《工业催化原理-英文课件Chapter21》由会员分享，可在线阅读，更多相关《工业催化原理-英文课件Chapter21（31页珍藏版）》请在金锄头文库上搜索。
1、Chapter 2Homogeneous catalysis,The catalyzed reaction which occur in a single phase such as liquid, solid, or gaseous, are termed homogeneous. Homogeneous catalyzed reaction especially refers to a reaction in liquid solution. It is popularly used in petrochemical engineering processes such as polyme
2、r synthesis and in biochemical processes.,Attractive features,high activity and high selectivity feasible to study reaction mechanism and kinetics important in preparing fine chemicals essential in biochemical system,Important homogeneous catalysts,Acid-Base catalyst Transition metal complexes catal
3、yst Enzyme catalyst Ionic liquid catalyst,2.1 Acid-Base catalysis,2.1.1 Definition and property Types of acid and base Arrhenius acid and base Brsted acid and base Lewis acid and base,2.1.1 Definition and property,Arrhenius acid and base HCl H+ + Cl- acid NaOH Na+ + OH- base In aqueous solution thos
4、e can be ionized to give positive hydrogen ions are termed acid while others give negative hydroxide ions refer to base.,2.1.1 Definition and property,Brsted acid and base NH4+(acid) NH3(base) + H+ B acid: substance which can release positive hydrogen ions (proton) or called as proton donor B base:
5、proton acceptor,2.1.1 Definition and property,Lewis acid and base F H F H F:B + :N:H F:B:N:H F H F H L acid: acceptor of electron pairAlCl3, SO3, Cu2+, H+ . L base: donor of electron pair OH-, NH3, C5H5N . Both Brsted and Lewis acid (base) are together termed as generalized acid (base).,2.1.1 Defini
6、tion and property,Strength of acid (or base) The strength of acid (base) is a representation of the ability of donating (accepting) proton for B acid (base) or the ability of accepting (donating) electron pair for L acid (base).,2.1.1 Definition and property,Hammett Index (哈明指数) H+ + B BH+ (acid) (i
7、ndicator, basic) (conjugate acid) Dissociate constant of BH+ Ka = aH+aB / aBH+ = aH+CBB / CBH+BH+ pKa = -lg Ka = -lg (aH+B / BH+) - lg (CB/CBH+) = H - lg (CB/CBH+),2.1.1 Definition and property,H = -lg (aH+B / BH+) Here H is defined as Hammett index to represent the magnitude of acid strength, a and
8、 are activity and activity coefficient respectively.,2.1.1 Definition and property,H+ + B BH+ For an indicator reacts with different acids, pKa is constant at a certain temperature. pKa = H - lg (CB/CBH+) = H + lg (CBH+ /CB) If CBH+ become larger then H become smaller or goes to more negative. This
9、means stronger ability of donating proton therefore the higher acid strength.,2.1.1 Definition and property,Since H = -lg (aH+B / BH+) In diluted acid solution, B = BH+ and aH+ = CH+ therefore H = -lg CH+ = pH,2.1.1 Definition and property,H+ + B BH+ Indicator B (a basic) will change its color when
10、it become BH+ in an acidic environment. At the turning point of an indicator (color has just changed) CB = CBH+ H = pKa By choosing different indicators, The acid strength can be measured according to above relationship.,2.1.1 Definition and property,Some indicators and its pKa values Neutral red 中性
11、红 6.8 yellow red Methyl red 甲基红 4.8 red yellow Dimethyl yellow 二甲基黄 3.3 yellow red Crystal violet 结晶紫 0.8 blue yellow Nitroaniline 苯胺 -0.29 brown violet Anthraquinone 蒽醌 -8.27 blue yellow,2.1.2 Reaction mechanism of acid-base catalysis,Example 1: acid catalyzed hydrolysis of ester R- COO - R + H2O R
12、COOH + ROH Example 2: ethanol dehydration to ethylene C2H5OH C2H4 + H2O,Catalyzed mechanism of ester hydrolysis,Proton tranfer H R-CO-OR + H3O+ R-CO-O+-R + H2O water molecule attack to form intermediate H H R-CO-O+-R + H2O R-C+OO-R R-CO-O+H2 + ROH O (slow rate-determining step) H H proton release R-
13、CO-O+H2 R-COOH + H+,Mechanism of ethanol dehydration,Proton tranfer H CH3-CH2-OH + H3O+ CH3-CH2-O+H + H2O water molecule removed H CH3-CH2-O+H CH3-C+H2 + H2O proton release (recovered) CH3-C+H2 H2C=CH2 + H+,Carbon Cation Mechanism,In above two examples, both reactions involve forming a positive carb
14、on ion intermediate in reaction steps so that termed as carbon cation mechanism The proton catalyst (H+) is recovered after the catalyzed circle and has not been consumed,2.1.3 Reaction kinetics of acid-base catalysis,Reaction catalyzed by acid and/or base A P rate equation -dA/dt = k0CA+ kH+CH+CA +
15、 kOH-COH-CA = (k0+kH+CH+kOH-COH-) CA = kCA overall rate constance k = k0+kH+CH+kOH-COH-,determining catalytic rate constant,case A: in acidic solution H+ OH- (for instance H+ = 0.1 N) k kH+CH+lg k = lg kH+ - pH lg k pH Slope equals 1, intercept is lg kH+,determining catalytic rate constant,case B: i
16、n basic solution OH- = kw / H+ = 10-14 / H+ k kOH-COH- lg k = lg kOH-+ lg kw + pH = lg kOH-14 + pH lg k pH Slope equals +1, intercept is lg kOH-14,determining catalytic rate constant,In the whole pH range lg k lg kH+ lg k0 pH lg kOH-14 Catalytic rate constant kH+ or kOH- can be determined from the intercept in the curve of lgk vs pH,
