Abstract
Beta沸石具有較大的孔洞及適中的酸性，故可廣泛的被應用於很多石化工業上。直接以水熱法製得的高矽鋁比之Beta沸石，結晶性與結晶產率均不佳。一般可試以脫鋁的方法，由低矽鋁比的樣品為起始物來製作。本研究旨在探討以不同脫鋁方法，包括酸萃取與Si取代等法，試由較低矽鋁比之Beta沸石製得較高矽鋁比者；尋求較適宜的脫鋁條件。再由beta沸石承載Pt後之TPR圖譜，了解脫鋁過程中缺陷位置之生成與消失。此TPR技術亦被用來探討Pt在Beta沸石中可能存在的物種與其分佈。 酸萃取法主要是將沸石置於不同濃度的無機酸水溶液中攪拌三天後，經過濾，水洗，乾燥與焙燒的步驟而得。經酸萃取法處理後，PQ公司生產之beta沸石的矽鋁比值可從11提高至150 以上，並導致沸石酸量減少，強酸座之比例則相對提高。XRD 測試結果顯示，以高至6 M 之硝酸或鹽酸處理，脫鋁後Beta沸石仍能保有其結晶結構，但熱穩定性並不因矽鋁比值之提高而增加。此是因脫鋁後缺陷位置增多之故。溫度對酸處理之效果的影響不甚明顯。脫鋁的程序探討，由酸處理後之樣品中有水合鋁離子與Al(H2O)4(SO4)+之物種存在，可說明其乃經鋁之萃取與水合之步驟。該水合鋁離子可被NaNO3交換出來，顯示其可取代沸石晶格之陽離子，而有平衡沸石結構電荷之作用。 矽取代法是以(NH4)2SiF6水溶液為矽源，在80℃下處理Beta沸石，以進行脫鋁和植矽的反應。此法的脫鋁效果受矽溶液之濃度的影響頗甚。當每克Beta沸石中SiO2添加量高於2.25 mmole時，由XRD可知沸石的結晶結構已受破壞。此是因反應過程中有強酸性的HF生成之故。利用此法將矽植入1 M HNO3處理後之Beta沸石中，矽鋁比可由90提高到127。由NMR光譜可知，在植矽的過程中水合鋁離子會被交換出來，加入之矽則會植入因脫鋁而產生的缺陷位置中。由於矽的填補，植矽前後樣品的結晶度並無明顯的差異。 白金離子以初濕含浸或離子交換法添加於Beta沸石後，由TPR (程溫還原， Temperature-Programmed Reduction)的測試結果可知，其可能存在的物種及其還原溫度分別為：PteO及PteO2 (沸石孔洞外表面之白金氧化物，Tr分別為-50 ℃與20 ℃)，PtoO及PtoO2 (孔洞內白金氧化物，Tr 分別於80 ~ 110 ℃與130 ℃)，平衡晶格電位之Pt4+離子(Pt4+ coordinated to Al-OH，Tr = 250 ℃)與Pt-(-O-Si≡)yn-y物種(和Si-OH鍵結之白金物種，n = 2或4，y = 1 ~ 4，Tr = 400 ~ 550 ℃)等。在不同脫鋁程度之beta沸石承載白金的樣品中，由Pt-(-O-Si≡)yn-y物種的存在與否，可知脫鋁後沸石結構上的變化。即沸石經酸處理後，結構上產生許多具有SiOH基團的缺陷位置，而使添加的白金可嵌入其中，產生Pt-(O-Si≡)yn-y (n = 2或4，y = 1 ~ 4)之物種。植矽後，則因植入之矽補到該hydroxyl nest中，所添加的白金則無法進入此種結構。故於TPR圖譜中看不到Pt-(O-Si≡)yn-y物種被還原的訊號。 由TPR探討製備條件對Beta沸石上之白金物種及其還原特性的影響則發現，(1)不同白金前驅物由於在水中解離出之白金錯離子的電性不同，其進入沸石孔道之難易程度依序為: PtCl4 > Pt(NH3)4(NO3)2 > H2Pt(OH)6。(2)一般而言，白金錯離子較偏好留在沸石之外表面。但提高白金溶液濃度，可使白金錯離子較易進入沸石孔道中。此錯離子一旦進入Beta沸石孔道後，會先和≡Si-OH基團作用，其餘則留在孔道中，再經焙燒後便會形成PtoOx。(3)沸石之陽離子形式不同，對白金物種的還原溫度亦有所影響。陽離子為H+形式時，因有穩定白金粒子的功能，需較高溫度才能使白金物種還原。再氧化效果亦受沸石陽離子不同而異。對Na+形式者而言，在再氧化的過程中，白金物種幾乎不再回到沸石孔道中。而在H+形式之沸石中，則當再氧化溫度為400 ℃時，即可使部分白金物種回到孔道中，而得和新鮮樣品相近的結構。當再氧化溫度為500 ℃時，不論陽離子為何種形式均可見有脫氧的現象。Beta zeolite has been widely used in many petrochemicalindustries due to its high Si/Al ratio, 12-ring pore opening andmoderate acidity. There is an optimum range in Si/Al for gettingBeta zeolite with good crystillinity and high crystal yield bydirectly hydrothermal synthesis. The alternative method forhighly siliceous zeolite Beta is post synthesis. In this work,we studied dealuminations of the Beta zeolite with acid leachingand Si-replacing.Prepared samples were characterized by ICP/AES,MAS NMR, NH3-TPD, XRD, etc. to realize the variation incomposition, acidity and thermal stability. Platinum were thenadded to the parent and dealuminated Beta zeolites. TPR(temperature-pregrammed reduction) technique was used to studythe reduction properties of all Pt/Beta zeolite samples.Acid-leached Beta zeolite were prepared by suspending zeolitepowders in inorganic acid (such as nitric acid, hydrochloricacid and sulfuric acid) solutions and stirred at roomtemperature for three days. Suspended solutions weresubsequently filtrated, washed, dried in 100 ℃ and calcined upto 540 ℃. The Si/Al of Beta zeolite can increase from 11 tohigher than 150 after leached with 4 M HNO3 solution. The acidamount and the weak acid sites decreased as Al content onframework decreased. The thermal stability of Beta zeolitebecome poor after dealumination due to a formation of defectsites. The Al(H2O)63+ species are found occluded in zeolitecavity and act as cations in acid-leached zeolites. Si-replacement of Beta zeolite was performed by stirring asuspension of Beta in (NH4)2SiF6 solution at 80 ℃. Theconcentration of (NH4)2SiF6 solution significantly affected theresulted Si/Al ratio. The zeolite structure damaged severely asthe Si amount added was higher than 2.25 mmole per gram Betazeolite due to a formation of HF during Si-treatment. Si/Alratio of acid-leached Beta zeolite can be increased from 90 to127 by further Si-replacement. In this treatment, the Al(H2O)63+ions were exchanged byNH4+ ions. And the Si added would insertinto the hydroxyl nest of dealuminated Beta zeolite. Itmaintained the crystillinity of dealuminated Beta zeolite due toSi insertion. Platinum ions were dispersed onto the startingand the treated samples through impregnation and/or ion-exchangeof aqueous Pt(NH3)4(NO3)2. Chemical environments of dispersedplatinum were characterized by TPR technique. Five majorsignals, in reduction temperature (Tr) regions around -50, 20,80, 130 and 480 ℃ were distinguished from these TPR traces andcharacterized as reductions of platinum oxides dispersed on theexternal surface of zeolite, PteO (Tr = -50 ℃) and PteO2 (Tr =20 ℃), platinum oxides occluded in channels (PtoO and PtoO2, Tr= 80 and 130 ℃, respectively), and Pt-(-O-Si()yn-y (Tr around480 ℃, n = 2 or 4, y = 1 ~ 4) complexes coordinated to externalsurface or defects of zeolite. From the variation in thecontribution of the 480 ℃ peak in TPR traces, a formation ofSi-OH silanol function groups on the zeolite surface during thedealumination treatment and an elimination of these groupsduring the silicon-insertion treatment are indicated. Thedistribution of platinum was found to vary with the cation form(H+ or Na+) of Beta zeolite, and the type and the concentrationof platinum precursor used in the preparation stage. The Pt(OH)62- anion tended to deposit on the external surface ofzeolite, while cations of PtCl4-x(H2O)xx+ and Pt(NH3)42+preferred to be occluded into zeolite channels and coordinatedto silanol groups on zeolite defects. When the concentration ofplatinum solution was high, Pt(NH3)42+ ions also tended to beoccluded into zeolite channels. The occluded and coordinated Ptsintered onto the external surface upon reduction. Upon areoxidation treatment at 400 ℃, the sintered platinum stayed onthe external surface if the zeolite is in the Na-form, but mightredisperse back into zeolite channels in the H-form zeolite.