Catalytic Depolymerisation of Rubber

Transcription

1 Catalytic Depolymerisation of Rubber Stephen Boothroyd, Rebecca Smith, Ezat Khosravi, Richard Thompson Department of Chemistry Durham University

2 Contents Motivation Metathesis Chemistry Basics Breaking down polybutadienes by Cross-Metathesis Test reaction on linear PB Test reaction model cross-linked PB Trials on SBR Too good to be true? Conclusions and outlook

3 Why? Main use of rubber is in tyres Millions of tyres are stockpiled each year due to difficulties in disposing of them Difficulties due to the high crosslink density Huge environmental pressure to make tyres reprocessable without compromising on safety, durability, energy efficiency

4 The idea Rubber contains double bonds which are available for further reaction n Use a cross metathesis reaction to break down the rubber network by reacting it with a small olefin

5 Cross Metathesis Rearranges carbon-carbon double bonds Grubbs Catalyst - ruthenium based metal centre [M] Reaction proceeds via a metallocyclobutane intermediate Number of chain ends is conserved!

6 Grubbs ruthenium catalysts Most widely used for metathesis reactions High functional group tolerance Low sensitivity to air and moisture scaleable! 2 nd generation is more active than the 1 st due to the N heterocyclic (NHC) ligand 1 st generation 2 nd generation

7 Experimental Approach To carry out cross metathesis reactions on models of increasing complexity/realism: 1. Linear 1,4-polybutadiene 2. Cross-linked 1,4-polybutadiene 3. Tyre rubber (SBR sheet)

8 Polybutadiene (PBD) Polymerise butadiene by 1,2- or 1,4-addition 1,4- PBD has the double bond in the polymer chain Model material contains a mixture of repeat units. 1,2-PBD cis 1,4-PBD trans 1,4-PBD Average M w 280,000 92% 1,4; 8%, 1,2PBD 36% cis 1,4 linkages

9 Method (E. Khosravi, R.F. Smith) Dissolve PBD and diester-olefin in DCM Grubbs ruthenium catalyst (1 mol% per repeat unit) Stir for 24 hours 2 ml ethyl vinyl ether added and stirred for another 2 hours Precipitate in methanol Filter solution Remove solvent from filtrate & dry

10 Test reaction on linear PBD: results Linear PBD can be broken down using the cross metathesis reaction + diester olefin Changing the diester olefin has little effect- used dimethyl fumarate and dimethyl maleate in reactions +? PBD can be broken down without any diester olefin +?

11 Test reaction on Model Cross-linked PBD Cross-link PBD with benzoyl peroxide Generate elastic network (rubber behaviour) Attempt catalysed breakdown Crosslinked Linear PBD G, G / Pa Breakdown Products / rad s -1

12 Catalysed breakdown of model rubber G1 + 1% DM diester Original PBD G1 only Mw / kg.mol-1 / rad.s-1 Rheology: Transform elastic solid to viscous liquid GPC: Massive reduction in molecular weight (200 kg/mol to 2 kg/mol) Both analyses: diester olefin has little effect on breakdown

13 NMR Chemical Analysis of Breakdown Products trans cis cis trans PBD trans cis cis trans Cross-linked PBD and 1 st generation Grubbs catalyst

14 SBR Styrene-butadiene rubber* Copolymer of styrene and butadiene Used in manufacturing tyres 1,4-PBD n Grubbs 2 nd generation catalyst *Sample provided by Stephen Millington, ARTIS

15 SBR Reacted SBR with dimethyl maleate and Grubbs 2 nd generation catalyst at 40 C Before reaction After ½ hour After 24 hours After reaction SBR in DCM (control), 24 hours at 40 C Straight after reaction After drying Before reaction After 24 hours After reaction SBR broken down to rubber crumb by CM Process also yields some soluble PBD of low mol wt.

16 Mass Loss Resulting following heptane swelling test on SBR Sample % weight loss Error SBR G2 10% DM 40 C 25 4 G2 10% DM 25 C G2 10% DF G2, no diester

17 Checking for catalyst in rubber 100 Ru L K Fe Zn SBR sheet fit 20HS fit Br Yield 10 Ru K X-ray Energy / kev PIXE Elemental analysis Found 1 Grubbs catalyst : 700 butadiene repeat units

18 Catalysts work too well? Catalysts necessary, but diester olefin optional why? Reaction must conserve chain ends Planned reaction Loop formation CM with 1,2 (vinyl) groups So not too good to be true making molecular lassos?

19 Conclusions Cross metathesis reaction can break down PBD to oligomers. SBR can be broken down using Grubbs catalyst and diester Main product = rubber crumb Also get ~15% PBD oligomer CM reaction has potential to be used to recycle rubber

20 Outlook / Future Directions Very promising route to PBD oligomer and/or crumb from rubber Need to optimise in a greener solvent Repolymerise and establish reprocessed rubber properties. Commercialise process industrial partners wanted! Smith, Boothroyd, Khosravi, Thompson, Green Chemistry. (ASAP) DOI: /C5GC03075G Free Access Until 11/5/16

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22 Rheology Deformation and flow behaviour Frequency sweeps, 20 C Gʹ=storage modulus, Gʺ= loss modulus 6 Cross-linked PBD log G ʹ 5 4 Gʹ/Gʺ 3 (Pa) 2 Cross-linked PBD log Gʺ Product log G ʹ log Ang. Freq (rad/s) Product log Gʺ

23 Gel permeation chromatography (GPC) Linear PBD before reaction After reaction Cross-linked PBD product Linear PBD product Retention volume/ ml

24 MALDI Cross-linked PBD has been broken down using the cross metathesis reaction

25 Reaction Reference PBD grade/ molecular weight(kda) Diester Type Grubbs Catalyst generation Double bond ratio of polymer:diester Outcomes DF 1 st 3:1 Major breakdown of PBD, mostly soluble in methanol DF 2 nd 3:1 Major breakdown of PBD, completely soluble in methanol st 1:0 Major breakdown of PBD but with large PDI DF 2 nd 3:1 Major breakdown of PBD, heating had no effect on the product Reaction Reference PBD grade/ molecular weight(kda) Diester Type Grubbs Catalyst generation Double bond ratio of polymer:diester Outcomes DF 1 st 3:1 Major breakdown of PBD, completely soluble in methanol DF 2 nd 3:1 Major breakdown of PBD, completely soluble in methanol DF 1 st 100:1 Major breakdown of PBD, mostly soluble in methanol DF 2 nd 100:1 Major breakdown of PBD, mostly soluble in methanol DF 1 st 200:1 Major breakdown of PBD, little difference to DM 1 st 3:1 Major breakdown of PBD, mostly soluble in methanol DM 2 nd 3:1 Major breakdown of PBD, mostly soluble in methanol st 1:0 Major breakdown of PBD, completely soluble 13 Cross-linked DF 1 st 100:1 Broken down to same extent as linear PBD 14 Cross-linked DM 1 st 100:1 Broken down to same extent as linear PBD 15 Cross-linked - 1 st 1:0 Broken down to same extent as linear PBD 16RT Rubber crumb DM 1 st 10:1 Appeared unchanged, Ru chemically bonded 16H Rubber crumb DM 1 st 10:1 Appeared unchanged, Ru chemically bonded 17RT Rubber crumb - - 1:0 Appeared unchanged 17H Rubber crumb - - 1:0 Appeared unchanged 18RT Rubber crumb DM 1 st 10:1 Appeared unchanged, Ru chemically bonded 18H Rubber crumb DM 1 st 10:1 Appeared unchanged, Ru chemically bonded 19 Rubber crumb DM 2 nd 10:1 Swelling tests show large weight loss 20RT SBR DM 2 nd 10:1 Crumb product 20H SBR DM 2 nd 10:1 Crumb product large weight loss in swelling test 21RT SBR - - 1:0 Swelled during reaction 21H SBR - - 1:0 Swelled during reaction 22 SBR DF 2 nd 10:1 Crumb product, Ru chemically bonded, large wt loss in swelling test 23 SBR - 2 nd 1:0 Crumb product DM 1 st 3:1 Completely broken down within 2 hrs DM 2 nd 3:1 Almost completely broken down within 2 hrs