Research articleSingle stage upgrading with the help of bifunctional catalysis of Pt supported on solid acid for converting product oil of triglyceride thermal cracking into drop-in fuel
Introduction
Facing threat to global climate change due to CO2 emission in modern epoch has driven many researchers to make their efforts towards replacing fossil-based energy with renewable one. If the perfect replacement of energy is achieved, it will be possible that atmospheric CO2 keep the current level (ca. 415 ppm) [1]. One of the approaches for the perfect replacement is to promote utilization of biomass as the alternative to the petrochemical feedstocks.
Decomposition of triglyceride ester bond is one of the potential technologies to produce drop-in fuel from biomass. It seems that industrially feasible technologies to decompose the ester bond are the catalytic hydrotreatment and the catalytic cracking, which had been utilized for existing petroleum refinery [[2], [3], [4], [5]]. The catalytic hydrotreatment is useful in eliminating sulfuric compounds from petroleum oil and NiMo sulfides supported on γ-alumina is employed as the catalyst. For applying the catalytic hydrotreatment to the ester bond decomposition, oxidative deactivation of the catalyst and leaching of sulfur into the product oil should be resolved [6]. The catalytic cracking could convert low-volatile fraction of the crude oil into gasoline fraction with the help of an acidic Y zeolite. However, a large amount of gaseous hydrocarbons are inevitably by-produced due to the high reaction temperature above 773 K [5]. As well as these catalytic technologies, non-catalytic thermal cracking could decompose the ester bond [7]. A major advantage of the thermal cracking is cost-reasonability caused by the simple process to construct. For the early research works, a variety of edible oils were employed for the thermal cracking test as a source of diesel-like fuel [[8], [9], [10]]. For instance, Schwab et al. showed in their research paper that product oil from the thermal cracking of soybean oil consisted of alkane, alkene and fatty acid, whose boiling ranges corresponded to diesel fraction [8]. Then, much attention was directed to oily waste as the source matter because edible oil not only is the costly feedstock but also brings about a competition with foods. For a research work by Itoh et al., 30% of hydrocarbons and 40% of fatty acids were yielded from waste cooking oil by its thermal cracking operated at 693 K [11]. Also, some researchers were interested in oil dregs and crops cake, which are waste from plant oil production [12,13]. Smets et al. reported that 35.7% of oil fraction was obtained from rapeseed cake by cracking it under inert atmosphere at 823 K for 30 min [11]. Recently, there are some research papers studying application of the thermal cracking into microalgae processing for production of drop-in fuel [[14], [15], [16]]. Maddi et al., described that oil fraction was extracted from oleaginous Chlorella sp. and Scenedesmus sp. by processing them with the simultaneous cracking-distillation technology termed as “Pyrolytic fractionation” [15]. It seemed that oil fraction produced from the algae cracking was not contaminated with nitrogen compounds from protein and oxygenates from carbohydrate.
However, for achieving industrial production of drop-in fuel by the triglyceride thermal cracking, its product oil must be upgraded with the help of catalytic reaction. One of the major components is straight-chain hydrocarbon that prevents the cold flow properties from meeting quality standard for diesel or aviation use [17]. Also, a large amount of alkenes contained in the product oil degrade the preservation stability. Moreover, it is hard to achieve the perfect decomposition of the ester bond because the triglyceride thermal cracking is operated with taking minimum by-production of gaseous hydrocarbons into consideration. Consequently, isomerization of the straight chain skeleton, hydrogenation of the double bond and decomposition of carboxyl group residual in fatty acids are key reactions for upgrading product oil of the triglyceride thermal cracking into drop-in fuel. These key reactions should be performed at the single stage without varying the operating condition, for the cost-reasonable upgrading.
For the present research work, the autoclave reaction tests to investigate catalysis of Pt supported on solid acid for model reactions of the single stage upgrading of the product oil from the triglyceride thermal cracking were carried out. Since one of the key reactions for the single stage upgrading is the alkane isomerization, Pt/solid acid was selected as a candidate for the catalyst. For the alkane isomerization, Pt catalyzes the primary dehydrogenation and the final hydrogenation, and solid acid protonates the olefinic intermediate into carbenium ion departing for its skeletal transformation. Thus, the model reaction primarily carried out is the catalytic isomerization of tetradecane. Experimental data from the primarily model reactions appreciated an upper limitation of the reaction temperature. According to some research paper discussing a reaction mechanism of the isomerization, it seems that the produced isomers are cracked undesirably to small molecules at the high reaction temperature [18,19]. Additionally, an effect of the initially charged hydrogen pressure on the tetradecane isomerization was examined because it is possible that the excess hydrogen supply inhibits the initial dehydrogenation step. Also, Y zeolite, mordenite and AlSBA-15 were used as the supporting material in order to examine effects of their acidity on the tetradecane isomerization. Then, catalytic hydroisomerization of 1-tetradecene was verified at the limited temperature and under. Finally, a mixture of 1-tetradecene and oleic acid as a model product oil of the triglyceride thermal cracking was employed for the autoclave test reaction. According to a review paper by Nakagawa et al., the metal-acid bifunctional catalysis required for isomerizing the straight chain alkanes could accelerate decomposition of carboxylic structure in fatty acids [20]. To the best of our knowledge, the present work is the first successful result for catalyzing simultaneously all the reactions required for upgrading the oil produced by thermal cracking of triglyceride.
Section snippets
Catalyst preparation
Commercially available Y zeolite and mordenite were employed as the supporting material without any pretreatment, because their protonation was completed as-received. Table S1 lists properties of the employed zeolites, in “Supplemental information”. Mesoporous silica SBA-15 was prepared in our laboratory, with referring a research paper by Xie and Fan [21]. Figs. S1 and S2 show properties of the prepared SBA-15, in “Supplemental Information”. The prepared SBA-15 was incorporated with Al at the
Operating condition for single stage upgrading
Isomerization of straight-chain alkane is one of key reactions for the single stage upgrading of product oil of triglyceride thermal cracking. According to some previous works for examining a reaction mechanism of the alkane isomerization catalyzed by Pt supported on solid acid [18,19], skeletal transformation of olefinic intermediate could be repeated three times, as shown in Fig. 2. According to the isomerization route, multi-branched isomers are cracked terminally into small molecules.
Conclusion
For studying single stage upgrading with the help of bifunctional catalysis of Pt/solid acid for product oil of the triglyceride thermal cracking, the model product oil consisting of 1-tetradecene and oleic acid was employed for autoclave test reaction. While hydroisomerization of 1-tetradecene and decomposition of carboxylic structure in oleic acid occurred simultaneously at 553 K, decomposition of the carboxylic structure gave priority to hydroisomerization of 1-tetradecene. Furthermore,
CRediT authorship contribution statement
Masato Kouzu: Conceptualization, Methodology, Writing - review & editing, Supervision. Tatsuhiko Kuwako: Investigation, Writing - original draft. Yoshimi Ohto: Investigation, Writing - original draft. Kensuke Suzuki: Investigation, Writing - original draft. Minato Kojima: Investigation, Validation.
Declaration of competing interest
To the best of our knowledge, our work described in the submitted manuscript is the first successful result for catalyzing simultaneously all the reactions required for upgrading the oil produced by thermal cracking of triglyceride into drop-in fuel.
Acknowledgement
The authors are thankful Dr. Shin-ya Yamanaka, who is an associate professor of Muroran Institute of Technology, for his support to measure the porosity of SBA-15. Also, we would like add a comment that characterization of SBA-15 by XRD and TEM was carried out in Nano-technology Research Center of Tokyo City University.
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