KAJIAN REAKSI SIKLISASI DAN HIDROGENASI (R)-(+)-SITRONELAL MENGGUNAKAN KATALIS BERBASIS ZnBr2/γ-Al2O3 dan Ni/ γ-Al2O3 (STUDY ON CYCLISATION AND HIDROGENATION OF (R)-(+)-SITRONELAL BASED ON ZnBr2/γ-Al2O3 dan Ni/ γ-Al2O3 CATALYSTS)
ELVINA DHIAUL IFTITAH, S.SI, M.SI, Prof.(ret). Dr. Muchalal, DEA,
2012 | Disertasi | S3 Ilmu KimiaHasil penelitian menunjukkan bahwa sitronelal hasil isolasi minyak sereh (Cimbopogon winterianus Java Type) dari Majingklak, Cilacap adalah senyawa enantiomer (R)-(+)-sitronelal. Kristalinitas, keasaman dan kandungan logam, luas permukaan spesifik dan porositas serta morfologi katalis ZnBr 2/γ-Al2O3 dan Ni/ γ-Al2O3 Oleh : Elvina Dhiaul Iftitah 07/263101/SPA/00175 Mentol dapat disintesis melalui dua tahap reaksi berurutan yaitu siklisasi (isomerisasi) selektif terhadap 3R-(+)-3,7-dimetil-6-okten-al ((R)-(+)-sitronelal) dengan katalis asam Lewis ZnBr2 menjadi (1S,2S,5R)-(-)-2-isopropenil-5-metilsikloheksanol ((-)-isopulegol) dan hidrogenasi (-)-isopulegol dengan katalis logam Ni menjadi (1S,2S,5R)-(-)-2-isopropil-5-metil-sikloheksanol ((-)-mentol). Hal menarik dalam sintesis isopulegol dan mentol adalah stereoselektivitas produk isopulegol dan mentol. Selektivitas reaksi terhadap produk dapat ditingkatkan dengan pemilihan katalis yang sesuai. Pemilihan katalis berbasis ZnBr2/γ-Al2O3 dan Ni/γ-Al2O3 diharapkan mampu menghasilkan produk (-)-mentol dari (R)-(+)-sitronelal melalui satu tahap reaksi dengan keberadaan bersama-sama situs asam dan logam yang stereoselektif. Tujuan utama penelitian ini adalah mengkaji stereoselektivitas dan mekanisme reaksi siklisasi dan hidrogenasi (R)-(+)-sitronelal dengan katalis berbasis ZnBr2/γ-Al2O3 dan Ni/γ-Al2O3. Penelitian dirancang sebagai penelitian eksperimental yang didukung dengan kajian teoritis kimia komputasi. Tahapan eksperimen di laboratorium meliputi isolasi (R)-(+)-sitronelal, preparasi dan karakterisasi katalis ZnBr2/γ-Al2O3, Ni/γ-Al2O3, Ni/ZnBr2/γ-Al2O3 dan ZnBr2/Ni/γ-Al2O3, serta uji aktivitas dan selektivitas katalis. Parameter yang dipelajari adalah pengaruh berbagai variasi jumlah penambahan senyawa prekursor, temperatur kalsinasi dan reduksi katalis, urutan pengembanan senyawa prekursor dalam katalis, jenis aliran/atmosfir gas, serta temperatur dan durasi reaksi katalitik terhadap aktivitas dan selektivitas reaksi siklisasi dan/atau hidrogenasi (R)-(+)-sitronelal. Mekanisme reaksi siklisasi dan hidrogenasi juga dipelajari baik tanpa maupun dengan menggunakan katalis. Kajian teoritis dilakukan dengan melakukan pemodelan (R)-(+)-sitronelal dan produk siklisasinya terhadap permukaan katalis γ-Al2O3 menggunakan metode perhitungan DFT/B3PW91/3-21-G. Kajian ini diharapkan dapat memberikan gambaran interaksi yang terjadi antara (R)-(+)-sitronelal dan produk siklisasinya dengan situs aktif di permukaan katalis. 2/γ-Al2O3, Ni/γ-Al2O3, Ni/ZnBr2/γ-Al2O3 dan ZnBr2/Ni/γ-Al2O3 dipengaruhi oleh variasi komposisi jumlah senyawa prekursor aktif yang ditambahkan, temperatur kalsinasi dan reduksi serta urutan penambahan senyawa prekursor aktif ke dalam katalis pengemban. Pemakaian katalis ganda ZnBr2/γ-Al2O3 dan Ni/γ-Al2O3 dalam sistim satu batch reaksi dengan perbandingan 1:1 memiliki selektivitas dan aktivitas lebih baik daripada pemakaian katalis ZnBr2/γ-Al2O3 dan Ni/γ-Al2O3 secara tunggal dalam dua tahap reaksi. Produk (-)-isopulegol dan (-)-mentol yang dihasilkan sebesar 37,81% dan 12,24% dengan produk samping sitronelol 20,58%. Sitronelol terbentuk sebagai produk hidrogenasi langsung (R)-(+)-sitronelal oleh peranan adanya gas H2 yang telah dialirkan pada jam ke-2 dan berlangsung selama 5 jam. Reaksi satu tahap siklisasi dan hidrogenasi sitronelal dengan katalis dwifungsi Ni-ZnBr2/γ-Al2O3 menghasilkan (-)- isopulegol 53,29% dan (-)-mentol 6,18%. Sedangkan reaksi satu tahap hidrogenasi langsung sitronelal dengan katalis tunggal Ni/γ-Al2O3 menghasilkan (-)-isopulegol 12,30% dan sitronelol 3,42%. Studi terhadap mekanisme reaksi siklisasi (R)-(+)-sitronelal di permukaan katalis γ-Al2O3 diperoleh bahwa reaksi berlangsung dengan mekanisme serentak (concerted reaction) melalui pembentukan senyawa transisi tanpa melalui pembentukan senyawa antara. Berdasarkan kajian teoritis dengan metode DFT/B3PW91/3-21-G diperoleh hasil bahwa stabilitas dan energi interaksi isomer (-)-isopulegol dengan situs aktif di permukaan γ-Al2O3 lebih rendah daripada interaksi dengan isomer isopulegol lainnya. Hal ini memberikan pengertian bahwa (-)-isopulegol teradsorpsi dan berinteraksi lebih kuat dengan permukaan γ-Al2O3.
The results showed that the citronella from citronella plant oils (Cimbopogon winterianus Java Type) from Majingklak, Cilacap is 2/γ-Al2O3 dan Ni/ γ-Al2O3 CATALYSTS By Elvina Dhiaul Iftitah 07/263101/SPA/00175 Menthol can be synthesized through two consecutive reaction steps, selective cyclization (isomerisation) of 3R-(+)-3,7-dimethyl-6-okten-al ((R)-(+)-sitronelal) with ZnBr2 catalyst to (1S,2S,5R)-(-)-2-isopropenil-5-metilsikloheksanol ((-)-isopulegol) and hydrogenation of (-)-isopulegol with Ni catalyst to (1S,2S,5R)-(-)-2-isopropil-5-metil-sikloheksanol ((-)-mentol). One of the most interesting in the synthesis of isopulegol and menthol is a stereoselectivity of their product. Stereoselectivity can be improved by selection of suitable catalysts. Selected catalyst based on ZnBr2/γ-Al2O3 and Ni/γ-Al2O3 could be expected to obtain (-)-menthol from (R)-(+)-sitronelal in one step reaction. The presence of both acid and metal sites in one batch reaction sistem is the best way for improve the stereoselectivity of reaction product. The primary objective of this research is to examine stereoselectivity and mechanism of cyclization and hydrogenation reaction toward (R)-(+)-sitronelal based on ZnBr2/γ-Al2O3 and Ni/γ-Al2O3 as catalysts. Study was designed as an experimental research study supported with theoretical computational chemistry approach. Experiments in the laboratory includes several stages such as determination of the absolute configuration of citronellal, preparation and characterization of catalysts and catalytic activity test. While the theoretical approach was done by modeling the interaction between (R)-(+)-citronellal and products of its cyclisation with the site active on γ-Al2O3 surface catalyst using DFT calculation methods. Characterization and catalytic activity tests were conducted on a series of single catalyst ZnBr2/γ-Al2O3, Ni/γ-Al2O3, dual catalysts ZnBr2/γ-Al2O3 and Ni/γ-Al2O3, and bifunction catalysts Ni-ZnBr2/γ-Al2O3 and ZnBr2-Ni/γ-Al2O3. Some of parameters which would be evaluated were the influence of a variation of precursor loading, the temperature of catalyst calcination and reduction, the sequence of precursor compound developing in the catalyst, the type of gas flow or gas atmosphere, as well as temperature and duration of catalytic cyclization and/or hydrogenation reaction of (R)-(+)-citronellal toward activity and selectivity of catalysts. Mechanism of cyclization and hydrogenation reactions was studied both without and with catalyst utilization, and supported by modeling the interaction between (R)-(+)-citronellal and cyclisation product with the γ-Al2O3 surface using DFT/B3PW91/3-21-G calculation method. enantiomerically pure compounds with absolute configuration of (R)-(+)-citronellal. Crystallinity, acidity and metal content, specific surface area and porosity and also morphology of the catalyst ZnBr2/γ-Al2O3, Ni/γ-Al2O3, Ni-ZnBr2/γ-Al2O3 and ZnBr2-Ni/γ-Al2O3 were influenced by variations in the composition of added the active precursor compounds, temperature of calcination and reduction as well as the order of addition of the active compound into the catalyst precursor bearers. Two-stage cyclization and hydrogenation reaction of (R)-(+)-citronellal with a single catalyst ZnBr2/γ-Al2O3 (Zn = 9.1%; temperature 150 o C, 5 hours of N2 gas flow) and Ni/γ-Al2O3 (Ni = 13.0% ; temperature 200 ° C, 2 hours of H2 gas flow) produced (-)-isopulegol 9.44% and (-)-menthol 5.05% respectively, meanwhile two-stage cyclization and hydrogenation reaction of (R)-(+)-citronellal in a single batch reaction using dual catalysts ZnBr2/γ-Al2O3 and Ni/γ-Al2O3 (ratio 1:1, temperature 200 ° C, with 1 hour of N2 and 5 hours of H2 gas flow) yield (-)-isopulegol 37.81% and (-)-menthol 12.24% with 20.58% sitronelol as by products. One stage cyclization and hydrogenation reaction of (R)-(+)-citronellal using bifunctional Ni-ZnBr2/γ-Al2O3 catalyst (temperature 120 o C, 4 hours of N2 gas flow and 20 hours of H2 gas flow) can produce (-)-isopulegol 53.29% and (-)-menthol 6.18%. Direct hydrogenation reaction of (R)-(+)-citronellal with Ni/γ-Al2O3 catalyst (temperature 120 °C, 5 hours of H2 gas flow) produced (-)-isopulegol 12.30% and sitronelol 3.42%. Mechanism of cyclization reaction of (R)-(+)-citronellal on γ-Al2O3 catalyst surface was proposed as simultaneous mechanism (concerted reaction) through the transition state without formation of intermediate states. Stereoselectivity of cyclisation reaction into (-)-isopulegol can be determined by calculating the interaction energy of (R)-(+)-citronellal and (-)-isopulegol modelled structures with the surface of γ-Al2O3 catalyst.
Kata Kunci : Katalis tunggal, katalis ganda, katalis dwifungsi, siklisasi, hidrogenasi, reaksi satu tahap, reaksi dua tahap.
