0.02 cm3 / g.

[0086] Surface area with NitrogenSurface area with nitrogen: determined according to the B.E.T. method (apparatus used SORPTOMATIC 1900 by Carlo Erba).EXAMPLESGeneral Depolymerization Procedure

[0087] General procedure for depolymerization test in a 500 ml round glass reactor

[0088] 30 g of real polymer plastic waste were loaded in a 500 mL round glass reactor having three necks equipped with thermocouple and nitrogen inlet.

[0089] The real polymer plastic waste used in the examples was analyzed and it resulted to be composed of about 97wt% of polyolefin in which the PP / PE ratio was about 30 / 70) with the residual containing traces of other common polymers (PET, PS, PA, PU) plus inorganic contaminants.

[0090] The solid catalyst (2.5wt% with respect to plastics) is then introduced in the proper amount into the glass reactor. Blank test without any catalyst can be also performed. Two glass condenser are connected in series and kept at 110°C and -8°C respectively using an oil bath (Cryostat Julabo). The reactor is placed in electrically heating system (mantle bath), and setting the desired power, the temperature was raised up to 450°C. The pyrolysis process takes place and the following experimental parameters are recorded:• L%, sum of the yield of liquid condensable at 110°C + liquid condensable at -8°C (with respect the polymer charged)• S%, yield of solid / waxy residue in the reactor, excluding catalyst (with respect to the polymer charged)• G% yield in gaseous products not condensable in both condensers (with respect the polymer charged)Example 1

[0091] SiC>2 modified with Tungstosilicic acid (TSA)

[0092] Reagents:1) Silicotungstic acid (or tungstosilicic acid, CAS 12027-38-2), formula H4[Si(W30io)4] • XH2O, MW 2878,31 g / mol, commercial product, by Sigma- Aldrich (alternative supplier Todini), labelled TSA2) Silica Sylopol XPO 2107, acquired from Grace, Hg Porosity 1.57 cm3 / g; D50 50-70 pm3) Deionized water or anhydrous methanol

[0093] Procedure

[0094] TSA acid (5.7 g) was dissolved at r.t. for 30 min into 75 mL of deionized water, obtaining a colorless solution. The latter was slowly added to silica Sylopol XPO 2107 (50.0 g), previously charged at room temperature into a IL round-bottom flask, equipped with mechanical stirrer. The PFR-value for this preparation was 0.96. The mechanical stirring was continued for 4 h at room temperature, thus all mixture was transferred into a rotary evaporator. The mixing last 12 h at room temperature by using the rotary evaporator. Drying step was carried out at 75°C for 5.5 h under vacuum at 30 mmHg. The final compound is a free flowing white powder (53.4 g).

[0095] Characterization :

[0096] W = 6.5%wt., Si = 40.5%wt.

[0097] Data on the efficiency of W fixation are reported in table 1. The so obtained catalyst was used in a depolymerization run carried out according to the general depolymerization procedure disclosed. The results are reported in Table 2.Example 2

[0098] Procedure

[0099] TSA acid (34.2 g) was dissolved at r.t. for 30 min into 450 mL of deionized water, obtaining a colorless solution. The latter was added dropwise in 65 min to silica Sylopol XPO 2107 (300.0 g), previously charged into a 3L rotavapor (rotational speed 35 rpm). The PFR-value for this preparation was 0.96 .The reaction mixture was stirred for additional 2h at room temperature at the same rotational speed, thus drying step followed. The temperature wasincreased from room temperature up to 80°C in 15 min. The drying step was carried out at 80°C for 8.0 h under vacuum at 30 mmmHg applying a rotational speed of 35 rpm. The final compound is a free flowing white powder (326.0 g).

[0100] Characterization :

[0101] W = 5.9%wt., Si = 38.0%wt., H2O = 3.3%wt.

[0102] Data on the efficiency of W fixation are reported in table 1. The so obtained catalyst was used in a depolymerization run carried out according to the general depolymerization procedure disclosed. The results are reported in Table 2.Example 3

[0103] Procedure

[0104] TSA acid (28.5 g) was dissolved at r.t. for 30 min into 75 mL of deionized water, obtaining a colorless solution. The latter was slowly added to silica Sylopol XPO 2107 (250.0 g), previously charged at room temperature into a 2L round-bottom flask, equipped with mechanical stirrer. The PFR-value for this preparation was 0.96 . The mechanical stirring was continued for 4 h at room temperature, thus all mixture was transferred into a rotary evaporator. The mixing last 12 h at room temperature by using the rotary evaporator. Drying step was carried out first at 75°C for 16.0 h under vacuum at 30 mmHg and then at 90°C for 8.0 h under vacuum at 30 mmHg. The final compound is a free flowing white powder (269.0 g).

[0105] Characterization :

[0106] W = 6.3%wt., Si = 38.7%wt., H2O = 2.6%wt.

[0107] Data on the efficiency of W fixation are reported in table 1. The so obtained catalyst was used in a depolymerization run carried out according to the general depolymerization procedure disclosed. The results are reported in Table 2.Example 4

[0108] Procedure

[0109] TSA acid (5.5 g) was dissolved at r.t. for 30 min into 75 mL of anhydrous methanol, obtaining a colorless solution. The latter was slowly added to silica Sylopol XPO 2107 (50.0 g), previously charged at room temperature into a IL round-bottom flask, equipped with mechanical stirrer. The PFR-value for this preparation was 0.96 . The mechanical stirring was continued for 4 h at room temperature, thus all mixture was transferred into a rotaryevaporator. The mixing last 12 h at room temperature by using the rotary evaporator. Drying step was carried out at 75°C for 5.5 h under vacuum at 30 mmHg. The final compound is a free flowing white powder (54.6 g)

[0110] Characterization :

[0111] W = 6.6%wt., Si = 43.0%wt., H2O = 2.9%wt.

[0112] Data on the efficiency of W fixation are reported in table 1. The so obtained catalyst was used in a depolymerization run carried out according to the general depolymerization procedure disclosed. The results are reported in Table 2.Comparative Example 1

[0113] Reagents:1) Silicotungstic acid (or tungstosilicic acid, CAS 12027-38-2), formula H4[Si(W30io)4] • XH2O, MW 2878,31 g / mol, commercial product, by Sigma- Aldrich (alternative supplier Todini), labelled TSA2) Silica ES70Y, acquired from PQ, Hg Porosity 1.68 cm3 / g; D50 90-110 pm3) Deionized water

[0114] Procedure

[0115] TSA acid (62.5 g) was dissolved at r.t. into 120 mL of deionized water, obtaining a colorless solution. The latter was stirred for 30 min at r.t. Silica ES70Y, (250.8 g), was suspended at r.t. into IL of deionized water into a 2L round-bottom flask, equipped with mechanical stirrer. The solution of tungstosilicic acid in water was slowly added on the slurry of silica in water: the resulting mixture was stirred for 12 h at r.t. The final slurry was filtered on a G4 frit, the white solid washed with deionized water, and subsequently dried under vacuum at 105°C / 24h. The final compound is a free flowing white powder (275.6 g): Si = 35.9%wt., W = 5.2%wt.Table 1

[0116] Table 1 shows that the method according to the present disclosure provides almost quantitative yield of W fixation at any content of W bonded. Conversely, when a different supportation technique is used, a much lower fixation yield for W is obtained.Table 2

[0117] When used in depolymerization of real plastic waste, the heteropolyacid catalysts prepared according to the method of the present disclosure perform at least at the same level as the catalyst obtained with a different supportation process.

Claims

CLAIMSWhat is claimed is:

1. A process for the preparation of a supported heteropolyacid catalyst comprising the steps of: a) contacting a solution of heteropolyacid (HP A) in a polar solvent with a solid inorganic support having a pore volume of at least 0.3 cm3 / g measured with liquid titration method and a particle size (D50 measured with laser diffraction) ranging from 5 to 200 pm using said solution and support in amount such that the ratio (PFR) between the volume of HPA solution and the total pore volume relative to the amount of support used is equal to, or lower than 1.20; b) drying the product obtained from previous step under the following conditions:- the product is kept in substantially continuous motion;- the drying is carried out under vacuum;- the drying temperature ranges from 40 to 150°C with the proviso that if the temperature chosen is higher than 90°C the drying time at that temperature is lower than 16 hours.

2. The process of claim 1 wherein the HPA has the formula HnfXMnCho], where X is a heteroatom selected from Si, P and As, M is a transition metal selected from W, Mo and V and n is a number balancing the remaining negative valences of oxygen atoms.

3. The process of claim 2 in which the transition metal compound is W or Mo and especially W.

4. The process according to any of the preceding claims 2-3 in which X is Si or P and especially Si.

5. The process according to any of the preceding claims in which the amount of HPA on the support, expressed by the amount of transition metal, ranges from 0.5 to 20wt% with respect to the total amount of supported catalyst.

6. The process according to any of the preceding claims in which the support is selected from inorganic oxides and more preferably selected from the group consisting of AI2O3,SiC>2 and TiCh and mixture thereof.

7. The process according to any of the preceding claims in which the support particles have a pore volume which is preferably in the range from 0.5 to 3.0 cm3 / g, more preferably in the range from 0.7 and 3.0 cm3 / g.

8. The process according to any of the preceding claims in which the support particles have a particle size (D50) measured with laser diffraction, ranging from 10 to 200 pm, more preferably from 20 to 110pm.

9. The process according to any of the preceding claims in which the polar solvent is selected from the group consisting of alcohols, Ci-Cs-carboxylic acids and inorganic aqueous acids such as dilute hydrochloric acid or sulfuric acid, water, aqueous ammonia or mixtures thereof.

10. The process according to claim 9 in which the polar solvent is water or methanol or mixture thereof.

11. The process according to any of the preceding claims in which the PFR is lower than 1.10 and more preferably kept in the range 0.8-1.0 and especially in the range 0.9-1.0.

12. The process according to any of the preceding claims in which the drying step (b) is carried out under vacuum at a pressure ranging from 5 to 600 mmHg, preferably from 10 to 400mmHg especially from 15 to 200 mmHg.

13. The process according to any of the preceding claims in which drying temperature ranges from 60 to 90°C and more preferably in the range 65-85°C.

14. The process according to claims 1 in which the drying temperature is higher than 90°C and the drying time, at that temperature, is lower than 16 hours, preferably lower than 13 hours and more preferably lower than 10 hours.

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