The Isomerization of Limonene Over the Ti-SBA-15 Catalyst—The Inﬂuence of Reaction Time, Temperature, and Catalyst Content

catalysts

Article The Isomerization of Limonene over the Ti-SBA-15 Catalyst—The Inﬂuence of Reaction Time, Temperature, and Catalyst Content

Monika Retajczyk and Agnieszka Wróblewska *

West Pomeranian University of Technology Szczecin, Faculty of Chemical Technology and Engineering, Institute of Organic Chemical Technology, Pulaskiego 10, 70-322 Szczecin, Poland; [email protected] * Correspondence: [email protected]

Received: 10 August 2017; Accepted: 12 September 2017; Published: 14 September 2017

Abstract: The isomerization of limonene over the Ti-SBA-15 catalyst, which was prepared by the hydrothermal method, was studied. The main products of limonene isomerization were terpinolene, α-terpinene, γ-terpinene, and p-cymene—products with numerous applications. The amount of these products depended on reaction time, temperature, and catalyst content. These parameters changed in the following range: reaction time 30–1380 min, temperature 140–160 ◦C, and catalyst content 5–15 wt %. Finally, the most favorable conditions for the limonene isomerization process were established: a reaction time of 180 min, temperature of 160 ◦C, and amount of the catalyst 15 wt %. In order to obtain p-cymene (dehydroaromatization product), the most favorable conditions are similar but the reaction time should be 1380 min. The application of such conditions allowed us to obtain the highest amounts of the desired products in the shortest time.

Keywords: limonene; isomerization; Ti-SBA-15 catalyst; p-cymene; α-terpinene; γ-terpinene; terpinolene

1. Introduction Due to restrictions associated with environmental protection, researchers increasingly use waste in their research as a source of organic compounds, which is beneﬁcial for the environment and has economic justiﬁcations. Orange peels are a residue from the process of orange juice production. About 19.8–33 million tons of oranges are produced each year, and 8 to 20 million tons of waste in the form of fruit peels result. Waste orange peels can be used as valuable feed additives for animals; however, during transport orange peels cause considerable corrosion damage to the trailers transporting them. The solution to this problem may be to use orange skins to obtain orange oil, in which the main component is an organic compound with numerous applications—limonene [1]. Orange oil is possible to obtain from orange peels with the help of hydrodistillation, steam distillation, or microwaves. The amount of limonene in orange oil is above 95%. Limonene is a cyclic terpene that is present in the form of two isomers, which are enantiomers to each other, but in the orange oil obtained from orange peels only R-(+)-limonene is present. The limonene obtained from waste orange peels is used in a wide range of everyday products. Furthermore, it is possible to transform this organic compound into many valuable products, for example in the oxidation and isomerization processes. Such transformations are valuable from the point of view of environmental protection, because we use a renewable raw material obtained from biomass. During the process of limonene isomerization, γ-terpinene, α-terpinene, and terpinolene can be obtained. Moreover, the dehydrogenation of products of limonene isomerization can also cause the

Catalysts 2017, 7, 273; doi:10.3390/catal7090273 www.mdpi.com/journal/catalysts Catalysts 2017, 7, 273 2 of 14 formation of p-cymene. The products of limonene isomerization are valuable compounds widely used as additives for food, cosmetics, or pharmaceuticals. P-cymene is the only naturally occurring form of cymene (an essential oil of caraway or thyme). This compound is a raw material for the synthesis of cresols. P-cymene is also used as the additive for masking odors in soaps and industrial products. Another application of this monoterpene is its transformation into aromatic monomers, such as terephthalic acid or dimethylstyrene. P-cymene is used as a solvent for dyes and varnishes, as a heat transfer agent, and as an additive in musk fragrances and perfumes. Moreover, p-cymene is applied as a ligand in olefine metathesis [2–4]. This compound is industrially produced in the Friedel–Craft reaction from toluene and isopropyl alcohol. In this reaction inorganic acids are used as catalysts, for example hydrochloric acid with AlCl3. The main disadvantages connected with the application of this catalyst are the corrosion of the apparatus and problems connected with the waste disposal after the reaction and catalyst separation. Many of these problems have been resolved by supporting the acid catalyst on a zeolite material or by application of mesoporous catalysts. The application of the mesoporous catalyst with the Si:Al ratio amounted to 93, allowed to receive p-cymene in 2 h at the temperature of 275 ◦C, with a selectivity of 75.3% and at the conversion of toluene 70.3% (the molar ratio of isopropanol to toluene amounted to 2:1). From p-cymene p-cymenesulfonic acid can also be obtained; this has similar properties to the p-toluenesulfonic acid compound, which is used, for example, in Fisher’s esterification reaction. The raw material for p-toluenesulfonic acid synthesis is toxic toluene but a good alternative to this raw material could be p-cymenesulphonic acid, which can be synthesized without the use of any organic solvent. The synthesis of p-cymenesulphonic acid was carried out in two stages. The first stage relied on the obtaining of p-cymene from limonene; the second relied on sulfonation of the received p-cymene. This method allowed us to obtain p-cymenesulfonic acid at a yield of 91% [5]. α-Terpinene is a natural and fragrant compound that is found in oranges, coriander, and oregano. Thanks to its refreshing fragrance, it is widely used in food products. α-Terpinene also found use as a fragrance compound in cosmetics and in household chemicals. Research carried out on Salmonella showed that this cyclic monoterpe does not exhibit mutagenic activity. α-Terpinene is also a component of tea tree oil and is responsible for this oil’s antioxidant properties, which allow it to be applied topically as an antimicrobial and anti-inflammatory agent, primarily used to treat skin complaints such as acne or mycosis. When α-terpinene is used as the additive for food, medicines, and cosmetics it also provides them with oxidative stability. Studies on α-terpinene demonstrated the effectiveness of this compound in the treatment of parasitic infection of Trypanosoma in horses. The thin plasma layer of α-terpinene is used to encapsulate solar cells, which protects them from atmospheric agents and moisture. Oxidation of α-terpinene, catalyzed by a photocatalyst supported on silica, leads to many valuable products such as ascaridole, isoascaridole, 4-isopropylbenzoic acid, and cuminaldehyde [6–10]. γ-Terpinene is a monocyclic compound that demonstrates antimicrobial properties. It is synthesized by certain plant species, including rice, and causes the destruction of the cell membrane of the bacterium that causes disease in this plant as well as other plant species. γ-Terpinene also has the potential to be used in treatment of atherosclerosis. It is known that LDL cholesterol is readily oxidized by free radicals, resulting in atherosclerotic lesions in the blood vessels. γ-Terpinene, thanks to its antioxidant properties, prevents the oxidation of LDL, because it has the ability to catch free radicals. Animal studies showed a significant decrease in total cholesterol of 18.3% and triglycerides of 30.3% in the serum blood of tested animals. Moreover, γ-terpinene is one of the two components in pine oil that exhibit acaricidal properties; thanks to this, pine oil can be used against mites that occur during food storage [11–13]. Terpinolene is used as a food additive, for example in baked products, ice cream, soft drinks, and candy. Moreover, terpinolene is a component in cleaning agents and deodorants. It is also used for the synthesis of terpineol, 1-methyl-4-isopropylidene, and cyclohexane-1-ol, and also in polymerization reactions that proceed in aqueous solution [14]. Terpinolene also exhibits a calming effect. The studies on this compound showed that terpinolene stimulates a different part of the brain than limonene. Catalysts 2017, 7, 273 3 of 14

Terpinolene inhalation affected the action of the nervous system and human psyche. As a consequence, terpinolene reduced tension and increased feelings of relaxation. Therefore, this compound could be used to treat mental disorders such as depression [15]. Terpinolene is an intermediary compound in the biosynthesis of Hinocitol (4-isopropyltropolone) by cypress cells. This compound has antitumor, antimicrobial, and antifungal properties. Studies on cypress tree cells suggest the possibility of using this compound for the synthesis of tropolone derivatives [16]. Terpinolene ozonized in the gaseous phase or treated with nitrate radicals is converted into a number of products. In the ozonolysis process it is converted to 4-methylcyclohex-3-en-1-one, 2-hydroxy-4-methylcyclohex-3-en-1-one, glyoxal, methylglyoxal, 3-oxobutanal, and 6-oxo-3-(propane-2-ylidene) heptanal, while in the presence of nitrate radicals it is converted to 2-hydroxy-4-methylcyclohex-3-en-1-one, glyoxal, methylglyoxal, and 4-oxopentanal [17]. Terpinolene, like γ-terpinene, also has the potential to treat atherosclerosis. This compound, in combination with β-carotene and α-tocopherol, effectively prevents the oxidation of LDL [18]. The scientiﬁc literature does not reveal much about the limonene isomerization process over heterogeneous catalysts. In 1963 Hunter et al. carried out the isomerization and disproportionation of limonene over a silica gel. The process was performed for 20 min at the temperature of 100 ◦C, and obtained 20% terpinolene, 18% α-terpinene, 10% γ-terpinene, and small amounts of isoterpinolene, terpinolene, and p-cymene. At these conditions the conversion of limonene amounted to 65%. The studies showed that the isomerization occurred immediately, while disproportion occurred in a small degree. These reactions can be explained by assuming that the carbonium ions are adsorbed onto the surface of the catalyst. Turkevitch and Smith observed that during the conversion of 1-butene to 2-butene hydrogen transfer takes place in the presence of the silica gel. Formation of limonene isomerization products works in the same way. Limonene “pulls out” the proton from the surface of the catalyst, which leads to the formation of an 8-menthenyl carbonium ion. The loss of hydrogen by this carbonium ion leads to α-terpinene and γ-terpinene formation. The hydrogen transfer is reversible, which indicates the presence of small amounts of dipentene during the isomerization of terpinolene, isoterpinolene, α-terpinene, and γ-terpinene over the silica [19]. Johnson used limonene anatase as the catalyst in the isomerization, prepared by the application of formic acid. This allowed him to obtain terpinolene with a selectivity of 70%, and the conversion of limonene amounted to 55%. This reaction was carried out at a temperature of 100 ◦C, for a reaction time of 4 h, under a nitrogen atmosphere and in the presence of a buffer (sodium acetate) [20]. Tanabe et al. used another method for the isomerization of limonene. These studies showed that in the presence of zinc oxide impregnated with aqueous H2SO4 and used as the catalyst, terpinolene was obtained with a selectivity of 78% and the conversion of limonene amounted to 65%. The reaction was performed at the relatively low temperature of 60 ◦C[14]. Dehydrogenation of limonene to p-cymene was also performed using a Pd/HZSM-5(258) catalyst, which contained silicon oxide and alumina (at the ratio of SiO2/Al2O3 amounted to 258), and palladium in the amount of 1 wt %. The use of this catalyst allowed the researchers to obtain p-cymene with a selectivity of 83% and 100% conversion of limonene. The reaction was performed at a temperature of 260 ◦C for 2 h, in the presence of n-dodecane and a nitrogen atmosphere. The use of a catalyst with a different ratio of SiO2/Al2O3—Pd/HZSM-5(5), caused the formation of a mixture of isomers (terpinenes and terpinolene) with a selectivity of 53%, and the conversion of limonene amounted to 91% [21]. Catrinescu et al. also carried out studies on the limonene isomerization process. In these studies betionites derived from Serra de Dentro were used as the catalysts. The examinations showed that nickel-modiﬁed betionites were the most active among the tested catalysts. This method produced the following products: 20% terpinolene, 12% α-terpinen, 4% γ-terpinene, and 8% isoterpinolene. The studies were performed at a temperature of 150 ◦C for 25 min and in the presence of n-dodecane. The obtained results showed that the transformation of limonene to p-cymene takes place via limonene isomers, as shown in Figure1[22]. Catalysts 2017, 7, 273 4 of 14 Catalysts 2017, 7, 273 4 of 15

-H2 gamma-terpinene limonene

-H2

terpinolene -H 2 p-cymene

alpha-terpinene

FigureFigure 1. 1.The The mechanism mechanismof of isomerizationisomerization and dehydrogenation of of limonene. limonene.

IsomerizationIsomerization of limoneneof limonene was was also also carried carried out with out ferrieritewith ferrierite H-FER H-FER (T) as the(T) catalyst as the (hydrogencatalyst forms(hydrogen of ferriarite forms z of Tosoh, ferriarite in which z Tosoh, the Si/Alin which ratio the amounted Si/Al ratio to amounted 8.9). As a to result 8.9). ofAs isomerization a result of performedisomerization at the performed temperature at ofthe 65 temperature◦C, a mixture of of 65 the °C, following a mixture products of the wasfollowing obtained: productsα-terpinene, was γ-terpinene,obtained: α terpinolene,-terpinene, γ and-terpinene, p-cymene. terpinolene, At this temperature and p-cymene. the conversionAt this temperature of limonene the achievedconversion 38% of limonene achieved 38% after the reaction time of 60 min [23]. after the reaction time of 60 min [23]. The catalytic conversion of limonene to p-cymene was also carried out by Martin-Luengo et al. The catalytic conversion of limonene to p-cymene was also carried out by Martin-Luengo et al. This group used sepiolite as the catalyst. Sepiolite is a clay (hydrated magnesium silicate) of natural This group used sepiolite as the catalyst. Sepiolite is a clay (hydrated magnesium silicate) of natural origin, cheap and easily obtainable in Madrid. In this material the magnesium layer is located origin, cheap and easily obtainable in Madrid. In this material the magnesium layer is located between between two layers of silicon oxide. Sepiolite consists mainly of magnesium, but some ions of twomagnesium layers of silicon and silicon oxide. are Sepiolite replaced consists by aluminum mainly ions. of magnesium, In this method but of some isomerization ions of magnesium microwave and siliconheating are was replaced used byand aluminum obtained products ions. In thisin a short method amount of isomerization of time. After microwave 5 min of isomerization heating was at used a andtemperature obtained products of 210 °C, in the a short amounts amount of product of time. were After as 5 minfollows: of isomerization 28% of α-terpinene, at a temperature 20% of γ- of ◦ 210terpinene,C, the amounts and 27% ofof productα-terpinolene. were asThe follows: same catalyst, 28% of αmodifi-terpinene,ed with 20% nickel, of γ obtained-terpinene, p-cymene and 27% of αwith-terpinolene. a selectivity The of 100% same and catalyst, 100% conversion modiﬁed of with limon nickel,ene, after obtained 20 min p-cymene of reaction with [24]. aThis selectivity reaction of 100%was and carried 100% out conversion without the of use limonene, of any solvents. after 20 minAnother of reaction achievement [24]. Thisof this reaction group was was the carried use in out withoutthe limonene the use isomerization of any solvents. of silica–alumina Another achievement catalysts with of different this group silica was content—from the use in the 1% limonene(SIRAL isomerization1) to 40% (SIRAL of silica–alumina 40). The studies catalysts showed with that differentwith some silica catalysts content—from (SIRAL 20 1%or SIRAL (SIRAL 40) 1) it towas 40% (SIRALpossible 40). to The obtain studies p-cymene showed with that a selectivity with some of 100% catalysts and (SIRALthe complete 20 or conversion SIRAL 40) of it limonene. was possible The to obtainuse of p-cymene a catalyst with in which a selectivity the silica ofcontent 100% amounted and the complete to 1% caused conversion formation of limonene.of α-terpinene The with use ofa a catalystselectivity in which of 85% the silicaand α content-terpinolene amounted with toa 1%selectivity caused of formation 15% after of α5 -terpinenemin of reaction, with a selectivitybut the ofconversion 85% and α of-terpinolene limonene amounted with a selectivity to 15% [25]. of 15% after 5 min of reaction, but the conversion of limoneneIn amountedthis work, we to 15% present [25]. important studies on the isomerization of limonene over one of the latestIn this titanium work, silicate we present mesoporous important Ti-SBA-15 studies catalyst on the isomerizations. The choice of of this limonene heterogeneous over one catalyst of the latestfor our research was dictated by the heterogeneous catalyst properties and the advantages of their use, titanium silicate mesoporous Ti-SBA-15 catalysts. The choice of this heterogeneous catalyst for our which are broadly described in the scientific literature. Homogeneous catalysts dissolve in reaction research was dictated by the heterogeneous catalyst properties and the advantages of their use, which and their active catalytic sites are more readily accessible, which allows a reaction to proceed in mild are broadly described in the scientiﬁc literature. Homogeneous catalysts dissolve in reaction and their conditions and with good selectivity of the appropriate product. However, these catalysts have active catalytic sites are more readily accessible, which allows a reaction to proceed in mild conditions limited application due to problems in separating products contaminated by unstable residuals and and with good selectivity of the appropriate product. However, these catalysts have limited application recycling expensive catalysts. The removal of residual catalytic species from the reaction mixture duealways to problems requires in separatingthe use of productsappropriate contaminated techniques, by such unstable as chromatography, residuals and recycling distillation, expensive or catalysts.extraction. The Heterogeneous removal of residual catalysts, catalytic especially species metals from the containing reaction mixturemesoporous always silica requires catalysts, the useare of appropriatemore durable techniques, and reusable such catalysts as chromatography, than homogeneou distillation,s catalysts. or extraction. In addition, Heterogeneous these catalysts catalysts,have a especially metals containing mesoporous silica catalysts, are more durable and reusable catalysts than

Catalysts 2017, 7, 273 5 of 14 homogeneous catalysts. In addition, these catalysts have a large surface area and sufficient inner space in theCatalysts pores, 2017 which, 7, 273 allows the reactants to access the catalytic sites and release catalytic products5 of 15 from pores. Mesoporous silica catalysts can be obtained in soft-templating processes, in which self-assembly of organiclarge surface molecules area and (surfactants sufficient andinner block space copolymers) in the pores, iswhich used allows to create the poresreactants of controlledto access the sizes. Thiscatalytic procedure sites forms and release an inorganic catalytic network products around from pores. the templatesMesoporous via silica a sol-gel catalysts process can be [26 obtained–30]. in soft-templating processes, in which self-assembly of organic molecules (surfactants and block Interest in mesoporous materials increased with the discovery of the mesoporous MCM-41 copolymers) is used to create pores of controlled sizes. This procedure forms an inorganic network material, which has a large surface area and a uniform and well-defined structure [31]. In 1998, around the templates via a sol-gel process [26–30]. as a resultInterest of the in substitution mesoporous of materials the usually increased usedsurfactants with the discovery with block of the copolymers, mesoporous highly MCM-41 ordered mesoporousmaterial, materialswhich has labeleda large surface with the area acronym and a uniform SBA were and obtained. well-defined The structure SBA-15 material[31]. In 1998, is similar as a to the MCM-41result of the material substitution because of itthe has usually a hexagonal used surfactants structure. with However, block copolymers, the undoubted highly advantage ordered of thismesoporous material is thematerials presence, labeled apart with from the theacronym hexagonally SBA were arranged obtained. mesopores, The SBA-15 of material significantly is similar smaller poresto thatthe MCM-41 account material for more because than 70% it has of a the hexagonal surface areastructure. of this However, material. the These undoubted micropores advantage are located of in thethis walls material of this is the type presence, of material apart andfrom are the a hexago combinationnally arranged of ordered mesopores, main pores of significantly [32]. The smaller advantage of SBA-15pores that is not account only for the more unique than construction 70% of the surface of this area material of this material. and better These stability micropores of its are structure located but alsoin the the possibility walls of this of type a safe of material application and are of a this combination material of because orderedit main does pores not [32]. exhibit The toxicity.advantage Also, the synthesisof SBA-15 is of not this only material the unique is ecologically construction friendly of this material in comparison and better to stability the MCM-41 of its structure material. but Our also the possibility of a safe application of this material because it does not exhibit toxicity. Also, the earlier studies related to MCM-41 and SBA-15 materials but containing titanium in the structure synthesis of this material is ecologically friendly in comparison to the MCM-41 material. Our earlier (Ti-MCM-41 and Ti-SBA-15) have shown that these materials can be very active catalysts in oxidation studies related to MCM-41 and SBA-15 materials but containing titanium in the structure (Ti-MCM- (also epoxidation) and isomerization processes. During these studies, the Ti-SBA-15 material showed 41 and Ti-SBA-15) have shown that these materials can be very active catalysts in oxidation (also betterepoxidation) stability and and better isomerization resistance processes. to titanium During leaching; these thus,studies, for the studiesTi-SBA-15 presented material inshowed this work, the Ti-SBA-15better stability catalyst and better was chosenresistance [33 to,34 titanium]. leaching; thus, for the studies presented in this work, theThe Ti-SBA-15 aim of this catalyst work was was chosen a preliminary [33,34]. study of the possibility of performing the isomerization of limoneneThe overaim of the this Ti-SBA-15 work was catalyst a preliminary and establishing study of the thepossibility ways of of transformation performing the ofisomerization limonene over thisof catalyst. limonene These over studiesthe Ti-SBA-15 have notcatalyst been and described establishing in the the literature ways of transfor data but,mation taking of limonene into account over the propertiesthis catalyst. of the These Ti-SBA-15 studies catalyst, have not thisbeen material described can in the have literature good catalytic data but, propertiestaking into inaccount this process. the Theproperties main parameters of the Ti-SBA-15 that have catalyst, an influence this material on thecan coursehave good of isomerization catalytic properties are: in the this reaction process. time, temperature,The main andparameters content that of the have catalyst. an influence The isomerization on the course was of isomerization performed without are: the any reaction solvent, time, so the proposedtemperature, isomerization and content method of the is environmentallycatalyst. The isom friendly.erization Thewas choiceperformed of these without parameters any solvent, allowed so us the proposed isomerization method is environmentally friendly. The choice of these parameters to establish the best conditions for performing limonene isomerization in search of the most desirable allowed us to establish the best conditions for performing limonene isomerization in search of the products: terpinolene, α-terpinene, γ-terpinene, and p-cymene. Research on this process should be most desirable products: terpinolene, α-terpinene, γ-terpinene, and p-cymene. Research on this developedprocess inshould the future, be developed taking in into the accountfuture, taking our preliminary into account studiesour preliminary presented studies in this presented work and in the numerousthis work applications and the numerous of the products applications of limonene of the products isomerization. of limonene isomerization.

2. Results2. Results and and Discussion Discussion TheThe performed performed research research allowed allowed us tous obtain to obtain terpinolene, terpinolene,α-terpinene, α-terpinene,γ-terpinene, γ-terpinene, and and p-cymene p- by thecymene process by ofthe limonene process of isomerization limonene isomerization and over theand Ti-SBA-15over the Ti-SBA-15 catalyst. catalyst. The reactions The reactions that proceeded that duringproceeded the studied during process the studied are presented process are in presented Figure2. in Figure 2.

FigureFigure 2. The2. The reactions reactions that that proceeded proceeded during the the process process of of limon limoneneene isomerization. isomerization.

The conversion of limonene was monitored during the process. It was carried out with The conversion of limonene was monitored during the process. It was carried out with appropriate appropriate values of process parameters such as reaction time, temperature, and catalyst content values(Table of process 1). During parameters the studies such changes as reaction in the time,yields temperature, of the appropriate and catalyst products content were also (Table observed1). During the studies(Table 2). changes in the yields of the appropriate products were also observed (Table2).

Catalysts 2017, 7, 273 6 of 14

Because the isomerization of limonene did not occur when the content of the catalyst amounted to 5 wt % at the temperatures of 140 and 150 ◦C, Tables1 and2 do not present results for these conditions.

Table 1. The inﬂuence of reaction time, catalyst content and temperature on the limonene conversion.

Reaction Time (min) 30 60 90 120 180 1380 Amount of the Ti-SBA-15 (5 wt %), temperature 160 ◦C Conversion of limonene (mol %) 3.13 7.25 13.82 13.99 20.46 86.31 Amount of the Ti-SBA-15 (10 wt %), temperature 140 ◦C Conversion of limonene (mol %) 1.72 2.38 3.19 6.46 9.09 56.08 Amount of the Ti-SBA-15 (10 wt %), temperature 150 ◦C Conversion of limonene (mol %) 3.01 6.75 8.93 11.82 15.88 61.62 Amount of the Ti-SBA-15 (10 wt %), temperature 160 ◦C Conversion of limonene (mol %) 9.25 21.56 23.56 29.14 31.61 96.41 Amount of the Ti-SBA-15 (15 wt %), temperature 140 ◦C Conversion of limonene (mol %) 1.79 5.00 7.48 11.02 17.22 73.86 Amount of the Ti-SBA-15 (15 wt %), temperature 150 ◦C Conversion of limonene (mol %) 10.70 17.60 24.57 31.74 41.28 88.83 Amount of the Ti-SBA-15 (15 wt %), temperature 160 ◦C Conversion of limonene (mol %) 34.92 57.78 71.85 75.84 87.84 98.80 Amounts of the Ti-SBA-15 catalyst: 0.25 g (5 wt % in relation to the mass of limonene), 0.5 g (10 wt % in relation to the mass of limonene) or 0.75 g (15 wt % in relation to the mass of limonene), temperatures: 140, 150 and 160 ◦C, and reaction time: 30, 60, 90, 120, 180 and 1380 min.

Table1 shows that for the catalyst content of 5 wt % and temperature of 160 ◦C the conversion of limonene rose during prolongation of the reaction time from 3.13 mol % (reaction time: 30 min) to 86.31 mol % (1380 min). For a catalyst content of 10 wt % an increase in the conversion of limonene with the increase of temperature and prolongation of the reaction time was also observed. The highest values of limonene conversion were observed for a temperature of 160 ◦C: at this temperature the conversion of limonene rose from 9.25 mol % to 96.41 mol %. The highest values of limonene conversion were observed for a catalyst content of 15 wt %. At the highest studied temperature (160 ◦C), the values of limonene conversion changed from 34.92 mol % to 98.80 mol %. These results show that an increase in the amount of the catalyst (at the same time as an increase in the number of active sites with Ti) caused an increase in the conversion of limonene because more limonene molecules undergo transformations in active centers of Ti. At the same time, large pores of the Ti-SBA-15 catalyst facilitate the access of organic particles to active centers of Ti and, moreover, the increase in temperature accelerates the diffusion of these molecules into the pores. Table2 presents the changes in values of yields of main products of limonene isomerization depending on the reaction time, temperature, and catalyst content. Also, for a better interpretation the obtained results, in Figures3–8 the changes in values of yields of products and conversion of limonene depending on the appropriate parameters are presented.

Table 2. The inﬂuence of reaction time, temperature, and catalyst content on the yield of main products of isomerization of limonene.

Reaction Time (min) 30 60 90 120 180 1380 Amount of the Ti-SBA-15 (5 wt %), temperature 160 ◦C Yield of α-terpinene (mol %) 0.00 1.79 2.66 3.76 5.79 31.54 Yield of γ-terpinene (mol %) 0.58 0.97 1.23 1.88 2.76 13.79 Yield of p-cymene (mol %) 0.18 0.27 0.38 0.52 0.85 10.38 Yield of terpinolene (mol %) 1.76 3.01 7.91 5.82 8.20 15.80 Catalysts 2017, 7, 273 7 of 14

Table 2. Cont.

Reaction Time (min) 30 60 90 120 180 1380 Amount of the Ti-SBA-15 (10 wt %), temperature 160 ◦C Yield of α-terpinene (mol %) 2.36 4.72 6.76 7.65 9.16 29.24 Yield of γ-terpinene (mol %) 1.28 2.40 3.28 3.68 4.33 13.63 Yield of p-cymene (mol %) 0.28 0.54 0.85 1.22 1.79 17.01 Yield of terpinolene (mol %) 3.86 7.04 9.56 10.38 12.62 6.95 Amount of the Ti-SBA-15 (10 wt %), temperature 150 ◦C Yield of α-terpinene (mol %) 0.75 1.87 2.26 3.02 4.22 4.44 Yield of γ-terpinene (mol %) 0.48 1.03 1.31 1.64 2.22 2.90 Yield of p-cymene (mol %) 0.18 0.32 0.49 0.64 0.94 14.38 Yield of terpinolene (mol %) 1.45 3.34 4.18 5.30 7.17 10.72 Amount of the Ti-SBA-15 (10 wt %), temperature 140 ◦C Yield of α-terpinene (mol %) 0.33 0.47 0.66 1.46 2.18 13.54 Yield of γ-terpinene (mol %) 0.25 0.32 0.43 0.84 1.8 6.71 Yield of p-cymene (mol %) 0.13 0.16 0.20 0.30 0.44 10.82 Yield of terpinolene (mol %) 0.71 0.99 1.40 2.78 3.98 16.93 Amount of the Ti-SBA-15 (15 wt %), temperature 160 ◦C Yield of α-terpinene (mol %) 11.96 20.82 25.91 28.30 31.33 13.20 Yield of γ-terpinene (mol %) 5.37 9.21 11.49 12.41 13.97 5.56 Yield of p-cymene (mol %) 1.01 2.66 4.88 7.19 12.45 50.66 Yield of terpinolene (mol %) 12.66 17.24 17.32 15.47 13.05 1.99 Amount of the Ti-SBA-15 (15 wt %), temperature 150 ◦C Yield of α-terpinene (mol %) 2.78 4.93 7.36 9.97 13.88 18.98 Yield of γ-terpinene (mol %) 1.46 2.46 3.55 4.68 6.17 9.42 Yield of p-cymene (mol %) 0.41 0.68 1.01 1.42 2.32 33.00 Yield of terpinolene (mol %) 4.58 7.40 10.09 12.63 15.40 10.91 Amount of the Ti-SBA-15 (15 wt %), temperature 140 ◦C Yield of α-terpinene (mol %) 0.33 1.05 1.68 2.63 4.43 19.58 Yield of γ-terpinene (mol %) 0.24 0.62 0.93 1.40 2.26 10.38 Yield of p-cymene (mol %) 0.12 0.24 0.32 0.44 0.70 17.26 Yield of terpinolene (mol %) 0.70 2.13 3.19 4.75 7.35 18.53 Amounts of the Ti-SBA-15 catalyst: 0.25 g (5 wt % in relation to the mass of limonene), 0.5 g (10 wt % in relation to the mass of limonene), or 0.75 g (15 wt % in relation to the mass of limonene), temperatures: 140, 150, and 160 ◦C, and reaction time: 30, 60, 90, 120, 180, and 1380 min.

Table2 shows that all the tested parameters had an influence on the yields of the main products. In all performed syntheses, the main products were: α-terpinene, γ-terpinene, terpinolene, and p-cymene. An increase in the amount of the catalyst caused an increase in the yields of α-terpinene, γ-terpinene, and terpinolene in a shorter time, and faster transformation of these limonene isomers to p-cymene. These results are similar to the results presented in the literature data, taking into account the products formed during the isomerization of limonene and a possible follow-up reaction in which p-cymene is formed. This allows us to assume that this reaction follows the mechanism outlined in the introduction to this article on the basis of the literature data [22]. Figure3 shows that the yields of isomers of limonene in the reaction mixture (for the isomerization performed at a temperature of 160 ◦C and the amount of the catalyst 15 wt %) decrease signiﬁcantly for a reaction in the range of 180–1380 min, and simultaneously the yield of p-cymene increases. This demonstrates the validity of the aforementioned reaction mechanism, according to which the ﬁrst limonene isomers are formed and are subsequently dehydrogenated to p-cymene. Reducing the yields of the isomerization products and simultaneously increasing the p-cymene yield may be attributed to the fact that isomerization products remain in the pores of the catalyst and do not diffuse outside the pores of the catalyst. In this way, the limonene isomerization products facilitate access to the active Catalysts 2017, 7, 273 8 of 14 centers of the Ti-SBA-15 catalyst and undergo further transformation, at the same time blocking the access of limoneneCatalysts 2017, 7 molecules, 273 to active centers. 8 of 15

Catalysts 2017, 7, 273 8 of 15

50.66 60.00

50.00 50.66 40.00 60.00 17.24 17.32 15.47 12.66 28.30 31.33 13.05 25.91 30.00 50.00 20.82 12.45 12.417.19 13.97 1.99 20.00 9.212.66 11.494.88 40.00 11.96 1.01 13.20 5.37 17.24 17.32 15.47 5.56 12.66 28.30 31.33 13.05 p-cymene limonene over Ti-SBA-15 (15 wt %) wt (15 Ti-SBA-15 over limonene Yield of products of isomerization of isomerization of products of Yield 10.00 25.91 30.00 20.82 12.45 7.19 13.97 alfa-terpinene 0.00 2.66 12.41 1.99 20.00 1.01 9.21 11.494.88 13.20 11.9630 5.37 60 90 120 180 1380 5.56 Reaction time (min) p-cymene limonene over Ti-SBA-15 (15 wt %) wt (15 Ti-SBA-15 over limonene

Yield of products of isomerization of isomerization of products of Yield 10.00 alfa-terpinene gamma-terpinene p-cymene terpinolene alfa-terpinene 0.00 Figure 3. The influence30 of 60 the reaction 90 time on 120 the yields 180of products 1380 of isomerization of limonene. Figure 3. The inﬂuence of the reactioneaction time time on (min the yields of products of isomerization of limonene. Amount of the catalyst was 0.75 gR (15 wt % in relation) to the mass of limonene), temperature was 160 Amount of the catalyst wasalfa-terpinene 0.75 g (15 wtgamma-terpinene % in relation top-cymene the mass ofterpinolene limonene), temperature was °C, and reaction time was 30, 60, 90, 120, 180, or 1380 min. 160 ◦C, and reaction time was 30, 60, 90, 120, 180, or 1380 min. FigureThe best 3. The results influence were of obtainedthe reaction for time the on catalyst the yiel dsTi-SBA-15 of products content of isom oferization 15 wt of % limonene. and at the temperatureAmount ofof the 160 catalyst °C. Under was 0.75 these g (15 conditions, wt % in relation the conversion to the mass of limonene),limonene temperaturewas close to was 90 160mol % The best results were obtained for the catalyst Ti-SBA-15 content of 15 wt % and at the temperature after°C, a reaction and reaction time time of 180 was min 30, 60,and 90, after 120, a 180, reaction or 1380 time min. of 1380 min almost all limonene underwent ◦ of 160 C.transformation. Under these At conditions, a lower process the temperature conversion (150 of °C), limonene limonene was conversion close after to 90 the mol reaction % after time a reaction time of 180of min1380The min andbest was afterresults below a reactionwere 90 molobtained % time (88.83 for of mol 1380the %),catalyst min the almostsame Ti-SBA-15 as allfor limonene140content °C, atof which underwent15 wt the % conversionand transformation. at the temperature of 160 °C. Under these conditions, the conversion of limonene was close to 90 mol % At a loweramounted process to temperature73.86 mol % (Figure (150 4).◦C), The limoneneresults show conversion that at the highest after content the reaction of the catalyst time the of 1380 min afternumber a reaction of active time centers of 180 in minthe catalystand after is aenough reaction for time transformation of 1380 min not almost only all of limonenelimonene underwentmolecules was below 90 mol % (88.83 mol %), the same as for 140 ◦C, at which the conversion amounted to transformation.but also of the products At a lower of processlimonene temperature isomerization. (150 Simultaneously,°C), limonene conversion the increase after in the temperature reaction time at 73.86 molofthe % 1380 (Figurehighest min 4wascontent). The below resultsof 90the mol catalyst show % (88.83 that accelerates mol at the %), highestthethese same reactions. content as for 140Probably, of °C, the at catalyst whichacceleration the the conversion number of the of active centers inamountedtemperature the catalyst to influences73.86 is mol enough the% (Figure limonene for transformation4). diffusionThe results rate show an notd thatalso only atthe the rate of highest limoneneof binding content of molecules organic of the catalyst molecules but the also of the productsnumber ofto the limonene active of active centers, isomerization. centers and inthe the dura catalyst Simultaneously,bility ofis enoughformed activefor thetransformation derivatives. increase in not temperature only of limonene at the molecules highest content but also of the products of limonene isomerization. Simultaneously, the increase in temperature at of the catalystthe highest accelerates content theseof the reactions.catalyst accelerates Probably, these acceleration reactions. Probably, of the temperature acceleration of inﬂuences the the limonenetemperature diffusion rateinfluences and alsothe limonene the rate diffusion of binding rate an ofd organic also the rate molecules of binding to of the organic active molecules centers, and the to the active centers, and the durability of formed active derivatives. 98.80 durability of formed active derivatives. 87.84 71.85 75.84 88.83 57.78 100.00 73.86 34.92 80.00 41.28 98.80 31.74 87.84 60.00 24.57 17.60 71.85 75.84 88.83 10.70 40.00 57.78 17.22 160 100.00 11.02 5.00 7.48 73.86 150 20.00 1.79 34.92 80.00 41.28 140 0.00 31.74 Temperature (°C) Temperature

Conversion of limonene (mol %) (mol limonene of Conversion 60.00 24.57 30 6017.60 90 120 180 1380 10.70 40.00 Reaction time (min) 17.22 160 11.02 20.00 1.79 5.00 7.48 150 140 0.00 Temperature (°C) Temperature Conversion of limonene (mol %) (mol limonene of Conversion 30 60 90140 150 120160 180 1380 Reaction time (min)

140 150 160

Figure 4. Inﬂuence of temperature and reaction time on the conversion of limonene. Amount of the catalyst was 0.75 g (15 wt % in relation to the mass of limonene), the temperature was 140, 150, or 160 ◦C, and the reaction time was 30, 60, 90, 120, 180, or 1380 min. Catalysts 2017, 7, 273 9 of 15 Catalysts 2017, 7, 273 9 of 14 Figure 4. Influence of temperature and reaction time on the conversion of limonene. Amount of the catalyst was 0.75 g (15 wt % in relation to the mass of limonene), the temperature was 140, 150, or 160 The yield°C, and of the terpinolene reaction time after was 30, a reaction60, 90, 120, time180, orof 1380 60 min. min (catalyst content 15 wt %, temperature ◦ 160 C) achievedThe yield a value of terpinolene of 17.24 after mol a %reaction and limonene time of 60 conversionmin (catalyst ofcontent 57.78 15 mol wt %, %; temperature after the reaction time of160 90 min°C) achieved the value a value of the of 17.24 yield mol of this% and compound limonene conversion was also of high 57.78 and mol amounted %; after the to reaction 17.32 mol % and a limonenetime of 90 conversionmin the value of of 71.85 the yield mol of % this (Figure compound5). was also high and amounted to 17.32 mol % and a limonene conversion of 71.85 mol % (Figure 5).

17.24 17.32 15.47 20.00 12.66 13.05 15.80 12.62 10.38 15.00 9.56 7.04 6.95 10.00 7.91 8.20 1.99 3.86 5.82 3.01 15 wt % 5.00 1.76 10 wt % Yield of terpinolene (mol %) (mol terpinolene of Yield 5 wt % 0.00 30 60 90 120 180 1380 Reaction time (min)

5 wt % 10 wt % 15 wt %

FigureFigure 5. Inﬂuence 5. Influence of the of amountthe amount of of catalyst catalyst and and thethe reac reactiontion time time on onthe theyield yield of terpinolene, of terpinolene, for the for the reaction carried out at 160 °C. The amount of catalyst was 5, 10, or 15 wt %, and the reaction time was reaction carried out at 160 ◦C. The amount of catalyst was 5, 10, or 15 wt %, and the reaction time was 30, 60, 90, 120, 180, or 1380 min. 30, 60, 90, 120, 180, or 1380 min. The highest yield of α-terpinene (31.33 mol %) was obtained for the amount of the catalyst of 15 Thewt highest% (temperature yield 160 of α°C)-terpinene and after the (31.33 reaction mol time %) amounted was obtained to 180 min for (Figure the amount 6). A similar of theyield catalyst of 15 wtof %this(temperature compound was 160also obtained◦C) and after after 1380 the min reaction but for smaller time amounts amounted of the to catalyst—5 180 min wt (Figure % 6). and 10 wt %, respectively: 31.54 mol % and 29.24 mol %. A similar yield of this compound was also obtained after 1380 min but for smaller amounts of the catalyst—5Catalysts 2017 wt, 7, %273 and 10 wt %, respectively: 31.54 mol % and 29.24 mol %. 10 of 15

31.33 28.30 25.91 29.24 35.00 31.54 20.82 30.00 25.00 11.96 13.20 20.00 9.16 15.00 6.76 7.65 4.72 10.00 2.36 5.79 15 wt % 3.76 1.79 2.66 10 wt % catalyst Content of 5.00 0.00 5 wt % 0.00 Yield of alpha-terpinene (mol %) (mol alpha-terpinene Yield of 30 60 90 120 180 1380 Reaction time (min) 5 wt % 10 wt % 15 wt %

FigureFigure 6. Inﬂuence 6. Influence of the of the amount amount of of thethe catalyst catalyst and and the the reaction reaction time on time the onyield the of yieldα-terpinene, of α-terpinene, for for the reactionthe reaction carried carried out out at at 160 160 ◦°C.C. The amount amount of of th thee catalyst catalyst was was 5, 10, 5, or 10, 15 or wt 15 %, wt and %, the and reaction the reaction time wastime 30, was 60, 30, 90, 60, 120, 90, 120, 180, 180, or 1380or 1380 min. min. After the reaction time of 180 min the yield of γ-terpinene was also the highest, amounting to 13.97 mol % (Figure 7). A very similar yield value of this compound is obtained for the catalyst content 5 wt % and 10 wt % but for a reaction time of 1380 min. Longer reaction times (for catalyst content 15 wt %) promoted formation of p-cymene. The highest yield of this compound was obtained after 1380 min and amounted to 50.66 mol % (Figure 8).

13.97 12.41 11.49 13.63 13.79 15.00 9.21

5.37 5.56 10.00

Yield of γ-terpinene (mol %) (mol γ-terpinene of Yield 4.33 3.28 3.68 2.40 5.00 1.28 2.76 15 wt % 1.88 0.58 0.97 1.23 10 wt % 5 wt % 0.00 30 60 90 120 180 1380 Reaction time (min)

5 wt % 10 wt % 15 wt %

Figure 7. Influence of the amount of the catalyst and the reaction time on the yield of γ-terpinene, for the reaction carried out at 160 °C. The amount of the catalyst was 5, 10, or 15 wt %, and the reaction time was 30, 60, 90, 120, 180, or 1380 min.

Catalysts 2017, 7, 273 10 of 15

31.33 28.30 25.91 29.24 35.00 31.54 20.82 30.00 25.00 11.96 13.20 20.00 9.16 15.00 6.76 7.65 4.72 10.00 2.36 5.79 15 wt % 3.76 1.79 2.66 10 wt % catalyst Content of 5.00 0.00 5 wt % 0.00

Yield of alpha-terpinene (mol %) (mol alpha-terpinene Yield of 30 60 90 120 180 1380 Reaction time (min) 5 wt % 10 wt % 15 wt %

Figure 6. Influence of the amount of the catalyst and the reaction time on the yield of α-terpinene, for Catalysts 2017, 7, 273 10 of 14 the reaction carried out at 160 °C. The amount of the catalyst was 5, 10, or 15 wt %, and the reaction time was 30, 60, 90, 120, 180, or 1380 min. γ AfterAfter the reaction the reaction time time of 180of 180 min min the the yield yield ofof γ-terpinene-terpinene was was also also the thehighest, highest, amounting amounting to to 13.97 mol13.97 % mol (Figure % (Figure7). A very7). A similarvery similar yield yield value value of this of compoundthis compound is obtained is obtained for for the the catalyst catalyst content 5 wt %content and 10 5 wt % % and but 10 for wt a % reaction but for a time reaction of 1380 time min.of 1380 Longer min. Longer reaction reaction times times (for (for catalyst catalyst content 15 wt %content) promoted 15 wt %) formation promoted formation of p-cymene. of p-cymene. The highest The highest yield yield of this of this compound compound was was obtainedobtained after 1380 minafter and 1380 amounted min and amounted to 50.66 to mol 50.66 % mol (Figure % (Figure8). 8).

13.97 12.41 11.49 13.63 13.79 15.00 9.21

5.37 5.56 10.00

Yield of γ-terpinene (mol %) (mol γ-terpinene of Yield 4.33 3.28 3.68 2.40 5.00 1.28 2.76 15 wt % 1.88 0.58 0.97 1.23 10 wt % 5 wt % 0.00 30 60 90 120 180 1380 Reaction time (min)

5 wt % 10 wt % 15 wt %

FigureFigure 7. Inﬂuence 7. Influence of theof the amount amount ofof the catalyst catalyst and and the the reaction reaction time timeon the on yield the of yield γ-terpinene, of γ-terpinene, for for thethe reaction reaction carried carried out out atat 160160 ◦°C.C. The The amount amount ofof th thee catalyst catalyst was was 5, 10, 5, or 10, 15 or wt 15 %, wt and %, the and reaction the reaction time was 30, 60, 90, 120, 180, or 1380 min. timeCatalysts was 2017 30, 60,, 7, 273 90, 120, 180, or 1380 min. 11 of 15

50.66

60.00

40.00 12.45 4.88 7.19 17.01 1.01 2.66 Yield of p-cymene (mol %) (mol Yield of p-cymene 20.00 0.28 0.54 0.85 1.22 1.79 10.38 15 wt % 0.18 0.27 0.38 0.52 0.85 10 wt % 5 wt % 0.00 30 60 90 120 180 1380 Content of catalyst Content of Reaction time (min) 5 wt % 10 wt % 15 wt %

FigureFigure 8. Inﬂuence 8. Influence of the of amount the amount of the of the catalyst catalyst and and the the reaction reaction timetime on on the the yield yield of ofp-cymene, p-cymene, for for the the reaction carried out at 160 °C. The amount of the catalyst was 5, 10, or 15 wt %, and the reaction reaction carried out at 160 ◦C. The amount of the catalyst was 5, 10, or 15 wt %, and the reaction time time was 30, 60, 90, 120, 180, or 1380 min. was 30, 60, 90, 120, 180, or 1380 min. A comparison of the yields of products depending on the conditions in which the performed Awere comparison obtained ofshows the yieldsthat of ofall productsthe isomers depending of limonene on the the highest conditions yields of in α which-terpinene the were performed were obtainedobtained. showsγ-Terpinene that and of allterpinolene the isomers yields ofwere limonene usually lower the highest than α-terpinene. yields of Thisα-terpinene indicates were that α-terpinene was the preferred product for a reaction time of 180 min, and prolongation of the obtained. γ-Terpinene and terpinolene yields were usually lower than α-terpinene. This indicates that reaction time caused a transformation of the products of limonene isomerization to p-cymene. α-terpinene was the preferred product for a reaction time of 180 min, and prolongation of the reaction time caused3. Experimental a transformation of the products of limonene isomerization to p-cymene.

3.1. Raw Materials For the synthesis of Ti-SBA-15 catalyst and the isomerization of limonene, the following raw materials were used: R-(+)-limonene (>93%, Sigma, St. Louis, MO, USA), hydrochloric acid (HCl, 37%, Hartim, Tyrol Austria), Pluronic P123 (Aldrich, MW = 5800, St. Louis, MO, USA) as a template, tetraethyl o-silicate (TEOS, 98%, Aldrich, St. Louis, MO, USA) as a silica source, and tetraisopropyl o-titanate (TiPOT, >98%, Merc, Schuchardt OHG Hohennbrunn) as a titanium source.

Catalysts 2017, 7, 273 11 of 14

3. Experimental

3.2. The Synthesis of the Ti-SBA-15 Catalyst The Ti-SBA-15 catalyst was synthesized by the direct method described by Berube et al. [35]. In this synthesis of the Ti-SBA-15, a Pluronic P123 triblock copolymer was used as the structure-directing agent, whereas the source of silica was o-tetraethyl silicate (TEOS). Ti-SBA-15 was obtained by the dissolving of the triblock copolymer in 37% hydrochloric acid at a temperature of 35 ◦C (with constant stirring). A silica source and tertiary isopropyl o-titanate were then added (the Si/Ti molar ratio amounted to 40). The receiving mixture was stirred for 24 h, maintaining a temperature of 35 ◦C. After this time, the reaction mixture was transferred to an autoclave, where it was kept for another 24 h at a temperature of 35 ◦C. The resulting precipitate was filtered off and then washed with deionized water and methanol on a filter. After drying at a temperature of 100 ◦C for 24 h, the product was calcined for 5 h at 550 ◦C. As a result, Ti-SBA-15 catalyst containing 2.5 wt % of titanium was obtained. A detailed description of this catalyst was presented in our previous work [36]. Additionally, for the obtained Ti-SBA-15 catalyst, ICP-AES analysis was performed and textural properties were also determined. For ICP-AES analysis, samples were dissolved by applying a four step procedure utilizing H2SO4, HF, HNO3, and H3PO4 in sequence. The mixtures were treated by acids in a microwave oven (Milestone MLS 1200 Mega, Milestone, Sorisole, Italy) with a power of 150–650 W (20 min for each step). ICP-AES analysis showed that the content of Ti in the catalysts amounted to 2.53 wt %. Textural properties were determined on the basis of nitrogen sorption at −196 ◦C (QUADRASORB evoTM Gas Sorption Surface Area and Pore Size Analyzer, Quantachrome Instruments, Boynton Beach, FL, USA). Before sorption measurements, all samples were outgassed at 250 ◦C for at least 20 h. The specific surface area (SBET) was calculated on the basis of the Brunauer–Emmett–Teller (SBET) equation and the multi-point method. The relative pressure range was selected on the basis of the linear section of the BET plot, i.e., in the relative pressure range (0.05–0.2). Pore volume distribution, micropore volume (Vmicro), and mean pore diameter (dm) were calculated by the DTF method. The total pore volume (Vtot) was obtained from the nitrogen volume, adsorbed at a relative pressure of 0.98. The studies on textural properties of the Ti-SBA-15 catalyst showed that this catalyst is characterized by the following textural 2 3 3 properties: SBET = 731 m /g, Vtot = 0.585 cm /g, Vmicro = 0.091 cm /g, and dm = 5.32 nm.

3.3. The Method of the Isomerization of Limonene over the Ti-SBA-15 Catalyst The studies on the inﬂuence of reaction time, temperature, and Ti-SBA-15 catalyst content on the course of limonene isomerization were carried out in a glass reactor with a capacity of 25 cm3. The reactor was equipped with a reﬂux condenser and a magnetic stirrer with a heating function. In the glass reactor were introduced: 5.00 g of limonene and 0.25 g, 0.5 g, or 0.75 g of the catalyst. The reactor was placed in the paw and then immersed in an oil bath before stirring was started (500 rpm). Limonene isomerization was tested at the temperatures of 160 ◦C, 150 ◦C, and 140 ◦C. Samples of the reaction mixture for analysis were taken at the following intervals of time: 30 min, 60 min, 90 min, 120 min, 180 min, and 1380 min. To separate the catalyst from the sample, the reaction mixture was centrifuged in a centrifuge. Catalysts 2017, 7, 273 12 of 14

3.4. The Identification Products by the Gas Chromatography Method The qualitative and quantitative analyses of the post-reaction mixtures were made by a gas chromatography method (GC), for which the FOCUS apparatus from Thermo (Waltham, MA, USA) was used. This apparatus is equipped with a flame-ionization detector (FID), a capillary column ZEBRON ZB-WAXplus type (0.32 mm × 30 m × 0.5 µm, filled with polyethylene glycol), and an autosampler. The parameters of the analyses were as follows: helium pressure 60 kPa, temperature of the sample chamber 240 ◦C, detector temperature 250 ◦C, ad thermostat temperature raised according to the following program: isothermally to 60 ◦C for 2 min, increase of temperature at a rate of 10 ◦C/min to 240 ◦C, isothermally to 240 ◦C for 4 min, and finally cooling to 60 ◦C. The samples for GC analysis were prepared in this way: first, the catalyst was separated from the post-reaction mixture with the help of a centrifuge; next, 0.250 g of the obtained solution was diluted with 0.750 g of acetone. The quantitative analyses were made with help of the external standard method.

4. Conclusions The Ti-SBA-15 catalyst enabled isomerization of limonene to several major products with a lot of applications. By selecting the appropriate reaction time, we can inﬂuence the formation of the appropriate products. An increase in the amount of catalyst used causes an increase in the yield of the obtained products and shortens the reaction time needed for obtaining appropriate products. It is worth noting that with the increase in the content of p-cymene in the reaction mixture, a decrease in the content of α-terpinene and γ-terpinene is observed. This conﬁrms the limonene isomerization mechanism that was shown in this work. Due to the low cost of limonene and the possibility of obtaining this substrate from waste orange peels, the limonene isomerization process is a useful way of obtaining terpinolene, α-terpinene, γ-terpinene, and p-cymene. This use of limonene is not often described in the literature because limonene is mainly oxidized to its oxygenate derivatives such as 1,2-epoxylimonene, perillyl alcohol, carvone, and carveol. The process of limonene isomerization does not require an oxidizing agent; it only used a titanium silicate catalyst, thus is cheaper than the process of limonene oxidation. The next advantage is lack of solvent, because isomerization can be performed without any solvent. The proposed method of limonene transformation into valuable compounds is an example of an ecofriendly process that reduces the cost of obtaining valuable organic reactants and allows us to utilize waste in the form of orange peels. It is also worth emphasizing that orange peels are a renewable source (biomass) of natural limonene.

Author Contributions: Monika Retajczyk performed the isomerizations in a laboratory. She prepared the GC analyses and took part in calculating the main functions describing the studied process and in the interpretation of data. She prepared the first, preliminary version of this manuscript. Agnieszka Wróblewska prepared the conception of isomerization of limonene over selected titanium silicate catalysts and designed the work. She took part in calculating the main functions describing the studied process on the basis of GC analyses and made a substantial contribution to the interpretation of data and selecting the appropriate parameters for the next stages of the study. She corrected the preliminary version of this manuscript, including the introduction, discussion of results, and conclusions. She prepared the final version of this manuscript and corrected the English. Conflicts of Interest: The authors declare no conflict of interest.

Abbreviations The following abbreviations are used in this manuscript:

Ti-SBA-15 Santa Barbara Amorphous-15 containing Ti Ti-MCM-41 Mobil Catalytic Material 41 containing Ti wt % weight percentage Vmicro micropore volume dm mean pore diameter Vtot total pore volume Catalysts 2017, 7, 273 13 of 14

References

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